JPH0546674B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a ceramic heater, and particularly to a ceramic heater that is effectively applied to glow plugs of diesel engines and the like.

[Prior art]

Glow plugs are used in diesel engines as starting parts at low temperatures, and glow plugs that heat quickly are required to improve engine starting performance. In order to meet this demand, the inventors added molybdenum silicide (MoSi ₂ ), which is an oxidation-resistant conductive ceramic, and silicon nitride (Si ₃ N, which is an insulating ceramic with a low coefficient of thermal expansion) to the outer periphery of the tip of the electrically insulating ceramic member. ₄ ) A glow plug was developed in which a heater was formed by sintering the mixed powder, and the heater was installed so that it was exposed to the atmosphere of the combustion chamber (Japanese Patent Application Laid-Open No. 1983-1999).
No. 126484). In this glow plug, the heater directly heats the inside of the combustion chamber, so it has excellent heat-up properties. In addition, MoSi ₂ , which is a component of the heater, gives the heater oxidation resistance, and Si _3, which has a low coefficient of thermal expansion,
_N4 gives the heater thermal shock resistance.

By the way, although this heater itself has very good thermal shock resistance, the joint between the insulating ceramic member that supports the heater and the heater formed around it has various coefficients of expansion, thermal conductivity, etc. There is a problem in that thermal stress is generated due to the difference in properties, which reduces thermal shock resistance.

As a countermeasure to this problem, the inventors constructed a heater with a central part and an outer peripheral part surrounding the central part with a U-shaped cross section, and both the central part and the peripheral part were made of molybdenum silicide (MoSi ₂ ) and silicon nitride (Si ₃ N ₄ ), the mixing ratio of both components is the same in both parts, and the specific resistance of both parts is adjusted by the particle diameter of both components, so that only the outer peripheral part is energized (Unexamined Japanese Patent Publication) (Sho 60-254586).

However, even in this ceramic heater, when the supporting member made of silicon nitride and alumina (Al ₂ O ₂ ) and the heater are sintered together, the optimal firing conditions for both members are different, so the excellent characteristics of both members are different. cannot be obtained at the same time.

Furthermore, the support member of a ceramic heater is housed and fixed in a housing made of metal, and the fixing method is generally to form a nickel plating layer on the surface of the support member and braze the support member to the housing via this plating layer. Measures will be taken. However, Si ₃ N ₄ --Al ₂ O ₃ based sintered bodies have a problem in that the adhesion of nickel plating is not good and sufficient bonding strength cannot be obtained.

[Problems to be solved by the present invention]

Therefore, the present invention provides a ceramic heater in which a heater made of a conductive ceramic sintered body is integrally sintered on the outer periphery of the tip of a support member made of an insulating ceramic sintered body, which has excellent bonding strength between the support member and the heater. It is an object of the present invention to provide a ceramic heater in which the supporting member and the heater can each exhibit their respective characteristics to the maximum, thereby solving the above-mentioned conventional problems.

Another object of the present invention is to provide a ceramic heater in which a support member and a housing are firmly connected.

[Means for solving problems]

In the ceramic heater of the present invention, both the support member and the heater are made of MoSi ₂ and Si ₃ N ₄ , and the mixing ratio of both components is the same for both the support member and the heater, and the support member and the heater are integrally sintered. The blending weight ratio of MoSi ₃ and Si ₃ N ₄ is 10 to 40:
The range should be between 60 and 90.

The supporting member consists of _two conductive MoSi particles and an insulating material.
It is surrounded by Si ₃ N ₄ particles and divided, creating an electrically insulated structure, and the heater uses Si ₃ N ₄ particles to
A conductive structure is formed in which _two MoSi particles are wrapped and _two MoSi particles are continuous.

A means of achieving the above-mentioned structure in the support member is to add and sinter a sintering aid that has poor wettability with MoSi ₂ or forms a glass layer with extremely low viscosity during sintering. As a result, the MoSi ₂ particles aggregate to form a sintered structure surrounded by Si ₃ N ₄ particles, as shown in the model in FIG. Examples of sintering aids include Al ₂ O ₃ , MgAl ₂ O ₄ , and others.
_Y2O3 may be used _.

The MoSi ₂ and Si ₃ N ₄ powders used for sintering the support member have an average particle diameter of almost the same for both components, or
Make the MoSi ₂ powder larger.

In the heater, the Si ₃ N ₄ powder used has a larger average particle diameter than the MoSi ₂ powder. By making the MoSi ₂ powder sufficiently small and large enough, the MoSi ₂ particles become Si ₃ N ₄ as shown in the model in Figure 3.
The continuous MoSi _particles surrounding the particles form a sintered structure with electrical conductivity. The specific resistance of the heater can be controlled by adjusting the relative particle diameters and amounts of MoSi ₂ and Si ₃ N ₄ . The average particle size of Si ₃ N ₄ is
It is desirable that it be about twice or more than that of MoSi ₂ .

[Effect]

In this way, in the present invention, the support member and the heater have substantially the same components and compositions, so when they are sintered together, both can be sintered under the same optimum conditions for the support member and the heater. Optimum characteristics are obtained, and both are strongly integrated. Furthermore, cracks do not occur at the joint due to the difference in coefficient of thermal expansion between the two.

Furthermore, the presence of MoSi ₂ improves the adhesion of the support member to the nickel plating, resulting in a good bonding force with the metal housing.

[Example 1] Hereinafter, the details of the present invention will be explained using the following examples and experimental examples in which the present invention is applied to a glow plug.

(1) As shown in FIG. 1, a plate-shaped protrusion 21 formed at the tip of a rod-shaped support member 2 having a square cross section is provided with a heater 1 having a U-shaped cross section surrounding it.
Tungsten conductive wires 3a and 3b whose tips are connected to the heater 1 are embedded in the support member 2.
A metal pipe 4 is attached to the outer periphery of the support member 2,
One end of a cylindrical metal housing 5 is joined to the pipe 4. The rear end of the energizing wire 3a is connected to the support member 2
It is connected to a metal cap 6 that extends to the base end of the cap and is fitted onto the base end, and is connected to a power source (not shown) via the cap 6 and a nickel wire 7. The rear end of the current-carrying wire 3b is connected to a metal sleeve.

Heater 1 and support member 2 are both MoSi ₂ 30%
(indicates weight %, same below) and Si ₃ N ₄ 70%
The support member 2 and the heater 1 are a sintered body of a mixture in which Y ₂ O ₃ and Al ₂ O ₃ are added in an amount of 7% and 3 _% , respectively, based on _the total amount of MoSi ₂ and Si 3 N 4 . It is sintered in one piece.

The average particle size of the MoSi ₂ powder is 0.9 μm in both support member 2 and heater 1, and the average particle size of the Si ₃ N ₄ powder is 0.6 μm in support member 2 and heater 1.
It is 13 μm.

The support member 2 and the pipe 4 are joined together by nickel plating the surface of the support member 2 and then brazing. Also pipe 4
and the housing 5 are joined together by brazing.

In the glow plug of the present invention having the above structure,
Support member 2 and heater 1 at 1560℃~1760℃, 500℃
Tests were conducted on the thermal shock resistance of the heater 1 and its joints, and the joint strength of the support member 2 and the pipe 4, which were integrally sintered by hot pressing at Kg/mm ² under 1 atmosphere of argon.

In addition, as a comparative material, support member 2 was made of 50% Si ₃ N ₄
−50% Al ₂ O ₃ and the other structure was the same as that of the present invention (Comparative Example 1), and supporting member 2 was 50%
MoSi ₂ −50% Al ₂ O ₃ and heater 1 is the first
It is composed of a central part corresponding to the protrusion 21 in the figure and a part surrounding it, and both parts are made into a sintered body made of MoSi ₂ and Si ₃ N ₄ with a common blending ratio, and by manipulating the particle size of the components. The center part is in a state close to insulation, and this heater is integrally sintered with the support member containing Al ₂ O ₃ , and the other parts have the same structure as the present invention (Comparative Example 2, Utility Model Application No. 60-254586) A similar test was also conducted for

Thermal shock resistance is tested by a spalling test, in which the glow plug is saturated with heat to a predetermined temperature by applying a voltage, and then the tip protruding from the pipe 4 is immersed in water at 20°C to determine whether or not there are any cracks on the surface. It was evaluated by investigating. The bonding strength was evaluated by applying pressure to the tip of the heater 1 in the direction of the pipe 4 and determining the limit point at which the heater 1 sank into the pipe 4. The results are shown in Tables 1 and 2. In Table 1, the x mark indicates the occurrence of cracks.

As is known from Table 1, in the present invention, the heater and its supporting member are made of the same material, so
Thermal stress caused by the difference in coefficient of thermal expansion and thermal conductivity between the two can be significantly reduced, and therefore the thermal shock resistance is greatly improved. In addition, as is known from Table 2, since MoSi ₂ is dispersed in the support member, the adhesion of the nickel plating to the support member is improved.
Therefore, the bonding strength between the support member and the metal pipe including the support member is significantly improved.

(2) The following experiment was conducted to investigate the insulation properties of the Si ₃ N ₄ -MoSi ₂ sintered body used as the support member.

Si ₃ N ₄ 70%, MoSi ₂ 30 with different average particle sizes
% of the mixed powder was added with varying amounts of a sintering aid, and then hot press fired.

That is, a mixed powder containing Si ₃ N ₄ and MoSi ₂ in the above ratio is mixed with a solvent such as ethanol,
After stirring, di-butyl phthalate as a plasticizer and poly-vinyl-butyral (degree of polymerization 1000) as a binder are added and further kneaded to prepare a slurry with a viscosity of 3 x 10 ⁴ to 10 x 10 ⁴ poise. death,
The thickness after drying is 0.7mm using the doctor blade method.
A ceramic green sheet was created. A plurality of these sheets were stacked and laminated at about 120°C, and then hot press firing was performed in Ar at 1700°C for 30 minutes and under a pressure of 500 kgf/cm ² to obtain a ceramic sintered body.

The specific resistance of the obtained ceramic sintered body was measured. The results are shown in Table 3.

As is known from the table, it is possible to control the resistivity by controlling the particle size of Si ₃ N ₄ and MoSi ₂ , but controlling the particle size is not sufficient to provide the necessary insulation for the support member.

As a support member, the resistivity is 10 ⁵ , preferably 10 ⁷
Ω・cm or more is required, and such insulation requires Al ₂ O ₃ of 3% or more, or
It can be seen that this becomes possible by adding 2% or more of MgAl ₂ O ₄ .

Note that the sintering aid is not limited to the above-mentioned ones, and those that have poor wettability with MoSi ₂ and good wettability with Si ₃ N ₄ may be used.

(3) Both the heater and supporting member are Si ₃ N ₄ −
A MoSi ₂ −Si ₃ N ₄ sintered body in the ceramic heater of the present invention, which is integrally sintered as a MoSi ₂ sintered body;
The heater is a MoSi ₂ −Si ₃ N ₄ sintered body and the support member is
Tested the firing conditions and properties of the Si ₃ N ₄ - Al _{2 O 3 sintered} body (comparison material) in a conventional ceramic heater that is integrally sintered as a Si ₃ N ₄ - _{Al 2} O 3 sintered body. did. The results are shown in Table 4.

As known from the table, 50%Si ₃ N ₄ −50%
The strength of Al ₂ O ₃ sintered bodies rapidly decreases when fired at temperatures above 1600°C. This strength decrease is caused by 3Si ₃ N ₄ +
The reaction becomes 2Al ₂ O ₃ →2Si ₄ Al ₂ O ₂ N ₆ +SiO _3.
It was found that this was caused by the progress of a solid solution reaction between Si ₃ N ₄ and Al ₂ O ₃ and the precipitation of SiO ₂ , which is an additional component, in the grain boundary layer. Therefore, in order to obtain the necessary strength when using this sintered body as a support member, a temperature of 1600℃ is required.
Should be fired below.

On the other hand, the Si ₃ N ₄ -MoSi ₂ sintered body has a hardness of 1640 yen even when hot pressed due to its difficult sinterability.
Firing is required at a temperature of 1680°C or higher, preferably 1680°C or higher.

Therefore, in conventional ceramic heaters,
It is extremely difficult to integrally sinter the Si ₃ N ₄ -Al ₂ O ₃ support member and the Si ₃ N ₄ -MoSi ₂ heater and simultaneously exhibit the excellent properties of both.

On the other hand, both the support member and the heater
In the ceramic heater of the present invention, which is made of Si ₃ N ₄ --MoSi ₂ and has the same composition, the excellent characteristics of both the support member and the heater can be maximized by integral sintering.

As explained above, according to the present invention, the support member and the heater can be integrally sintered and each can exhibit excellent characteristics. Furthermore, since the macroscopic physical properties of the supporting member and the heater, such as the coefficient of thermal expansion, can be made to match, even when cold and hot temperatures are repeated, cracks do not occur due to thermal stress, and excellent thermal shock resistance can be obtained. .

[Brief explanation of the drawing]

FIG. 1 is a longitudinal cross-sectional view of a glow plug equipped with a ceramic heater of the present invention, FIG. 2 is a model diagram showing the structure of the supporting member in the ceramic heater, and FIG. 3 is a structure of the heater part in the ceramic heater. FIG. 1... Heater, 2... Supporting member, 4... Metal pipe, 5... Metal housing.

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Claims (1)

[Scope of Claims] 1. A support member made of an electrically insulating ceramic sintered body, a heater made of a conductive ceramic sintered body formed on the outer periphery of the tip of the support member, an energizing means for energizing the heater, and a support member. The support member and the heater are made of a sintered body of a mixed powder of molybdenum silicide and silicon nitride having the same blending ratio, and are sintered together, and the support member is made of molybdenum silicide particles. The ceramic heater has a structure in which the molybdenum silicide particles surrounding the silicon nitride particles are mutually separated, and the heater has a structure in which molybdenum silicide particles surrounding the silicon nitride particles are continuous. 2 The weight ratio of molybdenum silicide and silicon nitride in the support member and heater is 10 to 40:90.
60. The ceramic heater according to claim 1, wherein the ceramic heater has a diameter of 60 to 60. 3. The ceramic heater according to claim 1, wherein the average particle diameter of the silicon nitride in the heater is larger than that of the molybdenum silicide. 4. The ceramic heater according to claim 1, wherein the support member is soldered to the housing via nickel plating applied to the surface thereof.