T. Gnanasekaran

Kinetics of thermal decomposition of Th(C2O4)2·6H2O

Abstract Thermal decomposition of thorium oxalate hexahydrate was investigated under nonisothermal and isothermal conditions in moist argon by using simultaneous thermogravimetric–differential thermal analyser. Nonisothermal experiments were carried out at various linear heating rates. Intermediates formed in each stage of the thermal decomposition of thorium oxalate hexahydrate were deduced. Kinetics of decomposition in each stage were evaluated from the dynamic TGA–DTA data by means of Coats and Redfern equation. The values of the activation energy (E) and the pre-exponential factor (A) of each stage of the thermal decomposition at various linear heating rates were calculated. These kinetic parameters were evaluated from isothermal experiments also and probable decomposition mechanism is proposed.

Phase diagram studies on the Na-Mo-O system

The phase diagram of the Na-Mo-O ternary system is of interest in interpreting the behaviour of structural materials in the sodium circuits of fast breeder reactors and sodium-filled heat pipes. Experiments involving heating of sodium oxide with molybdenum metal under vacuum, selective removal of oxygen from polymolybdates by reducing them under hydrogen and confirmation of the coexistence of various phase mixtures were conducted in the temperature range of 673 to 923 K. Phase fields involving molybdenum metal, dioxide of molybdenum and ternary compounds were derived from these results. The ternary phase diagram of the Na-Mo-O system was constructed and isothermal cross sections of the phase diagram are presented.

Differential scanning calorimetric studies on the phase diagram of the binary LiCl–CaCl2 system

Abstract The phase diagram of the binary LiCl–CaCl 2 system was investigated by differential scanning calorimetry (DSC). From the experimental results formation of a compound, LiCaCl 3 at 439±4°C is discerned. The compound undergoes peritectic decomposition at 490°C. The system shows a eutectic reaction between the compound and solid solution rich in LiCl at 485°C. Also, the system exhibits significant solid solubility of LiCl(s) in CaCl 2 (s) and vice versa. The complete phase diagram of this system is constructed and reported.

Threshold oxygen levels in sodium necessary for the formation of NaCrO2 in sodium-steel systems

Abstract A knowledge of the threshold oxygen level in liquid sodium necessary for the formation of NaCrO2 in sodium-steel systems is useful in the operation of fast breeder reactors. There is considerable discrepancy in the data reported in the literature. In order to resolve this, the problem was approached from two sides. Direct measurement of oxygen potential in the Na(l)-Cr(s)-NaCrO2(s) phase field using the galvanic cell In, In2O3/YDT/Na, Cr, NaCrO2 yielded: Δ G o2 = −800847 + 147.85 T J/mol O2 (657–825 K). Knudsen cell-mass spectrometric measurements were carried out in the phase field NaCrO2(s)-Cr2O3(s)-Cr(s) to obtain the Gibbs energy of formation of NaCrO2 as: ΔG o f,T(NaCrO2) = −870773 + 193.171 T J/mol (825–1025 K). The threshold oxygen levels deduced from G o f,T (NaCrO2) data were an order of magnitude lower than the directly measured values. The difference between the two sets of data as well as differing experimental observations from operating liquid sodium systems are explained on the basis of the influence of dissolved carbon.

Thermochemistry of binary Na-NaH and ternary Na-O-H systems and the kinetics of reaction of hydrogen/water with liquid sodium : a review

Abstract A review of the literature data on the binary Na–H and ternary Na–O–H systems has been carried out. Influence of dissolved oxygen on Sieverts constant for hydrogen in sodium is analysed and an expression for the variation of Sieverts constant with oxygen concentration is derived. Data on equilibrium hydrogen partial pressures over Na(l)–NaH(s) phase mixtures are assessed and an expression for variation of Gibbs energy of formation of NaH(s) with temperature is obtained. Analysis of the phase diagram and thermochemical information on the ternary Na–O–H system has been carried out. Kinetics of the reaction of water/steam and gaseous hydrogen with liquid sodium are also presented and the need to resolve the disagreement among the literature data is brought out.

On the phase relationships and electrical properties in the CaCl2CaH2 system

Abstract The phase diagram of the CaCl2ue5f8CaH2 system was investigated by using a differential scanning calorimeter and is reported. The hydride-ion conducting compound, CaHCI, is found to be peritectically melting. The total conductivity of CaCl2-5 mol% CaH2 solid electrolyte was measured by impedance spectroscopy and the electronic conductivity was determined by the DC polarisation technique in the temperature range of 673 to 796 K and hydrogen pressure range of 0.03–5 Pa. The activation energy for the ionic conduction was found to be 1.02 eV.

An electrochemical investigation of the thermodynamic properties of Na2Mo2O7 and Na2NiO3

Abstract The thermodynamic properties of the compounds Na 2 Mo 2 O 7 and Na 2 NiO 2 were determined by using the high temperature galvanic cells Pt,Ar + Co 2 + O 2 , Na 2 CO 3 |INASICONI|Na 2 Mo 2 O 7 + Na2MoO 4 , O 2 + CO 2 + Ar,Pt (I) and Pt,Ar, Ni + NiO + Na 2 NiO 2 |INASICON|Na 2 Mo2O 1 + Na 2 MoO 2 + MoO 2 , Ar,Pt (II) in the temperature range 650–800 K and 673–773 K, respectively. The reversible emf of cell-I, at P CO 2 = 112.6 and 282 Pa, and cell-II can be expressed as E I ′( P CO 2 = 112.6 Pa) = - 282.7 + 1.13T (± 2) mV, E I ′( P CO 2 = 282 Pa) = − 274.7 + 1.081 T (± 2) mV and E II = 1383 + -.230T (± 1.5) mV, respectively. The standard Gibbs energy of formation of Na 2 Mo 2 O 7 , and Na 2 NiO 2 from elements computed from the emfs of the cells and auxiliary information from the literature are expressed as Δ f G m °. 〈Na 2 Mo 2 O 7 〉 = −2240.0 + 0.618T (±2) kJ mol −1 and Δ f G m °〈Na 2 NiO 2 〉 = −684.1 + 0.257T (±7) kJ mol −1 , respectively.

Stability of ternary oxygen compounds of molybdenum in liquid sodium

Abstract The ternary system Na-Mo-O is of interest in sodium loops such as in the heat transport systems of LMFBRs. This paper reports the results of our investigations with a view to identifying the phases which could co-exist with liquid sodium and molybdenum. The limited number of studies reported in literature are first critically reviewed and shown to be inconsistent. Experiments involving in-sodium equilibrations, solid state reactions and pseudo-isopiestic equilibrations were carried out to understand the phase relations in the Na-Mo-O system. In addition to the above, oxygen potentials were measured in sodium to which molybdates of sodium were added, using a galvanic cell. From these results it could be deduced that Na 2 O(s) coexists with molybdenum and liquid sodium metal up to 681.1 K. Above this temperature the ternary oxygen compound, Na 4 MoO 5 (s) appears as the coexisting phase with the two metals. From the oxygen potential data, the Gibbs energy of formation of Na 4 MoO 5 (s) could be deduced.

Studies on the phase diagram of LiBr–SrBr2 system

Abstract Binary LiBr–SrBr 2 system was investigated using differential scanning calorimetry (DSC) and the equilibrium phases at different compositions were identified using X-ray diffraction (XRD). This system has a compound LiSr 2 Br 5 , and exhibits a eutectic reaction between this compound and LiBr at 434xa0°C and the eutectic has a composition of 35 mol% SrBr 2 . The compound LiSr 2 Br 5 undergoes peritectic decomposition at 484xa0°C. From the DSC and XRD results, phase diagram of the LiBr–SrBr 2 system is constructed.

Studies on the kinetics of oxidation of urania-thoria solid solutions in air

The oxidation behaviour of (UyTh1ˇy)O2 powders (ya 0:15; 0:30; 0:72 and 0:77) was studied by means of thermogravimetry. The oxidation was carried out under both isothermal and non-isothermal conditions. The limits of oxygen solubility in terms of O/M ratios of (UyTh1ˇy)O2 solid solutions, derived from the thermogravimetric data, were 2.08, 2.15, 2.35 and 2.36 for ya 0:15; 0:30; 0:72 and 0:77, respectively. A single-step oxidation was observed for (UyTh1ˇy)O2 solid solutions with ya 0:15‐0:77. The activation energy for the oxidation of (UyTh1ˇy)O2 powders with ya 0:15 and 0.30 (the final product is single-phase), was found to be (48.5 a 2.5) kJ mol ˇ1 . The activation energy for the oxidation of (UyTh1ˇy)O2 powders with ya 0:72 and 0.77 (the final product is a bi-phasic mixture) was found to be