全文获取类型
收费全文 | 1899篇 |
免费 | 274篇 |
国内免费 | 129篇 |
专业分类
化学 | 488篇 |
晶体学 | 7篇 |
力学 | 125篇 |
综合类 | 24篇 |
数学 | 216篇 |
物理学 | 405篇 |
无线电 | 1037篇 |
出版年
2024年 | 11篇 |
2023年 | 32篇 |
2022年 | 58篇 |
2021年 | 80篇 |
2020年 | 80篇 |
2019年 | 61篇 |
2018年 | 67篇 |
2017年 | 86篇 |
2016年 | 95篇 |
2015年 | 69篇 |
2014年 | 126篇 |
2013年 | 158篇 |
2012年 | 135篇 |
2011年 | 128篇 |
2010年 | 101篇 |
2009年 | 97篇 |
2008年 | 131篇 |
2007年 | 122篇 |
2006年 | 102篇 |
2005年 | 93篇 |
2004年 | 78篇 |
2003年 | 68篇 |
2002年 | 55篇 |
2001年 | 55篇 |
2000年 | 35篇 |
1999年 | 26篇 |
1998年 | 29篇 |
1997年 | 28篇 |
1996年 | 21篇 |
1995年 | 18篇 |
1994年 | 6篇 |
1993年 | 8篇 |
1992年 | 6篇 |
1991年 | 4篇 |
1990年 | 6篇 |
1989年 | 4篇 |
1988年 | 3篇 |
1987年 | 2篇 |
1986年 | 3篇 |
1985年 | 4篇 |
1984年 | 1篇 |
1982年 | 5篇 |
1981年 | 3篇 |
1979年 | 2篇 |
排序方式: 共有2302条查询结果,搜索用时 31 毫秒
151.
Prediction of flow stress at hot working condition 总被引:1,自引:0,他引:1
A mathematical model has been developed to determine flow stress at hot deformation condition. The proposed model is capable of including work softening due to dynamic phase transformations as well as the effect of temperature and strain rate variation on flow stress utilizing the additivity rule for strain. To verify the model, hot compression tests for two grades of steels has been carried out. The comparison between the experimental and theoretical results confirms the validity of the model. 相似文献
152.
We study the simultaneous one-dimensional flow of water and oil in a heterogeneous medium modelled by the Buckley-Leverett equation. It is shown both by analytical solutions and by numerical experiments that this hyperbolic model is unstable in the following sense: Perturbations in physical parameters in a tiny region of the reservoir may lead to a totally different picture of the flow. This means that simulation results obtained by solving the hyperbolic Buckley-Leverett equation may be unreliable.Symbols and Notation
f
fractional flow function varying withs andx
-
value off outsideI
-
value off insideI
-
local approximation off around¯x
-
f
–,f
+
values of
-
f
j
n
value off atS
j
n
andx
j
-
g
acceleration due to gravity [ms–2]
-
I
interval containing a low permeable rock
-
k
dimensionless absolute permeability
-
k
*
absolute permeability [m2]
-
k
c
*
characteristic absolute permeability [m2]
-
k
ro
relative oil permeability
-
k
rw
relative water permeability
-
L
*
characteristic length [m]
-
L
1
the space of absolutely integrable functions
-
L
the space of bounded functions
-
P
c
dimensionless capillary pressure function
-
P
c
*
capillary pressure function [Pa]
-
P
c
*
characteristic pressure [Pa]
-
S
similarity solution
-
S
j
n
numerical approximation tos(xj, tn)
-
S
1, S2,S
3
constant values ofs
-
s
water saturation
-
value ofs at
-
s
L
left state ofs (wrt.
)
-
s
R
right state ofs (wrt.
)
-
s
s for a fixed value of in Section 3
-
T
value oft
-
t
dimensionless time coordinate
-
t
*
time coordinate [s]
-
t
c
*
characteristic time [s]
-
t
n
temporal grid point,t
n=n t
-
v
*
total filtration (Darcy) velocity [ms–1]
-
W, , v
dimensionless numbers defined by Equations (4), (5) and (6)
-
x
dimensionless spatial coordinate [m]
-
x
*
spatial coordinate [m]
-
x
j
spatial grid piont,x
j=j x
-
discontinuity curve in (x, t) space
-
right limiting value of¯x
-
left limiting value of¯x
-
angle between flow direction and horizontal direction
- t
temporal grid spacing
- x
spatial grid spacing
-
length ofI
-
parameter measuring the capillary effects
-
argument ofS
-
o
dimensionless dynamic oil viscosity
- w
dimensionless dynamic water viscosity
-
c
*
characteristic viscosity [kg m–1s–1]
-
o
*
dynamic oil viscosity [kg m–1s–1]
-
w
*
dynamic water viscosity [k gm–1s–1]
-
o
dimensionless density of oil
-
w
dimensionless density of water
-
c
*
characteristic density [kgm–3]
-
o
*
density of oil [kgm–3]
-
w
*
density of water [kgm–3]
-
porosity
-
dimensionless diffusion function varying withs andx
-
*
dimensionless function varying with s andx
* [kg–1m3s]
-
j
n
value of atS
j
n
andx
j
This research has been supported by VISTA, a research cooperation between the Norwegian Academy of Science and Letters and Den norske stats oljeselskap a.s. (Statoil). 相似文献
153.
Summary A variant of the boundary element method, called the boundary contour method, offers a further reduction in dimensionality.
Consequently, boundary contour analysis of 2-D problems does not require any numerical integration at all. In a boundary contour
analysis, boundary stresses can be accurately computed using the approach proposed in Ref. [1]. However, due to singularity,
this approach can be used only to calculate boundary stresses at points that do not lie at an end of a boundary element. Herein,
it is shown that a technique based on the displacement/velocity shape functions can overcome this drawback. Further, the approach
is much simpler to apply, requires less computational effort, and provides competitive accuracy. Numerical solutions and convergence
study for some well-known problems in linear elasticity and Stokes flow are presented to show the effectiveness of the proposed
approach.
This research was supported in part by the 2004 Ralph E. Powe Junior Faculty Enhancement Award from Oak Ridge Associated Universities
and by the University of South Alabama Research Council. 相似文献
154.
Although, the effects of ultrasonic irradiation on multiphase flow through porous media have been studied in the past few
decades, the physics of the acoustic interaction between fluid and rock is not yet well understood. Various mechanisms may
be responsible for enhancing the flow of oil through porous media in the presence of an acoustic field. Capillary related
mechanisms are peristaltic transport due to mechanical deformation of the pore walls, reduction of capillary forces due to
the destruction of surface films generated across pore boundaries, coalescence of oil drops due to Bjerknes forces, oscillation
and excitation of capillary trapped oil drops, forces generated by cavitating bubbles, and sonocapillary effects. Insight
into the physical principles governing the mobilization of oil by ultrasonic waves is vital for developing and implementing
novel techniques of oil extraction. This paper aims at identifying and analyzing the influence of high-frequency, high-intensity
ultrasonic radiation on capillary imbibition. Laboratory experiments were performed using cylindrical Berea sandstone and
Indiana limestone samples with all sides (quasi-co-current imbibition), and only one side (counter-current imbibition) contacting
with the aqueous phase. The oil saturated cores were placed in an ultrasonic bath, and brought into contact with the aqueous
phase. The recovery rate due to capillary imbibition was monitored against time. Air–water, mineral oil–brine, mineral oil–surfactant
solution and mineral oil-polymer solution experiments were run each exploring a separate physical process governing acoustic
stimulation. Water–air imbibition tests isolate the effect of ultrasound on wettability, capillarity and density, while oil–brine
imbibition experiments help outline the ultrasonic effect on viscosity and interfacial interaction between oil, rock and aqueous
phase. We find that ultrasonic irradiation enhances capillary imbibition recovery of oil for various fluid pairs, and that
such process is dependent on the interfacial tension and density of the fluids. Although more evidence is needed, some runs
hint that wettability was not altered substantially under ultrasound. Preliminary analysis of the imbibition recoveries also
suggests that ultrasound enhances surfactant solubility and reduce surfactant adsorption onto the rock matrix. Additionally,
counter-current experiments involving kerosene and brine in epoxy coated Berea sandstone showed a dramatic decline in recovery.
Therefore, the effectiveness of any ultrasonic application may strongly depend on the nature of interaction type, i.e., co-
or counter-current flow. A modified form of an exponential model was employed to fit the recovery curves in an attempt to
quantify the factors causing the incremental recovery by ultrasonic waves for different fluid pairs and rock types. 相似文献
155.
Xianwen Lou Bas F. M. de Waal Lech‐Gustav Milroy Joost L. J. van Dongen 《Journal of mass spectrometry : JMS》2015,50(5):766-770
In matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry (MALDI TOF MS), analyte signals can be substantially suppressed by other compounds in the sample. In this technical note, we describe a modified thin‐layer sample preparation method that significantly reduces the analyte suppression effect (ASE). In our method, analytes are deposited on top of the surface of matrix preloaded on the MALDI plate. To prevent embedding of analyte into the matrix crystals, the sample solution were prepared without matrix and efforts were taken not to re‐dissolve the preloaded matrix. The results with model mixtures of peptides, synthetic polymers and lipids show that detection of analyte ions, which were completely suppressed using the conventional dried‐droplet method, could be effectively recovered by using our method. Our findings suggest that the incorporation of analytes in the matrix crystals has an important contributory effect on ASE. By reducing ASE, our method should be useful for the direct MALDI MS analysis of multicomponent mixtures. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
156.
试验采用磁化焙烧-磁选的方法来回收包头稀土尾矿中的Fe。考察了还原球团的粒径、焙烧温度、还原气体的成分、还原气气体流量、焙烧时间对于磁化焙烧回收Fe的影响。在焙烧温度为580℃,CO与CO2的浓度比为40∶60,还原气体气流量为1.2 L·min-1,焙烧时间为60 min的条件下对球团进行磁化焙烧,并将焙烧后的球团进行磨矿,使得-200目的物料占物料总重量的95%。将磨矿后的物料置于磁场强度为233 k A·m-1的磁选管中进行磁选,可以得到品位为60%左右的铁精矿,其铁回收率达到70%左右。 相似文献
157.
Richard Kueng Holger Rauhut Ulrich Terstiege 《Applied and Computational Harmonic Analysis》2017,42(1):88-116
We study the recovery of Hermitian low rank matrices from undersampled measurements via nuclear norm minimization. We consider the particular scenario where the measurements are Frobenius inner products with random rank-one matrices of the form for some measurement vectors , i.e., the measurements are given by . The case where the matrix to be recovered is of rank one reduces to the problem of phaseless estimation (from measurements ) via the PhaseLift approach, which has been introduced recently. We derive bounds for the number m of measurements that guarantee successful uniform recovery of Hermitian rank r matrices, either for the vectors , , being chosen independently at random according to a standard Gaussian distribution, or being sampled independently from an (approximate) complex projective t-design with . In the Gaussian case, we require measurements, while in the case of 4-designs we need . Our results are uniform in the sense that one random choice of the measurement vectors guarantees recovery of all rank r-matrices simultaneously with high probability. Moreover, we prove robustness of recovery under perturbation of the measurements by noise. The result for approximate 4-designs generalizes and improves a recent bound on phase retrieval due to Gross, Krahmer and Kueng. In addition, it has applications in quantum state tomography. Our proofs employ the so-called bowling scheme which is based on recent ideas by Mendelson and Koltchinskii. 相似文献
158.
159.
The transient recoverable deformation ratio after melt elongation at various elongational rates and maximum elongations was
investigated for pure polystyrene and for a 85 wt.% polystyrene/15 wt.% linear low density polyethylene (PS/LLDPE 85:15) blend
at a temperature of 170 oC. The ratio p of the zero shear rate viscosity of LLDPE to that of PS is p = 0.059 ≈ 1:17. Retraction of the elongated LLDPE droplets back to spheres and end-pinching is observed during recovery.
A simple additive rule is applied in order to extract the contribution of the recovery of the elongated droplets from the
total recovery of the blend. In that way, the recoverable portion of the PS/LLDPE blend induced by the interfacial tension
is determined and compared with the results of a theory based on an effective medium approximation. The effective medium approximation
reproduces well the time scale of the experimental data. In addition, the trends that the recoverable deformation increases
with elongational rate and maximum elongation are captured by the theoretical approach. 相似文献
160.
Relationship between the recovery factor and the viscous dissipation in a confined, impinging, circular jet of high-Prandtl number liquid 总被引:1,自引:0,他引:1
De-Yu Li Zeng-Yuan Guo Chong-Fang Ma 《International Journal of Heat and Fluid Flow》1997,18(6):585-590
The radial distribution of the recovery factor for a confined impinging jet of high-Prandtl number liquid is investigated by numerical approach with emphasis on its physical mechanism. The recovery factor is determined by the viscous dissipation and Prandtl number. Unlike the case of the gas jet impingement, the recovery factor in the region close to the stagnation point can be much larger than unity, while the recovery factor at the stagnation point approaches zero. The dependence of the recovery factor on the nozzle exit velocity profile, the jet Reynolds number, and the nozzle-to-plate spacing is examined. 相似文献