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   377793篇
  免费   3844篇
  国内免费   1131篇
化学   164462篇
晶体学   5374篇
力学   15531篇
综合类   7篇
数学   38461篇
物理学   109717篇
无线电   49216篇
  2021年   3495篇
  2020年   3775篇
  2019年   4241篇
  2018年   5720篇
  2017年   5870篇
  2016年   8060篇
  2015年   4368篇
  2014年   7658篇
  2013年   17087篇
  2012年   13091篇
  2011年   15664篇
  2010年   11960篇
  2009年   12446篇
  2008年   15111篇
  2007年   15626篇
  2006年   14510篇
  2005年   12757篇
  2004年   11901篇
  2003年   10689篇
  2002年   10592篇
  2001年   12293篇
  2000年   9545篇
  1999年   7633篇
  1998年   6594篇
  1997年   6403篇
  1996年   5921篇
  1995年   5219篇
  1994年   5137篇
  1993年   4979篇
  1992年   5405篇
  1991年   5514篇
  1990年   5232篇
  1989年   4959篇
  1988年   4638篇
  1987年   4424篇
  1986年   4111篇
  1985年   5140篇
  1984年   5181篇
  1983年   4311篇
  1982年   4315篇
  1981年   4019篇
  1980年   3907篇
  1979年   4151篇
  1978年   4122篇
  1977年   4114篇
  1976年   4062篇
  1975年   3878篇
  1974年   3776篇
  1973年   3816篇
  1972年   2730篇
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
211.
Itaconate (ITA) is an emerging powerhouse of innate immunity with therapeutic potential that is limited in its ability to be administered in a soluble form. A library of polyester materials that incorporate ITA into polymer backbones resulting in materials with inherent immunoregulatory behavior is developed. Harnessing hydrolytic degradation release from polyester backbones, ITA polymers result in the mechanism specific immunoregulatory properties on macrophage polarization in vitro. In a functional assay, the polymer-released ITA inhibits bacterial growth on acetate. Translation to an in vivo model of biomaterial associated inflammation, intraperitoneal injection of ITA polymers demonstrate a rapid resolution of inflammation in comparison to a control polymer silicone, demonstrating the value of sustained biomimetic presentation of ITA.  相似文献   
212.
In this paper, we propose a new peak-to-average power ratio (PAPR) reduction algorithm for OFDM transmission. The algorithm is based on simple amplitude predistortion of a subset of the input symbol blocks, the predistorted symbols being selected according to a metric which measures their contribution to the output signal samples of large. magnitude. This metric-based algorithm is simple, highly flexible, and can be implemented as a one-shot process, although its performance can be further improved by iterating the process one or more times.  相似文献   
213.
This paper addresses the impact of device macromodels on the accuracy of signal integrity and performance predictions for critical digital interconnecting systems. It exploits nonlinear parametric models for both single-ended and differential devices, including the effects of power supply fluctuations and receiver bit detection. The analysis demonstrates that the use of well-designed macromodels dramatically speeds up the simulation as well it preserves timing accuracy even for long bit sequences.  相似文献   
214.
Co-browsing is an activity in which a group of users navigate their way through of a set of Web pages together for a shared purpose. Effective co-browsing among users with different device capabilities requires a shared understanding of those Web pages. This paper demonstrates the concept of shared viewpoints (SVPs), and personal viewpoints (PVPs), for co-browsing, before detailing a framework for implementing these concepts. Finally, the effectiveness of the framework is presented through a perceptual experiment.  相似文献   
215.
This paper presents an analysis of Class D inverter when irregular driving patterns are given to the gate drive of the switch devices. The analysis has been carried out with focusing on the waveforms, harmonics, low-frequency components, output power, and equivalent dc resistance, which are numerically analyzed and discussed. Class D inverters with six different Q factors from 0.1 to 20 are analyzed about 2/sup 16/ driving patterns for each Q. Superior four models of the six inverters are built and tested in circuit experiments. The calculated waveforms are compared to the experimental results. Both of them are agreed well in time domain and frequency domain. Analytical results show a possibility of a novel control method with irregular driving patterns. In spite of discontinuous control, the output power or voltage can be strictly changed as if continuous using the selected driving patterns in some ranges.  相似文献   
216.
Formation of cobalt sulfide hollow nanocrystals through a mechanism similar to the Kirkendall Effect has been investigated in detail. It is found that performing the reaction at > 120 °C leads to fast formation of a single void inside each shell, whereas at room temperature multiple voids are formed within each shell, which can be attributed to strongly temperature‐dependent diffusivities for vacancies. The void formation process is dominated by outward diffusion of cobalt cations; still, the occurrence of significant inward transport of sulfur anions can be inferred as the final voids are smaller in diameter than the original cobalt nanocrystals. Comparison of volume distributions for initial and final nanostructures indicates excess apparent volume in shells, implying significant porosity and/or a defective structure. Indirect evidence for fracture of shells during growth at lower temperatures was observed in shell‐size statistics and transmission electron microscopy images of as‐grown shells. An idealized model of the diffusional process imposes two minimal requirements on material parameters for shell growth to be obtainable within a specific synthetic system.  相似文献   
217.
Using Java-based tools in multimedia collaborative environments accessed over the Internet can increase an application's client base. Most operating systems support Java, and its "compile once-run everywhere" architecture is easy to maintain and update. The Java-based tools presented here let users share Internet resources, including resources originally designed for single use.  相似文献   
218.
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   
219.
We have presented an overview of the literature and of our experimental data on the influence of electromagnetic radiation in the submillimeter spectral region on biological objects of molecular, cellular, and organism levels. Possible physical mechanisms involved and the reasons for limited development of the research in this spectral range are discussed.  相似文献   
220.
This paper proposes a method for designing a robust full-order observer for vector-controlled induction motors taking core loss into account. Although conventional research focuses on parameter identification, global stability of the identification remains questionable. Therefore, robustness against some parameters is required. This paper describes the design of a robust full-order observer which takes core loss into account, using both the gain-scheduled H/sub /spl infin// control and the linear matrix inequality technique. This design always results in a stable controller. The robustness of the proposed method against variations of resistances is evaluated by experiments.  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