Sintering of ultrafine SiC powders prepared by plasma CVD

Ultrafine SiC powders with a nanometre particle size were synthesized by r.f. plasma chemical vapour deposition (CVD) using a chemical system of SiH₄−C₂H₄−Ar. The powder was also ultrapure with a grade of 99.999% purity. The product was polytype 3C−SiC and black in colour, in spite of its high purity, because of its ultrafine size. Silicon carbide is a difficult ceramic to sinter; it is possible to sinter it to full density with the aid of sintering additives. Ultrafine and ultrapure SiC powders were hot-pressed without sintering additives in the present study, in order to investigate the sintering behaviour. The CVD powders proved sinterable to 88% theoretical density without sintering additives. The present experiments revealed that powder treatment before firing was a key technology when using ultrafine powders as starting materials in the sintering process. The sintering behaviour of the powder was characterized by a large shrinkage. Phase transformation was negligible after hot pressing at 2200°C for 30 min.     

Wet erosion behaviour of low SiC alumina-SiC nanocomposites

We have investigated the erosive wear behaviour of alumina and Al₂O₃-SiC nanocomposites with SiC content between 1 and 5%. Nanocomposites (grain sizes between 3.15 and 7.16 m) and alumina (grain size 4.43 m) were eroded with SiC particles using a custom-built erosive slurry wear tester. The erosion resistance of the nanocomposites increased slightly with decreasing grain size. Nanocomposites of all grain sizes showed better wear resistance than the alumina. Erosion resistance increases with SiC content, though this effect is not strong for SiC contents greater than 2%. These results are compared with related results from the literature.     

溶胶-共沉淀法制备PbSnO_3超细粉末

以Pb(NO3)2和SnCl4·5H2O为原料,采用溶胶一共沉淀法制备超细铅锡复合氧化物PbSnO3.并通过TG/DSC、XRD、SEM和EDS等技术对粉体的形成过程、粒度、形貌等方面进行了表征.结果表明:以NaOH为沉淀剂,并加入一定量的聚乙二醇为分散剂时,所得产物的粒度小,分散性好;PbSnO3的形成过程为:当pH值在8～9范围内,锡离子的水解产物和铅离子发生沉淀反应生成锡酸铅.     

Properties of ultrafine copper-containing powders prepared by a sonoelectrochemical method

In this paper, we analyze the factors that influence the particle size and morphology of copper powders prepared by electrodeposition from solution at increased current densities (0.5–1.0 A/cm²) under ultrasonication (sonoelectrochemical method). The cathode current density and current pulse duration are shown to have the most significant effect on the particle size of the powders. Reducing the current density and pulse duration leads to a reduction in particle size. Our results demonstrate that the method allows one to obtain copper powders with an average particle size of 100 nm exhibiting high antibacterial activity and capable of suppressing the growth of pathogenic bacteria.     

Ti-Al球磨粉反应等离子喷涂涂层的组织与性能

为制备Ti-Al金属间化合物复合涂层并研究其性能,以机械球磨的Ti-Al混合粉在Q235钢表面进行反应等离子喷涂实验,分别采用X射线衍射、扫描电子显微镜对涂层的成分、显微组织进行了分析,并测试了涂层的结合强度、显微硬度和耐腐蚀性能.结果表明:涂层由Al3Ti、TiN、Al2O3、少量TiAl与Ti3Al、以及残留的Al和Ti组成;球磨可促进喷涂时的反应,但喷涂时Al和Ti仍未完全反应,且在空气环境中喷涂容易氧化和氮化;涂层与基体之间是镶嵌式的机械结合,结合强度平均为30.24 MPa;涂层表面的显微硬度平均为206.1 HV,涂层的耐腐蚀性优于基体.总体上看,当球磨时间较长、电流较大、喷涂距离较大、气流量较小时,喷涂时的反应较充分,且涂层比较均匀、致密,其强度、硬度以及耐腐蚀性能较高.     

Characterization and adsorption properties of powders prepared from cactus pulp

Two cactaceous powders, labelled CACMM1 and CACMM2, are shown to contain calcium oxalate and to consist of globular or cubosome particles. The samples were characterized by X-ray diffractometry, spectroscopic, thermic, nitrogen and water adsorption methods and compared with other organic compounds. The pore systems have been analysed from adsorption isotherms, t-plots and pore size distribution curves. The bioadsorbents CACMM1 and CACMM2 exhibited low specific surface area values and presented pores in the mesopore range. Composite adsorption isotherms for the binary mixtures benzene-hexane and benzene-cyclohexane on cactaceous powders have been established and revealed preferential adsorption of hexane and cyclohexane in the respective system. Also the adsorption behavior of dyes from aqueous solution was studied to complete the diversity of adsorbate molecules. Linear adsorption isotherms were obtained up to 6 mM dye equilibrium concentration.     

Interaction of diamond with ultrafine Fe powders prepared by different procedures

We have studied the interaction of synthetic diamond crystals with ultrafine Fe powders during catalytic diamond gasification in a hydrogen atmosphere at 900°C. The Fe powders were prepared by three procedures: reduction of Fe₂O₃ nanopowder; evaporation using an ELV-6 electron accelerator, followed by condensation; and reduction of ferric chloride. The surface-processed diamond crystals were examined by electron microscopy. The results indicate that ultrafine powders produced by the first two procedures cause predominantly lateral etching of diamond. The Fe particles prepared by the third procedure penetrate into the bulk of diamond crystals and produce etch pits and “tunnels,” thereby markedly increasing the specific surface area of the crystals.     

Physicochemical properties of tungsten carbide powders prepared from ionic melts

Fine-particle tungsten carbide powders were prepared by high-temperature electrochemical synthesis, and their physicochemical properties were studied. The ceramics fabricated from these powders showed superior mechanical properties. The powders were found to exhibit high catalytic activity for hydrogen electrogeneration and electrodissolution.     

Synthesis and sintering properties of cerium oxide powders prepared from oxalate precursors

Fine cerium oxide powders obtained by low-temperature decomposition from oxalate and hydrazinate oxalate are compared. Studies of thermal behaviour and disagglomerating capability are given. The preparation of oxide powders by chemical and physical methods is able to give a dispersed suspension, and leads with appropriate recovery to increased green density and final density after sintering.     

Optical properties and characterization of zinc nitride nanoneedles prepared from ball-milled Zn powders

Zinc nitride nanoneedles (ZNNs) with diameters at stem and tip parts as 200-300 nm and 30-70 nm respectively have been prepared by the nitridation of ball-milled zinc powders at 600 °C for 120 min under NH₃ gas environment. The structural, compositional and morphological characterizations of the product were conducted by X-ray diffraction, energy dispersive X-ray spectroscopy, scanning electron microscopy and transmission electron microscopy. From transmission spectrum data, an indirect band gap of 2.72 eV has been calculated for ZNNs whereas photoluminescence studies exhibited a strong UV excitonic mission band at 395 nm as well as two weak defect related blue emissions at 453 and 465 nm. A vapor-solid (VS) process based growth mechanism for the formation of ZNNs has also been discussed briefly.     

Production and properties of ultrafine inorganic powders in an electric arc plasma

The article generalizes the results of investigations carried out by the present author in the field of plasmochemical production of ultrafine inorganic powders under conditions of electric arc low-temperature plasma. The results of a comprehensive study of specific chemical and physicochemical properties of synthesized ultrafine powders are considered.     

Study of the sintering properties of plasma synthesized ultrafine SiC powders

A study on the sintering of ultrafine SiC powders synthesized from elemental Si and CH₄ using radio frequency (r.f.) induction plasma technology is reported. The powder had a particle size in the range of 40 to 80 nm and was composed of a mixture of α and β-SiC. It was subjected to pressureless sintering in an induction furnace in the presence of different sintering aids. With the addition of B₄C (2.0 wt% B) by mechanical mixing, the powders could only be partially densified, with the highest value of 84.5% of theoretical density being achieved at 2170 °C for 30 min. Through the use of “in-flight” boron doping of the powder during the plasma synthesis step (1.65 wt % B), the ultrafine powder obtained could be densified to above 90% of its theoretical density at 2050 °C for 30 min. The addition of oxide sintering aids (7.0 wt % Al₂O₃ + 3.0 wt % Y₂O₃) by mehanical mixing produced sintered pellets of 95% of theoretical density at 2000 °C for 75 min. The Vicker’s microhardness of the sintered pellets in this case was as high as 31.2 GPa. In order to improve our understanding of the basic phenomena involved, extensive microstructural (scanning electron energy microscopy: SEM), physical (shrinkage, weight loss, porosity, hardness) as well as chemical analysis (prompt gamma neutron activation analysis (PGNAA), energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA)) was carried out. This helped establish a relationship between the properties of the as-synthesized powder and their sintering properties. The influences of sintering temperature, sintering time, additive concentration, and powder purity on the densification behaviour of the plasma-synthesized powders was investigated. The results were compared with data obtained using commercial powder. This revised version was published online in November 2006 with corrections to the Cover Date.     

Densification of ultrafine SiC powders

Recent results on the densification behaviour of ultrafine SiC powders (below 20 nm) are presented and compared with results on the densification of ultrafine silicon-based ceramic powders given in the literature. A study of different powder processing routes and their influence on the pore-size distribution is given. Pressureless sintered green bodies having pore sizes of about 20 nm show extreme coarsening without significant densification. The results indicate a significant influence of green density on shrinkage. Encapsulated hot isostatic pressing (HIPing) led to a reduction of pore size and to considerable density increase at temperatures below 1600 °C. But even then full density without extensive grain growth was difficult to achieve. The applied method to determine grain sizes (X-ray diffraction measurements, XRD, using the Scherrer formula, scanning electron microscopy, SEM, and transmission electron microscopy, TEM) gave similar results for TEM and SEM but lower values for XRD. A possible explanation is presented. Density and grain growth both during pressureless sintering and HIPing showed significant differences between samples with and without sintering additives (B and C). Whether or not the use of sintering agents is favourable in reaching high densities and fine grain sizes, is discussed. HIP densification was modelled assuming diffusion to be the dominant mechanism. Grain growth according to a t ^1/4 dependence and an activation energy of 6.8 eV was introduced into the model. Results on the properties (hardness, also at elevated temperatures, fracture toughness, bending and compression tests, thermal conductivity) of the hot isostatically pressed samples, are presented.     

退火处理对非晶态Ni-P合金超细粉结构及磁性能的影响

采用液相脉冲放电技术,在NiSO4溶液中用NaH2PO2为还原剂,制备出了Ni-P合金超细粉体材料。将Ni-P合金粉在200～450℃下退火处理,并通过X射线衍射(XRD)、透射电镜(TEM)和场发射扫描电镜(FESEM)对退火前后Ni-P合金粉的结构、形貌进行了分析。用振动样品磁强计(VSM)对Ni-P合金粉进行了磁性能分析。结果表明所制备的超细Ni-P合金粉为非晶结构,颗粒直径在300～500nm之间,颗粒之间紧密结合成链枝状形貌。Ni-P合金粉在250～280℃退火后具有良好的软磁性能,250℃退火后矫顽力降低至0.437×10-4T。     

Microstructure and mechanical properties of friction stir welded joints made from ultrafine grained aluminium 1050

In order to obtain ultrafine grained structure, commercially pure aluminium (Al 1050) plates were subjected up to 8 passes of Incremental Equal Channel Angular Pressing (IECAP) following route C. Plates in different stages of IECAP were joined using Friction Stir Welding (FSW). All welded samples were investigated to determine their mechanical properties and structure evolution in the joint zone. The joining process reduced mechanical strength of material in the nugget zone, which was explained by the grain growth resulting from temperature rise during FSW. Nevertheless, the obtained results are promising in comparison to other methods of joining aluminium.     

Microstructure and martensitic transformation in Si-coated TiNi powders prepared by ball milling

Si was coated on the surface of Ti–49Ni (at%) alloy powders by ball milling in order to improve the electrochemical properties of the Si electrodes of secondary Li ion batteries and then the microstructure and martensitic transformation behavior were investigated by means of scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and differential scanning calorimetry (DSC). Ti–Ni powders coated with Si were fabricated successfully by ball milling. As-milled powders consisted of highly deformed Ti–Ni powders with the B2 phase and amorphous Si layers. The thickness of the Si layer coated on the surface of the Ti–Ni powders increased from 3–5 μm to 10–15 μm by extending the milling time from 3 h to 48 h. However, severe contamination from the grinding media, ZrO₂ occurred when the ball milling time was as long as 48 h. By heating as-milled powders to various temperatures in the range of 673–873 K, the highly deformed Ti–Ni powders were recovered and Ti₄Ni₄Si₇ was formed. Two-stage B2–R–B19′ transformation occurred when as-milled Si-coated Ti–49Ni alloy powders were heated to temperatures below 873 K, above this temperature one-stage B2–B19′ transformation occurred.     

Investigation on crystalline phases and mechanical properties of TZP ceramics prepared from sol-gel powders

Y₂O₃-stabilized tetragonal zirconia polycrystalline (TZP) ceramics containing 1–5 mol% Y₂O₃ were prepared by hot pressing and pressureless sintering of sol-gel-derived powders. Sintered ceramics were evaluated for their density, grain and crystallite size, width of transformation zone, crystalline phases and mechanical properties. Variation in the values of fracture toughness and flexural strength has been explained on the basis of crystallite size and proportion of transformable tetragonal phase, which are influenced by the concentration of Y₂O₃ in TZP ceramics. Correlation of the data has indicated that the transformable tetragonal phase is the key factor in controlling the fracture toughness and strength of ceramics.