A branch-and-bound method for the minimum k−enclosing ball problem

The minimum k-enclosing ball problem seeks the ball with smallest radius that contains at least k of m given points. This problem is NP-hard. We present a branch-and-bound algorithm on the tree of the subsets of k points to solve this problem. Our method is able to solve the problem exactly in a short amount of time for small and medium sized datasets.     

现代分析仪器在高中化学教学中的应用*——以“防晒霜的防晒效果实验探究”为例

从高中化学原子、分子等抽象性的知识教学出发,以防晒霜的防晒效果实验探究为例,探讨了现代分析仪器在高中化学教学中的应用。通过利用现代分析仪器紫外可见分光光度计,结合紫外线变色球实验探究防晒霜的防晒效果,分析并认识防晒霜中化学防晒的机理。结果表明,防晒霜的厚度越大、SPF越高,防晒霜的防晒效果越好,并且防晒霜的防晒效果可以持续一段时间,但是不同类型的防晒产品存在一定差异。在化学教学中渗透现代分析仪器的应用,为学生进一步了解化学学科在改变学习方式、拓宽学科知识领域、推动自身全面与可持续发展方面打下坚实的基础。     

Comprehensive performance of a ball-milled La0.5Pr0.5Fe11.4Si1.6B0.2Hy/Al magnetocaloric composite

Due to the hydrogen embrittlement effect, La(Fe,Si)₁₃-based hydrides can only exist in powder form, which limits their practical application. In this work, ductile and thermally conductive Al metal was homogeneously mixed with La_0.5Pr_0.5Fe_11.4Si_1.6B_0.2 using the ball milling method. Then hydrogenation and compactness shaping of the magnetocaloric composites were performed in one step via a sintering process under high hydrogen pressure. As the Al content reached 9 wt.%, the La_0.5Pr_0.5Fe_11.4Si_1.6B_0.2H_y/Al composite showed the mechanical behavior of a ductile material with a yield strength of ~44 MPa and an ultimate strength of 269 MPa accompanied by a pronounced improvement in thermal conductivity. Due to the ease of formation of Fe-Al-Si phases and the several micron and submicron sizes of the composite particles caused by ball milling process, the magnetic entropy change of the composites was substantially reduced to ~1.2 J/kg· K-1.5 J/kg· K at 0 T-1.5 T.     

A zinc(Ⅱ) MOF based on secondary building units of infinite wavy-shaped chain exhibiting obvious luminescent sense effects

A new metal-organic framework (MOF),{[Zn_7 (BPS)_4 (OH)_6 (H_2O)_2]·5 H_2O]_n}(1), (H_2 BPS=4,4′-bibenzoic acid-2,2′-sulfone), based on a wavy and infinite chain-shaped secondary building units, has been synthesized under solvothermal conditions and characterized by single crystal X-ray diffraction and further confirmed by PXRD, TGA and IR spectrum. The solid-state emission spectra reveal that compound 1 presents strong luminescence emission bands at room temperature. The fluorescent properties of compound 1 in diverse organic solvents indicated that 1 has palpable luminescent sense effects for DMF and DMAC.     

柴油爆炸性能外场实验研究

通过?30 mm杀爆燃弹外场炮击实验，模拟车辆、装备油箱被炮火击中后二次爆炸场景，采用高速照相机、红外热成像仪分别记录引爆柴油过程和爆炸火球的温度场，对比评估普通柴油、含水型柴油和抑爆型柴油的爆炸特性。实验结果显示:炮弹射击油箱瞬间，柴油液滴被抛撒出油箱，与空气快速混合形成气溶胶，并在炸药能量作用下引发爆炸，形成爆炸火球；不同类型柴油的爆炸火球均经历3个发展阶段，但其尺寸、扩展速率和表面温度等有较大差别，普通柴油和含水型柴油的火球这3个参数比较接近，都大于抑爆型柴油；含水型柴油的油箱毁伤容积为108.00 dm3，远高于普通柴油的57.65 dm3和抑爆型柴油的38.15 dm3。研究表明，抑爆柴油中的高分子聚合物能起到较好的抑爆作用。     

RV减速器主支撑角接触球轴承混合润滑分析

针对RV减速器角接触球轴承承受预紧力、轴向力和径向力等联合外载荷作用的工况,分析得出了内、外圈滚道接触界面的接触区几何参数和接触载荷.在此基础上,综合考虑了角接触球轴承的接触区宏观几何、接触载荷、真实表面粗糙度、瞬态效应等因素,建立了角接触球轴承混合润滑数学模型,分析了在不同工况下角接触轴承的润滑状况及表面以下应力分布.结果表明:随着载荷的不断增加,钢球与内圈沟道之间的油膜厚度会不断减少,导致干接触面积迅速扩大,接触点表面以下最大应力增大;转速的增加会使油膜变厚,干接触面积缩小.该结果对角接触球轴承的实际工程应用具有重要借鉴意义.     

Azobenzene‐Functionalized Cage Silsesquioxanes as Inorganic–Organic Hybrid,Photoresponsive, Nanoscale,Building Blocks

Mono‐ and octa‐azobenzene‐functionalized cage silsesquioxanes were easily synthesized by the reaction of 4‐bromoazobenzene with monovinyl‐substituted octasilsesquioxane and cubic octavinylsilsesquioxane through the Heck coupling reaction. Excited‐state energies obtained from time‐dependent density functional theory (TDDFT) and the CAM‐B3LYP functional correlate very well with experimental trans–cis photoisomerization results from UV/Vis spectroscopy. These azobenzene‐functionalized cages exhibit good thermal stability and are fluorescent with maximum emission at approximately 400 nm, making them potential materials for blue‐light emission.     

以绕丹宁和噻唑烷-2,4-二酮为端基的不对称结构有机受体分子的设计合成与构性关系探讨

分别以绕丹宁和噻唑烷-2,4-二酮单元为端基、IDT为中心核设计合成了一个新型不对称结构的有机小分子受体IDT-2,并通过与两端均以绕丹宁或噻唑烷-2,4-二酮受体单元的对称小分子受体IDT-1和IDT-3进行对比,探讨了分子结构与性能之间的关系。研究发现,从IDT-1到IDT-3,随着两端的绕丹宁基团被噻唑烷-2,4-二酮基团逐步取代,这类小分子受体的吸收光谱显著蓝移,光学带隙E_g~(opt)逐步增大,LUMO和HOMO能级也逐渐抬升。随后我们分别以这三个小分子为受体、P3HT为给体共混构建活性层而制备了有机太阳能电池,结果表明,以两端均为绕丹宁单元的对称结构小分子受体IDT-1构建的电池器件具有最高的光电转换效率(PCE),相应的J_(sc)和FF值也最大,而V_(oc)则最低;而以两端均为噻唑烷-2,4-二酮基团的对称结构小分子受体IDT-3的电池器件,其V_(oc)最高,但其J_(sc)和FF则最低,PCE值也最小。对于IDT-2而言,由于分子只有一个绕丹宁单元被噻唑烷-2,4-二酮所取代,其V_(oc),J_(sc)和PCE均介于IDT-1与IDT-3之间。由此说明,尽管噻唑烷-2,4-二酮基团的引入能有效提升器件V_(oc),但却不利于改善其J_(sc)和FF,因此受体的分子设计中如何平衡电池器件的几种光伏性能参数而获得高的光电转换效率仍是十分重要的研究课题之一。     

基于双向长短期记忆网络的太赫兹光谱识别

特征提取是太赫兹光谱识别的关键处理步骤，通常利用降维方法作为特征提取手段。然而，当一些化合物的太赫兹光谱曲线整体差异度较小时，降维方法往往会缺失样本差异的重要特征信息，从而导致分类错误。如果不采用降维方法提取特征，传统机器学习分类算法对维数较高的原始太赫兹光谱数据又不能很好的分类。针对此问题，提出了一种基于双向长短期记忆网络（BLSTM-RNN）自动提取太赫兹光谱特征的识别方法。BLSTM-RNN作为一种特殊的循环神经网络，利用其LSTM单元可以有效解决原始太赫兹光谱数据维数较高使得模型难以训练问题。再结合模型的双向频谱信息利用架构模式，可以增强模型对复杂光谱数据自动提取有效特征信息的能力。采用三类、15种化合物太赫兹透射光谱作为测试对象，首先利用S-G滤波和三次样条插值对Anthraquinone，Benomyl和Carbazole等十五种化合物在0.9～6 THz内的太赫兹透射光谱数据进行归一化处理，然后通过构建一个具有双向长短期记忆的循环神经网络对太赫兹光谱的全频谱信息进行自动特征提取并利用Softmax分类器进行分类。通过试验优化网络结构和各项参数，最终获得了针对复杂太赫兹透射光谱数据的预测模型，并与传统机器学习算法SVM，KNN及神经网络算法MLP，CNN进行对比实验。结果表明，dataset-1和dataset-2分别作为差异度较大和无明显峰值特征的五种化合物太赫兹透射光谱数据集，其平均识别率分别为100%和98.51%，与其他方法相比识别率有所提高；最重要的是，dataset-3作为5种化合物谱线极为相似的太赫兹透射光谱数据集，其平均识别率为96.56%，与其他方法相比识别率提高显著；dataset-4作为dataset-1，dataset-2和dataset-3的透射光谱数据集集合，其平均识别率为98.87%。从而验证了BLSTM-RNN模型能自动提取有效的太赫兹光谱特征，同时又能保证复杂太赫兹光谱的预测精度。在选择模型训练优化算法方面，使用Adam优化算法要好于RMSProp，SGD和AdaGrad，其模型的目标函数损失值收敛速度最快。同时随着模型训练迭代次数增加，相似太赫兹透射光谱数据集的预测准确率也不断提升。可为复杂太赫兹光谱数据库的光谱识别检索提供一种新的识别方法。     

Conventional vs. Microwave- or Mechanically-Assisted Synthesis of Dihomooxacalix[4]arene Phthalimides: NMR,X-ray and Photophysical Analysis §

Direct O-alkylation of p-tert-butyldihomooxacalix[4]arene (1) with N-(bromopropyl)- or N-(bromoethyl)phthalimides and K₂CO₃ in acetonitrile was conducted under conventional heating (reflux) and using microwave irradiation and ball milling methodologies. The reactions afforded mono- and mainly distal di-substituted derivatives in the cone conformation, in a total of eight compounds. They were isolated by column chromatography, and their conformations and the substitution patterns were established by NMR spectroscopy (¹H, ¹³C, COSY and NOESY experiments). The X-ray structures of four dihomooxacalix[4]arene phthalimide derivatives (2a, 3a, 3b and 5a) are reported, as well as their photophysical properties. The microwave (MW)-assisted alkylations drastically reduced the reaction times (from days to less than 45 min) and produced higher yields of both 1,3-di-substituted phthalimides (3a and 6a) with higher selectivity. Ball milling did not reveal to be a good method for this kind of reaction.