A piezo-composite consists of a piezoelectric active phase and a passive plastic phase (1). In the 1-3 -configuration adopted in our case, piezoelectric rods parallely aligned in thickness direction are imbedded in a three-dimensional plastic matrix. The distance between the rods has to be chosen inferior to the half wave length of the shear wave in the matrix material ensuring that the whole compound is vibrating as a quasi-homogeneous material.
The global and local properties of the piezo-composite is determined by the intimate coupling of the active and the passive phase. By the proper choise of the ceramic material, the plastic material, the volume fraction of both materials as well as the geometrical dimensions of the structure the elastic and piezoelectric properties of composite were adjusted to the application. The material developped is characterized by a high electro-acoutic coupling coefficient (0,68), better acoustic matching to gas (acoustic impedance 8 MRayl in comparison to 35 MRayl of the piezo-ceramic) and a low mechanical quality factor.
The piezo-composites are fabricated according to the well-known dice-and-fill technique. A piezoelectric PZT-plate is sawed in parallel cuts, rotated at 90° and sawed again. For this procedure a diamand wire saw (minimum cutting width 100 µm) and a high-pecision wafer saw (minimum cutting width 10 µm) is available. The cutting depth is about 80 % of the plate thickness. After cleaning in an ultrasonic cleaning equipment and drying the ,ceramic-brush" is filled with a plastic material.
After a heat treatment of several hours the electrodes are deposited by sputtering a 50 nm base layer of Ni/Cr or Ni/Wi followed by 1.5 µm Au-layer generated by galvanization.
Air borne ultrasonic probes are applied in the detection of surface near cracks in metals or ceramics by generation of Rayleigh waves, the inspection of thin metal plates by Lamb wave generation, the detection of impact damages in composite materials and the imaging of surface structures.
1. R.. E. Newnham, D. P. Skinner and L. E. Cross in Mater. Res. Bull., 13 (1978) 525-53
Abstract Source:
Book of Abstracts, 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN: 87-986898-0-00
Full-Text Source:
Proceedings of the 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN:
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