Preparation of alumina-doped yttria-stabilized zirconia nanopowders by microwave-assisted peroxyl-complex coprecipitation

QI Liang(齐　亮), XU Ming-xia(徐明霞), TIAN Yu-ming(田玉明), ZHAO Jin-wei(赵今伟)

School of Materials Science and Engineering, Key Laboratory for Advanced Ceramics and Machining Technology of Ministry of Education, Tianjin University, Tianjin 300072, China

Received 10 April 2006; accepted 25 April 2006

Abstract: Alumina-doped yttria-stabilized zirconia (ADYSZ) nanopowders were prepared by microwave-assisted peroxyl-complex coprecipitation (MAPCC) using ZrOCl₂·8H₂O, Y₂O₃ and AlCl₃·6H₂O as starting materials, NH₃·H₂O as precipitant and H₂O₂ as complexant. The effects of adding H₂O₂ and microwave drying on the preparation and properties of ADYSZ were investigated. The precursors and nanopowders were studied by EDX, XRD, SEM and TEM techniques. The results show that the uniformity of component distribution within ADYSZ nanopowders is improved by adding appropriate dosage of H₂O₂. Complexing reaction between H₂O₂ and Zr⁴⁺ ion restrains the hydrolyzation and precipitation of Zr⁴⁺ ion. With the addition of H₂O₂, Al³⁺, Y³⁺ and Zr⁴⁺ ions can be precipitated synchronously in a relatively narrow range of pH value. H₂O₂ also improves the filterability of the wet precipitate. The highly hydrophilic precipitates can be quickly and effectively separated from aqueous solution. During microwave drying process, the moisture of wet precursors is selectively heated. Quick expansion of steam vapor within the wet colloidal particles causes the aggregations burst into numerous tiny lumps. Compared with oven drying, microwave drying can not only shorten drying time but also reduce aggregation intensity of the resultant ADYSZ nanopowders.

Key words: alumina-doping; yttria-stabilized zirconia; nanopowders; coprecipitation; hydrogen peroxide; microwave drying

1 Introduction

The widespread applications of various zirconia-based materials make zirconia a hotspot in materials science. In order to improve the physical and chemical properties of zirconia based-materials, many efforts including application of nano-scaled powders and adding different dopants to the matrix of zirconia-based materials have been made. Similarly, various approaches have been investigated to optimize the initial composition and synthesize monodisperse nanopowders with little aggregation.

Usually, substances like CaO[1], MgO[2], Y₂O₃[3] and CeO₂[4] were chosen as additives and added to the matrix of zirconia-based compounds. In recent years, more and more scholars have begun to focus their interesting on the alumina-doped yttria-stabilized zirconia (ADYSZ) for the great advantages of accelerating sintering[5,6], enhancing fracture toughness[7] and retarding low temperature degradation[8] brought by the trace addition of Al₂O₃. However, in most of these studies, raw materials were either expensive commercial products or fabricated by ball-milling method, and the cost as well as the properties of this kind of raw material are not so ideal.

Various methods, including hydrolysis[9,10], sol-gel[11], precipitation[12] , hydrothermal method[13], have been applied to synthesize different kinds of ZrO₂ nanopowders. Advantages and limitations of these methods were discussed in Ref.[14]. Among all kinds of methods, coprecipitation is the most common route to produce zirconia-based nanopowders. Compared with other synthesis methods, coprecipitation has many advantages such as simple requirement for synthesizing equipments, convenient manipulation. Nevertheless, the conventional coprecipitation method has the inherent disadvantage of inhomogeneity of resultant composition, especially when pH values of the precipitations of different initial ingredients are highly discrepant, efficiency is low in production and agglomeration is serious within resultant nanopowders. So, it is necessary to study an appropriate method to synthesize high performance zirconia-based nanopowders.

A novel modified coprecipitation method, called microwave-assisted peroxyl-complex coprecipitation (MAPCC), was put forward in this study. Effects of adding H₂O₂ and properties of alumina-doped yttria-stabilized zirconia (ADYSZ) nanopowders were investigated.

The starting materials in the study were zirconium oxychloride octahydrate (ZrOCl₂·8H₂O, AR grade), yttrium oxide (Y₂O₃, AR grade) and aluminium chloride hexahydrate (AlCl₃·6H₂O, AR grade). Ammonia solution (NH₃·H₂O, 6 mol/L) and hydrogen dioxide solution (H₂O₂, 30%(mass fraction)) were used as precipitant and complexant, respectively.

Appropriate dosage of ZrOCl₂·8H₂O was dissolved in de-ionized water, and formed 0.5 mol/L Zr⁴⁺ solution. Subsequently, Y₂O₃ and AlCl₃·6H₂O were added to Zr⁴⁺ solution. The molar fraction of Y₂O₃ in initial composition of yttria-stabilized zirconia (YSZ) compound was designed as 3%. The mass fraction of alumina, which acted as a dopant of 3%Y₂O₃(molar fraction) stabilized ZrO₂ (3YSZ) compound and was introduced by AlCl₃·6H₂O, ranging from 0 to 1.0% in 3YSZ matrix (0-1.0AD3YSZ). After the dissolution of starting materials, different dosages of H₂O₂, molar ratio of H₂O₂ to Zr⁴⁺ source ranging from 0 to 1.5, was dropwise added to the solution of starting materials with uninterrupted stirring of magnetic stirrer. With the addition of H₂O₂, pungent gas detected as chlorine (Cl₂) was gradually released from the solution of starting materials. After aging for a short period, the amount of released Cl₂ reduced and the 6 mol/L NH₃·H₂O was then dropwise added to the mixed solution with vigorous stirring. Simultaneously, the coprecipitation process began to proceed, and white flocculent precipitates were immediately generated from the mixed solution. The pH value of the reaction system was maintained about 8.5 during the whole course of reaction. This process is so-called peroxyl-complex coprecipitation (PCC). During the PCC process, the temperature of the reaction system was held at 40 ℃.

After aging for 1 h, till the pH value of the reaction system did not change, the resultant precipitates (aqueous precursors) were repeatedly washed with de-ionized water and filtered water. Filter cakes of the aqueous precursors, yielded from 0.5 mol ZrOCl₂·8H₂O and other corresponding additives, were dried in domestic microwave oven (Midea, 2.45 GHz, 320 W, 30 min) and traditional drying oven (80 ℃, 3 h), respectively. The dried precipitates made by different drying techniques were ball-milled and then calcined at 800 ℃ for 1 h. The whole process of PCC together with microwave drying was named after microwave-assisted peroxyl-complex coprecipitation (MAPCC).

The resultant ADYSZ nanopowders synthesized by MAPCC were uniaxially pressed at 120 MPa in a hard metal die with a diameter of 10 mm, and were cold isostatically pressed at 200 MPa, subsequently. The pressed pellets were sintered in air at different temperatures up to 1 500 ℃ for 2.5 h. The apparent density of the sintered pellets was measured by the ARCHIMEDE’s method, and the relative density was calculated using the theoretical density of 6.08 g/cm³.

To confirm the effect of H₂O₂ on the uniformity of component distribution, the compositional analysis of resultant ADYSZ nanopowders was done by using energy dispersive X-ray analysis (EDX, PHOENIX M/S EDAX, USA). The phase analysis of resultant ADYSZ nanopowders was done with X-ray diffractometer (XRD, PHILIPS PW 1 710, Netherlands) with Cu K_α radiation. Scanning electron microscopy (SEM, LEO 1530VP, Germany) was performed to observe the nature of agglomerates, surface morphology and approximate particle size in the resultant ADYSZ nanopowders exsiccated by different drying techniques. Finally, the mean size of resultant ADYSZ nanopowders was estimated from transmission electron microscopy (TEM, PHILIPS TECNAI F20, Netherlands).

The XRD patterns of 0.3%(mass fraction) Al₂O₃-doped 3%(molar fraction) Y₂O₃-stabilized ZrO₂ (0.3AD3YSZ) nanopowders, calcined at 800 ℃ for 1 h, synthesized by coprecipitation and PCC were shown in Fig. 1.

Fig.1 XRD patterns of 0.3AD3YSZ nanopowders synthesized by (a) coprecipitation and (b) PCC, calcined at 800 ℃ for 1 h

It can be found from Fig.1 that crystallinity of samples synthesized by PCC was better than that of samples synthesized by coprecipitation. Furthermore, characteristic reflections corresponding to monoclinic zirconia (m-ZrO₂) could be found in Fig.1(a). Whereas, absence of either m-ZrO₂ or other segregated substance was presented in Fig.1(b), and only reflections corresponding to tetragonal zirconia (t-ZrO₂) were observed in Fig.1(b), which implied homogeneous component distribution within 0.3AD3YSZ nanopowders synthesized by PCC.

The EDX analysis results on 0.3AD3YSZ nanopowders synthesized by PCC confirm the presence of both yttrium and zirconium along with small amount of aluminum. By analyzing the EDX patterns, the atomic ratios of yttrium to zirconium and aluminum to zirconium collected from several stochastic points in 0.3AD3YSZ nanopowders samples synthesized by PCC were well kept within solid solution of initial composition, rather than those in 0.3AD3YSZ nanopowders samples synthesized by coprecipitation. The possible reasons for the improvement of compositional homogeneity brought by PCC were as follows: the complexing reaction between H₂O₂ and Zr⁴⁺ restrained the hydrolyzation and precipitation of Zr⁴⁺[15], with the addition of H₂O₂, the pH values for the precipitations of Al³⁺, Y³⁺ and Zr⁴⁺ ions were relatively proximate, and the three kinds of ions could be precipitated synchronously in a relative narrow range of pH value, which resulted in the synchronous precipitations of Al³⁺, Y³⁺ and Zr⁴⁺ ions. Finally, the synchronous precipitations led to a more homogeneous component distribution.

Simultaneously, H₂O₂ also improved the filterability of the wet precipitates. The experimental data for the effects of H₂O₂ on the consumption of NH₃·H₂O and filterability of aqueous precursors, yielded from 0.5 mol ZrOCl₂·8H₂O and other corresponding additives, were summarized in Table 1.

Table 1 Effects of H₂O₂ on consumption of NH₃·H₂O and filterability of aqueous precursors

It can be seen from Table 1 that  the addition of H₂O₂ to the initial solution of starting materials led to the consumption of NH₃·H₂O reduce and the filterability of aqueous precursors improve, the time for filtration was almost halved. GAO et al[16] reported that Zr⁴⁺ could react with H₂O₂ and generated a complex compound with ligands of —OH and —OOH. Consequently, it can be inferred from Table 1 that the ligands of —OH and —OOH derived from the complexing reaction within H₂O₂ and Zr⁴⁺ reduced the consumption of NH₃·H₂O and made the highly hydrophilic precipitates be quickly and effectively separated from aqueous solution.

Moreover, H₂O₂ is a strong oxidizer, with the addition of H₂O₂, the following reaction takes place within the solution of starting materials:

2Cl^-+H₂O₂→Cl₂↑+2OH^-                                    (1)

As the solubility of chlorine (Cl₂) in de-ionized water was very little at room temperature, most of Cl^- was easily removed using PCC method. KATO et al[17] reported that the secondary particle size of hydrous zirconia synthesized from ZrOCl₂ solutions with the addition of metal chlorides increased as the number of Cl^- ions in the solution increased. Hence, the conclusion can be drawn that PCC method can also effectively reduce the scale of powders.

Microwave drying has many advantages such as great efficiency, little thermal wastage, and so on. In comparative experiments, when wet precursors, which were yielded from 0.5 mol ZrOCl₂·8H₂O and other corresponding additives, were dried in oven at 80 ℃, it took 3 or 4 h to get the wet precursors thoroughly dried. However, the drying time shortened to about 30 min when microwave was performed.

To investigate the influence of different drying technique on the microstructure features of ADYSZ nanopowders, SEM was performed in this research. Fig.2 shows the microstructure features of ADYSZ nanopowders yielded from precursors which were dried by oven and microwave. It can be found that samples in

Fig.2 SEM photographs of ADYSZ nanopowders yielded from precursors synthesized by PCC, dried by (a) oven (80 ℃, 3 h) and (b) microwave (2.45 GHz, 320 W, 30 min), calcined at 800 ℃ for 1 h

Fig.2(a) comprise considerably densely agglomerated particles compared with samples in Fig.2(b), and the scale of each tiny lump of samples in Fig.2(a) is larger than that of samples in Fig.2(b). By contrasting the two photographs in Fig.2, it can be drawn that, compared with oven drying, the state of agglomeration within samples is greatly lightened by microwave drying.

Different drying mechanisms cause distinct contrast between Fig.2(a) and Fig.2(b). Microwave drying has the characteristic of volumetric heating, and this kind of heating is selective. During microwave drying process, the moisture of wet precursors is selectively heated by the microwave radiation. Because of the quick expansion caused by the acute evaporation of exterior and interior moisture, most aggregation within the wet colloidal particles burst into numerous tiny lumps. The microwave drying effectively avoided the condensation polymerization of hydroxyl groups induced by traditional oven drying. The burst model of microwave drying is demonstrated in Fig. 3.

Fig. 4 shows a TEM photograph of the resultant ADYSZ nanopowders consisted of hexagonally shaped particles and relatively uniform size distribution with diameters approximately 30 nm.

After sintering at 1 385 ℃ for 2.5 h, the relative

Fig. 3 Expansion effects caused by steam within wet colloidal particles during microwave drying process

Fig.4 TEM photograph of ADYSZ nanopowders synthesized by MAPCC calcined at 800 ℃ for 1 h

density of the pressed pellets fabricated by 0.3AD3YSZ nanopowders reaches the maximum value of 98% of the theoretical density.

1) The addition of H₂O₂ in PCC route improves the filterability of the hydrous ADYSZ precursors and the uniformity of component distribution within resultant ADYSZ nanopowders. The reactions between H₂O₂ and Cl^- made chlorine be easily removed. By adding H₂O₂, Al³⁺, Y³⁺ and Zr⁴⁺ can be precipitated synchronously in a relatively narrow range of pH value, which results in the improvement of compositional uniformity.

2) Microwave drying can effectively reduce the aggregation intensity and shorten the drying period. During microwave drying process, the moisture within the wet precipitates is selectively heated and turned into vapor, subsequently. Burst of the aggregate wet colloidal particles caused by the acute evaporation of exterior and interior moisture is considered the primary reason for avoiding aggregation.

3) ADYSZ nanopowders with homogeneous composition, low aggregation and narrow size distribution ranging around 30 nm can be synthesized by MAPCC method. The resultant ADYSZ nanopowders have good sintering activity. Compared with powders synthesized by other methods, 0.3AD3YSZ can be densely sintered at a relatively low temperature of

1 385℃. The relative density of the pressed pellets fabricated by the 0.3AD3YSZ reaches the maximum value of 98% of the theoretical density.

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(Edited by CHEN Can-hua)

Foundation item: Project(043182311) supported by the Key Program of Scientific and Technical Research of Tianjin

Corresponding author: XU Ming-xia; Tel: +86-22-27890489; Fax: +86-22-27404724; E-mail: xumingxia@tju.edu.cn