全文获取类型
收费全文 | 7208篇 |
免费 | 245篇 |
国内免费 | 23篇 |
专业分类
化学 | 4743篇 |
晶体学 | 43篇 |
力学 | 230篇 |
综合类 | 2篇 |
数学 | 926篇 |
物理学 | 1532篇 |
出版年
2023年 | 54篇 |
2022年 | 178篇 |
2021年 | 199篇 |
2020年 | 165篇 |
2019年 | 174篇 |
2018年 | 183篇 |
2017年 | 177篇 |
2016年 | 318篇 |
2015年 | 221篇 |
2014年 | 304篇 |
2013年 | 479篇 |
2012年 | 525篇 |
2011年 | 587篇 |
2010年 | 348篇 |
2009年 | 299篇 |
2008年 | 457篇 |
2007年 | 413篇 |
2006年 | 336篇 |
2005年 | 281篇 |
2004年 | 258篇 |
2003年 | 229篇 |
2002年 | 170篇 |
2001年 | 122篇 |
2000年 | 89篇 |
1999年 | 87篇 |
1998年 | 55篇 |
1997年 | 44篇 |
1996年 | 54篇 |
1995年 | 41篇 |
1994年 | 58篇 |
1993年 | 55篇 |
1992年 | 46篇 |
1991年 | 43篇 |
1990年 | 26篇 |
1989年 | 34篇 |
1988年 | 23篇 |
1987年 | 25篇 |
1986年 | 17篇 |
1985年 | 32篇 |
1984年 | 33篇 |
1983年 | 17篇 |
1982年 | 24篇 |
1981年 | 18篇 |
1980年 | 31篇 |
1979年 | 16篇 |
1978年 | 14篇 |
1977年 | 17篇 |
1976年 | 22篇 |
1975年 | 15篇 |
1974年 | 14篇 |
排序方式: 共有7476条查询结果,搜索用时 12 毫秒
991.
Jang Hyun Jo 《代数通讯》2013,41(5):1577-1587
In case G is a finite group, there is a well-known criterion for projective modules: A ? G-module M is projective if and only if it is ? -free and has finite projective dimension. We first investigate whether only finite groups satisfy the above criterion. In the class of groups L H 𝔉, we conclude that this is true. Secondly, we consider the problem when a stably flat Γ-module is projective, where Γ is an arbitrary group. We show that if Γ is an L H 𝔉-group, then every stably flat cofibrant ? Γ-module is projective. 相似文献
992.
Leitão JM Esteves da Silva JC Girón AJ Muñoz de la Peña A 《Journal of fluorescence》2008,18(6):1065-1076
Excitation emission fluorescence matrices (EEMs) of Verapamil drug were obtained by direct and by derivatization fluorescence
spectroscopy. The fluorescence excitation and emission wavelengths were displaced to longer wavelengths and the fluorescence
intensity was enhanced upon derivation with respect to the native fluorescence of the drug. The complete EEM of the native
fluorescence of the drug and of the derivatization product were rapidly acquired by using a charged-coupled device detector
(CCD), which is advantageous in terms of speed in the analysis, with respect to the use of a conventional photomultiplier
detector. The EEMs were analyzed by several second-order multivariate calibration methods exploiting the second order advantage.
The three-dimensional decomposition methods used, based in different assumptions about the trilinearity of the three way data
structure under analysis, were parallel factor analysis (PARAFAC), bilinear least squares (BLLS), parallel factor analysis
2 (PARAFAC2) and multivariate curve resolution—alternating least squares (MCR-ALS). The determination was performed by using
the standard addition approach. The figures of merit of the PARAFAC and BLLS methods were calculated, obtaining a lower limit
of detection with the derivatization procedure, when compared with the direct measurement of the fluorescence of the drug.
In Verapamil drug the best estimations were found with the BLLS and the MCR-ALS models. In the quantification of Verapamil
in a pharmaceutical formulation the best estimation, when compared with the result obtained by the US Pharmacopeia high performance
liquid chromatography approach, was obtained by direct fluorescence spectroscopy with MCR-ALS and by derivatization fluorescence
spectroscopy with the PARAFAC2 model. 相似文献
993.
994.
In the phase separation occurring at the miscibility gap (at the spinodal region) of an alloy a discrete symmetry is spontaneously broken and a domain wall network is formed. Field theory simulations are often used to study the dynamics of topological defects networks appearing in different physical contexts. In this work, we focus on the dynamics of the two immiscible liquids appearing on the phase diagram of the Bi–Zn system, one of the basic systems of lead free solders. We use phase field simulations to quantitatively simulate the dynamics of the two liquids separation in the Bi–Zn system, at different temperatures and for different concentrations. We obtain the miscibility gap curve and the domain morphologies of the system as a function of time, temperature and component concentrations using simulations. 相似文献
995.
P. M.C. de Oliveira S. Moss de Oliveira D. Stauffer S. Cebrat A. Pękalski 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,63(2):245-254
We discovered a dynamic phase transition induced by sexual
reproduction. The dynamics is a pure Darwinian rule applied to diploid
bit-strings with both fundamental ingredients to drive Darwin's
evolution: (1) random mutations and crossings which act in the sense of
increasing the entropy (or diversity); and (2) selection which acts in
the opposite sense by limiting the entropy explosion. Selection wins
this competition if mutations performed at birth are few enough, and
thus the wild genotype dominates the steady-state population. By slowly
increasing the average number m of mutations, however, the
population suddenly undergoes a mutational degradation precisely at a
transition point mc. Above this point, the “bad”
alleles (represented by 1-bits) spread over the genetic pool of the
population, overcoming the selection pressure. Individuals become
selectively alike, and evolution stops. Only below this point, m < mc, evolutionary life is possible.
The finite-size-scaling behaviour of this transition is
exhibited for large enough “chromosome” lengths L, through
lengthy computer simulations. One important and surprising observation
is the L-independence of the transition curves, for large L.
They are also independent on the population size. Another is that mc is near unity, i.e. life cannot be stable with much more
than one mutation per diploid genome, independent of the chromosome
length, in agreement with reality. One possible consequence is that an
eventual evolutionary jump towards larger L enabling the storage
of more genetic information would demand an improved DNA copying
machinery in order to keep the same total number of mutations per
offspring. 相似文献
996.
D. O. Soares-Pinto I. S. Oliveira M. S. Reis 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,62(3):337-340
In this work we report Monte Carlo simulations of a 2D
Ising model, in which the statistics of the Metropolis algorithm
is replaced by the nonextensive one. We compute the magnetization
and show that phase transitions are present for q ≠ 1. A
q - phase diagram (critical temperature vs. the entropic
parameter q) is built and exhibits some interesting features,
such as phases which are governed by the value of the entropic
index q. It is shown that such phases favors some energy levels
of magnetization states. It is also shown that the contribution
of the Tsallis cutoff is capital to the existence of phase
transitions. 相似文献
997.
Marco A. S. Oliveira Itamar Borges Jr 《International journal of quantum chemistry》2019,119(8):e25868
Nitramine explosives can combine relative insensitivity to initiation and great energy content. In this work, based on a previous approach developed for nitroaromatic explosives, we propose four mathematical models to correlate impact sensitivity, given by the h50 value, to molecular charge properties. Fourteen cyclic nitramines were studied using Density Functional Theory (DFT). Six molecules of the set have measured h50 values, which were used to evaluate the sensitivity models. Converged DFT charge densities of the molecules were partitioned and analyzed according to the distributed multipole analysis (DMA) atom-centered method. The sensitivity models were based on the DMA electric multipole values. The electron withdrawing role of the nitro group and the strong polarization of the charges of the nitrogen atom in the amine group were clearly identified. The influence of the electronic properties on the sensitivity of the explosives was characterized by including in the sensitivity models the charge values of the nitro or the nitramine groups and electron delocalization, the latter quantified by the DMA quadrupole values of the ring atoms. Inclusion of electron delocalization effects can improve the prediction of h50 values for two out of the five strained-ring nitramines in the set. The charge values of the nitramine groups are the most important molecular property affecting the impact sensitivity. The h50 values of eight nitramine explosives of the set not available experimentally were computed. 相似文献
998.
999.
Mélanie Fonte Natália Fagundes Ana Gomes Ricardo Ferraz Cristina Prudêncio Maria João Araújo Paula Gomes Cátia Teixeira 《Tetrahedron letters》2019,60(17):1166-1169
A multi-step synthetic route towards N4,N9-disubstituted 4,9-diaminoacridines that, to the best of our knowledge, has no precedence in the literature, has been developed. The target structures are likely to reveal interesting biological activities in the near future, not only due to their mepacrine-like core, but also because they embed simultaneously the pharmacophores of chloroquine and primaquine, antimalarial drugs that act at different stages of malaria infection. 相似文献
1000.
Melanie Miller William E. Robinson Ana Rita Oliveira Nina Heidary Nikolay Kornienko Julien Warnan Inês A. C. Pereira Erwin Reisner 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(14):4649-4653
The integration of enzymes with synthetic materials allows efficient electrocatalysis and production of solar fuels. Here, we couple formate dehydrogenase ( FDH ) from Desulfovibrio vulgaris Hildenborough (DvH) to metal oxides for catalytic CO2 reduction and report an in‐depth study of the resulting enzyme–material interface. Protein film voltammetry (PFV) demonstrates the stable binding of FDH on metal‐oxide electrodes and reveals the reversible and selective reduction of CO2 to formate. Quartz crystal microbalance (QCM) and attenuated total reflection infrared (ATR‐IR) spectroscopy confirm a high binding affinity for FDH to the TiO2 surface. Adsorption of FDH on dye‐sensitized TiO2 allows for visible‐light‐driven CO2 reduction to formate in the absence of a soluble redox mediator with a turnover frequency (TOF) of 11±1 s?1. The strong coupling of the enzyme to the semiconductor gives rise to a new benchmark in the selective photoreduction of aqueous CO2 to formate. 相似文献