Ülkemizde alümina esaslı seramik malzemelerin kullanımı giderek yaygınlaşmaktadır. Yüksek alüminalı seramik malzemelerin üretiminde kullanılan alüminanın belirli özelliklere sahip olması gerekmektedir. Seydişehir’de üretilen alüminanın metalurjik spesifikasyonlarından dolayı seramik malzeme olarak kullanımı kısıtlanmaktadır. Seydişehir alüminası düşük sıcaklıkta yapılan kalsinasyondan dolayı yüksek oranlarda geçiş faz alüminası içermekte, dolayısıyla kararlı alfa faz yüzdesi düşük seviyelerde kalmaktadır. Çözümlendirmede kullanılan sodyum hidroksitten dolayı yüksek oranlardaki Na2O içeriği elektriksel özellikleri etkileyen temel faktör olmaktadır. Seramik değerli alumina özelliklerini etkileyen en önemli faktör ise Seydişehir alüminasının iri tanelerden oluşmasıdır. Bu çalışmada Seydişehir alüminasının slip döküm yöntemiyle şekillendirilebilmesi amacıyla karakterizasyonu, ıslahı, reolojik özellikleri, sinterlenmesi, fiziksel ve mekanik özellikleri çalışılmıştır. Faz analizi, kimyasal analiz, görünür yoğunluk ve tane boyut dağılımı tespit edildikten sonra yıkama, kalsinasyon ve öğütme işlemleri yapılmıştır. Daha sonra, ıslah edilen Seydişehir alüminasının reolojik özellikleri ve slip dökümü çalışılmıştır. Reolojik özellikler aşamasında zeta potansiyeli, elektrolit, viskozite, litre ağırlığı ve kolloidal davranış, slip döküm aşamasında, kuruma küçülmesi ve yaş yoğunluk çalışılmıştır. Çalışmanın sonraki aşamasında sinterleme işlemine geçilmiştir. Sinterleme çalışmalarında 1500 oC’den 1750 oC’ye kadar 5 farklı sıcaklıkta %1’den %3’e kadar Cr2O3, TiO2 ve Mg katkılarıyla sinterleme yapılmıştır. Daha sonra sinterlenen numunelerin fiziksel (yoğunluk, lineer çekme, porozite ve por boyut dağılımı) ve mekanik (eğme mukavemeti, serlik, kırılma tokluğu) özellikleri tespit edilmiş, mikroyapısal gelişimle beraber kırılma analizleri yapılmıştır.

In Turkey the ceramic industry has been growing rapidly. Especially, the applications of electrical ceramics, textile ceramics, traditional ceramics and refractories are important and large quantities of aluminas are being imported. In Seydişehir, the purpose of Bayer process is production of the metallurgical grade alumina. Owing to the nature of this of process Al(OH)3 is calcined at 1000 oC for dehydration of aluminum trihydroxide resulting that Seydişehir alumina powders have metastable transition phases. Also, Bayer-processed Seydişehir alumina powders have coarse particle sizes and high Na2O contamination. As a result, these properties make it non-ceramic grade. The first step of the process is characterization of Seydişehir aluminas. This step involves analyses of particle size, SEM, chemical composition, and XRD. Particle size and SEM analysis show that Bayer-processed Seydişehir alumina powders have broad range of particle size distribution from submicron to higher than 125 $mu m$. Chemical analysis shows that these aluminas have high soda contamination of about 0.3 percent. Also XRD analysis shows that Bayer-processed Seydişehir alumina powders contain both transition and alpha phases. The second step of the study is improvement of properties of these aluminas. For this purpose following operations are used: calcination, washing, grinding (with different techniques). With calcination, the XRD results show that all of the transition phases are converted to corundum at 1200 oC. By the help of washing, Na2O content of Bayer-processed Seydişehir alumina powders is reduced. Chemical analysis shows that Na2O content of aluminas washed in cold water is 0.11 percent. The last operation is grinding which is done by the help of attritory, planetary ball mill and vibratory grinding in dry and wet conditions before and after calcination. Particle size and SEM analysis show that grinding after calcination is the most effective one. It is observed that 4 hours of grinding after the calcination is enough for obtaining smaller particles than 10 $mu m$. The third step of the process is slip casting of Seydisehir aluminas. This step involves investigation of reological properties (viscosity, colloidal behaviour, etc…), molding properties and shirinkages. Viscosity of slurry, tixotropy, zeta potentials, and weight per liter of slurry are studied as rheological properties.Following the slip casting phase, sintering of the specimens were performed at five different temperatures between 1500 - 1750 oC with various sintering aid additives. Effects of Cr2O3 and MgO additions did not show any significant increase both for sintered density and strength, however TiO2 addition has increased the density and strength values up to 1650 oC. Further increase in the sintering temperature after this level has led to decrease in the strength values. The reason behind the decrease is determined as abnormal grain growth by microstructures studies. In the case of addition of 1 % TiO2 density of Seydişehir alumina has reached the value of 3.73 gr/cm3 after sintering at 1750 oC, and also 160 MPa flexural strength value was obtained after sintering at 1650 oC.Observation of the lower flexural strength values in comparison with the literature were mainly related to two reasons; one is abnormal grain growth in comparison to the samples without additives and second is retained bubbles during the slip casting process which are the main source of porosity. Actually, the elimination of this kind of porosity during sintering proved to be difficult. After the flexural strength measurements of the sintered samples at 1750 oC fracture toughness values were determined by indentation technique. Similar behavior was observed during fracture toughness measurements related to flexural strength measurements. The addition of 1 % TiO2 has decreased the fracture toughness value from 1.78 $MPam^{1/2}$ (no additives) to 1.40 $MPam^{1/2}$ . Behavior of crack propagation of the Seydisehir alumina samples sintered at 1750 oC was investigated via SEM supported studies and crack propagation routes were observed along the porosities, the grain boundries and the grains.