Pulse Calorimetry Research Articles

Heat capacity measurement of U1−yLayO2 (y=0.044, 0.090, 0.142) from 300 to 1500 K

Heat capacities of U1−yLayO2 were measured by means of direct heating pulse calorimetry in the temperature range from 300 to 1500 K. An anomalous increase in the heat capacity curve of each sample was observed similarly to the case of U1−yGdyO2, found recently in our laboratory. As the lanthanum content of U1−yLayO2 increased, the onset temperature of an anomalous increase in the heat capacity decreased and the excess heat capacity increased. The enthalpy of activation (ΔHf) and the entropy of activation (ΔSf) of the thermally excited process, which cause the excess heat capacity were obtained to be 2.14, 1.63 and 1.50 eV and 39.4, 34.2 and 31.8 J·K−1·mol−1 for U0.956La0.044O2, U0.910La0.090O2 and U0.858La0.142O2, respectively. The values at zero La content extrapolated by using the data of ΔHf and ΔSf for U1−yLayO2 were in good agreement with the experimental values of undoped UO2 so far reported, similarly to the case of Gddoped UO2. The electrical conductivities of U1−yLayO2 (y=0.044 and 0.142) were also measured as a function temperature. No anomaly was seen in the electrical conductivity curve. It may be concluded that the excess heat capacity originates from the predominant contribution of the formation of oxygen clusters and from the small contribution of the formation of electron-hole pairs.

Heat capacity measurements of U 0.80Zr 0.20 and U 0.80Mo 0.20 alloys from room temperature to 1300 K

The heat capacities and the electrical conductivities of U 0.80Zr 0.20 and U 0.80Mo 0.20 alloys were measured by means of direct heating pulse calorimetry in the temperature range from 300 to 1300 K and from 300 to 1150 K, respectively. Four peaks were observed in the heat capacity of U 0.80Zr 0.20 corresponding to four phase transitions. The transition temperatures were fairly in good agreement with those estimated from the phase diagram of the U-Zr system reported previously. Two peaks were also observed in the heat capacity of U 0.80Mo 0.20 corresponding to two phase transitions. The transition temperatures of U 0.80Mo 0.20 were in good agreement with those estimated from the phase diagram of the U-Mo system summarized previously. The thermal conductivity of U 0.80Zr 0.20 was calculated from the heat capacity obtained in this study and the thermal diffusivity data recently reported by Takahashi et al., and was very close to that of U 0.75Zr 0.25 experimentally determined by Leibowitz et al. The thermal conductivity of U 0.80Zr 0.20calculated from the electrical conductivity in this study on the basis of the Wiedemann-Franz's law was a little lower than that calculated from the heat capacity and the experimental values of U 0.75Zr 0.25 by Leibowitz et al. The thermal conductivity of U 0.80Mo 0.20 was also calculated from the electrical conductivity on the basis of the Wiedemann-Franz's law, and fell between the experimental values of U 0.77Mo 0.23 and U 0.88Mo 0.12.

Catalytic conversion of cyclohexylhydroperoxide to cyclohexanone and cyclohexanol

The low-conversion air oxidation of cyclohexane yields a mixture of cyclohexylhydroperoxide, cyclohexanone and cyclohexanol. The cyclohexyl-hydroperoxide is converted to additional cyclohexanone and cyclohexanol before the mixture is concentrated. Cobalt octoate is an active but short-lived catalyst for this transformation. Using a combination of pulse calorimetry and chemical luminescence techniques, a new family of long-lived catalysts has been discovered. These catalysts, based upon the (bis(2-pyridyl-imino)isoindolinato) ligand, are very active and long-lived, allowing cyclo-hexylhydroperoxide to be converted in a selective, low temperature process. The structure of one of these active catalysts, bis[bis((3-methyl-2-pyridyl)-imino)isoindolinato]cobalt( II), has been determined crystallographically. Under reaction conditions, it is likely that the active form of the catalyst has lost one ligand.

Heat capacity measurement of U1−yGdyO2 (0.00 ≦ y ≦ 0.142) from 310 to 1500 K

Heat capacity measurement of UO 2.004, U 0.956Gd 0.044O 2.000 U 0.927Gd 0.073O 1.998,U 0.101Gd 0.101O 2.001 and U 0.858Gd 0.142O 1.998 was conducted from 310 to 1500 K by means of direct heating pulse calorimetry. An increase in the heat capacity of each sample was observed at high temperatures and the excess heat capacity increased as the gadolinium content of U 1−y Gd y O 2 increased. The enthalpy of activation, ΔH f , of the thermally activated process which causes the excess heat capacity was obtained as 2.25, 1.88, 1.75,1.10 eV for U 0.956Gd 0.044O 0.073. U 0.927Gd 0.073O 1.998, U 0.899Gd 0.101O 2.001 and U 0.858Gd 0.142O 1.998, respectively. The enthalpy of activation for UO 2 was estimated to be about 3.1 eV by extrapolation to UO 2, which is in good agreement with that obtained by Szwarc for the excess heat capacity of UO 2. This value is higher than that of the thermally activated process for the formation of an electron-hole pair obtained by the reported theoretical calculation, but lower than that for the formation of Frenkel pairs of oxygen. It may be concluded that the excess heat capacity is originated from the predominant contribution of the formation of oxygen clusters and from the small contribution of the formation of electron-hole pairs.

Calorimetric study of the phase transition in NbO 2

The heat capacity and the electric conductivity of NbO 2 were measured by means of directheating pulse calorimetry from 400 to 1170 K. A heat-capacity anomaly and a rapid increase of electric conductivity were observed in the process of phase transition near 1080 K. The discrepancy among the transition temperatures reported by various authors was explained by the results of simultaneous measurement of the heat capacity and the electric conductivity. The entropy change due to the phase transition was calculated in terms of the 4d//-band electronic and the harmonic vibrational contributions, and a large contribution by the electronic entropy to the total entropy was suggested. The change of the electronic band structure of NbO 2 during the phase transition was discussed in comparison with that of VO 2.

Heat capacity and electrical conductivity of non-stoichiometric U 4O 9-y from 300 to 1200 K

The heat capacity and the electrical conductivity of non-stoichiometric U 4O 9-y with various compositions were measured simultaneously by direct heating pulse calorimetry from 300 to 1200 K. As well as the heat capacity anomaly due to the α-β transition around 350 K, two small heat capacity anomalies due to the β-γ transition were observed around 1000 and 1100 K, which are superimposed on a monotonie increase in the heat capacity above 800 K, presumably due to the onset of the γ- U 4 O 9- y - UO 2+ x transition. The change in the slope of the electrical conductivity curve as also observed at the phase transitions. The excess entropy due to the overall transition from α-U 4O 9to UO 2+x was evaluated to be 5.95 J K -1mol -1, which is in agreement with the value calculated on the assumption that the excess entropy consists of the contribution of the electronic disordering of U 4+ and U 5+ ions and that of the atomic disordering of oxygen atoms.

Pulse calorimetry and transient measurement of thermal properties at high temperatures

A system is described for the transient (subsecond) measurement of selected thermal and related properties of electrically-conducting substances in the temperature range 1500 K to the melting point of the specimen. The method is based on resistive self-heating of the specimen from room temperature to any desired high temperature in less than 1 s by the passage of an electrical current pulse through it; and on measuring and recording the experimental quantitites every 0.4 ms with a full-scale signal resolution of one part in 8000. The system has been used to measure heat capacity, electrical resistivity, hemispherical total emittance, normal spectral emittance, and the melting point of selected refractory elements and alloys. The results of preliminary experiments have shown the potential application of the system to measurements of temperatures and energies of solid–solid phase transformations and heat of fusion at high temperatures.

Specific Heat Determinations by Pulse Calorimetry Utilizing a Digital Voltmeter for Data Acquisition

A pulse calorimeter for measuring the specific heat of electrical conductors from 25 to 900°C is described. The technique employed was unique in that a digital voltmeter, which was capable of recording 400 readings per second to a readability of 0.1% of full scale, was utilized to measure the transient temperature and power signals requisite to the method. Using this technique, a calculated determinate accuracy of 99% was achieved. Measurements of the specific heat of high-purity iron from 25 to 900°C confirmed this calculated accuracy and were repeatable to within ±0.5%. The calorimeter was capable of measuring the specific heat rapidly at temperature intervals as small as 0.1 C°. Modifications are suggested to decrease the measurement errors to 0.5% and to increase the temperature range from −190 to 1400°C.