Mercury-Free Discharge Lamp

This invention pertains to electromagnetic discharge devices and, more particularly, is concerned with ultraviolet light sources.

Perhaps the most familiar electromagnetic discharge ultraviolet source is the common fluorescent lamp. Usually the lamp has a cylindrical envelope filled with low pressure neon and a small dose of metallic mercury. Voltage applied to electrodes within the envelope accelerates electrons which ionize the neon, initiating a discharge. Heat and electrons from the discharge vaporizes and excite the mercury which emits ultraviolet and visible radiation, with a strong ultraviolet line at 253.7 nm. A phosphor layer inside the envelope converts the ultraviolet to visible light.

Many modifications have been proposed to improve the conventional fluorescent lamp. Departing from a straight tube configuration, envelopes have been formed into toroids, spheriods, re-entrant cavities, and many other configurations. Beam shaping electrodes have been demonstrated, as have electrodeless discharges. Most of these modifications, however, call for mercury in the discharge medium.

Effort has also been made to improve the filling. For examples, U.S. Pat. No. 4,427,921 issued Jan. 24, 1984 to Proud et al for "Electrodeless Ultraviolet Light Source" disclosed fillings including I, HgI.sub.2, and CdI.sub.2, and U.S. Pat. No. 4,427,922 issued Jan. 24, 1984 to Proud et al for "Electrodeless Light Source" describes fillings including HgI, HgBr, and HgCl.

In the related art of high pressure mercury vapor lamps it has been known for a number of years to improve the visible output of such lamps by adding metal halides to a filling of mercury and inert gas. U.S. Pat. No. 3,586,898 "Aluminum Chloride Discharge Lamp" issued to Speroes and Simper divulges a filling of aluminum trichloride, mercury, and inert gas with the optional addition of aluminum tri-iodide. The lamp's envelope is either alumina or alumina coated quartz to avoid reaction between AlCl.sub.3 and SiO.sub.2.

Mercury and cadmium are known to accumulate in biological systems and are hazards to human health. While the dosage of these metals expected from individual lamps is likely to be below the threshold of harm, it would be desirable to avoid their use if an alternate efficient fill material were available.

Accordingly, it is an object of this invention to provide an efficient discharge ultraviolet light source having fillings free of mercury or cadmium. Another object is to provide an ultraviolet lamp source having greater luminosity than a mercury lamp of the same physical size.

Briefly a discharge lamp includes a discharge chamber filled with inert gas and a dose of aluminum trichloride which supports an electrical discharge and emits ultraviolet and visible light. The aluminum trichloride may be vaporized by the heat of the excited inert gas. In one embodiment the inert gas is neon at a pressure of about 2 torr and the aluminum trichloride has a vapor pressure of 1 torr. As a feature of the invention, the discharge chamber may be made of quartz internally coated with alumina silicate. Furthermore, the wall of the chamber may be coated with a layer of phosphor to convert the ultraviolet light to visible light. The lamp may be energized by radio frequency energy, or via internal electrodes.

FIG. 1 schematically represents a generalized ultraviolet source embodying the invention;

FIG. 2 is a spectrogram of ultraviolet and visible light emitted by the source of FIG. 1;

FIGS. 3 and 4 are examples of electrodeless lamps according to the invention; and

FIG. 5 is an electrode lamp according to the invention.

FIG. 1 shows a generalized high intensity, ultraviolet source 10 according to the invention. The source is characterized by a molecular discharge to produce intense ultraviolet radiation. The specific molecule is AlCl dissociated from aluminum trichloride (AlCl.sub.3). Mercury or cadmium is not used.

A vessel 11 defines a discharge chamber 12, which contains a filling 13 of aluminum trichloride vapor and one or more inert gases, preferably neon (Ne). Electrical energy from electrical source 14 is coupled into the discharge chamber. It has been found that when the pressures of the aluminum trichloride vapor and neon are within a broad range, the mixture can sustain an electrical discharge at moderate power densities (20-80 W/cm.sup.3). The pressure of the vapor can be in the range of 0.2 torr to 20 torr. The preferred pressures are 1 torr of AlCl.sub.3 vapor and 2 torr of Ne.

During discharge the components of the mixture become excited into a plasma state characterized by a high electron temperature. Several plasma reactions occur which produce ultraviolet and visible light.

The observed spectrum from such a plasma is depicted in FIG. 2. Radiation from excited states of the molecules AlCl.sub.3, AlCl.sub.2, and AlCl, and atomic Al, is observed. Plasma reactions which can account for these species include the dissociative attachment reactions;

AlCl.sub.3 +e.sup.- .fwdarw.AlCl.sub.2.sup.* +Cl.sup.-

AlCl.sub.2 +e.sup.- .fwdarw.AlCl*+Cl.sup.-

AlCl+e.sup.- .fwdarw.Al*+Cl.sup.-

AlCl.sub.2 +e.sup.- .fwdarw.Al*+Cl.sup.-.sub.2

Electron collisions with the neon, will produce excited states (Ne*) which can produce excitation exchange with concomitant dissociation similar to those depicted in (1):

AlCl.sub.3 +Ne*.fwdarw.AlCl.sub.2 *+Ne+Cl

AlCl.sub.2 +Ne*.fwdarw.AlCl*+Ne+Cl

AlCl+Ne*.fwdarw.Al*+Ne+Cl

AlCl.sub.2 +Ne*.fwdarw.Al*+Ne+Cl.sub.2

These reactions are reversible and are constantly occurring under equilibrium conditions.

Emission from the excited species (denoted by asterisks) in reactions (1) and (2), specifically from AlCl, pertains to the present invention. The ultraviolet band attributable to AlCl: A.sup.1 .pi..fwdarw..sub.X.sup.1 .SIGMA..sup.+ near 261.4 nm is a spectrally intense feature. This diatomic molecular band has a spectral bandwidth of approximately 28 times as large as the atomic Hg line at 253.7 nm. The peak intensity of the molecular band is less than that of atomic mercury, but the product of peak height times bandwidth (a measure of the UV energy output) is substantially greater in the molecular case.

The ultraviolet emission can, if so desired, be converted to visible light by phosphors surrounding the discharge chamber. This is, of course, the principle of fluorescent lamps. The diatomic AlCl ultraviolet emission is capable of exciting several types of phosphors including sodium salicylate. The polyatomic emission contributes to the visible light produced by the phosphors.

A quantitative assessment was made of the ultraviolet output from four mixtures containing AlCl.sub.3, Hg, HgI.sub.2, and I, each buffered by neon. The mixtures were added in sealed separate quartz vessels after the vessels were baked at 1000.degree. C. under a vacuum of 10.sup.-7 torr.

The lamps filled with either AlCl.sub.3 or Hg are approximately 2.5 times as efficient emitters as either of the HgI.sub.2 or I.sub.2 lamps. The results indicate that when the AlCl.sub.3 lamp runs at 22.7 W/cm.sup.3 the power normalized UV output (waves UV/watts RF) is similar to that of a high intensity Hg glow at 3.4 W/cm.sup.3.

It is important to note the units used for comparison of these lamps. The power UV output represents a measure of the relative efficiencies of the devices in watts of UV/watts of RF, which are approximately equal. However, the AlCl.sub.3 lamp constitutes a more intense UV source than does the Hg lamp. The intensity is defined as watts of UV/steradian. Because the lamps tested were of the same size, of these four lamps, the AlCl.sub.3 lamp is the most radiantly bright source of ultraviolet light, approximates six (6) times that of Hg. Radiant brightness is defined as watts of UV/steradian cm.sup.2 of emitting surface area. Thus, on AlCl.sub.3 lamp may be made more compact than a Hg lamp having the same ultraviolet power (stress compactness).

In addition to the UV ultraviolet emission from diatomic AlCl, polyatomic emission contributes significant continuum, as indicated in reactions (1) and (2). The photoptically corrected visible light output of the low pressure AlCl.sub.3 lamp was approximately 85% of the visible emission from Hg at these power levels for the lamp tested.

During the tests, a small amount of AlCl.sub.3 was heated in an auxiliary chamber at 100.degree. C. to provide a vapor pressure of about 1 torr. The auxiliary chamber is not necessary in commercial embodiments as a measured dose of AlCl.sub.3 may be sealed in the discharge chamber. When the source is energized, excited neon atoms heat and vaporize at least some of the AlCl.sub.3 to the preferred pressure without the need of an auxiliary chamber.

Commercial embodiments of the lamp may feature either electrodeless discharge or electroded discharge.

FIGS. 3 and 4 show examples of electrodeless discharge lamps. In FIG. 3 there is seen an electrodeless lamp 15 containing a filling 16. The electrodeless lamp 15 is supported within a coupling fixture 17 which couples power from a high frequency (RF) power source 18 to the filling of the electrodeless lamp. The electrodeless lamp forms a termination load for the fixture.

The electrodeless lamp 15 has a sealed discharge chamber 21 made of a suitable material which is transparent to ultraviolet radiation, for example, coated quartz or alumina. The filling 16 within the discharge chamber 21 in accordance with the present invention includes aluminum chloride and a buffer gas. The vapor pressure of the aluminum chloride after lamp warmup is preferably about 1 torr. The buffer gas such as argon, krypton, xenon, neon, or nitrogen has a pressure preferably about 2 torr.

The coupling fixture 17 includes an inner conductor 19 and an outer conductor 20 disposed around the inner conductor. The outer conductor 20 includes a conductive mesh 20a which acts as a conductor and provides shielding at the operating frequencies while permitting the passage of light radiated from the lamp 15. The lamp 15 is supported between a first metal electrode 22 at one end of the inner conductor 19 and a second metal electrode 23 connected to the outer conductor 20. The other ends of the inner and outer conductors are arranged in a coaxial configuration for coupling to the power source 18. In order to achieve electrodeless discharge it is necessary to employ RF power capable of penetrating the discharge chamber while being absorbed strongly in the low pressure discharge plasma contained therein. The power source 18 preferably is a source of continuous wave RF excitation in the range of from 902 to 928 MHz. Structural details of a similar discharge apparatus is disclosed in U.S. Pat. No. 4,427,920 issued Jan. 24, 1984 to Joseph M. Proud, Robert K. Smith, and Charles N. Fallier entitled "Electromagnetic Discharge Apparatus".

FIG. 4 is a schematic representation of an alternative embodiment of an electromagnetic discharge apparatus 24 in accordance with the present invention. The apparatus 24 includes an electrodeless lamp 25 having a discharge chamber 26 in the shape of a re-entrant cylinder providing a generally annular discharge region 27. The fill material of the lamp includes aluminum chloride as described hereinabove. The RF coupling arrangement includes a center electrode 29 disposed within the internal re-entrant cavity in the discharge chamber 26. An outer conductive mesh 30 surrounds the discharge chamber 26 providing an outer electrode which is transparent to radiation from the lamp. The center electrode 29 and outer mesh 30 are coupled by a suitable coaxial arrangement 31 to a high frequency power source 32. A radio frequency electric field is produced between the center electrode 29 and the outer mesh 30 causing ionization and breakdown of the fill material. Ultraviolet radiation at 261.4 nm is produced by the resulting glow discharge within the lamp as explained previously. Specific details of the structure of apparatus of this general type are shown in U.S. Pat. No. 4,266,167 which issued May 5, 1981, to Joseph M. Proud and Donald H. Baird entitled "Compact Fluorescent Light Source and Method of Excitation Thereof".

FIG. 5 shows an example of a lamp 33 utilizing an electroded discharge. The discharge chamber 34 contains a low pressure filling 35 of aluminum chloride and neon as described above. The two electrodes 36, 37 should be made of a noble metal or aluminum so not to react with the plasma. Electrodes 36, 37 may be coupled to line voltage. The structure is otherwise similar to high pressure metal arc mercury lamps such as disclosed in U.S. Pat. No. 4,158,789 issued June 19, 1979 to Scholz and Gardner.

The discharge chamber of each embodiment is a vessel made of heat resistant transparent material such as fused quartz, or alumina. If less expensive quartz is chosen, the plasma products of aluminum chloride will react with active silicon near the inner surface of the quartz vessel. This reaction, if unchecked, releases highly volatile silicon tetrachloride (SiCl.sub.4) and which eventually degrades the performance of the lamp. To prevent this, the inner walls of the discharge vessel may be precoated with a refractory material. During manufacture of the lamp the discharge chamber may be charged with a mixture of aluminum chloride and a buffer gas. A discharge is induced through the mixture intentionally causing a plasma reaction with the walls of the discharge vessel. A coating of aluminosilicate (3Al.sub.2 O.sup.. 2SiO.sub.2) is formed on the inner surface of the vessel. This method of depositing refractory coatings is disclosed in U.S. Pat. No. 4,436,762 issued Mar. 13, 1984 to W. P. Lapatovich, et al for "Low Pressure Plasma Discharge Formation of Refractory Coating".

The vessel is then evacuated to 10.sup.-7 torr and baked at 1000.degree. C. The vessel is then refilled with fresh aluminum chloride and inert gas and sealed.

An important feature of the invention is the complete elimination of mercury in discharge lamps. The toxic effects of mercury are cummulative and are a subject of environmental concern. This is not to say aluminum chloride is benign as it reacts with water or steam to produce heat, toxic and corrosive fumes. The products of reaction, such as hydrochloride acid are likely to promptly degrade. Another important aspect is obviating of lengthy positive column discharge lamps due to a high radiant intensity featured by the source. Thus the invention provides a compact ultraviolet source suitable for UV polymerization and other applications.

While there has been shown and described what are considered preferred embodiments of the present invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention as defined by the appended claims.