全文获取类型
收费全文 | 61959篇 |
免费 | 1709篇 |
国内免费 | 174篇 |
专业分类
化学 | 40020篇 |
晶体学 | 460篇 |
力学 | 1116篇 |
综合类 | 4篇 |
数学 | 10289篇 |
物理学 | 11953篇 |
出版年
2023年 | 417篇 |
2022年 | 606篇 |
2021年 | 861篇 |
2020年 | 935篇 |
2019年 | 956篇 |
2018年 | 1217篇 |
2017年 | 1117篇 |
2016年 | 2150篇 |
2015年 | 1722篇 |
2014年 | 1815篇 |
2013年 | 3857篇 |
2012年 | 3915篇 |
2011年 | 4194篇 |
2010年 | 2606篇 |
2009年 | 2352篇 |
2008年 | 3635篇 |
2007年 | 3567篇 |
2006年 | 3210篇 |
2005年 | 2926篇 |
2004年 | 2452篇 |
2003年 | 1986篇 |
2002年 | 1700篇 |
2001年 | 1312篇 |
2000年 | 1178篇 |
1999年 | 897篇 |
1998年 | 683篇 |
1997年 | 599篇 |
1996年 | 768篇 |
1995年 | 558篇 |
1994年 | 590篇 |
1993年 | 583篇 |
1992年 | 556篇 |
1991年 | 462篇 |
1990年 | 438篇 |
1989年 | 376篇 |
1988年 | 360篇 |
1987年 | 347篇 |
1986年 | 339篇 |
1985年 | 458篇 |
1984年 | 461篇 |
1983年 | 323篇 |
1982年 | 336篇 |
1981年 | 327篇 |
1980年 | 299篇 |
1979年 | 259篇 |
1978年 | 245篇 |
1977年 | 250篇 |
1976年 | 217篇 |
1975年 | 201篇 |
1973年 | 204篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
We consider a retrial queue with a finite buffer of size N, with arrivals of ordinary units and of negative units (which cancel one ordinary unit), both assumed to be Markovian arrival
processes. The service requirements are of phase type. In addition, a PHL,N bulk service discipline is assumed. This means that the units are served in groups of size at least L, where 1≤ L≤ N. If at the completion of a service fewer than L units are present at the buffer, the server switches off and waits until the buffer length reaches the threshold L. Then it switches on and initiates service for such a group of units. On the contrary, if at the completion of a service
L or more units are present at the buffer, all units enter service as a group. Units arriving when the buffer is full are not
lost, but they join a group of unsatisfied units called “orbit”. Our interest is in the continuous-time Markov chain describing
the state of the queue at arbitrary times, which constitutes a level dependent quasi-birth-and-death process. We start by
analyzing a simplified version of our queueing model, which is amenable to numerical calculation and is based on spatially
homogeneous quasi-birth-and-death processes. This leads to modified matrix-geometric formulas that reveal the basic qualitative
properties of our algorithmic approach for computing performance measures.
AMS Subject Classification: Primary 60K25 Secondary 68M20 90B22. 相似文献
82.
Given a constant of motion for the one-dimensional harmonic oscillator with linear dissipation in the velocity, the problem to get the Hamiltonian for this system is pointed out, and the quantization up to second order in the perturbation approach is used to determine the modification on the eigenvalues when dissipation is taken into consideration. This quantization is realized using the constant of motion instead of the Hamiltonian.
PACS: 03.20.+i, 03.30.+p, 03.65.−w,03.65.Ca 相似文献
83.
84.
Jaime Pfretzschner Pedro Cobo Francisco Simón María Cuesta Alejandro Fernández 《Applied Acoustics》2006,67(1):62-73
Microperforated panels (MPPs) coupled to a rigid wall have been proposed recently as an alternative to porous absorbers in situations having concerns with bacterial contamination and small particles discharge, like food, pharmaceutical and microelectronic industries. There exists also an increasing interest for MPP absorbers in the transportation industry and civil engineering. In general, an optimally designed MPP with good broadband absorption requires many submillimetric holes distributed over a panel of also submillimetric thickness. Such thin plates or foils become so fragile that they need to be protected from mechanical damage. In this paper, an alternative strategy is investigated which allows the design of MPPs with panels of millimetric thickness while maintaining their acoustic performance. These absorbers, named microperforated insertion units (MIUs), avoid the structural problems of the classical MPPs. An assessment of the sound absorption properties of these structures is presented. Comparisons between calculations and measurements are also made under two experimental conditions: plane waves at normal incidence (impedance tube) and free field (anechoic room). 相似文献
85.
Our object of study is the asymptotic behavior of the sequence of polynomials orthogonal with respect to the discrete Sobolev
inner product <formula> \langle f, g \rangle = ∈t_{E} f(ξ) \overline{g(ξ)} ρ(ξ) |d ξ|+ f(Z) A g(Z)^H, </formula> where E is a rectifiable Jordan curve or arc in the complex plane
f(Z) = (f(z_1), \ldots, f^{(l_1)}(z_1) , \ldots , f(z_m) , \ldots ,f^{(l_m)}(z_m)),
A is an M \times M Hermitian matrix, M l
1
+ ⋅s + l
m
+ m , |d ξ| denotes the arc length measure, ρ is a nonnegative function on E , and z
i
∈Ω,
i=1,2,\ldots,m , where Ω is the exterior region to E .
July 23, 1999. Dates revised: September 11, 2000 and February 16, 2001. Date accepted: February 26, 2001. 相似文献
86.
It was studied the tin phase composition of the electrodeposited alloys produced at different temperatures and current densities from an acid chloride/fluoride electrolyte and an acid chloride bath. Mössbauer spectra were consistent with a Ni3Sn4 intermetallic compound, dissolved tin, and a Cu?Sn intermetallic compound formed at the interface of the copper substrate. 相似文献
87.
The elastic constants of dilute alloys based on bcc metals have been calculated using the Green’s function method obtaining
explicit expressions for change in elastic constants in terms oft-matrix. The crystal impurity problem is discussed within an impurity model containing central and non-central force constant
changes extended up to second neighbours of the impurity. The effect of volume change on elastic constants and a contribution
from electron pressure term are considered. Numerical results for changes in three elastic moduli have been presented for
a number of dilute alloys based on Mo, Nb, W, Ta and V. 相似文献
88.
Oscillatory change of pH occurs during the chromate-catalyzed decomposition of hydrogen peroxide in a weakly acidic medium at elevated temperature and at high initial concentration of hydrogen peroxide. In a closed system, there are only two or three periods, but sustained oscillation occurs in a CSTR. In a CSTR bistability is also found. In closed systems the temperature exhibits a great maximum (up to 15°C increase), in a CSTR sustained oscillation occurs at a constant stationary temperature. 相似文献
89.
90.