Na-β''-Al2O3 electrolytes by microwave sintering precursors derived from the sol–gel method

Journal of Alloys and Compounds 497 (2010) 295–299Contents lists available at ScienceDirectJournal of Alloys andCompoundsj o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /j a l l c omSynthesis of Na-␤ -Al 2O 3electrolytes by microwave sintering precursors derived from the sol–gel methodJin Wang,Xiao-Ping Jiang,Xiao-Ling Wei,Hui Yang ∗,Xiao-Dong ShenCollege of Materials Science and Engineering,Nanjing University of Technology,Nanjing 210009,Jiangsu,Chinaa r t i c l e i n f o Article history:Received 24September 2009Received in revised form 1March 2010Accepted 2March 2010Available online 9 March 2010Keywords:Ceramics Sol–gelElectrochemical impedance spectroscopy X-ray diffractiona b s t r a c tNa-␤ -Al 2O 3electrolytes were synthesized by the microwave sintering assisted sol–gel technique in this study.The precursor powders and microwave-sintered pellets were systematically investigated by means of X-ray diffraction (XRD),scanning electron microscope (SEM),differential scanning calorimetry (TG–DSC)and Fourier transform infrared (FT-IR)spectroscopy.The ionic conductivity of the microwave-sintered pellets was also measured based on AC impedance spectroscopy.It was found that the calcining temperature of the precursor powders derived from the sol–gel method was the key factor to affect the purity of Na-␤ -Al 2O 3in the pellets.The sintered pellets prepared from the powders calcined at 850◦C exhibited a purity of 94.4%Na-␤ -Al 2O 3and the density of the sintered pellets was 98.91%of the theo-retical density of Na-␤ -Al 2O 3.In addition,the sample showed an ionic conductivity of 0.01085S cm −1at 300◦C by AC impedance measurement.© 2010 Elsevier B.V. All rights reserved.1.IntroductionSodium beta alumina is widely studied as a solid electrolyte in sodium/sulfur and sodium/metal chloride batteries,which are used as power sources in electric vehicles and other energy storage appli-cations [1,2].The two main subgroups of sodium beta alumina are generally found in materials of the type ␤-Al 2O 3with the empirical formula Na 2O ·11Al 2O 3,and in materials of the type ␤ -Al 2O 3iden-tified in the form Na 2O ·5.33Al 2O 3[3,4].␤ -Al 2O 3exhibits a higher sodium ionic conductivity and is the preferred phase for sodium battery electrolyte applications.Conventional synthesis of Na-␤ -Al 2O 3is carried out by solid-state reaction of ␣-Al 2O 3with Na 2CO 3and a small quantity of MgO and/or Li 2O as the stabilizer [3–6].The disadvantage of this method is the formation of two-phase mixture (␤-and ␤ -Al 2O 3)with relatively low conductivity [7,8].In addition to the conventional solid-state reaction,some soft-chemistry routes to synthesize ␤ -Al 2O 3have been reported,such as alkoxide hydrol-ysis [9],sol–gel processing [7,10–12],co-precipitation [13]and solution combustion techniques [14].Though the soft-chemistry routes were reported to yield high surface area powders that can be sintered at relatively low temperatures,the processes still require a long period of time at very high temperatures [8].The process of sintering at high temperature (∼1480◦C)with a long holding time is cost-and energy intensive.Furthermore,it may lead to sodium∗Corresponding author.Tel.:+862583587238;fax:+862583587238.E-mail address:yanghui@ (H.Yang).loss and exaggerated grain growth,which are deleterious to the ionic conductivity and the mechanical strength.This problem can be circumvented using microwaves as the energy source of the sintering [7,14].The use of microwaves as an energy source for processing ceramics has increased over the past two decades due to its rapid heating rates,volumetric heating,short holding time and low power requirements [15–18].Recently,Subasri [12]reported microwave processing to syn-thesize dense MgO stabilized Na-␤ -Al 2O 3in a short time.However,AC impedance measurements and the synthesis mecha-nisms related to the characteristics of the intermediate compounds have not been systematically reported.Such microwave processing of sodium beta alumina offers a novel and cost-effective method to synthesize phase-pure Na-␤ -Al 2O 3electrolytes.The present work thus applied the citrate complex-assisted microwave process to the synthesis of Na-␤ -Al 2O 3electrolytes.In this study,the effect of the synthesis conditions of the precursor powders on the purity of Na-␤ -Al 2O 3was systematically investigated.The conductivity of the sintered samples was determined by AC impedance measurements.2.Experimental proceduresThe sol–gel route was employed to synthesize the precursor powders of Na-␤ -Al 2O 3in this study.Appropriate amounts of aluminum nitrate (Al(NO 3)3·9H 2O,AR),sodium nitrate (NaNO 3,AR)and magnesium nitrate (Mg(NO 3)2·6H 2O,AR)were dissolved in de-ionized water to synthesize a product with the composition of Na 1.67Mg 0.67Al 10.33O 17.The saturated solution was stirred continuously and heated at 60◦C over a hot plate.Citric acid was added to the solution as the fuel and chelat-ing agent with an amount equal to the mole number of all the cations.The mixed solution was then stirred at 60◦C for several hours to form a yellowish sol,and then an appropriate amount of polyvinyl alcohol (PVA)solution (n PVA :n citrate =1:2)was0925-8388/$–see front matter © 2010 Elsevier B.V. All rights reserved.doi:10.1016/j.jallcom.2010.03.038296J.Wang et al./Journal of Alloys and Compounds497 (2010) 295–299Fig.1.Schematic of the industrial microwave oven:a,microwave cavity;b,fire-resistant cotton;c,fibrous alumina sheath;d,silicon carbide bars;e ceramic crucible; f,samples.added.Subsequently,the new mixed solution was heated at80◦C to form a trans-parent sticky gel.The resulting gel was dried at220◦C for10h and then ground completely to obtain the precursor powders.The precursor powders were then cal-cined at different temperatures from850◦C to1400◦C for2h in a muffle furnace in air.The resultingfine powders were then ball milled for5h with anhydrous alcohol and dried.Solid pellets of20mm in diameter were pressed at300MPa using cold isostatic pressure.An industrial microwave oven(2450MHz,MW-L0136V,China)(as shown in Fig.1)was used in the present work.The green pellets were placed in a ceramic crucible surrounded by an insulatingfibrous alumina sheath,which was enveloped withfire-resistant cotton.A large number of silicon carbide bars were inserted into the inter-space between the ceramic crucible and the insulatingfibrous alumina sheath for auxiliary heating.A schematic of the industrial microwave oven is pre-sented in Fig.1.The industrial microwave oven can provide complete control over all operational parameters.Consequently,the green pellets were sintered on a precise heating schedule pre-programmed in the system by adjusting the parameters of the input power.In this study,when the microwave oven was heated to approximately 1570◦C,a high-precision optical pyrometer was used for accurate temperature measurement.The heating rate was controlled in the range from10◦C min−1to 25◦C min−1and the power rating was operated in the range from500W to1500W.The crystal structure of the powders calcined at different temperatures and the sintered pellets(ground to powders again for measurements)was carried out by X-ray diffraction(XRD,Model X’TRAX)using nickelfiltered Cu-K␣radiation ( =0.15406nm)over the2Ârange from5◦to75◦.The cross-section microstruc-ture of the microwave-sintered pellets was observed using SEM(Model JSM-5900, JEP,Tokyo,Japan).In order to determine the mass loss steps and the crystallization temperatures of the precursor,differential scanning calorimetry(TG–DSC)analyses were performed on a TG–DSC analyzer(Model NETZSCH STA449C,Germany)from room temperature to1400◦C at a heating rate of10◦C min−1in air.The densities of the sintered pellets were measured by the Archimedes method using dimethylben-zene as the immersion medium.The Fourier transform infrared(FT-IR)spectra of the powders in KBr pellets were observed using an infrared spectrum analyzer(IR,Nexus 670,Nicolet,USA),in order to investigate the molecular structure of the powders. To determine the conductivity of the sintered pellets,AC impedance measurements of the sintered pellets were carried out with excitation potentials of20mV over a frequency range from1MHz to1Hz generated by an impedance analyzer(1260A Impedance analyzer,Solartron,UK)and an electrochemical interface(1287A Poten-tiostat,Solartron,UK)in the temperature range from room temperature to400◦C. The sintered pellets were coated with silver paste on their parallel surfaces and then sintered at550◦C for15min before the AC impedance measurements to ensure good bonding.3.Results and discussion3.1.TG/DSC and XRD of the precursorThe XRD patterns of the powders calcined at different tem-peratures,as described in Section2,are presented in Fig.2.They revealed that the powders calcined at850◦C were a mixture of mullite-like alumina(PDF number12-593)and a small amount ofMgAl2O4.However,the diffraction peak intensity was very low,Fig.2.XRD patterns of the powders calcined at different temperatures from850◦C to1400◦C for2h.suggesting its poor crystallinity.m-Al2O3,an intermediate phase prior to the formation of Na-␤ -Al2O3,can be observed in the pow-ders calcined at various temperatures from850◦C up to1000◦C, which was consistent with that reported by Subasri[12].It has been reported that m-Al2O3is a modification with a silica free mullite-like structure,and a recent NMR investigation indicated that the crystal structure of m-Al2O3is an orthorhombic one typi-fying two unequal Na sites within the unit cell[19].The formation temperature of m-Al2O3(∼850◦C)is lower than the crystallization temperature of␤ -Al2O3.Thus,the powders calcined at temper-atures below1000◦C were a mixture of m-Al2O3and MgAl2O4. With increasing the calcining temperature,MgAl2O4reacted with m-Al2O3by solid diffusion,and thus a mixture of␤-Al2O3and ␤ -Al2O3was obtained.As shown in Fig.2,when the calcining temperature was increased to1050◦C,the powders were a mix-ture of␤ -Al2O3and␤-Al2O3without the intermediate phase, m-Al2O3.However,the XRD pattern of the powders calcined at 1400◦C exhibits the characteristic peaks of␣-Al2O3,␤ -Al2O3and ␤-Al2O3.The phase transformation can be explained by the DSC/TG curves of the xerogel shown in Fig.3.Two exothermic peaks detected at392◦C and439◦C in the DSC curve may be ascribed to the combustion of the citrate and the oxidation of the polyvinyl alcohol,respectively.A large endother-mic peak appeared at about902◦C,which may result from the crystallization of m-Al2O3.A shift of52◦C in the crystallization temperature measured by DSC/TG and the transformation tem-peratures obtained from XRD may be attributed to the different soaking times and the hysteresis of the TG/DSC analyzer.The weakFig.3.DSC/TG curves of the xerogel.J.Wang et al./Journal of Alloys and Compounds 497 (2010) 295–299297Fig.4.XRD patterns of the sintered pellets prepared from powders calcined at different temperatures.exothermic peaks between 1000◦C and 1200◦C can be assignedto the transformation from m-Al 2O 3to ␤-Al 2O 3.Moreover,the exothermic peak at 1336◦C can be assigned to the transformation from ␤-Al 2O 3to ␣-Al 2O 3,which was consistent with the results obtained from XRD.The TG curve showed that complete decompo-sition of the xerogel was achieved at ∼457◦C with a total weight loss of 72.4%.The weight loss of 9.6%below 200◦C was attributed to the removal of water.However,the weight loss of 4.1%in the temperature range from 500◦C up to 1400◦C may be ascribed to the loss of Na 2O and the deprivation of CO 2on the surface of the powders.3.2.Effect of the calcining temperatureFig.4displays the XRD patterns of the sintered pellets pre-pared from the powders calcined at different temperatures.All the sintered pellets are a mixture of ␤-and ␤ -Al 2O 3(main phase).As shown in Fig.4,the sintered pellets prepared from the pow-ders calcined at 850◦C exhibited a remarkable diffraction peak of ␤ -Al 2O 3at 45.9◦.However,the extra diffraction peak at 44.5◦indi-cates the presence of ␤-Al 2O 3.When the calcining temperature of the precursor was 900◦C,the diffraction peak intensity of ␤ -Al 2O 3at 45.9◦decreased,but the diffraction peak intensity of ␤-Al 2O 3at 44.50◦was enhanced simultaneously.When the calcining tem-perature of the precursor was increased to 1000◦C,the diffraction peak intensity of ␤-Al 2O 3gradually sharpened.On the contrary,the diffraction peak intensity of ␤ -Al 2O 3decreased significantly.Thus,with increasing the calcining temperature of the powders,the con-tent of ␤ -Al 2O 3in the sintered pellets gradually decreased,while the content of ␤-Al 2O 3significantly increased.However,the XRD patterns of the pellet prepared from the pow-ders calcined at 1100◦C was found to almost coincide completely with that of the powders,but significantly differed from that of samples prepared from the powders calcined at the temperature range from 850◦C up to 1000◦C,as shown in Fig.4.Consequently,we assumed that the microwave sintering for the powders calcined at 1100◦C cannot prompt a transformation from ␤-to ␤ -Al 2O 3.Thus,the results suggested that the phase of the powders was cru-cial to the product of the microwave sintering samples.As shown in Fig.2,the powders calcined at the temperature range from 850◦C up to 1000◦C can be indexed to m-Al 2O 3,but the powders calcined at 1100◦C and above 1100◦C were identified as a mixture of ␤-and ␤ -Al 2O 3.Therefore,it can be speculated that the content of ␤ -Al 2O 3in the microwave-sintered pellets is related to the m-Al 2O 3phase in the precursor.Fig.5.FT-IR spectra of the (a)sintered pellets (ground to powders again for the mea-surement)prepared from the powders calcined at 850◦C and (b)powders calcined at 850◦C.Subasri [12]reported that each unit cell of m-Al 2O 3contains a spinel block and a mirror plane with the conduction of sodium ions,in contrast to three spinel block planes and a mirror plane in one unit cell of ␤ -Al 2O 3.Thereby,only a slight rearrangement of the structure is required to transform from the intermediate phase to the ␤ -Al 2O 3phase,which is crucial to prepare ␤ -Al 2O 3in a short time using microwaves.In order to investigate the difference between the structure of m-and ␤ -Al 2O 3,the FT-IR spectra of m-and ␤ -Al 2O 3were examined and are shown in Fig.5.The bands near 3500cm −1and 1600cm −1are due to the stretching vibra-tion of H 2O and the bending vibration of H 2O,respectively,which can verify the presence of chemically bonded H 2O molecules in the m-Al 2O 3structure.The bands from 650cm −1to 450cm −1can be attributed to the vibrations of the AlO 6octahedra.The bands from 950cm −1to 725cm −1can be assigned to the vibrations of the AlO 4tetrahedra.In addition,it can be observed that the peaks due to the AlO 4tetrahedra and the AlO 6octahedra for the two sam-ples are nearly similar,which indicates that the arrangement of the Al 2O 3spinel blocks is almost the same and the only differ-ence between two samples should be the number of spinel blocks in a unit cell.Thus,we assume another important factor assisting simultaneous microwave sintering of ␤ -Al 2O 3is the presence of chemically bonded H 2O molecules in the m-Al 2O 3structure,which has also been reported to promote microwave processing without the aid of any secondary heater [12].Consequently,the presence of m-Al 2O 3in the powders,the similar structure of m-and ␤ -Al 2O 3and the presence of chemically bonded H 2O molecules in the m-Al 2O 3structure may be the crucial causes that promote the transformation from the intermediate phase to the ␤ -Al 2O 3phase in a short time using microwaves.The relative content of the ␤ -Al 2O 3phase,F (␤ ),in the samples can be calculated according to the following equation [20]:F (␤)=0.85×I ␤ (I ␤␤ (1)where 0.85is the I -correction factor and relates to the relative intensities of the single-phase peaks as listed in the PDF cards of ␤-and ␤ -Al 2O 3,and I ␤ and I ␤are the peak intensities at 45.90◦and 44.50◦,respectively.The contents of ␤ -Al 2O 3in the pellets pre-pared from the powders calcined at different temperatures were calculated and are listed in Table 1.As shown in Fig.4,with increasing the calcining tempera-ture of the powders,the content of ␤ -Al 2O 3in the samples gradually decreased,while the content of ␤-Al 2O 3significantly increased.The sample prepared from the powders calcined at 850◦C showed the strongest peak intensity at 45.9,thus exhibit-298J.Wang et al./Journal of Alloys and Compounds 497 (2010) 295–299Table 1The relative contents of the ␤ -Al 2O 3phase in the sintered pellets prepared from the powders calcined at different temperatures (from 850◦C to 1000◦C).The calcining temperature of the powders (◦C)Relative content of ␤ -Al 2O 3phase (%)85094.4090088.2295073.58100069.98ing a high purity of 94.4%for ␤ -Al 2O 3.The bulk density of 3.2637g cm −3is approximately 98.91%of the theoretical den-sity of Na-␤ -Al 2O 3.The lattice parameters calculated from the XRD patterns were a =b =5.6159nm and c =33.5945nm,which are comparable to the standard data of Na 1.67Mg 0.67Al 10.33O 17(PDF number 35-0438,a =b =5.624nm,c =33.560nm),indicating its well-crystallized structure.As above mentioned,dense MgO sta-bilized Na-␤ -Al 2O 3can be obtained within a short time of 2h by microwave sintering precursors derived from the sol–gel method.3.3.Ionic conductivity of Na-ˇ -Al 2O 3Fig.6(a)–(c)displays the complex impedance spectra of the sintered pellets measured at 50◦C,150◦C and 300◦C in air,respec-tively.The shape of the experimental impedance diagrams was fairly similar to those obtained by previously reported literature [21,22],which exhibited the characteristic impedance spectra of polycrystalline electrolytes.As can be seen from the impedance spectra,the data in the high and intermediate frequency ranges can be approximately described by a depressed semicircle arc attributed to the crystal resistance and grain boundary resistance,and the inclined line at low frequencies can be ascribed to the elec-trode polarization.At high temperatures,the electrode polarizationeffect dominated at low frequencies.Because the sodium ion con-ductivity is extremely high at room temperature,the semicircle arc corresponding to the crystal resistance cannot be observed,except at extremely low temperatures.Moreover,the semicircle arc decreased with increasing temperature,which indicates that the crystal resistance and grain boundary resistance decreased with the increase in the temperature.The equivalent circuit for the Na-␤ -Al 2O 3electrolyte can be described as shown in Fig.6(d),where R i ,R b and R gb are the interface resistance,the bulk resistance and the grain boundary resistance,and C i ,C g and C gb are the inter-face capacitance,the geometric capacitance and the grain boundary capacitance,respectively.The crystal resistance and grain bound-ary resistance can be determined from the impedance spectra based on the electrical equivalent circuit.The total resistance of the sam-ple can be given by:R =R b +R gb(2)The curve fitting and resistance calculation were done by ZSim-pWin software,and then the total ionic conductivity at different temperatures can be obtained using the following equation: =L SR(3)where L and S represent the thickness of the sample and the electrode area of the sample surface,respectively.The total ionic conductivities of the samples measured at temperatures from 150◦C to 350◦C are listed in Table 2.The total ionic conductiv-ity measured at 300◦C was 0.01085S cm −1,which is comparable to 0.011S cm −1at 300◦C reported by Kalsi et al.[23].Fig.6(d)shows Arrhenius plots of the total ionic conductivity for the sintered pellet at temperatures from 30◦C to 400◦C in the form of ln( T )versus 1000/T .It can be observed that the total conductiv-ity increased significantly at lower temperatures,where the grainplex impedance spectra of the sintered pellet at different temperatures:(a)50◦C,(b)150◦C,(c)300◦C and (d)Arrhenius plots of the total conductivity of the sintered pellet (inset:the equivalent circuit for the Na-␤ -Al 2O 3electrolyte).J.Wang et al./Journal of Alloys and Compounds 497 (2010) 295–299299Table 2Total ionic conductivities of the sintered pellets prepared from the powders calcined at 850◦C.Measuring temperature (◦C)Total ionic conductivity, (×10−2S cm−1)1500.073862000.1962500.726300 1.0853501.457Fig.7.The cross-section micrograph of the microwave-sintered pellets from the powders calcined at 850◦C.boundary resistance dominated ionic transport.At higher temper-atures,it can also be observed that the total ionic conductivities appear to converge,where the crystal boundary resistance plays a more important role in the ionic transport than the grain boundary resistance.The temperature dependence of the conductivity can be described by the Arrhenius equation: =A Texp −E kT(4)where E is the activation energy for ionic migration;k is Boltz-mann’s constant;T is the absolute temperature;and A is the pre-exponential factor,which is a constant in a certain tempera-ture range.The activation energy of the sintered pellets,determined from the slope in Fig.6(d),was 0.08156eV,which was lower than the values reported in the literature [22].The difference may be partly due to the different purity of the ␤ -Al 2O 3phase in the pellets and the different conditions of the measurements.Nevertheless,the Arrhenius plots displayed similar temperature dependence.The cross-section micrograph of the sample,displayed in Fig.7,shows plate-like grains with a grain size of about 1–5␮m,but reveals some closed pores,indicating a not-fully compacted microstructure.This also implies that the ionic conductivity may be improved by increasing the density of the samples.4.ConclusionsMgO stabilized Na-␤ -Al 2O 3has been synthesized by the microwave sintering assisted sol–gel technique in a short period of 2h,with a bulk density up to 98.91%of the theoretical density.The presence of m-Al 2O 3in the precursor powders,the similar struc-ture of m-and ␤ -Al 2O 3and the presence of chemically bonded H 2O molecules in the m-Al 2O 3structure may be crucial to promot-ing a transformation from the intermediate m-Al 2O 3phase to the ␤ -Al 2O 3phase in a short time by microwave sintering.It is found that the calcining temperature of the precursor powders is the key factor affecting the purity of ␤ -Al 2O 3.The content of ␤ -Al 2O 3decreased with increasing calcining temperature of the precursor powders,due to soda loss and composition change.The sintered pellets prepared from the powders calcined at 850◦C exhibited a purity of 94.4%␤ -Al 2O 3.Microwave sintering of Na-␤ -Al 2O 3is a cost-effective and 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