凍結乾燥物から合成した Mg-Al スピネルの性質と焼結性  [in Japanese] Characterization and Sinterability of Mg-Al Spinel Powders Prepared with a Thermal Decomposition of a Freeze-dried Sulfate  [in Japanese]

硫酸塩混合水溶液を凍結乾燥し, 熱分解して得たスピネル粉末の性質とその焼結性を検討した.凍結乾燥物は樹枝状組織が放射状に配列した多孔質の球状粒子で, X線的には非晶質であり, 水和複合硫酸塩の結晶水が一部脱水したMgAl2(SO4)4・8-9H2Oに相当すると考えられた. これは1000℃で加熱すると直接スピネル単一相に変化した.1100°-1300℃仮焼で合成したスピネルは, 格子定数が8.082Åと完全結晶に近い値であるが, 見掛けのクリスタリットの大きさは600Å前後で小さく, 20-30m2/gの比表面積をもつ活性な粉末であった.1300℃で合成したスピネル粉末の初期焼結は, 体積拡散機構に支配され, 見掛けの活性化エネルギーとして127kcal/molの値が得られた. 1500℃, 2時間の焼成実験の結果, 合成したスピネル粉末は著しくち密化し, 1300℃で合成したものでは相対密度97%の焼結体が得られた.

The freeze-dried sulfate was prepared from the equi-mole mixed aqueous solution of magnesium and aluminum sulfates and calcined at 900°-1400°C for 1h. These calcined spinel powders were characterized and their sinterability was investigated. The results obtained were as follows:(1) The freeze-dried sulfate was amorphous and its composition was MgAl2(SO4)4⋅8-9H2O which corresponded to partially dehydrated MgAl2(SO4)4⋅22H2O.(2) The sulfate completely decomposed to spinel above the firing temperature of 1000°C.(3) Powders obtained from the calcination of sulfate at 1100°-1300°C were nearly perfect spinel crystals, showing that their lattice parameter was 8.082Å, but their apparent crystallite size was small, about 600Å, and their specific surface area was relatively large, 20-30m2/g.(4) Initial sintering of these specimens was mainly controlled by volume diffusion mechanism and the apparent activation energy of sintering for the specimen calcined at 1300°C was 127kcal/mol.(5) The bulk density of these speciments fired at 1500°C for 2h was higher than that of spinels prepared from oxide mixture, hydroxide mixture and coprecipitated materials reported previously and its maximum density was 3.47g/cm3, 97% of theoretical density, with specimen consisting of the powder calcined at 1300°C.