JPH07220859A - Ceramic heating element - Google Patents

Abstract

PURPOSE:To improve durability and reliability by setting the outermost periphery of the heating resistor layer A of the highest heating section to the specific & of the outer diameter of an electrically insulating ceramic sintered body B, locating the tip of the layer A on the inside from the tip of the sintered body B by the specific mm, and forming the cross section on the layer A on the sintered body B side in a circle. CONSTITUTION:The outermost periphery 9 of a heating resistor layer 3 corresponding to a U-shaped highest heating section 8 of a ceramic heating element 1 is set to 6-25% of the outer diameter 10 of an electrically insulating ceramic sintered body 2. The tip of the layer 3 is buried to the inside of the sintered body 2 by a distance L of 0.3-1.5mm from the tip of the sintered body 2, and a heating resistor layer 11 having the resistance lower than that of the layer 3 and nearly the same width is laminated at the end section of the layer 3. End sections of the layer 11 are made narrower in width than the layer 3, protruded from the end sections of the layer 3, and connected to electrode extracting layers 6, 7 via lead sections 4, 5. The tip of the sintered body 2 is formed into nearly a sphere, and the cross section where the layer 3 is buried is formed into a circle. The thermal shock resistance of l000 deg.C or above is provided, and durability and reliability can be improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a ceramic heating element for high temperatures of 1000 ° C. or higher, and particularly to a self-saturation type glow plug which rapidly preheats a sub combustion chamber at the time of starting a diesel engine or idling, and a liquid or gas. And various heaters using oil and gas, and heaters for heating and igniting such as instant water heaters.

[0002]

2. Description of the Related Art Nickel (Ni) is used in a sheath made of heat-resistant metal as a heater for heating and igniting glow plugs, various heaters, heaters, instantaneous water heaters, etc., which have been conventionally used to accelerate the starting of diesel engines. A sheathed heater has been used in which a heating resistor made of a high melting point metal wire mainly composed of chromium (Cr) or the like is filled and embedded together with heat resistant insulating powder.

However, since the sheathed heater transfers heat of the heat-generating resistor through the heat-resistant insulating powder filled in the sheath made of heat-resistant metal, heat conduction is poor and rapid temperature rise in a short time is difficult. In addition, there is a problem in that the durability is poor such as wear resistance, heat resistance, and corrosion resistance, and there is concern about safety such as reliable ignition.

Therefore, a heating resistor made of an inorganic conductive material is used as a heating element excellent in durability such as wear resistance, heat resistance, corrosion resistance and the like, which is capable of rapid temperature rise in a short time, and has excellent thermal conductivity. Ceramic heating elements embedded and integrated in an insulating ceramic sintered body have come to be widely used as glow heating heaters for internal combustion engines, as well as various heating and ignition heaters described above.

However, the ceramic heating element is
Due to the good thermal conductivity of the electrically insulating ceramic sintered body, the embedded state of the heating resistor greatly affects the temperature distribution of the heating region, and it is difficult to obtain a predetermined temperature range uniformly at a predetermined position. There has been a problem that cracks are easily generated in the ceramic heating element itself due to a difference in thermal expansion between the electrically insulating ceramic sintered body and the heating resistor, which easily causes disconnection.

In order to solve the above-mentioned problem, as shown in FIG. 4, heat generation of a refractory metal whose position and dimensions are specified in an electrically insulating ceramic sintered body 16 having high strength and excellent oxidation resistance. A ceramic heater 17 in which the resistor 15 is embedded has been proposed (see Japanese Utility Model Laid-Open No. 2-20293 and Japanese Utility Model Laid-Open No. 5-31191).

[0007]

In the ceramic heater 17, the temperature distribution of the heating portion is close to the design value, and the occurrence of cracks due to the difference in thermal expansion between the electrically insulating ceramic sintered body 16 and the heating resistor 15 is reduced. It is supposed to be.

However, the ceramic heater 17
When used as a ceramic heating element for high temperatures of 1000 ° C or higher, for example, in a glow plug of a diesel engine, the maximum heat generating part is designed within 5 mm from the tip to prevent reliable ignition and startability. Because heat is used, liquid fuel with a temperature extremely lower than the exothermic temperature may suddenly come into direct contact with the surface of the ceramic heating element of the highest exothermic part that glows red near the tip when starting, while other heating ignition Even when the heater for
Various media such as liquid or gas having a low temperature and fuel may directly contact the highest heat generating portion near the tip of the heating and ignition heater, which is heated and red.

In such a case, the temperature difference may exceed 950 ° C., and since the temperature difference often occurs locally near the highest heat generating portion at the tip, the temperature difference due to the temperature difference during operation is caused. Impact may be applied to the ceramic heating element, which may cause cracks in the ceramic heating element to cause the heating resistor itself to be disconnected, leaving anxiety about durability, as well as a problem of inferior safety and reliability of reliable ignition. there were.

[0010]

SUMMARY OF THE INVENTION The present invention has been developed in view of the above-mentioned drawbacks, and an object thereof is to maintain a low temperature liquid or gaseous fuel, a heating medium or the like while operating while maintaining tip heat generation and rapid temperature rising characteristics. Is abruptly directly contacted with the vicinity of the maximum heat generation part of the ceramic heating element and is locally subjected to a thermal shock that causes an extremely large temperature difference, cracks occur in the ceramic heating element and the heating resistor itself is disconnected. It is to provide a ceramic heating element that has high strength and withstands repeated heating / cooling during continuous operation for a long time, and has excellent durability and reliability such as wear resistance, heat resistance, and corrosion resistance. .

[0011]

A ceramic heating element of the present invention comprises a heating resistor layer made of, for example, two or more layers of inorganic conductive material formed by, for example, a screen printing method in an electrically insulating ceramic sintered body. A lead portion made of a wire material of a high melting point metal connected to the heating resistor layer and a layered electrode lead-out layer made of an inorganic conductive material connected to the lead portion are embedded in an electrically insulating ceramic sintered body. Integrated, at least the outermost periphery of the heating resistor layer surface of the highest heating portion,
A distance 1 of 6 to 25% with respect to the outer diameter of the electrically insulating ceramic sintered body is located inside the surface of the electrically insulating ceramic sintered body, and the tip of the heating resistor layer is made of the electrically insulating ceramic sintered body. It is located 0.3 to 1.5 mm inside from the tip, and is configured such that the cross-sectional shape of the tip side from at least the highest heat generating portion is circular, and at least the highest heat generating portion of the electrically insulating ceramic sintered body is It is more desirable to have a thermal shock resistance with a temperature difference of 1000 ° C. or more, or to have a surface roughness of 0.4 to 3.0 μm R _max .

[0012]

According to the ceramic heating element of the present invention, a heating resistor layer of an inorganic conductive material, a lead portion connected to the heating resistor layer, and a lead portion connected to the lead portion are provided in an electrically insulating ceramic sintered body. And the outermost periphery of the heating resistor layer of at least the highest heat generating portion is embedded at a distance 1 of 6 to 25% of the outer diameter of the electrically insulating ceramic sintered body. The inside of the body surface and the tip of the heating resistor layer should be located 0.3 to 1.5 mm inside the tip of the electrically insulating ceramic sintered body, and at least the cross section from the highest heat generating portion to the tip side. The shape is circular, and more preferably, at least the highest heating portion of the ceramic heating element has a thermal shock resistance with a temperature difference of 1000 ° C. or more, or has a surface roughness of 0.4 to 3.0 μm R _max. , Electric The strength of the insulating ceramic sintered body is maintained, electrically insulating ceramic sintered body which surrounds the heating resistor layer from good thermal conductivity is one which acts to reduce thermal shock.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A ceramic heating element of the present invention will be described below with reference to the drawings. FIG. 1 is a perspective view showing an embodiment of the ceramic heating element of the present invention, and FIG. 2 is a view showing a cross section of the highest heating portion of the ceramic heating element of the present invention.

In FIGS. 1 and 2, reference numeral 1 denotes an electrically insulating ceramic sintered body 2, a heating resistor layer 3 made of a layered inorganic conductive material, and a lead portion 4 connected to the heating resistor layer 3.
5 and the electrode extraction layers 6 and 7 connected to the lead portions 4 and 5.
Is a ceramic heating element in which is embedded.

In the ceramic heating element 1, at least the outermost periphery 9 of the heating resistor layer 3 corresponding to at least the highest heating portion 8 having a substantially U-shape has an outer diameter 10 of the electrically insulating ceramic sintered body 2. Only the distance 1 of 6 to 25%, and the tip of the heating resistor layer 3 is 0.3 from the tip of the electrically insulating ceramic sintered body 2.
Each of them is buried inside the surface of the electrically insulating ceramic sintered body 2 by a distance of up to 1.5 mm.

A heating resistor layer 11 having a resistance lower than that of the heating resistor layer 3 and having substantially the same width is laminated on each end of the heating resistor layer 3 made of a substantially U-shaped layered inorganic conductive material. The end portions of the heating resistor layer 11 are formed to be narrower than the width of the heating resistor layer 3 so as to project from the respective ends of the heating resistor layer 3, and tungsten (W) of tungsten is formed through the protruding heating resistor layer 11. The lead parts 4 and 5 made of wire are connected.

Next, at the other ends of the lead portions 4 and 5, an electrode lead-out layer 6 divided into a plurality of layered layers made of an inorganic conductive material,
7 are formed so as to be connected to each other and embedded, and a part of the electrode lead-out layers 6 and 7 is exposed on the outer peripheral surface of the ceramic sintered body 2, and the electrically insulating ceramic sintered body on the side of the heating resistor layer 3 is also formed. 2 has a substantially spherical tip, and at least the ceramic sintered body 2 in which the heating resistor layer 3 is embedded has a circular cross section.

The heating resistor layer 3 has two or more heating resistor layers formed by alternately laminating the heating resistor layers and the electrically insulating ceramic layers in the electrically insulating ceramic sintered body 2. May be.

The position of the outermost periphery 9 of the heating resistor layer 3 corresponding to at least the highest heat generating portion 8 is at least the outer diameter 1 of the electrically insulating ceramic sintered body 2 corresponding to the highest heat generating portion 8.
At a distance 1 of 6 to 25% with respect to 0 and the heating resistor layer 3
From the tip of the electrically insulating ceramic sintered body 2 to the tip of.
When it is located at a distance of 3 to 1.5 mm, it can generate heat at the tip and has a large thermal shock resistance with a temperature difference of 950 ° C. or more without impairing the rapid temperature rising characteristics, but has a high cooling cycle. Considering the durability in the additional durability test, the position of the outermost periphery 9 is 11 to 24%, and the position of the tip of the heating resistor layer 3 is preferably 0.5 to 1.3 mm.
In particular, considering the rate of resistance change, the position of the outermost periphery 9 is 16 to 19%, and the position of the tip of the heating resistor layer 3 is 0.
8 to 1.2 mm is most desirable.

On the other hand, the value of the surface roughness of the electrically insulating ceramic sintered body 2 corresponding to at least the highest heat generating portion 8 is
The smaller the size, the more desirable it is, but from the viewpoint of polishing cost, the surface roughness is preferably 0.4 to 3.0 μm R _max , and considering the strength, 0.6 to 1.8 μm R _max.
Is good, and considering the point of thermal shock resistance, 0.8-1.
Most preferred is 5 μm R _max .

Further, it is necessary to set the conduction resistance of the inorganic conductive material of the electrode lead-out layers 6 and 7 to be lower than that of the heating resistor layer 3 by about 100 to 160Ω, and the electrode lead-out layers 6 and 7 are divided into a plurality of parts. In addition, the electrode lead-out layers 6 and 7 may be provided in the same number as the heating resistor layer 3, or may be formed as a single layer by being electrically connected through a through hole or the like.

FIG. 3 is a cross-sectional view of essential parts showing an embodiment in which the ceramic heating element of the present invention is applied to a self-saturation type glow plug used for promoting the starting of a diesel engine.

In FIG. 3, in the electrically insulating ceramic sintered body 2, a heating resistor layer 3 made of two layers of inorganic conductive material, and lead portions 4 and 5 connected to each end of the heating resistor layer 3 are formed. , A cylindrical metal fitting 12 is externally fitted to the ceramic heating element 1 having a circular cross-sectional shape in which a plurality of divided electrode lead-out layers 6 and 7 connected to the respective lead parts 4 and 5 are embedded. The electrode lead-out layer 7 and the electrode lead-out metal fitting 13 exposed at one end of the ceramic heating element 1 are brazed to the electrode lead-out layer 6 exposed by polishing the side surface to form one electrode terminal.
Are connected to each other and led out as the other electrode terminal, assembled to the fitting 14 to be electrically connected, and a positive and negative electrodes are insulated from each other to form a self-saturation type glow plug.

The electrically insulating ceramic sintered body of the ceramic heating element of the present invention is a silicon nitride sintered body containing silicon nitride (Si ₃ N ₄ ) as a main component, which is excellent in oxidation resistance and strength at high temperatures. And sialon are preferred.

The main component of the heating resistor layer or the layered electrode lead-out layer made of an inorganic conductive material is a refractory metal such as tungsten (W), molybdenum (Mo), rhenium (Re) or tungsten-rhenium (W). In addition to alloys such as —Re), there are, for example, carbides or nitrides of Group 4a, Group 5a, and Group 6a such as tungsten carbide (WC), titanium nitride (TiN) and zirconium boride (ZrB ₂ ). In particular, tungsten carbide (WC) is most preferable in terms of ease of design and durability.

Furthermore, when the electrically insulating ceramic sintered body is a silicon nitride sintered body containing silicon nitride (Si ₃ N ₄ ) as a main component, the heating resistor layer or the electrode take-out layer is made of tungsten carbide ( WC) as a main component and silicon nitride (Si ₃ N ₃ ) as a main component of the electrically insulating ceramic sintered body.
₄ ) From the viewpoint of thermal expansion, it is preferable to appropriately add and mix powder.

That is, a heating resistor made of the inorganic conductive material
When the layer is formed by, for example, a screen printing method,
Considering the coefficient of thermal expansion, tungsten carbide (WC) is 65
~ 95 wt%, silicon nitride (Si₃N_Four) Is 5 to 35 weight
% Composition, especially carbonized tungsten.
75 to 90% by weight of silicon (WC), silicon nitride (Si
₃N _Four) 10 to 25% by weight of the composition for high temperature ceramic
(C) It is most preferable from the viewpoint of the coefficient of thermal expansion as a heating element.

Further, in order to prevent the heating resistor layer from being broken during manufacturing, the thickness of the heating resistor layer is preferably 2.3 to 150 μm at least in the highest heat generating portion, and particularly, the sintered integral layer. In order to prevent inconveniences such as cracks from occurring in the heat-generating resistor layer when the temperature is reduced, the thickness of the heat-generating resistor layer is at least 8 to 53 μm at the highest heat generating portion.
A range of m is desirable.

On the other hand, in the lead portion, tungsten (W), molybdenum (Mo), rhenium (R
In addition to e) and the like, alloys such as tungsten-rhenium (W-Re) can be cited. However, a wire rod made of tungsten (W), which has a low specific resistance and whose characteristics are hard to change even after firing and integration, is preferable, The resistance value is preferably about 80 to 120Ω, and the wire diameter is preferably about 6 to 15% of the outer diameter of the ceramic heating element.

In evaluating the ceramic heating element of the present invention, first, a high-purity silicon nitride (Si ₃ N ₄ ) powder is added.
A granulated product prepared by adding and mixing yttria (Y ₂ O ₃ ) or an oxide of a rare earth element as a sintering aid is used, and flat plate-shaped silicon nitride is used as a main component by a well-known molding method such as a press molding method. A ceramic molded body is manufactured.

Next, a solvent is added to a raw material powder prepared by mixing a predetermined amount of each fine powder of tungsten carbide (WC) and silicon nitride (Si ₃ N ₄ ) to prepare a paste, and the paste is prepared by screen printing based on the designed resistance value. A pattern having a substantially U-shaped pattern of various dimensions and a pattern having a projecting portion with a narrow width overlapped on an end of the substantially U-shaped pattern are sequentially formed on a surface of the ceramic molded body, and the pattern is formed from a side surface of the ceramic molded body. Various distances to the outermost periphery of the pattern were set.

On the other hand, for the electrode take-out layer, a heating resistance of the ceramic molded body was prepared by using a paste composed of 85% by weight of tungsten carbide (WC) and 15% by weight of fine powder of silicon nitride (Si ₃ N ₄ ). Four patterns each having a width of about 0.7 mm and a thickness of about 70 μm were formed on the end surface on the side opposite to the body layer in a predetermined arrangement in parallel to the side surface of the ceramic molded body, each having a width of about 0.7 mm and a thickness of about 70 μm.

In addition, boron nitride (BN) can be appropriately added to the paste for forming the heating resistor and the electrode lead-out layer to adjust the thermal expansion with the electrically insulating ceramic sintered body.

Next, between the ceramic molded body in which the heating resistor layer and the electrode lead-out layer are formed by printing on the same surface, and another ceramic molded body, a diameter of 0.
Heat generated in the above-mentioned U-shape through a low-resistance heating resistor layer formed by stacking a 25 mm tungsten (W) wire on the heating resistor layer made of a substantially U-shaped layered inorganic conductive material. It is sandwiched so that it is connected to the resistor layer and the electrode lead-out layer, respectively, and the same tungsten (W) wire as the above is sandwiched therewith with another ceramic molded body on which the heating resistor layer and the electrode lead-out layer are not formed by printing. 1 at a temperature of 1750 ° C. under a reducing atmosphere containing carbon (C)
It was baked under pressure for a time.

The ceramic sintered body having the two heating resistor layers thus obtained was ground around its circumference, and the tip on the heating resistor layer side was formed into a substantially spherical surface and processed into a circular cross section, and each embedded electrode was taken out. The end surface of the layer was exposed on the side surface of the cylinder, and a ceramic heating element having a diameter of about 3.5 mm and a length of about 54 mm was produced.

Next, a metal coating of nickel (Ni) or the like is formed on at least the exposed portion of the electrode heating layer of the ceramic heating element by a metallizing method or a plating method, and then one electrode exposed on the side surface of the ceramic heating element. A tubular metal fitting is externally fitted so as to be connected to the take-out layer, joined with silver brazing in a reducing atmosphere to form a negative electrode, and the other electrode take-out layer is a wire rod or a cap-like metal fitting. In the same manner as above, they were joined with silver brazing and connected as a positive electrode, and a positive and negative electrode was derived to produce a ceramic heating element for evaluation.

Next, using the ceramic heating element for evaluation, a direct heating voltage of 11 to 24 V was applied to reach the saturation temperature, and then the ceramic heating element was exposed from the cylindrical metal fitting in a non-contact manner with a radiation thermometer. After the surface temperature distribution from the tip of the is measured and the position of the highest heat generating portion is specified, the ceramic heat generating element for evaluation is radiographed by X-ray, and the photographic film is magnified with a projector to correspond to the highest heat generating portion. The dimension from the outermost periphery of the heating resistor layer at the position to the surface of the electrically insulating ceramic sintered body was measured, and the distance 1 to the outer diameter of the electrically insulating ceramic sintered body was calculated.

Further, the design value was confirmed by measuring the distance L from the tip of the ceramic heating element to the tip of the embedded heating resistor layer from the X-ray radiographic film in the same manner as described above.

On the other hand, the side surface including at least the highest heat generating portion of the ceramic heating element for evaluation is longitudinally aligned with JIS-B0.
The surface roughness was measured by R _max according to the standard of 601.

Based on the above results, a direct current voltage of 11 to 24 V was applied to the ceramic heating element for evaluation to generate heat, and it was confirmed that the set temperature was reached while the temperature of the highest heating part was measured without contact. After that, the ceramic heating element exposed from the cylindrical metal fitting is immersed in the molten solder bath three times, and the thermal shock of the temperature difference between the highest heating point and the solder bath is applied. Then, the presence or absence of cracks is visually inspected with a stereoscopic microscope. At the same time, the thermal shock resistance was evaluated by the temperature difference that did not generate cracks by detecting with the fluorescent penetrant flaw detection method.

A rapid heating characteristic was evaluated by using a part of the ceramic heating element for evaluation and applying a direct current voltage of 11 V to obtain the time from the application of electricity until the temperature reaches 800 ° C. did. Furthermore, a high load endurance test in which one cycle includes a step in which a direct current voltage of 11 to 24 V is applied to the ceramic heating element for evaluation for 5 minutes, the energization is stopped, and compressed air is blown for 2 minutes for forced cooling, When the resistance value of the ceramic heating element measured before and after the test was changed up to 10,000 cycles and changed by 20% or more after the durability test, it was judged as defective and the durability was evaluated. The above results are shown in Table 1.

[0042]

[Table 1]

The self-saturation glow plug incorporating the evaluation ceramic heating element was attached to a bench test apparatus of a 4-cylinder diesel engine, and the fuel was brought into direct contact with the exposed ceramic heating element while changing the fuel injection angle. As a result, a durability test was conducted in which the diesel engine was operated for 100 hours under conditions of maximum rotation speed and full load. However, it was confirmed that the diesel engine was operating normally, and a non-destructive inspection was conducted after the test. No abnormalities such as cracks were observed in any of the self-saturating glow plugs using the ceramic heating element according to the invention.

Even when the ceramic heating element of the present invention is used as a heater for heating and igniting, cracks are not generated due to thermal shock, and it is possible to securely ignite and ensure safety. confirmed.

[0045]

As described above, the ceramic heating element of the present invention has a heating resistor layer made of two or more layers of inorganic conductive material and connected to the heating resistor layer in an electrically insulating ceramic sintered body. The lead portion and the electrode lead-out layer connected to the lead portion are embedded, and at least the outermost periphery of the heating resistor layer corresponding to the highest heating portion is 6 to 25% of the outer diameter of the electrically insulating ceramic sintered body. And the tip of the heating resistor layer is 0.3 to 1.5 mm from the tip of the electrically insulating ceramic sintered body.
Since it is located inside the surface of the electrically insulating ceramic sintered body by a distance of at least, and the cross-sectional shape from the highest heat generating portion to the tip side is circular, at the same time, maintaining the tip heat generation, without impairing the rapid temperature rise characteristics, Even if the ceramic heating element receives a thermal shock that causes an extremely large temperature difference during operation,
A self-saturating glow for diesel engines that has excellent durability and reliability that can withstand thermal history during long-term continuous operation without cracking the ceramic heating element and ensure safety by igniting reliably. It is possible to obtain a ceramic heating element that is most suitable as a heater for various heating and ignition including plugs.

[Brief description of drawings]

FIG. 1 is a sectional view showing an embodiment of a ceramic heating element according to the present invention.

FIG. 2 is a view showing a cross section of the highest heat generating portion of the ceramic heat generating element according to the present invention.

FIG. 3 is a cross-sectional view of a main part showing an embodiment in which the ceramic heating element according to the present invention is applied to a self-saturation glow plug used to accelerate starting of a diesel engine.

FIG. 4 is a sectional view showing a main part of a conventional ceramic heater applied to a glow plug of a diesel engine.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Ceramic heating element 2 Electrically insulating ceramic sintered body 3 Heating resistor layer 4, 5 Lead part 6, 7 Electrode extraction layer 8 Maximum heat generating part 9 Outermost periphery 10 Outer diameter

Claims (3)

[Claims] 1. A heating resistor layer made of an inorganic conductive material comprising two or more layers, a lead portion connected to the heating resistor layer, and an electrode connected to the lead portion in an electrically insulating ceramic sintered body. A layer is embedded, and at least the outermost periphery of the heating resistor layer of the highest heat generating portion is separated from the surface of the electrically insulating ceramic sintered body by a distance 1 of 6 to 25% of the outer diameter of the electrically insulating ceramic sintered body. It is characterized in that the tip of the heating resistor layer is located inside and 0.3 to 1.5 mm inside the tip of the electrically insulating ceramic sintered body, and the cross-sectional shape of at least the highest heat generating portion from the tip side is circular. And a ceramic heating element. 2. An electrically insulating ceramic sintered body in which a heating resistor layer of the inorganic conductive material is embedded has at least the highest heat generating portion having a thermal shock resistance of 1000 ° C. or higher. Ceramic heating element. 3. The surface roughness of at least the highest heat generating portion of the electrically insulating ceramic sintered body in which the heat generating resistor layer of the inorganic conductive material is embedded is 0.4 to 3.0 μm R _max. The ceramic heating element according to claim 1.