1 INTRODUCTION 传感与检测技术.pptVIP

1.8 MATERIAL FOR SENSORS Dielectric material have covalent bonds, hence they are electrical insulators. Permittivity:介电常数 It is the ratio between the electric flux density and the electric field Vacuum 1.8 MATERIAL FOR SENSORS Ceramics (陶瓷) resist corrosion (), abrasion (), and high temperature. As base material for other sensors NTC thermistor Switching PTC thermistor pyroelectric (热电的) ceramics humidity sensor oxygen sensor gas sensors 1.8 MATERIAL FOR SENSORS Organic polymer (有机聚合物) macromolecules formed when many equal molecules called monomers (单体) bind together by covalent bonds. Bonded molecules can form linear or tridimensional (立体的) structures. Plastics result from adding filler (填充剂) to a polymer. Plastics are superb electrical insulators. Polymers can become conductive by adding to them relatively good conductors such as powered silver or carbon, as well as by adding different counterions (补偿离子) during polymer growing. 1.8 MATERIAL FOR SENSORS 1.8.2 Magnetic Materials The magnetic flux in vacuum is proportional to the applied magnetic field. All materials modify the magnetic flux to some extent, so that where M is the magnetic dipole moment per unit volume. 1.8 MATERIAL FOR SENSORS The magnetic properties of solids are related to the properties of the electrons in their atoms. Paramagnetic (顺磁的) materials have atoms or ions with incomplete electron shells. Diamagnetic (反磁的) materials have atoms or ions with complete electron shells. The magnetic induction in paramagnetic and diamagnetic materials is only slightly different from that in vacuum, and its amplitude is independent on that of the applied field. 1.8 MATERIAL FOR SENSORS Elementary magnetic dipoles in ferromagnetic (铁磁的) materials arise from elementary currents. A ferromagnetic material magnetizes because of two different processes: displacement and orientation. Domain: 磁畴 Figure 1.