Electric and Magnetic Properties of Low-Temperature Sintering Mn-Zn Ferrite by Addition of Lithium Borosilicate Glass

Abstract

Mn-Zn ferrites (spinel-type structure, cubic), which are sintered at 1250 to 1350 8C under a controlled partial oxygen pressure, are widely used in electronic applications such as transformers, choke coils, and noise ®lters because of their high permeabilities and low magnetic losses at high frequencies. Recently, low-temperature sintering of Mn-Zn ferrites have been developed to meet a demand for the miniaturization of electronic components [1]. The ferrite needs to be sintered at ,950 8C in order to co-heat with silver internal electrodes (T m 962 8C) during the manufacture of multilayer chip inductors and LC ®lters [2, 3]. The most important subject is the preparation of the reactive powder, which can be sintered at low temperatures. It has previously been shown that co-precipitated Mn-Zn ferrite powder treated for 30 min with a media agitation mill was highly sinterable; dense Mn-Zn ferrites could be fabricated by heating for 2 h at 900 8C in N2 [1]. However, the permeability at 100 kHz was 520 because of their small grain size ( 1.3 im). The purpose of the present study is to improve the electric and magnetic properties of low-temperature sintering Mn-Zn ferrites by adding lithium borosilicate glass. As-received co-precipitated Mn-Zn ferrite powder [4] (Toda Kogyo Co., Ltd.), with the composition of 51.4Fe2O3 24.3MnO 24.3ZnO (mol %) and lithium borosilicate glass (Senyo Glass Co., Ltd.), consisting of 3Li2O, 37B2O3 and 60SiO2 (mol %), were used as starting materials. To prepare reactive Mn-Zn ferrite powder, the former was ground for 30 min using a media agitation mill with zirconia balls (1 mm in diameter) and ethanol. The grinding conditions were the same as described in the earlier paper [1]. The latter, having a low melting point ( 850 8C) [5], was selected as a liquid-phase forming additive. It was ground and then screened (400 nominal open size). A weighed quantity of both powders was then mixed for 30 min in ethanol, using an agate mortar and pestle. The resulting wet powder was dried at 80 8C under a reduced pressure. Five specimens, denoted A through E, were chosen in this study (Table I). The specimens were uniaxially pressed into toroidal compacts (14 mm outer and 10 mm inner diameters) at 75 MPa. Densi®cation was performed for 12 h at 900 8C in N2; heating and cooling rates were 300 and 200 8C=h, respectively. Phases were identi®ed by X-ray diffraction (XRD) analysis using Mn-®ltered FeKa radiation. Bulk densities were measured by the Archimedes method and relative densities were calculated using the theoretical density (5:13 g=cm) of pure Mn-Zn ferrite with the same composition [1]. An electron probe micro-analyzer (EPMA), equipped with a wavelength dispersive X-ray (WDX) detector, was used for the microstructural observation and elemental distribution analysis of the sintered compacts. The sintered compacts with silver electrodes were subjected to the measurements of electrical resistivity r and complex impedance Z. The permeability i was measured in a magnetic ®eld of 0:8 A=m at 0.01 to 10 MHz with an impedance analyzer. The Curie temperature Tc was determined from the temperature dependence of i. All sintered compacts consisted of only Mn-Zn ferrites. The characteristics of Mn-Zn ferrites with and without glass addition are presented in Table I. The bulk and relative densities of ferrite A without glass addition were 4:96 g=cm and 96.6%, respectively. Ferrites with glass addition increased in density; ferrite B and ferrites C through E showed 97.5 and 98% of theoretical density, respectively. Microstructures were classi®ed into two groups: (1) ferrites A and B, and (2) ferrites C through E. The former consisted of ®ne grains (3 im) with small pores at grain boundaries (Fig. 1). On the other hand, the microstructures of the latter were