JP2985090B2 - High temperature electric insulating filler and sheath heater filled with the same - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to an electrical insulating filler having excellent electrical insulation resistance at high temperatures.

[Related Art] Magnesium oxide (MgO) is used as an insulating filler of a sheath heater. This is because MgO has the characteristic that the electrical insulation resistance at high temperatures is very high.

After the sheath heater is filled with MgO and then manufactured through rolling reduction, annealing, sealing, and bending, the physical properties of the filler change in the process.

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 heating wire is damaged or re-crushed at the time of molding after filling, resulting in a shortened life.

Regarding the prevention of re-crushing during molding after filling with electrofused magnesia, Dynamite Nobel Co., Ltd.
It describes a method of adding a refractory oxide additive to the article.
However, even in this method, re-crushing could not be sufficiently prevented, and a reduction in insulation resistance could not be sufficiently prevented.

Japanese Patent Publication No. 56-4033 proposes a method of adding silica or alumina having a particle size of 1 μ or less to calcined magnesia. However, remilling cannot be sufficiently prevented, and the amount of addition is 1%.
O There was a risk of impairing the original high insulation properties.

In addition, sintered magnesia is easy to manufacture, less re-crushed during molding after filling compared to electrofused magnesia, and although it has attracted attention in recent years, the spherical shape described by the present inventors in JP-A-62-90807 has been noted. Even with sintered magnesia, re-fracturing during molding after filling was small, but the insulation resistance was not yet large enough to make thin heaters or high-temperature heaters.

Also, it has been known that if the carbon content of the filler is large, the "blackening" phenomenon occurs in the magnesia powder and the life of the sheath heater is shortened, but in the case of electrofused magnesia, the carbon electrode is used during melting. Some carbon content was inevitable.

[Problems to be Solved by the Invention] The present invention uses a filler having a high insulation resistance, an extremely low carbon content and a mixture of silica powder, and reduces a change in physical properties such as re-crushing during molding of a sheath heater. By doing so, it is intended to provide an electric insulating filler having a high electric insulation resistance at a high temperature.

[Means for Solving the Problems] MgO that causes impurities in magnesia to be unevenly distributed in a crystal or a crystal grain boundary or to form a solid solution like calcium oxide, resulting in low insulation resistance of sintered MgO. The lattice distortion was removed by a heat treatment, and an electrically insulating filler obtained by mixing a silica powder with a sintered magnesia powder having a very low carbon content was obtained by the heat treatment. The sheath heater manufactured from the filler obtained above had extremely high electric insulation resistance at high temperatures.

That is, according to the present invention, a) the chemical composition is MgO ≧ 93 wt% CaO ≦ 1.5 wt% SiO ₂ ≦ 4 wt% Fe ₂ O ₃ + Al ₂ O ₃ ≦ 0.4 wt% B ₂ O ₃ ≦ 0.1 wt%, and b) MgO lattice strain is at 20 × 10 ^-4 or less c) carbon content of 180ppm or less d) filling property 1.90~2.30g / cc e) Igloss primary particle size in the powder is ≦ 0.3 wt% is 30mμ less, An electrically insulating filler which is filled prior to molding of a molded product, characterized by mixing 0.1 to 0.9 wt% of a water-repellent silica powder, a method for producing the same, and a molding process using the electrically insulating filler. Sheath heater.

In the present invention, when the chemical composition is within the above range, the electric insulation resistance of the sheath heater is extremely high, and when the chemical composition is out of the above range, the electric insulation resistance of the sheath heater becomes low. Disappears. Especially
CaO is desirably 1.2 wt% or less.

Further, in the present invention, the MgO lattice strain needs to be 20 × 10 ⁻⁴ or less, and if it is out of the range, the electric insulation resistance of the sheath heater becomes low, so that the practical use for high temperature is lost.

More preferably, the lattice distortion of MgO is 10 × 10 ⁻⁴ or less.

In the present invention, the filling property of the magnesia powder is 1.90.
It must be within the range of ~ 2.30 g / cc. If it exceeds 230 (g / cc), pulverization will occur during production, and if it is less than 1.90 g / cc, the withstand voltage will deteriorate. Disappears. More preferably, the filling property of magnesia powder is 2.00 to 2.
A range of 20 g / cc is desirable.

In the present invention, the carbon content of MgO is 180 ppm
If the temperature is higher than the above range, the resistance value of the sheath heater greatly decreases, the life is short, and practicality for high temperature use is lost. More preferably, the carbon content of MgO is 100 ppm or less.

The primary particle size of silica powder mixed with magnesia powder is 30
mμ = (0.03 μm) or less, and if it is out of the above range, the electric insulation resistance of the sheath heater becomes low, so that it is not practical for high temperature use. Preferably, the primary particle size of the silica powder is 12 mμ or less.

The specific surface area of the silica powder mixed with the magnesia powder should preferably be at least 84 m ² / g, and if it is outside the above range, the electrical insulation resistance of the sheath heater will be low, which is not preferable for high temperature use. More preferably, the specific surface area of the silica powder is 200 m ² / g or more, and more preferably, the specific surface area of the silica powder is 300 m ² / g or more.

In addition, the surface of the silica powder must have a cyanol group and have water repellency, which prevents water and the like from being introduced into the sheath when filling. However, a functional group having carbon such as an alkyl group is not preferable because it causes resistance deterioration.

The amount of silica powder mixed with magnesia powder is 0.1 to
The content must be 0.9 wt%, and if it is less than 0.1 wt%, the effect of reducing re-crushing during molding processing is lost, the electrical insulation resistance of the sheath heater is reduced, and the practical use for high temperatures is lost. On the other hand, if the content exceeds 0.9% by weight, excess silica powder is peeled or segregated at the time of filling, and the filling property of the raw material is reduced, the withstand voltage of the heater is deteriorated, and the practical use as a high temperature use is lost. Therefore, in the present invention, silica powder having a large specific surface area is 0.9 wt%.
The following mixing prevents crushing and does not impair the intrinsic properties of MgO. More preferably, the mixed amount of silica powder is 0.3 to 0.7 wt%.

The silica powder is desirably amorphous silica synthesized by a gas phase method.

Further, the lattice distortion of MgO of the above-mentioned magnesia powder taken out from the sheath heater manufactured from the magnesia powder of the present invention must be 15 × 10 ⁻⁴ or less, and when the above-mentioned range is not satisfied, the sheath heater is not heated. Since the electric insulation resistance is low, the practicability for high temperature use is lost. More preferably, the lattice distortion of MgO is 7 × 10 ⁻⁴ or less.

Magnesia powder used in producing such a filler for the sheath heater can be obtained by a general method, but can be more easily obtained by heat-treating magnesia powder at a maximum temperature of 1000 ° C. or more. It is.

Here, when the maximum temperature is 1000 ° C. or less, the effect of the heat treatment is small, and the temperature is preferably 1000 ° C. or more, preferably 1200 to 1400 ° C.

In the present invention, after contacting the acidic solution, the maximum temperature 1000
It is more preferable to perform the heat treatment at a temperature of not less than ° C.

Further, in the present invention, as long as the magnesia powder as the filler is within the above range, any of electrofused magnesia and sintered magnesia may be used, but sintered magnesia is preferable.

Also, the magnesia powder passed through a 420 μm sieve,
It is appropriate to collect a portion that does not pass through a μm sieve.
Further, the powder may contain other components such as an auxiliary agent such as ZrO _{2 to the} extent that there is no influence.

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 , and B ₂ O ₃ are obtained by hot-dissolving magnesia powder with an aqueous hydrochloric acid solution, and ZrO ₂ is a mixture of sodium carbonate and carbonate borate, alkali-melted, and then nitric acid aqueous solution. And heat-melted to 575-II made by Nippon Jarrell Ash.
It was measured using CPA.

The Ig-loss is the result of accurately weighing 10 g of a sample, placing it in a platinum crucible, and heating it in an electric furnace at 1000 ° C. for 1 hour, and showing the weight loss as a percentage of the original weight.

Further, the carbon content of the magnesia powder is obtained by blowing oxygen after blowing the sample powder at a temperature of 2300 ° C. in a nitrogen stream, and reacting the carbon with oxygen to form carbon dioxide. It was measured by infrared absorption. The measuring instrument is a Loec-CS44 type from Reco.

 The specific surface area of the powder was measured by the BET method.

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), (4.0.0), (each peak integration width of 4.2.0) was measured, ka _1, ka ₂ separate correction (Document 1),
Standard correction (Reference 1) is performed to determine the true half width. A Hall plot (Reference 2) is performed from the obtained half-value width, and a slope is obtained from linear regression by the least double method.
The value of 1/2 was regarded as lattice distortion. The standard sample was prepared by grinding a magnesia single crystal with an MgO purity of 99.9%,
m was heat-treated at 1300 ° C. for 5 hours. The measurement sample used had a particle size of 44 to 20 μm.

(Reference 1) "The measurement of particle size b
y the X-ray method '' by FWJones., Prpc.Roy.Soc., A
166,16 (1938).

(Reference 2) Hall, WH, Proc. Phys. Soc., A62., 741 (1
949).

The packing density and flow time of the powder are as per ASTM standar
American Boeh by the method specified in ds D 2755
The measurement was performed using an apparatus manufactured by Tool and Die Company.

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

The sheath heater used in Examples and Comparative Examples of the present invention was filled with magnesia powder in the gap between a nichrome wire having a wire diameter of 0.45 mm and an outer diameter of 8 mm and a length of 650 mm of an incoloy pipe.
After rolling reduction to 8 mm, annealing at 1050 ° C. for 30 minutes, a glass seal and a silicon seal were used.

Further, the insulation resistance test of the sheath heater filled with magnesia powder was performed up to 2000 times by applying ON voltage of 100 V and repeating ON for 10 minutes and OFF for 10 minutes.

Example 1 and Comparative Example 1 Predetermined 1 mm or less high-purity magnesia powder obtained by firing at a temperature of 2000 ° C. in a rotary kiln was sieved from 420 μm to 25 μm using a stainless steel wire mesh. Silica powder having a primary particle diameter of 12 mμ, a specific surface area of 200 m ² / g and having a silanol group on the surface was mixed with a 0.5 wt% mixer.

The chemical composition of this magnesia powder, particle size distribution, filling properties,
Table 1 shows the flow time, lattice strain, and initial insulation resistance. Further, a sheath heater was manufactured using the magnesia powder as a raw material, and the change with time of the insulation resistance under predetermined conditions is shown in FIG.

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

In FIG. 1, in Example 1 (A), the decrease in insulation resistance up to 2,000 repetitions was 30%, while in Comparative Example 1
(X) is 75%.

Example 2 A magnesia clinker fired at a temperature of 2000 ° C. in a rotary kiln was crushed, and a stainless steel wire mesh was used.
After sieving at 420 μm to 25 μm, the mixture was washed with an acidic solution having a pH of 3 or less. After heat-treating this at 1200 ° C., silica powder having a primary particle diameter of 12 μm, a specific surface area of 200 m ² / g and having a silanol group on the surface was mixed using a 0.5 wt% mixer.

The chemical composition of this magnesia powder, particle size distribution, filling,
Table 2 shows the flow time, lattice strain, and initial insulation resistance. Further, FIG. 1 shows the result of measuring the change with time of the insulation resistance under a predetermined condition under the condition that a sheath heater was manufactured using the magnesia powder as a raw material.

As shown in FIG. 1, Example 2 (B)
The decrease in insulation resistance up to 32 times is 32%.

Example 3 A sheath heater was prepared using magnesia powders having different carbon contents, and a life test was performed by the method described above.
The results are shown in Table 3. The chemical composition, particle size distribution, filling properties, and flow time are the same as in Example 1.

Example 4 A magnesia powder used in Example 1 was mixed with silica powder having a different primary particle diameter and specific surface area, and then a heater was prepared. The initial resistance (7 w / cm ² ) was measured by the method described above. The results are shown in Table 4.

Example 5 The magnesia powder used in Example 2 had a primary particle diameter of 12 m.
μ, silica powder having a specific surface area of 200 m ² / g and having a silanol group on the surface was added in the amount shown in Table 5 and mixed using a mixer. A heater was manufactured using this raw material, and the initial resistance (7 w / cm ² ) was measured by the method described above. The results are shown in Table 5.

Example 6 The filling density of the magnesia powder used in Example 1 was changed,
Primary particle size 12Emumyu, a silica powder having a specific surface area of 200m ² / g 0.5wt
%, A heater was prepared, and the initial resistance and the change with time were measured by the above-described method. The results are shown in Table 6.

Example 7 Magnesia powders having different lattice strains have a primary particle diameter of 12 m
After mixing 0.5% by weight of silica powder having a specific surface area of 200 m ² / g, a heater was prepared, and the initial insulation resistance was measured. The results are shown in Table 7. As the lattice distortion, the value of the magnesia powder used as the raw material and the value of the magnesia powder taken out from the heater are also shown. The chemical composition, particle size distribution, filling properties, and flow time are the same as in Example 1.

Example 8 After heating and cooling the magnesia powder of Example 1 at 1200 ° C. for 1 hour, 0.5 wt% of a silica powder having a primary particle diameter of 12 μm and a specific surface area of 200 m ² / g was mixed, and a heater was prepared. The insulation resistance was measured, and the results are shown in Table 8.

[Effects of the Invention] As described above, the electrically insulating filler of the present invention is extremely excellent as a raw material for a sheath heater, and the sheath heater manufactured from this material has not only an excellent initial insulation resistance but also an excellent insulation resistance. Deterioration of the heater is extremely small, and the service life of the heater becomes very long.

[Brief description of the drawings]

FIG. 1 shows Example 1 (A), Example 2 (B), Comparative Example 1.
It is a graph which shows the time-dependent change of the insulation resistance of (X).

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-62-90807 (JP, A) JP-A-1-166481 (JP, A) JP-A-2-307821 (JP, A) JP-A-3-307 210705 (JP, A) JP 56-4033 (JP, B2) (58) Fields investigated (Int. Cl. ⁶ , DB name) H01B 3/12 333 H05B 3/48 H05B 3/10 C04B 35/04 C01F 5/02

Claims (2)

(57) [Claims] 1. Magnesia a) The chemical composition is MgO ≧ 93 wt% CaO ≦ 1.5 wt% SiO ₂ ≦ 4 wt% Fe ₂ O ₃ + Al ₂ O ₃ ≦ 0.4 wt% B ₂ O ₃ ≦ 0.1 wt% b) The lattice distortion of MgO is 20 × 10 ^-4 or less c) The carbon content is 180 ppm or less d) The filling property is 1.90 to 2.30 g / cc e) The primary particle diameter is 30 mμ or less in the powder having Igloss ≦ 0.3 wt% An electric insulating filler which is filled prior to molding of a molded product, wherein 0.1 to 0.9 wt% of water-repellent silica powder is mixed. 2. A molded sheath heater characterized by being filled with the electrically insulating filler according to claim (1).