US3766095A

US3766095A - Compositions of matter containing ferromagnetic particles and nonferromagnetic aluminum particles in an elastic material

Info

Publication number: US3766095A
Application number: US00194943A
Authority: US
Inventors: S Mastrangelo
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1971-11-02
Filing date: 1971-11-02
Publication date: 1973-10-16
Anticipated expiration: 1990-10-16
Also published as: FR2213989A1; FR2213989B1

Abstract

COMPOSITIONS OF MATTER COMPRISING ALUMINUM PARTICLES, IRON OR OTHER FERROMAGNETIC PARTICLES AND AN ELASTIC BINDER FOR CEMENTING THE PARTICLES INTO A COHERENT MASS.

Description

0 1973 a s. v. R. MASTRANGELO 3,766,095

COMPOSITIONS OF MATTER CONTAINING FERROMAGNETIC, PARTICLES AND NONFERROMAGNETIC ALUMINUM PARTICLES IN AN ELASTIC MATERIAL Filed Nov. 2, 1971 FIG-1 FIG-Z INVENTOR SEBASTIAN V. R. MASTRANGELO ATTORNEY United States Patent O ABSTRACT OF THE DISCLOSURE Compositions of matter comprising aluminum particles, iron or other ferromagnetic particles and an elastic binder for cementing the particles into a coherent mass.

BACKGROUND OF THE INVENTION (1) Field of the invention (2) Description of the prior art Sawyers, McCarthy and Jacoby, in Technical Memorandum SCTM 293-60-52, Sandia Corp., Livermore, Calif. (1960), report that aluminum powder of commercial grade and known to contain magnetic metal in appreciable quantities exhibits switching properties when closely bound as finely divided material particles in a homogeneous mass by means of dielectric filler. Other than a suggestion that the magnetic material is probably iron, the identity, amount and purpose or efiect, if any, of the magnetic material are not disclosed.

Gibbons and Beadle disclose in Solid State Electronics, Pergamon Press 1964, vol. 7, pp. 785-797 that films of nickel oxide, films of other metallic oxides, anodized aluminum, and aluminum powder held in a suitable insulating binderas in Sawyers above, all have similar electrical switching properties. They list these properties of a typical switch operating between high and low resistance states referred to as OFF- and ON-states:

(1) It has an OFF-state in which its resistance is approximately 25 megohms.

(2) It has an ON-state in which its resistance is approx imately 100 ohms.

(3) The device may be turned from its OFF-state to its ON-state by application of a two-hundred volt pulse of 40 microseconds duration. The pulse source impedance should be high (approximately 100 thousand ohms).

(4,) The device may be turned from its ON-state to its OFF-state by application of a 150 milliamp current pulse, 10 nanoseconds in duration.

(5) Devices made to date have a limited lifetime. The

, maximum number of repetitive switching cycles obtained thus far is 1000. Furthermore, the device fails short; that is, it cannot be switched out of its ON- state with normal current amplitudes.

Two problems which commonly arise, therefore, in switching devices formed fiom aluminum and/or other materials in an insulating binder are: (1) high currents must normally flow during current pulsing in order to switch to the OFF-stateand (2) the maximum number of repetitive switching cycles is limited to about 1000 before the device fails short or burn-ON.

New compositions of matter have now been discovered which, when contacted with electrodes and activated by an electrical voltage pulse to a state of lower resistance, serves as a useful electrical switching device which minimizes these problems. Switching devices formed as described herein normally require only 0.1 to 10 milliamp current pulses to be turned from ON-state to OFF-state, and can be cycled about 10 times or more without failure.

SUMMARY OF THE INVENTION This invention is directed to compositions of matter comprising (a) aluminum particles, (b) iron or other ferromagnetic particles, and (c) an elastic binder for said particles, wherein the ratio of ferromagnetic to aluminum particles by weight is in the range of 1:6 to about 2:1 and the combined weights of the aluminum particles and the ferromagnetic particles is from about 40 to about of the total weight of said particles and the elastic binder.

DESCRIPTION OF THE DRAWINGS FIG. 1 is an activating circuit diagram. FIG. 2 is a diagram of a pulse circuit for repetitive cycling.

DESCRIPTION OF THE INVENTION It has been discovered that the incorporation of a ferromagnetic component in controlled amounts into a dispersion of an aluminum powder in an elastic binder greatly improves the performance of the resulting compositions when they are contacted with electrodes and activated by an electrical voltage pulse supplied through said electrodes to a state in which it can serve as a bistable switch.

Better performance is evident by direct comparison of switches or switching devices prepared from the compositions of this invention with the performance of those made without the addition of the specified amount of ferromagnetic powder. By comparison, after activation, the switches prepared from the compositions of this invention, as a result of the presence of the ferromagnetic component, turn OFF and stay OFF when they are switched OFF by a current-limited pulse and require less current to effect the transition. In life tests they do not burn-ON as readily. These advantages and others are attained by utilizing ferromagnetic particles in the compositions of the present invention.

Thus, a switching device made from the compositions of this invention can exist in any of three different states, a latent state, an ON-state, and an OFF-state. In the latent state, the resistance of the device is typically greater than 10 ohms; similarly, the resistance in the OFF-state is typically of the order of 10 ohms. However, in the ON-state, the resistance is typically only from 10 to 2.5 10 ohms, at least 10 times less than the latent or OFF-state.

As originally formed, a switching device of this in vention is in the latent state. In a manner described below, it can be altered from the latent state to the ON-state.

The transition from the latent state to the ON-state is called activation and is accomplished by applying What is called the activating voltage which is equal to or greater than a critical threshold or breakdown voltage. Typically the activating voltage is between and 400 volts per centimeter and it is applied as a brief pulse.

If the device is in the ON-state, it can be switched to the OFF-state, i.e., turned-OFF, by application of a small current-limited pulse of about 0.1 to 25 milliamperes, less than 10 milliamperes being preferred, and regulated such that at the end of the pulse the current drops very rapidly to a very low value. By a current-limited pulse is meant vIf the device is in the OFF-state, it can be switched to the ON-state by application of a voltage, i.e., turn-N voltage, typically between about volts and 225 volts and regulated such that at the end of the pulse the current drops comparatively slowly to a low value. By burn-ON is meant the transition to a state such that the device cannot be switched out of its ON-state. By a bistable switch is meant a switch that has two resistance states, an ON-state, and an OFF-state. Such a switch exhibits practically no other discrete resistance values in transit between the O'N- and OFF-states, as can be established by viewing on an oscilloscope screen the voltage across a small, fixed series resistor in response to an applied voltage pulse.

The novel compositions of this invention comprise:

(1)- non-magnetic metal particles of aluminum,

(2) ferromagnetic particles present in an amount such that the ratio of ferromagnetic to aluminum particles by weight is in the range of 1:6 to about 2:1, and

(3) an elastic binder for said particles.

Ferromagnetic metal powders useful in the compositions of this invention can be characterized according to methods described by Lark-Horovitz and Johnson, Solid State Physics, vol. 6, Part B, p. 204 (1959) Academic Press, as experiencing a measurable body force of a few milligrams in an imposed magnetic field. In general, the ferromagnetic material will have a saturation magnetization per unit volume of at least about 100, preferably at least 500 cgs. units of magnetic moment per unit volume. Ferromagnetic materials useful in this invention include saturation magnetizations 1752, 1446, and 512, respectively. Iron is preferred in making the compositions of this invention. 1

The ferromagnetic powder is selected to have an average particle size of 10 to 30 microns and preferably a narrow distribution of sizes around microns.

Ferromagnetic alloys having high saturation magnetization are also available in the desired powder form and particle size range and may be useful in this invention. Such alloys may contain other elements of the Periodic Table in addition to one or more of the three cited.

The aluminum metal powder component preferably meets the following general criteria: (1) an average particle size of from about 10 to 30 microns, more preferably a narrow distribution of sizes around 20 microns, (2) atomized powders that are granular in shape.

By the term elastic binder is meant an insulating material which is capable of elongation with substantial recovery of its original dimensions.

Preferably, the elastic binder (when tested without the aluminum and ferromagnetic particles) should be capable of being elongated at least 100% (A.S.T.M. D412 test), and still retract to less than 1.5 times its original length.

The elastic binder should be present in an amount such that the combined weights of the non-magnetic aluminum particles and the ferromagnetic particles comprise from 40 to 85 percent of the weight of said particles and the elastic binder. The elastic binder may be dissolved in a suitable carrier solvent and the aluminum and ferromagnetic particles added thereto to form a dope.

The nature of the elastic binder itself can vary widely and its composition is not critical provided it is sufiiciently elastic as defined. Binders with such elastic properties include natural rubber, synthetic polyisoprene rubber, elastomeric chloroprene polymers, fluorolefin elastomers, butadiene-styrene rubber, ethylene-propylene-nonconjugated diene rubbers, silicone rubbers and rubbery condensation polymers such as polyurethanes obtained by reaction of polyisocyanates with polyalkylene glycols. The elastic binder may also contain fillers, reinforcing agents or plasticizers commonly added to elastomers, providing the properties of the resultant binder remain within the limitations hereinbefore recited.

Still polymers with rigid molecular structure such as aromatic polyamides, polyimides and polystyrene result in switches that do not switch off. The elongations of such binders are about 60%, 8%, and 25-58%, respectively, all less than the specified lower limit of elongation.

For convenience in fabricating switches, by casting flexible sheets for example on which many switches can be formed side by side, it is desirable to handle fluid or fluidizable compositions from which the final switch composition can be formed in place. Accordingly, instead of the normally solid elastic binder by itself or in a solvent, there can be employed an elastic binder-forming material along with the aluminum and magnetic powder components.

Such elastic binder-forming material includes any one or more of the following:

(1) preformed polymer which can be further cured to an elastic binder, a curing agent, and optionally a carrier solvent as above,

(2) preformed polymer and optionally a carrier solvent,

said polymer being curable by heat or irradiation,

(3) polymer precursor, chemical agent to convert said precursor into elastic binder, and optionally an inert volatile solvent as thinner,

(4) liquid prepolymer, self-curing or containing a curing agent.

By carrier solvent as used herein is meant a liquid dispersion medium for transporting one or more substances, such as the particles of this invention, which also is capable of solubilizing other materials such as curing agent or chemical agent for polymerization if such be present, e.g., acetone, xylene, tetrahydrofuran, benzene, toluene, dimethylacetamide, ethyl ether, chloroform and dimethylformamide. Said carrier solvent need not be completely removed by subsequent treatment provided the required criteria for elongation and recovery are met by the resultant binder.

In making compositions of matter which lie within the spirit of this invention, dopes may therefore be used which are dispersions of aluminum and ferromagnetic particles in polymer solutions in volatile carrier solvents as mentioned above, e.g., a solution of hydrocarbon rubber in benzene or toluene. Another type of dope might also contain a reactant in addition to the solvent to promote further polymerization of an elastic binder forming mate rial that either may or may not yet be sufiiciently elastic to meet the required criteria for elongation and recovery; for example, a dope useful in making switches contains 20 wt. percent polyurethane rubber such as Adiprene C, a reaction product of diisocyanate and polyalkylene ether glycol in dimethylformamide containing 3.5 v./v. percent H O whereas other useful formulations include blends of powders in self-curing liquid prepolymers such as silicone rubbers. If desired, elastomers capable of undergoing further reaction, such as chain extension or crosslinking, to harden but still keep products elastic, can be cured in situ (in the presence of the metal components). For example, curing agents such as peroxides or sulfur for unsaturated systems represented by hydrocarbon rubbers (including natural and synthetic rubbers derived from olefins and polyolefins) can be incorporated into the compositions of this invention and subjected to curing conditions that are well known, for example, curing by heating. Alternatively, rubbers can be cured by irradiation under conditions known to the art for hardening them.

The order in which the components of the compositions of this invention are mixed is not critical nor is the temperature and pressure at which the mixing takes place. Normally the ferromagnetic metal powder and the alumlnum metal powder are first mixed together. Gentle mixing in a tumbler mixer is preferred to preserve the natural protective tarnish film of aluminum oxide which imparts a characteristically dull gray color to aluminum metal particles handled in air. The mixed ferromagnetic and aluminum metal powders are then blended with the elastic binder. Another satisfactory approach is to blend the powders separately, first one then the other, with elastic binder to form the compositions of this invention, the order of addition again not being critical.

According to this invention, the combined weights of the aluminum particles and the ferromagnetic particles comprise from about 40 to 85 percent of the total weight of said particles and the elastic binder. Compositions containing more than 85 percent generally contain insuflicient binder for mechanical strength. Percentages of 60-70% are preferred. Compositions containing less than 40 percent of the combined weight frequently burn-ON.

The ratio of the weight of the ferromagnetic powder to the weight of the aluminum powder must be in the range of about 1:6 to 2:1. The upper limit of 2:1 must be observed in order to avoid the burning-ON of switches, but not so closely at low combined particle weights near 40% as at high loadings near 85%. The preferred ratio is about 1.5: 1. At the lower limit of 1:6 switches are on the verge of failing to switch OFF, the current required to switch OFF approaches but is still less than the high current required without the ferromagnetic component present, and the switches at this ratio also tend to conduct along multiple paths between electrodes rather than a single path. Instead of acting as a bistable element such switches tend to develop and spend time in states of intermediate electrical resistance between the ON- and OFF-states upon being pulsed. Multiple paths can be detected by displaying the switch current, e.g., by taking a voltage signal from a fixed resistor in series with a switch, and displaying it on the screen of an oscilloscope as the switch is cycled between ON- and OFF-states by alternate application of current-limited and voltage pulses. If multiple paths exist, additional horizontal lines or steps will appear between the two widely separated horizontal lines or steps characteristic of the ON- and OFF-states during each switching cycle. Occasionally one or even several such lines of faint brightness may be seen indicative of a tendency to conduct along one or more conductive paths other than the activated path of lowest resistance. If all the intermediate lines are very faint, operation of a switch as a bistable element is usually not impaired; however, when one such line approaches the brightness of either the ON- or OFF-state, or becomes as bright as the OFF-state line, bistable switching may become uncontrollable.

As formed by solvent evaporation, melt techniques, or polymerization procedures, the compositions described herein typically have electrical resistivities greater than or ten billion ohm-centimeters before activation.

A switching device made from the compositions of this invention may be formed from a dope by shaping the dope, rendering it form-stable, and then applying two noncontiguous electrodes. The dope may be shaped by spreading it onto a substrate on which it remains when in use or from which it is removed before use. It may be spread onto the selected substrate by brushing, dipping, pouring use of a doctor-knife, and similar procedures. After the dope has been shaped, it is subjected to heat and/or vacuum to render it form-stable, that is, to remove volatile solvent and bring the properties of the elastic binder into the range hereinbefore recited.

Coated wires are made by using a wire as a substrate and dipping it into the dope. Either before or after the dope has been rendered form-stable, additional electrode or electrodes are placed in contact with it. The wire serves as one electrode, and each combination of the wire, switch material, and additional electrode serves as a switching device.

Fibers may be pulled from the dope of this invention. Either before or after being rendered form-stable, such fiber can be used to form a switch device by being cemented to two electrodes by the dope of this invention or any conductive cementing material.

Fiber bridges with a common terminal are made into switch arrays with one contact serving as a common ter-.

minal for several switches.

A preferred composition of this invention is made by mill-blending, e.g., in a two roll rubber mill, equal weights of aluminum powder, iron powder, and an elastic terpolymer of ethylene, propylene and an unconjugated diene, e.g., 1,4-hexadiene with sufiicient dicumylperoxide by weight of the terpolymer to effect curing. This and other similar terpolymers having suitable elastic properties are disclosed in US. 2,933,480. After'milling, the blend is hot-pressed into sheets which are cured. In the form of such sheets many applications in the computer or electronic fields are accommodated. For example, electrodes and printed circuitry may be formed on such sheets for use as read only memories and read-write memories. The sheets is easily cut into any sized or shaped smaller pieces for use as electronic circuit components in flip-flops or oscillators. Electrical contact with the sheet is made with painted electrodes or with suitable spring contact probes.

Glass, metal, plaster, rubber, wood and paper are satisfactory substrates for the compositions and dopes of this invention; preferences are for polyester film or no substrate at all.

Variation of the switch sheet or switch plate form includes a metal-backed switch plate made by casting a film of switch dope on a sheet of aluminum foil. The film dries to a reduced thickness and opposing spring contacts are aflixed. Other variations include a paper reinforced sheet made by padding various switch dopes on tissue paper, a plastic-backed switch sheet made by casting various switch dopes on a pressure-sensitive Mylar polyester film, and coated printed circuit boards, made by casting various switch dopes on printed circuit boards with or without printed circuits in place.

In order to make a useful bistable switch from a latent switch of the compositions of this invention between two electrodes, a voltage pulse must be applied to the switch composition to form a conductive path of less than one megohm resistance per centimeter. By application of such an activating voltage pulse specific resistance values of the initial ON-state can be attained ranging from Ohms to 250,000 ohms per centimeter. Once a conductive path has been established its resistance remains essentially unchanged during identification of the ON-state by any small testing or reading voltage not exceeding a voltage potential which produces enough current to cause a transition to the OFF-state, e.g., less than about 5 volts per centimeter.

The electrical resistance of the initial ON-state depends on the magnitude of the activating voltage as well as the nature, particle size, and amount of dispersed particles. In general the initial resistance is decreased by increasing the activating voltage above a critical threshold level for activation or by using larger particles. It can, however, also be decreased by reducing the size of a series resistor, nominally maintained at 330,000 ohms, which is used to limit the current which flows when the activating voltage pulse is applied. When the series resistor is reduced in size the rate of decay of the activating pulse may become so rapid that the switching device is not only activated to the ON-state to become a useful switch, but within the duration of the pulse passes through the ON-state and is left in the OFF-state at the end of the pulse. The reason for this will be better understood following a subsequent description of the nature of current-limited pulses required to turn off switches. Switches activated in this manner are as useful as those activated to an ON-state provided the switch is not impaired by an excessive surge of current. Thus, a switch with desired electrical properties within those practical with the materials used can be obtained from any variety of combinations of activating voltage, current, and particle size and amount of non-ferromagnetic aluminum and ferromagnetic powders.

Two terminal electrodes are needed to apply the activating voltage pulse. Electrode shape, size and form make little difference in switch performance. Silver, copper, and gold paints, copper wire (30 gauge and 18 gauge) straight pins, pressure-sensitive-backed metal foils, rounded spring-loaded pressure contacts and alligator clips have all been used successfully.

Between the two terminal electrodes standing oppositely across a 0.5 cm. sample path for example, a difference in electric potential or voltage of 150 to 400 volts is normally required to activate the switch. Higher voltages tend to produce ON-states of lower resistance but application of too high a voltage results in switches that will not turn- OFF. In a preferred manner a resistance of less than 250,000 ohms is attained by applying a voltage pulse which is limited so as to be nearly equal to the threshold voltage of the switch and relatively independent of variations in switch-forming compositions. Attempted activation with less than the threshold voltage may have deleterious effects. Incompleted paths may form which may in turn produce multiple paths when breakdown is finally reached or during switching operation.

The latent switches prepared from the compositions of this invention should therefore be activated by circuitry that will standardize and produce uniformity in switch characteristics and performance. A typical circuit for activation of a switching device, prepared from the compositions of this invention, from its latent state to its ON- state is shown in FIG. 1. This circuit is optimized for a switch with about 1 cm. spacing between electrodes. An initially open single-pole double throw switch 1 is thrown to terminal 2, allowing a source of electric potential 3 of 400 volts strength to energize a 0.001 ,uf. capacitor 4. Switch 1 is then thrown to make connection with terminal 5, whereupon the potential difference across latent switching device 6 rises at a rapid but controlled rate until its activation occurs.

Two means of control are provided. A parallel circuit path consisting of a 270,000 ohm resistor 7 provides a finite time constant for discharge of the energizing capacitor 4 since the latent switching device has too high a resistance to do so, typically ohms, before activation. Secondly, a 100 ,u/Lf. time delay capacitor 8 serves to slow down the rate of rise by receiving electrical charge flow from the energizing capacitor 4. The capacitor 8 establishes a time constant for potential difference rise determined by the product of the value of the adjacent 10,000 ohm resistor 9 and its own capacitance in farads equal to one microsecond. Within this order of time a threshold voltage between about 150 and 400 volts is thereby reached for the activation of the latent switching device, changing its electrical resistance from a high value typical of its latent state to a low value characteristic of its ON- state. Thereafter a 300,000 ohm resistor 10in series with the device limits the resultant increase in current through it as the potential difference that persists is rapidly dissipated. Finally, a 1N-4005 silicon diode 11 shorts out any reverse transient voltages that might develop.

If the potential difference is allowed to rise too rapidly, the voltage value may overshoot the threshold or breakdown voltage and produce a switch that will not turn- OFF.

If the current is not limited when the switching device is activated, it will tend to pass through the ON-state and be turned-OFF by the current surge. Sometimes actual destruction of the device will occur.

If the diode is omitted, the reverse transient voltages are capable of sometimes destroying the device.

By observing these criteria for composition preparation and switch activation, the current to switch OFF is greatly reduced and a degree of reliability is achieved which has been missing in switches made without the addition of a ferromagnetic component. The current needed'to' turn-OFFthe switchesis normallyE0;1 to :10 milliamperes and most frequently from 1.to 5 milliamperes, instead of 10 to 200 milliamperes characteristic of switches made without the addition of ferromagnetic powder. An ordinary switching circuit will not,however, sufiice unless it provides rapid decay of the trailing edge of the turn OFF pulse. This is evident, for instance, because an activated switching device will not turn-OFF in response to a 60 Hz. or even a 1000 Hz. pulse form.

A typical circuit useful for turning-OFF a switching device that is initially in its ON-state is shown in FIG. 2. Voltage source 21, when interrupted, results in rapid decay of circuit current. Voltage source 21 consists of a common Schmitt trigger circuit working off a sine wave generator into a one-shot multi-vibrator section and a coupling capacitor to provide a shaped current pulse as desired. Upon application of a first pulse, the switching device 22 is turned-OFF by the rapidly decaying current, but electrical charge tends to remain on both sides of the switch, the side toward the electrical ground as well as the side toward the voltage source 21. Such charge, if left unattended, may develop sufficient potential difference to turn-ON the switch again. Typical means for removing excess charge promptly is shown in FIG. 2. It may bleed through a parallel resistor 23 to ground. At the other side of the switch it may bleed through variable series resistor 24 to ground. Further difficulty may still arise because of differences in the time constants for bleeding off charge from both sides of the switch. It can be overcome by those skilled in the art by introducing a pulse time delay circuit 25, composed of an inductor and capacitors as shown for example, in series with an appropriate side of the switching device. A switch by-pass consisting of a 1N-4005 silicon diode 26 in series with a ten millihenry inductor 27, also, serves '-to eliminate transient voltages in the circuit.

The above circuit is not only useful for turning-OFF a switch, but can be used for repetitive cycling between ON- and OFF-states. This is possible because once a switch is turned-OFF it develops a much higher resistance than the internal resistance of voltage source 21. Hence, essentially the full voltage of voltage source 21 can then be made to appear across the switching device in its OFF- state. By voltage regulation the next pulse to be supplied is therefore adjusted in value to that required to turn-ON the switching device to complete a cycle between ON- and OFF-states. Repetitive cyclingrates may be varied from relative low frequencies to frequencies of 10,000 Hz. cycles or more and individual pulse widths from about 1 to 50 microseconds using a typical Schmitt trigger. Visual observation of alternating ON- and OFF-states may be followed by suitable use of an oscilloscope in testing the life of switching devices of this invention. Life test show that switching devices of this invention can be cycled over 10 times or more without failure using the circuitry of 'FIG. 2. 1

As stated above, the turn-OFF current can be as little as 0.1 milliampere. The extended current range makes possible a novel, three-lead, high speed switchingdevice with an isolated second lead. Preferably such a device comprises two bistable switches as described previously in series such that the first switch requires a current to turn-OFF by virtue of its improved composition, particle size, and shape, that is less than either the current to turn-OFF or the current that corresponds to the voltage to turn-ON the second switch. The first bistable switch must be turned-OFF initially. The characteristics of the two switches can be selected so that write (ON) and erase (OFF) pulses for the second switch pass through the first switch to reach the second switch, yet the first switch is always OFF whenever the ON or OFF condition of the second switch is read, i.e., determined using an isolated second lead between the two switches. Such a lead is electrically isolated from the write and erase circuits by the high resistance of the first switch when form a 1-2 mil thick film upon air drying at 40-50 C. for 24 hours. Two electrodes were painted and dried on the film said electrodes being applied as a conductive silver preparation containing silver powder and being The three-lead switch can be used, for example, to 5 spaced about 0.5 cm. apart and extending across the width store binary information, e.g., as a computer memory of the slide. The dope composition with the two applied element b ith r electrodes was then activated by an initial voltage pulse passing an electrical voltage pulse in series tin-ugh the and performed as a switch as recorded in Table 1 below:

lfirst and second switches, which first switch is turned- Comparative Example 1-A ON and then turned-igl g'N y i ffg i r gegf g g g dope was prepared and a switch made as in Example second switch 1s turne an e a l with the single exception of omitting component (c) of turned -ON by sa1 d first pul the said Example 1, and the performance of this switch by passing an electrical curre P 1!! 861165 T P 1s also recorded in Table I. It will be seen upon examid fi t and second switches, l nation of Table 1 data that the current to turn-OFF the turned-ON and then turned-OFF by s p W E ample 1 switch was one-ninth that required to tum-OFF second11 swttchdis left turned-OFF, 1.e., turned-OFF Y the switch of Example l-A.

said P so, an

Exam les 2,3 4 5and6 in a manner consistant with the binary form of mforma- F I tion to be stored as a conductive condition of the second We ad 9 f were Prepared as 1n Example 1 switch except that in each instance component (a) of Example 1 was replaced by: (2) solutions of chloro rene o1 mer Both the twoand three-lead switches utilizing the com p p y positions of this invention demonstrate more reliable XykPe (Du Pont Neoprene W Synthetlc rubber, 450% characteristics that have advantages in computer logic g g pg y f s p y 111 tetfahydroand memory systems as well as in modulation and conu R Adlprene C Polyurethane trol of other electrical devices. Other advantages are the 430%. fe block P' of P y y e and simplicity of design and fabrication, particularly the g f' m xylene 1102, 880% py). ease of interconnecting switches for use in high capacity cls'lATolybutadlene Polymer 1n Xylene (phllllps memories for the more advanced type of computers or etroleum Rubber], 540% g learning machines. Switching times are faster than a no), a p y l; of ethyle e, propylene and a microsecond and, "as a result of the low turn-OFF current, lri'hexadlene (Du Pont Nordel 1070 hydrocarbon banks of switches of this invention have extremely l ow m 46O' 48O% elongatlon) toluenepower requirements and high packing denslty qp g y- Comparative Examples 2-A, 3-A, 4-A, 5-A and 6-A These and other applications of the current switc prepared from the compositions of this invention are as gg' j gg gg g zfi l il a i g tg d fave6 switches electronic circuit elements in oscillators, multivibrators, tively except gt in a ch ins gglzz comI(:)lZI|1gent ,(esgeg; or flip-flops, relays, circuit breakers, flashers, electromc ommd p displays- EXAMPLES Fromthe results in Table 1, it can be seen that inl 40 corporatmg a ferromagnetic component into the composi- The following examples arg mtendedlto lietrneretllyl 111 1? tlOns according (:0 this invenltsion markedly decreases the trative of the invention an not in 1rn1 a ion ereo current requlre to turn-O F the switches, e.g., the Unless otherwise indicated, all quantities are by weight. smallest effective current to turn-OFF the switch of Ex- 1 ample 6, 0.29 ma. proved to be less than one-hundredth EI'KaITIPe 1 that of the 6-A switch. Reductions in current when A dope was prepared by mixing at room temperature utilizing compositions of this invention were noted in all and atmospheric pressure (a) 20 cc. of a 15% solution six comparisons.

TABLE l.-SWITCH PERFORMANCE Current limited pulse to Resistance Turn-OFF Actl- Voltage of the swich vating used to ON-State milli- Using composition ofvoltage Turn-ON (ohms) amperes Examplel 150 19.000 1. 88 gompamtive Example 1-A 5%, x m e goi rip fagive Example 2A 132 11 t ifig r t ii "Iiiriirhie'-AII .1 s 1 53 iii i ve'eanaan" 538 1 afi'gg ta efit rfiaa'srtasi B I 150 100 6 22'. 7 Example 6 150 180, 000 0. 29 Comparative Example 6A 140 95 500 36. 4

Resistance of each observed OFF-state was measured with a Simpson Volt-Ohmyst. and was found to be at least 10 ohms.

by weight of an elastomeric (250290% elongation) copolymer of vinylidene fluoride and hexafluoropropylene (Du Pont Viton A Fluoroelastomer) ll'l acetone, (b) 3 grams of nonleafing aluminum powder of average particle size 19.6 microns, by weight passing through a 325 mesh sieve (Alcan Aluminum Co. MD 2000 aluminum powder), and (c) 3 grams of iron powder of average particle size about 20 microns (Baker and Adamson 1807 iron). A sufficient quantity of the dope was deposited on a 1-inch x 3-inch glass microscope slide to Example 7 Three dopes were prepared by mixing at room temperature and atmospheric pressure (a) 20 cc. of a 15 solution by weight of natural rubber (500% elongation) in xylene, (b) 3 grams of non-leafing aluminum powder of average particle size 19.6 microns, 85 by weight passing through a 325 mesh sieve (Alcan Aluminum Co. MD 2000 aluminum powder), and (c) 1, 3, and 6 grams, respectively, of 200 mesh cobalt powder (City Chemical Co.). Switches were prepared and tested using these three 11 dopes as in Example 1 and their performances are shown by data of Table 2.

Example 8 Switches were prepared and tested as in Example 7 except that component (c) was 100 mesh cobalt powder (City Chemical Co.). Preformances are recorded by data of Table 2.

Example 9 Switches were prepared as in Example 7 except that component (c) was iron powder of average particle size about 20 microns (Baker and Adamson 1807 iron). Performances are recorded by data of Table 2.

Example 10 Switches were prepared as in Example 7 except that component (c) was 200 mesh iron powder (City Chemical Co.). Performances are shown by data of Table 2.

Example 11 TABLE 2.-SWITCH PERFORMANCE The life of an activated switch prepared from dope composition 6 was tested using the circuit-shown in FIG. 2. Voltage source 21 consisted of a Schmitt trigger delivering 200 volt DC pulses 1.0 microsecond in duration at a repetitive cycling rate of 1,000 Hz through'a 0.0003 ,uf. coupling capacitor. The-pulse time delay circuit 5 consisted of a 10 millihenry choke and ganged f. capacitors. The parallel chargebleed resistor 23 was about 6000 ohms and the series bleed'resistor 4 was 3500 ohms. The switch was cycled repetitively for3 hours or 10.8 million cycles. The switch was still running anddid not fall short or b O v 'j .1.: l

TABLE 3 M, Current limited Resist-, pulse to ance Turn-OFF Aeti- Voltage of the switch,

Using Example 11 vating used to n ON-State millicomposition voltage Turn-ON 4 (ohms) vemperes Current limited pulse to Ferro- Resist- Turn-OFF magnetic Acti- Voltage ance of the switch, Using composition 0! powder vating used to 0 ate Example- (grams) voltage Turn-0N ohms) amperes Resistance of each observed OFF-state was measured with 5 Simpson Volt-Ohmyst Percent Weight Dope (weight) ratio composition Al-l-Fe e:Al

1 a Al/LOFe 57 1:3 2. 6 A1 1.0 Fe 70 1:6 3. 1.5 [0.5 Fe 40 1:3

' 4- 3Al/0.5 Fe 54 1:6 5. 3 iii/2.0 Fe 63 2:3

- c A1/3-0 Fe 61 1.1 7 3 Al/6.0 Fe- 75 2:1

Y Switching performance of the switches utilizing the var? ious dope compositions are recorded in Table 3 which shows that switches made from compositions with weight ratios of iron to aluminum within the scope of this invention all require less than 5 milliamperes tum-OFF current.

Example 13 A mixture of (a) 100 grams of a terpolymer of ethylene, propylene and a 1,4-hexadiene (Du Pont Nordel 1070 in hydrocarbon rubber, 460-480% elongation) with 7.0 gm. dicumylperoxide added as curing agent, (b) 100 grams of nonleafing aluminum powder of average particle size 19.6 microns, by weight passing through a 325 mesh sieve (Alcan Aluminum Co. MD 2000 aluminum powder), and (c) grams of iron powder of average particle size about 20 microns (Baker and Adamson 1807 iron) was milled with 20 grams of milling oil (Sun Par Oil). The milling oil was then extracted with a mixed solvent composed of equal weights of perclene and ethanol. The mill-blended mixture wasthen heat-cured at C. for 30 minutes to 'form a 10 mil thick film about 12 inches in diameter. Two conductive silver paint elec' trodes were applied to the surface by brushing, and a volt age pulse of .400 volts was applied between the electrodes to form an activated switch of 200 ohmsre'sistance in its QN-state. The switch was operated successfully between OFF- and 'ON-states by'alternate applications of ac1ir' rent pulse 'of 20'milliamperesanda voltage pulse pf 200 volts 13 Example 14 A three-lead switch with an isolated second lead was built by placing the switch of Example 6 in ser es W1th the switch of Example 3. The latter switch requires 4.3 ma. to turn OFF and 65 volts to turn ON, so that the former switch which requires only 0.29 ma. (from a 35- 40 volt source) to turn OFF, remains OFF as the hlgh current switch is pulsed through the low current switch to either an ON or an OFF state. This keeps the pulse input terminal isolated so it cannot be read itself yet the state of the high current switch can always be read directly by a simple resistance measuring circuit placed across its terminals.

To demonstrate this a voltage pulse or write signal of 0.5 ma. derived from a voltage source of over 65 volts was applied through the low current switch of Example 6 to the high current switch of Example 3. A Simpson Volt- Ohmyst resistance meter was then connected in series with a blocking resistor of 100,000 ohms resistance and Comparative Example 15 Equal weights of Alcan Aluminum Co. aluminum powder MD 2100 and General Aniline and Films PQ-19 fine carbonyl iron powder were mixed together, fired in an argon-filled furnace at 1,200" C. until alloyed, and pulverized to pass a 325 mesh screen.

The procedure of Example 1 was repeated twice except that 4.5 grams and 7.5 grams, respectively, of the prepared aluminum/iron alloy powder were used instead of 3 grams each of separate aluminum and iron powder components (b) and (c) of Example 1, and polyurethane cpolymer in tetrahydroffuran (Du Pont Adiprene C polyurethane ru=bber 430% elongation) was substituted for component (a) of Example 1.

Two latent switches were prepared from these compositions as in Example 1 with two applied electrodes. They could not be activated to a state of low resistance by an initial voltage pulse in the range of 150 to 400 volts and did not perform as switches.

The foregoing detailed description has been given for clarity of understanding only and no unnecessary limitations are to be understood therefrom. The invention is not limited to exact details shown and described for obvious modifications will occur to one skilled in the art.

The embodiments of the invention in which an excluiive property or privilege is claimed are defined as folows:

1. A composition of matter comprising:

(a) aluminum particles,

(b) ferromagnetic particles, and

(c) an elastic binder for (a) and (b), wherein the ratio of ferromagnetic to aluminum particles by weight is in the range of 1:6 to about 2:1 and the combined weights of the aluminum particles and the ferromagnetic particles is from about 40 to about 85 percent of the total weight of said particles and the elastic binder.

2. A composition according to claim 1 wherein the ferromagnetic particles are selected from the group consisting of iron powders, cobalt powders, nickel powders and mixtures thereof.

3. A composition according to claim 2 wherein the ferromagnetic particles are iron powders.

4. A composition according to claim 2 wherein the ferromagnetic particles are cobalt powders.

5. A composition according to claim 2 wherein the ferromagnetic particles are nickel powders.

6. A composition according to claim 1 wherein the elastic binder is capable of being elongated at least 100 percent and still retract to less than 1.5 times its original length.

7. A composition according to claim 6 wherein the elastic binder is selected from the group consisting of natural rubber, synthetic polyisoprene rubber, elastomeric chloroprene polymers, fluoroolefin elastomers, butadienestyrene rubber, ethylene-propylene-non-conjugated diene rubbers, silicone rubbers, and polyurethane rubbers.

8. A composition according to claim 6 wherein the elastic binder is in a carrier solvent.

9. A composition according to claim 1 wherein the ratio of ferromagnetic to aluminum particles by weight is about 1.521.

10. A composition according to claim 1 wherein the combined weights of the aluminum particles and the ferromagnetic particles is from 60 to percent of the total weight of said particles and the elastic binder.

11. A composition according to claim 1 consisting essentially of (a) aluminum particles,

(b) iron particles, and

(c) terpolymer of ethylene, propylene and an unconjugated diene.

12. A composition according to claim 1 sheet form.

13. A composition according to claim 11 which is in sheet form.

14. A composition according to claim 13 wherein the composition has been hot-pressed into sheets.

15. A composition according to claim 1 which has been activated by the application thereto of a voltage pulse to form a conductive path of less than one megohm.

16. An activated composition according to claim 15 which has two resistance states and which is capable of being switched between these two resistance states.

which is in References Cited UNITED STATES PATENTS 3,571,777 3/1971 Tully 33820 3,685,028 8/ 1972 Wakabayashi et a1. '33820 X 3,562,187 2/1971 Abdella 252-513 OTHER REFERENCES Films Solid-State Electrons, Pergamon Press, vol. 7, pp. 785-797 (1964).

CHARLES E. VAN HORN, Primary Examiner US. Cl. X.R. 25262.54, 512; 338-20, 21; 340173.2