EP0729157B1 - Electrical conductor member such as a wire with an inorganic insulating coating - Google Patents

Description

BACKGROUND OF THE INVENTIONField of the Invention

The present invention relates to an electricallyconducting wire with an insulating coating made of aninorganic material. Such a wire is used for hightemperature operating conditions, e.g. as an insulatedlead wire or the like.

Description of the Background Art

An insulated conductor such as a wire or a member fora thermocouple is generally used in equipment such asheating equipment or fire alarm devices, which requiresafe operation at high operating temperatures. Such aninsulated wire is also employed in an automobile in anenvironment which is heated to a high temperature. Aninsulated wire of this type is generally formed by aconductor which is coated with a heat-resistant organicresin such as polyimide, fluororesin or the like.

Such a resin-coated wire can merely withstand atemperature of about 300°C at the most. However, a wirewhich is employed in a high vacuum apparatus, for example,must have high heat resistance against baking, etc., asmall emission characteristic as to absorbed gas and water for achieving and maintaining a high degree of vacuum,and a small emission of gases caused by thermaldecomposition. It is impossible to satisfy suchrequirements for heat resistance and a non-outgassingproperty with a conventional wire which is coated with anorganic material insulation.

When an insulated wire is used where a high heatresistance is required or in an environment requiring ahigh degree of vacuum, it is impossible to attain asufficient heat resistance nor the required non-outgassingproperty with only an organic coating. In that case,therefore, an insulated wire is comprising a conductorwhich passes through an insulator tube of ceramics, an MI(mineral insulated) cable comprising a conductor whichpasses through a tube of a heat-resistant alloy, such asstainless steel alloy, that is filled with fine particlesof a metal oxide such as magnesium oxide, or the like isgenerally used.

On the other hand, a glass braided tube insulatedwire employing an insulating member of glass fiber fabricor the like is known as an insulated, heat resistant,flexible wire.

Further, wires coated with in organic materials werestudied. As a result, wires have been proposed, one ofwhich is obtained by anodizing an aluminum (Aℓ) conductor for forming an Aℓ oxide layer on the outer wire surface,and another wire is obtained by mixing a frit prepared bymixing various metal oxides with each other and meltingand pulverizing the as-obtained mixture for forming aslip, applying this slip to a metal conductor and heatingand melting the same for forming a homogeneous compositemetal oxide layer or coating on the wire surface.

However, the wire with an aluminum oxide layer is notsuitable for use as a heat resistant wire since thistechnique is merely applicable to an aluminum conductorhaving a low melting point, while the as-formed film is soporous that the wire has an inferior moisture resistanceand a low breakdown voltage.

On the other hand, the wire with a composite metaloxide coating is applicable to a metal conductor of copper(Cu) or nickel (Ni) having a higher heat resistance. Inpractice, however, this technique is merely applicable toa metal composite oxide whose melting point is lower byabout 300 to 400°C than those of Cu and Ni since the metalcomposite oxide layer is formed through a melting process,and the heat resistance temperature is restricted belowthe just mentioned level. Further, the as-formed wire isinferior in flexibility since it is difficult to reducethe thickness of the film.

In the case of the MI cable, on the other hand, the overall diameter is increased as compared with theconductor diameter, leading to an inferior space factor.Thus, it is impossible to feed a high current.

In the glass braided tube insulated wire, further,fine glass powder is generated and the conductor isdisadvantageously exposed due to mesh displacement.

EP-A-0 494 424 discloses Ni or Ni alloy wires having aninner oxide layer of Ni or Ni alloy and an outerinsulating inorganic compound layer of SiO₂, Al₂O₃,MgO, ZrO₂ and mixtures thereof.

SUMMARY OF THE INVENTION

It is an object of the present invention to providean inorganic insulated member such as an electricconductor wire which has an excellent heat resistance andinsulability.

The inorganic insulating member or electrical conductorwire according to the present invention comprises aconductor of Ni or Ni alloy, an oxide layer of an oxideof Ni or Ni alloy on an outer surface of the conductor,said oxide layer being obtained by oxidizing theconductor in a vapor phase containing oxygen, an oxidelayer of silicon (Si) on an outer surface of the oxidelayer of Ni or Ni alloy, and an oxide layer of aluminum(Al) on an outer surface of the oxide layer of Si.

According to the present invention, the oxide layersof Aℓ and Si are oxide layers obtained by applying asolution prepared by hydrolyzing and polycondensingalkoxide of Aℓ or Si in a solvent, drying the same forallowing gelation, and thereafter heating the obtained gel.

According to the present invention, further, theoxide layers of Aℓ and Si have a melting point exceedingthat of Ni or Ni alloy.

The inorganic insulated member according to thepresent invention is applied to or used as a heatresistant wire or an incombustible wire at a hightemperature which does not permit using an organicinsulating material, for example. However, the presentinvention is not restricted to such a wire, but is alsoapplicable to another member such as a thermocouple.

The foregoing and other objects, features, aspectsand advantages of the present invention will become moreapparent from the following detailed description of thepresent invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a sectional view showing a wire inaccordance with present reference example 1 with anickel conductor core and two oxide layers;

Fig. 2. is a sectional view showing a wire inaccordance with present reference example 2 with anickel alloy core conductor and two oxide layers;

Fig. 3 is a sectional view showing a wire of thepresent invention with a nickel core conductor and three oxide layers; and

Fig. 4 is a sectional view showing a wire inaccordance with present reference example 3 with anickel alloy core conductor and two oxide layers.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Fig. 1 shows anNi core conductor 1 coated with aNioxide layer 2 formed around the core conductor. AnAℓoxide layer 3 is formed around theNi oxide layer 2. Theformation of these oxide layers will be described in moredetail below.

Fig. 2 shows anickel alloy conductor 11 first coatedwith a Ni alloy oxide layer 12 formed around theNi alloyconductor 11. ASi oxide layer 13 is formed around theNi alloy oxide layer 12.

In Fig. 3 the nickelNi core conductor 21 is firstcoated with aNi oxide layer 22 formed around theNi coreconductor 21. ASi oxide layer 23 is formed around theNi oxide layer 22. Then, anAℓ oxide layer24 is formed around theSi oxide layer 23.

In Fig. 4, a nickelalloy core conductor 31 is firstcoated with an Nialloy oxide layer 32 formed around theNialloy core conductor 31. Then, an Aℓ-Sicompositeoxide layer 33 is formed around the Nialloy oxide layer32.

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

Ni or Ni alloy is an inactive metal which has aninferior affinity for a metal oxide of Aℓ or Si. When asurface of Ni or Ni alloy is directly coated with such anAℓ or Si oxide, a rather poor adhesion is obtained and thecoating is immediately separated from the Ni or Ni alloy.In order to solve this problem, the present inventionteaches to first oxidize a core conductor of Ni or Nialloy in a vapor phase containing oxygen, so as to form anoxide layer of Ni or Ni alloy. The so formed nickel oxidelayer or Ni alloy oxide layer very strongly adheres to thesurface of the Ni or Ni alloy. This strong bonding is dueto the fact that the nickel oxide or the nickel alloyoxide has an excellent affinity for the nickel or nickelalloy. Additionally, the nickel or nickel alloy oxide hasa strong affinity to silicon oxide andhence also strongly bonds to the outer layer of Sioxide and to the conductor core. According to the presentinvention, therefore, the oxide layer of Si isnot separated for all practical purposes from theintermediate oxide layer, whereby an excellent flexibility is obtained when the inorganic insulating coating isapplied to a wire forming a core conductor, for example.

According to the present invention, the oxide layersof Aℓ and Si are obtained by applying a solutionprepared by hydrolyzing and polycondensing an alkoxide ofAℓ or Si in a solvent, drying the same for allowinggelation, and thereafter heating the so-obtained gel.

The Aℓ and Si oxide layers formed in theaforementioned manner have a melting point exceeding thatof the Ni or Ni alloy. Additionally, the Aℓ and Sioxide layers are formed without any melting process.

Therefore, the critical temperature to whichconductors or other members of the present invention withtheir inorganic insulating coatings may be exposed inoperation is not restricted by the melting point of theoxide layer. Rather, the present insulating members canbe heated to a temperature limited only by the meltingpoint of the Ni core or the Ni alloy core.

Further, the oxide layer formed in the aforementionedmanner has characteristics such as an extreme denseness, asmooth surface and a small adsorption of gases, e.g. steamor the like. Moreover, the present members have anexcellent insulability and a high moisture resistance.

Preferred embodiments have been produced as twoconductors C1 and C2 which were oxidized as follows.

(C1) An Ni wire of 0.5 mm is diameter consisting ofat least 99.9 % by weight of Ni, the remainder beingnatural impurities was heated in the atmosphere at 950°Cfor 10 minutes, to form a nickel oxide layer on thesurface of the wire.

(C2) An Ni alloy wire of 0.5 mm in diametercontaining 15 % by weight of Cr was heated in theatmosphere at 850°C for 30 minutes, to form an oxide layerof Ni alloy on the wire surface.

The following coating solutions L1, L2 and L3 wereprepared as follows:

(L1) A solution L1 was obtained by mixing tributoxyaluminum, triethanolamine, water, and isopropyl alcohol inmole ratios of 1:2:1:16. The mixture was hydrolyzed andpolycondensed at 50°C for 1 hour while stirring themixture.

(L2) A solution L2 was obtained by adding nitricacid to a mixed solution prepared by mixing tributylorthosilicate, water, and isopropyl alcohol in mole ratiosof 2:8:15 at a rate of 3/100 moles with respect totetrabutyl orthosilicate. The mixture was hydrolyzed andpolycondensed at 80°C for 2 hours while stirring themixture.

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

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

Reference Example 2: An oxidized nickel alloy conductor C2 wascoated with the coating solution L2 and heated at 500°Cfor 10 minutes. The coating and heating was repeated 10times, to form a Si oxide layer of 5 µm thickness on thefirst nickel alloy oxide layer.

Example 1: An oxidized nickel conductor C1 wascoated with the coating solution L2 and heated at 500°Cfor 10 minutes.

The coating and heating was repeated 5 times to forma Si-oxide layer having a thickness of 2.5 µm. Then, afurther coating operation was performed on the firstformed Si-oxide layer, with the coating solution L1. Thesample was again heated at 500°C for 10 minutes. Thecoating and heating was repeated 5 times to form an Aℓoxide layer of 2 µm thickness on the first formed Si oxidelayer of 2.5 µm thickness.

Reference Example 3: An oxidized conductor C2 was coated withthe coating solution L3 and heated at 500°C for 10minutes. The coating and heating was repeated 10 times toform an Aℓ-Si composite oxide layer of 6 µm in thickness.

Comparative Example 1: An aluminum wire was anodizedin a bath of sulfuric acid to form an Aℓ oxide layer of10µm thickness on the aluminum surface.

Comparative Example 2: An oxidized conductor C2 wascoated-with a slip which was prepared by mixing acommercially available frit (composite oxide of Ba, Ca, Tiand Si: GSP220A552 sold by Toshiba Glass Co., Ltd.) withwater. The wire coated with the slip was heated to 900°Cto form a homogenous metal composite oxide layer of 100 µmthickness through a melted state.

All the coating operations were, for example,performed by dipping the wire into the respective coatingsolution.

Test Results

Example	Breakdown Voltage	Flexibility
Reference Example 1	500 V	6D
Reference Example 2	600V	5D
1	800 V	8D
Reference Example 3	400 V	3D
Comparative Example 1	300 V	50D
Comparative Example 2	1200 V	1000D

The above Table shows the breakdown voltages and theflexibility values of the wires of Example 1 of theinvention, of Reference Examples 1 to 3 and of the two Comparative Examples. Theflexibility values were evaluated in terms of diameterratios, by winding the wires on circular cylinders of a prescribed diameter D and measuring the minimum diameterscausing no separation of the insulating inorganic compoundcoatings or layers from the conductor core. The diameterD was 0.5 mm.

The above Table shows that the wire of Example 1according to the present invention has a higherbreakdown voltage than the first Comparative Example and asuperior flexibility compared to both ComparativeExamples. However, the second Comparative Example has asubstantially higher breakdown voltage at the expense ofbeing very stiff.

As hereinabove described, the inorganic insulatingcoating on a conductor wire according to the presentinvention forms an insulating inorganic compound layerwhich is well bonded to the conductor core and has anexcellent heat resistance and insulability.

Although the present invention has been described andillustrated in detail, it is clearly understood that thesame is by way of illustration and example only and is notto be taken by way of limitation, the scope ofthe present invention being limited only by the terms ofthe appended claims.

Claims (13)

1. An insulated electrical conductor comprising a coreconductor (21) consisting essentially of a corematerial selected from the group consisting of Ni andNi alloy, a first oxide layer (22) consisting of anoxide of said core material formed on an outer surfaceof said core conductor (21) by oxidizing said coreconductor in a vapor phase containing oxygen, and asecond oxide layer (23) bonded to an outer surface ofsaid first oxide layer, said second oxide layerconsisting essentially of Si-oxide, characterized by furthercomprising a third oxide layer (24) on an outer surfaceof said second oxide layer (23), said third oxide layer(24) consisting essentially of Al-oxide.
2. The insulated electrical conductor according toclaim 1, wherein said second oxide layer (23)has a melting temperature greater than that of said corematerial (21).
3. The insulated electrical conductor according toclaim 1, wherein said core conductor (21)consists of Ni and trace amounts of naturally occurringimpurities.
4. The insulated electrical conductor according toclaim 1, wherein said core conductor (21)consists essentially of Ni and Cr.
5. The insulated electrical conductor according toclaim 4, wherein said core conductor (21)consists essentially of about 85 % Ni and about 15 % Cr.
6. A method of forming an insulated electrical conductor,comprising:

   (a) preparing a core conductor (21) of a corematerial selected from the group consisting of Ni andNi alloy;

   (b) forming a first oxide layer (22) on an outersurface of said core conductor (21) by oxidizing saidcore conductor in a vapor phase containing oxygen;

   (c) preparing a first coating solution by hydrolyzing andpolycondensing an alkoxide of Si in a solvent;

   (d) applying said first coating solution onto said firstoxide layer (22);

   (e) drying said first coating solution for gelling thesame; and

   (f) heating said first coating solution applied onto saidfirst oxide layer (22) to form a second oxide layer(23) on said oxide layer;

   characterized by further comprising:

   (g) preparing a second coating solution byhydrolyzing and polycondensing an alkoxide of Al in asolvent;

   (h) applying said second coating solution onto saidsecond oxide layer (23); and

   (i) heating said second coating solution to form athird oxide layer (24) on said second oxide layer (23).
7. The method of claim 6, further comprising repeatingsaid steps (d) to (f) and/or (g) to (i) a plurality oftimes, whereby successive coating films of said coatingsolution are applied one on top of another to form saidsecond and third oxide layers (22, 23).
8. The method of claim 6, wherein said step (f)does not involve a melting process.
9. The method of claim 6, wherein said second coatingsolution is prepared by forming a mixture of tributoxyaluminum, triethanolamine, water and isopropyl alcohol,and then hydrolyzing and polycondensing said mixture.
10. The method of claim 9, wherein the respectivemole ratio of said tributoxy aluminum, triethanolamine,water and isopropyl alcohol is 1:2:1:16, and saidhydrolyzing and polycondensing is carried out at 50°C for1 hour while stirring said mixture.
11. The method of claim 6, wherein said first coatingsolution is prepared by forming a mixture of tributylorthosilicate, water and isopropyl alcohol, adding nitricacid to said mixture, and then hydrolyzing andpolycondensing said mixture with said nitric acid addedthereto.
12. The method of claim 11, wherein the respectivemole ratio of said tributyl orthosilicate, water andisopropyl alcohol is 2:8:15, said nitric acid is added ata rate of 3/100 moles with respect to tributylorthosilicate, and said hydrolyzing and polycondensing iscarried out at 80°C for 2 hours while stirring saidmixture.
13. The method of claim 6, wherein said heating stepis carried out at about 500°C.

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