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 preparation of a set of samples under identical deposition conditions, but of various thicknesses, followed by the measurement of each sample separately is usually the choice. Nevertheless, it is desirable to monitor the film growth in situ to avoid the exposition of the material to external atmospheric contamination. This would allow a direct correlation between processing parameters and film properties as well to determine the most favourable conditions for the growth of high quality films. However, in-situ analysis techniques are rare for sputterdeposited films due to probe absorption and scattering by the sputtering gas and plasma environment. Under the operating pressures of sputter-deposition, X-ray radiation seems to be a logical choice to monitor the growth. It is known that photon scattering is relatively small in a gaseous atmosphere, when compared to that of electron beam probes, and photon are not sensitive to magnetic fields or charged particles. XRD with an X-ray tube poses some limitations for real-time investigations due to the measurement times required for serial data collection. Yet, Malhotra et al [55] and Lee et al [56] have developed sputtering deposition systems to be used into laboratory X-ray diffractometers. They have minimized data acquisition time through the use of optimised detectors. Alternatively, XRD with a synchrotron radiation source has been used since the 1990’s for in-situ investigations of film growth. Je et al [57] have reported a series of real-time synchrotron X-ray measurements to reveal the nature of the growth morphology of TiN films grown by radio frequency (RF). The chamber used during their work had only one magnetron available. Later, Matz et al [58] have developed a two-magnetron sputter deposition chamber for in-situ observation of film growth by synchrotron radiation scattering at ROBL/ESRF. Thus, the use of synchrotron radiation allows the in-situ monitoring of selected Bragg peaks or even whole diffraction patterns during the deposition process and important investigations have been made with this tool in the last few years [48, 59-61]. 1.2.5.1. Types of texture Here, a classification of the type of textures reported for films is given and various terminologies are discussed (random, fibre texture, and biaxial texture). Random: a homogeneous distribution of the diffracted intensity across pole figures is observed in the case of a random texture, since no restrictions are imposed on grain orientation. It can be expected that, for a sample with a fully random grain orientation, the intensity ratios of the different peaks observed in a θ/2θ XRD measurement correspond to ratios found in the powder diffraction database. Chapter 1 - Background and Literature Review 35
In-situ XRD studies during growth of Ni-Ti SMA films and their complementary ex-situ characterization Fibre texture: vapour deposited metallic films very often exhibit a strong crystallographic texture. In the case of a typical fibre texture, a particular (hkl) plane is preferentially aligned parallel to the film surface. The direction normal to the surface is generally referred to as the fibre axis. A dominating very intense peak is present at a θ/2θ XRD measurement, result of the diffraction from the lattice plane that is preferentially parallel to the surface of the film and, in pole figure measurements, a spot in the centre of the pole figure is observed [see Fig. 1.18(a)]. In some cases, a variant of the standard fibre texture is found, which is denominated tilted texture. The preferred orientation of the crystallites is inclined with respect to the sample surface. It means that the fibre axis is not perfectly perpendicular to the sample surface. In Fig. 1.18(b) it is shown a (200) pole figure for a MgO film, which was deposited without substrate rotation. The direction of the vapour flux is shown by the arrow [62]. The pole figure from Fig. 1.18(c) shows the (200) pole figure for a TiN/TiAlN multilayer coating deposited on WC-Co with substrate rotation [63]. Under such deposition conditions, circles centred near the middle of the pole figures are encountered in pole figure measurements. Due to the fact that the orientation of the grains is only subject to a single constraint, there is a remaining degree of freedom for rotation around the surface normal, leading in the circular region of highly diffracted intensity on the corresponding pole figure. (a) (b) (c) Fig. 1.18: Pole figures; (a) Permalloy (Ni 80 Fe 20 ) film exhibiting a strong fibre texture [64], (b) (200) pole figure for a MgO film (without substrate rotation - the direction of the vapour flux is shown by the arrow) [62] (c) (200) pole figure for TiN/TiAlN multilayer coating deposited on WC-Co (with substrate rotation) [63]. Biaxial texture, also known as in-plane texture for films deposited onto amorphous or polycrystalline substrates, and epitaxy or epitaxial alignment for films that are deposited or formed onto single crystal substrates. This type of texture is characterized by an orientation of Chapter 1 - Background and Literature Review 36
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RUI MIGUEL DOS SANTOS MARTINS IN-SI
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ACKNOWLEDGEMENTS The research work
Page 5 and 6: At CENIMAT, Luís Pereira also carr
Page 7 and 8: SUMÁRIO O fabrico de películas fi
Page 9 and 10: LIST OF SYMBOLS AND ABREVIATIONS α
Page 11 and 12: TABLE OF CONTENTS Introduction ....
Page 13 and 14: LIST OF FIGURES Fig. 1.1: Power/wei
Page 15 and 16: Fig. 2.3: Calculated integrated pho
Page 17 and 18: Fig. 3.25: Pole figures of Ni-Ti B2
Page 19 and 20: Fig. 3.66: SAED pattern obtained on
Page 21 and 22: LIST OF TABLES Tab. 1.1: Factors af
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