全文获取类型
收费全文 | 5496篇 |
免费 | 244篇 |
国内免费 | 52篇 |
专业分类
化学 | 4087篇 |
晶体学 | 40篇 |
力学 | 216篇 |
综合类 | 2篇 |
数学 | 791篇 |
物理学 | 656篇 |
出版年
2024年 | 6篇 |
2023年 | 43篇 |
2022年 | 63篇 |
2021年 | 306篇 |
2020年 | 207篇 |
2019年 | 230篇 |
2018年 | 219篇 |
2017年 | 168篇 |
2016年 | 292篇 |
2015年 | 202篇 |
2014年 | 210篇 |
2013年 | 535篇 |
2012年 | 360篇 |
2011年 | 385篇 |
2010年 | 238篇 |
2009年 | 204篇 |
2008年 | 259篇 |
2007年 | 261篇 |
2006年 | 207篇 |
2005年 | 183篇 |
2004年 | 164篇 |
2003年 | 141篇 |
2002年 | 145篇 |
2001年 | 54篇 |
2000年 | 65篇 |
1999年 | 44篇 |
1998年 | 31篇 |
1997年 | 43篇 |
1996年 | 33篇 |
1995年 | 25篇 |
1994年 | 38篇 |
1993年 | 31篇 |
1992年 | 31篇 |
1991年 | 27篇 |
1990年 | 41篇 |
1989年 | 25篇 |
1988年 | 22篇 |
1987年 | 25篇 |
1986年 | 27篇 |
1985年 | 32篇 |
1984年 | 19篇 |
1983年 | 17篇 |
1982年 | 25篇 |
1981年 | 20篇 |
1980年 | 13篇 |
1979年 | 16篇 |
1978年 | 11篇 |
1977年 | 12篇 |
1976年 | 9篇 |
1975年 | 6篇 |
排序方式: 共有5792条查询结果,搜索用时 421 毫秒
971.
Jai Han Seok Mohamed E. El-Khouly Yasuyuki Araki Kwang-Yol Kay 《Journal of organometallic chemistry》2009,694(12):1818-3862
Photoinduced electron transfer processes of two newly synthesized tetrads with a triphenylamine (TPA) as central building block, to which bisfullerenes (C60) and bisferrocenes (Fc) are covalently connected, have been studied. One of them has a TPA linked with one C60 moiety and two ferrocene moieties C60-TPA-(Fc)2 and another tetrad has a TPA linked with two C60 moieties and one ferrocene unit (C60)2-TPA-Fc. The photophysical properties of (C60)m-TPA-(Fc)n have been investigated by applying the picosecond time-resolved fluorescence and nanosecond transient absorption techniques in both polar and nonpolar solvents. The charge separation process via the excited singlet state of the C60 moiety of the C60-TPA-(Fc)2 is more efficient than that of the (C60)2-TPA-Fc. It is found that the ratio of Fc-donor to C60-acceptor affects charge separation efficiency via the excited singlet state of the C60 moiety. 相似文献
972.
Samir Mohamed El‐Moghazy Mohamed Abd El‐Azem Mohamed Marwa Fadel Mohamed Nadia Fayek Youssef 《中国化学会会志》2009,56(2):360-367
Reversed phase‐high performance liquid chromatography (RP‐HPLC), thin layer chromatography (TLC) densitometry and first derivative spectrophotometry (1D) techniques are developed and validated as a stability‐indicating assay of ezetimibe in the presence of alkaline induced degradation products. RP‐HPLC method involves an isocratic elution on a Phenomenex Luna 5μ C18 column using acetonitrile: water: glacial acetic acid (50:50:0.1 v/v/v) as a mobile phase at a flow rate of 1.5 mL/min. and a UV detector at 235 nm. TLC densitometric method is based on the difference in Rf‐values between the intact drug and its degradation products on aluminum‐packed silica gel 60 F254 TLC plates as stationary phase with isopropanol: ammonia 33% (9:1 v/v) as a developing mobile phase. On the fluorescent plates, the spots were located by fluorescence quenching and the densitometric analysis was carried out at 250 nm. Derivative spectrophotometry, the zero‐crossing method, ezetimibe was determined using first derivative at 261 nm in the presence of its degradation products. Calibration graphs of the three suggested methods are linear in the concentration ranges 1–10 mcg/mL, 0.1–1 mg/mL and 1–16 mcg/mL with a mean percentage accuracy of 99.05 ± 0.54%, 99.46 ± 0.63% and 99.24 ± 0.82% of bulk powder, respectively. The three proposed methods were successfully applied for the determination of ezetimibe in raw material and pharmaceutical dosage form; the results were statistically analyzed and compared with those obtained by the reported method. Validation parameters were determined for linearity, accuracy and precision; selectivity and robustness and were assessed by applying the standard addition technique. 相似文献
973.
Hisham Mohamed Megan Murray James N. Turner Michele Caggana 《Journal of chromatography. A》2009,1216(47):8289-8295
The isolation and analysis of circulating tumor cells (CTCs) from blood are the subject of intense research. Although tests to detect metastasis on a molecular level are available, progress has been hampered by a lack of tumor-specific markers and predictable DNA abnormalities. The main challenge in this endeavor is the small number of available cells of interest, 1–2 per mL in whole blood. We have designed a micromachined device to fractionate whole blood using physical means to enrich for and/or isolate rare cells from peripheral circulation. It has arrays of four successively narrower channels, each consisting of a two-dimensional array of columns. Current devices have channels ranging in width from 20 to 5 μm, and in depth from 20 to 5 μm. Several optimizations resulting in the fabrication of a total of 10 derivative devices have been carried out; only two types are used in this study. Both have increasingly narrower gap widths between the columns along the flow axis with 20, 15, 10, and 5 μm spacing all on one device. The first 20 μm wide segment disperses the cell suspension and creates an evenly distributed flow over the entire device, whereas the others were designed to retain increasingly smaller cells. The channel depth is constant across the entire device, the first type was 10 μm deep and the second type is 20 μm deep. When cells from each of eight tumor cell lines were loaded into the device, all cancerous cells were isolated. In mixing experiments using human whole blood, we were able to fractionate cancer cells without interference from the blood cells. Additionally, either intact cells, or DNA, could be extracted for molecular analysis. The ultimate goal of this work is to characterize the cells on the molecular level to provide non-invasive methods to monitor patients, stage disease, and assess treatment efficacy. Furthermore, this work will use gene expression profiles to gain insights into metastasis. 相似文献
974.
James R. Durig Arindam Ganguly Gamil A. Guirgis Stephen Bell Tarek A. Mohamed Todor K. Gounev 《Structural chemistry》2009,20(3):489-503
The infrared spectra (4,000–30 cm?1) of the gas and solid and the Raman spectrum of liquid 2,2-difluoroethanol as well as variable temperature infrared spectra of krypton/xenon solutions have been recorded. From all these data, two (Gg and Tg) out of the five possible stable conformers have been confidently identified. The order of the stabilities has been predicted to be Gg > Tg > Gt > Gg′ > Tt by utilizing ab initio MP2 (full) and DFT (B3LYP method) calculations, where the first indicator (capital letter) is in reference to rotation around the C–C bond (G = gauche or T = trans) and the second one (small letter) refers to the orientation of the hydroxyl group. The percentage of the minor conformer Tg, at ambient temperature, is estimated to be (16 ± 3%). The optimized geometries, fundamental frequencies, infrared intensities, Raman activities, and depolarization values as well as centrifugal distortion constants have been obtained from ab initio and density functional theory calculations by utilizing a variety of basis sets as well as those with diffuse functions. By utilizing the previously reported microwave rotational constants for two isotopomers of the Gg conformer combined with ab initio MP2(full)/6-311+G(d,p) predicted structural values, adjusted r 0 parameters have been obtained. The determined heavy atom distances (Å) for the Gg conformer are: C1–C2 = 1.510(3), C2–F4 = 1.371(3), C2–F5 = 1.362(3), C1–O3 = 1.412(3) Å and angles ∠O3C1C2 = 111.0(5), ∠F4C2C1 = 108.8(5), ∠F5C2C1 = 109.8(5), τF4C2C1O3 = 63.5(5), τF5C2C1O3 = 179.1(5)°. Barriers of internal rotation have been obtained and vibrational assignments for the Gg and Tg conformers are given. The five predicted centrifugal distortion constants compared to the experimental values are in reasonable agreement except for ?K, which appears to be in error. The results are discussed and the structural parameters compared to the corresponding ones for 2-fluoroethanol and 2,2,2-trifluoroethanol where those for the latter molecule have been redetermined. The currently determined heavy atom parameters are quite different from the earlier assumed values, which led to poor values of the six adjusted parameters. 相似文献
975.
976.
977.
Martin J. Hollamby Kieran Trickett Azmi Mohamed Stephen Cummings Rico F. Tabor Olesya Myakonkaya Sarah Gold Dr. Sarah Rogers Dr. Richard K. Heenan Dr. Julian Eastoe Prof. 《Angewandte Chemie (International ed. in English)》2009,48(27):4993-4995
Getting their feet wet : Low‐cost hydrocarbon surfactants act as fluid modifiers for supercritical carbon dioxide (scCO2). Increased terminal branching of the surfactant chains aids micelle formation (see middle picture: CO2 green), and more chains allows water to be incorporated (right, blue).
978.
Mohamed A. Ghanem Jean-Mathieu Chrtien Jeremy D. Kilburn Philip N. Bartlett 《Bioelectrochemistry (Amsterdam, Netherlands)》2009,76(1-2):115
We report the preparation, using electrochemical and solid-phase synthesis, and characterisation of a 26 member library of 13 dihydroxybenzene derivatives covalently attached to glassy carbon through ethylenediamine (EDA) and C6H4CH2NH linkers. First, Boc-protected EDA or Boc–NHCH2C6H4 were electrochemically attached to the GC surface. After Boc-deprotection, dimethoxybenzoyl chlorides were coupled to the EDA and C6H4CH2NH linkers using solid-phase synthesis followed by deprotection of the methoxy groups to give the corresponding dihydroxybenzene compounds. Surface coverage and electrochemical parameters of the dihydroxybenzene modified electrodes were evaluated in parallel using cyclic voltammetry. The mid-peak potentials, Emp, and surface coverages for the 13 dihydroxybenzene derivatives were found to be independent of the choice of linker. The mid-peak potentials of the immobilised dihydroxybenzene derivatives varied between 0.0 and 260 mV vs. SCE and their surface coverages varied between 0.07 and 1.1 nmol cm− 2, depending on the pattern of substitution of the dihydroxybenzene ring. The electrocatalytic activities of the library were evaluated for mediation of NADH oxidation, and the ortho-dihydroxybenzene derivatives were found to have higher catalytic activity. 相似文献
979.
Nagwa Burham Sami A. Azeem Mohamed F. El-Shahat 《Central European Journal of Chemistry》2009,7(4):945-954
A new solid — phase extraction sorbent was developed based on stepwise anchoring of two ligand molecules for the determination
of copper, zinc, lead and cadmium in drinking water by flame AAS. Amberlite XAD-2 functionalized with 4′-(2-hydroxyphenylazo)-3′-methyl-1′-phenyl-2′-pyrazolin-5′-one
(HPAPyr) was utilized for preconcentration/separation of these elements. The sorbent was prepared by two successive azo coupling
reactions. First, 2-aminophenol was anchored to the amino groups in the resin resulted from nitration followed by reduction.
Then, the resulted 2-aminophenol functionalized resin was further diazotized and coupled to the pyrazolone compound and the
final product HPAPyr-XAD-2 was characterized by IR and elemental analysis. The optimum pH range for sorption, shaking time,
exchange capacity, sample flow rate, preconcentration factor and interference from co-existing ions were investigated. All
metal ions were quantitatively desorbed from the resin by 4.5 mol L−1 nitric acid solution. The sorbent provides limit of detection within the range 0.9–3.3 μg L−1 and concentration factor up to 250. The procedure was validated by analysis of certified material NIST-SRM 1577b. Application
to drinking water showed satisfactory results with relative standard deviation RSD ≤ 8.5%.
相似文献
980.
Mohamed El Bachraoui 《Algebra Universalis》2009,60(4):425-438
A relation algebra is bifunctional-elementary if it is atomic and for any atom a, the element a;1;a is the join of at most two atoms, and one of these atoms is bifunctional (an element x is bifunctional if ’). We show that bifunctional-elementary relation algebras are representable. Our proof combines the representation theorems
for: pair-dense relation algebras given by R. Maddux; relation algebras generated by equivalence elements provided corresponding
relativizations are representable by S. Givant; and strong-elementary relation algebras dealt with in our earlier work. It
turns out that atomic pair-dense relation algebras are bifunctional elementary, showing that our theorem generalizes the representation
theorem of atomic pair-dense relation algebras. The problem is still open whether the related classes of rather elementary,
functional-elementary, and strong functional-elementary relation algebras are representable.
Received July 15, 2007; accepted in final form March 17, 2008. 相似文献