R. H. G. A. Kiminami

Combustion synthesized ZnO powders for varistor ceramics

Abstract Commercial ZnO varistor ceramics are multicomponent, with minor amounts of added oxides that play important roles, both in the strict electrical sense and for the control of the microstructure. The present work describes the straightforward combustion synthesis of pure and doped ZnO powders from stoichiometric mixtures of the relevant water soluble metal nitrates as cation precursors and urea as fuel. The mixtures were ignited at 500°C resulting in a dry, very fine powder. The as-prepared combustion products, characterized by XRD, SEM, TEM and BET, show high specific surface area, have very small particle sizes and are crystalline, with atomic level homogeneity. Implications on sintering and electrical behaviour are discussed.

Effect of heating conditions during combustion synthesis on the characteristics of Ni0.5Zn0.5Fe2O4 nanopowders

Ni-Zn ferrite powders were synthesized by combustion reaction. The effect of external conditions of heating on the characteristics of the resulting powders was evaluated. Two synthesis routes were studied. The first involved preheating on a hot plate at 300°C and subsequently heating in a muffle furnace at 700°C (RCPM). In the second route the powders (RCP) were heated directly to 600°C on a hot plate until self-ignition occurred. The resulting RCP products were evaluated before and after attritor milling in order to reduce the size and increase the uniformity of particles and/or agglomerates. The resulting powders were characterized by X-ray diffraction (XRD), BET, scanning electron microscopy (SEM), helium pycnometry, sedimentation and transmission electron microscopy (TEM). The results showed that it was possible to obtain Ni-Zn ferrite powders using both routes and that the second route (RCP) was the most favorable in terms of obtaining powders with high surface area. The efficiency of the grinding was confirmed by the reduction of the size of the particles.

Electrical properties of polycrystalline ZnO : Cu obtained from freeze-dried ZnO+copper(II) acetate powders

Highly homogeneous mixtures of powders containing ZnO and monohydrated copper(II) acetate, (CH3COO)2CuċH2O, AcCuH2O were obtained by freeze drying. Thermal analysis and high-temperature X-ray diffraction showed that pure freeze-dried AcCuH2O powder decomposed to CuO, in air, at temperatures above 325 °C. Due to the polymeric nature of AcCuH2O, the powder mixtures were compacted without pressing additives. Pellets with ZnO+x mol % Cu (x=0.05, 0.5, and 5.0) compositions were sintered in air from 750 to 1150 °C for 1 h. After sintering, the density, shrinkage and mass loss increased as the concentration of AcCuH2O increased. A microstructural analysis of samples sintered at 950 °C for 1 h revealed Cu-doped ZnO grains with Cu-clusters of x=5.0 mol % Cu; the J–E curves showed that both the breakdown electric field, Ebr, and the nonlinearity coefficient, α, increased as x increased from 0.05 to 0.5. Complex acceptor defects involving Cu+Zn in the grain boundary are believed to have compensated for the n-type conductivity of the ZnO grains, giving rise to high Ebr values.

Microwave fast sintering of submicrometer alumina

Commercially available alumina powder with high-purity submicrometer particle size and narrow particle size distribution was fully densified by a microwave hybrid fast firing technique. The alumina compacts were surrounded by susceptor material, which helped the heating of the samples, and sintered in a microwave oven at a frequency of 2.45 GHz and a power level of 1.8 kW. The sintered samples reached densities of 99% in sintering cycles of 30 to 40 minutes, a much shorter time than conventional sintering processes. The sintered samples showed uniform microstructures with powder particle size/average grain size rations higher than 1:2.

Sintering of Ni-Zn ferrite nanopowders by the constant heating rate (CHR) method

The constant heating rate method employed in sintering studies offers several advantages over the isothermal method, particularly the fact that all the parameters that describe the sintering phenomena can be obtained from a single sample. The purpose of this work is to determine the parameters of sintering kinetics of nanosized Ni-Zn ferrite powders synthesized by combustion reaction. The nonisothermal sintering method was studied using a constant heating rate (CHR). The Ni-Zn ferrite powders, with average particle size varying from 18 nm to 29 nm, were uniaxially pressed and sintered in an horizontal dilatometer at a constant heating rate of 5.0 °C/min from 600 °C up to complete densification, which was reached at 1200 °C. The compacts were characterized by scanning electron microscopy (SEM). Experimental results revealed three different sintering stages, which were identified through the Bannister Theory. The shrinkage and the shrinkage rate analyzed showed a viscous contribution in the initial sintering stage, which was attributed to the mechanism of structural nanoparticle rearrangement.

Argilas bentoníticas de Cubati, Paraíba, Brasil: Caracterização física-mineralógica

O Estado da Paraiba possui jazidas de bentonitas utilizadas comercialmente em uma vasta gama de setores tecnologicos. No entanto, esses jazimentos estao se exaurindo apos dezenas de anos de exploracao. Assim, este trabalho tem por objetivo a caracterizacao fisico-mineralogica de bentonitas recentemente descobertas no municipio de Cubati, PB. As amostras estudadas foram secas a 60 oC e caracterizadas por meio de fluorescencia de raios X, picnometria de He, determinacao da distribuicao de tamanho de particulas, difracao de raios X, analise termica diferencial e gravimetrica e microscopia eletronica de varredura. Os resultados evidenciaram que as amostras sao bentonitas policationicas, apresentando teores de MgO, CaO e K2O semelhantes aos de outras bentonitas sul-americanas e que sao constituidas por argilomineral esmectitico e por quartzo e caulinita. As amostras apresentaram fracoes de particulas abaixo a 2 μm variando entre 31 e 41%, estando, no entanto, em elevado estado de aglomeracao.

Sinterização de cerâmicas em microondas. Parte I: aspectos fundamentais

Processing of materials based on heating by microwave energy has gained increasing importance in many industrial applications due to its potential advantages over conventional heating methods. In the sintering of ceramic materials, the use of microwave energy decreases the processing time, saves energy and improves the microstructural homogeneity of ceramic bodies. These advantages have motivated various research groups around the world to study microwave sintering. However, the benefits deriving from the use of microwaves depend on the control and scientific understanding of the parameters and aspects of the process. This paper offers a review of the fundamental scientific aspects of microwave sintering of ceramics, focusing on the interaction between materials and microwaves and highlighting particular points of the hybrid sintering process and methodology.

Síntese e caracterização de nanopartículas de TiO2

Several methods of chemical synthesis have been developed and used to obtain powder for production of ceramic membranes. Amongst the alternative methods used in laboratory scale, the Pechini method has been used successfully for the preparation of several types of materials. The objective of this work is to synthesize and characterize TiO2 powders obtained by this method, aiming the preparation of ultra-filtration ceramic membranes. The powder has been characterized by gravimetric and differential thermal analysis, X-ray diffraction, infrared spectroscopy, nitrogen adsorption by BET, and scanning electron microscopy. The X-ray diffraction of the powders showed the presence of the anatase crystalline phase, with crystallite size 15 nm. The particle size calculated from the surface area was 19 nm and the powder morphology shows the presence of soft agglomerates. These results evidence that the Pechini method is interesting for the production of nanometric TiO2 appropriate for ceramic membranes preparation.

Microwave Fast Sintering of Ceramic Materials

Microwaves are electromagnetic waves with wavelengths ranging from 1m to 1 mm, which correspond to frequencies between 0.3 and 300 GHz. This frequency range lies just above radio waves and just below visible light on the electromagnetic spectrum (Katz, 1992). The possibility of processing ceramics by microwave heating was discussed over 50 years ago by Von Hippel (1954a), and experimental studies on microwave processing of ceramics were started in the mid 1960s by Tinga and Voss (Tinga & Voss, 1968). Since then, the results of many investigations into microwave sintering and joining of ceramics have been reported (Bykov et al., 2001). Activity in this field began to accelerate in the mid-1970s because of a shortage of natural gas, prompting the investigation of microwave heating and sintering of several ceramic materials in the late 1970s and 1980s.

Obtenção de mulita porosa a partir da sílica da casca de arroz e do acetato de alumínio (Porous mullite obtained using silica from rice husk and aluminum acetate)

O grande volume de producao de arroz no mundo gera uma enorme quantidade de rejeito na forma de cascas e de cinzas oriundas da queima da casca. Esses residuos sao fontes de poluicao e contaminacao, podendo agredir diretamente a saude da populacao. Assim, esse trabalho tem por objetivo a obtencao de mulita porosa a partir da silica da casca de arroz e do acetato de aluminio. O residuo foi fisica e quimicamente caracterizado e em seguida misturado com acetato de aluminio para a producao do precursor da mulita. O material precursor foi caracterizado termicamente e com base nos resultados foi calcinado a 850 oC e 1200 oC. Os corpos de prova foram queimados a 1500 oC e 1600 oC e caracterizados pela determinacao de sua porosidade aparente, absorcao de agua e densidade aparente e por difracao de raios X e microscopia eletronica de varredura. Os resultados indicaram que a silica e constituida por material amorfo e aglomerados micrometricos de particulas submicrometricas e nanometricas. Com base nos resultados conclui-se que a silica obtida da casca de arroz e o acetato de aluminio podem ser utilizados com sucesso para a producao de corpos muliticos porosos com porosidades superiores a 48%.