全文获取类型
收费全文 | 178篇 |
免费 | 5篇 |
专业分类
化学 | 151篇 |
数学 | 9篇 |
物理学 | 23篇 |
出版年
2023年 | 1篇 |
2022年 | 1篇 |
2021年 | 2篇 |
2019年 | 2篇 |
2016年 | 2篇 |
2015年 | 1篇 |
2014年 | 4篇 |
2013年 | 5篇 |
2012年 | 13篇 |
2011年 | 15篇 |
2010年 | 4篇 |
2009年 | 4篇 |
2008年 | 18篇 |
2007年 | 16篇 |
2006年 | 14篇 |
2005年 | 12篇 |
2004年 | 10篇 |
2003年 | 11篇 |
2002年 | 11篇 |
2001年 | 11篇 |
2000年 | 6篇 |
1999年 | 1篇 |
1998年 | 2篇 |
1996年 | 1篇 |
1994年 | 3篇 |
1993年 | 2篇 |
1992年 | 1篇 |
1991年 | 1篇 |
1989年 | 1篇 |
1987年 | 1篇 |
1986年 | 2篇 |
1985年 | 1篇 |
1984年 | 2篇 |
1977年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有183条查询结果,搜索用时 375 毫秒
121.
A. SH. Hussein Z. HassanS.M. Thahab S.S. NgH. Abu Hassan C.W. Chin 《Applied Surface Science》2011,257(9):4159-4164
The effect of Al mole fractions on the structural and electrical properties of AlxGa1−xN/GaN thin films grown by plasma-assisted molecular beam epitaxy (PA-MBE) on Si (1 1 1) substrates has been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and current-voltage (I-V) measurements. X-ray results revealed that the AlGaN/GaN/AlN was epitaxially grown on Si substrate. By applying Vegard's law, the Al mole fractions of AlxGa1−xN samples were found to be 0.11, 0.24, 0.30 and 0.43, respectively. The structural and morphology results indicated that there is a relatively larger tensile strain for the sample with the smallest Al mole fraction; while a smaller compressive strain and larger grain size appear with Al mole fraction equal to 0.30. The strain gets relaxed with the highest Al mole fraction sample. Finally, the linear relationship between the barrier height and Al mole fraction was obtained. 相似文献
122.
Pomerantz WC Yuwono VM Drake R Hartgerink JD Abbott NL Gellman SH 《Journal of the American Chemical Society》2011,133(34):13604-13613
We have examined the effect of β-peptide modifications on the propensity of these helical molecules to form lyotropic liquid crystalline (LC) phases in water. All of the β-peptides we have examined contain 10 residues. In each case, at least three residues are derived from trans-2-aminocyclohexanecarboxylic acid (ACHC), which strongly promotes folding to a 14-helical conformation. The structural features varied include the number of ACHC residues, the nature and spatial arrangement of charged side chains (cationic vs anionic), and the identity of groups at the β-peptide termini. We found that relatively small changes (e.g., swapping the positions of a cationic and an anionic side chain) could have large effects, such as abrogation of LC phase formation. The trends revealed by sequence-property studies led to the design of LC-forming β-peptides that bear biomolecular recognition groups (biotin or the tripeptide Arg-Gly-Asp). Structural analysis via circular dichroism and cryo-transmission electron microscopy revealed the existence of two different types of self-associated species, globular aggregates and nanofibers. Nanofibers are the predominant assembly formed at concentrations that lead to LC phase formation, and we conclude that these nanofibers are the functional mesogens. Overall, these studies show how the modularity of β-peptide oligomers enables elucidation of the relationship between molecular structure and large-scale self-assembly behavior. 相似文献
123.
Infection of cells by HIV depends upon profound structural rearrangements within the trimeric viral protein gp41. Critical to this process is the formation of a six-helix bundle in which a set of three N-terminal heptad repeat (NHR) helices assemble to form a core displaying long grooves that provide docking sites for three C-terminal heptad repeat (CHR) helices. We report experiments designed to discriminate between two alternative hypotheses regarding the source of affinity between individual CHR helices and the complementary groove: (1) affinity is dominated by interactions of a small cluster of side chains at one end of the CHR helix; or (2) affinity depends upon interactions distributed across the long CHR helix. We have employed two complementary experimental designs, and results from both favor the latter hypothesis. 相似文献
124.
Li Guo Weicheng Zhang Ilia A. Guzei Lara C. Spencer Samuel H. Gellman 《Tetrahedron》2012,68(23):4413-4417
Oligomers containing both α- and γ-amino acid residues in a 1:1 alternating pattern have recently been shown by several research groups to adopt helical secondary structures. We have begun to explore the folding behavior of oligomers with different α-residue/γ-residue backbone patterns. Previously we reported that the γ-amino acids bearing a cyclohexyl constraint at the Cβ–Cγ bond and a variable side chain at Cα strongly promote a helical conformation containing 12-atom CO(i)?H–N(i+3) hydrogen bonds for 1:1 α:γ backbones. Here we report synthesis and crystallographic analysis of 2:1 and 1:2 α/γ-peptides that adopt CO(i)?H–N(i+3) hydrogen-bonded helical conformations. 相似文献
125.
126.
The adsorption of thiophene (C4H4S) on the clean and sulfided Mo(100) crystal surface has been studied. A fraction of the adsorbed thiophene desorbs molecularly while the remainder decomposes upon heating, evolving H2 and leaving carbon and sulfur deposits on the surface. The reversibly adsorbed thiophene exhibits three distinct desorption peaks at 360, 230–290 and 163–174 K, corresponding to binding energies of 22, 13–16 and 7–9 kcal/mol respectively. Sulfur on the Mo(100) surface preferentially blocks the highest energy binding state and causes an increase in the amount of thiophene bound in the low binding energy, multilayer state. The thiophene decomposition reactions yield H2 desorption peaks in the temperature range 300–700 K. We estimate that 50–66% of the thiophene adsorbed to the clean Mo(100) decomposes. The decomposition reaction is blocked by the presence of c(2 × 2) islands of sulfur and is blocked completely at θs = 0.5, at which point thiophene adsorption is entirely reversible. 相似文献
127.
M.H. Farias A.J. Gellman G.A. Somorjai R.R. Chianelli K.S. Liang 《Surface science》1984,140(1):181-196
The chemisorption and reactivity of O2 and H2 with the sulfided Mo(100) surface and the basal (0001) plane of MoS2 have been studied by means of Thermal Desorption Spectroscopy (TDS), Auger Electron Spectroscopy (AES) and Low Energy Electron Diffraction (LEED). These studies have been carried out at both low (10?8–10?5Torr) and high (1 atm) pressures of O2 and H2. Sulfur desorbs from Mo(100) both as an atom and as a diatomic molecule. Sulfur adsorbed on Mo(100) blocks sites of hydrogen adsorption without noticeably changing the hydrogen desorption energies. TDS of 18O coadsorbed with sulfur on the Mo(100) surface produced the desorption of SO at 1150 K, and of S, S2 and O, but not SO2. A pressure of 1 × 10?7 Torr of O2 was sufficient to remove sulfur from Mo(100) at temperatures over 1100 K. The basal plane of MoS2 was unreactive in the presence of 1 atm of O2 at temperatures of 520 K. Sputtering of the MoS2 produced a marked uptake of oxygen and the removal of sulfur under the same conditions. 相似文献
128.
Arnold U Hinderaker MP Nilsson BL Huck BR Gellman SH Raines RT 《Journal of the American Chemical Society》2002,124(29):8522-8523
beta-Amino acids are incorporated into an enzyme by using the method of expressed protein ligation. In the resulting semisynthetic enzyme, an R-nipecotic acid-S-nipecotic acid module replaces Asn113 and Pro114 of ribonuclease A. The semisynthetic enzyme not only retains full catalytic activity but also gains conformational stability. Thus, structural elements can be replaced with foldameric equivalents to endow proteins with more desirable properties. 相似文献
129.
130.
Wafa I. Abdel-Fattah M.SH. Fayed SH.R. Gooda W.F.F. Mekky 《Journal of Sol-Gel Science and Technology》1998,13(1-3):981-985
-Eucryptite solid solution (LAS) Li2O : Al 2O3 : 3SiO2(113) was prepared via a sol-gel route. Complex solutions of lithium or aluminum salts in ethanol/water mixtures were catalysed by inorganic acids (catalyst/alkoxide ratio 0.1). Different ratios of H2SO4 catalyst were also used.TGA showed that the maximum % yield of LAS 113 occurred using H2SO4 catalyst. Above 400°C exothermic peaks occurred in DSC followed by the glass transition. XRD and IR showed a high crystallinity of -eucryptite solid solution using all acids at the 0.1 ratio with the highest crystallinity for the 0.05 H2SO4 ratio. SEM revealed similar grain sizes in spite of higher acid concentrations. XRD data indicated a change to a tetragonal modification with higher catalyst ratio. 相似文献