With the proliferation of unmanned aerial vehicles (UAVs) in both commercial and military use, the public is paying increasing attention to UAV identification and regulation. The micro-Doppler ...characteristics of a UAV can reflect its structure and motion information, which provides an important reference for UAV recognition. The low flight altitude and small radar cross-section (RCS) of UAVs make the cancellation of strong ground clutter become a key problem in extracting the weak micro-Doppler signals. In this paper, a clutter suppression method based on an orthogonal matching pursuit (OMP) algorithm is proposed, which is used to process echo signals obtained by a linear frequency modulated continuous wave (LFMCW) radar. The focus of this method is on the idea of sparse representation, which establishes a complete set of environmental clutter dictionaries to effectively suppress clutter in the received echo signals of a hovering UAV. The processed signals are analyzed in the time–frequency domain. According to the flicker phenomenon of UAV rotor blades and related micro-Doppler characteristics, the feature parameters of unknown UAVs can be estimated. Compared with traditional signal processing methods, the method based on OMP algorithm shows advantages in having a low signal-to-noise ratio (−10 dB). Field experiments indicate that this approach can effectively reduce clutter power (−15 dB) and successfully extract micro-Doppler signals for identifying different UAVs.
The development of environmentally friendly corrosion inhibitors has become a research hotspot. Aiming at the potential corrosion inhibition effect of perilla seed extract on Q235 carbon steel, the ...corrosion inhibition effect was quantitatively evaluated by various research methods, and the effective corrosion inhibition composition and mechanism were discussed. The research methods include potentiodynamic polarization curve method, HPLC-MS, FT-IR, XPS and chemical calculation. The experimental results show that the inhibitor prepared from perilla seed extract is a mixed inhibitor, and its adsorption behavior accords with Langmuir adsorption theory and its adsorption free energy is −22.70 kJ/mol. Combined with the experimental results and theoretical calculation, the effective corrosion inhibiting components are luteolin and apigenin. Theoretical calculation shows that both of them are adsorbed parallel to the surface of carbon steel to form thin films. The adsorption mechanism is that carbonyl O atoms in luteolin and apigenin hybridize with the 3 d empty orbit of Fe. From the point of view of quantum chemistry, the smaller the HOMO value and the energy gap value, the better the adsorption of corrosion inhibitor on the surface of carbon steel. From the point of view of molecular dynamics simulation, the greater the absolute value of adsorption energy, the better the adsorption of corrosion inhibitor on carbon steel surface.
Al-substituted Ni(OH)2 samples with Al3+/Ni2+ mole ratio of 0%, 10% and 20% have been prepared by a very facile chemical co-precipitation method. The microstructure of the prepared samples are ...analyzed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), thermo-gravimetric analysis (TGA), and Field emission scanning electron microscopy (FESEM). The results reveal that the pure Ni(OH)2 sample is β-Ni(OH)2 with nanosheets hierarchical structure; the sample with 10% Al is mixed phase α/β-Ni(OH)2 with hybrid nanosheets/nanoparticles hierarchical structure; the sample with 20% Al is α-Ni(OH)2 with irregular nanoparticles hierarchical structure. The lithium storage performances of the prepared samples are characterized by cyclic voltammograms (CV), electrochemical impedance spectroscopy (EIS), and charge–discharge tests. The results demonstrate that Al substitution could improve the lithium storage performances of nickel hydroxide. In particular, the mixed phase α/β-Ni(OH)2 with 10% Al exhibited the highest electrochemical activity, the best rate performance, and superior cycling stability. For example, after 30 charge/discharge cycles under a current density of 200 mA g−1, the mixed phase α/β-Ni(OH)2 with 10% Al can still deliver a specific discharge capacity of 964 mAh g−1, much higher than of for the α-Ni(OH)2 with 20% Al (681 mAh g−1) and the pure Ni(OH)2 (419 mAh g−1).
Lithium storage performances of nickel hydroxides with different phase structures and various hierarchical nanostructures prepared by a very facile chemical co-precipitation method. Display omitted
•Hierarchical structured nickel hydroxides are prepared by coprecipitation method.•The influence of Al substitution on the electrode performances has been studied.•Phase structure and morphology of nickel hydroxide are tuned by Al substitution.•Mixed phase nickel hydroxide with hierarchical nanosheets/nanoparticle morphology.•The mixed phase nickel hydroxide show high capacity and good cycling stability.
Femoral head varus is an important complication in intertrochanteric fracture patients treated with proximal femoral nail anti-rotation (PFNA) fixation. Theoretically, extending the length of the ...intramedullary nail could optimize fixation stability by lengthening the force arm. However, whether extending the nail length can optimize patient prognosis is unclear. In this study, a review of imaging data from intertrochanteric fracture patients with PFNA fixation was performed, and the length of the intramedullary nail in the femoral trunk and the distance between the lesser trochanter and the distal locking screw were measured. The femoral neck varus status was judged at the 6-month follow-up. The correlation coefficients between nail length and femoral neck varus angle were computed, and linear regression analysis was used to determine whether a change in nail length was an independent risk factor for femoral neck varus. Moreover, the biomechanical effects of different nail lengths on PFNA fixation stability and local stress distribution have also been verified by numerical mechanical simulations. Clinical review revealed that changes in nail length were not significantly correlated with femoral head varus and were also not an independent risk factor for this complication. In addition, only slight biomechanical changes can be observed in the numerical simulation results. Therefore, commonly used intramedullary nails should be able to meet the needs of PFNA-fixed patients, and additional procedures for longer nail insertion may be unnecessary.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Percutaneous transforaminal endoscopic discectomy (PTED) is widely used for the treatment of lumbar disc herniation. Facetectomy in PTED is necessary for accessing the intraspinal region and for ...decompressing the exiting nerve roots in patients who suffer from hypertrophy of the facet joints. However, this may increase morbidity in failed back surgery syndrome (FBSS) and has not been clearly elucidated.
A three-dimensional lumbosacral model was reconstructed and validated. And corresponding models after PTED with one-quarter and one-half excisions of the superior articular process were reconstructed. The maximum shear stress on the annulus in L5, von Mises stress of the facet cartilage, maximum principle capsular strain and deformation of the lumbosacral model were calculated using finite element methods.
Calculated results show no significant differences in the complete model and the model with one-quarter excision of the superior articular process, but all biomechanical indexes have been deteriorated under most of the loading conditions tested in the model with one-half excision of the superior articular process.
Less facetectomy is better because it may reduce the risk of biomechanical deterioration and consequently, that of FBSS.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Reducing the uncertainties in carbon balance assessment is essential for better pastureland management in arid areas. Climate forcing data are some of the major uncertainty sources. In this study, a ...modified Biome-BGC grazing model was driven by an ensemble of reanalysis data of the Climate Forecast System Reanalysis data (CFSR), the European Centre for Medium-Range Weather Forecasts Interim Reanalysis (ERA-Interim), and the Modern-Era Retrospective Analysis for Research and Applications (MERRA), to study the effect of climate change and grazing on the net ecosystem exchange (NEE) of the pasturelands in Central Asia. Afterwards, we evaluated the performance of corresponding climate datasets over four major pastureland types, and quantified the modeling uncertainty induced by climate forcing data. Our results suggest that (1) a significant positive trend in temperature and a negative trend in precipitation were obtained from the three climate datasets. The average precipitation is apparently higher in the CFSR and MERRA data, showing the highest temperature value among the data sets; (2) pasturelands in Central Asia released 2.10 ± 1.60 Pg C in the past 36 years. The highest values were obtained with the CFSR (−1.53 Pg C) and the lowest with the MERRA (−2.35 Pg C) data set; (3) without grazing effects, pasturelands in Central Asia assimilated 0.13 ± 0.06 Pg C from 1981–2014. Grazing activities dominated carbon release (100%), whereas climate changes dominated carbon assimilation (offset 6.22% of all the carbon release). This study offered possible implications to the policy makers and local herdsmen of sustainable management of pastureland and the adaptation of climate change in Central Asia.
(+)-Nootkatone is a highly valued sesquiterpenoid compound, exhibiting a typical grapefruit aroma and various desired biological activities for use as aromatics and pharmaceuticals. The high ...commercial demand of (+)-nootkatone is predominately met by chemical synthesis, which entails the use of environmentally harmful reagents. Efficient synthesis of (+)-nootkatone via biotechnological approaches is thus urgently needed to satisfy its industrial demand. However, there are only a limited number of studies that report the de novo synthesis of (+)-nootkatone from simple carbon sources in microbial cell factories, and with relatively low yield.
As the direct precursor of (+)-nootkatone biosynthesis, (+)-valencene was first produced in large quantities in Saccharomyces cerevisiae by overexpressing (+)-valencene synthase CnVS of Callitropsis nootkatensis in combination with various mevalonate pathway (MVA) engineering strategies, including the expression of CnVS and farnesyl diphosphate synthase (ERG20) as a fused protein, overexpression of a truncated form of the rate-limiting enzyme 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase (tHMG1), and downregulating the squalene synthase enzyme (ERG9). These approaches altogether brought the production of (+)-valencene to 217.95 mg/L. Secondly, we addressed the (+)-valencene oxidation by overexpressing the Hyoscyamus muticus premnaspirodiene oxygenase (HPO) variant (V482I/A484I) and cytochrome P450 reductase (ATR1) from Arabidopsis thaliana. However, (+)-valencene was predominantly oxidized to β-nootkatol and only minor amounts of (+)-nootkatone (9.66 mg/L) were produced. We further tackled the oxidation of β-nootkatol to (+)-nootkatone by screening various dehydrogenases. Our results showed that the short-chain dehydrogenase/reductase (SDR) superfamily dehydrogenases ZSD1 of Zingiber zerumbet and ABA2 of Citrus sinensis were capable of effectively catalyzing β-nootkatol oxidation to (+)-nootkatone. The yield of (+)-nootkatone increased to 59.78 mg/L and 53.48 mg/L by additional overexpression of ZSD1 and ABA2, respectively.
We successfully constructed the (+)-nootaktone biosynthesis pathway in S. cerevisiae by overexpressing the (+)-valencene synthase CnVS, cytochrome P450 monooxygenase HPO, and SDR family dehydrogenases combined with the MVA pathway engineering, providing a solid basis for the whole-cell production of (+)-nootkatone. The two effective SDR family dehydrogenases tested in this study will serve as valuable enzymatic tools in further optimizing (+)-nootkatone production.
Photochromic diarylethene derivatives, which can reversibly switch the fluorescence of adjacent fluorophores between the ON and OFF states under light irradiation, have been widely used to construct ...photoswitchable materials. Herein, eight dithienylethene (DTE) groups are integrated onto one polyhedral oligomeric silsesquioxane (POSS) core, obtaining a novel super molecular photoswitch. After being doped into conjugated polymer nanoparticles, the POSS‐DTE8 molecules show a higher contrast on/off photoswitching performance and a quicker responsive speed than free DTE molecules at same molar concentration of photochromic units. This enhanced photoswitching efficiency is attributed to the increased molecular interaction of the ring‐open form and lowered energy of the ring‐closed form of the DTE units on the POSS core, which is beneficial for the ring‐closing reaction and subsequent energy transfer between photoswitch and fluorophores. In addition, POSS‐DTE8 also exhibits good photomodulation behavior in the conjugated polymer film, giving it potential applications in optical devices.
Switch it up! A photoswitching molecule, POSS‐DTE8, was synthesized with eight dithienylethene (DTE) units integrated onto a polyhedral oligomeric silsesquioxane (POSS) core. Upon being doped into conjugated polymer nanoparticles with a low doping content (10 %), POSS‐DTE8 exhibits a much higher fluorescence contrast by photochemically modulating the emission compared with free DTE molecules with the same number of photochromic units (see figure).
•The Schrenk spruce tree-ring widths and PDSI values showed a significant positive correlation.•The extreme drought years were verified in the Tianshan Mountains.•The drought trends in different ...regions of Tianshan Mountains have been discovered and confirmed.
In Central Asia, the frequency and severity of droughts have a strong impact on the climate and lives of people. Therefore, it is essential to explore climate change, especially its impacts on the cycle and extent of drought occurrence. To determine drought conditions, the Palmer drought severity index (PDSI) is diffusely implemented and can be reconstructed using tree-ring chronologies. We collected Schrenk spruce (Picea schrenkiana) cores from six distinct areas in the Tianshan Mountains and used them to establish tree-ring chronologies. Correlation and reaction research revealed that the main impact on tree growth was humidity from previous years and the present growing season. To reconstruct the historical PDSI for the Tianshan Mountains, chronologies were applied. The reconstructed PDSI was considered to describe the climate change in the Tianshan Mountains because it captured severe drought events that were observed in other research findings. Furthermore, the spatial analysis results covered almost all of the Tianshan Mountains at 0.5° resolution. A large-scale drought event in the 1910s that was prevalent in the Tianshan Mountains was detected with tree rings. The extreme drought years of 1917, 1919, and 1944 detected on the chronologies were also specifically verified. A decadal analysis showed that the eastern and western Tianshan Mountains experienced continuous drought in the 1880s and during the 1810s ~ 1830s, the central and western Tianshan Mountains experienced continuous drought in the 1770s and 1640s, while it experienced a continuous wetting in the 1750s, and large-scale wetting occurred in Central Asia in the 2000s. This research offers a new perspective on the instability and spatial spread of drought in the Tianshan Mountains.
How to prevent the cracking of tunnel lining concrete under a high-temperature and low-humidity environment has gradually become a challenge faced by the engineering community. Actually, the concrete ...structure will be restrained, which easily leads to cracking. Aiming at this problem, a self-restraint device of concrete specimens was designed in this paper, which aims to more realistically simulate the restrained state of concrete structures during construction. SEM, EDS and XRD detection methods were used to study the macroscopic and microscopic properties of an early-age restrained concrete specimen under a high-temperature and low-humidity environment, and the results were compared with those of a non-restrained concrete specimen. The results show that the change in the internal relative humidity of the concrete was an extremely slow process, and the response rate of the internal humidity of the concrete was much slower than that of the temperature. A cubic curve model was used to fit the measured concrete damage degree with the loading age, and the fitting effect was good. Under the environment of high temperature and low humidity, the loading age from the 0.6th day to the 1st day was the period of a relatively large fluctuation in the concrete temperature and humidity, and the restraint would aggravate the damage of the concrete. The damage degree increased with the increase in the loading age, the microcracks gradually increased and, finally, macrocracks were formed. The restraint effect was to intensify the formation of microcracks, affect the hydration of the cement at the micro level and, finally, increase the risk of concrete cracking perpendicular to the restrained direction at the macro level. The research results may provide guidance for research on the cracking mechanism of tunnel lining concrete constructed under a high-temperature and low-humidity environment.
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