A-site Ca/Sr co-doping to optimize room-temperature TCR of La0.7Ca0.3-xSrxMnO3 films

Based on the analysis of crystal structure, Mn³⁺/Mn⁴⁺ pairs, distortion of MnO₆ octahedron, and electrical transport properties of La_1-xCa_xMnO₃ and La_1-xSr_xMnO₃ materials, room-temperature coefficient of resistivity (TCR) of La_0.7Ca_0.3-xSr_xMnO₃ (LCSMO) films was optimized by Ca/Sr co-doping at the A-site. LCSMO films are successfully fabricated on LaAlO₃ (100) substrates via facile spin coating technology. The microstructure of LCSMO films is characterized by X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, atomic force microscopy and X-ray photoemission spectroscopy. Results reveal that A-site Ca/Sr co-doping significantly influenced crystal structure, formation of Mn³⁺/Mn⁴⁺ pairs, and distortion of MnO₆ octahedron. The correlation between microstructure and electrical transport properties was explained through the phenomenological percolation model, double-exchange mechanism and Jahn-Teller effect. Furthermore, the TCR reached 10.2% K^-1 at 296.1 K in La_0.7Ca_0.18Sr_0.12MnO₃ films.     

Formation of single-crystalline BaTiO3 nanorods from glycolate by tuning the supersaturation conditions

We have studied peculiarities in the formation of single-crystalline barium titanate (BaTiO₃) nanorods from a glycolate-mediated complex via a single-step hydrothermal process under different supersaturation (S_R) conditions. X-ray diffraction (XRD) showed the formation of pure BaTiO₃ with an S_R of above 19. The tetragonality for the BaTiO₃ (c/a) reached 1.013 at S_R = 19–29 and dropped to 1.010 for S_R = 39. According to the transmission electron microscopy (TEM) and XRD analyses, the rod-shaped particles exhibited single crystallinity and crystal growth along the [001] plane. With scanning electron microscopy (SEM), the morphological evolution from a plate-shaped intermediate precursor (S_R = 6–9) to a rod-shaped product with an aspect ratio of 6–9 (S_R = 19–29), and to non-polar material with an irregular structure (S_R = 39), was observed. The negative slope, linear dependence of the particles’ width and length on the supersaturation level in the range S_R = 19–39 was established for the first time. The replacement of the prevailing crystallization mechanism from in-situ topotactic transformation into dissolution-precipitation above S_R = 19 was observed. It was shown that with a simple regulation of the S_R, the structural and morphological characteristics of the obtained BaTiO₃ nanoparticle can be effectively tuned.     

Grain boundary engineered,multilayer graphene incorporated LaCoO3 composites with enhanced thermoelectric properties

Enhancement of thermoelectric properties by virtue of decreased electrical resistance through grain boundary engineering is realised in this study. A robust strategy of optimisation of the transport properties by tuning the energy filtering effects at the interfaces by decreasing the interfacial electrical resistance is achieved in LaCoO₃ (LCO). This is accomplished by the incorporation of multilayer graphene within the parent LCO matrix containing multi-scale nano/micro grains. The present work has attained a substantial increment in electrical conductivity from a value of 96 Scm^-1 for bare LCO to ～5300 Scm^-1 at 750 K by incorporating 0.08 wt% multilayer graphene in LCO. No significant change in thermal conductivity is observed due to the presence of multilayer graphene in LCO. A zT of 0.33 at 550 K for 0.08 wt% multi-layer graphene incorporated LCO composite is achieved which is the highest thermoelectric figure of merit value for undoped LCO reported until now.     

Nonlinearity in regulating the metal to insulator transition of ReNiO3 towards low temperature range

Among the existing material family of the correlated oxides, the rare earth nickelates (ReNiO₃) exhibit broadly adjustable metal to insulator transition (MIT) properties that enables correlated electronic applications, such as thermistors, thermochromics, and logical devices. Nevertheless, how to accurately control the critical temperature (T_MIT) of ReNiO₃ via the co-occupation of the rare-earth elements is yet worthy to be further explored. Herein, we demonstrate the non-linearity in adjusting the T_MIT of ReNiO₃ towards lower temperatures via introducing Pr co-occupation within ReNiO₃ (e.g., Pr_xNd_1-xNiO₃ and Pr_xSm_1-xNiO₃) as synthesized by KCl molten-salt assisted high oxygen pressure reaction approach. Although the T_MIT is effectively reduced via Pr substitution, it does not strictly follow a linear relationship, in particular, when there is large difference in the ionic radius of the co-occupation rare-earth elements. Furthermore, the most significant deviation in T_MIT from the expected linear relationship appears at an equal co-occupation ratio of the two different rare-earth elements, while the abruption in the variation of resistivity across T_MIT is also reduced. The present work highlights the importance to use adjacent rare-earth elements with co-occupation ratio away from 1:1 for achieving more linear adjustment in designing the metal to insulator transition properties for ReNiO₃.     

An asymmetrically substituted dithieno[3,2-b:2',3'-d]pyrrole organic small-molecule hole-transporting material for high-performance perovskite solar cells

Hole-transporting materials play a vital role in terms of the performance of perovskite solar cells (PSCs). The dithieno[3,2-b:2',3'-d]pyrrole (DTP), an S,N-heterocyclic building block, has been proved to be desirable for molecular design of hole-transporting materials in PSCs. We developed an asymmetrically substituted DTP small-molecule (JW12) and a reference compound (JW11). The asymmetrical structure of JW12 leads to different absorption properties and electron distribution. The device in a planar n-i-p architecture using JW12 shows a much higher PCE (18.07%) than that based on JW11 (15.46%), which is also better than the device based on spiro-OMeTAD (17.47%). We hope our research can provide a new perspective in molecular design of organic HTMs for perovskite solar cells.     

Effects of various barium precursors and promoters on catalytic activity of Ba-Ti perovskite catalysts for oxidative coupling of methane

The influences of barium precursor and promoter type on the catalytic performance of perovskite catalysts in OCM reaction were studied. Catalysts (BaTiPO₃, P: promoter) were prepared by carbonate, hydroxide and propionate precursors of barium and SnCl₂ and CeO₂ as promoters by sol-gel method, tested in a fixed-bed microreactor and characterized by XRD, BET, CO₂-TPD, FT-IR and UV-Visible analysis. The experiment results showed that based on the extent of effect upon catalyst efficiency, the barium anions can be ranked as; propionate > carbonate > hydroxide, and the CeO₂ promoted catalysts were more active than the SnCl₂ promoted ones. The characterization results showed that the substitution of metal precursors caused formation of different phases with different particle sizes, influenced the basicity of the catalysts, resulted in the appearance of the peaks corresponding to different groups in IR spectroscopy, and shifted the absorption peaks in UV-Visible spectra. These results suggested that OCM reaction over perovskite catalysts is structure sensitive and depended on the type of used precursor and promoter.     

Influence of manganese substitution into the A-site of perovskite type Ca1−xMnxTiO3 ceramic

Ca_1−xMn_xTiO₃ (x = 0–1.0) perovskite ceramics were prepared by conventional solid state reaction. XRD was used to confirm the microcrystalline nature of the Ca_1−xMn_xTiO₃ crystals. For the x = 0 composition, the XRD patterns were those of a single orthorhombic perovskite while for x = 0.2–0.8, the XRD spectra were those of two orthorhombic perovskite phases: CaTiO₃ and MnTiO₃. For x = 1, XRD pattern was that of the MnTiO₃ phase only. The morphology and particle size of the grains of the different composition were observed using SEM. The size of the particles increased from 0.2 μm to 2–3 μm as x increased from 0 to 0.6. The room temperature dielectric constant at the frequency of 110 kHz for the x = 0.2 and x = 1.0 ceramics were ∼3.41 × 10⁴ and ∼4.99 × 10³, respectively. The ESR linewidth of samples increased with increasing manganese content due to the formation of magnetic cluster. Our ESR studies indicate that the manganese ions are in the Mn⁴⁺ state.     

La0.6Ca0.4CoO3, La0.1Ca0.9MnO3 and LaNiO3 as bifunctional oxygen electrodes

A series of perovskite catalysts was investigated for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in alkaline electrolyte and at room temperature, supplied by oxygen or air. A meniscus cell was used to screen-test candidate catalysts for their bifunctionality and assess their activity for ORR at 3 mm depth of immersion (DOI) in the electrolyte. Based on the meniscus data LaNiO₃, La_0.1Ca_0.9MnO₃ and La_0.6Ca_0.4CoO₃ were selected for further assessment in microelectrode and half-cell studies. Activity tests for the ORR and OER, Tafel slopes at high current densities and apparent activation energies for the ORR were determined using a microelectrode technique on samples of the selected perovskites, La_0.1Ca_0.9MnO₃, La_0.6Ca_0.4CoO₃ and LaNiO₃ with and without graphite support. Tafel slopes of ca. 120 mV per decade and apparent activation energies of approximately 18 kcal mol⁻¹ were measured at high cathodic current densities. Cycle-life and performance of La_0.1Ca_0.9MnO₃, La_0.6Ca_0.4CoO₃ and LaNiO₃-based gas-diffusion electrodes in half-cell configurations were tested at a constant current density of 25 mA cm⁻² with subsequent and intermittent polarizations. Similar activities resulted in the ORR, while increased numbers of cycles were observed for the La_0.1Ca_0.9MnO₃-based electrode. Furthermore, electrode material compositions, especially PTFE contents were optimized to conform to the establishment of the three phase interactions of the electrode structure. Transmission Electron microscopy (TEM) and BET-surface area analyses were carried out in order to find out the morphological and surface properties of the perovskite materials.     

Decomposition of nitric oxide over perovskite oxide catalysts: effect of CO2, H2O and CH4

Direct nitric oxide decomposition over perovskites is fairly slow and complex, its mechanism changing dramatically with temperature. Previous kinetic study for three representative compositions (La_0.87Sr_0.13Mn_0.2Ni_0.8O_3−δ, La_0.66Sr_0.34Ni_0.3Co_0.7O_3−δ and La_0.8Sr_0.2Cu_0.15Fe_0.85O_3−δ) has shown that depending on the temperature range, the inhibition effect of oxygen either increases or decreases with temperature. This paper deals with the effect of CO₂, H₂O and CH₄ on the nitric oxide decomposition over the same perovskites studied at a steady-state in a plug-flow reactor with 1 g catalyst and total flowrates of 50 or 100 ml/min of 2 or 5% NO. The effect of carbon dioxide (0.5–10%) was evaluated between 873 and 923 K, whereas that of H₂O vapor (1.6 or 2.5%) from 723 to 923 K. Both CO₂ and H₂O inhibit the NO decomposition, but inhibition by CO₂ is considerably stronger. For all three catalysts, these effects increase with temperature. Kinetic parameters for the inhibiting effects of CO₂ and H₂O over the three perovskites were determined. Addition of methane to the feed (NO/CH₄=4) increases conversion of NO to N₂ about two to four times, depending on the initial NO concentration and on temperature. This, however, is still much too low for practical applications. Furthermore, the rates of methane oxidation by nitric oxide over perovskites are substantially slower than those of methane oxidation by oxygen. Thus, perovskites do not seem to be suitable for catalytic selective NO reduction with methane.     

Deterioration behavior analysis of dysprosium and thulium co-doped barium titanate ceramics for multilayer ceramic capacitors

An accelerated testing method for barium titanate (BaTiO₃) dielectrics was proposed to elucidate deterioration behavior of dielectric constant based on the life-temperature relation. The accelerated degradation test (ADT) which was designed using various temperature ranges below and above Curie temperature (T_c) was focused on the optimized composition of dysprosium (Dy) and thulium (Tm) co-doped BaTiO₃. The statistical analysis of the failure time data was performed to determine the optimum distribution as a goodness-of-fitness test. A scale parameter (η) and activation energy (E_α) were calculated in order to predict the life time of the co-doped BaTiO₃, and there was difference between the expected life times according to the acceleration temperature rating of the ADT. The difference of deterioration mechanism around T_c could be deduced from the change of lattice parameter and polarization behavior. The drastic decrease of tetragonality and ferroelectric property caused by the phase transition of the co-doped BaTiO₃ was verified in the temperature above T_c. Accordingly, the acceleration factor over T_c should be considered as reliability study of the BaTiO₃ dielectrics for multilayer ceramic capacitors (MLCCs).