全文获取类型
收费全文 | 4918篇 |
免费 | 156篇 |
国内免费 | 12篇 |
专业分类
化学 | 3150篇 |
晶体学 | 33篇 |
力学 | 125篇 |
综合类 | 5篇 |
数学 | 560篇 |
物理学 | 1213篇 |
出版年
2024年 | 7篇 |
2023年 | 40篇 |
2022年 | 159篇 |
2021年 | 146篇 |
2020年 | 119篇 |
2019年 | 140篇 |
2018年 | 143篇 |
2017年 | 106篇 |
2016年 | 204篇 |
2015年 | 132篇 |
2014年 | 175篇 |
2013年 | 302篇 |
2012年 | 313篇 |
2011年 | 360篇 |
2010年 | 264篇 |
2009年 | 257篇 |
2008年 | 300篇 |
2007年 | 283篇 |
2006年 | 229篇 |
2005年 | 180篇 |
2004年 | 151篇 |
2003年 | 128篇 |
2002年 | 115篇 |
2001年 | 97篇 |
2000年 | 82篇 |
1999年 | 43篇 |
1998年 | 49篇 |
1997年 | 40篇 |
1996年 | 40篇 |
1995年 | 23篇 |
1994年 | 50篇 |
1993年 | 46篇 |
1992年 | 48篇 |
1991年 | 28篇 |
1990年 | 16篇 |
1989年 | 13篇 |
1988年 | 18篇 |
1987年 | 14篇 |
1986年 | 14篇 |
1985年 | 12篇 |
1984年 | 14篇 |
1983年 | 20篇 |
1982年 | 16篇 |
1981年 | 22篇 |
1980年 | 18篇 |
1979年 | 24篇 |
1978年 | 15篇 |
1977年 | 8篇 |
1976年 | 9篇 |
1973年 | 6篇 |
排序方式: 共有5086条查询结果,搜索用时 0 毫秒
101.
We expose and discussPenrose's thesis: Nature produces harnessable noncomputable processes, but none at the classical level. We then suggest a partial counterexample to it, based on aGedanken experiment about an undecidable family of integrable Hamiltonian systems that could lead to a sort of idealized solution to the Halting problem for Turing machines.1. Partially supported by a Fulbright/CNPq-Brazil grant. Permanent address after November 5, 1990: Center for the Study of Mathematical Theories of Communication, IDEA/School of Communications, Federal University of Rio de Janeiro, Av. Pasteur 250, 22290 Rio de Janeiro, RJ Brazil. 相似文献
102.
The complexes [Ru(salen)(NO)Cl] and [Ru(salen)(NO)(H(2)O)](+) were shown to release the nitrosyl ligand as nitric oxide upon exposure to visible light in organic and aqueous solutions respectively, by means of UV-visible, EPR, and FTIR spectroscopies. The former was prepared by a new synthetic route and had its structure determined by single-crystal X-ray diffraction. A crystal of the dichloromethane solvate is orthorhombic, space group Fdd2 (No. 43) and formula C(16)H(14)ClN(3)O(3)Ru.CH(2)Cl(2), with Z = 16 and cell parameters a = 25.489(4), b = 33.435(4), and c = 9.3716(9) A. The electronic absorption spectra of the complexes were calculated using the INDO/S method. The water-soluble complex is a potential drug for antitumoral phototreatment. 相似文献
103.
Rangel Elidiane C. Cruz Nilson C. da Kayama Milton E. Rangel Rita C. C. Marins Nazir Durrant Steven F. 《Plasmas and Polymers》2004,9(1):1-22
Diamond-like carbon (DLC) films were grown from radiofrequency plasmas of acetylene-argon mixtures, at different excitation powers, P. The effects of this parameter on the plasma potential, electron density, electron temperature, and plasma activity were investigated using a Langmuir probe. The mean electron temperature increased from about 0.5 to about 7.0 eV while the mean electron density decreased from about 1.2 × 109 to about 0.2 × 109 cm–3 as P was increased from 25 to 150 W. Both the plasma potential and the plasma activity were found to increase with increasing P. Through actinometric optical emission spectrometry, the relative concentrations of CH, [CH], and H, [H], in the discharge were mapped as a function of the applied power. A rise in [H] and a fall in [CH] with increasing P were observed and are discussed in relation to the plasma characteristics and the subimplantation model. The optical properties of the films were calculated from ultraviolet-visible spectroscopic data; the surface resistivity was measured by the two-point probe method. The optical gap, E
G, and the surface resistivity, s, fall with increasing P. E
G and s are in the ranges of about 2.0–1.3 eV and 1014–1016 /, respectively. The plasma power also influences the film self-bias, V
b, via a linear dependence, and the effect of V
b on ion bombardment during growth is addressed together with variation in the relative densities of sp2 and sp3 bonds in the films as determined by Raman spectroscopy. 相似文献
104.
Ana Serbanovic Manuel Nunes da Ponte Carlos A.M. Afonso 《Journal of organometallic chemistry》2005,690(15):3600-3608
In this work, osmium-catalyzed asymmetric dihydroxylation (AD) of methyl trans-cinnamate was studied. Osmium and chiral ligand catalysts were immobilized in ionic liquid only, without any other reaction solvents, while the recovery of the product was performed by extraction with supercritical CO2, and compared with results obtained by extractions with organic solvents such as hexane and diethyl ether. In supercritical CO2 extraction experiments, optimal extraction pressure was found and ionic liquid chosen, so that the highest reaction yields coupled with lowest osmium content in the crude product can be achieved. Finally, recycle experiments of the same (ionic liquid + catalytic system) mixture were successfully conducted. Application of ionic liquids and supercritical CO2 in osmium catalyzed AD allows for the isolation of the diol basically without contamination with osmium, in high yield and enantiomeric excess, and it makes possible the efficient reuse of ionic liquid solvent and the catalytic system. 相似文献
105.
The morphologically undifferentiated cells of nonregenerant callous tissue of Cereus peruvianus cultured in the original medium and in medium supplemented with tyrosine were used as an alkaloid source. Comparison of alkaloid
production by C. peruvianus plants and by callous tissues indicated that alkaloid levels were almost twice as high in callous tissues as in shoots of
C. peruvianus plants. The ratio of alkaloid concentration between mature plant and morphologically und ifferentiated cells of callous tissue
was 1∶1.7. A relationship between culture medium containing tyrosine and alkaloid production was also observed in the callous
tissues of C. peruvianus. Since increased alkaloid production may be induced by additional factors such as tyrosine, increasing levels of tyrosine
or other conditions of the culture medium may be considered factors for inducing higher alkaloid production by C. peruvianus callous tissues. 相似文献
106.
Ana Luiza Coeli Cruz Ramos Laiza Andrade Nogueira Mauro Ramalho Silva Ana Carolina do Carmo Mazzinghy Ana Paula Xavier Mariano Tssia Nunes de Albuquerque Rodrigues Ana Cardoso Clemente Filha Ferreira de Paula Angelita Cristine de Melo Rodinei Augusti Raquel Linhares Bello de Araújo Inayara Cristina Alves Lacerda Júlio Onsio Ferreira Melo 《Molecules (Basel, Switzerland)》2022,27(15)
Grumixama (Eugenia brasiliensis Lam.) is a native fruit of the Brazilian Atlantic Forest, belonging to the Myrtaceae family, which designatesthe most significant number of species with food potential. It stands out due to its phytochemical characteristics because of the presence of polyphenols and volatile organic compounds. Volatile compounds are substances released by foods that give off an aroma and influence flavor. Solid-phase microextraction is a technique that allows for low-cost, fast, and solvent-free extraction, has an affinity for numerous analytes, and is easily coupled to gas chromatography. The objectives of this work were to evaluate the efficiency of different fibers of SPME (solid-phase microextraction) in the extraction of volatile organic compounds from grumixama pulp; optimize a method for extraction time, temperature, and sample weight; and to determine the characteristic volatile profile of this fruit. For the extraction of volatile compounds, three fibers of different polarities were used: polar polyacrylate (PA) fibers, divinylbenzene/carboxyne/polydimethylsiloxane (DVB/CAR/PDMS) semipolar fibers, and polydimethylsiloxane/divinylbenzene (PDMS/DVB). Fourteen volatile organic compounds (VOCs) were identified by DVB/CAR/PDMS, six by PA, and seven by PDMS/DVB through solid-phase microextraction in the headspace mode (SPME-HS). Considering the total number of compounds identified, regardless of the fiber used, and the optimization of the method, Eugenia brasiliensis presented sesquiterpene fractions (85.7%, 83.3%, and 85.7% of total VOCs) higher than the monoterpene fractions (14.3%, 16.7%, and 14.3%) for DVB/CAR/PDMS, PA, and PDMS/DVB, respectively in its composition. In addition, it was possible to verify that the fiber DVB/CAR/PDMS presented a better efficiency due to the larger chromatographic area observed when the grumixama pulp was subjected to conditions of 75 °C, 2.0 g, and an adsorption time of 20 min. 相似文献
107.
Ana Ruas Angelica Graa Joana Marto Lídia Gonalves Ana Oliveira Alexandra Nogueira da Silva Madalena Pimentel Artur Mendes Moura Ana Teresa Serra Ana Cristina Figueiredo Helena M. Ribeiro 《Molecules (Basel, Switzerland)》2022,27(11)
Essential oils (EOs) and hydrolates (Hds) are natural sources of biologically active ingredients with broad applications in the cosmetic industry. In this study, nationally produced (mainland Portugal and Azores archipelago) EOs (11) and Hds (7) obtained from forest logging and thinning of Eucalyptus globulus, Pinus pinaster, Pinus pinea and Cryptomeria japonica, were chemically evaluated, and their bioactivity and sensorial properties were assessed. EOs and Hd volatiles (HdVs) were analyzed by GC-FID and GC-MS. 1,8-Cineole was dominant in E. globulus EOs and HdVs, and α- and β-pinene in P. pinaster EOs. Limonene and α-pinene led in P. pinea and C. japonica EOs, respectively. P. pinaster and C. japonica HVs were dominated by α-terpineol and terpinen-4-ol, respectively. The antioxidant activity was determined by DPPH, ORAC and ROS. C. japonica EO showed the highest antioxidant activity, whereas one of the E. globulus EOs showed the lowest. Antimicrobial activity results revealed different levels of efficacy for Eucalyptus and Pinus EOs while C. japonica EO showed no antimicrobial activity against the selected strains. The perception and applicability of emulsions with 0.5% of EOs were evaluated through an in vivo sensory study. C. japonica emulsion, which has a fresh and earthy odour, was chosen as the most pleasant fragrance (60%), followed by P. pinea emulsion (53%). In summary, some of the studied EOs and Hds showed antioxidant and antimicrobial activities and they are possible candidates to address the consumers demand for more sustainable and responsibly sourced ingredients. 相似文献
108.
A. L. Stepanov C. Marques E. Alves R. C. da Silva M. R. Silva R. A. Ganeev A. I. Ryasnyansky T. Usmanov 《Technical Physics》2006,51(11):1474-1481
Single-crystal Al2O3(0001) and Al2O3(1120) substrates are implanted by 160-keV Au+ ions with doses from 1015 to 1017 cm?2. Some of the implanted samples are air-annealed at 800–1200°C. The properties of the synthesized composite layers are studied by Rutherford backscattering and linear optical reflection measurements, and their nonlinear optical characteristics are examined by RZ-scanning using a picosecond Nd: YAG laser operating at a wavelength of 1064 nm. The Rutherford backscattering spectra indicate that the implanted impurity concentrates near the surface of the Al2O3. The formation of gold nanoparticles in the Al2O3 can be judged from the characteristic optical plasmon resonance band in the reflectance spectra of the samples irradiated to a dose higher than 6.0 × 1016 cm?2. The synthesized particles are shown to be responsible for nonlinear optical refraction in the samples. The nonlinear refractive index, n 2, and the real part of the third-order susceptibility, Rex(3), of the composite layers are determined. 相似文献
109.
110.