G.A. Rama Rao

Oxidation and hydrolysis kinetic studies on UN

Abstract The reaction of oxygen and water vapour with UN microspheres containing 0.78 and 10.9 mol% UO2 as impurity was studied under non-isothermal heating conditions in a thermobalance under different partial pressures of oxygen, a fixed pressure of water vapour in argon, and in air. Uranium mononitride was ultimately converted to U3O8, with the formation of UO2 and U2N3 as intermediates. The end product of pyrohydrolysis was UO2. The kinetic parameters were evaluated and the mechanism of the reaction was suggested. Different kinetic models were used to explain the oxidation behaviour of UN.

Structural and thermochemical studies on Cr2TeO6 and Fe2TeO6

Abstract Cr 2 TeO 6 and Fe 2 TeO 6 were prepared by the solid-state reaction route. The crystal structure was derived for both compounds from X-ray powder diffraction data. Cr 2 TeO 6 and Fe 2 TeO 6 are isostructural and have the trirutile structure. The Gibbs free energy of formation (Δ f G °) for Cr 2 TeO 6 and Fe 2 TeO 6 was obtained from vapor pressure data employing the Knudsen Effusion Mass Loss technique (KEML) and is given by the relation Δ f G° Cr 2 TeO 6 (s) =(−1651.6+0.5683T)±15 kJ/mol (1014–1100 K) Δ f G° Fe 2 TeO 6 (s) =(−1234.3+0.4729T)±15 kJ/mol (979–1052 K).

Thermochemical and kinetic studies on ThTe2O6

ThTe2O6 was prepared by a solid-solid reaction route and was characterised by X-ray diffraction and thermal methods. Thermal studies show that ThTe2O6 vaporizes incongruently as: ThTe2O6(s) ↔ ThO2(s) + 2TeO2(g). The vapour pressures of TeO2(g) were measured over ThTe2O6(S) + ThO2(s) and over TeO2(s, 1) in the temperature range 1000–1131 K employing Knudsen cell mass loss technique. From the partial pressures of TeO2(g), the standard Gibbs free energy of formation (ΔfG°) of ThTe2O1(S) was derived and it can be represented by the equation: ΔfG°〈ThTe2O6, s, T〉 ±21.0 kJ/mol= −1943.2+0.5506T (K), (1000 K < T < 1131 K). The kinetics of decomposition under isothermal heating conditions in flowing air were studied to determine the activation energy, reaction mechanism and rate constants.

Vaporization behaviour and Gibbs energy of formation of Ni2Te3O8, NiTe2O5 and Ni3TeO6

Abstract Three compounds Ni 2 Te 3 O 8 (s), NiTe 2 O 5 (s) and Ni 3 TeO 6 (s) in Ni-Te-O system were prepared by the solid state reaction route and characterised by X-ray powder diffraction and thermal methods. The standard Gibbs free energy of formation (▴ f G °) of Ni 2 Te 3 O 8 (s), NiTe 2 O 5 (s) and Ni 3 TeO 6 (s) were obtained by the vapour pressure measurement of TeO 2 (g) over these systems by employing the Knudsen Effusion Mass Loss (KEML) technique and which could be represented by the relations: ▴ f G °Ni 2 Te 3 O 8 (s)=(−1693.5+0.8677 T )±15 kJ/mole [973–1045 K] ▴ f G °NiTe 2 O 5 (s)=(−1000.7+0.5029 T )±25 kJ/mole [987–1018 K] ▴ f G ° Ni 3 TeO 6 (s)=(−1386.7+0.6880 T )±15 kJ/mole [1042–1129 K]

The oxidation of uranium monocarbide microspheres

Abstract The reaction of oxygen with UC microspheres was studied as a function of heating rate, partial pressures of oxygen and sample size under nonisothermal heating conditions. The ultimate product of the oxidation was U3O8 with the formation of UO2 as an intermediate. The kinetic parameters were evaluated and conditions suggested for ignition free bulk oxidation of UC.

Carbothermic reduction of (UO3 + C) microspheres to (UO2 + C) microspheres

Abstract Carbothermic reduction of UO3 gel microspheres containing uniformly dispersed carbon was carried out in order to obtain (UO2 + C) gel microspheres suitable for the preparation of UC or UN. The reaction was observed to proceed in two stages (i) the conversion of UO3 to U3O8 and (ii) the reduction of U3O8 to UO2 by reaction with carbon. Reaction conditions were optimised to suppress the side reaction of UO3 with ammonia present in the microspheres and also its thermal decomposition to facilitate its quantitative conversion to UO2 through reaction with carbon so that a predetermined C/UO2 mole ratio is obtained in the product (UO2 + C) microspheres. Kinetic parameters were evaluated for both the stages of conversion, under nonisothermal heating conditions in flowing argon as well as vacuum, by using thermogravimetric technique. Studies were also carried out to obtain kinetic parameters for the conversion of pure UO3 microspheres to U3O8.

Thermal stability and vapour pressure studies on UTe3O9(s) and UTeO5(s)

Abstract UTe 3 O 9 and UTeO 5 were prepared by the solid state reaction route and characterized by X-ray diffraction and thermal methods. Thermal and X-ray studies indicated that UTe 3 O 9 and UTeO 5 vaporize incongruently according to the reactions: UTe 3 O 9 (s)→UTeO 5 (s)+2TeO 2 (g); 3UTeO 5 (s)→U 3 O 8 (s)+3TeO 2 (g)+ 1 2 O 2 (g). Vapour pressures of TeO 2 (g) over the mixture of UTe 3 O 9 (s)+UTeO 5 (s) in the temperature range 888–948 K and UTeO 5 (s)+U 3 O 8 (s) in the temperature range 1063–1155 K were measured by employing the Knudsen effusion mass loss technique. From the partial pressures of TeO 2 (g) measured, the standard Gibbs energy of formation (Δ G f 0 ) of UTe 3 O 9 (s) and UTeO 5 (s) were obtained which could be represented by the following relations: Δ G f 0 UTe 3 O 9 (s)±30 kJ/mol=−2318+0.7981 T (K) (888–948 K); Δ G f 0 UTeO 5 (s)±25 kJ/mol=−1616+0.4006 T (K) (1063–1155 K). The kinetics of decomposition of UTe 3 O 9 and UTeO 5 under isothermal heating conditions in flowing air were studied to determine the reaction mechanisms, rate constants and activation energies.

Oxidation behaviour of U2N3

Abstract The kinetics of the oxidation of U 2 N 3 containing 0.48, 16.2 and 21.1 mol% of UO 2 was studied by thermogravimetry under isothermal as well as non-isothermal heating conditions to identify the intermediate steps and the mechanism of reactions. Kinetic parameters such as apparent activation energy and pre-exponential factor were calculated for the proposed mechanism. The influence of the presence of UO 2 in the matrix on the mechanism of the process is discussed.

Oxidation studies on UZr alloys

Abstract Oxidation of U-Zr alloys (10 to 90 at% Zr) in air was studied thermogravimetrically under isothermal as well as nonisothermal modes of heating. Increase in zirconium concentration led to decrease in reactivity with air. Based on the chemical analyses of the intermediate products, there appears to be a slight preferential oxidation of uranium in the matrix during the initial stages of the oxidation. The analysis of the intermediates by XRD indicated the formation of solid solution of UO 2 and ZrO 2 . Kinetic parameters were evaluated for the oxidation reactions and the mechanism of the process is suggested.

The oxidation of uranium-cerium mononitride microspheres

Abstract The reaction of oxygen with UN and (U,Ce)N microspheres containing 15 and 30 mole % of cerium was studied in 6.6 kPa pressure of oxygen, under non-isothermal heating conditions, using a thermobalance coupled to a gas chromatograph for evolved gas analysis. For uranium mononitride the final product was U 3 O 8 with UO 2 and U 2 N 3 as intermediate products. However, for (U,Ce)N containing 15 mole % of cerium, the final product of oxidation was MO 2+ x with sesqui-nitride as the intermediate product. The (U,Ce)N microspheres with 30 mole % of cerium ignited during the reaction forming a mixture of M 3 O 8 and MO 2+ x . The kinetic parameters were evaluated and the mechanism of the reaction was suggested to explain the catalytic action of cerium on the oxidation of nitride.