全文获取类型
收费全文 | 2036篇 |
免费 | 55篇 |
国内免费 | 202篇 |
专业分类
化学 | 1881篇 |
晶体学 | 8篇 |
力学 | 54篇 |
综合类 | 25篇 |
数学 | 50篇 |
物理学 | 275篇 |
出版年
2024年 | 5篇 |
2023年 | 41篇 |
2022年 | 109篇 |
2021年 | 112篇 |
2020年 | 69篇 |
2019年 | 51篇 |
2018年 | 37篇 |
2017年 | 72篇 |
2016年 | 89篇 |
2015年 | 58篇 |
2014年 | 67篇 |
2013年 | 203篇 |
2012年 | 148篇 |
2011年 | 106篇 |
2010年 | 65篇 |
2009年 | 101篇 |
2008年 | 117篇 |
2007年 | 125篇 |
2006年 | 102篇 |
2005年 | 80篇 |
2004年 | 87篇 |
2003年 | 63篇 |
2002年 | 57篇 |
2001年 | 19篇 |
2000年 | 25篇 |
1999年 | 29篇 |
1998年 | 27篇 |
1997年 | 28篇 |
1996年 | 19篇 |
1995年 | 23篇 |
1994年 | 26篇 |
1993年 | 14篇 |
1992年 | 27篇 |
1991年 | 13篇 |
1990年 | 26篇 |
1989年 | 11篇 |
1988年 | 18篇 |
1987年 | 6篇 |
1986年 | 2篇 |
1985年 | 2篇 |
1984年 | 6篇 |
1983年 | 3篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1978年 | 1篇 |
1962年 | 1篇 |
排序方式: 共有2293条查询结果,搜索用时 15 毫秒
61.
A one-dimensional stress-based elasticity model with limited strain extensibility is developed in this paper, based on thermodynamics arguments. Such nonlinear elastic models can be used to model certain rubber-like and biological materials with limiting chain extensibility. The derived constitutive function is a non-smooth piecewise expression, which can be regularized for numerical or physical considerations. This non-smooth constitutive expression is derived from a Gibbs potential. A three-dimensional extension of this stress-based model is also proposed in the paper. Some simple structural examples are investigated for a bar composed of this non-smooth elastic body. A homogeneous bar composed of this new class of nonlinear elastic material that is loaded is studied for different tension states, namely for concentrated or distributed axial loading. It is shown that the displacement limit extensibility can be observed at the structural scale, with finite or infinite axial load parameters. 相似文献
62.
This Trends article highlights the recent advances published between 2012 and 2015 in solid-state 17O NMR for organic and biological molecules. New developments in the following areas are described: (1) new oxygen-containing functional groups, (2) metal organic frameworks, (3) pharmaceuticals, (4) probing molecular motion in organic solids, (5) dynamic nuclear polarization, and (6) paramagnetic coordination compounds. For each of these areas, the author offers his personal views on important problems to be solved and possible future directions. 相似文献
63.
H.R. Manzano-Ayala 《Analytica chimica acta》2009,632(1):109-114
A simple and rapid method for the determination of DNA, involving the interaction between a surfactant, a long-wavelength fluorophor (LWF) and the nucleic acid, is presented. Different chemical systems based on the local effective charge of the surfactant/LWF system with DNA were tested, choosing cetyltrimethyl ammonium bromide (CTAB) and indocyanine green (ICG) for the development of the method. The fluorescence of ICG increases in the presence of CTAB, but it rapidly decreases in the presence of deoxyribonucleic acid. The initial reaction-rate (v0) and signal at a prefixed-time (ΔIF20) are monitored at 780 and 802 nm as excitation and emission wavelengths, respectively, using stopped-flow mixing technique, which makes the method applicable to automate routine analysis. Each measurement was obtained in about 30 s, being the integration time 0.1 s. The dynamic range of the calibration graph was 10-1500 ng mL−1, with a detection limit of 5 ng mL−1. The precision of the method, expressed as relative standard deviation, ranged between 2.1% and 4.5%. After a sample treatment consisting on a conventional extraction, the method was applied to the determination of DNA in several samples from different biological materials. 相似文献
64.
The cloud point extraction (CPE) preconcentration of ultra-trace amount of mercury species prior to reverse-phase high performance liquid chromatography (HPLC) with inductively coupled plasma mass spectrometry (ICP-MS) detection was studied. Mercury species including methyl-, ethyl-, phenyl- and inorganic mercury were transformed into hydrophobic chelates by reaction with sodium diethyldithiocarbamate, and the hydrophobic chelates were extracted into a surfactant-rich phase of Triton X-114 upon heating in a water bath at 40 °C. Ethylmercury was found partially decomposed during the CPE process, and was not included in the developed method. Various experimental conditions affecting the CPE preconcentration, HPLC separation, and ICP-MS determination were optimized. Under the optimized conditions, detection limits of 13, 8 and 6 ng l−1 (as Hg) were achieved for MeHg+, PhHg+ and Hg2+, respectively. Seven determinations of a standard solution containing the three mercury species each at 0.5 ng ml−1 level produced relative standard deviations of 5.3, 2.3 and 4.4% for MeHg+, PhHg+ and Hg2+, respectively. The developed method was successfully applied for the determination of the three mercury species in environmental water samples and biological samples of human hair and ocean fish. 相似文献
65.
Devsharan Verma 《Talanta》2009,78(1):270-65
The present paper deals with a new micro-extraction procedure for selective separation of Cr(VI) in the form of a metaloxy anionic species namely dichromate (Cr2O72−) with N1-hydroxy-N1,N2-diphenylbenzamidine (HOA) in to dichloromethane and its subsequent and rapid diffuse reflectance Fourier transform infrared spectroscopic (DRS-FTIR) determination employing potassium bromide matrix. The diffuse reflectance Fourier transform infrared spectroscopy gives both qualitative and quantitative information about the dichromate. The determination of chromium is based on the analytical peak selection, among the various vibrational peaks, at 902 cm−1. The micro-extraction was based on the liquid-liquid solvent extraction (LLSE) principle. The dichromate binds with the nitrogen and oxygen atoms of N1-hydroxy-N1,N2-diphenylbenzamidine (HOA) and forms 1:2, Cr(VI):HOA complex in 0.1 mol L−1 HCl medium. The formation of above complex, in the acidic medium, is confirmed by the appearance of chocolate-brown color in the micro-extract. The speciation studies of Cr(III) and Cr(VI) is done by conversion of Cr(III) into Cr(VI) utilizing H2O2 as an oxidizing agent. The chemistry of pure dichromate and that of its HOA complex is discussed. The limit of detection (LoD) and the limit of quantification (LoQ) of the method are found to be 0.01 μg g−1 Cr2O72− and 0.05 μg g−1 Cr2O72−, respectively. The standard deviation value and the relative standard value at a level of 10 μg Cr2O72−/0.1 g KBr for n = 10 is found to be 0.26 μg Cr2O72− and 2.6%, respectively. The relative standard deviation (n = 8 and 6) for the determination of dichromate (Cr2O72−) in real human biological fluid samples is observed to be in the range 3.1-7.8%. 相似文献
66.
The steady state tube furnace (Purser furnace, ISO TS 19700) has been developed specifically to replicate the generation of toxic products from real fires under different fire conditions on a bench-scale. Steady state burning is achieved by driving the sample into a furnace of increasing heat flux at a fixed rate and recording the product yields over a steady state period in the middle of the run. The furnace, sample, and effluent dilution chamber temperature profiles are presented to characterise the conditions in the apparatus. The distribution of smoke in the mixing chamber has been investigated to demonstrate the efficiency of mixing in the effluent dilution chamber. The heat flux applied to the sample at various points through the furnace has been measured, showing that conditions vary from those typical of pre-flaming to fully developed fires. An initial investigation of the repeatability and interlaboratory reproducibility has been undertaken, showing acceptable low levels of uncertainty in the toxic product yields. 相似文献
67.
17O solid state NMR of organic materials is developing rapidly. This article provides a snapshot of the current state of development of this field. The NMR techniques and enrichment protocols that are driving this progress are outlined. The 17O parameters derived from solid-state NMR experiments are summarized and the structural sensitivity of the approach to effects such as hydrogen bonding highlighted. The prospects and challenges for 17O solid-state NMR of biomolecules are discussed. 相似文献
68.
利用高分辨魔角旋转核磁共振(MAS 1H NMR)技术对腹腔注射不同剂量[2, 10, 50 mg/kg(体重)]的硝酸镧[La(NO3)3]和硝酸铈[Ce(NO3)3] 的雄性Wistar大鼠肝、肾组织的MAS 1H NMR谱进行比较分析, 研究了La(NO3)3和Ce(NO3)3的急性生物效应, 并结合模式识别技术对不同剂量La(NO3)3和Ce(NO3)3的急性生物效应进行了分类. 研究结果表明, La(NO3)3对大鼠的急性毒性主要表现为肝毒, Ce(NO3)3对大鼠肝、肾同时造成损伤. 该方法可用于其它稀土及金属化合物的毒性预测和毒理学研究. 相似文献
69.
70.
Fumi ShozuiKen’ichiro Matsumoto Ren MotohashiJian Sun Toshifumi SatohToyoji Kakuchi Seiichi Taguchi 《Polymer Degradation and Stability》2011,96(4):499-504
A poly(lactic acid) (PLA)-like terpolyester consisting of 96 mol% lactate (LA), 1 mol% 3-hydroxybutyrate and 3 mol% 3-hydroxyvalerate was produced in recombinant Escherichia coli LS5218 expressing LA-polymerizing enzyme (LPE). The strain was grown on glucose with a feeding of valerate as the monomer precursor. The glass transition and melting temperatures of the terpolyester were close to those of chemically synthesized poly(L-LA)s (PLLAs) having similar molecular weights. Additionally, a blend of the terpolyester, which was composed entirely of (R)-LA (D-LA) due to the strict enantiospecificity of LPE, with PLLA formed a stereocomplex with higher melting temperature (201.9 °C). These results indicate that the biological PLA-like polyester produced via this one-step microbial process has comparable thermal properties to chemically synthesized PLAs. 相似文献