全文获取类型
收费全文 | 4455篇 |
免费 | 334篇 |
国内免费 | 300篇 |
专业分类
化学 | 687篇 |
晶体学 | 19篇 |
力学 | 156篇 |
综合类 | 61篇 |
数学 | 386篇 |
物理学 | 707篇 |
综合类 | 3073篇 |
出版年
2024年 | 22篇 |
2023年 | 49篇 |
2022年 | 127篇 |
2021年 | 124篇 |
2020年 | 123篇 |
2019年 | 119篇 |
2018年 | 94篇 |
2017年 | 126篇 |
2016年 | 120篇 |
2015年 | 175篇 |
2014年 | 211篇 |
2013年 | 266篇 |
2012年 | 271篇 |
2011年 | 281篇 |
2010年 | 214篇 |
2009年 | 247篇 |
2008年 | 238篇 |
2007年 | 290篇 |
2006年 | 246篇 |
2005年 | 270篇 |
2004年 | 195篇 |
2003年 | 185篇 |
2002年 | 153篇 |
2001年 | 139篇 |
2000年 | 137篇 |
1999年 | 104篇 |
1998年 | 72篇 |
1997年 | 82篇 |
1996年 | 52篇 |
1995年 | 63篇 |
1994年 | 49篇 |
1993年 | 38篇 |
1992年 | 38篇 |
1991年 | 45篇 |
1990年 | 30篇 |
1989年 | 26篇 |
1988年 | 24篇 |
1987年 | 12篇 |
1986年 | 8篇 |
1985年 | 7篇 |
1984年 | 3篇 |
1983年 | 3篇 |
1982年 | 1篇 |
1981年 | 2篇 |
1980年 | 3篇 |
1978年 | 2篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1955年 | 1篇 |
排序方式: 共有5089条查询结果,搜索用时 812 毫秒
61.
The interaction of dimethyltin dichloride (Me2SnCl2) with calf thymus DNA was studied at 27 °C, pH 7.6 using various techniques including isothermal titration calorimetry (ITC) and UV-Vis, fluorescence and IR spectrophotometries. The binding isotherm and enthalpy curve for Me2SnCl2-DNA interaction was a biphasic transition process. This was determined by the analysis of the binding data with the Hill equation. The first phase of the enthalpy curve (exothermic process) was consistent with the first set of binding site, the second phase (endothermic process, less exothermicity) was consistent with second set of binding site from the cited interactions. Our results showed that the first set of binding sites is occupied by one mole of ligand bound per near 1 base pair of DNA. The DNA-ethidium bromide (EB) complex, in the presence of Me2SnCl2, caused the quenching of the fluorescence emission. The Scatchard plots illustrated a non-intercalating manner for such quenching. The DNA-EB complex results indicated that the binding of Me2SnCl2 is with the phosphate groups of DNA at low ligand concentrations (<9 mM). This was confirmed with the IR spectrophotometric spectra. However, the binding at higher ligand concentrations (>9 mM) was with the base groups of DNA. Therefore, these results suggest that the Me2SnCl2 binding to DNA at low concentrations occurs through an outside interaction by an exothermic process. However, the partial unfolding of the DNA caused at higher concentrations of Me2SnCl2 is through an endothermic process involving interactions with the base groups. 相似文献
62.
63.
J. L. Oscarson R. M. Izatt P. R. Brown Z. Pawlak S. E. Gillespie J. J. Christensen 《Journal of solution chemistry》1988,17(9):841-863
The aqueous reactions,
相似文献
64.
Fumio Kiyono Hideo Tajima Keiichi Ogasawara Akihiro Yamasaki 《Fluid Phase Equilibria》2005,230(1-2):90-98
A method for determining quadruple points of a two-component system containing a simple hydrate phase is proposed. This method utilizes the quasi-static change of the system along three-phase equilibrium lines and was proved to be able to determine the quadruple points as accurately as the conventional method. By using this method, even though some preparation is necessary, a quadruple point can be determined in just a single experimental run. The behavior of the system near the quadruple points was also examined experimentally, for both the quasi-static and the irreversible change cases. At the quadruple points, the temperature and pressure of the system were kept constant for a while, as at the triple point of water. In both cases, the representative point of the state of the system passed through the quadruple point on a p–T diagram. 相似文献
65.
聚合物与有机染料多层复合膜电致发光颜色的直流电压调制马於光,薛善华,黄劲松,田文晶,刘式墉,沈家骢,刘晓冬(吉林大学分子光谱与分子结构开放实验室,集成光电子学国家重点实验室,长春,130023)(白求恩医科大学,长春,130023)关键词聚合物,发光... 相似文献
66.
Thermal characteristics of polyurethane foams incorporated with phase change materials 总被引:1,自引:0,他引:1
Thermal energy storage plays an important role in heat management because of the demand for developed energy conservation, and has applications in diverse areas, from building heating/cooling systems which enable solar energy incorporation into the structure, to textiles and clothings providing an enhanced thermal comfort. In this study, we aimed to improve thermal characteristics of polyurethane rigid foams that have been widely used for thermal insulation as the ultimate energy savers due to their ability to form sandwich structures with various facer materials. Through a laboratory-scale work, two paraffin waxes acting as phase change materials, namely n-hexadecane and n-octadecane, each of which is capable of managing large heat storage/release, were directly incorporated into the polyurethane foams at different ratios. Polymerization modified by means of n-alkane addition could be achieved without any adverse effect. In order to determine both structural and thermal characteristics, seven types of foams produced were examined by FT-IR, SEM, DSC analyses, calorimeter bomb and mechanical tests. Results show that polyurethane foams can be designed as thermal insulators equipped with an improved buffering function against temperature changes. 相似文献
67.
Preparation and Characterization of Microencapsulated Hexadecane Used for Thermal Energy Storage 总被引:3,自引:0,他引:3
GuangLongZOU ZhiChengTAN XiaoZhengLAN LiXianSUN TaoZHANG 《中国化学快报》2004,15(6):729-732
Polyurea microcapsules about 2.5μm in diameter containing phase change material for thermal energy storage application were synthesized and characterized by interfacial polycondensation method with toluene-2,4-diisocyanate and ethylenediamine as monomers in an emulsion system. Hexadecane was used as a phase change material and OP, which is nonionic surfactant, and used as an emulsifier. The chemical structure and thermal behavior of the microcapsules were investigated by FTIR and thermal analysis respectively. The results show encapsulated hexadecane has a good potential as a solar energy storage material. 相似文献
68.
The effect of pressure on the dissociation constant of hydrofluoric acid was determined by using the indicator technique at 25°C at an ionic strength of 0.1m over a pressure range of 1 to 2000 atm. A value of 3.14 for pK
a
*
at I =0 was obtained by extrapolation to zero ionic strength at 1 atm. The pressure dependence yielded a partial molar volume change of –9.6 cm3-mol–1 and a compressibility change of — 35×10–3 cm3-mol–1 –atm–1 for the dissociation. The dependence of ionic strength on the association constant K
A
*
of NaF was studied at 25°C and 1 atm. Extrapolation to I=0 yielded a pK
A
*
of –0.78. The pressure dependence of K
A
*
gave a change of volume of 3.26 cm3-mol–1 and a change in compressibility of 6×10–3 cm3-mol–1-atm–1 for the formation of the ion pair. 相似文献
69.
The deformation processes in impact-modified PMMA, which deforms homogeneously, were determined by means of the stress/strain experiment (, ) with simultaneous lateral strain measurement (lat) in a wide range of strain rates () up to 105%/min (impact stress). The elastic, plastic cavitation and plastic shear processes were determined as a function of strain. Therefore we calculated the elastic strain (
el), the elastic volume expansion (
vol el), the cavitation strain (
cav), which is identical with the plastic volume expansion (
vol pl), the shear strain (
sh) and the energy densities (Wel, Wcav, Wsh) related to these three processes.For strains of 3 % onward it was found that plastic shear processes and plastic cavitation processes are responsible for a partial loss of elastically stored energy. Both plastic processes turn out to be mostly anelastic deformations, their amount depending strongly on the strain rate. The contributions of the processes to the total deformation of the unmodified PMMA in its strain range are similar to those of the impact-modified PMMA, and the high impact strength is caused by a shift of the catastrophic rupture to very high strains. 相似文献
70.
D. S. Robertson 《Speculations in Science and Technology》1998,21(3):187-197
It has been established from geological studies that change in the atmospheric content of carbon dioxide gas commenced about one hundred million years ago. The likely origin of this change is advanced as being the onset of the Brewer circulation caused by the rise in terrain induced by tectonic plate movement. It is demonstrated that tectonic plate movement can be affected by impacts from external bodies which penetrate the crust of the Earth. The consequences of the change in atmospheric concentration of carbon dioxide are proposed as first, extinctions and reductions in animal numbers, including primates, as a result of changes in body chemistry of these animals and second, a change in the rate of weathering of rocks giving rise to changes in the availability of chemicals such as calcium and potassium which are essential for plant and animal life. This latter change contributing to the extinctions and reductions in animal numbers. It is shown that the change in weathering can account for the rise to dominance of angiosperm plants. It is concluded that there were several simultaneous evolutionary environments on Earth which were a function of altitude which gave rise to a vertical variation in atmospheric content of carbon dioxide. This variation disappeared with rise of terrain and the onset of the Brewer circulation. Such changes are advanced and being much more important than any changes in temperature caused by greenhouse effects since the disappearance of atmospheric variations in carbon dioxide allowed animal migration. It is demonstrated that the conditions of extinction could be reintroduced by human activities. 相似文献
|