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X-Ray Diffraction

of In-situ Annealed

Electrodeposited Nanocrystalline

Co-3.2at.%P

Project report for the course

Materials Science: Characterization FFY155

Melina da Silva

(780516-5045) PhD-student at the Division of Materials Science and Engineering,

M-school, Chalmers University of Technology

Melina da Silva 780516-5045

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Abstract Electrodeposited nanocrystalline CoP is to be characterized by two XRD-techniques (conventional XRD, and in-situ heating XRD). The material’s main limiting factor with respect to the available techniques for this project is the small grain size (mean grain size in the as-deposited state is approximately 10 nm). The material has previously been studied by TEM (as part of my PhD-project) and FIM/TAP (in cooperation with another university), both in the as-deposited and annealed state. The aims of this study are to: 1. characterize the electrodeposit in the as-prepared state with respect to texture, and 2. study the phase transitions / texture changes when annealing up to ~500°C (the allotropic phase transformation hcp-Co → fcc-Co is expected at ~440°C).


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Index

1 Introduction.............................................................................................. 3

2 Characterization methods......................................................................... 4 2.1 X-Ray Photoelectron Spectroscopy (XPS)...................................................... 4

2.2 Auger Electron Spectroscopy (AES) ............................................................... 4

2.3 Infrared (IR) Spectroscopy .............................................................................. 5

2.4 Raman Spectroscopy ........................................................................................ 5

2.5 Neutron diffraction ........................................................................................... 6

2.6 Secondary Ion Mass Spectrometry (SIMS) .................................................... 6

2.7 Atom-Probe Field-Ion Microscopy (AP-FIM)................................................ 7

2.8 Scanning Probe Microscopy (SPM) ................................................................ 8

2.9 Electron Microscopy (EM)............................................................................... 8

2.9.1 Scanning Electron Microscopy (SEM)........................................................ 8

2.9.2 Transmission Electron Microscopy (TEM)................................................. 9

2.10 X-Ray Diffraction (XRD) ............................................................................... 11

3 Material .................................................................................................. 14

4 Experimental details............................................................................... 17

5 Results .................................................................................................... 19

6 Discussion and Conclusions .................................................................. 23

7 References.............................................................................................. 25

Acknowledgements………………………………………………….……..28


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1 Introduction Nanocrystalline materials (NCM) are commonly known as materials having grain sizes between 1 and 50 nm [1]. Due to their large interfacial volume fraction these materials exhibit exceptional macroscopic properties such as high hardness and wear resistance. As a consequence, they are strong contenders for protective coating applications replacing hard chromium layers which are harmful to the environment [2]. Furthermore, Co-based NCM have a higher saturation magnetization compared with Ni-based nanocrystalline alloys, which also makes them promising for soft magnetic applications [3]. To retain the excellent macroscopic properties of NCM at elevated temperatures, it is essential to preserve a stable nanocrystalline microstructure upon annealing. In some NCM abnormal grain growth occurs, resulting in an unstable microstructure at temperatures much lower than expected for normal grain growth. Electrodeposited nanocrystalline Co with an initial grain size of 20 nm, is stable up to 150°C [4, 5], whereas in nanocrystalline Ni abnormal grain growth sets in at temperatures as low as 110°C [6]. The thermal stability of nanocrystalline Ni has been improved by solute additions such as P, raising the temperature at which abnormal grain growth occurs to 240°C [7] and W, increasing its thermal stability up to 600°C [8]. A similar improvement of the thermal stability may be expected for nanocrystalline Co by addition of P. The thermal stability of a material depends on several factors such as initial grain size, texture, presence of solutes and second phases. The initial grain size of the material used for this study has been previously determined by transmission electron microscopy (TEM) dark field imaging. Field ion microscopy / tomographic atom probe (FIM/TAP) analysis shows the P-distribution in the as-deposited and annealed states. However, it is still unclear whether the electrodeposit is textured in the as-prepared state, and if there are any texture changes upon annealing. Furthermore, it is necessary to confirm the TEM results which show that only the hcp-Co phase is present in the as-deposited state, as well as to study if there are any phase formations / changes during annealing. The aims of this study are then to: 1. characterize the electrodeposit in the as-prepared state with respect to texture, and 2. study the phase transitions / texture changes when annealing up to ~500°C (the allotropic phase transformation hcp-Co to fcc-Co is expected at ~440°C). This will be done by use of X-ray diffraction (XRD).


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2 Characterization methods There are several methods available for characterization of materials in this course, and they will be described below. Special emphasis is made on two techniques: transmission electron microscopy (TEM) and X-ray diffraction (XRD). TEM is not used for this study, but some previous results will be presented in section 3 (the Material section). The only technique used for the present project work is XRD, the reason being that none of the other techniques available is suitable for studying nanocrystalline materials.

2.1 X-Ray Photoelectron Spectroscopy (XPS) X-Ray Photoelectron Spectroscopy, also called Electron Spectroscopy for Chemical Analysis (ESCA), is a surface sensitive technique (depth resolution ~10 nm) with a lateral resolution of 10 µm - 2 mm [9]. One of the main advantages of XPS is that in addition to indicating the chemical species present, it provides information about the chemical bonding in the material. Furthermore, it can be used both for conducting and insulating materials. The principle of XPS is based on the emission of electrons from atoms by absorption of X-rays [10]. A monochromatic X-ray beam irradiates the sample surface exciting electrons from the core levels, some of which escape the surface and pass through a hemispherical analyzer before they reach the detector. Additionally, the detector is connected to a multichannel analyzer. The current in the detector and the sample position give the intensity and the energy, respectively [10]. The limiting factor of this characterization method with respect to the material used for this study, i.e. nanocrystalline CoP, is the lateral resolution. Furthermore, the current studies of nanocrystalline CoP are focused on the bulk properties of the material, and not the surface properties.

2.2 Auger Electron Spectroscopy (AES) Auger Electron Spectroscopy is a surface sensitive technique (depth resolution ~5 nm) with a high lateral resolution (of 100 nm) [9]. The high lateral resolution is achieved by using a primary beam of highly energetic electrons [11]. The high surface sensitivity, high lateral resolution and possibility of performing depth profiling have made AES a very powerful technique. Applications are found in numerous fields such as metallurgy, corrosion, semiconductor technology, thin film analysis and fundamental surface physics [11]. The principle of AES is based on a competitive mechanism to the X-ray emission which will be described in section 2.10. Irradiation of a sample with a focused electron beam creates vacancies in the atoms, which return to the equilibrium state by either the Auger process or the emission of an X-ray. The emission of Auger electrons is caused by a


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higher energy electron dropping into a vacant level, transferring all or some of its energy to a third electron, and ejecting it into the vacuum [11]. As the energy of the Auger electrons is characteristic of the elements, it is possible to determine what species are present in a material by detecting the ejected electrons. Depth profiling is accomplished by sputtering the surface with e.g. Ar-ions and continuously recording the Auger peak intensities. The Auger process is dominant for light elements, making AES a better technique for these elements, while XPS is better for heavy elements (because X-ray emission is dominant for these elements). The limiting factor of AES with respect to nanocrystalline CoP is again the lateral resolution (the grains are ~10 nm in diameter in the as-deposited state). Also, as said before, it is the bulk properties which are interesting at this point, and not the surface chemistry.

2.3 Infrared (IR) Spectroscopy Infrared Spectroscopy has a lateral resolution of 15 µm and is used typically for identification of polymers, plastics, contaminants, organic films, fibers, and liquids [9]. The main advantages of this technique are that it provides chemical bonding information and in many cases also information about the chain structure, degree of branching, stereoregularity, types of end groups present, etc [12]. Limitations of IR Spectroscopy are for example that not all molecules are IR-active, resulting in that not all molecules can be identified with this technique, sample handling can be difficult, etc. [13]. The principle of IR Spectroscopy is based on the absorption of certain frequencies of IR radiation by the molecule corresponding to vibrational changes in it. The sample is subjected to polychromatic IR radiation which interacts with the vibrational energy levels of the molecules. The light from the source is split into two beams, one passes through the sample cell and the other through the reference cell. Most detectors used operate on the thermocouple principle, and if radiation has been absorbed the beams give rise to a pulsating or ac current from the detector to the amplifier. The ac part of the signal is amplified and the reference beam strength is matched to the sample beam strength [12]. IR Spectroscopy is a characterization technique for polymers and organic materials, not metallic materials. Therefore it is not suitable for characterization of nanocrystalline CoP.

2.4 Raman Spectroscopy Raman Spectroscopy is a characterization technique for identification of organic and inorganic materials by molecular chemical identification from vibrational spectra [9]. The lateral resolution of this technique is ~1 µm [9] and similarly to IR Spectroscopy it often gives information about the chain structure, degree of branching, geometric isomerism, conformation, crystallinity, etc [12]. Some of the limitations of this technique are that not all molecules are Raman-active (as was the case for IR Spectroscopy), some compounds fluoresce when irradiated by the laser beam, etc. [13]. IR and Raman Spectroscopy are complementary techniques.


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The principle of Raman Spectroscopy is based on the same physical phenomenon as IR Spectroscopy. However, the interaction between the incident radiation and the sample is different, in Raman Spectroscopy the sample is exposed to monochromatic radiation which is then scattered either elastically (Rayleigh scattering) or inelastically to produce Raman scattering. Since the Raman effect is very inefficient, very powerful light sources are required to generate enough signal, and high performance photomultiplier tubes are needed for detection. The detected signal is amplified by either a dc amplifier or pulse counting system [12]. As is the case for IR Spectroscopy, Raman Spectroscopy is not a characterization technique for crystalline metals. Hence, Raman Spectroscopy is not a suitable method for analysis of CoP.

2.5 Neutron diffraction Neutron diffraction provides similar structural information as electron diffraction. However, as neutron beams interact more strongly with nuclei than do X-rays, neutron diffraction is more useful than X-ray diffraction for determining proton positions. Neutrons interact weakly with matter and therefore have advantages in studying materials that are damaged by X-rays and in cases where a large penetration depth is desired. Neutrons are unique in that they have a magnetic moment and are therefore sensitive to magnetic ordering in a solid [14]. Some of the limitations of Neutron diffraction are the high costs, the large amount of sample needed, the relatively poor resolution, etc. [15] The principle of Neutron diffraction is based on the interaction of neutrons with the nuclei of atoms via the strong nuclear force. The amount by which atoms scatter neutrons, called the neutron cross section, varies randomly with atomic number. This means that neutrons are useful for studying light atoms, materials with different isotopes and samples that contain elements of similar atomic mass. Neutrons are produced by nuclear reactions in either a nuclear reactor or in an accelerator. Reactor sources produce a continuous spectrum of neutron energies and require a monochromator crystal to select a particular energy. Accelerator sources are usually operated in a pulsed mode and neutron wavelength is selected by time-of-flight methods, that is, data is taken at a fixed Bragg angle as a function of neutron energy [14]. Neutron diffraction is a suitable characterization technique for nanocrystalline CoP, but the high cost is not justified in this case as X-ray diffraction is a good enough and much more economic technique.

2.6 Secondary Ion Mass Spectrometry (SIMS) Secondary Ion Mass Spectrometry is a very sensitive technique for determination of element concentration (detection limit ~10-1-10-3 mole-ppm) and topographic distribution (in dynamic SIMS) of all elements in the periodic table (or isotopes) [16]. Conducting and some semiconducting materials can be analyzed with this technique, the lateral


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resolution typically being ~1 µm [9]. The high sensitivity and the possibility of doing depth profiling are some of the main advantages of this technique. There are two classes of SIMS, dynamic SIMS (depth profiling) and static SIMS (analysis of the outermost layers only). Some of the limitations of SIMS are that it is a destructive technique, and if the phases in the material are sputtered at very different rates (have different sputter yields) it can be difficult to do correct quantitative analysis [16]. The principle of SIMS is based on ion bombardment of solid surfaces and analysis of the sputtered ions according to their mass-to-charge ratio. The sample surface is bombarded by a focused ion beam (primary ions e.g. O2

+, O+, Ar+, N2+) so that the outermost atomic

layers of the sample are sputtered. The ejected secondary ions are separated according to their mass-to-charge ratio by a mass spectrometer, and then collected. Collection can be done in the form of quantifiable mass spectra, as in-depth or along-surface concentration, or as element distribution images of the surface [16]. The main limiting factor of SIMS with respect to characterization of nanocrystalline CoP is again the insufficient (lateral and depth) resolution. The best lateral resolution accomplished with SIMS is ~100 nm [16], and the grain size of as-deposited CoP is ~10 nm.

2.7 Atom-Probe Field-Ion Microscopy (AP-FIM) Atom-Probe Field-Ion Microscopy is a destructive characterization technique with the highest spatial resolution of all techniques (lateral resolution 1 nm and depth resolution 0.2 nm). All elements in the periodic table can be analyzed quantitatively with a sensitivity of up to 0.005 at.%. However, there are several limitations of this technique, the most important probably being that the sample has to have a certain mechanical strength and the shape of a sharp needle (tip radius <50 nm). Furthermore, a certain degree of electrical conductivity of the sample is required [17]. The principle of AP-FIM is based on the process of field evaporation of atoms from the sample tip when a high voltage is applied. Field evaporation of some surface atoms is induced by applying a short high-voltage pulse to the sample. The sample is cooled to cryogenic temperatures to impede field evaporation between pulses. If the tip in sharp enough and a sufficiently high voltage is applied, the surface atoms are removed by action of the field and become positively ionized. The ions are repelled from the tip and hit a earthed screen with an analyzing aperture. The mass of the atoms is identified by a time-of-flight spectrometer [17]. AP-FIM spectra or a composition profiles are then obtained. AP-FIM is an excellent technique for analysis of nanocrystalline materials, but sample preparation is tedious and time consuming. Therefore, this technique was not available for the projects in the course. Besides, three-dimensional atom probe analysis has already been performed on the material in cooperation with the University of Göttingen.


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2.8 Scanning Probe Microscopy (SPM) Scanning Probe Microscopy is a group name for several techniques such as Atomic Force Microscopy (AFM), Scanning Tunneling Microscopy (STM), and Magnetic Force Microscopy (MFM) [9]. The scanning probe microscope is used for studying surface properties of materials (lateral resolution of 1.5-5 nm [9]), and in some cases physical properties such as surface conductivity, static charge distribution, localized friction and magnetic fields can be measured [18]. Limitations of some of the techniques are that the area of interest cannot have too rough topography (must have less than 5 µm total relief [9]), the tip geometry can cause imaging artifacts, etc. [18]. The principle of SPM is based on that a small tip is scanned across the sample surface in order to construct a three dimensional image of the surface. Fine control of the scan is accomplished using piezoelectrically-induced motions. If the tip and the surface are both conducting, the structure of the surface can be detected by tunneling of electrons from the tip to the surface (STM). Any type of surface can be probed by the molecular forces exerted by the surface against the tip (AFM). The tip can be constantly in contact with the surface, it can gently tap the surface while oscillating at high frequency, or it can be scanned just minutely above the surface. By coating the tip with a magnetic material, the magnetic fields immediately above a surface can be imaged (MFM) [9]. SPM is a group of techniques used for studying the surface properties of materials. The study of nanocrystalline CoP is concentrated on the bulk properties, therefore STM is not a suitable characterization technique.

2.9 Electron Microscopy (EM) Electron Microscopes were developed to overcome the limitations of light microscopes, which can only achieve magnifications up to ~1000× and a resolution of ~0.2 µm (because the wavelength of visible light is 400-700 nm) [19]. Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) are the most important techniques belonging to this group, and are treated separately (special emphasis is made on TEM as there will be TEM results presented in section 3).

2.9.1 Scanning Electron Microscopy (SEM) In SEM the sample surface is bombarded with a convergent electron beam, which is scanned across the sample surface in a raster. The electron beam generates several types of signals (secondary electrons, backscattered electrons, X-rays, etc.), which are emitted from the area where the electron beam is impinging the sample. The induced signals are detected and the intensity of one of the signals (at the time) is amplified and used to modulate the intensity of a cathode ray tube (CRT) at each point [20]. A two dimensional image of the three dimensional sample surface can be generated by detection of secondary electrons (secondary electron imaging), or an elemental map can be generated by detection of X-rays (energy dispersive X-ray spectroscopy).


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The main advantages of SEM are that it has very high depth of field and imaging resolution compared to light microscopy [21]. This enables topographic imaging of rough sample surfaces such as fracture surfaces. Another advantage is that the magnification is obtained without the use of magnifying lenses (which always incorporate more or less significant defects), but it is given by the ratio between the scanned area on the sample surface and the CRT screen size [21]. The limiting factor for the resolution is the probe size, which can be as small as 4.5 nm for secondary electron imaging or ~1 µm for Energy Dispersive X-ray Spectroscopy (EDS) [9]. SEM is suitable for imaging of conducting materials, but non-conducting materials can be imaged too, if coated with a conducting (gold) layer.

2.9.2 Transmission Electron Microscopy (TEM) The principle of TEM is somewhat different than that of SEM. In this case the sample is made so thin (<100 nm) that the electron beam passes through it, either unaffected or scattered (elastically or inelastically). There are however things that SEM and TEM have in common, e.g. in both cases there are several signals produced by the interaction between the electron beam and the specimen. In Analytical Electron Microscopy (AEM) many of these signals are used, giving chemical information and a lot of other detail from the specimen [22].

Fig. 2.9.1: Sketch of the transmission electron microscope [22].


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A TEM consists of an electron gun and an assembly of electron lenses as seen in Fig. 2.9.1. The different parts can be divided into radiation source, illumination system, image forming system and projection system. The radiation system consists of the electron gun, which is made up of three parts: the filament (emits electrons and is held at the accelerating potential), the Wehnelt cylinder (is weakly biased and gathers the electrons), and an anode (held at ground potential and causes the electrons to accelerate). The gun forms an image of the emitted electrons, the cross over [23]. The illumination system usually consists of two condenser lenses, the first one is strong (collects electrons and focuses them) and the second one is weak (projects the image created by the first condenser lens onto the sample). To decrease the beam divergence the condenser lenses have condenser apertures. The image forming system consists of an objective lens (the most critical component of the TEM) and an objective lens aperture. All imaging errors, such as spherical aberration and astigmatism, are further magnified by the projection lenses in the projection system. The errors have to be corrected in order to give a good enough image at the end. This is for example done by making the objective lens as strong as possible (to make the spherical aberration as weak as possible) and by incorporating objective stigmators (to correct for the imaging astigmatism) [23, 24].

a) b)

Fig. 2.9.2: Ray diagrams showing the two basic modes of TEM, a) diffraction mode; and b) imaging mode [21].


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In Fig. 2.9.2a and b the ray diagrams for diffraction mode and imaging mode are shown. By changing the strength of the intermediate lens, i.e. one of the lenses in the projection system, it is possible to change between the two modes [21]. The selected area aperture (which is placed between the objective lens and the intermediate lens) is used in diffraction mode to chose a smaller area for diffraction, while the objective lens is used in image mode to chose one or several beams to form the image. If the transmitted beam is chosen it is called bright field imaging, and if a diffraction beam is used it is dark field imaging (one or several beams can be chosen). Sample preparation is crucial for TEM analysis, and it can be straight forward or very tedious, depending on the material to be studied. Normally, for bulk material sheets or foils are cut/thinned to a thickness of up to a few hundreds of mm. Then, discs of 2.5 or 3 mm in diameter are punched (for ductile materials) or cut with an ultrasonic disc cutter (for brittle materials). The next step is to make an area so thin that it can be passed through by the electron beam. This can be achieved in a number of ways such as electropolishing (using a suitable electrolyte, voltage, temperature, etc.), dimple grinding and ion milling (the sample is ground and milled in the center with grinding and polishing wheels, and then ion milled in the same area with a low angle of incidence), or with Focused Ion Beam (FIB). In the first two cases a hole is made in the center of the sample so that the material around the hole is thin enough for doing TEM work. In the last case the material is thinned with a focused ion beam at an edge of the sample until it is thin enough (<100 nm).

2.10 X-Ray Diffraction (XRD) X-Ray Diffraction techniques are some of the most useful in the characterization of crystalline materials, and some information can sometimes even be obtained on amorphous solids and liquids. Similar information can often be obtained by other diffraction methods such as electron diffraction and neutron diffraction, but the sample limitations are usually more severe and the equipment considerably more complicated and expensive. In general, the X-ray techniques are classified as single crystal or polycrystalline, depending on the kind of sample they are intended for [25]. One of the limitations of XRD is that all the elements that could be present in the analyzed material have to be known to characterize it correctly. The principle of XRD is based on the emission of (characteristic) X-rays when atoms return to the low-energy state from an excited energy state. Irradiation of the sample surface with a focused electron beam creates vacancies in the atoms, which return to the equilibrium state by either the emission of an X-ray or the Auger process [11]. As pointed out in section 2.2, the Auger process competes with the X-ray emission, the former being dominant for light elements. In addition to the characteristic X-rays (which contain valuable information about the analyzed material), X-rays are produced by rapid deceleration of the incoming electrons to the sample (bremsstrahlung radiation), forming a continuous background in the spectrum [26].


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Many different types of analysis can be conducted with XRD (type of sample is indicated as SC: single crystal, P: powder, PC: powder compact): crystal geometry (mainly SC), arrangement of atoms (SC or P), symmetry (SC), identification of compound (SC or P), crystal orientation (SC or PC), size of crystal (P), magnitude of strain (PC), amount of phase (P), change of state (SC or P), and crystal perfection (SC or P). Unfortunately, all types of analysis cannot be performed with the same method, e.g. crystal geometry measurements require moving crystal-spot patterns (SC), a computer positioned diffractometer (SC) or solution of d-spacing equations (P), while determination of atom arrangement requires analysis of diffracted intensities (SC) or refinement of the whole pattern (P) [25]. As the XRD method used for this project is a powder diffraction method, we will concentrate on powder diffractometers. X-Ray Powder Diffraction (XRPD) techniques are used to characterize samples in the form of loose powders or aggregates of finely divided material. There exist many different geometries for X-ray powder diffractometers, for example the Debye-Scherrer geometry, the Bragg-Brentano geometry, and the Guinier geometry. For each of these geometries there can be more than one variant, resulting in different diffractometers even if they are based on the same geometry [27]. In a diffractometer the intensity of a diffracted beam is measured directly, either by means of the ionization it produces in a gas or the fluorescence it produces in a solid. This is most often done with the aid of a counter (there are different types of counters such as the proportional counter, the Geiger counter and scintillation counter) [26]. In a diffractometer, monochromatic radiation is used and the X-ray detector (film or counter) is placed on the circumference of a circle centered on the powder specimen, see Fig. 2.10.1.

Fig. 2.10.1: Schematic figure of an X-ray diffractometer [26].


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The detector is rotated around the diffractometer circle measuring the intensity of the diffracted beam as a function of the diffraction angle (really as a function of 2θ). The collected information is plotted on a diffractogram such as the one shown in Fig. 2.10.2. The diffractogram is then compared to that of standard powders found in different databases, and indexed.

Fig. 2.10.2: Diffractogram of the powder pattern of NaCl using Ni filtered CuKαradiation [28].


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3 Material The material used for this study is nanocrystalline Co-3.2at.%P provided by Integran Technologies Inc., Toronto, Canada. It is produced by electrodeposition onto Ti-cathodes from a bath (containing cobalt sulphate, sodium chloride, boric acid, etc.) to a thickness of ~200 µm and then it is mechanically stripped from the substrate. For TEM investigations, 3 mm diameter disks were punched from the material and afterwards prepared by dimple grinding and ion milling. The material was thinned to a thickness of ~60 µm with a Buehler Minimet grinder-polisher prior to dimple grinding. Ion milling was carried out in a Gatan precision ion polishing system using an angle of incidence of 4° on each side, and a beam energy of 4 kV. The TEM investigations were performed using a Zeiss 912 Omega TEM with an integrated energy filter operating at an acceleration voltage of 120 kV. The grain size was measured by TEM dark field images. Figs. 3.1a and b show a zero loss bright field image and a dark field image of the same area, respectively. Fig. 3.1c shows the grain size histogram for Co-3.2at.%P. A mean grain size of 11.9 ± 3.2 nm was determined by measuring 417 grains in total. Evaluation of the grain size in Co-based nanocrystalline materials was found to be more difficult than for Ni-based materials (the study of both kinds of materials are part of my PhD) owing to the highly faulted microstructure. This characteristic appearance of nanocrystalline Co has been observed previously by other authors [4, 5, 29].

A diffraction pattern of the as-deposited material is shown in Fig. 3.2. It looks like a typical powder diffraction pattern, which is due the small grain size of material. As there are so many grains contributing to diffraction we get a ring pattern instead of a spot pattern. All rings are indexed in the figure, and correspond to the hcp-Co phase. This means that only the hcp-Co phase is present in the as-deposited state, as expected from the CoP phase diagram. The allotropic phase transformation hcp-Co → fcc-Co takes place at ~440°C for pure Co [30], and is expected at a similar temperature for Co-3.2at.%P.

0 5 10 15 20 25 300

5

10

15

20

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30

35

Freq

uenc

y [%

]

Grain size [nm]

c)a) b)

Fig. 3.1: a) TEM bright field image; b) TEM dark field image; and c) grain size histogram for as-deposited Co-3.2at.%P.


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The thermal stability of CoP is studied by TEM in-situ annealing, and comparing it with imaging and analysis of ex-situ annealed material. Differential Scanning Calorimetry (DSC) is performed in order to predict transformation temperatures, and the annealing experiments are done accordingly. A DSC-curve of Co-3.2at.%P shows that the microstructure is stable up to ~425°C, see Fig. 3.3. TEM bright field images of ex-situ annealed material are shown in Fig. 3.4 for different heating treatments.

Fig. 3.3: DSC-curve of Co-3.2at.%P, shown for the temperature range 350-525°C. Isochronal heating was performed from 50°C to 550°C with a rate of 10°C/min.

(1 0 -1 0) (0 0 0 2)

(1 0 -1 1)

(1 0 -1 2)

(1 1 -2 0) (1 0 -1 3)

(2 0 -2 0)

Indexing:

Fig. 3.2: Selected area diffraction pattern of Co-3.2at.%P. The rings are indexed, and correspond to hcp-Co.


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As seen in Fig 3.4a abnormal grain growth takes place in this material upon annealing at 440°C for 15 min. This means that only some grains start growing while the other grains remain unchanged. It is also notable that some of the grown grains have a very pronounced lamellar appearance. In Fig 3.4b an image is shown of a sample annealed at 460°C for 15 min, i.e. at a slightly higher temperature compared to the material in Fig. 3.4a. In this case abnormal grain growth is still visible, and the grown grains are even larger than in Fig. 3.4a.

b)Fig 3.4: a) Ex-situ annealed Co-3.2at.%P at 440°C for 15 min; and b) in-situ annealed Co-3.2at.%P at 460°C for 15 min imaged after cooling to room temperature.

a)


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4 Experimental details For XRD measurements a Siemens D5000 diffractometer (manufacturer: Bruker-AXS) was used, see Fig. 4.1a. The Siemens D5000 diffractometer has a large diameter goniometer (600 mm), low divergence collimator, and Soller slits. The instrument is useful for both powder and bulk materials. This diffractometer is best utilized for high-precision work [31]. Several attachments can be used with this diffractometer, and for the temperature runs an HTK-10 high temperature chamber was used (manufacturer: Anton Paar), see Figs. 4.1b and c.

A test run was performed on the material at room temperature in order to see if it was suitable for (variable temperature) XRD. CoKα radiation was used (λ = 1.5406 Å), which gave very bad results (very low signal to noise ratio). This is thought to be due to that the sample is a Co-based alloy and the radiation CoKα leading to fluorescence. Long Soller slits were combined with a flat secondary monochromator on the detector side in order to decrease this effect. No sample preparation was required, except for cutting the sheet material in a suitable size for mounting in the high temperature chamber, i.e. 0.9 mm × 1.0 mm, as the

Fig. 4.1: a) The Siemens D5000 diffractometer [31]; and the HTK-10 high temperature chamber shown b) from the outside, and c) from the inside (the arrow is pointing at the sample) [32].

a) b)

c)


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platinum plate on which the sample is placed has a width of 0.9 mm. The working principle of the heating assembly is that a thermocouple is placed under the platinum plate which is heated. The sample is placed (glued) on top of the platinum plate, as close to the thermocouple as possible. The heating chamber is connected to a pump making it possible to run experiments under vacuum (<10-2 atm). For the present study the measurements were done in vacuum with a tube voltage of 45 kV and a tube current of 45 mA. As there are heat losses during the heat transfer (from platinum plate to sample) the temperature chosen for the experiments should be set ~10% higher than desired. According to the DSC curve shown in Fig. 3.3 a transformation of some kind happens at ~440°C, and TEM analysis has shown that abnormal grain growth occurs at this temperature. Therefore, the temperatures chosen for the experiments were: room temperature (as-deposited state), 350°C (intermediate temperature, before the expected reactions occur), 450°C (peak temperature) and 500°C (further annealing). The set temperatures were then room temperature (~27°C is the lowest achievable temperature without external cooling), 385°C, 495°C, and 550°C. The temperature ramping program is sketched in Fig 4.2. A heating rate of 1°C/s was used for all the heating stages. At each temperature a 5 min holding time was set in order for the material to reach the desired temperature (except for room temperature for obvious reasons). The XRD measurements were done with a step size of 0.05° and a step time of 2 s, for 2θ = 40° - 100° (this range was chosen according to the standard diffractograms for the phases of interest). Each measurement took 40 min to complete.

0 30 60 90 120 150 1800

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ture

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27°C

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XRD -measurements

5 min holding time

Fig. 4.2: Graph of the ramping program for the variable temperature XRD run. The red sections mark the time while the measurements were running. At each temperature a holding time of 5 min was set to make sure that the material had reached the desired temperature before starting the measurement.


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5 Results The test run performed at room temperature (without the secondary monochromator) resulted in the diffractogram shown in Fig. 5.1. As seen, the signal-to-noise ratio is very low. However, it is seen that the hcp-Co phase only is present (as previously observed by TEM diffraction in Fig. 3.2).

The diffractogram from the temperature run at room temperature (with the secondary monochromator) is shown in Fig. 5.2. Here, the signal-to-noise ratio is clearly higher than in the diffractogram in Fig. 5.1. The powder standards are coded with different colors as shown in the figure, and we can see that the hcp-Co phase is dominant. Comparing the standards of hcp-Co (red) and fcc-Co (blue) it can be seen that there is only one reflection, the (200)-reflection at 2θ = 51.5°, that is unique for the fcc-Co phase, the rest coincide with reflections of the hcp-Co phase. There is no peak at this angle. However, there are other peaks clearly present which do not correspond to Co. These were found to correspond to Pt and Pt-Rh, which come from the platinum base plate to which the sample is glued, and not the analyzed sample.

Fig. 5.1: Diffractogram of the test-run on as-deposited nanocrystalline CoP at room temperature (green color). Superimposed on the measured diffractogram, the standard hcp-Co powder is shown in gray.


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Fig 5.2: X-ray diffractogram of CoP at room temperature. The powder standards shown are coded with different colors.

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Fig. 5.3: X-ray diffractogram of CoP at 350°C. The powder standards shown are coded with different colors.


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In Fig. 5.3 the X-ray diffractogram of CoP at 350°C is shown. As seen, the same peaks are found as in the case of CoP at room temperature. Furthermore, there are some additional peaks appearing which correspond to two types of cobalt oxide: CoO and Co3O4. The (200)-reflection of fcc-Co is still not visible, indicating that there is no phase transformation occurring at temperatures up to 350°C. In Fig. 5.4 the X-ray diffractogram of CoP at 450°C is shown, and again we observe reflections corresponding to hcp-Co, Pt, Pt-Rh, CoO, and Co3O4. The CoO-and Co3O4-peaks are however much more pronounced at this temperature. Additionally, the (200)-reflection of fcc-Co is visible, indicating that at this temperature some of the hcp-Co has transformed.

In Fig. 5.5 the X-ray diffractogram of CoP at 500°C is shown. Here, the peaks corresponding to the cobalt oxides have increased in intensity further, as well as the peak corresponding to the (200)-reflection of fcc-Co. Finally, in Fig. 5.6 all the acquired diffractograms are displayed together for comparison. The curves for higher temperatures have been shifted so that they can be distinguished more easily (therefore the background levels cannot be compared in this figure). Only the Co standards are shown, and it is clearly seen that the (200)-reflection of fcc-Co appears when annealing at 450°C, and grows with increased temperature.

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2θ [°]Fig 5.6: Comparison of X-ray diffractograms of CoP at different temperatures.

Standards for hcp-Co and fcc-Co are shown with colors as seen in the legend.

Co (hexagonal) standard Co (cubic) standard

(200)-reflection of fcc-Co


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6 Discussion and Conclusions XRD measurements on nanocrystalline CoP using CoKα radiation requires the use of a secondary monochromator in combination with the Soller slits. This is an effect of fluorescence, and gives a very low signal-to noise ratio in the measured diffractograms (this was seen by comparing Figs. 5.1 and 5.2). In the as-deposited state, i.e. at room temperature (or 27°C), only the hcp-Co phase was found in the material. In fcc-Co phase all reflections coincide with the hcp-Co reflections except for one, the (200) reflection found at 2θ = 51.5°. Apart from the positions of the peaks, which give enough information for us to identify the phases that are present, the intensities of the peaks can be analyzed. As seen in Fig. 5.2, the intensities of the three first peaks corresponding to the hcp-Co phase are different than those of the standard powder hcp-Co (in relation to each other). This is an indication of a preferential orientation of the nanocrystalline grains in the sample, which is not surprising bearing in mind that the material is electrodeposited on a substrate. Preferential orientation of grains give rise to texture, and depending on the total amount of grains that have specific orientations, the texture of the material is strong or weak. Unfortunately, the texture cannot easily be measured for all diffractometer geometries. For the Bragg-Brentano geometry, it is easier to say something about the texture in a quantitative way. However, in the present study this is not the case, and a quantitative measurement of texture cannot be done easily. What can be stated out of Fig. 5.2 is that there is a preferential orientation of the hcp-Co grains, with the (0001)-planes parallel to the sample surface. This is seen as the (0002)-reflection (the reflection at 2θ = 44.7°) has a higher intensity (relative to the other two reflections) than expected from the standard. If we compare the intensities of the three first hcp-Co peaks for the four temperatures, it is obvious that the relative intensity of the middle peak decreases with increasing temperature. This indicates that the degree of texture decreases as the temperature increases. Furthermore, fcc-Co is forming when annealing at temperatures as high as 450°C. This was expected, as the allotropic phase transformation from hcp-Co to fcc-Co happens at ~440°C for pure Co. What we can say in respect, is that if the addition of P shifts that transformation temperature, it does not seem to shift it to higher temperatures. Another thing that is observed in Fig. 5.6 is that the peaks shift to the left, i.e. to smaller 2θ angles, with increasing temperature. Smaller 2θ angle corresponds to smaller a diffraction angle, which means that the interplanar spacing (d) increases. Larger interplanar spacing means in turn a larger lattice parameter (and unit cell) of the material. This makes perfect sense as we know that material do expand when heated due to the asymmetrical shape of the potential energy curve as a function of interatomic distance [33].


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As observed already at 350°C, oxidation of Co takes place upon annealing, even if the experiments were conducted in vacuum. This has not been observed in TEM in-situ annealing nor ex-situ annealing in a tube type furnace operating at ultra-high vacuum. Therefore, it can be concluded that the vacuum in the high temperature chamber was not high enough to ensure that there is no Co-oxidation (Co seems to be highly reactive).


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Acknowledgements I would like to thank Vratislav Langer for his assistance with the XRD measurements and his invaluable help with the interpretation of my results.

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