Effect of interstitial boron and carbon on the hydrogenation properties of Ti25V35Cr40 alloy

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 1 1 9 7 5 e1 1 9 8 0

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Effect of interstitial boron and carbon on the hydrogenation properties of Ti25V35Cr40 alloy Chia-Chieh Shen a,b,c,*, Justin C.-P. Chou d, Hsueh-Chih Li b, Yuan-Pang Wu e, Tsong-Pyng Perng c,d,f a

Department of Mechanical Engineering, Yuan Ze University, Chung-Li, Taiwan Gradual School of Renewable Energy and Engineering, Yuan Ze University, Chung-Li, Taiwan c Fuel Cell Center, Yuan Ze University, Chung-Li, Taiwan d Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu, Taiwan e Materials and Electro-Optics Research Division, CSIST, Taoyuan, Taiwan f Department of Chemical Engineering and Materials Science, Yuan Ze University, Chung-Li, Taiwan b

article info

abstract

Article history:

Doping of interstitial elements B or C into a BCC-type Ti25V35Cr40 alloy to raise effective

Received 27 April 2010

desorption hydrogenation capacity was investigated. Ti25V35Cr40Mx alloys (M ¼ B or C and

Received in revised form

x ¼ 0, 0.1, 1, or 5) were prepared by arc-melting followed by homogenization treatment.

27 July 2010

X-ray diffraction shows that the as-cast specimens have a BCC structure, but they contain

Accepted 30 July 2010

some amount of precipitates that increases with the doping concentration of B and C.

Available online 15 September 2010

Doping-induced precipitates can be greatly eliminated by annealing treatment at 1200  C, indicating that B or C might have been partially dissolved into the interstitial sites in the

Keywords:

BCC lattice of matrix phase of specimens. With the doping of C, the second plateau pres-

TiVCr

sure of annealed specimens in the PCI curves at T ¼ 30  C significantly increases with the

Hydrogenation

amount of C, but the maximum hydrogenation capacity is reduced. On the other hand, the

Boron

second plateau pressure and maximum hydrogenation capacity are only slightly affected

Carbon

by the B doping. Under optimum doping conditions, the effective hydrogen desorption

Pressureecomposition isotherm

capacities are increased from 0.80 H/M of the sample without doping to 0.86 H/M and 0.87 H/M for Ti25V35Cr40B1 and Ti25V35Cr40C0.1, respectively. The improvement might be ascribed to the increase in second plateau pressure caused by less stable hydrogen atoms at the lattice sites of Ti25V35Cr40 containing interstitial B or C. ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Titanium is considered as one of the most promising candidates for application in hydrogen storage because titanium hydride (TiH2) has a hydrogen content of 4.2 wt%. The hydride is so stable that a temperature at around 700  C is required to dissociate hydride to release hydrogen gas over 105 Pa [1,2]. To lower the stability of TiH2 for application at room temperature, several Ti-based alloys have been developed by

substituting weak hydriding elements with Ti [3]. For example, TiFe (AB-type) shows maximum and reversible hydrogenation capacities of 1.9 wt% and 1.5 wt%, respectively. TiMn1.5 (AB2-type) has maximum and reversible hydrogen capacities of 1.9 wt% and 1.2 wt%, respectively. Recently, BCCtype TiV solid solution alloys have attracted great attentions because of its superior hydrogenation properties [4e9]. For instance, Ti25V35Cr40 alloy exhibits a maximum hydrogen capacity of 3.5 wt% (i.e., 80% of 4.2 wt% for TiH2) [8]. However,

* Corresponding author. Department of Mechanical Engineering, Yuan Ze University, Chung-Li, Taiwan. Tel.: þ886 3 4638800ext2447; fax: þ886 3 4558013. E-mail address: [email protected] (C.-C. Shen). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.07.166

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it gives a reversible effective desorption hydrogenation capacity of only 1.9 wt% under the condition of hydrogen pressures above 0.1 MPa at room temperature. Therefore, there is a challenge to promote reversible hydrogenation capacity of TiV alloys for practical application. For Ti25V35Cr40 alloy, it is realized that the maximum hydrogenation capacity of 3.5 wt% is attributed to the combination of low and high pressure plateaus in the pressureecomposition isotherm (PCI) curve, as reported previously by Okada et al. [8]. The PCI curve of Ti25V35Cr40 alloy is shown in Fig. 1. The low and high hydrogen plateaus at 30  C are 1 Pa and 0.5 MPa, respectively. The reversible hydrogenation capacity of 1.9 wt% is discharged only from the high pressure plateau. Accordingly, the study of how to release hydrogen from the low hydrogen plateau to increase the reversible hydrogenation capacity for Ti25V35Cr40 alloy is important for hydrogen economy, and is an interesting topic for metallurgy. In general, substitutional or interstitial alloying is widely proposed to solve for this problem. Previously, the benefits of using substitutional transition elements for titanium have been widely reported [4,6e11]. However, there are very few studies based on interstitial alloying method. Uno et al. studied first the effect of interstitial boron on the hydrogen solubility in BCC Ti37V38Mn25 and Ti31V39Cr30 alloys [12]. Later, Cho et al. reported the influence of interstitial boron on the hydrogen absorption/desorption property of Ti32V25Cr43 alloy [13]. Unfortunately, both cases showed that the maximum hydrogen storage capacity decreased with the addition of boron, probably due to the presence of micro-segregation in their specimens. Besides, a very small amount of boron or carbon has also been doped interstitially into TiFe that significantly promotes its hydrogen plateau pressure in our previous report [14]. Therefore, the aim of this study is to investigate the effect of doping with B or C and the annealing treatment on the hydrogenation properties such as kinetics and PCI curves of the Ti25V35Cr40 alloy.

2.

99.9%), Cr (purity 99.5%), B (purity 99.5%), and C (purity 99.5%) were mixed at the desired stoichiometries, and arc-melted on a water-cooled copper hearth in an argon atmosphere [6 N]. The ingots were turned and remelted five times for homogenization. They were further sealed with a quartz tube in vacuum (102 Torr) by a mechanical pump, and followed by annealing treatment at 1200  C for 2 h to eliminate segregation [15]. The annealed specimens were directly loaded into a reactor for hydrogenation experiment without pulverization since they are ductile. A typical Sievert’s apparatus was used for hydrogenation test, which has been described previously [2]. All of the specimens were activated at 400  C and hydrogen pressure of 4 MPa. To completely remove the residual hydrogen, the hydrided specimens were treated by a vacuum treatment at 700  C for 2 h. After two cycles of hydriding and dehydriding (i.e., pulverization), PeC isotherm measurements up to a maximum hydrogen pressure of 4 MPa were conducted at 30  C in a water bath. The effective hydrogen storage capacity was defined as the amount of hydrogen released at hydrogen pressures above 0.1 MPa at 30  C. X-ray diffraction (XRD) and optical microscopy (OM) were performed to examine the phase and microstructure, respectively.

3.

Results and discussion

3.1.

Ti25V35Cr40Bx series

The XRD patterns for as-cast and annealed Ti25V35Cr40Bx specimens are presented in Fig. 2(a) and (b), respectively. For as-cast Ti25V35Cr40Bx (x ¼ 1 or 5) specimens, they mainly consist of a single BCC matrix phase, but a small amount of

a

Material and methods

Ti25V35Cr40Mx alloys (M ¼ B or C and x ¼ 0, 0.1, 1, or 5) were prepared. Elemental powders of Ti (purity 99.7%), V (purity 7

H2 Pressure [Pa]

10

H

b

5

10

3

10

o

1

10

30 C Ti25V35Cr40

L

-1

10

0.0

0.4

0.8 1.2 1.6 H Content [H/M]

2.0

Fig. 1 e PeC isotherm curve of Ti25V35Cr40 at 30  C. L and H stands for the low plateau and high plateau, respectively. The low plateau indicated by dash line is obtained from Okada et al. [8].

Fig. 2 e XRD patterns of Ti25V35Cr40Bx alloys. (a) As-cast and (b) annealed at 1200  C for 2 h.

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Fig. 3 e Optical micrographs for annealed alloys. (a) Ti25V35Cr40, (b) Ti25V35Cr40B1, (c) Ti25V35Cr40C0.1, and (d) Ti25V35Cr40C5.

secondary phase is also present. After annealing treatment, the diffraction peaks from the secondary phase have almost disappeared for Ti25V35Cr40B1 and somewhat visible for Ti25V35Cr40B5. However, in the optical micrograph of annealed Ti25V35Cr40B1 (Fig. 3(b)), some small precipitates are visible at the grain boundaries. The amount of precipitates in the sample is probably below the detection limit of XRD. Based on the above results, boron is proved to be partially dissolved into the interstitial sites, but it may induce precipitation of a secondary phase if the doping amount of boron is over 1 at%. Fig. 4 shows the hydrogen absorption kinetics of annealed Ti25V35Cr40Bx (x ¼ 0, 0.1, 1, or 5) specimens at 30  C. Within a very short period of 10 s, all of the specimens have reached a hydrogenation capacity of 0.4 H/M. As mentioned

previously, hydrogen equilibrium pressure in the low plateau (a þ b hydrides) for the TiV alloys is so small that hydrogenation rate becomes very fast. Thus, hydrogen diffusion rate in this stage is not influenced by the boron atoms situated at the interstitial sites. However, the hydrogenation rate in the high plateau (b þ d hydrides) is very sensitive to the doping amount of boron. For the samples with doping amount of boron smaller than 1 at%, the hydrogenation rates are almost the same. However, the hydrogenation rate is seriously retarded for Ti25V35Cr40B5. The slow-down of kinetics might be partially ascribed to the diffusion barrier caused by the second phase [9]. Besides, the increment of hydrogen absorption pressure of alloy would cause the reduction of maximum temperature during quick hydrogenation, which would reduce the transfer of reaction heat from vessel to water bath. This phenomenon might also cause slow-down of kinetics.

2.0 o

7

30 C

10

o

30 C

1.2 0.8

Ti25V35Cr40 Ti25V35Cr40B0.1

0.4

Ti25V35Cr40B1

H2 pressure [Pa]

H content [H/M]

1.6 6

10

Ti25V35Cr40

5

10

Ti25V35Cr40B0.1 Ti25V35Cr40B1

Ti25V35Cr40B5

0.0

0

200

400

600

800

time [s] Fig. 4 e Hydrogen absorption kinetics of annealed Ti25V35Cr40Bx alloys after hydrogen activation treatment at 400  C.

Ti25V35Cr40B5

4

10

0.0

0.4

0.8

1.2

1.6

2.0

H content [H/M]

Fig. 5 e PeC isotherm curves of annealed Ti25V35Cr40Bx alloys.

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Table 1 e Effect of B and C on the hydrogenation characteristics of the annealed Ti25V35Cr40 at 30  C. Interstitial elementa

Solubilityb [at%]

Doping [at%]

Pdesorption [MPa]

Max. capacity [H/M]

Effective H desorption [H/M]

V

b-Ti e

e

0.16

1.61

0.80

B

<<0.1

e <<0.1

e 0.1 1.0 5.0

0.18 0.20 0.26

1.46 1.42 1.41

0.69 0.86 0.78

C

1.8

4.3

0.1 1.0 5.0

0.20 0.23 1.50

1.50 1.43 1.17

0.87 0.77 0.71

a Atomic radius: rB ¼ 0.088 nm and rC ¼ 0.077 nm. b From Refs. [1] and [16].

Ti25V35Cr40C5

(211)

a

(200)

The XRD patterns for as-cast and annealed Ti25V35Cr40Cx specimens are shown in Fig. 6(a) and (b), respectively. Again, the as-cast Ti25V35Cr40Cx (x ¼ 1 or 5) specimens are identified to be a BCC phase and along with a very small amount of secondary phase by XRD. After annealing, there is still a small amount of the secondary phase present in the specimens with x ¼ 1 and 5. From the OM observation (Fig. 3(c) and (d)), there are some precipitates situated in the matrix of Ti25V35Cr40C0.1 alloy and more in Ti25V35Cr40C5 alloy. Nevertheless, it demonstrates that carbon is dissolved more completely into the interstitial sites after annealing. Fig. 7 shows the hydrogen absorption kinetics of annealed Ti25V35Cr40Cx (x ¼ 0, 0.1, 1, and 5) specimens at 30  C. Within a very short period of 10 s, except for Ti25V35Cr40C5, all the specimens have a hydrogenation capacity of 0.4 H/M. For Ti25V35Cr40C5, the hydrogenation rates in the low and high plateaus are seriously retarded. It may be considered that hydrogen diffusion is retarded seriously either by the precipitates or carbon atoms situated in the interstitial sites. Fig. 8 shows the PCI curves of annealed Ti25V35Cr40Cx (x ¼ 0, 0.1, 1 or 5) specimens at 30  C. Their hydrogenation properties are also summarized in Table 1. With increasing the doping content of carbon in the sample, the desorption plateau pressure is increased, while the maximum hydrogenation capacity is simultaneously lowered. For example, the desorption pressure substantially increases from 0.16 MPa to 1.50 MPa with x ¼ 5 at%. Among them, the annealed Ti25V35Cr40C0.1 exhibits a maximum effective dehydrogenation capacity of 0.87 H/M, i.e., an increase of 9% hydrogen storage capacity.

The microstructural model for raising the effective hydrogen storage capacity in Ti25V35Cr40 due to doping of B or C can be schematically illustrated in Fig. 9. The lattice constant of annealed Ti25V35Cr40 derived from the XRD pattern is 0.303 nm. It is close to that of pure V (a ¼ 0.302 nm), but smaller than that of b-Ti (a ¼ 0.332 nm). This could be resulted from the substitution of larger Ti atoms (rTi ¼ 0.145 nm) with smaller atoms of V (rV ¼ 0.132 nm) and Cr (rCr ¼ 0.125 nm). As a result, the hydrogen desorption pressure of the annealed Ti25V35Cr40 alloy is similar to that of the VeH system, which has a flatten plateau with an ambient hydrogen desorption pressure at room temperature [17]. Moreover, considering the sizes of octahedral and tetrahedral sites in the BCC lattice using the hard sphere

(110)

Ti25V35Cr40Cx series

Microstructural model

o

Intensity

3.2.

3.3.

Ti25V35Cr40C1 o o

Ti25V35Cr40 20

b

30

40

Precipitate

50

60

70

80

50

60

70

80

2

Ti25V35Cr40C5

Intensity

Fig. 5 shows the PCI curves of annealed Ti25V35Cr40Bx (x ¼ 0, 0.1, 1, or 5) specimens at 30  C, and all of the hydrogenation properties are summarized in Table 1. With increasing the doping content of boron, the absorption pressure of high plateau is slightly increased. The increase of plateau pressure might be ascribed to modification of chemical potential at the interstitial sites due to the boron doping. It is unfortunate that the maximum hydrogenation capacity is simultaneously lowered. With the doping content of 1.0 at%, the desorption pressure of the high plateau increases from 0.16 MPa to 0.26 MPa. The effective dehydrogenation capacity increases from 0.80 H/M to 0.86 H/M, i.e., an increases of w8%.

Ti25V35Cr40C1 Ti25V35Cr40C0.1 Ti25V35Cr40

20

30

40

2 Fig. 6 e XRD patterns of Ti25V35Cr40Cx alloys. (a) As-cast and (b) annealed at 1200  C for 2 h.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 5 ( 2 0 1 0 ) 1 1 9 7 5 e1 1 9 8 0

2.0

11979

o

30 C

H content [H/M]

1.6 1.2

Ti25V35Cr40

0.8

Ti25V35Cr40C1

Ti25V35Cr40C0.1 Ti25V35Cr40C5

0.4 0.0

0

200

400

600

800

time [s] Fig. 7 e Hydrogen absorption kinetics of annealed Ti25V35Cr40Cx alloys after hydrogen activation treatment at 400  C.

model, both B and C atoms should situate only at the octahedral sites, as shown in Fig. 9. The solubility of B in either b-Ti or in V is extremely low, i.e., <<0.1 at%, as listed in Table 1. It is expected, therefore, that the annealed Ti25V35Cr40 has an extremely low solubility for B [1,16]. The amount of B occupying at the octahedral sites is easy to saturate at around 0.1 at%. Accordingly, the lattice constant of annealed Ti25V35Cr40 is not sensitive to the doping amount of B. Therefore, the maximum hydrogen absorption content in the annealed Ti25V35Cr40 will be leveled off beyond the doping concentration of 0.1 at%, as reported in Fig. 5 and Table 1. On the other hand, the carbon atom is smaller than boron atom. In addition, according to phase diagrams [1,16], the solubility of carbon element in either b-Ti alloy or V is larger than that of boron. For example, the solubilities of C in b-Ti and V are approximately 1.8 at% and 4.3 at%, respectively, as shown in Table 1. It is reasonably expectable that the amount of C dissolved into the matrix phase of the annealed Ti25V35Cr40 alloy is larger than that of B. The hydrogenation properties of the samples might be more affected by carbon than boron. For example, the blocking of hydrogen diffusion is more pronounced in Ti25V35Cr40C5 than in Ti25V35Cr40B5, presumably because a higher strain in the lattice is induced by

Fig. 9 e Microstructural model for doping of B or C into the octahedral sites of BCC Ti25V35Cr40.

more C atoms occupying at the interstitial sites. Hydrogen atoms at the interstitial sites become less stable because of the strain induced by the presence of B or C atoms. Therefore, the plateau pressure of Ti25V35Cr40Cx increases significantly with the doping of C, while the sites available for storing hydrogen atoms will decrease. Based on the above discussion, an 8e9% increase in the effective hydrogen storage capacity of the annealed Ti25V35Cr40 alloy was found with addition of 1 at % B and 0.1 at% C.

4.

Conclusion

In this study, interstitial doping by boron or carbon into Ti25V35Cr40 to raise effective hydrogenation capacity at room temperature was investigated. Annealing treatment at 1200  C for 2 h is effective to homogenize the doped alloy. All of the annealed specimens have a BCC structure, but some precipitates are also present. The effective reversible hydrogenation capacity of Ti25V35Cr40B1 and Ti25V35Cr40C0.1 are increased from 0.80 H/M to 0.86 and 0.87 H/M, respectively, leading to an 8e9% of increase in hydrogen storage capacity.

7

10

o

H2 pressure [Pa]

30 C

Acknowledgements

6

10

Ti25V35Cr40

5

10

Ti25V35Cr40C0.1

This work was supported by the Chung-Shan Institute of Science and Technology of Taiwan under contracts BV96D15P and BV97D11P and National Science Council of Taiwan under contract NSC-98-2218-E-155.

Ti25V35Cr40C1 Ti25V35Cr40C5

4

10

0.0

0.4

0.8

1.2

1.6

2.0

references

H content [H/M]

Fig. 8 e PeC isotherm curves of annealed Ti25V35Cr40Cx alloys.

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