Preparation of Al_2-xY_xW₃O₁₂ powders by citrate sol-gel process

ZHANG Hai-jun(张海军), ZHANG Qing(张 青), JIA Quan-li(贾全利), YE Guo-tian(叶国田)

High Temperature Ceramics Institute, Zhengzhou University, Zhengzhou 450052, China

Received 12 December 2007; accepted 23 April 2008

Abstract: Al_2-xY_xW₃O₁₂ (x=0.2, 0.5, 0.8, 1.0, 1.2, 1.5, 1.7 and 2.0) powders were synthesized by citrate sol-gel process. The concentration of species in a citric solution for preparing Al_2-xY_xW₃O₁₂ powders was calculated. The powders were characterized by differential thermal analysis(DTA), thermogravimetry(TG), X-ray diffractometry(XRD) and scanning electron microscopy(SEM), respectively. No solid solution of Al_2-xY_xW₃O₁₂ is formed with x values varying from 0 to 2.0. The maximum solid solubility of Y₂O₃ in Al₂W₃O₁₂ and Al₂O₃ in Y₂W₃O₁₂ is less than 0.5. Y₂W₃O₁₂ easily absorbs water in air and forms a composition of Y₂W₃O₁₂?3.2H₂O, and Al₂W₃O₁₂ forms Al₂W₃O₁₂?0.17H₂O in the same condition.

Key words: Al_2-xY_xW₃O₁₂; citrate sol-gel; preparation; hydration

1 Introduction

Most substances expand during heating. Recently, materials that exhibit negative thermal expansion(NTE) have attracted considerable interest because of the potential to prepare composite materials with a specific coefficient of thermal expansion, positive, negative, or zero in some specific temperature range. There are many materials that exhibit negative thermal expansion such as zeolites, b-quartz, some AMO₃ oxides (such as BaTiO₃, PbTiO₃) with the perovskite structure[1], compounds with general formula A₂(MO₄)₃ (A: trivalent cations and M: W⁶⁺, Mo⁶⁺, etc)[2-3], and compounds such as ZrMo₂O₈, HfMo₂O₈, Zn₂GeO₄[4] and Zr_1-xHf_xW₂O₈[5], ZrW₂O₈[6-7].

It is well known that the Pechini or citrate sol-gel process is usually considered to have the advantage of even mixing of starting materials, and easy control of the accurate composition of the final products[8-11]. To the knowledge of authors, there are still very few reports on the preparation of Y₂W₃O₁₂ and Al₂W₃O₁₂ powders by a homogeneous sol-gel process, as most of reported Y₂W₃O₁₂ and Al₂W₃O₁₂ powders are synthesized by classical solid-mixing-reaction method[12-16]. In this work, the citrate sol-gel process is used to prepare Al_2-xY_xW₃O₁₂ powders. A model is used to evaluate the influence of the pH value and citric acid content on the concentration of the cations citrate complexes in the starting solution. And the hydration of prepared Al₂W₃O₁₂ and Y₂W₃O₁₂ powders in air are also investigated.

2 Experimental

The raw materials utilized were analytical reagent grade yttrium nitrate (Y(NO₃)₃?6H₂O), aluminum nitrate (Al(NO₃)₃?9H₂O), ammonium tungstate ((NH₄)₅H₅- [H₂(WO₄)]₆?H₂O) and citric acid (H₃Cit). Stoichiometric amounts of yttrium nitrate and aluminum nitrate were dissolved in distilled water, and then citric acid was added. After complete mixing, ammonium tungstate and ammonia solution (about 1 mol/L) were added to get a homogenous transparent solution within a few seconds. The solution was slowly evaporated to form a highly viscous colloid, followed by heating in a temperature range of 120-140 ℃ for 24 h to obtain a dried gel. Finally, the dried gel precursor was calcined at 900 ℃ for 10 h to obtain Al_2-xY_xW₃O₁₂ powders.

Two series of specimens were prepared for XRD tests: 1) For series A, XRD characterization was immediately carried out as soon as the sintering process was finished in order to avoid the absorption of the water; 2) For series B, XRD tests were carried out after the prepared Al_2-xY_xW₃O₁₂ powders were held in air for about 6 months to study the moisture absorption properties of Al_2-xY_xW₃O₁₂ powders. The samples were characterized by PHILIPS (Model X’Pert PRO) diffractometer using Cu K_α radiation under 40 kV and 40 mA. The XRD data were collected in the 2θ range from 10? to 70? with a step of 0.016?.

Differential thermal analysis and thermogravimetric analysis of the precursor were carried out on a NETZSCH STA-449C Thermal Analysis System with a heating rate of 10 ℃/min in flowing air with sample mass of 30 mg. Platinum crucibles were used in TG-DTA analysis and the reference material was alpha-alumina. The powder morphology was observed with scanning electron microscopy(SEM) (Model JSM-5610LV, JEOL, Japan).

3 Calculation of concentration of yttrium and aluminum citrate complexes in Al³⁺-Y³⁺- H₃Cit solution

For Al_2-xY_xW₃O₁₂ preparation, there are several kinds of ions, such as, Y³⁺, Y(H₂Cit)²⁺, Y(OH)²⁺, [Al³⁺], Al(Cit), Al(HCit)⁺, Al(OHCit)^-, Al(OH)²⁺, Al(OH)₂⁺, H₃Cit, H₂Cit^-, HCit^2- and Cit^3- existing in Al³⁺-Y³⁺- H₃Cit solution. In order to achieve the complete homogeneity of yttrium and aluminum ions at atomic level and accurate control of the precursor composition, it is very important to increase the relative concentration of yttrium and aluminum citrate complexes, such as Y(H₂Cit)²⁺, Al(Cit) and Al(OHCit)^- and decreases the relative concentration of Al³⁺ and Y³⁺. In the present work, a theoretical model similar to that in Ref.[17] was used to calculate the concentration of yttrium and aluminum citrate complexes in Al³⁺-Y³⁺-H₃Cit solution under different conditions. The concentration of citric acid and pH value of the solution were taken into consideration.

For Al_1.0Y_1.0W₃O₁₂ preparation, the theoretically calculated yttrium and aluminum complex concentrations at different pH values and concentrations of citric acid are shown in Figs.1 and 2. The following phenomena can be seen from these figures.

1) The concentration of Y³⁺ and Al(Cit) ions decreases with pH values increasing till pH=5 at first, then the concentration of Y³⁺ ions increases evidently till pH=7. The concentration of Al(Cit) ions is about zero from pH=5 to 7. The concentration of Al(OHCit)^- ions increases with pH increasing. The concentration of Y(H₂Cit)²⁺ increases with pH increasing until pH=5, and then decreases at pH=7. The percentage of [Y(H₂Cit)²⁺] to total yttrium concentration at pH=7 is only about 50% with the 1?1 molar ratio of citric acid to the total cations (Fig.1).

2) The concentration of citric acid shows a little influence on the concentration of Y³⁺ and Y(H₂Cit)²⁺  ions. The concentration of Y³⁺ ion decreases with increasing the citric acid content at the same pH value.

3) The concentrations of Y(OH)²⁺, Al³⁺, Al(OH)²⁺, Al(OH)₂⁺, Al(HCit)⁺ ions are about lower than zero compared with those of Y(H₂Cit)²⁺, Al(Cit), Al(OHCit)^- in all conditions.

4) At pH=4 and [H₃Cit]_T/{[Y]_T+[Al]_T}=3, almost all yttrium and aluminum ions completely form yttrium citrate complex Y(H₂Cit)²⁺ and aluminum citrate complex Al(OHCit)^-, Al(Cit).

According to the theoretically calculation (Figs.1 and 2), at [H₃Cit]_T/{[Y]_T+[Al]_T}=3 and pH=4, the percentage of the yttrium and aluminum citrate complex would increase to about 100%, indicating that [H₃Cit]_T/ {[Y]_T+[Al]_T}=3?1 and pH=4 are suitable for Al_2-xY_x- W₃O₁₂ precursor synthesis. The calculation results are in agreement with the experimental phenomena, i.e. a transparent sol and gel with no precipitation is easily formed at the pH=4 and [H₃Cit]_T/{[Y]_T+[Al]_T}=3?1 in the experiment.

Fig.1 Evaluated concentration of aluminum and yttrium species in solution with 1?1 molar ratio of citric acid to total cations: (a) Al³⁺- H₃Cit; (b) Y³⁺-H₃Cit

Fig.2 Evaluated concentration of aluminum and yttrium species in solution with 3?1 molar ratio of citric acid to total cations: (a) Al³⁺- H₃Cit; (b) Y³⁺-H₃Cit

4 Results and discussion

4.1 TG-DTA analyses of precursor

The TG-DTA curves of the dry Al_1.0Y_1.0W₃O₁₂ precursor with pH=4 and [H₃Cit]_T/{[Y]_T+[Al]_T}=3?1 are shown in Fig.3. The mass loss can be divided into two temperature regions, namely, from room temperature to 225 ℃, and from 225 ℃ to 751 ℃. These two regions correspond to mass loss of about 40%, and 25% respectively. At higher temperature, there is almost no change in mass, which can be attributed to the formation of a pure oxide system. The exothermic peak in the first temperature region (about 225.0 ℃) can be contributed to the decomposition of the citrate Y(H₂Cit)²⁺, Al((OH)Cit^-), Al(Cit), (NH₄)₅H₅[H₂(WO₄)]₆?H₂O and the formation of the aluminum and yttrium carbonate. The broad exothermic peaks in the second temperature region from about 400 ℃ to 900 ℃ may be caused by the decomposition of the carbonate and the formation of Al₂W₃O₁₂ and Y₂W₃O₁₂.

z

4.2 XRD analyses of Al_2-xY_xW₃O₁₂ powders

The X-ray diffraction patterns of prepared Al_2-xY_xW₃O₁₂ calcined at 900 ℃ for 10 h are shown in Fig.4. It can be seen that, 1) pure Al₂W₃O₁₂ and Y₂W₃O₁₂ are prepared by citrate sol-gel methods; and 2) Al_2-xY_xW₃O₁₂ can not form a complete solid solution as x values vary continuously from 0.0 to 2.0. The solid solubility limit of Y₂O₃ in Al₂W₃O₁₂ is less than 0.5 (molar fraction), and so is that of Al₂O₃ in Y₂W₃O₁₂.

Fig.4 XRD patterns of Al_2-xY_xW₃O₁₂ precursors annealed at 900 ℃ for 10 h

The XRD patterns of series A and series B of Al₂W₃O₁₂ and Y₂W₃O₁₂ specimens are shown in Fig.5. It is shown that all the diffraction peaks of series B samples (even the sample re-annealed at 800 ℃) shift to high degree, indicating that the crystal cell volume of series B samples is less than that of series A samples. This result shows that water molecules can be present in the crystallographic voids and cause shrinkage of the structure. The difference in the volume of the hydrated (series B samples) and the unhydrated (series A samples) phase indicates that the water molecules cause shrinkage of the framework structure. It is possible that the Y–O–W linkages are bent away from 180? angle in the hydrated phase. As the water is lost, the Y–O–W angle approaches the 180? linkage angle.

Fig.5 XRD patterns of Al₂W₃O₁₂ and Y₂W₃O₁₂ under different condition: (a) Al₂W₃O₁₂; (b) Y₂W₃O₁₂

4.3 Moisture absorbing properties of Al₂W₃O₁₂ and Y₂W₃O₁₂ powders

TG-DTA analysis of B series Al₂W₃O₁₂ and Y₂W₃O₁₂ samples is shown in Fig.6. Based on calculation, the molecule number of absorbed water in B series Y₂W₃O₁₂ sample hold in air for 6 months is about 3.2, which is in good agreement with the results of KARMAKAR et al[13]. The molecule number of absorbed water in B series Al₂W₃O₁₂ sample is about 0.17.

Fig.6 TG-DTA results of B series Al₂W₃O₁₂ (a) and Y₂W₃O₁₂ (b) samples

4.4 SEM characterization of Al₂W₃O₁₂ and Y₂W₃O₁₂ powders

The SEM micrographs of Al₂W₃O₁₂ and Y₂W₃O₁₂ samples annealed at 900 ℃ for 10 h are shown in Fig.7. The particles of Al₂W₃O₁₂ and Y₂W₃O₁₂ powders are  widely distributed with the larger ones of more than 40mm and the smaller ones of just 3-5 mm. It is also seen from Fig.7 that the particle size of Al₂W₃O₁₂ is much larger than that of Y₂W₃O₁₂.

Fig.7 SEM photographs of Al₂W₃O₁₂ (a) and Y₂W₃O₁₂ (b) powder prepared at 900 ℃ for 10 h

5 Conclusions

1) The calculation results show that increasing the concentration of citric acid and pH values can increase the amount of yttrium and aluminum citrate, which plays a positive role in the formation of Al_2-xY_xW₃O₁₂ gel. The concentration of those different cation citrate complexes decided by citric acid and pH values in the starting solution influences the formation of sol and further affects precipitation and segregation during gelling and charring.

2) Al_2-xY_xW₃O₁₂ powders are formed by citrate sol-gel process using yttrium nitrate, aluminum nitrate, ammonium tungstate and citric acid as starting materials. The optimum conditions for Al_2-xY_xW₃O₁₂ powder synthesis are [H₃Cit]_T/{[Y] _T+[Al]_T}=3?1, pH=4 and calcining temperature of 900 ℃.

3) The maximum solid solubility of Y₂O₃ in Al₂W₃O₁₂ and Al₂O₃ in Y₂W₃O₁₂ is less than 0.5. Y₂W₃O₁₂ powders absorb much more water than Al₂W₃O₁₂ powders in air under the same condition. Y₂W₃O₁₂?3.2H₂O and Al₂W₃O₁₂?0.17H₂O form from Y₂W₃O₁₂ and Al₂W₃O₁₂ respectively after being left in air for 6 months.

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Foundation item: Project(0512002400) supported by the Fund for Distinguished Young Scholars of Henan Province, China

Corresponding author: ZHANG Hai-jun; Tel: +86-371-67761908, Fax: +86-371-67763822; E-mail: zhizdl@yahoo.com.cn, zhanghaijun@zzu.edu.cn

(Edited by YANG Bing)