Self-propagating High-temperature Synthesis, Microstructure and Mechanical Properties of TiC-TiB_2-Cu Composites

TiC-TiB2-Cu composites were produced by self-propagating high-temperature synthesis combined with pseudo hot isostatic pressing using Ti, B4C and Cu powders. The microstructure and mechanical properties of the composites were investigated. The X-ray diffraction （XRD） and scanning electron microscopy （SEM） results showed that the final products were only TiC, TiB2 and Cu phases. The clubbed TiB2 grains and spheroidal or irregular TiC grains were found in the microstructure of synthesized products. The reaction temperature and grain size of TiB2 and TiC particles decreased with increasing Cu content. The introduction of Cu into the composites resulted in a drastic increase in the relative density and flexual strength, and the maximum values were obtained with the addition of 20 wt pct, while the fracture toughness was the best when Cu content was 40 wt pct.

Dissolution-precipitation mechanism of self-propagating high-temperature synthesis of TiC-Cu cermets

The mechanism of self-propagating high-temperature synthesis （SHS） of TiC-Cu cermets was studied using a combustion front quenching method. Microstructural evolution in the quenched sample was observed using scanning electron microscope （SEM） with energy dispersive X-ray （EDX） spectrometry, and the combustion temperature was measured. The results showed that the combustion reaction started with local formation of Ti-Cu melt and could be described with the dissolution-precipitation mechanism, namely, Ti, Cu, and C particles dissolved into the Ti-Cu solution and TiC particles precipitated in the saturated Ti-Cu-C liquid solution. The local formation of Ti-Cu melt resulted from the solid diffusion between Ti and Cu particles.

Preparation of ZrB_2 Ceramics by Self-propagating High-temperature Synthesis and Hot Pressing Sintering

ZrB2 ceramics were prepared by self-propagating high-temperature synthesis（SHS） and were sintered by hot pressing（HP）.The effects of the granularities and doses of raw materials in Zr-B2O3-Mgon SHS process and product were investigated.XRD and combustion temperature curves prove that the ideal SHS reactants of Zr-B2O3-Mg are 50μm Zr powder,75μm B2O3 powder and 400μm Mg powder with 45% excessive.The particle sizes of SHS product,acid-leached product,sintered product are 2-5μm,0.5-2μm,2-10μm respectively.Chemical analysis indicates that the acid-leached product consists of ZrB2（94.59%）,ZrO2（3.87%）,and H3BO3（1.54%）,The sintered product has a relative density of 95.4%.

Microstructural Evolution During Self-propagating High-temperature Synthesis of Ti-Al System

In order to investigate the microstructural evolution during self-propagating high-temperature synthesis （SHS） of Ti-Al powder mixture with an atomic ratio of Ti： Al=1：1, a combustion front quenching method （CFQM） was used for extinguishing the propagating combustion wave, and the microstructures on the quenched sample were observed with scanning electron microscope （SEM） and analyzed with energy dispersive spectrometry （EDS）. In addition, the combustion temperature of the reaction was measured, and the phase constituent of the synthesized product was inspected by X-ray diffraction （XRD）. The results showed that the combustion reaction started from melting of the Al particles, and the melting resulted in dissolving of the Ti particles and forming of Al3Ti grains. As the Al liquid was depleted, the combustion reaction proceeded through solid-state diffusion between the solid Al3Ti and the solid Ti. This led to the forming of TiAl and Ti3Al diffusing layers. In addition, the combustion reaction is incomplete besides TiAl, there are a large amount of Ti3Al and TiAl3 and a small amount of Ti in the final product. This incompleteness chiefly results from the using of coarser Ti powder.

Effect of inner oxidant on self-propagating high-temperature synthesis of MnZn-ferrite powder

Using KClO3 as an inner oxidant, MnZn-ferrite powder was synthesized by a self-propagating high-temperature synthesis (SHS) process in normal air atmosphere. The effects of the inner oxidant on combustion temperature, combustion velocity, microstructure and the phase of the product were investigated by XRD and SEM,respectively. The results show that a highly ferritized powder can be obtained as well as the highest combustion temperature and the highest combustion velocity when the inner oxidant content m equals 54(k-16).

A Self-propagating high-temperature synthesis process for the fabrication of Fe（Cr）–Al2O3 nanocomposite

Self-propagating high-temperature synthesis(SHS)was used to fabricate a Fe(Cr)–Al2O3 nanocomposite.The composite was fabricated by the reactions between the powders of Fe,Fe2O3,Cr2O3,and Al.The effect of blending ratio and mechanical activation of the initial powders and the precursor compressing pressure on the microstructure of the final product was studied by optical microscopy,scanning electron microscopy,transmission electron microscopy,and X-ray diffraction.The significance of the effect of each of the aforementioned parameters on the quality of the composite(assessed by measuring the compressive strength and wear resistance)was determined using a full-factorial design of experiments method.The results showed that the best molar powder ratio that produced the most homogeneous product through a sustainable SHS reaction was Fe:Fe2O3:Cr2O3:Al=10:1:1:4.A lower Fe content caused the Fe(Cr)phase to melt and separate from the rest of the materials.

Kinetics and numerical simulation of self-propagating high-temperature synthesis in Ti–Cr–Al–C systems

In this paper, Ti–Cr–Al–C materials were investigated by self-propagating high-temperature synthesis（SHS） according to the experimental study and numerical simulation results. The highest adiabatic combustion temperature Tadof 2,467.45 K indicates that the2Ti–0Cr–Al–C is the highest exothermic reaction system in the Ti–Cr–Al–C system. The adiabatic combustion temperature decreases with the increase of the Cr content. And a higher exothermal reaction would result in higher porosity which is induced by the high temperature and pressure of C reducing atmosphere and Al vapor. Combustion characterization of the products shows that the geometrical alternating layers result in the high exothermal reaction and flame-front propagating velocity. The higher the Tadis, the thinner the layer is. To demonstrate the process of the microscopic characterization and show the detailed combustion process closed to the experimental observations, the flame-front propagating velocity and temperature distribution were simulated numerically.

Self-propagating high-temperature synthesis of ZrO_2 incorporated Gd_2Ti_2O_7 pyrochlore

In this research, Zr-doped Gd_2Ti_2O_7 pyrochlores, with the composition of Gd_2(Ti_(1-x)Zr_x)_2O_7, were firstly synthesized by self-propagating high-temperature synthesis plus quick pressing(SHS/QP) using CuO as the oxidant and Ti as the reductant. To improve the radiation resistance of titanate–pyrochlore, up to 35 at% Zr was incorporated to substitute the Ti site of Gd_2Ti_2O_7 pyrochlore(Gd_2(Ti_(0.75)Zr_(0.35))_2O_7). XRD and SEM microstructural characterizations showed the formation of a composite ceramic with the major pyrochlore phase and the minor Cu phase. The generated temperature of samples decreased from 1702to 1011 ℃ with increasing Zr content. The effects of sintering temperature and pressure time on phase composition and microstructure were systematically studied. Besides, the influence of thermal transmission on the whole combustion process was also explored. The pyrochlore-based waste form possessed high bulk density of 6.25 g/cm^3 and Vickers hardness of 10.81 GPa. The MCC-1 leaching test showed the normalized elemental leaching rates(42d) of Cu, Gd, and Zr are 1.27×10^(-2), 1.33×10^(-3), and 8.44×10^(-7)g·m^(-2)·d^(-1), respectively.

Sintering behavior of aluminum nitride powder prepared by self-propagating high-temperature synthesis method

Fully dense aluminum nitride(AIN) ceramics were synthesized by self-propagating high-temperature synthesis(SHS) method using AIN powder as raw material with Y2O3additive. The sintering behavior was studied at different sintering temperatures and additive contents. The change of phase compositions, secondary phase distributions and grain morphologies during sintering process were investigated. It is shown that fully dense ceramics using AIN powder prepared by SHS method can be obtained when the sintering temperature is above 1830 ℃. Both Y2O3content and sintering temperature have an important influence on the formation of Y-Al-O phase and grain shape. When Y2O3content is identified, the grain morphology converts from polyhedron into sphere-like shape with the rise of sintering temperature. At a certain sintering temperature,the grain size decreases with the increase in Y2O3content. The influencing mechanisms of different YAl-O secondary phases and sintering temperatures on the grain size and morphology were also discussed based on the experimental results.

Structural,magnetic and dielectric properties of nano-crystalline Ni-doped BiFeO_(3) ceramics formulated by self-propagating high-temperature synthesis

Ni-doped BiFeO_(3) powders with the composition BiFe_(1-x)Ni_(x)O_(3)(x=0.05,0.1 and 0.15)were prepared by a self-propagating high-temperature synthesis(SHS),using metal nitrates as oxidizers and glycine as fuel.The X-ray diffraction(XRD)patterns depict that Ni-doped BiFeO_(3) ceramics crystallize in a rhombhohedral phase.The scanning electron micrographs of Ni-doped BiFeO_(3) ceramics show a dense morphology with interconnected structure.It is found that,the room-temperature magnetization measurements in Ni-incorporated BiFeO_(3) ceramics give rise to nonzero magnetization.The magnetization of Ni-doped BiFeO_(3) ceramics is significantly enhanced when Ni doping concentration reaches to x=0.1 at 5 K.The variations of dielectric constant with temperature in BiFe_(0.95)Ni_(0.05)O_(3),BiFe_(0.9)Ni_(0.1)O_(3) and BiFe_(0.85)Ni_(0.15)O_(3) samples exhibit clear dielectric anomalies approximately around 450℃,425℃and 410℃respectively,which correspond to antiferromagnetic to paramagnetic phase transition of the parent compound BiFeO_(3).

Microstructure and properties of porous Si_(3)N_(4)ceramics by gelcasting-self-propagating high-temperature synthesis(SHS)

Porous silicon nitride ceramics have attracted a considerable attention due to their excellent overall performance,but poor porosity homogeneity and structural shrinkage induced by prolonged high temperature sintering limit its further application.Herein,as a three-in-one solution for the above issues,for the first time we develop a novel approach that integrates the merits of gelcasting-SHS(self-propagating high-temperature synthesis)to prepare porous Si_(3)N_(4)ceramics to simultaneously achieve high porosity,high strength,high toughness,and low thermal conductivity across a wide temperature range.By regulating the solid content,porous Si_(3)N_(4)ceramics with homogeneous pore structure are obtained,where the pore size falls inbetween 1.61 and 4.41 pm,and the elongated grains are interlaced and interlocked to form micron-sized coherent interconnected pores.At the same time,porous Si_(3)N_(4)ceramics with porosity of 67.83%to 78.03%are obtained,where the compressive strength reaches 11.79 to 47.75 MPa and fracture toughness reaches 1.20 to 6.71 MPa-m1/2.

IR-transparent MgO-Gd_(2)O_(3) composite ceramics produced by self-propagating high-temperature synthesis and spark plasma sintering

A glycine–nitrate self-propagating high-temperature synthesis(SHS)was developed to produce composite MgO-Gd_(2)O_(3) nanopowders.The X-ray powder diffraction(XRD)analysis confirmed the SHS-product consists of cubic MgO and Gd_(2)O_(3) phases with nanometer crystallite size and retains this structure after annealing at temperatures up to 1200℃.Near full dense high IR-transparent composite ceramics were fabricated by spark plasma sintering(SPS)at 1140℃and 60 MPa.The in-line transmittance of 1 mm thick MgO-Gd_(2)O_(3) ceramics exceeded 70%in the range of 4–5 mm and reached a maximum of 77%at a wavelength of 5.3 mm.The measured microhardness HV0.5 of the MgO-Gd2O3 ceramics is 9.5±0.4 GPa,while the fracture toughness(KIC)amounted to 2.0±0.5МPa·m1/2.These characteristics demonstrate that obtained composite MgO-Gd_(2)O_(3) ceramic is a promising material for protective infra-red(IR)windows.

High-efficiency Joining of C_f/Al Composites and TiAl Alloys under the Heat Effect of Laser-ignited Self-propagating High-temperature Synthesis

The aim of this study was to develop a high-efficiency joining method of Cf/Al composites and TiA l alloys under the heat effect of laser-ignited self-propagating high-temperature synthesis（SHS）. The SHS reaction of Ni–Al–Zr interlayer was induced by laser beam and acted as local high-temperature heat source during the joining. Sound joint was obtained and verified the feasibility of this joining method. Effect of filler metals on the joint microstructure and shear strength was evaluated. When the joining pressure was 2 MPa with additive filler metals, joint shear strength reached the maximum of 41.01 MPa.

Preparation of ZrB_(2)-MoSi_(2) high oxygen resistant coating using nonequilibrium state powders by self-propagating high-temperature synthesis

To achieve high oxygen blocking structure of the ZrB_(2)-MoSi_(2) coating applied on carbon structural material,ZrB_(2)-MoSi_(2) coating was prepared by spark plasma sintering(SPS)method utilizing ZrB_(2)-MoSi_(2) composite powders synthesized by self-propagating high-temperature synthesis(SHS)technique as raw materials.The oxygen blocking mechanism of the ZrB_(2)-MoSi_(2) coatings at 1973 K was investigated.Compared with commercial powders,the coatings prepared by SHS powders exhibited superior density and inferior oxidation activity,which significantly heightened the structural oxygen blocking ability of the coatings in the active oxidation stage,thus characterizing higher oxidation protection efficiency.The rise of MoSi_(2) content facilitated the dispersion of transition metal oxide nanocrystals(5-20 nm)in the SiO_(2) glass layer and conduced to the increasing viscosity,thus strengthening the inerting impact of the compound glass layer in the inert oxidation stage.Nevertheless,the ZrB_(2)-40 vol% MoSi_(2) coating sample prepared by SHS powders presented the lowest oxygen permeability of 0.3% and carbon loss rate of 0.29×10^(6)g·cm^(-2)·s^(-1).Owing to the gradient oxygen partial pressure inside the coatings,the Si-depleted layer was developed under the compound glass layer,which brought about acute oxygen erosion.

Magnetic properties of La-Zn substituted Sr-hexaferrites by self-propagation high-temperature synthesis

The La-Zn substituted SrM-type ferrites with the composition of Sr1-xLaxFe12-xZnxO19 （x=0-0.4） were prepared by self-propagating high-temperature synthesis （SHS）. The single SrM phase was detected by XRD in the as-received samples by controlling the Fe contents in the reagents. The substitution of La^3＋and Zn^2＋ obviously increased the magnetic properties of the as-prepared samples. The maximum improvements of Br, Hcb and （BH）m were 14.4%, 15.3% and 30.7%, respectively compared with that of the samples without La-Zn substitution. Microstructure observation by SEM showed that the SHS method benefited forming the better particle features and achieving the higher Hcj in comparison with the traditional firing method.

Preparation of Nano-sized Zirconium Carbide Powders through a Novel Active Dilution Self-propagating High Temperature Synthesis Method

High quality nano-sized zirconium carbide （ZrC） powders were successfully fabricated via a developed chemical active dilution self-propagating high-temperature synthesis （SHS） method assisted by ball milling pretreatment process using traditional cheap zirconium dioxide powder （ZrO2）, magnesium powder （Mg） and sucrose （C12H22Oll） as raw materials. FSEM, TEM, HRTEM, SAED, XRD, FTIR and Raman, ICP- AES, laser particle size analyzer, oxygen and nitrogen analyzer, carbon/sulfur determinator and TG-DSC were employed for the characterization of the morphology, structure, chemical composition and thermal stability of the as-synthesized ZrC samples. The as-synthesized samples demonstrated high purity, low oxygen content and evenly distributed ZrC nano-powders with an average particle size of 50nm. In addition, the effects of endothermic rate and the possible chemical reaction mechanism were also discussed.

Self-propagating high temperature synthesis of MoSi_2 matrix composites

MoSi2 is presently regarded as the most important material for electrical heating and as one with huge potential for high temperature structural uses. MoSi2 and MoSi2 matrix composites were prepared by self-propagating high temperature synthesis （SHS）. Pure MoSi2 was obtained and a compound of MoSi2 and WSi2was synthesized in the form of predominant solid solution （Mo,W）Si2. By adding aluminum of 5.5 at.% to Mo-Si, the crystal structure of MoSi2 changed into a mixture of tetragonal Cllb MoSi2and hexagonal C40 Mo（Si,Al）2. The （Mo,W）Si2-Mo（Si,Al）2-W（Si,Al）2 composite materials were synthesized by adding aluminum of 5.5 at.% to Mo-W-Si. However, if the amount of the added aluminum was not larger than 2.5 at.%, it did not have any significant effect. SHS is an effective technology for synthesis of MoSi2 and MoSi2 matrix composites.

Ultra-fast synthesis and thermodynamic analysis of MoAlB by self-propagating high-temperature combustion synthesis

MoAlB as a typical member of MAB phases has attracted much-growing attention due to its unique properties.However,the low production of MoAlB powders limits its further development and potential applications.In the present work,the ultra-fast preparation of high-purity MoAlB powders in a few seconds is achieved by self-propagating high-temperature synthesis(SHS)using a raw powder mixture at an atomic ratio of Mo:Al:B=1:1.3:1.SHS reaction mechanism is obtained by analyzing the corresponding composition changes of starting materials.Furthermore,the thermodynamic prediction for the SHS reaction is consistent with the present experiments,where the preparation of MoAlB also conforms to two common self-propagating conditions of the SHS.The enthalpy vs.temperature curve shows that the adiabatic temperature of the reaction decreases with the amount of excuse Al increasing but increases when pre-heating the reactants.Also,this thermodynamic calculation provides a new idea for the preparation of other MAB phases by the SHS.

Synthesis of SrTiO_3 for immobilization of simulated HLW by SHS

Strontium titanate synroc samples were synthesized by self-propagating high-temperature synthesis （SHS）. Sr directly took part in the synthesis process. As a result, the loading content issue is basically resolved. The products were characterized by density, microhardness X-ray diffraction, and scanning electron microscopy （SEM/EDS）. The leaching rate was measured by the method of PCT （product consistency test）. The results indicate that the Sr^2＋-SrTiO3 compound is of high density, low leach rate and high stability and the synthesis process is feasible in technology and economy. It can be concluded that the strontium titanate synroc is a perfect material to immobilize HLW.

Preparation of β-SiC by combustion synthesis in a large-scale reactor

The feasibility of 5 kg β-SiC synthesized in one batch was demonstrated through igniting the mixture of Si, C-black and polytetrafluoroethylene （PTFE） under different nitrogen pressures. The effect of experimental parameters, including the contents of PTFE, nitrogen pressure, preheating, and raw materials distribution forms were investigated. The results show that the products are β-SiC with equiaxed grains. The average grain size is less than 200 nm. The powders loaded loosely promote reaction heat dispersing, resulting in small grains. High purity β-SiC powders are obtained when the PTFE content is as low as 5wt%, which simplifies the process and decreases the cost effectively. The ceramic sintered from the obtained β-SiC powders presents the hardness of 22.20 GPa, the bending strength as high as 715.15 MPa and the fracture toughness of 8.179 MPa·m^1/2, which are higher than those of ceramics fabricated with α-SiC produced by combustion synthesis.