全文获取类型
收费全文 | 2429篇 |
免费 | 88篇 |
国内免费 | 9篇 |
专业分类
化学 | 1642篇 |
晶体学 | 13篇 |
力学 | 106篇 |
数学 | 347篇 |
物理学 | 418篇 |
出版年
2023年 | 28篇 |
2022年 | 33篇 |
2021年 | 54篇 |
2020年 | 59篇 |
2019年 | 58篇 |
2018年 | 42篇 |
2017年 | 29篇 |
2016年 | 71篇 |
2015年 | 64篇 |
2014年 | 60篇 |
2013年 | 102篇 |
2012年 | 147篇 |
2011年 | 171篇 |
2010年 | 84篇 |
2009年 | 72篇 |
2008年 | 152篇 |
2007年 | 137篇 |
2006年 | 149篇 |
2005年 | 127篇 |
2004年 | 68篇 |
2003年 | 74篇 |
2002年 | 49篇 |
2001年 | 42篇 |
2000年 | 26篇 |
1999年 | 23篇 |
1998年 | 32篇 |
1997年 | 21篇 |
1996年 | 23篇 |
1995年 | 18篇 |
1994年 | 17篇 |
1993年 | 22篇 |
1992年 | 22篇 |
1991年 | 19篇 |
1990年 | 14篇 |
1989年 | 16篇 |
1988年 | 13篇 |
1987年 | 21篇 |
1986年 | 26篇 |
1985年 | 15篇 |
1984年 | 23篇 |
1983年 | 16篇 |
1982年 | 20篇 |
1981年 | 29篇 |
1980年 | 29篇 |
1978年 | 22篇 |
1977年 | 24篇 |
1976年 | 22篇 |
1975年 | 17篇 |
1974年 | 12篇 |
1972年 | 12篇 |
排序方式: 共有2526条查询结果,搜索用时 15 毫秒
21.
Wang W Zhou Z Nandakumar K Xu Z Masliyah JH 《Journal of colloid and interface science》2003,259(1):81-88
The sliding velocity of glass beads on a spherical surface, made either of an air bubble or of a glass sphere held stationary, is measured to investigate the effect of surface mobility on the particle sliding velocity. The sliding process is recorded with a digital camera and analyzed frame by frame. The sliding glass bead was found to accelerate with increasing angular position on the collector's surface. It reaches a maximum velocity at an angular position of about 100 degrees and then, under certain conditions, the glass bead leaves the surface of the collector. The sliding velocity of the glass bead depends strongly on the surface mobility of a bubble, decreasing with decreasing surface mobility. By a mobile surface we mean one which cannot set up resistive forces to an applied stress on the surface. The sliding velocity on a rigid surface, such as a glass sphere, is much lower than that on a mobile bubble surface. The sliding velocity can be described through a modified Stokes equation. A numerical factor in the modified Stokes equation is determined by fitting the experimental data and is found to increase with decreasing surface mobility. Hydrophobic glass beads sliding on a hydrophobic glass sphere were found to stick at the point of impact without sliding if the initial angular position of the impact is less than some specific angle, which is defined as the critical sticking angle. The sticking of the glass beads can be attributed to the capillary contracting force created by the formation of a cavity due to spontaneous receding of the nonwetting liquid from the contact zone. The relationship between the critical sticking angle and the particle size is established based on the Yushchenko [J. Colloid Interface Sci. 96 (1983) 307] analysis. 相似文献
22.
Jacob de Kok†‡ Silvia E. Braslavsky‡ Carel J. P. Spruit† 《Photochemistry and photobiology》1981,34(6):705-710
Abstract— C-phycocyanin from Synechococcus sp. ( Anacystis nidulans ) shows photoreversible absorption changes when dissolved in buffer containing 75% ethylene glycol (vol/vol). Irradiation with red light (638 nm) causes a 7.5% decrease in absorbance around the absorption maximum (620 m), while the absorbance around 500 nm increases. Subsequent irradiation with green light (500 nm) partially reverses this change. Final photoreversibility at around 620 nm amounts to ca. 2.5% of the maximum absorbance. These reactions are ascribed to two interconvertible species PCr and PCg , the former with a higher absorbance in the red. the latter in the green. The rate of dark reversion from PCg to PCr is strongly enhanced by ferricyanide. It is proposed that with this reagent, dark reversion occurs via an oxidized form of PCg . Furthermore, ferricyanide in the presence of ethylene glycol is capable of reversibly oxidizing part of the chromophores of C-phycocyanin, presumably to a radical. In the absence of ethylene glycol, however, ferricyanide causes total irreversible bleaching of the pigment in the dark. The induced photoreversibility of C-phycocyanin is ascribed to the perturbing action on the protein structure by ethylene glycol in high concentrations. This solvent proved the most suitable perturbant of several compounds tested. 相似文献
23.
We prove the existence of uncountably many nonisomorphic topological projective planes, each universal in the sense that it contains an isomorphic copy of every pseudoline arrangement. 相似文献
24.
Chromatography has been an important tool in marine laboratories. Since the 1960s, marine laboratories have been involved in the analysis of polychlorinated biphenyls (PCBs), organochlorine pesticides (OCPs), polycyclic aromatic hydrocarbons (PAHs), and brominated flame retardants (BFRs). Column chromatography and liquid chromatography (LC) techniques have been used, mainly in the clean-up phase, while gas chromatography (GC) has been used extensively in the final determination of these contaminants. Developments have been observed from the use of packed GC columns, via capillary columns to the use of heart-cut multi-dimensional GC and comprehensive multi-dimensional GC. The progress made in interlaboratory studies and the availability of certified reference materials are discussed. 相似文献
25.
The defect rock-salt type materials Zr1?xS, Sc1?xS, and Lu1?xS have been shown to exhibit a complex variety of ordered superstructures. These superstructures are reviewed within the context of Landau's theory of symmetry and phase transitions, and it is concluded that there is a significant similarity in the transitions when they are considered from the point of view of reciprocal space. 相似文献
26.
The combination of flow injection analysis with chemiluminescent detection is shown to provide extremely selective and sensitive detection of insect pheromones which possess an aldehyde moiety. The flow injection analysis system provides reproducible control of both the reaction chemistry and the sample introduction process. Microliter volume samples can be precisely handled and analyzed with this experimental configuration. The detection system is based on the luciferase-catalyzed oxidation of reduced flavin mononucleotide which occurs in the presence of aldehydes with carbon backbones of between 14 and 16 carbons. A limit of detection of 3 fmol of tetradecyl aldehyde is demonstrated and the system is shown to be insensitive to the presence of various organic solvents up to concentrations of approximately 10%. The key experimental variables which control sensitive detection of pheromone at the femtomole level with be investigated and discussed. 相似文献
27.
Overgaard J Iversen BB Palii SP Timco GA Gerbeleu NV Larsen FK 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(12):2775-2786
The structure and detailed electron density distribution (EDD) of the large octanuclear chromium-wheel host complex [Cr8F8(tBuCO2)16] (1) has been determined from synchrotron X-ray structure factors collected at 16(5) K. The complex has a central cavity with a minimum entry distance between carbon atoms of the pivalate methyl groups (pivalic acid = tBuCO2H) of 4.027(4) A on one side of the molecule and 7.273(4) A on the other. The screened side of the molecule can be "opened" by rotation of methyl groups to create a strained host structure, which is compensated for by improved host-guest and host-solvent interaction. The EDD of the 272-atom complex (1144 e-) was determined by multipole modeling based on the experimental structure factors. 3d orbital populations on the Cr atoms and topological analysis of the EDD show that the covalent part of the metal-ligand interactions consists mainly of sigma donation from the ligands, but that overall the interactions are predominantly electrostatic. The electrostatic potential (EP) has been calculated from the experimental EDD. Knowledge of the geometry of the naked complex 1 as well as the EP in the central cavity of this molecule allows us to deduce which characteristic properties guest molecules must have to be accepted into the void. To probe these predictions, a series of complexes of 1 with different guest inclusions were synthesized (2 = 1 + N,N'-dimethylformamide (DMF), 3 = 1 + N,N'-dimethylacetamide (DMA), 4 = 1 + DMA + DMF, 5 = 1 + 2CH3CN), and their structures were examined by using X-ray diffraction data measured at 120(1) K. Results of these studies indicate that in the crystalline state, the optimal guest molecule should be linear and possess a permanent dipole. Attempts to crystallize the host complex with cations incorporated into the cavity were fruitless, although electrospray ionization mass spectrometry showed that a [1 + potassium]+ entity pre-exists in solution and can be transferred intact into the gas phase. 相似文献
28.
Jacob G. Reynolds 《Silicon Chemistry》2007,3(5):267-269
The Partial Charge Model was developed to predict the hydroxylation, polymerization, and precipitation of ions. The purpose
of this study is to evaluate the Partial Charge Model for describing the polymerization of silica in aqueous solutions. The
Partial Charge Model predicts the stability of ions and complexes based on the assumption that the stable species will have
the same electronegativity as the mean electronegativity of the solution. The silica system was chosen for model validation
because of the rare availability of self-consistent thermodynamic data on many dissolved but polymerized silicate anions,
including both linear and cyclical species. The electronegativity of each species was calculated using the Partial Charge
Model and the results were plotted against the stability constants for the ions. The silicate anions segregated into groups
on the plot based on the number of charges per silicon atom in the polymer. Plots of the log of the stability constant versus
the change in electronegativity produced a linear relationship for the silica polymers containing one negative charge per
silicon atom, which resulted in an r
2 of 0.9978. Thus, the Partial Charge Model successfully describes the thermodynamics of silica polymerization in aqueous solution
for species that are sufficiently alike, but was not accurate for all silica species. 相似文献
29.
Jacob Burbea 《Rendiconti del Circolo Matematico di Palermo》1978,27(2):259-269
LetD be a bounded plane domain (with some smoothness requirements on its boundary). LetB p(D), 1≤p<∞, be the Bergmanp-space ofD. In a previous paper we showed that the “natural projection”P, involving the Bergman kernel forD, is a bounded projection fromL p(D) ontoB p(D), 1<p<∞. With this we have the decompositionL p(D)=B p(D)⊕B q ⊥ (D,p –1+q –=1, 1<p< ∞. Here, we show that the annihilatorB q ⊥ (D) is the space of allL p-complex derivatives of functions belonging to Sobolev space and which vanish on the boundary ofD. This extends a result of Schiffer for the casep=2. We also study certain operators onL p(D). Especially, we show that , whereI is the identity operator and ? is an operator involving the adjoint of the Bergman kernel. Other relationships relevant toB q ⊥ (D) are studied. 相似文献
30.
Jackson P Srinivas R Blanksby SJ Schroder D Schwarz H 《Chemistry (Weinheim an der Bergstrasse, Germany)》2000,6(7):1236-1242
The cation [Si,C,O]+ has been generated by 1) the electron ionisation (EI) of tetramethoxysilane and 2) chemical ionisation (CI) of a mixture of silane and carbon monoxide. Collisional activation (CA) experiments performed for mass-selected [Si,C,O]+, generated by using both methods, indicate that the structure is not inserted OSiC+; however, a definitive structural assignment as Si(+)-CO, Si(+)-OC or some cyclic variant is impossible based on these results alone. Neutralisation-reionisation (+NR+) experiments for EI-generated [Si,C,O]+ reveal a small peak corresponding to SiC+, but no detectable SiO+ signal, and thus establishes the existence of the Si(+)-CO isomer. CCSD(T)@B3LYP calculations employing a triple-zeta basis set have been used to explore the doublet and quartet potential-energy surfaces of the cation, as well as some important neutral states. The results suggest that both Si(+)-CO and Si(+)-OC isomers are feasible; however, the global minimum is 2 pi SiCO+. Isomeric 2 pi SiOC+ is 12.1 kcal mol-1 less stable than 2 pi SiCO+, and all quartet isomers are much higher in energy. The corresponding neutrals Si-CO and Si-OC are also feasible, but the lowest energy Si-OC isomer (3A") is bound by only 1.5 kcal mol-1. We attribute most, if not all, of the recovery signal in the +NR+ experiment to SiCO+ survivor ions. The nature of the bonding in the lowest energy isomers of Si(+)-(CO,OC) is interpreted with the aid of natural bond order analyses, and the ground state bonding of SiCO+ is discussed in relation to classical analogues such as metal carbonyls and ketenes. 相似文献