AlN ceramics processed by aqueous slip casting
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Miranzo Instituto de Cerámica y Vidrio, Consejo Superior de Investigaciones Cientificas CSIC, Campus de Cantoblanco, 28049 Madrid, Spain
J.M.F. Ferreiraa) Department of Ceramics and Glass Engineering, Centro de Investigac¸a˜o em Materiais Ceraˆmicos e Compo´sitos, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal (Received 8 April 2005; accepted 10 March 2006)
Fully dense aluminium nitride (AlN) ceramics consolidated by aqueous slip casting were obtained after sintering at 1750 °C for 2 h. The obtained samples were characterized by Vickers hardness (1000 Hv), flexural strength (200 MPa) and thermal conductivity (115 W/mK). YF3 and CaF2 were used as sintering additives in total amounts ranging from 5 to 7 wt% in YF3/CaF2 weight ratios of 1.25, 1.5, and 2. The compatibility between the thermochemically treated AlN powder and the sintering additives in the aqueous suspensions was investigated by rheological and zeta potential measurements. Highly concentrated (50 vol%) and well-dispersed suspensions could be prepared and used to consolidate homogeneous green bodies by slip casting. The phosphate species used to protect the surface AlN particles against hydrolysis could no longer be detected at temperatures higher than 1400 °C. X-ray diffraction results revealed that all the new crystalline phases formed upon sintering belong to the system Al–Y–O–Ca, but their specific compositions as well as the thermal properties of the sintered materials were shown to depend on the total amount and ratio of sintering aids.
I. INTRODUCTION
Aluminium nitride is an interesting ceramic material due to its good thermal conductivity, low-dielectric loss, and thermal expansion behavior matching that of silicon.1,2 These properties are very important for applications as substrates for highly integrated microelectronic packaging and have motivated a large number of studies aiming at property optimization and establishing a better understanding of the relationships between processing, microstructural features, and final properties.3–5 Because of the covalent bonds, AlN has a low diffusivity and requires a high temperature (1900–2000 °C) for sintering. The use of sintering aids is the approach more extensively studied to enhance AlN densification at relatively lower temperatures. The additives more frequently used in the sintering of aluminium nitride are
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0301 2460 J. Mater. Res., Vol. 21, No. 10, Oct 2006 http://journals.cambridge.org Downloaded: 01 Aug 2014
Y2O3 and CaO,6–9 which provide low-melting point liquids in reaction with Al2O3 on the surface of AlN particles. These liquids crystallize on cooling to calcium aluminates for CaO additives and yttrium aluminates for the Y2O3 additive. However, since the presence of oxygen is detrimental for thermal conductivity, many efforts have been made to use sintering aids without oxygen in the composition.10–12 Other sintering conditions, such as atmosphere, furnace, sintering temperature and holdin
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