Effects of silver powder particle size on the microstructure and properties of Ag-Yb<Subscript>2</Subscript>O<Subscript>3</Subscript> electrical contact materials prepared by spark plasma sintering

Ag-Yb₂O₃ electrical contact materials were fabricated by spark plasma sintering (SPS). The effects of silver powder particle size on the microstructure and properties of the samples were investigated. The surface morphologies of the sintered samples were examined by optical microscope (OM), and the fracture morphologies were observed by scanning electron microscopy (SEM). The physical and mechanical properties such as density, electrical resistivity, microhardness, and tensile strength were also tested. The results show that the silver powder particle size has evident effects on the sintered materials. Comparing with coarse silver powder (5 μm), homogeneous and fine microstructure was obtained by fine silver powder (≤0.5 μm). At the same time, the electrical conductivity, microhardness, and tensile strength of the sintered samples with fine silver powder were higher than those of the samples with coarse silver powder. However, silver powder particle size has little influence on the relative densities, which of all samples (both by fine and coarse silver powders) is more than 95%. The fracture characteristics are ductile.     

Thermoelectric properties of n-type Bi<Subscript>2</Subscript>Te<Subscript>2.7</Subscript>Se<Subscript>0.3</Subscript> compounds prepared by spark plasma sintering

In this study, the thermoelectric properties of 0.1 wt.% Cdl₂-doped n-type Bi₂Te_2.7Sb_0.3 compounds, fabrieated by SPS in a temperature range of 250°C to 350°C, were characterized. The density of the compounds was increased to approximately 100% of the theoretical density by carrying out consolidation at 350°C. The Seebeck coefficient, thermal conductivity, and electrical resistivity were dependent on a hydrogen reduction process and the sintering temperature. The Seebeck coefficient and the electrical resistivity increased with the reduction process. Also, electrical resistivity decreased and thermal conductivity increased with sintering temperature. The results suggest that carrier density and mobility vary according to the reduction process and sintering temperature. The highest figure of merit, 1.93×10⁻³ K⁻¹, was obtained for the compound consolidated at 350°C for 2 min.     

放电等离子烧结的钛铝合金组织及氧化性能研究

研究了TiH2-45Al-0.2Si-5Nb未球磨和球磨两种粉末的放电等离子烧结组织特征以及经1000℃、100h高温氧化后的氧化性能.结果表明,未经球磨粉末的烧结组织由层片状TiAl和Ti3Al相组成,而经球磨粉末的烧结组织由细小的颗粒状TiAJ和Ti3Al相组成.球磨粉末的烧结组织氧化速度低于未球磨粉末的烧结组织,形成了连续的Al2O3和TiO2混合氧化物层,具有良好的高温抗氧化性.     

Thermodynamic optimization of the binary YbCl<Subscript>3</Subscript>-AECl<Subscript>2</Subscript> (AE=Mg,Ca, Sr,Ba) systems

By using the CALPHAD technique, an optimization of the binary YbCl₃-AECl₂ (AE=Mg, Ca, Sr, Ba) systems was carried out. From measured phase equilibrium data and experimental integral properties, the YbCl₃-AECl₂ phase diagrams were optimized and calculated. A set of thermodynamic functions was optimized based on an interactive computer-assisted analysis. The calculated phase diagrams and thermodynamic data are self-consistent.     

Microstructure and PTCR characteristics of porous BaTiO3-based ceramics prepared by adding activated carbon

As a pore precursor, activated carbon of different contents (0 to 30 vol%) was added to (Ba,Sr)TiO₃ powder. Porous (Ba,Sr)TiO₃ ceramics were prepared by pressureless sintering at 1350 °C for 1 h in air. The effects of the activated carbon content on the microstructure and positive temperature coefficient of resistivity (PTCR) characteristics in porous (Ba,Sr)TiO₃ ceramics were investigated. The porosity of the porous (Ba,Sr)TiO₃ ceramics increased from 6.5% to 33.5%. The PTCR jump of the porous (Ba,Sr)TiO₃ ceramics prepared with activated carbon was > 10⁵ and increased slightly with increasing activated carbon concentration. These results correspond to the Heywang model of the PTCR effect in (Ba,Sr)TiO₃ ceramics, suggesting that activated carbon is an effective additive for preparing porous BaTiO₃-based ceramics. The newly prepared (Ba,Sr)TiO₃ ceramics can be used as a gas sensor.     

Microstructure and microwave dielectric properties of ZnO-B2O3 added （Ca0.254Li0.19Sm0.14）TiO3 ceramics

The effects of ZnO-B2O3 (ZB2) on the sintering behavior and microwave dielectric properties of (Ca0.254Li0.19Sm0.14)TiO3 ceramics were investigated.The densities of the specimens reached the maximum value by adding 3 wt.% ZB2 and then decreased.The sintering temperature of the specimens was lowered from 1300 to 1100°C without degradation of the microwave dielectric properties.The (Ca0.254Li0.19Sm0.14)TiO3 + 3 wt.% ZB2 sintered at 1100°C for 3 h showed good microwave dielectric properties,εr = 108.2,Qf = 6545 GHz,and τf = 6.5 ppm/°C,respectively,indicating that ZB2 was an effective sintering aid to improve the densification and microwave dielectric properties of (Ca0.254Li0.19Sm0.14)TiO3 ceramics.     

Enhanced properties of nanostructured TiO<Subscript>2</Subscript>-graphene composites by rapid sintering

Despite of many attractive properties of TiO₂, the drawback of TiO₂ ceramic is low fracture toughness for widely industrial application. The method to improve the fracture toughness and hardness has been reported by addition of reinforcing phase to fabricate a nanostructured composite. In this regard, graphene has been evaluated as an ideal second phase in ceramics. Nearly full density of nanostructured TiO₂-graphene composite was achieved within one min using pulsed current activated sintering. The effect of graphene on microstructure, fracture toughness and hardness of TiO₂-graphene composite was evaluated using Vickers hardness tester and field emission scanning electron microscopy. The grain size of TiO₂ in the TiO₂-x vol% (x = 0, 1, 3, and 5) graphene composite was greatly reduced with increase in addition of graphene. Both hardness and fracture toughness of TiO₂-graphene composites simultaneously increased in the addition of graphene.     

Electrophoretic impregnation of porous anodizing layer by synthesized TiO<Subscript>2</Subscript> nanoparticles

This paper deals with the elaboration of a stable suspension of TiO₂ nanoparticles and their incorporation by electrophoretic deposition into pores of an anodized 5754 aluminum alloy. The as-synthesized TiO₂ nanopowder was characterized by the X-ray diffraction, scanning and transmission electron microscopy, energy dispersive X-ray spectroscopy and IR spectroscopy. During this work, both the transmission electron microscopy and particle analysis showed that the resulting particles had a narrow size distribution with a crystallite size of about 15 nm. The zeta potential and stability of TiO₂ nanoparticles dispersed with poly(acrylic acid) in an aqueous solution were also measured. A porous anodic film was synthesized in the phosphoric acid-base electrolyte and then filled by 15 nm TiO₂ particles via electrophoresis. In addition, the effect of poly(acrylic acid) and pH on the suspension stability has been investigated. It was also demonstrated that by adding glycine in buffered suspension gelating phenomenon can be avoided that inhibits the insertion of nanoparticles inside the pores of an anodic film. It was also noted that an applied electric field greatly influences the electrophoretic deposition process. The field emission gun-scanning electron microscopy observations showed that larger (125 nm in diameter) and linear (6 μm in length) pores are successfully filled in 5 min.     

Production of TiO<Subscript>2</Subscript> from CaTiO<Subscript>3</Subscript> by alkaline roasting method

CaTiO3 was decomposed by alkaline roasting method for the production of TiO2.The process included alkaline roasting, water leaching and acid leaching steps.In the alkaline roasting step, the factors such as roasting temperature and NaOH/CaTiO3 molar ratio were investigated and 99.5% TiO2 could be extracted from CaTiO3.In addition, it is believed that only ion-exchange between Ca2+ and Na+ takes place, while the structure of TiO 32-in CaTiO3 was not destroyed during the roasting process.In the acid leaching ...     

放电等离子烧结法制备Cu/金刚石复合材料的性能与显微组织

由于具备较高的热导率,铜/金刚石复合材料已成为应用于电子封装领域的新一代热管理材料。采用放电等离子烧结工艺（SPS）成功制备含不同金刚石体积分数的Cu/金刚石复合材料,研究复合材料的相对密度、微观结构均匀性和热导率（TC）随金刚石体积分数（50%、60%和70%）和烧结温度的变化规律。结果表明：随着金刚石体积分数的降低,复合材料的相对密度、微观结构均匀性和热导率均升高;随着烧结温度的提高,复合材料的相对密度和热导率不断提高。复合材料的热导率受到金刚石体积分数、微观结构均匀性和复合材料相对密度的综合影响。     

Influences of NiCO<Subscript>3</Subscript> content on the microstructure and magnetic properties of Ni<Subscript>0.5</Subscript>Zn<Subscript>0.5</Subscript>Fe<Subscript>2</Subscript>O<Subscript>4</Subscript> powders prepared by SHS

Stoichiometric Ni_0.5Zn_0.5Fe₂O₄ powders were produced by self-propagating high temperature synthesis (SHS). The effects of NiCO₃ content in the raw materials on the microstructure and magnetic properties of Ni-Zn ferrite powders were systematically studied. The Ni_0.5Zn_0.5Fe₂O₄ powders were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The magnetic properties of the powders were evaluated by vibrating sample magnetometry (VSM). The results show that the introduction of NiCO₃ into reactants improves the conversion percentage and refines the Ni_0.5Zn_0.5Fe₂O₄ particles. The increase of NiCO₃ content enhances the magnetic properties of Ni_0.5Zn_0.5Fe₂O₄. Particularly, the saturation magnetization reaches the maximum when the NiCO₃ content is 3 at.%.     

Photoresponse and donor concentration of plasma-sprayed TiO<Subscript>2</Subscript> and TiO<Subscript>2</Subscript>-ZnO electrodes

The photoelectrochemical characteristics of plasma-sprayed porous TiO₂, TiO₂-5%ZnO, and TiO₂-10%ZnO electrodes in 0.1 N NaOH solution were studied through a three-electrode cell system. The microstructure, morphology, and composition of the electrodes were analyzed using an electron probe surface roughness analyzer (ERA-8800FE), scanning electron microscopy, and x-ray diffraction. The results indicate that the sprayed electrodes have a porous microstructure, which is affected by the plasma spray parameters and composition of the powders. The TiO₂-ZnO electrodes consist of anatase TiO₂, rutile TiO₂, and Zn₂Ti₃O₈ phase. The photoresponse characteristics of the plasma-sprayed electrodes are comparable to those of single-crystal TiO₂, but the breakdown voltage is close to 0.5 V (versus that of a saturated calomel electrode). The short-circuit photocurrent density (J _SC) increases with a decrease of donor concentration, which was calculated according to the Gartner-Butler model. For the lowest donor concentration of a TiO₂-5%ZnO electrode sprayed under an arc current of 600 A, the short-circuit J _SC is approximately 0.4 mA/cm² higher than that of the TiO₂ electrodes under 30 mW/cm² xenon light irradiation. The J _SC increases linearly with light intensity. The original version of this paper was published as part of the DVS Proceedings: “Thermal Spray Solutions: Advances in Technology and Application,” International Thermal Spray Conference, Osaka, Japan, 10–12 May 2004, CD-Rom, DVS-Verlag GmbH, Düsseldorf, Germany.     

Fabrication and thermophysical properties of porous TiB2 ceramics fabricated by reactive spark plasma sintering

Titanium oxide (TiO₂) and boron carbide (B₄C) were added to TiB₂ raw powders to prepare porous TiB₂ ceramics by reactive spark plasma sintering, and the gas escape (such as CO and B₂O₃) resulted in higher porosity. X-ray Diffraction results indicated that the reduction reaction was completed after the reactive spark plasma sintering process. The porosity could be controlled by changing the ratio of synthesized TiB₂ to raw TiB₂ powders. The porosity of porous TiB₂ ceramics with 20 wt.% and 40 wt.% synthsized TiB₂ ceramics are 18.5% and 22.2%, respectively. The thermal diffusivity of the porous TiB₂ ceramics decreased with the porosity due to the low diffusivity behavior of gas and vacuum in pores, and the thermal conductivity for porous TiB₂ ceramics decreased as the temperature increased throughout the measured temperature range. The results here pointed to a potential method for fabricating porous TiB₂ ceramics with controllable thermophysical properties.     

MoSi_2/ZrB_2基超高温陶瓷SPS制备研究

ZrB2具有良好的抗氧化、抗热震和抗烧蚀性能。采用放电等离子体烧结(SPS)工艺,添加体积分数为10%~20%的MoSi2烧结助剂,选取不同的烧结参数,制备出超高温陶瓷成品。经测试,ZrB2-15%MoSi2(体积分数)体系的陶瓷致密度可达99.88%,维氏硬度可达1 612,通过SEM分析,该陶瓷具有典型的核(ZrB2)-壳(MoSi2、MoB)结构,可以作为高超声速飞行器热防护用陶瓷材料的候选方案。     

The microstructure and property of W/Ti multilayer composites prepared by spark plasma sintering

The W/Ti multilayer composites are prepared by spark plasma sintering. The original constituents are cold-rolled W foils with elongated grains and annealed Ti-1.5Al foils with fine equiaxed grains. After diffusion bonding, both elongated grains and fine equiaxed grains can be seen in the W layers. The diffusion rate of W in Ti is much faster than that of Ti in W due to different methods of diffusion. The detailed microstructure of W/Ti laminates can be divided into four types at different regions including pure W, mixture of α-W and β-Ti with light and dark lamellas, pure β-Ti and Ti-rich Widmanstäten α-β structure with alternative light and dark lamellas. Compared with the microhardness of original materials, the microhardness of W layers slightly decreases and the microhardness of Ti layers increases dramatically.     

Sintering characteristics and microstructure of WC-Co-VC/Cr<Subscript>3</Subscript>C<Subscript>2</Subscript> ultrafine cemented carbides

The sintering characteristics, microstructure, and mechanical properties of ultrafine WC-12%Co-0.2%VC/0.5%Cr₃C₂ cemented carbides were investigated. Dilatometric and differential thermal analyses (DTA) indicate that the compacts start to shrink at 600°C, the shrinkage rate peak is at 1190°C, and the liquid formation temperature is lower than the W-C-Co eutectic temperature (1330°C). Microstructure analysis results show that the cemented carbides with fine and homogeneous microstructure were obtained when sintered at 1430°C. Continuous and discontinuous grain growth was suppressed due to the synergistic action of VC/Cr₃C₂. The transverse rupture strength (TRS) of the samples reaches 4286 MPa, with the hardness HRA 92.1. The fine and homogeneous microstructure, alloy strengthening, and different phase constitutions of binder in the cemented carbides result in high hardness and TRS. Continuous and discontinuous grain growth was observed in the cemented carbide sintered at 1450°C, which results in significant decreases of hardness and TRS. It indicates that VC/Cr₃C₂ additions in the cemented carbides can only suppress the grain growth at a certain temperature.     

Hetero-Orientation Epitaxial Growth of TiO<Subscript>2</Subscript> Splats on Polycrystalline TiO<Subscript>2</Subscript> Substrate

In the present study, the effect of titania (TiO₂) substrate grain size and orientation on the epitaxial growth of TiO₂ splat was investigated. Interestingly, the splat presented comparable grain size with that of substrate, indicating the hereditary feature of grain size. In addition, hetero- and homo-orientation epitaxial growth was observed at deposition temperatures below 400 °C and above 500 °C, respectively. The preferential growth of high-energy (001) face was also observed at low deposition temperatures (≤?400 °C), which was found to result from dynamic nonequilibrium effect during the thermal spray deposition. Moreover, thermal spray deposition paves the way for a new approach to prepare high-energy (001) facets of TiO₂ crystals.