全文获取类型
收费全文 | 111615篇 |
免费 | 32137篇 |
国内免费 | 27817篇 |
专业分类
化学 | 56047篇 |
晶体学 | 1347篇 |
力学 | 5887篇 |
综合类 | 377篇 |
数学 | 11183篇 |
物理学 | 58289篇 |
无线电 | 38439篇 |
出版年
2024年 | 919篇 |
2023年 | 1995篇 |
2022年 | 2315篇 |
2021年 | 2478篇 |
2020年 | 2622篇 |
2019年 | 3718篇 |
2018年 | 3461篇 |
2017年 | 4333篇 |
2016年 | 4901篇 |
2015年 | 5905篇 |
2014年 | 5858篇 |
2013年 | 7751篇 |
2012年 | 8400篇 |
2011年 | 8998篇 |
2010年 | 11795篇 |
2009年 | 11774篇 |
2008年 | 5685篇 |
2007年 | 5066篇 |
2006年 | 4666篇 |
2005年 | 4466篇 |
2004年 | 5106篇 |
2003年 | 3958篇 |
2002年 | 3764篇 |
2001年 | 3848篇 |
2000年 | 3102篇 |
1999年 | 2956篇 |
1998年 | 2534篇 |
1997年 | 2269篇 |
1996年 | 2601篇 |
1995年 | 2941篇 |
1994年 | 2996篇 |
1993年 | 3118篇 |
1992年 | 2685篇 |
1991年 | 2341篇 |
1990年 | 1901篇 |
1989年 | 1995篇 |
1988年 | 1896篇 |
1987年 | 1149篇 |
1986年 | 1205篇 |
1985年 | 868篇 |
1984年 | 985篇 |
1982年 | 891篇 |
1981年 | 735篇 |
1980年 | 779篇 |
1979年 | 537篇 |
1978年 | 535篇 |
1977年 | 634篇 |
1976年 | 1043篇 |
1972年 | 539篇 |
1971年 | 445篇 |
排序方式: 共有10000条查询结果,搜索用时 15 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.
3.
4.
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
5.
6.
In the view of substrate availability, atomic efficiency and cost, directly using arenols as coupling partners in cross‐coupling, would be one of the most attractive goals. Up to date, many efforts have been made to activate the C—O bond of phenols with different strategies, for example, through in‐situ formed intermediates, through a catalytic reductive dearomatization‐condensation‐rearomatization sequence or catalytic deoxygenation. In this review, we summarized recent advances in cross‐couplings of arenols as the electrophiles via C—O activation. 相似文献
7.
8.
9.
We propose an analytical model to evaluate the lightpath blocking performance for a single ROADM node with intra-node add/drop contention,in which the number of lightpaths that can be added/dropped with the same wavelength is limited by the add/drop contention factor.Different models of traffic load per nodal degree are considered to validate the effectiveness of the analytical model.The simulation results show that the proposed analytical model is effective in predicting the performance for different values of add/drop contention factor C and for variable offered loads at the node.The add/drop contention factor shows an important impact on the lightpath blocking performance and properly raising the contention factor can significantly improve the lightpath blocking performance.When the add/drop contention factor C exceeds a certain level,the performance of a ROADM with intra-node contention is close to that of a contentionless ROADM. 相似文献
10.