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1 www.em3e.eu The EM3E Master is an Education Programme supported by the European Commission, the European Membrane Society (EMS), the European Membrane House (EMH), and a large international network of industrial companies, research centres and universities SILICON CARBIDE-BASED CAPILLARY MEMBRANES for GAS SEPARATION and WATER TREATMENT Done by –MENGISTU Meron Mulatu Supervisors: Dr. CERNEAUX Sophie & Dr. BERNARD Samuel 24 June-2014 European Master ERASMUS MUNDUS MASTER IN MEMBRANE ENGINEERING 2 The EM3E education programme has been funded with support from the European Commission. This publication reflects the views only of the author, and the Commission cannot be held responsible for any use which may be made of the information contained therein El programa de educación EM3E ha sido financiado con el apoyo de la Comisión Europea. Esta publicación es responsabilidad exclusiva de su autor, y la Comisión no se hace responsable del uso que pueda hacerse de la información aquí difundida. Le programme d'éducation EM3E a été financé avec le soutien de la Commission européenne. Cette publication n'engage que son auteur et la Commission ne peut être tenue responsable de tout usage qui pourrait être fait des informations qui y sont contenues. Die EM3E Bildungsprogramm wurde mit Unterstützung der Europäischen Kommission finanziert. Diese Veröffentlichung trägt allein der Verfasser, die Kommission haftet nicht für die weitere Verwendung der darin enthaltenen Informationen verantwortlich gemacht werden kann. Il programma di educazione EM3E è finanziato con il sostegno della Commissione europea. Questa pubblicazione riflette solo le opinioni dell'autore, e la Commissione non può essere ritenuta responsabile per qualsiasi uso che possa essere fatto delle informazioni in essa contenute. Образователната програма EM3E е финансиран с подкрепата на Европейската комисия. Тази публикация отразява само личните виждания на нейния автор и от Комисията не може да бъде държан отговорен за всяка употреба, която може да бъде извлечена от информацията, съдържаща се в него. Obrazovni program EM3E je financiran uz potporu Europske komisije. Ova publikacija odražava stavove samo autora, a Komisija ne može se smatrati odgovornim za bilo kakvu uporabu koji mogu biti izrađene od informacija sadržanih u njemu. Vzdělávací program EM3E byl realizován za finanční podpory Evropské komise. Tato publikace odráží pouze názory autora a komise nemůže být zodpovědná za jakékoli užití, které mohou být vyrobeny z informací v něm obsažených. 3 Det EM3E uddannelse er finansieret med støtte fra Europa-Kommissionen. Denne publikation forpligter kun forfatteren, og Kommissionen kan ikke holdes ansvarlig for nogen brug, der måtte blive gjort af oplysningerne heri. De EM3E opleiding werd gefinancierd met de steun van de Europese Commissie. Deze publicatie geeft de mening van de auteur weer en de Commissie kan niet verantwoordelijk voor het gebruik dat kan worden gemaakt van de daarin opgenomen informatie worden gesteld. EM3E haridusprogramm on rahaliselt toetanud Euroopa Komisjon. Väljaanne kajastab seisukohti autor, ja komisjon ei ole vastutav mis tahes kasutamise, mis võib olla valmistatud selles sisalduv teave. EM3E Koulutusohjelma on saanut tukea Euroopan komissiolta. Tästä julkaisusta vastaa ainoastaan sen laatija, eikä komissio ole vastuussa mahdollisesta käytöstä tehtävä sisältämien tietojen. Το πρόγραμμα EM3E εκπαίδευση έχει χρηματοδοτηθεί με την υποστήριξη της Ευρωπαϊκής Επιτροπής. Η παρούσα δημοσίευση δεσμεύει μόνο τον συντάκτη της και η Επιτροπή δεν μπορεί να θεωρηθεί υπεύθυνη για οποιαδήποτε χρήση που μπορεί να γίνει των πληροφοριών που περιέχονται σε αυτήν. A EM3E oktatási program már finanszírozott támogatásával az Európai Bizottság. Ez a kiadvány nézeteit tükrözi a szerző, és az Európai Bizottság nem vállal felelősséget bármilyen használata, amely lehet a benne foglalt információk. Tá an clár oideachais EM3E maoiníodh le tacaíocht ón gCoimisiún Eorpach. Léiríonn an foilseachán seo tuairimí an údair amháin, agus ní féidir leis an gCoimisiún a bheith ar siúl freagrach as aon úsáid a d'fhéadfaí a bhaint as an eolas atá ann. EM3E izglītības programma ir finansējusi Eiropas Komisija. Šī publikācija atspoguļo vienīgi autora uzskatus, un Komisijai nevar uzlikt atbildību par tajā var būt izgatavots no tajā ietvertās informācijas. 4 EM3E ugdymo programa buvo finansuojamas remiant Europos Komisijai. Šis leidinys atspindi tik autoriaus požiūrį, todėl Komisija negali būti laikoma atsakinga už bet kokį čia pateiktos informacijos panaudojimą. Il-programm ta 'edukazzjoni EM3E ġie ffinanzjat bl-appoġġ mill-Kummissjoni Ewropea. Din ilpubblikazzjoni tirrifletti l-opinjonijiet tal-awtur biss, u l-Kummissjoni ma tistax tinżamm responsabbli għal kwalunkwe użu li jista 'jsir mill-informazzjoni li tinsab fiha. Program edukacji EM3E został zrealizowany przy wsparciu finansowym Komisji Europejskiej. Projekt lub publikacja odzwierciedlają jedynie stanowisko ich autora i Komisja Europejska nie ponosi odpowiedzialności za jakiekolwiek wykorzystanie, które mogą być wykonane z informacji w nim zawartych. O programa de educação EM3E foi financiado com o apoio da Comissão Europeia. Esta publicação reflecte apenas as opiniões do autor, ea Comissão não pode ser responsabilizada por qualquer uso que possa ser feito da informação aqui contida. Programul de educație EM3E a fost finanțat cu sprijinul Comisiei Europene. Această publicație reflectă numai punctul de vedere al autorului, iar Comisia nu poate fi considerată responsabilă pentru orice utilizare care ar putea fi a informațiilor conținute de acestea. Vzdelávací program EM3E bol realizovaný za finančnej podpory Európskej komisie. Táto publikácia odráža iba názory autora a komisia nemôže byť zodpovedná za akékoľvek použitie, ktoré môžu byť vyrobené z informácií v ňom obsiahnutých. Program izobraževanja EM3E je financirana s strani Evropske komisije. Ta publikacija odraža le stališča avtorjev, in Komisija ne more biti odgovorna za kakršnokoli uporabo, ki se lahko iz informacij, ki jih vsebuje. Den EM3E utbildningsprogram genomförs med ekonomiskt stöd från Europeiska kommissionen. Denna publikation ansvarar endast upphovsmannen, och kommissionen kan inte hållas ansvarig för någon användning som kan göras av informationen däri. 5 ACKNOWLEDGEMENTS Glory be to the almighty God, who always guide me throughout my life. Next I offer my deep gratitude to my supervisors Dr. CERNEAUX Sophie and Dr. BERNARD Samuel who supported me throughout this project with a valuable comments and encouragement that enabled me to get a wider understanding of the subject. I also want to thank Mr. Anthony Ballestero for his help during the experimental work. Without their support it would have been impossible to accomplish it. It is an honour to thank EM3E, UNIZAR and IEM stuff members and laboratory technicians for their assistance and support to accomplish this thesis. Finally I would like to thank my family and all my friends who encouraged me thorough out the project. 6 Contents ACKNOWLEDGEMENTS ................................................................................................................................. 5 1. INTRODUCTION ..................................................................................................................................... 9 2. OBJECTIVES ......................................................................................................................................... 15 3. STRUCTURE and PROPERTY of SILICON CARBIDE ............................................................................... 16 3.1 Thermal Stability of SiC Polytypes ............................................................................................... 17 3.2 Crystallographic Properties ......................................................................................................... 17 4. SYNTHESIS METHOD for SiC-BASED NON-OXIDE CERAMICS .............................................................. 18 4.1 Powder Routes for SiC Synthesis ................................................................................................ 18 4.2 Molecular Routes for SiC Synthesis ............................................................................................ 20 4.2.1 Synthesis of Preceramic Polymer ........................................................................................ 22 4.2.2 Shaping and Cross-Linking .................................................................................................. 24 4.2.3 Pyrolysis .............................................................................................................................. 25 5. MACRO POROUS SiC MEMBRANE PROCESSING ROUTES ................................................................... 27 5.1 Partial Sintering Method ............................................................................................................. 27 5.2 Sacrificial Template Method ....................................................................................................... 28 5.3 Direct Foaming Method .............................................................................................................. 29 5.4 Bonding Method ......................................................................................................................... 30 6. PREPARATION of CERAMIC MEMBRANE ............................................................................................ 32 6.1 Extrusion ..................................................................................................................................... 32 6.2 Slip Casting .................................................................................................................................. 32 6.3 Tape Casting ................................................................................................................................ 33 6.4 Sintering ...................................................................................................................................... 34 6.4.1 Sintering Phenomena .......................................................................................................... 35 6.4.2 Sintering Stages ................................................................................................................... 36 7. CHARACTERIZATION TECHNIQUES ...................................................................................................... 38 7.1 Mercury Porosimetry .................................................................................................................. 38 7.2 Scanning Electron Microscopy (SEM) ......................................................................................... 38 7.3 Differential Scanning Calorimetry (DSC) ..................................................................................... 38 7.4 ThermoGravimetry Analysis (TGA).............................................................................................. 39 7.5 Fourier Transform InfraRed Spectroscopy (FTIR)........................................................................ 39 7.6 X-Ray Diffraction (XRD) ............................................................................................................... 39 7 7.7 Mechanical Strength ................................................................................................................... 39 8. EXPERIMENTAL ANALYSIS ................................................................................................................... 40 8.1 Preceramic Polymer Synthesis .................................................................................................... 40 8.1.1 Materials and Reactions ...................................................................................................... 40 8.1.2 Procedure ............................................................................................................................ 40 8.1.3 Results and Discussion ........................................................................................................ 43 8.2 SiC Macroporous Support Preparation ....................................................................................... 44 8.2.1 Materials ............................................................................................................................. 44 8.2.2 Procedures .......................................................................................................................... 45 8.2.3 Results and Discussion ........................................................................................................ 48 9. CONCLUSIONS and RECOMMENDATIONS .......................................................................................... 60 10. REFERENCES .................................................................................................................................... 62 I. List of Figures Figure 1. Schematic representation for definition of membrane ................................................................. 9 Figure 2. Size of separated substance and pore diameters of separation membranes ............................. 10 Figure 3. Schematic representation of multilayer inorganic membrane ................................................... 10 Figure 4. Reverse osmosis .......................................................................................................................... 11 Figure 5. Common SiC polytypes on [1120]plane, a)3C polytypes ( Zinc blend structure) b)6H polytypes c)2H polytype (Wurtizie structure) ..................................................................................................................... 16 Figure 6. Schematic diagram of powder route (left) and molecular route (right) for ceramic membrane synthesis ..................................................................................................................................................... 19 Figure 7. Silicon-based preceramic polymers ............................................................................................ 21 Figure 8. A general formula for preceramic polymer and different ceramics obtained through pyrolysis 21 Figure 9. Preceramic polymer synthesis routes from organo-chlorosilane ............................................... 23 Figure 10. Forming method for preceramic polymer ................................................................................. 25 Figure 11. Partial sintering .......................................................................................................................... 28 Figure 12. Sacrificial template synthesis method ...................................................................................... 29 Figure 13. Direct foaming method ............................................................................................................. 30 Figure 14. Bonding method ........................................................................................................................ 31 8 Figure 15. Slip casting process on a porous mold ...................................................................................... 33 Figure 16. Tape casting process ................................................................................................................. 34 Figure 17. Grain size and density or shrinkage of a precursor as a function of sintering temperature .... 37 Figure 18. 2-neck flask placed under dynamic vacuum and at 0oC ............................................................ 41 Figure 19. a. Solvent extraction under dynamic vacuum and at 60oC, b. After solvent extraction, yellow preceramic polymer and iced toluene are allowed to cool down and to warm up to room temperature 42 Figure 20. FTIR analysis of the five synthesized preceramic polymers Si/B/C and SiC precursor .............. 44 Figure 21. Mixer (DITO, Electrolux) ............................................................................................................. 45 Figure 22. Photographs of shaping equipments: a) roll-pressing (DITO SAMA) and b) hydraulic press used for vertical extrusion ................................................................................................................................... 46 Figure 23. A photograph of a roll-drier ....................................................................................................... 46 Figure 24. Temperature program used for sintering the prepared supports ............................................. 47 Figure 25. Photographs of sintered flat and capillary supports sintered at a) 1200°C in air, b) 1200°C in argon and c) 1600°C in argon. SEM picture d) represents the cross-section of a capillary sintered at 1200oC in air ............................................................................................................................................................ 48 Figure 26. XRD results of (a) SiC -500 and (b) SiC-800 powders ................................................................. 49 Figure 27. SEM images of the various synthesized supports after firing at different temperatures and atmospheres ............................................................................................................................................... 52 Figure 28. XRD results of MM01-1400-Air under air .................................................................................. 53 Figure 29. Pore size distribution of MM01-1400-air ................................................................................... 54 Figure 30. MM02 fired at different temperature ....................................................................................... 55 Figure 31. Pores size distributions of different compositions samples fired under Air at 1200oC ............. 56 Figure 32. Porosity and average pore diameter for MM02, MM04 and MM05 sintered at 1200oC under air .................................................................................................................................................................... 57 Figure 33. Porosity and average pore diameter of MM02 fired at 1200oC under air and argon ............... 58 Figure 34. Pore size distribution, porosity and pore diameter of MM03-900-Ar ....................................... 59 9 1. INTRODUCTION Membrane technology is an emerging technology that has become very important in the development of different industrial processes for sustainable growth. Membrane can be defined as a barrier that separates two phases, through a selective transport of one component over the other (figure 1). There are different ways to classify synthetic membranes, for instance depending on nature of the material; they can be grouped as inorganic such as ceramics or metals and organic such as polymers. On the other hand according to their shape they can be classified as tubular, flat and hollow fibres membranes (A.G. Fane, Rong Wang et al. 2011). Figure 1. Schematic representation for definition of membrane (Huang, Lee et al. 1994) IUPAC classification of membranes according to pore size are macroporous (>50nm), mesoporous (2-50nm) and microprous (<2nm). The pore size determines its industrial application (figure 2). Macroporous membranes are used for microfiltration and ultrafiltration processes, while mesoporous membranes can be implemented in ultrafiltration, nanofiltration and gas separation processes. Microporous and dense membranes are used for gas separation and industrial reaction process (Li 2007). 16 3. STRUCTURE and PROPERTY of SILICON CARBIDE Silicon carbide has different crystalline structures called polytypes that are built up by a stacking of identical SiC crystal at a different sequence (figure 5). Each polytype is stable at different temperatures. Generally the polytypes can be divided into β-SiC (Zinc blend) as a low temperature form (1500-1800oC) cubic phase and α-SiC as a high temperature (>1500°C) hexagonal phase (SHAFFER 1969). Figure 5. Common SiC polytypes on [1120]plane, a)3C polytypes ( Zinc blend structure) b)6H polytypes c)2H polytype (Wurtizie structure) (Jason Guth and Petuskey 1986) β-SiC is found in a cubic shape (3C) while different hexagonal (2H, 4H and 6H), rhombohedral (15R) and other shapes are categorized in α-SiC. The numbers indicate the stacking sequence repetition while C, H and R stand for cubic, hexagonal and rhombohedral shapes, respectively. For example, according to Ramsdell’s notation, on the (111) plane β-SiC has a stacking sequence of ABCABC…while for 2H also called wurtzite type, on the (0001) plane, it has a stacking sequence of ABAB… In all polytypes, the Si:C atomic ratio is 1:1and each atom is tetrahedrally bonded to a heterogeneous neighbouring atom (Inomata 1991). 17 3.1 Thermal Stability of SiC Polytypes The stability and /or formation of SiC polytpes depend on the temperature. At temperatures below 1400oC, 2H polytypes are formed. When the temperature increases up to 1500oC, 2H-structure will transform into another polytype but to a similar crystal appearance. If the temperature increases further, its crystal appearance will also transforms and 3C, 4H and 6H will be formed irreversibly (Inomata 1991). 3C is formed at early stage of crystallisation and when temperature reaches and exceeds 1600oC, it will be transformed to α-SiC polytypes (except 2H) by recrystallization. 4H has a probability of formation at approximately 2000oC and at temperatures ranges between 2200oC and 2600oC; 6H is more stable than the other polytypes. Though 15R has also a probability to be formed at temperatures greater than 2000oC, 6H is more stable compared to 15R (Inomata 1991). 3.2 Crystallographic Properties Highly pure 3C forms yellow transparent crystal, whereas the rest of polytypes are colorless. Depending on the presence of impure elements such as aluminum and nitrogen as a solid solution the crystal will be coloured green or blue. Under pressurized atmosphere and at 2830oC temperature SiC decomposes into graphite and molten silicon (Inomata 1991). Sintered SiC ceramics have a superior strength at high temperatures, thus they are considered as the most promising heat resistant materials. SiC has covalent bond with only 12% of ionicity. This covalent bond is the source for the high strength property. Fundamentally SiC is brittle; however sintered SiC is one of advanced materials that are used at high temperature of 1300oC and above (Inomata 1991). 18 4. SYNTHESIS METHOD for SiC-BASED NON-OXIDE CERAMICS 4.1 Powder Routes for SiC Synthesis Silicon carbide is conventionally synthesized following the Acheson’s method (A. G. Acheson, British Patent, 1892, 17911), in which silica sand or siliceous rocks are reacted with petroleum coke at high temperature (2100-2400oC) in an electric furnace according to reaction 2. Usually α-SiC is obtained by this method even though β-SiC can also be obtained by this method at lower temperature (1500-1800oC) (K. Yamada and Mohri 1991). Acheson’s method has drawbacks, starting from the high temperatures input, since impurities usually remain from the raw materials used such as Fe, Ti and present in siliceous rock. Indeed, these impurities will not be vaporized at high temperature so that they will be present in the synthesized SiC powders as impurities. In addition, the synthesis is done under carbon rich conditions; therefore after synthesis free carbon will also be available as an impurity. In addition as a result of insufficient mixing during reaction, silicon and silicon dioxide are both present in the final product, which results in abnormal grain growth (K. Yamada and Mohri 1991). Above all the mentioned quality problems it is also an environmentally hazardous process since at 1500oC and above there is NOx formation and emission (Riedel 1995). 19 Figure 6. Schematic diagram of powder route (left) and molecular route (right) for ceramic membrane synthesis (Riedel 1995) Particle size obtained by Acheson’s method varies widely, and by controlling temperature it is possible to obtain coarse α-SiC and fine β-SiC. Figure 6 shows a schematic diagram for ceramic synthesis through powder route. The step includes SiC powder mixing with different additives and solvents, followed by shaping and sintering. Ceramic membrane synthesized from a powder requires shaping and sintering at higher temperature (1000oC-2500oC) to obtain a porosity of 3550%. These steps will be discussed in chapter 6 (Riedel 1995; Greil 2000). 20 4.2 Molecular Routes for SiC Synthesis Report about production of non-oxide silicon carbide-based ceramic synthesized from molecular precursor started in the early 1960s. In 1970s, pyrolysis of preceramic polymers such as polysiloxane, polycarbosilane and polysilazanes to ceramic fibers of SiC/Si3N4 for a high temperature application was presented. Afterwards, numerous researches have been done on polymer derived ceramics (PDCs) and a significant progress has been achieved(Riedel 1995; Colombo, Mera et al. 2010). To obtain tailor made SiC-based ceramic it is important that the starting precursor or powder is highly pure and has a controlled composition at the atomic level. In the previous synthesis method of SiC, the powder method, in addition to the economic and environmental problems, it is impossible to control the composition and purity of the powders. By using polymer pyrolysis it is possible to synthesize highly pure non-oxide ceramic from a molecular precursor. Compared to the conventional powder synthesis method molecular precursor has many advantages. It is possible to control the microstructure such as homogeneity, multicomponent and pore size and porosity; and also membrane shapes such as films, continuous fibers, and monoliths are synthesized by the molecular methods that are difficult to obtain through the conventional methods. Specifically for polymer pyrolysis the synthesis is done at lower temperature (800oC-1200oC) that has benefit over the powder technology both economically and environmentally (Riedel 1995; L. V. Interrante, K. Moraes et al. 2002; Colombo, Mera et al. 2010). A preceramic polymer can be defined as an inorganic/organometallic system that when treated thermally can give ceramic with a controlled chemical composition and a closely defined nano structural organization at lower temperature 500oC-1500oC. Common types of organosilyl preceramic polymer are shown in figure 7 (Greil 2000). 21 Figure 7. Silicon-based preceramic polymers (Greil 2000) A general formula for organosilicon and the different possible ceramics obtained by pyrolysis can be seen in figure 8 (Greil 2000; Colombo, Mera et al. 2010). Where R, R’ =allyl, alkyl, aryl, arenyl and X can be NH (polysilazanes), CH2 (polycarbosilanes), O (Polysiloxanes), B (Polyborosilanes). Figure 8. A general formula for preceramic polymer and different ceramics obtained through pyrolysis (Greil 2000) X, R’ and R groups are important parameters in order to design and modify the preceramic polymer at a molecular level. For further modification of the preceramic polymer these groups are advantageous. For example, a polycarbosilane group that contains a reactive R group such as vinyl/allyl will allow further reaction such as hydroboration reaction for the incorporation of boron in final ceramics (Si/B/C). Similarly, hydroboration and thermal treatment of polyvinylsilazanes 22 result in S/B/C/N ceramics. Small amount of boron (<1 wt %) incorporation will increase the overall thermal stability and will thus help the sintering of the silicon carbide ceramic. Therefore incorporation of boron in the preceramic polymer enables the homogeneity of the final boron containing ceramic (Riedel 1995; Varaporn Suwanmethanond, Edward Goo et al. 2000; Alexis R. Puerta, Edward E. Remsen et al. 2003). The functional groups (R and R’) attached to silicon atoms will determine the thermal and chemical stability, and also the solubility and rheological property of the preceramic polymer. In addition, they also define the carbon content in the PDC. Therefore the final PDCs microstructure and thus the physical and chemical properties can be varied and modified to an extent that depends on the design of the polymer precursor (Colombo, Mera et al. 2010). R groups such as hydrogen, aromatic and aliphatic organic groups can be attached to silicon atoms. For further processing, the solubility, thermal stability and viscosity property of the polymers at different temperature are important factors (Colombo, Mera et al. 2010). Generally synthesis of ceramic from polymer precursor involves three steps (figure 6) (Greil 2000; Ralf Riedel, Emanuel Ionescu et al. 2008):  Preceramic polymer synthesis  Shaping and curing of the synthesized polymer and,  Then conversion of the preceramic polymer to ceramic by pyrolysis 4.2.1 Synthesis of Preceramic Polymer Organo-chlorosilane is most commonly used as monomer for the preceramic polymer synthesis. The different silicon based preceramic precursor synthesized from the monomer is shown in figure 9 (Greil 2000; Colombo, Mera et al. 2010). 23 Figure 9. Preceramic polymer synthesis routes from organo-chlorosilane (Colombo, Mera et al. 2010) Chlorosilane is mostly obtained from many silicone industries as a by-product; consequently it is easily available, inexpensive and can be purified by distillation. The preceramic polymer must fulfil some requirements such as appropriate solubility and rheological property for forming process, sufficiently high molecular weight to prevent volatilisation, and the presence of reactivity or functional groups for cross-linking and curing step (Greil 2000; Colombo, Mera et al. 2010). Poly(organosilanes) [-R,R’Si-]n Poly(organosilane) has inorganic Si-Si compounds attached to it. It has high thermal stability property and used for silicon carbide ceramic fiber manufacturing. It is mainly synthesised by dehalogenation reaction of organo-chlorosilane as shown in figure 9 (Greil 2000; Colombo, Mera et al. 2010). 24 Poly(organocarbosilane) [-RR’Si-CH2]n This group is primarily synthesised from poly(organosilane) (such as poly(methylsilane)) by thermal reorganization called Kumada rearrangement reaction (figure 9). Currently it is used for synthesising silicon carbide fiber in research issue because of its high ceramic yield. Currently different kinds of polycarbosilane are commercially available. Many types of poly(organocarbosilanes) have a complex structure having hyperbranched Si-Si and Si-C structure (Greil 2000; Colombo, Mera et al. 2010). Boron containing novel preceramic polymers precursors are under research and development for non-oxide ceramics synthesis. It was reported that incorporation of boron in SiC based ceramic improves sintering temperature. Therefore incorporation of boron to a preceramic polymer will improve the homogeneity of the ceramic. For instance it is reported that Si-B-C ceramic has exceptionally high thermal stability against decomposition, oxidation and crystallization up to 1600oC. Moreover it is developed that Si-B-C-N ceramic stability under inert atmosphere was up to 2000oC (Greil 2000; Colombo, Mera et al. 2010). 4.2.2 Shaping and Cross-Linking A preceramic precursor is a polymer in nature, therefore it obtains a unique characteristic so that it can be shaped in different methods such as fiber drawing and foaming, which are not easily exploited for ceramic powders. For example siloxanes were used to prepare highly porous ceramic foams by saturation with supercritical carbon dioxide, which is a well-known technology (Colombo, Mera et al. 2010). 25 Figure 10. Forming method for preceramic polymer (Colombo, Mera et al. 2010) The preceramic polymer can exist in liquid or in solid phase, if it is in solid phase it can be soluble in different organic solvents or can melt at lower temperature (<150oC). Figure 10 shows different preceramic polymer forming methods. To make forming possible typically a preceramic polymer that is curable and meltable solid or a dissolved solid, or a cross linkable liquid is used. After forming it is necessary to make the polymer maintain its shape by transforming it in to a thermoset. If the polymer has functional groups such as Si-OH, S-H, or Si-vinyl, spontaneous thermosetting will occur by addition or condensation reaction at low temperature (less than 200oC) called thermal cross-linking. The curing temperature can be further decreased by using catalysts (Colombo, Mera et al. 2010). Other curing method that was mostly used in the past is oxidative curing; however it has a disadvantage of excessive oxygen ≈15wt% in the final ceramic product that degrades its stability at high temperatures. Curing without oxygen contamination can also be done by electron beam or ᵞradiation; however it has depth limitation so that it is usually applicable in small dimensional objects such as fibers (Greil 2000). 4.2.3 Pyrolysis The conversion of organic to inorganic starts at 400oC and ends at temperature between 800oC and 1000oC. In addition to the inorganic ceramic the decomposition also results in release of gases mainly C6H6, CH4, CH3NH2, etc. This thermal decomposition brings volume shrinkage as high as 50%, that results in pore formation and also defects such as cracks. This brings a limitation for the synthesis of dense ceramic from a preceramic polymer through a direct conversion. Direct conversion is possible for small dimension pieces (few hundred micrometres such as fibers, coating 32 6. PREPARATION of CERAMIC MEMBRANE Generally there are three steps involved in ceramic membrane synthesis from SiC powder: (i) paste preparation, (ii) forming or shaping and (iii) consolidation of the green body by heat treatment. Paste preparation includes mixing of the silicon carbide powders with organic additives (plasticizer, pore former, lubricant, etc.) and solvent. According to the amount of the prepared paste, it will take few minutes to obtain a homogenized paste by mixing and pugging. Then next step will be preparation of the green body or ceramic precursor by using different shaping methods such as extrusion, roll-pressing or calandering, hot pressing, etc. Densification of the green body called sintering is done by appropriate heat treatment to yield a better mechanical strength (Li 2007; Ralf Riedel, Emanuel Ionescu et al. 2008). In this chapter different forming methods and sintering methods will be explained. 6.1 Extrusion Extrusion process is applicable both in large and small scales of different membrane support syntheses. Different kinds of inorganic membranes such as capillary tube, honey comb catalyst support and different structural ceramics are synthesized by extrusion process. The requirements of the paste to be extruded are to have a plastic behaviour so that it acts as a rigid solid and resists a small stress before deformation. In the process, a stiff paste is compacted and formed by forcing it slowly through a nozzle. An active separation layer can be deposited in the interior of the capillary tube by slip casting process (section 6.2). The limitation of this method is that cross sectional thickness of the capillary is usually larger than 0.5mm, though it is homogenous (Li 2007). 6.2 Slip Casting It is one of the most traditional and most commonly used techniques and ceramic preparation method. Slurry prepared from a well-mixed powder suspended in a solvent is poured into a porous support; the particles will aggregate at the surface of the support while the solvent will be absorbed by capillary suction into the pores (figure 15) (Li 2007). 33 Figure 15. Slip casting process on a porous mold (Li 2007) The slurry is characterized by its viscosity, particle diameter and concentration. A uniform thin layer can be obtained on the support depending on the induction time applied to the slurry and the drying schedule. Casting can be done under pressure or by ultrasonic frequency in order to improve the density of the layer and to avoid shrinkage that might be caused during casting (Ralf Riedel, Emanuel Ionescu et al. 2008). 6.3 Tape Casting Tape casting is also a well-known forming method especially for flat sheet membrane preparation. Basic principles of ceramic membrane prepartion by tape casting is shown in figure 16. The prepared slurry will flow from the reservoir on a movable carrier guided by a casting knife. The thickness of the flat sheet will be determined by the distance between the tip of the knife and the carrier. 34 Figure 16. Tape casting process (Li 2007) Viscosity of the slurry and carrier speed also controls the film thickness. The prepared film can be dried in the atmosphere or it can be placed in oven at a controlled temperature to avoid the degradation of the film. It is an advantageous technique for preparing large, flat and thin ceramic membranes, leading mainly to thickness between 250-1250µm (Li 2007; Ralf Riedel, Emanuel Ionescu et al. 2008). 6.4 Sintering Sintering is the process in which the formed green ceramic is converted into a dense ceramic by heat treatment. To synthesize a ceramic that has a specific property, different processing variables such as particle size and packing, composition, temperature and sintering atmosphere have their importance. They also have an influence on the microstructure and material property of the final ceramic (Lutgard C. De Jonghe and Rahaman 2003; Ralf Riedel, Emanuel Ionescu et al. 2008). Depending on the second phase that is formed during heating, sintering can be classified as solid state-sintering, liquid state-sintering and viscous sintering. For ceramic membrane solid sintering is the common one. In solid sintering the green ceramic is heated up to a temperature that is 0.5-0.9 of its melting point. The dominant mechanism is diffusion of atoms that results in grain growth and densification (Lutgard C. De Jonghe and Rahaman 2003). To enhance densification of ceramic, pressurized sintering can be implemented, in which an external pressure can be applied while heating. Hot pressing (HP) and hot isostatic pressing (HIP) 35 are the common types of pressure sintering. HP technique combines temperature and a uniaxial external pressure while in HIP pressurized gas and high temperature enhance densification. In general, pressure sintering increases the production cost, but dense and small grain size ceramic membrane can be obtained (Lutgard C. De Jonghe and Rahaman 2003; Ralf Riedel, Emanuel Ionescu et al. 2008). 6.4.1 Sintering Phenomena During sintering densification occurs through a process of diffusion that brings particle deformation and results in interface formation. For sintering to take place there are two conditions that must be satisfied, first free energy of the system must be lowered and second diffusion of atoms must occur. The first condition involves crystal grain boundary formation that reduces the particle surface area; as a result free energy of the system will be lowered. In the second case particle diffusion can be promoted by heating the material at high temperature (Lutgard C. De Jonghe and Rahaman 2003; Inomata 1991). Reduction in grain boundary and increment in diffusion promotes sintering and densification of a sample. Furthermore particle size, size distribution and purity of the sample also have an influence on sintering process. Grain boundary formation is directly related to the chemical bonding and structure of the material. For instance a sample with metallic bond can be densified due to its surface energy relaxation by joining with neighbouring particles and form grain boundary easily. However in case of SiC the presence of covalent bond would not allow surface energy relaxation and render impossible the achievement of dense and strong material even if it is sintered at very high temperature and for a long period of time. SiC powders lack the driving energy required for densification that makes them fundamentally non-sinterable. Therefore addition of sintering aids is necessary to reduce grain boundary energy that enhance sintering of SiC powders, to obtain an advanced SiC ceramic with a super mechanical strength at high temperature. Silica, boron and carbon, and aluminium oxides are common sintering aids that are used for SiC sintering (refer section 5.4). Incorporation of small amount of boron lowers sintering temperature, by lowering the grain boundary energy (Lutgard C. De Jonghe and Rahaman 2003; Inomata 1991). 36 6.4.2 Sintering Stages Depending on the microstructure, sintering can be divided into three idealized stages, initial stages, intermediate (thermolysis) and final stage. In initial stage, the main cause of material change is vaporisation of water that was combined chemically with the particles surface or from inorganic phases that contains water crystal. As a result of the vapour formed or due to the different thermal expansion coefficient of the phases present, stress will be caused. Therefore care must be taken as cracks might be formed. The adsorbed moisture may sustain in the membrane at temperatures exceeding 200oC (Lutgard C. De Jonghe and Rahaman 2003; Li 2007). Organic components such as dispersant, binders, lubricants, etc. are burnt out for further densification occurrence, this is called thermolysis. Membrane defects might be created by uncontrolled thermolysis and incomplete removal of binder. Therefore for the performance of the final synthesized membrane, it is vital to choose an appropriate binder and heating atmosphere so that the precursor can survive without being distorted, deformed or cracked (Li 2007). If the binder in the green ceramic is small, it would open pore channels during thermolysis that are sufficient enough for the transportation of vapours and gases from internal reaction zone to the membrane surface. The internal stress caused by the decomposition and vaporisation of the organic additives depends on the permeability, precursor size, gas evolution rate. For example in dense ceramic membrane formed by fine particles, the rate of gas permeation is very slow and the stress will be high. Generally for ceramic precursors that contain large amount of organic components, the duration of the thermolysis is longer. Carbon residues are expected when thermolysis is done in an inert atmosphere (without oxygen). This carbon residue serves as sintering aid by removing the adsorbed oxygen from the particle surface (Li 2007). Final sintering is assumed to occur in three stages as shown in figure 17: initial stage (I), intermediate stage (II) and final stage (III). When the temperature reaches the sintering temperature, mobility of atoms is achieved by diffusion and a sharp concave neck between the particles will start to form. This is the initial stage; as a result low densification will be achieved with a total porosity lowered by 12% (Lutgard C. De Jonghe and Rahaman 2003; Li 2007). 37 Figure 17. Grain size and density or shrinkage of a precursor as a function of sintering temperature (Li 2007) Intermediate stage covers the highest portion of densification in sintering process as can be seen in figure 17. This stage starts when the sharp curvature developed in the initial stage becomes moderate and a three dimensional pore channel is formed (Lutgard C. De Jonghe and Rahaman 2003). Density obtained ranges between 0.65-0.90 of the theoretical value. As sintering continues grain growth become significant (Li 2007). At the final stages, the channels will break down and isolated pores (closed voids) start to form. As a result of the closed pores grain boundaries will start to intersect with one another. Then the pores slowly shrink and large grain size starts to appear rapidly. Pore shrinkage continues and may disappear altogether (Lutgard C. De Jonghe and Rahaman 2003). In overall at relatively low temperature slow grain growth and a rapid densification are achieved (stage II) that is followed by little densification with a rapid grain growth at a relatively higher temperature (stage III) (Li 2007). 38 7. CHARACTERIZATION TECHNIQUES 7.1 Mercury Porosimetry Mercury porosimetry is a well-known membrane characterization method to determine pore size, porous volume and pore size distribution. Mercury is a non-wetting liquid, so that by applying a pressure it is forced to enter inside a pore. The relation between the applied pressure and pore size is explained by the Laplace equation (Li 2007). For this project micrometrics porosimetry equipment AutoPore IV 9500 was used. 7.2 Scanning Electron Microscopy (SEM) Morphology of a synthesized membrane surface and cross section can be seen in details using SEM. High energy electrons beam generated in the SEM will bombard the sample surface, and electrons emitted from the interaction generate image of the sample surface. The resolution of SEM is about 5nm to 100nm and with a magnification of 105 (RUSTE 2014). In this project HITACHI S4800 FEG equipment was used to analyse the synthesized membranes. The samples to be analyzed must be solid, with maximum height and thickness of2.5 each. Also the sample must be sputtered before analysis with a conductive layer (Pt) and thus pretreated under vacuum. 7.3 Differential Scanning Calorimetry (DSC) DSC is a thermal analysis used for characterizing the physical property of a sample when exposed to a temperature. The analysis can be isothermal for a specific time or with increasing temperature in a controlled atmosphere. Information obtained from DSC is the heat change of the sample with respect to temperature that are phase change properties such as glass transition, or decomposition (Jun Huang 2013). 39 7.4 ThermoGravimetry Analysis (TGA) TGA is also used for thermal characterization of a sample, by measuring the weight loss occurring during the decomposition of sample as a function of the temperature. It is used as an optimization of the polymer to ceramic conversion, by comparing the stability at high temperature and yield of ceramic (Daudon 2014). This technique is also used to evaluate the temperature of decomposition of the organic additives incorporated in the paste to establish the final heat treatment schedule. For this project ATG Q500 High resolution (TA instrument) was used for optimizing the preceramic polymers. 7.5 Fourier Transform InfraRed Spectroscopy (FTIR) FTIR is a characterization technique that gives information on the nature of the chemical bond present in the sample. The sample will be placed in the equipment and upon the infrared beam, interacts with it. Depending on the energy level of the chemical bonds, different vibrations bands are obtained that are characteristic of a typical function (DALIBART Michel 2000). 7.6 X-Ray Diffraction (XRD) XRD is a very important characterisation technique for identification of phase separation and crystal structure present in crystal powders or membranes (Li 2007). For this specific project, X’pert PRO XRD equipment was used for the analysis of SiC starting powders. 7.7 Mechanical Strength The mechanical strength of the synthesised silicon carbide membrane is analysed by a well-known three point bending test (LLOYD instrument). Test results from this analysis are Young’s modulus and tensile strength that defines the bending strength of the synthesised membrane (Li 2007). 40 8. EXPERIMENTAL ANALYSIS 8.1 Preceramic Polymer Synthesis 8.1.1 Materials and Reactions Polymer synthesis was carried out in a purified argon atmosphere by means of standard Schlenk manipulations and vacuum/argon-line techniques. Argon (>99.95%) was purified by passing through successive columns of phosphorous pentoxide, siccapentTM and BTS catalysts. The handling of the chemical products was made inside MBraun glove box (<<0.1 ppm H2O and O2, Germany) under argon atmosphere. Preceramic polymer containing boron, silicon and carbon (Si/B/C polymer) is synthesized through hydroboration reactions of a SiC precursor with a borane by using toluene as solvent. 8.1.2 Procedure The whole reaction is performed under inert or oxygen free atmosphere under a fume-hood and each chemical is used under an argon flow to avoid any degradation. The vacuum/argon ramp used during the syntheses is depicted in Figure 18. A 50mL 2-neck flask is connected to the ramp with the borane holder placed at the top. To remove air, the system is allowed to stay under dynamic vacuum for 30 minutes. An ice bath is also placed under the round-bottom flask to maintain the system at 0oC for 1-2 hours under dynamic vacuum, prior hydroboration. 41 Figure 18. 2-neck flask placed under dynamic vacuum and at 0oC A total of 50mL of toluene is poured with a syringe into the 2-neck flask and the borane holder flask under argon flow. Then 3mL of SiC precursor were injected with a syringe into the flask followed by injection of required amount of borane (table 1) that finally, was added drop by drop under stirring. The mixture is left at room temperature for three days to allow completion of the hydroboration reaction. 48 c) d) Figure 25. Photographs of sintered flat and capillary supports sintered at a) 1200°C in air, b) 1200°C in argon and c) 1600°C in argon. SEM picture d) represents the cross-section of a capillary sintered at 1200oC in air 8.2.3 Results and Discussion Prior to paste preparation SiC-500 and SiC-800 powders were characterized by XRD. α-SiC polytype, specifically 2H and 6H types, were identified as shown in figure 26. The particle diameter of SiC-500 and SiC-800 were 11.8-13.8µm and 5.5-7.5µm, respectively. a) 49 b) Figure 26. XRD results of (a) SiC -500 and (b) SiC-800 powders To understand the effect of temperature, sintering atmosphere, pore agent and bonding agent on the physico-chemical properties of the SiC supports, different compositions were prepared and sintered at different temperatures and under different atmospheres. The different compositions prepared in weight percentage (wt. %) are summarized in table 2. Table 2. Paste compositions Name SiC-500 SiC-800 Amijel corn starch and maize starch* Darvan Methocel Na2B4O7,10H2O MM01 --- 80 5 5 5 5 --- MM02 40 40 5 5 5 5 ---- MM03 32 32 5 5 5 5 16 MM04 41 41 5 2.5 5 5 ---- MM05 40 40 5 5* 5 5 ---- 50 MM01 is a 50g of paste composition that contains 40g of SiC-800 and each 2.5g of amijel, corn starch, darvan and methocel. While MM02 is a 100g paste that contains 40g of each SiC-500 and SiC-800 and 5g of each additive that are used in MM01. For MM03, a sodium borate bonding agent was used with a weight ratio of 2:8 with respect to the total SiC powders used that is 32g of each SiC powders and 16g of sodium borate. The additives used in MM03 were the same as in MM02 with respect to type and composition. To see the effect of pore forming agent three analyses were done. In addition to the first three paste compositions, the corn starch was reduced by half in MM04 sample keeping the other elements similar to MM02, so that the total paste prepared was 97.5g. Finally, MM05 was prepared by using maize starch as pore forming agent instead of corn starch. 100g was prepared and the paste composition was similar to MM02. To study the effect of temperature and firing atmosphere, sintering temperature range from 12001400oC under air flow and from 900-1600oC under argon flow were analyzed for the prepared paste compositions. For simplification, the paste sintered at different temperature and atmosphere is named as “paste name-temperature in oC-atmosphere”. a. MM01-1400-Air b. MM02-1200-Air 51 c. MM02-1600-Ar d. MM02-1200-Ar e. MM03-900-Ar 52 f. MM04-1200-Air g. MM05-1200-Air h. MM04-1600-Ar i. MM05-1600-Ar Figure 27. SEM images of the various synthesized supports after firing at different temperatures and atmospheres  Effect of temperature As stated earlier to analyze the effect of sintering temperature, a similar paste composition was sintered at different temperatures under similar atmosphere. Flat support of MM01 was placed at 1400oC under air (MM01-1400-air) (Figure 27-a). The sintered support looked white and seemed resistant to handling. XRD result of MM01-1400-air can be seen in figure 28. The presence of the 53 diffraction peak at 22o confirms that silica phase was formed during firing in air. The white color is thus characteristic of the presence of excess silica formed by oxidation of silicon carbide. Figure 28. XRD results of MM01-1400-Air under air 54 Figure 29. Pore size distribution of MM01-1400-air Figure 29 shows mercury porosimetry analysis for MM01-1400-air characterized by a narrow pores size distribution centered at 5µm. The average total porosity was 45% with an average pore diameter of 4.6µm. The SiC powder used for MM01 had relatively smaller size (~6.5µm) (refer section 8.2.3), thus results in a smaller pore diameter. On the other hand as discussed in literature review, silica formation occurs from 1100oC when SiC is sintered under air, and is often used as a binding agent. The small pore diameter obtained could be due to this excess of silica formed around the pores that covers the SiC particle surface. The presence of oxygen in SiC ceramic reduces its future application under high temperature and steam conditions. 55 Figure 30. MM02 fired at different temperature Pore size distributions of MM02 sintered under argon at 1200oC and 1600oC can be seen in figure 30. The porosity of MM02-1200-Ar and MM02-1600-Ar was almost similar around 50% with an average pore diameter of 4.1µm and 4.2µm, respectively. Though the difference in porosity and pore diameter is negligible, it can be noticed that a higher temperature under similar conditions results in low porosity and high pore diameter. Thus, higher temperature enhances sintering by accelerating diffusion of particles. From SEM image shown in figure 27 c and d, microstructure of MM02-1200-Ar and MM02-1600-Ar looks similar. In these two cases neck growth between the particles is not enough to be identified. Thus the sample was fragile and can be broken easily. A similar result is obtained for MM04 and MM05 that were sintered at different temperatures under inert. Thus adding a sintering additive is very important for SiC macroporous synthesis, for harsh industrial condition application.  Effect of pore forming agent MM02, MM04, and MM05 sintered at 1200oC under air were analyzed to see the effect of pore forming agent in the microstructure and porosity of SiC ceramics. The SEM images (figure 27b, f 56 and g) give evidence of a better particle growth occurrence in these three compositions compared to samples fired at similar temperature under argon. However from the SEM image alone it is difficult to see the difference that this paste compositions have. Figure 31. Pores size distributions of different compositions samples fired under Air at 1200oC Pore size distributions from mercury porosimetry are shown in figures 31. All the three compositions sintered at the mentioned temperature have a narrow pore size distribution centered at approximately 5μm for both MM02-1200-Air and MM05-1200-Air and 4μm for MM04-1200-Air. 57 Figure 32. Porosity and average pore diameter for MM02, MM04 and MM05 sintered at 1200oC under air The porosity and average pore size diameter are shown in figure 32. MM02 and MM05 containing 5wt.% of corn and maize starch, respectively (refer table 2), are characterized by a similar average pore size diameter. On the other hand, MM04 that contains only 2.5wt.% of corn starch is characterized by less porosity and smaller average pore diameter. This shows the significant effect of pore forming agent and that a higher content results in a larger pore diameters of porous SiC ceramics support keeping the other factors similar.  Effect of sintering atmosphere To understand the effect of atmosphere, MM02-1200-air and MM02-1200-Ar were compared. Comparing the two SiC ceramics, the one sintered under argon is fragile and can be broken easily, and also it has dust formed on its surface which might be the result of carbon ashes remaining after additives are burnt and decomposed during sintering. On the other hand, the one sintered under air is strong and is white in color (figures 25.a and b). The white color indicates the presence of silica, that is formed by oxidation of silicon at 1100-1400oC under oxygen (air). The amorphous silica helps the SiC particles to bind each other so that the overall macroporous material attains high mechanical strength. Consequently the SEM images seen in figures 27-b and d, showed a better 64 You Zhoua, Manabu Fukushimaa, et al. (2011). "Preparation and characterization of tubular porous silicon carbide membrane supports." Journal of Membrane Science 369(1-2): 112-118. Zhoua, Y., M. Fukushima, et al. (2011). "Preparation and characterization of tubular porous silicon carbide membrane supports." Journal of Membrane Science 369: 112-118.