A steady-state model for the resistivity of composites is presented, based on the idea that the resistance through a composite is the result of a series of a large number of resistors combined in series and parallel. There are three separate contributions to the resistance: constriction resistance at the contacts, tunneling resistance at the contacts, and the intrinsic filler resistance through each particle, with tunneling resistance generally dominating the magnitude of the overall resistance. The model predicts resistivity increases with increasing filler hardness and/or elastic modulus and insulating film thickness, while resistivity decreases with increasing particle size and intrinsic stress. The room-temperature dc resistivity behavior of conductor-filled silicone rubber composites was investigated to verify the model. Comparison of the model to this experimental data showed that good agreement could be obtained for filler materials in which the tarnish layer was a known quantity for a given powder; for other cases, the experimental values were higher than predicted.
The effect of glass addition on the properties of BaO-TiO,-WO, microwave dielectric material N-35, which has Q = 5900 and K = 35 at 7.2 GHz for samples sintered at 1360"C, was investigated. Several glasses including B,O,, SO,, 5Zn0-2B,03, and nine other commercial glasses were selected for this study. Among these glasses, one with a 5 wt% addition of B,O, to N-35, when sintered at 1200°C, had the best dielectric properties: Q = 8300 and K = 34 at 8.5 GHz. Both Q and K increased with firing temperature as well as with density. The Q of N-35, when sintered with a ZnO-B,O, glass system, showed a sudden drop in the sintering temperature to about 1000°C. The results of XRD, thermal analysis, and scanning electron microscopy indicated that the chemical reaction between the dielectric ceramics and glass had a greater effect on Q than on the density. The effects of the glass content and the mixing process on the densification and microwave dielectric properties are also presented. Ball milling improved the densification and dielectric properties of the N-35 sintered with ZnO-B,O,.
The effects of glass additions on the properties of (Zr,Sn)TiO, as a microwave dielectric material were investigated. The (Zr,Sn)TiO, ceramics with no glass addition sintered at 1360°C gave Q = 4900 and K = 37 at 7.9 GHz.Several glasses, including SiO,, B,O,, 5Zn0-2B,03, and nine commercial glasses, were tested during this study. Among these glasses, (Zr,Sn)TiO, sintered with ZnO-B,O,-SiO, (Corning 7574) showed more than 20% higher density than that of pure (Zr,Sn)TiO, sintered at the same temperature. A 5 w t % addition of SiO, to (Zr,Sn)TiO,, when sintered at 12OO0C, gave the best Q: Q = 2700 at 9 GHz.Results of XRD analysis and scanning electron microscopy and the effect of glass content are also presented.
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