全文获取类型
收费全文 | 361篇 |
免费 | 26篇 |
国内免费 | 44篇 |
专业分类
化学 | 173篇 |
晶体学 | 8篇 |
力学 | 6篇 |
数学 | 2篇 |
物理学 | 242篇 |
出版年
2023年 | 7篇 |
2022年 | 5篇 |
2021年 | 11篇 |
2020年 | 6篇 |
2019年 | 5篇 |
2018年 | 7篇 |
2017年 | 9篇 |
2016年 | 15篇 |
2015年 | 7篇 |
2014年 | 22篇 |
2013年 | 27篇 |
2012年 | 14篇 |
2011年 | 26篇 |
2010年 | 12篇 |
2009年 | 27篇 |
2008年 | 30篇 |
2007年 | 19篇 |
2006年 | 43篇 |
2005年 | 15篇 |
2004年 | 7篇 |
2003年 | 10篇 |
2002年 | 7篇 |
2001年 | 5篇 |
2000年 | 20篇 |
1999年 | 4篇 |
1998年 | 5篇 |
1997年 | 2篇 |
1996年 | 9篇 |
1995年 | 6篇 |
1994年 | 3篇 |
1993年 | 5篇 |
1992年 | 4篇 |
1991年 | 4篇 |
1990年 | 5篇 |
1989年 | 1篇 |
1988年 | 5篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 3篇 |
1984年 | 1篇 |
1979年 | 1篇 |
1976年 | 1篇 |
1975年 | 6篇 |
1974年 | 7篇 |
1972年 | 1篇 |
排序方式: 共有431条查询结果,搜索用时 15 毫秒
101.
Tomasz Czyszanowski Maciej Dems Hugo Thienpont Krassimir Panajotov 《Optical and Quantum Electronics》2007,39(4-6):427-433
A three-dimensional, full vectorial model has been applied for investigation the modal characteristics of a phosphide photonic-crystal
vertical-cavity surface-emitting diode laser. The photonic crystal has been defined as a regular, hexagonal net of holes,
etched in the upper, p-type Distributed Bragg Reflector of the laser. The electromagnetic field has been confined to the active region by a single
defect of the photonic crystal providing overlapping with carriers funneled by a tunnel junction. We have determined the range
of design parameters, which provide single mode operation. 相似文献
102.
C. Hugenschmidt K. Schreckenbach M. Stadlbauer B. Straßer 《Applied Surface Science》2006,252(9):3098-3105
The in-pile positron source NEutron induced POsitron source MUniCh (NEPOMUC) of the new Munich research reactor FRM-II is now operated at the nominal reactor power of 20 MW. Recently, intensity and positron beam profile measurements were performed at 30 eV and 1 keV, respectively. For this purpose, NaI-scintillators detect the 511 keV γ-radiation of positrons that annihilate at a removable target in the beam line. The beam profile is determined with a micro-channel plate detector and a CCD-camera. In the present arrangement of NEPOMUC's instrumentation the positron beam is connected to a coincident Doppler broadening (CDB) facility and to a positron induced Auger electron spectroscopy (PAES) analysis chamber. First experiments were carried out in order to show the performance of these new spectrometers. An overview of the positron beam facility is given and first experimental results of PAES are presented. 相似文献
103.
104.
Łukasz Tymecki Magdalena RejnisMarta Pokrzywnicka Kamil StrzelakRobert Koncki 《Analytica chimica acta》2012
Two compact optoelectronic fluorimetric devices operating according to the paired-emitter-detector-diode concept have been developed. The fluorimetric detector, fabricated of three light emitting diodes only, has been applied for the development of fluorimetric optosensor by further integration with sensing solid phase. In these investigations as a model analyte and as a model sensing layer useful for solid phase spectrometry, riboflavin and C18-silica have been chosen, respectively. Both developed analytical devices have been applied for non-stationary fluorimetric measurements performed under conditions of flow injection analysis. The presented flow-through detector and sensor operating under given flow conditions offer riboflavin determination in mg L−1 and μg L−1 ranges of concentration, respectively. 相似文献
105.
随着正电子发射断层扫描(PET)技术的进步, 高效18F标记反应显现出重大的研究价值. 同时, 在有机氟化学的快速发展下, 研究人员已开发出一系列产生二氟甲基自由基(·CHF2)的一碳合成子, 得以被引入到18F标记反应中. 本文从化学原理出发, 总结了近年来·CHF2合成子、 碳氢键活化反应及18F标记的研究, 并展望了未来的研究趋势. 相似文献
106.
Dong Wang D.D. Wang C.Y. Cao L.R. Huang 《Journal of magnetism and magnetic materials》2010,322(22):3642-3647
High purity Fe2O3/ZnO nanocomposites were annealed in air at different temperatures between 100 and 1200 °C to get Fe-doped ZnO nanocrystals. The structure and grain size of the Fe2O3/ZnO nanocomposites were investigated by X-ray diffraction 2θ scans. Annealing induces an increase of the grain size from 25 to 195 nm and appearance of franklinite phase of ZnFe2O4. Positron annihilation measurements reveal large number of vacancy defects in the interface region of the Fe2O3/ZnO nanocomposites, and they are gradually recovered with increasing annealing temperature. After annealing at temperatures higher than 1000 °C, the number of vacancies decreases to the lower detection limit of positrons. Room temperature ferromagnetism can be observed in Fe-doped ZnO nanocrystals using physical properties measurement system. The ferromagnetism remains after annealing up to 1000 °C, suggesting that it is not related with the interfacial defects. 相似文献
107.
Raphaël Hoareau 《Tetrahedron letters》2010,51(26):3353-3355
Due to an urgent need to image the peripheral benzodiazepine receptor (PBR) in living human subjects using positron emission tomography imaging, we had cause to prepare N-[(2-hydroxyphenyl)methyl]-N-(4-phenoxy-3-pyridinyl) acetamide (desmethyl-PBR28 (1)), the precursor to [11C]PBR28. Herein, we report a new synthesis of the precursor in which palladium-mediated reduction of the nitro pyridine to the corresponding amino pyridine, and subsequent reductive amination, can be achieved with decaborane in a convenient one-pot procedure. This procedure is operationally simpler than the current alternatives and provides high quality precursor suitable for use in clinical applications. 相似文献
108.
In the present Letter, a fast, automated, and reproducible method for the synthesis of S-[13N]nitrosothiols is reported. The labeling strategy is based on trapping in an anion exchange resin. The reaction with thiols in acidic media led to the formation of the desired nitrosothiols in short reaction times (60 s) with excellent radiochemical conversions (from 48.7% to 74.5%). Final radiotracers were purified by HPLC. Good radiochemical yields (from 33.8% to 60.6%, decay corrected) and radiochemical purities (>99%) were obtained in all cases. Stability of the labeled compounds was checked. 相似文献
109.
Koichi Kato Tatsuya Kikuchi Nobuki Nengaki Takuya Arai Ming-Rong Zhang 《Tetrahedron letters》2010,51(45):5908-5911
Tetrabutylammonium fluoride-promoted α-[11C]methylation of α-arlylesters was developed. The method was amenable to the remote-controlled synthesis of 11C-labeled ibuprofen. 相似文献
110.
We consider the problem min
i=1
m
(ai,x–biloga
i, z) subject tox 0 which occurs as a maximum-likelihood estimation problem in several areas, and particularly in positron emission tomography. After noticing that this problem is equivalent to mind(b, Ax) subject tox 0, whered is the Kullback-Leibler information divergence andA, b are the matrix and vector with rows and entriesa
i,b
i, respectively, we suggest a regularized problem mind(b, Ax) + d(v, Sx), where is the regularization parameter,S is a smoothing matrix, andv is a fixed vector. We present a computationally attractive algorithm for the regularized problem, establish its convergence, and show that the regularized solutions, as goes to 0, converge to the solution of the original problem which minimizes a convex function related tod(v, Sx). We give convergence-rate results both for the regularized solutions and for their functional values.The research of A. N. Iusem was partially supported by CNPq Grant No. 301280/86-MA. 相似文献