Ulrich Bast

Composite ultrasound transducer and method for manufacturing a structured component therefor of piezoelectric ceramic

A composite ultrasound transducer (array) contains piezoelectric ceramic transducer elements which radiate substantially in the longitudinal direction, and are embedded in a polymer matrix. The transducer elements have such a geometrical structure, and are arranged relative to each other, so that the development of oscillation modes perpendicular to the longitudinal direction of the transducer elements is suppressed. A method for manufacturing the composite ultrasound transducer includes the steps of filling a plastic form, having a negative of the desired shape of the transducer elements, with ceramic slurry, and pyrolyzing the plastic form after the slurry dries therein. The resulting voids between the hardened ceramic elements are filled with polymer to create the overall composite transducer. The transducer elements can thus be given an arbitrary shape and arrangement, such as hexagonal or irregular square structures, having a trapezoidal cross-section in planes parallel to the longitudinal axis of the transducer elements.

Characterization of Fatigue Mechanisms of Thermal Barrier Coatings by a Novel Laser-Based Test

The use of high performance ceramic thermal barrier coatings in stationary gas turbines requires fundamental knowledge of their fatigue behavior under high temperature gradients and thermal cycling. An experimental method based on rapid laser heating complemented with finite-element calculations was developed in order to identify the major damage mechanisms and to obtain a data set for reliability assessment of thermal barrier coatings for temperature and stress fields similar to gas turbine conditions.The observed failures are strongly related to the pretreatment procedures such as annealing under high temperature gradients and isothermal long-term oxidation. The vertical crack patterns observed close to the top surface of the Zirconia coating are generated at the moment of rapid cooling. These cracks are induced by high biaxial tensile stresses caused by the temperature gradient and the stress reversion after relaxation of compressive stresses at high temperatures. The long-term fatigue behavior is decisively determined by two processes:(i) The porous Zirconia loses its damage tolerant properties by densification.(ii) The growth of an oxide layer at the bond coat degrades adhesion and produces localized stress fields at the interface.Cyclic loads increase the length of existing in-plane cracks and delaminations rather than enlarging their number. Misfit of the crack flanks and wedge effects are the driving forces for continued crack propagation.These experimental results are discussed in terms of fracture mechanics.Copyright

Thermal Shock and Cyclic Loading of Ceramic Parts in Stationary Gas Turbines

A further increase in the efficiency of stationary gas turbines operating in the combined cycle process could save a huge amount of fuel costs while sharply reducing the output of environmental pollutants. Higher effective turbine inlet temperatures are therefore a requirement for the future. This goal can be reached by applying uncooled ceramic parts of high temperature resistance. Possible candidates are the combustion chamber and transition duct lining, stator vanes and rotor blades.