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BACKGROUND OF THE INVENTION
The present invention relates to a process for producing an electric
resistor designed for use as a conducting element on an electric circuit;
said resistor presenting a high conducting capacity selectable from within
a wide range and, more especially, being capable of varying its electrical
resistance as a function of the pressure exerted on the resistor itself.
Electrical resistors are known, substantially comprising a matrix formed
from flexible insulating material, e.g. synthetic plastic, and some sort
of powdered metal dispersed inside the said matrix. For producing the said
resistors, a number of processes have been proposed, all of which,
however, substantially come down to two basic types.
In one, the matrix consists of a sponge of insulating material defining a
number of cells, inside which the powdered material is dispersed by
passing a suitable liquid containing the suspended powder through the
sponge. In the other, the matrix material is liquified and blended
mechanically with the powdered material, so as to produce a mixture of
powdered material inside the liquid matrix material, which is then
solidified.
Resistors so formed present a number of drawbacks.
Firstly, they cannot be used as conducting elements on electric circuits,
due to the exceptionally high resistance they present when idle. Specific
conducting capacity is sufficiently high for this purpose only when the
resistors are subjected to fairly high pressure. On resistors of this
sort, electrical resistance decreases alongside increasing pressure, but,
when idle, with no external pressure applied, resistance is substantially
infinite. Secondly, the electrical characteristics of such resistors do
not remain constant throughout their working life, and are difficult to
repeat productionwise. For overcoming this drawback, processes have been
proposed whereby the powdered material dispersed inside the matrix is
produced by blending powders of specific types and grades, and specific
physical and chemical characteristics. Such processes, however, are both
complex and high-cost, due to the equipment involved, and the cost of raw
materials and processing for producing the required powdered material.
SUMMARY OF THE INVENTION
The aim of the present invention is to provide a process for producing
electric resistors of the aforementioned type, but involving none of the
aforementioned drawbacks; which process comprises a small number of easily
repeatable stages, and employs only low-cost, readily available raw
materials.
The said process is characterised by the fact that it consists in preparing
a homogeneous system comprising particles of a first electrically
conductive material arranged in substantially uniform manner inside a mass
of a second liquid material which, when solidified, is both flexible and
electrically insulating; and in solidifying the said mass of the said
second liquid material, so as to form a matrix for supporting the said
particles; throughout solidification of the said second liquid material, a
given pressure being applied on the system for the purpose of producing
triaxial precompression of the said second material when solidified.
For convenienty preparing the said homogeneous system, a structure of the
said particles is first formed; which structure statistically presents
each of the said particles arranged at least partially contacting the
adjacent particles with which it defines a number of gaps which are
subsequently injected with the said mass of the said second liquid
material.
The said process conveniently comprises at least a first stage, in which is
formed a mass of particles of the said first material; a second stage, in
which the said mass is compacted by subjecting it to a given pressure; a
third stage, in which the said mass is injected with the said second
material in its liquid form, so as to fill the said gaps between the said
particles and so form the said homogeneous system; and a fourth stage, in
which the said second material is solidified.
DESCRIPTION OF THE DRAWINGS
For clearly illustrating the structural characteristics of the electric
resistor according to the present invention, and the various stages in the
process for producing the same, both will now be described in more detail
with reference to the accompanying drawings, in which:
FIGS. 1 and 2 show two structural sections, to different scales, of a
portion of the resistor according to the present invention;
The graphs in FIGS. 3 to 6 show the variations in electrical resistance of
the resistor according to the present invention, as a function of the
pressure exerted on the resistor itself;
FIG. 7 shows a schematic diagram of a test circuit arrangement for plotting
the results shown in FIGS. 3 to 6;
FIGS. 8 to 12 show schematic diagrams of the basic stages in the process
for producing the electric resistor according to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
To enable a clearer understanding of the process according to the present
invention, a description will first be given of the structure of the
resistor so formed.
The structure of the resistor according to the present invention is as
shown in FIGS. 1 and 2, which show sections of a portion of the resistor
enlarged a few hundred times.
The said resistor substantially comprises a supporting matrix 1, formed
from flexible, electrically insulating material, and particles 2 of
electrically conductive material arranged in substantially uniform manner
inside corresponding cells 3 on the said matrix 1. As in the embodiment
shown, the said particles preferably consist of granules of electrically
conductive material. As shown in the larger-scale section in FIG. 2, at
least some (e.g. 50 to 90%) of the said cells communicate with one
another, and, in a number of cases, are exactly the same shape and size as
the granules contained inside. Other cells, on the other hand, are
slightly larger than the said granules, so as to form a minute gap 4
between at least part of the outer surface of the granule and the
corresponding inner surface portion of the respective cell.
The arrangement of cells 3, and therefore also of granules 2, inside matrix
1 is entirely random. Though the advantages of the resistor according to
the present invention are obtainable even if only a few of cells 3
communicate with one another, it is nevertheless preferable for most of
them to do so. For best results, the estimated percentage of communicating
cells is around 50-90%.
Though conducting granules 2 may be of any size, this conveniently ranges
between 10 and 250 microns. Likewise, granules 2 may be of any shape and,
in this case, are preferably irregular, as shown in FIGS. 1 and 2.
Matrix 1 may be formed from any type of electrically insulating material,
providing it is flexible enough to flex, when a given pressure is applied
on the resistor, and return to its original shape when such pressure is
released. Furthermore, the material used for the matrix must be capable of
assuming a first state, in which it is sufficiently liquid for it to be
injected into a granule structure statistically presenting each of the
said granules arranged at least partially contacting the adjacent granules
with which it defines a number of gaps; and a second state in which it is
both solid and flexible. The viscosity of the liquid material conveniently
ranges from 500 to 10,000 centipoise.
Matrix 1 may conveniently be formed from synthetic resin, preferably a
synthetic thermoplastic resin, which presents all the aforementioned
characteristics and is thus especially suitable for injection into a
granule structure of the aforementioned type.
Though the size of granules 2, which depends on the size of the resistor
being produced, is not a critical factor, the said granules are preferably
very small, ranging in size from 10 to 250 microns.
The conducting material used for the granules may be any type of metal,
e.g. iron, copper, or any type of metal alloy, or non-metal material, such
as graphite or carbon. The materials for matrix 1 and granules 2 may thus
be selected from a wide range of categories, providing they present the
characteristics already mentioned.
The material employed for matrix 1, which, as already stated, must be
flexible and insulating, is preferably, though not necessarily, so
precompressed inside matrix 1 itself as to exert sufficient pressure on
particles 2 to maintain contact between the same. It follows, therefore,
that each minute element of the said matrix 1 material is in a
sufficiently marked state of triaxial precompression as to exert on
adjacent elements, in particular particles 2, far greater stress, for
producing contact pressure between the surfaces of the said particles,
than if the said triaxial precompression were not provided for. As will be
made clearer later on, such a state of triaxial precompression is a direct
consequence of the process according to the present invention.
With the structure described and shown in FIGS. 1 and 2, the resistor
according to the present invention presents an extremely large number of
granules 2 of conducting material, which granules either contact one
another, or are separated from adjacent granules by extremely small gaps 4
which may be readily bridged when given pressure is applied on the
resistor. This results in the formation, inside the said structure, of a
number of electrical conductors, each consisting of a chain comprising an
extremely large number of granules 2, which are normally already arranged
contacting one another inside the said structure. Each of the said chains
may electrically connect end surfaces 5 and 6 on the resistor directly, as
shown by dotted line C1 in FIG. 1. Alternatively, chain may be formed
inside the resistor, as shown by dotted line C2 in FIG. 1, in which the
individual granules in the chain are partly arranged contacting one
another directly, and partly separated solely by gaps 4. The granules in
such chains may be brought into contact, as in the case of chain C1, by
subjecting surfaces 5 and 6 on the resistor to a given pressure sufficient
to flex the material of matrix 1 and so bridge the said gaps for bringing
the adjacent granules separated by the same into direct contact.
The process according to the present invention is as follows.
The first step is to prepare a homogeneous system comprising particles,
preferably granules, of a first electrically conductive material arranged
in substantially uniform manner inside a mass of a second liquid material
which, when solidified, is both electrically insulating and flexible. The
mass of the said second liquid material is then solidified to form a
supporting matrix for the granules. According to the present invention,
throughout solidification of the said second material, a given pressure is
applied on the system for the purpose of producing triaxial precompression
of the said second material when solidified. Such pressure, which is
maintained substantially constant throughout solidification, ranges from a
few tenths of a N/mm.sup.2 to a few N/mm.sup.2.
For forming the said homogeneous system, a granule structure is first
formed, which structure statistically presents each granule arranged at
least partially contacting the adjacent granules, with which it defines a
number of gaps which are then injected with the said second liquid
material. The said second material may be liquified by simply heating it
to a given temperature. For solidifying it, cooling is usually sufficient.
In the case of synthetic resins, however, these must be solidified by
means of curing.
The process according to the present invention may comprise the following
stages.
A first stage, in which a mass of electrically conductive granules 16 is
formed, for example, inside an appropriate vessel 15 (FIG. 8). For this
purpose, the granules, after being poured into the said vessel, are
vibrated so as to enable settling. The bottom of vessel 15 is conveniently
either porous or provided with holes for letting out the air or gas
trapped between the granules.
A second stage, as shown in FIG. 9, in which the mass of granules 16 is
compacted by subjecting it to a given pressure, e.g. by means of piston
17, applied in any appropriate manner on the upper surface of mass 16.
This produces a granule structure in which, statistically, at least part
of the surface of each granule is arranged contacting surface portions of
the adjacent granules, with gaps inbetween.
As shown in FIG. 9, piston 17 is conveniently provided with a tank 18
containing the said second material in liquid form; which liquid material
may be forced, e.g. by a second piston 19, through hole 20 into a chamber
21 defined between the upper surface of granules 16 and the lower surface
of piston 17, as shown clearly in FIG. 10. The said second liquid material
in tank 18 is a material which may be solidified and, when it is, is both
insulating and flexible. In the event the said material is liquified by
heating, appropriate heating means (not shown) are also provided for.
A third stage (FIGS. 10 and 11) in which piston 19 moves down and piston 17
up, so as to force a given amount of the said second liquid material
inside chamber 21 (FIG. 10). Piston 17 is then brought down for producing
a given pressure inside the liquid material in chamber 21 and so forcing
it to flow into the gaps between the granules in mass 16 and form, with
the said granules, the said homogeneous system. At the same time, any air
between the granules is expelled through the porous bottom of vessel 15.
The pressure produced by piston 17, at this stage, inside the liquid
material mainly depends on the size of the granules, the viscosity of the
liquid, the height of the granule mass being impregnated, and required
impregnating time.
Penetration of the liquid material inside the gaps in granule mass 16 has
been found to have no noticeable effect on the granule arrangement
produced in the compacting stage.
A fourth stage (FIG. 11) in which the homogeneous system of granules and
liquid material produced in the foregoing stage is substantially
solidified. This may be achieved by simply allowing the system to cool and
the said second liquid material to set. At this stage, changes may be
observed in the structure of the said second material due, for example, to
curing of the same.
It has been found necessary to dose the liquid material fed into chamber 21
prior to the injection stage, in such a manner as to ensure that it is
sufficient to impregnate only a large part of granule mass 16, leaving a
nonimpregnated layer 22 (e.g. of about 25%). In like manner, the liquid
material flowing inside the gaps between the granules is subjected solely
to atmospheric pressure through the porous bottom of vessel 15. The
granules, on the other hand, (be they impregnated or not), are subjected
to the pressure exerted by piston 17, as shown in FIG. 12. The said
pressure is applied evenly over all the contact points between adjacent
granules, and is what determines the specific electrical resistance of the
resulting material. That is to say, using the same type of granules and
liquid material, an increase in the said pressure results, within certain
limits, in a reduction of the specific electrical resistance of the
resulting material. The said presure must be maintained constant until the
liquid material has set, and must be at least equal or greater than the
compacting pressure applied in stage 2 (FIG. 9).
Though the said pressure may be selected from within a very wide range,
convenient pressure values have been found to range from a few tenths of a
N/mm.sup.2 to a few N/mm.sup.2. For resistors prepared as described in the
following examples, the following pressures were selected:
Example 1: 1.17 N/mm.sup.2
Example 2: 0.62 N/mm.sup.2
Example 3: 1.56 N/mm.sup.2
Example 4: 2.35 N/mm.sup.2
Example 5: 1.17 N/mm.sup.2
The mass of material so formed inside vessel 15 may be cut, using standard
mechanical methods, into any shape or size for producing the electric
resistor according to the present invention.
To those skilled in the art it will be clear that changes may be made to
both the resistor and the process as described and illustrated herein
without, however, departing from the scope of the present invention.
In particular, granules 2 arranged inside matrix 1 may be replaced by
particles of electrically conductive material of any shape or size, e.g.
short fibres.
For preparing the said homogeneous system comprising particles of a first
electrically conductive material distributed inside a mass of a second
liquid material which, when solidified, is both electrically insulating
and flexible, processing stages may be adopted other than those described
with reference to FIGS. 8 to 12.
The said homogeneous system, in fact, may be obtained by mixing the said
particles mechanically with the said second liquid material, using any
appropriate means for the purpose.
According to the aforementioned variation, throughout solidification of the
said second material, the said system is forced against a porous (or
punched) septum for letting out, through the said septum, at least part of
the said second liquid material. The pressure so produced may be
maintained until the said second material solidifies, so as to produce the
said triaxial precompression in the solidified said second material.
For achieving the said precompression, the said system may be spun
throughout solidification of the said second liquid material.
When incorporated in an electric circuit, performance of the resistor
according to the present invention is as follows.
If no external pressure is applied on the resistor, and end surfaces 5 and
6 are connected electrically via appropriate conductors, electric current
may be fed through the resistor as in any type of rheophore. The density
of the current feedable through the resistor has been found to be very
high, at times in the region of ten A/cm.sup.2. When idle, the resistance
of the resistor according to the present invention may, therefore, be low
enough to produce an electrical conductor capable of handling a high
current density, as required for supplying a circuit component or device.
A number of resistance values relative to resistors produced by
appropriately selecting the characteristics of the particles and the
material of matrix 1, and the parameters of the present process, are shown
in the Examples given later on.
Total resistance of the resistor so formed has been found to be constant,
and dependent solely on the structure of the resistor, in particular, the
number and size of communicating cells 3 in matrix 1, and the number of
gaps 4 separating adjacent granules 2.
By appropriately selecting the aforementioned parameters, some of which
depend on the process described, a resistor may be produced having a given
prearranged resistance. When pressure is applied perpendicularly to
surfaces 5 and 6, the electrical resistance measured perpendicularly to
the said surfaces is reduced in direct proportion to the amount of
pressure applied. FIGS. 3 to 6 show four resistance-pressure graphs by way
of examples and relative to four different types of resistors, the
characteristics of which will be discussed later on. As shown in the said
graphs, the fall in resistance as a function of pressure is a gradual
process represented by a curve usually presenting a steep initial portion.
Even very light pressure, such as might be applied manually, has been
found to produce a considerable fall in resistance. In the case of a
resistor having the resistance-pressure characteristics shown in FIG. 6,
starting resistance was reduced to less than one percent by simply
applying a pressure of around 1 N/mm.sup.2 (about 10 kg/cm.sup.2). With a
different structure and pressures of around 2 N/mm.sup.2 (about 20
kg/cm.sup.2), starting resistance may be reduced by 1/3 (as shown in the
FIG. 3 graph).
If the pressure applied on the resistor according to the present invention
is maintained constant (or zero pressure is applied), electrical
performance of the resistor has been found to conform with both Ohm's and
Joule's law. For application purposes, it is especially important to
prevent the heat generated inside the resistor (Joule effect) from
damaging the structure. This obviously entails knowing a good deal about
the thermal performance of the material from which the supporting matrix
is formed.
Assuming the resistor according to the present invention is capable of
withstanding an average maximum temperature of 50.degree. C., under normal
heat exchange conditions with an ambient air temperature of 20.degree. C.,
the density of the current feedable through the resistor ranges from 0.2
A/cm.sup.2 (Example 4) to 11 A/cm.sup.2 (Example 5) providing no external
pressure is applied.
In the presence of external pressure, such favourable performance of the
electric resistor according to the present invention is probably due to
improved electrical conductivity of granule chains such as C1 and C2 in
FIG. 1. In fact, as pressure increases, the conductivity of
contacting-granule chains (such as C1) increases due to improved
electrical contact between adjacent granules, both on account of the
pressure with which one granule is thrust against another, and the
increased contact area between adjacent granules. In addition to this,
granule chains such as C2, in which the adjacent granules are separated by
gaps 4, also become conductive when a given external pressure is applied
for bridging the gaps between adjacent pairs of otherwise non-conductive
granules.
Total electrical conductivity of the granule chains increases gradually
alongside increasing pressure by virtue of matrix 1 being formed from
flexible material, and by virtue of the said material being precompressed
triaxially. As a result, adjacent granules separated by gaps 4 are
gradually brought together, and the contact area of the granules already
contacting one another is increased gradually as flexing of the matrix
material increases. Each specific external pressure is obviously related
to a given resistor structure and a given total conducting capacity of the
same. When external pressure is released, the resistor returns to its
initial unflexed configuration and, therefore, also its initial resistance
rating.
In the said initial unflexed configuration, the electrical performance of
the material the resistor is made of has been found to be isotropic, in
the sense that the specific resistance of the material is in no way
affected by the direction in which it is measured. If, on the other hand,
the material the resistor according to the present invention is made of is
flexed by applying external pressure in a given direction, the specific
resistance of the material has been found to vary continuously in the said
direction, depending on the amount and direction of the flexing pressure
applied.
To illustrate the electrical performance of the resistor according to the
present invention, when subjected to varying external pressure, four
resistors featuring different structural parameters will now be examined
by way of examples.
A fifth example will also be examined in which the specific resistance of
the resistor according to the present invention is sufficiently low for it
to be considered a conductor.
EXAMPLE 1
A cylindrical resistor, 12.6 mm in diameter and 7.4 mm high was prepared,
as shown in FIGS. 8 to 12, using epoxy resin (VB-BO 15) for matrix 1.
Conducting granules 2 consisted of carbon powder ranging in size from 200
to 250 microns.
On resistors with granules of this sort, the matrix insulating material
injected between the granules occupies approximately 56.8% of the total
volume of the resistor. The resistor so formed was connected to the
electric circuit in FIG. 7, in which it is indicated by number 10. The
said circuit comprises a stabilized power unit 11 (with an output voltage,
in this case, of 4.5 V), a load resistor 12 (in this case, 10 ohm), and a
digital voltmeter 13, connected as shown in FIG. 7. Resistor 10 was
subjected to pressures ranging from 7.8.multidot.10.sup.-2 N/mm.sup.2 to
196.multidot.10.sup.-2 N/mm.sup.2.
Resistance was measured by measuring the difference in potential at the
terminals of resistor 12 using voltmeter 13, and plotted against pressure
as shown in the FIG. 3 graph. From a starting figure of 5.4 Ohm,
resistance gradually drops down to 1.78 Ohm as the said maximum pressure
is reached.
EXAMPLE 2
A cylindrical resistor, 12.6 mm in diameter and 7.2 mm high was prepared as
before using an alpha-cyanoacrylate-base resin for matrix 1 and carbon
granules ranging in size from 200 to 250 microns.
Once again, the resistor was connected to the FIG. 7 circuit, the
components of which presented the same parmeters as in Example 1. The
relative resistance-pressure graph is shown in FIG. 4, which shows a
resistance drop from 16 to 5.25 Ohm between the same minimum and maximum
pressures as in Example 1.
EXAMPLE 3
A tubular resistor with an outside diameter of 12.6 mm, an inside diameter
of 3.5 mm, and 5.4 mm high was prepared as before, using epoxy resin
(VB-BO 15) for the matrix and iron granules ranging in size from 50 to 150
microns. On resistors with granules of this sort, the matrix insulating
material injected between the granules occupies approximately 55% to the
total volume of the resistor. Resistance was again measured as shown in
FIG. 7, using a 1000 Ohm load resistor 12 and 4.5 V power unit 11.
Pressure was adjusted gradually from 59.multidot.10.sup.-2 N/mm.sup.2 to
7.22 N/mm.sup.2 to give the graph shown in FIG. 5, which shows a
resistance drop from 1790 to 493 Ohm between minimum and maximum pressure.
EXAMPLE 4
A 2.4 mm high tubular resistor having the same section as in Example 3 was
prepared as before using silicon resin for matrix 1 and iron granules
ranging in size from 50 to 150 microns.
Resistance was again measured on the FIG. 7 circuit, using a 100 Ohm load
resistor 12 and a 1.2 V power unit 11. Pressure was adjusted from
4.2.multidot.10.sup.-2 N/mm.sup.2 to 119.multidot.10.sup.-2 N/mm.sup.2 to
give the graph shown in FIG. 6, which shows a resistance drop from 1100 to
8.1 Ohm between minimum and maximum pressure.
EXAMPLE 5
A 3.4 mm high tubular resistor having the same section as in Example 4 was
prepared as before, using epoxy resin (VB-ST 29) for matrix 1 and tin
granules ranging in size from 50 to 200 microns.
Resistance, measured in the absence of external pressure between the two
bases of the tubular-section cylinder, was 0.08 Ohm. The specific
resistance of the resistor material, in this case, therefore works out at
0.27 Ohm.cm, which is low enough for the resistor to be considered a
conductor. Assuming heat (Joule effect) is dissipated by normal heat
exchange in air at a temperature of 20.degree. C., and the maximum
temperature withstandable by the resistor is 50.degree. C., the density of
the current feedable through this resistor is approximately 11 A/cm.sup.2.
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