Accepted Manuscript

Title: Investigation of Thermal Oxidation of Al2O3-CoatedSiC Powder

Author: Atanu Dey Nijhuma Kayal Atiar Rahaman MollaOmprakash Chakrabarti

PII: S0040-6031(14)00094-XDOI: http://dx.doi.org/doi:10.1016/j.tca.2014.03.011Reference: TCA 76800

To appear in: Thermochimica Acta

Received date: 19-12-2013Revised date: 14-3-2014Accepted date: 16-3-2014

Please cite this article as: A. Dey, N. Kayal, A.R. Molla, O. Chakrabarti, Investigationof Thermal Oxidation of Al2O3-Coated SiC Powder, Thermochimica Acta (2014),http://dx.doi.org/10.1016/j.tca.2014.03.011

This is a PDF file of an unedited manuscript that has been accepted for publication.As a service to our customers we are providing this early version of the manuscript.The manuscript will undergo copyediting, typesetting, and review of the resulting proofbefore it is published in its final form. Please note that during the production processerrors may be discovered which could affect the content, and all legal disclaimers thatapply to the journal pertain.

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Investigation of Thermal Oxidation of Al2O3-Coated SiC

Powder

Atanu Dey, Nijhuma Kayal, Atiar Rahaman Molla and Omprakash Chakrabarti*

CSIR-Central Glass and Ceramic Research Institute, 196 Raja S.C.Mullick Road,

Jadavpur, Kolkata-700 032, India

Abstract

SiC powder (-form) of sub-micrometer size (d50 of 0.65 m) was coated with a thin

layer (~12 nm) of boehmite by sol-gel technique. The final powder having ~ 24 wt%

boehmite was characterized by particle size and surface area determination, chemical

analysis, surface potential measurement and Fourier transform infrared (FTIR)

spectroscopic analysis. The thermal oxidation behaviour of the coated powder was

examined using the simultaneous TG/DTA technique in dry air. The results indicated

crystallization of mullite phase and the activation energy of crystallization was

determined to be 368.9±15.1 kJ mol-1. Synthesis of porous SiC ceramics of fine pore

structure (with average porosity and pore size ranging from 47-54% and 170-260 nm,

respectively) using thermal oxidation of coated powder was also demonstrated.

Key words: Oxidation kinetics, Sub-micron SiC powder, Thermal analysis, Al2O3

coating, Porous SiC

_______________

*Corresponding author. Tel.: +91-33-24733469/76/77

E-mail address: omprakash@cgcri.res.in

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1. Introduction

SiC ceramics have low bulk density, excellent mechanical and chemical

stability, high thermal conductivity and thermal shock resistance, and they can also

withstand high temperatures and hostile atmospheres. Processing routes are found to

retain or create porosity without sacrificing these special properties. One of the

important methods of producing porous SiC ceramics of pore size in micrometer scale

involves heating of a porous compact of SiC powders in air when oxidation derived

silica bridges the gap between the SiC particles at the contacting regions forming a

porous body.1-3 Al2O3 is added as a secondary phase in the starting mixture to produce

mullite for improvement of mechanical properties, corrosion and thermal shock

resistance of the porous ceramics.4-6 Porous SiC ceramics produced by the oxidation

bonding technique are promising filter materials and they can be used for removal of

dust and fine particles in the off-gas cleaning systems of many important industrial

processes (coal combustion and gasification processes for power generation, thermal

remediation of contaminated soils, incineration of hospital and industrial wastes, metal

smelting, manufacturing of cements, carbon blacks and glasses, etc.7-10). Efforts are

also made to bring the emissions of ultrafine particles ~ particles finer than 0.1 m ~

from industrial processes under the stringent regulatory control.11 Porous media

composed of agglomerates or granules of ceramic nanoparticles are successfully used

for filtration of submicron solid or liquid aerosol particles.12 In this respect

development of oxidation bonded porous SiC ceramics of very fine pore structure

using SiC-Al2O3 powders of sub-micrometer sizes assumes importance. For this

purpose complete understanding of oxidation behaviour of ultrafine SiC+Al2O3

powder systems becomes necessary. Al2O3 coating on SiC particles may serve as a

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way to distribute the secondary phase more uniformly in the system of SiC-Al2O3

powder of submicron sizes and makes possible the use of coated powder in oxidation

bonding of porous SiC of fine pore sizes. In this paper we present the results of an

investigation on Al2O3 coating of SiC powder of sub-micron size and the kinetic study

of oxidation of the coated powder.

2. Experimental Procedure

-SiC powder (Grade 059N/200; Lot # 050710C10; SiC: Min. 98 % w/w, Free

C: Max. 2.0% w/w, Oxygen: Max.1.1% w/w, Nitrogen: Max. 0.25% w/w, Metallic Si:

Max.0.03 % w/w; Superior Graphite, Chicago, IL, USA) with an average particle size

(d50) of 0.65 m and surface area of 11-16 m2 g-1 was used. Boehmite coating of SiC

powder was carried out following the method described by Yang and Shih.13 Aluminum

tri-sec-butoxide (Product No. 201073; Mol. Formula: C12H27AlO3; Purity: 97%; Sigma

Aldrich Co., MO, USA) was used as the precursor for boehmite. As-received SiC

powder was dispersed in de-ionized water with ultrasonication (Sonaprob PR-1000MP,

Oscar Ultrasonic Sakinaka, Mumbai, India) to disintegrate the agglomerates. The

suspension was heated to 90°C under constant magnetic stirring. 97% aluminum-tri-sec-

butoxide was then added and the molar ratio of alkoxide to water was kept at 1:150 in

order to ensure complete hydrolysis. After the alkoxide addition, HCl was continually

added to maintain the suspension pH at 3. After stirring for about half an hour the

suspension was centrifuged (REMI-PR-24, Remi Electronics Ltd., Vasai, India) and the

coated powder was obtained by repeatedly rinsing with water followed by drying and

grinding. In the present work the final powder with ~ 24% boehmite coating was

prepared using 12 g of aluminum tri-sec-butoxide and 8.8 g of SiC powder. The coated

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powder was characterized by measuring particle size (ZEN 3690, Malvern Instrument

Ltd., Worcestershire, U.K.) and, BET surface area (NOVA 4000e, Quantachrome

Instrument, FL, USA). Zeta potential measurement (ZEN 3690, Malvern Instrument

Ltd., Worcestershire, U.K.) of the coated and uncoated SiC powders was done and their

Fourier transform infrared transmission spectra (Nicolet 5700, Thermo Scientific, MA,

USA; wave number range of 400 to 4000 cm-1) were also recorded. The amount of

coating in the final powder was determined by wet chemical analysis method

(complexometric determination of aluminium). The oxidation experiment was carried

out using a simultaneous thermal analyzer (STA 449 F3 Jupiter, NETZSCH Geratbau

GmbH, Selb, Germany). DTA tests under non-isothermal conditions were performed

keeping a constant flow of dry air (XL Grade, BOC India Ltd., Kolkata, India; flow rate

of 60 ml min-1 under atmospheric pressure) at different linear heating rate (20 K min-1;

25 K min-1; 30 K min-1 and 40 K min-1).

3. Results and discussion

(1) Preparation of Boehmite Coated SiC Powder

The coating material was produced from the aluminium secondary butoxide

solution following the same process but without the presence of SiC. The particles so

produced were boehmite as was evident from the XRD pattern of the precipitate from

the aluminium secondary butoxide solution after drying at 100°C (Fig. 1). Formation

of boehmite was also reported in the experimental conditions similar to those of

present study.13 The average particle size of SiC powder before and after coating was

measure to be 400 and 479 nm, respectively, based on the number average and 517 and

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689 nm, respectively, based on the volume average (Fig. 2). The results of particle size

measurement tests indicated presence of very few free boehmite particles in the

suspension of coated particles. The mean particle size of boehmite was reported to be

around 80 nm.13 In the present study an average particle size of ~ 90 nm has been

obtained (Fig.2). None of the peaks centred on this average particle size was found in

case of coated powder. The average particle size of SiC increased after the coating

process. The individual boehmite particles possibly aggregated on SiC particles

increasing the average particle size after coating. Figure 3 shows the zeta potential

versus pH plots for the uncoated and coated SiC particles. The SiC particles exhibited

the isoelectric potential (IEP) at pH 4.6. With boehmite coating the IEP shifted to a

higher value at pH 9.4. The boehmite powders were reported to have an IEP at pH

9.8.14 The surface of boehmite has neutral surface sites and can react with either H+ or

OH- ions in acidic or basic solutions to produce dominant positive or negative surface

sites below or above the IEP, respectively. After coating SiC particles with boehmite

the surface was terminated with neutral surface sites and the behaviour of the coated

SiC suspension became similar to that of boehmite suspension. The zeta potential

measurement test further confirmed that the coating was successful as the surface

potential of SiC changed to that of boehmite after coating. The FTIR transmission

spectrum of the uncoated powder had the characteristic peak of SiC at 866 cm-1 (Fig.

4). For coated powder peaks appeared at 487, 534, 649, 717 and 877 cm-1. The band

position at 717 cm-1 indicated formation of the Al-O-Si linkage.15 The peaks at 487,

534 and 649 cm-1 were for boehmite.16 The peak at 877 cm-1 was similar to the

characteristic peak of SiC; Lee and Kim examined simultaneous hydrolysis and coating

of Al(OH)3 on SiC particles in water/aluminum isopropoxide solution containing SiC

and -Al2O3 (seeding material) particles and observed no peak of SiC in the FTIR

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spectrum of hydrolysed powder, inferring that the SiC particles were completely

embedded in SiC matrix.16 Hareesh observed that the characteristic peak of SiC

remained unchanged in the FTIR spectra of SiC powders coated with alumina

following hydrolysis of aluminum nitrate solution.17 Saraswati also observed that the

characteristic peak of SiC remained unchanged in the FTIR spectrum of boehmite

coated SiC whisker reinforced alumina composite after sintering at 1800°C.18 In

boehmite the aluminum ions can occupy two sub-lattices ~ tetrahedral sites and

octahedral sites and the band positions measured on coated powder in the range of 400

to 800 cm-1 are likely due to the stretching and bending of octahedrally coordinated

aluminum (AlO6 atomic group) while the band position at around 880 cm-1 wave

number is due to the stretching of tetrahedrally coordinated aluminum (AlO4 atom

group) of shorter Al-O bond length.19 The AlO4 band has probably been overlapped

with the characteristic peak of SiC in the coated powder.17 The SiC powder has surface

active silanol (Si-OH) groups and the coating takes place via the reaction of hydroxyl

group of silanol and the alkoxy group of Al-alkoxide. Initially an initial surface layer is

formed by chemical adsorption and the amount of reacted alkoxide is then increased by

starting the hydrolysis reaction. Hydrolysis reaction upon addition of water links the

alkoxide molecules by a polymerization type sol-gel reaction.20 FTIR analysis of the

boehmite coated SiC powder showed the nature of the coating phase. The experimental

results confirmed that the boehmite coating was in reality a thin and low density layer

of finely particulate crystallite material and it adhered fairly strongly to the underlying

SiC surface. The coating had a high specific surface area (a value of 64.18 m2 g-1 was

obtained by the BET surface area measurement) which showed the porous structure of

the coating. This open structure would facilitate ingress of other species of ionic

dimension to the underlying SiC surface. Though the coating was porous in nature, the

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overall electrical nature of the SiC particles was dominated by the adsorbed species

forming the coating making the surface electrical potential of the particles nearly equal

to that of the coating materials. Chemical analysis showed that the final powder had

22.54 wt% of boehmite. The thickness (d) of the coating layer was calculated using the

following equation:21

vs

a

Sm

md (1)

where, ma and ms represent the respective weight (in g) of the coating material (of

density g cm-3) and the coated particle (of specific surface area Sv m2 g-1). The

thickness of the boehmite coating was estimated to be 1.2 nm. The actual coating

thickness was larger than the estimated value as the actual coating was porous and

fully dense boehmite was assumed in the theoretical estimate. A similar discrepancy

was reported by other researchers in boehmite coating of ceramic powders who

obtained around ten times increase of actual coating thickness than the theoretically

estimated value.13,22 Taking this point into consideration the actual coating thickness in

the present work is expected to be around 12 nm.

(2) Oxidation of Boehmite Coated SiC Powder

The typical TG plot showing the change in weight during heating of the coated

SiC at a rate of 40 K min-1 in air is presented in Fig. 5. A gradual weight loss was

recorded on heating to 700°C. Above this temperature weight loss reached a maximum

value and a weight gain was observed at higher temperatures because of onset of

oxidation of SiC. The plateau value at maximum weight loss can be considered as a

reference when silica first forms as an oxidation product on the surface of SiC

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particles. The thermogram obtained in the DTA experiment shows an endothermic

peak ending at ~300°C. This is followed by a small endothermic peak at ~500°C, an

exothermic peak at ~800°C and another exothermic peak at 1297°C. The endothermic

peaks and the first exothermic peak fall in the weight loss region of the TG curve. The

second exothermic peak occurs in the weight gain region. Comparison of the

experimental results with the published literature data of thermal analysis of boehmite

helps to infer that the first endothermic peak is related to weight loss due to desorption

of physically bound water and the small endothermic peak results from release of

chemically bound water (the hydroxyl group) converting boehmite to -Al2O3.23

Published results of experiments conducted to monitor thermal desorption of gases

from SiC powder in oxygen atmosphere mentioned evolution of CO and CO2 at around

650°C.24 The first exothermic peak is thus possibly related to evolution of carbon oxide

from carbon impurities. Figure 6 shows the XRD patterns of coated SiC heat treated at

different temperatures in air. Although SiC might begin to oxidize at >800°C it became

noticeable only at 1100°C.In the sample heat-treated at 1100°C, a small diffusive

peak of -cristobalite (SiO2) could be detected. The mullite phase was first detected at

T=1250°C, indicating that the exothermic peak at 1297°C corresponds to mullite

crystallization reaction. The oxidation reaction of coated powder at T>1200°C is

therefore expected to be suppressed due to formation of mullite. Figure 7 shows the

DTA peak corresponding to mullite crystallization reaction at different heating rates. A

clear shifting of the peak to higher temperatures is visible with increase of heating

rates. Intervals of the initial peak temperature, Tin, maximum peak temperature, Tmax

and ending peak temperature, Ten, corresponding to different experimental heating rates

and calculated enthalpies are presented in Table 1. From the results of DTA

experiments done at different heating rates, the kinetic parameters of mullite

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crystallization reaction were determined. The areas under the exothermic peaks were

integrated using the trapezodial rule of numerical method. The ratio of the area under

the peak up to a certain value of abscissa, and the total area of the peak, represents the

crystallized fraction (x) of mullite corresponding to the instant (t) under consideration.

The relationship between crystallized fraction of mullite, x, and time under different

heating rates is shown in Fig. 8(a); the crystallization rate against time for different

heating rates is represented in Fig. 8(b). The rate of the crystallization reaction

increased with heating rate. The process of mullite crystallization can be compared

with several processes of phase transformation including the process of crystallization

of glass. The well-known Johnson-Mehl-Avrami theory describes an isothermal

crystallization process:26,27

])(exp[1 nktx (2)

where n is the Avrami exponent, k is the rate constant (k has the Arrhenius type of

temperature dependence: ]exp[ RTEkk o

), E is the activation energy and ko is the

frequency factor. Differentiation of Eq. (2) and further rearrangement give the rate of

crystallization of mullite:

)(xkfdt

dx (3)

where n

n

xxnxf

1

)]1ln()[1()(

(4)

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These equations are described for an isothermal crystallization process. But many

authors show that they can be applied to a non-isothermal process with satisfactory

results under certain boundary conditions.27 Taking logarithm of Eq.(3) and

rearrangement give:

)](ln[ln xfkRT

E

dt

dxo (5)

In a first approximation linearity between rate of crystallization and temperature

inverse was assumed for values of x in which the values of ln[kof(x)] remained

constant. The experimental values were adjusted to a straight line whose slope gave the

first value of activation energy (apparent activation energy). The value of ln[kof(x)]

corresponding to each value of x was calculated by this apparent activation energy

through Eq.(5). From the plots of ln[kof(x)] against x the intervals of x for which the

value of ln[kof(x)] was nearly constant were selected. Figure 9 shows the typical plot of

ln[kof(x)] versus x corresponding to the heating rate of 20 K min-1. With the

experimental data limited to this new interval, linear adjustment was carried out thus

obtaining a second value of the activation energy which was more accurate as the

condition of constancy for ln[kof(x)] was maintained to a greater extent. The repetition

of the procedure gave the more correct value of E. Figure 10 shows the plot of dt

dxln

against temperature inverse for different heating rates and all linear adjustment

correlation coefficients were close to 0.99. For each heating rate activation energy was

calculated and on an average it was equal to 368.9±15.1 kJ mol-1. In the new interval it

was verified that ln[kof(x)] was constant. Then rearrangement of Eq.(4) gives:

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Cxn

nxnko

)]1ln(ln[

1)1ln(lnln (6)

where C is a constant. Equation (6) is valid for any value of x contained in the new

interval. By writing Eq. (6) for two given values of crystallized fractions, x1 and x2 and

subtraction of expression obtained in each case gives:

)1ln(

)1ln(ln

1

1

1ln

2

1

2

1

x

x

n

n

x

x

(7)

Equation (7) on further rearrangement gives the expression for Avrami exponent:

1

11

22

1

2 }])1ln()1(

)1ln()1(][ln{

)1ln(

)1ln(ln[

xx

xx

x

xn (8)

The frequency factor, ko, can be determined from Eq.(6) by substituting the value of n

and the constant C by the value taken by function ln[kof(x)] in the constancy interval

for each heating rate. The kinetic parameters of mullite crystallization reaction

occurring during oxidation of coated powder were determined by these equations and

are listed in Table 1.

The temperature of mullite crystallization depends on the degree of mixing of

aluminium and silicon in the precursors. Natural precursors like aluminosilicate

minerals are advantageous for mullite production because of mixing of Al2O3 and SiO2

on a molecular scale. Li and Thomson studied the formation of mullite in a single-

phase aluminosilicate gel (monophasic precursor) in which Al2O3 and SiO2 were mixed

to achieve three dimensional molecular homogeneity.28 The crystallization of mullite

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occurred at temperatures as low as 940°C by a nucleation controlled process with

activation energy of 293-362 kJ mol-1 29 For similar reasons mullite forms in kaolinite

clays at low temperatures (980-1100°C) in which molecular mixing of Al2O3 and SiO2

is found in only one dimension.30-32 The wide range of mullite crystallization

temperature in kaolin is stated to be due to different kaolin samples having different

impurities or particle size distribution of starting materials. The activation energy of

mullite crystallization in kaolin is 1182 kJ mol-1 with an associated growth morphology

parameter (n) of 2, indicating a nucleation and two-dimension diffusion-controlled

growth process of mullite crystalls.32 When scale of homogeneity is in nanometer

range, i.e. from 1 to 100 nm in diphasic aluminosilicate gels, mullite formation can be

delayed to temperatures as high as 1350°C and mullite forms in this case by a

nucleation and growth process in the silica rich matrix (that fits Avrami model) with an

apparent activation energy of 1070±200 kJ mol-1.29,33 The mullite crystallization

temperatures above 1200°C are likely to be related to processing related alumina-silica

segregation and corresponding increase in diffusion path length. In case of a

monophasic precursor three dimensional molecular homogeneity minimizes the

diffusion path length and makes mullite formation no longer be a diffusion controlled

process. When mullite is produced through reaction of powder samples Al2O3 and

SiO2 (micron-sized powders) at temperatures above 1650°C, nucleation and growth of

mullite as an interfacial product can occur and the growth is controlled by

interdiffusion of aluminum and silicon ions through mullite layer.34,35 Because of low

diffusivities the Al2O3 and SiO2 mixed on a micrometer scale require high

temperatures for mullite formation. In the present work average activation energy of

368.9±15.1 kJ mol-1 was obtained and it can be attributed to some kind of molecular

structure with Al-O-Si bonding similar to that of mullite in the alumina-silica

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precursor. Li and Thomson observed that when the single phase aluminosilicate gels

were heat-treated at 900°C, the absorption peak at 850 cm-1, corresponding to Al-O-Al

stretching vibrations in tetrahedral Al, shifted to a higher frequency with increasing

temperature because of coupling of AlO4 tetrahedra with SiO4 tetrahedra.28 Moenke

also found a similar shift to a higher wavenumber in the absorption bands of SiO4

tetrahedra at 1100 cm-1 with formation of Al-O-Si bonds.36 For examination of

favourable molecular structure formation before mullite formation, the boehmite

coated SiC powder was heat-treated to 1000°C and characterized by FTIR

spectroscopic analysis. It was observed that the peak at around 880 cm-1,

corresponding to tetrahedrally coordinated aluminum (AlO4 atom group), shifted to a

higher frequency of 1106 cm-1 after the heat treatment, likely because of coupling of

the AlO4 tetrahedra with SiO4 tetrahedra (Fig.4). The mullite crystallization reaction in

a single-phase gel and boehmite coated SiC (of present study), therefore, has a low

activation energy due to formation of the favourable molecular structure before

mullitization. The low activation energy of mullite crystallization indicates possibility

of oxidation bonding of porous SiC at low temperature using the coated powder. To

realize the possibility compacts of coated powder at were heat treated at 1250 and

1350°C for 4 h in air and the oxidation bonded samples were characterized by

measuring porosity and pore size and examining the microstructure. The recorded

porosity of the SiC ceramics oxidation bonded at 1250 and 1300°C were 54% and

48%, respectively; microstructure examination revealed formation of interconnected

pores in the SiC ceramics. The image analysis results showed that the samples

prepared at 1250 and 1300°C had average pore sizes of 174 and 263 nm, respectively.

The typical microstructure of the porous ceramic sample prepared at 1250°C is shown

in Fig. 11. The experimental results demonstrated that SiC ceramics of high porosity

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(around 50%) and fine pore sizes (170-260 nm) could be synthesized using the

boehmite coated SiC powder of sub-micron size following a low temperature oxidation

bonding method.

4. Conclusions

The effect of boehmite coating on the thermal oxidation of boehmite coated

SiC powder was examined. With ~24 wt.% boehmite, a coating thickness of ~ 12 nm

was achieved and thermal oxidation of the coated powder was found to involve mullite

crystallization at a low temperature of around 1250°C because of atomic scale

arrangement of –Al-O-Si- groupings similar to that of mullite (before mullite

formation). The experimental rate data were fitted with the Johnson-Mehl-Avrami

transformation equations which suggested a low activation energy equal to 368.9±15.1

kJ mol-1 ~ similar in values obtained for mullite formation in monophasic

aluminosilicate precursor gels ~ likely because of mixing of Al2O3 and SiO2 on a

molecular scale. Porous SiC ceramics (of porosity and pore size of around 50% and

170-260 nm) were possible to be synthesized using the oxidation of coated powder at

low temperatures (1250-1300°C).

Acknowledgment

The authors are grateful for the financial support provided by CSIR-CGCRI

under the 12th Five Year Plan Project entitled “Advanced ceramic materials and

components for energy and structural application, CERMESA, ESC0104, (WP 1.2)”.

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One of the authors (AD) expresses grateful appreciation to CSIR-CGCRI for its

support to him under the CSIR Project.

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21. J. X. Zhang, L. Q. Gao, Nanocomposite Powders from coating with heterogeneous

nucleation processing, Ceram. Int. 27 (2001) 143-147.

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22. W. H. Shih, L. L. Pwu, A. A. Tseng, Boehmite coating as a consolidation and

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23. P. Alphonse and M. Courty, Structure and Thermal Behaviour of Nanocrystalline

Boehmite” Thermochim. Acta 425 (2005) 75-89.

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alumina nanocomposites, J. Am. Ceram. Soc. 78 (1995) 479-486.

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mullite, J. Am. Ceram. Soc. 74 (1991) 2460-2463.

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29. S. Sundaresan, I. A. Aksay, Mullitization of diphasic aluminosilicate gels, J. Am.

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30. G. Brindley, M. Nakahira, The kaolinite-mullite reaction series: I, A survey of

outstanding problems, J. Am. Ceram. Soc. 42 (1959) 311-324.

31. J. A. Pask, A. P. Tomsia, Formation of mullite from sol-gel mixtures and kaolinite,

J. Am. Ceram. Soc.74 (1991) 2367-2373.

32. Y. F. Chen, M. C. Wang, M. H. Hon, Phase transformation and growth of mullite

in kaoline ceramics, J. Eur. Ceram. Soc. 24 (2004) 2389-2397.

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gels, J. Am. Ceram. Soc. 71 (1988) 581-587.

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Figure Caption:

Fig.1. XRD pattern of the precipitate from the aluminium secondary butoxide solution

after drying at 100°C.

Fig.2. Particle size distribution of different particles: ● boehmite coating material; ▲

SiC powder; ■ SiC powder coated with ~24wt.% boehmite.

Fig.3. Zeta potential versus pH: ▲ SiC powder; ■ SiC powder coated with ~24wt.%

boehmite.

Fig.4. FTIR spectrum of the boehmite coated SiC powder within 400-3200 cm-1 after

heat treatment at 1000°C (top); also presented are the spectra of SiC powder (bottom)

and boehmite coated SiC powder (middle) without heat treatment.

Fig.5. TG/DTA curves of boehmite coated SiC at a heating rate of 40 K min-1.

Fig.6. X-ray diffraction patterns of boehmite coated SiC after heat treatment at

different temperatures; also plotted is the XRD pattern of SiC without heat treatment

(bottom). s = SiC (JCPDS Code: 01-075-0254), cr = SiO2 (JCPDS Code: 00-076-

0936), m = mullite (JCPDS Code: 00-015-0776).

Fig.7. DTA curves of SiC powder coated with ~24 wt.% boehmite at different heating

rates in a stream of air.

Fig.8. Plots showing (a) relationship between mullite crystallized fraction ‘x’ with time

‘t’ and (b) rate of mullite crystallization reaction versus time under different

experimental heating rates.

Fig.9. Plot of ln [kof(x)] versus x for the heating rate of 20 K min-1.

Fig.10. Plot of ln(dx/dt) versus absolute temperature inverse; open circle represents the

experimental data and dotted lines show the fitting curves.

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Fig.11. Microstructure of porous SiC ceramics sintered at 1250°C using boehmite

coated SiC powder; microstructure viewed under higher magnification is shown in the

inset.

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Highlights

Oxidation of Al2O3 coated 0.6 m SiC powder was studied by thermal analysis

method

Oxidation of coated powder involves crystallization of mullite at ~1300°C

Process of mullite crystallization has an activation energy of 369 kJ mole-1

Oxidation bonding of porous SiC is possible using coated powder at low

temperature

Oxidation bonded SiC ceramics have interconnected porosity and fine pore structure

Highlights (for review)

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Graphical Abstract (for review)

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Table 1: The kinetic parameters obtained for mullite crystallization reaction by

analyzing the exothermic peaks corresponding to different experimental heating

rates

Heating

rate

(K min-1

)

Temperatures relating to

the mullite crystallization

exotherm

Enthalpy

(mcal mg-1

)

Interval of

crystallized size

fraction for

constant

ln[k0f(x)]

Correlat

ion co-

efficient

(R2)

Activation

energy

(kJ mole-1

)

Abhrami

Exponent

(n)

Frequency

factor

(k0)

Tin Tmax Tend

20 1183.1 1294.4 1334.2 1.13 0.37-0.64 0.99 385.27 1.05±0.30 9.2×1017

25 1187.4 1305.9 1355.2 1.57 0.15-0.46 0.99 367.20 1.22±0.71 5.1×1018

30 1196.5 1317.9 1376.7 1.41 0.19-0.44 0.99 349.32 1.19±1.62 4.0×1016

40 1203.5 1323.7 1386.1 1.11 0.13-0.59 0.99 374.15 1.23±1.04 5.4×1019

Table(s)

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Figure(s)

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Figure(s)