In this paper, the fracture behavior of GaN piezoelectric semiconductor ceramics was investigated under combined mechanical and electric loading by using three-point bending tests and numerical analysis. The experimental results demonstrate that, in contrast to traditional insulating piezoelectric ceramics, electric current is a key factor in affecting the fracture characteristics of GaN ceramics. The stress, electric displacement, and electric current intensity factors were numerically calculated and then a set of empirical formulae was obtained. By fifitting the experimental data, a fracture criterion under combined mechanical and electrical loading was obtained in the form of an ellipsoid function of intensity factors. Such a fracture criterion can be extended to predict the failure behavior of other piezoelectric semiconductors or devices with a crack, which are useful in their reliability design and applications.
Piezoelectric semiconductor ceramics (PSCs) are semiconducting ceramic materials that have piezoelectric properties. Since the piezoelectric effect was discovered in ZnO and CdS semiconductors, the mechanical properties of PSCs have been intensively studied. In recent years, while considering the special interaction between the mechanical force and charge carrier of PSCs, numerous new PSC-based electromechanical devices have appeared, including ultrasonic transducers , sensors, and piezoelectric charge-coupled devices.
To develop the fracture criterion of PSCs, many theoretical efforts have been done. Usuallythe fracture criterion of insulating dielectric ceramics (22-24) is directly applied to analyze themulti-field fracture of PSCs, in which the effect of electric current field was ignored. The fractureprocess is thought to be controlled only by mechanical and electric field intensity factors. Howeverit is difficult to judge such a method because there is no available experimental evidence. Therefore, anurgent need is to experimentally investigate the effect of electric current on fracture of PSCs undera combined mechanical and electrical load and determine its fracture criterion, which can provide atheoretical basis for improving the reliability of PSC devices.
Fig1
The material that is used in this work is GaN, which is a new kind of electronic ceramics materials with piezoelectric and semiconductor properties. Due to the stable conductivity and excellent functional properties, GaN is a perfect candidate material of electromechanical, high-power and high-frequency devices for operation in extreme environments . Here, GaN ceramics were manufactured from pure GaN powder by vacuum hot pressing at 450 ℃ (with a density of 5.9 relative to water), which exhibits properties similar to that of N-type semiconductors with a Curie temperature of 265 ◦C. It is seen from Figure 1 that the X-ray diffraction pattern matches well with the standard spectrum of JCPDS fifile (i.e., No. 76-0703) of GaN.
