Hydrogen Diffusion Coefficient, Hydrogen Solution Coefficient and Hydrogen Permeability of Nb-TiNi Eutectic Alloy

In general, hydrogen permeabilityΦ of the alloy membrane is expressed as the product of the hydrogen diffusion coefficient D and the hydrogen solution coefficient K. Therefore, to improve the hydrogen permeability efficiently, the values of K and D should be separately considered. In the present study, hydrogen absorption and permeation behaviors of the Nb19Ti40Ni41 alloy consisting of the eutectic phase are investigated by measuring pressure-composition-isotherm (PCI) and by the hydrogen flow method and compared with those of palladium. The hydrogen absorption in the Nb19Ti40Ni41 alloy does not obey the Sieverts’ law in the pressure region of 0-1.0MPa at 523K, but it shows linear relationship between the difference in the square root of hydrogen pressure and hydrogen content between 0.1 and 0.4MPa. Although the value of D for the Nb19Ti40Ni41 alloy is considerably lower than that of palladium, its high K value enhances the hydrogen permeability Φ. It is suggested that the enhancement of D by microstructural control for Nb19Ti40Ni41 alloy is effective for improvement of Φ.


Introduction
Palladium-silver alloys have been used as hydrogen permeation membranes for separation and purification of hydrogen gas [1,2]. However palladium is expensive and a rare metal, so that it is urgent to develop new, low cost and high performance non-palladium-based hydrogen permeation alloys. Recently, the Nb-TiNi alloys which consist of the primary bcc-(Nb, Ti) and the eutectic {bcc-(Nb, Ti) + B2-TiNi} phases have attracted wide attention because of their high Φ values and excellent resistance to hydrogen embrittlement [3,4]. It has been explained that the hydrogen permeation is mainly determined by the primary phase, while the suppression of hydrogen embrittlement is mainly controlled by the eutectic one [5]. However it is not experimentally elucidated that the relationship of Φ, D and K in these alloys.
In general, the diffusion flux (J) of hydrogen in the metal membrane is given by Fick's first law as follows.
Where D represents the diffusion coefficient of hydrogen in solids, C u and C d are the hydrogen content in the solid just beneath the surfaces in the upstream and the downstream sides, respectively.
L is the thickness. The hydrogen concentration gradient is the driving force of the hydrogen diffusion. Hydrogen permeation thorough the metal membrane is schematically shown in Fig.1.

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Online:  When the hydrogen concentration is low, the relation between the equilibrium hydrogen content C and hydrogen partial pressure are given by Sieverts' law as follows: C = K×P 0. 5 (2) Then , the Fick's first law can be rewritten as follows, In practice, the membrane is used in the high hydrogen concentration region where the Sieverts' law is inapplicable. In such a case, if the hydrogen content is expressed as follows, C=K×P 0.5 +α (4) Then, the hydrogen flux is written as follows.
J =DK(∆P 0.5 )/L =Φ(∆P 0.5 )/L (5) Here, D and K are the hydrogen diffusion coefficient and the hydrogen solution coefficient, respectively. The product of them is hydrogen permeability (Φ). ∆P 0.5 and L are the difference of square root of the hydrogen pressure at both sides of the membrane and its thickness, respectively. If values of Φ and K are experimentally measured, then the value of D can be calculated by using Eq. (5). In the present study, K and Φ of the Nb 19 Ti 40 Ni 41 alloy consisting of the eutectic {bcc-(Nb, Ti) +B2-TiNi } phase [6] are determined by measuring the pressure-composition -isotherm (PCI) and by the gas flow method, respectively. Then, the value of D is evaluated by using Eq. (5). Furthermore, the values of D and K of the Nb 19 Ti 40 Ni 41 alloy are compared with those of palladium, and the controlling factor for hydrogen permeation of this alloy is discussed.

Experimental
Palladium and Nb 19 Ti 40 Ni 41 alloys (mol%) were used in the present study. Palladium (99.98 mass%) was purchased from Tanaka Co., Japan. The Nb 19 Ti 40 Ni 41 alloy was prepared by arc melting in a purified argon atmosphere using Nb (99.9 mass%), Ti (99.5 mass%) and Ni (99.9 mass%). The samples of 12mm in diameter and 0.75 mm in thickness were cut from this alloy ingot by a spark erosion wire-cutting machine for the hydrogen permeation experiments. The rectangular parallelepiped samples of 12 mm × 12mm × 0.75 mm were cut for hydrogen absorption experiments. Microstructural and structural examinations were carried out with a scanning electron microscope (SEM, JSM 5300) and an X-ray diffractometer (XRD, PANalytical X'Pert PRO),

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respectively. After both sides of these disks were polished using buff with 0.5µm Al 2 O 3 particles, palladium was coated with 190 nm thicknesses using a DC sputtering machine to avoid oxidation and to enhance the dissociation of hydrogen molecule on the disks. After the alloy disk was sealed by gaskets, both sides of the sample were evacuated using a diffusion pump below 3×10 -3 Pa, then heated to 673K and kept for 1.8 ks. Pure hydrogen gas (99.99999%) of 0.2-0.4 MPa and 0.1 MPa was introduced in the upstream side and in the downstream side, respectively. Hydrogen flux J passing through the disk was measured by a hydrogen flow meter (KOFLOC Model-3300), and the value of Φ was determined from the slope of the relation between J×L vs. ∆P 0.5 plot. PCI was measured using a Sieverts-type apparatus. The amount of absorbed hydrogen was calculated from the change of the pressure and the inner volume of the chamber. The temperature of the sample was controlled to an accuracy of ±1K during the measurement. Absorption experiments were carried out at 523K, and at pressures ranging from 0.01 to 1MPa.    The hydrogen flux J is related to hydrogen permeability Φ as shown in Eq. (5). The J×L values are plotted against ∆P 0.5 as shown in Fig. 5. The experimental data are on the straight lines passing through the origin. The gradient of these lines correspond to Φ at each temperature. Figure 6 shows an Arrhenius plot of the hydrogen permeability Φ for the Nb 19 Ti 40 Ni 41 alloy and palladium. The value of Φ increases with increasing temperature, and the Φ value of the Nb 19 Ti 40 Ni 41 alloy is lower than that of palladium from 523K to 673K.

Hydrogen permeability Φ, hydrogen solution coefficient K and hydrogen diffusion coefficient D of the b 19 Ti 40 i 41 alloy and palladium
The values of Φ, K and D for the Nb 19 Ti 40 Ni 41 alloy and palladium at 523 K are listed in Table 1.
The value of D for palladium is almost the same as that of Baykara [7].

Summary and Conclusion
The hydrogen concentration is approximated as C=KP 0.5 + α between 0.1 and 0.4MPa at 523K for the Nb 19 Ti 40 Ni 41 alloy. The value of hydrogen diffusion D for Nb 19 Ti 40 Ni 41 alloy is lower than that of palladium. The bcc-(Nb, Ti) phase which exhibits high hydrogen permeability and the B2-TiNi phase which is excellent resistance to hydrogen embrittlement but low hydrogen permeability are mixed alternately as lamellar microstructures. It is suggested that the enhancement of D by the microstructural control is effective for improvement of hydrogen permeability Φ for the Nb 19 Ti 40 Ni 41 alloy.