全文获取类型
收费全文 | 22608篇 |
免费 | 2342篇 |
国内免费 | 4938篇 |
专业分类
化学 | 26416篇 |
晶体学 | 138篇 |
力学 | 265篇 |
综合类 | 117篇 |
数学 | 426篇 |
物理学 | 2526篇 |
出版年
2024年 | 50篇 |
2023年 | 349篇 |
2022年 | 610篇 |
2021年 | 807篇 |
2020年 | 1206篇 |
2019年 | 1008篇 |
2018年 | 1029篇 |
2017年 | 949篇 |
2016年 | 1040篇 |
2015年 | 966篇 |
2014年 | 1450篇 |
2013年 | 2253篇 |
2012年 | 1262篇 |
2011年 | 1559篇 |
2010年 | 1186篇 |
2009年 | 1305篇 |
2008年 | 1384篇 |
2007年 | 1509篇 |
2006年 | 1403篇 |
2005年 | 1309篇 |
2004年 | 1292篇 |
2003年 | 990篇 |
2002年 | 616篇 |
2001年 | 488篇 |
2000年 | 498篇 |
1999年 | 422篇 |
1998年 | 381篇 |
1997年 | 333篇 |
1996年 | 308篇 |
1995年 | 299篇 |
1994年 | 250篇 |
1993年 | 198篇 |
1992年 | 192篇 |
1991年 | 140篇 |
1990年 | 112篇 |
1989年 | 102篇 |
1988年 | 84篇 |
1987年 | 57篇 |
1986年 | 48篇 |
1985年 | 53篇 |
1984年 | 48篇 |
1983年 | 26篇 |
1982年 | 37篇 |
1981年 | 35篇 |
1980年 | 26篇 |
1978年 | 27篇 |
1977年 | 29篇 |
1976年 | 36篇 |
1974年 | 38篇 |
1973年 | 24篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
采用固相反应法合成了单相的Ti_(1-x)(Hf_(0·919)Zr_(0·08))_xNiSn(x=0·00—0·15),并用放电等离子烧结方法制备出密实块体材料.研究了Hf和Zr同时在Ti位上的等电子合金化对Ti基半Heusler化合物热电性能的影响规律.结果表明:少量的Hf和微量的Zr在Ti位上的等电子合金化,显著地降低了体系的热导率κ,同时显著地提高了体系的Seebeck系数α.组成为Ti_(1-x)(Hf_(0·919)Zr_(0·08))_(0.15)NiSn的试样室温热导率为3·72W·m-1K-1,在700K时ZT值达到最大为0·56.与三元TiNiSn相比,在相同温度下ZT值的提高率为190%—310%. 相似文献
32.
本文用密度泛函理论(DFT)的总能计算研究了一氧化碳和氢原子在Ni(111)表面上p(2×2)共吸附系统的原子结构和电子态,结果表明CO和H原子分别被吸附于两个对角p(1×1)元胞的hcp和fcc位置.以氢分子和CO分子作为能量参考点,总吸附能为2.81 eV,相应的共吸附表面功函数φ为6.28 eV.计算得到的C—O,C—Ni和H—Ni的键长分别是1.19?, 1.96?和 1.71?,并且CO分子以C原子处于hcp的谷位与金属衬底原子结合.衬底Ni(111)的最外两层的晶面间距在吸附后的相对变化分别是
关键词:
Fisher-Tropsch反应
催化作用
Ni(111) p(2×2)/(CO+H)
共吸附 相似文献
33.
Microwave-induced organic reaction enhancement (‘MORE’) chemistry technique (open vessel; controlled microwave energy to stay below the boiling point of the reaction mixture) was used for the N-formylation of aliphatic and aromatic amines and amino heterocycles with aq formic acid (80%) on a multiple gram scale in a few minutes. 相似文献
34.
P.A. Ramachandran 《Numerical Methods for Partial Differential Equations》2006,22(4):831-846
Time‐dependent differential equations can be solved using the concept of method of lines (MOL) together with the boundary element (BE) representation for the spatial linear part of the equation. The BE method alleviates the need for spatial discretization and casts the problem in an integral format. Hence errors associated with the numerical approximation of the spatial derivatives are totally eliminated. An element level local cubic approximation is used for the variable at each time step to facilitate the time marching and the nonlinear terms are represented in a semi‐implicit manner by a local linearization at each time step. The accuracy of the method has been illustrated on a number of test problems of engineering significance. © 2005 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq 2006 相似文献
35.
Debendra K. Mohapatra Dhananjoy Mondal Mukund S. Chorghade Mukund K. Gurjar 《Tetrahedron letters》2006,47(52):9215-9219
The non-proteinogenic amino acid 3-hydroxy-4-methylproline (HMP) is an active constituent of some potent antimicrobials including echinocandins, nostopeptins, pneumocandins, sporiofungin and mulundocandins. A synthesis has been achieved in 10 steps with 29% overall yield; the Evans’ aldol reaction using Crimmins’ modified method was pivotal to the success of the strategy. 相似文献
36.
(S)-2-[2-(p-Tolylsulfinyl)phenyl]acetaldehyde reacts with different O-silylated ketenethioacetals in the presence of Yb(OTf)3 yielding β-hydroxythioesters in high yields and diastereoselectivities. The obtained compounds were readily transformed into β-hydroxyacids and their corresponding diols. These Mukaiyama aldol reactions are a direct evidence of the ability of the sulfinyl group to control 1,5- and 1,6-asymmetric induction processes. 相似文献
37.
A practical access to alkyl- and aryl-substituted (E)-2-(azidomethyl)alkenoates and related azido compounds from the corresponding allylic bromides in aqueous acetone is described. An alternative method to obtain the starting bromides based on heterogeneous catalysis under mild conditions was also investigated. 相似文献
38.
Cosmic dust and our origins 总被引:1,自引:0,他引:1
J Mayo Greenberg 《Surface science》2002,500(1-3):793-822
The small solid particles in the space between the stars provide the surfaces for the production of many simple and complex molecules. Processes involving the effects of ultraviolet irradiation of the thin (hundredth micron) mantles are shown to produce a wide range of molecules and ions also seen in comets. Some of the more complex ones inferred from laboratory experiments are expected to play an important role in the origin of life. An outline of the chemical evolution of interstellar dust as observed and as studied in the laboratory is presented. Observations of comets are shown to provide substantial evidence for their being fluffy aggregates of interstellar dust as it was in the protosolar nebula, i.e. the interstellar cloud which collapsed to form the solar system. The theory that comets may have brought the progenitors of life to the earth is summarized. 相似文献
39.
40.
1 INTRODUCTION Carbon nanotubes have many potential applica- tions due to their unique structures and properties[1~4]. Physicists have been studying their electrical, me- chanical and other properties since their discove- ries[5, 6]. Recently, chemists are interested in carbon nanotubes because their properties can be altered by chemical functionalization[7~14], and these functiona- lized nanotubes can undergo further chemical trea- tment. So the potential application range of such na- … 相似文献