Alkali Vapor PlasmasM. A. Hakeem and Harry S. Robertson Citation: Journal of Applied Physics 31, 2063 (1960); doi: 10.1063/1.1735499 View online: http://dx.doi.org/10.1063/1.1735499 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/31/11?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Terahertz radiation in alkali vapor plasmas Appl. Phys. Lett. 104, 191106 (2014); 10.1063/1.4876602 Microfabricated alkali atom vapor cells Appl. Phys. Lett. 84, 2694 (2004); 10.1063/1.1691490 Oscillations at the Anode of AlkaliVapor Arc Plasmas J. Appl. Phys. 39, 3849 (1968); 10.1063/1.1656864 Alkali Metal Vapor Spectral Lamps Rev. Sci. Instrum. 32, 688 (1961); 10.1063/1.1717470 Molecular Association in Alkali Halide Vapors J. Chem. Phys. 34, 558 (1961); 10.1063/1.1700983

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to

] IP: 147.143.2.5 On: Mon, 22 Dec 2014 02:00:42

JOURNAL OF APPLIED PHYSICS VOLUME 31. r-:UMBER II NOVEMBER, 1960

Letters to the Editor

Modification in... Technique for Growing Single Crystals of ZnF 2: Mn

L. M. MATARRESE AND F. L. HUGHES

U. S. Naval Research Laboratory, Washington 25, D. C. (Received July 26, 1960)

L ATELY there has been a considerable amount of experi­mental and the:lretical work on electron and nuclear mag­

netic resonance spectra involving single crystals of ZnF. contain­ing transition metal ions as dilute substitutional impurities.!,· Large single crystals of pure MnF2, FeF2, and CoF. have been obtaineds.4 from the melt by the moving-crucible technique,. usually in an atmosphere of anhydrous HF and in graphite crucibles with conical tips. In utilizing the same technique to grow single crystals of ZnF. with small additions of MnF2, we consistently obtained crystals of poor optical quality. Although the boules contained large crystals, they were uniformly clouded with striae of minute bubbles, except for the exterior, which con-. sisted of a thin shell of clear material 1-2 mm thick. The same difficulty has been experienced by others· in the growth of ZnF 2: Co and pure CoF 2 crystals. The purpose of this letter is to describe a modification to the growing procedure that has eliminated the difficulty for us in three consecutive trials.

The modification consists of placing a grooved and slotted insert of graphite into the crucible along with the starting powder. Figure 1 shows this insert and its location in the crucible. Not shown are the crucible cover and a graphite stick between the cover and insert, which restrains the insert from floating on the melt. We have obtained good results also with a crucible that has a tip with a square cross section and flat bottom, rather than the conical tip shown in the figure. The crystal grows into the grooves and slot of the insert, and all the material, including that in the crucible tip and that above the insert, is clear and bubble-free.

Crystals were grown in an apparatus similar to that which had been used in growing crystals of the pure fluorides.s" The crucible had an i.d. of i in., an o.d. of 1 in., and an interior length of 4 in. It was supported on a nickel elevating rod that traveled along the axis of a vertical tube furnace about 3 ft long. The furnace tube is a nickel pipe with an o,d. of 2.875 in. and an i.d. of 2.469 in. In the lower part of the furnace the o.d. was reduced to 2.75 in. in an effort to improve the thermal gradient in the crystallization region between the upper and lower parts of the furnace. The nickel pipe is provided with a closely fitting carbon liner of 2 in. i.d. in order to reduce the infiltration of impurities from the nickel pipe. The crucible was started in the upper part of the furnace where the temperature was about

FIG. 1. Graphite insert and its position in the crystal· growing crucible.

TARLE 1. Arc spectral analysis of a boule of ZnF"O.OI Mn.

Content, mole fraction Impurity Top Middle Bottom

Mn > 0.01 Ni < 0.01-0.1 Fe > 0.001-0.01 V, AI, Ca > 0.0001-0.001 Co 0.0001-0.001 Mg < 0.00001-0.0001 Si 0.00001-0.0001 Cu > 0.000001 Cr, Cb >

200°C in excess of the melting temperature, then lowered at the rate of 0.16 in./hr through a temperature gradient of about 150°C/in. near the middle of the furnace, where the temperature was maintained at the melting point of ZnF.(870°C). The run continued for 72 hr, after which the power was turned off and the furnace allowed to cool. During the entire run a slow stream of dry HF was passed through the furnace tube, from bottom to top.

Initially, the crucible was charged with a mixture of 99 parts of ZnF. and 1 part of MnF2 powder (cp grade). The ZnF. had been prepared by adding XX grade Horsehead ZnO (New Jersey Zinc Company) to cp semiconductor grade hydrofluoric acid and collecting the precipitate, which is ZnF2·4H.O. Dehydration was accomplished by a preliminary heating at 200°C of powder mix­ture under HF atmosphere in the same crucible and furnace used for crystal growing, just prior to a run.

Table I gives an idea of the segregation of impurities in a boule grown under the foregoing conditions, using a crucible with a square tip. Specimens were taken from the square bottom of the boule, the middle of one of the sections grown in a side groove, and the top edge of the section that grew above the insert. The symbols in the "Top" and "Bottom" columns refer to the in­equality or equality of the concentration of impurities in the top and bottom specimens compared to the impurity content of the middle specimen. The relatively large amounts of Ni and Fe evidently came from the furnace walls.

Our conjecture is that our previous failures with ZnF2 were caused by an inability to maintain the proper critical temperature distribution within the crucible in spite of the similarities of our apparatus to that which had produced good results in the past with the other fluorides.a .• The insert, we believe, aids materially in achieving this distribution by providing a sink for the heat of crystallization, thus providing at the liquid-solid interface a sufficiently large thermal gradient to eliminate such imperfections as bubbles.

1 M. Tinkham, Proc. Roy. Soc. (London) A236, 535, 549 (1956). 2 A. M. Clogston et aI., Phys. Rev. 117, 1222 (1960). , M. Griffel and J. W. Stout, J. Am. Chern. Soc. 72, 4351 (1950). : L .. M. Matarrese, Ph.D. dissertation, University of Chicago, Chicago,

Illmols, 1954. • D. Stockbarger, Rev. Sci. Instr. 7, 133 (1936). , E. Catalano (private communication).

Alkali Vapor Plasmas* M. A. HAKEEM AND HARRY S. ROBERTSON

Department of Physics, University of Miami, Coral Gables, Florida (Received May 26,1960)

T HE positive column of a glow discharge would be a con­venient laboratory plasma for a variety of experimental

purposes, if it were free of standing or moving striations. Mole­cular gases usually seem to show standing striations, and the noble gases and mercury vapor, as reported by Donahue and Dieke,! exhibit moving striations over the range of current and pressure ~ost convenient for many experimental purposes. Since the alkali metals have no neutral-atom metastable states, it was expected2 that alkali vapor glow discharges should be free of moving striations,

2063 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to

] IP: 147.143.2.5 On: Mon, 22 Dec 2014 02:00:42

2064 LETTERS TO THE EDITOR

\Ve have found, with the exceptions to he noted, no moving striations in alkali vapor positive columns. We have examined potassium, rubidium, and cesium, in the current range from 0 to 300 ma and the pressure range of about 0.01-2.0 mm Hg. Atom densities in this range are 3(10)14-3(10)16 cm-a. We have looked thoroughly over this range, both at the light emitted as a function of time and position, and at the tube voltage. We have used tubes of 1.8 cm and 3.0 cm diameter, 30 cm between tantalum electrodes. A pool of the metal is kept in a reservoir in a side arm of the sealed tube. The reservoir temperature, which is lower than that of the discharge tube, determines the vapor pressure in the tube. No metal was observed to have condensed on the walls of the discharge tube. No filler gas such as argon was used, and the tubes were sealed at a background gas pressure of about 10-8

mmHg. At the lowest operable pressures and at currents of 1 rna or less,

very faint standing striations were sometimes seen, with spacing approximately 1 mm. At higher currents and pressures, these were not present. Also at low pressures and low currents, transient oscillatory phenomena are sometimes observable in the light from near the edge of the negative glow and in the tube current. These oscillations are rarely encountered and are not as yet reproducible.

The only common, reproducible oscillation observed by us is associated with anode spots, which we have described elsewhere.' Tube voltage and light from the plasma within 1 cm of the anode show periodic oscillations, but the phototube shows clearly that the striation which moves into the positive column from the anode is very quickly attenuated, and that the plasma is essen­tially unperturbed by this anode spot oscillation. For sufficiently high or low currents, anode spot oscillations are absent, and both the tube voltage and light emission are free of periodic or large amplitude perturbations.

Cesium seems to be the most convenient alkali to use since it reaches an operable vapor pressure at a lower temperature than the others. Neither cesium nor rubidium attack or combine with the Pyrex tubes to an annoying extent, but potassium quickly turns the tubes dark brown, interfering with light measurements.

* This work is supported in part by funds provided by the U. S. Atomic Energy Commission.

1 T. M. Donahue and G. H. Dieke. Phys. Rev. 81, 248 (1951). 2 H. S. Robertson, Phys. Rev. 105,368 (1957). 3 H. S. Robertson and M. A. Hakeem, Plasma Phys. Bull. I, 2 (1960),

University of Miami. Copies may be obtained, as long as the supply lasts, by writing us.

Effect of Interelectrode Spacing on Cesium Thermionic Converter Performance

R. L. HIRSCH

Atomics International, Canoga Park, California (Received August 18, 1960)

I N the cesium thermionic converter, energy dissipating inter­actions occur between emitted electrons and the "plasma,"

thus reducing the net electrical output. These interactions can be suppressed by reducing either the cesium pressure or the dis­tance between electrodes. At very high emitter temperatures cesium is utilized primarily to neutralize space charge, and its effects on emitter work function are small. In this range the cesium pressure can be varied to approach emission limited performance. However, for operation in the moderate tempera­ture range (1400° to 2000°C), cesium may be utilized to modify emitter work function as well as to reduce space charge. In this mode of operation the cesium pressure is fixed by these require­ments, and departure from this optimum pressure results in drastic reductions in converter output. The interelectrode spacing then becomes a variable of great importance.

Experiments have been conducted with a variable spacing converter in this moderate temperature region, and the results affirm the importance of spacing. In this diode the plane-parallel

electrodes were fahricated to arc-cast molyhdenum. The emitting surface was the bottom of a circular cylinder, the sides of which were shielded in an effort to minimize edge currents. During op­eration, the interelectrode spacing could be varied from zero to one-tenth of an inch through the use of a micrometer drive. The cesium reservoir and anode temperatures could be independently varied and were controlled by gaseous forced convection. The temperatures at these points were continuously monitored by thermocouples and a chart recorder. The emitter was heated by electron bombardment, and its temperature was determined through the use of an optical pyrometer.

The results of the first phases of this work are shown in Figs. 1 through 3. Figure 1 shows the observed effects of spacing on specific power output. At each cathode temperature and spacing the load resistance, the cesium temperature, and the anode tem­perature were varied to yield maximum power output. Generally,

20,-__________________________________ -,

18

16

14

12

'" E ()

.....

a: w ~ 8 o Q.

U

ii: u 6 w Q. (/)

4

2

OL-____ L-____ ~ ____ ~ ____ ~ ____ ~ __ ~

o 15 30 45 60 75 SPACING - MILS

FIG. 1. The experimentally observed effects of electrode spacing on specific power output.

90

[This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ]

IP: 147.143.2.5 On: Mon, 22 Dec 2014 02:00:42