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General Sample preparation Crystal structure and microstructure

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Rare-earth-iron nanocrystalline magnets E.Burzo 1) , C.Djega 2) 1) Faculty of Physics, Babes-Bolyai University, Cluj-Napoca 2) Universite Paris XII, France. General Sample preparation Crystal structure and microstructure Magnetic properties of R 2 Fe 17 compounds

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slide1

Rare-earth-iron nanocrystalline magnetsE.Burzo1), C.Djega2)1)Faculty of Physics, Babes-Bolyai University, Cluj-Napoca2) Universite Paris XII, France

General

Sample preparation

Crystal structure and microstructure

Magnetic properties of R2Fe17 compounds

Magnetic properties of Sm-Fe-Si-C alloys

Mössbauer effects

Technical applications

slide2

Permanent magnets:

  • cobalt based: SmCo5, Sm2Co17
  • - high Curie temperatures
  • - good energy product
  • SmCo5 (BH)max 200 kJ/m3
  • Sm2Co17  240 kJ/m3
  • - very expensive
  • iron based: Nd-Fe-B
  • - low Curie points, TC
  • - high energy product at RT
  • (BH)max  360 kJ/m3
  • - high decrease of Energy Product with T
  • T<100 oC
  • - low cost
slide3

Directions of researches: nanocrystalline systems

  • iron based with rare-earths
  • iron based without rare-earth
  • spring magnets RCo5/α-Fe
slide4

Iron based magnets with rare-earth

  • R-Fe system
  • RFe2, RFe3, R6Fe23, R2Fe17
  • no RFe5 phases are formed
  • R2Fe17
  • Low Curie temperatures, Tc < 477 K for Gd2F17
  • High magnetization MFe2.1-2.2 B/atom
  • Planar anisotropy
  • Rombohedral structure: space group
  • Sm:6c, Fe:6c, 9d, 18f, 18h
  • different local environments
  • Increase Tcvalues by:
  • replacement of iron involved in negative exchange inteactions
  • increase volume: interstitial atoms (C,N)
  • Uniaxial anisotropy
slide6

4f-5d-3d Exchange interactions

Band structure calculations

M5d(0)=a MFe

slide7

Preparation. Crystal structure

High energy ball milling and annealing

Sm2Fe17-xSix; Sm2Fe17-xSixC for Ta>850 oC

Metastable Sm1-s(Fe,Si)5+2s P6/mmm type structure

s = 0.22 TbCu7 Ta= 650 oC-850 oC

s = 0.33 Sm2Fe17

s = 0.36-0.38 SmFe9 (new)

Carbonation: mixture of alloys and C14H10 powders 420 oC in vacuum

Grain sizes:

SmFe9-ySiy: 22-28 nm

SmFe9-ySiyC: 18-22 nm

Rietveld analysis

C 3f sites (1/2,0,0); (0,1/2,0); (1/2,1/2,0)

Sm at (0,0,0) is occupied by 0.64-0.62 atoms

Si 3g sites

slide8

R1-sM5+2s

P6/mmm

slide9

Electron microscopy:

distribution of elements

particle dimensions

SmFe8.75Si0.25

slide11

y=2

y=0

y=2

y=0

z=1

z=0

z=1

z=0

slide13

Curie temperature: effect of Si

  • Tc increases by Si substitution in noncarbonated samples
  • Tc decreases in carbonated sample
  • Volume effects:

Localized moment of iron moments

16.4 1:9

22.9 2:17

Molecular field approximation

25 1:9

30 2:17

Fe mainly localized magnetic behaviour

slide14

Intrinsic magnetic properties

  • Magnetic measurements: H ≤ 9T T≥4.2 K
  • initial magnetization curves
  • - inflection typical for pinning effects coherent precipitates with matrix
  • impede the motion of domain walls
  • Band structure calculations: LMTO-LDA method
  • MSm=-0.66 B/atom
  • MFe(6c) > MFe(18h)  MFe(18f) > MFe(9d)
  • Mean magnetic moment of Fe in field of 9T
  • increases with Si content
  • Noncarbonated:1.50 B (y=0.25); 1.75 (y=1.0)
  • Carbonated:1.88B (y=0.25); 1.97 (y=1.0)
  • Asymmetric filling of Fe 3d band by Si3p electrons
slide15

Mössbauer effect studies

  • SmFe9-xSixC P6/mmm type structure
  • Analysis of spectra
  • local environment
  • relationship between isomer shift, δ, and WSC volumes
  • x = 1.0 WSC volume in (Å3)
  • Sm1a (33.96); Fe2e (19.87); Fe3g (13.45); Fe6l (13.39)
  • Larger isomer shifts=larger WSC volumes
  • Statistical occupation of Si in the 3g site and random distribution in the 2e dumbbell atoms have been simulated by using an appropriate P6/mmm subgroup, P1 with a’=3a, b’=3a, c’=2c
  • 2e0 3g0 6l0
  • Six sextets:2e ; 3g ; 6l
  • 2e1 3g1 6l1
slide17

Mean 57Fe hyperfine fields decrease with Si content

Hhf2e > Hef6l > Hhf3g

correlate with number of NN Fe atoms

Mean isomer shifts:δ2e>δ3g>δ6l

δ2e, δ6l increase with Si substitution

δ3g remains nearly constant

preferred occupation of Si sites

(3g)

slide19

Technical parameters

  • Uniaxial anisotropy is induced in 1:9 phase
          • 2:17 phase
  • Coercive fields SmFe9-xSixC
  • X = 0.25 Hc=1.2 MA/m Ta=750 oC
  • x = 0.50 Hc = 1.04MA/m Ta=800 oC
  • The maximum in Hc values:
  • Too low Ta hinders the complete solid-state reaction for forming a perfect metastable phase responsible for magnetic hardening
  • Increasing Tc
  • - number of surface defects of hexagonal P6/mmm phase is reduced Hc
  • - the domain size increases Hc
slide20

2. Curie temperature increases

Tc  700 K for SmFe8.75Si0.25C

3. Induction resonance

Br  0.68 Bs

High energy product is expected with a smaller temperature coefficient than Nd-Fe-B alloys