全文获取类型
收费全文 | 3003篇 |
免费 | 43篇 |
国内免费 | 27篇 |
专业分类
化学 | 1263篇 |
晶体学 | 10篇 |
力学 | 90篇 |
数学 | 581篇 |
物理学 | 788篇 |
无线电 | 341篇 |
出版年
2021年 | 17篇 |
2019年 | 19篇 |
2018年 | 33篇 |
2017年 | 23篇 |
2016年 | 39篇 |
2015年 | 25篇 |
2014年 | 33篇 |
2013年 | 95篇 |
2012年 | 91篇 |
2011年 | 141篇 |
2010年 | 60篇 |
2009年 | 60篇 |
2008年 | 117篇 |
2007年 | 143篇 |
2006年 | 147篇 |
2005年 | 122篇 |
2004年 | 101篇 |
2003年 | 90篇 |
2002年 | 85篇 |
2001年 | 58篇 |
2000年 | 67篇 |
1999年 | 49篇 |
1998年 | 42篇 |
1997年 | 43篇 |
1996年 | 60篇 |
1995年 | 54篇 |
1994年 | 45篇 |
1993年 | 81篇 |
1992年 | 59篇 |
1991年 | 58篇 |
1990年 | 38篇 |
1989年 | 44篇 |
1988年 | 38篇 |
1987年 | 55篇 |
1986年 | 41篇 |
1985年 | 58篇 |
1984年 | 55篇 |
1983年 | 36篇 |
1982年 | 52篇 |
1981年 | 53篇 |
1980年 | 37篇 |
1979年 | 46篇 |
1978年 | 51篇 |
1977年 | 35篇 |
1976年 | 41篇 |
1975年 | 47篇 |
1974年 | 47篇 |
1973年 | 33篇 |
1972年 | 24篇 |
1969年 | 20篇 |
排序方式: 共有3073条查询结果,搜索用时 10 毫秒
131.
We study the stability of network communication after removal of a fraction q=1-p of links under the assumption that communication is effective only if the shortest path between nodes i and j after removal is shorter than al(ij)(a> or =1) where l(ij) is the shortest path before removal. For a large class of networks, we find analytically and numerically a new percolation transition at p(c)=(kappa(0)-1)((1-a)/a), where kappa(0) [triple bond] / and k is the node degree. Above p(c), order N nodes can communicate within the limited path length al(ij), while below p(c), N(delta) (delta<1) nodes can communicate. We expect our results to influence network design, routing algorithms, and immunization strategies, where short paths are most relevant. 相似文献
132.
We present measurements of equilibrium forces resulting from capillary condensation. The results give access to the ultralow interfacial tensions between the capillary bridge and the coexisting bulk phase. We demonstrate this with solutions of associative polymers and an aqueous mixture of gelatin and dextran, with interfacial tensions around 10 microN/m. The equilibrium nature of the capillary forces is attributed to the combination of a low interfacial tension and a microscopic confinement geometry, based on nucleation and growth arguments. 相似文献
133.
134.
Weixun Wang Bernard K. Choi Wenyu Li Zhege Lao Anita Y. H. Lee Sandra C. Souza Nathan A. Yates Timothy Kowalski Alessandro Pocai Lucinda H. Cohen 《Journal of the American Society for Mass Spectrometry》2014,25(4):614-625
Stromal cell-derived factor 1α (SDF-1α) or CXCL12 is a small pro-inflammatory chemoattractant cytokine and a substrate of dipeptidyl peptidase IV (DPP-IV). Proteolytic cleavage by DPP-IV inactivates SDF-1α and attenuates its interaction with CXCR4, its cell surface receptor. To enable investigation of suppression of such inactivation with pharmacologic inhibition of DPP-IV, we developed quantitative mass spectrometric methods that differentiate intact SDF-1α from its inactive form. Using top-down strategy in quantification, we demonstrated the unique advantage of keeping SDF-1α’s two disulfide bridges intact in the analysis. To achieve the optimal sensitivity required for quantification of intact and truncated SDF-1α at endogenous levels in blood, we coupled nano-flow tandem mass spectrometry with antibody-based affinity enrichment. The assay has a quantitative range of 20 pmol/L to 20 nmol/L in human plasma as well as in rhesus monkey plasma. With only slight modification, the same assay can be used to quantify SDF-1α in mice. Using two in vivo animal studies as examples, we demonstrated that it was critical to differentiate intact SDF-1α from its truncated form in the analysis of biomarkers for pharmacologic inhibition of DPP-IV activity. These novel methods enable translational research on suppression of SDF-1 inactivation with DPP-IV inhibition and can be applied to relevant clinical samples in the future to yield new insights on change of SDF-1α levels in disease settings and in response to therapeutic interventions. Figure
? 相似文献
135.
Dr. Lyle M. Gordon Jessica K. Román Dr. R. Michael Everly Michael J. Cohen Dr. Jonathan J. Wilker Dr. Derk Joester 《Angewandte Chemie (International ed. in English)》2014,53(43):11506-11509
Metastable precursors are thought to play a major role in the ability of organisms to create mineralized tissues. Of particular interest are the hard and abrasion‐resistant teeth formed by chitons, a class of rock‐grazing mollusks. The formation of chiton teeth relies on the precipitation of metastable ferrihydrite (Fh) in an organic scaffold as a precursor to magnetite. In vitro synthesis of Fh under physiological conditions has been challenging. Using a combination of X‐ray absorption and electron paramagnetic resonance spectroscopy, we show that, prior to Fh formation in the chiton tooth, iron ions are complexed by the organic matrix. In vitro experiments demonstrate that such complexes facilitate the formation of Fh under physiological conditions. These results indicate that acidic molecules may be integral to controlling Fh formation in the chiton tooth. This biological approach to polymorph selection is not limited to specialized proteins and can be expropriated using simple chemistry. 相似文献
136.
Noel D. Uri 《商业与工业应用随机模型》1996,12(1):45-61
This paper is concerned with the effect of natural resource scarcity on economic growth in the United States. After defining the notion of scarcity and introducing two measures of scarcity—unit costs and relative resource price—changes in resource scarcity trends for lead, zinc, nickel, aluminium, silver, iron and copper over the most recent three decades are investigated. Only for silver and iron is there any indication that such a change has occurred. For silver, the change is transitory. To the extent that change takes place, it has implications for future economic growth, i.e. natural resource scarcity and economic growth are interrelated. To see whether this is a relevant concern, subsequent to the examination of changing resource scarcity trends an objective effort is made to identify a long-run equilibrium relationship between natural resource scarcity and economic growth. Relying on cointegration techniques, only for the unit cost measure for lead and copper for one of the measures of cointegration is there a suggestion that natural resource scarcity has affected economic growth in the United States over the period 1889–1992. 相似文献
137.
Harry J. Whitlow Margaretha Andersson Mikael Hult Leif Persson Mohamed El Bouanani Mikael Östling Carina Zaring Nils Lundberg David D. Cohen Nick Dytlewski Peter N. Johnston Ian F. Bubb Scott R. Walker Erik Johanson Sture Hogmark P. Anders Ingemarsson 《Mikrochimica acta》1995,120(1-4):171-181
Recoil Spectrometry covers a group of techniques that are very similar to the well known Rutherford backscattering Spectrometry technique, but with the important difference that one measures the recoiling target atom rather than the projectile ion. This makes it possible to determine both the identity of the recoil and its depth of origin from its energy and velocity, using a suitable detector system. The incident ion is typically high-energy (30–100MeV)35C1,81Br or127I. Low concentrations of light elements such as C, O and N can be profiled in a heavy matrix such as Fe or GaAs. Here we present an overview of mass and energy dispersive recoil Spectrometry and illustrate its successful use in some typical applications. 相似文献
138.
The behavior of a terminally anchored freely-jointed bead-rod chain, subjected to solvent shear flow, was investigated via Brownian dynamics simulations. Previous calculations have been improved by computing the segment density and fluid velocity profiles self-consistently. The segment density distributions, components of the radius of gyration, and chain attachment shear and normal stresses were found to be sensitive to low values of shear rate. Additionally, it was found that the thickness of a model polymer layer was a strong function of the shear rate, and that the functional dependence on shear rate changed dramatically as the chain length increased. For the longest chains studied, the thickness of the model polymer layer first increased as the shear rate increased, passed through a maximum, and then decreased at high shear rates, in accordance with experimental results in theta solvents. These results suggest that a dilute or semi-dilute layer model may explain hydrodynamic behavior previously thought to be due to the entanglements that occur in dense surface bound polymer layers.Nomenclature
a
i
acceleration of bead i
-
b
radius of the beads
-
d
length of the rods connecting the chain beads
-
d
i
vector from bead i to bead i + 1
-
F
i
external force applied to bead i
-
F
i
b
external force on bead i due to Brownian motion of surrounding fluid
-
F
i
h
external force on bead i due to viscous drag
-
F
i
s
external force on bead i due to surface interactions
-
f
Stokes drag coefficient
-
Boltzmann's constant
-
L
h
effective hydrodynamic thickness
-
m
i
mass of bead i
-
N
number of beads on a model chain
-
n
number of chains anchored to the surface per unit surface area
-
P
segment density distribution P pressure
-
Q
flow in a tube with no surface bound polymer layer
-
Q
a
flow in a tube with a surface bound polymer layer
-
R
g
vector representation of the radius of gyration
-
R
tube radius
-
r
radial coordinate in the tube geometry
-
S
ij
pair hydrodynamic interaction tensor for beads i and j
-
T
i
internal chain force in rod i connecting beads i and i + 1
-
T
X
component of the surface attachment force in the direction of the fluid flow
-
T
y
component of the surface attachment force perpendicular to the surface
-
T
temperature
-
v
i
velocity of the center of mass of bead i
-
V
if
average fluid velocity at the location of bead i
-
v
if
0
fluid velocity in the absence of a polymer chain
-
v
if
perturbation to the fluid velocity due to hydrodynamic interactions
-
V
b
bead volume = 4 b
3/3
-
scalar fluid speed in the axial direction down the tube
-
x
axial coordinate in the tube geometry
Greek symbols w
apparent shear rate
-
fluid viscosity
-
polymer layer permeability
-
volume fraction of space occupied by chain beads
- (w)a
chain attachment stress perpendicular to the surface
- (w)a
chain attachment stress in the plane of the surface and in the direction of fluid flow 相似文献
139.
140.
Michal Firstenberg Kammasandra Nanjunda Shivananda Irit Cohen Olga Solomeshch Vladislav Medvedev Nir Tessler Yoav Eichen 《Advanced functional materials》2011,21(4):634-643
Sequence‐independent or “click”‐type chemistry is applied for the preparation of novel π‐conjugated oligomers. A variety of bi‐functional monomers for Wittig–Horner olefination are developed and applied in a sequential protection–deprotection process for the preparation of structurally similar π‐conjugated oligomers. Selected oligomers are incorporated as the organic semiconductors in light‐emitting diodes and a field‐effect transistor, demonstrating the potential of the approach. 相似文献