全文获取类型
收费全文 | 1008篇 |
免费 | 71篇 |
国内免费 | 12篇 |
专业分类
化学 | 581篇 |
晶体学 | 9篇 |
力学 | 33篇 |
数学 | 48篇 |
物理学 | 131篇 |
无线电 | 289篇 |
出版年
2023年 | 6篇 |
2022年 | 9篇 |
2021年 | 16篇 |
2020年 | 7篇 |
2019年 | 14篇 |
2018年 | 13篇 |
2017年 | 14篇 |
2016年 | 27篇 |
2015年 | 35篇 |
2014年 | 37篇 |
2013年 | 62篇 |
2012年 | 66篇 |
2011年 | 63篇 |
2010年 | 44篇 |
2009年 | 51篇 |
2008年 | 77篇 |
2007年 | 46篇 |
2006年 | 69篇 |
2005年 | 58篇 |
2004年 | 37篇 |
2003年 | 46篇 |
2002年 | 47篇 |
2001年 | 24篇 |
2000年 | 29篇 |
1999年 | 25篇 |
1998年 | 21篇 |
1997年 | 12篇 |
1996年 | 21篇 |
1995年 | 10篇 |
1994年 | 9篇 |
1993年 | 8篇 |
1992年 | 5篇 |
1991年 | 12篇 |
1990年 | 9篇 |
1989年 | 4篇 |
1988年 | 6篇 |
1987年 | 8篇 |
1985年 | 4篇 |
1984年 | 7篇 |
1983年 | 5篇 |
1982年 | 3篇 |
1981年 | 9篇 |
1977年 | 4篇 |
1973年 | 1篇 |
1971年 | 1篇 |
1970年 | 1篇 |
1968年 | 1篇 |
1967年 | 1篇 |
1966年 | 1篇 |
1900年 | 1篇 |
排序方式: 共有1091条查询结果,搜索用时 31 毫秒
181.
Hooi Ling Kee Jayeeta Bhaumik James R. Diers Pawel Mroz Michael R. Hamblin David F. Bocian Jonathan S. Lindsey Dewey Holten 《Journal of photochemistry and photobiology. A, Chemistry》2008,200(2-3):346-355
The photophysical properties of four imidazolium-substituted metalloporphyrins have been assessed to gain insights into the relative efficacy of the compounds for photodynamic therapy (PDT). A set of zinc(II), palladium(II), and chloro-indium(III) porphyrins all bear a net positive charge owing to the diethylimidazolium unit; one zinc chelate bears a negative charge owing to a bis(sulfobutyl)imidazolium unit. The photophysical properties of the cationic and anionic zinc porphyrins are very similar to one another in organic solvents, phosphate-buffered saline, and in the presence of bovine serum albumin. The properties of the zinc and palladium porphyrins bearing charged peripheral groups are generally similar to those of neutral analogs in organic solvents. The palladium porphyrin shows an essentially quantitative yield (≥0.99) of the triplet excited state compared to the zinc porphyrins (0.9), and all are quantitatively quenched (at the diffusion limit) by molecular oxygen in air-saturated fluid solution. If the rate constant and yield of quenching of the triplet excited state by energy or electron transfer to molecular oxygen is the same in the cellular environment as in solution, then these processes combined with the triplet yield contribute only a factor of 1.3 to the higher PDT activity of analogous palladium versus zinc porphyrins, which is much smaller than what is observed. Therefore, other factors such as transient reduction of the excited porphyrin or delivery to the target site must predominantly underlie the difference in PDT efficacy of these sensitizers. 相似文献
182.
Pyrolysis of the organometallic polymer [Ru(CO) 4] infinity affords metallic ruthenium nanofibers. The molecular structure, especially the presence of metal-metal bonds, appears to direct the aggregation of the metal atom chains upon loss of the carbonyl ligands. 相似文献
183.
Poulsen A William A Lee A Blanchard S Teo E Deng W Tu N Tan E Sun E Goh KL Ong WC Ng CP Goh KC Bonday Z 《Journal of computer-aided molecular design》2008,22(12):897-906
The Aurora family of serine/threonine kinases are mitotic regulators involved in centrosome duplication, formation of the
bipolar mitotic spindle and the alignment of the chromosomes along the spindle. These proteins are frequently overexpressed
in tumor cells as compared to normal cells and are therefore potential therapeutic oncology targets. An Aurora A high throughput
screen revealed a promising sub-micromolar indazole-benzimidazole lead. Modification of the benzimidazole portion of the lead
to a C2 linker with a phenyl ring was proposed to achieve novelty. Docking revealed that a conjugated linker was optimal and
the resulting compounds were equipotent with the lead. Further structure-guided optimization of substituents on the 5 & 6
position of the indazole led to single digit nanomolar potency. The homology between the Aurora A & Aurora B kinase domains
is 71% but their binding sites only differ at residues 212 & 217 (Aurora A numbering). However interactions with only the
latter residue may be used for obtaining selectivity. An analysis of published Aurora A and Aurora B X-ray structures reveals
subtle differences in the shape of the binding sites. This was exploited by introduction of appropriately sized substituents
in the 4 & 6 position of the indazole leading to Aurora B selective inhibitors. Finally we calculate the conformational energy
penalty of the putative bioactive conformation of our inhibitors and show that this property correlates well with the Aurora
A binding affinity. 相似文献
184.
Theoretical Modelling and Facile Synthesis of a Highly Active Boron‐Doped Palladium Catalyst for the Oxygen Reduction Reaction 下载免费PDF全文
Tat Thang Vo Doan Jingbo Wang Kee Chun Poon Desmond C. L. Tan Bahareh Khezri Prof. Richard D. Webster Prof. Haibin Su Prof. Hirotaka Sato 《Angewandte Chemie (International ed. in English)》2016,55(24):6842-6847
A highly active alternative to Pt electrocatalysts for the oxygen reduction reaction (ORR), which is the cathode‐electrode reaction of fuel cells, is sought for higher fuel‐cell performance. Our theoretical modelling reveals that B‐doped Pd (Pd‐B) weakens the absorption of ORR intermediates with nearly optimal binding energy by lowering the barrier associated with O2 dissociation, suggesting Pd‐B should be highly active for ORR. In fact, Pd‐B, facile synthesized by an electroless deposition process, exhibits 2.2 times and 8.8 times higher specific activity and 14 times and 35 times less costly than commercial pure Pd and Pt catalysts, respectively. Another computational result is that the surface core level of Pd is negatively shifted by B doping, as confirmed by XPS, and implies that filling the density of states related to the anti‐bonding of oxygen to Pd surfaces with excess electrons from B doping, weakens the O bonding to Pd and boosts the catalytic activity. 相似文献
185.
186.
Determination of haloacetic acids in water using layered double hydroxides as a sorbent in dispersive solid‐phase extraction followed by liquid chromatography with tandem mass spectrometry 下载免费PDF全文
Abdulnaser Alsharaa Muhammad Sajid Chanbasha Basheer Khalid Alhooshani Hian Kee Lee 《Journal of separation science》2016,39(18):3610-3615
In the present study, highly efficient and simple dispersive solid‐phase extraction procedure for the determination of haloacetic acids in water samples has been established. Three different types of layered double hydroxides were synthesized and used as a sorbent in dispersive solid‐phase extraction. Due to the interesting behavior of layered double hydroxides in an acidic medium (pH?4), the analyte elution step was not needed; the layered double hydroxides are simply dissolved in acid immediately after extraction to release the analytes which are then directly introduced into a liquid chromatography with tandem mass spectrometry system for analysis. Several dispersive solid‐phase extraction parameters were optimized to increase the extraction efficiency of haloacetic acids such as temperature, extraction time and pH. Under optimum conditions, good linearity was achieved over the concentration range of 0.05–100 μg/L with detection limits in the range of 0.006–0.05 μg/L. The relative standard deviations were 0.33–3.64% (n = 6). The proposed method was applied to different water samples collected from a drinking water plant to determine the concentrations of haloacetic acids. 相似文献
187.
188.
This study demonstrates a versatile and fast method for patterning three-dimensional (3D) monolithic microstructures made of multiple (up to 24 demonstrated) types of materials, all spatially aligned, inside a microchannel. This technique uses confocal scanning or conventional fluorescence microscopy to polymerize selected regions of a photocurable material, and microfluidics to automate the delivery of a series of washes and photocurable reagents. Upon completion of lithographic cycles, the aligned 3D microstructures are suitable for microfluidic manipulation and analysis. We demonstrated the fabrication of composite 3D microstructures with various geometries, size scales (up to 1 mm2), spatial resolution (down to 3 microm), and materials. For a typical multi-cycle process, the total fabrication time was tens of minutes, compared to tens of hours for conventional methods. In the case of 3D hydrogels, a potential use is the direct patterning of inhomogeneous 3D microenvironments for studying cell behavior. 相似文献
189.
Hydroxylated uroporphyrin I and urochlorin I derivatives formed by photochemical oxidation of uroporphyrinogen I were separated by high-performance liquid chromatography and fully characterized by electrospray ionization tandem mass spectrometry. The porphyrins and chlorins were identified by analysis of their product ion spectra with each hydroxylated derivative giving a characteristic collision-induced dissociation fragmentation pattern. The porphyrins and chlorins characterized were meso-hydroxyuroporphyrin I, alpha-hydroxypropionic acid uroporphyrin I, beta-hydroxypropionic acid uroporphyrin I, hydroxyacetic acid uroporphyrin I, trans-7-hydroxy-8-spirolactoneurochlorin I, cis-7-hydroxy-8-spirolactoneurochlorin I and trans- and cis-7,8-dihydroxyurochlorins I. 相似文献
190.
The reaction of [Cp*RuCl2]2 (1; Cp* = eta5-C5Me5) with tetraalkyldithiuram disulfides (R2NC(S)SS(S)CNR2, R = Me, Et), isopropylxanthic disulfide ([iPrOC(S)S]2), and bis(thiophosphoryl) disulfide ([(iPrO)2P(S)S]2) led to the isolation of dark-red crystalline solids of Cp*RuIVCl2(eta2-dithiolate) complexes [dithiolate = S2CNR2, DTCR (2a, R = Me; 2b, R = Et), S2COiPr (3), and S2P(iPrO)2 (4)]. Dichlorido substitution in 2 and 3 with DTCEt and S2COiPr anions yielded RuIV derivatives containing bis(DTC) and mixed DTC-dithiocarbonate ligands. These are the first organoruthenium complexes of such ligands. The reaction of monophosphines with 2a resulted in monochlorido substitution, whereas the analogous reaction with 3 resulted in displacement of both chlorido ligands and reduction of the metal center to RuII. Reduction at Ru was also observed in the reaction of 2a with [CpCr(CO)3]2. Of these complexes, only 2 and 3 are air-stable in the solid state for an extended period. All of the complexes have been spectrally characterized, and selected compounds are also crystallographically characterized. 相似文献