全文获取类型
收费全文 | 1957篇 |
免费 | 48篇 |
国内免费 | 6篇 |
专业分类
化学 | 1192篇 |
晶体学 | 10篇 |
力学 | 36篇 |
数学 | 193篇 |
物理学 | 580篇 |
出版年
2022年 | 12篇 |
2021年 | 21篇 |
2020年 | 24篇 |
2019年 | 12篇 |
2018年 | 19篇 |
2016年 | 28篇 |
2015年 | 27篇 |
2014年 | 36篇 |
2013年 | 73篇 |
2012年 | 91篇 |
2011年 | 132篇 |
2010年 | 49篇 |
2009年 | 46篇 |
2008年 | 95篇 |
2007年 | 95篇 |
2006年 | 104篇 |
2005年 | 104篇 |
2004年 | 99篇 |
2003年 | 90篇 |
2002年 | 85篇 |
2001年 | 55篇 |
2000年 | 54篇 |
1999年 | 16篇 |
1998年 | 18篇 |
1997年 | 20篇 |
1996年 | 34篇 |
1995年 | 17篇 |
1994年 | 35篇 |
1993年 | 37篇 |
1992年 | 39篇 |
1991年 | 37篇 |
1990年 | 25篇 |
1989年 | 23篇 |
1988年 | 14篇 |
1987年 | 11篇 |
1986年 | 19篇 |
1985年 | 23篇 |
1984年 | 18篇 |
1983年 | 20篇 |
1982年 | 10篇 |
1981年 | 11篇 |
1980年 | 14篇 |
1979年 | 13篇 |
1978年 | 12篇 |
1977年 | 9篇 |
1976年 | 10篇 |
1975年 | 14篇 |
1974年 | 11篇 |
1973年 | 23篇 |
1967年 | 10篇 |
排序方式: 共有2011条查询结果,搜索用时 15 毫秒
121.
Potential of coded excitation in medical ultrasound imaging 总被引:6,自引:0,他引:6
Misaridis TX Gammelmark K Jørgensen CH Lindberg N Thomsen AH Pedersen MH Jensen JA 《Ultrasonics》2000,38(1-8):183-189
Improvement in signal-to-noise ratio (SNR) and/or penetration depth can be achieved in medical ultrasound by using long coded waveforms, in a similar manner as in radars or sonars. However, the time-bandwidth product (TB) improvement, and thereby SNR improvement is considerably lower in medical ultrasound, due to the lower available bandwidth. There is still space for about 20 dB improvement in the SNR, which will yield a penetration depth up to 20 cm at 5 MHz [M. O'Donnell, IEEE Trans. Ultrason. Ferroelectr. Freq. Contr., 39(3) (1992) 341]. The limited TB additionally yields unacceptably high range sidelobes. However, the frequency weighting from the ultrasonic transducer's bandwidth, although suboptimal, can be beneficial in sidelobe reduction. The purpose of this study is an experimental evaluation of the above considerations in a coded excitation ultrasound system. A coded excitation system based on a modified commercial scanner is presented. A predistorted FM signal is proposed in order to keep the resulting range sidelobes at acceptably low levels. The effect of the transducer is taken into account in the design of the compression filter. Intensity levels have been considered and simulations on the expected improvement in SNR are also presented. Images of a wire phantom and clinical images have been taken with the coded system. The images show a significant improvement in penetration depth and they preserve both axial resolution and contrast. 相似文献
122.
In the development of therapeutic proteins, analytical assessment of structural stability and integrity constitutes an important activity, as protein stability and integrity influence drug efficacy, and ultimately patient safety. Existing analytical methodologies solely rely on relative changes in optical properties such as fluorescence or scattering upon thermal or chemical perturbation. Here, we present an absolute analytical method for assessing protein stability, structure, and unfolding utilizing Taylor dispersion analysis (TDA) and LED-UV fluorescence detection. The developed TDA method measures the change in size (hydrodynamic radius) and intrinsic fluorescence of a protein during in-line denaturation with guanidinium hydrochloride (GuHCl). The conformational stability of the therapeutic antibody adalimumab and human serum albumin were characterized as a function of pH. The simple workflow and low sample consumption (40 ng protein per data point) of the methodology make it ideal for assessing protein characteristics related to stability in early drug development or when having a scarce amount of sample available. 相似文献
123.
Frontispiece: Enantioselective Organocatalytic Cascade Approach to Different Classes of Benzofused Acetals
下载免费PDF全文
![点击此处可从《Chemistry (Weinheim an der Bergstrasse, Germany)》网站下载免费的PDF全文](/ch/ext_images/free.gif)
124.
125.
C. R.H. Bahl J. Garde K. Lefmann T. B.S. Jensen P.-A. Lindgård D. E. Madsen S. Mørup 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,62(1):53-57
In magnetic nanoparticles the uniform precession
(q = 0 spin wave) mode gives the predominant contribution
to the magnetic excitations. We have calculated the energy of the
uniform mode in antiferromagnetic nanoparticles with uncompensated
magnetic moments, using the coherent potential approximation. In
the presence of uncompensated moments, an antiferromagnetic
nanoparticle must be considered as a kind of a ferrimagnet. Two
magnetic anisotropy terms are considered, a planar term confining
the spins to the basal plane, and an axial term determining an
easy axis in this plane. Excitation energies are calculated for
various combinations of these two anisotropy terms, ranging from
the simple uniaxial case to the planar case with a strong
out-of-plane anisotropy. In the simple uniaxial case, the
uncompensated moment has a large influence on the excitation
energy, but in the planar case it is much less important. The
calculations explain recent neutron scattering measurements on
nanoparticles of antiferromagnetic α-Fe2O3 and NiO. 相似文献
126.
Robert Jensen Maciej Kocan Andrzej Swiech 《Proceedings of the American Mathematical Society》2002,130(2):533-542
We introduce the notion of a ``good" solution of a fully nonlinear uniformly elliptic equation. It is proven that ``good" solutions are equivalent to -viscosity solutions of such equations. The main contribution of the paper is an explicit construction of elliptic equations with strong solutions that approximate any given fully nonlinear uniformly elliptic equation and its -viscosity solution. The results also extend some results about ``good" solutions of linear equations.
127.
R.D. SuenramF.J. Lovas D.F. PlusquellicA. Lesarri Y. KawashimaJ.O. Jensen A.C. Samuels 《Journal of Molecular Spectroscopy》2002,211(1):110-118
The rotational spectrum of dimethyl methylphosphonate (DMMP) has been studied using a pulsed molecular beam Fourier transform microwave spectrometer. The spectrum is complicated by the internal rotation motions of the three methyl tops in the molecule as well as an interconversion motion of the two methoxy groups. Here, we present the microwave spectrum, the ab initio calculations, and the assignment of the rigid-rotor A-symmetry state of the molecule. The rotational constants for this state are A=2828.753(2) MHz, B=1972.360(3) MHz, and C=1614.267(2) MHz. In the following paper, a group theoretical analysis is developed for DMMP. The observed conformation of the molecule has no point-group symmetry and all three electric-dipole selection rules are active, with c-type transitions being the most intense. Ab initio calculations were carried out at both the Hartree-Fock 6-31G* and MP2/6-311G* levels of theory. These calculations indicate that two low-energy conformations are possible separated by energies of less than 170 cm−1. Furthermore, the calculated lowest energy conformer is in agreement with the one observed experimentally. The relative energies of the two low-energy conformers rise from 34 cm−1 at the HF level to 170 cm−1 at the MP2 level. 相似文献
128.
Renz M Blasco T Corma A Fornés V Jensen R Nemeth L 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(20):4708-4717
Sn-Beta is used as a heterogeneous catalyst for the Baeyer-Villiger reaction with hydrogen peroxide. Cyclic ketones are transformed into the corresponding lactones, while unsaturated ketones are oxidized to the corresponding unsaturated lactones with very high chemoselectivity. The catalyst is also selective for the oxidation of aromatic aldehydes with H2O2, producing the formate ester or the corresponding hydrolyzed product, that is the alcohol. Shape-selective oxidations are observed for isomeric reactants with different molecular shapes. The catalytic Sn sites have been characterized by 119Sn MAS-NMR spectroscopy, and tetrahedral incorporation into the zeolite framework has been demonstrated. In situ IR spectroscopy and 18O labeling experiments have shown that the oxidation mechanism involves an intermediate of the Criegee type. 相似文献
129.
Li W Nelson DP Jensen MS Hoerrner RS Cai D Larsen RD Reider PJ 《The Journal of organic chemistry》2002,67(15):5394-5397
3-Pyridylboronic acid was prepared in high yield and bulk quantity from 3-bromopyridine via a protocol of lithium-halogen exchange and "in situ quench". This technique was further studied and evaluated on other aryl halides in the preparation of arylboronic acids. 相似文献
130.
The established rates of glycoside hydrolysis reactions were analyzed using free energy relationship plots based on substituent constants that depend on whether the substituent is axial or equatorial. In all cases good correlations were found when assuming either that the transition state had a charged ring-oxygen atom or that it had a charged anomeric carbon atom. The spontaneous hydrolysis of 2,4-dinitrophenyl beta-glycopyranosides and the acidic hydrolysis of methyl beta-D-glycopyranosides were found to give a good correlation, when 100% charge at the ring-oxygen in the transition state of these reactions is assumed. The acidic hydrolysis of methyl alpha-glycopyranosides was found to give good correlations regardless of whether 100% charge at the ring-oxygen or 100% charge at the anomeric carbon was assumed. The findings clearly demonstrate how crucial the stereochemistry of even remote polar substituents is for their electronic effect on chemical reaction. 相似文献