全文获取类型
收费全文 | 9978篇 |
免费 | 1590篇 |
国内免费 | 678篇 |
专业分类
化学 | 3012篇 |
晶体学 | 42篇 |
力学 | 827篇 |
综合类 | 309篇 |
数学 | 3995篇 |
物理学 | 4061篇 |
出版年
2024年 | 31篇 |
2023年 | 152篇 |
2022年 | 472篇 |
2021年 | 468篇 |
2020年 | 324篇 |
2019年 | 267篇 |
2018年 | 213篇 |
2017年 | 378篇 |
2016年 | 432篇 |
2015年 | 350篇 |
2014年 | 599篇 |
2013年 | 819篇 |
2012年 | 617篇 |
2011年 | 588篇 |
2010年 | 556篇 |
2009年 | 647篇 |
2008年 | 635篇 |
2007年 | 636篇 |
2006年 | 538篇 |
2005年 | 466篇 |
2004年 | 377篇 |
2003年 | 390篇 |
2002年 | 344篇 |
2001年 | 275篇 |
2000年 | 292篇 |
1999年 | 225篇 |
1998年 | 219篇 |
1997年 | 184篇 |
1996年 | 150篇 |
1995年 | 92篇 |
1994年 | 86篇 |
1993年 | 92篇 |
1992年 | 45篇 |
1991年 | 41篇 |
1990年 | 41篇 |
1989年 | 23篇 |
1988年 | 21篇 |
1987年 | 16篇 |
1986年 | 30篇 |
1985年 | 21篇 |
1984年 | 24篇 |
1983年 | 3篇 |
1982年 | 9篇 |
1981年 | 12篇 |
1980年 | 7篇 |
1979年 | 9篇 |
1978年 | 6篇 |
1977年 | 9篇 |
1959年 | 4篇 |
1936年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
Ilyes Jedidi Zohra Ferhat‐Hamida Geneviève Delmas 《Journal of Polymer Science.Polymer Physics》2007,45(15):1932-1949
A linear Union Carbide PE (UC) has been analyzed by nonstandard calorimetry with a common DSC calorimeter and a Setaram C80 calorimeter. Nonstandard calorimetry entails using a low rate of heating (0.5–1 K/min), a small mass (0.5–1.5 mg), and an open cell (O‐cell) instead of the standard C‐cell. Events in O‐cells overlap less and occur with a faster kinetics than in C‐cells. PE crystals are nascent, solution‐grown (S‐grown), press‐grown (P‐grown), and strained by extrusion. In Part A, the traces show that the phase‐changes in the melt, previously observed in a C80 calorimeter (slow T‐ramp) and characterized by ΔHnetwork, can be observed with a common DSC in nonstandard conditions. In Part B, the difference between the C‐ and O‐cells and the changes in the main peak enthalpy (ΔHortho) are of interest. The main result is that, in O‐cells, the value of ΔHortho around Tortho, exceeds unambiguously in certain conditions ΔHortho found for perfect orthorhombic crystals. The main endotherm contains then another contribution, namely ΔHnetwork. Crystal reorganization during the slow T‐ramp is followed in the C‐ and O‐cells on S‐grown crystals. In O‐cells, lamellar thickening observed in the slow‐ramp is more extensive. The ease of phase‐change depends on the sample history. It is as follows: strained‐part extruded > nascent > S‐grown > P‐grown. Co‐operative chain motions are more hindered in the standard C‐cells than in the O‐cells. In Part C, lower values of m succeed in bringing phase‐changes in P‐grown (O‐cells) samples. The origin of the events is discussed: three processes are thought to contribute to the phase‐changes namely, melting of strained short‐range order, activation of vibrations in the CH2 groups, and fast decay of chain orientation which occurs simultaneously with melting. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1932–1949, 2007 相似文献
942.
R. K. Bayer 《Colloid and polymer science》1991,269(5):421-432
Melt-spinning experiments were carried out at high quenching rates. Mechanical properties (elongation at break, natural draw ratio, and elastic recovery) have been measured. Significant variations of these quantities were observed when extrusion conditions were changed. This has been attributed to different states of the entanglements within the melt, which are directly transferred into the solid state. This intercorrelation between melt and solid-state properties has been substantiated in the case of rapidly cooled samples, where a poor crystallization on one side and a simultaneous good conservation of melt history on the other side are provoked. 相似文献
943.
复杂通信网络的结构分解法及其在可靠性分析中的应用 总被引:2,自引:0,他引:2
对复杂通信网络拓扑结构的恰当描述与刻画是研究其可靠性及确定信号流在其中有效的传输方式的前提。通过引进适当的数据结构并采用递归分解的技巧,本给出了将任意复杂通信网络分解为环、弦、链等基本网络结构的某种组合与连接的结构分解法,并由此得到了研究网络可靠性的新的两阶段法,即先分析各基本结构的可靠性,再由此给出原网络可靠性的估计。 相似文献
944.
Y. Meyer小波的一般形式 总被引:1,自引:0,他引:1
首先我们证明了,如果尺度函数有紧支集,来自多尺度分析的小波函数的支集形式,然后我们证明了Y.Meyer小波的尺度函的一般形式。最后给出了它的另外两种形式和对应的Y.Meyer小波。 相似文献
945.
946.
将二维人工脑模型推广到三维,并给出了一个实际应用算例,还将二、三维模型的计算结果进行了对比,发现三维模型的计算效果明显优于二维模型。 相似文献
947.
综合评判的神经网络方法 总被引:21,自引:1,他引:20
综合评判的方法已经广泛运用到信息,决策,管理和控制等领域,但是它的两大难点-权重和单因素评价矩阵的确定越来越来明显地展现在人们面前,本文试图用神经网络方法来解决这两个难点,并从理论上证明了这种方法的收敛性。 相似文献
948.
为保障国家石油安全、保障国民经济安全运行,本文根据石油安全预警的特点,采用神经网络方法进行预警,设计了BP神经网络模型,应用BP神经网络建立了中国石油安全预警系统,并进行了预警研究和实证分析,得出我国石油安全属于重警区,需加强安全防范。 相似文献
949.
《Magnetic resonance in chemistry : MRC》2003,41(3):157-168
High‐level ab initio calculations of carbon–carbon coupling constants were carried out in tetrahedrane, prismane and cubane using the SOPPA (Second‐Order Polarization Propagator Approach) computational scheme, in good agreement with available experimental data. It was found that SOPPA performs perfectly well in combination with Dunning's correlation‐consistent basis sets augmented with inner core functions; however, no improvement was observed on adding tight s‐functions. The utmost importance of electronic correlation effects decreasing the total values of computed J(C,C) in the title compounds by a factor of ~2.0–2.5 was found. Unknown values of J(C,C) in the title polyhedranes were predicted with high reliability and the latter were treated in terms of s‐characters of carbon–carbon bonds based on the additive scheme of coupling pathways. All three compounds under study showed decreased s‐characters of their carbon–carbon bonds, which is the result of their remarkable steric strain. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
950.