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A quality management system has been designed for process analyses in wastewater plants. The quality management concept consists of a quality manual, quality control charts used together with certified reference materials, and proficiency testing. Certified reference materials have been used to eliminate basic analytical errors. Proficiency testing has been conducted by VKI, the Reference Laboratory for Environmental Chemistry, subcontracted by the Danish Environmental Protection Agency. In the first proficiency test the analytical quality was comparable in general to the quality of routine environmental laboratories. One of the simplified analytical systems resulted, however, in a low recovery of nitrate in wastewater. The COD results reported in the proficiency test were slightly higher than the theoretical values. 相似文献
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Darla Graff Thompson Jill C. Osborn Jon R. Schoonover 《Polymer Degradation and Stability》2006,91(12):3360-3370
A polyester polyurethane, was subjected to humid and dry aging conditions at 70 °C with 75% and 0% relative humidity, respectively. Differences in molecular weight and quasi-static tensile strength between humid- and dry-aged samples are attributed to hydrolysis of the humid-aged polymers. A phase-separation study was performed on selected samples from the aging matrix. Polymer samples were subjected to 110 °C for 10 min, by mixing the polyester (soft) and the polyurethane (hard) domains, then rapidly cooled to room temperature, initiating the phase-separation process. Uniaxial tension, dynamic shear and infrared spectra of these samples were measured as a function of time providing insight into the effects of hydrolytic degradation and the relationship of mechanical and molecular-level properties. An Avrami-type analysis shows two distinct processes whose characteristics vary as a function of increased hydrolysis. LA-UR 04-6447. 相似文献
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Stevens MM Allen S Sakata JK Davies MC Roberts CJ Tendler SJ Tirrell DA Williams PM 《Langmuir : the ACS journal of surfaces and colloids》2004,20(18):7747-7752
The coiled-coil protein motif occurs in over 200 proteins and has generated interest for a range of applications requiring surface immobilization of the constituent peptides. This paper describes an investigation of the environment-responsive behavior of a monolayer of surface-immobilized artificial proteins, which are known to assemble to form coiled-coil structures in bulk solution. An extended version of the quartz crystal microbalance (QCM-D) and surface plasmon resonance (SPR) are independently employed to characterize the adsorption of the proteins to a gold surface. The data suggest that the molecules arrange in a closely packed layer orientated perpendicular to the surface. QCM-D measurements are also employed to measure pH-induced changes in the resonant frequency (f) and the energy dissipation factor (D) of a gold-coated quartz crystal functionalized with the formed monolayer. Exposure of the protein monolayer to a pH 4.5 solution results in a shift of 43 Hz in f and a shift of -0.7 x 10(-6) in D as compared to pH 7.4. In contrast, increasing the pH to 11.2, results in f and D shifts of -17 Hz and 0.6 x 10(-6), respectively. The magnitude of the observed shifts suggests that the proteins form a rigid layer at low pH that can be hydrated to a fluid layer as the pH is increased. These observations correlate with spectroscopic changes that indicate a reduction in the helical content of the protein in bulk solutions of high pH. 相似文献
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Summary Molecular dynamics simulation and free energy perturbation techniques have been used to study the relative binding free energies of 8-methylpterins and 8-methyl-N5-deazapterins to dihydrofolate reductase (DHFR). Methyl-substitution at the 5, 6 and 7 positions in the N-heterocyclic ring gives rise to a variety of ring substituent patterns and biological activity: several of these methyl derivatives of the 8-methyl parent compounds (8-methylpterin and 8-methyl-N5-deazapterin) have been identified as substrates or inhibitors of vertebrate DHFR in previous work. The calculated free energy differences reveal that the methyl-substituted compounds are thermodynamically more stable than the primary compounds (8-methylpterin and 8-methyl-N5-deazapterin) when bound to the enzyme, due largely to hydrophobic hydration phenomena. Methyl substitution at the 5 and/or 7 positions in the 6-methyl-substituted compounds has only a small effect on the stability of ligand binding. Furthermore, repulsive interactions between the 6-methyl substituent and DHFR are minimal, suggesting that the 6-methyl position is optimal for binding. The results also show that similarly substituted 8-methylpterins and 8-methyl-N5-deazapterins have very similar affinities for binding to DHFR. The computer simulation predictions are in broad agreement with experimental data obtained from kinetic studies, i.e. 6,8-dimethylpterin is a more efficient substrate than 8-methylpterin and 6,8-dimethyl-N5-deazapterin is a better inhibitor than 8-methyl-N5-deazapterin. 相似文献
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Bhikhabhai R Sjöberg A Hedkvist L Galin M Liljedahl P Frigård T Pettersson N Nilsson M Sigrell-Simon JA Markeland-Johansson C 《Journal of chromatography. A》2005,1080(1):83-92
A new chromatography system, AKTAxpress (GE Healthcare, Amersham Biosciences, Uppsala, Sweden) has been designed to meet the demand for high-throughput purification of proteins in structural genomics and drug discovery. The system offers a number of automated multistep purification protocols for affinity-tagged proteins. All protocols start with affinity chromatography followed by combinations of desalting, ion exchange chromatography and gel filtration. As an option, tag removal can be included in the purification protocols. Up to 16 proteins can be purified per day and the yield can be as high as 50 mg of each protein at > 90% purity. To highlight the versatility of the system, this paper presents several case studies; purifications of hexahistidine- and glutathione S-transferase-tagged proteins using different protocols, automated on-column tag cleavage and optimization studies for a hexahistidine-tagged kinase. 相似文献
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Dative bonding of nitrogen-containing heterocycles offers a strategy for the controlled attachment of aromatic molecules to silicon surfaces. However, while scanning tunneling microscopy shows that pyridine on clean Si(100) initially binds via a dative bonding configuration, slow conversion to a more stable bridging state, destroying the aromaticity, is observed. To restrict adsorption to the dative bonded form, we investigated the interaction of pyridine with isolated reactive sites on partially H-terminated Si(100). While dative bonding on isolated clean dimers is observed, single dangling bonds remain unreacted. This selectivity can be accounted for by the ability of the Si-Si dimers to act as electron acceptors that stabilize the dative bonded species. This observation has important implications for the controlled positioning of single molecules on silicon via dative bonding. 相似文献
70.
The following experiment reinforces students working knowledge of statistics by utilizing the t test to compare the results of two independent methods for the determination of dissolved oxygen (DO). In this experiment students utilize a dissolved oxygen probe to determine the levels of DO in natural waters at two sampling locations while obtaining samples of water from the laboratory for analysis using the classic Winkler titration. The importance of using proper sampling methods and techniques to obtain representative samkples is a large focus of the prelaboratory discussion and is continually stressed during fieldwork. After analyzing the water samples by the DO mete and the Winkler titration, students pool the class data and are asked to determine if the two methods for dissolved oxygen agree at each sampling location. The students are then asked to determine if the DO levels at the different sampling locations are statistically different or not. The students are asked to consider why their results agree or differ from the theoretical value they calculate using Henrys law. 相似文献