Σύνθεση και μελέτη δομικών και μαγνητικών ιδιοτήτων νέων διαμεταλλικών ενώσεων σπάνιας γαίας - μεταβατικών μετάλλων για εφαρμογές μόνιμων μαγνητών σε υψηλές θερμοκρασίες

Abstract

The Sm2Co17-based magnets are known for their large maximum energy product (BH)max and high temperature operation. A series of recent studies has been focused on the composition adjustment of Sm(Co,Fe,Cu,Zr)7.5-8.5 magnets to optimize them for high temperature applications. In these magnets the desired magnetic properties are obtained after a long and complicated heat treatment which is required to develop the proper microstructure with the well known cellular/lamellar feature. The microstructure consists of Th2Zn17-type structure (S.G. R-3m) cells rich in Fe, cell boundaries of CaCu5-type structure (S. G. P6/mmm) rich in Cu and the Zr-rich "Zphase" lamellae, superimposed on the cellular structure perpendicularly to the hexagonal c-axis. The coercivity is generally attributed to a pinning type mechanism: due to the difference in the magnetocrystalline anisotropy and the domain wall energy of the 2:17 and 1:5 phases, the domain walls are pinned at the 1:5 phase cell boundaries. In this thesis there have been prepared and studied new magnetic materials, based on Sm-Co compositions with boron, which are characterized by very high coercive field at room and higher temperatures without the need of a long heat treatment. New compounds have been produced mainly as melt spun nanostructured ribbons, by using the melt-spinning technique, and as bulks, by using the arc-melting technique. A bulk precipitation hardened sample has been also studied. Crystal structure analysis revealed new aspects about the structure of Z-phase. On the same sample a viscosity analysis has been performed at room temperature and the domain wall width has been calculated. In detail, nanostructured/nanocomposite melt spun ribbons with Sm(CobalFe0.1Zr0.04Bx)7.5, με x=0.005, 0.01, 0.015 and Sm(CobalFe0.1Cu0.12Zr0.04Bx)7.5, με x=0.005, 0.01, 0.015, 0.025, 0.03, 0.04, 0.05 stoichiometries have been prepared by the melt-spinning technique in the range of wheel velocity 5 to 70 m/sec. The bulk samples have been produced by arc-melting of Sm, Co, Fe, Cu, Zr and Fe3B metals. The nanostructured magnetic melt spun ribbons with boron, are characterized by very high coercive fields while their microstructure is completely different than the well known cellular/lamellar microstructure. The coercive field is associated with the ultrafine microstructure and strong exchange coupling between the “hard-hard” and “hard-soft” nanograins. Structural and magnetic properties have been studied on melt spun ribbons as a function of cooling rate, stoichiometry and annealing conditions. Rietveld analysis revealed that the main phase of as spun ribbons is 1:7 while after annealing above 750 οC transforms to 2:17 and 1:5. In most of the cases a small amount of fcc-Co has been found mostly on the surface of the ribbons. As spun ribbons have very high coercive field iHc with the greatest value to be 38 kOe at room temperature while the reduced remanence mr is over 0.7. The annealed ribbons Sm(CobalFe0.1Cu0.12Zr0.04B0.04)7.5 at 39 m/sec have coercive field of about 5 kOe at 400 οC. The temperature variation does not deteriorate the coercive field. The ribbons Sm(Co0.69Fe0.1Cu0.12Zr0.04B0.05)7.5 at 40 m/sec after annealing at 850 οC for 10 min have high coercive field (16 kOe) and maximum energy product of 7.9 MGOe at room temperature. The as spun ribbons Sm(Co0.725Fe0.1Cu0.12Zr0.04B0.015)7.5 at 70 m/sec at 327 οC has (ΒΗ)max = 4.4 MGOe at reversed field of 1.4 kOe while Br is 5.6 kG. After demagnetization factor correction, by using Νd~0.555, the (ΒΗ)max is ~9.1 MGOe at 2.4 kOe, which is very high. Magnetic induction Br, at the same temperature, is 6.8 kG. At 377 οC, the (ΒΗ)max is 3.2 MGOe at 1.4 kOe and Br is 5.1 kG. After demagnetization factor correction, Νd~0.555, (ΒΗ)max is ~8.8 MGOe at 2.4 kOe while Br is 7.4 kG. These unique values are for the first time observed on not annealed or short time annealed nanostructured ribbons or powders. The precipitation hardened bulk sample with boron Sm(CobalFe0.1Cu0.12Zr0.04)7.5 has the well known celluler/lamellar microstructure with cell size of about 70 nm and lamella density of 0.06 nm-1. By analyzing the x-ray diffraction patterns by using the Rietveld method, the thermomagnetic curves and the selected area diffraction by using TEM, the 1:5, 2:17R and 6:23 phases were found Σ. Σ. Μακρίδης – Διδακτορική διατριβή vi to be the characteristic crystal structures. The 6:23 structure has been proposed for the first time to be the structure type of the lamellar phase. Hysteresis loops have been traced in the range of 4.2 K to 850 K. The magnetization fluctuations have also been studied as a function of time at room temperature under constant reversed field while recoil loops have been traced at the same fields. As a result, magnetic viscosity constants, fluctuation fields and thermal activation volumes have been calculated on the reversed field values. On the coercive field, the activation volume is V*=4?10-19 cm3 at 300 Κ. The domain wall width at the coercive field is close to 8 nm. As the temperature decreases from 250 Κ (Ηc~20 kOe) to 4.2 Κ (Ηc~50 kOe), the demagnetization curve has a characteristic small step due to the weaker exchange coupling between the phases. The coercive field has normal temperature dependence with Hc=8 kOe at 700 K. The magnetic viscosity study on the isotropic sample in parallel with the Rietveld analysis on the diffraction pattern is an original investigation. For the first time phase percentages of 1:5, 2:27 and 6:23 have been calculated while 6:23 has been identified as the structure of lamellar phase.

Οι μαγνήτες Sm2Co17 είναι γνωστοί για το μεγάλο ενεργειακό τους γινόμενο και τις υψηλές θερμοκρασίες λειτουργίας τους. Μία σειρά πρόσφατων μελετών έχει εστιαστεί στην ρύθμιση της στοιχειομετρίας Sm(Co,Fe,Cu,Zr)z για την βελτιστοποίηση των ιδιοτήτων των μαγνητικών υλικών ώστε να χρησιμοποιηθούν σε εφαρμογές υψηλών θερμοκρασιών. Σε αυτού του είδους τα υλικά οι απαιτούμενες μαγνητικές ιδιότητες επιτυγχάνονται μετά από μακρά και πολύπλοκη θερμική διεργασία που απαιτείται για να αναπτυχθεί η κατάλληλη μικροδομή, την γνωστή κυψελωτή/φολιδωτή (cellular/lamellar). Η μικροδομή αυτή αποτελείται από κελιά πλούσια σε Fe, με δομή τύπου Τh2Zn17 (Ο.Σ.Χ.: R-3m) στα όρια των κελιών η δομή είναι τύπου CaCu5 (Ο.Σ.Χ.: P6/mmm), πλούσια σε Cu ενώ κάθετα στον εξαγωνικό άξονα c αναπτύσσονται οι φολίδες της "Z- φάσης", πλούσιες σε Zr. Το υψηλό συνεκτικό πεδίο οφείλεται στο μηχανισμό του αγκυρώματος των τοιχωμάτων των μαγνητικών περιοχών στα όρια των κελιών της φάσης 1:5, λόγω της διαφοράς των ενεργειών των τοιχωμάτων των μαγνητικών περιοχών των φάσεων 2:17 και 1:5. Με την εργασία αυτή συντέθηκαν και μελετήθηκαν νέα μαγνητικά υλικά, βασισμένα σε ενώσεις Sm-Co με βόριο, τα οποία εμφανίζουν υψηλό συνεκτικό πεδίο σε κανονικές και υψηλές θερμοκρασίες, χωρίς να απαιτείται η κλασική διεργασία μαγνητικής σκλήρυνσης. Τα νέα υλικά παρασκευάστηκαν κυρίως ως νανοσύνθετες-νανοκρυσταλλικές ταινίες (ribbons), με την τεχνική απότομης ψύξης τήγματος (melt-spinning), είτε ως συμπαγή (bulk). Παράλληλα, μελετήθηκε συμπαγές δείγμα που υποβλήθηκε στην κλασική μαγνητική σκλήρυνση. Από την ανάλυση της δομής προέκυψαν νέα στοιχεία για τη δομή της Ζ-φάσης. Στο δείγμα αυτό επίσης, πραγματοποιήθηκε ανάλυση της μεταβολής της μαγνήτισης με τον χρόνο σε σταθερό πεδίο, μέσω της οποίας υπολογίστηκε στη θερμοκρασία δωματίου το μέσο πάχος των τοιχωμάτων των μαγνητικών περιοχών.