S.R. Dharwadkar

Thermodynamic stability of barium thorate, BaThO3, from a Knudsen effusion study

Abstract The Gibbs energy of formation of barium thorate was determined using the Knudsen effusion forward collection technique. The evaporation process could be represented by the equation BaThO 3 (s)=ThO 2 (s)+BaO(g) The vapour pressure of BaO(g) over the two-phase mixture of BaThO3(s) and ThO2(s) was obtained from the rate of effusion of BaO(g) and could be represented as ln (p/ Pa ) (±0.39)=−50526.5/T/ K +26.95 (1770≤T/ K ≤2136) The Gibbs energy of formation of BaThO3(s) could be derived from this data and represented as Δ f G°( BaThO 3 (s) )/ kJ mol −1 ±8.0=−1801.75+0.276T/ K

A microthermogravimetric system for the measurement of vapour pressure by a transpiration method

Abstract A novel automatic recording microthermogravimetric system for the measurement of vapour pressures at high temperatures by transpiration method was designed and fabricated. The performance of the system was checked by measuring the vapour pressure of anhydrous cadmium chloride in flowing dry argon in the temperature range 713–833 K. The vapour pressure of cadmium chloride in this range could be expressed as The enthalpy of sublimation of CdCl 2 derived from this equation at the mean temperature of investigation was found to be 167.2 ± 2.2 kJ mol −1 and compared very well with the value of 166.5 ± 4.7 kJ mol −1 reported by Skudlarski et al. (J. Chem. Thermodyn. 19 (1987) 857) from their recent Knudsen effusion mass spectrometric measurements. The present system can be used for measurement of vapour pressures up to 1425 K and is the first of its kind in which the mass loss due to vaporization is monitored continuously during the conventional transpiration experiment. The system described in this paper effects considerable saving of time in the experiment and minimizes the errors associated with mass measurements in the conventional transpiration method currently in use.

Lattice and grain boundary diffusion of chromium in superalloy Incoloy-800

The diffusion of chromium in Incoloy-800 has been studied by the serial sectioning technique using the radioactive tracer 51Cr in the temperature range 1060–1510 K for lattice diffusion and 775–1170 K for grain boundary diffusion. The lattice diffusion coefficient could be represented by the relation: DCr/Incoloy-800 = 3.24 × 10 −4 exp[(− 287.4 kJ/mol)/(RT)] m2/s. The grain boundary diffusion coefficients were evaluated for most of the experiments by Whipples method. For a few specimens, Suzuokas method was also applied. The two methods yielded values of grain boundary diffusion coefficients that were in good agreement with each other. The grain boundary diffusion coefficients could be represented by the equation: Dgb,Cr/Incoloy-800 = 5.80 × 10−5exp[(− 184.2 kJ/mol)/(RT)] m2/s. Segregation and mass transport along the grain boundaries have also been extensively studied by an autoradiographic technique. It has been observed that for small-grained specimens at low temperatures ( < 980 K) long-distance transport of the tracer atoms is mainly through the grain boundaries.

Gibbs energy of formation of barium thorate (BaThO3) by reactive carrier gas technique

Abstract The Gibbs energy of formation of BaThO3 was determined employing the heterogeneous reaction between the compound and water vapour involving the formation of gaseous barium hydroxide species according to the reaction BaThO3(s)xa0+xa0H2O(g)=ThO2(s)xa0+xa0Ba(OH)2(g). The vapour pressure of barium bearing species over the univariant mixture containing barium thorate and thorium dioxide as the condensed phases in equilibrium with a controlled pressure of water vapour was measured in the temperature range 1548–1683 K employing the automatic recording transpiration apparatus. The vaporization of BaThO3 was studied in the presence of flowing argon saturated with water vapour. The equilibrium constant of the above reaction could be expressed by the equation ln Kp (±0.03)=−20306/Txa0+xa05.37 (1548⩽T/K⩽1683). The Gibbs energy of formation for BaThO3 derived from these data could be expressed as ΔfG°〈BaThO3〉 (±38 kJ/mol)=−1775.8xa0+xa00.266T between 1548 and 1683 K.

Thermal decomposition of lanthanum formate

Abstract Thermal decomposition of lanthanum formate has been reinvestigated employing TG, DTA, GC, IR and X-ray diffraction techniques. Lanthanum formate was found to decompose according to the following scheme. The delineation of La2O(CO3)2 as an intermediate decomposition product is the main new finding in this investigation.

Thermodynamic stability of solid SrThO3

Abstract The Gibbs energy of formation of strontium thorate was determined by the Knudsen effusion forward collection technique. The evaporation process from a mixture of tungsten and strontium thorate in the Knudsen cell could be represented by the following heterogeneous equilibria: 5SrThO3(s)+W(s)=Sr2WO5(s)+5ThO2(s)+3Sr(g), 1670 Δ f G 0 ( SrThO 3 ( s )) (±5.0 kJ mol −1 )=−1953.6+0.367·T , (1670 Δ f G 0 ( SrThO 3 ( s )) (±7.0 kJ mol −1 )=−1960.2+0.369·T , (2135

Thermoanalytical investigation on the solid state synthesis of pure zirconium molybdate (ZrMo2O8)

Abstract The role of thermal techniques — thermogravimetry (TG) and differential thermal analysis (DTA) — in optimization of the experimental conditions in the synthesis of pure zirconium molybdate (ZrMo2O8) by the solid state reaction between the component oxides ZrO2 and MoO3 is highlighted. The two oxides differ in volatility by several orders of magnitude, the former being less volatile. The TG and DTA curves recorded for the mixture containing ZrO2 and MoO3 in the ratio 1:2 indicated that the reaction between the two oxides was completed at temperatures far below the melting point of pure MoO3, where its vaporization rate is also insignificant. The X-ray diffraction results for the mixtures heat treated at different temperatures showed that the solid state reaction between ZrO2 and MoO3 progressed at measurable rates at temperatures as low as 873 K and was completed in a few minutes between 925 and 975 K. The compound ZrMo2O8 formed and decomposed isothermally to ZrO2 by the loss of molybdenum-oxide-bearing vapour species above 1125 K.

The standard free energy of formation of ZrMo2O8 by the transpiration method

The standard free energy of formation for zirconium molybdate (ZrMo2O8) was derived from its vapour pressure measured in the temperature range 1029 to 1142 K employing the transpiration technique. ZrMo2O8 vaporizes incongruently according to the reaction n〈ZrMo2O8〉 → n〈ZrO2〉 + 2(MoO3)n, n = 3,4,5. The standard free energy of formation of ZrMo2O8 calculated from the partial pressure of (MoO3)3 in the vapour above ZrMo2O8 + ZrO2 mixture could be expressed by the relation ΔfG°〈ZrMo2O8〉 =(−2525.5±4.9) +(0.6115±0.0045)Tkj/mol (1029) < T/K < 1142). The values of ΔfG°〈ZrMo2O8〉 obtained in this work agree very well with those reported recently from solid electrolyte galvanic cell measurements.

A simple method of determining the activation energy of an isothermal solid-state decomposition reaction

A simple approach to determine the activation energy (E) of solid-state decomposition reactions is described. The activation energy is calculated from the slope of the logarithm of the maximum peak height of the isothermal DTA trace versus the reciprocal of the absolute temperature. The proposed method is applied in the study of the kinetics of thermal decomposition of cadmium carbonate. The activation energy calculated from this method (90.8±2.2 kJ mole−1) is in very good agreement with the value (87.5±2.5 kJ mole−1) obtained by the conventional method.RésuméOn décrit une méthode simple dapproximation pour déterminer lénergie dactivation (E) des réactions de décomposition en phase solide. Le calcul de lénergie dactivation seffectue à partir de la pente du logarithme de la hauteur maximale du pic de la courbe dATD isotherme en fonction de linverse de la température absolue. On a appliqué la méthode proposée lors de létude de la cinétique de la décomposition thermique du carbonate de cadmium. Lénergie dactivation calculée à partir de cette méthode (90.8±2.2 kJ mole−1) est en bon accord avec la valeur (87.5±2.5 kJ mole−1) obtenue par la méthode conventionnelle.ZusammenfassungEine einfache Annäherung zur Bestimmung der Aktivierungsenergie (E) von Festphasenzersetzungsreaktionen wird beschrieben. Die Aktivierungsenergie wird aus dem Anstieg des Logarithmus der maximalen Peakhöhe der isothermen DTA-Kurve als Funktion der reziproken absoluten Temperatur errechnet. Die vorgeschlagene Methode wird zur Untersuchung der Zersetzungskinetik von Cadmiumcarbonat eingesetzt. Die hiernach berechnete Aktivierungsenergie (90.8±2.2 kJ mol−1) ist in guter Übereinstimmung mit dem durch die konventionelle Methode erhaltenen Wert (87.5±2.5 kJ mol−1).РезюмеОписано простое приб лижение для определе ния энергии активации тв ердотельных реакций разложения. Э нергия активации выч ислялась их наклона кривой в коор динатах логарифм максимума в ысоты пика и обратной абсолютной температуры. Предлож енный метод был применен дл я изучения кинетики термического разлож ения карбоната кадми я. Вычисленная по этому методу энергия актив ация (90.8+2.2 кдж.моль−1) находится в хорошем согласии со значением 87.5± 2.5 кдж.моль−1, полученным обычным м етодом.

Partial phase diagram of CaO–TeO2 system

The partial phase diagram of BaO-TeO2 system was experimentally determined in the composition range 50-100 mol% TeO2 employing differential thermal analysis and x-ray diffraction techniques. Only three intermediate compounds BaTeO3, BaTe2O5 and BaTe4O9 melting congruently at 1000, 658 and 599 °C respectively, were observed. Each of these three line compounds exhibited two crystallographic phase transitions with corresponding transition temperatures at 802 and 982 °C, 608 and 648 °C, 576 and 591 °C respectively. The system exhibited three eutectic reactions i.e. between BaTeO3 and BaTe2O5 at 602 °C and 60 mol% TeO2 [L = BaTeO3(s) + BaTe2O5(s)], BaTe2O5 and BaTe4O9 at 596 °C and 76 mol% TeO2 [L = BaTe2O5(s) + BaTe4O9(s)] and another one between BaTe4O9 and TeO2 at 592 °C and 84.5 mol% TeO2 [L = BaTe4O9(s) + TeO2(s)].