US3422301A - Liquid hollow cathode lamp - Google Patents

Liquid hollow cathode lamp Download PDF

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US3422301A
US3422301A US560134A US3422301DA US3422301A US 3422301 A US3422301 A US 3422301A US 560134 A US560134 A US 560134A US 3422301D A US3422301D A US 3422301DA US 3422301 A US3422301 A US 3422301A
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lamp
hollow cathode
metal
coating
cathode
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Carl R Sebens
John W Vollmer
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Applied Biosystems Inc
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Perkin Elmer Corp
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    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01JELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J13/00Discharge tubes with liquid-pool cathodes, e.g. metal-vapour rectifying tubes
    • H01J13/02Details
    • H01J13/04Main electrodes; Auxiliary anodes
    • H01J13/06Cathodes
    • H01J13/08Cathodes characterised by the material
    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01JELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J17/00Gas-filled discharge tubes with solid cathode
    • H01J17/02Details
    • H01J17/04Electrodes; Screens
    • H01J17/06Cathodes
    • H01J17/066Cold cathodes
    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01JELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J2893/00Discharge tubes and lamps
    • H01J2893/0064Tubes with cold main electrodes (including cold cathodes)
    • H01J2893/0065Electrode systems
    • H01J2893/0066Construction, material, support, protection and temperature regulation of electrodes; Electrode cups
    • HELECTRICITY
    • H01ELECTRIC ELEMENTS
    • H01JELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J2893/00Discharge tubes and lamps
    • H01J2893/0072Disassembly or repair of discharge tubes
    • H01J2893/0073Discharge tubes with liquid poolcathodes; constructional details
    • H01J2893/0074Cathodic cups; Screens; Reflectors; Filters; Windows; Protection against mercury deposition; Returning condensed electrode material to the cathodic cup; Liquid electrode level control
    • H01J2893/0075Cathodic cups
    • H01J2893/0076Liquid electrode materials

Definitions

  • This invention relates to a new type of hollow cathode used in a lamp, especially useful as a light source in spectroscopic instruments of the atomic absorption type.
  • the sample is analyzed by determining the absorption (at a certain specific wavelength of radiation) caused by the atoms for which the analytical test is being made.
  • This technique is particularly useful for analyzing (both qualitatively and quantitatively) a sample containing one or more metals metallic compounds.
  • the metallic sample is converted into a salt (if necessary), then is dissolved in a liquid solvent (such as water), and is then vaporized in the flame of a burner, so that the sample is atomized.
  • the atomized sample is then irradiated with a light source which is of great intensity at least at one characteristic absorption ban-d of the metal for which the test is being made.
  • the light source itself preferably includes a relatively high concentration of the metal for which the test is being made.
  • the typical such light source is the hollow cathode lamp, in which a cup-shaped element (including at least a substantial percentage of the metal for which the test is intended) acts as the negative electrode of the lamp.
  • a cup-shaped element including at least a substantial percentage of the metal for which the test is intended
  • Both this hollow cathode and the positive electrode are hermetically sealed within a glass envelope in a low pressure atmosphere of an inert (noble) gas.
  • a hollow cathode should produce the desired spectral band at high intensity, without substantial intensity fluctuations with time, and have a long useful life.
  • the cathode material must be able to with stand the relatively high temperatures necessarily developed when electrical currents are passed therethrough (as is necessary to develop the high intensity radiation).
  • the cathode material should not boil, sublime, unduly sputter, decompose or change its relative composition (i.e., the various constituents should main tain substantially the same proportions before and after a reasonable period of use).
  • the cathode material should also be capable of being formed into the desired cup shape relatively readily.
  • the hollow cathode may be composed of the pure metal desired (such as copper, silver, and many other metals having the above-mentioned properties).
  • the material desired to be incorporated in the hollow cathode lacks one or more of the desired properties, other techniques must be utilized to obtain a satisfactory cathode including these materials.
  • One possible technique is the utilization of the desired metal in the form of a mixture or alloy (both terms being used in their broadest sense) with one or more other metals or other materials.
  • one technique is to form a close alloy (such as an intermetallic compound) with one or more of the metals, so as to raise its effective melting point above the 500 or 600 C. maximum operating temperature of the lamp.
  • a close alloy such as an intermetallic compound
  • Unfortunately such a technique generally reduces the intensity of the emitted radiation caused by the atoms of the first desired metal.
  • the invention avoids this disadvantageous effect by utilizing a relatively thin coating of the low melting point desired metal in the interior of a hollow cathode.
  • certain metals having melting points below the cathode temperatures during normal operation of the lamp, may nevertheless be util-izedin their pure state as a coating over interior walls of a modified hollow cathode cup.
  • certain types of pure metals may be retained within the cup despite the fact that the cathode temperature during operation is above the melting point of the particular metal.
  • the invention is particularly adaptable to those metals having a relatively low melting point but a substantially higher boiling point.
  • the metal although liquid, has a relatively low vapor pressure at lamp operating conditions, in order to maintain relatively low loss rates of the metal.
  • the invention is directly applicable to otherwise conventional hollow cathode lamps, one such lamp being shown for example in applicants co-pending application Ser. No. 510,754, filed Dec. 1, 1965 (in FIG. 4 thereof).
  • the invention requires merely the use of the correct material to form the hollow cathode, the relatively simple operation of turning in the-ends of the cylindrical wall forming the opening of the hollow cathode cup (e.g., by swaging), and the coating of the interior walls of the resulting modified hollow cathode cup with the desired metal (assuming it has the above-mentioned properties of low vapor pressure,
  • An object of the invention is therefore the provision of an improved hollow cathode lamp in which the emitting material of the cathode exhibits greatly enhanced brightness without materially reducing the life expectancy of the lamp.
  • Another object is the provision of a hollow cathode lamp having the above desired properties, in which the emitting material is in the molten state during operation of the lamp.
  • FIG. 1 is an enlarged vertical section through the longitudinal axis of the improved hollow cathode of the invention.
  • FIG. 2 is a vertical section taken on the line 2-2 in FIG. 1.
  • a completed hollow cathode assembly is shown generally at this cathode may be used in various known hollow cathode lamps, for example, the one shown in applicants above-mentioned co-pending application.
  • This hollow cathode assembly comprises the cathode holder or cup 12, including a narrower support portion 14 having at 16 a cylindrical aperture for receiving the supporting and electrical connecting pin of the lamp.
  • the larger body portion of the cathole holder 12 comprises a circumferential wall portion 18 surrounding the longitudinal (i.e., horizontal in the drawing) axis of the holder, thereby having a generally cylindrical interior wall at 20 enclosing the generally tubular interior volume of the holder.
  • the circumferential wall portion 18 is not cylindrical to its open (i.e., right-hand) end, but rather includes end portion 22 which is in-turned toward the longitudinal axis of the cup so as to form in-turned portion 24 of the interior wall.
  • Substantially the entire interior wall '20 is coated, as shown at 30, with the pure metal, the emission of which is desired to be obtained in the hollow cathode lamp.
  • this metallic coating is somewhat thicker at the lower part of interior wall portion 20 than the coating is (at 34) on the upper part of the interior wall (see FIG. 2).
  • This relatively greater thickness of the lower parts of the coating occurs since the metallic coating is molten during the use of the hollow cathode in the lamp (as will be more fully described hereafter), so that more of the metallic material tends to settle to the lower parts because of gravity.
  • the hollow cathode holder 12 forms part of the electrical circuit to the emissive coating 30, the holder is wholly composed of metal (which in itself is conventional).
  • the particular metal of which the holder 12 is composed should form no alloys having a relatively low melting point with the particular emissive coating metal employed in the particular hollow cathode assembly, even at the relatively elevated temperatures (e.g., up to about 500 C.) to which the assembly may be raised during operation.
  • the coating metal and the material of the cathode holder alloy at all, the resulting alloys should be of low content as to the coating metal. This avoids reducing the amount of this coated metal available for emission even after relatively long use.
  • a thin inter-layer of alloy, having a small proportion of the coated metal forms between the coating and the internal surface 20 of the cathode holder material.
  • the inturned end of portions 22 of walls 18 assist in maintaining the coating material 30 within the interior of the cathode when the coating is in molten state, even if the hollow cathode is tipped slightly, so as to lower its right-hand or open end.
  • Lamps utilizing hollow cathodes conforming generally to that shown in the drawing have been made and successfully tested, in which the coating (at 30) were, respectively, tin, gallium, and indium. Since each of the hollow cathodes is somewhat diflerent, each one will be described separately.
  • cathode cup is attached to a conventional stem assembly of a lamp by introducing the center pin thereof into cylindrical aperture 16 and crimping the surrounding support portion 14 thereon. Since the various assembly steps form no part of the present invention, a complete description is not included here, especially since this is fairly completely described in the above-mentioned c0- pending application.
  • Various conventional baking and flushing steps are normally performed. After such conventional steps, a small quantity of pure solid tin is added to the interior of the cup 12 (the cup being positioned with its longitudinal axis vertical, so that opening 28 is at the top).
  • the lamp After completion of assembly, out-gassing, and sealing of the entire lamp, the lamp (with the open end 28 of the hollow cathode still uppermost) is connected to a suitable source of electrical energy and run at slightly higher than normal operating conditions (in the particular lamp where 25 ma. is normal operating current, approximately 30 ma. is used during this step). Under such elevated temperature conditions the tin shot will melt so as to form a pool in the bottom of the cathode cup.
  • the entire lamp assembly is then tilted so that its axis (and therefore the longitudinal axis of the hollow cathode) is variously between about 45 and (relative to the vertical), and the lamp is then rolled about its longitudinal axis so as to cause the tin to wet the interior surface 20 of the titanium holder all the way up to the beginning of the turned-in (swaged) end portion 22; this point is indicated at 26 in FIG. 1.
  • the electrical energy source is then turned off, and the rotation of the lamp continued while the tin solidifies.
  • Tin lamps made according to the above description are normally operated between 20 and 30 ma. at about 200 volts -D.C. Lamps constructed and run in the disclosed manner exhibit a brightness of approximately thirty times that for conventional tin hollow cathode lamps (in the region of 2246 A., the atom line wavelength usually used in atomic absorption spectroscopy for tin).
  • the tin is present in the form of either pure tin machined or cast in a holder, the metal of which would alloy with the tin if the latter were molten (for example, a copper holder), or an alloy of tin which is solid throughout operation of the lamp.
  • the useable sensitivity and detection limit are of course slightly lower when the lamp is run in the range of 20-25 ma., which affords useful life in the many hundreds of hours (for example, 500-600 hours); however, reasonable use of the lamp at its maximum practical brightness of 40 ma. reduces its useful life only by a moderate factor (approximately to one half, or 250-300 hours).
  • the liquid tin film (at 30) will tend to be thicker at the bottom (i.e., at 32) than at the top 34 when the lamp is used in its normal position with its longitudinal axis horizontal. For this reason the coating will appear as shown in the drawing even when the lamp is cool between uses. It should be noted that the lamp should not be turned so that the open end of the hollow cathode (at 28) extends downward, either during use or immediately thereafter (before the cathode has cooled to below the melting temperature of the tin).
  • the gallium is introduced and cast into the columbium hollow cathode holder prior to assembly of the other parts of the hollow cathode lamp.
  • gallium has a melting point not much above room temperature
  • the gallium may be most easily introduced into the hollow cathode holder by warming the gallium to a temperature above 30 C., and by use of an eye dropper placing one drop (of approximately one-eighth inch diameter) in the columbium holder while the latter is positioned with its opening 28 uppermost.
  • the holder and drop of gallium are then placed in this same vertical position on a centrifuge.
  • the holder is covered (as with a Pyrex tube) and argon gas is used to continuously flush the assembly during the following steps.
  • the cathode With the centrifuge operating, the cathode is heated (with an induction heater) to approximately 800, the gallium thereby wetting the cylindrical walls 18 of the columbium holder and actually climbing up these walls toward the swaged, turned-in end portions 2'2.
  • the heat is removed (with the centrifuge still spinning).
  • the cathode assembly has cooled to about 100 C., it is removed from the centrifuge and any excess gallium is allowed to flow out by inverting the cathode holder.
  • the cathode assembly After the cathode assembly has cooled to room temperature, it may be crimped onto the center pin of the stem assembly of the lamp in a conventional manner and the rest of the lamp assembled in the conventional manner generally described above in Example I (argon being used as the flushing gas).
  • the lamp is then run in by repetitive operation under more or less normal operating conditions, but with complete evacuation and fresh filling with neon gas after each run.
  • the final filling is with neon at a few millimeters (of mercury) pressure.
  • Neon is preferred to argon as the filling gas because the latter has an absorption line very near one of the two most useful gallium lines, namely 2944 A.
  • Lamps made according to the above description are normally operated at 20 ma., and are preferably used with an atomic absorption spectrometer at either a 2874 or a 2944 A., wavelength setting (there also being substantially lower sensitivity absorption lines at, for example, 4033 and 4172 A.)
  • the nominal sensitivity in ppm. producing a 1% absorption reading was 2.3 p.p.m. at 2874 A. and 2.4 p.p.m. at 2944 A., with the above-mentioned Model 303 Perkin- Elmer atomic absorptionspectrophotometer, using a narrow UV3 (2 A.) slit setting. At relatively high concentrations of gallium and relatively large slit widths, the sensitivity at 2874 A.
  • the lower detection limit was 0.07 ppm. at 2874 A. (using the above spectrometer at a wide -UV5 (20 A.) slit setting, high noise suppression setting of 4, scale expansion of 30X, and a specially modified burner with a test sample containing one ppm. gallium). Liquid gallium hollow cathode lamps according to the above description have been successfully made and have shown relatively long, useful lives (i.e., over 1,000 hours).
  • Example III The technique for making a hollow cathode lamp according to the invention in which indium is the active coating within the cathode holder is very similar to the gallium lamp of Example II. The major difference is that a titanium holder is used (as in Example I, Tin). Specifically, the titanium holder is swaged heavily (as in the previous examples) after celaning; a solid piece of indium in the form of the Ms" diameter shot (cut to this size if necessary) is then placed in the cup, and the assembly placed with the open end (at 28) uppermost in a centrifuge and covered with a quartz tube. Argon is flowed over the cathode assembly within the tube (at about 5 cu. ft. per hour).
  • the titanium With the centrifuge operating, the titanium is heated to a dull orange heat (about 900 C.) with an induction heater. At this temperature the indium readily wets the titanium so as to climb up the now vertical interior surface 20 of the cylindrical wall 18. The centrifuging and heating is continued until the indium has climbed to at least point 26. at which time the heat is removed. The spinning of the centrifuge is continued until the indium has hardened, the argon gas flow being maintained until the temperature has closely approached room temperature.
  • the rest of the assembly steps of the lamp structure are conventional, being the same as those for the gallium lamp of Example II. It may be noted that in all of these techniques substantially all of the steps are done with argon as a forming gas surrounding the hollow cathode.
  • the lamp is evacuated and baked for a few hours at a few hundred degrees (C.) and then gradually cooled (this baking step being conventional and common to the manufacture of all of the exemplary lamps disclosed).
  • the lamps are then run in with neon gas, in a manner similar to the gallium lamps (with complete replacement of the neon gas after each run). After the last run the lamp is filled with fresh neon to a pressure of a few millimeters, and then finally sealed.
  • the lamps may be run at 30 ma. to completely melt the indium; and the lamp then rolled at betwen 45 and angle (in the same manner as explained in Example I for tin). As before, the rotation of the lamp should be continued after the turning off of the current until the lamp has cooled below the melting point of indium.
  • Indium lamps according to the above example when run at a current of 20 ma., have successfully undergone life tests of over 500 hours.
  • use with the previously mentioned spectrophotometer yielded a sensitivity of 0.9 ppm. indium (to produce 1% absorption) both at the 3256 A. (using a slit wid.h setting of UV3, which is equal to 2 A.) and at the 3039 A. (a UV4 slit setting of 7 A.) absorption lines.
  • the ultimate detection limit when using an indium lamp according to the invention is approximately 0.05 ppm. indium (using a scale expansion factor of 30X at a noise suppression setting of 4 with a test solution containing 0.2 ppm. indium). It is emphasized that this very good sensitivity is obtained at the normal operating current value of 20 ma.
  • the hollow cathode holder material should be chosen so that the coated metal forms with the material of the cathode holder neither a low melting point alloy nor any alloy having a relatively large amount of the coating metal, even at the moderately high temperatures reached during operation of the lamp (e.g,, 300 400 C.).
  • Such a thin intermediate layer assists both in forming the original coating 30 over the entire interior surface 20 of the cathode holder and in maintaining during use (when the coating is liquid) at least a thin coating even where (as at 34) gravity tends to cause the coating to run.
  • the lamps of the invention utilizing the tested-for metal in its pure molten state. Contrary to expectation, the molten coating does not form a single globule of molten material, even though the material is completely liquid under operating conditions, but rather maintains a relatively complete coating of the interior surface 20 of the cathode holder, with only a moderate tendency to thicken at the bottom (as indicated at 32).
  • the invention is not limited to these specific materials but is usable for making hollow cathode lamps with metallic coatings (at 30) of other materials having the desired properties of a boiling point substantially above the operating temperature of the cathode and a low or at least moderate vapor pressure at this operating temperature.
  • the material of the cathode holder should be chosen so that the interiorly coated metal does not readily alloy therewith to any great extent even at the somewhat elevated operating temperatures.
  • the material of the holder should be at least somewhat wettable by the metal coating so as to insure a relatively complete coating action and maintenance of this coating over at least a substantial area of the interior surface 20 of the cathode cup.
  • said hollow cathode being of generally cup shape, with the walls defining its interior having generally inturned wall portions adjacent to the open end thereof;
  • said generally cup-shaped hollow cathode comprises titanium; whereby alloying between said tin coating and said titanium hollow cathode is minimized, even at the moderately elevated operating temperature of said lamp.
  • said particular desired metal coating comprises substantially pure gallium.
  • said generally cup-shaped hollow cathode comprises columbium (niobium); whereby alloying between said gallium coating and said columbium hollow cathode is minimized, even at the moderately elevated operating temperature of said lamp.
  • said particular desired metal coating comprises substantially pure indium.
  • said generally cup-shaped hollow cathode comprises titanium; whereby alloying between said indium coating and said titanium hollow cathode is minimized, even at the moderately elevated operating temperature of said lamp.

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Description

Jan. 14, 1969 c. R. SEBENS ET AL 3,422,301
LIQUID HOLLOW CATHODE LAMP Filed June 24, 1966 INVENTORS. ari Selveus BY J Vollmer III'IWRNIY.
United States Patent 7 Claims This invention relates to a new type of hollow cathode used in a lamp, especially useful as a light source in spectroscopic instruments of the atomic absorption type.
In atomic absorption spectroscopy the sample is analyzed by determining the absorption (at a certain specific wavelength of radiation) caused by the atoms for which the analytical test is being made. This technique is particularly useful for analyzing (both qualitatively and quantitatively) a sample containing one or more metals metallic compounds. Usually the metallic sample is converted into a salt (if necessary), then is dissolved in a liquid solvent (such as water), and is then vaporized in the flame of a burner, so that the sample is atomized. The atomized sample is then irradiated with a light source which is of great intensity at least at one characteristic absorption ban-d of the metal for which the test is being made. Only radiation in the region of this characteristic wavelength which is passed through the sample is then allowed to affect a detector, which therefore yields a measurement of how much absorptoin has occurred. The detected intensity (as compared to the original source intensity, for example) therefore yields a quantitative measurement of the concentration of the particular metal for which the analysis is being conducted.
In order to irradiate the sample at high intensity in the narrow absorption band, the light source itself preferably includes a relatively high concentration of the metal for which the test is being made. At the present time the typical such light source is the hollow cathode lamp, in which a cup-shaped element (including at least a substantial percentage of the metal for which the test is intended) acts as the negative electrode of the lamp. Both this hollow cathode and the positive electrode are hermetically sealed within a glass envelope in a low pressure atmosphere of an inert (noble) gas.
Ideally a hollow cathode should produce the desired spectral band at high intensity, without substantial intensity fluctuations with time, and have a long useful life. Thus, the cathode material must be able to with stand the relatively high temperatures necessarily developed when electrical currents are passed therethrough (as is necessary to develop the high intensity radiation). In particular the cathode material should not boil, sublime, unduly sputter, decompose or change its relative composition (i.e., the various constituents should main tain substantially the same proportions before and after a reasonable period of use). Ideally the cathode material should also be capable of being formed into the desired cup shape relatively readily.
For those metals intrinsically having the desired properties (such as: a melting point above, say, about 500 C.; moderate vapor pressure in the neighborhood of this temperature; good machina'bility and other mechanical characteristics, and the like), the hollow cathode may be composed of the pure metal desired (such as copper, silver, and many other metals having the above-mentioned properties). When the material desired to be incorporated in the hollow cathode lacks one or more of the desired properties, other techniques must be utilized to obtain a satisfactory cathode including these materials. One possible technique is the utilization of the desired metal in the form of a mixture or alloy (both terms being used in their broadest sense) with one or more other metals or other materials.
For example if the particular desired metal has a relatively low melting point, one technique is to form a close alloy (such as an intermetallic compound) with one or more of the metals, so as to raise its effective melting point above the 500 or 600 C. maximum operating temperature of the lamp. Unfortunately such a technique generally reduces the intensity of the emitted radiation caused by the atoms of the first desired metal.
1 The invention avoids this disadvantageous effect by utilizing a relatively thin coating of the low melting point desired metal in the interior of a hollow cathode. Specifically it has been found that certain metals, having melting points below the cathode temperatures during normal operation of the lamp, may nevertheless be util-izedin their pure state as a coating over interior walls of a modified hollow cathode cup. Specifically it has been found that by turning in the ends of the integral walls of the hollow cathode cup (as by swaging), certain types of pure metals may be retained within the cup despite the fact that the cathode temperature during operation is above the melting point of the particular metal.
The invention is particularly adaptable to those metals having a relatively low melting point but a substantially higher boiling point. In particular it is important that the metal, although liquid, has a relatively low vapor pressure at lamp operating conditions, in order to maintain relatively low loss rates of the metal. The invention is directly applicable to otherwise conventional hollow cathode lamps, one such lamp being shown for example in applicants co-pending application Ser. No. 510,754, filed Dec. 1, 1965 (in FIG. 4 thereof). The invention requires merely the use of the correct material to form the hollow cathode, the relatively simple operation of turning in the-ends of the cylindrical wall forming the opening of the hollow cathode cup (e.g., by swaging), and the coating of the interior walls of the resulting modified hollow cathode cup with the desired metal (assuming it has the above-mentioned properties of low vapor pressure,
even though molten, at the operating temperatures of the lamp).
The relatively simple structural changes required by the invention lead to extremely significant improvements in the efiective brightness of the lamp. For example, hollow cathode lamps in which the metal coated on the interior of the modified hollow cathode cup was tin, have exhibited thirty times the brightness (over a relatively long and useful life) of conventional lamps, which latter typically utilize either pure tin run at relatively low currents to avoid melting or less frequently a tin alloy (also run solely as a solid).
An object of the invention is therefore the provision of an improved hollow cathode lamp in which the emitting material of the cathode exhibits greatly enhanced brightness without materially reducing the life expectancy of the lamp.
Another object is the provision of a hollow cathode lamp having the above desired properties, in which the emitting material is in the molten state during operation of the lamp.
Other objects and advantages of the invention will become obvious to one skilled in the art upon reading the following detailed description in conjunction with the accompanying drawing, in which:
FIG. 1 is an enlarged vertical section through the longitudinal axis of the improved hollow cathode of the invention; and
FIG. 2 is a vertical section taken on the line 2-2 in FIG. 1.
A completed hollow cathode assembly is shown generally at this cathode may be used in various known hollow cathode lamps, for example, the one shown in applicants above-mentioned co-pending application. This hollow cathode assembly comprises the cathode holder or cup 12, including a narrower support portion 14 having at 16 a cylindrical aperture for receiving the supporting and electrical connecting pin of the lamp. The larger body portion of the cathole holder 12 comprises a circumferential wall portion 18 surrounding the longitudinal (i.e., horizontal in the drawing) axis of the holder, thereby having a generally cylindrical interior wall at 20 enclosing the generally tubular interior volume of the holder. In distinction to conventional hollow cathode holders or cups, the circumferential wall portion 18 is not cylindrical to its open (i.e., right-hand) end, but rather includes end portion 22 which is in-turned toward the longitudinal axis of the cup so as to form in-turned portion 24 of the interior wall.
Substantially the entire interior wall '20 is coated, as shown at 30, with the pure metal, the emission of which is desired to be obtained in the hollow cathode lamp. As indicated at 32, this metallic coating is somewhat thicker at the lower part of interior wall portion 20 than the coating is (at 34) on the upper part of the interior wall (see FIG. 2). This relatively greater thickness of the lower parts of the coating occurs since the metallic coating is molten during the use of the hollow cathode in the lamp (as will be more fully described hereafter), so that more of the metallic material tends to settle to the lower parts because of gravity. Since the hollow cathode holder 12 forms part of the electrical circuit to the emissive coating 30, the holder is wholly composed of metal (which in itself is conventional). The particular metal of which the holder 12 is composed, however, should form no alloys having a relatively low melting point with the particular emissive coating metal employed in the particular hollow cathode assembly, even at the relatively elevated temperatures (e.g., up to about 500 C.) to which the assembly may be raised during operation. In fact, if the coating metal and the material of the cathode holder alloy at all, the resulting alloys should be of low content as to the coating metal. This avoids reducing the amount of this coated metal available for emission even after relatively long use. Thus only a thin inter-layer of alloy, having a small proportion of the coated metal forms between the coating and the internal surface 20 of the cathode holder material. The inturned end of portions 22 of walls 18 assist in maintaining the coating material 30 within the interior of the cathode when the coating is in molten state, even if the hollow cathode is tipped slightly, so as to lower its right-hand or open end.
Lamps utilizing hollow cathodes conforming generally to that shown in the drawing have been made and successfully tested, in which the coating (at 30) were, respectively, tin, gallium, and indium. Since each of the hollow cathodes is somewhat diflerent, each one will be described separately.
Example I.-Tin
cathode cup is attached to a conventional stem assembly of a lamp by introducing the center pin thereof into cylindrical aperture 16 and crimping the surrounding support portion 14 thereon. Since the various assembly steps form no part of the present invention, a complete description is not included here, especially since this is fairly completely described in the above-mentioned c0- pending application. Various conventional baking and flushing steps (as described in said co-pending application) are normally performed. After such conventional steps, a small quantity of pure solid tin is added to the interior of the cup 12 (the cup being positioned with its longitudinal axis vertical, so that opening 28 is at the top). It has been found that one or two round pellets or shot of one-eighth inch diameter (one of the forms in which pure tin is commercially available) is entirely satisfactory. The rest of the conventional assembly, flushing and baking steps are then finished (without allowing the tin shot to leave the hollow cathode cup). The completed lamp is then processed in a more or less conventional manner; this consists in operating the lamp for various relatively short periods of time while evacuating and replenishing the gas (preferably neon) between each run. After completion of this run-in, fresh neon at a few millimeters (of mercury) pressure is fed into the evacuated lamp, and the lamp is finally sealed.
After completion of assembly, out-gassing, and sealing of the entire lamp, the lamp (with the open end 28 of the hollow cathode still uppermost) is connected to a suitable source of electrical energy and run at slightly higher than normal operating conditions (in the particular lamp where 25 ma. is normal operating current, approximately 30 ma. is used during this step). Under such elevated temperature conditions the tin shot will melt so as to form a pool in the bottom of the cathode cup. The entire lamp assembly is then tilted so that its axis (and therefore the longitudinal axis of the hollow cathode) is variously between about 45 and (relative to the vertical), and the lamp is then rolled about its longitudinal axis so as to cause the tin to wet the interior surface 20 of the titanium holder all the way up to the beginning of the turned-in (swaged) end portion 22; this point is indicated at 26 in FIG. 1. The electrical energy source is then turned off, and the rotation of the lamp continued while the tin solidifies.
Tin lamps made according to the above description are normally operated between 20 and 30 ma. at about 200 volts -D.C. Lamps constructed and run in the disclosed manner exhibit a brightness of approximately thirty times that for conventional tin hollow cathode lamps (in the region of 2246 A., the atom line wavelength usually used in atomic absorption spectroscopy for tin). In such conventional lamps the tin is present in the form of either pure tin machined or cast in a holder, the metal of which would alloy with the tin if the latter were molten (for example, a copper holder), or an alloy of tin which is solid throughout operation of the lamp. The fact that the pure tin is liquid during the operation in the new lamp utilizing the hollow cathode of the invention thus yields substantial performance improvement. Atomic absorption spectroscopy tests of the inventive liquid tin hollow cathode lamp used with a Model 303 Perkin-Elmer atomic absorption spectrophotometer yielded a maximum sensitivity (for 1% absorption) of 1.2 p.p.m. of tin in the sample, with as little as 0.2 p.p.m. tin being detectable. For this maximum sensitivity, the liquid tin lamp was run at 40 ma.; and the instrument was set at 10 scale expansion, a noise suppression setting of 5, and a spectral slit width of 2 A., (slit position UV3). The useable sensitivity and detection limit are of course slightly lower when the lamp is run in the range of 20-25 ma., which affords useful life in the many hundreds of hours (for example, 500-600 hours); however, reasonable use of the lamp at its maximum practical brightness of 40 ma. reduces its useful life only by a moderate factor (approximately to one half, or 250-300 hours).
Because of the fact that the tin completely melts during operation (the interior of the hollow cathode reaches temperatures of approximately 300 C. at 20 ma), the liquid tin film (at 30) will tend to be thicker at the bottom (i.e., at 32) than at the top 34 when the lamp is used in its normal position with its longitudinal axis horizontal. For this reason the coating will appear as shown in the drawing even when the lamp is cool between uses. It should be noted that the lamp should not be turned so that the open end of the hollow cathode (at 28) extends downward, either during use or immediately thereafter (before the cathode has cooled to below the melting temperature of the tin).
Example II.Gallium Lamps having a hollow cathode in which gallium formed the interior coating have been successfully made, using a columbium (also known as niobium) hollow cathode cup, with which the gallium does not readily alloy. After thorough cleaning, the columbium hollow cathode holder is swaged heavily as indicated at 22 in the drawing. In distinction to the Example I technique for tin, the gallium is introduced and cast into the columbium hollow cathode holder prior to assembly of the other parts of the hollow cathode lamp. Since gallium has a melting point not much above room temperature, the gallium may be most easily introduced into the hollow cathode holder by warming the gallium to a temperature above 30 C., and by use of an eye dropper placing one drop (of approximately one-eighth inch diameter) in the columbium holder while the latter is positioned with its opening 28 uppermost. The holder and drop of gallium are then placed in this same vertical position on a centrifuge. The holder is covered (as with a Pyrex tube) and argon gas is used to continuously flush the assembly during the following steps. With the centrifuge operating, the cathode is heated (with an induction heater) to approximately 800, the gallium thereby wetting the cylindrical walls 18 of the columbium holder and actually climbing up these walls toward the swaged, turned-in end portions 2'2. When the gallium has climbed up the walls to about the point 26, the heat is removed (with the centrifuge still spinning). When the cathode assembly has cooled to about 100 C., it is removed from the centrifuge and any excess gallium is allowed to flow out by inverting the cathode holder.
After the cathode assembly has cooled to room temperature, it may be crimped onto the center pin of the stem assembly of the lamp in a conventional manner and the rest of the lamp assembled in the conventional manner generally described above in Example I (argon being used as the flushing gas). The lamp is then run in by repetitive operation under more or less normal operating conditions, but with complete evacuation and fresh filling with neon gas after each run. The final filling is with neon at a few millimeters (of mercury) pressure. Neon is preferred to argon as the filling gas because the latter has an absorption line very near one of the two most useful gallium lines, namely 2944 A.
Lamps made according to the above description are normally operated at 20 ma., and are preferably used with an atomic absorption spectrometer at either a 2874 or a 2944 A., wavelength setting (there also being substantially lower sensitivity absorption lines at, for example, 4033 and 4172 A.) The nominal sensitivity (in ppm. producing a 1% absorption reading) was 2.3 p.p.m. at 2874 A. and 2.4 p.p.m. at 2944 A., with the above-mentioned Model 303 Perkin- Elmer atomic absorptionspectrophotometer, using a narrow UV3 (2 A.) slit setting. At relatively high concentrations of gallium and relatively large slit widths, the sensitivity at 2874 A. decreases less rapidly than that at 2944 A.; additionally the 2874 A. line has somewhat less background noise. Therefore under many conditions the 2874 A. line is preferably used. The lower detection limit was 0.07 ppm. at 2874 A. (using the above spectrometer at a wide -UV5 (20 A.) slit setting, high noise suppression setting of 4, scale expansion of 30X, and a specially modified burner with a test sample containing one ppm. gallium). Liquid gallium hollow cathode lamps according to the above description have been successfully made and have shown relatively long, useful lives (i.e., over 1,000 hours).
Example III.-Indium The technique for making a hollow cathode lamp according to the invention in which indium is the active coating within the cathode holder is very similar to the gallium lamp of Example II. The major difference is that a titanium holder is used (as in Example I, Tin). Specifically, the titanium holder is swaged heavily (as in the previous examples) after celaning; a solid piece of indium in the form of the Ms" diameter shot (cut to this size if necessary) is then placed in the cup, and the assembly placed with the open end (at 28) uppermost in a centrifuge and covered with a quartz tube. Argon is flowed over the cathode assembly within the tube (at about 5 cu. ft. per hour). With the centrifuge operating, the titanium is heated to a dull orange heat (about 900 C.) with an induction heater. At this temperature the indium readily wets the titanium so as to climb up the now vertical interior surface 20 of the cylindrical wall 18. The centrifuging and heating is continued until the indium has climbed to at least point 26. at which time the heat is removed. The spinning of the centrifuge is continued until the indium has hardened, the argon gas flow being maintained until the temperature has closely approached room temperature.
The rest of the assembly steps of the lamp structure are conventional, being the same as those for the gallium lamp of Example II. It may be noted that in all of these techniques substantially all of the steps are done with argon as a forming gas surrounding the hollow cathode. After a leak check, the lamp is evacuated and baked for a few hours at a few hundred degrees (C.) and then gradually cooled (this baking step being conventional and common to the manufacture of all of the exemplary lamps disclosed). The lamps are then run in with neon gas, in a manner similar to the gallium lamps (with complete replacement of the neon gas after each run). After the last run the lamp is filled with fresh neon to a pressure of a few millimeters, and then finally sealed. To insure even coating of the indium on the interior surface of the titanium holder, the lamps may be run at 30 ma. to completely melt the indium; and the lamp then rolled at betwen 45 and angle (in the same manner as explained in Example I for tin). As before, the rotation of the lamp should be continued after the turning off of the current until the lamp has cooled below the melting point of indium.
Indium lamps according to the above example, when run at a current of 20 ma., have successfully undergone life tests of over 500 hours. At this same normal operating current, use with the previously mentioned spectrophotometer yielded a sensitivity of 0.9 ppm. indium (to produce 1% absorption) both at the 3256 A. (using a slit wid.h setting of UV3, which is equal to 2 A.) and at the 3039 A. (a UV4 slit setting of 7 A.) absorption lines. The ultimate detection limit when using an indium lamp according to the invention is approximately 0.05 ppm. indium (using a scale expansion factor of 30X at a noise suppression setting of 4 with a test solution containing 0.2 ppm. indium). It is emphasized that this very good sensitivity is obtained at the normal operating current value of 20 ma.
The three above examples and the results obtained therewith clearly show not only the feasibility, but the actual improvement occasioned by the invention. Specifically, using a hollow cathode holder of a material which does not substantially alloy with the desired emitting metal used as its coating (at 30), which holder has been modified by the inclusion of in-turned portions 22 allows the various coated metals on the interior surface 20 to be retained even at temperatures substantially above their melting points. As noted above, the invention may be used with such coatings formed of any metal which has a relatively low vapor pressure under the operating conditions of the lamp, even though it is liquid. As specifically pointed out previously, the hollow cathode holder material should be chosen so that the coated metal forms with the material of the cathode holder neither a low melting point alloy nor any alloy having a relatively large amount of the coating metal, even at the moderately high temperatures reached during operation of the lamp (e.g,, 300 400 C.). A tendency to form a thin intermediate layer of a relatively high melting alloy (at least at the somewhat higher manufacturing temperature), which alloy contains only a few percent (at most) of the coated metal, is actually advantageous. Specifically such a thin intermediate layer assists both in forming the original coating 30 over the entire interior surface 20 of the cathode holder and in maintaining during use (when the coating is liquid) at least a thin coating even where (as at 34) gravity tends to cause the coating to run.
Markedly greater brightness is obtained by the lamps of the invention utilizing the tested-for metal in its pure molten state. Contrary to expectation, the molten coating does not form a single globule of molten material, even though the material is completely liquid under operating conditions, but rather maintains a relatively complete coating of the interior surface 20 of the cathode holder, with only a moderate tendency to thicken at the bottom (as indicated at 32). Although three fully tested, specific examples of the invention have been given, (namely, with tin, gallium and indium as the coating metals), the invention is not limited to these specific materials but is usable for making hollow cathode lamps with metallic coatings (at 30) of other materials having the desired properties of a boiling point substantially above the operating temperature of the cathode and a low or at least moderate vapor pressure at this operating temperature. As above noted, the material of the cathode holder should be chosen so that the interiorly coated metal does not readily alloy therewith to any great extent even at the somewhat elevated operating temperatures. On the other hand, the material of the holder should be at least somewhat wettable by the metal coating so as to insure a relatively complete coating action and maintenance of this coating over at least a substantial area of the interior surface 20 of the cathode cup.
Because of its applicability to other metals, the invention is not limited to the specific examples hereinbefore described. Since various changes in the exemplary manufacturing techniques may be employed, the invention is not limited to any of the disclosed details, but rather is defined in the appended claims.
We claim:
1. In a hollow cathode lamp of the type in which the hollow cathode comprises at least one particular metal for which spectral lines are desired, such lamps being especially adapted for use in atomic absorption spectroscopy, the improvement comprising:
said hollow cathode being of generally cup shape, with the walls defining its interior having generally inturned wall portions adjacent to the open end thereof;
a thin coating of said particular desired metal substantially covering the entire interior surface of said walls at least up to said in-turned end portions,
said particular desired metal having a melting point below the normal operating temperature of said lamp, but having a boiling point substantially above said temperature and a relatively low vapor pressure thereat; said in-turned end portions of said wall thereby inhibiting loss of the liquid metal coating during operation of said lamp; whereby relatively high intensnity radiation at the spectral emission-absorption lines of said particular desired metal is obtained. 2. An improved hollow cathode lamp according to claim 1, in which:
said particular desired metal coating comprises substantially pure tin. 3. An improved hollow cathode damp according to claim 2, in which:
said generally cup-shaped hollow cathode comprises titanium; whereby alloying between said tin coating and said titanium hollow cathode is minimized, even at the moderately elevated operating temperature of said lamp. 4. An improved hollow cathode lamp according to claim 1, in which: said particular desired metal coating comprises substantially pure gallium. 5. An improved hollow cathode lamp according to claim 4, in which:
said generally cup-shaped hollow cathode comprises columbium (niobium); whereby alloying between said gallium coating and said columbium hollow cathode is minimized, even at the moderately elevated operating temperature of said lamp. 6. An improved hollow cathode lamp according to claim 1, in which:
said particular desired metal coating comprises substantially pure indium. 7. An improved hollow cathode lamp according to claim 6, in which:
said generally cup-shaped hollow cathode comprises titanium; whereby alloying between said indium coating and said titanium hollow cathode is minimized, even at the moderately elevated operating temperature of said lamp.
References Cited UNITED STATES PATENTS 2,161,790 6/1939 Abadie 3 l3-346 2,810,089 10/1957 MacNair 313346 X 2,847,605 8/1958 Byer 313346 3,089,054 5/1963 Walsh et al. 313218 X 3,286,119 11/1966 Sugawara et a1. 313-217 X FOREIGN PATENTS 977,545 12/ 1964 Great Britain.
JOHN W. HUCKERT, Primary Examiner.
J. R. SHEWMAKER, Assistant Examiner.
US. Cl. X.R. 313346

Claims (1)

1. IN A HOLLOW CATHODE LAMP OF THE TYPE IN WHICH THE HOLLOW CATHODE COMPRISES AT LEAST ONE PARTICULAR METAL FOR WHICH SPECTRAL LINES ARE DESIRED, SUCH LAMPS BEING ESPECIALLY ADAPTED FOR USE IN ATOMIC ADSORPTION SPECTROSCOPY, THE IMPROVEMENT COMPRISING: SAID HOLLOW CATHODE BEING OF GENERALLY CUP SHAPE, WITH THE WALLS DEFINING ITS INTERIOR HAVING GENERALLY INTURNED WALL PORTIONS ADJACENT TO THE OPEN END THEREOF; A THIN COATING OF SAID PARTICULAR DESIRED METAL SUBSTANTIALLY COVERING THE ENTIRE INTERIOR SURFACE OF SAID WALLS AT LEAST UP TO SAID-IN-TURNED END PORTIONS, SAID PARTICULAR DESIRED METAL HAVING A MELTING POINT BELOW THE NORMAL OPERATING TEMPERATURE OF SAID LAMP, BUT HAVING A BOILING POINT SUBSTANTIALLY ABOVE SAID TEMPERATURE AND A RELATIVELY LOW VAPOR PRESSURE THEREAT; SAID IN-TURNED END PORTIONS OF SAID WALL THEREBY INHIBITINH LOSS OF LIQUID METAL COATING DURING OPERATION OF SAID LAMP; WHEREBY RELATIVELY HIGH INTENSNITY RADIATION AT THE SPECTRAL EMISSION-ABSORPTION LINES OF SAID PARTICULAR DESIRED METAL IS OBTAINED.
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Publication number Priority date Publication date Assignee Title
US3474280A (en) * 1967-09-14 1969-10-21 Perkin Elmer Corp Lamps using spherical cathodes
US3898501A (en) * 1973-05-11 1975-08-05 Hitachi Ltd Light source lamp for atomic light absorption analysis

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US2161790A (en) * 1933-08-26 1939-06-13 Lumineles Electrode for vacuum tubes
US2810089A (en) * 1953-06-15 1957-10-15 Bell Telephone Labor Inc Cathodes for electron discharge devices
US2847605A (en) * 1954-11-18 1958-08-12 Byer Abner Albert Electrode for fluorescent lamps
US3089054A (en) * 1959-10-19 1963-05-07 Commw Scient Ind Res Org Atomic spectral lamps
GB977545A (en) * 1961-12-09 1964-12-09 Hitachi Ltd Improvements relating to the production of hollow cathodes
US3286119A (en) * 1963-05-08 1966-11-15 Hitachi Ltd Hollow cathode discharge tubes

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Publication number Priority date Publication date Assignee Title
US2161790A (en) * 1933-08-26 1939-06-13 Lumineles Electrode for vacuum tubes
US2810089A (en) * 1953-06-15 1957-10-15 Bell Telephone Labor Inc Cathodes for electron discharge devices
US2847605A (en) * 1954-11-18 1958-08-12 Byer Abner Albert Electrode for fluorescent lamps
US3089054A (en) * 1959-10-19 1963-05-07 Commw Scient Ind Res Org Atomic spectral lamps
GB977545A (en) * 1961-12-09 1964-12-09 Hitachi Ltd Improvements relating to the production of hollow cathodes
US3286119A (en) * 1963-05-08 1966-11-15 Hitachi Ltd Hollow cathode discharge tubes

Cited By (2)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US3474280A (en) * 1967-09-14 1969-10-21 Perkin Elmer Corp Lamps using spherical cathodes
US3898501A (en) * 1973-05-11 1975-08-05 Hitachi Ltd Light source lamp for atomic light absorption analysis

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