JPH01149303A - Method for fabricating vertically oriented molecule insulating layer - Google Patents

Abstract

PURPOSE:To lessen the dielectric loss by filling the space between facing conductors requiring insulating layer with a polar monomer, applying electrostatic pressure between the conductors, polymerizing monomers, and by decreasing the coefficient of linear expansion in the thickness direction of the insulating layer as well as increasing the thermal conductivity. CONSTITUTION:The space between facing conductors requiring insulating layer is filled with polar monomer, which has large dipole moment but small after polymerization and which can produce high-molecular polymer having high crystallinity, and the monomer are polymerized under electrostatic field by applying electrostatic voltage between the mating conductors. Thus provides high-molecular insulating layer in which the molecule axis (c axis) in the crystal is oriented in the thickness direction of the insulating layer. This an insulating layer is obtained, which has small coefficient of linear expansion in the thickness direction, a large thermal conductivity, and small dielectric loss.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention provides a polymer insulating layer for cryogenic temperatures that has a small coefficient of linear expansion in the thickness direction of the insulating layer, a high thermal conductivity, and a small dielectric loss. Regarding the manufacturing method.

[Conventional technology]

Conventionally, polyimide has been widely used as a polymeric insulating material for cryogenic temperatures. This is because polyimide has excellent not only heat resistance but also low-temperature mechanical properties, and in particular, the linear expansion coefficient of polyimide is 2.0X10-"K", which is higher than that of ordinary polymer materials. However, as seen in input/output cables for cryogenic devices, even if polyimide with such a low coefficient of linear expansion is used as an insulating material, it will not work as well as the conductor copper wire. Expansion rate (1,4x 1o
-G K-1) and the linear expansion coefficient of silicon of the contact substrate (7,
The slight difference in linear expansion coefficient between 6X10-1 and -1) causes a difference in thermal contraction at extremely low temperatures, causing contact failure (P.

Volume 25, page 107, (f5'85)). Insulating materials for cryogenic temperatures must not only have a low coefficient of linear expansion, but must also match the coefficient of linear expansion of conductors and substrates.

The application of superconductivity has increased from liquid helium temperature (4.2K) to liquid nitrogen temperature (7K) with the development of high-temperature superconducting ceramics.
It is expected that the temperature will be raised to 7K) and the activity will become even more active, but it will likely continue to be carried out at extremely low temperatures. Insulating materials for such superconductors are required not only to have a low coefficient of linear expansion in order to have low thermal strain, but also to have excellent thermal conductivity. This requirement for thermal conductivity is due to the fact that when some abnormality changes from a superconducting state to a normal conducting state, heat is suddenly generated and the normal conducting region expands. This is because it is necessary to dissipate heat quickly. Thermal conductivity of polyimide (Qf1
0155W/, K) is the thermal conductivity of polyethylene and polyoxymethylene ((LOO55, [10057W, respectively)
15K), which is extremely low at 175 to 172 or less, making it an excellent heat insulating material, but in terms of thermal conductivity, it is not preferred as an insulating material for superconductors.

In addition, polyimide has tan J of n, ooa (10
1Hz, 25℃) and polyethylene [LOOOl] and polyoxymethylene (LOOO5).
Moisture absorption rate is 2.7 inches (immersed for 24 hours), moisture absorption expansion rate is 2.7 inches.
Due to its large size of 1×10″I% RH-1 and its size changing due to moisture absorption, its use as an insulating material for extremely low temperatures is limited.

With the development of superconducting applications, there is a need for cryogenic polymer insulating materials with low coefficient of linear expansion, high thermal conductivity, and low dielectric loss to replace polyimide.

By stretching a crystalline polymer such as polyethylene or polyoxymethylene, a polymer material in which molecular chains are oriented in the -axis direction can be easily obtained. The coefficient of linear expansion in the stretching direction of such a -axis oriented polymer rapidly decreases with the stretching ratio and changes from zero to a negative value [Eng. 1M, Ward (Eng.).

M, wara), Developments in Oriented Polymers-1
in 0rientea Polymers -1)
, Applied Science (Appl, set, )
Co., Ltd., London (1982)); the thermal conductivity in the stretching direction increases with the stretching ratio (0.L,
Choi (c, p, ahoy) et al., [Journal of Polymer Science Polymer Physics Edition (J, Polym, 8ci, Poly
m, Phys, ? ! 11. ) Volume 18, No. 118
7 (1980)). For example, the linear expansion coefficient (room temperature) of high-density polyethylene is 1.2X10-' in the unstretched state, zero at a stretch ratio of 2 times, and -1.2 at a stretch ratio of 18 times.
It becomes X10-sK-1. Thermal conductivity (room temperature) is α0055 W/(from WIK at a stretching ratio of 25 times) in the unstretched state [
It becomes significantly larger to 115W/. For polyoxymethylene, linear expansion coefficient.

(room temperature) is zero at a stretching ratio of 8 times from aOX10''sK''''1 in the unstretched state, and -C0X10 at a stretching ratio of 20 times.
-@ becomes ``1.Thermal conductivity (at room temperature) is 103 at a stretching ratio of 8 times from CLOO57W/cvnK in an unstretched state.
It becomes large as W/cmK. However, the coefficient of linear expansion in the direction perpendicular to the stretching direction slightly increases with the stretching ratio, and the thermal conductivity in the direction perpendicular to the stretching direction decreases. Therefore, by using a stretch-oriented polymer or a laminate thereof, it is possible to reduce the linear expansion coefficient in the stretching direction or in the plane direction, and also to control the thermal conductivity, but it is possible to increase the thermal conductivity. On the contrary, it increases, and the thermal conductivity decreases.

The tan δ of oriented polymers also has anisotropy, and as seen in polyoxymethylene, the tan δ in the direction of one molecular chain is about 1/10 of the tan δ in the direction perpendicular to the molecular chain direction (A, tan δ). Tanaka et al., Journal of Polymer Science (J, P.
oxym, sat, ) A-2, Volume 8, No. 15
p. 85 (1970)). In this way, dielectric loss can be reduced depending on the orientation direction of the oriented polymer.

[Problem that the invention seeks to solve]

As a polymer insulating material for extremely low temperatures, it has a small coefficient of linear expansion in the thickness direction of the insulating layer, a high thermal conductivity, and a high thermal conductivity.
It is desirable that δ is small.

For this reason, it is sufficient for K to orient its molecular chains in the thickness direction of the insulating layer, but a specific method for realizing this has not been found so far.

The purpose of the present invention is to orient the intracrystalline molecular axis (C axis) of the crystalline polymer forming the insulating layer in the thickness direction of the insulating layer, so that the coefficient of linear expansion in the thickness direction of the insulating layer is small. An object of the present invention is to provide a method for producing a polymer insulating layer for cryogenic temperatures that has high thermal conductivity and low dielectric loss.

[Means for solving problems]

To summarize the present invention, the present invention relates to a method for producing a vertically aligned polymer insulating layer. In this case, the dipole moment is conversely small and the electrostatic field can be reduced by filling the polar monomer ÷-, which can produce a high-crystalline polymer, and applying an electrostatic voltage between the opposing conductors. The monomer is polymerized below.

The present invention differs from conventional techniques in that a polymer insulating layer oriented in the thickness direction of the insulating layer can be obtained.

It is desirable that the polar magnet has a large dipole moment in order to effectively achieve orientation by an electric field. For example, acrylonitrile (OH2, . . . CI!ON) is a polar monomer that has an extremely large dipole moment of 17.3D and is suitable for electric field alignment.

However, polyacrylonitrile obtained by polymerization has poor crystallinity, so it has the desired low coefficient of linear expansion, high thermal conductivity,
Low dielectric loss cannot be expected.

Polyvinylidene fluoride has a high crystallinity of about 50°, and its raw material monomer, vinylidene heptadide (C-OF,
) is 1. S 7 D, and it is expected that oriented polyvinylidene heptadide will be produced by polymerization under an electrostatic field. However, as polyvinylidene fluoride is known as a piezoelectric material, the molecular chains within the crystal form a planar zigzag structure, resulting in a large dipole moment and the ability to align the dipoles in the direction of the electric field under an electrostatic field. (poling). In other words, due to the electrostatic field, the molecular axis (C
The axis) may be oriented in the plane direction, but not in the thickness direction. In polyoxymethylene, the molecular chains have a helical structure within the crystal, and the dipoles cancel each other out, so they are not oriented by an electrostatic field. It also has a high crystallinity of 7D or more, and its raw material monomer formaldehyde (O
The dipole moments of IIEt-0) and trioxane (axo)s are as high as Z2.7D%2.08D, respectively, and they are the most suitable materials for producing oriented polymers by electric field orientation polymerization. In this way, a polymer material suitable for the present invention must have a high dipole moment of its monomer, a small dipole moment of the polymer obtained by polymerization, and a highly crystalline polymer. be.

By irradiating a single crystal of trioxane or tetraogysan (a40)+, which is a raw material monomer for polyoxymethylene, with radiation (gamma rays, etc.), the monomer's b-axis direction (dipole direction) and K molecular axis (c-axis ) is known to produce polyoxymethylene crystals oriented [B) 1 Hayashi et al., Journal of Polymer Science (J, Po1.ym, Sci.
) O, AA, p. 839 (1965): Mu, Tanaka (A, Tanaka) et al., Journal of Polymer Science (J.

Polym 8ai, )Mu-2, Volume 8, @1sas
(1970)). FIG. 1 shows the orientation state of monomers and produced polymers during gamma ray polymerization of a tetraoxymethylene single crystal. In polyoxymethylene, the molecular axis (C axis) within the polymer crystal is oriented in the direction of the dipole of the monomer. Therefore, if monomers are oriented and polymerized under an electrostatic field,
An insulating layer of polyoxymethylene in which the molecular axis (C axis) within the crystal is oriented in the direction of the electric field, that is, oriented in the thickness direction, is obtained.

The orientation effect due to the electric field is due to the balance between the thermal energy of the dipole and the energy due to the electric field, resulting in an increase in the degree of orientation (coa m>
is expressed by the Langevin function (F, Debye (
P, Debye), Polar-%L/Cuels (
polar uoxecuxes), Dover Publications, Inc.
Publication Inc., ) (192
9) ).

<aoa e)! coth x −x−”=s
L (x)X Col I! ! /k T where μ is the dipole moment), llf is the electric field applied to the dipole, k is Boltzmann's constant, and T is the absolute temperature. This function initially increases linearly with a slope of 115 for the curve, and shows a tendency to saturate, approaching perfect orientation from the table. For example, if the thermal energy and the energy due to the electric field are equal (x=
1), < cos # ) wα5. This condition amounts to applying an electric field of about 10 M'7/61H to a 1D dipole K at room temperature. If there is no strong interaction between dipoles, this value is approximately the same as the strength of the externally applied electric field, but such a strong electric field usually causes dielectric breakdown of the material, so it cannot be achieved in practice. However, as seen in the poling of polyvinylidene fluoride, when there is a strong interaction between dipoles, the local electric field directly applied to the dipoles becomes high, and the local electric field directly applied to the dipoles increases to 0.2~(L 5 MV /,,
It is possible to achieve sufficient orientation even with an external electric field of . As can be seen from the equation, it is better to suppress thermal energy in order to enhance the alignment effect caused by the electric field. That is, K indicates that it is more desirable to carry out electric field alignment polymerization at a low temperature. Furthermore, the lower the temperature, the higher the dielectric breakdown voltage, so a higher voltage can be applied.

It is known that formaldehyde, one of the monomers of polyoxymethylene, can be polymerized by γ-ray irradiation at liquid nitrogen temperature (-196°C) to obtain high-yield polyoxymethylene with good thermal stability and high molecular weight. [Y, Tsuku (
Y, Tsula), Journal of Polymers
Science (J.

Polym 8ci, ) Volume 49, Page 569 (19
61)].

Polymerization is said to proceed in a solid phase at 118°C, near the melting point (-92°C) of 4c+-o-cyformaldehyde ionized by γ-ray irradiation. When formaldehyde is ionized, it has a large dipole moment of approximately 67).Since polymerization proceeds at a low temperature of 118°C, the orientation effect due to the electric field is large. FIG. 2 shows the orientation state of monomers and produced polymers during γ-ray solid phase polymerization under an electrostatic field of formaldehyde. On the other hand, tetraoxymethylene has a small dipole moment of about 2D and undergoes γ-ray solid phase polymerization at a high temperature of 80 to 100°C, so the electric field alignment effect is small. Note that since radiation polymerization is non-catalytic polymerization, it is possible to prevent the contamination of polar low-molecular impurities and is an effective polymerization method for reducing dielectric loss.

〔Example〕

Hereinafter, the present invention will be explained in more detail with reference to Examples.
The invention is not limited to these examples.

FIG. 3 shows an example of electric field oriented radiation solid phase polymerization of formaldehyde. That is, FIG. 3 is a schematic diagram of an apparatus used in one example of electric field oriented radiation solid phase polymerization of formaldehyde, where 1 is a polymerization tube, 2 is a vacuum cock, and 5 is a /
1 - mesh seal, 4 is lead wire, 5 is electrode cell, 6
means formaldehyde. Formaldehyde gas generated by thermal decomposition of commercially available powdered paraformaldehyde is passed through a dry column filled with molecular sieves, dried, and liquefied and collected at -78°C (dry ice - ethanol) in a pressure-resistant glass container containing phosphorus pentoxide in advance. did. Into the polymerization tube 1 is placed an electrode cell 5 in which transparent electrodes are attached to opposite sides of the central part of a parallel plate-shaped glass, with an electrode spacing of 10 μm. It is now possible to apply more static voltage. Dehydrated and purified formaldehyde was filled into the polymerization tube 1 at -78° C. via the vacuum cock 2 by vacuum distillation to such an extent that the lower part of the electrode cell 5 was immersed in the liquefied formaldehyde. Due to surface tension, the gap between the stain electrodes is filled with liquefied formaldehyde. In this state, a static voltage of 5oon (electric field (L)
The polymerization tube 1 was cooled with liquid nitrogen (-196° C.) with the tube still covered with 5 MY15@) to solidify formaldehyde. For γ-ray irradiation, use 00"O, first at a dose rate of 5X10
'Only 196°C in solid phase at 50°C/h while applying electrostatic voltage.
I went for 0 minutes. Next, with the electrostatic voltage applied, the γ-ray irradiation was interrupted, and with the polymerization tube 1 raised above the liquid nitrogen level, irradiation was continued for another 15 hours at the same dose rate, and the measurement was carried out with the electrostatic voltage applied. 4.65X10'R irradiation was performed. In such an operation, the temperature of the polymerization tube 1 is gradually raised to room temperature, so formaldehyde is not rapidly vaporized due to reaction heat near the melting point of formaldehyde at which solid phase polymerization begins, and the yield is approximately 100%. Polyoxymethylene was obtained.

The orientation state of polyoxymethylene thus obtained in the gap between the transparent electrodes was qualitatively investigated by birefringence observation from the direction perpendicular to the surface of the transparent electrode plate. A birefringence phenomenon due to a collection of randomly oriented needle crystals was observed at the edge of the electrode cell without a transparent electrode, but no birefringence was observed at the center of the electrode cell with a transparent electrode. It was found that the molecular axis (C axis) within the crystal was oriented perpendicular to the electrode plate surface.

The thermal conductivity of this insulating layer was α05 W/, the coefficient of linear expansion was zero, and the tan δ was 1×10−” (101 Hz, 25° C.).

〔Effect of the invention〕

As explained above, in the present invention, the polar monomer filled between opposing conductors that require an insulating layer is polymerized under an electrostatic field by applying an electrostatic voltage between the opposing conductors, thereby forming an insulating layer. The molecular axis within the crystal (,c
A polymeric insulating layer in which the axes) are oriented is obtained. As a result, there is an advantage that a cryogenic polymer insulating layer having a small coefficient of linear expansion in the thickness direction of the insulating layer, a high thermal conductivity, and a small dielectric loss can be obtained. Such a polymeric material having a small coefficient of linear expansion in the thickness direction, a large thermal conductivity, and a small dielectric loss is effective if used as an edge material for superconducting applications.

In particular, it is effective when used in the insulating layer of a superconducting magnet, which is strongly required to have a small coefficient of linear expansion in the thickness direction and a high thermal conductivity in the thickness direction, since the quench phenomenon can be suppressed. In addition, the insulating layers of superconducting microstrip lines and superconducting cables, which require low dielectric loss in the thickness direction, high thermal conductivity in the thickness direction, and low linear expansion coefficient, and low linear expansion coefficient. It is effective if used as an insulating layer of a superconducting lead-out cable, which requires high thermal conductivity in the thickness direction.

[Brief explanation of the drawing]

Figure 1 shows the orientation state of monomers and produced polymers during γ-ray polymerization of tetraoxymethylene single crystals, and Figure 2 shows monomers and produced polymers during γ-ray solid-state polymerization under formaldehyde electrostatic field. FIG. 3 is a schematic diagram of an apparatus used in one example of electric field oriented radiation solid phase polymerization of formaldehyde. 1: Polymer tube, 2: Vacuum cock, 3: Hermetic seal, 4: Lead wire, S: Electrode cell, 6: Formaldehyde patent applicant Nippon Telegraph and Telephone Corporation

Claims (2)

[Claims] 1. The dipole moment of a monomer is large between opposing conductors that require an insulating layer, but when it is polymerized, a polymer with a small dipole moment and high crystallinity is produced. By filling it with a polar monomer that can produce
A method for producing a vertically oriented polymer insulating layer, comprising polymerizing the monomer under an electrostatic field. 2. 2. The method for producing a vertically aligned polymeric insulating layer according to claim 1, wherein the polymer forming the insulating layer is polyoxymethylene.