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Two commercial BaTiO3 based powders were selected, characterized and used for the fabrication of X7R dielectric ceramics. The effects of forming and sintering on the dielectric properties have been studied and processing parameters optimized. The annealing treatment in N2 atmosphere has been used in order to modify the dielectric properties of BaTiO3. The related dielectric properties have been tested and microstructures of BaTiO3 ceramics examined. The results show that by combining the optimized processing parameters such as cold isostatic pressure 225 MPa, sintering temperature from 1300 to 1400 °C and annealing treatment at 850 °C, a marked increase in dielectric constant from approximately 3000 to 3600 has been achieved with the commercial BaTiO3 based powder. This was accomplished while keeping the X7R capacitor requirements on the dissipation factor (

Over the past few decades, cemented carbides have been used in applications requiring high hardness, toughness, and strength. However, the metallic binder phase present in cemented ceramics can result in poor mechanical properties at high temperatures, and has proved susceptible to corrosion and erosion in aggressive environments. Metal free tungsten carbide ceramics developed by VM Advanced Carbides provide improved mechanical properties and chemical stability, and can be used in a variety of high temperature applications.

A mechanism for the toughening of brittle matrices with ductile particles is presented. It is shown that the constraints imposed on the particles by the rigid matrix suppress plastic deformation of the particles at crack tips so that the main contribution to composite toughness comes from ligament formation in the matrix and their fracture behind the advancing crack front. The incorporation of inherently tough second-phase particles into a brittle matrix does not therefore automatically lead to a large toughness improvement of the composite, if other requirements are not satisfied. As far as toughness is concerned, the most desirable composite is one consisting of a soft (low yield strength) particle strongly bonded to a brittle matrix and free from internal stresses. The mechanism of stress relaxation in a spherical particle embedded in a matrix of smaller thermal expansion coefficient is also described.

A toughening mechanism in composites, based on ligament zone formation behind the advancing crack front, is presented. In brittle matrix/ductile phase systems where there is a significant differenc...