全文获取类型
收费全文 | 42070篇 |
免费 | 7378篇 |
国内免费 | 6642篇 |
专业分类
化学 | 32206篇 |
晶体学 | 596篇 |
力学 | 2492篇 |
综合类 | 447篇 |
数学 | 4604篇 |
物理学 | 15745篇 |
出版年
2024年 | 86篇 |
2023年 | 757篇 |
2022年 | 1505篇 |
2021年 | 1484篇 |
2020年 | 1687篇 |
2019年 | 1653篇 |
2018年 | 1445篇 |
2017年 | 1424篇 |
2016年 | 2046篇 |
2015年 | 2157篇 |
2014年 | 2579篇 |
2013年 | 3185篇 |
2012年 | 3970篇 |
2011年 | 4067篇 |
2010年 | 2992篇 |
2009年 | 2760篇 |
2008年 | 3028篇 |
2007年 | 2643篇 |
2006年 | 2518篇 |
2005年 | 2033篇 |
2004年 | 1608篇 |
2003年 | 1344篇 |
2002年 | 1501篇 |
2001年 | 1275篇 |
2000年 | 928篇 |
1999年 | 925篇 |
1998年 | 646篇 |
1997年 | 552篇 |
1996年 | 545篇 |
1995年 | 473篇 |
1994年 | 418篇 |
1993年 | 341篇 |
1992年 | 288篇 |
1991年 | 249篇 |
1990年 | 190篇 |
1989年 | 174篇 |
1988年 | 144篇 |
1987年 | 105篇 |
1986年 | 92篇 |
1985年 | 64篇 |
1984年 | 50篇 |
1983年 | 41篇 |
1982年 | 21篇 |
1981年 | 32篇 |
1980年 | 15篇 |
1979年 | 9篇 |
1978年 | 4篇 |
1976年 | 4篇 |
1974年 | 6篇 |
1957年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
响应性凝胶及其在药物控释上的应用 总被引:19,自引:1,他引:19
综述近年来在响应性凝胶材料研究方面的进展,介绍了能感应pH、温度、光、电场以及生化物质等外界因素变化的响应性凝胶的结构特点与响应机理,同时介绍了此类凝胶应用于药物控制释放方面的研究近况。 相似文献
992.
993.
Hong Zheng Wei Yu Hon-bin Zhang Chi-xing Zhou 《高分子科学》2006,(1):1-11
Finite element method is used to simulate the high-speed melt spinning process,based on the equation system proposed by Doufas et al.Calculation predicts a neck-like deformation,as well as the related profiles of velocity,diameter, temperature,chain orientation,and crystallinity in the fiber spinning process.Considering combined effects on the process such as flow-induced crystallization,viscoelasticity,filament cooling,air drag,inertia,surface tension and gravity,the simulated material flow behaviors are consistent with those observed for semi-crystalline polymers under various spinning conditions.The structure change of polymer coils in the necking region described by the evolution of conformation tensor is also investigated.Based on the relaxation mechanism of macromolecules in flow field different types of morphology change of polymer chains before and in the neck are proposed,giving a complete prospect of structure evolution and crystallization of semi-crystalline polymer in the high speed fiber spinning process. 相似文献
994.
Melt extrusion was used to prepare binary nanocomposites of ethylene copolymers and organoclay and trinary nanocomposites of low-density polyethylene (LDPE), ethylene copolymer and organoclay. X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used to analyze the structure of the clay phase and the morphology of the nanocomposites. Influences of the comonomer in the copolymer and the content of the copolymer on the morphology of the resulting nanocomposites were discussed. The binary and the trinary composites may form intercalated or exfoliated structures depending on the interaction between the copolymer and the clay layers and the content of the copolymer. 相似文献
995.
Instead of environmentally toxic chromium oxidant, singlet oxygen generated photcchemically was used as environmentally friendly and benign oxidizing agent to accomplish the transformation of pseudodiosgenin diacetate to diosone efficiently in a low toxic and less expensive solvent acetone. Accordingly 16-dehydropregnenolone acetate(16-DPA), an important intermediate for preparation of steroidal drugs, was prepared in good yield (75%) when photoreaction was run in acetone/acetic anhydride/pyridine system. The mild reaction condition as well as simple and environmentally friendly process made the method commercially viable and important for production of 16-DPA in industrial scale. 相似文献
996.
An online UV photolysis and UV/TiO2 photocatalysis reduction device (UV–UV/TiO2 PCRD) and an electrochemical vapor generation (ECVG) cell have been used for the first time as an interface between high-performance liquid chromatography (HPLC) and atomic fluorescence spectrometry (AFS) for selenium speciation. The newly designed ECVG cell of approximately 115 L dead volume consists of a carbon fiber cathode and a platinum loop anode; the atomic hydrogen generated on the cathode was used to reduce selenium to vapor species for AFS determination. The noise was greatly reduced compared with that obtained by use of the UV–UV/TiO2 PCRD–KBH4–acid interface. The detection limits obtained for seleno-DL-cystine (SeCys), selenite (SeIV), seleno-DL-methionine (SeMet), and selenate (SeVI) were 2.1, 2.9, 4.3, and 3.5 ng mL–1, respectively. The proposed method was successfully applied to the speciation of selenium in water-soluble extracts of garlic shoots cultured with different selenium species. The results obtained suggested that UV–UV/TiO2 PCRD–ECVG should be an effective interface between HPLC and AFS for the speciation of elements amenable to vapor generation, and is superior to methods involving KBH4. 相似文献
997.
The vibrationally resolved mass analyzed threshold ionization (MATI) spectra of jet-cooled 7-azaindole have been recorded by ionizing via four different intermediate levels. The adiabatic ionization energy of this molecule is determined to be 65 462±5 cm−1, which is greater than that of indole by 2871 cm−1. The vibrational spectra of 7-azaindole in the S1 and D0 states have been successfully assigned by comparing the measured frequencies with those of indole as well as the predicted values from the ab initio calculations. Detailed analysis on the MATI spectra shows that the structure of the cation is somewhat different from that of this species in the neutral S1 state. 相似文献
998.
The reaction of Cu(I)(CH(3)CN)(4)BF(4) with 4-pyridylacrylic acid (4-HPYA) affords an unprecedented 8-fold interpenetrating diamondoid-like coordination polymer network [Cu(I)(3)(4-PYA)(2)(H(2)O)(BF(4)) ] (1) with a Cu(+)(4)(CO(2))(4) cluster as connecting node. The interpenetration in metal coordination polymers is the highest degree ever found within diamondoid nets containing a cluster as connecting note. Its fluorescent property was also reported. 相似文献
999.
1000.
Wei Sheng Feng Hong Wei Li Xiao Ke Zheng Sui Qing Chen 《中国化学快报》2007,18(12):1518-1520
Two new megastigmane O-glucopyranosides,named (2R,3R,5R,6S,9R)-3-hydroxy-5,6-epoxy-β-ionol-2-O-β-D-glucopyrano- side(1) and (2R,3R,5R,6S,9R)-3-hydroxyl-5,6-epoxy-acety-β-ionol-2-O-β-D-glucopyranoside(2) together with six known mega- stigmanes,were isolated from the leaves of Broussonetia papyrifera (Linn.) Vent.Their structures were established by chemical methods and spectroscopic techniques including 2D NMR. 相似文献