全文获取类型
收费全文 | 1403篇 |
免费 | 64篇 |
国内免费 | 12篇 |
专业分类
化学 | 838篇 |
晶体学 | 39篇 |
力学 | 70篇 |
综合类 | 2篇 |
数学 | 175篇 |
物理学 | 355篇 |
出版年
2023年 | 10篇 |
2022年 | 31篇 |
2021年 | 53篇 |
2020年 | 38篇 |
2019年 | 41篇 |
2018年 | 53篇 |
2017年 | 40篇 |
2016年 | 71篇 |
2015年 | 53篇 |
2014年 | 72篇 |
2013年 | 141篇 |
2012年 | 107篇 |
2011年 | 125篇 |
2010年 | 84篇 |
2009年 | 75篇 |
2008年 | 56篇 |
2007年 | 70篇 |
2006年 | 54篇 |
2005年 | 40篇 |
2004年 | 20篇 |
2003年 | 24篇 |
2002年 | 33篇 |
2001年 | 12篇 |
1999年 | 7篇 |
1998年 | 10篇 |
1997年 | 5篇 |
1996年 | 13篇 |
1995年 | 10篇 |
1994年 | 8篇 |
1993年 | 11篇 |
1992年 | 8篇 |
1991年 | 3篇 |
1990年 | 4篇 |
1989年 | 7篇 |
1988年 | 6篇 |
1987年 | 7篇 |
1986年 | 5篇 |
1985年 | 11篇 |
1984年 | 3篇 |
1982年 | 11篇 |
1981年 | 4篇 |
1980年 | 4篇 |
1979年 | 4篇 |
1976年 | 2篇 |
1975年 | 5篇 |
1974年 | 5篇 |
1973年 | 3篇 |
1971年 | 2篇 |
1969年 | 2篇 |
1964年 | 2篇 |
排序方式: 共有1479条查询结果,搜索用时 15 毫秒
991.
Hasan Demirkoparan 《International Journal of Non》2007,42(2):258-270
We study the effect of fiber recruitment on the mechanical response of a fiber reinforced non-linearly elastic tube that is both swollen and pressurized. Attention is restricted to cylindrically symmetric tube deformation. The constitutive model permits fibers to support tension, but not compression. While many combinations of pressure and swelling cause all of the fibers to be recruited for load support, both large swelling and large deswelling can give rise to fiber derecruitment at certain locations in the tube. This leads to less channel opening than would be the case if the fibers provided support while contracted. The transition between mechanically active and mechanically inactive fibers can be described in terms of the quasi-static motion of a fiber recruiting interface. 相似文献
992.
Eric E. Brown Prof. Wesley R. Burghardt Hasan Kahvand Prof. David C. Venerus 《Rheologica Acta》1995,34(3):221-234
Relaxation of the second normal stress difference (N
2) following step strain of a concentrated monodisperse polystyrene solution has been studied using mechanical and optical rheometry. Measurements of normal thrust in a parallel plate geometry are corrected for strain inhomogeneity and combined with independent measurements of the first normal stress difference (N
1) to determine N
2. Optical experiments were performed using a novel configuration where flow birefringence data collected using multiple light paths within the shear plane are combined with the stress-optical law to determine all three independent stress components for shearing deformations. This technique eliminates end effects, and provides an opportunity to oversample the stress tensor and develop consistency checks of experimental data. N
2 is found to be nonzero at all accessible times, and relaxes in roughly constant proportion to N
1. This reflects nonaffine distribution of chain segments, even well within the regime of chain retraction at short times. Data collected with the two techniques are reasonably consistent with each other, and with results of previous studies, generally lying between the predictions of the Doi-Edwards model with and without the independent alignment approximation. The normal stress ratio –N
2/N
1 shows pronounced strain thinning in the nonlinear regime. 相似文献
993.
Influence of the third monomer on lauryl methacrylate–methyl methacrylate emulsion terpolymerization
Rukhsana Shabnam Abu M. I. Ali Muhammad A. J. Miah Klaus Tauer Hasan Ahmad 《Colloid and polymer science》2013,291(9):2111-2120
An experimental study shows how the emulsion terpolymerization of lauryl methacrylate (LMA) and methyl methacrylate is influenced by the nature of the third monomer. The third monomer is either glycidyl methacrylate, hydroxyethyl methacrylate, hydroxypropyl methacrylate, or styrene. We report the synthesis of terpolymer particles with an appreciably high content of the very hydrophobic LMA (between 0.2515 and 0.238 molar fraction in the monomer mixture) in 60:40 weight water/ethanol mixture as the continuous phase, poly(vinyl pyrrolidone) as a polymeric steric stabilizer, and potassium peroxodisulfate as the initiator. The emulsion terpolymerization proceeds smoothly without the formation of coagulum and leads to particles with an average diameter clearly below 1 μm. We discuss the overall polymerization behavior regarding conversion–time curves, particle morphology, and glass transition temperature of the terpolymers in dependence of the lyophilicity/lyophobicity of the monomer mixture. 相似文献
994.
Emilia Kiersikowska Hasan Marai Grzegorz Wrzeszcz Ewa Kita 《Transition Metal Chemistry》2013,38(6):603-610
Tris-asparaginatochromium(III), [Cr(Asn)3]0 (where Asn forms a 5-membered chelate ring via amine nitrogen and α-carboxylate oxygen atoms) and its mono- and diaqua-derivatives were obtained, and their acid-catalyzed aquation was studied. The first reaction for [Cr(Asn)3]0 and [Cr(Asn)2(H2O)2]+ is the chelate ring opening at the Cr-NH2 bond, leading to metastable intermediates. Kinetics of these processes were studied spectrophotometrically in 0.1–1.0 M HClO4 at 303 and 333 K, respectively. A linear dependence of k obs on [H+], k obs = a + b[H+] was determined for both the complexes. Additionally, oxidation of chromium(III) to chromate(VI) by hydrogen peroxide was studied. The process proceeds through a chromium(V) intermediate, which is next transformed, in faster parallel steps into CrO4 2? and [Cr(O2)2]3? anions. The latter species, a chromium(V)-peroxo complex, is metastable under a large excess of H2O2. Kinetics of oxidation of [Cr(Asn)3]0 were studied at 298 K, at constant [OH?], within 0.2–1.0 M H2O2 range. A linear dependence of k obs on H2O2 was established. A mechanism is proposed, where the rate-determining step is an inner sphere 2-electron transfer within a precursor chromium(III) complex with coordinated O2H? anion of the [Cr(Asn)2(OH)(HO2)]? formula. EPR results provided clear evidence for formation of a relatively stable tetrakis(η 2-peroxo)chromate(V) complex, [Cr(O2)4]3?. 相似文献
995.
Jamil Ahmad Kalim Deshmukh 《International Journal of Polymer Analysis and Characterization》2013,18(4):287-296
PVA/TiO2 nanocomposite membranes developed were investigated for chemical, mechanical, and gas separation properties. PVA/TiO2 dispersion offers good optical property and less aggregation, as shown by UV-vis photospectroscopy. FT-IR spectra suggest strong interaction between PVA and TiO2. Mechanical properties of the composite membranes were enhanced by the addition of TiO2. Permeation results show that the addition of TiO2 up to 20 wt.% increased the selectivity of gas pairs O2/N2, H2/N2, H2/CO2, and CO2/N2 by 60%, 55%, 23%, and 26% respectively, with a corresponding decrease in the permeability. At higher loading of TiO2, a reverse trend was observed. 相似文献
996.
Ian W. J. Still S. Khaqan Hasan Kenneth Turnbull 《Phosphorus, sulfur, and silicon and the related elements》2013,188(1-2)
Abstract It has been shown that phosphorus pentasulfide (P4S10) is a mild, effective reagent for the reduction of sulfoxides to sulfides and that this conversion can be achieved selectively in the presence of other reactive groups such as ketone, ester, amide, nitro and halogen. This reagent can be used in similar fashion for the efficient reduction of sulfilimines (in both the N-p-toluenesulfonyl and free NH forms). Phosphorus pentasulfide also appears to be a promising reagent for reducing selenoxides but is ineffective in reducing sulfones, sulfinates, or sulfites. The mechanistic evidence presented indicates a probable four-centre (Wittig-like) intermediate or transition-state for the reduction by P4S10. 相似文献
997.
Hasan Mart 《高分子科学杂志,A辑:纯化学与应用化学》2013,50(9):1197-1206
The new Schiff base oligomer (oligo‐ortho‐chloroazomethinephenol) was synthesized by the condensation of ortho‐chloroaniline with oligosalicylaldehyde (OSA). Oligomer‐metal complexes of oligo‐ortho‐chloroazomethinephenol (OKAP) with Cu(II), Zn(II) and Co(II) were synthesized. The properties of OKAP and oligomer‐metal complexes were studied by elemental, UV‐Vis, 1H‐NMR, FT‐IR, magnetic susceptibility analyses. The number average molecular weight and mass average molecular weight OKAP were found to be 1494 g · mol?1 and 5418 g · mol?1, respectively. Elemental analyses of oligomer‐metal complexes suggest that the ratio of metal to oligomer is 1∶2. The results indicate that the OKAP coordinate through azomethine nitrogen and phenolic oxygen to the metal ions. Antimicrobial activity of OKAP was tested against S. cerevisiae, B. subtilis, E. coli, K. pneumoniae, M. luteus and S. aureus. The thermal stabilities of the OKAP and oligomer‐metal complexes were compared by thermogravimetric (TG) analyses. According to TG, OKAP, and oligomer‐metal complexes were stable against temperature and thermooxidative decomposition. The weight losses of OKAP and oligomer‐metal complexes were found to be at 400 and 800°C at 20.2 and 50.0 (OKAP), 17.1 and 41.1 (Cu(II)), 13.4 and 38.5 (Zn(II)), 18.3 and 68.2 (Co(II)), %, respectively. Based on half degradation temperature (T50%) parameters, Cu(II) and Zn(II) complexes were more resistant than the OKAP and Co(II) complex. 相似文献
998.
999.
Hasan Şahin Nurten Özsoy Berna Özbek Çelik Onur Koyuncu 《Natural product research》2020,34(5):621-628
AbstractPhytochemical composition of ethyl acetate fraction and total phenolic content, in vitro antioxidant, anti-inflammatory, antimicrobial activities of petroleum ether, chloroform, ethyl acetate and n-butanol fractions of the ethanol extract obtained from the subaerial parts of Scorzonera pygmaea Sibth. & Sm. (Asteraceae) were investigated. Nine compounds; scorzopygmaecoside (1), scorzonerol (2), cudrabibenzyl A (3), thunberginol C (4), scorzocreticoside I (5) and II (6), chlorogenic acid (7), chlorogenic acid methyl ester (8), 3,5-di-O-caffeoylquinic acid (9) were isolated and identified using spectroscopic methods. All substances were isolated for the first time from this species. Compounds 1 and 2 are new. The fractions showed high antioxidant capacity correlated with their phenolic content and no significant antimicrobial activity against tested bacteria and fungi. COX inhibition test was used to evaluate the anti-inflammatory activity and all the fractions showed low inhibition in comparison with indomethacin. 相似文献
1000.
Hasan Kırmızıbekmez Hilal Bardakcı Milena Masullo Özge Kamburoğlu Gonca Eryılmaz Galip Akaydın Erdem Yeşilada Sonia Piacente 《Helvetica chimica acta》2014,97(11):1571-1576
A new flavonol glycoside, quercetin 3‐O‐[6′′′‐O‐3,5‐dihydroxycinnamoyl‐β‐glucopyranosyl‐(1→2)]‐β‐galactopyranoside (named lilacifloroside; 1 ) and a new iridoid 2 (named asperulogenin), were isolated from the aerial parts of Asperula lilaciflora in addition to eight known secondary metabolites, i.e., quercetin, kaempferol, quercetin 3‐O‐β‐glucopyranosyl‐(1→2)‐β‐galactopyranoside, quercetin 3‐O‐β‐glucopyranosyl‐(1→2)‐arabinopyranoside, asperuloside, deacetylasperulosidic acid, asperulosidic acid methyl ester, and chlorogenic acid. The structures were elucidated on the basis of extensive 1D‐ and 2D‐NMR experiments as well as MS data. Compound 1 contains the rare 3,5‐dihydroxycinnamoyl moiety in its structure. This work constitutes the first phytochemical study of the title plant. 相似文献