全文获取类型
收费全文 | 912篇 |
免费 | 31篇 |
国内免费 | 7篇 |
专业分类
化学 | 520篇 |
晶体学 | 9篇 |
力学 | 39篇 |
数学 | 149篇 |
物理学 | 233篇 |
出版年
2023年 | 7篇 |
2022年 | 9篇 |
2021年 | 15篇 |
2020年 | 27篇 |
2019年 | 13篇 |
2018年 | 15篇 |
2017年 | 13篇 |
2016年 | 17篇 |
2015年 | 19篇 |
2014年 | 25篇 |
2013年 | 52篇 |
2012年 | 65篇 |
2011年 | 74篇 |
2010年 | 34篇 |
2009年 | 27篇 |
2008年 | 58篇 |
2007年 | 45篇 |
2006年 | 41篇 |
2005年 | 52篇 |
2004年 | 32篇 |
2003年 | 15篇 |
2002年 | 29篇 |
2001年 | 27篇 |
2000年 | 13篇 |
1999年 | 7篇 |
1998年 | 7篇 |
1997年 | 8篇 |
1996年 | 10篇 |
1995年 | 9篇 |
1993年 | 9篇 |
1992年 | 16篇 |
1991年 | 6篇 |
1989年 | 7篇 |
1987年 | 6篇 |
1986年 | 6篇 |
1985年 | 10篇 |
1984年 | 5篇 |
1983年 | 9篇 |
1982年 | 6篇 |
1981年 | 6篇 |
1980年 | 4篇 |
1979年 | 7篇 |
1978年 | 8篇 |
1977年 | 15篇 |
1976年 | 17篇 |
1975年 | 5篇 |
1974年 | 6篇 |
1973年 | 6篇 |
1972年 | 5篇 |
1971年 | 4篇 |
排序方式: 共有950条查询结果,搜索用时 31 毫秒
81.
Jesse R. Williams Masaaki Kobata Igor Pis Eiji Ikenaga Takeharu Sugiyama Keisuke Kobayashi Naoki Ohashi 《Surface science》2011,605(13-14):1336-1340
The surface structure of a single-crystal ZnO wafer was studied by angle-resolved x-ray photoelectron spectroscopy (ARXPS) using synchrotron radiation. As a result, well-defined x-ray photoelectron diffraction (XPD) patterns were obtained for the (0001) and (000) polar surfaces using the photoemission from the Zn 2p3/2 and O 1s core levels. The XPD patterns were indexed assuming forward scattering of photoelectrons by neighboring ions. Further, the XPD patterns for the (0001) and (000) surfaces were different from each other, indicating the possibility for using the XPD technique for polarity determination. 相似文献
82.
In order to improve the lateral resolution and extend the field of view of a previously reported 48 element 30 MHz ultrasound linear array and 16-channel digital imaging system, the development of a 256 element 30 MHz linear array and an ultrasound imaging system with increased channel count has been undertaken. This paper reports the design and testing of a 64 channel digital imaging system which consists of an analog front-end pulser/receiver, 64 channels of Time-Gain Compensation (TGC), 64 channels of high-speed digitizer as well as a beamformer. A Personal Computer (PC) is used as the user interface to display real-time images. This system is designed as a platform for the purpose of testing the performance of high frequency linear arrays that have been developed in house. Therefore conventional approaches were taken it its implementation. Flexibility and ease of use are of primary concern whereas consideration of cost-effectiveness and novelty in design are only secondary. Even so, there are many issues at higher frequencies but do not exist at lower frequencies need to be solved. The system provides 64 channels of excitation pulsers while receiving simultaneously at a 20–120 MHz sampling rate to 12-bits. The digitized data from all channels are first fed through Field Programmable Gate Arrays (FPGAs), and then stored in memories. These raw data are accessed by the beamforming processor to re-build the image or to be downloaded to the PC for further processing. The beamformer that applies delays to the echoes of each channel is implemented with the strategy that combines coarse (8.3 ns) and fine delays (2 ns). The coarse delays are integer multiples of the sampling clock rate and are achieved by controlling the write enable pin of the First-In-First-Out (FIFO) memory to obtain valid beamforming data. The fine delays are accomplished with interpolation filters. This system is capable of achieving a maximum frame rate of 50 frames per second. Wire phantom images acquired with this system show a spatial resolution of 146 μm (lateral) and 54 μm (axial). Images with excised rabbit and pig eyeball as well as mouse embryo were also acquired to demonstrate its imaging capability. 相似文献
83.
The palladium catalyzed asymmetric allylic sulfonylation reaction has been investigated employing β-hydroxy- and β-(o-diphenylphosphino)benzoyloxy (o-diphenyl phosphino)benzamides as chiral, non-racemic ligands. The bisphosphine β-benzoyloxybenzamide ligands proved to be the best ligands for this process. Competitive transition states for the (1S,2R)-norephedrine derived ligand 14 are compared and a rationale is provided for the observed enantioselectivities. 相似文献
84.
Treatment of β-d-ribose tetraacetate with trimethylaluminum gives α-3,5-O-acetyl-1,2-O-isopropylidene-d-ribofuranoside in excellent yield. This reaction allows for efficient and high-yielding installation of the 1,2-isopropylidene acetal (acetonide), which is difficult to prepare using more traditional acid-catalyzed methods. The reaction of trimethylaluminum with other per-acetylated carbohydrates is also described. 相似文献
85.
N.J. Mellors X. Zhao L.M. Simmons C.J. Quinn S.H. Kilcoyne 《Journal of magnetism and magnetic materials》2012
Ribbons of Fe100−xGax (x=15, 17.5, 19.5 and 22.5) were prepared by rapid solidification from the melt. 57Fe Mössbauer spectroscopy and high resolution neutron diffraction have revealed that Fe100−xGax alloys with x=15 and 17.5 have the disordered bcc (A2) structure even after annealing, but the alloy with x=19.5 developed the short-range ordered D03 phase when annealed. The x=22.5 alloys showed mainly D03 phase with a fraction of bcc phase. A fraction of the bcc phase transformed into D03 phase and the long-range ordering of D03 phase was improved after annealing. 57Fe Mössbauer spectra showed no observable L12 phase in any samples even though less than 1% volume of L12 phases has been found in the annealed samples by neutron diffraction. The additional absorption at hyperfine field of 25 T in x=22.5 samples was regarded as a result of imperfect D03 structure, rather than L12 phase. 相似文献
86.
Measuring modulated luminescence using non-modulated stimulation: ramping the sample period 总被引:1,自引:0,他引:1
N. R. J. Poolton L. Btter-Jensen C. E. Andersen M. Jain A. S. Murray A. E. R. Malins F. M. Quinn 《Radiation measurements》2003,37(6):588-645
Conventional methods of recording linearly modulated (LM) optically stimulated luminescence (OSL) require control over either the exciting light intensity, or the ability to pulse the source. For many light sources (e.g. constant-power CW lasers, arc lamps and synchrotrons) this can be problematic. Directly analogous results to LM-OSL can, however, be achieved with non-modulated excitation sources, by ramping the sample period (RSP) of luminescence detection. RSP-OSL has the distinct advantage over LM-OSL in that, since the excitation remains at full power, data accumulation times (that can be considerable) can be reduced by typically 50%. RSP methods are universally applicable and can be employed, for example, where the excitation source is constant heat, rather than light: here, iso-thermal decay of phosphorescence becomes recorded as a sequence of peaks, corresponding to de-trapping of charge from different defect levels, and is particularly useful for analysing shallow-trap effects. RSP methods are also useful in providing significant compaction of data sets, where signal analysis is required of overlapping systems having a wide range of decay kinetics. 相似文献
87.
David B. Redmond Charles M. Quinn Patrick W. Fowler 《Journal of mathematical chemistry》1998,23(3-4):263-276
Some new aspects of qualitative molecular-orbital theory are developed within the context of group theory and a symmetrized
Hückel approximation. Conclusions concerning the eigenvalue sums for sets of eigenfunctions of the same symmetries, the occurrence
of repetitions of symmetry-adapted projections and the relations between eigenvalues of Γg and Γu symmetries are exemplified using the 120 cage structure of Ih point symmetry. The implications of the orbit-by-orbit approach are outlined for larger clusters.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
88.
Catherine T. Quinn 《Journal of Geometry》2002,74(1-2):123-138
The André/Bruck and Bose representation ([1], [5,6]) of PG(2,q
2) in PG(4,q) is a tool used by many authors in the proof of recent results. In this paper the André/Bruck and Bose representation of
conics in Baer subplanes of PG(2,q
2) is determined. It is proved that a non-degenerate conic in a Baer subplane of PG(2,q
2) is a normal rational curve of order 2, 3, or 4 in the André/Bruck and Bose representation. Moreover the three possibilities
(classes) are examined and we classify the conics in each class.
Received 1 September 1999; revised 17 July 2000. 相似文献
89.
A design analysis of a class of six degree of freedom strain-gage dynamometers which are virtually uncoupled in each force and moment component is presented. The method of design is detailed and an optimal design algorithm is implemented. The dynamometer is made of six or more T-sections with thin webs and flanges called shear panel elements (SPE). Complete stress and buckling analyses are carried out for the SPE, and experiments confirm the predictions of the analyses. The optimal design method is illustrated with several case studies. A dynamometer has been built and used in laboratory and field experiments. 相似文献
90.
Dr. Michael Georg Hoesl M. Sc. Stefan Oehm Dr. Patrick Durkin Dr. Elise Darmon Dr. Lauri Peil Dr. Hans‐Rudolf Aerni Prof. Dr. Juri Rappsilber Prof. Dr. Jesse Rinehart Prof. Dr. David Leach Prof. Dr. Dieter Söll Prof. Dr. Nediljko Budisa 《Angewandte Chemie (International ed. in English)》2015,54(34):10030-10034
We have changed the amino acid set of the genetic code of Escherichia coli by evolving cultures capable of growing on the synthetic noncanonical amino acid L ‐β‐(thieno[3,2‐b]pyrrolyl)alanine ([3,2]Tpa) as a sole surrogate for the canonical amino acid L ‐tryptophan (Trp). A long‐term cultivation experiment in defined synthetic media resulted in the evolution of cells capable of surviving Trp→[3,2]Tpa substitutions in their proteomes in response to the 20 899 TGG codons of the E. coli W3110 genome. These evolved bacteria with new‐to‐nature amino acid composition showed robust growth in the complete absence of Trp. Our experimental results illustrate an approach for the evolution of synthetic cells with alternative biochemical building blocks. 相似文献