SYNTHESIS OF ALUMINUM BORATE POWDER, FABRICATION AND CHARACTERIZATION OF ALUMINUM BORATE-BASED CERAMICS

BAŞAK ÖZGÜR

SYNTHESIS OF ALUMINUM BORATE POWDER, FABRICATION AND CHARACTERIZATION OF ALUMINUM BORATE-BASED CERAMICS

Tezin Türü: Yüksek Lisans

Tezin Yürütüldüğü Kurum: Ankara Yıldırım Beyazıt Üniversitesi, Fen Bilimleri Enstitüsü, MALZEME MÜHENDİSLİĞİ ANABİLİM DALI, Türkiye

Tezin Onay Tarihi: 2022

Tezin Dili: İngilizce

Öğrenci: BAŞAK ÖZGÜR

Danışman: Cihangir Duran

Açık Arşiv Koleksiyonu: AVESİS Açık Erişim Koleksiyonu

Özet:

Aluminum borate is a suitable ceramic for structural applications due to its low thermal expansion coefficient (4.5×10^-6/°C), high elastic modulus (400 GPa), low density (2.93 gr/cm³). In particular, it is used as thermal insulator due to its low thermal conductivity (4 - 6 W/m.K up to 1000°C). However, its acicular particle shape makes densification difficult during sintering. In this thesis, Al₂O₃ and domestic B₂O₃source were utilized to synthetize Al₁₈B₄O₃₃powders by solid state calcination technique and then basic characterizations (phase formation, microstructural evolution, mechanical, thermal, dielectric properties) were completed. Seven formulations (10.7, 13.4, 18.1, 27.9, 34.4, 44, 54.1 wt% B₂O₃) were prepared and then calcined at 1300°C for 1h to synthesize Al₁₈B₄O₃₃ powders. Full Al₁₈B₄O₃₃formation was obtained at 44 wt% B₂O₃ (e.g., rather than the stoichiometric ratio of 13.17 wt% B₂O₃). Our results show that 44 wt% B₂O₃ (e.g., Al₂O₃/ B₂O₃ mole ratio of 0.87) was required to obtain a melt that was required to induce nucleation or crystallization of Al₁₈B₄O₃₃phase at calcination temperatures ³1200°C. Acicular Al₁₈B₄O₃₃ particles were fully obtained for the 44 wt% and 54.1 wt% B₂O₃ mixtures when Al₂O₃was all consumed. However, its acicular particle shape made densification difficult during sintering. Therefore, its densification was controlled by a composite approach with mullite due to its similar crystal structure. Al₁₈B₄O₃₃+2 wt% CaO (named as AB), %100 Al₁₈B₄O₃₃(named as PAB), %35 Al₁₈B₄O₃₃+%35 mullite+%30 glass composite (named as AMG) and %70 Al₁₈B₄O₃₃+%30 glass composite (named as ABG) were prepared by gel casting method. Glass and CaO were added to enhance densification by means of a liquid phase sintering mechanism. The PAB sample sintered at 1350^oC for 1h had a relative density of 59.7 %, but 2 wt% CaO addition improved densification to 95.4% (the AB sample). The AMG ceramics reached 94.8% relative density with near-zero open porosity and water absorption levels. The ABG ceramics reached a limited densification of 89.4%. Dielectric constant of the samples varied between the 4 and 6.6 at 5 MHz. The PAB ceramic had the lowest (4.05) and the ABG ceramic had the highest value (6.58) dielectric constant. Vickers hardness values were 924±118 for the AMG, 998±52 for the AB, 825±120 for the ABG and 98 ±10 for the PAB ceramics. The AB sample had a thermal conductivity of 7.26 W/m.K but the PAB sample had 2.7 W/m.K due to its low densification. The crystalline phases (e.g., Al₁₈B₄O₃₃and/or mullite) were embraced by amorphous glassy phase, which caused a sharp decrease in overall thermal conductivity (1.73 W/m.K) of the AMG and ABG composites. In this study, Al borate-glass (AB) and Al borate-mullite-glass (AMG) composites were investigated for the first time in literature. The ABG ceramics reached a limited densification of 89.4% due to insufficient filling of glass phase thru the porous network, but the AMG ceramics reached 94.8% relative density with near-zero open porosity and water absorption levels due to mullite addition. These results suggest that Al borate-based ceramics can be used for such applications as reinforcement material in metal matrix composites, filter, thermal insulator, substrate and radome materials.