全文获取类型
| 收费全文 | 267篇 |
| 免费 | 29篇 |
| 国内免费 | 23篇 |
专业分类
| 化学 | 152篇 |
| 晶体学 | 2篇 |
| 力学 | 12篇 |
| 综合类 | 5篇 |
| 数学 | 23篇 |
| 物理学 | 125篇 |
出版年
| 2023年 | 3篇 |
| 2022年 | 7篇 |
| 2021年 | 7篇 |
| 2020年 | 12篇 |
| 2019年 | 7篇 |
| 2018年 | 3篇 |
| 2017年 | 11篇 |
| 2016年 | 6篇 |
| 2015年 | 3篇 |
| 2014年 | 7篇 |
| 2013年 | 15篇 |
| 2012年 | 14篇 |
| 2011年 | 13篇 |
| 2010年 | 9篇 |
| 2009年 | 8篇 |
| 2008年 | 9篇 |
| 2007年 | 17篇 |
| 2006年 | 17篇 |
| 2005年 | 14篇 |
| 2004年 | 10篇 |
| 2003年 | 19篇 |
| 2002年 | 12篇 |
| 2001年 | 17篇 |
| 2000年 | 10篇 |
| 1999年 | 10篇 |
| 1998年 | 9篇 |
| 1997年 | 4篇 |
| 1996年 | 7篇 |
| 1995年 | 1篇 |
| 1994年 | 6篇 |
| 1993年 | 2篇 |
| 1992年 | 1篇 |
| 1991年 | 3篇 |
| 1990年 | 1篇 |
| 1989年 | 3篇 |
| 1988年 | 2篇 |
| 1987年 | 5篇 |
| 1984年 | 2篇 |
| 1982年 | 2篇 |
| 1981年 | 2篇 |
| 1980年 | 1篇 |
| 1979年 | 2篇 |
| 1977年 | 5篇 |
| 1975年 | 1篇 |
排序方式： 共有319条查询结果，搜索用时 16 毫秒
251.
对倾斜角为20°、24°、28°和32°的单头以及32°的双头周向重叠三分螺旋折流板换热器和弓形折流板换热器的传热和压降性能进行了测试,换热器采用公共壳体和可更换管芯结构。采用壳侧轴向雷诺数和轴向欧拉数分别作为反映壳侧流量和阻力系数的无因次参数。试验结果显示在试验范围内周向重叠三分螺旋折流板换热器壳侧换热系数、壳侧压降和综合性能指标都随着倾斜角增大而减小;倾斜角20°方案的性能指标最佳,其平均壳侧努塞尔数和轴向欧拉数与弓形折流板方案的数值之比分别为1.123和0.45;双头螺旋折流板方案的换热系数和压降都大于同样倾斜角的单头螺旋折流板方案,但两者的综合性能较接近。 相似文献
252.
利用PM3级别最大重迭对称性分子轨道法和最大键级杂化轨道方法,计算了系列烃类化合物的杂化轨道和电荷分布,拟合出计算C-H及C-C偶合常数的简单关系式. 研究了各种烃类分子中不同的C-H键和C-C键偶合常数,理论计算值和实验数据都较为符合. 进一步验证了直接键连1JCX偶合常数主要取决于偶合作用中的Fermi接触项,为从简单价键理论角度解释和计算1JCH和1JCC提供了一种简便直观的方法. 相似文献
253.
由于激光雷达光轴夹角的存在和光束质量分布的非均匀性, 导致理论回波信号与实际回波信号有较大差别, 需要对其进行校正. 基于数学推导和软件仿真, 给出了激光雷达重叠因子的数学表达式, 分析了光轴夹角导致的系统重叠因子在全程探测空间的变化情况; 分析了光束分布为高斯分布和均匀分布时的重叠因子变化情况; 对激光雷达的距离校正信号和Klett算法公式进行了重叠因子修正, 最后在对激光雷达系统参数标定的基础上, 在重叠区域利用修正的Klett公式对其所测得到的回波信号进行了修正, 在探测盲区利用斜率法修正距离校正信号, 进而得到了符合理论与实际情况的激光雷达在全程上校正的消光系数曲线.
关键词：
激光雷达
标定
重叠因子
消光系数 相似文献
254.
从理论上研究了高气压CO2激光器的宽带输出频谱特性.结果表明，在20个大气压下，CO2激光器有宽带输出，带宽高达5.0 cm－1（相当于150 GHz），从971 cm－1~976 cm－1，峰值在10R(16)线处，10R(14)线和10R(18)线差不多对称地分布其两侧.很小的增益差异（4.6%）造成了很大的能量输出差异（100多倍）.比较了谐振腔内有无色散元件时的输出频谱，理论证明了谐振腔内无任何色散元件时的宽带输出频谱是光滑连续的，而频率依赖的反馈引起了输出频谱很强的精细结构.理论计算结果与实验测量结果符合较好. 相似文献
255.
256.
Jiancheng Rao Liuqing Yang Xuefei Li Lei Zhao Shumeng Wang Junqiao Ding Lixiang Wang 《Angewandte Chemie (International ed. in English)》2020,59(41):17903-17909
The meta junction is proposed to realize efficient thermally activated delayed fluorescence (TADF) in donor–acceptor (D‐A) conjugated polymers. Based on triphenylamine as D and dicyanobenzene as A, as a proof of concept, a series of D‐A conjugated polymers has been developed by changing their connection sites. When the junction between D and A is tuned from para to meta, the singlet–triplet energy splitting (ΔEST) is found to be significantly decreased from 0.44 to 0.10 eV because of the increasing hole–electron separation. Unlike the para‐linked analogue with no TADF, consequently, the meta‐linked polymer shows a strong delayed fluorescence. Its corresponding solution‐processed organic light‐emitting diodes (OLEDs) achieve a promising external quantum efficiency (EQE) of 15.4 % (51.9 cd A?1, 50.9 lm W?1) and CIE coordinates of (0.34, 0.57). The results highlight the bright future of D‐A conjugated polymers used for TADF OLEDs. 相似文献
257.
Optical spectra (powder reflectance, UV/Vis/NIR region), and temperature dependent magnetic behavior (χ, μ/μB) were recorded for the series of anhydrous europium(III) phosphates EuIII3O3(PO4), EuIIIPO4, EuIII2P4O13, lt- and ht-EuIII(PO3)3, and EuIIIP5O14. By modeling within the AOM framework, the experimental data can be related to the ligand-field splitting experienced by the Eu3+ ions in the various mainly low-symmetry coordination environments. Our study confirms the well-established relation eσ(Eu3+–O2–) ~ d(Eu3+–O2–)–7.0 between the AOM parameter and the interatomic distance. In addition it is shown that eσ(Eu3+–O2–) depends strongly on the highly variable polarizability of the oxygen ligator atoms. This polarizability can be related to the optical basicity Λ of the various phosphates. 相似文献
258.
Despite great advances in X-ray absorption spectroscopy for the investigation of small molecule electronic structure, the application to biosystems of experimental techniques developed within this research field remains a challenge. To partially circumvent the problem, users resort to theoretical methods to interpret or predict the X-ray absorption spectra of large molecules. To accomplish this task, only low-cost computational strategies can be exploited. For this reason, some of them are single Slater determinant wavefunction approaches coupled with multiscale embedding techniques designed to treat large systems of biological interest. Therefore, in this work, we propose to apply the recently developed IMOM/ELMO embedding method to the determination of core-ionized states. The IMOM/ELMO technique resulted from the combination of the single Slater determinant Δself-consistent-field-initial maximum overlap approach (ΔSCF-IMOM) with the QM/ELMO (quantum mechanics/extremely localized molecular orbital) embedding strategy, a method where only the chemically relevant region of the examined system is treated at fully quantum chemical level, while the rest is described through transferred and frozen extremely localized molecular orbitals (ELMOs). The IMOM/ELMO technique was initially validated by computing core-ionization energies for small molecules, and it was afterwards exploited to study larger biosystems. The obtained results are in line with those reported in previous studies that applied alternative ΔSCF approaches. This makes us envisage a possible future application of the proposed method to the interpretation of X-ray absorption spectra of large molecules. 相似文献
259.
Wang Zhang Yuan Yongming Zhang 《Journal of polymer science. Part A, Polymer chemistry》2017,55(4):560-574
Nonconventional luminogens without classic conjugated structures have drawn increasing interests owing to their fundamental importance and promising applications. These luminogens generally bear such subgroups as tertiary amine, C = C, C≡N, C = O, OH, ether, and imide. The emission mechanism, however, remains under debate. Different assumptions like oxidation or acidification of tertiary amines, aggregation of C = O groups, as well as clustering and electron cloud overlap are proposed. Unlike concentration quenching and aggregation-caused quenching (ACQ) that are normally observed in traditional luminogens, many of these unorthodox luminogens exhibit unique aggregation-induced emission (AIE) characteristics, regardless of their molecular architectures. This review summarizes varying unorthodox luminogens with AIE features, aiming to outline the recent advances in this exciting area, with focus on the macromolecular systems. In light of the reported results, clustering-triggered emission mechanism, namely clustering of diverse subgroups with subsequent electron cloud overlap and conformation rigidification can well rationalize the photophysical behaviors of most systems. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 560–574 相似文献
260.
Arshad Mehmood Prof. Dr. Benjamin G. Janesko 《Angewandte Chemie (International ed. in English)》2017,56(24):6878-6881
Atomic partial charges are widely used to predict reactivity. Partial charge alone is often insufficient: the carbons of benzene and cyclobutadiene, or those of diamond, graphene, and C60, possess nearly identical partial charges and very different reactivities. Our atomic overlap distance complements computed partial charges by measuring the size of orbital lobes that best overlap with the wavefunction around an atom. Compact, chemically stable atoms tend to have overlap distances smaller than chemically soft, unstable atoms. We show here how combining atomic charges and overlap distances captures trends in aromaticity, nucleophilicity, allotrope stability, and substituent effects. Applications to recent experiments in organic chemistry (counterintuitive Lewis base stabilization of alkenyl anions in anionic cyclization) and nanomaterials chemistry (facile doping of the central atom in Au7 hexagons) illustrate this combination's predictive power. 相似文献