Synthesis and characterization of silver nanoparticles by sonoelectrodeposition

Shaped silver nanoparticles with sphere, wire and dendrite were prepared by sonoelectrochemical deposition from an aqueous solution of AgNO3 in the presence of ethylenediaminetetraacetic acid disodium salt （EDTA） and polyvinylpyrrolidone （PVP）. The diameter of spherical silver particles was about 30 nm. The diameter of the silver nanowires was also about 30 nm and the length was 200-900 nm. The dendrites were synthesized with the concentration of silver solution increasing. Silver nanoparticles were characterized by transmission electron microscope （TEM）, X-ray powder diffraction （XRD）, scanning probe microscope （SPM） and UV-vis absorption spectrum. XRD patterns revealed that silver particles were of face-centered cubic structure. UV-vis absorption spectra indicated that different morphology and size of silver particles could influence the optical properties.     

Synthesis and grain growth kinetics of in-situ FeAl matrix nanocomposites（ Ⅰ ）： Mechanical alloying of Fe-A1-Ti-B composite powder

Three nanocrystalline alloys, FesoAlso, Fe42.5Al42.5Ti5B10 and Fe35Al35Ti10B20 （molar fraction, %）, were synthesized from elemental powders by high-energy ball milling. The structural evolutions and morphological changes of the milled powders were characterized by X-ray diffractometry（XRD）, transmission electron microscopy（TEM） and scanning electron microscopy（SEM）. The effects of different Ti, B additions on the structure and phase transformation in these alloys were also discussed. It is observed that the diffusion of AI, Ti, B atoms into Fe lattice occurs during milling, leading to the formation of a BCC phase identified as Fe（Al） or Fe（Al, Ti, B） supersaturated solid solution. Fe-based solid solution with nanocrystalline structure is observed to be present as the only phase in all the alloy compositions after milling. Furthermore, the contents of Ti, B affect the formation of mechanical alloying products, changes in the lattice parameter as well as the grain size.     

Preparation of silver tin oxide powders by hydrothermal reduction and crystallization

Silver tin oxide composite powders were synthesized by the hydrothermal method with a silver ammine solution and a Na2SnO3 solution as raw materials. H2C2O4 was used as the co-precipitator of silver ions and tin ions. The co-precipitation conditions were investigated. The results show that the co-precipitate of Ag2C2O4 and Sn（OH）4 is available when the pH value of the solution is 4.27-8.36. Using the obtained precipitate as precursor,the reduction of Ag＋ and the crystallization of tin oxide were carried out simultaneously by the hydrothermal method and silver tin oxide composite powders were obtained. The composite powders were characterized by X-ray diffraction （XRD） analysis,scanning electron microscope （SEM）,and energy spectrum analysis. The results show that the silver tin oxide composite powders are small with a diameter of about 2 μm and with homogeneous distribution of tin.     

Microstructure characteristics of as-surface nanocrystallized 1420 aluminum alloy by high-energy shot peening

A nanostructured surface layer was fabricated on 1420 aluminum alloy by high-energy shot peening.Microstructures were characterized by X-ray diffractometer (XRD), transmission electron microscope (TEM) and high-resolution electron microscope(HRTEM), and microhardness measurement was conducted along the depth from top surface layer to matrix of the sample peened for 30 rain. The results show that a nanocrystalline layer about 20μm in thickness is formed on the surface of the sample after high-energy shot peening, in which the grain size is changed from about 20 nm to 100 nm. In the surface layer of 20-50μm in depth, the microstructure consists of submicron grains. The surface nanocrystallization is accomplished by dislocation slip. The microhardness of the top surface nanostructured layer is enhanced obviously after high-energy shot peening(HESP) compared with that of the coarse-grained matrix.     

Preparation and particle size characterization of Cu nanopartides prepared by anodic arc plasma

Copper nanoparticles were successfully prepared in large scale by means of anodic arc discharging plasma method in inert atmosphere. The particle size, specific surface area, crystal structure, and morphology of the samples were characterized by X-ray diffraction （XRD）, BET equation, transmission electron microscopy （TEM）, and the corresponding selected area electron diffraction （SAED）. The experimental results indicate that the crystal structure of the samples is fcc structure the same as that of the bulk materials. The specific surface area is 11 m^2/g, the particle size distribution is 30 to 90 nm, and the average particle size is about 67 nm obtained from TEM and confirmed from XRD and BET results. The nanoparticles with uniform size, high purity, narrow size distribution and spherical shape can be prepared by this convenient and effective method.     

Development of new type of wear and crack resistant hardfacing electrode

By using H08A bare electrode and the coating fluxes of ferrotitanium, rutile, graphite, calcium carbonate and calcium fluoride, a new type of wear and crack resistant hardfacing electrode was developed. The microstructure and wear properties of deposited layer were studied by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffractometry(XRD) and wear test. The results indicate that TiC particles are produced by direct metallurgical reaction between ferrotitanium or rutile and graphite during welding process. TiC particles with sizes in the range of 3 - 5μm are dispersed in the matrix of lath martensite and retained austenite. The deposited layer of the new type of hardfacing electrode possesses better wear and crack resistance than that of D618 and D667 hardfacing electrodes.     

Wear characteristics of spheroidal graphite roll WC-8Co coating produced by electro-spark deposition

Electro-spark deposition(ESD) was adopted for preparing high property coatings by depositing WC-8Co cemented carbide on an spheroidal graphite roll substrate.The microstructure and properties of the coating were investigated by X-ray diffraction(XRD), scanning electron microscopy(SEM) with energy dispersive X-ray(EDX) and ball-disc configuration wear tester.The results show that nanosized particles and amorphous structures prevail in the coating which is metallurgically bonded to the substrate.The microstructures of the transition zone include columnar structure and equiaxed structure.The primary phases of the coating contain W2C, W6C2.54, Fe3W3C, and Co3W3C.The results of abrasive test show that the coating has low friction coefficients(μaverage = 0.18) and the wear mechanisms are mainly abrasive wear, fatigue wear, and oxidation wear.The maximum microhardness value of the coating is about 17410 N/mm2.The study reveals that the electro-spark deposition process has better coating quality and the coating has high wear resistance and hardness.     

Structure and magnetic properties of nanocrystalline dysprosium powders

In this current study,nanocrystalline Dy powders were prepared by melt-spinning and subsequent highenergy ball-milling.The effect of ball-milling time on the structure and magnetic properties of the powders was studied.The crystal structure and microstructure of the melt-spun ribbons and ball-milled powders were observed by X-ray diffraction(XRD),scanning electron microscopy(SEM) and transmission electron microscopy(TEM).Magnetic measurement of all samples was performed with vibrating sample magnetometer(VSM).XRD results indicate that the average crystal grain size of the powders decreases from 90.09 nm of the ribbons to 10.42 nm of the4-h ball-milled powders.Further TEM observation shows that the grains are fine and uniform.The Neel temperature(T_N) decreases from 182 K of the ribbons to 172 K of the powders,while the Curie temperature(T_C) increases from100 to 130 K,demonstrating that the grain size has substantial influence on the magnetic transition process.Moreover,at 60 K,as the ball-milling time increases,the coercivity of the powders increases first,peaking at 0.48 T for 2-h milling,then drops again,while the remanence of the powders decreases monotonic ally.As a result,the powders milled for 2 h exhibit an optimal maximum energy product of 64.0 kJ·m~(-3),demonstrating the good potential of these powders as a permanent magnet at low temperatures.     

Structural characteristics and magnetic properties of bulk nanocrystalline Fe_(84)Zr_2Nb_4B_(10) alloy prepared by mechanical alloying and spark plasma sintering consolidation

Magnetic properties of Fe84Zr2Nb4B10 sample were investigated. The sample was produced from nanocrystalline powders made by the mechanical alloying (MA) and consolidation using the spark plasma sintering (SPS) technique. Effects of milling time on phase transformation, structural characteristics, and magnetic properties of powders were examined by X-ray diffraction (XRD), scanning electron microscopy (SEM), and physical property measure system (PPMS), respectively. Results show that nanostructured α-Fe supersaturated solid solution is obtained in the final MAed products. The saturation magnetization (Ms) increased with increasing milling time and became constant at 130 h, but the coercivity (Hc) increased firstly and then decreased. The consolidated bulk sample exhibited a high density of 6.893 g·cm-3, there was no phase change during SPS process, and the saturation magnetization and susceptibility of the SPSed bulk sample improved in comparison with the milled powders. The variation of magnetic parameters can be explained by nano-scale effect and Herzer model.     

Preparation of ultramicro molybdenum carbide powders and study on wear properties of their coating

Using specially designed mechanochemical ball-mill equipment, ultramicro molybdenum carbide (MoC) powders were prepared by high-energy ball milling from pure molybdenum powders in civil coal gas atmosphere at room temperature. The structure and the particle size of the powders were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), and Transmission Electron micros-copy (TEM). The results showed that after milling for 30 h, the MoC powders of hexagonal structure were obtained, and their average parti-cle size was around 100 nm. It was found that chemisorption of CO in coal gas onto the fresh molybdenum surfaces created by milling was the predominant processes during the solid-gas reaction, and the energy input due to the introduction of highly dense grain boundaries and lattice defects provided the activation energy for the transition from Mo-C chemisorption to MoC. A coating was formed on the 40Cr steel base using plasma spray by mixing Ni60 alloy powders and ultramicro MoC powders of 5 wt.%, 10 wt.%, and 15 wt.%, respectively. Coat-ing abrasion tests under the condition of dry-grinding, 2 h wear time, and 300 N load showed that the wear resistance property of the coating added with ultramicro MoC powders could be improved greatly, and the wear resistance property of the coating increased with the increase of ultramicro MoC content. The wear mechanisms of ultramicro MoC coating is mainly plough wear and flaking wear assisted. In the abra-sion process, the evenly distributed ultramicro MoC particles play a dispersion strengthening and self-lubricating role in the coating.     

纳米相包覆AgC5电触头材料

采用化学镀技术，在高能球磨所得纳米石墨粉上包覆银，制得纳米晶银／石墨包覆粉体，经冷压、烧结、复压工序，制备了AgC5触头材料。采用此新工艺制备的触头材料与常规工艺制得的相比，在力学和物理性能等方面有了较大的提高。分断实验表明：将纳米技术应用到银／石墨触头材料的制备中，使材料的抗电弧磨损性能有了较大的改善。     

Fabrication of Silver-Graphite Contact Materials Using Silver Nanopowders

Silver-graphite (AgC) composites are used in electrical switchgears as arcing as well as sliding contacts. AgC composite powders for electrical contact applications are conventionally prepared using micron-size silver powder. Present investigation is aimed at exploring the effect of nanosize silver powder, made by colloidal synthesis route, on the processing and properties of AgC contact materials. The AgC composite powders synthesized from micron-size and nanosize silver powders, respectively, are characterized for particle size distribution by dynamic light scattering technique, x-ray diffraction, and scanning electron microscopy. The bulk solid compacts produced by conventional powder metallurgy process of pressing, sintering, and repressing of AgC composite powders are evaluated for their density, microhardness, electrical conductivity, and microstructure. The study reveals that the use of nanosize silver powder not only leads to reduction in sintering temperature but also contributes in improving the properties of the AgC contact materials.     

新型AgC5电触头材料的性能及显微组织

采用高能球磨．还原剂液相喷雾化学包覆．粉末冶金工艺制备出新型银石墨电触头材料AgC5(质量分数)。经与烧结挤压和机械混粉工艺的同类触头相对比，该材料具有优异的机械物理性能。电磨损分断对比试验发现，与常规机械混粉同类触头相比该材料的耐电腐蚀性能提高了40％以上。利用扫描电镜和金相显微镜对AgC包覆粉体及烧结复压后触头显微组织进行了分析，发现微米尺寸的Ag颗粒呈絮凝状结构包覆在石墨片外，这种絮凝体内部孔洞尺寸细小且分布均匀；新工艺材料组织细腻，球磨石墨均匀分布在Ag基体上，弥散度较高。材料经电弧作用后的工作面用SEM和EDS分析发现：新工艺改善了Ag与C间的润湿性和物理结合强度，在电弧瞬时高温作用后，熔融Ag能够以珠状粘附在基体表面，有助于减少Ag液的喷溅损失。     

Microstructure evolution ofAZ91D induced by high energy shot peening

A nanostructured surface layer was fabricated on a AZ91D magnesium alloy by using a high-energy shot peening（HESP）. HESP induced structure along the depth of the treated sample surface layer was characterized by means of X-ray diffractometer （XRD）, transmission electron microscope（TEM） and high resolution transmission electron microscopc（HRTEM）. The experimental results show that a deformed layer of about 50 μm has formed after HESP treatment and the average grain size increases from about 40 nm in the surface layer to about 200 nm at the depth of 40 μm. The surface nanocrystallization can realize intercoordination of the dislocations slipping and dynamic recrystallization. The nanocrystalline grains have stacking faults and dislocation in their interiors. The microhardness of the top surface is about triplicate that of the coarse-grained matrix.     

纳米晶W粉和W-Ni-Fe预合金粉的制备

采用高能球磨法制备纳米晶W粉和W-Ni-Fe预合金粉，研究了不同的球磨材质包括硬质合金球(CCB)、钨球(TAB)和球磨转速、球料比及球磨时间等条件对球磨后粉末性能的影响。利用XRD，TEM和EDX分析球磨后粉末的晶粒尺寸、晶格畸变、形貌、结构变化及颗粒成分变化。结果表明：高能球磨法可制得10nm～80nm的W粉和W-Ni-Fe预合金粉，纳米级颗粒含量达80％以上。相同材质的钨球制得的纳米粉末综合性能较好。球磨过程中，粉末保持颗粒状结构，纳米级粉末颗粒形状最终趋于等轴化。     

高能球磨法制备纳米晶Ni粉的研究

采用高能球磨法制备出了纳米晶镍粉,并利用X射线衍射仪（XRD）、扫描电镜（SEM）、透射电镜（TEM）等分析检测手段,研究了该纳米晶镍粉末的结构、形貌和相的变化。结果表明,镍粉末平均粒度和晶粒度随球磨时间增加不断减小,而应变随球磨时间增加不断增大;当高能球磨54h后,球磨产物为FCC结构的鳞片状多晶体,晶粒度为17nm左右,应变为0.48%,颗粒尺寸为150～200nm;球磨时间增加至98h,产物中出现非晶相,但仍以多晶为主,晶粒尺寸为7nm,应变为1.24%,粉末团聚严重。     

Fabrication of nanocrystalline WC and nanocomposite WC–MgO refractory materials at room temperature

Reactant material powders of pure WO₃, Mg and graphite have been milled at room temperature using a high-energy ball mill. After a few kiloseconds of milling (11 ks), numerous fresh surfaces of the reactant materials are created as a result of the repeated impact and shear forces generated by the balls. After 86 ks of milling, a mechanical solid state reduction is successfully achieved between the fresh Mg and WO₃ particles to form a product of nanocrystalline mixture of MgO and W. A typical mechanical solid state reaction takes place between the W particles and graphite powders to obtain fine grains of nanocrystalline WC. Towards the end-stage of ball-milling (173 ks), the nanocrystalline MgO grains (10 nm) are embedded into the fine matrix of WC to form fine nanocomposite powders (1 μm in diameter) of WC–18% MgO material with spherical-like morphology. This composite powder was then consolidated under vacuum at 1963 K, with a pressure ranging from 19.6 to 38.2 MPa for 0.3 ks, using a plasma activated sintering method. In addition, pure nanocrystalline WC powders (7 nm in diameter) obtained by removing the MgO from the milled powders, using a simple leaching technique have been also consolidated by the same consolidation technique. The consolidation step does not lead to a dramatic grain growth and the compacted samples that are fully dense still maintain their unique nanocrystalline characteristics. The elastic properties and the hardness of both consolidated samples have been investigated. A model for fabrication of refractory nanocrystalline WC and nanocomposite WC–18% MgO materials at room temperature is proposed.     

Preparation of nanostructure silver powders by mechanical decomposing and mechanochemical reduction of silver oxide

The mechanical decomposing and mechanochemical reductions of silver oxide for preparation of nanocrystalline silver powders by high planetary ball mill was investigated. XRD and HRSEM techniques were used to characterize the structural evolution and morphological changes of products. The results show that the nanostructured silver with an average crystallite size of 14 nm and internal strain of 0.75% is synthesized by mechanical decomposing of Ag₂O after 95 h milling. While, the product of mechanochemical reduction of silver oxide using graphite after 22 h milling is nanostructured silver with an average crystallite size of 28 nm and internal strain of 0.44%.