Characteristic sample temperature and pressure during processing of titanium nitride combustion synthesis with liquid nitrogen

Titanium nitride synthesis has been investigated by sustaining a nitriding reaction of titanium powder compacts set in a closed vessel filled with liquid nitrogen. The characteristic features of the present combustion synthesis technology are the accelerating pressure in the vessel following sample heating through its combustion propagation, which results in a specific structure formation of the products. It has been confirmed in the present work that the pressure in the closed vessel was drastically increased (ca. 20 MPa s-1) by propagating the nitriding reaction. The product was shrunk and densified up to 99%. The product was identified as TiN0.87 without any trace of elemental titanium. © 1998 Chapman & Hall     

Structure of the combustion wave in the combustion synthesis of titanium carbides

From the recorded images of combustion wave propagation during the combustion synthesis of titanium carbide, real time combustion velocities have been determined with increase of density of the compact. Results demonstrate that the combustion velocity exhibits a maximum with increase of density of the compact, and the wave propagates in a steady-state manner. Real time temperature profiles suggest that adiabatic conditions exist during the steady-state propagation of the combustion wave. Analysis of the images for the propagation of the combustion wave suggest that density plays a major role in the nature of propagation and whether the combustion wave propagates or not. Pore structural analysis of the carbides indicate collapse of the original porosity of the carbons, and the collapse of porosity is attributable to an exothermic diffusional reaction occurring between liquid titanium and carbon forming a titanium carbide product layer.     

NiCrAl/YSZ/NiCrAl-B.e复合涂层对α+β型高温钛合金燃烧产物的影响

通过摩擦氧浓度方法点燃涂覆NiCrAl/YSZ/NiCrAl-B.e复合涂层的TC11钛合金,采用XRD,SEM,EDS及EPMA等微观表征方法对其燃烧产物进行分析,进而探讨复合涂层对燃烧行为的影响机理。结果表明:燃烧试样的基体从中心孔沿径向往外呈分区组织演变特征,中心孔处的钛合金基体是着火源;当燃烧程度较低时,YSZ中间层对钛合金燃烧行为的影响较小;当剧烈燃烧时,YSZ中间层通过分解反应而在钛合金熔体中大量溶解,为钛合金熔体提供了O和Zr,加速了Ti与O的快速结合,同时ZrTiO_4燃烧产物的阻氧能力比TiO_2差,从而大大促进了钛合金的扩展燃烧。     

Synthesis of Highly Dense β-SiC Pellet by Self Propagating High Temperature Synthesis

β-SiC has been synthesized using the process of self-propagating high temperature synthesis (SHS) (1). Ti was added as a binder and the effect of titanium addition on the reaction propagation has been studied. Temperature of combustion is considerably lowered by the addition of titanium and the incubation period leading to the melting of the silicon powders is reduced. The reaction propagated through the pellet faster and the product synthesized was dense and compact. The synthesized composite was characterized using X-ray, Raman spectroscopy, SEM and TEM. We proposed a model for the synthesis based on dissolution, diffusion and precipitation from the supersaturated solution. There was no indication of the formation of TiSi₂, during synthesis. An attempt is made to explain this observation based on free energy considerations.     

Microstructural evolution in the combustion synthesis of titanium carbide

Microstructural evolution in the combustion synthesis of titanium carbide was investigated using the combustion front quenching method and scanning electron microscopy. The results showed that the combustion reaction between titanium and carbon began with reaction diffusion of carbon into the surface layer of titanium powder, leading to the formation of a TiC shell around the titanium powder. The titanium powder coated with the TiC shell melted and became a molten core, as the carbon atoms diffused into the core through the TiC shell, TiC grains gradually crystallized. In the final products, the morphology and size of the initial titanium powder were maintained in the form of the TiC granule which was composed of isometrical TiC grains. In addition, a lamellar eutectic was present at the centre of some TiC granules. The microstructural evolution in the combustion synthesis of TiC has been described with a shell-core model.     

Numerical modeling of field-activated combustion synthesis process of the B4C system

A field-activation combustion synthesis process of the 4B + C reactive system was numerically simulated to investigate the effect of external field and porosity on the combustion reaction by an implicit difference method and a Gauss-Seidel iteration procedure. The new features of the model include a consideration of the melting of each constituent of the reactants and product and the inclusion of considerations involving porosity and dilution. The results show that the self-sustaining reactions are not possible until field-activated temperature is more than 1100 K, which agree with the theoretic calculation. As the reactant porosity values are decreased from 60% to 20%, the combustion velocity first increases because of an increase in the thermal conductivity. The combustion velocity, after reaching a maximum, decreases with a further decrease in the porosity because of the high value of the thermal conductivity of the reactants.     

A novel synthesis of B-Al-Ca-O-(N) glasses by self-propagating high-temperature combustion method

Glasses in the B-Al-Ca-O-(N) system have been prepared by self-propagating high-temperature combustion synthesis (SHS) directly from mixed powders of B, Al and CaO under pressurized dry-air of 3 MPa. The nitrogen content of the SHS product is 0.63 mass%, the value of which is about 30 times higher than that of conventional molten glass fabricated in air. Their initial reactants, B and Al when ignited, transform instantaneously into oxides, nitrides and oxynitride with a high exothermic heat of reaction. Sequentially the reacted products in a liquid state solidify into a glass material by rapid cooling. The glass forming region is similar to that of the same oxide glass system prepared by the conventional melting method.     

Effect of carbon sources on the combustion synthesis of TiC

The effect of carbon sources, i.e. graphite and amorphous carbon, on the reaction mechanism, product morphology, and the rate of combustion reaction between Ti and C to form TiC were studied. A reaction mechanism was proposed for each carbon source from the activation energy of combustion reaction. The microstructure and the composition of reaction products were also investigated. It was observed that graphite fissured in a layered form during the combustion reaction and the reaction between graphite and liquid titanium was accomplished mainly on the surface of the thinly fissured layer. Graphite was found to be more reactive with titanium and titanium carbide synthesized with graphite contains less amount of unreacted carbon and is more close to the stoichiometric TiC.     

Effects of calcium nitride and calcium carbonate gasifying agents on the combustion synthesis of Ni3Ti-TiC composites

The synthesis of nickel-titanium (NiTi) intermetallic composites is of considerable interest due to the ability to create a porous material with high strength and improved wear resistance. The effects of adding a carbon reactant to modify the reaction products and exothermicity have been studied using two reaction stoichiometries involving elemental nickel, titanium and carbon (graphite). The present study examines the synthesis of porous Ni₃Ti intermetallic composites in the presence of calcium nitride or calcium carbonate gasifying agent. Both gasifying agents show significant effects on the ignition characteristics of the reaction, burning velocities and TiC particle sizes present in the final product, but do not affect the combustion temperatures.     

A novel fabrication method for TiC–Al2O3–Fe functional material under centrifugal acceleration

Compacted powders of titanium (Ti) and carbon (C) in form of pellets were exposed to a massive amount of heat generated from the thermite reaction of Fe₂O₃ and Al in a graphite–steel tube mounted in a developed centrifugal accelerator machine. The centrifugal force facilitated the formation of multi-component products during the process. Titanium carbide (TiC) product is joined to an Al₂O₃–Fe layer, which are the products of the thermite reaction. The existence of centrifugal acceleration had a significant effect on both metallurgical alloying and mechanical interlocking between different layers of the sample to form a functional material. A mathematical model developed for this experiment to describe the speed rate of iron infiltration inside the TiC product as well as viscosity rate variation was presented. The composition, microstructure and mechanical properties confirmed the model.     

Microwave-hydrothermal process for the synthesis of rutile

The synthesis of pure rutile titanium dioxide is not an easy achievement, as the crystallization process generally leads to mixtures of two or even three phases; moreover the synthetic processes normally used by industry require harsh reaction conditions. We carried out the synthesis of titanium dioxide from an aqueous titanium tetrachloride solution under microwave irradiation in the reaction time range of 5-120 min. We mostly obtained mixtures of rutile and anatase, but obtained single-phase rutile after a 2-h treatment at 160 °C; transmission electron micrographs revealed well-dispersed spherical nanoparticles. We also investigated the effects of dilution and addition of a dispersant (polyvinylpyrrolidone) on phase crystallization and particle shape.     

Structurally stitched NCF CFRP laminates. Part 1: Experimental characterization of in-plane and out-of-plane properties

The insertion of local through-thickness reinforcements into dry fiber preforms by stitching provides a possibility to improve the mechanical performance of polymer-matrix composites perpendicular to the laminate plane (out-of-plane). Three-dimensional stress states can be sustained by stitching yarns, leading to increased out-of-plane properties, such as impact resistance and damage tolerance. On the other hand, 3D reinforcements induce dislocations of the in-plane fibers causing fiber waviness and the formation of resin pockets in the stitch vicinity after resin infusion which may reduce the in-plane stiffness and strength properties of the laminate.In the present paper an experimental study on the influence of varying stitching parameters on in-plane and out-of-plane properties of non-crimp fabric (NCF) carbon fiber/epoxy laminates is presented, namely, shear modulus and strength as well as compression after impact (CAI) strength and mode I energy release rate. The direction of stitching, thread diameter, spacing and pitch length as well as the direction of loading (which is to be interpreted as the direction of the three rail shear loading or the direction of crack propagation in case of mode 1 energy release rate testing) were varied, and their effect on the mechanical properties was evaluated statistically.The stitching parameters were found to have ambivalent effect on the mechanical properties. Larger thread diameters and increased stitch densities result in enhanced CAI strengths and energy release rates but deteriorate the in-plane properties of the laminate. On the other hand, a good compromise between both effects can be found with a proper selection of the stitching configurations.     

Investigation of joining Al–C–Ti cermets and Ti6Al4V by combustion synthesis

The paper has addressed a route for the welding of titanium alloy (Ti6Al4V) and Al–C–Ti powders by the combustion synthesis (CS) method. Al–C–Ti powders were compressed in the titanium alloy pipes with relative densities of 65%, and then the powder compact was sintered by two reaction mode at the same time as the annulus of titanium alloy and the synthesized product were joined. The paper has studied the effects of reaction mode and Al content in starting powders on the structure and property of the welded joints. And it has also discussed the microstructure of welded joints by laser-induced combustion synthesis (LCS). The mechanical properties of the welding seam have been also tested. The results show that LCS welding has realized fusion welding and the welding seam has good mechanical properties. Furthermore, SEM analysis has indicated that nano-size grains of TiC were formed in the joint layer.     

Influence of Ti Powder Characteristics on Combustion Synthesis of Porous NiTi Alloy

Porous NiTi shape memory alloy (SMA) is a novel biomedical material used for human hard tissue implant .The influence of elemental titanium powder characteristics such as powder morphology, particle size and specific surface area( SSA) on the minimal ignition temperature ,combustion temperature and final product of porous Ni-Ti SMA fabricated by combustion synthesis method was investigated in this paper by scanning electron microscopy (SEM) and laser diffraction.The preliminary data indicated that the titanium powder characteristics had a strong effect on combustion synthesis of porous NiTi SMA.     

Effect of aluminium addition on the combustion reaction of titanium and carbon to form TiC

Aluminium was incorporated into the reactant mixture with a molar ratio of Ti/C of 1.0 to study the effect of its addition on the combustion reaction between titanium and carbon to form TiC. Thermal analysis of the reactant mixture and component analysis of the reaction product suggest that the combustion reaction of the Ti-C-Al system proceeds in such a way that aluminium initially reacts with titanium to form titanium aluminide compounds of TiAl₃, Ti₂Al, and TiAl with heat evolution, and then the reaction between titanium and carbon and the decomposition of titanium aluminide to titanium and aluminium is subsequently followed. As the amount of aluminium incorporated was increased over the range of 0 to 40 wt%, the grain size of TiC decreased from approximately 15 m to 0.4 m. It was also observed that most of the aluminium in the TiC-Al composite was distributed on the surface of the spherical TiC grain.     

Combustion of TiO2-Mg and TiO2-Mg-C systems in the presence of NaCl to synthesize nanocrystalline Ti and TiC powders

The combustion process of TiO₂-Mg and TiO₂-Mg-C systems with sodium chloride as an inert diluent was investigated. The values of combustion parameters and temperature distribution on a high-temperature wave according to the amount of sodium chloride were obtained by the thermocoupling technique. The leading stages of combustion processes are found and the sizes of reactionary zones were estimated. It is shown that the introduction of NaCl in an initial mixture promotes the formation of a nanocrystalline structure of the final products. As a result, nanosized titanium, and titanium carbide powders have been successfully obtained.     

低温固相反应合成纳米级TiB2-TiC复合粉体

在Ti-B体系中引入PTFE作为反应促进剂, 实现了TiB₂-TiC粉体的低温固相合成。分别采用热分析仪、X射线衍射仪和场发射扫描电子显微镜, 测定了体系的反应温度, 表征了生成物的物相和微观形貌, 并对其反应过程和反应机理进行了分析。合成实验在氩气炉中进行, 结果表明: 当添加10wt% PTFE时, 能够在550℃通过固相反应制备出平均粒径小于400 nm的TiB₂-TiC复合陶瓷粉体。DTA测试表明固相反应合成过程主要包括两步: 首先, 在低温下PTFE和Ti发生反应并释放出大量的热, 然后, 诱发Ti和B的固相反应生成TiB₂。     

考虑离心作用的螺杆挤出机固体输送理论模型

在固体输送段,物料由散粒体逐渐被压缩,形成压力梯度。离心力对固体输送有较大的影响,由于离心力的作用,物料与机筒之间的径向作用力大于物料与螺杆之间的径向作用力。在固体输送段压力沿螺杆轴向逐渐增大,输送角逐渐减小。文中根据实验现象,考虑离心力,建立了可压缩体的固体输送物理模型,并求出数学模型的解析解。理论计算表明,即使机筒与固相的摩擦系数小于螺杆与固相的摩擦系数,仍会有固体输送。     

PTFE促发TiC陶瓷粉体低温固相合成研究

在Ti-C体系中引入PTFE(聚四氟乙烯树脂)作为反应促进剂, 实现了TiC粉体的低温固相合成。分别利用热分析仪、X-射线衍射仪和场发射扫描电子显微镜, 测定了体系的反应温度, 表征了生成物的物相和微观形貌, 并对其反应过程和反应机理进行了分析。结果表明: 当添加3wt% PTFE时, 能够在530℃通过燃烧合成制备平均粒径小于100 nm的TiC陶瓷粉体, 接近于利用Scherrer 公式取XRD最强衍射峰计算出的平均晶粒尺寸81 nm, 可以推测所合成的TiC颗粒为单晶颗粒。燃烧合成过程分为两个步骤: 首先, 在低温下PTFE和Ti发生反应并释放出大量的热; 然后, 诱发Ti和C的固相反应生成TiC。     

Effects of particle size of reactant on characteristics of combustion synthesis of TiC-Fe cermet

Four Ti-C-Fe powder mixtures, with a same molar ratio but a different particle size, were used for a combustion synthesis of TiC-Fe cermet to investigate effects of the particle size on the characteristics of the combustion synthesis. The results showed that the mixture with the finer Ti powder gave out a higher combustion temperature, a higher reaction velocity, a higher product density, a layer-shaped pore, and a greater size of TiC particles whether the Fe powder was finer or coarser. While in the case of the coarser Ti powder used, a small amount of residual phase remained in the product, and what is more interesting that the mixture with the finer Fe powder gave out a lower reaction velocity than that by the mixture with the coarser Fe powder. These effects were successfully explained with the previously proposed mechanisms of the combustion synthesis of TiC-Fe, and the mechanisms were proved to be valid thereby.