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At least 829 records · Page 46

Recent progress in half-Heusler thermoelectric materials

Highlights: • Summarize the recent progress and advances in HH thermoelectric materials. • Preparing nanocomposites could reduce thermal conductivity. • Introducing enhance phonon scattering could further reduce the thermal conductivity. • Forming ternary systems to reducing the cost effectively. • The new class of HHs presents another opportunity to further optimize the HH system. - Abstract: Half-Heusler (HH) thermoelectric (TE) materials have been attracting extensive research interest over the last two decades, owing to their thermal stability, mechanical strength, and moderate ZT. This material system are potential candidates for medium to high temperature applications, which is close to the temperature range of most industrial waste heat sources. In this mini-review article, we briefly summarize the recent progress and advances in HH thermoelectric materials. Some effectively available approaches, such as HH nanocomposites to reduce thermal conductivity, using larger atomic mass and size differences to enhance phonon scattering to further reduce the thermal conductivity, forming ternary systems following the cost effective approach. In addition, new thermoelectric HH members are also discussed in this article, which points out that many new HH compounds may be possible for TE applications.

36 MATERIALS SCIENCE↗

Influence of activation parameters on the thickness of MgO thin film on Ag-3Mg alloy and its secondary electron emission property

Highlights: • MgO films with controlled thicknesses on Ag-3Mg alloys were prepared by activation in low vacuum with oxygen. • Superior secondary electron yield was obtained for the Ag-3Mg alloy with proper thickness of MgO film. • Oxygen pressure during the activation process is crucial for obtaining good secondary electron yield for Ag-3Mg alloys. • The mechanism of secondary electron emission in MgO films is discussed. - Abstract: MgO thin films of different thicknesses were prepared on polished Ag-3wt%Mg alloy by an activation process at 500–650 °C under oxygen pressures of 0.5–10.0 Pa. The influence of the thickness of MgO film on the secondary electron yield of the Ag-3wt%Mg alloy is investigated. The alloy with an MgO film of moderate thickness of 65 ± 1 nm shows the highest secondary electron yield of 10. Oxygen pressure below 10.0 Pa is crucial for the formation of intact MgO film on the top surface. Computational simulation analysis reveals that the maximum yield occurs when the penetration depth of primary electrons is 13–18 nm less than the thickness of MgO film. Electron supply of this extra MgO film is allowed by an electron tunneling mechanism under certain bias voltage. The replenishment of new electrons to the electron deficient surface is crucial for obtaining optimized secondary electron yields for Ag-Mg alloy cathodes.

36 MATERIALS SCIENCE↗

Realization of strong violet and blue emissions from ZnO thin films by incorporation of Cu ions

Highlights: • A strong blue-violet co-emission occurs after Cu ions are doped into ZnO thin films.. • The violet emission centered at 430 nm is attributed to transitions of electrons from conduction band to Cu{sup +} acceptor levels. • The blue emission centered at 470 nm is attributed to transitions of electrons from Cu{sup 2+} donor levels to Cu{sup +} acceptor levels. • Both the violet and blue emissions decrease when Cu-doping concentration is higher than 2 at.%. - Abstract: In recent years, Cu-doped ZnO materials have attracted wide attention due to some unique properties, such as p-type conduction, room-temperature ferromagnetism and plentiful luminescence behavior. In this work, Cu-doped ZnO thin films with different doping levels were prepared by sol–gel method. The structural, morphological features and the composition of the samples were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray spectroscopy (EDX), respectively. The optical properties were analyzed by the UV–visible transmittance spectra and fluorescence spectra. The Cu-doping in a low concentration does not influence the crystal phase of wurtzite ZnO and has a very small impact on c-axis preferred orientation of ZnO thin films. With the increase of Cu-doping concentration, the Cu ions are gradually changed from dominant Cu{sup +} ions to dominant Cu{sup 2+} ions. Cu-doping in a low concentration caused strong blue-violet emissions in ZnO thin films. The violet emission is attributed to the transition of electrons from the conduction band to Cu{sup +} acceptor levels, and the blue emission is ascribed to the transition of electrons from Cu{sup 2+} donor levels to Cu{sup +} acceptor levels.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Correlation between structure and microwave dielectric properties of low-temperature-fired Mg2SiO4 ceramics

Highlights: • Mg{sub 2}SiO{sub 4}–xLiF ceramics could be sintered at 900 °C. • The microwave dielectric properties were significantly dependent on the structure. • The following optimum properties were achieved in the x = 8 wt% ceramics. • Temperature stability of Mg{sub 2}SiO{sub 4}–8 wt% LiF–6.0vol% CaTiO{sub 3} ceramics was obtained. • These ceramics also exhibited excellent chemical compatibility with Ag. - Abstract: Mg{sub 2}SiO{sub 4}–LiF ceramics were fabricated via a conventional solid-state process and sintered at 900 °C. The secondary phase, Li{sub 2}SiO{sub 3}, was observed with ≤6 wt% LiF, and the Li{sub 2}MgSiO{sub 4} phase was detected with ≥8 wt% LiF. The relative density and secondary phase can account for the decrease in the ε{sub r} with an increase in LiF. The degree of crystallization, unit cell volume, secondary phase, and microstructure correlated with the Q × f values. The value of the τ{sub f} was related to the distortions in the Mg(1)O{sub 6} octahedron and the secondary phase. The sample exhibited excellent properties for Mg{sub 2}SiO{sub 4}–8 wt% LiF with ε{sub r} = 6.1, Q × f = 109719 GHz, and τ{sub f} = − 61 ppm/°C. The temperature stability of the Mg{sub 2}SiO{sub 4}–8 wt%LiF–6 vol%CaTiO{sub 3} ceramics was obtained with ε{sub r} = 7.1, Q × f = 31091 GHz and τ{sub f} = − 2.0 ppm/°C. Mg{sub 2}SiO{sub 4}–LiF–CaTiO{sub 3} ceramics provide excellent chemical compatibility with Ag.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Research of sensing characteristic and dynamic measurement of graphene oxides modified flexible arrayed RuO2 chlorine ion sensor

Highlights: • Sensitivity of the flexible arrayed RuO2/GO chlorine ion sensor was improved with optimal GO concentration, which could increase the reaction area between the chlorine ion and chlorine sensing membrane. • The electrochemical impedance spectroscopy (EIS) and atomic force microscope (AFM) were utilized to explain the relationships among the electron transfer resistance, GO modified amount and sensitivity. • The 30 μL/min flow rate of dynamic measurement for flexible arrayed RuO2/0.010 wt% GO chlorine ion sensor was obtained the best sensitivity and linearity 66.673 mV/pCl and 0.998, respectively. - Abstract: In this study, we used the graphene oxide (GO) solution, in which the concentration was from 0.005 wt% to 0.500 wt%, to modify the flexible arrayed ruthenium dioxide (RuO{sub 2}) chlorine ion sensor. It is found that the flexible arrayed RuO{sub 2}/0.010 wt% GO chlorine ion sensor had the best sensitivity (44.445 mV/pCl) and the linearity was 0.971. The electrochemical impedance spectroscopy (EIS) and atomic force microscope (AFM) were utilized to explain the relationships among the electron transfer resistance, GO modified amount and sensitivity. After that, we used the flexible arrayed RuO{sub 2}/0.010 wt% GO chlorine ion sensor and the microfluidic measurement system to investigate the dynamic sensing characteristics with different flow rates from 5 μL/min to 40 μL/min. According to the experimental results, the best sensitivity and linearity of flexible arrayed RuO{sub 2}/0.010 wt% GO chlorine ion sensor were 66.673 mV/pCl and 0.992 at 30 μL/min.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Chemical bath deposition of single crystal SnS nanobelts on glass substrates

Highlights: • Single crystal SnS nanobelts were deposited by CBD method. • The morphologies of SnS nanobelts were regulated by mole ratios of precursors and pH levels. • The photo-electrical properties of SnS nanobelts were characterized. - Abstract: SnS nanobelt thin films were deposited on glass substrates in acidic solution by chemical bath deposition (CBD) method. The belt-like morphologies of as-deposited SnS thin films were characterized by scanning electron microscope (SEM) and transmission electron microscopy (TEM). X-ray diffraction (XRD) and Raman measurements were carried out to confirm the crystal structures and phase purities of SnS nanobelt thin films. The morphologies and phase purities of SnS thin films were influenced greatly by the tin and sulfur precursors. The bandgaps of SnS nanobelts were determined to be 1.39–1.41 eV by UV–vis absorption and photoluminescence (PL) spectra. Current-voltage ((IV)) and current-time ((I-T)) characteristics were studied to demonstrate the photoelectric performances of SnS nanobelt thin films.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Facile synthesis of the nitrogen-doped graphene quantum dots at low temperature for cellular labeling

Highlights: • The N-GQDs are prepared via a facile stirring method on a magnetic stirrer at low temperature. • The N-GQDs exhibit outstanding biocompatibility in the live HeLa cells for biological applications. • The live cells keep a 100% survival rate in the high N-GQDs concentration of 200 μg mL{sup −1} for 48 h. - Abstract: A facile one-step approach is developed for the large-scale synthesis of nitrogen doped graphene quantum dot (N-GQDs) at low temperature by using citric acid (CA) as the precursor. The emission wavelength of the N-GQDs is nearly excitation-dependent, which may mirror the effects from particles of different sizes and distribution of different emissive sites on nanoparticles. Moreover, the products are then tested with live Hela cells for the extraordinary biocompatibility. The cellular viability is remained to be 100% even after a 48 h exposure with the high N-GQDs concentration of 200 μg mL{sup −1} and this survival rate is really much higher than that have been reported previously. The as-prepared N-GQDs with negligible inherent toxicity exhibit excellent biological applications such as bio-imaging and cell tracking.

60 APPLIED LIFE SCIENCES↗

Recyclable magnetic Fe3O4@SiO2/β-NaYF4:Yb3+,Tm3+/TiO2 composites with NIR enhanced photocatalytic activity

Highlights: • Fe{sub 3}O{sub 4}@SiO{sub 2}/β-NaYF{sub 4}:Yb{sup 3+},Tm{sup 3+}/TiO{sub 2} composites have been successfully fabricated. • The upconversion of energy transfer from β-NaYF{sub 4}:Yb{sup 3+},Tm{sup 3+} to TiO{sub 2} was confirmed. • Fe{sub 3}O{sub 4}@SiO{sub 2}/β-NaYF{sub 4}:Yb{sup 3+},Tm{sup 3+}/TiO{sub 2} composites exhibit photocatalytic activity in NIR. • The composites can be easily magnetic separated from water after application. • The composites show good stability indicated from four recycled tests. - Abstract: To extend the activities of TiO{sub 2} to the near infrared (NIR) region and to separate and recycle photocatalysts easily is vital for the high harvest of solar energy for sustainable society. The magnetic NIR activated Fe{sub 3}O{sub 4}@SiO{sub 2}/β-NaYF{sub 4}:Yb{sup 3+},Tm{sup 3+}/TiO{sub 2} (FS/UC/T) composites have been synthesized by using the solvothermal method by adding Fe{sub 3}O{sub 4}@SiO{sub 2} and Degussa P25 into the mixed solution of NaOH, ethanol, oleic acid, Re(NO{sub 3}){sub 3} and NH{sub 4}F. Moreover, the obtained composites were explored for photocatalytic applications. It was found that FS/UC/T emit UV–vis light under irradiation of 980 nm laser and efficient energy transfer from β-NaYF{sub 4}:Yb{sup 3+},Tm{sup 3+} to TiO{sub 2} was verified. The photodegradation experiments for treating methylene blue (MB), rhodamine B (RhB), methyl orange (MO) and colorless phenol were carried out to evaluate the photocatalytic activities of the samples. The photodegradation rates reach 76.62%, 68.48%, 30.05% and 27.16% for MB, RhB, MO and phenol under irradiation of 980 nm laser, respectively, which suggested magnetic FS/UC/T composite is an advanced NIR-driven photocatalyst. Moreover, the magnetic photocatalyst showed easy separation and good stability in the recycled tests. It suggested a promising system to utilize the NIR energy for photocatalysis based on TiO{sub 2} and to utilize the magnetic for separation of photocatalysts, which will contribute to practical environmental remediation in the future.

36 MATERIALS SCIENCE↗

Effect of Eu/Ca ratio on the crystalline phase, morphology and luminescent properties of Ca2-1.5xEux(MoO4)2 (0 ≤ x ≤ 4/3) ceramics phosphors

Highlights: • Novel Ca{sub 2-1.5x}Eu{sub x}(MoO{sub 4}){sub 2} red phosphors have been proposed and fabricated. • The correlations between the microstructure and PL have been investigated. • This work may provide a guidance to design for molybdate phosphors. - Abstract: A series of Ca{sub 2-1.5x}Eu{sub x}(MoO{sub 4}){sub 2} (0 ≤ x ≤ 4/3) phosphors have been fabricated by a traditional solid-state reaction. The correlations between the crystal structure, luminescent properties and surface morphologies with the chemical composition of the as-fabricated Ca{sub 2-1.5x}Eu{sub x}(MoO{sub 4}){sub 2} crystals have been investigated in detail. The results show that the Ca{sub 2-1.5x}Eu{sub x}(MoO{sub 4}){sub 2} (0 ≤ x ≤ 1.3) phosphors possess tetragonal phase, whereas the obtained Ca{sub 2-1.5x}Eu{sub x}(MoO{sub 4}){sub 2} (x = 4/3) (i.e. Eu{sub 2}(MoO{sub 4}){sub 3}) phosphor exhibits monoclinic phase. The morphologies of crystals change gradually from quasi sphericity to tetragonal bipyramid owing to the preferential growth along c-axis induced by Eu{sup 3+} ions. The photoluminescence spectra reveal that the intensities of the characteristic emission and excitation peaks initially increase and then decrease with increasing x, given the maximum at x = 1/3. Our results indicate that the difference in the charge, electronegativity, as well as ionic radius between Ca{sup 2+} ions and Eu{sup 3+} ions lead to the variation of crystal symmetry, crystal phase, surface morphologies and luminescence of the final Ca{sub 2-1.5x}Eu{sub x}(MoO{sub 4}){sub 2} crystals.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

Sulvanite (Cu 3 VS 4 ) nanocrystals for printable thin film photovoltaics

Copper Vanadium Sulfide (Cu 3 VS 4 ), also known as sulvanite, has recently emerged as a suitable absorber material for thin film photovoltaics. The synthesis of Cu 3 VS 4 nanocrystals via a rapid solvothermal route is reported for the first time. The phase purity of the Cu 3 VS 4 nanocrystals has been confirmed by X-ray powder diffraction (XRD) and Raman spectroscopy, while the nanoparticle size, of about 10 nm, was evaluated by transmission electron microscopy (TEM). Successful ligand exchange with sulfide, an inorganic ligand, demonstrated that the nanoparticles are amenable to surface modifications, key element in solution processing. Further annealing of as-synthesized nanocrystals under a sulfur/argon atmosphere at 600 °C, rendered highly crystalline Cu 3 VS 4 powders exhibiting an impurity that could be potentially mitigated by annealing temperature optimization. Furthermore, Cu 3 VS 4 , formed solely from Earth-abundant elements, could provide an inexpensive, reliable approach to fabricating solution processed thin film photovoltaic absorbers.

36 MATERIALS SCIENCE↗

A rapid molecular precursor solid-state route to crystalline Fe 2 GeS 4 nanoparticles

Iron germanium sulfide (Fe 2 GeS 4 ) recently emerged as a potential thin film solar photovoltaic absorber. The introduction of the third element—germanium (Ge)—viewed as a solution for overcoming multiple barriers of a photovoltaic pyrite, confers stability to Fe 2 GeS 4 at elevated temperatures, typically required for accomplishing grain growth in Gen 2 thin film PV. A facile synthesis of Fe 2 GeS 4 nanoparticles from molecular precursors, comprising mechanical mixing of starting materials followed by a two-hour annealing in a sulfur-rich atmosphere is presented herein. Further processing of the resulting Fe 2 GeS 4 nanopowders at elevated temperatures demonstrates high thermal stability of Fe 2 GeS 4 (up to 500 °C), in comparison with pyrite, which shows onset of pyrrhotite upon heating above 160 °C. Based on the secondary crystalline phases formed, here we propose a mechanism of decomposition of Fe 2 GeS 4 at high temperatures. Films fabricated with Fe 2 GeS 4 were further annealed and revealed that Fe 2 GeS 4 withstands high temperatures in thin film.

36 MATERIALS SCIENCE↗

Impact of intravenous contrast used in computed tomography on radiation dose to carotid arteries and thyroid in intensity-modulated radiation therapy planning for nasopharyngeal carcinoma

The aim of this study was to investigate if intravenous contrast injection affected the radiation doses to carotid arteries and thyroid during intensity-modulated radiation therapy (IMRT) planning for nasopharyngeal carcinoma (NPC). Thirty consecutive patients with NPC underwent plain computed tomography (CT) followed by repeated scanning after contrast injection. Carotid arteries (common, external, internal), thyroid, target volumes, and other organs-at-risk (OARs), as well as IMRT planning, were based on contrast-enhanced CT (CE-CT) images. All these structures and the IMRT plans were then copied and transferred to the non–contrast-enhanced CT (NCE-CT) images, and dose calculation without optimization was performed again. The radiation doses to the carotid arteries and the thyroid based on CE-CT and NCE-CT were then compared. Based on CE-CT, no statistical differences, despite minute numeric decreases, were noted in all dosimetric parameters (minimum, maximum, mean, median, D05, and D01) of the target volumes, the OARs, the carotid arteries, and the thyroid compared with NCE-CT. Our results suggested that compared with NCE-CT planning, CE-CT scanning should be performed during IMRT for better target and OAR delineation, without discernible change in radiation doses.

61 RADIATION PROTECTION AND DOSIMETRY↗

Surpassing the conventional limitations of CO 2 separation membranes with hydroxide/ceramic dual-phase membranes

We report the development of a dual-phase membrane for CO 2 separation based on a molten hydroxide liquid phase and a nanoporous yttria-stabilized zirconia solid support phase, termed hydroxide/ceramic dual-phase (HCDP) membranes, that can operate in the flue gas temperature range of 250–650 °C. HCDP membranes demonstrate selectivity for CO 2 over N 2 greater than 1000 and CO 2 permeability of 1.78 ± 0.16×10 -10 mol m m –2 s –1 Pa –1 (5.32 × 10 5 barrer) at 550 °C with 20 vol% CO 2 , which is more than an order of magnitude greater than the best values for dual-phase and polymer membranes reported in the literature. Here, we also demonstrate using three different experimental methods that CO 2 absorption by molten hydroxides is reversible in the presence of water vapor at temperatures as low as 300 °C.

36 MATERIALS SCIENCE↗

Characterization of CO 2 flux through hollow-fiber membranes using pH modeling

CO 2 must be delivered efficiently during large-scale microalgal cultivation. Bubbleless mass-transfer via diffusion through hollow-fiber membranes (HFM) can achieve much higher CO 2 -transfer efficiency than traditional sparging systems. Here this study developed and used a model to compute accurate values for the CO 2 flux (J CO 2 ), overall mass-transfer coefficient (K L ), and overall volumetric mass-transfer coefficient (K L a) based on the rate of change of pH in batch experiments. A composite HFM comprised of two macroporous polyethylene layers and a nonporous polyurethane layer was tested for CO 2 transfer to a sodium carbonate solution using a range of total pressures, inlet CO 2 concentrations, and open-end versus closed-end modes of operation. The model accurately computed J CO2 and K L a for pH values above 8. Key trends are that (i) JCO2 and KLa increased with increasing average inlet CO 2 partial pressure; (ii) open-end HFMs performed better than closed-end HFMs when the supplied CO 2 was less than 100%; and (iii) the available membrane area used for CO 2 mass-transfer decreased as the inlet CO 2 partial pressure decreased due to depletion of CO 2 inside the membrane, especially for closed-end HFMs, since inert gases could not be vented.

42 ENGINEERING↗