全文获取类型
收费全文 | 1279篇 |
免费 | 28篇 |
国内免费 | 2篇 |
专业分类
化学 | 1055篇 |
晶体学 | 22篇 |
力学 | 4篇 |
数学 | 108篇 |
物理学 | 120篇 |
出版年
2023年 | 14篇 |
2022年 | 77篇 |
2021年 | 55篇 |
2020年 | 31篇 |
2019年 | 33篇 |
2018年 | 22篇 |
2017年 | 23篇 |
2016年 | 48篇 |
2015年 | 41篇 |
2014年 | 45篇 |
2013年 | 116篇 |
2012年 | 86篇 |
2011年 | 77篇 |
2010年 | 51篇 |
2009年 | 59篇 |
2008年 | 53篇 |
2007年 | 65篇 |
2006年 | 62篇 |
2005年 | 47篇 |
2004年 | 51篇 |
2003年 | 36篇 |
2002年 | 26篇 |
2001年 | 18篇 |
2000年 | 14篇 |
1999年 | 9篇 |
1998年 | 12篇 |
1997年 | 8篇 |
1996年 | 6篇 |
1995年 | 15篇 |
1994年 | 7篇 |
1993年 | 10篇 |
1992年 | 9篇 |
1991年 | 5篇 |
1990年 | 4篇 |
1989年 | 2篇 |
1988年 | 2篇 |
1987年 | 6篇 |
1986年 | 8篇 |
1985年 | 9篇 |
1984年 | 3篇 |
1983年 | 8篇 |
1982年 | 4篇 |
1981年 | 5篇 |
1980年 | 8篇 |
1979年 | 2篇 |
1978年 | 4篇 |
1977年 | 2篇 |
1974年 | 3篇 |
1962年 | 2篇 |
1937年 | 1篇 |
排序方式: 共有1309条查询结果,搜索用时 0 毫秒
81.
Ryszard Andruszkiewicz Ewa Liedke-Przybysz 《Phosphorus, sulfur, and silicon and the related elements》2013,188(12):2274-2278
Abstract Reaction of succinimide with diphenylchlorophosphate in the presence of lithium bis(trimethylsilyl)amide followed by aqueous work-up led to the formation of amidophosphate 3. The same reaction performed in the presence of sodium hydride yield N-(diphenylphosphoryl)succinimde 4, which undergoes nucleophilic ring opening with sodium alkoxides resulted in the formation of phosphoroamidate analogues 5 a-d in good yields. GRAPHICAL ABSTRACT 相似文献
82.
Agata Górniak Bożena Karolewicz Ewa Żurawska-Płaksej Janusz Pluta 《Journal of Thermal Analysis and Calorimetry》2013,111(3):2125-2132
The objective of the present investigation was to study the effect of eutectic formation on in vitro dissolution of simvastatin (SIM) released from mixtures with acetylsalicylic acid (ASA) prepared by a grinding method. SIM–ASA mixtures were characterized by means of differential scanning calorimetry (DSC), infrared spectroscopy (IR), X-ray powder diffractometry (XRPD), and in vitro dissolution tests. IR spectroscopy and XRPD studies indicated no interaction between SIM and ASA in the solid state. The DSC investigation has revealed that SIM and ASA form a simple eutectic system containing 66.6 % w/w of SIM at the eutectic point. In vitro dissolution studies of SIM and its mixtures with ASA were carried out. The eutectic mixture shows an appreciable increase in the dissolution rate in comparison with other ratios and SIM in 0.5 % w/v sodium lauryl sulfate. The dissolution enhancement of SIM was related to the effective wetting of the drug particles with a significantly reduced size released from eutectic composition. In conclusion, dissolution of SIM can be enhanced through eutectic formation with ASA by means of simple mechanical activation (a grinding method). 相似文献
83.
Dr. Xuecheng Chen Dr. Krzysztof Kierzek Karolina Wenelska Krzystof Cendrowski Jiang Gong Dr. Xin Wen Prof. Tao Tang Prof. Paul K. Chu Prof. Ewa Mijowska 《化学:亚洲杂志》2013,8(11):2627-2633
Core–shell‐structured mesoporous silica spheres were prepared by using n‐octadecyltrimethoxysilane (C18TMS) as the surfactant. Hollow mesoporous carbon spheres with controllable diameters were fabricated from core–shell‐structured mesoporous silica sphere templates by chemical vapor deposition (CVD). By controlling the thickness of the silica shell, hollow carbon spheres (HCSs) with different diameters can be obtained. The use of ethylene as the carbon precursor in the CVD process produces the materials in a single step without the need to remove the surfactant. The mechanism of formation and the role played by the surfactant, C18TMS, are investigated. The materials have large potential in double‐layer supercapacitors, and their electrochemical properties were determined. HCSs with thicker mesoporous shells possess a larger surface area, which in turn increases their electrochemical capacitance. The samples prepared at a lower temperature also exhibit increased capacitance as a result of the Brunauer–Emmett–Teller (BET) area and larger pore size. 相似文献
84.
The catalytic oxidative domino degradation of phenols was investigated. Hydrogen peroxide (30% aq.) was used as an oxidant and 2,2′-dinitro-4,4′-ditrifluoromethyldiphenyl diselenide 4e as a catalyst. The products were muconic acid 5, and muconolactones muconolactones—5-carboxymethylfuran-2(5H)-ones 7 and 9. Phenols with alkyl groups at 2 or 4 positions of the benzene ring were converted regioselectively to corresponding muconolactones substituted at alkenylene ring carbon atoms. The reaction mechanism is proposed. 相似文献
85.
The stereoselective synthesis of two epimeric penta-hydroxylated indolizidines was accomplished from 2,3:5,6-di-O-isopropylidene-α-d-mannofuranose and N-(2-methylpent-4-en-2-yl)hydroxylamine. The transformation of these substrates into the corresponding 7-oxa-1-azabicyclo[2.2.1]heptane by the intramolecular 1,3-dipolar cycloaddition was the key step of the synthesis. The adduct was transformed into the tricyclic ammonium salt by intramolecular N-alkylation. The tricyclic ammonium salt was converted to the target compounds by: (route 1) the catalytic hydrogenation; or (route 2) the reaction with sodium azide, followed by the enantioselective reduction of the resulting indolizidinone. 相似文献
86.
Ewa Gniazdowska Przemysław Koźmiński Leon Fuks 《Journal of Radioanalytical and Nuclear Chemistry》2013,298(2):1171-1177
Two types of technetium-99m complexes: (i) with the Hynic ligand linked to Substance P(1–11) and (ii) of the type ‘4 + 1’ consisting of tetradentate tripodal chelator tris(2-mercaptoethyl)-amine and monodentate isocyanide ligand previously coupled with Substance P(1–11), have been prepared on the n.c.a. scale. The obtained conjugates exhibit different lipophilicity and high stability in neutral aqueous solutions, even in the presence of excess concentration of histidine/cysteine competitive standard ligands. The conjugate (99mTc(NS3)(CN))2–SP containing two technetium-99m species in the molecule may be expected to be an extremely good diagnostic radiopharmaceutical. 相似文献
87.
Some symmetrical dimeric compounds containing biphenyl, biphenylcarboxylic acid or benzoiloxyphenyl moieties and polymethylene spacers were synthesised. The mesogenic properties of the synthesised compounds were investigated by optical microscopy, calorimetric and X-ray methods. It was shown that the location and direction of the ester bonds has a crucial significance in mesophase formation. 相似文献
88.
Tomasz Kowalkowski Hlanganani Tutu Leonard Mihaly Cozmuta Miroslav Sprynskyy Ewa M. Cukrowska Bogusław Buszewski 《International journal of environmental analytical chemistry》2013,93(10):797-811
The objectives of this study were to evaluate the mobility of heavy metals (HMs) in two types of soils (acidic forest soil and neutral agricultural soil) by leaching with calcium chloride solution in column experiments. The screening properties of neutral agricultural soil towards pollution by heavy metals (Ni, Cu, Zn and Cd) are approximately 10 times higher than those of acid forest soil. The neutral agricultural soil, polluted artificially by one pore volume (PV) of an HMs solution of concentration 200 mg L?1, can screen the leaching of these metals over several hundreds of years. The higher apparent desorption rate and per cent desorption of HMs (especially Cd) in acid forest soil indicated a higher potential of intensive migration of the metals across the profile and indicated potential risk of Cd pollution for this type of soil. The latest approach of artificial neural networks to describe transport of HMs in soil has been also evaluated. Using a simple three-layer perceptron topology with three hidden neurons, the experimental data could be simulated. The results suggested that the pH of soil is a major factor controlling the retention of the heavy metals in the soils. 相似文献
89.
Elwira Chrobak Ewa Michalik Joachim Kusz Maria Nowak Stanisaw Boryczka 《Acta Crystallographica. Section C, Structural Chemistry》2014,70(9):847-851
The lupane triterpenoid betulonic aldehyde [also known as betulonal; systematic name: lup‐20(29)‐en‐28‐al‐3‐one, C30H46O2] is a product of betulin oxidation. Crystals were obtained from hexane [form (I)] and dimethyl sulfoxide [form (II)] solutions. Forms (I) and (II) are both orthorhombic. The molecular geometric parameters in the two forms are similar, but the structures are different with respect to the crystal packing. Polymorph (I) contains two independent molecules in the asymmetric unit, while polymorph (II) contains only one molecule, which has a disordered aldehyde group [the disorder ratio is 0.769 (4):0.231 (4)]. In each molecule, the six‐membered rings have chair conformations, whereas the cyclopentane ring in each molecule adopts an envelope conformation. All the rings in the lupane nucleus are trans‐fused. The extended structures of both polymorphs are stabilized by weak intermolecular C—H...O and van der Waals interactions. Weak intramolecular C—H...O interactions are also observed. 相似文献
90.
Dr. Rebecca Walker Dr. Magdalena Majewska Dr. Damian Pociecha Dr. Anna Makal Prof. John MD Storey Prof. Ewa Gorecka Prof. Corrie T. Imrie 《Chemphyschem》2021,22(5):461-470
A selection of pyrene-based liquid crystal dimers have been prepared, containing either methylene-ether or diether linked spacers of varying length and parity. All the diether linked materials, CBOnO.Py (n=5, 6, 11, 12), exhibit conventional nematic and smectic A phases, with the exception of CBO11O.Py which is exclusively nematic. The methylene-ether linked dimer, CBnO.Py, with an even-membered spacer (n=5) was solely nematogenic, but odd-members (n=6, 8, 10) exhibited both nematic and twist-bend nematic phases. Replacement of the cyanobiphenyl fragment by cyanoterphenyl giving CT6O.Py, gave elevated melting and nematic-isotropic transition temperatures, and SmA and SmCA phases were observed on cooling the nematic phase. Intermolecular face-to-face associations of the pyrene moieties drive glass formation, and all these materials have a glass transition temperature at or above room temperature. The stability of the glassy twist-bend nematic phase allowed for its study using AFM, and the helical pitch length, PTB, was measured as 6.3 and 6.7 nm for CB6O.Py and CB8O.Py, respectively. These values are comparable to the shortest pitch of a twist-bend nematic phase measured to date. 相似文献