Consideration is given to accurate measurement of self-diffusivities of liquid elements (selected for the classlike structureproperties) and their temperature dependence). The measured diffusivities are interpreted in terms of diffusion mechanismand associated classes of liquid structures. The 'wall effect' believed to contaminate diffusivity measurements in capillaries isinvestigated. The convective contamination of the diffusivity measurements on Earth are characterized through numericalmodeling. The development of an efficient technique for dynamic in situ measurements of diffusivities in melts as a functionof temperature is discussed, as are the design and construction of flight-certified hardware for automated measurements ofdiffusivities at several temperatures per sample. (AIAA).
Franz Rosenberger, R. Michael Banish, J. Iwan D. Alexander, Lyle B. Jalbert and Ricky J. RobersonCenter for Microgravity and Materials Research
University of Alabama in Huntsville, Huntsville, Alabama 35899, USAPhone: (205) 890-6050, e-mail: [email protected]
Background
With increasing insight into transport and segrega-tion in solidification and crystal growth, reliable data fordiffusivities in liquid metals and semiconductor havebecome essential for guidance in process development.However, at this point even se//-diffusion in elementalliquids is not well understood, let alone binary and ternarydiffusion. In particular, there is little insight into thetemperature dependence of diffusivities and its correla-tion to the temperature-dependent liquid structure of anelement. Currently, the differences between several theo-retical predictions are often less than those between dif-ferent sets of data for the same system. Hence, for boththeoretical and technological developments, there is aclear need for diffusivity measurements of improved ac-curacy and precision for a large variety of elements overwide temperature ranges.
The exclusion of convectivc contamination in liquiddiffusion studies is difficult. Measurement techniques,such as nuclear magnetic resonance and inelastic neutronscattering, that probe the rapid molecular motion areinsensitive to convective contributions, but are not asprecise as macroscopic, averaging techniques. However,all macroscopic measurement techniques yield diffusiv-ity data prone to be contaminated by gravity-driven con-vection. The use of narrow capillaries to suppress con-vective transport has revealed poorly understood walleffects. Magnetic fields, widely used for suppressingconvection in conducting liquids, modify the diffusivemotion itself. Earlier liquid metal diffusion studies con-ducted on spacecraft have demonstrated the gain inpreci-sion resulting from the drastic reduction of convection ina low-gravity environment. However, by comparisonwith ground-based experiments, these data appear ratherinaccurate. Hence, there is a need for well defined liquiddiffusion studies under low gravity.
Objectives
The scientific objectives of this research project include the• accurate measurement of self-diffusivities of liquid ele-
ments (selected for their class-like structure proper-ties) and their temperature dependence;
• interpretation of the measured diffusivities in terms ofdiffusion mechanisms and associated classes of liquidstructures;
• investigation of the "wall effect" believed to contami-nate diffusivity measurements in capillaries;
• characterization of convective contamination of thediffusivity measurements on Earth through numericalmodelling (see AIAA 97-0571);
The technical objectives consist of the• development of an efficient technique for dynamic in-
situ measurements of diffusivities in melts as a func-tion of temperature;
• design and construction of flight-certified hardware forautomated measurement of diffusivities at several tem-peratures per sample.
Experiment Concept and Approach
We have developed a novel technique for the mea-surement of diffusivities in liquids. In this approach, asschematically depicted in Fig. la, the cylindrical diffu-sion sample consists of a pure material, with an activatedisotope, initially located at one end, as the diffusant. Thesample is positioned in a concentric isothermally heatedradiation shield, with two bores that act as radiationcollimators. These bores are located at x = L/6 and 5L/6to satisfy the requirement of the numerical scheme used toevaluate the diffusivity [Codastefano et al., Rev. Sci.Instr. 48 (1977) 1650]. After melting of the sample, theintensity of the radiation emitted through the collimatorsis monitored throughout the experiment with solid statedetectors and associated energy discrimination electron-ics. Corresponding signal traces ni(t) and na(t) obtainedat a given temperature are schematically plotted in Fig.Ib. The diffusivity D is calculated from the difference ofthe signals using Codastefano et al.'s relation
In [ni(t) - n2(t)] = Const -(n/L)2 D t .
Since the data are obtained in real time, diffusivities
Copyright €> 1997 by the American Institute of Aeronautics and Astronautics, Inc.All rights reserved.
RadioactiveTracer
RadiationShield
(a)
ni(t) n2(t) Time
Fig. 1. Schematic of real-time diffusivity measurement technique and signal output.
can be consecutively determined at several temperaturesduring the spreading of the concentration profile in onesample. This is advantageous for experiments underreduced gravity, particularly given the limited flight op-portunities and flight time of space craft.
By employing an isotope which emits photons at twosufficiently different energies and, thus, different self-absorption behavior, transport in the bulk of the sampleand near the container wall can be distinguished to some
extent. As an illustration of this approach, Fig. 2 presentsfor two different emission energies the fraction of the totalradiation measured outside a sample of In vs. the thick-ness of the sample. One sees that the 24 keV photonsreceived originate in essence only from a 300 Jim deepsurface layer. The 190 keV photons, on the other hand, stemfrom throughout the sample. Thus, using an appropriatedetector circuit with energy discrimination capability, somedistinction between transport near the wall of the samplecontainer and in the sample bulk can be made in real-time.
Detector/ SampleDiscriminator
0.5 1.0 1.5 2.0 2.5Thickness of Layer Considered [mm]
Fig. 2. Emission depth limitation in indium due self absorption.
Mounting cap
Welded aluminumcartridge
Boron nitride ampoulewith cap
Spring
Boron nitride plug
Radioactive section
Indium sample
1"
1 2 cm
Fig. 4. Cross-section through components of semi-automatic diffusivity measurement apparatus, (a) Cartridge with spring-loaded diffusion sample, (b) Complete payload with 1: Diffusion sample in cartridge inside heated isothermal liner/ radiationshield 2. 3: CdZnTe detector pairs with preamplifier/discriminator circuits 4. 5: Circuit boards for experiment control,programming and data storage. 6: Carousel with 4 additional sample cartridges. 7: Lead pig. 8: Sealed housing.
Hardware Development
In response to an imminent flight opportunity aboardMIR utilizing the Microgravity Isolation Mount (MIM),we have developed and constructed a low temperature(200 °C) version of our wide-temperature-range diffusionexperiment instrumentation to be flown later on either theShuttle or the Spacestation. Measurements to be con-ducted with In/In114m on the MIM in early 1997 underdefined residual accelerations will provide data for com-parison with our numerical modeling results as well asguidance for the planned high temperature experiments.Fig. 4 represents cross-sectional views of (a) the diffusionsample cartridge and (b) the diffusivity measurementapparatus with sample exchange carousel.
Diffusivity Measurements
Fig. 5 presents data obtained with the above lowtemperature apparatus for In/In'14m on Earth. The values
for the diffusivity given in Fig. 5b illustrate the highprecision of this novel diffusivity measurement tech-nique.
Choice of Elements
Based on diffraction data, one can distinguish be-tween elements that are known to or can be expected toundergo structural changes at temperatures above theirmelting points, and elements that do not undergo suchchanges. Such "second liquid structures" occur in ele-ments that are dominantly covalently-bound in their solidform. Diffusivity studies of a few of the elements withsuch transitions have revealed pronounced nonmonotonictemperature dependencies D(T). By analogy, we expectall other elements in this category to show structure-change revealing D(T)'s when measured over a widetemperature range. Hence, as indicated in the followingpartial Periodic Table, we have grouped, for the first time,the relevant elements into those with expected monotonic
16
<N 12
O
(a)
190 keV
1000 2000 3000 4000 5000Time [min]
D190 = (2.215 ± 0.018) 10-5 cm2/s-i
- D24 = (2.24 ± 0.02) TO-5 cm2/s
500 1000 ' 1500 2000Time [min]
Fig. 5. Ground-based results of diffusion measurements with In/In'Mm.
and nonmonotonic D(T) behavior, indicated by open andfull symbols, respectively. In addition to the behavioralpredictions, the table contains the melting and boilingpoints, the radioactive isotope suitable for diffusion stud-ies with their half-life and photon energy. If an isotopepossess two different photon energies, that is also indi-cated. Based on these consider-ations, we plan to studyIn, Na, Cd, Ga, Sn, Te and S.
Acknowledgments
This work has been supported by the Microgravity Science andApplication Division of the National Aeronautics and SpaceAdministration undercontractNASS-39716 and grant NCC8-99, and by the State of Alabama through the Alabama Super-computer Network and the Center for Microgravity and Mate-rials Research at the University of Alabama in Huntsville.
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