The dielectric properties of nanocomposite materials composed of nano-sized amorphous silica powder and micron-sized tungsten powder were studied as a function of composition and heat treatment temperature. The dielectric constant of the nanocomposites, at all compositions, is higher than that of a pure silica powder. The percolation threshold of this system is at about 20% vol. W. Above this composition a sharp increase in the dielectric constant occurs, which becomes even sharper after a heat treatment at 950°C for 1 hour. The dielectric constant of pure silica specimens, on the other hand, decreases after the heat treatments. ESR measurements show peaks associated with dangling bonds at the surfaces of the particles. The change of the intensity and location of the ESR peaks is correlated with the change of the dielectric constant of the powders due to the heat treatments.
IntroductionThe electrical properties of composite materials made of a mixture of conducting and insulating phases have been extensively studied, both theoretically [1-2] and experimentally [3][4][5][6][7][8][9][10][11] as a function of composition, frequency and temperature. The electrical properties of such materials can be altered by few orders of magnitude by changing the composition. At the percolation threshold a conducting path is formed throughout the material's volume [1] due to either physical contacts or short distances between neighboring conducting particles. Experimental data show that the percolation threshold can not be predicted theoretically in composite materials having particles with different sizes and shapes. Preliminary estimations of the effect of the size ratio between the SiO 2 and W particles predict a substantial increase in the dielectric constant of the composite material compared to pure SiO 2 powder. Our study was therefore focused on nanocomposite specimens, which consist of two constituents that differ in size, by a factor of 50. The goal of the study is to understand the enhancement of the dielectric constant of the W-SiO 2 nanocomposites. For this purpose, specimens were prepared with different compositions and exposed to various heat treatment temperatures. The dielectric constant was determined by capacitance measurements. The type and density of chargeable energy states were determined from ESR measurements. The SiO 2 /W materials system was chosen for their high stability against interdiffusion and interaction up to at least 1500°C, for their low thermal expansion coefficient, and for their relatively low price.Interface Controlled Materials. Edited by M. Rühle and H. GleiterThe raw materials of the studied specimens are SiO 2 and W powders. The pure SiO 2 powder, purchased from Aldrich Chemical Company, Inc., consists of amorphous particles with an average size of about 20nm. The pure tungsten powder (99.99%), purchased from ISKAR ltd., has an average grain size of about 1µm. The powders with various compositions were mixed for 12 hours in a Fritsch planetary ball milling system operated at 2300 rpm. G...