US2832705A - Process for improving the stability of base metal thermoelements - Google Patents

Process for improving the stability of base metal thermoelements Download PDF

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Publication number
US2832705A
US2832705A US571870A US57187056A US2832705A US 2832705 A US2832705 A US 2832705A US 571870 A US571870 A US 571870A US 57187056 A US57187056 A US 57187056A US 2832705 A US2832705 A US 2832705A
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Prior art keywords
coating
base metal
solution
wires
base
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Expired - Lifetime
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US571870A
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Seidl Alois
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Evonik Operations GmbH
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Degussa GmbH
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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01K—MEASURING TEMPERATURE; MEASURING QUANTITY OF HEAT; THERMALLY-SENSITIVE ELEMENTS NOT OTHERWISE PROVIDED FOR
    • G01K7/00—Measuring temperature based on the use of electric or magnetic elements directly sensitive to heat ; Power supply therefor, e.g. using thermoelectric elements
    • G01K7/02—Measuring temperature based on the use of electric or magnetic elements directly sensitive to heat ; Power supply therefor, e.g. using thermoelectric elements using thermoelectric elements, e.g. thermocouples
    • G01K7/04—Measuring temperature based on the use of electric or magnetic elements directly sensitive to heat ; Power supply therefor, e.g. using thermoelectric elements using thermoelectric elements, e.g. thermocouples the object to be measured not forming one of the thermoelectric materials
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N—ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10N10/00—Thermoelectric devices comprising a junction of dissimilar materials, i.e. devices exhibiting Seebeck or Peltier effects
    • H10N10/80—Constructional details
    • H10N10/85—Thermoelectric active materials
    • H10N10/851—Thermoelectric active materials comprising inorganic compositions
    • H10N10/854—Thermoelectric active materials comprising inorganic compositions comprising only metals
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
    • Y10T428/00—Stock material or miscellaneous articles
    • Y10T428/29—Coated or structually defined flake, particle, cell, strand, strand portion, rod, filament, macroscopic fiber or mass thereof
    • Y10T428/2913—Rod, strand, filament or fiber
    • Y10T428/2933—Coated or with bond, impregnation or core
    • Y10T428/294—Coated or with bond, impregnation or core including metal or compound thereof [excluding glass, ceramic and asbestos]
    • Y10T428/2958—Metal or metal compound in coating

Definitions

  • this object could be achieved by applying a special coating upon the wires or rods employed for such measuring instruments.
  • the special coatings according to the invention are not only stable in the furnace atmospheres usually encountered, including those of oxidising, as well as of reducing nature, but also especially those containing gaseous sulfur containing compounds.
  • the special coatings according to the invention are produced by applying a coating of mixed silicates, which preferably is thin, upon the surfaces of the wires or rods.
  • the mixed silicate coating according to the invention is applied as an essentially aqueous solution of an alkali metal silicate and of a water soluble compound of a second'metal, which is built into the coating achieved in addition to the alkali metal.
  • the water soluble additional metal salt is added directly to the alkali metal silicate solution applied to the wires or rods in the form of a water soluble alkali stable salt, for example, the additional metal can be in the form of an alkali stable complex salt.
  • Heavy metals primarily come into consideration as the additional metal which is to be introduced into the coating as a mixed silicate with the alkali metal.
  • multivalent heavy metals are employed which deliver ions in aqueous solution having at least two different valences.
  • Such metals for example, are arsenic, bismuth, chromium, copper, iron, manganese, molybdenum, nickel, lead, antimony and, preferably, cobalt or vanadium.
  • other metals which are monovalent, such as, silver or zinc can also be used.
  • mixtures of such additional metals can also be incorporated in the mixed silicate coating according to the invention.
  • the heat treatment of the coating according to the invention is eifected after it has been dried.
  • the heat treatment can be a special heat treatment or such heat treatment can occur when the thermoelement is first put into use. It is only essential that'such heat treatment is effected in a non-reducing atmosphere, as this is essential for the formation of the oxides of the base metals which are taken up by the coating and provide the excellent bond between the coating and the base metal.
  • the protective coating produced according to the invention can be maintained extraordinarily thin in com parison to normal enamel coatings.
  • the thickness of a coating according to the invention is not more than 50g.
  • the protective coating which is substantially impervious to gases, is extraordinarily elastic and can be applied to long lengths of wire which can then be wound on rolls without damage to the protective coating.
  • wires coated according to the invention can be welded together Without removal of the coating for the production of thermocouples Without causing any difficulties of contact at the weld.
  • thermocouples when the thermocouples are produced by welding precoated wires, the welded portion can subsequently also receive a protective coating according to the invention.
  • the coatings according to the invention can be applied to the wires or rods or thermocouples by the usual methods by dipping them in or spraying or painting them with the appropriate solution. It is essential, however, to select the coating thickness sufliciently thin that drop formation does not occur at the high temperatures at which the thermoelements coated therewith are used. After the silicate containing solutions are applied they are generally heated for one hour or more at temperatures of 200 C. and higher in a non-reducing atmosphere. The time and temperature of the heat treatment is so selected with reference to the base metal coated that a small quantity of oxide of the base metal is built into the silicate coating which is produced.
  • a portion of the water can be replaced by essentially inert water soluble organic solvents, such as, alcohols, aldehydes, ketones and the like.
  • the alkali metal silicate employed in the production of the coating solution is one in which the proportion of alkali metal oxide to silicon dioxide is 113.3 to 3.9.
  • Example 1 10 cc. commercial N21 O:SiO -:l:3.9)
  • thermocouple After removing the thermocouple from the solution, it was dried in air and then heated in air for 1 hour at 200 C.
  • Example 2 7 cc. of a 1% aqueous solution of cobalt nitrate .were mixed with -5 cc. of a 5% aqueous ammoniumnitrate solution and theresulting solution then slowly stirredinto .10 ccaofa water glass solution of 'the same compositionasjn Example 1.
  • the Wires which are to be protected, nickel Wires and nickel chromium Wires were spooled oil from rolls and passed through the solution and thereafter passed through adrying ovenmaintained at 200m 250 C. for aperiod-of 15 minutes and then Wound backon spools.
  • Example 3 A coating solution was prepared by mixing:
  • Example 5 A coating solution was prepared by mixing:
  • Example 7 A coating solution was prepared by mixing:
  • a process for increasing the chemical stability of base metal Wires and rods employed for thermoelectric measurement of temperatures which comprises applying a thin coating of an aqueous solution of an alkali metal sit containing a relatively small quantity of at least one water soluble heavy metal salt to the surface thereof and subsequently heating the coating to a temperature over 200 C. in a no reducing atmosphere until oxides formed from the base metal ,base aretaken up bysuch coating.
  • said :heavy metal salt is a salt of a multivalent heavy metal ,WhiCh delivers ions in aqueous solutions having at least 2 dificrent valences.
  • a process for increasing the chemical stability of base metal thermocouples which comprises applying .a thin coating of an aqueous solution of an alkali metal silicate containing a relatively small quantityvofatgleast one water soluble heavy metal salt tothe surface-thereof and subsequently heating the coating to a temperature over 200 C. in a non-reducing atmosphere untiloxides formed from the base metal base are taken up by such coating.
  • a base metal thermocouple the surfaces of which are protected by a coating produced by the process according to claim 5.

Description

United States atet Alois Seidl, Lam, Niederbayern, Germany, assignor to Deutsche Goldund Silber-Scheitleanstalt vormals Roessler, Frankfurt am Main, Germany No Drawing. Application March 16, 1956 Serial No. 571,370
Claims priority, application Germany March 22, 1955 6 Claims. (Cl. 148-414} and that as a result thereof their accuracy suffers during e extending use in addition to undergoing possible mechanical damage. For example, especially in sulfur containing atmospheres, base metal thermoelectric elements, such as, those of nickel, nickel-chromium or iron construction,
relatively rapidly become brittle and subject to breakage.
Previously, attempts have been made to overcome these difiiculties by the use of protective tubes which enclose the actual measuring elements. However, the difiiculty in selecting a suitable material for such tubes with reference to chemical stability, stability against changes in temperature and mechanical strength often provide new problems which prevented a satisfactory solution.
It is an object of the invention to provide a way to increase the stability of base metal wires or rods such as are used in the form of thermocouples for electrical measuring instruments so that they can be used for long periods of time Without deleterious attack upon the metal of the wires or rods and the accompanying reduction in accuracy of such measuring instruments.
According to the invention, it was found that this object could be achieved by applying a special coating upon the wires or rods employed for such measuring instruments. The special coatings according to the invention are not only stable in the furnace atmospheres usually encountered, including those of oxidising, as well as of reducing nature, but also especially those containing gaseous sulfur containing compounds.
The special coatings according to the invention are produced by applying a coating of mixed silicates, which preferably is thin, upon the surfaces of the wires or rods.
The mixed silicate coating according to the invention, contrary to the normal practices in applying enamel-like coatings, is applied as an essentially aqueous solution of an alkali metal silicate and of a water soluble compound of a second'metal, which is built into the coating achieved in addition to the alkali metal. Advantageously, the water soluble additional metal salt is added directly to the alkali metal silicate solution applied to the wires or rods in the form of a water soluble alkali stable salt, for example, the additional metal can be in the form of an alkali stable complex salt. After application of the coatings to the surfaces of the wires or rods, such coatings are subjected to a heat treatment at a temperature above 200 C. in a non-reducing atmosphere. During such heat treatment, oxides formed from the base metal migrate into the coating and are taken up thereby in a noticeable amount and this provides for an extraordinarily strong J ice bond and coalescence between the coating and the base metal.
Heavy metals primarily come into consideration as the additional metal which is to be introduced into the coating as a mixed silicate with the alkali metal. Preferably, multivalent heavy metals are employed which deliver ions in aqueous solution having at least two different valences. Such metals, for example, are arsenic, bismuth, chromium, copper, iron, manganese, molybdenum, nickel, lead, antimony and, preferably, cobalt or vanadium. However, other metals which are monovalent, such as, silver or zinc, can also be used. Of course, mixtures of such additional metals can also be incorporated in the mixed silicate coating according to the invention.
Preferably, the heat treatment of the coating according to the invention is eifected after it has been dried. The heat treatment can be a special heat treatment or such heat treatment can occur when the thermoelement is first put into use. It is only essential that'such heat treatment is effected in a non-reducing atmosphere, as this is essential for the formation of the oxides of the base metals which are taken up by the coating and provide the excellent bond between the coating and the base metal.
The protective coating produced according to the invention can be maintained extraordinarily thin in com parison to normal enamel coatings. Expediently, the thickness of a coating according to the invention is not more than 50g. As a result, the protective coating, which is substantially impervious to gases, is extraordinarily elastic and can be applied to long lengths of wire which can then be wound on rolls without damage to the protective coating. Furthermore, wires coated according to the invention can be welded together Without removal of the coating for the production of thermocouples Without causing any difficulties of contact at the weld. Of course,
when the thermocouples are produced by welding precoated wires, the welded portion can subsequently also receive a protective coating according to the invention.
The coatings according to the invention can be applied to the wires or rods or thermocouples by the usual methods by dipping them in or spraying or painting them with the appropriate solution. It is essential, however, to select the coating thickness sufliciently thin that drop formation does not occur at the high temperatures at which the thermoelements coated therewith are used. After the silicate containing solutions are applied they are generally heated for one hour or more at temperatures of 200 C. and higher in a non-reducing atmosphere. The time and temperature of the heat treatment is so selected with reference to the base metal coated that a small quantity of oxide of the base metal is built into the silicate coating which is produced.
In the production of the silicate coating solutions according to the invention a portion of the water can be replaced by essentially inert water soluble organic solvents, such as, alcohols, aldehydes, ketones and the like.
Preferably, the alkali metal silicate employed in the production of the coating solution is one in which the proportion of alkali metal oxide to silicon dioxide is 113.3 to 3.9.
The following examples will illustrate several modifications according to the invention:
Example 1 10 cc. commercial N21 O:SiO -:l:3.9)
7 cc. 8% aqueous ethyl alcohol 5 cc. 1% aqueous solution of Na Co(N0 water glass solution (d.=l.27;
After removing the thermocouple from the solution, it was dried in air and then heated in air for 1 hour at 200 C.
Example 2 7 cc. of a 1% aqueous solution of cobalt nitrate .were mixed with -5 cc. of a 5% aqueous ammoniumnitrate solution and theresulting solution then slowly stirredinto .10 ccaofa water glass solution of 'the same compositionasjn Example 1. The Wires which are to be protected, nickel Wires and nickel chromium Wires were spooled oil from rolls and passed through the solution and thereafter passed through adrying ovenmaintained at 200m 250 C. for aperiod-of 15 minutes and then Wound backon spools.
The following are further examplesof coating solutions according t0 the invention which can be applied to base metal wires or rods to be used for thermocouples or to the thermocouples themselves in a manner analogous to those describedin Examples 1 and 2:
Example 3 A coating solution was prepared by mixing:
10 cc. of a 0.05 aqueous sodium chromate solution 7 cc. of 0.l% aqueous Na Fe(CN) solution Example 5 A coating solution was prepared by mixing:
107cc. of a water glass solution (d.=1.33) 5 cc. of water 7 cc. of a 1% aqueous K Ni(CN) solution Example 6 A coating solution Was prepared by mixing 10 cc. of a Water glass solution (d.=1.33) with 5 cc. of Water, dissolving 0.1 gram of V 0 therein and adding 5 cc. of a 5% aqueous ethanol solution.
Example 7 A coating solution was prepared by mixing:
I claim:
1. A process for increasing the chemical stability of base metal Wires and rods employed for thermoelectric measurement of temperatures which comprises applying a thin coating of an aqueous solution of an alkali metal sit containing a relatively small quantity of at least one water soluble heavy metal salt to the surface thereof and subsequently heating the coating to a temperature over 200 C. in a no reducing atmosphere until oxides formed from the base metal ,base aretaken up bysuch coating.
2. A process according to claim 1 in which said :heavy metal salt is a salt of a multivalent heavy metal ,WhiCh delivers ions in aqueous solutions having at least 2 dificrent valences.
3. A process according to claim 1 in which said heavy metal salt is a cobalt salt.
4. A process according to claim 1 in which said coating is up to 50 thick.
5. A process for increasing the chemical stability of base metal thermocouples which comprises applying .a thin coating of an aqueous solution of an alkali metal silicate containing a relatively small quantityvofatgleast one water soluble heavy metal salt tothe surface-thereof and subsequently heating the coating to a temperature over 200 C. in a non-reducing atmosphere untiloxides formed from the base metal base are taken up by such coating.
6. A base metal thermocouple, the surfaces of which are protected by a coating produced by the process according to claim 5.
References Cited in the file of this patent UNITED STATES PATENTS 2,321,658 Chester June15, 1943

Claims (1)

1. A PROCESSS FOR INCREASING THE CHEMICAL STABILITY OF BASE METAL WIRES AND RODS EMPLOYED FOR THERMOELECTRIC MEASUREMENT OF TEMPERATURES WHICH COMPRISES APPLYING A THIN COATING OF AN AQUEOUS SOLUTION OF AN ALKALI METAL SILICATE CONTAINING A RELATIVELY SMALL QUANTITY OF AT LEAST ONE WATER SOLUBLE HEAVY METAL SALT TO THE SURFACE THEREOF AND SUBSEQUENTLY HEATING THE COATING TO A TEMPERATURE OVER 200*C. IN A NON-REDUCING ATMOSPHERE UNTIL OXIDES FORMED FROM THE BASE METAL BASE AE TAKEN UP BY SUCH COATING
US571870A 1955-03-22 1956-03-16 Process for improving the stability of base metal thermoelements Expired - Lifetime US2832705A (en)

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Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2978361A (en) * 1954-03-19 1961-04-04 Degussa Process for the surface treatment of metals
US3301702A (en) * 1963-05-09 1967-01-31 Allegheny Ludlum Steel Coating for the protection of ferrous base alloys at elevated temperatures
US3332796A (en) * 1961-06-26 1967-07-25 Philips Corp Preparing nickel ferrite single crystals on a monocrystalline substrate
US3818762A (en) * 1972-04-05 1974-06-25 Electro Nite Phase change detector cup
US5219617A (en) * 1989-09-19 1993-06-15 Michigan Chrome And Chemical Company Corrosion resistant coated articles and process for making same

Citations (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2321658A (en) * 1941-06-27 1943-06-15 Poor & Co Process for use in coating metal surfaces

Patent Citations (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2321658A (en) * 1941-06-27 1943-06-15 Poor & Co Process for use in coating metal surfaces

Cited By (6)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US2978361A (en) * 1954-03-19 1961-04-04 Degussa Process for the surface treatment of metals
US3332796A (en) * 1961-06-26 1967-07-25 Philips Corp Preparing nickel ferrite single crystals on a monocrystalline substrate
US3301702A (en) * 1963-05-09 1967-01-31 Allegheny Ludlum Steel Coating for the protection of ferrous base alloys at elevated temperatures
US3818762A (en) * 1972-04-05 1974-06-25 Electro Nite Phase change detector cup
US5219617A (en) * 1989-09-19 1993-06-15 Michigan Chrome And Chemical Company Corrosion resistant coated articles and process for making same
US5492766A (en) * 1989-09-19 1996-02-20 Michigan Chrome And Chemical Company Corrosion resistant coated articles and process for making same

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