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Materials Science and Engineering A 490 (2008) 457–464

Preparation and sintering behaviour of nanostructured alumina/titaniacomposite powders modified with nano-dopants

Yong Yang, You Wang ∗, Zheng Wang, Gang Liu, Wei TianDepartment of Materials Science, Harbin Institute of Technology, Harbin 150001, PR China

Received 27 October 2007; received in revised form 18 January 2008; accepted 18 January 2008

bstract

Nanostructured alumina/titania composite powders were prepared by doping with small amounts of nanosized zirconia and ceria. The nanosizedaw materials powders were reconstituted into nanostructured particles by ball milling, spray drying and heat treating. Then, the nanostructuredeconstituted powders were cool-isostatic pressed and pressureless-sintered into bulk ceramics. The phase composition and microstructures ofeconstituted powders and as-prepared ceramic composites were characterized by using X-ray diffractometer (XRD), scanning electron microscope

SEM) and energy-dispersive spectrometer (EDS). The sintering behaviour of the nanostructured ceramic composite powders and the effects ofano-dopants and sintering temperatures on the microstructures of the ceramic composites were investigated and discussed. It was found thatano-dopants could lower the sintering temperature and accelerate densification of ceramic composites.

2008 Elsevier B.V. All rights reserved.

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eywords: Nanostructured powder; Alumina/titania; Nano-dopant; Sintering; N

. Introduction

Ultrafine crystalline materials (grain size <1 �m), especiallyanocrystalline materials, have been drawing attention due to thexpectations of enhanced mechanical and functional properties1]. Moreover, in recent years, the nanocomposites, which wererepared by dispersing second-phase nanosized particles withinhe matrix grains and/or on the grain boundaries, have been a new

aterial design concept and significantly improved strength haseen achieved with moderate enhancement in fracture toughness2]. On the other hand, the nano/nano-type composites, whichere composed of the dispersoids and matrix grains both ofanometer size, showed additionally attractive functions due tohe peculiar role of nanosized phases in physical and mechanicalroperties [3].

Alumina-based ceramics are utilized in many areas of mod-rn industry due to their unique mechanical, electrical, and

ptical properties. The properties of alumina-based ceramicsepend much on the final microstructures which are insepara-ly influenced by the characteristics of the starting powders [4].

∗ Corresponding author at: Department of Materials Science, Harbin Institutef Technology, No. 92, West Da-Zhi Street, Harbin 150001, PR China.el.: +86 451 86402752; fax: +86 451 86413922.

E-mail address: wangyou@hit.edu.cn (Y. Wang).

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921-5093/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2008.01.068

eramic

lumina-based nanocomposite ceramics demonstrate novel andttractive properties compared with their microsized counter-arts [5,6].

The ideal choice for preparing nanoceramics is an optimiza-ion of a lower sintering temperature, a shorter sintering duration,nd use of the suitable additives. Many techniques have beensed to achieve this optimization. Such efforts are coupled withelection of finer starting powders, because finer powder sizere preferable for producing ceramics with finer final grain size7]. With the availability of nanocrystalline powders, numberf studies have been reported on sintering of nanocrystallineowders [8,9]. However, there is rare report on the sinteringehaviour of nanostructured powders, which are microsized par-icles composed of nanosized grains.

In addition, studies on the effects of additives on the sin-ering of alumina have been highlighted in the past. However,here is little report on the effect of additives on the sintering ofanocrystalline alumina [10].

In the present investigation, nanostructured alumina/titaniaAT) composite powders were prepared by doping withmall amounts of nanosized zirconia and ceria. The sintering

ehaviour of the nanostructured ceramic composite powders wasnvestigated. The effects of nano-dopants and sintering temper-tures on the microstructures of the ceramic composites werelso explored.

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58 Y. Yang et al. / Materials Science a

. Experimental procedure

Starting powders of high purity were Al2O3 (� and � phases,9.9% grade, Degussa Co. Ltd., Germany) with grain size of0–45 nm and TiO2 (anatase, 99.9% grade, Nanjing High Tech-ology of Nano Material Co. Ltd., China) with grain size of0–50 nm. These powders were blended uniformly to producepowder mixture with composition of 87 wt.% Al2O3 and

3 wt.% TiO2 with the addition of binder by wet ball milling.n addition, 7 wt.% ZrO2 (tetragonal phase, 99.9% grade, Cug-ano Material Manufacture Co. Ltd., Wu Han, China) withrain size of 20–50 nm and 6 wt.% CeO2 (cubic phase, 99.9%rade, Rare Chem. Co. Ltd., Hui Zhou, Guang Dong, China)ith grain size of 20–40 nm were added during mixing formodified nanostructured powder. The mixed powders were

hen reconstituted to form microsized particles with nanosizedrains. The process of reconstitution consists of spray dryinghe slurry of powder mixture and subsequently heat treatinghe as-prepared powders. Solid powders with appropriate par-icle size distribution, good flowability and low water ratioan be obtained by spray drying. After heat treating of recon-tituted powders, metastable phases in reconstituted powdersan transform to stable phases, e.g. �-Al2O3 and �-Al2O3 to-Al2O3 and anatase to rutile, therefore, large volume shrink-ge that may occur in the sintering process can be avoidable.n addition, the water and binder remaining in the reconsti-uted powders can be removed by heat treating; consequently,ores and cracks which are possibly formed in the sinter-ng process as a result of volatilization of water and binderan be avoided. Moreover, physical properties of reconstitutedowders, e.g. density, can be improved after heat treating.ubsequently, the reconstituted powders were compacted intoreen compacts in press machine under pressure of about0–60 MPa, after that the green compacts were cool-isostaticressed in rubber mould under 280 MPa for 2 min. At last, thereen compacts were pressureless-sintered at 1250 ◦C, 1350 ◦C,450 ◦C and 1550 ◦C for 1 h in air with the heating rate of◦C/min.

The phase composition of reconstituted powders and sintered

roducts was characterized by X-ray diffraction (XRD, D/max-B, Japan) with Cu K� radiation. A field-emission gun scanninglectron microscope (SEM, HITACHI-S570) equipped with X-ay energy-dispersive spectroscopy (EDS) was employed to

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ig. 1. XRD patterns of reconstituted powders: (a) alumina/titania (AT) and (b) alumin

gineering A 490 (2008) 457–464

haracterize the particle size, the morphology and the chemicalomposition of reconstituted powders and sintered products. Theintered samples were polished and thermally etched at 1200 ◦Cor 30 min in air. The density of sintered bodies was determinedsing the Archimedes method.

. Results and discussion

.1. Characterization of the nanostructured compositeowders

The starting nano-Al2O3 powder consisted of �-Al2O3 and �-l2O3 and the nano-TiO2 powder consisted entirely of anatase.-Al2O3 is a transition phase of �-Al2O3 to �-Al2O3. �-Al2O3,-Al2O3 and anatase are metastable phases, and volume shrink-ge will occur in the phase-transformation process. Therefore,eat treatment must be carried out for reconstituted powdersefore compaction, and �-Al2O3, �-Al2O3 and anatase willransform into �-Al2O3 and rutile, respectively after appro-riate heat treatment. Thereby, large volume shrinkage can bevoidable and pores and cracks will not form in the sinteringrocess. The average grain sizes of Al2O3, TiO2, ZrO2 andeO2 are 32 nm, 25 nm, 23 nm and 28 nm, respectively whichere calculated from the powders XRD patterns according toebye–Scherrer formula [11].The XRD patterns of reconstituted powders alumina/titania

nd alumina/titania/zirconia/ceria (ATD) after ball milling,pray drying and heat treating are shown in Fig. 1. Theetastable phase �-Al2O3, �-Al2O3 and anatase transformed

nto stable phase �-Al2O3 and rutile, respectively after heatreating. That would be advantageous to the sintering of ceramicomposites.

The SEM micrographs of surface and cross-section of recon-tituted powders are shown in Fig. 2. After ball milling, sprayrying and heat treating, spherical and compact powders hadeen obtained (Fig. 2a, b, e and f). Specific surface area of spher-cal powders is larger than that of other particle shapes, so it iseneficial to the sintering of ceramic composites. Fig. 2c, d, gnd h shows that every spherical solid particle consists of a great

ot of nanosized grains. That nanostructure is beneficial to thetorage, transportation of nanosized powders and to densifica-ion of bulk ceramics, and it may also have an effect of inhibitinghe growth of nanosized grains.

a/titania/zirconia/ceria (ATD) after ball milling, spray drying and heat treating.

Y. Yang et al. / Materials Science and Engineering A 490 (2008) 457–464 459

Fig. 2. SEM micrographs of the surface and cross-section of two reconstituted powders: (a), (c), (e) and (g) AT; (b), (d), (f) and (h) ATD; (a)–(d) surface micrographs;(e)–(h) cross-section micrographs; (c) high magnification of (a), (d) high magnification of (b), (e) AT, (f) ATD, (g) high magnification of (e) and (h) high magnificationof (f).

460 Y. Yang et al. / Materials Science and Engineering A 490 (2008) 457–464

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.2. Sintering behaviour of the nanostructured compositeowders

.2.1. Characterizations of phase composition andicrostructures of sintered bulk ceramicsThe XRD patterns of the ceramics (a) AT and (b) ATD sin-

ered at different temperatures (1250 ◦C, 1350 ◦C, 1450 ◦C and550 ◦C) are shown in Fig. 3. The reconstituted AT powdersonsist of �-Al2O3 and rutile. At the sintering temperatures of250 ◦C, 1350 ◦C and 1450 ◦C, the phase composition of ATulk ceramics is identical with that of AT-reconstituted powders,amely �-Al2O3 and rutile. However, there was a little Al2TiO5ormed in AT when sintered at 1550 ◦C. The reconstituted ATDowders consist of �-Al2O3, rutile, t-ZrO2 (tetragonal phase)nd CeO2 phases. At each sintering temperature, there wereome Ce0.75Zr0.25O2 solid solution and Ce2Ti2O7 solid solu-ion formed in ATD. The tetragonal ZrO2 phase was reserved inTD bulk ceramics sintered at various temperatures because of

he addition of CeO2 and TiO2 [12] and will be advantageous tomprovement of flexural strength and fracture toughness of bulkeramics. There was also a little Al2TiO5 formed in ATD whenintered at 1550 ◦C.

At 1550 ◦C, Al2TiO5 was formed in AT and ATD. In general,l2TiO5 can form through solid-state reaction between Al2O3

nd TiO2 above their eutectoid temperature 1280 ◦C accordingo the following reaction equation [13]:

-Al2O3 + R-TiO2 → �-Al2TiO5

Aluminium titanate, Al2TiO5, is known to be a promis-ng candidate material for the application fields of refractorynd engine components because of its high melting point,ow thermal expansion, excellent thermal shock resistance,nd low thermal conductivity. However, aluminium titanateas two fatal weaknesses. First, Al2TiO5 will decomposento �-Al2O3 and TiO2 (rutile) in the temperature range of50–1280 ◦C which is disadvantageous to densification of

ulk ceramics [14,15]. Second, at RT, there is much dif-erence in the thermal expansion coefficients of Al2TiO5eramics along the three crystalline axes (αa = 9.8 × 10−6 K−1,b = 20.6 × 10−6 K−1, αc = −1.4 × 10−6 K−1) [16]. Therefore,

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ifferent temperatures (1250 ◦C, 1350 ◦C, 1450 ◦C and 1550 ◦C).

he mechanical properties of polycrystalline Al2TiO5 are greatlyimited by grain-boundary microcracking that occurs as a resultf the large thermal anisotropy of individual grains during cool-ng after sintering. Large numbers of microcracking formed inulk ceramics will be detrimental to densification of ceramicsnd reduce mechanical properties of ceramics. Therefore, in theresent research to prepare Al2O3/TiO2 ceramic composites, itust avoid Al2TiO5 forming in bulk ceramics.The SEM micrographs of AT and ATD ceramics sintered

t different temperatures are shown in Fig. 4. Grain growth ofwo ceramics had occurred at 1250 ◦C (Fig. 4a and b). Grainizes of the spherical grains in two ceramics are in the range of00–300 nm. Although the grain boundary had formed, poresonnecting one another formed pore-meshwork. The structuref two bulk ceramics is loose. At this sintering temperature, theres no obvious difference between AT and ATD, and it just looksike that ATD is appreciably denser than AT.

The grain sizes of two ceramics increased to 300–400 nm at350 ◦C. Mass transfer between grains of AT at 1350 ◦C wasore than that at 1250 ◦C (Fig. 4a and c), but the mass transferas insufficient and there were also many pores between grains.owever, the microstructure of ATD at 1350 ◦C is obviouslyifferent from that of AT (see Fig. 4d); three different colourhases appeared in the SEM micrographs of ATD. Fig. 5 showshe EDS spectra of ATD ceramics sintered at 1350 ◦C. With theRD analyses, it can be seen that the black grains in the SEMicrographs (point I in Fig. 4d) are Al2O3; the gray grains (point

I in Fig. 4d) are TiO2; and the white grains (point III in Fig. 4d)re ZrO2 and CeO2. The self-existent region of CeO2 grainsannot be seen. Ce element was detected in the grain regions ofiO2 and ZrO2. There are two reasons: (1) ionic radius of Ce4+

0.097 nm) is much larger than that of Ti4+ (0.068 nm) [17–19],o it is difficult for Ce4+ to enter into the crystal lattice of TiO2o form CeO2–TiO2 solid solution. However, it is possible fori4+ to enter into the crystal lattice of CeO2 to form TiO2–CeO2olid solution. In the grain region of TiO2, a few Ti4+ entered

nto the crystal lattice of CeO2 to form TiO2–CeO2 solid solu-ion. The TiO2–CeO2 solid solution is a low melting point solidolution, which may transform to liquid phase during the sin-ering process [20,21]. And, there is liquid-state grain region

Y. Yang et al. / Materials Science and Engineering A 490 (2008) 457–464 461

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ig. 4. SEM micrographs of AT and ATD ceramics sintered at different temper450 ◦C, (f) ATD 1450 ◦C, (g) AT 1550 ◦C and (h) ATD 1550 ◦C.

f TiO2 that appeared in ATD at 1350 ◦C and 1450 ◦C (pointV in Fig. 4d and point III in Fig. 6d). (2) The XRD analy-is revealed the presence of ceria–zirconia solid solutions withhe composition Ce0.75Zr0.25O2 (Fig. 3). Ce4+ ions entered into

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he crystal lattice of ZrO2 [19,22,23]. Consequently, t-ZrO2 waseserved.

Most of Al2O3 grains of ATD grew into equiaxed grains at350 ◦C (Fig. 4d). And many white ZrO2 and CeO2 grains filled

462 Y. Yang et al. / Materials Science and Engineering A 490 (2008) 457–464

Fig. 5. EDS spectra of ATD ceramics sintered at 1350 ◦C: (a) point I, (b) point II and (c) point III (points I–III are shown in Fig. 4d).

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3The relative density of two ceramics sintered at different

temperatures is shown in Table 1. The relative density of twoceramics increases with increasing the sintering temperature

Table 1Relative density of two ceramics sintered at different temperatures

Sintering temperature (◦C)

ig. 6. SEM micrographs of AT and ATD ceramics sintered at different temper

n between Al2O3 and TiO2 grains, which made the structure ofTD denser than that of AT at the same temperature.

The grain sizes of two ceramics continued to grow at 1450 ◦Cnd 1550 ◦C. The mass transfer between matrix Al2O3 granulesf ATD spread, and the matrix granules contacted each other torow into larger grains. There was big difference between nano-cale grains and matrix granules. The diffusion rate of nano-scalerains was much slower than that of matrix granules; therefore,arge numbers of nano-scale grains were wrapped into the matrix

ranules (point I in Fig. 4f and point I in Fig. 4h). There are alsoome nano-dopant grains between matrix granules (point II inig. 4f). The matrix granules are larger and many fine grains arerapped into the matrix granules (Fig. 6d).

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: (a) AT 1350 ◦C, (b) AT 1450 ◦C, (c) ATD 1350 ◦C and (d) ATD 1450 ◦C.

.2.2. Relative density of sintered bulk ceramics

1250 1350 1450 1550

elative density of AT (%) 70.1 80.2 91.5 88.7elative density of ATD (%) 72.8 92.4 98.3 93.1

nd Engineering A 490 (2008) 457–464 463

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Y. Yang et al. / Materials Science a

rom 1250 ◦C to 1450 ◦C and then decreases at 1550 ◦C. Therere mainly changes of sizes and shapes of grains and poresccurring in the sintering process.

The SEM micrographs of AT and ATD ceramics sinteredt 1350 ◦C and 1450 ◦C are shown in Fig. 6. The pores in twoeramics reduced with increasing the temperature (Figs. 6 and 4).rain boundary had formed gradually, and grains grew up

hrough moving of grain boundary; consequently, the densitiesf two ceramics were enhanced. The pores in two ceram-cs are large at 1250 ◦C, and only a few grains contact eachther. The pores in ATD are less than that in AT. The relativeensities of AT and ATD at 1250 ◦C are 70.1% and 72.8%,espectively. The pores in two ceramics reduced with temper-ture up to 1350 ◦C, and the pores in ATD are obviously lesshan that in AT. Grain boundary between most of grains hadormed. The relative densities of AT and ATD at 1350 ◦C are0.2% and 92.4%, respectively. The pores in two ceramicseduced further at 1450 ◦C, and grains grew continually. Theelative densities of AT and ATD at 1450 ◦C are 91.5% and8.3%, respectively. It can be seen that ATD is quite dense at450 ◦C.

Grains of two ceramics grew continually with increasing tem-erature up to 1550 ◦C. The relative densities of AT and ATDt 1550 ◦C are 88.7% and 93.1%, respectively, which decreasedomparing with that at 1450 ◦C. Generally, the diffusion dis-ance increases and the diffusion rate decreases with increasingemperature. Grains grow unceasingly with further increase inemperature, but it is difficult to enhance the density of ceramicsurther. Although the densification rate of ceramics decrease inhe final stage of sintering, which means it is difficult to enhancehe density further in the final stage of sintering comparing withhat in the initial stage and middle stage of sintering, the densi-cation process will be progressing and the density would notecrease generally. But in the present research, the densitiesf two ceramics at 1550 ◦C decreased comparing with that at450 ◦C. Al2TiO5 formed in two ceramics at 1550 ◦C. Largeumbers of microcrack formed in ceramics because of thermaltress caused by Al2TiO5, and macrocrack formed when the den-ity of microcrack exceeded certain critical value. Consequently,t was difficult for bulk ceramics to get full densification. There-ore, the density of AT and ATD decreased comparing with thatt 1450 ◦C. Fig. 7 shows the macrocrack in the ATD ceramicst 1550 ◦C.

In addition, the relative density of ATD is higher than thatf AT at each sintering temperature (Table 1). That means theddition of nano-dopants enhanced the density of bulk ceramic.here are three possible reasons: (1) Figs. 4d, 5 and 6d show

hat nano-dopants ZrO2 and CeO2 are located between Al2O3nd TiO2 grains (point I in Fig. 6d) and in the grain bound-ries of matrix granules (point II in Fig. 6d). The nano-dopantsccumulated between matrix granules, making mass transfer ofrains accelerate and grain boundary form easily, and eliminat-ng the pores at grain boundaries. As a result, the density of

TD was enhanced. (2) It can be inferred from the XRD andDS analysis results that Ce4+ entered into the crystal latticef ZrO2 [22–24], and oxygen vacancies and substitution atomsrought about high-lattice distortion energy, consequently the

Fig. 7. Macrocrack in the ATD ceramics sintered at 1550 ◦C.

riving force of sintering was increased. Moreover, the poresrapped in grains may be eliminated though diffusion of oxy-en vacancies. (3) As discussed in Section 3.2.1, liquid phaseas likely to be present in the bulk ceramics at high temperature,hich made the sintering process change from solid phase sinter-

ng to solid/liquid phase sintering and granules rearrange easily4,25–30]. It also increased the mass-transfer routes and accel-rated the mass-transfer process. Therefore, the liquid phaseromoted the densification of bulk ceramics. In addition, we alsoonsider that it was easy for the liquid phase to fill in betweenhe matrix granules comparing with solid phase particles, con-equently the pores were eliminated effectively and the densityf bulk ceramics was enhanced.

. Conclusions

1) By ball milling, spray drying and heat treating, nano-Al2O3,nano-TiO2 and nano-dopants ZrO2 and CeO2 were reconsti-tuted into nanostructured AT and ATD composite powders.The nanostructured powders are advantageous to the com-paction and densification of ceramics.

2) The reconstituted AT powders consist of �-Al2O3 and rutile.The phase composition of AT bulk ceramics sintered at1250 ◦C, 1350 ◦C and 1450 ◦C is identical with that of AT-reconstituted powders. However, there was a little Al2TiO5formed in AT at 1550 ◦C. The reconstituted ATD powdersconsist of �-Al2O3, rutile, t-ZrO2 and CeO2 phases. At eachsintering temperature, there were some Ce0.75Zr0.25O2 solidsolution and Ce2Ti2O7 solid solution formed in ATD. Thetetragonal ZrO2 phase was reserved in ATD bulk ceramicssintered at various temperatures because of the addition ofCeO2 and TiO2 and will be advantageous to the improve-ment of flexural strength and fracture toughness of bulkceramics. There was also a little Al2TiO5 formed in ATDwhen sintered at 1550 ◦C.

ing the sintering temperature from 1250 ◦C to 1450 ◦C andthen decreases at 1550 ◦C. The nano-dopants lowered thesintering temperature and accelerated the densification ofbulk ceramic composites.

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