全文获取类型
收费全文 | 3516篇 |
免费 | 124篇 |
国内免费 | 15篇 |
专业分类
化学 | 2273篇 |
晶体学 | 21篇 |
力学 | 124篇 |
数学 | 678篇 |
物理学 | 559篇 |
出版年
2023年 | 19篇 |
2022年 | 85篇 |
2021年 | 91篇 |
2020年 | 38篇 |
2019年 | 76篇 |
2018年 | 49篇 |
2017年 | 70篇 |
2016年 | 115篇 |
2015年 | 103篇 |
2014年 | 95篇 |
2013年 | 211篇 |
2012年 | 196篇 |
2011年 | 211篇 |
2010年 | 126篇 |
2009年 | 115篇 |
2008年 | 209篇 |
2007年 | 174篇 |
2006年 | 198篇 |
2005年 | 172篇 |
2004年 | 124篇 |
2003年 | 130篇 |
2002年 | 90篇 |
2001年 | 51篇 |
2000年 | 44篇 |
1999年 | 48篇 |
1998年 | 36篇 |
1997年 | 36篇 |
1996年 | 46篇 |
1995年 | 39篇 |
1994年 | 38篇 |
1993年 | 41篇 |
1992年 | 30篇 |
1991年 | 21篇 |
1990年 | 23篇 |
1989年 | 21篇 |
1988年 | 20篇 |
1987年 | 23篇 |
1986年 | 17篇 |
1985年 | 46篇 |
1984年 | 35篇 |
1983年 | 41篇 |
1982年 | 29篇 |
1981年 | 23篇 |
1980年 | 31篇 |
1979年 | 24篇 |
1977年 | 24篇 |
1976年 | 20篇 |
1975年 | 18篇 |
1974年 | 14篇 |
1973年 | 15篇 |
排序方式: 共有3655条查询结果,搜索用时 46 毫秒
41.
Alejandro R. Engelmann Mario A. Natiello Mikael Hghede Erik Engdahl Erkki Brndas 《International journal of quantum chemistry》1987,31(5):841-845
In the present article we show how the bound states of the Coulomb potential may be associated with resonances that occur when this potential is perturbed by a barrier potential. The main idea is to trace the bound states on successive switching on of the barrier perturbation. It is found that those bound states that are localized inside the barrier are highly sensitive to variation with respect to the barrier height, whereas those that are localized outside are less sensitive. However, there are certain intervals for the barrier height when the role of being “a state localized inside the barrier” is shifted from one bound state to another. The result can be pictured as a “relay race,” where the “deliveries of the baton” are carried out over corresponding avoided crossings. The baton is ultimately handed over to a shape-type resonance state. 相似文献
42.
Neira-Carrillo A Yazdani-Pedram M Retuert J Diaz-Dosque M Gallois S Arias JL 《Journal of colloid and interface science》2005,286(1):134-141
The biopolymer chitosan was chemically modified by grafting polyacrylamide or polyacrylic acid in a homogeneous aqueous phase using potassium persulfate (KPS) as redox initiator system in the presence of N,N-methylene-bis-acrylamide as a crosslinking agent. The influence of the grafted chitosan on calcium salts crystallization in vitro was studied using the sitting-drop method. By using polyacrylamide grafted chitosan as substrate, rosette-like CaSO4 crystals were observed. This was originated by the presence of sulfate coming from the initiator KPS. By comparing crystallization on pure chitosan and on grafted chitosan, a dramatic influence of the grafted polymer on the crystalline habit of both salts was observed. Substrates prepared by combining sulfate with chitosan or sulfate with polyacrylamide did not produce similar CaSO4 morphologies. Moreover, small spheres or donut-shaped CaCO3 crystals on polyacrylic acid grafted chitosan were generated. The particular morphology of CaCO3 crystals depends also on other synthetic parameters such as the molecular weight of the chitosan sample and the KPS concentration. 相似文献
43.
Gancitano P Ciriminna R Testa ML Fidalgo A Ilharco LM Pagliaro M 《Organic & biomolecular chemistry》2005,3(13):2389-2392
Valuable organic compounds such as alpha-hydroxy acids are easily synthesised with relevant selectivity enhancement using a sol-gel hydrophobized nanostructured silica matrix doped with the organocatalyst TEMPO: A materials science based synthetic route which cannot be achieved via classical homogeneous synthesis. 相似文献
44.
Mass spectrometric identification and characterization of steroids using electrospray ionization and tandem mass spectrometry has advantages in drug testing and doping control analysis attributable to limitations of gas chromatography followed by electron ionization mass spectrometry. Steroids with an androstadiene-17beta-ol-3-one nucleus and double bonds located either at C-1 and C-4, C-4 and C-9, or C-4 and C-6 were used to determine characteristic fragmentation pathways. Diagnostic dissociation routes are proposed using deuterium labeling, MS3 experiments, and analyses of structurally closely related compounds. Steroids such as boldenone (androst-1,4-diene-17beta-ol-3-one) produced characteristic product ions at m/z 121, 135, and 147. Compounds with double bonds at C-4 and C-9 generated abundant product ions at m/z 145 and 147. Conjugated double bonds at C-4 and C-6 gave rise to an intense and characteristic signal at m/z 133. Stereochemical differentiation between 5alpha- and 5beta-isomers of androstan-17beta-ol-3-ones was possible because of significant differences in relative abundance of product ions generated by elimination of acetone from alpha,beta-saturated 3-keto steroids. 相似文献
45.
46.
A Trolox equivalent antioxidant capacity (TEAC) decolourisation assay was adapted to a flow injection analysis (FIA) system and a simple and rapid method for antioxidant activity evaluation was developed. To avoid the time consuming step of 2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonic acid)(ABTS) radical cation preparation by chemical oxidation of ABTS, as in the original TAEC assay, and hence, to shorten the analysis time, the ABTS radical cation was generated on-line by electrochemical oxidation of ABTS in the flow-through electrolysis cell forming a part of the FIA system. The proposed method was optimised with respect to a flow rate, injection volume and ABTS radical cation/carrier ratio. Under the optimised conditions linear calibration graphs for Trolox were obtained over the range 10-100 microM, with a limit of detection 1.6 microM. Good reproducibility (relative standard deviation 1.95%) and sample throughput (32 samples per hour) were achieved. The developed method was applied to the evaluation of the antioxidant activity of pure compounds and samples of some common beverages. In both cases a good correlation between the results obtained by the proposed method and TEAC values evaluated by the classic TAEC decolourisation assay was obtained (r(2)= 0.996 for pure compounds and r(2)= 0.957 for beverage samples). 相似文献
47.
Resinous materials, generally added to drying oils, were often employed as final protective films in paintings, both in tempera and oil techniques. Most of the resins used in fine arts come from exudates of plants. Natural resins are mainly composed by terpenoids. Among these resins, dammar, rosin, mastic, elemi gum, Venice turpentine were frequently employed in easel paintings. The aim of this work is the identification of natural resins in samples collected from works of art. In this paper we propose to use micro-FTIR spectrometry as a preliminary and non destructive technique to detect the presence of resinous materials in microsamples collected from paintings, followed by GC-MS analyses to obtain a precise identification of the resin. To reach this aim commercial resins, employed as standard, were analysed previously by micro-FTIR spectrometry and then analysed by GC-MS chromatography, searching characteristic chemical compounds used as "markers" of resins. The results obtained on standards allowed us to successfully analyse samples collected from two works of art: the "Portrait of a Young Gentleman", by Lorenzo Lotto (XVI century), and the "Madonna with the Infant Jesus and Saints": by Carlo Maratta (XVII secolo). 相似文献
48.
A new method of synthesizing the alkaloid aspidospermidine (1), based on building ring E on the pyridocarbazole [ABCD] ring structure, is reported. The preparation of the pyridocarbazole framework of Aspidosperma alkaloids is a new three-step synthetic application of 2-(1,3-dithian-2-yl)indoles. A tandem conjugate addition-alkylation reaction starting from indolyldithiane (4), 3-methylenelactam 6, and EtI yields the adduct 17. Treatment of lactam 17 with DIBALH leads to formation of the naphthyridoindole 18. Compound 18 isomerizes in aqueous AcOH to yield pyridocarbazole 3. Finally, closure of ring E and subsequent reduction of the dithiane ring produces aspidospermidine. Pyridocarbazoles 2 and 10 were prepared as models. 相似文献
49.
Three valency models for orthorhombic La2-x
Sr
x
CuO4 were investigated for increasing Sr concentrationsx (0x0.21): 1. Cu2+Cu3+, 2. apex O2–O– and 3. in-plane O2–O–. All calculations were done by using structural parameters valid for the temperature range from 10 to 22 K. We thereby calculated the electrostatic interaction energy which, next to ionization potentials and electron affinities, comprises a major of the binding energyE
B
of crystals. Second-order effects were accounted for by calculating the strength of ionic dipole moments induced by crystal electric fields at relevant lattice sites. Their largest strengths are comparable to the dipole moment of the water molecule. Three out of five dipoles in La2-x
Sr
x
CuO4 vanish during the transition from the orthorhombic to the tetragonal phase. The binding energy differences between the different models suggest that the system is in a state of model 1. However, the differences are very small, being in the order of 0.3 to 0.76 eV atx=0.13. 相似文献
50.
Mario Petrich 《Annali di Matematica Pura ed Applicata》1994,167(1):117-146
On the congruence lattice (2(5) of a completely regular semigroup S the following mappings are considered p: P and p: VP, whereP is any of the Green relationsH, L, R orD. The equivalence relationsP
andP
V induced by these maps represent the main object of study in the paper. The former is a complete -congruence whereas the latter is a complete congruence onC(S). In particularH
,H
V,L
V,R
V coincide with the kernel, trace, left trace and right trace relations onC(S), respectively. All essential properties known for the latter relations carry over to the new relationsP
andP
V. In addition, some interesting interplays of these provide for more richness in the theory of congruences on completely regular than is the case for the kernel-trace approach to congruences on regular semigroups. 相似文献