Microstructure study and hyper frequency electromagnetic characterization of novel hexagonal compounds

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Abstract

The physical characteristics and preparation of novel Z-type hexagonal compound, which have iron deficient composition of Ba3Co2(0.8−x)Cu0.4Zn2xFe23.5O41 (x≤0.30), was investigated. The results show that Zn has little effect on the morphology of compact and homogeneous ferrites with average 10–15 μm of lathy grain size, but make the formation temperature range narrow. In the range of Zn incorporation, the hexagonal cell lattice parameters (a and c) of ferrites reserve stable. However, the key magnetic and electrical properties, such as initial permeability, quality factor, resistivity, dielectric constant and magnetic hysteresis as well as Curie temperature were also characterized. The mechanisms involving in generating these variations were also discussed in this paper.

Introduction

In modern cost-driven applications, the use of miniaturization and lamination is desirable, which demands the use of low temperature sintering in fabrication of electronic components, such as inductors, chip beads, bead arrays, LC filters, etc. [1], [2], [3]. However, the traditional ferrites are far from to meet the property requirements of high performance, miniaturization size, light weight and low cost. For example, the magnetic loss of Ni–Zn–Cu system ferrite drastically increases in the range over 100 MHz [4], [5], which is well known as Snoek's limit [6]. This means that it is impossible to use this system ferrite as inductor materials in 300–1000 MHz region.

In order to solve this problem, ferroxplana type ferrites have been studied and developed. Among ferroxplana type ferrites, Z-type ferrite whose formula is Ba3Me2Fe24O41 (Me means divalent metal ions) has a hexagonal structure and a high resonance frequency of 3.4 GHz, duo to its basal plane anisotropy and large anisotropy field [7], [8]. Furthermore, Co2Z(Ba3Co2Fe24O41) has excellent performance (high initial permeability, high quality factor and low loss tangent) in hyper frequencies of 300–1000 MHz. Traditionally, the formation of Ba3Co2Fe24O41 involves in high temperature sintering up to 1300 °C. However, for lamination fabrication, it requires Z-type ferrite pastes be co-sintered at a relatively low temperature of 850–900 °C together with internal electrode materials (Ag or Ag–Pd alloy). How to lower the sintering temperature of Z-type ferrite has become of a key problem.

From the crystallographic point of view, Co2Z ferrites are among the most complex compound in the family of ferrites with planar hexagonal structure. The unit cell of a Co2Z ferrite contains 140 atoms and belongs to the P63/mmc space group. The hexagonal cell parameters, planar a and axial c, are 0.588 and 5.230 nm, respectively. The complexity of the structure mainly results from large Ba2+. Since the radii of Ba2+ ions are comparable to the O2− radius; they prefer the oxygen positions rather than the interstitial sites. Metal ions (Fe3+, Fe2+, Co2+, Cu2+ and Zn2+), however, are located in non-equivalent interstitial sites. Theoretically, the Co2Z can be treated as a sum of two simpler ferrites, namely of M-type (BaFe12O19) and Y-type (Ba2Co2Fe12O22). The extremely large elementary cell and the presence of ‘strongly anisotropic’ Co2+ ions also lead to the complexity of its magnetic properties [7], [8], [9].

45w>Zn, as a sole constituent, can effectively improve the properties of Z-type ferrite, but the sintering temperature of Ba3Co2(1−x)Zn2xFe24O41 is still high up to 1280 °C [7], [8]. The authors succeeded in incorporating Cu, as an effective constituent, into the Ba3Co2Fe24O41 and lowering the sintering temperature of Z-type ferrite to 1050–1130 °C [10]. In order to progressively improve Z-type ferrite progressively, just like Ni–Zn–Cu spinel ferrite, Zn and Cu were considered to incorporate at the same time. By this way, one kind of novel compound Ba3Co2(1−xy)Cu2yZn2xFe23.5O41, with low melting point and high performance should be expected to achieve. To facilitate the research, the optimal Cu content (y=0.20) was chosen in our work. To keep the soft magnetic characteristics and basal plane anisotropy of Z-type ferrite, the amount of Zn and Cu incorporation is generally less than 1.00 according to Jonker et al.'s research [8]. The iron deficient composition is used to improve electrical properties.

Section snippets

Sample preparation

Samples with the composition Ba3Co2(0.8−y)Cu0.4Zn2xFe23.5O41, where x=0.00, 0.05, 0.10, 0.15 and 0.20, were prepared by the solid-state reaction method. The raw materials, BaCO3, Co3O4, CuO, ZnO and Fe2O3, of high purity, were mixed in a ball mill for 4 h. The mixed powders were calcined at 1000–1050 °C/6 h in air. The resulting powders were pressed in a stainless-steel die under a pressure of about 40,000–60,000 N/m2 with 5 wt% PVA as lubricant. All pellet and toroidal samples were sintered at

Densification characterization

As mentioned above, sole Cu modified Z-type ferrite has been studied by choosing the sintering temperature and Cu content as experimental variables. A sintering study on the dwelling temperatures and amounts of Zn (fixed heating rate 4 °C/min and sintering temperature 1120 °C) was carried out to optimize the sintering density. The density data are plotted in Fig. 1.

As seen in Fig. 1, by incorporating Zn into ferrites with Cu modification, a high sintering density can be obtained at relatively low

Conclusions

  • 1.

    One kind of novel Z-type ferrite with the formula Ba3Co2(0.8−y)Cu0.4Zn2xFe23.5O41 can be synthesized at the temperature range of 1100–1130 °C. The calculated cell parameters of ferrites also show typical Z-type phase structure.

  • 2.

    The novel Z-type ferrite compound presents the compromise magnetic and electrical properties as following: initial permeability of about 9.0, quality factor of average 20, cut-off frequency of above 1 GHz, resistivity of about 3.21×107 Ω cm and dielectric constant of 19.

The

Acknowledgements

The authors are grateful to financial supports from DARPA (grants: DAAD16-00-C-9273, SPAWAR N66001-00-1-8928 and MDA 971-97-1-0003) and the Ceramic Technology Center, Motorola Inc., USA.

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