Formation of nanostructured YSZ/Ni anode with pore channels by plasma spraying

YSZ/Ni is the conventionally most used material for making the anode of a solid oxide fuel cell. Agglomerated nanostructured YSZ/NiO powders and plasma spray are applied to produce nanostructured YSZ/NiO coatings on porous support substrates. After reduction in an ambient atmosphere of 7% hydrogen and 93% argon at about 800 °C for 4 hours, a novel SOFC anode with nanostructured characteristics such as nano YSZ particles, nano Ni particles, nano pores and nano pore channels is produced. This new YSZ/Ni anode provides larger triple phase boundaries for hydrogen oxidation reactions. X-ray diffraction patterns of these YSZ/NiO coatings after 1 h of heat treatment at temperatures from 700 to 1100 °C are obtained and Scherrer analysis is conducted to study the effect of temperature on grain size. The results obtained from SEM, TEM, XRD and EDX measurements and analyses are presented in this investigation.     

Characterization and sinterability of oxide-dispersion strengthened nickel powder produced by mechanical alloying

Among the main requirements for the Ni/8% yttria stabilized zirconia (Ni/8YSZ) material, currently used for manufacturing solid oxide fuel cell (SOFC) anodes, fine homogeneous microstructure, considerable structural and mechanical stability, and sufficient gas permeability are of primary concern. In the present investigation, oxide-dispersion strengthened composite Ni powders containing 2, 5, and 10 vol.% 8YSZ were produced by mechanical alloying (MA) in air using a planetary milling machine and ZrO₂ milling media. The progress of the MA process was followed by particle size analysis, optical metallography, and x-ray diffraction (XRD) techniques. Results showed that dispersion of the oxide particles and structural refinement reached a significant point after milling for 180 h. The crystallite size and lattice distortion showed considerable dependence on the processing parameters. The mechanically alloyed powders were sintered at 1100° to 1350 °C. The mechanically alloyed powder containing 10 vol.% 8YSZ exhibited maximum densification. The minimum sintered density was observed for the composite powder containing 2 vol.% 8YSZ.     

纳米晶W-Ni-Fe复合粉末及其烧结过程中的固溶特性

采用溶胶喷雾干燥-热还原法制备了50W-35Ni-15Fe(质量分数,%,下同)纳米晶复合粉末。利用X射线衍射及微区组织的能谱(EDAX)分析,揭示了纳米晶W-Ni-Fe复合粉末及其烧结过程中的固溶特性。研究发现:纳米晶50W-35Ni-15Fe复合粉末中,W在γ-(Ni,Fe)相中的固溶度达到了15%,Ni、Fe元素在W中的固溶度达到了2.4%,形成了超饱和固溶体;随着温度的升高,W的超饱和固溶体W(Ni,Fe)发生Ni、Fe元素的脱溶。而W在γ-(Ni,Fe)相中的固溶度随温度的升高而进一步增加,在1300℃以后基本维持在29%左右;稀土La的加入在低温阶段对W在γ-(Ni,Fe)相中的固溶度影响不大,在1200℃以上使固溶度有一定的降低。     

In-situ metal binder-phase formation to make WC-FeNi Cermets with spark plasma sintering from WC,Fe, Ni,and carbon powders

High-density WC-FeNi ceramic-metal (cermet) composites were fabricated using liquid-phase spark plasma sintering/field-assisted sintering technology (SPS/FAST) with in-situ formation of metal binder phase. The precursor materials were micron-sized powders of WC, Fe, Ni, and C. A low melting point from a eutectic reaction of the powders enabled the in-situ formation of FeNi alloy and facilitates liquid-phase sintering of the WC. The carbon powder was added to stabilize the formation of the binder phase. Electron backscatter diffraction (EBSD) was performed to measure grain size and orientation. The composite exhibited a 99% theoretical density and a microstructure consisting of rounded and contiguous WC grains. The average grain size is 10.5 μm. The composite has a maximum hardness of 16.1 GPa. This research provides a fast and cost-effective approach to fabricate hard metals.     

Development and microstructure optimization of atmospheric plasma-sprayed NiO/YSZ anode coatings for SOFCs

In this paper, preparation and characterization of porous anode layers with uniform phase distribution are discussed for solid oxide fuel cell (SOFC) application. The Ni/8YSZ cermet coatings were fabricated by atmospheric plasma spray (APS) process using oxidized nickel coated graphite (Ni-graphite) and 8 mol% yittria — stabilized zirconia (8YSZ) blend as feedstock. To control the microstructure of the coating, the nickel coated graphite with low density was used as a starting feedstock instead of conventional pure nickel (Ni) powder. To balance the conductivity, uniform porosity, and structural stability of the coatings, the effects of process parameters such as hydrogen gas flow rate, stand off distance and pore formation precursor (graphite) addition on the microstructures of the resulting coatings are investigated. The results show that the anode coatings with high conductivity, structural stability and porosity could be deposited with moderate hydrogen gas flow rate and short stand off distance.     

化学镀法制备Cu/Al2O3复合粉体及其烧结行为

为了解决颗粒增强铜基复合材料中颗粒与铜基体相容性的问题,采用化学镀法使得颗粒表面金属化,通过XRD、SEM、EDS等技术研究了Al2O3纳米粉化学镀铜的工艺,并对Cu/Al2O3复合粉体的烧结行为做了初步探讨.结果表明:Al2O3粉前处理工艺、镀液的各种成分都对粉体表面铜的含量有影响,通过改变装载量可有效控制粉体表面Cu含量.实验中确定了最佳的镀覆工艺,并得到了颗粒较为弥散的Cu/Al2O3复合粉体烧结体.     

Fabrication of finely structured silicon-supported SOFC with anode deposited by multi-phase plasma spraying

The mechanically mixed NiO/YSZ powder was usually used as the anode material of atmospheric plasma sprayed (APS) solid oxide fuel cells (SOFC). Big particles and the non-uniform distribution of the pores were observed in the resultant anode layer. To overcome the limitations, a method of fabricating anode layer by multi-phase plasma spraying (MPS) was proposed in this paper. The NiO and YSZ powders were delivered into plasma jet by a separate injection, where nitrogen carrier was employed to feed micrometer-sized NiO powder and liquid carrier was to feed submicrometer-sized YSZ powder. Suspension plasma spraying (SPS) was applied to fabricate dense electrolyte layer. The microstructure and composition of coatings were characterized by SEM and EDS. The results showed that finely structured anode layer with small particle size (d ∼ 2 μm) was achieved by the MPS method. The MPS anode layer was porous with the porosity of 32.1% while the APS anode layer was 22.6%. Three kinds of elements (Ni, Y, Zr) were observed in the MPS anode layer and the NiO content was calculated to be 49.6 wt%. In the SPS process, the suspension flow rate was matched to the plasma gas flow rate to obtain proper injection condition.     

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.     

Synthesis of ultrafine/nanocrystalline W-(30-50)Cu composite powders and microstructure characteristics of the sintered alloys

Ultrafine/Nanocrystalline W-Cu composite powders with various copper contents (30, 40 and 50 wt.%) have been synthesized by sol-spray drying and a subsequent hydrogen reduction process. The powders were consolidated by direct sintering at temperatures between 1150 and 1260 °C for 90 min. The powder characteristics and sintering behavior, as well as thermal conductivity of the sintered alloys were investigated. The results show that the synthesized powders exist in ultrafine composite particles containing numerous nanosized particles, and the composition distributed very homogeneously. As the copper contents increase, the grain size of the powders decreases. The subsequent sintered parts show nearly full density with the relative density more than 99% at the temperature of 1250 °C. The sintered parts have very fine tungsten grains embedded in a bulk matrix. With increased copper contents, the tungsten grain size decreases and the microstructural homogeneity of the sintered alloys improves further. The thermal conductivity properties, while a little lower than that of the theoretical value, depend on the copper contents.     

Ni-GDC阳极的制备及其孔隙率研究

采用缓冲溶液法制备出NiO—GDC复合粉末，该粉末压制、烧结后再经氰气气氛还原得到Ni—GDC金属陶瓷阳极。使用XRD，TEM，SEM对NiO—GDC和Ni—GDC的物相、形貌进行了观察分析，测定了阳极还原前后的相对密度、总孔隙率及开孔隙率。实验结果表明，由于NiO转变为Ni，有O的火上，使得还原后试样的相对密度明显下降，而孔隙率由15．4％提高到29．6％，且开孔隙率达到23．1％，可满足阳极作为燃料扩散通道的作用。多孔结构的Ni—GDC金属陶瓷阳檄有望成为中混固体氧化物燃料电池的阳极材料。     

Catalysts characteristics of Ni/YSZ core-shell according to plating conditions using electroless plating

This study aims to develop an anode catalyst for a solid oxide fuel cell (SOFC) using electroless nickel plating. We have proposed a new method for electroless plating of Ni metal on yttria-stabilized zirconia (YSZ) particles. We examine the uniformity of the Ni layer on the plated core-shell powder, in addition to the content of Ni and the reproducibility of the plating. We have also evaluated the carbon deposition rate and characteristics of the SOFC anode catalyst. To synthesize Ni-plated YSZ particles, the plated powder is heat-treated at 1200 °C. The resultant particles, which have an average size of 50 μm, were subsequently used in the experiment. The size of the Ni particles and the Ni content both increase with increasing plating temperature and plating time. The X-ray diffraction pattern reveals the growth of Ni particles. After heat-treatment, Ni is oxidized to NiO, leading to the co-existence of Ni and NiO; Ni₃P is also observed due to the presence of phosphorous in the plating solution. Following heat treatment for 1 h at 1200 °C, Ni is mostly oxidized to NiO. The carbon deposition rate of the reference YSZ powder is ~135%, while that of the Ni-plated YSZ is 1%-6%.     

Preparation of nickel-coated tungsten carbide powders by room temperature ultrasonic-assisted electroless plating

In this paper nickel (Ni)-coated tungsten carbide (WC) composite powders have been synthesized by ultrasonic-assisted electroless plating with a simplified pretreatment at room temperature as the conventional sensitization and activation steps have not been employed. The growth mechanism of Ni layers and surface morphologies and composition of initial WC powders, pretreated WC powders and Ni-coated WC powders were analyzed by field emission scanning electron microscopy, and energy dispersion spectrometry. The results shows that uniform Ni-coated WC composite powders were successfully synthesized without conventional sensitization and activation steps by ultrasonic-assisted electroless plating at room temperature. The growth mechanism of Ni layers appears as follows: the surfaces of pretreated WC powders appear step-like defects which act as activated sites. Nucleation and the growth of nickel grains take place on the activated sites of pretreated WC powder, and the process repeats continuously on the lath particles with reticulate structure on the as-coated surfaces of previously deposited Ni-cells, finally Ni cells grow up and merge into a layer.     

化学镀Ni—P—聚四氟乙烯复合镀层及其性能

向一种含双络合剂的化学镍液中加入PTFE（聚四氟乙烯）浓缩乳液,成功获得了PTFE含量为25%-30%vol的Ni-P-PTFE复合镀层。应用X-Ray衍射仪、扫描电子显微镜、差热分析等对镀层组织结构和性能进行了测定。结果表明,这种镀层为非晶态结构,与基体结合良好,具有低的孔隙率和低的摩擦系数,是一种耐蚀、自润滑的表面复合材料镀层。     

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.     

Ni-P-PTFE-SiO2化学复合镀工艺研究

运用化学复合镀技术,在Ni-P合金镀液中添加PTFE及SiO2粒子进行试验,获得了Ni-P-PTFE-SiO2复合镀层.对复合镀的工艺及复合粒子PTFE和SiO2的添加量进行了研究分析;通过金相显微组织、显微硬度,结合强度和镀层孔隙率等测试对Ni-P-PTFE-SiO2化学复合镀层性能进行了表征.结果表明:当PTFE粒...     

The effect of sintering temperature on densification of nanoscale dispersed W–20–40%wt Cu composite powders

Nanoscale dispersed particles of W–20–40%wt Cu were synthesized using a chemical procedure including initial precipitating, calcining the precipitates and reducing the calcined powders. The powders were characterized using X-ray diffraction and map analyses. The effect of sintering temperature was investigated on densification and hardness of the powder compacts. Relative densities more than 98% were achieved for the compacts which sintered at 1200 °C. The results showed that in the case of W–20%wt Cu composite powders, the hardness of the sintered compacts increased by elevating the sintering temperature up to 1200 °C while for the compacts with 30 and 40%wt Cu, the sintered specimens at 1150 °C had the maximum hardness value. The microstructural evaluation of the sintered compacts by scanning electron microscopy showed homogenous dispersion of copper and tungsten and a nearly dense structure. A new proposal for the variation of the mean size and morphologies of W-particles with volume percent of copper melt within the composites has been suggested.     

Properties and fast low-temperature consolidation of nanocrystalline Ni-ZrO2 composites by high-frequency induction heated sintering

Nanopowders of Ni and ZrO₂ (11 nm and 90 nm, respectively) were synthesized from NiO and Zr by high energy ball milling. A highly dense nanostructured 2Ni-ZrO₂ composite was consolidated at low temperature by high-frequency induction heat sintering within 2 min of the mechanical synthesis of the powders (Ni-ZrO₂) with horizontal milled NiO + Zr powders under 500 MPa pressure. This process allows very quick densification to near theoretical density and prohibits grain growth in nano-structured materials. The grain sizes of Ni and ZrO₂ in the composite were calculated. Finally, the average hardness and fracture toughness values of nanostructured 2Ni-ZrO₂ composites were investigated.     

Chemical vapor deposition of C on SiO2 and subsequent carbothermal reduction for the synthesis of nanocrystalline SiC particles/whiskers

This study investigates chemical vapor deposition of C from CH₄ on particulate SiO₂ and subsequent carbothermal conversion of the resultant composite particles to SiC powders. Mass measurements, HR-TEM, SEM and XRD were used to characterize the products at various stages of the processes. It was found that oxide particles gained mass rapidly at 1300 K under CH₄ atmosphere owing to enhanced C uptake. Pyrolytic carbon layers 5-8 nm thick were deposited on SiO₂ particles. The coated powders with high C loadings (40-42.6 wt.% C) were converted to SiC under Ar flow in a temperature range of 1700-1800 K. Almost pure SiC powders containing a mixture of particles and whiskers of ~ 100 nm were synthesized at 1750 K for 45 min and at 1800 K for 30 min using the starting powder with 40 wt.% C. Whisker diameter increased with the C content of the coated powder. It was proposed that SiC whisker was grown by a vapor-solid mechanism. Equilibrium thermodynamic analysis by the method of minimization of Gibbs’ free energy predicted the reaction pathways to SiC and to the product species in the Si-O-C-Ar system. This study demonstrated that either C shell-SiO₂ core powders or SiC powders could be synthesized rapidly in the same reactor.     

Sintering of a nano-crystalline tungsten heavy alloy powder

A nano-crystalline Tungsten heavy alloy powder was obtained by mechanical alloying of elemental powders in a jar mill with a high ball to powder ratio. The chemical composition of the primary powder was 93 W-4.9Ni-2.1Fe (wt%). The mechanically alloyed powder had 22 nm sized tungsten crystallites distributed in an amorphous nickel base phase. Mechanical alloying reduced particle size of powders and also yielded to more uniform particles size distribution. Sintering behavior and microstructural development of that powder were studied and compared with a conventionally mixed powder. Mechanically stored energy and better distribution of primary elements in Nano-crystalline powder had decreased motivation energy of sintering and that powders showed more densification at relatively lower sintering temperatures. Sintering at low temperatures can depress grain growth during sintering and provide desirable properties. A transient intermetallic phase was formed in the nano-crystalline powder during sintering that has not been seen in conventionally mixed powders.     

Sinterability and microwave dielectric properties of nano structured 0.95MgTiO3-0.05CaTiO3 synthesised by top down and bottom up approaches

0.95MgTiO₃-0.05CaTiO₃ (MCT) nano powders were synthesised using sol-gel method and high energy ball milling (HEBM). Synthesised powders were characterised using X-ray diffraction analysis to ensure phase purity and HRTEM to determine the fine microstructural features like particle size, interplanar spacing, etc. The powder pellets were heat treated to study the sinterability and microwave dielectric properties and these properties were then compared with the microwave dielectric properties of micron sized sample. Nano powder synthesised using HEBM shows better dielectric properties, sinterability and gets densified to 90% of theoretical density (TD) at 1200 °C/2 h. Dielectric resonators prepared using chemically synthesised nano powder showed poor sinterability and microwave dielectric properties, but, dielectric properties of HEBM samples were very near to that of solid state synthesised samples. Sintered HEBM powders retain the microwave dielectric properties almost to the same level as the solid state synthesised powder with considerable lowering of sintering temperature.