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71.
The oxidation of cysteine by [Mo(CN)(8)](3-) in deoxygenated aqueous solution at a moderate pH is strongly catalyzed by Cu(2+), to the degree that impurity levels of Cu(2+) are sufficient to dominate the reaction. Dipicolinic acid (dipic) is a very effective inhibitor of this catalysis, such that with 1 mM dipic, the direct oxidation can be studied. UV-vis spectra and electrochemistry show that [Mo(CN)(8)](4-) is the Mo-containing product. Cystine and cysteinesulfinate are the predominant cysteine oxidation products. The stoichiometric ratio (Deltan(Mo(V))/Deltan(cysteine)) of 1.4 at pH 10.8 is consistent with this product distribution. At pH 1.5, the reaction is quite slow and yields intractable kinetics. At pH 4.5, the rates are much faster and deviate only slightly from pseudo-first-order behavior. With 2 mM PBN (N-phenyl-tert-butyl nitrone) present at pH 4.5, the reaction rate is about 20% less and shows excellent pseudo-first-order behavior, but the stoichiometric ratio is not significantly changed. The rates also display a significant specific cation effect. In the presence of spin-trap PBN, the kinetics were studied over the pH range 3.48-12.28, with [Na(+)] maintained at 0.09-0.10 M. The rate law is -d[Mo(V)]/dt = k[cysteine](tot)[Mo(V)], with k = {2(k(b)K(a1)K(a2)[H(+)] + k(c)K(a1)K(a2)K(a3))}/([H(+)](3) + K(a1)[H(+)](2) + K(a1)K(a2)[H(+)] + K(a1)K(a2)K(a3)), where K(a1), K(a2), and K(a3) are the successive acid dissociation constants of HSCH(2)CH(NH(3)(+))CO(2)H. Least-squares fitting yields k(b) = (7.1 +/- 0.4) x 10(4) M(-1) s(-1) and k(c) = (2.3 +/-0.2) x 10(4) M(-1) s(-1) at mu = 0.1 M (NaCF(3)SO(3)) and 25 degrees C. A mechanism is inferred in which k(b) and k(c) correspond to electron transfer to Mo(V) from the thiolate forms of anionic and dianionic cysteine. 相似文献
72.
High-performance liquid chromatography (HPLC) was combined with electrospray ionization tandem mass spectrometry (ESI-MS/MS) to develop a sensitive and selective method for the quantitative measurement of N(7)-(2-hydroxyethyl)guanine (N(7)-HEG) adducts in DNA obtained from ethylene oxide-exposed biological samples. Selected reaction monitoring (SRM) was used as the detection mode while the fragmentation product ion at m/z 152 generated from the precursor protonated N(7)-HEG (m/z 196) was monitored. The detection limits for N(7)-HEG were estimated by twofold serial dilution and determined to be 4 fmol in neat standard solution and 16 fmol when a matrix effect is considered. When the mass spectrometer was operated in the selected ion monitoring mode using only the first quadrupole (without MS/MS function), the detection limits increased to 128 fmol and 1 pmol (when matrix effect is considered), respectively. A good linear correlation (R(2) = 0.999) was observed for signal intensities obtained by injecting 16 fmol--33 pmol of N(7)-HEG into the HPLC/ESI-MS/MS system. Hep G2 cells were incubated for 8 h with medium containing various concentrations of ethylene oxide (ranging from 0.05 to 5.0 mM). A dose-response relationship was established, indicating that the adduct formation increases with the exposure level. The method shows potential, although the detection limit needs to be lowered for practical applications, for use in monitoring N(7)-HEG formation in other biological systems. 相似文献
73.
Campos LM Dang H Ng D Yang Z Martinez HL Garcia-Garibay MA 《The Journal of organic chemistry》2002,67(11):3749-3754
A detailed thermochemical analysis of the alpha-cleavage and decarbonylation reactions of acetone and several ketodiesters was carried out with the B3LYP/6-31G* density functional method. The heats of formation of several ground-state ketones and radicals were calculated at 298 K to determine bond dissociation energies (BDE) and radical stabilization energies (RSE) as a function of substituents. Results show that the radical-stabilizing abilities of the ketone substituents play a very important role on the thermodynamics of the alpha-cleavage and decarbonylation steps. An excellent correlation between calculated values and previous experimental observations suggests that photochemical alpha-cleavage and decarbonylation in crystals should be predictable from knowledge of excitation energies and the RSE of the substituent. 相似文献
74.
Institute of Chemistry, Moldavian SSR Academy of Sciences, Kishinev. Translated from Khimiya Prirodnykh Soedinenii, No. 5, pp. 759–760, September–October, 1988. 相似文献
75.
Hon Man Lee Chi Ying Lu Chih Yuan Chen Wen Ling Chen Hung Ching Lin Pei Ling Chiu Pi Yun Cheng 《Tetrahedron》2004,60(27):5807-5825
A series of new ethylene-bridged bis(imidazolium) halides with various N-substitutions were synthesized. Complexation of these imidazolium halides with Pd(OAc)2 produced new Pd(II) ethylene-bridged bis(carbene) complexes. Crystallographic analyses of some of the new imidazolium salts and Pd(II) complexes were determined. Applications of these seven-member palladacycles in Suzuki and Heck coupling reactions produced comparable catalytic activities to those of six-member analogs. 相似文献
76.
R A Wind N Zumbulyadis R H Young Y Hung L Li R H Nuttall G Maciel 《Solid state nuclear magnetic resonance》1992,1(2):55-65
We have investigated the EPR and DNP behavior of a molecularly doped polymer modeling those used to transport electronic charge in electrophotography. The EPR spectra show no evidence of the superexchange reported for a closely related system based on tri-p-tolylamine. The difference may be due to larger charge-transfer matrix elements in the latter system. An unambiguous interpretation of the observed 1H DNP was rendered difficult by the unanticipated asymmetry of the EPR spectra. We report extensive data on the "three-spin" effect evident in the DNP-enhanced 13C NMR spectra, and comment on its potential for characterizing polymer interfaces. 相似文献
77.
We have developed a method for fabricating microfluidic devices with multi-height structures using single step photolithography. The whole fabrication process is executed by conventional printed circuit board (PCB) technology without the need of having access to clean room facilities. Specifically designed "windows" and "rims" architectures were printed on films that were used as photomasks. Different levels of protruding features on the PCB master were produced by exposing a photomask followed by chemical wet etching. Poly(dimethylsiloxane) (PDMS) was then moulded against the positive relief master to generate microfluidic structures. In this report, we described the fabrication of a microfluidic device featured with a multi-height "sandbag" structure for particle entrapment and peripheral microchannels. Controlled immobilization of biological cells and immunocytochemcial staining assays were performed to demonstrate the applicability of the microfluidic device for cellular analysis. The integrity of the microdevice remained stable under applied pressure, indicating the robustness of the elastic PDMS structures for analytical operation. The simple microfabrication process requires only low-cost materials and minimal specialized equipment and can reproducibly produce mask lines of about 20 microm in width, which is sufficient for most microfluidic applications. 相似文献
78.
20-Deoxoluteone has been synthesized from sclareol. Sclareol was brominated with phosphorus tribromide to form a mixture of primary allyl bromides, from which, by the malonic synthesis, a mixture of bicyclogeranylgeranylacetic acids was obtained which was cyclized with fluorosulfonic acid to form a mixture of two diastereomeric -lactones. The predominating lactone was converted by successive reduction with lithium tetrahydroaluminate, oxidation with oxalyl chloride in dimethyl sulfoxide, reaction with methylmagnesium iodide, and oxidation by the chromium trioxide/pyridine complex into 20-deoxyluteone.Institute of Chemistry, Moldavian SSR Academy of Science, Kishinev. Translated from Khimiya Prirodnykh Soedinenii, No. 3, pp. 346–353, May–June, 1990. 相似文献
79.
SeongKu Kim Yu-Chueh Hung K. Geary Wei Yuan H.R. Fetterman D. Jin R. Dinu W.H. Steier 《Photonics Technology Letters, IEEE》2006,18(9):1055-1057
A variable optical attenuator (VOA) based on a metal-defined polymeric optical waveguide has been demonstrated for the first time. The metal film stressor deposited on top of the upper cladding layer not only produces the refractive index change within the core layer, but also acts as a thin-film heater allowing thermal tuning of the optical power within a metal-defined optical waveguide. Fabricated devices exhibit greater than 25 dB of optical attenuation with an applied electrical current of /spl sim/40 mA at 1550-nm wavelength. The switching speed of the VOA exhibits 800 /spl mu/s of rising and 720 /spl mu/s of falling time. 相似文献
80.
Fu S.-I. Cheng S.-Y. Chen T.-P. Lai P.-H. Tsai Y.-Y. Hung C.-W. Yen C.-H. Liu W.-C. 《Electron Devices, IEEE Transactions on》2006,53(11):2689-2695
A comprehensive study of emitter-ledge thickness of InGaP/GaAs heterojunction bipolar transistors (HBTs) has been undertaken. It is shown that the recombination rate and electron densities are drastically increased near the exposed base surface between the base contact and the emitter ledge. In contrast, the corresponding hole densities are decreased. If the emitter ledge is too thick, current will flow through the undepleted ledge, which increases the emitter-size effect. In contrast, if the emitter ledge is too thin, it may not effectively passivate the surface. Therefore, the thickness of the emitter ledge is a crucial issue and should be carefully considered. It is shown that, from simulated and experimental results, the optimum emitter-ledge thickness of InGaP/GaAs HBT is 100-200 Aring 相似文献