全文获取类型
收费全文 | 33905篇 |
免费 | 5320篇 |
国内免费 | 4814篇 |
专业分类
化学 | 19260篇 |
晶体学 | 277篇 |
力学 | 1389篇 |
综合类 | 311篇 |
数学 | 2917篇 |
物理学 | 8810篇 |
无线电 | 11075篇 |
出版年
2024年 | 142篇 |
2023年 | 866篇 |
2022年 | 1132篇 |
2021年 | 1445篇 |
2020年 | 1442篇 |
2019年 | 1480篇 |
2018年 | 1128篇 |
2017年 | 1159篇 |
2016年 | 1604篇 |
2015年 | 1785篇 |
2014年 | 1942篇 |
2013年 | 2432篇 |
2012年 | 2630篇 |
2011年 | 2694篇 |
2010年 | 2042篇 |
2009年 | 1984篇 |
2008年 | 2135篇 |
2007年 | 1936篇 |
2006年 | 1802篇 |
2005年 | 1632篇 |
2004年 | 1361篇 |
2003年 | 1396篇 |
2002年 | 1568篇 |
2001年 | 1377篇 |
2000年 | 936篇 |
1999年 | 778篇 |
1998年 | 495篇 |
1997年 | 403篇 |
1996年 | 400篇 |
1995年 | 321篇 |
1994年 | 280篇 |
1993年 | 219篇 |
1992年 | 207篇 |
1991年 | 210篇 |
1990年 | 177篇 |
1989年 | 101篇 |
1988年 | 93篇 |
1987年 | 54篇 |
1986年 | 64篇 |
1985年 | 69篇 |
1984年 | 39篇 |
1983年 | 35篇 |
1982年 | 10篇 |
1981年 | 10篇 |
1980年 | 6篇 |
1979年 | 6篇 |
1971年 | 1篇 |
1959年 | 1篇 |
1957年 | 3篇 |
1936年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 9 毫秒
11.
Xiao‐Dong Pan 《Journal of Polymer Science.Polymer Physics》2004,42(13):2467-2478
For properly chosen elastomer compounds, thermorheological characterization is combined with an examination of the variation of the wet sliding friction with temperature. A conceptual argument leads to the assumption that the wet sliding friction should maximize at the energy dissipation peak associated with the dynamic softening transition at a characteristic frequency determined by the sliding speed and the effective smallest surface asperity scale. The dynamic softening transition is characterized with the peak in tan δ/G′n, where tan δ is the loss tangent, G′ is the elastic modulus, and n is a constant between 0 and 1. The William–Landel–Ferry transform is uncritically applied for extrapolating the position of the peak in tan δ/G′n at high frequencies. Even based on the criterion of tan δ, the results obtained on a concrete surface indicate that the effective smallest asperity scale is of order of 100 μm. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2467–2478, 2004 相似文献
12.
Raymond Mackay Jie Zhang Qi Wu Yuzhuo Li 《Colloids and surfaces. A, Physicochemical and engineering aspects》2004,250(1-3):343-348
The longitudinal relaxation times (T1) of water in concentrated silica and alumina slurries were measured as a function of solids content. It was shown that the results could be fit very well with a two-phase fast-exchange model between free and surface-bound water. As expected, values of T1 for bound water were in the order of 20–2000 times lower than that for free water, indicating a higher effective viscosity of the surface-bound water. The strength of the interaction depended on the particular surface, and all of the aluminas examined interacted more strongly with water than the two silicas studied, which themselves differed considerably. The chemical mechanical polishing (CMP) removal rate of tantalum by silica slurries was shown to be directly correlated with the interaction parameters, derived from the NMR relation times rather than with total surface hydroxyl group concentration. 相似文献
13.
养殖池塘沉积物的重金属分布及污染特征 总被引:3,自引:0,他引:3
浙江省淅西北地区淡水养殖池塘沉积物重金属调查研究表明,养殖池塘沉积物中7种重金属元素分布不均衡,差异也较大,除Hg和Cr元素外,其余重金属元素已大大超过土壤本底值.虽然其实测含量都未超出国家规定的标准值范围,但沉积物中重金属元素对养殖水环境、养殖品种存在的潜在污染与危害值得关注.污染指数评价显示,养殖池塘沉积物中主要污染物质为Cu和Cr,而Hg、Pb污染程度相对较轻. 相似文献
14.
Zhen‐Feng Chen Hong‐Li Zhou Hong Liang Yan Li Ren‐Gen Xiong Xiao‐Zeng You 《应用有机金属化学》2003,17(11):883-884
The centrosymmetric binuclear structure of [Pb2(H‐Norf)2(ONO2)4]shows the geometry around each lead(II) atom to be distorted trigonal bipyramidal with Pb–O distances ranging from 2.357(3) to 2.769(4) Å. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
15.
16.
Xiao‐Hui Liu Yan‐Guo Li Ying Lin Yue‐Sheng Li 《Journal of polymer science. Part A, Polymer chemistry》2007,45(7):1272-1281
The reversible addition–fragmentation chain transfer (RAFT) polymerization of acrylonitrile (AN) mediated by 2‐cyanoprop‐2‐yl dithiobenzoate was first applied to synthesize polyacrylonitrile (PAN) with a high molecular weight up to 32,800 and a polydispersity index as low as 1.29. The key to success was ascribed to the optimization of the experimental conditions to increase the fragmentation reaction efficiency of the intermediate radical. In accordance with the atom transfer radical polymerization of AN, ethylene carbonate was also a better solvent candidate for providing higher controlled/living RAFT polymerization behaviors than dimethylformamide and dimethyl sulfoxide. The various experimental parameters, including the temperature, the molar ratio of dithiobenzoate to the initiator, the molar ratio of the monomer to dithiobenzoate, the monomer concentration, and the addition of the comonomer, were varied to improve the control of the molecular weight and polydispersity index. The molecular weights of PANs were validated by gel permeation chromatography along with a universal calibration procedure and intrinsic viscosity measurements. 1H NMR analysis confirmed the high chain‐end functionality of the resultant polymers. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 1272–1281, 2007 相似文献
17.
18.
用于频率合成器的低损耗声表面波滤波器 总被引:2,自引:2,他引:0
简述了产生声表面波滤波器插入损耗的主要机理和获得低损耗的原则;介绍了根据此原则对100~500MHz5种滤波器的研制过程和结果。实验表明,采用镜像阻抗连接换能器结构可达到3~4dB的低损耗,并成功地应用于频率合成器中。 相似文献
19.
关于布尔函数的二次逼近 总被引:2,自引:0,他引:2
本文首先利用矩阵形式引入了二阶walsh谱的概念并讨论了其性质,其次引入了二次bent函数的概念,同时论述了二次bent函数的密码学意义。 相似文献
20.
在本试验中,采用智能同步广播激励器和CCTV传输的1MHz标频,实现了中波同步广播同频保护率趋于0dB.并提出了自适应相位跟踪同步的概念。 相似文献