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Integral breadth calculation method
By using the lattice parameters, linewidths and mixing parameters of the assumed pseudo-Voigt profile of the Bragg lines, obtained for each Bragg reflection, we were able to calculate the average crystallographic domain size associated to the average diameter of the grains. The method we used is based on the integral breadth algorithm. The program calculates domain size and microstrain from input integral breadths of at least two physically broadened, due to the crystallite size, diffraction line profiles.
The integral breadth β is defined as the peak area divided by the maximum height of the peak. Is equivalent to the FWHM only for the symmetric Bragg peaks. In polycrystalline solids the lattice strains are not negligible and due to the method of synthesis, there are mechanical stresses induced in the plane of ribbons. Therefore, the Bragg peaks are rarely symmetric, especially in the case of nanocrystaline materials where the peaks are physically broadened. From this point of view it is a more accurate approach to use integral breadth instead of FWHM. Beyond the classic, so-called simplified, integral-breadth methods summarized in [58], the program calculates the rootmean-square strain (RMSS) and both surface- and volume-weighted domain sizes according to the ‘double-Voigt’ method [59,60], which is equivalent to the WarrenAverbach approach [61].
Experimental XRD Results
– Fe-rich batch
The structure of the samples was determined by performing XRD studies with the powder diffraction method in a Bragg-Brentano θ-2θ geometry using a Philips X’Pert diffractometer, at LPEC, Universite du Maine. The Cu Kα radiation wavelength of 1.54 Å was used for all powder diffraction experiments. The X-ray beam was incident to the ribbon plane. Figure 2.6 shows the X-ray spectra of 1a, 1b and 2a as-cast samples. It can be observed that the spectra of 1a and 2a samples exhibit sharp Bragg peaks, indicating the high degree of crystallinity in these samples. On the contrary, the as-cast 1b peaks exhibit large linewidths which indicate the lack of long range ordering, typical for low crystallinity as in nanocrystalline grains.
As-cast state characterization by Mössbauer spectroscopy
-Fe-rich batch
The Mössbauer spectrometry was done on the 1a, 2a and 1b samples, to complete the structural characterization of the as-cast state. We have performed 57Fe Mössbauer spectrometry in transmission geometry using a 57Co source in a Rh matrix. Figures 2.9, 2.10 and 2.11 present the Mössbauer spectra of the as-cast 1b, 1a and 2a samples, respectively, recorded at 300K and 77K.
The 300K and 77K spectra of 1b sample (Figure 2.9) exhibit broad magnetic sextets, typical of distributed Fe environments encountered in Fe-rich amorphous ribbons.
The shape of the spectra confirms the XRD results where an amorphous-like solid solution has been identified. Following these results, the spectra were fitted with a discrete hyperfine field distribution. The fitting procedure has been done using the Mosfit software [55]. The distribution contains 15 components with increasing hyperfine field values from 0 to 40 T. The average hyperfine fields obtained after fitting were 23.1 T at 300 K and 26.5 T at 77 K. These values are in agreement with other values of average hyperfine fields obtained on amorphous Fe-rich intermetallic ribbons [61]. The central lines of the broad, sextet, presents an asymmetry, more pronounced in the 77K spectrum.
This asymmetry has to be taken into account by involving a correlation between the 44 distribution of hyperfine field and the distribution of isomer shift. This correlation is explained by the different coordination of Fe sites, typical for a chemically disordered Fe environment, as in amorphous-like systems.
Introduction
Chapter 1: State of the art
Chapter 2: Synthesis and characterization of as cast FePtNbB ribbons
2.1 The Fe-Pt-Nb-B alloys
2.1.1 FePt binary phase diagram
2.1.2 Choice of chemical composition
2.2 Synthesis of the Fe-Pt-Nb-B ribbons
2.2.1 Choice of the synthesis method
2.2.2 The principle of method of the rapid solidification from the melt
2.2.3 Experimental procedure of synthesis
2.3 Structural characterization of as-cast ribbons
2.3.1 Experimental techniques of characterization
2.3.2. Elemental characterization of as-cast state by energy dispersive X-ray spectroscopy (EDX)
2.3.3. X-ray diffraction (XRD)
2.3.3.1. Fitting methods
2.3.3.2. Experimental XRD results
2.3.4. As-cast state characterization by Mössbauer spectrometry
2.3.5. Magnetic characterization of the as-cast state by Vibrating sample Magnetometry (VSM)
2.4 Thermal analysis
2.5 Formation and evolution of nanocrystalline magnetic phases in FePtNbB: in situ temperature dependent synchrotron XRD study
2.5.1 Characteristics of synchrotron radiation and schematics of synchrotron X-ray diffraction experiments
2.5.2 Temperature and pressure driven disorder-order phase evolution in FePtNbB
Chapter 3: Structural characterization of annealed Fe-Pt-Nb-B ribbons
3.1 Isothermal annealing and phase structure of Fe rich ribbons
3.1.1 Case of crystalline samples
3.1.2 Case of disordered sample
3.2 Isothermal annealing of Pt – rich alloys
3.2.1 Modifications in the chemical compositions of the FePtNbB alloys: structural effects
3.2.2 Choice of the annealing conditions
3.3. Phase structure of the annealed Pt – rich alloys
3.3.1 XRD studies
3.3.2 Transmission electron microscopy
3.3.3 Mössbauer spectrometry on annealed ribbons. Identification of the Phase structures
Chapter 4: Magnetic properties of the Fe51 Pt27 Nb2 BB20 and Fe 52 Pt28 Nb2 BB annealed ribbons.
Exchange spring effects and energy product for exchange coupled magnets.
4.1 Hysteresis loops for the Fe
B annealed ribbons
4.1.1 Fe51Pt27Nb2BBannealed ribbons 52 Pt28 Nb2 BB20
annealed ribbons
4.2 The exchange spring magnet criteria. Reversibility effects
Conclusions
Perspectives
References
Annex I
Annex II
Annex III
