J. Cent. South Univ. Technol. (2007)02-0176-05

DOI: 10.1007/s11771-007-0035-y                                                                                              

Preparation of AgSnO₂ composite powders by hydrothermal process

YANG Tian-zu(杨天足)¹, DU Zuo-juan(杜作娟)¹, GU Ying-ying(古映莹)²,

QIU Xiao-yong(邱晓勇)², JIANG ming-xi(江名喜)¹

(1. School of Metallurgical Science and Engineering, Central South University, Changsha 410083, China;

2. School of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China)

Abstract: Silver-tin oxide powders were synthesized by the hydrothermal method with Ag(NH₃)₂⁺ solution and Na₂SnO₃ solution as raw materials and Na₂SO₃ as reductant. The precipitation conditions of Na₂SnO₃ solution and the reduction conditions of Ag(NH₃)₂⁺ were also investigated. The powders prepared were characterized by differential thermal analysis (DTA), X-ray diffraction analysis (XRD), scanning electron microscope (SEM) and energy spectrum analysis. The results show that pH value of the solution is a key parameter in the formation of Sn(OH)₄ precipitate and the reduction reaction of Ag(NH₃)₂⁺ can release H⁺ ions, which results in synchronous precipitation of Sn(OH)₆^2- as Sn(OH)₄. The reduction of Ag(NH₃)₂⁺ and precipitation of Na₂SnO₃ occur simultaneously and the coprecipitation of silver and tin oxide is reached by the hydrothermal method. The silver-tin oxide composite powders have mainly flake shape of about 0.3 μm in thickness and there exists homogeneous distribution of tin oxide and silver in the powder synthesized.

Key words: silver-tin oxide powder; hydrothermal method; flake structure; coprecipitation                     

1 Introduction

The material made of silver and tin oxide(AgSnO₂), having some advantages, for example, excellent arc aggressiveness resistance, wear-resistance and resistance to fusion, has become the contact material that has the maximum possibility to replace AgCdO^[1]. AgSnO₂ has been the focus of researches in contact materials in recent years^[2-4]. It has got a rapid development and has gradually been applied to alternating current (AC) and direct current (DC) contactors, power relays and some low-voltage circuit breakers.

At present the preparation techniques for AgSnO₂ contact material mainly are powder metallurgical process and alloy internal oxidation process. Besides the above processes, there are reaction spray-on process^[5], chemical plating process^[6-8], reaction synthesis process^[9] and reaction ball milling technique^[10-11] that are used for preparation of the contact material. Each of these processes has its own advantages, but at the same time, some limitations exist as well^[12]. The hydrothermal method is one of the best wet chemical methods for the preparation of high quality oxide powder and the powder prepared by hydrothermal process has advantages such as well-defined grain, no aggregation and good dispersivity^[13]. In this study, the hydrothermal process was applied to prepare silver-tin oxide composite powders with Ag(NH₃)₂⁺ solution and Na₂SnO₃ solution as raw materials and Na₂SO₃ as the reductant. The obtained mixed solution was used as precursor. The reduction of Ag and the crystallization precipitation of SnO₂ were investigated simultaneously.

2 Experimental

2.1 Experimental procedure

Aqueous ammonia was added to AgNO₃ solution at certain concentration, with silver-ammonia complex ion being formed. Na₂SnO₃ in proportion was added to the above solution, with addition of Na₂SO₃ as reductant at the same time. The resulted mixed solution was taken as precursor, which was put into a high pressure reactor still to react for 4 h at 150 ℃, then AgSnO₂ composite powders were obtained. The powders were washed and filtered, and dried for 4 h at 100 ℃.

2.2 Characteristics

X-ray diffractometer (D/MAX-YA type) was used for determining the phases of the sample, Cu target, K_α, λ=0.154 056 nm; SEM (Japan JSM-5600LV) for observing the appearance and grain size of the sample; energy dispersive X-ray detector (EDX) (Japan JSM-5600LV) for analyzing chemical composition of the sample; KSCN titrimetric analysis for measuring the content of silver in the sample^[14]; and iodimetry for analyzing the content of Sn in the sample^[15].

3 Results and discussion

3.1 Precipitation of Sn(OH)₄

Na₂SnO₃ is a kind of compound with unstable chemical properties. It exists mostly in the form of Sn(OH)₆^2- in solutions. When it reacts with acid, especially weak acid, it is easy to form Sn(OH)₄ precipitate. The precipitate is very stable, with a solubility product constant of 1×10^-56. However, it can be decomposed into Sn⁴⁺ and H₂O in a strong acid solution. The relative reaction can be described as follows:

Sn(OH)₆^2- + 2H⁺ = Sn(OH)₄↓ + 2H₂O     (1)

Sn(OH)₄ + 4H⁺ = Sn⁴⁺ + 4H₂O        (2)

According to Eqns.(1) and (2), the total concentration ([Sn]_T) of tin in the solution, the pH value for Sn(OH)₄ precipitation can be figured out by the following equation:

[Sn]_T = 10^{(-4pH-1. 018 8)} + 10^{(2pH-22. 265 9)}      (3)

A diagram of pH value against lg[Sn]_T is constructed and the resulted curve is shown in Fig.1. The curve shows the relationship between [Sn]_T in the solution and the pH value, which illustrates the critical pH value for forming Sn(OH)₄ precipitates. The lowest point is the optimum pH value for Sn(OH)₄ precipitation.

Fig.1 Relation between [Sn]_T in solution and pH value at 25 ℃  

From Fig.1 it can be seen that when pH of Na₂SnO₃ solution is decreased, the precipitates of Sn(OH)₄ will be formed. And when the concentration of tin in feed solution is given, the precipitation rate of tin can be calculated from pH of the raffinate solution. Therefore, pH value of the solution is a key parameter for the precipitation of Sn(OH)₄.

3.2    Reduction of silver

The acid corresponding to sodium sulfite is H₂SO₃ which is a weak acid. In alkaline solution, SO₃^2- is the predominant species. The standard electrode potentials of SO₃^2- and Ag(NH₃)₂⁺ complex ion are as follows:

SO₄^2-+2H⁺+2e=SO₃^2-+H₂O,

           (4)

Ag(NH₃)₂⁺+e=Ag+2NH₃

          (5)

Obviously, SO₃^2- can reduce Ag(NH₃)₂⁺ to metallic silver. Theoretically, from Eqns.(4) and (5), when mole ratio of SO₃^2- to Ag(NH₃)₂⁺ is over 0.5, Ag(NH₃)₂⁺ can be reduced thoroughly by SO₃^2-. In this work, it is proved from the experiment that a complete reduction of Ag(NH₃)₂⁺ can be realized only when the mole ratio of SO₃^2- to Ag(NH₃)₂⁺ is over 1.

3.3 Preparation of AgSnO₂ composite powders                             

Silver ammine complex solution is made of AgNO₃ solution and aqueous ammonia. Then Ag(NH₃)₂⁺ solution is mixed with Na₂SnO₃ solution and Na₂SO₃ solution, with a colorless and clear solution being obtained. The solution obtained is used as precursor. Then the solution was pressured still and heated to 150 ℃ for 4 h. During thermal process, on one hand Ag(NH₃)₂⁺ is reduced by SO₃^2-, which can be described as:

2Ag(NH₃)₂⁺+SO₃^2-+H₂O=2Ag+SO₄^2-+2H⁺+4NH₃       (6)

On the other hand precipitation of Sn(OH)₄ takes place.

The pH value of the precursor is adjusted by controlling the addition of aqueous ammonia. According to Fig.1, when the pH value of the precursor is adjusted to the critical pH value for Sn(OH)₄ precipitation, H⁺ ions are released slowly with the progress of the reducing reaction, with synchronous precipitation of Sn(OH)₆^2- as Sn(OH)₄. Some studies have proved that Sn(OH)₄ can be dehydrated to form SnO₂ crystals under a hydrothermal condition^[16], which can be expressed by  the following reaction:

Sn(OH)₄=SnO₂+2H₂O         (7)

Therefore, during hydrothermal process, reduction of Ag(NH₃)₂⁺ to metallic silver, Sn(OH)₄ precipitation and de-hydration of Sn(OH)₄ to SnO₂ can be realized simultaneously. As a result, composite powders of silver and tin oxide evenly distributed are obtained.

When the concentration of tin is 0.01 mol/L in the precursor, the changes of pH, [Ag]_T and [Sn]_T after the hydrothermal reaction are listed in Table 1. It is seen from Table 1 that pH is decreased from 10.10 to 8.13 and [Sn]_T is decreased to 4.28×10^-5 mol/L which is close to the calculated value of 1.0×10^-6 mol/L.

From Table 1 it can be calculated that the precipitation rates of tin oxide and silver are 99.57% and 99.86%, respectively.

Table 1 Changes of pH, [Ag]_T and [Sn]_T after hydrothermal reaction

3.4 DTA analysis

Fig.2 shows the DSC diagram of AgSnO₂ powders. It is seen from Fig.2 that the curve has a heat absorption peak at 950 ℃, which indicates the melting point of silver. However, the other part of the curve presents no endothermic peak or exothermic peak. It can be considered that the SnO₂ has a thorough dehydration.

Fig.2 DSC diagram of AgSnO₂ powders

3.5 XRD analysis

Fig.3 shows the diffraction pattern of silver-tin oxide composite powders. In the pattern, there are four main peaks, which are corresponded to the crystal planes of (111), (200), (220) and (311) of the silver respectively. These peaks are sharp, and their positions are identical with the standard peak of Ag of cubic phase, indicating perfectly-grown silver particle with polycrystalline structure. Meanwhile, it is found that there appear heterophase peaks, which are corresponded to the diffraction peak of SnO₂ of tetragonal phase. These peaks are less sharp than the diffraction peaks corresponding to silver crystal. Compared with those reported in Ref.[17], the intensities of the peaks of SnO₂ are obviously lower. This may be resulted from the small size of SnO₂ crystal particles and the widening of the peaks^[18-19]. Therefore, under the prescribed conditions, silver and tin oxide composite powders can be prepared by hydrothermal method. The powders consist of matrix Ag and SnO_2, without other phases found in present condition of XRD.

Fig.3 XRD pattern of AgSnO₂ powders

3.6    SEM and EDX analysis

Fig.4 shows the micrograph of silver-tin oxide composite powders. It is observed that the powder prepared by the hydrothermal method has a flake structure with about 0.3 μm in thickness. The composition of a single particle is difficult to be quantitatively determined because of its small particle size. However, a back reflection contrast analysis indicates that these particles are of a same composition. Independent SnO₂ particles can not be observed from the appearance of the powders. The actual size and form of the SnO₂ particles in Ag matrix need to be studied further.

Fig.4 SEM image of AgSnO₂ powders

The results of energy dispersive X-ray analysis (as shown in Fig.5) of the composite powders show that the sample contains two kinds of elements, Ag and Sn. In the sample the content of SnO₂ is 9.65%(mass fraction), basically conforming to the actual addition amount of 10%. It is also proved by a chemical analysis as well. The results of surface scanning analysis of EDX are shown in Fig.6. It can be seen that Sn and O elements have a uniform distribution on the silver matrix.

Fig.5 Energy-dispersive X-ray analysis (EDX) spectrum of AgSnO₂ powders

According to the above analyses, the obtained sample is mainly in flake shape of about 0.3 μm in thickness with elements of silver, tin and oxygen being homogeneous distribution in the composite powders.

1) The relationship between tin concentration of tin(IV) and pH value is established from thermodynamic calculation. pH value is a key parameter in the precipitation of Sn(OH)₄. By controlling pH value of the solution, tin(IV) can be precipitated completely.

2) During hydrothermal process, Ag(NH₃)₂⁺ can be reduced by SO₃^2- effectively and Sn(OH)₄ precipitate can be dehydrated to form SnO₂ crystal. Owing to the reduction of Ag(NH₃)₂⁺, precipitation of Sn(OH)₄ and dehydration of Sn(OH)₄, pH value of the solution is decreased to some extent.

3) From the analysis of the composite powders by DTA, XRD, SEM and EDX, it is concluded that silver and tin exist in metallic silver and tin dioxide respectively. The composite powders are mainly in flake shape of about 0.3 μm in thickness, and silver, tin and oxygen in the composite powders are evenly distributed. During the hydrothermal process, the reduction of silver, precipitation of Sn(OH)₄ and dehydration of Sn(OH)₄ can be realized simultaneously.

Fig.6 Surface scanning analysis results of EDX in AgSnO₂ powders

(a) AgSnO₂ powders; (b) Ag elements; (c) Sn elements; (d) O elements

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Foundation item: Project(2001BA901A09)supported by the Key Program of Science and Technology Action of West China Development

Received date: 2006-06-24; Accepted date: 2006-09-02

Corresponding author: DU Zuo-juan, PhD candidate; +86-731-8836791; E-mail:cuckoo8211@163.com

(Edited by YANG Bing)