The system BeO-ThO
2 was studied for the purpose of realizing the secondary fixed point for temperatures over 2000°C in the perfect binary system and of taking some informations on both nuclear materials and their ceramic coatings in a nuclear reactor.
Two sorts of DTA apparatus were used. One of them was to obtain the transition temperature of BeO and the eutectic temperature and also to measure peak areas attending with eutectoid. The other one was to determine the liquidus temperature of this system.
Both oxides were mixed with acetone in an agate mortar, pressed under the pressure of 3t⋅cm
-2 into discs and calcined for 25hrs at 1350°C in dry air. Measurements were made in a tungsten furnace filled with He gas of 1 atm.
The phase diagram of the system BeO-ThO
2 was obtained as a perfect binary system. It was found that both of the end members of this system showed no solid solubility, that the system had only one eutectic point and that the liquidus curve of this system was concave upwards near the eutectic composition which suggested that a deviation from an ideal solution would be large near that composition.
As the results, following data were obtained. Both α-BeO and ThO
2 coexisted below 2107±15°C at which temperatures α→β transition of BeO occurred all over the compositions. Both β-BeO and ThO
2 coexisted between 2107±15 and 2175±8°C. Eutectic temperature was 2175±8°C and its composition was at 21±1 mol%ThO
2. The heats of fusion of pure BeO and ThO
2 which were very difficult to obtain were estimated from the liquidus lines, assuming the ideal solution law near the both end members, obtained as 7.6±2.1 and 14.7±2.2kcal⋅mol
-1, respectively.
The transition temperatures of BeO in the system BeO-ThO
2 were constant all over the compositions, though its reproducibility was slightly decreased by the addition of ThO
2 component. The eutectic temperature was obtained with a very stable and reliable precision, so it would be possible to use as a secondary fixed point of temperature near 2200°C, like the transition temperature of pure BeO might be used near 2100°C.
抄録全体を表示