Alloy ionizer fabrication Summary report

Fabrication methods of porous refractory ionizers from spherical powders of iridium-tungsten, and rhenium-tungste

Understanding the negative thermal expansion in planar graphite–metal composites

The addition of graphitic fibers and flakes as fillers is commonly used to control the thermal expansion of metals. Sintered metal matrix composites with a planar distribution of graphite flakes show a low or negative thermal expansion coefficient perpendicular to the orientation plane of the graphite (z-CTE). Since the metal matrix has a positive isotropic expansion and graphite has a high z-CTE, this effect cannot be explained by a simple model of stapled metal–graphite layers. Instead, a mechanical interaction between graphite and matrix must be considered. With neutron scattering measurements, we show that there is little or no strain of the graphite flakes caused by the matrix, which can be explained by the high modulus of graphite. Instead, we suggest that a macroscopic crumpling of the flakes is responsible for the low z-CTE of the composite. The crumpled flakes are thicker at low temperature and get stretched and flattened by the expanding matrix at high temperature, explaining the reduction in the thermal expansion across the orientation plane

Cross-sectional analysis of W-cored Ni nanoparticle via focused ion beam milling with impregnation

Tungsten and nickel bimetallic nanoparticle is synthesized by radio frequency thermal plasma process which belongs to the vapor phase condensation technology. The morphology and chemical composition of the synthesized particle were investigated using the conventional nanoparticle transmission electron microscopy (TEM) sample. A few part of them looked like core/shell structured particle, but ambiguities were caused by either TEM sample preparation or TEM analysis. In order to clarify whether a core/shell structure is developed for the particle, various methodologies were tried to prepare a cross-sectional TEM sample. Focused ion beam (FIB) milling was conducted for cold-compacted particles, dispersed particles on silicon wafer, and impregnated particles with epoxy which is compatible with electron beam. A sound cross-sectional sample was just obtained from cyanoacrylate impregnation and FIB milling procedure. A tungsten-cored nickel shell structure was precisely confirmed with aid of cross-sectional sample preparation method

Infiltration sintering of WCu alloys from copper-coated tungsten composite powders for superior mechanical properties and arc-ablation resistance

W70Cu30(W-30 wt.% Cu) alloys were fabricated using cold pressing and infiltration sintering methods from two types of powders, i.e., mixed copper-tungsten (M-Cu-W) powders and our newly developed copper-coated tungsten composite (Cu@W) powders. Microstructure, mechanical and arc-ablation properties of the W70Cu30 alloys were investigated, and the mechanism of enhanced physical/mechanical properties and arc-erosion resistance of the W70Cu30 alloys was discussed. For the W70Cu30 alloys prepared using the Cu@W powders, their physical properties, including hardness, electrical conductivity and relative density were much better than those prepared from the M-Cu-W powders. The W70Cu30 alloys fabricated from the Cu@W powders were free of cracks, and showed homogenous distributions of W and Cu network structures. Whereas for the alloys prepared from the M-Cu-W powders, segregation of Cu was observed and the segregation size was about 40–100 μm. Characterization of arc-erosion morphologies of the W70Cu30 alloys prepared with the Cu@W powders revealed the occurrence of evaporation of Cu phase; whereas that of W70Cu30 alloys prepared with the M-Cu-W powders revealed the occurrence of the sputtering of Cu. After arc breakdown for 200 times, mass loss of alloys made using the mixed powders was twice as much as those made using the coated composite powders. Based on the experimental results and theoretical analysis, an arc breakdown mechanism of the WCu-C alloys using the composite powders was proposed which is attributed to the formation of a homogeneous Cu-Cu network structure to uniformly disperse arc energy and dissipate the generated heat, thus prolonging the service life of the WCu alloy contacts

Mechanistic Undrestanding Of Amorphization In Iron-Based Soft Magnetic Materials

Iron-based magnetic alloys possess very good magnetic and mechanical properties. Among these alloys Fe-Si-B-based alloys show outstanding saturation magnetization and coercivity which makes them great candidates for many industrial applications. Addition of certain elements to the Fe-Si-B base is proven to improve the homogeneity and fineness of microstructure as well as enhance the magnetic behavior of these alloys. In this research work, we have studied the effect of adding copper and niobium to the Fe-Si-B base alloy. Previous studies have shown that magnetic alloys show better magnetic properties when their microstructure consists of nanocrystals embedded in an amorphous matrix. In order to reach amorphization, magnetic alloys are traditionally melted and then cooled down very fast to prevent crystallization and grain growth in their microstructure. However, there are several disadvantages associated with this method of fabrication, such as the limitation in thickness of the products. To solve this issue, we proposed a new method of fabrication for magnetic alloys where amorphization occurs through mechanical alloying, and the amorphous powder alloy that is produced by this process is then consolidated using a technique called spark plasma sintering finding appropriate mechanical alloying processing parameters to get an amorphous structure. Many different processing parameters were investigated, and the mechanical properties, microstructure, and magnetic properties of all samples were examined. The effect of spark plasma sintering processing parameters on samples sintered from the amorphous powders was then studied. Finally, the amount of energy introduced to the powder from the milling balls during the mechanical alloying process was calculated. We were able to find a trend between the energy introduced to the powder during the milling process and the amorphous structure of the milled powders. From our data, we draw an energy map that shows the window of total energy in which the powder, regardless of the mechanical alloying processing parameters under which it was milled, will show an amorphous structure. This area has not been explored for these magnetic alloys before, and this data can be used by researchers who are trying to obtain amorphization via the mechanical alloying process

Synthesis of Nano-dispersed TiO2 Zirconium Based Alloy by Mechanical Alloying

The present work aims to improve the mechanical properties of Zr- alloy (Fe-30-Ni-20-Mo-4 TiO2-2-Zr-44) (all are in wt. %) synthesized by mechanical alloying and consolidated by conventional sintering. A uniform dispersion of TiO2 tends to improve the mechanical properties of the present alloys. The current alloys show improvement of hardness value which is 1-1.5 times higher than that of the conventional zircaloy. For this purpose, pure elemental powders of Zr, Ni, Fe, Mo and TiO2 were blended and milled in a planetary ball mill for 20 h and sintered at 1400°C for 2 h in argon atmosphere. Phase evolution of milled powders at different stages of milling (0h, 1h, 5h, 10h, 15h and 20h) and after sintering of the current alloy were analyzed by X- ray diffraction techniques. The grain morphology or crystallite size of various stages of milled powders was characterized by scanning electron microscope. The crystallite size, lattice strain and lattice parameters were analyzed by Williamson-Hall method. The crystallite size decreases rapidly up to 5hours of milling and becomes almost constant with further milling. Addition of TiO2 (2 wt %) in the matrix improves the hardness. This is due to the high modulus of elasticity of TiO2

Stabilisation of Fe based nano-structures developed through mechanical milling

Synthesis of Fe based nanostructures developed by high energy ball milling has been carried out in the present study. Mixtures of Iron and Yttrium in the desired quantities were ball milled in stainless steel grinding media to prepare the required nanostructures. The powdered mixtures of 3 different compositions obtained were then pelletized in a pellet pressure machine. All the alloys were annealed at different temperatures up to 1200ºC under high purity argon atmosphere. XRD analysis and micro hardness measurement were carried out for all the specimens to analyze their stability after annealing and to investigate the grain size stabilization cased by adding minute amounts of Yttrium. Lattice strain and crystallite size values calculated by using Scherer formula were analysed. Stable crystallite size (less than 100 nm) and constant micro hardness of the annealed samples can dictate its usefulness for high strength and other functional applications

Materials review for improved automotive gas turbine engine

The potential role of superalloys, refractory alloys, and ceramics in the hottest sections of engines operating with turbine inlet temperatures as high as 1370 C is examined. The convential superalloys, directionally solidified eutectics, oxide dispersion strenghened alloys, and tungsten fiber reinforced superalloys are reviewed and compared on the basis of maximum turbine blade temperature capability. Improved high temperature protective coatings and special fabrication techniques for these advanced alloys are discussed. Chromium, columbium, molybdenum, tantalum, and tungsten alloys are also reviewed. Molbdenum alloys are found to be the most suitable for mass produced turbine wheels. Various forms and fabrication processes for silicon nitride, silicon carbide, and SIALON's are investigated for use in highstress and medium stress high temperature environments

WC – (Cu: AISI304) composites processed from high energy ball milled powders

Alternative binders to cobalt, based on stainless steel (SS, AISI304) and copper were investigated for tungsten carbide (WC) based cemented carbides. The binder content was fixed at 12 wt%, and the Cu:SS ratio varied in proportions of 0:1, 1:5, 1:2, 1:1, 1:0. High energy ball milling was applied to ensure high homogenization, nanometric particle size and mechanical alloying of binder elements in the powders' mixtures. To assess an adequate sintering route, wettability testing and constant heating rate dilatometry in vacuum were performed. The composites were analyzed in terms of their structural, microstructural and mechanical characteristics. The poor wettability of melted Cu on WC surfaces was increased by alloying it with SS and highly dense compacts could be successfully attained at reduced vacuum sintering temperatures with binders having a Cu:SS ratio equal to or lower than 1:2. The microstructures show secondary phases and significant grain coarsening during sintering, whereas the average grain size was kept in the nanometric range. The composites that attained almost full densification present high hardness, comparable to that of nanometric WC-12Co cemented carbides processed by similar routes, but lower toughness values.publishe

Principles and applications of CVD powder technology

Chemical vapor deposition (CVD) is an important technique for surface modification of powders through either grafting or deposition of films and coatings. The efficiency of this complex process primarily depends on appropriate contact between the reactive gas phase and the solid particles to be treated. Based on this requirement, the first part of this review focuses on the ways to ensure such contact and particularly on the formation of fluidized beds. Combination of constraints due to both fluidization and chemical vapor deposition leads to the definition of different types of reactors as an alternative to classical fluidized beds, such as spouted beds, circulating beds operating in turbulent and fast-transport regimes or vibro-fluidized beds. They operate under thermal but also plasma activation of the reactive gas and their design mainly depends on the type of powders to be treated. Modeling of both reactors and operating conditions is a valuable tool for understanding and optimizing these complex processes and materials. In the second part of the review, the state of the art on materials produced by fluidized bed chemical vapor deposition is presented. Beyond pioneering applications in the nuclear power industry, application domains, such as heterogeneous catalysis, microelectronics, photovoltaics and protection against wear, oxidation and heat are potentially concerned by processes involving chemical vapor deposition on powders. Moreover, simple and reduced cost FBCVD processes where the material to coat is immersed in the FB, allow the production of coatings for metals with different wear, oxidation and corrosion resistance. Finally, large-scale production of advanced nanomaterials is a promising area for the future extension and development of this technique