Preparation of AgSnO2 composite powders by hydrothermal process

J. Cent. South Univ. Technol. (2007)02- -0176- 05包SpringerDOI: 10.1007/s11771-007-0035-yPreparation of AgSnO2 composite powders by hydrothermal processYANG Tian-zu(杨天足)，DU Zuo-juan(杜作娟)', GU Ying-ying(古映莹)},QIU Xiao-yong(邱晓勇), JIANG ming-xi(江名喜) .(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 asraw 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 keyparameter in the formation of Sn(OH), precipitate and the reduction reaction of Ag(NH3)ht can rclease H+ ions, which results insynchronous precipitation of Sn(OH)。2~ as Sn(OH)4. The reduction of Ag(NH3)2 and precipitation of Na2SnO3 occur simultaneouslyand the coprecipitation of silver and tin oxide is reached by the bhydrothermal method. The silver-tin oxide composite powders havemainly flake shape of about 0.3 um in thickness and there exists homogeneous distribution of tin oxide and silver in the powdersynthesized.Key words: silver-tin oxide powder; hydrothermal method; flake structure; coprecipitationwas applied to prepare silver-tin oxide composite1 Introductionpowders with Ag(NH)2* solution and Na2SnO3 solutionas raw materials and Na2SO3 as the reductant. TheThe material made of silver and tin oxide(AgSnO2),obtained mixed solution was used as precursor. Thehaving some advantages, for example, excellent arcreduction of Ag and the crystallization precipitation ofaggressiveness resistance, wear-resistance and resistanceSnO2 were investigated simultaneously.to fusion, has become the contact material that has themaximum possibility to replace AgCdo". AgSnO2 has2 Experimentalbeen the focus of researches in contact materials inrecent years'- 4. It has got a rapid development and has2.1 Experimental proceduregradually been applied to alternating current (AC) andAqueous ammonia was added to AgNO3 solution atdirect current (DC) contactors, power relays and somecertain concentration, with silver-ammonia complex ionlow-voltage circuit breakers.being formed. Na2SnO3 in proportion was added to theAt present the preparation techniques for AgSnO2above solution, with addition of Na2SO3 as reductant atcontact material mainly are powder metallurgical processthe same time. The resulted mixed solution was taken asand alloy internal oxidation process. Besides the aboveprecursor, which was put into a high pressure reactor stillprocesses, there are reaction spray-on processsto react for 4 h at 150 C, then AgSnO2 compositechemical plating process6-8), reaction synthesis process'powders were obtained. The powders were washed andand reaction ball milling techniqueo that are used forfitered, and dried for4hat 100 C.preparation of the contact material. Each of theseprocesses has its own advantages, but at the same time,2.2 Characteristicssome limitations exist as well2l. The hydrothermalX-ray diffractometer (D/MAX- YA type) was usedmethod is one of the best wet chemical methods for thefor determining the phases of the sample, Cu target, Ku,preparation of high quality oxide powder and the powder2=0.154 056 nm; SEM (Japan JSM- -5600LV) foprepared by hydrothermal process has advantages suchobserving the appearance and grain size of the sample;as well-defined grain, no aggregation and goodene中国煤化工:ctor (EDX) (apandispersiviy!13!. In this study, the hydrothermal processJSNical composition of theYHCNMHGFoundation item: Project(2001BA901A09)supported by the Key Program of Science and Technology Action of West China DevelopmentReceived date: 2006 - 06- 24; Accepted date: 2006 -09 -02Corresponding anthor: DU Zuo-juan, PhD candidate; +86-731-8836791; E-mail:cuckoo8211@163.comYANG Tian-zu, et al: Preparation of AgSnO2 composite powders by hydrothermal process177sample; KSCN titrimetric analysis for measuring th3.2 Reduction of silvercontent of silver in the sampl4; and iodimetry forThe acid corresponding to sodium sulfte is H2SO3analyzing the content of Sn in the samplelS.which is a weak acid. In alkaline solution, SO3- is thepredominant species. The standard electrode potentials of3 Results and discussionSO32 and Ag(NH3)2 * complex ion are as follows:SO2- +2H*+2e=SO3-+H2O,3.1 Precipitation of Sn(OH)4Na2SnOz is a kind of compound with unstableφ°(S0 /so )=-0.93 V(4)chemical properties. It exists mostly in the form ofAgQNH3)2 +e=Ag+2NH3Sn(OH)2- in solutions. When it reacts with acid,especially weak acid, it is easy to form Sn(OH)4φ°(Ag(NHy)/Ag)=0.37 V(5)precipitate. The precipitate is very stable, with aObviously, SO3- can reduce Ag(NH3)2* to metallicsolubility product constant of 1X 10 56. However, it cansilver. Theoretically, fom Eqns.(4) and (5), when molebe decomposed into Sn* and H2O in a strong acidratio of SO32- to Ag(NH3)2* is over 0.5, Ag(NH3)2* cansolution. The relative reaction can be described asbe reduced thoroughly by SO3-. In this work, it isfollows:proved from the experiment that a complete reduction ofAg(NH3)2t can be realized only when the mole ratio ofSn(OH)。+ 2H* = Sn(OH)4↓+ 2H2O(1)SO3- to Ag(NH3)2* is over 1.Sn(OH)4+ 4H+ = Snt+ + 4H2OAccording to Eqns.(1) and (2), the total concentration3.3 Preparation of AgSnOz composite powdersSilver ammine complex solution is made of AgNO3([Sn]r) of tin in the solution, the pH value for Sn(OH)4precipitation can be figured out by the followingsolution and aqueous ammonia. Then Ag(NH)h2*equation:solution is mixed with Na2SnO3 solution and NazSO3solution, with a colorless and clear solution being[Sn]+= 10( 4p-1.18) + 102-22 2659)5(3)obtained. The solution obtained is used as precursor.A diagram of pH value against lg[Sn]r isThen the solution was pressured still and heated to 150constructed and the resulted curve is shown in Fig.1.C for 4 h. During thermal process, on one handThe curve shows the relationship between [Sn]r in theAg(NH3)2* is reduced by SO32，which can be describedsolution and the pH value, which ilustrates the criticalpH value for forming Sn(OH)4 precipitates. The lowest2Ag(NH3)2" +SO3-+H20=point is the optimum pH value for Sn(OH)4 precipitation.2Ag+SO42 +2Ht +4NH3(6)On the other hand precipitation of Sn(OH)4 takes place.The pH value of the precursor is adjusted bycontrolling the addition of aqueous ammonia. AccordingSolidto Fig.1, when the pH value of the precursor is adjustedto the critical pH value for Sn(OH)4 precipitation, Hions are released slowly with the progress of the-9卜reducing reaction, with synchronous precipitation ofAqueousSn(OH)2~ as Sn(OH)4. Some studies have proved that-12-Sn(OH)4 can be dehydrated to form SnO2 crystals under-152 0之468 1012 14a hydrothermal condition["6), which can be expressed bythe following reaction:HSn(OH)+-SnO2+2H2O(7)Fig.1 Relation between [Sn]r in solution and pH value at25 cTherefore, during hydrothermal process, reductionFrom Fig.1 it can be seen that when pH of Na2SnO3of中国煤化工Sn(OH)4 precipitationsolution is decreased, the precipitates of Sn(OH)4 will beand: SnO2 can be realizedformed. And when the concentration of tin in feed:MYH.CNM H Goste powders of sivesolution is given, the precipitation rate of tin can beand tin oxide evenly distributed are obtained.calculated from pH of the raffinate solution. Therefore,When the concentration of tin is 0.01 mol/L in thepH value of the solution is a key parameter for theprecursor, the changes of pH, [Ag]t and [Sn]r after thepreipistioegf Sn(OH)4.hydrothermal reaction are listed in Table 1. It is seen178J. Cent. South Univ. Technol. 2007, 14(2)from Table 1 that pH is decreased from 10.10 to 8.13reported in Ref.[17], the intensities of the peaks of SnO2and [Sn]r is decreased to 4.28X 10-5 mol/L which isare obviously lower. This may be resulted from the smallclose to the calculated value of 1.0X 10 6 mol/L.size of SnO2 crystal particles and the widening of theFrom Table 1 it can be calculated that thepeasl18-19.Therefore, under the prescribed conditions,precipitation rates of tin oxide and silver are 99.57% andsilver and tin oxide composite powders can be prepared99.86%, respectively.by hydrothermal method. The powders consist of matrixAg and SnO2, without other phases found in presentTable 1 Changes of pH, [Ag]r and [Sn]r after hydrothermalcondition of XRD.reactionStatepH[Ag]r/.Calculated[Sn]r/(111)1(mol!L7)(mol:L)(molL)。二sno,Beforehydrothermal 10.100.1260.01Afterhydrothermal 8.13 1.72X 10~41.0X10-6 4.28X 10-5(200)(311)3.4 DTA analysisFig.2 shows the DSC diagram of AgSnO2 powders.It is seen from Fig.2 that the curve has a heat absorption230106(7080peak at 950 C, which indicates the melting point of20/(°)silver. However, the other part of the curve presents noFig.3 XRD patterm of AgSnO2 powdersendothermic peak or exothermic peak. It can beconsidered that the SnO2 has a thorough dehydration.3.6 SEM and EDX analysisFig.4 shows the micrograph of silver -tin oxidecomposite powders. It is observed that the powderprepared by the hydrothermal method has a flakestructure with about 0.3 um in thickness. Thecomposition of a single particle is difficult to bequantitatively determined because of its small particle950 Csize. However, a back reflection contrast analysisindicates that these particles are of a same composition.Independent SnO2 particles can not be observed from theappearance of the powders. The actual size and form ofthe SnO2 particles in Ag matrix need to be studied200400600I 000further.Temperature/CThe results of energy dispersive X-ray analysis (asshown in Fig.5) of the composite powders show that theFig.2 DSC diagram of AgSnO2 powderssample contains two kinds of elements, Ag and Sn. In3.5 XRD analysisFig.3 shows the diffraction pattern of silver-tinoxide composite powders. In the pattrn, there are fourmain peaks, which are corresponded to the crystal planesof(111), (200), (220) and (311) of the silver respectively.These peaks are sharp, and their positions are identicalwith the standard peak of Ag of cubic phase, indicating中国煤化工perfectly-grown silver particle with polycrystallinestructure. Meanwhile, it is found that there appearHCNMHGheterophase peaks, which are corresponded to thediffraction peak of SnO2 of tetragonal phase. These20m9381 18/HAR/84peaks are less sharp than the diffraction peakscorrespending to silver crystal. Compared with thoseFig.4 SEM image of AgSnO2 powdersYANG Tian-zu, et al: Preparation of AgSnO2 composite powders by hydrothermal process179the sample the content of SnO2 is 9.65%(mass fraction),homogeneous distribution in the composite powders.basically conforming to the actual addition amount of10%. It is also proved by a chemical analysis as well.4 ConclusionsThe results of surface scanning analysis of EDX areshown in Fig.6. It can be seen that Sn and 0 elements1) The relationship between tin concentration ofhave a uniform distribution on the silver matrix.tin(IV) and pH value is established from thermodynamicAccording to the above analyses, the obtainedcalculation. pH value is a key parameter in thesample is mainly in flake shape of about 0.3 um inprecipitation of Sn(OH)4. By controlling pH value of thethickness with elements of silver, tin and oxygen beingsolution, tin(IV) can be precipitated completely.2) During hydrothermal process, Ag(NH3)2* can begreduced by SO3 effectively and Sn(OH)4 precipitatecan be dehydrated to form SnO2 crystal. Owing to thereduction of Ag(NH3)2 , precipitation of Sn(OH)4 anddehydration of Sn(OH), pH value of the solution is\gdecreased to some extent.3) From the analysis of the composite powders byDTA, XRD, SEM and EDX, it is concluded that silverSnand tin exist in metallic silver and tin dioxiderespectively. The composite powders are mainly in flake25shape of about 0.3 μm in thickness, and silver, tin andEnergykeVoxygen in the composite powders are evenly distributed.Fig.5 Energy-dispersive X -ray analysis (EDX) spectrum ofDuring the hydrothermal process, the reduction of silver,AgSnO2 powdersprecipitation of Sn(OH)4 and dehydration of Sn(OH)410 μm中国煤化工fHCNMHGFig.6 Surface scanning analysis results of EDX in AgSnO2 powders(a) AgSnO2 powders; (b) Ag elements; (c) Sn elements; (d) O elements180J. Cent. South Univ. Technol. 2007, 14(2)can be realized simultaneously.[1] LORRAIN N, CHAFFRON L, CARRY C, et al. Kinetics andformation mechanisms of the nanocomposite powder Ag-SnO2prepared by reactive milling[J]. Materials Science and Engineering A,References2004, 367: 1-8.[12] DU Zuo-juan, YANG Tian-zu, GU Ying-ying, et al. The advances of1] ZHANG Wang-sheng. Basic conditions of electrical contactspreparing methods and apication of AgSnO2 contact material[J].abroad[J]. 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