P型透明導電性酸化物の速度論・平衡論的安定性

摘要

p型透明導電性酸化物，CuAlO_2，CuGaO_2およびSrCu_2O_2について高温での速度論的安定性および平衡論的安定性を熱天秤・Ⅹ線回折測定および熱力学計算により評価した。空気中，高温ではCuが酸化されCuOとCuM_2O_4（M：Al，Ga）およびCuOとSr_(14)Cu_(24)O_(41)の混合物に分解することがわかった。本分解反応の速度は一次反応で表現でき，反応速度定数および活性化エネルギーを算出したところ，速度論的安定性が高い順にCuAlO_2，CuGaO_2，SrCu_2O_2となった。分解反応のエリンガム図を作成したところ，平衡論的に安定な温度・酸素分圧領域はCuAlO_2はCuGaO_2やSrCu_2O_2よりも広いこと，空気中でCuAlO_2，CuGaO_2，SrCu_2O_2は800℃，1200℃，1140℃以下では平衡論的に不安定であり，反応速度が充分に速ければ分解することがわかった。酸素分圧が低下すれば分解温度は低下し速度論的にp型透明酸化物が安定となることが期待できるが，SrCu_2O_2の場合は400℃，酸素分圧10~(-3)気圧下でも反応が速く，分解が観測された。%Stability of p-type transparent conducting oxides, CUAlO_2, CuGaO_2 and SrCu_2O_2, at high temperature has been studied by thermogravimetry, X-ray diffraction and thermodynamic calculation. It has been revealed that they decompose at high temperature in air according to the following chemical reaction formula; 4CuMO_2 + O_2 → 2CuO + 2CuM_2O_4 (M = Al, Ga) and 28SrCu_2O_2 + 17O_2 → 2Sr_(14)Cu_(24)O_(41)+8CuO. Assuming the first order reaction, kinetics of the chemical reactions and their activation energies have been evaluated from weight variation by time at the constant temperature measured by thermogravimetry. It was concluded that the order of the kinetic stability was CUAIO_2, CuGaO_2 and SrCu_2O_2. Ellingham diagram of the chemical reactions prepared in this study showed that thermodynamic stability of CuAlO_2 was higher than that of CuGaO_2 and SrCu_2O_2. It was also revealed that CUAIO_2, CuGaO_2 and SrCu_2O_2 are not thermodynamically stable in air below 800℃, 1200℃ and 1140℃, respectively, showing agreement with the weight increase due to the chemical reactions at constant temperatures in air. It can be expected that the p-type transparent conducting oxides would be kinetically stable with decreasing oxygen partial pressure, since temperatures where CuAlO_2, CuGaO_2 and SrCu_2O_2 decomposed decreased in the Ellingham diagram. However, decomposition of SrCu_2O_2 was observed even at 400 ℃ under oxygen partial pressure of 10~(-3) atm, which could be attributed to fast kinetics of the reaction even at 400 ℃ under such a low oxygen partial pressure.