全文获取类型
收费全文 | 4979篇 |
免费 | 292篇 |
国内免费 | 1798篇 |
专业分类
化学 | 4083篇 |
晶体学 | 108篇 |
力学 | 58篇 |
综合类 | 41篇 |
数学 | 2099篇 |
物理学 | 680篇 |
出版年
2023年 | 60篇 |
2022年 | 47篇 |
2021年 | 67篇 |
2020年 | 102篇 |
2019年 | 154篇 |
2018年 | 128篇 |
2017年 | 120篇 |
2016年 | 123篇 |
2015年 | 128篇 |
2014年 | 220篇 |
2013年 | 377篇 |
2012年 | 200篇 |
2011年 | 276篇 |
2010年 | 237篇 |
2009年 | 310篇 |
2008年 | 338篇 |
2007年 | 387篇 |
2006年 | 268篇 |
2005年 | 261篇 |
2004年 | 258篇 |
2003年 | 259篇 |
2002年 | 233篇 |
2001年 | 213篇 |
2000年 | 128篇 |
1999年 | 129篇 |
1998年 | 131篇 |
1997年 | 125篇 |
1996年 | 121篇 |
1995年 | 85篇 |
1994年 | 105篇 |
1993年 | 114篇 |
1992年 | 123篇 |
1991年 | 96篇 |
1990年 | 105篇 |
1989年 | 148篇 |
1988年 | 122篇 |
1987年 | 146篇 |
1986年 | 137篇 |
1985年 | 129篇 |
1984年 | 121篇 |
1983年 | 84篇 |
1982年 | 26篇 |
1981年 | 22篇 |
1980年 | 15篇 |
1979年 | 16篇 |
1978年 | 15篇 |
1977年 | 14篇 |
1976年 | 10篇 |
1975年 | 9篇 |
1974年 | 14篇 |
排序方式: 共有7069条查询结果,搜索用时 15 毫秒
61.
提出了一种评估结构疲劳可靠性的方法-风险预测方法。该方法利用结构在服股过役过程中的信息,在考虑了工程结构承采载荷的各态历经性的基础上,结合了结构元件寿命机分布形式和Miner线性累积损伤理论,以风险值来评价并监控结构元件在不同时间失效的危险性,并用算例和试验结果进行对比分析。 相似文献
62.
63.
Rupture of intracranial saccular aneurysms is the leading cause of spontaneous subarachnoid hemorrhage, which results in significant
morbidity and mortality. Although many have suggested that saccular aneurysms enlarge and rupture due to mechanical instabilities,
our recent nonlinear analyses suggest that at least certain classes of aneurysms do not exhibit a quasi-static limit point
instability or dynamic instabilities in response to periodic loading. Based on an increased understanding of the ubiquitous
role of growth and remodeling within the vasculature and recent histopathological data on saccular aneurysms, it is hypothesized
that a stress-mediated regulation of collagen turnover causes their enlargement. There is a need, however, for a theoretical
framework to explore this and competing hypotheses. In this paper, we present a 2-D constrained mixture model for growth and
remodeling of an ellipsoidally shaped saccular aneurysm and numerically simulate enlargement and changes in material symmetry
in the aneurysmal wall. Results suggest that ellipsoidal aneurysms tend toward spherical shapes, and a competition between
radial expansion and wall thickening plays a critical role in determining the stability of an enlarging lesion. 相似文献
64.
腐蚀磨损协同作用率的研究 总被引:7,自引:5,他引:7
提出了腐蚀与磨损“协同作用”的概念,并用于判断腐蚀与磨损协同作用的性质及大小和确定腐蚀磨损过程中材料损失的主要形式。介绍了协同作用率的测定方法,并以G105钢及化学镀Ni-P合金在5.0%NaCl溶液中的腐蚀腐损作为研究对象,研究了载荷、滑动速度对腐蚀磨损协同作用的影响。结果表明:协同作用率随载荷增大而增大,随滑动速度增大而减小。 相似文献
65.
66.
The macroscopic dynamics of a kinetic equation involving a model wave-particle collision operator of plasma physics is investigated.
The Chapman-Enskog asymptotics is first considered in the framework of a hydrodynamic scaling. The obtained macroscopic model
still involves a kinetic variable, the particle energy in the rest frame of the fluid, but shares similarities with the compressible
Navier-Stokes equation of gas dynamics. Then a diffusive scaling is examined under the hypothesis of small perturbations of
a global equilibrium. In this case, the macroscopic model couples the usual incompressible Navier-Stokes with a diffusion
equation for the energy distribution function of the particles, constrained by an extended version of the Boussinesq relation.
In both cases, the effect of a Lorentz force term is developed, in the perspective of plasma physical modelling.
Received June 16, 1997 相似文献
67.
《Phosphorus, sulfur, and silicon and the related elements》2013,188(11):2309-2317
Complexes of beryllium chloride and nitrate with (Me2N)2P(O)F were characterized in solution by multinuclear NMR spectroscopy and in some cases by IR spectroscopy and conductimetry. 31P and 19F NMR spectra were informative of changes associated with complex formation revealing resonances consistent with different species in solution and suggest an equilibrium between these species in both beryllium derivatives. These compounds show narrow lines in the solution 9Be NMR spectra, indicative of a highly symmetric environment for beryllium. The presence of the different species was more pronounced in beryllium chloride complexes. The results are compared to those reported in the literature for hexamethylphosphoramide (HMPA). 相似文献
68.
《Phosphorus, sulfur, and silicon and the related elements》2013,188(11):2349-2356
Seven new 1-fluorinated-4-phosphopyrazoles 2 have been prepared in 60–94% yield by cyclization of N-(2′,2′,2′-trifluoroethyl) phosphonyl β-hydrazones 1 with triethyl orthoformate in the presence of acetic acid. The IR data, the 1H, 13C, 19F, 31P chemical shifts and coupling constants of the compounds 2 are reported. 相似文献
69.
Chitosan is a natural based polymer obtained by alkaline deacetylation of chitin, exhibiting excellent properties such as non‐toxicity, biocompatibility and biodegradability. N‐Methylenephenyl phosphonic chitosan (NMPPC) is synthesized from chitosan by reacting with phenyl phosphonic acid using formaldehyde. The NMPPC was characterized by FTIR, 31P‐NMR, X‐ray diffraction, scanning electron microscopy, thermogravimeteric analysis and solubility studies. A significant decrease of molecular weight was observed in the NMPPC. The TGA studies suggested that NMPPC has less thermal stability than chitosan. The X‐ray diffraction analysis showed that NMPPC was amorphous in nature. The solubility property of the polymer was improved after the incorporation of a phenyl phosphonic group. 相似文献
70.
M. Rätzsch 《高分子科学杂志,A辑:纯化学与应用化学》2013,50(5):517-526
The investigations presented deal with the experimental results of the copolymerization of maleic anhydride (MAn) with alkenes. The course of the reaction is explained by the overall rate of the copolymerization (v Br), which correlates with the solution viscosity of the copolymer, and the dependence of the v Br maximum on the mole ratio of the monomers at constant total monomer concentration. The use of solvents with increasing donor power leads to increased complexing of the free MAn molecules and of the MAn radical chain ends. The results demonstrate that, for low 1-alkenes, the addition of the MAn chain radical is the rate-determining step of the copolymerization. As the substituents on the olefinic double bond become larger or the double bond shifts to the 1,2-position, the addition of MAn to the hydrocarbon radical becomes more and more the rate-determining step. On the other hand, an increase of the CT complexation of the MAn polymer radical by use of donor solvents decreases the alkene addition rate. 相似文献