全文获取类型
收费全文 | 2640篇 |
免费 | 128篇 |
国内免费 | 13篇 |
专业分类
化学 | 2159篇 |
晶体学 | 28篇 |
力学 | 56篇 |
数学 | 95篇 |
物理学 | 443篇 |
出版年
2023年 | 17篇 |
2022年 | 35篇 |
2021年 | 60篇 |
2020年 | 51篇 |
2019年 | 51篇 |
2018年 | 32篇 |
2017年 | 45篇 |
2016年 | 78篇 |
2015年 | 79篇 |
2014年 | 101篇 |
2013年 | 183篇 |
2012年 | 226篇 |
2011年 | 240篇 |
2010年 | 133篇 |
2009年 | 108篇 |
2008年 | 188篇 |
2007年 | 177篇 |
2006年 | 177篇 |
2005年 | 183篇 |
2004年 | 131篇 |
2003年 | 128篇 |
2002年 | 124篇 |
2001年 | 46篇 |
2000年 | 33篇 |
1999年 | 25篇 |
1998年 | 24篇 |
1997年 | 16篇 |
1996年 | 14篇 |
1995年 | 5篇 |
1994年 | 8篇 |
1993年 | 11篇 |
1992年 | 6篇 |
1991年 | 5篇 |
1990年 | 4篇 |
1989年 | 5篇 |
1988年 | 3篇 |
1987年 | 4篇 |
1986年 | 2篇 |
1984年 | 4篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1980年 | 1篇 |
1979年 | 2篇 |
1978年 | 1篇 |
1977年 | 2篇 |
1974年 | 3篇 |
1973年 | 1篇 |
1972年 | 1篇 |
1971年 | 1篇 |
1967年 | 1篇 |
排序方式: 共有2781条查询结果,搜索用时 327 毫秒
921.
Studies on the stereoselective synthesis of a pyrrolidinoisoquinoline derivative using acyliminium ion cyclization of hydroxylactams are described. Cyclization reaction of chiral hydroxylactams 2a~c, derived from (L)-tartaric acid and phenethylamine, afforded one diastereomer 3 under a variety of acidic conditions in moderate to good yields. 相似文献
922.
Polymer blend nanocomposites based on thermoplastic polyurethane (PU) elastomer, polylactide (PLA) and surface modified carbon nanotubes were prepared via simple melt mixing process and investigated for its mechanical, dynamic mechanical and electroactive shape memory properties. Chemical and structural characterization of the polymer blend nanocomposites were investigated by Fourier Transform infrared (FT-IR) and wide angle X-ray diffraction (WAXD). Loading of the surface modified carbon nanotube in the PU/PLA polymer blends resulted in the significant improvement on the mechanical properties such as tensile strength, when compared to the pure and pristine CNT loaded polymer blends. Dynamic mechanical analysis showed that the glass transition temperature (Tg) of the PU/PLA blend slightly increases on loading of pristine CNT and this effect is more pronounced on loading surface modified CNTs. Thermal and electrical properties of the polymer blend composites increases significantly on loading pristine or surface modified CNTs. Finally, shape memory studies of the PU/PLA/modified CNT composites exhibit a remarkable recoverability of its shape at lower applied dc voltages, when compared to pure or pristine CNT loaded system. 相似文献
923.
Byung Woo Yoo Heung Soo Baek Sam Rok Keum Cheol Min Yoon Ghil Soo Nam Sung Hoon Kim 《合成通讯》2013,43(23):4317-4322
The reductive cleavage of thiobenzoates promoted by samarium diiodide afforded various kinds of disulfides in good yields under mild and neutral conditions. 相似文献
924.
The alkylation and sulfonylation of 3-aryl-1-methyl-1,2,4-triazolin-5-ones (1) were studied with various alkyl halides and sulfonyl chlorides. The alkylation of 1 with methyl iodide and ethyl bromide afforded N-alkylated products, however with methyl 2-bromopropionate afforded O-alkylated products predominantly. The sulfonylation by methanesulfonyl chloride afforded a mixture of N-sulfonylated and O-sulfonylated products, while the sulfonylation by p-toluenesulfonyl chloride afforded mainly O-sulfonylated products. 相似文献
925.
Hyeong‐Ohk Bae Seung‐Yeal Ha Doheon Kim Yongsik Kim Hyuncheul Lim Jane Yoo 《Mathematical Methods in the Applied Sciences》2019,42(18):6029-6048
In this paper, we study stochastic aggregation properties of the financial model for the N‐asset price process whose dynamics is modeled by the weakly geometric Brownian motions with stochastic drifts. For the temporal evolution of stochastic components of drift coefficients, we employ a stochastic first‐order Cucker‐Smale model with additive noises. The asset price processes are weakly interacting via the stochastic components of drift coefficients. For the aggregation estimates, we use the macro‐micro decomposition of the fluctuations around the average process and show that the fluctuations around the average value satisfies a practical aggregation estimate over a time‐independent symmetric network topology so that we can control the differences of drift coefficients by tuning the coupling strength. We provide numerical examples and compare them with our analytical results. We also discuss some financial implications of our proposed model. 相似文献
926.
We present the optical coherence tomography (OCT) made to investigate the early dental caries in human teeth and compare its
results with those taken by conventional imaging modalities including light illuminating examination (LIE), digital intra-oral
radiography (DIOR), and electron probe micro analyzer (EPMA). Morphological features and caries-involved areas of the dental
structure were mainly investigated by LIE, DIOR, and OCT to study the infection of the caries lesion in pits and fissures.
The biochemical information acquired with EPMA and the morphological features taken with OCT in the early stage of caries
were compared and analyzed to present an objective and practical index for the degree of caries. The experimental results
allow us to conclude that OCT could be used to provide quantitative analysis of caries based on the reflectivity difference
in the specimen. 相似文献
927.
Conductivity stability at thermal environment of conductive polyaniline‐complexes/polyimide (PANI‐complexes/PI) blends, which were doped by camphorsulfonic acid (CSA) and dodecylbenzenesulfonic acid (DBSA), respectively, were investigated by conductivity measurements, electron spin resonance (ESR) spectra, differential and scanning thermometer (DSC). In the conversion process of PANI/Polyamic acid (PAA) to PANI/PI, the blend endeavored some kinds of alteration such as decomplexation of moisture and solvent, dissociation of dopant, crosslinking of PANI chain, and the imidization of PAA chain. PANI‐DBSA/PI showed higher thermal stability of conductivity than PANI‐CSA/PI, and both samples showed nearly linear decay of conductivity with increasing temperature showing greatly enhancement of conductivity stability. When they were exposed at near or over glass transition temperature, the conductivity decay became faster. The conductivity stability at base environment was also higher for PANI‐DBSA/PI due to difficulty in accessing of hydroxyl ion to PANI, which were resulted from dopant. DBSA‐doped blends showed increased polaron mobility and concentration at relatively high temperature, which led to extremely higher conductivity and its stability at high temperature. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
928.
The N4 donor ligand, Me2bpzb2? and a pyridine molecule are coordinated to the Zinc(II) so that the coordination geometry closely resembles a square‐pyramidal environment with a nitrogen atom of the pyridine ligand occupying the apical position. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
929.
Jihoon Na Jinchae Kim Eunseo Choi Seon Young Ryu Jae Ho Baek Byeong Ha Lee 《Optical and Quantum Electronics》2005,37(13-15):1251-1261
A spectral filtering method that can enhance the axial resolution and suppress sidelobes of the point spread function in Optical Coherence Tomography (OCT) system is demonstrated. A mechanically induced long-period fiber grating (MLPG) is used to tailor the spectral shape of a homemade Er-doped fiber amplified spontaneous emission (ASE) source. By using a MLPG, the high and narrow characteristic peak, centered at 1533 nm, of the ASE source was fairly well removed, which enabled us to use a broader spectral bandwidth of the source. The experimental results show that the proposed spectral filtering method can suppress the sidelobes in the axial point spread function up to 5.4 dB and thus enhance the axial resolution of an OCT system by four times. The OCT images of an extracted human premolar taken with and without the spectral filtering are presented and compared with each other. 相似文献
930.
Ok Doo Awh Jae Rok Kim Sung Chul Shim 《Journal of Radioanalytical and Nuclear Chemistry》1988,128(4):299-307
Four stereoisomeric99mTc complexes from meso and racemic 2,3-dimercaptosuccinic acids /DMSA/ were prepared controlling the pH and the molar ratio of ligand to tin/II/ chloride. The complexes showed different thermal stability and lipophilicity. 相似文献