Дослідження температурного коефіцієнта опору графіт-керамічних композиційних матеріалів із кварцовим наповнювачем різної дисперсності

Abstract

Представлено результати експериментального дослідження залежності електричного опору електропровідних графіт-керамічних композиційних матеріалів (ЕГККМ) із кварцовим наповнювачем від зміни температури у широкому діапазоні. Проаналізовано аналітичні залежності питомого електричного опору від деформованого стану кераміки в рамках теорії перколяції. Експериментально підтверджено вплив зміни об’єму кристалічного кремнезему при поліморфних перетвореннях на електричний опір композиту. Визначено раціональні технологічні параметри для виготовлення матеріалів із позитивним (ПТКО aR=0,01773) та негативним (НТКО aR=-0,00121) температурними коефіцієнтами опору.

The results of the experimental study of the dependence of resistance of electroconductive graphite-ceramic composite materials (EGCCM) with quartz filler on the temperature of a wide range (20–620OC) have been presented. The ratio of change in resistance to changes in temperature is generally called the temperature coefficient of resistance (TCR). Most of composite materials with significant (2–4 times) TCR are heterophase systems "conductor-insulator" with concentration of conductive phase close to percolation threshold. The effect of a sharp change of electrical resistance in the elastic deformation region due to the change in volumetric concentrations components, which for homogeneous materials is close to constant, has been already investigated for EGCCM in the literature. The analytical dependence of electrical resistance on strain state of ceramics has been analyzed in the theory of percolation. Accordingly in this study it was proposed to use polymorphism of silica in order to deform and/or change the concentration of electroconductive phase to modify the resistive characteristics of the material. EGCCM samples have been made by pressing in certain proportions kaolin, thermally expanded graphite (TEG) and silica fillers with different dispersion. Analysis of obtained microphotographs of the material shows that the structure of the material is highly porous and layered. Layered structure provides high sensitivity of resistive properties of the strain in the direction perpendicular to the layers. This is a relatively simple destruction of some electroconductive «bridges» between particles of the conductive phase and a reproduction of some others. Also microphotographs have showed that TEG particles in the process of forming create flat agglomerates that are evenly distributed on the volume of ceramic matrix with quartz filler. However, it is important to note that electroconductive «bridges» are contacts between individual agglomerates of TEG, and not contacts in the electroconductive agglomerate itself. The influence of various silica fillers on the porosity and electrical resistivity of EGCCM has been investigated. The effect of changing the volume of crystalline silica in the process of polymorph transformations on the electrical resistance of the composite has been experimentally testified. The impact of triple heating/cooling on the nature of resistance dependence on temperature has been experimentally investigated. The rational technological parameters for production of materials with positive (PTCR aR = 0,01773) and negative (NTCR aR = -0,00121) temperature coefficient of resistance have been defined.