FIXING APPARATUS AND ELECTROPHOTOGRAPHIC IMAGE FORMING APPARATUS HAVING CERAMIC HEATER HAVING ELECTRICALLY INSULATING LAYER WITH HIGH THERMAL CONDUCTIVITY BACKGROUND [0001] In electrophotographic image forming apparatuses such as facsimile machines, printers, and copy machines, toner is supplied to an electrostatic latent image formed on an image receptor to form a visible image on the image receptor, the visible image is transferred onto a recording medium, and then the unfixed transferred image is fixed onto the recording medium. [0002] A fixing process includes applying heat and pressure to an unfixed image. An on demand fixing (ODF)-type fixing apparatus enabling quick-heating has high thermal efficiency, thereby satisfying demand for high-speed printing and low- energy fixing. A fixing apparatus of this type includes a fixing belt and a pressing roller which are engaged with each other to form a nip portion. The fixing belt is located between a pressing member positioned inside the fixing belt and the pressing roller, and the nip portion is formed by mutually pressing the pressing member and the pressing roller with the fixing belt interposed therebetween. A planar shaped heater is located between the pressing member and the fixing belt to heat the fixing belt locally and directly at the nip portion. Thus, an unfixed image on the recording image transported through the nip portion is heated and pressed to be converted into a fixed image that is fixed on the recording medium. [0003] In an ODF method, as the planar shaped heater, a planar shaped ceramic heater including a ceramic substrate in the form of a long rectangle is used. In general, the length of a heating element pattern formed in the form of a line or band along a longitudinal direction of the substrate is greater than the maximum width of a recording medium, e.g., paper, transported in a direction perpendicular to the longitudinal direction of the heating element pattern. However, a so-called 'non-paper passing region overheating phenomenon' easily occurs in such a ceramic substrate and is a direct cause of the decrease in printing speed. To relieve the non-paper passing region overheating phenomenon, the temperature of the non-paper passing region may be lowered by installing a conductive plate with high thermal conductivity between the ceramic heater and the pressing member. Because the conductive plate with high thermal conductivity is installed using Al, Cu, or graphite in a sheet form, contact thermal resistance is generated between the heater and the conductive plate with high thermal conductivity. In addition, heat-resistant grease is generally applied to reduce friction between the inner surface of the fixing belt and the surface of the heater. However, the heat- resistant grease may permeate between the heater and the conductive plate with high thermal conductivity, thereby further increasing the contact thermal resistance. BRIEF DESCRIPTION OF THE DRAWINGS [0004] FIG.1 is a schematic configuration view of an electrophotographic image forming apparatus according to an example. [0005] FIG. 2 is a cross-sectional view of a fixing apparatus according to an example applicable to the image forming apparatus of FIG.1. [0006] FIG.3 is a cross-sectional view of a fixing apparatus according to another example applicable to the image forming apparatus of FIG.1. [0007] FIGS.4A, 4B, and 4C show a rear-surface-heating-type heater configured to heat an unfixed image on a recording medium according to an example and applicable to the fixing apparatus described above with reference to FIG.2 or 3. FIG.4A is a plan view illustrating an electrically insulating layer on a first surface, i.e., rear surface, of a substrate of the heater. FIG.4B is a plan view illustrating a sliding layer on a second surface, i.e., front surface, of the substrate of the heater. FIG.4C is a cross-sectional view take along line A-A of FIG.4A. [0008] FIG. 5 is a cross-sectional view of a front-surface-heating-type heater according to another example applicable to the fixing apparatus described above in FIG.2 or 3. [0009] FIG. 6 is a schematic conceptual diagram for describing a non-paper passing region overheating phenomenon occurring while an unfixed image is fixed by heating and pressing, according to an example. DETAILED DESCRIPTION [0010] Hereinafter, a ceramic heater including an electrically insulating layer with high thermal conductivity and a fixing apparatus and an electrophotographic image forming apparatus each including the same according to some examples will be described. [0011] FIG.1 is a schematic configuration view of an electrophotographic image forming apparatus according to an example. Referring to FIG.1, an image forming apparatus, for example, printer, may include a printing unit 100 configured to form a visible toner image on a recording medium P, e.g., paper, and a fixing apparatus 200 configured to fix the toner image on the recording medium P. The printing unit 100 of the present example may form a color toner image electrophotographically. [0012] The printing unit 100 may include a plurality of photosensitive drums 1, a plurality of developing devices 10, and a paper transport belt 30. The photosensitive drum 1 is an example of a photoconductor on which an electrostatic latent image is formed and may include a conductive metal pipe and a photosensitive layer formed on an outer circumferential surface thereof. The plurality of developing devices 10 respectively correspond to the plurality of photosensitive drums 1, and each developing device 10 supplies toner to the electrostatic latent image formed on each photosensitive drum 1 and develops the latent image to form a toner image on a surface of each photosensitive drum. Each of the developing devices 10 may be replaced independently of the plurality of photosensitive drums 1. In addition, each of the plurality of developing devices 10 may be in the form of a cartridge including the photosensitive drum 1. [0013] For color printing, the developing devices 10 may include a plurality of developing devices 10Y, 10M, 10C, and 10K configured to receive toner of yellow (Y), magenta (M), cyan (C), and black (K)s, respectively. The developing devices 10 may further include developing devices configured to receive toner of various colors such as light magenta and white in addition to the above-described colors. Hereinafter, an image forming apparatus including the developing devices 10Y, 10M, 10C, and 10K will be described. Unless otherwise specified, reference numerals with Y, M, C, or K respectively denote components for printing images by using toner of yellow (Y), magenta (M), cyan (C), and black (K) colors. [0014] The developing device 10 supplies toner accommodated therein to an electrostatic latent image formed on the photosensitive drum 1 and develops the electrostatic latent image into a visible toner image. The developing device 10 may include a developing roller 5. The developing roller 5 supplies toner in the developing device 10 to the photosensitive drum 1. A developing bias voltage may be applied to the developing roller 5. A regulating member (not shown) restricts the amount of toner that is supplied by the developing roller 5 to a developing region where the photosensitive drum 1 and the developing roller 5 face each other. [0015] In the case of a two-component developing method, a magnetic carrier and toner may be accommodated in the developing device 10. The developing roller 5 may be spaced apart from the photosensitive drum 1 by tens to hundreds of microns. Although not illustrated in the drawing, the developing roller 5 may include a magnetic roller arranged in a hollow cylindrical sleeve. Toner is attached to a surface of the magnetic carrier. The magnetic carrier is attached to the surface of the developing roller 5 and transported to the developing region where the photosensitive drum 1 and the developing roller 5 face each other. The toner excluding the magnetic carrier is supplied to the photosensitive drum 1 by developing bias voltage applied between the developing roller 5 and the photosensitive drum 1, and thus the electrostatic latent image formed on the surface of the photosensitive drum 1 is developed into a visible toner image. The developing device 10 may include an agitator (not shown) that mixes and agitates toner with the magnetic carrier and transport the resulting mixture to the developing roller 5. The agitator may be, for example, an auger, and the developing device 10 may be provided with a plurality of agitators. [0016] In a case where a one-component developing method that does not use a carrier, the developing roller 5 may be rotated while being in contact with the photosensitive drum 1. The developing roller 5 may also be rotated while being spaced apart from the photosensitive drum 1 by tens to hundreds of microns. The developing device 10 may further include a supply roller (not shown) configured to attach toner to the surface of the developing roller 5. A supply bias voltage may be applied to the supply roller. The developing device 10 may further include an agitator (not shown). The agitator may agitate toner to be frictionally charged. The agitator may be, for example, an auger. [0017] A charging roller 2 is an example of a charger configured to charge the photosensitive drum 1 to have a uniform surface potential. A charging brush, a corona charger, or the like may be used instead of the charging roller 2. [0018] A cleaning blade 6 is an example of a cleaning device configured to remove toner and impurities remaining on the surface of the photosensitive drum 1 after the transferring process of the toner images. Other forms of cleaning devices such as a rotary brush, and the like may also be instead of the cleaning blade 6. [0019] An example of a developing method of the image forming apparatus according to an example will be described in detail. However, the present disclosure is not limited thereto, and various developing methods may be employed. [0020] An exposer 20 emits light modulated to correspond to image information toward photosensitive drums 1Y, 1M, 1C, and 1K to form electrostatic latent images corresponding to images of yellow (Y), magenta (M), cyan (C), and black (K) colors on the photosensitive drums 1Y, 1M, 1C, and 1K, respectively. As the exposer 20, a laser scanning unit (LSU) using a laser diode as a light source or a light emitting diode (LED) using an LED as a light source may be used. [0021] The paper transport belt 30 supports and transports the recording medium P. The paper transport belt 30 may be supported by, for example, support rollers 31 and 32 and circulate. The recording medium P may be picked up one by one from a loading frame 50 by a pickup roller 51, transported by a transporting roller 52, and then attached to the paper transport belt 30, for example, by an electrostatic force. A plurality of transfer rollers 40 may be arranged at positions facing the photosensitive drums 1Y, 1M, 1C, and 1K, with the paper transport belt 30 arranged therebetween. The plurality of transfer rollers 40 are an example of transfer devices that transfer the toner images from the photosensitive drums 1Y, 1M, 1C, and 1K to the recording medium P supported by the paper transport belt 30. A transfer bias voltage is applied to the transfer rollers 40 to transfer the toner images to the recording medium P. A corona transfer unit or a pin scorotron-type transfer unit may be employed instead of the transfer roller 40. [0022] The fixing apparatus 200 may apply heat and/or pressure to the image transferred to the recording medium P to fix the transferred image to the recording medium P. The recording medium P having passed through the fixing apparatus 200 is discharged by a discharge roller 53. [0023] By the above example of a configuration, the exposer 20 forms electrostatic latent images by irradiating the photosensitive drums 1Y, 1M, 1C, and 1K with a plurality of light beams modulated to correspond to image information of respective colors. The plurality of developing devices 10Y, 10M, 10C, and 10K form visible toner images of Y, M, C, and K colors at surfaces of the photosensitive drums 1Y, 1M, 1C, and 1K, respectively, by respectively supplying toners of Y, M, C, and K colors to the electrostatic latent images formed on the photosensitive drums 1Y, 1M, 1C, and 1K. The recording medium P loaded on the loading frame 50 is supplied to the paper transport belt 30 by the pickup roller 51 and the transporting roller 52, and is held on the paper transport belt 30, for example, by an electrostatic force. The toner images of Y, M, C, and K colors are sequentially transferred onto the recording medium P transported by the paper transport belt 30, by the transfer bias voltage applied to the transfer rollers 40. In a case where the recording medium P passes through the fixing apparatus 200, the toner image is fixed on the recording medium P by heat and pressure. The recording medium P, on which the fixing process has been completed, is discharged by the discharge roller 53. [0024] Although the image forming apparatus illustrated in FIG. 1 employs a method of directly transferring the toner images formed on the photosensitive drums 1Y, 1M, 1C, and 1K to the recording medium P supported by the paper transport belt 30, other transferring methods may also be used. For example, a method of intermediately transferring the toner images developed on the photosensitive drums 1Y, 1M, 1C, and 1K to an intermedium transfer belt (not shown), and then transferring the transferred images to the recording medium P may also be employed. [0025] In a case where a monochromic image, e.g., an image of black color, the image forming apparatus may include the developing device 10K alone among the developing devices 10Y, 10M, 10C, and 10K. The paper transport belt 30 is optionally provided. The recording medium P is transported between the photosensitive drum 1K and the transfer roller 40, and the toner image formed on the photosensitive drum 1K may be transferred to the recording medium P by the transfer bias voltage applied to the transfer roller 40. [0026] The fixing apparatus 200 applies heat and pressure to the toner image to fix the toner image on the recording medium P. To improve a printing speed and reduce energy consumption, a portion to be heated of the fixing apparatus 200 may have a smaller thermal capacity. For example, an on demand fixing (ODF)- type fixing apparatus 200 including a thin film-type endless belt as the portion to be heated may be employed. Thus, temperature of the fixing apparatus 200 may be rapidly increased up to a fixable temperature, and a state in which image formation is possible after the image forming apparatus is powered on may be reached within a short period of time. Accordingly, a printer employing this fixing method may have a very short first print out time (FPOT) that is a time from an input of a print command to an output of an image of a first page. Power consumption in a standby state waiting for a print command and power consumption required during printing may be reduced in printers employing such a fixing method. [0027] FIG. 2 is a cross-sectional view of the fixing apparatus 200 according to an example that may be installed in the image forming apparatus of FIG.1. [0028] Referring to FIG.2, the fixing apparatus 200 includes a rotatable endless belt 210, a heating unit 400 provided inside the endless belt 210, and a pressing roller 230 provided outside the endless belt 210 to face the heating unit 400 and configured to form a fixing nip 201. The pressing roller 230 is arranged in contact with the heating unit 400 with the endless belt 210 interposed therebetween. The pressing roller 230 rotates by pressure mutually applied to the pressing roller 230 and the heating unit 400, thereby rotating the endless belt 210. The heating unit 400 is provided inside the endless belt 210, configured to face the pressing roller 230 to form the fixing nip 201, and configured to heat the endless belt 210 at the fixing nip 201. [0029] The heating unit 400 includes: a heating member having a recessed portion A at a position corresponding to the fixing nip 201; and a heater 300 for a fixing apparatus according to an example provided inside the recessed portion. [0030] As described above, the pressing roller 230 facing the heating unit 400 is provided outside the endless belt 210. The heating unit 400 and the pressing roller 230 are mutually pressed with the endless belt 210 interposed therebetween. For example, a pressing force acting towards the pressing roller 230 may be applied to both ends of the heating unit 400 in a width direction perpendicular to a rotation direction of the endless belt 210 by a first pressing device, e.g., spring 250. As shown in FIG.2, the spring 250 may press the heating unit 400 with a metal bracket 251 interposed therebetween. Also, a pressing force acting toward the heating unit 400 may be applied to the pressing roller 230 by a second pressing device, e.g., spring 231. The pressing roller 230 may rotate the endless belt 210. For example, the pressing roller 230 may be a pressing roller configured such that an elastic layer is formed on an outer circumferential surface of a metallic core. The pressing roller 230 may rotate the endless belt 210 while being pressed together with the heating unit 400 with the endless belt 210 interposed therebetween. The heating unit 400 forms the fixing nip 201 together with the pressing roller 230 and guides the endless belt 210 to rotate. A belt guide 240 may further be provided at an outer side of the fixing nip 201 to smoothly rotate the endless belt 210. The belt guide 240 and the heating unit 400 may be formed integrally or separately. [0031] The heating unit 400 includes: a pressing member 220 configured to face the pressing roller 230 and form the fixing nip 201; and a heater 300 configured to heat the endless belt 210 at the fixing nip 201. In the heating unit 400, the pressing member 220 configured to form the fixing nip 201 and the heater 300 may be formed integrally or separately. In general, a heat-resistant grease is applied between the inner surface of the endless belt 210 and the heater 300 to reduce friction of the inner surface of the endless belt 210 and the surface of the heater 300. [0032] FIG.3 is a cross-sectional view of a fixing apparatus according to another example. The fixing apparatus illustrated in FIG.3 is different from that described above in that a thermally conductive plate 260 is disposed between the heater 300 for a fixing apparatus and the endless belt 210. The thermally conductive plate 260 may be, for example, a thin metallic plate including Al, Cu, or the like or a thin graphite plate. By interposing the conductive plate with high thermal conductivity 260 between a plane formed by the pressing member 220 and the heater 300 and the endless belt 210, heat of the heater 300 may be uniformly transferred to the endless belt 210. In addition, the non-paper passing region overheating phenomenon may be relieved and fixability may further be improved by expanding a range of heat transfer to the recording medium P by setting a width of the conductive plate with high thermal conductivity 260 to be equal to or greater than a width N of the fixing nip 201. In this case, a heat-resistant grease may be applied between the endless belt 210 and the conductive plate with high thermal conductivity 260 to reduce contact thermal resistance between the heater 300 and the conductive plate with high thermal conductivity 260. [0033] FIGS. 4A, 4B, and 4C show a rear-surface-heating-type heater 300 configured to heat an unfixed image on a recording medium according to an example and applicable to the fixing apparatus described above with reference to FIG.2 or 3. FIG.4A is a plan view illustrating an electrically insulating layer 300e on a first surface, i.e., rear surface, of a substrate 300a of the heater 300. FIG. 4B is a plan view illustrating a sliding layer 300f on a second surface, i.e., front surface, of the substrate 300a of the heater 300. FIG.4C is a cross-sectional view take along line A-A of FIG.4A. The heater 300 of this example is a rear-surface- heating-type heater in which a heat generating layer 300b is formed on the first surface of a ceramic substrate 300a and a sliding layer 300f configured to slide in contact with the inner surface of the endless belt 210 is formed on the second surface opposite thereto. [0034] Referring to FIGS. 4A, 4B, and 4C, the heater 300 has a planar shaped electrically insulating substrate 300a having the first surface and the second surface facing each other. The electrically insulating substrate 300a is a ceramic substrate including or consisting of alumina (Al2O3) or aluminum nitride (AlN) or including the same as a main component. The substrate is an electrically insulating ceramic substrate with high thermal conductivity. A thickness of the substrate 300a may be adjusted from about 0.5 to about 1.0 mm to reduce thermal capacity and the substrate 300a may be in the form of a rectangle with a width of about 10 mm and a length of about 300 mm. [0035] A heat generating layer 300b is formed on the first surface of the substrate 300a in a pattern of a line or band along a longitudinal direction of the substrate 300a. The heater 300 further includes: an electrode unit 300c configured to supply electricity to the heat generating layer 300b; and an electrically conductive pattern 300d configured to connect the electrode unit 300c to the heat generating layer 300b. The electrode unit 300c and the electrically conductive pattern 300d may include or consist of Ag or an AgPt alloy or includes the same as a main component. The heat generating layer 300b includes a resistant component capable of generating heat upon receiving electricity from the electrode unit 300c. To this end, the heat generating layer 300b may include a silver-palladium (AgPd) alloy, a nickel-tin (NiSn) alloy, a ruthenium oxide (RuO2) alloy, or silver (Ag), or may include the same as a main component. The heat generating layer 300b is formed by applying the component thereto in the pattern of a line or band having a thickness of about 10 μm and a width of about 1 to about 5 mm by screen printing or the like and firing the component. The heat generating layer 300b may be formed in a parallel pattern including two longitudinal lines or bands or in a parallel pattern including three or more longitudinal lines or bands, as shown in FIG.4A. [0036] The heat generating layer 300b is coated, i.e., overcoated, with the electrically insulating layer 300e. The electrically insulating layer 300e may provide insulation between the heat generating layer 300b and an external conductive member, provide corrosion resistance for preventing change in resistance caused by oxidation of the heat generating layer 300b or the like, and prevent mechanical damage. The electrically insulating layer 300e may have a thickness of about 20 μm to about 100 μm, for example, about 40 μm to about 80 μm. [0037] Ceramic substrates formed of alumina (Al2O3) or aluminum nitride (AlN) are mainly used as substrates of fixing apparatuses due to relatively low price. However, particularly, an alumina substrate may cause serious non-paper passing region overheating because the substrate has a low thermal conductivity of about 25 W/m^ K. In addition, contact thermal resistance occurs in a fixing apparatus including the conductive plate with high thermal conductivity 260 and the contact thermal resistance becomes more serious by the heat-resistant grease. [0038] FIG. 6 is a schematic conceptual diagram for describing a non-paper passing region overheating phenomenon occurring while an unfixed image is fixed by heating and pressing. Referring to FIG. 6, a recording medium A or B passes through the heater 300 of the fixing apparatus by mutually pressing the rotating pressing roller 230 and endless belt 210. In FIG. 6, 'A' indicates wide paper and 'B' indicates narrow paper. [0039] As shown in FIG.6, a length of the heating element is greater than a width of the wide paper A, and thus temperature of both ends of the heater not in contact with the paper increases compared to the central region of the heater in contact with the paper because heat loss caused by contact with the paper does not occur at the both ends. This is called “non-paper passing region overheating”. In a case where a narrow paper B passes, a length of a heating portion of the non-paper passing region increases, and thus temperature of the non-paper passing region increases. In particularly, the overheating phenomenon occurring by using the ceramic heater including an Al ₂ O ₃ substrate 300a with low thermal conductivity is more serious than that using a ceramic heater including an AlN substrate 300a. In a case where printing is performed using narrow paper, a printing speed should be lowered or an idle process should be introduced to inhibit an increase in temperature of the non-paper passing region caused by the non-paper passing region overheating. In either case, a decrease in the printing speed is unavoidable. [0040] However, in the present example, the electrically insulating layer 300e is formed to have a thermal conductivity of 100 W/m^ K or more using AlN, SiC, BeO, BN, or the like having excellent insulating property and high thermal conductivity, instead of using a common glass material. Thus, the above- described non-paper passing region overheating phenomenon and decrease in the printing speed may be effectively prevented, and the contact thermal resistance between the conductive plate with high thermal conductivity 260 and the heater 300 may be maintained at a very low level. To this end, the electrically insulating layer 300e includes: 1 wt% to 40 wt% of a glass matrix; and 60 wt% to 99 wt% of thermally conductive particles distributed in the glass matrix including at least one type selected from aluminum nitride (AlN), silicon carbide (SiC), beryllium oxide (BeO), boron nitride (BN), graphite, and carbon nanotubes (CNT). [0041] According to another example, the electrically insulating layer 300e may include about 70 wt% to about 90 wt% of the thermally conductive particles and about 10 wt% to about 30 wt% of the glass matrix; or about 80 wt% to about 90 wt% of the thermally conductive particles and about 10 wt% to about 20 wt% of the glass matrix. Excellent insulating property and high thermal conductivity of 100 W/m^ K or more may be obtained using the electrically insulating layer 300e having the above-described configuration, thereby effectively preventing the non- paper passing region overheating, the decrease in the printing speed, and the increase in the contact thermal resistance between the conductive plate with high thermal conductivity 260 and the heater 300 while fixing an unfixed image. [0042] The heater 300 of this example is a rear-surface-heating-type in which the first surface of the substrate 300a provided with the heat generating layer 300b is a rear surface of the heater 300 and the opposite surface (second surface) is a front surface of the heater 300. The front surface of the heater 300 slides in a contact state with the inner surface of the endless belt 210. That is, the sliding layer 300f is formed on the second surface of the substrate 300a. The sliding layer 300f is a layer including an imide-based resin such as polyimide and polyamideimide as a main component. The sliding layer 300f has excellent heat resistance, lubricity, and abrasion resistance and smoothly slides in a contact state with the inner surface of the endless belt 210. [0043] The sliding layer 300f is formed by coating a solution prepared by dissolving a polyimide or a polyamideimide in an organic solvent such as N- methylpyrrolidone (NMP) or N,N-dimethylacetamide on the second surface of the substrate 300a by dip coating, spray coating, or screen printing, and drying and firing the coated solution. [0044] A thickness of the sliding layer 300f may be from about 5 μm to about 20 μm. As the thickness of the sliding layer 300f decreases, the substrate 300a may be exposed by abrasion of the fixing apparatus. In response to exposure of the ceramic substrate 300a, friction resistance increases and a driving torque increases, and the inner surface of the endless belt 210 may easily be worn. [0045] As the thickness of the sliding layer 300f increases, a surface temperature of the endless belt 210 decreases and fixability of a toner image deteriorate. The sliding layer 300f may be formed of glass. [0046] The surface of the ceramic substrate 300a may be pre-treated before forming the sliding layer 300f to increase adhesion between the sliding layer 300f and the ceramic substrate 300a. As one method therefor, surface polishing with sandpaper may be used. More particularly, by polishing a coating surface of the substrate 300a with sandpaper, oils and fats on the surface may be removed and scratches are formed on the surface, and thus the polyimide layer may be strongly adhered to the substrate 300a by anchor effect. In addition, a method of treating the surface of the substrate 300a with a blast, a method of removing fats; a chemical polishing method; or a method of spraying a silane-based coupling agent such as methyltriethoxysilane and ethyltrimethoxysilane or a titanium- based coupling agent such as tetraisopropoxytitanium onto the surface of the substrate 300a and drying the sprayed coupling agent may be used. Before spraying the coupling agent, the surface of the substrate 300a may be activated by corona discharge treatment. [0047] FIG.5 is a cross-sectional view of a front-surface-heating-type heater 300 according to another example applicable to the fixing apparatus described above in FIG.2 or 3. Referring to FIG.5, in a front-surface-heating-type heater 300 of this example, a sliding layer 300f is formed on an electrically insulating layer 300e formed on a heat generating layer 300b. In the front-surface-heating-type heater 300, the heat generating layer 300b faces the inner surface of the endless belt 210, i.e., a fixing nip. [0048] In this case, a thickness of the electrically insulating layer 300e is adjusted from about 30 μm to about 100 μm, e.g., from about 40 μm to about 80 μm, to obtain full insulation between the heat generating layer 300b and the inner surface of the endless belt 210 even in a case where the sliding layer 300f is worn. On the contrary, too thick electrically insulating layer 300e may deteriorate thermal conductivity to the inner surface of the endless belt 210, and thus the thickness of the insulating layer 300e may appropriately be about 100 μm or less. [0049] In general, in the front-surface-heating-type heater 300 of FIG.5 in which the heat generating layer 300b is formed on the second surface, i.e., front surface, of the substrate 300a, a distance between the heat generating layer 300b and the inner surface of the endless belt 210 is small, thereby providing higher heat transfer efficiency compared to the rear-surface-heating-type heater 300. [0050] Except for the elements described above, the same descriptions given above with reference to the rear-surface-heating-type heater 300 shown in FIG. 4 may be applied to the substrate 300a, the heat generating layer 300b, the electrode unit 300c, the electrically conductive pattern 300d, the electrically insulating layer 300e, and the sliding layer 300f of the front-surface-heating-type heater 300. [0051] Hereinafter, the present disclosure will be described in further detail with reference to the following comparative examples and examples. However, these examples are provided for illustrative purposes and are not intended to limit the scope. [0052] Comparative Example 1: Preparation of Ceramic Heater Including Existing Glass Insulating Layer Not Including Thermally Conductive Particles [0053] A rear-surface-heating-type ceramic heater illustrated in FIGS.4A, 4B, and 4C was prepared in the following order. [0054] A planar shaped alumina (Al ₂ O ₃ ) ceramic substrate 300a in a rectangular form having a thickness of about 1 mm and a width of about 300 mm was prepared. [0055] A heat generating layer 300b pattern was formed on an upper surface (first surface) of the ceramic substrate 300a using AgPd by screen printing. The heat generating layer 300b pattern was formed in a pattern of two bands each having a thickness of about 10 μm and a width of about 1.5 mm along a longitudinal direction of the ceramic substrate 300a as shown in FIG.4A. To this end, a paste for the heat generating layer 300b pattern was prepared by appropriately mixing particles of an AgPd alloy with a high Pd content, an ethyl cellulose binder, and an organic solvent and homogenizing the mixture. The paste was applied to the upper surface (first surface) of the ceramic substrate 300a by screen printing and dried for about 10 minutes at about 200 °C to planarize the screen-printed paste for a resistive heating element, i.e., the heat generating layer pattern, and evaporate the solvent. Then, the screen-printed pattern was fired at about 850 °C for about 1 hour to form the heat generating layer 300b pattern. [0056] Subsequently, an electrode unit 300c configured to supply electricity to the heat generating layer 300b pattern; and an electrically conductive pattern 300d configured to connect the electrode unit 300c to the heat generating layer 300b were formed by screen printing in a pattern as shown in the plan view of FIG.4A. The same paste as the paste for the heat generating layer 300b pattern was used and the electrode unit 300c and the electrically conductive pattern 300d had a thickness of about 10 μm and a width of about 1.5 to 2.5 mm in size. [0057] Then, an electrically insulating layer 300e was overcoated to a thickness of about 65 μm using a glass material to electrically insulate the heat generating layer 300b pattern, the electrode unit 300c, and the electrically conductive pattern 300d. To this end, a glass paste was prepared. [0058] The glass paste includes glass particles, an ethyl cellulose binder capable of suspending the glass particles to inhibit aggregation of the glass particles, and an organic solvent to adjust viscosity of the glass paste. The glass paste was coated on the upper surface (first surface) of the ceramic substrate 300a by screen printing to cover the heat generating layer 300b pattern, the electrode unit 300c, and the electrically conductive pattern 300d. In addition, the coated glass paste was dried in an oven for about 10 hours at about 200 °C to planarize the surface of the glass past by fluidity of the glass paste and evaporate the solvent. Thereafter, the dried glass paste was fired at a temperature of about 1,000 °C to 1,200 °C for about 60 minutes to form the electrically insulating layer 300e. The glass particles contained in the coated glass paste are melted and softened to be integrated by firing. By performing the firing in an air atmosphere containing oxygen, the ethyl cellulose binder was removed after being burned into carbon dioxide. In addition, the glass particles are integrated by softening and melting to form a dense glass layer. [0059] A conventional rear-surface-heating-type ceramic heater, in which the electrically insulating layer 300e was formed of a glass material, was obtained according to the above-described procedure. [0060] Example 1: Preparation of Novel Ceramic Heater Including Glass Insulating Layer Including Thermally Conductive Particles [0061] A glass paste for an electrically insulating layer 300e was prepared by further adding aluminum nitride (AlN) particles having an average particle diameter of about 200 μm to the glass paste prepared in Comparative Example 1 in addition to the glass particles, the ethyl cellulose binder, and the organic solvent to adjust viscosity of the glass paste, such that a weight ratio of the aluminum nitride particles to the glass particles was about 85 wt%: about 15 wt%. [0062] A rear-surface-heating-type ceramic heater in the form shown in FIGS.4A, 4B, and 4C was prepared in the same manner as in Comparative Example 1, except that the above-described glass paste was used to form the electrically insulating layer 300e. [0063] Examples 2 to 4: Preparation of Novel Ceramic Heater Including Glass Insulating Layer Including Thermally Conductive Particles [0064] Glass pastes for electrically insulating layers 300e were prepared by modifying the weight ratio of the aluminum nitride (AlN) particles: glass particles as shown in Table 1 below in addition to the glass particles, the ethyl cellulose binder, and the organic solvent to adjust the viscosity of the glass pastes. [0065] Rear-surface-heating-type ceramic heaters in the form shown in FIGS.4A, 4B, and 4C were prepared in the same manner as in Example 1, except that the above-described glass pastes were respectively used to form the electrically insulating layers 300e. [0066] The ceramic heaters obtained in Comparative Example 1 and Examples 1 to 4 were mounted on a commercially available HP Color LaserJet Enterprise M552 printer and degrees of non-paper passing region overheating were evaluated during printing with B5 and A5 paper. [0067] Table 1 [0068] Based on the results of Table 1, results as described below may be confirmed. [0069] Thermal conductivity in the longitudinal direction of the electrically insulating layers of the ceramic heaters according to Examples 1 to 4 significantly increased over 100 W/m ^ K compared to that of the ceramic heater of Comparative Example 1. Based thereon, in the printing test using B5 paper, while the non-paper passing region saturation temperature of the fixing apparatus was high (227 °C) in the case where the ceramic heater including the glass matrix insulating layer according to Comparative Example 1 was used, the non-paper passing region saturation temperature of the fixing apparatus significantly decreased (210 to 220 °C) in the case where the ceramic heaters including the electrically insulating layers with high thermal conductivity according to Examples 1 to 4 were used. [0070] Also, in the printing test using A5 paper, while the non-paper passing region saturation temperature of the fixing apparatus was high (228 °C) in the case where the ceramic heater including the glass matrix insulating layer according to Comparative Example 1 was used, the non-paper passing region saturation temperature of the fixing apparatus significantly decreased (212 to 221 °C) in the case where the ceramic heaters including the electrically insulating layers with high thermal conductivity according to Examples 1 to 4 were used. [0071] Therefore, by using the ceramic heaters including the electrically insulating layers with high thermal conductivity according to Examples 1 to 4, the printing speed of the printing process using narrow A5 paper may be increased by 40% or more. [0072] Based on the results described above, it was confirmed that the non-paper passing region overheating of the fixing apparatus, the decrease in the printing speed, and the increase in the contact thermal resistance between the conductive plate with high thermal conductivity and the heater while fixing an image may be effectively prevented by using the ceramic heater including the electrically insulating layer with high thermal conductivity according to the present disclosure. Therefore, a fixing apparatus and an image forming apparatus each including the above-described ceramic heater may be effectively used in high-speed printing and low-energy fixing methods. [0073] It should be understood that examples described herein should be considered in a descriptive sense and not for purposes of limitation. Descriptions of features or aspects within each example should typically be considered as available for other similar features or aspects in other examples. While one or more examples have been described with reference to the figures, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope as defined by the following claims.