Optimization of Finned-Tube Heat Exchanger in a Gravity-Assisted Separated Heat Pipe

Finned-tube heat exchanger(FTHE)is often used as an evaporator in commercial products of separated heat pipe(SHP).The working conditions of FTHE in gravity-assisted SHP are significantly different from those working in refrigerators and air conditioners.Although FTHE is widely used in commercial products of SHP,previous research on its characteristics is very limited.In this paper,a mathematical model for a SHP with FTHE as the evaporator and plate heat exchanger as the condenser is established and verified with experiments.Parametric analyses are carried out to investigate the influences of evaporator design parameters:air inlet velocity,number of tube rows,tube diameter,and fin pitch.With the increasing of air velocity,number of tube rows and tube diameter,and the decreasing of fin pitch,the heat transfer rate increases,while the energy efficiency ratio(EER)decreases monotonically.Using the total cost of the ten-year life cycle as the performance index,the structure parameters of the evaporator with a given heat transfer rate are optimized by the method of orthogonal experimental design.It is found that the total cost can differ as large as nearly ten times between groups.Among the three factors investigated,the number of tube rows has a significant impact on the total cost of the evaporator.With more tube rows,the total cost will be less.The impacts of fin pitch and tube diameter are insignificant.These results are of practical importance for the engineering design of FTHE in gravity-assisted SHP.

Unveiling localized electronic properties of ReS2 thin layers at nanoscale using Kelvin force probe microscopy combined with tip-enhanced Raman spectroscopy

Electronic properties of two-dimensional(2D) materials can be strongly modulated by localized strain. The typical spatial resolution of conventional Kelvin probe force microscopy(KPFM) is usually limited in a few hundreds of nanometers, and it is difficult to characterize localized electronic properties of 2D materials at nanoscales. Herein, tip-enhanced Raman spectroscopy(TERS) is proposed to combine with KPFM to break this restriction. TERS scan is conducted on ReS2bubbles deposited on a rough Au thin film to obtain strain distribution by using the Raman peak shift. The localized contact potential difference(CPD) is inversely calculated with a higher spatial resolution by using strain measured by TERS and CPD-strain working curve obtained using conventional KPFM and atomic force microscopy. This method enhances the spatial resolution of CPD measurements and can be potentially used to characterize localized electronic properties of 2D materials.

Unlocking the charge doping effect in softly intercalated ultrathin ferromagnetic superlattice

The electrolyte-assisted exfoliation strategy is widely employed to synthesize ultrathin two-dimensional(2D)materials.Yet,spins in 2D magnets are susceptible to the electrolyte due to the underlying charge doping effect.Hence,it is crucial to understand and trace the doping process during the delamination of 2D magnets.Taking the prototype Fe_(3)GeTe_(2),we utilized soft organic cations to exfoliate the bulk and obtain a freestanding organ-ic–inorganic hybrid superlattice with a giant electron doping effect as high as 6.9×10^(14)/cm^(2)(~1.15 electrons per formula unit).A remarkable ferromagnetic transition exceeding 385 K was revealed in these superlattices,together with unique anisotropic saturation magnetization.The doping enhanced the in-plane electron–phonon coupling and thus optimized originally poor indirect double-exchange scenario for spin electrons.The emerging vertical magnetization shift phenomenon served to evaluate the uniformity of charge doping.The above findings provide a new perspective for understanding the role of parasitic charge in 2D magnetism.

Thickness-dependent enhanced optoelectronic performance of surface charge transfer-doped ReS2 photodetectors

Surface charge transfer doping has been widely utilized to tune the electronic and optical properties of semiconductor photodetectors based on low-dimensional materials.Although many studies have been conducted on the performance(response time,responsivity,etc.)of doped photodetectors and their mechanisms,they merely examined a specific thickness and did not systematically explore the dependence of doping effects on the number of layers.This work performs a series of investigations on ReS_(2)photodetectors with different numbers of layers and demonstrates that the p-dopant tetrafluorotetracyanoquinodimethane(F_(4)-TCNQ)converts the deep trap states into recombination centers for few-layer ReS_(2)and induces a vertical p-n junction for thicker ReS_(2).A response time of 200 ms is observed in the decorated 2-layer ReS_(2)photodetector,more than two orders of magnitude faster than the response of the pristine photodetector,due to the disappearance of deep trap states.A current rectification ratio of 30 in the F_(4)-TCNQ-decorated sandwiched ReS_(2)device demonstrates the formation of a vertical p-n junction in a thicker ReS_(2)device.The responsivity is as high as 2,000 A/W owing to the strong carrier separation of the p-n junction.Different thicknesses of ReS_(2)enable switching of the prominent operating mechanism between transforming deep trap states into recombination centers and forming a vertical p-n junction.The thicknessdependent doping effect of a two-dimensional material serves as a new mechanism and provides a scheme toward improving the performance of other semiconductor devices,especially optical and electronic devices based on low-dimensional materials.

Luminescent ionic lattice occupation and wide tunable emission spectra of La_(2)MgZrO_(6):Bi^(3+),Eu^(3+)double perovskite phosphors for white light LED

La_(2)Mg_(1-x/2)Zr_(1-x/2)O_(6):xBi^(3+)(x=0.01-0.035,abbreviated as LMZ:Bi^(3+))and La_(2-y)Mg_(0.99)Zr_(0.99)O_(6):0.02Bi^(3+),yEu^(3+)(y=0.1-0.11,abbreviated as LMZ:Bi^(3+),Eu^(3+))double-perovskite phosphors were prepared through high-temperature solid-phase method.The emission spectrum of LMZ:xBi^(3+)(x=0.01-0.035)phosphors excited at 353 nm is asymmetric in the range between 375 and 650 nm,showing strong green light.There are two luminescent centers of[Mg1/Zr2-O_(6)]and[Mg2/Zr1-O_(6)]for Bi^(3+)occupation,which were analyzed through different excitation wavelengths,Gaussian fitting peaks,fluorescence decay curves and Rietveld refinement of powder X-ray diffraction data.Through deep study of the luminescent lattices in the LMZ matrix,the green to blue tunning-emission is observed by different excitation wavelengths.In addition,red emission is obtained by co-doping Bi^(3+)/Eu^(3+),and adjustable emission was investigated by changing the content of Eu^(3+)in the co-doped phosphor formulation,so it is converted from green emission to red emission.The above results demonstrate how to tune emission color by co-doping rare earth ions in the double perovskite phosphor,which is attractive for future applications.

Transforming bilayer MoS2 into single-layer with strong photoluminescence using UV-ozone oxidation

The use of single-layer MoS2 in light emitting devices requires innovative methods to enhance its low photoluminescence （PL） quantum yield. In this work, we report that single-layer MoS2 with a strong PL can be prepared by oxidizing bilayer MoS2 using W-ozone oxidation. We show that as compared to mechanically-exfoliated single-layer MoS2, the PL intensity of the single-layer MoS2 prepared by W-ozone oxidation is enhanced by 20-30 times. We demonstrate that the PL intensity of both neutral excitons and trions （charged excitons） can be greatly enhanced in the oxidized MoS2 samples. These results provide novel insights into the PL enhancement of single-layer MoS2.

Significant enhancement of blue photoluminescence intensity of Dy^(3+) modified Sr_(6)Ca_(4)(PO_(4))_(6)F_(2):Eu^(2+) phosphors

Blue-emitting phosphors Sr_(6)Ca_(4)(PO_(4))_(6) F_(2):Eu^(2+)(SCPF:Eu^(2+)),Sr_(6 )Ca_(4)(PO4)6 F_(2):Eu^(2+),Dy^(3+)(SCPF:Eu^(2+),Dy^(3+))and Sr_(6)Ca_(4)(PO4)6 F_(2):Eu^(2+),Dy^(3+),Si^(4+)(SCPF:Eu^(2+),Dy^(3+),Si^(4+)) with apatite structure were successfully synthesized by traditional solid-state reaction under reducing atmosphere.Eu^(2+),Dy^(3+) and Si^(4+)ions occupy the corresponding sites of Sr^(2+),Ca^(2+) and P5+.Strong broad blue photo luminescence band is exhibited in SCPF:Eu^(2+),Dy^(3+) phosphor ranging from 400 to 550 nm centered at 455 nm and Dy^(3+) ions are vital in creating traps.Emission intensity of Eu^(2+),Dy^(3+) co-doped SCPF:0.02 Eu^(2+),0.02 Dy^(3+) is about 1.8 times that of SCPF:0.02 Eu^(2+) and electron trap centers serve as energy transporting media.To further elucidate the formation and effect of the specific defect on the luminescence of SCPF:0.02 Eu^(2+),0.02 Dy^(3+) phosphor,the thermoluminescence properties,decay curves and thermal stability studies were performed while the Si^(4+)-P^(5+) charge compensated pho sphor SCPF:0.02 Eu^(2+),0.02 Dy^(3+),0.02 Si^(4+) was prepared as a contrast.All the results of present work indicate that Dy^(3+) co-doping can obviously enhance photoluminescence intensity of SCPF:0.02 Eu^(2+) by the electron traps generated by non-equivalence replacement of Dy^(3+)-Ca^(2+).

A dense graphene monolith with poloxamer prefunctionalization enabling aqueous redispersion to obtain solubilized graphene sheets

The realization of good aqueous dispersibility of commercial graphene products composed of exfoliated graphene sheets is of significance for downstream applications.However,the tap density of commercial graphene powder is quite low(0.03-0.1 kg/m3),meaning that 1 kg graphene powder occupies about 10-30 m3 in volume during transportation.And,the available content of commercial graphene dispersion/slurry in aqueous medium cannot exceed 5 wt%,although the density is high(≈1050 kg/m3).In this work,a graphene monolith was prepared by oven-drying of graphene sheets prefunctionalized with poloxamer surfactants.Our graphene monoliths not only have a high density(1500 kg/m^3) and high graphene content(≈10 wt%),but also a full capability to be completely redispersed(≈100%) in water by bath sonication to obtain solubilized graphene sheets,whose lateral size and thickness are unchanged compared to as-exfoliated ones.Moreover,a simple empirical method was proposed to predict the redispersion capability of graphene monoliths using different poloxamers by contact angle measurements.Our results provide a universal approach to make exfoliated graphene-based products with better downstream availability and lower transportation cost.