JPS62187156A - Manufacture of high insulation high alumina ceramic composition - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for producing a high alumina porcelain composition having high insulation properties, particularly as an insulator for spark plugs of internal combustion engines such as automobiles. It is used and effective.

[Conventional technology]

High alumina porcelain is chemically extremely stable, including heat resistance, and has excellent mechanical strength, so it is widely used as an electrical insulating material for internal combustion engine spark plugs and the like.

On the other hand, in recent years, as the performance of internal combustion engines has improved, the voltage required for spark plugs has tended to increase more and more, and there has been a demand for improvements in the withstand voltage characteristics of the insulators of spark plugs.

Conventionally, a method for manufacturing high alumina porcelain has been known, in which SiOz-MgO-CaO mixed metal oxide is added to alumina powder as a sintering aid and sintered. It is known that the sintering aid later forms grain boundaries with a low melting point and lowers the withstand voltage. The withstand voltage can be improved by eliminating the vitreous grain boundaries, which are the cause of this decrease in withstand voltage, but alumina powder alone does not cause sintering, and requires a very high sintering temperature, which increases the price limit. In addition to causing abnormal grain growth at high temperatures, there is a problem in that the strength of the sintered body is significantly reduced.

Therefore, the present invention focuses on the grain boundaries that cause a decrease in withstand voltage, and uses a small amount of sintering aid to appropriately promote alumina crystal growth, thereby reducing the proportion of grain boundaries that cause a decrease in withstand voltage. The purpose is to improve the withstand voltage by lowering the grain boundaries and forming the grain boundaries themselves to have a high melting point and be dense.

[Means for solving problems]

In order to achieve the above object, the present invention includes 0.2 to 2.5 wt of MgO as the first essential additive component.
%, y2o as the second essential additional component, La20:I
, one or more selected from ZrO□ in total from l to 10w
t%, and the remainder is A 1! A method for manufacturing a highly insulating, high alumina ceramic composition is adopted in which a raw material mixture consisting of 03 is formed into a desired shape and sintered at a temperature of 1650 to 1800° C. in a vacuum or hydrogen atmosphere.

[Effect]

According to the above means, the first and second essential additive components help form dense, high-melting-point grain boundaries, and the grain growth of alumina is moderate by sintering in the temperature range in vacuum or hydrogen atmosphere. is promoted, and the development of grain boundaries is suppressed.

〔Example〕

The present invention will be described in detail below based on examples thereof.

Purity 99.9% or higher, average particle size 0.25. A of crm
l 203, 99.9% purity as the first essential additive component
With the above, MgO1 with an average particle size of 0.1 μm and a second essential additive component X with a purity of 99.9% or more and an average particle size of 0.5
Yz 03 , L a 203 , Z r Oz in μm
Twelve kinds of raw material powders were prepared using the following methods, each having a weight ratio shown in Table 1.

Next, an appropriate amount of water was added to the mixed raw material powders, and the mixture was wet mixed and pulverized using an alumina ball.

1 of polyvinyl alcohol (PVA) for granulation after grinding.
A 3 wt % aqueous solution was added to the raw material powder in an amount of 10 wt %, mixed again, 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 produce a raw material powder with a uniform particle size.

This raw material powder was put into a mold and pressed at a surface pressure of 500 kg/cnl, and then placed in a pressure-resistant alumina container with a particle size of 0.
On top of which A & Z Off powder of 1 to 0.5 capacity is dispersed,
The compact was placed and calcined at 1000°C for about 2 hours, then heated to 1750°C at a rate of 100”C/hr in a vacuum furnace or hydrogen atmosphere furnace, and held at 1750°C for 2 hours. Then, samples 1 to 12 were prepared by lowering the temperature at 100°C/hr.

The bulk specific gravity of these sintered bodies 10 is 3.95 to 3.98,
For example, in sample 11kL1, the relative density (ratio of bulk specific gravity to true specific gravity) is 99.5%, almost no pores are included in the sintered body, and as shown in the schematic diagram of the structure in Figure 2, The grain boundary B
It was confirmed that the proportion of

Next, the withstand voltage, channeling resistance, and
The results of measuring bending strength will be explained. The withstand voltage was measured by polishing the sintered body 10 to a thickness of 1.0±0°95 mm using a polishing disk using diamond abrasive grains, etc.
The voltage resistance was actually measured using the withstand voltage measuring device shown in FIG. That is, the electrode 11 is attached to one surface of the sintered body 10 using a 4-electrode paste or the like by immersing it in silicone oil 12. Then, a probe 13 having a needle-shaped tip is fixed to the surface facing the electrode 11 of the sintered body 10, and in this state, between the electrode II and the probe 13, a constant voltage power source 14 is connected to an oscillator 15 and a coil 16. The high voltage generated by
The voltage was increased by Q, zk v every second at a frequency of cle/second, and the voltage at which the sample broke down was taken as the withstand voltage of the sintered sample.

Next, the channeling resistance was measured using a sintered body 10 with a diameter of 3 mm.
0"-0.05van, length 8.0±0.05mm, and evaluated using the channeling resistance evaluation device shown in FIG. The metal cap 19 is connected so that When subjected to creeping discharge, the weight change (loss of consumption) of the sintered body 10 before discharge and after 40 hours of discharge was measured, and the weight loss per unit of time was defined as channeling resistance.What is channeling resistance? , refers to a phenomenon in which alumina is damaged by creeping discharge that travels along the surface of a sintered body, and its practical meaning will be described later. It was processed into a shape of 3ml x 4cm x 40mm, and the three-point bending strength was measured using a span of 30 cranes.In Table 1, the conventional product was Alt 0395 with an average particle size of 2.5μm.
% and S i Ot M g OCa O-based auxiliary agent 5wt
% raw material powder was heated to 1650°C at a heating rate of 100°C/hr in an electric furnace, held at 1650°C for 2 hours, and then lowered at a rate of 100°C/hr. As is clear from the above, in comparison, the product of the present invention has a withstand voltage of 60% or more, a strength of 26% or more,
A 1/10 improvement in performance in channeling resistance was achieved. (Sample N11l, 3) In the conventional product, auxiliary agent (SiOz Mg0-Cab)
It is thought that the withstand voltage is low because it has a low melting point and there are many pores in the grain boundaries B.

This A1.0. In order to improve the withstand voltage of L, it is possible to eliminate this auxiliary agent, but without the auxiliary agent, sintering will not occur at low temperatures, and abnormal grain growth will occur when sintering at high temperatures. Therefore,
In the present invention, a small amount of MgO1, which is the first essential additive component as a grain growth inhibitor, and Y, which is the second essential additive component X, as an additive that can increase the melting point of grain boundaries, are added to fine grains of Al1tOx.
203, La.

One or more selected from Oz, Zr, Q□ is added and sintered at high temperature in vacuum or hydrogen atmosphere.

In this method, the grain size is determined by the effect that the grain growth rate is higher due to the influence of the atmosphere than when sintering in the normal atmosphere, the effect of MgO to moderately suppress it, and the high resistance of alumina grain boundaries. By suppressing the effect of the additive, the crystal grain size becomes appropriately large, as shown in the structure shown in FIG. 2, and the pores at the grain boundaries are reduced, resulting in a dense alumina sintered body. This alumina sintered body has a structure in which grain boundaries have a high melting point and can reduce factors such as pores and glass layers that lower the withstand voltage, so it has high withstand voltage performance.

Next, the relationship between the average alumina particle size, withstand voltage, and bending strength of this dense alumina sintered body was investigated, and the results are shown in FIG.

As is clear from the figure, as the average particle size increases, the withstand voltage improves and the bending strength decreases. For example, an insulator for a spark plug for an internal combustion engine is required not only to withstand voltage but also to have mechanical strength, and its bending strength must be 30 kg/- or more.

Therefore, the grain size of the alumina crystal grains needs to be 50 μm or less. However, when the grain size is 3 μm or less, the relative density is low and many pores are present at the grain boundaries, resulting in a decrease in withstand voltage. By controlling the particle size to 3 to 50 μm, an alumina sintered body with high voltage resistance and high strength was obtained. This grain size changes depending on the amount of MgO added, which has the effect of suppressing grain growth, and the sintering temperature, but sintering cannot be performed at a firing temperature of 1,650°C or lower, and abnormal grain growth occurs at a firing temperature of 1,800°C or higher.

On the other hand, when MgO is Q, 2wt% or less, abnormal grain growth cannot be suppressed, and the grain size becomes 50μ or more, which is 2.5wt%.
% or more, the sintered density does not increase. That is, when the particle size is 3 μm or less, many pores are generated and the withstand voltage does not increase.

If the essential additive component X in FIG. 2 is less than 1 wt%, high melting point grain boundaries will not be generated and high withstand voltage will not be obtained, and if it is more than 10 wt%, the sintered density will not increase. In other words, those with a large particle size (5
0 μm or more) and small pores (3 μm or less) exist unevenly, and at the same time, many pores are generated and the withstand voltage does not increase.

Therefore, the main component Al□03, the first. The weight ratio of the second essential additive component is preferably in the region shown by the shaded area in the ternary composition diagram of FIG.

Next, the second essential additive component Yt Ch , l, a
The functions of toj and zro□ will be explained in more detail. YZ O−+ and Lag Ch are each AlzOx
Y3 A 1 s OIt (melting point 1970°C) and LaAl01 (melting point 2110°C) are formed mainly at grain boundaries. Since ZrO has weak reactivity with Al2O2, a Zr0z phase is precipitated at and near grain boundaries.

(ZrOz has a melting point of 2675° C.) Among these, the reaction product of Y, oz and Al1202 is generated at a relatively low temperature, so it is possible to proceed with the sintering of Al1zOs at a low temperature. The reason why a high withstand voltage is obtained when a composition containing MgO and Y2O is fired at 1650 to 1750°C in a vacuum or hydrogen atmosphere is that in this firing temperature range,
This is thought to be because a very dense structure can be obtained by effectively utilizing the sintering promotion and sintering suppressing effects of i'z 02 and MgO, respectively. (Sample 3.11) One of Lag oj, Zr0z and A of MgO
1. The addition of O, (magnetic 4.l!15) to these and Al
Since the reaction of tos starts at a slightly higher temperature than that of yzO, the bulk specific gravity becomes slightly smaller when fired at 1650 to 1750°C. However, the withstand voltage is mainly due to ZrO present at grain boundaries.
Since the melting point of z, L a A 103 is sufficiently high, and the bulk specific gravity is also larger than that of conventional products, a sufficiently high value can be obtained.

When the amount of ZrO□ added exceeds 10 wt%, ZrO,
Since the material itself becomes ionic conductive at high temperatures, its withstand voltage decreases. However, it can be seen that adding ZrO in an appropriate amount of 19 wt % or less is particularly effective in improving strength and withstanding voltage. (Sample 1lkL5.Na
7) Also, YZ 0. .

If the addition amount of L at Os + Z r Oz is increased, the same capacity limit as Al1o* will be exceeded, and in the sintering process, an unreacted part of the auxiliary will remain and the fired product will become non-uniform. It stops working. The amount of additive added is l 9w.
A suitable value is t% or less; if it exceeds this, the sintered structure becomes non-uniform as described above and a sufficient withstand voltage cannot be obtained.

Auxiliary agents are Yt Owl, L az C1+, Z
If rOt is 19wt% or less (samples 1lh6 to 8), a sufficiently high withstand voltage can be obtained compared to the conventional example.

These results show that in order to obtain a sufficiently high withstand voltage, the alumina material itself should have a dense structure and the grain boundaries and their vicinity should be made of a material with a high melting point.

Generally, the size of crystal grains during sintering is important for the strength of alumina porcelain, and in order to improve the strength, manufacturing methods are being considered to form a dense structure using fine particles. Since materials that require strength have a large bonding energy, attempts have been made to obtain dense sintered bodies made of fine particles by heating them at very high temperatures or by adding large amounts of auxiliary agents.

According to the proposed method, the average particle diameter can be controlled by appropriately selecting the amount of Y2O3, La2O3, and ZrO, which form high-melting point material grain boundaries as auxiliary agents. An alumina porcelain composition with excellent strength can be obtained.

Next, a spark plug for an internal combustion engine in which the high alumina porcelain of the present invention is effectively used will be described in detail with reference to FIG. The creeping discharge type ignition plug 30 generates a discharge along the creeping discharge portion 34a of the insulator 34 between the center electrode 31 and the ground electrode 32 by the applied voltage from a coil that is placed in the cylinder of the engine and generates high voltage. This spark plug has the function of igniting the mixture of gasoline and air by causing a creeping discharge on the insulator, which burns the carbon, eliminates smoldering, and improves ignitability. be. When the fuel has a lower air-fuel ratio than the normal air-fuel ratio or other fuels that are difficult to ignite, good ignition cannot be achieved unless a voltage higher than normal is applied between the center electrode 31 and the ground electrode 32.

However, when the voltage exceeds a certain level, a portion of the insulator 34 is destroyed and a pinhole is created. In this state, a discharge that should have occurred between the ground electrode 32 and the center electrode 3I occurs between the housing 33 and the center electrode 31, causing a misfire.

Conventional insulator materials Al2O, SiO□-MgO
A material to which -CaO-based flux is added has many recesses called voids on the surface and many pores at grain boundaries. When a high voltage is applied to these defects, the potential concentrates on these defects, and the potential gradient causes a current to flow due to the easily mobile ions existing at the grain boundaries.The heat generated by this current further increases the number of mobile ions, which further increases the number of mobile ions. The cycle of current flowing and heat generation repeats. Destruction occurs when this calorific value exceeds the decomposition energy of alumina.

The inventors of the present invention discovered that fracture initially begins with the glass component at the grain boundaries, which led to the present invention. Voltage can be improved.

In addition, in the case of the above-mentioned creeping discharge type spark plug, the channeling properties of the insulator 34 are a problem, and Sing-M, which is used as a low melting point auxiliary material in conventional porcelain,
gO--CaO is melted and evaporated due to the discharge, and large grooves are formed, so that spark discharge occurs in the grooves and the gasoline ignitability deteriorates. On the other hand, according to the present invention, the grain boundaries can be formed densely with a high melting point, so there are few grooves formed and the ignitability does not deteriorate.

[Brief explanation of drawings]

Figure 1 shows the high alumina of the present invention! Figure 2 is an enlarged schematic diagram showing the structure of the high alumina porcelain, Figure 3 is a diagram showing the composition ratio of raw materials for porcelain, and Figure 4 is a diagram showing a measuring device for voltage resistance and channeling resistance. Fig. 5 is a characteristic diagram showing the relationship between average particle size, withstand voltage, and bending strength, and Fig. 6 shows the structure of a spark plug for internal combustion engines in which the high alumina porcelain of the present invention is effectively used. It is a partially broken sectional view.

Claims (1)

[Claims] 0.2-2.5w of MgO as the first essential additive component
t%, Y_2O_3, La_ as the second essential additive component
2O_3, one or more selected from ZrO_2 in total 1
~10wt%, with the remainder being Al_2O_3, is formed into a desired shape and sintered at a temperature of 1650 to 1800°C in a vacuum or hydrogen atmosphere.Highly insulating high alumina porcelain Method for producing the composition.