hydroxide in the cell is kept constant. Initially calibration was performed daily b u t the day-to-day variations were so small that calibration was reduced to twice weekly. The standing signal is dependent on both the humidity of the carrier gas and the presence of neutralbing components in the sample. Any change in standing signal is therefore an indication that the cell needs attention. This, together with the biweekly calibrations, will enable the life of the sodium hydroxide to be determined. I n this particular case we have found that three months is the average useful life but, of course, this will be dependent on the sample size used. The cell is used experimentally for measuring oxygen concentrations between 10
p.p.m. and 5OOO p.p.m., and the only maintenance required has been the renewal of the sodium hydroxide solution and the renewal of the molecular sieve because of moisture contamination by faulty samples. NOTE. It should be pointed out that Engelhard Industries, Inc., holds the patent for the Hersch cell. (U.S. Patent 2805701). ACKNOWLEDGMENT
We thank the Area Chief Chemist of the West Midlands Gas Board for his help and encouragement. We particularly thank T. E. Maltby for the excellent apparatus he constructed.
LITERATURE CITED
(1) “Advances in Analytical Chemistry
and Instrumentation,’ Vol. 111, p. 219, Wiley, New York, 1964. (2) Blakemore, G., Hillman, G., Analyst
90, 703-714 (1965). (3) British Patent No. 707323, April 14, 1954, (4) Bntish Patent No. 880965, Oct. 25, 1961, ( 5 ) Bntish Patent No. 913473 (U. S. Patent No. 3,223,597), Dec. 19, 1962. (6) Dimbst, M. Porter, P. E., Stross, F. H., ANAL.~ E E M .28, 290 (1956). (7) Hersch, P., Zbid., 32, 1030-4, (1960). (8) Hersch, P., Zmtrumat Practice, 1957, 882. (9) Phillips, T. R., Johnson, E. G., Woodward, H., Zbid., 36, 450 (1964).
We thank the Chairman of the West Midlands Gaa Board for permission to publish this paper.
Spectrographic Analysis of Semiconductor Filaments by an Exploding Wire Technique P.
L.
Goodfriend, H.
P. Woods,
and
L. J.
filaments produced Icomefromimportant inorganic materials have bebecause of the highN RECENT YEARS
strength composites that can be fabricated from them. For example, filaments of boron and of silicon carbide have yielded high-strength materials. Filaments of this type are quite small in diameter (2-4 mils), very hard, and do not lend themselves readily to analysis for trace elements. The desire to find possible correlations between the mechanical properties of such filaments and their impurity content has made a convenient method for detecting these trace elements desirable. It was decided that the difficulties in obtaining emission spectra inherent in sparking to filaments of a semiconductor might be avoided by obtaining the emission spectrum produced by electrically exploding an entire section of the filament. A number of spectrographic studies on atomic lines from exploding wires have been made but have been oriented toward temperature measurements (2) or the study of weak, “forbidden” lines (1). Experiments were made with boron filaments, both with and without a tungsten substrate. The filaments were placed in a holder which fixed their position with respect to the slit of the spectrograph, ensuring proper alignment. A 2-pf. capacitor, charged to 6000 volts was allowed to discharge through the filament by triggering a thyratron tube. The spectrograph used was a Jarrell-Ash F/6.3 plane grating instrument with a linear dispersion of 10 A./mm. There is sufficient light, however, for spectrographs of smaller speed to be used. Boron filaments similar to those used in the investigation
Parcell, Texaco Experiment Inc., Richmond, Va.
2WA
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250L
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Figure 1. Microphotometer tracing showing Fe impurity lines from the explosion of a boron filament without a metallic substrate
had a total impurity content of the order of lo00 parts per million. Iron waspresent in amounts of approximately 100 parts per million. These impurity concentrations were obtained by solids mess spectrometry. The rate of energy dissipation in the filament is important in producing the explosion of the wire. Two cases must be considered : discharge through filaments at room temperature and discharge through heated filaments. I n the first case there is a time delay between firing the triggered spark gap and the wire explosion. This arises because the filament has a high resistance initially and will permit only a feeble current to pass. As this current passes, however, the temperature rises. The resistance of semiconductors has the following temperature dependence :
R
=
R, exp ( - E / 2 k T )
where E is an energy depending upon the band gap or impurity levels and k is Boltzmann’s constant. Thus as the temperature rises, the resistance goes down and more current passes. A point is finally reached when enough current is passing to produce an explosion of the filament. Depending upon length, impurity concentrations, and other factors, the filament does not always explode; sometimes it merely breaks at points under strain. By a proper choice of conditions, however, the entire segment can be made to explode. I n the second case, where the filament is heated by a heat gun prior to discharging the capacitor, the initial conductivity becomes closer to that of a VOL 38, NO. 10, SEPTEMBER 1966
0
1433
metal and the explosion proceeds easily. If there is a metallic substrate, it is the substrate that determines the course of the explosion. A portion of a microphotometer tracing showing Fe impurity lines from the explosion of a boron filament without a metallic substrate is shown in Figure 1. Almost all the lines not marked are due to iron. In other spectral regions molecular bands due to BO appear as well BS atomic lines. Considerable work will be required if
the conditions for explosion are to be properly understood and controlled. At present the technique can only be used for qualitative analyeis and estimates of relative concentrations. The advantages of this technique are: only a small section of the filament in.) and hence only a very small amount of materialisrequired. No sample preparation is required. Thus there is no contamination introdud by grinding, diasolution, Or from graphite electrodes.
The technique is extremely rapid and convenient. UTERATURE CITED
(1) ~ ~ bL., ~F i ~~ 1. , M.,~ Z. Physik bp, 547 (1930). (2) Labuda, A. A. MWt*ov, E. G., Nekrsshevich, I. &.,Zmeat.Akad.Nauk., SSSR, Ser. F i t . 22,720 (1958).
(x
This work was partially supported by the
Air Force Materials Laboratory under Contract AF 33(615)3212.
Simplified Conversion of a Direct-Current Polarograph to a low Frequency, Alternating-Current Instrument Herman Beckman and William 0. Gauer, Agricultural Toxicology and Residue Research Laboratory, University of California, Davis, Calif. HE ALTERNATINQ current polaroT g r a p h i c method described in this article is limited only to the use of low frequency (SO c.P.s.), constant a.c. voltage of a relatively small value. The recorded curve for each reducible constituent undergoing analysis ap-
A. C. POWERSUPPLY PLUG
signed around the Sargent Model XV recording d.c. polarograph. The Model XV records d.c. polarograms by automatically scanning through a preset applied voltage range, with a sensitivity of from 1.OOO to 0.003 pa./-. The a.c. circuitry is based upon the pre-
pears aa a peak and is similsr in shape to a derivative curve of the d.c. step polarogram. For analytical purposes, the amplitude of the peak has a linear relationship to the concentration of the reducible species. The instrument conversion is de-
-7
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Figure 1.
1434
ANALYTICAL CHEMISTRY
Control panel modification of Sargent Model XV Polarograph
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