全文获取类型
收费全文 | 2451篇 |
免费 | 80篇 |
国内免费 | 1篇 |
专业分类
化学 | 1455篇 |
晶体学 | 27篇 |
力学 | 40篇 |
数学 | 362篇 |
物理学 | 648篇 |
出版年
2022年 | 48篇 |
2021年 | 72篇 |
2020年 | 28篇 |
2019年 | 47篇 |
2018年 | 69篇 |
2017年 | 50篇 |
2016年 | 150篇 |
2015年 | 92篇 |
2014年 | 108篇 |
2013年 | 249篇 |
2012年 | 112篇 |
2011年 | 138篇 |
2010年 | 100篇 |
2009年 | 99篇 |
2008年 | 119篇 |
2007年 | 85篇 |
2006年 | 87篇 |
2005年 | 65篇 |
2004年 | 71篇 |
2003年 | 63篇 |
2002年 | 41篇 |
2001年 | 42篇 |
2000年 | 31篇 |
1999年 | 27篇 |
1998年 | 24篇 |
1997年 | 28篇 |
1996年 | 22篇 |
1995年 | 26篇 |
1994年 | 14篇 |
1993年 | 14篇 |
1992年 | 15篇 |
1991年 | 21篇 |
1990年 | 21篇 |
1989年 | 20篇 |
1988年 | 13篇 |
1987年 | 26篇 |
1986年 | 17篇 |
1985年 | 23篇 |
1984年 | 36篇 |
1983年 | 13篇 |
1982年 | 21篇 |
1981年 | 20篇 |
1980年 | 25篇 |
1979年 | 18篇 |
1978年 | 14篇 |
1977年 | 16篇 |
1975年 | 14篇 |
1974年 | 9篇 |
1973年 | 9篇 |
1972年 | 8篇 |
排序方式: 共有2532条查询结果,搜索用时 997 毫秒
971.
Lidia?Najder-Kozdrowska Barbara?Pilawa Andrzej?B.?Wi?ckowski Ewa?Buszman Dorota?Wrze?niok 《Applied magnetic resonance》2009,36(1):81-88
DOPA–melanin (DOPA = 3,4-dihydroxyphenyl-alanine) complexes with Cu(II) cations were studied by electron paramagnetic resonance
(EPR) spectroscopy. After the addition of metal cations to the melanin polymer, the EPR spectra parameters were measured.
The axial g-factor values are g
|| = 2.20 and g ^ = 2.0 5 g_{ \bot } = 2.0 5 . Since $ g_{||} > g_{ \bot } > 2 $ g_{||} > g_{ \bot } > 2 , the ground state orbital of the unpaired electron of the Cu(II) complex is | x 2 - y 2
ñ \left| {{{x}}^{ 2} } \right. \, - \, \left. {{{y}}^{ 2} } \right\rangle . Square coordination of these cations in melanin is proposed. The influence of the Cu(II) concentration in solution used
during synthesis on the concentration of melanin radicals was investigated. Cu(II) cations causes the decrease in the paramagnetic
center concentration in melanin. The Cu(II) EPR spectra are not saturated within the microwave power range of 0.7–70 mW used
in our experiment. 相似文献
972.
Płuska M Czerwinski A Ratajczak J Katcki J Oskwarek L Rak R 《Micron (Oxford, England : 1993)》2009,40(1):46-50
The electron-microscope image distortion generated by electromagnetic interference (EMI) is an important problem for accurate imaging in scanning electron microscopy (SEM). Available commercial solutions to this problem utilize sophisticated hardware for EMI detection and compensation. Their efficiency depends on the complexity of distortions influence on SEM system. Selection of a proper method for reduction of the distortions is crucial. The current investigations allowed for a separation of the distortions impact on several components of SEM system. A sum of signals from distortion sources causes wavy deformations of specimen shapes in SEM images. The separation of various reasons of the distortion is based on measurements of the periodic deformations of the images for different electron beam energies and working distances between the microscope final aperture and the specimen. Using the SEM images, a direct influence of alternating magnetic field on the electron beam was distinguished. Distortions of electric signals in the scanning block of SEM were also separated. The presented method separates the direct magnetic field influence on the electron beam below the SEM final aperture (in the chamber) from its influence above this aperture (in the electron column). It also allows for the measurement of magnetic field present inside the SEM chamber. The current investigations gave practical guidelines for selecting the most efficient solution for reduction of the distortions. 相似文献
973.
Octadecyl (C18) groups are arguably the most popular ligands used for preparation of solid phase microextraction (SPME) devices. However, conventional C18-bonded silica particles are not fully compatible with the nearly 100% aqueous composition of typical biological samples (e.g., plasma, saliva, or urine). This study presents the first evaluation of thin-film SPME devices coated with special water-compatible C18-bonded particles. Device performance was assessed by extracting a mixture of 30 model compounds that exhibited various chemical structures and properties, such as hydrophobicity. Additionally, nine unique compositions of desorption solvents were tested. Thin-film SPME devices coated with C18-bonded silica particles with polar end-capping groups (10 µm) were compared with conventional trimethylsilane end-capped C18-bonded silica particles of various sizes (5, 10, and 45 µm) and characteristics. Polar end-capped particles provided the best extraction efficacy and were characterized by the strongest correlations between the efficacy of the extraction process and the hydrophobicity of the analytes. The results suggest that the original features of octadecyl ligands are best preserved in aqueous conditions by polar end-capped particles, unlike with conventional trimethylsilane end-capped particles that are currently used to prepare SPME devices. The benefits associated with this improved type of coating encourage further implementation of microextractraction as greener alternative to the traditional sample preparation methods. 相似文献
974.
Four new water-soluble derivatives of dibenzotetraaza[14]annulene have been synthesized, bearing meso substituents with different structures and dimensions: 3-(N,N,N-trimethylammonium)propyl, 3-(N-pyridinium-1-yl)propyl, 2-[3-(N,N,N-trimethylammonium)propoxy]benzoyl, and 2-[3-(N-pyridinium-1-yl)propoxy]benzoyl. The crystal structures of 3-(trimethylammonium)propyl and (N,N,N-trimethylammonium)propoxy]benzoyl derivatives were determined by single crystal X-ray analysis. According to the UV-vis titrations, thermal denaturation experiments, and ethidium bromide displacement assays, all compounds presented here interact strongly with double stranded (ct)-DNA. The product equipped with 3-(trimethylammonium)propyl pendant groups and two positive charges interacts with DNA in one dominant binding mode, whereas the other three derivatives revealed more complex mixed-type interactions. The results have been discussed in terms of dimensions, geometry, and electronic properties of the evaluated compounds, on the basis of corresponding crystallographic data. 相似文献
975.
The syntheses of positively and zwitterionically charged subphthalocyanines (SubPcs) are described for the first time. The SubPcs contain alkylated pyridinium substituents located either at the peripheral or at the axial positions of the macrocycle. The compounds were shown to be fairly water soluble. 相似文献
976.
Mateusz Kwaśnicki 《Potential Analysis》2009,31(1):57-77
We study the intrinsic ultracontractivity of the semigroup of the isotropic stable process on an unbounded open set. A two-sided
estimate of the first eigenfunction of the semigroup is derived. We obtain general geometric conditions for intrinsic ultracontractivity
which yield a necessary and sufficient condition for non-degenerate horn-shaped sets.
The research was supported by KBN grant 1 P03A 020 28. 相似文献
977.
Only two computer-controlled microsolenoid devices, namely two micropumps or one micropump and one microvalve, are sufficient for the construction of on-line dilution modules useful in several flow analytical systems for the calibration using single standard. Three simple constructions of such modules were tested and compared. The most promising is the one based on the concept of a microvalve controlling dilution ratio of the standard and a solenoid micropump playing a double role: solution pumping device and mixing segments homogenizer. All investigated modules were tested with paired emitter detector diode (PEDD) as photometric flow-through detector and bromothymol blue as a model analyte. The best module was implemented into more advanced flow-injection system dedicated for optical detection of alkaline phosphatase activity using UV-PEDD-based flow-through detector for the enzyme reaction product. 相似文献
978.
979.
Vapour–liquid equilibrium measurements for binary and ternary (carbon dioxide + β-myrcene and carbon dioxide + β-myrcene + hydrogen) systems have been carried out at 323.15 K and pressures in the range from 7 MPa to the critical pressure of the binary mixture and at pressures from 10 to 14 MPa for the investigated ternary systems. Samples from the coexisting phases were taken, and compositions were determined experimentally. Results were correlated using the Peng–Robinson and the Soave–Redlich–Kwong equations of state with the Mathias–Klotz–Prausnitz mixing rule. The set of interaction parameters for the employed equations of state and applied mixing rule for the system of CO2 + β-myrcene and of CO2 + β-myrcene + H2 were obtained. Additionally, the volume expansion of the liquid phase for the binary mixtures (carbon dioxide + β-myrcene and carbon dioxide + limonene) were measured at 323.15 K and at pressures from 4 MPa up to very close to the critical pressure of the mixture. The ratio of liquid phase total volumes at the given pressure and at 4 MPa was calculated. 相似文献
980.
Komsta Łukasz Sztanke Krzysztof Mączka Paulina Ucherek Marzena Skibiński Robert Gumieniczek Anna 《平面色谱法杂志一现代薄层色谱法》2009,22(5):327-331
JPC – Journal of Planar Chromatography – Modern TLC - The lipophilicity, log P, of 27 novel imidazo[2,1-c][1,2,4]triazine derivatives with strong biological activity has been determined... 相似文献