Ceramics International 49 (2023) 13363–13370

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Ceramics International
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A novel preparation method for elongated mullite using Zircon-Al2O3

modified SiO2 as silicon source for high-temperature functional ceramics
Haixiang Mai a, Fei Zhao a, b, *, Xianzhong Zhu a, d, Jiancheng An c, Weikang Lian c, Yang Hu a,
Xinhong Liu a, **
a
Henan Key Laboratory of High Temperature Functional Ceramics, Zhengzhou University, Zhengzhou, Henan, 450052, China
b
The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan, Hubei, 430081, China
c
Tongda Refractory Technologies, Co. LTD, 1Anningzhuang East Road, Haidian District, Beijing, 100085, China
d
China Petroleum Pipeline Research Institute Co. LTD, Langfang, Hebei, 065000, China

A R T I C L E I N F O A B S T R A C T

Handling Editor: Dr P. Vincenzini This work provides a novel route for the fabrication of mullite ceramics with excellent mechanical strength using
Zircon-Al2O3-modified SiO2 as silicon source. The results revealed that silicon sources had significant influences
Keywords: on the grain morphology and properties of the prepared mullite ceramics. A large number of short granular and
Modified SiO2 granular-shaped mullite grains were formed in the mullite ceramics directly prepared using γ-Al2O3, high-purity
Elongated mullite
quartz and zircon powder, and this is attributed to the very slow diffusion process between the pure Al2O3 and
SiO2-Rich liquid phase
SiO2 powders under solid state conditions. The melting temperature of the silicon source was obviously
High-temperature properties
decreased after the modification of an appropriate amount of Zr4+ and Al3+ in the SiO2 lattice. During the
sintering process, the Zircon-Al2O3-modified SiO2 was transformed into a transient liquid phase with low vis­
cosity at lower temperatures, and this promoted the anisotropic grain growth of the mullite. After sintering at
1650 ◦ C, elongated mullite was formed in-situ while the transient liquid phase was consumed, which enhanced
the high-temperature properties of the novel mullite ceramics.

1. Introduction improving the performances of mullite ceramics. Sintering additives

have been used as effective strategies for the syntheses of elongated
Owing to their high-temperature stability, low coefficient of thermal mullite ceramics. For example, V. Viswabaskaran et al. [10] prepared
expansion and good creep resistance at elevated temperature, mullite elongated mullite crystals using MgO as the additive, and the investi­
ceramics has been widely employed in high temperature industrial ap­ gation also showed that MgO promoted the liquid phase formation at
plications [1–3]. Several techniques have been developed for the syn­ high temperatures. The mullite grains grew and the mechanical strength
theses of high-purity mullites in recent years, such as the sol-gel method of the ceramics also increased with increasing MgO content (1–3 wt%).
[4], coprecipitation [5], and molten salt synthesis [6]. However, these H. Ye et al. [11] synthesized elongated iron-containing mullite solid
complex preparation processes limit their large-scale industrial appli­ solutions with an aspect ratio of 10.1 using α-Al2O3 and fused silica as
cations. Hence, traditional reaction sintering is still the most preferred the starting materials, and Fe2O3 powders (9 wt%) as an additive.
method considering the production cost involved and the feasibility of However, the majority of the sintering additives caused the formation of
the process. The reaction sintering of high-purity Al2O3 and SiO2 pow­ low-melting point phases, which did not favor the high-temperature
ders is mainly based on solid-state reactions and the slow interdiffusion performance of the mullite ceramics [12]. In contrast to the sintering
rates of Al2O3 and SiO2 generally leads to low mullite conversion effi­ additives, ZrO2 exhibits a high melting phase in an Al2O3–SiO2 system.
ciencies, loose ceramic structures, and fine granular or short columnar Zircon or ZrO2 is generally employed in mullite-zirconia ceramics, and
mullite grains [7–9], posing difficulties in achieving sintered mullite its incorporation effectively improves the thermal shock resistance,
ceramics with excellent high-temperature properties. flexural strength and fracture toughness of the ceramics [13–15].
Elongated grains strengthening and toughening is crucial for Notwithstanding, to the best of our knowledge, there are no existing

* Corresponding author. Henan Key Laboratory of High Temperature Functional Ceramics, Zhengzhou University, Zhengzhou, Henan, 450052, China.
** Corresponding author.
E-mail addresses: zhaofeiln@zzu.edu.cn (F. Zhao), liuxinhong@zzu.edu.cn (X. Liu).

https://doi.org/10.1016/j.ceramint.2022.12.211
Received 28 November 2022; Received in revised form 19 December 2022; Accepted 21 December 2022
Available online 22 December 2022
0272-8842/© 2023 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
H. Mai et al. Ceramics International 49 (2023) 13363–13370

literatures on the detailed investigations of the effects of zircon as an (abbreviated as ZA-modified SiO2) were synthesized using the basic
additive on the sintering properties and grain shapes of mullite ceramics. starting materials (γ-Al2O3, high-purity quartz and zircon). The modified
This is therefore our research motivation. SiO2 samples were prepared and compacted into cylinders after a
Reports have shown that a small quantity of ZrO2 and Al2O3 can thorough homogenization of the starting materials in a ball mill. The
promote SiO2-rich liquid phase formation due to the ternary eutectic green bodies were heated at 1550 ◦ C, and the modified SiO2 samples
point located at SiO2-rich area in a ternary Al2O3–SiO2–ZrO2 system were used as silicon sources after crushing and milling into fine powders
[16]. Thus, we herein report the modification of zircon and Al2O3 in (particle size< 45 μm).
silicon source based on the compositions of the eutectic point. The For comparison with the traditional methods, two groups of mullite
synthetic silicon source was transformed into a large amount of transient samples were prepared according to the stoichiometric ratio of mullite
liquid phase at lower temperatures, which effectively promoted the (3Al2O3⋅2SiO2), namely ZrMU-1 and ZrMU-2 samples. For the ZrMU-1
dissolution-precipitation rate and enhanced the anisotropic crystal group, the mullite was directly synthesized using γ-Al2O3, high-purity
growth of the mullite ceramics. Meanwhile, the ZrO2 and Al2O3 were quartz and zircon powder, while the raw materials for the ZrMU-2
transformed into high temperature phases in the ternary group were γ-Al2O3 and ZA-modified SiO2. Both the ZrMU-1 and
Al2O3–SiO2–ZrO2 system when the SiO2-liquid phase was consumed ZrMU-2 samples contained 1.8 wt% ZrO2. The two groups were fully
during the mullitization process. Moreover, we also investigated the mixed by ball-milling and then compacted at a uniaxial press of 120
effects of sintering temperatures and varying silicon sources on the MPa. The series of green bodies were sintered for 3 h between 1550 ◦ C
phase composition, development of mullite grains, and and 1600 ◦ C in order to evaluate the evolution of phase compositions
high-temperature performances of the prepared ceramics. and microstructures. The performances of the sintered mullite ceramics
at 1650 ◦ C for 6 h were investigated.
2. Experimental
2.2. Characterization
2.1. Raw materials and sample preparation
X-ray diffractometer (Philips X’Pert Pro, the Netherlands, Cu Kα ra­
Commercial γ-Al2O3 powder [≤74 μm, w (Al2O3) >98.9%, CHALCO
diation, XRD) was used to identify the phase compositions of sintered
Shandong Co. Ltd., China], high-purity quartz powder [≤74 μm, w
samples. The microstructures and elemental compositions of the sin­
(SiO2) >99.9%, Gucheng, Hubei Province, China], and zircon powder
tered samples were observed using a scanning electron microscope
[≤44 μm (ZrSiO4) >99.3%, Saint-Gobain Co. Ltd.,], were used as the
(Zeiss, Germany, EVO HD15, SEM) at 20 KV accelerating voltage
starting materials. The fabrication process chart of the modified SiO2 is
equipped with an energy-dispersive X-ray spectrometer (Oxford, UK,
shown in Fig. 1. Based on the composition of the eutectic point in
INCA2000, EDS). The average grain size of the mullite was obtained by
Al2O3–SiO2–ZrO2 ternary system [16], the Al2O3-modified SiO2
measuring the cross-section of over 100 grains of each sample.
(abbreviated as A-modified SiO2) and zircon-Al2O3-modified SiO2
The cold compressive strength (CCS, Ф20 mm × 20 mm) of the

Fig. 1. Fabrication process chart of the modified SiO2.

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prepared samples was measured by a digitally controlled tester with a Table 1

cross-head rate of 0.3 mm/min. The bulk density (BD) and apparent Lattice parameters of cristobalite in the different silicon sources.
porosity (AP) of the sintered samples were determined by the conven­ Silicon sources Lattice parameter
tional Archimedes drainage method. The cold modulus of rupture
a/nm b/nm c/nm V/nm3
(CMOR, 15 mm × 15 mm × 80 mm) was measured according to a three-
point bending mode using a span of 60 mm, and the loading speed was purity quartz 0.4973 0.4973 0.6924 0.1712
A-modified SiO2 0.4976 0.4976 0.6934 0.1717
0.5 mm/min [17]. According to the standard GB/T 30873–2014, the AZ-modified SiO2 0.4983 0.4983 0.6946 0.1725
thermal shock resistance (TSR) of the prepared mullite ceramics was
evaluated using the residual CMOR ratio (3 cycles, water cooling) before
and after thermal shock [18]. According to the standard GB/T SEM images and the corresponding EDS mappings of the different sili­
5989–2008, the refractoriness under load (RUL, Ф50mm × 50 mm, T0.5 con sources after sintering at 1590 ◦ C for 1 h. It is observed that the
was 0.5% of the deformation temperature, under a compressive load of granular-shaped cristobalite crystals formed SiO2 particles, and there is
0.2 MPa) of the sintered samples was examined to characterize the high no obvious liquid phase between the particles in the high-purity quartz
temperature mechanical properties of the obtained mullite ceramics. (SiO2) sample. The A-modified SiO2 and ZA-modified SiO2 exhibited
Quantitatively, the high temperature liquid viscosity of the different molten collapse state after sintering at 1590 ◦ C. The molten collapse area
silicon sources were indirectly characterized by the vertical projection of the ZA-modified SiO2 was much larger than that of the A-modified
area of the samples after cooling to room temperature [19]. SiO2 sample, indicating that a greater amount of rich-SiO2 liquids with
good fluidity was generated for the ZA-modified SiO2 at elevated tem­
3. Results and discussion peratures. As revealed by the EDS mappings, the Al and Zr elements
were embedded in the modified SiO2, and some of the ZrO2 particles
3.1. Effect of Al2O3/zircon-Al2O3 additives on silicon source agglomerated into larger particles. Based on the EDS results and the
lattice parameters of the different SiO2 sources presented in Table 1, it is
Fig. 2 shows the XRD patterns of the heated-treated high-purity believed that appropriate amounts of Zr4+ and Al3+ were incorporated
quartz, A-modified SiO2 and ZA-modified SiO2 at 1550 ◦ C. into the SiO2 framework, which contributed to the formation of lower
It can be seen from Fig. 2 that the three kinds of silicon sources viscosity liquid phases.
mainly exhibited cristobalite characteristics (PDF#39–1425) after heat
treatment. Only a small amount of mullite (PDF#15–0776) was present
in the modified SiO2 samples when Al2O3 was used as a dopant. Large 3.2. Evolution of phase compositions and microstructures
amounts of mullite phases in the modified SiO2 samples were difficult to
obtain with the addition of 6 wt% Al2O3 into the SiO2-rich system, which Since the conversion of the silicon sources from solid state to liquid
is much lower than the critical concentration of mullite nucleation [20]. state mainly occurs around 1500–1650 ◦ C, the difference in phase
Moreover, the cristobalite diffraction peaks were obviously weakened compositions and microstructures resulting from the liquid phases in the
and shifted to larger angle for the ZA-modified SiO2 source. Table 1 reaction systems with the different silicon sources is most obvious at the
shows the lattice parameters of the cristobalite from the different silicon same temperature interval [23,24]. Fig. 4 shows the XRD patterns of
sources, and they were obtained by the Whole Pattern Fitting and ZrMU-1 and ZrMU-2 samples sintered at different temperatures.
Rietveld Refinement method [21]. The lattice parameters of the cristo­ As depicted in Fig. 4 for both the ZrMU-1 and ZrMU-2 samples sin­
balite increased for the Al2O3 or ZrO2–Al2O3 modified in high-purity tered between 1500 ◦ C and 1550 ◦ C, mullite, zircon (PDF#06–0266) and
quartz due to the incorporation of Al3+ or Al3+-Zr4+ into the SiO2 lat­ obvious corundum (PDF#46–1212) phases are observed, indicating that
tice, which loosened the Si–O network structure and reduced the melting some of the pure SiO2 have been transformed into amorphous forms.
point of the silicon source. Compared with the ZrMU-1 sample, only the diffraction peaks of m-ZrO2
From the phase diagram studies, the addition of 6 wt% Al2O3 into the and mullite were observed in the XRD pattern of ZrMU-2 sample after
high-purity quartz rapidly decreased the liquid phase formation tem­ sintering at 1550 ◦ C. The zircon and corundum phases disappeared at
perature from 1720 ◦ C to 1590 ◦ C, and ZrO2 acted as a fluxing agent for lower temperatures, proving that the ZA-modified SiO2 in the system
the rich-SiO2 liquids [22]. Fig. 3 denotes the macroscopical photograph, was completely and rapidly consumed by the mullite formation reaction,
and the Al2O3-rich environment promoted the dissociation of zircon [25,

Fig. 2. XRD patterns of the different silicon sources.

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Fig. 3. Macroscopical photograph, SEM images and EDS mappings of the different silicon sources sintered at 1590 ◦ C for 1 h.

Fig. 4. XRD patterns of samples sintered between 1500 and 1650 ◦ C for 3 h: (a) ZrMU-1, (b) ZrMU-2.

26]. At 1600–1650 ◦ C, the mullite diffraction peaks were slightly 3.3. Structure and properties of the prepared mullite ceramics
enhanced, while all the zircon phases were dissociated to m-ZrO2 phase
in both samples. The SEM images of the prepared mullite samples after sintering at
Fig. 5 presents the SEM images of the ZrMU-1 and ZrMU-2 samples 1650 ◦ C are shown in Fig. 6.
sintered at 1500–1600 ◦ C for 3 h. Well-developed mullite grains were observed in both the prepared
For the ZrMU-1 sample, small granular-shaped mullite grains were mullite ceramics as displayed in Fig. 6. With the same composition, there
easily observed at 1500 ◦ C (Fig. 5a). As the sintering temperature was were significant differences between the microstructures of the samples
increased to 1600 ◦ C, there were observed gradual growths of the with different silicon sources. The mullite grains of the ZrMU-1 sample
granular-shaped mullite grains and the crystal boundaries became exhibited short columnar or granular-shapes and large numbers of
clearer. Different phenomena were observed in the ZrMU-2 sample. granular pores with large sizes. It was also observed that larger numbers
Obvious amorphous glass phases were seen (Fig. 5d), and some of the of elongated mullite with a large aspect ratio were formed in the ZrMU-2
fine granular-shaped mullite grains were surrounded by large amounts sample. According to statistical results, the prepared mullite grains had
of liquids at 1500 ◦ C. In comparison with Fig. 5a, it is observed that an average length of 12.32 μm and an average diameter of 1.76 μm. The
larger amounts of the ZA-modified SiO2 have been transformed into elongated columnar mullite grains were interlaced with each other to
rich-SiO2 liquid phase at a temperature of 1500 ◦ C. Furthermore, it was form a network structure, as exhibited by the ZrMU-2 sample (Fig. 6),
observed that some of the amorphous phase disappeared when the sin­ which improved the mechanical properties of the mullite ceramics
tering temperature was increased to 1550 ◦ C with the formation of [27–29].
larger mullite grains (Fig. 5e). Large elongated mullite grains were Table 2 lists the apparent porosity (AP), bulk density (BD) and RUL of
observed in Fig. 5f and some ZrO2 particles were found in the gaps the ZrMU-1 and ZrMU-2 samples sintered at 1650 ◦ C for 6 h, and the
around the mullite grains at 1600 ◦ C, which may have contributed to the results show that the density and high-temperature mechanical prop­
liquid phase was consumed during the mullite formation, while ZrO2 erties (RUL) of the prepared mullite ceramics were enhanced with the
was precipitated from the liquids. ZA-modified SiO2 as the silicon source. For the ZrMU-2 sample, the ZA-

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Fig. 5. SEM images of the ZrMU-1 samples sintered at (a) 1500 ◦ C, (b) 1550 ◦ C and (c) 1600 ◦ C for 3 h, and the ZrMU-2 samples sintered at (d) 1500 ◦ C, (e) 1550 ◦ C
and (f) 1600 ◦ C for 3 h.

Fig. 6. SEM microphotographs of the ZrMU-1 and ZrMU-2 samples sintered at 1650 ◦ C for 6 h.

modified SiO2 source was transformed into large amounts of SiO2-rich from the liquid, thereby losing its solvent role.
liquid phase at lower temperatures (Fig. 2), which was beneficial to the Sometimes, fracture toughness and thermal shock resistance (TSR)
improvement of the sintering performance of the mullite ceramics. The are more important than absolute strength in applications involving
RUL (T0.5) of both samples was above 1650 ◦ C, indicative of excellent high-temperature functional ceramics materials [32–35]. The fracture
high-temperature mechanical properties compared with reports from toughness of each sample can be qualitatively compared using the in­
literatures [30,31]. The transient SiO2-rich liquid phase of the obtained tegral area of the stress-strain curve [36,37]. Fig. 7 illustrates the
mullite was consumed by the mullitization, while ZrO2 was precipitated stress-strain curve of the different sintered samples under continuous

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Table 2 without elongated mullite. The improved TSR performance of the ZrMU-
Typical physical properties and refractoriness under load (RUL) of the prepared 2 sample correlates with its structure, which is the elongated mullite
mullite ceramics. form network structure (Fig. 6), and this prevents further propagation of
Samples AP/% BD/g⋅cm− 3
RUL (T0.5)/◦ C the cracks from thermal shock.
ZrMU-1 1.89 2.84 1650
ZrMU-2 1.22 2.94 1665 3.4. Discussions

The schematic diagrams of the microstructural development of the

different samples during sintering are given in Fig. 9. A number of ref­
erences reports [9,40] that mullite formations require high temperatures
and considerable time due to the very low diffusion coefficient of
alumina and silica, and that the activation enthalpy for the growth of
mullite grain is very high. The microstructures of the obtained mullite
were also controlled by the diffusion rates of Al2O3 and SiO2 particles
[8]. For the ZrMU-1 sample, the mullite formation can be explained by
the traditional solid state reaction mechanism. The mullitization first
occurred at the interface of the high-purity quartz/zircon and Al2O3
particles, and then mullite layers were formed around the residual SiO2
and zircon particles, which further prolonged the diffusion paths of Al3+
and Si4+ [41]. As the sintering temperature was increased, the unreacted
SiO2 was transformed to liquid state and then flowed to the gaps be­
tween the particles [42], leading to the formation of large numbers of
granular pores (as shown in Fig. 6). Therefore, the mullitization and
zircon decomposition were superimposed in a wide temperature range
for the reaction system of the ZrMU-1 sample. The small granular mullite
Fig. 7. Compressive stress-strain curves of the prepared samples and SEM of grains formation in the obtained mullite ceramics may be attributed to
their fractured surfaces. the slow mullitization rate and long diffusion paths for the silicon and
aluminum sources.
pressure. The ZA-modified SiO2 particles in the ZrMU-2 sample were
From the figure, it is observed that the CCS of the ZrMU-2 sample was completely transformed into the rich-SiO2 liquid phase at lower tem­
337.3 MPa, which is much higher than that of the ZrMU-1 (236.9 MPa), peratures (Fig. 2). The liquid phase had a lower viscosity due to the pre-
especially having the same heating-treat temperature and chemical modification of Zr4+ and Al3+ in the silicon source (Table 1). The Al2O3
composition. In the ZrMU-2 sample, the cracks needed a break across the particles were dissolved in the liquid at a fast rate to form aluminosili­
elongated mullite grains, which required a greater ultimate stress during cate liquid, and the liquid phase effectively accelerated the diffusion
the fracture process. The ZrMU-2 sample exhibited greater deformation rates of Al3+ and Si4+. The nucleation and growth of the mullite was
before destruction. This is mainly due to the fact that the elongated inititaed when the concentration of Al2O3 in the liquid phase exceeded
mullite expanded the propagation paths of the crack and increased the the critical concentration [20]. The mullite had the habit of preferred
energy loss [38,39]. This endowed the ZrMU-2 sample with good growth along the c-axis, but its morphology was also limited by the
toughening effects during strengthening. growth environment. The activation energy of the mullite crystal growth
The TSR of the prepared mullite ceramics was determined using the along the axial direction was reduced in the low viscosity liquid phase
residual strength ratio after three thermal cycles (ΔT = 1000 ◦ C, water environment provided by the melting of the ZA-modified SiO2. At the
cooling). Fig. 8 shows that the residual cold modulus of rupture (CMOR) same time, the transmission distance of Al3+ and Si4+ was also
and residual CMOR ratio. It is seen from the figure that the residual increased, providing substances for the anisotropic growth of the mullite
strength ratio of the ZrMU-2 sample was 72.55%, which is higher than grains. Therefore, the growth rate of the mullite grains along the length
that of the ZrMU-1 (55.64%). In addition, the residual CMOR value of direction was obviously higher than that along the diameter direction,
72.55 MPa was recorded for the ZrMU-2 sample after 3 cycles of thermal and the granular mullite gradually develops into elongated columnar
shocks. This is about 49.37% higher than that of the ZrMU-1 sample shape (as shown in Fig. 6). Moreover, the liquid phase effectively pro­
moted the rearrangement of particles, and reduced the formation of
pores during the sintering process, ultimately improving the density of
the prepared ceramics (Table 2). The excellent high-temperature per­
formances and mechanical strength of the ZrMU-2 mullite are attributed
to its high density, highly anisotropically-grown elongated crystals, and
interlocking network structure formed by the mutual interweaving and
squeezing of the crystals (Fig. 6). In addition, the Zircon additive mainly
acted as a flux in the SiO2-source, ensuring that some of the Zr4+ was
present in the liquid phase at high temperatures. It was difficult for Zr4+
to enter the mullite structure during the mullitization process due to its
large ionic radius and the mismatch of oxidation states [43]. With the
consumption of the SiO2-rich liquid in the reaction, ZrO2 was gradually
precipitated from the liquid phase and then crystallized into granular
grains [Figs. 5f -Fig.6], and lost its solvent role in the ZrO2-mullite
system.

4. Conclusions

Fig. 8. Thermal shock resistance (TSR) of the prepared mullite ceramics. Mullite ceramics with excellent high-temperature performances

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Fig. 9. Schematic diagrams of the sintering reaction process of (I) ZrMU-1 and (II) ZrMU-2: A–γ-Al2O3, S–quartz, S′ –modified SiO2, ZS-ZrSiO4, Z-ZrO2, P–pore,
M–mullite, L–SiO2 rich liquids.

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