JPS62296303A - Magnesia sintered powder for electric insulation - 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 3. Detailed Description of the Invention [Field of Industrial Application] The present invention relates to magnesia sintered powder for electrical insulation, which is mainly composed of magnesia and has excellent fire resistance. It is suitable as an insulating filler for heaters.

[Prior Art] Magnesia has the property of having very high high-frequency electrical insulation resistance and very high electrical insulation resistance at high temperatures. Conventionally, powder obtained by crushing electrofused magnesia has been used as an electrical insulating material, particularly as an insulating filler for sheath heaters. In addition, the digestibility of the powder is an important factor in the properties of the filler for sheath heaters. In other words, magnesia powder has excellent thermal conductivity and electrical insulation resistance, but it is highly hygroscopic, and the electrical insulation resistance deteriorates due to moisture absorption. Therefore,
As shown in Japanese Unexamined Patent Publication No. 57-189480, 3! Method of coating the surface of magnesia with Q2 etc. and Japanese Patent Publication No. 59-1119
As shown in 6, the method can be broadly divided into two methods: sealing the end of the heater with silicone resin or the like. In addition, there is a method that combines both methods, and a method described in Japanese Patent Publication No. 59-47870 is a method of coating with a water-repellent SiO2 layer. Further, in particular, electrofused magnesia is produced in large lumps due to its manufacturing method, so it has to be sized to be used as an insulating filler for fine sheath heaters, and the surface of the crushed particles is active and easily absorbs moisture. Therefore, it is necessary to coat the surface of the magnesia with silica and seal the ends of the heater.

[Problems to be Solved by the Invention] However, in the conventional method, the manufacturing cost is high and the digestion resistance of the filler is not necessarily satisfactory.

In addition, the conventional method using electrofused magnesia involves electromelting the magnesia clinker, which has manufacturing disadvantages such as increased manufacturing costs and the need for a complicated process of crushing and sizing lumpy electrofused magnesia. Moreover, the electrofused magnesia powder obtained in this manner has an angular surface, so when it is filled into a sheath, it tends to damage the heater and cause wire breakage.

[Means for Solving the Problems] The present invention particularly improves the drawbacks of the crushed electrofused magnesia powder and improves the sintered magnesia powder so that it is particularly suitable for use as an electrical insulating material. In other words, it is made of magnesia sintered powder with excellent digestion resistance, and has an insulation resistance of 600℃.
It is an electrically insulating material with a resistance of 107 Ω·cm or more.

The present invention provides magnesia powder with excellent digestion resistance, and it is desirable that the weight increase rate is 5 wt % or less, preferably 1 wt % or less, when the digestion resistance is measured by an autoclave method.

If this weight increase rate exceeds 5 wt%, the insulation resistance of the sheath heater will deteriorate significantly.

In the present invention, the magnesia sintered powder, which has excellent digestion resistance, preferably has a flow time of less than 240 seconds; if it is longer than that, there is a problem that the filling efficiency decreases.

Furthermore, the insulation resistance of the magnesia sintered powder of the present invention is 600
It has a resistance of 107 Ω·cm or more at a temperature of °C, and if it is less than that, it is difficult to put it into practical use as a filling material for a sheath heater. For this purpose, high purity magnesia sintered powder may be used.

The magnesia sintered powder mentioned here is cao in addition to MCl0.
, S i 02 , Fe2O3, Al2O3, B20i
In addition, the surface of the magnesia sintered powder may be coated with silica, zirconia, or the like.

Furthermore, the magnesia sintered powder of the present invention may be crushed so that its digestibility is 5 wt % or less in terms of weight increase rate.

If it exceeds 5 wt%, the life of the heater will be significantly shortened due to moisture absorption.

The manufacturing method is, for example, by adjusting the composition of high-purity magnesia to a particle size of 1 mm or less, which has been fired at a temperature of 1,600 to 1,800° C. or higher, and adjusting the particle size to a particle size of 500 to 25 μm by wind classification or the like.

Here, the purity of MqO is preferably 95 W t% or more, and among chemical components other than magnesia, CaO/S
i 02 ('E/L/ratio) is preferably 1.2[F, preferably 0.8 or less, and CaO/S i
As the −E ratio of 02 increases, the digestion resistance deteriorates, and as a result, the insulation resistance deteriorates more quickly. On the other hand, the content of SiO2 is desirably 0.6 wt% or more, particularly 1.5 wt% or more. Furthermore, the total content of other Fe2O3, Al2O3, and B2O3 is 1 wt% or less,
In particular, the content of Fe2O3 needs to be 0.5 wt% or less. Note that magnesia sintered bodies with chemical compositions outside this range will have a short heater life, which will impede practical use.

Conventionally, fused magnesia whose surface was coated with silica was compressed and reduced in diameter after being filled into a heater, a so-called swaging process, in which a crank was inserted through the cleavage, resulting in poor digestion resistance. However, since the present invention is a magnesia sintered body, calcium silicate, etc. and Calcijar compound (J silica) are present on the surface of the powder or in the so-called grain boundaries within the grains, and even during swaging process, cracks may occur. It is thought that even if cracks form in the cracks, the deterioration of the digestion resistance is suppressed due to the presence of Caljar silica compounds and silica on the surface.

Further, since the magnesia powder thus obtained does not necessarily have a spherical shape depending on the composition, the particle size of the magnesia sintered powder is particularly adjusted to be in a distribution range of 500 to 25 μm. Outside this distribution range, the flow time deteriorates.

The particle size distribution range is preferably 420 to 44 μm.
In particular, it is preferable that there be substantially no particles larger than 500 μm.

MCl01cao of the chemical composition of the example in the present invention,
5i02, Fe2O:+, Al2O3, and B2O3 were measured by hot dissolving magnesia powder in an aqueous hydrochloric acid solution, cooling it, and using ICAP type 575-II manufactured by Nippon Jarrell Ash.

In addition, the insulation resistance of magnesia powder at high temperatures is 10 mm in inner diameter.
After weighing and filling the sample into the gap between the metal pipe and the center rod with an outer diameter of 5 mm to a length of about 25 mm, the platinum wire was compressed at a pressure of 1.5 T/cm2. was attached and placed in an electric furnace, and the insulation resistance at each temperature was measured. The material of the metal pipe and center rod used was SUS 304.

Also, the powder flow time is AST)l 5tandar
'dD 2755 by the American Boeh Die Company.
It was measured using a device manufactured by Y.

The particle size distribution was determined by sieving using a JIS standard sieve.

In addition, the digestibility of the powder was determined by the Gakushuho (Japanese Science) Tatsukokai, No. 1.
Test method proposed by the 24 Committee Analysis Subcommittee) No.
The test was carried out in accordance with the magnesia clinker digestibility test method specified in Section 4.

The weight increase rate was determined when 20 g of sample m was held for 2 hours under conditions of water vapor pressure of 2.0 kgG/cm' and temperature of 133 to 134°C.

Example 1 High-purity magnesia powder of 1 mm or less and having the chemical composition shown in Table 1, which was fired in a 0-tally kiln at a temperature of 1800'C, was heated to 420') using a stainless steel wire mesh.
The powder was sieved by μm to obtain magnesia sintered powder having a size of 420 to 44 μm. Its particle size distribution is 420-250 μm 14
．． 0 wt%, 250-149 μm 48.4 wt%,
149-74μm 29.1wt%, 74-44μB,
It was 5wt%. Furthermore, the digestion resistance, chemical composition, flow time and 60'C of this magnesia sintered powder
Table 1 shows the insulation resistance in .

Example 2 High-purity magnesia powder having a chemical composition shown in Table 1 and having a size of 1 mm or less, which was fired at a temperature of 1900'C or more in a 0-tally kiln, was divided into particles of 500 μm or more and 44 μm using an air classifier.
The following were classified and removed.

The ratio of 500 to 44 μm in this magnesia sintered powder was 97.5 wt%, and 2.5 wt% was 44 to 25 μm. Table 1 also shows the digestion resistance, chemical composition, flow time, and insulation resistance at 600'C of this magnesia sintered powder.

Example 3 High-purity magnesia powder of 1 mm or less and having the chemical composition shown in Table 1 was calcined in a 0-tally kiln at a temperature of 1900'C or higher. The particle size force V5 of the sintered powder is 5
00~420μ0.8wt%, 420~250μm
17.3wt%, 250-149μm 30.8wt%
, 149-74 μm'28.5 wt%, 74-44 μm
22.3 wt%, 44-25 μm 0.3 wt%. This magnesia sintered powder's digestion resistance, chemical composition,
Table 1 shows the flow time and insulation resistance at 600°C.

Comparative Example 1 The digestion resistance, chemical composition, flow time, and insulation resistance at 600'C of commercially available fused magnesia powder were evaluated first.
Shown in the table. Of the chemical composition of this electrofused magnesia, S
The high iO2 is due to the surface being coated with silica to prevent magnesia from absorbing moisture. The particle size distribution of this fused magnesia is 420-250μm38.8wt%
, 250-149 μm29.8wt%, 149-74
μm 20.2wt%, 74-44μm 6.2wt%,
44 μm or less was found at 5.2 wt%.

Comparative Example 2 A high-purity magnesia powder of 1 nm or less having the chemical composition shown in Table 1, which was fired at a temperature of 1900° C. or higher in a 0-tally kiln, was classified to remove particles of 500 μm or more and 44 μm or less using an air classifier. 500~4 of this magnesia sintered powder
The proportion of 4 μm was 98.0 wt%. Table 1 also shows the digestion resistance, chemical composition, flow time, and insulation resistance at 600° C. of this magnesia sintered powder.

Comparative Example 3 The high-purity magnesia powder used in Example 1 was sieved at 800 μm, and the flow time under the sieve was measured to be 300 (sec).

Its particle size distribution is 29.8w with a proportion of 800-500μm
It was rare at t%.

Table 1 Example 4 The magnesia powder used in Example 1, Example 2, and Comparative Example 1 was sieved to obtain powder of 250 μm or more. This powder was crushed with a disk mill to obtain crushed powder having a particle size of 149 to 74 μm. The weight increase rate of this crushed powder is shown in Table 2.

In the actual manufacturing process of the sheathed heater, the sheath heater is not crushed to the extreme as in the crushing experiment conducted in this example, but the influence of cracks and the like during processing is clearly shown.

As explained above in Table 2, the magnesia sintered powder for electrical insulating materials of the present invention has significantly improved digestion resistance compared to conventional fillers for sheath heaters, and the magnesia powder does not deteriorate due to moisture absorption. This can be prevented and has the effect of significantly extending the life of the heater.

Claims (3)

[Claims] (1) Magnesia sintered powder for electrical insulation, which is excellent in digestion resistance and has an insulation resistance of 10^7 Ω·cm or more at 600°C. (2) Digestion resistance of magnesia sintered powder is 5 in terms of weight increase rate.
The magnesia sintered powder for electrical insulation according to claim (1), which has a content of not more than wt%. (3) The magnesia sintered powder for electrical insulation according to claim (1), which is further crushed magnesia powder, and whose digestion resistance is less than or equal to a weight increase rate of 5 wt%.