US3439209A - Positive column gas discharge lamp employing an alloy of two metals with impedance-free terminal connections - Google Patents

Description

Iv Apr-i115, 969 r ea. M. AHSMANN ETAL 3,439,209

POSITIVE COLUMN GAS DISCHARGE LAMP EMPLOYING AN ALLOY OF TWO METALS WITH IMPEDANCE-FREE TERMINAL CONNECTIONS Filed Aug. .15, 1966 Sheet 0f 2 FIG-.1

GERARDUS JMAJQ XEN I AGENT April 5, 1969 G. .1. M. AHSMANN ETAL 3, 39,

POSITIVE COLUMN GAS DISCHARGE LAMP EMPLOYING AN ALLOY OF TWO METALS WITH IMPEDANCE-FREE TERMINAL CONNECTIONS Filed Aug. 15, 1966 2 Sheet 4 8w com com o2 Q H E k F 1 1 United States Patent Office 3,439,209 Patented Apr. 15, 1969 US. Cl. 313-429 6 Claims ABSTRACT OF THE DISCLOSURE A positive-column gas-discharge lamp is disclosed which employs a rare gas at a pressure of 0.1 to 10 torr and an alloy of two metals having dilferent ionization potentials. Substantially impedance-free connections to an energy source are provided and the lamp is operated at a wall temperature at which at least of the metal having the lower ionization potential is ionized at the place of the positive column in each cross-sectional area of the lamp where the positive column is located while the density of the other metal has a value favorable to the radiation generation.

The invention relates to a device comprising a gas discharge lamp having a filament cathode, in which an arc discharge with radiation emission from the positive column occurs, the lamp containing, apart from a rare gas, at least two volatile metals, at least one of which is an alkalimetal. The invention furthermore relates to a lamp intended for use in such a device. The radiation emission may lie in the invisible part as well as in the visible part of the spectrum. The invisible radiation for example ultraviolet radiation, may be converted by luminescence into light.

In a known device the lamp filled with a rare gas, an alkali-metal and mercury, is operated so that all gas and vapour is ionised. This should provide a positive currentvoltage characteristic so that the complication of using a ballast can be avoided. The idea underlying this lamp is that the vapour pressure of the mercury is so low due to its being alloyed with an alkali-metal that by the depletion of the number of ionisible atoms the number of charge carriers does not or substantially not increase with an increase in current. However, since it is stated that also the rare gas is ionised, a positive characteristic can be obtained only when the pressure of the rare gas is very low, mm. or less. At this pressure, however, no positive column can appear, but there does occur a ditferent type of discharge, the light output of which is low. Moreover, the low gas pressure gives rise to difliculties in the ignition and to disintegration of the electrodes.

It is an object of the invention to give the conditions under which a positive current-voltage characteristic can be obtained of a lamp having more conventional discharge conditions and a higher light output.

In a device comprising a gas discharge lamp with a thermionic cathode, in which an arc discharge with radiation emission from the positive column occurs, said lamp containing, apart from a rare gas, at least two volatile metals, at least one of which is an alkali-metal, the two volatile metals are provided, in accordance with the invention, in the form of an alloy and the wall temperature under the operational conditions of the lamp is chosen so that the component of the alloy having the lower ionisation potential at the place of the positive column, is ionised for at least 20% on an average in the cross-sectional area of the lamp, whereas the density of the constituent of the higher ionisation potential has a value which is favourable for the radiation emission and the pressure of the rare gas is about 0.1 to 10 torr.

As is common practice in such lamps the raregas serves to enable the positive column to occur, to facilitate the ignition and to counteract the disintegration of the electrodes. The ionisation potential of the rare gases is always higher than that of the volatile metals, which will preferably be selected from the alkali-metals, cadmium, zinc and mercury.

The invention is based on the idea that due to the low pressure of the constituent of the lower ionisation potential a certain degree of saturation of the ionisation in the positive column is reached under the operational conditions. When the current is increased, the longitudinal voltage gradient in the positive column must increase, in order to raise the number of charge carriers, so that the characteristic becomes positive; this positive characteristic disappears only when the voltage gradient in the positive column has reached such a high value that also the second constituent, and, as the case may be, also the rare gas are ionised to a noticeable extent.

The advantage obtainable by a lamp in a device according to the invention consists not only in the positive current-voltage characteristic, due to which the series impedance may be omitted, but also in that the pressure of the constituent producing the radiation may be so chosen that the optimum value for the radiation generation is obtained. The diameter of the lamp should thereby be taken into account. In the lamps employed hitherto the vapour pressure of the radiation emitting metal also had to fulfill the conditions for current conduction, which means that the requirements for current conduction and radiation production had to be compromised.

The invention can be applied to lamps having mercury as the radiation producing element, in which one or more of the five alkali-metals caesium, rubidium, potassium, sodium and lithium can be added alone or in the form of an alloy. The invention may furthermore be applied to socalled sodium lamps, in which then the current conduction is mainly provided by caesium, rubidium, potassium or alloys thereof.

If any other metals are admixed, any influence on the vapour pressure of the metal providing mainly the current conduction and of the metal providing mainly the radiation has to be taken into account, while the additional metals may contribute to current conduction and to radiation emission.

0n the basis of the desired current density the pressure of the current conducting constituent can be determined by the formula:

wherein j is the current density in ampere/cm p is the pressure is the current-conducting constituent in ton and V is the drift velocity in km./ sec. of the electrons in the positive column. The term drift velocity is to be understood to mean the current density (j) divided by the total charge per cubic centimetre of the movable electrons.

By means of the data of the alloy of the radiation emitting constituent the temperature can be chosen so that the desired pressures are obtained.

The vapour pressure of the constituent of an alloy is, in general, the vapour pressure of the pure metal multiplied by the fraction of said constituent in the alloy, :1 coelfcient being added, which is the activity coefiicient, which may be higher or lower than 1. The activity coefiicients are known for many alloys.

In luminescent tubular lamps the choice of pressures and temperatures according to the invention may lead o materially higher wall temperatures than the conventional ones. In order to avoid high thermal losses it may be advisable to use an insulating envelope, such as is usually employed for sodium lamps.

Advantageous combinations for a lamp according to the invention are caesium or sodium as current-conducting constituent and mercury as a radiation emitting constituent, the rare gas being argon, krypton or xenon. A combination of caesium as the current-conducting constituent and of sodium as the radiation emitting constituent with argon, krypton or xenon as the rare gas also is advantageous.

In a lamp according to the invention the radiation generation will extend gradually from the portion around the axis towards the wall at an increase in current intensity and at an increase of the degree of ionisation. In many cases this appears first at the anode end of the discharge space. The most favourable compromise between the stability without ballast and a low self-absorption in the positive column is obtained when the boundary between the radiation generation throughout the sectional area and in the portion around the axis is located approximately at the middle of the discharge space.

The invention will be described more fully with reference to the drawing, in which- FIG. 1 shows a lamp according to the invention and FIG. 2 shows voltage-current curves at different temperatures for this lamp.

Referring to FIG. 1, the lamp comprises a hard-glass tube 1 having an inner diameter of 30 mms. and a length of 750 mms. The tube comprises two cathodes 2 and 3, surrounded by nickel rings 4 and 5 respectively. The tube has a filling of 3 mms. argon and furthermore a quantity of 200 mgs. of an alloy of 45 atom percent of mercury and 55 atom percent of caesium. At a wall temperature of 353 to 403 K. the pressure of the mercury is about 2 10- torr and that of the caesium 2 10 torr.

In FIG. 2 the voltage in volts of the lamp shown in FIG. 1 is illustrated at a direct-current discharge varying between about 100 and 500 ma. The measurement showed that the tube wall assumed the temperature indicated in the current-voltage characteristics; this temperature was controlled by a second tube 6 surounding the lamp and having a heating wire. It is apparent that the total voltage of the tube strongly increases with an increasing current and that the maximum in this characteristic appears at a current which increases with an increasing wall temperature.

The caesium portion may vary between 40 and 75 atom percent whilst the mercury portion is varied simultaneeously.

In the case of sodium as the current-conducting constituent in the tube of FIG. 1 the composition is 98 atom percent of sodium and 2 atom percent of mercury. The temperature is then about 483 K.

In both cases, of course, alternating current discharge may be used instead of direct-current discharge, which is more important for practical purposes.

In the case of sodium as a light-emitting gas, the alloy chosen contains less than 1% of caesium at a temperature of 483 K. to 543 K., the pressure of the caesium being 2 10- torr and the pressure of the sodium being 2 10 torr. The precise composition of the alloy and the choice of the temperature depend upon the requirements imposed: either maximum efiiciency or maximum light output or a compromise thereof.

What is claimed is:

1. A device comprising a gas discharge lamp having a thermionic cathode and emitting radiation from a positive column, the lamp containing a rare gas at a pressure of the order of 0.1 to 10 torr and an alloy of at least two volatile metals having different ionization potentials, at least one of said metals being an alkali-metal, and means to energize the lamp to produce therein a wall temperature at which at least 20% of the metal having the lower ioniaztion potential has a value favorable to the radiation column in each cross-sectional area of the lamp at said place while the density of the metal having the higher ionization potential has a value favorable to the radiation generation, said energizing means including means substantially free of impedance for connecting said lamp to an energy source.

2. A device as claimed in claim 1, in which the pressure p in torr of the current-conducting constituent fulfills the formula:

3i D 3L l.8 10 Vd 1) 9 1O Vd wherein j is the current density in ampere/cm. and V the drift velocity of the electrons in km./ sec. in the positive column.

3. A device as claimed in claim 1 in which the radiation emitting element is mercury and the current-conducting element is caesium, the quantity of the caesium being between 40 and atom percent of the alloy at a wall temperature of 353 to 403 K.

4. A device as claimed in claim 1 in which the radiation emitting element is mercury and the other constituent is sodium, with the composition of the alloy being 98 atom percent of sodium and 2 atom percent of mercury, at a wall temperature of about 483 K.

5. A device as claimed in claim 1 in which the radiation emitting element is sodium and the current-conducting element is caesium, the latter constituting less than 1% of the alloy at a wall temperature of 483 to 543 K.

6. A device as claimed in claim 1 in which at the anode end of the positive column the radiation is produced throughout the cross-sectional area and over part of the cross-sectional area at the cathode end.

References Cited UNITED STATES PATENTS 1,961,750 6/1934 Ewest et a1. 313-225 2,016,111 10/1935 Hitchcock 33-229 X 2,042,261 5/1936 Krefi't 313-225 2,733,371 1/1956 Campbell 313 3,219,869 11/1965 Schmidt 313-229 X JAMES W. LAWRENCE, Primary Examiner.

R. F. HOSSFELD, Assistant Examiner.

U.S. Cl. X.R. 313225, 228

U.S. DEPARTMENT OF COMMERCE PATENT OFFICE Washington, D.C. 20231 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,439 ,209 April 15 1969 Gerardus Josephur Marie Ahsmann et :11.

It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1 line 52 before "1/1000" insert l .e. Column 2 line 57 "is" should read of Column 4 line 14 "has a value favorable to the radiation" should read is ionized at the place of the positive line 21 "P should read P Signed and sealed this 7th day of April 1970.

(SEAL) Attest:

Edward M. Fletcher, Jr.

Attesting Officer Commissioner of Patents WILLIAM E. SCHUYLER, JR.

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