Studies on the Crystallographic Transition in BeO by Means of DTA

• Published in: Journal of the Ceramic Association, Japan Vol. 80; no. 920; pp. 142 - 146
• Main Authors: SATA, Toshiyuki, TAKAHASHI, Tatsuo
• Format: Journal Article
• Language: English; Japanese
• Published: The Ceramic Society of Japan 1972
• ISSN: 0009-0255; 1884-2127
• DOI: 10.2109/jcersj1950.80.920_142

The existence of the crystallographic transition in BeO was reported for the first time by Engberg et al. in 1958. After this report many researchers measured its transition temperature independently using different experimental methods. Values reported in the literatures, however, disagree with each other. In this work a DTA apparatus was devised to study the thermal changes up to 2400°C. BeO-transition temperature was measured precisely and enthalpy change of the transition was evaluated from the area of the peak in the DTA curve. The transition temperature was also examined for the BeO containing various oxides to study the effect of additives. Results are as follows; 1) Pure BeO: Measurements were made for the BeO of four grades of purities (99.9, 99.5, 99.2 and 99%). Measured transition temperatures agreed with each other within the experimental errors, and was determined to be 2107±7°C (IPTS 1968). In this measurement the calibrations of the thermocouple (W 3% Re-W 25% Re) were made using the melting points of Al2O3 (2053°C, IPTS 1968), Pt and Rh. The experimental precision and accuracy were ±3° and ±7°C, respectively. (Sample weight of 20-40mg, heating or cooling rate of 3°C/min and atmosphere of Ar containing a small amount of H2). The β→α transition took place in the temperature ranges about 20°C lower than the α→β transition range. 2) Enthalpy measurement: The enthalpy change in BeO transition was calculated to be 56±10cal/g (1.3kcal/mol) with reference to the heat of fusion for Al2O3 (260cal/g). This value is in good agreement with the one evaluated using the drop technique by Conway et al. 3) Effect of additives: Oxide additives (CaO, MgO, Al2O3 Y2O3, B2O3, ThO2, ZrO2, SiO2, ZnO, NiO, SrO, BaO) were used in this study. Among them, CaO and MgO contents were 1, 2, 5 and 10mol% and the others were 5mol% (mixing in acetone, pressing at 3t/cm2 and calcining for 25hrs at 1350°C). The X-ray patterns showed no solid solubility for all oxides. These transition temperatures were varied within the experimental errors. Only the BeO containing MgO additives showed the decreasing transition temperatures at the rate of about 1°C/mol. From the above results it follows that the BeO-transition temperature will be led to the possibility for the second fixed point of temperature over the Al2O3-point.