Powder preparation phase transformation

Homogeneous LaMn03 nanopowder with the size of 19-55 nm and with the specific surface area of 17-22 m2/g has been synthesized using a surfactant, sodium dodecyl sulphate (SDS) to prevent agglomeration [47], The sonochemically prepared LaMn03 showed a lower phase transformation temperature of 700°C, as compared to the LaMn03 prepared by other conventional methods which has been attributed to the homogenization caused by sonication. Also, a sintered density of 97% of the powders was achieved for the sonochemically prepared powders at low temperature than that of conventionally prepared powders. 

Equilibrium constants obtained for reactions involving metals were much less accurate because the experimental data on systems involving solids vary greatly. It is difficult to obtain good thermodynamic data on solids because of nonstoichiometric compositions, surface phases, and partial phase transformations. For example, Nunez et al. (8) found that the heat of formation of CuO depended on mode of preparation, state of subdivision, and previous heat treatment. Annealed, granular CuO was more stable by about 1 kcal/mole than finely divided powder. 

The importance of anatase crystallinity was emphasized in relation to the preparation and annealing procedures for achieving high photocatalytic performance [19,24,25,119,120]. The highest photoactivity always appeared at the onset of phase transformation of anatase to rutile. A linear relationship between photoactivity (normalized by the BET surface area) and crystallite size determined from FWHM of 101 diffraction line was reported for powders prepared from TTIP by either spray or gas pyrolysis, although the variations in BET surface area and crystallite size were limited to a narrow range, from 2.5 to 50 mVg and 10 to 33 nm [121],... 

The same acetate solution can be used for powder preparation. The zirconium acetate powder dried at 60°C for 4 days was crystallized into cubic Zr02 (by XRD) by calcination at 300 to 400°C. The phase transformation to tetragonal and to monoclinic with increasing... 

The diversity of sizes and shapes of ceramic powders prepared by homogeneous precipitation, phase transformation, or aerosol techniques are reviewed by Matijevic and Gherardi [163]. (See, e.g., Fig. 12.) The... 

Lin, C., Zhang, C. and Lin, J. (2007) Phase transformation and photoluminescence properties of nanocrystalline Zr02 powders prepared via the Pechini-type sol-gel process. Journal of Physical Chemistry C, 111, 3300-7. 

Zhang and Li [42] prepared ZnO nanoparticles by precipitation transformation method. Solution of zinc sulfate was reacted with sodium carbonate solution with vigorous stirring. To this, NaOH solution was added dropwise. The reaction was performed at temperatures from 25 to 70 °C. Washing, filtration and drying followed to obtain powder ZnO nanoparticles. Zn5(C03)2(0H)6 was formed as intermediate and phase transformation in presence of NaOH solution led to formation ZnO... 

Fluorhydroxyapatite solid solutions can be prepared by the sol-gel method [134,135]. Cheng et al. reported the control of fluoride content in fluorhydroxyapatite solid solutions by the amounts of triethanolamine (N(CH2CH20H)3) and trifluoroacetic acid (CF3COOH) in the mixed ethanol solutions of Ca(N03)2 and P0(CH2CH20H)x(0H)3 x with a Ca/P ratio of 1.67 [134]. After evaporation of the mixed ethanol solution at 150°C on a hot plate, the powder obtained, comprising a homogeneous mixture of calcium nitrate crystallites and amorphous calcium phosphates, is then heated at 500 or 900°C for 1 h to be transformed into the pure apatitic phase. 

It should be noted that it is possible to produce fully stabilized bodies with much higher fracture strengths than listed here but this requires the use of fine particle size, chemically prepared powders (3). The use of this type of material involves a number of penalties both in cost and processability that may be prohibitive for a high volume automotive application. In addition to the type of partially stabilized body described here, two other basic types of partially stabilized bodies have been reported (4, ). Both are classified as transformation toughened partially stabilized zirconias and involve different processing techniques to obtain a body with various amounts of a metastable tetragonal phase. While the mechanical properties of these materials have been studied extensively, little has been reported about their electrical properties or their stability under the thermal, mechanical and chemical conditions of an automotive exhaust system. 

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