首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   660篇
  免费   68篇
  国内免费   21篇
化学   412篇
力学   15篇
综合类   39篇
数学   248篇
物理学   35篇
  2024年   3篇
  2023年   7篇
  2022年   21篇
  2021年   18篇
  2020年   33篇
  2019年   30篇
  2018年   22篇
  2017年   36篇
  2016年   44篇
  2015年   24篇
  2014年   52篇
  2013年   65篇
  2012年   38篇
  2011年   45篇
  2010年   43篇
  2009年   46篇
  2008年   23篇
  2007年   28篇
  2006年   20篇
  2005年   22篇
  2004年   15篇
  2003年   14篇
  2002年   12篇
  2001年   15篇
  2000年   10篇
  1999年   9篇
  1998年   7篇
  1997年   2篇
  1996年   8篇
  1995年   1篇
  1994年   2篇
  1993年   2篇
  1992年   3篇
  1991年   1篇
  1990年   7篇
  1989年   4篇
  1988年   5篇
  1987年   1篇
  1986年   3篇
  1985年   1篇
  1984年   2篇
  1982年   1篇
  1979年   1篇
  1977年   1篇
  1976年   1篇
  1957年   1篇
排序方式: 共有749条查询结果,搜索用时 15 毫秒
71.
The ring‐opening metathesis polymerization (ROMP) of fatty acid functionalized norbornenes was explored in the presence of dichloro[1,3‐bis(2,4,6‐trimethylphenyl)‐2‐imidazolidinylidene](benzylidene)bis(3‐bromopyridine)ruthenium(II) ( C3 ) at room temperature. The investigated monomers were derived from fatty acids with different chain lengths (C6, C8, C10, C12, C14, C16, and C18) and can therefore contribute to the development of more sustainable, bio‐based polymeric materials. The polymerizations initiated by C3 proceeded in a living fashion with good initiation efficiency, and thus the synthesis of well‐defined polymers with narrow polydispersities was accomplished. All prepared polymers were fully characterized (GPC, DSC, TGA, NMR) and the results of these investigations are discussed within this contribution. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010  相似文献   
72.
As input flows of secondary raw materials show high volatility and tend to behave in a chaotic way, the identification of the main drivers of the dynamic behavior of returns plays a crucial role. Based on a stylized production-recycling system consisting of a set of nonlinear difference equations, we explicitly derive parameter constellations where the system will or will not converge to its equilibrium. Using a constant elasticity of substitution production function, the model is then extended to enable coverage of real world situations. Using waste paper as a reference raw material, we empirically estimate the parameters of the system. By using these regression results, we are able to show that the equilibrium solution is a Lyapunov unstable saddle point. This implies that the system is sensitive on initial conditions that will hence impede the predictability of product returns. Small variations of production input proportions could however stabilize the whole system.  相似文献   
73.
74.
In the Vehicle Routing Problem with Deliveries, Selective Pickups and Time Windows, the set of customers is the union of delivery customers and pickup customers. A fleet of identical capacitated vehicles based at the depot must perform all deliveries and profitable pickups while respecting time windows. The objective is to minimize routing costs, minus the revenue associated with the pickups. Five variants of the problem are considered according to the order imposed on deliveries and pickups. An exact branch-and-price algorithm is developed for the problem. Computational results are reported for instances containing up to 100 customers.  相似文献   
75.
高校人力资源管理中的模糊综合评判   总被引:6,自引:0,他引:6  
洪江如  赵斌 《运筹与管理》2001,10(2):163-166
人力资源管理是高校管理的重要组成部分。本简要介绍了高校人力资源的状况,讨论了模糊综合评判理论在人力资源管理中的应用。  相似文献   
76.
Allocation of tasks in IoT is an integral and critical approach to finding a perfect match between scheduled tasks of a particular application and Edge-based processing devices for instant response and efficient utilization of resources to make them renewable. We need a protocol to help optimize the problem of allocating processing devices to the tasks, as task allocation is considered an NP-hard problem to prevent problems with energy consumption and response time problems. For this, a hybrid bio-inspired Swarm-based approach will improve the solution to optimize the matching of a task to a particular device. This paper proposed a Meta-heuristic algorithm to optimize Energy and Time-delay for allocating tasks to the edge-based Processing device in IoT. The proposed algorithm called the Hybrid Artificial Bee Colony whales Optimization algorithm (HAWO) is formulated by integrating Artificial Bee Colony with the Whales Optimization algorithm to overcome the search process of an Artificial Bee Colony, which converges too soon due to the local search of Employee Bee phase and Onlooker Bee phase causing the problem of looping. From the simulation results conducted in Matlab, it is observed that the integrated HAWO method shows promising results in terms of Energy and Time Delay when compared with Artificial Bee Colony and Whales Optimization algorithms separately. Also, proposed method when compared with the benchmark work shows significant improvements of 50%, 25% and 60% in terms of Energy, Time Delay and Best cost, respectively.  相似文献   
77.
In this work, resol based binders were prepared with lignin, resorcinol and the bio-based aldehydic platform chemicals 5-hydroxymethylfurfural (HMF) and 2,5-diformylfuran (DFF). In preliminary studies, the solubility and stability of DFF in aqueous-alkaline conditions was investigated. Various binders were prepared to investigate the influence of the HMF-DFF ratio and the influence of the aldehyde concentration on the molar mass, rheological properties and curing characteristics of the final binders. It is shown that significantly higher molecular weights and viscosities are obtained by using DFF instead of HMF. The properties of the final binder could also be affected by the precise choice of DFF concentration, as this resulted in higher levels of crosslinking. This work offers a novel type of fully bio-based binder that consists of non-toxic components and is therefore less hazardous than some conventional binders.  相似文献   
78.
运筹学中有很多离散规划问题。其中的线性规划通常用分枝定界法或割平面法,还有图上作业法求解。不论哪种方法工作量都不小,而且效率低;至于非线性规划大都是用动态规划法求解,也很麻烦、耗时。对于大规模问题,不论线性或非线性离散规划,现有解法都受到问题规模的限制;还有资源分配和背包问题至今没有见到解决方法。本文就是为了解决这些问题,提出了相对差分搜索算法。通过5个算例和其它文献中的一些算例计算验证了本法简单、快速、有效和精确,尤其不受问题规模的限制是其最大的优点。  相似文献   
79.
For the system of multilayer dynamics of fluids in porous media, the second order upwind finite difference fractional steps schemes applicable to parallel arithmetic are put forward. Some techniques, such as calculus of variations, energy method, multiplicative commutation rule of difference operators, decomposition of high order difference operators and prior estimates are adopted. Optimal order estimates are derived to determine the error in the second order approximate solution. These methods have already been applied to the numerical simulation of migration-accumulation of oil resources.  相似文献   
80.
Abstract We analyze the efficiency of the international management of the Bay of Biscay anchovy. While a sharing agreement between France and Spain has been in place since 1992, the fish stock collapsed in 2005 and the fishery closed from 2005 to spring 2010. We consider differences in production technologies between both countries and calibrate our model using data from 1987 to 2009. Our results suggest two sources of rent dissipation under the existing sharing agreement: inefficient quota allocation and production inefficiencies due to inflexible national regulations. We discuss several alternatives to improve management.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号