CA2048418A1

CA2048418A1 - Electrically-conductive heating element

Info

Publication number: CA2048418A1
Application number: CA 2048418
Authority: CA
Inventors: Kouji Sakawaki; Yukinobu Watanabe; Yutaka Yamashita; Keiichiro Koba; Hirokatsu Kanda
Original assignee: Individual
Current assignee: Mitsui Mining Co Ltd
Priority date: 1990-08-03
Filing date: 1991-08-02
Publication date: 1992-02-04
Also published as: EP0469628A1; TW223729B

Abstract

ABSTRACT OF THE DISCLOSURE
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 mm, a thickness not greater than 1 mm and an aspect ratio of 10-5,000. The heating ele-ment is suitable for use as a ceramic heater.

Description

2~1~84~8 SPECIFICATION

TITLE OF THE INVENTION
ELECTRICALLY-CONDUCTIVE HEATING ELEMENT

BACKGROUND OF THE INVENTION

1. Field of the Invention 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 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 heat-ing. 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 20~8418 a borosilicate glass and alumina (Japanese Patent Ap-plication No. 20678/1987), etc.
In these ceramic heaters, a certain measure is taken to achieve a uniform heating temperature distrib-ution, for example, by forming a heating resistanceelement, 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 in-tensively 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 atmo-sphere 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 2~8~1~
-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.
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.

2~48~18 -- 4 -- `~

SUMMARY OF THE ~lVENT;CON
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 in-sulating layer integrated with the heating element and suited for use as a ceramic heater. ~ -;
The present inventors have carried out an ex- ~:
tensive investigation with a view toward overcoming the above-described problems. As a result, it has been found that a formed ceramic body having uniform elec-trical 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.
The present invention therefore provides an electrically-conductive heating element, which com-prises:

- 20~
- 5 - ~

loo 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-condu¢tivity-imparting material in the matrix, said graphite particles initially having a particle size of 1-lO0 ~m, a thickness not greater than 1 ~m 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. -The foliated fine graphite particles which have high cry~tallinity and are highly effective in impart-ing electrical conductivity are dispersed uniformly in the matrix. The electrically-conductive heating ele-ment according to the present invention is therefore an electrically-conducting heating element of the direct energization type, which has high electrical con-ductivity, can quickly respond to energization and is excellent in the temperature-raising characteristic, ¢an 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 2~4~

dimensions and weight. The electrically-conductive heating element is useful as a heater element for vari-ous electrical heaters and the like.
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 parti-cles and can improve the moisture resistance. The in-sulating 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 exam-ple, by baking an electrically-conductive paste or con-ducting metallization. It is therefore possible to provide a simplified process for the production of a heater~

BRIEF_DESCRIPTION OF THE DRAWINGS
FIG. 1 is a concept sketch showing one example of 2~8418 stacking of green sheets as heating layers and insulat-ing layers and one exampIe 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 il-lustrative, degreasing and sintering temperature pat-tern when green sheets are formed and then stacked and sintered into a heating element.
. . .
DETAILED DESCR~PTION OF THE INVENTION
AND THE P~ FR~LEK~QpIuNTs Examples of the ceramic which makes up the matrix of the electrically-conductive heating element accord-ing 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 de~ired ~or each product such as radiation property and thermal shock resistance, an appropriate ceramic can be selected from these ceramics. On the other hand, exam-ple~ o~ 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 composi-2048~

- 8 - ~

tion that is not softened to undergo deformation in shape at an application temperature (approximately 50-500C) 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 ex-pansion coefficient on the order of 40 x 10-7 (l/C) or smaller.
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 be-cause their combined use makes it possible to lower the -;
sintering temperature. on the other hand, the addi-tion 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.
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 ~m, a thickness not greater than 1 ~m and an aspect ratio of 10-5,000. More preferably, the particle size, 2~ 18 g . ~, thickness and aspect ratio is 1-50 ~m, not greater than 1 ~m and 200-3,000, respectively, and the average par-ticle size ranges from 10 ~m to 30 ~m or so. If the particle size of the foliated fine graphite particles becomes greater than 100 ~m, 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 ~m 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 par-ticles to break them up (see Japanese Patent Laid-Open No. 153810/1990). These foliated fine graphite parti-cles 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 natu-ral graphite, and have the special shape and high crys-20~841~

-- 10 -- ~, tallinity as described above. Owing to their high crystallinity, foliated fine graphite particles useful in the practice of the present invention have the char-acteristic property that they are resistant to oxida-tion even in an oxidizing atmosphere. For example, fo-liated fine graphite particles obtained from natural graphite mined in China have high crystallinity of de-veloped hexagonal graphite such that the lattice con-stant is about 0.67 nm, the crystallite thickness is approximately 70 nm and the crystallite size is about 100 n~. Incidentally, a variety of graphite particles -;
are available on the market. They can be classified in particle size, for example, to 1-30 ~m (15 ~m and smaller: 95%), 2-70 ~m (44 ~m and smaller: 95%) and 2-100 ~m (75 ~m 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 ~orm 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 2~84~

-- 11 -- . ~

thin thickness so that adjacent graphite particles tend to overlap, thereby making it possible to form electrically-conductive paths even at a low concentra-tion.
The electrically-conductive heating element ac-cording 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 col-lectively called a "matrix-forming raw material pow-der") which had been ground and sifted for particle ! .;
size adjustment in advance, foliated fine graphite par-ticles 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 ~m. In the thicknesswise direction, they are not damaged practically.
Although the matrix-forming raw material powder preferably has a particle size not greater than about 20~841~

- 12 - ~

loO ~m from the standpoint of mixing readiness with the foliated fine graphite particles, no particular limita-tion 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 heat-ing 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 discontinua- ;
tion 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 pow-der.
The foliated fine graphite powder employed as a raw material for the electrically-conductive heating element according to the present invention are in a fo-liated form having a high aspect ratio. When mixed with the matrix-forming raw material powder, the foli-ated fine graphite particles are free from such a phenomenon that the graphite particles alone would be separated or would be concentrated locally. The foli-20~4~8 - 13 - -~

ated 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 in-organic binder may also be added, as needed, as a form-ing aid upon mixing.
The resultant mixture of the matrix-forming raw material powder and the foliated fine graphite parti-cles 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 ~heets formed by the doctor blade method or calender roll method are stacked together, by slip casting, or by extrusion.
When a powder pressure forming method is employed by way of example, the forming pressure can be prefer-ably 2.9-98.1 MPa, especially 9.8-49.0 MPa or so. When green sheets are stacked together to conduct the form-ing, the mixture of the foliated fine graphite parti-cles 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 20~84~8 - 14 - -~

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.
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 there-fore arises. This applies equally to a sintering step which will be described next.
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 400C or lower, the preformed green body i8 sintered at a temperature of 450-1,500C. 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 2~41~

- 15 - ~

for example, it is necessary to set the sintering con-ditions at l,100-1500C for 0.5-5 hours and, where the glass component is contained in a large proportion, at 450-900C for 10 minutes to 1 hour. Although it is preferred to conduct the sintering in an inert gas at-mosphere, sintering in air is feasible where the pro-portion of the glass component in the matrix-forming raw material powder becomes 50 wt.% or higher because sintering at 900C or lower is Peasible so that there is no potential danger of oxidation of the mixed, foli-ated 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.
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 heat-ing element of the present invention can be obtained more efficiently in the form of an insulated, electrically-conductive heating element, in which a 2~4~4~8 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-for~ing 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 insu-- 10 lated, electrically-conductive heating element will also be referred to simply as an "electrically- -;
conductive heating element". This process is also ef-fective in preventing oxidation of the foliated fine graphite particles during sintering.
The insulating layer can also be formed in the following manner. For example, a mixture of the foli-ated 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 meth-od, the calender roll method or the like into a heating-layer-forming green sheet in which the foliated ~ine 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 20~84~8 formed, are stacked together to provide a preformed green body. The preformed green body is then sand-wiched 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. In-sulating 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 alterna-tive, a slurry of the matrix-fGrming raw material pow-der 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 coa~ed 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 20~84~8 - 18 - ~

or the unsintered green body or by dipping the electrically-conductive heating element or the un-sintered green body in the slurry, so that an insulat-ing layer is formed. The insulating layer is dried and then sintered. Where the slurry is coated on the un-sintered green body, the unsintered green body is also sintered concurrently with the sintering of the in-sulating layer. The thickness of the insulating layer varies depending on the voltage applied when the heat-ing 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.
Since the foliated fine graphite particles as an electrically conductive material are uniformly dis-persed 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 n.cm to 103 ~cm. By changing the amount of the foliated fine graphite particles to be added, the volume resistivity can be ad~u~ted as desired within the above range. Use of ~oliated fine graphite particles as an electrical-conductivity-imparting material permits the formation of many current flow paths despite the small volume oc-cupied by them and hence facilitates to develop elec-,, - .: . .

29~8~

trical 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 im-paired.
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-for~ing 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 baklng an electrically conductive paste or by metal-lization. Unlike conventional processes for the pro-duction of heaters, the present invention does not re-quire the step that an insulating layer made, for exam-' ' ' .~ .

20~8418 - 20 - ~

ple, 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 dimen-sions and weight of heaters.
The heating element according to the present in-vention can be easily energized by applying a voltage thereacross, and uniformly produces heat there-throughout. Noreover, it is possible to choose the shape, dimensions and volume resistivity as desired and, by adjusting the level of electricity to be sup- --plied, to control the heating temperature as desired.
Specifically, the heating element can be heated from room temperature to 600C 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 ~rom 10~1 n-cm to 10 n 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 ele-ments. Owing to the use of a low voltage, there is a smaller potential danger of electrification so that they are also advantageous from the standpo nt of safety. The electrically-conductive heating element 20~41~

- 21 - ~

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.
Electrically-conductive heating elements accord-ing to the present invention are useful as warming, cooking or drying heating elements or as heating ele-ments for fuel vaporizers.
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 measur-ing 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.
~he present invention will hereinafter be de-scribed more specifically by the following examples.
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 20~8418 - 22 - ~

glass-ceramic mixture was used as a matrix-forming raw material powder will be given as Examples 6-16 and Com-parative Examples 3-7.
Incidentally, the foliated fine graphite parti-cles employed in Examples 1-16 and ~omparative Examples 3-7 were prepared in the following manner.
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 800C in a nitrogen gas atmosphere and was maintained at that temperature, whereby ex-panded graphite particles were obtained. The expanded graphite particles were dispersed in water, to which ultrasonic waves whose frequency was 50 H~ were ap-plied. The expanded graphite particles were therefore broken up, whereby foliated fine graphite particles were obtained.
Example 1 To 100 g of "Petalite N-100" (trade name; product of Nishimura Togyo K.K.) whose particle size had been ad~usted to 250 ~m or smaller, 2.5 g of the foliated fine graphite particles having a thickness of about 0.1 ~m and an aspect ration of 100-500 were added. They - 23 - ~

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 NPa 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 3C per minute from room temperature to 1,300C
under a nitrogen gas atmosphere in an electric furnace.
After the preformed green body was fired further for 1 hour at 1,300C, it was cooled to 500C at a rate of 3c 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 150C, so that electrode-bearing surfaces were formed. ~he volume resistivity of the sample as mea~ured by the four-terminal method was 1.3 n-cm. To sample~ 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 400C and 500C in about 5 minutes and about 2 minutes, respectively. Continued energization allowed to stably maintain the samples at their respective temperatures.
Example 2 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:
Amount added (q) Volume resistivitv (ncm) 1.3 18 1.5 7.4 -1.8 3.0 2.0 1.7 2.3 0.9 15Example 3 A batch (300 g) proportioned and kneaded under the same conditions as in Example 1 was filled in a sguare cylindrical mold of 130 x 130 x 12 mm and pres-sure ~ormed under the pressure of ~.8 ~Pa. The preformed green body was fired under the same condi-tions 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 n-cm, respectively. The heating element was cut and polished into a sample of 113 x 120 2~84~8 - 25 - ~

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 220c in about lo 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 220C in all the nine regions, whereby the sample showed a uniform temperature dis-tribution.
Example 4 ;
Against the ~urface 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 149~m or smaller ltrade 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 1,100C in a nitrogen gas atmo-sphere 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 charac-teristics of the whole heating element were exactly the same as those of the sample produced in Example 3.
This sample was divided substantially equally 20~8418 into nine pieces, each of 39 x 39 x lo 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 mm3, followed by the measurèment of its volume resistivity by the two-terminal method. All the pieces had a resistivity of 0.8 n-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 410C in a~out 5 minutes. Each piece was successfully and stably maintained at the same tempera-ture for 30 minutes or longer.
Example 5 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 1,100C. The density and volume resistivity of the heating element were 1.7 g/cm3 and 2.9 ncm, 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 225C, respectively, when a voltage of 12 V was applied.
Comparative Example 1 A sintered body was obtained under the same con-2048~18 - 27 - ~

ditions as in Example 3 except for the use of commer-cial graphite powder (particle size: 1-S ~m, thickness:
0.2-0.6 ~m, 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 n.cm. In addition, the volume resistivities of pieces obtained by dividing the sample into 9 egual sections of 39 x 39 x 10 mm varied within a range of from 0.7 x 103 n.cm to l.S x 103 n.cm.
Comparative Example 2 Two sintered bodies were produced in a similar t .;
manner to Comparative Example 1 except that the amount of graphite powder was increased to 3.5 g. Their volume resistivities were 5.2 n.cm and 6.7 n.cm, respe¢tively, thereby indicating the occurrence of var-iations in properties despite their production under the same conditions.
Examples 6-16 Employed as raw materials were a borosilicate glas6 powder having properties of a softening point of 800C and a thermal expansion coefficient of 30 x 10-7/C and adjusted in particle size to an average partiale size of 3 ~m; foliated fine graphite particles adjusted in particle size to an average particle size of 20 ~m (particle size: 1-100 ~m, thickness: not 20~8~1~

- 28 - ~

greater than 1 ~m, aspect ratio: 10-5,000, average par-ticle size: 20 ~m); and, as ceramic powders, alumina, mullite and cordierite powders all adjusted in particle size to an average particle size of 2 ~m. Further, the matrix-forming raw material powder was added with an orqanic vehicle which had been prepared by dissolving ethylcellulose as a binder in ~-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. ! .;
In each example, the foliated fine graphite par-ticles 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 mix-ture 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 weigXt 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.
9y 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 20~84~8 - 29 - ~

sheet of the matrix-forming raw material powder was also formed as an insulating-layer-forming sheet.
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 l. The stacked layers were bonded together under pres-sure,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 atmo-sphere in accordance with the exemplary degreasing and firing temperature pattern depicted in FIG. 2.
Electrode-bearing surfaces were formed on both end ~urfaces of the thus-obtained ceramic heating ele-ment, 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 tlme 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 radiatlon thermometer. The results are shown in Table 1.
A heating element produced in a similar manner by using the heating-layer-forming sheets alone showed 20~8~

- 30 - ~

substantially the same characteristics as the heating element with the insulating layer formed thereon.
Electrode~bearing surfaces were also formed on both end surfaces of the electrically-conductive heat-ing 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 Mn at 300C and at least 3 Mn at 500C so that the heater had sufficient insulation.

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Comparative Examples 3-6 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 percent-age in Ta~le 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.
In Examples 6-16, the temperature became constant in about 30 seconds when the voltage of 50 V was ap-plied. 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 Exam-ples 4 and 6 because the content of the foliated fine graphite particles as an electrically conductive material was too much.
Although the invention has been described with pre~erred embodiments, it is to be understood that var-iations 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 pur-20~8~18 - 33 - ~

view 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 .OMEGA..cm to 103 .OMEGA..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 ele-ment.

4. The element of claim 3, whose volume resistivity ranges from 10-l .OMEGA..cm to 103 .OMEGA..cm.