Synthesis of Ti2SnC from Ti/Sn/TiC powder mixtures by pressureless sintering technique

Ti/Sn/TiC powder mixtures were first employed to synthesize Ti₂SnC powder by pressureless sintering in the temperature range of 950–1250 °C at vacuum atmosphere. Ti₂SnC began to form at 950 °C, its content increased with increasing temperature. High purity of Ti₂SnC was obtained by sintering the mixtures with deficient Sn and TiC at 1200 °C for 15 min. A reaction mechanism was proposed to explain the formation of Ti₂SnC. The Ti₂SnC powder was characterized by scan electron microscopy (SEM) and X-ray diffraction (XRD). Using the above mixtures and process, the Ti₂SnC ceramic powder can be obtained on a larger scale.     

Vacuum heat treatment of LiAlO2 densified silicon nitride ceramics

The aim of the present work has been to produce high-dense Si₃N₄ ceramics by a cheaper pressureless sintering method and then to attain vacuum heat treatment to remove residual grain boundary glass in gaseous form. LiAlO₂ was used as a sintering additive rather than using Li₂O, since its grain boundary glass is not stable above 1200 °C. LiAlO₂ was synthesised from 42% Li₂CO₃ and 58% Al₂O₃ powder mix reacting together at 1450 °C for 3 h in a muffle furnace. X-ray analysis showed that 95% LiAlO₂ was obtained. LiAlO₂ was milled and added to silicon nitride powder as a sintering additive. Hot-pressing and pressureless sintering of LiAlO₂ containing Si₃N₄ compacts were carried out at temperatures between 1450–1750 °C. The sintered samples were vacuum heat-treated at elevated temperatures under high vacuum to remove intergranular glass and to increase refractoriness of Si₃N₄ ceramics. Scanning electron microscope images and weight loss results showed that Li in grain boundary glass (Li–Al–Si–O–N) was successfully volatilised, and oxidation resistance of the sintered samples was increased.     

无压烧结Ti3SiC2/铝基复合材料组织与性能

MAX相具有独特的层状晶体结构,不但具备常用铝基复合材料外加陶瓷颗粒的性能特征,同时具有可与石墨媲美的摩擦性能.本文以Al粉、Si粉和典型MAX相Ti_3SiC_2为原料,采用冷压成型-无压烧结方法制备了Ti_3SiC_2/Al-Si复合材料,并通过金相显微镜、X射线衍射仪(XRD)、扫描电镜(SEM)、能谱仪(EDS)等分析手段,研究了烧结温度、Si元素含量对复合材料组织与性能的影响.研究表明:随着烧结温度从500℃提高到700℃,复合材料致密度先上升后下降,摩擦系数先降低后上升,硬度逐渐增大至最大值并基本保持稳定;随着Si质量分数从0增加到20.7%,复合材料的致密度逐渐降低,硬度逐渐增大,摩擦系数先降低后增大,晶粒尺寸随之下降,12.5%Si晶粒最为细小;烧结温度为650℃,Si元素质量分数为12.5%的铝基复合材料具有最低的摩擦系数0.18,相应的硬度为62 HV,致密度为92.12%.XRD物相和扫描电镜组织分析表明,复合材料的主要相组成为Al、Ti_3SiC_2,及由界面反应产生的Al_4C_3和Al的氧化产物Al_2O_3.     

Fabrication of silicon nitride nanoceramics—Powder preparation and sintering: A review

Fine-grained silicon nitride ceramics were investigated mainly for their high-strain-rate plasticity. The preparation and densification of fine silicon nitride powder were reviewed. Commercial sub-micrometer powder was used as raw powder in the “as-received” state and then used after being ground and undergoing classification operation. Chemical vapor deposition and plasma processes were used for fabricating nanopowder because a further reduction in grain size caused by grinding had limitations. More recently, nanopowder has also been obtained by high-energy milling. This process in principle is the same as conventional planetary milling. For densification, primarily hot pressing was performed, although a similar process known as spark plasma sintering (SPS) has also recently been used. One of the advantages of SPS is its high heating rate. The high heating rate is advantageous because it reduces sintering time, achieving densification without grain growth. We prepared silicon nitride nanopowder by high-energy milling and then obtained nanoceramics by densifying the nanopowder by SPS.     

Densification,microstructure and mechanical properties of multicomponent (TiZrHfNbTaMo)C ceramic prepared by pressureless sintering

A multicomponent (TiZrHfNbTaMo)C ceramic has been fabricated by pressureless sintering at temperatures from 2100 ℃ to 2500 ℃,using an equimolar multicomponent carbide powder synthesized by carbothermal reduction as the starting material.Influence of sintering temperature on densification,microstructure and mechanical properties of the ceramics was investigated.The relative density increases with increasing sintering temperature,and a nearly fully dense sample is achieved by pressureless sintering at 2500 ℃.Average grain size increases from 3.7 to 15.2 μm with increasing sintering temperature from 2300 to 2500 ℃.The (TiZrHfNbTaMo)C ceramic sintered at 2400 ℃ exhibits a single phase fcc structure with homogeneous chemical composition,an average grain size of 7.0 μm and a relative density of 96.5％,while its measured hardness is 33.2 GPa at 100 mN and 23.2 GPa at 9.8 N.     

Effects of weak boundary phases (WBP) on the microstructure and mechanical properties of pressureless sintered Al2O3/h-BN machinable composites

Al₂O₃/h-BN machinable composites were fabricated by pressureless sintering at 1750 °C for 2 h in nitrogen atmosphere. The relative density of the sintered composites decreased, while the porosity increased with increasing h-BN content. By adding 20 vol.% h-BN to the composites, the porosity increased up to 16.7%. The effects of weak boundary phases (WBP), including h-BN and pores, on the microstructure, mechanical properties and machinability of the composites were investigated. The flexural strength, fracture toughness, Young's modulus and hardness of the composites decreased with increasing WBP content. When WBP content increased up to 18.8 vol.%, the composites can be machined easily by cemented carbide drills. Furthermore, the machining mechanisms of the composites were investigated using Hertzian contact tests.     

CuO-TiO_2复合助剂低温烧结氧化铝陶瓷的机理(Ⅱ)

固定CuO(0.4%)和TiO₂(4%)的添加量、改变TiO₂(0--32%)和CuO(0--3.2%)的添加量(质量分数, 下同), 研究了CuO--TiO₂复合助剂对氧化铝陶瓷烧结性能、微观结构、物相组成以及烧结激活能的影响, 以揭示复合助剂的低温烧结机理。结果表明, 在1150--1200℃TiO₂固溶入Al₂O₃生成Al₂Ti₇O₁₅相, 并生成大量正离子空位提高了扩散系数, 从而以固相反应烧结的作用机理促进了氧化铝陶瓷的致密化; TiO₂在Al₂O₃中的极限固溶度为2%--4%, 超过固溶极限的TiO₂对陶瓷烧结没有促进作用; 添加适量的CuO(0.4%)可将TiO₂在Al₂O₃中的固溶温度降低到1100℃以下, 并以液相润湿作用促进氧化铝陶瓷的致密烧结。陶瓷烧结激活能的计算结果定量地印证了上述烧结机理; 当在Al₂O₃中添加4%的TiO₂和2.4%的CuO, 可将烧结激活能降低到54.15 kJ ? mol-1。     

Effects of talc and clay addition on pressureless sintering of porous Si<Subscript>3</Subscript>N<Subscript>4</Subscript> ceramics

Porous Si₃N₄ ceramics were successfully synthesized using cheaper talc and clay as sintering additives by pressureless sintering technology and the microstructure and mechanical properties of the ceramics were also investigated. The results indicated that the ceramics consisted of elongated β-Si₃N₄ and small Si₂N₂O grains. Fibrous β-Si₃N₄ grains developed in the porous microstructure, and the grain morphology and size were affected by different sintering conditions. Adding 20% talc and clay sintered at 1700°C for 2 h, the porous Si₃N₄ ceramics were obtained with excellent properties. The final mechanical properties of the Si₃N₄ ceramics were as follows: porosity, P ₀ = 45·39%; density, ρ = 1·663·g·cm⁻³; flexural strength, σ _b (average) = 131·59 MPa; Weibull modulus, m = 16·20.     

Potential use of only Yb2O3 in producing dense Si3N4 ceramics with high thermal conductivity by gas pressure sintering

Abstract

Yb₂O₃ is an efficient sintering additive for enhancing not only thermal conductivity but also the high-temperature mechanical properties of Si₃N₄ ceramics. Here we report the fabrication of dense Si₃N₄ ceramics with high thermal conductivity by the gas pressure sintering of α-Si₃N₄ powder compacts, using only Yb₂O₃ as an additive, at 1900 °C under a nitrogen pressure of 1 MPa. The effects of Yb₂O₃ content, sample packing condition and sintering time on the densification, microstructure and thermal conductivity were investigated. Curves of the density plotted against the Yb₂O₃ content exhibited a characteristic ‘N’ shape with a local minimum at 3 mol% Yb₂O₃ and nearly complete densification below and above this concentration. The effects of the sample packing condition on the densification, microstructure and thermal conductivity strongly depended on the Yb₂O₃ content. The embedded condition led to more complete densification but also to a decrease in thermal conductivity from 119 to 94 W m^-1 K^?1 upon 1 mol% Yb₂O₃ addition. The sample packing condition had little effect on the density and thermal conductivity (102–106 W m^?1 K^?1) at 7 mol% Yb₂O₃. The thermal conductivity value was strongly related to the microstructure.     

Potential use of only Yb2O3 in producing dense Si3N4 ceramics with high thermal conductivity by gas pressure sintering

Yb₂O₃ is an efficient sintering additive for enhancing not only thermal conductivity but also the high-temperature mechanical properties of Si₃N₄ ceramics. Here we report the fabrication of dense Si₃N₄ ceramics with high thermal conductivity by the gas pressure sintering of α-Si₃N₄ powder compacts, using only Yb₂O₃ as an additive, at 1900 °C under a nitrogen pressure of 1 MPa. The effects of Yb₂O₃ content, sample packing condition and sintering time on the densification, microstructure and thermal conductivity were investigated. Curves of the density plotted against the Yb₂O₃ content exhibited a characteristic ‘N’ shape with a local minimum at 3 mol% Yb₂O₃ and nearly complete densification below and above this concentration. The effects of the sample packing condition on the densification, microstructure and thermal conductivity strongly depended on the Yb₂O₃ content. The embedded condition led to more complete densification but also to a decrease in thermal conductivity from 119 to 94 W m^-1 K⁻¹ upon 1 mol% Yb₂O₃ addition. The sample packing condition had little effect on the density and thermal conductivity (102–106 W m⁻¹ K⁻¹) at 7 mol% Yb₂O₃. The thermal conductivity value was strongly related to the microstructure.     

Growth studies and characterization of silicon nitride thin films deposited by alternating exposures to Si2Cl6 and NH3

We deposited silicon nitride films by alternating exposures to Si₂Cl₆ and NH₃ in a cold-wall reactor, and the growth rate and characteristics were studied with varying process temperature and reactant exposures. The physical and electrical properties of the films were also investigated in comparison with other silicon nitride films. The deposition reaction was self-limiting at process temperature of 515 and 557 °C, and the growth rates were 0.24 and 0.28 nm/cycle with Si₂Cl₆ exposure over 2 × 10⁸ L. These growth rates with Si₂Cl₆ are higher than that with SiH₂Cl₂, and are obtained with reactant exposures lower than those of the SiH₂Cl₂ case. At process temperature of 573 °C where the wafer temperature during Si₂Cl₆ pulse is 513 °C, the growth rate increased with Si₂Cl₆ exposure owing to thermal deposition of Si₂Cl₆. The deposited films are nonstoichiometric SiN, and were easily oxidized by air exposure to contain 7-8 at.% of oxygen in the bulk film. The deposition by using Si₂Cl₆ exhibited a higher deposition rate with lower reactant exposures as compared with the deposition by using SiH₂Cl₂, and exhibited good physical and electrical properties that were equivalent or superior to those of the film deposited by using SiH₂Cl₂.     

Microstructure and mechanical properties of silicon nitride-titanium nitride composites prepared by spark plasma sintering

Si₃N₄-TiN composites were prepared by spark plasma sintering (conventional sintering (SPS1) and in situ reaction sintering (SPS2)). Homogeneous distribution of equiaxed TiN grains in Si₃N₄ matrix results in the highest microhardness (21.7 GPa) and bending strength (621 MPa) of sample SPS1 sintered at 1550 °C. Dispersion of elongated TiN grains in Si₃N₄ matrix results in the highest fracture toughness (8.39 MPa m^1/2) of sample SPS2 sintered at 1300 °C.     

Microstructure characterization of fibrous HAp-(20 vol.% t-ZrO2)/Al2O3-(25 vol.% m-ZrO2) composites by multi-pass extrusion process

Core-shell structured HAp-(t-ZrO₂)/Al₂O₃-(m-ZrO₂) composites were fabricated using a multi extrusion process. The shell of Al₂O₃-(m-ZrO₂) phases was selected due to their excellent biocompatibility and mechanical properties and the core was designed with t-ZrO₂ dispersed in the HAp matrix. The t-ZrO₂ and m-ZrO₂ particles (< 400 nm) were homogeneously dispersed in the HAp and Al₂O₃ phases, respectively. In the HAp-(t-ZrO₂) core region, a heavy strain field contrast was observed due to the mismatch of their thermal expansion coefficients. The values of relative density, bending strength and Vickers hardness of the third pass fibrous HAp-(t-ZrO₂)/Al₂O₃-(m-ZrO₂) composites, which were sintered at 1400 °C, were about 93%, 169 MPa, and 792 Hv, respectively.     

热压与放电等离子体烧结两种工艺制备Al2O3/Cu复合材料

由机械合金化法(MA)制得纳米级Al₂O₃颗粒弥散镶嵌于微米级Cu颗粒表面的复合粉末, 利用球形化工艺改善所制得复合粉的形貌及粒度范围, 分别采用热压法(HP)和放电等离子体烧结(SPS)法制备Al₂O₃/Cu复合材料。通过测试密度、 电导率、 抗弯强度及SEM复合粉形貌和烧结体断口分析、 微区成分分析, 对比研究了Al₂O₃质量分数分别为0%、 0.5%、 1.0%、 1.5%时Al₂O₃/Cu复合材料的物理、 力学和电学性能。结果表明: 不同制备工艺下随着Al₂O₃含量增加, 材料的抗弯强度先增后降, 电导率除受杂质影响外, 还受材料缺陷的影响, 故变化规律不明显, 对于Al₂O₃含量相同的Al₂O₃/Cu复合材料, 采用SPS法制备的复合材料的致密度、 抗弯强度及电导率均高于HP法; 在弯曲应力下两种制备方法所得复合材料均发生延性断裂。      

Densification and polymorphic transition of multiphase Y2O3 nanoparticles during spark plasma sintering

Multiphase (MP) monoclinic and cubic Y₂O₃ nanoparticles, 40 nm in diameter, were densified by spark plasma sintering for 5-15 min and100 MPa at 1000 °C, 1100 °C, and 1500 °C. Densification started with pressure increase at room temperature. Densification stagnated during heating compared to the high shrinkage rate in cubic single-phase reference nanopowder. The limited densification of the MP nanopowder originated from the vermicular structure (skeleton) formed during the heating. Interface controlled monoclinic to cubic polymorphic transformation above 980 °C led to the formation of large spherical cubic grains within the vermicular matrix. This resulted in the loss of the nanocrystalline character and low final density.     

Effect of Ni-modified α-Al2O3 prepared by sol–gel and solvothermal methods on the characteristics and catalytic properties of Pd/α-Al2O3 catalysts

In the present study, Ni-modified α-Al₂O₃ with Ni/Al ratios of 0.3 and 0.5 were prepared by sol–gel and solvothermal method and then were impregnated with 0.3 wt.% Pd. Due to different crystallization mechanism of the two preparation methods used, addition of nickel during preparation of α-Al₂O₃ resulted in various species such as NiAl₂O₄, mixed phases between NiAl₂O₄ and α-Al₂O₃, and mixed phases between NiAl₂O₄ and NiO. As revealed by NH₃-temperature programmed desorption, formation of NiAl₂O₄ drastically reduced acidity of alumina, hence lower amounts of coke deposited during acetylene hydrogenation was found for the Ni-modified α-Al₂O₃ supported catalysts. For any given method, ethylene selectivity was improved in the order of Pd/Ni–Al₂O₃-0.5 > Pd/Ni–Al₂O₃-0.3 > Pd/Ni–Al₂O₃-0  Pd/α–Al₂O₃-commercial. When comparing the samples prepared by different techniques, the sol–gel-made samples showed better performances than the solvothermal-derived ones.     

热压复合制备Ti-Al3Ti层状复合材料组织结构研究

采用热压复合工艺在不同工艺参数下制备了Ti-Al3Ti层状复合材料,利用SEM和EDS对材料的组织结构进行了观察,研究了热压复合工艺参数对Ti/Al扩散反应及反应层微观组织结构的影响规律.结果表明,在不同工艺参数下,Ti/Al叠层热压复合反应的初生相均为Al3Ti.600℃时Ti/Al叠层只发生少许反应,在界面处生成一薄层Al3Ti,650和700℃时,Al层完全反应,各层界面处结合状态良好,层间结合紧密.650℃时Al3Ti为唯一生成相,但700℃时,由于反应动力学的影响Al3Ti/Ti层之间有TiAl层生成,Ti-Al系金属间化合物的生成顺序为Al3Ti→TiAl.反应过程中液相的存在能够使Ti、Al持续紧密接触,加快反应速度,促进反应顺利进行.     

Microstructure and characterization of Ni-Co/Al2O3 composite coatings by pulse reversal electrodeposit

Ni-Co/Al₂O₃ composite coatings were obtained by pulse reversal electrodeposit (PRC) and direct current electrodeposit (DC). The microstructure of the coatings was characterized by means of SEM, XRD and TEM. Hardness, wear resistance and macro residual stress of coatings were also investigated. The results showed that the microstructure and performance of the coatings were significantly affected by the electrodeposit methods and the Al₂O₃ particles content. The PRC composite coatings exhibited compact surface, high hardness and excellent wear resistance. The macro residual stress of PRC composite coatings was lower than that of DC ones. With the increasing of Al₂O₃ particles content, the hardness and wear resistance of the composite coatings increased.     

Effect of Nb2O5 on the microstructure and mechanical properties of TiAl based composites produced by hot pressing

In situ composites of TiAl reinforced with Al₂O₃ particles are successfully synthesized from an elemental powder mixture of Ti, Al and Nb₂O₅ by the hot-press-assisted reaction synthesis (HPRS) method. The as-prepared composites are mainly composed of TiAl, Al₂O₃, NbAl₃, as well as small amounts of the Ti₃Al phase. The in situ formed fine Al₂O₃ particles tend to disperse on the matrix grain boundaries of TiAl resulting in an excellent combination of matrix grain refinement and uniform Al₂O₃ distribution in the composites. The Rockwell hardness and densities of TiAl based composites increase gradually with increasing Nb₂O₅ content, and the flexural strength and fracture toughness of the composites have the maximum values of 634 MPa and 9.78 MPa m^1/2, respectively, when the Nb₂O₅ content reaches 6.62 wt.%. The strengthening mechanism was also discussed.