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Optical and thermodynamic property measurements of liquid metals and alloys
Adv. Space Res. Vol. 11, No. 7, pp. (7)43—(7)52, 1991 Printed in GreatBritain. All rights reserved.
0273—1177/91 $0.00 + .50 Copyright © 199! COSPAR
OPTICAL AND THERMODYNAMIC PROPERTY MEASUREMENTS OF LIQUID METALS AND ALLOYS J. K. Richard Weber, Shankar Krishnan, RobertA. Schiffman and Paul C. Nordine Intersonics, inc., 3453 CommercialAvenue, Northbrook, IL 60062, US.A.
ABSTRACT Optical properties and spectral emissivities of liquid silicon, titanium, niobium, and zirconium were investigated by HeNe laser polarimetry at A = 632.8 run. The metals were of a high purity and, except for zirconium, clean. The more demanding environmental requirements for eliminating oxide or nitride phases from zirconium were not met. Containerless conditions were achieved by electromagnetic levitation and heating. CO 2 laser beam heating was also used to extend the temperature range for stable levitation and to heat solid silicon to form the metallic liquid phase. Corrections to previously reported calorimetric measurements of the heat capacity of liquid niobium were derived from the measured temperature dependence of its spectral emissivity. Property measurements were obtained for supercooled liquid silicon and supercooling of liquid zirconium was accomplished. The purification of liquid metals and the extension of this work on liquids to the measurement of thermodynamic properties and phase equilibria are discussed. INTRODUCTION
High temperature liquids achieve chemical and mechanical equilibrium rapidly and have smooth surfaces if they are clean. Vaporization of impurities and direct gasification of the condensed phase rather than contamination by residual gases is promoted at the high temperatures required to melt refractory materials. These qualities allow polarimetric measurements of intrinsic optical properties of high temperature liquids. Emissivity corrections to optical pyrometry can thus be determined to achieve accurate thermodynamic temperature measurements in container].ess experiments. The need for pure liquid metals in this optical property research required that container and ambient gas contamination of the specimens be eliminated or controlled. Reaction with containers was eliminated by the use of electromagnetic (EM) levitation and heating. Reaction with residual gas impurities was controlled by heating to sufficient temperatures in order that purification, rather than contamination, of the specimens occurred. It was possible to meet both of these requirements with liquid silicon, titanium, and niobium for which optical properties and spectral emissivities are reported. Research presently in progress on liquid zirconium is discussed and extensions of the work to studies of phase equilibria and thermodynamic properties are described. PURIFICATION AND REACTION WITH RESIDUAL GASES
Residual oxygen and nitrogen may react with metals to form condensed oxides and nitrides. For materials studies here , these phases can be removed at sufficiently high temperatures, by oxide vaporization and nitride decomposition reactions. Mass balances determine the conditions under which it is practical to purify the metals in this way. 3 liquid metal at 2000K that for also contains mole % nitrogen as of the0.1 metal nitride. The HertzConsider, example, a 11 cm diameter sphere mole/cm Knudsen equation yields the maximum rate of nitrogen gas loss from the specimen: n(N
t0”mole/hr 3),~, = 95,000 P(N2)/T where p(N 2) is the equilibrium nitrogen pressure, metal/metal nitride mixture. (7)43
(1) in atmospheres,
over the
J. K. R. Weber eta!.
(7)44
If p(N 2) = 5x10’ atm., at least one hour would be required to remove the nitride. Smaller loss rates would result from external gas transport limitations, nonunit nitride vaporization coefficients, and the nitride did not fully cover the liquid metal surface. In this example, partial pressures of reactive residual gases on the order of 10’ atm. are sufficient, and equilibrium vapor species pressures on the order of 106 atm. are necessary, to achieve containerless purification of liquid metals at useful rates. These pressures would be proportional in other cases to the initial contaminant concentration. Table 1 presents the temperatures at which volatile oxide or nitrogen pressures equal to 10’ atm occur for the metal—oxygen and metal—nitrogen systems relevant to the present work. The vapor pressures of the metals at this temperature and the melting temperatures of the metals are given. These calculations were based on a model in which the metal was assumed to dissolve no oxygen or nitrogen and to exist in equilibrium with the most reduced oxide or nitride for which thermodynamic data were available./l/ TABLE 1
Estimated Process Temperatures at~ihichMetal Oxide Vapor Pressures and Nitride Decomposition Pressures Exceed 10 Atmosphere Condensed Phases
Gaseous Oxides
Metal Melting Temp., K
Process Temp., K
Metal Vapor Pressure, Bar
Si, Si, Si Si02 3N4 Nb, NbO Ti, TiN TiO Zr, Zr0 2 Zr, ZrN
sio
1685
NbO, Nb02 TiO
2744 1943
1220 1328 2026 2220 2022
2 2.7Xl0~ 9.8XlO~ 9.7XlO 5 l.4Xl0~ 1.4X10
ZrO
2125
2152 2408
l.4Xl07 1.4X10
In practice, the residual gas (air) pressure was ca. 3 x 106 atm., and the oxides and nitrides could be removed at temperatures5 for which this model indicated equilibrium vapor species pressures Ca. 10 atm. The concentrations of oxygen and nitrogen dissolved in the metal were also made small enough that oxides and nitrides did not precipitate at reduced temperatures. EXPERIMENTAL Arrnaratus A schematic diagram of the apparatus is given in Figure la. The electromagnetic levitator was similar to the one described in our previous work./2/ It used a levitation coil made from 0.32 cm od copper tubing with four lower turns and two upper turns of inner turn, diameter approximately twice the specimen diameter. Power was supplied from a 5 kW, 0.45 MHz generator, through a 4:1 transformer. Continuous—wave CO~laser beam heating was used to obtain temperatures above those at which optimum levitation conditions occur. In this way, heating and levitation were decoupled, stability enhanced, and specimen distortion by the EN fields was minimized. Laser heating was also used to heat solid silicon specimens to the melting point so that EM levitation of the metallic liquid phase would occur. Figure lb. shows instruments for non—contact property measurements. Optical properties were determined by laser polarimetry at the H~Nelaser wavelength, 632.8 nm. Gaseous species concentrations were measured by laser-induced fluorescence (LIF) detection. Accurate specimen temperatures were obtained by optical pyrometry (at 650 run), in combination with corrections for specimen emissivity values that were calculated from the measured optical properties. Descriptions of the laser polarimetric/3-5/ and LIF/6,7/ equipment are given elsewhere. Materials and Gases The apparatus was operated under a flow of 100 sccm of pure argon gas at a pressure Ca. 30 torr. Argon purity was specified to be better than 10 ppm and an oxygen getter was u~edto reduce 02 content to less than 1 ppm. Leaks in the apparatus were 2 x 10 sccm and contributed about 70 ppm of air impurities to the gas. Specimens of about 0.5 cm diameter were prepared from high purity materials for which the metallic impurity content was less than 10 ppm. Two kinds of
(7)45
Property Measurements of Liquid Metals and Alloys
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Property Measurements of Liquid Metals and Alloys
(7)47
titanium, made by the electrolytic and Van Arkel (iodide transport) processes were used. Zr was also Van Arkel process material. The silicon was phosphorus doped, which increased its electrical conductivity so that EM levitation began to occur at high temperatures before the specimens melted. The phosphorus vaporized from the molten silicon so that re—levitation could only be achieved by laser melting of the silicon specimens. Optical Property Measurements The methods for interpretation of the laser laser light polarimetric measurements are 40. Linearly polarized incident is reflected by the surface described under investigation to obtain elliptically polarized light. The optical properties are determined from the measured polarization state of the reflected light. This yields the index of refraction, n, and the extinction coefficient, k, from which the normal spectral reflectance, R , of the material at the laser wavelength is obtained from the familiar relationship: R The normal spectral opaque materials: The normal
spectral
=
[(n
eiuittance,
1)2
—
e
,
R emittance
measured with an optical
+k2]/[(n
+ 1)2 +
is then
obtained
+e
was used
lc~] from Kirchoff’s
Law for
=1 to
correct
brightness
temperatures
pyrometer.
RESULTS Electrpmaqnetjp
Levitation
The solid metals were levitated in the EM coil. The suspended specimen remained quite stable when Nb or Zr were melted. Levitation of liquid silicon was also achieved when the solid specimens were laser—heated to the melting point. Electrolytic titanium released hydrogen upon melting, which caused instabilities and loss of the specimens. Instabilities also developed upon melting the Van Arkel process titanium, but stable levitation could be achieved by careful control of the EM power during melting. The maximum temperature obtained on the of levitated specimens, at the full EM and laser power, was greater for Nb than for Ti or Zr. High temperature processing of the liquid metals was necessary to remove surface phases formed from impurities in the metals. Temperatures sufficient to clean the specimens were possible with Si, Ti, and Nb, but not with Zr. Figure 2 shows examples of EM levitation-melted titanium. After levitation of the liquid was achieved, Ti and Zr remained liquid when the laser power was turned of f. It was not possible to solidify these liquids by reducing the EM power because levitation failed while the specimens were still liquid. However, measurements on levitated solid and liquid Nb were possible. Optical Properties Figures 3 - 5 present the optical properties and normal spectral emissivities of liquid silicon, titanium, and niobium versus specimen temperature. It can be seen that liquid silicon remained molten at temperatures significantly below its melting point. supercooling of liquid Zr was also indicated by the optical property and temperature measurements. However, the maximum temperature achieved with Zr (ca. 2200K) was not sufficient to clean the specimens. Work is continuing to measure optical properties of clean liquid Zr. The data for silicon include optical property results for the solid, which were measured by laser heating of a specimen supported on an aluminum oxide rod. It can be seen that the present results for silicon agree with data from the literature/8/, also given in Figure 4. The present emissivity values for liquid Ti are in fair agreement with values from the literature/9/. The previous results were obtained from the known melting temperature of Ti and measurements of the apparent temperature at which a levitated Ti specimen melted. Least squares fitting of the emittance vs temperature data for solid and liquid Nb yields: e(solid Nb) e(liquid
Nb)
0.355
=
=
0.749
0.000013 T
—
—
0.000156 T
J. K. R. Weber eta!.
(7)48
Pigure 2
Titanium specimens which have been levitated.
right: Heated to 2200K 7000 ppm airl.ak; Arkel Ti melted
with
700 ppm airleak;
Electrolytic and stuck to
Ti bloated coil;
Left
to
Heated to 1500K with with hydrogen;
van Arkel Ti
van
successfully
melted and cleaned.
At the melting point, 2750 ±lOK/9/ these equations give e = 0.315 for the solid and 0.322 for the liquid. These results agree within the measurement errors and are in good agreement with the reported value, 0.317, based on the apparent melting point of Nb/9/. Heat Catacitv of Liciuid Niobium The measurements of the heat capacity of liquid niobium reported by Cezairliyan/l0/, Bonnell/ll/, and Betz and Frohberg/l2/ are based on the assumption that the emissivity of the liquid is independent of temperature and equal to that observed at the melting point. This assumption underestimates the temperature interval used to calculate the heat capacity. Corrections based on the present emissivity measurements are made as follows. The constant emissivity assumption yields a calculated temperature, T~,which is related to the true temperature by the equation: l/T~— l/T
=
(C
2) ln(e,~/e),
where e~is the emissivity at the melting point and C2 that 2/C dT~/dT = (T 2) dln(e)/dT
=
1.4388 Cin°K. It follows
which equals 0.83 for liquid Nb at the melting point. Thi~ is the factor which corrects the previously calculated heat capacities for the temperature dependence of emissivity. Thus, the heat capacity of liquid Nb at the meltit~g point becomes 8.05 to 8.30 cal/znol°K rather than the 9.7 to 10.0 cal/md K reported /10-12/ in the literature and shows good agreement with the value, 8.0 cal/mol°K, estimated by Hultgren/l3/.
Property Measurements of Liquid Metals and
Alloys
(7)49
0.9 0.8 0.7 a
.
-‘
a
0.6. 0.5
‘U
0.4 0.3 0.2
-
0.1 0
1
800
I
1100
I
1300
1500
1700
1900
T/K Normal Spectral EmissMty of Silicon at 632.8 mm
versus Absolute Temperature 6.5 6. 5.5
a— a
a
a
a
54.5
.x
~.
C
3.5 3. 2.5 2.
S:-.
..
,
-.
.4
1.5
I,,,I..,.I.
1600
1650
1700
1750
T/K
1800
1850
1900
Normal Spectral n and k of Silicon at 632.8 nm versus Absolute Temperature (This work)
Figure 3
1950
J. K. R. Weber
(7)50
45
eta!.
a
-
2M.a&
.
£
t •_a_~~_a a
a
a
3.5
-
3. .x
2.5
.
.
a..
..
.
•
1.5
...
..
~
-
r.n
10.5 0
-
1900
_________
1950
1,1
2000
2050
2100
2150
2200
2250
2300
2350
T/K Normal Spectral n and k of Titanium at 632.8 nm Versus Absolute Temperature 0.5 0.45
-
0.4
-
0.35 a.•
‘ .4•~
•
~
...
•.
2200
2250
.
0.25 0.2
-
0.15
-
0.1
-
0.05
-
0 1900
1950
2000
2050
2100
2150
2300
2350
T/K Normal Spectral Emissivity of Titanium at 632.8 mm versus Absolute Temperature Figure 4 DISCUSS
ION
Purification of materials Brewer and Rosenblatt/14/ discussed the thermodynamics of metal purification by formation of volatile oxides. A large number of metals can be purified in this way. In practice, it is necessary to achieve sufficient oxide vaporization rates and the oxide molecule partial pressures are high enough that ultra-high vacuum techniques are not needed. The reason that Zr was not purified in our work was not that the residual gas levels were too high. Rather it was that high enough temperatures were not achieved.
Property Measurements ofLiquid Metals and Alloys
(7)51
0.5
0.4 .
0.3 ‘U
.
.
•
.~
~
•
-
a
-c 0.2
0.1
0
I
1800
I
2000
I
2200
I
2400
2600
2000
3000
T/K Normal Spectral ErnissMty of Niobium at 632.8 nm versus Absolute Temperature 5.5 5
.
aa’
4.5
.a
a —
a
a
a a——
4. C
~
a
-
3
-.
•
•
1
.
S
•
-
2 1800
~:.
•
I-n
.
2000
i... 2200
I 2400
_____
2600
2800
3000
T/K Normal Spectral n and k of Niobium at 632.8 nm Versus Absolute Temperature Figure 5 Further work is being directed to increase the specimen temperature rather than to improve the gas purity. Optical Prp~erties The emissivity of liquid Ti obtained in this work is 26% less than the 0.43 value given by Bonnell, et al./9/ This difference may be associated with specimen purity. A much wider range of values has been reported for the spectral emissivity of solid Ti and the poor reproducibility of this property is generally attributed to different levels of surface contamination of the specimens. Formation of oxides or nitrides at the surface is considered to increase the emittance.
(7)52
J. K. R. Weber eta!.
Oxide and nitride—free Ti were studied in this work and by Bonnell, et al., /9/ but significant differences likely remained in the concentration of impurities dissolved in the liquid. Further research is needed to determine if the emissivity of liquid Ti displays a significant dependence on the concentration of dissolved oxygen or nitrogen. Thermodynamic and Phase Diagram Measurements Spatially resolved laser induced fluorescence (LIF) allows the atomic concentration to be measured as a function of specimen temperature. The alloy: pure element vapor concentration ratios equals the elemental activities in the alloy. Accurate activity measurements are therefore possible in the liquid, when accurate optical property and temperature measurements are obtained by the methods described. By extrapolating these activity measurements to the liquidus, the liquidus temperature and component activities of the solid phases may also be obtained. Research to apply these methods for thermodynamic property and phase equilibria measurements is in progress. The measurement of vapor concentrations is made difficult by the limited optical access possible within the EM levitation coil. This was not the case in earlier successful measurements on aerodynamically levitated solids./6,7/ Containerless measurement of liquids temperatures should be possible in any system for which earth-based levitation of the liquid can be achieved The Authors wish to acknowledge the support for this work from the National Aeronautics and Space Agency.
REFERENCES 1.JANAF Thermal Chemical Tables
3rd.
edition
(1985).
2. R.A. Schiffman and P.C. Nordine in Materials Processing in the Reduced Gravi.ty Environment of SDace, Mat. Res. Soc. Symposium, R.H. Doremus and P.C. Nordine eds., ~, 339 (1987). 3.S. Krishnan, G.P. Hansen, R.H. Hauge and J.L. Margrave, High Temp. Sci., 17 (1990).
~,
4.S. Krishnan, PhD Thesis, Rice University, Houston, TX (1988). 5.J.K.R. Weber, R.A. Schiffman, S. Krishnan and P.C. European Symposium on Materials and Fluid Sciences Special Pub. SP—295, pp. 639—643.
Nordine, Proc. VIIth in Microgravity, ESA
6.P.C.
1 (1985).
Nordirte and R.A. Schiffman,
High Temp. Sci.,
~Q,
7.P.C. Nordine and R.A. Schiffman, Advanced Ceramic Mats. 8.K.W. Lange and H. Schenck,
Arch. Eisenhuttenw.
~,
~,
478 (1988).
611 (1968).
9. D.W. Bonnell, J .A. Treverton, A.J. Valerga and 3. L. Margrave, 5th symposium on Temperature,
Washington,
D.C., Jun 21—25, 1971. pp. 483—487.
10. A. Cezairliyan, Private communication, 1989. 11’.
D.W. Bonnell,
PhD Thesis,
Rice University,
Houston,
12. 0. Betz and M. G. Frohberg, Z. Metallkde. fl,
TX (1972).
451 (1980).
13. R. Hultgren et al, Selected Values of the Thermodynamic Properties of the Elements, ASM, Ohio, 1974 pp. 337. 14. L. Brewer and G. M. Rosenblatt, “Thermodynamics of Suboxide Vaporization”, Trans. Met. Soc. ~j, 1268—1271 (1961).
0273—1177/91 $0.00 + .50 Copyright © 199! COSPAR
OPTICAL AND THERMODYNAMIC PROPERTY MEASUREMENTS OF LIQUID METALS AND ALLOYS J. K. Richard Weber, Shankar Krishnan, RobertA. Schiffman and Paul C. Nordine Intersonics, inc., 3453 CommercialAvenue, Northbrook, IL 60062, US.A.
ABSTRACT Optical properties and spectral emissivities of liquid silicon, titanium, niobium, and zirconium were investigated by HeNe laser polarimetry at A = 632.8 run. The metals were of a high purity and, except for zirconium, clean. The more demanding environmental requirements for eliminating oxide or nitride phases from zirconium were not met. Containerless conditions were achieved by electromagnetic levitation and heating. CO 2 laser beam heating was also used to extend the temperature range for stable levitation and to heat solid silicon to form the metallic liquid phase. Corrections to previously reported calorimetric measurements of the heat capacity of liquid niobium were derived from the measured temperature dependence of its spectral emissivity. Property measurements were obtained for supercooled liquid silicon and supercooling of liquid zirconium was accomplished. The purification of liquid metals and the extension of this work on liquids to the measurement of thermodynamic properties and phase equilibria are discussed. INTRODUCTION
High temperature liquids achieve chemical and mechanical equilibrium rapidly and have smooth surfaces if they are clean. Vaporization of impurities and direct gasification of the condensed phase rather than contamination by residual gases is promoted at the high temperatures required to melt refractory materials. These qualities allow polarimetric measurements of intrinsic optical properties of high temperature liquids. Emissivity corrections to optical pyrometry can thus be determined to achieve accurate thermodynamic temperature measurements in container].ess experiments. The need for pure liquid metals in this optical property research required that container and ambient gas contamination of the specimens be eliminated or controlled. Reaction with containers was eliminated by the use of electromagnetic (EM) levitation and heating. Reaction with residual gas impurities was controlled by heating to sufficient temperatures in order that purification, rather than contamination, of the specimens occurred. It was possible to meet both of these requirements with liquid silicon, titanium, and niobium for which optical properties and spectral emissivities are reported. Research presently in progress on liquid zirconium is discussed and extensions of the work to studies of phase equilibria and thermodynamic properties are described. PURIFICATION AND REACTION WITH RESIDUAL GASES
Residual oxygen and nitrogen may react with metals to form condensed oxides and nitrides. For materials studies here , these phases can be removed at sufficiently high temperatures, by oxide vaporization and nitride decomposition reactions. Mass balances determine the conditions under which it is practical to purify the metals in this way. 3 liquid metal at 2000K that for also contains mole % nitrogen as of the0.1 metal nitride. The HertzConsider, example, a 11 cm diameter sphere mole/cm Knudsen equation yields the maximum rate of nitrogen gas loss from the specimen: n(N
t0”mole/hr 3),~, = 95,000 P(N2)/T where p(N 2) is the equilibrium nitrogen pressure, metal/metal nitride mixture. (7)43
(1) in atmospheres,
over the
J. K. R. Weber eta!.
(7)44
If p(N 2) = 5x10’ atm., at least one hour would be required to remove the nitride. Smaller loss rates would result from external gas transport limitations, nonunit nitride vaporization coefficients, and the nitride did not fully cover the liquid metal surface. In this example, partial pressures of reactive residual gases on the order of 10’ atm. are sufficient, and equilibrium vapor species pressures on the order of 106 atm. are necessary, to achieve containerless purification of liquid metals at useful rates. These pressures would be proportional in other cases to the initial contaminant concentration. Table 1 presents the temperatures at which volatile oxide or nitrogen pressures equal to 10’ atm occur for the metal—oxygen and metal—nitrogen systems relevant to the present work. The vapor pressures of the metals at this temperature and the melting temperatures of the metals are given. These calculations were based on a model in which the metal was assumed to dissolve no oxygen or nitrogen and to exist in equilibrium with the most reduced oxide or nitride for which thermodynamic data were available./l/ TABLE 1
Estimated Process Temperatures at~ihichMetal Oxide Vapor Pressures and Nitride Decomposition Pressures Exceed 10 Atmosphere Condensed Phases
Gaseous Oxides
Metal Melting Temp., K
Process Temp., K
Metal Vapor Pressure, Bar
Si, Si, Si Si02 3N4 Nb, NbO Ti, TiN TiO Zr, Zr0 2 Zr, ZrN
sio
1685
NbO, Nb02 TiO
2744 1943
1220 1328 2026 2220 2022
2 2.7Xl0~ 9.8XlO~ 9.7XlO 5 l.4Xl0~ 1.4X10
ZrO
2125
2152 2408
l.4Xl07 1.4X10
In practice, the residual gas (air) pressure was ca. 3 x 106 atm., and the oxides and nitrides could be removed at temperatures5 for which this model indicated equilibrium vapor species pressures Ca. 10 atm. The concentrations of oxygen and nitrogen dissolved in the metal were also made small enough that oxides and nitrides did not precipitate at reduced temperatures. EXPERIMENTAL Arrnaratus A schematic diagram of the apparatus is given in Figure la. The electromagnetic levitator was similar to the one described in our previous work./2/ It used a levitation coil made from 0.32 cm od copper tubing with four lower turns and two upper turns of inner turn, diameter approximately twice the specimen diameter. Power was supplied from a 5 kW, 0.45 MHz generator, through a 4:1 transformer. Continuous—wave CO~laser beam heating was used to obtain temperatures above those at which optimum levitation conditions occur. In this way, heating and levitation were decoupled, stability enhanced, and specimen distortion by the EN fields was minimized. Laser heating was also used to heat solid silicon specimens to the melting point so that EM levitation of the metallic liquid phase would occur. Figure lb. shows instruments for non—contact property measurements. Optical properties were determined by laser polarimetry at the H~Nelaser wavelength, 632.8 nm. Gaseous species concentrations were measured by laser-induced fluorescence (LIF) detection. Accurate specimen temperatures were obtained by optical pyrometry (at 650 run), in combination with corrections for specimen emissivity values that were calculated from the measured optical properties. Descriptions of the laser polarimetric/3-5/ and LIF/6,7/ equipment are given elsewhere. Materials and Gases The apparatus was operated under a flow of 100 sccm of pure argon gas at a pressure Ca. 30 torr. Argon purity was specified to be better than 10 ppm and an oxygen getter was u~edto reduce 02 content to less than 1 ppm. Leaks in the apparatus were 2 x 10 sccm and contributed about 70 ppm of air impurities to the gas. Specimens of about 0.5 cm diameter were prepared from high purity materials for which the metallic impurity content was less than 10 ppm. Two kinds of
(7)45
Property Measurements of Liquid Metals and Alloys
z
o
~ F U)
Lii
z Lii
NcD
0
otx
o. (I)
cna~ <0
z 0 I—
5
Lu -J
I—
U,x
w
-J LiJ
_________ _________
z 0
C
p
~
F
0
LI.
z~
~Li~ I—
0.U) S 1.4
CLI. .-4
ww NV)
dLJ.
~4I ~Cfl~
LU
Lu (1
0
K::~<~>
0~
I
z
___ L&J~
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C.)
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0 LU ~~10 ~I Nt____
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o_
~i
I
a_LU L05
/?
(7)46
J. K. R. Weberetal.
t~ L.
0)0)
JO
01 o~ a,
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0)
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0)
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Property Measurements of Liquid Metals and Alloys
(7)47
titanium, made by the electrolytic and Van Arkel (iodide transport) processes were used. Zr was also Van Arkel process material. The silicon was phosphorus doped, which increased its electrical conductivity so that EM levitation began to occur at high temperatures before the specimens melted. The phosphorus vaporized from the molten silicon so that re—levitation could only be achieved by laser melting of the silicon specimens. Optical Property Measurements The methods for interpretation of the laser laser light polarimetric measurements are 40. Linearly polarized incident is reflected by the surface described under investigation to obtain elliptically polarized light. The optical properties are determined from the measured polarization state of the reflected light. This yields the index of refraction, n, and the extinction coefficient, k, from which the normal spectral reflectance, R , of the material at the laser wavelength is obtained from the familiar relationship: R The normal spectral opaque materials: The normal
spectral
=
[(n
eiuittance,
1)2
—
e
,
R emittance
measured with an optical
+k2]/[(n
+ 1)2 +
is then
obtained
+e
was used
lc~] from Kirchoff’s
Law for
=1 to
correct
brightness
temperatures
pyrometer.
RESULTS Electrpmaqnetjp
Levitation
The solid metals were levitated in the EM coil. The suspended specimen remained quite stable when Nb or Zr were melted. Levitation of liquid silicon was also achieved when the solid specimens were laser—heated to the melting point. Electrolytic titanium released hydrogen upon melting, which caused instabilities and loss of the specimens. Instabilities also developed upon melting the Van Arkel process titanium, but stable levitation could be achieved by careful control of the EM power during melting. The maximum temperature obtained on the of levitated specimens, at the full EM and laser power, was greater for Nb than for Ti or Zr. High temperature processing of the liquid metals was necessary to remove surface phases formed from impurities in the metals. Temperatures sufficient to clean the specimens were possible with Si, Ti, and Nb, but not with Zr. Figure 2 shows examples of EM levitation-melted titanium. After levitation of the liquid was achieved, Ti and Zr remained liquid when the laser power was turned of f. It was not possible to solidify these liquids by reducing the EM power because levitation failed while the specimens were still liquid. However, measurements on levitated solid and liquid Nb were possible. Optical Properties Figures 3 - 5 present the optical properties and normal spectral emissivities of liquid silicon, titanium, and niobium versus specimen temperature. It can be seen that liquid silicon remained molten at temperatures significantly below its melting point. supercooling of liquid Zr was also indicated by the optical property and temperature measurements. However, the maximum temperature achieved with Zr (ca. 2200K) was not sufficient to clean the specimens. Work is continuing to measure optical properties of clean liquid Zr. The data for silicon include optical property results for the solid, which were measured by laser heating of a specimen supported on an aluminum oxide rod. It can be seen that the present results for silicon agree with data from the literature/8/, also given in Figure 4. The present emissivity values for liquid Ti are in fair agreement with values from the literature/9/. The previous results were obtained from the known melting temperature of Ti and measurements of the apparent temperature at which a levitated Ti specimen melted. Least squares fitting of the emittance vs temperature data for solid and liquid Nb yields: e(solid Nb) e(liquid
Nb)
0.355
=
=
0.749
0.000013 T
—
—
0.000156 T
J. K. R. Weber eta!.
(7)48
Pigure 2
Titanium specimens which have been levitated.
right: Heated to 2200K 7000 ppm airl.ak; Arkel Ti melted
with
700 ppm airleak;
Electrolytic and stuck to
Ti bloated coil;
Left
to
Heated to 1500K with with hydrogen;
van Arkel Ti
van
successfully
melted and cleaned.
At the melting point, 2750 ±lOK/9/ these equations give e = 0.315 for the solid and 0.322 for the liquid. These results agree within the measurement errors and are in good agreement with the reported value, 0.317, based on the apparent melting point of Nb/9/. Heat Catacitv of Liciuid Niobium The measurements of the heat capacity of liquid niobium reported by Cezairliyan/l0/, Bonnell/ll/, and Betz and Frohberg/l2/ are based on the assumption that the emissivity of the liquid is independent of temperature and equal to that observed at the melting point. This assumption underestimates the temperature interval used to calculate the heat capacity. Corrections based on the present emissivity measurements are made as follows. The constant emissivity assumption yields a calculated temperature, T~,which is related to the true temperature by the equation: l/T~— l/T
=
(C
2) ln(e,~/e),
where e~is the emissivity at the melting point and C2 that 2/C dT~/dT = (T 2) dln(e)/dT
=
1.4388 Cin°K. It follows
which equals 0.83 for liquid Nb at the melting point. Thi~ is the factor which corrects the previously calculated heat capacities for the temperature dependence of emissivity. Thus, the heat capacity of liquid Nb at the meltit~g point becomes 8.05 to 8.30 cal/znol°K rather than the 9.7 to 10.0 cal/md K reported /10-12/ in the literature and shows good agreement with the value, 8.0 cal/mol°K, estimated by Hultgren/l3/.
Property Measurements of Liquid Metals and
Alloys
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0.9 0.8 0.7 a
.
-‘
a
0.6. 0.5
‘U
0.4 0.3 0.2
-
0.1 0
1
800
I
1100
I
1300
1500
1700
1900
T/K Normal Spectral EmissMty of Silicon at 632.8 mm
versus Absolute Temperature 6.5 6. 5.5
a— a
a
a
a
54.5
.x
~.
C
3.5 3. 2.5 2.
S:-.
..
,
-.
.4
1.5
I,,,I..,.I.
1600
1650
1700
1750
T/K
1800
1850
1900
Normal Spectral n and k of Silicon at 632.8 nm versus Absolute Temperature (This work)
Figure 3
1950
J. K. R. Weber
(7)50
45
eta!.
a
-
2M.a&
.
£
t •_a_~~_a a
a
a
3.5
-
3. .x
2.5
.
.
a..
..
.
•
1.5
...
..
~
-
r.n
10.5 0
-
1900
_________
1950
1,1
2000
2050
2100
2150
2200
2250
2300
2350
T/K Normal Spectral n and k of Titanium at 632.8 nm Versus Absolute Temperature 0.5 0.45
-
0.4
-
0.35 a.•
‘ .4•~
•
~
...
•.
2200
2250
.
0.25 0.2
-
0.15
-
0.1
-
0.05
-
0 1900
1950
2000
2050
2100
2150
2300
2350
T/K Normal Spectral Emissivity of Titanium at 632.8 mm versus Absolute Temperature Figure 4 DISCUSS
ION
Purification of materials Brewer and Rosenblatt/14/ discussed the thermodynamics of metal purification by formation of volatile oxides. A large number of metals can be purified in this way. In practice, it is necessary to achieve sufficient oxide vaporization rates and the oxide molecule partial pressures are high enough that ultra-high vacuum techniques are not needed. The reason that Zr was not purified in our work was not that the residual gas levels were too high. Rather it was that high enough temperatures were not achieved.
Property Measurements ofLiquid Metals and Alloys
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0.5
0.4 .
0.3 ‘U
.
.
•
.~
~
•
-
a
-c 0.2
0.1
0
I
1800
I
2000
I
2200
I
2400
2600
2000
3000
T/K Normal Spectral ErnissMty of Niobium at 632.8 nm versus Absolute Temperature 5.5 5
.
aa’
4.5
.a
a —
a
a
a a——
4. C
~
a
-
3
-.
•
•
1
.
S
•
-
2 1800
~:.
•
I-n
.
2000
i... 2200
I 2400
_____
2600
2800
3000
T/K Normal Spectral n and k of Niobium at 632.8 nm Versus Absolute Temperature Figure 5 Further work is being directed to increase the specimen temperature rather than to improve the gas purity. Optical Prp~erties The emissivity of liquid Ti obtained in this work is 26% less than the 0.43 value given by Bonnell, et al./9/ This difference may be associated with specimen purity. A much wider range of values has been reported for the spectral emissivity of solid Ti and the poor reproducibility of this property is generally attributed to different levels of surface contamination of the specimens. Formation of oxides or nitrides at the surface is considered to increase the emittance.
(7)52
J. K. R. Weber eta!.
Oxide and nitride—free Ti were studied in this work and by Bonnell, et al., /9/ but significant differences likely remained in the concentration of impurities dissolved in the liquid. Further research is needed to determine if the emissivity of liquid Ti displays a significant dependence on the concentration of dissolved oxygen or nitrogen. Thermodynamic and Phase Diagram Measurements Spatially resolved laser induced fluorescence (LIF) allows the atomic concentration to be measured as a function of specimen temperature. The alloy: pure element vapor concentration ratios equals the elemental activities in the alloy. Accurate activity measurements are therefore possible in the liquid, when accurate optical property and temperature measurements are obtained by the methods described. By extrapolating these activity measurements to the liquidus, the liquidus temperature and component activities of the solid phases may also be obtained. Research to apply these methods for thermodynamic property and phase equilibria measurements is in progress. The measurement of vapor concentrations is made difficult by the limited optical access possible within the EM levitation coil. This was not the case in earlier successful measurements on aerodynamically levitated solids./6,7/ Containerless measurement of liquids temperatures should be possible in any system for which earth-based levitation of the liquid can be achieved The Authors wish to acknowledge the support for this work from the National Aeronautics and Space Agency.
REFERENCES 1.JANAF Thermal Chemical Tables
3rd.
edition
(1985).
2. R.A. Schiffman and P.C. Nordine in Materials Processing in the Reduced Gravi.ty Environment of SDace, Mat. Res. Soc. Symposium, R.H. Doremus and P.C. Nordine eds., ~, 339 (1987). 3.S. Krishnan, G.P. Hansen, R.H. Hauge and J.L. Margrave, High Temp. Sci., 17 (1990).
~,
4.S. Krishnan, PhD Thesis, Rice University, Houston, TX (1988). 5.J.K.R. Weber, R.A. Schiffman, S. Krishnan and P.C. European Symposium on Materials and Fluid Sciences Special Pub. SP—295, pp. 639—643.
Nordine, Proc. VIIth in Microgravity, ESA
6.P.C.
1 (1985).
Nordirte and R.A. Schiffman,
High Temp. Sci.,
~Q,
7.P.C. Nordine and R.A. Schiffman, Advanced Ceramic Mats. 8.K.W. Lange and H. Schenck,
Arch. Eisenhuttenw.
~,
~,
478 (1988).
611 (1968).
9. D.W. Bonnell, J .A. Treverton, A.J. Valerga and 3. L. Margrave, 5th symposium on Temperature,
Washington,
D.C., Jun 21—25, 1971. pp. 483—487.
10. A. Cezairliyan, Private communication, 1989. 11’.
D.W. Bonnell,
PhD Thesis,
Rice University,
Houston,
12. 0. Betz and M. G. Frohberg, Z. Metallkde. fl,
TX (1972).
451 (1980).
13. R. Hultgren et al, Selected Values of the Thermodynamic Properties of the Elements, ASM, Ohio, 1974 pp. 337. 14. L. Brewer and G. M. Rosenblatt, “Thermodynamics of Suboxide Vaporization”, Trans. Met. Soc. ~j, 1268—1271 (1961).