全文获取类型
收费全文 | 35613篇 |
免费 | 6462篇 |
国内免费 | 4707篇 |
专业分类
化学 | 13825篇 |
晶体学 | 287篇 |
力学 | 2161篇 |
综合类 | 263篇 |
数学 | 8567篇 |
物理学 | 21679篇 |
出版年
2024年 | 139篇 |
2023年 | 471篇 |
2022年 | 1194篇 |
2021年 | 1171篇 |
2020年 | 1274篇 |
2019年 | 1161篇 |
2018年 | 1123篇 |
2017年 | 1221篇 |
2016年 | 1534篇 |
2015年 | 1375篇 |
2014年 | 1945篇 |
2013年 | 3116篇 |
2012年 | 2082篇 |
2011年 | 2441篇 |
2010年 | 1928篇 |
2009年 | 2305篇 |
2008年 | 2519篇 |
2007年 | 2509篇 |
2006年 | 2204篇 |
2005年 | 1882篇 |
2004年 | 1557篇 |
2003年 | 1485篇 |
2002年 | 1386篇 |
2001年 | 1073篇 |
2000年 | 1133篇 |
1999年 | 880篇 |
1998年 | 842篇 |
1997年 | 616篇 |
1996年 | 413篇 |
1995年 | 373篇 |
1994年 | 315篇 |
1993年 | 291篇 |
1992年 | 284篇 |
1991年 | 220篇 |
1990年 | 240篇 |
1989年 | 229篇 |
1988年 | 199篇 |
1987年 | 186篇 |
1986年 | 164篇 |
1985年 | 167篇 |
1984年 | 154篇 |
1983年 | 83篇 |
1982年 | 122篇 |
1981年 | 113篇 |
1980年 | 97篇 |
1979年 | 102篇 |
1978年 | 82篇 |
1977年 | 81篇 |
1976年 | 75篇 |
1973年 | 62篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
通过测量光电流,直接观察了InGaN/GaN量子阱中载流子的泄漏程度随温度升高的变化关系。当LED温度从300K升高到360K时,在相同的光照强度下,LED的光电流增大,说明在温度上升之后,载流子从量子阱中逃逸的数目更多,即载流子泄漏比例增大。同时,光电流的增大在激发密度较低的时候更为明显,而且光电流随温度的增加幅度与激发光子的能量有关。用量子阱-量子点复合模型能很好地解释所观察到的实验现象。实验结果直接证明,随着温度的升高,InGaN/GaN量子阱中的载流子泄漏将显著增加,而且在低激发密度下这一效应更为明显。温度升高导致的载流子泄漏增多是InGaN多量子阱LED发光效率随温度升高而降低的重要原因。 相似文献
992.
芶清泉 《原子与分子物理学报》2011,28(5):785-788
我在一篇论文中提出了一个冷聚变的全原子论,阐明氘原子进入钯晶体间隙位置时扩展成为一个封闭的球形全原子,其价电子完全在球内,围绕氘核运动使氘核对邻近氘核的排斥作用被屏蔽掉,使氘核易于接近而产生聚变。对离子晶体两个负氘离子相互作用是怎样引起聚变的没做详细讨论,本文将对此问题进一步进行详细讨论。 相似文献
993.
《Molecular physics》2012,110(11-12):1213-1221
Thermodynamic models based on the van der Waals–Platteeuw statistical theory (Adv. Chem. Phys. 2, 1 (1959)) can be very accurate in describing hydrate equilibrium conditions, even for some occasions when multiple cavity occupancy occurs. These are cases outside the range of assumptions used for the development of the original statistical theory. However, during multiple cavity occupancy such models can perform poorly when calculating the cavity occupancies. This paper reports novel Grand Canonical Monte Carlo molecular simulations for the case of pure structure II N2 hydrate and compares the calculated cavity occupancies with experimental data and observe reasonable agreement. Also examined are the van der Waals–Platteeuw-based modifications that retained the single-occupancy assumption of the original theory and how they perform when predicting cavity occupancies and hydrate equilibrium pressures. 相似文献
994.
Material growth and device fabrication of terahertz quantum-cascade laser based on bound-to-continuum structure
下载免费PDF全文
![点击此处可从《中国物理 B》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The terahertz quantum-cascade laser (THz QCL) based on bound-to-continuum structure is demonstrated. The X-ray diffraction measurement of the material shows a high crystalline quality of the active region. A THz QCL device was fabricated with semi-insulating surface-plasmon waveguide. The test device is lasing at about 3 THz and operating up to 60 K. It shows a single frequency property under different drive currents and temperatures. At 9 K, the maximum output power is greater than 2 mW with a threshold current density of 159 A/cm2. 相似文献
995.
《International Journal of Approximate Reasoning》2014,55(7):1606-1608
This note replies to comments made on our contribution to the Low Quality Data debate. 相似文献
996.
《Journal of polymer science. Part A, Polymer chemistry》2018,56(9):977-985
Polymeric networks are produced by step‐growth polyaddition and co‐polyaddition reactions of 1‐ethylimidazoline in combination with various diisocyanates. Five aromatic, two aliphatic diisocyanates and a polyurethane prepolymer are used as particular reactant in N,N‐dimethylformamide as solvent at room temperature. Obviously, 1‐ethylimidazoline can serve as trifunctional monomer, which enables a crosslinking reaction with diisocyanates. Molecular structure elements of the polymeric networks were studied by solid state 13C‐NMR spectroscopy revealing that detailed molecular structure formations are determined whether aromatic or aliphatic diisocyanates are used. Quantum chemical calculations were used as supporting method to elucidate the complex reaction cascades. Hence, it can be shown that beside 3:1 stoichiometric structures 2:1 based structures are formed as well. These structures are observed as kinetically controlled products only when aromatic diisocyanate monomers are used. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 977–985 相似文献
997.
针对解决城市交通拥堵决策问题,首先给出了错误优化矩阵的概念,在此基础上引出错误矩阵方程的概念,利用消错理论中的错误优化矩阵方程,从错误优化的角度来研究并解决城市交通拥堵的决策方法.相应结合实际状况给出当前状态矩阵,从而进行下一步的求解,步步推理获得了决策人满意的方案集,为决策者提供最优建议. 相似文献
998.
We investigate the generalized polynomial Linard differential equations. Using the averaging theory of first and second order, we obtain the maximum number of limit cycles of the system. 相似文献
999.
《International journal of quantum chemistry》2018,118(15)
This article presents a discussion about the formalism, which might be associated to a general Quantum quantitative structure–properties relations operator, appearing in a Boltzmann‐like exponential form, which is based in turn on the definition of the concept of thermal voltage, applied to thermally scaled electronic density functions. Three practical numerical examples are presented, corresponding to the calculation of the polarization angle in assorted chiral molecules, the estimation of fish toxicity for perchlorobenzene within the set of chlorobenzenes and a typical quantum QSAR study on the popular Cramer steroid set. 相似文献
1000.
《International journal of quantum chemistry》2018,118(2)
We investigate the success of the quantum chemical electron impact mass spectrum (QCEIMS) method in predicting the electron impact mass spectra of a diverse test set of 61 small molecules selected to be representative of common fragmentations and reactions in electron impact mass spectra. Comparison with experimental spectra is performed using the standard matching algorithms, and the relative ranking position of the actual molecule matching the spectra within the NIST‐11 library is examined. We find that the correct spectrum is ranked in the top two matches from structural isomers in more than 50% of the cases. QCEIMS, thus, reproduces the distribution of peaks sufficiently well to identify the compounds, with the RMSD and mean absolute difference between appropriately normalized predicted and experimental spectra being at most 9% and 3% respectively, even though the most intense peaks are often qualitatively poorly reproduced. We also compare the QCEIMS method to competitive fragmentation modeling for electron ionization, a training‐based mass spectrum prediction method, and remarkably we find the QCEIMS performs equivalently or better. We conclude that QCEIMS will be very useful for those who wish to identify new compounds which are not well represented in the mass spectral databases. 相似文献