An Aluminum Foil OvenL. E. Prescott Citation: Review of Scientific Instruments 33, 485 (1962); doi: 10.1063/1.1717890 View online: http://dx.doi.org/10.1063/1.1717890 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/33/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Explosion of thin aluminum foils in air J. Appl. Phys. 96, 6061 (2004); 10.1063/1.1808482 Aluminum Foil by XPS Surf. Sci. Spectra 5, 4 (1998); 10.1116/1.1247850 Aluminum foil lined composite tubing AIP Conf. Proc. 361, 889 (1996); 10.1063/1.49983 Aluminum Foil Shield Tape Am. J. Phys. 30, vii (1962); 10.1119/1.1942169 Easily Mounted Aluminum Oxide Foils for Windows and Backings Rev. Sci. Instrum. 29, 380 (1958); 10.1063/1.1716202

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NOTES 485

FIG. 1. Schematic of the crossbar switch. The l-k resistor prevents relay arcing.

TYPICAL 20-"', IOO-WVOC TANTALUM CAPAC I TOR

New capacitors do not always retain their charged state and sometimes change from an initial uncharged state to a charged state. Both negative and positive charge-change rates occur for different capacitors. If the capacitors are successively charged with alternate polarity potentials, each less than the previous potential in absolute value, then the capacitors retain charge for long time periods and do not build up charge from the no-charge state.

Various commercial makes of both polarized and non­polarized capacitors were inspected for charge retentive properties. Of those tested, only polarized 100-wvdc tan­talum capacitors manufactured by the General Electric Company proved to be satisfactory (whereas the GE non­polarized units are not).

A further advantage of the use of tantalum capacitors is to reduce the component space and thereby to simplify the wiring system. The tantalum capacitors occupy a volume equal to approximately 3% of that for standard polysty­rene capacitors, and therefore can be wired directly to the relay terminals.

430

1180

FIG. 2. Schematic of the zero-setting control and function switch.

...... TElIT POIN<

J·2.CAP BANK

An anomalous characteristic of these tantalum capaci­tors was evident: If more than a few volts potential is applied across the capacitor, it permanently loses its charge retentive ability and behaves like a standard electrolytic capacitor. Therefore, care should be exercised not to exceed one volt potential.

In the present application, a circuit (Fig. 2) is used to ready the capacitors before a data collection run. This circuit impresses the following sequence upon the capaci­tors: -800,600, -400, 200, -100,50, - 20, and ° mv. All capacitors in the memory are first charged to -800 mv, then charged to the second potential in the sequence, and so on until the last potential (0 mv) is reached. After zero­setting the instrument, data are recorded and stored. They are later read out by a vacuum-tube voltmeter with a digital display. Individual capacitors deviate as much as 7% from linearity when all capacitors are charged by a saw-tooth wave. This condition can be corrected by adding compensation resistors to each capacitor j however, this has not been done here. The loss of charge for each capacitor with time is approximately 1% of the total charge per hour.

* Formerly with Rocketdyne, A Division of North American Aviation.

An Aluminum Foil Oven L. E. PRESCOTT

General Electric Research Laboratory, Schenectady, New York

(Received December 20, 1961)

ONE of the procedures required in high vacuum tech­nique is the bakeout of the various parts of the

vacuum system in order to drive off unwanted residues and gases from the glass walls and metal parts. Conventional ovens used for this purpose are usually rather bulky and expensive to build. There may be times when one builds an elaborate oven only to find that changes in the vacuum system would render it useless, or that the vacuum system must be confined to the limited space determined by the geometry of the oven. These limitations are easily over­come with the oven construction suggested here. The short construction time and the relatively low material costs make it possible to discard the oven after it is used a few times and still effect a savings in labor and material.

The oven construction is shown in Fig. 1. The materials used are aluminum foil and 0.080-in. thick Fiberfrax paper.! Asbestos paper has also been used with success-a thickness of 0.075 in. has proved satisfactory. The Fiber­frax is preferred, however, because it is easier to work with, and because it is somewhat lighter and more rigid.

The material is cut to size, a sheet of aluminum foil is placed on each side, and the three parts are stapled to­gether to form a single laminated sheet. The required number of sheets are made for the sides, top, and bottom

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4~6 NOTES

STAPLES

I

~ ALUMINUM FIBER

FOIL FRAX

FIG. 1. (a) An exploded view showing the construction of the oven. (b) Oven completely assembled with heating elements in place. (c) Enlarged cross section showing laminated structure.

of the oven. Wherever two sheets are to be joined, such as two sides or the top to the side, an additional inch is added to the dimensions for stapling. The heating elements are infrared quartz lamps,2 i in. in diameter and 191 in. long. The electrical connections are at each end, and project out about 1 in. on each side of the oven. Two of these are used in the oven shown which has the dimensions 9X16X22 in. The lamps are each rated at 1600 w at 230 to 250 v, but are operated at much lower power levels for an oven tempera­ture of 425°C, as the graph in Fig. 2 shows. The lamps can be connected either in series or parallel, but the parallel connection is preferred and used here because of the lower voltages involved. Power is fed through a 10-amp Variac to give control of the temperature. Power could also be fed through a thermostat control device that would turn the lamps on and off to maintain a constant temperature. By placing the heating elements as near to the bottom of the oven as practical, in this case 4 in., the most uniform tem­perature distribution is obtained. With the oven tempera­ture at 400a C the temperature gradient within the oven is about 0.5° lin., decreasing in the vertical direction. How­ever, near the heating elements at the bottom of the oven it is somewhat higher than this. The external surface temperature of the oven is 120°C, but very little radiant heat can be felt, even as close as an inch away. This is a big advantage when working in close quarters with the oven in operation.

Several of these ovens have been constructed. One which was built in this Laboratory had dimensions of 30 in. high, 18 in. wide, and 36 in. long. It had five heating ele­ments with an input power of 2.5 kw, giving an oven temperature of 390°C.

The oven described is very versatile and effective for vacuum system bakeout. It is not intended to replace

OVEN TEMPERATUREo:.-_---

OUTSIOE SURFACE TEMPERATURE

O·L-~~~~-4~070~~6~OO~~8~OO~~I~OOO~~1~20~O~"14~OO~ POWER IN WATTS

FIG. 2. Oven temperature and outside surface temperature as a function of input power to heating elements.

conventional ovens where ruggedness and durability are important, but for experimental setups (especially those of odd shapes) and systems that require occasional baking, this oven finds its most practical use.

I wish to express my thanks to K. B. Persson for his help and encouragement, and to W. R. Reed for information on the performance of the oven he built using this technique.

1 Obtained from the Carborundum Company, 1959 Buffalo Tr., Niagara Falls, New York.

2 May be obtained from the General Electric Company, Buffalo Service District, 98 Hydraulic Street, Buffalo, New York, Catalog No. 1600T3.

Simple Millimeter Wave Frequency Standard A. F. DIETRICH

Bell Telephone Laboratories, Holmdel, Nrw Jersey

(Received December 18, 1961)

H ARMONIC generation of microwave and millimeter wave frequencies! with semiconductor devices from

precisely controlled baseband frequencies results in a simple millimeter wave frequency standard. A single frequency or a series of marker frequencies can be generated. This fre­quency standard has been used for calibration of wave­meters in the 50- to 60-Gc frequency band at Holmdel.

A test setup which is used to provide check points for wavemeters in the 50- to 60-Gc band is shown by the block schematic of Fig. 1. A 20-v peak-to-peak 160-Mc sine wave signal is obtained by harmonic generation from 10 Mc using vacuum tubes. This signal is applied to a p-n junction charge storage diode mounted across RG-S2/u waveguide in the microwave pulse generator, Fig. 2. In this first wave­guide unit phase-locked microwave carrier pulses are

-IO-12GC WAVEGUIDE * SWITCH

TO DC COUPLED SCOPE

CALIBRATION POINTS

HARMONIC GENERATOR 50-60 Gc

NO FILTER WITH FILTER 51.2 Gc 56.fl Gc 52.0 52.8 53.6 54.4 55.2 56.0 56.8 57.6 58.4 59.2 60.0

FIG. 1. Block diagram of the 50-60 Gc band frequency standard test setup.

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