全文获取类型
收费全文 | 2286篇 |
免费 | 95篇 |
国内免费 | 115篇 |
专业分类
化学 | 568篇 |
晶体学 | 1篇 |
力学 | 159篇 |
综合类 | 4篇 |
数学 | 456篇 |
物理学 | 854篇 |
无线电 | 454篇 |
出版年
2024年 | 9篇 |
2023年 | 41篇 |
2022年 | 36篇 |
2021年 | 34篇 |
2020年 | 55篇 |
2019年 | 66篇 |
2018年 | 46篇 |
2017年 | 78篇 |
2016年 | 64篇 |
2015年 | 72篇 |
2014年 | 156篇 |
2013年 | 158篇 |
2012年 | 112篇 |
2011年 | 187篇 |
2010年 | 119篇 |
2009年 | 134篇 |
2008年 | 136篇 |
2007年 | 131篇 |
2006年 | 116篇 |
2005年 | 104篇 |
2004年 | 75篇 |
2003年 | 74篇 |
2002年 | 51篇 |
2001年 | 31篇 |
2000年 | 42篇 |
1999年 | 27篇 |
1998年 | 41篇 |
1997年 | 31篇 |
1996年 | 28篇 |
1995年 | 26篇 |
1994年 | 30篇 |
1993年 | 36篇 |
1992年 | 25篇 |
1991年 | 29篇 |
1990年 | 17篇 |
1989年 | 14篇 |
1988年 | 14篇 |
1987年 | 9篇 |
1986年 | 4篇 |
1985年 | 9篇 |
1984年 | 5篇 |
1983年 | 6篇 |
1982年 | 2篇 |
1981年 | 2篇 |
1980年 | 3篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1977年 | 1篇 |
1973年 | 4篇 |
排序方式: 共有2496条查询结果,搜索用时 0 毫秒
61.
62.
近年来,实验发现钛酸铅基材料具有负热膨胀性,且其热膨胀程度会受到掺杂元素的影响. 目前所研究的A位掺杂体系中,仅Cd原子掺杂能使钛酸铅负热膨胀性增强. 所以研究A位掺杂钛酸铅,比较Cd原子与其他原子在掺杂钛酸铅时化学键的异同,有助于深刻理解钛酸铅负热膨胀的本质. 本文利用第一性原理,分别优化了Sr、Ba、Cd掺杂钛酸铅的晶格常数,计算了它们的态密度和电荷密度. 结果表明Cd―O键的共价性强于Pb―O键,而Ba―O键和Sr―O键几乎呈离子性,Ba/Sr对Pb的替代削弱了化合物的共价性,降低了自发极化强度. 与实验测量的热膨胀系数对比可以发现,A位原子与氧原子之间的共价性增强,化合物负热膨胀程度升高;若A位原子与氧原子之间的共价性削弱,负热膨胀程度降低. 可见A位原子与氧原子之间的共价性影响了钛酸铅基化合物负热膨胀性. 相似文献
63.
为替代传统的贵金属基NOx储存还原(NSR)催化剂,本文设计并制备了不含贵金属的BaFeO3-x+Cu-ZSM-5 耦合催化剂,用于催化消除稀燃发动机尾气中的NOx. 在稀燃阶段,NO在BaFeO3-x催化剂上发生了氧化和储存反应;在富燃阶段,从BaFeO3-x催化剂中脱附出来未能消除的NOx被置于其后的Cu-ZSM-5催化剂进一步催化消除. 实验结果表明,BaFeO3-x+Cu-ZSM-5 耦合催化剂的工作温度窗口被拓宽到250-400 ℃,同时NOx消除性能得到了显著提高:NOx转化率最高可达98%,N2选择性接近100%. 相似文献
64.
Well‐oriented ZnO nanorods (NRs) arrays were grown on Si, alumina, quartz, and FTO substrates through a ZnO seed layer followed by low temperature wet chemical process. The influence of sputtered ZnO seed layer thickness (100, 50, 32, and 16 nm), annealing temperature and CuOx coverage on the characteristics of ZnO NRs were investigated in this study. The crystalline structural, chemical, morphological, optical, and electrical properties of ZnO NRs arrays were studied by X‐ray diffraction (XRD), field emission‐ scanning electron microscopy equipped by energy dispersive X‐ray spectroscopy (FE‐SEM/EDX), Raman scattering, UV/Vis ‐ near IR absorption spectroscopy and current‐voltage characteristic. XRD and Raman spectra measurement revealed that the synthesize ZnO displayed hexagonal wurtzite structure. The individual rod diameter, density, and orientation can be controlled by varying the seed layer thickness. The mean diameter and maximum length of ZnO NRs are around 55–66 nm and 282 nm, respectively. ZnO NRs/ ZnO thin film structure shows optical switching and negative differential resistance behavior as applicable to ON/OFF gate and memory devices. 相似文献
65.
66.
Rong Cheng 《Applicable analysis》2017,96(5):778-798
In this paper, we study the existence and concentration of positive solution of a class of coupled Schrödinger equations. We admit that the potentials may not be non-negative and suppose that the intersection of the sets has positive Lebesgue measure. By studying the modified functional of the associated functional carefully, we establish the existence of positive least energy solutions for the coupled Schrödinger system. Moreover, we prove the concentration phenomenon of the positive solution when the parameter goes to infinity. 相似文献
67.
A weighted (unweighted) graph is called equiarboreal if the sum of weights (the number) of spanning trees containing a given edge in is independent of the choice of edge. In this paper, we give some resistance characterizations of equiarboreal weighted and unweighted graphs, and obtain the necessary and sufficient conditions for -subdivision graphs, iterated double graphs, line graphs of regular graphs and duals of planar graphs to be equiarboreal. Applying these results, we obtain new infinite families of equiarboreal graphs, including iterated double graphs of 1-walk-regular graphs, line graphs of triangle-free 2-walk-regular graphs, and duals of equiarboreal planar graphs. 相似文献
68.
This work presents sufficient conditions for the existence of homoclinic solutions for second order coupled discontinuous systems of differential equations on the real line without the usual growth condition in the literature.The arguments apply the fixed point theory, Green's functions technique, -Carathéodory functions, lower and upper solutions and Schauder's fixed point theorem. 相似文献
69.
In this paper, the dynamics of a system of two van der Pol equations with a finite delay are investigated. We show that there exist the stability switches and a sequence of Hopf bifurcations occur at the zero equilibrium when the delay varies. Using the theory of normal form and the center manifold theorem, the explicit expression for determining the direction of the Hopf bifurcations and the stability of the bifurcating periodic solutions are derived. 相似文献
70.
Ali Emrouznejad Abdel Latef Anouze Emmanuel Thanassoulis 《European Journal of Operational Research》2010
Data Envelopment Analysis (DEA) is a nonparametric method for measuring the efficiency of a set of decision making units such as firms or public sector agencies, first introduced into the operational research and management science literature by Charnes, Cooper, and Rhodes (CCR) [Charnes, A., Cooper, W.W., Rhodes, E., 1978. Measuring the efficiency of decision making units. European Journal of Operational Research 2, 429–444]. The original DEA models were applicable only to technologies characterized by positive inputs/outputs. In subsequent literature there have been various approaches to enable DEA to deal with negative data. 相似文献