Molten salt synthesis of La0.8Sr0.2MnO3 powders for SOFC cathode electrode

For La_0.8Sr_0.2MnO₃ (LSM) perovskite, used as the cathode material for solid oxide fuel cells (SOFC), it is known that the formation of a triple-phase-boundary is restrained due to the formation of a second phase at the YSZ/electrode interface at high temperature. To decrease the 2^nd phase, lowering the sintering temperature has been used. LSM powder was synthesized by molten salt synthesis method to control its particle size, shape, and agglomeration. We have characterized the phase formation, particle size, shape, and sintering behavior of LSM in the synthesis using the variation of KCl, LiCl, KF and its mixed salts as raw materials. In the case of KCl and KCl-KF salts, the particle size and shape of the LSM was well controlled and synthesized. However, in the case of LiCl and KCl-LiCl salts, LiMnO_x as 2^nd phase and LSM were synthesized simultaneously. In the case of the mixed salt of KCl-KF, the growth mechanism of the LSM particle was changed from ‘diffusion-controlled’ to ‘reaction-controlled’ according to the amount of mixed salt. The sintering temperature can be decreased below 1000 °C by using the synthesized LSM powder.     

Corrosion properties of laser surface melted NiTi shape memory alloy

A Nd-YAG laser was used to melt the surface of NiTi shape memory alloy in air and argon environment respectively. The laser surface melted (LSM) layer is free of porosity and crack. The corrosion resistance of the LSM specimen in 3%NaCl solution has been improved significantly and is attributed to the increased amount of TiO₂ and Ti/Ni ratio on the surface.     

Powder processing and densification behaviour of alumina–high zirconia nanocomposites using chloride precursors

Al₂O₃–ZrO₂ composites having nominal equal volume fraction of Al₂O₃ and ZrO₂ were prepared from gel-precipitated powder, precipitated powder and washed precipitated powder. These different processing routes affected the crystallization temperature of the amorphous powder as well as the phase evolution of Al₂O₃ and ZrO₂ during calcination. The agglomerate size was largest for gel-precipitated powder (30 μm) and it was smallest for washed precipitated powder (19 μm). While gel-precipitated powder produce hard agglomerated powder (P_j = 110 MPa), washed precipitated powder produce soft agglomerates with low agglomeration strength (P_j = 70 MPa). Thus, washed precipitated powder could sinter to a high density at lower sintering temperature. The bending strength exhibits a semi logarithmic relationship with porosity. The hardness shows an increasing trend with sintering temperature.     

X-ray diffraction study on formation of ErNbO4 powder

The formation of ErNbO₄ powder, prepared by calcining an Er₂O₃ (50 mol%) and Nb₂O₅ (50 mol%) powder mixture at 1100 and 1600 °C for different durations, was investigated by using X-ray diffraction. The experimental results have displayed that although the solid-state reaction had started to some extent when the mixture was pre-calcined at 1100 °C for a duration of 13 h, the two original phases Er₂O₃ and Nb₂O₅ still dominated the mixture. When the duration of the calcination reaction was increased to 120 h at the same temperature, the resultant mixture experienced a nearly complete phase transformation. Accordingly, the ErNbO₄ phase was dominant phase in the mixture. Nevertheless, a small portion of the raw powder still existed in the mixture. When the calcining temperature was elevated to 1600 °C, ErNbO₄ powder with higher purity could be obtained for a relatively much shorter duration (only up to several tens of hours). A simple formation mechanism of ErNbO₄, an elevated-temperature-assisted solid-state chemical reaction: Er₂O₃+Nb₂O₅2ErNbO₄, is suggested. In addition, the present experimental results offer important evidence for the formation of the additional phase ErNbO₄ induced in Er:LiNbO₃ crystals by vapour transport equilibration (VTE) treatment.     

Properties of copper matrix reinforced with various size and amount of Al2O3 particles

Copper matrix was reinforced with Al₂O₃ particles of different size and amount by internal oxidation and mechanical alloying accomplished using high-energy ball milling in air. The inert gas-atomised prealloyed copper powder containing 1 wt.% Al as well as a mixture of electrolytic copper powder and 3 wt.% commercial Al₂O₃ powder served as starting materials. Milling of Cu-1 wt.% Al prealloyed powder promoted formation of fine dispersed particles (1.9 wt.% Al₂O₃, approximately 100 nm in size) by internal oxidation. During milling of Cu-3 wt.% Al₂O₃ powder mixture the uniform distribution of commercial Al₂O₃ particles has been obtained. Following milling, powders were treated in hydrogen at 400 °C for 1 h in order to eliminate copper oxides formed at the surface during milling. Compaction was executed by hot-pressing. Compacts processed from 5 to 20 h-milled powders were additionally subjected to high-temperature exposure at 800 °C in order to examine their thermal stability and electrical conductivity. Compacts of Cu-1 wt.% Al prealloyed powders with finer Al₂O₃ particles and smaller grain size exhibited higher microhardness than compacts of Cu-3 wt.% Al₂O₃ powder mixture. This indicates that nano-sized Al₂O₃ particles act as a stronger reinforcing parameter of the copper matrix than micro-sized commercial Al₂O₃ particles. Improved thermal stability of Cu-1 wt.% Al compacts compared to Cu-3 wt.% Al₂O₃ compacts implies that nano-sized Al₂O₃ particles act more efficiently as barriers obstructing grain growth than micro-sized particles. Contrary, the lower electrical conductivity of Cu-1 wt.% Al compacts is the result of higher electron scatter caused by nano-sized Al₂O₃ particles.     

Powder Size Influence on Tensile Properties and Porosity for PM Ti_2AlNb Alloy Prepared by Hot Isostatic Pressing

Pre-alloyed powder of Ti_2AlNb alloy was prepared by electrode induction gas atomization method, and the powder was screened into fi ve kinds of powder size distribution. Fully dense Ti_2AlNb alloy was prepared by powder metallurgy(PM) using hot isostatic pressing. The properties of pre-alloyed powder and PM Ti_2AlNb alloy were tested. Results show that mean grain size of PM Ti_2AlNb alloy is infl uenced by powder particle size, but particle size has no signifi cant infl uence on tensile properties. Finer Ti_2AlNb powder has low Argon gas bubble ratio and high oxygen content, and poor fl owability of fi ner powder increases the degree of diffculty during degassing. As a result, big pores( 50 μm) are observed in PM Ti_2AlNb alloy prepared by fi ner powder and cause plasticity loss of tensile properties. In order to get a better comprehensive properties of PM Ti_2AlNb alloy, powder with an average size(～ 100 μm) is suggested.     

Effects of TiC addition on formation of Ti3SiC2 by self-propagating high-temperature synthesis

Formation of Ti₃SiC₂ was conducted by self-propagating high-temperature synthesis (SHS) from both the elemental powder compacts of Ti:Si:C = 3:1:2 and the TiC-containing samples compressed from powder mixtures of Ti/Si/C/TiC with TiC content ranging from 4.3 to 33.3 mol%. The effect of TiC addition was studied on combustion characteristics and the degree of phase conversion. For the elemental powder compacts, with the progress of combustion wave the sample experiences substantial deformation, including axial elongation and radial contraction. The extent of sample deformation and flame-front propagation velocity were considerably reduced for the samples with TiC addition, because the dilution effect of TiC lowered the reaction temperature. Two reaction mechanisms were adopted to explain the formation of Ti₃SiC₂, one involving the reaction of a Ti–Si liquid phase with solid reactants for the elemental powder compact and the other dominated by the interaction of solid-phase species for the TiC-containing sample. For all products synthesized in this study, the XRD analysis identifies formation of Ti₃SiC₂ along with a major impurity TiC and a small amount of Ti₅Si₃. The resulting Ti₃SiC₂ is typically elongated grains. Based upon the XRD profile, the Ti₃SiC₂ content at a level of 71.5 vol.% was obtained in the product from the elemental powder compact. With the addition of TiC, an improvement in the yield of Ti₃SiC₂ was observed and an optimal conversion reaching 85 vol.% was achieved by the sample with 20 mol% of TiC. However, further increase of the TiC amount led to a decrease in the Ti₃SiC₂ content, because of the low reaction temperature around 1150 °C.     

Sintering characteristic of Al2O3-reinforced 2xxx series Al composite powders

Sintering characteristic of Al₂O₃-reinforced 2xxx series Al composite powders was investigated in order to obtain enhanced densification. In order to confirm the effect of the ceramic phase, Al composite powder, AMB 2905 (Al–3.2Cu–1.0Mg–5.0Al₂O₃), was used as the starting powder. Al blended powder, AMB 2712 (Al–3.8Cu–1.0Mg), was also used for comparison. The sintered density of the blended powder was about 93% of the theoretical value at 620 °C. The sintered density of the composite powder was about 95% at 630 °C. A small decrease in the density of each powder caused by swelling was observed after holding time of 10 min at the sintering temperature. After 20 min, the density slightly increased. The diffusion of the liquid phase was faster in the composite powder sintered specimen than in the blended powder sintered specimen. The liquid phase is thought to have infiltrated into the spaces between ceramic agglomerates. The results show that a greater amount of liquid phase is needed to enhance the sinterability of 2xxx series Al composite materials.     

硬质合金粉末表面电泳沉积制备金刚石涂层

采用电泳沉积法在硬质合金粉末表面涂覆金刚石涂层，分析硬质合金含量和MgCl₂·6H₂O含量对涂层沉积效果的影响，并对制备的涂层粉末进行性能表征。结果表明:电泳沉积法可实现硬质合金粉末表面涂覆金刚石涂层；硬质合金粉末为28.0 g，金刚石粉末为4.0 g，MgCl₂·6H₂O为1.0 g时，制备的金刚石涂层均匀且致密度好。      

Powder injection molding of WC–8%Co tungsten cemented carbide

An improved wax-based multi-component binder was developed for powder injection molding of tungsten cemented carbide. A critical powder loading of 65 vol.% and an ideal rheological properties were obtained by the feedstock based on the binder. An ideal control of carbon content was achieved by thermal debinding in 75 vol.%N₂/25 vol.%H₂ atmosphere, which balanced the decarbonization effect of H₂ and the carburation effect of N₂. Solvent debinding followed by subsequent thermal debinding could substantially increase the debinding rate, and it is more flexible and adjustable to debinding atmosphere. The transverse rupture strength, hardness and density of the as-sintered specimens made by an optimized powder injection molding process were 2500 MPa, HRA90 and 14.72 g cm⁻³ respectively. Good shape retention and ±0.02 mm dimension deviation were achieved.     

Influence of heat treatments and chemical composition on SCC susceptibility during repairing procedure of overlaying of Inconel 182 by laser surface melting

Abstract

A practical repairing technique using laser surface melting (LSM) was developed to remove the stress corrosion cracking (SCC) in overlaying of Inconel 182. Influence of microstructure of different heat treatments performed during repairing process on intergranular cracking/intergranular stress corrosion cracking (IGC/IGSCC) susceptibility was discussed. The intergranular precipitate was identified as M₂₃ C₆ by TEM. The microstructure with no intergranular precipitate and refiner sub-grain after LSM process shows excellent IGC/IGSCC resistance. The stress relief heat treatment induced severe microstructure of high IGC/IGSCC susceptibility, owing to the semicontinuous intergranular precipitation. The influence of Nb/C ratio on IGC/IGSCC susceptibility of three nickel based superalloys after LSM process was also investigated. For both of the Inconel 182 alloys with different Nb content, the microstructure after LSM process and following sensitisation treatment showed precipitation free grain boundary. The results of corrosion tests also indicated that the material with higher Nb/C ratio showed higher IGC/IGSCC resistance after LSM process and following sensitisation treatment.     

Double SHS of ZrB2 powder

Production of ZrB₂ powder through self-propagating high-temperature synthesis (SHS) from ZrO₂, Mg and H₃BO₃ mixture often leads to incomplete conversion. A new technique, called DSHS (double SHS) has been developed, wherein the reaction product of the first SHS is mixed with calculated amounts of Mg and H₃BO₃ powder and subjected to a second SHS. The ZrB₂ powder produced by DSHS technique yields increased conversion. The NaCl is used as a diluent during SHS to control the particle size of the product. The average particle size of SHS ZrB₂ powder found to be 75–125 nm in range, which decrease to 25–40 nm after DSHS.     

SOEC多层复合阳极的制备及性能研究

采用共沉淀-共沸蒸馏法合成锶掺杂的锰酸镧(LSM)粉体,在此基础上制备了LSM与钇稳定的氧化锆(YSZ)的复合材料,并研究了该材料应用于固体氧化物电解池(SOEC)阳极的性能.通过XRD、TEM、SEM等手段分析了该材料的化学稳定性及微观结构.通过动电位扫描以及电化学阻抗谱(EIS)研究了该阳极材料的电化学性能.TEM分析显示共沉淀-共沸蒸馏法在减小粉体粒径方面要优于传统的共沉淀方法.SEM结果显示经过1200 ℃,2 h的烧结后,复合阳极与电解质结合紧密,阳极材料内部孔隙均匀,YSZ与LSM两相各自形成连通的网络结构.对不同组成和不同结构的阳极复合材料的电化学性能进行了测试,结果显示多层的阳极结构增加了三相界面(TPB)的长度.     

Improving the performance of hydrogen storage electrodes based on mechanically alloyed Mg61Ni30Y9

Amorphous Mg₆₁Ni₃₀Y₉ powder was produced by mechanical alloying using a Retsch planetary ball mill under liquid nitrogen cooling. Additional gentle milling with graphite powder resulted in a thin graphite coating of powder particles. Further milling with a high energy SPEX mill transferred the alloy into a fully nanocrystalline state. The morphological and microstructural changes were followed by means of XRD, SEM, TEM and DSC. Hydrogen storage electrodes based on those alloy powders were fabricated and their cathodic and anodic polarization behaviour and their charge–discharge cycling behaviour in 6 M KOH solution were investigated. It was found that the alloy modification from a non-defective amorphous to a highly defective nanocrystalline state is more effective for improving the hydrogen sorption properties of the alloy than the graphite coating, but is detrimental for the alloy passivation. Accordingly, a SPEX-milled powder electrode exhibits with C_max = 570 mAh/g a higher maximum discharge capacity than a coated Retsch-milled powder electrode with C_max = 435 mAh/g, but degrades faster during repeated cycling. Using graphite powder supporting material for electrode preparation on a nickel foam carrier was found to be much more beneficial than nickel powder for achieving maximum discharge performance.     

Phase formation during mechanical alloying and subsequent low-temperature heat treatment of Al–27.4at%Fe–28.7at%C powders

Phase formation during high energy ball milling of a ternary elemental powder mixture with a composition of Al–27.4at%Fe–28.7at%C and during low temperature heat treatment of the milled powder was studied. It was found that an amorphous phase formed during prolonged milling. During heating the shorter time milled powder, Al and Fe reacted first, forming the AlFe phase and then at a higher temperature, AlFe reacts with Fe and C, forming the AlFe₃C_0.5 phase. During heating the longer time milled powder which contains a substantial amount of amorphous phase, the amorphous phase partially crystallizes first, forming the AlFe and AlFe₃C_0.5 phases, and then AlFe reacts with the remaining amorphous phase, forming the AlFe₃C_0.5 phase. Overall, mechanical alloying of Al, Fe and C elemental phases enables formation of an amorphous phase, while low temperature heat treatment of mechanically milled powder facilitates formation of AlFe and AlFe₃C_0.5 phases.     

Microstructure and electrochemical properties of mechanically ground Ce2Mg17 + x wt.% Ni (x = 0, 50, 100, 200) composites

The effects of mechanical grinding with or without nickel powder on microstructure and electrochemical properties of Ce₂Mg₁₇ hydrogen storage alloy in 6 M KOH solution were investigated. The microstructure and electrochemical properties depend greatly on the amount of nickel powder introduced during mechanical grinding. For the alloy ball-milled with nickel powder, the more nickel powder added, the more advantageous it is for the formation of a homogeneous amorphous structure, and the larger discharge capacity obtained. After 90 h ball-milling, the Ce₂Mg₁₇ + 200 wt.% Ni composite exhibited a large discharge capacity of 1014 mAh g(Ce₂Mg₁₇)⁻¹[338 mAh g(Ce₂Mg₁₇ + 200 wt.% Ni)⁻¹] at 303 K. The improvement of electrochemical capacity can be attributed to the formation of a homogeneous amorphous structure as well as the modification of the surface state by Ni addition.     

Deposition of HVAF-sprayed Ni-based amorphous metallic coatings

Ni₅₃Nb₂₀Ti₁₀Zr₈Co₆Cu₃ (at.%) amorphous alloy with high glass forming ability (GFA), capable of forming a wholly amorphous rod of 3 mm diameter by casting, was adopted to deposit amorphous metallic coatings by high velocity air fuel (HVAF) thermal spraying. The effects of powder feed rate and spraying distance on amorphous phase content and porosity of the coatings were investigated. It was indicated that an appropriate powder feed rate was desirable to produce a coating with high amorphous fraction, whereas a larger spraying distance led to a more dense coating. The corrosion resistance of the sprayed coatings was also examined in 1 M HCl aqueous solution.     

Ultrafine hardmetals prepared by WC–10 wt.%Co composite powder

Ultrafine tungsten carbide–cobalt (WC–10 wt.%Co) composite powder was synthesized via spray-drying and direct reduction and carburization process in vacuum, which includes precursor preparation by spray-drying of a suspension of ammonium metatungstate (AMT) and cobalt carbonate (CoCO₃), calcination to evaporate volatile components, formation of tungsten–cobalt mixed oxide powder (CoWO₄/WO₃), ball-milling with carbon black, and subsequent direct reduction and carburization reaction in vacuum. The synthesis temperature of WC–10 wt.%Co composite powder without η or graphite phases is lower than 1000 °C. The calculated particle size by BET test is 0.29 μm. Coarse WC powder (FSSS: 0.9 μm) and Co powder (FSSS: 1.0 μm) (WC:Co = 9:1 in mass) were added into the obtained WC–10 wt.%Co composite powder with addition of 30 wt.%, 50 wt.% and 70 wt.%, respectively. Results show that the hardmetal fabricated from 70 wt.% (WC–10 wt.%Co composite powder) + 30 wt.% (90 wt.%WC + 10 wt.%Co coarse powder) mixed powders exhibits a fine microstructure as well as optimum mechanical properties.     

Effect of ball-milling and shot-peening on Zr55Al10Ni5Cu30 alloys

Effect of ball-milling and shot-peening on a metallic glass Zr₅₅Al₁₀Ni₅Cu₃₀, which possesses a large supercooled liquid region, has been investigated by means of differential scanning calorimetry, x-ray diffractometry and transmission electron microscopy. Metallic glassy ribbons, powders and plates were prepared by melt-spinning, gas-atomizing and mold-clamp casting techniques, respectively. No structural changes were observed in both the ribbon and powder specimens by ball-milling for around 100 h; however, the powder specimens were crystallized by Fe contamination when they were ball-milled for 540 h. No structural evolution was also observed when the plate specimens were subjected to shot-peening, while crystallized plate specimens were easily amorphized by mild and short period shot-peening. These results imply high phase stability of the Zr₅₅Al₁₀Ni₅Cu₃₀ metallic glass against deformation.     

Effect of pre-aging pH on the formation of yttrium aluminum garnet powder (YAG) via the solid state reaction method

Yttrium aluminum garnet (YAG) powder was successfully synthesized by a novel solid state reaction method. The starting materials including: yttrium oxide (Y₂O₃), boehmite (AlOOH) and cerium chloride (CeCl₃·7H₂O) are pre-aged at pH 1, 2 and 3, respectively, before calcining. According to the experimental results, the case of pre-aging at pH 1 is the best condition for the formation of YAG powder. The particle size of Y₂O₃ and AlOOH are reduced by the pre-aging at pH 1. It induces to shorten the diffusion distance of the solid state reaction. The pre-aging at pH 1 process leads the formation of YAG powder via solid state reaction more easily. The emission intensity of the product was increased by the decrease of the pre-aging pH, the increase of the calcination temperature and the increase of the heating time.