Described is an attempt to determine critical wetting temperatures in monotectic systems and to investigate the wetting phase on container walls and the phase preferential wetting of a monotectic solid

Chang, K.

English

NASA Technical Reports Server

1982
NASA/ASEE SUIv_ER FACULTY RESEARCH FELLOWSHIP PROGRA_i
I_RSHALL SPACE FLIGHT CENTER
THE UNIVERSITY OF ALABA_A
SOLIDIFICATION STUDIES
OF r_0NOTECTIC SYSTEI,iS
Prepared by, Kichoon Chang, Ph.D.
Academic Rank, Associate Professor
University and Department, Alabama A&M University
Department of Chemistry
NASA/MSFC
Laboratory s
Divisions
Branch,
Space Science Laboratory
Space Processing
Solid State
MSFC Counterparts Donald O. Frazier, Ph.D.
Dates August 13, 1982
Contract No., NGT-Ol-O02-099
University of Alabama
XII-i
https://ntrs.nasa.gov/search.jsp?R=19830009100 2020-03-22T00:15:20+00:00Z
SOLIDIFICATION STUDIES
OF MONOTECTIC SYSTE_,S
By
Kichoon Chang
Associate Professor of Chemistry
Alabama A&M University
Normal, Alabama 35762
ABSTRACT
It has been suggested that at low or zero gravity critical
point wetting and thermal migration of second-phase droplets
due to interfacial tension gradient play major roles in phase
separation and solidification of a monotectic system.
Understanding of these roles requires estimation of inter-
facial tensions. However, solid-liquid interfacial tensions
are very difficult to measure. Ellipsometr_c techniques are
being investigated to measure liquid film thickness as a means
of determining the solld-liquid interfacial tensions at var-
temperatures and pressures.
Xll-ii
INTRODUCTION
Investigation of monotectlc and related systems has been
of considerable interest in recent years. However, one of
the first relatively detailed studies of a monotectic sys-
tem I, succlnonitrile - water, was reported in 1897.
In the last decade metallic monotectic systems have been
of renewed concern within NASA in view of recent space
processing experiment. 2 Currently a number of immiscible
organic binary mixtures are under investigation as models
for metallic pseudo binary systems.
In a monotectic system two component liquids are miscible
in any composition above an upper critical solution tempera-
ture. Below the critical temperature and between certain
composition limits two immiscible liquid phases are produced,
creating a miscibility gap. All such miscibility gaps are
two-phase regions. The two liquid phases generally differ
in density and separate by Stokes settling in a gravity
field.
At the monotectic temperature one of the liquid phases
decomposes into a solid phase and the other liquid phase.
In all known monotectlc phase diagrams the amount of the
solid phase produced is always greater than that of the
liquid phase produced. 3 This liquid phase solidifies at the
lower eutectic temperature.
XlI-I
A number of theories have been developed for explaining
the separation and solidification phenomena of binary systems.
The motion of bubbles in a temperature gradient in pure
liquids and binary liquids and Marangoni convection have
been studied by a number of workers. The velocity 4 of the
bubble was related to both the thermal variation of the sur-
face tension and the temperature gradient along which the
bubbles moved.
It was shown 5 that near the critical point one of the
critical phases completely wets the third phase and that at
some temperature below this point perfect wetting terminates
in a first-order transition of the surface.
If the third phase is a container wall which is wetted
preferentially by the minority phase, the phase separation
by the spreading minority phase, as the temperature is low-
ered, may be avoided if a crucible which is primarily wetted
by the majority phase is chosen. On the other hand, if the
majority phase begins to form solid at the monotectic tem-
perature, it will be preferentially wetted by the liquid of
the phase because of their closeness in composition.
Cahn 6 reports that perfect wetting of the monotectic solid
by the monotectic liquid occurs whenever the monotectic tem-
perature is close to the critical temperature.
In a study of monotectic systems including Al-In, Cu-Pb,
Cd-Ga and succinonitrile-water, Grugel and Hellawell 7 showed
that the solidification behavior of systems containing a
XII-2
liquid miscibility gap appeared to be related to the height
of the gap. They even modified the microstructure of the
solids by changing the height of the miscibility gap.
In certain binary solutions the lower of the two liquid
phases forms a layer which intrudes between the upper liquid
8
phase and the vapor. The thickness of the intruding wet-
ting layer was ellipsometrically measured and reported.
Pohl and Goldburg 9 verified experimentally a predicted
transition 5 from complete to partial wetting at a tempera-
ture away from the consolute temperature.
The Gibbs surface excess measurement for two miscibility
gap systems, ethyl sallcylate - diethylene glycol and hexane-
aniline, was reported by NASA workers, l0 Their measurement
of the surface excess by use of the Gibbs adsorption equation
led to the conclusion that the surface excess increases as
the binary solutions approach the conditions of the consolute
point. They also observed that near the consolute temperature
the surface-active components are switched if the two pure
components have a small surface tension difference. The
component switching will apparently affect the thickness of
the intruding layer and the liquid film thickness on the con-
tainer wall because of different molecular size and different
vapor pressure.
XII-3
RECENTWORKWITH MODELMATERIALS
Succinonitrile is a plastic crystal with a small entropy
of fusion and freezes llke metals, crystallizing with its
molecules in random orientations. The crystal grows dendri-
tically into undercooled melt like metals which have small
ll,12
entropies of fusion.
A monotectic system of succinonitrile and water mixture
has a convenient temperature range to work with for its
solution and solidification. Some of the properties includ-
1,11,13ing the phase diagram have been relatively well established.
The density difference between the two phases in the mis-
cibility gap is small and can be further minimized by adjust-
ing the amount of H20 and D20 to reduce the gravity effect
on the phase separation and solidification. Thus the system
continues to draw interest within NASA as a model for pseudo
metallic systems for investigating composite growth.
Diethylene glycol - ethyl salicylate system has also been
l0
studied for excess quantities measurement by NASA workers.
The same workers further investigated the miscibility gap
system for particle growth and thermal migration by means
14
of holographic techniques.
A number of other systems with miscibility _ap and metal-
like transparent organic compounds have been studied. 2'7'12'1_
Xll-4
CRITICAL-POINT WETTINGAND SPREADING
In solidification of monotectic systems in the absence
of gravitation the critical-point wetting 5 and thermal migra-
tion of droplets 4 appear to play major roles.
Generally, the surface free energy of the system is des-
cribed by Young's equation
for two immiscible liquids LI and L 2 in contact with a non-
deforming solid surface of the container. The contact angle
@ is between the liquld-liquid interface and the solid sur-
face. For all values of 8
In a temperature region below the critical temperature
the contact angle vanishes as the critical temperature is
reached. 5 One of the liquid phases, say, L I intrudes as a
wetting phase between the solid phase S and liquid phase L 2.
At the critical temperature, 8 = 0° and a perfect wetting
of the solid by liquid L I occurs. This perfect wetting may
be expressed in terms of the spreading coefficient of liquid L I
over solid S
spreading coefficient = _ _ _, -_,_z
which is greater than zero for a spontaneous spreading.
Thus, for the perfect wetting
XII-5
and Young's equation no longer holds and the contact angle
becomes indeterminate. The inequality sign has reversed.
As solidification occurs at the monotectic temperature
the solid S1 will be preferentially wetted by liquid LI,
resulting in a perfect wetting. For this situation, again
Under these conditions liquid L2 is a nonwetting phase and
monotectic composite can not be grown unless the growth
6
velocity is large enough to overcome the disjoining pressure.
However, Grugel and Hellawell 7 suggested that if the cri-
tical wetting temperature T w is above the monotectic tem-
perature Tm it may be possible to have steady-state composite
growth with solid SI being wetted also by liquid L 2. On
the other hand, with Tw lower than Tm for low miscibility
gap systems, liquid L 2 is separated due to the preferential
wetting of solid SI by liquid L I and high growth velocities
become necessary for composite growth.
XII-6
OBJECTIVE
As previously discussed, Cahn's theory 5 predicts that when
two fluid phases are near a critical point one of them must
completely wet any third phase, excluding the other fluid
from contact with the third phase. If the majority phase
wets the container preferentially the system is in stable
condition. Phase separation could then primarily result from
droplet migration due to temperature gradient effect on sur-
face tension.
On the other hand, if the minority phase wets the third
phase, the system is unstable and the minority phase drop-
lets will spread over the entire surface. In either case,
phase separation follows. Whether or not preferential or
perfect wetting of the monotectic solid phase S 1 occurs
when a monotectic reaction takes place is to be determined
by the location of the critical wetting temperature. 7
Thus, an attempt is being made to determine critical wet-
ting temperatures in monotectlc systems and to investigate
the wetting phase on the container walls and the phase pre-
ferentially wetting the monotectic solid. This will eventu-
ally lead to an examination of Young's equation in thse
critical wetting regions.
XII-7
PROCEDURES
Critical wetting occurs when the surface energies of the
three phases, two fluids and a solid, are balanced by the
equation, L_L _ _--_
for which the contact angle
proached zero.
of Young's equation has ap-
Measurement of _,L_ may be made by using a ring method
but the surface energies of the solid, _ and _ , can
not be measured directly.
An alternative is to estimate the surface energies from
liquid film thickness X on the surface in question.
If the surface excess of the vapor adsorbed from liquid
L1 or L 2 at a given temperature is substituted in the form
of an ideal gas in the Gibbs adsorption equation and inte-
grated over the vapor pressure range, the film pressure 77-
is obtained, _'PF Z. X(P)d .f
where _ is the surface free energy of the solid, v° the
molar volume of the vapor adsorbed at the standard tempera-
ture and pressure and _ the toal surface area of the ad-
sorbing solid.
Integration of X(p) over the vapor pressure range gives
_S - _" ". By repeating this process at a _iven temperature
')V
XII-8
for both liquid phases LI and L2 of the miscibility gap and
subtracting one film pressure from the other, the surface
tension of the solid (_) is eliminated but the desired
quantity, _L_- _,' which is indeed _-_ in the experi-
ment because of the equilibrium between the liquid and its
vapor, is obtained. With this value together with the meas-
ured quantity z_,L_, the contact angle _ of Young's equa-
tion can be calculated at the given temperature.
The entire process can be repeated at different tempera-
tures and the results are extrapolated to estimate the cri-
tical-point wetting temperature at which
The main task for integrating the equation to obtain the
film pressure is in determination of the liquid film thick-
ness as a function of vapor pressure at different temperatures.
Surface Viscosity Method
Migration of gas bubbles or liquid droplets in a liquid
4
phase is retarded by the viscosity of the liquid. On the
other hand, the film pressure T[ was shown to be related
to the surface viscosity _S by the equation 15
S
where _ is the surface viscosity with the activation en-
ergy for flow at zero pressure and c is a constant. The
accuracy of this equation was confirmed by the experimental
xIZ-9
work on surface films. 16 Thus a well established relationship
of the film pressure with the surface viscosity in terms of
experimental data will be useful in predicting one quantity
from the other.
More recently 17 it was shown for polymers that the thick-
ness adsorbed on the liquid-solid interface is proportional
to the intrinsic viscosity over a wide range of thickness in
theta solvents such as benzene, cyclohexane and methylethyl
ketone.
If the surface viscosity Vs is known, the film thickness
X may be calculated from the bulk viscosity _ by the equa-
tion _s
×=
which is a convenient relationship for experimental purposes
if surface viscosity can be measured without difficulty.
The measurement of surface viscosity is subject to subs-
tantial sensitivity limit. The surface canal method, though
more sensitive (_I0 -5 surface poise) than others such as
oscillating pendulum and rotational methods, is also limited
to surface concentrations which give appreciable film pres-
sures. Furthermore, the method, when applied to soluble ad-
sorbed films, suffers from the disadvantage that the film
pressure causes desorption and adsorption as well as sub-
phase drag.
Davies and Mayers 18 developed a circular viscous traction
XII-IO
viscometer which can be used for interracial viscosity meas-
urement with two circular rings held concentrically in the
interface. They were able to measure the surface viscosity
to an accuracy of 1 x 10 -4 surface poise.
Ellipsometric Method
Ellipsometry is sensitive and can detect film thickness
0 0
as thin as 0.i A and up to tens of thousands of A. The tech-
nique has been used for measuring oxide films and adsorption
of gases and liquids. 19'20
The basis of ellipsometry is the fact that the state of
polarization of a light beam is altered upon reflection from
a bare or film-covered surface. When the light reflects from
the specimen, the polarization of the light changes in accor-
dance with the specimen film thickness, the optical charac-
teristics of the film and the surface it is deposited on.
In actual use of the ellipsometer, the compensator (a
quarter wave plate) is set at a fixed angle and the polarizer
and analyzer circles are rotated until the reflected beam
from the specimen is extinguished.
From the angular settings of the polarizer and analyzer
at this condition two ellipsometric parameters required to
determine the state of polarization are obtained, The am-
plitude ratio and the phase difference. These parameters are
then used to determine the film thickness and refractive in-
dex of the film by using graphs, tables or computer programs
XII-II
provided the ootical constants of the substrate are known.
The refractive index is independent of the film thickness.
The computer programs are available at the Marshall Space
Center for calculating the optical constants (real and ima-
ginary values of the refractive index) and the film thick-
ness.
8
Kwon et al. used ellipsometry to measure the thickness
of the intruding layer of liquid with He-Ne laser at several
temperatures for certain solutions. The result was as pre-
dicted by de Gennes' long-range intermolecular potential. 21
In our preliminary investigation a cell of pyrex glass
was designed. Cells with specific angles 27 will be designed
and tested for maximum sensitivity in the thickness measure-
ment for monotectic systems.
XII-12
CURRENTINVESTIGATION
A pyrex glass cell was built and is being tested for
ellipsometric measurement. Several cells will be designed
with specific angles for maximum sensitivity and the in-
vestigation will be continued.
On the other hand, each layer of succinonitrile-water
miscibility gap is being analyzed for vapor composition by
slow distillation and condensation in a specially designed
cell below the critical temperature.
The result of this investigation and subsequent experi-
ments with monotectic systems will be reported when signi-
ficant amount of data are obtained and analyzed.
XII-13
REFERENCES
1. Schreinemakers, F. H. A., Zeitschr. f. Physik. Chemie,
Leipzig, 23, 417(1897).
2. Ang, C. Y. and Lacy, L. L., "Monotectic and Synthetic
Alloys, Experiment _IA-044," ASTP Preliminary Technical
Report, NASA TMX-64956, Sept., 1975! ibid, AIAA Progress
in Astronautics and Aeronautics, 52, 523(1977)I Lacy, L.
L. and Ang, C. Y., "A_oollo-Sovuz Test Project Summary
Science Report," I, NASA SP-412, 403(1977); Gelles, S. H.
and Markworth, A. J., "Space Processing Applications
Rocket Project, SPAR II Final Report," NASA TM78125,
Nov., 1977.
3. Chadwick, G. A., "Metallography of Phase Transformations,"
Crane, Russak & Co., Inc., New York. Chapter 4.
4. Young, N. 0., Goldstein, J. S. and Block, M., "The Motion
of Bubbles in a Veritcal Temperature Gradlent,"J. Fluid
Mech., 6, 350(1959).
5. Cahn, J. W., "Critical Point Wetting," J. Chem. Phys.,
666,3667(1977).
6. Cahn, J. W., "Monotectic Composite Growth," Met. Trans.
&, lOA, 119(1979).
Grugel, Ro N. and Hellawell, A., "Alloy Solidification
in Systems Containing a Liquid _iscibility Gap," Eet.
Trans. A, 12A, 669(1981).
Kwon, 0'D. et al, "Thickness of the Liquid-Vapor Wetting
Layer," Phys. Rev. Lett., 48, 185(1982).
Pohl, D. W. and Goldburg, W. I., "Wetting Transition in
" Phys. Rev. Lett , 48, llllLutidine - Water Mixtures,
(1982).
.
.
.
XIl-14
i0. Nishioka, G. M., Lacy, L, L. and Facemire, B. R., "The
Gibbs Surface Excess in Binary Miscibility Gap Systems,"
J, Colloid and Interface Sci., 80, 197(1981).
ii. Glickman, M. E., Schaefer, R. J. and Ayers, J. D.,
"Dendritic Growth - A Test of Theory," Met. Trans. A,
7A, 1  7(1976).
12. Jackson, K. A. and Hunt, J. D., "Transparent Compounds
That Freeze Like Metals," ACTA METALLURGICA, i_/, 1212
(1965), and references cited therein.
13. Gllckman, M. E. and Huang, S. C., "Convective Heat Trans-
fer During Dendritic Solidification," AIAA 16th Aerospace
Sciences Meeting, Huntsville, Ala., Jan., 1978, AIAA
Paper 78-220.
14. Lacy, L. L., Witherow, W. K., Facemlre, B. R., and Nishi-
oka, G. M., "Optical Studies of Model Binary Miscibility
Gap Systems," to be published.
15. Glasstone, S., Laidler, K. J. and Eyring, H., Theory of
Rate Processes, McGraw-Hill Book Co., Inc. 1941. p. 512.
16. Boyd, E. and Harkins, W. D., "Molecular Interactions in
Monolayers, Viscosity of Two-Dimensional Liquids and
" J Am. Chem. Soc., 61, 1188(1939).Plastic Solids, .
17. Chemistry and Physics of Interfaces, based on the Sympo-
sium on Interfaces, June, 1964, American Chemlcal Society
Publications, "Structure of Macromolecules as Liquid-
Solid Interfaces."
18. Davies, J. T. and Mayers, G. R. A., "Studies on the In-
terfacial Viscosities of Monolayers," Trans. Faraday Soc.,
56, 691(1960).
19. Archer, R. J., "Determination of the Properties of Films
on Silicon by the Method of Ellipsometry," J. Opt. Soc.
A___m.,52, 970(1962).
XII-15
20. ibid, "Measurement of the Physical Adsorption of Vapors
and the Chemisorption of Oxygen on Silicon by the Method
of Ellipsometry," Nat. Bur, Std., _isc. Publ, No. 256,
1964. p. 255.
21. De Gennes, P. G., "Some Effects of Long range Forces on
Interfacial Phenomena," J. Phys,-Lett. (Parish, 42,
L-377(1981).
22. McCrackin, F. L. et al., "Measurement of the Thickness
and Refractive Index of Very Thin Films and Optical
Properties of Surface by Ellipsometry," J. Res, Nat.
Bur, Std., 67A, 363(1963).
Xll-16


Solidification studies of monotectic systems

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server