How to select reaction temperature of hydrothermal method?
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Hydrothermal synthesis can be defined as a method of synthesis of single crystals that depends on the solubility of minerals in hot water under high pressure. The crystal growth is performed in an apparatus consisting of a steel pressure vessel called an autoclave, in which a nutrient is supplied along with wat
Hydrothermal Reaction Sintering of Processing Materials
Hydrothermal reaction sintering is one of the methods used to produce high-density oxide ceramics. A large variety of oxide ceramics have been processed by this method. Oxide powders were obtained by reactions between solutions or nonaqueous liquid and metals. Hydrothermal reaction sintering is a kind of reaction with hot isostatic processing. The hydrothermal reaction sintering of oxides is as follows:
a.
low-temperature sintering—able to make sintered body even if materials have high vapor pressure, decomposition, and/or transit,
b.
able to make a very fine grain-sized body,
c.
able to make high-purity body,
d.
able to make high-density body,
e.
able to make uniform microstructures
f.
able to save energy due to low-temperature sintering,
g.
able to control valency.
Factors, such as purity, grain size and shape of starting materials, reaction temperature, pressure, duration time, ratio of metal and water, and kinds of salt in solution, have made hydrothermal reaction sintering unclear. Several ceramic powders have been processed using this technique. The important ceramic materials obtained by this method are: magnetite, wüstite, monoclinic ZrO2, chromia, monoclinic HfO2, alumina, and so on [361–366]. Figure 10.79 shows the relation between pressure and grain size or relative density hydrothermal reaction sintering of magnetite [362]. Figure 10.80 illustrates the SEM fracture surface of high-density hydrothermal reaction sintered Cr2O3 (100 MPa, 1000°C for 3 h, 0.01 M HNO3 solution) with a relative density of 99.1% [363]. Table 10.7 gives selected results of hydrothermal reaction sintering of monoclinic ZrO2 for 3 h [362]. Yin et al. [367] have studied the low-temperature sintering and mechanical properties of ceria and yttria co-doped zirconia crystallized in supercritical methanol.