Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
  • H01B1/00—Conductors or conductive bodies characterised by the conductive materials; Selection of materials as conductors
• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
  • C04B35/01—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on oxide ceramics
  • C04B35/42—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on oxide ceramics based on chromites
• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
  • C04B35/50—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on rare-earth compounds
  • C04B35/505—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on rare-earth compounds based on yttrium oxide
• • C—CHEMISTRY; METALLURGY
  • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
  • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
  • C04B35/00—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products
  • C04B35/515—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics
  • C04B35/58—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on borides, nitrides, i.e. nitrides, oxynitrides, carbonitrides or oxycarbonitrides or silicides
  • C04B35/584—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on borides, nitrides, i.e. nitrides, oxynitrides, carbonitrides or oxycarbonitrides or silicides based on silicon nitride
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
  • H05B3/00—Ohmic-resistance heating
  • H05B3/10—Heating elements characterised by the composition or nature of the materials or by the arrangement of the conductor
  • H05B3/12—Heating elements characterised by the composition or nature of the materials or by the arrangement of the conductor characterised by the composition or nature of the conductive material
  • H05B3/14—Heating elements characterised by the composition or nature of the materials or by the arrangement of the conductor characterised by the composition or nature of the conductive material the material being non-metallic
  • H05B3/141—Conductive ceramics, e.g. metal oxides, metal carbides, barium titanate, ferrites, zirconia, vitrous compounds
• • H—ELECTRICITY
  • H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
  • H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
  • H05B3/00—Ohmic-resistance heating
  • H05B3/62—Heating elements specially adapted for furnaces

Definitions

• This invention relates to a heating element in which the production of heat is obtained by Joule effect resulting from the flow of an electric current, said element being capable of opera-ting within a range extending from room temperature to temperatures above 2000 C. and even more especially in an oxidizing atmosphere.
• electrically conductive heating elements which are designed in the form of Wires, cables, strips, plates, deposits and the like which are fabricated from metals such as molybdenum, tantalum, niobium, tungsten or from a metalloid such as graphite. These elements permit of satisfactory operation from room temperature to temperatures which can attain 2500 or even 3000" C. provided that work is carried out in a neutral atmosphere over a period of time which does not exceed approximately twenty hours. Consideration has also been given to the fabrication of heating elements from a compound such as silicon carbide which is formed by means of special binders and permits of satisfactory operation in time and in an oxidizing atmosphere but only on condition that the temperature does not exceed 1600 C.
• the present invention overcomes the disadvantages of the various solutions which have been proposed heretofore by means of a heating element which is conductive from room temperature up to temperatures of at least 2000 C. and permits utilization in an oxidizing atmosphere over very long periods of time.
• the invention consists in fabricating the heating element from a chromite of the lanthanide series combined with a variable proportion of at least one silicon compound which exhibits oxidation resistance.
• the chromite of the lanthanide series is lanthanum chromite whilst the silicon compound can be selected from molybdenum silicon MoSi silicon nitride Si N silicon carbide SiC or a mixture of zirconium boride ZrB and molybdenum silicide MoSi According to another preferred feature, the variable proportion of the silicon compound is within the range of 5 to 20% by weight.
• the invention therefore consists in combining with a highly refractory compound such as a chromite of the lanthanide series of elements and especially lanthanum chromite which is electrically conductive from room temperature to at least 2000 C. a suitable silicon compound whose intended functions are to reduce to a considerable extent the vapor pressure of the chromite which is employed in particular in an oxidizing atmosphere, to increase the thermal conduction of the chromite and therefore to make this latter less sensitive to thermal shocks without thereby modifying its oxidation resistance.
• a highly refractory compound such as a chromite of the lanthanide series of elements and especially lanthanum chromite which is electrically conductive from room temperature to at least 2000 C.
• a suitable silicon compound whose intended functions are to reduce to a considerable extent the vapor pressure of the chromite which is employed in particular in an oxidizing atmosphere, to increase the thermal conduction of the chromite and therefore to make this latter less sensitive to thermal shocks without thereby
• the silicon compound is chosen so as to ensure that, during the process of manufacture, the grains of the chromite which is employed are coated with a refractory silica film which prevents vaporization of the chromite and isolates this latter from the surrounding atmosphere which can especially be highly oxidizing.
• the heating element thus formed can be produced with equal ease in the form of bars, strips, rods, hairpin elements, plates, cylinders and the like While being suited in each case to a particular use and especially for furnaces of the tubular or parallelepipedal type employed in industrial laboratories, the susceptors of high-frequency circuits or alternatively other thermal applications involving either intermittent or continuous operation.
• heating elements which are produced in accordance with the invention are essentially as follows: these elements are electrically conductive from room temperature to a temperature which is higher than 2000 C. and consequently dispense with the need for ancillary circuits in furnaces or the like having either zirconium or thorium oxide elements in which the use of the heating element can be contemplated only at temperatures above 1100 to 1200 C.; these ancillary circuits are usually formed of silicon carbide and have to be protected above this threshold value in order to prevent melting.
• the heating elements of lanthanide chromite alloyed with a silicon compound can also readily be employed in an oxidizing atmosphere (also, of course, in a neutral atmosphere) for any length of time which may be necessary.
• the operation of the elements can be maintained at the maximum temperature during these periods of time without any difliculty.
• the manufacture of a heating element in accordance with the invention involves the following steps: in a first step, the manufacture of lanthanum chromite is carried out in a solar or plasma furnace starting from chromium oxide and lanthanum oxide which are mixed in stoichiometric proportions.
• the product obtained is then ground to a powder having a particle size between 1 and 5 microns to which is added in adequate proportions within the range of 5 to 20% a silicon compound which is preferably selected from the products referred to earlier.
• the mixture which is thus formed is then finally shaped in a mold within an isostatic press under a pressure within the range of 1.5 to 5 t./cm. in order to obtain a part having a density which varies between 90 and 95% of theoretical.
• the final step consists in carrying out a baking process within a gas furnace in a slightly oxidizing atmosphere with a level-temperature stage at 1650-1700? C. for a period of one to two hours; during this step, the partial conversion of the silicon compound to silica results in coating of the lanthanum chromite grains. Heating tests are than carried out with a temperature rise in successive stages of 100 to 200 C. per hour with a longer stage at 1600 C.
• a heating element having reduced vapor pressure I and improved thermal conductivity and electrically conductive from room temperature to at least 2000 C. in an oxidizing atmosphere and over a long period of operation comprising a mixture of lanthanum chromiteand from 5 to 20% by weight of at least one oxidation-resistant silicon compound selected from the group consisting of molybdenum silicide, silicon nitride, silicon carbide and a mixture of molybdenum silicide and zirconium boride.

Landscapes

• Engineering & Computer Science (AREA)
• Chemical & Material Sciences (AREA)
• Ceramic Engineering (AREA)
• Manufacturing & Machinery (AREA)
• Materials Engineering (AREA)
• Structural Engineering (AREA)
• Organic Chemistry (AREA)
• Ceramic Products (AREA)
• Resistance Heating (AREA)

Applications Claiming Priority (1)

Publications (1)

Family

ID=9062268

Family Applications (1)

Country Status (7)

Cited By (4)

Families Citing this family (2)

• 1970
  • 1970-10-05 FR FR7035867A patent/FR2109151A5/fr not_active Expired
• 1971
  • 1971-10-01 BE BE773401A patent/BE773401A/xx unknown
  • 1971-10-01 GB GB4573371A patent/GB1308437A/en not_active Expired
  • 1971-10-04 US US00186425A patent/US3730911A/en not_active Expired - Lifetime
  • 1971-10-04 IT IT70251/71A patent/IT943125B/it active
  • 1971-10-05 NL NL7113641A patent/NL7113641A/xx unknown
  • 1971-10-05 DE DE19712149764 patent/DE2149764A1/de active Pending

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