Contenido

Debido a las propiedades mecánicas superiores de los nanotubos de carbono, se han propuesto muchas estructuras que van desde artículos cotidianos como ropa y equipamiento deportivo hasta chaquetas de combate y ascensores espaciales.[20]​ Sin embargo, el ascensor espacial requerirá más esfuerzos para perfeccionar la tecnología de nanotubos de carbono, ya que la resistencia práctica a la tracción de los nanotubos de carbono debe mejorar mucho.[21]​.

En perspectiva, ya se han logrado avances sobresalientes. El trabajo pionero dirigido por Ray H. Baughman en el Instituto de Nanotecnología ha demostrado que los nanotubos de paredes simples y múltiples pueden producir materiales con una dureza sin igual en los mundos natural y artificial.[22]​[23]​.

Los nanotubos de carbono también son un material prometedor como bloques de construcción en materiales compuestos jerárquicos dadas sus excepcionales propiedades mecánicas (~1 TPa en módulo y ~100 GPa en resistencia). Los intentos iniciales de incorporar los CNT en estructuras jerárquicas (como hilos, fibras o películas[24]​) han llevado a propiedades mecánicas significativamente más bajas que estos límites potenciales. La integración jerárquica de nanotubos de carbono de pared múltiple y óxidos de metal/metal dentro de una única nanoestructura puede aprovechar la potencialidad de los compuestos de nanotubos de carbono para la descomposición del agua y la electrocatálisis.[25]​ Windle et al. han utilizado un método de hilado de deposición química de vapor (CVD) in situ para producir hilos de CNT continuos a partir de aerogeles de CNT cultivados mediante CVD.[26]​[27]​[28]​ Los hilos de CNT también se pueden fabricar extrayendo paquetes de CNT de un bosque de CNT y retorciéndolos posteriormente para formar la fibra (método de estirar y torcer, vea la imagen a la derecha). El grupo Windle ha fabricado hilos CNT con resistencias de hasta ~9 GPa en longitudes de calibre pequeñas de ~1 mm, sin embargo, se informaron resistencias de solo aproximadamente ~ 1 GPa en la longitud de referencia más larga de 20 milímetros[29]​[30]​ La razón por la cual las resistencias de las fibras han sido bajas en comparación con la resistencia de los CNT individuales se debe a que no se transfirió la carga de manera efectiva a los CNT constituyentes (discontinuos) dentro de la fibra. Una posible vía para paliar este problema es la reticulación covalente entre haces y entre CNT inducida por irradiación (o deposición) para "unir" eficazmente los CNT, con niveles de dosificación más elevados que den lugar a la posibilidad de obtener fibras compuestas de carbono amorfo/nanotubos de carbono.[31]​ Espinosa et al. desarrollaron hilos compuestos de DWNT-polímero de alto rendimiento retorciendo y estirando cintas de haces de DWNT orientados aleatoriamente y recubiertos con compuestos orgánicos poliméricos. Estos hilos de DWNT-polímero mostraron una energía de rotura inusualmente alta, de ~100 J-g-1 (comparable a la de uno de los materiales naturales más resistentes, la seda de araña),[32]​ y una resistencia de hasta ~1,4 GPa.[33]​ Se están realizando esfuerzos para producir compuestos de CNT que incorporen materiales de matriz más resistentes, como el Kevlar, para mejorar aún más las propiedades mecánicas respecto a las de los CNT individuales.

Debido a la gran resistencia mecánica de los nanotubos de carbono, se está investigando cómo tejerlos en la ropa para crear prendas a prueba de puñaladas y balas. Los nanotubos impedirían eficazmente que la bala penetrara en el cuerpo, aunque la energía cinética de la bala probablemente causaría fracturas óseas y hemorragias internas.[34]​.

Los nanotubos de carbono también pueden acortar los tiempos de procesamiento y aumentar la eficiencia energética durante el curado de composites mediante el uso de calentadores estructurados con nanotubos de carbono. El autoclave es el método de referencia para el curado de materiales compuestos, pero tiene un precio elevado y limita el tamaño de las piezas. Los investigadores calculan que el curado de una pequeña sección del fuselaje de fibra de carbono y epoxi del Boeing 787 requiere 350 GJ de energía y produce 80 toneladas de dióxido de carbono. Esto es aproximadamente la misma cantidad de energía que consumirían nueve hogares en un año.[35]​ Además, la eliminación de las limitaciones de tamaño de las piezas elimina la necesidad de unir pequeños componentes de composite para crear estructuras a gran escala. Esto ahorra tiempo de fabricación y da lugar a estructuras más resistentes.

Los calentadores estructurados con nanotubos de carbono prometen sustituir a los autoclaves y hornos convencionales para el curado de composites por su capacidad para alcanzar altas temperaturas a velocidades de rampa rápidas con una gran eficiencia eléctrica y flexibilidad mecánica. Estos calentadores nanoestructurados pueden adoptar la forma de una película y aplicarse directamente al composite. Esto da lugar a una transferencia de calor conductiva, a diferencia de la transferencia de calor convectiva que utilizan los autoclaves y los hornos convencionales. Lee et al. informaron de que sólo el 50% de la energía térmica introducida en un autoclave se transfiere al composite que se está curando, independientemente del tamaño de la pieza, mientras que en un calentador de película nanoestructurada se transfiere alrededor del 90% de la energía térmica, dependiendo del proceso.[36]​.

Lee et al. consiguieron curar con éxito materiales compuestos de calidad aeroespacial utilizando un calentador de CNT fabricado "empujando en dominó" un bosque de CNT sobre una película de teflón. A continuación, esta película se colocó sobre un preimpregnado OOA (tecnología y aplicación Out of Autoclave) de 8 capas. Se incorporó aislamiento térmico alrededor del conjunto. A continuación, todo el conjunto se envasó al vacío y se calentó con una fuente de alimentación de 30 V CC. Se realizaron pruebas mecánicas y de grado de curado para comparar los composites curados de forma convencional con su configuración OOA. Los resultados mostraron que no había diferencias en la calidad del composite creado. Sin embargo, la cantidad de energía necesaria para curar el composite OOA se redujo en dos órdenes de magnitud, de 13,7 MJ a 118,8 kJ.[37]​.

Sin embargo, antes de que los nanotubos de carbono puedan utilizarse para curar los fuselajes del Boeing 787, es necesario un mayor desarrollo. El mayor reto a la hora de crear calentadores estructurados con nanotubos de carbono es conseguir una dispersión uniforme de los nanotubos en una matriz polimérica para garantizar que el calor se aplique de manera uniforme. La elevada superficie de los nanotubos de carbono genera fuertes fuerzas de Van Der Waals entre cada uno de ellos, lo que provoca su aglomeración y hace que las propiedades caloríficas no sean uniformes. Además, la matriz de polímero debe elegirse cuidadosamente para que pueda soportar las altas temperaturas generadas y los ciclos térmicos repetitivos necesarios para curar múltiples componentes compuestos.

Mixtures

MWNTs were first used as electrically conductive fillers in metals, in concentrations up to 83.78% by weight (wt%). The MWNT-polymer composites reach conductivities of up to 10,000 S m at a loading of 10 wt%. In the automotive industry, CNT plastics are used in the electrostatic-assisted painting of mirror housings, as well as in fuel lines and filters that dissipate electrostatic charge. Other products include electromagnetic interference (EMI) shielding packages and silicon wafer carriers.[2]​.

For load-bearing applications, CNT powders are mixed with precursor polymers or resins to increase stiffness, strength, and toughness. These improvements depend on the diameter, aspect ratio, alignment, dispersion, and interfacial interaction of the CNTs. Pre-mixed resins and master batches employ CNT loadings of 0.1 to 20 wt.%. Nanoscale "stick-slip" between CNTs and CNT-polymer contacts can increase material cushioning and improve sporting goods, such as tennis rackets, baseball bats, and bicycle frames.[2]​.

CNT resins enhance fiber composites, including wind turbine blades and maritime security boat hulls that are manufactured by upgrading carbon fiber composites with CNT-enhanced resin. CNTs are implemented as additives in the organic precursors of stronger 1 μm diameter carbon fibers. CNTs influence the arrangement of carbon in the pyrolyzed fiber.[2]​

Faced with the challenge of organizing CNTs on a larger scale, hierarchical fiber composites are created by growing aligned forests on fibers of glass, silicon carbide (SiC), alumina and carbon, creating so-called "diffuse" fibers. CNT-SiC and CNT-alumina diffuse epoxy fabrics showed a 69% improvement in crack resistance (mode I) and/or interlaminar in-plane shear strength (mode II). Applications being investigated include lightning protection, de-icing, and aircraft structural health monitoring.[2]​.

MWNTs can be used as a flame retardant additive for plastics due to changes in rheology due to nanotube loading. These additives can replace halogenated flame retardants, which face environmental restrictions.[2]​.

CNT/concrete mixtures offer higher tensile strength and lower crack propagation.[38]​.

Buckypaper (aggregate of nanotubes) can significantly improve fire resistance thanks to efficient heat reflection[39]​.

Textiles

Previous studies on the use of CNTs for textile functionalization focused on spinning fibers to improve physical and mechanical properties.[40]​[41]​[42]​ Recently, much attention has been paid to the coating of fabrics with CNTs. Various methods have been employed to modify fabrics with CNTs. For example, Panhuis et al. produced smart e-textiles for human biomonitoring using a polyelectrolyte-based coating with CNTs.[43]​ Furthermore, Panhuis et al. dyed textile material by immersion in a poly(2-methoxyaniline-5-sulfonic acid) PMAS polymer solution or in a PMAS-SWNT dispersion with improved conductivity and capacitance with long-lasting performance.[44]​ In another study, Hu and co-workers coated single-walled carbon nanotubes with a simple “dip and dry” process for wearable electronics and energy storage applications.[45]​ In a recent study, Li and Their collaborators used an elastomeric separator and almost achieved a fully stretchable supercapacitor based on single-walled carbon nanotube macrofilms. Electrospun polyurethane was used, which provided good mechanical stretchability, and the entire cell achieved excellent charge and discharge stability.[46] CNTs have an aligned nanotube structure and a negative surface charge. Therefore, they have similar structures to direct dyes, so the exhaustion method is applied for the coating and absorption of CNT on the surface of the fiber for the preparation of multifunctional fabric including antibacterial, electrical conductive, flame retardant and electromagnetic absorption properties.[47][48][49]​.

Going forward, CNT[50]​ yarns and sheets manufactured by direct chemical vapor deposition (CVD) or spinning or drawing methods may compete with carbon fiber for high-end uses, especially in weight-sensitive applications requiring combined electrical and mechanical functionality. Research wires made from low-walled CNTs have achieved a stiffness of 357 GPa and a strength of 8.8 GPa for a gauge length comparable to that of millimeter CNTs in the wire. Centimeter-scale lengths only offer a gravimetric strength of 2 GPa, equal to that of Kevlar.[2]​.

Since the probability of critical failure increases with volume, the threads may never reach the strength of individual CNTs. However, the high surface area of ​​CNTs can provide interfacial coupling that mitigates these deficiencies. CNT threads can be knotted without loss of strength. Coating drawn CNT sheets with functional powder before twist insertion produces weaveable, braidable, and sewable yarns containing up to 95% by weight of powder. Its uses include superconducting wires, battery and fuel cell electrodes, and self-cleaning textiles.[2]​.

Aligned SWNT fibers, still impractical, can be fabricated by coagulation spinning CNT suspensions. Cheaper SWNTs or spun MWNTs are needed for commercialization.[2]​ Carbon nanotubes can be dissolved in superacids, such as fluorosulfuric acid, and stretched into fibers in dry wet jet spinning.[51]​.

Carbon nanotube springs

Stretched and aligned MWNT spring "forests" can achieve 10 times the energy density of steel springs, offering cyclic durability, temperature insensitivity, no spontaneous discharge, and no arbitrary discharge rate. It is expected that SWNT forests can store much more than MWNTs.[58]​.

Alloys

Adding small amounts of CNTs to metals increases tensile strength and modulus, with potential in aerospace and automotive structures. Commercial aluminum-CNT composites have strength comparable to that of stainless steel (0.7 to 1 GPa) with one-third the density (2.6 g cm-3), comparable to that of more expensive aluminum-lithium alloys.[2]​.

Coatings and films

CNTs can serve as a multifunctional coating material. For example, paint/MWNT mixtures can reduce biofouling of ship hulls by discouraging the adhesion of algae and barnacles. They are a possible alternative to paints containing biocides, dangerous for the environment.[59]​ The mixture of CNTs in anti-corrosion coatings for metals can improve the rigidity and resistance of the coating and provide a path for cathodic protection.[2]​.

CNTs are a cost-effective alternative to ITO for a number of consumer devices. In addition to cost, the flexible, transparent conductors of CNTs offer an advantage over fragile ITO (indium tin oxide) coatings for flexible displays. CNT conductors can be deposited from solution and patterned using methods such as screen printing. SWNT films offer 90% transparency and 100 ohms per square resistivity. These films are being developed for thin film heaters, for example to defrost windows or sidewalks.[2]​.

Carbon nanotube forests and foams can also be coated with various materials to modify their functionality and performance. Examples include silicon-coated CNTs to create flexible, high-energy-density batteries,[60]​ graphene coatings to create highly elastic aerogels[61]​, and silicon carbide coatings to create a strong structural material for high-aspect-ratio 3D microarchitectures.[62]​.

There is a wide range of methods to convert CNTs into coatings and films.[63]​.

A pulverized mixture of carbon nanotubes and ceramics demonstrates an unprecedented ability to resist damage while absorbing laser light. These coatings, which absorb the energy of high-power lasers without breaking down, are essential for optical power detectors that measure the output of such lasers. They are used, for example, in military equipment to defuse unexploded mines. The composite is made up of multi-walled carbon nanotubes and a silicon, carbon and nitrogen ceramic. Boron increases the breakdown temperature. Nanotubes and graphene-like carbon transmit heat well, while oxidation-resistant ceramics increase damage resistance. To create the coating, the nanotubes were dispersed in toluene, to which a transparent liquid polymer containing boron was added. The mixture was heated to 1100°C (2010°F). The result was ground to a fine powder, dispersed again in toluene and pulverized in a thin layer on a copper surface. The coating absorbed 97.5% of the light from a far-infrared laser and tolerated 15 kilowatts per square centimeter for 10 seconds. Damage tolerance is approximately 50 percent higher than similar coatings, for example, nanotubes alone and carbon paint.[64]​[65]​.

Radars operate in the microwave frequency range, which can be absorbed by MWNTs. Applying MWNTs to the aircraft would cause them to be absorbed by the radar and therefore appear to have a smaller radar cross section. One of these applications could be painting the nanotubes on the plane. Recently, the University of Michigan has studied the usefulness of carbon nanotubes as cloaking technology in airplanes. It has been discovered that, in addition to absorbing radar, nanotubes do not reflect or scatter visible light, so they are practically invisible at night, like if current stealth planes were painted black, but much more effectively. However, current manufacturing limitations make the production of nanotube-coated aircraft impossible. One theory to overcome these current limitations is to coat small particles with nanotubes and suspend the nanotube-coated particles in a medium such as paint, which can then be applied to a surface, such as a stealth aircraft.[66]​.

Se han fabricado transistores basados en nanotubos, también conocidos como transistores de efecto de campo de nanotubos de carbono (CNTFET), que funcionan a temperatura ambiente y son capaces de conmutar digitalmente utilizando un solo electrón.[69]​ Sin embargo, uno de los principales obstáculos para la realización de los nanotubos ha sido la falta de tecnología para su producción en masa. En 2001, investigadores de IBM demostraron que los nanotubos metálicos pueden destruirse y dejar los semiconductores para utilizarlos como transistores. Su proceso se denomina "destrucción constructiva", que incluye la destrucción automática de nanotubos defectuosos en la oblea "Oblea (electrónica)").[70]​ Sin embargo, este proceso sólo permite controlar las propiedades eléctricas a escala estadística.Los SWNT resultan atractivos para los transistores por su baja dispersión de electrones y su banda prohibida. Los SWNT son compatibles con las arquitecturas de los transistores de efecto de campo (FET) y los dieléctricos de alta k. A pesar de los avances logrados tras la aparición del transistor de CNT en 1998, cabe destacar un FET de efecto túnel con una oscilación subumbral de <60 mV por década (2004), un radio (2007) y un FET con una longitud de canal inferior a 10 nm y una densidad de corriente normalizada de 2,41 mA μm-1 a 0,5 V, superior a las obtenidas para dispositivos de silicio.

Sin embargo, el control del diámetro, quiralidad, densidad y ubicación sigue siendo insuficiente para la producción comercial. Los dispositivos menos exigentes de decenas a miles de SWNT son más prácticos de inmediato. El uso de matrices/transistores CNT aumenta la corriente de salida y compensa los defectos y las diferencias de quiralidad, mejorando la uniformidad y la reproducibilidad del dispositivo. Por ejemplo, los transistores que utilizan matrices CNT alineadas horizontalmente lograron movilidades de 80 cm V s, pendientes subliminales de 140 mV por década y relaciones de encendido/apagado de hasta 10. Los métodos de deposición de película CNT permiten la fabricación convencional de semiconductores de más de 10 000 dispositivos CNT por chip.

Los transistores de película fina (TFT) de CNT impresos resultan atractivos para accionar pantallas de diodos orgánicos emisores de luz, ya que presentan una movilidad superior a la del silicio amorfo (~1 cm V s) y pueden depositarse por métodos de baja temperatura y sin vacío. Se demostraron TFT de CNT flexibles con una movilidad de 35 cm V s y una relación de encendido/apagado de 6×10. Un FET vertical de CNT mostró una salida de corriente suficiente para accionar OLEDs a bajo voltaje, permitiendo la emisión rojo-verde-azul a través de una red transparente de CNTs. Se está estudiando la posibilidad de utilizar CNT en etiquetas de identificación por radiofrecuencia. Se demostró la retención selectiva de los SWNT semiconductores durante el recubrimiento por rotación y la reducción de la sensibilidad a los adsorbatos.La Hoja de Ruta Tecnológica Internacional para semiconductores sugiere que los CNT podrían sustituir a las interconexiones de Cu") en los circuitos integrados, debido a su baja dispersión, su alta capacidad de transporte de corriente y su resistencia a la electromigración. Para ello, se necesitan vías formadas por CNT metálicos densamente empaquetados (>10 cm) con baja densidad de defectos y baja resistencia de contacto. Recientemente se han demostrado interconexiones compatibles con semiconductores de óxido metálico complementario (CMOS) de 150 nm de diámetro con una resistencia de orificio de contacto de CNT de 2,8 kohmios en obleas de 200 mm de diámetro. Además, como sustitutos de los puntos de soldadura, los CNT pueden funcionar como conductores eléctricos y disipadores de calor en amplificadores de alta potencia.

Por último, se ha adaptado para su comercialización un concepto de memoria no volátil basada en interruptores electromecánicos individuales de barras cruzadas de CNT mediante el estampado de películas finas de CNT desordenadas como elementos funcionales. Para ello ha sido necesario desarrollar suspensiones ultrapuras de CNT que puedan recubrirse por rotación y procesarse en entornos industriales de sala blanca y, por tanto, sean compatibles con las normas de procesamiento CMOS.

Transistors

Carbon nanotube field effect transistors (CNTFET) can operate at room temperature and are capable of digital switching using a single electron.[69]​ A CNT logic circuit capable of performing useful work was demonstrated in 2013.[71]​ Among the main obstacles to nanotube-based microelectronics are the absence of technology for mass production, the density of circuits, the positioning of electrical contacts individual, the purity of the samples,[72]​ the control of the length, chirality and desired alignment, the thermal budget and the resistance of the contacts.

One of the main challenges was regulating conductivity. Depending on the subtle characteristics of the surface, a nanotube can act as a conductor or a semiconductor.

Another way to make carbon nanotube transistors has been to use random networks of them.[73]​ By doing so, all their electrical differences are averaged and large-scale devices can be produced at the wafer level.[74]​ This approach was first patented by Nanomix Inc.[75]​ (original application date June 2002[76]​). It was first published in academic literature by the United States Naval Research Laboratory in 2003 through an independent research paper. This approach also allowed Nanomix to fabricate the first transistor on a flexible and transparent substrate.[77]​[78]​.

Since the mean free path of electrons in SWCNTs can exceed 1 micrometer, long-channel CNTFETs exhibit nearly ballistic transport characteristics, resulting in high velocities. CNT devices are expected to operate at frequencies of hundreds of gigahertz.[79][80][81][82][83]​.

Nanotubes can grow on magnetic metal nanoparticles (Fe, Co), facilitating the production of electronic (spintronic) devices. In particular, control of current through a field-effect transistor using a magnetic field in a single-tube nanostructure has been demonstrated[84].

In 2001, IBM researchers demonstrated how metal nanotubes can be destroyed, leaving the semiconducting nanotubes for use as components. Using "constructive destruction", they destroyed defective nanotubes on the "Wafer (electronic)" wafer.[70] However, this process only allows control of electrical properties on a statistical scale. In 2003, room temperature ballistic transistors with ohmic metal contacts and a high kP dielectric were published, which generated between 20 and 30 times more current than the most advanced silicon Mosfets. Palladium is a metal with a high work function that presents contacts without a Schottky barrier) with semiconductor nanotubes with diameters > 1.7 nm.[85]​.

The potential of carbon nanotubes was demonstrated in 2003, when room temperature ballistic transistors with ohmic metal contacts and high kP dielectrics were published, showing an ON current 20-30 times higher than that of the most advanced Si MOSFETs. This represented an important advance in this field, since it was shown that CNT ones could potentially outperform Si ones. At that time, the formation of ohmic metal contacts was one of the main challenges. In this regard, it was shown that palladium, which is a metal with a high work function, presented contacts without a Schottky barrier with semiconductor nanotubes with diameters >1.7 nm.[86][87]​.

Thermal management

Large carbon nanotube structures can be used for thermal management of electronic circuits. A layer of carbon nanotubes about 1 mm thick was used as a special material to make refrigerators, this material has a very low density, ~20 times less weight than a similar copper structure, while the cooling properties are similar for the two materials.[96]​.

Buckypaper has characteristics suitable for use as a heat sink for chipboards, backlighting for LCD screens or as a Faraday cage.

One of the most promising applications of single-walled carbon nanotubes (SWNTs) is their use in solar panels, due to their strong UV/Vis-NIR absorption characteristics. Research has shown that they can provide a significant increase in efficiency, even in their current unoptimized state. The solar cells developed at the New Jersey Institute of Technology use a carbon nanotube complex, made up of a mixture of carbon nanotubes and carbon buckyballs (known as fullerenes) to form snake-shaped structures. Buckyballs trap electrons, but cannot make them flow.[97]​[98]​ If sunlight is added to excite the polymers, the buckyballs will trap the electrons. The nanotubes, which behave like copper wires, will then be able to flow electrons or current.[97] Additional research has been carried out into creating hybrid SWNT solar panels to further increase efficiency. These hybrids are created by combining SWNTs with photoexcitable electron donors to increase the number of electrons generated. The interaction between photoexcitable porphyrin and SWNTs has been found to generate pairs of electroholes on the surfaces of SWNTs. This phenomenon has been observed experimentally, and practically contributes to an increase in efficiency of up to 8.5%.[99]​.

Nanotubes can potentially replace indium tin oxide in solar cells as a transparent conductive film on solar cells to allow light to pass to the active layers and generate photocurrent.[100]​.

CNTs in organic solar cells help reduce energy loss (carrier recombination) and improve photooxidation resistance. Photovoltaic technologies may one day incorporate CNT-silicon heterojunctions to take advantage of the efficient generation of multiple excitons at p-n junctions formed within individual CNTs. In the short term, commercial PV may incorporate transparent SWNT electrodes.[2]​.

Además de poder almacenar energía eléctrica, se ha investigado el uso de nanotubos de carbono para almacenar hidrógeno y utilizarlo como fuente de combustible. Aprovechando los efectos capilares de los pequeños nanotubos de carbono, es posible condensar gases en alta densidad dentro de nanotubos de pared simple. Esto permite almacenar gases, sobre todo hidrógeno (H), a altas densidades sin que se condensen en un líquido. Potencialmente, este método de almacenamiento podría utilizarse en vehículos en lugar de los depósitos de combustible para un coche impulsado por hidrógeno. Uno de los problemas actuales de los vehículos de hidrógeno es el almacenamiento del combustible a bordo. Los métodos de almacenamiento actuales implican enfriar y condensar el gas H hasta un estado líquido para su almacenamiento, lo que provoca una pérdida de energía potencial (25-45%) en comparación con la energía asociada al estado gaseoso. El almacenamiento mediante SWNTs permitiría mantener el H en estado gaseoso, aumentando así la eficiencia del almacenamiento. Este método permite una relación volumen/energía ligeramente inferior a la de los vehículos de gas actuales, lo que permite una autonomía ligeramente inferior pero comparable.[101]​.

La eficacia del almacenamiento de hidrógeno mediante adsorción en nanotubos de carbono es un tema controvertido y objeto de frecuentes experimentos. La eficacia del almacenamiento de hidrógeno es fundamental para su uso como fuente primaria de combustible, ya que el hidrógeno sólo contiene aproximadamente una cuarta parte de la energía por unidad de volumen que la gasolina. Sin embargo, los estudios demuestran que lo más importante es la superficie de los materiales utilizados. Así, un carbón activado con una superficie de 2.600 m/g puede almacenar hasta un 5,8% p/p. (Porcentaje Peso a Peso (%P/P)[102]​ en Disoluciones) En todos estos materiales carbonosos, el hidrógeno se almacena por fisisorción a 70-90K.[103]​.

Experimental capacity

An experiment[104] attempted to determine the amount of hydrogen stored in CNTs using elastic recoil detection analysis (ERDA). CNTs (mainly SWNTs) were synthesized by chemical vapor deposition (CVD) and subjected to a two-stage purification process that included air oxidation and acid treatment. Flat, uniform disks were then formed and exposed to pressurized pure hydrogen at various temperatures. When the data was analyzed, it was found that the ability of CNTs to store hydrogen decreased as temperature increased. Furthermore, the highest hydrogen concentration measured was ~0.18%; significantly lower than what should be commercially viable hydrogen storage. Other experimental work performed using a gravimetric method also revealed that the maximum hydrogen uptake capacity of CNTs was as low as 0.2%.[105]

In another experiment, carbon nantubes were synthesized by CVD and their structure was characterized by Raman spectroscopy. Using microwave digestion"), the samples were exposed to different concentrations of acid and different temperatures for various periods of time in an attempt to find the optimal purification method for SWNTs of the diameter determined above. The purified samples were then exposed to hydrogen gas at various high pressures and their percent adsorption by weight was plotted. The data showed that hydrogen adsorption levels of up to 3.7% are possible with a very pure sample and under the right conditions. It is believed that the Microwave digestion helps to improve the hydrogen adsorption capacity of CNTs by opening the ends, allowing access to the internal cavities of the nanotubes.

Limitations on efficient hydrogen adsorption

The biggest obstacle to efficient hydrogen storage using CNTs is the purity of the nanotubes. To achieve maximum hydrogen adsorption, there must be a minimum of graphene, amorphous carbon, and metallic deposits in the nanotube sample. Current CNT synthesis methods require a purification step. However, even with pure nanotubes, the adsorption capacity is only maximized under high pressures, which are not desirable in commercial fuel tanks.

Varias empresas están desarrollando películas CNT transparentes y conductoras de electricidad y nanobuds") para reemplazar al óxido de indio y estaño (ITO) en LCD, pantallas táctiles y dispositivos fotovoltaicos. Las películas de nanotubos son prometedoras para su uso en pantallas de computadoras, teléfonos celulares, asistentes digitales personales y cajeros automáticos .[106]​ Los diodos CNT muestran un efecto fotovoltaico .

Los nanotubos de paredes múltiples ( MWNT recubiertos con magnetita ) pueden generar fuertes campos magnéticos. Avances recientes muestran que MWNT recubiertos con nanopartículas de maghemita se puede orientar en un campo magnético[107]​ y mejorar las propiedades eléctricas del material compuesto en la dirección del campo para su uso en escobillas de motores eléctricos .[108]​.

Una capa de nanotubos de pared simple ( SWNT ) enriquecidos con hierro al 29 % colocado encima de una capa de material explosivo como PETN puede encenderse con un flash de cámara normal.[109]​.

Los CNT se pueden usar como cañones de electrones en tubos de rayos catódicos (CRT) en miniatura en pantallas de alto brillo, baja energía y bajo peso. Una pantalla consistiría en un grupo de diminutos CRT, cada uno de los cuales proporciona los electrones para iluminar el fósforo "Fósforo (sustancia fosforescente)") de un píxel, en lugar de tener un CRT cuyos electrones se dirigen mediante campos eléctricos y magnéticos. Estas pantallas se conocen como pantallas de emisión de campo (FED).

Los CNT pueden actuar como antenas para radios y otros dispositivos electromagnéticos .[110]​.

Los CNT conductores se utilizan en escobillas "Escobilla (electricidad)") para motores eléctricos comerciales. Reemplazan al negro de humo tradicional. Los nanotubos mejoran la conductividad eléctrica y térmica porque se extienden a través de la matriz plástica de la escobilla. Esto permite que el relleno de carbón se reduzca del 30 % al 3,6 %, de modo que haya más matriz presente en la escobilla. Las escobillas de motor compuestas de nanotubos están mejor lubricadas (desde la matriz), funcionan más frías (tanto por una mejor lubricación como por una conductividad térmica superior), menos quebradizas (más matriz y refuerzo de fibra), más fuertes y moldeables con mayor precisión (más matriz). Dado que las escobillas son un punto crítico de falla en los motores eléctricos y además no necesitan mucho material, se volvieron económicas antes que casi cualquier otra aplicación.

Los cables para transportar corriente eléctrica pueden fabricarse a partir de nanotubos y compuestos de nanotubos y polímeros. Se han fabricado alambres pequeños con conductividad específica superior al cobre y al aluminio;[111]​[112]​ los cables no metálicos de mayor conductividad.

Los CNT están bajo investigación como alternativa a los filamentos de tungsteno en las bombillas incandescentes.

Interconnections

Metallic carbon nanotubes have sparked research interest for their applicability as very large scale integration interconnects (VLSI) due to their high thermal stability, high thermal conductivity and large current carrying capacity. an elevated temperature of 250 degrees Celsius (482 °F), eliminating the electromigration reliability concerns that plague Cu interconnects.[119]​ Recent modeling work comparing the two has shown that CNT package interconnects can potentially offer advantages over copper.[120]​[119]​ Recent experiments have demonstrated resistances as low as 20 ohms using different architectures,[121]​ measurements were shown. Detailed conductance measurements over a wide temperature range agree with the theory for a strongly disordered quasi-one-dimensional conductor.

Hybrid interconnects using CNT vias along with copper interconnects can offer advantages from a reliability and thermal management perspective.[122]​ In 2016, the European Union funded a €4 million, three-year project to evaluate the manufacturability and performance of composite interconnects using CNT and copper interconnects. The project, called CONNECT (CarbON Nanotube compositE InterconneCTs),[123]​ brings together the efforts of seven European industry and research partners around manufacturing techniques and processes that allow reliable carbon nanotubes to be used for on-chip interconnections in the production of ULSI microchips.

Electrical cables and wires

Electrical current-conducting wires can be made from pure nanotubes or from nanotube-polymer composites. It has already been shown that carbon nanotube cables can be used successfully for the transmission of energy or data.[124] Recently, small cables have been manufactured with a specific conductivity higher than that of copper and aluminum;[125]​[126]​ these cables are the carbon nanotube cables with the highest conductivity and also the non-metallic cables with the highest conductivity. Recently, it has been shown that carbon and copper nanotube composites have a current carrying capacity almost one hundred times greater than that of pure copper or gold.[127] In addition, the electrical conductivity of this compound is similar to that of pure copper. Thus, this carbon and copper nanotube composite (CNT-Cu) has the highest current carrying capacity observed among electrical conductors. Thus, for a given cross section of electrical conductor, the CNT-Cu composite can withstand and carry a current one hundred times higher than that of metals such as copper and gold.

El uso de CNT como soporte de catalizadores en pilas de combustible puede reducir potencialmente el uso de platino en un 60% en comparación con el negro de humo. Los CNT dopados pueden permitir la eliminación completa del Pt.[2]​.

supercapacitor

MIT's Electronics Research Laboratory uses nanotubes to improve supercapacitors. The activated carbon used in conventional ultracapacitors has many small hollow spaces of various sizes, which together create a large surface area for storing electrical charge. But since charge is quantized into elementary charges, i.e., electrons, and each elementary charge needs a minimum space, a significant fraction of the electrode surface is not available for storage because the void spaces are not compatible with the charge requirements. With a nanotube electrode, the gaps can be tailored to size (some too large or too small) and consequently the capacity must increase considerably.[128]​.

A 40 F supercapacitor with a maximum voltage of 3.5 V using SWNTs grown without binders or additives achieved an energy density of 15.6 Wh kg and a power density of 37 kW kg.[129] CNTs can be attached to the charge plates of the capacitors to dramatically increase the surface area and therefore the energy density.

Batteries

The interesting electronic properties of carbon nanotubes (CNTs) have shown promise in the battery field, where they are being experimented with as a new electrode material, particularly the anode of lithium-ion batteries.[130] This is because the anode requires a relatively high reversible capacity at a potential close to that of metallic lithium, and a moderate irreversible capacity, observed so far only for graphite-based compounds such as CNTs. They have been shown to greatly improve the capacity and cyclability of lithium-ion batteries, as well as the ability to be highly effective buffer components, alleviating battery degradation that is often due to repeated charging and discharging. Furthermore, electronic transport at the anode can be greatly improved by using highly metallic CNTs.[131]​.

More specifically, CNTs have shown reversible capacities of 300 to 600 mAhg, with some treatments for them showing these figures increasing up to 1000 mAhg.[132] Meanwhile, graphite, which is the material most used as an anode for these lithium batteries, has shown capacities of only 320 mAhg-1. By creating composites from CNTs, scientists see many possibilities to take advantage of these exceptional capabilities, as well as their excellent mechanical strength, conductivities, and low densities.[131]​.

MWNTs are used in lithium-ion battery cathodes.[133]​[134]​ In these batteries, small amounts of MWNT powder are mixed with active materials and a polymeric binder, such as 1 wt% CNT loading on LiCoO cathodes and graphite anodes. CNTs increase electrical connectivity and mechanical integrity, which improves load capacity and service life.[2]​.

A paper battery") is a battery designed to use a sheet of paper-thin cellulose (which is the main component of plain paper, among other things) infused with aligned carbon nanotubes. thin, indicating the need for paper batteries. Recently,[133] it has been shown that CNT-coated surfaces can be used to replace heavy metals in batteries.[136] More recently, functional paper batteries have been demonstrated, in which a lithium-ion battery is integrated into a single sheet of paper through a lamination process as a composite with LiTiO (LTO) or LiCoO (LCO). while CNT films act as current collectors for both the anode and cathode. These rechargeable energy devices show potential in RFID tags, functional packaging or new disposable electronic applications.[137]​.

Improvements in lead-acid batteries have also been demonstrated, based on research conducted by Bar-Ilan University using high-quality SWCNTs manufactured by OCSiAl. The study demonstrated an increase in the useful life of lead-acid batteries of 4.5 times and an increase in capacity of 30% on average and up to 200% at high discharge rates.[138][139]​.

CNTs can be used to transport and desalinate water. Water molecules can be separated from salt by forcing them through electrochemically robust nanotube networks with controlled nanoscale porosity. This process requires much lower pressures than conventional reverse osmosis methods. Compared to a simple "Membrane (object)") membrane, it operates at a temperature 20 °C lower and at a flow rate 6 times higher.[138]​[139]​[140]​ Membranes using aligned and encapsulated CNTs with open ends allow flow through the interior of the CNTs. Very small diameter SWNTs are necessary to reject salt at seawater concentrations. Portable filters containing CNT meshes can purify contaminated drinking water. These networks can electrochemically oxidize organic contaminants, bacteria and viruses.[2]​.

CNT membranes can filter carbon dioxide from power plant emissions.

CNTs can be filled with biological molecules, which helps biotechnology.

CNTs can store between 4.2 and 65% hydrogen by weight. If they can be mass produced economically, 13.2 liters of CNT could contain the same amount of energy as a 50-liter tank of gasoline.

CNTs can be used to produce nanowires from other elements or molecules, such as gold or zinc oxide. In turn, nanowires can be used to make nanotubes from other materials, such as gallium nitride. For example, gallium nitride nanotubes are hydrophilic, while CNTs are hydrophobic, so they can be used in organic chemistry.

• - on YouTube.

• - Applications of Carbon Nanotubes.

Contenido

Debido a las propiedades mecánicas superiores de los nanotubos de carbono, se han propuesto muchas estructuras que van desde artículos cotidianos como ropa y equipamiento deportivo hasta chaquetas de combate y ascensores espaciales.[20]​ Sin embargo, el ascensor espacial requerirá más esfuerzos para perfeccionar la tecnología de nanotubos de carbono, ya que la resistencia práctica a la tracción de los nanotubos de carbono debe mejorar mucho.[21]​.

En perspectiva, ya se han logrado avances sobresalientes. El trabajo pionero dirigido por Ray H. Baughman en el Instituto de Nanotecnología ha demostrado que los nanotubos de paredes simples y múltiples pueden producir materiales con una dureza sin igual en los mundos natural y artificial.[22]​[23]​.

Los nanotubos de carbono también son un material prometedor como bloques de construcción en materiales compuestos jerárquicos dadas sus excepcionales propiedades mecánicas (~1 TPa en módulo y ~100 GPa en resistencia). Los intentos iniciales de incorporar los CNT en estructuras jerárquicas (como hilos, fibras o películas[24]​) han llevado a propiedades mecánicas significativamente más bajas que estos límites potenciales. La integración jerárquica de nanotubos de carbono de pared múltiple y óxidos de metal/metal dentro de una única nanoestructura puede aprovechar la potencialidad de los compuestos de nanotubos de carbono para la descomposición del agua y la electrocatálisis.[25]​ Windle et al. han utilizado un método de hilado de deposición química de vapor (CVD) in situ para producir hilos de CNT continuos a partir de aerogeles de CNT cultivados mediante CVD.[26]​[27]​[28]​ Los hilos de CNT también se pueden fabricar extrayendo paquetes de CNT de un bosque de CNT y retorciéndolos posteriormente para formar la fibra (método de estirar y torcer, vea la imagen a la derecha). El grupo Windle ha fabricado hilos CNT con resistencias de hasta ~9 GPa en longitudes de calibre pequeñas de ~1 mm, sin embargo, se informaron resistencias de solo aproximadamente ~ 1 GPa en la longitud de referencia más larga de 20 milímetros[29]​[30]​ La razón por la cual las resistencias de las fibras han sido bajas en comparación con la resistencia de los CNT individuales se debe a que no se transfirió la carga de manera efectiva a los CNT constituyentes (discontinuos) dentro de la fibra. Una posible vía para paliar este problema es la reticulación covalente entre haces y entre CNT inducida por irradiación (o deposición) para "unir" eficazmente los CNT, con niveles de dosificación más elevados que den lugar a la posibilidad de obtener fibras compuestas de carbono amorfo/nanotubos de carbono.[31]​ Espinosa et al. desarrollaron hilos compuestos de DWNT-polímero de alto rendimiento retorciendo y estirando cintas de haces de DWNT orientados aleatoriamente y recubiertos con compuestos orgánicos poliméricos. Estos hilos de DWNT-polímero mostraron una energía de rotura inusualmente alta, de ~100 J-g-1 (comparable a la de uno de los materiales naturales más resistentes, la seda de araña),[32]​ y una resistencia de hasta ~1,4 GPa.[33]​ Se están realizando esfuerzos para producir compuestos de CNT que incorporen materiales de matriz más resistentes, como el Kevlar, para mejorar aún más las propiedades mecánicas respecto a las de los CNT individuales.

Debido a la gran resistencia mecánica de los nanotubos de carbono, se está investigando cómo tejerlos en la ropa para crear prendas a prueba de puñaladas y balas. Los nanotubos impedirían eficazmente que la bala penetrara en el cuerpo, aunque la energía cinética de la bala probablemente causaría fracturas óseas y hemorragias internas.[34]​.

Los nanotubos de carbono también pueden acortar los tiempos de procesamiento y aumentar la eficiencia energética durante el curado de composites mediante el uso de calentadores estructurados con nanotubos de carbono. El autoclave es el método de referencia para el curado de materiales compuestos, pero tiene un precio elevado y limita el tamaño de las piezas. Los investigadores calculan que el curado de una pequeña sección del fuselaje de fibra de carbono y epoxi del Boeing 787 requiere 350 GJ de energía y produce 80 toneladas de dióxido de carbono. Esto es aproximadamente la misma cantidad de energía que consumirían nueve hogares en un año.[35]​ Además, la eliminación de las limitaciones de tamaño de las piezas elimina la necesidad de unir pequeños componentes de composite para crear estructuras a gran escala. Esto ahorra tiempo de fabricación y da lugar a estructuras más resistentes.

Los calentadores estructurados con nanotubos de carbono prometen sustituir a los autoclaves y hornos convencionales para el curado de composites por su capacidad para alcanzar altas temperaturas a velocidades de rampa rápidas con una gran eficiencia eléctrica y flexibilidad mecánica. Estos calentadores nanoestructurados pueden adoptar la forma de una película y aplicarse directamente al composite. Esto da lugar a una transferencia de calor conductiva, a diferencia de la transferencia de calor convectiva que utilizan los autoclaves y los hornos convencionales. Lee et al. informaron de que sólo el 50% de la energía térmica introducida en un autoclave se transfiere al composite que se está curando, independientemente del tamaño de la pieza, mientras que en un calentador de película nanoestructurada se transfiere alrededor del 90% de la energía térmica, dependiendo del proceso.[36]​.

Lee et al. consiguieron curar con éxito materiales compuestos de calidad aeroespacial utilizando un calentador de CNT fabricado "empujando en dominó" un bosque de CNT sobre una película de teflón. A continuación, esta película se colocó sobre un preimpregnado OOA (tecnología y aplicación Out of Autoclave) de 8 capas. Se incorporó aislamiento térmico alrededor del conjunto. A continuación, todo el conjunto se envasó al vacío y se calentó con una fuente de alimentación de 30 V CC. Se realizaron pruebas mecánicas y de grado de curado para comparar los composites curados de forma convencional con su configuración OOA. Los resultados mostraron que no había diferencias en la calidad del composite creado. Sin embargo, la cantidad de energía necesaria para curar el composite OOA se redujo en dos órdenes de magnitud, de 13,7 MJ a 118,8 kJ.[37]​.

Sin embargo, antes de que los nanotubos de carbono puedan utilizarse para curar los fuselajes del Boeing 787, es necesario un mayor desarrollo. El mayor reto a la hora de crear calentadores estructurados con nanotubos de carbono es conseguir una dispersión uniforme de los nanotubos en una matriz polimérica para garantizar que el calor se aplique de manera uniforme. La elevada superficie de los nanotubos de carbono genera fuertes fuerzas de Van Der Waals entre cada uno de ellos, lo que provoca su aglomeración y hace que las propiedades caloríficas no sean uniformes. Además, la matriz de polímero debe elegirse cuidadosamente para que pueda soportar las altas temperaturas generadas y los ciclos térmicos repetitivos necesarios para curar múltiples componentes compuestos.

Mixtures

MWNTs were first used as electrically conductive fillers in metals, in concentrations up to 83.78% by weight (wt%). The MWNT-polymer composites reach conductivities of up to 10,000 S m at a loading of 10 wt%. In the automotive industry, CNT plastics are used in the electrostatic-assisted painting of mirror housings, as well as in fuel lines and filters that dissipate electrostatic charge. Other products include electromagnetic interference (EMI) shielding packages and silicon wafer carriers.[2]​.

For load-bearing applications, CNT powders are mixed with precursor polymers or resins to increase stiffness, strength, and toughness. These improvements depend on the diameter, aspect ratio, alignment, dispersion, and interfacial interaction of the CNTs. Pre-mixed resins and master batches employ CNT loadings of 0.1 to 20 wt.%. Nanoscale "stick-slip" between CNTs and CNT-polymer contacts can increase material cushioning and improve sporting goods, such as tennis rackets, baseball bats, and bicycle frames.[2]​.

CNT resins enhance fiber composites, including wind turbine blades and maritime security boat hulls that are manufactured by upgrading carbon fiber composites with CNT-enhanced resin. CNTs are implemented as additives in the organic precursors of stronger 1 μm diameter carbon fibers. CNTs influence the arrangement of carbon in the pyrolyzed fiber.[2]​

Faced with the challenge of organizing CNTs on a larger scale, hierarchical fiber composites are created by growing aligned forests on fibers of glass, silicon carbide (SiC), alumina and carbon, creating so-called "diffuse" fibers. CNT-SiC and CNT-alumina diffuse epoxy fabrics showed a 69% improvement in crack resistance (mode I) and/or interlaminar in-plane shear strength (mode II). Applications being investigated include lightning protection, de-icing, and aircraft structural health monitoring.[2]​.

MWNTs can be used as a flame retardant additive for plastics due to changes in rheology due to nanotube loading. These additives can replace halogenated flame retardants, which face environmental restrictions.[2]​.

CNT/concrete mixtures offer higher tensile strength and lower crack propagation.[38]​.

Buckypaper (aggregate of nanotubes) can significantly improve fire resistance thanks to efficient heat reflection[39]​.

Textiles

Previous studies on the use of CNTs for textile functionalization focused on spinning fibers to improve physical and mechanical properties.[40]​[41]​[42]​ Recently, much attention has been paid to the coating of fabrics with CNTs. Various methods have been employed to modify fabrics with CNTs. For example, Panhuis et al. produced smart e-textiles for human biomonitoring using a polyelectrolyte-based coating with CNTs.[43]​ Furthermore, Panhuis et al. dyed textile material by immersion in a poly(2-methoxyaniline-5-sulfonic acid) PMAS polymer solution or in a PMAS-SWNT dispersion with improved conductivity and capacitance with long-lasting performance.[44]​ In another study, Hu and co-workers coated single-walled carbon nanotubes with a simple “dip and dry” process for wearable electronics and energy storage applications.[45]​ In a recent study, Li and Their collaborators used an elastomeric separator and almost achieved a fully stretchable supercapacitor based on single-walled carbon nanotube macrofilms. Electrospun polyurethane was used, which provided good mechanical stretchability, and the entire cell achieved excellent charge and discharge stability.[46] CNTs have an aligned nanotube structure and a negative surface charge. Therefore, they have similar structures to direct dyes, so the exhaustion method is applied for the coating and absorption of CNT on the surface of the fiber for the preparation of multifunctional fabric including antibacterial, electrical conductive, flame retardant and electromagnetic absorption properties.[47][48][49]​.

Going forward, CNT[50]​ yarns and sheets manufactured by direct chemical vapor deposition (CVD) or spinning or drawing methods may compete with carbon fiber for high-end uses, especially in weight-sensitive applications requiring combined electrical and mechanical functionality. Research wires made from low-walled CNTs have achieved a stiffness of 357 GPa and a strength of 8.8 GPa for a gauge length comparable to that of millimeter CNTs in the wire. Centimeter-scale lengths only offer a gravimetric strength of 2 GPa, equal to that of Kevlar.[2]​.

Since the probability of critical failure increases with volume, the threads may never reach the strength of individual CNTs. However, the high surface area of ​​CNTs can provide interfacial coupling that mitigates these deficiencies. CNT threads can be knotted without loss of strength. Coating drawn CNT sheets with functional powder before twist insertion produces weaveable, braidable, and sewable yarns containing up to 95% by weight of powder. Its uses include superconducting wires, battery and fuel cell electrodes, and self-cleaning textiles.[2]​.

Aligned SWNT fibers, still impractical, can be fabricated by coagulation spinning CNT suspensions. Cheaper SWNTs or spun MWNTs are needed for commercialization.[2]​ Carbon nanotubes can be dissolved in superacids, such as fluorosulfuric acid, and stretched into fibers in dry wet jet spinning.[51]​.

Carbon nanotube springs

Stretched and aligned MWNT spring "forests" can achieve 10 times the energy density of steel springs, offering cyclic durability, temperature insensitivity, no spontaneous discharge, and no arbitrary discharge rate. It is expected that SWNT forests can store much more than MWNTs.[58]​.

Alloys

Adding small amounts of CNTs to metals increases tensile strength and modulus, with potential in aerospace and automotive structures. Commercial aluminum-CNT composites have strength comparable to that of stainless steel (0.7 to 1 GPa) with one-third the density (2.6 g cm-3), comparable to that of more expensive aluminum-lithium alloys.[2]​.

Coatings and films

CNTs can serve as a multifunctional coating material. For example, paint/MWNT mixtures can reduce biofouling of ship hulls by discouraging the adhesion of algae and barnacles. They are a possible alternative to paints containing biocides, dangerous for the environment.[59]​ The mixture of CNTs in anti-corrosion coatings for metals can improve the rigidity and resistance of the coating and provide a path for cathodic protection.[2]​.

CNTs are a cost-effective alternative to ITO for a number of consumer devices. In addition to cost, the flexible, transparent conductors of CNTs offer an advantage over fragile ITO (indium tin oxide) coatings for flexible displays. CNT conductors can be deposited from solution and patterned using methods such as screen printing. SWNT films offer 90% transparency and 100 ohms per square resistivity. These films are being developed for thin film heaters, for example to defrost windows or sidewalks.[2]​.

Carbon nanotube forests and foams can also be coated with various materials to modify their functionality and performance. Examples include silicon-coated CNTs to create flexible, high-energy-density batteries,[60]​ graphene coatings to create highly elastic aerogels[61]​, and silicon carbide coatings to create a strong structural material for high-aspect-ratio 3D microarchitectures.[62]​.

There is a wide range of methods to convert CNTs into coatings and films.[63]​.

A pulverized mixture of carbon nanotubes and ceramics demonstrates an unprecedented ability to resist damage while absorbing laser light. These coatings, which absorb the energy of high-power lasers without breaking down, are essential for optical power detectors that measure the output of such lasers. They are used, for example, in military equipment to defuse unexploded mines. The composite is made up of multi-walled carbon nanotubes and a silicon, carbon and nitrogen ceramic. Boron increases the breakdown temperature. Nanotubes and graphene-like carbon transmit heat well, while oxidation-resistant ceramics increase damage resistance. To create the coating, the nanotubes were dispersed in toluene, to which a transparent liquid polymer containing boron was added. The mixture was heated to 1100°C (2010°F). The result was ground to a fine powder, dispersed again in toluene and pulverized in a thin layer on a copper surface. The coating absorbed 97.5% of the light from a far-infrared laser and tolerated 15 kilowatts per square centimeter for 10 seconds. Damage tolerance is approximately 50 percent higher than similar coatings, for example, nanotubes alone and carbon paint.[64]​[65]​.

Radars operate in the microwave frequency range, which can be absorbed by MWNTs. Applying MWNTs to the aircraft would cause them to be absorbed by the radar and therefore appear to have a smaller radar cross section. One of these applications could be painting the nanotubes on the plane. Recently, the University of Michigan has studied the usefulness of carbon nanotubes as cloaking technology in airplanes. It has been discovered that, in addition to absorbing radar, nanotubes do not reflect or scatter visible light, so they are practically invisible at night, like if current stealth planes were painted black, but much more effectively. However, current manufacturing limitations make the production of nanotube-coated aircraft impossible. One theory to overcome these current limitations is to coat small particles with nanotubes and suspend the nanotube-coated particles in a medium such as paint, which can then be applied to a surface, such as a stealth aircraft.[66]​.

Se han fabricado transistores basados en nanotubos, también conocidos como transistores de efecto de campo de nanotubos de carbono (CNTFET), que funcionan a temperatura ambiente y son capaces de conmutar digitalmente utilizando un solo electrón.[69]​ Sin embargo, uno de los principales obstáculos para la realización de los nanotubos ha sido la falta de tecnología para su producción en masa. En 2001, investigadores de IBM demostraron que los nanotubos metálicos pueden destruirse y dejar los semiconductores para utilizarlos como transistores. Su proceso se denomina "destrucción constructiva", que incluye la destrucción automática de nanotubos defectuosos en la oblea "Oblea (electrónica)").[70]​ Sin embargo, este proceso sólo permite controlar las propiedades eléctricas a escala estadística.Los SWNT resultan atractivos para los transistores por su baja dispersión de electrones y su banda prohibida. Los SWNT son compatibles con las arquitecturas de los transistores de efecto de campo (FET) y los dieléctricos de alta k. A pesar de los avances logrados tras la aparición del transistor de CNT en 1998, cabe destacar un FET de efecto túnel con una oscilación subumbral de <60 mV por década (2004), un radio (2007) y un FET con una longitud de canal inferior a 10 nm y una densidad de corriente normalizada de 2,41 mA μm-1 a 0,5 V, superior a las obtenidas para dispositivos de silicio.

Sin embargo, el control del diámetro, quiralidad, densidad y ubicación sigue siendo insuficiente para la producción comercial. Los dispositivos menos exigentes de decenas a miles de SWNT son más prácticos de inmediato. El uso de matrices/transistores CNT aumenta la corriente de salida y compensa los defectos y las diferencias de quiralidad, mejorando la uniformidad y la reproducibilidad del dispositivo. Por ejemplo, los transistores que utilizan matrices CNT alineadas horizontalmente lograron movilidades de 80 cm V s, pendientes subliminales de 140 mV por década y relaciones de encendido/apagado de hasta 10. Los métodos de deposición de película CNT permiten la fabricación convencional de semiconductores de más de 10 000 dispositivos CNT por chip.

Los transistores de película fina (TFT) de CNT impresos resultan atractivos para accionar pantallas de diodos orgánicos emisores de luz, ya que presentan una movilidad superior a la del silicio amorfo (~1 cm V s) y pueden depositarse por métodos de baja temperatura y sin vacío. Se demostraron TFT de CNT flexibles con una movilidad de 35 cm V s y una relación de encendido/apagado de 6×10. Un FET vertical de CNT mostró una salida de corriente suficiente para accionar OLEDs a bajo voltaje, permitiendo la emisión rojo-verde-azul a través de una red transparente de CNTs. Se está estudiando la posibilidad de utilizar CNT en etiquetas de identificación por radiofrecuencia. Se demostró la retención selectiva de los SWNT semiconductores durante el recubrimiento por rotación y la reducción de la sensibilidad a los adsorbatos.La Hoja de Ruta Tecnológica Internacional para semiconductores sugiere que los CNT podrían sustituir a las interconexiones de Cu") en los circuitos integrados, debido a su baja dispersión, su alta capacidad de transporte de corriente y su resistencia a la electromigración. Para ello, se necesitan vías formadas por CNT metálicos densamente empaquetados (>10 cm) con baja densidad de defectos y baja resistencia de contacto. Recientemente se han demostrado interconexiones compatibles con semiconductores de óxido metálico complementario (CMOS) de 150 nm de diámetro con una resistencia de orificio de contacto de CNT de 2,8 kohmios en obleas de 200 mm de diámetro. Además, como sustitutos de los puntos de soldadura, los CNT pueden funcionar como conductores eléctricos y disipadores de calor en amplificadores de alta potencia.

Por último, se ha adaptado para su comercialización un concepto de memoria no volátil basada en interruptores electromecánicos individuales de barras cruzadas de CNT mediante el estampado de películas finas de CNT desordenadas como elementos funcionales. Para ello ha sido necesario desarrollar suspensiones ultrapuras de CNT que puedan recubrirse por rotación y procesarse en entornos industriales de sala blanca y, por tanto, sean compatibles con las normas de procesamiento CMOS.

Transistors

Carbon nanotube field effect transistors (CNTFET) can operate at room temperature and are capable of digital switching using a single electron.[69]​ A CNT logic circuit capable of performing useful work was demonstrated in 2013.[71]​ Among the main obstacles to nanotube-based microelectronics are the absence of technology for mass production, the density of circuits, the positioning of electrical contacts individual, the purity of the samples,[72]​ the control of the length, chirality and desired alignment, the thermal budget and the resistance of the contacts.

One of the main challenges was regulating conductivity. Depending on the subtle characteristics of the surface, a nanotube can act as a conductor or a semiconductor.

Another way to make carbon nanotube transistors has been to use random networks of them.[73]​ By doing so, all their electrical differences are averaged and large-scale devices can be produced at the wafer level.[74]​ This approach was first patented by Nanomix Inc.[75]​ (original application date June 2002[76]​). It was first published in academic literature by the United States Naval Research Laboratory in 2003 through an independent research paper. This approach also allowed Nanomix to fabricate the first transistor on a flexible and transparent substrate.[77]​[78]​.

Since the mean free path of electrons in SWCNTs can exceed 1 micrometer, long-channel CNTFETs exhibit nearly ballistic transport characteristics, resulting in high velocities. CNT devices are expected to operate at frequencies of hundreds of gigahertz.[79][80][81][82][83]​.

Nanotubes can grow on magnetic metal nanoparticles (Fe, Co), facilitating the production of electronic (spintronic) devices. In particular, control of current through a field-effect transistor using a magnetic field in a single-tube nanostructure has been demonstrated[84].

In 2001, IBM researchers demonstrated how metal nanotubes can be destroyed, leaving the semiconducting nanotubes for use as components. Using "constructive destruction", they destroyed defective nanotubes on the "Wafer (electronic)" wafer.[70] However, this process only allows control of electrical properties on a statistical scale. In 2003, room temperature ballistic transistors with ohmic metal contacts and a high kP dielectric were published, which generated between 20 and 30 times more current than the most advanced silicon Mosfets. Palladium is a metal with a high work function that presents contacts without a Schottky barrier) with semiconductor nanotubes with diameters > 1.7 nm.[85]​.

The potential of carbon nanotubes was demonstrated in 2003, when room temperature ballistic transistors with ohmic metal contacts and high kP dielectrics were published, showing an ON current 20-30 times higher than that of the most advanced Si MOSFETs. This represented an important advance in this field, since it was shown that CNT ones could potentially outperform Si ones. At that time, the formation of ohmic metal contacts was one of the main challenges. In this regard, it was shown that palladium, which is a metal with a high work function, presented contacts without a Schottky barrier with semiconductor nanotubes with diameters >1.7 nm.[86][87]​.

Thermal management

Large carbon nanotube structures can be used for thermal management of electronic circuits. A layer of carbon nanotubes about 1 mm thick was used as a special material to make refrigerators, this material has a very low density, ~20 times less weight than a similar copper structure, while the cooling properties are similar for the two materials.[96]​.

Buckypaper has characteristics suitable for use as a heat sink for chipboards, backlighting for LCD screens or as a Faraday cage.

One of the most promising applications of single-walled carbon nanotubes (SWNTs) is their use in solar panels, due to their strong UV/Vis-NIR absorption characteristics. Research has shown that they can provide a significant increase in efficiency, even in their current unoptimized state. The solar cells developed at the New Jersey Institute of Technology use a carbon nanotube complex, made up of a mixture of carbon nanotubes and carbon buckyballs (known as fullerenes) to form snake-shaped structures. Buckyballs trap electrons, but cannot make them flow.[97]​[98]​ If sunlight is added to excite the polymers, the buckyballs will trap the electrons. The nanotubes, which behave like copper wires, will then be able to flow electrons or current.[97] Additional research has been carried out into creating hybrid SWNT solar panels to further increase efficiency. These hybrids are created by combining SWNTs with photoexcitable electron donors to increase the number of electrons generated. The interaction between photoexcitable porphyrin and SWNTs has been found to generate pairs of electroholes on the surfaces of SWNTs. This phenomenon has been observed experimentally, and practically contributes to an increase in efficiency of up to 8.5%.[99]​.

Nanotubes can potentially replace indium tin oxide in solar cells as a transparent conductive film on solar cells to allow light to pass to the active layers and generate photocurrent.[100]​.

CNTs in organic solar cells help reduce energy loss (carrier recombination) and improve photooxidation resistance. Photovoltaic technologies may one day incorporate CNT-silicon heterojunctions to take advantage of the efficient generation of multiple excitons at p-n junctions formed within individual CNTs. In the short term, commercial PV may incorporate transparent SWNT electrodes.[2]​.

Además de poder almacenar energía eléctrica, se ha investigado el uso de nanotubos de carbono para almacenar hidrógeno y utilizarlo como fuente de combustible. Aprovechando los efectos capilares de los pequeños nanotubos de carbono, es posible condensar gases en alta densidad dentro de nanotubos de pared simple. Esto permite almacenar gases, sobre todo hidrógeno (H), a altas densidades sin que se condensen en un líquido. Potencialmente, este método de almacenamiento podría utilizarse en vehículos en lugar de los depósitos de combustible para un coche impulsado por hidrógeno. Uno de los problemas actuales de los vehículos de hidrógeno es el almacenamiento del combustible a bordo. Los métodos de almacenamiento actuales implican enfriar y condensar el gas H hasta un estado líquido para su almacenamiento, lo que provoca una pérdida de energía potencial (25-45%) en comparación con la energía asociada al estado gaseoso. El almacenamiento mediante SWNTs permitiría mantener el H en estado gaseoso, aumentando así la eficiencia del almacenamiento. Este método permite una relación volumen/energía ligeramente inferior a la de los vehículos de gas actuales, lo que permite una autonomía ligeramente inferior pero comparable.[101]​.

La eficacia del almacenamiento de hidrógeno mediante adsorción en nanotubos de carbono es un tema controvertido y objeto de frecuentes experimentos. La eficacia del almacenamiento de hidrógeno es fundamental para su uso como fuente primaria de combustible, ya que el hidrógeno sólo contiene aproximadamente una cuarta parte de la energía por unidad de volumen que la gasolina. Sin embargo, los estudios demuestran que lo más importante es la superficie de los materiales utilizados. Así, un carbón activado con una superficie de 2.600 m/g puede almacenar hasta un 5,8% p/p. (Porcentaje Peso a Peso (%P/P)[102]​ en Disoluciones) En todos estos materiales carbonosos, el hidrógeno se almacena por fisisorción a 70-90K.[103]​.

Experimental capacity

An experiment[104] attempted to determine the amount of hydrogen stored in CNTs using elastic recoil detection analysis (ERDA). CNTs (mainly SWNTs) were synthesized by chemical vapor deposition (CVD) and subjected to a two-stage purification process that included air oxidation and acid treatment. Flat, uniform disks were then formed and exposed to pressurized pure hydrogen at various temperatures. When the data was analyzed, it was found that the ability of CNTs to store hydrogen decreased as temperature increased. Furthermore, the highest hydrogen concentration measured was ~0.18%; significantly lower than what should be commercially viable hydrogen storage. Other experimental work performed using a gravimetric method also revealed that the maximum hydrogen uptake capacity of CNTs was as low as 0.2%.[105]

In another experiment, carbon nantubes were synthesized by CVD and their structure was characterized by Raman spectroscopy. Using microwave digestion"), the samples were exposed to different concentrations of acid and different temperatures for various periods of time in an attempt to find the optimal purification method for SWNTs of the diameter determined above. The purified samples were then exposed to hydrogen gas at various high pressures and their percent adsorption by weight was plotted. The data showed that hydrogen adsorption levels of up to 3.7% are possible with a very pure sample and under the right conditions. It is believed that the Microwave digestion helps to improve the hydrogen adsorption capacity of CNTs by opening the ends, allowing access to the internal cavities of the nanotubes.

Limitations on efficient hydrogen adsorption

The biggest obstacle to efficient hydrogen storage using CNTs is the purity of the nanotubes. To achieve maximum hydrogen adsorption, there must be a minimum of graphene, amorphous carbon, and metallic deposits in the nanotube sample. Current CNT synthesis methods require a purification step. However, even with pure nanotubes, the adsorption capacity is only maximized under high pressures, which are not desirable in commercial fuel tanks.

Varias empresas están desarrollando películas CNT transparentes y conductoras de electricidad y nanobuds") para reemplazar al óxido de indio y estaño (ITO) en LCD, pantallas táctiles y dispositivos fotovoltaicos. Las películas de nanotubos son prometedoras para su uso en pantallas de computadoras, teléfonos celulares, asistentes digitales personales y cajeros automáticos .[106]​ Los diodos CNT muestran un efecto fotovoltaico .

Los nanotubos de paredes múltiples ( MWNT recubiertos con magnetita ) pueden generar fuertes campos magnéticos. Avances recientes muestran que MWNT recubiertos con nanopartículas de maghemita se puede orientar en un campo magnético[107]​ y mejorar las propiedades eléctricas del material compuesto en la dirección del campo para su uso en escobillas de motores eléctricos .[108]​.

Una capa de nanotubos de pared simple ( SWNT ) enriquecidos con hierro al 29 % colocado encima de una capa de material explosivo como PETN puede encenderse con un flash de cámara normal.[109]​.

Los CNT se pueden usar como cañones de electrones en tubos de rayos catódicos (CRT) en miniatura en pantallas de alto brillo, baja energía y bajo peso. Una pantalla consistiría en un grupo de diminutos CRT, cada uno de los cuales proporciona los electrones para iluminar el fósforo "Fósforo (sustancia fosforescente)") de un píxel, en lugar de tener un CRT cuyos electrones se dirigen mediante campos eléctricos y magnéticos. Estas pantallas se conocen como pantallas de emisión de campo (FED).

Los CNT pueden actuar como antenas para radios y otros dispositivos electromagnéticos .[110]​.

Los CNT conductores se utilizan en escobillas "Escobilla (electricidad)") para motores eléctricos comerciales. Reemplazan al negro de humo tradicional. Los nanotubos mejoran la conductividad eléctrica y térmica porque se extienden a través de la matriz plástica de la escobilla. Esto permite que el relleno de carbón se reduzca del 30 % al 3,6 %, de modo que haya más matriz presente en la escobilla. Las escobillas de motor compuestas de nanotubos están mejor lubricadas (desde la matriz), funcionan más frías (tanto por una mejor lubricación como por una conductividad térmica superior), menos quebradizas (más matriz y refuerzo de fibra), más fuertes y moldeables con mayor precisión (más matriz). Dado que las escobillas son un punto crítico de falla en los motores eléctricos y además no necesitan mucho material, se volvieron económicas antes que casi cualquier otra aplicación.

Los cables para transportar corriente eléctrica pueden fabricarse a partir de nanotubos y compuestos de nanotubos y polímeros. Se han fabricado alambres pequeños con conductividad específica superior al cobre y al aluminio;[111]​[112]​ los cables no metálicos de mayor conductividad.

Los CNT están bajo investigación como alternativa a los filamentos de tungsteno en las bombillas incandescentes.

Interconnections

Metallic carbon nanotubes have sparked research interest for their applicability as very large scale integration interconnects (VLSI) due to their high thermal stability, high thermal conductivity and large current carrying capacity. an elevated temperature of 250 degrees Celsius (482 °F), eliminating the electromigration reliability concerns that plague Cu interconnects.[119]​ Recent modeling work comparing the two has shown that CNT package interconnects can potentially offer advantages over copper.[120]​[119]​ Recent experiments have demonstrated resistances as low as 20 ohms using different architectures,[121]​ measurements were shown. Detailed conductance measurements over a wide temperature range agree with the theory for a strongly disordered quasi-one-dimensional conductor.

Hybrid interconnects using CNT vias along with copper interconnects can offer advantages from a reliability and thermal management perspective.[122]​ In 2016, the European Union funded a €4 million, three-year project to evaluate the manufacturability and performance of composite interconnects using CNT and copper interconnects. The project, called CONNECT (CarbON Nanotube compositE InterconneCTs),[123]​ brings together the efforts of seven European industry and research partners around manufacturing techniques and processes that allow reliable carbon nanotubes to be used for on-chip interconnections in the production of ULSI microchips.

Electrical cables and wires

Electrical current-conducting wires can be made from pure nanotubes or from nanotube-polymer composites. It has already been shown that carbon nanotube cables can be used successfully for the transmission of energy or data.[124] Recently, small cables have been manufactured with a specific conductivity higher than that of copper and aluminum;[125]​[126]​ these cables are the carbon nanotube cables with the highest conductivity and also the non-metallic cables with the highest conductivity. Recently, it has been shown that carbon and copper nanotube composites have a current carrying capacity almost one hundred times greater than that of pure copper or gold.[127] In addition, the electrical conductivity of this compound is similar to that of pure copper. Thus, this carbon and copper nanotube composite (CNT-Cu) has the highest current carrying capacity observed among electrical conductors. Thus, for a given cross section of electrical conductor, the CNT-Cu composite can withstand and carry a current one hundred times higher than that of metals such as copper and gold.

El uso de CNT como soporte de catalizadores en pilas de combustible puede reducir potencialmente el uso de platino en un 60% en comparación con el negro de humo. Los CNT dopados pueden permitir la eliminación completa del Pt.[2]​.

supercapacitor

MIT's Electronics Research Laboratory uses nanotubes to improve supercapacitors. The activated carbon used in conventional ultracapacitors has many small hollow spaces of various sizes, which together create a large surface area for storing electrical charge. But since charge is quantized into elementary charges, i.e., electrons, and each elementary charge needs a minimum space, a significant fraction of the electrode surface is not available for storage because the void spaces are not compatible with the charge requirements. With a nanotube electrode, the gaps can be tailored to size (some too large or too small) and consequently the capacity must increase considerably.[128]​.

A 40 F supercapacitor with a maximum voltage of 3.5 V using SWNTs grown without binders or additives achieved an energy density of 15.6 Wh kg and a power density of 37 kW kg.[129] CNTs can be attached to the charge plates of the capacitors to dramatically increase the surface area and therefore the energy density.

Batteries

The interesting electronic properties of carbon nanotubes (CNTs) have shown promise in the battery field, where they are being experimented with as a new electrode material, particularly the anode of lithium-ion batteries.[130] This is because the anode requires a relatively high reversible capacity at a potential close to that of metallic lithium, and a moderate irreversible capacity, observed so far only for graphite-based compounds such as CNTs. They have been shown to greatly improve the capacity and cyclability of lithium-ion batteries, as well as the ability to be highly effective buffer components, alleviating battery degradation that is often due to repeated charging and discharging. Furthermore, electronic transport at the anode can be greatly improved by using highly metallic CNTs.[131]​.

More specifically, CNTs have shown reversible capacities of 300 to 600 mAhg, with some treatments for them showing these figures increasing up to 1000 mAhg.[132] Meanwhile, graphite, which is the material most used as an anode for these lithium batteries, has shown capacities of only 320 mAhg-1. By creating composites from CNTs, scientists see many possibilities to take advantage of these exceptional capabilities, as well as their excellent mechanical strength, conductivities, and low densities.[131]​.

MWNTs are used in lithium-ion battery cathodes.[133]​[134]​ In these batteries, small amounts of MWNT powder are mixed with active materials and a polymeric binder, such as 1 wt% CNT loading on LiCoO cathodes and graphite anodes. CNTs increase electrical connectivity and mechanical integrity, which improves load capacity and service life.[2]​.

A paper battery") is a battery designed to use a sheet of paper-thin cellulose (which is the main component of plain paper, among other things) infused with aligned carbon nanotubes. thin, indicating the need for paper batteries. Recently,[133] it has been shown that CNT-coated surfaces can be used to replace heavy metals in batteries.[136] More recently, functional paper batteries have been demonstrated, in which a lithium-ion battery is integrated into a single sheet of paper through a lamination process as a composite with LiTiO (LTO) or LiCoO (LCO). while CNT films act as current collectors for both the anode and cathode. These rechargeable energy devices show potential in RFID tags, functional packaging or new disposable electronic applications.[137]​.

Improvements in lead-acid batteries have also been demonstrated, based on research conducted by Bar-Ilan University using high-quality SWCNTs manufactured by OCSiAl. The study demonstrated an increase in the useful life of lead-acid batteries of 4.5 times and an increase in capacity of 30% on average and up to 200% at high discharge rates.[138][139]​.

CNTs can be used to transport and desalinate water. Water molecules can be separated from salt by forcing them through electrochemically robust nanotube networks with controlled nanoscale porosity. This process requires much lower pressures than conventional reverse osmosis methods. Compared to a simple "Membrane (object)") membrane, it operates at a temperature 20 °C lower and at a flow rate 6 times higher.[138]​[139]​[140]​ Membranes using aligned and encapsulated CNTs with open ends allow flow through the interior of the CNTs. Very small diameter SWNTs are necessary to reject salt at seawater concentrations. Portable filters containing CNT meshes can purify contaminated drinking water. These networks can electrochemically oxidize organic contaminants, bacteria and viruses.[2]​.

CNT membranes can filter carbon dioxide from power plant emissions.

CNTs can be filled with biological molecules, which helps biotechnology.

CNTs can store between 4.2 and 65% hydrogen by weight. If they can be mass produced economically, 13.2 liters of CNT could contain the same amount of energy as a 50-liter tank of gasoline.

CNTs can be used to produce nanowires from other elements or molecules, such as gold or zinc oxide. In turn, nanowires can be used to make nanotubes from other materials, such as gallium nitride. For example, gallium nitride nanotubes are hydrophilic, while CNTs are hydrophobic, so they can be used in organic chemistry.

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