Graphite fiber thermocouple device and method
BACKGROUND OF THE INVENTION 
     The invention relates to thermocouple devices used in measuring high temperatures. A thermocouple is defined as a device consisting of two dissimilar materials (usually metals) which when joined together generate a predictable voltage based on the temperature of their junction. 
     Work with carbon (carbon and graphite are used interchangeably) thermocouples has been recorded as early as 1881 continuing to the present. These early thermocouples evolved to the type described by Westbrook and Shepard, as in U.S. Pat. No. 2,946,385, whereby a graphite/graphite with an additive or doping agent thermocouple was made. The doping agent, boron, made a graphite of sufficient electrical valence difference that when joined with the undoped graphite a measureable Seeback voltage was created. Thermocouples have been described in Physiochemical Measurements at High Temperatures, 1959, Bockres, White, McKenzie, where the doping agent was clay. Thermocouples of this type suffer from inhomogenity, nonuniformity, and instability. Due to the low strength of the carbon materials, the physical size of these carbon thermocouples is much larger than typical metal thermocouples. Due to their physical size, they conduct heat away from the surroundings in which they are sensing. 
     A second approach to carbon thermocouples is described by Jamieson in U.S. Pat. No. 3,305,405. A material which yields graphite is pyrolized and deposited on a surface of boron nitride. A second layer of pyrolytic graphite which has seen a different heat history is then deposited. The different heat histories result in a sufficient electrical difference to generate a Seebeck voltage. This method of fabrication is difficult and batch-to-batch uniformity would be a problem. 
     Accordingly, an objective of the present invention is to provide a thermocouple which is stable and efficient for use in measuring high temperatures. 
     Another object of the present invention is to provide a thermocouple having a temperature sensing head is simple in construction and has a small mass to reduce conduction heat loss. 
     SUMMARY OF THE INVENTION 
     The above objectives are accomplished according to the instant invention by the discovery that a Seebeck effect voltage can ge obtained by joining pitch and PAN based graphite fibers. The Seebeck voltage is sufficiently high and sensitive to be used as a thermocouple. This Seebeck voltage is stable at high temperatures. The use of these fibers which are commercially available in large homogeneous lots eliminates the batch-to-batch differences inherent in previous graphite thermocouples. The use of commercially available fibers makes these graphite fiber thermocouples cost competitive with metal thermocouples. These fibers contain no doping agents which eliminates the possibility of voltage drift due to the diffusion of the doping agent at high temperatures. Due to their high strengths, graphite fiber thermocouples have been constructed which have a much reduced mass compared to previous carbon thermocouples. The reduced mass results in less heat conduction from the point of temperature measurement and reduces the complexity of the thermocouple insulating head design. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The construction designed to carry out the invention will hereinafter be described, together with other features thereof. 
     The invention will be more readily understood from a reading of the following specification and by reference to the accompanying drawings forming a part thereof, wherein an example of the invention is shown and wherein: 
     FIG. 1 is a partially cut-away schematic illustration of a graphite fiber thermocouple constructed according to invention without insulation. 
     FIG. 2 is a sectional view of a graphite fiber thermocouple with insulation and an optional protection sheath in accordance with the invention. 
     FIG. 3 is a sectional view taken along line 3--3 of FIG. 2. 
     FIG. 4 is a temperature voltage graph which illustrates the performance of a graphite fiber thermocouple made in accordance with invention. 
    
    
     DESCRIPTION OF A PREFERRED EMBODIMENT 
     The material used in making the thermocouples of this invention is carbon in the form of graphite. Graphite is the most temperature stable material known with a sublimation temperature of 4200° C. 
     In the decade of the 1960s it was discovered that graphite fibers could be produced by pyrolizing rayon or acrylic (PAN) fibers under specific conditions. These fibers had extremely high tensile strengths, and due to their low densities were ideally suited as the reinforcing member of composites. A large industry has developed producing graphite fibers for composites. In searching for a more economical graphite fiber, L. S. Singer discovered, as in U.S. Pat. No. 3,919,387, that a graphite fiber could be produced from a pitch containing liquid crystalline carbon (mesophase). Graphite fibers derived via the pyrolysis of PAN or rayon differ from those derived from pitch precursor in their electrothermal characteristics. Table 1 lists some of these differences. 
     
                       TABLE 1                                                     
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                      Electrical                                          
                                Thermal                                   
Type Fiber/                                                               
           Density    Resistivity                                         
                                Conductivity                              
Modulus    g/cm       u-ohm-m   cal/sec-cm K                              
______________________________________                                    
PAN-50 msi 1.6        18        0.16                                      
Pitch-50 msi                                                              
           1.9        13        0.25                                      
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     Scanning electron microphotes of the cross-section of PAN and pitch fibers show a macrostructural difference with the PAN graphite fiber exhibiting little structure texture, while pitch-based fibers have a radial structure. 
     Referring now in more detail to the drawings, FIG. 1 shows a graphite fiber thermocouple device A without insulation. Thermocouple A includes a strand or thread 10 of single or multiple filaments of a pitch-based graphite fibers, and a second thread or strand 12 comprised of single or multiple filaments of PAN or rayon based graphite fiber. The fibers are joined at a thermocouple junction by tying in a knot at 14. Intermingling of filaments, or use of a high temperature adhesive may also be utilized. The fiber threads 10, 12 are supported by blocks 16 and 18 which act as electrical insulation from ground. From the insulation blocks the graphite threads 10, 12 are insulated by any common insulating sheath 20 such as plastic or fiberglass until terminated at a detector means 22 for converting and reading out the electrical voltage signal as a temperature. The detector means may be any suitable device such as a potentiometer for converting the electromotive force developed by the graphite strands into a meter reading indicating temperature. 
     In FIG. 2, a thermocouple B is illustrated which shows a first strand 24 consisting of monofilament or multifilament pitch graphite fiber, and a second strand 26 consisting of a monofilament or multifilament PAN or rayon graphite fiber. The graphite strands 24 and 26 are separated by an insulation material 28 which has a higher electrical resistance than graphite fibers 24, 26. The insulation may be surrounded by an elongated housing in the form a thin-wall sheath 30. The strands 10, 12 may be encapsulated in a suitable insulation material such as Coltronics 931 graphite adhesive molding compound within sheath 30. This material fixes the strands in their prescribed spacing as can best be seen in FIG. 2. An insulation material that may be used without a sheath is carbon of a higher resistance than the graphite fibers. This carbon may be molded from powder, deposited by vapor deposition, or from the polymerization of a carbon-containing material. Other insulating materials can be magnesia (Mgo), alumina (Al O), berylia (BeO), and boron nitride (BN). If a sheath is desired it can be made from molybdenum, tantalum, platinum-rhodium, or Inconel 600. 
     FIG. 3 is a cross-sectional view of the thermocouple in FIG. 2 further illustrating the separation of the multifilament threads 24 and 26 extending longitudinally in the elongated housing 20. 
     FIG. 4 is a graphical representation of the electrical output of the thermocouple described in this invention. It is an important requirement of materials used in thermocouples that their output be linear. A suitable material for the first strand 10, 24 may be a pitch graphite fiber strand, 1000 filaments, identifiable as Thornel Type P--grade V50066 manufactured by the Union Carbide Company. The second strand 12, 26 may be a PAN graphite fiber strand, 1000 filaments, identifiable as Hercules AS4 manufactured by the Hercules Chemical Company. 
     Thus, it can be seen that a highly advantageous termocouple can be had in accordance with the invention using a simplified construction of graphite fibers for measuring temperatures as high as 3500 degrees C. which has not been heretofore obtainable. 
     While a preferred embodiment of the invention has been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims.