Study On Electrical Behaviors Of Metal-Oxide Ceramic/Silver Composite

In this paper, four different kinds of composites, which are composed of ZnO ceramic and silver (Ag or Ag₂O) and sintered at a low temperature (<1000℃), are prepared by normal ceramic techniques. The electrical behaviors and the related issues of the four composites are emphatically investigated and discussed. The results of the density and linear shrinkage of the samples show that Ag2O can enhance the sintering properties and improve the densification of the composites. In the composites doped large silver particles, there exist some small pores in the silver particles and gaps between the silver particles and ceramic grains. However, the microstructure of the composite doped Ag₂O is compact and no obvious pores and gaps is found by SEM in this composite, and the analysis of the XPS for this composite doped Ag2O indicates that the small silver particles deposit in the ceramic grain boundaries. The analysis of the XRD of all composites reveals that there are no chemical reactions between oxides and silver, and no other phases are found in the composites except of the ZnO ceramic phases and silver phases. The fact that the conductivities of every kind of composite increase with the content of silver (silver oxide) also further suggests there no chemical reactions between silver and other oxides. The I-V curves of composite samples display that the conductivities of every kind of composite would break, and the composite would become conductive material from dielectric one when the content of silver is 25%³5%(percent in weight) in the composite, and when the content of silver is 14%²5% in every kind of composite, the I-V curves of them is nonlinear corresponding to different nonlinear coefficients and nonlinear voltages. It is interesting that the nonlinear voltage of every kind of composite is variable with the change of the silver content in the composite. The practical significance of this observation is that the varistors with various standards can be achieved easily by just controlling the content of metal silver particles in varistor ceramic. The investigation of the dielectric behaviors of the three kinds of composites reveals that the dielectric constants of the composites can be enhanced by increasing the silver content in composites, especially, when the content of silver is about 20%³0%, the dielectric constants of every kind of composite are greatly improved with the increase of the silver content while the corresponding dielectric loss remains almost the same level. This fact means that the application of the ZnO ceramic/Ag composites is very hopeful in capacitor-varistor dual functional elements. At the same time, the effects of preparation conditions (such as sintering time) and work conditions (such as ambient temperature) on the properties of the composites mentioned above are also studies in this work. Equally, the AgZnO/AgZnO-CdO/AgCdO layered composite and the AgSnO2 graded composite are prepared by normal powder metallurgical method. The practical physic process of the electrical contact materials were considered more precisely than common those in this design method from the idea of the functionally graded material (FGM). As a new design method of the electrical contact, just some contact parameters and properties of the two composites, including the conductivity, thermal conductivity of the composites, elements distribution on the contact surfaces and the surface microstructure before and after eroded by arc, were investigated in this work.