全文获取类型
收费全文 | 73931篇 |
免费 | 10487篇 |
国内免费 | 6155篇 |
专业分类
电工技术 | 10800篇 |
技术理论 | 3篇 |
综合类 | 7917篇 |
化学工业 | 5252篇 |
金属工艺 | 2394篇 |
机械仪表 | 7524篇 |
建筑科学 | 4402篇 |
矿业工程 | 2570篇 |
能源动力 | 3414篇 |
轻工业 | 3390篇 |
水利工程 | 2039篇 |
石油天然气 | 2941篇 |
武器工业 | 1364篇 |
无线电 | 6595篇 |
一般工业技术 | 6767篇 |
冶金工业 | 2273篇 |
原子能技术 | 432篇 |
自动化技术 | 20496篇 |
出版年
2024年 | 525篇 |
2023年 | 1552篇 |
2022年 | 2765篇 |
2021年 | 2984篇 |
2020年 | 3234篇 |
2019年 | 2806篇 |
2018年 | 2487篇 |
2017年 | 3068篇 |
2016年 | 3440篇 |
2015年 | 3832篇 |
2014年 | 5535篇 |
2013年 | 5329篇 |
2012年 | 6134篇 |
2011年 | 6315篇 |
2010年 | 4386篇 |
2009年 | 4669篇 |
2008年 | 4194篇 |
2007年 | 4821篇 |
2006年 | 4043篇 |
2005年 | 3266篇 |
2004年 | 2714篇 |
2003年 | 2235篇 |
2002年 | 1835篇 |
2001年 | 1550篇 |
2000年 | 1342篇 |
1999年 | 916篇 |
1998年 | 825篇 |
1997年 | 701篇 |
1996年 | 564篇 |
1995年 | 496篇 |
1994年 | 416篇 |
1993年 | 289篇 |
1992年 | 262篇 |
1991年 | 187篇 |
1990年 | 183篇 |
1989年 | 140篇 |
1988年 | 113篇 |
1987年 | 65篇 |
1986年 | 44篇 |
1985年 | 33篇 |
1984年 | 35篇 |
1983年 | 38篇 |
1982年 | 24篇 |
1981年 | 17篇 |
1980年 | 18篇 |
1979年 | 19篇 |
1978年 | 10篇 |
1964年 | 11篇 |
1956年 | 9篇 |
1951年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
Motion of a stick-slip piezo actuator is generally controlled by the parameters related to its mechanical design and characteristics of the driving pulses applied to piezoceramic shear plates. The goal of the proposed optimization method is to find the driving pulse parameters leading to the fastest and the most reliable actuator operation. In the paper the method is tested on a rotary stick-slip piezo actuating system utilized in an atomic force microscope.The optimization is based on the measurement of the actuator response to driving pulses of different shapes and repetition frequencies at various load forces. To provide it, a computer controlled testing system generating the driving pulses, and detecting and recording the corresponding angular motion response of the actuator by a position sensitive photo detector (PSPD) in real time has been developed. To better understand and interpret the experimental results, supportive methods based on a simple analytical model and numerical simulations were used as well.In this way the shapes of the single driving pulses and values of the load force providing the biggest actuator steps were determined. Generally, the maximal steps were achieved for such a combination of the pulse shapes and load forces providing high velocities at the end of the sticking mode of the actuator motion and, at the same time, lower decelerations during the slipping mode.As for the multiple driving pulses, the pulse shapes and values of repetition frequency ensuring the sticking mode of the actuator motion during the pulse rise time together with the maximum average angular rotor velocity were specified. In this way the effective and stable operation conditions of the actuator were provided.In principle, the presented method can be applied for the testing and optimization of any linear or angular stick-slip actuator. 相似文献
43.
我国的军工科研所成立于上世纪,它以军工项目的科研、预研为主,为我国的国防军工提供研发产品。伴随着我国经济实力的增强和科技军事力量的增长,军工科研院所也转变了原有的工业专属性质,走上了军民两用、寓军于民的道路,并在时代的进步中成为了我国先进装备制造业的重要科研力量和科技创新支柱。凸现军事工业“高、精、尖”的特点,以项目管理的优化为具体策略进行军工科研开发工作。 相似文献
44.
We consider robust knapsack problems where item weights are uncertain. We are allowed to query an item to find its exact weight,where the number of such queries is bounded by a given parameter Q. After these queries are made, we need to pack the items robustly, i.e., so that the choice of items is feasible for every remaining possible scenario of item weights.The central question that we consider is: Which items should be queried in order to gain maximum profit? We introduce the notion of query competitiveness for strict robustness to evaluate the quality of an algorithm for this problem, and obtain lower and upper bounds on this competitiveness for interval-based uncertainty. Similar to the study of online algorithms, we study the competitiveness under different frameworks, namely we analyze the worst-case query competitiveness for deterministic algorithms, the expected query competitiveness for randomized algorithms and the average case competitiveness for known distributions of the uncertain input data. We derive theoretical bounds for these different frameworks and evaluate them experimentally. We also extend this approach to Γ-restricted uncertainties introduced by Bertsimas and Sim.Furthermore, we present heuristic algorithms for the problem. In computational experiments considering both the interval-based and the Γ-restricted uncertainty, we evaluate their empirical performance. While the usage of a Γ-restricted uncertainty improves the nominal performance of a solution (as expected), we find that the query competitiveness gets worse. 相似文献
45.
The paper proposes a limit analysis approach to define the ultimate load capacity of orthotropic composite laminates under biaxial loading and plane stress conditions. A lower bound to the collapse load multiplier is computed by solving a maximization nonlinear problem, according to the static theorem of limit analysis. To set up the optimization problem a stress field distribution is hypothesized at lamina level, moreover inter-lamina stresses are also considered. The effectiveness and validity of the proposed approach is shown by comparing the obtained numerical predictions both with available experimental data and with other numerical results carried out by means of a different numerical lower bound approach. 相似文献
46.
47.
This paper investigates the N-policy M/M/1 queueing system with working vacation and server breakdowns. As soon as the system becomes empty, the server begins a working vacation. The server works at a lower service rate rather than completely stopping service during a vacation period. The server may break down with different breakdown rates during the idle, working vacation, and normal busy periods. It is assumed that service times, vacation times, and repair times are all exponentially distributed. We analyze this queueing model as a quasi-birth–death process. Furthermore, the equilibrium condition of the system is derived for the steady state. Using the matrix-geometric method, we find the matrix-form expressions for the stationary probability distribution of the number of customers in the system and system performance measures. The expected cost function per unit time is constructed to determine the optimal values of the system decision variables, including the threshold N and mean service rates. We employ the particle swarm optimization algorithm to solve the optimization problem. Finally, numerical results are provided, and an application example is given to demonstrate the applicability of the queueing model. 相似文献
48.
To minimize the mass and increase the bearing failure load of composite doublelap bolted joints, a three-step optimization strategy including feasible region reduction,
optimization model decoupling and optimization was presented. In feasible region
reduction, the dimensions of the feasible design region were reduced by selecting dominant
design variables from numerous multilevel parameters by sensitivity analyses, and the
feasible regions of variables were reduced by influence mechanism analyses. In model
decoupling, the optimization model with a large number of variables was divided into
various sub-models with fewer variables by variance analysis. In the third step, the
optimization sub-models were solved one by one using a genetic algorithm, and the
modified characteristic curve method was adopted as the failure prediction method. Based
on the proposed optimization method, optimization of a double-lap single-bolt joint was
performed using the ANSYS® code. The results show that the bearing failure load increased
by 13.5% and that the mass decreased by 8.7% compared with those of the initial design of
the joint, which validated the effectiveness of the three-step optimization strategy. 相似文献
49.
Hai Huang Haichao An Haibo Ma Shenyan Chen 《International journal for numerical methods in engineering》2019,117(3):291-315
This work presents an engineering method for optimizing structures made of bars, beams, plates, or a combination of those components. Corresponding problems involve both continuous (size) and discrete (topology) variables. Using a branched multipoint approximate function, which involves such mixed variables, a series of sequential approximate problems are constructed to make the primal problem explicit. To solve the approximate problems, genetic algorithm (GA) is utilized to optimize discrete variables, and when calculating individual fitness values in GA, a second-level approximate problem only involving retained continuous variables is built to optimize continuous variables. The solution to the second-level approximate problem can be easily obtained with dual methods. Structural analyses are only needed before improving the branched approximate functions in the iteration cycles. The method aims at optimal design of discrete structures consisting of bars, beams, plates, or other components. Numerical examples are given to illustrate its effectiveness, including frame topology optimization, layout optimization of stiffeners modeled with beams or shells, concurrent layout optimization of beam and shell components, and an application in a microsatellite structure. Optimization results show that the number of structural analyses is dramatically decreased when compared with pure GA while even comparable to pure sizing optimization. 相似文献
50.