Showing papers on "Atmospheric temperature range published in 2019"

Linear-like lead-free relaxor antiferroelectric (Bi0.5Na0.5)TiO3–NaNbO3 with giant energy-storage density/efficiency and super stability against temperature and frequency

Abstract: A novel lead-free polar dielectric ceramic with linear-like polarization responses was found in (1 − x)(Bi0.5Na0.5)TiO3–xNaNbO3 ((1 − x)BNT–xNN) solid solutions, exhibiting giant energy storage density/efficiency and super stability against temperature and frequency. High-resolution transmission electron microscopy, Raman scattering and Rietveld refinements of X-ray diffraction data suggest that these property characteristics can be derived from temperature and electric field insensitive large permittivity as a result of relaxor antiferroelectricity (AFE) with polar nanoregions. Additionally, this feature intrinsically requires a high driving field for AFE to ferroelectric (FE) phase transitions due to large random fields. Measurements of temperature-dependent permittivity and polarization versus electric field hysteresis loops indicate that the high-temperature AFE P4bm phase in BNT was gradually stabilized close to room temperature, accompanying a phase transition from relaxor rhombohedral FEs to relaxor tetragonal AFEs approximately at x = 0.15–0.2. A record high of recoverable energy-storage density W ∼ 7.02 J cm−3 as well as a high efficiency η ∼ 85% was simultaneously achieved in the x = 0.22 bulk ceramic, which challenges the existing fact that W and η must be seriously compromised. Furthermore, desirable W (>3.5 J cm−3) and η (>88%) with a variation of less than 10% can be accordingly obtained in the temperature range of 25–250 °C and frequency range of 0.1–100 Hz. These excellent energy-storage properties would make BNT-based lead-free AFE ceramic systems a potential candidate for application in pulsed power systems.

High-entropy pyrochlores with low thermal conductivity for thermal barrier coating materials

Abstract: High-entropy pyrochlore-type structures based on rare-earth zirconates are successfully produced by conventional solid-state reaction method. Six rare-earth oxides (La2O3, Nd2O3, Sm2O3, Eu2O3, Gd2O3, and Y2O3) and ZrO2 are used as the raw powders. Five out of the six rare-earth oxides with equimolar ratio and ZrO2 are mixed and sintered at different temperatures for investigating the reaction process. The results demonstrate that the high-entropy pyrochlores (5RE1/5)2Zr2O7 have been formed after heated at 1000°C. The (5RE1/5)2Zr2O7 are highly sintering resistant and possess excellent thermal stability. The thermal conductivities of the (5RE1/5)2Zr2O7 high-entropy ceramics are below 1 W·m–1·K–1 in the temperature range of 300–1200°C. The (5RE1/5)2Zr2O7 can be potential thermal barrier coating materials.

Superior Thermal Stability of High Energy Density and Power Density in Domain-Engineered Bi0.5Na0.5TiO3-NaTaO3 Relaxor Ferroelectrics.

Abstract: Thermal-stable dielectric capacitors with high energy density and power density have attracted increasing attention in recent years. In this work, (1 - x)Bi0.5Na0.5TiO3-xNaTaO3 [(1 - x)BNT-xNT, x = 0-0.30] lead-free relaxor ferroelectric ceramics are developed for capacitor applications. The x = 0.20 ceramic exhibits superior thermal stability of discharged energy density (WD) with a variation of less than 10% in an ultrawide temperature range of -50 to 300 °C, showing a significant advantage compared with the previously reported ceramic systems. The WD reaches 4.21 J/cm3 under 38 kV/mm at room temperature. Besides, a record high of power density (PD ≈ 89.5 MW/cm3) in BNT-based ceramics is also achieved in x = 0.20 ceramic with an excellent temperature insensitivity within 25-160 °C. The x = 0.20 ceramic is indicated to be an ergodic relaxor ferroelectric with coexisted R3c nanodomains and P4bm polar nanoregions at room temperature, greatly inducing large maximum polarization, maintaining low remnant polarization, and thus achieving high WD and PD. Furthermore, the diffuse phase transition from R3c to P4bm phase on heating is considered to be responsible for the superior thermal stability of the high WD and PD. These results imply the large potential of the 0.80BNT-0.20NT ceramic in temperature-stable dielectric capacitor applications.

Improvement of Low-Temperature zT in a Mg3Sb2–Mg3Bi2 Solid Solution via Mg-Vapor Annealing

Abstract: Materials with high zT over a wide temperature range are essential for thermoelectric applications. n-Type Mg3 Sb2 -based compounds have been shown to achieve high zT at 700 K, but their performance at low temperatures ( 30 mm has a zT 0.8 at 300 K, which is comparable to commercial thermoelectric materials used at room temperature (n-type Bi2 Te3 ) while reaching zT 1.4 at 700 K, allowing applications over a wider temperature scale.

Realizing high thermoelectric performance in GeTe through decreasing the phase transition temperature via entropy engineering

Abstract: Entropy engineering is one of the powerful approaches to suppress phase transitions. GeTe has a very high thermoelectric performance at relatively high temperatures, but the low structure symmetry and phase transition in the low temperature range limits its performance stability for power generation applications. Therefore, the optimized electrical transport properties of GeTe in a low temperature range are expected for improving the structural symmetry via suppressing phase transition. Herein, the phase transition temperature for GeTe was successfully decreased by introducing high entropy via continuously multiple doping; the phase transition temperature is correspondingly reduced from 660 K to 523 K. The Seebeck coefficient was enhanced by the improved structural symmetry through enhancing band effective mass while the carrier concentration is maintained in an optimum range. A record-high power factor of ∼23 μW cm−1 K−2 was obtained at 300 K in the highest entropy sample. We found that the increased configurational entropy obtained by continuous, multiple doping produces short-range disordered microstructures, which lead to an ultralow lattice thermal conductivity of ∼0.4 W m−1 K−1. Combining the record high power factor and low thermal conductivity, a maximum ZT value of ∼2.1 at 800 K was achieved for the highest entropy species Ge0.84In0.01Pb0.1Sb0.05Te0.997I0.003. This study provides an effective path to enhance thermoelectric performances via introducing entropy engineering.

Review on the materials and devices for magnetic refrigeration in the temperature range of nitrogen and hydrogen liquefaction

Abstract: Magnetic refrigeration based on magnetocaloric effect (MCE) has become a promising alternative technique to the traditional gas-compression refrigeration due to its friendly environment and high energy efficiency. In addition to room temperature magnetic refrigeration, this novel technology can be applied at low temperature, especially for the potential applications in gas liquefaction. Therefore, attention has been paid to explore suitable materials with large MCE near the gas liquefaction temperature and develop low temperature magnetic refrigerators. Herein, the typical magnetocaloric materials and prototypes in the temperature range of nitrogen and hydrogen liquefaction are reviewed. Heavy rare earth intermetallic compounds are promising for low temperature magnetic refrigeration due to a low ordering temperature and large magnetic moments. For example, DyFeSi compound shows a large reversible MCE under a low field change of 1 T around TC = 70 K, which is near the liquefaction temperature of nitrogen (77 K). It has been proposed that a composite can be formed by a group of magnetic refrigeration materials with successive transition temperatures and nearly constant MCEs, therefore expanding the range of working temperature desirable for Ericsson-cycle magnetic refrigeration.

Realizing a stable high thermoelectric zT ∼ 2 over a broad temperature range in Ge 1−x−y Ga x Sb y Te via band engineering and hybrid flash-SPS processing

Abstract: We report a remarkably high and stable thermoelectric figure of merit zT close to 2 by manipulating the electronic bands in Ga–Sb codoped GeTe, which has been processed by hybrid flash-spark plasma sintering. According to the experimental results and first-principles calculations, the vast enhancement achieved in the thermopower due to codoping of Ga (2 mol%) and Sb (8 mol%) in GeTe is attributed to a concoction of reasons: (i) suppression of hole concentration; (ii) improved band convergence by decreasing the energy separation between the two valence band maxima to 0.026 eV; (iii) Ga predominantly contributing to the top of the valence band in Ga–Sb codoped GeTe, despite the Ga-induced resonance state not being located at a favorable position near the Fermi level; (iv) active participation of several bands increasing the hole carrier effective mass; (v) facilitating band degeneracy by reducing the R3m → Fm[3 with combining macron]m structural transition temperature from 700 K to 580 K. The synergy between these complementary and beneficial effects, in addition to the reduced thermal conductivity, enabled the flash sintered Ge0.90Ga0.02Sb0.08Te composition to not only exhibit a peak of zT of ∼1.95 at 723 K, but also to maintain/stabilize its high performance over a broad temperature range (600–775 K), thus making it a serious candidate for mid-temperature range energy harvesting devices.

Phonon hydrodynamics and ultrahigh-room-temperature thermal conductivity in thin graphite

Abstract: Allotropes of carbon, such as diamond and graphene, are among the best conductors of heat. We monitored the evolution of thermal conductivity in thin graphite as a function of temperature and thickness and found an intimate link between high conductivity, thickness, and phonon hydrodynamics. The room temperature in-plane thermal conductivity of 8.5-micrometer-thick graphite was 4300 watts per meter-kelvin-a value well above that for diamond and slightly larger than in isotopically purified graphene. Warming enhances thermal diffusivity across a wide temperature range, supporting partially hydrodynamic phonon flow. The enhancement of thermal conductivity that we observed with decreasing thickness points to a correlation between the out-of-plane momentum of phonons and the fraction of momentum relaxing collisions. We argue that this is due to the extreme phonon dispersion anisotropy in graphite.

Structural, luminescence and thermometric properties of nanocrystalline YVO 4 :Dy 3+ temperature and concentration series.

Abstract: We report systematic study of Dy3+-doped YVO4 nanophosphors synthesized via modified Pechini technique. Effect of calcination temperature and doping concentration on structure and luminescence has been investigated. XRD and Raman spectroscopy revealed preparation of single phase nanoparticles without any impurities. Synthesized nanopowders consisted of weakly agglomerated nanoparticles with average size about 50 nm. Photoluminescence spectra of YVO4:Dy3+ nanoparticles consisted of the characteristic narrow lines attributed to the intra-configurational 4f-4f transitions dominating by the hypersensitive 4F9/2–6H13/2 transition. The calcination temperature variation did not affect 4F9/2 lifetime, whereas increase of doping concentration resulted in its gradual decline. Potential application of YVO4:Dy3+ 1 at.% and 2 at.% nanopowders as ratiometric luminescence thermometers within 298–673 K temperature range was tested. The main performances of thermometer including absolute and relative thermal sensitivities and temperature uncertainty were calculated. The maximum relative thermal sensitivity was determined to be 1.8% K−1@298 K, whereas the minimum temperature uncertainty was 2 K.

Localized polarons and conductive charge carriers: Understanding CaCu 3 Ti 4 O 12 over a broad temperature range

Abstract: ${\mathrm{CaCu}}_{3}{\mathrm{Ti}}_{4}{\mathrm{O}}_{12}$ (CCTO) has a large dielectric permittivity plateau near room temperature due to several dynamic processes. Here, we consider the combined effects of localized charge carriers (polarons) and conductive charge carriers using a recently proposed statistical model [Phys. Rev. B 96, 054115 (2017)] to fit and understand its permittivity measured at different frequencies over a broad temperature range. We found that, at the lowest temperature, the small permittivity is related to frozen polarons, and the increase at higher temperatures is associated with the thermal excitation of polarons that gives rise to the Maxwell-Wagner effect. The final rapid increase at the highest temperature is attributed to thermally activated conductivity. Such an analysis enables us to separate the contributions from localized polarons and conductive charge carriers and quantify their activation energies, which also explains the permittivity plateau near room temperature. In particular, we show that the subtle balance between the number of activated polarons and their polarizability causes CCTO to have a permittivity plateau with small dielectric loss.

Double perovskite A2LaNbO6:Mn4+,Eu3+ (A = Ba, Ca) phosphors: potential applications in optical temperature sensing.

Abstract: Recently, much attention has been paid to Mn4+-doped phosphors due to their strong deep-red emissions which are in demand in white light-emitting diodes. However, a key challenge for the commercialization of Mn4+-doped phosphors is their low thermal stability caused by the thermal quenching of Mn4+ luminescence. Herein, a strategy of optical temperature sensing has been developed by specifically utilizing thermal quenching to explore the potential applications of Mn4+-doped phosphors in optical temperature sensing. In this work, we report two kinds of double perovskite type phosphors, Ba2LaNbO6 (BLN) and Ca2LaNbO6 (CLN), co-doped with Mn4+ and Eu3+. Through the study of temperature-dependent spectra in a large temperature range of 298–498 K, Mn4+ and Eu3+ yield different trends where the fluorescence intensity of Mn4+ ions decreases much more rapidly compared to that of Eu3+ ions as the temperature increases. Accordingly, based on the fluorescence intensity ratio (FIR) of the luminescence of Mn4+ and Eu3+, the optimal relative sensitivity of temperature sensing in the BLN and CLN matrices could reach 2.08% K−1 and 1.51% K−1, respectively. Finally, the application potential of Mn4+-doped phosphors in temperature sensing is confirmed by analyzing different temperature sensing results in the two matrices.

Highly reliable all-fiber temperature sensor based on the fluorescence intensity ratio (FIR) technique in Er3+/Yb3+ co-doped NaYF4 phosphors

Abstract: Accurate and remote temperature measurements in harsh environments are of great importance. The FIR technique is self-referenced and regarded as a promising method to improve the temperature accuracy and simplify the experimental devices. Herein, a point all-fiber temperature sensor based on the FIR technique has been developed. Er3+/Yb3+ co-doped NaYF4 phosphors (NPs) were used to fabricate a temperature sensing probe combined with silica fiber. Highly crystalline and pure hexagonal NPs were synthesized by a hydrothermal method. Intense green up-conversion luminescence was observed in the prototype sensor at an excitation power of 1 mW. The relationship between FIR value and temperature was investigated at the temperature range of 258–423 K. The maximum relative temperature sensitivity is 1.68% K−1 at 258 K. Preliminary experimental results indicate that the absolute error is ±1 K with a temperature uncertainty σ of 0.187 K and relative standard deviation of 0.133%, suggesting the high accuracy and reliability of the proposed all-fiber temperature sensor.

In situ characterization of the high pressure - high temperature melting curve of platinum.

Abstract: In this work, the melting line of platinum has been characterized both experimentally, using synchrotron X-ray diffraction in laser-heated diamond-anvil cells, and theoretically, using ab initio simulations. In the investigated pressure and temperature range (pressure between 10 GPa and 110 GPa and temperature between 300 K and 4800 K), only the face-centered cubic phase of platinum has been observed. The melting points obtained with the two techniques are in good agreement. Furthermore, the obtained results agree and considerably extend the melting line previously obtained in large-volume devices and in one laser-heated diamond-anvil cells experiment, in which the speckle method was used as melting detection technique. The divergence between previous laser-heating experiments is resolved in favor of those experiments reporting the higher melting slope.

High-Performance Pr3+-Doped Scandate Optical Thermometry: 200 K of Sensing Range with Relative Temperature Sensitivity above 2%·K-1.

Abstract: Nowadays, for lanthanide fluorescence thermometry, high relative temperature sensitivity (Sr) and wide sensing range are urgently required in practical applications. Herein, a Pr3+-doped scandate (Pr3+:CaSc2O4) luminescent thermometer is proposed, and the related crystal-field splitting of Pr3+ ions is systematically discussed. The 4f5d-4f and 4f-4f emissions of Pr3+ ions in CaSc2O4 basically present positive and negative correlation relationships with the increase in temperature, respectively. The different changing tendencies in relation to temperature for the two kinds of transitions are mainly derived from the effects of the thermally activated trap energy levels and the crossover process between 5d and 4f energy states of Pr3+ ions and are beneficial for the enhancement of relevant temperature sensitivities. The obtained experimental results manifest that Pr3+:CaSc2O4 owns a maximum relative temperature sensitivity Sr of 2.49%·K-1 (at 390 K) and a low temperature uncertainty (around 0.1 K from 275 to 490 K). Moreover, it is also able to keep a relatively high Sr (not lower than 2%·K-1) over a wide temperature sensing range (∼200 K), which is more excellent than those of reported luminescent thermometric materials (∼100 K). Hence, what discussed in this study might provide a useful design perspective for the exploration and development of high-performance luminescent thermometers with a wide applicable temperature range.

Ferromagnetism above 1000 K in a highly cation-ordered double-perovskite insulator Sr 3 OsO 6 .

Abstract: Magnetic insulators have wide-ranging applications, including microwave devices, permanent magnets and future spintronic devices. However, the record Curie temperature (TC), which determines the temperature range in which any ferri/ferromagnetic system remains stable, has stood still for over eight decades. Here we report that a highly B-site ordered cubic double-perovskite insulator, Sr3OsO6, has the highest TC (of ~1060 K) among all insulators and oxides; also, this is the highest magnetic ordering temperature in any compound without 3d transition elements. The cubic B-site ordering is confirmed by atomic-resolution scanning transmission electron microscopy. The electronic structure calculations elucidate a ferromagnetic insulating state with Jeff = 3/2 driven by the large spin-orbit coupling of Os6+ 5d2 orbitals. Moreover, the Sr3OsO6 films are epitaxially grown on SrTiO3 substrates, suggesting that they are compatible with device fabrication processes and thus promising for spintronic applications. Pursuing high Curie temperature magnetic insulators has been one of the extensively studied subjects due to their wide appeal for spintronic applications. Here the authors experimentally and theoretically demonstrate a record high Curie temperature over 1000 K in B-site ordered double-perovskite, Sr3OsO6.

Luminescence thermometry for in situ temperature measurements in microfluidic devices

Abstract: Temperature control for lab-on-a-chip devices has resulted in the broad applicability of microfluidics to, e.g., polymerase chain reaction (PCR), temperature gradient focusing for electrophoresis, and colloidal particle synthesis. However, currently temperature sensors on microfluidic chips either probe temperatures outside the channel (resistance temperature detector, RTD) or are limited in both the temperature range and sensitivity in the case of organic dyes. In this work, we introduce ratiometric bandshape luminescence thermometry in which thermally coupled levels of Er3+ in NaYF4 nanoparticles are used as a promising method for in situ temperature mapping in microfluidic systems. The results, obtained with three types of microfluidic devices, demonstrate that temperature can be monitored inside a microfluidic channel accurately (0.34 °C) up to at least 120 °C with a spot size of ca. 1 mm using simple fiber optics. Higher spatial resolution can be realized by combining luminescence thermometry with confocal microscopy, resulting in a spot size of ca. 9 μm. Further improvement is anticipated to enhance the spatial resolution and allow for 3D temperature profiling.