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Powder Technology 15

Study on the formation process of Al2O3–TiO2 composite powders

Shiquan Liu *, Wenhong Tao, Jia Li, Zhongxi Yang, Futian Liu

School of Materials Science and Engineering, Jinan University, Jinan, Shandong 250022, China

Received 24 August 2004; received in revised form 23 March 2005; accepted 18 May 2005

Available online 14 July 2005

Abstract

Fine particles of anatase were suspended in solutions of ammonium alum with Al2O3/TiO2 molar ratios from 0.1:1 to 7:1. By spray drying

the suspensions and calcining the spray-dried powders, Al2O3–TiO2 composite particles were obtained. The results show that after the spray

drying, coatings of ammomium alum are formed on the surface of the anatase particles, leading to composite precursor powders (CCPs) with

larger particle sizes. Upon calcining the CCPs, ammomium alum pyrolyzes to amorphous Al2O3 and anatase transforms into rutile. Both are

mainly responsible for the observed particle size reductions as well as the densification of each composite particle. The in-situ formed a-

Al2O3 and rutile may have higher reactivities, forming aluminum titanate at 1150 -C, about 130 -C lower than the theoretical temperature for

the formation of Al2TiO5 by solid reaction. The reaction between a-Al2O3 and rutile starts from the interface between the anatase and the

alum coating and mainly takes place in the single particles formed by spray drying. The molar ratio of Al2O3 to TiO2 influences the final

crystalline phases in the composite powders, but not stoichiometrically.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Al2O3–TiO2 composite particle; Aluminum titanate; Spray dyring; Calcination; Crystalline phase

1. Introduction Traditionally, Al O and TiO are ball-milled to form

Al2O3 ceramics have been widely used in many fields

because of their good mechanical properties, such as high

hardness and super abrasion resistance [1,2]. Recently,

researchers have tried to improve their performance through

additions of other oxides [3–6]. Xiao and his coworkers

found that protective coatings on metals, which were

prepared from Al2O3–TiO2 composite powders by flame-

spraying, have much better abrasion resistance than those

consisting of pure Al2O3 powders [6]. Wunderlich et al.

pointed out that nano-hybrid Al2O3–TiO2 might be applied

as catalysts [7]. In addition, ceramics made from Al2O3–

TiO2 composite powders show attractive perspectives.

Aluminum titanate ceramic is a good example. Owing to

its excellent thermal expansion behavior [8–10], it can be

used as thermal resistant materials, such as catalyst carriers

for purification of fume produced by cars, as containers and

tubes for storing or conveying high temperature steel liquid

and as protective tube for thermal couples, etc.

0032-5910/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.powtec.2005.05.048

* Corresponding author. Tel.: +86 531 2201692; fax: +86 531 7974453.

E-mail address: vctrliu@hotmail.com (S. Liu).

2 3 2

composite powder mixtures for the subsequent shaping and

sintering process [10]. Other authors have tried new

methods such as wet chemical synthesis and high temper-

ature oxidation [7,11,12]. We use spray drying, an industrial

technology, to form composite precursor powders (CPPs) of

ammonium alum and titania. After calcining the precursor

powders, Al2O3–TiO2 composite particles with different

crystalline phases are obtained. The most obvious character

of the spray drying is that the as-prepared powders are

uniformly spherical, which is beneficial to the following

conveying and shaping procedure in the manufacturing of

ceramic products. In addition, the spray drying introduces

less impurity and saves time and energy compared to the

traditional ball-milling. We also find that upon calcination

the in-situ formed components inside the CPPs show higher

chemical reactivities.

2. Experimental

Fine anatase powder produced by the sulfate process

(Yuxing Chemical Industrial Factory, Jinan) was suspended

5 (2005) 187 – 192

Table 1

Designed compositions of Al2O3–TiO2 composite powders

Sample no. AT1 AT2 AT3 AT4 AT5

Al2O3/TiO2 molar ratio 0.1:1 0.6:1 1:1 1.4:1 7:1

S. Liu et al. / Powder Technology 155 (2005) 187–192188

in the solutions of ammonium alum (Dagang Yizhong

Chemical Industrial Factory, Tianjing). The molar ratios of

Al2O3 to TiO2 (A/T ratio) were changed from 0.1:1 to 7:1

(see Table 1).

Spray drying was performed on a QP-3 spray-drier

(Research Institute of Chemical Industry, Beijing). The

conditions for the spray drying were as follows: pressure of

the atomizing air, 0.1–0.2 MPa; flow of feeding, 30 ml/min;

temperatures at the inlet and outlet of the spray-drier,

200T10 -C and 100T10 -C, respectively.Powders collected from the spray-drier were calcined in

an electrical furnace with a temperature precision of T5 -C.The temperatures were set at 700, 864, 1150 and 1368 -C,respectively, with a heating rate of 10 -C/min. The

calcined samples were cooled in air directly from high

temperatures.

X-ray diffractograms (XRD) were recorded on a D/max-

rA diffractometer (Rigaku, Japan) to identify the crystalline

phase in the calcined samples. A S-2500 Scanning Electron

Microscope (SEM) (Hitachi, Japan) equipped with an

Energy Dispersive Spectrum (EDS) analyzer (Oxford Co.,

Britain) was used to analyze the morphology and compo-

sition of the powders. The measurements of the powder

particle size and its distribution were performed on a FAM

Laser Particle Diameter Analyzer (Pike Instrument Co.,

Shanghai). Histograms of the particle size distribution

(PSD) based on the mass frequency were drawn according

to the original data. Modes, representing the values that

occur most frequently in the distributions, were labeled on

the histograms.

(a)

~200µm

Fig. 1. SEM graph (a) and EDS (b) of

3. Results and discussion

3.1. Formation of the composite precursor powders (CPPs)

upon spray drying

Most of the particles prepared by the spray drying are

dispersed and spherical (Fig. 1a).

A comparison among the mean particle sizes (d50) of the

original anatase and the spray-dried powder shows an

increase of d50 from 4.48 to 17.24 Am, indicating a

significant increment of the particle size. Taking AT3 as

an example, Figs. 2a and b depict the histograms of particle

size distribution of the original anatase powder and the

spray-dried powder. As compared with the columns in Fig.

2a, the heights of the first eight columns in Fig. 2b decrease

in contrast with increases of the height of the subsequent

columns, suggesting that after the spray drying, larger

particles are formed. It can also be seen that after the spray

drying, the mode shifts to a larger particle size value (Fig.

2b). Meanwhile, the particle size distribution becomes

broader, indicated by new columns in the size fractions

above 53.5 Am.

According to the mechanism of spray drying [6],

ammomium alum coating is supposed to be formed on the

surface of TiO2 particles. Moreover, EDS analysis (Fig. 1b)

reveals that these particles, bigger or smaller in size, all

contain elements of O, Al, S and Ti, indicating that spray

drying the suspension with anatase powder in the alum

solution successfully leads to the formation of composite

powder consisting of TiO2 and ammomium alum.

TGA analyses reveal that upon heating, the weight losses

of the CPPs depend on the A/T ratios (Fig. 3). The larger the

A/T ratio is, the more the weight loss is observed. Since the

weight loss is mainly due to the thermal decomposition of

alum in the CCP, this correlation between the weight losses

and the A/T ratios indicates that thicker alum coatings are

formed on the surface of anatase in the case of higher A/T

(b)

cps

15

10

5

0

1 2 3 4 5Energy (keV)

O

Al

S

Ti

the composite precursor powder.

21a

b

c

d

e

f

0

21

021

021

021

021

0

0.5-

1.9

1.9-

2.4

2.4-

3.0

3.0-

3.8

3.8-

4.8

4.8-

6.2

6.2-

7.9

7.9-

10.1

10.1

-13

13-1

6.7

16.7

-21.

621

.16-

28.1

28.1

-37.

637

.6-5

3.5

53.5

-87.

187

.1-1

859

Particle Size (µm)

Fre

quen

cy,

p

arti

cles

by

mas

s

mode

mode

mode

mode

mode

mode

Fig. 2. Histograms of the particle size distribution of the CCP and the

calcined samples of AT3.

Fig. 3. Weight losses versus A/T ratios.

S. Liu et al. / Powder Technology 155 (2005) 187–192 189

ratios. However, the correlation is not linear, because the

calculation of the weight loss of the CCP takes the mass of

inner anatase plus the alum coating as a whole, while the

former does not lose any weight except the adsorbed water

upon heating.

3.2. Reactions in the composite precursor powders (CPPs)

upon calcination

After being spray-dried, the CCP powders were first

calcined at 700 -C, 864 -C and 1150 -C for 2 h,

respectively. The results of the particle size analysis for

the calcined samples of AT3 are also depicted as histograms

in Fig. 2. Comparing Fig. 2c–e with Fig. 2b, we can see that

the modes of the calcined samples are shifting to smaller

size values, indicating that the powder particles are

becoming smaller upon increasing temperatures. Mean-

while, the PSDs are narrowing. The d50 values show

decreases from 17.24 for CCP to 15.12Y12.02Y7.03 Amafter calcinations. SEM observation reveals that most

particles in the sample calcined at 1150 -C for 2 h are still

spherical (Fig. 4a).

However, when the spry-dried powder was calcined at

1368 -C for 20 min, the histogram of PSD shows a

significant shift to the large size fractions, suggesting a

sharp increase of the particle size. The d50 value is increased

up to 26.07 Am, even larger than that for the spray-dried

powder. SEM observation reveals that particles in this case

are not spherical at all (Fig. 4b).

From the DTA curve of the spray-dried ammomium

alum powder (Fig. 5a), it can be seen that weight losses

mainly take place below 800 -C, no obvious weight

losses have been observed when the temperature is higher

than 1000 -C, suggesting that the pyrolysis of ammonium

alum is completed. That is to say, ammonium alum in

CCPs must have pyrolyzed to Al2O3. Similar results are

obtained for the CCPs of AT3 (Fig. 5b). Meanwhile, as it

will be illustrated later by the XRD results, some anatase

has transformed into rutile in the same temperature

region. since rutile is more densely compacted than

anatase, the phase transformation can result in a negative

volume change [13]. Therefore, it is supposed that the

decreases in the particle size under 900 -C are mainly

due to the pyrolysis of ammomium alum on the surface

of the anatase particles and the volume reduction caused

by the anatase-to-rutile transformation. At 1150 -C,densification which occurs inside each composite particle

should be mainly responsible for the particle size

reduction. At 1368 -C, possibly the inter-particle solid

Fig. 4. SEM graphs of the samples calcined at (a) 1150 -C and (b) 1368 -C.

Fig. 5. DTA-TG curves for (a) CCp of AT3, (b) spray-dried alum, (c)

annatase.

S. Liu et al. / Powder Technology 155 (2005) 187–192190

reactions and sintering effect finally lead to the non-

spherical morphology and the size increase of the

composite particles.

Fig. 6 shows the XRD patterns of the calcined AT3

powders. When the spray-dried powder is calcined at 700

-C, only the anatase phase is detected (Fig. 6a). Upon

raising the temperature to 864 -C, part of the anatase

transforms into rutile (Fig. 6b). This temperature is

significantly lower than that for the pure anatase-to-rutile

transformation [14,15]. This difference might be caused by

the composite effect of the two components in the CCP,

especially on the interface between the anatase and the alum

coating. It has been found that the incorporation of Al2O3

into anatase can accelerate the anatase-to-rutile transforma-

tion [16,17]. The reason is that Al3+ ions, with a valence

lower than Ti4+, can create oxygen vacancies due to the

necessity of charge balance, reducing the energy barrier for

the rearrangement of the Ti–O octahedra. Also this

composite effect may be responsible for the difference

between the thermal decomposition behaviors of a spray-

dried alum and the CCP. In our previous study [18], the

spray-dried ammomium alum powders were subsequently

pyrolyzed to crystalline Al2(SO4)3 and g- or u-Al2O3 in a

temperature region of ¨900 -C. However, for the CCP, as

indicated by the TGA result, the pyrolysis of ammonium

alum is completed under 864 -C. However, no similar

crystalline phases are detected, indicating that Al2O3 is in an

amorphous state.

With the temperature rising to 1150 -C, rutile is still the

main crystalline phase, but aluminum titanate (Al2O3ITiO2)

is detected along with a-Al2O3 (Fig. 6c) This temperature is

about 130 -C lower than the theoretical temperature for the

formation of Al2TiO5 through traditional solid reaction

between a-Al2O3 and rutile [10,19]. This might be due to

the higher reactivities of the rutile and a-Al2O3 formed in-

situ upon heating. Since the particles are still highly

dispersed (Fig. 4a), it is reasonable to believe that the

formation of Al2O3ITiO2 mainly takes place in every single

particle formed by the spray drying.

From Table 2, it can be seen that at a same calcining

temperature below 1150 -C, although the CCPs have

different A/T ratios, the crystalline phases in all calcined

Fig. 6. XRD patterns for the sample of AT3 calcined at : (a) 700 -C, 2 hr; (b) 864 -C, 2 hr; (c) 1150 -C, 2 hr; (d) 1368 -C, 20 min; (e) 1368 -C, 2 hr.

S. Liu et al. / Powder Technology 155 (2005) 187–192 191

samples are the same as those in AT3. However, when the

temperature is raised to 1368 -C, the reactions are

dominated by the formation of Al2O3ITiO2 as the main

crystalline phase at the loss of a-Al2O3 and rutile. The

final phases are dependent on the A/T ratios, but not

stoichiometrically. In the samples with A/T ratios smaller

than 1 (such as AT1 and AT2), a-Al2O3 totally disappears,

rutile, supposed as an unreacted component, still exists. It

is not difficult to understand that rutile co-exists with

Al2O3ITiO2 in these cases, because Al2O3 is relatively

inadequate for the formation of Al2O3ITiO2. But in AT3

with an A/T ratio of 1:1, rutile is also detected (Fig. 6d).

Even in the cases of AT4 and AT5, whose A/T ratios are

1.4 and 7, respectively, rutile is still found along with

unreacted a-Al2O3. Comparing the DTA curves in Fig. 6b

and c, we can see that each of them shows an

endothermal peak at exactly the same temperature,

1367.9 -C, indicative of the normal anatase-to-rutile

transformation. This proves that the inner part of the

anatase particles is still undergoing phase transformation,

although aluminum titanate (Al2O3ITiO2) has been iden-

tified to be formed at 1150 -C. It evidences that the

formation of Al2O3ITiO2 starts from the interface between

the anantase and the alum coatings inside the CCPs. Then

the completeness of the formation of Al2O3ITiO2 in each

Table 2

Crystalline phases in the calcined samples

Sample no. AT1 AT2 AT3 AT4 AT5

Calcining temperature/-C 700 Anatase Anatase Anatase Anatase Anatase

864 Anatase Anatase Anatase Anatase Anatase

Rutile Rutile Rutile Rutile Rutile

1150 Rutile Rutile Rutile Rutile Rutile

Al2O3ITiO2 Al2O3ITiO2 Al2O3ITiO2 Al2O3ITiO2 Al2O3ITiO2

a-Al2O3 a-Al2O3 a-Al2O3 a-Al2O3 a-Al2O3

1368 Al2O3ITiO2 Al2O3ITiO2, Al2O3ITiO2 Al2O3ITiO2 Al2O3ITiO2

Rutile Rutile Rutile Rutile Rutile

a-Al2O3 a-Al2O3

single particle relies on the solid diffusion and solid

reaction. Therefore, the thickness of the coating plays an

important role in the process of the formation of

Al2O3ITiO2. Increasing the A/T ratio results in a thicker

surface coating of alum on the surface of anatase. Then

the transformed rutile in the center part of a single CPP

particle is difficult to diffuse to the surface to react with

a-Al2O3. Consequently, rutile is still detected although it

is stoichiometrically inadequate for the formation of

Al2O3ITiO2. This result does not change when the soaking

time is prolonged from 20 min to 2 h at this temperature

(Fig. 6e).

4. Conclusions

1. The spray-dried composite precursor powder has a core–

shell structure with coatings of ammonium alum on the

surface of anatase particles. The thickness of the alum

coating increases with the increase of the Al2O3/TiO2

ratio.

2. Upon calcination of the spray-dried composite precursor

powder, pyrolysis of ammomium alum and the anatase to

rutile transformation as well as the densification of each

composite particle result in the particle size reductions.

S. Liu et al. / Powder Technology 155 (2005) 187–192192

3. During the calcination, ammonium alum pyrolyzes to

Al2O3 and anatase transforms into rutile. Al2O3ITiO2

forms at 1150 -C. The reaction between Al2O3 and rutile

mainly takes place in every single particle formed

through spray drying and starts from the interface.

4. The molar ratio of Al2O3 to TiO2 does not influence the

crystalline phases stoichiometrically in the final compo-

site Al2O3–TiO2 powders.

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