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Preparation and electromagnetic properties of ferrite–cordierite composites
July 2000
Materials Letters 44 Ž2000. 279–283 www.elsevier.comrlocatermatlet
Preparation and electromagnetic properties of ferrite–cordierite composites Zhenxing Yue ) , Longtu Li, Ji Zhou, Hongguo Zhang, Zhenwei Ma, Zhilun Gui State key Lab of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua UniÕersity, Beijing 100084, People’s Republic of China Received 15 August 1999; received in revised form 14 October 1999; accepted 29 December 1999
Abstract A series of novel composites consisting of NiCuZn ferrite and cordierite crystallites was prepared by sintering mixtures of NiCuZn ferrite nanosized particles and MgO–Al 2 O 3 –SiO 2 ŽMAS. glass powder. X-ray diffraction ŽXRD. results revealed that cordierite glass particles crystallize during sintering, resulting in the coexistence of NiCuZn ferrite with cordierite crystal phase in as-sintered composites. The resulting composites have tunable electromagnetic properties with high resonance frequencies, which may be used for multilayer chip inductors ŽMLCIs.. q 2000 Elsevier Science B.V. All rights reserved. Keywords: NiCuZn ferrites; Cordierite glass–ceramics; Ferrite–cordierite composites; Multilayer chip inductors
1. Introduction The multilayer chip inductor ŽMLCI. has recently been developed as one of the key surface mounting devices w1,2x. The key issue of fabrication of MLCIs is low-temperature sintered ferrites or low dielectric-constant ceramics, which are suitable for co-firing with silver internal electrode. Two materials systems, low-temperature sintered NiCuZn ferrites and glass–ceramics with low dielectric constant, are used for MLCIs at present w3–5x. The former has been used in the frequency range below 300 MHz, while the latter has found application in the frequency range higher than 1 GHz.
) Corresponding author. Tel.: q86-10-62784579; fax: q86-1062771160.
With development of communication technology, the high performance MLCIs used in the frequency range from 500 MHz to 2 GHz are especially required. In order to meet the requirement of these MLCIs with high inductance, high reliability and low cost, the materials must have the following properties: Ž1. good sinterability, substantially below the melting point of silver, Ž2. high permeability, and Ž3. low dielectric constant, assuring the high resonance frequency of MLCIs. The above-mentioned materials systems do not meet these requirements. Therefore, a new materials system must be developed as soon as possible. Further, with the development of high-density mounting technology, electromagnetic interference ŽEMI. between devices becomes a serious issue. Substrates with high permeability may decrease EMI between components in integrated circuits.
00167-577Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 0 . 0 0 0 4 5 - 8
280
Z. Yue et al.r Materials Letters 44 (2000) 279–283
Cordierite glass–ceramics that are based on the MgO–Al 2 O 3 –SiO 2 ŽMAS. formulation are usually used as substrates in integrated circuits and are important candidate materials for MLCIs used in high frequency range, because of their low dielectric constants and low densification temperatures w6,7x. However, the permeability of these glass–ceramics is unity because they are nonmagnetic. In the present study, the nanosized NiCuZn ferrite particles were incorporated into MAS glass, and sintered to form a composite consisting of ferrite and cordierite crystallites. This paper reports the preparation, crystallization process and electromagnetic properties of ferrite–cordierite glass–ceramics.
2. Experimental procedure 2.1. Synthesis of NiCuZn ferrite nanosized particles NiCuZn ferrite powder with composition ŽNi 0.25 Cu 0.25 Zn 0.50 .Fe 2 O4 was synthesized using sol–gel auto-combustion method. The details of the process were described previously in Ref. w8x. The analytical grade iron nitrate, nickel nitrate, zinc nitrate, copper nitrate, and citric acid were used as raw materials. Appropriate amount of nitrates and citric acid was first dissolved into deionized water to form a mixed solution. The molar ratio of nitrates to citric acid is 1:1. The pH value of solution was adjusted to about 7 using ammonia. Then, the mixed solution was poured into a dish and heated at 1358C under constant stirring to transform into a dried gel. Being ignited in air, the dried gel burnt in a self-propagating combustion manner to form loose powder.
solutions were mixed and stirred for 2 h at room temperature to form a mixed solution. The mixed solution was poured into a dish and dried at 808C for 12 h to transform into dried gel, then heated at 6008C for 4 h. After milling, a MAS glass powder was obtained for the preparation of composites. 2.3. Preparation of composites Two types of as-synthesized powders, NiCuZn ferrite nanosized particles and MAS glass powder, were mixed according to the molar ratio of 3:7, 5:5 and 7:3 Žnoted as C3, C5 and C7, respectively.. The mixed powder was ball-milled for 12 h. After drying, the powder was mixed with an appropriate amount of 5 wt.% polyvinyel alcohol as a binder, and granulated using a 60-mesh sieve. The powder was uniaxially pressed to form green toroidal and pellet specimens. After binder burnt out at 6008C for 1 h, the specimens were sintered at 700–9008C for 2 h. 2.4. Characterization The phase identification of the synthesized NiCuZn ferrite powder, MAS powder and composite samples were performed using X-ray diffraction ŽXRD. with Cu K a radiation. Transmission electron microscope ŽTEM. was used to observe the morphology of synthesized nanosized particles. Initial permeability and dielectric constant of sintered toroidal and pellet specimens were measured using HP4191A LCR and HP4194A impedance analyzers, respectively.
3. Results and discussion 2.2. Preparation of MAS glass powder A sol–gel process was utilized to prepare MAS glass powder in MSA system, as described in Ref. w10x. The composition of MAS glass powder is stoichiometric cordierite, Mg 2 Al 4 Si 5 O 18 , with 2.5 wt.% B 2 O 3 and 2.5 wt.% P2 O5 additives. AlŽNO 3 . 3 P 9H 2 O, MgŽCOOH. 2 P 4 H 2 O, HBO 3 and H 3 PO4 were dissolved in deionized water, and ethyl silicate ŽŽC 2 H 5 O.4 Si, TEOS. was dissolved into ethanol. The volume ratio of TEOS to ethanol was 1:4. Two
3.1. Character of synthesized ferrite and MAS glass powders XRD was performed on the synthesized NiCuZn ferrite powder, and the results showed that the asburnt powder is a single-phase NiCuZn ferrite with spinel structure w8x. This indicated that the non-crystalline precursor dried gel was directly transformed into crystalline NiCuZn ferrite after auto-combustion. No additional calcination was needed. The crys-
Z. Yue et al.r Materials Letters 44 (2000) 279–283
Fig. 1. TEM photograph of synthesized ferrite powder.
281
Fig. 2. XRD patterns of glass powder annealed at different temperatures indicating the crystallization process of a and m cordierite phases.
3.2. Phase structures of composites tallite size of the ferrite powder was measured using XRD broadening method and calculated from Scherrer equation, and was 42 nm. The TEM photograph of synthesized ferrite powder, shown in Fig. 1, shows that the powder is uniform in particle size. The particle size is about 40–50 nm, as measured using XRD method. This reveals that the synthesized powder is well-dispersed and no aggregates are formed during combustion process. Additionally, sintering experiment showed that this ferrite could be sintered at temperature lower than 9008C, indicating good sintering activity. DTA and TMA studies were performed on the MAS glass powder in our previous work w10x. The results revealed that the glass transition temperature ŽTg . is about 7008C, and the samples formed from this glass powder could be densified at 8808C, showing good sinterability. The thermal treatment at various temperatures was carried out for this glass powder to investigate the crystallization process. Fig. 2 shows the XRD patterns of glass powder annealed at different temperatures for 4 h. At temperature lower than 8008C, no crystalline phases were detected. At 8508C, a m-cordierite phase was crystallized from glass. With increasing temperature, m-cordierite transforms into a-cordierite. At 9008C, the transformation is finished. This indicates that a densified glass–ceramic with a-cordierite as crystalline phase was obtained by sintering at around 9008C.
Fig. 3 gives the XRD patterns for composites sintered at 9008C for 4 h. The XRD patterns of synthesized NiCuZn ferrite powder and thermally treated MAS powder are also shown in Fig. 3 for comparison. Fig. 3 indicates that two crystalline phases, NiCuZn ferrite and a-cordierite, coexist in all of three sintered composites. No chemical reaction between ferrite and MAS glass during sintering is detected. Above-mentioned results show that a series of expected composites consisting of ferrite and cordierite crystallites are obtained after sintering at lower temperature.
Fig. 3. XRD patterns of sintered ferrite, MAS glass and composites with ratios of ferrite to MAS glass of 3:7, 5:5 and 7:3 Žnoted as C3, C5 and C7 in plots..
282
Z. Yue et al.r Materials Letters 44 (2000) 279–283
of the embedded ferrite particles and nonmagnetic matrix, the effective permeability of composite can be described by w11x:
ms
Fig. 4. XRD patterns of composite ŽC5. sintered at different temperatures.
Fig. 4 gives the XRD patterns of composite ŽC5. sintered at different temperature. At temperature lower than 8008C, no crystallization of MAS glass, NiCuZn ferrite is the only crystalline phase. As temperature increases to 8508C, cordierite phase is crystallized from MAS glass, resulting in coexistence of two types of crystalline phases, ferrite and cordierite, in composite. Comparing with pure MAS glass, the presence of ferrite particles in glass does not change the crystallization behavior of MAS glass. 3.3. Magnetic properties of composites It has been shown that the initial permeability of NiCuZn ferrite prepared from the present synthesized powder is about 150, and the resonant frequency is about 20 MHz w9x. The permeability value of cordierite glass–ceramic is unity due to its inherent nonmagnetic nature. It is expected that the permeability value of glass–ceramic should be increased due to addition of ferrite particles. Fig. 5 shows the initial permeability vs. frequency for ferrite–cordierite composites with different molar ratios of ferrite to MAS glass. As expected, the permeability values of composites increase with increasing ferrite content. On the other hand, the permeability values of composites are dramatically reduced relative to that of NiCuZn ferrites. This may be attributed to the presence of nonmagnetic cordierite phase, which reduce the interaction between magnetic particles. For the composite material consisting
mB y 1 q1 d 1 q mB D
where m and m B are the effective permeability of the composite material and ferrite particles, respectively. D is the average size of the ferrite particles, and d the average thickness of the nonmagnetic layer between two ferrite particles. From above equation, the effective permeability of composite is determined by m B and the gap parameter drD. The gap parameter drD is related to the ferrite volume loading a by: 3
a
ž
1q
/
1
. a Therefore, the effective permeability of composite is reduced with the decrease of ferrite volume loading. The increase of resonant frequencies of composites may be attributed to the following facts. Firstly, the nanosized ferrite particles possess the higher resonant frequency than that of coarse-grain ferrites due to lacking the contribution of domain wall. Secondly, dispersion of ferrite particles in non-magnetic glass–ceramic matrix reduces the particle–particle interaction, resulting in decrease in permeability values and increase in resonant frequency. As the ferrite content is increased to 70 mol%, the permeability value increases and resonant frequency reD
s
Fig. 5. Initial permeability vs. frequency for ferrite–cordierite composites.
Z. Yue et al.r Materials Letters 44 (2000) 279–283
283
glass powder. After being sintered at 9008C, acordierite was crystallized from glass matrix, resulting in coexistence of NiCuZn ferrite and a-cordierite in sintered composites. No chemical reaction between two constituents during sintering was detected, and the presence of ferrite particles does not affect the crystallization process of MAS glass. The composite, which has permeability value higher than 3, low dielectric constant and high resonant frequency, was obtained. This composite can be used for MLCIs in high frequency range. Fig. 6. Dielectric constant vs. frequency for composites, NiCuZn ferrite and MAS glass–ceramic.
duces because of enhancement of the particle–particle interaction. 3.4. Dielectric constants of composites Dielectric constant vs. frequency for composites, NiCuZn ferrite and MAS glass–ceramic are shown in Fig. 6. The dielectric constant values of composites increase with increasing ferrite content. However, the effective permittivity of the composite is much lower than that calculated from logarithmic law. In fact, for composites consisting of conducting particles dispersed in an insulating matrix, the polarization of surface charges induced on conducting particles increases the permittivity values w12,13x. For the present composites, ferrite particles can be considered as conducting phase relative to MAS matrix, because the ferrites have smaller resistivity value than the cordierite glass–ceramics.
4. Conclusions The composites consisting of NiCuZn ferrite and cordierite were prepared through sintering the mixtures of NiCuZn ferrite nanosized particles and MAS
Acknowledgements This work was supported by the High Technology Research and Development Project of the People’s Republic of China.
References w1x R.J. Charles, A.R. Achuta, U.S. Pat. No. 4966625 Ž1990.. w2x H. Watanabe, Y. Kanagawa, T. Suzuki, T. Nomura, U.S. Pat. No. 4956114 Ž1990.. w3x M. Fujimoto, J. Am. Ceram. Soc. 77 Ž11. Ž1994. 2873. w4x J.-Y. Hsu, W.-S. Ko, H.-D. Shen, C.-J. Chen, IEEE Trans. Magn. 30 Ž6. Ž1994. , 4875. w5x T. Nakamura, J. Magn. Magn. Mater. 168 Ž3. Ž1997. 285. w6x S.H. Knickerbocker, A.H. Kumar, L.W. Herron, Am. Ceram. Soc. Bull. 72 Ž1. Ž1993. 90. w7x R.R. Tummala, J. Am. Ceram. Soc. 74 Ž5. Ž1991. 895. w8x Z. Yue, L. Li, J. Zhou, H. Zhang, Z. Gui, Mater. Sci. Eng., B 64 Ž1999. 68. w9x Z. Yue, J. Zhou, H. Zhang, Z. Gui, L. Li, J. Magn. Magn. Mater., in press. w10x Z. Yue, J. Zhou, H. Zhang, Z. Gui, L. Li, J. Mater. Sci. Lett., in press. w11x T. Tsutaoka, M. Ueshima, T. Tokunaga, T. Nakamura, K. Hatakeyama, J. Appl. Phys. 78 Ž6. Ž1995. 3983. w12x A. Dias, R.L. Moveira, N.D.S. Mohallem, J. Magn. Magn. Mater. 172 Ž1997. L9. w13x T.J. Tiske, H.S. Gokturk, D.M. Kalyon, J. Mater. Sci. 32 Ž1997. 5551.
Materials Letters 44 Ž2000. 279–283 www.elsevier.comrlocatermatlet
Preparation and electromagnetic properties of ferrite–cordierite composites Zhenxing Yue ) , Longtu Li, Ji Zhou, Hongguo Zhang, Zhenwei Ma, Zhilun Gui State key Lab of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua UniÕersity, Beijing 100084, People’s Republic of China Received 15 August 1999; received in revised form 14 October 1999; accepted 29 December 1999
Abstract A series of novel composites consisting of NiCuZn ferrite and cordierite crystallites was prepared by sintering mixtures of NiCuZn ferrite nanosized particles and MgO–Al 2 O 3 –SiO 2 ŽMAS. glass powder. X-ray diffraction ŽXRD. results revealed that cordierite glass particles crystallize during sintering, resulting in the coexistence of NiCuZn ferrite with cordierite crystal phase in as-sintered composites. The resulting composites have tunable electromagnetic properties with high resonance frequencies, which may be used for multilayer chip inductors ŽMLCIs.. q 2000 Elsevier Science B.V. All rights reserved. Keywords: NiCuZn ferrites; Cordierite glass–ceramics; Ferrite–cordierite composites; Multilayer chip inductors
1. Introduction The multilayer chip inductor ŽMLCI. has recently been developed as one of the key surface mounting devices w1,2x. The key issue of fabrication of MLCIs is low-temperature sintered ferrites or low dielectric-constant ceramics, which are suitable for co-firing with silver internal electrode. Two materials systems, low-temperature sintered NiCuZn ferrites and glass–ceramics with low dielectric constant, are used for MLCIs at present w3–5x. The former has been used in the frequency range below 300 MHz, while the latter has found application in the frequency range higher than 1 GHz.
) Corresponding author. Tel.: q86-10-62784579; fax: q86-1062771160.
With development of communication technology, the high performance MLCIs used in the frequency range from 500 MHz to 2 GHz are especially required. In order to meet the requirement of these MLCIs with high inductance, high reliability and low cost, the materials must have the following properties: Ž1. good sinterability, substantially below the melting point of silver, Ž2. high permeability, and Ž3. low dielectric constant, assuring the high resonance frequency of MLCIs. The above-mentioned materials systems do not meet these requirements. Therefore, a new materials system must be developed as soon as possible. Further, with the development of high-density mounting technology, electromagnetic interference ŽEMI. between devices becomes a serious issue. Substrates with high permeability may decrease EMI between components in integrated circuits.
00167-577Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 0 . 0 0 0 4 5 - 8
280
Z. Yue et al.r Materials Letters 44 (2000) 279–283
Cordierite glass–ceramics that are based on the MgO–Al 2 O 3 –SiO 2 ŽMAS. formulation are usually used as substrates in integrated circuits and are important candidate materials for MLCIs used in high frequency range, because of their low dielectric constants and low densification temperatures w6,7x. However, the permeability of these glass–ceramics is unity because they are nonmagnetic. In the present study, the nanosized NiCuZn ferrite particles were incorporated into MAS glass, and sintered to form a composite consisting of ferrite and cordierite crystallites. This paper reports the preparation, crystallization process and electromagnetic properties of ferrite–cordierite glass–ceramics.
2. Experimental procedure 2.1. Synthesis of NiCuZn ferrite nanosized particles NiCuZn ferrite powder with composition ŽNi 0.25 Cu 0.25 Zn 0.50 .Fe 2 O4 was synthesized using sol–gel auto-combustion method. The details of the process were described previously in Ref. w8x. The analytical grade iron nitrate, nickel nitrate, zinc nitrate, copper nitrate, and citric acid were used as raw materials. Appropriate amount of nitrates and citric acid was first dissolved into deionized water to form a mixed solution. The molar ratio of nitrates to citric acid is 1:1. The pH value of solution was adjusted to about 7 using ammonia. Then, the mixed solution was poured into a dish and heated at 1358C under constant stirring to transform into a dried gel. Being ignited in air, the dried gel burnt in a self-propagating combustion manner to form loose powder.
solutions were mixed and stirred for 2 h at room temperature to form a mixed solution. The mixed solution was poured into a dish and dried at 808C for 12 h to transform into dried gel, then heated at 6008C for 4 h. After milling, a MAS glass powder was obtained for the preparation of composites. 2.3. Preparation of composites Two types of as-synthesized powders, NiCuZn ferrite nanosized particles and MAS glass powder, were mixed according to the molar ratio of 3:7, 5:5 and 7:3 Žnoted as C3, C5 and C7, respectively.. The mixed powder was ball-milled for 12 h. After drying, the powder was mixed with an appropriate amount of 5 wt.% polyvinyel alcohol as a binder, and granulated using a 60-mesh sieve. The powder was uniaxially pressed to form green toroidal and pellet specimens. After binder burnt out at 6008C for 1 h, the specimens were sintered at 700–9008C for 2 h. 2.4. Characterization The phase identification of the synthesized NiCuZn ferrite powder, MAS powder and composite samples were performed using X-ray diffraction ŽXRD. with Cu K a radiation. Transmission electron microscope ŽTEM. was used to observe the morphology of synthesized nanosized particles. Initial permeability and dielectric constant of sintered toroidal and pellet specimens were measured using HP4191A LCR and HP4194A impedance analyzers, respectively.
3. Results and discussion 2.2. Preparation of MAS glass powder A sol–gel process was utilized to prepare MAS glass powder in MSA system, as described in Ref. w10x. The composition of MAS glass powder is stoichiometric cordierite, Mg 2 Al 4 Si 5 O 18 , with 2.5 wt.% B 2 O 3 and 2.5 wt.% P2 O5 additives. AlŽNO 3 . 3 P 9H 2 O, MgŽCOOH. 2 P 4 H 2 O, HBO 3 and H 3 PO4 were dissolved in deionized water, and ethyl silicate ŽŽC 2 H 5 O.4 Si, TEOS. was dissolved into ethanol. The volume ratio of TEOS to ethanol was 1:4. Two
3.1. Character of synthesized ferrite and MAS glass powders XRD was performed on the synthesized NiCuZn ferrite powder, and the results showed that the asburnt powder is a single-phase NiCuZn ferrite with spinel structure w8x. This indicated that the non-crystalline precursor dried gel was directly transformed into crystalline NiCuZn ferrite after auto-combustion. No additional calcination was needed. The crys-
Z. Yue et al.r Materials Letters 44 (2000) 279–283
Fig. 1. TEM photograph of synthesized ferrite powder.
281
Fig. 2. XRD patterns of glass powder annealed at different temperatures indicating the crystallization process of a and m cordierite phases.
3.2. Phase structures of composites tallite size of the ferrite powder was measured using XRD broadening method and calculated from Scherrer equation, and was 42 nm. The TEM photograph of synthesized ferrite powder, shown in Fig. 1, shows that the powder is uniform in particle size. The particle size is about 40–50 nm, as measured using XRD method. This reveals that the synthesized powder is well-dispersed and no aggregates are formed during combustion process. Additionally, sintering experiment showed that this ferrite could be sintered at temperature lower than 9008C, indicating good sintering activity. DTA and TMA studies were performed on the MAS glass powder in our previous work w10x. The results revealed that the glass transition temperature ŽTg . is about 7008C, and the samples formed from this glass powder could be densified at 8808C, showing good sinterability. The thermal treatment at various temperatures was carried out for this glass powder to investigate the crystallization process. Fig. 2 shows the XRD patterns of glass powder annealed at different temperatures for 4 h. At temperature lower than 8008C, no crystalline phases were detected. At 8508C, a m-cordierite phase was crystallized from glass. With increasing temperature, m-cordierite transforms into a-cordierite. At 9008C, the transformation is finished. This indicates that a densified glass–ceramic with a-cordierite as crystalline phase was obtained by sintering at around 9008C.
Fig. 3 gives the XRD patterns for composites sintered at 9008C for 4 h. The XRD patterns of synthesized NiCuZn ferrite powder and thermally treated MAS powder are also shown in Fig. 3 for comparison. Fig. 3 indicates that two crystalline phases, NiCuZn ferrite and a-cordierite, coexist in all of three sintered composites. No chemical reaction between ferrite and MAS glass during sintering is detected. Above-mentioned results show that a series of expected composites consisting of ferrite and cordierite crystallites are obtained after sintering at lower temperature.
Fig. 3. XRD patterns of sintered ferrite, MAS glass and composites with ratios of ferrite to MAS glass of 3:7, 5:5 and 7:3 Žnoted as C3, C5 and C7 in plots..
282
Z. Yue et al.r Materials Letters 44 (2000) 279–283
of the embedded ferrite particles and nonmagnetic matrix, the effective permeability of composite can be described by w11x:
ms
Fig. 4. XRD patterns of composite ŽC5. sintered at different temperatures.
Fig. 4 gives the XRD patterns of composite ŽC5. sintered at different temperature. At temperature lower than 8008C, no crystallization of MAS glass, NiCuZn ferrite is the only crystalline phase. As temperature increases to 8508C, cordierite phase is crystallized from MAS glass, resulting in coexistence of two types of crystalline phases, ferrite and cordierite, in composite. Comparing with pure MAS glass, the presence of ferrite particles in glass does not change the crystallization behavior of MAS glass. 3.3. Magnetic properties of composites It has been shown that the initial permeability of NiCuZn ferrite prepared from the present synthesized powder is about 150, and the resonant frequency is about 20 MHz w9x. The permeability value of cordierite glass–ceramic is unity due to its inherent nonmagnetic nature. It is expected that the permeability value of glass–ceramic should be increased due to addition of ferrite particles. Fig. 5 shows the initial permeability vs. frequency for ferrite–cordierite composites with different molar ratios of ferrite to MAS glass. As expected, the permeability values of composites increase with increasing ferrite content. On the other hand, the permeability values of composites are dramatically reduced relative to that of NiCuZn ferrites. This may be attributed to the presence of nonmagnetic cordierite phase, which reduce the interaction between magnetic particles. For the composite material consisting
mB y 1 q1 d 1 q mB D
where m and m B are the effective permeability of the composite material and ferrite particles, respectively. D is the average size of the ferrite particles, and d the average thickness of the nonmagnetic layer between two ferrite particles. From above equation, the effective permeability of composite is determined by m B and the gap parameter drD. The gap parameter drD is related to the ferrite volume loading a by: 3
a
ž
1q
/
1
. a Therefore, the effective permeability of composite is reduced with the decrease of ferrite volume loading. The increase of resonant frequencies of composites may be attributed to the following facts. Firstly, the nanosized ferrite particles possess the higher resonant frequency than that of coarse-grain ferrites due to lacking the contribution of domain wall. Secondly, dispersion of ferrite particles in non-magnetic glass–ceramic matrix reduces the particle–particle interaction, resulting in decrease in permeability values and increase in resonant frequency. As the ferrite content is increased to 70 mol%, the permeability value increases and resonant frequency reD
s
Fig. 5. Initial permeability vs. frequency for ferrite–cordierite composites.
Z. Yue et al.r Materials Letters 44 (2000) 279–283
283
glass powder. After being sintered at 9008C, acordierite was crystallized from glass matrix, resulting in coexistence of NiCuZn ferrite and a-cordierite in sintered composites. No chemical reaction between two constituents during sintering was detected, and the presence of ferrite particles does not affect the crystallization process of MAS glass. The composite, which has permeability value higher than 3, low dielectric constant and high resonant frequency, was obtained. This composite can be used for MLCIs in high frequency range. Fig. 6. Dielectric constant vs. frequency for composites, NiCuZn ferrite and MAS glass–ceramic.
duces because of enhancement of the particle–particle interaction. 3.4. Dielectric constants of composites Dielectric constant vs. frequency for composites, NiCuZn ferrite and MAS glass–ceramic are shown in Fig. 6. The dielectric constant values of composites increase with increasing ferrite content. However, the effective permittivity of the composite is much lower than that calculated from logarithmic law. In fact, for composites consisting of conducting particles dispersed in an insulating matrix, the polarization of surface charges induced on conducting particles increases the permittivity values w12,13x. For the present composites, ferrite particles can be considered as conducting phase relative to MAS matrix, because the ferrites have smaller resistivity value than the cordierite glass–ceramics.
4. Conclusions The composites consisting of NiCuZn ferrite and cordierite were prepared through sintering the mixtures of NiCuZn ferrite nanosized particles and MAS
Acknowledgements This work was supported by the High Technology Research and Development Project of the People’s Republic of China.
References w1x R.J. Charles, A.R. Achuta, U.S. Pat. No. 4966625 Ž1990.. w2x H. Watanabe, Y. Kanagawa, T. Suzuki, T. Nomura, U.S. Pat. No. 4956114 Ž1990.. w3x M. Fujimoto, J. Am. Ceram. Soc. 77 Ž11. Ž1994. 2873. w4x J.-Y. Hsu, W.-S. Ko, H.-D. Shen, C.-J. Chen, IEEE Trans. Magn. 30 Ž6. Ž1994. , 4875. w5x T. Nakamura, J. Magn. Magn. Mater. 168 Ž3. Ž1997. 285. w6x S.H. Knickerbocker, A.H. Kumar, L.W. Herron, Am. Ceram. Soc. Bull. 72 Ž1. Ž1993. 90. w7x R.R. Tummala, J. Am. Ceram. Soc. 74 Ž5. Ž1991. 895. w8x Z. Yue, L. Li, J. Zhou, H. Zhang, Z. Gui, Mater. Sci. Eng., B 64 Ž1999. 68. w9x Z. Yue, J. Zhou, H. Zhang, Z. Gui, L. Li, J. Magn. Magn. Mater., in press. w10x Z. Yue, J. Zhou, H. Zhang, Z. Gui, L. Li, J. Mater. Sci. Lett., in press. w11x T. Tsutaoka, M. Ueshima, T. Tokunaga, T. Nakamura, K. Hatakeyama, J. Appl. Phys. 78 Ž6. Ž1995. 3983. w12x A. Dias, R.L. Moveira, N.D.S. Mohallem, J. Magn. Magn. Mater. 172 Ž1997. L9. w13x T.J. Tiske, H.S. Gokturk, D.M. Kalyon, J. Mater. Sci. 32 Ž1997. 5551.