全文获取类型
收费全文 | 1303篇 |
免费 | 47篇 |
国内免费 | 16篇 |
专业分类
化学 | 936篇 |
晶体学 | 4篇 |
力学 | 41篇 |
数学 | 172篇 |
物理学 | 213篇 |
出版年
2023年 | 3篇 |
2022年 | 68篇 |
2021年 | 50篇 |
2020年 | 49篇 |
2019年 | 49篇 |
2018年 | 33篇 |
2017年 | 33篇 |
2016年 | 75篇 |
2015年 | 38篇 |
2014年 | 44篇 |
2013年 | 136篇 |
2012年 | 88篇 |
2011年 | 74篇 |
2010年 | 65篇 |
2009年 | 42篇 |
2008年 | 52篇 |
2007年 | 56篇 |
2006年 | 52篇 |
2005年 | 49篇 |
2004年 | 36篇 |
2003年 | 31篇 |
2002年 | 31篇 |
2001年 | 11篇 |
2000年 | 15篇 |
1999年 | 11篇 |
1998年 | 17篇 |
1997年 | 7篇 |
1996年 | 6篇 |
1995年 | 5篇 |
1994年 | 10篇 |
1993年 | 12篇 |
1992年 | 7篇 |
1991年 | 5篇 |
1989年 | 6篇 |
1988年 | 3篇 |
1987年 | 6篇 |
1986年 | 8篇 |
1985年 | 10篇 |
1984年 | 10篇 |
1983年 | 8篇 |
1982年 | 9篇 |
1981年 | 4篇 |
1980年 | 6篇 |
1979年 | 4篇 |
1978年 | 7篇 |
1976年 | 3篇 |
1975年 | 5篇 |
1973年 | 2篇 |
1967年 | 2篇 |
1938年 | 3篇 |
排序方式: 共有1366条查询结果,搜索用时 0 毫秒
991.
Optical supermicrosensor responses for simple recognition and sensitive removal of Cu (II) Ion target 总被引:1,自引:0,他引:1
The field of optical chemosensor technology demands a simple yet general design for fast, sensitive, selective, inexpensive, and specific recognition of a broad range of toxic metal ions. The suitable accommodation of chromogenic receptors onto ordered porous carriers have led to selective and sensitive chemosensors of target species. In this study, we offer real evidence on the potential use of two- and three-dimensional (2D and 3D) ordered supermicroporous monoliths as selective shape and size carriers for immobilizing the chromogenic probe. Among all the chemosensors, 3D supermicropore has exhibited easy accessibility of target ions, such as ion transports and high affinity responses of receptor-metal analyte binding events. This leads to an optical color signal that is easily generated and transduced even at trace levels of Cu(II) target ions. The supermicrosensors have shown the ability to create Cu(II) ion-sensing responses up to nanomolar concentrations (∼10−9 mol/dm3) with rapid response time (in the order of seconds). Supermicrosensors have the ability to create easily modified sensing systems with multiple regeneration/reuse cycles of sensing systems of Cu(II) analytes. The simple treatment using ClO4− anion as a stripping agent has removed effectively the Cu(II) ions and formed a “metal-free” probe surface. The supermicrosensors have exhibited the specificity behavior permitting Cu(II) ion-selective determination in real-life samples, such as in wastewater, despite the presence of active component species. Extensive analytical results indicate that the use of the supermicrosensor as Cu(II) ion strips for field screening can be a time- and cost-alternative tool to current effective laboratory assays. 相似文献
992.
Ismail I. Althagafi Amr S. Abouzied Thoraya A. Farghaly Nadia T. Al‐Qurashi Mohammad Y. Alfaifi Mohamed R. Shaaban Mohamed R. Abdel Aziz 《Journal of heterocyclic chemistry》2019,56(2):391-399
A facile access to novel bis‐(indoline‐2,3‐dione) was achieved via reactions of isatin with 1,3‐dibromopropane. The utility of the versatile bis‐(indoline‐2,3‐dione) in the design of new multifunctional building blocks using condensation with hydrazine derivatives was demonstrated. Moreover, a new series of bis‐thiazoles and bis‐thiazol‐4(5H)‐ones were synthesized by the reaction of bis‐thiosemicarbazone derivative with various derivatives of hydrazonoyl halides. The calculations derived from X‐ray diffraction patterns indicated the nanosize of the newly designed compounds. The spectral data of the formed compounds were established their structures. Also, the cytotoxic activity of the produced derivatives was screened against line MCF‐7 (breast cancer cell). It was found that four derivatives from nine investigated compounds showed activity more potent than the standard drug used by 20 times in some cases. 相似文献
993.
Richard J. Cremlyn Brian B. Dewhurst Ismail Khattak 《Phosphorus, sulfur, and silicon and the related elements》2013,188(3):325-332
Abstract Cholesteryl N-phenylphosphoramidic chloride has been converted to the corresponding phosphoramidic hydrazide and azide. The former compound was characterised by the preparation of a number of hydrazones, while the latter has been reacted with norbornene, dimethylsulphoxide, and triphenylphosphine. However, the azide did not react with decane, o-anisole, or butylamine. 17-Oxoandrost-5-ene-3β-yl N-phenylphosphoramidic chloride was prepared and was converted to the hydrazide, but a pure sample of the azide could not be isolated. Cholesteryl N-phenyl phosphoramidic triphenylphosphinimine has been reacted with eight carbonyl compounds and the structures of the products investigated. Cholesteryl N-cyclohexylphosphoramidic chloride was converted to the azide and the triphenylphosphinimine; the latter was reacted with acetone and p-nitrobenzaldehyde. Cholesteryl phosphorodichloridate has been condensed with phenol, p-nitrophenol, and p-methoxyphenol to give the corresponding O-arylphosphorochloridates. The O-phenyl and O-p-methoxyphenyl phosphorochloridates have been converted to the corresponding azides, but the azide from the p-nitrophenyl derivative could not be isolated. The reactions of cholesteryl phosphorodichloridate with diethylamine, hydrazine, and sodium azide have also been examined; and cholesteryl phosphorodichloridothioate has been condensed with aniline and benzylamine. 相似文献
994.
Abdel Haleem M. Hussein Mohamed A. M. Gad-Elkareem Abu-Bakr A. A. M. El-Adasy Ismail M. Othman 《Phosphorus, sulfur, and silicon and the related elements》2013,188(9):2263-2280
N-1-Naphthyl-3-oxobutanamide (1) reacts with arylidinecyanothioacetamide 2a–c in ethanol/piperidine solution under reflux to yield the pyridine-2(1H)-thiones 6a–c. Compound 6a reacts with α-haloketones 7a–e to give the 6-thio-N-1-naphthyl-nicotinamides derivatives 8a–e, which cyclized to thieno[2,3-b]pyridine derivatives 9a–e. The reaction of compound 9a with hydrazine hydrate and formamide gives the thieno[2,3-b]pyridine carbohydrazide derivative 10 and pyridothienopyrimidine derivative 11, respectively. Reaction of 9a with benzoyl isothiocyanate gave thiourea derivative 12. Compound 12, upon treatment with alcoholic NaOH, gave pyridothienopyrimidine 13. Saponifications of 9a gave the amino acid 15, which affords 16 when refluxed in Ac2O. Treatment of compound 16 with AcONH4/AcOH gave 17. Diazotization and self-coupling of 9b gave the pyridothienotriazine 18. Also, diazotization of the ortho-aminohydrazide 10 give the corresponding azide 19, which was subjected to Curtius rearrangement in boiling xylene to give imidazothienopyridine 20. Reaction of 10 with either formic acid or triethylorthoformate and phenyl isothiocyanate gave the corresponding pyridothienotriazepines 22 and 23, respectively. The interaction of 10 with acetylacetone furnished the pyrazolyl derivative 24. The structures of the synthesized compounds were established from their analytical and spectral data. 相似文献
995.
Ayman M. Atta Saeed A. Sayed Abdelfatah B. Farag Husein S. Ismail Zakaa M. Mohamed Ahmed M. Eraky 《Journal of Dispersion Science and Technology》2013,34(9):1285-1295
Crosslinked acrylonitrile/acrylamidoxime/2-acrylamido-2-methylpropane sulfonic acid (AN/AAx/AMPS) based hydrogels was prepared by radical solution polymerization technique. The structures of hydrogels were characterized by FTIR analysis and the results were consistent with the expected structures. These hydrogels were used for the separation of Cd(II), Cu(II), and Fe(III) ions from their aqueous solutions. The influence of the uptake conditions such as pH, time, and initial feed concentration on the metal ion binding capacity of hydrogel was also tested. The selectivity of the hydrogel toward the different metal ions tested was Cd(II)>Fe(III)>Cu(II). The recovery of metal ions was also investigated in acid media. 相似文献
996.
The reaction of 1-amino-5-(4-methylbenzoyl)-4-(4-methylphenyl)pyrimidin-2(1H)-one/-thione (1a,b) with ethyl acetoacetate (EA) afforded moderate to good yields (59–63%) of ethyl 2-methyl-4-(4-methylbenzoyl)-5-(4-methylphenyl)-7-oxo/-thioxo-3,3a-dihydropyrazolo[1,5-c]pyrimidine-3-carboxylate
(2a,b). The newly synthesized compounds were characterized by elemental analyses, IR, 1H and 13C NMR spectral data. All were compared with their previous analogues. The reaction mechanism of 1 with EA was studied by means of the B3LYP/6-31G(d,p) method. In addition, for reactants Fukui functions were performed using
the data calculated with the Becke3–Lee–Yang–Parr (B3LYP) hybrid function. 相似文献
997.
Córdova A Engqvist M Ibrahem I Casas J Sundén H 《Chemical communications (Cambridge, England)》2005,(15):2047-2049
The intrinsic ability of amino acids to catalyze the asymmetric formation of carbohydrates, which enzymes have mediated for millions of years, with significant amplification of enantiomeric excess suggests a plausible ancient catalytic process for the evolution of homochirality. 相似文献
998.
Gui‐Ling Zhao Dr. Farman Ullah Dr. Luca Deiana Shuangzheng Lin Dr. Qiong Zhang Junliang Sun Prof. Dr. Ismail Ibrahem Dr. Pawel Dziedzic Armando Córdova Prof. Dr. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(5):1585-1591
The first examples of one‐pot highly chemo‐ and enantioselective dynamic kinetic asymmetric transformations (DYKATs) involving α,β‐unsaturated aldehydes and propargylated carbon acids are presented. These DYKATs, which proceed by a combination of catalytic iminium activation, enamine activation, and Pd0‐catalyzed enyne cycloisomerization, give access to functionalized cyclopentenes with up to 99 % ee and can be used for the generation of all‐carbon quaternary stereocenters. 相似文献
999.
The suitability of Syrian Portland cement for disposal of solidified low-level radioactive waste was assessed by measuring the leaching rate of 134Cs. In ordinary cement concrete, a leaching rate of 1.309 × 10?3 g/cm2 per day was measured. Mixing this concrete with microsilica reduced significantly the leaching rate to 3.106 × 10?4 g/cm2 per day for 1% mixing, and to 9.645 × 10?5 g/cm2 per day for 3% mixing. It was also found that the application of a latex paint reduced these leaching rates by about 10%. These results, along with mechanical strength tests (under radiation exposure, high temperature, long water immersion and freeze–thaw cycling) indicate that Syrian Portland cement is suited for the disposal of low-level radioactive waste. 相似文献
1000.
Gregory B. Kharas Lujain A. Alyahya Sara M. Rocus Aisha Ismail Rosio Juarez Alfreda Kavaliauskaite 《高分子科学杂志,A辑:纯化学与应用化学》2016,53(12):725-728
Novel trisubstituted ethylenes, oxy ring-disubstituted propyl 2-cyano-3-phenyl-2-propenoates, RPhCH = C(CN)CO2C3H7 (where R is 4-methoxy-2-methyl, 4-methoxy-3-methyl, 3-ethoxy-4-methoxy, 3,4-dibenzyloxy, 3-benzyloxy-4-methoxy, 4-benzyloxy-3-methoxy) were prepared and copolymerized with styrene. The monomers were synthesized by the piperidine catalyzed Knoevenagel condensation of ring-substituted benzaldehydes and propyl cyanoacetate and characterized by CHN elemental analysis, IR, 1H- and 13C-NMR. All the ethylenes were copolymerized with styrene (M1) in solution with radical initiation (ABCN) at 70°C. The composition of the copolymers was calculated from nitrogen analysis, and the structures were analyzed by IR, 1H and 13C-NMR, GPC, DSC, and TGA. Decomposition of the copolymers in nitrogen occurred in two steps, first in the 200–500°C range with residue (3–22% wt.), which then decomposed in the 500–800°C range. 相似文献