全文获取类型
收费全文 | 1581篇 |
免费 | 18篇 |
国内免费 | 13篇 |
专业分类
化学 | 1010篇 |
晶体学 | 6篇 |
力学 | 28篇 |
综合类 | 1篇 |
数学 | 340篇 |
物理学 | 227篇 |
出版年
2021年 | 16篇 |
2020年 | 12篇 |
2018年 | 12篇 |
2016年 | 26篇 |
2015年 | 16篇 |
2014年 | 21篇 |
2013年 | 80篇 |
2012年 | 70篇 |
2011年 | 91篇 |
2010年 | 47篇 |
2009年 | 40篇 |
2008年 | 73篇 |
2007年 | 72篇 |
2006年 | 78篇 |
2005年 | 80篇 |
2004年 | 74篇 |
2003年 | 59篇 |
2002年 | 67篇 |
2001年 | 23篇 |
2000年 | 24篇 |
1999年 | 18篇 |
1998年 | 8篇 |
1997年 | 19篇 |
1996年 | 24篇 |
1995年 | 17篇 |
1994年 | 13篇 |
1993年 | 16篇 |
1992年 | 21篇 |
1991年 | 18篇 |
1990年 | 22篇 |
1989年 | 16篇 |
1988年 | 14篇 |
1987年 | 18篇 |
1986年 | 15篇 |
1985年 | 28篇 |
1984年 | 40篇 |
1983年 | 18篇 |
1982年 | 29篇 |
1981年 | 21篇 |
1980年 | 15篇 |
1979年 | 28篇 |
1978年 | 31篇 |
1977年 | 21篇 |
1976年 | 24篇 |
1975年 | 25篇 |
1974年 | 21篇 |
1973年 | 21篇 |
1972年 | 14篇 |
1971年 | 8篇 |
1968年 | 11篇 |
排序方式: 共有1612条查询结果,搜索用时 15 毫秒
1.
Pu Duan Bo Zhi Luke Coburn Christy L. Haynes Klaus Schmidt-Rohr 《Magnetic resonance in chemistry : MRC》2020,58(11):1130-1138
The composition of fluorescent polymer nanoparticles, commonly referred to as carbon dots, synthesized by microwave-assisted reaction of citric acid and ethylenediamine was investigated by 13C, 13C{1H}, 1H─13C, 13C{14N}, and 15N solid-state nuclear magnetic resonance (NMR) experiments. 13C NMR with spectral editing provided no evidence for significant condensed aromatic or diamondoid carbon phases. 15N NMR showed that the nanoparticle matrix has been polymerized by amide and some imide formation. Five small, resolved 13C NMR peaks, including an unusual ═CH signal at 84 ppm (1H chemical shift of 5.8 ppm) and ═CN2 at 155 ppm, and two distinctive 15N NMR resonances near 80 and 160 ppm proved the presence of 5-oxo-1,2,3,5-tetrahydroimidazo[1,2-a]pyridine-7-carboxylic acid (IPCA) or its derivatives. This molecular fluorophore with conjugated double bonds, formed by a double cyclization reaction of citric acid and ethylenediamine as first shown by Y. Song, B. Yang, and coworkers in 2015, accounts for the fluorescence of the carbon dots. Cross-peaks in a 1H─13C HETCOR spectrum with brief 1H spin diffusion proved that IPCA is finely dispersed in the polyamide matrix. From quantitative 13C and 15N NMR spectra, a high concentration (18 ± 2 wt%) of IPCA in the carbon dots was determined. A pronounced gradient in 13C chemical-shift perturbations and peak widths, with the broadest lines near the COO group of IPCA, indicated at least partial transformation of the carboxylic acid of IPCA by amide or ester formation. 相似文献
2.
An optimal control model of exhaustible resources is used to clarify the long run relationship between mineral rent and depletion cost at the industry level. A standard first order condition of the time rate of change of rents is reformulated to reveal that rent data may be used to help forecast the rise in extraction costs resulting from resource depletion. This application of the theory of exhaustible resources is illustrated using historical mineral industry rent and extraction cost data. A forecast of U.S. coal extraction costs, following the method proposed in this paper, suggests that future rates of extraction cost increases will be similar to rates experienced in the past. 相似文献
3.
4.
Starting with a partition of a rectangular box into subboxes, it is shown how to construct a natural tetrahedral (type-4) partition and associated trivariate C 1 quintic polynomial spline spaces with a variety of useful properties, including stable local bases and full approximation power. It is also shown how the spaces can be used to solve certain Hermite and Lagrange interpolation problems. 相似文献
5.
We examine to what extent finite-dimensional spaces defined on locally compact subsets of the line and possessing various weak Chebyshev properties (involving sign changes, zeros, alternation of best approximations, and peak points) can be uniformly approximated by a sequence of spaces having related properties. 相似文献
6.
7.
Peter G. Doyle Colin Mallows Alon Orlitsky Larry Shepp 《Israel Journal of Mathematics》1989,67(1):34-42
Letn cells be arranged in a ring, or alternatively, in a row. Initially, all cells are unmarked. Sequentially, one of the unmarked
cells is chosen at random and marked until, aftern steps, each cell is marked. After thekth cell has been marked the configuration of marked cells defines some number of islands: maximal sets of adjacent marked
cells. Let ξ
k
denote the random number of islands afterk cells have been marked.
We give explicit expressions for moments of products of ξ
k
’s and for moments of products of 1/ξ
k
’s.
These are used in a companion paper to prove that if a random graph on the natural number is made by drawing an edge betweeni≧1 andj>i with probabilityλ/j, then the graph is almost surely connected ifλ>1/4 and almost surely disconnected ifλ≦1/4. 相似文献
8.
Efficiencies of the maximum pseudolikelihood estimator and a number of related estimators for the case-cohort sampling design in the proportional hazards regression model are studied. The asymptotic information and lower bound for estimating the parametric regression parameter are calculated based on the effective score, which is obtained by determining the component of the parametric score orthogonal to the space generated by the infinite-dimensional nuisance parameter. The asymptotic distributions of the maximum pseudolikelihood and related estimators in an i.i.d. setting show that these estimators do not achieve the computed asymptotic lower bound. Simple guidelines are provided to determine in which instances such estimators are close enough to efficient for practical purposes. 相似文献
9.
10.
J. P. Bucher D. C. Douglass P. Xia B. Haynes L. A. Bloomfield 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1991,19(4):251-254
A new apparatus for measuring the magnetic properties of metal clusters has been constructed. The technique involves the conventional Stern-Gerlach deflection scheme together with modern pulsed laser vaporization source technology and time of flight mass spectrometry. High field seeking monodirectional deflections have been measured for cobalt clusters containing between 40 and 400 atoms. The measured magnetic moments per atom are found to be lower than the known values for the bulk. Special attention has been given to velocity measurements of the metal clusters and the carrier gas atoms in the beam. The residence time of the particles in the source cavity has been measured. 相似文献