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蛍石型化合物の比熱測定

Measurement of specific heat on fluorite compounds

廣岡 瞬 ; 松本 卓; 加藤 正人   ; 土持 亮太; 小笠原 誠洋*

Hirooka, Shun; Matsumoto, Taku; Kato, Masato; Tsuchimochi, Ryota; Ogasawara, Masahiro*

蛍石型の結晶構造を有するアクチニド酸化物やCaF$$_{2}$$などでは、融点近傍においてブレディッグ転移に起因する比熱の急上昇とピークが現れる。これは核燃料の物性として非常に重要な現象と考えられるが、アクチニド酸化物ではその現象が2000$$^{circ}$$C付近から起こるため測定が難しく、報告値にもバラツキきが大きい。本研究では、融点が1500$$^{circ}$$C以下と比較的測定が容易なCaF$$_{2}$$, SrF$$_{2}$$及び固溶の影響を評価するために(Ca,Sr)F$$_{2}$$を用いて比熱の測定を行い、これらの比較的高温領域における比熱の挙動について評価を行った。単体と比べて融点が低くなる(Ca,Sr)F$$_{2}$$では、比熱の急上昇の開始温度が低く、ピーク温度も低くなった。UO$$_{2}$$, PuO$$_{2}$$, (U,Pu)O$$_{2}$$の高温比熱については報告例が少なくバラツキも大きいが、融点との関係について同様の傾向が予想される。このように、蛍石型化合物の固溶体の高温比熱については、ノイマン-コップ則の適用が難しい可能性があることが示唆された。

Fluorite compounds such as actinide oxide and CaF$$_{2}$$ perform a rapid increase and a peak in the specific heat which are attributed by Bredig transition. The behavior is important as a nuclear fuel; however, the fact that the behavior occurs above 2000$$^{circ}$$C makes the experiment difficult, and the reported data are scattered. In this work, measurement of specific heat was carried out on CaF$$_{2}$$, SrF$$_{2}$$ and (Ca,Sr)F$$_{2}$$ for the relatively lower melting temperature of below 1500$$^{circ}$$C. (Ca,Sr)F$$_{2}$$ which has lower melting temperature than CaF$$_{2}$$ and SrF$$_{2}$$ showed the rapid increase of specific heat and the peak at lower temperature. It is expected that UO$$_{2}$$, PuO$$_{2}$$ and MOX that the high temperature data is limited and scattered could show the similar relationship between the specific heat and the melting temperature. Therefore, the results of the measurements on the fluorite compounds suggest the possibility that the Kopp's law is not applicable to the solid solution of fluoride materials at high temperature.

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