全文获取类型
收费全文 | 314308篇 |
免费 | 9731篇 |
国内免费 | 10719篇 |
专业分类
化学 | 153992篇 |
晶体学 | 4393篇 |
力学 | 11087篇 |
综合类 | 937篇 |
数学 | 33436篇 |
物理学 | 84667篇 |
综合类 | 46246篇 |
出版年
2021年 | 3079篇 |
2020年 | 3362篇 |
2019年 | 3205篇 |
2018年 | 3877篇 |
2017年 | 4348篇 |
2016年 | 5852篇 |
2015年 | 5262篇 |
2014年 | 7074篇 |
2013年 | 14075篇 |
2012年 | 13091篇 |
2011年 | 15358篇 |
2010年 | 11913篇 |
2009年 | 11755篇 |
2008年 | 15425篇 |
2007年 | 15425篇 |
2006年 | 14545篇 |
2005年 | 13102篇 |
2004年 | 11112篇 |
2003年 | 9193篇 |
2002年 | 9178篇 |
2001年 | 9921篇 |
2000年 | 8118篇 |
1999年 | 5364篇 |
1998年 | 3775篇 |
1997年 | 3645篇 |
1996年 | 3757篇 |
1995年 | 3321篇 |
1994年 | 3416篇 |
1993年 | 3261篇 |
1992年 | 3682篇 |
1991年 | 3562篇 |
1990年 | 3379篇 |
1989年 | 3245篇 |
1988年 | 3107篇 |
1987年 | 3091篇 |
1986年 | 2989篇 |
1985年 | 3922篇 |
1984年 | 3967篇 |
1983年 | 3324篇 |
1982年 | 3554篇 |
1981年 | 3287篇 |
1980年 | 3078篇 |
1979年 | 3319篇 |
1978年 | 3499篇 |
1977年 | 3487篇 |
1976年 | 3491篇 |
1975年 | 3217篇 |
1974年 | 3398篇 |
1973年 | 3298篇 |
1972年 | 2532篇 |
排序方式: 共有10000条查询结果,搜索用时 156 毫秒
41.
Cavitation erosion is caused in solids exposed to strong pressure waves developing in an adjacent fluid field. The knowledge of the transient distribution of stresses in the solid is important to understand the cause of damaging by comparisons with breaking points of the material. The modeling of this problem requires the coupling of the models for the fluid and the solid. For this purpose, we use a strategy based on the solution of coupled Riemann problems that has been originally developed for the coupling of 2 fluids. This concept is exemplified for the coupling of a linear elastic structure with an ideal gas. The coupling procedure relies on the solution of a nonlinear equation. Existence and uniqueness of the solution is proven. The coupling conditions are validated by means of quasi‐1D problems for which an explicit solution can be determined. For a more realistic scenario, a 2D application is considered where in a compressible single fluid, a hot gas bubble at low pressure collapses in a cold gas at high pressure near an adjacent structure. 相似文献
42.
43.
44.
45.
46.
Experimental and Theoretical Studies on the Rearrangement of 2‐Oxoazepane α,α‐Amino Acids into 2′‐Oxopiperidine β2,3,3‐Amino Acids: An Example of Intramolecular Catalysis
下载免费PDF全文
![点击此处可从《Chemistry (Weinheim an der Bergstrasse, Germany)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Dr. Diego Núñez‐Villanueva Dr. M. Ángeles Bonache Laura Lozano Dr. Lourdes Infantes Prof. José Elguero Prof. Ibon Alkorta Prof. M. Teresa García‐López Dr. Rosario González‐Muñiz Dr. Mercedes Martín‐Martínez 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(6):2489-2500
Enantiopure β‐amino acids represent interesting scaffolds for peptidomimetics, foldamers and bioactive compounds. However, the synthesis of highly substituted analogues is still a major challenge. Herein, we describe the spontaneous rearrangement of 4‐carboxy‐2‐oxoazepane α,α‐amino acids to lead to 2′‐oxopiperidine‐containing β2,3,3‐amino acids, upon basic or acid hydrolysis of the 2‐oxoazepane α,α‐amino acid ester. Under acidic conditions, a totally stereoselective synthetic route has been developed. The reordering process involved the spontaneous breakdown of an amide bond, which typically requires strong conditions, and the formation of a new bond leading to the six‐membered heterocycle. A quantum mechanical study was carried out to obtain insight into the remarkable ease of this rearrangement, which occurs at room temperature, either in solution or upon storage of the 4‐carboxylic acid substituted 2‐oxoazepane derivatives. This theoretical study suggests that the rearrangement process occurs through a concerted mechanism, in which the energy of the transition states can be lowered by the participation of a catalytic water molecule. Interestingly, it also suggested a role for the carboxylic acid at position 4 of the 2‐oxoazepane ring, which facilitates this rearrangement, participating directly in the intramolecular catalysis. 相似文献
47.
Su X. F. Zhang Y. L. Maximov A. L. Zhang K. Xin Q. Feng C. Q. Bai X. F. Wu W. 《Russian Journal of Applied Chemistry》2020,93(1):127-136
Russian Journal of Applied Chemistry - Nanoscale ZSM-5 (AlZ5-D) and Ga-substituted ZSM-5 zeolites ((Al,Ga)Z5-D and GaZ5-D) were synthesized by a green dry-gel conversion strategy. For comparison,... 相似文献
48.
49.
通过对电子阅览室读者满意度的问卷调查和对调查数据的分析,对电子阅览室服务质量进行了综合评价,旨在进一步完善电子阅览室的各项工作,从而提升读者对图书馆的满意度. 相似文献
50.
提出了工程造价专业建设目标,对工程造价专业人才培养模式、教学计划、教学手段和课程考核等方面的改革进行全面阐述,结合当前工程造价专业改革的现状,提出了具有较强实用性和可行性的工程造价专业教学改革方案。 相似文献