kinetics of physical vapor transport at low pressure under microgravity conditions: i. dcmf flight...

Journal of Crystal Growth 193 (1998) 80—89

Kinetics of physical vapor transport at low pressureunder microgravity conditions

I. DCMF flight hardware and experimental conditions

M. Piechotka!, E. Kaldis!,*, G. Wetzel!, H.-J. Schneider", W. Buff", M. Tardy", H. Stanna"

! Swiss Federal Institute of Technology, ETH/HPF, CH-8093 Zurich, Switzerland" Oerlikon-Contraves AG, CH-8052 Zurich, Switzerland

Received 15 December 1997

Abstract

An autonomously operating apparatus has been developed for the measurement of the transport rate of mercuriciodide vapor. It was flown aboard the Space Shuttle STS-77 mission in May 1996. The results of the diffusion coefficientmeasurement facility (DCMF) space experiment provide a breakthrough in the understanding of the transport phe-nomena during the physical vapor transport under low total pressures (0.5—3.5 mbar). The comparison of the transportdata in microgravity and on earth could reliably be made only due to the sophisticated hardware development. Thedesign, construction and the technical performance of the apparatus are presented in Part I whereas the major results arediscussed in Part II. ( 1998 Elsevier Science B.V. All rights reserved.

1. Introduction

Based on an analysis of the Spacelab-1 materialsscience experiments performed till the middle of80’s, recommendations have been made for futurespace experimentation [1]. This also brought upthe idea to perform precise measurements of thetransport rate (TR) during the physical vaportransport (PVT) in low pressure systems (total pres-sure within a fraction of a mbar). The DCMFproposal was finally shaped and submitted to theSwiss Bundesamt fur Bildung und Wissenschaft(BBW) in 1987 [2]. As discussed in more detail

*Corresponding author.

in Part II of the present paper, such measurementswere necessary in order to clear the persisting un-certainty as to whether or not convection, aside ofdiffusion and Stefan flow, could be one of the mech-anisms contributing to the vapor transport alonga temperature gradient in low pressure systems.Convection, if present, is expected to contributeto the net mass transport rate on earth but not inmicrogravity environment. Thus, differences in TRbetween earth and space should reveal convection.However, such measurements constitute an ap-preciable challenge due to the limitations inherentto space experimentation (power, size, weight) aswell as the fact that the anticipated contributions ofconvection to the net mass transport rate mightbe very low in these diluted gas systems where

0022-0248/98/$ — see front matter ( 1998 Elsevier Science B.V. All rights reserved.PII: S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 3 4 9 - 2

Rayleigh and Grashof numbers are very low.Therefore, a novel method has been developedat the ETH Zurich which ensures high precisionin situ measurements of the transport rate ofmercuric iodide vapor both on earth and in micro-gravity.

2. Principle of the measurements

Up to now, weighing (in situ or post-process) wasused for the measurement of the mass transportrate in closed ampoules. However, apart of its obvi-ous inapplicability in microgravity environment,weighing has an estimated accuracy of approxim-ately$15% which is too low to detect small con-vective contributions to the mass transport. In theDCMF method [3], the displacement velocity�0

(cm/s) of the evaporating source material is mea-sured along the ampoule axis (Fig. 1). The fluxdensity J

0(mol/cm2 s) along the measurement path

is directly proportional to �0:

J0"�

0(o/M), (1)

where o is the density of the evaporating materialand M its molecular weight. If the evaporatinginterface moves without changing its shape, both�0and J

0are equal at any point of the interface and

the flux J (mol/s) can be calculated from

J"J0S, (2)

where S (cm2) represents the area of the evaporat-ing interface. As evidenced in a series of laboratory

Fig. 1. Schematic diagram of the DCMF ampoule and the tem-perature distribution along the furnace axis.

measurements [3], a nearly flat evaporating inter-face could be maintained within a period of timelong enough to make TR measurements at varyingsupercooling in narrow ampoules (i.d. 7 mm) forvelocities around 1]10~6 cm/s which are typicalfor vapor growth.

The original ultimate goal of the DCMF experi-ment was to assess the difference between the diffu-sion coefficient of mercuric iodide calculated fromthe kinetic theory of gases and the effective diffu-sion coefficients obtained from the PVT data mea-sured in microgravity and on earth. This differencerepresents the contribution of convection to theoverall mass transport rate. For this purpose, notonly the flux J but also the total pressure P shouldbe measured in each ampoule at 120°C, the temper-ature at which the transport proceeds. Suchmeasurements, originally foreseen in the DCMFproject, have not yet been done. If the necessaryfunding is secured, they will be performed in thefuture. However, the contribution of convection tothe net mass transport rate can be inferred from thealready available data without the determination ofJ and P. For this purpose, it is sufficient to comparethe velocity values �

0obtained in one and the same

ampoule on earth and in space (see Part II of thisarticle).

In these comparative measurements, the repro-ducibility and not the absolute accuracy is of majorimportance. Two factors may diminish the re-producibility in the DCMF method: (1) inhomo-geneous source ingot, and (2) nonisothermal axialtemperature distribution along the measurementpath. The method of the source formation used forthe DCMF experiment (see Section 5) ensuresa compact, homogeneous source ingot with welldefined interface. The temperature along themeasurement path (16 mm long) is kept constantto within $0.1 K. Thus, there were no changesobserved in the interface shape originating fromeither of the two factors. This holds for all theampoules filled with HgI

2and 3 mbar of argon

where the interface velocity does not exceed1]10~6 cm/s due to the presence of the inert gas. Italso does for the ampoules filled with mercuriciodide of medium purity and stoichiometry used inthe breadboard phase of the DCMF hardware de-velopment. In all these cases, a reproducibility of

M. Piechotka et al. / Journal of Crystal Growth 193 (1998) 80–89 81

$3% has been demonstrated at high supercoolings(around *¹"16 K) [4]. Among the DCMF flightampoules filled without addition of an inert gasusing a special procedure (see Section 5) ensuringa very high grade of purity and stoichiometry ofmercuric iodide, a relatively large change in theinterface shape was found in one ampoule (d55)for reasons to be discussed below (Section 6) andthe corresponding results had to be interpretedtaking this effect into account.

The transport rate is inversely proportional tothe distance between the source and the deposit.This distance would remain constant when the freesurface area and the density of the condensate wereall the time equal to that of the evaporating source.For various reasons this is usually not the case ina tube with a constant cross-section. The majorreason is that the condensate is always an agglom-erate of crystallites with size and location depend-ing on the distribution of the actual supercoolingwithin the deposition zone. At low supercoolings,loosely located and sizable crystallites are formedwhereas small densely packed crystallites prevail athigh supercoolings. In order to mitigate this vari-ation, the ampoule geometry shown in Fig. 1 wasused in the DCMF experiment. The source mate-rial evaporates in a narrow cylindrical part of theampoule and condenses on a flat glass plate (here-after referred to as the sink) at the opposite am-poule end. Since the area of the sink (154 mm2) isfour times larger than that of the cross-section ofthe source tube (38.5 mm2), the changes in the posi-tion of the condensate interface are four timessmaller than those of the evaporating interface andcan be neglected. This arrangement also ensuresa minimum thermal gradient across the deposit, nodiffusive limitations in the vapor supply to the sinkand more homogeneous thickness distribution ofthe deposit along the sink.

3. Design of the DCMF apparatus

In the preliminary phase of the laboratory testsat the ETH Zurich, in which the feasibility of themethod had to be proven, cylindrical ampouleswith a constant diameter were used in a simpletwo-zone, thermostate-controlled furnace and �

0

was measured by means of a 4000-element linearCCD array [3]. The temperature stability of$0.03 K and a linear resolution of better than10 lm have been demonstrated. The major chal-lenge in accommodating this well working labora-tory method to the space conditions was to achievean exact reproducibility of the thermal conditionsbetween the earth and microgravity environmentkeeping the temperature accuracy and stability aswell as the linear resolution close to the level reach-ed in the laboratory apparatus.

The DCMF space apparatus has been designedfor a standard get away special (GAS) containerused in the Space Shuttle flights by NASA forautonomous experiments on the open cargo bay.The engineering design and manufacturing wasdone by the Space Division of the Oerlikon-Con-traves AG (Zurich, Switzerland). The apparatusconsists of six single experiment (SE) units. Thecross-section of a SE unit is shown in Fig. 2a andthe overall view of the DCMF/GAS in Fig. 2b. Theampoule with mercuric iodide is fixed at its ends ina furnace with thin semi-transparent film heaters.The furnace has four independently controlledheating zones (Fig. 3) with integrated temperaturesensors. An additional passive sensor (sensor5 — thin film PT-100) is attached directly to theampoule at the source, another (sensor 6) is embed-ded into the centre of the sink plate. These twosensors monitor the actual temperature at theselocations which is slightly different from thetemperature set at the corresponding heaters (seeSection 4).

The measurement of the interface velocity wasdone by means of a high-resolution (2048 pixels)CCD camera (EG&G) at the bottom of each SEunit. A dedicated algorithm has been developed fora high-resolution edge recognition of the interface($3 lm within the view field of 16 mm). The evap-orating interface was illuminated with a flash lampthrough one of the two prisms (Fig. 2) and its imagetransferred to the CCD camera through the otherone. One measurement was done every minute.After 6 min an average and standard deviationwere calculated and stored by an on-board com-puter together with the maximum/minimum valuesas well as plenty of housekeeping data (date/time,temperature at all the furnace sensors, power

82 M. Piechotka et al. / Journal of Crystal Growth 193 (1998) 80–89

Fig. 2. (a) Cross-section of a single experiment (SE) unit of the DCMF (the getter rod is not visible in this view). (b) Six SE units wereplaced in a get away special during the space shuttle flight in May 1996.

Fig. 3. Arrangement of the heating zones around the ampoulewith mercuric iodide in a SE unit. The active sensors (S

1—S

4) for

the temperature control are integrated into the heaters. Twoadditional temperature sensors monitor the temperature of thesource material (S

5) and of the deposit (S

6).

consumption, temperature at various locations ofGAS, etc.).

4. Thermal performance of the space apparatus

One of the major difficulties in the design ofthe DCMF originated from the anticipated strongvariation in the environment temperature duringthe space shuttle flights (several tens of K in the


Fig. 4. Temperature and power record during a single measurement run of the DCMF (microgravity experiment in this example).¹

1—¹

6denote temperatures measured at sensors S

1—S

6of Fig. 3.

worst case) and the severe power limitations of anautonomous experiment.

Loss of static vacuum in the furnace leading toa prompt deterioration of the vacuum thermal in-sulation and increased power consumption wasexperienced in the past space experiments on phys-ical vapor transport of organic solids (PVTOS).The loss of thermal insulation started to be signifi-cant when the pressure in the furnace exceeded0.65 mbar [5]. Thus, each of the DCMF furnaceswas surrounded by semitransparent radiationshields and operated under dynamic vacuum ofbetter than 0.01 mbar maintained by a getter rod(ST172, SAES Getters SA, Italy) integrated intoeach SE. Due to a very careful choice of the con-struction materials (minimum outgasing) and elim-ination of leak sources, this vacuum level could bekept for more than three months and resulted ina very good reproducibility of the thermal condi-tions on earth and in microgravity. Temperatureresolution of 0.02 K at absolute accuracy of betterthan$0.1 K was routinely achieved.

The temperature distribution along a DCMFampoule is shown in Fig. 1. The source evaporateswithin an isothermal (¹

4$0.1 K) zone. There is

a small temperature hump (¹4#2.0 K) in order to

prevent condensation of the vapor at the placewhere the ampoule diameter extends from 7 to15 mm. Supercooling, i.e. the temperature drop*¹"¹

4!¹

#, is imposed at the sink plate. In

a measurement run, ¹4

is kept constant whereas¹

#decreases in several steps from 1.5 to 16 K.

Fig. 4 shows the temperature and power record ina single measurement run. It is seen that the actualtemperature of the source ¹

4(sensor ¹

5) is slightly

lower than the setting (¹1) at the source heater and

that there is a radial temperature gradient along thesink plate (¹

6!¹

4). For this reason the actual

supercooling *¹ is calculated from the difference*¹"¹

5!¹

6rather than *¹@"¹

1!¹

4(super-

cooling setting).Fig. 4 also reveals appreciable changes in power

supply during the run. The power supply level wasused as an indicator of the vacuum level within


Table 1Comparison of the thermal conditions during the DCMF runs (run A on earth prior to the flight, run B in microgravity, runC post-flight on earth). x stays for (¹

1—¹

4). “Vac” denotes vacuum ampoules and “Ar” the ampoules, to which 3 mbar of argon was

added. For the location of the temperature sensors see Fig. 3

Ampoule Parameter Earth (A) Space (B) Earth (C)

51 vac ¹5

0.2 0 0.255 vac ¹

5!0.1 0 0

67 vac ¹5

!0.2 0 0.05

52 Ar ¹5

!0.3 0 !0.256 Ar ¹

50.1 0 0.1

57 Ar ¹5

0.2 0 0.15

51 vac (¹5!¹

6)"f (¹

1!¹

4) 1.02 x!3.2 1.02x!1.3 1.02x!1.2

55 vac (¹5!¹

6)"f (¹

1!¹

4) 1.03x!1.9 1.03x!1.9 1.03x!1.9

67 vac (¹5!¹

6)"f (¹

1!¹

4) 1.03x!1.8 1.00x!1.2 1.03x!1.5

52 Ar (¹5!¹

6)"f (¹

1!¹

4) 1.03x!1.6 1.03x!1.5 1.03x!1.3

56 Ar (¹5!¹

6)"f (¹

1!¹

4) 1.03x!2.2 1.01x!2.2 1.04x!2.3

57 Ar (¹5!¹

6)"f (¹

1!¹

4) 1.03x!1.8 1.00x!1.8 1.04x!2.1

51 vac (¹6!¹

4)"f (¹

1!¹

4) 3.173!0.029x 0.916!0.008x 0.790!0.014x

55 vac (¹6!¹

4)"f (¹

1!¹

4) 1.311!0.021x 1.267!0.018x 1.257!0.024x

67 vac (¹6!¹

4)"f (¹

1!¹

4) 0.995!0.018x 0.913!0.015x 0.900!0.019x

52 Ar (¹6!¹

4)"f (¹

1!¹

4) 1.125!0.024x 1.289!0.019x 0.979!0.021x

56 Ar (¹6!¹

4)"f (¹

1!¹

4) 1.524!0.022x 1.648!0.018x 1.541!0.024x

57 Ar (¹6!¹

4)"f (¹

1!¹

4) 0.778!0.019x 1.023!0.014x 0.868!0.019x

each SE unit under given temperature conditions.However, the relatively large fluctuations seen inFig. 4 were specific for the space experiment andare the result of a strong variation ($4 K) in theenvironment temperature while the space shuttlecircled the earth. On earth, much lower changes inthe power consumption were observed and corre-lated only with the changes in the supercoolingsetting. A post-flight measurement of the pressurein all SE units confirmed a vacuum level of severalmicrobars (at room temperature) which was suffi-cient to maintain a good thermal insulation duringthe flight experiment. It is worth noting, that thisvacuum level was found five months after theDCMF had been shipped to NASA which indicatesthe excellent efficiency of the getter. Thus, getterpumping can be used with advantage in all spaceexperiments where application of mechanicalpumps is not possible due to a very compact con-struction of the unit.

Table 1 shows a comparison of the thermal con-ditions in three consecutive measurement runsdone with six ampoules, three of them containingonly mercuric iodide (hereafter referred to as“vacuum ampoules”), the other three 3 mbar ofargon in addition to mercuric iodide (hereaftercalled “argon ampoules”). Three parameters arecompared: (1) actual temperature of the sourcezone (¹

5), (2) relationship between the actual and

apparent supercooling, (¹5!¹

6)"f (¹

1!¹

4),

and (3) relationship between the radial temper-ature gradient within the sink and the apparentsupercooling, (¹

6!¹

4)"f (¹

1!¹

4). Even

though the setting ¹1

in the source zone was keptstrictly the same in all the runs, ¹

5slightly changed.

Since the largest difference between earth andmicrogravity did not exceed 0.3 K and the changesdid not correlate with the changes in the interfacevelocity, their effect is negligibly small and one cansafely assume that the actual temperature of the


evaporating interface was the same in all theruns. The same applies to parameters (2) and (3) forall but one ampoule (vacuum ampoule d51) wherethe actual supercooling in the first measurementrun differed significantly (by 2 K) from that in thetwo subsequent runs. As one can infer fromTable 1, the reason for this disparity was a changein the temperature at the centre of the sink plate(¹

6) whose origin is unclear at present. Thus, this

ampoule has been neglected in the final discussionof the results even though the corresponding velo-city data follow the trend found in all the otherampoules.

5. Procedure for filling the ampoules with mercuriciodide

High-quality mercuric iodide crystals are grownunder possibly pure and stoichiometric conditionsand one is interested in examining the convectivecontributions under such conditions. Creation ofa highly pure and stoichiometric growth environ-ment is a very challenging task in the case of mer-curic iodide due to the strong relationship betweenits purity (hydrocarbons, hereafter HC) and non-stoichiometry [6]. The method for the synthesisand purification developed at the ETH Zurich [7]ensures the highest degree of purity and stoichio-metry of the material. This method has further beenmodified to meet the requirements of the DCMFexperiment. The basic idea of the method is chem-ical purification of the elements from HC prior tothe synthesis of mercuric iodide, which then pro-ceeds in the gas phase. The intermediate mercuriciodide product is compensated for the excess ofmercury (which is inherently present due to thekinetics of the synthesis processes), multiply sub-limed in a HC-free vacuum environment and sub-limed directly into the DCMF ampoules. Fig. 5shows the filling procedure for the DCMF experi-ment. In step 1, all the six DCMF ampoules areattached to a HC-free pump station (membranepump for rough vacuum combined with a magneti-cally beared turbomolecular pump with a coldtrap) together with two larger sealed glass am-poules containing mercuric iodide. The DCMFampoules are then outbacked at the pressure of

around 10~8 mbar at ¹"165°C for 4 days andsealed off. In the next step, this glass setup (approx-imately 2 m long) is placed in a multizone furnaceand the final refinement of mercuric iodide takesplace by sublimation from the first into the secondampoule, the former being sealed off upon comple-tion of this step. In step 3, sublimation into thelower three DCMF ampoules takes place. Due tothe small diameter of the ampoules, the long vaportransport path and temperature limitations(¹)150°C) it takes about two weeks to sublimeapproximately 14 g of the material into each ofthese three ampoules simultaneously. This very lowsublimation rate was very probably the reason fora much better separation of impurities, as can bededuced from the results of run A (compare Sec-tion 6). After the completion of the sublimation andsealing off of the first three vacuum ampoules, theremaining ampoules are connected to the pumpstation, again evacuated down to 10~8 mbar, filledwith 3 mbar of argon at room temperature andsealed off. Now the sublimation of mercuric iodideproceeds into the remaining three DCMF am-poules which takes 3—4 weeks due to the decreasedtransport rate in the presence of argon. In the finalstep the ampoules are sealed off.

After the filling process, the entire mercuric iod-ide sublimated into the DCMF ampoules is locatedwithin the broader part of the ampoules (the futuresink zone during the measurement runs). For thetransport rate measurements, the material has to betransported into the source zone and to condensethere as a compact ingot with a well-defined inter-face. This is achieved using a very steep tem-perature gradient (approximately 80 K/cm) androtation/pulling of the ampoule. Since there is onlya poor control of the total amount of the materialsublimated into the DCMF ampoules during thefilling process, the interface of the formed ingotusually lies above the reference mark and somematerial has to be transported back to the sink inthe final step (pre-evaporation) in order to opena measurable gap for the CCD-array. This finalstep is made in the same laboratory furnace as thepre-evaporation.

This procedure for filling the DCMF ampoulesprocedure is very time-consuming (up to threemonths) but results in an exceptionally pure and


Fig. 5. Schematic diagram showing the filling procedure of the DCMF ampoules. (1) Ampoule with mercuric iodide charge; (2) am-poule for the final refinement of mercuric iodide; (3) connector to the pump station; (4) break-seal; (5, 6) DCMF ampoules. For the sakeof simplicity, only four instead of six DCMF ampoules are shown.

stoichiometric material as evidenced by the veryhigh transport rates never achieved with mercuriciodide up to now.

6. Interface shape

Unfortunately, due to technical problems result-ing in a very tight schedule of the DCMF deliveryto NASA, there was no time to check the interfaceshape just before the flight, i.e. after the firstmeasurement run on earth (run A). Three weekswould be necessary for disassembling and re-as-sembling of the six single experiment units forvisual inspection. On the other hand, the X-raytransmission tomography, which has been usedafter runs B (microgravity) and C (post-flight onearth) to visualise the interfaces of mercuric iodidein situ (see Part II), was considered as too risky forthe electronic circuits of DCMF to be done prior tothe flight experiment.

X-ray tomography done after the flight (run B)revealed, however, that a change in the interfaceshape of the source material took place in someampoules.

Fig. 6 shows a photograph of the vacuum am-poule d55 after the final pre-evaporation step andjust before it has been mounted into the DCMF.This nearly flat source interface turned into anirregular cone in the course of the DCMF measure-ments. The explanation for this change is to besought in the very high interface velocity (�

0"7]

10~6 cm/s at *¹"16 K in run A) reached bymercuric iodide in this ampoule. At this highvelocity, limitations in the radial heat transfer setin due to the appreciable heat of evaporation ofmercuric iodide (110.4 kJ/mol [8]) combined withthe extremely low heat conductivity (0.1/0.4 W/m K depending on the crystallographic orienta-tion [9]) of the solid. As a result, the evaporationrate at the ampoule wall is higher than thatalong the ampoule axis and a conical interface


Fig. 6. DCMF ampoule d55 after filling and pre-evaporation of mercuric iodide.

develops. This phenomenon is more pronouncedin furnaces with radiation shields, such as the flightDCMF furnace. Since there is variation in bothinterface velocity �

Rand flux density J

Ralong the

ampoule radius when the evaporating interfacechanges its shape, J cannot be calculated fromEqs. (1) and (2) using the values �

0measured solely

along the ampoule axis. However, as it has beenevidenced by X-ray tomography in Part II, afterthe interface adjusted to the thermal conditionsof the DCMF furnace in run A, no further signifi-cant changes occurred in the two subsequent runs(with �

0+4]10~6 cm/s at maximum). Thus, the

flux could still be calculated from Eq. (2) for runsB and C.

This extreme case was found only in this particu-lar DCMF ampoule. In all other cases either noneor negligibly small curvature of the interface de-veloped.

7. Summary

A sophisticated apparatus has been developedfor fully automated in situ measurements of thevapor transport rate in PVT of mercuric iodideboth on earth and space. Due to a very goodreproducibility of the thermal conditions as well asstrictly controlled filling and source formation pro-cedure a reliable comparison of the transport ratein microgravity and on earth could be made for thefirst time.

Acknowledgements

The DCMF project was financially supported bythe PRODEX of the Swiss Bundesamt fur Bildungund Wissenschaft (BBW) and the European SpaceAgency (ESA). The contribution of several ESA


officers, especially Drs. P. Behrmann, O. Minster,W. Riesselmann, M. Chofflet, H.U. Walter and G.Seibert are acknowledged with great pleasure.Special thanks are owed to Mr. B. Nussberger whoexcellently did a lot of the very difficult glassblowery work. Thanks are also due to Dr. A. Ross-berg for performing a part of the laboratory tests inthe breadboard development phase.

References

[1] E. Kaldis, Materials science on board Spacelab-1. An over-view, in: Materials Science under Microgravity, ESA SP-222, 1984, p. XVII.

[2] E. Kaldis, Proposal to BBW for measurements of the appar-ent diffusion coefficient of HgI

2vapours, 1987.

[3] M. Piechotka, E. Kaldis, P. Behrmann, H. Stanna, Proc. 7thInt. Symp. on Materials and Fluid Sciences in Microgravity,Oxford, UK, 10—15 September 1989, ESA SP-295, 1990, p.587.

[4] M. Piechotka, E. Kaldis, A. Rossberg, E. Wetzel, Technicalassistance in DCMF flight preparation and vapour growthstudies, ESTEC Contract No. 10989/94/NL/JS, Final Re-port, ETH Zurich, Zurich, 1995, vol. 1A, p. 14.

[5] M.K. Debe, E.L. Cook, R.J. Poirier, G.J. Follet, L.R. Miller,S.M. Spiering, J. Vac. Sci. Technol. A 5 (1987) 2406.

[6] M. Piechotka, E. Kaldis, Monatsh. Chem. 118 (1987)137.

[7] M. Piechotka, Mater. Sci. Eng. Rep. R 18 (1997) 1.[8] M. Piechotka, E. Kaldis, J. Less Common Met. 115 (1986)

315.[9] A. Burger, S.H. Morgan, E. Silberman, D. Nason,

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