La doped SrTiO3 Based Oxide Thermoelectrics

In this project, the thermoelectric properties of La-doped Sr3Ti2O7, Ca3Ti2O7 and SrTiO3 ceramics sintered in air and N2/5%H2 have been investigated. Different defect chemistry models were studied in an attempt to improve the thermoelectric performance of La-doped SrTiO3 and related systems. For La-doped Sr3Ti2O7 Ruddlesden-Popper (RP) ceramics, the three starting nominal compositions, (Sr1-xLax)3Ti2O7 (electronic donor-doping), (Sr1-3y/2Lay)3Ti2O7 (A-site vacancies) and (Sr1-zLaz)3Ti2-z/4O7 (B-site vacancies) were sintered under air and flowing N2/5%H2 at 1773 K. The La-doped air sintered ceramics were all off-white/yellow in appearance and electrical insulators with low bulk conductivity and a high activation energy, Ea, confirming that solid solubility of La was small and that electronic (donor-doping) compensation does not exist for La-doping of ceramics sintered in air. Processing ceramics under reducing atmosphere is sufficient to form dark single-phase samples for the x series (electronic donor-doping series) up to (Sr0.95La0.05)3Ti2O7 (x = 0.05), indicating that reducing conditions and oxygen-loss from the Sr3Ti2O7 lattice are conducive towards electronic La-doping in Sr3Ti2O7-δ ceramics and to extend solid solubility. In all N2/5%H2 sintered samples, an insulating surface layer associated with SrO volatilization and oxygen up-take (during cooling) from the sintering process occurred that, unless removed, masked the conductive nature of the ceramics. In the bulk, significantly higher power factors were obtained for ceramics that were phase mixtures containing highly conductive (Sr, La)TiO3-δ, ST. This highlights the superior power factor properties of reduced perovskite-type ST compared to reduced RP-type Sr3Ti2O7 and serves as a warning for the need to identify low levels of highly conducting perovskite phases when exploring rare-earth doping mechanisms in RP-type phases. For La-doped SrTiO3, the favoured mechanism for doping was through the formation of A-site vacancies independent of P_(O_2 ). Samples with A-site vacancies (Sr1-3y/2LayTiO3) had the highest electrical conductivity for the same La content (i.e. 10 at. %) sintered at 1773 K, independent of P_(O_2 ). In the Sr1-3y/2LayTiO3 system, air sintered ceramics were metrically cubic for 0.1 ≤ y < 0.30, tetragonal with short range strontium vacancy, VSr, ordering for 0.30 ≤ y < 0.50, then orthorhombic with long range ordering of VSr for y ≥ 0.50 by X-ray powder and electron diffraction at room temperature. For samples reduced in N2/5%H2, compositions with 0.1 ≤ y ≤ 0.50 were metrically cubic. Short range VSr ordering and an orthorhombic structure with long range VSr ordering were observed for y = 0.50 and 0.63, respectively. Samples with y = 0.15 sintered in N2/5%H2 revealed the largest dimensionless thermoelectric figure-of-merit (ZT = 0.41 at 973 K) reported for n-type SrTiO3 based materials, suggesting that the accommodation of La through the formation of VSr accompanied by reduction in N2/5%H2 represents a new protocol for the development of oxide-based thermoelectrics

Volumetric chemical imaging by clearing-enhanced stimulated Raman scattering microscopy

Three-dimensional visualization of tissue structures using optical microscopy facilitates the understanding of biological functions. However, optical microscopy is limited in tissue penetration due to severe light scattering. Recently, a series of tissue-clearing techniques have emerged to allow significant depth-extension for fluorescence imaging. Inspired by these advances, we develop a volumetric chemical imaging technique that couples Raman-tailored tissue-clearing with stimulated Raman scattering (SRS) microscopy. Compared with the standard SRS, the clearing-enhanced SRS achieves greater than 10-times depth increase. Based on the extracted spatial distribution of proteins and lipids, our method reveals intricate 3D organizations of tumor spheroids, mouse brain tissues, and tumor xenografts. We further develop volumetric phasor analysis of multispectral SRS images for chemically specific clustering and segmentation in 3D. Moreover, going beyond the conventional label-free paradigm, we demonstrate metabolic volumetric chemical imaging, which allows us to simultaneously map out metabolic activities of protein and lipid synthesis in glioblastoma. Together, these results support volumetric chemical imaging as a valuable tool for elucidating comprehensive 3D structures, compositions, and functions in diverse biological contexts, complementing the prevailing volumetric fluorescence microscopy

Lead Free Multilayer Piezoelectric Actuators by Economically New Approach

The replacement of lead zirconate titanate ceramics (PZTs), with a lead-oxide (PbO)-free alternative is the subject of intense investigation worldwide. In this research, a cheap reliable methodology for the fabrication of multilayers (MLs) of lead-free potassium sodium niobate (KNN)-based ceramics is presented without the need for vacuuming and/or hot isostatic pressing of the tape. The thickness per active layer was 193 μm and 102 μm for the 10 and 16 layer MLs actuators, respectively. Effective d33 (piezoelectric coefficient), effective d33∗ (electrostrain coefficient), bi-polar strain (Smax), max displacement, dielectric constant (ϵr) and loss (tanδ) are 2500 pC/N, 4604 pm/V, 0.17%, 2.2 μm, 1812 and 2% at 1 kHz, respectively, where the effective d33 or d33∗ is the total output of all layers together. The ultimate target materials for potential substitution with KNN-based ceramics and MLs are commercial PZT-4 and PZT-8

Crystal Structure, Phase Transitions and Photoferroelectric Properties of KNbO3-Based Lead-Free Ferroelectric Ceramics: A Brief Review

Ferroelectric KNbO3 (KN) ceramics were first fabricated in the 1950s, however, their use in commercial technical applications has been hampered by inherently challenging processing difficulties. In the early 1990s, the interest in KN ceramics was revived by the pursuit of Pb-free piezoceramics. More recently the search for inexpensive photovoltaic materials alternative to Si prompted bandgap engineering studies in KN-based solid solutions. If the ferroelectric and piezoelectric properties of KN-based ceramics are now well established, the understanding of chemical doping on the bandgap of KN-based ceramics is still in its infancy. Here we provide a brief review on the current understanding of the structure-property relationships in this class of materials, which successively covers crystal structures, structural phase transitions, lattice dynamics, polarization, solid solutions and bandgap engineering of KN

High-Figure-of-Merit Thermoelectric La-Doped A-Site-Deficient SrTiO3 Ceramics

The structure and thermoelectric (TE) properties of La-doped, A-site-deficient SrTiO3 (Sr1–3x/2LaxTiO3) ceramics sintered in air and N2/5% H2 have been investigated. Air-sintered ceramics with 0.10 ≤ x 0.50 are orthorhombic with an a–a–c+ tilt system and long-range VA ordering. x = 0.15 sintered in N2/5% H2 shows the largest dimensionless TE figure-of-merit ZT = 0.41 at 973 K reported for n-type SrTiO3-based ceramics, suggesting that the accommodation of La through formation of (VSr) coupled with reduction in N2/5% H2 represents a new protocol for the development of oxide-based thermoelectrics

All-Inorganic Perovskite Solar Cells With Both High Open-Circuit Voltage and Stability

Metal halide perovskite solar cells based on all-inorganic CsPbBr3 have attracted considerable attentions recently, due to their high open-circuit voltage and good stability. However, the fabrication of CsPbBr3 film is limited by the poor solubility of cesium precursors in organic solvents by the one-step method. Here, we successfully fabricated CsPbBr3 film solar cells by employing colloid nanocrystal. The effects of technique parameters, including purification times, anneal temperatures, and spin-coating times on film morphology, optical spectra, and device performance are investigated in detail. The highest power conversion efficiency of 4.57% has been achieved based on a large open-circuit voltage of 1.45 V and a large short-circuit current of 9.41 mA cm−2. A large open-circuit voltage results from the reduced non-radiative energy loss channels and defect states while a large short-circuit current is related to the high conductivity induced by the removal of organic ligands with the increased nanocrystal electronic coupling. Furthermore, excellent stability in air is disclosed on the unencapsulated device suggesting the enormous potential for developing high open-circuit photovoltaic devices with high stability in future

Energy storage properties in Nd doped AgNbTaO3 lead-free antiferroelectric ceramics with Nb-site vacancies

It is crucial to discover lead-free materials with ultrahigh recoverable energy density (Wrec) that can be employed in future pulse power capacitors. In this work, a high Wrec of 4.51 J/cm3 was successfully obtained in lead-free Nd-doped AgNb0.8Ta0.2O3 antiferroelectric ceramics at an applied electric field of 290 kV/cm. It is discovered that Nd doping paired with Nb-site vacancies could stabilize the antiferroelectric phase by lowering the temperatures of the M1–M2 and M2–M3 phase transitions, which leads to higher energy storage efficiency. Furthermore, Nd and Ta co-doping will contribute to the electrical homogeneity and low electrical conductivity, resulting in large breakdown strengths. Aliovalent doping in Ag-site with Nb-site vacancies serves as a novel strategy for the construction of AgNbO3-based ceramics with excellent energy storage performance

Effects of Processing Strategies and La + Sm Co-Doping on the Thermoelectric Performance of A-Site-Deficient SrTiO3-δ Ceramics

The effect of calcining in either air (VSTO-A) or 5% H2/N2 (VSTO-H) on the thermoelectric performance of La and Sm co-doped A-site-deficient Sr1-3x/2Lax/2Smx/2TiO3-δ ceramics is reported. All calcined powders were sintered 6 h in 5% H2/N2 at 1773 K to ≥96% relative density. All peaks in X-ray diffraction patterns indexed as a cubic perovskite phase. Scanning electron microscopy revealed grain sizes ~14 and ~10 μm for VSTO-A and VSTO-H ceramics, respectively. x = 0.30 showed the lowest k (2.99 W/m.K at 973 K) for VSTO-A, whereas x = 0.20 had the lowest (2.67 W/m. K at 973 K) for the VSTO-H ceramics. x = 0.30 VSTO-A showed a thermoelectric figure of merit, ZT = 0.25 (at 973 K), whereas the maximum ZT = 0.30 (at 973 K) was achieved for x = 0.20 VSTO-H ceramics, demonstrating that thermoelectric properties are optimized when all processing is carried out in 5% H2/N2

Microstructure and electrical properties of Nb‐doped SrTiO3‐BiFeO3 based lead‐free ceramics

In this work, Nb-doped 0.75SrTiO3-0.25BiFeO3 (ST-BF) lead-free ceramics are designed and synthesized using a conventional solid-state reaction method. The influence of Nb doping on the microstructure, dielectric, and electrical properties are systematically investigated. With the increase of Nb concentration, the crystal structure of ST-BF remains pseudo-cubic as exhibited in the X-ray diffraction patterns. The grain size is found to increase from 0.33 to 6.23 μm, and then decrease to 1.88 μm by Nb doping, along with a clear heterogeneous core–shell microstructure. A relatively low dielectric loss (∼0.1, at 1 kHz) and a stable dielectric constant (∼700, at 1 kHz) are obtained for the 0.03 Nb-doped ST-BF composition at room temperature. Impedance spectroscopy analysis shows that Nb doping in ST-BF increases the total resistivity, forming an electrically conductive core and a nonconductive shell, with enhanced activation energy. The results may provide a feasible approach to develop novel ST-based lead-free dielectric ceramics for capacitor application

Cold sintering of microwave dielectric ceramics and devices

Microwave (MW) dielectric ceramics are used in numerous electronic components for modern wireless communication systems, including antennas, resonators, capacitors and filters. However, to date, MW ceramics are manufactured by an energy-intensive, conventional high-temperature (> 1000 °C) sintering technology and thus cannot be co-sintered with low melting point and base electrodes (Ag, Al, etc., < 1000 °C), nor directly integrated with polymers (< 200 °C). Cold sintering is able to densify ceramics at < 200 °C via a combination of external pressure and a transient liquid phase, reducing the energy consumed and facilitating greater integration with dissimilar materials. This review outlines the basics of MW ceramics alongside the mechanism of cold sintering. Recent developments in cold sintering of MW ceramics, composites and devices are described, emphasizing new materials and progress towards component/device fabrication. Future prospects and critical issues for advancing cold-sintered MW materials and devices, such as unclear mechanism, low Q × f values and poor mechanical properties, are discussed