全文获取类型
收费全文 | 86866篇 |
免费 | 1953篇 |
国内免费 | 1475篇 |
专业分类
化学 | 30735篇 |
晶体学 | 871篇 |
力学 | 7318篇 |
综合类 | 60篇 |
数学 | 33006篇 |
物理学 | 18304篇 |
出版年
2023年 | 223篇 |
2022年 | 321篇 |
2021年 | 368篇 |
2020年 | 499篇 |
2019年 | 433篇 |
2018年 | 10727篇 |
2017年 | 10508篇 |
2016年 | 6465篇 |
2015年 | 1300篇 |
2014年 | 867篇 |
2013年 | 998篇 |
2012年 | 4616篇 |
2011年 | 11354篇 |
2010年 | 6202篇 |
2009年 | 6556篇 |
2008年 | 7098篇 |
2007年 | 9215篇 |
2006年 | 750篇 |
2005年 | 1690篇 |
2004年 | 1824篇 |
2003年 | 2203篇 |
2002年 | 1291篇 |
2001年 | 418篇 |
2000年 | 470篇 |
1999年 | 370篇 |
1998年 | 338篇 |
1997年 | 280篇 |
1996年 | 344篇 |
1995年 | 233篇 |
1994年 | 178篇 |
1993年 | 167篇 |
1992年 | 124篇 |
1991年 | 123篇 |
1990年 | 99篇 |
1989年 | 103篇 |
1988年 | 96篇 |
1987年 | 75篇 |
1986年 | 85篇 |
1985年 | 69篇 |
1984年 | 54篇 |
1983年 | 39篇 |
1982年 | 42篇 |
1981年 | 44篇 |
1980年 | 47篇 |
1979年 | 44篇 |
1914年 | 45篇 |
1913年 | 40篇 |
1912年 | 40篇 |
1909年 | 41篇 |
1908年 | 40篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
Yinghong Sheng Jerzy Leszczynski Thuc-Quyen Nguyen Anu Bamgbelu 《Structural chemistry》2007,18(6):827-832
A theoretical study using density functional theory was performed to understand the structure/property relationship of the
cationic conjugated polyelectrolytes, poly[9,9-bis-(6′-N,N,N-trimethylammonium) hexyl] fluorene-alt-4,7-(2,1,3-benzothiadiazole)] (PFBT-X, where X = Br). The torsion angle between the fluorene and benzothiadiazole units in the PFBT monomer was found to substantially affect
the structural and electronic properties of the cationic PFBT monomer. The changes of geometrical parameter, HOMO and LUMO energy levels, and band gap, as well as the absorption maximum
are discussed in terms of the torsion in the PFBT monomer structure. For comparison, its neutral analogue, the monomer of
poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT) was also studied. The length of conjugation backbone was also examined. 相似文献
952.
Electrodeposition of nanocrystalline zinc from acidic sulfate solutions containing thiourea and benzalacetone as additives 总被引:1,自引:0,他引:1
Mou Cheng Li Li Li Jiang Wen Qi Zhang Yu Hai Qian Su Zhen Luo Jia Nian Shen 《Journal of Solid State Electrochemistry》2007,11(4):549-553
Nanocrystalline zinc coatings were produced by pulse electrodeposition in acid sulfate bath containing thiourea and benzalacetone
additives and characterized by X-ray diffraction and scanning electron microscopy techniques. The influence of benzalacetone
concentration and pulse peak current density on the grain size and crystallographic orientation of zinc deposits was investigated.
Zinc electrodeposited from additive-free solutions or with one of the two additives is not composed of nanosized crystals.
The mixture additives of thiourea and benzalacetone give rise to the formation of particle-like nanocrystalline zinc with
a (10ī1) random orientation. A change in peak current density from 2 to 1 A/cm2 only increases the grain size from 60 to 62 nm. 相似文献
953.
Su-Hong Ge Xin-Lu Cheng Xin-Xing Wang Guang-xing Dong Gui-hua Sun 《Structural chemistry》2007,18(6):985-991
Some explosives are stable molecules with large energy barriers to chemical reaction, and in shock or impact initiation, a
sizable amount of phonon energy must be converted to the molecular internal higher vibrations by multiphonon up pumping. To
investigate the relationship between impact sensitivities and energy transfer rates, the number of doorway modes of explosive
molecules is estimated by a simple theory in which the rate is proportional to the number of normal mode vibrations. We evaluated
frequencies of normal mode vibrations of 13 explosive molecules which are CHNO nitramine-contained and have not been analyzed
previously. The number of doorway modes in the regions of 200–700 cm−1 was evaluated by the direct counting method. For more clear investigation of the relationship we have classified these 13
nitramine explosive molecules, by the number of nitramine group they contained, into two groups. There are eight molecules
that contained one nitramine group and five molecules that contained poly-nitramine groups. It is found that the number of
doorway modes shows a linearly correlation to the impact sensitivities derived from drop hammer tests. This result is in agreement
with that of several previous works. Besides, it is also noted in our study that in those nitramine explosives molecules with
similar molecular structure (similar number nitramine group they contained) and similar molecular weight, the correlation
between the sensitivity and the number of doorway modes is higher. We found that the vibrational frequency of ω corresponds
to nitro group motions of every molecule is contributed to the number of doorway modes in the regions of 200–700 cm−1. 相似文献
954.
100公里以上的大气分子离子主要为NO~+。它的辐射特点及它与电子、原子或分子的相互作用,对于理解大气的化学过程具有特别重要的意义。为了研究这些过程,确定NO~+分子离子基态及其各个激发态的分子势能函数是非常重要的。精确的X~1Σ~+,A~1Ⅱ和a~3∑~+势能曲线已发表;基于光电子谱的研究发现了NO~+的其它激发态,但对于这些激发态的研究尤其是势能函数的研究不多。本文研究并导出NO~+的基态和10个激发态的势能函数。 相似文献
955.
采用差示扫描量热法(DSC)、X射线衍射(XRD)和透射电镜(TEM)研究了Y对Nd-Fe-Al-Ni非晶合金热稳定性和晶化行为的影响。结果表明,淬火态的Nd60Fe20Al10Ni10-xYx(x=0,2)合金基本为非晶组织同时还含有少量的淬态相,Y的加入抑制了淬态相的析出。加入Y后,非晶合金的晶化开始温度和晶化峰值温度都向高温方向移动,证明其热稳定性提高。Y的加入改变了合金的晶化方式和最终晶化产物,使非晶基体中析出的晶化相更加弥散圆整细小。并且Y具有在化学上钝化氧杂质的作用,从而抑制了氧的有害作用。利用Kissinger方程获得了Nd60Fe20Al10Ni8Y2非晶合金的晶化开始和晶化峰值激活能分别为1.21和1.16 eV。 相似文献
956.
基于芘的荧光熄灭的单质碘荧光敏感膜的研究 总被引:8,自引:0,他引:8
单质碘熄灭固定于增塑的PVC膜中的芘的荧光,且这种熄灭作用可逆。本据此研制了用于测定单质碘浓度的荧光敏感膜,最佳膜组成为2~4mg芘、50mgPVC粉、100mg邻苯二甲酸二异辛酯,测定碘的浓度范围为2.26×10^-5~1.04×10^-3mol/L。此膜测定单质碘的重现性好,响应时间小于40s。除Fe^3+、Bi^3+外,其它常见离子均无干扰。将此膜用于食盐中碘的测定,结果令人满意。 相似文献
957.
The inducing method for preparing Ag-micelle solution with the use of mixed solvent/nonsolvent, and the morphological characterization of the generated metal–micelles were investigated and reported in this paper. In this method, an Ag containing metal chelate polymer (MCP) raw solution was preprepared by dissolving poly(vinyl acetate) (PVAc)–silver nitrate (AgNO3) MCP in conc. formic acid, and a mixed solvent of HCOOH/H2O with specific water composition was then added to induce the micellization of the MCP chain. The critical water concentration (CWC) that was needed for inducing the formation of the Ag-micelles, and the water concentration at which the flocculation of the Ag-micelles occurred in micellar solution, were studied by measuring the transmittance of the dilute MCP solution; the results showed that a long-lasting MCP solution with stable micelles might be prepared by using a H2O/HCOOH solvent of specific weight ratio 1:1.2. The effect of the AgNO3 concentration on the morphology of the Ag-micelles was also investigated by transmission electron microscopy (TEM). At AgNO3 concentration below 0.5 wt%, the Ag-micelles displayed a variety of core-shell structure; but as the AgNO3 concentration was increased to 1.0–2.0 wt%, micelles that had Ag-solid embedded in the micellar core were observed. 相似文献
958.
Binary excess molar volumes, V
m
E, have been evaluated from density measurements, using a vibrating tube densimeter over the entire composition range for binary
liquid mixtures of ionic liquids 1-ethyl-3-methyl-imidazolium diethyleneglycol monomethylethersulphate [EMIM]+[CH3(OCH2CH2)2OSO3]− or 1-butyl-3-methyl-imidazolium diethyleneglycol monomethylethersulphate [BMIM]+[CH3(OCH2CH2)2OSO3]− or 1-methyl-3-octyl-imidazolium diethyleneglycol monomethylethersulphate [MOIM]+[CH3(OCH2CH2)2OSO3]−+methanol and [EMIM]+[CH3(OCH2CH2)2OSO3]−+water at 298.15, 303.15 and 313.15 K. The V
m
E values were found to be negative for all systems studied. The V
m
E results are explained in terms of intermolecular interactions and packing effects. The experimental data were fitted by the
Redlich-Kister polynomial. 相似文献
959.
Coordination reactions of copper(II) ions and their effect on non-covalent interactions in uridine (Urd) or uridine 5′-monophosphate (UMP) systems with nucleosides (Ado, Cyd, Thd) and nucleotides (AMP and CMP) in aqueous solutions have been studied. At high pH the effective coordination centers are deprotonated N(3) atoms from Urd and Thd, whereas at low pH, the N(3) atoms of pyrimidine nucleosides are blocked for coordination and the metallation sites are endocyclic
nitrogen atoms from Ado, Cyd, AMP and CMP. Moreover, at low pH, the main reaction center in nucleotide solutions is the phosphate group. The NMR study has proven the
occurrence of non-covalent ion-dipole interactions and stacking interactions in the systems considered. Introduction of a
copper ion in the majority of systems causes the disappearance of weak interactions between ligands. The structures of the
complexes in solution have been inferred from the equilibrium study: an analysis of the pH range of their occurrence with
respect to the pH range of deprotonation of particular groups in the compounds studied, using Vis, EPR and 13C as well as 31P NMR spectral analysis. 相似文献
960.
We present a method for computing classical Newtonian trajectories that minimize the path length or transit time from reactant
to product. Our approach is based on a generalization of the fast-marching method, which allows us to construct the solution
of the Hamilton-Jacobi equation for the action that optimizes the desired quantity. The resulting “reactive paths” can be
interpreted as reaction coordinates but, unlike more conventional choices, they contain dynamical information about the chemical
system of interest. 相似文献