Sintering of aluminium nitride with calcium compounds.

Author(s)

André Luiz Molisani

• Document type: Master's Dissertation
• Press: São Paulo.
• Institution: Universidade de São Paulo (USP). Escola Politécnica (EP/BC)
• Defense date: 2004-11-23

Examining board members

Helio Goldenstein; Ana Helena de Almeida Bressiani; Humberto Naoyuki Yoshimura

• Advisor: Helio Goldenstein

Abstract

Aluminum nitride (AlN) presents high thermal conductivity, beyond several physical properties, that make it an excellent candidate to substitute alumina (Al2O3) and the berylia (BeO) in the manufacturing of high performance electronic devices. The route of production of AlN with high thermal conductivity is established in literature, with Y2O3 and CaO the most used sintering additives. However, the studies on this ceramics clarify only partially the mechanisms involved in its sintering. The general aim of this work was to study the possible mechanisms related in the sintering of AlN. The specific objective was to understanding the influence of the amount of CaCO3 and CaO in the densification of AlN. The densification behavior of AlN with 0.5%, 1%, 2%, 4%, and 8% in weight of CaO, added as CaCO3 and calcined CaO, was studied by sintering both in dilatometer and in an oven with tungsten resistive elements between 1100ºC and 2000ºC. The sintered bodies were analyzed by scanning electronic microscopy, microanalysis by energy dispersive spectrometry, X-ray difratometry, and chemical analysis by ICP-AES. The same general tendencies in densification were observed in samples with CaCO3 and calcined CaO between 1100ºC and 1800ºC. The addition of small amounts of both additives (0.5% CaO) strongly enhanced the sinterability of AlN. With increasing amount of both additives, calcium aluminates richer in CaO were formed between 1300ºC and 1600ºC, as expected. However, above this temperature range, it was observed the tendency of the formation of CA phase, independent of the additive type and content, showing that the CA phase is more stable in high temperatures in AlN than the others calcium aluminates predicted by CaO - Al2O3 system. As a rule, the densification of the AlN samples with CaCO3 and calcined CaO additions was influenced by the second-phase evolution path, liquid phase content, and the formation of large pores. The formation of CaO rich phases (C3A and C12A7) promoted the formation of liquid phase in low temperatures, which caused a fast initial densification of the AlN samples with 2% to 8% CaO, with both additives, below 1600ºC. The increasing additive content also favored the densification of theses samples by the formation of a higher amount of liquid phase, but it caused the formation of higher fractions of large pores. The AlN samples with 0.5% and 1% CaO, with both additives, presented lower fractions of large pores, however they presented Al2O3 rich aluminate phases (CA2 and CA6), which melt at higher temperatures. Thus, only above 1600ºC these samples presented rapid densification because of the formation of liquid phase and the low fraction of large pores. On the other hand, the samples with 2% to 8% CaO presented slow densification because of the difficult of the elimination of the large pores, even sintering at high temperatures (> 1800ºC). The large pores were gradually eliminated from the microstructure as a consequence of grain growth, which occurred mainly during the liquid phase sintering. (AU)