The combustion synthesis of refractory nitrides

A theoretical analysis of the formation of refractory metal nitrides through the combustion of metal powders in a nitrogen atmosphere is presented. Thermodynamic and kinetic criteria are employed to determine the dependence of the process on the nitrogen pressure and on the porosity of the compacts. For metals which can dissolve large amounts of nitrogen, the relative contributions of the processes of nitrogen dissolution and nitride phase formation are analysed.     

The combustion synthesis of refractory nitrides

The self-sustaining combustion of niobium in a nitrogen atmosphere was investigated. The effect of initial porosity, nitrogen pressure, and the amount of added diluent on the combustion process and on the nature of the products of combustion was determined. Auxiliary investigations on the interaction between niobium and nitrogen were made under isothermal conditions. Results of X-ray analyses of these investigations are compared to those obtained under self-propagating reaction conditions.     

Modeling of thermal processes in laser sintering of reactive powder compositions

A two-dimensional mathematical model describing a combination of the processes of self-propagating high-temperature synthesis and selective laser sintering of powder compositions (it is controlled by the laser-radiation energy) has been proposed. The model makes it possible to evaluate the correspondence of the geometric characteristics of the system to the values of the velocities of a moving laser spot at which the layer poured completely reacts in the vicinity of the spot. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 78, No. 6, pp. 42–48, November–December, 2005.     

Estimation of thermal expansion behaviour of some refractory carbides and nitrides

Lattice parameters of some refractory carbides and nitrides were estimated up to 2400 K using an empirical approach. The computed lattice parameters are in very close agreement (±0.5%) with the values calculated from experimental thermal expansion data reported in the literature. This empirical approach with modifications may be applicable to the prediction of the thermal expansion behaviour of other classes of high temperature materials. This revised version was published online in November 2006 with corrections to the Cover Date.     

Investigation of mass and heat transfer processes in laser alloying of iron from predeposited coatings of refractory metals

The laws of mass and heat transfer in laser alloying of Armco iron from predeposited coatings of molybdenum and chromium were determined experimentally.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 58, No. 3, pp. 380–385, March, 1990.     

Ion-beam-defect processes in group-III nitrides and ZnO

Recently, there has been much interest in wide band-gap wurtzite semiconductors such as group-III nitrides (GaN, AlGaN, and InGaN) and ZnO. Ion-beam-defect processes are considerably more complex in these wurtzite semiconductors than in the case of both elemental and group-III-V cubic semiconductors. This brief review focuses on our recent studies of the following aspects of ion-beam-defect processes: (i) effects of implanted species and the density of collision cascades, (ii) the nature of ion-beam-produced planar defects in GaN, (iii) defect production in GaN by swift heavy ions, (iv) blistering of H-implanted GaN, (v) electrical isolation of GaN and ZnO, (vi) the effect of Al and In content on defect processes in III-nitrides, and (vii) structural damage in ZnO with an intriguing effect of the formation of an anomalous defect peak. Emphasis is given to unusual ion-beam-defect processes and to the physical mechanisms underlying them.     

Modification of the LSMPS method for the conservation of the thermal energy in laser irradiation processes

In this work, new least-square moving particle semi-implicit (LSMPS) formulations for the modeling of the heat conduction in laser irradiation processes for both thick blocks and thin plates are developed. These new LSMPS formulations guarantee the conservation of the total thermal energy during the heat exchange between particles. The conservation of the thermal energy in the LSMPS method was implemented together with multiresolution techniques for the discretization of the domain with particles of different sizes so that a better characterization of the thermal gradients in the vicinity of the laser beam can be obtained. The simulation of laser irradiation processes for thin plates is still very challenging for particle methods with spherical particles and this is essentially because it is difficult to accommodate a minimum number of particles along the thickness direction without increasing considerably the resolution or the number of particles in the entire plate. In order to overcome this difficulty, a new multiresolution method based on particles with ellipsoidal shapes was also developed for a more efficient modeling of the laser irradiation in thin plates. By conducting the heat conduction simulations, in which the standard LSMPS method can provide accurate temperature distribution and by comparing the results with an analytical solution, it was confirmed that the proposed method is as accurate as the standard LSMPS method. Moreover, the heat conduction with an external heat source, in which the total thermal energy is not conserved by using the standard LSMPS method, was successfully simulated by using the proposed method. The simulations of laser irradiations were also conducted, and the validity of the proposed method has been confirmed by comparing numerical results with experimental data.     

Interferometric technique for the subsecond measurement of thermal expansion at high temperatures: Applications to refractory metals

A high-speed interferometric technique has been developed at the National Institute of Standards and Technology to measure thermal expansion of metals between room temperature and temperatures primarily in the range 1500 K to near their melting points. The basic method involves resistively heating the specimen from room temperature up to and through the temperature range of interest in less than 1 s by passing an electrical current pulse through it and simultaneously measuring, with submillisecond resolution, the specimen temperature by means of a high-speed photoelectric pyrometer and the shift in the fringe pattern produced by a Michelson-type interferometer. The polarized beam from a He-Ne laser in the interferometer is split into two components, one which undergoes successive reflections from highly polished flats on opposite sides of the specimen and one which serves as the reference beam. The linear thermal expansion of the specimen is determined from the cumulative fringe shift corresponding to each measured temperature. The technique is capable of measuring linear thermal expansion with a maximum estimated uncertainty which ranges from about 1% at 2000 K to approximately 2% at 3600 K. Measurements have been performed on the refractory metals, niobium, molybdenum, tantalum, and tungsten, yielding thermal expansion data in the temperature range 1500 K up to near their respective melting points. Also, the technique has been used to follow the rapid dimensional changes that occur during solid-solid phase transformations; in particular, the transformation in iron has been studied.Invited paper presented at the Tenth International Thermal Expansion Symposium, June 6–7, 1989, Boulder, Colorado, U.S.A.     

Transient behaviors in thermal radiation characteristics of heat-resisting metals and alloys in oxidation processes

A high-speed spectrophotometer system is developed to study radiation characteristics of materials. The system allows measurements of the spectra at wavelengths of 0.35–10m repeatedly with a period of less than 1 s. It is applied to the study of transient behaviors in reflection characteristics of heat-resisting alloys and the constituent transition metals in air-oxidation processes at high temperatures. An interference phenomenon due to the multiple reflection at the upper and lower boundaries of the oxide film is observed in the diffuse reflection spectra of oxidizing rough-finished surfaces as well as in the specular reflection spectra of oxidizing specular-finished surfaces. The phenomenon is found to be fairly reproducible and consistent over all the materials investigated. It is attributed to the interference and diffraction of radiation at three-dimensional nonparallel film elements of the polycrystalline oxide grains. A possibility is suggested for the theoretical modeling of radiation characteristics of real surfaces in the actual environments of industry.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.     

Modeling thermal and electrical processes in a semiconductor module with a temperature constraint

A method of combined investigation is proposed for thermal and electrical process in semiconductor modules with a temperature constraint.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 48, No. 3, pp. 483–489, March, 1985.     

Schottky contacts of refractory metal nitrides on gallium nitride using reactive sputtering

Schottky contacts of refractory metal nitrides formed by reactive sputtering on n-type gallium nitride (GaN) were electrically evaluated, including film resistivity, Schottky characteristics and thermal stability. For the metal nitrides of TiN_x, MoN_x and ZrN_x, resistivities of 108.3, 159.0 and 270.0 μΩcm were obtained, respectively. Current-voltage (I-V) characteristics showed that the ideality factor varied from 1.03 to 1.16, while the Schottky barrier height (SBH) varied from 0.66 to 0.79 eV for the three kinds of Schottky contacts. Especially for the ZrN_x contact, the ideality factor and SBH were improved after annealing at 800 °C for 30 s. Schottky contact utilizing a refractory metal nitride on GaN shows its potential to develop thermally stable GaN devices.     

Studying the coefficient of thermal conductivity for liquid metals

We discuss a method for an experimental study of the coefficient of thermal conductivity, specific heat capacity, and the specific electrical resistance of metals in the solid and liquid states, as well as the data that we have obtained with respect to the indicated properties of Bi, In, Cd, and Pb.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 57, No. 6, pp. 892–896, December, 1989.     

Optimization of thermal processes using an Eulerian formulation and application in laser surface hardening

A systematic design approach has been developed for thermal processes combining the finite element method, design sensitivity analysis and optimization. Conductive heat transfer is solved in an Eulerian formulation, where the heat flux is fixed in space and the material flows through a control volume. For constant velocity and heat flux distribution, the Eulerian formulation reduces to a steady-state problem, whereas the Lagrangian formulation remains transient. The reduction to a steady-state problem drastically improves the computational efficiency. Streamline Upwinding Petrov–Galerkin stabilization is employed to suppress the spurious oscillations. Design sensitivities of the temperature field are computed using both the direct differentiation and the adjoint methods. The systematic approach is applied in optimizing the laser surfacing process, where a moving laser beam heats the surface of a plate, and hardening is achieved by rapid cooling due to the heat transfer below the surface. The optimization objective is to maximize the rate of surface hardening. Constraints are introduced on the computed temperature and temperature rate fields to ensure that phase transformations are activated and that melting does not occur. Copyright © 2000 John Wiley & Sons, Ltd.