Correlation Between Anisotropic Green Microstructure of Spherical-Shaped Alumina Particles and Their Shrinkage Behavior Zongwen Fu, ‡ Pit Polfer, § Torsten Kraft, § and Andreas Roosen ‡,† ‡ Department of Materials Science, Glass and Ceramics, University of Erlangen-Nuremberg, Martensstrasse 5, Erlangen 91058, Germany § Fraunhofer-Institute for Mechanics of Materials IWM, W€ ohlerstrasse 11, Freiburg 79108, Germany Dimensional control is one of the basic problems in ceramic processing, especially for tape cast ceramic sheets which are used to build up multilayer structures for high-integrated com- ponents. Uncontrolled anisotropic shrinkage can cause geo- metrical distortion and circuit failure of multilayer ceramics during sintering. The understanding of the relationship between green tape microstructure and shrinkage anisotropy is of great importance for further miniaturization of multi- layer devices. In this study, alumina powders with spherical particle shape were used to cast green tapes. The microstruc- ture as well as the pore orientation and the shrinkage behav- ior were analyzed. According to the sintering theory, grain growth and pore elimination are the two most important mechanisms to describe sintering shrinkage. In this work, three-dimensional shrinkage behavior of tape cast alumina powders with spherical particle shape was investigated and correlated with pore orientation in the microstructure. Specifi- cally, the reason for the different shrinkage in z-direction compared to the lateral shrinkage is in focus. The study is based on experiments as well as on mathematical visualiza- tion. I. Introduction D URING sintering of tape cast products, which require high dimensional accuracy for the use of high-inte- grated components, an anisotropic shrinkage can be observed, 1–4 which is attributed to an anisotropic green microstructure. During tape casting, nonspherical particles are oriented caused by shearing and constrained drying 5–7 resulting in a higher sintering shrinkage in the thickness direction than in the lateral direction. However, samples composed of spherical particles also exhibit anisotropic shrinkage behavior during sintering. 8,9 In this study, tapes composed of almost spherical particles were investigated experimentally and analytically. To achieve a better under- standing for the reason of the higher shrinkage in the z-direction compared to the lateral shrinkage, the shrinkage behavior was analyzed after binder removal and sintering at different temperatures in all three spatial directions. The correlation between anisotropic shrinkage and microstruc- ture of the green tape concerning pore orientation was discussed. II. Experimental Procedure (1) Slurry Preparation, Tape Casting, and Thermal Treatment A nearly spherical alumina powder SP (d 50 : ~3 lm; Sumito- mo Chemical Co., Ltd., Tokyo, Japan) with a narrow Gauss- ian shaped particle size distribution (d 10 : ~2 lm, d 90 : ~8 lm) was used in this study. Table I shows the exact composition of the tape casting slurry used in this study and also the density of the compo- nents. The slurry was based on organic solvents containing 68 mass% ethanol and 32 mass% toluene. The solid load- ing of SP-powder in the slurry was kept at 28 vol%. Men- haden Fish oil (Kellogg Co., Buffalo, NY), polyvinyl butyral (B-98; Solutia Inc., St. Louis, MO) and alkyl benzyl phthalate (Santicizer S261A; Ferro Corp., Cleveland, OH) were used as dispersant, binder, and plasticizer, respectively. The slurry preparation and tape casting process were described in detail by Fu et al. 6 A silicon-coated PET film (Mitsubishi Plastics, Inc., Tokyo, Japan) with a thickness of ~100 lm was used as tape carrier. To obtain tapes with a homogenous tape thickness, a tape casting machine with a fixed double chamber casting head was used maintaining an invariable low hydrostatic pressure. 10 The front and rear doctor blades were adjusted to a gap height of 900 and 1100 lm, respectively, resulting in dried tapes with a thick- ness of 250  10 lm. According to Watanabe et al. 11 increasing shear rates above ~12.5 s 1 does not result in an increased particle orientation degree, thus the casting speed was set to 700 mm/min, resulting in a shear rate of ~13.0 s 1 . At this shear rate the slurry exhibits a viscosity of 3.7 Pas. The dried green tapes were removed from the PET carrier tape and cut to the desired sample dimension with a hot knife (Groz-Beckert KG, Albstadt, Germany) at 60°C. The samples were then placed on smooth, high-purity alumina setters (Ker- afol GmbH, Eschenbach, Germany) for binder burnout (BBO). Debinding took place in air with a heating rate of 2 K/ min up to 450°C for 1 h, followed by prefiring at 1600°C with a holding time of 1 h and a heating rate of 4 K/min. At this temperature the SP-powder exhibits a linear shrinkage less than 4%, which is sufficient to determine the anisotropy of shrinkage, but low enough to preserve the microstructure of the green tape for characterization; prefiring also gives the tape sufficient strength for the preparation of cross sections. To improve the densification, the prefired tapes were subsequently sintered at 1600°C for 10 h as well as at 1730°C for 5 h and for 15 h, respectively. To ensure free sintering the setters used during thermal treatments from 1600°C to 1730°C were coated with a thin layer of Al 2 O 3 parting sand with a mean particle size of 150 lm. The theoretical density of ceramic tapes before and after firing was calculated using the theoretical density val- ues shown in Table I. The bulk density of green tapes was determined by measuring the weight, the area by scanning the D. Bouvard—contributing editor Manuscript No. 36075. Received December 15, 2014; approved February 17, 2015. † Author to whom correspondence should be addressed. e-mail: andreas.roosen@fau.de 3438 J. Am. Ceram. Soc., 98 [11] 3438–3444 (2015) DOI: 10.1111/jace.13567 © 2015 The American Ceramic Society J ournal