High temperature chlorination kinetics of a niobium pyrochlore

The chlorination kinetics of a niobium (Cb) pyrochlore has been studied between 1873 and 2223 K, the chlorine concentration in helium varying between 0 and 20 pct. The pyrochlore was subjected to a preliminary thermal treatment at 1473 K in order to remove fluorine which escaped under the form of niobium oxyfluorides. This left NaNbO₃, CaNb₂O₆ and residual refractory oxides. The large chlorination reaction rate difference between NaNbO₃ and CaNb₂O₆ made possible the definition of distinct chlorination reaction rates for these constituents. It was found that the decomposition of CaNb₂O₆ is the controlling step in the chlorination of this constituent, while Nb₂O₅ (NbO₂ + NbO₂ at the prevailing temperatures) chlorination is very fast. The reaction is second order with respect to CaNb₂O₆ concentration and first order with respect to chlorine partial pressure between 1873 and 2023 K, a distinct reaction rate equation being obtained at 2223 K. Reaction rate constants have been calculated and vary between 3 and 10 moleJ.kg ·min for the temperature range considered. The NaNbO₃ reaction rate is first order with respect to total Nb₂O₅ concentration and 2.5 order with respect to chlorine partial pressure for the temperature range covered (1973 to 2223 K). Reaction rate constants are much higher than in the former case, being respectively 148 (1873 K), 214 (2023 K) and 518 (2223 K) mole/kg-min. Reaction orders may be affected by an error varying between 16 and 40 pct. The reaction rate constants are found accurate within 40 pct for CaNb₂O₆ and 25 pct for NaNbO₃.     

Chlorination kinetics of aluminum bearing minerals

The chlorination of domestic low-grade bauxite and kaolin clay was studied to determine the potential of these materials as sources of aluminum chloride for the chlorination/electrowinning route to aluminum metal. The reactivity of pure alumina was found to be representative of bauxite chlorination. Kaolin chlorinated in a complex manner at a much slower rate. Gamma alumina, which can be prepared, from bauxite or clay, offers the best potential for chlorination.     

The kinetics of diffusion of hydrogen in niobium

The kinetics of hydrogen diffusion in niobium were studied in the temperature ranges of 200° to 300°C and 600° to 700°C by gas-metal absorption experiments. The hardness profile was correlated with the concentration of hydrogen on specimens of various grain sizes. It was found that the diffusion kinetics of both temperature ranges could be represented by a single Arrhenius-type rate equation. The activation energy and the preexponential factor were found to be 10.000±320 cal per mole, and 1.8±1.0×10⁻² cq cm per sec., respectively. No observable effect of grain size was found on the diffusivity.     

Nb(C,N)沉淀析出动力学计算

对动力学模型进行了优化,对模型中的常数做了修正,引入界面能修正因子为0.38,推导出了动力学计算中的常数B的理论值为1.365×1010;对含Nb贝氏体钢980℃和950℃不同应变速率和应变量计算表明,随着应变速率和应变量的增加,析出时间是减少的,并且最快析出温度在930℃.     

Absorption kinetics and mechanisms of carbon monoxide in liquid niobium

Absorption kinetics and mechanisms of carbon monoxide in liquid niobium were investigated in the temperature range of 2700 to 3000 K in samples levitated in a CO/Ar stream. The carbon and oxygen dissolution in liquid niobium from CO gas is an exothermic process, and the solubilities of carbon and oxygen (Ce, Oe in at. pct) are related to temperature (T in kelvin) and partial pressure of CO (P _co in atm) as follows:

Sintering kinetics of niobium carbide powders of nonstoichiometric composition

Conclusions The sintering densification kinetics of NbC_x powders depends to a large extent on their degree of dispersion and nonstoichiometry; a characteristic feature of the densification kinetics is that the main contribution to densification, which grows further with increasing degree of dispersion and defectiveness (activity) of NbC_x phases, is made by the initial stage of sintering, apparently involving threshold mechanisms of mass transport, which are activated by oxycarbide decomposition processes in the temperature range 1600–2420°K. Because of this, in the sintering of NbC_x and ZrC_x phases the principal mechanisms of mass transport change in different temperature ranges.Translated from Poroshkovaya Metallurgiya, No. 5 (185), pp. 47–50, May, 1978.     

Nitridation kinetics of niobium in the temperature range of 873 to 1273 K

The kinetics of Nb (Cb) nitridation and ε-NbN growth obey a parabolic relationship and their temperature dependence can be expressed ask _p = 5.19 × 10⁻⁷ exp (−125,500/RT) and (k _{p ξ} = 1.15 × 10⁻⁴ exp (−61,500/RT), respectively, with the activation energies in joules/mole. The nitrogen diffusion coefficient in niobium, obtained from microhardness traverses, is given byD = 1.02 × 10⁻⁵ exp (−77,000/RT). A diffusion model accounting for the partition of nitrogen between ε-NbN and Nb is proposed. The total nitrogen uptake calculated from the model is compared to that obtained experimentally.     

Oxidation kinetics of niobium in the temperature range of 873 to 1083 K

Oxidation kinetics of niobium (columbium) in oxygen are investigated in the temperature range of 873 to 1083 K. The observed abnormal dependence of the linear oxidation rate on temperature is suggested to be due to the relative amount of NbO₂ in the oxide scale. The amount of oxygen dissolved in the metal is calculated and it constitutes a small fraction of the total oxygen uptake. From hardness measurements, the oxygen diffusion coefficient in Nb can be expressed as:D = 2.72 × 10⁻³ exp ( − 95.4/R T) with the activation energy in kjoule/mole.     

The mass transfer kinetics of niobium solution into liquid steel

Niobium cylinders were immersed into liquid steel and their mass-transfer rates measured under dynamic conditions. The cylinders were suspended from a load cell, and the apparent weight of the cylinder as well as temperatures at various locations in the immersed specimens were measured continuously during immersion and recorded with a data acquisition system. From the weight measurements, the mass-transfer rate was deduced. A steel shell period and free dissolution period were identified. During the steel shell period, a shell of frozen steel wraps the cylinder following its initial immersion. When niobium cylinders were immersed into liquid steel with low superheat, a reaction was detected at the steel shell/niobium interface. This reaction took place during the later stages of the steel shell period. The intermetallic compounds Fe₂Nb and Fe₂Nb₃ were identified as reaction products. The mass transfer which takes place during the free dissolution period was found to be exothermic, and the rate-controlling step was found to be liquid phase diffusion through a mass-boundary layer. The apparent activation energy was estimated to be 172 (±15) kJ/mol. This uncommonly high value of apparent activation energy is best explained on the basis of the macroexothermic reaction which takes place as niobium dissolves into liquid steel. Formerly Postdoctoral Fellow, Department of Metallurgy and Materials Science, University of Toronto     

Kinetics of the vapor phase chlorination of niobium oxychloride by phosgene

The kinetics of the homogeneous gas phase reaction between niobium oxychloride and phosgene was studied between 380° and 450°C in a constant volume reactor. The reaction was found to be essentially irreversible with the only detected reaction products being niobium pentachloride and carbon dioxide. The data obtained were fit by an elementary second-order rate equation. Values of the Arrhenius frequency factor and activation energy were found to be 1.33 × 10⁶ liter per g mol sec and 21.9 kcal per g mol with standard deviations of 0.05 × 10⁶ and 1.4, respectively.     

The effect of transformation stresses on the devitrification kinetics of a minory phase in liquid phase sintered ceramics

The crystallization of minory amorphous constituents in liquid phase sintered ceramics, for example in Si₃N₄, is usually accompanied by a volume change. The resulting mismatch between crystallizing second phase inclusions and the surrounding matrix of the primary phase leads to the formation of transformation stresses. The strain energy stored in the stress field reduces the thermodynamic driving force of crystallization. The coupling of crystallization, stress formation and relaxation is modelled. The extended duration of the crystallization process due to an intermediate stress induced decrease of the crystallization rate is assessed. The properties of amorphous grain boundary films are discussed with respect to stress relaxation and creep resistance at high temperatures.     

On the rule of additivity in phase transformation kinetics

It is commonly held that a sufficient condition for the rule of additivity to be valid is that the transformation rate depend only on temperature and volume fraction. This is not true in general.     

Chlorination kinetics of ZnO with Ar-Cl2-O2 gas and the effect of oxychloride formation

The chlorination rate of ZnO with Ar-Cl₂-O₂ gas was measured from 1023 K to 1273 K and the effects of temperature and partial pressures of chlorine and oxygen were investigated. The rate-determining step of chlorination was considered to be the dissociation of intermediate between ZnO and Cl₂ from linear relationship between reciprocal values of reaction rate and partial pressure of chlorine. The activation energy of chlorination was 58.2±2.5 kJ/mol. This comparatively low activation energy as a chemically controlled reaction was consistent with estimated rate-determining step of the dissociation of unstable compound. Increasing the partial pressure of oxygen slightly increased the chlorination rate, and this effect is considered to be caused by the increase in the formation rate of zinc oxychloride. To clarify the formation of zinc oxychloride, the equilibrium between Ar-Cl₂-O₂ gas and ZnO was investigated by the transpiration method at 1073 K. Calculated partial pressures of ZnOCl from experimental results were in the same order with or one order of magnitude larger than those estimated from reported Gibbs energy of formation of ZnOCl. Zinc oxychloride formation in ZnO chlorination must be taken into consideration as well as ZnCl₂ and Zn₂Cl₄ formation.