29 research outputs found
Studies on the ignition behaviour of boron powder
The ignition behaviour of boron powder, prepared through electrowinning process, was studied by using thermogravimetry coupled with simultaneous differential thermal analysis (TG-SDTA). The dependence of the inception of the ignition reaction on the partial pressure of oxygen, particle size of the boron powder and heating rate was investigated. It was observed that all these factors affect the ignition temperature. Boron powder with a mean particle size of about 10 mu m was found to be susceptible to ignition in oxygen even at 783K. In general, the susceptibility to ignition was found to vary inversely
with the degree of crystallinity. Presence of carbon was found to retard the oxidation of boron and raise the ignition temperature. These results are useful in safe handling and storage of finely divided boron powder and in the subsequent production of boron carbide from it. (C)
2009 Elsevier B.V. All rights reserved
Surface studies on uranium monocarbide using XPS and SIMS
The air-exposed surfaces of sintered and arc-melted UC samples were examined by XPS and SIMS. XPS results indicate that the surface is covered with a very thin layer of UO2 mixed with free carbon, which would have formed along with the oxide during the reaction between UC and oxygen or moisture. From the SIMS depth profile of oxygen, the thickness of the oxide layer is found to be approximately 10 nm. The SIMS oxygen images of the surface as a function of etching time reveal that the surface of UC consists of a top layer of adsorbed moisture/oxygen; this contamination layer is followed by a layer containing uranium oxide, uranium hydroxide and free carbon and then grain boundary oxide and finally bulk UC. The behaviour of sintered and arc-melted samples is similar
Stabilities of ternary carbides UWC<sub>1.75</sub> and UWC<sub>2</sub>
The methane-hydrogen gas equilibration technique has been used to measure the chemical potential of carbon associated with two three-phase fields of the system U-W-C in the temperature range 973 to 1173 K. By combining the values of the chemical potential of carbon in the three-phase fields UC + W + UWC<sub>1.75</sub> and UC + UWC<sub>1.75</sub> + UWC<sub>2</sub> obtained in this study with the data on the Gibbs energy of formation of UC available in the literature, expressions for the Gibbs energies of formation of the two ternary carbides were derived: Δ<sub>f</sub>G° (UWC<sub>1.75</sub>) = -131, 600 - 30.0 T (± 8000) J mol-1 Δ<sub>f</sub>G° (UWC<sub>2</sub>) = - 144, 800 - 32.0 T (± 10.000) J mol<sup>-1</sup> .Although estimates of Gibbs energies of formation of the two ternary carbides UWC<sub>1.75</sub> and UWC<sub>2</sub> have been reported, there have been no previous experimental determinations of thermodynamic properties of these compounds
Stabilities of ternary carbides UWC1.75 and UWC2
The methane-hydrogen gas equilibration technique has been used to measure the chemical potential of carbon associated with two three-phase fields of the system U-W-C in the temperature range 973 to 1173 K. By combining the values of the chemical potential of carbon in the three-phase fields UC + W + UWC1.75 and UC + UWC1.75 + UWC2 Obtained in this study with the data on the Gibbs energy of formation of UC available in the literature, expressions for the Gibbs energies of formation of the two ternary carbides were derived:
Delta(f)G degrees [UWC1.75] = -131, 600 - 300 T (+/-8000) J mol(-1)
Delta(f)G degrees [UWC2] = -144, 800 - 32.0 T (+/- 10,000) J mol(-1)
Although estimates of Gibbs energies of formation of the two ternary carbides TSWC1.75 and UWC2 have been reported, there have been no previous experimental determinations of thermodynamic properties of these compounds
Chemical potential of carbon in the system U-Pu-C-O-N: measurements and calculation
The carbon potential of (U,Pu) mixed carbides with Pu/(U + Pu) ratios of 0.55 and 0.70 was measured in the temperature range 973 to 1173 K by employing a methane-hydrogen gas equilibration technique. The technique was validated by measuring the Gibbs energy of formation of WC. The compatibility of the mixed carbides with the stainless steel clad was analysed by using the measured carbon potentials. The carbon potentials of mixed carbides of other compositions were calculated theoretically in order to assess their compatibility. The calculations assume ideal solution behavior for all the solid solutions present in the U-Pu-C-O-N system
Gibbs energies of formation of chromium carbides
The carbon potentials corresponding to the two-phase mixtures Cr + Cr23C6, Cr23C6 + Cr7C3, and Cr7C3 + Cr3C2 in the binary system Cr-C were measured in the temperature range 973 to 1173 K by using the methane-hydrogen gas equilibration technique. Special precautions were taken to prevent oxidation of the samples and to minimize thermal segregation in the gas phase. The standard Gibbs energies of formation of Cr23C6, Cr7C3, and Cr3C2 were derived from the measured carbon potentials. These values are compared with those reported in the literature. The Gibbs energies obtained in this study agree well with those obtained from solid-state cells incorporating CaF2 and ThO2(Y2O3) as solid electrolytes and sealed capsule isopiestic measurements reported in the literature
Determination of the thermodynamic stability of TiB2
The standard free energy of formation of titanium boride (TiB2) Was measured by the Electro Motive Force (EMF) method (by using yttria doped thoria (YDT) as the solid electrolyte). Two galvanic cells viz. Cell (I): Pt, TiB2 (s), TiO2 (s), B (s) vertical bar YDT vertical bar NiO (s), Ni (s), Pt and cell (II): Pt, TiB2 (s), TiO2 (s), B (s) vertical bar YDT vertical bar FeO (s). Fe (s), Pt were constructed in order to determine the Delta(f)G degrees, of TiB2. Enthalpy increments on TiB2 were measured by using inverse drop calorimetry over the temperature range 583-1769 K. The heat capacity, entropy and the free energy function have been derived from these experimental data in the temperature range 298-1800 K. The mean value of the standard enthalpy of formation of TiB2 (Delta H-f(298)degrees (TiB2)) was obtained by combining these Delta(f)G degrees, values and the free energy functions of TiB2 derived from the drop calorimetry data. The mean values of Delta H-f(298)degrees (TiB2) derived from the Delta(f)G degrees, data obtained from cell I and II were -322 +/- 1.2 kJ mol(-1) and -323.3 +/- 2.1 kJ mol(-1), respectively. These values were found to be in very good agreement with the assessed data. (C) 2009 Elsevier B.V. All rights reserved
Gibbs energies of formation of UMoC1.7 and UMoC2
The chemical potentials oi carbon associated with two three-phase fields in the system U-Mo-C were measured by using the methane-hydrogen gas equilibration technique in the temperature range 973 to 1173K. The technique was validated by measuring the standard Gibbs energy of formation of Mo2C. From the experimentally measured values of the chemical potential of carbon in the ternary phase fields UC+Mo+UMoC1.7 and UC+UMoC1.7+UMoC2 and data for UC from the literature, the Gibbs energies of formation of the two ternary carbides were derived
Gibbs energies of formation of chromium carbides
The carbon potentials corresponding to the two-phase mixtures Cr + Cr23C6, Cr23C6 + Cr7C3, and Cr7C3 + Cr3C2 in the binary system Cr-C were measured in the temperature range 973 to 1173 K by using the methane-hydrogen gas equilibration technique. Special precautions were taken to prevent oxidation of the samples and to minimize thermal segregation in the gas phase. The standard Gibbs energies of formation of Cr23C6, Cr7C3, and Cr3C2 were derived from the measured carbon potentials. These values are compared with those reported in the literature. The Gibbs energies obtained in this study agree well with those obtained from solid-state cells incorporating CaF2 and ThO2(Y2O3) as solid electrolytes and sealed capsule isopiestic measurements reported in the literature