JPH0717459B2 - Method for producing highly insulating and highly alumina-based porcelain composition - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a high-alumina porcelain composition having a high insulating property, and in particular, an insulator for a spark plug for an internal combustion engine of an automobile or the like, and a semiconductor. It is effectively used as an insulating substrate for products and the like.

[Prior Art] High-alumina porcelain is chemically extremely safe 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, the inventors of the present invention have studied the mechanism of the dielectric breakdown in detail for the purpose of improving the withstand voltage.
It became clear that it was caused by the combination of two causes.

That is, the first cause is a recess B called a void where dielectric breakdown occurs on the surface of the sintered body as shown in the sintering vs. composition diagram of FIG. 6, and pores C existing in the grain boundary A inside the sintered body. An electric field is concentrated on the defect (1) by applying a high voltage, and the concentrated electric field causes an ion current to flow, resulting in dielectric breakdown.

The second cause is the conventionally used SiO ₂ --MgO--C.
The aO-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 experimental observations, the present inventors focus on improving the grain boundary structure of the sintered body and aim to enhance the insulation resistance.

[Means for solving problems]

95 to 99% by weight of alumina fine particles with a particle size of 0.6 to 2 μm, and 1 to 5% by weight of A, LUMINA ultrafine particles with a particle size of 0.5 μm or less, M
Mixed raw material powder with at least one additive selected from gO, Y ₂ O ₃ , ZrO ₂ and La ₂ O ₃ and 0.1 to 0.4% by weight (1 to 8% by weight based on the alumina ultrafine powder particles). The method for producing a highly insulating and highly alumina-based porcelain composition is characterized in that a sintered body is formed by forming and sintering.

[Action]

According to the first aspect of the present invention, by increasing the melting point of the grain boundary formed after sintering of the alumina ultrafine particles having a grain size of 0.5 μm or less, it is possible to suppress melting of the grain boundary and increase the withstand voltage,
According to the second aspect of the invention, in addition to the above configuration, MgO, Y ₂ O
By adding at least one of ₃ , ZrO ₂ and La ₂ O ₃ , it is possible to suppress the generation of defects such as voids that are likely to occur by the method of the first invention, and further increase the withstand voltage.

〔The invention's effect〕

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

〔Example〕

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

97 wt% of the main component consisting of Al ₂ O ₃ fine particles with a purity of 99.8% or more and an average particle size of 0.6 μm, the sintering temperature is very low at about 1300 ° C., and the surface has an active average particle size of 0.15 μm. A low-temperature sinterable alumina ultrafine powder (manufactured by Daimei Chemical Co., Ltd.) was added as a raw material powder containing 3% by weight of a sub-ingredient, and an appropriate amount of water was added to the mixture. .
10 wt% of polyvinyl alcohol (PVA) for granulation after crushing
% Aqueous solution was ignited at 10 wt% with respect to the raw material powder, remixed, and then dried. Next, this dry granulation raw material is further roughly crushed,
A raw material powder A having a uniform particle size was produced by sieving with a 60-mesh mesh.

This raw material powder A 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 0.1 to 0.5 mm of Al ₂ O ₃ powder 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 ² 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 by a withstand voltage measuring device shown in FIG. An electrode 11 is attached to one surface of the sintered body 10 with a conductive paste or the like and immersed in a silicon oil 12. Then, a probe 13 having a needle-like tip is fixed to the facing surface of the electrode 11 of the sintered body 10, and in this state, a constant voltage power supply is provided between the electrode 11 and the probe 13.
High voltage generated by oscillator 15 and coil 16 from 14
While monitoring with the high-voltage probe 17 and the oscilloscope 18, the voltage was increased by 0.2 KV per second at a frequency of 30 cycle / sec, and the breakdown voltage of the sample was defined as the withstand voltage of the sintered sample.

As a result, it was 33 KV / mm in the above example. On the other hand, for comparison, 95 wt% alumina with an average particle size of 2.5 μm and C
The value of 24 KV / mm was produced by the same method as described above from the raw material powder consisting of 5 wt% of aO-MgO-SiO ₂ based flux, and that of the present invention showed an improvement in withstand voltage of nearly 40%.

The reason why the withstand voltage is improved in this way is that the flux existing at the grain boundaries is a glassy flux of SiO ₂ -MgO-CaO system, and it is due to Joule heat generation by mobile ions, electrons, etc. contained in the glass when a high voltage is applied. In contrast to the fact that it easily melts (melting point of conventional flux, 1200 to 1300 ° C), the present invention allows low-temperature sinterable alumina ultrafine powder particles to perform the function of conventional flux and perform sintering, and at the same time It is considered that after that, it becomes an alumina sintered body, and the melting point can be increased (melting point 2050 ° C), so that the melting point at the grain boundary can be significantly increased.

Next, in the above examples, even if only low-temperature sinterable alumina ultrafine particles are added as a subcomponent, a high voltage is shown as compared with the conventional product, but the sintering temperature of low-temperature sinterable alumina ultrafine particles is the main component. Since it is significantly different from the fine alumina particles, some pore structure is generated during sintering. In order to suppress this, high melting point MgO (melting point 2800 ° C), Y ₂ O ₃ (melting point 2400 ° C), ZrO ₂
It was found that the densification of the sintered body can be achieved by adding one of (melting point 2765 ° C) to the low temperature sinterability alumina in an amount of 1 to 8 wt% (0.1 to 0.4 wt% as a whole). . The results are shown in FIG. As can be seen from the figure, it was clarified that the addition amount of these additives was 1 to 8 wt% with respect to the low temperature sinterable alumina, and the density was high and the withstand voltage was also high. This is because the additive and the low temperature sinterable alumina fine powder particles are changed to a high melting point compound, so that the sintering rate can be slower than the case of sintering by adding only the low temperature sinterable alumina fine powder particles, It is considered that when the sintering was performed only with the low-temperature sintered alumina, the sintering rate was too fast, and thus the voids generated could be reduced. Fig. 4 shows the results obtained by setting the particle size of the main component, alumina fine powder, to 0.6 μm, and adding 1,3,5 wt% of low-temperature sinterable alumina fine powder, which is a subcomponent of each particle size, to it. When 1 to 5 wt% of low-temperature sinterable alumina fine particles with a particle size of 0.1 to 0.5 μm were added, a high withstand voltage was obtained in all cases, but the effect was greater with 3 to 5 wt%. Further, the smaller the particle size of the low temperature sinterable alumina, the higher the withstand voltage, but the lower limit of what is currently obtained is about 0.10 μm.

Y ₂ O ₃ as an additive, ZrO ₂ , MgO in the sintering process,
Low temperature sinterability 0.1 ~ in order to have sinterability equivalent to Al ₂ O ₃
It is preferably 0.5 μm. In addition to Y ₂ O ₃ , ZrO ₂ and MgO, La ₂ O ₃ has the same effect.

Next, the structure of the 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 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 is placed in the cylinder of the engine and generates a high voltage. Hold. 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 and the ground electrode. However, if the voltage exceeds a certain level, the insulator 24
Part of it is destroyed and pinholes occur. 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 flux of SiO ₂ —MgO—CaO system to Al ₂ O ₃ which is a conventional insulator material has a concave portion B called a void on the surface as shown in FIG.
There are many. When a high voltage is applied to this, the potential concentrates on these defects, and the potential gradient causes a current to flow due to the easily moving ions existing in the grain boundaries A and pores C, and the heat generated by this current further increases the number of mobile ions. By doing so, 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 have found that the fracture starts from the glass component that becomes the first grain boundary, and as a result, the grain boundary is made to have a higher melting point material,
In addition, as a result of studying the additive for densely sintering Al ₂ O, it came to the idea that low-temperature sinterable alumina ultrafine particles are used instead of the conventional glass-based flux. This material alone
Since it can be sintered at about 1300 ° C, it works like a conventional flux, and once sintered, it has a melting point similar to that of the conventional product, so the withstand voltage can be higher than that of the conventional product. Moreover, since the addition amount is small, the cost does not increase significantly.

[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 FIGS. 3 and 4 are FIG. 5 is a characteristic diagram showing the results of measuring the relative density and withstand voltage of each sintered body, and FIG. 5 is a semi-finished structure showing the structure of a spark plug in which the highly insulating and highly alumina-based ceramic composition according to the production method of the present invention is preferably used. FIG. 6 is a schematic view showing the structure of a conventional high alumina ceramic composition. A ... Grain boundary.

Claims (1)

[Claims] 1. Alumina fine powder particles 95-99 having a particle size of 0.6-2 .mu.m.
1% to 5% by weight of alumina ultrafine particles with a particle size of 0.5 μm or less
%, And 0.1 to 0.4% by weight of at least one additive selected from Mgo, Y ₂ O ₃ , ZrO ₂ and La ₂ O ₃ (1 to 8% by weight with respect to the alumina ultrafine powder particles). 1. A method for producing a highly insulating and highly alumina-based porcelain composition, which comprises forming and sintering the mixed raw material powder of 1. to obtain a sintered body.