全文获取类型
收费全文 | 699篇 |
免费 | 30篇 |
国内免费 | 59篇 |
专业分类
化学 | 271篇 |
晶体学 | 2篇 |
力学 | 205篇 |
数学 | 130篇 |
物理学 | 180篇 |
出版年
2024年 | 4篇 |
2023年 | 31篇 |
2022年 | 28篇 |
2021年 | 25篇 |
2020年 | 36篇 |
2019年 | 30篇 |
2018年 | 18篇 |
2017年 | 37篇 |
2016年 | 46篇 |
2015年 | 34篇 |
2014年 | 43篇 |
2013年 | 39篇 |
2012年 | 32篇 |
2011年 | 43篇 |
2010年 | 30篇 |
2009年 | 33篇 |
2008年 | 42篇 |
2007年 | 37篇 |
2006年 | 18篇 |
2005年 | 28篇 |
2004年 | 21篇 |
2003年 | 17篇 |
2002年 | 7篇 |
2001年 | 5篇 |
2000年 | 12篇 |
1999年 | 9篇 |
1998年 | 9篇 |
1997年 | 9篇 |
1996年 | 8篇 |
1995年 | 10篇 |
1994年 | 8篇 |
1993年 | 4篇 |
1992年 | 1篇 |
1991年 | 3篇 |
1990年 | 2篇 |
1989年 | 2篇 |
1988年 | 8篇 |
1986年 | 2篇 |
1985年 | 10篇 |
1983年 | 1篇 |
1981年 | 2篇 |
1971年 | 1篇 |
1957年 | 3篇 |
排序方式: 共有788条查询结果,搜索用时 15 毫秒
31.
《Journal of Coordination Chemistry》2012,65(4):505-521
Abstract The grinding of a 2: 1 molar ratio mixture of isonitrosoacetylacetone and 1,3-diaminopropan-2-ol led to formation of the tribasic ligand (H3L), (1) with two oxime groups and a flexible alcoholic backbone. The 1:2 molar ratio reaction of (1) with CuX2 produced the planar dinuclear complexes LCu2(X) nH2O; × = acetate (2), phenylacetate (3), formate (4), monochloroacetate (5), dichloroacetate (6), trichloroacetate (7), benzoate (8), and p-hydroxybenzoate (9); n = 1 for (2) and (8); n = 2 for (3)-(7); and n = 4 for (9). The copper(II) ions are bridged by the carbox-ylate and the alcoholic oxygen. The strong antiferromagnetic interactions in (2)-(9) are impeded in (5)-(7) by the chloroacetate bridge withdrawing electron density from the carboxylate. The latter bridge is replaced by picrate in the 1:1 molar ratio reaction of (2) with picric acid (10). The 1:1 molar ratio reaction of (1) with copper(II) acetate produced the tetranuclear [HLCu]2[LCu2(OAc)] 5H2O (11), whereas the 2:1 molar ratio reaction, similar to the reaction which led to (8), produced HLCu (12). The latter complex reacted (1:1 molar ratio) with either copper(ll) acetate or nickel(II) acetate to produce complexes (2) and the heterodinuclear LNi-Cu(OAc) 2H2O (13), respectively. Similar reactions with (11) gave the same complexes (2) and (13). The acid adducts of (9) with p-hydroxybenzoic acid (14) and LCu2(X)-HX (15); × = p-aminobenzoic acid were isolated. The cobalt(II) analogue of the mononuclear (12), HLCo 2H2O (16) was obtained from the 1:1 molar reaction of (1) with cobalt(II) acetate. The supramolecular structure of (11), (12) and (16) took place via intermolecular hydrogen bonding of the alcoholic proton with the oximato oxygen of the adjacent molecule which mediated electron density and allowed for a magnetic exchange interaction. The suggested structures of the ligand and metal complexes are in accordance with analytical, spectral and magnetic moment data. 相似文献
32.
M. Pereyaslavets I. Ogawa T. Idehara S. Mitsudo 《International Journal of Infrared and Millimeter Waves》1999,20(8):1453-1468
The excitation and transmission losses of a dielectric-lined waveguide have been calculated with real, experimentally measured output beams of a submillimeter wave gyrotron. It is shown that for not-so-long transmission lines the coupling losses far exceed the transmission losses in the waveguide. 相似文献
33.
E. K. Boukas J. Yang Q. Zhang G. Yin 《Journal of Optimization Theory and Applications》1996,91(2):347-361
In this paper, we consider a periodic preventive maintenance, repair, and production model of a flexible manufacturing system with failure-prone machines, where the control variables are the repair rate and production rate. We use periodic preventive maintenance to reduce the machine failure rates and improve the productivity of the system. One of the distinct features of the model is that the repair rate is adjustable. Our objective is to choose a control process that minimizes the total cost of inventory/shortage, production, repair, and maintenance. Under suitable conditions, we show that the value function is locally Lipschitz and satisfies an Hamilton-Jacobi-Bellman equation. A sufficient condition for optimal control is obtained. Since analytic solutions are rarely available, we design an algorithm to approximate the optimal control problem. To demonstrate the performance of the numerical method, an example is presented.Research of this author was supported by the Natural Sciences and Engineering Research Council of Canada, Grant OGP0036444.Research of this author was supported in part by the University of Georgia.Research of this author was supported in part by the National Science Foundation, Grant DMS-92-24372. 相似文献
34.
The optimal attitude control problem of spacecraft during its solar arrays stretching process is discussed in the present
paper. By using the theory of wavelet analysis in control algorithm, the discrete orthonormal wavelet function is introduced
into the optimal control problem, the method of wavelet expansion is substituted for the classical Fourier basic function.
An optimal control algorithm based on wavelet analysis is proposed. The effectiveness of the wavelet expansion approach is
shown by numerical simulation.
This work is supported by the National Natural Science Foundation of China. 相似文献
35.
The paper deals with trajectory tracking for a flexible spacecraft, subject to a gravity-gradient disturbance, under parameter uncertainties. The controls are gas jets and reaction wheels, and the measured variables describe the attitude and angular velocity of the rigid part. The flexible dynamics is treated as an additional disturbance acting on a rigid structure. First, an adaptive control is designed with only the gravity-gradient disturbance acting on the spacecraft; second, it is proved to be effective also in the presence of disturbance due to the flexibility, provided that appropriate robustness conditions on the controller gains are satisfied. These conditions use partial knowledge of the parameters describing the elastic dynamics. Simulations show the good performance of such control scheme and demonstrate its applicability even in the presence of input saturation. 相似文献
36.
We consider two-stage tandem queueing systems attended by two specialized and one flexible server, where all servers have time varying rates. Assuming exponential processing times and linear holding costs, we derive properties of server allocation policies that minimize expected costs over an infinite time horizon. 相似文献
37.
I.M. KrishnakumarBeena Mathew 《European Polymer Journal》2002,38(9):1745-1752
To correlate the coupling difficulty and nature of the support in solid-phase peptide synthesis, a set of difficult peptides were built under identical conditions on rigid and hydrophobic divinyl benzene-crosslinked polystyrene (DVB-PS) and flexible and polar 1,4-butanediol dimethacrylate-crosslinked polystyrene (BDDMA-PS) resins of same functional capacity (∼2 mmol/g). The difficult coupling stages observed on each support were identified by monitoring the acylation time, coupling yield and deprotection efficiency. The yield and purity of the peptides prepared on BDDMA-PS were higher than when DVB-PS support was used. 相似文献
38.
39.
One of the most well-known situations in which nonlinear effects must be taken into account to obtain realistic results is
the rotating beam problem. This problem has been extensively studied in the literature and has even become a benchmark problem
for the validation of nonlinear formulations. Among other approaches, the substructuring technique was proven to be a valid
strategy to account for this problem. Later, the similarities between the absolute nodal coordinate formulation and the substructuring
technique were demonstrated. At the same time, it was found the existence of a critical angular velocity, beyond which the
system becomes unstable that was dependent on the number of substructures. Since the dependence of the critical velocity was
not so far clear, this paper tries to shed some light on it. Moreover, previous studies were focused on a constant angular
velocity analysis where the effects of Coriolis forces were neglected. In this paper, the influence of the Coriolis force
term is not neglected. The influence of the reference conditions of the element frame are also investigated in this paper. 相似文献
40.
Tsuyoshi Katayama 《Queueing Systems》2007,57(4):169-178
We consider a multi-class priority queueing system with a non-preemptive time-limited service controlled by an exponential
timer and multiple (or single) vacations. By reducing the service discipline to the Bernoulli schedule, we obtain an expression
for the Laplace-Stieltjes transform (LST) of the waiting time distribution via an iteration procedure, and a recursive scheme
to calculate the first two moments. It is noted that we have to select embedded Markov points based on the service beginning
epochs instead of the service completion epochs adopted for most of M/G/1 queueing analyses. Through the queue-length analysis, we obtain a decomposition form for the LST of the waiting time in
each queue having the exhaustive service.
相似文献