RESUMO A MP é constituída de, pelo menos, três etapas: obtenção do pó, conformação e sinterização. Na etapa de obtenção dos pós uma técnica de amplo uso é a Moagem de Alta Energia (MAE) que além de propiciar a quebra das partículas preexistentes, também pode melhorar a dispersão dos componentes. Um material obtido pela técnica é WC-Co, o metal duro, que apesar de ter propriedades interessantes, como por exemplo, alta dureza e resistência à compressão, não consegue manter tais características quando colocado em trabalho em altas temperaturas, como o corte na usinagem. Como alternativa ao sistema compósito WC-Co, estuda-se a substituição do cobalto pelo níquel, entretanto, o compósito WC-Ni apresenta uma baixa dureza quando comparado ao WC-Co. Para minimizar tal efeito, o presente estudo investigou a possibilidade da adição do óxido de alumínio (Al2O3) aos pós de WC e Ni. O estudo estabeleceu a influência da moagem de alta energia (MAE) nas características dos pós compósitos de WC-Ni-Al2O3 com 5 e 10%p de Al2O3. Fez-se moagem de alta energia (400 rpm) em via úmida pelos tempos de 1, 2, 4, 10 e 20h. As amostras foram caracterizadas por: DRX, particulometria a laser e MEV/EDS. Os resultados mostraram-se promissores uma vez que houve redução de até 50% do tamanho mediano de partícula com os tempos estudados, bem como, formação de partículas compósitas.

ABSTRACT The PM consists of at least three stages: obtaining the powder, consolidation and sintering. In the stage of obtaining the powders, a widely used technique is the High Energy Ball Milling (HEM) which, besides providing the breakdown of the pre-existing particles, can also improve the dispersion of the components. A material obtained by the technique is WC-Co, the hard metal, which despite having interesting properties, such as, for example, high hardness and resistance to compression, cannot maintain such characteristics when put to work in the presence of high temperatures, such as cutting in machining. As an alternative to the WC-Co composite system, the replacement of cobalt by nickel is studied, however, the WC-Ni composite has a low hardness when compared to the WC-Co. To minimize this effect, the present study investigated the possibility of adding aluminum oxide (Al2O3) to WC and Ni powders. The study established the influence of high energy ball milling (MAE) on the characteristics of the composite powders WC-Ni-Al2O3 with 5 and 10% w of Al2O3. HEBM (400 rpm) was carried out in a wet way, and samples were taken at 1, 2, 4, 10 and 20h. The samples were characterized by: XRD, laser granulometry and SEM / EDS. The results were promising since there was a reduction of up to 50% in the median particle size with the studied times, as well as the formation of composite particles.

The use of niobium containing materials' has gained much attention of the scientific community in the late years due its various applications in diverse fields. NbC is a highly versatile material. Copper addition may alter several of its properties, such as morphology, crystal structure etc. as well as enhance its catalytic behavior. Nanostructured NbC with copper addition synthesis' presented here had the precursor [(NH4)3[NbO(C2O4)3]xH2O] as starting material, which was doped with Cu(NO3)2 at 5% and 10% (molar) ratios. Doped NbC was obtained via gas solid reaction in fixed bed reactor at lower temperature (980°C) and with shorter reaction time (2h) than traditional methods. Reaction products' were characterized by XRD, crystal sizes were estimated according to HWL method, and SEM, XRF, BET and laser particle size analysis were performed. XRD indicated the formation of NbC and Cu phases with cubic crystal structure of ~20nm. SEM showed slight morphological change upon increasing copper content, indicating a less porous structure, which is consistent with BET data (43.7m2/g for 5%Cu-NbC and 37m 2/g for the 10% Cu-NbC). Crystal size calculations showed that increasing dopant content particle sizes were also increased, probably due to the presence of the dopant, in some extent, in the crystal structure.

WC/Co is widely used as cutting tools, because has a unique combination of high strength, hardness, toughness, and moderate stiffness, especially with fine grained WC and finely distributed cobalt. WC/Co powder mixture sinters by different methods such as vacuum sintering, microwave sintering and SPS. High pressure high temperature (HPHT) sintering is a proposed method that can result in better distribution of cobalt and avoid undesirable phases by using high pressure, high temperature and very short sintering time. In this study, a powder mixture of WC- 10 wt% Co was sintered by HPHT at 1500 to 1900ºC under a pressure of 7.7 GPa for 2 minutes. Microstructural/structural analyses were performed by SEM/EDS. Hardness and compression test were also done to obtain the effect of sintering parameters. It was found that HPHT sintering method can be used to produce WC/Co hardmetal with low sintering time and high production rate. It was realized that increasing sintering temperature in HTHP sintering method results in increasing density but hardness and compression strength increase by increasing sintering temperature up to 1800 ºC and then decrease.

The objective of this work is obtaining W-30 wt% Cu composite powder from ammonium paratungstate hydrated (APT) and hydrated copper nitrate and investigating the influence of some production process parameters of W- 30 wt% Cu in particles form which results in high sinterability and densification of W-30 wt% Cu composite. To achieve this objective, the powder was obtained by manually mixing the APT and copper nitrate and reducing under a hydrogen atmosphere at 800 °C and then compacting at 500 MPa and sintering in a tubular furnace at 1200 °C for 60 minutes. The obtained materials were characterized by XRD, SEM, EDS, particle size analysis, density and microhardness measurements. The composite powder showed a good homogenization of Cu in W with very fine and agglomerated particles and a mean crystallite size of 25.64 nm. SEM coupled with EDS with mapping analysis revealed a homogeneous distribution of Cu and W in the sintered sample. The composite had a relative density of 96.77% and a microhardness of 523.66 HV. Therefore the method of obtaining the composite powder is feasible to sinter W-30 wt% Cu powder due to a greater dispersion and homogenization of phases and the average particle size.

The present paper's objective was to synthesize and characterize both the oxalic niobium precursor (NH4)3NbO(C2O4)3.H2O, and the product of its doping with Cu and calcining, CuNb2O6. In order to obtain the niobium precursor, commercially available niobium oxide (Nb2O5) was subject to fusion with potassium bisulfate (KHSO4). Once leached with water, the powder was complexed with oxalic acid and ammonium oxalate. The as produced material was manually mixed with copper nitrate Cu(NO3)2.2H2O (25% Cu molar) and calcined at 1000°C in a muffle furnace. CuNb2O6 was then obtained. The precursor was characterized by XRD, SEM, FT-IR, XRF, TG/DTG. The calcination product was characterized by XRD, XRF and SEM. Results show that single phase CuNb2O6 could be obtained by this method without Nb2O5 contamination.

São apresentadas as propriedades dielétricas de cerâmicas diatomita-titânia sinterizadas. Foram determinadas capacitância específica, fator de dissipação, fator de qualidade e constante dielétrica como função da temperatura de sinterização, percentagem de titânia e freqüência. O coeficiente de temperatura e a capacitância são medidos como função da freqüência. Além da corrente de fuga, a influência da tensão dc aplicada sobre a resistência de isolamento e a rigidez dielétrica foram estudadas. Os resultados mostram que as composições diatomita-titânia podem ser usadas como um dielétrico alternativo.

Dielectric properties of sintered diatomite-titania ceramics are presented. Specific capacitance, dissipation factor, quality factor and dielectric constant were determined as a function of sintering temperature, titania content and frequency; the temperature coefficient of capacitance was measured as a function of frequency. Besides leakage current, the dependence of the insulation resistance and the dielectric strength on the applied dc voltage were studied. The results show that diatomite-titania compositions can be used as an alternative dielectric.