Dissolution of Alumina, Sintering, and Crystallization in Glass Ceramic Composites for LTCC Ralf Mu¨ller,* ,w Robert Meszaros, z Burkhard Peplinski, Stefan Reinsch, Markus Eberstein, y and Wolfgang A. Schiller BAM Federal Institute for Materials Research and Testing, 12484 Berlin, Germany Joachim Deubener Clausthal University of Technology, Institute of Non-Metallic Materials, 38678 Clausthal-Zellerfeld, Germany Sintering and microstructure evolution of alkali-free calcium– alumo–borosilicate glass/a-Al 2 O 3 composites (mean particle size ca. 2 lm) for low-temperature cofired ceramics were stud- ied during heating at 5 K/min by heating microscopy, thermal analysis (DTA), X-ray diffraction (XRD), and electron micros- copy (SEM). Composites fully densify at  8301C, not essen- tially influenced by the dissolution of alumina and glass crystallization. Thus wollastonite, as first crystalline phase, was detectable at 8401C. Above 9001C, a pronounced crystal- lization of anorthite is evident, reaching 60 wt% at 10501C. Rietveld analyses of XRD data revealed that anorthite precip- itates at the expenses of alumina, which declines from  33 to o10 wt%, and wollastonite, which fully declines from its max- imum of  19 wt%. Based on XRD, we discuss the evolution of crystal mass fractions, the residual glass composition, the glass viscosity, and the effective shear viscosity of the composites under study during heating. I. Introduction L OW temperature cofired ceramics (LTCC) are progressively used in ceramic packaging of microelectronic components for telecommunication, automotive, and medical applications. 1–3 Many LTCC are based on boro-silicate glass ceramic compos- ites (GCC) containing up to 55 vol% of dispersed ceramic particles. The glass matrix easily enables low temperature cofiring with low-resistance metals such as silver, glass crystal- lization is utilized to ensure postfiring stability, and the large variety of utilizable glasses and ceramic fillers in different ratios permits the continuous tuning of LTCC properties in wide ranges. 4 This diversity, as well as the complex sintering and crystalli- zation phenomena during GCC firing, however, makes it often difficult to match the desired processing requirements and ma- terial properties. Driven by the need for a targeted approach, related research has been intensified in recent years, addressing aspects like glass viscosity modeling, 5 mechanism of sintering, 6 modeling of constrained sintering, 7–9 self-constrained LTCC, 10,11 steric effects of dispersed alumina on sintering, 12 crystallization of LTCC glasses, 13–17 and silver dissolution. 18,19 These aspects can be additionally influenced by the dissolu- tion of dispersed alumina particles and related phase boundary reactions. Such phenomena were demonstrated by Schiller et al. 20 for K 2 O–B 2 O 3 –SiO 2 glass/alumina composites, in which potassium-enriched phase-boundary layers of a few micrometer in thickness surrounded the alumina particles after firing at 9251C for 5 h. Alumina dissolution was later utilized by Jean et al. 14 to inhibit cristobalite crystallization in (wt%) 4Na 2 O– 2Al 2 O 3 –13B 2 O 3 –81SiO 2 glass/alumina composites possessing mean particle sizes of D G  7 mm and D C  3 mm for glass and alumina, respectively. Imanaka et al. 15 also prevented the crys- tallization of cristobalite by alumina dissolution for (wt%) 0.2 CaO–0.4K 2 O–3.8Na 2 O–2.2Al 2 O 3 –12.9B 2 O 3 –80.5SiO 2 glass/al- umina composites (D C  3 mm, D G  4 mm). He found an Al-en- riched layer thickness of  3 mm after firing at 8001C for 1 h, which corresponds to an aluminum diffusion coefficient of  10 12 cm 2 /s. Park and Lee 17 observed aluminum diffusion distances of 30 mm in (wt%) 1.1K 2 O–6.3Na 2 O–2.4Al 2 O 3 –17.5 B 2 O 3 –72.1SiO 2 glass powder compacts (D C  4 mm, D G  1.2 mm) sintered at 9001C for 10 min on alumina substrates. Al- umina dissolution can even hinder densification, as reported by Fang and Jean 21 for K 2 O–CaO–SrO–BaO–B 2 O 3 –SiO 2 glass/al- umina composites (not fully disclosed glass composition, D G  2.5 mm) in the case of  20 vol% of very small alumina particles (D C  0.05 mm). Although alkali metal ions have been addressed in Jean et al., 14 Fang et al., 21 and Schiller et al. 20,22 for the governance of alumina dissolution and Al-enriched layer formation, Tram- osljika et al. 19 detected substantial alumina dissolution in CaO– Al 2 O 3 –B 2 O 3 –SiO 2 glass/alumina composites (not fully disclosed glass composition). Rietveld analysis of X-ray diffraction (XRD) data of samples, heated at 0.5–10 K/min to selected temperatures and quenched in air, revealed that the alumina weight fraction decreased from 36 to 30 wt% between 8201 and 9001C. Because previous studies refer to differently composed glasses, different particle sizes, and different annealing sched- ules, no general conclusion can be made about the influence of alumina dissolution on sintering and crystallization. The present paper aims to elucidate these effects in the case of alkali-free CaO–Al 2 O 3 –B 2 O 3 –SiO 2 glass/alumina composites for LTCC application. II. Experimental Procedure (1) Samples For the present study, we prepared composites of 75 vol% al- kali-free 38CaO–4.7Al 2 O 3 –7.3B 2 O 3 –50SiO 2 glass (mol% batch) and 25 vol% commercial alumina powders (Martinswerk, Berg- heim, Germany, DS-6, D C  5 mm, r C  3.96 g/cm 3 , 99.8 wt% Al 2 O, o0.1 wt% Na 2 O). Glass batches were inductively melted in 2.4 l Pt crucibles at 14301C for 6 h (EMA-TEC, Sondershau- sen, Germany). The melt was then poured onto a steel mold and subsequently quenched in water. J. Ferreira—contributing editor Supported by the Deutsche Forschungsgemeinschaft (DFG). *Member, The American Ceramic Society. w Author to whom correspondence should be addressed. e-mail: ralf.mueller@bam.de z On a trainee leave from Faculty of Materials Technology, Georg-Simon-Ohm-Univer- sity of Applied Science, 90489 Nu¨ rnberg, Germany. y Present address: W. C. Heraeus GmbH, 63450 Hanau, Germany. Manuscript No. 25480. Received November 11, 2008; approved March 11, 2009. J ournal J. Am. Ceram. Soc., 92 [8] 1703–1708 (2009) DOI: 10.1111/j.1551-2916.2009.03089.x r 2009 The American Ceramic Society 1703