Ceramic insulated electrical conductor wire and method for manufacturing such a wire

A ceramic insulated wire has a conductor core of copper or copper alloy, a stainless steel layer around the conductor core and a chromium oxide film (2A) around the stainless steel layer. The chromium oxide film (2A) is surrounded by an outer ceramic insulator formed by a vapor deposition method. Cladding the conductor core with stainless steel is done by inserting the core lengthwise into a stainless steel pipe, plastically working the resulting composite body to provide a desired size, and oxidizing the stainless steel which contains sufficient chromium for the formation of the chromium oxide film to have a thickness within the range of 10 nm to 1000 nm. The outer ceramic insulator formed by vapor deposition is made of Al.sub.2 O.sub.3, SiO.sub.2, AlN and Si.sub.3 N.sub.4 which provide an excellent heat resistance while the chromium oxide film substantially increases the bonding strength.

FIELD OF THE INVENTION 
The present invention relates to ceramic insulated electrical conductor 
wires used, for example, in vacuum devices, on combustion engines, and the 
like where such wires are exposed to high operating temperatures. The 
present invention further relates to a method of manufacturing such 
ceramic insulated electrical conductor wires. 
BACKGROUND INFORMATION 
Bare uninsulated electric wires passing through ceramic bead insulators and 
electric wires having an oxide film formed by anodization or 
electrophoretic deposition around a conductor mainly formed of aluminum, 
have been known as insulated electric wires used in vacuum devices and in 
other high operating temperatures devices. 
However, manufacturing of such insulated electric wires which are made by 
passing bare copper wires through ceramic bead insulators, takes much time 
and labor, since the bare copper wire must be passed through ceramic bead 
insulators one by one. 
By anodization or electrophoretic deposition, oxide films could be formed 
only around conductors mainly formed of aluminum. The insulated electric 
wires manufactured by such method have a rough surface and many voids in 
the insulating outer films. Hence, when such insulated electric wires were 
used in vacuum devices, it took much time to evacuate the vacuum devices, 
because of gases such as air adsorbed at the surface of the voids. 
Further, the reduced pressure was not low enough due to slow leak 
problems. As a result, the attained vacuum was not high enough for many 
purposes. 
Electric wires coated by resin including fluorine such as 
tetrafluoroethylene are used where high heat resistance is not very 
important. These wires are not suitable for high operating temperatures. 
Vacuum devices requiring a high vacuum are subjected to degassing by a 
baking process, so as to improve the evacuation efficiency. 
However, when electric wires coated with a resin including fluorine, are 
used at a temperature of at least 260.degree. C., the resin is decomposed, 
generating gas and lowering the vacuum and the dielectric breakdown 
voltage. Therefore, the use of such electric wires is limited to 
applications not requiring a high heat resistance. 
U.S. Pat. No. 3,222,219 (Saunders et al.), issued on Dec. 7, 1965, 
discloses a ceramic coated electrically conductive wire and method for 
making such a wire in which a good adhesion of the ceramic coating to the 
metal substrate is obtained by the solution of the metal oxide, formed in 
the initial stages of curing, by the glassy phase, to form a saturated 
interfacial layer of this metal oxide in the glassy phase at the metal 
ceramic interface. The glassy phase at the metal ceramic interface is part 
of the coating which also includes a crystalline phase. This combination 
of a glassy phase with a crystalline phase in the coating provides a good 
flexibility and contributes to the bonding between the oxidation resistant 
conductor and the ceramic coating. While chromium oxide may be contained 
in the glassy phase, it does not exhibit its inherent nature in this type 
of glassy phase forming part of the ceramic insulating coating. Such a 
structure does not suggest the intentional formation of a chromium oxide 
layer on the oxidation resistant conductor core as taught by the present 
invention. A re-melting temperature of the glassy phase in the ceramic 
coating is about 700.degree.-800.degree. C. Accordingly, the ceramic 
coated conductor wire disclosed in U.S. Pat. No. 3,222,219 may not be used 
at a high temperature above 700.degree. C. 
If a ceramic insulator directly formed on the conductor core is mainly 
formed of copper by vapor deposition, the conductor core is not 
sufficiently bonded to the ceramic insulator, since the affinity between 
the copper and the ceramic insulator is low. The invention wants to avoid 
this problem. 
SUMMARY OF THE INVENTION 
Therefore, it is one object of the present invention to provide a ceramic 
insulated electrical conductor wire having a superior heat resistance, no 
voids nor any unevenness in the surface of its insulating film, and which 
can be manufactured easily. 
Another object of the present invention is to provide a method for 
manufacturing a ceramic insulated wire as just described. 
The ceramic insulated wire in accordance with the present invention 
comprises an electrical conductor core formed of copper or a copper alloy, 
a stainless steel layer provided around the conductor core, a chromium 
oxide layer having a thickness in the range from 10 nm to 1000 nm on the 
stainless steel layer and a ceramic insulator on said chromium oxide layer 
bonded to said stainless steel layer through said chromium oxide layer. 
The stainless steel layer and the chromium oxide layer neutralize and 
thereby prevent any effects of a low affinity between the copper conductor 
and the ceramic insulator, whereby the bonding strength is improved to 
secure the outer ceramic insulator to the copper conductor through the 
chromium oxide layer and the stainless steel layer. 
The method of manufacturing the ceramic insulated wire in accordance with 
the present invention comprises the steps of covering said copper core 
conductor with a stainless steel coating containing chromium sufficient 
for forming a chromium oxide layer on said stainless steel coating, 
oxidizing the stainless steel coating covering the copper conductor core, 
at a temperature in the range of 200.degree. C. to 620.degree. C. in the 
presence of a partial pressure of oxygen not higher than 200 Torr, to form 
said chromium oxide layer on the surface of the stainless steel coating, 
and then forming a ceramic insulator on the chromium oxide layer by vapor 
deposition. 
The ceramic insulated wire in accordance with the present invention has, on 
the copper core, a stainless steel layer covered on its radially outer 
surface with a chromium oxide layer bonded to the outer ceramic insulator. 
The chromium oxide coating on the stainless steel has a stabilizing 
passivation function which assures an excellent bonding of the outer 
ceramic insulation layer to the conductor wire by preventing adverse low 
affinity effects between the copper conductor and the ceramic insulation. 
Provided the chromium oxide layer has the above thickness of 10 to 1000 
nm, bonding strengths within the range of 180 kgf/mm.sup.2 to 200 
kgf/mm.sup.2 have been achieved, whereby the chromium oxide film is 
firmly bonded to the outer ceramic insulator and to the stainless steel 
layer which is thus firmly in contact with the outer ceramic insulator. 
These features also improve the flexibility of the ceramic insulated wire. 
The stainless steel used according to the invention includes austenitic 
stainless steels such as SUS 304, SUS 316, ferritic stainless steel such 
as SUS 430, and martensitic stainless steel such as SUS 410. The reference 
characters SUS 304, SUS 316, SUS 430 and SUS 410 are types of stainless 
steels defined by Japanese Industrial Standard (JIS). 
Preferably, the stainless steel layer has a first cross-sectional area. The 
copper conductor and the stainless steel layer together have a second 
cross-sectional area. The ratio between the first and second 
cross-sectional areas is within the range of 5 to 70%. If the ratio is 
less than 5%, the surface of the conductor portion may not be covered 
uniformly with the stainless steel layer. If the ratio exceeds 70%, the 
conductivity of the ceramic insulated wire itself is reduced, since the 
stainless steel has a low conductivity. 
According to the invention, the present ceramic-insulated wire having a 
copper core conductor and a stainless steel layer around the copper core 
conductor and a ceramic insulator, is produced by the following steps. 
First, the stainless steel layer is formed around the copper conductor by 
cladding. Second, a chromium oxide layer is formed on the stainless steel 
layer. Third, the ceramic insulator is formed around the chromium oxide 
layer by vapor deposition. 
To perform the cladding, preferably, the copper core conductor is inserted 
lengthwise into a stainless steel tube to form a composite body which is 
then subjected to plastic working such as forging or stamping, wire 
drawing and the like to reduce the initial outer diameter of the composite 
body down to a practically useful size, depending on the use for which the 
present wires are intended. 
It is an advantage of the present invention, that the outer ceramic 
insulator is stable even at a high temperature, whereby the ceramic 
insulated wire of the present invention does not generate gas derived from 
the decomposition of the insulator even when it is used at a high 
operating temperature, e.g. in an engine compartment, in a vacuum device 
and the like. Further, the invention prevents lowering the dielectric 
breakdown voltage even at these high operating temperatures. The term high 
temperature here means a temperature not lower than 300.degree. C. and up 
to 1000.degree. C., near the melting point of copper. 
Therefore, the ceramic insulated wire in accordance with the present 
invention can be used in vacuum devices which require a high heat 
resistance for the wiring used therein. 
Al.sub.2 O.sub.3, SiO.sub.2 and Si.sub.3 N.sub.4, and AlN for example, are 
members of a group known as ceramics that are preferably used for the 
purpose of the present invention because these ceramics are superior both 
in their insulating quality and heat resistance. 
The outer ceramic insulator film on the ceramic insulated wire of the 
invention is formed by vapor deposition. Any of the following types of 
vapor deposition are suitable for the present purposes, namely chemical 
vapor deposition, plasma enhanced chemical vapor deposition, ion plating, 
sputtering, vacuum deposition, and cluster ion beam deposition. These 
depositions of the ceramic insulator on the chromium oxide film of the 
stainless steel are flat with a smooth surface and with a uniform 
thickness of the ceramic insulator throughout its extent. Such an even or 
smooth surface of the ceramic insulator without any voids in which air 
could be contained is an important advantage because it does not take a 
long time to pump down a vacuum device in which the present conductors are 
used. 
Preferably, the thickness of the ceramic insulator film on the ceramic 
insulated wire of the present invention, is in the range of 2 .mu.m to 10 
.mu.m. If the film is thinner than 2 .mu.m, the dielectric breakdown 
voltage is too low. If the film thickness exceeds 10 .mu.m, cracks may 
possibly occur in the insulator film causing peeling of the insulator 
film. 
Forming the ceramic insulator film on the wire of the present invention by 
vapor deposition has yet another advantage due to the fact that the 
handling of the vapor deposition is easier compared with the conventional 
operation of passing bare copper wires through beads formed as ceramic 
insulators.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Embodiment 1 
One embodiment of the ceramic insulated wire in accordance with the present 
invention shown in FIG. 1 is manufactured in the following manner. 
A stainless steel layer 2 is provided on the copper or copper alloy wire 
core 1 by a cladding method, to provide a composite wire body. The wire 
body has a diameter of 2 mm. The stainless steel layer 2 contains 
sufficient chromium, preferably within the range of 12 to 20% by weight of 
the stainless steel to form a film or coating 2A of chromium oxide 
(Cr.sub.2-x O.sub.3 ; x&lt;0.077) on the surface of the steel cladding by 
oxidizing the stainless steel cladding under controlled oxidizing 
conditions at a temperature within the range of 200.degree. C. to 
600.degree. C. and at an oxygen partial pressure of less than 200 Torr. 
Under these oxidizing conditions the chromium oxide film or coating 2A 
that is intentionally formed on the stainless steel surface, provides a 
passivation film 2A which greatly enhances the bonding of the ceramic 
electrical insulation outer layer to the stainless steel cladding and thus 
to the conductor core. Bonding strengths within the range of 180 
kgf/mm.sup.2 to 200 kgf/mm.sup.2 have been achieved according to the 
invention. The oxidizing step is continued until the chromium oxide layer 
has a thickness within the range of about 10 nm to about 1000 nm. If the 
chromium content of the stainless steel is less than 12% wt., it is 
difficult to form a suitable chromium oxide layer. If the chromium content 
in the stainless steel is more than 20 % wt., the stainless steel layer 
becomes fragile. 
After completion of the oxidizing, a ceramic insulating layer 3 is formed 
on the chromium oxide film or coating 2A by vapor deposition. FIG. 2 shows 
the steps of oxidizing and forming the ceramic insulating film around the 
chromium oxide film of the wire vapor deposition. 
As shown in FIG. 2, the wire is transported from station to station as 
indicated by the arrows. The wire passes from a cladding station 4A to an 
oxidizing station 4 and then to a pressure adjustment zone 5. Thereafter, 
the wire is transmitted from the pressure adjustment zone 5 to a thin film 
forming zone 6 where any of the above mentioned vapor depositions is 
performed, to produce the ceramic insulating film 3 on the chromium oxide 
or layer 2A of the wire. 
Thereafter, the wire is transmitted from the thin film forming zone 6 to a 
pressure adjustment zone 7 and then to a winding mechanism 8. 
The following Table 1 supports the above disclosed oxidizing conditions, 
the thickness of the chromium oxide film or coating 2A on the stainless 
steel cladding 2, and the bonding strength. 
TABLE 1 
______________________________________ 
Thickness Adhesiveness 
Conditions for Oxidizing 
of Between Chromium 
Stainless Steel Chromium Oxide Film And 
Oxygen Partial 
Oxide Ceramic Insulating 
Temperature 
Pressure Film Film 
______________________________________ 
1 100.degree. C. 
10 Torr 6 nm 60 kgf/mm.sup.2 
2 200.degree. C. 
150 Torr 12 nm 180 kgf/mm.sup.2 
3 400.degree. C. 
10 Torr 20 nm 200 kgf/mm.sup.2 
4 600.degree. C. 
1 Torr 50 nm 200 kgf/mm.sup.2 
5 620.degree. C. 
1 Torr 50 nm 200 kgf/mm.sup.2 
6 600.degree. C. 
250 Torr 2000 nm 15 kgf/mm.sup.2 
7 650.degree. C. 
1 Torr 4000 nm 10 kgf/mm.sup.2 
8 Thickness of a Naturally 
5 nm 50 kgf/mm.sup.2 
Formed Chromium 
Oxide Film 
9 600.degree. C. 
5 Torr 200 nm 200 kgf/mm.sup.2 
10 600.degree. C. 
40 Torr 1000 nm 180 kgf/mm.sup.2 
______________________________________ 
As is apparent from Table 1, when the thickness of the chromium oxide film 
or coating is outside of the range from 10 nm to 1000 nm, the adhesiveness 
between the chromium oxide film 2A and the ceramic insulating layer 3 is 
remarkably degraded. If the thickness of the chromium oxide layer is 
larger than 1000 nm, cracks are generated in the chromium oxide film 2A. 
The crack portions do not contribute to adhesion between the chromium 
oxide film 2A and the ceramic insulating layer 3. Therefore, if the 
thickness of the chromium oxide layer exceeds 1000 nm, the adhesiveness or 
bonding strength is decreased. 
A chromium oxide film may be naturally formed on the surface of stainless 
steel containing chromium. However, the thickness of the chromium oxide 
layer naturally formed is only about 5 nm and hence cannot provide any 
sufficient adhesiveness. The thickness of the chromium oxide layer cannot 
exceed about 5 nm unless the stainless steel is positively oxidized under 
the conditions taught by this invention. 
By oxidizing the stainless steel layer, the chromium oxide layer is formed. 
As is apparent from the Table 1, the thickness of the chromium oxide layer 
depends on the temperature and the partial pressure of oxygen and on the 
time of exposure to the oxidizing condition. An exposure time within the 
range of 10 to 60 minutes has been found to be adequate. If the 
temperature is lower than 200.degree. C., the thickness of the chromium 
oxide film tends to be smaller than 10 nm. If the temperature exceeds 
620.degree. C., the thickness of the chromium oxide film 2A becomes 
thicker than 1000 nm. If the partial pressure of oxygen is higher than 200 
Torr, the thickness of the chromium oxide layer exceeds 1000 nm. 
The ratio of the cross-sectional area of the stainless steel including the 
chromium oxide film or coating to the total cross-sectional area of the 
copper core and the stainless steel with its oxide coating is within the 
range of 5 to 70%, preferably within the range set forth below in Table 2 
which also shows the methods of forming the insulator film 3, the ceramic 
insulator film material, and the insulator film thickness. 
The following tests were made on the ceramic insulated wires formed as 
described above. Namely: 
(1) Flexibility test. The ceramic insulated wires are wound around a bar 
having a diameter of 6 mm. Wires which as a result of this winding do not 
have any cracks nor peelings of the ceramic insulating film, received a 
"passing grade". The wires are inspected for cracks and peelings in the 
ceramic insulating film through a stereo microscope at a magnification of 
fifteen. 
(2) Breakdown Voltage Test. This test is carried out in accordance with the 
metal-foil method of JIS C 3003 (Japanese Industrial Standards), and the 
dielectric breakdown voltages of these ceramic insulated wires are 
measured. The metal-foil method is a test for measuring the breakdown 
voltage by wrapping a conductor with a metal foil in tight contact and by 
applying an AC voltage between the conductor and the metal foil. The 
ceramic insulated wire was operated at 500.degree. C. for 60 minutes and 
then, the breakdown voltage test was performed at room temperature. 
(3) Heat test. The ceramic insulated wires were heated to 500.degree. C. 
and maintained at this temperature for 60 minutes. Those wires exhibiting 
neither cracks nor any peeling of the ceramic insulating film and having 
no change in the dielectric breakdown voltage received a "passing grade". 
Tests (1), (2) and (3) were also made on a conventional electric wire 
coated with resin including fluorine and on an electric wire on which the 
ceramic insulating film is directly formed around the copper wire, as 
examples for comparison, which are represented as No. 8 and No. 9, 
respectively, in Table 2. 
TABLE 2 
__________________________________________________________________________ 
Ratio of Cross- 
Sectional Area of Film 
Stainless Steel thickness 
to Total Cross- (.mu.m) of 
Stainless 
Sectional Area 
Film Ceramic Breakdown 
Steel 
of Copper and 
Forming 
Film Outer voltage 
Heat 
No Remarks Material 
Stainless Steel 
Method Material 
Insulator 
Flexibility 
(V) Resistance 
__________________________________________________________________________ 
1 Examples SUS 304 
36% plasma CVD 
SiO.sub.2 
3 passed 
400 passed 
2 of the SUS 316 
28% ion plating 
Al.sub.2 O.sub.3 
4 passed 
400 passed 
3 Present SUS 430 
44% sputtering 
SiO.sub.2 
3 passed 
400 passed 
4 Invention 
SUS 410 
20% plasm CVD 
Si.sub.3 N.sub.4 
3 passed 
400 passed 
5 SUS 304 
18% ion plating 
Al.sub.2 O.sub.3 
3 passed 
400 passed 
6 SUS 316 
36% sputtering 
Al.sub.2 O.sub.3 
5 passed 
400 passed 
7 SUS 304 
20% plasm CVD 
SiO.sub.2 
5 passed 
500 passed 
8 Prior -- -- -- Resin 
200 passed 
1500 failed 
Art Including 
Fluorine 
9 For -- -- plasma CVD 
SiO.sub.2 
Immeasurable: 
Comparison No Film is Formed 
10 Examples SUS 304 
38% ion plating 
AlN 5 passed 
400 passed 
of the 
Present 
Invention 
__________________________________________________________________________ 
As shown in Table 2, the insulated electric wires No. 1 to No. 7 and No. 
10, which are the embodiments of the present invention, have passed the 
flexibility test and the heat test. 
The wire coated with resin including fluorine, which is a prior art example 
represented as No. 8, passed the flexibility test but failed in the heat 
test. 
The wire No. 9 on which the ceramic insulating film is directly formed 
around the copper wire, which is an example for comparison, could not be 
subjected to the flexibility test and the heat test, since no film could 
be formed as the ceramic was easily peeled off from the conductor portion. 
In view of the foregoing, the ceramic insulating wires of the present 
invention are superior in flexibility and in heat resistance. 
Therefore, when the ceramic insulated wires of the present invention are 
used as electric wires in vacuum devices, the vacuum devices may be heated 
to a high temperature. Consequently, the pressure of the vacuum devices 
can be decreased. In addition, the ceramic insulated wires can be used 
where flexibility is required. 
The ceramic insulated wires No. 1 to No. 7 and No. 10 which are the 
embodiments of the present invention, have dielectric breakdown voltages 
not lower than 400 V. Therefore, the ceramic insulated wires of the 
present invention have preferable dielectric breakdown voltages required 
for insulated electric wires intended for use under high operating 
temperatures. 
Embodiment 2 
An electric wire as long as 20 m is placed in a vacuum chamber, and the 
time required for pumping down to the vacuum of 10.sup.-5 Torr was 
measured, for each of the insulated electric wires No. 1 to No. 7 of the 
present invention. 
The time for pumping down was 1 hour and 25 minutes for each of the 
insulated wires No. 1 to No. 7 of the present invention positioned in the 
vacuum chamber, and there was no significant difference in the pumping 
down time in comparison with the condition in which there is no insulated 
wire in the vacuum chamber. Thus, the time of pumping down can be reduced 
compared to conventional vacuum systems, wherein conventionally coated 
electric wires are used. 
The above mentioned austenitic stainless steels SUS 304 and SUS 316, 
ferritic stainless steel SUS 430, and martensitic stainless steel SUS 410 
are taken from Japanese Industrial Standards JIS G4303-1991. 
Although the present invention has been described with reference to 
specific example embodiments, it will be appreciated that it is intended 
to cover all modifications and equivalents within the scope of the 
appended claims.