Elsevier

Ceramics International

Volume 42, Issue 15, 15 November 2016, Pages 17303-17309
Ceramics International

Electromagnetic properties of ferrite tile absorber as a function of compaction pressure

https://doi.org/10.1016/j.ceramint.2016.08.026Get rights and content

Abstract

In this study, the influence of compaction pressure on the electromagnetic properties of Cu-doped NiZn ferrites was investigated. The evolution of the imaginary part – μ″ of the complex magnetic permeability was analysed as a function of compaction pressure of the green bodies, and the relative density and average grain size of the sintered pieces. The results show that μ″ highly depends on the sintered microstructure and that there is threshold value of the average grain size (~20 to 25 µm) as of which the electromagnetic properties of these kinds of materials significantly worsened. A linear relationship was observed between permeability and grain size, taking into account the two magnetization mechanism contributing to the complex permeability: spin rotation and wall motion, and an empirical polynomial equation has been proposed to satisfactorily predict the evolution of the imaginary part – μ″ of the complex magnetic permeability with a given compaction pressure and sintered relative density.

Introduction

Ferrites are iron-containing, non-conducting ceramics which exhibit magnetic properties. The general formula is MFe2O4, where M is one of the following divalent ions (or a combination thereof): zinc, nickel, copper, manganese, etc. [1].

Polycrystalline ferrite has been extensively used in many electronic devices because of its high permeability in the RF frequency region, high electrical resistivity, mechanical hardness and chemical stability [2]. One of the main applications of the Cu-doped NiZn soft ferrites is the production of specimens which prevent possible interferences between electronic devices, allowing their correct operation without either disrupting, or being disrupted by, any electronic devices in their immediate surroundings [3], [4]. They are also used for the fabrication of anechoic and semi-anechoic chambers [5], [6].

The value of the imaginary part of the relative magnetic permeability (μ″), within the material’s operative frequency range, is one of the parameters used in the quality control of the electromagnetic properties during production of this type of soft ferrite [7]. This parameter not only depends on the composition of the synthesized ferrite [8], but also to a great extent on the microstructure of the final piece [9], such as its relative density or final porosity, grain boundary nature, pore size and grain size distribution, etc. [10], [11], [12], [13], [14]. Indeed, this microstructural dependence is so critical that an excellent raw material may result in a final piece with very poor magnetic properties if, for example, the microstructure of this final piece ends up being heterogeneous in microstructure, or if a second phase precipitates at the grain boundaries [15]. Meanwhile, green processing can significantly affect the microstructure and microstructure evolution of the green bodies, meaning a green heterogeneous compact will undergo more heterogeneous densification and microstructure development during sintering [16].

The overall goal of the green-forming method is to produce a bulk particulate compact with a green density that is suitable for achieving the desired fired density, sufficient strength to survive the firing process and as few flaws as possible [17]. An accurate control of the ferrite microstructure and a good understanding of how these microstructural parameters change with the modification of the processing variables is imperative for the formation of homogeneous green bodies of high green density. This understanding would also allow the design of sintering conditions that would enable retention of the fine grain microstructure, achieving near theoretical density.

In a previous paper [18], we studied the effect of the variation of two thermal cycle parameters, sintering temperature and sintering time, on the value of the imaginary part of Cu-doped Ni-Zn ferrites. In this paper, we build on this research by systematically analysing the effect of varying compaction pressure on the green microstructure and microstructure evolution during sintering, and the effect of these latter on the value of the imaginary part – μ″ (at a frequency of 107 Hz). This knowledge could shed light on the microstructure–properties relationship of these ceramic materials, information that can undoubtedly be of great relevance to improve the performance of these electromagnetic absorbers.

Section snippets

Experimental procedures

The raw material used in this work was the same as that used in the aforementioned previous paper [18]. The chemical composition of the ferrite was (Cu0.12Ni0.23Zn0.65)Fe2O4. The ferrite powder, supplied by Fair-Rite Products Corp., consisted of granules with an average size of 175 µm, made up of particles with an average size of 2.1 µm, and narrow particle-size distribution (around 4 µm), and with a true density of 5380 kg/m3 (experimentally determined on a helium pycnometer). As before, ferrite

Results and discussion

The SEM micrographs in Fig. 1 show, at 100× magnification, the morphology of the green ferrite microstructures, compacted at the six maximum pressures established for the study. As can be observed, only the higher pressures are high enough to allow the breaking down of the spray-dried granules. A higher magnification of the samples (Fig. 2 at 4000×) revealed a thin and highly homogeneous particle size distribution of the ferrite powder, with an average particle size of around 1–2 µm.

Fig. 3

Conclusions

In this study, the influence of compaction pressure on the imaginary part – μ″ of the complex magnetic permeability was systematically analysed. For each given sintering time, increases in μ″ were observed for increasing compaction pressures was in the 50–150 MPa range, while an almost constant value was obtained for higher compaction pressures. It was also observed that μ″ increases with increasing G50 (regardless of the compaction pressure) up to G50 values of around 20–25 µm, defining a single

Acknowledgements

This study has been conducted with funding from the project P1·1B2012-13, within the scope of the Pla de Promoció de la Investigació de la Universitat Jaume I 2012.

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