INVESTIGATION OF SUBSTRATE SURFACE EFFECTS ON KINETICS OF THIN MG-NI, MG AND MG-TI FILMS HYDROGENATION

Transkriptas

1 VYTAUTAS MAGNUS UNIVERSITY Martynas LELIS INVESTIGATION OF SUBSTRATE SURFACE EFFECTS ON KINETICS OF THIN MG-NI, MG AND MG-TI FILMS HYDROGENATION Summary of doctoral dissertation Physical science, Physics (02 P) Kaunas, 2012

2 Doctoral dissertation was prepared in at Vytautas Magnus university and Lithuanian energy institute Scientific supervisor: prof. habil. dr. Liudvikas Pranevičius (Vytautas Magnus university, physical science, physics 02 P) Consultant: dr. Darius Milčius (Lithuanian energy institute, technology science, material engineering 08T) Dissertation is defended at Vytautas Magnus university in the council of Physical science: Chairman of the council prof. habil. dr. Gintautas Kamuntavičius (Vytautas Magnus university, physical science, physics 02 P) Members: prof. habil. dr. Arvaidas Galdikas (Kaunas university of technology, physical science, physics 02 P) doc. dr. Saulius Mickevičius (Vytautas Magnus university, physical science, physics 02 P) prof. habil. dr. Romualdas Navickas (Vilniaus Gediminas technical university, technology science, Electrical and electronic engineering 01 T) dr. Vitas Valinčius (Lithuanian energy institute, technology science, Energy and thermoengineering 06 T) Opponents: prof. habil. dr. Julius Dudonis (Kaunas university of technology, physical science, physics 02 P) prof. habil. dr. Audrius Maruška (Vytautas Magnus university, physical science, chemistry 03 P) Doctoral dissertation is defended at open session of the council of Physics at 20-th of December, At 12:00 a.m. in 220 auditorium Address: Vileikos st. 8, LT-44404, Kaunas, Lithuania Summary of doctoral dissertation is sent at 20-th of November, Doctoral dissertation can be viewed at the libraries of Vytautas Magnus university and Lithuanian energy institute 2

3 VYTAUTO DIDŽIOJO UNIVERSITETAS Martynas LELIS PADĖKLIUKŲ PAVIRŠIAUS ĮTAKOS PLONŲ MG- NI, MG IR MG-TI DANGŲ HIDRINIMO KINETIKAI TYRIMAS Daktaro disertacijos santrauka Fiziniai mokslai, fizika (02 P) Kaunas,

4 Disertacija rengta metais Vytauto Didžiojo universitete ir Lietuvos energetikos institute Mokslinis vadovas: prof. habil. dr. Liudvikas Pranevičius (Vytauto Didžiojo universitetas, fiziniai mokslai, fizika 02 P) Konsultantas: dr. Darius Milčius (Lietuvos energetikos institutas, technologijos mokslai, medžiagų inžinerija 08T) Disertacija ginama Vytauto Didžiojo universiteto Fizikos mokslo krypties taryboje: Pirmininkas prof. habil. dr. Gintautas Kamuntavičius (Vytauto Didžiojo universitetas, fiziniai mokslai, fizika 02 P) Nariai: prof. habil. dr. Arvaidas Galdikas (Kauno technologijos universitetas, fiziniai mokslai, fizika 02 P) doc. dr. Saulius Mickevičius (Vytauto Didžiojo universitetas, fiziniai mokslai, fizika 02 P) prof. habil. dr. Romualdas Navickas (Vilniaus Gedimino technikos universitetas, technologijos mokslai, Elektros ir elektronikos inžinerija 01 T) dr. Vitas Valinčius (Lietuvos energetikos institutas, technologijos mokslai, Energetika ir termoinžinerija 06 T) Oponentai: prof. habil. dr. Julius Dudonis (Kauno technologijos universitetas, fiziniai mokslai, fizika 02 P) prof. habil. dr. Audrius Maruška (Vytauto Didžiojo universitetas, fiziniai mokslai, chemija 03 P) Disertacija bus ginama viešame Fizikos mokslo krypties tarybos posėdyje 2012 m. gruodžio mėn. 20 d., 12 val., 220 auditorijoje Adresas: Vileikos g. 8, LT-44404, Kaunas, Lietuva Disertacijos santrauka išsiuntinėta 2012 m. lapkričio mėn. 20 d. Disertaciją galima peržiūrėti Vytauto Didžiojo universiteto ir Lietuvos energetikos instituto bibliotekose 4

5 REZIUMĖ Didžiojoje dalyje mokslinių publikacijų, kurios aprašo metalų hidridų sintezę plonų dangų pavidalu, dėmesys yra koncentruojamas tik į garinamos dangos elementinę sudėtį ir kitas jos savybes, o dangų sintezei naudojami padėkliukai yra tik įvardijami visiškai nedetalizuojant nei jų paviršinių savybių, nei galimos sąveikos su formuojama hidrido danga. Todėl šiuo darbu buvo siekta eksperimentiškai patikrinti, ar skirtingos padėkliukų paviršinės savybės turi realios įtakos plonų metalinių dangų hidridų formavimuisi ir ar vien skirtingų padėkliukų ir skirtingo jų paruošimo metodų naudojimas yra pakankamas veiksnys siekiant modifikuoti pasirinktos medžiagos plonų dangų hidrinimosi savybes. Visus darbo metu vykdytus tyrimus galima suskirstyti į tris pagrindines eksperimentų grupes, kurios pateiktos 3.2, 3.3 ir 3.4 disertacijos skyriuose, ir vieną papildomą parengiamąją eksperimentų grupę, kuri aprašyta 3.1 disertacijos skyriuje. Norint tirti skirtingų padėkliukų paviršinių savybių įtaką metalų hidridų dangų formavimuisi, pirmiausia buvo pasirinkti du oponuojančių savybių padėkliukai: Si ir išplėstinis polytetrafluoroetilenas (eptfe). Kaip žinia, dalis tyrėjų, atliekančių plonų dangų sintezę, prieš garindami dangas naudoja įvairias plazmines technologijas, skirtas padėkliukų nuvalymui. Todėl šiame darbe taip pat buvo nuspręsta panaudoti dviejų skirtingų tipų plazmos generavimo šaltinius ir ištirti, kaip jie pakeičia Si ir eptfe padėkliukų paviršių savybes. Atlikus išsamią, skirtingais plazmos režimais paveiktų ir neveiktų padėkliukų paviršinių savybių analizę, buvo nustatyta, kad gautos 3 poros skirtingų paviršinių savybių Si ir eptfe padėkliukų (iš viso 6 skirtingų savybių padėkliukai): pirmą porą sudarė plazma nepaveikti Si ir eptfe padėkliukai, o kitas dvi poras dviem skirtingais plazmos režimais paveikti Si ir eptfe padėkliukai. Taip pat parodyta, kad plazmos poveikis padėkliukams yra kompleksinis ir priklauso nuo naudojamų plazmos parametrų. Todėl bendru atveju, padėkliukų paveikimas plazma negali būti vadinamas padėkliukų nuvalymu, nes priešingai nei įprasta manyti, padėkliukų paveikimas plazma negarantuoja paviršiaus nuvalymo nuo anglies, deguonies ir kitų priemaišų. Antrojoje tyrimų dalyje, gautieji 6 skirtingų paviršinių savybių padėkliukai buvo naudojami magnio nikelio plonų dangų hidrido sintezei. Kaip atskleidžiama literatūros 5

6 analizės dalyje, magnio nikelio hidrido plonos dangos turi didelį potencialą būti panaudotos įvairiuose pažangiųjų technologijų įrenginiuose, įskaitant vandenilio jutiklius, protingus langus ir saulės elementus. Tam, kad būtų realizuotas šis potencialas, būtina tiksliai kontroliuoti, kokią hidrido fazę Mg-Ni dangos suformuos hidrinimo metu ir koks bus jos stabilumas. Šio disertacijos rengimo metu parodyta, kad Mg-Ni dangų hidrinimosi savybės, įskaitant ir preferencinį konkrečių kristalinių hidrido fazių formavimąsi, gali būti keičiamos pakeičiant tik dangų sintezei naudojamus padėkliukus ar jų paviršines savybes. Stebimi procesai buvo paaiškinti remiantis skirtingo stiprumo dangos-padėkliuko sąveika ir iš to kylančiais vidiniais įtempiais bei hidrido fazės plėtimosi apribojimais. Trečiojoje tyrimų dalyje, buvo siekiama patikrinti, ar nustatytas reiškinys, kad padėkliuko savybių pakeitimas gali paveikti Mg-Ni dangų hidrinimo savybes, gali būti panaudojamas kitų metalų hidridų formavimosi savybių pakeitimui, konkrečiai šiuo atveju Mg-Ni dangos pakeistos Mg ir Mg-Ti dangomis. Atliki tyrimai su Mg ir Mg-Ti dangų sinteze ir hidrinimu naudojant skirtingų paviršinių savybių Si ir eptfe padėkliukus parodė, kad pirminiais duomenimis, padėkliukų savybių keitimas turi poveikį ir šių dangų hidrinimo savybėms. Remiantis gautais pastebėjimais ir pristatytu aiškinamuoju modeliu, pateiktos rekomendacijos, kokių savybių padėkliukai yra tinkamiausi Mg pagrindo hidridų plonų dangų formavimui. Tačiau tam, kad iškeltos prielaidos būtų visiškai patvirtintos arba paneigtos, ši sritis (su Mg ir Mg-Ti dangomis) reikalauja atskirų išsamių tyrimų. Disertaciją sudaro įvadas, trys pagrindiniai skyriai, darbo išvados, literatūros sąrašas ir informacija apie autoriaus mokslinius darbus paskelbtus disertacijos rengimo metu. Bendra disertacijos apimtis 171 puslapiai, 52 paveikslai ir 16 lentelių. 6

7 1. Introduction Scientific background and relevance of the work Metal hydrides were discovered in the beginning of the nineteenth-century but for a long time remained interesting only for the fundamental research [1]. However, the development of fundamental understanding and experimental observations of the metal hydrides properties revealed their relevance to the industrial applications. For the last decades a lot of the research was focused to use metal hydrides in energy related applications [2-6]. It was shown that advanced solid state hydrogen storage systems potentially can become one of the key elements for the future energy systems, meanwhile rechargeable nickel-metal hydride batteries are already widely spread in the global market and successfully competes with the Li based alternatives [7]. Ball milling is the main synthesis technology of metal hydrides in energy (hydrogen) storage sector, however, applications of thin films in metal hydride research studies have gained separate interest because most of the Physical Vapour Deposition (PVD) technologies allow synthesizing nanostructured materials in much more controllable way in comparison to the chemical methods or semi-mechanical ball milling technique [8]. In the fundamental research field the usage of thin films provides opportunity for scientists to synthesise and analyse complex multilayered structures. Apart from that thin films are useful in research of hydrogen adsorbtion-desorbtion, surface oxidation and other processes which can be advantageously simplified to the one-dimensional processes [9]. Furthermore, synthesis of the metallic films and its subsequent hydrogenation opened up possibilities for the in-situ and ex-situ optical spectroscopy and electrical resistivity measurements which are difficult or even impossible to implement for the ball milled powders [10-11]. Indeed, then J. N. Huiberts et. al. synthesized and hydrided yttrium and lanthanum films they observed that transition from dihydride to trihydride leads to the significant changes in optical and electrical properties of the films [12]. The later transition from the conductive metallic mirror state to transparent semiconductor or dielectric hydride phase was also observed for other metal hydrides including magnesium-rare-earth and magnesiumtransition-metal hydrides [13-16]. These observations were important not only from the fundamental point of view but also opened up new areas for the metal hydride film applications such as hydrogen sensors and switchable mirror devices [17-21]. 7

8 Mg 2 NiH 4 hydride has theoretical hydrogen content of 3.6 wt.%, good hydrogen absorption - desorbtion cycle stability [22, 23], moderate decomposition temperature [24, 25] and exhibits optical switching phenomena then produced in thin film form [15, 18]. Therefore it was a target of a lot of research both in hydrogen storage and metalhydride-based devices (hydrogen sensors, switchable mirrors and recently even as possible candidate to replace silicon in solar cells) [26]. Mg 2 NiH 4 has three crystal phases. Low temperature phases (below 510 K) LT-1 and LT-2 both have monoclinic structure respectively without and with microtwinings; and high temperature (HT) phase has FCC structure [27-28]. H. Blomqvist and D. Noréus demonstrated that at room temperature Mg 2 NiH 4 powders of LT-1 phase has brownish-grey colour meanwhile LT-2 phase powders has characteristic orange colour [29]. When heating across the LT HT transition at 510 K, both phases transforms to HT phase and change their colour to black. Cooling down of HT phase Mg 2 NiH 4 powders leads to the transition to LT-2 phase and reappearance of orange colour independent of what phase powders were before heating up. At the same article authors demonstrate that both conductivity and crystal structure of Mg 2 NiH 4 powders is affected by the applied mechanical stress [29]. All these features of Mg 2 NiH 4 hydride and particularly its phase changes resulting into different conductivity and optical properties boosted research for Mg 2 NiH 4 synthesis in thin film form. It was expected that hydrogenation properties of Mg 2 Ni alloy thin films will be similar to powders, whereas experiments showed that situation with thin films is more complicated. Contrary to expectation experimental results has shown that hydrogenation of Mg 2 Ni films formed on quartz and calcium fluorite substrates results into pseudo cubic phase (instead of LT-1 for hydrogenation temperatures below 510 K) which remains unchanged over all temperature range from room temperature up to its decomposition at around 550 K [30, 31]. R. Gremaud et al. investigated optical properties of Mg y Ni 1 y H x gradient thin films which were hydrided at room temperature and also reported yellow-to-red colours suggest that it could have been LT-2 or pseudo cubic phase (experimentally measured hydride structure was not reported) [32]. But P.Jain et al. showed that formation of crystalline monoclinic Mg 2 NiH 4 phase is also possible [33] whereas R.J. Westerwaal et al. claimed that in film form Mg 2 NiH 4 hydride is amorphous [34, 35]. Interestingly, later group has 8

9 demonstrated that hydrogenation of Mg 2 Ni alloy starts by formation of α hydride phase (Mg 2 NiH 0.3 solid solution) at the interface with the substrate followed by the β hydride phase which also forms at the interface and proceeds to the film surface [34]. As far as it was found during literature analysis there have been no systematic approaches to investigate what factors introduced by magnetron sputtering technique or thin film interaction with substrate are responsible for the dissimilarities between Mg 2 NiH 4 hydride properties in film and powder forms. However, it is possible that it is substrate and its interaction with the thin film that becomes one of the main factors affecting hydrogenation properties. Experimental observations indicate that when hydrogen enters the film the in-plane expansion is limited and this introduces the inplane stress which changes the thermodynamic properties compared to the corresponding bulk material [36, 37]. Another group demonstrated that hydrogenation of Mg thin films deposited on porous and non-porous substrates can have effect for the micro-strains inside the film which alters Mg film hydrogenation properties [38]. Aim and tasks of the work Considering the provided background the main aim of this research work was to test experimentally if using dissimilar substrates and varying their pretreatment conditions alone could have significant effects to change hydrogenation properties of selected materials in thin film form. Special attention was paid to analyse how different substrates helps to keep thin film integrity and how substrate can affect formation of selected crystal phases. In order to achieve these goals following tasks have been formulated: 1. To treat Si and expanded polytetrafluoroethylene (eptfe) substrates with different plasma modes and to have throughout characterisation of their surface properties before and after plasma treatment. 2. To deposited metallic Mg-Ni thin films on differently pretreated Si and eptfe substrates and to analyse their properties including microstructure characterisation and crystal phase identification. 3. To hydrogenate Mg-Ni thin films in hydrogen atmosphere (elevated pressure and temperature) and to analyse if different substrates and their pretreatment 9

10 methods have induced any changes Mg 2 NiH 4 hydride crystal phase formation and/or other properties. 4. To test the observed regularities and presumptions by using different hydrogen absorbing material: Mg and Mg-Ti thin films. Scientific novelty and practical value of the work As it can be seen from the task list, the main hydrogen absorbing material which was used in this research was Mg 2 Ni alloy and its corresponding hydride Mg 2 NiH 4. Based on the current knowledge from powder analysis Mg 2 NiH 4 hydride thin films has high potential to be used for various practical devices and applications such as hydrogen storage, batteries, hydrogen sensors, smart windows devices and photovoltaic cells. However, the observed experimental differences between Mg 2 NiH 4 in ball milled powder and thin film form limits the exploitation of this potential. Therefore, this work is not just one of the first coherent researches on how different substrates can affect formation of selected metal hydrides but it also is the first attempt to control properties of Mg 2 NiH 4 thin films by changing its substrate in order to make it more suitable for the demanding applications. The second aspect describing the novelty of the work is the fact that as far as it was found during literature analysis there were no attempts to synthesise metal hydrides onto eptfe substrates which has high surface area, low surface energy, flexible and soft surface which can be useful minimising strains which are characteristic for metal hydride thin films. It is also important to note, that successful usage of flexible eptfe substrates for synthesis of metal hydride films opens new possibilities for them to be used at the novel non-constant shape devices and instruments. Finally, this work experimentally demonstrates that despite the fact that substrate pretreatment with plasma is widely spread practice in synthesis of metal hydride thin films (and in thin film deposition in general) it is not always beneficial and it not necessarily removes all carbon and oxygen contaminants from the substrate surface. Therefore, potential positive or negative effect of plasma pretreatment must be justified individually according to the particular material which is being deposited, the used substrate and different aspect of plasma process itself. 10

11 2. Methodology Fig Scheme of the experimental program. From earlier studies it is know that by depositing Mg-Ni thin films on quartz substrates and performing their subsequent hydrogenation it is possible to form transparent orange coloured films consisting of nanocrystaline pseudo-cubic crystal phase Mg 2 NiH 4 hydride [39]. Therefore, the first stage of current research (the scheme of the whole experimental program is presented in Fig. 2.1.) was dedicated to find what experimental parameters of Mg-Ni thin film deposition as well as hydrogenation are mostly suitable to form high quality (in terms of homogeneity, phase purity and structural integrity) Mg 2 NiH 4 hydride thin films. Accordingly, during the first stage of the research metallic Mg-Ni thin films of varying Mg:Ni ratio were deposited on quartz substrates by co-deposition from two in depended magnetrons. Then all Mg-Ni films were hydrogenated under different conditions (duration, hydrogen pressure and temperature) and by qualitatively analysing whole set of the hydrogenated films the most suitable experimental parameters were selected. The second stage of the research was designated for deposition of metallic Mg-Ni thin films on different substrates. In total a set of 6 substrates having different surface 11

12 properties were used: i) Si and eptfe substrates cleaned only with the cloth rinsed in ethyl alcohol (Si also was prewashed in 99.9 % purity acetone in ultrasonic bath), ii) Si and eptfe substrates before Mg-Ni thin film deposition treated with plasma generated by direct current power source (later abbreviated as DC plasma), and iii) Si and eptfe substrates before Mg-Ni thin film deposition treated with plasma generated by pulsed direct current power source (later abbreviated as PDC plasma). By deposition of thin films some of the substrate surface properties are concealed by the film, therefore, in order to have detailed characterisation of the substrates the experiments of second stage are split into two parts. First of all, Si and eptfe substrates were cleaned with rinsed cloth, treated under selected plasma conditions (one pair was kept without plasma treatment) and without depositing any film analysed by using various analytical techniques: gas phase composition during plasma treatment was analysed by RGA, chemical surface composition analysis was performed with XPS, qualitative micrometeric surface topography analysis was done with SEM, qualitative and quantitative nanometeric surface topography analysis was done with AFM. The second part of the experiments had uninterrupted sequence of substrate treatment with selected plasma mode and Mg-Ni thin film deposition. After deposition various properties of Mg-Ni films were analysed by contact surface nanoprofiler (thickness measurement), XPS and EDS (elemental and chemical composition), XRD (crystal structure), SEM and AFM (qualitative and quantitative surface topography). During the third stage Mg-Ni thin films deposited on different substrates were hydrogenated in the custom build cell and afterwards analysed by different methods with the goal to determine if the existing differences of substrate surface properties have had any influence to the hydrogenation of the Mg-Ni films and formation of the hydride phase. The last stage was appointed to test the identified consistent pattern of substrate effects with different hydrogen adsorbing material Mg and Mg-Ti thin films. These films were also deposited on differently pretreated Si and eptfe substrates and hydrogenated under the same conditions as Mg-Ni thin films. The differences (in crystal structure and microstructure of the film) of thin films deposited on variously pretreated substrates were quantitatively identified by XRD and SEM. These results were combined with ones observed for the Mg-Ni films and final conclusions are drawn. 12

13 3. Main results Screening and selection of Mg-Ni thin film deposition and hydrogenation parameters As it can be seen from the chemical formulas of Mg 2 Ni and its corresponding hydride Mg 2 NiH 4 the steheometric ratio of Mg:Ni is 2:1. In the phase diagram of the Mg-Ni system (which can be found at [40]) it is seen that close to the stecheometric ratio of 2:1 there is a peritectic point, which means that at the exact ratio of 2:1 we can form not only Mg 2 Ni but also a small fraction of MgNi 2 which is intrusive as it does not adsorbing hydrogen. To avoid this formation traditionally Mg-Ni material is enriched in Mg. In such case a small portion of Mg is formed but it adsorbs hydrogen therefore is less problematic than MgNi 2. However, material synthesis by magnetron sputtering can have slightly different behaviour comparing to the traditional heating-melting-cooling methods, therefore in this work a wider range of Mg-Ni compositions around the stechemetric ratio was tested. Fig Mg concentration dependence on the Ni magnetron current In order to synthesise Mg-Ni thin films with different ratios we have fixed the magnetron current of Mg at I Mg = 0.5 A and varied the magnetron current of Ni in the range of I Ni = A. In all of these experiment Ar working gas pressure was kept constant at 0.6 Pa. Additionally thin films with the same set of Mg and Ni magnetron currents were deposited for 15, 20, 25 and 30 min which depending on the Ni magnetron current resulted in the thickness change from roughly nm up to nm. Afterwards all as-deposition Mg-Ni thin films were analysed by XRD, 13

14 SEM and EDS techniques. The received results for the deposition of Mg-Ni thin films can be summarised as follows: It was experimentally demonstrated that by using constant Mg magnetron current of I Mg = 0.5 A, the required Mg:Ni ratio can be achieved by varying only working current of Ni magnetron I Ni (Fig. 3.1). In Mg concentration region of %, the dependence of Mg concentration from Ni current I Ni can be approximated linearly by (3.1) equation: c Mg% (I Ni ) = 84, ,44571 I Ni (3.1) From the analysed currents the closest approach to the stecheometric ratio is achieved when I Ni = 0.25 A. All Mg-Ni thin films whose deposition time was shorter than 25 min had amorphous structure (even with substrate temperature increased up to 100 C and 200 C) but for longer deposition times XRD analysis shows formation of Mg 2 Ni crystal phase. It is expected that crystallisation is induced by the internal strains which are becoming larger as film gets thicker. During hydrogenation process three in depended parameters can be controlled: i) hydrogen pressure, ii) temperature and iii) hydrogenation duration. All of the above mentioned Mg-Ni films were hydrogenated using 20 bar hydrogen pressure and 180 C and 100 C temperature. Hydrogenation duration was 48 hours for the lower temperature and 24 hours for the higher temperature experiments. First evaluation of all hydrogenated thin films was qualitative optical investigation on how thin films have kept their integrity. It was observed that all thin films those deposition duration was min (thickness of nm) have had full or partial fracture of the films. Analogous fracture of the films was also observed for all samples whose were deposited by using higher than 0.2 Ni magnetron current, i.e. samples with I Ni 0,25 A (Ni concentration higher than 32.3 at. %). Closer investigation of the fractured films revealed that looking in front of the light some of them were not homogeneous and had two areas types. The dominating area was black and not transparent, but some regions of the samples had claret colour and were transparent to light. Later zones were mostly visible for samples those Ni magnetron deposition current was A I Ni A (30.7 % C Ni 34 %) and deposition time does not exceeded 25 min. These observations can be explained based on two causes. First of all, 14

15 it is know than as thin film gets thicker higher internal strains build up, therefore film cracking and fracturing can be seen as the result of structure relaxation. Secondly, as it was already mentioned above close to the stechemetric ratio of 2:1 Mg-Ni system has peritectic point, this means that even small segregation of Mg and Ni phases can lead to the formation of hydrogen adsorbing Mg 2 Ni alloy and not adsorbing MgNi 2 alloy. During hydrogenation Mg 2 Ni tends to expand significantly, meanwhile MgNi 2 stays unchanged, this builds up additional stresses to the structure which also relaxe by fracturing. Amongst the rest of the hydrogenated thin films fracturing was observed only for several samples which were deposited by 20 min process (thickness of nm) or Ni magnetron current I Ni = A (Ni concentration 30.7 %), meanwhile the rest of them were undamaged. Qualitative optical analysis also showed that the most promising sample was deposited by using 15 min deposition time with magnetron currents of I Mg = 0.5 A and I Ni = 0.2 A and hydrogenated at 180 C for 24 hours. This film has completely changed its outlook from mirrored metallic to transparent orange. The film was optically homogeneous and structurally integral. XRD analysis of this film confirmed that it had almost completely transformed to pseudo cubic Mg 2 NiH 4 phase with only small fraction of Mg 2 NiH 0.3 which was absent than hydrogenation time was increased to 72 hours. Accordingly, for the Mg-Ni thin film synthesis and hydrogenation experiments the following experimental parameters were chosen: deposition time 15 min; Ar gas pressure during deposition 0.6 Pa; magnetron currents I Mg = 0.5 A and I Ni = 0.2 A; hydrogen pressure 20 bar, temperature 180 C; hydrogenation time 72 hours. Characterisation of plasma treated and untreated Si and eptfe substrate surfaces As it was already mentioned above, in order to check if usage of different substrates have effects on Mg 2 NiH 4 hydride formation properties we have chosen two types of substrate with completely opposing properties. First of them was polished prime grade silicon wafers with (111) surface orientation received from Siegert Consulting e.k. In contrast to quartz and CaF 2, silicon wafers have flat and periodical surface even at the atomic scale which is guaranteed by (111) crystalline orientation. Furthermore, it is known that at elevated temperatures magnesium and silicon can form 15

16 magnesium silicates [41] which can suppose much higher Mg-Ni film and substrate interaction comparing to film-substrate pairs used in earlier studies [39]. For the second substrate we were looking for substrate with as low surface energy as possible. We have chosen to use eptfe from W.L.Gore & Associates Inc. eptfe is widely known for very low free surface energy and apart from that it also i) exhibits very good temperature stability in -268 C to +315 C temperature range; ii) is dimensionally stable with no aging or degradation; iii) chemically inert resistant to all media in the 0-14 ph range, except molten alkali metals and element fluorine; iv) possible operating pressure from vacuum up to 200 bar. The combination of low surface energy, chemical inertness of the eptfe substrate as well as its flexible microstructure is expected to result in much smaller micro-strains which are formed in Mg-Ni-H films. For both types of substrates three different surface pre-treatment procedures have been applied. For the first group of samples Si and Expanded PTFE substrates were washed in acetone in ultrasonic bath, dried out under dry air flow and then wiped out with cloth rinsed in ethyl alcohol. Substrates for the second group of samples in addition to washing as described above before film deposition were affected by DC power source generated plasma (no contact with air between plasma treatment and thin film deposition); i.e. constant negative potential of 900 V was applied to the substrate holder for 15 min at mbar pressure prior to the film deposition. DC power source ignited plasma and constantly attracted positive Ar + ions which induced mainly ballistic interaction with the substrate. Substrates for the third group were washed and treated with pulsed DC power source generated plasma. In pulsed DC regime power source used long 650 V negative potential with shorting out and reversing the target voltage to roughly 100 V for 5μs at 20-kHz rate. This reversal of voltage periodically attracts electrons from plasma and prevents charge build-up on the sample holder as well as substrate surfaces and increases process efficiency. Exposure of substrate for electron bombardment also give significant rise in temperature and can induce modification in electronic structure of substrate which is usually short term for metals and semiconductors and long term polymeric materials [42, 43]. The choice of plasma based pretreatment of the substrate was impelled by literature analysis as such methods are quite often used in thin film deposition experiments, therefore it is important to know if it has any effect on hydrogenation properties. 16

17 Fig SEM images of eptfe substrates: a) without plasma treatment, b) DC plasma treated, c) PDC plasma treated. As it might have been expected prime grade silicon wafers with and without plasma treatment were too flat for SEM to reveal any topographic details, therefore Si surface was analysed with AFM (quantitative AFM data is provided in the summarised Table 3.1). Meanwhile, SEM images of eptfe substrates with and without plasma treatment revealed extensive changes which are induced by bombardment with ions and electrons (Fig. 3.2). In order to determine elemental and chemical changes in the surfaces of Si and eptfe under plasma treatment additional analysis with XPS was performed. For Si substrates elemental composition at the top surface and subsurface was determined by using XPS depth profiling (with Ar ion gun, Fig. 3.3) and angle resolved XPS (AR- XPS) techniques. For eptfe substrates because of the non flat surfaces none of the later techniques could have been applied but valuable information was extracted from C1s peak fitting (Fig. 3.4). All experimental data of analysis of Si and eptfe substrates surface properties before and after plasma pretreatment is summarised in Table

18 Fig XPS depth profiles of Si surfaces: a) without plasma treatment, b) DC plasma treated, c) PDC plasma treated. Fig C1s peak fitting of eptfe substrates: a) without plasma treatment, b) DC plasma treated, c) PDC plasma treated. 18

19 Table 3.1. Summarised experimental results of analysis of Si and eptfe substrates surface properties before and after plasma pretreatment. Elemental Substrate mode information Plasma Topography (chemical) Remarks composition Si eptfe - DC PDC - DC PDC Relatively flat. Surface roughness: Ra = 0,2 nm, Rq = 0,4 nm Relatively flat. Surface roughness: Ra = 0,4 nm, Rq = 0,7 nm Relatively flat. Surface roughness: Ra = 0,2 nm, Rq = 0,3 nm Porous structure consisting of 1-5 μm size knots connected by dense network of nm diameter strings Porous structure, damage level: weak (local damage of individual strings (fully ruptured 10-15% of strings) and knots) Porous structure, damage level: strong (massive damage of strings (fully ruptured 90-95% of strings) and knots) Si 45,0 % O 39,5 % C 15,5 % Si 34,1 % O 62,8 % C 2,0 % Ni 1,1 % Si 28,6 % O 61,4 % C 4,5 % Ni 5,5 % O 5,0 %, F 48,4 % C 46,6 % (38,44 % C-C, 9,97 % C-O, 3,98 % C=O, 4,69 % O=C-O, 40,44 % CF2, 2,49 % CF3) O 4,3 %, F 52,9 %, Ni 0,2 %, C 42,6 % (26,64 % C-C, 9,03 % C-O, 6,44 % C=O, 5,79 % O=C-O, 48,87 % CF2, 3,22 % CF3) O 6,3 % F 53,8 % Ni 0,7 % C 39,2 % (19,65 % C-C, 10,78 % C-O, 8,22 % C=O, 7,63 % O=C-O, 50,29 % CF2, 3,44 % CF3) - Concentrations of C and O can be overestimated due to the contamination introduced during transfer from the magnetron sputtering vacuum system to the XPS chamber - Concentrations of C and O can be overestimated due to the contamination introduced during transfer from the magnetron sputtering vacuum system to the XPS chamber 19

20 Investigation of substrate surface effects on hydrogenation properties of Mg-Ni thin films SEM images of metallic Mg-Ni films deposited on Si substrates are shown in Fig From these images we can identify that plasma treatment has observable effect on film morphology. In Fig. 3.5 a we can see that Mg-Ni film is rugged having small ( nm) ridges. Similar ridges just of smaller dimensions (20-30 nm) still can be identified in Fig. 3.5 c and are completely absent in Fig. 3.5 e. These observations indicate that plasma treatment improves film homogeneity, augments its density and makes film flatter which means it repeats silicon substrate surface better than plasma untreated sample. Such changes can be attributed to the removal of organic contamination from the surface and/or strengthening of the film-substrate bond. The qualitative statements of augmented film density are supported by the quantitative film thickness measurement data observed with stylus profiler (Fig. 3.6). The cross-section images of Mg-Ni films deposited on Si reveal that in all cases it has characteristic columnar structure (Fig. 3.5). The diameter of individual columns are smallest ( 70.5 nm) for Mg-Ni deposited on untreated Si and progressively increases for DC ( 80.0 nm) and PDC plasma treated ( 85.7 nm) samples. If we would assume that substrate temperature increases significantly during plasma treatment then such tendency agrees with film growing model proposed J.A.Thorton [44]. Nevertheless, in contrast to R.J.Westerwaal et.al. who also investigated structure of Mg-Ni film on Si substrate [34] at the available SEM resolution we did not noticed that Mg-Ni films would have clear smaller interfacial grains. SEM images of Mg-Ni films deposited on eptfe substrates are provided in Fig Images reveal that plasma treatment of eptfe substrates not only brakes structural strings but also significantly improves film adhesion to the eptfe which results in better surface coverage with the film. This can be explained by the extremely low surface energy of untreated eptfe and its increase induced by possible bond braking, crosslinking, carbonisation and other events which are present during plasma treatment. It was not possible to use surface profiler for films deposited on eptfe substrates therefore the film thickness was roughly estimated by comparing SEM images of uncoated and coated eptfe. The thickness of the film deposited on plasma untreated eptfe was 200 nm, and for the plasma treated eptfe thickness was roughly 350 nm. 20

21 Fig SEM images of as deposited Mg-Ni thin films on Si substrates: a) surface of Mg-Ni thin film on plasma not treated Si, b) cross section of Mg-Ni thin film on plasma not treated Si, c) surface of Mg-Ni thin film on DC plasma treated Si, d) cross section of Mg-Ni thin film on DC plasma treated Si, e) surface of Mg-Ni thin film on PDC plasma treated Si, f) cross section of Mg-Ni thin film on PDC plasma treated Si (insets in a, c, and e figures show corresponding qualitative and quantitative AFM data). 21

22 Fig Film thickness measurement results for Mg-Ni films deposited on Si substrate after different pre-treatment. Fig SEM images of as deposited Mg-Ni thin films on eptfe substrates: a) Mg-Ni thin film on washed eptfe, b) Mg-Ni thin film on DC plasma treated eptfe and c) Mg-Ni thin film on PDC plasma treated eptfe substrates. 22

23 Elemental composition measurements by EDS revealed that for all samples Mg and Ni atomic concentration respectively were close to 71 at. % and 29 at. %. X-ray diffraction analysis of as-deposited Mg-Ni thin films on both Si and eptfe substrates has revealed that in all cases deposited Mg-Ni film is formed in amorphous phase. After hydrogenation surface characteristics of Mg-Ni thin films deposited on Si substrates have changed drastically (Fig. 3.8). For the substrate which did not had any plasma pre-treatment (Fig. 3.8 a) we can see that the film is flatter and probably more homogeneous than it was before the hydrogenation, meanwhile SEM images of Mg-Ni films on plasma pre-treated Si substrates (Fig. 3.8 b and c) presuppose that during hydrogenation they have undergone through much higher mass transport phenomena which induced various structures at the surface and therefore it is reasonable to assume that these two samples have adsorbed more hydrogen than the first one. Also by the surface morphology changes (signs of expansion) we can predict that plasma treatment was favourable for the formation of magnesium nickel hydride. Fig SEM images of as hydrogenated Mg-Ni thin films on Si substrates: a) Mg-Ni thin film plasma not treatred Si, b) Mg-Ni thin film on DC plasma treated Si, c) Mg-Ni thin film on PDC plasma treated Si (insets in figures show corresponding qualitative and quantitative AFM data). 23

24 Fig SEM images of as hydrogenated Mg-Ni thin films on eptfe substrates: a) Mg- Ni thin film on plasma not treated eptfe, b) Mg-Ni thin film on DC plasma treated eptfe, c) Mg-Ni thin film on PDC plasma treated eptfe. Looking at SEM images of hydrogenated Mg-Ni films which were deposited on eptfe substrates we can see that the crosscut dimensions of strings in Fig. 3.9 a is roughly 1.5 times bigger than in Fig. 3.7 a and this reveal high level of thin film expansion. The measured crosscut dimension increase of films deposited on plasma treated eptfe samples is slightly smaller 1.3 times. These results show that Mg-Ni films deposited on eptfe substrate can absorb hydrogen and expand significantly in all directions without braking or falling apart from the substrate. When metallic film is deposited on hard flat substrate (such as Si) during the hydrogenation and/or dehydrogenation processes it is hard to avoid film cracking and falling off of the substrate [38]. Therefore, later results of eptfe substrate usage for hydride film formation can be useful in trying to find proper substrates for the hydrides for functional devices such as hydrogen sensors. 24

25 Fig X-ray diffractograms of hydrogenated Mg-Ni thin films deposited on Si substrates with different pre-treatment. X-ray diffraction data for hydrogenated Mg-Ni films on Si and eptfe substrates are shown in Fig and Fig respectively. As a general trend we can notice that XRD diffractograms of hydrogenated samples on differently pretreated Si substrates differ meanwhile, those for the expanded PTFE substrates looks rather similar. The diffraction patterns of hydrogenated Mg-Ni films deposited on untreated and PDC plasma treated Si substrates have all of the strongest peaks related to the α-phase Mg 2 NiH 0.3 hydride (solid solution phase). The diffraction pattern of hydrogenated Mg- Ni film deposited on DC plasma treated Si substrate has several additional peaks those interpretations and indexing is not straightforward. However, there are several factors which allow assuming that later sample (DC plasma treated) has a coexistence of two crystal phases, namely α hydride phase Mg 2 NiH 0.3 and monoclinic low temperature β hydride phase Mg 2 NiH 4. First of all, to advocate such assumption we can notice the presence of the same peaks as in former (untreated and PDC plasma treated) samples which were already attributed to the Mg 2 NiH 0.3 phase. In powder diffraction case monoclinic Mg 2 NiH 4 LT phase is easily recognized by its characteristic duplet peaks at roughly 23.3 and 24.1 degrees which are coming from (-1 1 2) and (1 1 2) crystallographic planes respectfully. In our case for the thin film sample we observe 25

26 only single peak at 23.3 degree and there is no second peak at 24.1 degree, but this can be explained by the preferential crystal structure orientation of Mg 2 NiH 4 crystal phase. The preferential orientation of thin films deposited by magnetron sputter technique is a well known phenomena which can limit observable X-ray diffraction peaks down to a single or several reflections from closely oriented planes [45-47]. In the case of monoclinic Mg 2 NiH 4 crystal phase the orientations of (-1 1 2) and (1 1 2) crystallographic planes are differing significantly and this can explain the absence of several intensive peaks of monoclinic Mg 2 NiH 4 phase which are present in the corresponding powder diffraction patterns at 24.1 (1 1 2), 39.1 (0 2 4), 39.5 (2 2 0) and 40 (2 0 4) degrees. The provided assumption of presence of monoclinic Mg 2 NiH 4 phase is further supported by the experimental observation that both of the strongest additionally present peaks at roughly 23.3 and 37.8 degrees can be respectively attributed to the reflections from relatively close crystallographic planes (-1 1 2) and (-2 0 4). The same is applied for the appearance of smaller peaks at 19.5 (larger shoulder), 31.4, 32.9, 48.7 and 55.7 degrees. Furthermore, the intensive peak at 37.8 degree only can be attributed to the monoclinic Mg 2 NiH 4 phase as none of the Mg, Ni, Mg 2 Ni, MgNi 2, Mg 2 NiH 4, Mg 2 NiH 0.3 or corresponding oxides phases have X-ray diffraction peaks at this angle. Summarizing observations from Fig we can see that according to the crystal phase information the highest hydrogen content (partial beta hydride) is received for the hydrogenated Mg-Ni film which was deposited on DC plasma treated Si substrate, whereas films deposited on other substrates has less hydrogen containing phase. This is in good agreement with predictions which were made looking at SEM images (Fig. 3.8). As it can be seen from Fig all three Mg-Ni films deposited on eptfe substrate during hydrogenation formed pseudo cubic Mg 2 NiH 4 hydride, i.e. different crystal structure (in comparison to films deposited on Si) even if the hydrogenation conditions were fixed for all six samples. The observed cubic phase of Mg 2 NiH 4 in general is similar to those reported in our earlier studies with films deposited on quartz and CaF 2 substrates [30]. 26

27 Fig X-ray diffractograms of hydrogenated Mg-Ni thin films deposited on eptfe substrates with different pre-treatment. It is important to point up that β magnesium nickel hydride is formed only for the sample which was deposited on DC plasma pre-treated substrate and no clear signs crystalline β hydride formation was observed for other two samples which were deposited on differently pretreated Si substrates. Another striking result is that after DC plasma pre-treatment we were able to form low temperature monoclinic Mg 2 NiH 4 hydride which was not observed on eptfe as well as on quartz substrate. The observed results (especially for Si substrates) indicate that probably there is a range of interface region conditions under which we witness best formation of crystalline Mg 2 NiH 4 hydride as well as preference for monoclinic or cubic hydride phase formation. These conditions can include several factors amongst those we can think about: i) proper adhesion strength which leads to the certain amount of in-plane stress during hydrogenation, ii) selected topography and temperature of substrate surface which leads to preferential growth of certain microstructures (for example columns of optimal diameter and/or formation of smaller grain interfacial zones), iii) presence of particular crystal phases at the interface (for example magnesium silicates) and/or elements which could act as hydrogenation catalyst and/or centres for hydride 27

28 0 phase nucleation. Other factors can be equally important though more experimental data and systematic studies are needed in order point out most important contributors as well as to test if these statements are also valid for other hydrogen absorbing materials. Investigation of substrate surface effects on hydrogenation properties of Mg and Mg- Ti thin films Si and eptfe substrates with the same plasma pretreatment were also used for the deposition of Mg and Mg-Ti thin films which were hydrogenated in 20 bar hydrogen pressure at 180 C temperature for 72 hours. From whole set of the samples crystal phase of MgH 2 hydride was formed only in one sample Mg thin film on plasma not treated Si substrate (Fig a). X X - Mg I - MgH2 Intensity, arb unt. X X c b X I I X a Diffraction angle, 2 theta Fig XRD patterns of hydrogenated Mg thin films deposited on Si substrates with different pre-treatment: a) Mg thin film on plasma not treatred Si, b) Mg thin film on DC plasma treated Si, c) Mg thin film on PDC plasma treated Si. The explanation of such result is based on the scientific publications reporting that at low temperatures Mg quickly reacts with hydrogen and covers surface by thin layer of the MgH 2 crystal phase which in turn blocks further formation of the magnesium hydride as hydrogen diffusion in MgH 2 phase is highly limited [48, 49] and 28

29 experimental observations that Mg thin film on plasma not treated Si substrate has smallest diameter columnar structure (Fig. 3.13). The smallest dimensions of the columnar structure results in the highest specific surface (both internal and external) of Mg thin film which react with hydrogen and forms significant amount of MgH 2 which is observable by XRD technique. If these assumptions are correct than it follows that in order to have more expressed formation of MgH 2 crystal phase it is necessarily to synthesise Mg coating with as small columns as possible. According to the classical models of thin film microstructure formation [50] this can be achieved by limiting surface diffusion of the sputtered atoms during thin film deposition. And there are two basic ways to limit diffusion to lower down the substrate temperature and/or to use substrate which would interact stronger (possibly formation of chemical bonds) with the Mg thin film. Keeping in mind that plasma treatment increases the substrates temperature the temperature factor also explains why Mg thin films on plasma treated Si substrate have larger columns and why their XRD does not show formation of crystalline MgH 2 phase. On the other hand, the observation that no MgH 2 crystal phase was formed on the eptfe substrate can be explained by the second factor weak interaction between sputtered atoms and substrate which result in quite high surface diffusion during deposition. With addition of the Ti fraction into Mg thin films it was expected to improve its hydrogenation properties and to achieve higher fraction of MgH 2 phase. However, after hydrogenation XRD analysis revealed that crystalline MgH 2 phase was not formed independently what substrate was used. Such result was explained by the experimental observation that in contrast to pure Mg films (Fig. 3.13); Mg-Ti thin films have had more dense structure without any signs of high specific surface area (Fig. 3.14). 29

30 Fig SEM images of metallic (left column) and hydrogenated (right column) Mg thin films on differently pretreated Si substrates: a and b Mg thin film on plasma not treatred Si, c and d Mg thin film on DC plasma treated Si, e and f Mg thin film on PDC plasma treated Si. 30

31 Fig SEM images of metallic (left column) and hydrogenated (right column) Mg- Ti thin films on differently pretreated Si substrates: a and b Mg-Ti thin film on plasma not treatred Si, c and d Mg-Ti thin film on DC plasma treated Si, e and f Mg-Ti thin film on PDC plasma treated Si. 31

32 4. Conclusions 1. During the presented research it was determined that if Si substrates are put into vacuum chamber with a base vacuum pressure of 8 x 10-6 Pa and then treated with DC and PDC plasma (Ar gas purity class - 5.0, working pressure 2 Pa) after plasma treatment C concentration at the surface is reduced close to the 0 %, but O concentration correspondingly increases from 39.5 % (Si without plasma treatment) up to 62.8 % and 61.4 %. At the same time oxygen penetration depth increases from roughly 2 nm up to 3-4 nm. These results indicate that common practice (which is met in scientific publications) to name plasma pretreament as substrate cleaning from contamination should be addressed critically and effects of plasma treatments should be analysed individually for each set of plasma conditions and substrate. 2. XPS analysis revealed that difference of integral amounts of O between plasma treated and not treated eptfe substrates is small less than 1.3 %. But C1s peak fitting showed that after plasma treatment the amount of C=O and O=C-O bonds increases up to 2 times. Furthermore, it was observed that there is significant difference in C bonds distribution between DC and PDC treated eptfe sample: the relative difference for C-C bond is 35 %, for C-O 19 %, C=O 28 %, O=C-O 32 %, CF 2 3 %. These observations confirmed the statements that PDC plasma affects dielectrics and semiconductors in a different way than does the DC plasma. 3. Mg-Ni thin films were deposited on 6 substrates with different surface properties. After deposition all of these samples were hydrogenated at the same conditions (180 C temperature, 20 bar hydrogen pressure) but crystal phase formation was differing: i) for Mg-Ni thin films deposited on plasma not treated and PDC plasma treated Si substrates after hydrogenation only Mg 2 NiH 0,3 solid solution phase was registered, ii) Mg-Ni thin film deposited on the DC plasma treated Si substrate after hydrogenation has formed Mg 2 NiH 0,3 and Mg 2 NiH 4 crystal phases, iii) all Mg-Ni thin films deposited on eptfe substrates after hydrogenation has transformed into pseudo cubic Mg 2 NiH 4 hydride phase. These results show that the usage of particular substrate can be consider as a way to control hydride phase formation during hydrogenation of Mg-Ni thin films. 4. The low temperature (< 200 C) hydrogenation experiments of Mg and Mg-Ti thin films deposited onto 6 substrates with different surface properties does not confirm 32