全文获取类型
收费全文 | 4904篇 |
免费 | 397篇 |
国内免费 | 177篇 |
专业分类
化学 | 3541篇 |
晶体学 | 68篇 |
力学 | 156篇 |
综合类 | 14篇 |
数学 | 352篇 |
物理学 | 1347篇 |
出版年
2024年 | 5篇 |
2023年 | 56篇 |
2022年 | 79篇 |
2021年 | 131篇 |
2020年 | 126篇 |
2019年 | 142篇 |
2018年 | 140篇 |
2017年 | 111篇 |
2016年 | 211篇 |
2015年 | 206篇 |
2014年 | 250篇 |
2013年 | 376篇 |
2012年 | 387篇 |
2011年 | 441篇 |
2010年 | 301篇 |
2009年 | 289篇 |
2008年 | 343篇 |
2007年 | 298篇 |
2006年 | 278篇 |
2005年 | 239篇 |
2004年 | 171篇 |
2003年 | 169篇 |
2002年 | 153篇 |
2001年 | 82篇 |
2000年 | 87篇 |
1999年 | 56篇 |
1998年 | 48篇 |
1997年 | 37篇 |
1996年 | 37篇 |
1995年 | 24篇 |
1994年 | 22篇 |
1993年 | 22篇 |
1992年 | 16篇 |
1991年 | 21篇 |
1990年 | 20篇 |
1989年 | 15篇 |
1988年 | 10篇 |
1987年 | 12篇 |
1986年 | 7篇 |
1985年 | 6篇 |
1984年 | 4篇 |
1983年 | 5篇 |
1982年 | 4篇 |
1981年 | 7篇 |
1980年 | 3篇 |
1977年 | 6篇 |
1976年 | 3篇 |
1975年 | 3篇 |
1972年 | 4篇 |
1971年 | 3篇 |
排序方式: 共有5478条查询结果,搜索用时 265 毫秒
51.
52.
Jeong JO Byun J Jeon ES Gwon HC Lim YS Park J Yeo SJ Lee YJ Kim S Kim DK 《Experimental & molecular medicine》2002,34(4):278-284
Direct injection of the vascular endothelial growth factor (VEGF) gene plasmid DNA into the myocardium was shown to induce development of new blood vessels to increase the circulation in the heart of patients with coronary artery diseases. However, such angiogenic gene therapy (via naked DNA) was limited by low level of gene expression. Furthermore, the temporal and spatial characteristics of VEGF gene transfer in the heart are not known. In this study, we demonstrated that a plasmid vector, containing the human cytomegalovirus immediate early (HCMV IE) promoter and enhancer, induces greater expression of gene in the rat heart monitored by gene fused to the chloramphenicol acetyl transferase (CAT) reporter, than four different viral and cellular promoters. Interestingly, expression of VEGF121 protein showed an earlier peak, a shorter duration, and a wider distribution than that of CAT only. Therefore, a plasmid vector with an HCMV IE promoter/enhancer provides clear advantages over other previously developed plasmids. Furthermore, expression profile of VEGF121 gene may provide useful information in the design of angiogenic gene therapy in the heart. 相似文献
53.
A microfluidic apparatus capable of creating continuous microscale cylindrical polymeric structures has been developed. This system is able to produce microstructures (e.g. fibers, tubes) by employing 3D multiple stream laminar flow and "on the fly"in-situ photopolymerization. The details of the fabrication process and the characterization of the produced microfibers are described. The apparatus is constructed by merging pulled glass pipettes with PDMS molding technology and used to manufacture the fibers and tubes. By controlling the sample and sheath volume flow rates, the dimensions of the microstructures produced can be altered without re-tooling. The fiber properties including elasticity, stimuli responsiveness, and biosensing are characterized. Responsive woven fabric and biosensing fibers are demonstrated. The fabrication process is simple, cost effective and flexible in materials, geometries, and scales. 相似文献
54.
Jeong Hyun Kim Seung-Hyun Kang Il Nam Jung Kwan Mook Kim Bok Ryul Yoo Myong Euy Lee 《Journal of Sol-Gel Science and Technology》2004,32(1-3):31-35
Reaction of (triphenylmethyl)silanetriol (1) with cyclopentadienyltitanium trichloride (CpTiCl3) in the presence of triethylamine as a HCl scavenger gave both compounds of a partial open-cage type {[Ph3CSiO(OH)]3(Ph3CSiO3/2)(CpTiO3/2)4} (2) and cube type (Ph3CSiO3/2CpTiO3/2)4 (3). The 1:1 reaction of 1 and CpTiCl3 in toluene solvent at reflux temperature for 3 d afforded compounds 2 (22%) and 3 (36%). When 1 is reacted with a 1.5 fold excess of CpTiCl3 under the same conditions, compound 3 was obtained in high yield (81%) along with 2 in trace quantities. Compounds 2 and 3 were fully characterized by the analyses of 1H, 13C, 29Si NMR, IR, and FAB MS data. The solid-state structure of 3 was determined by a single crystal X-ray diffraction study. Compound 3 had shown to have catalytic activity for the oxidation of alkenes such as 1-octene, cyclooctene, and norbornene with t-butyl hydrogen peroxide. The effect of solvent was observed in this epoxidation reaction. The order of reactivity were decreased as follows: CHCl3 > hexane THF. 相似文献
55.
Ming Guo Jue Wang Man Xiong Ronghui Wu Xuejuan Yang Jianzhong Zhou Minghui Li Jiajun Li 《大学化学》1986,35(12):237-246
In order to improve university laboratory teaching and practical education under the background of information technology, as well as overcome the difficulty for practical training, develop a virtual simulation experiment platform for rapid pyrolysis of natal materials and high-performance liquid chromatographic (HPLC) detection of its components. The experiment integrates the actual operation process of the plant to produce cleaved products by rapid pyrolysis of biomass with HPLC detection. The virtual experiment is divided into biomass sample pretreatment, rapid biomass pyrolysis, bio-oil extraction, and HPLC determination of complex components, making the biomass cleavage workshop "real" through simulation. This virtual simulation experiment fully integrates the utilization of biomass resources and basic chemistry courses (such as organic chemistry, instrumental analysis, chemical engineering principles, etc.), which are widely involved in agricultural and forestry majors with advantageous characteristics value. 相似文献
56.
Solid-solid transformation mechanism of Ga Keggin-type ion intercalated clay into sodalite has been clarified by X-ray diffraction (XRD), FT-IR spectroscopy, and X-rayabsorption spectroscopy (XAS). To follow the structural evolution precisely by XANES and EXAFS, the XAS active element containing polycation [Ga13O4(OH)24(H2O)12]7+ was intercalated into montmorillonite (MMT). FT-IR and XAS spectra confirm that the formation of sodalite framework is initiated by the delocalized rearrangement between silicate networks and collapsed interlayered Ga species, and is followed by the incorporation of Al in the octahedral sheet of clay (MMT). According to the XAS studies, it is found that the formation of Ga(Al)-O-Si species is strongly influenced by the trivalent cations, which are rapidly changed in the environment, followed by reaction with the silicate network of clay during the solid-solid transformation. 相似文献
57.
Li Yan Liang Shao Ping Ren Yi Quan Zheng Man Geng Lu Guangzhou Institute of Chemistry Chinese Academy of Sciences Guangzhou China Key Laboratory of Polymer Materials for Electronics Guangdong China Graduate School of the Chinese Academy of Sciences Beijing China 《中国化学快报》2007,18(2):225-228
A series of novel liquid crystalline epoxides with lateral substituents were cured with anhydrides and the cure kinetics was investigated by non-isothermal DSC technique. The results showed that the lengths of lateral substituents have great effect on the value of Ea. The curing reaction became less active, when the liquid crystalline epoxides have long lateral substituents and were controlled by diffusion at the late stage of cure. A nematic structure was observed by POM and XRD. 相似文献
58.
Jeong Hwan Koh 《Tetrahedron》2004,60(34):7405-7410
A catalytic oxidative polycyclization reaction initiated by the carbocyclization of 1,5-dienes with Pd(II) is reported. Trapping of a putative carbocation with suitable functional groups (phenols, alkenes, alcohols, sulfonamide), or rearrangement protocols (Pinacol) yields poly-cyclic products in good yields and in excellent diastereoselectivities. Turnover of the intermediate Pd-C bond is via β-H elimination. 相似文献
59.
Using column-switching liquid chromatography/tandem mass spectrometry (LC-MS/MS), we developed an improved analytical method of urinary estriol glucuronides. This new method is derived predominantly from maternal and fetal precursors in pregnancy. We used in the following procedure: first, we filtered urine samples with a membrane filter. Next, we directly injected the 50 microL aliquot of urine samples onto a pre-column. Then, after activating the column-switching valve, we backflushed the loaded samples onto the C(18) analytical column. Urine samples can be assayed within 20 min without any sample preparation steps. We monitored separated estriol glucuronides by negative electrospray ionization (ESI) and selected-reaction monitoring (SRM). The calibration range of estriol-3-glucuronide (E3-3G) and estriol-16-glucuronide (E3-16G) was 0.1-20 microg/mL and the linearity of the method was 0.9984 for E3-3G and 0.9987 for E3-16G. The limits of detection at a signal-to-noise (S/N) ratio of 3 were 10 ng/mL (E3-3G) and 5 ng/mL (E3-16G). The analytical recovery was over 85% and, in general, inter-day and intra-day variability for precision and accuracy were less than 10%. When applied to a pregnancy urine sample to biomedical monitoring of the function of the maternal/fetal unit, the proposed method allowed rapid and sensitive screening for the detection of E3-3G and E3-16G. 相似文献
60.
Heterobimetallic alkoxides are broadly recognized as versatile precursors for luminescence materials, and efforts are being made to develop novel routes by applying the concept of geometrical molecular design, for their synthesis and to design a single source precursor suited to photoluminescent materials. Novel and new series of bimetallic alkoxides has been prepared by metathesis route. They exhibit a lower sensitivity towards hydrolysis and so they are easier to handle as compared to other alkoxides. All the compounds were characterized by elemental analysis, FT-IR and multinuclear NMR spectroscopies. FT-IR revealed that the molecular structure of these metal alkoxides was retained to a large extent in 4 : 1 halogenated alcohol-benzene solution. The heteronuclear NMR spectroscopy provided useful information about chemical shifts for better understanding the likely structure based on interactions with their coordinate metals. The mass spectra show similar types of fragmentation pattern. SEM-EDS analyses showed consistency with the formulation. XRD patterns show an enhanced homogeneity at high temperature. TGA measurements show that thermal decomposition occured in steps that depended entirely on the chemical compositions and the synthesis routes. SEM observation reveals that the morphology and particle size strongly depend on synthesis routes for their precursors. 相似文献