Effects of precursors on the synthesis of Si3N4 powders formed by carbothermal reduction-nitridation of sepiolite
Başlık çevirisi mevcut değil.
- Tez No: 400348
- Danışmanlar: DR. TOM J. DAVIES
- Tez Türü: Doktora
- Konular: Metalurji Mühendisliği, Seramik Mühendisliği, Metallurgical Engineering, Ceramic Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1998
- Dil: İngilizce
- Üniversite: University of Manchester Institute of Science and Technology (UMIST)
- Enstitü: Yurtdışı Enstitü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 150
Özet
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Özet (Çeviri)
Sepiolite of Turkish origin was used as a Si precursor in the syntheses of siliconnitride (Si3N4) powders by a carbothermal reduction nitridation (CRN) process aftermixing with several reducing agents i.e. charcoal, carbon black and petroleum coke asdiscrete particles and acrylonitrile as an intercalation medium. Selective leaching wasused to purify as-received sepiolite mineral. Silicon nitride powders of various qualitieshaving different morphologies, ranging from long whiskers to equiaxed fine particleswere obtained from sepiolite polyacrylonitrile (PAN) and sepiolite-discrete carbonparticle mixture. The ?/ß ratio and secondary phase content of the powders after CRNwere found to depend on several factors in nitridation such as, temperature, time,heating rate and on the physicochemical properties of the precursor used such as,surface area and mixing condition of the reactants. The type of reducing agent affectedthe yield, phase content and morphology of the powders in CRN. An increase inspecific surface area of the precursor resulted in increased reaction rate in CRN. Finegrain size, high specific surface area (up to 11.5m2/g) powders of mainly ß-Si3N4 wereobtained using PAN as a carbon source. Silicon nitride whiskers of sub-micronthickness and hundreds micron in length were synthesised when PAN was replaced withdiscrete charcoal particles. The yield of ?-Si3N4 also increased (up to 43wt.% of totalsilicon nitride content) using sepiolite-charcoal or sepiolite-carbon black mixtures bycarefully adjusting the experimental variables during purification of the mineral andduring nitridation of the precursor. The best results in nitridation in terms of the Si3N4yield were obtained in the temperature range (1300-1450oC), reaction time (4-16h), N2-flow rate (1000-2000ml/min) and heating rate (150-300oC) depending on the type ofsepiolite and reducing agent used. Some of the product powders after nitridation weresuitable for pressureless sintering due to presence of retained intermediate oxides andnitrides in the starting powders. Dense bodies (above 80% T.D.) were obtained bypressureless sintering of the cold isostatically-pressed green compacts using a powderbed method after 1h firing at 1700oC in a nitrogen atmosphere. It is suggested thatsepiolite-PAN mixture after stabilisation and also the Si3N4 powders from the CRNprocess could have commercial use.
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