US2979631A - Process for the production of ion-emitting surfaces, particularly for halogen leak detectors - Google Patents

Description

Aplll 11, MOESTA PROCESS FOR THE PRODUCTION OF ION-EMITTING SURFACES. PRTICULARLY FOR HALOGEN LEAK DETECTORS Filed May 14, 1958 INVENTOR. Hass o M0 csfa.

United States Patent() Hasso Moesta, Koln-Weidenpesch, Germany, assignor to y National Research Corporation, Cambridge, Mass., a corporation of Massachusetts Filed May 14, 1958, Ser. No. 735,327

1 Claim. (Cl. 313-63) Ion sources are known in which suitable materials, chiefly alkali or alkaline earth metals and/or their compounds, are used as ion-forming activated materials, which are suspended in a suitably shaped storage chamber. Such a chamber, preferably a ceramic tube, is heated by a special heating device,perhaps by a current-carrying wire coil.

In a known method of performance the activated materials to be volatilized are deposited on the material of a small ceramic tube. The tube is then surrounded with aheating coil, which causes the evaporation of the volatile substances. At a slight distance from this evaporation set-up formed from the small ceramic tube and the heating coil belonging to it there is another coil which serves as the anode.r

In another known construction of an ion source alkali or alkaline earth metals are evaporated from a storage chamber further removed from the anode, so that the atoms of the metal vapor formed at the aperture are ionized by the perforated or porous anode surface.

Special problems arise with the use of ion sources in so-called halogen leak detectors as they are used in chemical apparatus for the locating of'leaks. In these instruments an ion source is necessary which is resistant to air penetration with suflicient temporary stability. The ion source must, moreover, show a high sensitivity to halogens so that very small traces provoke at once a marked change of ion emission. On operating such a halogen leak detector ions are released by the ion source and are absorbed by a preferably cold cathode. If halogen vapor (for example CFzCl-z) is formed around such an ion source, the stream of ions is altered and an attached meter indicates the presence of the halide. For detection a vacuum must iirst be produced in the apparatus' and after this the outer surfaces of the seal are swept for possible leak points with a halogen containing test gas, for example CF2C12. Under the effect of the surrounding air test gas diffuses through the unsealed points linto the inside of the apparatus and is passed on into the attached leak detector tube with the ion source. There the iniiuence of the halogen vapor produces a correspondnig change of emission at the anode. In this way leaks in extensive and complicated apparatus of the large chemical industries can be located in a short time. t

A known method of making an ion source for halogen leak detector is shown in the sketch. In a small ceramic tube 1 which contains a suitable supply of activated material (for example potassium compounds) is arranged a PatentedvApr. 11, 1961 in a discharge gap which the ions arising from the yanode areabsorbed by a cold cathode 4. The method of "operating suchaion source is as follows: First the vaporized metallic atoms emerge from the surface of the small ceramic tube and diffuse subsequently, preferably along the grain boundaries of the polycrystalline metal casing of the covering 3, to its outer surface where in the charging field ions of the activated substance are formed.

The duration and sensitivity of such an ion source depend now on the size and the time constant of the diffusion process of the metal atoms through the covering 3. In the practical production of such ion sources great diiiiculties arise since the structure of the grain boundary of the platinum sheet varies from lot to lot and since likewise varying porosity acts in a similar way. After the discovery that the elfectiveness of such ion sources depends on the diffusion of the activated material along the grain boundaries and through porous places, porous sintered metal has been used for the covering 3. Such porous sintered shapes of platinum are uneconomical.

The present new invention discloses a process for the production of ion-emitting surfaces, especially for halogen leak detectors, in which the activated substance, vaporizing from a supply source, passes through a plate-shaped anode and comprises depositing foreign atoms in measured amount on the crystalline getterer of the anode sheets and in the crystalline interstices. By this preparative process the structure of the grain boundaries of the anode plates are so aiected that practically with no waste there can be accomplished the production of ion sources which show a high and uniform emission of ions and marked sensitivity of the electrode toward halogens. The foreign atoms which are to be embedded must be so supplied that an expansion of the gettering structure and of the ntergranular space occurs. They are furthermore so selected that the process of causing the appropriate sensitivity at the anode is not lessened. With an anode plate of platinum the deposit of carbon as the foreign atoms has worked well.

In a particularly favorable embodiment of the new process the carbon atoms are embedded in the platinum Y sheet by thermal diffusion. For this purpose the platinum sheet, designed as a tube, is mounted on a suitable carbon arc lamp, and it is embedded for a certain period at elevated temperature, preferably in a reducing atmosphere. Temperatures between about 500 C. and 1000 C. preferably about 700 C. can be suitably employed. In place of deposition in the reducing atmosphere, embedding can be done in a vacuum. According to the duration of the annealing, annealing temperature and raw material, one obtains different high ion' currents from the ion source produced and different high diffusion coefficients of the activated material through the anode metal. These parameters can be varied within broad limits and thus it is possible to produce ion sources which are adjusted to the most different requirements with respect to duration, yield, and sensitivity. fy

What is claimed is:

A detector for a linely divided atmospheric substance capable of inducing the formation of positive ions at a conductive surface comprising an electrical discharge device including a pair of electrodes, means for causing a quantity of an atmosphere containing said substance to pass between said electrodes, means for imposing an electric potential difference between said electrodes, and means for detecting changes in the current between said electrodes caused by the formation of positive ions at one of said electrodes due to the presence of said substance, the electrode at which said ions are formed comprising a platinum sheet-like anode having adjacent thereto a ceramic container for activated material, said anode having carbon embedded therein so that said activated maten'al may penetrate said anode through interstices present in said platinum carbon material.

References Cited in the tile of this patent UNITEDfsTATE-s PATENTS yYllilliams Feb. 14, 1950 Rice Apr. 24, 1951 Morgan Sept. 29, 1953 De Juren Aug. 16, 1955 Zemany Apr. 17, 1956 Bouty July 10, 1956 Bouty May 20, 1958

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