全文获取类型
收费全文 | 16917篇 |
免费 | 419篇 |
国内免费 | 117篇 |
专业分类
化学 | 12018篇 |
晶体学 | 98篇 |
力学 | 330篇 |
数学 | 2921篇 |
物理学 | 2086篇 |
出版年
2022年 | 86篇 |
2021年 | 140篇 |
2020年 | 167篇 |
2019年 | 169篇 |
2018年 | 136篇 |
2017年 | 130篇 |
2016年 | 278篇 |
2015年 | 303篇 |
2014年 | 309篇 |
2013年 | 717篇 |
2012年 | 726篇 |
2011年 | 948篇 |
2010年 | 473篇 |
2009年 | 419篇 |
2008年 | 867篇 |
2007年 | 887篇 |
2006年 | 910篇 |
2005年 | 974篇 |
2004年 | 866篇 |
2003年 | 721篇 |
2002年 | 633篇 |
2001年 | 208篇 |
2000年 | 203篇 |
1999年 | 173篇 |
1998年 | 168篇 |
1997年 | 217篇 |
1996年 | 272篇 |
1995年 | 180篇 |
1994年 | 176篇 |
1993年 | 165篇 |
1992年 | 166篇 |
1991年 | 154篇 |
1990年 | 151篇 |
1989年 | 135篇 |
1988年 | 146篇 |
1987年 | 155篇 |
1986年 | 121篇 |
1985年 | 269篇 |
1984年 | 239篇 |
1983年 | 186篇 |
1982年 | 255篇 |
1981年 | 208篇 |
1980年 | 265篇 |
1979年 | 231篇 |
1978年 | 235篇 |
1977年 | 245篇 |
1976年 | 206篇 |
1975年 | 167篇 |
1974年 | 160篇 |
1973年 | 154篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
Mario Wolf Alexey Rybakov Richard Hinterding Armin Feldhoff 《Entropy (Basel, Switzerland)》2020,22(11)
Besides the material research in the field of thermoelectrics, the way from a material to a functional thermoelectric (TE) module comes alongside additional challenges. Thus, comprehension and optimization of the properties and the design of a TE module are important tasks. In this work, different geometry optimization strategies to reach maximum power output or maximum conversion efficiency are applied and the resulting performances of various modules and respective materials are analyzed. A Bi2Te3-based module, a half-Heusler-based module, and an oxide-based module are characterized via FEM simulations. By this, a deviation of optimum power output and optimum conversion efficiency in dependence of the diversity of thermoelectric materials is found. Additionally, for all modules, the respective fluxes of entropy and charge as well as the corresponding fluxes of thermal and electrical energy within the thermolegs are shown. The full understanding and enhancement of the performance of a TE module may be further improved. 相似文献
992.
Interactive music uses wearable sensors (i.e., gestural interfaces—GIs) and biometric datasets to reinvent traditional human–computer interaction and enhance music composition. In recent years, machine learning (ML) has been important for the artform. This is because ML helps process complex biometric datasets from GIs when predicting musical actions (termed performance gestures). ML allows musicians to create novel interactions with digital media. Wekinator is a popular ML software amongst artists, allowing users to train models through demonstration. It is built on the Waikato Environment for Knowledge Analysis (WEKA) framework, which is used to build supervised predictive models. Previous research has used biometric data from GIs to train specific ML models. However, previous research does not inform optimum ML model choice, within music, or compare model performance. Wekinator offers several ML models. Thus, we used Wekinator and the Myo armband GI and study three performance gestures for piano practice to solve this problem. Using these, we trained all models in Wekinator and investigated their accuracy, how gesture representation affects model accuracy and if optimisation can arise. Results show that neural networks are the strongest continuous classifiers, mapping behaviour differs amongst continuous models, optimisation can occur and gesture representation disparately affects model mapping behaviour; impacting music practice. 相似文献
993.
994.
Richard A. Fleming 《Macromolecular Symposia》1992,57(1):75-93
The technology of separating plastics from the municipal waste stream for reuse is reviewed, with emphasis on the key role played by the bottle sorting step. Experience gained in the first year's operation of two large commercial recycling plants provides a better understanding of practical problems encountered, their relative importance, and technical advances needed for improved operation. It is concluded that purity of feedstock and processing rate are two critical independent variables requiring technical and business understanding and management. Hypothetical examples illustrate the interplay of these with dependent variables of yield and sorting cost. 相似文献
995.
New 19F NMR methodology reveals structures of molecules in complex mixtures of fluorinated compounds
Although the number of natural fluorinated compounds is very small, fluorinated pharmaceuticals and agrochemicals are numerous. 19F NMR spectroscopy has a great potential for the structure elucidation of fluorinated organic molecules, starting with their production by chemical or chemoenzymatic reactions, through monitoring their structural integrity, to their biotic and abiotic transformation and ultimate degradation in the environment. Additionally, choosing to incorporate 19F into any organic molecule opens a convenient route to study reaction mechanisms and kinetics. Addressing limitations of the existing 19F NMR techniques, we have developed methodology that uses 19F as a powerful spectroscopic spy to study mixtures of fluorinated molecules. The proposed 19F-centred NMR analysis utilises the substantial resolution and sensitivity of 19F to obtain a large number of NMR parameters, which enable structure determination of fluorinated compounds without the need for their separation or the use of standards. Here we illustrate the 19F-centred structure determination process and demonstrate its power by successfully elucidating the structures of chloramination disinfectant by-products of a single mono-fluorinated phenolic compound, which would have been impossible otherwise. This novel NMR approach for the structure elucidation of molecules in complex mixtures represents a major contribution towards the analysis of chemical and biological processes involving fluorinated compounds. 19F-centred NMR structure determination protocol alleviates the need for compound separation. Disinfection byproducts of chloramination were unraveled by analyzing the reaction pathways of a single fluorinated molecule. 相似文献
996.
Roselis A. Landaeta Aponte Andreas Luxenburger Scott A. Cameron Alex Weymouth-Wilson Richard H. Furneaux Lawrence D. Harris Benjamin J. Compton 《Molecules (Basel, Switzerland)》2022,27(7)
Bile acid receptors have been identified as important targets for the development of new therapeutics to treat various metabolic and inflammatory diseases. The synthesis of new bile acid analogues can help elucidate structure–activity relationships and define compounds that activate these receptors selectively. Towards this, access to large quantities of a chenodeoxycholic acid derivative bearing a C-12 methyl and a C-13 to C-14 double bond provided an interesting scaffold to investigate the chemical manipulation of the C/D ring junction in bile acids. The reactivity of this alkene substrate with various zinc carbenoid species showed that those generated using the Furukawa methodology achieved selective α-cyclopropanation, whereas those generated using the Shi methodology reacted in an unexpected manner giving rise to a rearranged skeleton whereby the C ring has undergone contraction to form a novel spiro–furan ring system. Further derivatization of the cyclopropanated steroid included O-7 oxidation and epimerization to afford new bile acid derivatives for biological evaluation. 相似文献
997.
Shane M. Hickey Shani K. Tripcony Rui Li Richard J. Williams 《Supramolecular chemistry》2015,27(5-6):425-435
Using the rigid norbornane scaffold, a series of low-molecular-weight organogelators has been synthesised and evaluated. Three separate compounds (16, 19 and 20) were identified as organogelators in three aromatic organic solvents (PhMe, anisole and o-xylene). The formation of fibrillar assemblies at nanometre level was confirmed using atomic force microscopy and transmission electron microscopy. 相似文献
998.
Perchlorate ion (ClO4−) is an environmental contaminant of growing concern due to its potential human health effects, impact on aquatic and land animals, and widespread occurrence throughout the United States. The determination of perchlorate cannot normally be carried out in the field. As such, water samples for perchlorate analysis are often shipped to a central laboratory, where they may be stored for a significant period before analysis. The stability of perchlorate ion in various types of commonly encountered water samples has not been generally examined—the effect of such storage is thus not known. In the present study, the long-term stability of perchlorate ion in deionized water, tap water, ground water, and surface water was examined. Sample sets containing approximately 1000, 100, 1.0, and 0.5 μg l−1 perchlorate ion in deionized water and also in local tap water were formulated. These samples were analyzed by ion chromatography for perchlorate ion concentration against freshly prepared standards every 24 h for the first 7 days, biweekly for the next 4 weeks, and periodically after that for a total of 400 or 610 days for the two lowest concentrations and a total of 428 or 638 days for the high concentrations. Ground and surface water samples containing perchlorate were collected, held and analyzed for perchlorate concentration periodically over at least 360 days. All samples except for the surface water samples were found to be stable for the duration of the study, allowing for holding times of at least 300 days for ground water samples and at least 90 days for surface water samples. 相似文献
999.
Soft single-photon ionisation (SPI)–time-of-flight mass spectrometry (TOFMS) has been used to investigate the effect of different
cigarette-lighting devices on the chemical composition of the mainstream smoke from the first cigarette puff. Lighting devices
examined were a Borgwaldt electric lighter, a propane/butane gas lighter, a match, a candle, and the burning zone of another
cigarette. To eliminate the effects of the different masses of tobacco burnt by use of the different lighting methods a normalisation
procedure was performed which enabled investigation of changes in the chemical patterns of the resulting smoke. When another
cigarette was used as the lighting device, elevated levels of ammonia and other nitrogen-containing substances were observed.
These are high in the sidestream smoke of the cigarette used for lighting and would be drawn into the mainstream smoke of
the cigarette being lit. In contrast, smoke from the cigarette lit by the electric lighter contained slightly higher normalised
amounts of isoprene. Lighting the cigarette by use of a candle resulted in larger amounts of substances, e.g. benzene, which
most probably originated from thermal decomposition of wax. The composition of the first puff of smoke obtained by use of
the three lighting methods with open flames (gas lighter, match, and candle) was usually similar whereas the composition of
the smoke produced by use of the electric lighter and the cigarette as the lighter were more unique. The chemical patterns
generated by the different lighting devices could, however, be separated by principal-component analyses. Two additional test
series were also studied. In the first the cigarette was lit with an electric lighter, then extinguished, the ash was cut
off, and the cigarette was re-lit. In the second the cigarette was heated in an oven to 80 °C for 5 min before being lit.
These treatments did not result in changes in the chemical composition compared with cigarettes lit in the ordinary way.
Figure Time-of-flight mass spectrometry (TOFMS) has been used to investigate the effect of different cigarette-lighting devices on
the chemical composition of the mainstream smoke from the first cigarette puff 相似文献
1000.
Goodchild I Collier L Millar SL Prokes I Lord JC Butts CP Bowers J Webster JR Heenan RK 《Journal of colloid and interface science》2007,307(2):455-468
The surface, phase and aggregation behaviour of mixtures of 1-alkyl-3-methylimidazolium halide, [C(n)mim]X, where n is the alkyl chain length, with water has been explored using a variety of methods. Critical micelle concentrations (cmc) and micelle structures have been determined for aqueous [C(n)mim]Br solutions for n=2, 4, 6, 8, and 10. Small-angle neutron scattering (SANS) measurements reveal that for the n=8 and 10 systems, at concentrations just above the cmc, small near-spherical aggregates exist, which, after initial growth, possess core radii (aggregation numbers) at intermediate concentrations of 10.5+/-0.5 Angstrom (22+/-2) and 13.2+/-0.5 Angstrom (40+/-3), respectively, for n=8 and n=10. Towards higher concentrations, the aggregates appear to grow, with the aggregates in the [C(10)mim]Br system becoming increasingly elongated (prolate) with increasing concentration. No evident aggregates are formed in the systems with n=2 and 4. In the n=6 system, it appears that oblate aggregates with radius approximately 9 Angstrom form at the cmc and that the radius increases with increasing concentration. For longer alkyl chain lengths, at high concentrations lyotropic mesophases form in some systems. The mesophase region for the [C(8)mim]Cl system has been explored across the composition range using X-ray diffraction and (2)H NMR spectroscopy. Both techniques suggest that a major hexagonal phase with lattice parameter of 29.5+/-0.5 Angstrom coexists with a minor lamellar phase (23.5+/-0.3 Angstrom) or possibly a second hexagonal phase (27.1+/-0.4 Angstrom). The area per adsorbed molecule at the surface of [C(8)mim]Br solutions has been measured as a function of concentration using neutron reflectometry. A minimum in the area per molecule behaviour is coincident with a minimum identified in the surface tension isotherm occurring close to the cmc. The data suggest depletion of [C(8)mim]Br from the surface region occurs at concentrations immediately above the cmc. 相似文献