Kinetics of a non-catalytic gas-solid chemical reaction under SHS-like conditions

Nitration of metallic tantalum under self-propagating high-temperature synthesis (SHS) conditions (i.e. rapid heating rates 2000–3000 Ks^–1, and short heating periods 2–100 s) has been studied. Phase analysis and microstructural characterization have been performed. A two-phase mixture region of solid solution TaN_x and tantalum subnitride, namely Ta₂N, was observed in the nitrided tantalum at temperatures of 1600–2600 K and a partial nitrogen pressure of 0.1–0.8 MPa. It was also found that tantalum subnitride phase formed before the formation of Ta-N solid solution throughout the sample during nitration. A theoretical study was conducted in order to determine the intrinsic reaction kinetics of a typical combustion synthesis reaction. A new model was developed to explain all experimental observations. The computer simulation results are in good agreement with experimental data. The results indicate that at sufficiently large heating rates the product layer does not necessarily act as a diffusion barrier that prevents further uptake of nitrogen. The results are helpful in developing an understanding of the mechanism of powder-based SHS.     

Shock synthesis and synthesis-assisted shock consolidation of suicides

Shock-induced chemical synthesis and synthesis-assisted consolidation of high-temperature materials (suicides) were investigated. Niobium, molybdenum, and titanium powders mixed with silicon powders were chosen as reactant materials for shock-induced synthesis of silicides. In parallel experiments, these reactant materials were also respectively mixed with inert intermetallic compound powders of NbSi₂, MoSi₂, and Ti₅Si₃ in different proportions and were shock consolidated. Shock processing was carried out using a modification of the experimental set-up developed by Sawaoka and Akashi. The shock waves were generated in the materials by the impact of a flyer plate at a velocity of 2 km sec^–1. An explosive plane-wave generator was used to initiate the main explosive charge to accelerate the flyer plate. The passage of shock waves of sufficient pressure and temperature induced a highly exothermic and self-sustaining reaction between reactant materials. The shock-synthesized intermetallic compounds and the heat of reaction enhanced bonding between inert matrix materials. The proportion of reactant powder mixtures blended with inert intermetallic materials plays a very important role in the synthesis-assisted consolidation process. Characterization of compacts was done by optical microscopy, scanning electron microscopy, and X-ray diffraction. A preliminary analysis of shock-induced chemical reactions is conducted; it predicts a 30% increase in shock pressure and shock-wave velocity over those in unreacted powders. For shock synthesis, the profuse formation of voids indicates that melting of the material occurred; in contrast, unreacted regions did not exhibit porosity.     

Structure refinement for tantalum nitrides nanocrystals with various morphologies

Tantalum nitrides (TaN_x) nanocrystals with different phase and morphology have been synthesized through homogenous sodium reduction under low temperature with the subsequent annealing process under high vacuum. The crystal structures of tantalum nitrides were determined by Rietveld refinement based on the X-ray diffraction data. The morphologies of various tantalum nitrides nanocrystals in high quality were analyzed through the electron microcopies examinations. The spherical TaN nanoparticles, cuboidal TaN_0.83 and TaN_0.5 nanocrystals have been selectively prepared at different annealing temperatures. In addition, the specific surface areas of the tantalum nitrides nanocrystals measured by BET method were around 9.87–11.64 m² g^?1, indicating that such nano-sized tantalum nitrides could be suitable for capacitor with high specific capacitance.     

Nanostructured tantalum nitride films as buffer-layer for carbon nanotube growth

Tantalum nitride (TaN_x) films are usually used as barriers to the diffusion of copper in the substrate for electronic devices. In the present work, the TaN_x coating plays an extra role in the iron catalyzed chemical vapor deposition production of carbon nanotubes (CNT). The CNTs were grown at 850 °C on TaN_x films prepared by radio frequency magnetron sputtering. The correlation between the CNT morphology and growth rate, and the pristine TaN_x film nature, is investigated by comparing the evolution of the nano-composition, roughness and nano-crystallinity of the TaN_x films both after annealing and CVD at 850 °C.     

Structural modification of tantalum crystal induced by nitrogen ion implantation

This paper investigates the effect of nitrogen ion implantation on tantalum surface structure. In this experiment, nitrogen ions which had an energy of 30 keV and doses of 1 × 10¹⁷ to 10 × 10¹⁷ ions cm^?2 were used. X-ray diffraction analysis (XRD) was applied for both the metallic Ta substrate and the study of new structures that have been created through the nitrogen ion implantation. Atomic force microscopy (AFM) was also used to check the roughness variations prior to and also after the implantation phase. The experimental results show the formation of hexagonal tantalum nitride (TaN_0.43) in addition to the fact that by increasing the ion dose, the nitrogen atoms occupy more interstitial spaces in the target crystal. The nitride phase also seen for 3 × 10¹⁷ and 5 × 10¹⁷ ions cm^?2, while it disappeared for higher dose of 7 × 10¹⁷ and 1 × 10¹⁸ ions cm^?2. The FWHM of the dominant peak of tantalum nitride suggest the growth of the crystallite’s size, which is in agreement with the AFM results of the grains.     

Thermodynamical approach to the brittle fracture of dry plasters

The evolution of the fracture toughness, K _lc, and fracture energy, G _lc, of set plasters was determined on notched beams as a function of sample porosity, P, and characteristic size, W. Toughness was found to decrease with decreasing crack width. For set plasters of 57.7% porosity, the lowest toughness measured was K _lc=0.13 MPa m^1/2 for a crack width of 0.2 mm. For this crack width, fracture toughness and fracture energy linearly changed with porosity: K _lc=0.5 1–1.3 P) MPa m^1/2 and G _lc= 13.47 (1–1.12 P) Jm^–2. Dense plasters were more difficult to break than porous ones. The fracture energies were affected by the velocity of the fracture propagation, which induces damaging and multicracking of the material, so that the roughly calculated chemical surface energy of set plaster was too high. After correction it was estimated to be 0.4 J m ^–2. Finally, because toughness increased with increasing sample size, it was concluded that fracture toughness and energy were not intrinsic parameters of the material. On the other hand, for our sample porosities and sizes, the reduced rupture force, F _rupt W ^–0.65 is a constant and seems to be a characteristic parameter of the mechanical resistance of set plaster beams.     

Carbon fibre-reinforced silicon nitride composite

The processing of silicon nitride reinforced with carbon fibre was studied. The problems of physical and chemical incompatibility between carbon fibre and the silicon nitride matrix were solved by addition of a small amount of zirconia to the matrix and by low-temperature hot-pressing. The composite material possesses a much higher toughness than hot-pressed silicon nitride. Its work of fracture increased from 19.3 J m^–2 for unreinforced Si₃N₄, to 4770 J m^–2; its fracture toughness,K _lc , increased from 3.7 MN m^–3/2 for unreinforced material, to 15.6 MN m^–3/2. The strength remains about the same as unreinforced Si₃N₄ and the thermal expansion coefficient is only 2.51×10^–6 ° C^–1 (RT to 1000° C). It is anticipated that this composite may be promising because of its mechanical and good thermal shock-resistance properties.     

The sintering of combustion-synthesized titanium diboride

A comparative study of the sinterability of combustion-synthesized titanium diboride was conducted over the temperature range of 1800 to 2100° C. During the initia! sintering stage, the densification rate was slightly higher in the combustion-synthesized than in the commercially obtained titanium diboride. For sintering times of > 30 min, however, the shrinkage rates for both types of powders were the same. The activation energy for the late sintering stage was 774 ± 46 kJ mol^–1, consistent with 8 volume diffusion mechanism, end was the same for both combustion-synthesized and commercial powders. The microstructures of sintered specimens with initial particle size below 1O µm exhibited a grain size ranging from 5 to over 40 µm after 30 min of sintering. The addition of 5 wt% NbB₂ to the combustion-synthesized resulted in enhanced shrinkage during the initial sintering stage, but did not affect later stage kinetics. Various amounts of additives of CrB₂, NiB and TiC had no effect on early and late stage sintering kinetics, with the exception of 50 wt% TiC which appreciably inhibited densification.     

Stability and degradation of plasma deposited boron nitride thin films in ambient atmosphere

Boron nitride (BN) thin films were deposited at 296 K, 398 K, 523 K and 623 K by low power radio frequency plasma enhanced chemical vapor deposition with nitrogen (N₂) and hydrogen diluted diborane (15% B₂H₆ in H₂) source gases. Fourier transform infrared and UV–visible spectroscopies were used to investigate the stability and degradation of BN films under ambient air conditions. The action of moisture on the films is reduced with increasing substrate temperature (T_s) to the detriment of the film growth rate. This has been interpreted as related to the decrease in porosity and relative volume fraction of B–O containing disordered tissue at higher T_s. The thickness of the unstable films increases logarithmically with the air exposure time. Parallel to this, although the E₀₄ gap increases logarithmically with time, the Tauc gap remains the same. The increase of subgap absorptions and the decrease of Tauc slope with time indicate reduction of structural order. Crystallites of ammonium borate hydrates, the main product of the chemical reactions, are initially formed within the bulk. At a later time, as a result of increased porosity and disorder, the film thickness decreases while the islands of micro-crystallites rapidly grow above the surface of the film. Stability dependence on other deposition parameters was also studied: it is found that the 1260/1360 cm⁻¹ (O–B–O/B–N) infrared peak area ratio plays an indicator role to reveal the stability of BN films.     

Preparation of turbostratic and cubic boron-nitride films by electron-cyclotron-resonance,plasma-assisted,chemical vapour deposition

Boron-nitride films were prepared on single-crystal silicon substrates by electron-cyclotron-resonance (ECR) plasma-assisted, chemical vapour deposition (CVD), using B₂H₆ and nitrogen as the source gases, without intentional heating of the substrate. Turbostratic boron-nitride (t-BN) films were obtained at a maximum deposition rate of 1.8 nms^–1. A cubic boron nitride (c-BN) phase formed in the t-BN films after application of a radio-frequency (r.f.) bias to the substrates at a voltage of 156–172 V and at a maximum deposition rate of 0.08 nms^–1.     

Wettability of aluminium nitride by tin–aluminium melts

The wettability of aluminium nitride by Sn–Al melts was studied by the sessile drop method in a vaccum of 2 × 10^–3 Pa at 1100 °C over the whole concentration region. The minimum interval on the contact-angle concentration dependence curve was observed at intermediate composition. For comparison, experiments were also performed on porous AlN. Wetting of porous nitride is worse than the dense nitride. The results have been analysed on the basis of the relation between wettability and the chemical interface reactivity in solid–liquid metal systems.     

Sintering and characterization of fully dense aluminium nitride ceramics

Aluminium nitride ceramics with no sintering additives could be densified to close to theoretical density (99.6% theoretical) by pressureless sintering of tape-cast green sheets at 1900 °C for 8 h. The thermal conductivity and bending strength of the specimens were 114 Wm^–1 K^–1 and 240 MPa, respectively. The effect of Y₂O₃ additive on sinterability, thermal conductivity and microstructure of aluminium nitride ceramics was investigated. Thermal conductivity increased with increasing amount of Y₂O₃ additive, sintering temperature and holding time at the sintering temperature. Samples with a thermal conductivity up to 258 Wm^–1 K^–1 were fabricated by elimination of the grain-boundary phase.     

Thermodynamics of the Chemical Vapor Deposition of Carbides in the System TaBr5–CCl4–Cd

Thermodynamic analysis of the chemical vapor deposition (CVD) of tantalum carbides in the TaBr₅–CCl₄–Cd system is carried out. The equilibrium composition in the system is calculated as a function of temperature and ratios of reagents. The optimal CVD conditions for tantalum carbides with particular deviations from stoichiometry are assessed.     

Recovery of stoichiometry of Ta2O5 prepared by KrF excimer laser CVD from tantalum methoxide using microwave discharge of oxygen gas

A flow of oxygen gas activated by passing through a microwave discharge plasma in a 100 and 200 W field was introduced into a deposition chamber for tantalum oxide deposition by KrF laser photolysis of Ta(OCH₃)₅, and the change of the non-stoichiometry of deposit was examined under the laser condition of 120 Hz and 200 JM^–2. The stoichiometry of the deposit could be improved to a value of about 90% under the condition of rather low supply rate of Ta(OCH₃)₅ (100 mg h^–1). An effect of post-treatment laser following chemical vapour deposition (CVD) by KrF laser and/or microwave discharge of oxygen gas was also investigated, and it was found that activated oxygen species formed by KrF laser irradiation in an oxygen gas atmosphere which passed through a microwave discharge was effective in enhancing the oxidation of non-stoichiometric tantalum oxide prepared by KrF laser CVD.     

Oxidation kinetics of hexagonal boron nitride powder

The isothermal oxidation of hexagonal boron nitride powders was carried out at 900–1050°C in dry oxygen and air. The oxidation kinetics were found to obey linear rate law and were described by the surface chemical reaction-controlled shrinking cylindrical model. The apparent activation energies were found to be 298 and 330 kJ mol^–1in dry oxygen and air, respectively. The oxygen partial pressure dependence of the oxidation rate constant at 1000 °C is well represented by a Langmuir-type equation. Microscopic examination of the oxidized sample after removal of the oxide scale with water suggested that the rate of attack by oxygen was determined by an anisotropy due to the crystallographic direction, similar to oxidation in graphite. The volatilization of B₂O₃ was observed only in dry oxygen and obeyed a linear rate law, and was found not to affect the oxidation reaction.     

Effect of active metal coatings on the mechanical properties of silicon nitride-based ceramics

The effects of titanium, zirconium, hafnium and tantalum coatings on the mechanical properties of three silicon nitride ceramics were studied. The titanium coatings was found to cause a 50% decrease in the four-point bend strength of one of the silicon nitride ceramics while the effects of the zirconium, hafnium and tantalum coatings on all three silicon nitride ceramics were moderate. The reactions at a high temperature (940–980°C) between titanium and the grain-boundary glassy phase was the major cause for the degradation of the ceramic properties by the titanium coating. Residual tensile stress developed at the reaction interface replaced the glassy grain-boundary phase. Analytical electron microscopy showed the formation of a 180 nm thick Ti₅Si₃ layer and the crystallization of the amorphous grain-boundary phase. An indentation technique was used to measure qualitatively the residual stress developed at the reaction interface.     

I.r. spectra resolution in fluorinated silicon nitride films

The broad I.r. Si-N band (700–1200 cm^–1) in fluorinated silicon nitride films has been resolved. I.r. spectra of the samples have been fitted using Gaussian curves centred in positions determined from the maxima of the negative second derivative of the digitized spectra. In silicon nitride films deposited by plasma-assisted chemical vapour deposition from SiH₄-NF₃-NH₃-N₂ gas mixtures-SiF_n radicals (n=1–3) incorporated in their structures have been detected. The relative concentrations of the different fluorine radicals present in the network depend on the NF₃ flow ratio used. As the flow ratio increases above 0.5, -SiF and-SiF₂ concentrations in the film reach the steady state. However, an appreciable increase in-SiF₃ and [-SiF₂-] _n concentrations has been observed.     

High-temperature thermoelectric properties of the sintered Bi2Sr2Ca1 − xYxCu2Oy (x = 0 – 1)

Electrical conductivity and Seebeck coefficient were measured in a temperature range of 320–1073 K for sintered samples of Bi₂Sr₂Ca_{1 – x} Y _x Cu₂O _y (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0). It has been found that the conduction behavior changes from n-type metallic to p-type semiconducting with increasing yttrium concentration. The power factors were in a range of 1.7–3.0 × 10^–5 Wm^–1 K^–2 for the sample with x = 0.8, being maximized by the optimization of the yttrium concentration. The thermal conductivity for the sample with x = 0.8 was 0.73 Wm^–1 K^–1 at 310 K, and decreased with increasing temperature. The values of thermoelectric figure of merit were estimated to be in a range of 3.4–4.8 × 10^–5 K^–1 at temperatures of 320–673 K for the sample with x = 0.8.     

Preparation of ultradispersed magnesium nitride by the electric-arc low-temperature plasma technique

The conditions are investigated for the preparation of ultradispersed magnesium nitride (UDMN) from elemental magnesium and nitrogen by the electric-arc low-temperature plasma (LTP) technique. Thermodynamic calculations are carried out with the objective of finding the values of the equilibrium constant for the synthesis reaction of magnesium nitride from its elements in the temperature range 298–1800 K as a function of the aggregation state of the components. By means of a cold wall (CW) plasma-chemical reactor (PCR), ultradispersed product (UDP) is synthesized with a specific surface of 33 m²g^–1 containing up to 56% of Mg₃N₂; the use of a warm wall (WW) PCR allows the preparation of UDP with a Mg₃N₂ content of up to 73% and a specific surface reaching 184 m²g^–1. The dominant role of the radial temperature gradient in the PCR in the synthesis of UDMN with maximal degree of nitride formation and maximal specific surface values is experimentally demonstrated. The UDMN exhibits a high chemical activity; heat-treatment at 900 K in an inert atmosphere decreases substantially its chemical affinity to water vapour and oxygen in the air.     

Chemical Vapor Deposition of Boron Nitride in the B–N–H–He–O System

Thermodynamic analysis of boron nitride (cubic, hexagonal, and wurtzite forms) chemical vapor deposition in the B–N–H–He–O system was carried out for temperatures from 300 to 2100 K, a total pressure of 1.33 Pa, residual pressures from 1.33 × 10^–5 to 0.133 Pa, and a wide range of He : B₃N₃H₆ ratios. The conditions for the deposition of c-BN, h-BN, or mixtures of BN and B₂O₃ (solid or liquid) were established. Oxygen impurities are shown to have a significant effect on the temperature stability limits of the condensed phases involved.