Electrical overstress pulse protection

An electrical overstress composite of conductor/semiconductor particles including particles in the 100 micron range, micron range, and submicron range, distributed in a densely packed homogeneous manner, a minimum proportion of 100 angstrom range insulative particles separating the conductor/semiconductor particles, and a minimum proportion of insulative binder matrix sufficient to combine said particles into a stable coherent body.

SUMMARY OF THE INVENTION 
The present invention relates to the protection of electrical and 
electronic circuits from high energy electrical overstress pulses that 
might be injurious or destructive to the circuits, and render them 
non-functional, either permanently or temporarily. In particular, the 
invention relates to a composition and formulation of materials which can 
be connected to, or incorporated as part of an electrical circuit, and are 
characterized by high electrical resistance values when exposed to low or 
normal operating voltages, but essentially instantaneously switch to low 
electrical impedance values in response to an excessive or overstress 
voltage pulse, thereby shunting the excessive voltage or overstress pulse 
to ground. 
These materials and circuit elements embodying the invention are designed 
to respond substantially instantaneously to the leading edge of an 
overstress voltage pulse to change their electrical characteristics, and 
by shunting the pulse to ground, to reduce the transmitted voltage of the 
pulse to a much lower value, and to clamp the voltage at that lower value 
for the duration of the pulse. The material is also capable of 
substantially instantaneous recovery to its original high resistance value 
on termination of the overstress pulse, and of repeated responses to 
repetitive overstress pulses. For example, the materials of the present 
invention can be designed to provide an ohmic resistance in the megohm 
range in the presence of low applied voltages in the range of 10 to more 
than 100 volts. However, upon the application of a sudden overstress pulse 
of, for example, 4,000 volts, the materials and circuit elements of the 
invention essentially instantaneously drop in resistance, and within a 
nanosecond or two of the occurrence of the leading edge of the pulse, 
switch to a low impedance shunt state that reduces the overstress pulse to 
a value in the range of a few hundred volts, or less, and clamps the 
voltage at that low value for the duration of the pulse. In the present 
description, the high resistance state is called the 37 off-state", and 
the low resistance condition under overstress is called the 37 on-state". 
In general, the present materials constitute a densely packed intimate 
mixture and uniform dispersion of 100 micron range, micron range, and 
submicron range electrically conductive and semiconductive particles 
supported in fixed spaced relation to each other in an electrically 
insulative binder or matrix. As currently understood, these particles 
should embody a homogeneously dispersed mixture of particles wherein the 
intrinsic electrical conductivities of some of the particles are 
significantly disparate from others of the particles, preferably 
characterized as conductor and semiconductor particles. Further, as 
currently understood, there should be an interfacial spacing between these 
particles of the order of 20 to 200 angstroms, or so. In order to obtain 
that spacing, a small amount of 100 angstrom range insulative particles is 
preferably dispersed in the mixture of conductive and semiconductive 
particles to function as spacers. Thus, when this composite of particulate 
materials is densely packed, the micron range particles tend to occupy the 
major voids left by the closely packed 100 micron range particles, and the 
submicron range particles tend to occupy the lesser voids left by the 
closely packed micron range particles, with the 100 angstrom range 
insulative particles separating many of those particles. The residual 
voids between the particles are filled with the aforesaid electrically 
insulative binder or matrix, preferably a thermoset resin, although other 
insulative resins, rubbers and other materials can be employed. 
In the above-described composite material, it is believed that an important 
feature in attaining the desired electrical properties is the formation of 
the particulate composition into a dense and compact mass, as free of 
voids as possible, and wherein the particles are packed in as dense a 
configuration as possible and as permitted by the aforesaid spacer 
particles, in the manner described above. Optimumly, the density of the 
entire composite composition, particulate and matrix, should be within a 
few percent of the theoretical density for the materials used, preferably 
within about 1-3%, thereby attaining the interparticulate packing and 
spacing as above-specified over the entire volume of the composite. 
As currently understood, the high ohmic resistance for the composite at low 
applied voltages, is obtained by the uniform conduction discontinuities or 
gaps between the spaced conductive/semiconductive particles, while the low 
resistance conductivity of the composite in response to a high voltage 
electrical overstress pulse, is obtained predominantly by 
quantum-mechanical tunneling of electrons across the same angstrom range 
gaps between adjacent conductive and/or semiconductive particles. Pursuant 
to this interpretation of the operation of the composite, the role of the 
insulative spacer particles and the insulative resin matrix is not to 
supply a high resistance material, but simply to provide non-conductive 
spacing between the conductive and semiconductive particles, and to bind 
the composite into a coherent mass. Consistent with that understanding of 
the invention, the volume proportion of insulative spacer particles and of 
insulative resin in the composite should optimumly be the minimum quantity 
of each consistent with obtaining the desired spacing, and consistent with 
imparting structural integrity to the composite. Likewise, in accordance 
with this understanding of the invention, it is desirable, and perhaps 
important to the proper functioning of the invention, that the conductive 
and semiconductive particles be relatively free of insulative oxides on 
their surfaces, because these insulative oxides only add to the 
interfacial spacing between the conductive/semiconductive materials of the 
particles, when it is important that the spacing be minimized, and they 
unnecessarily impede the quantum-mechanical tunneling. 
When the teachings of the present invention are employed and practiced with 
maximum effect, one obtains an electrical overstress pulse responsive 
material, which, on the one hand, provides high (megohm range) resistance 
values to applied low voltage currents of the order of up to 100 volts, or 
so, but on the other hand, responds essentially instantaneously to the 
leading edge of an overstress voltage pulse of the order of several 
thousand volts or more, by becoming electronically conductive to clamp 
that voltage pulse within a few nanoseconds to a maximum value of several 
hundred volts or less and to maintain that clamp for the duration of the 
overstress pulse, and to return immediately to its high ohmic value on 
termination of the overstress pulse. By proper adjustment of the 
composition of the composite, desired off-state resistances and desired 
on-state clamping voltages can be selected as desired for a particular use 
or environment. 
The present invention resides in the electrical overstress composite 
material, its composition, and its formulation. The physical structure of 
its use in a particular environment is not part of this invention, and 
such are known in the art and are readily adapted to, and designed for the 
specific environment of use. Obviously, as a bulk electrical resistance 
material, the prepared composite may be formed by compression molding in 
an elongate housing, and may be provided with conductive terminal end 
caps, as is conventional for such resistors. Alternatively, the prepared 
composite may be formed by conventional extrusion molding about a center 
conductor and encased within a conductive sheath or sleeve, so that an 
overstress pulse on the center conductor would be shunted through the 
composite to the outer sheath which, in use, would be grounded. Also, the 
composite may be incorporated into structural circuit elements, such as 
connectors, plugs and the like. 
The prior art contains teachings of electrical resistance composites 
intended for purposes similar to that of the present invention, but they 
differ from the present invention and do not accomplish the same results. 
U.S. Pat. No. 2,273,704 to R. O. Grisdale discloses a granular composite 
material having a non-linear voltage-current characteristic. This patent 
discloses a mixture of conductive and semiconductive granules that are 
coated with a thin insulative film (such as metal oxides), and are 
compressed and bonded together in a matrix to provide stable, intimate and 
permanent contact between the granules. 
U.S. Pat. No. 4,097,834 to K. M. Mar et al. provides an electronic circuit 
protective device in the form of a thin film non-linear resistor, 
comprising conductive particles surrounded by a dielectric material, and 
coated onto a semiconductor substrate. 
U.S. Pat. No. 2,796,505 to C. V. Bocciarelli discloses a non-linear 
precision voltage regulating element comprised of conductor particles 
having insulative oxide coatings thereon that are bound in a matrix. The 
particles are irregular in shape, and are point contiguous, i.e. the 
particles make point contact with each other. 
U.S. Pat. No. 4,726,991 to Hyatt et al. discloses an electrical overstress 
protection material, comprised of a mixture of conductive and 
semiconductive particles, all of whose surfaces are coated with an 
insulative oxide film, and which are bound together in an insulative 
matrix, wherein the coated particles are in contact, preferably point 
contact, with each other. 
Additional patents illustrative of the prior art in respect to this general 
type of non-linear resistor are U.S. Pat. No. 2,150,167 to Hutchins et 
al., 2,206,792 to Stalhana, and 3,864,658 to Pitha et al. 
Within the teachings of the prior art, and particularly in the aforesaid 
Hyatt et al. patent, is the ability to create composite materials that are 
capable of responding substantially instantaneously to an electrical 
overstress pulse of several thousand volts, and clamping the voltage of 
the pulse to a relatively low value, of several hundred volts. However, in 
order to attain that goal following the teachings of said Hyatt et al. 
patent, it is necessary to design the composite material in a manner that 
provides a very low resistance of only a few hundred or a few thousand 
ohms in the off-state. Such a device obviously would have very limited 
application. Following said Hyatt et al. patent teachings, if the 
composite composition is altered to increase the off-state resistance to 
the megohm range, the on-state clamping voltage in response to an 
electrical overstress pulse is increased to substantially over 1000 volts. 
This dichotomy or contradiction in results stems from the understanding 
expressed in said patent that high off-state resistance is a function of 
the inclusion of high proportions of insulation material in the composite. 
However, the high proportion of insulation material interferes with the 
quantum-mechanical tunneling effect on which the on-state low clamping 
voltage characteristic depends. 
In accordance with the present invention, it is discovered that a consonant 
effect of both off-state high resistance and on-state low clamping voltage 
can be obtained. As currently understood, it appears that the key to these 
consonant effects is the presence of a minimum proportion of insulative 
material in the composite, including the 100 angstrom range spacer 
particles and binder, with a high proportion of conductive/semiconductive 
particles, and a densely packed, uniform, and essentially homogeneous 
distribution of the conductive/semiconductive components throughout the 
composite, with the density of the entire composite approaching the 
theoretical density for the materials used. It is currently believed that 
the consonant results are obtained under these circumstances, because: on 
the one hand, the conductive/semiconductive particles are in large part 
separated from each other by uniformly distributed insulative spacer 
particles, to limit or avoid long conductive chains of contiguous 
conductor/semiconductor particles, thereby providing the high off-state 
resistance; and on the other hand, the minimal quantity of uniformly 
distributed insulative spacer particles and of binder results in the 
uniform closely spaced separation of the densely packed 
conductor/semiconductor particles, thereby providing for efficient 
quantum-mechanical tunneling throughout all portions of the composite on 
the occurrence of an electrical overstress pulse. 
It is accordingly one object of the present invention to provide a 
composite material that is responsive to electrical overstress pulses for 
protecting electrical circuits and devices. 
Another object of the present invention is to provide such a composite 
material which provides a large ohmic resistance to normal electrical 
voltage values, but in response to an electrical overstress voltage pulse 
substantially instantaneously switches to a low impedance. 
Still another object of the present invention is to provide such a 
composite material which, when coupled to ground, shunts the pulse to 
ground and clamps the overstress voltage pulse at a low value. 
And still another object of the present invention is to provide such a 
composite material which returns to its initial state promptly after 
termination of the overstress voltage pulse, and will similarly respond 
repetitively to repeated overstress voltage pulses. 
Other objects and advantages of the present invention will become apparent 
to those skilled in the art from a consideration of the illustrative and 
preferred embodiments of the invention described in the detailed 
description of the invention set forth below.

DETAILED DESCRIPTION OF THE INVENTION 
In the practice of the present invention, the key electrical ingredient of 
the composite is a mixture of conductor/semiconductor particles, 
constituting from about 55 to about 80%, and preferably from about 60 to 
about 70%, by volume of the composite. Considered individually, conductive 
particles may comprise from about 20 to about 60%, preferably from about 
25 to about 40%, by volume of the composite; and semiconductive particles 
may comprise from about 10 to about 65%, preferably from about 20 to about 
50%, by volume of the composite The insulative components of the 
composite, i.e. the binder and the insulative separating particles, may 
comprise from about 20% to about 45%, preferably from about 30 to about 
40%, by volume of the composite. The insulative separating particles are 
most preferably about 1% by volume of the composite, although they may be 
a few percent, and for special purposes up to as much as about 5% by 
volume. These composite composition parameters are depicted in the 
three-coordinate triangular graph of FIG. 1. 
As explained above, it is believed that the maximum benefits of the 
invention are obtained by use of a minimum percent of insulative particles 
and matrix binder, consistent with obtaining the desired angstrom range 
separation of conductor/semiconductor particles and securing the composite 
in a stable coherent body. At the present time, extremely good results are 
experienced with approximately 30% by volume of binder, and 1% by volume 
of 100 angstrom range insulative particles. 
The presently preferred conductor particulate material utilized in the 
practice of the present invention are nickel powders and boron carbide 
powders. For most composites, it is preferred to use a mixture of two 
different forms of nickel: the first is a carbonyl nickel, reduced by ball 
milling in large measure to its ultimate particles of highly structured 
(i.e. irregular angular shape) balls of about 2-3 microns; the second is a 
spherical nickel ranging in size between 40 and 150 microns. The carbonyl 
nickel used is from Atlantic Equipment Engineers, marketed as Ni228, and 
the larger nickel particles are from the same company, marketed as Ni227. 
The boron carbide used is one supplied by Fusco Abrasive, and has a median 
particle size of about 0.9 micron. 
Obviously, numerous other conductive particle materials can be used with, 
or in place of the preferred materials, it being desirable and important 
for optimum results, however, to provide a proper distribution of particle 
sizes in the composite in order to obtain the dense particulate packing 
described above. Among the conductive materials that may be employed are 
carbides of tantalum, titanium, tungsten and zirconium, carbon black, 
graphite, copper, aluminum, molybdenum, silver, gold, zinc, brass, 
cadmium, bronze, iron, tin beryllium, and lead. As stated above, it is 
important that these conductive particles be free of insulative or high 
resistance surface oxides, or the like, for purposes of the present 
invention. Accordingly, for some of the more reactive materials it may be 
necessary to specially remove oxide coatings, and to keep the particles 
under a protective atmosphere until formulated in the composite. 
The presently preferred semiconductor particulate material utilized in the 
practice of the present invention is silicon carbide. In addition, zinc 
oxide in combination with bismuth oxide has been used in place of the 
silicon carbide. The silicon carbide used in the practice of the invention 
is Sika grade, polyhedral or 37 blocky" in form, with a particle size 
range of about 1 to 3 microns, supplied by Fusco Abrasive, Inc. The zinc 
oxide and bismuth oxide were obtained form Morton Thiokol, Inc. and had 
particle sizes, for zinc oxide, in the range of 0.5 to 2 microns, and for 
bismuth oxide, about 1 micron. 
Obviously, numerous other semiconductor particulate materials can be used 
with, or in place of the preferred materials, it being desirable and 
important for optimum results, however, to provide a proper distribution 
of particle sizes in the composite in order to obtain the dense 
particulate packing described above. Among the semiconductor materials 
that may be employed are; the oxides of calcium, niobium, vanadium, iron 
and titanium; the carbides of beryllium, boron and vanadium; the sulfides 
of lead, cadmium, zinc and silver; silicon; indium antimonide; selenium. 
lead telluride; boron; tellurium; and germanium. 
The preferred insulative spacing particle is a fumed colloidal silica, 
marketed as Cab-O-Sil by Cabot Corporation. Cab-O-Sil is a chain of highly 
structured balls approximately 20-100 angstroms in diameter. 
One binder or matrix material that has been used is a silicone rubber 
marketed by General Electric Company as SE63, cured with a peroxide 
catalyst, as for example Varox. Obviously, other insulating thermosetting 
and thermoplastic resins can be used, various epoxy resins being most 
suitable. It is desired that the binder resistivity range from about 1012 
to about 10.sup.15 ohms per cm. 
The composites of the present invention are preferably compounded and 
formulated in the following manner, described with reference to the 
above-identified preferred ingredients. Initially, the two nickel 
components are ball milled individually for two purposes--first, to remove 
oxide films from their surfaces, and second, to break up any agglomerates 
and reduce the nickel powders essentially to their ultimate particle 
sizes, particularly the carbonyl nickel (Ni228) which otherwise exists as 
highly structured balls agglomerated into long chains several hundred 
microns long. The two nickel powders are then ball milled together (if two 
nickel powders are used) to distribute the smaller micron sized carbonyl 
nickel particles uniformly over the surfaces of the much larger (100 
micron range) nickel particles (Ni227). In so doing, the smaller 
structured nickel particles tend to adhere to, or embed in the surface of 
the larger nickel particles. Then, the boron carbide, colloidal silica and 
semiconductor particulate are combined with the nickel by hand mixing. The 
prepolymer matrix or binder material is introduced first into a 
mixer--preferably, for example, a C. W. Brabender Plasticorder mixer, with 
a PLD 331 mixing head, which provides a relatively slow speed, high shear 
(greater than 1500 meter-grams) kneading or folding type of mixing action 
to expell all air. While the mixer is operating, the entire premixed 
powder or particulate charge is added gradually. Then, the mixer is 
operated until the mixing torque curve asymptotically drops to a stable 
level, indicating that essentially complete homogeneity of the mix has 
been obtained, the Varox or other curing catalyst is then added and 
thoroughly mixed into the composite. Whereupon, the composite is ready for 
molding, extruding or other forming operation, as appropriate. 
In the foregoing procedure, there is no preferential coating of any of the 
particulate components with the colloidal silica; the silica is merely 
distributed throughout the mix. The close packing of the particulate 
materials results from several factors: 1. The use of a minimum proportion 
of binder or matrix material; 2. The proportions of different sized 
particulates adapted to fill the voids between an array of essentially 
contiguous larger particles with smaller particles; and 3. The mixing by 
high shear kneading action, continued sufficiently to produce an 
essentially homogeneous composite, whereby the proportioned size 
distribution of particles is forced to occupy the minimum volume of which 
it is capable. The resultant composite material obtains a density of only 
1 or 2% less than the theoretical density for the ingredients employed. 
An idealized illustration of the composite structure is depicted at FIG. 2. 
The largest particles are designated by the numeral 21, and represent the 
100 micron range nickel particles. In some instances adjacent points are 
separated by the 100 angstrom range colloidal silica particles 24. The 
larger voids between contiguous particles 21 contain the next smaller 
particles, the micron range particles 22, e.g. the carbonyl nickel, the 
bismuth oxide, and/or the silicon carbide particles. The smaller voids 
contain the submicron range particles, such as the boron carbide and the 
zinc oxide particles, depicted by numeral 23. Interposed and separating 
many of the aforesaid conductor/semiconductor particles are the colloidal 
silica particles 24. The remainder of the voids is filled with the matrix 
resin binder. As stated, the depiction in FIG. 2 is idealized, and it is 
simplified. To facilitate the illustration, the voids between particles 21 
are left somewhat open and are not shown loaded with micron and submicron 
particles. Also, statistically it is apparent that some proportion of 
conductor/semiconductor particles will be in conductive contact with each 
other; but with a large number of particles occupying a relatively large 
volume compared to the sizes of the particles, it is apparent that there 
will be frequent insulative particle interruptions, and the conductive 
chains of particles will be relatively short in relation to the macro 
system as a whole. 
An illustrative use of the composite material is depicted in FIG. 3. A 
section of a coaxial cable 31 is shown, containing a center conductor 32, 
a dielectric 34 surrounding the conductor 32, and a conductive braided 
sleeve 33 overlying the dielectric 34. The braided sleeve is grounded, as 
indicated at 35. A small segment of the dielectric 34 is replaced by the 
section 36 formed from the composite of the present invention, and secure 
electrical contact is maintained between the conductor 32 and the 
composite, and between the braid 33 and the composite. Under normal 
working conditions, the composite 36 presents a very high resistance from 
the conductor 32 to the braid 33, and therefore signals on conductor 32 
are essentially unaffected. However, if a high voltage overstress pulse 
appears on conductor 32, its presence will immediately switch composite 36 
to the on-state, thereby immediately shunting the pulse to ground and 
clamping the pulse at a low voltage value, to protect the circuit or 
device to which the cable is connected. 
In order to illustrate the present invention, further, the following 
specific examples are provided, showing specific illustrative composite 
formulations and the electrical properties thereof, specifically the 
response to an overstress pulse and the normal operating resistance. 
EXAMPLES 1-3 
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Vol. Percent 
Formulation Ex. 1 Ex. 2 Ex. 3 
______________________________________ 
Carbonyl nickel (Ni228) (micron range) 
7.8 9.0 -- 
Nickel (Ni227) (100 micron range) 
23.5 27.0 36.0 
Silicon Carbide (micron range) 
9.5 -- -- 
Boron carbide (submicron range) 
21.7 10.0 3.0 
Zinc oxide (submicron range) 
-- 19.6 28.3 
Bismuth oxide (micron range) 
-- 1.3 1.6 
Colloidal silica (20 to 100 angstrom 
4.8 1.0 1.0 
range) 
Silicone rubber binder (SE63) 
32.6 32.0 30.0 
Actual density 4.05 4.98 5.28 
Theoretical density 4.06 5.01 5.34 
Electrical Characteristics 
Thickness of sample (mils) 
55 50 180 
Overstress pulse (volts) 
4800 4800 4800 
Clamping value (volts) at time 
from leading edge of pulse 
0 nanoseconds 458 280 385 
50 nanoseconds 438 263 376 
100 nanoseconds 428 237 372 
500 nanoseconds 405 228 350 
1.0 microseconds 405 222 350 
2.0 microseconds 400 228 350 
3.0 microseconds 396 228 340 
Resistance in megohms at 10 volts 
2.2 1.7 3.5 
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From the foregoing examples it will be appreciated that an electrical 
overstress protection device can be provided, wherein an overstress pulse 
of thousands of volts is clamped essentially instantaneously to values of 
a few hundred volts, and maintained at that value. Further, the normal 
operating resistance value of the overstress responsive device is in the 
megohm range. Obviously, by varying the components and proportions of the 
composite material within the principles and concepts of the invention, 
the values of the electrical parameters can be altered and tailored to the 
needs of a specific environment, system or purpose. 
By way of comparison, reference is made to the materials in the 
above-mentioned prior art patent to Hyatt et al. 4,726,991. Therein, two 
specific composite compositions are set forth at col. 9, lines 20 to 24. 
The components of the composite are there specified in weight percent. For 
comparison purposes they are here converted to volume percent. 
EXAMPLES 4 and 5 
______________________________________ 
Ex. 4 Ex. 5 
Composition Wt. % Vol. % Wt. % Vol. % 
______________________________________ 
Carbonyl nickel 
12 3.2 22.5 6.1 
Silicon Carbide 
56 40.6 43 32 
Colloidal silica 
2 2.1 2.5 2.7 
Epoxy binder 30 53.9 32 59.2 
______________________________________ 
It will be immediately apparent that the prior art composites use a much 
greater percent of insulation material (binder plus colloidal silica), and 
a much lesser volume percent of conductor particles, than is used in the 
practice of the present invention. Although not stated in the patent, 
these compositions in the prior patent provide excessively high clamping 
voltages, in excess of 1800 volts per millimeter of thickness of composite 
material. 
Referring to FIG. 5 of said Hyatt et al. patent, while it depicts an 
overstress clamping voltage of less than 200 volts for a composite 
material, what is not stated in the patent is that this result was not 
obtained with the composites described above at Examples 4 and 5, and that 
the resistance of the FIG. 5 material in response to a normal operating 
voltage of 10 or 20 volts, or so, was less than 20,000 ohms. 
It will thus be appreciated that in accordance with the teachings of the 
present invention, a composite of particulate components in a binder 
matrix is provided, which is capable of providing a high resistance at 
relatively low operating voltages, and a low impedance in response to a 
high voltage electrical overstress pulse to clamp the overstress pulse at 
a low voltage. The specific low voltage resistance and overstress clamping 
voltage can be varied and tailored to a specific need by appropriate 
selection of the composite ingredients and proportions. Accordingly, while 
the invention is described herein with reference to several specific 
examples and specific procedures, these are presented merely as 
illustrative and as preferred embodiments of the invention at this time. 
Modifications and variations will be apparent to those skilled in the art, 
and such as are within the spirit and scope of the appended claims, are 
contemplated as being within the purview of the present invention.