Classifications

• • 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
• • 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
  • H05B3/10—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

Definitions

• This invention relates to heating elements of the kind including an electrically conductive metal oxide film on an electrically insulating substrate.
• Such devices are known, and may for example consist of a thin film of tin oxide deposited on a glass substrate by means of pyrolitic deposition.
• Such thin film heating elements are to be used in electrical appliances such as cooktops, it is desirable that they be capable of operating at high temperatures, up to 650°C. In applications such as electric kettles where the heating element is small, the element must be capable of handling high power densities, of the order of 10- 20 Watts cm "2 .
• Prior art devices have not proved satisfactory in these conditions. It has been found by the present applicants that tin oxide layers tend to become unstable with increasing temperature, due to the tendency for the oxide to change state. It has also been found that where fluorine is employed as an electron donor or conductivity carrier the properties of the film change irreversibly with increasing temperature, apparently due to the fluorine tending to leave the film at temperatures above 400°C.
• US Patent No. 4,889,974 of Auding, et al. describes thin film elements intended for temperatures beyond 600°C, using oxide films doped at high levels with pairs of compensating foreign atoms.
• the metal oxide films are doped with, maximally, 10 mol % of each of the foreign atoms compensating each other in pairs, the quantity of said acceptor-forming elements and said donor- forming elements differing maximally by 10%.
• the Auding patent describes the use of indium, boron, aluminium or zinc as the acceptor- forming dopant, and antimony or fluorine as the donor-forming dopant.
• a metal oxide layer of satisfactory stability in high power density applications may be obtained by doping with at least one and preferably two rare earth elements.
• the rare earth dopants are preferably cerium and lanthanum. Preferably these two rare earths are present in substantially equal concentrations.
• the presence of the rare earth dopants in the thin film layer has been found by the present applicants to have the effect of stabilising the oxidation state of the metal.
• the invention resides in a thin film electrical heating element including a layer of an electrically conducting metal oxide on an electrically insulating substrate, said metal oxide layer being doped with at least one rare earth element.
• the metal oxide is deposited on the substrate by pyrolysis of an organometallic base solution containing the at least one rare earth element.
• the metal oxide layer is tin oxide and contains two rare earth elements such as cerium and lanthanum.
• This aspect of the invention provides a thin film heating element which is capable of withstanding power densities of up to 10-20 Watts cm “" and/or temperatures in excess of 600°C.
• the invention resides in a method for the manufacture of a thin film heating element including the step of depositing a layer of metal oxide onto an electrically insulating substrate by pyrolysis of an organometallic base solution containing at least one rare earth element.
• the base solution contains both cerium and lanthanum in concentrations up to 5 mol %.
• Fig. 1 is graph showing the power dissipation versus time relationship for a thin film heating element made according to the invention.
• Fig. 2 shows the relationship between temperature and power at steady state for five elements having power ratings between 500 and 1330 watts.
• concentrations in the pyrolysis solution of 0.01 mol % preferred concentrations of each of the cerium and lanthanum are between approximately 1.25 mol % and approximately 3.75 mol %.
• concentrations in the pyrolysis solution of 0.01 mol % preferred concentrations of each of the cerium and lanthanum are between approximately 1.25 mol % and approximately 3.75 mol %.
• concentration of these rare earths will be chosen as that which contributes to film stability at the power densities for which the film is intended. Best results for films intended for operation at 20 Watts cm " " have been obtained using equal concentrations of approximately 2.5 mol %.
• the film is preferably doped with substantially equal quantities of donor and acceptor elements, the preferred dopants being antimony and zinc.
• concentrations of both antimony and zinc will be influenced by the resistivity which is required. We have found base solution concentrations for these materials in the region of 2.8 mol % to be suitable for heating element applications.
• a useful characteristic of such films in their application as heating elements arises from the positive temperature coefficient resistance of the film. This enables elements to be produced which are self-regulating, in that they will initially operate at a higher wattage and, with increasing temperature, stabilise at the lower design wattage.
• Suitable substrates include glass ceramics, silicon nitrides and other ceramic substrates as well as metallic substrates coated with high-temperature stable, electrically-insulating materials.
• the preferred substrate temperatures for applying the base solution with dopants range from 500 to 750°C.
• post annealing at approximately 600°C for at least one hour is carried out to assist in stabilising the film.
• Films according to this invention were manufactured from a solution using the spray pyrolysis process.
• monobutyl tin trichloride was used as a base solution, with 2.8 mol % antimony chloride, 2.8 mol % zinc chloride, 2.5 mol % cerium and 2.5 mol % lanthanum.
• These films were fabricated with effective resistances of 26 ohm, 30 ohm and 45 ohm to enable heaters of 2.2 kW, 1.8 kW and 1.2 kW respectively to be used, powered by a 240V mains supply voltage.
• the films were selectively deposited using high temperature masking inks which were removed by brushing after deposition of the film.
• the films deposited had a high degree of transparency.
• the resistive properties of the heating elements remained unchanged after 3500 cycles (40 minutes on and 20 minutes off) at 650°C.
• Fig. 1 shows the typical behaviour of the elements, where power dissipation is plotted against time of operation.
• the dissipation of the element commences at a high level and decreases as the resistance of the element increases with temperature, until a steady state condition is achieved at the design power consumption.
• power dissipation will temporarily increase, assisting in achieving rapid heating.
• Fig. 2 shows the relationship between temperature and power at steady state for five elements having power ratings between 500 and 1330 watts.
• Sheet resistances varying from around 60 ohms to above 400 ohms have been fabricated by varying the number of spray passes.
• the thin film thickness could be varied between 2000 Angstrom Units to around 14000 Angstrom Units by varying the number of spray passes.
• the films were deposited on various substrates including glass ceramics, alumina, silica glass and silicon nitride.
• films made in accordance with the invention may be used in low temperature applications, such as comfort heating, refrigerating defrost, and general heating.
• Heating elements of tubular shape manufactured using the above technology can be used in heat exchangers for flow applications, air-conditioning re-heaters, hair dryers, washing and drying appliances, and can also be used as radiating surfaces.

Landscapes

• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Ceramic Engineering (AREA)
• Resistance Heating (AREA)
• Surface Heating Bodies (AREA)

Priority Applications (7)

Applications Claiming Priority (2)

Publications (1)

Family

ID=3810212

Family Applications (1)

Country Status (10)

Cited By (5)

Families Citing this family (2)

Citations (5)

Family Cites Families (1)

• 1998
  • 1998-09-18 AU AUPP5995A patent/AUPP599598A0/en not_active Abandoned
• 1999
  • 1999-09-17 WO PCT/AU1999/000791 patent/WO2000018189A1/en not_active Application Discontinuation
  • 1999-09-17 KR KR1020017003505A patent/KR20010079859A/ko not_active Application Discontinuation
  • 1999-09-17 CN CNB998107727A patent/CN1146301C/zh not_active Expired - Fee Related
  • 1999-09-17 CA CA002344486A patent/CA2344486A1/en not_active Abandoned
  • 1999-09-17 BR BR9913812-3A patent/BR9913812A/pt not_active IP Right Cessation
  • 1999-09-17 NZ NZ510655A patent/NZ510655A/en unknown
  • 1999-09-17 ID IDW20010871A patent/ID29148A/id unknown
  • 1999-09-17 JP JP2000571716A patent/JP2002525829A/ja active Pending
  • 1999-09-17 EP EP99947118A patent/EP1120014A4/en not_active Withdrawn

Patent Citations (5)

Non-Patent Citations (2)

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