全文获取类型
收费全文 | 1593篇 |
免费 | 55篇 |
国内免费 | 5篇 |
专业分类
化学 | 1057篇 |
晶体学 | 16篇 |
力学 | 36篇 |
数学 | 197篇 |
物理学 | 347篇 |
出版年
2023年 | 41篇 |
2022年 | 45篇 |
2021年 | 52篇 |
2020年 | 46篇 |
2019年 | 46篇 |
2018年 | 41篇 |
2017年 | 22篇 |
2016年 | 48篇 |
2015年 | 45篇 |
2014年 | 50篇 |
2013年 | 83篇 |
2012年 | 83篇 |
2011年 | 128篇 |
2010年 | 46篇 |
2009年 | 62篇 |
2008年 | 80篇 |
2007年 | 79篇 |
2006年 | 63篇 |
2005年 | 48篇 |
2004年 | 55篇 |
2003年 | 28篇 |
2002年 | 29篇 |
2001年 | 20篇 |
2000年 | 24篇 |
1999年 | 13篇 |
1998年 | 13篇 |
1997年 | 17篇 |
1996年 | 15篇 |
1995年 | 11篇 |
1994年 | 15篇 |
1993年 | 23篇 |
1992年 | 8篇 |
1991年 | 14篇 |
1990年 | 12篇 |
1988年 | 12篇 |
1987年 | 13篇 |
1986年 | 8篇 |
1985年 | 9篇 |
1984年 | 16篇 |
1983年 | 11篇 |
1982年 | 9篇 |
1981年 | 9篇 |
1980年 | 10篇 |
1979年 | 10篇 |
1978年 | 11篇 |
1977年 | 9篇 |
1976年 | 10篇 |
1975年 | 7篇 |
1974年 | 6篇 |
1973年 | 7篇 |
排序方式: 共有1653条查询结果,搜索用时 0 毫秒
991.
Andréa Cynthia Santos Christophe Duhamel Lorena Silva Belisário Lucas Moreira Guedes 《Journal of Heuristics》2012,18(4):657-675
Wireless Sensor Networks are used in several practical applications such as environmental monitoring and risk detection. In this work, we deal with the problem of organizing the network topology into clusters in order to minimize the total energy consumption. The problem is modeled as an Independent Dominating Problem with Connecting requirements. We first present a state-of-the-art on the problems to optimize energy consumption in WSN. Then, we propose a mixed integer linear programming formulation, constructive heuristics, a local search procedure, and a GRASP-based metaheuristic. Results are provided for large scale WSN instances. 相似文献
992.
Holcomb RE Mason LJ Reardon KF Cropek DM Henry CS 《Analytical and bioanalytical chemistry》2011,400(1):245-253
There is increasing interest in using microalgae as a lipid feedstock for the production of biofuels. Lipids used for these
purposes are triacylglycerols that can be converted to fatty acid methyl esters (biodiesel) or decarboxylated to “green diesel.”
Lipid accumulation in most microalgal species is dependent on environmental stress and culturing conditions, and these conditions
are currently optimized using slow, labor-intensive screening processes. Increasing the screening throughput would help reduce
the development cost and time to commercial production. Here, we demonstrated an initial step towards this goal in the development
of a glass/poly(dimethylsiloxane) (PDMS) microfluidic device capable of screening microalgal culturing and stress conditions.
The device contained power-free valves to isolate microalgae in a microfluidic growth chamber for culturing and stress experiments.
Initial experiments involved determining the biocompatibility and culturing capability of the device using the microalga Tetraselmis chuii. With this device, T. chuii could be successfully cultured for up to 3 weeks on-chip. Following these experiments, the device was used to investigate
lipid accumulation in the microalga Neochloris oleabundans. It was shown that this microalga could be stressed to accumulate cytosolic lipids in a microfluidic environment, as evidenced
with fluorescence lipid staining. This work represents the first example of microalgal culturing in a microfluidic device
and signifies an important expansion of microfluidics into the biofuels research arena. 相似文献
993.
The fragmentation reaction of bis-nor-seco-CB[10] with 3,5-dimethylphenol (3) delivers methylene bridged glycoluril pentamer 5 in 81% yield. The host-guest recognition properties of the previously known tetramer 4 and those of pentamer 5 and hexamer 6 toward cationic guests in water are used to delineate some important features of the binding of acyclic CB[n]-type receptors. 相似文献
994.
Auer S Nirschl M Schreiter M Vikholm-Lundin I 《Analytical and bioanalytical chemistry》2011,400(5):1387-1396
Nanomolar quantities of single-stranded DNA products ~100 nucleotides long can be detected in diluted 1% serum by surface plasmon resonance (SPR) and film bulk acoustic resonators (FBARs). We have used a novel FBAR sensor in parallel with SPR and obtained promising results with both the acoustic and the optical device. Oligonucleotides and a repellent lipoamide, Lipa-DEA, were allowed to assemble on the sensor chip surfaces for only 15 min by dispensing. Lipa-DEA surrounds the analyte-binding probes on the surface and effectively reduces the non-specific binding of bovine serum albumin and non-complementary strands. In a highly diluted serum matrix, the non-specific binding is, however, a hindrance, and the background response must be reduced. Nanomolar concentrations of short complementary oligos could be detected in buffer, whereas the response was too low to be measured in serum. DNA strands that are approximately 100 base pairs long at concentrations as low as 1-nM could be detected both in buffer and in 1% serum by both SPR and the FBAR resonator. 相似文献
995.
Hermi F. Brito Jukka Hassinen Jorma Hölsä Högne Jungner Taneli Laamanen Mika Lastusaari Marja Malkamäki Janne Niittykoski Pavel Novák Lucas C. V. Rodrigues 《Journal of Thermal Analysis and Calorimetry》2011,105(2):657-662
The details of the mechanism of persistent luminescence were probed by investigating the trap level structure of Sr2MgSi2O7:Eu2+,R3+ materials (R: Y, La-Lu, excluding Pm and Eu) with thermoluminescence (TL) measurements and Density Functional Theory (DFT)
calculations. The TL results indicated that the shallowest traps for each Sr2MgSi2O7:Eu2+,R3+ material above room temperature were always ca. 0.7 eV corresponding to a strong TL maximum at ca. 90 °C. This main trap
energy was only slightly modified by the different co-dopants, which, in contrast, had a significant effect on the depths
of the deeper traps. The combined results of the trap level energies obtained from the experimental data and DFT calculations
suggest that the main trap responsible for the persistent luminescence of the Sr2MgSi2O7:Eu2+,R3+ materials is created by charge compensation lattice defects, identified tentatively as oxygen vacancies, induced by the R3+ co-dopants. 相似文献
996.
Lucas Bomfim Bolzon Alexandre G. S. Prado 《Journal of Thermal Analysis and Calorimetry》2011,106(2):427-430
The protonation and deprotonation of the Nb2O5 surface has been followed in order to understand the reactions of surface of this catalyst. The simultaneous potentiometric
and conductometric titrations had been carried by using 50 mL of water suspension of Nb2O5 40 g L−1. The oxide was entirely deprotonated when adding 0.4 mL NaOH 1 mol L−1, and later titrated with 0.1 mol L−1. The titration had supplied K
1 and K
2 and the obtained values were 3.24 × 10−3 and 4.17 × 10−8, respectively. The zero point charge was pHpcz = 4.94. The thermodynamic studies were carried out by using 50 mL of a 40 g/L Nb2O5 aqueous suspension with the pH adjusted to pHPZC value. The suspension was titrated with 0.5 mol/L of HNO3 or NaOH for protonation or deprotonation studies, respectively, in an isoperibol calorimeter CSC ISC-4300. Thus, the obtained
thermodynamic values of the protonation and deprotonation of Nb2O5 were Δdp
G = −37.60 kJ/mol, Δdp
H = −23.72 kJ/mol and ΔdpS = 47 J/(mol K). 相似文献
997.
López E Lucas JM de Andrés J Albertí M Bofill JM Bassi D Aguilar A 《Physical chemistry chemical physics : PCCP》2011,13(35):15977-15984
The association reactions of benzene molecules with alkali ions M(+) (Li(+), Na(+) and K(+)) under single collision conditions have been studied using a radiofrequency-guided-ion-beam apparatus and mass spectrometry characterization of the different adducts. Cross-section energy dependences for [M-C(6)H(6)](+) adduct formation have been measured at collision energies up to 1.20 eV in the center of mass frame. All excitation functions decrease when collision energy increases, showing the expected behaviour for barrierless reactions. From ab initio chemical structure calculations at the MP2(full) level, the formation of the adducts makes evident the alkali ion-benzene non-covalent chemical bond. The cross-section energy dependence and the role of radiative cooling rates and unimolecular decomposition on the stabilization of the energized collision complex are also discussed. 相似文献
998.
Albertí M Huarte-Larrañaga F Aguilar A Lucas JM Pirani F 《Physical chemistry chemical physics : PCCP》2011,13(18):8251-8258
The specific influence of X(-) ions (X = F,Cl, Br, I) in the solvation process of halide-benzene (X(-)-Bz) ionic heterodimers by Ar atoms is investigated by means of molecular dynamic (MD) simulations. The gradual evolution from cluster rearrangement to solvation dynamics is discussed by considering ensembles of n (n = 1-15 and n = 30) Ar atoms around the X(-)-Bz stable ionic dimers. The potential energy surfaces employed are based on an atom/ion-atom and atom/ion-bond decomposition, which has been developed previously by some of the authors. The outcome of the dynamics is analyzed by employing radial distribution functions (RDF) and tridimensional (3D) probability densities. 相似文献
999.
Vilcachagua JD Ducati LC Rittner R Contreras RH Tormena CF 《The journal of physical chemistry. A》2011,115(7):1272-1279
Interesting insight into the electronic molecular structure changes associated with substituent effects on the Fermi contact (FC) and paramagnetic spin-orbit (PSO) terms of (1)J(CF) NMR coupling constants (SSCCs) in o-X-, m-X-, and p-X-fluorobenzenes (X = NH(2); NO(2)) is presented. The formulation of this approach is based on the influence of different conjugative and hyperconjugative interactions on a second-order property, which can be qualitatively predicted if it is known how they affect the main virtual excitations entering into that second-order property. A set of consistent approximations are introduced in order to analyze the behavior of occupied and virtual orbitals, which define some experimental trends for (1)J(CF) spin-spin coupling constants. In addition, DFT hybrid functionals were used, and a similar degree of confidence to compute the (1)J(CF) with those observed for the SOPPA(CCSD) method was obtained. The (1)J(CF) SSCCs for ezetimibe, a commercially fluorinated drug used to reduce cholesterol levels, were measured and DFT-calculated, and the qualitative approach quoted above was applied. As a byproduct, a possible method to determine experimentally a significant PSO contribution to (1)J(CF) SSCCs is discussed. 相似文献
1000.
Beyhan SM Götz AW Ariese F Visscher L Gooijer C 《The journal of physical chemistry. A》2011,115(9):1493-1499
Isoflavones are known to show fluorescence with intensities that depend strongly on the solvent properties and exhibit Stokes' shifts as large as 1.4 eV. While some of this behavior can be explained by (excited state) deprotonation, this mechanism does not apply for all isoflavones. The aim of this study is to computationally and experimentally investigate the reasons for this anomalous behavior of neutral isoflavones, taking the daidzein molecule as a model compound. We find that the absence in fluorescence in aprotic solvents and the weak fluorescence in protic solvents can be explained by a change of order of the lowest singlet states in which a fluorescent charge-transfer state lies below the nonfluorescent locally excited state in water but not in acetonitrile. The large Stokes' shift is partly due to a significant rotation among the chromone-phenyl bond in the excited state. 相似文献