全文获取类型
收费全文 | 3350篇 |
免费 | 99篇 |
国内免费 | 194篇 |
专业分类
化学 | 1086篇 |
晶体学 | 2篇 |
力学 | 178篇 |
综合类 | 24篇 |
数学 | 1609篇 |
物理学 | 744篇 |
出版年
2024年 | 2篇 |
2023年 | 40篇 |
2022年 | 40篇 |
2021年 | 42篇 |
2020年 | 42篇 |
2019年 | 61篇 |
2018年 | 62篇 |
2017年 | 83篇 |
2016年 | 89篇 |
2015年 | 93篇 |
2014年 | 127篇 |
2013年 | 350篇 |
2012年 | 154篇 |
2011年 | 208篇 |
2010年 | 157篇 |
2009年 | 228篇 |
2008年 | 189篇 |
2007年 | 206篇 |
2006年 | 190篇 |
2005年 | 161篇 |
2004年 | 136篇 |
2003年 | 105篇 |
2002年 | 124篇 |
2001年 | 77篇 |
2000年 | 80篇 |
1999年 | 88篇 |
1998年 | 70篇 |
1997年 | 69篇 |
1996年 | 63篇 |
1995年 | 55篇 |
1994年 | 50篇 |
1993年 | 41篇 |
1992年 | 43篇 |
1991年 | 17篇 |
1990年 | 16篇 |
1989年 | 11篇 |
1988年 | 9篇 |
1987年 | 9篇 |
1986年 | 6篇 |
1985年 | 12篇 |
1984年 | 5篇 |
1983年 | 7篇 |
1982年 | 2篇 |
1981年 | 5篇 |
1980年 | 2篇 |
1979年 | 3篇 |
1978年 | 5篇 |
1977年 | 4篇 |
1976年 | 3篇 |
1975年 | 2篇 |
排序方式: 共有3643条查询结果,搜索用时 62 毫秒
91.
92.
Nobuyuki Akai Satoshi Kudoh Masao Takayanagi Munetaka Nakata 《Journal of photochemistry and photobiology. A, Chemistry》2002,150(1-3):93-100
Rotational isomerization of 2-chlorobenzaldehyde in low-temperature rare-gas matrices has been investigated by vibrational and electronic spectroscopies with aids of the density functional theory (DFT) and configuration interaction single (CIS) calculations. Infrared spectrum of the less stable O-cis isomer, produced from the more stable O-trans isomer upon UV irradiation, is measured with an FT-IR spectrophotometer. The enthalpy difference between the O-cis and O-trans isomers is estimated to be 9.7±0.2 kJ mol−1 from the temperature dependence of the infrared band intensities. Analyses of the infrared and electronic absorption spectral changes after UV irradiation and the phosphorescence spectra measured at various excitation wavelengths suggest that the rotational isomerization occurs via the intersystem crossing from S1 to T1. 相似文献
93.
The photodissociation of methyl iodide in various matrices at low temperature was studied. The observed Raman spectra excited by 514.5 nm laser radiation showed that there were two different photolytically produced iodine species isolated in the matrices after illumination by a medium pressure mercury lamp. One species which was dominant at lower iodine concentrations and exhibited a progression with an ωe of 201 cm?1, belonged to the matrix isolated iodine monomer (I2). The other species, which was dominant at higher iodine concentrations with an ωe of approximately 180 cm?1, belonged to the iodine aggregate ((I2)n). Five progressions of resonance Raman or resonance fluorescence of these two species were also observed in the other matrices. The iodine aggregate in the methyl iodide matrix at 77 K was formed in a crystalline structure, while the photolytically generated iodine aggregate from CH3I/Ar (2/3) matrix at 10 K, after illumination with a mercury lamp, was in amorphous form. The rearrangement of photolytically produced iodine aggregate in methyl iodide matrix was observed as a function of the duration of illumination. Local heating effects of the laser radiation might induce the iodine monomer to aggregate in matrices. The photodissociation mechanism of methyl iodide in matrices is also proposed. 相似文献
94.
95.
The volumetric properties of two reservoir fluid samples collected from one condensate gas well and one natural gas well were measured under four groups of temperatures, respectively, with pressure up to 116 MPa. For the two samples examined, the experimental results show that the gas compressibility factor increases with the increase of pressure. But the influence of the temperature is related to the range of the experimental pressure. It approximately decreases with the increase of temperature when the pressure is larger than (45 to 50) MPa, while there is the opposite trend when the pressure is lower than (45 to 50) MPa. The dew point pressure was also determined for the condensate gas sample, which decreases with the increase of temperature. The capabilities of four empirical correlations and a thermodynamic model based on equation of state for describing gas compressibility factor of reservoir fluids under high pressure were investigated. The comparison results show that the thermodynamic model recommended is the most suitable for fluids whatever produced from high-pressure reservoirs or conventional mild-pressure reservoirs. 相似文献
96.
The longitudinal motions and vertical accelerations of a floating torus as well as wave motion inside the torus are studied by model tests in regular deep-water waves. Comparisons are made with linear and partly with second-order potential-flow theory for the smallest examined experimental wave height-to-wave length ratio 1/120. Reasonable agreement is obtained, in particular for the linear problem. The importance of 3D flow, hydroelasticity and strong hydrodynamic frequency dependency is documented. Experimental precision errors and bias errors, for instance, due to tank-wall interference are discussed. Numerical errors due to viscous effects are found to be secondary. Experiments show that the third and fourth harmonic accelerations of the torus matter and cannot be explained by a perturbation method with the wave steepness as a small parameter. 相似文献
97.
The earlier reported matrix system methylacetate/ H2O/Ar has been reinvestigated by IR spectroscopy. The complex band pattern of the ester ν c=0 and the water ν3, ν1 and ν2 modes can be explained by the coexistence of 1:1, linear 1:2 and linear 2:1 H-bonded complexes. The study of methyl benzoate/H2O/Ar supports the proposed interpretation. 相似文献
98.
It is well-known that matrix effects in high performance liquid chromatography coupled to electrospray ionization mass spectrometry (HPLC-ESI-MS) can seriously compromise quantitative analysis and affect method reproducibility. Paired ion electrospray ionization (PIESI) mass spectrometry is an approach for analyzing ultra-low levels of anions in the positive ion mode. This approach uses a structurally optimized ion pairing reagent to post-column associate with the anionic analyte, subsequently forming positively charged complexes. These newly formed complex ions are often more surface-active as compared to either the native anion or the ion pairing reagent. No studies have examined whether or not the PIESI approach mitigates matrix effects. Consequently, a controlled study was done using five analytes in highly controlled and reproducible synthetic groundwater and urine matrices. In addition, two different mass spectrometers (linear ion trap and triple quadrupole) were used. Compared to the negative ion mode, the PIESI-MS approach was less susceptible to matrix effects when performed on two different MS platforms. Using PIESI-MS, less dilution of the sample is needed to eliminate ionization suppression which, in turn, permits lower limits of detection and quantitation. 相似文献
99.
《Optimization》2012,61(3):497-510
In this note, by making use of the concept of unbounded approximate Hessian matrices, we present second order optimality conditions for a constrained mathematical programming problem with data whose gradient map is continuous, but not necessarily locally Lipschitz. 相似文献
100.
《Comptes Rendus Mecanique》2014,342(12):671-691
The paper extends the use of the PGD method to viscoelastic evolution problems described by a large number of internal variables and with a large spectrum of relaxation times. The internal variables evolution is described by a set of linear differential equations that involve many time scales. The feasibility and the robustness of the method are discussed in the case of a polymer in a non-equilibrium state under creep and cyclic loading. The relationships between different time scales (loading and internal variables) are also discussed. 相似文献