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   1509148篇
  免费   32107篇
  国内免费   8205篇
化学   652743篇
晶体学   20205篇
力学   74348篇
综合类   104篇
数学   239962篇
物理学   366382篇
无线电   195716篇
  2021年   14716篇
  2020年   17349篇
  2019年   17516篇
  2018年   13290篇
  2016年   29620篇
  2015年   21754篇
  2014年   32750篇
  2013年   78550篇
  2012年   39205篇
  2011年   35406篇
  2010年   39210篇
  2009年   44214篇
  2008年   38839篇
  2007年   35765篇
  2006年   42957篇
  2005年   34810篇
  2004年   36616篇
  2003年   34638篇
  2002年   35610篇
  2001年   34391篇
  2000年   30897篇
  1999年   28636篇
  1998年   27329篇
  1997年   27203篇
  1996年   26825篇
  1995年   24613篇
  1994年   24067篇
  1993年   23462篇
  1992年   23039篇
  1991年   23293篇
  1990年   22049篇
  1989年   21693篇
  1988年   20857篇
  1987年   19617篇
  1986年   18426篇
  1985年   24977篇
  1984年   26168篇
  1983年   22168篇
  1982年   23622篇
  1981年   22814篇
  1980年   22076篇
  1979年   21886篇
  1978年   23197篇
  1977年   22745篇
  1976年   22349篇
  1975年   21040篇
  1974年   20645篇
  1973年   21178篇
  1972年   15368篇
  1967年   13124篇
排序方式: 共有10000条查询结果,搜索用时 371 毫秒
961.
A birth-process approach to Moranda's geometric software-reliability model   总被引:1,自引:0,他引:1  
To alleviate some of the objections to the basic Jelinski Moranda (JM) model for software failures, Moranda proposed a geometric de-eutrophication model. This model assumes that the times between failures are statistically-independent exponential random variables with given failure rates. In this model the failure rates decrease geometrically with the detection of a fault. Using an intuitive approach, Musa, Iannino, Okumoto , see also Farr , derived expressions for the mean and the intensity functions of the process N (t) which counts the number of faults detected in the time interval [O, t] for the Moranda geometric de-eutrophication model. N (t) is studied as a pure birth stochastic process; its probability generating function is derived, as well as its mean, intensity and reliability functions. The expressions for the mean and intensity functions derived by MIO are only approximations and can be quite different from the true functions for certain choices of the failure rates. The exact expressions for the mean function and the intensity function of N (t) are used to find the optimum release time of software based on a cost structure for Moranda's geometric de-eutrophication model.  相似文献   
962.
A 1.5-μm eye-safe, 3-D scanning, and compact Mie LIght Detection and Ranging (LIDAR) is presented. The transmitter of the LIDAR is based on a KTA optical parameter oscillator (OPO) resonator. For detecting return signals, an InGaAs APD is used. The all solid-state OPO laser transmitter has the feature of small volume and lightweight, which allows a 165-lb compact eye-safe scanning LIDAR to be constructed. A system simulation using our own model was conducted to direct the system development. A method to solve the problem with small active area APD detectors was developed and described. The preliminary field-test measurement results indicated that the LIDAR has the capability to detect aerosols and clouds in lower atmospheres up to three dimensions.  相似文献   
963.
Methods for phase stabilization of ammonium nitrate were sought for in order to considerably expand the application area of this oxidizing agent in various-purpose self-combustible formulations, including that in a new generation of gas-generating formulations for automobile air bags. New methods for stabilization of ammonium nitrate were studied and, in particular, a search was made for organic compounds that can stabilize ammonium nitrate even at their low content. The mechanism of phase state stabilization of ammonium nitrate by compounds of this kind was examined.  相似文献   
964.
Cellular neural networks (CNNs) are dynamical systems, described by a large set of coupled nonlinear differential equations. The equilibrium-point analysis is an important step for understanding the global dynamics and for providing design rules. We yield a set of sufficient conditions (and a simple algorithm for checking them) ensuring the existence of at least one stable equilibrium point. Such conditions give rise to simple constraints, that extend the class of CNNs, for which the existence of a stable equilibrium point is rigorously proved. In addition, they are suitable for design and easy to check, because they are directly expressed in term of the template elements.  相似文献   
965.
Optical interconnection technology on the printed circuit board level is a key technology for future microelectronic equipment. The consideration of functional, technological, and economical requirements results in a hybrid solution, where electrical and optical interconnects are integrated into one substrate called electrical optical printed circuit board. The significant part of the entire design process for electrical optical printed circuit boards is marked by the design supporting modelling and simulation of optical interconnects. Based on an abstract model for an entire optical interconnect a simulation model for optical multimode-waveguides is presented, taking into account all significant waveguide properties. Apart from that, the modeling of active components (laser- and photo-diodes) is addressed.  相似文献   
966.
967.
We present a 3-12 GHz compact mixer-based reflectometer (CMR) and horn antenna system, and demonstrate its use in detecting backscatter signals with a free space calibration procedure. We evaluate the frequency-domain performance of the CMR-antenna system for measuring the complex reflection coefficient of a dielectric slab and compare it with that of a commercial vector network analyzer (VNA)-antenna setup. Time-domain responses are also investigated, and show the effectiveness of this calibration method. This low-cost, compact system eliminates the need for traditional mechanical standards and a VNA, is effective in reducing reflection artifacts, and allows for flexibility in the placement of reference planes; thus it is well suited for array-based imaging applications  相似文献   
968.
In this paper, we consider the problem of designing optimal asynchronous wake-up schedules to facilitate distributed power management and neighbor discovery in multihop wireless networks. We first formulate it as a block design problem and derive the fundamental trade-offs between wake-up latency and the average duty cycle of a node. After the theoretical foundation is laid, we then devise a neighbor discovery and schedule bookkeeping protocol that can operate on the optimal wake-up schedule derived. To demonstrate the usefulness of asynchronous wake-up, we investigate the efficiency of neighbor discovery and the application of on-demand power management, which overlays a desirable communication schedule over the wake-up schedule mandated by the asynchronous wake-up mechanism. Simulation studies demonstrate that the proposed asynchronous wake-up protocol has short discovery time which scales with the density of the network; it can accommodate various traffic characteristics and loads to achieve an energy savings that can be as high as 70 percent, while the packet delivery ratio is comparable to that without power management.  相似文献   
969.
970.
In colloidal suspensions of silica, particles undergo constant collisions. By controlling various parameters, the repulsive barrier can be reduced, thereby substantially increasing the number of collision‐induced dimerization events. Xia and co‐workers report on p. 1627 that the dimers could be arrested and then permanently fixed by introducing a small amount of fresh tetraethylorthosilicate into the colloidal suspension, with monodisperse dimer yields of up to 50 %. This yield could be increased to 80 % by centrifugation in a density gradient medium. When fresh tetraethylorthosilicate is introduced into a colloidal suspension of silica spheres, it hydrolyzes and condenses in situ to arrest and fix the dimers resulting from constant collisions between the spheres. By optimizing the experimental parameters (including the length of aging time) and the diameter of the silica spheres, as well as the concentrations of counterions, water, and ammonia, it is possible to routinely produce monodisperse dimers with a yield as high as 50 %. When combined with centrifugation using a density gradient medium, the yield of such dimers could be further increased to 80 %. It is believed that this method will provide a simple and versatile approach to the high‐volume production of dimers from spherical colloids composed of different materials. These dimers may find widespread use in a range of applications such as fabrication of photonic crystals and fundamental studies related to colloidal science.  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