International Journal of Inorganic Materials 3 (2001) 1189–1192

Liquid phase sintering of NiCuZn ferrite and its magnetic properties

*Yuh-Ruey Wang , Sea-Fue WangDepartment of Materials and Minerals Resources Engineering National Taipei University of Technology, 1, Sec. 3, Chung-Hsiao E. Rd., Taipei,

Taiwan

Abstract

The influence of glass additives on the densification, microstructural evolution and magnetic properties of NiCuZn ferrites wereinvestigated. Two glass systems including PbO?SiO and PbO?B O were used to reduce the sintering temperature of a NiCuZn ferrite.2 2 3

The PbO?SiO glass system was shown to be an effective additive to obstruct the grain boundary movement and prevent the abnormal2

grain growth. In addition, it significantly lowers the sintering temperature of the ferrite. Ferrites sintered with PbO?SiO have higher2

resistivity, higher Q and higher H compared with those of PbO?B O systems. These results also indicated that the ratio of modifier toc 2 3

glass-former did not significantly change the densification behavior, microstructural evolution and magnetic properties of the NiCuZnferrite. 2001 Elsevier Science Ltd. All rights reserved.

Keywords: Soft ferrite; Liquid phase sintering; Magnetic properties

1. Introduction applications. Typical fluxes used for liquid phase sinteringfor NiCuZn ferrites include V O , Na O, PbO, Bi O , and2 5 2 2 3

The surface mounting devices, such as multilayer chip 3NiO?V O [5,6]. In order to investigate the influence of2 5

beads or inductors, have been developing rapidly for the glass additives on the densification, microstructuralelectronic applications. Among available materials, poly- evolution and magnetic properties of NiCuZn ferrites,crystalline ferrites have been used widely because of their different ratios of the modifier /glass-former for PbO?SiO2

high permeability in the RF frequency region, high electri- and PbO?B O were used as sintering aids for a commer-2 3

cal resistivity, and environmental stability [1]. Ag is cial NiCuZn ferrite. Effects of the flux systems on thesuggested as an internal electrode for multilayer ferrites densification behavior of the powders were investigateddue to its high conductivity that provides the components through sintering studies. Electrical resistivity and mag-with a higher quality factor, Q. Also, the Ag paste has netic properties such as permeability, quality factor andmuch lower cost compared with that of the Ag/Pd paste. hysteresis loop of the sintered parts were measured andTherefore, the ferrite is required to sinter at temperatures analyzed.below 9508C. There has been a growing interest in NiCuZnferrites for the applications of producing the multilayer-type chips mainly because these oxides can be sintered at 2. Experimental procedurerelatively low temperatures with a wide range of com-position [2]. In addition, NiCuZn ferrites have better A NiCuZn ferrite (Ni /Cu/Zn50.38:0.12:0.50) preparedproperties at high frequencies than MnZn ferrite and lower from the solid state reaction was used during this study.densification temperatures than NiZn ferrites. The powders have a particle size of 0.73 mm from SEM

2 21Several approaches were attempted to reduce the sinter- micrographs and surface area of 7.60 m g using BETing temperature, including (a) the reduction of the particle method. The 2PbO?SiO , 5PbO?SiO , 2PbO?B O , and2 2 2 3size of the ferrite [3], (b) the incorporation of dopants into 5PbO?B O glasses were prepared by mixing stoichio-2 3the ferrite lattice [4], and (c) the usage of sintering metric amounts of PbO and SiO or B O (Fisher Sci-2 2 3additives which change the sintering mechanism (liquid entific, reagent grade) for 24 h in a vibratory mill. Afterphase sintering) [2,5]. Among them, the use of sintering drying, these powders were transferred to a Pt crucible,additives was found to be an effective method in practical reacted at 8508C, and quenched. The powder was then

dry-milled and sieved though a 325 mesh screen.*Corresponding author. (Ni Cu Zn )Fe O powders were mixed with 30.38 0.12 0.50 2 4

1466-6049/01/$ – see front matter 2001 Elsevier Science Ltd. All rights reserved.PI I : S1466-6049( 01 )00123-4

1190 Y.-R. Wang, S.-F. Wang / International Journal of Inorganic Materials 3 (2001) 1189 –1192

wt% of 2PbO?SiO , 5PbO?SiO , 2PbO?B O , and 5PbO?2 2 2 3

B O , respectively, methyl alcohol and an iron ball ([552 3

mm) media, using a ball mill for 2 h. The slurries werethen dried in a 1508C oven for 24 h. Then the as-obtainedpowders were mixed with 3.5 wt% of a 15% PVA solutionand then pelletized into toroid- and disc-shapes using an

22uniaxial pressure of 5 tons cm . The samples were thenheat treated at 5508C for 2 h to eliminate the PVA,followed by sintering at temperatures between 850 and

2110508C for 2 h (heating rate5108C min ). Densities ofspecimens were measured by employing the liquid dis-placement method. Microstructure of sintered surfaceswere investigated using a scanning electron microscopy(Joel JSM-T330A) at 20 kV. For the magnetic properties,initial permeability (m) and quality factor (Q) weremeasured using a HP4194 impedance analyzer at a fre-quency of 1 MHz on toroid-shape samples. B–H curve is Fig. 2. Initial permeability (m ) at 1 MHz for ferrite with various fluxes1

performed using a magnetic hystersis loop tracer sintered at the temperatures ranging from 850 to 10508C.

(Yokagawa 3257) at a voltage of 20 V.

the densification behavior of NiCuZn ferrite. The micro-3. Results and discussion structure study indicates that an uniform microstructure

with a grain size ranging from 1 to 2 mm was observed forThe sintered densities of (Ni Cu Zn )Fe O NiCuZn ferrites sintered with 2PbO?SiO or 5PbO?SiO0.38 0.12 0.50 2 4 2 2

powders with 3 wt% of 2PbO?SiO , 3 wt% of 5PbO?SiO , glasses. However, for ferrites sintered with 2PbO?B O , or2 2 2 3

3 wt% of 2PbO?B O , or 3 wt% of 5PbO?B O sintered 5PbO?B O , the microstructures showed a rapid grain2 3 2 3 2 3

at temperatures ranging from 850 to 10508C are shown in growth for a limited number of grains up to (¯15 mm)Fig. 1. For pure (Ni Cu Zn )Fe O powders, it much larger than the average grain size population (of the0.38 0.12 0.50 2 4

requires sintering temperature of 12508C in order to order of 1.5 mm). It seems that PbO?SiO glass is an223achieve the density of 5.00 g cm (¯97.2% theoretical effective additive to obstruct the grain boundary motion

density). Powders with PbO?SiO , or PbO?B O additives and prevent the exaggerated grain growth.2 2 3

significantly reduce their sintering temperature down to Figs. 2 and 3 depict the initial permeability (m ) andi

10008C. The lower sintering temperatures are due to the quality factors (Q) at 1 MHz for ferrite sintered withliquid phase formation during sintering. Results in Fig. 1 various fluxes at the temperatures ranging from 850 toalso showed that the ratio of modifier /glass-former (i.e. 10508C. The results revealed that the initial permeabilityPbO:SiO , or PbO:B O ratio) did not significantly modify increases with the sintering temperature for all cases. It2 2 3

Fig. 1. Densification curves for the (Ni Cu Zn )Fe O powder sintered with different flux systems.0.38 0.12 0.50 2 4

Y.-R. Wang, S.-F. Wang / International Journal of Inorganic Materials 3 (2001) 1189 –1192 1191

Fig. 3. Quality factor (Q ) at 1 MHz for ferrite with various fluxes sintered at the temperatures ranging from 850 to 10508C.1

seems that neither the PbO:SiO nor the PbO:B O ratio This may be due to the low reactivity nature of PbO?SiO2 2 3 2

significantly impacted on the permeability of NiCuZn glass and the consequent fine grain size of the sinteredferrite. The increase in initial permeability is closely ferrite.correlated to the densification and grain size of the Table 1 presents the magnetic parameters from hyster-ceramics. Ferrites with higher density and larger average esis loops obtained at 258C for ferrites sintered withgrain size possess a higher initial permeability. An increase PbO?SiO and PbO?B O flux systems at 10008C. A2 2 3

in the density of ceramics not only results in the reduction decreasing tendency of coercivity (H ) and remanence (B )c r

of demagnetizing field due to the presence of pores but are observed for the samples with larger grains. It could bealso raises the spin rotational contribution, which in turn explained by the corresponding facility to domain wallincreases the permeability [7]. For a pure motions. Also, a larger saturation flux density (B ) iss

(Ni Cu Zn )Fe O material sintered at 12508C for observed for ferrites with an increasing average grain size,0.38 0.12 0.50 2 4

2 h, the initial permeability is 387 and the quality factor is which is due to the presence of a multidomain grain6.3 with an average grain size of ¯30 mm. Deducting structure and hence the increase in the number of dipolesfrom the effects of densification and grain size, the dilution per unit volume.of the initial permeability by the presence of fluxes isapparent. Also, the permeability could be diluted by thecontinuous second phases along the grain boundary [8].

As revealed in Fig. 3, the dependence of quality factor 4. Summaryon the sintering temperature behaved differently from thatof the permeability shown in Fig. 2. It appears that quality The PbO?SiO glass system is an effective additive to2

factors of the ceramics are not coincident with the sintered obstruct the grain boundary movement and can prevent thedensity values. The quality factor reduced significantly as exaggerated grain growth for (Ni Cu Zn )Fe O0.38 0.12 0.50 2 4

the sintered density raised. This is probably due to the ferrite. It reduces the sintering temperature of the ferritereduction of DC resistivity with sintering temperatures. and results in ceramics of high Q and larger H . Thec

Among the flux systems; the ferrites sintered with 2PbO? results also showed that the ratio of modifier to glass-SiO have the best quality factor, as indicated in Fig. 3. former (i.e. PbO:SiO , or PbO:B O ratio) did not sig-2 2 2 3

Table 1Magnetic parameters obtained from hysteresis loops for ferrites sintered with various flux systems at 10008C

Sample Sintering Density B (G) B (G) H (Oe)r s c23temperature (g cm )

3 wt% 2PbO?SiO 10008C/2 h 5.04 3049 3675 2.792

3 wt% 5PbO?SiO 10008C/2 h 5.04 2957 3635 3.172

3 wt% 2PbO?B O 10008C/2 h 5.06 2706 3680 1.792 3

3 wt% 5PbO?B O 10008C/2 h 4.99 2807 3696 1.962 3

Pure ferrite 12508C/2 h 4.94 2060 4241 0.41

1192 Y.-R. Wang, S.-F. Wang / International Journal of Inorganic Materials 3 (2001) 1189 –1192

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