全文获取类型
收费全文 | 4007篇 |
免费 | 429篇 |
国内免费 | 316篇 |
专业分类
化学 | 2627篇 |
晶体学 | 27篇 |
力学 | 197篇 |
综合类 | 20篇 |
数学 | 489篇 |
物理学 | 1392篇 |
出版年
2023年 | 52篇 |
2022年 | 119篇 |
2021年 | 106篇 |
2020年 | 134篇 |
2019年 | 99篇 |
2018年 | 101篇 |
2017年 | 95篇 |
2016年 | 132篇 |
2015年 | 138篇 |
2014年 | 165篇 |
2013年 | 237篇 |
2012年 | 328篇 |
2011年 | 295篇 |
2010年 | 218篇 |
2009年 | 194篇 |
2008年 | 256篇 |
2007年 | 212篇 |
2006年 | 194篇 |
2005年 | 168篇 |
2004年 | 131篇 |
2003年 | 115篇 |
2002年 | 103篇 |
2001年 | 89篇 |
2000年 | 102篇 |
1999年 | 72篇 |
1998年 | 66篇 |
1997年 | 55篇 |
1996年 | 75篇 |
1995年 | 56篇 |
1994年 | 60篇 |
1993年 | 53篇 |
1992年 | 54篇 |
1991年 | 36篇 |
1990年 | 29篇 |
1989年 | 41篇 |
1988年 | 37篇 |
1987年 | 31篇 |
1986年 | 41篇 |
1985年 | 30篇 |
1984年 | 35篇 |
1983年 | 26篇 |
1982年 | 20篇 |
1981年 | 14篇 |
1980年 | 22篇 |
1979年 | 12篇 |
1978年 | 11篇 |
1977年 | 15篇 |
1976年 | 17篇 |
1975年 | 17篇 |
1973年 | 9篇 |
排序方式: 共有4752条查询结果,搜索用时 15 毫秒
961.
Summary: In this paper, unsaturated groups were introduced into poly(lactic acid) (PLA) for fabricating PLA‐based nanospheres with carbon‐carbon double bonds as functional groups. The morphology dependencies on fabrication conditions, including the fabrication time as well as the stirring rate, were also investigated.
962.
Wideband multiwavelength erbium-doped fiber ring laser with frequency shifted feedback 总被引:7,自引:0,他引:7
Wideband multiwavelength erbium-doped fiber ring lasers with frequency shifted feedback are described. The use of an intra-cavity gain flattening filter (GFF) was proposed in order to increase the lasing spectral bandwidth, leading to a demonstration of 34 lasing wavelengths in 28 nm bandwidth in C-band. The GFF induced spectral output power fluctuation is discussed. Multiwavelength operation was also demonstrated for the first time in L-band, where wideband laser operation was obtained without a GFF. Optical bistability and Kerr effect induced pulsation were determined to be limiting factors to stable operation range in this kind of multiwavelength lasers. 相似文献
963.
Shengyun Zhu Jiazheng Zhu Minamisono T. Matsuta K. Yongjun Xu Fukuda M. Mihara M. Zhiqiang Wang Sato K. Akai H. Chaofan Rong Chengjie Chu Jun Chen Hailong Luo 《Hyperfine Interactions》2001,136(3-8):149-153
An on-line time differential perturbed angular correlation technique has been developed with the light probe nucleus 19F. The first on-line TDPAC measurement was carried out to determine the field gradient in Cd and quadrupole moment of 19F (0.197 MeV: I
π = 5/2+, T1/2 = 89 ns).
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
964.
965.
Previous investigations have indicated that the finite number of balls can cause the bearing stiffness to vary periodically. However, effects of unbalanced force in a rotor–bearing system on the bearing stiffness have not received sufficient attention. The present work reveals that the unbalanced force can also make the bearing stiffness vary periodically. The parametric excitations from the time-varying bearing stiffness can cause instability and severe vibration under certain operating conditions. Therefore, the determination of the operating conditions of parametric instability is crucial to the design of high speed rotating machinery. In this paper, an extended Jones–Harris stiffness model is presented to ascertain the stiffness of the angular contact ball bearing considering five degrees of freedom. Stability analysis of a rigid rotor–bearing system is performed utilizing the discrete state transition matrix (DSTM) method. The effects of unbalanced force, bearing loads and damping on the instability regions are discussed thoroughly. Investigations mainly show that the time-varying bearing stiffness fluctuates sinusoidally due to finite number of balls and unbalanced force. The locations and widths of the instability regions caused by these two parametric excitations differ distinctly. Unbalanced force could change the widths of the instability regions, but without altering their central positions. The axial and radial loads of the bearing only change the positions of the instability regions, without affecting their widths. Besides, damping can reduce the widths of the instability regions. 相似文献
966.
The new technique of contact etching has been utilized to study the bulk morphology of polyethylene terephthalate by the echant, n-propylamine. A variety of films and fibers with different mechanical and thermal histories have been subjected to contact etching. The sample surfaces have been studied principally by scanning electron microscopy. A network superstructure with its characteristic dimension (thickness) of from 700 to 3000 Å, depending on sample history, has been observed. In the oriented samples the network superstructure aligns perpendicular to the direction of sample orientation. A simple model is proposed to describe the network superstructure which is believed to be moderately crystalline. 相似文献
967.
In this paper we investigate the influence of microstructure on the CH4 adsorption behavior of deep coal. The coal microstructure is characterized by N2 adsorption at 77 K, scanning electron microscopy (SEM), Raman spectroscopy, and Fourier transform infrared spectroscopy (FT-IR). The CH4 adsorptions are measured at 298 K at pressures up to 5.0 MPa by the the volumetric method and fitted by the Langmuir model. The results show that the Langmuir model fits well with the experimental data, and there is a positive correlation with surface area, pore volume, ID/IG, and CH4 adsorption capacity. The burial depth also affects the methane adsorption capacity of the samples. 相似文献
968.
M. Lavagnini F. Pfuner R. Monnier L. Degiorgi H.-M. Eiter L. Tassini B. Muschler R. Hackl J.-H. Chu N. Ru K.Y. Shin I.R. Fisher 《Physica B: Condensed Matter》2012,407(11):1864-1867
We summarize our recent efforts in investigating the charge–density-wave (CDW) state of the rare-earth tri-tellurides RTe3 by means of infrared and Raman techniques. We identify the CDW gap, as order parameter of the broken-symmetry ground state, as well as the collective mode of the CDW condensate. 相似文献
969.
Kuei-Yi Chu Shiou-Ying Cheng Meng-Hsueh Chiang Yi-Jung Liu Chien-Chang Huang Tai-You Chen Chi-Shiang Hsu Wen-Chau Liu Wen-Yu Cheng Bin-Cian Lin 《Superlattices and Microstructures》2011,50(4):289-295
The characteristics of InGaP/InGaAs/GaAs dual channel pseudomorphic high electron mobility transistors (DCPHEMTs) with different graded triple delta-doped sheets are investigated and experimentally demonstrated. Based on a two-dimensional simulator of ATLAS, the band diagrams, electron densities and DC characteristics of studied devices are comprehensively analyzed. Due to the use of properly graded triple delta-doped sheets, good pinch-off and saturation characteristics, improved transport properties and wide current swing are obtained. For comparison, a practical DCPHEMT with good device performances is fabricated as well. It is found that the simulated data are in good agreement with experimental results. 相似文献
970.
James Shaw-Stewart Bryan Chu Thomas Lippert Ylenia Maniglio Matthias Nagel Frank Nüesch Alexander Wokaun 《Applied Physics A: Materials Science & Processing》2011,105(3):713-722
Laser-induced forward transfer (LIFT) has been investigated for bilayer transfer material systems: silver/organic film (Alq3 or PFO). The LIFT process uses an intermediate dynamic release layer of a triazene polymer. This study focuses on the effect of introducing a controlled donor–receiver substrate gap distance and the effect of doing the transfer at reduced air pressures, whilst varying the fluence up to ∼200 mJ/cm2. The gap between ‘in-contact’ substrates has been measured to be a minimum of 2–3 μm. A linear variation in the gap width from ‘in contact’ to 40 μm has been achieved by adding a spacer at one side of the substrate–substrate sandwich. At atmospheric pressure, very little transfer is achieved for Alq3, although PFO shows some signs of successful doughnut transfer (with a large hole in the middle) in a narrow fluence range, at gaps greater than 20 μm. For the transfer of Ag/PFO bilayers at atmospheric pressure, the addition of a PFO layer onto the receiver substrate improved the transfer enormously at smaller gaps and higher fluences. However, the best transfer results were obtained at reduced pressures where a 100% transfer success rate is obtained within a certain fluence window. The quality of the pixel morphology at less than 100 mbar is much higher than at atmospheric pressure, particularly when the gap width is less than 20 μm. These results show the promise of LIFT for industrial deposition processes where a gap between the substrates will improve the throughput. 相似文献