全文获取类型
收费全文 | 41170篇 |
免费 | 7705篇 |
国内免费 | 6346篇 |
专业分类
化学 | 31045篇 |
晶体学 | 848篇 |
力学 | 2395篇 |
综合类 | 493篇 |
数学 | 4536篇 |
物理学 | 15904篇 |
出版年
2024年 | 102篇 |
2023年 | 821篇 |
2022年 | 1334篇 |
2021年 | 1474篇 |
2020年 | 1772篇 |
2019年 | 1873篇 |
2018年 | 1520篇 |
2017年 | 1496篇 |
2016年 | 2001篇 |
2015年 | 2171篇 |
2014年 | 2399篇 |
2013年 | 3146篇 |
2012年 | 3904篇 |
2011年 | 3841篇 |
2010年 | 2817篇 |
2009年 | 2864篇 |
2008年 | 3166篇 |
2007年 | 2679篇 |
2006年 | 2584篇 |
2005年 | 2248篇 |
2004年 | 1739篇 |
2003年 | 1372篇 |
2002年 | 1390篇 |
2001年 | 1134篇 |
2000年 | 916篇 |
1999年 | 790篇 |
1998年 | 543篇 |
1997年 | 461篇 |
1996年 | 424篇 |
1995年 | 375篇 |
1994年 | 332篇 |
1993年 | 290篇 |
1992年 | 212篇 |
1991年 | 201篇 |
1990年 | 152篇 |
1989年 | 105篇 |
1988年 | 99篇 |
1987年 | 65篇 |
1986年 | 68篇 |
1985年 | 75篇 |
1984年 | 44篇 |
1983年 | 49篇 |
1982年 | 22篇 |
1981年 | 27篇 |
1980年 | 17篇 |
1979年 | 24篇 |
1978年 | 12篇 |
1977年 | 12篇 |
1975年 | 8篇 |
1957年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 296 毫秒
991.
The retention and selectivity of the chromatographic separation of basic (cationic) analytes on a polybutadiene-coated zirconia (PBD-ZrO2) stationary phase have been studied in greater detail than in previous studies. These separations are strongly influenced by the chemistry of the accessible surface of zirconia. In the presence of buffers which contain hard Lewis bases (e.g., phosphate, fluoride, carboxylic acids) zirconia's surface becomes negatively charged due to adsorption of the buffer anion at the hard Lewis acid sites. Consequently, under most conditions (e.g., neutral pH), cationic analytes undergo both hydrophobic and cation-exchange interactions. This mixed-mode retention process generally leads to greater retention factors for cations relative to those on silica-based reversed phases despite the lower surface areas of the zirconia phase, but, more importantly, adsorption of hard Lewis bases can be used to control the chromatographic selectivity for cationic analytes on these zirconia-based stationary phases. In contrast to our prior work, here we show that when mixed-mode retention takes place, both retention and selectivity are easily adjusted by changing the type of hard Lewis base buffer anion, the type of buffer counter-ion (e.g., sodium, potassium, ammonium), the pH, and the ionic strength of the eluent as well as the type and amount of organic modifier. 相似文献
992.
Jun Yue Bai Liang Wang Hong Jing Wang Peng Fei Huang Yu Qing Zhao Sheng Di Fan 《Mikrochimica acta》2006,156(3-4):321-326
The electrochemical behavior of epinephrine (EP) at a mercaptoacetic acid (MAA) self-assembled monolayer modified gold electrode
was studied. The MAA/Au electrode is demonstrated to promote the electrochemical response of epinephrine by cyclic voltammetry.
The possible reaction mechanism is also discussed. The diffusion coefficient D of EP is 6.85 × 10−6 cm2 s−1. In 0.1 mol L−1 phosphate buffer (pH 7.20), a sensitive oxidation peak was observed at 0.177 V, and the peak current is proportional to the
concentration of EP in the range of 1.0 × 10−5–2.0 × 10−4 mol L−1 and 1.0 × 10−7–1.0 × 10−6 mol L−1. The detection limit is 5 × 10−8 mol L−1. The modified electrode is highly stable and can be applied to the determination of EP in practical injection samples. The
method is simple, quick, sensitive and accurate. 相似文献
993.
Frank Seela Ping Ding Xiaohua Peng Henning Eickmeier Hans Reuter 《Acta Crystallographica. Section C, Structural Chemistry》2007,63(10):o600-o602
The title compound, 2,4‐diamino‐5‐bromo‐7‐(2‐deoxy‐2‐fluoro‐β‐d ‐arabinofuranosyl)‐7H‐pyrrolo[2,3‐d]pyrimidine, C11H13BrFN5O3, shows two conformations of the exocyclic C4′—C5′ bond, with the torsion angle γ (O5′—C5′—C4′—C3′) being 170.1 (3)° for conformer 1 (occupancy 0.69) and 60.7 (7)° for conformer 2 (occupancy 0.31). The N‐glycosylic bond exhibits an anti conformation, with χ = −114.8 (4)°. The sugar pucker is N‐type (C3′‐endo; 3T4), with P = 23.3 (4)° and τm = 36.5 (2)°. The compound forms a three‐dimensional network that is stabilized by several intermolecular hydrogen bonds (N—H...O, O—H...N and N—H...Br). 相似文献
994.
Ab initio molecular orbital theory and density functional theory have been used to study nine isomers of N7 ionic clusters with low spin at the HF/6-31G*, MP2/6-31G*, B3LYP/6-31G*, and B3LYP/6-311(+)G* levels of theory. All stationary
points are examined with harmonic vibrational frequency analyses. Four N7
+ isomers and five N7
− isomers are determined to be local minima or very close to the minima on their potential-energy hypersurfaces, respectively.
For N7
+ and N7
−, the energetically low lying isomers are open-chain structures (C
2
v
and C
2
v
or C2). The results are very similar to those of other known odd-number nitrogen ions, such as N5
+, N9
+, and N9
−, for which the open-chain structures are also the global minima. This research suggests that the N7 ionic clusters are likely to be stable and to be potential high-energy-density materials if they could be synthesized.
Received: 16 July 2001 / Accepted: 8 October 2001 / Published online: 21 January 2002 相似文献
995.
The one-electron oxidized linear pentanuclear nickel complexes [Ni(5)(tpda)(4)(H(2)O)(BF(4))](BF(4))(2) (1) and [Ni(5)(tpda)(4)(SO(3)CF(3))(2)](SO(3)CF(3)) (2) have been synthesized by reacting the neutral compound [Ni(5)(tpda)(4)Cl(2)] with the corresponding silver salts. These compounds have been characterized by various spectroscopic techniques. Compound 1 crystallizes in the monoclinic space group P2(1)/n with a = 15.3022(1) A, b = 31.0705(3) A, c = 15.8109(2) A, beta = 92.2425(4) degrees, V = 7511.49(13) A(3), Z = 4, and compound 2 crystallizes in the monoclinic space group C2/c with a = 42.1894(7) A, b = 17.0770(3) A, c = 21.2117(4) A, beta = 102.5688(8) degrees, V = 14916.1(5) A(3), Z = 8. X-ray structural studies reveal an unsymmetrical Ni(5) unit for both compounds 1 and 2. Compounds 1 and 2 show stronger Ni-Ni interactions as compared to those of the neutral compounds. 相似文献
996.
Aromatic molecular "clips" bearing two symmetrically bound platinum moieties have been prepared. The molecular "clip" 4 readily self-assembled with linear linkers such as 4,4'-bipyridyl, 1,4-bis[2-(4-isocyano-3,5-diisopropylphenyl)ethynyl]benzene, and nicotinic acid to form molecular rectangles. The overall dimensions of the rectangle 7 were 7.3 Angstroms x 15.3 Angstroms. The molecular "clip" also self-assembled with tritopic pyridyl and isocyanide ligands to form trigonal prismatic frameworks. The characterization of the supramolecules by multinuclear NMR, electrospray mass spectrometry, and X-ray crystal structures is also reported. 相似文献
997.
We synthesized dialkoxy-substituted poly[phenylene vinylene]s (dROPPV-1/1, 0.2/1, and 0/1) consisting of two repeating units with different side-chain lengths (methoxy and 3,7-dimethyloctyloxy). These polymers can serve as a model system to clarify roles of aggregates (the sites with ground-state interchain interactions) and the independent chain segments in the well-packed chains (the chain segments that are compactly packed without interaction) in the emission mechanism of conjugated polymers. Due to the packing of polymer chains, films of all of these polymers are accessible to interchain excitations, after which excitons can re-form to result in delayed luminescence. Besides, some chains form aggregates so that the delayed luminescence is no more the ordinary single-chain emission but red-shifted and less structured. Not only the re-formation of these indirect excitons but also the aggregation of chains are facilitated in the polymers with short methoxy side groups, revealing that both packing and aggregation of chain segments require a short spacing between polymer chains. However, the incorporation of other side chains such as the 3,7-dimethyloctyloxy group to dROPPVs is necessary for the formation of aggregates because these long branched side chains can reduce the intrachain order imposed by the short methoxy groups, which accounts for the absence of aggregate emission in the well-studied poly[2,5-dimethoxy-1,4-phenylene vinylene]. This study reveals that the well-packed chains do not necessarily form aggregates. We also show that the photophysical properties and the film morphology of conjugated polymers can be deliberately controlled by fine-tuning of the copolymer compositions, without altering the optical properties of single polymer chains (e.g., as in dilute solutions). 相似文献
998.
999.
Jun?ZhangEmail author Pei-Hong?Deng Yong-Lan?Feng Yun-Fei?Kuang Jin-Jun?Yang 《Mikrochimica acta》2004,147(4):279-282
A carbon black microelectrode modified by -MnO2 has been prepared. The electrocatalytical oxidation of ascorbic acid (AA) at this microelectrode was investigated. The 2nd-order linear scan voltammograms of AA are recorded from –0.5 to 0.5V (vs. SCE). The relationship between the oxidation peak current of AA and its concentration in the range of 1.0×10–64.0×10–3molL–1 is linear. The detection limit (3) was found to be 6.0×10–7molL–1. Also, the determination of AA in samples is evaluated, and the results are satisfactory. 相似文献
1000.
以P123 嵌段共聚物为模板剂, 3-三甲基丙基氯化铵三甲氧基硅烷(TMAPS)为修饰剂, 酸性条件下一步法直接合成了有机季铵基团功能修饰的SBA-15, 并通过XRD、TEM、N2吸附-脱附、Raman 光谱等对功能化样品的结构和性质进行了表征, 对一步法合成TMAPS 修饰的SBA-15 的可能反应机理进行了探讨. 修饰后的SBA-15 仍然保持了二维六方特征介孔结构, 随着TMAPS负载量的增大, SBA-15 孔道有序度下降, 孔径、孔容和比表面积也随之下降. 有机季铵基团在SBA-15 孔道表面均匀分散, 可与HAuCl4通过快速离子交换制备Au 颗粒高度分散的Au-SBA-15. 相似文献