Ultrafine oxide powders prepared by electron beam evaporation

The surface properties and microstructure of ultrafine oxide powders prepared by vapour phase condensation have been examined by several techniques (electron microscopy, X-ray diffraction, nitrogen adsorption isotherms, infra-red absorption of surface species). All the powders had a very small particle size (<20 nm), and gave type II adsorption Isotherms — typical of non-porous adsorbents. From infra-red absorption studies of water adsorbed on the surface of the oxides, an unusually relaxed and unreactive surface was indicated. The compaction and sintering characteristics of the powders were exceptional, and made them highly suited for ceramic fabrication. The contrasting behaviour of an ultrafine magnesia, consisting of porous pseudomorphic agglomerates, is demonstrated.     

机械球磨3%C-Cu复合粉末的微观组织

为了获得具有良好微观组织的C-Cu复合粉末,以利于后续的压制、烧结和挤压等工艺,用机械球磨方法制备了3%C-Cu(质量分数)复合粉末.运用扫描电镜、背散射和X射线衍射等分析手段研究了该复合粉末的微观组织随球磨时间的演变规律.实验结果表明,随着机械球磨时间的增加,Cu颗粒由树枝状转变为层片状、块状,最后转变为近球形.球磨2 h,复合粉末中的石墨衍射峰消失.随着球磨的进行,复合粉末中Cu的微观应变逐渐增大.经3 h的机械球磨获得了晶粒尺寸约为20 nm的Cu纳米晶,说明该方法可以有效地细化晶粒组织.     

机械球磨Cu-15%Cr复合粉末的致密化工艺研究

探讨机械球磨Cu—15％Cr复合粉末经真空烧结后采用热静液挤压致密化的可能性．研究了机械球磨时间、烧结温度、保温时间和挤压温度等工艺参数对材料致密化的影响．结果表明，热静液挤压工艺可以有效的促进机械球磨复合粉末的致密，所获得的材料具有优异的组织性能．     

Incontinuous grain growth in cobalt nanocrystalline powders prepared by high-energy mechanical milling

Cobalt nanocrystalline powders with the average grain size of about 17 nm were prepared by high-energy mechanical milling. Grain growth in highly pure and particle-containing nanocrystalline Co powders were investigated respectively by a series of annealing experiments at different temperatures. The characteristics of incontinuous grain growth were found in both the pure and the particle-containing nanocrystalline powders. It is proposed by the authors that the sharp increase in nanograin size in the transition between the low and high temperature regions is a result of enhanced grain growth promoted by the stored energy as a supplied driving force, based on which rapid grain growth occurs through a particular dominant mechanism of nanograin rotations in the pure nanocrystalline powders, and that through off-pinning of grain boundaries in the particle-containing nanocrystalline powders.     

机械合金化制备Cu-Cr-C复合粉体

以Cu、Cr、C粉末为原料,采用机械合金化方法制备CuCr-C复合粉体,其中Cr、C的添加量按照Cr_3C_2质量分数为5%来计算;利用X射线衍射(XRD)和扫描电镜(SEM)研究机械合金化过程中粉末的物相和微观形貌,并结合能谱仪(EDS)面扫描得到粉末的元素微观分布。结果表明:随着机械合金化的进行,C、Cr和Cu形成Cu-Cr-C过饱和固溶体,随着球磨时间的延长,粉末粒径细化,颗粒形态由片状向球状发展;球磨30 h后,生成Cr_3C_2增强相,粉末细化趋势变缓并逐渐产生团聚,故原位生成Cr_3C_2的最佳球磨时间为30 h。     

Low temperature dielectric studies of zinc oxide (ZnO) nanoparticles prepared by precipitation method

Using zinc nitrate as a precursor and NaOH starch as a stabilizing agent, hexagonal zinc oxide (ZnO) nanoparticles has been synthesized by precipitation method. The transmission electron microscopy (TEM) images show particles of nearly uniform spherical size of around 40 nm. The infrared spectroscopy (FT-IR) measurement reveals the peak at 500 cm^?1, corresponding to the Zn–O bond. Dielectric studies of ZnO nanoparticles show frequency dependence dielectric anomaly at low temperature (85–300 K). Results reveal that the capacitance and loss tangent decrease with the frequency while these parameters improve with the increasing of temperature. The increase of a.c. conductivity with the temperature indicates that the mobility of charge carriers is responsible for hopping and electronic polarization in ZnO nanoparticles.     

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.     

Sub-micron sized Al2TiO5 powders prepared by high-energy ball milling

High energy ball milling to obtain ultrafine aluminium titanate particles has been investigated. Tempered steel has been selected as material for the containers and balls because the desirable properties of aluminium titanate are not degraded by small amounts of Fe₂O₃. The starting powders have been milled during different periods (1–60 h) and the evolution of the morphology and crystallinity of the treated powders as well as the extent of contamination from the milling media have been characterised. Different experimental techniques, X-ray diffraction, BET-analysis, chemical analysis, scanning electron microscopy and low and high resolution transmission electron microscopy have been used. High energy ball milling has been proved to be an efficient route to obtain submicron sized (50–100 nm) aluminium titanate powders, but further milling of the powders is accompanied by contamination from the milling media and the formation of hard agglomerates.     

Microstructural and chemical properties of AlN-Cu nanocomposite powders prepared by planetary ball milling

Microelectronic circuits require contact with a high thermal conductivity, and controlled low coefficients of thermal expansion packaging materials for the high performance and reliability of the semiconductor chips. The present study was carried out to investigate the microstructural and chemical properties of AlN-Cu nanocomposite powders created by planetary ball milling. X-ray diffraction and scanning electron microscopy analysis of the obtained powders were performed. The residual oxygen and carbon in ball milled AlN-Cu powders were analyzed. AlN-Cu composite powders of 15 μm size and copper crystallites of 25 nm were obtained after milling for 8 h. The amount of residual oxygen was considerably reduced after exposure to hydrogen reduction treatment at 600°C for 2 h. More significantly, 97% of the residual carbon was removed regardless of milling time and alloy composition. Furthermore, residual carbon was removed as a gaseous mixture of carbon monoxide and carbon dioxide.     

Melting of iron nanoparticles embedded in silica prepared by mechanical milling

For decades, experimental studies on the size-dependent melting of metals are regretfully limited to some eight archetypal examples. In this work, to expand this slim range of materials, the melting behavior of Fe nanoparticles embedded in SiO₂ prepared by using mechanical milling are investigated. Effects of factors in sample preparation on the size, isolation and thermal stability of Fe nanoparticles are systematically studied. On this basis, the size-dependent melting of Fe is successfully traced: for Fe nanoparticles with a diameter of about 15 nm, the melting point depression is 30 °C in comparison with bulk Fe, in accordance with our recent theoretical prediction.     

Detonation synthesis of zinc oxide nanometer powders

Novel zinc oxide nanometer powders have been synthesized via detonation reaction with respect to the presence of an energetic precursor, such as lithium nitrate and zinc nitrate. The detonation products of emulsion explosives were characterized by powder X-ray diffraction. Transmission electron microscopy was used to observe the structures. Zinc oxide with primary particles of diameters from 20 to 50 nm and a distribution with proportional spacing of spherical morphologies were found. The oxides produced by this cheap method affirmed the validity of detonation synthesis of nanosize powders. It is concluded that unconventional emulsion explosives are designed uniquely for synthesis of the nanometer zinc oxide.     

Boron-doped zinc oxide thin films prepared by sol-gel technique

Multilayer transparent conducting boron-doped zinc oxide films have been prepared on glass substrates by the sol gel dip coating process. Zinc acetate solutions of 0.4 M in isopropanol stabilized by diethanolamine and doped with boron tri-i-propoxide were used. Each layer was fired at 400–650^∘C in a conventional furnace for 30 min. Selected samples were vacuum annealed at 400–450^∘C for 1 h to improve their electrical properties. The electrical resistivity curve with doping shows a minimum around 0.8 at.%. Excess boron caused a drop of the carrier mobility without acting as donors. Post-deposition annealing sequence was crucial for dopant partial regeneration. Films with an average optical transmittance exceeding 90% can be achieved reproducibly.     

Luminescent properties of YAG:Ce phosphor powders prepared by hydrothermal-homogeneous precipitation method

Y_3 − xCe_xAl₅O₁₂ (YAG:Ce³⁺) phosphor powders were successfully prepared by hydrothermal-homogenous precipitation (HHP) method, under mild conditions with inexpensive aluminum and yttrium nitrates as the starting materials and urea as homogenous precipitant. The pure YAG crystalline phase could be formed after hydrothermal treatment at 100 °C for 4 h and 240 °C for 20 h and postannealing process at 1200 °C for 2 h. All of the as-prepared YAG:Ce³⁺ powders did not have the CeO₂ phase. The photoluminescence spectrum of crystalline YAG:Ce³⁺ phosphors showed the emission intensity of phosphor increased with increasing the annealed temperature and reached its maximum as the molar fraction of cerium ion was 0.10, and also showed the maximum emission wavelength nearly unchanged with the calcination temperature and cerium doping concentration.     

不同Sol-Gel前躯体体系制备纳米ZnO粉体的研究

为研究不同溶胶前躯体体系对制备纳米ZnO粉体粒径大小的影响,采用溶胶-凝胶法分别研究了硬脂酸体系、草酸体系、高分子网络体系制备纳米ZnO粉体,并利用TG-DTA、XRD、TEM对其结构、形貌和粒径进行了表征.在3种体系中,高分子网络体系所得粉体粒径最小,粒径在30～40nm,粒径大小均匀,团聚较少.硬脂酸体系得到ZnO粉末粒径较小,范围分布在30～50nm,分散性较好.草酸体系制得ZnO粉末粒径相对较大,在40～60nm,有轻微团聚.     

Impact of the SiC addition on the morphological,structural and mechanical properties of Cu-SiC composite powders prepared by high energy milling

The effect of the SiC content on the microstructural, mechanical, and magnetic properties of Cu(1 − x)SiC(x) composite powders (x = 0, 2, 10 and 15 wt%) prepared by high energy milling for 30 h was investigated. The results showed that Cu particles were severely deformed and formed plate like particles of different sizes, while SiC particles were fragmented and embedded in the Cu phase, thus, forming composite particles. As the SiC content increased, the average particle size decreased from 40.75 to 12.84 µm. Besides, XRD data showed a decrease in the crystallite sizes of the Cu phase (from 23.66 to 21.56 nm), accompanied by an increase in the lattice micro-strain (from 0.41 to 0.46%). Changes in the lattice parameters of the Cu phase were observed. The Vickers microhardness were measured in compacted powder samples and reached a maximum value of 135.22 HV for the sample with 15 wt% SiC. The samples showed hysteresis magnetic behavior at 300 K, and with a maximum saturation magnetization of 0.123 emu/g. The weak magnetic signal is mainly due to Co impurities present in the WC from the milling media.     

Studies on structural and electrical properties of copper-doped zinc oxide powders prepared by a solid state method at high temperatures

Abstract

The synthesis, crystal structure and electrical conductivity properties of Cu-doped ZnO powders (in the range of 0.25 – 15 mole %) is reported. I-phase samples, which were indexed as single phase with a hexagonal (wurtzite) structure in the Cu-doped ZnO binary system, were determined by X-ray diffraction. The limit solubility of Cu in the ZnO lattice at this temperature is 5 mole % at 1000°C. The impurity phase was determined as CuO when compared with standard XRD data using the PDF program. We focused on single I-phase ZnO samples which synthesised at 1000°C because the limit solubility range is widest at this temperature. It was observed that the lattice parameters a increased and c decreased with Cu doping concentration. The morphology of the I-phase samples was analysed with a scanning electron microscope. The electrical conductivity of the pure ZnO and single I-phase samples were studied using the four-probe dc method at temperatures between 100 and 950°C in an air atmosphere. The electrical conductivity values of pure ZnO and 5 mole % Cu-doped ZnO samples at 100°C were 2 × 10^?6 and 1.4 × 10^?4 ohm^?1 cm^?1, and at 950°C they were 1.8 and 3.4 ohm^?1 cm^?1, respectively. In other words, the electrical conductivity slightly increased with Cu doping concentration. Also, it was observed that the activation energy of the I-phase samples was decreased with Cu doping concentration.     

溶胶-凝胶法制备c轴取向生长ZnO薄膜

采用sol-gel法,在普通载玻片上使用旋转涂覆技术生长了具有c轴择优取向生长的ZnO薄膜.采用热分析、XRD、SEM等手段对薄膜样品进行了表征.热分析结果表明,二水醋酸锌-乙醇胺-乙二醇甲醚体系sol-gel的热分解过程与纯二水合乙酸锌的分解过程大相径庭.ZnO薄膜的sol-gel分解趋于在较窄的温度范围内一步完成.样品在XRD图谱中表现出明显的c轴择优取向.此外,SEM照片也表明:ZnO薄膜样品中间区域和边缘区域的表面形貌相差甚远.探讨了预热处理温度、退火温度等工艺条件对ZnO薄膜的结构性能的影响,最佳的预热处理温度被认为在ZnO相完全生成温度附近.     

NiAl-Al2O3 intermetallic matrix composite prepared by reactive milling and consolidation of powders

Reactive milling of nickel oxide and aluminium powders corresponding to the stoichiometric reaction 3NiO + 5Al resulted in the formation of intermetallic matrix composite NiAl-Al₂O₃, with 28 wt% of alumina. Prolongation of the milling process allowed obtaining the microstructure with nanosize range of crystallites of both phases, as showed XRD measurements and TEM observations. The refinement of microstructure was accompanied with an increase of lattice strain as a result of ball milling. The particles size and morphology changed from several tens of micrometers and polyhedron shape observed immediately after the reaction took place, to several micrometers and spherulitic shape after long-term milling. Two consolidation techniques of nanocomposite powders were applied: explosive compaction and hot-pressing under high pressure. Both methods allowed obtaining the samples of high density (up to 99% of theoretical one) and microhardness above 13 GPa. Simultaneously, a nanocrystalline structure of the material was preserved.