Solid-state reactions pressure effects

Hydrothermal routes Under ambient conditions, the low reaction temperature and fast precipitation rate have deleterious effect on the crystallization and optical performance of rare earth vanadate nanomaterials. Referring to traditional solid-state reactions, bulk YV04 Eu phosphors require a calcinations temperature above 1300 K, but it is too high for the preparation of nanomaterials. Alternatively, hydrothermal routes could provide the adequate energy for solution phase reactions, which have been widely described in preparation of ceramic powders. The high pressure and temperature largely promote the dissolution-reprecipitation process, so as to decrease the lattice defects of NCs. With fine modulation, this method is also efficient to produce nano-sized crystals. 

Some typical examples of the first two cases are listed in Tables 3 and 4, respectively. Since solid-state reactions and transformations are accelerated at elevated temperatures, high pressure synthesis almost automatically assumes treatments at elevated temperatures. Reaction times are usually short, 0.5-Ih. However, discrimination between these kinetic effects and thermodynamical stabihty should always be kept in mind. 

The major mechanical forces that affect chemical processes—coupling P->C—are mechanical stresses or pressures that deform crystal lattices, affect the solid density and chemical potentials, and cause disaggregation or aggregation of solid particles on a macroscopic scale. The reverse coupling of the chemical effects on solids—C- P -includes a very broad category of chemical reactions in a solid state, reactions of mineral or biogenic solids with waters and atmospheric gases, and corrosion of metals. 

A valuable approach for measuring thermal degradation kinetic parameters is controlled-transformation-rate thermal analysis (CRTA) - a stepwise isothermal analysis and quasi-isothermal and quasi-isobaric method. In this method, some parameters follow a predetermined programme as functions of time, this being achieved by adjusting the sample temperature. This technique maintains a constant reaction rate, and controls the pressure of the evolved species in the reaction environment. CRTA is, therefore, characterised by the fact that it does not reqnire the predetermined temperature programmes that are indispensable for TG. This method eliminates the nnderestimation and/or overestimation of kinetic effects, which may resnlt from an incomplete understanding of the kinetics of the solid-state reactions normally associated with non-isothermal methods. 

Arsenious oxide in the solid state is not affected by oxygen under ordinary conditions,4 but if subjected at high temperature to oxygen under pressure oxidation to the pentoxide results. Thus, if heated at 400° to 480° C. with oxygen at pressures of 130 to 180 atm., oxidation occurs, but is incomplete the amount oxidised increases with the temperature.3 According to Razuvaev and Malinovskil,6 at 200° to 300° C. the optimum pressure for oxidation by air is 60 to 80 atm., and under these conditions the reaction is complete in about 20 minutes finely divided iron has a wreak and copper a strong catalytic effect. In the presence of potassium iodide or activated carbon, a suspension of the oxide in water is oxidised by air or oxygen at 130° to 140° C. and 4 to 5 atm. pressure.7 The usually accepted reaction is... 

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