全文获取类型
收费全文 | 4248篇 |
免费 | 557篇 |
国内免费 | 598篇 |
专业分类
化学 | 3140篇 |
晶体学 | 41篇 |
力学 | 331篇 |
综合类 | 60篇 |
数学 | 475篇 |
物理学 | 1356篇 |
出版年
2024年 | 9篇 |
2023年 | 71篇 |
2022年 | 110篇 |
2021年 | 146篇 |
2020年 | 186篇 |
2019年 | 156篇 |
2018年 | 163篇 |
2017年 | 149篇 |
2016年 | 204篇 |
2015年 | 172篇 |
2014年 | 230篇 |
2013年 | 311篇 |
2012年 | 300篇 |
2011年 | 413篇 |
2010年 | 278篇 |
2009年 | 272篇 |
2008年 | 291篇 |
2007年 | 259篇 |
2006年 | 207篇 |
2005年 | 174篇 |
2004年 | 132篇 |
2003年 | 108篇 |
2002年 | 97篇 |
2001年 | 90篇 |
2000年 | 75篇 |
1999年 | 96篇 |
1998年 | 72篇 |
1997年 | 60篇 |
1996年 | 61篇 |
1995年 | 64篇 |
1994年 | 59篇 |
1993年 | 52篇 |
1992年 | 52篇 |
1991年 | 37篇 |
1990年 | 46篇 |
1989年 | 33篇 |
1988年 | 25篇 |
1987年 | 31篇 |
1986年 | 27篇 |
1985年 | 17篇 |
1984年 | 8篇 |
1983年 | 11篇 |
1982年 | 13篇 |
1981年 | 5篇 |
1980年 | 7篇 |
1979年 | 6篇 |
1977年 | 3篇 |
1976年 | 2篇 |
1975年 | 2篇 |
1957年 | 5篇 |
排序方式: 共有5403条查询结果,搜索用时 31 毫秒
991.
Tiopronin (TP) is a synthetic thiol compound without chromophore. By optimizing the chromatographic conditions and sample preparation processes, an improved LC‐MS/MS analytical method without derivatization has been developed and validated to determine TP concentrations in human plasma. After reduction with 1,4‐dithiothreitol, plasma samples were deproteinized with 10% perchloric acid. The post‐treatment samples were analyzed on a C8 column interfaced with a triple quadrupole tandem mass spectrometer in negative electrospray ionization mode. Methanol–5 mmol/L ammonium acetate (20:80, v/v) was used as the isocratic mobile phase. The assay was linear over the concentration range of 40.0–5000 ng/mL. The intra‐ and inter‐day precisions were within 12.9% in terms of relative standard deviation and the accuracy within 5.6% in terms of relative error. This simple and sensitive LC‐MS/MS method with short analytical time (3.5 min each sample) was successfully applied to the pharmacokinetic study of TP in healthy Chinese male volunteers after an oral dose of 300 mg TP. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
992.
993.
994.
CdHgTe/SiO2 nanoparticles were prepared by SiO2 capping on the surface of CdHgTe QDs. The characteristics, such as optical spectra, photostability, size and cell toxicity
were investigated. The dynamic distribution of CdHgTe/SiO2 nanoparticles was in vivo monitored by near infrared fluorescence imaging system. CdHgTe/SiO2 nanoparticles acted as a novel fluorescence probe have a maximum fluorescence emission of 785 nm and high photo-stability.
The hydrodynamic diameter of CdHgTe/SiO2 nanoparticles could be adjusted to 122.3 nm. Compared to CdHgTe QDs, inhibitory effects of CdHgTe/SiO2 nanoparticles on proliferation of HCT116 cells decreased to a certain extent. CdHgTe/SiO2 nanoparticles had their specific dynamic distribution behavior, which provided new perspectives for bio-distribution of nanoparticles. 相似文献
995.
When the silicon material is irradiated by laser, it absorbs the laser energy leading to the temperature rise and the thermal stress. The damage effect includes melting, vaporation and thermal stress damage. Once the thermal stress exceeds the stress strength the crack will initiate. The silicon surface cracks induced by a millisecond laser are investigated. The experimental results show that three types of cracks are generated including cleavage crack, radial crack and circumferential crack. The cleavage crack is located within the laser spot. The radial crack and circumferential crack are located outside the laser spot. A two-dimensional spatial axisymmetric model of silicon irradiated by a 1064 nm millisecond laser is established. To assess what stresses generate and explain the generation mechanism of the different cracks, the thermal stress fields during laser irradiation and the cooling process are obtained using finite element method. The radial stress and hoop stress within the laser spot are tensile stress after the laser irradiation. The temperature in the center is the highest but the thermal stress in the center is not always highest during the laser irradiation. The cleavage cracks are induced by the tensile stress after the laser irradiation. The radial crack and the circumferential crack are generated during the laser irradiation. 相似文献
996.
Xuewen Wu Dalian Ding Haiyan Jiang Xiaowei Xing Suping Huang Hong Liu Zhedong Chen Hong Sun 《Journal of nanoparticle research》2012,14(1):708-13
Hydroxyapatite nanoparticles (nHAT) are known to have excellent biocompatibility, and have attracted increasing attention
as new candidates of non-viral vectors for gene therapy. In our previous studies, nHAT carrying a therapeutic gene and a reporter
gene were successfully transfected into the spiral ganglion neurons in the inner ear of guinea pigs in vivo as well as in
the cultured cell lines, although the transfection efficiencies were never higher than 30%. In this study, the surface modification
of nHAT with polyethylenimine (PEI) was made (PEI–nHAT, diameter = 73.09 ± 27.32 nm) and a recombinant plasmid carrying enhanced
green fluorescent protein (EGFP) gene and neurotrophin-3 (NT-3) gene was constructed as pEGFPC2–NT3. The PEI modified nHAT
and the recombinant plasmid was then connected to form the nHAT-based vector–gene complex (PEI–nHAT–pEGFPC2–NT3). This complex
was then placed onto the intact round window membranes of the chinchillas for inner ear transfection. Auditory brainstem response
(ABR) was tested to evaluate auditory function. Green fluorescence of EGFP was observed using confocal microscopy 48 h after
administering vector–gene complexes. There was no significant threshold shift in tone burst-evoked ABR at any tested frequency.
Abundant, condensed green fluorescence was found in dark cells on both sides of the crista and around the macula of the utricle.
Scattered EGFP signals were also detected in vestibular hair cells, some Schwann cells in the cochlear spiral ganglion region,
some outer pillar cells in the organ of Corti, and a few cells in the stria vascularis. The density of green fluorescence-marked
cells was obviously higher in the vestibular dark cell area than in other areas of the inner ear, suggesting that vestibular
dark cells may have the ability to actively engulf the nHAT-based vector–gene complexes. Considering the high transfection
efficiency in the vestibular system, PEI–nHAT may be a potential vector for gene therapy of inner ear diseases, especially
vestibular disorders, and deserves further study. 相似文献
997.
In this paper, a novel four dimensional hyper-chaotic system is coined based on the Chen system, which contains two quadratic terms and five system parameters. The proposed system can generate a hyper-chaotic attractor in wide parameters regions. By using the center manifold theorem and the local bifurcation theory, a pitchfork bifurcation is demonstrated to arise at the zero equilibrium point. Numerical analysis demonstrates that the hyper-chaotic system can generate complex dynamical behaviors, e.g., a direct transition from quasi-periodic behavior to hyper-chaotic behavior. Finally, an electronic circuit is designed to implement the hyper-chaotic system, the experimental results are consist with the numerical simulations, which verifies the existence of the hyper-chaotic attractor. Due to the complex dynamic behaviors, this new hyper-chaotic system is useful in the secure communication. 相似文献
998.
Transparent phosphate glass ceramics co-doped with Er3+ and Yb3+ in the system P2O5Li2OCaF2TiO2 were successfully synthesized by melt-quenching and subsequent heating. Formation of the nanocrystals was confirmed by X-ray powder diffraction. Judd–Ofelt analyses of Er3+ ions in the precursor glasses and glass ceramics were performed to evaluate the intensity parameters Ω2,4,6. Under 975 nm excitation, intense upconversion (UC) and infrared emission (1545 nm) were observed in the glass ceramics by efficient energy transfer from Yb3+ to Er3+. The luminescence processes were explained and the emission cross section was calculated by Fuchtbauer–Ladenburg (F–L) formula. The results confirm the potential applications of Er3+/Yb3+ co-doped glass ceramics as laser and fiber amplifier media. 相似文献
999.
G. Ruano J.C. Moreno-López M.C.G. Passeggi R.A. Vidal J. Ferrón M.Á. Niño R. Miranda J.J. de Miguel 《Surface science》2012,606(5-6):573-579
The growth of ultrathin epitaxial layers of aluminum fluoride on Cu(100) has been studied by a combination of surface science techniques. Deposition at room temperature results in step decoration followed by the formation of dendritic two-dimensional islands that coalesce to form porous films. Ultrathin layers (up to 2 monolayers in thickness) are morphologically unstable upon annealing; de-wetting takes place around 430 K with the formation of three-dimensional islands and leaving a large fraction of the Cu surface uncovered. Films several nanometers thick, on the contrary, are stable up to ca. 730 K where desorption in molecular form sets on. The effect of electron irradiation on the AlF3 has also been characterized by different spectroscopic techniques; we find that even small quantities of stray electrons from rear electron beam heating can provoke significant decomposition of the aluminum fluoride, resulting in the release of molecular fluorine and the formation of deposits of metallic aluminum. These features make AlF3 an interesting material for spintronic applications. 相似文献
1000.
改进型Czerny-Turner成像光谱仪光学系统设计方法 总被引:8,自引:2,他引:6
像散是目前影响Czerny-Turner结构成像光谱仪空间分辨率最大的像差。首先引入柱面反射镜,利用光焦度衡量像散大小,推导出易于计算的像散校正公式,有效地校正了像散。给出准直镜到光栅距离的计算方法,有效校正了成像光谱仪边缘视场像差。给出了成像光谱仪像面倾角的计算方法,实现了宽波段的像差校正。最终利用上述方法设计了一套用于115~200nm的改进型Czerny-Turner成像光谱仪,焦距f′=48mm,F数为5.0,全视场、全波段调制传递函数(MTF)在0.7以上。全波段光谱分辨率为0.22nm,像面大小为8mm×7mm。设计方法适用于多种结构要求的成像光谱仪。 相似文献