Materials and Their Parameters

Date: 2015-10-07; view: 463.

Fig.1. Left, in a uniformly heated material, the electrons and positively charged ions are uniformly distributed. Right, distribution of electrons and positively charged ions as it is influenced over a thermal gradient. Electrons concentrate at cold end of the specimen to cause a gradient of electrical charge.

An important factor in the growth of thermoelectric technology is the ability to adjust the number of free electrons in semiconductor materials. The importance of this is due to two basic relationships: First, the output voltage of any thermoelectric material is inversely proportional to the number of free electrons in that material, and, second, the conductivity of the material is directly proportional to the number of free electrons. Thus, insulators containing 10¹⁰ electrons per cubic centimeter generate output voltages of about 10,000 micro­volts per degree centigrade of temperature difference between the hot and cold ends; offsetting this, however, is the fact that they have an extremely high internal resistance. On the other hand, the metals give output voltages of about 5 microvolts per degree, but have ex­tremely low internal resistance. Therefore, to obtain maximum power output or optimum efficiency from a thermoelectric material, the electron density must be adjusted to an acceptable compromise value between high voltage and high electrical conductivity. This is essential to the production of useful power since a combination of high voltage and low current or of low voltage and high current result in little power. The compromise is shown by the efficiency curves in Fig.2, which indicate that the optimum electron density is about 10¹⁹ free electrons per cubic centimeter, value well within the range of good-conducting semiconductors and one that affords output volt­ages of about 175 microvolts per degree C. Some typical materials that demonstrate acceptable efficiency are zinc antimony, lead telluride, bismuth telluride, and germanium telluride.

Fig.2. Curves showing the relationship between density of free electrons in a material and conductivity and thermoelectric output voltage. Optimum density for maximum power output is about 10¹⁹ electrons per cubic centimeter.

In thermoelectric generators built for practical uses, it is desirable to use a number of different thermoelectric materials, to take advan­tage of the fact that each has its best range of operating temperatures. This contributes to the increased efficiency that is possible when genera­tors are operated at high temperature. To cover low temperatures, say up to 600 degrees C, several semiconductors have proved satisfactory.

However, to go higher, say into the 1,000 degree C range, semi­conductors are no longer suitable, since at these temperatures they become "intrinsic"; that is, the heat input causes both positive and negative electrical charges to migrate in equal numbers and so no out­put voltage is possible. As an extreme example, Fig.3 shows how bismuth telluride's output voltage falls to zero at 150 degrees C.

Obviously, at higher temperatures materials are required that are free of this behavior. A promising approach is the use of insulator ma­terials that have been modified to become good thermoelectric materi­als. This is particularly interesting since many insulators do not become intrinsic conductors in the 1,000 degree C range. As an illustration of this modification, pure nickel oxide is normally an insulator, but if it is modified by the addition of three percent of lithium, its resistivity de­creases to about 0.01 ohm-centimeters. As explanation for this, in nor­mal nickel oxide the nickel has a valence of plus two but the addition of lithium causes the appearance of nickel with valence of plus one. The material's greatly increased conductivity is brought about by an exchange of charges between plus-one nickel and plus-two nickel. Through simi­lar modifications, other materials are being developed for use at higher temperatures. For example, this approach led to one of the newest mixed valence materials, samarium sulphide, which has a good figure of merit at temperatures as high as 1,100 degrees C.