由有机和无机金属前躯体自蔓延高温合成碳包覆磁纳米粒子(英文)

介绍了一项由自蔓延高温合成(SHS)碳包覆磁纳米粒子的系统研究。采用还原剂NaN3和三种不同氧化剂-聚四氟乙烯、六氯乙烷和六氯苯,实施了SHS制备。研究了金属前躯体(Fe(CO)5或K3[Fe(CN)6])对产物的得率、反应热、形貌、结构和磁性能的影响。结果表明:有机铁前躯体和C2Cl6氧化剂反应体系可获得磁性最佳、得率最高的产物。     

Effect of substitution of Si by Al on microstructure and synthesis behavior of Ti5Si3 based alloys fabricated by mechanically activated self-propagating high-temperature synthesis

The effect of substitution of Si by Al and mechanical activation on microstructure, phase composition, ignition and combustion temperature of Ti₅Si₃ based alloys and composites that were prepared by mechanically activated self-propagating high-temperature synthesis (MASHS) method was investigated. For this purpose elemental powders of titanium, silicon and aluminum were mixed according to the 5Ti + 3(1 − X)Si + 3XAl formula, where X = 0, 0.2, 0.4, 0.6. The samples were characterized by X-ray diffraction (XRD) analytical technique and scanning electron microscope (SEM) equipped with an energy-dispersive spectrum (EDS) analyzer. The results have shown that formation of Ti₅Si₃ during milling stage is postponed by adding Al into the system. Presence of Al in the Ti–Si system have a significant effect on the phase composition of the final products. Substitutional solid solution of Ti₅(Si, Al)₃ and Ti₅Si₃–Ti₃Al composite are formed by increasing Al amount in the system. Furthermore combustion temperature and crystallites size of Ti₅Si₃ is reduced with addition of Al into the Ti–Si system. Moreover, solubility of Al in Ti₅Si₃ is increased with enhancing the X up to 0.4, after that, the solubility of Al in Ti₅Si₃ is ceased, due to achieving the solubility limit of Al in the Ti₅Si₃. The average crystallites size of Ti₅Si₃ are decreased with increasing milling time prior to the reaction.     

Gas-phase and heat-exchange effects on the ignition of high- and low-exothermicity porous solids subject to constant heating

This article investigates the ignition of low-exothermicity reactive porous solids exposed to a maintained source of heat (hotspot), without oxygen limitation. The gas flow within the solid, particularly in response to pressure gradients (Darcy’s law), is accounted for. Numerical experiments related to the ignition of low-exothermicity porous materials are presented. Gas and solid products of reaction are included. The first stage of the paper examines the (pseudo-homogeneous) assumption of a single temperature for both phases, amounting to an infinite rate of heat exchange between the two. Isolating the effect of gas production and flow in this manner, the effect of each on the ignition time is studied. In such cases, ignition is conveniently defined by the birth of a self-sustained combustion wave. It is found that gas production decreases the ignition time, compared to equivalent systems in which the gas-dynamic problem is effectively neglected. The reason for this is quite simple; the smaller heat capacity of the gas allows the overall temperature to attain a higher value in a similar time, and so speeds up the ignition process. Next, numerical results using a two-temperature (heterogeneous) model, allowing for local heat exchange between the phases, are presented. The pseudo-homogeneous results are recovered in the limit of infinite heat exchange. For a finite value of heat exchange, the ignition time is lower when compared to the single-temperature limit, decreasing as the rate of heat exchange decreases. However, the decrease is only mild, of the order of a few percent, indicating that the pseudo-homogeneous model is in fact a rather good approximation, at least for a constant heat-exchange rate. The relationships between the ignition time and a number of physico-chemical parameters of the system are also investigated.     

Mechanical-activation-assisted combustion synthesis of SiC

Mechanical-activation-assisted combustion synthesis of SiC has been conducted with PVC as promoters. The mechanical activation of the Si-C reactants through high-energy attrition milling could result in substantial decrease of the ignition temperature and the incubation time for the Si-C combustion reaction. Ultra-fine β-SiC powders with equiaxed grains were synthesized at the preheating temperature as low as 1050 °C. The specific surface area (SSA) of the combustion synthesized SiC powders was 4.36 m²/g, and the average particle size was less than 5 μm.     

A numerical study of the heterogeneous porosity effect on self-propagating high-temperature synthesis (SHS)

Self-propagating high-temperature synthesis (SHS), or the so-called micropyretic/combustion synthesis, is a technique whereby a material is synthesized by the propagation of a combustion front across a powder. Heterogeneous distributions of porosities are common during self-propagating high-temperature synthesis when powders are pressed and the conventional modeling treatments thus far have only considered uniform systems. Heterogeneities in the porosity are thought to result in local variations of such thermophysical/chemical parameters for the reactants as density and thermal conductivity further changing the combustion temperature, the propagation velocity, and the propagation pattern of a combustion front. This study investigates the impact of porosity variations during self-propagating high-temperature synthesis with Ti + 2B. In addition, the simulations for the propagation of combustion fronts across a non-uniform compact where the porosity is monotonically decreased or increased along the specimens due to die wall friction are also carried out.     

Synthesis of nanometric MoNbW alloy using self-propagating high-temperature synthesis

Nanometric powders of stoichiometric compositions in the Mo-Nb-W ternary system were produced by Self-propagating High-temperature Synthesis (SHS) using magnesium to reduce their oxides, in presence of sodium chloride. The influence of Mg and adiabatic temperature on the phase compositions of the final products were determined. After reaction, samples were analyzed by XRD, SEM, and BET.Results demonstrate the possibility to obtain high purity nanostructured products in the 20–800 nm range, with an average equivalent diameter, from specific surface measurements, of 66 nm. For moderate excess Mg amounts, due to the low Mg boiling temperature, niobium is not completely reduced and forms complex compounds with MgO, whereas reduction is near-complete for the highest Mg amounts.     

Parameters investigation during simultaneous synthesis and densification WC–Ni composites by field-activated combustion

The field-activated, and pressure-assisted combustion synthesis (FAPACS) process, which combines the simultaneous synthesis and densification of materials, was utilized to produce WC–Ni composites from powdered reactants, mixtures of tungsten, carbon and nickel. These reactants were subjected to high DC currents and uniaxial pressures. Under these conditions, a reaction is initiated by field activation and completed within a short period of time. Several experimental parameters, such as pulse current, power-controlled mode, temperature-increasing rate, maximum temperature and pressure during FAPACS on the relative densities of products were studied. Finally, the material with nearly complete density was fabricated. The percentage of the total shrinkage occurring before and during the synthesis reaction and addition densification was measured. The relative density of the end product and Vickers microhardness measurement (at 50 kg force) on the dense sample is 99.2% and 1424 kg mm⁻², respectively.     

In situ synthesis of titanium diboride composites through volume combustion

Abstract

Hard in situ synthesis of TiB₂–Fe₂B metal matrix composite (MMC) has been synthesised by volume combustion synthesis (VCS) reactions of Fe–FeTi–FeB system. VCS samples were characterised by SEM, EDX, XRD and DTA. Results show that it is possible to synthesise in situ structured MMC samples (with TiB₂ and Fe₂B phases) by VCS. Metallographic investigations show that Fe₂B and TiB₂ are found dispersed throughout the metal matrix, and other borides are present in microlevel patches dispersed in a eutectic matrix. The Fe–TiB₂ composites sintered at temperature of 1200°C consist of three different regions, i.e. α-Fe, TiB₂ and Fe₂B regions. The increase in sintering temperature to 1400°C leads to a hypereutectic microstructure of the Fe–B binary system having TiB₂ grains uniformly distributed throughout the matrix. A semiliquid phase sintering occurred by increasing eutectic phase transformation temperatures to 1400°C, which increased the efficiency of VCS. On the other hand, increasing sintering time from 1 to 3 h decreased the volume fraction of α-Fe and increased the volume fraction TiB₂ phase.     

Development of a thermal barrier material using combustion synthesis

The present investigation employs combustion synthesis as a method to produce a functionally graded Ni₃Al/Al₂O₃+TiB₂ composite material for use as a thermal barrier system for nickel-based alloys at elevated temperatures. Starting materials were Ni, Al, TiO₂ and B₂O₃ in powder form. Adiabatic thermodynamic calculations used to determine the maximum theoretical temperature reached during combustion suggest that up to 1600 K may be reached in the Ni+Al metallic layer, easily sufficient to initiate the ceramic-based reaction. The latter reaction is predicted to reach 3000 K. Experiments were first conducted in an induction furnace to establish conditions necessary for combustion to occur. Subsequent experimentation, with applied pressure during combustion, was conducted in a Gleeble 1500 thermomechanical test unit modified to accept the samples of interest. Characterisation of the combustion products by means of hardness measurements, X-ray diffraction, scanning electron microscopy and electron probe microanalysis confirmed that the products were Ni₃Al and Al₂O₃+TiB₂. Also, the mechanical integrity was unchanged after 10 thermal cycles in the modified Gleeble unit. Finally, the coating thickness required to keep a Ni-based substrate below 850°C in a 1100°C environment is estimated to be 1.8 mm, based on thermal conductivity calculations using a finite element method.     

Fabrication of bulk Al2O3 dispersed ultrafine-grained Cu matrix composite by self-propagating high-temperature synthesis casting route

Bulk Al₂O₃ dispersed ultrafine-grained (UFG) Cu matrix composite has been fabricated by self-propagating high-temperature synthesis (SHS) casting route. The microstructures and mechanical properties of the as-fabricated materials have been investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis, and microhardness measurements and compression tests, respectively. The results show that the as-fabricated material has small amount of entrapped Al₂O₃ particles in uniform microstructure Cu having a grain size ranging from 200-500 nm with an elevated compressive yield stress (298 MPa) and an improved microhardness value (1.06 GPa). The possible strengthening mechanism of the product has been discussed.     

Synthesis and formation mechanisms of molybdenum silicides by mechanical alloying

The synthesis and formation of MoSi₂, Mo₅Si₃, and Mo₃Si compounds by the mechanical alloying of MoSi powder mixtures has been investigated. Ball-milling experiments were conducted for the composition range of 10–80 at.% Si. The formation of molybdenum silicides, especially MoSi₂, during mechanical alloying and the relevant reaction rates markedly depended on the powder composition. The spontaneous formation of MoSi₂ during mechanical alloying at 67 at.% Si (MoSi₂ stoichiometry) proceeded by a mechanically-induced self-propagating reaction (MSR), the mechanism of which is analogous to that of the self-propagating high-temperature synthesis (SHS). At the compositions of 54 and 80 at.% Si, however, the formation of MoSi₂ proceeded by the gradual formation of both the and /gb phases instead of the MSR mode. The formation of Mo₅Si₃ during mechanical alloying was characterized by a slow reaction rate as the reactants and product coexisted over a long period. The milling of Mo-rich powder mixtures up to 150 h did not lead to the direct formation of Mo₃Si. The Mo₃Si phase appeared only after brief annealing at temperatures of 800°C and above.     

Effects of Nanocrystalline Powders Additions on the Characteristics of Combustion Process, Phase- and Structure-Formation, and Properties of SHS Alloys on Titanium Carbide Base

A study was made of the effect of different nanocrystalline powders additives on the macrokinetics of combusting Ti-Cr-C-Ni mixtures as well as on the composition, structure, and physical-mechanical properties of SHIM-SB alloy produced by the power SHS-compaction technique. An additive of nanocrystalline powder is found to result in 2- to 5-fold modification of alloy structure and, in most cases, in improving physical and mechanical properties (bending strength, hardness, microhardness, and crack growth resistance).     

Combustion Synthesis of Aluminoborate Glass Matrices

Combustion synthesis or self-propagating high-temperature synthesis (SHS) has been used for the first time to produce glass–ceramic materials. The materials produced by the technique have a glassy matrix (aluminoborate glass) and crystalline TiB₂ particles, of about 0.5 m. The combustion characteristics and microstructures of the synthesized materials are presented.     

Solution combustion synthesis of LiMn2O4 fine powders for lithium ion batteries

In this work, fine powders of spinel-type LiMn₂O₄ as cathode materials for lithium ion batteries (LIBs) were produced by a facile solution combustion synthesis using glycine as fuel and metal nitrates as oxidizers. Single phase of LiMn₂O₄ products were successfully prepared by SCS with a subsequent calcination treatment at 600–1000 °C. The structure and morphology of the powders were studied in detail by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The electrochemical properties were characterized by galvanostatic charge–discharge cycling and cyclic voltammetry. The crystallinity, morphology, and size of the products were greatly influenced by the calcination temperature. The sample calcined at 900 °C had good crystallinity and particle sizes between 500 and 1000 nm. It showed the best performance with an initial discharge capacity of 115.6 mAh g⁻¹ and a capacity retention of 93% after 50 cycles at a 1 C rate. In comparison, the LiMn₂O₄ sample prepared by the solid-state reaction showed a lower capacity of around 80 mAh g⁻¹.     

In-situ combustion synthesis and densification of TiC–xNi cermets

The combustion synthesis reaction was combined with quasi-isostatic pressing (QIP) technique to fabricate full density TiC–xNi composites in a single processing operation. Combustion wave velocity and temperature of Ti–C–Ni were measured and the microstructure of the product was characterized by X-ray diffraction and scanning electron microscopy. With increasing Ni content in TiC–xNi, both the combustion wave velocity and temperature decrease. The Ni additive, mainly as a diluent and the binder of TiC grains in a matrix, formed a quasi-continuous phase enveloping spheroidal TiC particles and brought about a grain size decrease from 9 to 1 μm. TiC-20 wt% Ni cermet produced by the combustion synthesis/quasi-isostatic pressing process under 160 MPa for 20 s show near full density, high hardness and transverse rupture strength (1024.2 MPa).     

SHS/PHIP法合成TiB2陶瓷的研究

采用自蔓延高温合成结合准热等静压（SHS／PHIP）工艺制备了TiB2陶瓷材料,并进行了X射线衍射（XRD）、扫描电镜（SEM）、力学性能等实验研究。XRD结果表明反应生成了高纯度的TiB2,没有其他相生成,这与热力学计算结果相吻合,材料SEM分析发现,TiB2陶瓷颗粒形貌为近等轴状,尺寸细小且较均匀,在TiB2陶瓷颗粒间还存在“烧结颈”,由于SHS反应所产生的高温,造成了TiB2陶瓷颗粒表面的圆化。合成产物的致密度较低,气孔主要存在于TiB2陶瓷颗粒交界处,较低的致密性导致了材料具有较低的硬度、弯曲强度和断裂韧性等力学性能。     

Layered growth of Ti2AlC and Ti3AlC2 in combustion synthesis

In Ti-Al-C system two ternary carbides of Ti₂AlC and Ti₃AlC₂ were prepared by combustion synthesis. Laminated grain morphology and a terraced structure consisting of several parallel laminated layers were observed. On the basis of SEM and TEM results, a layered growth mechanism was proposed to describe the formation of the terraced structure. In this mechanism, the ternary carbide grains will undergo a preferential growth, i.e. each layer grows fast and expands quickly in the basal plane and all the layers are successively stacked along the normal direction identical to the c-axis in the hexagonal structure. This preferential growth characterized by the terraced structure was widely observed in this study and hence may be a common behavior during the growth of Ti₂AlC and Ti₃AlC₂ crystals in combustion synthesis.     

An investigation of the synthesis of nickel aluminides through gasless combustion

The formation of nickel aluminidms by the thermal explosion mode of gasless combustion synthesis was investigated for Ni-Al powders ranging in composition from 5 to 3O at% Al. Compound formation was found to take place sequentially starting with the most aluminium-rich and ending with AlNi₃ as the predominant compound in the product. Compounds formed through both solid- and liquid-sate reactions, with the relative contribution of each depending on the rate of heating of the powders to the reaction temperature. The effect of the particle size of nickel on these reactions vvasalso investigated for powders with average diameters from 14 to 58m.     

Facile synthesis of nanocrystalline zinc ferrite via a self-propagating combustion method

Macroporous nanocrystalline zinc ferrite with single spinel-phase was prepared by a facile self-propagating combustion method using zinc nitrate, iron nitrate and glycine. The as-prepared ZnFe₂O₄ were characterized by X-ray diffraction (XRD) analysis, N₂ adsorption, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED) and energy dispersive X-ray spectrum (EDS). The magnetic properties of the prepared ZnFe₂O₄ were also studied.     

Mechanically activated combustion synthesis of Ti3AlC2/Al2O3 nanocomposite from TiO2/Al/C powder mixtures

Ti₃AlC₂/Al₂O₃ nanocomposite powder was synthesized by mechanical-activation-assisted combustion synthesis of TiO₂, Al and C powder mixtures. The effect of mechanical activation time of 3TiO₂-5Al-2C powder mixtures, via high energy planetary milling (up to 20?h), on the phase transformation after combustion synthesis was experimentally investigated. X-ray diffraction (XRD) was used to characterize as-milled and thermally treated powder mixtures. The morphology and microstructure of as-fabricated products were also studied by scanning electron microscopy (SEM) and field-emission gun electron microscopy (FESEM). The experimental results showed that mechanical activation via ball-milling increased the initial extra energy of TiO₂-Al-C powder mixtures, which is needed to enhance the reactivity of powder mixture and make it possible to ignite and sustain the combustion reaction to form Ti₃AlC₂/Al₂O₃ nanocomposite. TiC, AlTi and Al₂O₃ intermediate phases were formed when the initial 10?h milled powder mixtures were thermally treated. The desired Ti₃AlC₂/Al₂O₃ nanocomposite was synthesized after thermal treatment of 20?h milled powder and consequent combustion synthesis and FESEM result confirmed that produced powder had nanocrystalline structure.