Sol-gel processing and phase characterization of Al2O 3 and Al2O3/SiC nanocomposite powders

Article abstract of DOI:10.1016/j.materresbull.2005.10.014

Monolithic Al₂O₃ and Al₂O₃/SiC nanocomposite powders were prepared by sol-gel processing. The process involved the precipitation of Al(NO₃)₃·9H₂O with NH₄OH in excess water to form boehmite (AlOOH). XRD indicates that the subsequent thermal reaction proceeds by the phase transformation sequence AlOOH, γ-, δ-, θ-, to α-Al₂O₃. The ²⁷Al NMR spectra indicate a gradual increase in the proportion of Al in the tetrahedral sites of the γ-, δ- and θ-Al ₂O₃ formed at increasing calcination temperatures. Complete transformation to octahedral Al (α-Al₂O₃) is marked by the abrupt disappearance of tetrahedral Al. Al₂O ₃/SiC nanocomposite powders were prepared by adding α-SiC powder to the boehmite precursor at the precipitation stage. Upon heating, the ²⁹Si NMR spectra of the Al₂O₃/SiC powders reveal α-SiC, Al₂O₃·xSiO₂ and SiO₂ phases. Stable α-Al₂O₃ and α-Al₂O₃/SiC nanocomposite powders are formed at 1200 and 1300 °C, respectively. It appears likely that the presence of SiC modifies the thermal behaviour of the Al₂O₃ in the nanocomposites by stabilising the Al₂O₃ phases with concomitant oxidation of SiO₂.

Full text of DOI:10.1016/j.materresbull.2005.10.014

Materials Research Bulletin 41 (2006) 791–803
www.elsevier.com/locate/matresbu
Sol–gel processing and phase characterization of Al2O3 and
Al2O3/SiC nanocomposite powders
b
Sukanda Jiansirisomboon a, , Kenneth J.D. MacKenzie
*
a Department of Physics, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand
b School of Chemical and Physical Sciences, Victoria University of Wellington, P.O. Box 600, Wellington, New Zealand
Received 24 June 2005; received in revised form 30 September 2005; accepted 10 October 2005
Available online 4 November 2005
Abstract
Monolithic Al2O3 and Al2O3/SiC nanocomposite powders were prepared by sol–gel processing. The process involved the
precipitation of Al(NO3)3ꢀ9H2O with NH4OH in excess water to form boehmite (AlOOH). XRD indicates that the subsequent
thermal reaction proceeds by the phase transformation sequence AlOOH, g-, d-, u-, to a-Al2O3. The 27Al NMR spectra indicate a
gradual increase in the proportion of Al in the tetrahedral sites of the g-, d- and u-Al2O3 formed at increasing calcination
temperatures. Complete transformation to octahedral Al (a-Al2O3) is marked by the abrupt disappearance of tetrahedral Al. Al2O3/
SiC nanocomposite powders were prepared by adding a-SiC powder to the boehmite precursor at the precipitation stage. Upon
heating, the 29Si NMR spectra of the Al2O3/SiC powders reveal a-SiC, Al2O3ꢀxSiO2 and SiO2 phases. Stable a-Al2O3 and a-Al2O3/
SiC nanocomposite powders are formed at 1200 and 1300 8C, respectively. It appears likely that the presence of SiC modifies the
thermal behaviour of the Al2O3 in the nanocomposites by stabilising the Al2O3 phases with concomitant oxidation of SiO2.
# 2005 Elsevier Ltd. All rights reserved.
Keywords: A. Ceramics; A. Composites; B. Sol–gel chemistry; C. Differential scanning calorimetry (DSC); C. Nuclear magnetic resonance
(NMR); C. X-ray diffraction
1. Introduction
Sol–gel powder processing involves the precipitation of metal oxide particles from solution. These form true
colloidal dispersions called sols. Upon destabilisation of the sol by changing the composition of the dispersing
solution, aggregation occurs to form a rigid network called a gel. The use of a sol–gel synthesis route would appear to
have many advantages over the conventional (solid state) ceramic processing route. These include a fine particle size
distribution, the ability to produce homogeneous precursors dispersed at the molecular level, and the possibility of
controlling the chemical purity and crystallinity. These factors allow the precipitation of ceramics and novel materials
such as nanocomposites in the form of spherical powders, fibres, coatings or monoliths. The technology is versatile
because it uses mainly aqueous dispersions, and it is also energy-efficient because the reactions occur at low
temperature. The sol–gel process has proved useful for preparing monolithic Al2O3 powder, and has also been used to
produce Al2O3/SiC nanocomposite powders. The most common starting precursor for preparing Al2O3 powder is
boehmite (AlOOH) [1–8], but other precursors may be used, for example, aluminium nitrate [9–12], aluminium
* Corresponding author. Tel.: +66 53 943376; fax: +66 53 357512.
E-mail address: sukanda@chiangmai.ac.th (S. Jiansirisomboon).
0025-5408/$ – see front matter # 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2005.10.014

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S. Jiansirisomboon, K.J.D. MacKenzie / Materials Research Bulletin 41 (2006) 791–803
isopropylate [13], aluminium sec-butoxide [13–16], aluminium chloride [13], aluminium sulphate [13,17,18], and
aluminium acetate [19].
Many previous investigations have explored the textures and structures of the dried gel obtained from the various
precursors. The materials produced range from fibrous gels to mono-sized spheres. The structures include all the
varieties of the aluminium trihydroxides [Al(OH)3] and monohydroxides [AlOOH], with crystalline or amorphous
structures. Many investigations have focused on the chemical processes which take place, including hydrolysis and
peptisation [20]. It was found that the acidic amorphous gels obtained from alumina salts transform into pseudo-
boehmite [AlOOH] and bayerite [Al(OH)3] by a dissolution–crystallisation process, whenever they are washed in
water to eliminate anionic impurities [21]. The conversion at neutral pH of the aqueous hydrolysis products of
aluminium alkoxide to bayerite has also been investigated at room temperature [22,23]. Hydrolysis at 20 8C produced
an amorphous monohydroxide which can be preserved by drying the precipitates after hydrolysis. Ageing this
hydrolysis product in excess water at 20 8C transforms it into crystalline bayerite by a dissolution–crystallisation
process, whereas ageing at 80 8C converts it into crystalline boehmite. Seeding boehmite with both a-Al2O3 and g-
Al2O3 crystals has been investigated as a means of improving the transformation kinetics and thus decreasing the
transformation temperature [2,5]. The use of seed crystals may be also improve densification at lower sintering
temperatures than for the unseeded gels; for example, seeded boehmite gel-derived a-Al2O3 sinters to about 99%
density at 1150 8C [2], whereas temperatures >1600 8C are needed for unseeded materials [24].
Various chemical precursors have been used for the preparation of sol–gel Al2O3 powder. One of the most popular
Al2O3 starting materials for the preparation of sol–gel Al2O3/SiC nanocomposite powder is aluminium nitrate
(Al(NO3)3) [11,25–27], with an addition of ammonium hydroxide (NH4OH) to control the pH [19,25–28]. In the
present work, Al(NO3)3ꢀ9H2O was used as the starting material for sol–gel Al2O3 and Al2O3/SiC nanocomposite
powders. The phase transformations as a function of temperature were fully characterized using X-ray diffractrometry
(XRD) and solid-state nuclear magnetic resonance with magic-angle spinning (MAS NMR).
2. Experimental
2.1. Sol–gel processing
The processing flow chart for producing monolithic sol–gel Al2O3 and Al2O3/SiC nanocomposite powder is shown
in Fig. 1. Sol–gel Al2O3 powder was precipitated from Al(NO3)3ꢀ9H2O (Aldrich, UK) using NH4OH (Aldrich, UK),
with the nanocomposite Al2O3/SiC powders obtained by the addition of 5% by volume of a-SiC powder (Lonza-
UF45, Germany) to the reaction mixture. A solution of 0.5 M Al(NO3)3ꢀ9H2O was prepared by dissolving 100.5 g
Al(NO3)3ꢀ9H2O in 500 ml distilled water and a solution containing three parts of NH4OH to one part of distilled water
was added as the precipitant. The SiC starting material had a particle diameter of 20–200 nm and was ultrasonically
dispersed in 500 ml distilled water for at least 1 h to break up any agglomerates before being vigorously stirred into the
alumina sol using a magnetic stirrer.
Fig. 2 shows a schematic diagram of sol–gel processing system. To precipitate the sol–gel Al2O3/5 vol%SiC
precursors, the solutions of Al(NO3)3ꢀ9H2O and NH4OH were peptised from individual separatory funnels into a well-
dispersed SiC mixture with continuous magnetic stirring to homogenize the system. Similarly, the solutions were
peptised into 500 ml distilled water to prepare sol–gel monolithic Al2O3. The pH was controlled at ꢁ9 by the addition
of NH4OH solution [12]. Peptisation was continued until the Al(NO3)3ꢀ9H2O solution was consumed, which took
approximately 1 h 30 min.
After precipitation, the precursor was filtered under reduced pressure and the cake-like precipitate was washed with
distilled water before ageing at room temperature in a bath of distilled water at 100 8C. After processing, the mixture
was filtered again and finally oven-dried at 85 8C overnight. The aim of the above procedure was to achieve an
amorphous boehmite (AlOOH) or boehmite/SiC mixture using the simplest and most rapid route. This boehmite was
then used as a starting material for preparing monolithic Al2O3 or Al2O3/SiC nanocomposite by heating at calcination
temperatures determined using a differential scanning calorimeter (DSC). The precipitates were calcined for 2 h at the
selected temperature at a heating rate of 5 8C/min in argon in order to prevent oxidation of the SiC particles. The
calcination temperatures ranged between 300–1200 and 300–1300 8C for monolithic Al2O3 and Al2O3/SiC
nanocomposite precipitates, respectively.

S. Jiansirisomboon, K.J.D. MacKenzie / Materials Research Bulletin 41 (2006) 791–803
793
Fig. 1. Schematic flow chart of the fabrication steps used for the sol–gel preparation of the nanocomposite powders.
2.2. Characterization of sol–gel products
DSC analysis of the as-precipitated sol–gel Al2O3 and Al2O3/5 vol%SiC nanocomposite powders was performed at a
heating rate of 5 8C/min. X-ray diffractrometry (Philips PW1729) with Cu Ka1 radiation generated at 35 kV was used to
determined phase composition of the powders. Solid-state 27Al and 29Si NMR spectroscopy was used to characterize the
phases present in the powders and to estimate their relative concentrations. The spectra were acquired at 11.7 T using a
Varian Unity 500 spectrometer with a 5 mm Doty MAS probe spun at 10 kHz. The 27Al NMR spectra were obtained at
130.244 MHz using a 158 pulse of 1 ms with a recycle delay of 1 s. These spectra were referenced to the secondary
standard of the AlO6 resonance of Y3Al5O12 at 0.7 ppm with respect to Al(H2O)43+. The 29Si NMR spectrawere obtained
at 99.297 MHz using a 908 pulse of 6 ms with a recycle delay of 300 s and were referenced to tetramethylsilane (TMS).
3. Results and discussion
3.1. Differential scanning calorimetry (DSC)
Fig. 3 shows the DCS traces of the precipitated sol–gel monolithic Al2O3 and Al2O3/SiC nanocomposite powders.
By superimposing baselines tangentially to the curves, the approximate starting temperatures of the different thermal
events were found to be 300, 420, 740 and 920 8C. The thermal event at ꢁ300 8C is associated with the removal of the
structural water from the aluminium hydroxide [29] and the possible decomposition of ammonium nitrate by-product

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S. Jiansirisomboon, K.J.D. MacKenzie / Materials Research Bulletin 41 (2006) 791–803
Fig. 2. Schematic diagram of the sol–gel processing system.
of the precipitation reaction in the monolithic Al2O3 [9]. The higher-temperature thermal events (>420 8C) are
associated with crystallisation of the amorphous solid and subsequent transformation of the various metastable forms
of alumina (g-, d- and u-Al2O3).
Since the DSC apparatus used in this study could not be operated at temperatures greater than 1100 8C, the upper
calcination temperatures for boehmite were taken from the literature. The dehydration sequence of alumina hydrates
has been summarised by Gitzen [30] on the basis of many reports [31–34] and suggests that the complete
transformation of boehmite to stable a-Al2O3 occurs at >1000 8C. In the present study, a calcination temperature of
1200 8C was initially used [15,35,36], but was increased to 1300 8C following Arakelyan and Chistyakova [34], in the
absence of XRD evidence of a-Al2O3 formation.
Fig. 3. DSC traces of sol–gel monolithic AlOOH and AlOOH/SiC nanocomposite powders determined at a heating rate of 5 8C/min.

S. Jiansirisomboon, K.J.D. MacKenzie / Materials Research Bulletin 41 (2006) 791–803
795
Fig. 4. XRD traces of sol–gel Al2O3 powders calcined for 2 h in ambient atmosphere: (a) unheated, (b) 300 8C, (c) 420 8C, (d) 740 8C, (e) 920 8C
and (f) 1200 8C (the sharp peaks in (a) marked by * correspond to NH4NO3; b: boehmite; g, d, u: metastable Al2O3 phases; a: a stable Al2O3 phase).
3.2. X-ray diffractrometry (XRD)
XRD was used to determine the simplest and fastest sol–gel procedure for preparing boehmite (AlOOH)
precursors. Fig. 4 shows a set of XRD traces of a washed, aged and dried Al2O3 precipitate calcined at various
temperatures. The broad peaks of boehmite (Fig. 4(a)) indicate an amorphous structure, with sharp peaks arising from
residual ammonium nitrate. The latter could be removed by aging at 100 8C prior to washing with distilled water, but
although this procedure produced the required boehmite phase, these processes are however time-consuming. The
chemical reactions involved in the formation of the precipitate are summarised in Eqs. (1)–(3). This synthesis route
was also used for preparing the sol–gel Al2O3/SiC nanocomposite powders:
ꢃ
AlðNO3Þ þ 3NH4OH þ xH2OagedT ꢂ!; pH ꢄ 93NH4NO3 þ AlðOHÞ þ xH2O
(1)
(2)
(3)
100
C
3
3
3NH4NO3 þ AlðOHÞ þ xH2Owashed; filtered
ꢂ!
AlðOHÞ3
3
dehydrated
AlðOHÞ3 ꢂ! AlOOH þ H2O
To study the high temperature transformation sequences leading to the most desirable stable a-Al2O3 sol–gel powder,
theprecipitatedmonolithicAl2O3 andAl2O3/5 vol%SiCnanocompositeswerecalcinedattemperaturesbetween300and
1300 8C as suggested by the DSC and literature data. Fig. 4 shows the XRD traces of the precipitated Al2O3 powder and

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Fig. 5. XRD traces of sol–gel Al2O3/SiC nanocomposite powder calcined for 2 h in Ar: (a) unheated, (b) 300 8C, (c) 420 8C, (d) 740 8C, (e) 920 8C,
(f) 1200 8C and (g) 1300 8C (s: SiC; b: boehmite; g, d, u: metastable Al2O3 phases; a: a stable Al2O3 phase; *: aluminosilicate). The other
background peaks in (g) indicate metastable u-Al2O3.
sol–gel Al2O3 powder calcined at 300–1200 8C for 2 h in ambient atmosphere. The precipitated Al2O3 powder is mainly
˚
˚
˚
orthorhombic boehmite (space group Cmcm: a 2.87 A, b 12.20 A, c 3.69 A) with a small amount of ammonium nitrate
impurity (Fig. 4(a)). After calcination at 300 8C, ammonium nitratewas no longer present and the powder consists solely
of amorphous boehmite (Fig. 4(b)). On increasing the temperature to 420 8C, the first indication of metastable g-Al2O3 is
found. The peaksshown in Fig. 4(c) arise from poorly-crystallineg-Al2O3 with a spinelstructureof cubicclose-packed O
(ICDD 29-0063). The g-Al2O3 phase is more distinct in the sample calcined at 740 8C (Fig. 4(d)). After calcination at
920 8C (Fig. 4(e)), a mixture of metastable poorly-crystalline d- and u-Al2O3 is detected by XRD. d- and u-Al2O3 are also
of spinel-structurewith cubic close-packed O (ICDD 04-0877 and 11-0517, respectively). Fig. 4(f) shows the XRD trace
of the Al2O3 sample calcined at 1200 8C. The extremely sharp peaks arise solely from corundum (crystalline a-Al2O3).
Table 1
Summary of phases present in sol–gel Al2O3 and Al2O3/SiC nanocomposite powders calcined at different temperatures, as detected by XRD
Calcination temperature (8C)
Sol–gel Al2O3
Sol–gel Al2O3/SiC
As-precipitated
AlOOH + NH4NO3
AlOOH
AlOOH + a-SiC
AlOOH + a-SiC
300
420
g-Al2O3
g-Al2O3
g-Al2O3 + a-SiC
g-Al2O3 + a-SiC
740
920
d-, u-Al2O3
a-Al2O3
g-Al2O3 + a-SiC
d-, u-Al2O3 + a-SiC
a-Al2O3 + a-SiC + (d-, u-Al2O3 + Al2O3ꢀxSiO2)a
1200
1300
–
a
Indicates minor phases.

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797
This is the final product of the thermal transformation sequence of boehmite and involves a change in the O packing from
ccp to hcp, the Al remaining in octahedral sites throughout [37].
Fig. 5 shows XRD traces of precipitated and calcined sol–gel Al2O3/5 vol%SiC nanocomposite powders heated
between 300 and 1300 8C for 2 h in Ar. Fig. 5(a) shows the XRD trace of the precipitated sol–gel Al2O3/SiC powder
containing amorphous boehmite and a-SiC. These phases remain after the powder is calcined at 300 8C (Fig. 5(b)).
Fig. 5(c)–(e) show the diffraction traces of sol–gel Al2O3/SiC powder calcined at 420, 740 and 920 8C. The traces are
similar in most respects, indicating the presence of poorly-crystalline g-Al2O3 and a-SiC phases and suggest that the
Fig. 6. 27Al MAS NMR spectra of sol–gel Al2O3 powders calcined for 2 h in ambient atmosphere: (a) unheated, (b) 300 8C, (c) 420 8C, (d) 740 8C,
*
(e) 920 8C, and (f) 1200 8C, b: boehmite and a: corundum. indicates spinning sidebands.

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boehmite is transformed to metastable g-Al2O3 above 300 8C, the transition being completed at about 920 8C. The g-
Al2O3 diffraction peaks become more distinct as the calcination temperature increases. Fig. 5(f) shows the diffraction
trace of powder calcined at 1200 8C, indicating the presence of metastable d-Al2O3, u-Al2O3, and a-SiC. The
diffraction trace of a sample calcined at 1300 8C (Fig. 5(g)) reveals a combination of crystalline a-Al2O3 and a-SiC,
together with relatively low-intensity peaks associated with an aluminosilicate phase (Al2O3ꢀxSiO2), and the
metastable aluminas previously detected at 1200 8C. Table 1 summarises all the XRD phases present in sol–gel Al2O3
and sol–gel Al2O3/5 vol%SiC nanocomposite powders.
Fig. 7. 27Al MAS NMR spectra of sol–gel Al2O3/SiC nanocomposite powders calcined for 2 h in ambient atmosphere: (a) unheated, (b) 300 8C, (c)
*
420 8C, (d) 740 8C, (e) 920 8C, (f) 1200 8C and (g) 1300 8C, b: boehmite and a: corundum. indicates spinning sidebands.

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799
3.3. Nuclear magnetic resonance (NMR)
Fig. 6(a)–(f) show the 27Al MAS NMR spectra of sol–gel Al2O3 powder calcined at various temperatures. The 27Al
NMR resonances at approximately +8 and +14 ppm correspond to the octahedral AlO6 sites of boehmite and
corundum, respectively [38–40]. The resonance at +65 ppm arises from the tetrahedral AlO4 groups of the metastable
transition alumina phases g-, d-, and u-Al2O3 [38,40]. Fig. 6(a) shows the 27Al NMR spectrum of the as-precipitated
sol–gel Al2O3 powder, in which the intense resonance at ꢁ+8 ppm results from the octahedral AlO6 sites of boehmite.
On heating this powder at 300 8C, the AlO6 resonance remains dominant, but a small fraction (about 7%) has been
converted to AlO4 (the peak at +63 ppm in Fig. 6(b)). Fig. 6(c)–(e) show the 27Al NMR spectra of the powders calcined
at 420, 740 and 920 8C. Heating the powder in this temperature range gradually increases the relative intensity of the
AlO4 resonance with a concomitant decrease in the intensity of the boehmite AlO6 resonance. After calcination at
1200 8C, only the intense octahedral resonance of a-Al2O3 at ꢁ+14 ppm is detected by 27Al NMR (Fig. 6(f)).
Fig. 7(a)–(g) show the 27Al NMR spectra of sol–gel Al2O3/5 vol%SiC powder calcined at various temperatures. The
27Al NMR spectrum of the as-precipitated sol–gel Al2O3/SiC powder (Fig. 7(a)) exhibits an intense octahedral Al peak at
ꢁ+8 ppm corresponding to boehmite, similar to the peak detected in monolithic precipitated Al2O3. The 27Al NMR
spectrum of a composite precipitate calcined at 300 8C (Fig. 7(b)) shows that the relative intensity of the AlO6 resonance
has decreased to about 82% and the relative intensity of the tetrahedral AlO4 resonance has increased to about 18%. The
increase in the proportion of AlO4 in the composite at this temperature is greater than for monolithic Al2O3 without SiC
(7%). At calcination temperatures of 420 and 740 8C (Fig. 7(c) and (d), respectively), the relative intensities of the AlO4
and AlO6 resonances are similar to those of monolithic powders without SiC. In samples calcined at 740, 920 and
Fig. 8. Changes in 27Al NMR site occupancy as a function of calcination temperature: (A) sol–gel Al2O3 and (B) sol–gel Al2O3/SiC powders.

800
S. Jiansirisomboon, K.J.D. MacKenzie / Materials Research Bulletin 41 (2006) 791–803
1200 8C, the relative intensities of the tetrahedral and octahedral Al resonances in the composite remain stable at about
30% AlO4 and about 70% AlO6 (Fig. 7(d)–(f)). The 27Al NMR spectrum of the composite powder calcined at 1300 8C
(Fig. 7(g)) shows that the tetrahedral resonance of the transition alumina has almost entirely disappeared, being replaced
by the octahedral resonance associated with a-Al2O3. This spectrum consists principally of AlO6 (a-Al2O3) with only a
very small tetrahedral contribution from untransformed transition alumina.
The changes in 27Al NMR site occupancy on heating of both sol–gel monolithic Al2O3 and sol–gel Al2O3/SiC
powders, estimated by integration of the respective resonances, are summarised in Fig. 8(A) and (B). Heating to
progressively higher temperatures produces a gradual increase in the proportion of Al in the tetrahedral sites (g-, d- and
Fig. 9. 29Si MAS NMR spectra of sol–gel Al2O3/SiC powders calcined for 2 h in Ar: (a) unheated, (b) 300 8C, (c) 420 8C, (d) 740 8C, (e) 920 8C, (f)
1200 8C and (g) 1300 8C.

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801
u-Al2O3). The complete transformation to octahedral Al (a-Al2O3) is marked by the abrupt disappearance of
tetrahedral Al. The temperature at which complete transformation occurs in the composite (1300 8C) is at least 100 8C
higher than for the corresponding monolithic Al2O3 (1200 8C).
Fig. 9(a)–(g) show the 29Si NMR spectra of sol–gel Al2O3/5 vol%SiC nanocomposite powders calcined at various
temperatures. The 29Si NMR resonances at approximately ꢂ14, ꢂ20 and ꢂ23 ppm arise from a-SiC [40–42]. The
resonances between ꢂ76 and ꢂ91 ppm are due to aluminosilicate phases (Al2O3ꢀxSiO2) [42,45], while that at
ꢂ110 ppm corresponds to silica (SiO2) [40,43,44], formed by surface oxidation of SiC particles during calcination.
Even though calcination of the sol–gel Al2O3/SiC powder was carried out in an Ar atmosphere, it is possible that
sufficient O was present as an atmospheric impurity to oxidise the SiC to a small amount of SiO2 [45] which then
reacted with the Al2O3 to form an Al2O3ꢀxSiO2 phase. The possible reactions are summarised in Eqs. (4)–(6):
SiCðsÞ þ 2O2ðgÞ ! SiO2ðsÞ þ CO2ðgÞ "
SiCðsÞ þ 1:5O2ðgÞ ! SiO2ðsÞ þ COðgÞ "
Al2O3ðsÞ þ xSiO2ðsÞ ! Al2O3ꢀxSiO2ðsÞ
(4)
(5)
(6)
Fig. 9(a) shows the 29Si NMR spectrum of precipitated sol–gel Al2O3/5 vol%SiC, indicating that 88% of the Si occurs
as a-SiC and 12% of the Si is present as amorphous SiO2, even though this sample was not exposed to high temperature.
The presence of this SiO2 may arise from nucleation of amorphous SiO2 during sol–gel processing. After calcination at
300 8C, the fraction of Si present as amorphous SiO2 increases to 25% and the Si in a-SiC is correspondingly reduced to
75% (Fig. 9(b)). Fig. 9(c) shows that this phenomenon continues on heating at 420 8C, the Si fraction in amorphous SiO2
increasing in this case to 41%. Heating at 740 8C (Fig. 9(d)) causes an amorphous Al2O3ꢀxSiO2 phase to nucleate. At this
temperature, the Si content of both the amorphous SiO2 and Al2O3ꢀxSiO2 phases is 38%. The fraction of Si in the
amorphous phase increases to 51% in the powder calcined at 920 8C (Fig. 9(e)), but subsequently decreases to an
approximately constant value of 42% when the powder is calcined at 1200 and 1300 8C (Fig. 9(f)–(g)). The changes in Si
site occupancy in the heated sol–gel Al2O3/SiC powders are summarised in Fig. 10.
The 27Al and 29Si NMR data for the sol–gel samples suggest that the presence of SiC during the calcination process
modifies the thermal behaviour of Al2O3 in the composite powders. For instance, corundum (a-Al2O3) forms at a
higher temperature in the composite than in monolithic Al2O3. In Al2O3, metastable g-, d- and u-Al2O3 gradually form
with increasing calcination temperature, with a corresponding monotonic increase in the fraction of tetrahedral AlO4,
up to the formation at 1200 8C of corundum which contains only octahedral AlO6. By contrast, the presence of SiC
stabilises the concentrations of transition aluminas which remain more or less static over the temperature range 420–
Fig. 10. Changes in 29Si NMR site occupancy as a function of calcination temperature of sol–gel Al2O3/SiC nanocomposite powder.

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1200 8C. Even when corundum finally forms at 1300 8C, it does so sluggishly and a small amount of transition alumina
still remains after 2 h at this temperature, as shown in Figs. 7(g) and 8(B). Changes in the relative intensity of SiC
NMR peaks during calcinations suggest that a relationship exists between the presence of a-SiC and the kinetics of the
Al2O3 phase transformations. This is also seen in the apparent protection of the SiC from further oxidation to SiO2 over
the temperature range in which the transition aluminas are stabilised.
4. Conclusion
This work describes a sol–gel process for the preparation of Al2O3 and Al2O3/SiC nanocomposite powders from
AlOOH precursors. DSC results indicate that on heating, the precursors undergo thermal events at 300, 420, 740 and
920 8C for both powders. Higher calcination temperatures (1200 and 1300 8C) were also employed. XRD and 27Al
MAS NMR results reveal that the stable alumina form (a-Al2O3) is formed at 1200 and 1300 8C in the monolithic
Al2O3 and Al2O3/SiC nanocomposite powders, respectively. The 29Si MAS NMR spectra clearly indicate that
amorphous SiO2 is always present throughout the precipitation and calcination stages in samples containing
polycrystalline a-SiC. Moreover, undesirable amorphous Al2O3ꢀxSiO begins to form in these samples at about 920 8C.
Acknowledgements
SJ would like to thank the Thailand Research Fund for financial support and Department of Materials, Oxford
University, UK for providing laboratory facilities.
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