全文获取类型
收费全文 | 848篇 |
免费 | 17篇 |
国内免费 | 9篇 |
专业分类
化学 | 432篇 |
晶体学 | 5篇 |
力学 | 7篇 |
数学 | 159篇 |
物理学 | 271篇 |
出版年
2021年 | 5篇 |
2020年 | 10篇 |
2018年 | 13篇 |
2017年 | 10篇 |
2016年 | 10篇 |
2015年 | 18篇 |
2014年 | 23篇 |
2013年 | 40篇 |
2012年 | 24篇 |
2011年 | 36篇 |
2010年 | 23篇 |
2009年 | 27篇 |
2008年 | 26篇 |
2007年 | 20篇 |
2006年 | 23篇 |
2005年 | 26篇 |
2004年 | 22篇 |
2003年 | 23篇 |
2002年 | 22篇 |
2001年 | 17篇 |
2000年 | 20篇 |
1999年 | 15篇 |
1998年 | 14篇 |
1997年 | 25篇 |
1996年 | 23篇 |
1995年 | 20篇 |
1994年 | 27篇 |
1993年 | 22篇 |
1992年 | 28篇 |
1991年 | 17篇 |
1990年 | 15篇 |
1989年 | 16篇 |
1988年 | 6篇 |
1987年 | 16篇 |
1986年 | 6篇 |
1985年 | 9篇 |
1984年 | 6篇 |
1983年 | 12篇 |
1982年 | 11篇 |
1981年 | 5篇 |
1980年 | 8篇 |
1978年 | 7篇 |
1977年 | 6篇 |
1976年 | 5篇 |
1975年 | 10篇 |
1974年 | 12篇 |
1973年 | 7篇 |
1969年 | 4篇 |
1966年 | 20篇 |
1965年 | 39篇 |
排序方式: 共有874条查询结果,搜索用时 27 毫秒
51.
Marek Mac Tomasz Uchacz Tomasz Wróbel Andrzej Danel Ewa Kulig 《Journal of fluorescence》2010,20(2):525-532
Novel fluorescing dyes 1,3,4-triphenyl-6-(1,4,7,10-tetraoxa-13-aza-cyclopentadec-13-ylmethyl)-1H-pyrazolo[3,4-b]quinoline (K1) and 2-[(2-hydroxyethyl)-(1,3,4-triphenyl-1H-pyrazolo[3,4-b]quinolin-6-ylmethyl)-amino]ethanol (L1) have been synthesized and investigated by the means of steady state and time-resolved fluorescence techniques. These compounds
act as sensors for the fluorescence detection of small inorganic cations (lithium, sodium, barium, magnesium and calcium)
in solvents of different polarities (THF and acetonitrile). The mechanism, which allows application of these compounds as
sensors, is an electron transfer from the electro-donative part of molecule to the acceptor part (fluorophore), which is retarded
upon complexation of the electro-donative part by inorganic cations. We found that crown ether-containing compound is very
sensitive to the addition of any investigated ions but amino alcohol-containing one exhibits better selectivity to the addition
of two-valued cations. Two kinds of the complexes (LM+ and L2M+) were found in the investigated systems. In addition, the dyes may be used as fluorescence indicators in solvents of lower
polarity like tetrahydrofuran. 相似文献
52.
Phase transitions of [M(H2O)6](MnO4)2, where M=Mg, Ni, Zn and Cd, have been studied at 100–400 K by differential scanning calorimetry (DSC). One solid–solid phase transition for the compounds with M=Mg and Ni and two solid–solid phase transitions for these with M=Zn and Cd have been discovered. Additionally, three phase transitions, connected with three-stage melting process of these compounds, were also found. Thermodynamic parameters of the all detected phase transitions were calculated. 相似文献
53.
Rü diger Gö bel Saharon Shelah 《Proceedings of the American Mathematical Society》2002,130(3):673-674
If and are groups and is a normal subgroup of , then the -closure of in is the normal subgroup of . In particular, is the -radical of . Plotkin calls two groups and geometrically equivalent, written , if for any free group of finite rank and any normal subgroup of the -closure and the -closure of in are the same. Quasi-identities are formulas of the form for any words in a free group. Generally geometrically equivalent groups satisfy the same quasi-identities. Plotkin showed that nilpotent groups and satisfy the same quasi-identities if and only if and are geometrically equivalent. Hence he conjectured that this might hold for any pair of groups. We provide a counterexample.
54.
It is well-known, that the ring of polynomial invariants of the alternating group has no finite SAGBI basis with respect to the lexicographical order for any number of variables . This note proves the existence of a nonsingular matrix such that the ring of polynomial invariants , where denotes the conjugate of with respect to , has a finite SAGBI basis for any . 相似文献
55.
Understanding and controlling solid-state morphologies and molecular conformations is the key to optimizing the properties of materials. As an example for the influence of small chemical changes on solid-state structures, we studied oligo(m-phenylene ethynylene) foldamers, where the introduction of an endo-methyl group induces a transition from an extended all-transoid to a helical all-cisoid conformation. The resulting structural changes were analyzed by X-ray diffraction (XRD), polarized optical microscopy (POM), and low-dose high-resolution electron microscopy (LD-HREM) over several length scales from the molecular to the mesoscopic level. The strong tendency of the endo-methyl oligomer 1 to form stable compact helices in solution resulted in round droplets with an ordered hexagonal columnar (Col(ho)) liquid crystalline structure, where shrinkage during the crystallization resulted in the formation of a banded texture. On the other hand, the endo-hydrogen oligomer 2 exhibited a very different morphology; its extended linear shape was maintained during crystallization and resulted in an extended lamellar structure, which was determined by a compromise between crystalline packing and minimization of the surface area. Another pronounced difference between both molecular structures was the ability of the extended lamellar "crystals" to bend, whereas the helices form either straight or disordered domains. In addition, both materials exhibit strong surface effects, which extend considerably inside the droplet and induce uniform bending of the supramolecular structures. 相似文献
56.
Jaroszyński J Andrearczyk T Karczewski G Wróbel J Wojtowicz T Papis E Kamińska E Piotrowska A Popović D Dietl T 《Physical review letters》2002,89(26):266802
We report on the observation of the Ising quantum Hall ferromagnet with Curie temperature T(C) as high as 2 K in a modulation-doped (Cd,Mn)Te heterostructure. In this system field-induced crossing of Landau levels occurs due to the giant spin-splitting effect. Magnetoresistance data, collected over a wide range of temperatures, magnetic fields, tilt angles, and electron densities, are discussed taking into account both Coulomb electron-electron interactions and s-d coupling to Mn spin fluctuations. The critical behavior of the resistance "spikes" at T-->T(C) corroborates theoretical suggestions that the ferromagnet is destroyed by domain excitations. 相似文献
57.
Link JM Reyes M Yager PM Anjos JC Bediaga I Göbel C Magnin J Massafferi A de Miranda JM Pepe IM dos Reis AC Carrillo S Casimiro E Cuautle E Sánchez-Hernández A Uribe C Vazquez F Agostino L Cinquini L Cumalat JP O'Reilly B Ramirez JE Segoni I Butler JN Cheung HW Gaines I Garbincius PH Garren LA Gottschalk E Kasper PH Kreymer AE Kutschke R Bianco S Fabbri FL Zallo A Cawlfield C Kim DY Rahimi A Wiss J Gardner R Kryemadhi A Chung YS Kang JS Ko BR Kwak JW Lee KB Park H Alimonti G Boschini M 《Physical review letters》2002,88(16):161801
A high statistics measurement of the Lambda(+)(c) lifetime from the Fermilab fixed-target FOCUS photoproduction experiment is presented. We describe the analysis technique with particular attention to the determination of the systematic uncertainty. The measured value of 204.6 +/- 3.4 (stat) +/- 2.5 (syst) fs from 8034 +/- 122 Lambda(+)(c)-->pK(-)pi(+) decays represents a significant improvement over the present world average. 相似文献
58.
Vyvey K Chmel S Neyens G Hübel H Balabanski DL Borremans D Coulier N Coussement R Georgiev G Nenoff N Pancholi S Rossbach D Schwengner R Teughels S Frauendorf S 《Physical review letters》2002,88(10):102502
The quadrupole moment of the 11(-) isomer in 196Pb has been measured by the level mixing spectroscopy method. This state has a pi(3s(-2)(1/2)1h(9/2)1i(13/2))11(-) configuration which is involved in most of the shears band heads in the Pb region. The first directly measured value of Q(s)(11(-)) = (-)3.41(66) b, coupled to the previously known quadrupole moment of the nu(1i(-2)(13/2))12(+) isomer allows us to estimate the quadrupole moment of the 16(-) shears band head as Q(s)(16(-)) = -0.32(10) b. The experimental values are compared to tilted axis cranking calculations, giving insight into the validity of the additivity approach to couple quadrupole moments and on the amount of deformation in the shears bands. 相似文献
59.
K. Gäbel Ch. Bergmann E. Fill E. Förster I. Uschmann 《Applied physics. B, Lasers and optics》1993,56(1):3-7
Photo-resonant pumping of X-ray lasers requires close coincidence of a strong emission line with a suitable absorbing transition. We use a high-resolution crystal spectrometer to verify five coincidences, one of which has apparently not yet appeared in published work. All but one of the coincidences are found to lie within one linewidth. With respect to X-ray laser photo-pumping three of the five coincidences seem to be promising candidates. 相似文献
60.
Lattice constant, Curie temperature, and electrical conductivity of CdCr2Se4:In single crystals have been measured after heat treatments of the crystals in Se atmosphere and under streaming hydrogen. By these treatments, the concentration of the Se vacancies and of the charge carrier concentration is altered drastically. The lattice constant as well as the magnetic ordering temperature have been found not to be affected by these heat treatments.Since the Se vacancies act as doubly changed donors, the electrical conductivity is strongly dependent on the concentration of the Se vacancies. A resistivity anomaly and large magnetoresistance are observed only in crystals with considerable Se deficiency. From these results it is concluded that the magnetoresistance is caused by hopping conduction between donor sites partly emptied by compensating A-site vacancy acceptors. Large magnetoresistance is found in samples with considerable Se deficit because only in this case the conduction at lower temperatures is dominated by the impurity band. 相似文献