David Henry Pearce

Routes to net shape electroceramic devices and thick films

Abstract The net shape fabrication of a range of electroceramic devices and thick films is described. The fabrication routes involve producing homogeneous and formable ceramic dough using a viscous polymer processing technique, with various subsequent shape-forming operations to produce devices with sizes ranging from tens of millimetres to tens of microns. The advantages of these processing routes are discussed and examples of large (>1 mm) planar and 3-D components (hemispheres, tubes and helices) and structures with small (

Processing and properties of silver/PZT composites

Abstract Commercial PZT powders were processed with various levels of silver (I) oxide powder and the effects on sintering behaviour, mechanical and electrical properties analysed. It was found that silver oxide additions significantly accelerate the sintering behaviour of soft doped PZT, but not hard doped PZT. The resulting metallic silver behaves as a heterovalent acceptor ion dopant, thereby depressing the soft piezoelectric properties such as coupling coefficients and dielectric constant in the poled direction. Above the saturation point of Ag in PZT, however, other effects begin to occur, resulting in a partial recovery of the dielectric constant. Silver additions were also found to increase the fracture strength of PZT, and values of over 170 MPa were measured. With sufficient additions the material formed after sintering was shown to be conductive.

Pressureless sintering and characterization of Al2O3-platelet-reinforced barium-magnesium aluminosilicate glass-ceramic composites

Abstract The fabrication of Al2O3-platelet-reinforced (BMAS) glass-ceramics via a low-cost pressureless sintering route has been considered. Two new compaction techniques were introduced for the fabrication of this type of composite: pressure filtration and rotary forging. For comparison, conventional processing using uniaxial and isostatic die cold-pressing was considered also. Pressure filtration did not yield improved results in comparison with dry compaction techniques in terms of final density. Rotary forging, on the contrary, was shown to be an attractive compaction technique since it allows the fabrication of nearly fully dense composites using forming pressures 10 times lower than those employed using conventional die-pressing. Composites containing up to 20 vol% of platelets could be densified fully under pressureless conditions at temperatures of 980°C by exploiting the viscous flow of the amorphous matrix.

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.

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.

Multi-phase piezoelectric structures through co-extrusion

A novel processing route has been developed which will allow electrode structures to be applied around and within structures of lead zirconate titanate (PZT) before sintering, without the use of expensive metal electrodes such as palladium or platinum. The processing methods which have been applied to obtain these results have involved viscous processing and co-extrusion. Deagglomerated pastes of PZT and different levels of silver (I) oxide have been formed and co-extruded into cylindrical sections to demonstrate the feasibility of the technology. The microstructural and functional properties of these structures are shown, and the potential for the technology to generate complex novel piezoelectric devices illustrated.

Characterisation of a pressureless sintered sapphire fibre reinforced mullite matrix composite

Abstract The mechanical and elastic properties of a unidirectional monofilament sapphire fibre reinforced mullite matrix composite have been evaluated. Youngs modulus measurements made using Forsters technique indicate an increase in stiffness in line with fibre additions. Flexural strength testing of the composite has shown a failure stress of 475 MPa for a fibre volume fraction of 0.115. The fracture behaviour indicates that failure occurs mainly by mixed mode cracking (delamination), with crack deflection around the fibre reinforcements. Push-through indentation testing results have indicated a strong fibre—matrix interface, the strength being comparable to that of the matrix material shear strength. Thus the material has yet to be optimised in terms of composite behaviour. Potential methods of improving the material are suggested.

Thermoschockbeständigkeit und das Verhalten von SiC-faserverstärkten Glas-Matrix-Verbundwerkstoffen bei einer thermischen Wechselbeanspruchung

ZusammenfassungDas Schädigungsverhalten von kommerziellen SiC-Nicalon™ faserverstärkten Glas-Matrix-Verbundwerkstoffen bei einer Thermoschockbelastung und thermischen Wechselbeanspruchung in einer oxidierenden Atmosphäre wurde untersucht. Die Thermoschockprüfung bestand aus Abkühlen von hohen Temperaturen (590°C–710°C) auf Raumtemperatur in einem Wasserbad. Bei der Wechselbeanspruchung wurde der Werkstoff in Luftatmosphäre von einer hohen Temperatur (T=700°C) in Raumtemperaturumgebung eine definierte Anzahl von Zyklen bewegt. Zerstörende sowie zerstörungsfreie Prüfverfahren wurden zur Werkstoffcharakterisierung angewendet, mit dem Ziel die Unterschiede im Werkstoffverhalten bei verschiedenen Bedingungen der thermischen Belastung zu finden.Schädigung in Form von Matrixrissen wurde nach der Thermoschockbelastung beim Abkühlen von mittleren Temperaturen, ca. 660°C, festgestellt. Die Schädigung wuchs mit der steigenden Anzahl von Thermoschockzyklen, was durch einen Abfall des Elastizitätsmoduls bei gleichzeitigem Anstieg der inneren Reibung mittels mechanischer Kraftresonanzmethode zerstörungsfrei nachgewiesen werden konnte. Bei der Wechselbeanspruchung kam es zur Werkstoffdegradation aufgrund der Porositätsbildung in Folge eines länger andauernden Einflusses von hohen Temperaturen. Die Möglichkeiten der Heilung von Mikrorissen, die bei der Thermoschockbelastung entstanden sind, wurden untersucht. Hierzu wurde der beanspruchte Werkstoff einer optimierten Wärmebehandlung (Tempern), die zum viskosem Fließen des Glasmatrix führte, ausgesetzt.AbstractThe damage evolution of commercially available SiC-Nicalon™ fiber-reinforced glass matrix composites under thermal shock and thermal cycling conditions in oxidizing atmospheres was investigated. The thermal shock tests involved quenching the samples from high temperatures (590–710°C) to room temperature in a water bath. For the thermal cycling tests the samples were quickly alternated between high temperature (T=700°C) and room temperature air for different number of cycles. Both destructive and non-destructive techniques were employed to characterize the samples and to detect differences in behavior for the various thermal loading conditions. In thermally shocked samples, damage in the form of matrix microcracks was induced by quenching from intermediate temperatures, e.g. 660°C. The extent of damage increased with the number of thermal shock cycles, as detected by a decrease in the Young’s modulus and a simultaneous increase in the internal friction measured non-destructively be a mechanical force resonance technique. In thermally cycled samples, material degradation was ascribed to porosity formation in the matrix as a consequence of the extended exposures at high temperatures. With increasing number of cycles, also interfacial oxidation was detected. An attempt was made also to explore the possibility of healing the induced microcracks in thermally shocked samples by an optimized post-thermal shock heat-treatment (annealing) schedule, exploiting the viscous flow of the glass matrix.