Download pdf - RareEarth-Doped SrTiO3 PerovskiteFormationfromXerogelsstrontium titanate. Finally, thermogravimetry diﬀerential scanning analyses, in a pure CO 2 atmosphere until 1300 C, were performed

International Scholarly Research NetworkISRN CeramicsVolume 2012, Article ID 926537, 6 pagesdoi:10.5402/2012/926537

Research Article

Rare Earth-Doped SrTiO3 Perovskite Formation from Xerogels

Anastasia Rocca,1 Antonio Licciulli,1 Monia Politi,2 and Daniela Diso3

1 Department of Innovation Engineering, University of Salento, Via per Monteroni, 73100 Lecce, Italy2 Research Centre ENEL, Litoranea Salentina Brindisi-Casalabate, Cerano, 72020 Tuturano, Italy3 Salentec srl, Via dell’Esercito, 73020 Cavallino, Italy

Correspondence should be addressed to Anastasia Rocca, [email protected]

Received 23 August 2012; Accepted 21 September 2012

Academic Editors: O. Dymshits and S. Gutzov

Copyright © 2012 Anastasia Rocca et al. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

A synthesis process of rare earth doped SrTiO3 by modified sol-gel technique is described. Impervious strontium titanate dopedwith rare earth was prepared by gelification and calcination of colloidal systems. Powders of thulium substituted strontium titanate(SrTi1−xTmxO3-δ, where x = 0.005; 0.02; 0.05) were obtained through cohydrolysis of titanium, strontium, and thulium precursorsby sol-gel method. The xerogel obtained from the evaporation of solvents was milled and calcinated at 1100◦C to give a reactivepowder. Pure and doped SrTiO3 dense disks were formed by uniaxial pressing. Thermogravimetry (TGA), differential scanningcalorimetry (DSC) analysis, X-ray diffractometry (XRD), and scanning electron microscopy (SEM) have been used to study themicrostructural evolution of amorphous xerogel into crystalline reactive and sinterable powders. Hardness was measured for eachmembrane by a Vickers microindenter. Dilatometric and TGA-DSC in pure CO2 flow tests have been performed to evaluate,respectively, the thermal and chemical stability of the material. The optimized preparation route has allowed to synthesize highlyreactive easy sintering powders used for fully densified, impervious ceramics with high thermal and chemical stability at hightemperature.

1. Introduction

Perovskite is an ideal face-centered cubic crystal structure,which has ABX3 stoichiometry; where A is a bigger cation(such as Na1+, K1+, Ca2+, Sr2+, and Ba2+), B is a smallercation (such as Ti4+, Nb5+, Mn4+, and Zr4+), and X is ananion (such as O2−, F1−, and Cl1−). In this paper, strontiumworks as A cation, titanium as B cation, and oxygen asanion. The Ti-ion is surrounded by the octahedron of theoxygen anions and therefore exhibits a 6-fold coordinationwhile the strontium cation shows a 12-fold coordination[1–4]. The individual ionic radii of the constituents canlead to a distortion that modifies the cubic elementary cellinto tetragonal, rhombohedral, or orthorhombic structures,which are responsible for numerous properties of perovskitematerials. The rare earth titanate perovskites exhibit goodthermodynamic stability over large temperature ranges andinteresting transport properties which make them suitablefor various applications in electronics, ionic membranes,solid ion conductors, and sensing devices [5–7]. For SrTiO3,

there are three possible crystal structures: up to 65 K it isorthorhombic, going to tetragonal from 65 K to 104 K whereit changes again into the cubic structure [8]. This structureis maintained up to the melting point (2083◦C), and itsdensity is 5.12 g/cm3 [9]. Strontium titanate is characterizedby relatively low electronic and ionic conductivity in awide range of temperatures [10, 11]. The ionic conductivityin SrTiO3 could be improved by increasing the oxygenvacancy concentration by acceptor doping of the Ti-site[12]. SrTiO3 acceptor doping with trivalent elements on theTi site promotes p-type conduction and increases the levelof ionic conductivity [13]. Incorporation of protons intothe perovskite lattice can be viewed in terms of acid/basechemistry, and the basicity of the B-site (and A-site) cationalso will influence the proton concentration [14]. However,increased basicity can lead to reaction with CO2 to obtain thecorresponding decomposition products [14, 15]. The effectof different acceptor dopants, such as Co and Ga, on theB-site was evaluated [16, 17]. The ionic conductivity resultsone order of magnitude higher than that of undoped SrTiO3

2 ISRN Ceramics

−50

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−40

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Figure 1: TGA (solid line) and DSC (dashed line) profiles for doped strontium titanate powders SrTiO3 with 2 mol% Tm (STO 2% Tm).

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SrTiO3

(110)(111)

(200)(210)

(211) (220)

2θ (deg)

T = 1100◦C

T = 920◦C

T = 800◦C

Figure 2: XRD patterns of calcined STO as a function of calciningtemperature.

[18]. In this paper, a synthesis of SrTiO3 powders by modifiedsol-gel technique has been presented. A complete ceramicprocessing route to dense perovskite material is proposedfocusing on the influence of Tm dopant, chosen for itsbasicity, on the microstructure and properties of sinteredstrontium titanate. Finally, thermogravimetry differentialscanning analyses, in a pure CO2 atmosphere until 1300◦C,were performed to evaluate chemical stability of perovkitedense ceramics.

2. Experimental

2.1. Membrane Preparation. The doped perovskites stron-tium titanate (SrTi1−xTmxO3-δ, x = 0.005; 0,02; 0,05),that were abbreviated as STO x∗100 Tm, were preparedusing sol-gel method. A 6 wt% alcoholic sol of titaniumdioxide (TiO2) was prepared using titanium tetraisopropox-ide Ti(OC3H7)4 (TPOT) as precursor (28 wt%), propanol

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SrTiO3

2θ (deg)

T = 1500◦C

T = 1350◦C

T = 1200◦C

T = 1100◦C

Figure 3: XRD patterns of STO at room temperature after sinteringprocess at different temperatures.

(58 wt%) as solvent, hydrochloric acid HCl (13 wt%) ascatalyst, and distilled water (1 wt%). Part of the propanolwas mixed with TPOT, and the remaining part was addedto distilled water and HCl. Both mixtures were stirred for20 min before combining. The alcoholic sol was stirredfor 24 h at room temperature to ensure complete mixingand hydrolysis. Then, strontium acetate [Sr(CH3CO2)2] andthulium nitrate pentahydrate [Tm(NO3)3·5H2O] at differentmolar fraction were added. The dried alcogel was handmilled in a mortar and calcined at 1100◦C for 2 h witha heating rate of 3◦C/min. After thermal treatment, thepowders were milled, in distilled water, by centrifugal ballmill (RETSCH mod.S2) for 2 hours. Polyvinyl alcohol (PVA,87–89% hydrolysed, Mw ∼130000, Sigma-Aldrich) 2% inweight was added as binder for pressure forming. PVA wasmixed with distilled water at 80◦C and stirred to dissolve itcompletely. The milled suspension was dried at 70◦C, anda granulated material ready to press was obtained after few

ISRN Ceramics 3

SrTiO3

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2θ (deg)

STO 5% Tm 1100◦C

STO 2% Tm 1100◦C

STO 0.5% Tm 1100◦C

(a)

0100020003000400050006000700080009000

10000110001200013000

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2θ (deg)

SrTiO3

STO 5% Tm 1350◦C

STO 2% Tm 1350◦C

STO 0.5% Tm 1350◦C

(b)

Figure 4: X-ray diffraction patterns of doped strontium titanate samples (a) calcined at 1100◦C and (b) sintered at 1350◦C.

Table 1: Particles size calculated by Sherrer’s equation versuscalcination temperature.

Calcination temperature(◦C)

Crystallite size(nm) (±10%)

800 28.6

920 39.9

1100 49.9

1200 54.4

1350 68.6

1500 70.7

Table 2: Average pore dimension on sintered samples at 1350◦Cmeasured by mercury porosimeter.

SampleAverage pore diameter

(μm)

STO 0.5% Tm 5.40

STO 2% Tm 0.26

STO 5% Tm 0.04

Table 3: Vickers microhardness of doped perovskites samples.

SampleVickers hardness

(GPa)

STO 0.5% Tm 1350◦C 9.61 ± 0.86STO 2% Tm 1350◦C 9.76 ± 0.56STO 5% Tm 1350◦C 9.16 ± 0.73

minutes of mortar grinding. Green bodies, made by uniaxialpressing at 278 MPa, were heated (heating rate of 1◦C/min)to 500◦C for 60 min for binder burnout and sintered at1350◦C for 75 min at the rate of 2.9◦C/min.

2.2. Characterization. The samples were characterized bythermogravimetry (TGA) and differential scanning calor-imetry (DSC) using Simultaneous Thermal Analyzer (STA409 C, NETZSCH). Tests were performed in air, with aheating rate of 10◦C/min, in Pt crucibles, up to 1200◦C. Theevolution of perovskite phase with calcination temperaturewas studied by X-ray diffractometry (XRD, DiffractometerRigaku Ultima), and the Sherrer’s equation was used tocalculate the crystallite size [19] taking the full width athalf maximum (FWHM) of (110) reflection for strontiumtitanate as follows:

d = kλ(β cos θ

) , (1)

where d is the crystallite size, k is a constant (0.9 assumingthat the particles are spherical), λ is the wavelength of X-ray radiation (λ = 1.5406 Å), β = FWHM, and θ is theangle of diffraction. The pore size distribution was measuredwith the mercury intrusion porosimeter PASCAL 140 and240 by Thermofinnigan whereas the morphological charac-terization of the dense ceramic membrane was performedby SEM (Zeiss EVO 40). Moreover, thermomechanical anal-yses were performed with optical dilatometer (HorizontalOptical Dilatometer ODLT, Expert System Solutions) toevaluate plastic deformation of sample at high temperature.Simultaneous thermal analyzer (STA 409 C, NETZSCH) wasused to test the chemical stability of the doped powderstreated at 1350◦C in pure 1 atm CO2 flow. Measurementswere carried out from room temperature up to 1300◦C intoPt crucibles. The heating rate was 10◦C/min and the CO2flow was 50 mL/min. Finally, Vickers microhardness (LeicaVMHT microdurometer) tests were performed to evaluatethe mechanical properties of perovskites [20]. A load of 9.8 Nwas applied for 30 seconds on a mirror finished surface ofthe sample. At least 10 indentations for each sample wereperformed.

4 ISRN Ceramics

(a) (b)

(c) (d)

(e) (f)

Figure 5: SEM micrographs of the samples sintered at 1350◦C: (a) and (b) STO 0.5% Tm; (c) and (d) STO 2% Tm; (e) and (f) STO 5% Tm.

3. Results and Discussion

As a result of the addition of strontium acetate solutionto the titania colloidal suspension, a gel is formed in 2-3hours. The gelling of the solution can be attributed to thechange of stability occurring when strontium acetate wasadded to the sol which contains the highly protonated titaniananoparticles with PH ≈ 1. The PH increases up to 5-6.Destabilized titania sol turns to an homogemeous opalescentalcogel that is then dried to a xerogel. Thermogravimetric(TGA) and differential scanning calorimetry (DSC) anal-yses were carried out to characterize the microstructuralevolution of the xerogel to the perovskite powders duringthe calcination process. In Figure 1, TGA-DSC results ofstrontium titanate xerogels at Tm 2% wt have been reported.

No relevant difference has been observed by changing therare earth concentration. The main weight loss occurredin the range of 25–500◦C. The DSC analysis recordedthree endothermic events in the range 25–190◦C due tothe vaporization of water and decomposition of organics.This corresponds to a weight loss of about 27% on TGAcurve. The exothermic peaks from 320 to 500◦C can beattributed to the dissociation of the acetate and alkoxideorganic groups and correspond to 10% weight loss. Theweight loss between 500 and 630◦C is attributed to residualcarbon burnout. Above 1000◦C, the sample became stablesuffering few percentages weight loss. The total weight loss isof about 48%. Figure 2 shows the X-ray diffraction patternsof strontium titanate samples. XRD analyses were performedon undoped perovskite powders [STO] calcined at different

ISRN Ceramics 5

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CT

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−1)

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Figure 6: Coefficient of thermal expansion (CTE) ofSTO Tm0.5% 1350◦C (75 min) measured by optical dilatometry.

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STO Tm2% 1350 ◦C

Figure 7: TGA-DSC curves in CO2 flow for STO Tm2% 1350◦C.

temperatures 800◦C, 920◦C, and 1100◦C for 2 hours. XRDspectra show that in the range of 800–1100◦C nucleation andgrowth of SrTiO3 perovskite crystals take place. XRD peaksdetected at 800◦C are attributed to rutile titania crystalsthat dissolve upon reaction with other matrix element athigher temperature. The cubic lattice of SrTiO3 can beeasily identified from 920◦C. At 920◦C, XRD curve showsthe onset of crystallization of perovskite phase at 1100◦C,the crystallization is ongoing until T = 1500◦C for whichunidentified crystalline phases were found (Figure 3). Theaverage crystallite size increases with calcination temperature(Table 1). Data reported in Table 1 have been calculatedby the Scherrer’s formula, which provides a lower valueof the particle size [19]. The thermal analysis and XRDresults suggested the choice of T = 1100◦C as calcinationtemperature and of T = 1350◦C as sintering temperature.In Figure 4, XRD patterns for doped compositions arereported at sintering temperature of 1100◦C and of 1350◦C.

Diffraction spectra on SrTi1−xTmxO3 samples at variousdopant content show that they form a solid solution, with thesame perovskite structure of pure SrTiO3. The increase of Tmconcentration causes no phase change, suggesting that thedoping concentration is under the isomorphic substitutionlimit of thulium (Tm) in strontium titanate lattice. Sinteredsamples are very hard and compact. In Figure 5, thereare SEM micrographs of samples sintered at 1350◦C asfunction of doping thulium concentration. A dense structurewith residual and isolated pores is obtained for all REconcentrations. In Table 2, the average pore sizes obtainedusing mercury intrusion porosimetry are reported. The porespresence does not affect the imperviousness of membranes,as will be shown by vacuum seal tests (10−6 mbar for2 h). Moreover, in Figure 5(f) it can be observed the smalland uniform grain structure. The three types of samplesshowed similar thermal expansion coefficient. In Figure 6,the coefficient of thermal expansion (CTE) is reported forof STO 0.5%Tm from room temperature up to 1150◦C. Themeasured thermal expansion coefficient is in good agreementwith literature [21, 22]. The average thermal expansioncoefficient from room temperature to 1150◦C is about11·10−6◦C−1. The heating and cooling curves are linear andoverlap. In conclusion, no deformation or shrinkage occursat high temperature, that is, the samples have a good thermalstability up to 1150◦C. The chemical stability in pure CO2flow at atmospheric pressure has been demonstrated until1300◦C by thermogravimetry differential scanning analysis.TGA-DSC curves in Figure 7 show no weight change andthe trend of the heat flow is nearly linear. In addition thereare no endoexothermic peaks associated to decompositionreactions. All samples doped with different percentages ofthulium have the same behavior in CO2 atmosphere at hightemperature. Finally, hardness for each doped membranessintered at 1350◦C was measured with Vickers microinden-ter. The results obtained are summarized in Table 3; thevalues calculated are comparable with literature data thatattested Vickers hardness at 7.77 GPa for undoped sampleSrTiO3, in the same experimental condition [23].

4. Conclusions

Using sol-gel method, we prepared highly reactive andhomogeneus perovskite SrTiO3 powders. Homogeneousalcogels were obtained, dried, calcinated, milled, and gran-ulated. Dense materials with homogenous microstructureswere prepared by uniaxial pressing of granules derived fromcalcined powders and sintering at 1350◦C. Rare earth dopingof cubic lattice of strontium titanate was achieved withoutany phase segregation. Dense and hard ceramic disks wereobtained with high thermal and chemical stability at hightemperature from doped and undoped system. Rare earthdopants do not affect stability and thermostructural prop-erties of perovskite. Developed materials and process can betherefore regarded as interesting candidate for applicationrequiring solid ionic conductivity at high temperature andaggressive chemical environment.

6 ISRN Ceramics

Acknowledgments

The authors acknowledge the financial support of ENELand are grateful to D. Cannoletta from University of Salentofor XRD analysis and to Dr. A. Chiechi from Salentec fordilatometric measurements.

References

[1] Y. Yang, Y. Sun, and Y. Jiang, “Structure and photocatalyticproperty of perovskite and perovskite-related compounds,”Materials Chemistry and Physics, vol. 96, no. 2-3, pp. 234–239,2006.

[2] F. S. Galasso, Perovskites and High Tc Superconductors, Gordonand Breach, New York, NY, USA, 1990.

[3] L. M. Feng, L. Q. Jiang, M. Zhu, H. B. Liu, X. Zhou, and C. H.Li, “Formability of ABO3 cubic perovskites,” Journal of Physicsand Chemistry of Solids, vol. 69, no. 4, pp. 967–974, 2008.

[4] A. S. Bhalla, R. Guo, and R. Roy, “The perovskite structure—areview of its role in ceramic science and technology,” MaterialsResearch Innovations, vol. 4, no. 1, pp. 3–26, 2000.

[5] V. Ravikumar, R. P. Rodrigues, and V. P. Dravid, “An inv-estigation of acceptor-doped grain boundaries in SrTiO3,”Journal of Physics D, vol. 29, no. 7, pp. 1799–1806, 1996.

[6] M. Leonhardt, J. Jamnik, and J. Maier, “In situ monitoring andquantitative analysis of oxygen diffusion through Schottky-barriers in SrTiO3 bicrystals,” Electrochemical and Solid-StateLetters, vol. 2, no. 7, pp. 333–335, 1999.

[7] C. Tragut and K. H. Härdtl, “Kinetic behaviour of resistiveoxygen sensors,” Sensors and Actuators, vol. 4, no. 3-4, pp. 425–429, 1991.

[8] S. N. Ruddlesden and P. Popper, “The compound Sr3TiO7 andits structure,” Acta Crystallographica, vol. 11, article 54, 1958.

[9] R. Wurm, O. Dernovsek, and P. Greil, “Sol-gel derived SrTiO3and SrZrO3 coatings on SiC and C-fibers,” Journal of MaterialsScience, vol. 34, no. 16, pp. 4031–4037, 1999.

[10] T. Kawada, T. Watanabe, A. Kaimai, K. I. Kawamura, Y. Nigara,and J. Mizusaki, “High temperature transport properties inSrTiO3 under an oxygen potential gradient,” Solid State Ionics,vol. 108, no. 1–4, pp. 391–402, 1998.

[11] W. Huang and S. Gopalan, “Bi-layer structures as solid oxidefuel cell interconnections,” Journal of Power Sources, vol. 154,no. 1, pp. 180–183, 2006.

[12] S. Steinsvik, T. Norby, and P. Kofstad, Electroceramics IV,Volume 2, Edited by R. Waser, S. Hoffmann, D. Bonnenberg,C. Hoffmann, Augustinus Buchhandlung, Aachen, Germany,1994.

[13] T. Norby and Y. Larring, “Concentration and transport ofprotons in oxides,” British Ceramic Proceedings, vol. 56, pp. 83–93, 1996.

[14] K. D. Kreuer, “On the development of proton conductingmaterials for technological applications,” Solid State Ionics, vol.97, no. 1–4, pp. 1–15, 1997.

[15] K. Li, Ceramic Membranes for Separation and Reaction, JohnWiley & Sons, 2007.

[16] J. Robertson, “Energy levels of point defects in SrTiO3 andrelated oxides,” Journal of Applied Physics, vol. 93, no. 2, pp.1054–1059, 2003.

[17] R. K. Astala and P. D. Bristowe, “Ab initio calculations ofdoping mechanisms in SrTiO3,” Modelling and Simulation in

Materials Science and Engineering, vol. 12, no. 1, pp. 79–90,2004.

[18] S. Hui and A. Petric, “Electrical conductivity of yttrium-dopedSrTiO3: influence of transition metal additives,” MaterialsResearch Bulletin, vol. 37, no. 7, pp. 1215–1231, 2002.

[19] B. D. Cullity, Elements of X-Ray Diffraction, Addison-Wesley,2nd edition, 1978.

[20] AA.VV., Annual Book of ASTM Standards, E92-82, ASTM,Philadelphia, Pa, USA, 1995.

[21] O. A. Marina, N. L. Canfield, and J. W. Stevenson, “Thermal,electrical, and electrocatalytical properties of lanthanum-doped strontium titanate,” Solid State Ionics, vol. 149, no. 1-2,pp. 21–28, 2002.

[22] Y. Somiya, A. S. Bhalla, and L. E. Cross, “Study of (Sr, Pb)TiO3ceramics on dielectric and physical properties,” InternationalJournal of Inorganic Materials, vol. 3, no. 7, pp. 709–714, 2001.

[23] S. Yamanaka, K. Kurosaki, T. Maekawa, T. Matsuda, S. I.Kobayashi, and M. Uno, “Thermochemical and thermophysi-cal properties of alkaline-earth perovskites,” Journal of NuclearMaterials, vol. 344, no. 1–3, pp. 61–66, 2005.

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