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91.
[structure: see text] An efficient, enantioselective synthesis of the C1-C12 fragment 2 of borrelidin is presented. Construction of the "skipped" polymethylene chain of 2 was accomplished by iteration of Myers' alkylation, while formation of the C3 stereocenter was achieved by Roush's asymmetric allylboration methodology. 相似文献
92.
A new material loss technique for the experimental determination of the free evaporation rate of a metal is presented. This technique is based on the study of tip profile changes. The tip radius variation is a consequence of the simultaneous action of free evaporation and surface self-diffusion. The rate of free evaporation is related to the tip radius evolution, so the vapor pressure and heat of evaporation can be determined by measuring in situ the radius changes with time. The method permits an easy control of the cleanliness of the surface by field electron microscopy, and should enable one to study the influence of adsorption on evaporation. The method is tested with molybdenum tips. The variation of the radius of the tips, heated in ultra-high vacuum, is determined by field electron and scanning microscope measurements. Vapor pressures and heat of evaporation values obtained are in agreement with values obtained by other techniques. 相似文献
93.
Third-harmonic generation from ZnO films deposited by MOCVD 总被引:2,自引:0,他引:2
C.Y.?LiuEmail author B.P.?Zhang N.T.?Binh Y.?Segawa 《Applied physics. B, Lasers and optics》2004,79(1):83-86
Third-harmonic generation (THG) was studied from ZnO thin film deposited by the metalorganic chemical vapor deposition (MOCVD) technique on sapphire substrates at different temperatures. A strong THG signal was obtained from the film deposited at an appropriate temperature. The dependence of THG on the deposition temperatures was discussed. A third-order susceptibility (3)=3.77×10-12 esu was deduced for a film deposited at 250 °C. This value is similar to that observed from ZnO nanocrystalline films fabricated by pulse-laser deposition (PLD). We conclude that film structure and the crystalline quality is main factors determined the THG in the film. PACS 42.55. Sa; 77.80.-e; 42.55.Px 相似文献
94.
A new mechanism for electron emission from planar cathodes is described. The theoretical analysis shows that, with an ultrathin wide band-gap semiconductor layer (UTSC) on a metal, the surface barrier is lowered to approximately 0.1 eV due to the creation of a space charge induced by the electrons injected from the metal. The barrier height depends mostly on the UTSC thickness and not on the state of the surface, as in thermionic and field emissions. This mechanism explains the measured stable emission at 300 K and 10(-7) Torr, with a threshold field of only approximately 50 V/&mgr;m, from these solid-state field-controlled emitters. 相似文献
95.
Ahmet Ocak Akdemir Ho Duy Binh Donal O'Regan Anh Tuan Nguyen 《Mathematical Methods in the Applied Sciences》2023,46(1):1076-1095
In this paper we investigate the Cauchy problem for a fractional diffusion equation and the time-fractional derivative is taken in the Caputo type sense. We give a representation of solutions under Fourier series and analyze initial value problems for the semi-linear fractional diffusion equation with a memory term. We also discuss the stability of the fractional derivative order for the time under some assumptions on the input data. Our key idea is to use Mittag-Leffler functions, the Banach fixed point theorem, and some Sobolev embeddings. 相似文献
96.
Excited‐State Multiple Proton Transfer Depending on the Acidity and Basicity of Mediating Alcohols in 7‐Azaindole–(ROH)2 (R=H,CH3) Complexes: A Theoretical Study 下载免费PDF全文
Long‐range proton transfer plays an important role in many chemical and biological phenomena. It has recently been reported that the rate of excited‐state multiple proton transfer depends on the acidity and basicity of mediating alcohols in the H‐bonded wire. The excited‐state triple proton transfer in 7‐azaindole complexes through cyclic H‐bonded wires was theoretically studied to investigate rates depending on the mediating alcohols. This study showed that the acidity and basicity of alcohols collectively functioned to assist proton transfers depending on the paths; the proton transfers of protolytic and solvolytic paths were assisted by the pull‐behind effect and the push‐ahead effect, respectively. Both proton‐donating and accepting abilities of alcohols in the H‐bonded wire can accumulate to help proton transfer, and the strong acidity and basicity of the alcohols with relatively small structural changes in the wire have larger impacts on reducing the activation energies than those of alcohols that trigger proton transfer. 相似文献
97.
Binh Khanh Mai Yongho Kim 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2015,127(13):4018-4023
Two‐state reactivity (TSR) is often used to explain the reaction of transition‐metal–oxo reagents in the bare form or in the complex form. The evidence of the TSR model typically comes from quantum‐mechanical calculations for energy profiles with a spin crossover in the rate‐limiting step. To prove the TSR concept, kinetic profiles for C H activation by the FeO+ cation were explored. A direct dynamics approach was used to generate potential energy surfaces of the sextet and quartet H‐transfers and rate constants and kinetic isotope effects (KIEs) were calculated using variational transition‐state theory including multidimensional tunneling. The minimum energy crossing point with very large spin–orbit coupling matrix element was very close to the intrinsic reaction paths of both sextet and quartet H‐transfers. Excellent agreement with experiments were obtained when the sextet reactant and quartet transition state were used with a spin crossover, which strongly support the TSR model. 相似文献
98.
The Kinetic Isotope Effect as a Probe of Spin Crossover in the CH Activation of Methane by the FeO+ Cation 下载免费PDF全文
Binh Khanh Mai Prof. Dr. Yongho Kim 《Angewandte Chemie (International ed. in English)》2015,54(13):3946-3951
Two‐state reactivity (TSR) is often used to explain the reaction of transition‐metal–oxo reagents in the bare form or in the complex form. The evidence of the TSR model typically comes from quantum‐mechanical calculations for energy profiles with a spin crossover in the rate‐limiting step. To prove the TSR concept, kinetic profiles for C? H activation by the FeO+ cation were explored. A direct dynamics approach was used to generate potential energy surfaces of the sextet and quartet H‐transfers and rate constants and kinetic isotope effects (KIEs) were calculated using variational transition‐state theory including multidimensional tunneling. The minimum energy crossing point with very large spin–orbit coupling matrix element was very close to the intrinsic reaction paths of both sextet and quartet H‐transfers. Excellent agreement with experiments were obtained when the sextet reactant and quartet transition state were used with a spin crossover, which strongly support the TSR model. 相似文献
99.
N. T. Binh Y. Truong N. T. Ngo L. N. Sieu P. D. Hien 《Journal of Radioanalytical and Nuclear Chemistry》2000,244(1):103-107
Concentrations of elements in fine and coarse PM10 samples collected in Ho Chi Minh City were determined by INAA for the purpose of characterising air pollution sources using multivariate receptor modeling techniques. Seven sources common to coarse and fine samples were identified. Resuspended soil dust is dominant in the coarse samples accounting for 41% of the particulate mass. In the fine samples, vehicle emissions and coal burning are most important accounting for about 20% each. Although a great number of elements were included in the input data for receptor modeling, the interpretation of emission sources was not always straightforward. Information on other source markers were needed. Therefore, a polarography method was used for quantifying lead, and recently, ion chromatography method became available for quantifying secondary sulphates, nitrates and other water soluble ions. 相似文献
100.
Thai Binh Le Peter C. Hauser Thi Ngoc Mai Pham Thi Lan Phuong Kieu Thi Phuong Quynh Le Quoc Anh Hoang Dinh Chi Le Thi Anh Huong Nguyen Thanh Duc Mai 《Electrophoresis》2020,41(23):1980-1990
In this study, the development of our purpose-made capacitively coupled contactless conductivity detection (C4D) for CE is reported. These systems have been employed as a simple, versatile, and cost-effective analytical tool. CE-C4D devices, whose principle is based on the control of the ion movements under an electrical field, can be constructed even with a modest financial budget and limited infrastructure. A featured application was developed for quality control of antimicrobial drugs using CE-C4D, with most recent work on determination of aminoglycoside and glycopeptide antibiotics being communicated. For aminoglycosides, the development of CE-C4D methods was adapted to two categories. The first one includes drugs (liquid or powder form) for intravenous injection, containing either amikacin, streptomycin, kanamycin A, or kanamycin B. The second one covers drugs for eye drops (liquid or ointment form), containing either neomycin, tobramycin, or polymyxin. The CE-C4D method development was also made for determination of some popular glycopeptide antibiotics in Vietnam, including vancomycin and teicoplanin. The best detection limit achieved using the developed CE-C4D methods was 0.5 mg/L. Good agreement between results from CE-C4D and the confirmation method (HPLC- Photometric Diode Array ) was achieved, with their result deviations less than 8% and 13% for aminoglycoside and glycopeptide antibiotics, respectively. 相似文献