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961.
At room temperature, cobalt oxide cations directly convert methane to methanol with high selectivity but very low efficiency. Two potential intermediates of this reaction, the [HO-Co-CH(3)](+) insertion intermediate and [H(2)O-Co=CH(2)](+) aquo-carbene complex are produced in a laser ablation source and characterized by electronic and vibrational spectroscopy. Reaction of laser-ablated cobalt cations with different organic precursors seeded in a carrier gas produces the intermediates, which subsequently expand into vacuum and cool. Ions are extracted into a time-of-flight mass spectrometer and spectra are measured via photofragment spectroscopy. Photodissociation of [HO-Co-CH(3)](+) in the visible and via infrared multiple photon dissociation (IRMPD) makes only Co(+) + CH(3)OH, while photodissociation of [H(2)O-Co=CH(2)](+) produces CoCH(2)(+) + H(2)O. The electronic spectrum of [HO-Co-CH(3)](+) shows progressions in the excited state Co-C stretch (335 cm(-1)) and O-Co-C bend (90 cm(-1)); the IRMPD spectrum gives ν(OH) = 3630 cm(-1). The [HO-Co-CH(3)](+)(Ar) complex has been synthesized and its vibrational spectrum measured in the O-H stretching region. The resulting spectrum is sharper than that obtained via IRMPD and gives ν(OH) = 3642 cm(-1). Also, an improved potential energy surface for the reaction of CoO(+) with methane has been developed using single point energies calculated by the CBS-QB3 method for reactants, intermediates, transition states and products. 相似文献
962.
Rodrigues AC Nascimento ML Bragatto CB Souquet JL 《The Journal of chemical physics》2011,135(23):234504
Conductivity data of the xAgI(1 - x)AgPO(3) system (0 ≤ x ≤ 0.5) were collected in the liquid and glassy states. The difference in the dependence of ionic conductivity on temperature below and above their glass transition temperatures (T(g)) is interpreted by a discontinuity in the charge carrier's mobility mechanisms. Charge carrier displacement occurs through an activated mechanism below T(g) and through a Vogel-Fulcher-Tammann-Hesse mechanism above this temperature. Fitting conductivity data with the proposed model allows one to determine separately the enthalpies of charge carrier formation and migration. For the five investigated compositions, the enthalpy of charge carrier formation is found to decrease, with x, from 0.86 to 0.2 eV, while the migration enthalpy remains constant at ≈0.14 eV. Based on these values, the charge carrier mobility and concentration in the glassy state can then be calculated. Mobility values at room temperature (≈10(-4) cm(2) V(-1) s(-1)) do not vary significantly with the AgI content and are in good agreement with those previously measured by the Hall-effect technique. The observed increase in ionic conductivity with x would thus only be due to an increase in the effective charge carrier concentration. Considering AgI as a weak electrolyte, the change in the effective charge carrier concentration is justified and is correlated to the partial free energy of silver iodide forming a regular solution with AgPO(3). 相似文献
963.
Excited-state quantum mechanics/molecular mechanics molecular dynamics simulations are performed, to examine the solvent effects on the fluorescence spectra of aqueous formaldehyde. For that purpose, the analytical energy gradient has been derived and implemented for the linear-response time-dependent density functional theory (TDDFT) combined with the effective fragment potential (EFP) method. The EFP method is an efficient ab initio based polarizable model that describes the explicit solvent effects on electronic excitations, in the present work within a hybrid TDDFT/EFP scheme. The new method is applied to the excited-state MD of aqueous formaldehyde in the n-π* state. The calculated π*→n transition energy and solvatochromic shift are in good agreement with other theoretical results. 相似文献
964.
Ribeiro da Silva MA Lobo Ferreira AI Cimas Á 《The Journal of organic chemistry》2011,76(10):3754-3764
Thermodynamic properties of 3- and 4-phenoxyphenol have been determined by using a combination of calorimetric and effusion techniques as well as by high-level ab initio molecular orbital calculations. The standard (p° = 0.1 MPa) molar enthalpies of formation in the condensed and gas states, Δ(f)H(m)°(cr or l) and Δ(f)H(m)°(g), at T = 298.15 K, of 3- and 4-phenoxyphenol were derived from their energies of combustion in oxygen, measured by a static bomb calorimeter, and from the enthalpies of vaporization or sublimation derived respectively by Calvet microcalorimetry for the 3-phenoxyphenol and by Knudsen effusion technique for the 4-phenoxyphenol. The theoretically estimated gas-phase enthalpies of formation were calculated from high-level ab initio molecular orbital calculations at the G3(MP2)//B3LYP level of theory. Furthermore, this composite approach was also used to obtain information about the gas-phase acidities, gas-phase basicities, proton and electron affinities, adiabatic ionization enthalpies, and, finally, O?H bond dissociation enthalpies. The good agreement between the G3MP2B3-derived values and the experimental gas-phase enthalpies of formation for the 3- and 4-phenoxyphenol gives confidence to the estimate concerning the 2-phenoxyphenol isomer, which was not experimentally studied, and to the estimates concerning the radical and the anion. Additionally, the experimental values of gas-phase enthalpies of formation were also compared with estimates based on the empirical scheme developed by Cox. 相似文献
965.
966.
Corrêa da Costa R Buffeteau T Del Guerzo A McClenaghan ND Vincent JM 《Chemical communications (Cambridge, England)》2011,47(29):8250-8252
The Ru(bipy)(3) dication is efficiently and reversibly transferred into perfluorocarbons due to the formation of a highly fluorophillic hydrogen-bonded fluorous carboxylate-carboxylic acid counter-anion, whilst retaining key luminescence and photosensitizer characteristics, for example in singlet oxygen production. 相似文献
967.
da Silva AS Fernandes FC Tognolli JO Pezza L Pezza HR 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2011,79(5):1881-1885
This article describes a simple, inexpensive, and environmentally friendly method for the monitoring of glyphosate using diffuse reflectance spectroscopy. The proposed method is based on reflectance measurements of the colored compound produced from the spot test reaction between glyphosate and p-dimethylaminocinnamaldehyde (p-DAC) in acid medium, using a filter paper as solid support. Experimental designs were used to optimize the analytical conditions. All reflectance measurements were carried out at 495 nm. Under optimal conditions, the glyphosate calibration graphs obtained by plotting the optical density of the reflectance signal (AR) against the concentration were linear in the range 50-500 μg mL(-1), with a correlation coefficient of 0.9987. The limit of detection (LOD) for glyphosate was 7.28 μg mL(-1). The technique was successfully applied to the direct determination of glyphosate in commercial formulations, as well as in water samples (river water, pure water and mineral drinking water) after a previous clean-up or pre-concentration step. Recoveries were in the ranges 93.2-102.6% and 91.3-102.9% for the commercial formulations and water samples, respectively. 相似文献
968.
R. A. G. Silva A. T. Adorno A. G. Magdalena T. M. Carvalho M. Stipcich A. Cuniberti M. L. Castro 《Journal of Thermal Analysis and Calorimetry》2011,103(2):459-463
In this study the effect of Ag additions on the thermal behavior of the Cu–22.55 at.%Al alloy was studied using electrical
resistivity measurements, in situ X-ray diffractometry, differential scanning calorimetry, and optical microscopy. The results
indicated that Ag additions do not change the phase transformations sequence in the studied alloys, but modify its critical
temperatures due to a change on entropy of system. It was verified that at the cooling rate of 10 K/min the decomposition
of β phase into (α + γ1) is incomplete, but for lower cooling rates than 1.0 K/min this reaction is completed. 相似文献
969.
Luiz K. C. de Souza Juliana J. R. Pardauil José R. Zamian Geraldo N. da Rocha Filho Carlos E. F. da Costa 《Journal of Thermal Analysis and Calorimetry》2011,106(2):355-361
Al-modified MCM-41, La-modified MCM-41, and Ce-modified MCM-41 mesoporous materials were prepared with different molar ratios
(Si/M = 10; 25; 50; 100 and 200) at room temperature. The materials were characterized using XRD, BET–BJH, and TG–DTA. The
XRD showed four peaks, due to the ordered hexagonal array of parallel silica tubes, which could be indexed as (100), (110),
(200), and (210), assuming a hexagonal unit cell. The surface area decreased as the concentration of the metal incorporated
in the material increased. The thermal stability of the materials was around 650 °C. The CeO2 phase made the mass transfer process more difficult, hindering Hofmann degradation and favoring oxidation. 相似文献
970.
Hellyda K. T. A. Silva Thiago Chellappa Fabíola C. Carvalho Edjane F. B. Silva Tarcísio A. Nascimento Antônio S. Araújo Valter J. FernandesJr. 《Journal of Thermal Analysis and Calorimetry》2011,106(3):731-733
Biodiesel is defined as a mixture of mono- or di-alquil esters of vegetable oil or animal fats. During long-term storage,
oxidation caused by contact with air (autoxidation) presents a legitimate concern in relation to monitoring and maintaining
fuel quality. Extensive oxidative degradation may compromise the quality by adversely affecting kinematic viscosity, acid
value, or peroxide value. The oxidation susceptibility of biodiesel, due to the presence of triacilglycerides of poly-unsaturated
fatty acids, was evaluated in this study. Samples of sunflower, castor, and soybean biodiesels were obtained through the transesterification
reaction, with the intention of achieving the thermal stability study through thermogravimetrical analyses and differential
scanning calorimetry high pressure. It was furthermore observed through thermogravimetry and pressure differential scanning
calorimetry curves that castor biodiesel exhibited the highest thermal and oxidative stability. 相似文献