Study of self-propagating high-temperature synthesis of aluminium nitride using a laser monitor

This study focused on the synthesis of aluminium nitride (AlN) by combusting aluminium nanopowder in air. To investigate the combustion of aluminium nanopowder, a copper bromide laser monitor was used. The optical system equipped with brightness amplification allowed the elimination of the background lighting effect and enabled the high time-resolved recording of the process. In particular, the laser monitor enabled us to detect changes in the morphology and optical properties of the surface of the aluminium nanopowder sample as well as to observe the propagation of the combustion waves in spite of the intense background lighting during combustion. The main time parameters of the combustion of aluminium nanopowder in air were determined. To improve and facilitate the processing of laser monitored high-speed video recordings, we proposed to analyse the time dependence of the intensity of the output signal of the laser monitor. The dependence was used to successfully detect the occurrences of all combustion waves and describe their dynamics. The time dependence also favourably represented the evolution of the reflection coefficient of the combustion products of aluminium nanopowder. This is the first time that this property of aluminium nanopowder has been investigated. The reflection coefficient evolution coupled with video recordings of the sample surface development during the combustion of nanopowder could be used to control the combustion process.     

Preparation of ZrC by self-propagating high-temperature synthesis

ZrC fine powder has been prepared by self-propagating high-temperature synthesis (SHS) based on exothermic reduction reaction of ZrO₂–C–Mg. The combustion temperature observed was 1979 K. The effects of Mg content and particle size on the combustion temperature and chemical composition of the product were investigated. The reducing agent Mg plays an important role on the purity of ZrC powder obtained by SHS process. Post-heat treatment was applied to decrease the oxygen content of the final product further.     

Obtaining aluminomagnesia spinel by self-propagating high-temperature synthesis

The conditions for forming a solid solution of aluminomagnesia spinel with magnesium oxide in the regime of self-propagating high-temperature synthesis and the properties of the resultant refractory are investigated. The main parameters of the synthesis are determined. Test batches of specimens are used to study the main physicomechanical properties.Translated from Ogneupory, No. 12, pp. 10 – 12, December, 1994.     

Controlled nitrogen content synthesis of hafnium carbonitride powders by carbonizing hafnium nitride for enhanced ablation properties

In this study, a new method of carbonizing hafnium nitride was proposed to synthesize ultrahigh-temperature hafnium carbonitride (HfC_xN_y) powders. The new method helps to maintain both the purity of phases and control the content of nitrogen in the HfC_xN_y. The results show that the as-prepared HfC_xN_y powders have a single phase, with an average particle size of approximately 2 $\mu m$, and Hf, C and N are evenly distributed. Moreover, the microstructures, phase compositions, ablation properties and mechanism of the HfC_0.62N_0.38 composites under a plasma ablation environment were studied in detail. The results show that the HfC_0.62N_0.38 composites exhibited excellent ablation resistance at 3073 K for 60 s and the ablation mechanism of HfC_0.62N_0.38 can be identified as HfC_0.62N_0.38→HfC_xO_y→HfO₂. The mass ablation rate of the HfC_0.62N_0.38 composite is evaluated to be 1.36 mg/cm²∙s, which is lower than that of HfC ceramics. Our work is intended to provide new insight regarding the development of ultrahigh-temperature ceramics and widen their applications.     

Production of low-porosity products by forced self-propagating high-temperature synthesis-compaction

A two-velocity quasistationary model for the combustion of low-gas compositions in a press mold subjected to a constant load is used to analyze the feasibility of obtaining high-density materials through various unilateral extrusion schemes. Structural transitions in systems with a low-melting component are examined in the region of the liquid phase. It is shown that obtaining compacted products depends on the amount of the liquid phase and the relationship between the densities of the initial and final materials. Formulas are obtained for estimating the required loads for production of low-porosity products. The effect of a low-melting additive on the formation of composites durign forced compacting in self-propagating high-temperature synthesis (SHS) is analyzed. Translated fromFizika Goreniya i Vzryva, Vol. 34, No. 4, pp. 43–48, July–August 1998.     

Acoustic emission during self-propagating high-temperature synthesis

It was established experimentally that during SHS, reaction systems (Ni-Al, Ti-B, Mo-B, etc.) generated acoustic oscillations in the frequency range from 5 Hz to 1.1 MHz with a pulse power of up to 17 W. It was found that the combustion of different systems is characterized by an individual set of dynamic parameters of acoustic emission in the modes of low ordered discrete pulses and highly ordered self-oscillations. It is shown that the spatial zone of acoustic emission is localized near the combustion wave. Analysis of the acoustic emission mechanisms of SHS is performed.     

自蔓延高温合成镧掺杂钡铁氧体改性研究

采用白蔓延高温合成法制备了La掺杂的M型钡铁氧体BaLaxFe12-xO19（x=0,0．3,0.4,0．6）。用X射线衍射仪、振动样品磁强计和扫描电镜对粉末产品的结构与磁学性能进行了观测。系统地研究了La掺杂对M型钡铁氧体结构和磁性能的影响。实验结果表明,随着掺杂量（x）的增大,矫顽力He增大,而饱和磁化强度M6和剩余磁化强度Mr均减小。当掺杂量x=0．3时,钡铁氧体具有最佳的综合磁性能。     

Preparation of nano-size ZrB2 powder by self-propagating high-temperature synthesis

Preparation of nano-size ZrB₂ powder by SHS has been investigated. Zr and B elemental powders were mixed with 10–50 wt.% NaCl, and prepared pellets were reacted under argon. Adiabatic temperatures were calculated by HSC software. Increasing NaCl content led to a continuous decrease in adiabatic temperatures and reaction wave velocity. Products were subjected to XRD, SEM and FESEM analyses. Average crystallite size of ZrB₂, which was 303 nm without NaCl, decreased to 32 nm with 40% NaCl addition. Distinct decrease in ZrB₂ particle size was also observed from SEM analyses. 30% NaCl addition was found to be optimum for ensuring a stable SHS reaction and providing the formation of nano-size ZrB₂ particles. It was revealed from particle size distribution measurements that ZrB₂ powder obtained by 30 wt.% NaCl addition contained particles mostly finer than 200 nm. A mechanism, similar to solution-precipitation was proposed for the particle size refining effect of NaCl.     

X-ray radiation in self-propagating high-temperature synthesis processes

Emission effects of heterogeneous combustion in the region of ionization radiation are studied. By an example of a Ti-B powder system, it is demonstrated that the processes of self-propagating high-temperature synthesis in the thermal explosion regime is accompanied by “soft” X-ray radiation with the quantum energy estimated as ≈5 keV. Key words: heterogeneous combustion, X-ray radiation. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 6, pp. 127–129, November–December, 2008.     

Features of self-propagating high-temperature synthesis of spinel pigments

The method of self-propagating high-temperature synthesis (SHS) was used to synthesize spinel ceramic pigments. Producing aluminum-nickel and aluminum-cobalt pigments in a finely dispersed state is a complex stepwise (combined) process involving a reduction stage and simultaneous natural air filtration. In the case of small-diameter samples, a flat combustion front is observed and in large samples, the front is formed of many hot spots. The dependence of combustion rate on porosity is presented. The maximum combustion rate of these systems are recorded for porosity of 50–60%. Powdered SHS pigments were obtained for the first time.     

Effect of preheating on synthesis of tantalum nitride by self-propagating combustion

An experimental investigation of self-propagating high-temperature synthesis (SHS) of tantalum nitride (TaN) was conducted with tantalum compacts in nitrogen of 0.27–1.82 MPa. Effects of sample density, nitrogen pressure, and preheating temperature on the flame-front propagation velocity, combustion temperature, degree of conversion, and product composition were studied. Results showed that the SHS process of the tantalum/nitrogen reaction was characterized by the steady propagation of a planar combustion front, followed by a prolonged afterburning reaction. The flame-front velocity increased with nitrogen pressure, but decreased with sample density. Preheating the sample prior to ignition contributed higher combustion temperatures, thus leading to an increase in the conversion percentage. For the unpreheated samples, the conversion increased significantly with nitrogen pressure and reached around 80% at 1.82 MPa of N₂. With preheating temperatures between 150 and 300 °C, the conversion was increased by about 15% when compared with that without preheating. The nitride phase TaN was identified by XRD as the dominant composition in the combustion product.     

Structure and phase composition of zinc sulfide produced by self-propagating high-temperature synthesis

The structure of polycrystalline ZnS produced by self-propagating high-temperature synthesis is investigated. The monolithic specimens produced are a mixture of wurtzite and sphalerite. In the cross section of the specimens there are three distinct zones: an outer zone of fine crystals and zones of acicular and equiaxial crystals. Changing heat-exchange conditions, one can affect the dimensions of the second and third zones. Introduction of the disperser NH₄Cl into the starting mixture changes the crystallization mechanism and produces powder ZnS of a hexagonal modification with a varying grain size. Experimental results are explained within the framework of a two-phase model of crystallization and Ostwald’s phase rule. Translated fromFizika Goreniya i Vzryva, Vol. 34, No. 1, pp. 39–44, January–February, 1998.     

Aspects of making a superconducting yttrium ceramic by self-propagating high-temperature synthesis

Macrokinjetic features are considered for the combustion in oxygen of BaO₂–Cu–Y₂O₃ mixtures, which produces the high-temperature superconductor yttrium barium cuprate YBa₂Cu₃O_7–x. Studies have been made on how the initial temperature affects the combustion temperature and rate, and the critical temperature for self-ignition has been determined. Ultrasonic activation of the initial powders has an advantageous effect on the product quality. Thermal analysis has been applied to the activated mixtures, which indicates the reason for the rise in burning wave propagation rate and increase in conversion to the superconducting phase.Chernogolovka. Translated from Fizika Goreniya i Vzryva, Vol. 29, No. 2, pp. 62–67, March–April, 1993.     

Features of self-propagating high-temperature synthesis in a titanium-based system

The combustion temperature and rate have been measured and the product structure has been examined to define the interactions in a system designed to produce a porous permeable material having a heterogeneous framework.Barnaul. Translated from Fizika Goreniya i Vzryva, Vol. 29, No. 4, pp. 37–42, July–August, 1993.     

Ultrafast synthesis and pressureless densification of multicomponent nitride and carbonitride ceramics

High-entropy nitride and carbonitride ceramics have received wide attention for their excellent properties such as high hardness, high melting point, and high wear resistance, but the susceptibility of nitrides to decomposition at high sintering temperatures has rendered their densification challenging. In this work, four multicomponent nitrides and five multicomponent carbonitrides (containing five to seven cations) were prepared through ultrafast high-temperature sintering. While only (Cr_1/7Zr_1/7Nb_1/7Hf_1/7Ta_1/7Ti_1/7V_1/7)N formed a single phase among the nitrides, all carbonitride systems formed a single-phase high-entropy solid solution with a relative density exceeding 92%. The polyanionic structure of carbonitrides is responsible for their high configurational entropy, which in turn results in their high solid solubility. Although carbonitrides showed higher hardness and modulus than nitrides with the same cations, their fracture toughness was lower. Among carbonitrides with different C/N ratios, the system with a C/N ratio of 5:5 showed the highest solid solubility and best overall mechanical properties.     

Electroimpulsive activation of self-propagating high-temperature synthesis in powder mixtures

The effect of frequency electroimpulsive treatment on the self-propagating high-temperature synthesis reaction in a powder mixture is studied with the use of the Ni-Al system as an example. The longitudinal direction of the electric field applied to the sample and of the electric current relative to the direction of combustion-wave propagation was used. It is established that under the action of electrotreatment, the linear combustion rate increases by a factor of 1.3-1.6, the structure is transformed, and the completeness of chemical transformation increases. It is shown that the effects observed are of a nonthermal nature. Translated fromFizika Goreniya i Vzryva, Vol. 36, No. 4, pp. 133–136, July–August, 2000. This work was supported by the Russian Foundation for Fundamental Research (Grant No. 99-03-32465).