全文获取类型
收费全文 | 30570篇 |
免费 | 2773篇 |
国内免费 | 951篇 |
专业分类
电工技术 | 1280篇 |
综合类 | 1602篇 |
化学工业 | 6521篇 |
金属工艺 | 1145篇 |
机械仪表 | 1296篇 |
建筑科学 | 1711篇 |
矿业工程 | 450篇 |
能源动力 | 901篇 |
轻工业 | 4734篇 |
水利工程 | 589篇 |
石油天然气 | 764篇 |
武器工业 | 136篇 |
无线电 | 2485篇 |
一般工业技术 | 3737篇 |
冶金工业 | 1273篇 |
原子能技术 | 265篇 |
自动化技术 | 5405篇 |
出版年
2024年 | 103篇 |
2023年 | 397篇 |
2022年 | 1006篇 |
2021年 | 1400篇 |
2020年 | 1060篇 |
2019年 | 950篇 |
2018年 | 1060篇 |
2017年 | 1138篇 |
2016年 | 1045篇 |
2015年 | 1344篇 |
2014年 | 1805篇 |
2013年 | 2399篇 |
2012年 | 2537篇 |
2011年 | 2778篇 |
2010年 | 2272篇 |
2009年 | 2225篇 |
2008年 | 2023篇 |
2007年 | 1890篇 |
2006年 | 1495篇 |
2005年 | 1105篇 |
2004年 | 769篇 |
2003年 | 608篇 |
2002年 | 509篇 |
2001年 | 349篇 |
2000年 | 293篇 |
1999年 | 232篇 |
1998年 | 264篇 |
1997年 | 211篇 |
1996年 | 156篇 |
1995年 | 134篇 |
1994年 | 93篇 |
1993年 | 82篇 |
1992年 | 58篇 |
1991年 | 53篇 |
1990年 | 48篇 |
1989年 | 46篇 |
1988年 | 32篇 |
1987年 | 31篇 |
1986年 | 31篇 |
1985年 | 35篇 |
1984年 | 24篇 |
1983年 | 20篇 |
1982年 | 21篇 |
1981年 | 21篇 |
1980年 | 18篇 |
1979年 | 16篇 |
1978年 | 24篇 |
1977年 | 8篇 |
1976年 | 28篇 |
1973年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 312 毫秒
61.
The new emerging area of Enterprise Wide Optimization (EWO) has focused the attention in effectively solving the combined production/distribution scheduling problem. The importance of logistic activities performed in multi-site environments comes from the relative magnitude of the associated transportation costs and the good chance of getting large savings on such expenses. This paper first develops an exact MILP mathematical formulation for the multiple vehicle time-window-constrained pickup and delivery (MVPDPTW) problem. The approach is able to account for many-to-many transportation requests, pure pickup and delivery tasks, heterogeneous vehicles and multiple depots. Optimal solutions for a variety of benchmark problems with cluster/random distributions of pickup and delivery locations and limited sizes in terms of customer requests and vehicles have been discovered. However, the computational cost exponentially grows with the number of requests. For large-scale m-PDPTW problems, a local search improvement algorithm steadily providing a better solution through two evolutionary steps is also presented. A neighborhood structure around the starting solution is generated by first allowing multiple request exchanges among nearby trips and then permitting the reordering of nodes on every individual route. If a better set of routes is found, both steps are repeated until no improved solution is discovered. Compact MILP mathematical formulations for both sub-problems have been developed and solved through an efficient branch-and-bound algorithm. A significant number of large-scale m-PDPTW benchmark problems, some of them including up to 100 transportation requests, were successfully solved in reasonable CPU times. 相似文献
62.
Vanesa Rodríguez-Amor Juan P. Fernández-Blázquez Antonio Bello Ernesto Pérez María L. Cerrada 《Polymer Bulletin》2008,60(1):89-96
Summary The effect of molecular weight on the uniaxial orientation process has been analyzed in two samples of thermotropic poly(diethylene
glycol p,p′-bibenzoate), evaluating the influence of that parameter on the type of orientation obtained. Several strain rates and deformation
temperatures have been tested in order to map out the conditions for obtaining the two different kinds of orientation. The
results show that in the lower molecular weight PDEB sample (Mw=31200) it is rather easy to get exclusively perpendicular “anomalous” orientation, with the molecular axes aligned transversely
to the stretching direction. However, it is extremely difficult to obtain 100% parallel “normal” orientation. On the contrary,
either type of orientation or a mixture of them can be easily developed for the higher molecular weight PDEB sample (Mw=102900). 相似文献
63.
Carlos E. Esparza Martha P. Guerrero-Mata Roger Z. Ríos-Mercado 《Computational Materials Science》2006,36(4):457-467
A numerical optimization technique based on gradient-search is applied to obtain an optimal design of a typical gating system used for the gravity process to produce aluminum parts. This represents a novel application of coupling nonlinear optimization techniques with a foundry process simulator, and it is motivated by the fact that a scientifically guided search for better designs based on techniques that take into account the mathematical structure of the problem is preferred to commonly found trial-and-error approaches. The simulator applies the finite volume method and the VOF algorithm for CFD analysis. The direct gradient optimization algorithm, sequential quadratic programming (SQP), was used to solve both a 2D and a 3D gating system design problems using two design variables. The results clearly show the effectiveness of the proposed approach for finding high quality castings when compared with current industry practices. 相似文献
64.
65.
Carlos Fernandes Agostinho C. Rosa 《Soft Computing - A Fusion of Foundations, Methodologies and Applications》2008,12(10):955-979
Mate selection plays a crucial role in both natural and artificial systems. While traditional Evolutionary Algorithms (EA)
usually engage in random mating strategies, that is, mating chance is independent of genotypic or phenotypic distance between
individuals, in natural systems non-random mating is common, which means that somehow this mechanism has been favored during
the evolutionary process. In non-random mating, the individuals mate according to their parenthood or likeness. Previous studies
indicate that negative assortative mating (AM)—also known as dissortative mating—, which is a specific type of non-random mating, may improve EAs performance by maintaining the genetic diversity of the
population at a higher level during the search process. In this paper we present the Variable Dissortative Mating Genetic Algorithm (VDMGA). The algorithm holds a mechanism that varies the GA’s mating restrictions during the run by means of simple rule
based on the number of chromosomes created in each generation and indirectly influenced by the genetic diversity of the population.
We compare VDMGA not only with traditional Genetic Algorithms (GA) but also with two preceding non-random mating EAs: the
CHC algorithm and the negative Assortative Mating Genetic Algorithm (nAMGA). We intend to study the effects of the different methods in the performance of GAs and verify the reliability of
the proposed algorithm when facing an heterogeneous set of landscapes. In addition, we include the positive Assortative Mating Genetic Algorithm (pAMGA) in the experiments in order test both negative and positive AM mechanisms, and try to understand if and when negative
AM (or DM) speeds up the search process or enables the GAs to escape local optima traps. For these purposes, an extensive
set of optimization test problems was chosen to cover a variety of search landscapes with different characteristics. Our results
confirm that negative AM is effective in leading EAs out of local optima traps, and show that the proposed VDMGA is at least
as efficient as nAMGA when applied to the range of our problems, being more efficient in very hard functions were traditional
GAs usually fail to escape local optima. Also, scalability tests have been made that show VDMGA ability to decrease optimal
population size, thus reducing the amount of evaluations needed to attain global optima. We like to stress that only two parameters
need to be hand-tuned in VDMGA, thus reducing the tuning effort present in traditional GAs and nAMGA. 相似文献
66.
María J. Martín David E. Singh J. Carlos Mourio Francisco F. Rivera Ramn Doallo Javier D. Bruguera 《Parallel Computing》2003,29(11-12):1763
The aim of this work is to provide a high performance air quality simulation using the STEM-II (Sulphur Transport Eulerian Model 2) program, a large-scale pollution modeling application. First, we optimize the sequential program with the aim of increasing data locality. Then, we parallelized the program using OpenMP directives for shared memory systems, and the MPI library for distributed memory machines. Performance results are presented for a SGI O2000 multiprocessor, a Fujitsu AP3000 multicomputer and a Cluster of PCs. Experimental results show that the parallel versions of the code achieve important reductions in the CPU time needed by each simulation. This will allow us to obtain results with adequate speed and reliability for the industrial environment where it is intended to be applied. 相似文献
67.
LIJuan HUAYu-lin WANGChang-sheng XIONGShao-zhen 《半导体光子学与技术》2004,10(1):41-43,47
The white organic light emitting device (OLED) with single-structure using a polymer blend as the light emitting layer is fabricated. Heat treatment is used to control the ratio between the intensities of main electroluminescent spectral peaks. The electroluminescent spectrum of our device is quite similar to that of white inorganic LED produced by Nichia Corporation after being annealed, and its turn-on voltage can be decreased by 1V. 相似文献
68.
69.
70.
Angela M. Zapata-Anzola Carlos A. Rivera-Rondón Jhon Ch. Donato-Rondón 《Lakes & Reservoirs: Research and Management》2006,11(1):29-38
Guatavita Lake is a small, sheltered tropical high mountain lake located in the Colombian Andes, with a closed watershed and a maximum depth of 25 m. It is the freshwater source for human consumption in nearby small towns, as well as being a site of cultural value for the country, as it was a sacred place to indigenous peoples until about five centuries ago. As the structure and function of this aquatic ecosystem is poorly understood, this study provides initial knowledge on its phytoplankton biomass dynamics, which should be useful in designing efficient management plans with environmental baseline information for similar lakes elsewhere. Physical and chemical data, and photosynthetic pigment concentrations, were measured for the period November 1999–November 2000 at the central vertical axis of Guatavita Lake. The vertical profile of the chlorophyll‐a concentration was closely related to the dissolved inorganic nitrogen concentrations and the thermal stratification characteristics. The maximum chlorophyll‐a concentration in the metalimnion was recorded for the thermal stratification period (November 1999–June 2000). Deepening of the maximum chlorophyll‐a concentration began in February, continuing to June, when it reached its lowest value at the 15 m depth. The phytoplankton biomass values showed an increment within the entire water column at the beginning of the mixing period (July). The relation between the metalimnetic peak of chlorophyll‐a and the dissolved inorganic nitrogen concentration suggests the growth of the phytoplankton community is limited mainly by the availability of nitrogen. 相似文献