Phase structure and microwave dielectric properties of Zr(Zn1_3Nb2_3)(x)Ti2-xO6 ceramics

Phase structure and microwave dielectric properties of Zr(Zn 1/3Nb 2/3)x Ti 22x O 6(0.2£x £0.8)ceramicsBin Tang •Shengquan Yu •Hetuo Chen •Shuren Zhang •Xiaohua ZhouReceived:30August 2012/Accepted:20October 2012/Published online:1November 2012ÓSpringer Science+Business Media New York 2012Abstract The phase structure,morphology and micro-wave dielectric properties of Zr(Zn 1/3Nb 2/3)x Ti 2-x O 6(0.2B x B 0.8)ceramics were investigated as a function of the amount of (Zn 1/3Nb 2/3)4?substitution by the solid-state reaction technique.X-ray diffraction analysis showed the co-existence of the solid solution ZrTiO 4(x =0.2)or ZrTi 2O 6(x =0.4–0.5)as the main phase and TiO 2as the second phase,and the single-phase region for substituted ZrTi 2O 6was obtained with x C 0.5.As increasing x from 0.2to 0.8,the grain growth was promoted,moreover the dielectric constant e r and the temperature coefficient of resonant frequency s f dropped from 53.8to 39.2and from ?106.2to -25.5ppm/°C respectively,and the Q 9f values increased from 33,200to 43,200GHz.Ceramics with good microwave dielectric properties:e r =41.7,Q 9f =42,100GHz and s f =-15.5ppm/°C was obtained at x =0.6.1IntroductionWith the operating frequency of wireless communication expanding to microwave frequency,dielectric ceramics have been widely used in microwave components such as resona-tors,duplexers and so on.It is because these ceramics have excellent microwave dielectric properties:high dielectric constant (e r ),high quality factor (Q 9f)and near-zerotemperature coefficient of resonant frequency (s f )[1].The applications of such functional ceramics have realized the miniaturization and the cost reduction of microwave equip-ments.In particular,the high dielectric constant ensures the miniaturization because of the physical length of a dielectricresonator in proportion to1=ffiffiffiffie r p ,and the high quality factor is good for microwave selectivity.The near-zero temperature coefficient of resonant frequency means the high temperature stability of the equipments.Kinds of ceramics materials were reported to be suitable for such applications,for example Mg(Z r0.05Ti 0.95)O 3–SrTiO 3,Ba 2Ti 9O 20,BaO(Zn l/3Ta 2/3)O 3,(Zr 0.8Sn 0.2)TiO 4,BaO–Nd 2O 3–TiO 2[2–6].Zirconium titanate based ceramics,showing the high dielectric constant,high Q 9f value and unique compo-sition dependence of the temperature coefficient of reso-nant frequencies,have been researched extensively [5].(Zr 0.8Sn 0.2)TiO 4ceramics is a typical representative and has been widely used as dielectric resonators,but its sin-tering temperature about 1,600°C is very high ,so it is difficult to obtain its dense microstructure by the solid-phase reaction technique without sintering aids such as ZnO,NiO,CuO,V 2O 5and Bi 2O 3[7,8].On the other hand,to substitute Zr 4?ions or Ti 4?ions in ZrTiO 4by other ions could also affect the densification process and microwave dielectric properties,for example one-third of Zn 2?ions and two-thirds of Nb 5?or Ta 5?ions could co-substitute Zr 4?ions in ZrTiO 4crystal structure [9,10].Zr 1-x (Zn 1/3Ta 2/3)x TiO 4ceramics was sintered well at 1,300°C and had good microwave dielectric properties with e r =42.5,Q 9f =40,200GHz and s f =?l.1ppm/°C at x =0.3[9].Moreover,RiaziKhoei et al.[11]studied (1-x)ZrTi 2O 6–xZnNb 2O 6ceramics and the single-phase region for solid solution ZrTiO 4was formed because (Zn 1/3Nb 2/3)4?randomly occupied both of Zr 4?and Ti 4?ions sites.And inB.Tang ÁS.Yu (&)ÁH.Chen ÁS.Zhang ÁX.ZhouState Key Laboratory of Electronic Thin Films and Integrated Devices,University of Electronic Science and Technology of China,Chengdu 610054,Chinae-mail:yu.s.quan@;yyuu@ B.Tange-mail:tangbin@J Mater Sci:Mater Electron (2013)24:1475–1479DOI 10.1007/s10854-012-0955-7the research of Mn-modified(1-x)ZrTi2O6–xZnNb2O6 ceramics,(Zn1/3Nb2/3)4?was found to preferentially substituted the sites of Ti4?ions in ZrTi2O6crystal struc-ture,and then the sites of Zr4?ions[12].In this work, Zr(Zn1/3Nb2/3)x Ti2-x O6ceramics was prepared for further studying the homogeneity range of the substitution of(Zn1/ 3Nb2/3)4?for Ti4?ions sites in ZrTi2O6crystal structure. The microwave dielectric properties of Zr(Zn1/3Nb2/ 3)x Ti2-x O6ceramics were also carefully investigated.2ExperimentalThe samples used in this study were prepared by the con-ventional solid-state reaction technique.For the synthesis of Zr(Zn1/3Nb2/3)x Ti2-x O6(0.2B x B0.8)powders,ZrO2, ZnO,Nb2O5and TiO2of[99%purity were mixed and ball-milled in a nylon jar with zirconia balls and deionized water for10h,and after drying these powders were cal-cined at1,150°C for3h.Then the calcined powders were re-milled for5h with0.3wt%ter,with6wt% PVA as binder,thefine powders were pressed into cylin-drical samples with15mm in diameter and7mm in thickness under a pressure of20MPa.At last,these sam-ples were sintered at1,260°C for6h in air.The bulk densities of the sintered samples were measured by the Archimedes method.The phase composition was identified by X-ray diffraction(XRD)using Cu K a radiation (Phlips x’pert Pro MPD).Microstructure observation was conducted on the surface of samples by using scanning electron microscopy(SEM,FEI Inspect F).The dielectric characteristics at microwave frequencies were measured by the Hakki–Coleman dielectric resonator method in the TE011 mode using a network analyzer(Agilent Technologies E5071C)[13].The s f values were determined from the dif-ference between the resonant frequencies obtained at25and85°C using the equation:s f¼ðf t2Àf t1Þ=ðf t1Âðt2Àt1ÞÞ,where f t1and f t2are the resonant frequencies at t1=25°Cand t2=85°C,respectively.3Results and discussionFigure1shows the XRD patterns of Zr(Zn1/3Nb2/ 3)x Ti2-x O6(0.2B x B0.8)ceramics with0.3wt% MnCO3sintered at1,260°C for6h in air.For the sample with x=0.2,the main phase was indexed to ZrTiO4in an orthorhombic a-PbO2structure[14],and a significant amount of the rutile type TiO2phase was also observed, which was left from the raw materials.As increasing x further to0.4,the remaining TiO2phase gradually decreased,and the desired single phase region for the solid solution ZrTi2O6was developed when the value of x was in the range from0.5to0.8.The solid solution Zr(Zn1/3Nb2/ 3)x Ti2-x O6was only formed in the samples with high ZnNb2O6content,which should be attributed to that the activation energy for the formation of the solid solution was decreased when the content of ZnNb2O6was increased due to its relatively low sintering temperature about 1,150°C.Table1displays the lattice parameters and unit cell volume of the solid solution ZrTi2O6phase in the samples with x=0.4–0.8.The lattice parameters for a-axis,b-axis and c-axis were all increased with increasing the content of ZnNb2O6,especially for the b-axis,its increase was more noticeable,and thus the unit cell volume was also increased.The reason for this should be the substitution of(Zn1/3Nb2/3)4?(the average r6=0.673A˚, Fig.1The XRD patterns of Zr(Zn1/3Nb2/3)x Ti2-x O6(0.2B x B0.8) ceramics with0.3wt%MnCO3sintered at1,260°C for6h in air Table1The lattice parameters and unit cell volume of the solid solution ZrTi2O6phase in Zr(Zn1/3Nb2/3)x Ti2-x O6(0.4B x B0.8) ceramics with0.3wt%MnCO3sintered at1,260°C for6h in air Sample Lattice parameters Unit cell volume(A˚3) a-axis(A˚)b-axis(A˚)c-axis(A˚)x=0.4 4.7379 5.5119 5.0138130.93x=0.5 4.7445 5.5288 5.0246131.80x=0.6 4.7506 5.5419 5.0276132.36x=0.7 4.7577 5.5506 5.0347132.96x=0.8 4.7605 5.5612 5.0386133.39the superscript number is the coordination number)for Ti4?ions(r6=0.605A˚)in ZrTi2O6crystal structure.And no evidence implied that(Zn1/3Nb2/3)4?would occupy the sites of Zr4?.To investigate the relationship between the composi-tional ratio and the microstructure,the SEM photographs of Zr(Zn1/3Nb2/3)x Ti2-x O6(0.2B x B0.8)ceramics with0.3wt%MnCO3sintered at1,260°C for6h in air with(a)x=0.2,(b)x=0.4,(c)x=0.6and(d)x=0.8are shown in Fig.2.With increasing the content of ZnNb2O6, there was a noticeable change that the grain growth was promoted strongly.And the average grain size was approximately increased from about2l m to6l m.In addition,with the grain growth,the pores,observed in the sample with x=0.2,were quickly eliminated in the sample with x=0.4.Generally,the pure Zr–Ti based ceramics were difficult to be densified below1,600°C. However,Zr(Zn1/3Nb2/3)x Ti2-x O6ceramics with x=0.4–0.8could be sintered well at1,260°C and all of these samples showed compact microstructure with crystal grains in dense contact.It was because ZnNb2O6lowered the sintering temperature and improved the densification via the liquid-phase mechanism[15].And the reason for the grain growth was also because of this.Figure3a shows the apparent densities of Zr(Zn1/3Nb2/ 3)x Ti2-x O6(0.2B x B0.8)ceramics with0.3wt% MnCO3sintered at1,260°C for6h in air.When the value of x was increased from0.2to0.5,the densities increased noticeably from4.68to4.97g/cm3,which was mainly attributed to the change in the molar mass of the starting formulation.Moreover,the elimination of pores and the grain growth could also promote the increase of density. When the value of x was increased further to0.8,the densities started to become saturated and showed the maximum apparent density5.00g/cm3with x=0.6.And there was a slight drop of the densities,which might be caused by the abnormal grain growth.The dielectric constant of Zr(Zn1/3Nb2/3)x Ti2-x O6 (0.2B x B0.8)ceramics with0.3wt%MnCO3sintered at 1,260°C for6h in air is shown in Fig.3b.The change of the dielectric constant displayed an opposite changing trend compared to the change of the apparent densities,and the dielectric constant showed a continuous downtrend from53.8to39.2.Therefore,it could be inferred that the change of the dielectric constant was determined not by the densities as usual,but mainly by the gradual decrease of the second phase TiO2with a high e r[100[16]and the more and more substitution of the composite ions(Zn1/ Fig.2The SEM photographs of Zr(Zn1/3Nb2/3)x Ti2-x O6(0.2B x B0.8)ceramics with0.3wt%MnCO3sintered at1,260°C for6h in air with a x=0.2,b x=0.4,c x=0.6and d x=0.83Nb 2/3)4?in ZrTi 2O 6crystal structure.Zhang et al.[17]had suggested that (Zn 1/3Nb 2/3)4?substituted for Ti 4?ions in Ba 3Ti 4-x (Zn 1/3Nb 2/3)x Nb 4O 21ceramics caused the decrease of dielectric constant because the covalency of Zn–O and Nb–O has more stronger covalency bond than that of Ti–O bonds in the AO 6octahedra.Hence,it is not hard to understand why the dielectric constant dropped continuously in the single phase region with x C 0.5.For the Q 9f values of Zr(Zn 1/3Nb 2/3)x Ti 2-x O 6(0.2B x B 0.8)ceramics with 0.3wt%MnCO 3sintered at 1,260°C for 6h in air as shown in Fig.3c,the increase of the Q 9f values was divided into two stages:the first stage for a rapid increase (x =0.2–0.5)corresponding to the two phase region and the second stage for a slow increase (x =0.5–0.8)corresponding to the single phase region.As we know,besides that the lattice vibration modes cause the main microwave dielectric loss,the additional phases,the poros-ity,the crystal defects and the average grain size contribute to the microwave dielectric loss.For the first stage,the rapid increase of the Q 9f values from 33,200to 40,900GHz was due to the decrease of the second phase TiO 2and the improvement of the microstructure,for example,the elimi-nation of pores and the moderate grain growth.For the sec-ond stage,the increase of the Q 9f values from 40,900to 43,200GHz was attributed to the strengthening of covalency bonds caused by the substitution of the composite ions (Zn 1/3Nb 2/3)4?into ZrTi 2O 6crystal structure as mentioned above.Figure 3d shows the temperature coefficient of resonant frequency of Zr(Zn 1/3Nb 2/3)x Ti 2-x O 6(0.2B x B 0.8)ceramics with 0.3wt%MnCO 3sintered at 1,260°C for 6hin air.The s f values varied towards the negative direction from ?106.2ppm/°C (x =0.2)to 225.5ppm/°C (x =0.8).For the samples with x =0.2–0.5,their s f values were the mutual offset effect between the solid solution ZrTiO 4(x =0.2)or ZrTi 2O 6(x =0.4–0.5)and the second phase TiO 2,and the s f values declined quickly to 210.2ppm/°C (x =0.5)mainly because of the decrease of the second phase TiO 2with s f [?400ppm/°C [16].For the single phase region with x =0.5–0.8,the slow decrease of the s f values was related to the change of crystal structure of the solid solution ZrTi 2O 6caused by the more and more substitution of (Zn 1/3Nb 2/3)4?.As reported by Collar et al.[18],the octahedra tilting affected the s f value,and the s f value moved towards negative when the tilting increased.In the present single phase region,as the quantity of substitution of (Zn 1/3Nb 2/3)4?for Ti 4?increasing,the degree of tilting on oxygen octahedra was increased,and the structures of Zr(Zn 1/3Nb 2/3)x Ti 2-x O 6(x =0.5–0.8)ceramics turned to be more distorted.As a result,the s f values decreased with increasing x.At last,the microwave dielectric ceramics with the composition Zr(Zn 1/3Nb 2/3)0.6Ti 1.4O 6?0.3wt%MnCO 3was sintered well at 1,260°C for 6h in air and it had good microwave dielectric properties:e r =41.7,Q 9f =42,100GHz and s f =215.5ppm/°C.4ConclusionZr(Zn 1/3Nb 2/3)x Ti 22x O 6(0.2B x B 0.8)ceramics were prepared by the traditional ceramic technology.ThephaseFig.3The apparent densities and microwave dielectric properties of Zr(Zn 1/3Nb 2/3)x Ti 2-x O 6(0.2B x B 0.8)ceramics with 0.3wt%MnCO 3sintered at 1,260°C for 6h in airstructure,morphology and the microwave dielectric prop-erties of these ceramics were investigated as a function of the amount of(Zn1/3Nb2/3)4?substitution.X-ray diffraction analysis confirmed the co-existence of the solid solution ZrTiO4(x=0.2)or ZrTi2O6(x=0.4–0.5)as the main phase and TiO2as the second phase.The single-phase region for the substituted ZrTi2O6was obtained with x C0.5.As increasing x from0.2to0.8,the grain growth was promoted via the liquid-phase mechanism,moreover the dielectric constant e r and the temperature coefficient of resonant frequency s f dropped from53.8to39.2and from ?106.2to-25.5ppm/°C respectively,and 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