US3002121A - Electroluminescent device - Google Patents

Description

Sept. 26, 1961 A. BRAMLEY ET AL 3,002,121

ELECTROLUMINESCENT DEVICE Filed April 20, 1956 2 Sheets-Sheet 1 I NV E N T0138 Jenny flramey A 154%)" firamiey .ATTORNEYS P 1961 A. BRAMLEY ETAL 3,002,121

ELECTROLUMINESCENT DEVICE Filed April 20, 1956 2 Sheets-Sheet 2 INIE VTORS J m 574m 52702216 ATTORNEYS 3,002,121 ELECTROLUMINESCENT DEVICE Arthur Bramley and Jenny Bramley, both of Van Houten Ave., Passaic, NJ.

Filed Apr. 20, 1956, Ser. No. 579,570 9 Claims. (Cl. 313-108) The present invention relates to devices responsive to voltage which may be employed as voltmeters or voltage or transient indicators.

The present application is a continuation-in-part of our copending application Serial No. 328,757, filed December 30, 1952, for Electroluminescent Voltage Device.

A purpose of the invention is to produce a voltmeter or voltage indicator of very high impedance.

A further purpose is to produce a voltmeter of extremely rugged construction.

A further purpose is to render phosphor dispersed in a semitransparent dielectric layer luminous by applying a voltage across conducting layers on opposite sides of the dielectric layer, one of the conducting layers preferably being semitransparent, henceforth referred to as multilayer element.

A further purpose is to apply a mirror on the conducting layer opposite to the semitransparent conducting layer to direct most of the light through the semitransparent conducting layer.

A further purpose is to render an electroluminescent, semitransparent dielectric layer with impurity centers dispersed therein luminous by applying a voltage across conducting layers on opposite sides of the dielectric layer, one of the conducting layers preferably being semitransparent.

A further purpose is to employ a dielectric having a phosphor content of between 1 and 50 percent of the weight of the mixture and preferably about 30 percent.

A further purpose is to use an electroluminescent dielectric with impurity centers dispersed therein, having a breakdown voltage greater than 30,000 volts per centimeter.

A further purpose is to employ the dielectric in a thickness between 0.01 millimeter and 0.2 millimeter.

A further purpose is to use a semitransparent conductor layer on the side on which the light is to be observed.

A further purpose is to provide a protecting dielectric layer over the semitransparent conducting layer.

A further purpose is to measure the light given oif by the phosphor by a photometer.

A further purpose is to interpose a series impedance between the multilayer element of the invention and the source of voltage.

A further purpose is to connect the voltage being indicated to the multilayer element through a variable im pedance in parallel with the multilayer element of the invention and further to interpose a series impedance between the multilayer element and the source of voltage.

A further purpose is to connect the voltage being indicated to the multilayer element through a Thyrite whose impedance varies with the voltage impressed across the Thyrite element placed in parallel with the multilayer element of the invention and further to interpose a series impedance between the multilayer element and the source of voltage.

A further purpose is to connect the voltage being indicated to the multilayer element through a photosensitive element receiving light from the multilayer element.

A further purpose is to provide a maintaining voltage across the multilayer element and the photoconductive element to maintain luminescence once it is initiated.

A further purpose is to modulate the voltage applied .to the conducting layers.

A further purpose is to modulate the frequency of finely divided and the like.

Patented Sept. 26, 1961 ice alternating current applied across the conducting layers.

A further purpose is to obtain longer life and greater resistance to shock and vibration of the voltage indicator at a voltage less than 1000 volts.

Further purposes appear in the specification and in the claims.

In the drawings we have chosen to illustrate a few only of the numerous embodiments in which our invention may appear, selecting the forms shown from the standpoints of convenience in illustration, satisfactory operation and clear demonstration of the principles involved.

FIGURE 1 is a front elevation of the multilayer element used as a basic constituent of the voltage indicator or transient indicator of the invention.

FIGURE 2 is a fragmentary enlarged section of FIG- URE l on the line 2-2.

FIGURES 3 to 10 are views corresponding to FIG- URE 2 but illustrating variations in application.

The present invention is designed to function as a voltage responsive device. It may be used as a voltmeter, or as indicator of voltage such as a transient indicatcr or an emergency voltage alarm to indicate that some structure not intended to be energized has become energized as due to a short circuit.

The invention has many advantages over the cathode ray tube, and over ex sting voltmeters of the standard moving coil type. It is extremely rugged, and not sensitive to vibration or shock. It has a high impedance, making it valuable for many applications where a high impedance device is desired. The space requirement is not large and particularly the depth requirement is small, thus making it possible to reduce greatly the depth at present required by the oscilloscope. v V

in all of the forms of the invention, the fundamental characteristics present in the device of FIGURES 1 and 2 are employed. A dielectric layer 20 extends suitably over the whole area of the screen, panel or viewer 19 and has a thickness which is preferably in therange between 0.01 millimeter and 0.2 millimeter. The dielectric is very desirably of a character which has a high breakdown voltage, that is higher than 30,000 volts per centimeter. The device operates at a voltage below the breakdown voltage, and, if the structural requirements permit, the thickness may be even less than 0.01 millimeter with corresponding reduction in voltage, or it may be greater than 0.2 millimeter with corresponding increase in voltage.

The dielectric 20 must, of course, be provided with impurity centers, which contribute to the activation of the dielectric so that the material can become electroluminescent. The impurity centers may also function to induce a state of semiconductivity in the dielectric either per se or as a result of strong electric fields or of radiation. V

An electroluminescent material is defined as a material capable of emitting radiation under the action of a strong electric field below its breakdown potential.

The dielectric is semitransparent, and by that it is meant that it is at least semitransparent, since it may of course be fully transparent. I

Various materials for the dielectric may be used, depending on the particular requirements. Glass is suitable, permissibly glass having a low melting point and being Sheets of semitransparent crystalline salts may also be used, of which zinc sulphide is a suitable example. Transparent plastic or resin dielectrics may be employed, of which vinyl resin and shellac are examples respectively.

Dispersed throughout the dielectric 20 may be a phosphor, which may in the most general form be any phosphor which is energized by subjecting it to the electric field here used. Obviously a wide variety of phosphors are available, and any suitable material can be employed, the examples given merely being suggestive. Where the dielectric is a vitreous material such as glass, Zinc silicate, or zinc beryllium silicate, magnesium activated, are desirable phosphors. If a crystalline salt dielectric, such as Zinc sulphide or zinc sulphoselenide is used, the dielectric impregnated with copper or manganese activator to act as impurity centers will suitably be its own phosphor. For dispersion in a plastic, such as vinyl resin, or a resin, such as shellac, the phosphor will suitably be zinc sulphide, lead and copper activated.

The concentration of phosphor dispersed in the dielectric will vary with the particular service requirements and in general should be between 1 and 50 percent of the weight of the mixture of dielectric and phosphor. The lower concentrations of phosphor are usually less desirable because the light intensities are lower, although they are suitable for indicating purposes where the pres-- ence of light alone is sufficient. The higher concentrations of phosphor are less desirable because they tend to lower the dielectric properties and interfere somewhat with the transparency. For most purposes it is best to use a concentration of phosphor of about 30 percent by weight.

The character of phosphor will depend also on the time of the repeat cycle. If the device is simply an indicator it may be desirable to use persistent phosphor, but in applications having short shift-over time, phosphors of short duration will be preferable. Phosphors of various durations as well known in the art are applicable in the present invention.

On one side of the dielectric layer 2%} containing the dispersed phosphor is placed a conducting layer 21. Where the luminescence is to be viewed from the edge of the dielectric it may be permissible to employ an opaque conducting layer 21, but in most cases the light from the phosphor should pass through the conducting layer 21, and the conducting layer 21 should be semitransparent. By this it is meant that it should be at least semitransparent, although it may permissibly be transparent.

The manner of applying the conducting layer 21 to the dielectric will depend upon the character of the dielectric and the character of the conducting layer. Evaporated metallic layers such as aluminum, silver and gold are suitable for application to any of the dielectrics, the order of thickness being centimeters so that the metallic layers are semitransparent. In the case of coatings on glass, semitransparent conducting layers are made as described in Leverenz, Luminescence of Solids (1950) 471; Pittsburgh Plate Glass Company Technical Glass Bulletin No. on Nesa Coated Glass; Corning Glass Works Bulletin on E. H. Coated Glass and T. W. Littleton U.S. Patent 2,118,795, granted May 24, 1938. in one procedure tin chloride is used to deposit tin oxide. Any other suitable character of conducting layer 21 may be used.

On the opposite side of the dielectric layer 2th from the conducting layer 21, a conducting layer 22 is placed, which may suitably be of the same character as the conducting layer 21, except that, since it does not ordinarily need to be semitransparent, it may be thicker, there being no exact limit on thickness. For example, it could be as thick as inch, in which case it would provide a structural support for the device. In this case, of course, the conducting layer 22 would be a metallic sheet or plate, such as copper, stainless steel, aluminum, or other suitable electrically conducting structural metal or alloy.

It is preferred to provide a mirror surface on the face of the conducting layer 22 which is directed toward the dielectric layer so that substantially all light will be reflected out through the conducting layer 21. For this purpose it is preferable to use an evaporated layer of aluminum or silver or a highly polished surface on a sheet of stainless steel, silver or gold, or a chemically deposited silver mirror surface.

As a protection against damage to the conducting layer 21 which is exposed toward the observer, and also to protect the observer against electric shock, a semitransparent dielectric layer 23 is applied on the side of the conducting layer 21 remote from the dielectric layer 20. The dielectric layer 23 will preferably be thin so as not to cut off much light, and will desirably be a glass sheet or a plastic layer of transparent dielectric such as polystyrene, methylmethacrylate, urea-formaldehyde or polyvinylchloride.

The element as shown in FIGURES 1 and 2, which is employed with some modifications in the other forms, is for the sake of convenience referred to elsewhere herein as a multilayer element, and consists of the electroluminescent dielectric layer Ztl with impurity centers dispersed therein, and the conducting layers 21 and 22 on the opposed sides thereof.

The voltage to be measured, indicated, or otherwise employed, or a definite fraction thereof, is applied bctween the conducting layers 21 and 22 and should not exceed the breakdown voltage of the dielectric 29 in the particular thickness used, but should be greater than the normal cut-off voltage, that is, the minimum voltage which will render luminous the electroluminescent dielectric with impurity centers dispersed therein.

It will be evident that the device of FIGURES 1 and 2 can extend over considerable areas, the only limitations being increase in capacity and increase in leakage current, and can be mounted substantially fiat against or adjoining walls, without requiring the depth away from the observer which is necessary in an oscilloscope.

In using the device as a voltmeter, the voltage being indicated or measured is applied by leads 24' and 25 respectively to the conducting layers 21 and 22. The pulse may be either A.C. or DC, providing it rises above the normal cut-oif voltage, that is, the voltage necessary to make the phosphor luminous under the impedance conditions present. Above the normal cut-off voltage the quantity of light is roughly proportional to the square of the voltage and therefore is used as a measure of voltage. A photometer 26 is employed, which in the simple form comprises a photocell pick-up 27 in series with a battery 23 and a current measuring instrument 30 of desired sensitivity, desirably a micrometer of moving coil type. If the photometer operates on alternating current, an oscilloscope is preferably used at 30 and an alternating current source is used instead of the battery. It is best to use the device of the invention, as shown in FIGURE 4-, in series with an impedance 31 so that the pulse being measured is impressed on the series combination of the multilayer element and the impedance, and the applied voltage is divided between the multilayer element and the impedance, only a fraction of the totalt voltage being impressed across the multilayer elemen In the embodiments of the voltmeter shown in FIG- URES 3, 4 and 4a the multilayer element is again composed of two conducting layers 21 and 22 with an electroluminescent dielectric 20, with impurity centers dispersed therein, sandwiched between the two conductors.

The multilayer element itself has a very high impedance, and therefore if the impedance 31 is to function as a voltage divider it must be an extremely high impedance.

In the embodiment of the voltage indicator shown in FIGURE 4a, the multilayer element is used in parallel with a variable impedance 39 whose impedance falls as the voltage across it is increased. Such impedances are described in the literature in 13 Oscillographer 15 (Number 3, July-August 1952) and are sold under trade names such as Thyrite and Varister. In this device the voltage indicated is impressed across an impedance 40, suitably a resistor, in series with a multilayer element which is in parallel with the impedance 39. The multilayer element has a terminal 42 connected to the conductor 21 and a terminal 25 connected to the conductor 22. A suitable lead 24' from one side of the source of voltage being indicated is connected to one terminal of the impedance 40, the other terminal of which is connected through the lead 41 to the terminal 42. The terminals of the variable impedance 39 are connected to the terminal 42 and 25 respectively of the multilayer element. This circuit device reduces the possibility of the voltage across the multilayer element exceeding the breakdown voltage because at high voltages the impedance of the variable impedance 39 falls. Therefore the voltage across the multilayer element does not increase as rapidly as it would if the variable impedance 39 in the parallel path were not present. The same function may be achieved by using for the variable impedance 39 a photoconductor, which is positioned to receive a fraction of the radiation from the multilayer element comprising 23, 21, 20, and 22. By choosing a photoconductor with suitable impedance ratio to that of the multilayer element, the impedance of the'photoconductor 39 can be made to fall below that of the multilayer element as the voltage across the multilayer element becomes excessive. As a consequence, the radiation emitted cannot become higher than that for safe operation of the multilayer element.

The invention is applicable as shown in FIGURES 5 and 6 to a specialized type of voltmeter known as a transient voltage indicator. A lead 24 from one side of the voltage being measured is connected to one terminal of photocell 32, the other terminal of the photocell being connected by lead 33 to conducting layer 21. Instead of the photocell 32, a photoconductive element 32 may be employed, as shown in FIGURE 6. In either of the forms of FIGURES 5 and 6, the light produced in the electroluminescent dielectric with impurity centers dispersed therein passes through the conducting layer 21 and reduces the impedance of the photoconductive element 32 or 32'. If all applied voltage ceases, the light will be extinguished, but if some applied voltage remains, even though it is not at the peak, the voltage applied -across the multilayer element is a larger proportion of the total voltage because the photoconductive element reduces its impedance when it is illuminated.) Therefore if the remaining voltage applied across the multilayer element is somewhat greater than the cut-off voltage, the luminescence will remain.

In the form of FIGURE 7 the transient indicator may also trigger and remain luminous. In this form in addition to the lead 24' applied to the input side of the photoresponsive device 32', a lead 34 from a suitable source through impedance 35 is applied to the input side of the photo-responsive device to introduce a maintaining voltage across to the lead 25. In View of the high impedance of the photo-responsive device 32 when the electroluminescent dielectric with impurity centers dispersed therein is not luminous, the maintaining voltage between the leads 34 and 25 is insuflicient to permit the mutilayer element to be luminous, but when the transient voltage across between leads 24 and 25 renders the dielectric luminous, the light received in photo-responsive device 32 reduces its impedance to such an extent that a higher fraction of the maintaining voltage is applied across the multilayer element, and this being above the cut-off voltage, maintains the multilayer element luminous indefinitely.

The impedance 35 prevents the transient from being dissipated in the maintaining circuit.

In many cases it is necessary to record not a single voltage transient but a multiplicity of them. In these cases it is convenient to set up, instead of an array of single transient voltage indicators, such as shown in FIG- 6 URE 7, a panel embodying all the indicators in one unit as shown in FIGURE 9.

Various embodiments are possible for the generalization of the multilayer shown in FIGURE 2, which multilayer is limited to the impression of a single voltage at any given instant. Two simple possibilities are shown in FIGURES 8 and 10.

As mentioned in connection with FIGURE 4a, a suitable variable impedance such as a Thyrite can be connected across the terminals 25 and 42 or possibly between the terminals 24 and 25.

In the form of FIGURES 8, 9 and 10, the conducting layer on one side of the dielectric is not continuous, but consists of a series of separate strips 21' side by side and desirably parallel. They are insulated from one another by the intervening portions 20 of the dielectric layer 20. The strips 21 will very desirably be semitransparent although since light can pass through the spaces between the strips it will be evident that light is visible at the front surface of 23 even though the strips 21' are opaque.

A lead 36 is connected to each of the strips 21'. It serves to apply the voltage to the strips 21' while the other voltage terminal is connected to the lead 38 connected to the common conductor 22'.

Any suitable way of applying the strips can be employed, one method being to evaporate a suitable metal such as aluminum, gold or silver on the dielectric through a mask consisting of separated ribs which prevent the evaporated metal from coating the areas between the strips.

On the opposite side of the dielectric layer 20 containing the dispersed electroluminescent phosphor is placed a conducting layer 22'.

It is desirable to be able to apply different voltages between dilferent conducting strips 21' and continuous conducting layer 22', and this can readily be done with step by step switching, the applied voltage being progressively difierent, either to allow for differences in leakage currents, or to obtain different colors from illumination of the phosphors or different intensities of phosphor illumination. Thus a mixture of phosphors may be employed, which in some areas are energized by a lower voltage which favors luminescence by one phosphor and in other areas are energized by a higher voltage which causes luminescence of another phosphor. In this way voltage modulation is employed on the different strips. In some instances a change in intensity or a change in hue is preferably introduced by difference in frequency. In this case the frequencies applied to the different terminals (which may be the terminals 36 of FIGURES 8, 9 and 10) may vary to create different intensities or different color effects in different areas, the differences in intensities and color effects being obtained either from the same phos phor or preferably from a mixture of phosphors.

The invention is applicable for presentation of color by a screen, as already explained, through modulation of voltage or frequency. Color presentation is also accomplished, as shown in FIGURE 10, by dividing the dielectric layer 20 into a series of strips 20 29 and 20 arranged side by side but including only dispersed phosphor which radiates in dififerent colors in the different strips. Thus strips 20 may have phosphor fluorescing red, strips 20 fluorescing blue and strips 20 fluorescing green, and these strips recur in repeated cycles over the entire screen. Each conducting strip 21" extends along one of the dielectric strips 20 20 or 20 having the particular color of phosphor luminescence.

It will thus be evident that the invention makes it possible for the designer to create screens of a wide variety of sizes and contours adapted to many uses in signal communication and amusement devices.

Although the embodiment shown in FIGURE 10 has been discussed with the dielectric 20' in the form of strips 20 20 and 20 having phosphors which radiate in different colors dispersed in them; in this embodiment the strips 20 20 and 20 which luminesce in difierent colors, are composed of electroluminescent semitransparent dielectrics with impurity centers, the dielectrics and impurity centers in the different strips being selected so that the strips luminesce in the desired colors.

The structures indicated in FIGURES 3 to 7 for the single unit voltage or transient voltage indicator can be readily adapted to the more complex structure of the multiple line radiating elements of the multilayer structure of FIGURES 8 and 10.

The structure, as shown for example in FIGURE 8, can be used with a photoresponsive device 26', which can be either a photometer or a photoconductive element, in series with a voltage source 28 and a current indicating meter 30. By placing the photo-responsive device so that each unit receives radiation only from a definite region of the multilayer excited by a voltage impressed between a definite strip 21 and the common conductor 22, the radiation from each definite strip, and hence the voltage impressed upon it can be evaluated separately. Thus a multiple unit based on the single unit shown in FIGURE 3 can be constructed, as in FIGURE 9.

In view of our invention and disclosure variations and modifications to meet individual whim or particular need will doubtless become evident to others skilled in the art, to obtain all or part of the benefits of our invention without copying the structure shown, and we therefore claim all such insofar as they fall within the reasonable spirit and scope of our claims.

Having thus described our invention what we claim as new and desire to secure by Letters Patent is:

1. In an electroluminescent voltage device, an electroluminescent semitransparent layer having a breakdown voltage exceeding 30,000 volts per centimeter, comprising electroluminescent phosphor particles embedded in a semiconductor making contact with the phosphor particles, the phosphor particles constituting between 1 and 50 percent by weight of the total of phosphor and semiconductor, and means for applying a voltage differential less than'the breakdown voltage across the semitransparent layer and thereby producing light.

2. A voltage device of claim 1, in which the semitransparent layer consists essentially of electroluminescent phosphor particles dispersed in a semiconductor composed of a dielectric having distributed impurity centers which induce the state of semiconductivity in the dielectric.

3. A voltage device according to claim 2, in which there are conducting layers on either side of the electroluminescent layer across which the voltage is applied.

4. A voltage device according to claim 3, in which at least one of the conducting layers is semit-ransparent.

5. A voltage device according to claim 3, in combination with an impedance having the property of reducing as the voltage increases, connected in shunt across theconducting layers in parallel with the voltage device.

6. A voltage device according to claim 5, in combination with a second impedance in series with the parallel branches consisting of the first impedance and the semiconductor electroluminescent layer.

7. A voltage device according to claim 1, having in a single panel an array of electrically separate electroluminescent voltage devices each according to claim 1.

8. A screen comprising a semitransparent dielectric layer, the dielectric layer including electroluminescent phosphor particles embedded in a glass dielectric of low melting point and of finely divided particles including impurity centers, which induce a state of semiconductivity as a result of radiation, and means for applying 21 voltage across the electroluminescent layer and thereby producing light.

9. A screen according to claim 8, in which the means for applying a voltage across the electroluminescent layer includes an electrically conducting layer on each side of the semi-transparent dielectric layer and in contact therewith, at least one of the conducting layers being semitr-ansparent.

References tlited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Marshall et al.: Quarterly Rep. #3 Fellowship on Computer Components #347, Mellon Inst. of Industrial Research, 1951.

Claims (1)

1. IN AN ELECTROLUMINESCENT VOLTAGE DEVICE, AN ELECTROLUMINESCENT SEMITRANSPARENT LAYER HAVING A BREAKDOWN VOLTAGE EXCEEDING 30,000 VOLTS PER CENTIMETER, COMPRISING ELECTROLUMINESCENT PHOSPHOR PARTICLES EMBEDDED IN A SEMICONDUCTOR MAKING CONTACT WITH THE PHOSPHOR PARTICLES, THE PHOSPHOR PARTICLES CONSTITUTING BETWEEN 1 AND 50 PERCENT BY WEIGHT OF THE TOTAL OF PHOSPHOR AND SEMICONDUCTOR, AND MEANS FOR APPLYING A VOLTAGE DIFFERENTIAL LESS THAN THE BREAKDOWN VOLTAGE ACROSS THE SEMITRANSPARENT LAYER AND THEREBY PRODUCING LIGHT.

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