Patent Grant
10299317
The present invention relates to a ceramic heater used, for example, as an ignition or flame detection heater for combustion type onboard heating apparatus, an ignition heater for various combustion apparatuses such as kerosene fan heater, a heater for glow plug of automobile engine, a heater for various sensors such as oxygen sensor, or a heater for measuring instrument; and a glow plug provided with the same.
A heater used in such applications as glow plug of automobile engine includes a resistor including a heat-generating portion, a lead, and an insulating base. The materials for them are selected and the shapes of them are designed such that the resistance of the lead is lower than that of the resistor.
Here, a junction between the resistor and the lead is a point of change in shape at which the resistor and the lead having different shapes are connected to each other, or a point of change in material composition at which the resistor and the lead having different material compositions are connected to each other. Thus, modifications are made such as increasing the junction area in order to reduce the effect caused by a difference in thermal expansion produced by heat generation or cooling during use. For example, there is known a heater in which the interface between a resistor 3 and each lead 8 is tilted when being seen in a cross section parallel to the axial direction of the lead as shown in
PTL 1: Japanese Unexamined Patent Application Publication No. 2002-334768
PTL 2: Japanese Unexamined Patent Application Publication No. 2003-22889
In recent years, in order to optimize a combustion state of an engine, a driving method has been employed in which a control signal from an ECU is pulsed.
Here, a square wave is often used as a pulse. A high-frequency component is present in a rising portion of the pulse, and the high-frequency component propagates on a surface portion of a lead. However, when a joint portion (connection portion) is formed such that end surfaces of a lead and a resistor having different impedances are opposed to each other, a portion of the high-frequency component impedance of which portion cannot be matched at the connection portion is reflected and diffused at the connection portion, and dissipated as a Joule heat. Thus, heat is locally generated in the connection portion. However, when the interface between each lead 8 and the resistor 3 is flat as shown in
In addition, even when pulse drive is not employed and DC drive is employed, the same problem arises. In other words, since circuit loss is decreased in a recent ECU, a high current flows through a resistor at start of an engine operation for the purpose of quick temperature rise. Therefore, rising at which power inrushes is steepened like a square wave of a pulse, and high power including a high-frequency component rushes into the heater. Thus, the same problem arises.
The present invention has been conceived of in view of the above-described problems of the related art, and an object thereof is to provide a highly-reliable and durable heater in which even when a high current flows through a resistor, occurrence of a micro crack in a connection portion between the resistor and a lead, development of a crack at an interface, and change in the resistance value of the heater are suppressed, and a glow plug provided with the same.
Solution to Problem
A heater according to the present invention is a heater including: an insulating base; a resistor buried in the insulating base; and a lead buried in the insulating base and connected at a front end side thereof to the resistor. A connection portion is provided such that an end surface of the resistor and an end surface of the lead are opposed to each other, and a boundary between the resistor and the lead has a curved shape when the connection portion is seen in a cross section perpendicular to the axial direction.
In addition, the present invention is a glow plug including any described heater having the above-described configuration; and a metallic retaining member which is electrically connected to the lead and retains the heater.
According to the heater of the present invention, even when a high-frequency component propagates along the surface of the lead, occurrence of a micro crack in the connection portion between the resistor and the lead, development of a crack in the boundary surface, and change of the resistance value of the heater are suppressed, and the resistance value of the heater is stabilized over a long period of time. Thus, the reliability and the durability of the heater are improved.
Hereinafter, examples of embodiments regarding a heater according to the present invention will be described in detail with reference to the drawings.
The heater 1 of the embodiment is a heater which includes an insulating base 9, a resistor 3 buried in the insulating base 9, and a lead 8 which is buried in the insulating base 9 and connected at a front end side thereof to the resistor 3. The heater 1 includes a connection portion 2 where the resistor 3 and the lead 8 overlap each other in a direction perpendicular to the axial direction of the lead 8, and the boundary between the resistor 3 and the lead 8 has a curved shape when the connection portion 2 is seen in a cross section perpendicular to the axial direction.
The insulating base 9 in the heater 1 of the embodiment is formed, for example, in a bar shape. The insulating base 9 covers the resistor 3 and the lead 8. In other words, the resistor 3 and the lead 8 are buried in the insulating base 9. Here, the insulating base 9 is preferably made of ceramics. Thus, the insulating base 9 is able to resist higher temperatures than metals, and hence it is possible to provide a heater 1 having further improved reliability in quick temperature rise. Specific examples thereof include ceramics having electrical insulating properties such as oxide ceramics, nitride ceramics, and carbide ceramics. Particularly, the insulating base 9 is preferably made of silicon nitride ceramics. This is because silicon nitride, which is a principal component, is good in terms of high strength, high toughness, high insulating properties, and heat resistance. It is possible to obtain the silicon nitride ceramics, for example, by mixing 3 to 12% by mass of a rare earth element oxide such as Y2O3, Yb2O3, or Er2O3 as a sintering aid, 0.5 to 3% by mass of Al2O3 with silicon nitride as the principal component, further mixing SiO2 therewith such that an SiO2 amount contained in a sintered body is 1.5 to 5% by mass, molding the mixture into a predetermined shape, and then conducting firing through hot pressing at, for example, 1650 to 1780° C.
In addition, when one made of silicon nitride ceramics is used as the insulating base 9, it is preferred that MoSiO2, WSi2, or the like is mixed and dispersed therein. In this case, it is possible to make the coefficient of thermal expansion of the silicon nitride ceramics as the base material to be close to the coefficient of thermal expansion of the resistor 3, and thus it is possible to improve the durability of the heater 1.
The resistor 3 includes a heat-generating portion 4 which is a region in which heat is particularly generated. When the resistor 3 has a linear shape as shown in
In addition, when the resistor 3 has a folded shape as shown in
One containing a carbide, a nitride, a silicide, or the like of W, Mo, Ti, or the like as a principal component may be used as the resistor 3. When the insulating base 9 is the above material, tungsten carbide (WC) among the above-described materials is good as the material of the resistor 3 in that the difference in coefficient of thermal expansion from the insulating base 9 is small, in having a high heat resistance, and in having a low specific resistance. Furthermore, when the insulating base 9 is made of silicon nitride ceramics, the resistor 3 preferably contains, as a principal component, WC which is an inorganic conductor, and the amount of silicon nitride added thereto is preferably equal to or greater than 20% by mass. For example, in the insulating base 9 made of silicon nitride ceramics, tensile stress is generally applied to a conductor component which is to be the resistor 3, since the conductor component has a higher coefficient of thermal expansion than that of silicon nitride. On the other hand, when silicon nitride is added to the resistor 3, it is possible to make the coefficient of thermal expansion of the resistor 3 to be close to the coefficient of thermal expansion of the insulating base 9 and to alleviate stress caused by a difference in coefficient of thermal expansion in temperature rise or temperature fall of the heater 1.
In addition, when the amount of silicon nitride contained in the resistor 3 is equal to or less than 40% by mass, it is possible to make the resistance value of the resistor 3 relatively small and stabilize the resistance value. Therefore, the amount of silicon nitride contained in the resistor 3 is preferably 20% by mass to 40% by mass. More preferably, the amount of silicon nitride is 25% by mass to 35% by mass. Moreover, instead of silicon nitride, boron nitride may be added in an amount of 4% by mass to 12% by mass as a similar additive to the resistor 3.
In addition, the thickness of the resistor 3 (the thickness in the up-down direction shown in
The same material as that of the resistor 3 containing a carbide, a nitride, a silicide, or the like of W, Mo, Ti, or the like as a principal component may be used for the lead 8 which is connected at the front end side thereof to the end portion of the resistor 3. Particularly, WC is preferred as the material of the lead 8 in that the difference in coefficient of thermal expansion from the insulating base 9 is small, in having a high heat resistance, and in having a low specific resistance. In addition, when the insulating base 9 is made of silicon nitride ceramics, the lead 8 preferably contains, as a principal component, WC which is an inorganic conductor, and silicon nitride is preferably added thereto in an amount of equal to or greater than 15% by mass. It is possible to make the coefficient of thermal expansion of the lead 8 to be closer to the coefficient of thermal expansion of the insulating base 9 as the amount of silicon nitride is increased. In addition, when the amount of silicon nitride is equal to or less than 40% by mass, the resistance value of the lead 8 is decreased and stabilized. Therefore, the amount of silicon nitride is preferably 15% by mass to 40% by mass. More preferably, the amount of silicon nitride is 20% by mass to 35% by mass. It should be noted that the resistance value of the lead 8 per unit length may be made lower than that of the resistor 3 by making the amount of the forming material of the insulating base 9 smaller than that of the resistor 3, or by making the cross-sectional area of the lead 8 larger than that of the resistor 3.
The connection portion 2 is provided such that the resistor 3 and the lead 8 overlap each other in the direction perpendicular to the axial direction of the lead 8. It should be noted that the connection portion 2 refers to a region where the interface between the resistor 3 and the lead 8 is present, when being seen in a cross section parallel to the axis direction of the lead 8. For example, as shown in
Furthermore, the boundary between the resistor 3 and the lead 8 has a curved shape when the connection portion 2 is seen in a cross section perpendicular to the axial direction. In other words, the boundary surface between the resistor 3 and the lead 8 is a curved surface.
With such a configuration, a portion of a high-frequency component having propagated along the surface of the lead 8 the impedance of which portion cannot be matched at the connection portion 2 between the lead 8 and the resistor 3 is reflected and diffused at the connection portion 2, and dissipated as a Joule heat, and heat is locally generated in the connection portion 2. At that time, when the boundary between the resistor 3 and the lead 8 connected to each other has a curved shape, it is possible to make the directions of stress within the boundary surface, which is caused due to the fact that the coefficient of thermal expansion of the lead 8 is different from the coefficient of thermal expansion of the resistor 3, to be different from each other. Therefore, regardless of pulse drive or DC drive, even when rising at which power inrushes is steepened, occurrence of a micro crack in the connection portion 2 between the lead 8 and the resistor 3 is suppressed, a crack occurring in the boundary surface between the lead 8 and the resistor 3 is restrained from developing immediately, and the resistance value of the heater 1 is stabilized over a long period of time.
In other words, even with a driving method in which a control signal from an ECU is pulsed, occurrence of a micro crack in the connection portion 2 between the lead 8 and the resistor 3 is suppressed, a crack does not develop immediately in the boundary surface between the lead 8 and the resistor 31, and the resistance value of the heater 1 is stabilized over a long period of time.
In addition, even when pulse drive is not employed and DC drive is employed, the same advantageous effects are obtained. Specifically, when a high current is passed through the resistor at start of an engine operation for the purpose of quick temperature rise, rising at which power inrushes is steepened like a square wave of a pulse, and high power including a high-frequency component rushes into the heater. However, even when high power including a high-frequency component rushes into the heater, occurrence of a micro crack in the connection portion 2 between the lead 8 and the resistor 3 is suppressed, a crack does not develop immediately in the boundary surface between the lead 8 and the resistor 31, and the resistance value of the heater 1 is stabilized over a long period of time.
In addition, in the heater 1 shown in
As described above, with the configuration in which even though the steps are provided, the boundary between the resistor 3 and each lead 8 joined to each other has a curved shape when the connection portion 2 is seen in a cross section perpendicular to the axial direction, a structure is provided in which a shield is provided at 90□°□ for each step, and thus it is possible to further suppress a crack.
Furthermore, in the heater 1 shown in
Particularly, when a high DC current is passed through the resistor 3 at start of an engine operation for the purpose of quick temperature rise, rising at which power inrushes is steepened like a square wave of a pulse, and high power including a high-frequency component rushes into the heater. However, by making the rear end side of the connection portion 2 to have such a structure (have a curved shape so as to be convex at the lead 8 side), even when high power including a high-frequency component rushes into the heater, occurrence of a micro crack in the connection portion 2 between each lead 8 and the resistor 3 is suppressed, a crack does not develop immediately in the boundary surface between each lead 8 and the resistor 31, and the resistance value of the heater 1 is stabilized over a long period of time.
Furthermore, the cathode side of the heater 1 is grounded and a high DC current is passed through the resistor 3 at start of an engine operation for the purpose of quick temperature rise, a potential difference rapidly occurs between the anode side and the cathode side, electrons momentarily and rapidly flows in from the grounded cathode side, and thus the temperature rises at the cathode side earlier than at the anode side. Because of this, by making not only the connection portion 2 at the anode side but also the connection portion 2 at the cathode side to have such a structure (have a curved shape so as to be convex at the lead 8 side), heat is transmitted to the center of the heater and is distributed such that the center side is hot. By so doing, compressive stress is applied from the insulator, thus no crack occurs along the boundary surface between each lead 8 and the resistor 3, and the resistance value of the heater 1 is stabilized over a long period of time.
It should be noted that even with a driving method in which a control signal from an ECU is pulsed, the same advantageous effects are obtained.
Meanwhile, as shown in
When a short time elapses after start of passing of current, generation of heat is started from the heat generation region at the front end side of the heater 1 to cause the temperature to be higher than that of the connection portion 2, and the temperature of the resistor 3 becomes high earlier than each lead 8. Here, since the boundary between the resistor 3 and each lead 8 at least at the front end side of the connection portion 2 when being seen in a cross section perpendicular to the axial direction has a curved shape so as to be convex at the resistor 3 side, when heat of the resistor 3 is transmitted to the lead 8 side, the heat is transmitted such that the resistor 3 encompasses the lead 8. Thus, compressing stress, not tensile stress, is applied to the interface portion, and it is possible to suppress a crack in the interface.
Particularly, the following advantageous effects are obtained when the boundary between the resistor 3 and each lead 8 at the rear end side of the connection portion 2 (the lead 8 side) when being seen in a cross section perpendicular to the axial direction has a curved shape so as to be convex at the lead 8 side as shown in
In an initial stage when a high DC current is passed through the resistor 3 at start of an engine operation for the purpose of quick temperature rise, rising at which power inrushes is steepened like a square wave of a pulse, and high power including a high-frequency component rushes into the heater 1. Even when the high power including the high-frequency component rushes into the heater 1, occurrence of a micro crack in the connection portion 2 between each lead 8 and the resistor 3 is suppressed, and a crack does not develop immediately in the boundary surface between each lead 8 and the resistor 3. In addition, when, after start of passing of current, a short time elapses and generation of heat is started from the heat generation region at the front end side of the heater 1 to cause the temperature to be higher than that of the connection portion 2, the temperature of the resistor 3 becomes high earlier than each lead 8, and thus it is possible to alleviate stress.
As described above, it is possible to suppress occurrence of a micro crack in the connection portion 2, thus a crack odes not develop along the boundary surface, and the resistance value of the heater 1 is stabilized over a long period of time.
In addition, as shown in
Particularly, as shown in
In addition, as shown in
It should be noted that the metallic retaining member 7 (sheath metal fitting) is a metallic cylindrical body which retains the heater 1, and is joined to one of the leads 8 which is drawn out to the side surface of the ceramic base 9, by a solder material. In addition, the wire is joined to the other lead 8 drawn out to the rear end of another ceramic base 9. Thus, even when long-term use is made while ON/OFF is repeated in an engine at a high temperature, the resistance of the heater 1 is not changed. Therefore, it is possible to provide a glow plug which has good ignitability at any time.
Next, a method for manufacturing the heater 1 according to the embodiment will be described.
The heater 1 according to the embodiment may be formed by, for example, an injection molding method or the like using molds having the shapes of the resistor 3, the lead 8, and the insulating base 9.
First, a conductive paste which contains conductive ceramic powder, a resin binder, and the like and is to be the resistor 3 and the lead 8 is prepared, and a ceramic paste which contains insulating ceramic powder, a resin binder, and the like and is to be the insulating base 9 is prepared.
Next, a molded body of the conductive paste having a predetermined pattern which is to be the resistor 3 (a molded body a) is formed by an injection molding method or the like using the conductive paste. Then, in a state where the molded body a is retained within a mold, the conductive paste is injected into the mold to form a molded body of the conductive paste having a predetermined pattern which is to be the lead 8 (a molded body b). Thus, a state is provided in which the molded body a and the molded body b connected to the molded body a are retained within the mold.
Next, in the state where the molded body a and the molded body b are retained within the mold, a portion of the mold is replaced with a mold for molding the insulating base 9, and then the ceramic paste which is to be the insulating base 9 is injected into the mold. Thus, a molded body of the heater 1 (a molded body d) in which the molded body a and the molded body b are covered with a molded body of the ceramic paste (a molded body c) is obtained.
Next, the obtained molded body d is fired, for example, at a temperature of 1650° C. to 1800° C. under a pressure of 30 MPa to 50 MPa, whereby it is possible to produce the heater 1. The firing is preferably conducted in a non-oxidizing gas atmosphere such as hydrogen gas.
Heaters according to examples of the present invention were produced as follows.
First, injection molding of a conductive paste containing 50% by mass of tungsten carbide (WC) powder, 35% by mass of silicon nitride (Si3N4) powder, and 15% by mass of a resin binder was conducted within a mold to produce a molded body a which is to be a resistor.
Next, in a state where the molded body a was retained within a mold, the above conductive paste which is to be the lead was injected into the mold to be connected to the molded body a, to form a molded body b which is to be the lead. At that time, as shown in
Next, in a state where the molded body a and the molded body b were retained within a mold, injection molding of a ceramic paste containing 85% by mass of silicon nitride (Si3N4) powder, 10% by mass of an oxide (Yb2O3) of ytterbium (Yb) as a sintering aid, and 5% by mass of tungsten carbide (WC) for making a coefficient of thermal expansion to be close to those of the resistor and each lead was conducted within the mold. By so doing, a molded body d was formed which has a configuration in which the molded body a and the molded body b are buried in a molded body c which is to be an insulating base.
Next, the obtained molded body d was placed into a cylindrical mold made of carbon, and then sintered by conducting hot pressing at 1700° C. under a pressure of 35 MPa in a non-oxidizing gas atmosphere composed of nitrogen gas to produce a heater. A metallic cylindrical retaining member was soldered to a lead end portion (terminal portion) exposed on the surface of the obtained sintered body, to produce a glow plug.
A pulse pattern generator was connected to an electrode of the glow plug, a rectangular pulse having an applied voltage of 7 V, a pulse width of 10 μs, and a pulse interval of 1 μs was continuously passed therethrough. After 1000 hours elapsed, the change rate of the resistance value before and after the current passing ((resistance value after current passing−resistance value before current passing)/resistance value before current passing) was measured. The results are shown in Table 1.
As shown in Table 1, in Sample number 1, the location where heat was generated most was a connection portion between the lead and the resistor. When a pulse waveform flowing through the heater of Sample number 1 was checked with an oscilloscope in order to check a conduction state, rising of the pulse was not steepened unlike an input waveform, and 1 μs was taken until reaching 7V, and the waveform was wavy with overshoot.
This is thought that in the heater of Sample number 1, a high-frequency component contained in a rising portion of the pulse was reflected at the boundary surface between the lead and the resistor, since its impedance was not matched at the boundary surface. In addition, the reason why the location in the heater where heat was generated most was the connection portion between the lead and the resistor is thought to be that heat was locally generated in the connection portion between the lead and the resistor due to the reflection of the high-frequency component.
Furthermore, the resistance change in Sample number 1 between before and after the current passing was 55% and very great. Thus, when the connection portion between the lead and the resistor in Sample number 1 was observed with a scanning electron microscope after the pulse passing, it was confirmed that a micro crack occurred in the boundary surface from an outer peripheral direction toward the inside.
Meanwhile, in Sample numbers 2 to 4, the location where heat was generated most was the resistor heat-generating portion at the heater front end. When a pulse waveform flowing through the heater was checked with an oscilloscope in order to check a conduction state, rising of the pulse was substantially the same as an input waveform.
This shows that the current was able to flow through the connection portion between the lead and the resistor without abnormally generating heat in the connection portion.
In addition, the resistance changes in Sample numbers 2 to 4 between before and after the current passing were equal to or less than 5% and were small. When the connection portion between the lead and the resistor in these sample numbers was observed with a scanning electron microscope after the pulse passing, no micro crack was observed.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2011-099603 | Apr 2011 | JP | national |
This application is a continuation of U.S. application Ser. No. 14/113,922 filed Oct. 25, 2013, which claims the benefit of PCT Patent Application No. PCT/JP2012/061374 filed Apr. 27, 2012, which claims priority to JP 2011-099603 filed Apr. 27, 2011, the entire contents of all of which are expressly incorporated by reference herein as if fully set forth.
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