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Solubility of hydrogen in Ti3Cu
Journal of the Less-Common
57
Metals, 128 (1987 f 57 - 64
SOLUBILITY OF HYDROGEN IN Ti,Cu N. RAJALAKSHMP,
U. V. VARADA
RAJUb and K. V. S. RAMA RAOa
aDepartment of Physics, Indian Institute of Technology,
Madras 600 036 (India)
bMaterials Science (India)
of Technology,
Research
Centre,
Indian Institute
Madras 600 036
(Received April 27, 1986)
Summary Hydrogen absorption has been carried out on the intermetallic TisCu system in the temperature range 575 “C < T < 850 “C and in the pressure range 0.001 bar
58
2. Experimental details The alloy was prepared by arc melting under an argon atmosphere the weighed quantities of the constituent elements of 99.99% purity. The ahoy buttons were turned upside down and remelted. The homogeneity of the alloy was ensured by repeated meltings. The sample was then annealed for 72 h at about 750 “C. X-ray powder patterns established the existence of a single tetragonal phase with lattice parameters a = 4.168 a and c = 3.567 a in agreement with the literature values [ 10, 111. There was no evidence for the presence of either titanium-rich or titanium-poor phases or any other phases. The unit cell of Ti&u with the atom positions is shown in Fig. 1. Pressure-composition-temperature relationships were studied using a conventional volumetric apparatus [ 121. The experimental set-up was tested by measuring the absorption isotherms of the Pd-H system. The results were in good agreement with the literature values [13]. The TisCu sample was kept in a molybdenum capsule and then inserted into a quartz tube which in turn was connected to the experimental set-up. The sample was heated by a wire wound resistance furnace, controlled to kO.5 K using a proportional temperature controller. The sensitivity of the system was increased by reducing the dead volume using a quartz rod within the reactor tube. The whole system was evacuated to a pressure of 1O-6 Torr and following each measurement the sample was degassed by evacuation to 10e6 Torr by using an oil diffusion pump.
Fig. 1. Sketch of the tetragonal D4h1 (P4/mmm), one molecule %), Ti(I1) = lc (I/;, %, 0).
unit cell of Ti$u: 0, Cu; l, Ti(1); 0, Ti(I1). Space group per unit cell. Cu = la (0, 0, 0), Ti(1) = 2e (0, !A, %) (95, 0,
59
3. Activation of the samples Titanium and titanium-based alloys are initially inactive for the absorption of hydrogen because of the presence of a protective oxide- film on the metal surface. The method used to produce an active sample is to first hydride the alloy at high pressures and temperatures. This causes the alloy to fracture into a number of small pieces. The fresh metal surface exposed by this procedure is active for the absorption of hydrogen. A Ti,Cu alloy button weighing approximately 30 g was cut by the spark erosion technique into small pieces and then by a diamond-wheel cutter to obtain even smaller pieces. About 1 g of the sample was used for measuring the absorption isotherms. The apparatus was evacuated to lop6 Torr and the sample was degas& at 800 “C for 24 h. It was then activated for absorption by loading it with hydrogen (purified by an Ag-Pd tube) at P = 0.925bar, T = 800 "C and t = 2 h followed by slow cooling to room temperature over a period of 15 h. A few more absorption-desorption cycles produced an active sample.
4. Results and discussion The overall view of the absorption isotherms is shown in Fig. 2. The pressure first increases as the concentration of hydrogen is increased. The pressure-concentration curve then turns sharply, becoming nearly parallel to the concentration axis, indicating a two-phase region. The curve subsequently continues upwards until it again turns sharply and becomes nearly parallel to the x axis wherein the second two-phase region is reached. After passing through this region, the curve rises steeply upwards. For simplicity, the different phase regions are denoted here as (Y, fl and y. In the o-phase region, the increase in pressure with concentration of hydrogen (Izu/nTi,cu) shows an ideal dilute solution behaviour where Sievert’s law is obeyed. The plateau pressures of the cx+ p and /3+ y transformations at 600 “C are 0.4 bar”2 and 0.5 bar1’2 respectively. The highest hydrogen concentration nu in TisCu is 3.3 (nn/nn?cu ) at T = 600 ‘C, p = 0.9bar”2 in the temperature and pressure ranges studied. The critical temperature T, and pressure P, for the (x -+ 0 transformation lie between 700 - 725 “C and 0.7 - 0.75 bar”2 respectively. The T, and P, for /I+ y are 625 - 650 “C and 0.6 - 0.65 bar”2 respectively. The kinetics of absorption of hydrogen in Ti,Cu in the regions of temperature and pressure where equilibrium pressure is dependent on the concentration are shown in Fig. 3. It is seen from the plot that the period needed to attain equilibrium is only a few minutes. However, a drastic change in the absorption kinetics was observed at a pressure just above the decomposition pressure, indicating a two-phase region. The time required for the attainment of equilibrium was around 2 - 3 h in the two-phase regions. As no concentration dependent measurements were taken near these plateau
60
0.8
-
0.6
0.4
0.8
1.2
1.6
2.0
2.4
2.8
3.2
3.f
Fig. 2. Hydrogen absorption isotherms in T&&u.
1 = BM’C
PI 589Iorr
0
I 1
I
2
1
3
1
I
L
4 5 6 Time In mlnutor
.
I
m
I
30
-
Fig. 3. Kinetics of hydrogen absorption at pressure PH, = 589 Torr and temperature 2’ = 850 “C.
pressure regions owing to the sluggishness of the decomposition reactions, the constant hydrogen pressure observed at the beginning of the transformation under absorptive conditions is assumed to be the plateau pressure.
61
From the solubility data shown in Fig. 2, the relative partial molar enthalpy A& and the relative partial molar entropy ASH of solution of hydrogen in Ti&u are evaluated. These quantities are related to the hydrogen equilibrium pressure PH*by lnPu2=
2A& -
--
RT
2A& (1)
R
is The plot of In PH, us.l/T for different hydrogen concentrations shown in Fig. 4. Here the two-phase regions are not marked as no concentration dependent measurements were taken near the plateau pressure regions. The dependence of ARn and As, on the concentration of hydrogen in TisCu is shown in Fig. 5. These quantities are evaluated by a least-square fit. The value obtained for the partial molar enthalpy of solution at infinite dilution hHG in Ti,CuH,(x + 0) is comparable with the value obtained in several other systems with titanium, particularly in Ti$, as can be seen from Table 1. This may be because the two systems are isostructural. The variation in the relative partial molar enthalpy with hydrogen content for the Ti,Cu alloy (see Fig. 5) shows that as hydrogen is added to the alloy AA, remains almost constant at -38 kJ (mol H)-’ in the a-phase region. A& remains constant over the two-phase region (a, + p) and then decreases to -57 kJ (mol H)-’ at a hydrogen content corresponding to the second phase p of the pressure-composition isotherms (see Fig. 2). In the /3 + y transition region corresponding to a hydrogen content of 2.0 to 2.7, the value remains constant at -57 kJ (mol H))‘. In the y region, however, it increases to a value of -34 kJ (mol H)-‘.
0001 I 085
0.9
0.95
1.0 --
1.05 lOOOK T
1.1
1.15
12
Fig. 4. Logarithmic plot of PH, as a function of the reciprocal temperature for different equilibrium hydrogen concentrations x = nH/nTi,Cu in Ti&!u H,.
62
Fig. 5. Relative partial molar enthalpy solution as a function of x = nH/na3Cu.
A& and relative partial molar entropy As, of
The variation of the relative partial molar entropy A& with composition is similar to that of enthalpy. Therefore both these entropy and enthalpy terms follow closely the changes in In P with hydrogen content (see Fig. 2). Consequently the changes in the free energy A cn with composition are not only owing to enthalpy but also owing to the changes in the entropy term. The origin of the variations in Ail, is uncertain. However, it is possible that a change in the electronic properties of the system is responsible, since electronic contributions are more important than lattice contributions. The hydrogen absorption studies in TiCu, Ti,Cu, Ti*Pd and Ti,Cu,.,Pdo.s have been carried out by Kadel and Weiss [ 121. They have reported a stepwise decomposition of TiCu and Ti&u into copper and y-titanium
63 TABLE 1 Thermodynamic
data of titanium and TijM (M = Al, Sn, P, Sh, Ga, Cu) alloys
Metal
AH;;
Structure
Reference
(kJ (mol H)-I) &Ti &Ti Ti3Al Ti$n Ti3p (Y-TigSb Ti3Ga
Hexagonal Cubic Hexagonal Hexagonal Tetragonal Cubic Hexagonal Tetragonal
Ti3cu
-44.4 -54.4 -53.1 -50.6 -43.1 -55.1 -51.3 -38.8
f 0.8 + 0.8
f 1.5 + 4.0
14 15 1 2 3 6 9 This work
hydride. The present work in TisCu revealed the existence of three phases 01, fl and y. Since TiCu, Ti,Cu and TisCu belong to the same tetragonal crystal structure, it can be expected that similar stepwise decomposition also occurs in Ti&u. Such probable decompositions are TisCu + g
H2 -
0.6 TisCuH,. 1 + H2 2 Ti,Cu + $
H2 -
((IIphase)
Ti3CuH1_, Ti$u Ti&uH,.,
0.75 Ti‘2 CuH,., + H2 2 2.95 TiCuH,_65 + H, 2
(a! + fl phase)
+ TiH,_, (P phase)
T~CUH~.~~+ TiH2 TiCuHse6
(0 + Y phase)
(Y phase)
In order to identify the above phases, X-ray studies are being undertaken. Furthermore, the nuclear magnetic resonance of hydrogen is also being studied in this system with a view to finding out the occupancy of hydrogen in the T&u lattice.
Acknowledgments One of the authors (KVSRR) is grateful to Professor Dr. Alarich Weiss, Technische Hochschule, Darmstadt, F.R.G., for introducing him to the area of hydrogen absorption in metals. N. Rajalakshmi wishes to thank the Indian Institute of Technology, Madras for financial support. The authors wish to thank Mr. A. G. Venugopal, Glass Blowing Section, Indian Institute of Technology, Madras for helping in the fabrication of the experimental facility.
64
References 1 P. S. Rudman, J. J. Reilly and R. H. Wiswall, Ber. Bunsenges Phys. Chem., 81 (1977) 76. 2 P. S. Rudman, J. J. Reilly and R. H. Wiswall, Ber. BunsengesPhys. Chem., 82 (1978) 611. 3 U. Halter, M. Mrowietz and Al. Weiss, J. Less-Common Met., 118 (1986) 343. 4 R. L. Beck, Investigation of Hydriding Characteristics of Intermetallic Compounds, DRI 2059, 1962 (Denver Research Institute, University of Denver). 5 J. B. Vetrano, G. L. Guthrie and H. E. Kissinger, Phys. Lett. A, 26 (1967) 45. 6 K. H. J. Buschow, P. C. P. Bouten and A. R. Miedema, Rep. Progr. Phys., 45 (1982) 937. 7 K. V. S. Rama Rao, M. Mrowietz and AI. Weiss, Ber. Bunsenges Phys. Chem., 86 (1982) 1135. 8 K. V. S. Rama Rao, H. Strum, B. Eischner and Al. Weiss, Phys. Lett., 102 (1983) 492. 9 M. Mrowietz and Al. Weiss, Ber. Bunsenges Phys. Chem., 89 (1985) 49. 10 N. Karlasson, J. Inst. Met., 79 (1951) 391. 11 W. B. Pearson, Handbook of Lattice Spacings and Structures of Metals and Alloys, Vol. 2, Pergamon, Oxford, 1967. 12 R. Kadel and Al. Weiss, Ber. Bunsenges Phys. Chem.. 82 (1978) 1290. 13 J. Simons and T. B. Flanagan, J. Phys. Chem., 69 (1965) 3773. 14 P. Dantzer, C. J. KIeppa and M. E. Melnichak, J. Chem. Phys., 64 (1976) 139. 15 D. F. Lynch and J. Tanaka, Scripta. Metall.. 13 (1979) 599.
57
Metals, 128 (1987 f 57 - 64
SOLUBILITY OF HYDROGEN IN Ti,Cu N. RAJALAKSHMP,
U. V. VARADA
RAJUb and K. V. S. RAMA RAOa
aDepartment of Physics, Indian Institute of Technology,
Madras 600 036 (India)
bMaterials Science (India)
of Technology,
Research
Centre,
Indian Institute
Madras 600 036
(Received April 27, 1986)
Summary Hydrogen absorption has been carried out on the intermetallic TisCu system in the temperature range 575 “C < T < 850 “C and in the pressure range 0.001 bar
58
2. Experimental details The alloy was prepared by arc melting under an argon atmosphere the weighed quantities of the constituent elements of 99.99% purity. The ahoy buttons were turned upside down and remelted. The homogeneity of the alloy was ensured by repeated meltings. The sample was then annealed for 72 h at about 750 “C. X-ray powder patterns established the existence of a single tetragonal phase with lattice parameters a = 4.168 a and c = 3.567 a in agreement with the literature values [ 10, 111. There was no evidence for the presence of either titanium-rich or titanium-poor phases or any other phases. The unit cell of Ti&u with the atom positions is shown in Fig. 1. Pressure-composition-temperature relationships were studied using a conventional volumetric apparatus [ 121. The experimental set-up was tested by measuring the absorption isotherms of the Pd-H system. The results were in good agreement with the literature values [13]. The TisCu sample was kept in a molybdenum capsule and then inserted into a quartz tube which in turn was connected to the experimental set-up. The sample was heated by a wire wound resistance furnace, controlled to kO.5 K using a proportional temperature controller. The sensitivity of the system was increased by reducing the dead volume using a quartz rod within the reactor tube. The whole system was evacuated to a pressure of 1O-6 Torr and following each measurement the sample was degassed by evacuation to 10e6 Torr by using an oil diffusion pump.
Fig. 1. Sketch of the tetragonal D4h1 (P4/mmm), one molecule %), Ti(I1) = lc (I/;, %, 0).
unit cell of Ti$u: 0, Cu; l, Ti(1); 0, Ti(I1). Space group per unit cell. Cu = la (0, 0, 0), Ti(1) = 2e (0, !A, %) (95, 0,
59
3. Activation of the samples Titanium and titanium-based alloys are initially inactive for the absorption of hydrogen because of the presence of a protective oxide- film on the metal surface. The method used to produce an active sample is to first hydride the alloy at high pressures and temperatures. This causes the alloy to fracture into a number of small pieces. The fresh metal surface exposed by this procedure is active for the absorption of hydrogen. A Ti,Cu alloy button weighing approximately 30 g was cut by the spark erosion technique into small pieces and then by a diamond-wheel cutter to obtain even smaller pieces. About 1 g of the sample was used for measuring the absorption isotherms. The apparatus was evacuated to lop6 Torr and the sample was degas& at 800 “C for 24 h. It was then activated for absorption by loading it with hydrogen (purified by an Ag-Pd tube) at P = 0.925bar, T = 800 "C and t = 2 h followed by slow cooling to room temperature over a period of 15 h. A few more absorption-desorption cycles produced an active sample.
4. Results and discussion The overall view of the absorption isotherms is shown in Fig. 2. The pressure first increases as the concentration of hydrogen is increased. The pressure-concentration curve then turns sharply, becoming nearly parallel to the concentration axis, indicating a two-phase region. The curve subsequently continues upwards until it again turns sharply and becomes nearly parallel to the x axis wherein the second two-phase region is reached. After passing through this region, the curve rises steeply upwards. For simplicity, the different phase regions are denoted here as (Y, fl and y. In the o-phase region, the increase in pressure with concentration of hydrogen (Izu/nTi,cu) shows an ideal dilute solution behaviour where Sievert’s law is obeyed. The plateau pressures of the cx+ p and /3+ y transformations at 600 “C are 0.4 bar”2 and 0.5 bar1’2 respectively. The highest hydrogen concentration nu in TisCu is 3.3 (nn/nn?cu ) at T = 600 ‘C, p = 0.9bar”2 in the temperature and pressure ranges studied. The critical temperature T, and pressure P, for the (x -+ 0 transformation lie between 700 - 725 “C and 0.7 - 0.75 bar”2 respectively. The T, and P, for /I+ y are 625 - 650 “C and 0.6 - 0.65 bar”2 respectively. The kinetics of absorption of hydrogen in Ti,Cu in the regions of temperature and pressure where equilibrium pressure is dependent on the concentration are shown in Fig. 3. It is seen from the plot that the period needed to attain equilibrium is only a few minutes. However, a drastic change in the absorption kinetics was observed at a pressure just above the decomposition pressure, indicating a two-phase region. The time required for the attainment of equilibrium was around 2 - 3 h in the two-phase regions. As no concentration dependent measurements were taken near these plateau
60
0.8
-
0.6
0.4
0.8
1.2
1.6
2.0
2.4
2.8
3.2
3.f
Fig. 2. Hydrogen absorption isotherms in T&&u.
1 = BM’C
PI 589Iorr
0
I 1
I
2
1
3
1
I
L
4 5 6 Time In mlnutor
.
I
m
I
30
-
Fig. 3. Kinetics of hydrogen absorption at pressure PH, = 589 Torr and temperature 2’ = 850 “C.
pressure regions owing to the sluggishness of the decomposition reactions, the constant hydrogen pressure observed at the beginning of the transformation under absorptive conditions is assumed to be the plateau pressure.
61
From the solubility data shown in Fig. 2, the relative partial molar enthalpy A& and the relative partial molar entropy ASH of solution of hydrogen in Ti&u are evaluated. These quantities are related to the hydrogen equilibrium pressure PH*by lnPu2=
2A& -
--
RT
2A& (1)
R
is The plot of In PH, us.l/T for different hydrogen concentrations shown in Fig. 4. Here the two-phase regions are not marked as no concentration dependent measurements were taken near the plateau pressure regions. The dependence of ARn and As, on the concentration of hydrogen in TisCu is shown in Fig. 5. These quantities are evaluated by a least-square fit. The value obtained for the partial molar enthalpy of solution at infinite dilution hHG in Ti,CuH,(x + 0) is comparable with the value obtained in several other systems with titanium, particularly in Ti$, as can be seen from Table 1. This may be because the two systems are isostructural. The variation in the relative partial molar enthalpy with hydrogen content for the Ti,Cu alloy (see Fig. 5) shows that as hydrogen is added to the alloy AA, remains almost constant at -38 kJ (mol H)-’ in the a-phase region. A& remains constant over the two-phase region (a, + p) and then decreases to -57 kJ (mol H)-’ at a hydrogen content corresponding to the second phase p of the pressure-composition isotherms (see Fig. 2). In the /3 + y transition region corresponding to a hydrogen content of 2.0 to 2.7, the value remains constant at -57 kJ (mol H))‘. In the y region, however, it increases to a value of -34 kJ (mol H)-‘.
0001 I 085
0.9
0.95
1.0 --
1.05 lOOOK T
1.1
1.15
12
Fig. 4. Logarithmic plot of PH, as a function of the reciprocal temperature for different equilibrium hydrogen concentrations x = nH/nTi,Cu in Ti&!u H,.
62
Fig. 5. Relative partial molar enthalpy solution as a function of x = nH/na3Cu.
A& and relative partial molar entropy As, of
The variation of the relative partial molar entropy A& with composition is similar to that of enthalpy. Therefore both these entropy and enthalpy terms follow closely the changes in In P with hydrogen content (see Fig. 2). Consequently the changes in the free energy A cn with composition are not only owing to enthalpy but also owing to the changes in the entropy term. The origin of the variations in Ail, is uncertain. However, it is possible that a change in the electronic properties of the system is responsible, since electronic contributions are more important than lattice contributions. The hydrogen absorption studies in TiCu, Ti,Cu, Ti*Pd and Ti,Cu,.,Pdo.s have been carried out by Kadel and Weiss [ 121. They have reported a stepwise decomposition of TiCu and Ti&u into copper and y-titanium
63 TABLE 1 Thermodynamic
data of titanium and TijM (M = Al, Sn, P, Sh, Ga, Cu) alloys
Metal
AH;;
Structure
Reference
(kJ (mol H)-I) &Ti &Ti Ti3Al Ti$n Ti3p (Y-TigSb Ti3Ga
Hexagonal Cubic Hexagonal Hexagonal Tetragonal Cubic Hexagonal Tetragonal
Ti3cu
-44.4 -54.4 -53.1 -50.6 -43.1 -55.1 -51.3 -38.8
f 0.8 + 0.8
f 1.5 + 4.0
14 15 1 2 3 6 9 This work
hydride. The present work in TisCu revealed the existence of three phases 01, fl and y. Since TiCu, Ti,Cu and TisCu belong to the same tetragonal crystal structure, it can be expected that similar stepwise decomposition also occurs in Ti&u. Such probable decompositions are TisCu + g
H2 -
0.6 TisCuH,. 1 + H2 2 Ti,Cu + $
H2 -
((IIphase)
Ti3CuH1_, Ti$u Ti&uH,.,
0.75 Ti‘2 CuH,., + H2 2 2.95 TiCuH,_65 + H, 2
(a! + fl phase)
+ TiH,_, (P phase)
T~CUH~.~~+ TiH2 TiCuHse6
(0 + Y phase)
(Y phase)
In order to identify the above phases, X-ray studies are being undertaken. Furthermore, the nuclear magnetic resonance of hydrogen is also being studied in this system with a view to finding out the occupancy of hydrogen in the T&u lattice.
Acknowledgments One of the authors (KVSRR) is grateful to Professor Dr. Alarich Weiss, Technische Hochschule, Darmstadt, F.R.G., for introducing him to the area of hydrogen absorption in metals. N. Rajalakshmi wishes to thank the Indian Institute of Technology, Madras for financial support. The authors wish to thank Mr. A. G. Venugopal, Glass Blowing Section, Indian Institute of Technology, Madras for helping in the fabrication of the experimental facility.
64
References 1 P. S. Rudman, J. J. Reilly and R. H. Wiswall, Ber. Bunsenges Phys. Chem., 81 (1977) 76. 2 P. S. Rudman, J. J. Reilly and R. H. Wiswall, Ber. BunsengesPhys. Chem., 82 (1978) 611. 3 U. Halter, M. Mrowietz and Al. Weiss, J. Less-Common Met., 118 (1986) 343. 4 R. L. Beck, Investigation of Hydriding Characteristics of Intermetallic Compounds, DRI 2059, 1962 (Denver Research Institute, University of Denver). 5 J. B. Vetrano, G. L. Guthrie and H. E. Kissinger, Phys. Lett. A, 26 (1967) 45. 6 K. H. J. Buschow, P. C. P. Bouten and A. R. Miedema, Rep. Progr. Phys., 45 (1982) 937. 7 K. V. S. Rama Rao, M. Mrowietz and AI. Weiss, Ber. Bunsenges Phys. Chem., 86 (1982) 1135. 8 K. V. S. Rama Rao, H. Strum, B. Eischner and Al. Weiss, Phys. Lett., 102 (1983) 492. 9 M. Mrowietz and Al. Weiss, Ber. Bunsenges Phys. Chem., 89 (1985) 49. 10 N. Karlasson, J. Inst. Met., 79 (1951) 391. 11 W. B. Pearson, Handbook of Lattice Spacings and Structures of Metals and Alloys, Vol. 2, Pergamon, Oxford, 1967. 12 R. Kadel and Al. Weiss, Ber. Bunsenges Phys. Chem.. 82 (1978) 1290. 13 J. Simons and T. B. Flanagan, J. Phys. Chem., 69 (1965) 3773. 14 P. Dantzer, C. J. KIeppa and M. E. Melnichak, J. Chem. Phys., 64 (1976) 139. 15 D. F. Lynch and J. Tanaka, Scripta. Metall.. 13 (1979) 599.