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排序方式: 共有49条查询结果,搜索用时 15 毫秒
21.
G. E. Stratakos N. V. Vlassopoulos N. K. Uzunoglu C. N. Capsalis 《International Journal of Infrared and Millimeter Waves》1995,16(8):1367-1392
Millimeter wave frequencies are proposed as carriers of microcell future mobile systems. High frequencies in the region of 60–62 Ghz are suggested. A serious difficulty in this context is the lack of sufficient gain in active devices. Both on receiver and transmitter units this difficulty is faced. Mixing, amplification, carrier recovery and modulation / demodulation subsytems are required to be operational at these higher frequencies. Presently the MMIC 's show also limited performance. In this report an alternative approach active processing technique is examined based on an injection locking in Gunn oscillator. A non-linear analysis is applied to determine the fundamental properties of injection locking in Gunn oscillator performance under the influence of an external signal. Details of waveguide action is taken into account by solving the corresponding boundary condition problem. Finally experimental results are presented for a 30GHz oscillator. 相似文献
22.
Izgi T Alver O Parlak C Aytekin MT Senyel M 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2007,68(1):55-62
FT-IR and (1)H, (13)C, DEPT, HETCOR, COSY, and NOESY NMR spectra of 2-(1-cyclohexenyl)ethylamine (CyHEA) have been reported for the first time. The vibrational frequencies and (1)H, (13)C NMR chemical shifts of CyHEA (C(8)H(15)N) have been calculated by means of the Hartree-Fock (HF), Becke-Lee-Yang-Parr (BLYP) and Becke-3-Lee-Yang-Parr (B3LYP) density functional methods with 6-31 G(d) and 6-31 G(d,p) basis sets, respectively. The comparison between the experimental and the theoretical results indicates that density functional B3LYP method is superior to the scaled HF and BLYP approach for vibrational frequencies and predicting NMR properties. 相似文献
23.
Enantiospecific Synthesis of (R)‐3‐Amino‐4‐(2,4,5‐trifluorophenyl)butanoic Acid Using (S)‐Serine as a Chiral Pool 下载免费PDF全文
Aytekin Köse Özlem Gündoğdu Derya Aktaş Meryem Fıstıkçı Ramazan Altundaş Hasan Seçen Yunus Kara 《Helvetica chimica acta》2015,98(2):260-266
Starting from (S)‐serine, a new method was developed for the synthesis of the β‐amino acid part of sitagliptin in ten steps and with an overall yield of 30%. The crucial step of the synthesis was the ring opening of N‐ and O‐protected (R)‐aziridin‐2‐methanol with (2,4,5‐trifluorophenyl)magnesium bromide to give N‐ and O‐protected (R)‐2‐amino‐3‐(2,4,5‐trifluorophenyl)propan‐1‐ol. 相似文献
24.
Rıdvan Baldık Hüseyin Aytekin Mustafa Erer 《Journal of Radioanalytical and Nuclear Chemistry》2011,289(2):297-302
In this work, the radionuclide activity concentrations of 226Ra, 232Th and 40K in surface soils and radon levels in dwellings of Karabük, Turkey were determined in order to evaluate the environmental
radioactivity. Concentrations of 226Ra, 232Th and 40K radionuclides were determined using gamma spectrometry with using HPGe detector. The etch track detectors (CR-39) were used
to determine the distribution of radon concentrations. The average activity concentrations for 226Ra, 232Th and 40K were found as 21.0, 23.5 and 363.5 Bq kg−1, respectively. The calculated average annual effective dose equivalent from the outdoor terrestrial gamma radiation from
226Ra, 232Th and 40K is 53.5 μSv y−1. The average radon concentration and annual effective dose equivalent of 222Rn in Karabük dwellings were obtained 131.6 Bqm−3 and 3.32 mSv y−1, respectively. The evaluated data were compared with the data obtained from different countries. 相似文献
25.
Chitosan (CH) is a biopolymer with biocompatible, biodegradable, and bioactive properties. N,N,N‐trimethyl chitosan chloride (TMC) is a quaternized form of CH that is highly cationic and more water soluble than unmodified CH. The physiological activities of CHs with different molecular weights (Mw) and degrees of TMCs quaternization were investigated in U937 and 3T3‐L1 cell lines. 1H‐NMR spectrometry and size exclusion chromatography were used for characterization of the biopolymers. The half inhibition concentration (IC50) of DPPH‐radical‐scavenging activity was below 0.9 mg/m with quaternized CHs. The IC50 values of chitooligosaccharides, low‐ and medium‐molecular‐weight CHs were 8.4, 10.9, and 13.9 mg/ml, respectively. High‐molecular‐weight CHs and TMCs showed apoptotic activity on U937 cells. T41, a TMC of 549 kDa with a 41% degree of quaternization (DQ), yielded 30.7% apoptotic cell death in U937 at 20 µg/ml and effectively repressed cell differentiation and triglyceride accumulation in 3T3‐L1. Depolymerized CHs reduced triglyceride accumulation but also caused cell differentiation. TMCs showed repressor activity to both cell differentiation and triglyceride accumulation. Increasing the molecular weight of CHs and TMCs generally resulted in increased physiological activity. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
26.
In this work, we have developed an analytical model of a multilayer porous material based on the transfer matrix method to predict the absorption behavior at plane wave incidence. The aim of this study is to modify/tune the sound absorption coefficient of a felt to obtain an improved absorbing performance in the mid frequency range without increasing its weight. To achieve this target, the developed model has been used to find the best combination of each layer type and thickness. The analytical results were validated by test results. 相似文献
27.
S. E. Lagaras N. K. Uzunoglu 《International Journal of Infrared and Millimeter Waves》2008,29(6):596-607
The maritime environment is a very unique system with respect to the environmental conditions that affect the transmission
of electromagnetic energy, and especially the part of the spectrum that falls in the infrared band. This paper proposes an
end-to-end model for the detection of ships at the environmental conditions of the eastern Mediterranean Sea, with the use
of a Forward Looking Infrared (FLIR) system. The model: a) covers the issues regarding the detection and classification of
simple model targets in various states of temperature conditions, by applying the Moser and O’Neill criteria, b) takes into
account existing environmental conditions, by using data obtained from the Hellenic National Meteorological Service (HNMS)
and the British Atmospheric Data Centre (BADC) for the years 2004–05, and c) uses the LOWTRAN 7 code for the calculation of
the transmission through the maritime atmosphere. In the calculation of the atmospheric transmission, turbulence is taken
into account through the introduction of a Modulation Transfer Function (MTF), to model the specific phenomenon. The FLIR
system is also modeled as a series of MTF functions, with each one of those covering the characteristics of major subsystems
of the detection device. Finally, the observer’s MTF is included in the model, and the probability of completing the specific
task is calculated and presented for a range of observer to target distances.
LOWTRAN computer model generated by the U.S. Air Force Geophysics Laboratory, is now in its 7th version and may be purchased
in a PC version from the ONTAR Corporation, 9 Village Way North Andover 01845, Massachusetts.
MATLAB and SIMULINK are registered trademarks of The Math Works Inc. 3 Apple Hill Drive, Natick 01760-2098 Massachusetts.
S. E. Lagaras is currently pursuing the PhD degree in Electrical Engineering at the National Technical University of Athens. 相似文献
28.
Queuing systems with finite buffers are reasonable models for many manufacturing, telecommunication, and healthcare systems. Although some approximations exist, the exact analysis of multi‐server and finite‐buffer queues with general service time distribution is unknown. However, the phase‐type assumption for service time is a frequently used approach. Because the Cox distribution, a kind of phase‐type distribution, provides a good representation of data with great variability, it has a vast area of application in modeling service times. The research focus is twofold. First, a theoretical structure of a multi‐server and finite‐buffer queuing system in which the service time is modeled by the two‐phase Cox distribution is studied. It is focused on finding an efficient solution to the stationary probabilities using the matrix‐geometric method. It is shown that the stationary probability vector can be obtained with the matrix‐geometric method by using level‐dependent rate matrices, and the mean queue length is computed. Second, an empirical analysis of the model is presented. The proposed methodology is applied in a case study concerning the geriatric patients. Some numerical calculations and optimizations are performed by using geriatric data. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
29.
In the presence of BF3, a series of symmetrical and unsymmetrical ethers reacted with epichlorohydrin and 2‐[(benzyloxy)methyl]oxirane, two terminal epoxides, to afford 1‐alkoxy‐3‐chloropropan‐2‐ol and 1‐alkoxy‐3‐(benzyloxy)propan‐2‐ol. The cleavage of unsymmetrical ethers occurred via an SN2 or SN1 mechanism. Secondary epoxides did not give similar ring‐opening products. 相似文献
30.