全文获取类型
收费全文 | 41965篇 |
免费 | 6328篇 |
国内免费 | 8305篇 |
专业分类
化学 | 31168篇 |
晶体学 | 911篇 |
力学 | 2400篇 |
综合类 | 925篇 |
数学 | 5609篇 |
物理学 | 15585篇 |
出版年
2024年 | 84篇 |
2023年 | 543篇 |
2022年 | 1122篇 |
2021年 | 1245篇 |
2020年 | 1334篇 |
2019年 | 1321篇 |
2018年 | 1175篇 |
2017年 | 1368篇 |
2016年 | 1536篇 |
2015年 | 1749篇 |
2014年 | 2122篇 |
2013年 | 2936篇 |
2012年 | 3317篇 |
2011年 | 3579篇 |
2010年 | 2827篇 |
2009年 | 2916篇 |
2008年 | 3316篇 |
2007年 | 3004篇 |
2006年 | 2751篇 |
2005年 | 2489篇 |
2004年 | 2025篇 |
2003年 | 1539篇 |
2002年 | 1546篇 |
2001年 | 1325篇 |
2000年 | 1334篇 |
1999年 | 1074篇 |
1998年 | 736篇 |
1997年 | 641篇 |
1996年 | 649篇 |
1995年 | 547篇 |
1994年 | 520篇 |
1993年 | 514篇 |
1992年 | 463篇 |
1991年 | 379篇 |
1990年 | 329篇 |
1989年 | 328篇 |
1988年 | 271篇 |
1987年 | 210篇 |
1986年 | 205篇 |
1985年 | 197篇 |
1984年 | 161篇 |
1983年 | 100篇 |
1982年 | 108篇 |
1981年 | 92篇 |
1980年 | 81篇 |
1979年 | 77篇 |
1978年 | 48篇 |
1976年 | 40篇 |
1974年 | 47篇 |
1973年 | 49篇 |
排序方式: 共有10000条查询结果,搜索用时 922 毫秒
161.
162.
163.
The growth mechanism of the peritectic η phase involving the peritectic reaction and peritectic transformation in Cu-70%Sn alloy was investigated under directional
solidification. The results show that a major growth mechanism in thickening of the peritectic η-layer is not the peritectic reaction but the peritectic transformation. The transformation temperature and isothermal time
play crucial roles in determining the volume fraction and the thickness of the peritectic η phase. With the increase of the temperature and isothermal time, the volume fraction of the peritectic η phase increases. The regressed data show that the relationship between the thickness of η phase (Δx) and the transformation temperature (T) meets the following equation In Δx=6.5−1673 1 / T. Additionally, there exists a relationship between the thickness of the η phase (Δx) and the isothermal time (t) at the 9 mm solidification distance below the peritectic reaction interface, Δx=0.72t
1/2, which is consistent with the theoretical model.
Supported by the National Science Foundation of China (Grant No. 50395102) 相似文献
164.
The Kirchoff-Love plate theory and electroelasticity theory are combined to simulate the dynamic behaviors of the trimorph ring transducers under different boundary conditions. The transducer consists of an isotropic elastic ring laminated between two identical piezoelectric rings. Their electric current response, resonant frequencies, antiresonant frequencies and electromechanical coupling coefficients (EMCCs) are theoretically formulated and studied by numerical simulation. Also, the resonant frequencies and their corresponding mode shape are simulated by the finite element modelling to verify the theoretical results. Finally, to obtain the maximum energy conversion efficiency, the dynamic EMCC is optimized by varying the proportion of piezoelectric and elastic parts. It is shown that the dynamic EMCC depends on geometric thickness and radii ratios. Optimum settings for a particular transducer to reach the maximum dynamic EMCC are found for different boundary conditions. The trimorph ring transducer for the fixed inner and free outer surfaces boundary condition has slightly lower resonant and anti-resonant frequencies, and larger EMCCs than that for the free inner and fixed outer surfaces boundary condition does. 相似文献
165.
We study maps from a 2‐surface into the standard 2‐sphere coupled with Born‐Infeld geometric electromagnetism through an Abelian gauge field. Such a formalism extends the classical harmonic map model, known as the σ‐model, governing the spin vector orientation in a ferromagnet allows us to obtain the coexistence of vortices and antivortices characterized by opposite, self‐excited, magnetic flux lines. We show that the Born‐Infeld free parameter may be used to achieve arbitrarily high local concentration of magnetic flux lines that the total minimum energy is an additive function of these quantized flux lines realized as the numbers of vortices antivortices. In the case where the underlying surface, or the domain, is compact, we obtain a necessary sufficient condition for the existence of a unique solution representing a prescribed distribution of vortices antivortices. In the case where the domain is the full plane, we prove the existence of a unique solution representing an arbitrary distribution of vortices and antivortices. Furthermore, we also consider the Einstein gravitation induced by these vortices, known as cosmic strings, establish the existence of a solution representing a prescribed distribution of cosmic strings cosmic antistrings under a necessary sufficient condition that makes the underlying surface a complete surface with respect to the induced gravitational metric. © 2003 Wiley Periodicals, Inc. 相似文献
166.
167.
药用植物化学分类法与红外指纹图谱的相关性研究 总被引:27,自引:6,他引:21
本文首次采用傅里叶变换红外光谱法(FTIR)对五加科、桔梗科、木兰科、樟科、豆科、蕨类等科目中的典型药用植物进行了系统研究,比较了各科内植物的异同,并且讨论了相同植物不同药用部位、不同采药时间对其红外谱图的影响。结合药典中关于药物主体有效成分的记载,分析了各药用植物谱图中主要成分的特征基团,对主要吸收峰进行了指认。在传统形态比较分类方法的基础上,FTIR可以成为一种快速、可靠、客观、有效的化学分类学的补充方法。 相似文献
168.
1689-nm diode lasers used in medical apparatus have been fabricated and characterized. The lasers had pnpn InP current confinement structure, and the active region consisted of 5 pairs of InGaAs quantum wells and InGaAsP barriers. 相似文献
169.
PURE STATE APPROACH TO C(x)_αZ_nLIBINGREN;LINQING(InstituteofMathematics,AcademiaSinica,Beijing100080,China.Projectsupportedbyth?.. 相似文献
170.
A europium complex Eu (DBM)3 TPPO (Eu tris(benzoylmethide)-(triphenylphosphine oxide)) and silicon nanoparticles have been hybridized.The hybridization can evidently change the photoluminescence (PL) characteristics of the Eu complex in the following aspects:under an excitation of 390nm,the intensity of the PL peak at 611nm due to the ^5Du-^7F2 transition of the Eu^3 ions has been increased by 30%,and thc integrated PL intensity in the visible range has been increased by nearly 3 times;the PL excitation efficiency beyond 440nm has been improved cvidently;the peak in the PL excitation spectrum shifts from 408nm to 388nm,and the PL decay time decreases from 2.07 to 0.96μs,The experimental results indicatde that in the PL process,the photoexcited energy may transfer from the silicon nanoparticlcs to the Eu^3 ions. 相似文献