TEM and STEM investigations of SrO-doped Sr(Ti,Nb)O3-δ thermoelectrics

Sr(Ti1-xNbx)O3-δ solid solutions are promising materials for n-type high-temperature thermoelectrics1. In our study 10 mol% of SrO excess was added to stoichiometric composition with x=0.2 in order to introduce Ruddlesden-Popper (RP) type-planar faults2,3 into the material, thus minimizing thermal conductivity. TEM and STEM were used to study possible ordering and/or distribution of Nb on Ti sites in the perovskite structure. All results were obtained in a Jeol ARM-200F with a CFEG and Cs probe corrector. HAADF imaging was performed at angles from 70 to 175 mrad, while ABF imaging from 11 to 23 mrad. EDXS spectra were acquired using JEOL Centurio Dry SD100GV SDD Detector. RP planar faults, as viewed along [001] zone axis, are shown in HRTEM micrograph in figure 1. The commonly observed number of perovskite unit cells between the planar faults is >2, which corresponds to various homologous compounds with the formula Srn+1(Ti,Nb)nO3n+1. However, solid solution Sr(Ti,Nb)O3-type grains with no RP faults can also be observed (bottom inset in Fig. 1). A HR HAADF STEM image of ordered RP faults (Fig. 2) shows that while the measured intensities of individual Sr atomic columns along a single fault do not scatter significantly, the (Ti,Nb)O atom columns exhibit quite large differences in measured intensities, thus indicating significant variation in Nb and Ti content within a single atom column. Quantitative analysis of measured intensities is in progress. The comparison between simultaneously acquired HAADF and ABF images of a single RP fault is shown in figure 3. While pure oxygen atomic columns cannot be resolved in the HAADF image, they can be readily observed using ABF imaging. The positions of oxygen atom columns along the planar faults are in full agreement with the structural model of a RP planar fault. Additional information on Nb distribution within perovskite matrix/RP faults was obtained by EDXS. While low magnification EDXS mappings show enrichment of Sr at RP faults accompanied by a corresponding decrease in Ti and Nb content, atom-resolved EDXS mappings confirm that individual mixed (Ti,Nb)O atom columns contain different Nb content (annotated atom column). Additionally, the spot EDXS line analysis (net counts) again shows much larger scatter in accumulated net counts for Ti as compared with Sr. The results being presented clearly show that no Nb is incorporated into the SrO RP faults and that the Nb is inhomogeneously incorporated within (Ti,Nb)O atom columns

Supplementary data for the article: Samphao, A.; Kunpatee, K.; Prayoonpokarach, S.; Wittayakun, J.; Švorc, L.; Stankovic, D. M.; Zagar, K.; Ceh, M.; Kalcher, K. An Ethanol Biosensor Based on Simple Immobilization of Alcohol Dehydrogenase on Fe3O4"Au Nanoparticles. Electroanalysis 2015, 27 (12), 2829–2837. https://doi.org/10.1002/elan.201500315

Supporting information for: [ https://doi.org/10.1002/elan.201500315]Related to published version:[http://cherry.chem.bg.ac.rs/handle/123456789/2034

TEM and STEM investigations of Sr(Ti,Nb)O3-δ thermoelectric with the addition of CaO and SrO

It is known that thermoelectric properties, i.e. figure of merit ZT of oxide-based polycrystalline thermoelectric materials can be improved by introducing planar faults into the microstructure of these materials. It is assumed that in-grown planar faults will reduce thermal conductivity without reducing electrical properties which would consequently increase the ZT value. In order to successfully tailor thermoelectric properties of chosen thermoelectric materials, it is prerequisite to know the structure and chemical composition of introduced planar faults. This is why we used HR TEM and HAADF STEM imaging with EDXS in order to study structure and chemical composition of the Ruddlesden-Popper-type (RP) planar faults1,2 in Sr(Ti,Nb)O3-d (STNO) thermoelectric material with the addition of SrO and/or CaO. All results were obtained in a Jeol ARM-200F with a CFEG and Cs probe corrector. HAADF imaging was performed at angles from 70 to 175 mrad (ADF from 42 to 168 mrad). EDX spectra were acquired using JEOL Centurio Dry SD100GV SDD Detector. TEM bright-field images of pure STNO showed that the STNO solid solution grains contained no planar faults of any kind. Furthermore, the interfaces between the grains were clean with no observable interface phase. However, when SrO and/or CaO were added to the STNO, various nanostructured features were observed. In SrO-doped STNO, one can observe three distinctly different regions, i.e. the STNO solid solution, the regions with ordered SrO faults and the region with a network of random SrO planar faults (Figure 1). In the ordered regions one SrO layer is always followed by two perovskite STNO blocks, which corresponds to the Sr3(Ti1-xNbx)2O7 RP-type phase in which Nb and Ti occupy the same crystallographic site. While the measured HAADF intensities across Sr atomic columns at the RP fault do not scatter significantly, the mixed (Ti1-xNbx)O6 atom columns on the other hand exhibit significant differences in measured intensities thus indicating variation in Nb and Ti content within a single mixed atom column (Figure 2). Semi-quantitative HAADF STEM of the perovskite matrix, i.e. the comparison of measured integrated intensities of the atom columns with the calculated intensities showed that that the Nb content on the Ti sites within the perovskite structure varied from app. X=0.05 to X=0.35 (from Sr(Ti0.95Nb0.05)O3-d to Sr(Ti0.65Nb0.35)O3-d). When RP-type planar faults are isolated they run parallel to the {001} low-index zone axes of the perovskite structure. A similar structural phenomenon was observed in STNO with excess of CaO. Again, ordered and/or random 3D networks of RP-type planar faults were observed in the STNO grains (Figure 3). In very thin regions of CaO-doped STNO specimen many orthogonal loops of RP faults were observed that were not detected in SrO doped STNO (Figure 4). The EDX analysis from a single fault and from the matrix showed higher concentration of Ca at the fault. This is in agreement with previously reported investigations3 since smaller Ca ions are easier incorporated at the RP fault than in the perovskite matrix. The TEM and STEM investigations thus confirmed that the addition of SrO and/or CaO to the STNO perovskite solid solution is structurally compensated via the formation of RP-type planar faults within the STNO grains. Finally, thermoelectric measurements confirmed that the existence of RP-type faults in the perovskite STNO matrix reduced the thermal conductivity of this oxide thermoelectric material

Determination of structure and chemistry of long-persistence strontium aluminate phosphor compounds in aberration-corrected tem/stem

Representing a source of short-term stored energy, strontium aluminate phosphor compounds of nominal stoichiometry (SrO)•(Al2O3)2 co-doped with 1 mol% Eu2+ and 1 mol% Dy3+ (SA2ED) exhibit long persistence that is even further extended by the incorporation of boron1. To elucidate the effect of boron on afterglow persistence, we synthesized the phosphor powders using a sol-gel (i.e., modified Pechini) method2 and investigated the chemistry and structure by applying high-resolution STEM imaging, energy dispersive X-ray (EDX) spectroscopy, and electron energy-loss spectroscopy (EELS). Large single-crystal grains were analyzed from as-reduced powders suspended on carbon-coated lacey formvar on copper support grids. Individual crystalline particles were tilted onto a low-index [0001] zone axis and imaged in both high resolution TEM and STEM, using a JEOL JEM-ARM 200CF, equipped with a cold field-emission tip and a probe-side Cs aberration corrector. High-angle annular dark-field (HAADF) images were formed using an annular detector with an inner diameter of 70 mrad and an outer diameter of 175 mrad, while annular bright-field (ABF) images were obtained from an annular detector of 11-mrad inner diameter and 23-mrad outer diameter. EDX spectra were collected using a JEOL Centurio Dry SD100GV SDD detector. EELS analysis was enabled by a Gatan GIF Quantum ER spectrometer. Rietveld refinement of XRD spectra obtained from the powders revealed a mixture of (SrO)4•(Al2O3)7, (SrO)•(Al2O3)2 , and (SrO)•(Al2O3)6 phases. Single crystal particles of the (SrO)•(Al2O3)6 phase were the most stable and allowed for tilting onto the [0001] zone axis for qualitative identification of the atomic columns in HAADF and ABF micrographs. Quantitative image simulations of the measured intensities are in progress. Local variations were observed in the energy loss near-edge fine structure of the B-K, O-K, Al-L2,3 edges

Structural features and thermoelectric properties of Al-doped (ZnO)(5)In2O3 homologous phases

International audienceIn this work, we investigated the influence of Al doping on the structure of the (ZnO)(5)In2O3 homologous phase and the thermoelectric characteristics of (ZnO)(5)(In1-xAlx)(2)O-3 ceramics for x=0, 0.01, 0.03, 0.05, 0.1, and 0.2, prepared using a classic ceramic procedure and sintering at 1500 degrees C for 2 hours. The Al substituted for In on both the primary sites in the Zn-5(In1-xAlx)(2)O-8 homologous phase, the octahedral sites in the basal-plane inversion boundaries and the trigonal bi-pyramidal sites in the zig-zag inversion boundaries, which resulted in a uniformly increased shrinkage of the unit cell with the additions of Al. The a and c parameters were reduced for x=0.2 by a maximum 0.8%. All the samples had similar microstructures, so the differences in the TE characteristics mainly resulted from the effects of the substitution of Al for In, decreasing the charge-carrier concentration and affecting their mobility. Slightly improved TE characteristics were only observed for Al additions with x=0.01-0.05, while larger additions of Al only resulted in a reduced electrical conductivity and decreased ZT values in comparison to the un-doped composition

Structural features and thermoelectric properties of Al-doped (ZnO)(5)In2O3 homologous phases

In this work, we investigated the influence of Al doping on the structure of the (ZnO)(5)In2O3 homologous phase and the thermoelectric characteristics of (ZnO)(5)(In1-xAlx)(2)O-3 ceramics for x=0, 0.01, 0.03, 0.05, 0.1, and 0.2, prepared using a classic ceramic procedure and sintering at 1500 degrees C for 2 hours. The Al substituted for In on both the primary sites in the Zn-5(In1-xAlx)(2)O-8 homologous phase, the octahedral sites in the basal-plane inversion boundaries and the trigonal bi-pyramidal sites in the zig-zag inversion boundaries, which resulted in a uniformly increased shrinkage of the unit cell with the additions of Al. The a and c parameters were reduced for x=0.2 by a maximum 0.8%. All the samples had similar microstructures, so the differences in the TE characteristics mainly resulted from the effects of the substitution of Al for In, decreasing the charge-carrier concentration and affecting their mobility. Slightly improved TE characteristics were only observed for Al additions with x=0.01-0.05, while larger additions of Al only resulted in a reduced electrical conductivity and decreased ZT values in comparison to the un-doped composition