Multi-Field Fracture Behaviors Of Piezoelectric Semiconductors

Piezoelectric semiconductors(PSCs),as a new kind of intelligent electronic materials with both piezoelectric and semiconductive properties,have had a wide range of applications in electromechanical coupling devices.However,PSCs display brittle characteristics and are highly sensitive to deformation,inclusions and defects.This leads to complex failure behaviors in multi-field environments,which has a great influence on the structure,reliability and service ability of their devices.In this thesis,taking GaN as an example,fracture and failure behaviors of piezoelectric semiconductors are systematically investigated under combined mechanical-voltage-current loadings.To analyze and clarify deformation and fracture mechanisms of PSCs under coupled electromechanical loads,a multi-field fracture experimental system is developed and a nonlinear coupling iterative method is proposed.Based on the experimental and numerical results,both the multi-field fracture criterion and a failure analysis system of PSCs are established.It provides a basis for reliability design and structural analysis of PSC devices under multi-field conditions.(1)According to the material properties and multi-field working conditions of PSCs,experimental research methods of electromechanical fracture are built up,and a typical and standard sample for multi-field fracture experiment is designed.Based on the experimental methods,a multi-field fracture testing system for PSCs is developed,and a series of auxiliary experimental devices and insulative facilities are designed.In this system,the key technologies of coupled loading of mechanical force-electric current-strong electric field are solved,and in addition,the remote loading,control of each physical field and automation of data measurement are realized.According to the special characteristics of PSCs,a specialized polarization device is designed by using the interlayer polarization method and a new crack prefabrication technology is developed,which make the experimental platform hold the ability of polarization and crack prefabrication.(2)Based on the strict nonlinear theory of PSCs,a nonlinear coupling iterative method of “elastic-semiconductor” is proposed.By means of general finite element analysis software,the numerical iterative solution of elastic-semiconductor boundary value problems is obtained.In addition,effectiveness of the iterative calculation method is verified by relevant experimental methods.Using this iterative method,the electromechanical coupling characteristics of a typical GaN PSC device are deeply studied.It is found that the current transport behavior of PSCs can be effectively adjusted and controlled by mechanical loads.The effects of different mechanical and electrical boundary conditions on the space charge region of Schottky junction of PSCs are analyzed,which provides a basis for the structure analysis of PSCs in multi-field conditions.(3)By using experimental tests and numerical simulation,the effects of an applied DC electric field and current on the strength of depoled GaN PSC ceramics are investigated.It is shown that the bending strength decreases with the electric field.When the field strength exceeds a certain range,there is almost no change of the bending strength,and however,current causes the bending strength to decrease continuously.Under a coupled mechanical-voltage-electrical current load,the corresponding stress and electric fields and carrier distribution in specimens are analyzed based on the finite element method.Combined with simulated and experimental results under different loads,the influence mechanism of electric field and current on the bending properties of PSCs is analyzed.In addition,the influences of polarization on electromechanical properties are systematically tested and simulated.It is shown that there is a close correlation between polarization and electromechanical properties.That is,polarization treatment can improve the mechanical and electrical characteristics,especially for the current transport performance of PSCs.It is expected that the new discovery can provide support for the design and manufacture of PSC devices.(4)Based on the multi-field fracture experimental system,fracture toughness of GaN PSCs is determined,and their fracture behavior under combined mechanical and electric loading is studied.The results show that current is a key factor in affecting the fracture characteristics of PSCs,and the electric field can toughen the materials.This is completely different from the fracture characteristics of traditional piezoelectric ceramics.Under combined mechanical-voltage-electrical current loading,the corresponding physical field distributions in specimens are simulated,and the fracture mechanism of PSCs is analyzed.The stress,electric displacement and electric current intensity factors are numerically calculated and then a set of empirical formulae are obtained.By fitting the experimental data,a fracture criterion under combined electrical and mechanical loading is established.The relationship between fracture behavior and current density of PSCs is clarified.The multi-field fracture model and failure analysis system are constructed,which provides a basis for the reliability design and evaluation of PSC devices.