Surface pressure crystalline particles

The surface pressures for small crystals are in general much larger than the equivalent pressures for liquid droplets because the surface tensions of crystalline solids are usually much greater than those of liquids. As an example, consider an NaCl nanocrystal with r, = 5 nm for the (100) face for which tr,- = 413 dyne cm (Tasker, 1979), The surface pressure A/ = 2(415)/(5)(10) dynes/cm or 1660 bars. Thus the internal pressures of nanometer-size solid particles in equilibrium with atmospheric pressure vapors may reach thousands of bars. However, small crystals formed under dynamic gas-phase conditions in which their shape Is controlled by heat and mass transfer may not have time to attain the equilibrium shape, so the use of (9.31) and (9.32) may not be quantitatively correct in practical applications. 

Precipitation from solution under hydrothermal conditions has been widely used for the synthesis of fine crystalline particles of various oxides [178-180]. The process involves heating reactants, such as metal salts, oxide, hydroxide, or even metal powder, in the form of solution or suspension, at certain temperatures. Water is the most widely used solvent. In this case, the precipitation temperatures are set between the boiling and critical points of water, i.e., 100-374 °C, while the pressures are up to 22.1 MPa, which is the vapor pressure of water at the critical point. Due to the presence of high pressures, hardened steel autoclaves are usually used to carry out hydrothermal reactions. The autoclaves have inner surfaces of which are lined with a plastic, such as Teflon, to prevent corrosion of the vessels. Similar to chemical precipitation method, hydrothermal synthesis also offers almost unlimited flexibility in combination of types and concentrations of starting reactants, additives, pH levels, temperatures, time durations, and so on. 

In general, pyrolants composed of a polymeric material and AN particles are smokeless in character, their burning rates are very low, and their pressure exponents of burning rate are high. However, black smoke is formed as i decreased and carbonaceous layers are formed on the burning surface. These carbonaceous layers are formed from the undecomposed polymeric materials used as the matrix of the pyrolant. When crystalline AN particles are mixed with GAP, GAP-AN pyrolants are formed. Since GAP burns by itself, the GAP used as a matrix for AN particles decomposes completely and bums with the oxidizer gases generated by the AN particles. 

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