全文获取类型
收费全文 | 3352篇 |
免费 | 801篇 |
国内免费 | 1263篇 |
专业分类
化学 | 2451篇 |
晶体学 | 85篇 |
力学 | 259篇 |
综合类 | 143篇 |
数学 | 702篇 |
物理学 | 1776篇 |
出版年
2024年 | 19篇 |
2023年 | 80篇 |
2022年 | 96篇 |
2021年 | 111篇 |
2020年 | 73篇 |
2019年 | 107篇 |
2018年 | 122篇 |
2017年 | 138篇 |
2016年 | 108篇 |
2015年 | 108篇 |
2014年 | 211篇 |
2013年 | 152篇 |
2012年 | 184篇 |
2011年 | 178篇 |
2010年 | 163篇 |
2009年 | 162篇 |
2008年 | 206篇 |
2007年 | 178篇 |
2006年 | 194篇 |
2005年 | 172篇 |
2004年 | 195篇 |
2003年 | 179篇 |
2002年 | 139篇 |
2001年 | 146篇 |
2000年 | 137篇 |
1999年 | 125篇 |
1998年 | 143篇 |
1997年 | 179篇 |
1996年 | 166篇 |
1995年 | 159篇 |
1994年 | 132篇 |
1993年 | 101篇 |
1992年 | 143篇 |
1991年 | 112篇 |
1990年 | 112篇 |
1989年 | 90篇 |
1988年 | 55篇 |
1987年 | 66篇 |
1986年 | 34篇 |
1985年 | 45篇 |
1984年 | 43篇 |
1983年 | 47篇 |
1982年 | 17篇 |
1981年 | 28篇 |
1980年 | 13篇 |
1979年 | 6篇 |
1978年 | 7篇 |
1977年 | 4篇 |
1958年 | 4篇 |
1954年 | 3篇 |
排序方式: 共有5416条查询结果,搜索用时 15 毫秒
991.
992.
以介孔碳纳米线为基体, 通过电化学方法制备了新型聚苯胺/介孔碳纳米线(PANI/MCFs)复合材料, 采用SEM和TEM等手段对样品的结构和形貌进行了表征. 结果表明, 聚苯胺均匀附在介孔碳纳米线表面, 并填充到纳米线介孔孔道中. 将复合材料组装成三电极体系超级电容器, 用循环伏安、 恒流充放电和交流阻抗等方法对材料的电化学性能进行了测试. 结果显示, 在1 mol/L H2SO4溶液中, 复合材料的比电容达到391 F/g, 其循环稳定性也得到显著提高. 相似文献
993.
994.
Revealing the physical nature of vortex wall(VW) behavior in magnetic nanostructures has been of great importance for future device concepts. Here we introduce the superior properties of VW in a notched FeNi nanowire under the action of an electronic current. The pinning-dependent VW propagation is demonstrated by a successive in-field magnetic force microscopy, an anisotropic magnetoresistance measurement, as well as micromagnetics.Based on the developed method, the propagation of VW can be effectively captured by monitoring the change of magnetoresistance in the FeNi nanowire, which sheds light on the development of future spin-based devices. 相似文献
995.
针对高阶正交幅度调制和大线宽相干光正交频分复用(CO-OFDM)系统,提出了一种基于广义回归神经网络(GRNN)的非线性均衡算法。将接收端进行相位噪声恢复之后的批量数据作为训练数据样本,通过训练学习得到GRNN的唯一参数平滑因子,然后对测试数据进行非线性均衡。对传输速率为50Gb/s,传输距离为100km的CO-OFDM系统进行了仿真验证。仿真结果表明,在大线宽和高阶调制下,GRNN非线性均衡算法对系统非线性损伤的补偿效果优于相应反向传播神经网络(BPNN)非线性均衡算法,且其训练运行时间远小于BPNN。GRNN非线性均衡算法能极大促进CO-OFDM系统在中长距离光纤传输中的应用。 相似文献
996.
本文用高精度的量子力学ab initio方法计算了氦原子与一氧化碳分子相互作用各向异性势能面,通过三重激发校正耦合簇、二次组态相互作用等方法和不同基组的计算结果比较,并采用BSSE方法消除了基组重叠误差,得到了氦原子与一氧化碳分子体系相互作用各向异性势,然后采用精确度较高的密耦(Close-Coupling)近似方法,研究了氦原子与一氧化碳分子碰撞的散射截面,通过计算得出了该体系碰撞激发微分截面和分波截面,计算得到的微分截面数据与实验值符合较好,说明本文得到的势能面是准确的. 相似文献
997.
Molecular dynamics simulations are performed to investigate the liquid–liquid phase transition(LLPT) and the spatial heterogeneity in Al–Pb monotectic alloys. The results reveal that homogeneous liquid Al–Pb alloy undergoes an LLPT,separating into Al-rich and Pb-rich domains, which is quite different from the isocompositional liquid water with a transition between low-density liquid(LDL) and high-density liquid(HDL). With spatial heterogeneity becoming large, LLPT takes place correspondingly. The relationship between the cooling rate, relaxation temperature and percentage of Al and the spatial heterogeneity is also reported. This study may throw light on the relationship between the structure heterogeneity and LLPT, which provides novel strategies to control the microstructures in the fabrication of the material with high performance. 相似文献
998.
In nanomaterials, optical anisotropies reveal a fundamental relationship between structural and optical properties, in which directional optical properties can be exploited to enhance the performance of optoelectronic devices. First principles calculation based on density functional theory(DFT) with the generalized gradient approximation(GGA) are carried out to investigate the energy band gap structure on silicon(Si) and germanium(Ge) nanofilms. Simulation results show that the band gaps in Si(100) and Ge(111) nanofilms become the direct-gap structure in the thickness range less than 7.64 nm and7.25 nm respectively, but the band gaps of Si(111) and Ge(110) nanofilms still keep in an indirect-gap structure and are independent on film thickness, and the band gaps of Si(110) and Ge(100) nanofilms could be transferred into the direct-gap structure in nanofilms with smaller thickness. It is amazing that the band gaps of Si~((1-x)/2)Ge~xSi~((1-x)/2)sandwich structure become the direct-gap structure in a certain area whether(111) or(100) surface. The band structure change of Si and Ge thin films in three orientations is not the same and the physical mechanism is very interesting, where the changes of the band gaps on the Si and Ge nanofilms follow the quantum confinement effects. 相似文献
999.
Structural,optical,and electrical properties of Cu-doped ZrO_2 films prepared by magnetron co-sputtering 下载免费PDF全文
Copper(Cu)-doped ZrO_2(CZO) films with different Cu content(0 at.%~ 8.07 at.%) are successfully deposited on Si(100) substrates by direct current(DC) and radio frequency(RF) magnetron co-sputtering. The influences of Cu content on structural, morphological, optical and electrical properties of CZO films are discussed in detail. The CZO films exhibit ZrO_2 monocline(ˉ111) preferred orientation, which indicates that Cu atoms are doped in ZrO_2 host lattice. The crystallite size estimated form x-ray diffraction(XRD) increases by Cu doping, which accords with the result observed from the scanning electron microscope(SEM). The electrical resistivity decreases from 2.63 ?.cm to 1.48 ?·cm with Cu doping content increasing, which indicates that the conductivity of CZO film is improved. However, the visible light transmittances decrease slightly by Cu doping and the optical band gap values decrease from 4.64 eV to 4.48 eV for CZO films. 相似文献
1000.
研究决策信息以区间数形式给出的方案决策问题。考虑到区间数非均匀分布的特点,为避免区间极端值在区间数确定中产生较大误差,本文将中位数概念运用到区间数的确定上,提出OIP(Ordered Interval Point)有序中位算子。取单位区间单调函数(BUM函数)为二分之一所表示的值为权重,将区间数确定为一个实数,并研究算子单调性和有界性的初等运算性质。通过比较OIP算子与COWA算子对态度参数和区间长度的反应灵敏度,获得了在一定条件下OIP算子对态度参数反应更稳健,对区间长度反应更灵敏的结论。最后用算例证明该算子的可行性和有效性。 相似文献