JPH0717436B2 - Method for producing highly insulating and highly alumina-based porcelain composition - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a high-alumina porcelain composition having high insulation properties, and is particularly used as an ignition plug for an internal combustion engine of an automobile or the like. It is valid.

[Prior Art] High-alumina porcelain is chemically extremely stable including heat resistance, has excellent mechanical strength, and its manufacturing method is suitable for mass production. Has been widely used as.

On the other hand, in recent years, the required voltage for the spark plug has been increasing more and more due to higher performance of the internal combustion engine, and it has been required to improve the withstand voltage characteristic of the insulator of the spark plug.

[Problems to be solved by the invention]

In contrast, conventional SiO ₂ -MgO-CaO used for spark plugs
It is known that, in a sintered body having a glassy flux of the system, the flux is reduced by increasing the alumina content to improve the withstand voltage, but it cannot be said to be sufficient for the above requirements. is the current situation.

Therefore, as a result of detailed study of the mechanism of the dielectric breakdown for the purpose of improving the withstand voltage, the present inventors have found that the dielectric breakdown occurs due to the combination of the following two causes.

That is, the first cause is the voids C called voids where dielectric breakdown occurs on the surface of the sintered body and the pores C existing in the grain boundaries A inside the sintered body as shown in the sintered body structure diagram of FIG. An electric field is concentrated on such defects as a high voltage is applied, and an ion current flows due to this concentrated electric field, leading to dielectric breakdown.

The second cause is the previously used SiO ₂ -MgO-Ca.
The O-based sintering aid forms a grain boundary A having a relatively low melting point, and is easily melted by the ion current to cause dielectric breakdown.

Based on the results of the above-described experimental observations, the present inventors focus on the improvement of the grain boundary structure of the sintered body and aim to enhance the insulation resistance.

[Means for solving problems]

Y ₂ O ₃ , at least one additive selected from Z _r O ₂ and La ₂ O ₃ , or at least one additive selected from Y ₂ O ₃ , Z _r O ₂ and La ₂ O _3. In the case of forming a sintered body by molding and firing a mixed raw material powder composed of any one of solid solution complex oxides of alumina and alumina powder having a particle size of 0.5 μm or less, the content as the additive is included. The amount is 0.5 with respect to the sintered body.
Method for producing a highly insulating high alumina ceramic composition, which is a 10 wt%, _{or, Y 2 O 3, Z r} O 2, La least one additive from the selected ₂ O _3, or, either of Y ₂ O _3, Z _r O _2, at least one additive was La selected from ₂ O ₃ solid solution composite oxide of alumina, and sintering aid SiO ₂ -CaO-MgO-based When a mixed raw material powder composed of alumina powder having a particle diameter of 0.5 μm or less is molded and fired into a sintered body, the content of the additive is 0.5 with respect to the sintered body.
To 4.5% by weight, and the content as the sintering aid is 0.5 to 4.5% by weight, based on the sintered body, for producing a highly insulating and highly alumina-based porcelain composition. The method is adopted.

[Action]

According to the above means, the particle size of the raw material powder of alumina is 0.5μ.
When it is m or less, voids in the sintered body and voids on the surface can be eliminated, and the additive selected from Y ₂ O ₃ , ZrO ₂ and La ₂ O ₃ is a grain in the sintered body structure. It is possible to increase the melting point of the boundary portion and suppress the generation of ion current.

〔The invention's effect〕

Therefore, according to the present invention, the insulation resistance of the high-alumina porcelain composition can be greatly improved, and it can be effectively used as an insulator for parts to which a high voltage is applied.

〔Example〕

 Hereinafter, the present invention will be described in detail based on examples.

93 wt% of the main component consisting of Al ₂ O ₃ with a purity of 99.9% or more and a particle size of 0.25 μm, SiO ₂ -MgO-CaO system sintering aid powder (composition ratio SiO ₂ 50%, MgO 10%, CaO 40%) 2 wt% and Y ₂ O ₃ powder 5 wt% as an additive of the present invention were mixed with an appropriate amount of water, and wet mixing and grinding were performed using alumina balls. For pulverization after granulation, 10 wt% aqueous solution of polyvinyl alcohol (PVA) was added to the raw material powder in an amount of 10 wt%, re-mixed, and then dried. Next, this dry granulated raw material was further coarsely pulverized and passed through a sieve with a mesh size of 60 mesh to prepare raw material powder having a uniform particle size.

This raw material powder is put into a mold and pressed at a surface pressure of 500 kg / cm ² to form a disk shape with a diameter of 30 mm, and then a powder of Al ₂ O _{3 having} a powder diameter of 0.1 to 0.5 mm is dispersed in a heat-resistant alumina container. The molded product is placed on top of it, heated up to 1650 ° C at a heating rate of 100 ° C / hr in an electric furnace, kept at 1650 ° C for 2 hours, and then cooled at 100 ° C / hr to obtain a bulk specific gravity of 3.72. A sintered body having a g / cm ³ and a relative density (ratio of the bulk specific gravity to the true specific gravity) of 94% was obtained (structure chart is shown in FIG. 1).

Next, this sintered body was polished to a thickness of 1.0 ± 0.05 mm using a polishing machine using diamond abrasive grains and the withstand voltage was measured with a withstand voltage measuring device shown in FIG. That is, the electrode 11 is attached to one surface of the sintered body 10 by a conductive paste or the like and immersed in the silicon oil 12. And the electrode 11 of the sintered body 10
Probes with needle-like tips on opposite surfaces of the
13 is fixed, and in this state, the high voltage generated by the oscillator 15 and the coil 16 from the constant voltage power supply 14 between the electrode 11 and the probe 13 is monitored at the high voltage probe 17 and the oscilloscope 18 for 30 cycles / sec. The breakdown voltage of the sample was defined as the withstand voltage of the sintered sample by increasing the voltage by 0.2 kv per second at the frequency.

Table 1 shows the main component of alumina and SiO ₂ -Ca as a sintering aid.
5 is an experimental result showing the withstand voltage of a sintered body sample prepared from raw material powders in which the contents of additives of O-MgO, Y ₂ O ₃ , ZrO ₂ , and La ₂ O ₃ were changed.

The conventional product is 95 wt% alumina with an average particle size of 2.5 μm and SiO ₂ -MgO
-CaO-based sintering aid (SiO ₂ 50%, MgO 10%, CaO 40%) 5 wt% was prepared by the same method as above. The present invention has a maximum resistance of more than 40%. Improvement of voltage has been achieved (Sample Nos. 2 and 6).

Here, it was confirmed that when the grain size of alumina exceeds 0.5 μm, the relative density decreases and the withstand voltage does not improve. Moreover, when the SiO ₂ -MgO-CaO system sintering aid is 5 wt% or more,
The sinterability was poor and a dense sintered body could not be obtained, and it became clear from the table that it was a little too high at 5 wt%, and it is preferably about 4.5 wt% or less. It should be noted that this SiO ₂ —MgO—CaO based sintering aid may be omitted. The Y ₂ O _3, ZrO _2, the addition amount of the additive of La ₂ O ₃ has no effect of improving the withstand voltage is below 0.5 wt%, SiO ₂
-With the addition of the MgO-CaO system sintering aid, the withstand voltage begins to decrease when it exceeds 5 wt%. If SiO ₂ -MgO-CaO system sintering additive is not added, these additives are 10wt%.
The withstand voltage does not decrease even if added to a certain degree.

Additives such as Y ₂ O ₃ , ZrO ₂ and La ₂ O ₃ may be added to the raw material powder as their own powder, but once these are added to Al
_It may be added to the raw material powder as a solid solution mixed oxide with ₂ O _3, and by doing so, the above additives can be mixed with good dispersibility when the amount of the additives is very small.

Next, the structure of an ignition plug for an internal combustion engine, in which the highly insulating and highly alumina-based ceramic composition of the present invention is preferably used, will be described with reference to FIG.

The spark plug 20 is placed in the cylinder of the engine and has a function of igniting a mixture of gasoline and air by causing discharge between the center electrode 21 and the ground electrode 22 by an applied voltage from a coil that generates a high voltage. . Since the housing 23 has the same potential as the ground electrode 22, the insulator 24 is placed so that the discharge between the housing 23 and the center electrode 21 does not occur. If the fuel is smaller than the normal air-fuel ratio or other fuel that is difficult to ignite, good ignition cannot be generated unless a higher voltage than usual is applied between the center electrode 21 and the ground electrode 22. However, when the voltage exceeds a certain level, a part of the insulator 24 is destroyed and a pinhole is generated. In this state, a discharge that should occur between the ground electrode 22 and the center electrode 21 is generated between the housing 23 and the center electrode 21, causing misfire.

As described above, the material obtained by adding a SiO ₂ -MgO-CaO-based sintering aid to Al ₂ O ₃ which is a conventional insulator material, as described above, has a concave portion B called a void and a grain boundary as shown in FIG. There are many pores C in the. When a high voltage is applied to this, a potential concentrates on these defects, and the potential gradient causes a current to flow due to the easily moving ions existing in the grain boundary A, and the heat generated by this current further increases the number of mobile ions. Then, a cycle in which an electric current further flows to generate heat is repeated. When this calorific value becomes larger than the decomposition energy of alumina, destruction occurs.

The present inventors found that the fracture first started from the glass component at the grain boundary, and as a result of examining the additive which makes the grain boundary a high melting point material and sinters Al ₂ O ₃ densely as a result, The present invention has been achieved by reducing the particle size of alumina and adding additives such as Y ₂ O ₃ , ZrO ₂ , and La ₂ O ₃ .

[Brief description of drawings]

FIG. 1 is a schematic diagram showing the structure of a highly insulating and highly alumina-based porcelain composition produced by the production method of the present invention, FIG. 2 is a system diagram showing a withstand voltage measuring device, and FIG. 3 is the production of the present invention. FIG. 4 is a partial cross-sectional view showing the structure of an ignition plug in which a highly insulating and highly alumina-based ceramic composition is preferably used, and FIG. 4 is a schematic diagram showing the structure of a conventional highly-alumina-based ceramic composition. A ... Grain boundary.

 ─────────────────────────────────────────────────── ─── Continuation of front page (56) Reference JP-A-59-156961 (JP, A) JP-A-56-96777 (JP, A) JP-A-60-21854 (JP, A) JP-A-48- 91112 (JP, A)

Claims (2)

[Claims] 1. A _{Y 2 O 3, Z r O} 2, La 2 O 3 , at least one additive was more chosen, or selected from _{Y 2 O 3, Z r O} 2, La 2 O 3 At least one additive and one of solid solution composite oxides of alumina, and a mixed raw material powder composed of alumina grain-penetrating powder having a particle size of 0.5 μm or less is molded and fired to obtain a sintered body, The content as the additive is 0.5 with respect to the sintered body.
-10% by weight, a method for producing a highly insulating and highly alumina-based porcelain composition. Wherein _{Y 2 O 3, Z r O} 2, La 2 O 3 , at least one additive was more chosen, or selected from _{Y 2 O 3, Z r O} 2, La 2 O 3 A mixed raw material powder composed of at least one additive and a solid solution composite oxide of alumina, a SiO ₂ -CaO-MgO-based sintering aid, and an alumina powder having a grain size of 0.5 μm or less. In forming a sintered body by molding and firing, the content as the additive is 0.5 with respect to the sintered body.
To 4.5% by weight, and the content as the sintering aid is 0.5 to 4.5% by weight, based on the sintered body, for producing a highly insulating and highly alumina-based porcelain composition. Method.