JP2010146832A - Resistance heating element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a resistance heating element showing NTC characteristics, having the absolute value of a negative resistance temperature coefficient which is larger than that of graphite, and showing resistivity on the same level as that of graphite. <P>SOLUTION: This resistance heating element contains carbon and silicon carbide showing conductivity. In this resistance heating element, since silicon carbide is added to carbon, NTC characteristics are exhibited, and the absolute value of a negative resistance temperature coefficient is larger than that of graphite. The resistivity of the resistance heating element does not become excessively higher than original resistivity of graphite. For example, if the resistance heating element is used for a heating device to fix a toner image formed on the surface of a recording material such as paper, a temperature in a portion where the recording material does not pass through (paper non-passing portion) can be prevented from rising. A part of or all of the carbon contained in the resistance heating element is preferably graphite, and the resistance heating element preferably contains 20 mass% of silicon carbide. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention generally relates to a resistance heating element, and more specifically, to a heating device or the like for fixing a toner image formed on the surface of a recording material such as paper used in a copying machine, a printer, or the like. The present invention relates to a resistance heating element used.

  2. Description of the Related Art Conventionally, as a heating device for fixing a toner image, a heating device that heats a recording material as a material to be heated by a heating body having a resistance heating element that generates heat by power feeding is used. In such a heating apparatus, various improvements have been made in order to prevent a temperature rise in a portion where the recording material does not pass (non-sheet passing portion).

  For example, in a heating device disclosed in Japanese Patent Application Laid-Open No. 2004-234997 (hereinafter referred to as Patent Document 1), a resistance heating element used in the heating device includes graphite (graphite).

  According to Patent Document 1, graphite exhibits an NTC characteristic (Negative Temperature Coefficient: a negative resistance temperature characteristic in which electric resistance decreases as temperature rises) below a certain temperature, and PTC characteristic above that temperature. (Positive Temperature Coefficient: a positive resistance temperature characteristic in which electrical resistance increases as temperature rises). The temperature is called an inflection point temperature and is about 700 ° C. Since graphite exhibiting such properties is included in the resistance heating element, the resistance heating element described in Patent Document 1 can exhibit NTC characteristics at an inflection point temperature or lower. Thereby, since the calorific value of the resistance heating element portion corresponding to the non-sheet passing portion can be reduced by the NTC characteristic, the temperature rise of the non-sheet passing portion can be suppressed.

  Further, in the resistance heating element disclosed in Patent Document 1, since the sheet resistance of graphite is large, silver or palladium showing PTC characteristics is contained in order to reduce the total resistance of the resistance heating element.

  By the way, a general carbon heating element has a negative temperature coefficient, that is, exhibits an NTC characteristic, so that the electrical resistance value is lower than the electrical resistance value at room temperature at a high temperature. For this reason, depending on the rate of change of the electrical resistance value, it may be difficult to automatically control the temperature of the carbon heating element.

  In order to solve this problem, a manufacturing method for obtaining a carbon-based heating element having a temperature coefficient substantially close to zero, or a fixing device including a heating element whose components are adjusted to have a desired electric resistance value For example, Japanese Patent Application Laid-Open No. 2001-15250 (hereinafter referred to as Patent Document 2) or International Publication No. WO2005 / 124471 (hereinafter referred to as Patent Document 3) discloses a heater for heating.

  Patent Document 2 or Patent Document 3 describes a carbon-based heating element including graphite or amorphous carbon, and a metal or metalloid compound as a conductive inhibitor such as boron nitride or silicon carbide.

Japanese Patent Laid-Open No. 7-53265 (hereinafter referred to as Patent Document 4) discloses a conductive silicon carbide ceramic material having a very low temperature dependency of electrical resistance and having a stable conductive performance. The conductive silicon carbide ceramic material is made of a material having an absolute value of a resistance temperature coefficient from room temperature to 500 ° C. of 0.2% / ° C. or less.
JP 2004-234997 A JP 2001-15250 A International Publication No. WO2005 / 124471 Pamphlet JP-A-7-53265

  The resistance heating element disclosed in Patent Document 1 contains silver or palladium in order to reduce the total resistance of the resistance heating element. Graphite shows NTC characteristics, whereas silver or palladium shows PTC characteristics. Therefore, the resistance heating coefficient containing graphite and silver has a resistance temperature coefficient of −240 ppm / ° C., and the resistance heating element containing graphite and palladium has a resistance temperature coefficient of −460 ppm / ° C. The absolute value is smaller than. That is, the resistance heating element disclosed in Patent Document 1 has a problem that the absolute value of the negative resistance temperature coefficient is small, in other words, the NTC characteristic is weaker than that of graphite.

  Since the carbon-based heating element disclosed in Patent Document 2 or Patent Document 3 includes a conductivity-inhibiting substance for adjusting the specific resistance of the heating element, the adjusted specific resistance value is higher than the original specific resistance of graphite. It gets quite expensive. In addition, this carbon-based heating element does not exhibit remarkable NTC characteristics than graphite, that is, the absolute value of the negative resistance temperature coefficient is not larger than that of graphite.

  Note that Patent Document 4 aims to obtain a conductive silicon carbide ceramic material having an extremely small resistance temperature coefficient, exhibits NTC characteristics as a conductive member such as a heating element, and has a negative resistance. There is no mention of obtaining a silicon carbide material having a large absolute value of the temperature coefficient.

  Accordingly, an object of the present invention is to provide a resistance heating element that exhibits NTC characteristics, has a larger absolute value of a negative resistance temperature coefficient than graphite, and exhibits a specific resistance comparable to that of graphite.

  The resistance heating element according to the present invention includes carbon and silicon carbide exhibiting conductivity.

  In the resistance heating element of the present invention, since silicon carbide exhibiting conductivity is added to carbon, NTC characteristics are exhibited, and the absolute value of the negative resistance temperature coefficient is larger than that of graphite. Further, the specific resistance of the resistance heating element of the present invention is not excessively higher than the specific resistance of graphite. Therefore, for example, when the resistance heating element of the present invention is used in a heating device or the like for fixing a toner image formed on the surface of a recording material such as paper, a portion through which the recording material does not pass (non-paper passing portion) Can be significantly suppressed.

  Part or all of the carbon contained in the resistance heating element of the present invention is preferably graphite.

  The resistance heating element of the present invention preferably contains 20% by mass or more of silicon carbide. By including 20% by mass or more of silicon carbide, the specific resistance of the resistance heating element and the absolute value of the negative resistance temperature coefficient can be controlled within an appropriate range in order to achieve the above-described effects.

  As described above, according to the present invention, it is possible to obtain a resistance heating element that exhibits NTC characteristics, has an absolute value of a negative resistance temperature coefficient larger than that of graphite, and exhibits a specific resistance comparable to that of graphite. For example, when the resistance heating element of the present invention is used in a heating device or the like for fixing a toner image formed on the surface of a recording material such as paper, the portion where the recording material does not pass (non-sheet-passing portion) rises. The temperature can be significantly suppressed.

  A resistance heating element according to one embodiment of the present invention includes carbon and silicon carbide in which nitrogen is dissolved as an example of silicon carbide exhibiting conductivity.

  In the resistance heating element according to one embodiment of the present invention, since silicon carbide that exhibits conductivity by dissolving nitrogen in solid solution is added to carbon, it exhibits NTC characteristics and has a resistance temperature more negative than graphite. The absolute value of the coefficient is large. Further, the specific resistance of the resistance heating element of the present invention is not excessively higher than the specific resistance of graphite. Therefore, for example, when the resistance heating element of the present invention is used in a heating device or the like for fixing a toner image formed on the surface of a recording material such as paper, a portion through which the recording material does not pass (non-paper passing portion) Can be significantly suppressed.

The resistance heating element of the present invention preferably contains 20% by mass or more, more preferably 50% by mass or more of silicon carbide. In order to achieve the above-mentioned effect by including 20% by mass or more of silicon carbide, the specific resistance of the resistance heating element is 25 × 10 ^{−5 to} 5 × 10 ⁻¹ Ωcm at room temperature of 25 ° C. The temperature coefficient of resistance in the range of 125 ° C. can be controlled in the range of −1000 to −5000 ppm / ° C.

  The carbon contained in the resistance heating element of the present invention acts to relatively reduce the specific resistance of the entire resistance heating element. The carbon contained in the resistance heating element may be amorphous or crystalline carbon obtained by carbonizing an organic resin. The carbon contained in the resistance heating element may be graphite powder. Furthermore, the carbon contained in the resistance heating element may be a mixture of amorphous or crystalline carbon obtained by carbonizing an organic resin and graphite powder.

  When amorphous or crystalline carbon obtained by carbonizing an organic resin is used as the carbon contained in the resistance heating element, the organic resin preferably has a high carbonization rate, specifically, polyvinyl chloride. Thermosetting of any thermoplastic resin such as polyacrylonitrile, polyvinyl alcohol, polyvinyl chloride-polyvinyl acetate copolymer, polyamide, phenol resin, furan resin, epoxy resin, unsaturated polyester resin, polyimide, etc. For example, a functional resin. The organic resin may be prepared in a powder form or may be prepared in a paste form. The temperature for carbonizing the organic resin is preferably 700 ° C. or higher.

  In order to reduce the specific resistance of the resistance heating element, it is preferable to use graphite powder as part or all of the carbon contained in the resistance heating element. The graphite powder may be general graphite powder, but natural graphite powder is preferably used in order to reduce the specific resistance.

The specific resistance of carbon contained in the resistance heating element is 1 × 10 ⁻⁵ to 1 × 10 ⁻² Ωcm at a normal temperature of 25 ° C., and the temperature coefficient of resistance in the range of 25 to 125 ° C. is −800 to −3000 ppm / ° C. preferable.

  When graphite powder is used as the carbon contained in the resistance heating element, the particle diameter of the graphite powder is preferably 10 μm or less, and more preferably 5 μm or less, from the standpoint of making the resistance heating element raw material into a compact or paste. .

The conductive silicon carbide contained in the resistance heating element of the present invention acts to relatively increase the absolute value of the negative resistance temperature coefficient of the entire resistance heating element. Conductive silicon carbide is prepared as a raw material in the form of a powder. The specific resistance of the silicon carbide powder is preferably −1000 to −6000 ppm / ° C. in the range of 1 × 10 ^{−3 to} 10 Ωcm and 25 to 125 ° C. at a normal temperature of 25 ° C. The silicon carbide powder preferably has a particle size of 10 μm or less, and more preferably 5 μm or less, from the standpoint of easily forming the resistance heating element as a molded body or paste.

  The crystal form of the silicon carbide powder is not limited, and may be α-type or β-type. As a method for producing the silicon carbide powder, any one of the Atchison method, the silica reduction method, the gas phase method, and the liquid phase method may be employed.

  In order to control the absolute value of the negative temperature coefficient of resistance of silicon carbide to be a target value, the crystal form of silicon carbide and the element that is solid-solved so as to exhibit conductivity are appropriately selected. In general, in a semiconductor such as silicon carbide, the larger the energy gap between a donor level or an acceptor level formed by a solid solution element and a conductor or a valence band, the higher the resistance temperature coefficient of the material. The absolute value increases. In addition, the greater the energy gap, the greater the specific resistance of the material. In this case, the energy gap of silicon carbide is determined by the crystal form of silicon carbide and the elements dissolved in silicon carbide.

  Elements dissolved in silicon carbide are aluminum (Al), boron (B), phosphorus (P), nitrogen (N), gallium in consideration of the absolute value of the negative resistance temperature coefficient and the value of specific resistance. (Ga), beryllium (Be), chromium (Cr), titanium (Ti), scandium (Sc), vanadium (V) and the like are preferable, and aluminum (Al), boron (B), phosphorus (P), nitrogen (N ) Is more preferable. As a method for dissolving an element in the silicon carbide powder, there are a method of treating the silicon carbide powder at a high temperature in an atmosphere containing the element to be dissolved, a method of irradiating the silicon carbide powder with ions of the element to be dissolved.

  The resistance heating element of the present invention can be obtained by adhering either organic resin or graphite powder or a mixture of both organic resin and graphite powder and conductive silicon carbide to a substrate in the form of a solid material. Alternatively, it may be baked after being applied on the substrate in the form of a paste.

  Specifically, the form of the solid material is either an organic resin or graphite powder, or both organic resin and graphite powder and conductive silicon carbide powder are dry-mixed with a Henschel mixer or the like, an organic binder It is produced by adding a solvent and a plasticizer, kneading with a kneader, etc., extruding, and firing. In this case, instead of using conductive silicon carbide powder, normal silicon carbide powder is used, and the firing step after extrusion molding is performed in a nitrogen atmosphere, so that nitrogen is dissolved in silicon carbide to form silicon carbide. You may provide electroconductivity.

  Specifically, the method of baking after coating on the substrate in a paste form is either an organic resin or a graphite powder, or both an organic resin and a graphite powder, and a conductive silicon carbide powder, etc. Add the organic binder and solvent to the dry-mixed mixture, knead with three rolls, etc., and adjust the viscosity of the paste-like form on the substrate by screen printing method, dipping method, etc. It is the method of baking after apply | coating to. In this case, instead of using conductive silicon carbide powder, normal silicon carbide powder is used, and the baking process after applying the paste-like form on the substrate is performed in a nitrogen atmosphere, so that nitrogen is added to silicon carbide. May be dissolved to impart conductivity to silicon carbide.

  Examples of the organic binder include ethyl cellulose and polyvinyl alcohol. After adding the organic binder to either the organic resin or the graphite powder, or both the organic resin and the graphite powder, and the conductive silicon carbide powder, the organic binder may be removed in a degreasing step in the subsequent step, It does not have to be removed. When the organic binder is not removed, it is preferable to reduce the specific resistance of the remainder by performing a carbonization treatment by heat treatment at a high temperature in a non-oxidizing atmosphere in a subsequent step.

  Examples of the solvent include water, ethanol, toluene, methyl ethyl ketone, pyrrolidone and the like. The type of solvent used is selected in consideration of the solubility of the organic binder and plasticizer used and the dispersibility of the raw material powder.

  Examples of the plasticizer include glycerin, polyalkyl glycol, and polyethylene glycol.

  Examples of the molding method for producing the resistance heating element in the form of a solid material include a doctor blade molding method, an injection molding method, and a press molding method in addition to the extrusion molding method. When an extrusion molding method or a doctor blade molding method is employed as the molding method, it is preferable to add a solvent or a plasticizer to the raw material powder. However, when an injection molding method or a press molding method is employed, the raw material powder It is not necessary to add a solvent or a plasticizer.

  The molded body must be fired in a non-oxidizing atmosphere so as not to vaporize carbon. The firing temperature is preferably in the range of 700 to 2300 ° C. A firing temperature exceeding 2300 ° C. is not preferable because silicon carbide decomposes. However, when firing is performed in a pressurized atmosphere, the firing temperature may be slightly higher than 2300 ° C. In addition, you may produce the resistance heating element of the form of a solid substance by drying a molded object, without baking. The thickness of the solid resistance heating element obtained is preferably in the range of 0.01 to 2 mm.

  Samples were prepared as examples of the resistance heating element of the present invention.

Example 1
First, a silicon carbide powder having an average particle diameter of 5 μm is held for 1 hour in a nitrogen gas atmosphere having an atmospheric pressure of 1 to 10 atm and a temperature of 2000 ° C., and the nitrogen is solid-dissolved in the silicon carbide powder. A silicon carbide powder was prepared. On the other hand, graphite powder having an average particle size of 5 μm was prepared.

  Next, the graphite powder and the conductive silicon carbide powder were dry-mixed with a Henschel mixer so that the content of the conductive silicon carbide in the finally obtained resistance heating element was 20 to 80% by mass.

  Thereafter, polyvinyl alcohol as an organic binder, water as a solvent, and glycerin as a plasticizer were added to the obtained mixed powder and kneaded with a kneader.

  The obtained kneaded material was extruded using an extruder to produce a molded body.

  The obtained molded body was placed in a firing furnace and fired by holding it in a nitrogen gas atmosphere at a temperature of 1700 ° C. for 2 hours.

When the resistivity and the temperature coefficient of resistance of the sample of the resistance heating element of the present invention thus manufactured were measured, the resistivity of the resistance heating element at a room temperature of 25 ° C. was 5 × 10 ^{−4 to} 5 × 10 ⁻² Ωcm. The temperature coefficient of resistance in the range of 25 to 125 ° C was within the range of -1200 to -4500 ppm / ° C.

(Example 2)
First, a silicon carbide powder having an average particle diameter of 5 μm is held for 1 hour in a nitrogen gas atmosphere having an atmospheric pressure of 1 to 10 atm and a temperature of 2000 ° C., and the nitrogen is solid-dissolved in the silicon carbide powder. A silicon carbide powder was prepared.

  Next, a conductive silicon carbide powder, a paste-like polyimide resin as an organic resin, and a solvent so that the content of the conductive silicon carbide in the finally obtained resistance heating element is 20 to 80% by mass As a result, pyrrolidone was kneaded with three rolls.

  The paste thus obtained was applied onto a substrate by a screen printing method, and baked by maintaining in a nitrogen gas atmosphere at a temperature of 1500 ° C. for 2 hours.

When the specific resistance and the temperature coefficient of resistance of the sample of the resistance heating element of the present invention thus manufactured were measured, the specific resistance of the resistance heating element at room temperature of 25 ° C. was 5 × 10 ⁻³ to 1 × 10 ⁻¹ Ωcm. The temperature coefficient of resistance in the range of 25 to 125 ° C. was within the range of −1000 to −4000 ppm / ° C.

(Comparative example)
For comparison, when a specific resistance and a temperature coefficient of resistance were measured for a commercially available carbon sheet, the specific resistance of the resistance heating element at room temperature of 25 ° C. was within the range of 5 × 10 ^{−4 to} 5 × 10 ⁻³ Ωcm, and 25 to 25 ° C. The temperature coefficient of resistance in the range of 125 ° C. was within the range of −800 to −1200 ppm / ° C.

  From the above results, the samples of Examples 1 and 2 of the present invention exhibit NTC characteristics, have a larger absolute value of the negative temperature coefficient of resistance than a commercially available carbon sheet made of graphite, and are commercially available carbon sheets. It can be seen that this is a resistance heating element exhibiting a specific resistance of the same level.

It should be considered that the embodiments and examples disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is shown not by the above embodiments or examples but by the scope of claims, and is intended to include all modifications and variations within the meaning and scope equivalent to the scope of claims. .

Claims (3)

  A resistance heating element comprising carbon and silicon carbide exhibiting conductivity.   The resistance heating element according to claim 1, wherein a part or all of the carbon is graphite.   The resistance heating element according to claim 1 or 2 containing 20 mass% or more of said silicon carbide.