J.E. Holmes

Processing effects on the microstructure and dielectric properties of Barium Strontium Titanate (BST) ceramics

Barium Strontium Titanate (BST) ferroelectric thick films have been investigated as potential candidates for use in frequency agile microwave circuit devices. Powder processing techniques such as screen-printing have been used to make BST thick films. However, due to the interactions between the BST and substrates such as alumina, the sintering temperatures for the BST thick films are limited and the resultant films are difficult to achieve full densification. In this paper, the effects of different powder processing conditions (calcination, sintering temperature and time) on the sintering behaviour and dielectric properties of the BST ceramics have been investigated. The dielectric behaviour of the ceramics has been correlated with composition and microstructural features such as chemical homogeneity, grain size and domain wall movements.

Dielectric and mechanical losses in (Ba,Sr) TiO3 systems

In the application of tuneable microwave devices of ferroelectric (BaSr)TiO3 systems the two critical parameters needed for optimal device performance are high tunability and low dielectric loss. The dielectric loss of the materials is strongly dependent on microstructure. This paper is concerned with an investigation of the variation in the dielectric and mechanical losses in BaxSr1 − xTiO3 systems (x = 0.5, 0.6, 0.7 and 1.0) with microstructure (grain sizes from 1 μm to 50 μm). The magnitude of the loss peak and sharpness of the anomaly in the dielectric constant/elastic modulus observed for the phase transitions in BaxSr1 − xTiO3, depend not only on the composition and but also on the grain size. A relaxation peak has been observed in large grain material, which is indication of interactions between different configurations of domain walls and the diffusion of oxygen vacancies in the domains.

Novel piezoelectric structures for sensor and actuator applications

Abstract The piezoelectric behaviour of ferroelectric ceramics is commonly utilised for sensors and actuators. Simple shapes (bimorph beams, moonies and rainbows), thin films, composite and layered structures have been used thus far. Processing technologies developed at the University of Birmingham allow the production of novel complex shapes which, as well as improving the mechanical strength of the ceramic, may well allow the application of piezoelectric elements into areas previously unexplored. This paper concentrates on circular cross-sectional shapes in the form of helical springs, and describes the preliminary investigation carried out to characterise the response of springs to both dynamic and static impulses. The application of such geometries to sensor and actuator devices is discussed and possible areas of further work to realise the potential of the processing technology are also outlined.

YBCO thick films for high Q resonators

Three dimensional structures can have high geometric factors and have thus been favoured in applications requiring high Q resonant elements. In this paper the surface resistance of YBCO thick film materials measured in the frequency range 1-10 GHz in a variety of resonant structures is discussed. The power dependence of the materials in the 1 to 2 GHz region is also examined and at these frequencies it is possible to attain Qs of over 150000 using thick film materials.

Dielectric helical resonators

This paper discusses the possibility of constructing a resonant element from dielectric material shaped into a helix. The modeling conducted shows that the mode associated with ceramic quarter-wave resonators is present within a coiled dielectric helix, the properties of which are strongly dependent upon the pitch and the thickness of the coils. Some of the helices studies in this paper possess high-quality factors due to the low filling factors associated with the resonant mode. It is intended to use these initial results as design criteria to compare the dielectric helix with existing resonators. The modeling conducted suggests that the analogy with a conductive helix does not apply.

Novel piezoelectric structures for sensor applications

Piezoelectric ceramic devices have been formed into the helical spring shapes from tubular extrudates. These structures are shown to possess low compliance and low natural resonant frequencies. Equations are presented allowing the prediction of the resonant frequencies of the devices; these are shown to be in good agreement with measured responses. Using the design criteria a device has been constructed to exhibit a low fundamental resonance and a clear spectrum up to 500 Hz. The frequency response is shown for this device and compared to a conventional electromagnetic geophone. The results show that the device acts as a displacement sensor providing an output charge of 0.427 mC/m with no observed spurious resonances.

Demonstration of HTS microwave sub-systems with a pulse tube cryocooler

Abstract A special pulse tube cryocooler was designed and fabricated. Acceleration measurements show that the vibration of this cooler is much smaller than that of a Stirling machine. Two HTS microwave devices have been integrated with this cooler, which operate successfully as practical HTS sub-systems.