全文获取类型
收费全文 | 36614篇 |
免费 | 6853篇 |
国内免费 | 10213篇 |
专业分类
化学 | 27609篇 |
晶体学 | 1161篇 |
力学 | 2339篇 |
综合类 | 933篇 |
数学 | 4811篇 |
物理学 | 16827篇 |
出版年
2024年 | 115篇 |
2023年 | 478篇 |
2022年 | 1331篇 |
2021年 | 1358篇 |
2020年 | 1395篇 |
2019年 | 1330篇 |
2018年 | 1278篇 |
2017年 | 1486篇 |
2016年 | 1485篇 |
2015年 | 1808篇 |
2014年 | 2144篇 |
2013年 | 2958篇 |
2012年 | 3069篇 |
2011年 | 3243篇 |
2010年 | 2848篇 |
2009年 | 2930篇 |
2008年 | 3154篇 |
2007年 | 3000篇 |
2006年 | 2818篇 |
2005年 | 2505篇 |
2004年 | 1960篇 |
2003年 | 1451篇 |
2002年 | 1534篇 |
2001年 | 1405篇 |
2000年 | 1344篇 |
1999年 | 875篇 |
1998年 | 562篇 |
1997年 | 490篇 |
1996年 | 466篇 |
1995年 | 395篇 |
1994年 | 399篇 |
1993年 | 326篇 |
1992年 | 306篇 |
1991年 | 217篇 |
1990年 | 182篇 |
1989年 | 189篇 |
1988年 | 121篇 |
1987年 | 109篇 |
1986年 | 103篇 |
1985年 | 79篇 |
1984年 | 79篇 |
1983年 | 55篇 |
1982年 | 54篇 |
1981年 | 56篇 |
1980年 | 31篇 |
1979年 | 32篇 |
1978年 | 16篇 |
1976年 | 10篇 |
1974年 | 16篇 |
1965年 | 12篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
A (w,r) cover‐free family is a family of subsets of a finite set such that no intersection of w members of the family is covered by a union of r others. A (w,r) superimposed code is the incidence matrix of such a family. Such a family also arises in cryptography as the concept of key distribution pattern. In the present paper, we give some new results on superimposed codes. First we construct superimposed codes from super‐simple designs which give us results better than superimposed codes constructed by other known methods. Next we prove the uniqueness of the (1,2) superimposed code of size 9 × 12, the (2,2) superimposed code of size 14 × 8, and the (2,3) superimposed code of size 30 × 10. Finally, we improve numerical values of upper bounds for the asymptotic rate of some (w,r) superimposed codes. © 2004 Wiley Periodicals, Inc. 相似文献
52.
基于自适应差分量化理论,提出了在不增加转换位数的情况下提高模数转换器(ADC)动态范围的一种设计方法。该方法通过量化输入与预测的差值来获得预测增益,从而扩大ADC的动态范围;通过差分量化值与预测值相加得到输出信号,并将该分别采用LMS和RLS算法进行处理.得到输入信号的预测值.计算机模拟结果证实,上述两种算法均能使模数转换器的动态范围提高约25dB.比较而言,RLS算法收敛速度更快,收敛性更好,但计算量较大. 相似文献
53.
54.
Neodymium doped bismuth layer structure ferroelectrics (BLSFs) ceramics CaBi4−xNdxTi4O15 (x=0, 0.25, 0.50, 0.75) were prepared by solid-state reaction method. X-ray diffraction pattern showed that single phase was formed when x=0-0.75. The refined lattice parameters showed that a (b) axes decrease at x=0.25 and increase with more Nd3+ dopant. The effects of Nd3+ doping on the dielectric and ferroelectric properties of CaBi4Ti4O15 ceramics are studied. Nd3+ dopant decreased the Curie temperature linearly, and the dielectric loss, tan δ, as well. The remnant polarization of Nd3+ doped CaBi4Ti4O15 ceramics was increased by 80% at x=0.25, while more Nd3+ dopant decreased the remnant polarization. CaBi3.75Nd0.25Ti4O15 ceramics had the largest piezoelectric constant d33. The structure and properties of CaBi4−xNdxTi4O15 ceramics showed that Nd3+ may occupy different crystal locations when Nd3+ content x is less than 0.25 and more than 0.50. 相似文献
55.
YBa2Cu3O7-x (YBCO) bulk is one of the most important high temperature superconducting materials for magnetic levitation applications, because it has a high magnetic irreversibility field Hirr at liquid nitrogen temperature, and can grow into large grains. The levitation force of YBCO bulk is proportional to the critical current density (Jc) and grain radius (r)[1], so higher Jc and r are appreciated for YBCO bulks to achieve higher levitation force. Now single-domain YBCO bulks up to s… 相似文献
56.
ZHU Yabin ZHOU Yueliang LIU Zhen WANG Shufang CHEN Zhenghao U Huibin YANG Guozhen XlAO Ling Ren Hongtao JIAO Yulei Zheng Minghui 《中国科学G辑(英文版)》2004,47(2):165-172
The irreversibility field (Hirr) of Y-based superconductor is much higher than that of Bi-based superconductor. Y-based superconductor is capable of maintaining stable electrical currents in high magnetic field and electric field, so it is a better suited mate-rial for electric-current applications. Commonly, the Y-based tapes comprise a YBCO thick film deposited on a flexible substrate, typically with an intermediate buffer layer, and an overcoat of noble metal. In this process, the interm… 相似文献
57.
本文考察了包括平面上的各种广义 Cantor集 ,Sierpinski集和包括某些连续不可微曲线在内的广义 Sierpinski集 .由相似变换 ,导出了它们的级数表达式 ,并利用它和字符串空间的对应关系 ,计算出它们的Hausdorff维数 相似文献
58.
Deposition of Hydrogen-Free Silicon Nitride Thin Films by Microwave ECR plasma Enhanced Magnetron Sputtering at Room Temperature 下载免费PDF全文
Hydrogen-free silicon nitride (SiNx) films were deposited at room temperature by microwave electron cyclotron resonance (MW-ECR) plasma enhanced unbalance magnetron sputtering system. Both Fourier-transform infrared spectroscopy and x-ray photoelectron spectroscopy are used to study the bonding type and the change of bonding structures of the silicon nitride films. The results indicate that the chemical structure and composition of SiNx films deposited by this technique depend strongly on the N2 flow rates, the stoichiometric SiNx film, which has the highest hardness of 22.9 GPa, could be obtained at lower N2 flow rate of 4 sccm. 相似文献
59.
一类混合型微分差分方程的周期解 总被引:1,自引:0,他引:1
利用Fenchel变换,我们推出一类微分差分方程存在周期解等价于某泛函具有临界点,并求出方程具有周期解的充分条件. 相似文献
60.