全文获取类型
收费全文 | 9907篇 |
免费 | 507篇 |
国内免费 | 209篇 |
专业分类
化学 | 6233篇 |
晶体学 | 96篇 |
力学 | 414篇 |
综合类 | 25篇 |
数学 | 1574篇 |
物理学 | 2281篇 |
出版年
2023年 | 90篇 |
2022年 | 146篇 |
2021年 | 211篇 |
2020年 | 239篇 |
2019年 | 265篇 |
2018年 | 212篇 |
2017年 | 174篇 |
2016年 | 304篇 |
2015年 | 312篇 |
2014年 | 350篇 |
2013年 | 640篇 |
2012年 | 641篇 |
2011年 | 733篇 |
2010年 | 462篇 |
2009年 | 408篇 |
2008年 | 525篇 |
2007年 | 485篇 |
2006年 | 496篇 |
2005年 | 444篇 |
2004年 | 374篇 |
2003年 | 318篇 |
2002年 | 290篇 |
2001年 | 202篇 |
2000年 | 180篇 |
1999年 | 132篇 |
1998年 | 94篇 |
1997年 | 91篇 |
1996年 | 96篇 |
1995年 | 78篇 |
1994年 | 102篇 |
1993年 | 88篇 |
1992年 | 88篇 |
1991年 | 70篇 |
1990年 | 61篇 |
1989年 | 56篇 |
1988年 | 48篇 |
1987年 | 46篇 |
1986年 | 49篇 |
1985年 | 74篇 |
1984年 | 82篇 |
1983年 | 51篇 |
1982年 | 79篇 |
1981年 | 67篇 |
1980年 | 77篇 |
1979年 | 60篇 |
1978年 | 67篇 |
1977年 | 59篇 |
1976年 | 57篇 |
1975年 | 45篇 |
1973年 | 48篇 |
排序方式: 共有10000条查询结果,搜索用时 437 毫秒
81.
82.
83.
84.
K. Najim L. Pibouleau M. V. Le Lann 《Journal of Optimization Theory and Applications》1990,64(2):331-347
Optimization techniques are finding increasingly numerous applications in process design, in parallel to the increase of computer sophistication. The process synthesis problem can be stated as a largescale constrained optimization problem involving numerous local optima and presenting a nonlinear and nonconvex character. To solve this kind of problem, the classical optimization methods can lead to analytical and numerical difficulties. This paper describes the feasibility of an optimization technique based on learning systems which can take into consideration all the prior information concerning the process to be optimized and improve their behavior with time. This information generally occurs in a very complex analytical, empirical, or know-how form. Computer simulations related to chemical engineering problems (benzene chlorination, distillation sequence) and numerical examples are presented. The results illustrate both the performance and the implementation simplicity of this method.Nomenclature
c
i
penalty probability
-
cp
precision parameter on constraints
-
D
variation domain of the variablex
-
f(·)
objective function
-
g(·)
constraints
-
i,j
indexes
-
k
iteration number
-
N
number of actions
-
P
probability distribution vector
-
p
i
ith component of the vectorP as iterationk
-
r
number of reactors in the flowsheet
-
u(k)
discrete value or action chosen by the algorithm at iterationk
-
u
i
discrete value of the optimization variable in [u
min,u
max]
-
u
min
lowest value of the optimization variable
-
u
max
largest value of the optimization variable
-
Z
random number
-
x
variable for the criterion function
-
xp
precision parameter on criterion function
-
W(k)
performance index unit output at iterationk
-
0, 1
reinforcement scheme parameters
-
p
sum of the probability distribution vector components 相似文献
85.
J. -F. Le Gall 《Probability Theory and Related Fields》1988,78(3):389-402
Summary We consider the following heat conduction problem. Let K be a compact set in Euclidean space 3. Suppose that K is held at the temperature 1, while the surrounding medium is at the temperature 0 at time 0. Following Spitzer we investigate the asymptotic behaviour of the integral E
K
(t) which represents the total energy flow in time t from the set K to the surrounding medium 3–K. An asymptotic expansion is given for E
K
(t) which refines a theorem due to Spitzer. This expansion also verifies and improves a formal calculation of Kac. Similar results are proved in higher dimensions. Up to the constant m(K), the quantity E
K
(t) can be interpreted as the expected value of the volume of the Wiener sausage associated with K and a d-dimensional Brownian motion. This point of view both plays a major role in the proofs and leads to a probabilistic interpretation of the different terms of the expansion. 相似文献
86.
87.
88.
The solid diffusion coefficient of lithium-ion in LiCoO2 cathode material has been investigated by the capacity intermittent titration technique (CITT) at different voltages and at different charge/discharge cycles. By SEM, XRD and FTIR techniques, the structure of LiCoO2 was studied before and after charge-discharge cycles, and the relationship between solid diffusion coefficient and crystal structure was further discussed. CITT results show that the value of Li+ solid diffusion coefficient of LiCoO2 is about 10-12 cm2·s-1. During the whole charge-discharge cycles, the Li+ solid diffusion coefficient decreased within the voltage of 4.0~4.3 V, which is attributed to the change of the structure of LiCoO2. 相似文献
89.
The thiochlorides Mo6Cl10Y (Y = S, Se, Te) have been prepared; they are isostructural with Nb6I11, space group Pccn, and have four formula units per unit cell. The X-ray structure of Mo6Cl10Se has been determined from three-dimensional single-crystal counter data and refined to a final R value of 0.053 for 3350 independent reflections. The most important result concerning this structure is a statistical distribution of the Se atom on the unit (Mo6X′8) with : so the compound Mo6Cl10Se must be formulated . The diamagnetic and dielectric behavior of these new thiochlorides is discussed. 相似文献
90.
Bombek S Lenarsic R Kocevar M Saint-Jalmes L Desmurs JR Polanc S 《Chemical communications (Cambridge, England)》2002,(14):1494-1495
An electrophilic amination of halogenated phenols with diisopropyl diazenedicarboxylate in the presence of ZrCl4 as a Lewis acid, accompanied by a halogen migration, was demonstrated for the first time; the fluorine, chlorine, bromine, or iodine atom migrated during the amination process under mild reaction conditions. 相似文献