Thermal decomposition, of freeze-dried

Grain growth of zinc oxide occurs at much lower temperatures than in titania, so that production of fairly crystalline ZnO nanopowders needs special precautions. The thermal decomposition of freeze-dried Zn(N03)2 at 260 to 270°C results in the formation of poorly crystalline, but coarse-grained ZnO powders. " At the same time, insulation of ZnO particles in the inert NaCl matrix by the method described before - led to no change in the size of the ZnO particles. The ZnO particles were obtained from freeze-dried precipitate of zinc hydroxide of 15 to 20 nm, and this size was maintained at least up to 600°C. "... 

The cryochemical method is also rather useful for the preparation of various metastable compounds. Thus thermal decomposition of freeze-dried oxalate complexes resulted in the formation of metastable monoclinic CuNb20g and CuTa20g with trirutile structure. An unusual polymorph of FeOOH—akageneite—was found during analysis of freeze-dried hydrated ferric oxides obtained from different precursors under various precipitation conditions. Soft oxidation of... 

Another solution-based method for material synthesis is based on the freezedrying of frozen aqueous solutions of components, followed by thermal decomposition of freeze-dried precursors [10]. The nitrates of a number of cations that are readily available and easily soluble in water are also used in this method. However, freezing of the nitrate solutions of transition metals and rare earth elements or their soluble complex compounds is accompanied by their vitrification [12]. Freeze-... 

Solid state reactant Thermal decompositions of a solid Oxidation or reduction of a solid Precipitation Solution heating or cooling Evaporative salting-out Chemical reaction with insoluble product Hydrothermal synthesis Forced insolubility Dissolution reprecipitation Evaporative Condensation Gas phase reaction with solid product Thermal decompositions Oxidation or reduction reactions Combination reactions with a solid product Solvent removal Spray drying Freeze drying Spray roasting Sol-gel synthesis Melt solidification... 

Freeze drying is used to dry either salt solutions or ceramic suspensions in a gentle way, avoiding thermal decomposition of the metal salts and aggregation of the particles. There are fovir steps in fieeze drying ... 

After a mixture of oxides is produced by the thermal decomposition of the freeze dried and calcined powder, fiorther solid-solid reactions can take place. Solid—solid reactions operate by the mechanisms discussed in Chapter 5, which include solid state diffusion and chemical reaction. Diffusion in ceramic solids is always ionic in nature and depends on defect or hole diffusivity, as well as electron conductivity. Once the ionic reactants are in close association, chemical reactions can take place, giving mixed oxides like BaTiOs and PbZrOs. 

Recent studies have demonstrated that the chemical homogeneity of the freeze-dried precursor in many cases has no decisive influence on its thermolysis mechanism. It is well known that thermal decomposition of solid solutions of salts can lead to the formation of individual oxides instead of complex oxide compounds. Similar phenomena were observed during thermal decomposition of amorphous freeze-dried precursors in spite of their high chemical homogeneity. " Meanwhile, the small grain size of thermolysis products and their high reactivity resulted in the fast synthesis of complex oxides soon after thermal... 

A simple and efficient method to obtain considerably smaller particles with a narrow size distribution from usual precursors was proposed by Ding et al. and Shi et al. Precursors obtained by freeze-drying of coprecipitated hydroxides (or by other chemical synthesis methods) are processed in a planetary mill with excess NaCl. Such a treatment reduces the size of the soft agglomerates formed during precipitation and insulates these aggregates from each other. The formation of complex oxide particles during thermal decomposition of the mixture occurs... 

Two-line ferrihydrite, for use as a catalyst, can also be synthesized by thermal decomposition of iron penta carbonyl, Fe(CO)5, in a stream of moist air at 500°C (Kosowski, 1993 Zhao et al.l993). It is a free flowing, reddish brown powder with a much lower bulk density than freeze-dried ferrihydrite prepared as above, probably because of much weaker aggregation. Small changes in the reaction conditions (water content of the air, duration of heating) may induce hematite formation. An analogous recipe involved slow thennal decomposition of trinuclear aceto-hy-droxy Fe "-nitrate for 20-40 hr in air in an attempt to simulate the red pigment on the Martian surface (Morris et al. 1991). 

Bellini, J.V., Machado, R., Morelli, MP. et al (2002) Thermal, structural and morphological characterisation of freeze-dried copper(ll) acetate monohydrate and its solid decomposition products. Mater. Res., 5,453-457. 

---