Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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SINTERLESS ~INC OXIDE ~ARISTOR DEVICES
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Background o~ the Invention
Zinc oxide ~aris~ors of the tY~pe used with surge
arrester devices are currently prepared by batch mixing
and ball milling techniques wherein the zinc oxide powder
is combined with certain additive metal compounds for
pressing and sintering into solid discs . After sintering
the discs are then coated ~ith a layer of conductive
metal to provide electrodes to the discs. Pressing and
sintering the discs into pr~scribed cylindrical configura-
tions requires a yood deal of preparation time and contributes
to the overall varistor costsO
The electrical properties of the varistors, for
example, the exponent n and resistance R must be uniform from
batch to batch to ensure uniformity of electrical character-
istics æmong the variou varistors. Removing -the varistors
having unacceptably low exponent values causes a decrease in
the material's operating eficiency since defective varistor
discs quite often must be discarded. Methods for currently
reclaiming rejected ~inc oxide varistor discs require that
the discs be completely reprocessed including costly re-
pressing and resintering operations.
The purpose of this invention is to provide means for
forming inexpensive ~inc oxide varistor electrical elements
from rejected varistor discs in one embodiment and for form-
ing high purity zinc oxide varistor discs from high purity
starting materials in another embodiment~
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Brief Description of the DraWings
~ IGURE 1 is a side view of a flame spray coating
apparatus for use within the invention;
FIGURE 2 is a top perspective view of one embodiment
of a sinterless zinc oxide varistor manufactured by the
methods of the invention;
FIGURE 3 is a top perspective view of the varistor
of FIGURE 2 in the fo.rm of a trimmable resistor;
FIGURE 4 is a top perspective view of a zinc oxide
Yaristor fuse device;
FIGURE 5 is a front perspective view of a high purity
zinc oxide varistor manufackured by a plasma spray tech-
ni~ue; and
FIGURE 6 is a side view of a plasma spray apparatus
for manufac~uring the varistor of FIGURE 5.
Description of the Preferred Embodiment
FIGURE 1 shows a flame spraying apparatus 14 of the
type utilizing a high tempexature flame 15 and a special
nozzle 16 wherein an oxyhydrogen gas flame is used for the
purpose of directing the pOWder stream 13 onto the surface
of substrate 11. The metal oxide varistor components 13'
are propelled by combustible gases through pipe 19 and
tubing 17 up into manifold ~J where they become heated upon
further passage through nozzle 16 and flame 15. The heated
powder stream 13 is caused to impinge upon a rotating
-25 tubular substrate 11 such that the powder 13' within con-
tainer 18 forms an oxide coating 12. The oxide coating
tightly adheres to the substrate by becoming fused thereon
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in the deposition pxocess. ~ coated substrate 10 conkaining
an oxide coating 12 is shown in FIG~RE 2 wherein the sub-
strate 11 also contains a ~etal coating 20 at each end of
the substrate to provide electrical access with the oxide
coating. The substrate 11 comprises a refractory tubing.
The refractory matertal can compri'se a silica or alumina
compound or a ceramic or porcelain material. The metal
coating generally compxises alumin~m metal deposited by
the same flame spray techni~ues as for the oxide coating
12. The metal coating can be applied directly on top of
the oxide coating or upon the substrate contingent with the
oxide materialO
FIGURE 3 contains a coated substrate 10 similar to
the coated substrate of FIGURE 2 with part of the oxide
layer removed b~ an abrasive grit blasting technique so
that the remaining oxide 12 exhibits a continuous long
filament of very thin cross section. The embodiment of
FIGURE 3 comprises a trimmable non-linear resistor device
having a very high resistance value ~nd containing elec-
trodes at each end formed by the metal coa~ing 20.
A varistor fuse device is provided by the coated
substrate 10 of FIGURE 4 wherein the oxide material 12 is
deposited upon a flat substrate 8 and contains a region 9
o~ reduced thickness. The metal coating 20 at each end of
the coated substrate provides electrodes to the device and
the reduced thickness 9 is designed to melt and break
contact between the electrodes when the power capacity of
the device is exceeded. The embodiment of FIGURE 4 therefore
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5D3717
combines non-linear resis~ive properties due to the operating
characteristics of the oxide coating 12 with a fusible feature
to protect sensitive circuit elements from conditions of ex-
cess current as well as excess voltage.
S The flame spray apparatus of FIGURE 1 is particula~ly
use~ul for reclaiming the zinc oxide powder ~rom rejected
YaristOrs. The varistors are crushed, pulverized, and classi-
fied through a 200 mesh sieve to provide pinhole free, tightly
adherent coatings upon ~lame deposition.
A sinterless zinc oxide varistor 21 having exceptional
electrical properties is shown in FIGURE 5 and consists o
a highly pure zinc oxide disc con~aining a deposited metal
coating 20 at both ends for providing electrical cOntact with
the zinc oxide material. A ceramic collar 22 is provided
lS around the periphery of the varistor in order to prevent
varistor current from transferring in the vicinity of the
periphery rathex than through the bulk cross section of the
varistor. The highly purified zinc oxide composition is
achieved by the vapor deposition process depicted in FIGURE
6. The highly puriied materials are obtained by in situ
oxidation of the nitrates or carbonates of the constituen s.
Other materials such as oxalates, acetates and other anions
of organic acids, which thermally decompose to their oxides,
can also be employed. The zinc oxide precursor comprises
zinc nitrate and other materials such as bismuth, titanium,
magnesium and cobalt are provided by either their respective
carbonates or nitrates. In order to ensure sufficient oxygen
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5D3717
to completely oxidize the carbonates and nitrates of the
constituents, the materials, in powder form, are entrained
within a stream.of pure oxygen. In order to energize the
plasma discharge an ionizable and inert gas, such as argon,
is also transported through the plasma nozzle 16 along with
the powder constituents. A first tube 28 is connected with
a source of highly purified oxygen gas, a second tube 2g i~
connected with a highly puri~ied ta~k of argon and a third
tube 30 is connected to a containex having the mixed oxide
precursors. The mixing between the oxygen and the precursor
materials occurs within the manifold 25 and the mixed oxygen,
argon and precursor materials transmit through nozzle 16
wherein a plasma carries the powder 13 to a rotating support
24. The powder precursor materials can be directly entrained
lS within tha oxygen gas by passing the oxygen gas through the
container and forcing the precursor powders through manifold
25 and nozzle 16. The varistor 21 is built up upon the
rotating support 24 of the coating table assembly 23. The
height and diameter of the varistor can accurately be gaged
by the speed of rotation in the direction of the indicating
arrow, the location of the nozzle relative to the rotating
support and both the flow rate and concentration of the
precursor powder. The coating table comprises the rotating
support which is connected by gears or puIies to shaft 27
of motor 26 and is supported upon a base member 28. When
ultrapure zinc o~i.de varistors are desired the precursor
- materials can be formed from ultrapure liquid compounds ~hich
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react with the oxygen en-training gas to form the respective
oxides insitue. The zinc oxide precursor would then be a
zinc chloride solu~ion and the bismuth precursor, for
example, would comprise bismuth trichloride.
When formlng ultrapure zinc oxide varistor material
having exceptionally good electric:al characteristics the
plasma spray technique of FIGURE 6 is preferred. When
relatively ine~pensive æinc oxide varistors or varistor
devices are ~o be manufactured the flame spray technique
of FIGURE 1 is preferLed since the flame process does not
readily lead to ultrapure materials.
One o~ the advantages of the method of the instant
invention is the ability to make non-linear resistors by
calcining the chemical constituents consisting of ~inc
oxide and various additives without first having to sinter,
the constituents., This is a valuable feature since calcining
can be achieved in air by heating at relatively low tempera-
tures compared to the higher sintering temperatures. In
some instances partial calcining can be achieved during the
actual flame spray process.
The method of the instant invention also allows
varistors to be manufactured having pred~termined and carefully
controlled resistance values. Zinc oxide materials having
very high resistance for example can be combined with
materials having low resistance values to provide any inter-
mediate range of desired resistance ~o the varistor devices.
Although the methods of the invention are disclosed for
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manu~acturing zinc oxide varistors ~or surge arres~er
applications this is by way of example only. The varistors
manufactured by the method~ of the invention find applica-
tion wherever 2inc oxide var.istors may be employed.
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