JPH03250504A - High temperature electric insulating filler and sheath heater filled therewith - Google Patents

Abstract

PURPOSE:To reduce insulation resistance deterioration to lengthen the life of a heater greatly by mixing a prescribed amount of silica powder of prescribed characteristics with magnesia powder of prescribed composition and prescribed characteristics. CONSTITUTION:The chemical composition of magnesia is shown in Formula I to Formula VI. Water repellent silicon powder whose primary grain size is 3mmu or less is mixed at 0.1-0.9wt.% in the magnesia powder having the characteristics shown in Formula VII-Formula IX to be used as an electric insulation filler for a sheath heater. The sheath heater filled with this filler is not only good in initial insulation resistance but also the deterioration of the insulation resistance is extremely little and the life of the heater can be very much lengthened.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an electrically insulating filler having excellent electrical insulation resistance at high temperatures.

[Prior art] Magnesium oxide (
MgO) is used. This is because MgO has an extremely high electrical insulation resistance at high temperatures.

A sheath heater is manufactured by filling it with MgO and then performing rolling, annealing, sealing, and bending, but the physical properties of the filler change during this process.

The conventionally used electrofused magnesia is obtained in large lumps due to its manufacturing method, so in order to use it as an insulating filler for thin sheath heaters, it must be crushed and sized, and the crushed particles are angular. Because it has a shape,
Not only was it difficult to fill, but the heating wires were damaged or re-fractured during the molding process after filling, resulting in a shortened service life.

Regarding the prevention of re-crushing during molding after filling with electrofused magnesia, Dynamite Nobel Co., Ltd. published Japanese Patent Application Laid-Open No. 51-150.
No. 094 describes a method for adding refractory oxide additives. However, even with this method, re-fracture could not be sufficiently prevented, and a decrease in insulation resistance could not be sufficiently prevented.

Japanese Patent Publication No. 5B-4033 proposes a method of adding silica or alumina of 1μ or less to calcined magnesia, but not only does it not sufficiently prevent re-grinding, but the amount added is only 1%, which does not interfere with MgO's original high insulating properties. There was a risk of sexual deterioration.

In addition, sintered magnesia is easy to manufacture and requires less re-crushing during molding after filling compared to fused magnesia, and has attracted attention in recent years. Even with sintered magnesia, there is little re-fracture during molding after filling, but the insulation resistance is still not large enough to make thin heaters or high-temperature heaters.

It is also known that if the filler has a high carbon content, the magnesia powder will undergo a "blacking" phenomenon, which will shorten the lifespan of the sheath heater. Some amount of carbon content was unavoidable.

[Problems to be Solved by the Invention] The present invention uses a filler that has high insulation resistance, extremely low carbon content, and is mixed with silica powder, thereby minimizing changes in physical properties such as re-crushing during molding of the sheath heater. By doing so, it is intended to provide an electrically insulating filler having high electrical insulation resistance at high temperatures.

[Means for solving the problem] Impurities in magnesia may be unevenly distributed within the crystals or grain boundaries, or may be dissolved in solid solution like calcium oxide, resulting in sintered MgO
Through heat treatment, we were able to remove the lattice distortion of MgO that causes low insulation resistance, and through heat treatment, we were able to obtain an electrical insulating filler made by mixing silica powder with sintered magnesia powder, which has an extremely low carbon content. . The sheath heater manufactured from the filler obtained above had extremely high electrical insulation resistance at high temperatures.

That is, the present invention provides a) chemical composition of ■ MgO≧93νt% ■ CaO≦1.5 wt% ■ 5iOz≦4 vt% ■ FezO3+Al2O3≦0.4 wt% ■ B2O
3≦0.1wt% ■ Igloss≦0.3wt%, b) MgO lattice strain is 20X10” or less C) Carbon content is 180pp- or less d) Filling property is 1.90 to 2.
30 g/cc of powder with a primary particle size of 30 (-μ) or less and water-repellent silica powder at 0.1 = 1. Owt
% of the electrically insulating filler, a method for producing the same, and a sheath heater using the electrically insulating filler.

In the present invention, when the chemical composition is within the above range, the electrical insulation resistance of the sheath heater is extremely high, and when the chemical composition is outside the above range, the electrical insulation resistance of the sheath heater becomes low, making it impractical for high temperature applications. It disappears. Especially C
It is desirable that aO is i, 2vt% or less.

Further, in the present invention, it is necessary that the MgO lattice strain is 20x10' or less, and if it is out of this range, the electrical insulation resistance of the sheath heater becomes low, making it impractical for high temperature applications.

More preferably, the lattice strain of MgO is IOX 10'' or less.

In addition, in the present invention, the filling property of magnesia powder is 1.9
Must be in the range of 0 to 2.30 g/cc, 2,3
If it exceeds 0 <g/Qc,>, pulverization will occur during production,
Moreover, if it is less than 1.90 g/cc, the withstand voltage deteriorates, and both become impractical for high temperature applications. More preferably, the magnesia powder has a filling property of 2. oO~2.20g/CC,
A range of is desirable.

Further, in the present invention, the carbon content of MgO is 180
It is necessary that the temperature is less than pp. If it exceeds this value, the resistance value of the sheath heater will decrease significantly and its life will be short, making it impractical for high temperature applications. More preferably, the carbon content of MgO is 1100 pp or less.

The primary particle size of silica powder mixed with magnesia powder is 30
It must be less than mμ-(0.03μ■), and if it is out of the above range, the electrical insulation resistance of the sheath heater will become low, making it impractical for high-temperature applications. Preferably, the primary particle size of the silica powder is 12 mμ or less.

The specific surface area of the silica powder to be mixed with the magnesia powder must preferably be 84112/g or more, and if it is out of the above range, the electrical insulation resistance of the sheath heater will become low, making it undesirable for high-temperature applications. More preferably, the specific surface area of the silica powder is 200 m/g or more, and it is further desirable that the specific surface area of the silica powder is 30011+2/g or more.

Furthermore, the surface of the silica powder must have a shear tool group and be water repellent, which prevents moisture from being brought into the sheath during filling. However, carbon-containing functional groups such as alkyl groups are undesirable because they cause resistance deterioration.

Also, the amount of silica powder mixed with magnesia powder is 0.1~
The content needs to be 0.9 wt%, and if it is less than 0.1 wt%, the effect of reducing re-fracture during molding is lost, the electrical insulation resistance of the sheath heater becomes low, and it becomes impractical for high-temperature applications. On the other hand, if it exceeds 1. Ovt%, excess silica powder may peel off or segregate during filling, and the filling properties of the raw material may deteriorate, resulting in poor dielectric strength of the heater, making it impractical for high-temperature applications. Therefore, in the present invention, by mixing 0.9% or less of silica powder with a large specific surface area, crushing is prevented and the original properties of MgO are not impaired. More preferably, the amount of silica powder mixed is 0.3 to 0.71 ilt%.

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

In addition, the lattice strain of MgO in the magnesia powder taken out from the sheath heater manufactured from the magnesia powder of the present invention must be 15x IQ-4 or less, and if it is out of the above range, the electricity of the sheath heater will be affected. Since the insulation resistance becomes low, it becomes impractical for high temperature applications. Furthermore, MgO
It is more desirable that the lattice strain is 7×10 − or less.

The magnesia powder used to manufacture the filling material for such sheath heaters can be obtained by general methods, but it can be obtained more easily by heat-treating magnesia powder at a maximum temperature of 1000°C or higher. It is.

Here, the effect of heat treatment is small when the maximum temperature is below 1000°C, and above 1000°C, preferably 1200 to 1400°C.
is desirable.

In the present invention, the maximum temperature after contacting the acidic solution is 100%.
It is more preferable to perform the heat treatment at 0° C. or higher.

Further, in the present invention, the magnesia powder serving as the filler may be either fused magnesia or sintered magnesia as long as it falls within the above range, but sintered magnesia is preferred.

In addition, the magnesia powder passed through a 420μ intestinal sieve, and 2
It is appropriate to collect the portion that does not pass through a 5μ sieve. Further, the powder may contain other components such as auxiliary agents such as Z ro2 to the extent that they do not have any adverse effects.

[Examples] Hereinafter, the present invention will be specifically explained using Examples and Comparative Examples.

Among the chemical compositions of Examples in the present invention, MgO1CaO
1SiO2, Fe2e3, Al2O3, and BzO3 are prepared by dissolving magnesia powder in a hydrochloric acid aqueous solution and then Z r0
In No. 2, a mixture of sodium carbonate and borate carbonate was melted in an alkali, then hot-dissolved in an aqueous nitric acid solution, and measured using an ICPA manufactured by Nippon Jarrell Ash Co., Ltd., 575-II.

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

Further, the carbon content of the magnesia powder is determined by heating the sample rice to a temperature of 2300° C. in a nitrogen stream and then blowing oxygen into it to react the carbon with oxygen to convert it into carbon dioxide. It was measured by infrared absorption.

The measuring instrument was a Leco-CS 44 model manufactured by Leco Co., Ltd.

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

Lattice strain is measured by X-ray diffraction (IR-IA model manufactured by Rigaku Corporation)
40kV, 20mA, l/4deg/a from 1 coil
M under the condition of s, time constant 5sec
(1,1,1), (2,0,0), (2,2,
0), (3,1,1), (2,2゜2), (4,0
, 0), (4, 2, 0), and
Separate correction of kαl and kα2 (Reference 1) and standard correction (Reference 1) are performed to determine the true half-width. A Hall plot (Reference 2) was performed from the obtained half-value width, the slope was determined by linear regression using the least squares method, and the value of 1/2 of the slope was taken as the lattice distortion. The standard sample has MgO purity of 99.
．． After crushing 9% magnesia single crystal, 44~20
The sample μ■ was heat-treated at 1300° C. for 5 hours. The measurement samples also used had a particle size of 44 to 20 μm.

(Reference 1 above) r The measurement
of particle 5ize by the
X-ray methodJ by FV,J
ones,, Prpc, Roy, Soc,, A 16
6.1B (1938).

(Reference 2 above) Hall, W. H., Proc, Ph.
ys, 5ocA82. ．． 741 (1949).

In addition, the packing density and flow time of the powder comply with ASTM 5.
Boeh Tool and Die in the United States according to the method specified in standards D 2755.
The measurement was performed using a device manufactured by the company.

Particle size distribution was determined using a JIS standard sieve.

In addition, the sheath heater used in the examples and comparative examples of the present invention was made by filling the gap between a nichrome wire with a wire diameter of 0.45 av and an Incoloy vibrator with an outer diameter of 8I111s and a length of 6501mm with magnesia powder, and then up to 6.8cm. The diameter was reduced by rolling, annealed at 1050° C. for 30 minutes, and then sealed with glass and silicon.

Furthermore, the insulation resistance test of the sheath heater filled with magnesia powder was performed by applying 100V and 20 minutes 0N to 10 minutes OF.
F was repeated up to 2000 times.

Example 1 and Comparative Example 1 High-purity magnesia powder having a predetermined Ig+v or less obtained by firing at a temperature of 2000° C. in a 0-tally kiln was sieved using a stainless steel wire mesh at a size of 420μ to 25μ. In addition, the secondary particle diameter is 12 mμ, and the specific surface area is 200 2
/g of silica powder having silanol groups on the surface.
Mixing was performed using a 5vt% mixer.

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

In addition, as Comparative Example 1, the above measured values of magnesia powder used as a raw material are also listed.

In Table 1 and FIG. 1, the reduction in insulation resistance of Example 1(A) up to 2000 repetitions is 30%, while that of Comparative Example 1(x) is 75%.

Example 2 Magnesia clinker fired at 2000°C in a 0-tally kiln was crushed, sieved from a 420μ layer to a 25μ layer using a stainless wire mesh, and then washed with water with an acidic solution having a pH of -3 or less. . After heat-treating this at a temperature of 1200℃, the -order particle size was 121μ and the specific surface area was 200w2.
/g of silica powder with silanol groups on the surface.
Mixed using a wt% mixer.

The chemical composition, particle size distribution, filling property, flow time, lattice strain, and initial insulation resistance of this magnesia powder are shown in Table 2. Furthermore, a sheath heater was made using this magnesia powder as a raw material, and the change in insulation resistance over time was measured under predetermined conditions. The results are shown in FIG.

As shown in Table 2 and Figure 1, Example 2 (B) was repeated 200 times.
The decrease in insulation resistance up to 0 times is 32%.

Example 3 Sheathed heaters were manufactured using magnesia powders having different carbon contents, and life tests were conducted using the method described above. The results are shown in Table 3.

Note that the chemical composition, particle size distribution, filling property, and flow time are the same as in Example 1.

Table 3 Example 4 After mixing silica powder with different primary particle diameter and specific surface area with the magnesia powder used in Example 1, a heater was made,
Measure the initial resistance (7w/cm2) using the method described above,
The results are shown in Table 4.

Table 4 Example 5 The magnesia powder used in Example 2 had a particle diameter of 12 m.
Silica powder having μ, specific surface area of 200 μm and 21 g and having silanol groups on the surface was added in the amount shown in Table 5, and mixed using a mixer. A heater is made using this raw material,
The initial resistance (7v/c@2.) was measured using the method described above, and the results are shown in Table 5.

Table 5 Example 6 The packing density of the magnesia powder used in Example 1 was changed, -
After mixing 0.5vt% of silica powder with a secondary particle size of 121μ and a specific surface area of 200m'/g, a heater was prepared, and the initial resistance and its change over time were measured using the method described above.The results are shown in Table 6. It was shown to.

Table 6 Example 7 Magnesia powder with different lattice strains has a primary particle diameter of 11 sμ
After mixing 0.5vt% of silica powder with a specific surface area of 200m27g, a heater was prepared, and the initial insulation resistance was measured.
The results are shown in Table 7. In addition, for the lattice strain, the value of the magnesia powder used as a raw material and the value of the magnesia powder taken out from the heater are listed together. In addition, chemical composition, particle size distribution,
The filling properties and flow time are the same as in Example 1.

Example 8 The magnesia powder of Example 1 was heated at 1200°C for 1 hour.
After cooling, -order particle size 12+μ, specific surface 1II200
After mixing 0.5 wt % of 12/g silica powder, a heater was prepared and the initial insulation resistance was measured. The results are shown in Table 8.

[Effects of the Invention] As explained above, the electrical insulating filler of the present invention is extremely excellent as a raw material for sheathed heaters, and sheathed heaters manufactured from it not only have excellent initial insulation resistance but also have low insulation resistance. Deterioration is extremely small, and the lifespan of the heater is extremely long.

[Brief Description of the Drawings] Figure 1 shows Example 1 (A), Example 2 (B), and Comparative Example 1 (
It is a graph which shows the time-dependent change of the insulation resistance of x).

Claims (1)

[Claims] (1) Magnesia a) Its chemical composition is (1) MgO≧93wt% (2) CaO≦1.5wt% (3) SiO_2≦4wt% (4) Fe_2O_3+Al_2O_3≦0.4wt% ( 5) B_2O_3≦0.1wt% (6) Igloss≦0.3wt%, b) MgO lattice strain is 20×10^-^4 or less c) Carbon content is 180ppm or less d) Filling property is 1. An electrically insulating filler characterized in that 0.1 to 0.9 wt % of water-repellent silica powder having a primary particle diameter of 30 mμ or less is mixed with a powder of 90 to 2.30 g/cc. (2) A sheath heater, characterized in that it is filled with the electrically insulating filler according to claim (1).