全文获取类型
收费全文 | 1968篇 |
免费 | 70篇 |
国内免费 | 9篇 |
专业分类
化学 | 1430篇 |
晶体学 | 9篇 |
力学 | 44篇 |
数学 | 232篇 |
物理学 | 332篇 |
出版年
2023年 | 25篇 |
2022年 | 53篇 |
2021年 | 76篇 |
2020年 | 50篇 |
2019年 | 43篇 |
2018年 | 30篇 |
2017年 | 39篇 |
2016年 | 82篇 |
2015年 | 71篇 |
2014年 | 76篇 |
2013年 | 136篇 |
2012年 | 146篇 |
2011年 | 132篇 |
2010年 | 96篇 |
2009年 | 103篇 |
2008年 | 148篇 |
2007年 | 121篇 |
2006年 | 98篇 |
2005年 | 82篇 |
2004年 | 71篇 |
2003年 | 47篇 |
2002年 | 59篇 |
2001年 | 27篇 |
2000年 | 23篇 |
1999年 | 15篇 |
1998年 | 8篇 |
1997年 | 15篇 |
1996年 | 16篇 |
1995年 | 10篇 |
1994年 | 11篇 |
1993年 | 12篇 |
1992年 | 12篇 |
1991年 | 9篇 |
1990年 | 7篇 |
1989年 | 6篇 |
1988年 | 6篇 |
1987年 | 7篇 |
1986年 | 5篇 |
1985年 | 5篇 |
1984年 | 8篇 |
1983年 | 4篇 |
1982年 | 8篇 |
1981年 | 6篇 |
1980年 | 5篇 |
1979年 | 6篇 |
1978年 | 5篇 |
1977年 | 7篇 |
1974年 | 4篇 |
1962年 | 2篇 |
1937年 | 2篇 |
排序方式: 共有2047条查询结果,搜索用时 62 毫秒
941.
Felipe Augusto Ferrari Fernanda Lopes Motta Reinaldo Gaspar Bastos Maria Helena Andrade Santana 《Applied biochemistry and biotechnology》2013,170(6):1491-1502
Hyaluronic acid (HA) is a biopolymer with important applications in the pharmaceutical, medical, and cosmetic fields. This work explores the potentialities of a cylindrical polyurethane foam dowel with central aeration as a novel packed bed bioreactor for the production of HA. The goals were to provide a large surface area for oxygen transfer through the patches of liquid film that form in the pores of the foam in which cell proliferation and HA production occur and to easily recover the HA produced. The resulting yields of HA/cell were higher than 1, and the produced HA was completely recovered by pressing the foam. The external conditions that inhibit catabolism, the deviation of energetic metabolism toward the production of HA, were modulated by aeration and the initial glucose concentration. The production of HA was reproducible in 12 successive fermentation cycles. These findings contribute to the development of efficient strategies for the controlled production and recovery of HA. 相似文献
942.
In this short note, we develop the Stienstra–Beukers theory of supercongruences in the setting of the Catalan–Larcombe–French
sequence. We also give some applications to other sequences. 相似文献
943.
Domingos SR Silva PS Buma WJ Garcia MH Lopes NC Paixão JA Silva MR Woutersen S 《The Journal of chemical physics》2012,136(13):134501
We have observed large second-order nonlinear optical and vibrational circular dichroism (VCD) responses in a charge-transfer-type L-Histidinium salt. Using X-ray Diffraction, VCD spectroscopy, and time-dependent density functional theory to characterize the compound, we employ a two-level model to explain and quantify the strongly enhanced optical signals. We find that both linear and nonlinear optical responses are greatly enhanced by a single low-lying charge-transfer state. 相似文献
944.
de Souza Merli LA de Medeiros VP Toma L Reginato RD Katchburian E Nader HB Faloppa F 《Photochemistry and photobiology》2012,88(5):1293-1301
The low level laser therapy (LLLT) has been used as an option to accelerate the regeneration of bone tissue. In this study, both femurs of male Wistar rats (30 animals) were injured with a drill and the effect of LLLT using a laser diode (100 mW at 660 nm) in the bone matrix on the left paw measured. LLLT effect on the healing bone tissue matrix was evaluated by a combination of immunohistochemical histomorphometry, confocal immunofluorescence microscopy and isolation and characterization of glycosaminoglycans. Histomorphometric analysis showed that LLLT increased bone matrix and showing more organized. Alcian Blue and PAS staining seems to suggest differential glycosaminoglycans and glycoproteins. The data showed increased expression of chondroitin sulfate and hyaluronic acid, after reduction as the LLLT and mature bone, resembling the expression of osteonectin and biglycan. The difference in expression of siblings (DMP‐1, OPN and BSP) is in accordance with the repair accelerated bone formation after the application of LLLT as compared with control. The expression of osteonectin and osteocalcin supports their role in bone mineralization protein, indicating that LLLT accelerates this process. The overall data show that LLLT bone changes dynamic array, shortening the time period involved in the bone repair. 相似文献
945.
N-acetylgalactosamine kinase is a member of the GHMP family of small molecule kinases which catalyses the ATP-dependent phosphorylation
of N-acetylgalactosamine. It is highly similar in structure and sequence to galactokinase. Alteration of galactokinase at
a key tyrosine residue (Tyr-379 in the human enzyme) has been shown to dramatically enhance the substrate range of this enzyme.
Here, we investigated the substrate specificity of the wild type N-acetylgalactosamine kinase and demonstrated that it can
also catalyse the phosphorylation of N-acetylglucosamine and N-acetylmannosamine. In human N-acetylgalactosamine kinase, the
equivalent residue to Tyr-379 in galactokinase is Phe-444. Alteration of this residue did not result in dramatic changes to
the specificity of the enzyme. The more relaxed substrate specificity of N-acetylgalactosamine kinase, compared to galactokinase,
can be explained by the greater flexibility of a glycine rich loop in the active site of the enzyme. These results suggest
that N-acetylgalactosamine kinase is a potential biocatalyst for the phosphorylation of N-acetyl sugars. However, it is unlikely
that it will be possible to further broaden the substrate range by alteration of Phe-444. 相似文献
946.
Helena Nilsson Sylvain Galland Per Tomas Larsson E. Kristofer Gamstedt Tommy Iversen 《Cellulose (London, England)》2012,19(3):751-760
In this study, the importance of hemicellulose content and structure in chemical pulps on the property relationships in compression
molded wood pulp biocomposites is examined. Three different softwood pulps are compared; an acid sulfite dissolving grade
pulp with high cellulose purity, an acid sulfite paper grade pulp and a paper grade kraft pulp, the latter two both containing
higher amounts of hemicelluloses. Biocomposites based the acid sulfite pulps exhibit twice as high Young’s modulus as the
composite based on paper grade kraft pulp, 11–12 and 6 GPa, respectively, and the explanation is most likely the difference
in beating response of the pulps. Also the water retention value (WRV) is similarly low for the two molded sulfite pulps (0.5 g/g)
as compared to the molded kraft pulp (0.9 g/g). The carbohydrate composition is determined by neutral sugar analysis and average
molar masses by SEC. The cellulose supramolecular structure (cellulose fibril aggregation) is studied by solid state CP/MAS
13C-NMR and two forms of hemicellulose are assigned. During compression molding, cellulose fibril aggregation occurs to higher
extent in the acid sulfite pulps as compared to the kraft pulp. In conclusion, the most important observation from this study
is that the difference in hemicellulose content and structure seems to affect the aggregation behaviour and WRV of the investigated
biocomposites. 相似文献
947.
Lopes JG de Oliveira LF Santos PS 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2001,57(3):399-404
A more detailed investigation of the squaric acid aggregate within mordenite was undertaken with the use of Raman spectroscopy. The previous reported investigation was limited to the carbonyl stretching region in the IR. In the present work the entire region from 500 to 2000 cm(-1) was investigated, revealing rather substantial vibrational shifts of the oxocarbon ring modes in the aggregate. Comparison of such shifts with those observed for the squaric acid (H2Sq)/4,4'-bipyridine (Bipy) charge transfer (CT) complex reveals that the interaction is much stronger in the aggregate, a clear effect of the restrict geometry. On the other hand, the shifts observed for the CO stretching modes are rather modest. The comparison of the ring modes present in the Raman spectra of squaric acid, potassium hydrogen squarate, potassium squarate, H2Sq/Bipy and squaric acid aggregate in mordenite strongly suggests that in the latter hydrogen bonded species are present. 相似文献
948.
Oualid Talhi Guido R. Lopes Sérgio M. Santos Diana C. G. A. Pinto Artur M. S. Silva 《Journal of Physical Organic Chemistry》2014,27(9):756-763
The diastereoselective photodependent isomerization equilibrium of E/Z‐1,3‐ditolyl‐5‐[3‐oxobenzofuran‐2(3H)‐ylidene]imidazolidine‐2,4‐dione ( 5 ) is reported. Both diastereomers E-5 and Z-5 are stereochemically stable in solid state but show significant photosensibility in solutions of halogenated solvent. The photoisomerization equilibrium of E/Z‐ 5 is therefore deduced from the 1H NMR profile after visible‐light irradiation of both E-5 and Z-5 samples. The results of the kinetic study, monitored by UV‐HPLC, reveal that the E/Z equilibrium is diastereoselective and photodependent, being the transformation E ? Z proceeding faster than that of Z ? E, and the E/Z ratio at the equilibrium depends on the used solvent, light source, and temperature. Both diastereomers are visible‐light photosensitive tending to coexist together in equilibrium solutions at a determined ratio, which is always in favor of the Z‐product assuming a minimum thermodynamic energy and an increased entropy of the system. Time‐dependent density functional theory calculations suggest that the photoisomerization mechanism proceeds via a conical intersection involving the first‐excited state: Upon irradiation, the E-5 isomer is excited to the S1 potential energy surface, where it relaxes through rotation of the C=C bond and reaches a conical intersection with the ground‐state potential energy surface, thus yielding the Z-5 isomer. Copyright © 2014 John Wiley & Sons, Ltd. 相似文献
949.
Helena M. Lovick 《Tetrahedron letters》2009,50(9):1016-270
A series of chiral guanidines were synthesized and shown to efficiently catalyze the aza-Henry reaction. Modifications of the catalyst structure revealed important selectivity trends as well as an intriguing reversal in stereoselectivity with bisguanidine variants. These compounds were applied to the aza-Henry reaction between N-Boc imines and nitroalkanes generating the β-nitroamines in up to 77% ee and up to 20:1 diastereoselectivity. 相似文献
950.
We present the dynamics of a series of three paramagnetic molecules of different volume, mass, and shape in amorphous glass‐forming polymer poly(isobutylene) (PIB) as investigated by means of electron spin resonance (ESR) technique. The reorientation behavior of spin probes is related to the ortho‐positronium (o‐Ps) annihilation in PIB from positron annihilation lifetime spectroscopy (PALS) and the extracted free volume information. It is also related to the dynamic data of PIB from broadband dielectric spectroscopy (BDS), neutron scattering (NS), and nuclear magnetic resonance (NMR) spectroscopy from literature. In the case of the smallest spin probe, 2,2,6,6‐tetramethyl‐1‐piperidinyloxy (TEMPO), a discontinuous course of the spectral parameter 2Azz′ versus T dependence was observed and the subsequent phenomenological model‐free analyses of the spectral parameter, 2Azz′ versus T, as well as of the correlation time, τc, versus 1/T plots provided the characteristic ESR temperatures ( , T50G, ) and (T, T, T). These characteristic ESR temperatures were found to be consistent with the characteristic PALS temperatures: T, T = T from temperature dependences of the mean o‐Ps lifetime, τ3, or the width of o‐Ps lifetime distribution, σ3, respectively. In addition, the relationships between the spin probe size, V, and the free volume hole size distributions gn(Vh) at the characteristic ESR temperatures indicate the significant influence of the free volume fluctuation at the crossover from slow to rapid regime as well as within the rapid motional regime. On the other hand, the two larger spin probes exhibit a rather continuous 2Azz′–T plots with the respective T50G's lying in the vicinity of T independently of their volume, mass and shape, suggesting the common origin of underlying process controlling this T50G transition. Finally, these mutual PALS and ESR findings were compared with the known dynamic behavior of PIB which suggest that the dynamics of the TEMPO and the larger spin probes are related to free volume fluctuation associated with primary α ‐ and secondary β processes, respectively. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1058–1068, 2009 相似文献