全文获取类型
收费全文 | 3856篇 |
免费 | 170篇 |
国内免费 | 8篇 |
专业分类
化学 | 2705篇 |
晶体学 | 54篇 |
力学 | 136篇 |
数学 | 357篇 |
物理学 | 782篇 |
出版年
2023年 | 23篇 |
2022年 | 52篇 |
2021年 | 87篇 |
2020年 | 79篇 |
2019年 | 66篇 |
2018年 | 67篇 |
2017年 | 61篇 |
2016年 | 135篇 |
2015年 | 109篇 |
2014年 | 158篇 |
2013年 | 237篇 |
2012年 | 332篇 |
2011年 | 313篇 |
2010年 | 212篇 |
2009年 | 174篇 |
2008年 | 260篇 |
2007年 | 254篇 |
2006年 | 223篇 |
2005年 | 189篇 |
2004年 | 167篇 |
2003年 | 125篇 |
2002年 | 139篇 |
2001年 | 67篇 |
2000年 | 76篇 |
1999年 | 39篇 |
1998年 | 22篇 |
1997年 | 23篇 |
1996年 | 21篇 |
1995年 | 15篇 |
1994年 | 19篇 |
1993年 | 24篇 |
1992年 | 13篇 |
1991年 | 15篇 |
1990年 | 19篇 |
1989年 | 22篇 |
1988年 | 14篇 |
1987年 | 16篇 |
1986年 | 10篇 |
1985年 | 9篇 |
1984年 | 11篇 |
1983年 | 8篇 |
1982年 | 8篇 |
1981年 | 10篇 |
1980年 | 16篇 |
1979年 | 15篇 |
1978年 | 19篇 |
1977年 | 11篇 |
1976年 | 9篇 |
1975年 | 8篇 |
1973年 | 6篇 |
排序方式: 共有4034条查询结果,搜索用时 15 毫秒
991.
This work describes the conditions, in terms of dimensions and geometry, to guide a dc glow discharge preferentially through a microchannel in simple networks. Two- and three-channel microfluidic structures were studied. A preference towards a wider channel, in a two-channel network, was observed when the difference in width was at least 18% and the length was at least 10 mm. In a three-channel structure, a change in glow discharge intensity was observed when the network was downscaled from a pathlength of 2 to 0.5 cm. While the intensity within the path with fewer turns decreased with a reduction in size, the intensity of the path with greater number of turns increased. 相似文献
992.
Kim K Yang H Park SH Lee DS Kim SJ Lim YT Kim YT 《Chemical communications (Cambridge, England)》2004,(13):1466-1467
A new electrochemical DNA detection method using double-stranded probes and competitive hybridization reaction offers highly selective discrimination of single base mismatch without post-hybridization washing. 相似文献
993.
Formation constants for the 1:1 complexation equilibrium reactions between β-cyclodextrin and the drugs, clonazepam, delorazepam, diazepam (Valium), flurazepam (Dalmane), lorazepam, medazepam, nitrazepam, nordiazepam, oxazepam, and temazepam, were measured by using circular dichroism spectropolarimetry. The range in values is almost one order of magnitude, suggesting a strong structural dependence in the interlocking between the host and different guests. Enthalpy and entropy data for the lorazepam interaction are consistent with data from several earlier studies. Direct determinations of diazepam and flurazepamm in pharmaceutical products are within ±2% of the prescribed amounts. 相似文献
994.
Teik-Cheng Lim 《Journal of mathematical chemistry》2006,39(1):177-186
The use of differential equations for modeling chemical systems and solving by numerical approaches (e.g. finite difference
methods) are prevalent in chemistry-related problems. As an extension to the direct use of Pascal’s Triangle to obtain the
forward and backward difference equations to partial differentials by Lim [Mathematical Medley 31 (2004) 2], this paper proposes
the use of binomial coefficient to generate central difference equations to odd-ordered partial differentials in a single-step
operation. All finite difference equations to partial differentials shown herein display finite series of palindromic coefficients
with alternating signs 相似文献
995.
Zirconia is known to be one of the best materials for the chromatographic support due to its excellent chemical, thermal, and mechanical stability. In this work, we report preparation and use of 9-O-(phenyloxycarbonyl)quinine-bonded carbon-clad zirconia (QNCZ) as a chiral stationary phase (CSP) for separation of N-(2,4-dinitrophenyl) (DNP)-amino acids (AAs) enantiomers in reversed-phase liquid chromatography. Retention and enantioselectivity of the QNCZ CSP were compared with those of quinine 3-triethoxysilylpropylcarbamate-coated zirconia (QNZ) and quinine 3-triethoxysilylpropylcarbamate-bonded silica (QNS). The QNCZ CSP showed in general the better enantioselectivity for most of the amino acids studied. 相似文献
996.
This paper describes the development of a mass spectrometer-based, intelligent, programmable, sample-selection data acquisition system with two unique features. One is that the system allows automatic determination of the mass to charge ratio (m/z) of an unknown compound and the utilization of the molecular ion information to perform selective ion monitoring (SIM) experiments for quantitation. The other is its decision-making capability to select intelligently different samples and perform different experiments during data acquisition. These features were demonstrated by the application of the system to simultaneous screening for the microsomal stability and metabolite profiling of adatanserin. In this application, the data acquisition system continuously calculated the peak areas of adatanserin from SIM analyses of a batch of microsomal incubates stopped at various time points. Once the peak area of adatanserin had dropped to an arbitrarily predefined 60% of the initial value, the system made a decision to perform metabolite profiling of the sample. This decision initiated a series of automated operations, such as selecting a sample for re-analysis, changing the data acquisition time and liquid chromatographic gradient and switching the SIM mode to the data-dependent product ion scanning mode. The completed analysis of the batch of samples provided information both on the microsomal stability and on the metabolic profile of adatanserin. This simultaneous approach to investigating microsomal stability and metabolite profiling significantly increases the throughput for drug discovery support. 相似文献
997.
Kim WS Kim J Park JK Mukamel S Rhee SK Choi YK Lee JY 《The journal of physical chemistry. B》2005,109(7):2686-2692
We investigate the structures, NMR chemical shifts, absorption spectra, frontier molecular orbitals, and transition density matrices of pi-stacked polyfluorenes by ab initio calculations. For F1-F4, we consider two different conformations, syn and anti. The simulated 1H NMR chemical shifts are in good agreement with the previous experiment, and the significantly upfielded chemical shifts explain that the fluorene moieties are stacked on each other. It is found that the relative stability for syn and anti conformers is almost equivalent in B3LYP calculations; however, the syn conformer becomes much more stable than the anti conformer in MP2 calculations, which is consistent with the experimental finding that only the syn conformers are relevant. The vertical detachment energy, which is linearly proportional to the ionization potential, shows the same size dependence as the previous experiment. The electron attachment energy decreases exponentially as the size increases, which implies that the electron transport would be possible even for long chains such as F3 and F4. This was evident from the frontier molecular orbitals (HOMO and LUMO). Also, it is found that the syn conformers are very favorable for electron transport through the pi-stacked fluorene moieties. 相似文献
998.
Chitosan-alginate polyelectrolyte complex (PEC) have been prepared in situ in beads and microspheres. This study examines the preparation of suitable chitosan-alginate coacervates for casting into homogeneous PEC films for potential applications in packaging, controlled release systems and wound dressings. Coacervation between chitosan and alginate was rapid, but the rate may be controlled with the addition of water miscible organic solvents. Compared with ethanol and PEG200, acetone was the more promising solvent moderator. Suspensions of fine, uniformly dispersed coacervates were produced by a dropwise addition of 0.25% w/v chitosan solution (solvent: 1: 1 v/v of 2% acetic acid and acetone) into 0.25% w/v sodium alginate solution in water under rapid agitation. The PEC films were transparent and flexible. They exhibited high permeability to water vapor, but resisted complete dissolution in 0.1 M HCI, distilled water and pH 7.4 phosphate buffer solution. Microscopic heterogeneity in the films could be reduced by immersion in aqueous media, but this was accompanied by modifications in the thickness, permeability and mechanical property of the films. 相似文献
999.
Single-stage anaerobic codigestion for mixture wastes of simulated Korean food waste and waste activated sludge 总被引:2,自引:0,他引:2
Nam Hyo Heo Soon Chul Park Jin Suk Lee Ho Kang Don Hee Park 《Applied biochemistry and biotechnology》2003,107(1-3):567-579
Korean food waste was treated with a single-stage anaerobic codigester (SSAD) using waste activated sludge (WAS) generated
from a municipal wastewater treatment plant. The stability and performance of the system was analyzed. The C/N ratio was improved
with increasing food waste fraction of feed mixture. The pH, alkalinity, and free ammonia nitrogen concentration were the
parameters used to evaluate the digester’s stability. The experimentally determined values of the parameters indicated that
there were no methane inhibitions in the digester. Digester performance was determined by measuring the total chemical oxygen
demand TCOD), volate solids (VS) removal, methane content in biogas, methane production rate (MPR), and specific methane productivity.
Methane content in biogas and MPR were significantly dependent on hydraulic retention time (HRT) and ratio of food waste to
WAS. The methane content in biogas decreased at shorter HRT or higher organic loading rate (OLR) with increased food waste
fraction. Concerning the performance of the codigester, the optimum operating condition of the SSAD was found to be at an
HRT of 10 d with a feed mixture ratio of 50% food waste and 50% WAS. A TCOD removal efficiency of 53.6% and a VS removal efficiency
of 53.7% were obtained at an OLR of 5.96 kg of TCOD/(m3·d) and 3.14 kg of VS/(m3·d), respectively. A maximum MPR of 1.15 m3 CH4/(m3·d) and an SMP of 0.37 m3 CH4/kg of VSfeed were obtained at an HRT of 10 d with a methane content of 63%. 相似文献
1000.
The in vitro metabolism of toremifene has been studied in liver microsomal preparations from rat, mouse and human sources using high-performance liquid chromatography-electrospray ionisation mass spectrometry (HPLC-ESIMS). The metabolites detected were N-desmethyltoremifene (m/z 392), 4-hydroxytoremifene (m/z 422), 4'-hydroxytoremifene (m/z 422) and toremifene N-oxide m/z 422). In addition, a new polar metabolite with a protonated molecule at m/z 422 has been detected in all three species. The compound was identified by tandem MS-MS as alpha-hydroxytoremifene, an analogue of alpha-hydroxytamoxifen. The results showed that alpha-hydroxylation is a common feature of tamoxifen and toremifene metabolism and that alpha-hydroxytamoxifen is unlikely to be the reactive metabolite responsible for the hepatocarcinogenesis in rat, as widely believed. 相似文献