全文获取类型
收费全文 | 2779篇 |
免费 | 57篇 |
国内免费 | 263篇 |
专业分类
化学 | 1466篇 |
晶体学 | 5篇 |
力学 | 182篇 |
综合类 | 2篇 |
数学 | 255篇 |
物理学 | 1189篇 |
出版年
2024年 | 10篇 |
2023年 | 62篇 |
2022年 | 43篇 |
2021年 | 43篇 |
2020年 | 51篇 |
2019年 | 59篇 |
2018年 | 47篇 |
2017年 | 57篇 |
2016年 | 81篇 |
2015年 | 76篇 |
2014年 | 95篇 |
2013年 | 171篇 |
2012年 | 204篇 |
2011年 | 253篇 |
2010年 | 188篇 |
2009年 | 194篇 |
2008年 | 179篇 |
2007年 | 190篇 |
2006年 | 163篇 |
2005年 | 151篇 |
2004年 | 138篇 |
2003年 | 80篇 |
2002年 | 68篇 |
2001年 | 61篇 |
2000年 | 46篇 |
1999年 | 42篇 |
1998年 | 50篇 |
1997年 | 39篇 |
1996年 | 32篇 |
1995年 | 39篇 |
1994年 | 23篇 |
1993年 | 27篇 |
1992年 | 20篇 |
1991年 | 21篇 |
1990年 | 10篇 |
1989年 | 14篇 |
1988年 | 9篇 |
1987年 | 7篇 |
1986年 | 5篇 |
1985年 | 8篇 |
1984年 | 10篇 |
1983年 | 3篇 |
1982年 | 3篇 |
1981年 | 6篇 |
1980年 | 2篇 |
1979年 | 6篇 |
1978年 | 4篇 |
1977年 | 4篇 |
1976年 | 3篇 |
1974年 | 2篇 |
排序方式: 共有3099条查询结果,搜索用时 15 毫秒
71.
The effect of the chemical structure of styrene-based epoxides, namely, styrene epoxide (SE), α-methylstyrene epoxide (MSE), p-methylstyrene epoxide (pM-SE) and α-methyl-p-methylstyrene epoxide (pM-MSE), in conjunction with TiCl4, on the initiation efficiency (Ieff) in the carbocationic polymerization of isobutylene (IB) was investigated. SE yielded living polymerization, but the initiation efficiency was low when compared to MSE (Ieff=8% and 35%, respectively). pM-SE led to non-living IB polymerization, while pM-MSE revealed linear Mn-conversion plot and narrow MWD with a non-linear first order rate plot. Among the epoxides investigated, MSE was the best initiator to scale up the one-step synthesis of polyisobutylenes (PIBs) carrying one primary hydroxyl head group and one tertiary chloride end group. The hydroxyl functionality of these PIBs determined by 1H-NMR was Fn=1.09±0.16 from 24 experiments. 相似文献
72.
Lukasz Kraszkiewicz 《Tetrahedron》2004,60(41):9113-9119
Two ‘model’ deactivated arenes, benzoic acid and nitrobenzene, were effectively monoiodinated within 1 h at 25-30 °C, with strongly electrophilic I+ reagents, prior prepared from diiodine and various oxidants (CrO3, KMnO4, active MnO2, HIO3, NaIO3, or NaIO4) in 90% (v/v) concd sulfuric acid (ca. 75 mol% H2SO4). Next, an I2/NaIO3/90% (v/v) concd H2SO4 exemplary system was used to effectively mono- or diiodinate a number of deactivated arenes. All former papers dealing with the direct iodination of deactivated arenes are briefly reviewed. 相似文献
73.
Christophe Hoarau Alexis Du Fou de Kerdaniel Pierre Grandclaudon Francis Marsais 《Tetrahedron letters》2005,46(49):8573-8577
The efficient and regioselective palladium-catalyzed C-2 arylation of ethyl 4-oxazolecarboxylate 1 with phenyliodide is described. The different parameters (solvent, base, ligand and catalyst) for the optimal conditions of this arylation process have been screened. 相似文献
74.
《Arabian Journal of Chemistry》2022,15(11):104302
Traditional Chinese medicine (TCM) is the key to unlock treasures of Chinese civilization. TCM and its compound play a beneficial role in medical activities to cure diseases, especially in major public health events such as novel coronavirus epidemics across the globe. The chemical composition in Chinese medicine formula is complex and diverse, but their effective substances resemble “mystery boxes”. Revealing their active ingredients and their mechanisms of action has become focal point and difficulty of research for herbalists. Although the existing research methods are numerous and constantly updated iteratively, there is remain a lack of prospective reviews. Hence, this paper provides a comprehensive account of existing new approaches and technologies based on previous studies with an in vitro to in vivo perspective. In addition, the bottlenecks of studies on Chinese medicine formula effective substances are also revealed. Especially, we look ahead to new perspectives, technologies and applications for its future development. This work reviews based on new perspectives to open horizons for the future research. Consequently, herbal compounding pharmaceutical substances study should carry on the essence of TCM while pursuing innovations in the field. 相似文献
75.
The analytical performance of a glow discharge quadrupole mass spectrometer is demonstrated using sintered tungsten as an example. The inherent problem of molecular interferences in glow discharge mass Spectrometry has been considerably reduced using neon as a complementing discharge gas. Particular attention has been paid to time dependences. Analytical figures of merit are presented. 相似文献
76.
用直接酯化法合成了聚对苯二甲酸乙二酯(PET)。对缩聚反应温度、真空度及稳定剂等合条件对产物结晶速率的影响进行了研究。以ΔTc=Tc-Tg和ΔTco=Tco-Tg表征样品的结晶速率。结果表明,真空度一定,反应温度高则样品结晶速率慢;温度一定,真空度高样品结晶速率也慢。这两个因素的影响均是不可忽视的。 相似文献
77.
Summary Procedures for the quantitative analysis of industrial effluents which involve concentration by solvent extraction or the
purge-and-trap method are time-consuming, labor-intensive, and prone to error. Direct aqueous injection gas chromatography
using an electron-capture detector for the analysis of volatile halocarbons at the ppb level is in routine use in many laboratories.
We now discuss the development of a similar protocol for the analysis of volatile polar organics such as acetone, methyl ethyl
ketone, methyl isobutyl ketone, and tretrahydrofuran using a flame-ionization detector. 相似文献
78.
The morphological changes with different compositions of casting solvents in the membranes of sulfonated polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene (SEBS) were investigated with small-angle X-ray scattering (SAXS) and transmission electron microscope (TEM). Using the single solvent of tetrahydrofuran (THF) and the solvent mixtures of methanol/THF with different compositions, the casting polymer solutions were prepared for the membrane fabrication. The proton conductivity and the methanol permeability of membranes were measured, and the effect of morphological changes on the casting solvents was discussed through Flory–Huggins theory. It was found that the sulfonated SEBS membranes were transformed from well ordered lamellar to disordered co-continuous structure and the morphological difference caused abrupt enhancement in the proton conductivity and the methanol permeability as the concentration of methanol in the mixed solvents (MeOH/THF) increased. 相似文献
79.
Using a very simple trial function and unperturbed electron densities calculated by a new procedure, the frequency-dependent dipole polarizability () of Ne, Ar, Kr and Xe has been calculated in the range 0 0.45 a.u., by a Karplus-Kolker-type variation-perturbation method. Results progressively worsen for larger systems so that, for Xe, (0) is only 75% of the experimental value. Probable reasons for this are discussed. 相似文献
80.
《Electroanalysis》2006,18(11):1131-1134
The direct electrochemistry of glucose oxidase (GOD) was revealed at a carbon nanotube (CNT)‐modified glassy carbon electrode, where the enzyme was immobilized with a chitosan film containing gold nanoparticles. The immobilized GOD displays a pair of redox peaks in pH 7.4 phosphate buffer solutions (PBS) with the formal potential of about ?455 mV (vs. Ag/AgCl) and shows a surface‐controlled electrode process. Bioactivity remains good, along with effective catalysis of the reduction of oxygen. In the presence of dissolved oxygen, the reduction peak current decreased gradually with the addition of glucose, which could be used for reagentless detection of glucose with a linear range from 0.04 to 1.0 mM. The proposed glucose biosensor exhibited high sensitivity, good stability and reproducibility, and was also insensitive to common interferences such as ascorbic and uric acid. The excellent performance of the reagentless biosensor is attributed to the effective enhancement of electron transfer between enzyme and electrode surface by CNTs, and the biocompatible environment that the chitosan film containing gold nanoparticles provides for immobilized GOD. 相似文献