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In this work, the influence of SiO2 additions in leucite ceramics on the bulk linear thermal expansion coefficient (TEC) especially during the phase transition, has been studied. Thermal expansion and x-ray diffraction measurements at high temperatures were carried out to characterize the tetragonal-cubic phase transition. TEC for reference and SiO2added leucite samples exhibited similar behavior as a function of temperature. Before and after the phase transition, the TEC values were similar to those observed in non-SiO2added samples, whereas during the phase transition, a maximum TEC value was observed and it tends to decrease as the SiO2 addition increases. This behavior could be caused by the formation of an intermediate phase with an extremely high TEC (70
10–6  C1) during the phase transformation. Furthermore, the results suggest that as the intermediate phase is partially suppressed via SiO2 addition, the cubic phase can be partially stabilized at temperatures as low as 200  C.

I. INTRODUCTION

Leucite powders (KAlSi2O6) have been extensively used in dental applications to reduce the mismatch between the bulk linear thermal expansion coefficient (TEC) of porcelain and metal reinforcement,1–7 thus making their fusion and the enhancement of mechanical strength possible.8 However, leucite powders have rarely been used as a bulk material, mainly because they exhibit a phase transition with different TEC values.8,9 It is reported that the tetragonal leucite (I/41a, No88, a = 13.090, and c = 13.750) with TEC  20
10–6  C1 is stable from room temperature (RT) up to approximately 600  C,5,8,10 whereas cubic leucite (Ia-3, No206, a = 13.574), with TEC  8
10–6  C1, is stable over 650  C.8,10,11 Leucite structure is composed of an aluminosilicate framework, which is a linkage of four- and six-membered rings of the (Si,Al)O4 tetrahedra. The framework contains cation sites of two different sizes: one has 12-coordination (W-sites) and the other has six-coordination (S-sites).9 Normally, K+ ions occupy the W-sites, but depending on the nature of the sample, synthetic or natural, sometimes K+ ions can be slightly displaced from the W-sites, changing the thermal expansion coefficient in leucite ceramics. A representation of cubic leucite structure is shown in Fig. 1. The key feature of this complex structure is the channels parallel to directions.11 a)

Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0241 J. Mater. Res., Vol. 24, No. 6, Jun 2009

http://journals.cambridge.org

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Some works report the existence of an intermediate tetragonal phase (ITP) during the phase transition9,11,12; however, there are no reports about physical or mechanical properties of this phase. The identification of this ITP by diffraction techniques, especially by XRD, is difficult because the main difference with the tetragonal I41/a is the ordering of the Si-Al tetrahedra; Gro¨gel et al. found the existence of this ITP by neutron diffraction of a natural leucit