Materials Science Forum Vols. 534-536

Abstract: Recently alkali oxide materials, such as sodium - potassium niobate have drawn much attention due to their ultrasonic applicability and are also considered as promising candidates for a piezoelectric lead-free system. However, it is difficult to sinter such NKN-based materials via conventional sintering process. Therefore, in this study, dense 0.95(Na0.5K0.5)NbO3-0.05LiTaO3 (NKN-5LT) ceramics were developed by conventional sintering process. Sintering temperature was lowered by adding Li2O as a sintering aid. The electrical properties of NKN-5LT ceramics were investigated as a function of Li2O concentration. At the addition of 1 mol% Li2O, electromechanical coupling factor (kP) and piezoelectric coefficient (d33) of NKN-5LT ceramics were found to reach the highest values of 0.37 and 250 pC/N, respectively.

Abstract: A steel/cemented carbide couple is selected to generate a tough/hard two layers material. The sintering temperature and composition are chosen according to phase equilibria data. The choice of optimal sintering conditions needs experimental studies. First results evidence liquid migration from the hard layer to the tough one, leading to porosity in the hard region. The study of microstructure evolution during sintering of the tough material (TEM, SEM, image analysis) evidences the coupled mechanisms of pore reduction and WC dissolution, and leads to temperature and time ranges suitable to limit liquid migration. The sintering of the two layer material is then shown to need further compromises to avoid interface crack formation due to differential densification.

Abstract: Two approaches for the fabrication of tailored powder composites with specially distributed pore-grain structure and chemical composition are investigated. Electrophoretic Deposition (EPD) followed by microwave sintering is employed to obtain functionally graded materials (FGM) by in-situ controlling the deposition bath suspension composition. Al2O3/ZrO2 and zeolite FGM are successfully synthesized using this technique. In order to fabricate an aligned porous structure, unidirectional freezing followed by freeze drying and sintering is employed. By controlling the temperature gradient during freezing of powder slurry, a unidirectional ice-ceramic structure is obtained. The frozen specimen is then subjected to freeze drying to sublimate the ice. The obtained capillary-porous ceramic specimen is consolidated by sintering. The sintering of the graded structure is modeled by the continuum theory of sintering.

Abstract: Most materials produced today are monolithic structures that are heat treated to perform a particular function. Laser Powder Deposition (LPD) is a technology capable of modifying a metallic structure by adding the appropriate material to perform a desired function (e.g., wear and corrosion resistance). LPD offers a unique fabrication technique that allows the use of soft (tough) materials as base structures. Through LPD a hard material can be applied to the base material with little thermal input (minimal dilution and heat-affected-zone {HAZ}), thus providing the function of a heat treatment or other surface modifications (e.g., carburizing, nitriding, thermal spray and electroplating). Several materials (e.g., Stellite 6 &21, 316 SS, 420 SS, M4, Rex 20, Rex 121, 10V, AeroMet 100, CCW+, IN 625 and IN 718) have been deposited on to carbon steel (4140, 4340, 1566, 1018) substrates to provide various functions for a number of industrial applications. These surface modifications have been evaluated through standard wear testing (ASTM G-65), surface hardness (Rc), micro-hardness (vickers), and optical microscopy. The results from these evaluations will be presented along with several industrial application case studies.

Abstract: Thermoelectric elements using environment-friendly materials with high thermoelectric conversion efficiency and of these thermoelectric elements can be increased by using a structure combining n-type and p-type semiconductors. From the above point of view, attention was directed at ZnO as a candidate n-type semiconductor material and investigations were made. As the result, a dimensionless figure of merit ZT close to 0.28 (1073K) was obtained for specimens produced by the PCS (Pulse Current Sintering) method with addition of specified quantities of TiO2, CoO, and Al2O3 to ZnO. It was found that the interstitial TiO2 in the ZnO restrains the grain growth and CoO acts onto the bond between grains.

Abstract: The behavior of stoichiometric and near-stoichiometric NiAl at plasma spray deposition, without and with a bond coat, for coating layers realization on a low alloyed steel substrate has been investigated and is presented. In all variants, NiAl particle melting and their welding at the impact with substrate were observed. Furthermore, a relatively compact and adherent coating layer was formed and the NiAl was found to maintain its stability. These are all important factors for assuring the coating layer oxidation and corrosion resistance. Good results from the coating structure and adherence to the substrate points of view were obtained for the 45:55 Ni:Al composition, without a bond coat but adopting an Ar protective surrounding of plasma jet. The high resistance to corrosion of 45:55 NiAl composition was further validated through corrosion tests.

Abstract: Bismuth telluride material was fabricated using hot extrusion. The crystal structure was identified by X-ray diffraction and Scanning electron microscopy. The oriented texture in extrusion direction was formed. All the samples showed high mechanical strength. The thermoelectric properties were determined by measuring electrical resistivity, Seebeck coefficient and thermal conductivity. The influence of extrusion temperature on the thermoelectric performence was studied. The variation of figure with temperature was also studied. The oriented texture seems promising to improve thermoelectric properties and mechanical strength.

Abstract: Fundamental studies on the thermoelectricity have been mainly done in the pseudo binary systems of Mg2Si – Mg2Ge – Mg2Sn. In recent years, their thermoelectricity is revisited because of light-weight, low initial const and short turning back time in addition to high potential in figureof- merit for ZT approaching to unity or more. Conventional melting and solidification, or, normal PM routes fail in precise, wide-range control of chemical composition and microstructure control. New PM route via bulk mechanical alloying is developed to fabricate the solid solution semiconductive materials with Mg2Si1-xGex and Mg2Si1-ySny for 0 < x, y < 1 and to investigate their thermoelectric materials. Since Mg2Si is n-type and both Mg2Ge and Mg2Sn are p-type, pntransition takes place at the specified range of germanium content, x, and tin content, y. Through optimization of chemical composition, solid-solution type thermoelectric semi-conductive materials are designed both for n- and p-type materials. In addition, appropriate doping condition can be searched in the specified range of x and y.

Abstract: The encapsulated induction melting and hot pressing were employed to prepare the Fedoped CoSb3 skutterudites and their thermoelectric properties were investigated. Single phase δ- CoSb3 was successfully obtained by the subsequent heat treatment at 773K for 24 hours. Iron atoms acted as electron acceptors by substituting cobalt atoms. Thermoelectric properties were remarkably improved by the appropriate doping. Co0.7Fe0.3Sb3 was found as an optimum composition for best thermoelectric properties in this work.

Abstract: Ni-doped CoSb3 was prepared by the encapsulated induction melting and hot pressing, and its doping effects on the thermoelectric properties were investigated. Single phase δ-CoSb3 was successfully obtained by the subsequent heat treatment at 773K for 24 hours. Nickel atoms acted as electron donors by substituting cobalt atoms. Thermoelectric properties were remarkably improved by the appropriate doping.