全文获取类型
收费全文 | 3614篇 |
免费 | 99篇 |
国内免费 | 28篇 |
专业分类
化学 | 2278篇 |
晶体学 | 32篇 |
力学 | 78篇 |
综合类 | 1篇 |
数学 | 663篇 |
物理学 | 689篇 |
出版年
2023年 | 25篇 |
2022年 | 98篇 |
2021年 | 90篇 |
2020年 | 83篇 |
2019年 | 94篇 |
2018年 | 83篇 |
2017年 | 79篇 |
2016年 | 146篇 |
2015年 | 129篇 |
2014年 | 146篇 |
2013年 | 283篇 |
2012年 | 223篇 |
2011年 | 238篇 |
2010年 | 170篇 |
2009年 | 166篇 |
2008年 | 206篇 |
2007年 | 199篇 |
2006年 | 161篇 |
2005年 | 135篇 |
2004年 | 107篇 |
2003年 | 79篇 |
2002年 | 87篇 |
2001年 | 33篇 |
2000年 | 43篇 |
1999年 | 31篇 |
1998年 | 24篇 |
1997年 | 30篇 |
1996年 | 41篇 |
1995年 | 32篇 |
1994年 | 25篇 |
1993年 | 24篇 |
1992年 | 26篇 |
1991年 | 13篇 |
1990年 | 23篇 |
1989年 | 20篇 |
1988年 | 22篇 |
1987年 | 17篇 |
1986年 | 12篇 |
1985年 | 21篇 |
1984年 | 31篇 |
1983年 | 17篇 |
1982年 | 21篇 |
1981年 | 15篇 |
1980年 | 24篇 |
1979年 | 16篇 |
1978年 | 25篇 |
1977年 | 27篇 |
1976年 | 14篇 |
1973年 | 8篇 |
1968年 | 9篇 |
排序方式: 共有3741条查询结果,搜索用时 15 毫秒
991.
Marek Biskup Lincoln Chayes Roman Kotecký 《Communications in Mathematical Physics》2003,242(1-2):137-183
We study the formation/dissolution of equilibrium droplets in finite systems at parameters corresponding to phase coexistence. Specifically, we consider the 2D Ising model in volumes of size L
2
, inverse temperature >
c
and overall magnetization conditioned to take the value m
L
2
–2m
v
L
, where
c
–1
is the critical temperature, m
=m
() is the spontaneous magnetization and v
L
is a sequence of positive numbers. We find that the critical scaling for droplet formation/dissolution is when v
L
3/2
L
–2
tends to a definite limit. Specifically, we identify a dimensionless parameter , proportional to this limit, a non-trivial critical value
c
and a function such that the following holds: For <
c
, there are no droplets beyond log L scale, while for >
c
, there is a single, Wulff-shaped droplet containing a fraction
c
=2/3 of the magnetization deficit and there are no other droplets beyond the scale of log L. Moreover, and are related via a universal equation that apparently is independent of the details of the system. 相似文献
992.
Pescitelli G Berova N Xiao TL Rozhkov RV Larock RC Armstrong DW 《Organic & biomolecular chemistry》2003,1(1):186-190
A phenyl-substituted chiral dihydrofuroangelicin, 4-methyl-8-(2-E-phenylethenyl)-8,9-dihydro-2H-furo[2,3-h]- 1-benzopyran-2-one, synthesized in racemic form, has been resolved by HPLC chiral separation, and its absolute configuration determined by the non-empirical exciton chirality method. The solution conformation has been investigated through NMR and molecular modeling methods: two minima found by molecular mechanics and DFT methods are in keeping with observed 1H-1H 3J coupling constants and NOE effects. The experimental CD spectrum for the second eluted enantiomer shows a positive couplet between 230 and 350 nm (amplitude A = + 15.7); by application of the exciton chirality method, the absolute configuration of this enantiomer at C8 is determined as (S). The experimental spectrum is in very good agreement with the one evaluated by means of DeVoe coupled-oscillator calculations, using the DFT calculated geometries. 相似文献
993.
Rispens MT Meetsma A Rittberger R Brabec CJ Sariciftci NS Hummelen JC 《Chemical communications (Cambridge, England)》2003,(17):2116-2118
Two crystal structures of PCBM, obtained from different crystallisation solvents, are presented; a proposed link with solvent dependence of the efficiency of MDMO-PPV:PCBM solar cells is described. 相似文献
994.
995.
Roman ?ermák Martin Obadal Petr Poní?il Martina Polášková Karel Stoklasa Ane?ka Lengálová 《European Polymer Journal》2005,41(8):1838-1845
The paper studies the effects of mould temperature and holding pressure on the structure of neat and β-nucleated isotactic polypropylenes. Commercially available isotactic polypropylene was modified with a β-specific nucleator based on N,N′-dicyclohexylnaphthalene-2,6-dicarboxamide in the concentration of 0.03 wt.%. From both the original material (α-iPP) and the β-nucleated material (β-iPP), dog bone-shaped test specimens were injection-moulded, using two sets of processing parameters. In the T-set the mould temperature was varied within the range of 40-120 °C in 10 °C steps, while in the P-set the holding pressure was changed from 5 to 13 MPa in 1 MPa steps. Other processing parameters were kept on the same level. Polarized-light microscopy showed a strong effect of mould temperature on the morphology of α-iPP specimens; the skin thinned out and the spherulite size increased with mould temperature rise. On the other hand, in the case of β-iPP only the skin thickness was correspondingly affected, while the spherulite size remained virtually constant, independent of the mould temperature changes. At the same structure level, both α-iPP and β-iPP specimens were insensitive to holding pressure variations. Polymorphic composition derived from wide-angle X-ray scattering displayed similar range of changes induced by variations of the processing parameters for both materials. The increase of mould temperature positively influenced the crystallinity and the β-form content, particularly in the skin of specimens. On the contrary, higher holding pressure depressed the crystallinity proportionally within the bulk of specimens. 相似文献
996.
Budnik BA Tsybin YO Håkansson P Zubarev RA 《Journal of mass spectrometry : JMS》2002,37(11):1141-1144
Ionization energies (IE) of [M + zH](z+) (z+) electrospray-produced polypeptides were determined by electron ionization in a Penning cell of 4.7 and 9.4 T Fourier transform mass spectrometers. For z = 1+ and substance P, the found IE value of 11.0 +/- 0.4 eV is in agreement with that obtained earlier for ions generated with matrix-assisted laser desorption/ionization. For higher z, the following values were found: 11.7 +/- 0.3 eV for 2+ of [Arg-8]-vasopressin, 11.1 +/- 0.6 eV for 2+ of substance P, 12.2 +/- 0.7 eV for 2+ of renin substrate, 13.3 +/- 0.4 eV for 3+ of B-chain of insulin and 14.6 +/- 0.6 eV for 4+ and 15.1 +/- 0.4 eV for 5+ of melittin. It was found that 90% of existing IE data on polypeptides in the 1.0-3.5 kDa mass range are described with 相似文献
997.
Constituents of Osmanthus Absolute, 6th Communication. (7 S , 10 S , 5 E ) - and (7 R , 10 S , 5 E )-2,6,10-Trimethyl-7,10-epoxy-2,5,11-dodecatriene Two novel sesquiterpenoid oxides 1a and 1b from Osmanthus absolute have been identified. Their structural proof is based on pectral data and synthesis starting from the known methyl [5-methyl-5-vinyl-tetrahydrofur-1-yl] ketones ( 4a and 4b , respectively), whose configuration is well established. The thus obtained compounds 1a / 1b identical with the natural products, were accompanied by their corresponding 6-methylidene isomers 3a and 3b which could not be detected in the natural substrate. 相似文献
998.
Appelqvist LA Addis P Björkhem I Bosset JO Caboni MF Dutta P Grandgirard A Guardiola F Hau LB Nielsen JH Hugget A Hwang LS Kumpulainen J McCluskey S Ohshima T Przbylski R Sevanian A Yan P 《Journal of AOAC International》2004,87(2):511-519
A compilation of literature data on the content of cholesterol oxidation products (COP) in various food products and in blood demonstrates a large variation in content in products or tissues of very similar nature when analyzed in different laboratories according to a large number of methods. The lack of validated, internationally recognized methodology with published accuracy and precision has so far hindered such assessments. Hence an interlaboratory comparision of methodologies of COP analysis was undertaken on egg yolk powders (EYP), whole milk powders (WMP), skim milk powders (SMP), and lard (L). Each product type had one fresh sample (low) and one aged (high) in COP contents. A total of 17 sets of results on WMP, 15 on SMP and EYP, and 13 on L were compared. Overall results (mg/kg sample) varied extensively: Fresh EYP 0.72-265, aged EYP 2.51-361; fresh WMP 0.02-18.1, aged WMP 0.02-26.9; fresh SMP 0.02-6.51, aged SMP <0.01-6.51; fresh L 0.18-97, aged L 4.15-452. Some results were questioned, viz., those from laboratories not indicating substantial differences between samples "low" and "high" in total COP. Others were excluded because of lack of verification of identity of gas chromatographic peaks by mass spectrometry. Then a more narrow range of core results (mg/kg sample) was observed: Fresh EYP 5.69-29.5 sample, aged EYP 11.8-79.0; fresh WMP 0.12-1.76, aged WMP 1.17-13.7; fresh SMP <0.30-<1.21, aged SMP 0.30-2.26; fresh L 0.18-5.07, aged L 94.4-231. At a workshop discussing the results, numerous recommendations were made toward more reliable methodology for determination of COP in foods. 相似文献
999.
Charge sensitivity analysis (CSA ) is carried out for model toluene–vanadium pentoxide chemisorption complexes involving the two-pyramidal model of the active site on the (010)—V2O5 surface. Maps of the electrostatic potential around the adsorbate and the substrate cluster are used to rationalize energetical preferences of alternative perpendicular and parallel arrangements of the toluene ring relative to the pyramid bases, known from previous SCF MO studies. The minimum-energy coordinates (MEC ) in the electron population space are determined from the CSA semiempirical, finite difference atomic hardness matrix for the actual SCF MO charges in the chemisorption clusters. They represent collective charge displacements which minimize the system energy per unit change in the oxidation state of a specified atom, thus providing a convenient diagnostic tool for testing the alternative charge rearrangements and range of perturbations due to the chemisorption bond or changes in the cluster environment. The MEC relaxed hardnesses diagnose mode stabilities and together with the MEC topologies identify the most probable locations of the adsorbate activation. Finally, the atomic Fukui function indices are used to explore trends in the distribution of the external charge transfer due to the system environment. © 1995 John Wiley & Sons, Inc. 相似文献
1000.