2CaO・P_2O_5-3CaO・P_2O_5共晶組成融液の一方向凝固  [in Japanese] Unidirectional Solidification of the 2CaO・P_2O_5-3CaO・P_2O_5 Eutectic  [in Japanese]

人工骨材料などとして有用なCaO-P2O5系の高強度多結晶体を, 融液の一方向凝固により製造する基礎的条件を明らかにするために, 2CaO・P2O5-3CaO・P2O5共晶組成及びその近傍の組成の融液を白金るつぼ中でその底部から上方に向けて一方向に凝固した. その結果, 共晶組成の融液を1350℃以下の温度で溶融し, 同組成の微結晶集合体を種子結晶として, 20℃/cmの温度こう配下で2-20mm/hの速度 (R) で一方向に凝固すると, 気孔を含まず2CaO・P2O5結晶のマトリックス中に葉片状の3CaO・P2O5結晶が凝固進向方向に平行に一定の間隔 (λ) で並んだ多結晶体が得られることを見いだした. λとRの間には, 他の共晶組成の場合と同様にλ∝R-1/2の関係が認められた. ただし, これらの凝固物中には熱応力によって生じたと考えられるき裂がいくらか認められた. 初めの融液の組成が共晶組成からずれている場合には, そのずれが0.5wt%程度であっても粒状の結晶や気孔を含んだ凝固物しか得られず, また融液上部の最高加熱温度を1400℃以上とした場合には上部の結晶の配列が著しく乱れた凝固物しか得られなかった. これらの原因はともに融液中に組成的過冷が生じたためと推定した.

Calcium phosphate ceramics with high mechanical strength are of potential use as artificial bones. Fundamental conditions for preparing this kind of ceramics by unidirectional solidification of their melts were investigated. Starting materials with nominal compositions of the 2CaO⋅P2O5-3CaO⋅P2O5 eutectic and its neighbors (Table 1) were prepared by melting the mixtures of reagent grade chemicals of CaHPO4⋅2H2O and CaCO3 in a platinum crucible at 1400°C for 30min and casting them into another platinum crucible. The starting materials were remelted except for their lowest part about 15mm thick in the platinum crucible placed in a temperature gradient furnace (Fig. 2) and unidirectionally solidified upwards under various conditions (Table 2). The lowest part of the starting material left unmolten acted as seed crystals. When the starting material with the exact eutectic composition (E+1.0P2O5 in Table 3) were first remelted below 1350°C and then unidirectionally solidified at rates of 2-20mm/h under a thermal gradient of 20°C/cm (d-d′′′ in Table 2), ingots with regular structure were obtained (Fig. 5, E+1.0P2O5), In these ingots, lamellar α- and β-3CaO⋅P2O5 crystals were aligned parallel to the solidification direction in a α-2CaO⋅P2O5 crystal matrix. The crystallographic direction perpendicular to the (002) plane of the α-2CaO⋅P2O5, and those perpendicular to the (113) and (220) planes of the α- and β-3CaO⋅P2O5 crystals, respectively, coincided all with the solidification direction (Fig. 6), The lamellar spacing λ decreased from 16.5 to 6.0μm with increasing solidification rates R from 2 to 20mm/h according to the relation λ∝R-1/2, like for other eutectic systems (Fig. 7). The ingots were pore-free but contained some microcracks, as shown in Fig. 8. These microcracks arose probably from thermal stresses due to a large difference in thermal expansion between the 2CaO⋅P2O5 and 3CaO⋅P2O5 crystals, which were formed during cooling of the solidified ingots (Fig. 9 and Table 5). When the composition of the starting material deviated from the eutectic even by 0.5%, the resultant unidirectionally solidified ingots showed irregular structure and contained many pores (Table 3 and Fig. 5). The compositional deviation were found to occur when the starting material was prepared by melting above 1400°C, where the P2O5 constituent tended to vaporize separately from the melt. The formation of the irregular strlucture was attributed to a constitutional supercooling of the melts at the front of the growing crystals (Figs. 10 and 11).