EP0082627B1

EP0082627B1 - An improved fluid permeable porous electric heating element

Info

Publication number: EP0082627B1
Application number: EP82306494A
Authority: EP
Inventors: James Francis Pollock; Roy Frederick Preston; Vincent Keith Hewinson; John Malcolm North
Original assignee: UK Atomic Energy Authority
Current assignee: UK Atomic Energy Authority
Priority date: 1981-12-14
Filing date: 1982-12-06
Publication date: 1988-01-27
Also published as: GB2111809B; DE82627T1; GB2111809A; DE3278066D1; JPS58112296A; EP0082627A1; JPH0416914B2

Description

This invention relates to a fluid permeable, fibrous electric heating element.
It is known from UK patent specification GB 2 056 829A to make a fluid permeable electric heating element from a permeable body of carbon fibres in a random arrangement and joined together by carbon at points of contact, this fibrous body being produced by carbonisation of an acrylic-fibre felt-like material. A desired resistivity of the element may be achieved by coating the carbon fibers with a layer of carbon of lower resistivity than that of the fibres, the coating layer being applied by plasma activated vapour deposition (PAVD). Alternatively the carbon fibres may be coated by PAVD with silicon carbide and the carbon fibres then removed by oxidation to leave tubular silicon carbide fibres. The silicon carbide may then be subjected to a heat treatment at a temperature between 1200 and 1900°C to achieve a desired resistivity. The two types of heating element so produced are of substantially uniform chemical nature, consisting entirely of carbon in the first case, and silicon carbide in the second case.
The invention provides a fluid permeable electric heating element comprising fibres of carbon in a random arrangement, the carbon fibres being joined together by carbon at points of contact to form a self-supporting body, and characterised by a layer of an electrically conducting material consisting principally of free silicon coating the carbon fibre body.
The coating desirably includes a dopant material for modifying its electrical resistivity, for example phosphorus, arsenic or nitrogen.
The following description describes heating elements as defined above; and also describes heating elements from which the carbon fibres have been removed by oxidation to leave tubular fibres of silicon. The present invention relates to the former type of heating element.
The invention also provides a process for making a fluid permeable, electric heating element, the process comprising forming a permeable body consisting of carbon fibres in a random arrangement and joined together at points of contact to form a self-supporting body, and characterised by coating the carbon fibre body by decomposition of a silicon-containing gas so as to provide a coating on the fibres consisting principally of free silicon, and then heating the coated carbon fibre body at about 600°C to 1400°C so as to provide a desired electrical resistivity of the element.
The heating element of the invention may be used for example in a heat transfer system comprising a circuit adapted to contain a fluid, a heating element of the invention extending in a part of the circuit, and a heat transfer portion of the circuit for transferring heat from the fluid. The heat transfer portion may comprise a portion of a device for heating a material such for example as, a plastics material, a metal, paper, a textile, or a chemical substance.
Examples of porous carbon fibre electric heating elements and their method of manufacture are described in British Patent Specification Nos. 1466240 (United States Patent No. 3943330), 1503644, and 1600253 (United States Patent No. 4257157), which may be referred to for further details.
It is frequently necessary to heat a fluid, either a gas or a liquid, electrically and to utilise a closed loop fluid circulating system to transfer this heat to other fluids or plant in a heat exchange system. A particular example is the use of a heated liquid (e.g. oil or water) for the purpose of controlling the temperature of moulds, dies, extruders and calenders used in the plastics industry. In existing designs of such temperature control units immersion electric elements of the metal sheathed type have been used, the heat generated in these elements being transferred by conduction through the metal sheath wall to the surrounding liquid which flows past the element. There is a limit to the heat transfer coefficient which can be used with this type of element if oil breakdown or element burnout is to be avoided, e.g. 1-20 watts/cm2 of the element surface area. This factor exerts a consequent effect on the metal sheath size, and the size and weight of the heating system especially at high power ratings.
Furthermore, the response of such units to a requirement for a change in the rate of heat transfer to the fluid is comparatively slow due to the appreciable heat capacity of the heating element itself, to the need to limit the element centre temperature to avoid melting the electrical conductor, and to the need to avoid high surface temperatures of the element sheath. This slow response can have a controlling influence on production cycle times when such temperature control units are used to heat moulds and dies in a production run of identical components.
The invention largely overcomes these limitations by providing a fluid permeable fibrous electric heating element which may be used to replace such a sheathed metal element. A fluid can be circulated through the body of the permeable heating element instead of over the external surface only. Power densities exceeding 1 kW/cm³ of heating element material can then be attained, and this leads not only to a reduction in size and weight of a heating unit for a given duty but also a decrease in the response time such that the system can react virtually instantaneously to a demand for a large step increase in the heat generated and transferred to the circulating fluid.
A suitable permeable heating element might have a voidage of 50-98% and a bulk density of 50-750 kg/m³, and consist of a matrix of fibres of a diameter in the range 5-300 microns and a coating thickness in the range 1-30 microns, a preferred range being 4-20 microns, the space between the fibres providing the voidage. A thermal barrier/dispenser may be used in conjunction with the heating element to produce a uniform fluid flow (see U.K. Patent No. 1 466 240), especially where there is a hydrostatic pressure head variation over the heating element entry surface.
It may be desirable to maintain a heat generation rate and a fluid flow rate through the voidage in the heating element which restricts the fluid temperature rise across the heating element to not more than about 50°C, but this is not essential and temperature rises of 2-300°C or more are possible with a heating element wall thickness of 2-15 mm for an annular heating element. The heating element may have a predetermined electrical resistivity at a particular temperature, and a particular temperature coefficient of resistance, and may be suitable for mains voltage operation without the need for a transformer.
The invention will now be further described by way of example only with reference to the accompanying drawing which shows a diagrammatic representation of a plasma activated vapour deposition apparatus.
The porous electric heating element may be made from a precursor in the form of a permeable fibrous carbon electric heating element comprising carbon fibres, made for example as described in the aforementioned patent Specifications, and coated with material conveniently applied by a plasma assisted deposition process (hereinafter referred to as PAVD). The electrical properties of this deposited material, which may be modified after deposition by appropriate heat treatments may largely determine the electrical operating characteristics of the porous electric heating element. The permeable fibrous carbon precursor on which the material is deposited by the PAVD process may be retained, or may be removed from beneath the deposited material by means of an oxidation process.
In the PAVD process a plasma is used as a medium for chemical reaction, and deposition of a coating on a substrate is achieved through the decomposition of a gas in an electrically-induced plasma. An example of the plasma assisted deposition process is described in British Patent Specification No. 2056829A, and related techniques are disclosed in the paper "Codeposition of Glassy Silica and Germania inside a Tube by Plasma Activated CVD" by D. Kuppers et al, Journal of the Electrochemical Society Vol. 123 No. 7 pages 1079-1082 July 1976.
One form of apparatus for performing the PAVD coating process is shown in the drawing to which reference is now made. In the drawing a fibrous carbon permeable heating element 200 is supported horizontally by an alumina tube 201 inside the central region along the bore of a cylindrical silica tube 202 having enlarged ends 204, 205, the alumina tube 201 extending axially inside the silica tube 202 from an end cap 203 which closes the enlarged end 204. An outlet pipe 206 from the enlarged end 204 is connected through a vacuum-type valve 208 to a vacuum pump 209. The other enlarged end 205 is closed by an inlet end cap 210 which has a feed pipe 212 to a mercury pressure gauge 214, and an inlet 216 connected to a manifold 218 fed by gas supply lines 220, 222, and 224 each controlled by a respective valve 221, 223, and 225. A silica vessel 226 of annular form is movable along the outside of the tube 202 and is shown surrounding the central region of the tube 202. The vessel 226 supports an electrically isolated susceptor 228 (e.g. graphite), and is evacuated down to about 1 to 10 Torr through a vacuum valve 230 by B'vacuum pump 232. The vessel 226 is itself surrounded by a helical water-cooled copper coil 236 having eight turns and which is coupled to a high frequency power supply 238 for working in the frequency range of 10^4-10⁸ Hertz. Earth connections 240, 241, 242 are made to the heating element 200, the end cap 203, and the enlarged end 204 respectively. The high frequency generator 238 is similarly earthed by a connection 246. It may in some instances be advantageous to bias the connection 240 positively or negatively to earth and to connect the element 200 to the connection 240 by a metallic conductor strip (not shown).
In operation, the tube 202 is evacuated down to about 1-10-² Torr by the vacuum pump 209, and then filled with argon through the gas supply line 220 to a vacuum pressure inside the tube 202 of between 10-^z-100 Torr. The high frequency power supply 238 is energised typically at an anode voltage of 1-4 kV and 4×10⁵ Hertz, and a plasma is created about the carbon precursor 200 whilst the susceptor 228 is heated by electrical induction and heats the carbon precursor 200. After about 15 to 30 minutes when the carbon precursor 200 has reached thermal equilibrium and its surfaces have been cleaned by ion bombardment from the ionised gaseous argon, the selected reactant gas or gases are introduced through the appropriate gas supply lines 222 and/or 224 into the tube 202, and the pressure inside the tube 202 maintained in the range 10-¹ to 100 Torr by adjustment of the appropriate valve 223, 225 and use of the vacuum valve 208 and the vacuum pump 209. A coating of the selected material is now applied to the surfaces of the fibres of the carbon precursor 200 by the reactant gases, and the thickness of the material deposited is proportional to time. For example a coating of 1 to 20 11m might be deposited in about 24 hours. The valves 221, 223, and 225 are then closed, the high frequency power supply 238 switched off, whilst the tube 202 continues to be evacuated by the vacuum pump 209 to allow the coated element 200 to cool under vacuum conditions (e.g. 10-² Torr), after which the coated precursor 200 is removed from the tube 202.
Subsequently the original fibrous carbon precursor 200 on which the coating has been deposited may either be retained, or removed by a heat treatment comprising oxidation in air at a temperature above 300°C so that a structure of tubular fibres of the deposited material remains. A suitable heat treatment may be used to adjust the electrical conductivity and temperature coefficient of resistance of the deposited material to desired values. In cases where the fibrous carbon precursor 200 is retained, this heat treatment must be carried out in an inert environment since it would normally take place above the carbon oxidation threshold temperature of about 300°C. When the fibrous carbon precursor is removed by oxidation and the coating is subjected to a heat treatment to adjust its electrical characteristics, this heat treatment may be effected before or after the carbon precursor is removed, or simultaneously with removal.
The coating of the fibrous carbon precursor 200 may be performed in the apparatus of the drawing using a gas containing silicon e.g. silane, as the reactant gas to deposit a coating containing silicon on the fibrous carbon element 200. Phosphine gas may be used as a dopant together with the silane if desired to assist in modifying the coating's electrical conductivity by co-depositing phosphorus with silicon in the coating. Other suitable dopants may be aluminium, or boron, or arsenic, or nitrogen, or oxygen, the purpose of the dopant being to increase the electrical conductivity of the deposited material by providing an electron donor or acceptor material, so that the final desired electrical resistivity of the heating element can be obtained at lower heat treatment temperatures than would otherwise be required. The heat treatment of the coating to modify its electrical resistivity is normally carried out at 800°-1400°C to achieve a particular resistivity of the deposited coating at a selected temperature. In many instances the use of phosphine gas as a dopant allows the heat treatment temperature to achieve a particular coating resistivity to be reduced to about 600-1100°C.
Typical fabrication procedure parameters used are given below:-
The fibrous carbon precursor is placed in the tube 202 and the tube 202 subsequently evacuated down to a vacuum pressure of about 10-² Torr. Argon is admitted through the gas supply line 220 to a vacuum pressure inside the tube 202 of about 1.0 Torr. The high frqeuency generator 238 is subsequently energised, and after about one hour, the silane gas to deposit the silicon is introduced into tube 202 through the gas supply line 222, and a mixture of phosphine/argon is introduced into the tube 202 through the gas supply line 224, the vacuum pressure inside the tube 202 being maintained at about 0.9 Torr. After a suitable time interval (e.g. 48 hours) depending on the thickness of coating required (typically 0.8 grammes/hour are deposited), the valves 221, 223 and 225 are closed, the tube 202 evacuated to about 10-² Torr and the high frequency generator 238 switched off to allow the coated precursor to cool under vacuum conditions. The coated precursor is subsequently removed from the tube 202 and cut into appropriate lengths prior to oxidation to remove the fibrous carbon precursor and/or such further heat treatment as may be required to modify the electrical resistivity of the coating.
An oxidation process to remove the fibrous carbon precursor may be carried out at any convenient temperature above about 300°C, but the time taken to oxidise the carbon can be reduced by increasing the oxidation temperature. Too high an oxidation temperature may however have an effect on the electrical properties of the coating which is not desired, and this possibility arises as a result of the nature of the material deposited during the PAVD coating process.
Heat treatment of the deposited material changes the electrical properties of the coating to a degree which is influenced by the heat treatment temperature and time. Thus it is possible to either heat treat in an inert atmosphere at a relatively high temperature to obtain a selected resistivity, and to follow this by oxidation at a lower temperature, or to oxidise at a relatively low temperature and to follow this process with one or more heat treatment stages in an inert or oxidising atmosphere to adjust the coating resistivity to the desired value. In some cases depending on the PAVD coating process parameters employed it will be possible to carry out a one stage heat treatment process which both removes the fibrous carbon precursor by oxidation and adjusts the resistivity of the coating to a desired value. Additionally some of the free silicon may become oxidised if the oxidation temperature is very high.
The heat treatment time and temperature to produce a required coating resistivity depend not only on the PAVD coating parameters, for example the gas composition and flow rates and RF conditions, but also on the electrical properties of the carbon precursor on which the coating is deposited. The electrical resistivity of the fibrous carbon precursor during the coating process is a function not only of the carbonisation temperature of the carbon precursor but also of the RF field conditions through its effect on the temperature of the carbon precursor during the coating process. It has been found that the composition of the material deposited during the coating process and its electrical properties is influenced by both the RF field voltage and the carbonisation temperature to which the carbon precursor was subjected, and that the heat treatment procedure required to obtain a particular electrical resistivity of the deposited material depends on its composition. Consequently some combinations of PAVD parameters and carbon precursor carbonisation temperatures can allow a given resistivity of the deposited material to be obtained together with a low temperature coefficient of resistance of the deposited material after heat treatment, whereas others may not. Furthermore the use of a dopant such as phosphorus in the PAVD coating process changes the relationship between resistivity, heat treatment conditions, and temperature coefficient of resistance.
Removing the phosphorus from the coating gases and substituting arsenic or boron for phosphorus also changes the composition of the material deposited and affects the heat treatment process required to attain a particular resistivity.
An example of the process, using nitrogen rather than phosphorus as dopant, is as follows:-

Example:

Heating element containing nitrogen-Doped silicon

As a general rule increasing the time and the temperature of heat treatment of the coating up to a heat treatment temperature of about 1400°C, reduces the electrical resistivity of the coating provided material is not lost by evaporation.
When the heating element is to be used over a relatively wide temperature range (e.g. 20-200°C), the temperature coefficient of resistance (TCR) of the element may become of importance. By appropriate selection of the carbonisation temperature, the coating conditions and the heat treatment temperature, heating elements having a required electrical resistivity and TCR may be made.
Although the coating of the fibrous carbon has been described in relation to PAVD, it will be understood that appropriate alternative coating processes may be used.
The permeable electric heating elements may be of alternative shapes or sizes to those aforedescribed, and may be used to heat either a gas or a liquid. It will be understood that where the carbon fibres in the fibrous carbon precursor are contiguous, the coatings about the fibres may merge together at these positions of contiguity. Hence if the coated precursor is oxidised, the tubular fibres that remain may be integral with closely adjacent tubular fibres at the positions of contiguity. Otherwise the coating material deposited substantially replicates the geometry and fibre distribution pattern of the original fibrous carbon precursor.
For some applications the fibrous carbon precursor need not be removed, but may be allowed to remain in situ during any heat treatment of the coating which should then be carried out in an inert atmosphere. The electrical resistivity of the heating element would then be a combination of the resistivity of the fibrous carbon precursor and the selected resistivity of the coating. The electrical resistivity of the coating may be the same or greater than that of the carbon precursor, but in some cases it may be desirable that the electrical resistivity of the heating element is determined mainly by the coating, so that under operating conditions the coating carries the majority of the electric current. The electrical resistivity of the fibrous carbon precursor should, therefore, be substantially greater than the electrical resistivity of the coating after the final heat treatment to adjust the electrical resistivity of the coating has been completed.
A relatively high electrical resistivity of the fibrous carbon precursor may be achieved by selecting a relatively low carbonisation temperature, e.g. below about 800°C, and this implies that the PAVD coating parameters must be selected so as to ensure that the heat treatment required to achieve a desired coating resistivity may be carried out at a temperature which is preferably below the carbonisation temperature of the fibrous carbon precursor.

Claims

1. A fluid permeable electric heating element comprising fibres of carbon in a random arrangement, the carbon fibres being joined together by carbon at points of contact to form a self-supporting body (200), and characterised by a layer of an electrically conducting material consisting principally of free silicon coating the carbon fibre body.

2. A process for making a fluid permeable, electric heating element, the process comprising forming a permeable body (200) consisting of carbon fibres in a random arrangement and joined together at points of contact to form a self-supporting body, and characterised by coating the carbon fibre body by decomposition of a silicon-containing gas so as to provide a coating on the fibres consisting principally of free silicon, and then heating the coated carbon fibre body at about 600°C to 1400°C so as to provide a desired electrical resistivity of the element.