Investigation of hydrogenation kinetics of magnesium and magnesium alloys in the ionized reactive atmosphere ; Magnio ir jo lydinių hidrinimo jonizuotų reaktyvių dujų aplinkoje kinetikos tyrimas
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2008
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01 janvier 2008
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VYTAUTAS MAGNUS UNIVERSITY Irmantas BARNACKAS INVESTIGATION OF HYDROGENATION KINETICS OF MAGNESIUM AND MAGNESIUM ALLOYS IN THE IONIZED REACTIVE ATMOSPHERE Summary of Doctoral Dissertation Physical sciences, Physics (02 P)
Kaunas, 2008
The work was performed at the Vytautas Magnus University and the Lithuanian Energy Institute 2004-2008. The right of doctoral studies was granted to Vytautas Magnus University jointly with the Institute of Physics on July 15, 2003, by the decision No. 926 of the Government of the Republic of Lithuania. Scientific Consultant: Prof. habil. dr. Liudvikas PPRANEVIČIUS (Vytautas Magnus University, Physical Sciences, Physics 02P). Council of Physical sciences trend: Chairman: Prof. habil. dr. Gintautas P. KAMUNTAVIČIUS (Vytautas Magnus University, Physical Sciences, Physics 02P) Members: Prof. habil. dr. Julius DUDONIS (Kaunas University of Technology, Physical Sciences, Physics 02P) Prof. Giedrius LAUKAITIS (Kaunas University of Technology, Physical Sciences, Physics 02P) Doc. dr. Valdas GIRDAUSKAS (Vytautas Magnus University, Physical Sciences, Physics 02P) Dr. Sigitas RIMKEVIČIUS (Lithuanian Energy Institute, Technological Science, Thermo engineering 06T) Official Opponents: Prof. habil. dr. Romualdas NAVICKAS (Vilnius Gediminas Technical University, Technological Science, Electrical and Electronic Engineering 01T) Prof. habil. dr. Alfonsas GRIGONIS (Kaunas University of Technology, Ph sical Sciences, Ph sics 02P The official defence of the dissertation will be held at 12.30 .m. on the 11th of Se tember, 2008, in the Conference hall of V tautas Ma nus Universit . Address: Daukanto 28, LT - 44248 Kaunas, Lithuania. Tel.: +370 37 32 79 09 Summary of the dissertation has been sent on 8thof August 2008. The dissertation is available at the Vytautas Magnus University (K. Donelaičio g. 52, Kaunas LT 44248); the Lithuanian National M. Mavydo (Gedimino pr. 51, Vilnius LT 01504); and the Institute of Physics (Gotauto 12, Vilnius LT- 2600) libraries.
VYTAUTO DIDIOJO UNIVERSITETAS Irmantas Barnackas MAGNIO IR JO LYDINIŲHIDRINIMO JONIZUOTŲ REAKTYVIŲDUJŲAPLINKOJE KINETIKOS TYRIMAS Daktaro disertacijos santrauka Fiziniai mokslai, fizika (02P) Kaunas, 2008
Disertacija rengta 20042008 metais Vytauto Didiojo universitete ir Lietuvos Energetikos institute. Doktorantūros ir daktaro mokslų laipsnio teikimo teisė suteikta Vytauto Didiojo Universitetui kartu su Fizikos institutu 2003 m. liepos mėn. 15 d. Lietuvos Respublikos Vyriausybės nutarimu Nr. 926. Mokslinis vadovas: Prof. habil. dr. Liudvikas Pranevičius (Vytauto Didiojo universitetas, fiziniai mokslai, fizika 02P) Disertacija ginama Vytauto Didiojo universiteto Fizikos mokslo krypties taryboje: Pirmininkas: Prof. habil. dr. Gintautas P. KAMUNTAVIČIUS (Vytauto Didiojo universitetas, fiziniai mokslai, fizika 02P) Nariai: Prof. habil. dr. Julius DUDONIS (Kauno technologijos universitetas, fiziniai, mokslai, fizika 02P); Prof. Giedrius LAUKAITIS (Kauno technologijos universitetas, fiziniai, mokslai, fizika 02P); Doc. dr. Valdas GIRDAUSKAS (Vytauto Didiojo universitetas, fiziniai mokslai, fizika 02P). Dr. Sigitas RIMKEVIČIUS (Lietuvos Energetikos Institutas, technologijos mokslai, Termoininerija 06T) Oficialieji oponentai: Prof.habil.dr. Romualdas NAVICKAS (Vilniaus Gedimino technikos universitetas, technologijos mokslai, elektros ir elektronikos ininerija 01T) Prof. habil. dr. Alfonsas GRIGONIS (Kauno technologijos universitetas, fiziniai, mokslai, fizika 02P) Disertacija bus ginama vieame posėdyje, kurisįvyks 2008 m. rugsėjo 11d., 12.30 val., Vytauto Didiojo universiteto Maojoje salėje. Adresas: Daukanto g. 28, LT - 44248 Kaunas, Lietuva Tel.: +370 37 32 79 09 Disertacijos santrauka isiuntinėta 2008 m. rugpjūčio mėn. 8 d. Su disertacija galima susipainti Lietuvos nacionalinėje M. Mavydo, Vytauto Didiojo universiteto ir Fizikos instituto bibliotekose.
Introduction Actuality of dissertation. energetic needs are mainly covered Nowadays, by fossil energies leading to pollutant emissions mostly responsible for global warming. Among the different possible solutions for the greenhouse effect reduction, hydrogen has been proposed for energy transportation. Indeed, H2 can be seen as a clean and efficient energy carrier. However, beside the difficulties related to hydrogen production, efficient high capacity storage is still to be developed [1]. Mobile applications in combination with hydrogen fuel cell systems require sustainable storage materials that contain large amount of hydrogen. Furthermore, low decomposition temperatures and fast kinetics for adsorption and desorption of hydrogen are required. Hydrogen can be stored as a compressed gas, as liquid in cryogenic tanks or absorbed in solids. Many metals and alloys are able to store large amounts of hydrogen. This latter solution is of interest in terms of safety, global yield and long time storage. However, to be suitable for applications, such compounds must present high capacity, good reversibility, fast reactivity and sustainability [2]. One of the most attractive means of hydrogen storing lies in the form of metal hydrides. The most experimental works relative with hydrogen storage for using metal hydrides, are made in the USA, Japan and European Union countries. The metal hydrides can store 2 -7 % weight percent of hydrogen. The experimental part of the presented work was done at the Materials Testing and Research Laboratory of Lithuanian Energy Institute (LEI) and partly at the Physical Metallurgical laboratory of Poitiers University (France). Some measurements had been performed in the Rossendorf-Dresden research center (Germany). The research work was motivated and partially financially supported by the research projects performed with the Sandia National Laboratories and Lithuanian Science Foundation. Scientific novelty and practical meaning of the work.Many advantages such as lightweight, great abundance, low-cost and high hydrogen storage capacity have made Mg-based alloys the most promising hydrogen storage materials. In general, the magnesium hydride, MgH2(which contains 7.6 wt% hydrogen per mass), is made by hydriding process under 400 K. In practice, however, such a way to produce MgH2is not effective, because high reaction temperature (~ 673 K) and slow hydridingdehydriding kinetics limit its applications and hence seriously hinder its development. Recent experimental results [3,4] of synthesis, structure and thermal properties of magnesium alanate, Mg(AlH4)2, which contains 9.3 wt% H, exhibit promising features as
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hydrogen storage material. However, hydrogen behavior in MgAl alloys, used as the initial material for hydrogenation, becomes complicated because besides the physical processes related to long-range mass-transport phenomena and formation of new phases, the chemical processes related to hydrogenation and dehydrogenation processes are involved [5,6]. In the experimental work Mg and MgAl thin films were fabricated using physical vapor deposition (PVD) technologies as nontraditional and new nanotechnology methods for designing high performance hydrogen storage materials. The physical vapor deposition technologies allow the formation of metal, alloy and chemical compounds with strictly controlled composition, microstructure and stoichiometry at low temperatures. Hydrogenation of Mg and MgAl thin films was performed employing plasma immersion ion implantation technologies. Hydrogen can be introduced in a strictly controlled manner employing plasma immersion ion implantation technique. Additional ion bombardment modifies new phases synthesis kinetics. It is important that hydriding process may be performed at low pressure and the temperature of dehydriding may be in the range from 353 to 423 K. The followingaimsand goals were set for the given thesis: 1. The production of nanocrystaline metals and their alloys used for hydrogen storage, employing physical vapor deposition methods. The PhD work is designed for the research and development of new multifunctional nanostructured materials, which technical characteristics are conditioned by these processes: hydrogen absorption, dissociation, diffusion, synthesis of hydrides (hydriding process) and decomposition of hydrides, hydrogen desorption (dehydriding process), using combined magnetron sputtering/plasma hydriding processes. 2. The hydriding of nanocrystaline metals and their alloys employing non-equilibrium plasma processes, which would help to overcome thermodynamic limitation in mechanism of H-saturation and synthesing these nanomaterials at low temperature (less than 1000C) using ion beam/plasma technologies. 3. Selection of catalysts to increase kinetics of hydrogen absorption-desorption, conditioned by surface processes of hydrogen dissociation, employing decomposition of catalysts using formation of nanocrystalline compositions from gas phase in reactive plasma. The maingoalswhich must be solved: 1. The search of new technological processes to form metal hydrides for hydrogen storage applications.
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2. To employ plasma immersion ion implantation technology for the hydrogenation of Mg and MgAl thin films fabricated by magnetron sputtering. 3. To employ the reactive deposition technology for the hydrogenation of Mg and MgAl materials. 4. To do the synthesis of Mg and MgAl using combined magnetron sputtering/plasma hydriding processes (reactive sputtering); 5. To study the kinetics of plasma hydrogenation of Mg and MgAl films using plasma immersion ion implantation process. 6. the properties of the produced thin films using combinedTo characterize methods of analysis employing XRD, SEM, AFM, NRA, ERDA, GDOES, thermal desorbtion and four probes techniques. 7. To suggest mathematical model in an attempt to explain the obtained experimental results. The statements presented for defense: 1. Ion beam/plasma technologies are new and perspective in the area of new materials used for hydrogen storage because these technologies allow to control growing film properties and structure. 2. The use of highly non-equilibrium processes in the hydriding technologies opens new possibilities to overcome thermodynamic limitations in mechanism of H-saturation and the use of metastable metals (alloys), as nanocrystalline materials; 3. For the first time the synthesis of Mg and MgAl hydrides were performed and kinetics of hydriding/dehydriding was analyzed using the plasma immersion hydrogen ion implantation; 4. For the first time chemical compound Mg(AlH4)2was synthesized using the deposition of Mg and Al atoms from gas phase in hydrogen plasma (reactive deposition with simultaneously hydrogen implantation). 5. The mathematical model which describes the kinetics of hydrogen atoms accumulation in a material during implantation as a function of incident ions flux, irradiation density and the properties of the implanted atoms reemission was presented. Approbation of the research results.The results of research were presented in the 5 publications. The main theses of the research were presented at 12 national and Lithuanian conferences. Structure of the dissertation.This dissertation consists of introduction, four chapters, conclusions, list of references (138 entries) and list of scientific publications. The material of the dissertation is presented in 140 pages, including 98 figures and 9 tables.
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Content of the dissertation
ion ntroductIof the research, definition of thepresents the relevance research, aim and objectives, survey of the scientific novelty and practical value of dissertation, defensive propositions. Chapter 1.publications related to the theme ofGives the view of relevant dissertation. A Hydrogen Energy System.two-thirds of primary energy today isAbout used directly as transportation and heating fuels. Any discussion of energy-related issues, such as air pollution, global climate change, and energy supply security, raises the issue of future use of alternative fuels. Hydrogen offers large potential benefits in terms of reduced emissions of pollutants and greenhouse gases and diversified primary energy supply. Like electricity, hydrogen is a premium-quality energy carrier, which can be used with high efficiency and zero emissions. Of course, the introduction of hydrogen as a fuel and energy carrier presents, besides unquestionable advantages, several problems in developing the required technologies for the following: production, storage, transportation and utilization [7]. Hydrogen storage in metals and their alloys. In 1866, Graham reported that Pd could absorb large amounts of hydrogen, and since this discovery substantial work has been devoted to the subject of hydrogen in metals, alloys and intermetallic compounds. Many metals form hydrides, but most of them do not fulfill the requirements for commercial use as hydrogen storage material, i.e. high reversible storage capacity and fast kinetics at ambient temperatures [8]. Metal hydrides can be divided into three groups as illustrated in Fig. 1.
Fig 1. Classification of metal hydrides [9] The binary or elemental hydrides can be additionally divided into the following subgroups:ionic hydrides,formed by the reaction of s-elements and 8
hydrogen,metallic hydrideswhich are formed through the reaction of most of the transition elements and hydrogen and thecovalent hydrides, hydrogen forming molecules or polymeric structures together with the p-elements. The intermetallic hydridesare based on intermetallic AxBByalloys. A (found to the left in the periodic table) is a strong hydride former while B is not. By combining different (A and B) elements and varying the individual ratio, thermodynamic and kinetic properties can be adjusted. Examples of this type of hydrides are the AB, AB2 and AB (laves-phase)5 compounds. The last groups of metal hydrides are thecomplex metal hydrides. This type of compound is characterized by a metal or transition metal covalently bonded to hydrido ligands, forming negatively charged complexes [10]. Many metals and alloys react with hydrogen to form metal hydrides according to the reaction (1): M + x/2 H2 ↔MHx (1) here, M is a metal, MHX the hydride and x the ratio of hydrogen to metal, is H/M. In most cases, the reaction is exothermic and reversible. By application of heat, hydrogen is desorbed again [10]. Metal hydrides can form when metals interact with hydrogen molecules and atoms. The reaction between gas phase H2 a metal surface are and schematically illustrated in the Fig. 2 where the one-dimensional Lennard-Jones potential of atomic H and molecular H2is shown [11-13].
Fig. 2. Schematic of potential energy curves of hydrogen approaching a metal [11-13]
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Far from the surface the two lines are separated by the hydrogen dissociation energy which is 218 kJ/mol H. A H2molecule moving towards the surface will at some point feel a weak attractive force in the range of approx. 0-20 kJ/mol H (van der Waals forces) corresponding to molecular physisorption. The physisorption energy is typically of the order EPhys ≈10 kJ/mol. In this process, a gas molecule interacts with several atoms at the surface of a solid. Closer to the surface, the hydrogen has to overcome an activation barrier for dissociation and formation of the hydrogen metal bond. This process is called chemisorptions. The chemisorptions energy is typically of the order EChem ≈50 kJ/mol-H. The chemisorbed hydrogen atoms may have a high surface mobility, interact with each other, and form surface phases at sufficiently high coverage. In the next step, the chemisorbed hydrogen atom can jump in the subsurface layer and finally diffuse on the interstitial sites through the host metal lattice. At a small hydrogen to metal ratio (H/M < 0.1), the hydrogen is exothermically dissolved in the metal (solid-solution,α-phase). The metal lattice expands proportional to the hydrogen concentration by approximately 2-3Ǻ3 per hydrogen atom. At greater hydrogen concentrations in the host metal (H/M > 0.1), a strong hydrogen-hydrogen interaction becomes important because of the lattice expansion, and the hydride phase (β-phase) nucleates and grows. The hydrogen concentration in the hydride phase is often found to be H/M 1. = The volume expansion between the coexistingα- andβ-phases corresponds, in many cases, to 10-20% of the metal lattice [11-13]. The thermodynamic aspects of hydride formation from gaseous hydrogen are described by pressure-composition isotherms (Fig. 3).
Fig. 3. Pressure composition isotherms and Van't Hoff plot [12] When solid solution and hydride phases coexist, there is a plateau in the isotherms, the length of which determines the amount of hydrogen stored. In the pureβ-phase, the hydrogen pressure rises steeply with the concentration. 10
The two-phase region ends in a critical point, TC. The equilibrium pressure, peq, is related to the changesΔH andΔS in enthalpy and entropy, respectively, as a function of temperature by the Vant Hoff equation (pe0q is initial equilibrium pressure): ln⎛⎜⎝⎜ppe0q⎟⎟=Δ⎞HR⋅1−ΔS eq⎠T R (2) whereΔHandΔSrepresent the enthalpy and the entropy change, respectively, Ris the ideal gas constant andTthe temperature. If the logarithm of the plateau pressure is plottedvs1/T, a straight line is obtained (Vant Hoff plot) as seen in Fig. 3. [14] Some alkali and alkali-earth metal hydrides and their complex hydrides have very high hydrogen storage capacities and reversibility. Unfortunately, most of them have decomposition temperatures that are too high. This must be overcome before these hydrides can be considered seriously as practical hydrogen storage materials for on-board applications. More detailed description of possibilities and properties of light and complex metal hydrides used for hydrogen storage are discussed in dissertation. Plasma immersion ion implantation process to obtain metal hydrides: theory and model. Plasma immersion ion implantation (PIII) has been shown to be an effective surface modification and materials fabrication technique [15,16]. . There are different ways allowing modifying surface properties during adsorption (in-situ). The most universal method is simultaneous adsorption and ion irradiation. In this case, not only the interaction effects modify properties and structure of surfaces. Energetic ions in dependence of their energy penetrate into the bulk of material and saturate it by the incident particles. The penetration depth of incident ions depends on the energy of ions and can be calculated analytically or extracted from the commercial code TRIM. For example, forH+with energy 100 eV the mean penetration depth is 10 nm. Plasma hydriding is essentially non-equilibrium process. Ions extracted from plasma are accelerated and bombard surface of alloy. The schematic presentation of the plasma hydriding is shown in Fig. 4. The use of plasma is potentially economically more desirable but results in significant effects that are not negligible under high-flux, low-energy ion irradiation. Plasma includes individual types of particles, e.g. electrons, ions and neutrals in the form of non-excited and excited atoms, molecules and radicals. Dense plasmas are usually produced by the application of a sufficient high level of energy, e.g. in the form of arcs, sparks or glow discharges [17-19].
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