Electroluminescent Device

Abstract

An electroluminescent panel or sheeting requiring minimum power for acceptable brightness and including luminescent phosphors imbedded in a dielectric medium between a pair of electrodes connected to an AC source, the medium being selected to have low electrical loss as determined by the ratio of its dissipation factor to its dielectric constant at the operation frequency of the source. The panel is connected with an inductor in a resonant circuit with the frequency of the AC source being adjusted to the resonant frequency of the circuit.

Description

This invention relates to electroluminescent devices.

Electroluminescent devices basically comprise luminescent phosphors suspended in a dielectric medium sandwiched between a pair of electrodes which are connected to a source of alternating current. Electroluminescent devices for use as large area panels or signs, for example, have not enjoyed widespread use for a variety of reasons, a principal one being that in order to achieve an acceptable level of brightness a large amount of power is required. Even if the power cost were not a significant factor, the large amount of power produces excessive heat with consequent rapid degradation of the components of the panels so that their useful life is shortened to such an extent that they are economically unacceptable for commercial use except as low-power, low-light-intensity units for such use as night lights and the like.

The power requirements for an electroluminescent panel can be reduced by treating the panel as a capacitor and connecting it with a properly matched inductor in a resonant circuit with the source being adjusted to the resonant frequency of the circuit. This arrangement eliminates the reactive load on the source due to the capacitance of the electroluminescent panel and thus lowers the power requirement.

The use of an electroluminescent panel as part of a resonant circuit has been resorted to by the prior art but heretofore the art has not realized that unexpectedly superior results can be achieved through a careful selection of the dielectric medium supporting the luminescent phosphors in the electroluminescent panel. For example, the prior art has relied on dielectric suspension media having high dielectric constants such as cyanoethylated resins apparently because such substances require less power than low dielectric constant materials when the panel is directly connected to an AC source in a conventional circuit. Materials having low dielectric constants have been generally shunned because of the relatively much higher power requirement when used in a conventional circuit.

I have discovered that previously discarded dielectric media which may have low dielectric constants produce results superior to the media usually employed heretofore provided also that the dissipation factor of a medium is of a magnitude such that the ratio of the dissipation factor (D) to the dielectric constant (K) of the medium does not exceed a predetermined low value. When the D/K factor is low, the electrical loss due to the pure resistance of the dielectric medium is also low so that when an electroluminescent panel utilizing such a medium is connected in a resonant circuit operating at resonance, not only is the capacitive reactance of the panel cancelled out but because of the low electrical loss of the medium, the panel requires less power to produce an acceptable level of brightness than has heretofore been achieved by panels of the prior art.

Thus the object of the invention is to provide an electroluminescent panel or sheeting and driving means therefore which operate at a high level of efficiency to provide acceptable light levels with minimum power requirements and consequent longer life than has been possible heretofore.

Referring now to the drawings wherein:

FIG. 1 is an enlarged broken vertical cross sectional view of an electroluminescent panel which may incorporate the features of the invention;

FIG. 2 is a circuit diagram of an oscillator which may be utilized to drive the panel of FIG. 1 connected in a series resonant circuit;

FIG. 3 is a circuit diagram of a modified oscillator drive means for the panel of FIG. 1; and

FIG. 4 is a graphical representation of the power requirements for operating at various frequencies electroluminescent panels utilizing different selected dielectric media.

Referring now to the drawings and particularly to FIG. 1, the numeral 10 designates a typical electroluminescent sheeting or panel comprising an integral substrate 12 which may be a flexible film. For purposes of the invention, the film is selected to have the electrical loss characteristics on the order of that inherent in polystyrene or polypropylene film. Applied to one side of the film 12 is a dielectric medium 14 having characteristics in accordance with the present invention as will be further described hereinafter. Imbedded in the dielectric medium 14 are luminescent phosphor crystallites 16 which preferably have their electrical axes parallel to each other and generally perpendicular to the outer or light emitting surface of the sheeting or panel. Applied to the upper surface of the dielectric material and to the lower surface of the substrate 12 are respective electrodes 17, 18, at least one of which is of light transmitting material as, for example, a transparent thin film. Connected to the electrode layers 17, 18 are respective conductors 20, 22 leading to a source of alternating current 24 and connected in series between the source 24 and the electrodes is an inductor 26.

Referring now to FIG. 2, the electroluminescent panel 10 is depicted as comprising a capacitor 28 in series with a resistance 30 which is the normal resistive load or electrical loss of the dielectric medium. A second parallel capacitor and resistance 32, 34, representing the capacitive reactance and resistance of the luminescent phosphor materials is shown connected to the resistance 30 and connected in series with the foregoing components is the inductor 26 already mentioned in connection with FIG. 1.

The panel 10 may be operated by an oscillator circuit including three transistors 36, 38 and 41. The transistors 36 and 38 form a part of the series output circuit and require phase inversion which is provided by a driver transformer 40 having split secondary windings 42, 44. The transistor 38 operates as an emitter follower and provides gain of somewhat less than unity while the transistor 36 is connected in common emitter configuration and provides substantial voltage gain. In the load part of the circuit, the inductor 26 is connected in series with the panel 10 and cancels the reactive part of the load. A feed back winding 46 is employed with the inductor 26 to maintain the circuit at resonance at all times and to sustain self-oscillation.

In lieu of the series arrangement of FIG. 2 a parallel arrangement as shown in FIG. 3 may be employed. The oscillator of FIG. 3 is a self-tuned amplifier arranged to provide an exciting voltage between the base and emitter of the transistor 48 that is approximately 180.degree. out of phase with respect to the alternating voltage developed between the collector and emitter of the transistor. The phase relation counteracts the phase reversal produced by the amplifying operation and enables the exciting voltage to have the polarity required to generate the amplifier output. The frequency of oscillation is set so that the reactive load due to the capacitance of the panel 10 is cancelled, or if desired the panel can be operated at a predetermined frequency by employing extra capacitance or inductance in series or parallel as the case may be with the load. As with FIG. 2, a feedback winding 46 is provided to sustain oscillation.

Using known functions, it can be shown that the electrical loss in a dielectric material is R.sub.d = 9 Dd/ 5 fkA .times. 10.sup.10 ohms,

where:

D = dissipation factor of the dielectric media;

d = thickness of the dielectric media in meters;

f = the operating frequency in Hertz;

K = dielectric constant of the media;

A = useful area in square meters.

Where the physical dimensions of the panel and the frequency of the electric drive means are constant, the electrical loss is proportional to the factor D/K. In accordance with the invention, with the D/K factor of the dielectric medium as small as possible when an electroluminescent panel employing such a medium is connected in a resonant circuit operating at resonance, surprisingly low power is required to operate the panel at an acceptable degree of brightness despite the fact that the dielectric medium may have a lower dielectric constant than had heretofore been considered feasible. In accordance with the invention, the dielectric constant is not the controlling factor but rather the ratio of the dissipation factor of the medium to its dielectric constant which ratio I have determined should not exceed 0.003 at the operating frequency of the source.

Dissipation factors and the dielectric constants are known or ascertainable for substantially all dielectric materials and provided the dissipation factor is sufficiently low with respect to the dielectric constant that a maximum D/K value of 0.003 at the operating frequency is not exceeded, any dielectric material may be used so long as it also possesses other necessary characteristics suiting it to use as a component of electroluminescent sheeting. Some of these characteristics are cohesiveness and adhesiveness, low moisture absorption, workability, curability and light transmitting and abosrbing qualities. Dielectric materials having these qualifications are: polystyrene resins, polytetrafluoroethylene, polypropylene and polysulfones.

Referring now to FIG. 4, there is illustrated there the plots of volt ampere power requirements to product 50 foot-Lamberts of brightness at the indicated frequencies for panels of identical dimensions but using different dielectrics. The graph designated by the numeral 50 illustrates the power requirements for a panel employing a high dielectric constant material composed of a cyanothylated resin having a D/K ratio of 0.0043 and supporting a copper activated chlorine co-activated, green emitting electroluminescent phosphor. The dielectric media and the phosphor particles were mixed and coated to a thickness of 35 microns on a conductive substrate serving as one electrode. The panel is 0.1 square meter and after the application of a second electrode the panel had a capacitance of 0.32 microfarads. The essentially straight line of the graph 50 was obtained with the panel directly connected to an alternating current source and shows that about 723 volt amps are required to produce a light output of 50 foot-Lamberts at 1,000 Hertz. Cyanoethylated resin has a high dielectric constant and the power requirements of 723 volt amps at 1,000 Hertz is substantially less than the power requirements shown by graph 52 for similarly connected and constructed panel employing a low dielectric constant medium composed of polystyrene resin. As can be seen, the polystyrene resin requires 3,200 volt amps at 1,000 Hertz to produce the same light intensity as was produced by 723 volt amps for the cyanoethylated resin and this clearly demonstrates why the prior art selected the latter in preference to the former.

In addition to a low dielectric constant, polystyrene resin also has a low dissipation factor with a ratio of D/K equal to 0.00013 as compared to 0.0043 for the cyanoethylated material. Thus when the same panel employing polystyrene resin as the dielectric medium was placed in a resonant circuit with the parameters of the circuit, including an inductor of 365 millihenries, being selected to produce resonance at 1,000 Hertz there is, as indicated by the graph 54, a marked and dramatic drop in the power requirements to 42.7 volt amps to produce the same 50 foot-Lamberts of light output as was produced by 3,200 volt amps for the directly connected panel.

Though connecting a panel using cyanoethylated material in a resonant circuit would also lower the power requirements, experiments have shown that this would at best lower the power to between 70 and 80 volt amps which is not as great an improvement as achieved by the polystyrene resin with its significantly lower D/K factor.

In another experiment, an epoxy resin having a D/K factor equal to 0.0025 was used as a suspension medium for phosphor particles. The resin was coated to a dry thickness of 25 microns on a polyester film having a thickness of 12 microns and a D/K factor of 0.0016. The panel had an area of 0.1 sq. meter and when electroded on both sides had a capacitance of 0.087 micro farads. It was found that in a direct connection with an AC source, 2,650 volt amps were required at 1,000 Hertz to produce a light output of 50 foot-Lamberts whereas when the same panel was connected in a resonant circuit with an inductance equal to 292 millihenries, 58.7 volt amps were required to produce the same foot-Lamberts of light intensity.

When materials were used having better D/K factors than those used in the example immediately above, the results were markedly improved. For example, a polystyrene resin having a D/K factor equal to 0.00013 was used in place of the epoxy on a polypropylene film having a D/K factor equal to 0.00014. All dimensions and other materials were the same and when this panel was electroded it had a capacitance equal to 0.06 microfarads. When used in a direct circuit 3,830 volt amps were required to produce a light output of 50 foot-Lamberts but when an inductor equal to 420 millihenries was added to the circuit only 43.8 volt amps were required to produce the same light output.

As a final example illustrating again the significance of the low D/K factor, when a 2-micron-thick electroluminescent layer was evaporated directly on a 6-micron-thick polypropylene film having an area of 0.1 sq. meter, the panel thus produced had a capacitance of 0.28 microfarads and required 820 volt amps to produce 50 foot-Lamberts of light output at 1,000 Hertz. When a 90 millihenry inductor was added to the circuit, the volt amp requirement for the same light output dropped to 12.7.

Though one specific electroluminescent panel has been shown and described, it should be understood that the invention is not limited to this type of panel but may be used with the same results with panels or sheeting of other configurations. What is claimed is:

Claims

1. In combination, an electroluminescent panel comprising electroluminescent material including a dielectric medium between a pair of spaced electrodes, said dielectric medium having low electrical loss as determined by a ratio of its dissipation factor to its dielectric constant not in excess of 0.003 and including material selected from the group consisting of polystyrene, polytetrofluroethylene, polypropylene, polysulfone, polyester and epoxy, said panel being connected with an inductor in a resonant circuit, and an alternating current source connected in driving relationship to said resonant circuit and operating at the resonant frequency of said circuit.

2. In combination, an electroluminescent panel comprising electroluminescent material including a dielectric medium between a pair of spaced electrodes, said dielectric medium having low electrical loss as determined by a ratio of its dissipation factor to its dielectric constant not in excess of 0.003 and being selected from the group consisting of polystyrene, polytetrofluroethylene, polypropylene, polysulfone, polyester and epoxy, said panel being connected with an inductor in a resonant circuit, and an alternating current source connected in driving relationship to said resonant circuit and operating at the resonant frequency of said circuit.

3. The electroluminescent sheeting of claim 2 wherein said dielectric medium comprises flexible film and a resin applied to said film, said film having a D/K ratio not in excess of 0.0016 and selected from the group consisting of polystyrene, polypropylene, polytetrofluroethylene, and polyester and said resin having a D/K ratio not in excess of 0.003 and selected from the group consisting of epoxy, polystyrene, and polyester resin.