Sol-Gel Synthesis of Mullite Coating

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Optimizing synthesis parameters of Mullite Coating.

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Sol-Gel Synthesis of Mullite Coating:

Sol-Gel Synthesis of Mullite Coating Abhivadan Wananje BT06MME055 DEPARTMENT OF METALLURGICAL AND MATERIALS ENGINEERING VISVESVARAYA NATIONAL INSTITUTE OF TECHNOLOGY NAGPUR – 440010 1

Introduction:

Introduction Silicon based Ceramics for High Temperature Applications (in order of 1200 o C) Under dry oxidation conditions protective Silica layer forms- SiC + 2O 2  SiO 2 + CO 2 In high velocity water vapor environment volatilization of Silica layer to gaseous Silicic acid [1] - SiO 2 + 2H 2 O  Si(OH) 4 ↑ To protect Silica layer Environmental Barrier Coatings is being applied, eg . Mullite , YSZ, BSAS. 2 Ref. [1] : Lee K.N., Current status of environmental barrier coatings for Si-Based Ceramics, Surface and Coatings Technology 133-134 (2000 )

Mullite (3Al2O3.2SiO2):

Mullite (3Al 2 O 3. 2SiO 2 ) A solid solution intermediate phase of alumina and silica system Excellent Coefficient of Thermal Expansion match of Mullite with SiC Low thermal conductivity Low activity of silica Low permeability to oxygen and steam Chemical compatibility with SiC and SiO 2 3

Phase Diagram:

Phase Diagram 4 Ref. [2]: Frederic J. Klug, Svante Prochazka , Alumina-Silica Phase Diagram in the Mullite Region, J. Am. Cerum. Soc., 70 [lo1 750-59 (1987) Alumina – Silica Phase Diagram [2]

PowerPoint Presentation:

Compound Mullite Composition 3Al 2 O 3 ·2SiO 2 Melting point (°C) ~ 1850 Density (g cm −3 ) 3.2 Linear thermal expansion (×10 −6  °C −1 ) 4.5 Thermal conductivity (kcal m −1  h −1  °C −1 ) 6 (at Room Temperature) Strength (MPa) 200 Fracture toughness K Ic (MPa√m) 2.5 Properties [3] 5 Ref. [3]: H. Schneider, J. Schreuer , B. Hildman Structure and Properties of Mullite , Journal of the European Ceramic Society 28(2008)

Sol-Gel Technique:

Sol-Gel Technique Formation of an oxide network through polycondensation reactions of a molecular precursor in a liquid 6

Process:

Process Advantages Cheap and low temp. operation good chemical homogeneity and stoichiometry easy processing of multi-component or complex- shaped systems 7

Reaction Mechanism:

Reaction Mechanism Hydrolysis: M-(OR) X + H-O-H  HO-M-(OR) X-1 + H-OR M-(OR) X + R-O-H  RO-M-(OR) X-1 + H-OR M = Si, Al, Ti, La, Y, etc. R = CH 3 , C 2 H 5 , C 3 H 7 , etc.   Condensation: (OR) X-1 -M-OH + RO-M-(OR) X-1  (RO) X-1 -M-O-M-(OR) X-1 + ROH   (OR) X-1 -M-OH + HO-M-(OR) X-1  (RO) X-1 -M-O-M-(OR) X-1 + HOH 8

Effects of Sol-Gel Synthesis Process Parameters [4]:

Effects of Sol-Gel Synthesis Process Parameters [4] Solvents Catalysts Time Agitation Temperature pH of Solution 9 Ref. [4]: Pirjo Kortesuo, Manja Ahola, Minna Kangas, Antti Yli-Urpo, Juha Kiesvaara, Martti Marvola; Effect of sol-gel synthesis parameters , International Journal of Pharmaceutics 221 (2001) 107 – 114

Reaction Mechanism[5]:

Reaction Mechanism [5] - Precursors: metal alkoxides and metal chlorides(as they readily reacts with water) e.g. TEOS Colloid contain broad range of solid(or liquid) particles dispersed in various degrees in liquid - Sedimentation and centrifugation Drying and firing 10 Ref. [5 ]: Yen-Yu Chen, Wen-Cheng J. Wei, Formation of mullite thin film via a sol-gel process with polyvinylpyrrolidone additive, Journal of the European Ceramic Society 21 (2001) 2535–2540

Experimental Procedure[5]:

Experimental Procedure [5] Procedure 9 gm (0.069 mol) of Aluminium Chloride (AlCl 3 ) was dissolved in 65 ml Propan-2-ol .   10 ml (0.94 g/cc) Tetra ethyl orthosilicate (TEOS) was added as a cross linking agent and the resulting mixture was stirred for 24 hours for dissolution of AlCl 3 . The pH of the solution was then adjusted using ammonia to get solutions at 2 , 3 & 4 pH respectively. The gels synthesized at 2 pH, 3 pH and 4pH were dried at 120 0 C . 11 Ref. [5 ]: Yen-Yu Chen, Wen-Cheng J. Wei, Formation of mullite thin film via a sol-gel process with polyvinylpyrrolidone additive, Journal of the European Ceramic Society 21 (2001) 2535–2540

PowerPoint Presentation:

The crushed powders were calcined in SiC furnace at three different temperatures- 1000 0 C, 1100 0 C & 1200 0 C in order to get Mullite powder. For calcination, Nickel crucibles were used. pH Calcination Temperature ( 0 C) 2 1000 1100 1200 3 1000 1100 1200 4 1000 1100 1200 12

XRD Results:

XRD Results Phase evolution of 2pH powder at different Temperature 13

XRD Results:

XRD Results Phase Evolution of 3 pH powder at different Temperature 14

XRD Results:

XRD Results Phase evolution of 4 pH powder at different Temperature 15

Observations:

Observations The amorphous nature of the mullite powders in 2pH and 3pH powders till calcination temperature of 1100 0 C. At 1200 0 C, increased crystallinity w ere observed in all the cases Exact composition was observed in 4pH-1200 with Al:Si ratio 3:1 . As the temperature increases, the crystalline nature and the purity of mullite also increases. 16

XRD Results:

XRD Results Phases formed at different pH values, calcined at 1000 o C 17

XRD Results:

XRD Results Phases formed at different pH values, calcined at 1100 o C 18

XRD Results:

XRD Results Phases formed at different pH values, calcined at 1200 o C 19

Observations:

Observations From the above comparative studies, the following observations can be made: With increase in pH , the crystalline nature also increases. At 2pH and 3pH, semi-crystalline nature of graphs A t 4pH complete crystalline morphology is observed at all calcination temperature Increase in pH resulted in reducing the mullitization temperature 20

Average crystal size of all the powder sample:

Average crystal size of all the powder sample pH Calcination Temperature ( 0 C) Average Crystalline Size (nm) 2 1000 20.50 1100 17.66 1200 75.51 3 1000 24.95 1100 30.74 1200 82.15 4 1000 68.95 1100 106.85 1200 116.85 21

Study of mullite coatings:

Study of mullite coatings Mullite of 4pH-1200 0 C powder selected Sol-gel coating was obtained by dipping Coated sample was dried in oven at 120 0 C. Dried sample was fired in a furnace at 1200 o C for 4 hours. Further characterization of coatings was done using X-Ray Diffraction and Scanning electron microscopy (SEM). 22

XRD pattern of mullite coating on SiC base:

XRD pattern of mullite coating on SiC base 23

SEM micrographs of SiC samples:

SEM micrographs of SiC samples 24 uncoated SiC samples coated SiC samples

CONCLUSIONS:

CONCLUSIONS At 1200 o C silica disappeared and transformation to mullite was complete. With increase in calcination temperature, the proportion of mullite phase increased. The crystallite size increased with increase in the calcination temperature. The crystallite sizes in all the powders were seen to be in the nanoscale range; from 17 to 116 nm With increase in pH from 2 to 4, mullitization temperature reduced. The surface characteristics of the substrate have a profound influence on the quality of the coating. Mullite coatings were formed on the SiC substrates with a precursor gel of pH value 4. 25

REFERENCES:

REFERENCES Lee K.N., Current status of environmental barrier coatings for Si-Based Ceramics, Surface and Coatings Technology 133-134[1-7] (2000) Frederic J. Klug, Svante Prochazka , Alumina-Silica Phase Diagram in the Mullite Region, J. Am. Cerum. Soc., 70 [10] 750-59 (1987) H. Schneider, J. Schreuer , B. Hildman Structure and Properties of Mullite , Journal of the European Ceramic Society 28[329-344] (2008) Pirjo Kortesuo, Manja Ahola, Minna Kangas, Antti Yli-Urpo, Juha Kiesvaara, Martti Marvola; Effect of sol-gel synthesis parameters , International Journal of Pharmaceutics 221 [107 – 114 ]( 2001) Yen-Yu Chen, Wen-Cheng J. Wei, Formation of mullite thin film via a sol-gel process with polyvinylpyrrolidone additive, Journal of the European Ceramic Society 21 [2535–2540 ] (2001 ) 26

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