PALLADIUM RARE-EARTH METAL ALLOYS ADVANCED MATERIALS FOR HYDROGEN POWER ENGINEERING

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1 PALLADIUM RARE-EARTH METAL ALLOYS ADVANCED MATERIALS FOR HYDROGEN POWER ENGINEERING Natalia Kol chugina a, Gennady Burkhanov a, Natalia Roshan a, Nikolai Korenovskii a, Dmitry Slovetskii b, Evgenii Chistov b a Baikov Institute of Metallurgy and Materials Science, Russian Academy of Sciences, Leninskii pr. 49, Moscow, Russia, natalik@ultra.imet.ac.ru b Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, Leninskii pr. 29, Moscow Russia Abstract Alloys of the Pd-Y and Pd-Y-Me systems showing promise as membrane materials for preparation of high-purity hydrogen were studied. It was shown that the following conditions should be satisfied to prepare high-quality alloys exhibiting high service properties: (1) the use of high-purity components (whose purity is no less than wt %), in particular, high-purity Y prepared by vacuum distillation and (2) and holding the reached purity in the final product. It was suggested a cycle of manufacturing operations, which includs the preparation of a vacuumtight foil of 50 µm thick as the final stage. The hydrogen-permeability of the alloys was measured at different temperatures and hydrogen pressures. The alloy of the optimum composition Pd-8Y-Me in the annealed state exhibits the following mechanical properties: HV = 75 kg/mm 2, σ u = 58 kg/mm 2, and δ = 20%. Its hydrogen-permeability (Q H2 ) measured as a function of the temperature is m 3 /m 2 hmpa 0.5 at C, respectively. 1. INTRODUCTION Hydrogen of no less than vol.% purity is a principal power media for hydrogen power engineering. A single method for the preparation of high purity hydrogen consists in its separation from vapour-gas mixtures via the selective diffusion of hydrogen through a palladium membrane. The rate of hydrogen diffusion and the strength and stability during the operation in aggressive gases are important characteristics of palladium membranes. The increase in the strength, plasticity, and hydrogen-permeability of membrane alloys can be reached by alloying palladium with rare-earth metals. In recent years, the ever-growing interest of investigators to the study of rare-earth metals (REM) is observed owing to the unique properties of these metals and their potential use in electronics, laser technology, space technology, medicine, and many other high-technology applications. The application of REM in palladium-rare-earth membranes is among them. The effeciency of the works, first of all, is due to the use of high-purity materials. In this case, an accidental effect of impurities on the properties can be eliminated and true properties of materials can be attained. 2. PURIFICATION OF RARE-EARTH METALS A technology for purification of rare-earth metals (REM) by vacuum distillation (Y, Pr, Gd, Tb, Lu) and sublimation (Sc, Dy, Ho, Er, Tb, Tm) and optimum regimes of the purification have been developed at the Baikov Institute of Metallurgy and Materials Science [1]. The purification is performed at a residual pressure of Pa in a resistor furnace equipped (Fig. 1) with a graphite heater (3). A purified metal (6) is evaporated from a tantalum crucible (2) and deposited on a water-cooled (4) copper condenser (5) in the form of a druse of small crystal growing together (for some REM, the growth of single crystals is possible). The spacing between the condenser and molten metal is equal to mm; the spacing can be varied with graphite tables (1); (7) is an aluminum oxide spacer. 1

2 The melting chamber is evacuated with roughing-down and diffusion pumps. Molybdenum cylindrical shields are placed between the graphite heater and furnace walls. On the basis of a performed thermodynamic analysis, we preliminarily estimated the efficiency of separation of principal impurities (depending on the amount of evaporated substance) and optimized the temperature conditions of the distillation process for some of REM. The total scheme of the process consists in the evaporation of volatile impurities at the beginning stage of distillation, evaporation of the basis metal, and removal low-volatile impurities with a residual metal. According to laser mass spectrometry data, the content of impurities decreases substantially after the purification and content of matrix metal reaches wt %. The distilled (sublimed) REM are characterized by low contents of interstitial and metallic impurities; for example, after the purification, the contents of oxygen, nitrogen, and carbon in erbium decrease by 35, 50, and 12 times [2], respectively. Figure 2 shows the appearance of distilled REM; Table 1 lists the impurity composition of distilled yttrium. Figure 1. Scheme of the furnace used for the vacuum distillation of REM in the Baikov Institute of Metallurgy and Materials Science. Figure 2. Apperance of distilled rare-earth metals. The purity of starting components and the preparation method for high-purity alloys play an important role in reaching preset properties. 2

3 The preparation of the metals in the high-purity state allows one to study their true magnetic, electrical, absorption, thermal, etc. properties, to find distinctive features of their characteristics, and subsequently to use in developing new alloys and materials based on them. Table 1 Impurity Composition of Distilled Y Element ppm Element ppm Element ppm H <0.1 Zn <0.02 Pr 0.4 Li <0.01 Ga <0.01 Nd <0.1 Be <0.01 Ge <0.03 Sm <0.1 B <0.005 As <0.01 Eu <0.05 C 4 Se <0.03 Gd 5 N 0.2 Br <0.02 Tb <0.04 O 20 Rb <0.02 Dy <0.1 F <0.007 Sr <0.03 Ho <0.05 Na <0.02 Y Matrix Er <0.2 Mg 0.7 Zr <0.03 Tm <0.05 Al 10 Nb <0.02 Yb <0.2 Si 5 Mo <0.1 Lu <0.1 P <0.01 Ru <0.06 Hf <0.2 S <0.02 Rh <0.02 Ta <0.1 Cl <0.04 Pd <0.07 W <0.2 K 0.03 Ag <0.04 Re <0.2 Ca 0.1 Cd <0.1 Os <0.2 Sc <0.02 In <0.03 Ir <0.1 Ti <0.2 Sn <0.1 Pt <0.2 V <0.01 Sb <0.03 Au <0.1 Cr <0.03 Te <0.05 Hg <0.3 Mn <0.01 I <0.02 Tl <0.2 Fe 2 Cs <0.02 Pb <0.2 Co <0.01 Ba <0.07 Bi <0.1 Ni <0.06 La <100 Th <0.1 Cu <0.1 Ce 20 U < PALLADIUM-RARE-EARTH ALLOYS ADVANCED MEMBRANE MATERIALS Hydrogen of no less than vol.% purity is a principal power media for hydrogen power engineering. A single method for the preparation of high purity hydrogen consists in its separation from vapour-gas mixtures via the selective diffusion of hydrogen through a palladium membrane. The rate of hydrogen diffusion and the strength and stability during the operation in aggressive gases are important characteristics of palladium membranes. The presence of hydride phases and their mutual transformations lead to the failure of diffusion membranes made of pure (non-alloyed) palladium after several heating-cooling cycles in a hydrogen atmosphere. That is why, the problem of the development of strength palladium-based alloys whose hydrogenpermeability is higher than that of pure palladium is of high-priority task. Moreover, membranes comprise foils of a micron-sized thick; thus, the alloys intended for these membranes should exhibit, along with the high hydrogen permeability, the high mechanical both strength and plasticity, i.e., should correspond to the Pd-based solid-solution range. The increase in the strength, plasticity, and hydrogen-permeability of membrane alloys can be reached by alloying palladium with the formation of solid solutions. 3

4 The formation of wide ranges of palladium-rare-earth metal (REM) solid solutions with all rare-earth metals (to wt %), except La and Nd (to 2 wt %), is an interesting feature of palladium in spite the fact that factors determining the rare-earth metal-palladium interaction are unfavourable. Earlier, we have studied Pd-based alloys containing La or Nd to 2 wt % and Sm or Lu to 15 %. It was shown that the presence of the rare-earth metal additions increases substantially the rate of hydrogen mobility and increases substantially the strength of palladium on retention of the adequate plasticity. For example, the ultimate strength of palladium alloyed with 7-9 wt % Lu or Sm is higher than that of pure palladium by a factor of 3; in this case, the plasticity of alloyed palladium exhibits only 10-% decrease. The alloys Pd-8 wt % Lu and Pd-12 wt % Sm exhibited the highest rate of hydrogen mobility at 300 C. However the alloying with Sm increases markedly the oxidation ability of Pd, whereas Lu exhibiting the higher corrosion resistance is very expensive metal [3]. A number of works reporting about the high hydrogen permeability of palladium-yttrium alloys (the optimum compositions is Pd-7 wt % Y) is available in the literature [4]. The origin of the phenomenon is not explained. Because of this, the study of Pd-Y system alloys in the range of solid solutions is of practical and theoretical interest. The phase diagram of the Pd-Y system is rather complex (Fig. 3) [5]. It is characterized by the formation of seven intermetallic compounds. The maximum solubility of yttrium in palladium at 1200 C (the eutectic temperature) is about 12 wt %. Palladium is virtually insoluble in yttrium. 4

5 Figure 3. Phase diagram of the Pd-Y system. Since palladium-rare-earth metal alloys exhibit the capability to oxidation and formation of complex impurity inclusions, the chemical purity of initial components, in particular, rare-earth metals and the possibility to hold purity (during the preparation) in the final product play an important role for the preparation of high-quality alloys. As starting materials, we used palladium of wt % purity and yttrium purified by double vacuum distillation of wt % purity. 5

6 The Pd-based alloys were prepared by arc melting in a purified helium atmosphere (under an excess pressure) using a non-consumable tungsten electrode and a water-cooled bottom. We prepared palladium-based alloys containing 6, 8, and 10 wt % Y as well as alloys Pd-6 wt % Y- 0.5 wt % Me VIII and Pd-8 wt % Y-0.5 wt % Me VIII. A metal of Group VIII of the Periodic Table was added as a stabilizing addition. The weight of ingots is equal to 30 g. To reach the uniform composition of the alloys, the ingots were melted twice. The composition of alloys was studied by chemical analysis. Plane blanks 180 g in weight for the subsequent rolling to foils were melted using the aforementioned ingots, the arc furnace and a water-cooled bottom designed at the Baikov Institute of Metallurgy and Materials Science, RAS. The plane blanks were subjected to hammering to a total reduction of area of 50-75% to 3-4 mm thick (depending on the alloy composition) and subsequent vacuum annealing (the vacuum is higher than 10-4 mmhg) at 900 C for min (depending on the plate thickness). A foil 50 µm in thickness was produced by 7 passes. We measured the mechanical properties and hydrogen permeability for all of the alloys. The hardness of (HV) the annealed alloys (in vacuum at 900 C for 3 h) is equal to 65 and 85 kg/mm 2 for Pd-6 wt% Y and Pd-10 wt% Y, respectively. Before the measuring of the hydrogen permeability, the foils were tested for the tightness using diffusions cells. The hydrogen permeability was measured using original equipment [6] - test bed including a volumetric measuring cell containing a membrane foil made of a palladium-based alloy as a principal element. The diameter of operating surface of the membrane is 20 mm. Table 2 shows the specific hydrogen permeability of the Pd-based alloys. Specific hydrogen permeability Q H2 (m 3 /m 2 different temperatures Table 2 h MPa 0.5 ) of the Pd-based alloys measured at Figure 4 shows the plotted temperature dependences for the alloys. The hydrogen permeability of palladium-yttrium alloys as a function of the temperature was found to exceed that of widely used (in other countries) alloy Pd-23% Ag and the Pd-6 % In- 0.5% Ru alloy by a factor of 2-3. The unstable operation in hydrogen atmosphere of the binary Pd-Y was corrected using Me VIII alloying. For example, the alloy of the optimum composition Pd-8Y-Me in the annealed state exhibits the following mechanical properties: HV = 75 kg/mm 2, σ u = 58 kg/mm 2, and δ = 20%. Its specific hydrogen-permeability (Q H2 ) measured as a function of the temperature is m 3 /m 2 hmpa 0.5 at C. 6

7 The Pd-Y-Me alloys exhibiting the high hydrogen-permeability combined with the high mechanical properties shows promise as materials for diffusion hydrogen purification devices whose productivity reaches tens thousands m 3 /h. Figure 4. Temperature dependences of the specific hydrogen permeability of different Pd-based alloys. Alloy of optimum composition Pd-(6-7%)Y was used to prepare 50 µm-thick foil, from which a membrane 50 mm in diameter was cut. A filtering element foil (membrane) + stainless- steel wall was manufactured using diffusion welding at 900ºC for 40 min at a 2-kg loading. It is tested now. Some principal components of diffusion element containing a palladium-based membrane and operating in an apparatus for production of high-purity hydrogen are shown in Fig. 5. 7

8 METAL 2006 Figure 5. Some components of diffusion membrane element. The diffusion element was developed and designed by investigators from the Topchiev Institute of Petrochemical Synthesis and A.A. Baikov Institute of Metallurgy and Materials Science (in cooperation), Russian Academy of Science, Moscow (E.M. Chistov, D.I. Slovetskii, N.R. Roshan). Apparatus containing a number of the diffusion elements is shown in Fig. 6. Figure 6. Apparutus for diffusion purification of hydrogen BIBLIOGRAPHY 1. Burkhanov, G.S., Chistyakov, O.D., Kolchugina, N.B. High-purity rare-earth metals for research and technology. Elements, 1998, vol. 7, number 1, pages Chistyakov, O.D., Burkhanov, G.S., Kolchugina, N.B., Panov, N.N. Purification of rare-earth metals by solidification from vapor, Vysokochistye veshchestva, 1994, number 3, pages Mishchenko, A.P., Roshan, N.R., Korenovskii, N.L., et al. In Abstracts of papers from Russian Conf. Membranes-95, Moscow,

9 4. R.C.J. Wileman, D. Doyle, I.R. Harris, Neue Folge. Zeit. Phys. Chem, 1998, B184, pp Phase diagrams of binary metallic systems, Handbook. Ed. By N.P. Lyakishev, Moscow: Mashinostroenie, 1996, vol. 3, part 1, 871 p. 6. Burkhanov, G.S., Roshan, N.R., Korenovskii, N.L., Slovetskii, D.I., Chistov, E.M. Palladiumyttrium alloys adavanced materials for diffusion purification of hydrogen. In Abstracts of papers from Russian Conf. Membranes-2004, Moscow, 2004, p