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The solubility of tantalum in eight liquid rare-earth metals
108
JOURNAL
THE SOLUBILITY METALS*
D.
H.
DENNISON,
OF
M.
TANTALUM
J. TSCHETTER
AND
IN
K.
EIGHT
A.
OF THE LESS-COMMON
LIQUID
GSCHNEIDNER,
METALS
RARE-EARTH
JR.
Institute for Atomic Research and Departments of Metallurgy and Chemistry, Iowa State University, Ames, Iowa (U.S.A.) (Received
August
a+th, 1965)
SUMMARY
The solubilities of tantalum in liquid gadolinium, terbium, dysprosium, holmium, erbium and thulium have been determined from the melting point of the rareearth metal to N 1800°C, to N Igoo’C for scandium, and to N 20oo’C for lutetium. The change of the transition and melting points by tantalum has been measured, and the partial molal heats of solution of tantalum in the rare-earth metals calculated from these data. The liquid solubility of tantalum was found to depend upon the atomic size difference between the rare-earth solvent and the tantalum solute, i.e. the smaller the size difference the greater the solubility at a given common temperature (e.g. 1700°C). These results also suggest that the atomic size factor curves presented by STRAUSS, WHITE AND BROWN for liquid alloys should be modified.
INTRODUCTION
Much of the rare-earth metallurgy is based on the use of tantalum crucibles. Spectrochemical analyses have indicated that tantalum is picked up by the rareearth metals, especially scandium and the heavier lanthanide metal+, however, no quantitative data exist. Because of this lack of information, this study was begun. Furthermore, in our high-temperature enthalpy study (from room temperature to above the melting point) it is necessary to know the quantity of tantalum dissolved in these liquid metals, so that a correction can be made for this contamination, which in turn dictated the choice of rare-earth solvents. EXPERIMENTAL
Samples of the rare-earth metals were sealed in thick-wall tantalum crucibles and heated to a temperature above their melting points for one hour. During this heating period the liquid rare-earth metals became saturated with tantalum. Several preliminary experiments showed that saturation was reached between 20 and 40 min; thus the one-hour holding time was sufficient to establish equilibrium conditions for *
Contribution
No.
1782.
Work
was performed
Energy Commission. J. Less-Common
Metals, IO (1965) 108-115
in the Ames
Laboratory
of the U.S.
Atomic
SOLUBILITY OF TANTALUM
ro9
IN EIGHT LIQUID RARE-EARTH METALS
all samples. Temperatures were determined by using an optical pyrometer sighting into “black-body holes” drilled into the bottoms of the crucibles. A sample was heated in a vacuum of 10-5 torr by using an induction furnace. When the one-hour holding period was completed the specimen was dropped into a water-cooled copper receptacle. The thick-wall tantalum crucible was machined-off, and the rare-earth-tantalum alloy was then chemically analyzed for tantalum. It was assumed that during the quenching period, the dissolved tantalum did not have time to precipitate and settleout in the bottom of the crucible. That is, the tantalum content of the quenched alloy is representative of the liquidus composition at the particular temperature from which the alloy was quenched. The rare-earth metals were prepared in this laboratory by the metallothermic reduction techniquel.2. The impurity contents of these metals are listed in Table I. The 99.9+:/o pure tantalum rod, from which the crucibles were machined, was obtained from Fansteel metallurgical Corp.
CHEMICAL
AND SPECTROSCOPIC
ANALYSIS
OF THE RARE-EARTH
METALS
SC
H C N 0 F Mg Al Si Ca SC Ti Cr Fe Ni Cu Y S111 Eu (;d Tb Dy Er Till Yb Lu Ta
0.0018
0.0048 0.04" 0.0045
0.0175 0.0005 0.07
<0.0003
0.04
0.012
0.002
0.002 -_
0.006
0.005 0.005 0.03 0.005 0.005
0.003
0.003 0.0012
0.007
0.03
-.
0.0045
0.006
0.02
0.004 0.013 0.003 0.04 0.02
-
-
<0.002
<0.05 <0.05 co.225
-
0.0068 0.0054 -
0.005
0.031
0.02
0.04 -
0.0005 o.oorg <0.0005
0.01
-
0.02
0.03
(0.02
(0.04
--
-
-
0.002
-
(0.01
-
-
0.005 0.05
The melting points and transition temperatures of six of the rare-earth metals (Gd, Tb, Dy, Ho, Er Tm) saturated with tantalum were determined by differential thermal analysis using platinum/platinum-r3O/b rhodium thermocouples. The melting points of the scandium and lutetium alloys were determined by means of an optical pyrometer using the method of PIRANI AND ALTERTHUM~. J. Less-Commow
Metals,
x0
(x965) 108-r
15
110
D. H. DENNISON,
M. J. TSCHETTER,
K. A. GSCHNEIDNER,
JR.
Each rare-earth-tantalum alloy sample of approximately 0.5 to 0.8 g was weighed and treated with 75-100 ml of 6 M hydrochloric acid. After the reaction had subsided, the insoluble tantalum residue was filtered off and washed with water. The filtrate was reduced to about 40 ml in volume by boiling and then diluted to volume in 50 ml volumetric flasks for subsequent determination of the rare-earth concentration. The wet filter paper containing the tantalum was placed in a platinum dish and decomposed with concentrated sulfuric acid followed by nitric acid and hydrogen peroxide. After decomposition and oxidation of the filter paper, 50 ml of 48% hydrofluoric acid and 20 ml of concentrated sulfuric acid were cautiously added to complete the t~talum dissolution. The solution was heated until heavy fumes of sulfuric acid evolved for at least 15 min. After cooling, the sides of the platinum dish were washed with concentrated sulfuric acid. About 0.5 g of ammonium persulfate was added to oxidize any remaining organic impurity and again heat was applied until heavy fumes evolved from the solution for IO min. When the solution was just cool enough to handle, it was transferred to a 5o-ml volumetric flask and stoppered. Because the content of tantalum present in the rare-earth varied from 0.10% to possibly IO%, the rare-earth concentration was also determined in order to obtain an estimate of the amount of tantalum present. The amount of rare earth was determined by a direct titration4 with 0.05 M EDTA at a pH of five. Xylenol orange was used as indicator in a hexamethylenetetramine buffered solution. The tantalum was determined spectrophotometrically according to the procedure of WATERBURY AND BRICKER~, by using hydroquinone as the chromogenic reagent in a sulfuric acid media, However, the method of sample treatment described here eliminated the need for the extraction step included in their procedure. A standard curve ranging from 30 to 300 pg of tantalum per ml was made for the tantalum determination. This two step dissolution for the rare-earth-tantalum alloys has proved to be the most advantageous method of dissolution. It provides an easy separation and simple dissolution. Qualitative tests indicate that the amount of rare earth carried over in the sulfate portion is limited to trace amounts. Semi-quantitative tests on the hydrochloric acid portion reveal 0.01 y0 or less tantalum present. RESULTS
The thermal analysis results are summarized in Tables II and III, in which TABLE
II
TRANSITION RARE-EARTH __-._
Metal
SC Gd Tb DY HO
TEMPERATURES ALLOY
OF THE
PURE
RARE-EARTH
METAL
AND
THE
SATURATED
-Pwz metal
Saturated with Ta
Temp. (“C)
Ref.
1335 1258-+2 1287+3 138418 1428f ‘4
GSCHNEIDNERG this paper this paper GSCHNEIDNER~ Gs~~NE~~NER~
J. Less-Common
Metals, IO (1965) IO%1x5
-~
Temp. (“C)
Ref. -
1373iIo 1236f5 Iz9IiS 1377f8 -
MARDON eta1.7 this pptper this pitper this pqer -
TANTALUM-
SOLr!BILITY OF TANTALUMIN EIGHT LIQUID RARE-EARTHMETALS
III
are compared the transition (Table II) and melting temperatures (Table III) of the rare-earth metals saturated with tantalum to those of the pure metals. These data indicate that the addition of tantalum raises the transition temperature of scandium, and lowers that of gadolinium (Table II). For terbium and dysprosium the transition temperature is unchanged within experimental error from that of the pure metal by the addition of tantalum. The effect of tantalum on the melting point of all these eight rare-earth metals (Table III) is quite difficult to assess, because in general the two sets of melting-point values agree with each other within experimental error. The data for scandium, erbium and lutetium suggest that the melting point is fowered by tantalum forming a eutectic at low tantalum concentrations. The melting point of scandium saturated with tantalum determined by MARDONand co-workers7 is in very good agreement with our value. TABLE XELTING
III TEMPERATURES
AND
SOLUBILITY
OF
TANTALUM
AT
THE
MELTING
TEMPERATURE
OF
SATURATEDTANTALUIVL-RARE-EARTHALI,OY ....___~_ nr
eta1
-_-
Pure metul
?‘enlp.
Saturated with Ta Rej.
w/ SC
7539
SC
Gil ‘I% 1))
Ho Er
Till T-11
1312+2 ‘35’14 ‘409 ‘470
GSCHNEID~~ER~
this paper this paper
Temp. WI
Composztion (at.“/b Ta)
15i9+r5 752’$r5
3.LfO.2
GSCHNEIDNER"
I31’ztS 1354+9 ‘408rt9 14681t9
1g22
GSCHNEIDWER~
1490+-m
‘545i 15 rG75115
GSCHNEIDNER~
GSCHNEIDNER~
x548* 15 IGGO-I_15
this paper
0.06~0.02 O.I=j~O.O3
0.19~0.06 0.36io.04 044f=J7 O.i>j+o.o j 1.4-&O.’
Ref.
this paper Xtuwox
this this this this this this this
et al.7
paper paper paper paper paper paper paper
-
The liquidus data are listed in Table IV and shown in Figs. I and 2. From the melting point data given in Table III and the liquidus points, we have determined the eutectic compositions in the lutetium-and scandium-tantalum systems to be 1.qk0.1 at.?; and 3.220.2 at.“,d, respectively. For the other rare-earth metals the compositions, which correspond to the intersection of the liquidus curves with the eutectic (peritectic) temperatures, are listed in Table III. The error was estimated by taking into account the variation in the eutectic (peritectic) temperature and the spread of the liquidus data. The reliability of these solubility data is dependent primarily on the accuracy of the temperature readings and the chemical analyses. The accuracy of a temperature reading is estimated to be between 0.5 and I.o”/~.The accuracy of chemical analysis is estimated to be less than f 5’+; of the tantalum content. The liquidus data were fitted by the least-squares method to the equation:
where N is the atomic fraction of tantalum, T the absolute temperature, and A and U experimental constants. These constants are listed in Table V, along with the partial molal heats of solution of tantalum in the rare-earth metals at infinite dilution. These
D. H. DENNISON,
112 TABLE
M. J, TSCHETTER,
METALS
Tantalum
content
Rare-earth
Tem$erature
metal
PC)
(“~0
(wt.%)
lat.%)
SC
1572 1605 1613
1845 1878 1886
3.22
1659 1667 1672 1800 1812 1888
1932 1940 1945 2073 2085 2161
11.8 12.1 12.7 16.2
1381 1502 1584 1674 ‘772
1654 1775 1857 1947 2045
1431 1474 1605
1704 1747 1878
‘705 1746 1615
1978 2019
0.35 0.46 0.56
1888
0.41
1693 1762 1830
1971 2035 2103 1800
0.54 0.76 0.87
Gd
Tb
=Y
Ho
1527 1576 1651
Er
Tm
Lu
1751 1773
1929 2024 2046
‘576 1617
1849 18go
1677 1719 1781
1950 1992 2054
1662
1935 1976 2002 2012 2084
1703 1729 1739 1811 1862
molal
heats
of solution
equilibrium
AH
5.07 0.07 0.11 0.17 0.22
0.39 0.22 0.20
0.34 0.19 0.18 0.31 0.40
Metals,
IO
0.47 0.62 0.79 0.99 0.51 0.71 0.82 1.13 1.24 0.72 0.84 1.11 1.25 1.50
I.34 1.60
I.34 I.45 I.69 I.91 2.31
I.39 1.50 I.75 I.97 2.39 2.43 3.35 3.91
2.35 3.28 3.79
(ARm Ta) were
calculated
Iiq. r.e.~ (r.e.,
=
(1965)
0.79 0.46
I.34
-
Rd(ln d+.
J. Less-Common
0.49 0.36 0.48 0.68
0.77 0.90 I.19
Ta~,@Ta~in
=AH’+s+AH%a
17.7 0.09 0.13 0.20 0.26
0.55 0.77 0.89 I.22
2135 2244 2310
1971 2037
4.39 4.88
0.50 0.51 0.68 0.87 I .08
1803 1878
1530 1605 1656
3.32 3.49 4.59 4.36 4.II
15.5 14.7 15.6 17.2
1849 1924 2010 2081
‘737 1808
for the
JR.
IV
SOLUBILITY OF TANTALUM IN RARE-EARTH
partial
K. A. GSCHNEIDNER,
108%115
N)
rare-earth
from
the van’t
metal)
Hoff
expression
:
(2)
SOLUBILITY OF TANTALUM IN EIGHT LIQUID RARE-EARTH
__.
0
04
Q6 ATOMIC
1.2 PERCENT
1.6 TANTALUM
METALS
113
20
Fig. I. Solubility of tantalum in liquid rare-earth metals.
I
I
L
1.4?.I% 1600 Lu +Ta 1500 &Sc+Ta
14000L.~
2
4
6
I
I
2
4
--I
6
ATOMIC PERCENT TANTALUM
Fig. 2. Lutetium-rich end and scandium-rich tantalum phase diagrams.
end of the lutetium-tantalum
and scandium-
where AHmrB is the heat of fusion of tantalum and R is the gas constant. AHmra was calculated by multiplying the estimated entropy of fusion of tantalum (1.76 e.u.)* by the temperature which corresponds to mid-point of the temperature range over which the solubility data have been determined. We have assumed that these rare-earth metals are not soluble in solid tantalum over the temperature range of these measureJ. Less-Common
Metals, IO (1965) 108-115
D. H. DENNISON,
114 TABLE
K. A. GSCHWEIDNER, JR.
V
SOLUBILITY POINT
M. J. TSCHETTER,
CONSTANTS,
TEMPERATURE,
HEAT
A x 10-3
SC Gd Tb
-2.317
-5.769 -4.801 - 6.446 -4.651 -5-727
Dy Ho Er Tm Lu
ments.
OF SOLUTION,
AND PARTIAL
MOLAL
B
-0.2079 0.3356 0.0471 0.9729 0.2167
0.8884 I. Icq o-8935
ESTIMATED
HEAT
HEAT
OF FUSION
OF TANTALUM
AT
MID-
OF SOLUTION
AH (kcal/male)
Mid-$oint tern+. (OK)
Af+rra
AR%%
10.60 26.40
2003
3.53 3.26 3.28 3.51 3.42 3.39 3.44 3.74
7.07 23.14 x8.69 25.99 17.87
1850
21.97
1862
29.50
1996
21.29 26.21
1941
274’
x9.52
24.42
2123
192.5
(kcallmole)
In view of the results reported by SPEDDING AND DAANE* we believe that this is a reasonable assumption.
(kcal /mole)
22.82 ::::8’
and by HABERMANN
AND DAANE** DISCUSSION
creasing solvents
The solubility of tantalum in the liquid lanthanide metals increases with inatomic number of the lanthanide at a given temperature common to all the (Fig. I), This behavior is quite different from that observed for magnesium
%I,
SIZE
I.1 FACTOR,
1.3 RB/R4
Fig. 3. Temperature coefficient of liquid solubility vs. size factor. 0, Tantalum in liquid rareearth metals; LI, data points taken from STRAUSS, WHITE AND BROWNIE. * No change in the lattice parameter of tantalum dendrites grown from liquid La, Ce, Pr, Gd, Dy and Er compared to that of pure tantaluml. ** No change in the superconducting transition of a tantalum dendrite grown from liquid yttrium compared to that of pure tantalum*. J. Less-Common
Metals,
IO (1965)
108-115
SOLURILITY
OF TANTALUM
in some solid
lanthanide
IN EIGHT LIQUID RARE-EARTH metals 10. In the study
JOSEPH AND GSCHNEIDNER~~ ubilities
we must take into account
and solute, ever,
noted
but also the lattice
the shear
modulus
expect
this factor
appear
that the observed
decreases
with
to be important solubilities
depend
primarily
of a high-melting
metal
size differences
pointed
eqn. (I) vs. the size factor
the two
liquid
points BROWN the
(solid
plotted
for
curve,
they
curve
behavior tantalum points
lines)
(as shown was noted alloys
lie slightly
Thus,
V, do not
show
were
drawn
constants
are in a region by when
the the
was compared
lines)
of STRAUSS, WHITE alloys
B solubility to the data
to the left of the curve
off
be more
constant given
drawn
lie
data
and suggest
might
which
by
of eqn.
that
constant
of
systems.
Although
the
STRAUSS-WHITEthe adjustment
appropriate. (I)
ago
AND BROWNIE and
points. the
years
dependent
of the A solubility
to best fit their
of few points
dashed
not
it would
for the solution
were approximately
A plot
lanthanide-tantalum
how-
a systematic
Several
is shown in Fig. 3 for the eight rare-earth-tantalum
which
the
alloys, we would
alloys.
for the solubility.
solvent
metals.
liquid
sol-
solvent
to scandium.
in Table
above
Also shown in Fig. 3 are some of the data points the curves
these
solid
between
and, therefore,
out that the solubility
in a low-melting
between
size difference For
alloys
observed
upon the size difference,
to lutetium
size as noted
STRAUSS and co-workers11,12
is zero
the
for the lanthanide-tantalum
from gadolinium
the solvent
to explain
of the solvent.
modulus)
solubility constants, summarized
The
upon
rigidity
115
of the magnesium-lanthanide
in order
not only the atomic
(or rigidity
as we proceed
variation
that
METALS
A
of
similar
for the rare-earth-
STRAUSS~~, i.e. the rare-earth
by STRAUSS based
on approximately
ten systems.
KEFERENCES I 2
3 4 5 6
F. H. SPEDDING AND A. H. DAANE, Progu. h’ucl. Energy, Ser. V, I (1956) 413. .4. H. DAANE, in F. H. SPEDDING AND A. H. DAANE (eds.), The Rare Earths, Wiley,
New York, 1961, p. 102. M. PIRANI AND H. ALTERTHUM, Z. Elektrochem., 29 (1923) 5. J. KINNUNEN AND B. WENNERSTAND, Chem.-Analyst, 46 (1957) gz. G. R. WATERBURY AND C. E. BRICKER, Anal. Chem., 29 (1957) 1474. K. A. GSCHNEIDNER, JR., Application of vacuum metallurgy to the purification of rare-earth metals, Eighth Conf. on Vacuum Metallurgy, June 21-23, 1965, New York; to be published in Conf. Proc. P. G. MARDON, 1. L. NICHOLS, 1. H. PEARCE AND D. M. POOLE, n’atuve, 189 (1961) 566. K. A. GSCHNEI~NER, JR., SolidState Phys., 16 (1964) 275. C. E. HABERMANN AND A. H. DAANE, 1. Less-Common Metals, 5 (1961) 134. Ii. R. JOSEPH AND K. A. GSCHNEIDNE-R, JR., Solid solubility 07 r&g&i~~ in some lanthanide metals, Trans. AIME, 233 (Nov., 1965). S. W. STRAUSS, J. L. WHITE AND B. F. BROWN, Acta Met., 6 (1958) 604. S. W. STRAUSS, Acta Met., IO (1962) 171.
J. Less-Common
Metals, IO (1965) 108%r15
JOURNAL
THE SOLUBILITY METALS*
D.
H.
DENNISON,
OF
M.
TANTALUM
J. TSCHETTER
AND
IN
K.
EIGHT
A.
OF THE LESS-COMMON
LIQUID
GSCHNEIDNER,
METALS
RARE-EARTH
JR.
Institute for Atomic Research and Departments of Metallurgy and Chemistry, Iowa State University, Ames, Iowa (U.S.A.) (Received
August
a+th, 1965)
SUMMARY
The solubilities of tantalum in liquid gadolinium, terbium, dysprosium, holmium, erbium and thulium have been determined from the melting point of the rareearth metal to N 1800°C, to N Igoo’C for scandium, and to N 20oo’C for lutetium. The change of the transition and melting points by tantalum has been measured, and the partial molal heats of solution of tantalum in the rare-earth metals calculated from these data. The liquid solubility of tantalum was found to depend upon the atomic size difference between the rare-earth solvent and the tantalum solute, i.e. the smaller the size difference the greater the solubility at a given common temperature (e.g. 1700°C). These results also suggest that the atomic size factor curves presented by STRAUSS, WHITE AND BROWN for liquid alloys should be modified.
INTRODUCTION
Much of the rare-earth metallurgy is based on the use of tantalum crucibles. Spectrochemical analyses have indicated that tantalum is picked up by the rareearth metals, especially scandium and the heavier lanthanide metal+, however, no quantitative data exist. Because of this lack of information, this study was begun. Furthermore, in our high-temperature enthalpy study (from room temperature to above the melting point) it is necessary to know the quantity of tantalum dissolved in these liquid metals, so that a correction can be made for this contamination, which in turn dictated the choice of rare-earth solvents. EXPERIMENTAL
Samples of the rare-earth metals were sealed in thick-wall tantalum crucibles and heated to a temperature above their melting points for one hour. During this heating period the liquid rare-earth metals became saturated with tantalum. Several preliminary experiments showed that saturation was reached between 20 and 40 min; thus the one-hour holding time was sufficient to establish equilibrium conditions for *
Contribution
No.
1782.
Work
was performed
Energy Commission. J. Less-Common
Metals, IO (1965) 108-115
in the Ames
Laboratory
of the U.S.
Atomic
SOLUBILITY OF TANTALUM
ro9
IN EIGHT LIQUID RARE-EARTH METALS
all samples. Temperatures were determined by using an optical pyrometer sighting into “black-body holes” drilled into the bottoms of the crucibles. A sample was heated in a vacuum of 10-5 torr by using an induction furnace. When the one-hour holding period was completed the specimen was dropped into a water-cooled copper receptacle. The thick-wall tantalum crucible was machined-off, and the rare-earth-tantalum alloy was then chemically analyzed for tantalum. It was assumed that during the quenching period, the dissolved tantalum did not have time to precipitate and settleout in the bottom of the crucible. That is, the tantalum content of the quenched alloy is representative of the liquidus composition at the particular temperature from which the alloy was quenched. The rare-earth metals were prepared in this laboratory by the metallothermic reduction techniquel.2. The impurity contents of these metals are listed in Table I. The 99.9+:/o pure tantalum rod, from which the crucibles were machined, was obtained from Fansteel metallurgical Corp.
CHEMICAL
AND SPECTROSCOPIC
ANALYSIS
OF THE RARE-EARTH
METALS
SC
H C N 0 F Mg Al Si Ca SC Ti Cr Fe Ni Cu Y S111 Eu (;d Tb Dy Er Till Yb Lu Ta
0.0018
0.0048 0.04" 0.0045
0.0175 0.0005 0.07
<0.0003
0.04
0.012
0.002
0.002 -_
0.006
0.005 0.005 0.03 0.005 0.005
0.003
0.003 0.0012
0.007
0.03
-.
0.0045
0.006
0.02
0.004 0.013 0.003 0.04 0.02
-
-
<0.002
<0.05 <0.05 co.225
-
0.0068 0.0054 -
0.005
0.031
0.02
0.04 -
0.0005 o.oorg <0.0005
0.01
-
0.02
0.03
(0.02
(0.04
--
-
-
0.002
-
(0.01
-
-
0.005 0.05
The melting points and transition temperatures of six of the rare-earth metals (Gd, Tb, Dy, Ho, Er Tm) saturated with tantalum were determined by differential thermal analysis using platinum/platinum-r3O/b rhodium thermocouples. The melting points of the scandium and lutetium alloys were determined by means of an optical pyrometer using the method of PIRANI AND ALTERTHUM~. J. Less-Commow
Metals,
x0
(x965) 108-r
15
110
D. H. DENNISON,
M. J. TSCHETTER,
K. A. GSCHNEIDNER,
JR.
Each rare-earth-tantalum alloy sample of approximately 0.5 to 0.8 g was weighed and treated with 75-100 ml of 6 M hydrochloric acid. After the reaction had subsided, the insoluble tantalum residue was filtered off and washed with water. The filtrate was reduced to about 40 ml in volume by boiling and then diluted to volume in 50 ml volumetric flasks for subsequent determination of the rare-earth concentration. The wet filter paper containing the tantalum was placed in a platinum dish and decomposed with concentrated sulfuric acid followed by nitric acid and hydrogen peroxide. After decomposition and oxidation of the filter paper, 50 ml of 48% hydrofluoric acid and 20 ml of concentrated sulfuric acid were cautiously added to complete the t~talum dissolution. The solution was heated until heavy fumes of sulfuric acid evolved for at least 15 min. After cooling, the sides of the platinum dish were washed with concentrated sulfuric acid. About 0.5 g of ammonium persulfate was added to oxidize any remaining organic impurity and again heat was applied until heavy fumes evolved from the solution for IO min. When the solution was just cool enough to handle, it was transferred to a 5o-ml volumetric flask and stoppered. Because the content of tantalum present in the rare-earth varied from 0.10% to possibly IO%, the rare-earth concentration was also determined in order to obtain an estimate of the amount of tantalum present. The amount of rare earth was determined by a direct titration4 with 0.05 M EDTA at a pH of five. Xylenol orange was used as indicator in a hexamethylenetetramine buffered solution. The tantalum was determined spectrophotometrically according to the procedure of WATERBURY AND BRICKER~, by using hydroquinone as the chromogenic reagent in a sulfuric acid media, However, the method of sample treatment described here eliminated the need for the extraction step included in their procedure. A standard curve ranging from 30 to 300 pg of tantalum per ml was made for the tantalum determination. This two step dissolution for the rare-earth-tantalum alloys has proved to be the most advantageous method of dissolution. It provides an easy separation and simple dissolution. Qualitative tests indicate that the amount of rare earth carried over in the sulfate portion is limited to trace amounts. Semi-quantitative tests on the hydrochloric acid portion reveal 0.01 y0 or less tantalum present. RESULTS
The thermal analysis results are summarized in Tables II and III, in which TABLE
II
TRANSITION RARE-EARTH __-._
Metal
SC Gd Tb DY HO
TEMPERATURES ALLOY
OF THE
PURE
RARE-EARTH
METAL
AND
THE
SATURATED
-Pwz metal
Saturated with Ta
Temp. (“C)
Ref.
1335 1258-+2 1287+3 138418 1428f ‘4
GSCHNEIDNERG this paper this paper GSCHNEIDNER~ Gs~~NE~~NER~
J. Less-Common
Metals, IO (1965) IO%1x5
-~
Temp. (“C)
Ref. -
1373iIo 1236f5 Iz9IiS 1377f8 -
MARDON eta1.7 this pptper this pitper this pqer -
TANTALUM-
SOLr!BILITY OF TANTALUMIN EIGHT LIQUID RARE-EARTHMETALS
III
are compared the transition (Table II) and melting temperatures (Table III) of the rare-earth metals saturated with tantalum to those of the pure metals. These data indicate that the addition of tantalum raises the transition temperature of scandium, and lowers that of gadolinium (Table II). For terbium and dysprosium the transition temperature is unchanged within experimental error from that of the pure metal by the addition of tantalum. The effect of tantalum on the melting point of all these eight rare-earth metals (Table III) is quite difficult to assess, because in general the two sets of melting-point values agree with each other within experimental error. The data for scandium, erbium and lutetium suggest that the melting point is fowered by tantalum forming a eutectic at low tantalum concentrations. The melting point of scandium saturated with tantalum determined by MARDONand co-workers7 is in very good agreement with our value. TABLE XELTING
III TEMPERATURES
AND
SOLUBILITY
OF
TANTALUM
AT
THE
MELTING
TEMPERATURE
OF
SATURATEDTANTALUIVL-RARE-EARTHALI,OY ....___~_ nr
eta1
-_-
Pure metul
?‘enlp.
Saturated with Ta Rej.
w/ SC
7539
SC
Gil ‘I% 1))
Ho Er
Till T-11
1312+2 ‘35’14 ‘409 ‘470
GSCHNEID~~ER~
this paper this paper
Temp. WI
Composztion (at.“/b Ta)
15i9+r5 752’$r5
3.LfO.2
GSCHNEIDNER"
I31’ztS 1354+9 ‘408rt9 14681t9
1g22
GSCHNEIDWER~
1490+-m
‘545i 15 rG75115
GSCHNEIDNER~
GSCHNEIDNER~
x548* 15 IGGO-I_15
this paper
0.06~0.02 O.I=j~O.O3
0.19~0.06 0.36io.04 044f=J7 O.i>j+o.o j 1.4-&O.’
Ref.
this paper Xtuwox
this this this this this this this
et al.7
paper paper paper paper paper paper paper
-
The liquidus data are listed in Table IV and shown in Figs. I and 2. From the melting point data given in Table III and the liquidus points, we have determined the eutectic compositions in the lutetium-and scandium-tantalum systems to be 1.qk0.1 at.?; and 3.220.2 at.“,d, respectively. For the other rare-earth metals the compositions, which correspond to the intersection of the liquidus curves with the eutectic (peritectic) temperatures, are listed in Table III. The error was estimated by taking into account the variation in the eutectic (peritectic) temperature and the spread of the liquidus data. The reliability of these solubility data is dependent primarily on the accuracy of the temperature readings and the chemical analyses. The accuracy of a temperature reading is estimated to be between 0.5 and I.o”/~.The accuracy of chemical analysis is estimated to be less than f 5’+; of the tantalum content. The liquidus data were fitted by the least-squares method to the equation:
where N is the atomic fraction of tantalum, T the absolute temperature, and A and U experimental constants. These constants are listed in Table V, along with the partial molal heats of solution of tantalum in the rare-earth metals at infinite dilution. These
D. H. DENNISON,
112 TABLE
M. J, TSCHETTER,
METALS
Tantalum
content
Rare-earth
Tem$erature
metal
PC)
(“~0
(wt.%)
lat.%)
SC
1572 1605 1613
1845 1878 1886
3.22
1659 1667 1672 1800 1812 1888
1932 1940 1945 2073 2085 2161
11.8 12.1 12.7 16.2
1381 1502 1584 1674 ‘772
1654 1775 1857 1947 2045
1431 1474 1605
1704 1747 1878
‘705 1746 1615
1978 2019
0.35 0.46 0.56
1888
0.41
1693 1762 1830
1971 2035 2103 1800
0.54 0.76 0.87
Gd
Tb
=Y
Ho
1527 1576 1651
Er
Tm
Lu
1751 1773
1929 2024 2046
‘576 1617
1849 18go
1677 1719 1781
1950 1992 2054
1662
1935 1976 2002 2012 2084
1703 1729 1739 1811 1862
molal
heats
of solution
equilibrium
AH
5.07 0.07 0.11 0.17 0.22
0.39 0.22 0.20
0.34 0.19 0.18 0.31 0.40
Metals,
IO
0.47 0.62 0.79 0.99 0.51 0.71 0.82 1.13 1.24 0.72 0.84 1.11 1.25 1.50
I.34 1.60
I.34 I.45 I.69 I.91 2.31
I.39 1.50 I.75 I.97 2.39 2.43 3.35 3.91
2.35 3.28 3.79
(ARm Ta) were
calculated
Iiq. r.e.~ (r.e.,
=
(1965)
0.79 0.46
I.34
-
Rd(ln d+.
J. Less-Common
0.49 0.36 0.48 0.68
0.77 0.90 I.19
Ta~,@Ta~in
=AH’+s+AH%a
17.7 0.09 0.13 0.20 0.26
0.55 0.77 0.89 I.22
2135 2244 2310
1971 2037
4.39 4.88
0.50 0.51 0.68 0.87 I .08
1803 1878
1530 1605 1656
3.32 3.49 4.59 4.36 4.II
15.5 14.7 15.6 17.2
1849 1924 2010 2081
‘737 1808
for the
JR.
IV
SOLUBILITY OF TANTALUM IN RARE-EARTH
partial
K. A. GSCHNEIDNER,
108%115
N)
rare-earth
from
the van’t
metal)
Hoff
expression
:
(2)
SOLUBILITY OF TANTALUM IN EIGHT LIQUID RARE-EARTH
__.
0
04
Q6 ATOMIC
1.2 PERCENT
1.6 TANTALUM
METALS
113
20
Fig. I. Solubility of tantalum in liquid rare-earth metals.
I
I
L
1.4?.I% 1600 Lu +Ta 1500 &Sc+Ta
14000L.~
2
4
6
I
I
2
4
--I
6
ATOMIC PERCENT TANTALUM
Fig. 2. Lutetium-rich end and scandium-rich tantalum phase diagrams.
end of the lutetium-tantalum
and scandium-
where AHmrB is the heat of fusion of tantalum and R is the gas constant. AHmra was calculated by multiplying the estimated entropy of fusion of tantalum (1.76 e.u.)* by the temperature which corresponds to mid-point of the temperature range over which the solubility data have been determined. We have assumed that these rare-earth metals are not soluble in solid tantalum over the temperature range of these measureJ. Less-Common
Metals, IO (1965) 108-115
D. H. DENNISON,
114 TABLE
K. A. GSCHWEIDNER, JR.
V
SOLUBILITY POINT
M. J. TSCHETTER,
CONSTANTS,
TEMPERATURE,
HEAT
A x 10-3
SC Gd Tb
-2.317
-5.769 -4.801 - 6.446 -4.651 -5-727
Dy Ho Er Tm Lu
ments.
OF SOLUTION,
AND PARTIAL
MOLAL
B
-0.2079 0.3356 0.0471 0.9729 0.2167
0.8884 I. Icq o-8935
ESTIMATED
HEAT
HEAT
OF FUSION
OF TANTALUM
AT
MID-
OF SOLUTION
AH (kcal/male)
Mid-$oint tern+. (OK)
Af+rra
AR%%
10.60 26.40
2003
3.53 3.26 3.28 3.51 3.42 3.39 3.44 3.74
7.07 23.14 x8.69 25.99 17.87
1850
21.97
1862
29.50
1996
21.29 26.21
1941
274’
x9.52
24.42
2123
192.5
(kcallmole)
In view of the results reported by SPEDDING AND DAANE* we believe that this is a reasonable assumption.
(kcal /mole)
22.82 ::::8’
and by HABERMANN
AND DAANE** DISCUSSION
creasing solvents
The solubility of tantalum in the liquid lanthanide metals increases with inatomic number of the lanthanide at a given temperature common to all the (Fig. I), This behavior is quite different from that observed for magnesium
%I,
SIZE
I.1 FACTOR,
1.3 RB/R4
Fig. 3. Temperature coefficient of liquid solubility vs. size factor. 0, Tantalum in liquid rareearth metals; LI, data points taken from STRAUSS, WHITE AND BROWNIE. * No change in the lattice parameter of tantalum dendrites grown from liquid La, Ce, Pr, Gd, Dy and Er compared to that of pure tantaluml. ** No change in the superconducting transition of a tantalum dendrite grown from liquid yttrium compared to that of pure tantalum*. J. Less-Common
Metals,
IO (1965)
108-115
SOLURILITY
OF TANTALUM
in some solid
lanthanide
IN EIGHT LIQUID RARE-EARTH metals 10. In the study
JOSEPH AND GSCHNEIDNER~~ ubilities
we must take into account
and solute, ever,
noted
but also the lattice
the shear
modulus
expect
this factor
appear
that the observed
decreases
with
to be important solubilities
depend
primarily
of a high-melting
metal
size differences
pointed
eqn. (I) vs. the size factor
the two
liquid
points BROWN the
(solid
plotted
for
curve,
they
curve
behavior tantalum points
lines)
(as shown was noted alloys
lie slightly
Thus,
V, do not
show
were
drawn
constants
are in a region by when
the the
was compared
lines)
of STRAUSS, WHITE alloys
B solubility to the data
to the left of the curve
off
be more
constant given
drawn
lie
data
and suggest
might
which
by
of eqn.
that
constant
of
systems.
Although
the
STRAUSS-WHITEthe adjustment
appropriate. (I)
ago
AND BROWNIE and
points. the
years
dependent
of the A solubility
to best fit their
of few points
dashed
not
it would
for the solution
were approximately
A plot
lanthanide-tantalum
how-
a systematic
Several
is shown in Fig. 3 for the eight rare-earth-tantalum
which
the
alloys, we would
alloys.
for the solubility.
solvent
metals.
liquid
sol-
solvent
to scandium.
in Table
above
Also shown in Fig. 3 are some of the data points the curves
these
solid
between
and, therefore,
out that the solubility
in a low-melting
between
size difference For
alloys
observed
upon the size difference,
to lutetium
size as noted
STRAUSS and co-workers11,12
is zero
the
for the lanthanide-tantalum
from gadolinium
the solvent
to explain
of the solvent.
modulus)
solubility constants, summarized
The
upon
rigidity
115
of the magnesium-lanthanide
in order
not only the atomic
(or rigidity
as we proceed
variation
that
METALS
A
of
similar
for the rare-earth-
STRAUSS~~, i.e. the rare-earth
by STRAUSS based
on approximately
ten systems.
KEFERENCES I 2
3 4 5 6
F. H. SPEDDING AND A. H. DAANE, Progu. h’ucl. Energy, Ser. V, I (1956) 413. .4. H. DAANE, in F. H. SPEDDING AND A. H. DAANE (eds.), The Rare Earths, Wiley,
New York, 1961, p. 102. M. PIRANI AND H. ALTERTHUM, Z. Elektrochem., 29 (1923) 5. J. KINNUNEN AND B. WENNERSTAND, Chem.-Analyst, 46 (1957) gz. G. R. WATERBURY AND C. E. BRICKER, Anal. Chem., 29 (1957) 1474. K. A. GSCHNEIDNER, JR., Application of vacuum metallurgy to the purification of rare-earth metals, Eighth Conf. on Vacuum Metallurgy, June 21-23, 1965, New York; to be published in Conf. Proc. P. G. MARDON, 1. L. NICHOLS, 1. H. PEARCE AND D. M. POOLE, n’atuve, 189 (1961) 566. K. A. GSCHNEI~NER, JR., SolidState Phys., 16 (1964) 275. C. E. HABERMANN AND A. H. DAANE, 1. Less-Common Metals, 5 (1961) 134. Ii. R. JOSEPH AND K. A. GSCHNEIDNE-R, JR., Solid solubility 07 r&g&i~~ in some lanthanide metals, Trans. AIME, 233 (Nov., 1965). S. W. STRAUSS, J. L. WHITE AND B. F. BROWN, Acta Met., 6 (1958) 604. S. W. STRAUSS, Acta Met., IO (1962) 171.
J. Less-Common
Metals, IO (1965) 108%r15