PTC composite material

[0001] [Technical field to which the invention belongs] The present invention relates to PTC (positive temperature coefficient of resistance) composite materials that are advantageously used in current limiting components and the like that control accident currents. [0002] [Conventional technology] In a certain temperature range, the electrical resistance of PTC material increases rapidly with increasing temperature. Thus, PTC materials are used, for example, as current limiting components to control fault currents in circuits. [0003] The most well-known PTC material is barium titanate-type ceramic, whose electrical properties change at its Curie point. However, since the resistivity at room temperature (room temperature resistivity) is high, the power loss becomes large when this PTC material is used. Moreover, the manufacturing cost is high. As a result, the industry has begun searching for other substances that exhibit PTC properties. As a result, it was found that the composite material made of the polymer matrix and the conductive filler exhibits the same type of characteristics as the PTC characteristics of the barium titanate type ceramic. [0004] For example, a mixture of a specific proportion of a crystalline polymer that acts as an insulator (eg polyethylene) and conductive particles that act as conductive pathways formed in this polymer matrix (eg carbon particles) is at very low room temperature. It exhibits electrical resistance (30 ° C) and behaves as a conductor when it undergoes an insulator-conductor transition. Specifically, the coefficient of thermal expansion of the crystalline polymer is much higher than that of the conductive particles, so that when the composite is heated and the crystalline polymer melts, the crystalline polymer expands dramatically. As a result, the conductive particles forming the conductive path in the polymer are separated from each other at the melting point temperature of the polymer, the conductive path is cut, and the electrical resistance of the composite material increases sharply. That is, this composite material exhibits behavior as a PTC material (PTC behavior). [0005] However, when organic substances such as the polymers mentioned above are used as a matrix for PTC composite materials, the heat resistance of the organic substances is generally low, so that the composite material exerts its intended action when the high temperature caused by the accident current continues for a long time. The problem arises that it cannot be shown. In the conventional polymer composite material, since the resistance of the material after the trip state does not return to the initial resistance, there is also a problem that highly reliable repetitive operation cannot be performed. Therefore, it is difficult to rely on these composites for delicate circuit applications. [0006] Research is also being conducted on composite materials consisting of silica-type matrices such as quartz and cristobalite and conductive particles. However, like barium titanate ceramics, these materials have high room temperature resistivity and therefore cause large power loss. Moreover, the transition temperature (ie, trip point temperature) of these materials exceeds 200 ° C, which makes them unsuitable for use in certain circuit applications. [0007] [Problems to be Solved by the Invention] Considering the problems of the prior art described above, the present invention reconstitutes with low room temperature resistivity, large resistivity jumps at transition temperature, transition temperature below 200 ° C, high heat resistance, and low power loss. We have succeeded in providing a usable PTC composite material. [0008] [Means for solving problems] According to the present invention, (i)<u style="single">B, Al, Ti, and W</u>A matrix of ceramic materials having one of a cristobalite crystal structure and a tridimite crystal structure to which at least one oxide of the above is added, and (ii) a conductive phase dispersed throughout the matrix, said conductive phase. metal<u style="single">, And silicide</u>Including at least one of<u style="single">At the same time, the transition temperature is 200 ° C or less.</u>PTC composites are provided. The ceramic material is a cristobalite phase SiO₂, Tridimite phase SiO₂, Cristobalite phase AlPO₄, Tridimite phase AlPO₄It is preferably one of them. [0009] When the above-mentioned materials are added to the matrix phase in an amount of preferably 0.1 mol% to 20 mol%, the transition temperature (that is, trip point temperature) of the materials is lowered to a level of 200 ° C or less, while the room temperature resistivity is 1 Ωcm. Maintained below. In addition, the "resistivity jump" at the trip point temperature of the material (ie, the increase in resistivity at the trip point temperature) is at least 10 times, preferably at least 100 times, more preferably at least 1000 times, most preferably at least 10000. Double (10 in some cases)⁹Double). [0010] The conductive phase of the PTC material generally takes the form of particles selected from the materials described above. Preferred conductive materials are<u style="single">Nb, Ta, and Mo</u>It is a silicide of. These materials increase the cycle life of PTC materials. This is because the silicide forms a strong chemical bond with the matrix material and thus enhances the overall strength of the composite material.<u style="single">Nb, Ta, and Mo</u>The silicide is most preferred because it is stable in hot air and therefore does not easily deteriorate during high temperature operations used to form the composite (eg, the composite can be debindered in air). [0011] The average particle size of the conductive particles forming the conductive phase in the composite material is preferably 5 to 100 μm, more preferably 20 to 60 μm. If the average particle size is less than 5 μm, the room temperature resistivity of the resulting material tends to be too high and the resistivity jump at trip point temperature tends to be too small. However, when the average particle size exceeds 100 μm, the stress caused by the thermal expansion mismatch between the matrix material and the conductive phase becomes too large, and the cycle life of the composite material is shortened. It is preferred that 10-45% by volume of the conductive phase be present (with respect to the total volume of the composite), more preferably 20-35% by volume. [0012] Further, since the voids substantially act as an insulating phase in the material, the relative density of the PTC composite is preferably 90% or more in order to reduce the room temperature resistivity of the composite. It is preferable to leave a small amount of bubbles in the matrix because crack formation and growth due to thermal stress during operation can be prevented. Further, the expansion of the material at the trip point temperature of the composite material is preferably 0.2% to 1.4% (volume%). If the volume expansion is less than 0.2%, the composite does not show a sufficient resistivity jump at the trip point temperature. If the volume expansion exceeds 1.4%, stress cracks at the interface between the matrix and the conductive phase can occur in the composite. [0013] Further, the composite material is preferably produced at a firing temperature at least 20 ° C. lower than the melting point of the material having the lowest melting point contained in the conductive phase of the composite material. By doing so, the positions of the conductive particles constituting the conductive phase are maintained even during the firing operation. If the conductive particles were allowed to melt and agglomerate, a relatively high conductivity region would be formed penetrating the composite and thus the high temperature resistivity of the composite could be significantly reduced. In addition, the molten conductive particles can leak out of the matrix, making it difficult to control the volume ratio of the conductive particles in the composite. [0014] For a more complete understanding of the essence and purpose of the invention, see the following detailed description of embodiments of the invention, along with accompanying drawings. [0015] BEST MODE FOR CARRYING OUT THE INVENTION In the PTC composite material of the present invention, one of the cristobalite crystal structure and the tridimite crystal structure (preferably the cristobalite phase SiO).₂, Tridimite phase SiO₂, Cristobalite phase AlPO₄, Tridimite phase AlPO₄A ceramic material having one of these) is used as the matrix. These materials undergo dramatic volume expansion at a particular temperature as the crystal structure of the material changes from one phase / structure to another. These transition temperatures are generally in the range 220-250 ° C. [0016] The present invention takes advantage of this dramatic volume expansion that occurs with the matrix materials described above. Specifically, this matrix material exhibits a relatively low room temperature resistivity when combined with the conductive phase material described above, so that current can flow through it through the conductive phase. However, when an overcurrent condition occurs and the internal temperature of the composite rises to the crystal transition temperature of the matrix, the matrix material expands rapidly, disrupting the conductive path formed by the conductive phase. As a result, the resistivity of the entire composite increases rapidly, which causes the composite to exhibit PTC behavior. [0017] The idea of using a cristobalite matrix / conductive particle composite in this regard is disclosed in NGK Insulators, Ltd., US Patent Application No. 09/035074, but the temperature at which crystal structure transitions occur is subject to some circuit applications. Too expensive for. Therefore, according to the present invention, in this matrix material,<u style="single">B, Al, Ti, and W</u>At least one of these oxides is preferably added in an amount of 0.1 mol% to 20 mol%. This additive lowers the transition temperature of cristobalite to 130 ° C to 180 ° C (additive-free cristobalite phase SiO)₂And additive-free cristobalite phase AlPO₄The phase transition temperature of is about 245 ° C and 220 ° C, respectively), and the transition temperature of tridimite is lowered to 110 ° C to 150 ° C (additive-free tridimite phase SiO).₂And additive-free tridimite phase AlPO₄The phase transition temperature of is about 180 ° C and 160 ° C, respectively), which makes this composite suitable for a wider range of electronic applications. Preferred additives are Ti, Al, B and W, which are most effective in lowering the crystal structure transition temperature of the matrix phase. Preferred composites are 2-6 mol% TiO₂Cristobalite phase SiO added₂, And 1-5 mol% tungstate (Na)₂WO₄, K₂WO₄, CsWO₄Etc.) added tridimite phase SiO₂including. [0018] The amount added to the matrix material is preferably 0.1 mol% to 20 mol%. When the amount of additive is less than 0.1 mol%, the transition temperature of the matrix material does not decrease much. If the amount added exceeds 20 mol%, it becomes difficult to maintain the crystalline phase of the original matrix material. Additives can be used individually or in combination, as long as the material is converted to oxides during debindering or sintering, oxides, carbonates, sulphates, chlorides or nitrates. Can be added to the composite material in the form of. When W is used as an additive, it is in the form of tungstate (Na) to ensure reaction with the matrix material.₂WO₄, K₂WO₄, CsWO₄Etc.) must be added. [0019] All of the matrix materials described above have a relatively high melting point (eg, cristobalite phase SiO).₂1730 ° C), therefore, it has better heat resistance than the polymer matrix used in the prior art. Therefore, even if a high current flows through the composite for a long time, there is no concern that the composite will melt or ignite during use. The preferred crystalline phase described above can be introduced into the composite at the raw material stage or can be produced in the composite during calcination. For example, cristobalite phase SiO₂Can be obtained by calcining quartz at a high temperature. Cristobalite can also be obtained by calcining quartz at low temperatures in the presence of alkali or alkaline earth metals that stabilize cristobalite. [0020] The conductive phase is produced in the composite material by adding conductive particles to the matrix raw material which is an insulator. Conductive particles<u style="single">Nb, Ta, and Mo</u>Is preferably contained. To keep the room temperature resistivity of the PTC material below 1.0 Ωcm and thus reduce the power loss of the PTC material, the room temperature resistivity of the conductive particles is 10.^-3Must be less than or equal to Ω cm. Therefore 10^-3Materials with room temperature resistivity greater than Ωcm shall not be used as conductive particles in the PTC material of the present invention. [0021] [0021] The average particle size of the conductive particles is preferably 5 to 100 μm, more preferably 20 to 60 μm. It is important that the average particle size is at least 5 μm in order to keep the room temperature resistivity of the composite sufficiently low and to keep the resistivity jump at the trip point temperature sufficiently large. When the average particle size of the conductive particles exceeds 100 μm, the thermal expansion mismatch between the matrix and the conductive phase becomes large, so that the cycle life of the composite is reduced. Therefore, as the average particle size of the conductive particles increases, the PTC effect increases and the room temperature resistivity decreases, but the cycle life of the composite material decreases. [0022] It is also important to maintain the volume ratio of conductive particles to the total volume of the composite at 10-45% by volume. If the volume ratio of the conductive particles is lower than 10%, it becomes difficult to make the room temperature resistivity of the composite material acceptablely low. If the volume ratio of the conductive particles is higher than 45%, it becomes difficult to divide the conductive phase in the composite material at the trip point temperature. In such cases, it becomes difficult to obtain at least 10 times the resistance jump generally required for most electronic applications. [0023] The amount of conductive particles added depends on the average particle size of the matrix particles and the conductive particles. When the average particle size of the conductive particles is 0.1 to 10 μm, the amount of the conductive particles is preferably 25 to 40% by volume of the total volume of the PTC composite material, and the average particle size of the conductive particles is 5 to 50 μm. When it is, it is preferably 20 to 35%. [0024] When silicide is used as conductive particles, it is preferable to form a solderable composite electrode layer on the outer surface of the PTC body. The composite electrode layer contains a matrix material and a conductive component. The matrix material of the composite electrode layer is preferably the same material as the matrix of the PTC body in order to prevent a mismatch of thermal expansion between the composite electrode layer and the PTC body under the composite electrode layer. The conductive component of the composite electrode layer shall be one of Co, Cr, Fe, Mo, Nb, Ni, Pt, Rh, Ti, W, Zr, or an alloy of one or several of these metals. Is preferable. [0025] The conductive component is 30 to 90% by volume of the total amount of the composite electrode material. If it is less than 30% by volume, the conductivity of the composite electrode material becomes insufficient. Since the thermal expansion of the matrix material is larger than that of the conductive component, the thermal expansion of the composite electrode decreases as the amount of the conductive component increases. In order to maintain the thermal expansion of the composite electrode close to the thermal expansion of the PTC body beneath it, the conductive component must not exceed 90% by volume of the composite electrode. Due to the presence of less expansive conductive components, the thermal expansion of the composite electrode layer is always smaller than that of the underlying PTC body. [0026] The thickness of the composite electrode should not exceed 50% of the thickness of the PTC body. Since the coefficient of thermal expansion of the composite electrode layer is smaller than that of the PTC body, if the electrode layer is too thick, the expansion of the PTC body at the transition temperature is limited, which adversely affects the device performance. [0027] In the present invention, it is preferable to produce the PTC composite material by firing at a temperature at least 20 ° C. lower than the melting point temperature of the material having the lowest melting point contained in the conductive particles. This prevents the conductive particles from melting during firing and keeps the conductive particles inside the composite. Allowing the conductive particles to melt and leak out of the composite sintered body makes it difficult to maintain the intended ratio of conductive particles to matrix material. Furthermore, allowing the conductive particles to melt during firing also allows the conductive particles to agglomerate, which is very low and cannot be split even when the matrix material undergoes dramatic volume expansion. Conductive paths of resistance occur in the composite. [0028] Therefore, when a single material is used for the conductive particles, the composite material must be fired at a temperature at least 20 ° C lower than the melting point temperature of the conductive particles. On the other hand, when using a mixture of different conductive particles, the composite must be fired at a temperature at least 20 ° C lower than the melting point temperature of the material with the lowest melting point used in the conductive particles. .. [0029] The PTC composite must be sintered at a sufficiently high temperature for a sufficient period of time in order for the relative density of the final sintered body to be at least 90%, more preferably at least 95%. When the relative density is less than 90%, the characteristic stability of this PTC composite is reduced under the condition that the overcurrent suppression operation is repeated. This is because the room temperature resistivity of the material tends to change after each trip operation. When designing precision electronic circuits, designers also require stable room temperature resistivity after repeated trips. On the other hand, bubbles in the matrix of less than 5% are preferable because they have an effect of preventing crack formation and growth. Bubbles can be created by adding a pore-forming agent such as organic powder to the raw material or by using a matrix material having a particle size of 0.5 to 20 μm. [0030] When using a composite electrode layer, it is possible to form a composite electrode layer on an unfired PTC body and then co-fire this component in order to achieve good bonding between the electrode layer and the PTC body beneath it. preferable. Preferably, the electrode layer component is mixed with other conventional additives to form a slurry, which is then applied to both sides of the PTC body. This part is then sintered as described above. In order to preserve the particle morphology of the composite electrode layer, the melting point of the conductive component must be higher than the sintering temperature. [0031] For certain applications, such as sheet steel PTC components that require flexibility for mounting, polymer components that function as part of the PTC composite matrix described above are added to give the PTC composite flexibility. Is desirable. In such cases, the polymer can be used with thermosetting resins such as phenol, epoxy, urea, melamine, polyester, alkyd, diallyl phthalate, silicone resins (eg polymethylsiloxane, polyphenylsiloxane), and polyvinyl chloride, poly. It can be selected from thermoplastic resins such as vinyl acetate, polyvinyl chloride, acrylic resin, polyethylene, polypropylene, polystyrene, nylon, polytetrafluoroethylene, polybutylene terephthalate, polyphenylene sulfide, polyamideimide, and polyimide. These polymeric materials help strengthen the matrix and increase cycle life. Unlike polymer-based PTC materials, the polymers used in the composites of the invention do not need to expand upon melting. Therefore, various polymer materials (eg, highly heat resistant polymers) can be selected. [0032] This polymer should be added in an amount of 5-40% by volume based on the total volume of the composite. There is no flexibility effect on polymers below 5% by volume, and polymers above 40% by volume significantly reduce resistivity jumps as the overall expansion of the composite decreases as the amount of ceramic matrix decreases. [0033] By using the above-mentioned dopant materials, the PTC composites of the present invention can be used at temperatures lower than 200 ° C., and thus many of the above-mentioned polymer materials can be used. Many of these polymer materials cannot be used without the dopant material, as PTC composites without dopant materials exhibit trip point temperatures above 200 ° C above the maximum allowable temperature of the polymer. [0034] Among the above-mentioned polymer materials, silicone-based resins are preferable. This is because the silicone-based resin does not change into a flammable material when decomposed, and easily forms a chemical bond with the matrix ceramic material to enhance the strength of the entire PTC composite material. [0035] Next, a preferred method for forming the PTC composite material of the present invention will be described. [0036] Desired crystal structure (eg cristobalite phase SiO)₂) Can be pre-existing in the matrix raw material or can be generated in the matrix during the firing of the PTC composite. Cristobalite phase SiO₂Is formed by calcining quartz powder at high temperatures or by calcining quartz powder in the presence of alkali metals or alkaline earth metals to convert the quartz powder to cristobalite. In this case, the obtained cristobalite phase SiO₂Cristobalite SiO with an average particle size of 5 μm or less₂Get the powder. When quartz is used as the starting material for the matrix, the quartz powder is ground in a wet pot mill to give the quartz powder with an average particle size of 0.5-2 μm. [0037] The raw materials can be mixed in a wet or dry ball mill to obtain a mixture, depending on the flow characteristics of the raw materials. If quartz is used as the starting material, then an alkali metal or alkaline earth metal is added to the mixture to allow the quartz to cristobalite phase SiO during the firing phase.₂To be converted to. These materials also function as sintering aids during firing. [0038] The resulting mixture is then press molded to give an initial molding material. If unpressurized sintering is used in the firing step, the mixture is then subjected to cold hydrostatic compression. [0039] Next, the molded product is sintered. The compact, which is simply press-molded, is preferably subjected to hot hydrostatic pressure press molding at 1100 to 1500 ° C. for 1 to 5 hours in a non-oxidizing atmosphere. The hydrostatic pressure press-molded molded product is preferably sintered at normal pressure at 1200 to 1800 ° C. for 1 to 5 hours in a non-oxidizing atmosphere. Other details of the methods used to form PTC materials can be found in US Simultaneous Application No. 09/035074. [0040] The non-oxidizing atmosphere used in the sintering process discussed above was primarily designed to prevent oxidation of the conductive particles, but the slight oxidation of the conductive particles further enhances the PTC effect. I found out. Specifically, when using silicide conductive particles, N₂, H₂, Ar and the like are used, and it is preferable to adjust the oxygen partial pressure in the reducing gas during sintering. This allows a thin oxide layer to be formed around the conductive particles, which is effective in enhancing the PTC effect of the complex. [0041] Although not completely elucidated, it is believed that this thin oxide layer does not affect the conductivity of the conductive particles at room temperature, as the conductive particles are pressed against each other by the insulating matrix. However, at the transition temperature of the matrix, it is clear that this thin oxide layer helps break the conductive links that penetrate the complex. [0042] Any known method can be used to control the oxygen partial pressure of the sintered gas. As an example, there is a method of bubbling a reducing gas for a predetermined time in a water bath (maintained at a predetermined temperature). The oxygen partial pressure is preferably maintained at a dew point of 77 ° C to 20 ° C. [0043] The thickness of the thin oxide layer must be at least 0.01 μm and not more than 1.0 μm. When this layer is less than 0.01 μm, no substantial enhancement of the PTC effect is seen. If this layer is thicker than 1.0 μm, the room temperature resistivity will be greater than acceptable levels. [0044] [Example] (Example 1) A variety of conductive materials were used to form a conductive phase in the resulting PTC body. Table 1 shows the average particle size of each material. An air classifier was used to sort the conductive particles based on the average particle size shown in Table 1. Quartz powder (average particle size 4 microns), 0.5 mol% alumina powder (average particle size 2 microns) and 0.4 mol% LVDS₃The resulting mixture was calcined at 1450 ° C for 8 hours in a ball mill with alumina-added cristobalite phase SiO.₂Was formed. The material was then ball milled to an average particle size of 1.1 microns and mixed with the conductive particles, organic binder and distilled water shown in Table 1. The mixture was then vacuum kneaded to form a clay, which was then vacuum extruded to form a green sheet. The green sheet was then sintered in a non-oxidizing atmosphere (hydrogen) at 1400 ° C. for 4 hours. [0045] Each sintered body was then processed into a 5 × 30 mm square column with electrodes and tested for room temperature resistivity and high temperature resistivity using the DC4 probe method. The results are shown in Table 1. [0046] Furthermore, each test sample was repeatedly placed in a trip state, and it was determined whether or not the room temperature resistivity was restored after each trip state, and the cycle life of each sample was tested. The cycle test was performed by subjecting the sample to a thermal cycle between 30 ° C and 250 ° C and repeated until the room temperature resistivity increased by more than 200%. The results are shown in Table 1. [0047] Cristobalite Phase AlPO₄In these examples using the tridimite phase SiO, the suitable precursor material was calcined at 1220 ° C for 5 hours.₂In the case of, a suitable precursor material was calcined at 1400 ° C for 8 hours. In Examples 1-1 to 1-3, 32% by volume of the conductive powder was added, and in the remaining Examples, 28% by volume of the conductive powder was added. [0048] [table 1]<img file="JP4393003B2_D0001.tif" />[0049] Examples 1-1 and 1-2 in Table 1 show that it is important to use conductive particles with an average particle size of at least 5 μm. In the case of Example 1-1 in which the average particle size of the conductive particles was 1 μm, the room temperature resistivity was 62.5 Ωcm. On the other hand, in the case of Example 1-2 in which the average particle size of the conductive particles was 5 μm, the room temperature resistivity was 0.82 Ωcm. [0050] Examples 1-8 and 1-9 show that the cycle life is reduced from more than 10 to 1 when the average particle size of the conductive particles exceeds 50 μm. The reason for this is that large conductive particles generate a large stress at the interface between the matrix and the conductive particles. [0051] In Examples 1-5 to 1-7, extremely low room temperature resistivity is realized when the average particle size of the conductive particles is 20 to 40 μm, and the material trip point temperature (that is, the crystal structure transition / transformation temperature of the matrix). ) At 10⁷It shows that a double resistivity jump is presented. [0052] (Example 2) The amount of TiO shown in Table 2₂1 was repeated using TiO as an additive.₂Added cristobalite phase SiO₂A powder was formed. [0053] [Table 2]<img file="JP4393003B2_D0002.tif" />[0054] Examples 2-3 to 2-7 are cristobalite phase SiO₂2 to 6 mol% of TiO to lower the α β phase transition temperature₂Indicates that is valid. Examples 2-8 have more than 7 mol% TiO₂Shows that part of the matrix is converted to rutile, which weakens PTC behavior. [0055] (Example 3) The following Examples 3-1 to 3-8 show the effect of adding the additive to the matrix by the method described above. [0056] The additives shown in Examples 3-1 to 3-6 in Table 3 were added, and Example 1-5 was repeated. Examples 3-7 and 3-8 are the same as in Example 1-5, but the tridimite phase SiO as the matrix material.₂And cristobalite phase AlPO₄The difference is that In Examples 3-1 to 3-8, Na and K were added to stabilize the crystal structure of the matrix and promote sintering. [0057] [Table 3]<img file="JP4393003B2_D0003.tif" />[0058] [0058] Examples 3-1 and 3-2 have at least 0.1 mol% Al.₂O₃It has been shown that the transition temperature of the PTC composite can be significantly reduced when is added to the matrix material as an additive. On the other hand, Examples 3-4 and 3-5 have an Al content of more than 20 mol%.₂O₃Is added to the matrix material as a dopant, indicating that the resulting composite does not exhibit any effective PTC behavior. This is a large amount of Al₂O₃Is to convert the matrix into mullite, which does not show PTC behavior. [0059] (Example 4) Examples 4-1 to 4-12 are PTC composite sample similar to the samples of Examples 1 to 3, except that a polymer material forming a part of the matrix is added. 27% by volume NbSi in each example₂Was added. [0060] The results in Table 4 show that adding at least 5% by volume of polymer to the matrix increases the overall density of the composite and thus reduces room temperature resistivity. Increasing density also improves the cycle life of composites, as higher density materials have higher toughness. The presence of the polymeric material reduces the resistivity jump, as the addition of the polymeric material to the matrix reduces the amount of ceramic material in the matrix. Therefore, no more than 40% by volume polymer material should be added. [0061] The results in Table 4 also indicate that polymers with inadequate thermal stability, such as epoxies, exhibit inadequate cycle life and should therefore be avoided. The most preferred polymer material is a silicone resin that exhibits high heat resistance and binds well to the ceramic material of the matrix. [0062] [Table 4]<img file="JP4393003B2_D0004.tif" />[0063] (Example 5) Examples 5-1 to 5-6 are PTC composite sample similar to the samples of Examples 1 to 4, but MoSi as conductive particles.₂(Grain size = 35 μm), Cristobalite type SiO as a matrix₂Is using. Further, these samples were prepared using a sintered gas having an adjusted oxygen partial pressure. The "oxygen partial pressure" condition in Table 5 reflects the temperature of the water bubbling the sintered gas and the time when water vapor was added to the sintered gas. [0064] [Table 5]<img file="JP4393003B2_D0005.tif" />[0065] These results indicate that the thin oxide layer on the surface of the conductive particles enhances the PTC effect of the PTC body. Examples 5-6 show that room temperature resistivity becomes unacceptably high when the thickness of the oxide layer exceeds 1.0 μm. [0066] Although the present invention has been described with respect to specific preferred embodiments and accompanying drawings, those skilled in the art will not be limited to this preferred embodiment and will deviate from the scope of the invention as defined in the claims. It can be understood that various modifications and the like can be implemented in this embodiment without any need. [Simple explanation of drawings] FIG. 1 is a graph showing the temperature dependence of the electrical resistance of the PTC material of Example 1-5 based on the present invention.