OF THE INVENTION In general, sheet-shaped heating units have a configuration wherein a nichrome wire as a heating member, which is coated with an insulating material, is continuously arranged in the groove of a sheet in a bended form and electrodes are connected to each end of the nichrome wire. In the sheet-shaped heating unit of this configuration, while the nichrome wire itself is heated to a high temperature, other portions than the nichrome wire are heated to a relative low temperature only by irradiation or conduction. In other words, the sheet-shaped heating unit using a nichrome wire is of a sectional heating type in which only a part is heated to a high temperature; therefore, if the insulating coat covering the nichrome wire is of organic materials such as wood or resin, it can be easily ignited or sparked to cause firing.

In order to prevent this problem, a ceramic material with excellent heat-resistant and fireproof properties is sometimes used as an insulating coat which, however, increases the weight of the sheet-shaped heating unit and also makes it difficult to achieve the bended shape

of the nichrome wire. Inorganic fibers such as glass fiber can be consider as an alternative but they have many limits in aspects of design, weight, cost, productivity, etc.

Meanwhile, in addition to the nichrome wire, a carbon-based heating member such as carbon fiber is sometimes used for construction of a heating unit. Where a carbon fiber is made in a linear form, it results in the same problem as in the linear nichrome wire, as mentioned above. As such, the carbon fiber is generally made in a woven stuff for preparation of a heating unit ; however, a mono filament in the carbon fiber stuff is thin, whereby it can be readily cut due to the potential difference and cannot be stably used for a long time.

An alternative heating unit, prepared by a method wherein carbon blacks are mixed with a binder and the resulting mixture is molded in the shape of sheet, is often used as a sheet- shaped heating unit. However, in order for the carbon blacks to serve as a heating member, a large amount thereof must be mixed with the binder, thereby deteriorating the mechanical properties such as the bending strength and causing difficulty in molding the heating member.

The most serious problem is in that the electrical property of carbon black can easily disappear because a resin surrounds carbon black itself to isolate them from one another.

As another approaching, Japanese Patent Laid-open gazette No. Sho 57-151186 provides a sheet-shaped heating unit that is prepared by impregnating a carbon fiber stuff with a thermosetting resin; however, this heating unit has demerits of a low bend ability and limited shapes.

Under this current circumstances, there is a strong need for a sheet-shaped heating unit having a good moldability, excellent mechanical properties such as being capable of being easily bended, thermal uniformity, excellent heating ability at a low voltage, and good electrical stability.

SUMMARY OF THE INVENTION Accordingly, the object of the present invention is to solve the aforementioned problems and at the same time meet the requirements in the prior art. That is, an object of the present invention is to provide a sheet-shaped heating unit comprising a heating complex which is prepared by dispersing a small amount of carbon nanomaterials in a matrix resin and which exhibits an excellent heating ability and good pliability, and porous, planar electrodes which are attached to both planes of the heating complex and which provide an electrical stability and high heating efficiency based upon electric current.

To accomplish the foregoing object and advantages, the present invention provides a sheet-shaped heating unit comprising (a) a heating complex in which anisotropic carbon nanomaterials of the diameter of several nm-hundreds ofnm and the length of several, um ~ hundreds of pm are dispersed in an electrically insulating matrix resin in the amount of 2-40% by weight based upon the total weight of the heating complex, the carbon nanomaterials being in electrical contact with one another, and (b) porous, planar electrodes attached to both planes of the heating complex.

In accordance with the present invention, very fine carbon nanomaterials are dispersed in the insulating matrix resin under the specific condition that the carbon nanomaterials are in electrical contact with one another, whereby the sheet-shaped heating unit can exhibit a higher heat-radiation rate efficiency than the prior art heating unit even by addition of a relatively small amount of additives, i. e. , carbon nanomaterials and can also be easily molded into various structures. Furthermore, since the carbon nanomaterials also act as a reinforcing agent, the mechanical properties such as the bending strength of the sheet-shaped heating unit are improved. In other words, the prior art sheet-shaped heating unit in which conductive carbon

particles are dispersed in a polymer matrix achieves a desired resistance for heating only when a large amount of carbon particles are contained in the heating unit, resulting in the difficulty of molding and the deterioration of mechanical properties. Conversely, the sheet-shaped heating unit according to the present invention exhibits electrical conductivity comparable to that of the prior art sheet-shaped heating unit by dispersing only a small amount of carbon nanomaterials in a matrix resin under a specific condition, and also does not suffer deterioration of its mechanical properties, while maintaining moldability and pliability owing to the use of a small amount of additives, i. e. , carbon nanomaterials. Moreover, the porous, planar electrodes which are attached to both planes of the heating complex are electrically stable because they are separated from each other by the heating complex, whereas electrodes in prior art heating units are sometimes attached to the same plane of a heating complex. Moreover, the porous, planar electrodes are pliable owing to their planar structure. Also, the porous, planar electrode provides a high heat- radiation rate efficiency because the electrodes are spaced apart from each other by a heating complex in a short distance, and can be easily made at low cost.

DETAILED DESCRIPTION OF THE INVENTION Anisotropic carbon nanomaterials used in the sheet-shaped heating unit of the present invention are not particularly limited, if they are anisotropic carbon materials having the diameter of several nm-hundreds ofnm and the length of several llm ~ hundreds of pm, and include preferably single-layered wall carbon nanotube, multilayered wall carbon nanotube, carbon nanofiber, single-oriented graphite, and the like.

For instance, the carbon nanotube has a diameter of 1-500 mu and a length of several pm ~ hundreds of Fm and thus has a high anisotropy in view of structure. Moreover, the carbon nanotube has a conductivity of ~104 Q, which means that the carbon nanotube is a conductor.

The carbon nanotube is mechanically rigid (approximately 100 times more rigid than steel) and chemically stable and also has excellent thermal conductivity of 2000 W/mK. In addition, the carbon nanotube is hollow and thus has a lower density compared to graphite or carbon fibers as general carbon materials. Due to the above properties, the carbon nanotube has a large L/R (length/radius) ratio of 100-10, 000 and can act as a conductor in series with other carbon nanotubes when a current is fed thereto. Accordingly, even when a small amount of carbon nanotubes is used, the resulting heating unit can exhibit similar electrical conductivity to other heating units containing a large amount of different carbon materials. A carbon nanofiber (CNF) and a single-oriented graphite as well as a multilayered wall carbon nanotube (MWNT) and a single-layered wall carbon nanotube (SWNT) can be adapted for the carbon nanotube in the present invention.

The addition amount of carbon nanomaterials is, as mentioned above, 2-40% by weight on the basis of the total weight of a heating complex, the heating complex comprising carbon nanomaterials and a matrix resin. If the addition amount is less than the lowest range, it is difficult to achieve the desired electrical connection. Conversely, if the addition amount is more than the highest range, it is difficult to expect the satisfied heat-radiation efficiency for use as a heating unit because the conductivity becomes excessively high. It is preferably in the range of 4-30% by weight, more preferably 4-15% by weight. However, so long as the electrical connection of carbon nanomaterials to one another can be achieved, a smaller amount of carbon nanomaterials may be used in order to accomplish the object of the present invention.

The term"electrical contact" (sometimes, referred to as"electrical connection") as described in this disclosure means the condition wherein a carbon nanomaterial is in physical contact with other carbon nanomaterials to conduct electricity therebetween. This term simultaneously means the condition wherein a carbon nanomaterial is spaced apart from other

carbon nanomaterials but the distance therebetween is in the range allowing electron tunneling to occur in view of quantum mechanics. Such electrical connection can be achieved by uniformly dispersing fine carbon nanomaterials in a resin for matrix in the range as described above. Conversely, where isotropic carbon nanomaterials or isotropic general carbon materials instead of anisotropic carbon nanomaterials are employed, the satisfying electric connection cannot be achieved in the above range of additional amount.

A matrix used in the present invention is not particularly limited so long as it is a material which is thermo-conductive and thermo-stable and in which fine carbon nanomaterials can be dispersed under a specific condition that the carbon nanomaterials are in electrical contact with one another. Such a material includes, for example, but is not limited to polymer materials meeting the above requirements, and ceramic materials. Examples of the polymer material include, but are not limited to polyethylene (PE), polypropylene (PP), polycarbonate (PC), polymethyl methacrylate (PMMA), phenol resins, urea resins, melamine resins, silicone resins, epoxy resins, or modified resins, copolymers and blends thereof, etc.

Among them, the silicone resins and modified resins thereof are especially preferable in consideration that they are easy moldability and carbon nanomaterials can be easily dispersed in them in the electrical contact condition, which was ascertained by inventors of the present invention. The reason that the silicone resins have a high dispersion ability as to anisotropic carbon nanomaterials is not verified. It appears that carbon nanomaterials generally tend to aggregate rather than disperse in general resins due to a high affinity thereof, but the property balance between a silicone resin and carbon nanomaterials harmonizes repulsion with affinity, and also a high viscous silicone resin allows carbon nanomaterials to be dispersed therein during the preparation procedure with the carbon nanomaterials being in electrical contact with one another. Such a silicone resin and modified resins thereof include, for example, but are not

limited to silicon rubber resins, methylphenyl silicone resins, modified alkyd resins, silicone- modified epoxy resins, silicone-modified alkyl resins, silicone-modified urethane resins, silicone-modified polyester resins, silicone-modified alkyl resins, methyl silicone resins, and the like.

The thickness of a heating complex is not particularly limited and, for example, approximately 2 mm to 40 mm. However, if the thickness is excessively thin, it is difficult to control the heating temperature of the heating complex. Conversely, if the thickness is excessively thick, the heating rate is decreased, which is not desired.

In the sheet-shaped heating unit according to the present invention, porous, planar electrodes are attached to both planes of a heating complex, in an attachment or insertion manner, in which current is fed through the electrodes to induce heating of the heating complex when current is applied thereto. The porous, planar electrode provides current to a large area of the heating complex so that it can increase the ratio of heat-radiation efficiency versus current.

Also, the porous, planar electrode can be applied in various forms due to its structural pliability.

The term"porous"used herein means that a plurality of holes are perforated through an electrode plate. The type of holes is not particularly limited and includes, for example, rectangle, square, regular triangle, right-angled triangle, rhombus, pentagon, hexagon, etc. The dimension of holes can become a factor for determining the range of heating temperature when the resistance is fixed, thus it can be properly determined to fit to the resistance.

In FIG. 1, there is illustrated a porous, planar electrode through which a plurality of hexagonal holes are perforated, according to an embodiment of the present invention. The porous, planar electrode 100 of FIG. 1 looks like a honeycomb because a plurality of hexagonal holes 120 are formed on an electrode plate 110. In FIG. 2, there is illustrated a porous, planar

electrode on which a plurality of triangle holes as well as hexagonal holes are formed, according to another embodiment of the present invention. The porous, planar electrode 102 of FIG. 2 generally has the shape corresponding to a heating complex of tetragon shape. In FIG. 3, there is illustrated a porous, planar electrode 104 in which a plurality of square holes 124 are perforated through an electrode plate 114, according to a further another embodiment of the present invention. The porous, planar electrode 104 of FIG. 3 shows the net shape. However, the configurations of porous, planar electrodes 100,102, 104 in FIGS. 1-3 are only exemplary according to the present invention, and many other configurations are available for the present invention.

A porous, planar electrode serves to induce the heating of a heating complex when current is applied thereto and thus is not particularly limited so long as it is of a material having a high conductivity. Such a highly conductive material includes, for example, but is not limited to metal plates such as aluminum plate, copper plate, silver plate, aluminum alloy plate, etc. and conductive composite plates of polymer or ceramic material containing a large amount of conductive materials such as silver particles, copper particles, carbon powders, surface-treated carbon nanotubes, etc.

The thickness of a porous, planar electrode is not particularly limited and, for example, approximately 0.1 mm to 0. 8 mm. However, if the thickness is excessively thin, a short circuit may occur. Conversely, if the thickness is excessively thick, it costs a great deal to prepare it, which is not desired.

A porous, planar electrode can be connected to a heating complex in an attachment manner and/or insertion manner.

In an embodiment, an insulating material may be laminated to the outer surface of a

porous, planar electrode (in the case of insertion manner, a heating complex) so as to prevent electricity leakage resulting from the exposure thereof to the exterior. Such an insulting material includes, for example, epoxy resin, polyvinylchloride (PVC), acryl-butadiene-styrene resin (ABS), polyethyleneterephthalate (PET), polypropylene (PP), etc.

In FIG. 4, there is illustrated a sheet-shaped heating unit 400 in which porous, planar electrodes are connected to a heating complex in an attachment manner. Referring to FIG. 4, the sheet-shaped heating unit 400 is configured such that two honeycomb-shaped, planar electrodes 100, 100'as shown in FIG. 1 are attached to both surfaces of a planar heating complex 200 and two insulators 300,300'cover the upper and lower surfaces of the electrodes 100,100'and heating complex 200. It should be noted that the heating complex 200 is not limited to a planar shape.

In comparison with the heating unit 400 of FIG. 4, FIG. 5 shows a sheet-shaped heating unit 500 in which two general planar electrodes 510 are attached to only one surface of a heating complex 520 and two insulators 530,530'cover the upper and lower surfaces of the heating complex 520. In the planar heating unit 500 as shown in FIG 5, silver paste is generally used as a raw material for these electrodes 510, but the silver paste is expensive, e. g., about US$ 900/kg.

Furthermore, a large amount of silver is wasted during the manufacture procedure of electrode because silver microparticles are dispersed in a resin solution containing solvent, then the solvent should be rapidly removed for drying, which causes the cost of silver paste electrode to increase.

Conversely, the sheet-shaped heating unit 400 according to the present invention, as shown in FIG. 4, can be prepared by forming a plurality of holes on low-priced metal plates, i. e., perforation, and also these holes serve to effectively diffuse the heat generated from a heating

complex. Moreover, two electrodes are placed on both planes of a heating complex but not on one plane thereof to prevent short circuit from occurring, which makes the safe design of product possible in view of electricity. Also, where it is impossible to design a specific configuration of electrode with silver paste, the electrode pattern according to the present invention may be printed on a heating complex using the silver paste.

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a schematic view of a porous, planar electrode useful for a sheet-shaped heating unit in accordance with an embodiment of the present invention, in which a plurality of hexagonal holes are perforated orderly through an electrode plate ; FIG. 2 is a schematic view of a porous, planar electrode useful for a sheet-shaped heating unit in accordance with another embodiment of the present invention, in which a plurality of hexagonal holes and triangle holes are perforated orderly through an electrode plate to have the shape corresponding to a square heating complex; FIG. 3 is a schematic view of a porous, planar electrode useful for a sheet-shaped heating unit in accordance with a further another embodiment of the present invention, in which a plurality of square holes are perforated orderly through an electrode plate ; FIG. 4 is a schematic view of a sheet-shaped heating unit according to an embodiment of the present invention which includes the porous, planar electrode of FIG. 2 as a constitutional element; FIG. 5 is a schematic view of a sheet-shaped heating unit of the prior art which includes general silver paste electrodes as constitutional elements.

Designation of the reference numerals

100,102, 104: porous, planar electrode 110,112, 114: electrode plate 120,122, 124: holes 200,520 : heating complex 300,530 : insulator 400,500 : sheet-shaped heating unit 510 : silver paste electrode DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in more detail by EXAMPLES, but the scope of the present invention is not limited thereto.

EXAMPLE 1 Carbon nanotubes (Nanotech Corporation, Korea) having the diameter of approximately 30-50 nm and the length of several pun ~ tens of pm were used as a carbon nanomaterial. These carbon nanotubes were prepared by a conventional thermal decomposition method in which ferrocen as a metallic catalyst is dissolved in benzene as a carbon stock and the resulting solution is heated at high temperature of 1000°C in an electric furnace to synthesize carbon nanotubes. 10 g of the carbon nanotubes was mixed with 100 g of silicone resin (Dongyang Silicone Corporation, Korea), to which 1 g of DCP (dicumyl peroxide, Aldrich Corporation) was then added as a curing agent and stirred in an internal mixer for 10 minutes.

The resulting product was made to the shape of sheet having the thickness of 4 mm using a rolling mill. The cross-linkage rheology at 160°C was measured about some of the mixture using a rubber rheometer, thereafter the mixture was cross-linked to a sheet form at 160°C using

an oil pressure press. The sheet prepared thus was tailored to the size of 15 cm x 20 cm to make a planar heating complex.

Meanwhile, an aluminum plate of 0.2 mm thickness was perforated using a boring machine to make a plurality of hexagonal holes with the major axis of 1.5 cm at intervals of 0.5 cm, up and down, left and right, respectively. Two porous, planar electrodes as shown in FIG. 1, prepared thus, were attached to both planes of the planar heating complex, then PET sheet (thickness: 0.1 mm) as an insulating sheet was covered thereon to make an sheet-shaped heating unit as shown in FIG. 4.

The surface resistivity and heating temperature of the sheet-shaped heating unit were measured by the methods as below, and the results are described in TABLE 1.

(1) Surface resistivity test: the surface resistivity was been measured using a resistivity tester (Upteck LP212).

(2) Heating temperature test: an adapter supplying DC 12 V was connected to a sheet-shaped heating unit, 2 minutes after which the heating temperature was measured using a digital surface temperature tester (GLas-Col).

EXAMPLE 2 A heating complex and a sheet-shaped heating unit were prepared, respectively, in the same manner as in EXAMPLE 1 except for using 4 g of carbon nanotubes and the measurement was conducted. The results are described in TABLE 1 below.

EXAMPLE 3 A heating complex and a sheet-shaped heating unit were prepared, respectively, in the same manner as in EXAMPLE 1 except for using 20 g of carbon nanotubes and the

measurement was conducted. The results are described in TABLE 1 below.

EXAMPLE4 A heating complex and a sheet-shaped heating unit were prepared, respectively, in the same manner as in EXAMPLE 1 except for using 40 g of carbon nanotubes and the measurement was conducted. The results are described in TABLE 1 below.

COMPARATIVE EXAMPLE 1 An experiment was repeated in the same manner as in EXAMPLE 1 except for making thin electrode bars with a width of 0.5 cm and a length of 20 cm using silver paste (7466DB@, Daejoo Fine Chemical) and then fabricating a sheet-shaped heating unit with the configuration (the distance between electrodes : about 13 cm) as shown in FIG. 5. The heating temperature of the sheet-shaped heating unit was measured according to the method of EXAMPLE 1 and the result is described in TABLE 1 below.

COMPARATIVE EXAMPLE 2 An experiment was repeated in the same manner as in EXAMPLE 2 except for using a silver paste electrode of COMPARATIVE EXAMPLE 1 to fabricate a sheet-shaped heating unit as shown in FIG. 5 and measure the heating temperature of which the result is described in TABLE 1 below.

COMPARATIVE EXAMPLE 3 An experiment was repeated in the same manner as in EXAMPLE 3 except for using a silver paste electrode of COMPARATIVE EXAMPLE 1 to fabricate a sheet-shaped heating unit as shown in FIG. 5 and measure the heating temperature of which the result is described in TABLE 1 below.

COMPARATIVE EXAMPLE 4 An experiment was repeated in the same manner as in EXAMPLE 4 except for using a silver paste electrode of COMPARATIVE EXAMPLE 1 to fabricate a sheet-shaped heating unit as shown in FIG. 5 and measure the heating temperature of which the result is described in TABLE 1 below.

[TABLE 1] Electrical resistivity (Q-cm) Heating temperature (°C) EXAMPLE 1 210 160~180 EXAMPLE2580170-180 EXAMPLE3 110 160-175 EXAMPLE 413 COMPARATIVE EX. 1-28-36 COMPARATIVE EX. 2 - 27#34 COMPARATIVE EX. 3 - 28#40 COMPARATIVE EX. 4 26~43

As seen in TABLE 1 above, the sheet-shaped heating units (COMPARATIVE EXAMPLES 1-4) which were made employing silver paste electrodes and had the configuration of FIG. 5 radiated heat at the range of 25~45°C, regardless of their electrical resistivity differencesg which results iìom a low voltage of 12 V and small amount of current owing to a long distance of 13 cm between both electrodes. In order to overcome such problem, in another experiment, a heating unit was fabricated in which more than ten silver paste electrodes are attached to a heating complex at intervals of 10 cm, respectively, and the same experimental procedure as in COMPARATIVE EXAMPLE 1 was repeated. The result showed the similar heating temperature approaching those of EXAMPLES 1-4. However, as the distance between electrodes shortens, a possibility of spark-occurring increases, which is not desirable in view of electrical stability. Moreover, necessity for many electrodes increases the manufacturing cost.

Conversely, where porous, planar electrodes were used according to the present invention, the resulting heating unit radiated heat around 170°C except EXAMPLE 4. That is because both electrodes are spaced apart from each other at the distance of 4 mm, as being identical to the thickness of a heating complex, which allows a high-temperature heating to occur even at a low voltage. Meanwhile, the heating temperature can be controlled by the size of holes formed on an electrode plate. In the case of EXAMPLE 4, the heating temperature could not be measured because of some problems in view of electricity. More specifically, since the addition amount of carbon nanotubes was beyond a specific range, the electrical resistivity became so small that the current characteristic exhibited more than the resistivity characteristic, thereby showing circuit short.

INDUSTRIAL APPLICABILITY As described above, the sheet-shaped heating unit according to the present invention comprises a matrix resin having good moldability and mechanical property and anisotropic carbon nanomaterials which are dispersed in the matrix resin in the electrical contact condition, so that it can exhibit excellent electrical and heating characteristics. Furthermore, the sheet- shaped heating unit comprises two porous, planar electrodes, as being attached to both planes of a heating complex, which provide a high heat-radiation capacity at lower cost and more stable condition compared to prior arts using silver paste or copper electrodes.

As the present invention may be embodied in several forms without departing from the spirit or essential characteristics thereof, it should also be understood that the above-described examples are not limited by any of the details of the foregoing description, unless otherwise specified, but rather should be construed broadly within its spirit and scope as defined in the appended claims, and therefore all changes and modifications that fall within the meets and bounds of the claims, or equivalences of such meets and bounds are therefore intended to be embraced by the appended claims.