Thermocouples have been employed commercially for years as accurate temperature measurement sensors. Thermocouples are applicable over a wide range of temperatures (from -200.degree. C. to 1800.degree. C.) and are particularly useful for determining temperatures which extend far beyond the range of liquid-in-gas thermometers.
With the development of techniques for applying thin, thermocouple films, it was discovered that thin film thermocouples have several advantages over standard wire thermocouples for measuring solid surface temperatures.
Such advantages include 1) low mass, which leads to extremely rapid response; 2) thin film thickness, which does not affect the convective heat transfer characteristics of the surface compared to conventional wire sensors and 3) small size of the thermocouple films, which is quite compatible with intricate fabrication techniques in electronics applications. Such advantages have permitted adaptation of new thin film thermocouples to conventional, standard wire temperature sensing devices for use in molten metal fabrication, ceramic kilns, furnaces and interior hardware in gas turbine and internal combustion engines.
Conventional high output thermocouple devices have been constructed of bismuth-antimony intermetallics which have a voltage output (Seebeck coefficient) of 105 .mu.V/.degree. C. (see, P. A. Kinzie, "Thermocouple Temperature Measurement," John Wiley, p. 135, New York, N.Y., 1973). Recent developments have focused on narrow-bandgap semiconductors such as lead-selenium, bismuth-tellurium and antimony-tellurium.
More contemporary applications of intermetallic thermocouples have attempted to make use of the thermoelectromotive force (Seebeck coefficient) of specific alloys in transforming thermal energy, particularly solar radiation, into electrical energy. In these applications, not only the Seebeck coefficient, but also the electric and thermal conductivities, are critical. The alloys first employed in solar electrical devices proved commercially impractical. Because the earliest predecessors to contemporary thermocouple alloys had low efficiencies and a lower ratio of electrical energy output obtained to the input of thermal energy, the use of such alloys in solar electric applications was limited to scientific and experimental use.
In an effort to overcome the limitations of the earlier alloys for use in solar electrical devices and the like, artisans attempted to develop thermocouple alloys having higher Seebeck coefficients. For example, U.S. Pat. No. 2,229,482 to Telkes discloses a zinc-antimonide thermocouple alloy having a Seebeck coefficient ranging from 114 .mu.V/.degree. C. to 265 .mu.V/.degree. C. Although these intermetallic, thermocouple compositions exhibited improved Seebeck coefficients over previously known alloys, the use of these compositions in advanced technology applications is limited due to their composition's still relatively low thermal conductivity.
U.S. Pat. No. 3,086,068 to Charland et al. discloses two processes used in preparing thermoelectric material: 1) single crystal growth and 2) casting. Several drawbacks of the thermoelectric materials exist. Namely, the single crystal growth is difficult and costly and the casting process produces polycrystalline materials having a coarse grain structure too brittle for commercial use.
Attempting to overcome these disclosed limitations, U.S. Pat. No. 3,086,068 to Charland et al. teaches a process for preparing antimony intermetallic thermoelectrics from powdered intermetallic materials by hot-pressing. The resulting thermoelectrics exhibit sufficient stability and much higher Seebeck coefficients than were previously available. Representative Seebeck coefficients range from 157.8 .mu.V/.degree. C. to 210 .mu.V/.degree. C. The disclosed intermetallics are limited to use in thermocouples not requiring higher Seebeck coefficients.
In an alternative process in U.S. Pat. No. 3,182,391 to Charland et al., a metal powder sintering process results in a zinc-antimonide thermoelectric material with a Seebeck coefficient of not more than 206 .mu.V/.degree. C. As is true of previous zinc-antimonide systems, this Seebeck coefficient is still relatively small.
U.S. Pat. No. 3,900,603 to Rittmayer discloses low output thermoelectric generators having thermocouple legs of different conductance type. The disclosure discusses a method of vapor-depositing thermoelectric leg material upon a semiconductor carrier, and thereafter tempering to transform the amorphous phase of the thermoelectric semiconductor to a crystalline phase. The pre-tempered semiconductor has a Seebeck coefficient of 820 .mu.V/.degree. C. and after heat treatment has a Seebeck coefficient of 146 .mu.V/.degree. C. The disclosed pre-tempered semiconductors, although they possess relatively high Seebeck coefficients, have not been successful as a thermoelectric power material due to their high electrical resistivity (25-100 .OMEGA..multidot.cm) Only after tempering when the Seebeck coefficient has effectively diminished to less than one fourth of its value in the amorphous phase, are the tempered semiconductors useful as thermocouples.