Abstract:
A method for fabricating a thermocouple capable of long-term operation in high temperature, hostile environments without significant signal degradation or shortened thermocouple lifetime due to heat induced brittleness.

Description:
GOVERNMENT RIGHTS 
       [0001]    The United States Government has certain rights in this invention pursuant to Contract No. DE-AC07-05-ID14517, between the United States Department of Energy and Battelle Energy Alliance, LLC. 
     
    
     TECHNICAL FIELD 
       [0002]    This invention relates to a thermocouple design and fabrication and more specifically to a high temperature thermocouple capable of long-term operation in hostile high temperature environments without significant signal degradation. 
       BACKGROUND OF INVENTION 
       [0003]    The accurate measurement of temperatures between 1100° C. and 1700° C. is important to the safe, efficient and economical operation of many industries such as electrical power production, processing and refining of chemicals, the fabrication of steel and other metals, and production of glass and ceramic materials. Accurate temperature measurement over time can also be critical to the operation of industrial machinery such as jet engines, nuclear reactors, gasification units, incinerators, and gas turbines. In such temperature environments, thermocouples are the most widely used industrial temperature sensors because they are rugged, affordable and accurate—at least initially. 
         [0004]    Unfortunately, after installation all commercial thermocouples are unstable in this temperature range and prone to decalibration or “drift,” providing increasingly unreliable and unpredictable readings as they age. As operating temperatures and thermal cycles increase, the performance of these thermocouples decreases. Together, these factors often result in costly redundant instrument clusters, sensor failures, downtime and potential accidents due to undetected overheating. For temperatures above 1100° C. in radiation environments, such as in high-temperature nuclear test reactors, conventional thermocouples are incapable of stable and accurate operation. 
         [0005]    The thermocouple of the present invention overcomes the two most critical thermocouple issues plaguing high-temperature operations-signal drift and sensor longevity. The first problem with all conventional thermocouple sensors is that they are subject to decalibration. The uncertainties surrounding decalibration are difficult to quantify, but elevated temperatures and longer operating times inevitably result in increasingly unreliable measurements. Standard thermocouples drift appreciably after a few hundred hours of use, making accurate temperature measurement and high-temperature process control difficult without frequent sensor change out. For high-temperature nuclear applications there are even greater limitations. Currently there are no high-temperature thermocouples capable of withstanding neutron flux in nuclear fission reactors or, potentially one day, fusion reactors. The key to minimizing drift lies in selecting thermocouple materials with properties that do not interact with each other or appreciably change during use. 
         [0006]    A second problem is that prolonged heating, contamination, and thermal cycling increases brittleness and fragility and shortens thermocouple life. Most metals, including those used in thermocouples, become brittle with high temperature exposure; as a result, they can fail due to mechanical stresses induced by vibrations, expansion, and contraction. Heat from welding to form standard thermocouple junctions also can lead to mechanical failure from embrittlement. Compatibility of component metals at high temperature and improved joining methods are essential to improved thermocouple durability. 
         [0007]    It is an object to the invention to provide a high temperature thermocouple capable of operating in hostile environments for a long period of time without significant signal degradation. 
         [0008]    It is another object of the invention to provide a method of fabricating a high temperature thermocouple capable of operating in hostile environments for a long period of time without significant signal degradation. 
         [0009]    It is still a further object of the invention to provide a thermocouple capable of operating in a temperature range of 1100° C. to 1700° C. in a radiation environment. 
         [0010]    Additional object, advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following and practice of the invention. 
       SUMMARY OF THE INVENTION 
       [0011]    The present invention relates to a design and method for fabricating a thermocouple capable of operating for long periods of time in a high temperature, hostile environment without significant signal degradation. The invention includes providing two dissimilar thermoelement wires having high melting temperatures, high ductility and low neutron cross-sections. The thermoelement wires are joined at one end by swaging a metallic tube over the ends of the thermoelement wires to form a thermocouple junction. In one embodiment of the invention the thermoelement wires and junction with the insulation are heated to remove moisture and impurities. The swaged thermocouple junction helps avoid failure due to embrittlement associated with conventional junction forming techniques. This junction is also resistant to breakage due to metal fatigue and brittleness cause by operating at high temperatures for long periods of time. The invention utilizes insulating materials that are not reactive with the thermoelement wires or metallic sheath. In one embodiment of the invention a less porous insulating material is used to provide an interface with the insulating material surrounding the thermocouple junction. A metallic sheath is loaded with the insulating materials, thermoelement wires and thermocouple junction, and then the metallic sheath is swaged to provide intimate contact between the insulating materials and the thermoelement wires and thermocouple junction. The interface between the insulating materials is determined and the less porous insulating material is physically removed by cutting the thermocouple sheath at a location between the junction and the interface. The sheath is then capped to provide a leak-tight thermocouple. The thermocouple is then heated to a temperature for a sufficient time to stabilize grain structure. 
         [0012]    In one embodiment of the invention, the thermoelement wires are a doped molybdenum and niobium/zirconium alloy. For example the dopants of the molybdenum thermoelements are tungsten, potassium and silicon in amounts typically ranging from 100 to 300 parts per million. The niobium/zirconium alloy is 99% by weight niobium and 1% by weight zirconium. The metallic sheath is also comprised of the same niobium/zirconium alloy. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    Illustrative and presently preferred exemplary embodiments of the invention are shown in the drawings in which: 
           [0014]      FIG. 1  is a photograph showing a thermocouple junction comprising a metallic tube swaged onto thermoelement wires; 
           [0015]      FIG. 2  is a photograph showing an electrical insulating material having two interior passageways; 
           [0016]      FIG. 3  is a photograph showing a second electrical insulating material having a hollowed out void to receive the swaged junction; 
           [0017]      FIG. 4  is a sectional drawing illustrating the thermoelement wires, thermocouple junction and insulating materials; 
           [0018]      FIG. 5  is a sectional drawing illustrating the thermoelement wires, thermocouple junction and insulating materials within a metallic sheath; 
           [0019]      FIG. 6  is a graph showing the measured temperature over a period of time of the present invention and conventional Type K and Type N thermocouples. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    Referring now to the drawings in which like numerals represent like elements throughout the several views, the preferred embodiment of the present invention will be described. 
         [0021]    The present invention employs a thermoelement wire combination of doped molybdenum and niobium/1% zirconium alloy with a sheath of the same niobium/1% zirconium. The molybdenum is doped with the elements tungsten, potassium, and silicon (typically dopants are present in the range of 100-300 ppm). Molybdenum is an excellent refractory metal but recrystallizes upon heating above 1200° C. The doped molybdenum selected for the present invention remains ductile after heating for 12 hours at 1800° C. Niobium has excellent ductility, a high melting temperature, and low neutron absorption. Alloying the niobium with zirconium increases its recrystallization temperature. Importantly, molybdenum and niobium are both less expensive than the metals used for conventional high-temperature tungsten/rhenium or platinum/rhodium thermocouples. Hence, in a preferred embodiment of the invention, these thermoelement materials are selected based on factors such as cost, melting temperature, ductility, and low neutron cross-section. 
         [0022]    During fabrication, care must be taken to avoid contaminating the thermoelement wires, metallic junction tube, and sheath. This is accomplished by cleaning these components with a solvent, such as acetone, and handling the components with gloves. 
         [0023]    Referring now to  FIG. 1  a photograph of a metallic tube swaged onto the thermoelement wires to form a thermocouple junction is shown. The use of swaged metallic tubes, such as tantalum tubes, to form the junction eliminates the requirement of a welded junction, which have a tendency to become brittle and mechanically unstable at higher temperatures. 
         [0024]    In developing the present invention, hafnium dioxide (HfO 2 ) insulation was found to be an excellent pairing with doped molybdenum and niobium-zirconium alloy. In fact, tests indicate no discernible interactions up to at least 1600° C. (and material properties suggest that higher temperatures are viable). The chemical stability of the thermocouple/insulation pairing contributes to signal stability of the present invention. The insulating material of a thermocouple must have high electrical resistivity and must not interact chemically with the thermoelement wires or sheath materials. Even small changes in thermoelement composition can result in decalibration. This is often due to temperature-induced migration of impurities from the sheath and insulation to the thermoelement wires. For standard thermocouples, metal/insulation interactions are one cause of drift, but at 1600° C., reaction kinetics make material interactions difficult to avoid. Clearly, minimizing component material interactions aids in minimizing thermocouple drift. 
         [0025]    The electrical insulating materials are heated in a desiccator at a high temperature for a sufficient time to outgas impurities and moisture. Preferably, the insulation is heated to a temperature of at least 120° C. for at least twenty four hours. Heating at a higher temperature will reduce the period of time needed for heating the insulating material. 
         [0026]      FIG. 2  shows the insulating material having two interior passageways through which the ends of the thermoelement wires are threaded.  FIG. 3  is a photograph of a second insulating material in which a void has been created by hollowing out an interior portion of the insulating material. The void is formed to receive the thermocouple junction. 
         [0027]    A sectional drawing of the invention is shown in  FIG. 4  which illustrates the doped molybdenum thermoelement wire  12  and the niobium-1% zirconium thermoelement wire  14  that have been threaded though the first insulation material  16  passageways  18  and  20 . The first insulating material is in contact with the second insulating material  22 . The first insulating material  16  has a void  23  formed to receive the junction after the wires are threaded into passageways  18  and  20 . The second insulating material  22  has a void  24  formed therein for receiving the thermocouple junction  26 . 
         [0028]      FIG. 5  is a sectional drawing showing the first and second insulating materials  16  and  22  respectively, containing the thermoelement wires  12  and  14  within a metallic sheath  28 . In contact with the second insulating material  22  within the sheath  28  is a third insulating material  30 . Sheath  28  is comprised of a material that does not react with the insulating materials. Preferably sheath  28  is a niobium/zirconium alloy, such as a 99% niobium/1% zirconium alloy. The third insulating material  30  is comprised of a material that is less porous than the first and second insulating material  16  and  22 . For example, the third insulating material may be comprised of aluminum oxide (hereafter “alumina”). 
         [0029]    After insulators are threaded onto the thermoelement wires, the assembly is loaded into the Nb1% Zr sheath. A piece of hafnium dioxide (hereafter “hafnia”) insulating material  22  with a void  24  is used to cover the junction. Then, a piece of alumina insulation  30  is placed above the hafnia insulation  22  to facilitate swaging. Experience indicates that the less porous alumina prevents undesirable crushing of the junction and thermoelement wires observed when only hafnia is placed within the sheath. The sheath containing the junction, wire, and insulation assembly is placed in an oven and baked for at least 8 hours at 120 C. 
         [0030]    The sheath  28  containing the thermoelement wires and insulating materials is then swaged to provide intimate contact between the thermoelement wires  12  and  14 , thermocouple junction  26  and the insulating materials  16  and  22 . The swaging step results in a repositioning of the interface between the second insulating material  22  and third insulating material  30 . The interface between the second and third insulating materials can be ascertained by means of an x-ray imaging system or other radiography techniques. In the present invention, the thermocouple is cut orthogonally to its longitudinal axis at a location between the junction and this interface between the insulating materials. The end is then prepared for receiving an end cap (not shown) by sanding the end of the sheath flat. The end cap consists of a niobium plug that is then laser welded to the cut end of the sheath. 
         [0031]    The thermocouple is then checked for leak-tightness using a helium leak detection system. As a final step in the fabrication of the present thermocouple invention, a length that will encompass the anticipated length that will see a temperature gradient during its planned operating conditions is inserted into a tube furnace and heat treated at an appropriate temperature above the anticipated service temperature, for a sufficient duration to ensure assure stable response. The thermocouple is typically heated at least 100° C. above its planned service temperature for at least 3 hours. 
         [0032]    Long duration tests in a high-temperature furnace at the Idaho National Laboratory (INL) demonstrate that the present invention remains stable with less than 2% drift after operating 4,000 hours at 1200° C. (see  FIG. 6 ). Compare this with the leading commercially available Type N and K thermocouples often used for this temperature range that began to drift beyond 2% after only 200 hours at temperature. The present invention offers a twenty-fold performance improvement in resistance to drift over this current technology. Similarly, less than 2% drift was observed in present invention tested at 1400° C. for 4,000 hours, well beyond the use range for Type N and K thermocouples. 
         [0033]    For temperature ranges from 1100° C. to 1700° C., the present invention competes well with more expensive tungsten-rhenium or platinum-rhodium thermocouples that are prone to failure after prolonged temperature exposure and repeated thermal cycling. The enhanced performance of the present invention is due to the ductility of its component metals and the unique thermoelement joining method that results in a rugged sensor, offering significantly longer, more stable service. The result is fewer open-circuit failures (from fractures in the thermoelement wires or at the junction) common to high-temperature thermocouples. Finally, because the present invention is made from metals with very low thermal neutron cross-sections, it can be used in nuclear reactors without suffering decalibration due to neutron-induced transmutation. 
         [0034]    At temperatures above 1100° C., the present invention is a superior replacement for currently installed Type K and N thermocouples with improved reliability, accuracy and longevity. At higher temperatures (1100° C. to 1700° C.) the invention offers even more advantages and can be more durable and less prone to drift than competing Type B, C, D, R, or S thermocouples. All of this can be achieved for a lower projected per-unit price. The thermocouple of the present invention performs well in this critical gap where other sensors often fail. Furthermore, the invention is well-suited for control of long-duration, high-temperature processes with frequent thermal cycling that often causes competing sensors to fail. For the nuclear industry, the present invention thermocouple offers clear advantages for affordable and reliable in-pile high-temperature monitoring. 
         [0035]    In compliance with the statute, the invention has been described in language more or less specific as to structural and methodical features. It is to be understood, however, that the invention is not limited to the specific features shown and described, since the means herein disclosed comprise preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims appropriately interpreted in accordance with the doctrine of equivalents.