The main goal of this project is to examine whether the coatings of current carbon fibres are fully adapted to the thermostable resins (bismaleimides) used for medium-range temperature structural applications.
The aim of the project is to increase the frequency range for which piezoceramics can replace single crystals in bulk and surface acoustic wave (SAW) devices. Piezoceramics offer advantages over single crystals such as tailoring of properties through changes in chemical composition and relative ease of fabrication. However, at present, their use is limited to frequencies below 20 MHz for bulk devices due to the low mechanical strength 0.05 to 0.1 mm thick samples and to the coarse grain structure which leads to inhomogeneous properties in this thickness range. For SAW devices coarse grains lead to high propagation losses and limit their use to less than 100 MHz.
A number of ways to obtain fine grain and high density ceramics were investigated. It was shown that this could be achieved by hydrothermal synthesis of lead titanate leading to grain size of less than 1 um, or milling a mixed oxide powder for a longer time, followed by reducing the sintering temperature leading to PZT ceramics with grain size of less than 2 um.
Using a postsinter hot isostatic pressing (HIP) process it was possible to reduce the porosity to less than 1% and as low as 0.1% was achieved by optimizing the HIP conditions. The reduction in porosity was accompanied by an improvement in the dielectric and piezoelectric properties.
The surface acoustic wave (SAW) properties of various ceramics were measured. PZT materials have good SAW properties, but only their high propagation losses limit them to < 30 MHz. Modified lead titanate compositions have better temperature stability and lower propagation loss, making them suitable for devices in the range 20-150 MHz. The properties were further improved through compositional studies and by producing fine grained, high density materials. In addition, ceramics prepared from hydrothermal powders had improved SAW properties compared to the same composition prepared by the mixed oxide process.
The fine grained ceramics allowed higher frequency bulk devices to be obtained. This was demonstrated in 2 devices, a 15 MHz focused transducer, which gave improved axial resolution, and a linear array in which finer structure allowed an operating frequency of 20 MHz to be achieved.
The development of these new materials will allow the extension of product ranges and increase competitiveness on the world market.