全文获取类型
收费全文 | 7307篇 |
免费 | 812篇 |
国内免费 | 500篇 |
专业分类
化学 | 1853篇 |
晶体学 | 40篇 |
力学 | 1574篇 |
综合类 | 73篇 |
数学 | 2064篇 |
物理学 | 1636篇 |
无线电 | 1379篇 |
出版年
2024年 | 17篇 |
2023年 | 65篇 |
2022年 | 103篇 |
2021年 | 135篇 |
2020年 | 190篇 |
2019年 | 156篇 |
2018年 | 190篇 |
2017年 | 218篇 |
2016年 | 275篇 |
2015年 | 272篇 |
2014年 | 329篇 |
2013年 | 531篇 |
2012年 | 403篇 |
2011年 | 438篇 |
2010年 | 315篇 |
2009年 | 349篇 |
2008年 | 412篇 |
2007年 | 469篇 |
2006年 | 405篇 |
2005年 | 377篇 |
2004年 | 321篇 |
2003年 | 300篇 |
2002年 | 272篇 |
2001年 | 238篇 |
2000年 | 252篇 |
1999年 | 213篇 |
1998年 | 194篇 |
1997年 | 144篇 |
1996年 | 137篇 |
1995年 | 139篇 |
1994年 | 118篇 |
1993年 | 101篇 |
1992年 | 74篇 |
1991年 | 71篇 |
1990年 | 59篇 |
1989年 | 44篇 |
1988年 | 39篇 |
1987年 | 37篇 |
1986年 | 28篇 |
1985年 | 25篇 |
1984年 | 26篇 |
1983年 | 10篇 |
1982年 | 22篇 |
1981年 | 24篇 |
1980年 | 12篇 |
1979年 | 11篇 |
1978年 | 18篇 |
1977年 | 7篇 |
1976年 | 13篇 |
1972年 | 6篇 |
排序方式: 共有8619条查询结果,搜索用时 10 毫秒
151.
The solubilities of o-, m- and p-xylene in water were measured at 25.0°C up to 250, 385, and 50 MPa, respectively. The solubility increased with increasing pressure up to 120 MPa (50 MPa for p-xylene) and then decreased. The reaction volumes, Vo accompanying the dissolution at 0.1 MPa were estimated as –3.6±0.5, –3.4±0.5, and –4.1±0.5 cm3-mol–1 for o-, m-, and p-xylene, respectively, from the pressure dependences of the solubilities. The limiting partial molar volumes, of p- and o-xylene in water under high pressure were estimated from Vo and the molar volume of the xylene. The partial molar volumes decreased with increasing pressure. The reaction volume for the formation of intra-molecular pairwise hydrophobic interaction between the methyl groups, as proposed by Ben-Naim, is discussed for the Vo of p- and o-xylene at 0.1 MPa. 相似文献
152.
The diffusion of six azo and five anthraquinone derivatives through nylon 6, poly(ethylene terephthalate) and secondary cellulose acetate films were studied under high hydrostatic pressures of up to 3000 bar and at temperatures 80–130 °C, by analyzing the diffusion profiles yielded in a stacked multiple film, placed in the solution of the diffusant. It was found that the diffusion coefficient,D, of the diffusant decreased with increasing pressure, giving a linear relationship between InD and the pressure, the slope of which gave the activation volume for the diffusion,V
. It was revealedV
increased linearly with increasing intrinsic molecular volume of the diffusant,V
w
, the slopes being different between the azo and the anthraquinone derivatives. The ratio ofV
toV
w
(V
/V
w
) ranged from 0.13 to 0.93, depending in a sensitive manner on the degree of swelling of the polymer matrix which in turn was varied by the solvent. The overall results could be explained in accordance with the formulation,V
f, local +V
=V
w
, whereV
f, local represents the free volume contribution. It was proposed thatV
w
is increased by solvation when the solvent is good for the diffusant. 相似文献
153.
In ternary aqueous solutions, hydrophobic solutes such as alcohols tend to aggregate with surfactants to form mixed micelles. These systems can be studied by meas of the functions of transfer of hydrophobic solutes from water to aqueous solutions of surfactant. These thermodynamic functions often go through extrema in the critical micellar concentration (CMC) region of the surfactant. A simple model based on interactions between surfactant and hydrophobic solute monomers, on the distribution of the hydrophobic solute between water and the micelles and on the shift in the CMC induced by the hydrophobic solute, can simulate the magnitude and trends of the transfer functions using parameters which are mostly derived from the binary systems. In order to check the model more quantitatively, volumes and heat capacities of transfer of alcohols from water to aqueous solutions of a nonionic surfactant, octyldimethylamine oxide, were measured. A quantitative agreement was achieved with three adjustable parameters. Good fits are also obtained for the transfers to the ionic surfactants, octylamine hydrobromide and sodium dodecylsulfate. When the equilibrium displacement contribution is small, the distribution constants and the partial molar properties of the alcohols in the micellar phase agree well with the parameters obtained with similar models. 相似文献
154.
The connection between the dielectric and calorimetric relaxation behaviours of synthetic polyisoprene Cariflex IR 305 is studied. A similar comparison of dielectric and dilatometric results was described in [1]. The heat capacity was measured during heating of samples prepared with different thermal history. Experimental results were compared with the heat capacity curves calculated for a model based on the multiparameter theory of Kovacs et al. [4]. The model considers the relaxation system as being composed of a set of subsystems characterized by different relaxation times. The distribution of relaxation times and their temperature dependence were taken from the diclectric measurement. The relaxation time of a subsystem from posed to depend, not only on the actual, temperature of the sample, but also on the deviation of this subsystem from equilibrium, or alternatively, on the deviation of the system as a whole. The comparison between the measured and modeled curves shows that both influences must be taken into account in order to explain the experimental results.Dedicated to Prof. Dr. W. Pechhold on the occasion of his 60th birthday. 相似文献
155.
The coordination around the thorium(IV) ion in aqueous perchlorate, chloride and nitrate solutions has been determined from large angle X-ray scattering measurements. In perchlorate solutions, where inner-sphere complexes are not formed, the first coordination sphere contains 8.0±0.5 water molecules with Th-H2O bond lengths of 2.485 Å. In chloride solutions inner-sphere complexes are formed, which lead to an increase in the coordination number. In nitrate solutions the nitrate ions are bonded as bidentate ligands to the thorium ion. The bond lengths are similar to those found in crystalline hydrates of thorium nitrate. The coordination numbers found for thorium(IV) in solution are compared with previously reported values for lower charged ions of similar size.On leave from Department of Inorganic Chemistry Royal Institute of Technology S-10044 Stockholm Sweden 相似文献
156.
Summary The role of the volume of the sample and the sample vial in equilibrium headspace-gas chromatography is discussed. A new term, thesample phase fraction (S) is introduced. It is shown that if the value of S is kept constant, the vial's volume has no influence on the sensitivity of the headspace analysis (which is proportional to the concentration of the analyte in the headspace). In a given headspace sampling system, concentration of the compound of interest in the headspace (c
G
*
) at equilibrium is related to the value of S: a higher S will increase c
G
*
. However, the influence is important only in the case of low distribution coefficients: in the case of higher distribution coefficients this influence is negligible. This conclusion is also true for small changes in the sample volume in duplicate analyses: exact reproducibility of the sample volume is important only in the case of low distribution coefficient values. 相似文献
157.
Excess volumes V
E
of binary liquid mixtures of quinoline with alkanols have been determined from densities at 30°C as a function of composition. The excess volumes are negative over the whole mole fraction range for all the mixtures and decrease with increasing length of alkanol (C1–C10). The VE data have been analyzed in terms of an approach which uses graph theoretical connectivity parameters of the third degrees for two components. The analysis gives information regarding associated species in the pure state and in the mixture. It is suggested that, in the mixture state, no change occurs in the association of alkanols. 相似文献
158.
159.
本文选取了子宫内膜癌患者108例作为观察组,另选取子宫内膜良性病变患者105例作为对照组,均行DCE-MRI检查后发现,观察组容量转移常数(Ktrans)、速率常数(Kep)、血管外细胞外间隙容积比值(Ve)高于对照组(P<0.05);Ktrans、Kep、Ve值联合诊断子宫内膜癌的曲线下面积(AUC)为0.841;K... 相似文献
160.