Taizo Hibino

Promotion of ZnAl2O4 formation by LiF: Part 1Effect of alumina and compaction pressure on zinc aluminate formation in the presence of lithium fluoride

The effects of the particle size of AI2O3 on the formation of ZnAI2O4 in the presence of LiF were investigated. The promotion of ZnAI2O4 formation depended on the dissolution rate of Al2O3 into an intermediate phase which contained LiF, ZnO and AI2O3, i.e. the dissolution of Al2O3 was a key step for the promotion of the reaction. In order to promote the reaction it was necessary that the intermediate phase containing AI2O3 spread over the whole of the specimen before a dense ZnAI2O4 layer started to grow around the AI2O3 particles. Rapid dissolution of Al2O3 of a finer particle size than 10μm was preferable for the promotion of ZnAI2O4 formation in the presence of LiF. Interference in ZnAI2O4 formation by the formation of a byproduct (LiAI5O8) was found in specimens containing a starting AI2O3 of grain size larger than 10μm.The formation of ZnAl2O4 in the presence of LiF was affected by the various states of Al2O3 and by the compaction pressure. The rate curves showed a typical sigmoid form and were analysed by the Hancock and Sharp method. The results indicated the presence of a main process controlled by nucleation which is followed by a diffusion-controlled process. The effects of the aggregation or agglomeration state of Al2O3 powder were examined with various Al2O3 powders, with compaction pressures of up to 60 M Pa on the sample pellets. The amount of ZnAl2O4 decreased with an increase in the degree of agglomeration or aggregation when the firing temperature was around 600° C, but no effect was observed at higher temperatures. Similar effects were also observed for the compaction pressures. These effects are interpreted as the results of the degree of formation of a liquid intermediate phase.

Effects of LiF on the Formation of BaTiO3 from BaCO3 and TiO2

炭酸バリウムと酸化チタンからメタチタン酸バリウムを合成する反応に対する弗化リチウムの反応促進作用を研究した. 弗化リチウムを添加すると炭酸バリウムの分解反応およびメタチタン酸バジウムの生成反応が促進されるのが認められた. 炭酸バリウム, 酸化チタンおよび弗化リチウムの三成分が共存する場合の炭酸バリウムの分解反応はこれら三成分が同時に相互作用することが必要であることを示した. この三成分の同時相互作用の際, リチウムイオンはバリウムイオンより強く炭酸塩に作用し, 酸化チタンとの反応により分解するのを促進していることを認めた. また弗素イオンはこの分解反応を促進する効果があることがわかった. 三成分の同時相互作用により生成した分解反応の中間生成物であるチタン酸リチウムと弗化バリウムはさらに反応してメタチタン酸バリウムを生成することを示した. 弗化リチウムの作用下でのメタチタン酸バリウムの生成は, このメタチタン酸リチウムと弗化バリウムの反応および弗化バリウムの弗素の作用下での炭酸バリウムと酸化チタンの直接反応の同時進行によるとして説明できる.

Reaction of ZnO with Polished Sapphire Surfaces

Many studies have been made on powder reactions by applying Jander’s equation and its revised form.1 Important assumptions in Jander’s equation are that the diffusing species completely covers the second type of particle and the reaction proceeds by the diffusion of the first into the second component particles forming a uniform product layer. Of course, the determination of the rate controlling diffusing species is one of the problems, but the nature of the contacts between reactant powder particles is the most important problem that should be considered. Komatsu discussed the probability that the particles of diffusing species surround the central particles.2 The authors showed that the contact surface area was reduced at least to about 5% of ideal contact surface in fine powder mixtures.3