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Improvement in high temperature stability of Pd coating on Nb by Nb2C intermediate layer
International Journal of Hydrogen Energy 32 (2007) 615 – 619 www.elsevier.com/locate/ijhydene
Improvement in high temperature stability of Pd coating on Nb by Nb2C intermediate layer Yuji Hatano ∗ , Keita Ishiyama, Hirofumi Homma, Kuniaki Watanabe Hydrogen Isotope Research Center, University of Toyama, Toyama 930-8555, Japan Received 17 May 2006; accepted 3 June 2006 Available online 2 August 2006
Abstract Niobium subcarbide (Nb2 C) was chosen as a material for non-porous intermediate layer to improve the high temperature durability of Pd–Nb composite membranes for hydrogen separation. A layer of Nb2 C was prepared between Nb substrate and thin Pd films (100 nm), and the stability of Pd coating at elevated temperatures (573–773 K) was examined by hydrogen absorption experiments. Hydrogen permeability through the Nb2 C layer appeared to be sufficiently high, and no noticeable deterioration was observed in hydrogen absorption rate under as-prepared conditions. The degradation in coating effect of Pd at elevated temperatures was substantially mitigated by Nb2 C layer. Such improved durability was ascribed to retardation of open porosity development by Nb2 C caused as a consequence of impeded interdiffusion between Pd and Nb. 䉷 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Hydrogen; Purification; Separation; Membrane; Palladium; Niobium; Durability
1. Introduction Hydrogen separation membranes are key components of membrane reactors for hydrogen production via, for example, steam reforming of natural gas [1]. Group V metals (V, Nb and Ta) have higher hydrogen permeabilites, lower prices and richer natural resources than Pd which has been extensively used as a membrane material. The surfaces of group V metals are reactive not only for hydrogen but also for other impurities such as oxygen, and hence it is common to prepare thin Pd films on the surfaces of permeation membranes as protection layers against oxidation [2–12]. The hydrogen permeability of such composite membranes, however, declines at elevated temperatures (> 673 K) due to the degradation in coating effect caused by interdiffusion between Pd and base metals [6,7]. Edlund et al. [6,7] developed porous oxide intermediate layers to prevent the interdiffusion between Pd and V. The Pd layers employed in their study, however, were rather thick (25 m), and hence the obtained values of permeability were significantly smaller than the intrinsic value of V. In addition, the amount of Pd necessary to prepare the composite membrane was comparable with conventional Pd membranes. ∗ Corresponding author. Tel.: +81 76 4456928; fax: +81 76 4456931.
E-mail address: [email protected] (Y. Hatano).
In the present study, layers of Nb2 C were prepared between thin Pd films (100 nm) and Nb substrates, and the high temperature durability of Pd coating against interdiffusion was examined by hydrogen absorption experiments. The carbide was chosen as a material for non-porous intermediate layer because it can be prepared rather easily and in low cost by, for example, heating the substrate material in hydrocarbon gases. In addition, it has been known that carbide layers formed on Nb and V are permeable for hydrogen [13–15]. 2. Experimental Disk type specimens (10 mm in diameter and 0.5 mm in thickness) were prepared from a cold-worked rod of pure Nb supplied by Nilaco (99.9 mass% purity). Their surfaces were polished with abrasive papers and finished with Al2 O3 powder of 0.06 m. A portion of specimens were carburized by heating at 1173 K for 30 min in a mixture gas of CH4 (10%) and H2 (90%) at atmospheric pressure. Then these specimens were heated in vacuum at 1373 K for 5 h to saturate the bulk by carbon. The diffusion coefficient of carbon, DC , in Nb at 1373 K can be evaluated from the data reported by Power and Doyle [16] to be 2 × 10−12 m2 s−1√ . Hence, the mean diffusion length during this heat treatment, 2DC tH , where tH is the duration of heat
0360-3199/$ - see front matter 䉷 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2006.06.045
Y. Hatano et al. / International Journal of Hydrogen Energy 32 (2007) 615 – 619
The specimens were also examined by means of XRD and scanning electron microscopy (SEM) combined with energydispersive X-ray analysis (EDX). 3. Results and discussion Fig. 1(a) shows typical hydrogen absorption curves observed at 573 K for Pd/bare-Nb. In the case of the specimen under asprepared conditions (i.e. no heat treatment), the pressure of hydrogen dropped rapidly due to the absorption by the specimen. The rate of hydrogen absorption, however, slightly decreased after heating at 573 K, and the extent of reduction clearly increased with increasing heating temperature. The degradation appeared to be ended by heating at 648 K, and no further decline in the absorption rate was observed by heating at higher temperatures. The typical results for Pd/Nb2 C–Nb are shown in Fig. 1(b). The hydrogen absorption rate of the specimen under asprepared conditions was comparable with or even slightly higher than that of Pd/bare-Nb. These observations clearly show 14 12
648K 673K
10 H2 pressure (Pa)
treatment, is calculated to be 0.27 mm and larger than a half of specimen thickness. Analysis by means of X-ray diffraction (XRD) showed the presence of Nb2 C, while no peak of NbC was observed. The thickness of Nb2 C layer was evaluated to be about 300 nm from the mass gain due to carburization. In this evaluation, the contribution of carbon in solid solution phase to mass gain was assumed to be negligible since the solubility limit of carbon in Nb is small at 1373 K [17]. An accurate value of solubility limit at this temperature, however, is not available. Hence, the thickness of Nb2 C layer thus evaluated may be slightly overestimated. Thin Pd films whose thickness was 100 nm were prepared on both surfaces of carburized and non-carburized specimens by vacuum deposition in a separate apparatus. The specimens were installed in the vacuum chamber and heated at 1000 K for 4 h to remove oxide films formed on the surfaces during the transportation of specimens; dissolution of oxygen into the bulk by heating led to removal of oxide films. The pressure of residual gases during heating was 10−6 Pa. After cooling down the specimens to room temperature, Pd was deposited onto the surfaces up to 100 nm at the rate of 0.05 nm s−1 . The specimens without carbide layers are hereafter denoted as Pd/bare-Nb, and those with Nb2 C layer as Pd/Nb2 C–Nb. It is known that the preparation of Pd films of such thickness does not deteriorate the total permeability of composite membrane based on group V metal [3]. The stability of Pd coating was examined in a temperature range from 573 to 773 K in the manner described below. First, the specimens were heated in vacuum at a given temperature for 1 h. After cooling down to room temperature, the specimens were exposed to air for 1 h. Then the durability of Pd coating was examined by measuring hydrogen absorption rate at 573 K. The hydrogen absorption experiments were carried out in a closed vacuum system, and hence the rate of hydrogen absorption was evaluated from that of pressure drop. Details of the apparatus used and experimental procedures are described elsewhere [18]. The initial pressure of H2 gas was adjusted to be 13.3 Pa. In preliminary hydrogen absorption experiments carried out for pure Pd and Nb under the above-mentioned conditions, the Pd specimen absorbed hydrogen rapidly, whereas the Nb specimen showed no noticeable hydrogen absorption due to the presence of oxide film. These observations indicate that the hydrogen absorption rate of the Pd-coated Nb specimens should decline substantially if Nb appears on the specimen surfaces due to interdiffusion between Nb and Pd. The above-mentioned value of initial H2 pressure (13.3 Pa) is significantly smaller than the upstream H2 pressures commonly employed in the permeation experiments in this field [3–12]. In general, however, the rates of hydrogen absorption and permeation become more sensitive to surface states as pressure decreases [19]. Hence, the experiments in the low H2 pressure region appeared to be suitable to examine the stability of coating effects. Some specimens were subjected for hydrogen absorption experiments for several times. The hydrogen concentration in the specimen, however, never exceeded 2 at%.
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Fig. 1. Typical hydrogen absorption curves obtained for (a) Pd/bare-Nb and (b) Pd/Nb2 C–Nb at 573 K. The temperatures indicated in the figures are those of heat treatments in vacuum.
Y. Hatano et al. / International Journal of Hydrogen Energy 32 (2007) 615 – 619
Nb,
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Fig. 2. Typical examples of SEM images of (a) Pd/bare-Nb and (b) Pd/Nb2 C–Nb after heating in vacuum at 773 K for 4 h.
Fig. 3. Typical XRD patters of (a) Pd/bare-Nb and (b) Pd/Nb2 C–Nb obtained with Cu K X-rays after heating in vacuum at 873 K for 3 h. The incident angle of X-rays to the specimen surfaces was adjusted to be 1.5◦ .
that the permeability of hydrogen in Nb2 C is sufficiently high, and Nb2 C intermediate layers of the present thickness (about 300 nm) do not deteriorate the total permeability of composite membrane. In contrast to Pd/bare-Nb, no noticeable decline in the hydrogen absorption rate was observed even after heating at 623 K. Although the reduction in the absorption rate was observed at heating temperatures above 648 K, the extent of reduction was significantly smaller than that of Pd/bare-Nb. Observations by SEM revealed that the Pd layer became porous after heat treatments. Figs. 2(a) and (b) show SEM images of Pd/bare-Nb and Pd/Nb2 C–Nb after heating at 773 K for 4 h as typical examples. In the case of Pd/bare-Nb, holes whose diameter was 50–900 nm were formed in high areal density. Point analyses by means of EDX showed that the concentration ratios of Pd to Nb, [Pd]/[Nb], in holes were significantly smaller than those in matrix. Such holes were also observed in the Pd layer of Pd/Nb2 C–Nb, but the density and the maximum size of holes were significantly smaller than Pd/bare-Nb. The formation of intermetallic compound was observed by XRD analysis after heating Pd/bare-Nb at 873 K. The diffraction pattern after heating for 3 h is shown in Fig. 3(a)
as an example. The peaks of Pd which were observed before heating disappeared completely, and new peaks of Pd2 Nb appeared. In the case of Pd/Nb2 C–Nb, the peaks of Pd still remained as shown in Fig. 3(b), and no peaks of intermetallic compounds were observed. These results indicate that the rate of interdiffusion between Pd and Nb in Pd/Nb2 C–Nb was smaller than that in Pd/bare-Nb. The above-mentioned observations showed that the improved durability of Pd coating effects obtained for Pd/Nb2 C–Nb is due to the mitigation of porosity development during vacuum heating caused by reduced interdiffusion rate between Pd and Nb. Such development of open porosity should result in the appearance of Nb on the surfaces and formation of Nb oxides by subsequent exposure to air at room temperature. As described in Section 2, Nb oxides strongly impede hydrogen absorption at 573 K, and hence the regions on specimen surfaces covered by Nb oxides should not contribute to hydrogen absorption under the present conditions. Consequently, the rate of hydrogen absorption decreases with progress of porosity development. The extent of porosity development of Pd/bare-Nb, however, cannot be explained by interdiffusion via vacancy mechanism
Y. Hatano et al. / International Journal of Hydrogen Energy 32 (2007) 615 – 619
even with taking account of enhancement by hydrogen. Iida et al. [20] have measured the self-diffusion coefficient for pure Nb and Nb–H alloys ([H]/[Nb] = 0.05.0.34) and observed the significant enhancement of diffusion by hydrogen-induced vacancies. The obtained self-diffusion coefficient of Nb, however, was still small [20], and the value at 773 K and [H]/[Nb]=0.05 was evaluated to be 3 × 10−29 m2 s−1 by extrapolating their data to this temperature. The hydrogen concentration in Nb was less than 2 at% in the present study as mentioned in Section 2, and hence the Nb self-diffusion coefficient is considered to be smaller. Consequently, the mean diffusion length during heating appears to be radically smaller than the thickness of Pd layer; for example, the mean diffusion length during heating at 773 K for 1 h was evaluated to be 1×10−13 m even at [H]/[Nb]=0.05. Hydrogen concentration in Pd layers was evaluated to be much less than that in Nb (below 0.1 at%) because of smaller heat of hydrogen solution. Hence, the influence of hydrogen appears to be negligible. In the case of pure Pd, the self-diffusion coefficient at 773 K can be evaluated to be 2 × 10−23 m2 s−1 from the data reported by Peterson [21]. The mean diffusion length under the above-mentioned conditions is calculated to be 2 nm and still much smaller than the thickness of Pd layer. Therefore, fast diffusion through defects such as dislocations and grain boundaries appears to play a decisive role in the interdiffusion between Pd and Nb. The density of such defects could be sensitively dependent on heat treatment conditions. As described in Section 2, the heat treatment at 1373 K for 5 h was carried out in the preparation of Pd/Nb2 C–Nb to saturate the bulk with carbon. Namely, the density of defects in Nb substrate of Pd/Nb2 C–Nb should be smaller than that of Pd/bare-Nb. In order to separate the effect of Nb2 C layer on interdiffusion from that of the heat treatment, the durability of Pd layer was examined also with a fully recrystallized Nb specimen. A ribbon of Nb (0.1 mm in thickness) was ohmically heated in vacuum (10−7 Pa) at temperatures above 2300 K for 2.8 h and then a specimen whose size was 10×10×0.1 mm3 was cut from the ribbon. This heat treatment resulted in the recrystallization of the specimen with the development of equiaxed crystal grains of 0.1–1 mm size appearing along the specimen surfaces. After preparing Pd films, the hydrogen absorption experiments were carried in the above-mentioned manner to examine the durability of Pd layer. The results obtained for this specimen, hereafter denoted as Pd/RX-Nb, is shown in Fig. 4. Although the hydrogen absorption rate of Pd/RX-Nb was smaller than that of Pd/bare-Nb under as-prepared conditions, Pd/RX-Nb showed higher absorption rate after heating (see Fig. 1(a)). Namely, the extent of degradation appears to be reduced by the recrystallization. It should be noted that the small hydrogen absorption rate observed for Pd/RX-Nb under as-prepared conditions can be ascribed to the presence of Nb oxides in the interface between Pd and Nb. As described in Section 2, the specimens were heated to 1000 K in vacuum before the preparation of Pd films to remove the oxide films from the surfaces. The hydrogen absorption experiments for the specimens without Pd coating, however, showed that the oxide films formed on the recrystallized specimens could not be removed completely at this temperature. Namely, the hydrogen
14 648K
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618
8 6 573K
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Fig. 4. Hydrogen absorption curves of Pd/RX-Nb obtained at 573 K. The temperatures indicated in the figures are those of heat treatments in vacuum.
absorption rate of recrystallized specimen was significantly smaller than that of the specimen prepared from the coldworked Nb rod after heating in vacuum at 1000 K. The influence of this oxide film on interdiffusion between Pd and Nb, however, appears to be small as described below. For quantitative comparison of the durability, the absorption coefficient, , of H2 was evaluated for Pd/bareNb, Pd/Nb2 C–Nb and Pd/RX-Nb from the initial slopes of absorption curves shown in Figs. 1 and 4. Here the absorption coefficient is defined as the probability for the absorption of a H2 molecule into the specimen at a single collision to the surface. The absorption rate in the initial stage is described as dN P0 = A √ , (1) dt t=0 2mkT where N is the number of hydrogen molecules absorbed, t the elapsed time, A the surface area of specimens, k the Boltzmann constant, P0 the initial pressure of hydrogen, and m the mass of a H2 molecule. The values of thus obtained are plotted against heating temperature in Fig. 5. The values of decreased with increasing heating temperature for all types of specimens. Although the heat treatment conditions in the preparation of Pd/Nb2 C–Nb was more moderate than those of Pd/RX-Nb, Pd/Nb2 C–Nb gave the highest values of at all heating temperature examined; the values for Pd/Nb2 C–Nb were higher than those for Pd/RX-Nb by three times or more. In addition, SEM observations showed that the extent of porosity development in Pd layer of Pd/RX-Nb was comparable with that of Pd/bareNb after heating at 773 K for 4 h. The formation of intermetallic compounds, Pd2 Nb, was also observed for Pd/RX-Nb after heating at 873 K for 3 h. It was therefore concluded that not the heat treatment but the Nb2 C layer played a dominant role in the improved durability observed for Pd/Nb2 C–Nb. Namely, the preparation of Nb2 C intermediate layer could improve the high temperature durability of Pd coating without deterioration in hydrogen permeability.
Y. Hatano et al. / International Journal of Hydrogen Energy 32 (2007) 615 – 619
Science and Technology of Japan, No. 17560613. The authors express their sincere thanks to Professor A. Livshits and Dr. A. Busnyuk of Bonch-Bruyevich University for fruitful discussion, and to Professor K. Ikeno of University of Toyama for the use of SEM.
10-2 Pd/Nb2C-Nb Absorption coefficient,
-3
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Fig. 5. Change of H2 absorption coefficient, , of Pd/bare-Nb, Pd/Nb2 C–Nb and Pd/RX-Nb with heating temperature.
As described in Section 1, Edlund et al. [6,7] used porous oxide intermediate layers to prevent the interdiffusion between V substrate and rather thick Pd layers (25 m). In general, the preparation of thin Pd layer on porous materials is difficult. Hence, non-porous intermediate layer employed in the present study has advantage in this point. For further improvement of high temperature stability, the thickness of Nb2 C layers has to be optimized. The quantitative evaluation of hydrogen permeability through Nb2 C layer is necessary for this purpose. The optimizations of the thickness of Pd layer and preparation conditions are also required. 4. Conclusions Intermediate layers of Nb2 C were prepared between thin Pd films and Nb substrates, and its effect on the high temperature durability (573–773 K) of Pd films was examined by hydrogen absorption experiments. The following conclusions were derived. (1) No noticeable deterioration in hydrogen absorption rate was induced by the preparation of Nb2 C intermediate layer due to sufficiently high hydrogen permeation rate through the Nb2 C layer. (2) Hydrogen absorption rates of specimens without intermediate layers were sharply declined by heating at temperatures above 648 K, while such degradation was substantially mitigated by Nb2 C intermediate layer. (3) Such improvement in high temperature durability of Pd coating by the Nb2 C layer was ascribed to the retardation of porosity development in Pd layer caused by impeded interdiffusion between Pd and Nb. Acknowledgements This study was supported in part by a Grant-in-Aid for Scientists Research (C) of Ministry of Education, Culture, Sports,
[1] Oertel M, Schmitz J, Weirich W, Jendryssek-Neumann D, Schulten R. Steam reforming of natural gas with integrated hydrogen separation for hydrogen production. Chem Eng Technol 1987;10:248–55. [2] Makrides AC, Wright MA, Jewett DN. Separation of hydrogen by permeation. US Patent 1967; 3350845, 7 November. [3] Nishimura C, Komaki M, Amano M. Hydrogen permeation characteristics of vanadium–nickel alloys. Mater Trans JIM 1991;32(5):501–7. [4] Amano M, Komaki M, Nishimura C. Hydrogen permeation characteristics of palladium-plated V–Ni alloy membranes. J LessCommon Met 1991;172–174:727–31. [5] Buxbaum RE, Marker TL. Hydrogen transport through non-porous membranes of palladium-coated niobium, tantalum and vanadium. J Membr Sci 1993;85:29–38. [6] Edlund DJ, Friesen D, Johnson B, Pledger W. Hydrogen-permeable metal membranes for high-temperature gas separations. Gas Sep Purif 1994;8(3):131–6. [7] Edlund DJ, McCarthy J. The relationship between intermetallic diffusion and flux decline in composite-metal membranes: implications for achieving long membrane lifetime. J Memb Sci 1995;107:147–53. [8] Buxbaum RE, Kinney AB. Hydrogen transport through tubular membranes of palladium-coated tantalum and niobium. Ind Eng Chem Res 1996;35:530–73. [9] Peachey NM, Snow RC, Dye RC. Composite Pd/Ta metal membranes for hydrogen separation. J Membr Sci 1996;111:123–33. [10] Moss TS, Peachey NM, Snow RC, Dye RC. Multilayer metal membranes for hydrogen separation. Int J Hydrogen Energy 1998;23(2):99–106. [11] Takano T, Ishikawa K, Matsuda T, Aoki K. Hydrogen permeation of eutectic Nb–Zr–Ni alloy membranes containing primary phases. Mater Trans 2004;45(12):3360–2. [12] Paglieri SN, Birdsell SA, Snow RC, Smith FM, Tewell CR. Development of palladium composite membranes for hydrogen separation. In: Chandra D, Bautista RG, Schlapbach L, editors, Advanced materials for energy conversion, vol. II. Warrendale, PA: The Minerals, Metals and Materials Society, 2004. p. 219–24. [13] Ohyabu N, Nakamura Y, Nakahara Y, Livshits A, Alimov V, Busnyuk A. et al., Effects of thin films on inventory, permeation and re-emission of energetic hydrogen. J Nucl Mater 2000;283–287:1297–301. [14] Nakamura Y, Livshits AI, Nakahara Y, Hatano Y, Busnyuk A, Ohyabu N. Hydrogen absorption capability of a niobium panel for pumping neutral atoms in divertor region. J Nucl Mater 2005;337–339:461–5. [15] Hatano Y, Livshits A, Nakamura Y, Busnyuk A, Alimov V, Hiromi C. et al., Influence of oxygen and carbon on performance of superpermeable membranes. Fusion Eng Design 2006;81:771–6. [16] Powers RW, Doyle MV. Diffusion of interstitial solutes in the group V transition metals. J Appl Phys 1959;30(4):514–24. [17] Huang W. Thermodynamic evaluation of Nb-C system. Mater Sci Technol 1990;6:687–94. [18] Hayakawa R, Hatano Y, Pisarev A, Watanabe K. Barrier effect against hydrogen ingress by Ti segregating to surface of V–Ti alloy. Phys Scr 2004;T108:38–41. [19] Ali-Khan I, Dietz KJ, Waelbroeck FG, Wienhold P. The rate of hydrogen release out of clean metallic surfaces. J Nucl Mater 1978;76&77: 337–43. [20] Iida T, Yamazaki Y, Kobayashi T, Iijima Y, Fukai Y. Enhanced diffusion of Nb in Nb–H alloys by hydrogen-induced vacancies. Acta Mater 2005;53:3083–9. [21] Peterson NL. Isotope effects in self-diffusion in palladium. Phys Rev 1964;136(2A):A568–74.
Improvement in high temperature stability of Pd coating on Nb by Nb2C intermediate layer Yuji Hatano ∗ , Keita Ishiyama, Hirofumi Homma, Kuniaki Watanabe Hydrogen Isotope Research Center, University of Toyama, Toyama 930-8555, Japan Received 17 May 2006; accepted 3 June 2006 Available online 2 August 2006
Abstract Niobium subcarbide (Nb2 C) was chosen as a material for non-porous intermediate layer to improve the high temperature durability of Pd–Nb composite membranes for hydrogen separation. A layer of Nb2 C was prepared between Nb substrate and thin Pd films (100 nm), and the stability of Pd coating at elevated temperatures (573–773 K) was examined by hydrogen absorption experiments. Hydrogen permeability through the Nb2 C layer appeared to be sufficiently high, and no noticeable deterioration was observed in hydrogen absorption rate under as-prepared conditions. The degradation in coating effect of Pd at elevated temperatures was substantially mitigated by Nb2 C layer. Such improved durability was ascribed to retardation of open porosity development by Nb2 C caused as a consequence of impeded interdiffusion between Pd and Nb. 䉷 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Hydrogen; Purification; Separation; Membrane; Palladium; Niobium; Durability
1. Introduction Hydrogen separation membranes are key components of membrane reactors for hydrogen production via, for example, steam reforming of natural gas [1]. Group V metals (V, Nb and Ta) have higher hydrogen permeabilites, lower prices and richer natural resources than Pd which has been extensively used as a membrane material. The surfaces of group V metals are reactive not only for hydrogen but also for other impurities such as oxygen, and hence it is common to prepare thin Pd films on the surfaces of permeation membranes as protection layers against oxidation [2–12]. The hydrogen permeability of such composite membranes, however, declines at elevated temperatures (> 673 K) due to the degradation in coating effect caused by interdiffusion between Pd and base metals [6,7]. Edlund et al. [6,7] developed porous oxide intermediate layers to prevent the interdiffusion between Pd and V. The Pd layers employed in their study, however, were rather thick (25 m), and hence the obtained values of permeability were significantly smaller than the intrinsic value of V. In addition, the amount of Pd necessary to prepare the composite membrane was comparable with conventional Pd membranes. ∗ Corresponding author. Tel.: +81 76 4456928; fax: +81 76 4456931.
E-mail address: [email protected] (Y. Hatano).
In the present study, layers of Nb2 C were prepared between thin Pd films (100 nm) and Nb substrates, and the high temperature durability of Pd coating against interdiffusion was examined by hydrogen absorption experiments. The carbide was chosen as a material for non-porous intermediate layer because it can be prepared rather easily and in low cost by, for example, heating the substrate material in hydrocarbon gases. In addition, it has been known that carbide layers formed on Nb and V are permeable for hydrogen [13–15]. 2. Experimental Disk type specimens (10 mm in diameter and 0.5 mm in thickness) were prepared from a cold-worked rod of pure Nb supplied by Nilaco (99.9 mass% purity). Their surfaces were polished with abrasive papers and finished with Al2 O3 powder of 0.06 m. A portion of specimens were carburized by heating at 1173 K for 30 min in a mixture gas of CH4 (10%) and H2 (90%) at atmospheric pressure. Then these specimens were heated in vacuum at 1373 K for 5 h to saturate the bulk by carbon. The diffusion coefficient of carbon, DC , in Nb at 1373 K can be evaluated from the data reported by Power and Doyle [16] to be 2 × 10−12 m2 s−1√ . Hence, the mean diffusion length during this heat treatment, 2DC tH , where tH is the duration of heat
0360-3199/$ - see front matter 䉷 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2006.06.045
Y. Hatano et al. / International Journal of Hydrogen Energy 32 (2007) 615 – 619
The specimens were also examined by means of XRD and scanning electron microscopy (SEM) combined with energydispersive X-ray analysis (EDX). 3. Results and discussion Fig. 1(a) shows typical hydrogen absorption curves observed at 573 K for Pd/bare-Nb. In the case of the specimen under asprepared conditions (i.e. no heat treatment), the pressure of hydrogen dropped rapidly due to the absorption by the specimen. The rate of hydrogen absorption, however, slightly decreased after heating at 573 K, and the extent of reduction clearly increased with increasing heating temperature. The degradation appeared to be ended by heating at 648 K, and no further decline in the absorption rate was observed by heating at higher temperatures. The typical results for Pd/Nb2 C–Nb are shown in Fig. 1(b). The hydrogen absorption rate of the specimen under asprepared conditions was comparable with or even slightly higher than that of Pd/bare-Nb. These observations clearly show 14 12
648K 673K
10 H2 pressure (Pa)
treatment, is calculated to be 0.27 mm and larger than a half of specimen thickness. Analysis by means of X-ray diffraction (XRD) showed the presence of Nb2 C, while no peak of NbC was observed. The thickness of Nb2 C layer was evaluated to be about 300 nm from the mass gain due to carburization. In this evaluation, the contribution of carbon in solid solution phase to mass gain was assumed to be negligible since the solubility limit of carbon in Nb is small at 1373 K [17]. An accurate value of solubility limit at this temperature, however, is not available. Hence, the thickness of Nb2 C layer thus evaluated may be slightly overestimated. Thin Pd films whose thickness was 100 nm were prepared on both surfaces of carburized and non-carburized specimens by vacuum deposition in a separate apparatus. The specimens were installed in the vacuum chamber and heated at 1000 K for 4 h to remove oxide films formed on the surfaces during the transportation of specimens; dissolution of oxygen into the bulk by heating led to removal of oxide films. The pressure of residual gases during heating was 10−6 Pa. After cooling down the specimens to room temperature, Pd was deposited onto the surfaces up to 100 nm at the rate of 0.05 nm s−1 . The specimens without carbide layers are hereafter denoted as Pd/bare-Nb, and those with Nb2 C layer as Pd/Nb2 C–Nb. It is known that the preparation of Pd films of such thickness does not deteriorate the total permeability of composite membrane based on group V metal [3]. The stability of Pd coating was examined in a temperature range from 573 to 773 K in the manner described below. First, the specimens were heated in vacuum at a given temperature for 1 h. After cooling down to room temperature, the specimens were exposed to air for 1 h. Then the durability of Pd coating was examined by measuring hydrogen absorption rate at 573 K. The hydrogen absorption experiments were carried out in a closed vacuum system, and hence the rate of hydrogen absorption was evaluated from that of pressure drop. Details of the apparatus used and experimental procedures are described elsewhere [18]. The initial pressure of H2 gas was adjusted to be 13.3 Pa. In preliminary hydrogen absorption experiments carried out for pure Pd and Nb under the above-mentioned conditions, the Pd specimen absorbed hydrogen rapidly, whereas the Nb specimen showed no noticeable hydrogen absorption due to the presence of oxide film. These observations indicate that the hydrogen absorption rate of the Pd-coated Nb specimens should decline substantially if Nb appears on the specimen surfaces due to interdiffusion between Nb and Pd. The above-mentioned value of initial H2 pressure (13.3 Pa) is significantly smaller than the upstream H2 pressures commonly employed in the permeation experiments in this field [3–12]. In general, however, the rates of hydrogen absorption and permeation become more sensitive to surface states as pressure decreases [19]. Hence, the experiments in the low H2 pressure region appeared to be suitable to examine the stability of coating effects. Some specimens were subjected for hydrogen absorption experiments for several times. The hydrogen concentration in the specimen, however, never exceeded 2 at%.
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Fig. 1. Typical hydrogen absorption curves obtained for (a) Pd/bare-Nb and (b) Pd/Nb2 C–Nb at 573 K. The temperatures indicated in the figures are those of heat treatments in vacuum.
Y. Hatano et al. / International Journal of Hydrogen Energy 32 (2007) 615 – 619
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Fig. 2. Typical examples of SEM images of (a) Pd/bare-Nb and (b) Pd/Nb2 C–Nb after heating in vacuum at 773 K for 4 h.
Fig. 3. Typical XRD patters of (a) Pd/bare-Nb and (b) Pd/Nb2 C–Nb obtained with Cu K X-rays after heating in vacuum at 873 K for 3 h. The incident angle of X-rays to the specimen surfaces was adjusted to be 1.5◦ .
that the permeability of hydrogen in Nb2 C is sufficiently high, and Nb2 C intermediate layers of the present thickness (about 300 nm) do not deteriorate the total permeability of composite membrane. In contrast to Pd/bare-Nb, no noticeable decline in the hydrogen absorption rate was observed even after heating at 623 K. Although the reduction in the absorption rate was observed at heating temperatures above 648 K, the extent of reduction was significantly smaller than that of Pd/bare-Nb. Observations by SEM revealed that the Pd layer became porous after heat treatments. Figs. 2(a) and (b) show SEM images of Pd/bare-Nb and Pd/Nb2 C–Nb after heating at 773 K for 4 h as typical examples. In the case of Pd/bare-Nb, holes whose diameter was 50–900 nm were formed in high areal density. Point analyses by means of EDX showed that the concentration ratios of Pd to Nb, [Pd]/[Nb], in holes were significantly smaller than those in matrix. Such holes were also observed in the Pd layer of Pd/Nb2 C–Nb, but the density and the maximum size of holes were significantly smaller than Pd/bare-Nb. The formation of intermetallic compound was observed by XRD analysis after heating Pd/bare-Nb at 873 K. The diffraction pattern after heating for 3 h is shown in Fig. 3(a)
as an example. The peaks of Pd which were observed before heating disappeared completely, and new peaks of Pd2 Nb appeared. In the case of Pd/Nb2 C–Nb, the peaks of Pd still remained as shown in Fig. 3(b), and no peaks of intermetallic compounds were observed. These results indicate that the rate of interdiffusion between Pd and Nb in Pd/Nb2 C–Nb was smaller than that in Pd/bare-Nb. The above-mentioned observations showed that the improved durability of Pd coating effects obtained for Pd/Nb2 C–Nb is due to the mitigation of porosity development during vacuum heating caused by reduced interdiffusion rate between Pd and Nb. Such development of open porosity should result in the appearance of Nb on the surfaces and formation of Nb oxides by subsequent exposure to air at room temperature. As described in Section 2, Nb oxides strongly impede hydrogen absorption at 573 K, and hence the regions on specimen surfaces covered by Nb oxides should not contribute to hydrogen absorption under the present conditions. Consequently, the rate of hydrogen absorption decreases with progress of porosity development. The extent of porosity development of Pd/bare-Nb, however, cannot be explained by interdiffusion via vacancy mechanism
Y. Hatano et al. / International Journal of Hydrogen Energy 32 (2007) 615 – 619
even with taking account of enhancement by hydrogen. Iida et al. [20] have measured the self-diffusion coefficient for pure Nb and Nb–H alloys ([H]/[Nb] = 0.05.0.34) and observed the significant enhancement of diffusion by hydrogen-induced vacancies. The obtained self-diffusion coefficient of Nb, however, was still small [20], and the value at 773 K and [H]/[Nb]=0.05 was evaluated to be 3 × 10−29 m2 s−1 by extrapolating their data to this temperature. The hydrogen concentration in Nb was less than 2 at% in the present study as mentioned in Section 2, and hence the Nb self-diffusion coefficient is considered to be smaller. Consequently, the mean diffusion length during heating appears to be radically smaller than the thickness of Pd layer; for example, the mean diffusion length during heating at 773 K for 1 h was evaluated to be 1×10−13 m even at [H]/[Nb]=0.05. Hydrogen concentration in Pd layers was evaluated to be much less than that in Nb (below 0.1 at%) because of smaller heat of hydrogen solution. Hence, the influence of hydrogen appears to be negligible. In the case of pure Pd, the self-diffusion coefficient at 773 K can be evaluated to be 2 × 10−23 m2 s−1 from the data reported by Peterson [21]. The mean diffusion length under the above-mentioned conditions is calculated to be 2 nm and still much smaller than the thickness of Pd layer. Therefore, fast diffusion through defects such as dislocations and grain boundaries appears to play a decisive role in the interdiffusion between Pd and Nb. The density of such defects could be sensitively dependent on heat treatment conditions. As described in Section 2, the heat treatment at 1373 K for 5 h was carried out in the preparation of Pd/Nb2 C–Nb to saturate the bulk with carbon. Namely, the density of defects in Nb substrate of Pd/Nb2 C–Nb should be smaller than that of Pd/bare-Nb. In order to separate the effect of Nb2 C layer on interdiffusion from that of the heat treatment, the durability of Pd layer was examined also with a fully recrystallized Nb specimen. A ribbon of Nb (0.1 mm in thickness) was ohmically heated in vacuum (10−7 Pa) at temperatures above 2300 K for 2.8 h and then a specimen whose size was 10×10×0.1 mm3 was cut from the ribbon. This heat treatment resulted in the recrystallization of the specimen with the development of equiaxed crystal grains of 0.1–1 mm size appearing along the specimen surfaces. After preparing Pd films, the hydrogen absorption experiments were carried in the above-mentioned manner to examine the durability of Pd layer. The results obtained for this specimen, hereafter denoted as Pd/RX-Nb, is shown in Fig. 4. Although the hydrogen absorption rate of Pd/RX-Nb was smaller than that of Pd/bare-Nb under as-prepared conditions, Pd/RX-Nb showed higher absorption rate after heating (see Fig. 1(a)). Namely, the extent of degradation appears to be reduced by the recrystallization. It should be noted that the small hydrogen absorption rate observed for Pd/RX-Nb under as-prepared conditions can be ascribed to the presence of Nb oxides in the interface between Pd and Nb. As described in Section 2, the specimens were heated to 1000 K in vacuum before the preparation of Pd films to remove the oxide films from the surfaces. The hydrogen absorption experiments for the specimens without Pd coating, however, showed that the oxide films formed on the recrystallized specimens could not be removed completely at this temperature. Namely, the hydrogen
14 648K
12
673K 773K
10 H2 pressure (Pa)
618
8 6 573K
623K
4 2 No heat treatment 0 0
1000
2000 3000 Time (s)
4000
5000
Fig. 4. Hydrogen absorption curves of Pd/RX-Nb obtained at 573 K. The temperatures indicated in the figures are those of heat treatments in vacuum.
absorption rate of recrystallized specimen was significantly smaller than that of the specimen prepared from the coldworked Nb rod after heating in vacuum at 1000 K. The influence of this oxide film on interdiffusion between Pd and Nb, however, appears to be small as described below. For quantitative comparison of the durability, the absorption coefficient, , of H2 was evaluated for Pd/bareNb, Pd/Nb2 C–Nb and Pd/RX-Nb from the initial slopes of absorption curves shown in Figs. 1 and 4. Here the absorption coefficient is defined as the probability for the absorption of a H2 molecule into the specimen at a single collision to the surface. The absorption rate in the initial stage is described as dN P0 = A √ , (1) dt t=0 2mkT where N is the number of hydrogen molecules absorbed, t the elapsed time, A the surface area of specimens, k the Boltzmann constant, P0 the initial pressure of hydrogen, and m the mass of a H2 molecule. The values of thus obtained are plotted against heating temperature in Fig. 5. The values of decreased with increasing heating temperature for all types of specimens. Although the heat treatment conditions in the preparation of Pd/Nb2 C–Nb was more moderate than those of Pd/RX-Nb, Pd/Nb2 C–Nb gave the highest values of at all heating temperature examined; the values for Pd/Nb2 C–Nb were higher than those for Pd/RX-Nb by three times or more. In addition, SEM observations showed that the extent of porosity development in Pd layer of Pd/RX-Nb was comparable with that of Pd/bareNb after heating at 773 K for 4 h. The formation of intermetallic compounds, Pd2 Nb, was also observed for Pd/RX-Nb after heating at 873 K for 3 h. It was therefore concluded that not the heat treatment but the Nb2 C layer played a dominant role in the improved durability observed for Pd/Nb2 C–Nb. Namely, the preparation of Nb2 C intermediate layer could improve the high temperature durability of Pd coating without deterioration in hydrogen permeability.
Y. Hatano et al. / International Journal of Hydrogen Energy 32 (2007) 615 – 619
Science and Technology of Japan, No. 17560613. The authors express their sincere thanks to Professor A. Livshits and Dr. A. Busnyuk of Bonch-Bruyevich University for fruitful discussion, and to Professor K. Ikeno of University of Toyama for the use of SEM.
10-2 Pd/Nb2C-Nb Absorption coefficient,
-3
Pd/RX-Nb
10
Pd/bare-Nb 10-4
References
10-5
10-6 No heat treatment 10-7 500
619
550
600 650 700 Heating temperature (K)
750
800
Fig. 5. Change of H2 absorption coefficient, , of Pd/bare-Nb, Pd/Nb2 C–Nb and Pd/RX-Nb with heating temperature.
As described in Section 1, Edlund et al. [6,7] used porous oxide intermediate layers to prevent the interdiffusion between V substrate and rather thick Pd layers (25 m). In general, the preparation of thin Pd layer on porous materials is difficult. Hence, non-porous intermediate layer employed in the present study has advantage in this point. For further improvement of high temperature stability, the thickness of Nb2 C layers has to be optimized. The quantitative evaluation of hydrogen permeability through Nb2 C layer is necessary for this purpose. The optimizations of the thickness of Pd layer and preparation conditions are also required. 4. Conclusions Intermediate layers of Nb2 C were prepared between thin Pd films and Nb substrates, and its effect on the high temperature durability (573–773 K) of Pd films was examined by hydrogen absorption experiments. The following conclusions were derived. (1) No noticeable deterioration in hydrogen absorption rate was induced by the preparation of Nb2 C intermediate layer due to sufficiently high hydrogen permeation rate through the Nb2 C layer. (2) Hydrogen absorption rates of specimens without intermediate layers were sharply declined by heating at temperatures above 648 K, while such degradation was substantially mitigated by Nb2 C intermediate layer. (3) Such improvement in high temperature durability of Pd coating by the Nb2 C layer was ascribed to the retardation of porosity development in Pd layer caused by impeded interdiffusion between Pd and Nb. Acknowledgements This study was supported in part by a Grant-in-Aid for Scientists Research (C) of Ministry of Education, Culture, Sports,
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