Uilame Umbelino Gomes

The influence of the dispersion technique on the characteristics of the W-Cu powders and on the sintering behavior

Abstract The influence of the dispersion technique on the sintering behavior of W–Cu composites is investigated. The dispersion of the copper phase and the fineness of the tungsten phase mostly influence the sinterability of W–Cu powders. By using high energy milling, particles containing very fine tungsten grains embedded in copper, called composite particles, can be produced. A powder consisting of composite particles has an optimal Cu dispersion and a very fine tungsten phase. This kind of powder improves significantly the sinterability of the W–Cu system in solid and in liquid states. Relative densities above 94% can be obtained easily for such powders at temperatures above the Cu melting point and for Cu contents as low as 19% in volume, even for short sintering times. A homogeneous fine structure is obtained. The sintering kinetics of such powders is described.

Thermal decomposition of illite

The effect of heat treatment on illite in air at temperatures ranging from 750 to 1150 °C was studied using the Mossbauer effect in 57Fe. The dependence of the Mossbauer parameters and relative percentage of the radiation absorption area was measured as a function of the firing temperature. The onset of thermal structural decomposition occurred at 800 °C. With rising temperature, the formation of hematite (Fe2O3) increased at the expense of the silicate mineral.

Microstructural and electrical properties of sintered tungsten trioxide

Tungsten trioxide sintered wafers were prepared from WO3 powder obtained when ammonium paratungstate is decomposed in air at moderate temperature. Two wafer series of five samples were sintered under the same conditions in the temperature range 600–1000 °C. One of these wafers series was submitted to a subsequent annealing at 700 °C under a hydrogen atmosphere. All samples were characterized at room temperature by X-ray diffraction and electrical measurements. X-ray spectra show that WO3 ceramic presents a mixture of the triclinic and monoclinic phases before the reduction process. After the reduction process, WO2 and four hydrogen tungsten bronze phases are present in wafers. Capacitance measurements showed that the samples submitted only to the sintering process changed the dielectric constant with the frequency according to the Debye model. The reduced WO3 shows a semiconductor behavior, as determined by electrical resistivity measurements.

Two-Step Sintering Applied to Ceramics

During the process of sintering of ceramics, it is necessary to apply high temperature owing the high melting point of the raw materials. In general, a ceramist, wishing to produce a material with particular properties, must identify the required microstructure and then design processing conditions that will produce this required microstructure (Lutgard et al., 2003). One of the options to adapt the microstructure is a technique called two step sintering (TSS), this technique has been applied to the sintering of ceramic oxides to achieve full density without grain growth in final stage of sintering without loss densification (Chen & Wand, 2000). The two-step sintering process consists in to heat a ceramic body to a peak temperature (T1) to achieve an intermediate density and then the temperature is reduced to a dwell temperature (T2), which is held till full density is achieved. To succeed in two-step sintering, a sufficiently high relative density (70% or greater) needs to be achieved at T1 (Chen & Wang, 2000 & Chen, 2000). Once this critical density is reached, a lower temperature, T2, used for the isothermal hold will be sufficient to achieve full density. Difference between kinetics of grain boundary diffusion and grain boundary migration is used to obtain almost full dense, nanostructured ceramics. During the last stage of the sintering occur the grain growth in materials, this implicate in final properties, like mechanical resistance, density, ionic and electrical conductivity and others (Robert et al., 2003). The two-step sintering has been applied in many mateirals with the main goal of avoiding the grain growth in final stage of sintering, the results show the TSS is a technique efficient for it. Some application for two-step sintering are materials which need high density and small grain size, for example electrolytes of solid oxide fuel cell, as ceramics based in Y2O3 and CeO2, both with and without doppant (Wang et al., 2006; Wright, 2008 & Lapa, 2009). Others examples in which TSS are used also as nanostructural fosterite (Fathi, et al., 2009), alumina-zirconia ceramics (Wang et al., 2008), TiBaO3 and Ni-Cu-Zn Ferrite (Wang et al., 2006), ZnO (Shahraki et al., 2010). In this cases, the researchs are getting the relative density higher than 97% and the size grains in level sub-micrometer.

The Influence of the Milling Media Environment on the Characteristics of a W-Cu Composite Powder

W-Cu composite powders were prepared by high energy milling under two milling environments: cyclohexane and air. Composite particles are formed in both cases. The W particles are fragmented and embedded into the Cu particles. Both, W and Cu, are heavily strained, mainly in the first hours of milling. The composite powder has high homogeneity and is much finer than the original Cu powder. The mean particle size of the powders milled in both conditions is very close, but the wet milling was near 25% longer than dry milling and the size distribution is wider. This is consequence of the higher milling intensity of dry milling.

Estudo comparativo da queima rápida com a queima tradicional nas propriedades de materiais cerâmicos de base argilosa

In industrial processes that use high temperatures, greater fire great can reduce the cost of production and increase productivity. The use of faster and more efficient fire cycles has been little investigated by the structural ceramic industry in Brazil. As clay materials are submitted to high temperatures, pores are sealed by mass transport mechanisms and a series of thermally activated processes involving the initial constituents. The reactions that take place during firing depend on the nature and composition of the clay materials used. The firing cycle must be tuned to assure the technological performance required for each product, but at the same time minimize the consumption of energy. Samples were pressed under 25 MPa, dried and fired in an electric furnace. The heating rates selected were 2 °C/min (slow cycle) and 20 °C/min (fast firing). The holding time at the sintering temperatures (950 °C and 1050 °C) were null. This paper presents for the first time the use of a fast fire rate for raw materials using a traditional slow cycle and fast cycle were tested from physical and mechanical properties of the burned pieces were measured, with emphasis on water absorption and linear shrinkage tests. Microstructural analysis was performed using X-ray diffraction. The results show that is possible to obtain products with equivalent water absorption values and in enhanced mechanic properties when is used the fast firing.

Influence of the Addition of Rare-Earth Elements on the Sintering of Cemented Carbide

This work presents the results of a study concerning the influence of the addition of rareearth elements (La2O3 and CeO2) on the sintering of the WC/10Co cemented carbide. Several WC/10Co mixtures containing up to 3 wt.% rare-earth of the cobalt phase were prepared. Specimens were uniaxially pressed at 200 MPa, and sintered in a vacuum furnace at 1400 °C during 60 minutes. The sintering behaviour was accompanied by the linear shrinkage, density, and mechanical strength. The development of the microstructure was followed by XRD and SEM. The results showed that the sintering behaviour of the WC/10Co cemented carbide was influenced by adding of rare-earth element. In addition, the lanthanium oxide addition was more effective on the improvement of the physical-mechanical properties of the studied carbide.

Characterization of Prealloyed Nb–Ta–Al Powders for Capacitors

Abstract This paper describes the production of various prealloyed Nb–Ta–Al powders through the simultaneous aluminothermic reduction of Nb 2 O 5 and Ta 2 O 5 and subsequent comminution, with the objective of producing highly porous anodes for application in electrolytic capacitors. The morphological, chemical, and electrical characteristics of the powders are described. These powders, which have low production costs, have low apparent density and high specific surface as well as a high specific charge of 12,992 to 25,253μF · V · g −1 .

Study on sintered anodes of Nb–Ta–Al aluminothermic alloys

Abstract This work presents the results of a study concerning the morphological and electrical properties of sintered anodes of Nb–Ta–Al aluminothermic alloys. These alloys have potential application in electrolytic capacitors. The compacting behavior of the Nb–Ta–Al powders was evaluated during uniaxial pressing in a steel die. Anodes with a low green density (compacting pressure below 60 MPa) were sintered in vacuum ( ∼10 −4 Pa) at temperatures between 1300 and 1600°C for 15, 40 and 60 min. Significant changes of the morphological and electrical properties occurred during sintering. The Nb–Ta–Al electrolytic capacitors gave high specific charge from 14310 to 27807 μ FV/g.

Improved, Photon Conversion Efficiency of (SnO2) Doped Cesium Oxide (Cs) Nanofibers for Photocatalytic Application Under Solar Irradiation

We report the synthesis of high quality Tin Oxide (SnO2) doped Cesium Oxide (Cs) nanofibers by hydrothermal method at room temperature. Composition, structure and morphology of the nanofibers were analyzed by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). XRD assessed the crystal structure of the nanoplatelets which identified peaks associated with (110), (101) and (200) planes of hexagonal wurtzite-type SnO2 with lattice constants of a = b = 3.249 A and c = 5.219 A. XRD results also indicated that the crystalline properties of the doped samples were improved without affecting the parent lattice. The morphological and optical properties (SnO2) doped (Cs) nanosamples were characterized by HRTEM and UV-vis spectroscopy. The IR results showed high purity of products and indicated that the nanoplatelets are made up of Sn-O2 and Cs bonds. Absorption spectra exhibited an upward shift in characteristic peaks caused by the doping material, suggesting that crystallinity of both doped compounds is improved due to specific doping level. The Photoluminescence (PL) spectra are dominated by a strong narrow band edge emission tunable in the blue region of the visible spectra indicating a narrow size distribution of SnO2/Cs nanofibers.