PhD Thesis_RuiMSMartins.pdf - RUN UNL

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In-situ XRD studies during growth of Ni-Ti SMA films and their complementary ex-situ characterization Different experiments were performed on naturally oxidized Si(100) substrates and on TiN buffer layers with thickness ≈ 215 nm. An example of a Ni-Ti graded film deposited on a naturally oxidized Si(100) substrate without applying V b is given in section 3.5.1. It concerns a film (total thickness of ≈ 420 nm) with a Ti-rich composition in the central part (ranging from 50 to ≈ 60 at%) and a near equiatomic composition in the extremities, following four distinct deposition periods (different Ti magnetron powers). During the initial deposition step (near equiatomic composition), the Ni-Ti B2 phase starts by stacking onto (h00) planes on the naturally oxidized Si(100) substrate due to the presence of the native silicon oxide (2-3 nm). The in-situ XRD studies [evolution of the a o values of B2 phase as calculated from d (200) ] have shown also a considerable reduction of compressive stresses with increasing thickness. These observations are in agreement with the previous study of the growth of near equiatomic Ni-Ti films (143 min deposition - see Fig. 3.1). In that experiment, with increasing film thickness a crossover of the crystallographic orientation into a (110) preferred orientation is observed (≈ 30 min after deposition start), which is the more densely packed crystallographic plane. The preferential stacking of B2 phase on (110) planes starts at the moment the stabilization of a 0 of B2 phase as calculated from d (200) is detected (significant decrease of compressive stress). However, for the present sample, after this initial deposition period of ≈ 30 min, the Ti co-sputtering power was increased, first to 20 W and later to 30 W, leading to a gradual increase of the Ti content and the precipitation of the Ti-rich phase Ti 2 Ni. During the precipitation process the a 0 value of Ti 2 Ni, as calculated from d (333/511) , slightly decreases in a continuous way. Ishida et al. [164] observed that when an amorphous Ti-rich Ni-Ti film is crystallized (Ti-48.2 at. % Ni in the study), the microstructure changes in the sequence of (1) Guinier- Preston (GP) zones, (2) GP zones and Ti 2 Ni precipitates within Ni-Ti grains, (3) Ti 2 Ni precipitates within Ni-Ti grains and (4) Ti 2 Ni precipitates along grain boundaries. The microstructure of a sample annealed at 500°C for 60 min was reported to have a dispersion of Ti 2 Ni inside the Ni-Ti grains, as well as GP zones. The Ti 2 Ni has partial coherency with the matrix and the misfit is considerably relaxed. They found the close relationship between the Ti 2 Ni precipitates and the Ni-Ti matrix, and observed some similarity between the units cells of both phases. A high magnification image of the interface in their study revealed that some of the {110} planes are continuous through the precipitate and the matrix. Since the d-spacing of Ti 2 Ni(400) is 0.283 nm and that of B2(100) is 0.301 nm, if the Ti 2 Ni precipitate is coherent with the matrix at the beginning of the precipitation process, a tensile stress field is formed around the precipitate. In the present study, it is not possible to compare directly the results Chapter 4 – Discussion 181
In-situ XRD studies during growth of Ni-Ti SMA films and their complementary ex-situ characterization with what has been obtained by Ishida et al. [164] because here the Ni-Ti film is crystallized during the deposition process. Nevertheless, it should be considered that the continuous slight decrease of the a 0 value of Ti 2 Ni, as calculated from d (333/511) , could also be associated with the formation of a stress field around the precipitate. During the last deposition period (without Ti co-sputtering), Ti 2 Ni dissolves and, thus, plays the role of a reservoir for the formation of the B2 phase now preferentially stacking onto (110). As suggested by the depth profile results of the atomic concentrations (Fig. 3.64) this dissolution process is a consequence of the shift of the global composition towards Ni-richer, ending up at the near equiatomic composition. Since no relevant peak intensity changes could be discerned by in-situ XRD during the annealing period, the main progress to reach the near equiatomic composition occurs certainly during this last deposition step. During this step, a significant increase of the a 0 value of the Ti 2 Ni is observed and for the B2 phase the increase is slightly higher for the value calculated from d (110) . The variation observed for the a 0 value of the B2 phase could be related with the incorporation of Ti in the matrix. It should be also considered that if this dissolution process leads to the formation of GP zones, compressive stress fields in the matrix around them are produced. Following these studies [using a naturally oxidized Si(100) substrate], a TiN film with a topmost layer formed mainly by oriented grains has been used as substrate. It has been shown before (during the deposition of near equiatomic Ni-Ti films) that TiN acts not only as a diffusion barrier, but also induces different crystallographic orientations on the B2 phase. The in-situ XRD results have shown a preferential growth of oriented grains of the B2 phase from the beginning of the deposition, with a constant growth rate during the whole deposition for a Ni-Ti film deposited on this type of buffer layers [Fig. 3.31(a)]. However, it is also important to know what would be the trend of the B2 preferred orientation for an increase of the Ti content during the deposition of graded films (e.g. half near equiatomic and half Ti-rich), since the precipitation of Ti 2 Ni would be expected in the Ti-rich film fraction. This is a typical question that can be answered taking profit of the unique equipment installed at ROBL. In section 3.5.2, the effect of the controlled change of the Ti:Ni ratio during the deposition of Ni-Ti films on the TiN buffer layer (with a V b of −45 V) was tested. The precipitation of the Ti 2 Ni phase in the second step of the deposition (due to the increase of the power of the Ti target) did not affect the preferential growth of oriented grains of the Ni-Ti B2 phase (Fig. 3.73). Chapter 4 – Discussion 182
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RUI MIGUEL DOS SANTOS MARTINS IN-SI
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ACKNOWLEDGEMENTS The research work
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At CENIMAT, Luís Pereira also carr
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SUMÁRIO O fabrico de películas fi
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LIST OF SYMBOLS AND ABREVIATIONS α
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TABLE OF CONTENTS Introduction ....
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LIST OF FIGURES Fig. 1.1: Power/wei
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Fig. 2.3: Calculated integrated pho
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Fig. 3.25: Pole figures of Ni-Ti B2
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Fig. 3.66: SAED pattern obtained on
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LIST OF TABLES Tab. 1.1: Factors af
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In-situ XRD studies during growth o
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