Self-propagating High-temperature Synthesis Method Research Articles

NiTi-SiC composite coating on Ti6Al4V alloy produced by SHS using induction heating

Abstract In this study, NiTi and NiTi-XSiC (1 wt%, 2 wt%, and 4 wt%) powders were coated on the Ti6Al4V surface by the self-propagating high-temperature synthesis (SHS) method using induction heating. Optic microscope and a scanning electron microscope (SEM) for the microstructural properties of coatings, energy dispersive spectrometry (EDS) for element distribution analysis, and X-ray diffractogram (XRD) for phase component analysis were carried out. The microhardness difference between the coating and the substrate was assessed by using the Vickers hardness scale. The microhardness increased with the addition of SiC, and the highest hardness value was 1416 Hv and detected in the sample NiTi–SiC 4 wt%. Phases formed by diffusion during solidification in the compounds Ti, Al, NiTi, NiTi2, NiAl, Ni3Al, Al3Ti, and TiO2, respectively.

Preparation, structure and photocatalytic activity of nanocomposites based on cerium oxide

Mesoporous CeO2 and MgO powders and nanocomposites based on them were synthesized by the method of self-propagating high-temperature synthesis, and the mutual influence of magnesium oxide and cerium oxide on the crystal structure, microstructure, and morphology of the obtained materials was studied. It has been established that CeO2 is formed on the surface of magnesium oxide, while the developed surface of the material is preserved. It has been established, that the sizes of CeO2 crystallites in the composition of nanocomposites change insignificantly and range from 6.5 to 7.4 nm, while the values of the specific surface area and average pore diameter of the studied samples depend on the composition and vary in the ranges of 19–41 m2/g and 11.9‒19 nm, respectively. The highest efficiency of photodegradation of dyes of acid telon blue and direct bright blue is observed for samples of MgO–CeO2 (30 mol.%) and MgO–CeO2 (50 mol.%) ‒ 98.5 and 92.5 %, respectively, taking into account the effect of direct photolytic decomposition under the influence of ultraviolet radiation.

Features of compaction in the combustion process of a system based on (Ni‒Al)/(SiO<sub>2</sub>, Al<sub>2</sub>O<sub>3</sub>, 3Al<sub>2</sub>O<sub>3</sub>∙2SiO<sub>2</sub>)

The possibility of using the method of self-propagating hightemperature synthesis to obtain high-temperature cermet materials and non-removable layer compounds based on (NiAl)/oxide, where the oxides are SiO2, Al2O3 and 3Al2O3·2SiO2, is investigated. The thermodynamic analysis of gorenje reactions was carried out and the properties of the synthesized products were investigated. An alternative method of creating a layered material with different content of oxide phases opens up wide possibilities for its use as heat insulators and special elements for high-temperature applications. Ill.8. Ref. 16. Tab. 1.

Ni-Al Self-Propagating High-Temperature Synthesis Catalysts in Dry Reforming of Methane to Hydrogen-Enriched Fuel Mixtures

The worldwide increase in demand for environmentally friendly energy has led to the intensification of work on the synthesis of H2-containing fuel. The dry reforming of methane has become one of the most important avenues of research since the consumption of two greenhouse gases reduces the rate of global warming. A study of NiAl composite materials as catalysts for methane reforming has been carried out. Self-propagating high-temperature synthesis (SHS) has been used to produce NiAl catalysts. Comparative studies were carried out regarding the dry reforming and partial oxidation of methane, as well as catalysts prepared using the impregnation (IM) and SHS methods. A catalyst with 29% Ni and 51% Al after SHS contains the phases of NiAl and NiAl2O4, which are active phases in the dry reforming of methane. The optimal crystal lattice parameter (for the maximum possible conversion of CO2 and CH4) is 3.48–3.485 Å for Al2O3, which plays the role of a catalyst carrier, and 1.42 Å, for NiAl2O4, which plays the role of a catalyst. The aim of the work is to develop a new and efficient catalyst for the dry reforming of methane into a synthesis gas, which will further promote the organization of a new era of environmentally friendly energy-saving production methods.

Study of the TIG weldability of aluminum matrix composite material AMg2-TiC

Subject of research: Weldability of the composite alloy AMg2-TiC.
 Purpose of research: Matrix alloy AMg2 is often used in welded joints. Reinforcement with a highly dispersed ceramic phase of titanium carbide leads to an increase in the hardness and strength of the alloy; therefore, it is necessary to study how the composite material behaves during and after welding.
 Methods and objects of research: The object of study is the AMg2-TiC composite alloy obtained by the method of self-propagating high-temperature synthesis. Microstructural analysis was carried out on a Jeol JSM 6390A scanning electron microscope. The elemental composition was determined by X-ray microanalysis on a Jeol JSM 6390A scanning electron microscope with a Jeol JED-2200 attachment. The hardness of the samples was determined on a hardness tester ZIP TK=2M using the Brinell method according to GOST 9012-59. Weldability tests were carried out by argon-arc welding using an Everlast Power TIG 210 EXT welding inventory and filler materials that most closely match the chemical composition of the matrix alloy.
 Main results of research: This paper shows the possibility of welding a composite material using the TiG method. The weldability of the composite was rated as satisfactory. Highly dispersed particles of the reinforcing phase do not dislocate from the area of ​​the base metal to the area of ​​the weld. The hardness of the composite material after argon-arc welding does not fall, but remains at the same level. The porosity in the weld zone of the composite is less than in the weld zone of the matrix alloy.

Effect of the Mass Fraction of NiTi-TiB2 SHS-Particles on the Phase Composition, Structure, and Mechanical Properties of Inconel 625-NiTi-TiB2 Composites Produced by Direct Laser Deposition.

This paper studies the impact of the mass fraction of NiTi–TiB2 particles obtained by the method of self-propagating high-temperature synthesis (SHS) on the phase composition, structure, and mechanical properties of composites made by direct laser deposition from an Inconel 625–NiTiz–TiB2 powder mixture. Composites were obtained from a powder mixture with the mass fraction of particles at 5–10 wt%, and they consisted of an Inconel 625 metal matrix wherein ceramic inclusions of titanium diboride TiB2 were distributed. Increasing the mass fraction of SHS-produced NiTi particles from 30 to 95 wt% led to the emergence of a NiTi intermetallide phase in the matrix material as well as an increase in the average TiB2 particle size and formation of their agglomerates. In addition, an increase in the microhardness of the materials was observed. The graph of tensile strength of Inconel 625–NiTi–TiB2 samples has a parabolic shape with a maximum at 1000 MPa (when the mass fraction of SHS-produced NiTi–TiB2 particles is at 30 wt%). A further increase in the mass fraction of NiTi–TiB2 led to a decrease in the tensile strength down to 400 MPa. Here the deformation of samples decreases linearly as the ratio of composite particles in the initial mixture increases. From a comparative analysis of the results obtained, the optimal mass fraction of composite NiTi–TiB2 particles in the Inconel 625-NiTi–TiB2 powder mixture was found to be 5 wt%.

THE INFLUENCE OF ULTRASOUND ACTIVATION ON MICROSTRUCTURE AND HARDNESS OF POROUS Ni-Ti SHAPE MEMORY ALLOYS

A Ni-Ti shape memory alloy with a porous structure is obtained by the method of self-propagating high-temperature synthesis (SHS) and investigated the possibility of changing its microstructure and hardness. The research aimed to study the changes in the microstructure and hardness of the Ni-Ti shape memory alloy with a porous structure when the Ni and Ti metal powders are preliminarily subjected to ultrasound activation for different periods. The microstructure of the alloy surfaces was studied with using a Hitachi scanning electron microscope (SEM), and the hardness was measured with a Brinell electronic hardness tester. The results of the experiments showed that the porosity and hardness of the Ni-Ti alloy enlarged with an increase in the time of ultrasound activation of the components. The innovative aspect of this study is that prior to the synthesis of powder elements with a purity of Ni 99.9% and Ti 99.9% with an average particle size of 40 μm, they were previously subjected separately to ultrasound activation at different periods.

Investigation of possibility of fabrication of long-length samples of Ti3AlC2–Al MAX-cermet by the SHS method with spontaneous infiltration by aluminum melt

The article discusses the features of combining the self-propagating high-temperature synthesis (SHS) of the Ti3AlC2 MAX phase porous skeleton with infiltration by aluminum melt in a spontaneous mode in order to obtain enlarged samples of Ti3AlC2–Al ceramic-metal composite (MAX cermet) in an air atmosphere. A new scheme was developed for obtaining long-length SHS cermet samples from a bulk density charge with spontaneous infiltration by melt in the same direction with the combustion wave movement, which makes it possible to regulate the time gap between the end of the Ti3AlC2 synthesis and the beginning of the spontaneous pore filling with aluminum melt. This technology was used to obtain a Ti3AlC2 SHS skeleton with a total length of 250 mm and a diameter of 22–24 mm where the depth of infiltration with pure aluminum was about 110 mm, and impregnation with the Al–12%Si alloy was 130 mm. The paper provides comparative data on density, microstructure, and phase composition at different areas along the length of MAX cermet samples obtained. It was found that infiltration with pure aluminum destroys the Ti3AlC2 MAX phase to transform it into a mixture of TiC + TiAl3 phases in the SHS cermet, and 12 % Si added to the Al melt promote Ti3AlC2 preservation in the cermet to a some extent. Instead of MAX cermet samples with the target composition of Ti3AlC2–Al and Ti3AlC2–(Al–12%Si), long-length samples of SHS cermets with a different actual phase composition were obtained: TiC–TiAl3–Al and TiC–Ti3AlC2–TiAl3–(Al–12%Si), respectively, where the Ti3AlC2 MAX phase either practically absent or present in small quantities. The average hardness values of TiC–TiAl3–Al and TiC–Ti3AlC2–TiAl3–(Al–12%Si) SHS cermets wereHB= 640 and 740 MPa, density ρ = 2.88÷3.16 g/cm3 and 3.03÷3.13 g/cm3, and residual porosityП= 17.0÷24.6 % and 17.6÷20.3 %, respectively.

Fabrication and oxidation resistance of the non-stoichiometric tantalum-hafnium carbonitride

This research was conducted to obtain non-stoichiometric tantalum-hafnium carbonitride powder of the Fm3m (225) structural type using a combination of mechanical activation (MA) and self-propagating high-temperature synthesis (SHS) methods. Mechanical activation for 60 min in a low-energy mode (347 rpm) forms Ta/Hf/C composite particles 1 to 20 μm in size with a layered structure and contributes to a uniform distribution of elements. SHS of a mechanically activated Ta + Hf + C mixture in a nitrogen atmosphere (0.8 MPa) leads to the formation of a single-phase tantalum-hafnium carbonitride powder with the Ta0.25Hf0.75C0.5N0.3 composition where particles feature by a ≪spongy≫ morphology with pores and caverns and consist of submicron grains. Spark plasma sintering (SPS) was used to obtain a bulk sample of tantalum-hafnium carbonitride with a grain size of 3 to 5 μm, relative density of 98.2 Ѓ} 0.3 %, hardness of 19.8 Ѓ} 0.2 GPa, and crack resistance of 5.4 Ѓ} 0.4 MPa・m1/2. The kinetics of (Ta,Hf)CN oxidation at 1200 °C in air is described by a parabolic law suggesting the formation of an Hf6Ta2O17 + mHfO2 oxide layer with a low oxygen diffusion rate where the oxidation rate is 0.006 mg/(cm2・s). A (Ta,Hf)CN oxidation mechanism is proposed, which states that Ta2O5 and HfO2 are formed on the surface of grains at the first stage that react with each other at the second stage to form a Hf6Ta2O17 homologous superstructure and monoclinic HfO2. CO, CO2, NO and NO2 gaseous oxidation products are released with the formation of pores and cracks.

Improved oxidation protective ability of SHS powder-synthesized ZrB2-MoSi2-SiC-Si coating on carbon/carbon composites

ZrB2-MoSi2-SiC-Si coatings were prepared by gaseous silicon infiltration on the carbon/carbon composites, and the powders synthesized by the self-propagating high-temperature synthesis method (SHS) were used as the raw materials to benefit the oxidation protective ability of the coatings. The ZrB2-MoSi2-SiC-Si coatings prepared from commercial powders (CP coated samples) were used as a comparison. After sintering, the structure of the coating prepared with SHS powders is denser than the CP coated sample with a density of 3.5 g/cm3. After 62 h of oxidation tests at 1500 °C, the mass loss of the SHS coated sample was only 0.28 %, which was 32 % lower than that of the CP coated sample. The ability of SHS coatings to exhibit better oxidation protection is attributed to the higher oxidation stability and sintering activity of the SHS powders than that of commercial ones.

Kinetics of the Synthesis of Aluminum Boride by the Self-Propagating High-Temperature Synthesis Method

The influence of certain factors on the kinetics of the process of obtaining aluminum borides (burning rate, ingot formation, and phase separation) was investigated. In this study, we report the registration of diboride using the SHS protocol. The synthesis of aluminum diboride from boric anhydride occurred by the aluminothermic method. The initial components were boron trioxide and aluminum in the form of powders. Researchers paid special attention to the degree of grinding of the charge fluxing substances. The influence this had on the rate of development of the degree of charge concentration was studied. To calculate the degree of charge, a composition was chosen according to the speed obtained from a number of experiments where melting was carried out with the following charge densities in g/cm3: 0.80; 1.08; 1.18; 1.74. The method of melting was ignition from above. The experimental results allowed us to conclude that the nature of the change in the combustion rate of the system, where there was an excess of the reducing agent in the charge, is the same. An increase in the combustion rate, where there was an excess of aluminum of up to 20%, was likely due to the fact that the reaction area of the charge components increased. In addition, an increase in speed can be explained by a decrease in heat losses due to a reduction in the melting time. With an increase in excess aluminum above 20% of the stoichiometry, the observed decrease in the combustion rate can be explained by a decrease in the specific heat of the process due to the melting of the excess aluminum, which played the role of a ballast.

The production of corrosion alloy by self-propagating high-temperature synthesis

The subject of research: Self-propagating high-temperature synthesis (SHS) is an energetically low-cost method for obtaining materials with desired properties.
 The purpose of research: Development and approbation of the method of self-propagating high-temperature synthesis.
 Methods and objects of research: For the synthesis of a corrosion-resistant material, a thermite mixture of Al (75 %) and Fe2O3 (25 %) and the addition of MoO3 with a variable mass fraction of 10 %, 15 %, 20 %, 25 % of thermite were taken. The composition and structure of the synthesized material were determined by XRD and metallography methods. Based on the properties of the physical properties of the obtained material (porosity, electrical conductivity) and its phase composition (MoO2, Al2O3, FeMoO4), samples obtained with a 15 % addition of MoO3 were selected for testing for corrosion resistance.
 Results of research: The paper presents the results of approbation of obtaining a corrosion-resistant material by the SHS method. Conducted tests for corrosion resistance established: the rate of continuous corrosion of the synthesized material (MoO3 15 %) was 0.2 mm / year, which is close in value to corrosion-resistant steels; high resistance of the synthesized material to technical salt when tested for 14 days with a concentrated solution of the reagent 0.18 ml * eq / l. The listed properties of the synthesized material confirm the possibility of obtaining corrosion-resistant materials by the SHS method, including those for operation in the Arctic.

Effects of reactants proportions on features of in-situ magnesiothermic self-propagating high temperature synthesized boron carbide powder

The effects of reactant proportions were investigated on features (phase composition, micromorphology and crystal development) of B4C (boron carbide) powder synthesized by in-situ magnesiothermic SHS (self-propagating high temperature synthesis) method, which was based on the perspective of thermodynamic design. The results showed that the reactant proportions were the fundamental reasons affecting the phase composition of the products during the reaction process. Mg addition could decrease the Mg3B2O6/MgO mass ratio of the SHS products. Moreover, C addition would increase the C/B mass ratio of the leached products. When the C molar ratio was 0.2, free boron appeared in the leached products, and the composition of boron carbide was B13C2. When the C molar ratio ≥0.6, the composition of boron carbide changed into B4C, while free carbon began to appear. The lattice parameters a and c of boron carbide crystal dropped with the increase of the C/B mass ratio from 0.1 to 0.3. Meanwhile, carbon atoms gradually entered the boron carbide unit cell. When the C/B mass ratio exceeds 0.3, the lattice parameters tended to be constants of a = 5.608 Å, c = 12.039 Å, while free carbon began to appear in the leached products.

Metamorphosis of the properties of the gas-phase fraction of ZnS:Mn obtained by the method of self-propagating high-temperature synthesis from the charge with a different Zn/S ratio

Performed in this work is a comparative analysis of photoluminescence, photoluminescence excitation and EPR spectra of gas-phase fraction of powdery ZnS:Mn obtained using the self-propagating high-temperature synthesis from the charge with a different Zn/S ratio. It was found that the size of particles in the synthesized materials was with in the range 10–300 nm. The size that is comparable with the Debye screening length (L). The luminescence and other characteristics of the studied material were found to differ significantly from the bulk ones. This finding has been explained by a unique energy structure of crystallites. Zn/S ratio has been shown to impact significantly on the crystallites sizes and the amount of incorporated Mn. The largest crystallite sizes and the high Mn content were observed in the powder synthesized with a stoichiometric Zn/S ratio, while the minimum particle sizes - in the powders synthesized with S excess. At sulfur excess, the size of particles is close to the Debye length, and carriers can freely migrate in the conduction band to the surface, where mainly nonradiative recombination occurs. The stoichiometric Zn/S ratio or Zn excess in the charge results in the increase of the particle sizes up to the values 2L ≤ d≤3L. Such materials are characterized by minimization of non-radiative losses. This finding opens new possibilities for ZnS application.

Novel Pathway for the Combustion Synthesis and Consolidation of Boron Carbide.

A novel pathway for the magnesiothermic reduction of boron oxide and magnesium dodecaboride (MgB12) in the presence of carbon by a self-propagating high-temperature synthesis method was proposed that was aimed at the direct preparation of boron carbide nanopowder. The combined utilization of two boron sources, boron oxide and MgB12, allowed tailoring the overall caloric effect of the process, increasing the yield of the target product and lessening the laborious leaching process. In addition, it is an alternative way to utilize magnesium borides, which are inevitable side products at boron production. Multivariate thermodynamic calculations performed in the B2O3-MgB12-Mg-C system allowed estimating equilibrium compositions of the products and deducing the optimum composition of the initial mixture for obtaining B4C. For the latter, the adiabatic temperature (Tad) is 2100 °C, which is theoretically enough for the implementation of the self-propagating reaction. The combustion reaction was shown to be extremely sensitive to the initial mixture composition, external pressure, as well as sample diameter (heat losses). It proceeds in self-oscillatory mode and leads to the product of a layered macrostructure. The combustion product was then consolidated by the spark plasma sintering technique at different conditions. Vickers microhardness was measured, and the wear erosion behavior was examined. The variation in lattice parameters of boron carbide reflected the influence of synthesis, sintering and erosion conditions on the ordering/disordering of the boron carbide structure.

Analytical control for the content of small amounts of oxygen in aluminum nitride powders using a «Metavak-AK» analyzer

A method for prompt and reliable control of low oxygen content in aluminum nitride powder using a modern gas analyzer «Metavak-AK» operating in automatic mode and never used previously for this object is presented. Aluminum nitride was obtained under conditions of pilot production at ISMAN by the method of self-propagating high-temperature synthesis (SHS). A program for operation of a «Metavac-AK» analyzer has been developed for oxygen determination in AlN samples and optimal conditions of analysis were specified: reductive melting of a sample in a graphite crucible at a furnace temperature of 2700°C in the presence of a combined Sn/Ni flux in a helium flow. The total oxygen content in aluminum nitride powders and the relative standard deviation ranged within 1.01 – 1.18% and 1.75 – 9.87%, respectively.

Azide self-propagating high-temperature synthesis of highly dispersed TiN–SiC ceramic nitride-carbide powder composites

The study covers the method of azide self-propagating high-temperature synthesis (SHS-Az) to obtain a highly dispersed TiN–SiC ceramic composite with a theoretical ratio of nitride and carbide phases from 1 : 4 to 4 : 1 (in moles) using the combustion of the corresponding composition of powder reagent mixtures: NaN3 sodium azide, (NH4)2TiF6, (NH4)2SiF6 and Na2SiF6 halide salts, titanium, silicon and carbon in a nitrogen gas atmosphere. Thermodynamic calculations using the Thermo computer program showed that the optimum nitrogen pressure in the reactor is about 4 MPa, and the final composition of SHS-Az products can be completely different depending on the composition of reagents: it may include only target phases (TiN–SiC), contain silicon nitride and free carbon phases impurities (TiN–SiC–Si3N4–C) along with the target phases or consist only of nitride and free carbon phases (TiN–Si3N4–C). It was found that only target TiN and SiC phases are formed when using halide salt (NH4)2TiF6, at any ratio of nitride and carbide phases in the final powder composition. In cases where halide salts (NH4)2SiF6 and Na2SiF6 are used, target TiN and SiC phases are synthesized with an increased titanium content in reagents, i.e. only when composites of the 2TiN–SiC and 4TiN–SiC with an increased content nitride phase are obtained. Experimental studies of combustion products using scanning electron microscopy, energy dispersion analysis and X-ray phase analysis showed that they differ significantly from the theoretical compositions of products by the completely absent or significantly reduced SiC phase content in the final composition of powder composites synthesized during the combustion of bulk charge with carbon, and at the same time the absence of free carbon in the final composition of powder composites obtained. This difference is explained by the fact that when the combustion of a silicon and carbon powder mixture is initiated, silicon nitride is synthesized at the first stage with the temperature rising to high values of about over 1900 °C, at which the synthesized Si3N4 dissociates, and then at the second stage the resulting silicon reacts with carbon to form SiC that is more stable at high temperatures. But during combustion, very small light particles of carbon black (soot) may be removed (blown out) from a burning highly porous charge sample of bulk density by gases released at the first stage of combustion and not participate in the transformation of Si3N4 into SiC. In this regard, in case of low-carbon charge combustion, silicon carbide either does not form at all, or it is formed in small quantities compared to the theoretically possible amount, and Si3N4 silicon nitride remains the main component of the composite. A noticeable amount of SiC is formed only when burning high-carbon charges, but this amount is significantly less than the possible theoretical one, and the difference between them is replaced by the silicon nitride content. Therefore, it was experimentally shown for the first time that the SHS process can be used to obtain composites of highly dispersed ceramic powders TiN–Si3N4 and TiN–Si3N4–SiC consisting of a mixture of nanoscale (less than 100 nm) and submicron (100 to 500 nm) particles with a relatively low content of free silicon admixture (less than 1.4 %).

ПОЛУЧЕНИЕ МЕТАЛЛОМАТРИЧНЫХ СПЛАВОВ ДЛЯ ИЗНОСОСТОЙКИХ ЭЛЕКТРОИСКРОВЫХ ПОКРЫТИЙ

The study objective is to obtain and study metal matrix alloys based on NiAl intermetallic compound with different concentrations of Ni for forming wear-resistant coatings on steel 45. 
 The problems to which the paper is devoted are: obtaining electrode materials based on NiAl intermetallic compound with different concentrations of Ni and studying their structure; studying the mass transfer during electrospark deposition on steel 45; studying the comparative wear resistance of the coatings obtained. 
 Research methods: electrode alloys are obtained by the method of liquid-phase self-propagating high-temperature synthesis; electro spark alloying is used to create wear-resistant coatings; the microstructure of the obtained alloys is studied by the method of metallographic analysis.
 The novelty of the work: for the first time, the influence of different Ni concentration values on the wear resistance of NiAl electrospark coatings on steel 45 is studied.
 Study results: the microstructure of the obtained alloys consists of doped Cr and Co grains of NiAl matrix with a different Ni/Al ratio, along the boundaries of which the compounds of all alloy components are located. When forming coatings by electrospark method, time dependences of changes in anode erosion, cathode gain and mass transfer coefficient are obtained. 
 Conclusions: NiAl metal-based alloys with different concentrations of Ni were smelted using a liquid-phase SHS method using charge consisting of metal oxides and mineral concentrates containing tungsten and zirconium; the microstructure of the obtained alloys consists of doped Cr and Co grains baseds on NiAl, along which boundaries compounds of all the constituent components of the alloy are concentrated, including Zr and W. Experimental results of anode erosion, cathode gain, and mass transfer coefficient were obtained while forming the coatings with the alloys using electrospark method, and it was found during wear resistance tests that with an increase of Ni concentration in the alloy, the wear resistance increases.

Self-Propagating High-Temperature Synthesis of Magnesium Aluminate Spinel Using Mg-Al Alloy.

In this study, magnesium aluminate spinel (MgAl2O4) was synthesized by a self-propagating high-temperature synthesis method using Mg–Al alloy with a Mg/Al mass ratio of 50:50 as raw material. Synthesized MgAl2O4 was characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy, UV–vis diffused reflectance spectroscopy, photoluminescence, and thermogravimetric differential scanning calorimetry techniques. The results show that synthesized products are of high purity and excellent crystallinity. However, the particle size is not uniform and there is obvious agglomeration. The crystallite size of spinel phase is calculated to be 37.78 nm. In the UV band, the synthesized MgAl2O4 has a certain absorption capacity, and the extrapolated band gap is 4.02 eV. The synthesis mechanism was studied, and continued rupture and growth of the oxidation layer is thought to be responsible for grain refinement.

Densification mechanism during hot-pressing of single-phase zirconium boride powders with different dislocation density

ZrB2 powders with different dislocation density were synthesised by the self-propagating high-temperature synthesis (SHS) and carbothermal reduction (CTR) method. The defect structure was analysed by transmission electron microscopy (TEM), and the dislocation density was determined though the convolutional multiple whole profile fitting (CMWP-fit). The SHS process exhibited a very high rate of heating and cooling, which introduces a higher dislocation density (8.68 × 1015 m-2) due to thermal stresses caused by the volume change of crystal, and the crystal grew spirally. Comparably, the CTR process depicted a low rate of heating and cooling. The time was sufficient for crystal nucleation and growth, thus the dislocation density of crystal was lower (6.49 × 1011 m-2), and the crystal grew layer-by-layer. By mixing with the SHS and CTR powders, 5 kinds of powders with different dislocation density were prepared, followed by hot-pressing densification. The dislocation density of powder was calculated and result shows that with the increasing of the content of SHS powder from 0 to 1 the dislocation density increases from 6.49 × 1011 m-2 to 8.68 × 1015 m-2, meanwhile the relative density of ZrB2 ceramics increased from 82.1% to 98.1%. The hot-pressing densification can be divided into two stages. In the pre-sintering stage, with increasing of dislocation density of the powders, the densification mechanism changes from bulk diffusion to grain boundary diffusion, and all are of the dislocation-climb mechanism in the post-sintering stage.