The behaviour of silica during the carbothermal reduction nitriding process at
temperatures between 1300-1500°C was studied by means of X-ray diffraction and scanning
electron microscope analysis. The experimental runs were allowed to proceed up to 1 h in presence
of nitrogen flow. The following mechanism of reduction nitriding of silica which was based on the
experimental observation was proposed. Initially the impurity of the starting material is reduced
before 1300°C. SiO2 was reduced into SiO gas phase by active carbon and it was vaporised out of
the mixture. The nucleation of α-Si3N4 was formed vapour-gas reaction took place and deposited on
the surface of the mixture as well as around the reaction crucible. In the third stage, α-Si3N4
transforms to one dimensional direction which was β-Si3N4 particle. This was followed by the
formation of SiC at temperature above 1450°C.
Bu çalışmada, farklı türlere ait süt (koyun ve inek), yoğurt (koyun ve inek) ve kremaların (inek ve koyun; çiğ, olgunlaştırılmış ve pastörize krema) bazı fizikokimyasal ve mikrobiyolojik özellikleri araştırılmıştır. Koyun sütünün inek sütünden daha fazla yağ, protein ve mineral madde içeriğine sahip olduğu belirlenmiştir (P<0.05). Koyun ve inek pH değerleri arasındaki fark önemsizken (P>0.05), koyun sütü titrasyon asitliği inek sütünden daha yüksek bulunmuştur (P<0.05). İnek yoğurdu kurumadde, yağ oranı ve titrasyon asitliği değeri koyun yoğurdundan daha düşük (P<0.05), pH değerlerinin ise farklı olmadığı belirlenmiştir (P>0.05). Koyun sütünden elde edilen kremaların kuru madde ve yağ değerlerinin inek sütünden elde edilen kremalardan daha yüksek olduğu (P<0.001), pH ve titrasyon asitliği değerlerinin aynı olduğu belirlenmiştir (P>0.05). En yüksek pH ve en düşük titrasyon asitliği değerleri çiğ kremada tespit edilmiştir (P<0.001). Olgunlaştırılmış kremalara ait E.coli ve Koliform bakteri sayılarının çiğ süt ve çiğ kremalardan daha yüksek olduğu belirlenmiştir. Yoğurt ve pastörize kremalarda E.coli ve Koliform bakteriler bulunmamıştır. Sonuç olarak bu çalışmada kullanılansüt, yoğurt ve kremaların Türk Gıda Kodeksi Tebliği’ne uygun olduğu ve sadeyağ üretiminde kullanılan yoğurt ve pastörize kremaların Gıda Güvenliği ’ne uygunluğu belirlenmiştir.
In the present work, laboratory experiments were carried out on the sintering of silicone nitride powder without any sintering additives. Silicone nitride powder was produced at own laboratory conditions by carbothermal reduction-nitriding of silica of which was presented elsewhere. It consists of mainly -Si3N4 (83%) and -Si3N4 (17%) phases with a small amount of minor constituents. Before compaction procedure, the powder was ball milled for 2 h and then 2 gr. of the sample was compacted uniaxially at a pressure of 25 MPa. The green compact was placed into furnace in a graphite crucible. The sintering process was carried out in a programmable muffle furnace at 1700 °C and held for 1 h. with a 20 °C/min. heating rate. The sintered compacts were analyzed using X-ray, SEM, SEM-EDS techniques. The results showed that the formation of liquid phase can accelerate the densification of the compact. The densification of the pure SIALON can be increased by adding of sintering aids that accelerates the formation of liquid phases in the sample. The densified sialons give better hardness and thermal properties of the sample. It was well demonstrated that for the densification of sialons large amount of liquid phase forming oxides are needed to add to initial composition. It is also suggested that there is linear relationships between densification and amount of liquid phase formed in the compact.
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