Kendiliğinden ilerleyen yüksek sıcaklık sentezi yöntemi ile B4C üretiminde katalizörlerin etkilerinin araştırılması
The investigation of effects of catalysts on the production of B4C via self propagating high temperature synthesis
Yazar: HASAN ÖZER
Danışman: PROF. DR. ONURALP YÜCEL
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Metalurji ve Malzeme Mühendisliği Ana Bilim Dalı / Üretim Metalurjisi ve Teknolojileri Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2014
Konu: Metalurji Mühendisliği = Metallurgical Engineering
Dizin: Bor = Boron ; Bor bileşikleri = Boron compounds ; Bor karbür = Boron carbide ; Borik oksit = Boric oxide ; Kendiliğinden yanma = Spontaneous coubustion ; Seramik malzemeleri = Ceramic materials ; Seramik tozları = Ceramic powders ; Seramik yakıtlar = Ceramic fuels ; Yüksek sıcaklık = High temperature ; Yüksek sıcaklık malzemeleri = High temperature materials

Özet:
Kendiliğinden ilerleyen yüksek sıcaklık sentezi (Self-propagating high temperature synthesis: SHS) bir çok ileri teknoloji seramiğin üretiminde kullanılmaktadır. Bu yöntemde; metaloksit ve metalik tozlar istenen nihai ürünü verecek ve tepkimenin serbest enerjisi negatif olacak bileşimde ve stokiyometride karıştırılır. Tekime koruyucu gaz atmosferinde gerçekleştirilebildiği gibi açık atmosferde de gerçekleştirilebilmektedir. Sadece tepkimeyi başlatmak üzere bir varyak yardımı ile sisteme enerji verilmektedir. Reaksiyonun serbest enerji değişimi negatif olduğu için ısı veren tepkime esnasında yüksek miktarda enerji açığa çıkar ve bu durum tepkimenin sürekli devam ederek girenlerin tamamının ürüne dönüşmesini sağlar.
SHS yöntemiyle üretim genellikle iki aşamalıdır ve tepkime sonrası oluşan yan ürünler asit liçi ile çözeltiye alınarak istenen ürün saflaştırılmaktadır. Bu tez çalışmasında, önemli bir ileri teknoloji seramik malzeme olan bor karbürün (B4C) SHS yöntemi ile üretilmesine çeşitli katalizör malzemelerin etkileri araştırılmıştır.
Deneylerde bor kaynağı olarak bor oksit (B2O3), redükleyici olarak magnezyum metali (Mg), karbon kaynağı olarak karbon karası ve katalizör olarak da potasyum klorat (KClO3), potasyum hidrojen sülfat (KHSO4), magnezyum sülfat hepta hidrat (MgSO47H2O), sodyum klorür (NaCl), magnezyum oksit (MgO) ve sodyum sülfat (Na2SO4) kullanılmıştır. Öncelikle, 1:3:0,8 ve 1:2,5:0,8 (mol:mol:mol) olmak üzere iki farklı B2O3:Mg:C başlangıç karışımına ağırlıkça % 5 oranında katalizör maddeler ilave edilerek deneyler yapılmıştır. Deneyler silindirik bakır pota içerisinde gerçekleştirilmiştir. Elde edilen SHS ürünleri stokiyometrik oranın % 50 fazlası HCl ile çözümlendirme yapılarak MgO ve oluşan diğer oksitler çözeltiye alınmış ve B4C elde edilmeye çalışılmıştır. Elde edilen ara ve nihai ürünlerin karakterizasyonu için X-ışınları, yüzey alanı (BET), karbon ve oksijen analizi ile kimyasal analiz yöntemleri kullanılmıştır.
X-ışınları difraksiyon analizi (XRD) ile SHS ürünlerinde ağırlıklı olarak MgO fazının bulunduğu ve indirgenemeyen B2O3'ün serbest yapıda veya MgO ile tepkimeye girerek magnezyum borat [Mg3B2O6/Mg3(BO3)2] yapısını oluşturduğu belirlenmiştir. Yine X-ışınları analizi ile filtre keklerinde bor karbürün yanında serbest karbonun varlığı da tespit edilmiş, karbon analizi ile bu serbest karbonun yaklaşık ağırlıkça % 25'e kadar çıkabildiği belirlenmiştir. Yapılan oksijen analizlerinde filtre keklerinde % 1 ile % 2 oranlarında oksijenin mevcut olduğu saptanmıştır.
Liç çözeltilerine yapılan kimyasal analizlerde yüksek oranda borun çözünerek yaklaşık % 20-25 oranında verim kaybına sebebiyet verdiği saptanmıştır. Bunun nedeni olarak B2O3'ün tamamen indirgenememesi ve Mg3(BO3)2 yapısının oluşması gösterilebilir.
Yüzey alanı analizlerinde MgO, Na2SO4 ve MgSO4.7H2O katkılı SHS deneylerinde konvansiyonel yöntemle veya katalizör kullanılmadan yapılan SHS deneylerine oranla daha yüksek yüzey alanına sahip bor karbür tozları üretildiği belirlenmiştir.
Değişen oranlarda MgO ve MgSO4.7H2O katalizör katkısının etkisi ikinci grup deneylerde incelenmiştir. Özellikle %5 ve %7 MgO katkılı deneylerde filtre kekleri yüzey alanının yaklaşık 30 m2/g civarında olduğu tespit edilmiştir.

Abstract:
Self-propagating high-temperature synthesis (SHS), is a relatively novel and simple method for producing certain advanced ceramic, composites and intermetalic materials. This process has received considerable attention as alternative to conventional furnace technology. The principle concept of this process is that, such as burning of coil or wood, when initiated, a highly exothermic reaction can become self sustaining and will propagate through the reactant mixture in the form of combustion wave. For this happen, chemical reaction must have a relatively high activation energy and must also generate sufficient amount of heat for the continuity of the combustion vawe. 
The SHS method has been used for synthesise advanced materials and inorganic compouns for more than a century. However, the systematic study of this method was discovered by Merzhanov and his associates in 1967 in former Russia. Actually more than 500 compounds such as borides, carbides, nitrides, aluminides silicides, hydrides, intermetallics, carbonitrides, cemented carbides, chalgogenides, binary compounds and composites have been synthesising by this process. These materials can be applied in industry as resistive heating elements, powders for further processing, abrasives, cutting tools, poolishing powders, shape memory alloys, steel processing additives, high temperature intermetalic compounds, electrodes for electrolysis of corrosive media, coatings for containment of liquid metals or corosive media, thin film and coating and composite materials etc.
When compared to the processes for synthesis advanced materials like conventional furnace process or reaction in shock and detonation waves etc., the SHS method has very important andvanteges. The products obtained with this method are more pure due to the generated high temperature, wich can volatilize low boiling point empurities; the simplicity of the process avoid the need for expensive processing equipment and facilities; because the exotermic reaction time is very short, the operating and processing costs are low; formation of metastable phases are possible due to high thermal gradients and rapid cooling. 
One of the methods to produce iron free boron carbide powders (B4C) is the SHS process. This process comprises reaction of anhydrous boric acid powders (B2O3) with magnesium powders as reducing agent and carbon black powders as carburizer. Process is followed by leaching the reaction products in an aqueous hydrochloric acid media. In this study, effects of various additives as catalyst in the SHS stage on the production of B4C were investigated. Definite proportions of catalysts, potassium chlorate (KClO3),  potassium hydrogen sulphate (KHSO4), magnesium sulphate hepta hydrate (MgSO4.7H2O), sodium chloride (NaCl), magnesium oxide (MgO) and sodium sulfate (Na2SO4), were mixed with initial SHS charge mixtures containing B2O3, magnesium and carbon black powders. Two different molar ratios were used for initial SHS mixtures as 1 B2O3 - 3 Mg - 0.8 C and 1 B2O3 - 2.5 Mg - 0.8 C. After the SHS step, an HCl leaching process was applied to the reaction products. Leach cakes were dried and characterized by using XRD (X-Ray Diffraction Spectroscopy), surface area measurement-BET and chemical analysis methods. The obtained results were compared with the literature.
Boron carbide (B4C) is the fourth hardest material after diamond, boron carbo nitride (heterodiamond, BC2N) and cubic boron nitride  and it is even harder than diamond and cubic boron nitride above 1300 oC. Due to its excellent hardness, low density (2,52 g/cm3), good chemical resistence and very good neutron absorbtion cross-section, B4C is used in many areas such as high performance body tiles, abrasive tools, lightweight armor body, ceramic composites, control and shielding material in nuclear industry etc. 
B4C can be synthesized by several methods, such as direct reaction of carbon with boron, carbothermic reduction of boron oxide, reduction of BCl3 by CH4 with laser (1500 oC). B4C nanoparticles can be produced by using BBr3 and CCl4 as the reactants and metallic Na as the coreductant and by a reaction of boron from thermal decomposition of MgB2 with carbon nano tubes. Each proces has some advanteges and disadvanteges. B4C powders with high purity can be synthesized from elemantal boron and carbon directly, but the high cost of these elements make this process inattractive. The main industrial method for the production of B4C is the carbothermal reduction of boron oxide and/or boric acid in a batch electric arc or resistance furnace. However, the process requires high temperarature, also the process is expensive and time consuming. Because the product is in coarse grained, it needs crushing and intensively milling to produce powder and then milled powder requres an acid leaching for purification. Laser process requires expensive equipment and the productivity is low. The row material for sol-gel method are very expensive and the process is inconvenient.
Comparing with the other methods, production of B4C powders by using SHS method provide an attractive low cost and simple alternative. Here we report our investigations about the effect of various addives as catalyst in SHS stage of magnesiothermic synthesis of B4C. 
In the SHS experiments, B2O3 (98.60% pure and particle size < 53 μm), Mg (99.7% pure and partical size < 150μm), carbon black powders (99.8% pure and surface area with 27 m2/g) and technical powders of additive catalyst were used. B2O3 powder was obtained by the calcination of 99.5% boric acid  (H3BO3 Eti Holding Inc.) in a Ni crucible at 1073 K for 2 hours followed by milling and sieving.
The initial mixtures were prepared from dried powders at molar ratios of B2O3:Mg:C as 1:3:0.8 and 1:2.5:0.8 and firstly 5% of additive catalyst. The powder mixtures (100 g) were charged into a spherical Cu crucible, which has  10 mm thickness, 38 mm inner diameter and 145 mm height, and compacted with vibration. W resistance wire was placed on the surface of mixture and then the reaction was realized by passing electricity through the resistance wire. The obtained SHS products were discharged and milled below 45 μm and then leached (25 g) with aqueous 150% of the stochiometric HCl (353 K, 60 min, 400 rpm, 1/10 solid/liquid ratio). Leach cakes were dried and characterized by using X-ray difractometer (Rigaku miniflex powder XRD analyzer, Cu Kα X-ray tube (30 kV; 15 MA)). B and Mg contents of samples were measured by using atomic absorbtion spectrometer (Perkin Elmer Analyst 800) and C content was measured with ELTRA CS-800 and O content was measured with ELTRA ONH 2000. Particle size of products was measured by using BET absorption method (Quantachrome Nova 2200e).
Oxygen content was also analysed in some leached products and was found as 1.41 and 2.59 wt.%. It was seen that small amount of magnesium borate structure was still present in the leached products. According the XRD and chemical analysis between 15 and 35 wt. % of free carbon was still present in the leached products too and the highest amount of B4C in leach cakes was nearly 80 wt.%.
As it is shown in XRD pattern, Mg3(BO3)2 (magnesium borate) occured in SHS stage as a byproduct and it caused the lost of boron to the leach solution and decreased the yield of B4C.
Specific surface area of the commercial B4C was given as 4.92 m2/g and the B4C produced without additives via SHS as 15.56 m2/g. According to result of our surface area mesurement on leached SHS product we can say that with the addition of catalyst via SHS method , it is possible to produce B4C powder with larger specific surface area than the commercial method and  than SHS method without catalyst addition.
According to the XRD-pattern and chemical analysis results, there is an important amount of free carbon still exists in leached product. Further studies should be considered to obtain less or no free carbon in leached product with the less addition of carbon black powder to the initial SHS mixture.