全文获取类型
收费全文 | 38883篇 |
免费 | 5567篇 |
国内免费 | 3018篇 |
专业分类
化学 | 8256篇 |
晶体学 | 105篇 |
力学 | 4192篇 |
综合类 | 840篇 |
数学 | 21918篇 |
物理学 | 12157篇 |
出版年
2024年 | 63篇 |
2023年 | 359篇 |
2022年 | 551篇 |
2021年 | 801篇 |
2020年 | 1002篇 |
2019年 | 986篇 |
2018年 | 956篇 |
2017年 | 1287篇 |
2016年 | 1434篇 |
2015年 | 1138篇 |
2014年 | 1978篇 |
2013年 | 2936篇 |
2012年 | 2002篇 |
2011年 | 2262篇 |
2010年 | 2052篇 |
2009年 | 2413篇 |
2008年 | 2577篇 |
2007年 | 2719篇 |
2006年 | 2323篇 |
2005年 | 2045篇 |
2004年 | 1845篇 |
2003年 | 1698篇 |
2002年 | 1728篇 |
2001年 | 1329篇 |
2000年 | 1195篇 |
1999年 | 1145篇 |
1998年 | 1005篇 |
1997年 | 868篇 |
1996年 | 747篇 |
1995年 | 667篇 |
1994年 | 481篇 |
1993年 | 407篇 |
1992年 | 344篇 |
1991年 | 312篇 |
1990年 | 225篇 |
1989年 | 168篇 |
1988年 | 182篇 |
1987年 | 143篇 |
1986年 | 129篇 |
1985年 | 155篇 |
1984年 | 134篇 |
1983年 | 79篇 |
1982年 | 108篇 |
1981年 | 117篇 |
1980年 | 82篇 |
1979年 | 93篇 |
1978年 | 51篇 |
1977年 | 40篇 |
1976年 | 40篇 |
1973年 | 13篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
151.
Fang‐Jung Huang Tzong‐Liu Wang 《Journal of polymer science. Part A, Polymer chemistry》2004,42(2):290-302
Liquid‐crystalline, segmented polyurethanes with methoxy–biphenyl mesogens pendant on the chain extender were synthesized by the conventional prepolymer technique and esterification reaction. Two, side‐chain, liquid‐crystalline (SCLC) polyurethanes with mesogens having spacers of six and eight methylene units were prepared. The structures of the mesogenic units and SCLC polyurethanes were confirmed by Fourier transform infrared spectroscopy and 1H NMR. Polymer properties were also examined by solubility tests, water uptakes, and inherent viscosity measurements. Differential scanning calorimetry studies indicated that the transition temperature of the isotropic to the liquid‐crystalline phase decreased with increasing spacer length. Wide‐angle X‐ray diffraction (WAXD) studies revealed the existence of liquid‐crystalline phases for both SCLC polyurethanes. Polarized optical microscopic investigations further confirmed the thermotropic liquid‐crystalline behaviors and nematic mesophases of both samples. Thermogravimetric analysis displayed better thermal stabilities for both SCLC polymers and indicated that the presence of mesogenic side chains may increase the thermal stability of segmented polyurethanes. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 290–302, 2004 相似文献
152.
Alexandros A. Skordos Ivana K. Partridge 《Journal of Polymer Science.Polymer Physics》2004,42(1):146-154
This article presents a new methodology for the quantitative determination of the progress of the curing reaction of a thermosetting resin, using the results of electrical impedance spectroscopy. The method is an extension of the use of the imaginary impedance maximum as a reaction progress indicator and is based on the demonstration of a close correlation between the reaction rate, as measured by conventional differential scanning calorimetry, and the rate of change of the value of the imaginary impedance spectrum maximum. Tests on a commercial aerospace epoxy resin under both isothermal and dynamic heating conditions with calorimetry and impedance spectroscopy have demonstrated the validity of the method and set the accuracy limits involved. This technique can be used as a real-time online control tool for thermoset composite manufacturing. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 146–154, 2004 相似文献
153.
介绍了一台10mm口径两级双程离轴放大系统,实现了对5mm×5mm口径光束的激光放大,耦合系统采用高功率LDA紧密侧面直接抽运棒状Nd:YLF方式。分析并实验研究了在不同抽运电流、放大脉冲与放大器LDA抽运时刻的不同延时及不同注入能量条件下,放大系统及光束每次放大时放大特性的规律。实验得到:在放大系统5mm×5mm软光阑处注入1.58mJ能量时,放大系统可输出129.2mJ能量,能量提取效率达到19.5%,满足该系统的设计指标。 相似文献
154.
155.
156.
针对受参数不确定和外扰影响的混沌Lorenz系统,提出一种基于径向基函数(RBF)神经网 络的滑模控制方法.基于被控系统在不稳定平衡点处状态误差的可控规范形,设计滑模切换 面并将其作为神经网络的唯一输入.单入单出形式的RBF控制器隐层只需7个径向基函数,网 络的权值则依滑模趋近条件在线确定.仿真表明该控制器对系统参数突变和外部干扰具有鲁棒性,同时抑制了抖振.
关键词:
混沌控制
滑模
径向基函数神经网络
Lorenz系统 相似文献
157.
158.
与传统时域有限差分算法相比,采用以伪谱方法离散Maxwell微分方程为基础的时域伪谱(PSTD)算法计算大的电尺度电磁场时域问题,将大大提高计算效率,降低内存需求。为了拓宽PSTD算法的应用,近年来,基于网格插值方法的非均匀时域伪谱算法得到了发展。研究的重点是算法中非均匀网格技术的实现及其在时域瞬态脉冲电磁场模拟和高功率超宽带脉冲技术方面的应用。以高斯脉冲为激励源,用该算法计算了多层介质的反射和透射,并通过超宽带脉冲穿墙实验对这一方法的应用进行了验证。模拟和实验结果具有较好的一致性。 相似文献
159.
Differential game formulations provide an adequate basis for a guidance law synthesis against highly maneuvering targets. This paper deals with a guidance law based on a linear-quadratic differential game formulation. This guidance law has many attractive properties: it is continuous, linear with respect to the state variables, and its gain coefficients can be precalculated offline. Nevertheless, due to the lack of hard control constraints in the formulation, the magnitude of the control can exceed the admissible level imposed by the nature of the problem. In this paper, the upper bound of the interceptor control is obtained depending on the system parameters and the penalty coefficients of the game performance index. It is shown that the interceptor can guarantee an arbitrarily small miss distance without exceeding the control constraints if it has sufficient maneuverability and if the penalty coefficients are chosen properly. By manipulating the penalty coefficients, it is possible to reduce significantly the maneuverability requirements compared to the case of zero interceptor penalty coefficient. 相似文献
160.
Defining speed of diffusion as the amount of time it takes to get from one penetration level to a higher one, we introduce a dynamic model in which we study the link between pricing policy, speed of diffusion, and number of competitors in the market. Our analysis shows that, in the case of strategic (oligopolistic) competition, the speed of diffusion has an important influence on the optimal pricing policy. In particular, we find that higher speeds of diffusion create an incentive to strategically interacting firms to lower their prices. 相似文献