全文获取类型
收费全文 | 1851篇 |
免费 | 98篇 |
国内免费 | 452篇 |
专业分类
化学 | 2221篇 |
晶体学 | 20篇 |
力学 | 5篇 |
综合类 | 14篇 |
数学 | 1篇 |
物理学 | 140篇 |
出版年
2024年 | 1篇 |
2023年 | 16篇 |
2022年 | 23篇 |
2021年 | 44篇 |
2020年 | 43篇 |
2019年 | 50篇 |
2018年 | 45篇 |
2017年 | 72篇 |
2016年 | 91篇 |
2015年 | 78篇 |
2014年 | 91篇 |
2013年 | 198篇 |
2012年 | 121篇 |
2011年 | 133篇 |
2010年 | 117篇 |
2009年 | 131篇 |
2008年 | 135篇 |
2007年 | 137篇 |
2006年 | 100篇 |
2005年 | 140篇 |
2004年 | 102篇 |
2003年 | 79篇 |
2002年 | 65篇 |
2001年 | 39篇 |
2000年 | 45篇 |
1999年 | 42篇 |
1998年 | 32篇 |
1997年 | 30篇 |
1996年 | 23篇 |
1995年 | 37篇 |
1994年 | 32篇 |
1993年 | 13篇 |
1992年 | 20篇 |
1991年 | 17篇 |
1990年 | 8篇 |
1989年 | 15篇 |
1988年 | 9篇 |
1987年 | 6篇 |
1986年 | 5篇 |
1985年 | 4篇 |
1984年 | 1篇 |
1983年 | 3篇 |
1982年 | 2篇 |
1980年 | 1篇 |
1979年 | 2篇 |
1978年 | 1篇 |
1976年 | 1篇 |
1974年 | 1篇 |
排序方式: 共有2401条查询结果,搜索用时 16 毫秒
91.
The self-assembly of a novel series of donor-acceptor interaction-driven artificial hetero-duplexes in organic media has been described. Four linear compounds 1a-1d, bearing two to five electron rich 1,5-dioxynaphthalene units connected by the tetra(ethylene glycol) linker, respectively, have been prepared and used as donors, while eight compounds 2a-2d, 13-16, bearing one to four electron deficient pyromellitic diimide, 1,4,5,8-naphthalene-tetracarboxydiimide, or perylene-3,4,9,10-tetracarboxydiimide units, respectively, have been used as acceptors. The structure of the hetero-duplexes has been characterized by the 1H NMR, UV-vis spectroscopy and vapor pressure osmometry. It is revealed that the binding stability of the duplexes vary greatly, depending on the length and structure of the monomers and also the solvent, and hetero-duplex 1d·2d displays a maximum association constant of ca. 1.0×104 M−1 in chloroform. 相似文献
92.
Zhang De-Qing Wang Shao-Jun Sun Hong-Shan Wang Xiu-Li Cao Mao-Sheng 《Journal of Sol-Gel Science and Technology》2007,41(2):157-161
Lead zirconate titanate nanopowders Pb(Zr0.52Ti0.48)O3 (PZT) were prepared by modified sol-gel process in ethylene glycol system with zirconium nitrate as the zirconium source.
The research showed that it was critical to add lead acetate after the reaction of zirconium nitrate and tetrabutyl titanate
in ethylene glycol system for preparing PZT of exact titanium content. The reaction mechanisms of the sol synthesis, preparation
of xerogel and agglutinating process were characterized through using FT-IR, NMR, TG-FTIR, and GC-MS. The experiment proved
that ethylene glycol system did not rely on hydrolysis and condensation reactions in the process of the sol formation, but
on the formation of chain or network large molecules from complexation of ethylene glycol and all Ti and minor Pd, Zr. In
the preparation of xerogel, the complexation reaction was so completed that it formed large molecules network composed of
metal and dioxyethyl. Bulk weight loss happened before 350°C in the process of sintering xerogel to prepare PZT nanopowders.
Volatile matters and vapor phase decomposition resultants were primarily oxy-compounds including ethylene glycol, aldehyde-ketone
compounds, carbon dioxide and nitrate radical conversion matters. After 350°C, primary vapor phase decomposition resultants
were carbon dioxide and minor carbonyl compound. 相似文献
93.
Silica gel impregnated with poly(ethylene glycol) of different molecular mass (400, 1000, 1540, 4000, and 5500) was investigated for salting-out thin-layer chromatography of 15 mixed aminocarboxylato Co(III) complexes using eight ammonium sulphate solutions as mobile phases. Regularities established earlier for non-impregnated adsorbents are also valid in this work. Poly(ethylene glycol) of high molecular mass increases the hydrophobicity of the adsorbent. Positive linear dependence of RM values and of salting-out efficiency on average poly(ethylene glycol) molecular mass was usually observed. In contrast with non-impregnated silica gel, separation was achieved between complexes with the smallest hydrocarbon groups. 相似文献
94.
95.
Diblock copolymers composed of poly(oxy-ethylene) (POE) and poly(dl-lactic acid) segments were synthesized by anionic polymerization of d,l-lactide using the oxyanion formed by reaction of the monohydroxyl monomethoxy-poly(ethylene glycol) on sodium hydride. For comparison, a similar copolymer was prepared by using tin octoate to catalyze the lactide polymerization. The copolymers were used to make nanoparticles, which were stored at 4 °C. After a few months under these storage conditions, a dramatic decrease of the poly(ethylene glycol) content was observed, however, the mean diameter of the nanoparticles was not affected. The degradation of the nanoparticles was investigated in vitro under conditions selected to mimic physiological conditions. Changes of characteristics were monitored by 1H NMR, SEC, DLLS and CZE on nanoparticles and/or on the degradation by-products dissolved in the ageing medium. According to their nanometric dimensions, the microparticles degraded very slowly and there was no difference in behaviour between the sodium hydride and the stannous octoate-derived copolymers. 相似文献
96.
Verheyen S. Blaton N. Kinget R. Van den Mooter G. 《Journal of Thermal Analysis and Calorimetry》2003,73(2):563-576
The effect of grinding on the physical properties and pharmaceutical performance of solid dispersions made of poly(ethylene)
glycol 6000 (PEG6000) and temazepam or diazepam was studied using differential scanning calorimetry (DSC), X-ray powder diffraction
and dissolution experiments. DSC-analysis of flash-cooled dispersions revealed that amorphous PEG present immediately after
grinding crystallised upon aging mainly into the twice folded modification and to a small extent into the extended form. DSC-analysis
of dispersions kept in the slab form for 1 month and subsequently ground, revealed that in the abscence of the grinding impulse
crystallisation of PEG6000 takes place in the same way as in dispersions ground immediately after preparation and then aged
for 1 month. Grinding solid dispersions immediately after preparation resulted in superior dissolution properties compared
with solid dispersions kept in the monolith-slab form and subsequently ground. This difference in dissolution properties was
found to be attributed to the drug and not to the polymer, more precisely, it was suggested that the drug particle size in
ground dispersions was smaller than in dispersions kept in the slab form and subsequently ground. These findings suggest that
grinding of solid dispersions immediately after preparation is the preparation method of choice instead of liquid filling
of hard gelatin capsules resulting in monoliths.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
97.
M. M. R. Talukder M. M. Zaman Y. Hayashi J. C. Wu T. Kawanishi 《Applied biochemistry and biotechnology》2007,141(1):77-83
The thermostability of Cromobacterium viscosum lip ase (EC 3.1.1.3) entrapped in AOT (sodium bis-[2-ethylhexyl] sulfosuccinate) reverse micelles was in creased by the addition
of short-chain polyethylene glycol (PEG 400). Two different approaches were considered: (1) the determination of half-life
time and (2) the mechanistic analysis of deactivation kinetics. The half-life of lipase entrapped in AOT/isooctane reverse
micelles with PEG 400 at 60°C was 28h, ninefold higher than that in reverse micelles without PEG 400. The lip ase entrapped
in both reverse micellar systems followed a series-type deactivation mechanism involving two first-order steps. The deactivation
constant for the first step at 60°C in PEG containing reverse micelles was 0.055 h11, 11-fold lower than that in reverse micelles without PEG, whereas it remained almost constant for the second step. The inactivation
energy of the lip ase entrapped in reverse micelles with and without PEG 400 was 88.12 and 21.97 kJ/mol, respectively. 相似文献
98.
L. Hernandez M. Rudolph R. Lammertink J. Kornfield C. Zurita F. A. Gomez 《Chromatographia》2007,65(5-6):299-303
Vancomycin (Van) from Streptomyces orientalis has been derivatized with polyethylene glycol [PEG; PEG-550 (1), 750 (2), 1,100 (3), 2,000 (4), 5,000 (5), and 8,000 (6) g mol−1] at the N-terminus of the glycopeptide backbone and their binding to d-Ala-d-Ala terminus peptides assessed using affinity capillary electrophoresis (ACE). Utilizing ACE, a plug of Van-PEG and non-interacting
standards are injected and electrophoresed. Analysis of the change in the relative migration time ratio of the Van-PEG species,
relative to the non-interacting standards, as a function of the concentration of peptide, yields a value for the binding constant
(K
b). Values of K
b for N-acetyl-d-Ala-d-Ala, 7 to the Van-PEG derivatives are weaker than those for N
α,N
ε-diacetyl-Lys-d-Ala-d-Ala, 8 (for example, values of K
b for 7-1 and 8-1 are 1.8 and 47.7 × 103 M−1, respectively). These results demonstrate that derivatization of Van with PEG has little effect on the affinity of d-Ala-d-Ala peptide ligands to it. The findings further prove the versatility of ACE and its ability to estimate binding parameters
of ligands to antibiotics. 相似文献
99.
100.
The kinetics and mechanism of thermal polymerization of acrylonitrile initiated by Mn(III) pyrophosphate — poly(ethylene glycol) (PEG, molecular weight 6000) redox system in aqueous sulfuric acid medium was studied in the temperature range 30–60°C. The overall rates of polymerization and the disappearance of Mn3+ were determined. The polymerization was initiated by the organic free radical produced from the Mn3+-PEG reaction and the termination was by the metal ions. The rate of polymerization of acrylonitrile was found to be directly proportional to the square of the monomer concentration and first power of PEG concentration, and inversely proportional to the concentration of Mn3+. The rate of manganic ion disappearance was found to be directly proportional to manganic ion concentration and PEG concentration, and independent of the monomer concentration. Based on these observations, a plausible reaction scheme was suggested and suitable kinetic expressions were evaluated. 相似文献