Electrically conductive heating element

Abstract

 Electrical conductivity excellent for an electrically-conductive heating element can be provided by uniformly dispersing, in a matrix made of a ceramic or the like, foliated fine graphite particles having a particle size of 1-100 µm, a thickness not greater than 1 µm and an aspect ratio of 10-5,000. The heating element is suitable for use as a ceramic heater.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The present invention relates to an electrically-conductive heating element suitable for use in a ceramic heater. The heating element can produce heat by direct energization, and is usable in a wide range of industrial and civil fields.

2. Description of the Related Art

[0002] Most of ceramic heaters employed these days are of the type that electricity is fed to a metallic resistance heating element embedded in a matrix made of a ceramic to obtain thermal energy by resistance heating. Such ceramic heaters are known to include those having a metallic resistance heating member of tungsten or molybdenum embedded in a matrix composed principally of alumina, those containing a metallic resistance heating element such as palladium or platinum embedded in a matrix composed principally of cordierite, those having a metallic resistance heating element made of copper and embedded in a matrix composed principally of a borosilicate glass and alumina (Japanese Patent Application No. 20678/1987), etc.

[0003] In these ceramic heaters, a certain measure is taken to achieve a uniform heating temperature distribution, for example, by forming a heating resistance element, which is in the form of a wire, strip or the like, into a wavy, spiral or tortuous shape and then arranging it uniformly. Heat is however produced intensively near the heating resistance element only, so that they are still insufficient to provide a uniform heating temperature distribution. These heaters also have unsolved problems such as the fact that their heating response is slow because heat must be conducted through a thick matrix and, in addition, high- temperature firing and adjustment of the firing atmosphere are needed upon production of ceramic heaters. It is therefore recently attempted to obtain a heating element, which permits production of uniform heat therethroughout, by adding an electrically-conductive material such as carbon to a heat-resistant ceramic. A carbon material such as graphite powder is generally used as an electrically-conductive material. Graphite powder, which has conventionally been employed as an electrically-conductive material, can be obtained by mechanically comminuting natural or synthetic graphite or by subjecting carbon black to graphitization. It is, however, difficult to uniformly disperse such a carbon material in a raw ceramic batch, resulting in serious problems such that substantial variations may occur in electrical conductivity among materials to be obtained and the electrical conductivity may not be uniform throughout the product to be formed.

[0004] Various processes have heretofore been attempted with a view toward overcoming such problems and hence obtaining a ceramic material having uniform electrical conductivity, including, for example, a process in which, after carbon and an inorganic material are kneaded and heated in advance, the resultant mass is crushed into powder and the powder so prepared is again kneaded as a pre-treated raw material, followed by forming and sintering (Japanese Patent Laid-Open No. 217668/1984) and another process in which, in order to improve the integrity between a ceramic and a carbon material filled therein, the ceramic is nitrided while being sintered (Japanese Patent Laid-Open No. 19505/1985). Even when these processes are followed, one or more problems still arise, for example, the need for a more complex process for the production of a ceramic heater and/or the imposition of a limitation to particular ceramic materials.

SUMMARY OF THE INVENTION

[0005] An object of the present invention is to overcome the above-described problems, and hence to provide a heating element - which features the possibility of production of heat by direct energization, quick response to energization, excellent thermal shock resistance and production of uniform heat and requires only an easy production process - and an electrically-conductive heating element having an electrical insulating layer integrated with the heating element and suited for use as a ceramic heater.

[0006] The present inventors have carried out an extensive investigation with a view toward overcoming the above-described problems. As a result, it has been found that a formed ceramic body having uniform electrical conductivity can be obtained by adding, as an electrically-conductive material, foliated fine graphite particles having a high aspect ratio to a ceramic, an inherent electrical insulator, or the like, and, subsequent to formation of the resultant mass into a green body, sintering the green body, leading to the completion of the present invention.

[0007] The present invention therefore provides an electrically-conductive heating element, which comprises:

100 parts by weight of a matrix composed of a ceramic, a glass or a glass-ceramic mixture; and

0.5-10 parts by weight of foliated fine graphite particles uniformly distributed as an electrical-conductivity-imparting material in the matrix, said graphite particles initially having a particle size of 1-100 /.Lm, a thickness not greater than 1 /.Lm and an aspect ratio of 10-5,000. The electrically-conductive heating element may optionally includes an insulating layer composed of the same material as the matrix and provided integrally on a surface of the element.

[0008] The foliated fine graphite particles which have high crystallinity and are highly effective in imparting electrical conductivity are dispersed uniformly in the matrix. The electrically-conductive heating element according to the present invention is therefore an electrically-conducting heating element of the direct energization type, which has high electrical conductivity, can quickly respond to energization and is excellent in the temperature-raising characteristic, can produce uniform heat upon application of a low voltage, and has excellent heat resistance. It can be formed into an electrically-conductive heating element of a desired shape. It is therefore possible to meet the demands for heaters, such as a reduction in both dimensions and weight. The electrically-conductive heating element is useful as a heater element for various electrical heaters and the like.

[0009] Further, the optional formation of the insulating layer on the electrically-conductive heating element can provide electrical insulation and, moreover, can prevent oxidation of the foliated fine graphite particles and can improve the moisture resistance. The insulating layer is therefore effective in prolonging the service life of the electrically-conductive heating element as a heater. Since the insulating layer uses the same batch as the matrix, which is a base member of a main body of the heating element, the main body of the heating element and the insulating layer are not separated due to any difference in thermal expansion coefficient when used as a heater. Furthermore, the electrically-conductive heating element can be easily formed into a heater by mounting electrodes, for example, by baking an electrically-conductive paste or conducting metallization. It is therefore possible to provide a simplified process for the production of a heater.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] 

FIG. 1 is a concept sketch showing one example of stacking of green sheets as heating layers and insulating layers and one example of formation of insulating paste layers, upon production of a heating element with the insulating layers formed thereon; and

FIG. 2 is a diagrammatic representation of an illustrative, degreasing and sintering temperature pattern when green sheets are formed and then stacked and sintered into a heating element.

DETAILED DESCRIPTION OF THE INVENTION AND THE PREFERRED EMBODIMENTS

[0011] Examples of the ceramic which makes up the matrix of the electrically-conductive heating element according to the present invention include oxide ceramics such as alumina, silica-alumina, cordierite, mullite, petalite, titania and zirconia; non-oxide ceramics such as silicon nitride and silicon carbide; and mixtures thereof. Depending on properties and performance desired for each product such as radiation property and thermal shock resistance, an appropriate ceramic can be selected from these ceramics. On the other hand, examples of the glass include silicate glasses such as borosilicate glass, aluminosilicate glass and soda line glass; and oxynitride glasses. It is necessary to choose, from these glasses, a glass having a composition that is not softened to undergo deformation in shape at an application temperature (approximately 50-500 C) when employed as a heater. From the standpoint of avoiding breakage due to thermal shocks, it is desirable for these raw materials to have a thermal expansion coefficient on the order of 40 x 10-⁷ (1/ " C) or smaller.

[0012] Although it is possible to use either a ceramic or a glass singly, the addition of a glass to a ceramic is advantageous in view of the production process because their combined use makes it possible to lower the sintering temperature. On the other hand, the addition of such a glass component results in an electrically-conductive heating element having a lower withstandable maximum temperature. It is therefore necessary to suitably determine the proportion of a glass, which is to be added, in view of the application purpose and production conditions.

[0013] In the electrically-conductive heating element according to the present invention, the foliated fine graphite particles added as an electrical-conductivity-imparting material are graphite particles having the very special shape that they have a particle size of 1-100 µm, a thickness not greater than 1 µm and an aspect ratio of 10-5,000. More preferably, the particle size, thickness and aspect ratio is 1-50 µm, not greater than 1 µm and 200-3,000, respectively, and the average particle size ranges from 10 µm to 30 µm or so. If the particle size of the foliated fine graphite particles becomes greater than 100 µm, it will be difficult to uniformly disperse the foliated fine graphite particles in the matrix-forming raw material powder. On the other hand, particle sizes smaller than 1 /.Lm make it difficult to form electrically-conductive paths or make it necessary to increase the amount of the foliated fine graphite particles to be used so that electrically-conductive paths can be formed, thereby making it difficult to obtain a dense, sintered body. Such foliated fine graphite particles can be prepared, for example, by dispersing expanded graphite particles - which have been obtained by causing natural graphite to expand in accordance with acid treatment, heat treatment or the like - in an aqueous solvent and then applying ultrasonic waves to the expanded graphite particles to break them up (see Japanese Patent Laid-Open No. 153810/1990). These foliated fine graphite particles have been formed into powder in such a state as being separated between layers while maintaining the crystalline form of the starting graphite such as natural graphite, and have the special shape and high crystallinity as described above. Owing to their high crystallinity, foliated fine graphite particles useful in the practice of the present invention have the characteristic property that they are resistant to oxidation even in an oxidizing atmosphere. For example, foliated fine graphite particles obtained from natural graphite mined in China have high crystallinity of developed hexagonal graphite such that the lattice constant is about 0.67 nm, the crystallite thickness is approximately 70 nm and the crystallite size is about 100 nm. Incidentally, a variety of graphite particles are available on the market. They can be classified in particle size, for example, to 1-30 µm (15 µm and smaller: 95%), 2-70 µm (44 _/.Lm and smaller: 95%) and 2-100 µm (75 µm and smaller: 95%). They have a thickness substantially equal to or about a half of the particle size, so that they look as if they have a block-like shape. When graphite particles of such a block-like shape are used, it is difficult to form electrically-conductive paths by using them in a small amount. Use of such graphite particles in an increased amount to form electrically-conductive paths, however, leads to problems such that a dense, sintered body can hardly be obtained. On the other hand, foliated fine graphite particles usable in the present invention have a very thin thickness so that adjacent graphite particles tend to overlap, thereby making it possible to form electrically-conductive paths even at a low concentration.

[0014] The electrically-conductive heating element according to the present invention can be produced, for example, as will be described next. To 100 parts by weight of a ceramic powder, a glass powder or a ceramic-glass mixture (which will hereinafter be collectively called a "matrix-forming raw material powder") which had been ground and sifted for particle size adjustment in advance, foliated fine graphite particles having the above-described shape were added as an electrically-conductive material in an amount of 0.5-10 parts by weight, preferably 1-5 parts by weight. They were then mixed using a conventional powder mixer such as a kneader, a Henschel mixer, or a double-cone or twin-cylinder blender. The foliated fine graphite particles are somewhat damaged and shortened in the lengthwise direction in the course of the mixing, but most of the foliated fine graphite particles remain within the range of 1-100 /.Lm. In the thicknesswise direction, they are not damaged practically.

[0015] Although the matrix-forming raw material powder preferably has a particle size not greater than about 100 µm from the standpoint of mixing readiness with the foliated fine graphite particles, no particular limitation is imposed on the particle size of the matrix-forming raw material powder. It is only necessary to use a matrix-forming raw material powder of a suitable particle size in accordance with the mixing and forming methods to be used and properties sought for the heating element to be produced. If foliated fine graphite particles are added in an amount smaller than 0.5 part by weight, they cannot exhibit sufficient electrical-conductivity-imparting effect because of discontinuation of electrically-conductive paths. On the other hand, amounts greater than 10 parts by weight impair the density of a heating element to be formed because of a reduction in the number of points of contact with the particles of the matrix-forming raw material powder.

[0016] The foliated fine graphite powder employed as a raw material for the electrically-conductive heating element according to the present invention are in a foliated form having a high aspect ratio. When mixed with the matrix-forming raw material powder, the foliated fine graphite particles are free from such a phenomenon that the graphite particles alone would be separated or would be concentrated locally. The foliated fine graphite particles therefore permit uniform dispersion so that a uniform, distributed sate can be maintained not only in the green body but also in the sintered body. Further, water or an organic or inorganic binder may also be added, as needed, as a forming aid upon mixing.

[0017] The resultant mixture of the matrix-forming raw material powder and the foliated fine graphite particles are next formed into desired shape and dimensions by a forming method, for example, by a powder pressure forming method such as uniaxial pressure forming or cold isostatic pressing, by a forming method in which green sheets formed by the doctor blade method or calender roll method are stacked together, by slip casting, or by extrusion.

[0018] When a powder pressure forming method is employed by way of example, the forming pressure can be preferably 2.9-98.1 MPa, especially 9.8-49.0 MPa or so. When green sheets are stacked together to conduct the forming, the mixture of the foliated fine graphite particles and the matrix-forming raw material powder are kneaded with an organic vehicle. To provide a heating layer for an electrically-conductive heating element, the above-prepared mass is then formed by the doctor blade method or the calender roll method into a green sheet in which the foliated fine graphite particles as one of the raw materials are uniformly dispersed in the matrix-forming raw material powder. A plurality of such green sheets, the number of said green sheets depending on the specification of each product to be fabricated, are stacked together and pressure-bonded under heat to laminate them.

[0019] Depending on the forming method, the foliated fine graphite particles are somewhat damaged or broken in the course of the formation. Even when the foliated fine graphite particles are broken in this stage, electrically-conductive paths to be formed will not be in a disconnected form. Practically, no problem therefore arises. This applies equally to a sintering step which will be described next.

[0020] After the formation, the preformed green body is adjusted in shape and dimensions by cutting, grinding or the like as needed. Subsequent to degreasing at a temperature of 400 C or lower, the preformed green body is sintered at a temperature of 450- 1,500` C. The degreasing temperature and sintering conditions can be set suitably in accordance with the kinds of the binder and matrix-forming raw material powder used, the shape of the preformed green body, etc. When the matrix-forming raw material powder is a silica-alumina ceramic for example, it is necessary to set the sintering conditions at 1,100-1500 C for 0.5-5 hours and, where the glass component is contained in a large proportion, at 450-900 C for 10 minutes to 1 hour. Although it is preferred to conduct the sintering in an inert gas atmosphere, sintering in air is feasible where the proportion of the glass component in the matrix-forming raw material powder becomes 50 wt.% or higher because sintering at 900 C or lower is feasible so that there is no potential danger of oxidation of the mixed, foliated fine graphite particles. The density of the preformed green body after the sintering, namely, the density of the electrically-conductive heating element may be 1.85-2.20 g/cm³ or so.

[0021] The electrically-conductive heating element is generally used in a form with an insulating layer formed on a surface thereof in order to improve its electrical insulation, moisture resistance, etc. This insulating layer can be formed, for example, by baking a glaze or a low-melting glass on the surface of the electrically-conductive heating element obtained by the sintering. However, the electrically-conductive heating element of the present invention can be obtained more efficiently in the form of an insulated, electrically-conductive heating element, in which a main body of the heating element and an insulating layer are firmly united together into an integral body, by covering a surface of the preformed green body with a layer composed of an organic vehicle component and the matrix-forming raw material powder - which has not been added with the foliated fine graphite powder as a conductivity-imparting material - before the sintering of the preformed green body and then sintering the thus-covered green body. Hereinafter, such an insulated, electrically-conductive heating element will also be referred to simply as an "electrically-conductive heating element". This process is also effective in preventing oxidation of the foliated fine graphite particles during sintering.

[0022] The insulating layer can also be formed in the following manner. For example, a mixture of the foliated fine graphite particles and the matrix-forming raw material powder is kneaded with an organic vehicle. The resulting mass is formed by the doctor blade method, the calender roll method or the like into a heating-layer-forming green sheet in which the foliated fine graphite particles are uniformly distributed in the matrix-forming raw material powder. A plurality of such green sheets, the number of said green sheets being dependent on the specification of a product to be formed, are stacked together to provide a preformed green body. The preformed green body is then sandwiched between insulating-layer-forming green sheets which have been prepared in a similar manner and which are composed of an organic vehicle component and the matrix-forming raw material powder not added with the foliated fine graphite powder as a conductivity-imparting material. The resultant assembly is pressure-bonded under heat, whereby the preformed green body and the green sheets are laminated together. Insulating paste layers composed of the matrix-forming raw material power and the organic vehicle are formed by a method such as screen printing on end and side surfaces of the preformed green body at areas where the surfaces are not used as electrode terminal attachment portions. The preformed green body with the insulating paste layers is then sintered. As a further alternative, a slurry of the matrix-forming raw material powder which has not been added with the foliated fine graphite particles as a conductivity-imparting material is prepared with an adjusted viscosity. The slurry is coated on an electrically-conductive heating element, which has been obtained in advance by sintering, or an unsintered green body, for example, by spraying the slurry onto the electrically-conductive heating element or the unsintered green body or by dipping the electrically-conductive heating element or the unsintered green body in the slurry, so that an insulating layer is formed. The insulating layer is dried and then sintered. Where the slurry is coated on the unsintered green body, the unsintered green body is also sintered concurrently with the sintering of the insulating layer. The thickness of the insulating layer varies depending on the voltage applied when the heating element is used as a heater. For example, for voltages up to about 100 V, 0.2 mm or so is sufficient as the thickness of the insulating layer.

[0023] Since the foliated fine graphite particles as an electrically conductive material are uniformly dispersed in the electrically-conductive heating element according to the present invention, the formed body has uniform conductivity therethroughout and its volume resistivity is in the range of from 10-¹ Ω·cm to 10³ Ω·cm. By changing the amount of the foliated fine graphite particles to be added, the volume resistivity can be adjusted as desired within the above range. Use of foliated fine graphite particles as an electrical-conductivity-imparting material permits the formation of many current flow paths despite the small volume occupied by them and hence facilitates to develop electrical conductivity, because the foliated fine graphite particles have a high aspect ratio. High conductivity can therefore be obtained by adding the foliated fine graphite particles in a small amount, thereby bringing about the advantage that the characteristic features of the matrix-forming raw material powder are not impaired.

[0024] In the electrically-conductive heating element with the insulating layer formed of the matrix-forming raw material powder, the composition of the insulating layer is the same as that of the matrix-forming raw material powder employed as a base material for the heating element. While employed as a heater, the main body of the heating element and the insulating layer therefore remain free from separation which would take place if there were any substantial difference in thermal expansion coefficient between the main body of the heating element and the insulating layer. Further, the electrically-conductive heating element with the insulating layer formed thereon can be used easily as a heater by mounting electrodes thereon, for example, by baking an electrically conductive paste or by metallization. Unlike conventional processes for the production of heaters, the present invention does not require the step that an insulating layer made, for example, of alumina is provided around a heating element. The present invention therefore makes it possible not only to simplify the production process for heaters but also to meet the demand for reductions in the dimensions and weight of heaters.

[0025] The heating element according to the present invention can be easily energized by applying a voltage thereacross, and uniformly produces heat therethroughout. Moreover, it is possible to choose the shape, dimensions and volume resistivity as desired and, by adjusting the level of electricity to be supplied, to control the heating temperature as desired. Specifically, the heating element can be heated from room temperature to 600 C or so in 10 minutes after the initiation of its energization at a voltage of from about several volts to about 100 V, and can be maintained in a stably heated state. In particular, those having a low volume resistivity on the order of from 10-¹ Q'cm to 10 Q'cm can produce heat at a low voltage of from about several volts to about 40 V, so that they can be used as small, low-power, heating elements. Owing to the use of a low voltage, there is a smaller potential danger of electrification so that they are also advantageous from the standpoint of safety. The electrically-conductive heating element according to the present invention can be easily formed into a heater element by mounting electrode thereon, for example, by baking an electrically conductive paste or by metallization.

[0026] Electrically-conductive heating elements according to the present invention are useful as warming, cooking or drying heating elements or as heating elements for fuel vaporizers.

[0027] As has been described above, the electrically-conductive heating elements of the present invention feature the use of the particular foliated fine graphite particles. It is, however, not fully clear how much the initial shape of the foliated fine graphite particles is retained in the heating elements. It may, however, be possible to estimate it by measuring the characteristic electrical conductivity, which has been achieved for the first time by the use of the foliated fine graphite particles, in relation to the content of the graphite particles.

[0028] The present invention will hereinafter be described more specifically by the following examples.

[0029] Examples in which a matrix-forming ceramic was used as a matrix-forming raw material powder will be described as Examples 1-5 and Comparative Examples 1-2. On the other hand, examples in which a glass or a glass-ceramic mixture was used as a matrix-forming raw material powder will be given as Examples 6-16 and Comparative Examples 3-7.

[0030] Incidentally, the foliated fine graphite particles employed in Examples 1-16 and Comparative Examples 3-7 were prepared in the following manner.

[0031] Natural flake graphite powder mined in China was treated with a mixed acid of sulfuric acid and nitric acid (sulfuric acid:nitric acid = 11:1 by weight) into an intercalation compound. After being washed with water and then dried, the intercalation compound was rapidly heated to 800 C in a nitrogen gas atmosphere and was maintained at that temperature, whereby expanded graphite particles were obtained. The expanded graphite particles were dispersed in water, to which ultrasonic waves whose frequency was 50 Hz were applied. The expanded graphite particles were therefore broken up, whereby foliated fine graphite particles were obtained.

Example 1

[0032] To 100 g of "Petalite N-100" (trade name; product of Nishimura Togyo K.K.) whose particle size had been adjusted to 250 µm or smaller, 2.5 g of the foliated fine graphite particles having a thickness of about 0.1 µm and an aspect ration of 100-500 were added. They were mixed and kneaded for 5 minutes in a kneader. Fifty grams of the resultant mass were pressure formed under a pressure of 4.9 MPa in a cylindrical mold whose diameter was 48 mm, whereby a preformed green body was obtained. The preformed green body was heated at a rate of 3° C per minute from room temperature to 1,300 C under a nitrogen gas atmosphere in an electric furnace. After the preformed green body was fired further for 1 hour at 1,300 C, it was cooled to 500° C at a rate of 3° C per minute. The thus-fired body was then allowed to cool down to room temperature. The resultant, electrically-conductive heating element had a density of 1.9 g /cm³ and had been fully sintered. From the heating element, a rectangular parallelopipedal sample of 25 x 38 x 4.5 mm was cut out. A sinterable Ag paste was coated on both longitudinal end surfaces and then dried at 150°C, so that electrode-bearing surfaces were formed. The volume resistivity of the sample as measured by the four-terminal method was 1.3 Ω·cm. To samples identical to the above sample, voltages of 12 V and 18 V were applied, respectively, so that the samples were energized by currents of 2.7 A and 4.6 A, respectively. The samples were heated in toto to about 400 C and 500 C in about 5 minutes and about 2 minutes, respectively. Continued energization allowed to stably maintain the samples at their respective temperatures.

Example 2

[0033] Electrically-conductive heating elements were produced in a similar manner to Example 1 except that the amount of the foliated fine graphite particles added was changed and the forming pressure was raised to 9.8 MPa. The volume resistivities of the heating elements so obtained were as follows:

Example 3

[0034] A batch (300 g) proportioned and kneaded under the same conditions as in Example 1 was filled in a square cylindrical mold of 130 x 130 x 12 mm and pressure formed under the pressure of 9.8 MPa. The preformed green body was fired under the same conditions as in Example 1, whereby an electrically-conductive heating element was obtained. The density and volume resistivity of the heating element were 2.2 g/cm³ and 0.8 Ω·cm, respectively. The heating element was cut and polished into a sample of 113 x 120 x 10 mm. A voltage of 13 V was applied at an inter-electrode distance of 113 mm so that a current of about 10 A was allowed to pass across the sample. The sample was then heated to 220 C in about 10 minutes and was stably maintained at the same temperature. Further, the surface temperature of the sample was measured in equally-divided nine regions. The surface temperature was approximately 220 C in all the nine regions, whereby the sample showed a uniform temperature distribution.

Example 4

[0035] Against the surface of an electrically-conductive heating element produced under the same conditions as in Example 3, a glaze formed of 60 g of a frit adjusted to 149µm or smaller [trade name: "3127", product of Ferro Enamels (Japan) Limited] and 40 g of water was sprayed. After the glaze was dried, the glazed heating element was heated at 1,100°C in a nitrogen gas atmosphere to bake the glaze onto the heating element. The resultant, surface-coated, electrically-conductive heating element was insulated at the surface thereof, but the volume resistivity of the energization characteristics of the whole heating element were exactly the same as those of the sample produced in Example 3.

[0036] This sample was divided substantially equally into nine pieces, each of 39 x 39 x 10 mm. An electrically-conductive Ag paste was baked on each of the pieces. Terminals are attached to each piece (at an inter-terminal distance of 39 mm), followed by the measurement of its volume resistivity by the two- terminal method. All the pieces had a resistivity of 0.8 Ω·cm. When a voltage of 7 V was applied to each piece to energize it at a current of 10 A, each piece was heated to 410° C in about 5 minutes. Each piece was successfully and stably maintained at the same temperature for 30 minutes or longer.

Example 5

[0037] An electrically-conductive heating element was obtained in a similar manner to Example 1 except that "Cordierite N-53" (trade name; product of Nishimura Togyo K.K.) was used in place of "petalite N-10" and the firing temperature was lowered to 1,100°C. The density and volume resistivity of the heating element were 1.7 g/cm³ and 2.9 O_.cm, respectively. An energization test was also conducted under the same conditions as in Example 1. As a result, the current level and heating temperature were 1.2 A and 225 C, respectively, when a voltage of 12 V was applied.

Comparative Example 1

[0038] A sintered body was obtained under the same conditions as in Example 3 except for the use of commercial graphite powder (particle size: 1-5 µm, thickness: 0.2-0.6 µm, aspect ratio: 2-8) in place of the foliated fine graphite particles. The volume resistivity of a sample of 120 x 120 x 10 mm was as high as 1.2 x 10³ Ω·cm. In addition, the volume resistivities of pieces obtained by dividing the sample into 9 equal sections of 39 x 39 x 10 mm varied within a range of from 0.7 x 10³ Ω·cm to 1.5 x 10³ Ω·cm.

Comparative Example 2

[0039] Two sintered bodies were produced in a similar manner to Comparative Example 1 except that the amount of graphite powder was increased to 3.5 g. Their volume resistivities were 5.2 Ω·cm and 6.7 Ω·cm, respectively, thereby indicating the occurrence of variations in properties despite their production under the same conditions.

Examples 6-16

[0040] Employed as raw materials were a borosilicate glass powder having properties of a softening point of 800 C and a thermal expansion coefficient of 30 x 10-⁷/°C and adjusted in particle size to an average particle size of 3 µm; foliated fine graphite particles adjusted in particle size to an average particle size of 20 µm (particle size: 1-100 µm, thickness: not greater than 1 µm, aspect ratio: 10-5,000, average particle size: 20 µm); and, as ceramic powders, alumina, mullite and cordierite powders all adjusted in particle size to an average particle size of 2 µm. Further, the matrix-forming raw material powder was added with an organic vehicle which had been prepared by dissolving ethylcellulose as a binder in a-terpinol. The resultant mixture was kneaded by a three-roll mill, followed by adjustment to a suitable viscosity. The mixture so prepared was employed as an insulating paste.

[0041] In each example, the foliated fine graphite particles were added in the corresponding proportion shown as an outer percentage in Table 1 to form a homogeneous mixture. Added next to 100 parts by weight of the mixture were 16 parts by weight of an acrylic resin, 3 parts by weight of dibutyl phthalate, 22 parts by weight of toluene and 48 parts by weight of ethanol. The resulting mixture was mixed for 24 hours in a polyethylene-made pot mill with alumina-made balls filled therein, whereby a homogeneous slurry was prepared.

[0042] By the doctor blade method, a green sheet of 0.3 mm in thickness was formed as a heating-layer-forming sheet from the slurry. Similarly, a green sheet of the matrix-forming raw material powder was also formed as an insulating-layer-forming sheet.

[0043] As is illustrated in FIG. 1, three heating-layer-forming sheets 1 were stacked, and one insulating-layer-forming sheet 2 was superposed on each of the top and bottom of the stacked heating-layer-forming sheets 1. The stacked layers were bonded together under pressure into a preformed green body of 100 x 50 mm. An insulating paste layer 3 was formed on each side wall of the preformed green body. The assembly so formed was degreased and sintered in the environmental atmosphere in accordance with the exemplary degreasing and firing temperature pattern depicted in FIG. 2.

[0044] Electrode-bearing surfaces were formed on both end surfaces of the thus-obtained ceramic heating element, whereby a heater was formed. A voltage of 50 V was applied to the heater so that the heater was energized and heated. The electrical resistance at that time and the temperature of the surface of the heating element at the time of energization and heat production were measured by means of a non-contact type radiation thermometer. The results are shown in Table 1.

[0045] A heating element produced in a similar manner by using the heating-layer-forming sheets alone showed substantially the same characteristics as the heating element with the insulating layer formed thereon.

[0046] Electrode-bearing surfaces were also formed on both end surfaces of the electrically-conductive heating element having the insulating layer thereon, so that a heater was produced. The heater was energized across both terminals. When the heater reached a predetermined temperature and the temperature became stable, an insulation resistance test was conducted. As a result, the insulation resistance was at least 800 MΩ at 300 C and at least 3 MΩ at 500 C so that the heater had sufficient insulation.

Comparative Examples 3-6

[0047] In each comparative example, the same raw materials as in Comparative Examples 6-16 was used. The foliated fine graphite particles were added in the corresponding proportion indicated as an outer percentage in Table 1. Then, the procedures of Example 6-16 were followed to produce a formed product. Measurement results of its characteristics are shown in Table 1.

[0048] In Examples 6-16, the temperature became constant in about 30 seconds when the voltage of 50 V was applied. The samples of these examples therefore showed sufficient characteristics as heaters. In contrast, the samples of Comparative Examples 3 and 5 did not permit energization because of the low contents of the foliated fine graphite particles as an electrically conductive material. Further, it was unable to obtain a dense, sintered product in each of Comparative Examples 4 and 6 because the content of the foliated fine graphite particles as an electrically conductive material was too much.

[0049] Although the invention has been described with preferred embodiments, it is to be understood that variations and modifications may be resorted to as will be apparent to those skilled in the art. Such variations and modifications are to be considered within the purview and the scope of the claims appended hereto.

Claims

1. An electrically-conductive heating element comprising:

100 parts by weight of a matrix composed of a ceramic, a glass or a glass-ceramic mixture; and

0.5-10 parts by weight of foliated fine graphite particles uniformly distributed as an electrical-conductivity-imparting material in the matrix, said graphite particles initially having a particle size of 1-100 µm, a thickness not greater than 1 µm and an aspect ratio of 10-5,000.

2. The element of claim 1, whose volume resistivity ranges from 10-¹ Ω·cm to 10³ Ω·cm.

3. The element of claim 1, further comprising an insulating layer composed of the same material as the matrix and provided integrally on a surface of the element.

4. The element of claim 3, whose volume resistivity ranges from 10-¹ Ω·cm to 10³ Ω·cm.

Drawing