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The influence of microstructure on the microwave absorption of Co–U hexaferrites
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 310 (2007) 2558–2560 www.elsevier.com/locate/jmmm
The influence of microstructure on the microwave absorption of Co–U hexaferrites Darja Lisjaka,, Vladimir B. Bregara,b, Miha Drofenika a
Jozˇef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia b Iskra Feriti d.o.o., Stegne 29, 1000 Ljubljana, Slovenia Available online 20 November 2006
Abstract Co–U hexaferrites with the composition Ba4Co2Fe36O60 were prepared with high-energy milling or chemical co-precipitation followed by reaction sintering at 1250 1C. The preparation method significantly influenced the density of the ceramics, i.e., 4.2 g/cm3 when prepared with high-energy milling, and 3.6 g/cm3 when prepared with chemical co-precipitation. The absorption increased with the density of the sample, while at the same time the absorption range decreased. 90% absorption within a broad-frequency range was determined for the 2.5-mm thick absorbers. r 2006 Elsevier B.V. All rights reserved. PACS: 75.50.G Keywords: Hexaferrites; Microwave absorption; Microstructure
1. Introduction The importance of microwave absorbers is increasing in line with the exploitation of microwaves and the increasing levels of microwave pollution. Among the ferrite materials, hexaferrites exhibit the highest magnetic losses with microwaves. These materials include Y-hexaferrites and other hexaferrites that contain Co, and all of them exhibit planar magnetocrystalline anisotropy. So far, only Y-, Co–M and Co–Z hexaferrites have been considered as possible candidates for microwave absorbers [1]. Their crystal structure [2] is composed of three basic structural blocks: S, R and T. A specific combination of these blocks results in a specific hexaferrite type: (RS)2 ¼ M, (TS)3 ¼ Y, and (RSTS)2 ¼ Z. In the search for new microwave-absorbing materials, it is necessary to investigate the other types of hexaferrites: W, X or U. Co–U hexaferrites possess a very similar structure, (RSRSTS)3, to that of Co–Z hexaferrites. Corresponding author. Tel.: +386 1 4773 872; fax: +386 1 4773 875.
E-mail address: [email protected] (D. Lisjak). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.866
Therefore, it is expected that Co–U hexaferrites would also demonstrate a significant absorption of microwaves in the lower frequency range. Here we report, for the first time, on the microwave absorption of Co–U hexaferrites with the composition Ba4Co2Fe36O60. 2. Experimental Co–U hexaferrites with the composition Ba4Co2Fe36O60 were prepared as described elsewhere [3] with high-energy milling (HEM) or chemical co-precipitation (CC) followed by reaction sintering at 1250 1C. The ceramics were polished and thermally etched at 1200 1C for observation of their microstructures with a scanning electron microscope (SEM Jeol-5800). Their densities were determined with the Archimede’s method using Hg as an immersion liquid. The permeability and permittivity of the samples were obtained from the S-parameters measured with a vector network analyzer (Anritsu 37369C) at 0.4–18 GHz. A coaxial sample holder with an inner diameter of 3 mm and an outer diameter of 7 mm was used. The microwave absorption of the ceramics was calculated for the case of a
ARTICLE IN PRESS D. Lisjak et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 2558–2560
2559
metal-backed sample by using Eq. (1) [4], where R is reflection, m is permeability, e is permittivity, f is frequency, d is thickness and c is speed of light. " pffiffiffiffiffiffiffi # pffiffiffiffiffi i m= tanð2pfdc1 mÞ 1 RðdBÞ ¼ 20 log pffiffiffiffiffiffiffi pffiffiffiffiffi i m= tanð2pfdc1 mÞ þ 1
(1)
3. Results and discussion The preparation method significantly influenced the density of the ceramics, i.e., 4.2 g/cm3 for the HEM samples, and 3.6 g/cm3 for the CC samples. Regardless of the preparation method, an exaggerated grain growth occurred during the sintering, which resulted in a bimodal grain-size distribution. As shown in Fig. 1, grains with diameters larger than 10 mm were embedded in a matrix of micron-sized grains. Despite the similar grain morphology, the samples exhibited different absorption behaviors (Fig. 2). In general, a higher absorption (except for the 3.5-mm absorber) with a maximum at lower frequencies was calculated for the HEM samples. The HEM samples also had a higher density than the CC samples. Because of this higher density, the HEM samples also exhibited higher permeability (Fig. 3) and permittivity in the measured frequency range: e ¼ 14.5–i0.5 for the HEM sample and e ¼ 10.5–i0.1 for the CC sample. In Fig. 3, two distinctive contributions to the magnetic losses can be observed: the one at frequencies up to 2 GHz, related to the domain-wall relaxation (DWR), and the one at higher frequencies, related to the ferromagnetic resonance (FMR). Obviously, similar grain morphology also resulted in a similar domain structure for both samples. Consequently, the DWR
Fig. 2. Calculated reflection (absorption) for a metal-backed sample with different thickness d. The dotted line is for the HEM sample and the full line is for the CC sample.
Fig. 3. The permeability spectra of the HEM and CC ceramics.
contribution was similar for both samples. The FMR contribution was much more dispersed for the CC sample due to the higher porosity. Pores can be regarded as dielectric inclusions in a ferrite matrix and represent an inhomogeneity in the magnetic field of the sample that results in broadening and a frequency shift of the FMR [5]. Consequently, a broader absorption range and, at the same time, a lower absorption with higher porosity can be expected.
4. Conclusions
Fig. 1. SEM images of the etched surfaces of the Co–U hexaferrites prepared by the HEM and CC methods.
Co–U hexaferrites with the composition Ba4Co2Fe36O60 having sintered densities of 4.2 and 3.6 g/cm3 were prepared at 1250 1C via high-energy milling and chemical co-precipitation, respectively. The densities of the ceramics
ARTICLE IN PRESS 2560
D. Lisjak et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 2558–2560
had a significant influence on their permeability vs. frequency behavior and consequently on their microwave absorption. The pores decreased the absorption and broadened the frequency range of the absorption. Therefore, the porosity/density needs to be optimized for the optimum absorption of microwaves. The maximum absorption is expected for the 3.5-mm thick absorber from the low-density sample, while the broadest frequency range, with 90% absorption, is expected for the 2.5-mm thick absorber.
References [1] D. Autissier, A. Podembski, C. Jacquiod, J. Phys. IV France 7 (1997) C1-409; M. Matsumoto, Y. Miyata, J. Appl. Phys. 79 (1996) 5486. [2] E. Pollert, Prog. Crystal Growth and Charact. 11 (1985) 155. [3] D. Lisjak, D. Makovec, M. Drofenik, J. Mater. Res. 19 (2004) 2462. [4] Y. Naito, K. Suetake, IEEE Trans. Microwave Theor. Tech. MTT- 19 (1971) 65. [5] B. Lax, K.J. Button, Microwave Ferrites and Ferrimagnetics, McGraw-Hill, New York, 1962.
Journal of Magnetism and Magnetic Materials 310 (2007) 2558–2560 www.elsevier.com/locate/jmmm
The influence of microstructure on the microwave absorption of Co–U hexaferrites Darja Lisjaka,, Vladimir B. Bregara,b, Miha Drofenika a
Jozˇef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia b Iskra Feriti d.o.o., Stegne 29, 1000 Ljubljana, Slovenia Available online 20 November 2006
Abstract Co–U hexaferrites with the composition Ba4Co2Fe36O60 were prepared with high-energy milling or chemical co-precipitation followed by reaction sintering at 1250 1C. The preparation method significantly influenced the density of the ceramics, i.e., 4.2 g/cm3 when prepared with high-energy milling, and 3.6 g/cm3 when prepared with chemical co-precipitation. The absorption increased with the density of the sample, while at the same time the absorption range decreased. 90% absorption within a broad-frequency range was determined for the 2.5-mm thick absorbers. r 2006 Elsevier B.V. All rights reserved. PACS: 75.50.G Keywords: Hexaferrites; Microwave absorption; Microstructure
1. Introduction The importance of microwave absorbers is increasing in line with the exploitation of microwaves and the increasing levels of microwave pollution. Among the ferrite materials, hexaferrites exhibit the highest magnetic losses with microwaves. These materials include Y-hexaferrites and other hexaferrites that contain Co, and all of them exhibit planar magnetocrystalline anisotropy. So far, only Y-, Co–M and Co–Z hexaferrites have been considered as possible candidates for microwave absorbers [1]. Their crystal structure [2] is composed of three basic structural blocks: S, R and T. A specific combination of these blocks results in a specific hexaferrite type: (RS)2 ¼ M, (TS)3 ¼ Y, and (RSTS)2 ¼ Z. In the search for new microwave-absorbing materials, it is necessary to investigate the other types of hexaferrites: W, X or U. Co–U hexaferrites possess a very similar structure, (RSRSTS)3, to that of Co–Z hexaferrites. Corresponding author. Tel.: +386 1 4773 872; fax: +386 1 4773 875.
E-mail address: [email protected] (D. Lisjak). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.866
Therefore, it is expected that Co–U hexaferrites would also demonstrate a significant absorption of microwaves in the lower frequency range. Here we report, for the first time, on the microwave absorption of Co–U hexaferrites with the composition Ba4Co2Fe36O60. 2. Experimental Co–U hexaferrites with the composition Ba4Co2Fe36O60 were prepared as described elsewhere [3] with high-energy milling (HEM) or chemical co-precipitation (CC) followed by reaction sintering at 1250 1C. The ceramics were polished and thermally etched at 1200 1C for observation of their microstructures with a scanning electron microscope (SEM Jeol-5800). Their densities were determined with the Archimede’s method using Hg as an immersion liquid. The permeability and permittivity of the samples were obtained from the S-parameters measured with a vector network analyzer (Anritsu 37369C) at 0.4–18 GHz. A coaxial sample holder with an inner diameter of 3 mm and an outer diameter of 7 mm was used. The microwave absorption of the ceramics was calculated for the case of a
ARTICLE IN PRESS D. Lisjak et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 2558–2560
2559
metal-backed sample by using Eq. (1) [4], where R is reflection, m is permeability, e is permittivity, f is frequency, d is thickness and c is speed of light. " pffiffiffiffiffiffiffi # pffiffiffiffiffi i m= tanð2pfdc1 mÞ 1 RðdBÞ ¼ 20 log pffiffiffiffiffiffiffi pffiffiffiffiffi i m= tanð2pfdc1 mÞ þ 1
(1)
3. Results and discussion The preparation method significantly influenced the density of the ceramics, i.e., 4.2 g/cm3 for the HEM samples, and 3.6 g/cm3 for the CC samples. Regardless of the preparation method, an exaggerated grain growth occurred during the sintering, which resulted in a bimodal grain-size distribution. As shown in Fig. 1, grains with diameters larger than 10 mm were embedded in a matrix of micron-sized grains. Despite the similar grain morphology, the samples exhibited different absorption behaviors (Fig. 2). In general, a higher absorption (except for the 3.5-mm absorber) with a maximum at lower frequencies was calculated for the HEM samples. The HEM samples also had a higher density than the CC samples. Because of this higher density, the HEM samples also exhibited higher permeability (Fig. 3) and permittivity in the measured frequency range: e ¼ 14.5–i0.5 for the HEM sample and e ¼ 10.5–i0.1 for the CC sample. In Fig. 3, two distinctive contributions to the magnetic losses can be observed: the one at frequencies up to 2 GHz, related to the domain-wall relaxation (DWR), and the one at higher frequencies, related to the ferromagnetic resonance (FMR). Obviously, similar grain morphology also resulted in a similar domain structure for both samples. Consequently, the DWR
Fig. 2. Calculated reflection (absorption) for a metal-backed sample with different thickness d. The dotted line is for the HEM sample and the full line is for the CC sample.
Fig. 3. The permeability spectra of the HEM and CC ceramics.
contribution was similar for both samples. The FMR contribution was much more dispersed for the CC sample due to the higher porosity. Pores can be regarded as dielectric inclusions in a ferrite matrix and represent an inhomogeneity in the magnetic field of the sample that results in broadening and a frequency shift of the FMR [5]. Consequently, a broader absorption range and, at the same time, a lower absorption with higher porosity can be expected.
4. Conclusions
Fig. 1. SEM images of the etched surfaces of the Co–U hexaferrites prepared by the HEM and CC methods.
Co–U hexaferrites with the composition Ba4Co2Fe36O60 having sintered densities of 4.2 and 3.6 g/cm3 were prepared at 1250 1C via high-energy milling and chemical co-precipitation, respectively. The densities of the ceramics
ARTICLE IN PRESS 2560
D. Lisjak et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 2558–2560
had a significant influence on their permeability vs. frequency behavior and consequently on their microwave absorption. The pores decreased the absorption and broadened the frequency range of the absorption. Therefore, the porosity/density needs to be optimized for the optimum absorption of microwaves. The maximum absorption is expected for the 3.5-mm thick absorber from the low-density sample, while the broadest frequency range, with 90% absorption, is expected for the 2.5-mm thick absorber.
References [1] D. Autissier, A. Podembski, C. Jacquiod, J. Phys. IV France 7 (1997) C1-409; M. Matsumoto, Y. Miyata, J. Appl. Phys. 79 (1996) 5486. [2] E. Pollert, Prog. Crystal Growth and Charact. 11 (1985) 155. [3] D. Lisjak, D. Makovec, M. Drofenik, J. Mater. Res. 19 (2004) 2462. [4] Y. Naito, K. Suetake, IEEE Trans. Microwave Theor. Tech. MTT- 19 (1971) 65. [5] B. Lax, K.J. Button, Microwave Ferrites and Ferrimagnetics, McGraw-Hill, New York, 1962.