全文获取类型
收费全文 | 1383篇 |
免费 | 58篇 |
国内免费 | 4篇 |
专业分类
化学 | 941篇 |
晶体学 | 4篇 |
力学 | 19篇 |
数学 | 132篇 |
物理学 | 349篇 |
出版年
2023年 | 7篇 |
2022年 | 21篇 |
2021年 | 26篇 |
2020年 | 23篇 |
2019年 | 16篇 |
2018年 | 19篇 |
2017年 | 13篇 |
2016年 | 48篇 |
2015年 | 36篇 |
2014年 | 46篇 |
2013年 | 61篇 |
2012年 | 76篇 |
2011年 | 94篇 |
2010年 | 47篇 |
2009年 | 56篇 |
2008年 | 80篇 |
2007年 | 75篇 |
2006年 | 95篇 |
2005年 | 78篇 |
2004年 | 60篇 |
2003年 | 39篇 |
2002年 | 46篇 |
2001年 | 29篇 |
2000年 | 21篇 |
1999年 | 12篇 |
1998年 | 17篇 |
1997年 | 15篇 |
1996年 | 15篇 |
1995年 | 14篇 |
1994年 | 22篇 |
1993年 | 41篇 |
1992年 | 35篇 |
1991年 | 12篇 |
1990年 | 14篇 |
1989年 | 6篇 |
1988年 | 6篇 |
1987年 | 5篇 |
1986年 | 10篇 |
1985年 | 13篇 |
1984年 | 13篇 |
1983年 | 6篇 |
1982年 | 14篇 |
1981年 | 4篇 |
1979年 | 10篇 |
1978年 | 7篇 |
1977年 | 7篇 |
1975年 | 3篇 |
1974年 | 3篇 |
1973年 | 8篇 |
1970年 | 5篇 |
排序方式: 共有1445条查询结果,搜索用时 15 毫秒
51.
J. L. Martínez Vidal A. Garrido Frenich T. López López I. Martínez Salvador L. Hajjaj el Hassani M. Hassan Benajiba 《Chromatographia》2005,61(3-4):127-131
An analytical method for determining residues of twenty pesticides by liquid chromatography (LC) coupled to electrospray ionisation (ESI) tandem mass spectrometry (MS-MS) in eight commodities, cucumber, tomato, pepper, green bean, eggplant, zucchini, melon and watermelon, has been developed and validated. On one hand, calibration curves prepared in solvent were compared with calibration curves prepared in a blank matrix extract of each target matrix. On the other hand, calibration curves and recoveries for each commodity were compared. Cucumber was selected as potential reference matrix for the target vegetables. 相似文献
52.
Salvador Guardiola Monica Varese Xavier Roig Macarena Snchez-Navarro Jesús García Ernest Giralt 《Chemical science》2021,12(14):5164
Peptides are a rapidly growing class of therapeutics with various advantages over traditional small molecules, especially for targeting difficult protein–protein interactions. However, current structure-based methods are largely limited to natural peptides and are not suitable for designing bioactive cyclic topologies that go beyond natural l-amino acids. Here, we report a generalizable framework that exploits the computational power of Rosetta, in terms of large-scale backbone sampling, side-chain composition and energy scoring, to design heterochiral cyclic peptides that bind to a protein surface of interest. To showcase the applicability of our approach, we developed two new inhibitors (PD-i3 and PD-i6) of programmed cell death 1 (PD-1), a key immune checkpoint in oncology. A comprehensive biophysical evaluation was performed to assess their binding to PD-1 as well as their blocking effect on the endogenous PD-1/PD-L1 interaction. Finally, NMR elucidation of their in-solution structures confirmed our de novo design approach. In silico design of heterochiral cyclic peptides that bind to a specific surface patch on the target protein (PD-1, in this case) and disrupt protein–protein interactions. 相似文献
53.
An experimental study of the thermal decomposition of powdered cellulose and pine sawdust has been performed. The influence of the heating rate on the temperature profiles in the sample and on the solid conversion rate has been studied. A mathematical model without adjustable parameters has been used to calculate the temperature at different points in the solid bed and the average total solid conversion. The experimental results have been compared with those calculated by the model. A good agreement has been obtained for sawdust. Some differences are observed for cellulose at high heating rates, and the influence of the thermal conductivity and the reaction heat on the results has been analysed. 相似文献
54.
Osuna RM Ortiz RP Ruiz Delgado MC Sakamoto Y Suzuki T Hernandez V López Navarrete JT 《The journal of physical chemistry. B》2005,109(44):20737-20745
In this Article, we report on the synthesis and full characterization of three perfluorinated oligothiophenes, ranging in length from the trimer to the pentamer (PF-nT, with n = 3, 4, or 5). The differential pulse voltammetry (DPV) analysis of the compounds showed that they can be both oxidized and reduced (i.e., they display a dual or amphoteric electrochemical behavior), with the reduction peaks positively shifted relative to those of the corresponding unsubstituted oligothiophenes. The electrochemically determined energy gaps are in agreement with those measured from the UV-vis-NIR absorption spectra in solution. The conjugational properties have been investigated by means of FT-Raman spectroscopy, both as pure solids and as dilute solutes in CH(2)Cl(2), revealing that: (i) pi-conjugation does not still reach saturation with chain length for the longest oligomer, and (ii) conformational distortions from a nearly coplanar arrangement of the successive thiophene units upon solution are not too large. DFT and TDDFT quantum chemical calculations have been performed, at the B3LYP/6-31G level, to assess information about the optimized molecular structure, equilibrium atomic charges distribution, energies and topologies of the frontier molecular orbitals (MO) around the gap, vibrational normal modes associated with the most outstanding Raman scatterings, and vertical one-electron excitations that give rise to the main optical absorptions. 相似文献
55.
Monitoring multi-class pesticide residues in fresh fruits and vegetables by liquid chromatography with tandem mass spectrometry 总被引:6,自引:0,他引:6
Frenich AG Vidal JL López TL Aguado SC Salvador IM 《Journal of chromatography. A》2004,1048(2):199-206
A new analytical method was developed using liquid chromatography with tandem mass spectrometry for the routine analysis of 31 multi-class pesticide residues and applied to approximately 50 fresh fruit and vegetable samples (green bean, cucumber, pepper, tomato, eggplant, watermelon, melon and zucchini). Extraction of the pesticides with ethyl acetate was carried out. The optimal ionisation conditions were selected for each pesticide in the same run. The procedure was validated and the values of some merit figures, such as recovery, precision, linear range, detection limit and quantification limit for each pesticide were calculated together with its calculated expanded uncertainty (U). The average recoveries in cucumber obtained for each pesticide ranged between 74 and 105% at two different fortification levels (n = 10 each) that ranged between 9 and 250 ng g(-1) (depending on the pesticide). The uncertainty associated to the analytical method was lower than 23% for all compounds tested. The calculated limits of detection and quantitation were typically <1 ng g(-1) that were much lower than the maximum residue levels established by European legislation. 相似文献
56.
Two series of dendrimers containing a single ferrocene unit located in the focal point of these macromolecules have been synthesized and characterized. The first series of dendrimers has considerable lipophilic character, with tert-butyl ester groups located in their peripheral regions. In contrast, the second series of dendrimers was obtained by the hydrolysis of these peripheral ester groups, yielding water-soluble dendrimers with carboxylic acid groups in their surfaces. The electrochemical properties of these macromolecules were strongly affected by the dendritic groups attached to the ferrocene subunits. Host-guest interactions between the water-soluble dendrimers and the well-known receptor beta-cyclodextrin were also investigated. The dendritic groups were found to hamper markedly the formation of inclusion complexes between the cyclodextrin receptor and the dendrimer's ferrocene unit. 相似文献
57.
X. Bokhimi A. Morales E. Ortíz T. López R. Gómez J. Navarrete 《Journal of Sol-Gel Science and Technology》2004,29(1):31-40
Sol-gel titania was sulfated by using sulfuric acid as hydrolysis catalyst, or by impregnating with ammonium sulfate fresh samples prepared with nitric acid or ammonium hydroxide as hydrolysis catalyst. Samples were characterized with X-ray powder diffraction, infrared spectroscopy, thermogravimetry and atomic absorption spectroscopy. Sulfate ions were found anchored to brookite and anatase phases, because they have short O—O atomic bond lengths slightly larger than the largest O—O bond length of sulfate ion. Since the shortest O—O atomic bond in anatase is smaller than the one in brookite, the sulfate ions are then less deformed when they are anchored to anatase, and consequently more stable. Therefore when the sample temperature is raised, the brookite with sulfate ions was transformed mainly to anatase and not into rutile, which is the most probably transformation when these ions are not involved. Sulfate ions also hindered anatase and brookite crystallite growing and stabilized the crystalline structure of anatase. When the sulfate ions are lost the crystalline anatase phase is transformed into rutile, leaving a large number of vacancies that favored atom diffusion and consequently the growing of rutile crystallites. The crystalline evolution of the samples as a function of the annealing temperature is almost independent of the sulfating method. 相似文献
58.
Gloria C. Lin Maria L. Tsoukas Margaret S. Lee Salvador González Chitralada Vibhagool R. Rox Anderson Nikiforos Kollias 《Photochemistry and photobiology》1998,68(4):575-583
In an ideal world, photodynamic therapy (PDT) of abnormal tissue would reliably spare the surrounding normal tissue. Normal tissue responses set the limits for light and drug dosimetry. The threshold fluence for necrosis (TFN) was measured in normal skin following intravenous infusion with a photosensitizer, benzoporphyrin derivative monoacid ring A (BPD-MA) Verteporfin as a function of drug dose (0.25-2.0 mg/kg), wavelength of irradiation (458 and 690 nm) and time interval (0–5h) between drug administration and irradiation. The BPD-MA levels were measured in plasma and skin tissue to elucidate the relationship between TFN, drug kinetics and biodistribution. The PDT response of normal skin was highly reproducible. The TFN for 458 and 690 nm wavelengths was nearly identical and the estimated quantum efficiency for skin response was equal at these two wavelengths. Skin phototoxicity, quantified in terms of 1/ TFN, closely correlated with the plasma pharmacokinetics rather than the tissue pharmacokinetics and was quadratically dependent on the plasma drug concentration regardless of the administered drug dose or time interval between drug and light exposure. This study strongly suggests that noninvasive measurements of the circulating drug level at the time of light treatment will be important for setting optimal light dosimetry for PDT with liposomal BPD-MA, a vascular photosensitizer. 相似文献
59.
An analytical study on the use of graphite-epoxy composite (GEC) electrodes for differential pulse anodic stripping voltammetry (DPASV) of heavy metals is presented. This study is accompanied by microscopic observations of the electrode surface before and after the stripping step in comparison to glassy carbon electrode. GEC electrodes show much better accumulation properties and consequently acceptable behaviour which makes them suitable as working electrodes in the DPASV of heavy metals. Lead determination in real water samples in a batch system as well as some preliminary results in a flow-through system are presented. The detection limits in batch measurements were 100ppb for Cd, 10pb for lead and 50ppb for copper. The detection limit for lead in the flow-through system was similar to that in the batch. The results obtained show that these low cost and easy to prepare materials can be of interest in future research concerning stripping techniques of heavy metals and other analytes. 相似文献
60.
Canac Y Conejero S Donnadieu B Schoeller WW Bertrand G 《Journal of the American Chemical Society》2005,127(20):7312-7313
(Amino)(silyl)carbenes, prepared by substitution reactions at a carbene center, can survive several days at 0 degrees C. These species are not push-pull carbenes as their phosphino analogues and therefore are excellent ligands for transition-metal centers. 相似文献