Nanocrystalline Alloys - Features

“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”

Nanocrystalline alloys:I. Crystallization

M. Miglierini et al.

Department of Nuclear Physics and Technology Slovak University of Technology

Ilkovicova 3,812 19 Bratislava, SlovakiaE-mail: [email protected]://www.nuc.elf.stuba.sk/bruno


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 e 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.


• 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 Phys 64 (1988) 6044

• 1988: NANOPERM: FeMB(Cu) where M = Zr, Mo, Ti, Nb, Hf, …

• Suzuki K, Kataoka N, Inoue A, et al.Mater Trans JIM 31 (1990) 743

• 1998: HITPERM: FeCoZrB(Cu)• Willard M A, Laughlin D E, McHenry M E, et al.

J Appl Phys 84 (1998) 6773

Nanocrystalline Alloys - Features

ferritesFe-Co

HITPERM

Si steel

Co-am

Fe-am

nc-FINEMET

NANOPERM

A. Makino, A. Inoue and T. MasumotoMater Trans JIM 36 (1995) 924


Possible Applications of Nanocrystalline Alloys

magnetic shielding

transformersensors

ribbons

core


Preparation of Nanocrystalline Alloys

• 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 m 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

inductioncoil

melt

quenching wheel

melt-spunribbon

tube

planar flow

casting

amorphous ribbon


Structures from a Melt

Starting material(melt)

quasicrystallinecrystalline amorphous

nanocrystalline

Conditions (quenching rate, composition, …)

• Ordered structure– periodicity– long range order

• Disordered structure– short range order– no translation symmetry

annealing


• 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)

Characterization of Nanocrystalline Alloys

200 250 3000

20

40

60

80

100

120ta =440o C

as-quenched

ta = 250o C

ta = 350o C

spec

ific

mag

netiz

atio

n (A

m2 /

kg)

temperature (K)

Miglierini M et al. J Appl Phys 85 (1999) 1014

ED

XTEM

TEM

STM

35 40 45 50 55

2 (o)

XRD

400 500 600 700 800

heat

flow

(a.u

.)

temperature (°C)

DSC


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.


0 10 20 30

P(B

)

B (T)

0 1 2

P(

)

(mm/s)

Structural Arrangement and Mössbauer Spectra

crystalline(ordered structure)

hyperfine parameters

non-magnetic

magnetic

B

amorphous(disordered structure)

B

AM

AM CR

CR

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.


-5 0 5

0.95

1.00

rela

tive

tran

smis

sion

velocity (mm/s)0 10 20 30

P(H)

H (T)

-5 0 5

0.95

1.00

rela

tive

tran

smis

sion

velocity (mm/s)0 10 20 30

P(H)

H (T)

FINEMETFe73.5Nb3Cu1Si13.5B

9

NANOPERMFe80Mo7Cu1B12

Fe-SiFe-Si

bcc- Febcc- Fe

Mössbauer Spectra of Nanocrystalline Alloys (295 K)

Fe on A site Fe on D site Si on D site

Miglierini M J Phys Condens Matter 6 (1994) 1431

Miglierini M and Grenèche J-M J Phys Condens Matter 9 (1997) 2303


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

diffusion-like precrystallization effects

normal grain-growth-like formation of-Fe nanocrystallites in amorphous matrix

diffusion controlled grain-growth ofalready created -Fe nanocrystallites

diffusion controlled nucleation and growth-like precipitation of -Fe(Mo)

structural relaxation

RT 300 400 500 600 700

A B C D E FG H

I

J K

L

M

heat

pow

er

Tx1

temperature ( oC)

Tx1 460oCMiglierini M et al. phys stat sol (b) 243 (2006) 57

Fe76Mo8Cu1B15


750 oC

550 oC

TEM and XRD

470 oC

100 200 300 400 500 600 700

heat

pow

er

temperature (o C)

650 oC

Tx1 450oC

• Tx1 = 450 oC

450 oC

Miglierini M et al. phys stat sol (b) 243 (2006) 57

Fe76Mo8Cu1B15


Mössbauer Spectrometry

• 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

Fe76Mo8Cu1B15


300 K 77 K


Fitting Model

Fe80Mo7Cu1B12 440oC/1h

Miglierini M and Grenèche J-M J Phys Condens Matter 9 (1997) 2303, 2321Miglierini M and Grenèche J-M Hyperfine Interact 113 (1998) 375

crystalline

interface

amorphous

-5 0 5

0.95

1.00

rela

tive

tran

smis

sion

velocity (mm/s)

10nm

HREM

0 10 20 30 40

IF CRP(H

)

hyperfine field (T)

AM

295 K


100 200 300 400 500 600 700

heat

pow

er

temperature (o C)

Transmission Mössbauer Spectrometry (295 K)

600 oC

550 oC

510 oC

450 oC

• bulk• Tx1 = 450 oC (?)


410 oC

Fe76Mo8Cu1B15


600 oC

100 200 300 400 500 600 700

heat

pow

er

temperature (o C)

Conversion Electron Mössbauer Spectrometry (295 K)

550 oC

510 oC

450 oC

• surface• Tx1 = 450 oC

Miglierini M et al. Hyperfine Int 165 (2005) 75

410 oC

Fe76Mo8Cu1B15


40 45 50

(deg)

600 oC

XRD – Peak Decomposition

100 200 300 400 500 600 700

heat

pow

er

temperature (o C)

• Tx1 = 450 oC40 45 50

(deg)

550 oC

40 45 50

(deg)

510 oCMiglierini M et al. phys stat sol (b) 243 (2006) 57

40 45 50

(deg)

450 oC

40 45 50

(deg)

410 oC

Fe76Mo8Cu1B15


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 nanocrystals450 500 550 600

0

10

20

30

40

50

60

XRD TMS CEMS

A CR (%

)

temperature ( oC)


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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Nanocrystalline Alloys - Features