US2094102A - Thermoelectric apparatus - Google Patents

Description

Sept 28, 1937. G. R. FITTERER THERMOELECTRIC APPARATUS I I Filed Nov. 25, 1932 .2 sh 'eis-sheet 1 I I Carbonp x w w Q S x955 95am skim Qk Temperarure C INVENTOR Geokge fF. F/fferer ATTORNEY "Sept. 28, 1937. e. R. FITTERER 2,094,102

THERMOELECTRI-C APPARATUS Filed Nov. 25, 1932' 2 Sheets-Sheet 2 ffil/ dwelt? meazzs J4 12 6 07 Jmdwn INVENTOR George R F/fferer .nTTORNEY birle 2! Patented Sept. 28, 1937.

UNITED STATES PATENT OFFICE THERMOELECTRIC APPARATUS Application November 25, 1932, Serial No. 644,243

. 18 Claims.

This invention is related to the thermoelectric art, and particularly to improvements in thermocouples.

It is known that heating certain dissimilar materials in contact with each other develops an electromotive force across the heated interface. Where such couples" form part of an electrically conducting circuit, in which interfering temperature effects are avoided, this thermoelectric potential is useful for various purposes. Certain applications of thermocouples, such as for measuring temperature, are so exacting that elements of desired quality heretofore have been diificult to provide and for some purposes even impossible to w obtain. Thus it is customary to use at least three different kinds of couples to measure the temperature range from normal up to that of molten steel. Up to about 300 C. resort is had to a base metal couple of copper or of iron against constantan (an alloy of about 60 percent copper with about 40 percent nickel); at more elevated temperature up to about 1000 C. a couple of chromel (an alloy of approximately percent chromium with about 90 percent nickel) 25 against alumel (an alloy containing approximately 2 percent aluminum with about 90 percent nickel); and at yet higher temperatures a noble metal couple of platinum against an alloy of platinum with rhodium. 30 Many such' couples are readily subject to chemical attack or to physical change so that out of their appropriate limits of temperature they exhibit characteristic defects, such as irregular sensitivity measured in microvolts change of electric potential with one degree change of temperature. Y Moreover, even the noble metal couples are limited to use below about 1500 C. At higher temperatures recourse is had to less accurate methods, such as those of optical pyrometry. Such difficulties as inapplicability to extensive temperature ranges, substantial variability in rate of developing thermoelectric potential, and lack of reproducibility are attributable to chemical and physical changes in the thermoelements. These 45 changes are of such character as to be exceedingly difficult to control. Not only the conventional thermocouples, but proposed substitutes have encountered these defects of lack of stability. Moreover, many proposed thermoelements are hardly amenable to manufacture into thermocouples with suificient ruggedness for practical use.

It is an object of this invention generally to improve these conditions and to overcome these problems. Particularly these objects comprise the provision of couples that develop electric potential at substantially constant rates, and over extended ranges of temperature; that develop high potential and high sensitivity; that are capable of rugged construction for various commercial conditions; that yield reproducible results even at 5 elevated temperatures; or, even under severe conditions are relatively stable, physically and chemically.

This invention is set forth in the following description and particularly pointed out in the ap- 1 pended claims. Illustrative of this invention are the accompanying drawings, in which:

Fig. 1 presents a chart of values of various thermoelectric couples in comparison with a carbon-silicon carbide couple of the present invention;

Fig. 2 illustrates schematically a typical arrangement of a thermocouple embodying this invention,

Fig. 3 illustrates apparatus embodying a carhon-silicon carbide thermocouple according to the present invention; and

Fig. 4 shows in simple arrangement a carbon tube containing in thermocouple relationship a silicon carbide rod in accordance with this inven- 5 tion.

Reference is made to the accompanying drawings illustrative of this invention, wherein Fig. 1 presents a, chart representative of the thermoelectricpower of a carbon-silicon carbide couple of this invention, in comparison with couples of other materials as indicated, with temperatures as abscissae and electric potential in millivolts as ordinates. From the curve it is seen that this C-SiC couple produces a relatively high potential even at low temperatures. Also its sensitivity or rate of increase of potential with temperature rise is both high and constant even up to temperatures of the order of 2200 C.

Fig. 2 shows schematically a typical thermocouple having a heated thermojunction of dissimilar conductors l and 2, the cooler portions of which are in electrical circuit with some e1ectro= sensitive apparatus, such for instance as meter 3. Or various regulatory apparatus similarly may be in electro-responsive relation to the couple.

Fig. 3 shows conventionally a form of heating apparatus in which furnace l0 contains saturated silicon carbide or carborundum resistors or heatdeflecting elements H as an element of its structure exposed to heat. In the form shown. these are electrically connected for generating heat. An element H is hollowed sufficiently to contain a thermoelement such as carbon rod l3, insulated therefrom at the outer end and in electrical and from otherwise.

thermal contact therewith at the inner end or hot junction I 2. Elements l4 cool the outer ends or cool junction. An electric circuit through I I, I2 and I3 is completed through an electric measuring instrument l5 responsive to changes in E. M. F. or temperature difference between the hot junction I 2 and the cool end.

Fig.4 illustrates in simple manner a form of this invention in which a carbon tube closed at one end and designated as 20 contains an element 2|, for example a rod, in thermoelectric contact at the closed end of the tube, and insulated there- Element 2| for example is essentially a saturated silicon carbide rod, which is negative relatively to the carbon. Conventional conductor 22 completes the electric circuit of the thermocouple.

This invention is predicated on my discovery that these and other objects may be attained by thermocouples containing essentially certain relatively stable non-metallic conductors, to be described more particularly. For illustration reference is made to preferred embodiments of this invention, but without implying limitations beyond those inherent in the claims and the prior art, since this invention may be practiced in principle by other specific means.

This invention comprises thermocouples in which at least one element contains essentially certain saturated compounds that are relatively stable in air at elevated temperatures. Such are carbides of elements of the fourth group of the periodic system, of the iron group (iron, cobalt, nickel), of chromium, of molybdenum, and of tungsten.

For example, a couple of carbon against silicon carbide (CSiC) produces a remarkably high electromotive force over an exceedingly wide range of temperature. In this couple the SiC element is negative relatively to the carbon. This E. M. F. is considerably higher than is obtained in the usual base metal couples, is approximately thirty to thirty-five times as high as that of platinum-platinum rhodium couples, and is substantially constant from very low temperatures up to temperatures far above the limits of Pt-PtRh couples. sistant to chemical attack and is readily adaptable to withstand direct exposure to media that are corrosive to former thermocouples. The rate of change of E. M. F. with temperature is reversible, high and constant even up to temperatures of the order of 2200 C. It is approxi-. mately 300 microvolts for every degree Centigrade temperature change. In comparison, the sensitivity of base metal couples is of the order of 40 to microvolts, and of the noble metal couple Pt-PtRh is only about 10 microvolts.

Considerable latitude is permissible in the manufacture of this thermocouple. The carbon element may be indifferently amorphous or graphitic carbon. Practical considerations may lead to preference for one or the other under particular circumstances, but thermoelectric characteristics arescarcely different between the two forms. This is shown in Fig. 1. Concerning the silicon carbide element the product known commercially as carborundum suitably produces a high E. M. F. I

Commercial practice as to carborundum affords .many convenient forms and processes for em-' bodying silicon carbide in a thermocouple. The relatively high E. M. F. of such a couple as this renders less significant than in former couples the presence of extraneous material. Hence various bonding agents may be used for either or plicable.

Moreover, this couple is extremely re-' both the carbon and the carborundum, having in mind the retention of proper electrical conductivity and the requirements of the couple for particular applications.

The utility of such a couple is extensive, and many practical applications will occur to those familiar with the properties of the thermoelements. Because of the high E. M. F. available, measuring instruments may be of more rugged construction than heretofore. Similarly a given instrument may be considerably more sensitive. Thus, with sensitivity of about 300 microvolts, temperatures upward of 2000 C. may be ascertained. At 1700 C. I find the accuracy within one or two degrees Centigrade. Moreover, the potential immediately developed by the couple may be magnified or used in various regulatory apparatus, including those of thermionic or lightsens'itive nature.

In form of construction this thermocouple has been found practical as a rod of one element con tained in a tube of the other element, with their temperature responsive ends in contact but insulated otherwise. At the constant-temperature end, a cooling coil of conventional form is ap- Which element is placed outside will depend on the various conditions of use. Thus for example, for measuring temperatures of molten steel or slag the carbon element may constitute the tube as shown in Fig. 4 and be placed directly in the liquid. This facilitates quick response of the thermocouple. Similarly for measuring molten glass temperature, though particular conditions may alter this arrangement. For measuring the temperatures of hot gases, as for example coke oven or gas furnace or regenerator gases, either element may'be outside. IIn other instances carborundum already is an element of the apparatus, such as furnace resistors or heatdeflecting baflies or walls of furnaces and the like; so the carbon element may be suitably arranged to contact with such element, either continuously or intermittently. For example, a carborundum resistor may be hollowed sufficiently to permit containing the carbon element in thermocouple relationship. In some instances it may be desirable to protect the couple with the usual protecting tube. In certain apparatus where the walls are fusible, as glass, the thermocouple or the outer element may be sealed in.

For illustration, particular reference has been made to a couple of carbon against silicon carbide, but the invention is not to be thus restricted. Nor is it restricted to temperature measurement norto high temperature applications.

Further exemplification of this invention is found in a thermocouple of carbon against certain other saturated carbides. Thus, titanium carbide against carbon forms a thermocouple which, like silicon carbide, exhibits relatively high stability and constant thermoelectric power. Likewise zirconium carbide against carbon is representative of this invention Further examples are found in couples of carbon against saturated carbide of molybdenum or of tungsten.

An illustrative variation of this invention is obtained with a thermocouple of carbon against the product of carburizing an alloy of suitable metals, as carburizing chromel (nickel and chromium). This product 'is saturated in the sense used in this specification, that further heating with carbon produces no substantial change in composition, as evidenced by thermoelectric constancy at a given temperature. The

product is stable in the sense of being resistant physically as well as chemically on heating appropriately in such gases as air, carbon oxide or nitrogen. As used in this specification carburized mixtures of metals includes'mixtures of stable saturated carbides as well as stable carburized alloys. Suitable bonds may be used. The manufacture may comprise compacting the carbides or sintering them.

Although for illustration reference has been directed particularly to couples of carbon against saturated stable carbides, it is contemplated that for some purposes there may be replacement of these individual thermoelements. An example is a couple having carbon replaced. Thus for many purposes platinum against the carbide constitutes a thermocouple of considerable utility. This is so particularly since it is contemplated under this invention that these thermocouples may be utilized at relatively low temperatures, even though the sign or direction of thermo-electric potential be reversed. Or the carbon thermoelement may be replaced by carbide, as for example athermocouple of silicon carbide against tungsten carbide. In this instance tungsten carbide is selected as having a relatively low E. M. F. so that its difierence against silicon carbide will be considerable.

In replacing the carbide thermoelement, the benefits of this invention may be obtained by substituting nitrogen or boron for some or all of the carbon of the carbide, having in mind that constancy of composition under the conditions of production and use is requisite where it is desired to obtain reproducibility and substantially constant thermoelectric rate.

In the claims the term metal is intended to include the element silicon.

According to the provisions of the patent statutes, I have explained the principle and mode of operation of my invention, and have illustrated and described what I now consider to represent its best embodiment. However, I desire to have it understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically illustrated and described.

What is claimed is:

1. A thermocouple comprising essentially saturated carbide of metal as thermoelement against saturated carbide of other metal as thermoelement a silicon carbide thermoelement herein comprised being negative to carbon thermoelectrically above room temperatures.

2. A thermocouple comprising saturated tungsten carbide as thermoelement against saturated silicon carbide as thermoelement a silicon carbide thermoelement herein comprised being negative to carbon thermoelectrically above room temperatures;

3. A thermocouple comprising a thermoelement selected from a group consisting of boride, essentially saturated carbide and nitride of metal selected from a group consisting of metal of the fourth group of the periodic table, chromium, molybdenum, tungsten, iron, cobalt and nickel a silicon carbide thermoelement herein comprised being negative to carbon thermoelectrically above room temperatures.

4. A thermocouple comprising carbon as thermoelement against a thermoelement which above normal temperatures is negative to carbon thermoelectrically and consists essentially ofsaturated carbide of metal selected from metal of the fourth group of the periodic table, chromium, molybdenum, tungsten, iron, cobalt and nickel a silicon carbide thermoelement herein comprised being negative to carbon thermoelectrically above room temperatures.

5. A thermocouple comprising a thermoelement which consists essentially of saturated compound selected from a group consisting of metal carbide, borlde, and nitride, said thermoelement being free from tantalum carbide a silicon carbide thermoelement herein comprised being negative to carbon thermoelectrically above room temperatures.

6. A thermocouple comprising a thermoelement which consists essentially of saturated carbide of metal selected from a group consisting of silicon, titanium, germanium, zirconium, chromium, molybdenum, tungsten, iron, cobalt and nickel a silicon carbide thermoelement herein comprised being negative to carbon thermoelectrically above room temperatures.

7. A thermocouple comprising a thermoelement of carbon against a thermoelement which consists essentially of saturated, stable carbide, said thermocouple being free from tantalum carbide asilicon carbide thermoelement herein comprised being negative to carbon thermoelectrically above room temperatures.

8. A thermocouple comprising a thermoelement which consists essentially of a mixture of saturated carburized metals, said. thermolement being free from tantalum carbide a silicon carbide thermoelement herein comprised being negative to carbon thermoelectrically above room temperatures.

9. A thermocouple comprising a thermoelement which consists essentially of saturated carbide against a thermoelement selected from a group consisting of carbon, carbide, boride and nitride, said thermocouple being free from tantalum carbide a silicon carbide thermoelement herein comprised being negative to carbon thermoelectrically above room temperatures.

10. A thermocouple comprising essentially saturated silicon carbide as a thermoelement a silicon carbide thermoelement herein comprised being negative to carbon thermoelectrically above room temperatures.

11. A thermocouple comprising essentially saturated silicon carbide against carbon a silicon carbide thermoelement herein comprised being negative to carbon thermoelectrically above room temperatures.

12. A thermocouple comprising carbon against carborundum the carborundum element being negative to the carbon thermoelectrically above room temperatures.

13. A thermocouple comprising carbon against a bonded thermoelement consisting essentially of saturated silicon carbide a silicon carbide thermoelement herein comprised being negative to carbon thermoelectrically above room temperatures.

14. Heating apparatus comprising saturated silicon carbide as an element of its structure exposed to heat, and means to contact a co-operative thermoelement with the silicon carbide element to form a thermojunction, and means comprising the thermojunction thus formed to complete a thermocouple responsive to temperature in the apparatus a silicon carbide thermoelement herein comprised being negative to carbon thermoelectrically above room temperatures.

15. A furnace having saturated silicon carbide resistors and means to contact a co-operating thermoelement with the silicon carbide for thermoelectric response to the temperature thereof, and an electric circuit comprising the thermocouple thus formed a silicon carbide thermoelement herein comprised being negative to carbon thermoelectrically above room temperatures.

16. A furnace having saturated silicon carbide resistors, and means to contact carbon with said resistors for thermoelectric response to the temperature thereof, and an electric circuit comprising the thermocouple thus iormed a silicon carbide thermoelement herein comprised being negative to carbon thermoelectrically above room temperatures.

17. A furnace having saturated silicon carbide as a heat-deflecting member, and means to embody carbon therein as thermoelement to form a 15 couple responsive to temperatures in the furnace a silicon carbide thermoelement herein comprised being negative to carbon thermoelectrically above room temperatures.

18. Heating apparatus comprising saturated silicon carbide as an element of its structure contributing to heating, and means to contact a co-operative thermoelement with the silicon carbide element to form a thermojunction, and

means comprising the thermojunction thus formed to complete a thermocouple responsive to temperature in the apparatus a silicon carbide thermoelement herein comprised being negative to carbon thermoelectrically above room temperatures.

GEORGE R.

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