Ceram.Int.,2014,40,5307–Fe掺杂ZnO粉末的制备、介电性质及红外发射率研究

CERAMICSINTERNATIONALAvailable online at Ceramics International 40(2014)5307–5311Preparation,dielectric property and infrared emissivity of Fe-doped ZnOpowder by coprecipitation method at various reaction timeXiaolei Su n ,Yan Jia,Xiaoqin Liu,Junbo Wang,Jie Xu,Xinhai He,Chong Fu,Songtao LiuCollege of Mechanical &Electronic Engineering,Xi'an Polytechnic University,Xi'an Shaanxi 710048,ChinaReceived 12August 2013;received in revised form 14October 2013;accepted 23October 2013Available online 1November 2013AbstractFe-doped ZnO powders have been synthesized by the coprecipitation method at 6001C with various reaction time,using zinc nitrate as the staring material,urea as the precipitator,and ferric nitrate as the doping source,respectively.The phase and morphology of the prepared powders have been characterized by X-ray diffraction and scanning electron microscopy,respectively.It was found that the prepared product synthesized for 1h had a pure ZnO wurtzite structure and was a ZnO(Fe)solid solution powder.The real part,imaginary part,and dielectric loss of complex permittivity of prepared powders in the frequency range of 8.2–12.4GHz decreased with increasing reaction time.The average infrared emissivities of prepared powders at the waveband range of 8–14μm increased with extending reaction time.&2013Elsevier Ltd and Techna Group S.r.l.All rights reserved.Keywords:ZnO;Dielectric property;Infrared emissivity1.IntroductionZinc oxide (ZnO)is one of the most important and versatile semiconductors with a direct wide band gap of 3.37eV at room temperature.It has also many excellent properties,such as,nontoxic,cheap,unique optical,and electrical proper-ties,which has wide applications in the solar cells,sensors,photocatalysis,transparent thin film transistors [1–3].In recently years,ZnO has recently attracted increasing interest due to its remarkable microwave absorbing property and infrared shielding property.Zhuo et al.[4]synthesized intriguing ZnO dendritic nanos-tructures by a two-step chemical vapor deposition (CVD)process.The RL (re flection loss)value of the composite with 50vol%ZnO dendritic nanostructures reached À42dB at 3.6GHz with a thickness of 5.0mm.Chen et al.[5]fabricated the tetrapod-like ZnO nanostructures by rapid heating metal Zn pellet at 9001C in air,which showed the good microwave absorption property in the frequency range of 2–18GHz.Wang et al.[6]synthesized Al-doped ZnO (AZO)powders by a coprecipitation method.The real part (ε′)and imaginary part(ε″)of complex permittivity of the prepared AZO powders have been improved in the frequency range of 8.2–12.4GHz due to the generation of Al Zn defects.Zhou et al.[7]prepared microwave absorption coating using tetra-needle-like ZnO whiskers as the main functional agent.It showed the excellent microwave absorption property in the frequency range of 8–18GHz because of the formation of semi-conductive networks,diffuse re flections,multipoles,and the multi-interfaces.In addition,ZnO shows also the low infrared emissivity at the waveband range of 8–14μm.Yao et al.[8]synthesized Zn 0.99Àx Mn 0.01Co x O (x ¼0.00,0.01,0.03,and 0.05)powders at different calcination temperatures by a solid-state reaction and the best infrared emissivity value was 0.754.Dongmei et al.[9]fabricated AZO thin films by a sol-gel method and studied the effect of Al concentration and temperature on the infrared emissivity of AZO films at the waveband range of 8–14μm.The prepared AZO film showed the low infrared emissivity.Zhang et al.[10]synthesized the Co-doped ZnO powder by the conventional solid state reaction.The prepared powders presented the low infrared emissivity at the waveband range of 3–5μm and 8–14μm and the excellent microwave absorption property in the frequency range of 8.2–12.4GHz.Therefore,because the Fe element belongs also to the VIII group,it presents the same effects on the microstructure,/locate/ceramint0272-8842/$-see front matter &2013Elsevier Ltd and Techna Group S.r.l.All rights reserved./10.1016/j.ceramint.2013.10.107nCorresponding author.Tel.:þ862962335812;fax:þ862962335812.E-mail address:suxlei@ (X.Su).dielectric property and infrared emissivity of ZnO powder as the Co element possibly.Compared with Co-doped ZnO powder,Fe-doped ZnO powder has the good magnetic property and optical property besides microwave wave absorp-tion property and infrared shielding property,which is a multifunctional material [11].In addition,because the density of Fe element is lower than that of Co element,the Fe-doped ZnO powder become a lighter mass shielding material than Co-doped ZnO powder possibly.In the paper,Fe-doped ZnO powders have been synthesized by coprecipitation method at 6001C with various reaction time,using zincnitrate as the staring material,urea as the precipitator and ferric nitrate as the doping source,respec-tively.The dielectric property in the frequency range of 8.2–12.4GHz and the average infrared emissivity at the waveband range of 8–14μm have been determined.The effects of Fe-doping on the microstructure,dielectric property and infrared emissivity of ZnO powder have been studied.2.Experimental 2.1.Sample preparationFe-doped ZnO powders were synthesized by the coprecipi-tation method,using zinc nitrate (Zn(NO 3)2Á6H 2O)as the staring material,urea (CO(NH 2)2)as the precipitator and ferric nitrate (Fe(NO 3)3Á9H 2O)as the doping source,respectively.All the chemical reagents were analytical grades and were used without further puri fication.First,Zn(NO 3)2Á6H 2O,Fe(NO 3)3Á9H 2O and CO(NH 2)2were separately dissolved in de-ionized water.Second,appro-priate amounts of Fe(NO 3)3Á9H 2O solutions were added into the Zn(NO 3)2Á6H 2O solution according to the experimental design.In our study,the molar ratio of Fe to (Zn þFe),n Fe /(n Fe þn Zn ),was 0.06.Finally,CO(NH 2)2solution was slowly added into above mixture solutions.The as-prepared solutions were continually stirred at 1001C for 1h to yield precipitates.After filtration,the precipitates were washed with distilled water and ethanol for several times,and then dried in air at 1201C in stove.Fe-doped ZnO powders were obtained after the calcination of precipitates at 6001C for 1h,2h,3h,and 4h,respectively.2.2.Microstructure,infrared emissivity and dielectric property characterizationThe phases of the as-prepared powders were identi fied by X-ray diffraction (XRD,X'Pert PRO MPD,Cu K α).The 99.99%Si (a ¼5.43088Å)was used as an inner standard.The morphology of the prepared powders was investigated by scanning electron microscope (SEM,JSM-6360LV,JEOL,Tokyo,Japan).The average infrared emissivities of prepared powders at the waveband of 8–14μm were determined by the reference's method [10].The permittivities of prepared pow-ders in the frequency range of 8.2–12.4GHz were determined by the reference's method [12],in which the measuringsamples were prepared by mixing the produced powders with paraf fin in a mass ratio of 10:90.3.Results and discussionThe XRD patterns of prepared Fe-doped ZnO powders at 6001C with various reaction time are shown in Fig.1.It can be seen that the synthesized powders had a hexagonal wurtzite structure belonging to the C 46v space group (P 63mc )according JCPDS card.When the reaction time is 1h,no impurity phase is observed and the prepared product is a ZnO (Fe)solid solution powder.However,when the reaction time is up to 2h,ZnFe 2O 4impurity phase appeared.The reason is that the more and more Fe atoms entering ZnO crystal and substituting Zn atoms of ZnO lattice precipitated from ZnO lattice with increasing reaction time.Therefore,the Fe con-centration is higher than the solubility limit of Fe in ZnO in some areas of ZnO crystal [13].The ZnFe 2O 4impurity has been generated by the following reactions:4Fe (precipitated)þ3O 2-2Fe 2O 3(1)Fe 2O 3þZnO -ZnFe 2O 4(2)In addition,the main peak intensities of ZnO phase at 2θ¼31.781,34.381,and 36.281increase with extending reaction time,indicating that the prepared Fe-doped ZnO powder presents the better crystallization.According to related study [14],Fe doping can inhibit ZnO crystallization.Con-versely,the precipitation of Fe atoms from ZnO lattice leads to the better crystallization.The lattice constants a and c calculated by Bragg formula of all samples corresponding to ZnO doped by Fe are shown in Table 1.According to related study [14],when the Fe atoms entered ZnO lattice,the Fe atoms occupied Zn site in ZnO lattice and substituted Zn ions preferentially.Because the ion radius of Fe 3þ(0.645Å)is smaller than that of Zn 2þ(0.74Å),the decrease of lattice constant of ZnO is caused by the substitution of Zn atoms of ZnO lattice by Fe atoms.So theFig.1.XRD patterns of the prepared Fe-doped ZnO powders at 6001C with various reaction time.X.Su et al./Ceramics International 40(2014)5307–53115308lattice constant of the Fe-doped ZnO powder synthesized for 1h is less than the standard lattice constant values (a ¼3.2524Åand c ¼5.2071Å)of ZnO.According to above analysis,because the more and more Fe atoms precipitated from ZnO lattice with extending reaction time,the increase of lattice constant of prepared Fe-doped ZnO with increasing reaction time is expected.Fig.2shows SEM photos of the prepared Fe-doped ZnO powders at 6001C with various reaction time.It is observed that the synthesized ZnO powders have fine spherical particles and present narrow particle size distribution.The average sizes of Fe-doped ZnO powders synthesized at 6001C for 1h,2h,3h,and 4h are about 300nm,800nm,1.5μm,and 2μm,respectively,showing that the powder sizeincreases gradually with increasing reaction time.The reason isthat the precipita-tion of Fe atoms from ZnO lattice improves the grain growth of ZnO.Fig.3shows real part ε′,imaginary part ε″,and dielectric loss tg δof complex permittivity as a function of frequency inthe frequency range of 8.2–12.4GHz for the prepared Fe-doped ZnO powders at 6001C with various reaction time.It can be seen that dielectric properties of prepared powders decreased with increasing reaction time.Because the Fe atom presents trivalent state usually and Fe 3þis a stable state,there are three electrons bonded with the other elements.Synchro-nously,when the Fe atoms entered the ZnO lattice,the Fe atoms substituted Zn atoms in ZnO crystal preferentially and the Fe Zn defects have been generated.Therefore,there are bound electrons around Zn Fe defects in the ZnO crystal.Under the alternating electromagnetic field,these bound electrons will migrate to and fro to form relaxation polarization and loss,thus leading to the higher ε″and tg δ,respectively.The more the substitutions have been generated,the higher the ε″and tg δare.Additionally,impurity at interfaces is another important factor affecting loss tangent of Fe-doped ZnO powders.Because the high surface area supplies opportunities for impurities to be distributed,the additives are mostly positioned at grain bound-aries or crystal surfaces in crystal materials [15].Some impurity atoms congregated near the grain boundary also,which may cause impurity conduction.Interfacial conduction and impurities play an important role in increasing the ε″and tg δof the prepared Fe-doped ZnO powders.Additionally,the charge accumulation at materials interfaces induced the interfacial polarization,Maxwell –Wagner effects [16],which may partly explain the phenomena observed above.Interfacial polarity causes a build-up of charges near heterogeneities and grain boundaries in polycrystalline materials.This kind of polarity may contribute also to the ε″and tg δ.Because the Fe-doped ZnO powder synthesized for 1h has the most amounts of Fe ZnTable 1lattice constant and infrared emissivity of prepared Fe-doped ZnO powders at Fig.2.SEM photos of the prepared Fe-doped ZnO powders at 6001C with various reaction time;(a)1h;(b)2h;(c)3h;(d)4h.X.Su et al./Ceramics International 40(2014)5307–53115309defects,the most amounts of bound electrons and the highest surface area,the ε′,ε″,and tg δhave the highest values,ε′E 3.3,ε″E 0.82,and tg δE 0.25.Therefore,the Fe-doped ZnO powder synthesized at 6001C for 1h shows the best dielectric property in the frequency range of 8.2–12.4GHz.In addition,the average infrared emissivities of Fe-doped ZnO powders prepared at 6001C with various reaction time at the waveband range of 8–14μm have been presented,as shown in Table 1.It can be seen that the infrared emissivities of prepared Fe-doped ZnO powders increase with increasing reaction time.According to above analysis,the bound elec-trons have been generated arounds Fe Zn defects in ZnO crystal.Because Fe-doped ZnO is an ionic crystal and relative vibration between different ions will produce a certain change in the electric dipole moment,which will produce the interac-tion between long optical wave and infrared radiation and then infrared radiation is produced and absorbed.Additionally,the high surface area improves also the production and absorption of infrared radiation.Therefore,the most amounts of Fe Zn defects,the most amounts of bound electrons and the highest surface area lead to the best infrared emissivity (0.84)at the waveband range of 8–14μm for the Fe-doped ZnO powder synthesized at 6001C for 1h.4.ConclusionsFe-doped ZnO powders have been prepared successfully by coprecipitation method at 6001C for 1h,2h 3h,and 4h,using zinc nitrate as the staring material,urea as the pre-cipitator,and ferric nitrate as the doping source,respectively.XRD analysis reveals that the sample synthesized for 1h had a pure ZnO wurtzite structure and is ZnO(Fe)solid solution powder.SEM photos indicate that the prepared Fe-doped ZnO powders have narrow size distribution and show the particle size increases with increasing reaction time due to the precipitation of Fe from ZnO crystal.The ε′,ε″,and tg δin the frequency range of 8.2–12.4GHz and infrared emissivity at the waveband range of 8–14μm of 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