Contenido
El propósito de la activación de EMC es fundamentalmente causar la destrucción de la estructura cristalina del material procesado, para convertirlo en un material amorfo.[38] Aunque este cambio aumenta la reactividad química del material procesado, no se produce ninguna reacción química "durante" el proceso de activación de EMC.
En su forma más simple, la mecanoquímica puede definirse como "un campo que estudia las reacciones químicas iniciadas o aceleradas por la absorción directa de energía mecánica".[15] Más técnicamente, se puede definir como una rama de la química que se ocupa de la "transformación química y fisicoquímica de sustancias en todos los estados de agregación producida por el efecto de la energía mecánica".[65] La IUPAC no contiene una definición estándar del término "mecanoquímica", sino que define una ""reacción" mecanoquímica" como una reacción química "inducida por la absorción directa de energía mecánica", al tiempo que señala "corte, estiramiento y molienda son métodos típicos para la generación mecano-química de propiedades reactivas".[66][67].
Más estrictamente, activación mecánica fue un término definido por primera vez en 1942 como un proceso que implica un aumento en la capacidad de reacción de una sustancia que permanece químicamente sin cambios.[68] Aún más estrictamente, la activación de EMC es una forma especializada de activación mecánica limitada a la aplicación de molienda de bolas de alta energía (HEBM) a materiales cementosos. De manera más restringida, la activación de EMC implica el uso de pulverización vibratoria, y usando exclusivamente sus propios medios de molienda.[38] Como se indica en un libro de texto académico de 2023 limitado a la mecanoquímica, la activación de EMC ha "demostrado de manera impresionante" sus efectos al provocar un cambio en la reactividad del material de cemento alternativo y las características físicas resultantes del hormigón.[15].
Thermodynamic justification
More specifically, the HEBM process can be described as increasing the chemical reactivity of a material by increasing its chemical "potential" energy. In EMC activation, the transferred mechanical energy is stored in the material as defects caused by the destruction of the crystalline structure of the material. Therefore, the process transforms the state of solid substances into other thermodynamically and structurally more "unstable" states, which allows an explanation of this greater reactivity as an increase in their Gibbs energy:[69].
In its simplest form, the HEBM process causes the destruction of crystalline bonds to increase the reactivity of a material.[70] From the thermodynamic perspective, any subsequent chemical reaction can decrease the level of excess energy in the activated material (i.e., as a reactant) to produce new components comprising a lower chemical energy and a more stable physical structure. On the contrary, to convert the preprocessed material into a more reactive physical state, the disorder generated during the HEBM process can be justified as equivalent to a “decrystallization” (and therefore an increase in entropy) that in part produces an increase in volume (due to the decrease in apparent density). A reverse process, sometimes called "relaxation", can be almost immediate (10 to 10 seconds) or take much longer (e.g. 10 seconds).[71] Ultimately, any general "retained" thermodynamic effect can be justified on the basis that any such "reverse" process is incapable of reaching an ideal thermodynamic end state on its own. As a result, in the course of mechanical activation of minerals, reverse "relaxation" processes cannot completely decrease the Gibbs free energy that has been created. Therefore, energy remains in the material, which is stored in the "defects" created in the crystal lattice.[72][73].
In general, the HEBM process generates a net thermodynamic effect:[74][75][76].
• - Structural disorder implies an increase in both entropy and enthalpy and therefore stimulates the properties of the crystal according to thermodynamic modifications. Only a small fraction (approximately 10%) of the excess enthalpy of the activated product can be considered as increased surface area.
• - Instead, most of the excess enthalpy and changed properties can be attributed primarily to the development of thermodynamically unstable states in the material lattice (and not as a reduction in particle size).
• - Since the activated system is unstable, the activation process is reversible, resulting in deactivation, recrystallization, entropy loss, and energy production by the system. That reverse process ("relaxation") continues until a thermodynamic equilibrium, but ultimately can never reach an ideal structure (i.e., one free of defects).
• - A more complete description of such an "activation" process also includes enthalpy, by which, according to the Gibbs-Helmholtz equation, the Gibbs free energy between the activated and non-activated solid state can be represented as:.
Where disorder in crystals is low, it is very small (if not negligible). On the contrary, in highly deformed and disordered crystals, the values of can have a significant impact on the Gibbs free energy generated.
Leaving aside the heat generated during the process due to effects caused during the activation process such as friction, the excess Gibbs free energy retained in the activated material can be justified due to two changes, namely, an increase in the specific surface area (), and of the defects of the crystal structure ().[77][76] In successful HEBM processes such as EMC activation:[78][79].
The relatively low value of () compared to the high value of () serves to further distinguish the HEBM process from general grinding or "milling" (where, instead, the only objective is to increase the surface area of the processed materials), allowing an explanation of the change in the entropy of the pulverized material in the form of elastic energy (stored in defects of the crystal lattice that can take years to "relax"), which is the "source of excess energy and Gibbs enthalpy".[77] Regarding enthalpy, four descriptors can be outlined to give an overview of the total change during said activation process:[78][80][81].
Because most of the energy required during the EMC activation process is absorbed by (), it is trivial. Therefore, the main functions for the enthalpy change are approximated by:.
In EMC activation, the above terms and are considered particularly noteworthy due to the nature of the observed changes in physical structure.[38] Therefore, the change in enthalpy caused during EMC activation can be approximated by:[80][81].
From the above thermodynamic construction, EMC activation results in a highly "amorphous" phase that can be justified as a large increase in and also a large increase in .[38][80][81] The benefits of EMC activation are large in , meaning that its reactivity is less dependent on temperature. In terms of the thermodynamic momentum of any reaction, the overall enthalpy of a reactant is not dependent on temperature, meaning that a material that has been subjected to HEBM with a corresponding elevation of can react at a lower temperature (since the "activated" reactant is less dependent on temperature for its progression). Furthermore, an EMC reaction can exhibit physical mechanisms at extremely small scales "with the formation of thin layers of SiO (Silicon(IV) oxide")", with the suggestion that EMC activation increases the proportion of favorable reaction zones.[45] Other studies have determined that the HEBM process can significantly reduce the temperature required for a subsequent reaction to occur (up to a three-fold reduction), so that a major component of the overall reaction dynamics is initiated in a "phase nanocrystalline or amorphous" to exhibit "unusually low or even negative values of the apparent activation energy" necessary to cause a chemical reaction.[82].
In general, EMCs are likely to be less dependent on temperature for the progression of a chemical pathway (see previous section on pozzolanic reactions), which may explain why EMCs provide “self-healing” benefits even at the low temperatures recorded in the Arctic.[83][84].
Physical justification (amorphization)
The large changes in , more particularly in the resulting values of and , provide insight into the effectiveness of EMC activation. Amorphization of crystalline material under high pressure conditions "is a rather unusual phenomenon" for the simple reason that "most materials actually undergo the reverse transformation from amorphous to crystalline under high pressure conditions."[85] Amorphization represents a highly distorted "periodicity" of the lattice element of a material, comprising a relatively high Gibbs free energy.[72][74] In fact, amorphization can be compared to a state "almost-melted".[73][75].
In all, like other HEBM processes, EMC activation causes crystal destruction due to extremely violent and disruptive factors occurring at the nanoscale of the material being processed.[86] Although short-lived and highly focal, the processes repeat at high frequency: therefore, those factors are believed to mimic the pressures and temperatures found deep within the Earth to cause the required phase change.[2] For example, Peter Thiessen") developed the "magma-plasma model" which assumes that localized temperatures (greater than 10 Kelvin) can be generated at the various impact points to induce a momentary excited plasma state in the material, characterized by the ejection of electrons and photons along with the formation of excited fragments (see diagram above).[87] Experimental data collected since 1975 on the generation of localized cracks, itself itself an important component of EMC activation, have confirmed elevated temperatures in this process.[88].
Vibratory ball mills (MVB)
For EMC activation, the HEBM method used is a vibrating ball mill (MVB).[38] An MVB uses a vertical eccentric drive mechanism to vibrate a closed chamber up to many hundreds of cycles per minute. The chamber is filled with the material that is processed along with specialized objects called "grinding media." In their simplest format, these media can be simple balls made of special ceramic. In practical terms, EMC activation deploys various grinding media of different sizes, shapes and compounds to achieve the required mechanochemical transformation.[4].
It has been suggested that an MVB will grind 20 to 30 times more than a rotary ball mill, reflecting that the mechanism of an MVB is especially efficient.[89].
In simple terms, the compressive force acting between two "identical" balls colliding in an MVB can be expressed as:[90].
It can be seen that an increase in impact speed increases . The size and mass of the grinding media also contribute to the efficiency of the process. The denominator term incorporates , meaning that the nature of the material used for the grinding media is an important factor (it is ultimately squared in , so its negative value is of no consequence). More fundamentally, due to the rapid vibration, a high acceleration is imparted to the grinding media, whereby continuous, brief and sharp impacts on the cement result in a rapid reduction in particle size.[89] Furthermore, high pressures and shear stress facilitate the required phase transition to an amorphous state both at the point of impact and during the transmission of shock waves that can produce pressures even higher than the impact itself.[86].
For example, the contact time of a two-ball collision can be as short as 20 μs, generating a pressure of 3.3 GPa "Pascal (unit)") upward and with an associated ambient temperature rise of 20 Kelvin.[86] Due to the short duration of the impact, the rate of change in momentum is significant, generating a shock wave of a duration of only 1-100 μs but with an associated pressure of 10 GPa upward and a focal and very localized temperature (that is, on a nanoscopic scale) up to several thousand degrees Kelvin.[86] To put this in context, a pressure of 10 GPa is equivalent to that exerted by about 1,000 kilometers of seawater. As another example, the impact of two identical 2.5 cm diameter steel balls with a speed of 1 m/s will generate a collision with an energy density of more than 10 joules "Joule (unit)")/m; and with alumina balls of the same 2.5 cm diameter and a speed of 1 m/s an even higher energy density is generated.[90] Collisions occur on a very short time scale and therefore the "rate of energy release over the relatively small contact area can be very high".[90].
Scientific background of EMC activation:.