DE69502270T2 - Electrical conductor element such as a wire with an inorganic insulating coating - Google Patents

Description

Background of the invention Field of the invention

The present invention relates to an electrically conductive wire having an insulating sheath made of an inorganic material. Such wires are used for high temperature operating conditions, e.g. as an insulated connecting wire or the like.

Description of the technical background

An insulated conductor such as a wire or part for a thermoelectric element is generally used in devices such as heaters or fire alarm devices which must ensure safe operation at high operating temperatures. Such an insulated wire is also used in an automobile in an environment heated to a high temperature. An insulated wire of this type is generally constituted by a conductor coated with a heat-resistant organic resin such as polyimide, fluororesin or the like.

Such a resin-coated wire can only withstand a temperature of about 300ºC at the most. However, a wire used in, for example, a high vacuum apparatus must have high heat resistance against burnout and others, as well as low emission property of absorbed gas and water so as to achieve and maintain a high vacuum, and must have low emission of gases generated by thermal decomposition. It is impossible to satisfy such requirements for heat resistance and non-outgassing properties with a conventional cable coated with an insulation made of organic material.

It is impossible to achieve sufficient heat resistance or the required non-outgassing properties with an organic coating when an insulated wire is used where high heat resistance is required or in an environment requiring high vacuum. Therefore, in this case, generally used are wires comprising a conductor passing through a ceramic insulator tube or an MI (mineral insulated) cable comprising a conductor passing through a tube made of a heat-resistant alloy such as a stainless steel alloy filled with fine particles of a metal oxide such as magnesium oxide or the like.

Also known is a wire with a braided glass sheath, which uses an insulating part made of glass fiber cloth or the like and which serves as an insulated, heat-resistant flexible cable.

Furthermore, wires coated with inorganic materials have been studied. As a result, wires have been proposed, one of which is obtained by anodizing an aluminum (Al) conductor to form an Al oxide layer on the outer cable surface, and another wire is obtained by mixing a melt prepared by mixing various metal oxides and melting and pulverizing the mixture thus obtained to form a slurry, applying this slurry to a metal conductor, and heating and melting it to form a homogeneous composite metal oxide layer or coating on the wire surface.

However, the wire with an aluminum oxide layer is not suitable for use as a heat-resistant cable because this technique is only applicable to an aluminum conductor with a low melting point, while the film thus formed is so porous that the cable is not resistant to moisture and has a low breakdown voltage.

On the other hand, the wire with a composite metal oxide coating is also applicable to metal conductors made of copper (Cu) or nickel (Ni) which have higher heat resistance. In practice, however, this technique is only applicable to a composite metal oxide whose melting point is about 300-400ºC lower than that of Cu and Ni, because the composite metal oxide layer is formed by a melting process and the temperature at which the heat resistance is achieved is limited below the value just described. Furthermore, the wire thus formed is inferior in flexibility because it is difficult to reduce the thickness of the film.

On the other hand, in the case of MI cable, the total diameter increases compared to the conductor diameter, resulting in a disadvantageous space factor. Consequently, it is impossible to supply a high current.

In the insulated wire with a glass braid sheath, fine glass powder continues to be generated and the conductor is disadvantageously subject to detachment of the braid.

EP-A-0 494 424 discloses wires made of nickel or nickel alloy, with an inner oxide layer made of nickel or nickel alloy and an outer insulating layer made of an inorganic composition of SiO₂, Al₂O₃, MgO, ZrO₂ and mixtures thereof.

Summary of the invention

It is an object of the present invention to provide an inorganic insulating article such as an electric conductor cable which has excellent heat resistance and insulation ability. The inorganic insulating article or electric conductor cable according to the present invention comprises a conductor made of nickel or nickel alloy, an oxide layer made of an oxide made of nickel or nickel alloy on an outer surface of the conductor, the oxide layer being obtained by oxidizing the conductor in a vapor phase containing oxygen, an oxide layer made of silicon (Si) on an outer surface of the oxide layer of Ni or Ni alloy, and an oxide layer made of aluminum (Al) on an outer surface of the oxide layer of Si.

According to the present invention, the oxide layers of Al and Si are obtained by using a solution prepared by hydrolyzing and polycondensing alkoxides of Al or Si in a solvent, drying it to allow gelling and then heating the gel obtained.

According to the present invention, the oxide layers of Al and Si further have a melting point exceeding that of Ni or Ni alloy. The inorganic insulated article according to the present invention is used as or for a heat-resistant wire or a high-temperature indecomposable wire, which, for example, does not allow the use of an organic insulated material. However, the present invention is not limited to such wires, but is also applicable to other articles such as a thermocouple.

The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.

Short description of the drawings

Fig. 1 is a sectional view showing a wire according to the present Reference Example 1, having a conductor core made of nickel and two oxide layers;

Fig. 2 is a sectional view showing a wire according to the present Reference Example 2, having a core conductor made of a nickel alloy and two oxide layers;

Fig. 3 is a sectional view showing a wire according to the present invention having a core conductor made of nickel and three oxide layers; and

Fig. 4 is a sectional view showing a wire according to the present Reference Example 3, having a core conductor made of a nickel alloy and two oxide layers.

Description of the preferred embodiments

Fig. 1 shows a Ni core conductor 1 coated with the Ni oxide layer 2 formed around the core conductor. An Al oxide layer 3 is formed around the Ni oxide layer 2. The formation of these oxide layers is described in more detail below.

Fig. 2 shows a nickel alloy conductor 11 which is first coated with a nickel alloy oxide layer 12 which is applied around the nickel alloy conductor ration 11. A Si oxide layer 13 is formed around the Ni alloy oxide layer 12.

In Fig. 3, the Ni core conductor 21 is first coated with a Ni oxide layer 22, which is formed around the nickel core conductor 21. A Si oxide layer 23 is formed around the Ni oxide layer 22. Then, an Al oxide layer 24 is formed around the Si oxide layer 23.

In Fig. 4, a nickel alloy conductor 31 is first coated with a Ni alloy oxide layer 32 formed around the nickel alloy core conductor 31. Then, an Al-Si composite oxide layer 33 is formed around the Ni alloy oxide layer 32.

According to the present invention, first, a first oxide layer of Ni or a Ni alloy is formed on an outer surface of a conductor of Ni or a Ni alloy by oxidizing the conductor in a vapor phase containing oxygen. Then, a second oxide layer of Si is formed on the first oxide layer.

Ni or Ni alloy is an inactive metal which has a low affinity to a metal oxide of Al or Si. When a surface of Ni or Ni alloy is directly coated with such Al or Si oxide, rather poor adhesion is obtained and the coating is soon separated from the Ni or Ni alloy. To solve this problem, the present invention teaches to first oxidize a core conductor of Ni or Ni alloy in a vapor phase containing oxygen to form an oxide layer of Ni or Ni alloy. The nickel oxide layer or Ni alloy oxide layer thus formed adheres very strongly to the surface of the Ni or Ni alloy. This strong bond is based on the fact that the nickel oxide or the oxide of the nickel alloy has an excellent affinity to the Ni or Ni alloy. Furthermore, the nickel or nickel alloy oxide has a strong affinity to silicon oxide and thus strong bonds to the outer layer of Si oxide and to the conductor core. Consequently, according to the present invention, the oxide layer of Si does not separate from the intermediate oxide layer in practical use, whereby excellent flexibility is obtained when the inorganic insulation coating is applied to a wire constituting, for example, a core conductor.

According to the present invention, the oxide layers of Al and Si are obtained by using a solution prepared by hydrolyzing and polycondensing alkoxides of Al or Si in a solvent, drying it to perform gelation and subsequently heating the gel thus obtained.

The Al and Si oxide layers formed in the above-mentioned manner have a melting point that exceeds the melting point of Ni or Ni alloy. Furthermore, the Al and Si oxide layers were formed without any melting process.

Thus, the critical temperature to which the conductor or other articles of the present invention, together with their inorganic insulating coatings, may be exposed in service is not limited by the melting point of the oxide layer. Rather, the present insulating members may be heated to a temperature limited only by the melting point of the Ni core or the Ni alloy core.

Furthermore, the oxide layer formed in the above-mentioned manner has properties such as extreme density, a smooth surface and low adsorption of gases, e.g. steam or the like. The present articles also have excellent insulating properties and high moisture resistance.

Preferred embodiments were prepared as the two conductors C1 and C2 which were oxidized as follows.

(C1) A Ni wire of 0.5 mm diameter consisting of at least 99.9 wt% Ni, the balance being natural impurities, was heated in the atmosphere at 950ºC for 10 minutes to form a nickel oxide layer on the surface of the wire.

(C2) A Ni alloy wire with a diameter of 0.5 mm, containing 15 wt% of Cr, was heated in the atmosphere at 850 °C for 30 minutes to form an oxide layer of the Ni alloy on the wire surface.

The coating solutions L1, L2 and L3 were prepared as follows:

(L1) A solution L1 was obtained by mixing tributoxy aluminum, triethanolamine, water and isopropyl alcohol in molar ratios of 1:2:1:16. The mixture was hydrolyzed and polycondensed at 50°C for one hour with stirring.

(L2) A solution L2 was prepared by adding nitric acid to a premixed solution prepared by mixing tributyl orthosilicate, water and isopropyl alcohol in molar ratios of 2:8:15, at a rate of 3/100 moles with respect to the tetrabutyl orthosilicate. The mixture was hydrolyzed and polycondensed with stirring for 2 hours at 80°C.

(L3) A solution L3 was obtained by mixing solutions L1 and L2 with a mass ratio of 80:20.

Reference Example 1: An oxidized nickel conductor C1 was coated with the coating solution L1 and heated at 500ºC for 10 minutes. The coating and heating was repeated 10 times to form an Al oxide layer of 4 μm thickness on the first nickel oxide layer.

Reference Example 2: An oxidized nickel alloy conductor C2 was coated with the plating solution L2 and heated at 500ºC for 10 minutes. The plating and heating were repeated 10 times to form a Si oxide layer of 5 µm thickness on the first nickel alloy oxide layer.

Example 1: An oxidized nickel conductor C1 was coated with the coating solution L2 and heated at 500ºC for 10 minutes.

The coating and heating were repeated five times to form a Si oxide layer with a thickness of 2.5 µm. Then, another coating operation was performed with the coating solution L1 on the first-formed Si oxide layer. The sample was again heated at 500°C for 10 minutes. The coating and heating were repeated five times to form an Al oxide layer of 2 µm thick on the first-formed Si oxide layer of 2.5 µm thick.

Reference Example 3: An oxidized conductor C2 was coated with the coating solution L3 and heated at 500°C for 10 minutes. The coating and heating were repeated ten times to form an Al-Si composite oxide layer of 6 µm thick.

Comparative Example 1: An aluminum wire was anodized in a bath of sulfuric acid to form an Al oxide layer of 10 μm thickness on the aluminum surface.

Comparative Example 2: An oxidized conductor C2 was coated with a slurry prepared by mixing a commercially available melt (composite oxides of Ba, Ca, Ti and Si: GSP220A552 purchased from Tos hiba Glass Co., Ltd.) with water. The wire coated with the slip was heated to 900ºC to form a homogeneous metal composite oxide layer of 100 µm thick through a molten state.

All coating activities were carried out by immersing the wire in the respective coating solution. Test results

The above table shows the breakdown voltages and the flexibility values of the wires of Example 1 of the invention, Reference Examples 1 to 3 and the two Comparative Examples. The flexibility values were evaluated in terms of the diameter ratio obtained by winding the wire on circular cylinders having a prescribed diameter D and measuring the smallest diameter that does not cause separation of the insulating inorganic coating compositions or layers from the conductor core. The diameter D was 0.5 mm.

The above table shows that the wire of Example 1 according to the present invention has a higher breakdown voltage than the first comparative example as well as superior flexibility compared to both comparative examples.

However, the second comparison example has a much higher breakdown voltage, at the expense of being very stiff.

As described above, the inorganic insulating layer on a conductor wire according to the present invention forms an insulating inorganic compound layer which is very well bonded to the conductor core and has excellent heat resistance and insulation ability.

Although the present invention has been described and illustrated in detail, it is to be understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the scope of the present invention being limited only by the appended claims.

Claims (13)

1. An insulated electrical conductor comprising a core conductor (21) consisting essentially of a core material selected from the group consisting of Ni and Ni alloy, a first oxide layer (22) consisting of an oxide of the core material which has been produced on an outer surface of the core conductor (21) by oxidizing the core conductor in an oxygen-containing vapor phase, and a second oxide layer (23) bonded to an outer surface of the first oxide layer, the second oxide layer consisting essentially of Si oxide, characterized in that it additionally comprises a third oxide layer (24) on an outer surface of the second oxide layer (23), the third oxide layer (24) consisting essentially of Al oxide. 2. Insulated electrical conductor according to claim 1, wherein the second oxide layer (23) has a melting temperature which is higher than that of the core material (21). 3. An insulated electrical conductor according to claim 1, wherein the core conductor (21) consists of Ni and trace amounts of naturally occurring impurities. 4. An insulated electrical conductor according to claim 1, wherein the core conductor (21) consists essentially of Ni and Cr. 5. An insulated electrical conductor according to claim 4, wherein the core conductor (21) consists essentially of about 85% Ni and about 15% Cr. 6. A method of making an insulated electrical conductor, comprising: (a) manufacturing a core conductor (21) from a core material selected from the group consisting of Ni and Ni alloy; (b) forming a first oxide layer (22) on an outer surface of the core conductor (21) by oxidizing the core conductor in an oxygen-containing vapor phase; (c) preparing a first coating solution by hydrolyzing and polycondensing an alkoxide of Si in a solvent; (d) applying the first coating solution to the first oxide layer (22); (e) drying the first coating solution to cause it to gel; (f) heating the first coating solution applied to the first oxide layer (22) to form a second oxide layer (23) on the oxide layer, characterized in that it further comprises: (g) preparing a second coating solution by hydrolyzing and polycondensing an alkoxide of Al in a solvent; (h) applying the second coating solution to the second oxide layer (23) and (i) heating the second coating solution to form a third oxide layer (24) on the second oxide layer (23). 7. The method of claim 6, further comprising repeating steps (d) to (f) and/or (g) to (i) several times, whereby successive coating films of the coating solution are applied one on top of the other to form the second and third oxide layers (23, 24). 8. The method of claim 6, wherein step (f) does not involve a melting process. 9. The method of claim 6, wherein the second coating solution is prepared by forming a mixture of tributoxyaluminum, triethanolamine, water and isopropyl alcohol and then hydrolyzing and polycondensing the mixture. 10. The process according to claim 9, wherein the respective molar ratio of the tributoxyaluminum, triethanolamine, water and isopropyl alcohol is 1:2:1:16 and the hydrolyzing and polycondensing is carried out for 1 hour at 50°C while stirring the mixture. 11. The method of claim 6, wherein the first coating solution is prepared by forming a mixture of tributyl orthosilicate, water and isopropyl alcohol, adding nitric acid to the mixture, and then hydrolyzing and polycondensing the mixture with the added nitric acid. 12. The process according to claim 11, wherein the respective molar ratio of the tributyl orthosilicate, water and isopropyl alcohol is 2:8:15, the nitric acid is added in an amount of 3/100 mole based on the tributyl orthosilicate, and the hydrolysis and polycondensation are carried out for 2 hours at 80°C while stirring the mixture. 13. The method of claim 6, wherein the heating step is carried out at about 500°C.