Precursor characteristics

Numerous investigators have attempted to control the precursor structure and related solution chemistry effects with varying degrees of success, to influence subsequent processing behavior, such as crystallization tempera-ture.40-42,78,109 110 Particular attention has been given to precursor characteristics such as structural similarity to the desired product and the chemical homogeneity of the precursor species. For multicomponent films, this latter factor is believed to influence the interdiffusional distances associated with the formation of complex crystal structures, such as perovskite compounds. Synthetic approaches have been geared toward the preparation of multimetal species with cation stoichiometry identical to that of the desired crystalline phase.40 42 83 84... 

Mixed-ligand precursors are also frequently employed in CSD processing. For example, titanium tetraisopropoxide, which is too reactive to be directly employed in most CSD routes, may be converted into a more suitable precursor by a reaction with either acetic acid or acetylacetone (Hacac). Such reactions are critical in dictating precursor characteristics and have been studied extensively. - Using these reactions, crystalline compounds of known stoichiometry and structure have been synthesized that may subsequently be used as known precursors for film fabrication.Mixed-hgand molecules (carboxylate-alkoxide and diketonate-alkoxide ) represent complexes that are not easily hydrolyzed. A typical structure for one of these compounds is shown in Figure 27.3e. 

To understand why solution precursor characteristics can have such a significant impact on microstructural evolution, it is informative to consider how various features of the amorphous film, which are retained from the solution precursor, might influence the magnitude of AG,. As suggested in Figure 27.5, the free energy of CSD-derived amorphous films is expected to... 

Figure 2.32 Effect of precipitation conditions and precursor characteristics on (A) particle morphology and (B) composite particle microstructure in spray pyrolysis. (From Ref. 89.)...

Precursor Characteristics. The precursors used in the sonochemical synthesis were chosen due to their ability to readily decompose to produce metallic clusters, as well as their cost and commercial availability. For example, Fe(CO)5 is a liquid that can readily decompose at its boiling point to Fe clusters (29-30). The chromium and molybdenum precursors were chosen for similar reasons. In the case of the chem... 

Barium titanate powders were produced using either an amorphous hydrous Ti gel or anatase precursor in a barium hydroxide (Ba(OH)2) solution via a hydrothermal technique in order to discern the nucleation and formation mechanisms of BaTi03 as a function of Ti precursor characteristics. Isothermal reaction of the amorphous Ti hydrous gel and Ba(OH)2 suspension is believed to be limited by a phase boundary chemicd interaction. In contrast, the proposed BaTi03 formation mechanism from the anatase and Ba(OH>2 mixture entails a dissolution and recrystallization process. BaTi03 crystallite nucleation, studied using high resolution transmission electron microscopy, was observed at relatively low temperatures (38°C) in the amorphous hydrous Ti gel and Ba(OH)2 mixture. Additional solution conditions required to form phase pure crystallites include a C02-free environment, temperature >70°C and solution pH >13.4. Analysis of reaction kinetics at 75°C was performed using Hancock and Sharp s modification of the Johnson-Mehl-Avrami approach to compare observed microstructural evolution by transmission electron microscopy (7). 

Ti Precursor Characteristics. The phase of the as-received hydrous Ti gel was both XRD and TEM amorphous. Heat treatment of the as-received gel at 130°C produced a mixture of an amorphous phase with anatase. Increase in the reaction temperature to the 150°C hydrothermal treatment produced anatase crystallites, whereas calcination at 600°C produced a rutile phase. Further investigation proceeded with only the as-received gel and hydrothermally treated precursor, hereafter referred to as the anatase precursor. Observation of the precursor morphology by TEM indicated a fine gel structure and a lOnm granular nature for the as-received hydrous gel and anatase precursor, respectively. 

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