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General Sample preparation Crystal structure and microstructurePowerPoint Presentation

General Sample preparation Crystal structure and microstructure

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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

- 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

- Directions of researches: nanocrystalline systems
- iron based with rare-earths
- iron based without rare-earth
- spring magnets RCo5/α-Fe

- 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 MFe2.1-2.2 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

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

R1-sM5+2s

P6/mmm

- 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

- 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:1.88B (y=0.25); 1.97 (y=1.0)
- Asymmetric filling of Fe 3d band by Si3p electrons

- 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

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)

- 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

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

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