Showing papers on "Atmospheric temperature range published in 2017"

Enhanced energy storage properties in La(Mg1/2Ti1/2)O3-modified BiFeO3-BaTiO3 lead-free relaxor ferroelectric ceramics within a wide temperature range

Abstract: A new ternary lead-free (0.67-x)BiFeO3-0.33BaTiO3-xLa(Mg1/2Ti1/2)O3 ferroelectric ceramic exhibited an obvious evolution of dielectric relaxation behavior. A significantly enhanced energy-storage property was observed at room temperature, showing a good energy-storage density of 1.66 J/cm3 at 13 kV/mm and a relatively high energy-storage efficiency of 82% at x = 0.06. This was basically ascribed to the formation of a slim polarization-electric field hysteresis loop, in which a high saturated polarization Pmax and a rather small remnant polarization Pr were simultaneously obtained. Particularly, its energy storage properties were found to depend weakly on frequency (0.2 Hz–100 Hz), and also to exhibit a good stability against temperature (25 °C–180 °C). The achievement of these characteristics was attributed to both a rapid response of the electric field induced reversible ergodic relaxor to long-range ferroelectric phase transition and a typical diffuse phase transformation process in the dielectric maxima.

Relaxor ferroelectric 0.9BaTiO3–0.1Bi(Zn0.5Zr0.5)O3 ceramic capacitors with high energy density and temperature stable energy storage properties

Abstract: A relaxor ferroelectric ceramic for high energy storage applications based on 0.9BaTiO3–0.1Bi(Zn0.5Zr0.5)O3 (0.9BT–0.1BZZ) was successfully fabricated via a conventional solid-state method. The sintered samples have a perovskite structure with a pseudocubic phase, showing a moderate dielectric constant (500–2000), low dielectric loss (tan δ < 0.15) and highly diffusive and dispersive relaxor-like behavior. The weak dielectric nonlinearity exhibits a dielectric constant change of ∼10% as the bias electric field increases from 0 kV cm−1 to 40 kV cm−1. Extra slim polarization–electric field loops accompanying the slow decrease of breakdown strength from 266.5 kV cm−1 to 217.7 kV cm−1 are observed in a measured temperature range of 30–150 °C. A maximum energy density of 2.46 J cm−3 was obtained at the electric field of 264 kV cm−1 close to the breakdown strength at ambient temperature. Temperature stability of both energy density and energy efficiency exists in a wide temperature range, which makes BT–BZZ ceramics promising candidates for high power electric applications.

Thermal stability of WS2 flakes and gas sensing properties of WS2/WO3 composite to H2, NH3 and NO2

Abstract: We report on the fabrication and on the morphological, structural, chemical and the electrical characterization of WS2 thin films sensors prepared by drop casting a commercial solution of dispersed few-layers WS2 flakes on Si3N4 interdigitated substrates and annealing the films in air at 150 °C, 250 °C and 350 °C. Thermal stability of WS2 in air at different annealing temperatures has been investigated by X-ray photoemission spectroscopy, scanning electron microscopy, X-ray diffraction and by simultaneous thermal analysis techniques. We found that WS2 is not stable in air and partially oxidizes to amorphous WO3 in the annealing temperature range 25 °C–150 °C. The oxidation of WS2 in air at 250 °C and 350 °C yields a composite crystalline WS2/WO3 hierarchical structure characterized by the presence of surface oxygen and sulphur vacancies. The contribution of each phase of the WS2/WO3 composite to the overall chemoresistive gas response utilizing H2 (1–10 ppm), NH3 (1–10 ppm) and NO2 (40 ppb–1 ppm) gases in dry air carrier is presented and discussed. WS2/WO3 composite films show excellent gas sensing properties to reducing (H2, NH3) as respect to oxidizing (NO2) gases at 150 °C operating temperature. In this work we found low detection limits of 1 ppm H2, 1 ppm NH3 and 100 ppm NO2 in dry air carrier, among the smallest so far ever reported for transition metal dichalcogenides. Furthermore, the sensor doesn’t show any cross sensitivity effects to both H2 and NH3 when exposed to water vapor. Outstanding reproducibility responses, by exposing the 150 °C annealed film to dynamic and cumulative gas pulses where obtained utilizing H2 gas.

Structural and Photophysical Properties of Methylammonium Lead Tribromide (MAPbBr3) Single Crystals

Abstract: The structural and photophysical characteristics of MAPbBr3 single crystals prepared using the inverse temperature crystallization method are evaluated using temperature-dependent X-ray diffraction (XRD) and optical spectroscopy. Contrary to previous research reports on perovskite materials, we study phase transitions in crystal lattice structures accompanied with changes in optical properties expand throughout a wide temperature range of 300–1.5 K. The XRD studies reveal several phase transitions occurred at ~210 K, ~145 K, and ~80 K, respectively. The coexistence of two different crystallographic phases was observed at a temperature below 145 K. The emission peaks in the PL spectra are all asymmetric in line shape with weak and broad shoulders near the absorption edges, which are attributed to the Br atom vacancy on the surface of the crystals. The time-resolved PL measurements reveal the effect of the desorption/adsorption of gas molecules on the crystal surface on the PL lifetimes. Raman spectroscopy results indicate the strong interplays between cations and different halide atoms. Lastly, no diamagnetic shift or split in emission peaks can be observed in the magneto-PL spectra even at an applied magnetic field up to 5 T and at a temperature as low as 1.5 K.

NaYF4:Er3+,Yb3+/SiO2 Core/Shell Upconverting Nanocrystals for Luminescence Thermometry up to 900 K

Abstract: The rapid development of nanomaterials with unique size-tunable properties forms the basis for a variety of new applications, including temperature sensing. Luminescent nanoparticles (NPs) have demonstrated potential as sensitive nanothermometers, especially in biological systems. Their small size offers the possibility of mapping temperature profiles with high spatial resolution. The temperature range is however limited, which prevents use in high-temperature applications such as, for example, nanoelectronics, thermal barrier coatings, and chemical reactors. In this work, we extend the temperature range for nanothermometry beyond 900 K using silica-coated NaYF4 nanoparticles doped with the lanthanide ions Yb3+ and Er3+. Monodisperse ∼20 nm NaYF4:Yb,Er nanocrystals were coated with a ∼10 nm silica shell. Upon excitation with infrared radiation, bright green upconversion (UC) emission is observed. From the intensity ratio between 2H11/2 and 4S3/2 UC emission lines at 520 and 550 nm, respectively, the tempe...

2D Crystals Significantly Enhance the Performance of a Working Fuel Cell

Abstract: 2D atomic crystals such as single layer graphene (SLG) and hexagonal boron nitride (hBN) have been shown to be “unexpectedly permeable” to hydrogen ions under ambient conditions with the proton conductivity rising exponentially with temperature. Here, the first successful addition of SLG made by a chemical vapor deposition (CVD) method is shown to an operational direct methanol fuel cell significantly enhancing the performance of the cell once the temperature is raised above 60 °C, the temperature at which the proton conductivity of SLG is higher than the Nafion membrane on which it is mounted. Above this temperature, the resistance to proton transport of the system is not affected by the graphene but the barrier properties of graphene inhibit methanol crossover. The performance of the fuel cell is shown to increase linearly with coverage of SLG above this temperature. Results show that the maximum power density is increased at 70 °C by 45% in comparison to the standard membrane electrode assembly without graphene. In addition, a membrane with CVD hBN shows enhanced performance across the entire temperature range due to better proton conductivity at lower temperatures.

High Thermoelectric Performance in Electron-Doped AgBi3S5 with Ultralow Thermal Conductivity

Abstract: We report electron-doped AgBi3S5 as a new high-performance nontoxic thermoelectric material. This compound features exceptionally low lattice thermal conductivities of 0.5–0.3 W m–1 K–1 in the temperature range of 300–800 K, which is ascribed to its unusual vibrational properties: “double rattling” phonon modes associated with Ag and Bi atoms. Chlorine doping at anion sites acts as an efficient electron donor, significantly enhancing the electrical properties of AgBi3S5. In the carrier concentration range (5 × 1018–2 × 1019 cm–3) investigated in this study, the trends in Seebeck coefficient can be reasonably understood using a single parabolic band model with the electron effective mass of 0.22 me (me is the free electron mass). Samples of 0.33% Cl-doped AgBi3S5 prepared by spark plasma sintering show a thermoelectric figure of merit of ∼1.0 at 800 K.

Thermoelectric properties of CuGa1−xMnxTe2: power factor enhancement by incorporation of magnetic ions

Abstract: Chalcopyrite CuGaTe2 is under research for its high thermoelectric performance. Different routes have been investigated recently for enhancing its thermoelectric parameters. In this work we report the synthesis of chalcopyrite CuGa1−xMnxTe2 (x = 0.0, 0.01, 0.02, and 0.03) by a solid state method and through which an enhanced power factor was obtained. The samples were characterized for electrical, thermal and thermoelectric transport properties in the temperature range 325–870 K after performing stability analysis using TG-DTA data. XRD patterns confirm a phase pure tetragonal structure for all nominal compositions with the space group I2d. The electrical conductivity σ increases drastically by Mn2+ doping which increases the hole carriers, while the Seebeck coefficient S still retains large positive values. As a result, the power factor of CuGa0.99Mn0.01Te2 reaches 1.55 mW K−2 m−1 at 718 K. Calculations using the relationship of S and ln σ suggest that the power factor observed for Mn-doped samples is higher than that expected for CuGaTe2 with optimized carrier concentration, suggesting that the Mn-doping brings additional effects other than simple carrier tuning. The total thermal conductivity is reduced by Mn doping, with a minimum thermal conductivity of 1.6 W m−1 K−1 for the x = 0.01 sample. The maximum value for ZT reached at 870 K was 0.83, which is more than 40% enhancement as compared to that of pure CuGaTe2. Strong interactions between the magnetic moments of Mn and charge carriers are inferred by the large negative Weiss temperature in the magnetic susceptibility and distinct anomalous Hall effect, the latter of which develops in accordance with the increase of magnetization at low temperature. These results suggest that the carrier–magnetic moment interaction plays an essential role in the enhancement of the thermoelectric properties of CuGa1−xMnxTe2.

Synthesis of Mn2+:Zn2SiO4–Eu3+:Gd2O3 nanocomposites for highly sensitive optical thermometry through the synergistic luminescence from lanthanide-transition metal ions

Abstract: In this work, Mn2+:Zn2SiO4–Eu3+:Gd2O3 nanocomposites were fabricated following a multi-step solution route. The fluorescence intensity ratio of Mn2+ to Eu3+ in these composites exhibits remarkable temperature dependence, owing to the diverse thermal quenching behaviors of Mn2+ and Eu3+ ions. Using these nanomaterials to perform thermometry, excellent temperature sensitivity is achieved in the room temperature range. Specifically, the values of relative temperature sensitivity are all beyond 2.5% K−1 in the whole range of 303–323 K (i.e. 30–50 °C), and reach as high as 3.05% K−1 at 303 K, remarkably higher than those of other inorganic optical thermometric materials at a similar temperature. And the absolute sensitivities are all beyond 0.0089 K−1 in this temperature range, which is also a relatively high value. These results indicate the Mn2+:Zn2SiO4–Eu3+:Gd2O3 nanocomposites to be very promising nano-thermometric materials.

Resonant Nonplasmonic Nanoparticles for Efficient Temperature-Feedback Optical Heating

Abstract: We propose a novel photothermal approach based on resonant dielectric nanoparticles, which possess imaginary part of permittivity significantly smaller as compared to metal ones. We show both experimentally and theoretically that a spherical silicon nanoparticle with a magnetic quadrupolar Mie resonance converts light to heat up to 4 times more effectively than similar spherical gold nanoparticle at the same heating conditions. We observe photoinduced temperature raise up to 900 K with the silicon nanoparticle on a glass substrate at moderate intensities (<2 mW/μm2) and typical laser wavelength (633 nm). The advantage of using crystalline silicon is the simplicity of local temperature control by means of Raman spectroscopy working in a broad range of temperatures, that is, up to the melting point of silicon (1690 K) with submicrometer spatial resolution. Our CMOS-compatible heater–thermometer nanoplatform paves the way to novel nonplasmonic photothermal applications, extending the temperature range and si...

A family of doped lanthanide metal–organic frameworks for wide-range temperature sensing and tunable white light emission

Abstract: A family of lanthanide MOFs (LnL) with high thermal and air stability have been successfully synthesized. Based on this robust framework, a series of binary and ternary co-doped LnMOFs, EuxTbyL (y = 1−x) and EuxTbyGd1−x−yL, are achieved for use as ratiometric temperature sensors and white-light-emitting materials. In a binary co-doped system, Eu0.0066Tb0.9934L and Eu0.0089Tb0.9911L show good linear responses to temperature with high sensitivities over a very wide range (from 77 K to 450 K), of which Eu0.0066Tb0.9934L exhibits a maximum relative sensitivity (Sm) of 3.76% K−1 at 450 K. This value is comparable to those of other excellent LnMOF thermometers reported recently whereas the response temperature range is greatly enlarged. The ternary mixed LnMOFs based on energy transfer of different lanthanide ions as ratiometric luminescent thermometers are firstly investigated, in which Eu0.013Tb0.060Gd0.927L displays an extremely high sensitivity (Sm = 6.11% K−1), representing one of the largest values reported so far in mixed LnMOFs. Furthermore, by virtue of carefully adjusting the composition of the mixed LnMOFs and the wavelength of excitation, the emission color can be systematically modulated and a tunable white light emission material, Eu0.0062Tb0.0087Gd0.9851L, is successfully developed.

Optimization of highly sensitive YAG:Cr3+,Nd3+ nanocrystal-based luminescent thermometer operating in an optical window of biological tissues.

Abstract: Luminescent and temperature sensitive properties of YAG:Cr3+,Nd3+ nanocrystals were analyzed as a function of temperature, nanoparticle size, and excitation wavelength. Due to numerous temperature-dependent phenomena (e.g. Boltzmann population, thermal quenching, and inter-ion energy transfer) occurring in this phosphor, four different thermometer definitions were evaluated with the target to achieve a high sensitivity and broad temperature sensitivity range. Using a Cr3+ to Nd3+ emission intensity ratio, the highest 3.48% K-1 sensitivity was obtained in the physiological temperature range. However, high sensitivity was compromised by a narrow sensitivity range or vice versa. The knowledge of the excitation and temperature susceptibility mechanisms enabled wise selection of the spectral features found in luminescence spectra for a temperature readout, which enabled the preservation of relatively high temperature sensitivity (>1.2% K-1 max) and extended the temperature sensitivity range from 100 K to 850 K. The size of the nanophosphors had negligible impact on the performance of the studied materials.

Multiferroic properties and structural features of M -type Al-substituted barium hexaferrites

Abstract: Precise studies of the crystal and magnetic structures of M-type substituted barium hexaferrites BaFe12–x Al x O19 (0.1 ≤ x ≤ 1.2) have been performed by powder neutron diffraction in the temperature range 300–730 K. The electric polarization and the magnetization, and also the magnetoelectric effect of the compositions under study have been studied in electric (to 110 kV/m) and magnetic (to 14 T) fields at room temperature. The spontaneous polarization and significant correlation between the dielectric and magnetic subsystems have been observed at room temperature. The magnetoelectric effect value is, on average, about 5%, and it increases slightly with the aluminum cation concentration. The precise structural studies made it possible to reveal the cause and the mechanism of formation of the spontaneous polarization in M-type substituted barium hexaferrites BaFe12–x Al x O19 (x ≤ 1.2) with a collinear ferromagnetic structure.

Structure–property relationships of low sintering temperature scheelite-structured (1 − x)BiVO4–xLaNbO4 microwave dielectric ceramics

Abstract: A series of (1 − x)BiVO4–xLaNbO4 (0.0 ≤ x ≤ 1.0) ceramics were prepared via a solid state reaction method. A scheelite-structured solid solution was formed for x ≤ 0.5 but for x > 0.5, tetragonal scheelite, monoclinic LaNbO4-type and La1/3NbO3 phases co-existed. As x increased from 0 to 0.1, the room temperature crystal structure gradually changed from monoclinic to tetragonal scheelite, associated with a decrease in the ferroelastic phase transition temperature from 255 °C (BiVO4) to room temperature or even below. High sintering temperatures were also found to accelerate this phase transition for compositions with x ≤ 0.08. Temperature independent high quality factor Qf >10000 GHz in a wide temperature range 25–140 °C and high microwave permittivity er ∼76.3 ± 0.5 was obtained for the x = 0.06 ceramic sintered at 800 °C. However, small changes in composition resulted in a change in the sign and magnitude of the temperature coefficient of resonant frequency (TCF) due to the proximity of the ferroelastic transition to room temperature. If TCF can be controlled and tuned through zero, then (1 − x)BiVO4–xLaNbO4 (0.0 ≤ x ≤ 1.0) is a strong candidate for microwave device applications.

Temperature-Dependent Photoluminescence of Cesium Lead Halide Perovskite Quantum Dots: Splitting of the Photoluminescence Peaks of CsPbBr3 and CsPb(Br/I)3 Quantum Dots at Low Temperature

Abstract: We investigated the temperature-dependent photoluminescence (PL) properties of colloidal CsPbX3 (X = Br, I, and mixed Br/I) quantum dot (QD) samples in the 30–290 K temperature range. Temperature-dependent PL experiments reveal thermal quenching of PL, blue shifting of optical band gaps, and line width broadening for all CsPbX3 QD samples with increasing temperature. Interestingly, side-peak emissions that are spectrally separated from the excitonic PL peaks were observed for both CsPbBr3 and CsPb(Br/I)3 QD samples at temperatures below ∼250 K. The side-peak emission for the CsPbBr3 QD sample is located at a lower energy compared to the band-edge peak, whereas that of the Br-rich CsPb(Br/I)3 alloy QD sample is located at a higher energy than that of the band-edge peak. We found that the CsPbBr3 QDs have two emissive states, a band-edge state, and one involving shallow defects, which can be spectrally separated by narrowing the emission line widths at low temperature. In the case of the Br-rich CsPb(Br/I)3...

Ultra-high Seebeck coefficient and low thermal conductivity of a centimeter-sized perovskite single crystal acquired by a modified fast growth method

Abstract: A centimeter-sized organic–inorganic hybrid lead-based perovskite CH3NH3PbI3 (MAPbI3) single crystal was obtained by using a modified fast and inverse-temperature growth method. The optical properties of this single crystal at room and low temperatures were studied in terms of optical absorption and photoluminescence measurements. The single crystal exhibited optical properties with a band-gap of 1.53 eV, which is comparable to a reported value. The temperature-dependent UV-vis spectra of this perovskite single crystal showed a unique structural phase transition as the temperatures varied. The thermoelectric properties of this MAPbI3 single crystal were studied, showing that the Seebeck coefficient of 920 ± 91 μV K−1 almost remained unchanged from room temperature to 330 K and it progressively increased with the increase in temperature and reached 1693 ± 146 μV K−1 at 351 K. In contrast, there was no very clear trend for thermal conductivities with changes in temperature. The thermal conductivities were maintained between 0.30 and 0.42 W m K−1 in the temperature range of 298–425 K. These thermoelectric characteristics would be useful for potential thermoelectric applications if the electrical conductivity of this crystal is improved by tuning its composition.

High energy storage density over a broad temperature range in sodium bismuth titanate-based lead-free ceramics.

Abstract: A series of (1-x)Bi0.48La0.02Na0.48Li0.02Ti0.98Zr0.02O3-xNa0.73Bi0.09NbO3 ((1-x)LLBNTZ-xNBN) (x = 0-0.14) ceramics were designed and fabricated using the conventional solid-state sintering method. The phase structure, microstructure, dielectric, ferroelectric and energy storage properties of the ceramics were systematically investigated. The results indicate that the addition of Na0.73Bi0.09NbO3 (NBN) could decrease the remnant polarization (P r ) and improve the temperature stability of dielectric constant obviously. The working temperature range satisfying TCC 150 °C ≤±15% of this work spans over 400 °C with the compositions of x ≥ 0.06. The maximum energy storage density can be obtained for the sample with x = 0.10 at room temperature, with an energy storage density of 2.04 J/cm3 at 178 kV/cm. In addition, the (1-x)LLBNTZ-xNBN ceramics exhibit excellent energy storage properties over a wide temperature range from room temperature to 90 °C. The values of energy storage density and energy storage efficiency is 0.91 J/cm3 and 79.51%, respectively, for the 0.90LLBNTZ-0.10NBN ceramic at the condition of 100 kV/cm and 90 °C. It can be concluded that the (1-x)LLBNTZ-xNBN ceramics are promising lead-free candidate materials for energy storage devices over a broad temperature range.

Revisiting the Temperature Dependent Ionic Conductivity of Yttria Stabilized Zirconia (YSZ)

Abstract: The temperature dependent conductivity of yttria stabilized zirconia (YSZ) exhibits a bending in Arrhenius' plots which is frequently discussed in terms of free and associated oxygen vacancies. However, the very high doping concentration in YSZ leads to such a strong defect interaction that the concept of free vacancies becomes highly questionable. Therefore, the temperature dependent conductivity of YSZ is reconsidered. The conductivity of YSZ with different doping concentration was measured in a broad temperature range. The data are analyzed in terms of two different barrier heights that have to be passed along an average path of an oxygen vacancy in YSZ (two barrier model). For 8–10 mol% yttria, the two barriers are in the range of 0.6 eV and 1.1–1.2 eV, respectively. The conductivity and thus the barrier heights also depend on the cooling rate after a high temperature pre-treatment. This indicates that different frozen-in distributions of dopants affect the vacancy motion by different energy landscapes. Temporarily existing defect configurations, possibly with a strong effect of repulsive oxygen vacancy interaction, are suggested as the reason of high barriers. Future dynamic ab-initio calculations may reveal whether this modified model of the YSZ conductivity is mechanistically meaningful.

Anomalously temperature-dependent thermal conductivity of monolayer GaN with large deviations from the traditional 1 / T law

Abstract: Efficient heat dissipation, which is featured by high thermal conductivity, is one of the crucial issues for the reliability and stability of nanodevices. However, due to the generally fast $1/T$ decrease of thermal conductivity with temperature increase, the efficiency of heat dissipation quickly drops down at an elevated temperature caused by the increase of work load in electronic devices. To this end, pursuing semiconductor materials that possess large thermal conductivity at high temperature, i.e., slower decrease of thermal conductivity with temperature increase than the traditional $\ensuremath{\kappa}\ensuremath{\sim}1/T$ relation, is extremely important to the development of disruptive nanoelectronics. Recently, monolayer gallium nitride (GaN) with a planar honeycomb structure emerges as a promising new two-dimensional material with great potential for applications in nano- and optoelectronics. Here, we report that, despite the commonly established $1/T$ relation of thermal conductivity in plenty of materials, monolayer GaN exhibits anomalous behavior that the thermal conductivity almost decreases linearly over a wide temperature range above 300 K, deviating largely from the traditional $\ensuremath{\kappa}\ensuremath{\sim}1/T$ law. The thermal conductivity at high temperature is much larger than the expected thermal conductivity that follows the general $\ensuremath{\kappa}\ensuremath{\sim}1/T$ trend, which would be beneficial for applications of monolayer GaN in nano- and optoelectronics in terms of efficient heat dissipation. We perform detailed analysis on the mechanisms underlying the anomalously temperature-dependent thermal conductivity of monolayer GaN in the framework of Boltzmann transport theory and further get insight from the view of electronic structure. Beyond that, we also propose two required conditions for materials that would exhibit similar anomalous temperature dependence of thermal conductivity: large difference in atom mass (huge phonon band gap) and electronegativity (LO-TO splitting due to strong polarization of bond). Our study offers fundamental understanding of phonon transport in monolayer GaN, and the insight gained from this study is of great significance for the design and search of materials superior for applications in nano- and optoelectronics in terms of high-performance thermal management.

Elastic properties, thermal stability, and thermodynamic parameters of MoAlB

Abstract: MoAlB is the first and, so far, the only transition-metal boride that forms alumina when heated in air and is thus potentially useful for high-temperature applications. Herein, the thermal stability in argon and vacuum atmospheres and the thermodynamic parameters of bulk polycrystalline MoAlB were investigated experimentally. At temperatures above 1708 K, in vacuum and inert atmospheres, this compound incongruently melts into the binary MoB and liquid aluminum metal as confirmed by differential thermal analysis, quenching experiments, x-ray diffraction, and scanning electron microscopy. Making use of that information together with heat-capacity measurements in the 4–1000-K temperature range—successfully modeled as the sum of lattice, electronic, and dilation contributions—the standard enthalpy, entropy, and free energy of formation are computed and reported for the full temperature range. The standard enthalpy of formation of MoAlB at 298 K was found to be −132 ± 3.2 kJ/mol. Lastly, the thermal conductivity values are computed and modeled using a variation of the Slack model in the 300–1600-K temperature range.

Duplex structure in K0.5Na0.5NbO3-SrZrO3 ceramics with temperature-stable dielectric properties

Abstract: Temperature-stable dielectric properties have been developed in the 0.86 K0.5Na0.5NbO3-0.14SrZrO3 solid solution system. High dielectric permittivity (e′ = 2310) with low loss sustained in a broad temperature range (−55–201 °C), which was close to that of the commercial BaTiO3-based high-temperature capacitors. Transmission electron microscopy with energy dispersive X-ray analysis directly revealed that submicron grains exhibited duplex core-shell structure. The outer shell region was similar to the target composition, whilst a slightly poor content of Sr and Zr presented in the core region. Based on Lichtenecker’s effective dielectric function analysis along with Lorentz fit of the temperature dependence of dielectric permittivity, a plausible mechanism explaining the temperature-stable dielectric response in present work was suggested. These results offer an opportunity to achieve the X8 R specification high-temperature capacitors in K0.5Na0.5NbO3 based materials.

Extremely low thermal conductivity and high thermoelectric performance in liquid-like Cu2Se1−xSx polymorphic materials

Abstract: Recently, copper chalcogenides Cu2−xδ (δ = S, Se, Te) have attracted great attention due to their exceptional thermal and electrical transport properties. Besides these binary Cu2−xδ compounds, the ternary Cu2−xδ solid solutions are also expected to possess excellent thermoelectric performance. In this study, we have synthesized a series of Cu2Se1−xSx (x = 0.2, 0.3, 0.5, and 0.7) solid solutions by melting the raw elements followed by spark plasma sintering. The energy dispersive spectroscopy mapping, powder and single-crystal X-ray diffraction and X-ray powder diffraction studies suggest that Cu2Se and Cu2S can form a continuous solid solution in the entire composition range. These Cu2Se1−xSx solid solutions are polymorphic materials composed of varied phases with different proportions at room temperature, but single phase materials at elevated temperature. Increasing the sulfur content in Cu2Se1−xSx solid solutions can greatly reduce the carrier concentration, leading to much enhanced electrical resistivity and Seebeck coefficients in the whole temperature range as compared with those in binary Cu2Se. In particular, introducing sulfur at Se-sites reduces the speed of sound. Combining the strengthened point defect scattering of phonons, extremely low lattice thermal conductivities are obtained in these solid solutions. Finally, a maximum zT value of 1.65 at 950 K is achieved for Cu2Se0.8S0.2, which is greater than those of Cu2Se and Cu2S.

Electronic and optical properties of BaTiO3 across tetragonal to cubic phase transition: An experimental and theoretical investigation

Abstract: Temperature dependent diffuse reflectance spectroscopy measurements were carried out on polycrystalline samples of BaTiO3 across the tetragonal to cubic structural phase transition temperature (TP). The values of various optical parameters such as band gap (Eg), Urbach energy (Eu), and Urbach focus (E0) were estimated in the temperature range of 300 K to 480 K. It was observed that with increasing temperature, Eg decreases and shows a sharp anomaly at TP. First principle studies were employed in order to understand the observed change in Eg due to the structural phase transition. Near TP, there exist two values of E0, suggesting the presence of electronic heterogeneity. Further, near TP, Eu shows metastability, i.e., the value of Eu at temperature T is not constant but is a function of time (t). Interestingly, it is observed that the ratio of Eu (t=0)/Eu (t = tm), almost remains constant at 300 K (pure tetragonal phase) and at 450 K (pure cubic phase), whereas this ratio decreases close to the transition temperature, which confirms the presence of electronic metastability in the pure BaTiO3. The time dependence of Eu, which also shows an influence of the observed metastability can be fitted with the stretched exponential function, suggesting the presence of a dynamic heterogeneous electronic disorder in the sample across TP. First principle studies suggest that the observed phase coexistence may be due to a very small difference between the total cohesive energy of the tetragonal and the cubic structure of BaTiO3. The present work implies that the optical studies may be a sensitive probe of disorder/heterogeneity in the sample.