A ceramic heater and a method of manufacture thereof

Abstract

 This invention provides a ceramic heater of current self-control type, which has no gap in the boundary between the enclosure 1 and the filler 2, and which has reduced manufacturing cost, a stable strength and an improved heating performance. The invention also provides a method of manufacturing such a ceramic heater. In this ceramics heater, porous ceramic fillers 2, 3 are filled inside the end portions of the enclosure 1 made of a dense ceramics. The metal coil 10 consisting of the heater coil 6 and the current control coil 9 connected in series with each other is installed inside the enclosure 1, with the end surfaces 15 of the enclosure 1 hermetically closed by the sealing films 4. To prevent corrosion of the metal coil 10, the low heat conductivity ceramics 25 made from ceramic whiskers and/or ceramic powder is disposed in the central portion of the enclosure 1 and an N₂ gas is sealed therein. The heater coil 6 is arranged in contact with the inner wall surface 7 of that portion of the enclosure 1 where the filler 2 is located, and the current control coil 9 is spaced from the inner wall surface 8 of the enclosure 1 so as to be heat-insulated.

Description

[0001] The present invention relates to a ceramic heater that forms a ceramic glow plug used with diesel engines and also to a method of manufacturing the ceramic heater.

[0002] A ceramic glow plug conventionally has its heater portion consisting of a high-melting point metal such as tungsten and an Si₃N₄ formed integral during a hot-press process. A conventional current self-control type glow plug is not satisfactory in terms of heat durability, strength against thermal shocks and high-temperature resistance when it is used in high-temperature combustion chambers in heat-insulated engines. Because the self-control type glow plug is made by connecting a heating coil made from a tungsten wire and a resistor coil made from a nickel wire, both with different resistance-temperature coefficients, there are drawbacks of complex structure, high cost and insufficient strength.

[0003] In the conventional current self-control type glow plug, the metal coil such as tungsten wire is embedded in ceramics and sintered during the forming process, so that the sintering requires application of pressure. Hence, a hot-press, i.e., a single axis pressure sintering is usually performed. The metal coil embedded in the ceramics is therefore limited in the shape and takes a two-dimensional structure, giving rise to a problem that a gap is formed between the wall surface of the ceramic enclosure and the metal coil, reducing the heat conductivity and the quick-heating capability of the ceramic heater.

[0004] In the ceramics glow plug whose heater portion consists of a tungsten wire and an Si₃N₄ formed integral during the hot-press process, a gap is formed in the boundary between the tungsten wire and the Si₃N₄, resulting in ingress of water or oxygen through the gap to cause corrosion. To prevent entry of water or oxygen from the gap between the tungsten wire and the Si₃N₄ requires that the gap be sealed. For this purpose, a glass sealing may be employed but this increases the number of steps in the manufacture process, lowering the reliability.

[0005] In a glow plug disclosed in Japanese Utility Model No. 15077/1989, the heater portion is formed of a sintered ceramics that consists of a heater coated with a ceramic coating layer applied by a vapor deposition method.

[0006] In a ceramics heater disclosed in Japanese Patent Laid-Open No. 157084/1989, a heating resistor wire formed of a high melting point metal such as tungsten (W), molybdenum (Mo) and rhenium (Re) or their alloys is embedded in an Si₃N₄ sintered body and an intermediate layer, which is made from the same metal elements as the major constituent elements of the resistor wire or non-oxidized ceramics such as nitrides, carbides, silicides or silicified carbide of these metals, is formed over the surface of the resistor wire.

[0007] Among the conventional current self-control type glow plugs is one disclosed by Japanese Patent Laid-Open No. 157423/1983. This uses two kinds of material, that is, a sheath type resistor having a greater positive resistance-temperature coefficient than the heater tungsten coil is used as a current control element and connected in series with the heater coil in the glow plug, with the heater coil embedded in a bar-shaped ceramics, to improve the heat conductivity and self-control the electricity supplied to the heating coil thereby improving the heating characteristics and preventing overheating of the heater portion.

[0008] A self-control type glow plug disclosed in Japanese Patent Publication No. 34052/1992, for example, has a current control resistor connected in series with a heating body to control the heating body temperature when a current is being supplied and the temperature is rising. In this glow plug, the heating coil and the resistor coil are connected in series and embedded in a ceramic sintered material, thus forming an integral ceramic heater. The heating coil is made from a tungsten-rhenium alloy wire which has a positive resistance-temperature coefficient of less than four times, and the resistor coil is made of a pure tungsten wire or a pure molybdenum wire.

[0009] A sheathed glow plug disclosed in Japanese Patent Publication No. 19404/1985 is a self-control type sheathed glow plug, in which a heating coil and a resistor coil are directly connected to each other between the inner bottom of a heat resisting, bottomed metal tube and a center electrode, with the winding pitch of the resistor coil made dense in an area close to a mounting fitting on the central electrode side and coarse in an area close to the heating body side.

[0010] A glow plug for diesel engine disclosed in the Japanese Patent Laid-Open No. 141424/1987 has a cylindrical ceramic heater mounted at the front end portion of a hollow holder. The ceramic heater consists of a thin plate insulator made of insulating ceramics and a thin plate resistor made of conductive ceramics stacked over the side surfaces and one end portion of the thin plate insulator, these laminated thin plate insulator and resistor being bent widthwise to form a cylindrical body. A protective pipe of conductive ceramics is fitted over the outer circumference of the rear end of the cylindrical body. The protective pipe and the cylindrical body are then sintered to be formed as an integral one-piece structure.

[0011] A primary aim of this invention is to solve the above-mentioned problems.

[0012] The present invention provides a ceramic heater that includes: fillers disposed inside an enclosure; a heater coil disposed inside a front end portion of the enclosure where one of the fillers is located; and a current control coil arranged inside a central portion of the enclosure; said ceramics heater being characterized in: that the enclosure is made from a dense ceramics; that the fillers consist of porous ceramics contained in end portions of the enclosure and a low heat conductivity ceramics contained in a central portion of the enclosure; that the heater coil and the current control coil are formed of one and the same metal coil; that the heater coil is arranged in contact with an inner wall surface of that portion of the enclosure where the fillers are contained; and that the current control coil is spaced from an inner wall surface of that portion of the enclosure where the low heat conductivity ceramics is contained, so that the current control coil is heat-insulated.

[0013] The present invention also provides a ceramic heater which is characterized in:
   that different kinds of fillers are disposed inside an enclosure;
   that a current control coil is disposed in an intermediate portion of the enclosure having a higher heat insulation capability;
   that a heater portion is disposed in a front end portion of the enclosure having a lower heat insulation capability to increase the amount of heat that an be dissipated;
   that a current flowing through the heater portion and the current control portion is controlled to ensure an optimum heating value, so as to form the ceramic heater as a current self-control type; and
   that the fillers adhere to the enclosure so that no gap is formed in the boundary between the enclosure and the fillers.

[0014] The ceramic heater with this construction can improve the heat conductivity, prevent ingress of water or oxygen from the boundary between the metal coil and the enclosure ceramics in the heater portion, reduce manufacturing cost and assure a stable strength and improved reliability.

[0015] The present invention also provides a ceramic heater which includes:
   an enclosure made of a fine ceramics and open at both ends thereof;
   fillers made of a porous ceramics and disposed inside the end portions of the enclosure;
   a low heat conductivity ceramics arranged inside the central portion of the enclosure and containing an N₂ gas sealed therein; and
   a metal coil consisting of a first portion arranged in contact with an inner wall surface of the front end portion of the enclosure where the filler is disposed, and a second portion disposed in the portion of the enclosure where the N₂ gas is sealed in such a way that this portion of the metal coil is spaced from an inner wall surface of the enclosure so as to be heat insulated.

[0016] In this ceramic heater, the portion of the metal coil arranged in contact with the inner wall surface of the front end portion of the enclosure where the filler is disposed constitutes a heater coil, and the second portion of the metal coil spaced from the inner wall surface of the enclosure where the low heat conductivity ceramics is disposed constitutes a current control coil. Because the heater coil and the current control coil are formed of a single, continuous metal coil of, say, a tungsten wire, the metal coil can easily be made at low cost by using a ceramic coil making jig, which has a through-hole in the center and spiral grooves on its outer circumferential surface. The metal coil has improved electrical reliability and durability as there are no joints or connections in the coil.

[0017] When a current is supplied to the metal coil, the heat generated by the heater coil heats the enclosure and is dissipated outside. On the other hand, the heat generated by the current control coil is insulated by the low heat conductivity ceramics, raising the temperature of the current control coil. As the temperature of the current control coil increases, its resistance also increases, limiting the current flowing through the current control coil or the whole metal coil. That is, an elevated temperature of the current control coil increases its resistance and reduces the current flowing through the metal coil, thus self-controlling the amount of heat dissipated from the enclosure to an optimum level.

[0018] The enclosure is formed of Si₃N₄ and its end surfaces are hermetically closed with sealing films to seal the interior of the enclosure. The sealing films are made from glass or the same kind of ceramics as that of the enclosure.

[0019] The metal coil is made from a tungsten wire, whose surface is coated with a ceramic film formed by the chemical vapor deposition (CVD) or with an organic silicon polymer-converted ceramic film.

[0020] The porous ceramics is a non-contracting ceramics that is made by sintering the material containing Si and Ti for reaction. The non-contracting ceramics contains Si₃N₄, TiN, TiO₂, and TiON.

[0021] The low heat conductivity ceramics is made from ceramic whiskers and/or ceramic powder and contains Si₃N₄ and metal nitrides or metal oxides.

[0022] The present invention also provides a method of manufacturing a ceramic heater including the steps of: making an enclosure and a metal coil; installing the metal coil inside the enclosure; and filling fillers inside the enclosure; said ceramic heater manufacturing method being characterized by the steps of: making the enclosure from a dense ceramics; making the metal coil from a high-melting point metal wire; installing the metal coil inside the enclosure in such a way that the metal coil is in contact with an inner wall surface of one end portion of the enclosure and is spaced from an inner wall surface of an intermediate portion of the enclosure; filling a material containing Si and Ti inside the one end portion of the enclosure; filling a low heat conductivity ceramics inside the intermediate portion of the enclosure and filling a material containing Si and Ti inside the other end portion of the enclosure; baking the enclosure together with the filling materials and the metal coil for reaction in an N₂ atmosphere to convert the Si/Ti material into a porous ceramics containing Si₃N₄ and TiN; and making the porous ceramics adhere to the dense ceramics of the enclosure.

[0023] The present invention further provides a ceramic heater manufacturing method, which is characterized by the steps of:
   making an enclosure from an Si₃N₄ dense ceramics;
   coating the surface of a metal coil made of a high-melting point metal wire with a ceramics having almost the same thermal expansion coefficient as the coil;
   installing the metal coil inside the enclosure in such a way that the coil contacts the inner wall surface of one end portion of the enclosure and is spaced from the inner wall surface of an intermediate portion of the enclosure;
   filling a material containing Si and Ti inside the one end portion of the enclosure;
   filling a ceramic whisker or a porous ceramics inside the intermediate portion of the enclosure to form a low heat conductivity member therein;
   filling a material containing Si and Ti inside the other end portion of the enclosure;
   baking the enclosure together with the filler materials and the metal coil for reaction in an N₂ atmosphere to convert the Si/Ti material into a porous ceramics containing Si₃N₄, TiN, TiO₂ and TiON; and
   making the porous ceramics adhere to the dense ceramics of the enclosure.

[0024] If the enclosure is made of an Si₃N₄ dense ceramics, TiN is generated from the reaction of Si and Ti during the baking process. Because TiN expands during baking, the porous ceramics generated from baking and containing TiN slightly expands, causing the sintered Si₃N₄ to adhere to the enclosure Si₃N₄. Further, if baking is performed without applying pressure, the expansion of TiN prevents formation of any gap in the boundary between the enclosure and the filler. Hence, the metal coil, even if formed in a three-dimensional shape, can be maintained in good contact with the enclosure, improving the heat conduction efficiency and permitting a fast temperature rise at the heater portion.

[0025] Furthermore, because the ends of the enclosure are open, it is possible to easily seal an N2 gas in the enclosure, particularly in the intermediate portion of the enclosure, during the baking process by closing the end surfaces of the enclosure with sealing films such as ceramics. Since an N₂ gas can be sealed in the enclosure during the baking process, the number of manufacturing steps can be reduced, lowering the cost. Moreover, the sealed N₂ gas improves corrosion resistance and durability of the metal coil, which is made, for example, of a tungsten wire.

[0026] In this ceramic heater, because the heater coil, which forms the heater portion, is placed in contact with the inner wall surface of the enclosure, the temperature of the whole enclosure rises uniformly. The construction of the heater portion, in which the heater coil is put in contact with the inner wall surface of the enclosure and embedded in the filler, offers a very good heat conduction, improving the fast-heating performance of the heater portion, that is, assuring a quick temperature rise upon energization.

[0027] Rather than using the above-mentioned enclosure which is open at both ends, it is possible to employ a dense ceramic enclosure which is closed at one end and open at the other end and is formed with fine holes in the wall thereof. In this case, after the material containing Si and Ti is sintered and converted into a non-contracting ceramics, the fine holes in the enclosure need to be hermetically closed with sealing films in a vacuum.

[0028] Preferred embodiments of the present invention will be described hereinbelow by way of example only with reference to the accompanying drawings, in which:

Figure 1 is a cross section of a ceramic heater as one embodiment of this invention.

Figure 2 is a cross section of a ceramic heater as another embodiment of this invention;

Figure 3 is a plan view of one embodiment of an enclosure of the ceramic heater shown in Figure 2;

Figure 4 is a plan view of the enclosure of Figure 3 with slits closed by a sealing film; and

Figure 5 is a graph showing temperature rises of the ceramic heater of this invention and the conventional glow plug in connection with an energizing time.

[0029] First, an embodiment of the ceramic heater according to this invention is explained by referring to Figure 1. This ceramic heater, which is incorporated in a ceramic glow plug of current self-control type built into a diesel engine, consists of: a pipe-shaped enclosure 1 made from dense ceramic with open ends; fillers 2, 3 made from porous ceramics and filled inside the ends of the enclosure 1; a sealing film 4 attached to the ends of the enclosure 1 to seal the interior of the enclosure 1; a low heat conductivity ceramics 5 installed in the central portion of the enclosure 1 and sealing an N₂ gas; and a metal coil 10 installed inside the enclosure 1. The fillers 2, 3 are filled in the enclosure 1 in such a way as to contact the inner wall surfaces 7, 14 of the enclosure 1.

[0030] The metal coil 10 is a resistor wire made from a high melting point metal such as tungsten and consists of a heater coil 6 and a current control coil 9. The heater coil 6 is installed in the front end part of the enclosure 1 where the filler 2 is filled, in such a way that the heater coil contacts the inner wall surface 7 of the enclosure 1. The current control coil 9 is installed in a part of the enclosure where the N₂ gas is sealed, in such a way that it is spaced from the inner wall surface 8 of the enclosure 1. Further, the metal coil 10 also has a connecting wire 11 that connects one end of the heater coil 6 and one end of the current control coil 9; a connecting wire 12 that connects to the other end of the heater coil 6, extends through the interior of the enclosure 1 and projects from the porous ceramics filler 3; and a connecting wire 13 that connects to the other end of the current control coil 9, extends through the interior of the enclosure 1 and protrudes from the porous ceramics filler 3.

[0031] The surface of the metal coil 10 is coated with a ceramic film. The porous ceramics forming the fillers 2, 3 contains Si and Ti which, when baked, are transformed into Si₃N₄ and TiN, respectively. TiN is further transformed into more stable materials TiO₂ and TiON through oxidation. The sealing film 4 attached to the ends 15 of the enclosure 1 and to the end surface of the fillers 2, 3 are made from glass material such as polymer precursor or the same ceramics as that of the enclosure 1. The enclosure 1 is formed of a dense Si₃N₄ ceramics. The low heat conductivity ceramics 5 is made of either SiC whisker, Si₃N₄ whisker, Al₂O₃ whisker, Al₂O₃-SiO₂ whisker, ceramic powder or Si₃N₄ porous material. It is preferred from the standpoint of heat insulation that the ceramics around the current control coil 9 be made from a material having as low a heat conductivity as air. Further, during the baking process, an N₂ gas is sealed in a part of the enclosure 1 where the low heat conductivity ceramics 5 is located in order to prevent oxidation of the metal coil 10.

[0032] One end of the enclosure 1 is secured to a hollow body that has an electrode mounted in a hollow portion thereof through an insulator such as insulating bushing. The hollow body is made from a metal such as heat resistant alloy and has a thread for mounting to other component. The end of the connecting wire 13 is connected to the electrode and the end of the connecting wire 12 is connected to the hollow body. Therefore, in this ceramic heater, electric current flows from the electrode to the connecting wire 13 to the current control coil 9 to the connecting wire 11 to the heater coil 6 to the connecting wire 12 to the hollow body.

[0033] This ceramic heater can be manufactured as follows. First, the enclosure 1 is made from a dense ceramics and the surface of the metal coil 10 of, say, tungsten wire is covered by the chemical vapor deposition (CVD) with ceramics, such as SiC, that has a thermal expansion coefficient almost equal to that of the metal coil 10. Next, the ceramics-coated metal coil 10 of tungsten wire is installed inside the enclosure 1 so that it contacts the inner wall surface 7 of one end portion of the enclosure 1 but does not contact the inner wall surface 8 of the intermediate portion of the enclosure 1. A material containing Si and Ti is filled inside the end of the enclosure 1, a material containing a low thermal conductivity ceramics 9 such as SiC whisker and Si₃N₄ porous material is filled inside the intermediate portion of the enclosure 1, and a material containing Si and Ti is filled inside the other end. After this, the enclosure 1 filled with the Si/Ti-containing material and with the low heat conductivity ceramic material is baked for reaction in the presence of N₂ gas to transform the material containing Si and Ti into porous ceramics such as Si₃N₄, TiN, TiO₂ and TiON, thereby converting the materials into fillers 2, 3. This permits the porous ceramics to adhere to the dense ceramics that forms the inner wall surfaces 7, 14 of the enclosure 1 and particularly the heater coil 6 to adhere to the inner wall surface 7 of the enclosure.

[0034] Because the ends of the enclosure 1 are open, N₂ gas is sealed in the enclosure 1, particularly in the low heat conductivity ceramics 5 filled in the intermediate portion of the enclosure 1 during baking. Further, the end surfaces 15 of the enclosure 1 are coated with the sealing films 4, made of the same kind of ceramics as that of the enclosure 1, to seal the interior of the enclosure 1. Therefore, compared with the conventional heaters, this ceramic heater formed in a way mentioned above has improved adherence of the heater coil 6 and the filler 2 to the inner wall surface 7 of the enclosure 1, resulting in an improved heat conduction, which in turn allows for an immediate temperature rise. Another advantage of this ceramic heater is that the manufacturing process has a fewer number of fabrication steps and therefore is simple, reducing the cost and improving reliability of the ceramic heater built into the ceramic glow plug.

[0035] Next, an example embodiment of this ceramic heater is explained below.

[0036] First, a tungsten (W) wire 0.2 mm in diameter was wound into a coil 3.5 mm in outer diameter. The coiled tungsten wire was then covered over its surface with SiC by the CVD. With the SiC-coated tungsten wire coil arranged in a porous mold such as plaster mold for slip cast, a 85:15 Si/Ti slurry was poured in a specified amount into the porous mold. Next, with the slurry solidified to a certain degree, an SiC whisker in a slurry state is poured onto the semi-solidified slurry in the porous mold. With the SiC whisker slurry solidified to a certain degree, the above-mentioned Si/Ti slurry was poured in a specified amount onto the SiC whisker n the porous mold. The water in the slurry was absorbed through the porous mold until the slurry solidified completely. With this process, a molded body was obtained in which a material containing Si and Ti was put around the tungsten coil, with the SiC whisker disposed in the intermediate portion of the enclosure.

[0037] Then, the molded body was taken out of the porous mold and dried in the presence of N₂ gas. The dried molded body was inserted into the enclosure 1, a pipe-shaped cylinder made of dense Si₃N₄ with a relative density of more than 99%. The outer diameter of the molded body and the inner diameter of the enclosure 1 were almost equal with virtually no gap between them. The molded body inserted into the enclosure 1 was baked in a furnace that contained 5 atm of N₂ gas heated up to 1,400°C to convert the Si and Ti components of the material into the baked nitrides such as Si₃N₄ and TiN, i.e., porous ceramics fillers 2, 3. At the same time, the SiC whisker was transformed into a low heat conductivity ceramics member 5 having N₂ gas sealed therein. No gap was formed in the boundary between the baked body and the enclosure 1 because of the TiN expansion of about 0.2% during the baking process. Next, the end surfaces 15 of the enclosure 1 and the porous ceramics fillers 2, 3 are coated by the CVD with Si₃N₄ to completely seal N₂ gas in the intermediate portion of the enclosure 1.

[0038] In another example, rather than being coated with SiC by CVD, the tungsten wire coil was immersed in a toluene solution of polycarbosilane, an organic silicon polymer, and was heated to a specified temperature in the presence of N₂ gas to convert the organic silicon polymer into Si₃N₄. The above process was repeated five times to form a ceramic film about 50 µm thick over the surface of the tungsten wire coil. This ceramics-coated tungsten wire coil was used to fabricate the ceramic heater in the same process as the first embodiment.

[0039] In a further example, a slurry of only Si instead of a Si/Ti mixture was used for filling the interior of the end portions of the enclosure 1 and, for the intermediate portion of the enclosure 1, the same SiC whisker slurry as in the first embodiment was used to manufacture the ceramic heater in the same process as the first embodiment. In this case, the porous ceramics that forms the fillers 2, 3 was a porous Si₃N₄.

[0040] A further example used a 80:20 Si/Si₃N₄ slurry instead of the Si/Ti slurry for filling the interior of the end portions of the enclosure 1. The same process as the first embodiment was followed to fabricate the ceramic heater. In this case, the porous ceramics forming the fillers 2, 3 was a porous Ni₃N₄.

[0041] Still another example used an Si slurry instead of the Si/Ti slurry for filling the interior of the end portions of the enclosure 1. For filling the intermediate portion of the enclosure 1, an Si slurry was also used instead of SiC whisker slurry. A ceramic heater was fabricated in the same process as the first embodiment. In this case, the porous ceramics forming the fillers 2, 3 was a porous Si₃N₄ and the porous ceramics forming the low heat conductivity ceramic member 5 was an Si₃N₄ porous material.

[0042] The ceramic heater according to this invention can be fabricated in ways described in the above embodiments. Although there are slight variations in temperature vise time with respect to current supply time, almost similar effects can be produced.

[0043] Next, by referring to Figure 2 through 5, another embodiment of the ceramic heater and its manufacturing method according to this invention will be described.

[0044] The ceramic heater is used with a ceramics glow plug built into a diesel engine and is formed as a two-layer structure consisting of an inner shell and an outer shell 21. The ceramics of the inner shell in which a metal coil 30 as a heater wire is embedded uses a ceramics that expands during baking. The outer shell 21 is formed into a pipe-shaped shell made from a dense ceramics which contains fillers 22, 23 of non-contracting ceramics in the end portions thereof. In the central portion of the outer shell 21 is installed a low heat conductivity ceramics member 25. The metal coil 30 is installed in the fillers 22, 23 and the low heat conductivity ceramic member 25 inside the outer shell 21.

[0045] The outer shell 21 is made from a dense ceramics such as silicon nitride Si₃N₄ and has one end thereof formed as a closed end 35 and the other end as an open end. Further, the outer shell 21 is formed with fine holes 36 such as slits piercing through the wall and running in the longitudinal direction or holes piercing through the wall. Through these fine holes gases present in the interior of the outer shell are evacuated to create a vacuum inside during the manufacture process. With the fine holes 36 formed in the outer shell 21, it is possible to sinter the Si and Ti materials installed in the outer shell 21 while applying pressure three-dimensionally during gas-pressure baking.

[0046] The non-contracting ceramics that forms the fillers 22, 23 filling the interior of the end portions of the outer shell 21 is made by baking the material containing Si and Ti and transforming them into a porous ceramics containing Si₃N₄, TiN, TiO₂ and TiON. The volume of Si and Ti before backing is virtually equal to that of Si₃N₄, TiN, TiO₂ and TiON combined after baking. In other words, the material of the fillers does not contract when subjected to baking. Hence, no gap is formed in the boundary between the inner wall surfaces 27, 34 of the dense Si₃N₄ outer shell 21 and the outer surfaces of the fillers 22, 23.

[0047] Further, the low heat conductivity ceramics 25 installed in the central portion inside the outer shell 21 is made from a ceramic powder, ceramic fibers or ceramic whiskers, all made of Si₃N₄ and metal nitride or oxide. That is, the low heat conductivity ceramics 25 is made of such material as SiC whisker, Si₃N₄ whisker, Al₂O₃ whisker, Al₂O₃-SiO₂ whisker, ceramic powder or Si₃N₄ porous material. It is desired from the standpoint of heat insulation that the ceramics around the current control coil 29 be formed of a material with a low heat conductivity almost equal to that of air.

[0048] The metal coil 30 is made of a high-melting point metal wire such as a tungsten wire, which is formed into a three-dimensional coil. A portion of the coil arranged in contact with the inner wall surface 27 of the outer shell 21 constitutes a heater coil 6 and a portion of the coil spaced from the inner wall surface 34 of the outer shell 21 constitutes the current control coil 29. Further, the metal coil 30 has a connecting wire 31 that connects one end of the heater coil 26 and one end of the current control coil 29; a connecting wire 32 that connects to the other end of the heater coil 26, extends along the inside of the outer shell 21 and projects from the filler 23 of the non-contracting ceramics; and a connecting wire 33 that connects to the other end of the current control coil 29, extends along the inside of the outer shell 21 and projects from the filler 23 of the non-contracting ceramics.

[0049] This ceramic heater is assembled into a ceramic glow plug. One end of the outer shell 1 is secured to a hollow body that has an electrode mounted in a hollow portion thereof through an insulator such as insulating bushing. The hollow body is made from a metal such as heat resistant alloy and has a thread for mounting to other component. The end of the connecting wire 33 is connected to the electrode and the end of the connecting wire 32 is connected to the hollow body. Therefore, in this ceramic heater, electric current flows from the electrode to the connecting wire 33 to the current control coil 29 to the connecting wire 31 to the heater coil 26 to the connecting wire 32 to the hollow body.

[0050] Next, another embodiment of the ceramic heater manufacturing method is explained. First, an outer shell 21 made of a dense ceramics is formed closed at one end 35 and open at the other end and, as shown in Figure 3, is formed with fine holes 36, such as slits that pierce through the wall and extend in the longitudinal direction and holes that pierce through the wall. A three-dimensionally wound metal coil 30 is made from, say, a tungsten wire. Next, the tungsten wire coil 30 is installed inside the outer shell 21 in such a way that it contacts the inner wall surface 7 of one end portion of the outer shell 21 and is spaced from the inner wall surface 28 of the intermediate portion of the outer shell. A material containing Si and Ti is filled inside one end portion of the outer shell 21; a material containing a low heat conductivity ceramic member such as ceramic powder is filled inside the intermediate portion of the outer shell 21; and a material containing Si and Ti is filled inside the other end portion of the outer shell 21. Then, the outer shell filled with the Si/Ti material and the low heat conductivity ceramics member 25 is sintered in an N₂ atmosphere at a gas pressure of less than 9.9 kgf/cm² to convert the Si/Ti material into a porous ceramics containing Si₃N₄, TiN, TiO₂ and TiON, i.e., into a non-contracting ceramics, to form the fillers 22, 23 and cause the non-contracting ceramics to adhere to the dense ceramics of the outer shell 21.

[0051] The ceramics heater is then placed in a vacuum and, as shown in Figure 4, the fine holes 36 formed in the wall of the outer shell 21 are sealed with a sealing film 24 such as glass and brazing filler metal to hermetically enclose the interior of the outer shell 21, thus forming the ceramic heater. Hence, compared with the conventional ceramic heaters, the ceramic heater of this invention that is incorporated into the ceramics glow plug can be manufactured easily at reduced cost and has higher reliability and stable characteristics.

[0052] In this ceramic heater, the heater coil 26 which constitutes a heater portion is in contact with the inner wall surface 27 of the outer shell 21 and is embedded in the non-contracting ceramic filler 22 that contacts the inner wall surface 27 of the outer shell 21. This construction allows the entire outer shell 21 to be heated uniformly and assures a very good heat conductivity of the heater portion, offering an improved heating performance that allows the heater portion to be heated quickly upon energization. Figure 5 shows a graph illustrating the relation between the temperature (°C) of the heater portion and the energizing time (second). In the graph, the ceramic heater of this invention is represented by a broken line and the conventional glow plug by a solid line. Figure 5 shows that the heater of this invention can be heated to a specified temperature in a shorter energizing time than is required with the conventional heater, demonstrating the superior heating characteristic.

Claims

1. A ceramic heater that includes:
      fillers (2, 3, 5, 22, 23, 25) disposed inside an enclosure (1, 21);
      a heater coil (6, 26) disposed inside a front end portion of the enclosure where one of the fillers is located; and
      a current control coil (9, 29) arranged inside a central portion of the enclosure;
   said ceramics heater being characterized in:
      that the enclosure is made from a dense ceramics;
      that the fillers consist of porous ceramics (2, 3, 22, 23) contained in end portions of the enclosure and a low heat conductivity ceramics (5, 25) contained in a central portion of the enclosure;
      that the heater coil and the current control coil are formed of one and the same metal coil (10, 30);
      that the heater coil (6, 26) is arranged in contact with an inner wall surface of that portion of the enclosure where the fillers (2, 22) are contained; and
      that the current control coil (9, 29) is spaced from an inner wall surface of that portion of the enclosure where the low heat conductivity ceramics (5, 25) is contained, so that the current control coil is heat-insulated.

2. A ceramic heater according to claim 1, wherein the enclosure is formed of an Si₃N₄ ceramics, the fillers (2, 3, 22, 23) are porous ceramics containing Si and Ti, and the inner wall surface of the enclosure (1, 21) and the outer circumferential surfaces of the fillers (2, 3, 22, 23) closely adhere to each other with no gap therebetween.

3. A ceramic heater according to claim 2, wherein an N₂ gas is sealed in the central portion of the enclosure (1, 21) where the low heat conductivity ceramics (5, 25) is contained.

4. A ceramic heater according to any one of claim 1 to 3, wherein the enclosure (1) is open at both ends, which are closed by sealing films (4) to seal the interior of the enclosure.

5. A ceramic heater according to claim 4, wherein the sealing films (4) are formed of glass or the same kind of ceramics as that of the enclosure.

6. A ceramic heater according to any one of claim 1 to 3, wherein the enclosure (21) is formed of a dense ceramics and is closed at one end and open at the other end, and the porous ceramics contained in the end portions of the enclosure (21) are a non-contracting ceramics.

7. A ceramic heater according to any one of claim 1 to 6, wherein the metal coil is made from a tungsten wire.

8. A ceramic heater according to any one of claim 1 to 7, wherein a surface of the metal coil is coated with a ceramic film deposited by the chemical vapor deposition or with an organic silicon polymer-converted ceramic film.

9. A ceramic heater according to any one of claim 1 to 8, wherein the porous ceramics are a non-contracting ceramics made by sintering Si and Ti for reaction and the non-contracting ceramics contains Si₃N₄, TiN, TiO₂ and TiON.

10. A ceramic heater according to any one of claim 1 to 9, wherein the low heat conductivity ceramics (5, 25) is made from ceramic whiskers and/or ceramic powder.

11. A ceramic heater according to any one of claim 1 to 10, wherein the low heat conductivity ceramics (5, 25) is made from Si₃N₄ and a metal nitride or a metal oxide.

12. A method of manufacturing a ceramic heater including the steps of:
      making an enclosure (1, 21) and a metal coil (10, 30);
      installing the metal coil (10, 30) inside the enclosure; and
      filling fillers inside the enclosure (1, 21);
   said ceramic heater manufacturing method being characterized by the steps of:
      making the enclosure from a dense ceramics;
      making the metal coil (10, 30) from a high-melting point metal wire;
      installing the metal coil inside the enclosure in such a way that the metal coil is in contact with an inner wall surface of one end portion of the enclosure and is spaced from an inner wall surface of an intermediate portion of the enclosure;
      filling a material containing Si and Ti inside the one end portion of the enclosure; filling a low heat conductivity ceramics (5, 25) inside the intermediate portion of the enclosure and filling a material containing Si and Ti inside the other end portion of the enclosure;
      baking the enclosure together with the filling materials and the metal coil for reaction in an N₂ atmosphere to convert the Si/Ti material into a porous ceramics containing Si₃N₄ and TiN; and
      making the porous ceramics adhere to the dense ceramics of the enclosure.

13. A ceramic heater manufacturing method according to claim 12, wherein after the material containing Si and Ti is sintered for reaction, the end surfaces of the enclosure (1) are coated with sealing films (4) to seal the interior of the enclosure.

14. A ceramic heater manufacturing method according to claim 12 or 13, wherein before the metal coil is installed inside the enclosure, the surface of the metal coil is coated with a ceramics having almost the same thermal expansion coefficient as the metal coil.

15. A ceramic heater manufacturing method according to any one of claim 12 to 14, wherein the enclosure (21) is made of a dense ceramics, is closed at one end and open at the other end, and is formed with fine holes (36) passing through the wall thereof; wherein after the material containing Si and Ti is filled inside the enclosure (21) and is sintered for reaction and converted into a porous ceramics, i.e., a non-contracting ceramics, the fine holes (36) in the enclosure (21) are hermetically closed in a vacuum by the sealing films (24) and also the sealing films (24) that seals the one end surface of the enclosure (21).

16. A ceramic heater manufacturing method according to claim 15, wherein the porous ceramics is a non-contracting ceramics containing Si₃N₄, TiN, TiO₂ and TiON, the non-contracting ceramics adheres to the Si₃N₄ dense ceramics of the enclosure (21), and the fine holes (36) formed in the enclosure (21) are hermetically closed by the sealing films (24) made of Si₃N₄ in a vacuum.

Drawing