U.S. patent number 3,929,660 [Application Number 05/364,596] was granted by the patent office on 1975-12-30 for arc-extinguishing materials.
This patent grant is currently assigned to Square D Company. Invention is credited to Joseph M. Khalid.
United States Patent |
3,929,660 |
Khalid |
December 30, 1975 |
Arc-extinguishing materials
Abstract
Arc-extinguishing materials are selected in accordance with
criteria such as first ionization potential of each of the
constituents, carbon content, boiling, sublimation, or
decomposition temperature, endothermic character of decomposition,
electronegativity of decomposition products, rate of reformation,
dimensional and chemical stability, ease of application, cost,
toxicity, and coefficient of thermal conductivity.
Inventors: |
Khalid; Joseph M. (Cedar
Rapids, IA) |
Assignee: |
Square D Company (Park Ridge,
IL)
|
Family
ID: |
25769334 |
Appl.
No.: |
05/364,596 |
Filed: |
May 29, 1973 |
Current U.S.
Class: |
524/404; 524/405;
524/408; 524/409; 524/431; 524/432; 524/435; 218/117; 218/158;
218/150 |
Current CPC
Class: |
H01H
33/76 (20130101); A62C 3/16 (20130101) |
Current International
Class: |
H01H
33/76 (20060101); H01H 33/70 (20060101); A62C
3/00 (20060101); A62C 3/16 (20060101); H01B
003/02 () |
Field of
Search: |
;252/63.2,63.5,63.7
;200/151 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Bailar et al., Comprehensive Inorganic Chem., 1973, pp. 880-882.
.
Hawley, Condensed Chemical Dictionary, 8th Ed., (1971). .
Grant, Hackh's Chemical Dictionary, 4th Ed., (1969). .
The Merck Index, (1968), 8th Ed..
|
Primary Examiner: Sebastian; Leland A.
Assistant Examiner: Hunt; B.
Attorney, Agent or Firm: Rathbun; Harold J. Kettelson;
Ernest S.
Claims
I claim:
1. An arc-extinguishing material consisting of a compound
comprising a primary ingredient having arc-extinguishing
properties, and a binder, said primary ingredient including an
element having a first ionization potential no less than 7.54
electron volts and selected from a class consisting of silver,
tantalum, copper, rhenium, tungsten, boron, palladium, tellurium,
zinc and selenium, the relative proportions of said primary
ingredient and said binder being a silicone resin, the carbon atoms
of said total compound being no greater than fifteen percent of the
total concentration of atoms thereof.
2. An arc-extinguishing material as set forth in claim 1, wherein
said compound has a boiling temperature no greater than
2,000.degree. C, and said element is selected from a class
consisting of silver, tellurium, zinc and selenium, the boiling
points of such elements in pure form being less than 2,000.degree.
C.
3. An arc-extinguishing material as set forth in claim 1, wherein
said compound has a boiling temperature no greater than
2,000.degree. C, and said element is selected from a class
consisting of tantalum, copper, tungsten, boron and palladium, the
boiling points of such elements in pure form being greater than
2,000.degree. C, said binder and said selected element comprising
said compound having a boiling temperature no greater than
2,000.degree. C.
4. An arc-extinguishing material consisting of a compound
characterized by a negligible rate of reformation after
decomposition, said compound being boron oxide and a silicon based
resin.
5. An arc-extinguishing material consisting of a compound
characterized by a negligible rate of reformation after
decomposition, said compound being boric acid and a silicone based
resin.
6. An arc-extinguishing material as set forth in claim 1, wherein
said primary ingredient comprises at least 40 percent by weight of
said compound.
7. An arc-extinguishing material as set forth in claim 1, wherein
said primary ingredient comprises at least 55 percent by weight of
said compound.
8. An arc-extinguishing material as set forth in claim 1, wherein
said primary ingredient comprises about 56 percent by weight of
said compound.
9. An arc-extinguishing material consisting of a compound
characterized by negligible rate of reformation after
decomposition, a first ionization potential no less than 7.54
electron volts, a boiling temperature no greater than 2,000.degree.
C, for which decomposition is an endothermic process, having
thermal conductivity of at least one-thousandth of a calorie per
second per square centimeter per degree Centigrade per centimeter,
said compound comprising a primary ingredient and a binder, said
primary ingredient including an element selected from a class
consisting of silver, tantalum, copper, rhenium, tungsten, boron,
palladium, tellurium, zinc and selenium, said binder being a
silicone resin, the carbon content and proportion thereof relative
to said primary ingredient being such that the carbon atoms are no
greater than 15 percent of the total atoms of said compound.
Description
This invention relates to arc-extinguishing materials usable as
molding compositions for molding arc shields, arc chute frame
components, cases of electrical devices, and the like and also
usable in coating form for coating surface portions of a member
wholly or partly defining an arc chamber in which separable
contacts of an electrical circuit interrupting device are
disposed.
Arc interruption requires the establishment of conditions promoting
a higher rate of recombination than the rate of ionization. The
problem reduces to one of recognizing the basic factors that
control these two rates.
One of the factors which should be considered in the selection of
arc-extinguishing materials is the first ionization potential of
each of the constituents of the material. The first ionization
potential of an element is the amount of energy required to remove
one electron away to infinity from a neutral atom of that element.
For silver, the first ionization potential is 7.54 electron volts,
while the first ionization potential for aluminum is 5.98 electron
volts. To appreciate the significance of a difference of one
electron volt between the first ionization potentials of two
elements, consider that the temperature difference between two
particles having an energy difference of one electron volt is
7730.degree. C.
Another factor which should be considered in the selection of
arc-extinguishing materials is electronegativity of decomposition
products. This is a measure of the affinity of an atom of an
element for electrons. As an arc current approaches zero it would
be desirable to have an electronegative gas in the arc chamber that
would capture the cooling and slowing electrons. Such electrons
would otherwise be accelerated by the rising recovery voltage and
cause further ionization and possible reignition.
Yet another factor which should be considered in the selection of
arc-extinguishing materials is the boiling sublimation, or
decomposition temperature. During the few microseconds preceding
and following the passing of the current through zero in an
alternating current system, the arc column temperature decreases
rapidly until it reaches the temperature of the arc chamber walls,
which act as a constant temperature sink. The higher the boiling
temperature of the material of the arc chamber walls is, the lower
the recombination rates will be.
Endothermic processes have their primary importance at current
zero, particularly in the mode of arc interruption wherein a
post-arc current flows. Under this latter condition the energy
being absorbed in an endothermic process taking place in the arc
chamber on a modest scale may approximate the energy being put into
the chamber by the post-arc current. This effect, complemented by a
favorable boiling temperature, significantly contributes to the
success of arc extinction.
If an arc-extinguishing material decomposes to yield free carbon,
this carbon may be oxidized to carbon dioxide. The process is
exothermic, and its timing is bad, because it is as the arc current
approaches zero and the temperature drops and recombination of ions
and dissociated molecules starts to take place that the exothermic
formation of carbon dioxide occurs. Deposits of free carbon can
also cause tracking and dielectric failure. However, to limit
arc-extinguishing materials to those containing no carbon would too
severely limit the choice of materials. Moreover, hydrogen has been
found to be a desirable component of the gas medium surrounding an
arc, especially when an application makes it desirable to force
motion of the arc by a transverse magnetic field, and carbon and
hydrogen generally occur together. It has been found that the
number of carbon atoms as a percentage of the total number of atoms
in an arc-extinguishing material should not exceed a value of about
fifteen percent.
An endothermic process of decomposition is exothermic on formation.
Therefore, it is desirable to select materials having formulas such
that the probability of reformation after decomposition is small.
Aluminum oxide (Al.sub.2 O.sub.3) and boron oxide (B.sub.2 O.sub.3)
are materials with such formulas, because the probability of two
aluminum or baron atoms colliding simultaneously with three oxygen
atoms, or with one molecule and one atom of oxygen, is small.
However, aluminum oxide is undersirable as an arc-extinguishing
material because the ionization potential of aluminum is low and
the boiling temperature of aluminum oxide is too high. In this
respect, the present invention is a departure from the prior art as
represented by U.S. Pat. Nos. 2,768,264, and 3,071,666 which tout
aluminum oxide as a good arc-extinguishing material.
SUMMARY OF CRITERIA
In accordance with the invention, the first ionization potential of
each of the constituents of a suitable arc-extinguishing material
should be equal to or greater than 7.54 electron volts, which is
the first ionization potential of silver. The atomic concentration
of carbon in the material should not be greater than about 15 per
cent. The boiling, sublimation, or decomposition temperature of the
material should be as low as possible consistent with other
requirements, preferably below 2,000.degree. C. Decomposition of
the material should be a strongly endothermic process, the more
endothermic the better. The products of decomposition of the
material should be as electronegative as possible consistent with
other requirements. The material should have a formula such that
the rate of reformation after decomposition is vanishingly small
near the zero point of the alternating current, such as that for
B.sub.2 O.sub.3 or H.sub.3 BO.sub.3. The material should be
non-toxic, non-caustic, non-inflammable, easy to process and apply,
and dimensionally and chemically stable under operating conditions.
Further, it should have good thermal conductivity for an
electrically insulating material, preferably greater than 0.001
calories per second per square centimeter per degree Centigrade per
centimeter.
Representative Materials
In general the oxides, borides, borates, silicates, and the
ammonium complexes of the elements having a first ionization
potential equal to or greater than that of silver (7.54eV) are
suitable arc-extinguishing materials, and so are their hydrated
forms. A preferred selection of such elements, with their first
ionization potentials shown in parentheses, is tantalum (7.70eV),
copper (7.72eV), cobolt (7.86eV), rhenium (7.87eV), iron (7.90eV),
tungstem (7.98eV), silicon (8.15eV), boron (8.29eV), palladium
(8.30eV), antimony (8.64eV), tellurium (9.01eV), zinc (9.39), and
selenium (9.75eV). These substances may also be used as fillers in
suitable resins provided that the atomic carbon concentration does
not exceed the nominal limit of fifteen percent. Silicone resins
are suitable. The use of resins becomes mandatory when the filler
material cannot be used in its pure form. For example, pure silicon
dioxide cannot be used because its boiling temperature is too high,
and pure boric acid cannot be used because it is water soluble and
therefore dimensionally unstable, but these materials are suitable
when used in suitable resins.
Beryllium (9.32eV), arsenic (9.81eV), and mercury (10.43eV) have
first ionization potentials higher than that of silver, but these
elements are toxic and therefore undesirable. Sulphates,
phosphates, nitrates, and halogens are also undesirable as
toxic.
Silver, platinum, and gold oxides, borates and silicates, with or
without ammonium complexes, would be suitable arc-extinguishing
materials except for high cost.
In particular, a material composed of 55.8 per cent by weight
Firebrake ZB and 44.2 per cent Sylgard 182 has been found to be a
very suitable arc-extinguishing material. Firebrake ZB is made by
United States Borax and Chemical Corporation of Los Angeles,
Calif., and has a formula 2ZnO.3B.sub.2 O.sub.3.31/2H.sub.2 O.
Sylgard 182 is made by Dow Corning Corporation of Midland, Mich.
and is a dimethyl silicone resin having a basic monomer represented
by the following structural formula:
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