Thermodynamics of Nanoscale Calcium and Strontium Titanate Perovskites
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UC Davis UC Davis Previously Published Works Title Thermodynamics of Nanoscale Calcium and Strontium Titanate Perovskites Permalink https://escholarship.org/uc/item/36t333ss Journal Journal of the American Ceramic Society, 96(11) Author Sahu, Sulata Kumari Publication Date 2013-08-22 Peer reviewed eScholarship.org Powered by the California Digital Library University of California J. Am. Ceram. Soc., 1–7 (2013) DOI: 10.1111/jace.12622 © Journal 2013 The American Ceramic Society Thermodynamics of Nanoscale Calcium and Strontium Titanate Perovskites Sulata K. Sahu, Pardha S. Maram, and Alexandra Navrotsky† Peter A. Rock Thermochemistry Laboratory and NEAT ORU, University of California, Davis, California 95616 The surface enthalpies of nanocrystalline CaTiO3 and SrTiO3 anhydrous surfaces were determined using high-temperature perovskites were determined using high-temperature oxide melt solution calorimetry and water adsorption microcalorimetry. solution calorimetry in conjunction with water adsorption calo- rimetry. The nanocrystalline samples were synthesized by a II. Experimental Methods hydrothermal method and characterized using powder X-ray dif- fraction, FTIR spectroscopy, and Brunauer–Emmett–Teller sur- (1) Hydrothermal Synthesis of ATiO3 [A = Ca, Sr] face area measurements. The integral heats of water vapor Phases adsorption on the surfaces of nanocrystalline CaTiO3 and Nanocrystallites of ATiO3 [A = Ca, Sr] were synthesized 15 SrTiO3 are À78.63 Æ 4.71 kJ/mol and À69.97 Æ 4.43 kJ/mol, using a modified procedure described by Srdić et al. respectively. The energies of the hydrous and anhydrous (Fig. 1). The synthesis was carried out in two steps. In the 2 2 surfaces are 2.49 Æ 0.12 J/m and 2.79 Æ 0.13 J/m for first step, Ti(OC4H9)4 (98+%; Alfa Aesar, Ward Hill, MA) 2 2 CaTiO3 and 2.55 Æ 0.15 J/m and 2.85 Æ 0.15 J/m for was dissolved in ethanol (99.5%; Aldrich, St. Louis, MO), in SrTiO3, respectively. The stability of the perovskite compounds a nitrogen filled glove box. The solution was hydrolyzed with in this study is discussed according to the lattice energy and deionized water. The obtained white titania sol was added to tolerance factor approach. The energetics of different perovsk- a 5N NaOH (EMD Chemicals; Fisher Scientific, Pittsburgh, ites suggest that the formation enthalpy becomes more exother- PA) solution under vigorous stirring to precipitate a titanium mic and surface energy increases with an increase in ionic hydroxide gel. An aqueous solution containing A2+ ions radius of the “A” site cation (Ca, Sr, and Ba), or with the tol- [from Sr(NO3)2 (99.0%; Alfa Aesar) and Ca(NO3)2Á4H2O erance factor. PbTiO3 shows a lower surface energy, weaker (99.0%; Alfa Aesar)] was slowly added to the prepared slurry water binding, and a less exothermic enthalpy of formation under vigorous stirring. Reaction between the titanium than the alkaline-earth perovskites. hydroxide particles and the corresponding ions was carried out for 1 h at 438 K for CaTiO3 and 453 K for SrTiO3 in a sealed teflon-lined 45 mL autoclave (Parr Instruments, I. Introduction Moline, IL). The obtained powders were washed several EROVSKITES are a versatile group of compounds, and one times with distilled water to expel sodium ions, and then with P of the most studied ceramic families, as they form a diver- absolute ethanol to decrease the degree of agglomeration by 1–5 sity of materials with various potential applications. CaTiO3 removing free water. The washed and dried powders were and SrTiO3 have attracted considerable attention because they subjected to heat treatments at various temperatures to display numerous outstanding physical properties and poten- obtain materials with varying crystallite size. tial for practical applications.6–8 These materials are of much interest as dielectrics in ceramic capacitors, in dye-sensitized solar cells, for bone implants, and for use in various other (2) Characterization fields.9–11 Hence, it is important to understand their surface Powder XRD patterns of the samples were collected using a thermodynamic properties, which directly influence their syn- Bruker AXS D8 Advance diffractometer (CuKa radiation), thesis, processing, sintering, and function. Although theoreti- (Bruker-AXS Inc., Fitchburg, WI). Data were recorded from cal calculations of the surface energy of CaTiO3 and SrTiO3 10° to 70° (2h) at a step size of 0.02° (2h) and a collection have been made, no experimentally determined surface energy time of 2 s/step. The compositions of the synthesized samples data have been reported. Over the past decade, high-tempera- were measured using wavelength dispersive electron probe ture oxide melt solution calorimetry has been shown to be a microanalysis with a Cameca SX100 (Gennevilliers, France) powerful and convenient method for studying the surface ener- instrument operated at an accelerating voltage of 15 kV, a gies of numerous nanophase oxides.12–14 beam current of 20 nA, and a beam size of 1 lm. Sample This work presents the formation enthalpies and surface homogeneity was analyzed using backscattered electron energies of nanoscale perovskite oxides, CaTiO3 and SrTiO3, images. For the microprobe analysis, CaTiO3 and SrTiO3 and compares their energetics to isostructural BaTiO3 and powders were pelletized and sintered at 1373 K. The sintered PbTiO3. The samples were synthesized by a hydrothermal pellets were polished and carbon coated. CaTiO3 and SrTiO3 method and annealed at different temperatures to produce standards were used to measure Sr, Ca, and Ti contents. The nanocrystalline powders with different grain sizes. Composi- sample compositions were calculated from an average of 10 tion, crystallite size, and surface area were determined using data points per sample. electron microscopy, X-ray diffraction (XRD), and N2 Surface areas of the nanocrystalline CaTiO3 and SrTiO3 adsorption experiments. Enthalpies for the hydrous and were determined by N2 adsorption using the Brunauer– Emmett–Teller (BET) method at 77 K. Ten-point nitrogen adsorption isotherms were collected in a relative pressure range of p/p0 = 0.05–0.3 (where, p0 = saturation pressure) using a Micromeritics ASAP 2020 surface area and porosity T. Vanderah—contributing editor analyzer (Micromeritics Instrument Corp., Norcross, GA). Before analysis, the samples were degassed under vacuum at 300°C for 2 h. The uncertainties in the BET surface area Manuscript No. 33323. Received June 5, 2013; approved August 22, 2013. measurements were propagated from fitting a straight line to † 0 0 Author to whom correspondence should be addressed. e-mail: [email protected] 1/[Q(p /p1)] (Q = quantity adsorbed, mmol/g) versus p/p 1 2 Journal of the American Ceramic Society—Sahu et al. water vapor adsorption was measured at 298 K using a Seta- ram Calvet-type Sensys calorimeter (Setaram Instruments, Caluire, France) coupled with the Micromeritics ASAP 2020 instrument. The calorimeter was calibrated against the enthalpy of fusion of gallium metal. The instrumental design is described in detail elsewhere.17 This technique enables pre- cise gas dosing, volumetric detection of the amount of adsorbed water, and simultaneous measurement of the heat effect. The sample was placed in one side of a specially designed silica glass fork tube so that it could fit into the twin chambers of the microcalorimeter, with the empty tube serving as a reference. The experiment comprised three steps. In the first step, the sample was degassed under vacuum (<0.3 Pa) for 3 h at 573 K for CaTiO3 and 673 K for SrTiO3. The second step involved measurement of the free volume of the sample tube with helium, and the BET surface area of the sample by nitrogen adsorption. In the third step, a water adsorption calorimetry experiment was programmed 2 in incremental dose mode to provide ~2 lmol of H2O/nm per dose. The surface area of each sample used was 2 m2. The integral enthalpy of water adsorption and surface cover- age were calculated from an average of three experiments. A Fig. 1. Experimental procedures for the synthesis of ATiO3 2+ 2+ correction from a blank run was applied to account for (A = Sr and Ca ). water adsorbed onto the instrument manifold, and onto the walls of the sample tube. using the Micromeritics software. The total amount of water (chemisorbed and physisorbed) on the nanocrystalline sam- ples was determined on a set of five samples, each 10–15 mg, (4) High-Temperature Oxide Melt Solution Calorimetry by thermogravimetric analysis using a Netzsch STA 449 The drop solution enthalpies of CaTiO3 and SrTiO3 samples system (Netzsch GmbH, Selb, Germany). The sample was of different crystallite sizes were measured in a custom-made heated in a platinum crucible from 303 to 1173 K in an oxy- isoperibol Tian-Calvet twin microcalorimeter (University of gen atmosphere at 10 K/min. A buoyancy correction was California, Davis, CA) described previously.12,13 The calo- made by subtracting the baseline collected by running an rimeter assembly was flushed with oxygen at 43 mL/min. identical Differential Thermal Analysis/Differential Scanning Oxygen was bubbled through the solvent at 4.5 mL/min to Calorimetry (TGA/DSC) scan with an empty platinum cruci- aid dissolution and maintain oxidizing conditions. Pellets of ble. The water content was determined from the TGA approximately 5 mg were loosely pressed, weighed, and weight-loss curve. The gases that evolved during the thermal dropped from room temperature into 3Na2OÁ4MoO3 molten analysis were analyzed by infrared spectroscopy (IR) using a solvent at 975 K. Measurements were repeated 8–10 times Bruker Equinox 55 FTIR spectrometer (range 400– for each sample to achieve statistically reliable data. The cal- 4000 cmÀ1) coupled to the TGA/DSC by a transfer line orimeter was calibrated against the heat content of 5 mg pel- heated to 423 K. lets of high purity a-Al2O3 (99.997%; Alfa Aesar) dropped into an empty crucible.