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101.
Jifei Jia Liwu Lin Jianyi Shen Zhusheng Xu Tao Zhang Dongbai Liang Yi Chen 《中国科学B辑(英文版)》1998,41(6):606-615
CO adsorption microcalorimetry was employed in the study of γ-Al2O3-supported Pt, Pt-Sn and Pt-Fe catalysts. The results indicated that the initial differential heat of CO adsorption of the
Pt/γ-Al2O3 catalyst was 125 kJ/mol. As CO coverage increased, the differential heat of adsorption decreased. At higher coverages, the
differential heat of adsorption decreased significantly. 60% of the differential heat of CO adsorption on the Pt/γ-N2O3 catalyst was higher than 100 kJ/mol. No significant effect on the initial differential heat was found after adding Sn and
Fe to the Pt/γ-Al2O3 catalyst. The amount of strong CO adsorption sites decreased, while the portion of CO adsorption sites with differential
heat of 60–110 kJ/mol increased after increasing the Sn or Fe content. This indicates that the surface adsorption energy was
changed by adding Sn or Fe to Pt/γ-N2O3. The distribution of differential heat of CO adsorption on the Pt-Sn(C)/γ-Al2O3 catalyst was broad and homogeneous. Comparison of the dehydrogenation performance of C4 alkanes with the number of CO adsorption sites with differential heat of 60–110 kJ/mol showed a good correlation. These results
indicate that the surface Pt centers with differential heats of 60–110 kJ/mol for CO adsorption possess superior activity
for the dehydrogenation of alkanes.
Project supported by FORD and the National Natural Science Foundation of China (Grant No. 09412302) and the Transcentury Training
Program Foundation for the Talents by The State Education Commission of China. 相似文献
102.
Density functional calculations are employed to explore the mechanisms of all elementary reaction steps involved in the catalytic cycle of pyruvate decarboxylase (PDC). Different models are constructed for mimicking the involvement of some key residues in a certain step. The effect of the protein framework on the potential energy profiles of active site models is approximately modeled by fixing some freedoms, based on the crystal structure of the PDC enzyme from Saccharomyces cerevisiae (ScPDC). Our calculations confirm that Glu51 is the most important residue in the formation of the ylide and the release of acetaldehyde via the proton relay between Glu51, N1', and the 4'-amino group of thiamine diphosphate. The presence of Glu477 and Asp28 residues makes the decarboxylation of lactylthiamin diphosphate (LThDP) an endothermic process with a significant free energy barrier. The protonation of the alpha-carbanion to form 2-(1-hydroxyethyl)-thiamin diphosphate is found to go through a concerted double proton transfer transition state involving both Asp28 and His115 residues. The final step, acetaldehyde release, is likely to proceed through a concerted transition state involving carbon-carbon bond-breaking and the deprotonation of the alpha-hydroxyl group. The decarboxylation of LThDP and the protonation of the alpha-carbanion are two rate-limiting steps, relative to the facile occurrence of the ylide formation and acetaldehyde release. The catalytic roles of residues Glu51, Glu477, Asp28, and Gly417 in the active site of ScPDC in individual steps elucidated from the present study are in good agreement with those derived from site-directed mutagenesis. 相似文献
103.
Xin Ge Shenghua Hu Qing Sun. Jianyi ShenSchool of Chemistry Chemical Engineering Nanjing University Nanjing China 《天然气化学杂志》2003,(2)
Dehydrogenation of ethane to ethylene in CO2 was investigated over CeO2/γ-Al2O3 catalysts at 700℃ in a conventional flow reactor operating at atmospheric pressure. XRD, BET and microcalori-metric adsorption techniques were used to characterize the structure and surface acidity/basicity of the CeO2/γ-Al2O3 catalysts. The results show that the surface acidity decreased while the surface basicity increased after the addition of CeO2 to γ-A12O3. Accordingly, the activity of the hydrogenation reaction of CO2 increased, which might be responsible for the enhanced conversion in the dehydrogenation of ethane to ethylene. The highest ethane conversion obtained was about 15% for the 25%CeO2/γ-Al2O3. The selectivity to ethylene was high for all the CeO2,γ-A12O3 and CeO2/γ-Al2O3 catalysts. 相似文献
104.
采用共沉淀法合成了Ni/Fe摩尔比为1、2、3、6和10的NiFe水滑石。热重-差热结果显示NiFe水滑石的分解有二个过程,分别对应于200℃和300℃的吸热峰。经400℃焙烧后,NiFe水滑石转变为具有较高表面积的NiFeO混合氧化物(约100 m2/g),其XRD结果仅显示NiO的衍射峰;经800℃焙烧后,NiFeO仍有15 m2/g左右的表面积。穆斯堡尔谱测定表明NiFeO混合氧化物较Fe2O3易还原。 相似文献
105.
Mingliang Ye Hanfa Zou Zhengdeng Lei Renan Wu Zhen Liu Jianyi Ni 《Chromatographia》2001,53(7-8):425-430
Summary Acidic and neutral compounds have been separated by strong anion-exchange capillary electrochromatography (SAXCEC) dynamically
modified by addition of sodium dodecylsulfate (SDS). It was found that separation of neutral solutes by SAXCEC without addition
of SDS is difficult because of the weak interaction of the solutes with the hydrophilic packing surface. The hydrophobicity
of the packing surface increases on addition of SDS to the mobile phase, however, and the capacity of separating neutral solutes
increases. Acidic solutes are retained, mainly because of the ion-exchange properties of this system. The influence of mobile
phase composition, e.g. SDS concentration, ionic strength, and organic modifier fraction, on the retention of acidic and neutral
solutes was investigated. Three acids and five neutral solutes were separated in only 5 min under optimized conditions, because
the direction of the strong electroosmotic flow (EOF) was the same as that of electrophoretic migration of the acids under
the conditions used. The repeatability of this system in terms of migration time relative standard deviation (RSD) is good—less than 0.48% for 10 consecutive runs of all the solutes tested. Column efficiencies for acids were > 125000 plates
m−1; those for neutral solutes varied from 25000 to 100000 plates m−1. 相似文献
106.
The normal-to-local transition for the bending modes of acetylene is considered a prelude to its isomerization to vinylidene. Here, such a transition in fully deuterated acetylene is investigated using a full-dimensional quantum model. It is found that the local benders emerge at much lower energies and bending quantum numbers than in the hydrogen isotopomer HCCH. This is accompanied by a transition to a second kind of bending mode called counter-rotator, again at lower energies and quantum numbers than in HCCH. These transitions are also investigated using bifurcation analysis of two empirical spectroscopic fitting Hamiltonians for pure bending modes, which helps to understand the origin of the transitions semiclassically as branchings or bifurcations out of the trans- and cis-normal bend modes when the latter become dynamically unstable. The results of the quantum model and the empirical bifurcation analysis are in very good agreement. 相似文献
107.
Li J Wang Y Jiang B Ma J Dawes R Xie D Bowman JM Guo H 《The Journal of chemical physics》2012,136(4):041103
We report a chemically accurate global potential energy surface for the HOCO system based on high-level ab initio calculations at ~35,000 points. The potential energy surface is shown to reproduce important stationary points and minimum energy paths. Quasi-classical trajectory calculations indicated a good agreement with experimental data. 相似文献
108.
The authors report extensive high-level ab initio studies of the first excited (A??(2)A(')) state of HO(2). A global potential energy surface (PES) was developed by spline-fitting 17?000 ab initio points at the internal contracted multireference configuration interaction (icMRCI) level with the AVQZ basis set. To ascertain the spectroscopic accuracy of the PES, the near-equilibrium region of the molecule was also investigated using three interpolating moving least-squares-based PESs employing dynamically weighted icMRCI methods in the complete basis set limit. Vibrational energy levels on all four surfaces agree well with each other and a new assignment of some vibrational features is proposed. In addition, the dynamics of both the forward and reverse directions of the H+O(2)(a??(1)Δ(g))?OH+O reaction (J=0) were studied using an exact wave packet method. The reactions are found to be dominated by sharp resonances. 相似文献
109.
We report on an experimental 3×3 thermo-optical switch on silicon on insulator. By controlling a single combined phase shifter, light from any input waveguide can be directed to any output waveguide, showing a simple control method and highly integrated structure as compared to the conventional multiway optical switches. Furthermore, the proposed optical switch can be generalized to be a 1×N and N×N optical switch without an extra phase shifter. The switch is fabricated by complementary metal oxide semiconductor technology. By experiment, full 3×3 switching functionality is demonstrated at a wavelength of 1.55 μm, with an average cross talk of -11.1 dB and a power consumption of 97.5 mW. 相似文献
110.