全文获取类型
收费全文 | 38796篇 |
免费 | 281篇 |
国内免费 | 324篇 |
专业分类
化学 | 16094篇 |
晶体学 | 357篇 |
力学 | 1742篇 |
综合类 | 12篇 |
数学 | 10592篇 |
物理学 | 10604篇 |
出版年
2021年 | 155篇 |
2020年 | 158篇 |
2019年 | 148篇 |
2018年 | 1302篇 |
2017年 | 1549篇 |
2016年 | 888篇 |
2015年 | 703篇 |
2014年 | 634篇 |
2013年 | 1212篇 |
2012年 | 3446篇 |
2011年 | 2730篇 |
2010年 | 2101篇 |
2009年 | 1800篇 |
2008年 | 940篇 |
2007年 | 956篇 |
2006年 | 942篇 |
2005年 | 4707篇 |
2004年 | 4205篇 |
2003年 | 2558篇 |
2002年 | 752篇 |
2001年 | 477篇 |
2000年 | 274篇 |
1999年 | 321篇 |
1998年 | 227篇 |
1997年 | 205篇 |
1996年 | 213篇 |
1995年 | 160篇 |
1994年 | 170篇 |
1993年 | 177篇 |
1992年 | 324篇 |
1991年 | 314篇 |
1990年 | 254篇 |
1989年 | 225篇 |
1988年 | 231篇 |
1987年 | 240篇 |
1986年 | 190篇 |
1985年 | 240篇 |
1984年 | 217篇 |
1983年 | 178篇 |
1982年 | 188篇 |
1981年 | 199篇 |
1980年 | 187篇 |
1979年 | 201篇 |
1978年 | 193篇 |
1977年 | 182篇 |
1976年 | 226篇 |
1975年 | 200篇 |
1974年 | 169篇 |
1973年 | 171篇 |
1972年 | 114篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
241.
Low molecular weight tri-podal biphenyl- and benzoate-type mesogens [C6H5C6H4O(CH2)5SiMe2CH2CH2SiMe2]3CH (4), [C11H23O(C6H4)2O(CH2)5SiMe2]3CH (5) and [MeOC6H4OC(O)C6H4O(CH2)5SiMe2]3CH (6) (C6H4 = 1,4-phenylene) were obtained, from branched silyl substituted methane precursors [CH2CH(Me)2Si]3CH (1) and (HMe2Si)3CH (2). The biphenyl-containing ones (4) and (5) were converted into terminal alkenes, which were subsequently hydrosilylated with poly(methylsiloxanes). The polymer derived from (5) exhibited mesomorphic properties. Such systems have the potential to significantly increase the density of liquid crystal rod-like structures in side chains of linear polymers (or dendritic liquid crystal polymers). 相似文献
242.
Hainer?Wackerbarth Rodolphe?Marie Mikala?Grubb Jingdong?Zhang Allan?G.?Hansen Ib?Chorkendorff Claus?B.?V.?Christensen Anja?Boisen Jens?UlstrupEmail author 《Journal of Solid State Electrochemistry》2004,8(7):474-481
We provide a comprehensive study of single- (ss) and double-strand (ds) oligonucleotides with either 25 or 10 bases or base pairs (bp) immobilized on polycrystalline and single-crystal Au(111) surfaces. The study is based on X-ray photoelectron spectroscopy, cyclic and differential pulse voltammetry, interfacial capacitance data, and electrochemical scanning tunnelling microscopy (in situ STM). The sequences used were the 25-bp sequence from the BRCA1 gene (25-mer), while the 10-bp oligonucleotides contained solely linear adenine and thymine sequences. The oligonucleotides were modified by the dimethoxytrityl group (DMT) via a disulfide group [DMT-S-S-ss25-mer and DMT-S-S-ds(AT)10], a pure disulfide group (A10-S-S-T10), or a thiol group [HS-ss25-mer and HS-ds-(AT)10], all via a hexamethylene linker. The overall pattern suggests strategies for controlled adsorption of DNA-based molecules and recognition of complementary strands or other molecules. 相似文献
243.
Haijun HaoHerbert W. Roesky Yuqiang DingChunming Cui Mark SchormannHans-Georg Schmidt Mathias NoltemeyerBoris ?emva 《Journal of fluorine chemistry》2002,115(2):143-147
β-Diketiminato magnesium fluoride [{CH(CMeNAr)2}Mg(μ-F)(THF)]2·toluene (Ar=2,6-i-Pr2C6H3, 2·toluene) was synthesized. The molecular structure of 2 revealed for the first time, the double fluoro bridge feature between the two magnesium atoms with a typical MgF bond length (average 1.95 Å). 相似文献
244.
Konrad Kowalski Janusz Zakrzewski S?awomir Domaga?a 《Journal of organometallic chemistry》2006,691(18):3902-3908
The synthesis of 1′-ethynyl-2,5-dimethylazaferrocene is reported along with its cyclic voltammetry measurements and the X-ray structure determination of its W(CO)5-complex. Basic coordination and organic chemistry of 1′-ethynyl-2,5-dimethylazaferrocene is also presented via derivatives incorporating cis-Pt(dppe) and Si(Et)3 units. 相似文献
245.
The antitumor activity of (1----3)-beta-D-glucans was tested in order to clarify its conformation-dependent response together with conformational elucidation by carbon-13 nuclear magnetic resonance (13C-NMR) spectroscopy. It was shown that the following three conformations, single chain, single helix and triple helix, are readily distinguished by the high-resolution solid-state 13C-NMR method. It turned out that preparations of linear (1----3)-beta-D-glucans of a triple helical conformation were ineffective in the inhibition of tumor growth. These linear (1----3)-beta-D-glucans were converted to an effective form in the inhibition of tumor growth when they were lyophilized from dimethyl sulfoxide (DMSO) solutions as a result of a conformational change from the triple helical to the single chain forms. They were not effective, however, when assayed in DMSO solution. In contrast, it was found that a branched (1----3)-beta-D-glucan is effective not only in either saline solutions of the triple helical sample or the lyophilized sample from DMSO, but also in DMSO solution. The aforementioned drastic change in antitumor activity was interpreted in terms of resulting conformational changes as analyzed by the 13C-NMR method. 相似文献
246.
Perfluoromethyl Element Ligands. XXIX. Preparation and Spectroscopic Investigation of M(CO)4L2 Complexes (M ? Cr, Mo, W; L ? Me2PSMe, Me2PSeMe, (CF3)2PSMe, (CF3)2PSMe) The complexes M(CO)4L2 (see Inhaltsübersicht) have been prepared by the reaction of tetracarbonyl norbornadiene metal compounds M(CO)4NBD with L at room temperature or 35°C, respectively. The cis-complexes formed in the first step undergo rearrangement to trans-isomers at higher temperatures. New compounds have been characterized by analytical and spectroscopic (IR, NMR, MS) methods. 相似文献
247.
H. R. Eschnauer F. Alt J. Messerschmidt G. T?lg 《Fresenius' Journal of Analytical Chemistry》1989,332(8):874-879
Summary Little is known about selenium contents in alcoholic beverages, especially in wine, because no reliable determination method was available so far. A procedure for the detection of selenium in the pg- and low ng-range was developed, which uses the hydride-generation-condensation-AAS (HGC-AAS). Selenium contents in 103 German wines of the year vintages 1950 until 1985 were determined. In mean they range between 0.29 and 0.79 g/l Se and show a certain dependence from the geological soil formation and the wine species. Corresponding vineyard soils from Ingelheim (Rheinhessen, FRG) contain 0.18–0.44 mg/kg Se, mean 0.24 mg/kg Se, 10% of it is available for plants. The distribution of selenium in grape berries shows selenium-rich seeds and — with red wine species — selenium-rich skins. Higher selenium contents by contamination were observed near an industrial location. The behaviour of selenium in the vegetation cycle is pursued. The selenium depletion during fermentation may amount to 60%. All results are summarized in the so-called trace element vinogram. Selenium hence is a regular and natural constituent part of wine.
Herrn Prof. Dr. R. Neeb zum 60. Geburtstag gewidmet 相似文献
Herrn Prof. Dr. R. Neeb zum 60. Geburtstag gewidmet 相似文献
248.
H. L. L. M. Van Leengoed N. van der Veen A. A. C. Versteeg R. Ouellet J. E. van Lier W. M. Star 《Photochemistry and photobiology》1993,58(4):575-580
Six sulfonated metallophthalocyanines, chelated with either aluminum or zinc and sulfonated to different degrees, were studied in vivo for their photodynamic activity in a rat skin-fold chamber model. The chamber, located on the back of female WAG/Rij rats, contained a syngeneic mammary carcinoma implanted into a layer of subcutaneous tissue. Twenty-four hours after intravenous administration of 2.5 μmol/kg of one of the dyes, the chambers received a treatment light dose of 600 J/cm2 with monochromatic light of 675 nm at a power density of 100 mW/ cm2. During light delivery and up to a period of 7 days after treatment, vascular effects of tumor and normal tissue were scored. Tumor cell viability was determined by histology and by reimplantation of the chamber contents into the skin of the same animal, either 2 h after treatment or after the 7 day observation period. Vascular effects of both tumor and subcutaneous tissue were strongest with dyes with the lowest degree of sulfonation and decreased with increasing degree of sulfonation. Tumor regrowth did not occur with aluminum phthalocyanine mono- and disulfonate and with zinc phthalocyanine monosulfonate. With the protocol that was used, complete necrosis without recovery was only observed when reimplantation took place at the end of the 7 day follow-up period. Reimplantation 2 h after treatment always resulted in tumor regrowth. At this interval, the presence of viable tumor cells was confirmed histologically. In general tumor tissue vasculature was more susceptible to photodynamic damage than vasculature of the normal tissue. The effect on the circulation of both tumor and normal tissue increased with decreasing degree of sulfonation. Based on this study, the photodynamic effects using the six sulfonated metallophthalocyanines on the vasculature can be ranked from high to low as: AlPcS2= ZnPcS1 > AIPcS1 > AIPcS4 > ZnPcS2 > ZnPcS4. 相似文献
249.
The catalytic system methylaluminoxane (MAO) and bis(n-butylcyclopentadienyl)zirconium dichloride ((nBuCp)2ZrCl2) was immobilized on commercial silica, silica-alumina and aluminophosphate calcined at different temperatures. The properties of the supports were determined by using N2 adsorption-desorption isotherms at 77 K, FT-IR spectroscopy and SEM. After aluminium and zirconium impregnation, the catalysts were analyzed by ICP-AES, FT-IR and UV-vis spectroscopy. Ethylene polymerizations were carried out in a Schlenk tube at 70 °C and 1.2 bar of ethylene pressure. The polyethylene obtained was characterized by GPC, DSC and SEM.Catalysts supported on silica-alumina exhibited higher polymerization activity than those supported on silica and aluminophosphate. Besides, the activity of MAO/(nBuCp)2ZrCl2 catalytic system supported on silica-alumina and aluminophosphate decreased strongly with support calcination temperature, while remained almost constant when silica was employed as support. All these experimental features suggest a role of the support acid properties and hydroxyl group population in the generation of active polymerization species. 相似文献
250.
The I-E response of the liquid membrane of the calcium selective electrode is studied under constant or linearly varying current and voltage. An increase in the membrane resistance, recorded when an electrical current crosses the membrane, is due to the outflow of Cl? ions initially present in the membrane. When calcium ions are replaced by alkaline ions inside the membrane at constant current, the decrease of the membrane resistance due to an ion exchange is in agreement with the conductivity measurements (Part II). When the applied voltage is imposed besides the ion exchange one must take into account the interfacial overpotential to explain the important rectification effect observed. The interfacial transfer constant rate of alkaline ions seems greater than that of Ca2+ ion. 相似文献