全文获取类型
收费全文 | 445篇 |
免费 | 49篇 |
国内免费 | 36篇 |
专业分类
化学 | 290篇 |
晶体学 | 5篇 |
力学 | 7篇 |
综合类 | 3篇 |
数学 | 56篇 |
物理学 | 169篇 |
出版年
2023年 | 2篇 |
2022年 | 9篇 |
2021年 | 8篇 |
2020年 | 16篇 |
2019年 | 7篇 |
2018年 | 9篇 |
2017年 | 6篇 |
2016年 | 15篇 |
2015年 | 17篇 |
2014年 | 12篇 |
2013年 | 19篇 |
2012年 | 17篇 |
2011年 | 20篇 |
2010年 | 13篇 |
2009年 | 19篇 |
2008年 | 26篇 |
2007年 | 25篇 |
2006年 | 21篇 |
2005年 | 29篇 |
2004年 | 20篇 |
2003年 | 15篇 |
2002年 | 14篇 |
2001年 | 15篇 |
2000年 | 13篇 |
1999年 | 16篇 |
1998年 | 7篇 |
1997年 | 7篇 |
1996年 | 8篇 |
1995年 | 4篇 |
1994年 | 7篇 |
1993年 | 10篇 |
1992年 | 7篇 |
1991年 | 5篇 |
1990年 | 4篇 |
1989年 | 4篇 |
1988年 | 4篇 |
1987年 | 4篇 |
1986年 | 7篇 |
1985年 | 6篇 |
1984年 | 9篇 |
1983年 | 4篇 |
1982年 | 3篇 |
1980年 | 3篇 |
1979年 | 4篇 |
1978年 | 5篇 |
1977年 | 6篇 |
1976年 | 6篇 |
1975年 | 6篇 |
1974年 | 6篇 |
1972年 | 6篇 |
排序方式: 共有530条查询结果,搜索用时 15 毫秒
511.
Shirley K. García-Castillo Sonia Sánchez-SáezEnrique Barbero 《Mechanics Research Communications》2011,38(5):404-407
The purpose of this paper is to evaluate the behaviour of structural thin plates made of 7075-T6 aluminium alloy under normal impact of spherical steel projectiles, when they are subjected to uniaxial in-plane tensile preload. Also, nonloaded plates were tested under high-velocity impact. The impact and residual velocity were measured in all tests. From the relationships between the impact and residual velocities, the ballistic limit was estimated by a least-squares method. No significant differences were found between the ballistic limits under the two loaded conditions. Above the ballistic limit in the preloaded plates, unstable cracks that can generate the catastrophic failure of the plate were observed. At similar velocities, this phenomenon was not observed in nonloaded plates. 相似文献
512.
Holographic pixel super-resolution in portable lensless on-chip microscopy using a fiber-optic array
Bishara W Sikora U Mudanyali O Su TW Yaglidere O Luckhart S Ozcan A 《Lab on a chip》2011,11(7):1276-1279
We report a portable lensless on-chip microscope that can achieve <1 μm resolution over a wide field-of-view of ~ 24 mm(2) without the use of any mechanical scanning. This compact on-chip microscope weighs ~ 95 g and is based on partially coherent digital in-line holography. Multiple fiber-optic waveguides are butt-coupled to light emitting diodes, which are controlled by a low-cost micro-controller to sequentially illuminate the sample. The resulting lensfree holograms are then captured by a digital sensor-array and are rapidly processed using a pixel super-resolution algorithm to generate much higher resolution holographic images (both phase and amplitude) of the objects. This wide-field and high-resolution on-chip microscope, being compact and light-weight, would be important for global health problems such as diagnosis of infectious diseases in remote locations. Toward this end, we validate the performance of this field-portable microscope by imaging human malaria parasites (Plasmodium falciparum) in thin blood smears. Our results constitute the first-time that a lensfree on-chip microscope has successfully imaged malaria parasites. 相似文献
513.
Fister TT Seidler GT Shirley EL Vila FD Rehr JJ Nagle KP Linehan JC Cross JO 《The Journal of chemical physics》2008,129(4):044702
New theoretical and experimental investigations of the occupied and unoccupied local electronic densities of states (DOS) are reported for alpha-Li(3)N. Band-structure and density-functional theory calculations confirm the absence of covalent bonding character. However, real-space full-multiple-scattering (RSFMS) calculations of the occupied local DOS find less extreme nominal valences than have previously been proposed. Nonresonant inelastic x-ray scattering, RSFMS calculations, and calculations based on the Bethe-Salpeter equation are used to characterize the unoccupied electronic final states local to both the Li and N sites. There is a good agreement between experiment and theory. Throughout the Li 1s near-edge region, both experiment and theory find strong similarities in the s-and p-type components of the unoccupied local final DOS projected onto an orbital angular momentum basis (l-DOS). An unexpected, significant correspondence exists between the near-edge spectra for the Li 1s and N 1s initial states. We argue that both spectra are sampling essentially the same final DOS due to the combination of long core-hole lifetimes, long photoelectron lifetimes, and the fact that orbital angular momentum is the same for all relevant initial states. Such considerations may be generally applicable for low atomic number compounds. 相似文献
514.
515.
516.
O'Connell MA de Cuendias A Gayet F Shirley IM Mackenzie SR Haddleton DM Unwin PR 《Langmuir : the ACS journal of surfaces and colloids》2012,28(17):6902-6910
Evanescent wave cavity ring-down spectroscopy (EW-CRDS) has been employed to study the interfacial adsorption kinetics of coumarin-tagged macromolecules onto a range of functionalized planar surfaces. Such studies are valuable in designing polymers for complex systems where the degree of interaction between the polymer and surface needs to be tailored. Three tagged synthetic polymers with different functionalities are examined: poly(acrylic acid) (PAA), poly(3-sulfopropyl methacrylate, potassium salt) (PSPMA), and a mannose-modified glycopolymer. Adsorption transients at the silica/water interface are found to be characteristic for each polymer, and kinetics are deduced from the initial rates. The chemistry of the adsorption interfaces has been varied by, first, manipulation of silica surface chemistry via the bulk pH, followed by surfaces modified by poly(L-glutamic acid) (PGA) and cellulose, giving five chemically different surfaces. Complementary atomic force microscopy (AFM) imaging has been used for additional surface characterization of adsorbed layers and functionalized interfaces to allow adsorption rates to be interpreted more fully. Adsorption rates for PSPMA and the glycopolymer are seen to be highly surface sensitive, with significantly higher rates on cellulose-modified surfaces, whereas PAA shows a much smaller rate dependence on the nature of the adsorption surface. 相似文献
517.
Shirley LawNathan Reading 《Journal of Combinatorial Theory, Series A》2012,119(3):788-824
We define and study a combinatorial Hopf algebra dRec with basis elements indexed by diagonal rectangulations of a square. This Hopf algebra provides an intrinsic combinatorial realization of the Hopf algebra tBax of twisted Baxter permutations, which previously had only been described extrinsically as a Hopf subalgebra of the Malvenuto-Reutenauer Hopf algebra of permutations. We describe the natural lattice structure on diagonal rectangulations, analogous to the Tamari lattice on triangulations, and observe that diagonal rectangulations index the vertices of a polytope analogous to the associahedron. We give an explicit bijection between twisted Baxter permutations and the better-known Baxter permutations, and describe the resulting Hopf algebra structure on Baxter permutations. 相似文献
518.
Analyses of cytokinins are very important in both plant physiological and biomedical research as they are implicated in many biological processes. Reliable, sensitive, selective and inexpensive methods that are flexible and designed for automation are required for these analyses. This review addresses the advances made in the separation and determination of cytokinins by CE as well as the other applications of CE (i.e., determination of dissociation constants and complexation constants of cytokinins). The various CE modes used to separate the compounds and the quantification strategies are examined. Special attention is also focused on those aspects that improve on the sensitivity and/or selectivity, such as sample extraction and preconcentration, on-line preconcentration techniques (stacking), and/or specific detectors (e.g., MS). With the coupling to the preconcentration techniques and certain detection systems, numerous CE methods can potentially be adapted for the analysis of cytokinins in complex biological samples. Therefore, we would anticipate wider applications of CE methods in the near future for cytokinin analyses, which should facilitate a decrease in analysis cost and should help to improve analysis efficiency. 相似文献
519.
Patterson-Elenbaum S Stanley JT Dillner DK Lin S Traficante D 《Magnetic resonance in chemistry : MRC》2006,44(8):797-806
13C NMR Substituent chemical shift (SCS) increments have been determined for the carbonyl carbon of a variety of substituted benzaldehydes and acetophenones. The 13C NMR chemical shift of the carbonyl carbon can be predicted for many di- and trisubstituted benzaldehydes and acetophenones through simple additivity of the SCS increments. The magnitude and sign of the SCS increments have been explored using Hartree-Fock 6-31G* calculations to determine the natural atomic charges of the carbonyl carbon. When a substituent capable of intermolecular hydrogen bonding is present, deviations from additivity on the order of 2 ppm are observed in dilution experiments; deviations of up to 6 ppm can result from intramolecular hydrogen bonding. 相似文献
520.
Permethrin is the most popular synthetic pyrethroid insecticide used in agriculture and public health. For the assessment
of human exposure to permethrin, a competitive indirect enzyme-linked immunosorbent assay (ELISA) for the detection of the
glycine conjugate of a major metabolite, cis-/trans-3-(2,2-dichlorovinyl)-2,2-dimethylcyclopropane-1-carboxylic acid (DCCA), of permethrin was developed based on a polyclonal
antibody. An assay based on an antibody with a high sensitivity was optimized and characterized. The IC50 value and the detection range for trans-DCCA–glycine, in the assay buffer were 1.2 and 0.2−7.0 μg/L, respectively. The antibody recognized trans-DCCA–glycine and the mixture of cis-/trans-DCCA–glycine with an isomer range from 30:70 to 50:50 nearly equally. Little or no cross-reactivity to permethrin and its
other free metabolites or glycine conjugates was measured. The integration of the ELISA and solid-phase extraction which was
used to reduce the matrix effect from human urine samples provided for analysis of total cis-/trans-DCCA–glycine at low parts per billion levels in the samples. The limit of quantitation of the target analyte was 1.0 μg/L
in urine with a limit of detection of 0.1 μg/L in buffer. This assay might be a useful tool for monitoring human exposure
to permethrin. 相似文献