全文获取类型
收费全文 | 71427篇 |
免费 | 6372篇 |
国内免费 | 4324篇 |
专业分类
化学 | 38004篇 |
晶体学 | 348篇 |
力学 | 4738篇 |
综合类 | 1045篇 |
数学 | 19016篇 |
物理学 | 18972篇 |
出版年
2023年 | 692篇 |
2022年 | 1043篇 |
2021年 | 2196篇 |
2020年 | 1819篇 |
2019年 | 1819篇 |
2018年 | 1541篇 |
2017年 | 1956篇 |
2016年 | 2279篇 |
2015年 | 2220篇 |
2014年 | 2669篇 |
2013年 | 4514篇 |
2012年 | 3368篇 |
2011年 | 3580篇 |
2010年 | 3257篇 |
2009年 | 4380篇 |
2008年 | 4529篇 |
2007年 | 4666篇 |
2006年 | 3795篇 |
2005年 | 2812篇 |
2004年 | 2534篇 |
2003年 | 2570篇 |
2002年 | 4855篇 |
2001年 | 2241篇 |
2000年 | 1725篇 |
1999年 | 1492篇 |
1998年 | 1416篇 |
1997年 | 1016篇 |
1996年 | 1004篇 |
1995年 | 900篇 |
1994年 | 839篇 |
1993年 | 768篇 |
1992年 | 723篇 |
1991年 | 580篇 |
1990年 | 438篇 |
1989年 | 366篇 |
1988年 | 382篇 |
1987年 | 324篇 |
1986年 | 321篇 |
1985年 | 444篇 |
1984年 | 322篇 |
1983年 | 203篇 |
1982年 | 380篇 |
1981年 | 528篇 |
1980年 | 496篇 |
1979年 | 538篇 |
1978年 | 421篇 |
1977年 | 330篇 |
1976年 | 291篇 |
1974年 | 96篇 |
1973年 | 162篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Dr. Zaifa Shi Dr. Yihuang Jiang Dr. Jingxiong Yu Prof. Shanjun Chen Prof. Jun Chen Prof. Zichao Tang Prof. Lansun Zheng 《Chemistry (Weinheim an der Bergstrasse, Germany)》2022,28(8):e202103546
At present, the reactivity of cyclic alkanes is estimated by comparison with acyclic hydrocarbons. Due to the difference in the structure of cycloalkanes and acycloalkanes, the thermodynamic data obtained by analogy are not applicable. In this study, a molecular beam sampling vacuum ultraviolet photoionization time-of-flight mass spectrometer (MB-VUV-PI-TOFMS) was applied to study the low-temperature oxidation of cyclopentane (CPT) at a total pressure range from 1–3 atm and low-temperature range between 500 and 800 K. Low-temperature reaction products including cyclic olefins, cyclic ethers, and highly oxygenated intermediates (e. g., ketohydroperoxide KHP, keto-dihydroperoxide KDHP, olefinic hydroperoxides OHP and ketone structure products) were observed. Further investigation of the oxidation of CPT – electronic structure calculations – were carried out at the UCCSD(T)-F12a/aug-cc-pVDZ//B3LYP/6-31+ G(d,p) level to explore the reactivity of O2 molecules adding sequentially to cyclopentyl radicals. Experimental and theoretical observations showed that the dominant product channel in the reaction of CPT radicals with O2 is HO2 elimination yielding cyclopentene. The pathways of second and third O2 addition – the dissociation of hydroperoxide – were further confirmed. The results of this study will develop the low-temperature oxidation mechanism of CPT, which can be used for future research on accurately simulating the combustion process of CPT. 相似文献
2.
3.
4.
针对圆柱形膨胀腔消声器三维建模及声学性能分析问题, 提出一种基于切比雪夫变分原理的耦合声场建模方法, 建立三维圆柱形膨胀腔消声器理论模型并搭建试验台架, 传递损失试验结果验证了理论模型的准确性. 将膨胀腔消声器内部声场分解为多个子声场, 基于子声场间压力与质点振速连续性条件, 推导声场耦合变分公式, 构建子声场拉格朗日泛函. 将子声场声压函数展开为切比雪夫-傅里叶级数形式, 通过瑞利-里兹法求解膨胀腔消声器频率、声压响应及传递损失. 计算并对比分析扩张比、扩张腔长度、进出口管偏置对膨胀腔消声器消声性能的影响. 结果表明: 扩张比增大会有效提高消声器在低频段的消声性能, 进出口管的偏置对消声器消声性能影响很小. 相似文献
5.
A numerical model was developed and validated to investigate the fluid–structure interactions between fully developed pipe flow and core–shell-structured microcapsule in a microchannel. Different flow rates and microcapsule shell thicknesses were considered. A sixth-order rotational symmetric distribution of von Mises stress over the microcapsule shell can be observed on the microcapsule with a thinner shell configuration, especially at higher flow rate conditions. It is also observed that when being carried along in a fully developed pipe flow, the microcapsule with a thinner shell tends to accumulate stress at a higher rate compared to that with a thicker shell. In general, for the same microcapsule configuration, higher flow velocity would induce a higher stress level over the microcapsule shell. The deformation gradient was used to capture the microcapsule's deformation in the present study. The effect of Young's modulus on the microcapsule shell on the microcapsule deformation was investigated as well. Our findings will shed light on the understanding of the stability of core–shell-structured microcapsule when subjected to flow-induced shear stress in a microfluidic system, enabling a more exquisite control over the breakup dynamics of drug-loaded microcapsule for biomedical applications. 相似文献
6.
《Operations Research Letters》2022,50(3):274-280
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. 相似文献
7.
We propose a conjecture on the relative twist formula of l-adic sheaves, which can be viewed as a generalization of Kato—Saito's conjecture. We verify this conjecture under some transversal assumptions. We also define a relative cohomological characteristic class and prove that its formation is compatible with proper push-forward. A conjectural relation is also given between the relative twist formula and the relative cohomological characteristic class. 相似文献
8.
In this article, we construct and analyze a residual-based a posteriori error estimator for a quadratic finite volume method (FVM) for solving nonlinear elliptic partial differential equations with homogeneous Dirichlet boundary conditions. We shall prove that the a posteriori error estimator yields the global upper and local lower bounds for the norm error of the FVM. So that the a posteriori error estimator is equivalent to the true error in a certain sense. Numerical experiments are performed to illustrate the theoretical results. 相似文献
9.
10.
In this work, a vanillin complex is immobilized onto MCM-41 and characterized by FT-IR, X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, thermogravimetric analysis, and BET techniques. This supported Schiff base complex was found to be an efficient and recoverable catalyst for the chemoselective oxidation of sulfides into sulfoxides and thiols into their corresponding disulfides (using hydrogen peroxide as a green oxidant) and also a suitable catalyst for the preparation of 2,3-dihydroquinazolin-4(1H)-one derivatives in water at 90°C. Using this protocol, we show that a variety of disulfides, sulfoxides, and 2,3-dihydroquinazolin-4(1H)-one derivatives can be synthesized in green conditions. The catalyst can be recovered and recycled for further reactions without appreciable loss of catalytic performance. 相似文献