全文获取类型
收费全文 | 47973篇 |
免费 | 5753篇 |
国内免费 | 2540篇 |
专业分类
化学 | 6124篇 |
晶体学 | 112篇 |
力学 | 2972篇 |
综合类 | 673篇 |
数学 | 11723篇 |
物理学 | 11346篇 |
无线电 | 23316篇 |
出版年
2024年 | 114篇 |
2023年 | 466篇 |
2022年 | 835篇 |
2021年 | 1087篇 |
2020年 | 1265篇 |
2019年 | 892篇 |
2018年 | 919篇 |
2017年 | 1476篇 |
2016年 | 1799篇 |
2015年 | 1812篇 |
2014年 | 3276篇 |
2013年 | 3177篇 |
2012年 | 3323篇 |
2011年 | 3361篇 |
2010年 | 2596篇 |
2009年 | 2761篇 |
2008年 | 3225篇 |
2007年 | 3279篇 |
2006年 | 2905篇 |
2005年 | 2529篇 |
2004年 | 2306篇 |
2003年 | 2162篇 |
2002年 | 1761篇 |
2001年 | 1379篇 |
2000年 | 1178篇 |
1999年 | 1036篇 |
1998年 | 855篇 |
1997年 | 758篇 |
1996年 | 677篇 |
1995年 | 634篇 |
1994年 | 428篇 |
1993年 | 386篇 |
1992年 | 286篇 |
1991年 | 275篇 |
1990年 | 182篇 |
1989年 | 110篇 |
1988年 | 140篇 |
1987年 | 91篇 |
1986年 | 82篇 |
1985年 | 83篇 |
1984年 | 63篇 |
1983年 | 42篇 |
1982年 | 56篇 |
1981年 | 48篇 |
1980年 | 31篇 |
1979年 | 39篇 |
1978年 | 24篇 |
1977年 | 13篇 |
1976年 | 9篇 |
1974年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
为满足便携式投影仪的市场需求,设计了一种基于LED微型阵列的投影系统。该系统由显示单元和投影物镜构成。采用尺寸为12 mm×9 mm的自发光LED微型阵列作为系统的显示单元,利用光学设计软件设计了投影物镜。投影物镜采用反远距光学结构,全视场角为80°,焦距为8 mm,属于强光、广角镜头。在空间频率20 lp/mm处,该物镜的调制传递函数大于0. 85,畸变小于2%,符合投影系统的设计要求。该投影系统具有体积小,结构简单,投影效果好,易加工等诸多优势,可为第三代投影技术的发展提供参考。 相似文献
992.
In this paper we study the domain of the generator of stable processes, stable-like processes and more general pseudo- and integro-differential operators which naturally arise both in analysis and as infinitesimal generators of Lévy- and Lévy-type (Feller) processes. In particular we obtain conditions on the symbol of the operator ensuring that certain (variable order) Hölder and Hölder–Zygmund spaces are in the domain. We use tools from probability theory to investigate the small-time asymptotics of the generalized moments of a Lévy or Lévy-type process , for functions f which are not necessarily bounded or differentiable. The pointwise limit exists for fixed if f satisfies a Hölder condition at x. Moreover, we give sufficient conditions which ensure that the limit exists uniformly in the space of continuous functions vanishing at infinity. As an application we prove that the domain of the generator of contains certain Hölder spaces of variable order. Our results apply, in particular, to stable-like processes, relativistic stable-like processes, solutions of Lévy-driven SDEs and Lévy processes. 相似文献
993.
Dr. Yi Luo Dr. Stef Smeets Dr. Zhendong Wang Prof. Junliang Sun Prof. Weimin Yang 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(9):2184-2188
A new germanosilicate zeolite named SCM-15 (Sinopec Composite Material No. 15), the first zeolite containing a 3-dimensional (3D) channel system with interconnected 12-, 12-, and 10-ring channels (pore sizes: 6.1×7.2, 6.1×7.4, and 5.2×5.9 Å), has been synthesized using neutral 4-pyrrolidinopyridine as organic structure-directing agents (OSDAs). Its structure has been determined by combining single-crystal electron diffraction (SCED) and synchrotron powder X-ray diffraction (SPXD) data. The unique open framework structure of SCM-15 is related to that of FOS-5 ( BEC ), ITQ-7 ( ISV ), PKU-16 ( POS ), ITQ-26 ( IWS ), ITQ-21, Beta polymorph B, and SU-78B, since all these framework structures can be constructed from similar chains which are connected through shared 4-ring or double 4-ring (d4r) units. Based on this relation, six topologically reasonable 3D large or extra-large pore hypothetical zeolites are predicted. 相似文献
994.
The gravitational Szekeres differential system is completely integrable with two rational first integrals and an additional analytical first integral. We describe the dynamics of the Szekeres system when one of these two rational first integrals is negative, showing that all the orbits come from the infinity of and go to infinity. 相似文献
995.
996.
Surasak Seesukphronrarak Shinichi Kawasaki Shigeki Kuwata Toshikazu Takata 《Journal of polymer science. Part A, Polymer chemistry》2019,57(24):2602-2605
Two novel 9,9‐difunctionalized fluorene‐type monomers, 9,9‐bis(4‐hydroxyphenyl‐ and 4‐aminophenyl)‐2,3:6,7‐dibenzofluorenes, are synthesized by the reaction of dibenzenzofluorenone with phenol and aniline. These monomers are used for the preparation of polyester and polyimide as the typical polymers to evaluate the property change such as thermal stability caused by the benzene rings fused to the fluorene skeleton with keeping good solubility, in comparison with the polymers derived from simple fluorenone. In fact, these two new polymers have the fairly enhanced thermal stability and refractive index value along with satisfactory solubility in organic solvents, enough to emphasize the fusion effect. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 2602–2605 相似文献
997.
We report the results of our investigation of magnetization and heat capacity on a series of compounds CeYxNiGe2 () under the influence of external magnetic field. Our studies of the thermodynamic quantity on these compounds indicate that magnetic frustration persists in Ce0.9Y0.1NiGe2, as also reported for the parent compound CeNiGe2. The weak signature of this frustration is also noted in Ce0.8Y0.2NiGe2, whereas, it is suppressed in Ce0.6Y0.4NiGe2. Heat capacity studies on Ce0.9Y0.1NiGe2 and Ce0.8Y0.2NiGe2 indicate the presence of a new magnetic anomaly at high field which indicates that quantum criticality is absent in these compounds. However, for Ce0.6Y0.4NiGe2 such an anomaly is not noted. For this later compound, the magnetic field (H) and temperature (T) dependence of heat capacity and magnetization obey scaling above critical fields. However, the obtained scaling critical parameter (δ) is 1.6, which is away from mean field value of 3. This deviation suggests the presence of unusual fluctuations and anomalous quantum criticality in these compounds. This unusual fluctuation may arise from disorderness induced by Y-substitution. 相似文献
998.
In this paper, we study the following Klein–Gordon–Maxwell system Using variational methods, we obtain the existence of ground state solutions under some appropriate assumptions on and . 相似文献
999.
In this study, batch experiments were conducted to investigate the performance of microscale Fe/Cu bimetallic particles-air-persulfate system (mFe/Cu-air-PS) for p-nitrophenol (PNP) treatment in aqueous solution. The results indicate that toxic and refractory PNP in aqueous solution could be decomposed effectively and transformed into lower toxicity intermediates. 相似文献
1000.
网络时代计算机通信与控制技术应用在各行各业之中,在获得这项技术带来的便利之时,其可靠性也受到人们的关注,并成为计算机发展的重要研究课题。本文首先对其可靠性理论进行了探讨,并重点分析了影响其可靠性的因素,从而提出了提升计算机通信与控制系统的策略。 相似文献