The effect of the particle size and the morphology of alumina powders on the processing of green bodies by electrophoretic deposition

The electrophoretic deposition (EPD) method was investigated for the preparation of compacts from nanosized alumina powders. Ethanol suspensions of commercially available nanosized transition alumina and α-alumina powders were used. In addition, submicron powders were also considered for comparison. Besides the influence of operational pH change, the influence of the addition of three different surfactants – two branched polyethylene imines with molecular masses 1800 and 10,000, and citric acid—on the zeta-potential of the particles in ethanol and afterwards on the EPD was investigated.The packing density of the green parts prepared by the EPD of nanosized α-alumina powders at 30 V/cm was low (the green density was 27 vol. % of the theoretical density—TD), and a bimodal pore size distribution was observed. In contrast, the packing density in deposits made with the transition alumina nanopowder was much higher (the green density was 42 vol. % of the TD) and a monomodal distribution of pores was observed. In general, for all the deposits made from nanosized alumina powders, the particle-packing density was found to be lower than for those made from the submicron alumina powders (a green density equal to 59 vol. % of the TD).     

离心法制备氧化铝-氧化锆泡沫陶瓷过滤器

通过在发泡聚苯乙烯模板内的离心成型方法来制备Al2O3-ZrO2泡沫陶瓷过滤器,测量了浆料在不同pH值下的Zeta电位,分析了浆料的固相含量对离心时间的影响。观察了不同离心加速度和浆料固相含量下产物生坯密度的变化,测量了最终产物的烧结密度、收缩率、孔隙率和压缩强度,并通过扫描电镜观察了烧结试样的显微结构。试验结果表明,在pH=10时,浆料具有较好的分散性。在较低的固相含量和较高的离心加速度下,浆料的分离现象明显,生坯中出现密度梯度。但在高的固相含量(50%,体积分数)下,分离现象被抑制,孔筋具有较高的生坯密度(61.5%)和烧结密度(91.1%),最终产物的孔隙率在75.3%～83.1%,压缩强度为2.07～3.82 MPa。     

Processing of concentrated aqueous Si3N4 slips stabilized with tetramethylammonium hydroxide

The optimization of the dispersion of 43 vol% aqueous Si₃N₄/sintering aid slurries with tetramethylammonium hydroxide to achieve high packing slipcast compacts and subsequent high sintered densities was investigated. Slips containing 43 vol% solids were obtained by dispersing most of the solid (35 vol%) at high pH values (9.7 to 12.3) with different amounts of tetramethylammonium hydroxide and with subsequent addition of the remaining solid up to 43 vol%. The influence of pH on the viscosity and on the amount of tetramethylammonium ions adsorbed, and in solution, of 43 vol% slips was studied. The minimum viscosity was observed in the 9.7 to 10.2 pH range. This pH range seemed to be an intermediate situation between high ionic strength (pH higher than 10.2) and low electrostatic repulsion (pH lower than 9.7). The maximum sintered density at 1650 °C, 97.5% of theoretical density (TD), was obtained from bars of green density, 55.6% TD, prepared from slips with the minimum viscosity (44 mPa/s).     

粉末粒径对高速压制Ti-6Al-4V合金性能的影响(英文)

采用高速压制(HVC)技术成形平均粒径分别为103、66和44μm的3种Ti6Al4V合金粉末,然后将合金粉末进行真空烧结,考察粉末粒径对成形效果和烧结体性能的影响。结果表明,细粉末比粗粉末更难压制,获得的压坯密度也更低。但是细粉末压坯的烧结密度明显高于粗粉末压坯的烧结密度。与粒径为103和66μm的粉末相比,粒径为44μm的粉末的压坯密度最低,在冲击能量为913J时其相对密度为85.1%。然而在1300°C烧结2.5h后,粒径为44μm的粉末压坯的烧结密度最高,其相对密度达到98.2%。而且,粒径为44μm的烧结试样具有最高的硬度和压缩强度,分别为HV354和1265MPa。     

水和乙醇悬浮液等离子喷涂SOFC电解质层的工艺及涂层特点比较

选用水和乙醇两种溶剂配置悬浮液,研究了分散剂PAA含量对(Ce0.80Gd0.20)O1.9(GDC)悬浮液粘度的影响和pH值对GDC颗粒Zeta电位的影响。采用固相质量分数为10%的GDC悬浮液等离子喷涂制备固体氧化物燃料电池电解质层。利用X射线衍射、扫描电镜和电子探针分别分析了喷涂前后GDC的相结构、电解质层微观组织及化学成分。研究结果表明:水悬浮液优化参数PAA质量分数为2.5%,pH=10;乙醇悬浮液优化参数为:PAA质量分数为2.0%,pH=10。在相同喷涂工艺下,乙醇悬浮液得到的电解质层结构更精细,由图像分析法得水和乙醇悬浮液得到电解质层的孔隙率分别为5.64%和1.25%。GDC喷涂前后没有相结构的变化,但是晶粒有长大,并且在喷涂过程中氧化铈发生烧损,水和乙醇悬浮液制备的电解质层中氧化铈分别烧损了15.8%和16.8%。     

不同粒径Ti粉的高速压制行为和烧结性能

以平均粒径为150,75,48和38μm的4种Ti粉为原料(依次定义为A,B,C和D粉末),采用高速压制技术进行成形,考察粉末粒径对压坯密度、最大压制力和脱模力的影响,进一步研究粉末的高速压制特性和压坯的烧结性能.结果表明,高速压制的压坯密度与粉末粒径和松装密度有关.冲击能量较小时,压坯密度主要取决于松装密度,而冲击能量较高时,则主要取决于粉末粒径.在冲击能量≤761 J下成形时,具有最大松装密度的B粉末所获得的压坯密度最高;进一步增大冲击能量,平均粒径最大的A粉末所获得的压坯密度最高.粉末粒径对压坯密度和最大压制力具有相似的影响,并且4种粉末的最大压制力和压坯密度之间的关系均符合黄培云压制方程;但粉末粒径对脱模力无明显影响.试样的烧结密度随粒径的细化而增加,同时伴随着不同程度的晶粒长大.4种压坯经1250℃真空烧结后,最终均获得了近全致密的试样.     

W-Cu电子封装材料的气密性

分析了传统熔渗法生产的ＷＣｕ材料气密性差的工艺因素,采用加入一定数量的诱导铜的工艺方法进行压型,通过调整成型压力,使生坯中的Ｗ含量达到电子封装ＷＣｕ１５含钨量的标准,其余的Ｃｕ在熔渗时渗入。实验表明,加入１％～２．５％的诱导铜的生坯在１３５０℃熔渗１．５ｈ,其气密性可以达到满意的效果。     

Densification behavior of nanocrystalline W–Ni–Fe composite powders prepared by sol-spray drying and hydrogen reduction process

This paper studied the densification behavior of nanocrystalline composite powders of 93W–4.9Ni–2.1Fe (wt.%) and 93W–4.9Ni–2.1Fe–0.03Y synthesized by sol-spray drying and hydrogen reduction process. The X-ray diffraction (XRD) analysis showed that γ-(Ni, Fe) phase was formed in the final obtained powders. Powders morphology characterized by scanning electron microscope (SEM) showed that the 93W–4.9Ni–2.1Fe nanocrystalline composite powders exhibited larger agglomeration and grain size compared with the 93W–4.9Ni–2.1Fe–0.03Y nanocrystalline composite powders. Both kinds of green compacts can obtain full density if sintered at 1410 °C for 1 h. When sintering temperature was above 1410 °C, the sintering density for both compacts decreased rapidly. In addition, the sintering density, densification rate and grain coarsening rate of 93W–4.9Ni–2.1Fe compacts were higher than those of 93W–4.9Ni–2.1Fe–0.03Y. The effect of trace yttrium on the densification behavior of nanocrystalline composite powders was also discussed.     

A novel technique for making open-cell AI203-ZrO2 ceramic foam with plant seed template

The aim of the present research is to provide a technique for preparing open-cell AI203-ZrO2 ceramic foams with uniform cell size. This technique used plant seeds to array templates and centrifugal slip casting to obtain cell struts with high packing density. Aqueous Al2O3-ZrO2 slurries with up to 50 vol.% solid contents were prepared and the rheological characteristic of the slurries was investigated. Consolidation was performed at an acceleration of 2,860 g for 60 rain. The effect of the characteristic of plant seeds on the drying behavior of Al2O3- ZrO2 green compact was analyzed. The effects of the solid contents of slurries on segregation phenomena ofAl2O3 and ZrO2 particles and green compact uniformity were investigated. The compressive stress-strain curve and deformation behavior of Al2O3-ZrO2 ceramic foams prepared using plant seed template were analyzed. The results showed segregation phenomenon is negligible for highly stable slurry with 50 vol.% solid loading. The prepared cell struts of Al2O3-ZrO2 foams have high green density (61.9% TD), sintered density (99.1% TD) and homogeneous microstructure. When sintered at 1,550 ℃ for 2 h, the cell size of Al2O3-ZrO2 foam is approximately uniform and the diameter is about 1.1 mm. The porosity and compressive strength of sintered products is 66.2% and 5.86 MPa, respectively.     

Suspension Plasma Spraying of YPSZ Coatings: Suspension Atomization and Injection

Among processes evaluated to produce some parts of or the whole solid-oxide fuel cell, Suspension Plasma Spraying (SPS) is of prime interest. Aqueous suspensions of yttria partially stabilized zirconia atomized into a spray by an internal-mixing co-axial twin-fluid atomizer were injected into a DC plasma jet. The dispersion and stability of the suspensions were enhanced by adjusting the amount of dispersant (ammonium salt of polyacrylic acid, PAA). A polyvinyl alcohol (PVA) was further added to the suspension to tailor its viscosity. The PVA also improved the dispersion and stability of the suspensions. The atomization of optimized formulations is described implementing Weber and Ohnesorge dimensionless numbers as well as gas-to-liquid mass ratio (ALR) value. Drop size distributions changed from monomodal distributions at low We to multimodal distributions when We number increases. The viscosity of the suspensions has a clear influence on the drop size distribution and suspension spray pattern. The secondary fragmentation of the drops due to the plasma jet was evidenced and the final size of the sheared drops was shown to depend on the characteristics of the suspension. Rather dense zirconia coatings have been prepared, which is a promising way to produce electrolyte.     

GELCASTING OF NANO-SIZE Y-TZP

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Improvement of physical properties of Cu-infiltrated W compacts via electroless nickel plating of primary tungsten powder

In the present research, tungsten particles were coated using nickel/nickel-phosphorus electroless plating technique. The coated tungsten powders were pressed under constant pressure to achieve compact material of cylindrical shape with same porosity. Then, attained compacts were infiltrated/penetrated by liquid copper under the hydrogen atmosphere in order to obtain W-15 wt.% Cu composites. The coated/uncoated powders as well as its infiltrated compacts were characterized by optical microscopy (OM) as well as scanning electron microscopy (SEM), EDS and XRD methods. The microstructure, relative density and specific resistivity of composites were compared. The microstructural observations revealed that the infiltration behavior can be improved in the compacts prepared by both nickel and nickel-phosphorus coated tungsten powders, in comparison with uncoated ones. In addition, it was found that relative density may be raised from < 85% to > 95% by nickel electroless plating, that leads to decrease specific resistivity from 6 to 4 µΩ cm. Enhancement of electrical conductivity of infiltrated W-15 wt.% Cu compacts prepared by electroless nickel coated tungsten powders was related to its higher density.     

α-Al2O3纳米粒子抛光浆料稳定性及其对蓝宝石抛光性能的影响

目的制备分散稳定性良好的α-Al2O3纳米粒子抛光浆料,提高对蓝宝石的化学机械抛光性能。方法将α-Al2O3分散在硅溶胶、氧化铈溶胶、水等不同分散介质中,于不同pH值、不同硅溶胶浓度及硅溶胶粒径等条件下制备出α-Al2O3纳米粒子的抛光浆料,考察抛光浆料的稳定性及抛光浆料对蓝宝石化学机械抛光性能的影响。采用Zeta电位仪测量抛光浆料中α-Al2O3的电势,进而对其分散稳定性进行分析。采用原子力显微镜(AFM)和分析天平分别对蓝宝石表面粗糙度(Ra)和材料去除速率(MRR)进行评价。结果分散介质为硅溶胶时,抛光浆料的稳定性及对蓝宝石的抛光性能较好。当抛光浆料pH值为10时,其分散稳定性较好,且化学腐蚀与机械研磨达到动态平衡,抛光浆料对蓝宝石的抛光性能较好。随着α-Al2O3浓度的增大,浆料的抛光性能呈现先增加后降低的趋势,当α-Al2O3的质量分数为10.0%时,抛光浆料对蓝宝石的抛光性能较好。当硅溶胶的质量分数为0.02%时,抛光浆料的分散稳定性及对蓝宝石的抛光性能较好。随着硅溶胶粒径的增加,抛光浆料的稳定性及对蓝宝石的抛光性能逐渐变差,所以选择最小粒径5 nm的硅溶胶作分散介质。即在10.0%的α-Al2O3、0.02%粒径为5 nm的硅溶胶、pH值为10等条件下的抛光浆料稳定性较好,该浆料对蓝宝石抛光的材料去除速率为15.16 nm/min,抛光后的表面粗糙度为0.272 nm,满足蓝宝石后续外延工艺要求。结论适宜浓度的硅溶胶能明显改善α-Al2O3抛光浆料的分散稳定性,分散效果明显优于水或氧化铈溶胶作分散介质,且对蓝宝石的抛光性能得到显著提高。     

The effects of nanoparticle addition on SiC and AlN powder–polymer mixtures: Packing and flow behavior

The development of methods to increase sintered density and improve dimensional tolerances is a crucial issue in powder metallurgy and ceramic processing. Increasing the packing density of starting powders is one effective route to achieve high sintered density and dimensional precision. The current paper presents an in-depth study on the effect of nanoparticle addition on the powder content of SiC and AlN powder–polymer mixtures. In particular, bimodal mixtures of nanoscale and sub-micrometer particles were found to have significantly increased powder volume fraction (solids loading) in the mixtures for injection molding. This observation to increasing packing density by using nanoparticles is surprising and novel since nanoparticles are known to inherently exhibit poor packing behavior. Additionally, for a given volume fraction of powder, the bimodal μ-n suspensions had a lower viscosity at any shear rate compared to the monomodal μ-suspensions. The ability to lower the suspension viscosity by adding nanoparticles to micron-sized particles has important implications for processing of particulate suspensions by powder injection molding (PIM), extrusion, slip casting and tape casting. Samples made from bimodal powders exhibited slower polymer removal during debinding and higher densification with lower shrinkage on sintering compared to the corresponding samples made from monomodal powder mixtures.     

Dispersing WC–Co powders in aqueous media with polyethylenimine

The use of aqueous WC–Co suspensions for powder metallurgical processing of hard metal powders is expected to have environmental as well as economic advantages over the currently used systems with organic solvents. Without a suitable dispersing agent, however, poor mixing and aggregation of the different particulate constituents will occur in water and requirements on homogeneity and colloidal stability of the hard metal slurry cannot be met. We evaluated the effect of polyethylenimine (PEI) on the flow properties of concentrated aqueous WC–Co suspensions. The investigated parameters were polymer concentration, polymer molecular weight, pH, powder solubility, and ageing time. We found that simple mixing of powder, water, and PEI (at concentrations above 0.3 wt%) is sufficient to obtain well-dispersed, low-viscous suspensions without the need for pH adjustment or further additives. Almost identical flow behavior was observed over the investigated pH range 7–10.8. Effects of ageing and powder solubility on the colloidal stability were negligible over the investigated time span of 36 h. Adsorption of PEI with M_w=10,000 results in thinner adsorbed layers as compared to PEI with M_w=25,000, which is reflected by a lower suspension viscosity at high solids loading (40 vol%).     

The oxidation behavior of gas-atomized Al and Al alloy powder green compacts during heating before hot extrusion and the suggested heating process

The oxidation behavior of gas-atomized Al and Al alloy powder green compacts during heating prior to hot extrusion compaction was studied at laboratory and industrial scales by TGA, DSC, DTA, EDX, TEM and XRD methods. The effect of the heating of green compacts on the mechanical properties of the powder-extruded samples was assessed. Significant oxidation of Al and Al alloy powder green compacts takes place in the solid state during heating in air. The onset and intensity of oxidation were affected by the Mg content, the surface area of the powder and the volume of the powder green compacts. An exothermic heat associated with the oxidation of Al and Al alloy powders resulted in intense overheating of bulky powder green compacts during heating in air. The samples extruded from the powder green compacts heated in air exhibited reduced strength. The loss in strength was especially pronounced in the case of Mg-containing Al alloy powders. Mg diffuses from a powder metallic core toward the native Al₂O₃ surface layer present on as-atomized Al alloy powders; it reacts with oxygen present in air and in the Al₂O₃ surface layer where the MgO phase forms, eventually resulting in the depletion of Mg from the powder core. Materials extruded from Al powders depleted of Mg do not exhibit effective Al-Mg solid solution strengthening or strengthening by Mg-containing precipitates. Economically viable approaches to avoiding the detrimental effects of powder oxidation during the heating of green compacts prior to hot working consolidation are discussed.     

The sintering behavior of quasi-spherical tungsten nanopowders

In this work, the sintering behavior of quasi-spherical tungsten nanoparticles was investigated by analysis the sintered compacts obtained at different sintering temperatures and dwell time, and the influence of microstructures on the density and Vickers microhardness of sintered products was also studied. Experimental results show that particle shape and size distribution are critical to the sintering activity and mechanical properties of obtained compacts. 91.3% of theoretical density (TD) of the compact could be obtained at low sintering temperature of 1500 °C, and the highest hardness of 606 VHN could be achieved when sintered at 1100 °C due to formation of uniform, densely packed sintered compacts with grain size of 235.7 nm. Importantly, unusual linear correlation between grain size and relative density was observed in our experiment, and a cut-off point exists at 85.6% of TD. The kinetic analysis revealed that surface diffusion is responsible for the mass transport during the initial sintering stage.     

水和乙醇悬浮液等离子喷涂SOFC (Ce0.80Gd0.20)O1.9电解质层的比较

选用水和乙醇两种溶剂配置悬浮液,研究了分散剂含量对(Ce_0.80Gd_0.20)O_1.9 (GDC)悬浮液粘度的影响和pH值对GDC颗粒Zeta电位的影响。结果表明：水悬浮液优化参数是：PAA含量为2.5 wt.%,pH=10;乙醇悬浮液优化参数为：PAA含量2.0 wt.%,pH=10。采用GDC悬浮液等离子喷涂制备固体氧化物燃料电池电解质层。利用XRD、SEM和电子探针分别分析了喷涂前后GDC的相结构、电解质层微观组织及化学成分。研究表明：GDC喷涂前后没有相结构的变化,乙醇悬浮液得到的电解质层结构更精细,水和乙醇悬浮液得到电解质的孔隙率分别为5.64 ％和1.25 %,但是氧化铈有烧损,水和乙醇悬浮液电解质层氧化铈分别烧损了15.8 %和 16.8 %。     

彩色铝粉Fe2O3/SiO2/Al的液相沉积制备与表征

采用分步液相沉积法在片状铝粉表面包覆SiO2与Fe2O3双层薄膜制备了具有金属光泽的彩色铝粉,以彩度与光泽度为主要评价指标,探讨Fe2O3用量与pH值对铝粉包覆效果的影响,利用原子力显微镜(AFM)、扫描电子显微镜(SEM)和X射线衍射(XRD)等手段对包覆前后的铝粉进行表征.结果表明:当SiO2包覆量为20％(质量分数)时,随着Fe2O3膜厚的增加,铝粉颜色由浅黄→金黄→红黄过渡,光泽度下降;当m(Fe2O3)∶m(SiO2)∶m(Al)=0.75∶0.2∶1时,铝粉具有良好的彩度与光泽度,铝粉表面粗糙度由5.05 nm增大到20.6 nm;反应过程pH值对Fe2O3包覆效果有重要影响,pH值宜控制在3.9～4.0;表面活性剂PVP的加入有利于提高Fe2O3膜层与SiO2和Al基体的结合力,从而提高颜色稳定性.     

Dispersion of concentrated aqueous neodymia-yttria-alumina mixture with ammonium poly(acrylic acid) as dispersant

A stable aqueous slurry using ammonium polyacrylic acid polyelectrolyte as dispersant and a neodymia-yttria-alumina mixture was prepared as the starting powder. The effect of the polyelectrolyte concentration and the pH of the slurry on the stability of the suspension is studied, and the optimal pH value and the amount of dispersant needed to obtain a stable slurry were determined. Highly consistent slurries with optimal pH and dispersant concentration were prepared by ball milling. The rheological behavior of the slip with different solid loading (48-58 wt.%) has been studied by measuring the viscosity and shear stress as a function of shear rate. Slip with solid loadings of 53 wt.% shows near-Newtonian behavior but becomes non-Newtonian with typical shear-thinning behavior above this solid loading value. The density and microstructure of the cast product bears a direct relationship to the state of the slip induced by variation of the pH and the concentration of the dispersant as well as by solid loading. Transparent Nd:YAG ceramics were obtained by sintering of compacts prepared from optimized slurries at 1750 °C in vacuum.