全文获取类型
收费全文 | 102765篇 |
免费 | 9609篇 |
国内免费 | 17998篇 |
专业分类
化学 | 71839篇 |
晶体学 | 2741篇 |
力学 | 1405篇 |
综合类 | 615篇 |
数学 | 4985篇 |
物理学 | 22308篇 |
综合类 | 26479篇 |
出版年
2024年 | 299篇 |
2023年 | 1478篇 |
2022年 | 2996篇 |
2021年 | 3014篇 |
2020年 | 3414篇 |
2019年 | 3018篇 |
2018年 | 2558篇 |
2017年 | 3443篇 |
2016年 | 3644篇 |
2015年 | 3441篇 |
2014年 | 4725篇 |
2013年 | 7832篇 |
2012年 | 6316篇 |
2011年 | 7077篇 |
2010年 | 5907篇 |
2009年 | 7005篇 |
2008年 | 6711篇 |
2007年 | 7354篇 |
2006年 | 6726篇 |
2005年 | 5992篇 |
2004年 | 5443篇 |
2003年 | 4647篇 |
2002年 | 3877篇 |
2001年 | 3200篇 |
2000年 | 3078篇 |
1999年 | 2585篇 |
1998年 | 2033篇 |
1997年 | 1892篇 |
1996年 | 1604篇 |
1995年 | 1515篇 |
1994年 | 1363篇 |
1993年 | 1201篇 |
1992年 | 1054篇 |
1991年 | 815篇 |
1990年 | 583篇 |
1989年 | 523篇 |
1988年 | 433篇 |
1987年 | 282篇 |
1986年 | 209篇 |
1985年 | 174篇 |
1984年 | 131篇 |
1983年 | 56篇 |
1982年 | 100篇 |
1981年 | 130篇 |
1980年 | 104篇 |
1979年 | 94篇 |
1978年 | 70篇 |
1977年 | 73篇 |
1976年 | 43篇 |
1973年 | 38篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
961.
Fused-sphere surfaces can be used to mimic a molecular boundary associated with a constant value of the electron density. The simplest of such fused-sphere models are constructed by using the atomic radii for the spherical isodensity surfaces of individual atoms. In this work, we discuss the extension of this model to molecules containing atoms beyond the second row. In these many- electron systems, the computation of electron densities is usually simplified by adopting a pseudopotential (or effective-core potential) approach. Here, we discuss the performance of large- and small-core pseudo-potential calculations as a tool to derive atomic radii. Our results provide an optimum set of variable radii that can be used to build fused-sphere surfaces. This continuum of surfaces provides a simple approximation to the low-electron-density regions around molecules with heavy atoms. 相似文献
962.
在自制的硅基纤维素-三(3,5-二甲基苯基氨基甲酸酯)高效液相色谱手性固定相上(HPLC-CSP),优化了1-(6'-甲氧基萘)乙醇氢酯基化反应产物-萘普生甲酯手性分离的条件,测定了相应的一系列不对称氢酯基化反应产物的对映体过剩值(e.e.值)。结果表明,在CDMPC-CSP手性柱上用HPLC测定此类不对称催化反应的光学产率,评价催化剂体系的手性选择是一种非常理想的方法。 相似文献
963.
SAPO—11分子筛用i—Pr2NH导向合成及其酸性研究 总被引:3,自引:0,他引:3
以AlPO4、H3PO4、SiO2等为原料采用单一模板剂iPr2NH合成了SAPO11分子筛,利用Avrami方程等对合成过程进行了定量描述。还利用红外(IR)、程序升温脱附(TPD)、线性丁烯骨架异构反应催化活性测试等手段表征了SAPO11的酸性。结果表明,合成过程随温度有明显变化,晶化阶段的活化能远高于核化阶段的值。可用Avrami方程的参数来表征合成过程的难易程度。分子筛中存在B酸和L酸位,主要在中等强度范围,从而对线性丁烯骨架异构反应有较好的催化活性。提高分子筛中硅含量可明显增大酸位浓度,酸强度也有轻度增强。 相似文献
964.
《Surface and interface analysis : SIA》2003,35(12):991-997
In x‐ray photoelectron spectroscopy (XPS) the Auger parameter is often used to study the electronic properties of elements, particularly in insulator materials, because this parameter is assumed to be independent of charging effects. In this paper we report on subtle differences in sample structure and experimental conditions for which the sample potential may not remain constant during the measurements for some spectrometers or experimental arrangements; for such conditions the Auger parameter is not independent of charging. We compare a series of measurements with insulating plate substrates of Al2O3 on which different amounts of SnO2 and Au were deposited. X‐ray photoelectron spectra were collected for different conditions of the sample that was placed either grounded or left floating on a metallic sample holder during measurement. It is found that the Auger parameter is independent of the experimental conditions for Au but substantial differences were found for deposited SnO2. Surprisingly, measurement artifacts due to charging appeared in the Auger parameter for Sn when the sample holder was grounded but not when it was left floating. In the grounded samples differences up to 0.6 eV in the Auger parameter for Sn were found with respect to the actual value of this parameter measured with substrates where charging effects were not significant. Because no differences in peak broadening have been observed under different measurement conditions, it has been assumed that the shift was not caused by a conventional differential charging phenomenon. Considering the different response of the substrate and the deposited layer on stabilizing the charge when the sample is grounded, we have worked out a possible explanation to account for the observed artifacts. Instrumental specifications should be optimized very carefully, especially if (as here) relatively high charging shifts point to a non‐optimum self‐biasing of the surface potential at the insulating samples. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
965.
Study on the Phase Diagram of CsCl-CeCl3-HCl(11%)-H2O System and the Properties of the Compounds 总被引:2,自引:0,他引:2
The equilibrium solubility of CsCl-CeCl3-HCl(11%)-H2O qua-ternary system at 25℃ has been determined by the physicchemical analysis method ,and the phase diagram was plotted, Two new double salts 3CsCl.CeCl3.3H2O and CsCl.CeCl3.4H2O obtained from the complicated system were identified and characterized by XRD,TG-DTA ,DSC,UV and fluorescence spectroscopy, Studies on the fluorescence excitation and emission show that 3CsCl.CeCl3.3H2O and CsCl.CeCl3.4H2O have upconversion luminescence of infrared-visible range,and the upconversion emission intensity increases with the increase of ratio of CeCl3 in CsCl. 相似文献
966.
Ahmed M. Mayouf Yong-Tae Park 《Journal of photochemistry and photobiology. A, Chemistry》2002,150(1-3):115-123
The UV photolysis of 2′-chloro-4-R-benzanilides in acetonitrile solution under nitrogen atmosphere leads to the formation of intramolecular photocyclization products (9-R-phenanthridin-6(5)-one, 20–30%), along with the minor photoreduction, photo-Fries, and intramolecular photosubstitution products. The photoreaction of 2′-chloro-4-R-benzanilide in acetonitrile solution containing 10% of water or aqueous sodium hydroxide however gives 2-(4-R-phenyl)-1,3-benzoxazole as a major product (30%) along with the photoreduction, photo-Fries and intramolecular photocyclization products. 相似文献
967.
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
968.
氧化锌脱硫中氢和氧的双气氛效应及动力学研究 总被引:4,自引:0,他引:4
用热重法研究了氧化锌脱硫中氢氧的气氛效应以及氢氧共同存在下氧化锌脱硫的微观动力学行为。实验温区为200 ℃~320 ℃,氧化锌粒度为100目~120目。研究结果表明,在0%~40%的体积浓度范围内,氢可促进脱硫反应进行;氧则由于会引起氧化锌表面析硫使得脱硫过程随其浓度的变化复杂化。氢氧双气氛下,氧化锌脱硫动力学行为可用改良收缩核模型进行描述,表面反应活化能和固体扩散活化能分别为14.96 kJ/mol和46.77 kJ/mol。 相似文献
969.
用手性的丙烯酸薄荷醇酯与原位生成的1,3-偶极试剂发生Diels-Alder反应, 得到一系列新的手性2,4-二酯基四氢吡咯衍生物,经氢化铝锂还原得到纯的手性 2,4-二羟甲基吡咯衍生物。并首次发现,在用Oxone氧化烯时,2,4-二羟甲基四 氢吡咯衍生物能有效地诱导烯烃发生环氧化反应,转化率达到定量。 相似文献
970.
A superoxochromium(III) ion, CraqOO2+, acts as a catalyst for the co-oxidation of alcohols and nitrous acid with molecular oxygen according to the stoichiometry: CH3OH+HNO2+O2→CH2O + NO3− + H2O+H+. The kinetics are second order in [HNO2] and independent of the concentrations of the superoxochromium catalyst, substrate, and O2. The proposed mechanism features the disproportionation of HNO2 to NO and NO2, both of which react rapidly with CraqOO2+. The CraqOO2+/NO reaction generates another equivalent of NO2 and a mole of CraqO2+, the active oxidant. The two-electron oxidation of the alcohol by CraqO2+ produces Craq2+, which reacts rapidly with O2 to regenerate the catalyst, CraqOO2+. The NO2/CraqOO2+ reaction yields the peroxynitrato complex, CraqOONO22+, in a dead-end equilibrium process that has no effect on the catalytic reaction. The disproportionation of NO2 yields the final nitrogen-containing product, NO3−, and regenerates an equivalent of HNO2. Under a fixed set of conditions, the relative catalytic efficiency (CE) of CraqOO2+ decreases as its concentration increases owing to the competition between O2 and CraqOO2+ for the intermediate Craq2+. 相似文献