BaTiO powder

Powder Preparation. The goal in powder preparation is to achieve a ceramic powder which yields a product satisfying specified performance standards. Examples of the most important powder preparation methods for electronic ceramics include mixing/calcination, coprecipitation from solvents, hydrothermal processing, and metal organic decomposition. The trend in powder synthesis is toward powders having particle sizes less than 1 p.m and Httie or no hard agglomerates for enhanced reactivity and uniformity. Examples of the four basic methods are presented in Table 2 for the preparation of BaTiO powder. Reviews of these synthesis techniques can be found in the Hterature (2,5). 

Problem 1 assumes that the BaTiOs powder is monosized. This is not actually the case, the powder used has a geometric mean size of 0.71 jum and a geometric standard deviation of 1.6. Determine the isothermal shrinkage at 1200°C of this BaTiOs sample during both initial and intermediate stage sintering. 

Li Wenjun, Shi Erwei and Zheng Yanqing, Preparation of BaTiO, powder by hydrothermal synthesis. J. the Chinese Ceramic Society, 27(6),1999 pp. 714-719. 

Anderson. D.A. et al.. Surface chemistry effects on ceramic processing of BaTiO, powders, in Ceramic Transactions, Messing, G.L., Fuller, R.R., and Housner, H, eds., American Ceramic Society, Columbus, OH, 1988, p. 485, cited after [412]. 

Figure 8 Transmission electron micrographs of BaTiOs powder (a) as prepared and (b) calcined at 700°C.

Hydrothermal synthesis of BaTiOs powders has been extensively studied [181-186]. One example is to use the reaction between Ti02 gels or fine anatase particles and Ba(OH)2 in a strongly alkaline solution (pH 12-13) at 150-200 °C. The... 

Kim GD, Park JA, Lee EIL, Lee DA, Moon JW, Kim JD (1999) Synthesis and sintering of BaTiOs powders by the glycine-nitrate process using metal caibonate and alkoxide. J Ceram Soc Jpn 107 691-696... 

The electrodes, which are necessary for measurements at frequencies lower than 100 MGz, can lead to appearance of the space charges near the electrodes, which obscures the obtained physical information. Latter space charge is especially high in thin films, where the special type and geometry of electrodes have to be chosen to accurately measure permittivity. In Fig. 2.6, we report the dielectric constant of BaTiOs powder measured by certain refined (for powders) method [22]. The measured dielectric constant has maximum near 70 nm, which defines the critical size for this case. The difference between latter and other values obtained by lattice constants measurements (see previous section) could originate from the influence of electrodes as weU as from different quality of the samples investigated by different authors. In the case of the Aims, the influence of substrate should also be taken into account. 

The above considered core-shell model gives an approximate description of radiospectroscopy spectra size effects in the cases, when it is difficult to find the coordinate dependence of resonant field. For the ferroics this problem can be solved more accurately. Really, the phase transitions, which are characteristic for ferroics (see Chap. 1), generate the order parameter. This parameter depends on the new phase symmetry, its crystalline field consfanfs and nanoparticles size. In the majority of cases, the latter constants determine the resonant fields and size effects in the corresponding spectra of ferroics. Below we calculate the EPR spectra for this case on the example of nanosize ferroelectric BaTiOs powders. 

Barium titanate. The continuous miniaturization of integrated electronic devices forces to manufacture active and passive components of smaller size, including multilayer capacitors (MLCCs) [285,286]. This accounts for a significant breakthrough in the synthesis of ultrafine BaTiOs powders in recent years [287]. 

Ultrafme pure BaTi03 powders were obtained by a modified oxalate precipitation method as described previously [13], The powders had a specific surface area of 57 m g and the particle size was nearly spherical from 20 to 30 nm. The main impurities contained in the powders were at the following levels 0.04 wt% Sr, 0.02 wt% Na, and 0.006 wt% K. The Ba/Ti atomic ratio was 1 0.003 for all the powders. The X-ray diffraction (XRD) patterns of nanocrystalline powders apparently correspond to a pseudo-cubic structure without peak splitting of lines such as (002) and (200). We also used Raman spectra to detect local symmetry of the nanocrystalline powder samples. It showed that the local symmetry in the nanopowder appears to be a cubic structure even at a very low temperature of 123 K. Therefore, XRD patterns and Raman spectra revealed that the BaTiOs powder exhibited the commonly reported pseudocubic perovskite structure. 

C.Pithan, D.Hemiings, R. Waser Progress in the Synthesis of Nanocrystalline BaTiOs Powders for MLCC International Journal of Applied Ceramic Technology 2 (1), (2005), 1-14. 

The alkoxides are dissolved in a mutual solvent (e.g., isopropanol) and refluxed for 2 h prior to hydrolysis. While the solution is vigorously stirred, drops of deionized, triply distilled water are slowly added. The reaction is carried out in a C02-free atmosphere to prevent the precipitation of barium carbonate. After drying the precipitate at 50°C for 12 h in a helium atmosphere, a stoichiometric BaTiOs powder with a purity of more than 99.98% and a particle size of 5-15 nm (with a maximum agglomerate size of < 1 p.m) is produced. Dopants can be incorporated uniformly into the powders by adding a solution of the metal alkox-ide prior to hydrolysis. 

Hydrothermal BaTiOs powders, particularly very fine powders (less than — 100 nm) prepared at lower temperatures, show some structural characteristics that are not observed for coarser powders prepared by solid-state reaction at higher temperatures. X-ray diffraction reveals a cubic structure that is normally observed only at temperatures above the ferroelectric Curie temperature of 125-130°C. The possible causes for the apparent cubic and nonferroelectric structure are not clear and have been discussed in detail elsewhere (74). They include the idea of a critical size for ferroelectricity and, particularly for powders prepared by precipitation from solution, the presence of a high concentration of point defects due to hydroyxl groups in the structure. 

The BaTiOs powders thermally treated at 700 °C for 2h were maintained in cubic phase and the average size was 25 nm. The samples were then heated in a furnace at 1100, 1150, 1200, 1250,1275, and 1300 °C for 2h, with a heating rate of 5°Cmin . Longer dwell times, 10 and 15h, were also used for the 1150°C sintering temperature. 

Finally, different thermal treatments were carried out on the xerogels prepared by the different routes at 300, 500, 700, 950, and 1150 °C for 1 h. Also, undoped BaTiOs powders were prepared with chelating agents, using a similar procedure, as a reference. 

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