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Nanocrystalline Alloys - Features

Nanocrystalline alloys: I. Crystallization M. Miglierini et al. Department of Nuclear Physics and Technology Slovak University of Technology Ilkovicova 3,812 19 Bratislava, Slovakia E-mail: marcel.miglierini@stuba.sk http://www.nuc.elf.stuba.sk/bruno.

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Nanocrystalline Alloys - Features

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  1. Nanocrystalline alloys:I. CrystallizationM. Miglierini et al.Department of Nuclear Physics and Technology Slovak University of TechnologyIlkovicova 3,812 19 Bratislava, SlovakiaE-mail: marcel.miglierini@stuba.skhttp://www.nuc.elf.stuba.sk/bruno

  2. Nanocrystalline alloys prepared by controlled annealing from rapidly quenched amorphous ribbons exhibit an interesting class of materials from the point of view of their magnetic properties [1]. Resulting magnetic parameters, which are superior to those of conventional transformer steels and/or amorphous materials, are ensured by a presence of crystalline grains several nanometres in size embedded in the amorphous residual phase [2]. Magnetic parameters of amorphous alloys are frequently deteriorated in the process of their practical employment by elevated temperature especially during prolonged operational times. On the other hand, nanocrystalline alloys are in fact already partially crystallized and from this point of view their structure is more resistant to such external effects and that is why it is more stable. Nevertheless, because the excellent magnetic behaviour of nanocrystalline alloys depends strongly on the amount and size of the crystalline grains, the process of crystallization should be known. [1] K. Suzuki, A. Makino, A. Inoue, T. Masumoto, J. Appl. Phys. 70 (1991) 6232. [2] G. Herzer, Phys. Scr. T49 (1993) 307. The following slide shows a comparison of some magnetic parameters (magnetic permitivity me versus saturation magnetization Bs) for different types of magnetic materials used for, e.g. the production of cores of magnetic circuits. The main three types of compositions which yield nanocrystalline alloys are also listed.

  3. Nanocrystalline Alloys - Features nc-FINEMET NANOPERM Co-am HITPERM Fe-am Fe-Co ferrites Si steel • nanocrystalline alloys • good soft magnetic properties • thermal stabilization of the structure as compared to amorphous alloys • 1988: FINEMET: FeCuNbSiB • Yoshizawa Y, Oguma A, Yamauchi K J Appl Phys64 (1988) 6044 • 1988: NANOPERM: FeMB(Cu) where M = Zr, Mo, Ti, Nb, Hf, … • Suzuki K, Kataoka N, Inoue A, et al.Mater Trans JIM31 (1990) 743 • 1998: HITPERM: FeCoZrB(Cu) • Willard M A, Laughlin D E, McHenry M E, et al.J Appl Phys84 (1998) 6773 A. Makino, A. Inoue and T. Masumoto Mater Trans JIM 36 (1995) 924

  4. Possible Applications of Nanocrystalline Alloys core ribbons magnetic shielding transformer sensors

  5. Preparation of Nanocrystalline Alloys tube melt induction coil melt-spunribbon quenching wheel • production of an amorphous precursor • mixing of appropriate amounts of pure elements with subsequent melting • rapid quenching of the melt ( ~106 K/min)  method of planar flow casting • result: ribbon up to several cm wide and typically about 20 mm thick • check of composition (OES ICP) and amorphicity (XRD) • (nano)crystallization • check of crystallization behaviour by DSC (onset of crystallization, first crystallization peak) • choice of temperature of annealing • annealing (in vacuum) for typically 1 hour at the selected temperature • characterization of the resulting structural and magnetic properties planar flow casting amorphous ribbon

  6. Ordered structure periodicity long range order Disordered structure short range order no translation symmetry Structures from a Melt annealing Starting material (melt) Conditions (quenching rate, composition, …) crystalline quasicrystalline amorphous nanocrystalline

  7. Characterization of Nanocrystalline Alloys XRD DSC TEM XTEM STM ED • structural characterization • DSC (differential scanning calorimetry) • evolution of structure with temperature • XRD (X-ray diffraction) • crystalline phases, relative fraction of crystallites and amorphous rest • TEM (transmission electron microscopy) • including HREM (high resolution TEM) and XTEM (cross-sectional TEM) • type and size of (nano)crystals • STM (scanning tunnelling microscopy) • including AFM (atom force microscopy) • surface features • ED (electron diffraction) • structural ordering of phases • magnetic properties • magnetic measurements • 57Fe Mössbauer spectroscopy (TMS + CEMS) • simultaneous information on both structural arrangement and magnetic behaviour (hyperfine interactions) Miglierini M et al. J Appl Phys85 (1999) 1014

  8. Mössbauer spectrometry is a very sensitive tool for the study of both structural arrangement and hyperfine interactions (magnetic ordering) in nanocrystalline alloys [3]. Tthe FINEMET-type alloys, which are very frequently studied because their macroscopic properties are beneficial for practical applications [4] exhibit rather complicated Mössbauer spectra. They consist of several sextets of narrow lines ascribed to different crystallographic positions in the Fe-Si lattice which are superimposed upon a broadened signal which belongs to the amorphous rest of the original precursor [5]. Evaluation of such spectra is pretty complicated and, unfortunately, prevents from acquiring more detail information related to such phenomena as for example interfacial regions [6]. In order to benefit from its diagnostic potential, it is useful to investigate such materials whose Mössbauer spectra are reasonably simple. This is the situation for example in NANOPERM-type alloys which crystallize into bcc-Fe, the latter being a calibration material for Mössbauer spectrometry. Thus, here we concentrate on the Fe-Mo-Cu-B system which belongs to the NANOPERM family. [3] H. Bremers, O. Hupe, C. E. Hofmeister, O. Michele and J. Hesse: J. Phys.: Condens. Matter 17 (2005) 3197. [4] T. Liu, Z. X. Xu and R. Z. Ma, J. Magn. Magn. Mat. 152 (1996) 365. [5] T. Pradell, N. Clavaguera, J. Zhu and M. T. Clavaguera-Mora: J. Phys.: Condens. Matter 7 (1995) 4129. [6] J. M. Grenèche and A. Slawska-Waniewska, J. Magn. Magn. Mat. 215-216 (2000) 264.

  9. Structural Arrangement and Mössbauer Spectra AM CR non-magnetic D D AM CR magnetic B B Mössbauer spectra of an ordered structure (crystallites) exhibit narrow lines which lead to single values of the spectral parameters. Due to non-unique positions of resonant atoms in a disordered structure the spectral lines are broad and, consequently, distributions P() and P(B) of the spectral parameters must be considered. amorphous (disordered structure) crystalline (ordered structure) hyperfine parameters

  10. Mössbauer Spectra of Nanocrystalline Alloys (295 K) Fe-Si Fe on A site Fe on D site Si on D site bcc- Fe FINEMET Fe73.5Nb3Cu1Si13.5B9 Miglierini M J Phys Condens Matter6 (1994) 1431 NANOPERM Fe80Mo7Cu1B12 Miglierini M and Grenèche J-M J Phys Condens Matter9 (1997) 2303

  11. Annealing of the Amorphous Precursor • DSC  continuous heating (temperature ramp of 10 K/min) • choice of annealing temperatures (B-M, A = as-quenched) => sample preparation • onset of crystallization identified at Tx1 Tx1 460oC Fe76Mo8Cu1B15 structural relaxation Miglierini M et al. phys stat sol (b)243 (2006) 57 diffusion-like precrystallization effects normal grain-growth-like formation ofa-Fe nanocrystallites in amorphous matrix diffusion controlled grain-growth ofalready created a-Fe nanocrystallites diffusion controlled nucleation and growth-like precipitation of g-Fe(Mo)

  12. TEM and XRD 550 oC 650 oC 470 oC 750 oC 450 oC Fe76Mo8Cu1B15 • Tx1= 450 oC Miglierini M et al. phys stat sol (b)243 (2006) 57 Tx1 450oC

  13. evolution of Mössbauer spectra with temperature of annealing ta transmission Mössbauer spectra are plotted upside-down to enable 3D mapping temperature of measurement 300 K and 77 K 300 K Mössbauer Spectrometry Fe76Mo8Cu1B15 77 K Miglierini M et al. phys stat sol (b)243 (2006) 57

  14. Fitting Model HREM 10nm Miglierini M and Grenèche J-M J Phys Condens Matter9 (1997) 2303, 2321 Miglierini M and Grenèche J-M Hyperfine Interact113 (1998) 375 Fe80Mo7Cu1B12440oC/1h crystalline interface amorphous 295 K

  15. Transmission Mössbauer Spectrometry (295 K) 510 oC 550 oC 600 oC 450 oC 410 oC Miglierini M et al. phys stat sol (b)243 (2006) 57 • bulk • Tx1= 450 oC (?) Fe76Mo8Cu1B15

  16. Conversion Electron Mössbauer Spectrometry (295 K) 550 oC 510 oC 600 oC 450 oC 410 oC Miglierini M et al. Hyperfine Int165 (2005) 75 • surface • Tx1= 450 oC Fe76Mo8Cu1B15

  17. XRD – Peak Decomposition 550 oC 510 oC 600 oC 450 oC 410 oC Miglierini M et al. phys stat sol (b)243 (2006) 57 • Tx1= 450 oC Fe76Mo8Cu1B15

  18. Summary • structure of nanocrystalline alloys • (nano)crystallites • residual amorphous matrix • interface = surface of crystalline grains + crystal-to-amorphous matrix region • crystallization • first at the surface • progress of crystallization is more rapid at the surface • identification of crystallinephase • amount of nanocrystals

  19. Mössbauer spectroscopy contributes to the study of nanocrystalline alloys from several viewpoints. First, it is possible to identify the structural arrangement from a very first look at a Mössbauer spectrum (e.g., onset and progress of crystallization). Crystalline phases are characterized by narrow and usually well separated lines whereas the amorphous residual phase exhibits broad patterns due to its disordered nature. Signal from resonant atoms located at the interfacial regions can be also distinguished. The latter two contributions are described by the help of distributions of hyperfine parameters through which information on both topological and chemical short-range order can be derived. The fraction (and/or type) of the crystalline phase(s) can be readily obtained from the spectral parameters. Second, magnetic order of the system under the study is also directly followed from changes of the spectral line shapes, viz. (broadened) doublet vs. sextet. This can be studied as a function of annealing temperature (i.e., crystalline contents), measuring temperature, and/or composition. More details can be found in another presentation. In this presentation, we have shown that the crystallization of amorphous precursors for the preparation of nanocrystalline alloys proceeds more rapidly on the surface of the rapidly quenched ribbons than in their bulk. In doing so, we have employed CEMS and TMS, respectively. The crystalline content was determined also from XRD and the results coincide well with those from TMS. The temperature of the onset of crystallization Tx1 determined from DSC is somewhat higher than that from XRD, TEM and MS due to different regime of annealing (continuous during DSC and isothermal during the preparation of the samples).

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