全文获取类型
收费全文 | 232535篇 |
免费 | 32420篇 |
国内免费 | 25342篇 |
专业分类
化学 | 119223篇 |
晶体学 | 1889篇 |
力学 | 11203篇 |
综合类 | 1352篇 |
数学 | 22551篇 |
物理学 | 65654篇 |
无线电 | 68425篇 |
出版年
2024年 | 839篇 |
2023年 | 4760篇 |
2022年 | 6538篇 |
2021年 | 8137篇 |
2020年 | 8055篇 |
2019年 | 7450篇 |
2018年 | 6912篇 |
2017年 | 6669篇 |
2016年 | 9357篇 |
2015年 | 10056篇 |
2014年 | 12356篇 |
2013年 | 15948篇 |
2012年 | 18673篇 |
2011年 | 19281篇 |
2010年 | 14308篇 |
2009年 | 14246篇 |
2008年 | 14991篇 |
2007年 | 13725篇 |
2006年 | 13072篇 |
2005年 | 11398篇 |
2004年 | 8630篇 |
2003年 | 7161篇 |
2002年 | 6647篇 |
2001年 | 5748篇 |
2000年 | 5261篇 |
1999年 | 5396篇 |
1998年 | 4566篇 |
1997年 | 3974篇 |
1996年 | 3995篇 |
1995年 | 3536篇 |
1994年 | 3075篇 |
1993年 | 2655篇 |
1992年 | 2318篇 |
1991年 | 1957篇 |
1990年 | 1605篇 |
1989年 | 1244篇 |
1988年 | 972篇 |
1987年 | 798篇 |
1986年 | 745篇 |
1985年 | 658篇 |
1984年 | 470篇 |
1983年 | 408篇 |
1982年 | 321篇 |
1981年 | 230篇 |
1980年 | 169篇 |
1979年 | 106篇 |
1978年 | 100篇 |
1976年 | 95篇 |
1975年 | 94篇 |
1973年 | 93篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
291.
292.
Chin‐Ping Yang Ruei‐Shin Chen Kuei‐Hung Chen 《Journal of polymer science. Part A, Polymer chemistry》2003,41(7):922-938
To investigate the position and amount of the CF3 group affecting the coloration of polyimides (PIs), we prepared 2,2‐bis[4‐(4‐amino‐2‐trifluoromethylphenoxy)phenyl]hexafluoropropane ( 2 ) with four CF3 groups with 2‐chloro‐5‐nitrobenzotrifluoride and 2,2‐bis(4‐hydroxyphenol)hexafluoropropane. A series of soluble and light‐colored fluorinated PIs ( 5 ) were synthesized from 2 and various aromatic dianhydrides ( 3a – 3f ). 5a – 5f had inherent viscosities ranging from 0.80 to 1.19 dL/g and were soluble in amide polar solvents and even in less polar solvents. The glass‐transition temperatures of 5 were 221–265 °C, and the 10% weight‐loss temperatures were above 493 °C. Their films had cutoff wavelengths between 343 and 390 nm, b* values (a yellowness index) ranging from 5 to 41, dielectric constants of 2.68–3.01 (1 MHz), and moisture absorptions of 0.03–0.29 wt %. In a comparison of the PI series 6 – 8 based on 2,2‐bis[4‐(4‐aminophenoxy)phenyl]hexafluoropropane, 2,2‐bis[4‐(4‐amino‐2‐trifluoromethylphenoxy)phenyl]propane, and 2,2‐bis[4‐(4‐aminophenoxy)phenyl]propane, we found that the CF3 group close to the imide group was more effective in lowering the color; this means that CF3 of 5 , 7 , and 8f was more effective than that of 6c . The color intensity of the four PI series was lowered in the following order: 5 > 7 > 6 > 8 . The PI 5f , synthesized from diamine 2 and 4,4′‐hexafluoroisopropylidenediphthalic anhydride, had six CF3 groups in a repeated segment, so it exhibited the lightest color among the four series. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 922–938, 2003 相似文献
293.
Fang Wu Yaojiang Zhang Zaw Zaw Oo Erping Li 《Antennas and Propagation Magazine, IEEE》2005,47(4):110-118
The Multilevel Fast Multipole Method (FMM) is a well-established method and can be applied to solve electromagnetic (EM) scattering problems. Compared with other traditional methods, it requires less computational time and memory. However, constrained by a single processor's speed and memory limitations, the problem size that can be solved by serial implementation is still relatively small. For a million-unknown target, the computational time on a single processor is intolerable, and memory could be easily exhausted. Parallel-computing technology, which can utilize multiprocessors, provides an efficient way to solve electrically large-scale EM problems. This paper will focus on discussing the parallel methodologies applied to a multilevel FMM code, as well as demonstrating the computational efficiency of the parallel approach. 相似文献
294.
稀疏过程在破产问题中的应用 总被引:5,自引:0,他引:5
本讨论一类人寿保险的风险过程,其中保单到达服从齐次Poisson过程。而描述退保及索赔发生的计数过程分别为这一过程的q-稀疏与p-稀疏.对此模型给出其破产概率的具体上界,并与其它一类风险模型进行比较. 相似文献
295.
本文提出的MMD算法用于提高模型区别错误信息和正确信息的能力.利用该算法在对模型的参数进行重估计时.涉及到复杂的目标函数的梯度运算.击运用矩阵运算使得梯度运算变得简单明了,因此本文给出了MMD算法下的HMM参数重估计的矩阵表示形式并给出了证明. 相似文献
296.
Complex impedance spectra were obtained on a crystal of CaCu3Ti4O12 (CCTO) from 289 to 456 K. As in the case of ceramic CCTO, these spectra can be interpreted as arising from a conducting material containing insulating barriers. This is then further evidence for the existence of planar defects within crystals of CCTO that act as insulating barriers and produce the large dielectric constant through a space charge mechanism. 相似文献
297.
We present the cosmological solutions in the six-dimensional Einstein-gauge theory to which is added a matter term. The general behavior of the scale factors of the ordinary (R3) and extra (R2) dimensions can be depicted in four stages. Stage 1: Both R3 and R2 expand. Stage 2: R2 is a constant and R3 is an inflationary solution. Stage 3: R2 recollapses towards its minimum value, R3 will continue to increase monotonically. Stage 4: R2 stabilizes at RKK and R3 expands in the usual FRW way, while the four-dimensional cosmological constant becomes zero. 相似文献
298.
激光晶体YAG中Er3+的辐射跃迁 总被引:2,自引:0,他引:2
本文讨论和分析了激光晶体YAG中Er3+离子的激光上能级的4S3/2、4I11/2和4I13/2辐射跃迁的有关因素。 相似文献
299.
In this paper we verified the submodeling technique applied in the thermomechanical reliability assessment of a flip-chip BGA under accelerated thermal cycling test conditions. Since the steady-state creep model was implemented for the solder bump to better represent its realistic mechanical behavior, submodeling procedures developed specifically for path-dependent thermomechanical problems were considered. A detailed global model for the flip-chip BGA was built up to verify submodeling solutions. This model also served as a benchmark to examine solution discrepancies caused by different simplifications of the global model. 相似文献
300.
Hao Chen 《IEEE transactions on information theory / Professional Technical Group on Information Theory》2005,51(5):1854-1856
We prove a result which reduces the computation of the linear complexity of a sequence over GF(pm) (p is an odd prime) with period 2n (n is a positive integer such that there exists an element bisinGF(pm), bn=-1) to the computation of the linear complexities of two sequences with period n. By combining with some known algorithms such as the Berlekamp-Massey algorithm and the Games-Chan algorithm we can determine the linear complexity of any sequence over GF(pm) with period 2tn (such that 2 t|pm-1 and gcd(n,pm-1)=1) more efficiently 相似文献