Upload
basharat-ahmad
View
215
Download
0
Embed Size (px)
Citation preview
7/31/2019 Co-Ti Substituted BaHex
1/7
7/31/2019 Co-Ti Substituted BaHex
2/7
the hexaferrite family for high-frequency applica-
tion [9,10]; however, few investigations have been
made on M-type hexaferrite, especially those
sintered at temperatures below the melting pointof silver.
The M-type barium ferrite BaFe12O19 has strong
uniaxial magnetocrystalline anisotropy and is
usually used as permanent magnetic materials.
However, the strong uniaxial magnetocrystalline
anisotropy can be reduced by substitution of Fe3+
cations which distribute on five distinct crystal-
lographic sites. Different cations have been used to
substitute the Fe3+ to make the barium ferrite
suitable for magnetic recording and microwave
applications [1113]. The typical example is the
substitution of Fe3+ by Co2+ and Ti4+, which
undergoes a magnetic anisotropy change from
uniaxial to planar when the CoTi substitution
amount is 0.91.20 [14]. Complex permeability of
CoTi substituted barium ferrites have been
investigated by Autissier et al. using classical
ceramics method with sintering temperature
123012901C [15]. In this paper, the microstruc-
tures and high-frequency magnetic properties of
CoTi substituted barium ferrites prepared by
chemical coprecipitation and low-temperature
sintering with Bi2O3 doping are presented.
2. Experimental procedure
BaFe122xCoxTixO19 particles with different
target compositions were prepared by a modified
flux method which combines the chemical copre-
cipitation process and the synthesis from salt melts
[11,16]. An aqueous solution of the metal chlorides
containing Ba2+, Fe3+, Co2+ and Ti4+ in the
ratio required for the ferrite was stirred into anexcess of an aqueous solution of NaOH and
Na2CO3. All the reagents were of analytical grade.
A suspension containing intermediate precipitates
was formed during mixing. The product of
coprecipitation was filtered off, washed thor-
oughly, dried and mixed with NaCl. When heated
at 8109501C, ferrite particles crystallized from the
NaCl matrix. When the salts were dissolved in
water, BaFe122xCoxTixO19 particles were ob-
tained. The ferrite particles were then mixed with
an appropriate amount of Bi2O3. For electromag-
netic properties measurement, pellets (10 mm
diameter, 0.71.0 mm thickness) and toroidal
samples (20 mm outside diameter, 10 mm insidediameter, about 13 mm thickness) were pressed
and then sintered at 9001C and 9501C in air.
The identification of the crystalline phases for
ferrite particles was carried out by X-ray diffrac-
tion (XRD). To obtain detailed information on the
morphology, and to verify the size of the
BaFe122xCoxTixO19 particles, a set of micro-
graphs were taken by transition electron micro-
scope (TEM). Scanning electron microscope
(SEM) was used to observe the microstructures
of sintered ferrites. The high-frequency magnetic
properties were measured using an HP4291B RF
impedance/materials analyzer from 10 MHz to
1.8 GHz. The electrical resistivity of all ferrite
samples was determined with an HP4140B meter
using silver contacts.
3. Results and discussion
3.1. Structure and morphology
The XRD patterns of particle samples with CoTi substitution amount x 1:20 obtained by
chemical coprecipitation process and synthesis
from NaCl melts at different temperatures are
shown in Fig. 1. As can be seen, the only crystal-
line phase that can be detected by XRD is an M-
type hexaferrite for all particle samples, no other
phases were apparently detectable. With the
increase of heat-treatment temperature from
8101C to 9501C, the relative intensity increases
while peaks become sharper, indicating an increase
of particle size. The temperature of 8101
C, which isslightly higher than the melting point (8011C) of
NaCl, is much lower than (above 10001C) what is
needed for complete reaction in classical ceramics
method. Only the M-type hexaferrite phase
formed at 8101C was thought to be from the
contribution of liquid phase formation of NaCl
which could make the metal ions diffusing more
easily.
Fig. 2 shows the morphologies of BaFe9.6Co1.2-
Ti1.2O19 particles prepared at 8101C (a) and 9501C
C. Wang et al. / Journal of Magnetism and Magnetic Materials 257 (2003) 100106 101
7/31/2019 Co-Ti Substituted BaHex
3/7
(b) taken by TEM. It is clear that barium ferrite
particles are hexagonal platelet crystals, and high-
er temperature corresponds to larger average size
and more regular particle shape with a more
perfect crystalline phase. The particle sizes are
ranged in 40100 nm and 50200 nm for samples
prepared at 8101C and 9501C, respectively.
The microstructures and morphologies of sin-
tered barium ferrites were studied by SEM. Fig. 3shows the micrographs of samples sintered at
9001C for 5 h with CoTi substitution amount x
1:20: As can be seen, the microstructure of a
sample sintered with Bi2O3 addition (b) is very
different from that without Bi2O3 (a), although
both the sintered samples were based on the same
ferrite particles obtained at 9501C for 2h. When
sintered without Bi2O3 addition, sample (a) is
composed of small grains with little porosity,
indicating little change of particle size compared
with the original barium ferrite particles. Whereas
the sample (b) that sintered with 2 wt% Bi2O3doping is composed of much larger grains with a
grain size distribution of 16 mm. The more and
larger pores were formed due to the irregular
alignment of larger hexagonal platelets. Effects of
temperatures for barium ferrite particles fabrica-
tion on microstructures of sintered samples canalso be concluded from (b)(d) micrographs in
Fig. 3. It can be seen that higher particle fabrica-
tion temperature produced larger size grain and
more regular grain shape when sintered at 9001C
for 5 h with the same amount of Bi2O3 addition.
3.2. High-frequency magnetic properties
To investigate the effect of Bi2O3 content on the
high-frequency magnetic properties of CoTi
substituted barium ferrites, ceramic samples wereprepared by mixing the CoTi substituted barium
ferrite particles which were fabricated at 9501C for
2 h, with various amount of Bi2O3 and sintered at
9501C for 5 h. The CoTi substitution amount was
x 1:20 for all samples. Fig. 4 depicts the
frequency dependence of complex permeability of
sintered ferrites, where m0 is the real part and m00 is
the imaginary part. Results revealed the tendency
of an increase in initial permeability with the Bi2O3content. For instance, the sample without Bi2O3
20 30 40 50 60 70
0
20
40
60
80
100BaFe12 O1927-1029
2 (Degrees)
Intensity(%)
950 Cx2h
900 Cx2h
850 Cx2h
810 Cx2h
Intensity(Arb.
Units)
Fig. 1. XRD patterns of CoTi substituted barium ferrite
particles fabricated at different temperatures.
Fig. 2. TEM micrographs of the barium ferrite particles
fabricated at 8101C (a) and 9501C (b).
C. Wang et al. / Journal of Magnetism and Magnetic Materials 257 (2003) 100106102
7/31/2019 Co-Ti Substituted BaHex
4/7
has a permeability m0 of about 6; however, sample
with 2 wt% Bi2O3 content has a permeability m0 of
above 14 in a wide frequency range. Increasing the
Bi2O3 content from 2 to 5 wt%, there is a littleenhancement of both m0 and m00: Comparing with
the frequency of about 1 GHz for a sample without
Bi2O3, permeability m0 of samples with Bi2O3
doping starts to decrease from 400 to 500 MHz.
Permeability m00 has a maximum value at about
1 GHz for samples with Bi2O3 doping and above
1.8 GHz which is out of the measurement range for
a sample without Bi2O3 doping. This behavior
indicates a decreasing effect of Bi2O3 doping on
cut-off frequency while the permeability m0 can be
improved effectively in lower frequency rangebelow the magnetic resonant frequency.
Fig. 5 shows the effect of particles fabricating
temperature on high-frequency magnetic proper-
ties of CoTi substituted barium ferrites sintered
at 9001C for 5 h with CoTi substitution x 1:20
and Bi2O3 content of 2 wt%. Obviously, the
magnetic properties are promoted significantly by
using the barium particles fabricated at higher
temperature. The value of initial permeability m0
rises from 8 (for 8101C) to 12.5 (for 9501C). The
frequency where m0 has the maximum value shifts
lower when the temperature for particles fabrica-
tion increases. The permeability m00 has little
change in the frequency region below 100 MHz,but has a larger value corresponding to larger
magnetic loss in relative higher frequency region of
3001000 MHz with increasing the particles fabri-
cation temperature.
The CoTi substitution dependence of complex
permeability is given in Fig. 6. Samples were
sintered at 9001C for 5h with Bi2O3 content of
2 wt%, and barium particles used were prepared at
9501C for 2 h. As it shows, the magnetic resonant
frequency has been greatly affected by the CoTi
substitution. With the increase of CoTi substitu-tion amount, the permeability m0 decreases and the
magnetic resonant frequency increases. Barium
ferrites with initial permeability m0 > 10 and
magnetic resonant frequency >1 GHz could be
obtained when the CoTi substitution amount was
x 1:2021:30:
It is known that the permeability of a poly-
crystalline ferrite can be described as the super-
position of two different magnetizing mechanisms:
spin rotation and domain wall motion [17]. Globus
Fig. 3. SEM micrographs of barium ferrites sintered at 9001C for 5 h without Bi2O3 doping with particles fabrication temperature
9501C (a); with 2 wt% Bi2O3 and particles fabrication temperature 9501C (b); with 2 wt% Bi2O3 and particles fabrication temperature
8501C (c); with 2 wt% Bi2O3 and particles fabrication temperature 8101C (d).
C. Wang et al. / Journal of Magnetism and Magnetic Materials 257 (2003) 100106 103
7/31/2019 Co-Ti Substituted BaHex
5/7
suggested that the domain wall motion was
affected by the grain size and could be enhanced
with the increase of grain size [18]. Comparing the
magnetic properties with the microstructures
indicated in Fig. 3, it manifests that the increase
in initial permeability is closely correlated to thedensification and grain size of sintered ferrites. In
case of ferrites composed with small grains, the
relatively larger volume fraction of grain boundary
and defects, where domain wall pinning could
occur, would decrease the contribution of domain
wall motion. In addition, small grains would make
magnetization decreased due to the existence of
spin canting layer [19], which also could increase
the effective magnetic anisotropy field. On the
contrary, since the melting temperature of Bi2O3 is
8251C, when samples with Bi2O3 doping were
sintered at above 9001C, higher density could be
obtained through particle re-arrangement, solu-
tion re-precipitation and solid skeleton processes
by formation of the Bi2O3 liquid phase [20]. An
increase in sintered density of ferrites not onlycauses the reduction of the demagnetizing field due
to pores but also raises the spin rotational
contribution, which in turn increases the perme-
ability [21]. Also, as the grain size was increased
with Bi2O3 doping, the multi-domain grains
appeared which could result in higher permeability
values due to the domain wall motion [22]. The
CoTi substitution dependence of high-frequency
magnetic properties was mainly due to the change
of the magnetocrystalline anisotropy field, which is
1E7 1E8 1E9
0
4
8
12
16
20
'
Frequency (Hz)
Bi2O3 wt%0 23 45
1E7 1E8 1E9
0
4
8
12
16
20
"
Frequency (Hz)
Bi2O3 wt%
02345
Fig. 4. Effect of Bi2O3 content on the complex permeability.
1E7 1E8 1E9
0
2
4
6
8
10
12
14
16
'
Frequency (Hz)
950 C
850 C
810 C
1E7 1E8 1E9
0
2
4
6
8
10
12
14
16
"
Frequency(Hz)
950 C
850 C
810 C
Fig. 5. Effect of magnetic particles fabrication temperature on
the complex permeability.
C. Wang et al. / Journal of Magnetism and Magnetic Materials 257 (2003) 100106104
7/31/2019 Co-Ti Substituted BaHex
6/7
in proportion to the magnetic resonant frequency.
In addition, the electrical resistivity for all sintered
samples was measured using silver contacts, and
was found to be above 108O cm. This high
electrical resistivity could reduce the eddy current
losses in the high-frequency range. All the electro-
magnetic behaviors of CoTi substituted barium
ferrites showed the potential to be used for MLCIsor multi-layer chip beads (MLCBs) in the hyper-
frequency range.
4. Summary
CoTi substituted barium ferrites Ba-
Fe122xCoxTixO19 were prepared by chemical
coprecipitation and low-temperature sintering
with Bi2O3 doping, their microstructures and
high-frequency magnetic properties were investi-
gated. Experimental results showed that CoTi
substituted barium ferrite particles with average
size range 40200 nm could be obtained whenheated in NaCl matrix at 8109501C, and high
temperature for barium ferrite particles fabrication
resulted in high permeability for sintered barium
ferrites. High-frequency magnetic properties could
be improved obviously with Bi2O3 doping. The
initial permeability m0 > 14 of sintered ferrites
could be obtained with Bi2O3 content varying
from 2 to 5 wt% compared with m0 6 without
Bi2O3 doping when sintered at 9501C for 5 h .
Barium ferrites with initial permeability m0 > 10;
magnetic resonant frequency >1 GHz were ob-
tained when it was sintered at 9001C with 2 wt%
Bi2O3 doping in the CoTi substitution of 1.20
1.30. The electrical resistivity of all sintered ferrite
samples was above 108O cm.
Acknowledgements
This work was supported by the High Tech-
nology Research and Development Project of
Peoples Republic of China (Grant No.
2001AA320502).
References
[1] H.M. Sung, C.J. Chen, W.S. Ko, H.C. Lin, IEEE Trans.
Magn. 30 (1994) 4906.
[2] H.G. Zhang, L.T. Li, Z.W. Ma, J. Zhou, Z.X. Yue,
Z.L. Gui, J. Magn. Magn. Mater. 218 (2000) 67.
[3] K. Hirota, T. Aoyama, S. Enomoto, M. Yoshinaka,
O. Yamaguchi, J. Magn. Magn. Mater. 205 (1999) 283.
[4] J. Smit, H.P.J. Wijn, Ferrites, Philips Technical Library,
Eindhoven, 1959, p. 285.[5] P. Allegri, D. Autissier, T. Taffary, Key Eng. Mater.
132136 (1997) 1424.
[6] R.C. Pullar, S.G. Appleton, A.K. Bhattacharya, J. Mater.
Sci. Lett. 17 (1998) 973.
[7] J.L. Snoek, Physica 14 (1948) 207.
[8] M. Sugimoto, in: E.P. Wohlfarth (Ed.), Ferromagnetic
Materials, Vol. 3, 1982, p. 394.
[9] I.G. Chen, S.H. Hsu, Y.H. Chang, J. Appl. Phys. 87 (2000)
6247.
[10] O. Sakaguchi, T. Kagotani, D. Book, H. Nakamura, S.
Sugimoto, M. Okada, M. Homma, Mater. Trans. JIM 37
(1996) 878.
1E7 1E8 1E9
0
2
4
6
8
1012
14
16
18
'
Frequency (Hz)
x = 1.20
x = 1.25
x = 1.30
1E7 1E8 1E9
0
2
4
6
8
10
12
14
16
18
"
Frequency (Hz)
x = 1.20
x = 1.25
x = 1.30
Fig. 6. CoTi substitution dependence of complex permeability.
C. Wang et al. / Journal of Magnetism and Magnetic Materials 257 (2003) 100106 105
7/31/2019 Co-Ti Substituted BaHex
7/7
[11] Z. Yang, H.X. Zeng, M.L. Yan, S.L. Geng, J. Magn. Soc.
Jpn. 13S1 (1989) 363.
[12] T. Fujiwara, IEEE Trans. Magn. 21 (1985) 1480.
[13] H. Kojima, in: E.P. Wohlfarth (Ed.), Ferromagneric
Materials, Vol. 3, 1982, p. 305.[14] D.J. de Bitetto, J. Appl. Phys. 35 (1964) 3482.
[15] D. Autissier, A. Podembski, C. Jacquiod, J. Phys. IV 7
(1997) 409.
[16] C.S. Wang, F.L. Wei, M. Lu, D.H. Han, Z. Yang,
J. Magn. Magn. Mater. 183 (1998) 241.
[17] J.P. Bouchaud, P.G. Zerah, J. Appl. Phys. 67 (1990) 5512.
[18] A. Globus, J. Phys. Suppl. C1(ICF-3) (1977) 1.
[19] X. Batlle, X. Obradors, M. Medarde, J. Rodriguez-
Carvajal, M. Pernet, M. Vallet-Regi, J. Magn. Magn.
Mater. 124 (1993) 228.[20] S.F. Wang, Y.R. Wang, T.C.K. Yang, C.F. Chen, C.A.
Lu, C.Y. Huang, J. Magn. Magn. Mater. 220 (2000) 129.
[21] J.J. Shrotri, S.D. Kulkarni, C.E. Deshpande, S.K. Date,
Mater. Chem. Phys. 59 (1999) 1.
[22] T. Nakamura, J. Magn. Magn. Mater. 168 (1997) 265.
C. Wang et al. / Journal of Magnetism and Magnetic Materials 257 (2003) 100106106