全文获取类型
收费全文 | 131604篇 |
免费 | 35260篇 |
国内免费 | 32575篇 |
专业分类
化学 | 62065篇 |
晶体学 | 1955篇 |
力学 | 6876篇 |
综合类 | 1432篇 |
数学 | 13353篇 |
物理学 | 63182篇 |
无线电 | 50576篇 |
出版年
2024年 | 967篇 |
2023年 | 1960篇 |
2022年 | 2891篇 |
2021年 | 2810篇 |
2020年 | 2897篇 |
2019年 | 3947篇 |
2018年 | 3743篇 |
2017年 | 5046篇 |
2016年 | 5102篇 |
2015年 | 6579篇 |
2014年 | 6677篇 |
2013年 | 8850篇 |
2012年 | 9596篇 |
2011年 | 10454篇 |
2010年 | 13774篇 |
2009年 | 13837篇 |
2008年 | 7837篇 |
2007年 | 7224篇 |
2006年 | 6672篇 |
2005年 | 6378篇 |
2004年 | 6505篇 |
2003年 | 5019篇 |
2002年 | 4789篇 |
2001年 | 4787篇 |
2000年 | 4176篇 |
1999年 | 3406篇 |
1998年 | 2690篇 |
1997年 | 2404篇 |
1996年 | 2722篇 |
1995年 | 3080篇 |
1994年 | 3091篇 |
1993年 | 3239篇 |
1992年 | 2813篇 |
1991年 | 2438篇 |
1990年 | 1987篇 |
1989年 | 2096篇 |
1988年 | 1990篇 |
1987年 | 1248篇 |
1986年 | 1273篇 |
1985年 | 909篇 |
1984年 | 1035篇 |
1982年 | 936篇 |
1981年 | 774篇 |
1980年 | 806篇 |
1979年 | 558篇 |
1978年 | 544篇 |
1977年 | 643篇 |
1976年 | 1045篇 |
1972年 | 539篇 |
1971年 | 449篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
1.
Within the framework of the effective-mass approximation and the dipole approximation, considering the three-dimensional confinement of the electron and hole and the strong built-in electric field(BEF) in strained wurtzite Zn O/Mg0:25Zn0:75O quantum dots(QDs), the optical properties of ionized donor-bound excitons(D+, X)are investigated theoretically using a variational method. The computations are performed in the case of finite band offset. Numerical results indicate that the optical properties of(D+, X) complexes sensitively depend on the donor position, the QD size and the BEF. The binding energy of(D+, X) complexes is larger when the donor is located in the vicinity of the left interface of the QDs, and it decreases with increasing QD size. The oscillator strength reduces with an increase in the dot height and increases with an increase in the dot radius. Furthermore, when the QD size decreases, the absorption peak intensity shows a marked increment, and the absorption coefficient peak has a blueshift. The strong BEF causes a redshift of the absorption coefficient peak and causes the absorption peak intensity to decrease remarkably. The physical reasons for these relationships have been analyzed in depth. 相似文献
2.
We investigate the effect of dopant random fluctuation on threshold voltage and drain current variation in a two-gate nanoscale transistor. We used a quantum-corrected technology computer aided design simulation to run the simulation (10000 randomizations). With this simulation, we could study the effects of varying the dimensions (length and width), and thicknesses of oxide and dopant factors of a transistor on the threshold voltage and drain current in subthreshold region (off) and overthreshold (on). It was found that in the subthreshold region the variability of the drain current and threshold voltage is relatively fixed while in the overthreshold region the variability of the threshold voltage and drain current decreases remarkably, despite the slight reduction of gate voltage diffusion (compared with that of the subthreshold). These results have been interpreted by using previously reported models for threshold current variability, load displacement, and simple analytical calculations. Scaling analysis shows that the variability of the characteristics of this semiconductor increases as the effects of the short channel increases. Therefore, with a slight increase of length and a reduction of width, oxide thickness, and dopant factor, we could correct the effect of the short channel. 相似文献
3.
就标准厚度卡片在霍尔传感器的定标方面的应用作了有益的探讨。在纵向测量微小长度时,霍尔片处于磁场边缘位置,其对磁场的梯度更大,即灵敏度更高,且靠中心位置的线性更好。 相似文献
4.
Polypyrrole Shell@3D‐Ni Metal Core Structured Electrodes for High‐Performance Supercapacitors 下载免费PDF全文
Gao‐Feng Chen Yu‐Zhi Su Pan‐Yong Kuang Zhao‐Qing Liu Dao‐Yi Chen Xu Wu Nan Li Prof. Shi‐Zhang Qiao 《Chemistry (Weinheim an der Bergstrasse, Germany)》2015,21(12):4614-4621
Three‐dimensional (3D) nanometal films serving as current collectors have attracted much interest recently owing to their promising application in high‐performance supercapacitors. In the process of the electrochemical reaction, the 3D structure can provide a short diffusion path for fast ion transport, and the highly conductive nanometal may serve as a backbone for facile electron transfer. In this work, a novel polypyrrole (PPy) shell@3D‐Ni‐core composite is developed to enhance the electrochemical performance of conventional PPy. With the introduction of a Ni metal core, the as‐prepared material exhibits a high specific capacitance (726 F g?1 at a charge/discharge rate of 1 A g?1), good rate capability (a decay of 33 % in Csp with charge/discharge rates increasing from 1 to 20 A g?1), and high cycle stability (only a small decrease of 4.2 % in Csp after 1000 cycles at a scan rate of 100 mV s?1). Furthermore, an aqueous symmetric supercapacitor device is fabricated by using the as‐prepared composite as electrodes; the device demonstrates a high energy density (≈21.2 Wh kg?1) and superior long‐term cycle ability (only 4.4 % and 18.6 % loss in Csp after 2000 and 5000 cycles, respectively). 相似文献
5.
6.
Understanding and control of the surface properties such as molecular orientations are of great importance in numerous applications of ionic liquids. However, there remain discrepancies among the previous experimental and theoretical studies on the surface orientation and structures of room temperature ionic liquids(RTIL) systems. In this article, the orientation of 1-butyl-3-methylimidazolium([bmin]) cation at the air/liquid interface of a characteristic RTIL, 1-butyl-3-methylimidazolium hexafluorophosphate([bmim][PF6]), was investigated by the sum frequency generation vibrational spectroscopy(SFG-VS). Detailed polarization and experimental configuration analyses of the SFG-VS spectra showed the possibility of a small spectral splitting in the CH3 symmetric stretching region, which can be further attributed to the probable existence of multiple orientations for the interfacial [bmim] cations. In addition, the(N)–CH3 vibrations were absent, ruling out the prediction by several recent molecular dynamics simulations which state that portions of the [bmim] cations orient with a standing-up(N)–CH3 group at the ionic liquid surface. Hence, new realistic theoretical models have to be developed to reflect the complex nature of the ionic liquid surface. 相似文献
7.
针对现代消费类电子产品快速更新换代的现状, 介绍了该类产品的自动化生产线成组技术,阐述了通过度量消费类电子产品相似性和派生性特征,对产线进行成组编码的原理,研究产线工位单元的建模方法,描述了建模内容,并通过有无装载板的产线结构,论述了产线成组编码集成的途径和方法. 相似文献
8.
9.
10.
为实现电源设备乃至通信机房的少人或无人值守和集中维护,必须建立一套完善的电信局动力环境集中监控系统。文中结合番禺区电信局的实际,讨论了提高系统运行可靠性的几点措施,包括硬件方面与软件方面的手段,关键在于加强系统的运行管理。 相似文献