全文获取类型
收费全文 | 585篇 |
免费 | 33篇 |
专业分类
化学 | 503篇 |
晶体学 | 1篇 |
力学 | 4篇 |
数学 | 38篇 |
物理学 | 72篇 |
出版年
2024年 | 2篇 |
2023年 | 7篇 |
2022年 | 6篇 |
2021年 | 11篇 |
2020年 | 10篇 |
2019年 | 15篇 |
2018年 | 9篇 |
2017年 | 3篇 |
2016年 | 21篇 |
2015年 | 19篇 |
2014年 | 25篇 |
2013年 | 28篇 |
2012年 | 43篇 |
2011年 | 50篇 |
2010年 | 42篇 |
2009年 | 26篇 |
2008年 | 49篇 |
2007年 | 41篇 |
2006年 | 29篇 |
2005年 | 38篇 |
2004年 | 31篇 |
2003年 | 28篇 |
2002年 | 12篇 |
2001年 | 3篇 |
2000年 | 5篇 |
1999年 | 7篇 |
1998年 | 4篇 |
1997年 | 5篇 |
1996年 | 5篇 |
1995年 | 3篇 |
1994年 | 1篇 |
1993年 | 2篇 |
1991年 | 1篇 |
1990年 | 2篇 |
1989年 | 3篇 |
1988年 | 3篇 |
1987年 | 2篇 |
1986年 | 4篇 |
1984年 | 3篇 |
1983年 | 5篇 |
1982年 | 2篇 |
1981年 | 2篇 |
1980年 | 4篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1970年 | 1篇 |
1969年 | 1篇 |
排序方式: 共有618条查询结果,搜索用时 614 毫秒
541.
Jelka Šuštar-Vozlič Katja Rostohar Andrej Blejec Petra Kozjak Zoran Čergan Vladimir Meglič 《Analytical and bioanalytical chemistry》2010,396(6):2031-2041
In order to comply with the European Union regulatory threshold for the adventitious presence of genetically modified organisms
(GMOs) in food and feed, it is important to trace GMOs from the field. Appropriate sampling methods are needed to accurately
predict the presence of GMOs at the field level. A 2-year field experiment with two maize varieties differing in kernel colour
was conducted in Slovenia. Based on the results of data mining analyses and modelling, it was concluded that spatial relations
between the donor and receptor field were the most important factors influencing the distribution of outcrossing rate (OCR)
in the field. The approach for estimation fitting function parameters in the receptor (non-GM) field at two distances from
the donor (GM) field (10 and 25 m) for estimation of the OCR (GMO content) in the whole receptor field was developed. Different
sampling schemes were tested; a systematic random scheme in rows was proposed to be applied for sampling at the two distances
for the estimation of fitting function parameters for determination of OCR. The sampling approach had already been validated
with some other OCR data and was practically applied in the 2009 harvest in Poland. The developed approach can be used for
determination of the GMO presence at the field level and for making appropriate labelling decisions. The importance of this
approach lies in its possibility to also address other threshold levels beside the currently prescribed labelling threshold
of 0.9% for food and feed. 相似文献
542.
Klementina Punik renar Alexandra Aulova Dimitrios N. Bikiaris Dimitra Lambropoulou Katja Kuzmi
Lidija Fras Zemlji
《Molecules (Basel, Switzerland)》2021,26(14)
In this work, the modification process of poly(lactic acid) (PLA) with metal-based nanoparticle (NPs) additives (Ag, ZnO, TiO2) at different loading (0.5, 1.0, and 2.5 wt%) and by melt-mix extrusion method followed by film formation as one of the advantageous techniques for industrial application have been investigated. PLA nanoparticle composite films (PLA-NPs) of PLA-Ag, PLA-ZnO, PLA-TiO2 were fabricated, allowing convenient dispersion of NPs within the PLA matrix to further pursue the challenge of investigating the surface properties of PLA-NPs reinforced plastics (as films) for the final functional properties, such as antimicrobial activity and surface mechanical properties. The main objective was to clarify how the addition of NPs to the PLA during the melt extrusion process affects the chemistry, morphology, and wettability of the surface and its further influence on the antibacterial efficiency and mechanical properties of the PLA-NPs. Therefore, the effect of Ag, ZnO, and TiO2 NPs incorporation on the morphology (SEM), elemental mapping analysis (SEM-EDX), roughness, surface free energy (SFE) of PLA-NPs measured by goniometry and calculated by OWRK (Owens, Wendt, Rabel, and Kaelble) model was evaluated and correlated with the final functional properties such as antimicrobial activity and surface mechanical properties. The developed PLA-metal-based nanocomposites, with improved mechanical and antimicrobial surface properties, could be used as sustainable and biodegradable materials, offering desirable multifunctionalities not only for food packaging but also for cosmetics and hygiene products, as well as for broader plastic products where antimicrobial activity is desirable. 相似文献
543.
Rajic Z Zorc B Raic-Malic S Ester K Kralj M Pavelic K Balzarini J De Clercq E Mintas M 《Molecules (Basel, Switzerland)》2006,11(11):837-848
3,5-Disubstituted hydantoin (1,3-imidazolidinedione) derivatives 5a-h were prepared by base induced cyclization of the corresponding N-(1-benzotriazolecarbonyl)-L- and D-amino acid amides 4a-h. Compounds 5a-h were evaluated for their cytostatic and antiviral activities. Among all the compounds evaluated, 3-benzhydryl-5-isopropyl hydantoin (5a) showed a weak but selective inhibitory effect against vaccinia virus (EC(50) = 16 microg/mL; selectivity index: 25). 3-Cyclohexyl-5-phenyl hydantoin (5g) showed inhibitory activity against cervical carcinoma (HeLa, IC(50) = 5.4 microM) and breast carcinoma (MCF-7, IC(50) = 2 microM), but also cytotoxic effects towards human normal fibroblasts (WI 38). On the contrary, the 3-benzhydryl-5-phenyl substituted hydantoin derivative 5h showed moderate inhibitory activity towards HeLa, MCF-7, pancreatic carcinoma (MiaPaCa-2), lung carcinoma (H 460) and colon carcinoma (SW 620) (IC(50) = 20-23 microM), but no effect on WI 38. 相似文献
544.
545.
The electroporative transfer of gene DNA and other bioactive substances into tissue cells by electric pulses gains increasing importance in the new disciplines of electrochemotherapy and electrogenetherapy. The efficiency of the electrotransfer depends crucially on the adsorption of the gene DNA and oligonucleotides to the plasma cell membranes. Here it is shown that the adsorption of larger oligonucleotides such as fragments (ca. 300 bp) of sonicated calf-thymus DNA, to anionic lipids of unilamellar vesicles (diameter Phi=300+/-90 nm) is greatly enhanced by divalent cations such as Ca(2+)-ions. Applying centrifugation, bound and free DNA are monitored optically at the wavelength lambda=260 nm. Using arsenazo III as a Ca(2+)-indicator and atomic absorption spectroscopy (AAS), Ca(2+)-titrations of DNA and vesicles yield the individual equilibrium constants of Ca(2+)- and DNA-binding not only for the binary complexes: Ca/lipids, Ca/DNA and DNA/lipids, respectively, but also for the various processes to form the ternary complex DNA/Ca/lipids. The data provide the basis for goal-directed optimization protocols for the adsorption and thus efficient electrotransfer of oligonucleotides and polynucleotides into cells. 相似文献
546.
A mild, thermal Alder-ene reaction of enallenes has been developed. The allenic double bond acts as the "ene" and generates a carbon-carbon bond to an unactivated olefinic "enophile" in DMF at 120 degrees C to give [n.3.0] bicyclic systems (n = 3-5) in good yields. Except for a minor [2 + 2] cycloaddition byproduct, the reaction proceeded with complete atom economy, as there is no requirement of a catalyst or additional reactants, and no waste products are formed in the process. 相似文献
547.
Chelated amino acid ester enolates are excellent nucleophiles for allylic alkylations. With these enolates, even terminal pi-allyl palladium complexes react without significant isomerization. This allows a transfer of the cis-olefin geometry from the substrate into the product. Chiral substrates also show a reasonably good 1,5-induction. 相似文献
548.
Samarra FI Deecke VB Vinding K Rasmussen MH Swift RJ Miller PJ 《The Journal of the Acoustical Society of America》2010,128(5):EL205-EL210
This study reports that killer whales, the largest dolphin, produce whistles with the highest fundamental frequencies ever reported in a delphinid. Using wide-band acoustic sampling from both animal-attached (Dtag) and remotely deployed hydrophone arrays, ultrasonic whistles were detected in three Northeast Atlantic populations but not in two Northeast Pacific populations. These results are inconsistent with analyses suggesting a correlation of maximum frequency of whistles with body size in delphinids, indicate substantial intraspecific variation in whistle production in killer whales, and highlight the importance of appropriate acoustic sampling techniques when conducting comparative analyses of sound repertoires. 相似文献
549.
Para-nitroaniline derivatives with peripheral 1,2- and 1,3-diol functionalities [O(2)N-C(6)H(4)-NR(1)-CH(2)CH(OH)CH(2)OH; O(2)N-C(6)H(4)-NR(1)-CH(CH(2)OH)(2); R(1) = -H, -CH(3)] covalently bonded to the amino group are esterified with various para-substituted phenylboronic acids [R(2)-C(6)H(4)-B(OH)(2); R(2) = -OCH(3), -CH(3), -H, -Br, -CHO, -NO(2), -B(OH)(2)], and the solvatochromic properties of these esters are investigated in 33 solvents of different polarity. To interpret the solvent effects, the established linear solvation energy (LSE) multiparameter equations of Kamlet-Taft and the improved Catalán scales are used. Although the boron atom is separated by two or three sp(3)-hybridized carbon atoms from the actual chromophore, solvation effects have a significant positive solvatochromic effect on the nitroaniline unit (R(1) = -CH(3)) as result of the solvent acting as a donor at the boron atom. The influence of the substituent R(2) on the coefficient b of the LSE relationship according to Kamlet-Taft and Catalán, which reflects the quantitative influence of the hydrogen-bonding acceptor or the electron-pair donor capacity of the solvent on the position of the UV-vis absorption maximum, can be determined via a linear Hammett relationship [b = f(σ(p))]. The interpretation of the effects is based on the electronic influence of the solvated boronic acid ester unit on the 4-nitroaniline group, predominantly through inductive interactions. 相似文献
550.
In this article a complete classification of tetravalent arc-transitive graphs of order twice a product of two primes is given. 相似文献