Kimyasal buhar biriktirme yöntemi ile volfram çekirdekli bor fiber üretiminde sistem tasarımı
System design in tungsten-cored boron fiber production by chemical vapor deposition
Yazar: SELİM ERTÜRK
Danışman: PROF. DR. İSMAİL DUMAN
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı
Derece: Doktora, 2012
Konu: Metalurji Mühendisliği = Metallurgical Engineering
Dizin: Metal matrisli kompozitler = Metal matrix composites

Özet:
Bor, dünya kaynaklarının büyük bir kısmı ülkemiz topraklarında bulunan bir elementtir. Ancak, ülkemizde endüstiriyel boyutlarda üretilen en uç bor ürünü borik asitten öteye gidememektedir. Laboratuvar ölçekli çalışmalarda üretilen en uç bor ürünü de çinko borat (ODTÜ) ve sodyum bor hidrürdür (TÜBİTAK MAM). Bor'la birlikte volfram metali de ülkemizde yüksek oranda bulunan ancak değerlendirilmeyen bir başka metaldir. Bu iki elementin bir araya getirilmesi sonucu elde edilen bir ürün olan bor fiberi geniş bir spektrumdaki kullanım alanı (nükleer teknolojiler, otomotiv, havacılık, uzay sanayi, spor gereçleri, vd.) ile yüksek katma değer ortaya koymaktadır.
Metaller ya tek başlarına yada birden fazla metal veya metal alaşımının birbirlerinin içine ilave edilmesi yoluyla kompozit yapılar oluşturarak kullanılırlar. Bor fiberi; başta alüminyum olmak üzere çeşitli metalik yapıların içine döşenerek metal matriksli bor fiber kompozit malzemelerin üretiminde kullanılmakta olup en önemli kullanım alanı havacılık sektörüdür. Gerek üretim koşullarının karmaşıklığı ve ekipmanların hassaslığı, gerekse elde edilen yan ürünlerin (reaksiyon gazlarının; bortriklorür, dikloroboran, hidroklorik asit gazı, reaksiyona girmemiş hidrojen) toksik ve idare edilmesi zor gazlar olması bor fiber üretimini zorlaştırmaktadır.Bor fiberler, 10-15 mikron çaplı volfram filaman üzerine, kimyasal buhar biriktirme yöntemi ile bor kaplanması sonucu üretilen (100 – 140 mikron çaplı) çok özel ipliklerdir. Bor fiber üretimi; bortriklorür gazının 800 – 1250ºC'ye ısıtılmış volfram telin üzerine sıcak filaman kimyasal buhar biriktirme yöntemi olarak tanımlanan bir yöntemle bortriklorür ile hidrojen gaz karışımının birlikte verilmesi ve bortriklorür gazının redüklenerek volfram filamanın üzerine bor birikmesi ile gerçekleşir.
Bu çalışmada, özel olarak tasarlanmış, birbirine akuple edilmiş 3 farklı bölgeye sahip, sıcak filaman kimyasal buhar biriktirme (SFKBB – HFCVD) sistemi ile çok ince (≤10 µm) volfram filaman üzerine bortriklorür ve hidrojen gazlarının reaksiyonu sonucu bor biriktirilmesi için kullanılabilecek bir sistem tasarlanmıştır. Tasarlanan sistemde birbirine akuple edilmiş 30 ve 150 cm uzunluğunda, 10 mm iç çaplı 12 mm dış çaplı reaktörler ve özel olarak tasarlanmış kontaklar kullanılmıştır. Reaksiyon sıcaklığı kontaklara konulan civa/civa-indiyum amalgamı üzerinden volfram telin indirekt olarak ısıtılması ile sağlanmıştır.Tasarlanan sistemde hem laboratuvar ölçekli hem de pilot ölçekli hücreler kullanılarak sürekli akış halinde, volfram çekirdekli bor fiberi ve bor karbür kaplı bor fiberi üretimi gerçekleştirilmiştir. Üretilen fiberlerin karakterizasyonuna yönelik olarak SEM ve EDS analizleri ile bor/bor karbür kaplamanın ve volfram – bor ara yüzeyinin mikroyapı incelemesi (kesit ve boyuna) ve çap ölçümleri, Raman Spektroskopisi ve X-Işınları Difraksiyonu (XRD) analizi ile bor tabakasının faz analizi yapılmıştır.Gerçekleştirilen tasarım çalışmaları ve deneysel çalışmalar neticesinde aşağıda sıralanan sonuçlar elde edilmiştir.
•	Bor fiberi ve bor karbür kaplı bor fiberi üretimi için en uygun yöntem kimyasal buhar biriktirme yöntemidir.
•	Volfram çekirdeğe elektrik vermek için kullanılan civa veya civa amalgamı hareket halindeki filamana yapışmaktadır. Civa yerine amalgam kullanılarak viskozite düşürülmüş ancak yine de yapışma gözlemlenmiş ve teknik nedenlerden ötürü amalgam yerine civa kullanımı tercih edilmiştir.
•	Bortriklorür ile hidrojen gazlarının reaksiyonu sonucunda tungsten altlık üzerine bor biriktirilmesi, deneylerin gerçekleştirildiği bütün sıcaklıklarda mümkün olmuştur.
•	10-15 mikron çaplı volfram filaman üzerine farklı koşullarda bor kaplanarak 65-100 mikron çaplı bor fiberi üretimi gerçekleştirilmiştir.
•	Bor fiberi üzerine 3-5 mikron et kalınlığında bor karbür kaplanarak bor karbür kaplı bor fiber üretimi başarıyla gerçekleştirilmiştir.
•	Bor fiberinin kimyasal yapısı, reaksiyon sıcaklığına direkt olarak bağlıdır. T<900°C durumunda oluşan yapı amorf olurken bu sıcaklığın üzerinde kristalin yapı oluşmaktadır.
•	Genel olarak W ile B arasında reaksiyon ara yüzeyinde bir katman oluşur. Gerçekleştirilen deneylerde bu ara fazın W2B17 olduğu tespit edilmiştir.
•	Son olarak hem laboratuvar ölçekli hem de pilot ölçekli sistemde kontinü olarak bor fiberi üretilebilmiştir.

Abstract:
Elemental boron is a light, refractory material with high hardness, high corrosion resistance and high mechanical strength. It has high neutron absorption capacity and exhibits semiconducting behavior. Because of these attractive properties, it can be used in significant applications such as the production of protective and refractory coatings, fiber-reinforced composite materials, and electronic components as well as in the area of nuclear protection. The majority of world's boron resources are found on Turkey. However, boric acid is the most valuable boron product that can be produced in industrial scale in our country. Laboratory scale production of boron products are zinc borate (produced in Middle East Technical University / METU) and sodium boron hydride (produced in The Scientific and Technological Research Council of Turkey – Marmara Research Center / TUBITAK-MRC). Tungsten is another metal that Turkey has high amount of resources but not processed efficiently. Boron fiber, a high added value product, obtained by combining these two elements, has a wide range of usage (nuclear technologies, automotive industry, aviation industry, aerospace industry, sports equipments, etc.).As a result of improved technological possibilities, metals are not only used by themselves but also used as combined togehter, and either added or reinforced to each other to form composite structures. By combining two or more materials together, it is possible to make advanced composite materials which are lighter, stronger, and stiffer than any other structural materials and have found applications in various areas (Vinson,1975). Boron fibers are used in the production of metal matrix composite materials mostly aluminium is the matrix material and the most important usage of this composite is the aviation industry. The complexity and sensitivity of the conditions of its production equipment, as well as by-products obtained (reaction gases; borontrichloride, dichloroborane, hydrochloric acid gas, unreacted hydrogen) gases are toxic and difficult to handle that makes it difficult for the production of boron fiber. Boron fibers are special yarns, having a diameter of 100-140 microns that are produced by chemical vapor deposition (CVD) process on tungsten filament (generally tungsten is chosen as substrate and has a diameter of 10-15 microns) as the core material. Boron fiber production; borontrichloride gas 800 - 1250ºC hot filament chemical vapor deposition method on a heated tungsten wire is defined as a method of hydrogen gas mixture borontrichloride and borontrichloride can be accompanied by the accumulation of gas takes place on reduction on tungsten filament. Boron fibers have an average tensile strength of 3.4*109 – 3.5*109 N/m2 and an elastic modulus of 4.0*1011 – 4.2*1011 N/m2. The density of boron fiber is around 2.6 g/cm3.Chemical vapor deposition is the dominating technique for the deposition of thin boron, boron carbide and boron nitride films as well as for the production of boron and boron carbide fibers. The CVD technique used for the production of high-purity boron involves the reduction of boron halides (generally BCl3 or BBr3) by hydrogen over a hot metal substrate (Vandenbulke and Vuillard, 1977; Sekine et al., 1989; Haupfear and Schmidt, 1994; Sezgi et al., 1997).There is no printed research about the production of boron fiber in Turkish literature. The only work is about the kinetic investigation on the boron fiber production carried by Prof. Dr. Timur DOĞU and his co-workers in Middle East Technical University within the frame work of the TUBITAK. Their work was mainly subjected to thermodynamics and kinetics of the reaction. They used tungsten as substrate material on a fixed position and their results does not contain precise information about the industrial working conditions. In their work, the experiments were carried out at high temperature (1000-1300°C), and and a FTIR spectrometer was placed to the exhaust of the system to obtain the content and composition of the gases leaving the system simultaneously. According to their results, BHCl2 is formed as a by product from the reaction of borontrichloride and hydrogen and this is not a desired situation. Chemical composition and phase of the coated material was not investigated, only reaction kinetics is considered and thermodynamic calculations were done.
The first notification of the BHCl2 formation is noticed by Carlton and co-workers in 1970. They used borontrichloride as boron source and reduce it by hydrogen to obtain boron on tungsten substrate between 800 and 1400°C. After this work, Pollard and Michaelidis (1984) showed that BHCl2 has a great influence on the reaction mechanism by controlling the reaction rate on the surface. Also, they showed that some amount of BCl is also formed as by product. According to their results, they developed a model including reaction kinetics, hydrodynamics and heat transfer of CVD system. Sekine and co-workers calculated the activation energy of the system as 127.9 kJ/mol at low temperature and 17.1 kJ/mol at high temperature. According to the activation energy they specify that reaction is controlled by mass transfer. They indicate that tungsten and boron has an interaction between and WB is formed at the interface. Morphology of the boron by CVD is investigated by Carlsson in 1980. Temperature is the main parameter affecting the morphology whether it is crystalline or amorphous. In literature, it is showed that at low temperatures amorphous phase and at high temperatures crystalline phase is obtained. Carlsson showed that regardless from the morphology, reaction rate is only proportional with temperature and can be written as k=1.59108e(-20070/T), where activation energy is 20070 kj/mol and also when the reaction rate is higher than that value, coated material has amorphous structure. Crystalline structure has 3 different phases ( – rombohedral, 
 – rombohedral and  – tetragonal). Thermodynamically  – rombohedral is the stable phase, however,  – rombohedral and  – tetragonal phase is obtained as meta-stable at low temperature and high temperature respectively.
According to Vandelbulcke ve Vuillard (1932), when Stefan-Maxwell equation is used in boron fiber production by CVD with the reaction of BCl3 and H2  reaction has 3 main steps. These are;
1.	Mass transfer – interface equilibrium,
2.	Mass transfer – surcafe kinetics,
3.	Surface kinetics.
Boron and boron carbide fibers are the main ingredient of the metallic, ceramic or polymer matrix composites, which is called as advanced composties. The physical properties of advanced composites are superior to the metallic structural materials when they are compared according to their density. Their performance is very high due to their density. Thus, they are generally used in the production of lightweight structural compounds. The success that has been made in the development of boron composites, metal matrix composites, and more recently, in carbon-carbon composites has shown the ever increasing potential of composite materials. Boron fiber, especially in the production of polymer and metal matrix composite materials, is preferred when high strength and low weight is required.
In this study, a specially designed hot-filament chemical vapor deposition (HFCVD) system is used to deposit boron on a very thin tungsten filament by the reaction of hydrogen and boron-trichloride. Both lab-scale and pilot-scale cells are prepared and tungsten-cored boron fiber was produced. A glass reactor used as CVD medium 
(30 cm and 150 cm length and 10 mm ID / 12 mm OD) was designed to investigate the optimum production conditions of boron fiber production. Reaction temperature is controlled by heating the tungsten substrate (wire) via mercury and mercury-indium amalgam. Experiments were carried out at atmospheric pressure and at a reaction temperature range of 800-1250°C with different inlet reactant concentrations. SEM and EDS analyses of fibers produced, were conducted along with microstructure investigations (both cross-sectional and longitudional) and diameter measurements of boron coating and tungsten-boron intermediate, and phase analyses of boron layer with Raman Spectra and XRD techniques.
Results of the designed system and experiments can be summarized as;
•	The most suitable method for the production of boron fiber and boron carbide coated boron fiber is chemical vapor deposition.
•	Mercury in contact, which is used to conduct the electricity to the tungsten wire sticks on to filament. Viscosity of the mercury is decreased by adding indium however adhesion of mercury still continues. Because of the technical limitations, mercury is used instead of amalgam.
•	Reduction of borontrichloride with hydrogen on to tungsten filament was accomplished at all temperatures in which experiments were carried out.
•	65-100 microns diameter boron fiber production on to 10-15 microns diameter tungsten wire was succeeded at different conditions.
•	Boron carbide coated boron fiber production was successfully achieved by coating 3-5 microns thick boron carbide layer on to boron fiber.
•	Chemical structure of the boron fiber is strictly connected with reaction temperature. Structure is amorphous tt temperatures below 900°C while it is crystalline at higher temperatures.
•	Generally there is an interaction with substrate and coating at the interface. In our experiments, W2B17 phase was obtained at the interface.
•	Finally, both laboratory and industrial scale production of boron fiber and boron carbide caoted boron fiber was successfully achieved.