JPH0416914B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to fluid permeable or permeable fibrous electrical heating elements. According to British Patent Specification GB2 056 829A, a fluid-permeable electrical heating element is made from a permeable body consisting of randomly arranged carbon fibers bonded together by carbon at the contact points, the fiber body being made of acrylic. It is made by carbonizing a fibrous, felt-like material. The desired resistivity of the electrothermal element is obtained by depositing on the carbon fiber a carbon layer having a resistivity lower than that of the carbon fiber. The adhesion layer is created by plasma activated vapor deposition (PAVD). Alternatively PAVD silicon carbide to carbon fiber
The carbon fiber is then removed by oxidation leaving a tubular silicon carbide fiber. The silicon carbide is heat treated at a temperature of 1200 to 1900 degrees Celsius to obtain the desired resistivity. The two types of heating elements thus produced have substantially uniform chemistry, the first consisting of carbon and the latter consisting of silicon carbide. The invention consists of carbon fibers randomly arranged and bonded together by carbon at contact points to form a self-supporting body, and features a layer of electrically conductive material consisting primarily of free silicon covering the carbon fiber body. A fluid permeable electric heating element is provided. Here, "free silicon" refers to silicon that is not chemically bonded, that is, not chemically bonded to any other element. The coating contains a dopant material that changes its electrical resistivity, such as phosphorous, arsenic or nitrogen. The heating element described above will be described in detail below, and the heating element will also be described in which the carbon fibers are removed from the heating element by oxidation to leave the tubular silicon fibers behind. The present invention relates to heating elements of the previous type mentioned above. The present invention provides a method for making a fluid permeable electrothermal element. This process forms a permeable body consisting of carbon fibers randomly arranged and bonded together at contact points to form a self-supporting body, characterized by coating the carbon fiber body by decomposition of a silicon-containing gas. A coating consisting primarily of free silicon is formed on the fiber, and the coated carbon fiber body is heated to about 600 to 1400 degrees Celsius to obtain the desired electrical resistivity. The heating element of the present invention is used, for example, in a heat transfer device system equipped with a flow path for accommodating a fluid, and the heating element of the present invention extends in a portion of the flow path, and the heat transfer surface portion of the flow path is Transfer heat from a fluid.
The heat transfer surface portion forms part of a device for heating materials such as, for example, plastic materials, metals, paper, textiles, or chemicals. Porous heating elements are described, for example, in British Patent No. 1466240 (U.S. Pat.
No. 3,943,330), UK Patent No. 1,503,644, and UK Patent No. 1,600,253 (US Pat. No. 4,257,157). It is often necessary to electrically heat a fluid, either a gas or a liquid, and use a closed loop fluid circulation system to transfer this heat to other fluids or heat exchange system plants. Specific examples include the use of heated liquids (eg, oil and water) to control the temperature of molds, dies, extruders, and polishers used in the plastics industry. Existing designs of such temperature control units utilize metal sheathed transparent electrical elements, in which the heat generated in these elements is transferred by conduction through the metal sheath walls to the surrounding liquid (through the elements). flow). If oil breakdown and element burning are to be avoided, there are limits to the heat transfer coefficients that can be used with this type of element, such as 1-20 watts per square ^centimeter of element surface area. This factor affects the size of the metal sheath and, especially for high power ratings, the size and weight of the heating device. Furthermore, the response of these units to the requirement to vary the rate of heat transfer to the fluid is due to the significant heat capacity of the heating element itself and the need to limit the temperature at the center of the element to prevent melting of the electrical conductors. This results in a relatively slow response due to the need to prevent the surface temperature of the element sheath from increasing. This slow response creates a control effect on the production cycle time when such temperature control units are used to heat molds and dies when producing the same part. The present invention overcomes these limitations by using a fluid-permeable fibrous electrical heating element in place of a sheathed metal element. The fluid circulates through the body of the fluid-permeable heating element rather than flowing over the outer surface of the element. In this way, power densities of more than 1 KW per cm ³ of heating element material can be obtained, which not only leads to a reduction in the size and weight of the heating unit for a given load, but also contributes to a reduction in response time. It is thus possible to meet the demand virtually immediately for a significant increase in the heat transfer to the circulating fluid. Void existence ratio of a suitable fluid-permeable heating element (void means the space between fibers, and void existence ratio means the proportion of the total volume that is not occupied by fibers)
is 50 to 98% and is the bulk density
50 to 750 Kg/m ³ and consists of a matrix of fibers with a diameter of 5 to 300 microns and a wall thickness of 1 to 30 microns, preferably 4 to 20 microns, with the spaces between the fibers forming voids. A thermal barrier/dispenser may be used in conjunction with the heating element to ensure uniform fluid flow, especially if the hydrostatic pressure varies with respect to the fluid introduction surface of the heating element (see British Patent No. 1466240). ). It is desirable, and important Instead, in the case of an annular heating element, a temperature increase of 200 to 300°C or more can be obtained with a wall thickness of the heating element of 2-15 mm. The heating element has a certain electrical resistivity at a certain temperature and a certain low temperature coefficient of resistance and is suitable for operation at mains voltage without the need for a transformer. Hereinafter, the present invention will be explained in more detail with reference to the accompanying drawings. The porous electrical heating element is made from a precursor in the form of a permeable carbon fiber electrical heating element consisting of carbon fibers, made for example as disclosed in the above-mentioned patent, and plasma assisted. A substance is deposited by a vapor deposition process (hereinafter referred to as PAVD). The electrical properties of this deposited material, which may be modified by suitable heat treatment after deposition, primarily determine the electrical operating properties of the porous electric heating element. The permeable carbon fiber precursor to which material is deposited by the PAVD process may be retained or removed from beneath the deposited material by an oxidation process. In the PAVD process, a plasma is used as the medium for the chemical reaction, and the deposition of a coating on a material is accomplished by the action of electrically induced gas decomposition in the plasma. An example of a plasma-assisted deposition process is UK Patent No. 2056829A
The technology related to this was disclosed in 1976
Journal of the Electrochemistry in July
Society, Vol. 123, No. 7, pp. 1079-1082, by D. Kuppers et al.
Codeposition of Glassy Silica and Germania inside the tube
Germania inside a Tube by Plasma
One type of apparatus for carrying out the PAVD coating process is shown in the accompanying drawings and is described below. In the accompanying drawings, a fiber carbon permeable heating element 200 is shown. is horizontally supported along the bore of a cylindrical silica tube 202 having large diameter ends 204, 205 in its central region by an alumina tube 201, with the alumina tube 201 having large diameter ends 204, 205.
Close end cap 203 to silica tube 202
It extends axially inside. A discharge pipe 206 from the large diameter end 204 is connected to a vacuum pump 209 via a vacuum type valve 208. The other large diameter end 20
5 is closed by an inlet end cap 210, which end cap 210 is equipped with a mercury pressure gauge 21.
4 and an inlet 216 connected to the manifold 218.
8 is supplied with gas from gas supply lines 220, 222 and 224 controlled by respective valves 221, 223 and 225, respectively. A silica container 226 in an annular configuration is shown movable along the outer surface of the tube 202 and surrounding the central region of the tube 202 . This container 226 supports an electrically insulated susceptor 228 (e.g. graphite) and a vacuum pump 228.
32 via vacuum valve 230 from about 1 to
Exhausts up to 10Torr. The container 226 itself is surrounded by an 8-turn helical water-cooled copper coil 236, which is connected to a high frequency power source 238 operating in the 10 ₄ -10 ₆ Hz frequency range. Heating element 200, end cap 203 and large diameter end 20
4 to earth connections 240, 241 and 24
2 are formed respectively. The high frequency power source 238 is similarly grounded via the connecting line 246. In some cases, it may be convenient to bias connection 240 positively or negatively with respect to ground and to connect element 200 to connection 240 by a piece of metal conductor (not shown). In operation, tube 202 is evacuated to approximately 1 to 10 ^-2 Torr by vacuum pump 209 and then evacuated to approximately 1 to 10 ^-2 Torr through gas supply line 220 into tube 202.
It is filled with argon at a vacuum pressure of 100 Torr.
The radio frequency power supply 238 is typically energized with an anode voltage of 1 to 4 KV and a frequency of 4×10 ⁵ Hz;
A plasma is then formed around the carbon precursor 200 while the susceptor 228 is heated by electrical induction to heat the carbon precursor 200 . After about 15 to 30 minutes, carbon precursor 20
When 0 has reached thermal equilibrium and its surface has been cleaned by ion bombardment from ionized argon gas, the selected reactant gas is passed through appropriate gas supply lines 222 and/or 224 to tube 202.
A valve 22 is introduced into the pipe 202 to ensure that the pressure within the pipe 202 is adequate.
3,225 and the use of vacuum valve 208 and vacuum pump 209 to maintain a range of 10 ^-1 to 100 Torr. A coating of the selected material is now deposited by the reactive gas on the surface of the fiber of the carbon precursor 200, and the thickness of the deposited material is proportional to time. For example, a coating of 1 to 20 μm is approximately 24
It will be attached in time. Valves 221, 223, and 225 are then closed and RF power supply 238 is turned off, while tube 202 is turned on by vacuum pump 209 so that coated element 200 can be cooled under vacuum conditions (eg, 10 ^-2 Torr). The tube 202 continues to be evacuated, after which the coated precursor 200 is removed from the tube 202. After that, the fiber-like carbon precursor 200 with the coating attached may be held as it is, or
Alternatively, it may be removed by heat treatment by oxidation in air at temperatures above 300°C, leaving behind a tubular fiber structure of the deposited material. Appropriate heat treatments are performed to adjust the electrical conductivity and temperature coefficient of resistance of the deposited material to desired values. If the fibrous carbon precursor 200 is to be retained, this heat treatment must be performed in an inert environment. This is because this heat treatment is typically performed at a temperature above the carbon oxidation threshold temperature of about 300°C. When the fibrous carbon precursor is removed by oxidation and the coating is heat treated to adjust its electrical properties, this heat treatment may be performed before or after the carbon precursor is removed. It may be removed or removed at the same time. Formation of a coating on the fiber-like carbon precursor 200 is carried out using a silicon-containing gas such as silane gas as a reaction gas in the apparatus shown in the accompanying drawings so as to adhere a silicon-containing coating to the fiber-like carbon element 200. If desired, phosphine gas may be used as a dopant with silane to help modify the electrical conductivity of the coating by co-depositing phosphorus with the silicon of the coating. Other suitable dopants include aluminum, boron, arsenic,
Nitrogen or oxygen, the purpose of this dopant is
The electron donor enhances the electrical conductivity of the deposited material by providing an acceptor material to achieve the desired final electrical resistivity of the heating element at a lower heat treatment temperature than would be required without the use of a dopant. The aim is to make sure that Heat treatment of the coating to modify the electrical resistivity of the coating is typically carried out at 800° C. to 1400° C. to obtain a specific resistivity of the coating at a selected temperature. In many cases, by using phosphine gas as a dopant, the heat treatment temperature for a particular film resistivity can be lowered to about 600 to 1100°C. Typical manufacturing parameters used are as follows. Fiber-like carbon element pre- cursor range Carbonization temperature 600-1800℃ Pre-cursor shape range Length: 5-1000mm Outer diameter: 5-500mm Inner diameter: 1-499mm Density: 50-750Kg/m ³ PAVD film formation conditions Gas Composition Range Argon 100% Silane 100% Phosphine/Argon 0.1/100-10/100% Gas flow rate: ml/min Range Argon 10-500 Silane/Ethylene 1-500 Phosphine/Argon 1-100 Film-forming gas pressure: Torr 10 ^{- 1} -100 Effective value of high frequency voltage applied to the coil (kv)
1-5 The fiber-like carbon precursor is attached to the tube 2 shown in the attached figure.
02, then the tube 202 is approximately
It is evacuated to a vacuum pressure of 10 ^-2 Torr. Then,
Argon is introduced via gas supply line 220 until the vacuum pressure within tube 202 is approximately 1.0 Torr.
Radio frequency power supply 238 is then energized, and after approximately one hour, silane gas for depositing silicon is introduced into tube 202 via gas supply line 222, and a phosphine/argon mixture is introduced through gas supply line 224. is introduced into the tube 202 through the tube 202.
The vacuum pressure inside is maintained at approximately 0.9Torr. After an appropriate time interval (e.g. 48 hours) based on the required coating thickness (typically 0.8 grams per hour deposited), the valves 221, 22
3 and 225 are closed, and the tube 202 is approximately
Exhaust to 10 ^-2 Torr and high frequency power supply 238
is turned off to allow the coated precursor to cool under vacuum conditions. The coated precursor is then removed from tube 202, cut to length, and then oxidized to remove the fibrous carbon precursor and as needed to modify the electrical resistivity of the coating. A further heat treatment is performed. Although the oxidation process to remove the fibrous carbon precursor is carried out at convenient temperatures of about 300° C. or higher, the time required to oxidize the carbon can be reduced by increasing the oxidation temperature. . However, if the oxidation temperature is too high, it may undesirably affect the electrical properties of the coating, a risk that is caused by the nature of the materials deposited during the PAVD coating process. When the deposited material is heat treated, the electrical properties of the coating change to some extent based on the heat treatment temperature and time. Therefore, the heat treatment can be carried out in an inert atmosphere at a relatively high temperature to obtain a selected resistivity and then the oxidation can be carried out at a lower temperature, or alternatively the oxidation can be carried out at a lower temperature. This may be followed by one or more heat treatment steps in an inert or oxidizing atmosphere to adjust the resistivity of the coating to the desired value. In some cases, based on the PAVD coating process parameters used, a single step heat treatment is performed to oxidize away the fibrous carbon precursor and adjust the resistivity of the coating to the desired value. be able to. Furthermore, if the oxidation temperature is very high, some free silicon will also be oxidized. The heat treatment time and temperature to obtain the desired film resistivity depend not only on the PAVD film formation parameters such as gas composition and flow rate and RF conditions, but also on the carbon precursor to which the film is deposited. It also depends on electrical characteristics. The electrical resistivity of a fibrous carbon precursor during the coating process is not only a function of the carbonization temperature of this carbon precursor, but also a function of the RF field conditions that affect the temperature of the carbon precursor during the coating process. be. The composition and electrical properties of the material deposited during the coating process are influenced both by the RF field voltage and the carbonization temperature experienced by the carbon precursor, and also by the specific electrical resistance of the deposited material. It has been found that the heat treatment procedure required to obtain the yield is tailored to the composition of the material. Therefore, for certain combinations of PAVD parameters and carbonization temperature of the carbon precursor, it is possible to obtain a given resistivity of the deposited material and to obtain a low temperature coefficient of resistance of the deposited material after heat treatment; This does not happen with other combinations. Additionally, the use of dopants such as phosphorous in the PAVD coating process changes the relationship between resistivity, heat treatment conditions, and temperature coefficient of resistance. Also, if phosphorus is removed from the film-forming gas and replaced with arsenic or boron, the composition of the deposited material will change and the heat treatment process required to obtain a specific resistivity will be affected. . An example of a method using nitrogen instead of phosphorus as a dopant is shown below. Example Heat generating element containing silicon doped with nitrogen Fiber-like carbon precursor Carbonization temperature of element 725℃ Density 53.5Kg/m ³ PAVD film formation conditions Gas composition Argon 100% Silane 100% Dopant: N ₂ 100% gas Flow rate: ml/min Argon 214 Silane 20.7 Dopant 19.9 Film-forming gas pressure: Torr 0.9 Effective value of high-frequency voltage applied to the coil (kv) 3.5 Heat treatment conditions Oxidation temperature (°C) 800 Time (hours) 16 Next heat treatment (°C) None Time (hours) Electrical characteristics after heat treatment (heating element) Temperature (°C) 247 Resistance at this temperature (ohms) 28.8 Resistance ratio (R20/R200) 1.55 Power at 247°C (kw) 1.79 Voltage (volts) 227 Element length (mm) 14.2 Generally, the time and temperature of the heat treatment of the film is approximately 1400 mm.
Increasing the heat treatment temperature to 0.degree. C. lowers the electrical resistivity of the coating, assuming no material is lost by evaporation. Relatively wide temperature range (e.g. 20°C to 200°C)
The temperature coefficient of resistance (TCR) of the heating element becomes important when it is necessary to use the heating element over a temperature range of By appropriately selecting the carbonization temperature, film formation conditions, and heat treatment temperature, a heating element having the required electrical resistivity and TCR can be produced. Although fibrous carbon coating has been described in connection with PAVD, it will be appreciated that other suitable coating processes may be used. Fluid-permeable electrical heating elements may be of other shapes or sizes than those described above and may be used to heat gases or liquids. Additionally, the fluid-permeable heating element described above may be used for other purposes of heating fluids. It will be appreciated that when the carbon fibers of the fibrous carbon precursor are in contact with each other, the coatings on the fibers bond together at these contact locations. Thus, when the coated precursor is oxidized, the tubular fibers become integral with their intimate tubular fibers at these contact locations. Otherwise, the deposited coating material will have substantially the same shape and fiber distribution pattern as the original fibrous carbon precursor. In some applications, the fibrous carbon precursor does not need to be removed and can be left in place during heat treatment of the coating, but the heat treatment must be conducted in an inert atmosphere. The electrical resistivity of the heating element is therefore a combination of the resistivity of the fibrous carbon precursor and the selected resistivity of the coating. The electrical resistivity of the coating is the same as or higher than that of the carbon precursor, but in some cases the electrical resistivity of the heating element is such that the coating conducts most of the current in the operating state of the heating element. It is desired that the electrical resistivity of the device be determined primarily by the coating. Therefore, the electrical resistivity of the fibrous carbon precursor must be substantially greater than the electrical resistivity of the coating after the final heat treatment that adjusts the electrical resistivity of the coating is completed. By selecting a relatively low carbonization temperature, for example below about 800°C, the electrical resistivity of the fibrous carbon precursor can be made relatively high;
This means that the PAVD coating parameters must be selected such that the heat treatment required to obtain the desired coating resistivity is preferably performed at a temperature below the carbonization temperature of the fibrous carbon precursor.

[Brief explanation of drawings]

The accompanying figure is a schematic diagram of a plasma-operated deposition apparatus. 200... Heating element (pre-cursor) 201... Alumina tube, 202... Cylindrical silica tube, 203,2
10... End cap, 208, 230... Vacuum type valve, 209, 232... Vacuum pump, 214... Mercury pressure gauge, 218... Manifold, 220, 22
2,224...Gas supply pipe line, 221,223,2
25... Valve, 226... Silica container, 228... Susceptor, 236... Copper coil, 238... High frequency power supply.

Claims (1)

Claims: 1. In a fluid-permeable electrical heating element comprising carbon fibers arranged randomly and bonded together by carbon at points of contact to form a self-supporting body, the layer of electrically conductive material is attached to the carbon fiber body. A fluid-permeable heating element characterized in that it consists primarily of free silicon coated with. 2. A method for manufacturing a fluid-permeable heating element consisting of carbon fibers arranged randomly and bonded together at contact points to form a self-supporting body, comprising coating the carbon fiber body by decomposition of a silicon-containing gas. A coating consisting primarily of free silicon is created on the fiber, and the coated carbon fiber body is then heated to approximately 600°C.
A method for manufacturing a fluid-permeable electric heating element, characterized by heating the electric heating element at a temperature of 1400°C to create a desired electrical resistance of the electric heating element.