Preparation of hexaferrite multilayers by tape casting

Tape casting technics were used to realize a screen with continuous change in characteristics. Different hexaferrite compositions owned by the Ba3C02-~Zn~Fe24041 (Z) family were synthetized by coprecipitation method and calcined at 1200 "C in air. Then a slurry containing this powder is slip casted using tape casting. The tapes are dried under magnetic fied in order to orient the magnetic platelet-shaped particles. Multi-sheets realized with different tapes by hot pressing (65 "C, 4 MPa) are sintered at 1270 "C in air. The coefficient of reflexion measured on single composition multi-sheets (CO~Z, Col.5Zn0,~Z or CoZnZ) is compared to the computed value of a multi-composition multi-sheet which is C02Z/C0l ~ g z ~ ~ g z / c o z n Z . 1Introduction. All the hexafemtes of Z family (Ba3C02-~Zn~Fe24041) present a hexagonal cristallie structure. The x value has an important effect on the magnetic anisotropy, on the resonance fiequency and on initial permeability. All compositions have an easy plane of magnetization perpendicular to the c axis. So it is very interesting to align this easy plane parallel to the tape to improve permeability of the material. If tapes corresponding to Werent x values in hexafemte formula are superimposed and co-sintered, the coefficient of reflexion is expected to cover a broader frequency range. aSvnthesis of the herafememte ~owder. The studied Z hexafemtes correspond to x between 0 and 1. The particles are prepared by coprecipitation chemical method. This process leads to particles with contolled size and morphology. This technics involves the dissolution of metallic salts (FeCl3, CoC12, ZnC12, BaC12). A mixture of iron oxides, hydroxides and carbonate precipitates in an alkaliie bath consisting of Na2C03. The obtained powder is washed in water to eliminate residual chlorides. Then, the particles are calcined at 1200 OC in air in order to transform into oxide. Z and Y hexafemte phases are observed by X-ray diffraction (figure 1) on powder after calcination. Particles present a platelet shape as observed by SEM (figure 2). Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:19937123 JOURNAL DE PHYSIQUE IV bPreuaration of the slum. Hexaferrite particles (71.8 WOh) with dispersent (Emphos PS21A, 0.5wOh) are ball-milled for 24 h in solvent (MEK/EtOH, 28.2 WO?). A second stirring is achieved during 3 hours after addition of binders and plasticizer (Rhovinal + DBP + PEG, 11.9 w.%). Then, the balls are removed and the slurry is deairing slowly during at least 1 hour. cSku castinf aeneral urinciules. The general principles of the process are schematized on figure 3. Taue casting. The slurry is casted on a capton support using a classical tape casting apparatus (figure 4). Tape thickness depends on container moving speed and on depth of adjustable slit. The different parameters used in this work are the following (table 1): This technics allows us to align hexafemte particles by a mechanical effect. Orientation degree as high as 70 % (X-Ray determination method) is obtained at this stage. table 1: parameters of slip casling experiments. dMapnetic orientation. Tape width 60to315 mm The tape is dried under a rotating magnetic field, just after casting, in order to improve the orientation of hexaferrite platelets. The magnetic field intensity is about 0.06 T and the exposure time exceeds 1 hour. After drying, the orientation degree attains 98 %. eRealization of the multi-sheets. Depth of slit 0.1 to 2 mm Five to ten tapes (300 pm thick each) of one or different compositions are hot pressed (65OC, 4 MPa) to obtain a multi-sheet of a few millimeters thick. Moving speed 1.85 cm/s fSintmenna Multilayers are sintered in air at 1270 OC in order to transform into Z hexaferrite phase. 3Characterization of the multi-sheet. The density, measured by hydrostatic method in bromobenzen, is about 88 % of theoritical density. Interfaces are no longer observed by optical micrography after sintering. In the case of multicomposition materials, microprobe analysis shows a continuous variation of composition (figure 5). The coefficierit of reflexion is measured in an APC7 coaxial line on a toroldal sample placed in front of a short-circuit. The computed attenuation of a multi-sheet CoZnZICo 1.5Zn0.5lCo2Z is compared with measured values on single composition (figure 6). The computed curve corresponding to a multi-composition materials with each layers at matching frequency thickness (Co2Z: 1.82 mm, Col.SZnO.5Z: 2.12 rnm, CoZnZ: 2.73 mm) covers the curves of the single-compositions. Nevertheless, the absolute value of attenuation is lower. It will be very interesting to perform calculation from other values of thickness (different from matching frequency thickness) of singlecomposition samples in order to optimize the multi-composition material. 4Conclusion. In this paper we have shown that we can obtain well-oriented hexaferrite multi-composition multisheets with tape casting method followed by drying under magnetic field. A gradient of composition was observed in the material thickness. Tape casting technics could be successfully applied to elaboration of many other kinds of fimctionnally gradient materials. 786 JOURNAL DE PHYSIQUE IV Figure 1 : X-Ray diffraction pattern on hexafemte powder after calcination. Figure 2: SEM micrograph of hexafemte particles. I POWDER SYNTHESIS I I I SLURRY PREPARATION I I 1 R I SLIP CASTING I I MULTISHEET PRESSING I Fimre 3: Slip casting generd principles