Inverse Gradient Research Articles

Temperature uniformity characteristics of array jet impingement cooling with the maximum cross-flow scheme

Temperature uniformity of the impingement cooling is commonly needed in many fields in order to enhance performance, reliability, and to ensure material quality. In particular, operating life of the hot components in gas turbine depends heavily on how uniformly they are being cooled to mitigate the development of thermal stresses. However, there are rather limited literatures addressing these, and the cooling uniformities of jet impingement in extreme conditions, such as the cooling of turbine vane and combustor liner, are seldom discussed. This study focused on the temperature uniformity of array jet impingement cooling with the maximum crossflow scheme, where the detailed distribution of the nonuniformly cooled areas on the target wall, as well as their correlations with the impinging flow field, and the effects of typical geometrical and fluid parameters on the uniformity were investigated. Results indicated that the arising of the high temperature zones and inverse temperature gradient pairs around them, essentially induced by the entrainment vortex and counter rotating vortex pair in the streamwise and spanwise directions respectively, are the sources of temperature nonuniformities over the impingement target surface. Moreover, increasing the impingement Reynolds number or shortening the jet-to-target spacing for higher jet Reynolds numbers could improve the temperature uniformity.

Analyzing the Inlet Gas Void Fraction on the Flow Characteristics for a Multiphase Pump Based on Cavitation Cases

Inlet gas void fraction (IGVF) affects the cavitation evolution in a multiphase pump and easily results in a drop of the head and efficiency when cavitation is more serious. In this paper, a numerical method was performed to qualitatively and quantitatively analyze the effect of the inlet gas void fraction on the pressure and velocity characteristics of the multiphase pump at different cavitation stages. The results show that with the increase of IGVF and the development of cavitation, the pressure in the impeller flow passage is reduced, and the pressure corresponding to the cavitation region drops sharply to the saturated vapor pressure. With the decrease of the cavitation coefficient, and due to the expulsion effect of the cavitation bubbles, the relative velocity in the cavitation region becomes larger. Because of the large pressure gradient at the end of the cavitation bubbles, the kinetic energy of the fluid is insufficient to overcome the effect of the inverse pressure gradient, resulting in a backflow vortex. Investigations on cavitation evolution in the multiphase pump at different IGVFs are of great significance for improving its performance.

Analysis of a hypersaline drought-prone estuary reveals low density and diversity of fish eggs and larvae

We analyzed fish eggs and larvae in an estuary under severe drought conditions. We detected an inverse salinity gradient, with values increasing from the mouth to the upper estuary. Egg densities decreased from the estuarine mouth to the upstream areas following the salinity increase for all three mesh net sizes. This pattern was also found for the density of larvae, which decreased in estuarine regions with hypersalinity (38 to 62). The low diversity constituted only nine fish species, which were classified as anadromous (Anchoa hepsetus), estuarine and marine (Bathygobius soporator, Hippocampus reidi, Eucinostomus sp., and Diapterus auratus), marine estuarine-opportunist (Caranx latus and Bardiella rochus), and marine stragglers (Echeneis naucrates and Haemulon sp.). In addition, we observed an oversimplification of the assemblage to include stress-tolerant estuarine and marine species. Our baseline results suggest that this hypersaline estuarine ecosystem has lower densities and diversity than a healthy mangrove system.

Cesium doping for improving performance of inverse-graded 2D (CMA)<sub>2</sub>MA<sub>8</sub>Pb<sub>9</sub>I<sub>28</sub> perovskite film and solar cells

Compared with three-dimensional (3D) perovskites, two-dimensional (2D) perovskites have excellent environmental stability. However, the efficiency of 2D perovskite solar cells is still lower than that of their 3D counterparts owing to the poor carrier transport. In order to improve the efficiency of 2D perovskite solar cells, the cesium (Cs) doping 2D (CMA)<sub>2</sub>MA<sub>8</sub>Pb<sub>9</sub>I<sub>28</sub> films with an inverse gradient structure (small <i>n</i> quantum well (QW) located at the surface and large <i>n</i> QW at the bottom) are prepared. The inverse gradient QW structure has the advantages of being more moisture-resistant (small <i>n</i> QW protects the vulnerable large <i>n</i> QW from being attacked by water molecules) and favoring self-driven charge transport (type-II band alignment between different phases). The results show that the quality and thermal stability of 2D (CMA)<sub>2</sub>MA<sub>8</sub>Pb<sub>9</sub>I<sub>28</sub> film can be effectively improved by using Cs doping. The SEM images show that the film grains become larger, and the surface is smoother and more compact with CsI content increasing. When CsI increases to 15%, the surface becomes coarse. From the XRD, it can be seen that the crystallinity of perovskite is improved with CsI doping, and it reaches saturation when CsI content increases to 10%. The PL intensity of the film with 5% CsI is higher than the others’, implying a relatively low non-radiative recombination loss and low defect state in that film. Therefore, the minority carrier average lifetime of film doped with 5% CsI is the longest. The absorption is almost unchanged when CsI is doped. The thermal stability of film can be effectively improved when CsI exceeds 10%. Considering the SEM images, crystallinity, PL intensity, light absorption and thermal stability of perovskite, the optimized CsI doped concentration is 10% in our work. Finally, the highest efficiency of (CMA)<sub>2</sub>MA<sub>8</sub>Pb<sub>9</sub>I<sub>28</sub> perovskite solar cells doped with 10% CsI content reaches to 14.67%, with a short-circuit current density (<i>J</i><sub>SC</sub>) of 23.16 mA/cm<sup>2</sup>, an open-circuit voltage (<i>V</i><sub>oc</sub>) of 1.05 V and a fill factor (FF) of 60.75%. The efficiency of the undoped cells is 10.06%, which is lower than that of CsI doped cells (10%). Therefore, CsI doping is an effective method to further improve the efficiency and thermal stability of 2D perovskite solar cells.

PS-BPC02-7: RADIAL-DIGITAL PULSE WAVE VELOCITY: STIFFNESS OF SMALL CONDUIT ARTERIES INCREASES AFTER NITROGLYCERIN ADMINISTRATION

Objective: The alteration of the physiological stiffness gradient within larger vessels would increase the transmission of greater pulsatility to the microcirculation, thus explaining the damage to the pressure-sensitive organs. We aimed to describe the response of small conduit arteries of the hand (radial-digital PWV, rd-PWV) following the pharmacological alteration of the stiffness gradient using nitroglycerin. Design and method: Simultaneous application of piezoelectric sensors (Complior) at the level of the carotid (c), the radial artery (r) and the tip of the index finger (d) was used to calculate the rd-PWV by the following formula (rd-PWV = cd-cr direct distances/cd-cr transit times). Rd-PWV was obtained before and 4 minutes after a sublingual administration of 0.4 mg of NTG, in healthy adults (controls, n = 36) and patients with moderate chronic kidney disease (CKD, n = 30). Changes in arterial stiffness pre-post NTG and comparisons between the two groups were analyzed respectively with paired or independent samples t-tests. Results: Compared to controls, the CKD group was older (69 ± 15 years vs. 41 ± 20 years) and more hypertensive. Despite similar cd-PWV and cr-PWV, rd-PWV was significantly lower in CKD than in controls (3.6 ± 1.4 m/s vs. 4.5 ± 1.8 m/s, p = 0.024), but this difference faded (p = 0.145) after adjustment for age. Post NTG, brachial blood pressure decreased and heart rate increased similarly between groups (see Table 1). Cr-PWV decreased after NTG (both groups p < 0.001) while rd-PWV increased in controls (from 4.62 ± 1.49 m/s to 5.94 ± 2.29 m/s, p < 0.001) and in CKD (from 3.71 ± 1.60 m/s to 5.18 ± 2.12 m/s, p < 0.001), in a similar extent (p = 0.111 for interaction term). Cd-PWV significantly increased post-NTG only in the CKD group (p = 0.009). Conclusions: Again, we observed opposing changes in the regional stiffnesses of two distinct vascular territories following the administration of a vasodilator. Here, the two segments were mid-size and small conduit muscular arteries. This technique, adapted by our team for short peripheral arterial segments, may broaden our understanding of the consequences of the inversion of the stiffness gradient.

Effect of cavitation evolution on power characteristics of tidal current turbine

With the development of large-scale tidal current turbines and the increase in tidal current velocity, the possibility of cavitation increases. Furthermore, unsteady cavitation is a complicated multiphase flow that causes power degradation of tidal current turbine blade. There has been no comprehensive investigation of it so far. In this study, the blade captured power is obtained at different cavitation numbers using the Schnerr–Sauer cavitation model. The numerical uncertainty for the mesh and the time step is calculated by the grid convergence index method. It has been shown that, when the cavitation number is 5 and 2, cavitation has no effect on the blade power. With the decrease in the cavitation number, the rise in cavitation intensity occurs when the vapor distribution area stretches from the blade tip to the blade root and from the leading edge to the trailing edge, respectively. With a fall in cavitation number to 1.3, the vapor volume fraction rises, and the viscosity of the mixed phase reduces, resulting in a reduction in viscous power. When the cavitation number is 0.8, there exists a larger region in which an absolute value of minimum pressure coefficient is less than the cavitation number, a smaller blade load is present, and the pressure difference power is substantially decreased. Because of the huge inverse pressure gradient created by cavitation, the negative pressure difference power is generated, resulting in a decline of the blade power coefficient to 14%, when the cavitation number is 0.5.

Structure Guided Multiparameter Waveform Inversion With Attenuation Compensation in Viscoacoustic Medium

Full waveform inversion (FWI) has been proved as an effective method for subsurface parameter estimation by iteratively reducing the residual between the predictions and the observations. In recent years, FWI has been widely used due to the increasing computing power. However, seismic waves usually suffer from energy dissipation and phase distortion during their propagation in the anelastic medium, which will decelerate the convergence rate of FWI and make the inversion processing even more time-consuming. In this letter, we propose a structure guided FWI method with attenuation compensation to estimate velocity model and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> model simultaneously. We refer to the proposed method as SGQFWI. With the help of a fractional decoupled viscoacoustic equation, we introduce the attenuation compensation mechanism into full waveform inversion iterative algorithm, which can effectively compensate the deep weak amplitude, so as to obtain a more accurate inversion gradient and improve the accuracy and efficiency of inversion. Additionally, the structure regularization is added to model update, which helps to retrieve local detailed information more efficiently. Benefit from the incorporation of the priori structure constraint and attenuation compensation, this method has the ability to invert velocity and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> model with improved efficiency and resolution. Synthetic and field data examples are adopted to further verify the effectiveness of the proposed method.

An Adaptive Domain Adaptation Method for Rolling Bearings’ Fault Diagnosis Fusing Deep Convolution and Self-Attention Networks

Intelligent fault diagnosis methods based on deep learning have attracted significant attention in recent years. However, it still faces many challenges, including complex and variable working conditions, noise interference, and insufficient valid data samples. Therefore, a novel deep transfer learning bearing fault diagnosis model is designed in this work by fusing time-frequency analysis, residual network (ResNet) and self-attention mechanism (SAM). A multiscale time-frequency feature map (MTFFM) and global statistical feature matrix (GSFM) of vibration signals are first constructed using wavelet packet transform (WPT). A deep feature extraction network combining ResNet and SAM networks is then designed to realize the fused extraction of local and global time-frequency features. Finally, we construct a joint loss function by the combination of multi-kernel maximum mean discrepancy (MK-MMD) and the domain adversarial neural network (DANN) to optimize the depth feature extraction network, which improves the cross-domain invariance and fault state discrimination of depth features. The proposed optimization method fully exploits the advantages of high-dimensional spatial distribution difference evaluation and gradient inversion adversarial strategy. Its effectiveness is demonstrated through variable working condition transfer fault diagnosis tasks, showing superior performance compared with other intelligent fault diagnosis methods.

4D Seismic Monitoring with Diffraction-Angle-Filtering for Carbon Capture and Storage (CCS)

Carbon capture and storage (CCS) is a key technology that directly removes industry driven CO2 to achieve carbon neutrality. In the process of CCS, it is necessary to monitor whether injected CO2 is properly stored and not leaking. The behavior of CO2 can be investigated using a 4D seismic survey that compares seismic data before and after injection. We proposed a two-step monitoring with diffraction-angle filtering (DAF) to effectively locate the CO2 plume. Because DAF allows us to control wavenumber components, the gradient of full-waveform inversion (FWI), which is the first step, is composed of low-wavenumber components, and reverse time migration (RTM) for seismic imaging is carried out with high-wavelength components. To verify our method, we implemented FWI and RTM with and without DAF using the velocity model in the Volve oil field in the North Sea. Numerical examples show that the CO2 plume is properly detected from the difference between baseline and post-injected survey and the extension of the reflective boundary is improved compared to the results of the conventional method. With our proposed method, local minima problem is mitigated in FWI, and the boundaries between layers can be clearly distinguished in RTM.

Distribution and transport of Fish larvae at the entrance of the Gulf of California (September, 2016)

Fish larvae distribution and transport of dominant species were analyzed at the entrance to the Gulf of California in September 2016. Using the Sea Surface Temperature (SST) gradient, a frontal system was defined at the entrance to the Gulf. The most relevant gradient (∼0.06 °C km−1) in the region was transverse to the gulf axis (W-E), from south of Cabo San Lucas to the mainland, south of the Pescadero Basin. This SST gradient was associated with an anticyclonic flow (∼0.4 m s−1) that was connected with a cyclonic eddy outside the Gulf. Fish larvae of mesopelagic species such as Vincigueria lucetia and Benthosema panamense were widely distributed and increasing in abundance from north to south, while the mesopelagic Bregmaceros Bathymaster and the pelagic Auxis spp. showed an inverse abundance gradient. Most of these larvae were absent from the zone of strong SST gradient. A Lagrangian particle tracking model applied at the sampling stations where the fish larvae were collected, showed that during the first 5 days of trajectory, the transport was weak. But for the next 5 days, most of the particles were trapped by mesoscale flows that dominated the region. An unexpected result was that no particles crossed the transverse front, coinciding with the absence or low abundance of larvae there. The particles near to this front were transported west out the Gulf. We concluded that when the mesoscale flows are extended along the transverse axis of the Gulf entrance, these might trap inert particles and zooplankton organisms with little mobility, such as fish larvae in early phases of development, preventing their transport along the Gulf parallel axis (NW-SE), and therefore, affecting the species distribution.

Hyphenated liquid chromatography - diode array detection - charged aerosol detection - high resolution - multistage mass spectrometry with online hydrogen/deuterium exchange: One stop solution for pharmaceutical impurity profiling

Hyphenation of different analytical techniques has always been advantageous in structural characterization as it saves time, money and resources. In the pharmaceutical sector, chromatography-based impurity profiling, including identification, characterization, and quantification in drug substances or finished products, is of utmost importance to comply with quality, patient safety and regulatory requirements. These impurities are monitored using LC-UV/DAD and identified and/or characterized using HRMS and MS/MS. LC analysis usually yields the area percent purity of the targeted peak, however, this is not sufficient for pharmaceutical purposes; where the regulatory requirement is to report impurities in percent weight by weight. Unfortunately, the non-availability of impurity standards and relative response factors at an early stage of drug development, risks the product quality due to the inability of the method to differentiate percent purity, and percent weight by weight. Hence, there is a need for a distinctive way of determining the relative response factor. In the current study, a unique hyphenation has been employed by integrating LC with DAD, CAD, and HRMSn with hydrogen-deuterium exchange. The LC flow, post-DAD detection has been diverted to CAD with an inverse gradient for relative response factor determination and MS Orbitrap for exact mass, and MSn fragmentation. A separate infusion pump has been incorporated to infuse D2O on a need basis, which can perform partial hydrogen deuterium exchange for determining the number of labile hydrogens in the impurity structure. This hyphenation has been validated with four model compounds and a total of nineteen chromatographic peaks. The technique provides ample information for their qualitative analysis along with percent weight-by-weight values, which fulfils the regulatory requirements and can be used as one-stop solution for impurity profiling.

Memory of crystallization in the melt of commercial linear low density polyethylenes processed in an open twin-screw extruder

Compared with a constant recrystallization rate of commercial linear low density polyethylenes (LLDPE) as a function of melt temperature, copolymers reprocessed in an open twin-screw extruder, display the expected strong melt-memory features upon melting followed by recrystallization. Precipitation from dilute solution had the same effect on melting-recrystallization of commercial copolymers. In the twin-screw melt re-processing, a small content of long-chain branches and crosslinks are incorporated to the copolymer's structure, but especially a further consumption of additives uncovers self-nucleation features that speed up recrystallization and are related to the major copolymer microstructure. The observed difference in re-crystallization behavior between metallocene type and Ziegler-Natta (ZN) type copolymers, both with equivalent average comonomer content and molecular mass, is due to the inter-chain comonomer distribution. While for metallocene-made and low density polyethylenes, below a critical melt temperature the recrystallization rate increases continuously with decreasing melt temperature, in broadly distributed ZN copolymers there is an inversion of the temperature gradient of the rate at the onset of liquid-liquid phase separation (LLPS). The mass fraction of highly branched molecules required to observe the onset of LLPS via melting followed by recrystallization was evaluated from the TREF profiles, and the effect of self-nucleated melts at different levels on the overall spherulitic morphology was observed via polarized optical microscopy.

Trends in school-neighbourhood inequalities and youth obesity: Repeated cross-sectional analyses of the public schools in the state of California.

It is currently unknown whether the relationship between affluence of school neighbourhoods and prevalence of youth overweight/obesity is uniform across demographic subgroups and areal context in the United States. We examined association between school-neighbourhood income tertiles and school-level overweight/obesity (OVOB) prevalence, using data on body mass index of fifth, seventh, and nineth graders who attended public schools in California in 2001 and 2010 (n=1 584 768), using multiple logistic regression models. Overall, OVOB prevalence was higher in lower-income school neighbourhoods, with a steeper income-OVOB gradient for girls. Among boys, the gradient became steeper in 2010 than 2000. Among Asian and White girls, the negative gradients were steepest in rural areas. For African-American students in all areas and Latino boys in rural areas, there was less clear evidence of inverse income-OVOB gradients. Addition of fast-food restaurant availability to the models did not change the observed inverse school-neighbourhood income-obesity gradients. The findings suggest the needs to investigate reasons for this variability with consideration to combinations of sociodemographic, economic, and environmental risk factors that may contribute to disparities in childhood obesity.

A federated learning attack method based on edge collaboration via cloud

AbstractFederated learning (FL) is widely used in edge‐cloud collaborative training due to its distributed architecture and privacy‐preserving properties without sharing local data. FLTrust, the most state‐of‐the‐art FL defense method, is a federated learning defense system with trust guidance. However, we found that FLTrust is not very robust. Therefore, in the edge collaboration scenario, we mainly study the poisoning attack on the FLTrust defense system. Due to the aggregation rule, FLTrust, with trust guidance, the model updates of participants with a significant deviation from the root gradient direction will be eliminated, which makes the poisoning effect on the global model not obvious. To solve this problem, under the premise of not being deleted by the FLTrust aggregation rules, we construct malicious model updates that deviate from the trust gradient to the greatest extent to achieve model poisoning attacks. First, we utilize the rotation of high‐dimensional vectors around axes to construct malicious vectors with fixed orientations. Second, the malicious vector is constructed by the gradient inversion method to achieve an efficient and fast attack. Finally, a method of optimizing random noise is used to construct a malicious vector with a fixed direction. Experimental results show that our attack method reduces the model accuracy by 20%, severely undermining the usability of the model. Attacks are also successful hundreds of times faster than the FLTrust adaptive attack method.

Global biogeography of ant social parasites: Exploring patterns and mechanisms of an inverse latitudinal diversity gradient

AbstractAimOne of the most consistent global biogeographic patterns is the latitudinal diversity gradient where species richness peaks within the equatorial tropics and decreases towards the poles. Here, we explore the global biogeography of socially parasitic ant species, which comprises the most diverse group of social parasites in the Hymenoptera. We test the biogeographic hypothesis that ant social parasites are distributed along an inverse latitudinal diversity gradient (iLDG) by peaking in diversity outside of the equatorial tropics, which would contrast with the biogeographic pattern observed in free‐living, non‐parasitic ant species.LocationGlobal.TaxonAnts (Hymenoptera: Formicidae).MethodsWe assembled a comprehensive biogeographic dataset consisting of 6001 geographic distribution records for all 371 taxonomically described socially parasitic ant species. We used phylogenetic and taxonomic studies to estimate the number of independent evolutionary origins of ant social parasitism to directly compare species richness with the number of species representing independent evolutionary origins of social parasitism across a latitudinal gradient. In addition, we compared ant social parasite diversity across biogeographic regions using rarefaction to account for different sampling efforts. Finally, we tested for a correlation between latitude and the proportion of ant social parasite species within regional ant faunae.ResultsThe geographic distribution records and the inferred 91 independent evolutionary origins of socially parasitic life histories in ants show that both species richness and the number of species representing independent evolutionary origins of social parasitism peak in the northern hemisphere outside of the equatorial tropics. Based on rarefaction curves, northern latitude regions harbour the most ant social parasite species, but the diversity of independent evolutionary origins is not significantly different between northern and southern hemispheres. The proportion of ant social parasite species within regional faunae is tightly correlated with latitude only in the northern hemisphere.Main conclusionsThe iLDG of ant social parasites contrasts with the biogeographic pattern observed in free‐living, non‐parasitic ant species and appears to be driven by large species radiations as well as by the presence of specialized life histories exclusive to the northern hemisphere.

Aerosol Oxidative Potential in the Greater Los Angeles Area: Source Apportionment and Associations with Socioeconomic Position.

Oxidative potential (OP) has been proposed as a possible integrated metric for particles smaller than 2.5 μm in diameter (PM2.5) to evaluate adverse health outcomes associated with particulate air pollution exposure. Here, we investigate how OP depends on sources and chemical composition and how OP varies by land use type and neighborhood socioeconomic position in the Los Angeles area. We measured OH formation (OPOH), dithiothreitol loss (OPDTT), black carbon, and 52 metals and elements for 54 total PM2.5 samples collected in September 2019 and February 2020. The Positive Matrix Factorization source apportionment model identified four sources contributing to volume-normalized OPOH: vehicular exhaust, brake and tire wear, soil and road dust, and mixed secondary and marine. Exhaust emissions contributed 42% of OPOH, followed by 21% from brake and tire wear. Similar results were observed for the OPDTT source apportionment. Furthermore, by linking measured PM2.5 and OP with census tract level socioeconomic and health outcome data provided by CalEnviroScreen, we found that the most disadvantaged neighborhoods were exposed to both the most toxic particles and the highest particle concentrations. OPOH exhibited the largest inverse social gradients, followed by OPDTT and PM2.5 mass. Finally, OPOH was the metric most strongly correlated with adverse health outcome indicators.

A protocol for setting-up robust hydrophobic interaction chromatography targeting the analysis of intact proteins and monoclonal antibodies.

Hydrophobic interaction chromatography (HIC) is a chromatographic technique that mainly targets the separation of biomolecules (intact proteins, monoclonal antibodies, etc.) based on the difference in surface hydrophobicity while applying non-denaturing conditions. This protocol paper provides guidelines for setting-up robust HIC analysis and considers the instrument configuration, mobile-phase and sample preparation, as well as chromatographic conditions and settings. The separation of a mixture of intact proteins and monoclonal antibodies is demonstrated by applying conventional HIC conditions, that is, using a mildly hydrophobic (C4) stationary phase in combination with an inverse ammonium sulphate gradient dissolved in aqueous phosphate buffer. The effect of sample-preparation conditions on sample breakthroughs is presented. Finally, good run-to-run repeatability (relative standard deviation<2%) is demonstrated for five different columns obtained from three different column lots, considering chromatographic retention, peak width, peak area and column pressure.

Structural Control of Molybdenum Silicide by Electrolytic Silicification of a Mo Substrate

β-MoSi2 is one of the expected silicide candidates for thermoelectric material because of its semi-conductive and metastable characteristics. However, it is not easy to fabricate β-MoSi2 phase under low temperature condition as easily anticipated from the equilibrium Mo-Si binary phase diagram. In this study, the formation of β-MoSi2 by electrochemical silicification of a Mo substrate with a thickness of 0.5 mm in CaCl2-based melts containing SiO2 has been confirmed. Throughout XRD analysis, the formation of metastable β-MoSi2 phase was identified on a Mo substrate by potentiostatic electrolysis below 973 K. The results of TEM combined with EDS analysis at the Mo/β-MoSi2 interface showed the inverse concentration gradients of Mo and Si in the width of ca. 25 nm. In addition, we examined the growth process of β-MoSi2 by using a partially Pt-coated Mo substrate since the reduction of Si ions does not proceed at the Pt-coated area. The cross-sectional SEM image of the substrate demonstrated that a homogeneous film of β-MoSi2 was grown with almost the same thickness on both sides of the Mo substrate and the melt region. The results indicated that the growth of β-MoSi2 was caused by the mutual diffusion of Mo and Si atoms.

A numerical study of the rapid deflagration-to-detonation transition

This paper describes numerically the rapid deflagration-to-detonation transition (DDT) in detail in a high-frequency pulse detonation rocket engine. Different from traditional DDT, reactants are injected into the chamber from near the open end and travel toward the closed end. Previous experiments have implied that the gasdynamic shock by injecting in a confined space and the intensive turbulence generated by the high-speed jet play important roles in the detonation initiation, but explanations of how, when, and where the detonation is generated were not presented clearly due to the limitation of experimental observation. In this work, high-resolution two-dimensional simulations are performed to investigate this process employing a physical model similar to the experimental configuration. A new mechanism manifesting itself as a complicated vortex–flame interaction is found for the flame transition from a laminar to compressible or choking regime. It is discovered that the gasdynamic shock, after reflecting from the end wall, triggers the detonation through the gradient of reactivity with the hot spot formed by the collision of the shock and the flame. A dimensionless criterion defined by the ratio of the acoustic speed to the inverse gradient of the ignition delay time is applied to further describe the spontaneous wave propagation from the perspective of chem-physical dynamics. This criterion quantitatively gives a good prediction of the propagating mode from the subsonic deflagration to a developing detonation, even in such a complex scenario as encountered in this work.

Identifying the surface properties of Ti3C2Tx MXene through transmission electron microscopy

The titanium carbide Ti 3 C 2 T x (T x = surface functional groups) MXene exhibits promising physical properties and chemical activity as one of the most commonly and extensively investigated materials, but the participating surface structures and properties have yet to be clarified. Here, we reveal the superficial nature of etching-processed Ti 3 C 2 T x using transmission electron microscopy/spectroscopy, first-principles calculation, and molecular dynamics simulation. An O-rich, but C-deficient, diffusion layer (approximately 3 nm) with an inverse gradient distribution is identified. Associated with the superficial substitution of O for C, an amorphous overlayer (approximately 2 nm) containing oxyfluoride of Ti, Al, and C is also revealed. The stable diffusion layer has a high density of states at the Fermi level and a low shear modulus to bulk modulus ratio, implying that the formation of the diffusion layer is not detrimental to Ti 3 C 2 T x ’s conductivity and ductility. These findings are significant for improving oxidation strategies and understanding the structural properties of this material. • O-rich, but C-deficient, diffusion layer and oxyfluoride overlayer are identified • Diffusion layer preserves layered structure, and oxyfluoride overlayer is amorphous • Electron transfer is found between Ti and C and between Ti and O in diffusion layer • Stable diffusion layer is no harm to conductivity and ductility of Ti 3 C 2 T x Wang et al. reveal the atomic configurations and electrical and mechanical properties of superficial Ti 3 C 2 T x MXene using transmission electron microscopy, first-principles calculation, and molecular dynamics simulation. The discovery of the diffusion layer and oxyfluoride overlayer is critical for improving oxidation strategies and understanding the structural properties of this material.