全文获取类型
收费全文 | 5353篇 |
免费 | 850篇 |
国内免费 | 599篇 |
专业分类
化学 | 3702篇 |
晶体学 | 61篇 |
力学 | 312篇 |
综合类 | 38篇 |
数学 | 618篇 |
物理学 | 2071篇 |
出版年
2024年 | 18篇 |
2023年 | 117篇 |
2022年 | 144篇 |
2021年 | 185篇 |
2020年 | 207篇 |
2019年 | 159篇 |
2018年 | 168篇 |
2017年 | 145篇 |
2016年 | 238篇 |
2015年 | 237篇 |
2014年 | 317篇 |
2013年 | 352篇 |
2012年 | 475篇 |
2011年 | 475篇 |
2010年 | 355篇 |
2009年 | 332篇 |
2008年 | 325篇 |
2007年 | 322篇 |
2006年 | 290篇 |
2005年 | 254篇 |
2004年 | 196篇 |
2003年 | 138篇 |
2002年 | 158篇 |
2001年 | 124篇 |
2000年 | 107篇 |
1999年 | 120篇 |
1998年 | 108篇 |
1997年 | 97篇 |
1996年 | 95篇 |
1995年 | 70篇 |
1994年 | 82篇 |
1993年 | 55篇 |
1992年 | 56篇 |
1991年 | 57篇 |
1990年 | 35篇 |
1989年 | 52篇 |
1988年 | 40篇 |
1987年 | 35篇 |
1986年 | 14篇 |
1985年 | 14篇 |
1984年 | 9篇 |
1983年 | 8篇 |
1982年 | 4篇 |
1981年 | 8篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1957年 | 2篇 |
排序方式: 共有6802条查询结果,搜索用时 15 毫秒
51.
52.
Difference-frequency ultrasound generation from microbubbles under dual-frequency excitation 下载免费PDF全文
The difference-frequency (DF) ultrasound generated by using parametric effect promises to improve detection depth owing to its low attenuation, which is beneficial for deep tissue imaging. With ultrasound contrast agents infusion, the harmonic components scattered from the microbubbles, including DF, can be generated due to the nonlinear vibration. A theoretical study on the DF generation from microbubbles under the dual-frequency excitation is proposed in formula based on the solution of the RPNNP equation. The optimisation of the DF generation is discussed associated with the applied acoustic pressure, frequency, and the microbubble size. Experiments are performed to validate the theoretical predictions by using a dual-frequency signal to excite microbubbles. Both the numerical and experimental results demonstrate that the optimised DF ultrasound can be achieved as the difference frequency is close to the resonance frequency of the microbubble and improve the contrast-to-tissue ratio in imaging. 相似文献
53.
开展了铝单丝在负极性电流脉冲作用下电爆炸特性的研究.利用皮秒激光探针,搭建了阴影、纹影和干涉的光学诊断平台,得到了不镀膜铝丝典型的能量沉积过程,在电压崩溃时刻其沉积能量为2.4 eV/atom.为了增加金属丝内的沉积能量,开展了相同电参数及金属丝尺寸下的镀膜铝丝电爆炸实验,其沉积能量可达到5 eV/atom,实现了在电压崩溃之前铝丝完全气化(完全气化所需能量为4 eV/atom).阴影图像展示了高密度丝核区域的膨胀过程,不镀膜铝丝平均膨胀速度为2.2 km/s,而镀膜铝丝因为沉积能量大,其膨胀速度约为不镀膜铝丝的2.3倍,高密度区域膨胀速度为5 km/s.由于阴影不能反映低密度等离子体的膨胀,开展了平行双丝实验,通过测量自发光辐射,估算了低密度等离子体的膨胀速度.利用条纹相机拍摄了不镀膜铝丝电爆炸过程中自发光区域的图像.纹影图像清晰地展示了不镀膜铝丝在电爆炸过程中形成的核冕结构,而镀膜铝丝电爆炸过程中核冕结构得到了一定程度的抑制.从干涉图像计算了相移,在轴对称假设下对相移进行阿贝尔逆变换,重构了三维的铝原子数密度分布. 相似文献
54.
55.
Huimin He Fengman Liu Haiyun Xue Haidong Wang Delong Qiu 《Fiber and Integrated Optics》2016,35(5-6):212-229
With the development of the multicore processor, the bandwidth and capacity of the memory, rather than the memory area, are the key factors in server performance. At present, however, the new architectures, such as fully buffered DIMM (FBDIMM), hybrid memory cube (HMC), and high bandwidth memory (HBM), cannot be commercially applied in the server. Therefore, a new architecture for the server is proposed. CPU and memory are separated onto different boards, and optical interconnection is used for the communication between them. Each optical module corresponds to each dual inline memory module (DIMM) with 64 channels. Compared to the previous technology, not only can the architecture realize high-capacity and wide-bandwidth memory, it also can reduce power consumption and cost, and be compatible with the existing dynamic random access memory (DRAM). In this article, the proposed module with system-in-package (SiP) integration is demonstrated. In the optical module, the silicon photonic chip is included, which is a promising technology to be applied in the next-generation data exchanging centers. And due to the bandwidth–distance performance of the optical interconnection, SerDes chips are introduced to convert the 64-bit data at 800 Mbps from/to 4-channel data at 12.8 Gbps after/before they are transmitted though optical fiber. All the devices are packaged on cheap organic substrates. To ensure the performance of the whole system, several optimization efforts have been performed on the two modules. High-speed interconnection traces have been designed and simulated with electromagnetic simulation software. Steady-state thermal characteristics of the transceiver module have been evaluated by ANSYS APLD based on finite-element methodology (FEM). Heat sinks are placed at the hotspot area to ensure the reliability of all working chips. Finally, this transceiver system based on silicon photonics is measured, and the eye diagrams of data and clock signals are verified. 相似文献
56.
It is well known that for a Brownian motion, if we change the medium to be inhomogeneous by a measure μ, then the new motion(the time-changed process) will diffuse according to a different metric D(·, ·).In 2009, Kigami initiated a general scheme to construct such metrics through some self-similar weight functions g on the symbolic space. In order to provide concrete models to Kigami’s theoretical construction, in this paper,we give a thorough study of his metric on two classes of fractals of pr... 相似文献
57.
Lin Ziyang Jue Shi Xiang Ying L. P. Shi Wang Jian Z. X. Liang Z. R. Qiu 《Physics letters. A》2000,270(6):207-330
We observed an azo-isomerization-induced Freedericksz transition of the 5CB liquid crystal doped with 1% double-azo. In experiment, we found that the Freedericksz transition threshold occurred at optical intensity level of 0.3 mW/mm2 for the 12.5 μm-thickness planar alignment doped 5CB, which is four orders of magnitude lower as that for the pure 5CB. Using optical phase retardation, we measured the relations between the Freedericksz transition threshold and the pump beam polarization. The maximum twist angle of liquid crystal molecules under different pump beam intensity is given. The experiment indicated that trans–cis isomerization plays an important role in the decrease of the Freedericksz transition threshold. 相似文献
58.
The electronic structures of the laser crystal MgF2:V2+ were obtained with the spinpolarized MS-X
method. The calculated value of 10Dq and the energies of the lowest doublet states were given for the first time. The relations between the laser properties and the electronic structures were discussed. 相似文献
59.
B. Yang J. Zhou H. Hu L. Li J. Qiu J. Guo P. He J. Lu C. Ye 《Applied magnetic resonance》1995,9(3):379-388
Four Ib-type synthetic diamond crystals were studied by dynamic nuclear polarization (DNP)-enhanced high resolution solid state13C nuclear magnetic resonance (NMR) spectroscopy. The home built DNP magic-angle-spinning (MAS) NMR spectrometer operates at a field strength of 1.9 T and the highest DNP enhancement factor of synthetic diamonds came near to 103. Comparing with Ib-type natural diamonds, the13C NMR linewidths of synthetic diamonds in static spectra are broader. The13C spin-lattice relaxation time and DNP polarization time of synthetic diamond are shorter than those of Ib-type natural diamond. From the hyperfine structure of the DNP enhancement curve, four kinds of nitrogen-centred free radicals could be identified in synthetic diamond. 相似文献
60.
Yikai Chen Douguo Zhang Dong Qiu Liangfu Zhu Sisheng Yu Peijun Yao Pei Wang Hai Ming Ramachandram Badugu Joseph R. Lakowicz 《Laser \u0026amp; Photonics Reviews》2014,8(6):933-940
The unique optical properties of Tamm plasmons (TPs) – such as flexible wavevector matching conditions including inplane wavevector within the light line, and existing both S‐ and P‐polarized TPs − facilitate them for direct optical excitation. The Tamm plasmon‐coupled emission (TPCE) from a combined photonic–plasmonic structure sustaining both surface plasmons (SPs) and TPs is described in this paper. The sensitivity of TPCE to the emission wavelength and polarization is examined with back focal plane imaging and verified with the numerical calculations. The results reveal that the excited probe can couple with both TPs and SPs, resulting in surface plasmon‐coupled emission (SPCE) and TPCE, respectively. The TPCE angle is strongly dependent on the wavelength allowing for spectral resolution using different observation angles. These Tamm structures provide a new tool to control the optical emission from dye molecules and have many potential applications in fluorescence‐based sensing and imaging. 相似文献