Barium titanyl oxalate

The thermal decomposition of barium titanyl oxalate tetrahydrate, BaTi0(C204)2.4H20, occurs in three stages [105] (i) dehydration, (ii) decomposition of the anhydrous oxalate to the carbonate, and (iii) decomposition of the carbonate forming barium titanate. Isothermal ar-time curves for stage (ii), 509 to 599 K in vacuum, derived from separate measurements of pressures of evolved CO and COj, were deceleratory and superimposable up to ar= 0.3. CO evolution was slower beyond ar= 0.3 and a diffusion mechanism was proposed, , = 189 kJ mol . 

Fig. 5.16 Temperature dependence of transformation percentage, weight loss rate and thermal effects during barium titanyl-oxalate decomposition with heating rate of 480 °C/h [291]...

The temperature of calcination is the most sensitive parameter for the particles size. The details of reaction mechanism of BaTiOs formation from barium titanyl oxalate have been studied a lot by thermochemical methods (TGA, DTA), X-ray diffraction, gas chromatography analysis and infrared spectroscopy (FTIR). The decomposition of barium titanyl oxalate proceeds in four stages as it is shown in Fig. 5.16. 

The summary of barium titanyl oxalate decomposition for different heating rates can be represented by the equations of mass balance (as in Fig. 5.17), which show the increasing role of oxygen with increasing heating rate (Table 5.9). 

An isothermal mode of process does not allow easy control of particle size distribution and agglomerate size and strength. Alternatively, the heating rate during calcination is an effective control parameter for the particle size distribution during non-isothermal reaction with controlled rate [281, 282, 291]. Thermal decomposition of barium titanyl oxalate BaTi0(C204)2 was performed in a wide... 

Fig. 5.17 Temperature dependence of gas volume released during thermal decomposition of barium titanyl oxalate under heating rates (a) 50, (b) 100, (c) 300 °C/h, (d) rate controlled regime [283]...

Table 5.9 Analysis of gas release on barium titanyl oxalate decomposition...

Fig. 5.18 Specific surface area of barium titanate nanocrystalline powder vs. heating rate. Data corresponds to 100 % decomposition of barium titanyl oxalate (a), at various temperatures (b) [281]...

Fig. 5.19). The input barium titanyl oxalate powder has specific surface area 1 m /g. Therefore, the coefficient of refining rox/rut reaches 10 0 times on oxalate decomposition. Using more dispersed oxalate, however, is not reasonable due to the small particles coalescence on heating, and therefore, the oxalate grinding has almost no effect on the end of the BaTiOs synthesis. The morphology of nanoparticles depends on the gas release rate during the decay of oxalate, and hence the heating rate determines density of nucleation and nuclei coalescence probability. In addition, the increase in heating rate leads to a change in the mechanism of oxalate oxidation as described above. Structurally barium titanyl oxalate crystal transforms to the microreactor - particles of resin-like phase, size and activity of which can be flexibly controlled by the heating rate. The general view of the reactor is shown in Fig. 5.20.

Fig. 5.20 Microreactor for synthesis of barium titanate nanoparticles - former particle of barium titanyl-oxalate of 1.5 p,m size including BaTiOs nuclei...

Vasyl kiv, O.O., Ragulya, A.V., Skorokhod, V.V. Synthesis and sintermg of nanocrystaUme barium titanate powder under nonisothermtd conditions. II. Phase analysis of the decomposition products of barium titanyl-oxalate and the synthesis of barium titanate. Powder MetaU. Metal Ceram. 36(5-6), 277-282 (1997)... 

Gallagher, P.K., Schrey, F. Thermal decomposition of some substituted barium titanyl oxalates and its effect on the semiconducting properties of the doped materials. J. Am. Ceram. Soc. 46, 567-573 (1963)... 

Wada, S., Narahara, M., Hoshina, T, Ktikemono, H., Tsurumi, T Preparation of nm-sized BaTiOs particles using a New 2-step thermal decomposition of barium titanyl oxalate. J. Mater. Sd. 38, 2655-2660 (2003)... 

Figure 8 Static yield stress of urea coated barium titanyl oxalate /silicone oil suspension is plotted as a function of applied electric field for two solid concentrations. Inset logarithm of the current density J plotted as a function of iE. Reproduced with pennission from W. Wen, X. Huang, S. Yang, K. Lu, and P. Sheng, Nature Materials, 2(2003)727...

Figure 7-3 (author s unpublished experimental data) demoiKtrates the results of the impedance analysis for a medium composed of insulating-base silicon oil with characteristic resistance 10 ohm and capacitance 5 10" F with added 25% urea-coated barium titanyl oxalate conductive particles with characteristic resistance 10 ohm and capacitance 10" F at 25 °C. The pure base oil shows a single relaxation, and particle-containing samples show relaxations at high frequency due to the conductive particles and at low frequency due to the base fluid in the complex modulus plot. 

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