NiZnCu ferrite composites

Journal of Alloys and Compounds 470 (2009) 269–272

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Low-temperature cofiring behavior of ZnTiO3 dielectrics/NiZnCu ferrite composites Xiangchun Liu a,b,∗ , Feng Gao b , Jiaji Liu b , Changsheng Tian b a b

Department of Materials Science and Engineering, Xi’an University of Science and Technology, Xi’an 710054, People’s Republic of China School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, People’s Republic of China

a r t i c l e

i n f o

Article history: Received 4 July 2007 Received in revised form 3 February 2008 Accepted 11 February 2008 Available online 2 April 2008 Keywords: Layered structures Electrical properties Interface Sintering

a b s t r a c t The low-temperature cofiring compatibility between ferrite and dielectric materials is the key issue in the production process of multilayer chip LC filters. In this paper, no camber and cracks multilayer composite samples were obtained by using restricted shrinkage sintering process. The cofiring interface, ionic interdiffusion between constituents and dielectric properties of cofired composites were investigated. The results show that interfacial reactions occur at the interface, which can strengthen combinations between ferrite layers and dielectric layers. Iron and titanium have a wide diffusion range. Due to their low-firing characteristics and realizable cofiring compatibility, ZnTiO3 dielectrics/NiZnCu hexagonal ferrite can serve as the promising medium materials in the multilayer LC filter. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Recently, the surface mount technology (SMT) has been rapidly developed for miniaturization of electric devices such as multilayer ceramic capacitor (MLCC) and multilayer chip inductor (MLCI). The trend of electronics has been for an increasingly compact design. Some characterizations of cofired devices such as ceramic-filled glass electronic package and varistor–capacitor cofired multilayer device have been reported [1,2]. Multilayer chip LC filters have been developed as a type of promising surface mounting device (SMD). They are made by stacking ferrite layers and dielectric layers with internal electrode pastes, and then cofired. The sintering temperature of ferrite and dielectrics must be below 900 ◦ C so as to cofiring with Ag internal electrode. The properties of devices depend on the properties of the sintered ferrites, dielectrics and the quality of the interface. On the one hand, interfacial diffusion is inevitable, which will affect properties near the interface. On the other hand, the mismatch in sintering kinetics between different ceramics may cause residual stresses or cracks. Therefore, the matching and compatibility during low-temperature cofiring process is the key to achieve multilayer chip composites. Zinc titanates and their modified systems, with relatively low sintering temperature and good dielectric properties, have been provided as suitable candidates for low-temperature cofiring compatibility (LTCC) [3–5]. NiCuZn ferrite, which has excellent magnetic

∗ Corresponding author. Tel.: +86 13891801847. E-mail address: [email protected] (X. Liu). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.02.049

permeability and can be sintered at low temperature, is an important material using for producing low-temperature sintered multilayer chip inductor (MLCI) [6–8]. They are promising materials for the multilayer chip LC filters. In our previous work, the sintering temperatures of ZnTiO3 and NiCuZn ferrite have been lowered to 900 ◦ C [4,9]. So in the present study, the multilayer components were prepared by cofiring ZnTiO3 (abbreviated ZT) dielectrics and NiCuZn (abbreviated NZC) ferrite. The cofiring interface, ionic interdiffusion between constituents and dielectric properties of cofired composites were investigated. 2. Experimental procedure NZC ferrite and ZT powders were synthesized by solid-state reaction method combined with a chemical processing. The first, A.R zinc hydroxide carbonate (Zn5 (CO3 )2 ·(OH)6 ) was heat treated at 350 ◦ C for 2 h in air to obtain ZnO with high active energy as a starting material. Then, 99% pure reagent-grade NiO, Fe2 O3 , CuO and high active ZnO nanopowder, anatase nanopowder (10–30 nm), according to the composition of (Ni0.8 Cu0.12 Zn0.12 )Fe1.96 O4 and ZnTiO3 , respectively, were weighted and mixed for 12 h in a planetary ball mill. The mixtures were calcined (NZC: 720 ◦ C for 4 h, ZT: 750 ◦ C for 2 h), and then sintering additives (V2 O5 and B2 O3 for ZT [4], Bi2 O3 for NZC), solvent, dispersant, binder, and plasticizer were mixed into them and were milled for 48 h again to prepared the slurries. After being sifted out and eliminated the air bubbles, the slurries were tape-casted into green sheets. The multilayer samples were assembled by stacking green sheets of ferrite and dielectric layers alternately, and each green sheet was about 100 ␮m thick. The binder was burnt out before sintered by heating up very slowly to 350 ◦ C, and dwelling for 6 h at this temperature. The samples were sintered at 900 ◦ C in air for 4 h using free sintering (conventional sintering method) and restricted shrinkage sintering, respectively. Restricted shrinkage sintering is sintering the multilayer ceramic green body with no-shrinkage alumina substrate on both sides [10,11]. During firing, the multilayer ceramic in middle layer can be bonded by the sintered rigid board. Therefore, the ceramic green sheets shrink in the z direction, not in the x–y directions, due to the

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Fig. 2. Density and shrinkage of ZT ceramics with V2 O5 and B2 O3 additions vs. sintering temperature.

phase decomposition of ZnTiO3 [12]. The density of samples sintered at 900 ◦ C reaches 95.7% theoretical density which satisfies requirement of LTCC [12]. The density and shrinkage of NZC reach saturation at 900 ◦ C as shown in Fig. 3, the density attains 96.1% theoretical density which can serve as the medium materials in LTCC. 3.2. Interfacial diffusion

Fig. 1. ZT/NZC cofired composite sintered by free sintering method and restricted shrinkage sintering process.

constraining effects of the no-shrinkage alumina substrate layer [10,11]. The result of using restricted shrinkage sintering is obtaining the no camber and cracks multilayer composite samples, as are shown in Fig. 1. To investigate the interaction of the two materials, the 50/50 (wt%) mixture (designated as HH) of the ZT dielectrics and NCZ ferrite powder was prepared and sintered at 900 ◦ C for 4 h. Interfacial diffusion was examined by scanning electron microscopy (SEM, JEOL JSM-5800, Japan) and energy-dispersive spectroscopy (EDS). The crystalline structure of HH samples was investigated using XRD (X’ Pert MPD PRO, Holland). The dielectric properties were measured by impedance analyzer (HP-4294A) at different frequencies.

SEM micrographs of the interfacial microstructure of ZT/NZC composite sintered at 900 ◦ C for 4 h are shown in Fig. 4. It can be seen that the interface is in good connection. An exaggerated grain growth was observed nearby the interface of the samples, and a few closed pores come into being in them. The layer thickness of exaggerated grain growth was estimated about 30–50 ␮m. The grains of rutile phase appeared in the ferrite region. From above results, it is believed that the interfacial diffusion and reactions occurred. The interfacial diffusion of Ti4+ in the dielectric region adjacent to the interface into ferrite led to the form of rutile. At the same time, the interfacial diffusion of Ti4+ resulted in the deviation from stoichiometric ZnTiO3 compound. The decomposition of ZnTiO3 then was induced, as shown in following reaction: ZnTiO3 → Zn2 TiO4 + TiO2

(1)

Zn2 TiO4 has a cubic spinel crystal structure, which is easy to be appeared when the sintering temperature is higher than 900 ◦ C in ZnTiO3 .

3. Results and discussion 3.1. Low-temperature sintering of ZT and NZC Figs. 2 and 3 show the density and shrinkage of ZT and NZC, respectively. The sintering temperatures of the zinc titanate ceramics are effectively reduced from 1100 to 875 ◦ C by V2 O5 and B2 O3 additions [12]. It can be seen from Fig. 2 that the density and shrinkage of ZT samples increased as the sintering temperature increasing from 875 to 900 ◦ C. After reaching a maximum value at 900 ◦ C, the density and shrinkage decreased with the increase of sintering temperature from 900 to 950 ◦ C. Then, as the sintering temperature increased to 1000 ◦ C, the densities of the ceramics enhanced again. The reason for the densification decrease can be attributed to the

Fig. 3. Density and shrinkage of NZC ceramics with Bi2 O3 additions vs. sintering temperature.

X. Liu et al. / Journal of Alloys and Compounds 470 (2009) 269–272

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Fig. 4. SEM micrographs of the interfacial structures of ZT/NZC composite: (a) NZC and (b) ZT.

To study the interfacial main element distribution, EDS analysis was done along the line shown in Fig. 4, and Fig. 5 shows the result. It can be seen from Fig. 5 that titanium, iron and nickel have a wide diffusion range. For titanium and iron, it is revealed that there are two plates regions of constant composition operated by a slopping region of changing composition. It is similar to the results observed in Gao et al.’s study [9]. The constant composition regions correspond to the ionic concentration in dielectric and ferrite. The sloping region is the ionic interdiffusion area across the interface between ZT and NZC. Variation of atomic distribution around the interface indicates new compound or new phase may form during cofiring. 3.3. Interfacial reaction To study the interfacial reactions during sintering process, ZT, NZC and HH were sintered at 900 ◦ C for 4 h. Fig. 6 shows the XRD patterns for ZT, NZC, and HH composites. It can be seen that the characteristic peaks of ZnTiO3 disappeared in HH, at the same time, TiO2 appeared. Considering the variation of atomic distribution around the interface, it can be concluded that interfacial reactions occur during sintering process between NZC and ZT cofiring system. Analyzing the XRD pattern of HH, it is found that a new spinel-type composition formed, which was not spinel NZC or spinel Zn2 TiO4 . The d-spacing values of NZC, HH and Zn2 TiO4 are shown in Table 1. The d-spacing and cell parameter values of HH were found close to NZC and Zn2 TiO4 . So it can be concluded that the new phase in

Fig. 5. Element distribution at the interface in sintered samples.

Fig. 6. XRD patterns of ZT, NZC and HH sintered at 900 ◦ C.

HH is spinel-type solid solution of NZC and Zn2 TiO4 . The reactions can promote interfacial combination. According to Fig. 4 and above analysis, the layer structure of ZT/NZC can be shown schematically as in Fig. 7. The layer of interfacial diffusion and interfacial reaction

Fig. 7. Schematic interface layer structure of NZC and ZT cofiring samples.

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Table 1 The d-spacing and cell parameter of NZC, HH and Zn2 TiO4 Composition

Crystal structure

˚ d-Spacing (A)

NZC HH Zn2 TiO4

Spinel Spinel Spinel

2.97733 3.00814 2.9948

˚ Cell parameter (A) 2.53450 2.56115 2.5539

1.70793 1.72851 1.7290

1.61325 1.62900 1.6301

1.48004 1.49547 1.4974

8.364 8.422 8.470

be attributed to that the interfacial diffusion and interfacial reaction became more severe when the sintering temperature was increased. In addition, Fig. 8 indicates a decreasing tendency of all the dielectric constants with increasing measuring frequency. Even at a high frequency, the εr value of cofiring samples is presumed to be about 13–16. Similarly, the dielectric loss tangent of cofiring samples decreased gradually with frequency increasing. NZC/ZT layer composite sintered at 900 ◦ C exhibited the dielectric properties: εr = 15.1, tan ı = 2.91 × 10−3 . Moreover, much smaller tan ı value at a high frequency can be presumed from the decreasing tendency of tan ı with the increasing value of measuring frequency [13]. 4. Conclusions In summary, the multilayer components were prepared by cofiring ZnTiO3 dielectrics and NiCuZn ferrite. The cofiring interface and ionic interdiffusion between the constituents were investigated. And the ionic diffusion across the interface was observed by SEM and EDS. Iron and titanium diffused more significantly. Interfacial reactions occurred in ZT/NZC cofiring system, which can strengthen combination between dielectric layer and ferrite layer. Due to their dielectric performance, low-firing characteristics, and realizable cofiring compatibility, ZnTiO3 dielectrics/NiZnCu hexagonal ferrite can serve as the promising medium materials in the multilayer LC filter. Acknowledgments The work was supported by National Natural Science Foundation of China (Project 60501015) and the Doctorate Foundation of Northwestern Polytechnical University under Grant CX200408. References Fig. 8. Dielectric properties of NZC/ZT cofired composite.

was constructed by two layers; the one composed by ZnTiO3 , spineltype solid solution and rutile was close to ZT, the other was close to NZC, composed by NZC, spinel-type solid solution and rutile was close to ZT. This interface layer structure can enhance interfacial combination. 3.4. Dielectric property Fig. 8 shows the dielectric properties of NZC/ZT cofired composite. The dielectric constants decrease and the dielectric loss tangent increase with the sintering temperature increased. This result can

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