Thermodynamics in the design of longlife, hightemperature devicesR. J. Zollweg Citation: Review of Scientific Instruments 57, 1438 (1986); doi: 10.1063/1.1138566 View online: http://dx.doi.org/10.1063/1.1138566 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/57/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Conceptual design study of small long-life PWR based on thorium cycle fuel AIP Conf. Proc. 1615, 61 (2014); 10.1063/1.4895862 Design study of long-life PWR using thorium cycle AIP Conf. Proc. 1448, 101 (2012); 10.1063/1.4725443 Methods for Accelerated Life Evaluation of LongLife Cryocoolers AIP Conf. Proc. 710, 1239 (2004); 10.1063/1.1774811 Long-life space reactor for photon propulsion AIP Conf. Proc. 608, 596 (2002); 10.1063/1.1449777 A highly stable longlife GaPGaAlP heterojunction cold cathode Appl. Phys. Lett. 34, 545 (1979); 10.1063/1.90881

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holes through the mounting plate are concentric with the plate's contacts so that the contacts are in the form of con­ducting annuli. Each sample has a pattern of solid contacts identical to that of the mounting plate. The sample is posi­tioned on the plate so that its contacts are aligned over the conducting annuli and then the vacuum is applied. The de­posited contact pattern on the mounting plate includes con­ductors extending from the conducting annuli to a second set of contacts near the periphery of the plate. The peripheral contacts are used to connect to lead wires which, in turn, are connected to the appropriate measurement devices.

The described mounting method has been used to make

van der Pauw measurements (Ref. I) on thin (17 pm) sili­con carbide platelets. The van der Pauw technique requires a pattern of four contacts. For this application, gold contacts were used and were arranged in a square pattern with 2.5-mm center-to-center spacing. The contact size on the sample was 0.5 mm2

• The mounting method has been used for mak­ing electrical measurements over a wide temperature range (77 to 1000 K) and does not introduce distortions into the magnetic field during Hall measurements.

IL. J. van der Pauw. Philips Res. Rep. 13. I (1958).

Thermodynamics in the design of long-Ufe, high-temperature devices R. J. Zollweg

Westinghouse R&D Center. Pittsburgh, Pennsylvania 15235

(Received 18 November 1985; accepted for publication 12 February 1986)

The application of chemical thermodynamics to the design oflong-life, high-temperature devices is advocated and illustrated by its use in selecting the material components for a high-temperature lead vapor Raman cell. It is shown that the reduction of alumina and of a caIcia-alumina sealing frit by tantalum lead to equilibrium aluminum and calcium partial pressures orders of magnitude greater than those over pure alumina at the desired ceU operating temperature.

Many factors must be considered in choosing materials to fabricate a high-temperature cell. Cell windows must have adequate diameter, thickness, and transparency. The me­chanical properties of the components must be adequate with compatible thermal expansion coefficients. Obviously, the components must be chemically compatible with each other and with the intended fiU materials in condensed and vapor form. However, the latter is not always so obvious, especially if the device is to be operated at unusually high temperatures and for a life of five years or so. To build and test a device for such a period of time is both expensive and time consuming, while techniques of accelerated testing re­main questionable until validated.

In this paper we advocate the application of chemical thermodynamic equilibrium calculations to this kind of problem to ( I.) identify potential reaction problems, includ­ing those that may not appear until. testing has proceeded for a year or more, (2) choose between alternate materials in order to design a superior product in the first place, and (3) avoid surprises. We shall iUustrate this application by way of the components selected for a 1200 ·C lead vapor Raman cell with sapphire windows to transmit intense UV laser radi­ation.'

While thermodynamics can be of great heIp in the de­sign of high-temperature devices, their results must be un­derstood and carefully evaluated. Over the long haul, one expects thermodynamic equilibrium to be reached. How­ever, kinetics can sometimes take a long time such that pre­dictions of precise life may not be quantitative. One needs to

consider the validity of the thermochemical data and whether all possible reactions are considered. Some of the data may not be available and may have to be estimated. Thus, such calculations must not be viewed as a panacea but rather as a powerful tool and guide. Thermodynamic design may be viewed as the intelligent use of our vast, available data base.

The specific device to be discussed here is an all-hot cell to be operated at 1200 to 1300 °C to obtain sufficiently high lead vapor pressure plus an inert buffer gas. Sapphire win­dows are required to admit the intense beam of near UV laser radiation which is to be downshifted to the blue-green in the Raman cell for underwater communication purposes. The sapphire windows can be sealed to a refractory metal tube with suitable sealing materials and technique. Tantalum and niobium have thermal expansion coefficients similar to sap­phire (perpendicular to the c axis), but niobium has been reported to react with lead at about 800 °C, whereas tanta­lum was reported to be compatible at least to 1000°C.2

Therefore, tantalum was chosen as the tube material for ini­tial work. Compatibility with lead to 1200 DC is being investi­gated. The compatibility of tantalum with alumina (sap­phire), with the sealing material, and these materials with condensed and vaporized lead can be explored by chemical equilibrium caJ.culations and alternatives identified where incompatibility is found.

Chemical equilibrium calculations were made for some­what arbitrary quantities of tantalum, alumina, a calcia-alu­min a sealing frit, lead, and an argon buffer gas at a total

1438 Rev. Sci.lnstrum. 57 (7), July 1986 0034-6748/86/071438-03$01.30 © 1986 American Institute of Physllcs 1438 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

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pressure of one atmosphere, and a range of temperatures encompassing sealing and expected cell operating tempera­tures. These were done with the Westinghouse computer code and data base developed by R. W. Liebermann, which minimizes the free energy of a mixture of species which can include both gases and condensed phases. The present calcu­lations neglect interactions in the condensed phase except for the known CaO· A120 3 condensed phase complex. (Other activity coefficients were assumed equal to one.) These results show that tantalum reacts with alumina in a manner similar to that found for niobium. 3 That is, a small part of the tantalum reacts with the alumina to form con­densed phase Ta20 5 and small quantities ofTa02 and TaO in the vapor phase. Al appears in the vapor phase at an equilib­rium partial pressure that is almost two orders of magnitude greater than the total of all vapor species expected over pure alumina (mainly Al and 0 plus some suboxides). When a calcia-alumina sealing frit is used with the tantalum and alumina, then Ca vapor is present at equilibrium with a par­tial pressure about 40 times that of the Al and also 40 times that over all the expected vapor partial pressures over pure calcia. In this calculation we neglected other oxides, added to the frit in limited quantities to control seal morphology, as these may be more strongly bound. Figure 1 shows the calcu-

10 -5 I-~\--:I"......:c-,-,--+_.Increased Vapor Pressures of AI and Ca because rJ Ta

Reduction of Alf 3 or caO· Alf 3

10-6 !-\r-':-\I-\--+-:::-::­\A-~-T~CaO

TaO 2

10-9

10 -10 !---+--+-+-~~"---+--+-----i

Ca

10-11 '-:-_--'-_____ OL-'---..L---'-___ '-_--'

0.40.5 0.6 0.7 0.8 0.1 1.0 HXXl/PK

FIG. 1. Compares the total equilibrium partial pressures over pure Alp, (~AI20,). principally the elements, and over pure CaO (~CaO) with the high partial pressures ofCa, AI, and Ta02 which result from the Ta reduc­tion of AloO, and of CaO . A120 3 • For an all-alumina cell sealed with a cal­cia-alumina frit, the partial pressures resulting from the cell components will be approximately ~ CaO plus ~AI20,.

1439 Rev. Sci.lnstrum., Vol. 57, No.7, July 1986

lated partial pressures in atmospheres as a function of tem­perature together with the desired cell operating tempera­ture and the sealing temperature for the calcia-alumina-type sealing frits.

The above calculations show that Ca should be the most volatile undesired species in the cell which will, in time, de­posit on the colder surfaces. The Ca should not deposit on the windows if they are kept warmer than other cell parts. The calcium will, however, alloy with the lead and might, in time, reduce the lead vapor pressure unless sufficient excess lead is present. That problem could be reduced by making a cell without tantalum, that is, going to a totally alumina cell.

One should still consider the compatibility of lead with the cell components. With sufficient lead added to the cell, most of it will remain in the condensed phase, unreacted according to our calculations. There will be the desired atomic lead vapor together with a small dimer concentration (about 0.1 % ). In addition there will be a small amount of PbO in the vapor phase (6 X 10- 10 atm.). The PbO partial pressure exists even when tantalum is not present, but is then over an order of magnitude lower at operating temperature as shown in Fig. 2. The concern here is whether the vapor phase lead oxide reaches the sapphire window and dissolves or reacts with the sapphire. Crystals of alumina with signifi­cant quantities of PbO, such as PbO . 6Al20 3 are known in

I I I

10-? ,

1\\ 'W \ "b

\ "".0\-\ ....

'" \ \ ~

-0 \ '" '3 C! _\ y .., \ -~ \ ""0

~ \ "".0\-'!, -0

10-6

~\ C$

"b \ v> \

~\ C$ \

\ \

\ \ \ \

\

1 \ \

10-10

0.5 0.6 0.7 0.8 0.9 1.0 l(XXl/T oK

FIG. 2. Compares the calculated PbO partiaJ pressure when lead, alumina, tantaJum, and calcia-alumina are present with that when only lead and alu­mina are present in the cell. The risk of reaction with the sapphire window is less in the latter case.

Notes 1439

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the phosphor literature. 4 If such complex compounds form on the surface, then with thermal cycling, the differential thermal expansion mismatch may cause crazing or cracking, which could, at minimum, destroy the window transmission over a period of time.

The results obtained above illustrate the power of doing relatively simple thermodynamic equilibrium calculations using presently known data to consider the probable conse­quence of various alternative materials chosen for the high­temperature, lead vapor Raman cell. We have not consid­ered the possibility of creep and other mechanical properties, grain growth in the sealing frit, condensed phase interac­tions, and the effects of impurities. The latter two could be estimated and included at least in some approximation if the work were extended.

Low-noise Han probe preampUfier Steven W. Smith

Support for some of this work was provided by the Na­val Ocean Systems Center under contract N66001-84-C-0238. I am indebted to R. W. Liebermann for use of the code and data base developed by him and for instructions on their use. Discussions with C. S. Liu and I. Liberman regarding the Raman cell have also been helpful.

'R. Burnham and E. J. Schimitschak. Laser Focus 17. 54 (1981). 2Technicalliterature supplied by KBI Div .• Cabot Corp., Reading, PA and other niobium and tantalum material suppliers. evidently based on unpub­lished work done at Fansteel Metals, North Chicago, IL.

3L. N. Grossman, J. Chern. Phys. 44. 4127 (1966). 4F. Kroger. Some Aspects of the Luminescence of Solids (Elsevier. New York, 1948). p. 86.

Department of Radiology. University of Utah Medical Center. Salt Lake City, Utah 84112

(Received 24 January 1986; accepted for publication 20 March 1986)

A low-noise preamplifier circuit for use with HaHtron HR-66 Han effect probes in the frequency range dc to 100 kHz, shows a wideband noise of 6- and 2-mG rms for bandwidths of dc to 100 kHz and dc to 100 Hz, respectively. Below 100 Hz the noise is due to the l/fnoise from the Hall probe, while white preamplifier noise dominates above 100 Hz. Above 1 kHz, preamplifier noise exceeds the noise of the Hall probe alone by a factor of five. dc drift of the system is approximately 10 mG in 24 h.

Hall effect probes are rugged, small, and inexpensive devices for measuring magnetic fields over a wide frequency range that includes dc. They do not significantly distort the mag­netic field being measured and they have a linear response over more than six orders of magnitude. A major disadvan­tage of these devices is their low sensitivity; typical Hall probes produce an output signal of less than 100 flY per gauss. Amplification of these small signals requires a special preamplifier with low noise, low dc drift, and high common mode rejection to reduce stray signal pickup.

The schematic of a useful preamplifier is shown in Fig. 1. The Halltron HR-66 1 is typical of commerical Hall probes. The LM7805 voltage regulator and 15-0 resistor form a 300-mA constant current source to drive the Hall probe. Ql and the zener diode bias the output voltage of the Hall probe near ground potential. The 15-0 resistor across the voltage terminals of the Hall probe is recommended by the manufacturer for increased linearity. The LM394 tran­sistor pair and the LF353 dual operational amplifier are the active dements in the preamplifier. This circuit is based on a previously reported2 low-noise preamplifier with modifica­tions for dc coupling and increased collector current to pro­vide lower transistor noise. For the component values shown, the preamplifier has a voltage gain of 133 from dc to 100 kHz. A differential input amplifier follows the preampli­fier and provides for dc offset and gain adjustment. The cir-

cuit shown has a voltage gain of 10, resulting in a system sensitivity of 100 mY IG at the output of the differential am­plifier.

The preamplifier uses both differential input and out­put, so that the signal is never connected to ground. In fact, the preamplifier circuit does not even contain a ground con­nection. This allows the preamplifier to be placed near the Han probe with a shielded four-conductor cable connecting the preamplifier to the differential amplifier. Since the signal is ungrounded until it passes through the differential ampli­fier, common mode signals are eliminated whether they en­ter the system through the HaJJ probe, preamplifier, or the interconnecting cable. Also note that no trimming is re­quired in the preamplifier, which simplifies its placement near the measurement site. The use of a dual transistor and dual operational amplifier ensures low dc drift from tem­perature variations. Low preamplifier noise is obtained by using the LM394, one of the lowest noise transistor pairs presently available.

The preamplifier is constructed in an 8 X 4 X 2-cm alu­minum case that also provides a heat sink for the 10 W dissi­pated by the LM7805 and Q1. The Hall probe measures 5 X 7 X 1 mm and is attached to a small heat sink to increase thermal stability. A steel.-cased LM394 transistor pair is used; however, a plastic DIP package is available for in­stances where the magnetic field must not be disturbed. Car-

1440 Rev. Sci.lnstrum. 57 (7), July 1986 0034·6748/86/071440-02$01.30 © 1986 Amerl'can Institute of Physics 1440 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

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