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Synthesis and orientation of barium hexaferrite ceramics by magnetic alignment
Journal of Magnetism North-Holland
and Magnetic
Materials
SYNTHESIS AND ORIENTATION BY MAGNETIC ALIGNMENT Denis
413
83 (1990) 413-415
OF BARIUM HEXAFERRITE
CERAMICS
AUTISSIER
Commissariar
tr I’Energie Atomique,
Cenrre de BruyPres-le-Charel,
B.P. 12, 91680 Bruy&es-le-Chatel,
Frunce
Particles of Ba,Mn,Zn,_,Fe,,0,2 with planar structure were prepared by chemical precipitation. They were processed by sleep casting in presence of a magnetic field. The degree of alignment was improved by a special sintering treatment. By this procedure an alignment as high ai 99.9% is obtained.
1. Introduction
A Relative
intensities
BaFeaO, Most of the common ferrites have a cubic crystal structure, but in 1957, Jonker et al. [l] discovered a new class of magnetic materials having a hexagonal crystal structure. They are in the ternary BaO-MeO-Fe,O, compositional diagram. The symbol Me represents a divalent metal ion like Co, Zn, Mn or others. These magnetic compounds, now known as hexagonal ferrites, exhibit magnetic properties at high frequencies. Several phases were identified, and we studied the following composition: Ba,Zn,_,Mn,Fe,,O,,. The x values has an important effect on the magnetic anisotropy, on the resonance frequency and on the initial permeability. Best permeability results were obtain with x = 0.4. This composition, with an easy plane of magnetization perpendicular to the c axis, must be completely aligned by means of a static magnetic field alternatively applied in two directions in the plane of easy magnetization. 2. Powder preparation Hexagonal ferrites are usually prepared by the ceramic method: stoichiometric amounts of basic components are ground, and fired at high temperature (1200-1300 o C) to obtain the desired phase. The powders are then ground in order to reduce the particle size. Haneda et al. used the coprecipitation method for the preparation of BaFe,,O,, [2] and Leccabue et al. for the preparation of Zn,W [3]. The first grinding and the very high temperature can be avoided by using a chemical coprecipitation method. It consists in dissolving a stoichiometric amount of starting components (FeCl,, MnCl,, ZnCO,, BaCO,) in hot water (80” C). Addition of this solution to a solution of a basic salt (Na,CO,) with a pH higher than 10 induces the precipitation.The coprecipitated powders were heated at different temperatures in air. The sam0304-8853/90/$03.50
(North-Holland)
0 Elsevier
Science Publishers
B.V.
Y BaCOa Fe&,
c z
S S,S’:spinels
? P .g :
0
*
500
Fig. 1. Phases present subjected to different
1000
T’C
in samples of coprecipitated heat treatment, determined diffraction.
powder, by X-ray
ples were characterized by X-ray diffraction, and the different phases are represented in fig. 1. The samples with the best characteristics were obtained at 1000 o C. In this case the presence of ZnFe,O,
Fig. 2. SEM observation
of hexaferrite loOo”c.
platelets,
calcinated
at
D. Autissier
414
/ Synthesis and orientutron of barrum hemferrite
Electra magnet ’
Matrix
I
elat ive ra lues
Ferrite I
Hz500
‘:, ?
0 =
Oe
J
Fig. 3. Apparatus used for crystallite orientation.
was reduced to less than 2%, and the saturation magnetization was higher than 30 emu/g. The morphology and the sizes of the particles were determined by SEM observations (fig. 2). T hey are platelet relatively well disagglomerated hexagonal crystals, 3-4 pm in sizes.
I
I 0
99.9
99.99
Degree 3. Orientation
and sintering
Fig. 5. Initial
The particles were ground for 24 h in water and cast in a plaster matrix. This matrix was rotated between the poles of a stationary electromagnet. The apparatus schedule is shown in fig. 3. Fields of approximately 500 Oe are used for the orientation procedure. After a 24 h drying, the sample is treated as indicated in fig. 4. The final orientation degree depends upon the time of sintering. It was determined by X-ray diffraction and we represent the dependence of the initial permeability versus degree of alignment in fig. 5.
if
alignmen?
permeability versus degree non-oriented sample).
of alignment
(I =
An orientation as high as 99.9% was obtained. The initial permeability depends also upon the density of the samples. as reported in fig. 6.
I
Tot
1
1170
4175
5.1-O’
Density Fig. 4. Sintering
treatment
of hexaferrites.
Fig. 6. Initial permeability
versus density
D. Autissier / Synthesis and orientation of barium hexaferrite
415
4. Conclusion
References
The improvement of the magnetic properties strongly depends upon the structural quality of the ceramic, either orientation and density. The orientation due only to magnetic alignment is above 70%, but the special sintering treatment allows a good recrystallization and improves the degree of alignment. The dimensions of the ceramic grains are larger than 500 pm.
[l] G.H. Jonker, H.P.J. Wijn and P.B. Braun, Phil. Tech. Rev. 18 (1956) 145. [2] K. Haneda, C. Miyakawa and H. Kojima, J. Am. Ceram. sot. 51 (1974) 354. [3] F. Leccabue, R. Panizzieri, G. Salviati, G. Albanese and J.L. Sanchez Llamazares, J. Appl. Phys. 59 (1986) 2114.
and Magnetic
Materials
SYNTHESIS AND ORIENTATION BY MAGNETIC ALIGNMENT Denis
413
83 (1990) 413-415
OF BARIUM HEXAFERRITE
CERAMICS
AUTISSIER
Commissariar
tr I’Energie Atomique,
Cenrre de BruyPres-le-Charel,
B.P. 12, 91680 Bruy&es-le-Chatel,
Frunce
Particles of Ba,Mn,Zn,_,Fe,,0,2 with planar structure were prepared by chemical precipitation. They were processed by sleep casting in presence of a magnetic field. The degree of alignment was improved by a special sintering treatment. By this procedure an alignment as high ai 99.9% is obtained.
1. Introduction
A Relative
intensities
BaFeaO, Most of the common ferrites have a cubic crystal structure, but in 1957, Jonker et al. [l] discovered a new class of magnetic materials having a hexagonal crystal structure. They are in the ternary BaO-MeO-Fe,O, compositional diagram. The symbol Me represents a divalent metal ion like Co, Zn, Mn or others. These magnetic compounds, now known as hexagonal ferrites, exhibit magnetic properties at high frequencies. Several phases were identified, and we studied the following composition: Ba,Zn,_,Mn,Fe,,O,,. The x values has an important effect on the magnetic anisotropy, on the resonance frequency and on the initial permeability. Best permeability results were obtain with x = 0.4. This composition, with an easy plane of magnetization perpendicular to the c axis, must be completely aligned by means of a static magnetic field alternatively applied in two directions in the plane of easy magnetization. 2. Powder preparation Hexagonal ferrites are usually prepared by the ceramic method: stoichiometric amounts of basic components are ground, and fired at high temperature (1200-1300 o C) to obtain the desired phase. The powders are then ground in order to reduce the particle size. Haneda et al. used the coprecipitation method for the preparation of BaFe,,O,, [2] and Leccabue et al. for the preparation of Zn,W [3]. The first grinding and the very high temperature can be avoided by using a chemical coprecipitation method. It consists in dissolving a stoichiometric amount of starting components (FeCl,, MnCl,, ZnCO,, BaCO,) in hot water (80” C). Addition of this solution to a solution of a basic salt (Na,CO,) with a pH higher than 10 induces the precipitation.The coprecipitated powders were heated at different temperatures in air. The sam0304-8853/90/$03.50
(North-Holland)
0 Elsevier
Science Publishers
B.V.
Y BaCOa Fe&,
c z
S S,S’:spinels
? P .g :
0
*
500
Fig. 1. Phases present subjected to different
1000
T’C
in samples of coprecipitated heat treatment, determined diffraction.
powder, by X-ray
ples were characterized by X-ray diffraction, and the different phases are represented in fig. 1. The samples with the best characteristics were obtained at 1000 o C. In this case the presence of ZnFe,O,
Fig. 2. SEM observation
of hexaferrite loOo”c.
platelets,
calcinated
at
D. Autissier
414
/ Synthesis and orientutron of barrum hemferrite
Electra magnet ’
Matrix
I
elat ive ra lues
Ferrite I
Hz500
‘:, ?
0 =
Oe
J
Fig. 3. Apparatus used for crystallite orientation.
was reduced to less than 2%, and the saturation magnetization was higher than 30 emu/g. The morphology and the sizes of the particles were determined by SEM observations (fig. 2). T hey are platelet relatively well disagglomerated hexagonal crystals, 3-4 pm in sizes.
I
I 0
99.9
99.99
Degree 3. Orientation
and sintering
Fig. 5. Initial
The particles were ground for 24 h in water and cast in a plaster matrix. This matrix was rotated between the poles of a stationary electromagnet. The apparatus schedule is shown in fig. 3. Fields of approximately 500 Oe are used for the orientation procedure. After a 24 h drying, the sample is treated as indicated in fig. 4. The final orientation degree depends upon the time of sintering. It was determined by X-ray diffraction and we represent the dependence of the initial permeability versus degree of alignment in fig. 5.
if
alignmen?
permeability versus degree non-oriented sample).
of alignment
(I =
An orientation as high as 99.9% was obtained. The initial permeability depends also upon the density of the samples. as reported in fig. 6.
I
Tot
1
1170
4175
5.1-O’
Density Fig. 4. Sintering
treatment
of hexaferrites.
Fig. 6. Initial permeability
versus density
D. Autissier / Synthesis and orientation of barium hexaferrite
415
4. Conclusion
References
The improvement of the magnetic properties strongly depends upon the structural quality of the ceramic, either orientation and density. The orientation due only to magnetic alignment is above 70%, but the special sintering treatment allows a good recrystallization and improves the degree of alignment. The dimensions of the ceramic grains are larger than 500 pm.
[l] G.H. Jonker, H.P.J. Wijn and P.B. Braun, Phil. Tech. Rev. 18 (1956) 145. [2] K. Haneda, C. Miyakawa and H. Kojima, J. Am. Ceram. sot. 51 (1974) 354. [3] F. Leccabue, R. Panizzieri, G. Salviati, G. Albanese and J.L. Sanchez Llamazares, J. Appl. Phys. 59 (1986) 2114.