全文获取类型
收费全文 | 20957篇 |
免费 | 3276篇 |
国内免费 | 1946篇 |
专业分类
化学 | 15114篇 |
晶体学 | 244篇 |
力学 | 1173篇 |
综合类 | 160篇 |
数学 | 2588篇 |
物理学 | 6900篇 |
出版年
2024年 | 40篇 |
2023年 | 407篇 |
2022年 | 659篇 |
2021年 | 770篇 |
2020年 | 922篇 |
2019年 | 834篇 |
2018年 | 719篇 |
2017年 | 614篇 |
2016年 | 998篇 |
2015年 | 1009篇 |
2014年 | 1105篇 |
2013年 | 1591篇 |
2012年 | 1754篇 |
2011年 | 1745篇 |
2010年 | 1194篇 |
2009年 | 1157篇 |
2008年 | 1345篇 |
2007年 | 1180篇 |
2006年 | 1041篇 |
2005年 | 975篇 |
2004年 | 748篇 |
2003年 | 656篇 |
2002年 | 662篇 |
2001年 | 553篇 |
2000年 | 461篇 |
1999年 | 482篇 |
1998年 | 292篇 |
1997年 | 264篇 |
1996年 | 287篇 |
1995年 | 255篇 |
1994年 | 211篇 |
1993年 | 184篇 |
1992年 | 153篇 |
1991年 | 126篇 |
1990年 | 127篇 |
1989年 | 85篇 |
1988年 | 82篇 |
1987年 | 63篇 |
1986年 | 59篇 |
1985年 | 67篇 |
1984年 | 45篇 |
1983年 | 28篇 |
1982年 | 27篇 |
1981年 | 34篇 |
1980年 | 20篇 |
1979年 | 15篇 |
1977年 | 13篇 |
1976年 | 19篇 |
1975年 | 16篇 |
1974年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 328 毫秒
281.
为了解佛山市产母、新生儿血铅水平及其相关影响因素 ,采用电感耦合高频等离子体发射光谱法 ,于 2 0 0 2年 5~ 9月 ,取本院产科产母静脉血及其新生儿脐血 1 5 2例 ,进行血铅、钙、锌、铁、铜和锰水平测定 ,按妇幼保健管理卡内容 ,进行相关记录。结果表明 ,产母血铅水平90 1± 3 4 8μg/L ,BPb≥ 1 0 0 μg/L者占 2 0 3 9% ;新生儿脐血铅水平 89 2± 3 1 6μg/L ,铅中毒率为2 2 3 7% ;母子血铅值呈显著正相关 (r=0 85 7,P <0 0 1 ) ,产母血铅值与其血钙、铁值显著负相关 (r=0 2 66,-0 3 0 1 ,P <0 0 5 ) ;城郊产母及新生儿血铅水平显著高于城区。揭示市郊孕妇、新生儿血铅较高 ,血钙、锌、铁含量影响血铅水平 相似文献
282.
The structures, stabilities, thermodynamic quantities, dissociation energies, infrared spectra, and electronic properties of LiOH hydrated by up to seven water molecules are investigated by using the density-functional theory and the M?ller-Plesset second-order perturbation theory (MP2). Further accurate analysis based on the coupled-cluster theory with singles, doubles, and perturbative triples excitations agrees with the MP2 results. The Li-OH stretch mode significantly shifts with the increase of water molecules, and it eventually disappears upon dissociation. It is revealed that seven water molecules are needed for the stable dissociation of LiOH (as a completely dissociated conformation), in contrast to the cases of RbOH and CsOH which require four and three water molecules, respectively. 相似文献
283.
Lam WH Jia G Lin Z Lau CP Eisenstein O 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(12):2775-2782
Theoretical calculations on the metathesis process, [Tp(PH3)MR(eta 2-H[bond]CH3)] --> [Tp(PH3)M(CH3)(eta 2-H[bond]R)] (M=Fe, Ru, and Os; R=H and CH3), have been systematically carried out to study their detailed reaction mechanisms. Other than the one-step mechanism via a four-center transition state and the two-step mechanism through an oxidative addition/reductive elimination pathway, a new one-step mechanism, with a transition state formed under oxidative addition, has been found. Based on the intrinsic reaction coordinate calculations, we found that the trajectories of the transferring hydrogen atom in the metathesis processes studied are similar to each other regardless of the nature of reaction mechanisms. 相似文献
284.
The high-performance liquid chromatographic behavior of some sulfonamides and quinolones was studied on a p-tert-butyl-calix[6]arene-bonded silica gel stationary phase. The effect of mobile phase variables such as methanol content, ionic strength and pH on their chromatographic behavior was investigated. The retention behavior of sulfonamides on the stationary phase was compared with that on both Zorbax C18-bonded silica gel and γ-(ethylenediamino)propyltriethoxylsilane-bonded silica gel (diamino-bonded phase). The retention mechanism of sulfonamides and quinolones on the stationary phase was also discussed. The results indicate that the stationary phase behaves as a reversed-phase packing and its separation selectivity is much better than that of not only Zorbax C18 phase but also diamino-bonded phase. Some sulfonamides and quinolones were separated on the stationary phase, but the separation of sulfonamides is far more successful. 相似文献
285.
Yonghua Wang Ke-Li Han Sheng-Li Yang Ling Yang 《Journal of Molecular Structure》2004,710(1-3):215-221
Cytochrome P450 3A4 (CYP3A4), a major member of cytochrome P450 isoenzymes, metabolizes the majority of steroids in 6β-position. For the purpose of determining requisite structural features of a series of structurally related steroids for CYP3A4-mediated metabolism, three-dimensional pharmacophore modeling as well as electrotopological state map were conducted for 15 steroids. Though prior studies speculated that the chemical reactivity of the allylic 6β-position might have a greater influence on CYP3A4 selective 6-hydroxylation than steric constraints in the enzyme, our results reveal that for CYP3A4 steroidal substrates, it is not the chemical reactivity of atoms at 6β-site, but the pharmacophoric features, i.e. the two hydrophobic rings together with two H-bond donors, that act as the key factors responsible for determining the CYP3A4 selective 6-hydroxylation of steroids. 相似文献
286.
Myint U Khin Mg Myoe Ba Han J. Tölgyessy 《Journal of Radioanalytical and Nuclear Chemistry》1995,201(2):97-103
A simple radiometric flow injection analyzer with NaI(Tl) detector was used for the study of some experimental parameters of137Cs and60Co determination. A sample transfer system was developed employing both merging zone and sample injection principle. 相似文献
287.
Lu Y Xiong H Jiang X Xia Y Prentiss M Whitesides GM 《Journal of the American Chemical Society》2003,125(42):12724-12725
Asymmetric dimers consisting of gold microcrystals and spherical silica colloids have been fabricated by depositing thin films of gold onto the spherical colloids to form half-shells, followed by annealing at elevated temperatures. The capability and feasibility of this procedure have been demonstrated with silica and titania beads of 0.2-2 mum in diameter and gamma-Fe2O3/polystyrene@SiO2 core-shell particles 0.5 mum in size. The dimensions of gold microcrystals could be conveniently varied in the range of 100-650 nm by controlling the thickness of gold films and/or the diameter of the spherical colloids. This method provides another route to asymmetric dimers made of colloidal particles that could be different in size, chemical composition, surface functionality, density or sign of surface charge, bulk property, or a combination of these properties. 相似文献
288.
Qingde Zhang Yisheng Tan Caihong Yang Yizhuo Han Jun Shamoto Noritatsu Tsubaki 《天然气化学杂志》2007,16(3):322-325
The attractive utilization route for one-step catalytic oxidation of dimethyl ether to dimethoxymethane was successfully carried out over the H3PW12O40(40%)/SiO2 catalyst, modified by Cs, K, Ni, and V. The Cs modification of H3PW12O40(40%)/SiO2 gave the most promising result of 20% dimethyl ether conversion and 34.8% dimethoxymethane selectivity. Dimethoxymethane could be synthe- sized via methoxy groups decomposed from dimethyl ether through the synergistic effect between the acid sites and the redox sites of Cs modified H3PW12O40(40%)/SiO2. 相似文献
289.
Marek Da?browski 《Tetrahedron letters》2005,46(24):4175-4178
The metalation of (quasi)alkoxy-substituted dibromobenzenes C6H3(OR)Br2 with lithium diisopropylamide (LDA) has been investigated. For 1-(quasi)alkoxy-3,5-dibromobenzenes (R = Me, TMS), different selectivities were observed depending on reaction conditions and the size of the alkoxy group. The methoxy group was an effective ortho-director whereas this was not the case for the bulky trimethylsilyloxy group. The metalation of related 2,5-dibromoanisole was also examined showing a significant meta-directing effect by the methoxy group. The thermal stability of aryllithium intermediates is significantly lower when lithium is flanked by a bromine and a methoxy group, whereas 4-(quasi)alkoxy-2,6-dibromoaryllithiums are less labile. 相似文献
290.
考虑到手性毛细管电泳分离机理的复杂性,提出了不依赖于统计模型的二进制编码遗传算法多目标优化策略。根据实验参数的多少及其重要性,采用均匀设计安排完成初步实验。并用Derringer功效函数作为多指标同时优化的评价指标,以取得较高的分离度和较快分离速度;通过对实验参数给予编码,转化为二进制字符串,根据得到的总功效函数值大小进行遗传算法操作,产生新的一组实验条件。重复上述过程直到得到最佳分离条件为止。此方法成功地应用于华法令对映体的毛细管电泳分离条件优化。 相似文献