Search

Experiments on R1234yf flow boiling in a horizontal 2.01 mm copper tube under hypergravity are conducted to investigate the effect of gravity on two-phase flow boiling heat transfer coefficient (HTC). The experimental range of parameters is gravity levels of 1–3.16 g, mass fluxes of 365 and 570 kg m−2 s−1, saturation pressures of 0.62 and 0.70 MPa, heat fluxes of 47.6 and 97.5 kW m−2, and vapor qualities up to 0.985. Based on the experimental data, the effects of gravity, quality, heat flux, mass flux, and saturation pressure on the HTCs were analyzed. The results show that the HTCs increase with increasing gravity levels for the qualities up to around 0.7, and the gravity effects become indistinct and inconsistent when the quality is high (x > 0.8). The trends of the HTC variation with quality and the effects of the heat flux, mass flux and saturation pressure on the HTCs are similar under different gravity levels. When x 0.7 or higher, and they occur earlier when the gravity level is higher. The comparison of the experimental data with existing saturated flow boiling HTCs for normal gravity shows that the Fang (2013b) correlation for R134a performs best with a mean absolute deviation (MAD), and that all of the top four correlations in prediction accuracy under-predict the experimental HTCs, which is consistent with the experimental results that hypergravity enhances flow boiling heat transfer.

Among various earth-abundant and noble metal-free catalysts for oxygen reduction reaction (ORR), Mn and N co-doped carbon (Mn-N-C) is highly desirable and promising, which, however, suffer from the limited amount of active sites largely abating its ORR catalytic performance. Herein we demonstrate an effective strategy to elevate the ORR active site density by designing a helical graphitized carbon tubes to highly disperse the single Mn atomic sites coordinated with nitrogen. The obtained polydopamine-modified helical MnNC-PDA-700 catalyst shows excellent ORR electrocatalytic performance with a half-wave potential of 0.87 V and extra-high Zn-air battery power density of 122.7 mW cm−2, which are comparable to and even higher than those of Pt/C. Such an excellent electrocatalytic performance is attributed to the much enhanced amount of MnN4 active sites created on the helical graphitized carbon tubes owing to high surface area helical structure of the carbon tubes and the strong bonding of polydopamine molecules onto the tubes. The density functional theory (DFT) calculations further confirm that the MnN4 sites are the origin of the superior ORR activity via a 4e− pathway in alkaline media.

Hepatic stellate cells (HSCs) play a crucial role in the progression of liver fibrosis, which can be considered as the specific therapeutic target of anti-fibrotic treatment. Targeted induction of HSCs to hepatocytes via delivery of clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (dCas9) system holds promise for hepatic fibrosis treatment. Our study here revealed that CRISPR/dCas9-VP64 system encapsulated in AML12 cell-derived exosomes could efficiently and successfully be delivered into the HSCs. In turn, the CRISPR/dCas9-VP64 system loaded in the exosomes can be efficiently released into the HSCs. As a proof-of-concept study, gRNA against hepatocyte nuclear factor 4α (HNF4α) together with the delivery of CRISPR/dCas9-VP64 system induced the HSCs to hepatocyte-like phenotype. In conclusion, our study here revealed that CRISPR/dCas9-VP64 system encapsulated in AML12 cell-derived exosomes could be functional in HSCs, emerging as a gene therapy strategy for hepatic fibrosis.

Platinum (Pt) is an efficient catalyst for hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR), but the debate of the relevance between the Pt particle size and its electrocatalytic activity still exist. The strong metal-support interaction (SMSI) between the metal and carrier causes the charge transfer and mass transport from the support to the metal. Herein, Pt species (0.5 wt.%) with various particle sizes supported on carbon nanotubes (CNTs) have been synthesized by a photo-reduction method. The ~1.5 nm-sized Pt catalyst shows much higher HER performance than the counterparts in all pH solutions, and the mass activity of it is even 23–36 times that of Pt/C. While for ORR, the ~3 nm-sized Pt catalyst exhibits the optimal performance, and the mass activity is 3 times and even 16 times that of Pt/C in acidic and alkaline media, respectively. The high HER and ORR performances of the ~1.5 nm- and ~3 nm-sized Pt catalysts benefit from the SMSI between Pt and the CNTs matrix and the higher ratio of face sites to edge sites, which is meaningful for the design of efficient electrocatalysts for renewable energy application.

With the progress in science and technology, non-oxide ceramics (NOCs) are playing a vital role in various fields at high temperature. However, under harsh service conditions, NOCs tend to be confr...

The development of smart grid calls for pervasive deployment of current measurement in the power transmission and distribution system, serving for monitoring, control and protection purpose. However, traditional methods fail to measure current with bandwidth from direct current to kHz and are inconvenient to install or dismount. In this paper, a novel current measurement method with a clamp-like structure is proposed to alleviate the magnetic interference from random sources. This method is based on a special magnetic sensor array which consists of three pairs of sensors. In each pair, the sensitive directions of the sensors are opposite. Besides, positioning calibration of the conductor under measurement is considered in order to reduce the error contributed by the displacement uncertainty of the conductor. The proposed design is tested by numerical simulation and finite element analysis (FEA). For peak current up to 500 A, the maximum error is 0.6532%. A prototype is then implemented and tested in laboratory experiments to demonstrate the performance of the proposed approach for measurement and protective purpose. From the results, the maximum error is 1.73%, which is compatible with standard IEC 60044-8:2002 accuracy classes for protective electronic current transformer.

Aluminum nitride (AlN) with a combination of very high thermal conductivity and excellent electrical insulation properties exhibits wide applications. However, it is quite sensitive to a moist envi...

The Dirac-like cones underlie many unique properties of photonic crystals (PhCs). This paper aims to design fabrication-friendly PhCs with Dirac-like cones for transverse magnetic (TM) modes and transverse electric (TE) modes at different specific frequencies. By maximizing the minimum of a collection of the local density of states corresponding to different judiciously selected sources, this paper demonstrates that Dirac-like cones formed by the degeneracy of a doubly degenerate mode and a single mode at different desired frequencies are successfully obtained. The exotic wave manipulation properties associated with Dirac-like cones, such as cloaking, wavefront shaping and tunneling through bent channels, are exhibited based on the optimized structures. This paper also demonstrates that the proposed method could be used for the design of PhCs with one Dirac-like cone at ω , and one monopolar band at 2 ω at the Γ point, and PhCs with third order Dirac-like cones, which have potential applications in nonlinear optics. All topological patterns of the optimized PhCs are reported and have regular and smooth features, meaning they can be readily fabricated.

With an auxiliary weak external magnetic field, we reexamine the fundamental thermodynamic function, Gibbs free energy $G(T,h)$, to study phase transitions in classical spin lattice models. A cross derivative, i.e., the second-order partial derivative of $G(T,h)$ with respect to both temperature and field, is calculated to precisely locate the critical temperature, which also reveals the nature of a transition. The strategy is efficient and universal, as exemplified by the five-state clock model, two-dimensional (2D) and 3D Ising models, and the $XY$ model, no matter if a transition is trivial or exotic with complex excitations. More importantly, other conjugate pairs could also be integrated into a similar cross derivative if necessary, which would greatly enrich our vision and means to investigate phase transitions both theoretically and experimentally.

With superior electrical and thermal properties, aluminum nitride (AlN) exhibits wide application. However, AlN is rather oxygen-sensitive and tends to be oxidized at high temperature. The surface oxidation of AlN remains a major challenge, while the underlying physics of AlN surface oxidation is still elusive. Here, First-principles calculations were performed to study wurtzite AlN(0001) surface oxidation process. The adsorption energy of oxygen was calculated to be site-dependent on the surface with varying O coverage. Calculation indicates that oxygen atoms are preferentially adsorbed at the hollow site (H3) of the AlN(0001) surface regardless of the O coverage. N

Oxygen evolution reaction (OER) is a pivotal electrochemical reaction process for many renewable energy technologies. Due to the sluggish OER kinetics, searching for efficient low-cost non-precious metal catalysts is one of the crucial but very challenging steps. Herein, V2C MXene synergistically coupled with hypophosphite-intercalated FeNi (oxy)hydroxide (H2PO2−/FeNi-LDH-V2C) electrocatalyst is synthesized. The H2PO2−/FeNi-LDH-V2C exhibits excellent OER performance with an overpotential of 250 mV (η10) and small Tafel slope of 46.5 mV dec−1 in 1.0 M KOH electrolyte, and excellent rechargeable Zn-air battery performance with superior open circuit potential (1.42 eV), power density (137 mW cm−2) and well durability. The strong interaction and electronic coupling with prominent charge-transfer between FeNi-LDHs and V2C MXene endow the composite significant OER performance and structural stability, and the adsorption/desorption balance for the OER reaction pathway, eventually promoting the intrinsic activity. This work demonstrates the great promise of MXene-based nanohybrids as advanced electrocatalysts for renewable energy applications.

Developing highly efficient noble metal-free electrocatalysts for hydrogen oxidation reaction (HOR) and oxygen reduction reaction (ORR) in alkaline exchange membrane fuel cells (AEMFCs) is of great importance for the large-scale applications, which, however, still remains a great challenge. Here we present the synthesis of SnO2 and CeO2 nanoparticle-decorated mesoporous ZSM-5 (denoted as SnCe-ZSM) via electrostatic interaction, which, surprisingly, not only exhibits excellent catalytic activity towards HOR, but also largely enhance ORR performance. The strong electronic interaction between SnO2 and CeO2 nanoparticles synergistically leads to the electron-enrichment on SnO2 and oxygen vacancy accumulation on CeO2, which respectively enable the hydrogen and oxygen adsorption/activation concurrently, finally resulting in the significant enhancements of the electrocatalytic activity towards both HOR and ORR. Additionally, the SnCe-ZSM based zinc-air battery exhibits a remarkably high power density of 98 mW/cm2 and excellent stability during discharging/charging cyclic tests for 60 h. The SnCe-ZSM electrocatalyst provides a significant paradigm for designing bifunctional composite catalysts for applications in clean energy devices.

The realization of unidirectional acoustic transmission (UAT) has recently aroused great attention owing to the versatile possibility in acoustics-based applications. This paper extends the bi-directional evolutionary structural optimization (BESO) method to the design of phononic crystal (PC) for achieving UAT. The optimization objective is to enlarge the minimum imaginary part of wave vectors along Γ-X while keep that along Γ-M less than the constraint value. We systematically studied the design of symmetric and asymmetric PCs at various frequencies. Numerical examples demonstrate that the proposed optimization algorithm is effective for creating the partial band gap at the specified frequency. The UAT with high rectifying efficiency is then successfully realized by placing the optimized PC in a bend wave guide. The results also show that the asymmetric PCs are more favorable for the design of broadband UAT devices compared with symmetric ones.

We have demonstrated the fabrication of homogeneously distributed In0.3Ga0.7N/GaN quantum nanodisks (QNDs) with a high density and average diameter of 10 nm or less in 30-nm-high nanopillars. The scalable top-down nanofabrication process used biotemplates that were spin-coated on an In0.3Ga0.7N/GaN single quantum well (SQW) followed by low-damage dry etching on ferritins with 7 nm diameter iron cores. The photoluminescence measurements at 70 K showed a blue shift of quantum energy of 420 meV from the In0.3Ga0.7N/GaN SQW to the QND. The internal quantum efficiency of the In0.3Ga0.7N/GaN QND was 100 times that of the SQW. A significant reduction in the quantum-confined Stark effect in the QND structure was observed, which concurred with the numerical simulation using a 3D Schrodinger equation. These results pave the way for the fabrication of large-scale III–N quantum devices using nanoprocessing, which is vital for optoelectronic communication devices.

The propagation of evanescent waves inside phononic band gaps is important for the design of phononic crystals with desirable functionalities. This paper extends the bi-directional evolutionary structure optimization (BESO) method to the design of phononic crystals for maximizing spatial decay of evanescent waves. The optimization objective is to enlarge the minimum imaginary part of wave vectors at a specified frequency. The study is systematically conducted for both out-of-plane and in-plane waves at various frequencies. Numerical examples demonstrate that the proposed optimization algorithm is effective for designing phononic crystals with maximum spatial decay of evanescent waves. Various topological patterns of optimized phononic crystals are given. The results also show that the proposed optimization algorithm can successfully overcome difficulties in opening band gap at desirable frequencies, especially for the complete band gap and multiple band gaps.

Topological photonics revolutionizes some of the traditional approaches to light propagation and manipulation, and it provides unprecedented means for developing novel photonic devices. Recently discovered higher-order topological phases go beyond the conventional bulk-edge correspondence for photonic crystals and introduce novel opportunities for topological protection. Here, we introduce an intelligent numerical approach for inverse design of higher-order photonic topological insulators with great flexibility for controlling both topological edge and topological corner states. In particular, we consider the second-order photonic topological insulator and design several structures supporting both edge and corner states at different frequencies. By carefully programming these structures, we suggest a novel approach for topological routing of the edge and corner states by changing the operational frequency. Our finding paves the way to integrated topological photonic devices with novel functionalities., 12 pages, 5 figures

The interfacial effects between noble metals and supports should be well understood for the design of efficient and low-cost catalysts for HCHO removal. Two Ag/CeO2 catalysts were prepared by using two CeO2 supports with well-defined crystal planes: a nanorod enclosed with the {1 1 0} and {1 0 0} crystal planes (Ag/CeO2-rod) and commercial CeO2 enclosed with the {1 1 1} crystal plane. The reactivity results demonstrated that Ag/CeO2-rod was a highly efficient active catalyst, which completely oxidized HCHO at 80 ℃. Compared to commercial CeO2, CeO2 nanorod had more oxygen vacancies and surface-active oxygen species, which were further promoted by the Ag-CeO2 interface. The HCHO could be quickly adsorbed and oxidized at the Ag-CeO2 interface, which led to the high reactivity for HCHO oxidation. This article not only proposes the reaction mechanism of HCHO oxidation but also provides ideas for the design of additional highly efficient and low-cost catalysts.

Developing high-efficiency hydrogen evolution reaction (HER) catalysts is of great interest for the large-scale high-purity hydrogen production, which, unfortunately, is largely blocked by the sluggish kinetics of the electro-chemical dissociation step of water molecules. Herein, a MOF-derived method has been developed to prepare Ru nanoparticles-anchored sponge-like WNO embedded in N-doped carbon layers (Ru@WNO-C). The fabricated catalyst Ru@WNO-C (Ru: 0.9 wt%) shows excellent HER catalytic activity and long-term stability with the overpotentials of 172, 358 and 24 mV at 10 mA/cm2 in 0.5 M H2SO4, 1 M Na2SO4 and 1 M KOH, respectively. More impressively, the overpotentials, especially in neutral medium, decrease markedly during the durability tests owing to the catalyst structure activation after residual Zn species leaching. The enhanced catalytic activity of Ru@WNO-C during operation is mainly attributed to the in-situ electron donation from Ru to W, leading to the easier cleavage of highly polarized H‒OH bond of water molecules and thus endowing Ru@WNO-C with excellent HER catalytic performance in non-acidic solution.

This paper provides a study on a novel octagonal multi-cell tube with functionally graded thickness (FGT) under multiple loading angles. First, comparative analysis on the FGT tube and the counterpart tube with uniform thickness(UT) under multiple loading angles reveal that the energy absorption is more superior for the FGT tube when the loading angle exceeds the lower bound of the transition range of the UT tube. Second, parametric study on the FGT tube indicates that thickness gradient exponent and thickness range have significant effect on its crashworthiness. Third, multiobjective optimizations of the FGT tube are conducted, aiming to maximize specific energy absorption(SEA) and minimize initial force(IPF) under multiple loading angles, based upon the Non-dominated sorting genetic algorithm(NAGA-Ⅱ) and RBF technique. The optimized FGT tube demonstrates better crashworthiness than the UT tube in all design cases. These findings can provide valuable guidelines for the design of multi-cell tube with functionally graded thickness under multiple loading angles.

Nanofluid boiling is an important research area of nanofluids, which provides many opportunities to explore new frontiers but also poses great challenges. This paper presents a comprehensive review on the nanofluid heat transfer (HT) and critical heat flux (CHF) of pool boiling and flow boiling. The research results in the literature previously reviewed are briefly summarized. An emphasis is put on the recent progresses in the nanofluid HT and CHF of pool boiling and flow boiling. It is also included comparing developments and research results of the nanofluid HT and CHF between pool boiling and flow boiling. The important achievements, inconsistence, and contradictions of the existing research results are identified and discussed in detail. Topics worthy of attention for future studies are suggested.

The saturated flow boiling heat transfer of nitrogen (N 2 ) in a vertical upward 11.9 mm inner diameter stainless steel tube was experimentally investigated. The heat transfer coefficient (HTC) was measured as a function of vapor quality. Totally 414 experimental data points were obtained, with the parameter range of vapor quality from 0.01 to 0.99, pressure from 0.84 to 2.29 MPa, heat flux from 9 to 98.8 kW/m 2 , and mass flux from 110 to 800 kg/m 2 s. The effects of vapor quality, heat flux, mass flux, and pressure on the HTC are explored. Four flow patterns are identified by analyzing the HTC data, including bubbly flow, annular flow with liquid in the core and vapor between the liquid and the tube wall, dispersed flow, and mist flow. The trend of the HTC along the tube length (with vapor quality) is explained in relation to flow patterns. The flow and heat transfer characteristics in the experiments are compared with those in horizontal macro-tubes. Substantial differences between these two flow directions are revealed. For upward flow in vertical macro-tubes, it is fairly commonly seen that there are two occurrences of critical heat flux (CHF) along a uniformly heated channel. The mechanisms resulting in this phenomenon are found. The experimental data are compared with the correlations of saturated flow boiling heat transfer coefficient. The best one has a mean absolute deviation of 16.6%.

For the realization of topologically protected wave propagation, engineering artificial structures of topological insulators (TIs) inspired by quantum mechanics has attracted extensive attention over the last few years. However, the existing structures commonly compose of a regular shape of inclusions for the limited arrangements in the unit cell. This paper develops a topology optimization approach for designing novel structures of acoustic TIs. With the mechanism of acoustic TIs based on the quantum spin Hall effect (QSHE), topology optimization first seeks sonic crystals (SCs) with double Dirac cones of dipolar and quadrupolar modes. Then, topology optimization creates topological trivial and non-trivial SCs by breaking the double Dirac cones, which freely assemble into acoustic TIs. The optimized structures exhibit the typical functionalities of acoustic TIs, the robust propagation and spin-locked unidirectional transmission of topological edge states, verifying the effectiveness of the proposed approach.

Vibrations propagating underground may have harmful effects on adjacent buildings and human health. Recently, the notion of phononic crystals has been introduced and applied to isolate vibrations below the ground. In this paper, considering the viscoelastic material model, the complex band diagrams are suggested to analyse the transportation of vibrations in the composite foundation consist of soil and different underground structures. In the complex band diagram, the minimum imaginary part of the wave vectors is taken as the index to measure the ability of the pile group to weaken vibrations. Several types of underground barriers, including circular piles, continuous walls, and multi-layer piles, are analysed. The result shows that multi-layer piles can induce local resonance and achieve an attenuation zone at the low-frequency level. The attenuation effect of periodic barriers is then verified by transmission models. The results show that the complex band diagrams can quantitatively evaluate the isolation effect of periodic barriers, which is beyond the capability of classic band diagrams.

Higher-order topological insulators (TIs) develop the conventional bulk-boundary correspondence theory and increase the interest in searching innovative topological materials. To realize a higher-order TI with a wide passband of one-dimensional (1D) and two-dimensional (2D) transportation modes, we design three-dimensional non-trivial and trivial sonic crystals whose combination mimics the Su–Schrieffer–Heeger model. The topological boundary states can be found at the interfaces, including the zero-dimensional corner state, 1D hinge state, and 2D surface state. The fabricated sample with the bent two-dimensional and one-dimensional acoustic channels exhibits the multidimensional sound propagation and verifies the mode transition among the complete bandgap, hinge mode, and surface mode. The bandwidth of the single-mode hinge state achieves a large relative bandwidth of 9.1% in which sound transports one-dimensionally without significant leak into the surfaces or the bulk. The higher-order topological states in the study pave the way for sound manipulation in multiple dimensions.

Accurate information about electric current is a key parameter in smart grid. Non-contact measurements are desirable and state of the art. One notable problem with the non-contact current measurement is that they are based on the relationship of electric current and the magnetic field generated by the current, which is susceptible to the superposition of unwanted magnetic field on the magnetic field under measurement. In this paper, a novel adaptive filter with three-sensor array to reduce the effect of the unwanted magnetic field, is proposed. Testing is performed to quantify the consequence of cross talk in alternating current measurement by means of numerical simulations and finite element analysis (FEA) method. The proposed method demonstrates that when the peak value of the current under measurement is from 500 A to 1000 A with interference current changing in a certain range, error between the processed magnetic field and magnetic field under measurement is less than 3 % in numerical simulations and less than 4 % FEA simulations.

One drawback of contactless current measurement by magnetic sensor is that its detected results can be easily affected by the superposition of the external magnetic interference. Damping the effect of the noise is complex and delicate without the knowledge of the interference. The usage of magnetic shielding is one potent way to compensate the shortcoming. However, complete magnetic shielding fails to provide the shielding for the shielding only happens within the material. Therefore, in this paper, a separated double-layer structure with the gap is proposed for large current measurement with strong magnetic noise from random places. The ambition is to keep the targeted magnetic field within the shielding stable even when exposed to strong magnetic interference environment. Parameters of the designed structure are investigated and the validation of the structure is confirmed by finite element analysis method when the magnitude of the current under measurement ranging up to 1000 A. From FEA method, when the interference is at arbitrary places around the conductor, the standard deviation without and with noise is less than 0.13, authenticating the efficacy of this shielding structure.

To design an efficient catalyst for HCHO oxidation, it requires good understanding of the correlation between structural properties of catalyst and the reaction mechanism. We investigated performance of commercial CeO2 (CeO2-C) and nanorod CeO2 (CeO2-R) and the corresponding Au/CeO2 catalysts in HCHO oxidation. Reactivity tests showed that Au/CeO2-R is highly efficient and stable in HCHO oxidation at room temperature, which is attributed to the highly active Au-CeO2-R interface. The strong interaction between Au and CeO2-R greatly promotes the creation of oxygen vacancies and Au3+ near the interface, which promotes the formation of formate. The conversion of formate can follow both a decomposition and a direct oxidation pathway, which is remarkably enhanced by the coexistence of Au3+ and oxygen vacancies. In addition, formate accumulation and Au particles aggregation led to deactivation. The presence of humidity promotes the oxidation of formate, which improves the reactivity and stability of Au/CeO2 in HCHO oxidation.

Temperature-dependent radiative and non-radiative dynamics of photoexcited carriers were studied in In0.3Ga0.7N nanodisks (NDs) fabricated from quantum wells (QWs) by neutral-beam etching using bio-nano-templates. The NDs had a diameter of 5 nm, a thickness of 2 and 3 nm, and a sheet density of 2 × 1011 cm–2. The radiative decay time, reflecting the displacement between the electron and hole wavefunctions, is about 0.2 ns; this value is almost constant as a function of temperature in the NDs and not dependent on their thickness. We observed non-exponential decay curves of photoluminescence (PL) in the NDs, particularly at temperatures above 150 K. The thermal activation energies of PL quenching in the NDs are revealed to be about 110 meV, corresponding to the barrier heights of the valence bands in the disks. Therefore, hole escape is deemed responsible for the PL quenching, while thermal activation energies of 12 meV due to the trapping of carriers by defects were dominant in the mother QWs. The above-mentioned non-exponential PL decay curves can be attributed to variations in the rate of hole escape in the NDs because of fluctuations in the valence-band barrier height, which, in turn, is possibly due to compositional fluctuations in the QWs. We found that non-radiative trapping, characteristic of the original QW, also exists in about 1% of the NDs in a form that is not masked by other newly formable defects. Therefore, we suggest that additional defect formation is not significant during our ND fabrication process.