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排序方式: 共有242条查询结果,搜索用时 15 毫秒
41.
This paper reports the feasibility of nano-oxide precipitate formation in Fe–Cr alloy by ion implantation synthesis. High contents of Al+ and O+ ions were implanted into thin films of high purity Fe–10%Cr alloy at room temperature and were studied by transmission electron microscopy (TEM) and atom probe tomography (APT). In contrast, to the common two-stage implantation/annealing scheme of precipitate ensemble synthesis by ion beams, cluster formation took place at the implantation stage in our study, requiring no subsequent high-temperature annealing. The post-implantation microstructural examination revealed in the as-implanted thin foil an array of precipitates with diameters in the range of 3–30?nm. The precipitate number density distribution was found to depend on the foil thickness. The precipitate enrichment with both Al and O was confirmed by the energy-filtered TEM analysis. Judging from the electron diffraction pattern and high-resolution TEM analysis, the crystal lattice of precipitates corresponds to some cubic modification of aluminium-rich oxide or pure aluminium oxide. The precipitate lattice alignment with the host matrix was revealed for at least a part of precipitates. The analysis of APT data using cluster detection algorithm indicates the presence of local zones enriched in Al and O, even in those areas of as-implanted samples where no clusters were visible by TEM. 相似文献
42.
Pédrot M Dia A Davranche M Bouhnik-Le Coz M Henin O Gruau G 《Journal of colloid and interface science》2008,325(1):187-197
Organic or inorganic colloids play a major role in the mobilization of trace elements in soils and waters. Environmental physicochemical parameters (pH, redox potential, temperature, pressure, ionic strength, etc.) are the controlling factors of the colloidal mobilization. This study was dedicated to follow the colloid-mediated mobilization of trace elements through time at the soil/water interface by means of an experimental approach. Soil column experiments were carried out using percolating synthetic solutions. The percolated solutions were ultrafiltrated with various decreasing cutoff thresholds to separate the different colloidal phases in which the dissolved organic carbon and trace element concentrations were measured. The major results which stem from this study are the following: (i) The data can be divided into different groups of organic compounds (microbial metabolites, fulvic acids, humic acids) with regard to their respective aromaticity and molecular weight. (ii) Three groups of elements can be distinguished based on their relationships with the colloidal phases: the first one corresponds to the so-called "truly" dissolved group (Li, B, K, Na, Rb, Si, Mg, Sr, Ca, Mn, Ba, and V). The second one can be considered as an intermediate group (Cu, Cd, Co, and Ni), while the third group gathers Al, Cr, U, Mo, Pb, Ti, Th, Fe, and rare earth elements (REE) carried by the organic colloidal pool. (iii) The data demonstrate that the fulvic acids seem to be a major organic carrier phase for trace elements such as Cu, Cd, Co, and Ni. By contrast, the trace elements belonging to the so-called colloidal pool were mostly mobilized by humic acids containing iron nanoparticles. Lead, Ti, and U were mobilized by iron nanoparticles bound to these humic acids. Thus, humic substances allowed directly or indirectly a colloidal transport of many insoluble trace elements either by binding trace elements or by stabilizing a ferric carrier phase. (iv) Finally, the results demonstrated also that REE were mostly mobilized by humic substances. The REE normalized patterns showed a middle REE downward concavity. Therefore, as previously shown elsewhere humic substances are a major control of REE speciation and REE fractionation patterns as well since the humic substance/metal ratio was the key parameter controlling the REE pattern shape. 相似文献
43.
Leduc AM Salameh A Soulivong D Chabanas M Basset JM Copéret C Solans-Monfort X Clot E Eisenstein O Böhm VP Röper M 《Journal of the American Chemical Society》2008,130(19):6288-6297
The surface complex [([triple bond]SiO)Re([triple bond]CtBu)(=CHtBu)(CH2tBu)] (1) is a highly efficient propene metathesis catalyst with high initial activities and a good productivity. However, it undergoes a fast deactivation process with time on stream, which is first order in active sites and ethene. Noteworthy, 1-butene and pentenes, unexpected products in the metathesis of propene, are formed as primary products, in large amount relative to Re (>1 equiv/Re), showing that their formation is not associated with the formation of inactive species. DFT calculations on molecular model systems show that byproduct formation and deactivation start by a beta-H transfer trans to the weak sigma-donor ligand (siloxy) at the metallacyclobutane intermediate having a square-based pyramid geometry. This key step has an energy barrier slightly higher than that calculated for olefin metathesis. After beta-H transfer, the most accessible pathway is the insertion of ethene in the Re-H bond. The resulting pentacoordinated trisperhydrocarbyl complex rearranges via either (1) alpha-H abstraction yielding the unexpected 1-butene byproduct and the regeneration of the catalyst or (2) beta-H abstraction leading to degrafting. These deactivation and byproduct formation pathways are in full agreement with the experimental data. 相似文献
44.
A rational basis for the axial ligand effect in C-H oxidation by [MnO(porphyrin)(X)]+ (X = H2O, OH-, O2-) from a DFT study 总被引:1,自引:0,他引:1
Oxyl radical character in the MnO group of the title system is shown from a density functional theory study to be essential for efficient C-H cleavage, which is a key step in C-H oxidation. Since oxyl species have elongated Mn-O bonds relative to the more usual oxo species of type MnO, the normal expectation would be that high trans-influence ligands X should facilitate oxyl character by elongating the Mn-O bond and thus enhance both oxyl character and reactivity. Contrary to this expectation, but in line with the experimental data (Jin, N.; Ibrahim, M.; Spiro, T. G.; Groves, J. T. J. Am. Chem. Soc. 2007, 129, 12416), we find that reactivity increases along the series X = O(2-) < OH(-) < H2O for the following reasons. The ground-state singlet (S) is unreactive for all X, and only the higher-energy triplet (T) and quintet (Q) states have the oxyl character needed for reactivity, but the higher trans-influence X ligands are also shown to increase the S/T and S/Q gaps, thus making attainment of the needed T and Q states harder. The latter effect is dominant, and high trans-influence X ligands thus disfavor reaction. The higher reactivity in the presence of acid noted by Groves and co-workers is thus rationalized by the preference for having X = H2O over OH(-) or O(2-). 相似文献
45.
Poater A Solans-Monfort X Clot E Copéret C Eisenstein O 《Journal of the American Chemical Society》2007,129(26):8207-8216
Density functional theory (DFT, B3PW91) calculations have been carried out on the reactivity of ethene with model systems M(NR)(=CHCH3)(X)(Y) for M = Mo or W, R = methyl or phenyl, X = CH2CH3, OCH3, or OSiH3, and Y = CH2CH3, OCH3, or OSiH3, which are representative of experimental olefin metathesis catalysts, and the results are compared to those previously obtained for Re(CCH3)(=CHCH3)(X)(Y). The general pathway comprises four steps: olefin coordination, [2+2] cycloaddition, cycloreversion, and olefin de-coordination. Two key factors have been found to control the detailed shape of the energy profiles: the energy of distortion of the tetrahedral catalyst and the stability of the metallacycle intermediate, which is controlled by the M-C bond strength. The efficiency has been evaluated by calculating the turnover frequency (TOF) based on the steady-state approximation, and the most striking feature is that the unsymmetrical catalysts (X not equal to Y) are systematically more efficient for all systems (Mo, W, and Re). Overall, the Re complexes have been found to be less efficient than the Mo and W catalysts, except when Re is unsymmetrically substituted: it is then calculated to be as efficient as the best Mo and W catalysts. 相似文献
46.
Collinet P Sabban F Cosson M Farine MO Villet R Vinatier D Mordon S 《Photochemistry and photobiology》2007,83(3):647-651
The goal of this study was to assess the interest of photodynamic diagnosis (PDD) for laparoscopic detection of peritoneal micro metastasis in ovarian carcinoma. Using an experimental animal model, intraperitoneal injection of aminolevulinic acid (ALA) and hexylester of aminolevulinic acid (He-ALA) were compared in order to improve laparoscopic detection of ovarian peritoneal carcinomatosis. Twenty-one 344 Fischer female rats received an intra peritoneal injection of 106 NuTu-19 cells. At day 22, carcinomatosis with micro peritoneal metastasis was obtained. Rats were randomized in three groups concerning intra peritoneal injection before laparoscopic staging: 5-ALA hydrochloride, HE-ALA and sterile water. Using D Light system, laparoscopic peritoneal exploration was performed with white light (WL) first and then with blue light (BL). The main objective was to assess feasibility and sensibility of laparoscopic PDD for nonvisible peritoneal micro metastasis of ovarian cancer. The main parameter was the confirmation of neoplasic status of fluorescent foci by histology. Concerning PDD after intraperitoneal injection of 5-ALA, mean values of lesions seen is higher than without fluorescence (32 vs 20.7; P = 0.01). Using He-ALA, mean values of detected lesions is higher than without fluorescence (42.9 vs 33.6; P < 0.001). Neoplasic status of fluorescent foci was confirmed in 92.8% of cases (39/42). Using 5-ALA, fluorescence of cancerous tissue is significantly higher than that of normal tissue in all the rats (ratio 1.17) (P = 0.01). With He-ALA, intensity of fluorescence is significantly higher in cancerous tissue compared to normal tissue, irrespective of the rat studied (ratio 1.22; P < 0.001). 相似文献
47.
Dominique Collin Günter K. Auernhammer Odile Gavat Philippe Martinoty Helmut R. Brand 《Macromolecular rapid communications》2003,24(12):737-741
We describe the preparation and characterization of uniaxial magnetic gels. Fibril formation of the embedded magnetic particles generates easily detectable magnetic and optical anisotropies. A finite magnetization is frozen‐in and leads to a ferromagnetic‐like response in small homogeneous external magnetic fields. We present, for the first time, frequency dependent measurements of the shear modulus G′. Despite their optical and magnetic anisotropy, the gels are mechanically isotropic.
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50.
Camille Piovesan Anne-Laurence Dupont Isabelle Fabre-Francke Odile Fichet Bertrand Lavédrine Hervé Chéradame 《Cellulose (London, England)》2014,21(1):705-715
Two di-alkoxysilanes, with (AMDES, aminopropylmethyldiethoxysilane) or without (DMDES, dimethyldiethoxysilane) an amine function, and a tri-alkoxy aminosilane (APTES, aminopropyltriethoxysilane) as well as their mixtures were introduced in paper as fiber strengthening agents. The polymerization and copolymerization of these polysiloxanes in the paper were investigated. In all the cases where APTES was present, the formation of networks was established by measuring the soluble fraction amount extracted from the treated papers. A slight decrease of the opacity of the paper sheets when AMDES was part of the treatment was noted. The presence of APTES reduced this opacity loss. The study of the physicochemical properties of the treated paper (mechanical strength and alkalinity) demonstrated that, besides the required deacidification feature, the different treatments allowed an efficient strengthening of the cellulose fibers to various extents. Contact angle measurements indicated a decrease of the hydrophilic character of papers treated with the mixture APTES/AMDES and the occurrence of a hydrophobic character of the papers treated with APTES alone. These results were consistently obtained for both spray and immersion treatment processes. 相似文献