Tham khảo tài liệu 'thermal analysis of polymeric materials part 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 286 4 Thermal Analysis Tools Change of Electrical Conductivity with T 1 n number of charge carriers Electrical Conductivity _ Y A 7 J e charge ol carriers neh ị mobility of carriers a. Metals R Ro Rth T n 1 22 cm 2 specific resistance 10-5-10-6fìcm Rth T is close to linear with temperature b. Semiconductors _ x n K ltd cm 2 specific resistance 10 2 - 10 9 ÍÌCII1 Typical semiconductors N nr tn 9rXnt the specific resistance of insulators is 1 p 3 to 1 o ữcm Fig. increasing atomic vibration amplitudes at higher temperatures. The resistance increases almost linearly with temperature. The best-known resistance thermometers are made of platinum. They are also used for the maintenance of the international temperature scale ITS 90 as shown in Appendix 8. Over a wide temperature range the change of its resistance is per degree. In order to make a platinum resistance thermometer a wire is wound noninductively. Most conveniently the total resistance is made to be 0 at K. Under this condition the resistance of the thermometer will change by about 0 K 1. The best precision that has been achieved with platinum resistance thermometers is K at 530 K and K at K. It decreases to K at 1700 K. Semiconductor properties are also summarized in Fig. . The conduction mechanism of semiconductors is more complicated. At low temperatures semiconductors have a very high resistance because their conductance band is empty of electrons. As the temperature increases electrons are promoted out of the relatively low-lying valence band into the conductance band or they may also be promoted from impurity levels into the conductance band. It is also possible that positive holes created by the electrons promoted out of the valence band carry part or all of the current. All these effects increase the conductance with increasing temperature by creating mobile charge carriers that more than compensate the decrease in mobility with increasing temperature. .
