JP2638052B2 - High temperature electrical insulating filler and method for producing the same - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an electric insulating filler containing sintered magnesia (MgO) as a main component and having excellent electric insulation resistance at a high temperature, and a method for producing the same. It is suitable as an insulating filler for heaters.

[Prior art and problems] MgO is characterized by having a very high electric insulation resistance at high temperatures, and is used as an electric insulating material, particularly as an insulating filler of a sheath heater.

Since conventionally used fused magnesia is obtained in a large lump due to its manufacturing method, it must be crushed and sized to be used as an insulating filler for a thin sheath heater, and the crushed grains are square. In addition, it is difficult to fill, and the heater is damaged at the time of molding after filling, resulting in shortening of the life.

Sintered magnesia is easy to manufacture and has attracted attention in recent years, but the sintered magnesia described in JP-A-62-90807 and JP-A-62-268002 does not satisfy the demand for high insulation resistance. Was.

The spherical sintered magnesia described in Japanese Patent Application Laid-Open No. 62-90807 by the present inventors was suitable for producing a thin heater, but still had low insulation resistance.

[Problems to be Solved by the Invention] The present invention has a high electric insulation resistance at a high temperature, a spherical particle shape, and when used as an insulating material of a sheath heater, does not damage the heater at the time of forming the heater. An object of the present invention is to provide a spherical high-temperature electric insulating filler for high temperature and a method for producing the same.

[Means for Solving the Problems] It is known that sintered magnesia has a solid solution of CaO in MgO, and the solid solution of CaO causes distortion of the lattice of MgO, and the lattice constant of MgO is reduced. growing.

It is presumed that the low insulation resistance of sintered magnesia is caused by the solid solution of CaO and the lattice distortion of MgO caused by the solid solution.

The present invention intends to provide an electric insulating filler having a high electric insulation resistance at high temperature by precipitating the solid solution CaO at a grain boundary by heat treatment and dissolving and removing the CaO with an acidic solution. It is.

[Means for Solving the Problems] The electrically insulating filler for solving the above problems is a magnesia sintered powder, a) a chemical composition i) MgO ≧ 93 wt% ii) 0.3 ≦ CaO ≦ 1.5 wt% iii) SiO _{2 ≦ 4wt% iv) Fe 2} O 3 + Al 2 O 3 ≦ 0.4wt% v) B is a _{2 O 3 ≦ 0.1wt% vi)} Igloss ≦ 0.3wt%, b) the lattice constant of the MgO is 4.2130A less c) It is an electrically insulating filler with a lattice strain of MgO of 7.0 × 10 ^-4 or less.

When the chemical composition of the magnesia sintered powder is within the above range, the electric insulation resistance is sufficiently high, and when the chemical composition is out of the above range, the insulation resistance becomes low, so that the practical use as a high-temperature filler is lost.

In a method for producing such an electrically insulating filler, magnesia powder is heat-treated at a maximum temperature of 1000 ° C. or higher, then brought into contact with an acidic solution, and then sufficiently washed with water.

In the production of the electric insulating filler of the present invention, among the spherical powder having a predetermined composition sintered in a rotary kiln or the like, 42
It is appropriate to collect a portion that passes through a 0 μm sieve and does not pass through a 25 μm sieve. Further, the powder may contain an auxiliary agent such as ZnO ₂ or the like in a range where other components do not affect the powder.

The production method is preferably 9 to 25 μm in particle size.
After the powder adjusted to 5 wt% or more is heat-treated at 1000 ° C. or more in a heating furnace such as a rotary kiln, the powder is brought into contact with an acidic solution, then sufficiently washed with water, filtered and dried.

The magnesia powder of the present invention preferably has an insulation resistance of 1.8 × 10 ⁹ Ω · cm or more after being kept at 800 ° C. for 24 hours. More preferably, it is 2.5 × 10 ⁹ Ω · cm or more.

Furthermore, it is well known that the flow time of the filler is an important factor in manufacturing, especially work efficiency when filling sheath heater powder, and the flow time is 200 sec / 100 gr.
Below, in particular, it is industrially significant to be 180 sec / 100gr or less.

In the present invention, the packing density of the filler is 2.10 to 2.
It is preferably 35 (g / cc). If it is 2.10 (g / cc) or less, the life of the insulation resistance is greatly reduced. If it exceeds 2.35 (g / cc), the insulation resistance is low from the beginning. Will not be able to serve.

In the present invention, the lattice constant of MgO is 4.2130A or less, Mg
The lattice distortion of O needs to be 7 × 10 ⁻⁴ or less,
Outside of this range, the insulation resistance deteriorates in any case, making it unsuitable for high temperature use. More preferably, the lattice distortion of MgO is 5 × 10 ⁻⁴ or less.

In the production method of the present invention, the maximum temperature is less than 1000 ℃, the effect of heat treatment is small, 1000 ℃ or more, preferably 1200 ~
1400 ° C is desirable.

Next, it is brought into contact with an acidic solution for 1 minute or more, separated from the solution, washed sufficiently with water, filtered and dried.

That is, the magnesia sintered powder having few impurities on its surface has excellent insulation resistance at high temperatures without impairing the fluidity described above.

EXAMPLES Hereinafter, the present invention will be specifically described with reference to Examples and Comparative Examples.

MgO, CaO, Si among the chemical compositions of the examples of the present invention
_{O 2, Fe 2 O 3,} Al 2 O 3, B 2 O 3 is then heat treated magnesia powder with aqueous hydrochloric acid, and ZrO ₂ is _{Na 2 CO 3, Na 2 B} 2 O 7 · 10H 2 O
After alkali-melting, it was dissolved in a nitric acid aqueous solution by heating, and then measured using ICAP manufactured by Nippon Jarrell Ash 575-II.

Igloss is a method in which 10 g of a sample is precisely weighed, placed in a platinum crucible, placed in an electric furnace, and the weight loss after 1000 ° C. × 1 hour is represented by the weight percentage of the weight loss.

The high-temperature insulation resistance in magnesia of the embodiment of the present invention is obtained by compressing and filling an insulating filler in a gap between a metal pipe having an inner diameter of 10 mm and a center rod having an outer diameter of 5 mm at a pressure of 1.5 T / cm ² to a length of about 25 mm. Then, a platinum wire was attached and placed in an electric furnace, and the insulation resistance at each temperature was measured (Fettery method). The material of the metal pipe and the center rod used is sus.304.

The lattice strain was measured by X-ray diffraction (IR-1A, manufactured by Rigaku Denki) under the conditions of 40 kV, 20 mA, 1/4 deg / mm, and time constant of 5 sec. (1.1.1), (2.0.0), 2.2.0), (3.1.
1), (2.2.2), Shin performed (4.4.0) (ka ₁ measures the integration width of each peak 4.2.0), ka ₂ separate correction (Document 1), Standard Correction (Document 1) Find the half-value width of A Hall plot (Reference 2) was performed from the obtained half width, a slope was obtained from linear regression by the least squares method, and a value of 1/2 of the slope was defined as lattice distortion.

As a standard sample, a magnesia single crystal having an MgO purity of 99.9% was pulverized, and a material having a size of 44 to 20 μm which was heat-treated at 1300 ° C. for 5 hours was used. The measurement sample used also had a particle size adjusted to a range of 44 to 20 μm.

(1) "The measurement of particle size"
by X-ray method '' by FWJones., Prpc.Roy.Soc., A16
6,16 (1938).

(Reference 2) Hall, WH, Proc. Phys. Soc., A62., 741
(1949).

In addition, measurement of lattice constant was performed using silicon as an internal standard.
It was obtained from Bragg's equation. MgO (3.1.1), (2.2.0),
From the peaks of (3.1.1), (2.2.2), and (4.2.0), read the diffraction angle from the half-value width midpoint method, correct the separation of kα ₁ and kα ₂ , and calculate the lattice constant by the least square method. I asked.

In addition, metal pipes (cells) filled with the insulating filler
It was placed in an electric furnace at 800 ° C., and the change with time in the insulation resistance was measured.

FIG. 1 and FIG. 2 show the resistance values after firing at 800 ° C. for 24 hours or after firing for 72 hours.

The tap density and flow time of the powder are ASTM stand
American Boe by the method specified in ards D 2755
h Measured using a tool and tool from Die Company.

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

 The amounts (%) of the components shown in the examples are% by weight.

Example 1 and Comparative Example 1 A high-purity magnesia powder of 1 mm or less fired at a temperature of 2000 ° C. in a rotary kiln using a stainless steel wire mesh,
Screened from 420 μm to 25 μm. This was fired at a maximum temperature of 1200 ° C. using a rotary kiln.

2.5 kg of the above magnesia powder was placed in a 0.2N hydrochloric acid solution 10 and stirred for 5 minutes. The supernatant was discarded, water 10 was added, and the mixture was washed twice with stirring and vacuum filtered. Add 5 more water to the cake.
Was washed with water and dried in hot air at 120 ° C.
(Hereinafter, this treatment method is referred to as pickling with water.) The chemical composition, particle size distribution, flow time and initial insulation resistance of this magnesia powder, and the change with time of resistance at 800 ° C. are shown in Table 1 and FIG.

Further, as Comparative Example 1, the above measured values of magnesia powder used as a raw material are also shown.

Example 2 The magnesia powder sieved to 420 to 25 μm used in Example 1 was put into a box-type electric furnace, fired at each temperature of 800 to 1400 ° C., and then left in the furnace to room temperature. It was washed with acid water. FIG. 2 shows the change over time in the resistance of the magnesia powder at 800 ° C., and plots the values 24 hours and 72 hours after the change with respect to the heat treatment temperature.

It is shown that the resistance value is twice or more higher than that of a commercial electromagnet (for high temperature) shown in Comparative Example.

Example 3 The particle size distribution was adjusted from the sample used in Example 1 to prepare 1 to 5 samples having different packing densities by the same method as in Example 2, and the insulation was performed at 800 ° C. after 24 hours and 240 hours. The resistance was examined, and the results are shown in Table 2 and FIG.

[Effects of the Invention] As described above, the electric insulating filler of the present invention is 800
It has high insulation resistance at a high temperature of not less than ℃ and excellent flow characteristics, and is excellent as an insulating filler for a sheath heater. In addition, as shown in FIGS. 1 and 2, the deterioration of the insulation resistance is small, and the life of the heater is prolonged.

[Brief description of the drawings]

FIG. 1 shows the change over time of the insulation resistance at 800 ° C. in Example 1 and Comparative Example 1. Fig. 2 shows the heat treatment temperature and 800 magnesia powder after pickling.
Shows the resistance at ° C. Open circles indicate 24 Hr values and black circles indicate 72 Hr values. FIG. 3 shows the insulation resistance after 24 hours and 240 hours at 800 ° C. The symbols in the figure are as follows. White circles after 24 hours Black circles after 240 hours

Claims (2)

(57) [Claims] 1. Magnesia sintered powder a) Chemical composition i) MgO ≧ 93 wt% ii) 0.3 ≦ CaO ≦ 1.5 wt% iii) SiO ₂ ≦ 4 wt% iv) Fe ₂ O ₃ + Al ₂ O ₃ ≦ 0.4 wt% v) B ₂ O ₃ ≤ 0.1 wt% vi) Igloss ≤ 0.3 wt%, b) The lattice constant of MgO is 4.2130A or less c) The lattice distortion of MgO is 7.0 × 10 ^-4 or less Electrical insulation filler 2. A method for producing an electrically insulating filler, comprising: subjecting magnesia powder to a heat treatment at a maximum temperature of 1000 ° C. or higher, bringing the magnesia powder into contact with an acidic solution, and then thoroughly washing with water.