一种基于FCOS神经网络的小建筑物目标检测方法北大核心

提出了一种基于FCOS神经网络的小建筑物目标检测算法,针对FCOS模型在特征提取阶段提取到的小建筑物目标特征较少问题,引入多尺度检测和可变形卷积方式,加强网络对小建筑物目标的特征提取能力,并通过改进后的SGE注意力机制降低特征图中的干扰噪声权重。改进后的网络可以提取到更多的小建筑物目标特征,对环境干扰噪声的鲁棒性更强。在自己搭建的数据集上进行了实验测试,结果表明,在相同环境下网络改进后建筑物的整体检测准确率提升了1.7%,其中对小建筑物目标提升了3.6%,减少了小建筑物目标漏检、误检的问题。     

The phosphorescence emission in undoped lead-halide Cs4PbBr6 single crystals at low temperature

Recently, all-inorganic cesium lead-halide perovskites have shown their promise for light emission applications, due to the excellent optical performance. Herein, we report that the initially nonphosphorescent undoped lead-halide Cs₄PbBr₆ single crystals (SCs) exhibit an ultralong phosphorescence emission under X-ray excitation at low temperatures. It is shown that the dramatic change has been taken place in radioluminescence spectra and the broad-band emission gradually appeared with the decrease of temperature. Below 210 K, the radioluminescence spectra can be deconvoluted into one narrow peak located at 530 nm and two broad peaks centered at 595 nm and 672 nm respectively. Subsequently, the time-dependent radioluminescence spectra in undoped lead-halide Cs₄PbBr₆ SCs were investigated. The ultralong phosphorescence emission can persist over 120 min at 70 K. We consider that ultralong phosphorescence originates from defect-related emission. To the best of our knowledge, our finding is the first time that undoped Cs₄PbBr₆ SCs exhibit the phosphorescence emission, which will offer a paradigm to motivate revolutionary applications on perovskite.     

The formed surface characteristics of SiCf/SiC composite in the nanosecond pulsed laser ablation

For the advantages of high-temperature resistance, corrosion resistance and ultra-high hardness, SiC_f/SiC composite is becoming a preferred material for manufacturing aero-engine parts. However, the anisotropy and heterogeneity bring great challenges to the processing technology. In this study, a nanosecond pulsed laser is applied to process SiC_f/SiC composite, where the influence of the scanning speed and laser scanning direction to the SiC fibers on the morphology of ablated grooves is investigated. The surface characteristics after ablation and the involved chemical reaction of SiC_f/SiC are explored. The results show that the increased laser scanning speed, accompanied by the decreasing spot overlap rate, leads to the less accumulation of energy on the material surface, so the ablation effect drops. In addition, for the anisotropy of the SiC_f/SiC material, the obtained surface characteristics are closely dependent on the laser scanning direction to the SiC fibers, resulting in different groove morphology. The element composition and phase analysis of the machined surface indicate that the main deposited product is SiO₂ and the carbon substance. The results can provide preliminary technical support for controlling the machining quality of ceramic matrix composites.     

Laser damage resistance of plasma-sprayed alumina and honeycomb skeleton/alumina composite ceramic coatings

Al₂O₃ and honeycomb skeleton-Al₂O₃ composite coatings on Titanium alloy (Ti–6Al–4V) were prepared by atmospheric plasma spraying. A laser ablation experiment on as-sprayed coatings was performed. In this paper, the laser damage resistance, microstructure, phase composition of Al₂O₃ coatings were examined. 3D Dimensional Confocal Microscopy, Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD), and Energy Dispersive Spectrometry (EDS) characterized the laser damage morphology, microstructure, phase composition, and element analysis, respectively. The influence of the honeycomb skeleton on the laser ablation damage on as-sprayed coatings was investigated by a comparative analysis of the laser damage morphology with different laser ablation times and gas flow. The results show that the honeycomb skeleton raises thermal conductivity and thermal diffusivity. Moreover, a “tower”-like dendrite was generated during the laser irradiation of the composite coating. The honeycomb skeleton refined the structure, suppressed crack propagation, and reduced the influence of gas flow on cracks. Under the same experimental laser ablation parameters, the laser damage area of the honeycomb skeleton-Al₂O₃ composite coating was smaller than that of the Al₂O₃ coating. It was demonstrated that the laser damage resistance of the honeycomb skeleton-Al₂O₃ composite coating was superior to that of the Al₂O₃ coating.     

Ultralight and robustly compressible silica aerogel enhanced by AC/C sponge with high oil/water separation

Journal of Porous Materials - In recent years, oil spills and industrial organic pollutants have caused irreparable damage to the environment and biological ecosystems. Therefore, the treatment of...     

Enhancement of oxygen/pressure sensing performance of Eu3+-doped YSZ phosphors via Bi3+ sensitization

The effects of the incorporation of a Bi³⁺ sensitizer on the phosphorescence properties and oxygen partial pressure sensitivity of the Eu³⁺ doped yttria stabilized zirconia (YSZ) phosphors were studied using a lifetime-based optical measurement system. Two series of YSZ: Eu phosphors were investigated in this work: Eu_0.01Bi_xY_0.07-xZr_0.92O_1.96 substitutional series and Eu_0.01Bi_xY_0.07Zr_0.92-xO_1.96-0.5x additive series. The phosphorescence intensity of the additive-series phosphors was enhanced by 47% excited at 405 nm with a Bi³⁺ concentration of 2 mol% due to the energy transfer between Bi³⁺ and Eu³⁺. In contrast, the phosphorescence intensity of the substitutional-series phosphors decreased as the Bi³⁺ concentration increased. The phosphorescence lifetimes for both series phosphors were highly sensitive to oxygen partial pressure at elevated temperatures. With increasing Bi³⁺ concentration, the oxygen sensitivities of both series were enhanced initially, which was related to the increment of concentration dependent non-radiative decay via cross-relaxation between Bi³⁺ and Eu³⁺. With 1 mol% Bi³⁺ doping, the oxygen sensitivity was enhanced by 28% and 12% for substitutional-series and additive-series phosphors, respectively. As the Bi³⁺ concentration further increased, the oxygen sensitivities of both series declined, which was attributed to the energy transfer between Bi³⁺, the formation of Bi³⁺ aggregates as well as the increase of the Eu³⁺ site symmetry. The results of this study not only provided valuable references for phosphor thermometry, but also offered new ideas for developing high-temperature non-contact pressure sensors.     

Comparison of ignition,injection and micro - Explosion characteristics of RP-3/ ethanol and biodiesel / ethanol mixed drops

The ignition, injection, and micro-explosion characteristics of aviation fuel (RP-3)/ethanol mixed droplets and biodiesel/ethanol mixed droplets at different proportions under high temperature conditions (420 °C) were compared using an experimental setup. A device for measuring small droplet volumes was designed using an infusion set and different types of needles, and a corresponding equation was established. Mixed droplets suspended on high-temperature resistance nichrome wire with a diameter of 0.2 mm were heated by sending them to a position approximately 2 mm from the forklift preheating plug using a moving rail. SLR and high-speed cameras were used to observe the flame structure as well as the injection and micro-explosion of the mixed droplets during combustion, respectively. Expansion, injection, and micro-explosion were observed in the biodiesel/ethanol mixed droplet experiments when the biodiesel content was 60%. Although the micro-explosion of mixed droplets of aviation fuel/ethanol was not observed, expansion and ejection of the droplets were observed. Image Pro-plus software was used to calculate the diameters at different times in the combustion cycle of the droplets. Through this analysis, the occurrence of micro-explosion was described, and a model for the calculation of micro-explosion strength was established.     

面向云计算应用层演化的隐私保护方法研究

云计算应用层中的组合服务具有演化属性,因此,隐私数据在服务组合过程中,用户的隐私数据可能会因为服务或服务流程的演化而暴露。根据服务演化的特征,以描述逻辑为基础,提出了一种面向云计算应用层演化的隐私保护方法。对隐私协议进行形式化描述;根据服务的演化特征,对服务的演化进行监控,保证满足用户的隐私需求;利用实例研究证明该方法的正确性与可行性。     

Highly efficient chemical mechanical polishing method for SiC substrates using enhanced slurry containing bubbles of ozone gas

In this study, a highly efficient method for chemical mechanical polishing (CMP) of silicon carbide (SiC) substrates using enhanced slurry was proposed and developed. The enhanced slurry contains bubbles of ozone gas generated by ozone gas generator in pure water mixed with a conventional commercially available slurry. Therefore, the enhanced slurry has an oxidizing effect on the Si-face of SiC substrates. To confirm the effectiveness of bubbles enclosing ozone gas, both nano-indentation test and X-ray photoelectron spectroscopy (XPS) analysis were conducted. As a result, the hardness decrease of the Si-face of the SiC substrate was confirmed through the nano-indentation test, and the generation of reaction products was confirmed on Si-face of SiC substrate in the XPS analysis. According to a series of experimental results of our proposed highly efficient CMP method for SiC substrates, the removal rate can be increased when the enhanced slurry was applied, comparing with that for the not only conventional commercially available slurry but also commercially available dedicated slurry.     

预压力对压电驱动履带移动系统的性能影响

为了提高夹心式压电驱动移动系统的机械输出性能，提出了一种U型预压力调节机构，并开展了预压力对夹心式压电驱动履带移动系统输出性能影响关系的实验研究。首先，针对U型预压力调节机构的安装对夹心式压电振子的振动特性影响关系开展了有限元仿真分析，发现U型预压力调节机构始终处在夹心式压电振子的两相工作模态振动节点位置；其次，开展了夹心式压电驱动履带移动系统的原理样机的预压力调节测试实验，确定了系统的最大输出牵引力及其所对应的最佳预压力；最后，在最佳预压力工作状态下，开展了原理样机的牵引力特性、越障性能以及模拟月壤环境下的运动特性实验。研究结果表明，在最佳预压力工作状态下，夹心式压电驱动履带移动系统的机械输出性能最佳，为其进一步在月面巡视器上的应用提供了技术支持和试验基础。