Sintering and joining of low temperature co-fired tungsten and aluminum oxide

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2006.Includes bibliographical references (p. 181-189).Conventional methods used to fabricate co-fired tungsten/alumina composites usually rely on high temperature processing (>1500C). As it would be beneficial or even necessary for some applications to produce such composites at relatively low firing temperatures, low-temperature processing techniques and the attendant knowledge of processing-property relationships need to be developed. In this thesis, a set of experiments and simulations are performed to obtain a better understanding of sintering and joining of the tungsten/alumina system processed at temperatures near or below 12000C. The technique of activated sintering for tungsten is investigated, whereby a minimal content of additives enables low firing temperatures through a change in the sintering mechanism for tungsten. Tungsten compacts produced by this method are found to sinter only to the "initial stage" and are characterized by high residual porosity level. Hardness and fracture toughness of such partially-sintered materials are examined experimentally and analytically, and dependence of mechanical properties on the relative particle neck size is observed. Various studies are carried out to examine both fundamental and practical aspects of joining co-fired tungsten/alumina.(cont.) First, contributions to adhesion of co-sintered bilayers are studied where the properties of the tungsten layer are controlled using the process of activated sintering. Using a bending delamination test, improvements in sintered density of tungsten are found to increase the adhesive strength of the system only up to a point, beyond which shrinkage mismatch compromises the intrinsic toughness of the interface. A study of low-temperature co-fired tungsten/alumina is then focused on composite shells for an investment casting application. The influences of various processing parameters in a slurry-based route on the sintering and adhesion properties of tungsten/alumina are investigated. Binder content, stucco sand application, and powder characteristics are among the parameters found to critically control the quality of tungsten/alumina shells produced. Finally, the feasibility of several joining strategies, which involve the use of chemical additives, is examined on co-fired tungsten/alumina compacts processed at low temperatures. Some bonding techniques are verified to help improve the bonding of the co-sintered composites.by Yuttanant Boonyongmaneerat.Ph.D

Magnetic Material with Large Magnetic-Field-Induced Deformation

A magnetic materials construct and a method to produce the construct are disclosed. The construct exhibits large magnetic-field-induced deformation through the magnetic-field-induced motion of crystallographic interfaces. The construct is a porous, polycrystalline composite structure of nodes connected by struts wherein the struts may be monocrystalline or polycrystalline. If the struts are polycrystalline, they have a bamboo microstructure wherein the grain boundaries traverse the entire width of the strut. The material from which the construct is made is preferably a magnetic shape memory alloy, including polycrystalline Ni-Mn-Ga. The construct is preferably an open-pore foam. The foam is preferably produced with a space-holder technique. Space holders may be dissolvable ceramics and salts including NaAlO2

Recent Developments in Ni-Mn-Ga Foam Research

Grain boundaries hinder twin boundary motion in magnetic shape-memory alloys and suppress magnetic-field-induced deformation in randomly textured polycrystalline material. The quest for high-quality single crystals and the associated costs are a major barrier for the commercialization of magnetic shape-memory alloys. Adding porosity to polycrystalline magnetic-shape memory alloys presents solutions for (i) the elimination of grain boundaries via the separation of neighboring grains by pores, and (ii) the reduction of production cost via replacing the directional solidification crystal growth process by conventional casting. Ni-Mn-Ga foams were produced with varying pore architecture and pore fractions. Thermo-magnetic training procedures were applied to improve magnetic-field-induced strain. The cyclic strain was measured in-situ while the sample was heated and cooled through the martensitic transformation. The magnetic field-induced strain amounts to several percent in the martensite phase, decreases continuously during the transformation upon heating, and vanishes in the austenite phase. Upon cooling, cyclic strain appears below the martensite start temperature and reaches a value larger than the initial strain in the martensite phase, thereby confirming a training effect. For Ni-Mn-Ga single crystals, external constraints imposed by gripping the crystal limit lifetime and/or magnetic-field-induced deformation. These constraints are relaxed for foams

A study of tin-free activation process for electroless nickel plating on polyurethane foam

Abstract -In the present study, a tin-free activation process has been studied for electroless nickel plating on polyurethane (PU) foam. The effect of HCl concentration (2-6 molar), PdCl 2 concentration (0.2-1.8 g/l), and the type of reducing agents (sodium hypophosphite and hydrazine) on the properties of the activated surface and the deposition of Ni were investigated by scanning electron microscope (SEM), energy dispersive X-ray spectrometer (EDX), X-ray photoelectron spectroscopy (XPS). The use of HCl concentration ≥ 6 M resulted in partial destruction of the PU foam. More uniform coating and higher amount of Ni deposit were obtained with the higher amount of PdCl 2 during activation. The XPS results revealed that the chemical state of nickel on the Ni/PU foam were Ni 2+

Superhydrophobic surface modification for corrosion protection of metals and alloys

Modifying a surface to achieve a superhydrophobic characteristic has increasingly become an attractive approach to protect oxidation of metals and alloys. This review comprehensively discusses the state of the art of superhydrophobic surface modification with a particular focus on the corrosion inhibiting quality. Generally, there are two main approaches to induce superhydrophobicity, namely application of low surface energy coating and roughening of a surface to a binary micro/nano-rough structure. These strategies, which are often combined and found to complement one another, have been successfully demonstrated to provide corrosion-resistant improvement with moderate durability for a number of metals and alloys, including stainless steels, aluminum, magnesium, titanium, copper, and zinc

TEPNET: An industry cluster initiative for advancement of electroplating activities in Thailand

TEPNET is a new cluster initiative from the electroplating industry in Thailand. It is build up in a triple helix structure from industry, government agencies and institutes

Increasing Magnetoplasticity in Polycrystalline Ni-Mn-Ga by Reducing Internal Constraints Through Porosity

Foams with 55% and 76% open porosity were produced from a Ni-Mn-Ga magnetic shape-memory alloy by replication casting. These polycrystalline martensitic foams display a fully reversible magnetic-field- induced strain of up to 0.115% without bias stress, which is about 50 times larger than nonporous, fine-grained Ni-Mn-Ga. This very large improvement is attributed to the bamboolike structure of grains in the foam struts which, due to reduced internal constraints, deform by magnetic-field-induced twinning more easily than equiaxed grains in nonporous Ni-Mn-Ga

Investigation of electrodeposited Ni-based coatings for biodiesel storage

Biodiesel commonly experiences oxidative and hydrolytic degradation, leading to problems of low storage stability and corrosion of fuel containers. This study investigates the fabrication and use of electrodeposited nickel and nickel-tungsten alloys as potential coating materials that effectively protect steel-based biodiesel containers from corrosion. Through long-term static immersion, surface analyses of the coatings, and assessments of the biodiesel's acidity, it is determined that the electrodeposited nickel is well compatible with biodiesel, whereas the addition of tungsten deteriorates the corrosion resistance of nickel. Tight control of the tungsten content in nickel deposits is required to enhance both mechanical integrity and corrosion resistance of the deposits exposed to biodiesel environments.Electroplating Biofuel Oxidation stability Corrosion