Study of multilayer piezoelectric transformers

Abstract

The work presented in this thesis is related to the fabrication and characterization of two novel piezoelectric transformers. The new working principles of them have been demonstrated successfully. Lead zirconate titanate is used in this project for fabricating multilayer piezoelectric transformers (MPTs). After fabrication, the performances of these transformers are measured and modelled. Both the equivalent circuit simulation and finite-element analysis are used in the modelling. The first transformer is a thin rectangular bar-shaped bilayer piezoelectric transformer operating in the bending vibration mode. It is used for voltage step-up and its size is ~31.6 mm x 9.1 mm x 0.75 mm. The transformer is polarized in the thickness direction. It is composed of 2 input sections and 1 output section. The input sections are located at both ends of the length of the transformer with bilayer structures and the output section is located at the centre part with a single layer structure. By using the bilayer structure, a piezoelectric transformer with bending vibration mode can be realized. Its keff and power transmission efficiency can be maintained at a high value by using a thin plate. The second piezoelectric transformer is a ring-shaped multilayer type. As the number of layers in the multilayer ceramic structure increases, residual porosity tends to appear in the structure. The pores are created in the binder burnout process when solvents and binders are released from the ceramic green body. Without a well-controlled compaction technique, defects between sheets in the stacked body often exist, leading to delamination problems. Also it is difficult to fabricate a ring-shaped multilayer structure without cracks, especially near the inner surface. Due to difference between the thermal expansion coefficient of the mould and the ceramic green body, large internal stress is often induced in the ceramic green body during hot pressing and cracks are initiated during sintering. In this work, using a specially designed mould and by adjustment of the hot pressing conditions, the internal stress induced during hot pressing has been reduced effectively. In particular, the hot pressing process is divided into two steps and two different sizes of inner shaft for the mould are used in order to reduce clamping of the inner shaft by the ceramic green body. Therefore, the inner shafi can be released easily and the delamination problem can be alleviated. Guided by the result of thermogravimetric analysis (TGA) and by putting the green sample in a stainless steel mould during the binder burnout process, pores have been greatly reduced. Scanning electron microscopy and X-ray diffraction are used to study the cross-sectional fractured surface of the transformer. The resulting multilayer transformer do not have cracks, pores and delamination. The working principles of the two transformers have been demonstrated. However, the efficiency of these MPTs still needs to be further improved. The experimentally measured characteristics are found to agree quite well with modelling results.