Pirinç kabuğundan silisyum nitrür tozunun sentezi

thumbnail.default.alt
Tarih
1993
Yazarlar
Kuşkonmaz, Nilgün
Süreli Yayın başlığı
Süreli Yayın ISSN
Cilt Başlığı
Yayınevi
Fen Bilimleri Enstitüsü
Institute of Science and Technology
Özet
 Bu çalışmada Trakya(Enez) yöresine ait pirinç kabuk ları kullanılarak silisyum Nitrür (Si"N, ) toz sentezi yapılmıştır. Pirinç kabukları, 7DDQC'de argon gazı altında piro- liz işlemi sonunda öğütülerek <53 fim tane boyutunda piralize pirinç kabuğu tozu (% 50 C, % 47.9 5iD") şeklin de Si-N, toz sentez hammaddesi olarak hazırlanmıştır. Piralize pirinç kabuğunun deneyler için özel olarak hazırlanan grafit reaktör içinde optimum nitrürleşme ve riminin 50L/saat azot akışı altında olduğu saptanmıştır. Pirolize pirinç kabuğundan silisyum karbür (SiC) fazı oluşmadan Si-N, fazının oluşumu için çıkılabilecek maksimum reaksiyon sıcaklığının 1400DC olduğu, 1450 C de ise Si-N, fazının yanısıra SiC fazının ortaya çıktığı ve reaksiyon başlagıcında oluşan Si,l\l, fazının fazla karbonun etkisi ile kararlılığını kaybederek SiC fazına dönüştüğü belirlenmiştir. Pirolize pirinç kabuğunun reaktöre şarjının toz ( < 53 fim)veya pelet (+ 3.1 5-4.75-+4.75-S.1 5 mm) olması nın 1400 C ve 50. L/saat azot akışı altında iki saat nit- rürleme sonunda elde edilen Si-N, oluşum veriminde (% 79(Ağırlıkça) ) etkili olmadığı saptanmıştır. Çekirdeklestirici Si-N, "toz ilavesinin, optimal şartlarda (14QD C, 50L/saat N") Si-N. oluşum verimine bir etkisinin olmamasına rağmen üretilen Si3N, 'U viskır formundan eş eksenli bir yapıya dönüştürdüğü1 gözlenmiştir. 1400DC de elde edilen Si-N, viskırlarının dikdörtgen ke sitli olduğu, sıcaklığın 1450 C'ye çıkması durumundan, SiC fazının çökelmesinde etkili olan Fe-Si sıvı fazının kürecikler şeklinde ortaya çıktığı, ve 1500 C. de elde edilen SiC viskırlarının ise dairesel kesitli olduğu be lirlenmiştir.
Silicon Nitride is a material used in high technology applications. This is mainly due to oxidation resistance at elevated temperatures low density, high hardness and strength of the material. These properties make silicon nitride an ideal candidate to be used as high temperature turbine components, bearings and cotting tools. World production of Si.,N, pouders are unavailable, but it is estimated that anual production world wide of Si3N, powder is about 5D0 metric tons. Many of the manufactures of Si"N, products make their own powder but there is apparently no domestic commercial supplier of Si,N,. Powder can be purchased from sources in Germany 3apan, Sweden and the United Kingdom. Preces vary from $/kg to 350 $/kg depending on specified particle size and purity. There are several routes available for the produc tion of silicon nitride powders; these include: 1)- Direct nitridation of silicon with nitrogen 2)- Vapour phase reactions between silicon halides and ammonia 3)- Reaction between silica and ammonia k)- Carbotermal reduction and nitridation of silica with nitrogen. Direct silicon nitridation is widely used because it is the simplest method of silicon nitride formation. However, it is difficult to control particle shape and size, as well as the high a -phase content, because of an intrinsic reaction mechanism in the silicon and nitrogen. vıı Vapor phase reaction has been widely used in the semiconductor industry as a synthesis method for pure silicon nitride. The silicon imide decomposition met hod is a recent development in which pure a -silicon nitride can be obtained. Carbothermal reduction and nitridation of silicon dioxide is of commerical interest since this method yields powder with small particle size and high purity, therefore, specific emphasis is given to the development of this method since 1970's. According to these studies, main parameters effecting Si-N., formation in SiG2-N"-C system are found to be reaction temperature, N2 gas flow rate specific surface of silica and C powder, and particle size, Si,N. addition as seeding additive, existence of Fe impurity, C/Si02 ratio in the initial mixture. Various raw materials can be used as silica and carbon source; some Df them are: sublimated silica, amporphous silica, silica precipitate from geothermal waters, graphite, and strach. The use of rice husk as C and Silica source is itageous in producing a -5i,N, pa.cle size, since it provides Homo mixture with very fine particle size. advantageous in producing a -5i,N, powder with small particle size, since it provides Homogenous C+SiD" Production of Si"N, powder is of significant im portance for Turkey, since this would not only.create an apportunity for enterprise but would also develop an understanding for high technology ceramics. Therefore the scope of this thesis is to develop a method for carbothermal reduction and subsequent nitri dation of rice huks obtained from Enez/Trakya region. As can be seen at the flowchart in figure, pyrolized rice husks are subjected to a nitridation process in a kk kVA Tammann furnace under N" gas. The effects of reaction temperature and duration N? flow rate, charging way of pyrolized rice husk to vm the graphite reactor, addition of seeding Si-N, powder on Sİ..N, recovery and product morphology were examined. Also, formation mechanisms of Si,N, was determined according to CO measurements of exhaust gases and to weight losses during the reaction, and the effects of impurity Fe on Si.,N, - SiC equilibrium was investigated. As a result of the research presented in this desertation, the Enez/Trakya rice husk was characterised and found to be fiberous material composed of amorphors silica. It was shown that, through pyrolysis at 7DD C under Ar gas, the rice husk can be turned in to a raw material containing % 47.9 SiO- and 50 % C to be used in Si"N, synthesis. It was unterstood that whether Si-N, or SiC forms is related to N? gas partial pressure. Nitridation yield was found to be related to the l\L flow gase up to a certain value after which the yield was constant. The optimum conditions for Si_l\l, formation were deter mined to be 1400 C temperature and 2 h. duration at 50 L/h flow rate of N". Above 1400 C partly turns into SiC due to the presence of excess carbon; Si,N. + 3C * 3SİC + 2N" 3 4 2 Since carbon is already present in the raw material, it was found that the particle shape of the raw material, whether powder or pellete (+ 3.15-4.75, + 4.75-5.6 mm) does not have influence on the 5i,N, formation yield. ieding the charge with Si-N, at 1400DC and 50 l/h )w, on the other hand, effects the product Set gas floi morphology but not the formation yield. When the CO concentration in exhaust gas was monitered, it was observed that CO concentration increases suddenly during heating up to 1400 C and then decreases in 2 steps in the case of no seeding and decreases linearly in the case of seeding. The sudden increase was attributed to the reduction of SiO^. The first step in CO concentration of nucleoids and the secent Step being the growth of the nuclei. In the case of seeding, the CO concentration decrease was uniform till stabilization since the nuclei were already present. IX Rice Husk I Pyroliysis Process 7G0DC 1 hour Argon gas Milling 53 Jim Pelletizing % 1 PUA solution Carbothermal Reduction and Nitridation 1350° - 15DD°C ^N2- Deoxidation (0.1 ppm D2) (100 ppm 0") Decarburization 700°C, 3 hours X-ray diffraction: Recovery analysis 1 Purificaiton from amorphous Silica Pure 5iJ\l, powder Air % 20 NaOH solution FIGURE: 1. Flowchart of Si-N, Powder Production From Rice Husk. The weight lass during the recduction was found to be more than the theoretical valve. This was attributed to the formation and evoparation of SiO before the tem perature reaches to thermodynamically feasible valve. It was found that the morphology of the product, whether whisker or powder; is related to reactant gas partical pressure and the seeding additive. It was interesting to note that the whiskers formed at T+QOaC one Si,N, but above 1^00 C, impurity iron forms Fe-Si intermetallic compound which disolves carbon and then helps the precipitation of SiC.
Açıklama
Tez (Doktora)-- İTÜ Fen Bil.Enst., 1993
Anahtar kelimeler
Pirinç-metal, Sentez, Silisyum nitrür, Brass, Synthesis, Silicon nitride
Alıntı