全文获取类型
收费全文 | 88662篇 |
免费 | 15558篇 |
国内免费 | 22077篇 |
专业分类
化学 | 64343篇 |
晶体学 | 2776篇 |
力学 | 5807篇 |
综合类 | 2300篇 |
数学 | 12812篇 |
物理学 | 38259篇 |
出版年
2024年 | 165篇 |
2023年 | 1123篇 |
2022年 | 2576篇 |
2021年 | 2882篇 |
2020年 | 2988篇 |
2019年 | 3017篇 |
2018年 | 2737篇 |
2017年 | 3509篇 |
2016年 | 3337篇 |
2015年 | 4261篇 |
2014年 | 5125篇 |
2013年 | 6887篇 |
2012年 | 7315篇 |
2011年 | 7879篇 |
2010年 | 6922篇 |
2009年 | 6914篇 |
2008年 | 7821篇 |
2007年 | 6960篇 |
2006年 | 6635篇 |
2005年 | 5815篇 |
2004年 | 4446篇 |
2003年 | 3351篇 |
2002年 | 3270篇 |
2001年 | 3078篇 |
2000年 | 3078篇 |
1999年 | 2038篇 |
1998年 | 1339篇 |
1997年 | 1143篇 |
1996年 | 1122篇 |
1995年 | 950篇 |
1994年 | 1048篇 |
1993年 | 931篇 |
1992年 | 834篇 |
1991年 | 641篇 |
1990年 | 587篇 |
1989年 | 581篇 |
1988年 | 458篇 |
1987年 | 361篇 |
1986年 | 331篇 |
1985年 | 304篇 |
1984年 | 236篇 |
1983年 | 203篇 |
1982年 | 193篇 |
1981年 | 143篇 |
1980年 | 134篇 |
1979年 | 121篇 |
1978年 | 68篇 |
1976年 | 62篇 |
1974年 | 62篇 |
1973年 | 59篇 |
排序方式: 共有10000条查询结果,搜索用时 171 毫秒
31.
32.
针对智能化柔性制造过程存在的再制造过程,对中小型设备生产车间及其仓储转运的模型构建、分析方法、以及参量相关性提出了新的解决思路.为了最大限度的减少建设周期、合理化投资成本、以及优化生产流程,建立了考虑非结构性的多级生产流程的数学模型,以完成度、完工时间和调度频次为目标函数分析生产要素间的相关性.实验结果表明,融合再加工过程的智能化柔性制造单元中,生产要素为非线性相关,个别要素具备对称性,为实际工况中的生产改进及优化提供了一定理论基础. 相似文献
33.
Single Crystal to Single Crystal (SC‐to‐SC) Transformation from a Nonporous to Porous Metal–Organic Framework and Its Application Potential in Gas Adsorption and Suzuki Coupling Reaction through Postmodification 下载免费PDF全文
Dr. Rupam Sen Debraj Saha Dr. Subratanath Koner Dr. Paula Brandão Dr. Zhi Lin 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(15):5962-5971
A new amino‐functionalized strontium–carboxylate‐based metal–organic framework (MOF) has been synthesized that undergoes single crystal to single crystal (SC‐to‐SC) transformation upon desolvation. Both structures have been characterized by single‐crystal X‐ray analysis. The desolvated structure shows an interesting 3D porous structure with pendent ?NH2 groups inside the pore wall, whereas the solvated compound possesses a nonporous structure with DMF molecules on the metal centers. The amino group was postmodified through Schiff base condensation by pyridine‐2‐carboxaldehyde and palladium was anchored on that site. The modified framework has been utilized for the Suzuki cross‐coupling reaction. The compound shows high activity towards the C?C cross‐coupling reaction with good yields and turnover frequencies. Gas adsorption studies showed that the desolvated compound had permanent porosity and was microporous in nature with a BET surface area of 2052 m2 g?1. The material also possesses good CO2 (8 wt %) and H2 (1.87 wt %) adsorption capabilities. 相似文献
34.
35.
36.
37.
38.
A self-propelled object coupled with an enzyme reaction between urease and urea was investigated at the air/aqueous interface. A plastic object that was fixed to a urease-immobilized filter paper was used as a self-propelled object, termed a urease motor, placed on an aqueous urea solution. The driving force of the urease motor is the difference in the surface tension around the object. Oscillatory motion or no motion was triggered depending on the initial pH of the urea solution. Both the frequency and maximum speed of the oscillatory motion varied depending on the initial pH of the water phase. The mechanisms underlying the oscillatory motion and no motion were discussed in relation to the bell-shaped enzyme activity of urease in the enzyme reaction and the surface tension around the urease motor. 相似文献
39.
40.
In the present research, hierarchical structure observation and mechanical property characterization for a type of biomaterial are carried out. The investigated biomaterial is Hyriopsis cumingii, a typical limnetic shell, which consists of two different structural layers, a prismatic "pillar"structure and a nacreous "brick and mortar" structure. The prismatic layer looks like a "pillar forest" with variationsection pillars sized on the order of several tens of microns.The nacreous material looks like a "brick wall" with bricks sized on the order of several microns. Both pillars and bricks are composed of nanoparticles. The mechanical properties of the hierarchical biomaterial are measured by using the nanoindentation test. Hardness and modulus are measured for both the nacre layer and the prismatic layer, respectively.The nanoindentation size effects for the hierarchical structural materials are investigated experimentally. The results show that the prismatic nanostructured material has a higher stiffness and hardness than the nacre nanostructured material.In addition, the nanoindentation size effects for the hierarchical structural materials are described theoretically, by using the trans-scale mechanics theory considering both strain gradient effect and the surface/interface effect. The modeling results are consistent with experimental ones. 相似文献