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51.
Jesús M. Ruiz Gonzalez Marcos Loroo Tania Crdova Gabriel Chuchani 《Journal of Molecular Structure》2005,732(1-3):55-61
Møller-Plesset MP2/6-31G method was used to examine the gas-phase elimination of 2-substituted alkyl ethyl N,N-dimethylcarbamates. The results of these calculations support a concerted non-synchronous six-membered cyclic transition state mechanism for carbamates containing a Cβ–H bond at the alkyl side of the ester. These substrates produce the N,N-dimethylcarbamic acid and the corresponding olefin. The unstable intermediate, N,N-dimethylcarbamic acid, rapidly decomposes through a four-membered cyclic transition state to dimethylamine and CO2 gas. Correlation of the logarithm of theoretical rate coefficients against original Taft's σ* values gave an approximate straight line (ρ*=−1.39, r=0.9558 at 360 °C). In addition to this fact, when log krel is plotted against the theoretical log krel for 2-substituted ethyl N,N-dimethylcarbamates a reasonable straight line (r=0.9919 at 360 °C) is obtained, suggesting similar mechanism. 相似文献
52.
Rutile Ni
x
Ti1-3x
Sb2x
O2 solid solution nanoparticles were synthesized by a sol-gel route using propylene oxide as a gelation agent. Titanium oxide
nanopowder and 12% TiCl3 solution were used as the source for titanium to investigate the influence of the titanium precursors on the formation of
the target materials. It was found that the nanoparticles prepared using 12% TiCl3 solution showed a much lower phase formation temperature (700°C) as compared to those prepared from TiO2 nanoparticles (1000°C). This lower phase formation temperature allowed a substantial reduction of the aggregation of the
particles during calcination leading to the formation of nearly mono-dispersed nanoparticles of about 20 nm. The results of
this work show that the epoxide assisted sol-gel method is capable to produce titanium-based ternary oxide solid solution
nanoparticles, owing to the formation of a highly homogeneous precursor gel intermediate. 相似文献
53.
Marcos Mandado Christian Van Alsenoy Paul Geerlings Frank De Proft Ricardo A Mosquera 《Chemphyschem》2006,7(6):1294-1305
A Hirshfeld decomposition scheme of the Hartree-Fock total molecular energy into atomic energies is presented. The calculations are performed by direct numerical integration and the results are compared for a set of 28 molecules containing different kinds of atoms. The calculated atomic energies show a strong dependency on changes of atomic electron population and hybridization. Linear correlations are found between the energy and the population for H, these being related to the electronegativity of this atom and to the external potential created by the remaining atoms. The proposed energy partitioning scheme appears to be useful for studies such as proton acidity, the anomeric effect and group transferability, and allows atomic virial ratios to be obtained. Finally, the atomic potential energies are found to mimic trends based on exact expressions as well as trends displayed by molecular quantities, thus lending credibility to the partitioning scheme used. 相似文献
54.
Comprehensive two-dimensional liquid chromatography-size-exclusion chromatography (LC x SEC) was investigated as a tool for the characterization of functional poly(methyl methacrylate) (PMMA) polymers. Ultraviolet-absorbance and evaporative light-scattering detection (ELSD) were used. A simple method to quantify ELSD data is presented. Each data point from the ELSD chromatogram can be converted into a mass concentration using experimental calibration curves. The qualitative and quantitative information obtained on two representative samples is used to demonstrate the applicability of LC x SEC for determining the mutually dependent molar-mass distributions (MMD) and functionality-type distributions (FTD) of functional polymers. The influence of the molar mass on the retention behavior in LC was investigated using LC x SEC for hydroxyl-functional PMMA polymers. The critical conditions, at which retention is--by definition--independent of molar mass, were not exactly the same for PMMA series with different end-groups. Our observations are in close agreement with theoretical curves reported in the literature. However, for practical applications of LC x SEC it is not strictly necessary to work at the exact critical solvent composition. Near-critical conditions are often sufficient to determine the mutually dependent distributions (MMD and FTD) of functional polymers. 相似文献
55.
Rodrigues e Silva AA da Silva Góes AJ de Lima WT de Souza Maia MB 《Chemical & pharmaceutical bulletin》2003,51(12):1351-1355
The search for new anti-inflammatory drugs has been constant in several research centers. The use of the Bioisostery concept allows the elaboration of new bioactive compounds with different properties through the introduction of substitute groups in one or more positions of a main molecule with known biological activity. Preliminary works accomplished at our laboratory with 2,4-thiazolidinedione isosters demonstrated inhibitory activity on edema formation for N-tryptophyl-5-(3,5-di-tert-butyl-4-hydroxybenzylidene)-2,4-thiazolidinedione (GS28) and N-tryptophyl-5-(3,5-di-tert-butyl-4-hydroxybenzylidene) rhodanine (GS26). We verified the antiedematogenic and ulcerogenic activity of these two compounds in Wistar rats. The carrageenan induced paw edema suffered significant (p<0.05) inhibition (28.36% on average) for GS28 (100 mg/kg; v.o.) during the entire time of the experiment. GS26 (50 and 100 mg/kg; v.o.) significantly inhibited (p<0.05) the paw edema dextran induced (22.1 and 27.8%, for the respective doses) after 180 min. The compounds GS26 and GS28 did not show ulcerogenic activity on gastric mucous. The results suggest antiedematogenic action for both compounds without the appearance of gastric lesions. 相似文献
56.
The use of aerosol produced in a nebulization chamber is proposed as an alternative to gas sample capture in flow systems. This paper describes the coupling of a sampling interface with a flow system, for in situ gas monitoring. Aspects related with the behavior of aerosol formation and gas solubilization in liquid drops are discussed. The method is applied to the determination of residual lime in acidic soils. Aliquots of 5.0 ml of 1.0 mol l−1 HCl were mixed with soil samples (1 g). The CO2 released from these samples was captured by a nebulized aerosol and determined conductivity. The analytical curve from 1.0×10−2 to 5.0×10−2 mol kg−1 CaCO3 was ploted applying the matrix matching approach. This proposition, allowed an increase in the sensibility with detection limit of 6.0×10−3 mol kg−1. The precision was good (R.S.D. <3%) for an analytical frequency of 22 determinations per hour. A fair agreement, at 95% confidence level, was found between the results from the proposed method and certified values of the investigated samples. 相似文献
57.
Jose′ L. F. C. Lima Antoacutenio O. S. S. Rangel M. Renata S. Souto 《Fresenius' Journal of Analytical Chemistry》1997,358(5):657-662
A flow injection system was developed for the sequential determination of total nitrogen and phosphorus in digests of vegetables
using potentiometric and spectrophotometric detection systems, respectively. A tubular ammonium selective electrode with a
sensor system composed of nonactin/monactin in tris(2-ethylhexyl) phosphate was used. The selectivity limitations of this
electrode were overcome by the inclusion of a gas-diffusion unit in the system that separated ammonium from the rest of the
sample matrix and allowed the determination of total nitrogen and phosphorus by the partition of the sample plug between two
streams.
The results obtained with the developed FIA system were in good agreement with those of the reference methods. Sampling rates
from 40 to 60 samples per hour and relative standard deviations below 3.5% were achieved.
Received: 17 October 1996/Revised: 21 November 1996/Accepted: 27 November 1996 相似文献
58.
Inductively coupled plasma optical emission spectrometry (ICP-OES) was applied to the analysis of major and minor elements of Ni-Nb alloys obtained by aluminothermic reduction process. Digestion of samples was made using a mixture of HF+HNO3. Minor and trace elements were determined without matrix separation. The precision for all constituents was <3%. Recoveries for the analyte-spiked samples were 95%. 相似文献
59.
The L-dopa is the immediate precursor of the neurotransmitter dopamine. Unlike dopamine, L-dopa easily enters the central nervous system and is used in the treatment of Parkinson’s disease. A sensitive and selective method is presented for the voltammetric determination of L-dopa in pharmaceutical formulations using a carbon paste electrode modified with trinuclear ruthenium ammine complex [(NH3)5RuIIIORuIV(NH3)4ORuIII(NH3)5]6+ (Ru-red) incorporated in NaY zeolite. The parameters which influence on the electrode response (paste composition, potential scan rate, pH and interference) were also investigated. The optimum conditions were found to an electrode composition (m/m) of 25% zeolite containing 6.7% Ru, 50% graphite and 25% mineral oil in acetate buffer at pH 4.8. Voltammetric peak currents showed a linear response for L-dopa concentration in the range between 1.2×10−4 and 1.0×10−2 mol l−1 (r=0.9988) with a detection limit of 8.5×10−5 mol l−1. The variation coefficient for a 1.0×10−3 mol l−1 L-dopa (n=10) was 5.5%. The results obtained for L-dopa in pharmaceutical formulations (tablet) was in agreement with compared official method. In conclusion, this study has illustrated that the proposed electrode modified with Ru-red incorporated zeolite is suitable valuable for selective measurements of L-dopa. 相似文献
60.
Flow injection system with gas diffusion for the sequential determination of total nitrogen and phosphorus in vegetables 总被引:1,自引:0,他引:1
Jose´ L. F. C. Lima Antoacutenio O. S. S. Rangel M. Renata S. Souto 《Analytical and bioanalytical chemistry》1997,358(5):657-662
A flow injection system was developed for the sequential determination of total nitrogen and phosphorus in digests of vegetables
using potentiometric and spectrophotometric detection systems, respectively. A tubular ammonium selective electrode with a
sensor system composed of nonactin/monactin in tris(2-ethylhexyl) phosphate was used. The selectivity limitations of this
electrode were overcome by the inclusion of a gas-diffusion unit in the system that separated ammonium from the rest of the
sample matrix and allowed the determination of total nitrogen and phosphorus by the partition of the sample plug between two
streams.
The results obtained with the developed FIA system were in good agreement with those of the reference methods. Sampling rates
from 40 to 60 samples per hour and relative standard deviations below 3.5% were achieved.
Received: 17 October 1996/Revised: 21 November 1996/Accepted: 27 November 1996 相似文献