Ni64Zr36 (M = V, Nb) thin films for hydrogen purification

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Multi-Layered Ni64Zr36/M/Ni64Zr36 (M=V, Nb) Thin Films for Hydrogen Purification Deok-hwan Yun , Sung Bum Park , Yong-il Park PII: DOI: Reference:

S0040-6090(19)30624-8 https://doi.org/10.1016/j.tsf.2019.137596 TSF 137596

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Thin Solid Films

Received date: Revised date: Accepted date:

3 August 2018 21 August 2019 24 September 2019

Please cite this article as: Deok-hwan Yun , Sung Bum Park , Yong-il Park , Multi-Layered Ni64Zr36/M/Ni64Zr36 (M=V, Nb) Thin Films for Hydrogen Purification, Thin Solid Films (2019), doi: https://doi.org/10.1016/j.tsf.2019.137596

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Highlights 

Significantly improved hydrogen permeability at low temperature region

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Negative activation energy for hydrogen permeation due to the V/Nb mid-layer

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Hydrogen permeability reached to the permeability level of Pd at 373K

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Inter-diffusion between layers at high temperature prevents further permeability increase

Multi-Layered Ni64Zr36/M/Ni64Zr36 (M=V, Nb) Thin Films for Hydrogen Purification Deok-hwan Yun, Sung Bum Park, Yong-il Park* Department of Advanced Materials Engineering, Kumoh National Institute of Technology,61 Daehak-ro, Gumi, Gyeongbuk 39177, Republic of Korea * Corresponding author, Tel: +82-54-478-7743, Email: [email protected]

Abstract For hydrogen purification at low temperature, multi-layered hydrogen-permeable thin films with amorphous Ni64Zr36/M/Ni64Zr36 (M=V, Nb) structure were developed and the microstructure and hydrogen permeation performance were examined. The multi-layered thin films were deposited through DCmagnetron sputtering using Ni64Zr 36 and V/Nb targets. The overall thickness was adjusted to 2 μm, and that of the internal V or Nb layer was varied between 0.5 and 1.6 μm. The hydrogen permeability was measured in the range of 373 ~ 573K. All the multi-layered thin films showed negative activation energy for hydrogen permeation indicating an increasing permeability with a decreasing temperature. The calculated negative activation energy, - 0.43 ~ - 0.60 kJ/mol for Ni64Zr36/V/Ni64Zr36 thin films and - 1.19 ~ - 3.90 kJ/mol for Ni64Zr36/Nb/Ni64Zr36 thin films, and the increased hydrogen permeation at low temperature region is obviously due to the V or Nb mid-layer. The observed hydrogen permeability of the prepared thin films was higher than the reported value of amorphous Ni64Zr 36 alloy membrane at 573K and reached the permeability of Pd at 373K. Keywords: hydrogen purification; hydrogen permeability; Nickel-Zirconium; thin film; multilayer

1. Introduction The use of fossil fuels is constantly increasing; however, their reserves are limited. Fossil fuels not only emit pollutants; their prices also greatly fluctuate. Accordingly, hydrogen is being spotlighted as the clean fuel of the future and as the main raw material for industry because compared with fossil fuels, hydrogen has lower production costs and exists in nearly infinite amounts [1]. Therefore, constant research is being conducted on the separation and purification of hydrogen existing in the air and sea. One method used is the permeation-catalysis separation process of metallic membranes, which is efficient and has advantages in terms of the selectivity and economic feasibility of materials. In addition, metallic separation membranes have a compact structure, which allows selective permeation and grant them high applicability. The metallic separation membranes commercialized thus far generally use Pd, which has excellent hydrogen storage and permeation properties, and also is an expensive noble metal. Accordingly, constant research has been dedicated to the commercialization of hydrogen purification membranes with the same properties as the hydrogen-permeable membranes using Pd or its alloys [2-4]. It has been reported that the metallic hydrogen permeable membranes rely on a surface catalysis reaction that separates a hydrogen atom into electrons and protons [5]. The protons then move across the interior of the metallic permeable

membrane to the surface of the opposite side, reuniting with electrons, yielding an increase in hydrogen permeation. At this point, the passage of large molecules such as CO, CO2, N2, and O2 is restrained, allowing only hydrogen to pass through, thereby resulting in high hydrogen selectivity. In addition, hydrogen-permeable membranes using Pd or its alloys exhibit excellent surface activation, which facilitates the dissociation of hydrogen molecules, yielding high hydrogen permeability. However, reactions with hydrogen molecules and impurities take precedence, weakening the dissociation characteristics of the membrane. In addition, the high hydrogen solubility causes hydrogen embrittlement, reducing the durability because of membrane deterioration [6]. The hydrogen permeability of such single-element pure crystalline metals depends on their lattice structure, deformities, and reactivity with hydrogen. Metals with a body-centered cubic (BCC) structure such as Fe, Nb, V and Ta exhibit excellent hydrogen permeability properties, and metals with a face-centered cubic (FCC) structure such as Ni and Pd exhibit comparatively lower hydrogen permeability. This behavior is caused by diffusivity differences, which arise as a result of the lattice density [7]. Amorphous hydrogen-permeable membranes have been reported to exhibit better mechanical and structural properties as well as higher strength, ductility, corrosion resistance, and hydrogen solubility than crystalline hydrogen-permeable membranes. Moreover, it has also been reported that their hydrogen embrittlement can be reduced due to the high content of open lattices. Amorphous alloys in hydrogenpermeable membranes exhibit properties that are equivalent to or better than those of Pd membranes; thus, many studies have been carried out on this topic. Recently, research on hydrogen-permeable membranes using alloys of Ni with Nb, V, and Zr (which exhibit excellent hydrogen solubility properties) have been reported [8-10]. Amorphous Ni-Nb-Zr alloys (created using the melt-spinning process presented by the Chandra group) were reported to exhibit hydrogen permeabilities of 10 -8~10-9 mol/m‧ s‧ Pa0.5 at 573~673K. However, the efficiency of the continuous hydrogen permeation decreases over time [11]. In addition, the hydrogen permeability of the amorphous Ni64HfxZr 64-x alloys reported by Hara et al. [12] decreased with decreasing Zr content and increasing Hf content. The amorphous Ni 64Zr36 alloy without Hf exhibited the highest hydrogen permeability. The hydrogen permeabilities of the Pd-coated Ni64Zr36 and Ni64Zr26Nb10 alloys researched by Dolan et al. [13] were 2.6 × 10-9 and 1.9 × 10-9mol/m‧ s‧ Pa0.5 at 773K, respectively. In this study, with the objective of obtaining high-purity hydrogen by improving production costs and hydrogen permeability at low temperature, multi-layered hydrogen-permeable thin films based on Ni64Zr36/M/Ni64Zr36 (M=V, Nb) alloys were deposited on porous nickel supports (PNS) using the DCmagnetron sputtering process. In the suggested multi-layered structure, the Ni64Zr36 alloy, which exhibits excellent hydrogen dissociation ability, was placed on the surface, and V and Nb, which exhibit excellent hydrogen solubility and diffusion speed at low temperature, were placed in the middle. Since V and Nb have been reported to have hydrogen solubilities more than 1000-times higher than that of Pd, and also the activation energies for hydrogen permeation have negative values. That is, the hydrogen permeabilities of these metals increases as temperature decreases in contrast to Pd, Pt and Ni-Zr alloys, which show relatively sharp decrease of hydrogen permeabilities with a decreasing temperature [14]. The amorphous Ni64Zr36 alloy target with hydrogen dissociation properties similar to those of Pd was used to create hydrogen-permeable thin films with excellent performance at low temperatures under 573Kand appropriate performance-durability-cost relationships. The schematic illustration of the Ni64Zr36/M/Ni64Zr36 (M=V, Nb) is shown in Fig. 1.

Fig. 1. Schematic illustration of the multi-layered Ni64Zr36/M/Ni64Zr36 (M=V, Nb) thin films.

2. Experimental Procedure 2.1 Deposition of the thin films The multi-layered Ni64Zr36/M/Ni64Zr36 (M=V, Nb) thin films were prepared using Ni64Zr36, V, and Nb targets and deposited on porous nickel supports (PNS) through DC-magnetron sputtering. The PNS was uniaxially pressurized with nickel powder (Sigma Aldrich, 97%) having a particle size of 50 μm for 1 minute at a pressure of 300 MPa at room temperature using a stainless molding die of 2.0 cm diameter. Then, it was sintered in an argon atmosphere at 650 oC for 4 hours. The sintered PNS was polished using a sand paper (# 100, 200, 400, 800, 1000, 1200, 1500, 2000) on an auto polishing machine at room temperature and finally with 1 µm - and 0.3 µm-sized-abrasive alumina powder for 1 hour to obtain a mirror surface. After mirror polishing, it was cleaned using an ultrasonic cleaner. The PNS size used for the permeation test was 2.0 cm in diameter and 1.5 mm in thickness. As for each sputtering target, Gun 1 was equipped with a V or Nb target (99.9%, Plasmaterials, Inc., 5.08 cm - ϕ) and Gun 2 was equipped with a Ni64Zr36 target (99.8%, Plasmaterials, Inc., 5.08 cm - ϕ). During deposition, the substrate temperature was kept at 25 oC. The deposition thickness of each material was controlled using the sputtering rate listed in Table 1. The total thickness was adjusted to 2 µm and the thin film thickness was measured using postdeposition FE-SEM cross sectional images. The specimens were cooled with liquid nitrogen and immediately broken to observe the cross sections. The sample numbers and the measured thicknesses are listed in Table 2.

Table 1. DC-magnetron sputtering conditions. Target Initial vacuum (Torr)

Ni64Zr36 8.0 × 10-7

V 8.0 × 10-7

Nb 8.0 × 10-7

Operation vacuum (Torr)

4.2-4.3 × 10-3

4.2-4.3 × 10-3

4.2-4.3 × 10-3

DC Power (W)

100

100

100

Argon flow (sccm) Deposition rate (nm/min)

20 8

20 5.3

20 11.1

Target diameter(cm)

2.0

2.0

2.0

Target to substrate distance(mm)

50

50

50

Process gas

Argon 5.0

Argon 5.0

Argon 5.0

substrate temperature(℃)

25

25

25

Table 2. Hydrogen permeability of Ni64Zr36/M/Ni64Zr36 (M=V, Nb) thin films. Sample

V-type

Nb-type

V-25 V-50 V-80 Nb-25 Nb-50 Nb-80

M / Total thickness (%, M=V, Nb) 25.4 49.2 79.2 26.2 49.8 79.7

Measured Thickness (nm) Ni64Zr36 V orNb Ni64Zr36 765 494 206 763 494 212

521 971 1534 537 975 1645

764 506 196 744 487 206

H2 Permeability at 373K (nmol/m‧ s‧ Pa0.5) 0.72 0.56 0.50 0.72 0.56 0.50

Ea (kJ/mol) -0.50 -0.60 -0.43 -1.19 -3.16 -3.90

2.2 Analysis

To verify the crystallinity of the fabricated thin films, X-ray diffraction (XRD, Rigaku Corp. Model D/max-2500) using a Cu Kα target was employed, and the analysis was performed in the range of 2θ = 20°~85° with a rate of 0.5°/min. In addition, to verify the composition and determine the surface micromorphology before and after hydrogen permeation, both the surface and cross section were examined using field-emission scanning electron microscopy (FE-SEM, JEOL model JSM 6500F) and energydispersive X-ray spectrometry (EDS, Oxford Instruments, INCA Energy). For hydrogen permeability measurement, Sievert-type hydrogen-permeation test system (Nara Cell Tech Corp., Korea) was used as shown in Fig. 2(a). The coin-shaped sample is mounted as shown in Fig. 2(b) and hydrogen is injected in the PNS direction. The operating system is shown in the figure, and measurements were made using Gas Chromatograph (GC-2010 Plus, Shimadzu). The equipment included a mass flow controller (MFC), pressure control device, heating chamber, and STS permeation cell. To minimize the possible development of oxides after deposition of the thin films during the evaluation of the hydrogen permeation characteristics, an activation treatment was performed under a hydrogen atmosphere for more than 1 hour at greater than 573K. During this treatment, the heating rate was set below 5 K/min to prevent cracking caused by thermal shock. During the hydrogen permeability test, the hydrogen permeating through the membrane together with Ar, which is a carrier gas, was monitored using gas chromatography (GC). The value measured by GC was used to determine the concentration of hydrogen gas. The concentration of hydrogen was measured using a thermal conductivity detector (TCD), which utilizes the thermal conductivity differences between the measuring components and Ar in its measurement. The hydrogen permeability of the produced coinshaped permeable membrane was evaluated by fixing the hydrogen pressure at 760 Torr (1 atm) and taking temperature as a variable. Table 3 lists the hydrogen permeation test conditions.

(a)

(b) Fig. 2. Schematic of (a) Sievert-type hydrogen-permeation test system and (b) test cell.

Table 3. The used conditions for the hydrogen permeation test. Test gas

Ni64Zr36/V/Ni64Zr36 H2 (99.999%)

Ni64Zr36/Nb/Ni64Zr36 H2 (99.999%)

Flow gas

Ar (99.995%)

Ar (99.995%)

Flow rate (sccm)

40

40

Temperature range (K) Gas pressure difference (Torr)

373-573 760

373-573 760

3. Results and Discussion 3.1 Microstructure Fig. 3 and 4 present XRD patterns of the fabricated Ni64Zr36/M/Ni64Zr36 (M=V, Nb) thin films; the peaks assignable to Ni64Zr36 and V are unidentifiable for each part indicating that the thin films do not have a crystalline structure but an amorphous one or a mix of crystalline and amorphous structure. Only the strong Ni and Ni3C characteristic peaks by PNS were identified. The Ni-Zr alloy of the Nb-type hydrogen-permeable thin films exhibits a broad peak at 2θ = 27°~40° in Fig. 3. A Nb halo pattern is also observed around 2θ = 37°, and this halo pattern has an almost identical form as that of the peak observed in Ni-Nb-Zr amorphous alloys.

Fig. 3. XRD patterns of (a) PNS and (b) V-80, (c) V-50, (d) V-25.

Fig. 4. XRD patterns of (a) PNS and (b) Nb-80, (c) Nb-50, (d) Nb-25. As mentioned in the experimental section, the PNS substrates were fabricated by SPS method. Unlike the conventional powder-sintered substrate, the pores are narrow and long in shape and have a very uniform distribution. As shown in the SEM image in Fig. 5, the width of the pores is in the range of 0.8 ~ 1.5 µm, which can be sufficiently covered by a 2- µm -thick film.

Fig. 5. FE-SEM surface images of the polished PNS substrates. SEM images of the fabricated thin films’ surfaces and cross-sections are presented in Fig. 6 and 7. Any pin-holes or cracks, internal defects are not observed. The films have compact three-layered structure in which each upper, middle and lower layer is clearly divided. The observed columnar structures can be explained by Thornton model [15,16]. The Ar pressure was 0.01 Torr (10 micron) in this study, which is similar pressure with that used in the model. Thus, a section of 0.01 Torr of Ar pressure and 0.20 of Ts(298K)/Tm(1480K) for Ni64Zr36 (Ts: substrate temperature, Tm: melting point) [17] are applied for the thin films fabricated in this study. It has been reported that when the ratio of Ts/Tm reaches about 0.2 to 0.4, a columnar shape occurs during the sputtering process [16]. Therefore, applying 0.01 Torr of Ar pressure and 0.2 of Ts/Tm to the Thornton model corresponds to the obvious columnar structure between 3C and 3B, and this structure can be seen in the FE-SEM cross-sectional images of Fig. 7. Although the hydrogen permeability of the thin films deposited with this columnar shape is expected to be increased due to the formation of defects in the columnar structure, they were reported to have a lower hydrogen purification rate than conventional crystalline membranes with fully dense granular structures. [18,19] On the other hand, in a previous study on multi-layered Ni-Zr films formed by SPS, it was found that hydrogen-embrittlement easily breaks the Zr-rich film in hydrogen activation process even under low hydrogen pressure. In this study, Nb-80 and Nb-50 with high Nb content were broken after hydrogen permeation test. However, such fracture was not observed for Nb-25 and V-type thin films.

Fig. 6. FE-SEM surface images of the multi-layered hydrogen permeable thin films deposited on the PNS; (a) Nb-25, (b) Nb-50, (c) Nb-80, (d) V-25, (e) V-50, (f) V-80. (Scale bar denotes 100 nm.)

Fig. 7. FE-SEM cross-sectional images of (a) V-25, (b) V-50, (c) V-80 (d) Nb-25, (e) Nb-50, (f) Nb-80. (Scale bar denotes 1 μm.) 3.2 Hydrogen permeability Fig. 8 shows an Arrhenius plot of the obtained hydrogen permeabilities of the fabricated thin films. The hydrogen permeability at 373K and the activation energy (Ea), which is calculated from the slope of the fitting lines in Fig. 8, are listed in Table 2. According to Matsumura et el. [20], the hydrogen permeability (Q) can be expressed as a function of the diffusion coefficient (D) and the hydrogen solubility (S). This relationship can be summarized as following equations (1)~(5).

Q = DS (1) Since both D and S follow thermal activation process, D = D0 exp(Ed/RT) (2) S = S0 exp(Es/RT) (3) where, D0 is a pre-exponential factor for hydrogen diffusion, S0 is a pre-exponential factor for hydrogen solution. Therefore, temperature dependence of Qis, Q = Q0 exp(Ea/RT) (4) where, Q0 is a pre-exponential factor. Thus, activation energy for hydrogen permeability is, Ea = Ed + Es(5) Therefore, Ea can be expressed as a sum of the activation energy for the diffusion Ed, and the activation energy for the hydrogen solution Es. The activation energy of the multi-layered thin films is expected to be significantly affected by the mid-layers which have high hydrogen solubility. All the measured hydrogen permeability of the Nb-type thin films increased with decreasing temperature, presenting opposite activation energy to those reported in Pd membrane [21] and amorphous Ni64Zr 36 thin film [22]. That is, the activation energies of Nb-type thin films decreased from -1.19 kJ/mol to -3.90 kJ/mol as Nb layer thickness increased from Nb-25 to Nb-80, showing a decrease in hydrogen permeability in elevating temperature. V-type thin films show also negative but relatively low activation energy, varies from -0.43 to -0.60 kJ/mol, thus the hydrogen permeabilities almost independent on temperature. As previously mentioned, for pure V and pure Nb, the activation energies for hydrogen permeation have negative values. Therefore, the observed negative activation energies of the V- and Nb-type thin films are obviously attributable to the V and Nb mid-layers. Therefore, all the hydrogen permeabilities of the V- and Nb-type thin films below 523K showed higher values than that of the amorphous Ni 64Zr36 membrane reported by Hara et al. [22]. For the V-type thin films, as the V-layer thickness increased, the hydrogen permeability also increased and reached to 7.2 x 10-10 mol/m‧ s‧ Pa0.5 at 373K. This is because the hydrogen diffusion is faster in V than in amorphous Ni64Zr36. Since the total permeable membrane thickness is fixed at 2 μm, as the thickness of the V layer increases - simultaneously that of Ni64Zr36 decreases - effect of V becomes greater. Therefore, V-80 exhibit higher hydrogen permeability than V-50 and V-25, simply because they have low V content. However, it was found that as the Nb layer’s thickness in Nb-type thin films decreased from Nb-80 to Nb-25, the apparent hydrogen permeability increased and the activation energy increased. The maximum permeability of Nb-25 was 2.1 x 10 -9 mol/m‧ s‧ Pa0.5 at 373K. The permeability and activation energy change induced by the inter-diffusion at interfaces can be considered as a reasonable cause. According to Yamaura et al. [23], Ni-Nb or Ni-Nb-Zr alloys are easily formed and their hydrogen permeabilities are lower than those of the Ni64Zr36 and pure Nb. Therefore, these layers possibly act as the aforementioned walls preventing diffusion. After measuring the hydrogen permeability for 3 h at 373-573 K, it was predictable that partial inter-diffusion would occur in the interface between the Ni64Zr36 layer and Nb layer. In order to check the inter-diffusion of Nb-type thin films, EDS line mapping was performed for Nb-25 and Nb-50 and the results are presented in Fig. 9. It is obvious that inter-diffusion occurred at the Ni64Zr36Nb interfaces from the EDS result. In particular, the diffusion of Nb into the entire depth of the relatively thin Ni64Zr36 layer was noticeable in Nb-50. However, partial diffusion was observed in Nb-25 with a relatively thick Ni64Zr36 layer. This finding indicates that the hydrogen permeability of Nb-80 and Nb-50 could be lower than that of the Nb-25 because the catalytic performance depending on Ni64Zr36 layers could be seriously damaged by the formation of Ni-Nb or Ni-Nb-Zr alloys which can work as diffusion barrier. Most of the Nb-type thin films with thick Nb layers fractured at 373~423K because of hydrogen embrittlement after performing hydrogen permeability measurements. In particular, Nb-80 became

extremely brittle at temperatures below 423K by hydrogen embrittlement, and it was unable to endure just 0.1 MPa pressure difference. In fact, below 171 °C, the BCC-phase Nb transformed into FCC NbH2, in which H enters the tetrahedral sites, causing hydrogen embrittlement. Similarly, below 493K, V changes into a V2H or VH2, in which H enters the tetrahedral sites of V [24]. However, unlike in the aforementioned Nb-type thin films, no fracture occurred in the V-type thin films in this study.

Fig. 8. Hydrogen permeability of the fabricated thin films.

Fig. 9. EDS line mapping results (a) before and (b) after hydrogen permeation test for Nb-25, and (c)

before and (d) after hydrogen permeation test for Nb-50. 4. Conclusions Hydrogen-permeable thin films with a multi-layered structure of Ni64Zr36/M/Ni64Zr 36 (M=V, Nb) were successfully fabricated through DC-magnetron sputtering using Ni64Zr36 alloy and pure V/Nb targets. The obtained thin films with thickness of about 2 μm showed defect-free columnar structure and amorphous phase. After exposed to hydrogen at 373~423K, most of the Nb-type thin films became extremely brittle and fractured by hydrogen embrittlement. However, no fracture occurred in the V-type thin films. All the fabricated thin films showed negative activation energies for hydrogen permeation indicating an increasing permeability with decreasing temperature. The observed negative activation energies and the increased hydrogen permeation performance at low temperature under 573K is obviously due to V/Nb mid-layer. This interesting thermal behavior is expected to be widely applied to low-temperature hydrogen purification researches. The hydrogen permeabilities of the fabricated thin films were superior to that of the amorphous Ni 64Zr36 membrane under 573 K and reached to the permeability level of Pd at 373K. The maximum hydrogen permeability of the Nb-type thin films was 2.1 x 10-9 mol/m‧ s‧ Pa0.5 at 373K for Nb-25. For the samples with thicker Nb layer, a significant Nb diffusion into Ni 64Zr36 layers seems to prevent further permeability by formation Ni-Nb or Ni-Nb-Zr diffusion barrier. For the V-type thin films, as the V-layer thickness increased, the permeability also increased and reached to 7.2 x 10-10 mol/m‧ s‧ Pa0.5 at 373K for V-80. Acknowledgement This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (NRF-2016R1D1A1B01011819).

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