12
19
S. Öztürk , D.D Çakıl, K. İcin
Karadeniz Teknik Üniversitesi
Metalurji ve Malzeme Mühendisliği

Presentation Plan
1- MOTIVATION
Why did we carry out this study?
2-MARKETING OF Sr-HEXAFERRITE
• Sr-hexaferrite marketing in World and Turkey,
• Celestite marketing of Turkey
3-APPLICATIONS
Over 1,200 Applications
4- PROPERTIES
• Crystal structures of hexaferrites
• Magnetic properties of Sr-hexaferrite
5-PRODUCTION METHOD
• Conventional ceramic method
• Wet chemical methods
6- EXPERIMANTAL
• Prepared of Sr-hexaferrite powders
• Structural, thermal and magnetic characterization of magnetic powder
2
7- RESULTS
• Morphological
• Structural
• Thermal
• Magnetic

Celestite
3
Mill scale
 Use of celestite ore for the magnet production
 Dissemination of selestit ore use in Turkey
 Reducing process temperature below 1100 °C
 Recycling of steel mill scale
 Increasing magnetic properties and decreasing production cost

4
➢
➢

5

6
Applications

7
Applications

Properties
This section describes
• crystal structure,
• molecular arrangement
• magnetic properties of different types of ferrite.
8
Switching of magnetic easy-axis using crystal orientation for large perpendicular coercivity in CoFe2O4 thin film

9
Properties
• In 1950’s Magnetite (Fe3O4 ) lodestone , the first magnetic material was known .
• Ferrites are iron containing complex oxides that crystallize in the form of a cubic structure composed of different transition metals ( d-block elements ; group 3 to 12).
• Each corner of a ferrite unit cell consists of a ferrite molecule
.

Properties
10
There are two types of ferrites
• Soft ferrites Low Coercivity means the material's magnetization can easily reverse direction without dissipating much energy ( hysteresis losses ), while high resistivity prevents eddy currents in the core.
• Hard ferrites High Coercivity means the materials are very resistant to becoming demagnetized, as in Permanent Magnet.
Due to high magnetic permeability , these are called Ceramic magnets .

11
Properties
According to their different crystal types, ferrites can be classified into four groups,
• spinel,
• garnet,
• magnetoplumbite or hexaferrite
• orthoferrite

Properties
• There is a kind of crystal structure with the same structure as the mineral spinel (MgAl2O4), which is called spinel structure.
• Analogous to the mineral spinel, the spinel has the general formula
MeFe2O4 where Me is the divalent metal ion like Mn2+, Ni2+, Cu2+,
Co2+, Fe2+ or more often, combinations of these .
• The spinel lattice is composed of a close-packed oxygen arrangement in which 32 oxygen ions form a unit cell that is the smallest repeating unit in the crystal network
• Between the layers of oxygen ions, there are interstices that may accommodate the metal ions .
12

13
Properties
The interstices are not all the same; some which we will call
A (tetrahedral) sites are surrounded by or coordinated with
4 nearest neighboring oxygen ions whose lines connecting their centers form a tetrahedron.

Properties
• B (octahedral) sites are coordinated by 6 nearest neighbor oxygen ions whose center connecting lines describe an octahedron.
• The unit cell contains eight formula units MeFe2O4.
In a full unit cell there are 64 - A sites, of which 8 are occupied by divalent metal ions, and
32 - B sites, of which 16 are occupied by trivalent metal ions .
• If in the process of preparation, the ratio of metal ions to oxygen ions is too small, some of the B sites will be unoccupied.
• These sites are then referred to as vacancies
14

Properties
•
The crystal structure is that of the garnet mineral, Mn3Al2Si3O12.
•
The magnetic garnets include Fe3+ instead of Al and Si, and a rare earth cation (R) substitutes Mn ,
• to give the general formula
R3Fe5O12 .
15

16
Properties
• The crystal structure has cubic symmetry and is relatively complex.
• In contrast with spinels, the oxygen sublattice is not a closepacked arrangement
• it is better described as a polyhedral combination.
• Three kinds of cation sites exist in this structure: dodecahedral (eight fold), octahedral (six fold), and tetrahedral (four fold) sites.

Properties
• Rare earth cations, R, occupy the largest, dodecahedral sites, while Fe3+ cations distribute among the tetra and octahedral places.
• There are
 16 octahedral,
 24 tetrahedral
 16 dodecahedral sites in a unit cell containing 8 formula units.
• One formula unit, 3M2O35Fe2O3 is distributed as follows:
 3M2O3-dodecahedral,
 3Fe2O3-tetrahedral,
 2Fe2O3-octahedral
17

18
Properties
The group of ferrites possessing hexagonal crystal structure is referred to hexagonal ferrites. Six types of hexagonal ferrites are distinguished and indicated as M, W,
Y, X, Z and U

19
Properties
•
They correspond to (MO + MeO)/Fe2O3 ratios of 1:6, 3:8, 4:6,
4:14, 5:12 and 6:18 respectively .
•
Where
M can be the ions Ba, Sr, Pb, Ca, L a, etc. whilst Me is a transition cation (Zn, Mg, Mn, Co, etc.) or a combination of cations as it would occur in spinels.

20
Properties
• The crystalline structure of the hexagonal ferrites is the result of a close packing of oxygen ion layers.
• The divalent and trivalent metallic cations are located in interstitial sites of the structure, while the heavy Ba or Sr ions enter substitutionaly the oxygen layers.

Properties
• All the known hexagonal ferrites have a crystalline structure which can be described as a superposition of three fundamental structural blocks namely
• S, R and T.
• The S*, R* and T* are the rotational symmetry of S, R, and T at 180 º around the hexagonal c-axis.
21

Properties
• The repeating unit ‘S’ has composition of either [Me 22+ Fe 43+ O 8 ] 0
(S 0 ) or [Fe 63+ O 8 ] 2+ (S 2+ ) with neutral or uncompensated charge of
+2 per subunit, respectively.
22

Properties
• The ‘R’ sub-unit has the composition [Me 2+ Fe 63+ O 11 ] 2 − whereas the
‘T’ unit is [Ba 22+ Fe 83+ O 14 ] 0 .
• The sub-unit ‘R’ combines with ‘S 2+ ’ to give the neutral block (RS), with the total composition MeFe
12
O
19
(M-phase) . Similarly, ‘T’ subunit combines with the S 0 to give the neutral block (TS), with the total composition Ba
2
Me
2
Fe
12
O
22
(Y-phase)
23

24
Properties

25
Properties
• M-type hexaferrite is a solid solution written in molecular form MeO·6Fe
2
O
3 or MeFe
12
O
19 and possesses the same structure as the natural mineral magnetoplumbite
• Where, Me can be the divalent ions Ba 2+ , Sr 2+ or
Pb 2+ .
• The magnetoplumbite structure can be built up from spinel blocks of two oxygen layers being blocks
S and S* which are connected by a block R containing barium or strontium ion .

26
Properties
• The M-type hexaferrite crystallizes in a hexagonal structure with 64 ions per unit cell on 11 different symmetry sites .
• The unit cell contains 38 oxygen ions,
24 ferric ions and 2 Me ions (Me =
Ba2+, Sr 2+ , Pb 2+ and La 3+ .

27
Properties
• The 24 ferric ions are distributed over five distinct sites i.e. 2a, 2b, 4f
1
, 4f
2 and
12k.
• Out of these five, 2a, 4f
2 and 12k are octahedral , 4f
1 is tetrahedral and the last in which the ferric ion is surrounded by five oxygen atoms forming a trigonal bipyramid (2b) site .

28
Properties
The 12 Fe 3+ are arranged as:
• 6 Fe 3+ are in 12k site having the spin up ,
• 2 ions in 4f
2 and 4f
1 having spin down
• 1 ion in 2a and 2b site having spin up .
 So the 8 Fe 3+ are in the upward direction and
4 in the downward direction.

29
Properties
 So 4 upward and downward cancel each other and the net moment is obtained of 4 Fe 3+ per formula units.
 According to the configuration of Fe 3+ , there are 5 unpaired electrons in the 3d orbital, each
Fe 3+ ion has the magnetic moment of 5 μB and the total moment is 20 μB per formula unit

30
Properties
 The molecular unit of W ferrite is composed of
• two S blocks and
• one R block , so it is similar to the M structure but not identical.
 There are now two S blocks above and below the R block , but again there is a mirror plane in the R block and the unit cell consists of two molecular W units to give SSRS*S*R*.

Properties
31
• The molecular unit of Y-type hexaferrite is one S and one T unit , with a total of six layers,
• the unit cell consists of three of these units.
• The T block does not have a mirror plane , and therefore a series of three T blocks is required to accommodate the overlap of hexagonal and cubic close packed layers
• With the relative positions of the barium atoms repeating every three T blocks .
• This gives the unit cell formula as simply
3(ST).

32
Properties
• The Z - unit is composed of Y + M ,
• therefore consists of ST + SR , with a mirror plane in the R block and a repeat distance of
11 oxygen layers .
• Therefore, two molecular units are required to form a single unit cell of Z - ferrite, one rotated 180
° around the c-axis relative to the other,
• to give STSRS*T*S*R*,

Properties
Magnetic moment depends on the electronic configuration and the distribution of the substituted ions at different sites in the crystal structure
33
• S block = 2↓ tetrahedral and 4↑ octahedral = 2↑
• R block = 1↑ trigonal bipyramidal and 3↑ 2↓ octahedral = 2↑
• T block = 2↓ tetrahedral and 4↑ 2↓ octahedral = 0

Properties
The resulting magnetization (M) at a temperature (T) of
BaFe12O19 per formula unit can be approximated by simple summation according to the formula
Where, σi stands for the magnetic moment of the i-Fe 3+ ion.
Assuming a magnetic moment of 5 μB per Fe 3+ ion at 0K ( μB is the Bohr magneton) the net magnetization is of 20
μB per formula unit of barium hexaferrite
34

Properties
Many variations seen in the magnetic properties of the hexagonal ferrites with
• Temperature
• Composition
But it also means that to calculate the magnetic moment of a compound the exact positions of all the cations must first be known. However, the contribution towards the moment of each site in a compound can still be summed up as
M = 1↑ trigonal bipyramidal + 7↑ 2↓ octahedral + 2↓ tetragonal = 4↑
W=1↑ trigonal bipyramidal + 11↑ 2↓ octahedral + 4↓ tetragonal = 6↑
X = 2↑ trigonal bipyramidal + 10↑ 4↓ octahedral + 2↓ tetragonal = 6↑
Y = 8↑ 2↓ octahedral + 4↓ tetrahedral = 2↑
Z = 1↑ trigonal bipyramidal + 15↑ 4↓ octahedral + 6↓ tetragonal = 6↑
U = 2↑ trigonal bipyramidal + 22↑ 6↓ octahedral + 8↓ tetragonal = 10↑
35

Properties
Structure of Hexagonal
Strontium Ferrite
• Hexagonal Strontium Ferrite has a magnetoplembite crystal structure with easy magneto crystalline anisotropy field.
Substitution for Fe and Sr is an effective method for tailoring the magnetic properties of these ferrites.
• The oxygen atoms are closed packed with the Sr and Fe ions in the interstitial sites.
•
There are ten layers of oxygen atoms along the C-axis .
• The structure is build up from smaller units:
 a cubic s, having the spinal- type structure and
 a hexagonal block R, containing the
Sr ions.
36

37
Properties
SrFe
12
O
19 powders with a narrow size distribution is a promising material for industrial applications due to;
• its good magnetic,
• electrical,
• mechanical and
• magneto-optical properties
• perfect thermal
• chemical stability.

38
Properties
Magnetic Properties of Hexagonal Strontium Ferrite
The magnetic properties of Sr-hexaferrite such as saturation magnetization (Ms), remanence (Mr) and coercivity (Hc) are determined from the hysteresis loop. The shape and width of the hysteresis loop of a ferrite not only depends upon the chemical composition but also on various factors such as
• porosity,
• size, and
• shape of the pores and
• crystals.
• Strontium hexaferrite is required to have low coercivity and moderate saturation magnetization and remanence for its use in magnetic recording media .

39
Properties
The coercivity of M-type hexaferrites depends on;
• the particle size,
• porosity and
• the magneto crystalline anisotropy.
The material having smaller particle size and large magneto crystalline anisotropy has high value of coercive force and vice versa.

40
It is known that the electrical, optical and magnetic properties of ferrites are very sensitive to the
• particle sizes,
• shape and
• degree of crystallinity.
At present, tremendous efforts have been made in improving their magnetic capabilities by using different synthesis methods . The main classification of production metdos is
• Conventional Ceramic method
• Wet chemical method.

41
Wet chemical method
Conventional ceramic method
Mechanical mixing of raw materials; difficult to get complete homogeneity.
Chemical of mixing of the raw materials results in a homogeneous mixture.
Single phase ferrite formation and small grain size can be easily obtained.
Possibility of some phase segregation cannot be ruled out
No impurity pick-up or material loss during processing.
There is a possibility of impurity pick-ups and loss of material during the grinding process.
Lower temperature processing, shorter sintering duration required and no specific heating and or cooling rate is required.
The raw materials are in the form nitrate, citrate, acetates and chlorides.
Processing requires a higher temperature and longer durations due to lower reactivity of the starting oxides, the heating and/or cooling rates control the particle size.
The raw materials are always in the form of oxides or carbonates

42
Most common chemical methods:
• hydrothermal processes
• chemical co-precipitation
• micro emulsion
• sol-gel
• pyrolisis of aerosol
• mechanochemical method
• sol-gel auto combustion
• microwave combustion method
• ionic coordination reaction technique
• solvothermal method
• sonochemical method
• glass-ceramic processing
• spray pyrolysis
• pulsed laser ablation
• cryochemical method
• colloidal synthesis
• reverse micelle technique
• sparkplasma sintering
• novel chitosan method

43
Experimantal Studies
First Stage
Pure strontium carbonate from celestite ore;

44
Experimantal Studies
Second Stage

45
Results (Mophological, SEM)
Fine particle
SrCO
3
20 min milled, 400 rpm
• Flue dust have large particle size
• SrCO
3 has small particle size
• Not observing agglomeration
Coarse particle
Flue dust

Results (Mophological, SEM)
400 rpm, 20 min
400 rpm,
16h
When the miling time increases;
• Decreasing particle size with 12 to 0,2 µm.
• Particle size become homogeneous
• Particle shape become homogeneous at max. milling time compared with min. milling time
• Increasing agglomeration
400 rpm,
32h
46
Important results to produce high performance magnet.

Results (Mophological, BET)
Specific surface area of milled particle spesiwas determined by BET (Brunauer –Emmett–Teller) analysis in Recep Tayyip Erdoğan University depending on the grinding time. (Degasing temperature is 150 °C)
0.5338 m 2 /g
8.7495 m 2 /g
47
400 rpm, 20 min
400 rpm,
32h

48
Results (Mophological, BET)
Milling time
(Hour)
0.3
1
2
4
8
16
32
Spe. Sur. Area
(m
2
/g)
0,5338
0,930
1,1022
1,130
4,0232
6,0805
8,7495

49
Results (Structure, XRD)
Selestit Ore
Flue Dust
(annealed 900
°C, 1h)
Magnetic mineral has been converted to hematite mineral with 100% at normal atmospheric conditions.

50
Results (DTA)
• DSC analysis was used to determine the calcination temperature.
• The mixtures prepared at a ratio of 1: 6 (SrCO
3
:Fe
2
O
3
) were heated to 1100 at atmospheric conditions to determine phase transitions.
Heating rate (25 °C)
Onset and offset point of the reaction decreased with incerasing milling time.
Onset point
Offset point
32 h milled
Min. onset point to form of SrFe12O19 is 392 °C.
The reaction is completed at min. offset point 728 °C.

51
Results (Structure, XRD)
Effect of chemical composition
X=5.5-6.5
20 min. milled
Calcination temperature and time are kept constant at
975
°C, 1h
α-Fe
2
O
3 increasing
Optimum chemical composition
α-Fe
2
O
3 increasing

52
Results (Magnetic, VSM)
Effect of chemical composition
Chemical composition of the prepared mixtures affected magnetic properties because the prpportion of the phases (SrFe
12
O
19
Fe
2
O
3
) in structure change the these properties. and
Chemical
Comp.
Hc,
Oe
Mr, emu/g
Ms, emu/g
1:5.5
1189 17.3
26.1
1:6
1:6.5
3586 17.8
790 14.2
26.9
21.0

53
Results (Structure, XRD)
Effect of calcination temperature
The proportion of the SrFe
12
O
19 hard magnetic phase is increased with increasing calcination temperature. On the contrary, Fe
2
O
3 phases in the structure are decreased with increasing temperature.
60 min milled
Calcination time and chemical composition is kept constant, 1h and 1:6 respectively.
α-Fe
2
O
3 decreasing

54
Results (Magnetic, VSM)
Effect of calcination temperature
Calcination temperature affected magnetic properties due to the proportion of the phases (SrFe the these properties.
12
O
19 and Fe
2
O
3
) in structure change
Calc. Temp.
Hc,
Oe
950 4180
975
1000
4180
1788
Mr, emu/g
18
22
25
Ms, emu/g
29.5
35.7
39.3
Mr and Ms values of different temperature calcinated are increased with increasing temperature. But coercivities of the powder are decreased with increasing temperature.

55
Results (Structure, XRD)
Effect of milling time The proportion of the
SrFe
12
O
19 hard magnetic phase is decreased with increasing milling time up to 16 h. On the contrary,
Fe
2
O
3 phases in the structure are decreased with increasing milling time.
Calcination temperature, time and chemical composition is kept constant, 975,
1h and 1:6 respectively.
α-Fe
2
O
3 decreasing

56
Results (Magnetic, VSM)
Effect of milling time
Effect of particle shape and size on the magetic properties
Milling time
20 min
1 h
2 h
4 h
8 h
Hc,
Oe
3747
3666
3427
4705
4862
Mr, emu/g
11
14
16
15
17
Ms, emu/g
19
22
27
24
28
16 h 5501 28 47
32 h 5422 30 49
Mr and Ms values are continuously increased with increasing milling time. But coercivities of the powder are not changed with milling time up to 2 h, after incerased with milling time.

57
Results (Structure, XRD)
Effect of washing with HCl, 24 h
Calcination temperature, time and chemical composition is kept constant, 975,
1h and 1:6 respectively.
NOT OBSERVING SIGNIFICANT
CHANGE.
Effect of particle shape and size on the magetic properties

58
Results (Magnetic, VSM)
Effect of washing with HCl, 24 h
Hc,
Oe
16 h milled 5501 washed 5580
Mr, emu/g
28
26
Ms, emu/g
47
44
The acid washing of the calcined powders increases the coercivity and causes the other magnetic properties to decrease. Because HCl acid dissolves iron oxide minerals.

12
19
S. Öztürk 1 , D.D Çakıl 1 , K. İcin 1 , R. Sezer 1,2 , U. Topal 3 , B. Öztürk 1
Karadeniz Teknik Üniversitesi
Metalurji ve Malzeme Mühendisliği