Rare Earth-Doped SrTio₃ Perovskite Formation from Xerogels

Abstract

A synthesis process of rare earth doped SrTiO₃ by modified sol-gel technique is described. Impervious strontium titanate doped with rare earth was prepared by gelification and calcination of colloidal systems. Powders of thulium substituted strontium titanate (SrTi_1−𝑥Tm_𝑥O₃-δ, where 𝑥=0.005; 0.02; 0.05) were obtained through cohydrolysis of titanium, strontium, and thulium precursors by sol-gel method. The xerogel obtained from the evaporation of solvents was milled and calcinated at 1100°C to give a reactive powder. Pure and doped SrTiO₃ dense disks were formed by uniaxial pressing. Thermogravimetry (TGA), differential scanning calorimetry (DSC) analysis, X-ray diffractometry (XRD), and scanning electron microscopy (SEM) have been used to study the microstructural evolution of amorphous xerogel into crystalline reactive and sinterable powders. Hardness was measured for each membrane 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 highly reactive easy sintering powders used for fully densified, impervious ceramics with high thermal and chemical stability at high temperature.

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 smaller cation (such as Ti⁴⁺, Nb⁵⁺, Mn⁴⁺, and Zr⁴⁺), and 𝑋 is an anion (such as O²⁻, F¹⁻, and Cl¹⁻). In this paper, strontium works as A cation, titanium as B cation, and oxygen as anion. The Ti-ion is surrounded by the octahedron of the oxygen anions and therefore exhibits a 6-fold coordination while the strontium cation shows a 12-fold coordination [1–4]. The individual ionic radii of the constituents can lead to a distortion that modifies the cubic elementary cell into tetragonal, rhombohedral, or orthorhombic structures, which are responsible for numerous properties of perovskite materials. The rare earth titanate perovskites exhibit good thermodynamic stability over large temperature ranges and interesting transport properties which make them suitable for 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 is orthorhombic, going to tetragonal from 65 K to 104 K where it changes again into the cubic structure [8]. This structure is maintained up to the melting point (2083°C), and its density is 5.12 g/cm³ [9]. Strontium titanate is characterized by relatively low electronic and ionic conductivity in a wide range of temperatures [10, 11]. The ionic conductivity in SrTiO₃ could be improved by increasing the oxygen vacancy concentration by acceptor doping of the Ti-site [12]. SrTiO₃ acceptor doping with trivalent elements on the Ti site promotes p-type conduction and increases the level of ionic conductivity [13]. Incorporation of protons into the perovskite lattice can be viewed in terms of acid/base chemistry, and the basicity of the B-site (and A-site) cation also will influence the proton concentration [14]. However, increased basicity can lead to reaction with CO₂ to obtain the corresponding decomposition products [14, 15]. The effect of different acceptor dopants, such as Co and Ga, on the B-site was evaluated [16, 17]. The ionic conductivity results one order of magnitude higher than that of undoped SrTiO₃ [18]. In this paper, a synthesis of SrTiO₃ powders by modified sol-gel technique has been presented. A complete ceramic processing route to dense perovskite material is proposed focusing on the influence of Tm dopant, chosen for its basicity, on the microstructure and properties of sintered strontium titanate. Finally, thermogravimetry differential scanning analyses, in a pure CO₂ atmosphere until 1300°C, were performed to evaluate chemical stability of perovkite dense ceramics.

2. Experimental

2.1. Membrane Preparation

The doped perovskites strontium titanate (SrTi_1−𝑥Tm_𝑥O₃-δ, 𝑥=0.005; 0,02; 0,05), that were abbreviated as STO_x*100 Tm, were prepared using sol-gel method. A 6 wt% alcoholic sol of titanium dioxide (TiO₂) was prepared using titanium tetraisopropoxide Ti(OC₃H₇)₄ (TPOT) as precursor (28 wt%), propanol (58 wt%) as solvent, hydrochloric acid HCl (13 wt%) as catalyst, and distilled water (1 wt%). Part of the propanol was mixed with TPOT, and the remaining part was added to distilled water and HCl. Both mixtures were stirred for 20 min before combining. The alcoholic sol was stirred for 24 h at room temperature to ensure complete mixing and hydrolysis. Then, strontium acetate [Sr(CH₃CO₂)₂] and thulium nitrate pentahydrate [Tm(NO₃)₃·5H₂O] at different molar fraction were added. The dried alcogel was hand milled in a mortar and calcined at 1100°C for 2 h with a heating rate of 3°C/min. After thermal treatment, the powders were milled, in distilled water, by centrifugal ball mill (RETSCH mod.S2) for 2 hours. Polyvinyl alcohol (PVA, 87–89% hydrolysed, 𝑀_𝑤∼130000, Sigma-Aldrich) 2% in weight was added as binder for pressure forming. PVA was mixed with distilled water at 80°C and stirred to dissolve it completely. The milled suspension was dried at 70°C, and a granulated material ready to press was obtained after few minutes of mortar grinding. Green bodies, made by uniaxial pressing at 278 MPa, were heated (heating rate of 1°C/min) to 500°C for 60 min for binder burnout and sintered at 1350°C for 75 min at the rate of 2.9°C/min.

2.2. Characterization

The samples were characterized by thermogravimetry (TGA) and differential scanning calorimetry (DSC) using Simultaneous Thermal Analyzer (STA 409 C, NETZSCH). Tests were performed in air, with a heating rate of 10°C/min, in Pt crucibles, up to 1200°C. The evolution of perovskite phase with calcination temperature was studied by X-ray diffractometry (XRD, Diffractometer Rigaku Ultima), and the Sherrer’s equation was used to calculate the crystallite size [19] taking the full width at half maximum (FWHM) of (110) reflection for strontium titanate as follows: 𝑑=𝑘𝜆,𝛽cos𝜃 (1) where 𝑑 is the crystallite size, 𝑘 is a constant (0.9 assuming that the particles are spherical), 𝜆 is the wavelength of X-ray radiation (𝜆=1.5406 Å), β = FWHM, and 𝜃 is the angle of diffraction. The pore size distribution was measured with the mercury intrusion porosimeter PASCAL 140 and 240 by Thermofinnigan whereas the morphological characterization of the dense ceramic membrane was performed by SEM (Zeiss EVO 40). Moreover, thermomechanical analyses were performed with optical dilatometer (Horizontal Optical Dilatometer ODLT, Expert System Solutions) to evaluate plastic deformation of sample at high temperature. Simultaneous thermal analyzer (STA 409 C, NETZSCH) was used to test the chemical stability of the doped powders treated at 1350°C in pure 1 atm CO₂ flow. Measurements were carried out from room temperature up to 1300°C into Pt crucibles. The heating rate was 10°C/min and the CO₂ flow was 50 mL/min. Finally, Vickers microhardness (Leica VMHT microdurometer) tests were performed to evaluate the mechanical properties of perovskites [20]. A load of 9.8 N was applied for 30 seconds on a mirror finished surface of the sample. At least 10 indentations for each sample were performed.

3. Results and Discussion

As a result of the addition of strontium acetate solution to the titania colloidal suspension, a gel is formed in 2-3 hours. The gelling of the solution can be attributed to the change of stability occurring when strontium acetate was added to the sol which contains the highly protonated titania nanoparticles with PH ≈ 1. The PH increases up to 5-6. Destabilized titania sol turns to an homogemeous opalescent alcogel that is then dried to a xerogel. Thermogravimetric (TGA) and differential scanning calorimetry (DSC) analyses were carried out to characterize the microstructural evolution of the xerogel to the perovskite powders during the calcination process. In Figure 1, TGA-DSC results of strontium titanate xerogels at Tm 2% wt have been reported. No relevant difference has been observed by changing the rare earth concentration. The main weight loss occurred in the range of 25–500°C. The DSC analysis recorded three endothermic events in the range 25–190°C due to the vaporization of water and decomposition of organics. This corresponds to a weight loss of about 27% on TGA curve. The exothermic peaks from 320 to 500°C can be attributed to the dissociation of the acetate and alkoxide organic groups and correspond to 10% weight loss. The weight loss between 500 and 630°C is attributed to residual carbon burnout. Above 1000°C, the sample became stable suffering few percentages weight loss. The total weight loss is of about 48%. Figure 2 shows the X-ray diffraction patterns of strontium titanate samples. XRD analyses were performed on undoped perovskite powders [STO] calcined at different temperatures 800°C, 920°C, and 1100°C for 2 hours. XRD spectra show that in the range of 800–1100°C nucleation and growth of SrTiO₃ perovskite crystals take place. XRD peaks detected at 800°C are attributed to rutile titania crystals that dissolve upon reaction with other matrix element at higher temperature. The cubic lattice of SrTiO₃ can be easily identified from 920°C. At 920°C, XRD curve shows the onset of crystallization of perovskite phase at 1100°C, the crystallization is ongoing until 𝑇=1500°C for which unidentified crystalline phases were found (Figure 3). The average crystallite size increases with calcination temperature (Table 1). Data reported in Table 1 have been calculated by the Scherrer’s formula, which provides a lower value of the particle size [19]. The thermal analysis and XRD results suggested the choice of 𝑇=1100°C as calcination temperature and of 𝑇=1350°C as sintering temperature. In Figure 4, XRD patterns for doped compositions are reported at sintering temperature of 1100°C and of 1350°C. Diffraction spectra on SrTi_1−𝑥Tm_𝑥O₃ samples at various dopant content show that they form a solid solution, with the same perovskite structure of pure SrTiO₃. The increase of Tm concentration causes no phase change, suggesting that the doping concentration is under the isomorphic substitution limit of thulium (Tm) in strontium titanate lattice. Sintered samples are very hard and compact. In Figure 5, there are SEM micrographs of samples sintered at 1350°C as function of doping thulium concentration. A dense structure with residual and isolated pores is obtained for all RE concentrations. In Table 2, the average pore sizes obtained using mercury intrusion porosimetry are reported. The pores presence does not affect the imperviousness of membranes, as will be shown by vacuum seal tests (10⁻⁶ mbar for 2 h). Moreover, in Figure 5(f) it can be observed the small and uniform grain structure. The three types of samples showed similar thermal expansion coefficient. In Figure 6, the coefficient of thermal expansion (CTE) is reported for of STO_0.5% Tm from room temperature up to 1150°C. The measured thermal expansion coefficient is in good agreement with literature [21, 22]. The average thermal expansion coefficient from room temperature to 1150°C is about 11·10^-6°C⁻¹. The heating and cooling curves are linear and overlap. In conclusion, no deformation or shrinkage occurs at high temperature, that is, the samples have a good thermal stability up to 1150°C. The chemical stability in pure CO₂ flow at atmospheric pressure has been demonstrated until 1300°C by thermogravimetry differential scanning analysis. TGA-DSC curves in Figure 7 show no weight change and the trend of the heat flow is nearly linear. In addition there are no endoexothermic peaks associated to decomposition reactions. All samples doped with different percentages of thulium have the same behavior in CO₂ atmosphere at high temperature. Finally, hardness for each doped membranes sintered at 1350°C was measured with Vickers microindenter. The results obtained are summarized in Table 3; the values calculated are comparable with literature data that attested Vickers hardness at 7.77 GPa for undoped sample SrTiO₃, in the same experimental condition [23].

Table 1 

Particles size calculated by Sherrer’s equation versus calcination temperature.

Table 2 

Average pore dimension on sintered samples at 1350°C measured by mercury porosimeter.

Table 3 

Vickers microhardness of doped perovskites samples.

Figure 1 

TGA (solid line) and DSC (dashed line) profiles for doped strontium titanate powders SrTiO3 with 2 mol% Tm (STO_2% Tm).

Figure 2 

XRD patterns of calcined STO as a function of calcining temperature.

Figure 3 

XRD patterns of STO at room temperature after sintering process at different temperatures.

(a)

(b)

Figure 4 

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

(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.

Figure 6 

Coefficient of thermal expansion (CTE) of STO_Tm0.5%_1350°C (75 min) measured by optical dilatometry.

Figure 7 

TGA-DSC curves in CO2 flow for STO_Tm2%_1350°C.

4. Conclusions

Using sol-gel method, we prepared highly reactive and homogeneus perovskite SrTiO₃ powders. Homogeneous alcogelswere obtained, dried, calcinated, milled, and granulated. Dense materials with homogenous microstructures were prepared by uniaxial pressing of granules derived from calcined powders and sintering at 1350°C. Rare earth doping of cubic lattice of strontium titanate was achieved without any phase segregation. Dense and hard ceramic disks were obtained with high thermal and chemical stability at high temperature from doped and undoped system. Rare earth dopants do not affect stability and thermostructural properties of perovskite. Developed materials and process can be therefore regarded as interesting candidate for application requiring solid ionic conductivity at high temperature and aggressive chemical environment.

Acknowledgments

The authors acknowledge the financial support of ENEL and are grateful to D. Cannoletta from University of Salento for XRD analysis and to Dr. A. Chiechi from Salentec for dilatometric measurements.