全文获取类型
收费全文 | 1859篇 |
免费 | 23篇 |
国内免费 | 10篇 |
专业分类
化学 | 1279篇 |
晶体学 | 27篇 |
力学 | 23篇 |
数学 | 136篇 |
物理学 | 427篇 |
出版年
2023年 | 9篇 |
2021年 | 12篇 |
2020年 | 16篇 |
2019年 | 17篇 |
2018年 | 13篇 |
2017年 | 21篇 |
2016年 | 20篇 |
2015年 | 24篇 |
2014年 | 33篇 |
2013年 | 62篇 |
2012年 | 71篇 |
2011年 | 88篇 |
2010年 | 54篇 |
2009年 | 63篇 |
2008年 | 115篇 |
2007年 | 125篇 |
2006年 | 114篇 |
2005年 | 109篇 |
2004年 | 104篇 |
2003年 | 90篇 |
2002年 | 66篇 |
2001年 | 51篇 |
2000年 | 44篇 |
1999年 | 31篇 |
1998年 | 15篇 |
1997年 | 26篇 |
1996年 | 20篇 |
1995年 | 19篇 |
1994年 | 34篇 |
1993年 | 15篇 |
1992年 | 27篇 |
1991年 | 23篇 |
1990年 | 22篇 |
1989年 | 13篇 |
1988年 | 21篇 |
1987年 | 18篇 |
1986年 | 18篇 |
1985年 | 32篇 |
1984年 | 32篇 |
1983年 | 16篇 |
1982年 | 27篇 |
1981年 | 14篇 |
1980年 | 16篇 |
1979年 | 21篇 |
1978年 | 20篇 |
1977年 | 25篇 |
1976年 | 13篇 |
1975年 | 19篇 |
1974年 | 11篇 |
1965年 | 4篇 |
排序方式: 共有1892条查询结果,搜索用时 15 毫秒
991.
Tetramethylammonium hydroxide (TMAH) thermochemolysis of 2-arylcoumaran lignin model compounds 总被引:1,自引:0,他引:1
Ken-ichi Kuroda Akiko Nakagawa-izumi Tatsuya Ashitani Koki Fujita 《Journal of Analytical and Applied Pyrolysis》2009,86(1):185-191
Phenolic 2-arylcoumarans 1–6 were used to examine the behaviors of β-5 subunits in lignin during tetramethylammonium hydroxide (TMAH) thermochemolysis. Products were monitored by gas chromatography/mass spectrometry. The process predominantly provided dimeric products with the opened hydrofuran ring. Substituent changes at the γ-position of ring A and at the 5-position of ring B had a large effect on the product compositions. 2-Arylcoumarans 1 and 6 with the γ-CH2OH substituent predominantly gave 2,3,3′,4′-tetramethoxystilbenes involving the elimination of the γ-CH2OH substituent, while 2–5 with the γ-CH3 substituent gave a mixture of 2,3,3′,4′-tetramethoxy-α-methylstilbenes and α-methoxy-α-(3′,4′-dimethoxyphenyl)-β-(2,3-dimethoxyphenyl)propanes. Substituent –CHCHCH3 on ring B remained unaffected. Substituents –CHCHCH2OH and –COOH on ring B produced the corresponding methyl ether and ester, respectively, by methylation. The –CHCHCHO substituent on ring B was converted to the –CHO substituent. 相似文献
992.
We combine two well-known results by Mader and Thomassen, respectively. Namely, we prove that for any k-connected graph G (k≥4), there is an induced cycle C such that G−V(C) is (k−3)-connected and G−E(C) is (k−2)-connected. Both “(k−3)-connected” and “(k−2)-connected” are best possible in a sense. 相似文献
993.
Ohta A Danev R Nagayama K Mita T Asakawa T Miyagishi S 《Langmuir : the ACS journal of surfaces and colloids》2006,22(20):8472-8477
An aggregation behavior of potassium N-acyl phenylalaninate in dilute aqueous solution was investigated. It was found that this surfactant formed large aggregates at lower concentrations, which were then transformed to micelles at higher concentrations. Fluorescence intensity measurements using a probe were used to examine the effects of alkali concentration, acyl chain length, and solvent isotope on the aggregation behavior. The influence of the alkali concentration suggested that formation of an acid-soap dimer brought about the construction of the large particles at very dilute concentrations. Increases in both the acyl chain length and replacement of H(2)O with D(2)O resulted in stronger hydrophobic interactions; consequently, the large aggregate formation was enhanced. This aggregation behavior has not been observed when racemic modification of N-acyl phenylalaninate has taken place. By using cryo-transmission electron microscopy (TEM) with a Zernike differential contrast phase plate, it was found that the large aggregates were tubes with bilayer structures, which were then transformed into spherical micelles via threadlike micelles with increasing concentration due to a drastic increase in the concentration of ionic species in the aggregate. 相似文献
994.
Nanosecond time-resolved coherent anti-Stokes Raman spectroscopy is used to investigate the shock-induced liquid-solid phase transition and crystallization of liquid benzene. Temporal evolution of the Raman shift of the ring-breathing and C-H stretching modes is investigated. A metastable supercompressed state and a liquid-solid phase transition are observed under shock compression. Time-resolved Raman spectra reveal that the liquid state is initially a metastable state and rapidly transforms to the solid state within 25 ns under shock compression at 4.2 GPa. 相似文献
995.
Imura T Ohta N Inoue K Yagi N Negishi H Yanagishita H Kitamoto D 《Chemistry (Weinheim an der Bergstrasse, Germany)》2006,12(9):2434-2440
Self-assembling properties of "natural" glycolipid biosurfactants, mannosyl-erythritol lipids A and B (MEL-A, MEL-B), which are abundantly produced from yeast strains, were investigated by using the fluorescence-probe method, dynamic light-scattering (DLS) analysis, freeze-fracture transmission electron microscopy (FF-TEM), and synchrotron small/wide-angle X-ray scattering (SAXS/WAXS) analysis, among other methods. Both MEL-A and MEL-B exhibit excellent self-assembly properties at extremely low concentrations; they self-assemble into large unilamellar vesicles (LUV) just above their critical-aggregation concentration (CAC). The CAC(I) value was found to be 4.0x10(-6) M for MEL-A and 6.0x10(-6) M for MEL-B. Moreover, the self-assembled structure of MEL-A above a CAC(II) value of 2.0x10(-5) M was found to drastically change into sponge structures (L3) composed of a network of randomly connected bilayers that are usually obtained from a complicated multicomponent "synthetic" surfactant system. Interestingly, the average water-channel diameter of the sponge structure was 100 nm. This is relatively large compared with those obtained from "synthetic" surfactant systems. In addition, MEL-B, which has a hydroxyl group at the C-4' position on mannose instead of an acetyl group, gives only one CAC; the self-assembled structure of MEL-B seems to gradually move from LUV to multilamellar vesicles (MLV) with lattice constants of 4.4 nm, depending on the concentration. Furthermore, the lyotropic-liquid-crystal-phase observation at high concentrations demonstrates the formation of an inverted hexagonal phase (H2) for MEL-A, together with a lamella phase (L(alpha)) for MEL-B, indicating a difference between MEL-A and MEL-B molecules in the spontaneous curvature of the assemblies. These results clearly show that the difference in spontaneous curvature caused by the single acetyl group on the head group probably decides the direction of self-assembly of glycolipid biosurfactants. The unique and complex molecular structures with several chiral centers that are molecularly engineered by microorganisms must have led to the sophisticated self-assembling properties of the glycolipid biosurfactants. 相似文献
996.
Slepkov AD Hegmann FA Tykwinski RR Kamada K Ohta K Marsden JA Spitler EL Miller JJ Haley MM 《Optics letters》2006,31(22):3315-3317
We present ultrafast z-scan measurements of the two-photon absorption (TPA) spectra of a pair of two-dimensionally conjugated quadrupolar donor/acceptor (D/A) chromophores. The all-donor substituted species displays a peak TPA cross section [sigma(2)=520+/-30 GM] that is more than twice that of the D-A species [sigma(2)=240+/-20 GM]. Unlike previous structure-property studies that have evaluated TPA behavior for D/A molecules through the comparison of dipolar and quadrupolar compounds, both molecules investigated herein are quadrupolar, ultimately providing a more consistent evaluation of the effects of donor and/or acceptor substitution on the TPA of conjugated chromophores. 相似文献
997.
We develop a theory for shrinkage dynamics of a vesicle interacting with surfactant molecules. A stepwise shrinkage is formulated
in such a way that it consists of two processes. One is a nucleation process of a pore under increasing of the membrane tension.
The other is a closure process of the pore due to the line tension of the pore edge after leakage of the inner fluid. We carry
out numerical simulations and show that the results agree with experiments semi-quantitatively. An analytical study is also
carried out to understand the periodic shrinkage. 相似文献
998.
We examined the influence of nanocrystalline particle size on the cubic-to-tetragonal phase transition of barium titanate (BaTiO3). In-situ X-ray diffraction (XRD) and differential scanning calorimetry (DSC) were used in combination with electron microscopy to study the evolution of lattice structure and phase-transformation behavior with heat treatment and particle growth for BaTiO3. The lattice constants were obtained for various polycrystal orientations in the diameter size range 10–90 nm. In-situ XRD results during the crystallization of amorphous 4BaTiO3-SiO2 revealed a critical size of ∼25 nm for the cubic-to-tetragonal phase transition of BaTiO3 at room temperature. 相似文献
999.
1000.
Akira Katayama Takehiro Ohta Yuko Wasada‐Tsutsui Tomohiko Inomata Tomohiro Ozawa Takashi Ogura Hideki Masuda 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(33):11401-11406
Reported here is the N2 cleavage of a one‐electron oxidation reaction using trans‐[Mo(depe)2(N2)2] ( 1 ) (depe=Et2PCH2CH2PEt2), which is a classical molybdenum(0)‐dinitrogen complex supported by two bidentate phosphine ligands. The molybdenum(IV) terminal nitride complex [Mo(depe)2N][BArf4] ( 2 ) (BArf4=B(3,5‐(CF3)2C6H3)4) is synthesized by the one‐electron oxidation of 1 upon addition of a mild oxidant, [Cp2Fe][BArf4] (Cp=C5H5), and proceeds by N2 cleavage from a MoII‐N=N‐MoII structure. In addition, the electrochemical oxidation reaction for 1 also cleaved the N2 ligand to give 2 . The dimeric Mo complex with a bridging N2 is detected by in situ resonance Raman and in situ UV‐vis spectroscopies during the electrochemical oxidation reaction for 1 . Density‐functional theory (DFT) calculations reveal that the unstable monomeric oxidized MoI species is converted into 2 via the dimeric structure involving a zigzag transition state. 相似文献