ZrC Powders Research Articles

Oxidation Kinetics of Zirconium Carbide at Relatively Low Temperatures

The isothermal oxidation of ZrC powders was carried out at relatively low temperatures of 380° to 550°C at oxygen pressures of 1.3, 2.6, and 7.9 kPa under a static total pressure of 39.5 kPa, achieved by mixing with argon, using an electromicrobalance. The oxidation kinetics are described by the diffusion‐controlled Jander's equation, following rapid oxidation in the early stage. Two activation energies were obtained: 138 kJ · mol−1 below about 470°C and 180 kJ · mol−1 above that temperature. The high‐ and low‐temperature oxidation mechanisms are discussed in connection with the crystallization of cubic ZrO2, accompanied by the generation of cracks, as well as the formation of carbon in the hexagonal diamond form in the product phase.

Quick Analysis of Free Carbon in Metal Carbide by Plasma Oxidation

Plasma oxidation combined with emission monitoring was applied to the quantitative analysis of free carbon in ZrC powder. The emission was monitored with an optical color analyzer and was calibrated with standard samples of ZrO2+ C mixtures. Oxidation rates of the free and the combined carbons are so different that it is possible to estimate the amount of the former from the emission. No chemical treatment is necessary.

Reaction between TiC or TiN Powder and High Temperature-High Pressure Water

The reactions of ZrC or ZrN powder and H2O have been studied at 190°-400°C under 100MPa. The reaction of ZrC with H2O above 240°C for 3h yielded monoclinic ZrO2 and CH4, while the reaction of ZrN above 190°C for 3h yielded monoclinic ZrO2, NH3 and H2. The oxidation rate calculated from the wegiht gain or the integrated peak intensity of the X-ray diffraction was compared with various model functions. A nucleation-controlled model was the best fitted one, while a core-shrinking model fitted as well. This suggested that the reaction was controlled by phase boundary reactions. TEM observation showed that the ultrafine particles about 10nm nucleated on the surface of ZrN or ZrC grains of about 1-3μm. These ZrO2 fine particles did not from a protective layer on ZrN or ZrC particles, but formed clusters of particles because of large volume expansions about 40% during the oxidation. An Arrhenius plot of the rate constants gave apparent activation energies of 99kJ/mol and 78kJ/mol for the oxidation of ZrC and ZrN by H2O, respectively, on the basis of the nucleation-controllod model. The difference of about 20kJ/mol in the activation energies for ZrC and ZrN migth be explained by the difference in the formation energy between the C-H bonding and N-H bonding.

Reactions between ZrC or ZrN Powder and High Temperature-High Pressure Water

Reactions between ZrC or ZrN Powder and High Temperature-High Pressure Water

マグネシウム還元法によるジルコニアからの炭化ジルコニウム超微粉体の合成

The preparation of ZrC powder from ZrO2 by reduction with Mg has been investigated from 500° to 1000°C for 2 to 5 molar ratio of Mg/ZrO2 under a flow of methane. ZrC was formed above 650°C. The reaction finished within 30min above 750°C.The ZrC powder obtained at 1000°C was ultrafine (below 0.1μm) and had a C/Zr ratio of 0.96. The contents of 0 and Mg in the ZrC powder were 2.9 and 0.4wt%, respectively. The content of free carbon in the ZrC powder became 0.4wt% by decreasing the concentration of CH4 to 40%. The apparent activation energy for the formation of ZrC was approximately 83.3KJ/mol (750°-900°C).

A study of the reactions between ZrC and some metal oxides

High temperature chemical reactions of ZrC powder or ZrC coating on alumina spheres with CeO 2, UO 2 and SrO were studied. CeO 2 reacts with ZrC at 1473–2073 K to give Ce 2O 3 and Ce 2Zr 2O 7. Virtually no reactions were observed in UO 2-ZrC mixtures at 1673–1973 K. SrO reacts with ZrC above 1273 K to form SrZrO 3, Sr vapor and, presumably, CO. The SrZrO 3 formation on ZrC coated spheres contained in a bed of SrO powder was rapid at 1673 K. All particles suffered serious damages on the ZrC coatings by the reaction with SrO. Though the Sr concentration in the ZrC was below the detection limit, Sr was found to be distributed within the alumina kernel.

Solubility study in the systems PuCZrC and PuCTaC

Pressed pellets of PuCZrC and PuCTaC powder mixtures were sintered and subjected to several annealing and arc-melting treatments to obtain mutual solution of the constituents. The products were studied by X-ray diffraction and electron microprobe analysis (EMPA). It was found that up to 23 mol.% ZrC and 17 mol.% TaC dissolved in the PuC phases of the samples. The ZrC dissolved up to 24 mol.% PuC, the TaC up to 67 mol.% PuC. The lattice parameter-concentration relationships for the two pseudobinary systems were derived using the concentrations determined by EMPA for the various phases of the samples.

Cesium solubility, diffusion and permeation in zirconium carbide

To compare the relative effectiveness of ZrC vis-a-vis SiC as a fission product barrier in fuel structures for high temperature gas cooled reactor (HTGR) applications, a series of cesium infusion experiments on various ZrC powders, and ZrC coated graphite structures was performed to study the cesium solubility, diffusivity, and permeability of this coating material. The ZrC powder results yield a solubility of Cs in ZrC, S(ppm wt) = (1.7 × 10 −6) exp[229 kJ/RT], over the temperature range 1485–1896 K. The diffusion coefficient of Cs in ZrC is 10 −18–10 −16 m 2/s over a similar temperature interval. The activation energy of diffusion is estimated to be ≈ 50 kJ/mole. The results of experiments in which both SiC and ZrC coated graphite samples were exposed to cesium are more difficult to interpret. The results support the conclusions of the ZrC powder experiments that ZrC is comparable to SiC as a diffusion barrier to cesium.

Arc Melting and Homogenization of ZrC and ZrC+B Alloys

A description is given of the methods used to arc-melt and to homogenize near-stoichiometric ZrC and ZrC-boron alloys, giving attention to the oxygen contamination problem. The starting material for the carbide preparation was ZrC powder with an average particle size of 4.6 micron. Pellets weighing approximately 3 g each were prepared at room temperature from the powder by the use of an isostatic press operated at 50,000 psi. These pellets were individually melted in an arc furnace containing a static atmosphere of purified argon. A graphite resistance furnace was used for the homogenization process.

Sintering and recrystallization of ZrC-ZrB2 compacts

1. By using finely-divided starting materials, it is possible substantially to improve the sinterability of ZrC, ZrB2, and cermets based on these compounds. The addition of ZrB2 to ZrC or of ZrC to ZrB2 has a very beneficial effect on sinterability. By sintering ZrC and ZrB2 powders with a specific surface area of 4–5 m2/g at 2300–2400°K it is possible to obtain specimens with a total porosity of not more than 2–4%. 2. To describe the process of collective recrystallization over a wide range of temperatures and holding times, Eq. (4) should be used instead of Eq. (1), which is normally employed for this purpose. 3. Grain growth is very intense in ZrC, slightly less intense in ZrB 2, and much less pronounced in ZrC-ZrB 2 cermets. Both the grain size and energy of activation of the cermets diminish with increasing amount of the second component.