Microstructure characterization and properties of chemically synthesized Co2Z hexaferrite

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Abstract

Powders having stoichiometric composition of Co2Z (Ba3Co2Fe24O41) were prepared by gel self-propagating method and processed at different temperatures to study phase evolution process. Unlike other ferrites, Co2Z hexaferrite was not synthesized directly by gel self-propagating technique, but was formed gradually from some intermediate phases due to its complicated crystalline structure. After Z-type phase formed at relatively low temperatures of 1200°C, the powders were annealed at temperatures lower than 1200°C. It was found that, once formed, this phase was quite stable. At the same time, the morphology and distribution of hexagonal particles was improved, and ceramic samples thus achieved from the relative powders presented excellent ultrahigh frequency magnetic properties.

Introduction

Recently, with the rapid development of information and communication technology, multi-layer chip inductors (MLCI) applied in the hyper frequency regions have been rapidly developed as surface mounting devices (SMD).1, 2 MLCI is produced by coating ferrite and internal electrode pastes alternately and then co-firing. Since the multi-layer technology becomes mature, the properties of MLCI largely depend on the properties of the low temperature sintered ferrites. However, the low-fired ferrite has been a key problem of MLCI production. Much work has focussed on NiZn system ferrite3, 4, 5 applied in frequency region of less than 300 MHz. As to hyper frequency regions (300–1000 MHz), there has been no ideal material so far. As an attempt, Co2Z hexaferrite is expected to fill in this gap.

Crystallographically, Co2Z (Ba3Co2Fe24O41) comprises basic units of hexagonal barium and cubic spinel ferrite in certain combination like other hexaferrites, such as BaM, Co2Y and Co2W.6, 7 Due to the sharing of many peak positions, the signals of small amounts of these compounds are extremely difficult to distinguish from the major phases in the very convoluted and similar XRD spectra of the hexagonal ferrite.8 Studies on phase evolution of Co2Z become extremely important.

However, its special plain-hexagonal structure is very complicated and only formed at rather high temperature6, 7 (1200–1400°C). Furthermore, the purity and stability of Z-type phase during sintering at low temperature is also a key problem.

In order to meet the increasingly demand for hyper-frequency MLCI, developing Co2Z system ferrites become increasingly urgent. Various chemical methods have been proposed and proved to be effective to reduce the sintering temperature and get good properties for hexaferrites, such as BaM (BaFe12O19), Co2Y (Ba2Co2Fe12O22) and Co2W (BaCo2Fe16O27).9, 10, 11 But for Co2Z, there has no been an effective way to prepare Co2Z at low temperature. The authors developed a novel and economical method, namely of gel self-propating technique, to prepare ultra-fine and highly reactive powders. Consequently, the powders were used to synthesize Co2Z ferrite at relatively low temperatures. Furthermore, a few issues involved in phase evolution, characterization and stability of Co2Z hexaferrite were mainly discussed in this paper.

Section snippets

Powder preparation

All reagents were chosen AR grades.

Stoichiometric amount of the starting materials including barium nitrate, cobalt nitrate, iron nitrate and citric acid monohydrate in proper molar ratio were dissolved in distilled water, respectively. Then the former salt solution was dissolved in the citric acid solution: a few minutes of moderate heating and actuating were necessary. When the blended solution cooled to room temperature, it was titrated quantitatively by concentrated ammonia solution until

Phase evolution and characterization

The XRD patterns of gel and powders treated between 700 and 1200°C are shown in Fig. 1. As to the self-propagating powder, a few kinds of single compounds can be identified as BaFe2O4, CoFe2O4 (spinel ferrite), α-Fe2O3 and this situation remained up to 900°C or so until BaM phase formed. However, at 1100°C the peaks of most BaM have been either replaced or hidden by the appearance of small amount of Co2Y. The coexistence of these two phases of BaM and Co2Y continued up to about 1180°C. By

Conclusions

  • 1.

    Unlike spinel ferrites, Co2Z hexaferrite with planar hexagonal structure could not be synthesized directly by gel self-propagating method, but could be formed gradually by reaction between some intermediate phases, such as spinel, BaFe2O4, BaM and Co2Y.

  • 2.

    Co2Z hexaferrite can steadily exist and take on more homogeneous distribution in typical hexagonal particle when subjected to different annealing treatment, although the grain size increased obviously. This makes it hopeful to be adopted in

Acknowledgements

The authors were indebted to the financial support from the High Technology and Development Project of PR China (Grant: 715-Z33-006-0050), National Natural Science Foundation of PR China (Grant: 59995523) and the Ceramic Technology Center, Motorola, Inc., USA.

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