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
  • H05B3/20—Heating elements having extended surface area substantially in a two-dimensional [2D] plane, e.g. plate-heater
  • H05B3/22—Heating elements having extended surface area substantially in a two-dimensional [2D] plane, e.g. plate-heater non-flexible
  • H05B3/26—Heating elements having extended surface area substantially in a two-dimensional [2D] plane, e.g. plate-heater non-flexible heating conductor mounted on insulating base
  • H05B3/265—Heating elements having extended surface area substantially in a two-dimensional [2D] plane, e.g. plate-heater non-flexible heating conductor mounted on insulating base the insulating base being an inorganic material, e.g. ceramic
• • 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
  • H05B3/148—Silicon, e.g. silicon carbide, magnesium silicide, heating transistors or diodes

Definitions

• the invention relates to a silicon heating element as described in YU patent application No. 1097/85 dated June 28, 1985 and to a method of its production.
• the conventional electric heater with wire heating element has a low efficiency particularly in open constructions (where is no forced transfer from source to load) and at low and medium heating temperatures (up to 400°C) .
• the essential cause is a high heater temperature (about 800°C) in relation to the desired working temperature: a great part of heat en ⁇ ergy is lost as the high temperature of the heater radiates the heat all around and in consequence only a part of it rea ⁇ ches up to the useful location.
• heating elements are made of alloy Cr-Ni-Fe-Al with a resistance of about 0.04 Ohm/cm.
• a relatively low spe- cific electric resistance and a poor thermal conductivity of such heating material determine the manner of its use: the heater is made in form of wire coil that must be electrically isolated with robust ceramics or an equivalent isolation material, which in turn has also a low thermal conductivity.
• the element is protected by a metal covering.
• Such an element has a great mass and because of the poor thermal con ⁇ ductivity it has to be heated to about 800°C to enable the heat to reach the heating location as quick as possible.
• the high temperature of the heating element induces the isotropic expansion of the heat by radiation and results in a low de ⁇ gree of efficiency. It particularly relates to smaller hea ⁇ ting units e.g. household heating appliances.
• the thermal resistance of the material itself for the active part of the heating element as low as possible; b) the mass of the heating element as small as possible; c) thermal resistance from the source up to the load also as low as possible; d) the difference between the working temperature and the heater temperature as low as possible.
• the conventional wire heater can be easily constructed by using simple technologies (e.g. winding a wire into coil).
• simple technologies e.g. winding a wire into coil.
• the energy saving has not been a motivation for a more intensive development of a new, energy saving type of heatig element. Only in the past few years the energy saving has become imperative to the modern technology and a more significant energy saving can no more be achieved by use of a conventional heater.
• the work on development of a heating element in the world that will allow a more economi ⁇ cal energy consumption may be expected to take a more inten ⁇ sive move.
• the invention proposes a new heating element made of doped polycrystalline or amorphous silicon which is heated up to a temperature of 400°C.
• Polycrystalline or amorphous silicon have a thermal conductivity about 5 times that of the wire material of a conventional heater and therefore the heating energy can be directed to the consumption place at minimal
• An element which consists of an electrically isolating support with a high thermal conductivity spread with an active layer of doped polycrystalline or amorphous sili ⁇ con and surface electrodes to which the working electric supply is connected.
• An element which after fabrication is installed on a sup ⁇ port made of highly thermal conductive metal (Al, Cu or similar) that directs the conduction of heat from the heat source to the heat load.
• the invention is significant for two essential reasons:
• TITUTE SHEET 1 Polycrystalline or amorphous silicion are 50 to 100 times cheaper than semiconducting monocrystalline silicon.
• Polycrystalline or amorphous silicion can be laid on the support in thin films (ranging from 0,1 to 1 ⁇ m of film thickness). Using the selective etching technology (as known in the semiconductor technology) such films can be shaped into surface structures of more suitable forms (e.g. meanders) and thus provide a better freedom in the choice of satisfactory resistance of the element.
• the other characteristics of the polycrystalline or amorphous silicon are very similar to those of the semiconductive mono- crystalline one. This relates especially to the high thermal conductivity and the metallurgical alloying characteristics which are the main reasons why silicon has been selected as a new material.
• the material for conventional heaters has a defin- ed electric resistivity which is unchangeable
• the spec ⁇ ific silicon resistivity can be changed by several or ⁇ ders of magnitude either in the course of fabrication or during the thermal treatment taking place afterwards. This allows to obtain the necessary electric resistance of the heater with its minimum mass.
• the silicon thermal resistance is 5 times lower than the material for the conventional heater wire.
• the silicon can be perfectly alloyed with metals and consequently connecting leads can be made using other material which is welded or hard soldered to the layer of the active
• SUBSTITUTE SHEET part of the heater The welding or alloying characteris ⁇ tics of silicon can be used for a directed conduction of heat towards the heated media at a minimal loss of en ⁇ ergy. In conventional wire heaters this is not possible.
• the silicon can be covered with a thin dioxide film.
• This is a layer of quartz glass that protects the silicon layer effectively from a further oxidation and passivates its surface.
• the dioxide film grows directly from the silicon and the risk of the production of cracks is therefore minimized.
• the temperature coefficient of the silicon electric resistance is positive: its resistance increases with increasing temperature.
• This characteristic of the sili ⁇ con heating element can be used as an inherent overload protection.
• a part of the heater can be used also as a temperature sensor.
• the layer of polycrystalline or amorphous silicon maintains the positive temperature coefficient of electric resis ⁇ tance up to the temperature of 600-800°C dependent to the doping level.
• the above mentioned characteristics of silicon are used to a maximum extent by this invention in order to obtain an opti ⁇ mized silicon heating element.
• the heating element uses all stated advantages of such material or, as may be necessary, some of them only.
• Fig.1 shows a schematic illustration of variant 1 of the silicon heating element according to the invention.
• the layer B of either polycrystalline or amorphous silicon is spread on a ceramic tile A.
• the layer B is doped during spreading or is laid without dopant and the doping follows afterwards.
• Con ⁇ tacts C are placed at the ends of the tile and alloyed on the silicon layer B.
• the surface has no contour, and the suitable electric resistance is obtained through the shape of the tile A, thickness of the silicon layer B and the doping level of the layer.
• the silicon heating element of a new, energy saving type as described in Fig.1 allows directing the heat energy from the heat source toward the heat load, noted due to the fact that on an electric isolated support A of a suitable surface con ⁇ tour and a high thermal conductivity a layer of polycrystal- line or amorphous silicon B is spread, which is doped either in the course of spreading or after the completion of spread ⁇ ing of silicon layer, by semiconducting dopants of the p- or n-type in a concentration higher than 10 /cc on which layer at the end of ground, highly temperature resistant contacts C are alloyed for electric supply.
• Fig.2 represents a schematic illustration of variant 2 of the proposed heater: unlike variant 1, here a layer C r of high- melting metal (Mo, Ta, W or similar) with contact contour (Fig.2) is first laid on the ends of the support. A layer of silicon B is laid over the entire surface and then etched by
• Fig.3 illustrates a variant as in Fig.2, but the contour of the silicon layer B is shaped using the method of photolito- graphy by masking and etching into a meander or a similar suittable contour of desired form in order to obtain a higher electric resistance of the heater when required (e.g. work with higher voltage and small intensity heating current).
• Fig.4 shows a variant of heater according to Figs.1,2 and 3 with a protective layer of silicon dioxide D over the doped layer B of polycrystalline or amorphous silicon.
• This layer is obtained by oxidizing the silicon layer B at high tempera- ture (1000°C) in an oxidizing water vapour atmosphere so that the conducting layer is covered by an insulating layer D of silicon dioxide which passivates and protects the active heating layer and thus increases its safety and durability.
• This technological step takes place simultaneously with dif- fusing the dopant into the silicon layer namely with its dis ⁇ tribution and homogenisation in the case when doping is ef ⁇ fected during the layer spreading. Therefore the layer pro ⁇ tection by oxidation does not require a separate process step.
• Fig.5 illustrates a variant of a heater installed on a metal support E of high thermal conductivity (e.g. Cu, Al or simi ⁇ lar) to form a heating unit.
• a metal support E of high thermal conductivity e.g. Cu, Al or simi ⁇ lar
• the support E directs the conduction of heat energy from the heat source to the heat load.
• the two heating elements have been assembled in a "sandwich” type construction by alloying the connection contacts and form a unit of series or parallel configuration in order to adjust the supply voltage and to obtain necessary power.
• Such "sandwich” construction is installed in a support made of high thermal conducting metal in a closed form.
• the silicon film is laid on the support which should be an electric insulator and an excel ⁇ lent thermal conductor at the same time. Due to high work temperature a ceramic material for the support is suitable. However, ceramic is a poor thermal conductor. Until a few years ago the only ceramic material that, as to the thermal conductivity, stood out among others was beryllium oxide (BeO) . Its conductivity is approximately equal to that of pure aluminium. Unfortunately, BeO is a highly toxic material and its use is therefore limited.
• BeO beryllium oxide
• the silicon heating element of the invention represents a brand new and original component and its industrial applica ⁇ tion can consequently be assumed with a high degree of proba- bility.
• the application shall certainly take place in two ways:
• the element can be applied at temperatures up to 600°C it is expected its application will extend not only to household heaters (electric radiators), cookers, and all electrothermic appliances of wide use but also to some specific professional appliances.
• this element allows not only the possibility to save energy due to a higher efficiency but also significant savings of material in the production of se ⁇ veral appliances due to the distributed application of the new heating element.
• it can be inserted directly into the heating volume or in its close vicinity i.e. just in the place where heating is necessary and conse- quently eliminates robust insulators and transfer means which represent today a great deal of mass of such appliances.

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

Applications Claiming Priority (2)

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• 1989
  • 1989-12-27 YU YU247189A patent/YU247189A/xx unknown
• 1990
  • 1990-12-21 CA CA 2047193 patent/CA2047193A1/en not_active Abandoned
  • 1990-12-21 EP EP19910901791 patent/EP0460175A1/de not_active Withdrawn
  • 1990-12-21 JP JP3502213A patent/JPH05500435A/ja active Pending
  • 1990-12-21 WO PCT/EP1990/002272 patent/WO1991010336A1/en not_active Ceased

Non-Patent Citations (1)

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