全文获取类型
收费全文 | 3440篇 |
免费 | 896篇 |
国内免费 | 594篇 |
专业分类
化学 | 600篇 |
晶体学 | 56篇 |
力学 | 384篇 |
综合类 | 67篇 |
数学 | 1534篇 |
物理学 | 2289篇 |
出版年
2024年 | 30篇 |
2023年 | 49篇 |
2022年 | 72篇 |
2021年 | 68篇 |
2020年 | 124篇 |
2019年 | 102篇 |
2018年 | 136篇 |
2017年 | 147篇 |
2016年 | 171篇 |
2015年 | 132篇 |
2014年 | 211篇 |
2013年 | 288篇 |
2012年 | 215篇 |
2011年 | 243篇 |
2010年 | 198篇 |
2009年 | 218篇 |
2008年 | 259篇 |
2007年 | 230篇 |
2006年 | 194篇 |
2005年 | 215篇 |
2004年 | 179篇 |
2003年 | 200篇 |
2002年 | 159篇 |
2001年 | 143篇 |
2000年 | 158篇 |
1999年 | 124篇 |
1998年 | 111篇 |
1997年 | 108篇 |
1996年 | 71篇 |
1995年 | 57篇 |
1994年 | 47篇 |
1993年 | 43篇 |
1992年 | 29篇 |
1991年 | 18篇 |
1990年 | 14篇 |
1989年 | 19篇 |
1988年 | 15篇 |
1987年 | 15篇 |
1986年 | 15篇 |
1985年 | 18篇 |
1984年 | 19篇 |
1983年 | 7篇 |
1982年 | 14篇 |
1981年 | 6篇 |
1980年 | 5篇 |
1979年 | 10篇 |
1978年 | 4篇 |
1977年 | 3篇 |
1976年 | 4篇 |
1974年 | 4篇 |
排序方式: 共有4930条查询结果,搜索用时 15 毫秒
151.
如何实现高精度的测量是现代制造业及微电子技术领域的热点问题之一. 基于微纳米测头的三坐标测量机是当前实现高精度测量的重要手段. 随着测量尺寸的减小, 常用的纳米/微纳尺度的测头与待测表面之间形成静态接触, 其表面相互作用成为了影响其测量精度和可靠性的关键因素之一. 本文基于一种触发式振动测头, 研究了其动力学模型, 并通过对测头纳米尺度表面相互作用的理论分析及数值模拟, 确立了测头振动参数与表面相互作用之间的关联. 实验研究表明, 参数优化后的谐振微纳测头能有效抑制表面作用带来的干扰, 提高测量精度. 相似文献
152.
Khang Hoang 《固体物理学:研究快报》2016,10(12):915-918
Erbium (Er) doped GaN has been studied extensively for optoelectronic applications, yet its defect physics is still not well understood. In this work, we report a first‐principles hybrid density functional study of the structure, energetics, and thermodynamic transition levels of Er‐related defect complexes in GaN. We discover for the first time that ErGa–CN–VN, a defect complex of Er, a C impurity, and an N vacancy, and ErGa–ON–VN, a complex of Er, an O impurity, and an N vacancy, form defect levels at 0.18 eV and 0.46 eV below the conduction band, respectively. Together with ErGa–VN, a complex of Er and an N vacancy which has recently been found to produce a donor level at 0.61 eV, these defect complexes provide explanation for the Er‐related defect levels observed in experiments. The role of these defects in optical excitation of the luminescent Er center is also discussed. 相似文献
153.
李玉山 《原子与分子物理学报》2015,32(6)
基于Thomas-Fermi半经典近似方法研究了谐振子势阱约束下任意维理想玻色气体的玻色-爱因斯坦凝聚(BEC).导出了玻色气体的BEC转变温度、基态粒子占据比例、内能和热容量等物理量的解析表达式,讨论了空间维度和谐振子势阱的影响.以二维和三维玻色系统为例,数值计算了上述热力学量,并与解析结果进行了对比,二者获得了较好的吻合. 相似文献
154.
以第一性原理和变分原理为基础,给出了氩原子基态波函数的一种解析表达式,计算了基态氩原子(含类氩离子)的能量,导出了所涉及的所有积分的解析表达式.对氩原子,所得到的能量理论值与实验值的相对误差为0.22%. 相似文献
155.
采用基于密度泛函理论(DFT)的平面波超软赝势方法(PWPP), 利用Material studio 计算N, Fe, La三种元素掺杂引起的锐钛矿TiO2晶体结构、能带结构和态密度变化. 并通过溶胶-凝胶法制得锐钛矿型本征TiO2, N, Fe共掺杂TiO2和N, Fe, La共掺杂TiO2; 用X射线衍射和扫描电镜表征结构; 紫外-可见分光光度计检测TiO2对甲基橙的降解效率变化. 计算结果表明, 由于N, Fe, La三掺杂TiO2的晶格体积、键长等发生变化, 导致晶体对称性下降, 光生电子-空穴对有效分离, 同时在导带底和价带顶形成杂质能级, TiO2禁带宽度由1.78 eV变为1.35 eV, 减小25%, 光吸收带边红移, 态密度数增加, 电子跃迁概率提升, 光催化能力增加. 实验结果表明: 离子掺杂使颗粒变小, 粒径大小: 本征TiO2>N/Fe_TiO2>N/Fe/La_TiO2, 并测得N/Fe/La_TiO2发光峰425 nm, 能隙减小, 光催化能力比N/Fe_TiO2强, 增强原因是杂质能级和电子态数量增加引起. 相似文献
156.
Consider first passage percolation on with passage times given by i.i.d. random variables with common distribution F. Let be the time from u to v for a path π and the minimal time among all paths from u to v. We ask whether or not there exist points and a semi‐infinite path such that for all n. Necessary and sufficient conditions on F are given for this to occur. When the support of F is unbounded, we also obtain results on the number of edges with large passage time used by geodesics. © 2014 Wiley Periodicals, Inc. Random Struct. Alg., 47, 414–423, 2015 相似文献
157.
Luke Gordon Joel B. Varley John L. Lyons Anderson Janotti Chris G. Van de Walle 《固体物理学:研究快报》2015,9(8):462-465
Achieving high levels of n‐type conductivity in AlN and high Al‐content nitride alloys is a long standing problem; significant decreases in conductivity are observed as the Al content is increased, a phenomenon that has been attributed to donors such as oxygen or silicon forming DX centers. We address this problem through a comprehensive first‐principles hybrid density functional study of potential n‐type dopants, identifying SN and SeN as two elements which are potential shallow donors because they do not undergo a DX transition. In particular, SN is highly promising as an n‐type dopant because it also has a low formation energy and hence a high solubility. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim) 相似文献
158.
Synthesis,spatial structure and spectral properties of pyrylo‐4 (thio) squaraines variously substituted in cyclobutene moiety 下载免费PDF全文
Vladimir V. Kurdyukov Aleksei I. Tolmachev Marina L. Dekhtyar Yurii G. Vlasenko Aleksandr N. Chernega 《Journal of Physical Organic Chemistry》2015,28(7):452-459
Synthetic routes have been developed to a number of (thio) squaraine dyes containing the residues of CH‐acids at the central cyclobutene ring. The electronic and spatial structure as well as the chemical conversions and optical behaviour of the compounds obtained have been studied both theoretically and by X‐ray diffraction analysis, 1H NMR and electronic spectroscopy. As shown, the electronic nature and sterical characteristics of the central ring substituents give rise to some general conformational features and crystal packing regularities and also govern the spectral position of the first π–π* absorption band. The structure–property relationships established in the study provide guidance for the purposeful design of deeply coloured (thio) squaraines. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
159.
Single crystals of a new organic material, 4-hydroxy-l-proline-l-tartaric acid (HPTA) adduct, were grown by slow evaporation of an aqueous solution containing equimolar concentrations of 4-hydroxy-l-proline and l-tartaric acid at room temperature. Formation of new crystal has been confirmed by single crystal X-ray diffraction and NMR spectroscopic techniques. The single crystal XRD analysis shows that a molecule of 4-hydroxy-l-proline and a molecule of free l-tartaric acid both of which are interlinked to each other by two types of H-bonding interactions, namely O(6)–H(6)⋯O(2) and N(1)–H(1B)⋯O(10). The title compound (HPTA) crystallizes in monoclinic crystal system with non-centrosymmetric space group P21. The suitability of the crystal for optical applications was studied by UV–Vis–NIR spectroscopy. Fourier transform infrared (FT-IR) spectral analysis was used to confirm the presence of various functional groups in the grown crystals. The thermal stability of the compound was investigated by thermogravimetric and differential thermal analyses. The dielectric constant and dielectric loss of the crystal were studied as a function of frequency and temperature. The fluorescence spectral analysis of the title crystal shows an indigo emission. The second order nonlinear optical (NLO) property of the crystal was confirmed by the modified Kurtz–Perry powder second harmonic generation (SHG) test. 相似文献
160.
Large Hexagonal Bi‐ and Trilayer Graphene Single Crystals with Varied Interlayer Rotations 下载免费PDF全文
Zheng Yan Yuanyue Liu Long Ju Zhiwei Peng Dr. Jian Lin Dr. Gunuk Wang Dr. Haiqing Zhou Changsheng Xiang E. L. G. Samuel Carter Kittrell Dr. Vasilii I. Artyukhov Prof. Feng Wang Prof. Boris I. Yakobson Prof. James M. Tour 《Angewandte Chemie (International ed. in English)》2014,53(6):1565-1569
Bi‐ and trilayer graphene have attracted intensive interest due to their rich electronic and optical properties, which are dependent on interlayer rotations. However, the synthesis of high‐quality large‐size bi‐ and trilayer graphene single crystals still remains a challenge. Here, the synthesis of 100 μm pyramid‐like hexagonal bi‐ and trilayer graphene single‐crystal domains on Cu foils using chemical vapor deposition is reported. The as‐produced graphene domains show almost exclusively either 0° or 30° interlayer rotations. Raman spectroscopy, transmission electron microscopy, and Fourier‐transformed infrared spectroscopy were used to demonstrate that bilayer graphene domains with 0° interlayer stacking angles were Bernal stacked. Based on first‐principle calculations, it is proposed that rotations originate from the graphene nucleation at the Cu step, which explains the origin of the interlayer rotations and agrees well with the experimental observations. 相似文献