全文获取类型
收费全文 | 4279篇 |
免费 | 518篇 |
国内免费 | 457篇 |
专业分类
化学 | 4171篇 |
晶体学 | 48篇 |
力学 | 101篇 |
综合类 | 50篇 |
数学 | 56篇 |
物理学 | 828篇 |
出版年
2024年 | 12篇 |
2023年 | 81篇 |
2022年 | 183篇 |
2021年 | 258篇 |
2020年 | 219篇 |
2019年 | 184篇 |
2018年 | 134篇 |
2017年 | 175篇 |
2016年 | 212篇 |
2015年 | 167篇 |
2014年 | 200篇 |
2013年 | 462篇 |
2012年 | 269篇 |
2011年 | 212篇 |
2010年 | 190篇 |
2009年 | 214篇 |
2008年 | 186篇 |
2007年 | 222篇 |
2006年 | 189篇 |
2005年 | 172篇 |
2004年 | 167篇 |
2003年 | 131篇 |
2002年 | 98篇 |
2001年 | 93篇 |
2000年 | 74篇 |
1999年 | 89篇 |
1998年 | 79篇 |
1997年 | 67篇 |
1996年 | 75篇 |
1995年 | 57篇 |
1994年 | 57篇 |
1993年 | 57篇 |
1992年 | 34篇 |
1991年 | 21篇 |
1990年 | 31篇 |
1989年 | 21篇 |
1988年 | 24篇 |
1987年 | 17篇 |
1986年 | 14篇 |
1985年 | 14篇 |
1984年 | 9篇 |
1983年 | 7篇 |
1982年 | 11篇 |
1981年 | 7篇 |
1980年 | 11篇 |
1979年 | 13篇 |
1978年 | 8篇 |
1977年 | 6篇 |
1976年 | 6篇 |
1975年 | 4篇 |
排序方式: 共有5254条查询结果,搜索用时 31 毫秒
101.
102.
利用原位红外技术研究了CH3OH、CO、O2等在Pd/γAl2O3上的吸附情况及CH3OH和O2共吸附时表面物种的变化,结果表明:表面纯净的Pd/γAl2O3催化剂对甲醇具有很好的解离吸附性能,在无氧条件下,CH3OH可在其上脱氢产生HCHO及CO。Pd对O2的吸附力强,氧均以解离方式吸附于Pd/γAl2O3上。当吸附的[CH3O]与[O]在Pd/γAl2O3上相互作用时,可产生HCHO、CO、甲二氧基、甲酸根等中间物种 相似文献
103.
K对Mn—Co—O的结构及氧化活性的影响 总被引:1,自引:0,他引:1
利用XRD,TPD,TFD-MS及催化氧化反应等实验技术,研究了K对Mn-Co-O的结构及氧化活性的影响。XRD结果表明,Mn-Co-O系已形成反尖晶石型的MnCo_2O_4结构;K-Mn-Co-O系中除MnCo_2O_4型结构外,还有新相KMnO_2存在。根据O_2的TPD-MS及吡啶的TPD结果,Mn-Co-O中添加适量的K能提高供氧活性和增加供氧数目,但酸中心数目减少,强度降低。K对Mn-Co—O催化剂氧化活性的影响随反应物分子结构的不同而异,对乙酸乙脂、苯甲酰氯等极性有机物的完全氧化反应,K能提高氧化活性,但对苯、正己烷等非极性有机物的完全氧化反应,K使其活性降低。 相似文献
104.
Summary In this work, an optimization procedure for gradient RPLC separation, using ternary mobile phases, is described. This procedure requires eight preliminary experiments in gradient elution mode to predict the retention surface for each solute over the whole triangular space. This is followed by computerized calculations to determine the best ternary gradient elution profile with respect to both selectivity and analysis time. The efficiency of this procedure from the point of view of rapidity and of accuracy, is illustrated for the specific separation of twelve phenyl urea herbicides. 相似文献
105.
Proton NMR was used to study the complexation reaction between silver ion and hexathia-18-crown-6 in a number of binary mixed solvents of dimethyl sulfoxide with acetonitrile and methanol. Formation constants for the resulting 1:1 complexes in different solvent mixtures were determined by computer fitting of the chemical shift-mole ratio data. The influence of solvent composition on the stability of the resulting complex is discussed. The exchange kinetics of Ag+-hexathia-18-crown-6 in 70-30 wt.% dimethyl sulfoxide-acetonitrile and 75-25 wt.% dimethyl sulfoxide-methanol were studied by proton NMR line-shape analysis. In both solvent mixtures, the exchange of thiacrown ether between the free and complexed sites was found to proceed via a dissociative pathway. The exchange rates and the activation parameters E
a, H
, S, and G for the ligand exchange were determined and the influence of solvent properties on these parameters discussed. 相似文献
106.
Dean C. Luehrs Richard E. Brown Kalpana A. Godbole 《Journal of solution chemistry》1989,18(5):463-469
A linear free energy relationship was found betweenv
max, the energy of the absorption maximum of chloride, bromide, or iodide ion in various solvents, and the solvatochromic parameter which is a measure of the hydrogen bond donor ability of the solvent. The relationships are: for chloride,v
max=6.99+50.16 kK; for bromide,v
max=5.52+45.20 kK; for iodide,v
max=5.02+40.33 kK. The correlation ofv
max for iodide ion with gave a larger correlation coefficient and smaller standard deviation than a correlation with the E
T
(30) parameter of Dimroth. From the values ofv
max for iodide ion and the solvated electron in liquid ammonia at 25°C, these correlations indicate a preliminary value of 0.00 for the parameter of liquid ammonia. This had not been reported before and is lower than expected from a general knowledge of the physical and chemical properties of liquid ammonia but is consistent with a more detailed consideration and recent gas phase measurements of hydrogen bond donor strength. 相似文献
107.
Rong Zhang Qiao Bai Ling Xu Yan Hui Wang Department of Life Science Technology Beijing University of Chemical Technology Beijing China Institute of Materia Medica Chinese Academy of Medical Sciences & Peking Union Medical College Beijing China 《中国化学快报》2007,18(6):656-658
2-Substituted-2,3-dihydro-4(1H)-quinazolinones were obtained in high yields by condensation of anthranilamide with aryl, alkyl or heteroaryl aldehydes or ketones in the refluxing 2,2,2-trifluoroethanol without any catalyst. 相似文献
108.
A comparative study was performed on thermal stability of mesophilic and thermophilic α-amylases, and the effects of various
denaturing agents, organic solvents, and stabilizers were investigated. As expected, the thermophilic enzyme showed higher
resistance toward denaturation in water as its natural medium, but such a difference could not be detected in nonaqueous environments.
Furthermore, stability of these molecules was improved by including various stabilizing agents. Of the compounds tested, sorbitol
provided the highest degree of protection, which was found to be owing to its effect on increasing T
m and its ability in totally preventing deamidation of amino acid residues in the protein molecules. 相似文献
109.
采用密度泛函方法在B3LYP/6-31+G**水平上研究了2',3'-二脱氧-2',3'-二去氢鸟嘌呤核苷分子(D4G)的构象. 分别研究在气相中的孤立分子和一水合物异构体的相对稳定性和异构体之间的相互转变过程, 分析了水分子的参与对D4G异构体的相对稳定性和几何结构参数以及自然电荷的影响. 结果表明, 孤立的D4G分子在气相中存在8种稳定构象, 其中构象d4g-2是所有构象中最稳定的, 气相中D4G主要以d4g-2存在. 气相中各构象的相对稳定性为: d4g-2>d4g-1>d4g-5>d4g-3>d4g-6>d4g-4>d4g-8>d4g-7. 计算得到的各构象键长和键角数据与实验值接近. 一个水分子的加入对D4G分子的构型参数有所影响, 基本不改变D4G分子各构象的稳定性顺序, 但构象转变的能垒有所提高. 氢键在分子构象中发挥了重要作用. 相似文献
110.
混合溶剂火焰原子吸收法直接测定原油中的镍刘广东袁存光张丙华刘文钦(石油大学(华东),山东东营257062)彭力(中国石油天然气总公司环保处,北京100724)关键词原油,混合溶剂,无机盐标准,原子吸收法,镍测定原油中,微量元素镍、钒、铁、铜的存在对... 相似文献