全文获取类型
收费全文 | 2396篇 |
免费 | 61篇 |
国内免费 | 19篇 |
专业分类
化学 | 1463篇 |
晶体学 | 20篇 |
力学 | 87篇 |
数学 | 457篇 |
物理学 | 449篇 |
出版年
2021年 | 24篇 |
2020年 | 27篇 |
2019年 | 34篇 |
2018年 | 24篇 |
2017年 | 16篇 |
2016年 | 41篇 |
2015年 | 48篇 |
2014年 | 34篇 |
2013年 | 136篇 |
2012年 | 79篇 |
2011年 | 99篇 |
2010年 | 59篇 |
2009年 | 37篇 |
2008年 | 69篇 |
2007年 | 89篇 |
2006年 | 102篇 |
2005年 | 120篇 |
2004年 | 92篇 |
2003年 | 96篇 |
2002年 | 81篇 |
2001年 | 50篇 |
2000年 | 46篇 |
1999年 | 27篇 |
1998年 | 21篇 |
1997年 | 34篇 |
1996年 | 43篇 |
1995年 | 35篇 |
1994年 | 34篇 |
1993年 | 35篇 |
1992年 | 42篇 |
1991年 | 37篇 |
1990年 | 28篇 |
1989年 | 33篇 |
1988年 | 30篇 |
1987年 | 18篇 |
1986年 | 25篇 |
1985年 | 42篇 |
1984年 | 35篇 |
1983年 | 35篇 |
1982年 | 35篇 |
1981年 | 33篇 |
1980年 | 37篇 |
1979年 | 26篇 |
1978年 | 37篇 |
1977年 | 27篇 |
1976年 | 34篇 |
1975年 | 29篇 |
1974年 | 40篇 |
1973年 | 40篇 |
1972年 | 18篇 |
排序方式: 共有2476条查询结果,搜索用时 15 毫秒
981.
Fermi's golden rule is used to develop relationships between rate constants for electron transfer in donor-bridge-acceptor and electrode-bridge-acceptor systems and resistances across metal-bridge-electrode and metal-bridge-tip junctions. Experimental data on electron-transfer rates through alkanethiolate, oligophenylene, and DNA bridges are used to calculate the electronic coupling matrix element per state through these moieties. The formulation is then used to predict the resistance of these bridges between two gold contacts. This approach provides a straightforward method for experimentalists to assess the self-consistency between intramolecular electron-transfer rate constants and low-bias resistances measured for molecularly bridged junctions between two metallic contacts. Reported resistances for alkanethiolate bridges vary by a factor of 20, with predicted resistances falling within this range. However, comparisons between carboxylato and directly linked alkanethiolate bridges suggest differences between the coupling at the interface to either the redox center or the gold electrode in such systems. Calculated resistances for oligophenylene bridges are close to those measured experimentally in a similar oligophenylene system. 相似文献
982.
Self-assembling cyclic D,L-peptide nanotubes are electronically insulating and possess wide band gaps (E(g) > 4 eV). Our ab initio electronic structure calculations indicate that the presence of aromatic rings in the side chain of peptide nanotubes significantly reduces the band gap. We investigate the conductivity of the modified peptide nanotubes through calculations of the electron tunneling probability. The electron tunneling probability through a molecule depends on the length of the molecule, L, as e-beta(E)L, where the tunneling beta(E)-factor is strongly energy-dependent. We have calculated beta(E) in three types of peptide nanotubes that have different sequences of amino acid residue, (L-Gln, D-Ala)(4), (L-Gln, D-Leu)(4) and (L-Gln, D-Phe)(4), using the complex band structure approach. We find large beta values near midgap, making these peptide nanotubes a poor tunneling conductor. 相似文献
983.
We report experiments on dendritic molecules with integrated conjugated chromophores that provide microscopic mechanistic information about their solvation dynamics. The fluorescence of a series of immobilized dendritically organized oligothiophenes is studied as they are exposed to good solvents. Initially, the pi-stacking of the oligothiophene units in the dendrimer is destabilized, but full separation of the oligothiophene dendrons takes a time that is orders of magnitude longer due to barriers to torsional motion of the ester linkages. The metastable state prior to separation of the conjugated segments exhibits solution-like spectroscopy but low fluorescence quantum yield relative to the fully solvated segments. This species may play an important role in the photophysics of conjugated oligomer and polymer films. Unusual non-exponential kinetics for the oligothiophene separation step are observed and can be understood in terms of energy transfer among the dendrons. 相似文献
984.
The circular dichroism (CD) spectrum of a large biological molecule represented as an array of interacting chromophores is investigated. The configuration-averaged Green function formalism is developed to describe the CD and absorption spectra. The perturbation theory expansion is derived for absorption and CD spectra in the case of strong interaction of chromophores with their environment (solvent and/or internal dynamics) compared to their interaction with each other. We apply this formalism to study CD spectra of DNA hairpins. 相似文献
985.
Ruiz-Salvador AR Almora-Barrios N Gómez A Lewis DW 《Physical chemistry chemical physics : PCCP》2007,9(4):521-532
Computational methods are described that model accurately the structure of hydrated Ca-bearing zeolites. Using Goosecreekite as a model system we probe the influence of framework ordering, cation siting and hydration of pores on the structure and its stability. We develop a methodology which allows the location of Al within the framework to be determined together with the position of extra-framework cations, in a stepwise fashion, progressing from an anhydrous model, via a dielectric continuum model, to finally, a fully atomistic model of the water in the intrazeolite pore space. Our methods reveal the complex interplay of short- and long-range interactions on the optimal structure of such materials. 相似文献
986.
Zhang J Reimer MT Ji QC Chang MS El-Shourbagy TA Burke S Schwartz L 《Analytical and bioanalytical chemistry》2007,387(8):2745-2756
During stent development, accurate monitoring of the drug concentration in animal tissues can provide critical information
on how the drug is released into the circulation and the surrounding tissues. To establish the relationship between the drug
concentration and the distance from the stent to the target tissue, a comprehensive strategy was developed for sample collection,
sample homogenization and sample storage as well as sample analysis. This strategy was developed with the analytical chemists
and animal surgical specialists working together as a team. The optimized sampling process was designed to yield a representative
sample, appropriately located and of an appropriate size. The sampling process was also designed to eliminate the potential
for carryover and cross-contamination. During sample processing, the analyte solution was spiked into blank tissues using
a sharp needle and a gas-tight syringe to prepare tissue quality control samples. These tissue quality controls were then
used to evaluate the stability of the drug in solid tissue and homogenate, the homogenization carryover, the cross-contamination
and the recovery of the drug during method validation and to monitor the overall process of drug analysis of the swine tissues.
This thorough strategy has been applied to the accurate determination of zotarolimus in swine tissues for regulated toxicology
studies. The entire process was controlled, including precise tissue sampling, compound-based tissue homogenization, method
validation, and the application of the method to regulated toxicokinetics studies. The results demonstrate that analytical
chemistry concepts can be successfully integrated into toxicokinetics studies in order to collect precise samples and obtain
meaningful results. The strategy can be applied to similar toxicokinetics studies of locally administrated drugs in tissues. 相似文献
987.
988.
In this note, we show that if
is a π-partial character of the π-separable group
is a chain of normal subgroups of G, and H is a Hall π-subgroup of G, then
has a Fong character α
Irr(H) such that for every subgroup
, every irreducible constituent of α
H∩N
is Fong for N. We also show that if
is quasi-primitive, then for every normal subgroup M of G the irreducible constituents of
are Fong for M.
Received: 21 July 2006 Revised: 17 January 2007 相似文献
989.
Arehart KH Kates JM Anderson MC Harvey LO 《The Journal of the Acoustical Society of America》2007,122(2):1150-1164
Noise and distortion reduce speech intelligibility and quality in audio devices such as hearing aids. This study investigates the perception and prediction of sound quality by both normal-hearing and hearing-impaired subjects for conditions of noise and distortion related to those found in hearing aids. Stimuli were sentences subjected to three kinds of distortion (additive noise, peak clipping, and center clipping), with eight levels of degradation for each distortion type. The subjects performed paired comparisons for all possible pairs of 24 conditions. A one-dimensional coherence-based metric was used to analyze the quality judgments. This metric was an extension of a speech intelligibility metric presented in Kates and Arehart (2005) [J. Acoust. Soc. Am. 117, 2224-2237] and is based on dividing the speech signal into three amplitude regions, computing the coherence for each region, and then combining the three coherence values across frequency in a calculation based on the speech intelligibility index. The one-dimensional metric accurately predicted the quality judgments of normal-hearing listeners and listeners with mild-to-moderate hearing loss, although some systematic errors were present. A multidimensional analysis indicates that several dimensions are needed to describe the factors used by subjects to judge the effects of the three distortion types. 相似文献