Alloy for hollow cathode lamp

3,487,254

Dec. 30, 1969

J. W. VOLLMER

ALLOY FOR HOLLOW CATHODE LAMP

Filed Jan. 16, 1969

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INVENTOR.

JOHN W· VOLLMER

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ATTORNEY

3,487,254

Patented Dec. 30, 1969

United States Patent Office

3,487,254

ALLOY FOR HOLLOW CATHODE LAMP John W. Vollmer, Danbury, Conn., assignor to The

Perkin-Elmer Corporation, Norwalk, Conn., a corporation of New York

Filed Jan. 16,1969, Ser. No. 791,637

Int. Cl. HOlj 17/04, 61/06

U.S. CI. 313—218 3 Claims

ABSTRACT OF THE DISCLOSURE

In a hollow cathode lamp of the type useful as a spectral radiation source for atomic absorption spectrometers, the interior of the hollow cathode is coated with a tellurium-tin alloy. The preferred alloy is tin telluride (of 51.8% tellurium and 48.2% tin, by weight). This alloy is less brittle, exhibits both lower vapor pressure and a higher melting point at lamp operating temperature, and has lower electrical resistance than pure tellurium. The resulting lamp may be used as a spectral source for both tellurium and tin.

This invention relates to an improved hollow cathode used in a lamp, of the type especially useful as a light source in spectroscopic instruments of the atomic absorption type. More particularly this invention concerns an improved hollow cathode for a lamp, which is useful primarily for atomic absorption measurements of both tellurium and tin.

In atomic absorption spectroscopy the sample (containing one or more metals) 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. Usually the metallic sample (typically in the form of a dissolved salt) is vaporized in the flame ot a burner, so that the sample is atomized, and then irradiated with a light source of great intensity at least at one characteristic absorption band of the tested-for-metal. Only transmitted radiation in the region of this characteristic wavelength is then allowed to reach a detector, which therefore yields a measurement of how much absorption has occurred. The detected intensity (as compared to the original source intensity, for example) yields a quantitative measurement of the absorption and therefore the concentration of the particular metal for which the analysis is being conducted. A description of atomic absorption spectroscopy and spectrometers is contained in United States Patent No. 2,847,899, issued Aug. 19, 1958 to A. Walsh.

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. A 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. A typical such hollow cathode lamp is shown in applicant’s United States Patent No. 3,361,925, issued Jan. 2, 1968. Preferably the hollow cathode assembly of the present invention is incorporated in a shielded hollow cathode lamp of the type having an “open” generally cylindrical, shield of insulating material substantially coaxial with the cathode, as shown, for example, at 60 in FIGURE 1 of applicant’s United States Patent No. 3,390,297, issued on June 25, 1968.

As noted in applicant’s said Patent No. 3,361,925, the actively radiating part of the hollow cathode should have certain desirable physical characteristics to provide high intensity spectral radiation, to have a long useful life, and to simplify manufacture. In addition to having such desirable mechanical properties and to having physical and _r chemical stability at all tempertures to which it is likely ° to be exposed in use, the actively emitting cathode material must also have certain desirable electrical characteristics. in particular, this cathode material should have an electrical resistance of a convenient value (i.e., rela₁₀ tively low) under the actual operating conditions of the lamp, and the resistance-versus-temperature curve (over the range from room temperature through operating temperature) should be “smooth.”

Pure tellurium is an extremely brittle metallic sub15 stance, which exhibits relatively high vapor pressure at the temperatures normally reached within the “hollow” of the cathode lamps of the type noted above. In addition to these non-ideal physical characteristics, the electrical resistivity of tellurium is substantially higher than the low 20 ideal value. By utilizing an alloy of tellurium, in particular tin-telluride, applicant provides a material of substantially increased ductility, reduced vapor pressure at operating temperatures, and reduced electrical resistance as compared to pure tellurium. Additionally, the melting 25 point of this alloy is higher than that of pure tellurium, so that the hollow cathode lamp can be operated at higher currents (and therefore yield higher radiation intensity). Additionally, the use of this alloy in the interior of the hollow cathode enables a lamp to be utilized not only as 30 a tellurium radiation source but also as a tin radiation source.

An object of the invention is the provision of an improved hollow cathode assembly for use in a lamp for emitting radiation in the characteristic spectrum of tel35 lurium.

A similar object is the provision of such a hollow cathode assembly, in which the tellurium-containing substance has greater ductility, reduced vapor pressure at operating temperature, lower electrical resistance, and higher melt40 ing point than previously utilized pure tellurium, thereby obtaining, respectively, greater ease of manufacture, longer life at a given operating current, lessened voltage requirements for the lamp current supply, and higher practical operating currents (and therefore intensity).

Another object of the invention is the provision of an improved spectral radiation source lamp, which provides radiation not only in the characteristic spectrum of tellurium, but also in the characteristic spectrum of another metal, for example tin, so that the same lamp may 50 be utilized in the analysis of either of such metals.

Other objects and advantages of the invention will become obvious to one skilled in the art on reading the following detailed description of an exemplary embodiment of the invention in conjunction with the accom55 panying drawing in which:

The sole figure is an enlarged vertical cross-section through the improved hollow cathode assembly, incorporating in its interior a tellurium alloy according to the invention.

®θ The drawing shows a finished hollow cathode assembly according to the invention prior to its being incorporated into a lamp of the type shown, for example in the above mentioned U.S. Patent No. 3,361,925 or preferably of the type shown in FIGURE 1 of U.S. Patent ⁶⁵ No. 3,390,297. The hollow cathode assembly 20 of the instant drawing includes a conventional cathode cup or holder 22, which is preferably made of copper in the combination of the present invention. The closed (left hand in the drawing) end of the holder 22 has a narrowed ⁷⁰ portion 24, having a central narrow aperture 25 for press fitting or crimping upon the cathode pin of hollow

3,487,254 cathode lamp (as shown in either of applicant’s above mentioned two patents). The wider open-cup portion (right-hand in the drawing) of 26 of the cathode holder will contain a generally cylindrical insert or slug 28, having a generally cylindrical central aperture or „ “hollow” at 29. This insert 29 comprises the material which determines the actual radiation characteristics of the lamp, and in the preferred embodiment of the invention will be made of tin-telluride, as more explicitly described hereinafter.

A major point of novelty of the present invention is the constituency of the actively emitting substance of the insert 28. Specifically by utilizing tin-telluride for the substance, an improvement is obtained in both ease of manufacture and in the characteristics of the lamp (in- 75 eluding intensity of the radiation) for use as a source of tellurium spectral radiation. Additionally, the lamp is a highly satisfactory (in fact, improved) source for tin spectral radiation. An exemplary technique for making a hollow-cathode assembly, according to the inven- 20 tion is as follows.

A suitable amount (about 5 grams being required for each insert) of tin-telluride may be made by alloying tin (48.2 percent by weight) with a chemically equivalent amount of tellurium (51.8 percent by weight) in an 25 inert atmosphere (for example, an argon forming gas in a carbon crucible % of an inch internal diameter) by means of an induction heater. If a single 5 gram batch is thus prepared, a slug or insert of tin-telluride with a % inch external diameter and about inch long 30 will be formed. This slug is faced off and drilled (¼ inch diameter) so as to form the central longitudinal bore or hollow 29 of the drawing to a depth of approximately % of an inch. The insert or slug is then placed within the copper cathode holder 22 which has a mating 35 internal diameter of slightly greater than the % inch external diameter of the slug 28. The extreme right hand part of the wide portion 26 of the cathode holder (which has been up to this point merely an extension of the cylindrical portion 26) is now swaged as indicated at 36 40 so as to secure the insert 28 within the cathode holder. The finished cathode assembly as shown in the drawing will then be attached to the cathode pin of the lamp of the type shown in either of the above mentioned U.S. Patents (and preferably of the type shown in FIGURE _4g1 of U.S. Patent No. 3,390,297), as more fully explained in both these patents.

Hollow cathode lamps (of the type shown in FIGURE 1 of said U.S. Patent No. 3,390,297) in which the cathode assembly is as shown in the instant drawing and as de- 5Q scribed herein above have been successfully tested and are currently being manufactured for commercial utilization. It has been found that neon is the preferable fill gas for the finished lamp, since it yields higher intensities for both the tellurium spectral radiation (as measured at the 2143 55 A. line for tellurium) and for tin (at both the 2246 A. and the 2863 A. tin lines) than the other conventional fill gas, argon. A neon fill gas pressure of approximately 8 torr has proved highly satisfactory. Lamps according to the invention have been successfully run for over 200 60 hours at 30 milliamps (6 ampere hours). Lamps manufactured using the cathode assembly according to the invention were operable satisfactorily at higher currents (30 milliamps) than previous lamps using an internal coating or insert (28) in the interior of the cathode holder of pure tellurium (about 18 milliamps being the maximum practical with such prior lamps). Thus, the reduced vapor pressure and increased melting temperature of the tintelluride insert yielded improved operating characteristics, in addition to simplifying manufacture and lowering the risk of cracking during use (because of the improved ductility of tin-telluride compared to the brittle tellurium). A further advantage is the dual purpose nature of the finished lamp, lamps according to the invention actually having higher tin spectral radiation intensity than previous lamps, in which the interior of the cathode is molten tin (during operation).

Thus, applicant has provided an improvement in the cathode assembly for a spectral lamp, having improved stability, greater intensity, simpler manufacture, and dual utility (for both tellurium and tin spectral radiation). Thus, the invention accomplishes the purposes desired, although only relatively simple steps are required to manufacture the improved hollow cathode assembly. Although a specific exemplary embodiment of the invention has been fully disclosed, the invention itself is defined by the scope of the appended claims.