全文获取类型
收费全文 | 67673篇 |
免费 | 10692篇 |
国内免费 | 8313篇 |
专业分类
化学 | 49375篇 |
晶体学 | 909篇 |
力学 | 3711篇 |
综合类 | 586篇 |
数学 | 7825篇 |
物理学 | 24272篇 |
出版年
2024年 | 124篇 |
2023年 | 1214篇 |
2022年 | 1856篇 |
2021年 | 2328篇 |
2020年 | 2625篇 |
2019年 | 2555篇 |
2018年 | 2147篇 |
2017年 | 2095篇 |
2016年 | 3113篇 |
2015年 | 3149篇 |
2014年 | 3781篇 |
2013年 | 4967篇 |
2012年 | 6057篇 |
2011年 | 6131篇 |
2010年 | 4369篇 |
2009年 | 4117篇 |
2008年 | 4593篇 |
2007年 | 4119篇 |
2006年 | 3741篇 |
2005年 | 3195篇 |
2004年 | 2609篇 |
2003年 | 2161篇 |
2002年 | 2146篇 |
2001年 | 1731篇 |
2000年 | 1434篇 |
1999年 | 1463篇 |
1998年 | 1118篇 |
1997年 | 1066篇 |
1996年 | 1020篇 |
1995年 | 919篇 |
1994年 | 781篇 |
1993年 | 684篇 |
1992年 | 577篇 |
1991年 | 500篇 |
1990年 | 431篇 |
1989年 | 318篇 |
1988年 | 225篇 |
1987年 | 214篇 |
1986年 | 195篇 |
1985年 | 175篇 |
1984年 | 112篇 |
1983年 | 107篇 |
1982年 | 67篇 |
1981年 | 53篇 |
1980年 | 39篇 |
1977年 | 25篇 |
1976年 | 28篇 |
1975年 | 26篇 |
1973年 | 23篇 |
1972年 | 21篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
241.
设u(z)是单位圆内的实值调和函数 ,若 p_平均Mp(r ,u) =12π∫2π0|u(reiθ) |pdθ1 p <∞ ,则称u(z) ∈hp( 1
相似文献
242.
研究了基于级联二阶非线性的铌酸锂波导全光波长变换器的特性.首先从耦合模方程出发,比较了数值分析结果与小信号近似分析的结果.其次在数值分析基础上,分析了铌酸锂晶体的温度变化导致相应基频光波波长与极化反转光栅周期的变化关系.最后分析了在不同相互作用长度下,转换的光波功率与有效基频光波波长带宽、温度调谐带宽、极化反转光栅周期带宽等关系,以对全光波长变换器件进行优化设计
关键词:
级联二阶非线性
波长变换
准位相匹配
铌酸锂光波导 相似文献
243.
ICP-AES测定饮用水源中的Cu、Mn、Pb、Cd、Zn 总被引:7,自引:2,他引:5
用ICP-AES法同时测定饮用水源中的Cu、Mn、Pb、Cd、Zn等重金属元素,具有基体效应小、测量范围宽等优点。检出限为0.2-4.0μg/L,回收率为91.5%-103.9%,相对标准偏差为0.29%-1.5%,测定密码样与实际样品,结果令人满意。 相似文献
244.
Yah ZHAO Xiao Qing LIU Hong You ZHU Zi Ming YANG Rong HUANG 《中国化学快报》2006,17(12):1543-1546
Bridged bis(β-cyclodextrin)s (CDs), as a very important family of CD derivatives, have been known that they can significantly alter the molecular binding ability and selectivity toward a variety of guests in comparison with parent cyclodextrin. Their two… 相似文献
245.
Qiu Hong WU He Xiang WANG Kun GAO State Key Laboratory of Applied Organic Chemistry College of Chemistry Chemical Engineering Lanzhou University Lanzhou 《中国化学快报》2006,17(2)
Many Ligularia species have long been used as traditional folk medicine for theirantibiotic, antiphlogistic and antitussive activities1, and were found to be an importantsource of eremophilane. In the course of our search for bioactive sesquiterpenoids, w… 相似文献
246.
247.
连通图的离散度是用s(G)来表示的,s(G)=max{ω(G-S)-|S|:ω(G-S)>1,SV(G)}.给出了两个完全图乘积的和一个完全图与路的乘积的离散度.还给出了两个完全图乘积的坚韧度. 相似文献
248.
Y.-Q. Peng J.-H. Yang F.-P. Lu Q.-S. Yang H.-W. Xing X.-S. Li C.-A. Song 《Applied Physics A: Materials Science & Processing》2007,86(2):225-229
Based on the assumption of Gaussian energy distributions of the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO), analytical expressions of generalized Einstein relation in chemically doped organic semiconductor are developed, by approximation of Coulomb traps with a rectangle potential well. Numerical calculations show that traditional Einstein relations do not hold for chemically doped organic semiconductors. Similar to physical doping, the dependence of diffusion coefficient to mobility D/μ ratio on the carrier concentration has a maximum. An essential difference between chemical doping and physical doping is that, the D/μ ratio in chemically doped organic semiconductors depends not only on carrier concentration and doping concentration, but also on the applied electric field. PACS 71.20.Rv; 72.90.+y; 73.50.-h 相似文献
249.
Yong LU Hong WANG Ye LIU Ming Yuan HE 《中国化学快报》2006,17(10):1397-1400
Recently, miniature H2 generator to power fuel cells for portable/micro electronic devices and passenger propulsion has been the focus of intense research activities1-3. One of the strategies is to find simple CO-free H2 production with novel microreactor… 相似文献
250.
Methods to synthesize magnetic Fe3O4 nanoparticles and to modify the surface of particles are presented in the present investigation. Fe3O4 magnetic nanoparticles were prepared by the co-precipitation of Fe3+ and Fe2+, NH3·H2O was used as the precipitating agent to adjust the pH value, and the aging of Fe3O4 magnetic nanoparticles was accelerated by microwave (MW) irradiation. The obtained Fe3O4 magnetic nanoparticles were characterized by Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), X-ray powder diffraction (XRD) and vibrating sample magnetometer (VSM). The average size of Fe3O4 crystallites was found to be around 8–9 nm. Thereafter, the surface of Fe3O4 magnetic nanoparticles was modified by stearic acid. The resultant sample was characterized by FT-IR, scanning electron microscopy (SEM), XRD, lipophilic degree (LD) and sedimentation test. The FT-IR results indicated that a covalent bond was formed by chemical reaction between the hydroxyl groups on the surface of Fe3O4 nanoparticles and carboxyl groups of stearic acid, which changed the polarity of Fe3O4 nanoparticles. The dispersion of Fe3O4 in organic solvent was greatly improved. Effects of reaction time, reaction temperature and concentration of stearic acid on particle surface modification were investigated. In addition, Fe3O4/polystyrene (PS) nanocomposite was synthesized by adding surface modified Fe3O4 magnetic nanoparticles into styrene monomer, followed by the radical polymerization. The obtained nanocomposite was tested by thermogravimetry (TG), differential scanning calorimetry (DSC) and XRD. Results revealed that the thermal stability of PS was not significantly changed after adding Fe3O4 nanoparticles. The Fe3O4 magnetic fluid was characterized using UV–vis spectrophotometer, Gouy magnetic balance and laser particle-size analyzer. The testing results showed that the magnetic fluid had excellent stability, and had susceptibility of 4.46×10−8 and saturated magnetization of 6.56 emu/g. In addition, the mean size d (0.99) of magnetic Fe3O4 nanoparticles in the fluid was 36.19 nm. 相似文献