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Superconductivity in the technetium-titanium alloy system
Journal of the Less-Common Metals, 44 (1976) 177 - 181 @ Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
SUPERCONDUCTIVITY SYSTEM *
177
IN THE TECHNETIUM-TITANIUM
ALLOY
C. C. KOCH Metals and Ceramics Division, Oak Ridge National Laboratory, (U.S.A.)
Oak Ridge, Tenn. 37830
(Received May 22, 1975)
Summary A survey of superconducting transition temperatures and crystal structures has been made for the Tc-Ti alloy system. The highest transition temperature in this system (10.89 K) occurs in the h.c.p. technetium-rich solid solution. The Tc-Ti system is qualitatively similar to the Re-Ti system and follows the generally observed dependence of transition temperatures on average group number and crystal structure.
Introduction Technetium has the second highest superconducting transition temperature for an element (exceeded only by niobium) at -7.8 K [l]. However, data on superconducting properties of technetium-base alloys are limited compared with the extensive investigations of niobium-base systems [ 21. No superconductivity data have been reported for the Tc-Ti alloy system. The crystal structures in four Tc-Ti alloys have been reported by Darby et al. [3], but no phase equilibrium diagram exists. As part of a continuing study of superconductivity in technetium alloys, superconducting transition temperatures and structural data are presented for the Tc-Ti alloy system. Experimental Alloys were prepared by arc melting weighed portions of pure technetium and titanium metals. The preparation and analysis of the pure technetium metal has been previously described in detail [4]. Pure titanium metal was obtained from the New Jersey Zinc Company in the form of iodideprocess crystal bar. Analysis of the titanium metal has been reported [ 51. The impurity content of the starting materials is summarized in Table 1. * Research sponsored by the Energy Research and Development Administration under contract with the Union Carbide Corporation.
178 TABLE
1
Impurity content of Ti and Tc melt stock Impurity
Ti (ppm)
2r
180-
&
Si Fe Ni Cu MO W H2 N2 02
C
700
3 200 <4 3 4 2
330 -
10 50
15
After remelting and turning the alloy buttons at least six times the final melt was cast into a 3 mm diam. cylindrical copper mold. The superconducting transition temperatures, T,, were determined by an inductive technique on the as-cast cylindrical samples. The temperature Was measured with a calibrated Ge thermometer and a pure lead sample was kept in the measuring coil so that its T, gave an additional calibration check on each measurement. Most of the alloys were subsequently annealed at temperatures from 700 to 1500 “C in a Ta element resistance furnace under a vacuum of -lo-’ Torr. T, values were also measured after annealing. Optical metallography was carried out on many of the samples to assist in their characterization. Microhardness measurements were made on the metallographic samples. Crystal structures and lattice parameters were determined by X-ray diffraction in a Debye-Scherrer camera, and lattice parameters were calculated using the Nelson-Riley extrapolation function. The estimated accuracy of the lattice parameters is +O.OOl A. Crushed powders were used for the brittle alloys and slivers were cut from the ingots for the ductile samples. Results and discussion The T, and structural data are presented in Table 2. The T, results are also plotted against atomic percent. technetium (and average group number) in Fig. 1. Consistent with other transition metals to the left of Tc in the Periodic Table [6], the addition of Ti to Tc increases the T, in the h.c.p. solid solution. The Te5 at.% Ti alloy is already two-phase; i.e., h.c.p. + x(cu-Mn). The breadth (onset through completion) of the superconductingto-normal transition in this alloy must reflect the T, of the h.c.p. and x-phases
179 TABLE 2 Superconducting aoyS Alloy
transition temperature, T,
Condition
(at.%)
and structure of technetium-titanium Structure
T, (K)
Tc- 3Ti Tc- 5Ti
As-cast As-cast, 1500 a C anneal
10.89 - 10.23 10.56 - 9.40 10.16 - 9.40
h.c.p. h.c,p. + or-Mn h.c.p. + e-Mn
Tc-12.5Ti
As-cast, 1500 ’ C anneal As-cast, 1500 o C anneal
10.10 9.50 9.87 8.10
a-Mn + h.c.p. a-Mn + h.c.p. wMn a-Mn
As-cast As-cast, 1000 ‘C anneal
6.00 < 1.76 < 1.70
As-cast, 700 ’ C anneal
< 1.70 < 1.70
Tc-15Ti Tc25Ti Tc- 33Ti Tc50Ti
-
7.90 7.90 7.20 - 7.73 4.20
Lattice parameter (A)
Hardness
(kg/mm2) 325 k 52 427 + 28
a, = 9.500
960 + 36 1041 + 25
a, = 9.512
b.c.c. b.c.c. CsCl
a, = 3.083
b.c.c. C&l
a0 = 3.098 a, = 3.091
529 k 29
526 + 15 524 * 15
Tc-65Ti
As-cast
2.65 -
1.85
b.c.c.
Tc-75Ti
As-cast
2.75 -
2.65
b.c.c.
a, = 3.181
443 + 12
Tc85Ti
As-cast
1.79 -
1.70
b.c.c.
a,, = 3.221
433 + 10
as well as compositional gradients. Annealing at 1500 o C sharpens the transition to some extent but the width remains large. Optical metallography indicated that the 5% Ti and 12.5% Ti alloys were two-phase both before and after annealing. The 15% Ti alloy was essentially single phase x-structure after annealing. The x-type phases have been found to have a favorable structure-type for superconductivity, and the Tc-Ti x-phase with a T, range of 8 - 10 K puts it among the highest T, transition metal x-phases [ 71. The values of T, decrease as the Ti concentration increases and the crystal structure changes to the b.c.c. or CsCl form. The Tc-33 at.% Ti and Tc-50 at.% Ti samples were not superconducting down to at least -1.7 K. Both samples had the ordered b.c.c. (CsCl) structure after annealing. The as-cast alloys did not exhibit superlattice reflections, giving strong indication that there is an order-disorder transformation below the melting point in this composition range. The average group number is unfavorable for superconductivity in these alloys and the low T, values (<1.7 K) are consistent with other similar alloy systems [2]. The Ti-rich b.c.c. solid solution alloys show a broad peak in T, at -25 at.% Tc. The influence of Tc additions on the T, of Ti is like that of the isoelectronic elements Mn and Re [8]. The magnitude of the T, changes is similar, but the peak in T, due to Tc additions occurs at a higher value of average group number (-4.7) than the corresponding peak due to Mn or Re additions (-4.4). Atomic volumes calculated from the lattice parameters are plotted against composition in Fig. 2. The solid line represents a linear interpolation
180 A.G.N 4.0
P c”
4.3
4.6
4.9
5.2
5.5
/
i
5.6
6.1
6.4
7.0
6.7
8.0 6.0
ATOMIC
Fig. 1. Superconducting
0
10
20
30
40
ATOMIC
%
TECHNETIVM
transition temperature,
50
60
70
80
90
T,, vs. atomic percent. technetium.
100
% TECHNETIUM
Fig. 2; Atomic volume vs. atomic percent. technetium.
between the atomic volume of Ti and Tc (Vegard’s “Law”). The Tc-rich (h.c.p., x-phases ) alloys exhibit a slight positive deviation from linearity but the C&l and b.c,c. alloys show large negative deviations. These negative deviations are found in alloy systems which favor ordering; ie., unlike nearest neighbor atoms. The T, results qualitatively reflect the structural behavior. in that it has been shown empirically [9 ] that, other factors being constant, in general, a larger atomic volume favors higher T,. The tr~sition-temperat~e dependence on concentration and crystal structure in the Tc-Ti alloy system is similar to that previously reported in the Re-Ti [8, lo] and Mn-Ti systems (at the b.c.c. Ti-rich end) [8]. Although technetium is in the same group of the Periodic Table as Re its alloying behavior can be markedly different. For example, the CrsSi-type and CsCltype phases formed in technetium-transition metal alloys have no counterparts as equilibrium structures in Re alloys [ 31, No evidence of the CsCl (ie., ordered b.c.c.) phase has been found in the Re-Ti alloys, in contrast to the Tc-Ti system. The lattice parameters determined for the Tc-12.5 at.% Ti alloy (xphase) and the Tc-50 at.% Ti alloy (CsCl-phase) are, respectively, 0.8 and 0.6% smaller than those reported by Darby et al. [3]. These small differences may be at~butable to imp~ity levels in the alloys.
181
Conclusions Superconducting transition temperatures and structural determinations have been made on ten compositions of the Tc-Ti alloy system. Titanium increases the T, of the Tc h.c.p. solid solution, and the x-phase has a relatively high T,. The b.c.c. Ti-rich alloys have low T, values comparable with the Ti-rich Ti-Mn and Ti-Re alloys, while the CsCl-type phase alloys near the center of the system have T, values <1.7 K. With the exception of the ordered b.c.c. (CsCl-type) phase the TcTi system is qualitatively similar to the Re-Ti system. Acknowledgements The author would like to thank P. J. Jones and J. 0. Scarbrough technical assistance.
for
References 1 S. T. Sekuia, R. H. Kernohan and G. R. Love, Phys. Rev., 155 (1967) 364; A. L. Giorgi and E. G. Szklarz, J. Less-Common Met., 11 (1966) 455. 2 B. W. Roberts, Superconductive Materials and Some of Their Properties, Nat. Bur. Stand. Tech. Note 482, May, 1969. 3 J. B. Darby, D. J. Lam, L. J. Norton and J. W. Downey, J. Less-Common Met., 4 (1962) 558. 4 C. C. Koch and G. R. Love, J. Less-Common Met., 12 (1967) 29. 5 G. D. Kneip, J. 0. Betterton and J. 0. Scarbrough, Phys. Rev., 130 (1963) 1687. 6 C. C. Koch, W. E. Gardner and M. J. Mortimer, in K. D. Timmerhaus, W. J. O’Sullivan and E. F. Hammel (eds.), Low Temperature PhysicsLT-13, Vol. 2, Plenum Press, New York, 1974, p. 595. 7 B. W. Roberts, Superconductive Intermetahic Compounds, in J. H. Westbrook (ed.), Intermetallic Compounds, Wiley, New York, 1966. 8 B. T. Matthias, V. B. Compton, I-L Suhl and E. Corenzwit, Phys. Rev., 115 (1959) 1597. 9 W. S. DeSorbo, Phys. Rev., 130 (1963) 2177. 10 B. T. Matthias, V. B. Compton and E. Corenzwit, J. Phys. Chem. Solids, 19 (1961) 130.
SUPERCONDUCTIVITY SYSTEM *
177
IN THE TECHNETIUM-TITANIUM
ALLOY
C. C. KOCH Metals and Ceramics Division, Oak Ridge National Laboratory, (U.S.A.)
Oak Ridge, Tenn. 37830
(Received May 22, 1975)
Summary A survey of superconducting transition temperatures and crystal structures has been made for the Tc-Ti alloy system. The highest transition temperature in this system (10.89 K) occurs in the h.c.p. technetium-rich solid solution. The Tc-Ti system is qualitatively similar to the Re-Ti system and follows the generally observed dependence of transition temperatures on average group number and crystal structure.
Introduction Technetium has the second highest superconducting transition temperature for an element (exceeded only by niobium) at -7.8 K [l]. However, data on superconducting properties of technetium-base alloys are limited compared with the extensive investigations of niobium-base systems [ 21. No superconductivity data have been reported for the Tc-Ti alloy system. The crystal structures in four Tc-Ti alloys have been reported by Darby et al. [3], but no phase equilibrium diagram exists. As part of a continuing study of superconductivity in technetium alloys, superconducting transition temperatures and structural data are presented for the Tc-Ti alloy system. Experimental Alloys were prepared by arc melting weighed portions of pure technetium and titanium metals. The preparation and analysis of the pure technetium metal has been previously described in detail [4]. Pure titanium metal was obtained from the New Jersey Zinc Company in the form of iodideprocess crystal bar. Analysis of the titanium metal has been reported [ 51. The impurity content of the starting materials is summarized in Table 1. * Research sponsored by the Energy Research and Development Administration under contract with the Union Carbide Corporation.
178 TABLE
1
Impurity content of Ti and Tc melt stock Impurity
Ti (ppm)
2r
180-
&
Si Fe Ni Cu MO W H2 N2 02
C
700
3 200 <4 3 4 2
330 -
10 50
15
After remelting and turning the alloy buttons at least six times the final melt was cast into a 3 mm diam. cylindrical copper mold. The superconducting transition temperatures, T,, were determined by an inductive technique on the as-cast cylindrical samples. The temperature Was measured with a calibrated Ge thermometer and a pure lead sample was kept in the measuring coil so that its T, gave an additional calibration check on each measurement. Most of the alloys were subsequently annealed at temperatures from 700 to 1500 “C in a Ta element resistance furnace under a vacuum of -lo-’ Torr. T, values were also measured after annealing. Optical metallography was carried out on many of the samples to assist in their characterization. Microhardness measurements were made on the metallographic samples. Crystal structures and lattice parameters were determined by X-ray diffraction in a Debye-Scherrer camera, and lattice parameters were calculated using the Nelson-Riley extrapolation function. The estimated accuracy of the lattice parameters is +O.OOl A. Crushed powders were used for the brittle alloys and slivers were cut from the ingots for the ductile samples. Results and discussion The T, and structural data are presented in Table 2. The T, results are also plotted against atomic percent. technetium (and average group number) in Fig. 1. Consistent with other transition metals to the left of Tc in the Periodic Table [6], the addition of Ti to Tc increases the T, in the h.c.p. solid solution. The Te5 at.% Ti alloy is already two-phase; i.e., h.c.p. + x(cu-Mn). The breadth (onset through completion) of the superconductingto-normal transition in this alloy must reflect the T, of the h.c.p. and x-phases
179 TABLE 2 Superconducting aoyS Alloy
transition temperature, T,
Condition
(at.%)
and structure of technetium-titanium Structure
T, (K)
Tc- 3Ti Tc- 5Ti
As-cast As-cast, 1500 a C anneal
10.89 - 10.23 10.56 - 9.40 10.16 - 9.40
h.c.p. h.c,p. + or-Mn h.c.p. + e-Mn
Tc-12.5Ti
As-cast, 1500 ’ C anneal As-cast, 1500 o C anneal
10.10 9.50 9.87 8.10
a-Mn + h.c.p. a-Mn + h.c.p. wMn a-Mn
As-cast As-cast, 1000 ‘C anneal
6.00 < 1.76 < 1.70
As-cast, 700 ’ C anneal
< 1.70 < 1.70
Tc-15Ti Tc25Ti Tc- 33Ti Tc50Ti
-
7.90 7.90 7.20 - 7.73 4.20
Lattice parameter (A)
Hardness
(kg/mm2) 325 k 52 427 + 28
a, = 9.500
960 + 36 1041 + 25
a, = 9.512
b.c.c. b.c.c. CsCl
a, = 3.083
b.c.c. C&l
a0 = 3.098 a, = 3.091
529 k 29
526 + 15 524 * 15
Tc-65Ti
As-cast
2.65 -
1.85
b.c.c.
Tc-75Ti
As-cast
2.75 -
2.65
b.c.c.
a, = 3.181
443 + 12
Tc85Ti
As-cast
1.79 -
1.70
b.c.c.
a,, = 3.221
433 + 10
as well as compositional gradients. Annealing at 1500 o C sharpens the transition to some extent but the width remains large. Optical metallography indicated that the 5% Ti and 12.5% Ti alloys were two-phase both before and after annealing. The 15% Ti alloy was essentially single phase x-structure after annealing. The x-type phases have been found to have a favorable structure-type for superconductivity, and the Tc-Ti x-phase with a T, range of 8 - 10 K puts it among the highest T, transition metal x-phases [ 71. The values of T, decrease as the Ti concentration increases and the crystal structure changes to the b.c.c. or CsCl form. The Tc-33 at.% Ti and Tc-50 at.% Ti samples were not superconducting down to at least -1.7 K. Both samples had the ordered b.c.c. (CsCl) structure after annealing. The as-cast alloys did not exhibit superlattice reflections, giving strong indication that there is an order-disorder transformation below the melting point in this composition range. The average group number is unfavorable for superconductivity in these alloys and the low T, values (<1.7 K) are consistent with other similar alloy systems [2]. The Ti-rich b.c.c. solid solution alloys show a broad peak in T, at -25 at.% Tc. The influence of Tc additions on the T, of Ti is like that of the isoelectronic elements Mn and Re [8]. The magnitude of the T, changes is similar, but the peak in T, due to Tc additions occurs at a higher value of average group number (-4.7) than the corresponding peak due to Mn or Re additions (-4.4). Atomic volumes calculated from the lattice parameters are plotted against composition in Fig. 2. The solid line represents a linear interpolation
180 A.G.N 4.0
P c”
4.3
4.6
4.9
5.2
5.5
/
i
5.6
6.1
6.4
7.0
6.7
8.0 6.0
ATOMIC
Fig. 1. Superconducting
0
10
20
30
40
ATOMIC
%
TECHNETIVM
transition temperature,
50
60
70
80
90
T,, vs. atomic percent. technetium.
100
% TECHNETIUM
Fig. 2; Atomic volume vs. atomic percent. technetium.
between the atomic volume of Ti and Tc (Vegard’s “Law”). The Tc-rich (h.c.p., x-phases ) alloys exhibit a slight positive deviation from linearity but the C&l and b.c,c. alloys show large negative deviations. These negative deviations are found in alloy systems which favor ordering; ie., unlike nearest neighbor atoms. The T, results qualitatively reflect the structural behavior. in that it has been shown empirically [9 ] that, other factors being constant, in general, a larger atomic volume favors higher T,. The tr~sition-temperat~e dependence on concentration and crystal structure in the Tc-Ti alloy system is similar to that previously reported in the Re-Ti [8, lo] and Mn-Ti systems (at the b.c.c. Ti-rich end) [8]. Although technetium is in the same group of the Periodic Table as Re its alloying behavior can be markedly different. For example, the CrsSi-type and CsCltype phases formed in technetium-transition metal alloys have no counterparts as equilibrium structures in Re alloys [ 31, No evidence of the CsCl (ie., ordered b.c.c.) phase has been found in the Re-Ti alloys, in contrast to the Tc-Ti system. The lattice parameters determined for the Tc-12.5 at.% Ti alloy (xphase) and the Tc-50 at.% Ti alloy (CsCl-phase) are, respectively, 0.8 and 0.6% smaller than those reported by Darby et al. [3]. These small differences may be at~butable to imp~ity levels in the alloys.
181
Conclusions Superconducting transition temperatures and structural determinations have been made on ten compositions of the Tc-Ti alloy system. Titanium increases the T, of the Tc h.c.p. solid solution, and the x-phase has a relatively high T,. The b.c.c. Ti-rich alloys have low T, values comparable with the Ti-rich Ti-Mn and Ti-Re alloys, while the CsCl-type phase alloys near the center of the system have T, values <1.7 K. With the exception of the ordered b.c.c. (CsCl-type) phase the TcTi system is qualitatively similar to the Re-Ti system. Acknowledgements The author would like to thank P. J. Jones and J. 0. Scarbrough technical assistance.
for
References 1 S. T. Sekuia, R. H. Kernohan and G. R. Love, Phys. Rev., 155 (1967) 364; A. L. Giorgi and E. G. Szklarz, J. Less-Common Met., 11 (1966) 455. 2 B. W. Roberts, Superconductive Materials and Some of Their Properties, Nat. Bur. Stand. Tech. Note 482, May, 1969. 3 J. B. Darby, D. J. Lam, L. J. Norton and J. W. Downey, J. Less-Common Met., 4 (1962) 558. 4 C. C. Koch and G. R. Love, J. Less-Common Met., 12 (1967) 29. 5 G. D. Kneip, J. 0. Betterton and J. 0. Scarbrough, Phys. Rev., 130 (1963) 1687. 6 C. C. Koch, W. E. Gardner and M. J. Mortimer, in K. D. Timmerhaus, W. J. O’Sullivan and E. F. Hammel (eds.), Low Temperature PhysicsLT-13, Vol. 2, Plenum Press, New York, 1974, p. 595. 7 B. W. Roberts, Superconductive Intermetahic Compounds, in J. H. Westbrook (ed.), Intermetallic Compounds, Wiley, New York, 1966. 8 B. T. Matthias, V. B. Compton, I-L Suhl and E. Corenzwit, Phys. Rev., 115 (1959) 1597. 9 W. S. DeSorbo, Phys. Rev., 130 (1963) 2177. 10 B. T. Matthias, V. B. Compton and E. Corenzwit, J. Phys. Chem. Solids, 19 (1961) 130.