全文获取类型
收费全文 | 3719篇 |
免费 | 452篇 |
国内免费 | 1001篇 |
专业分类
化学 | 3072篇 |
晶体学 | 102篇 |
力学 | 114篇 |
综合类 | 95篇 |
数学 | 310篇 |
物理学 | 1479篇 |
出版年
2024年 | 10篇 |
2023年 | 25篇 |
2022年 | 61篇 |
2021年 | 53篇 |
2020年 | 78篇 |
2019年 | 67篇 |
2018年 | 55篇 |
2017年 | 85篇 |
2016年 | 94篇 |
2015年 | 106篇 |
2014年 | 140篇 |
2013年 | 238篇 |
2012年 | 232篇 |
2011年 | 248篇 |
2010年 | 215篇 |
2009年 | 211篇 |
2008年 | 229篇 |
2007年 | 273篇 |
2006年 | 275篇 |
2005年 | 214篇 |
2004年 | 223篇 |
2003年 | 211篇 |
2002年 | 227篇 |
2001年 | 179篇 |
2000年 | 135篇 |
1999年 | 129篇 |
1998年 | 124篇 |
1997年 | 140篇 |
1996年 | 114篇 |
1995年 | 127篇 |
1994年 | 105篇 |
1993年 | 94篇 |
1992年 | 107篇 |
1991年 | 84篇 |
1990年 | 81篇 |
1989年 | 54篇 |
1988年 | 41篇 |
1987年 | 17篇 |
1986年 | 8篇 |
1985年 | 10篇 |
1984年 | 4篇 |
1982年 | 5篇 |
1981年 | 3篇 |
1980年 | 8篇 |
1979年 | 13篇 |
1978年 | 4篇 |
1976年 | 2篇 |
1975年 | 2篇 |
1974年 | 2篇 |
1973年 | 6篇 |
排序方式: 共有5172条查询结果,搜索用时 15 毫秒
91.
负载稀土型酸活化膨润土的酯化催化活性 总被引:7,自引:0,他引:7
负载稀土型酸活化膨润土的酯化催化活性李永绣,冯天泽,胡平贵,何小彬,辜子英,廖荣辉(南昌大学化学系南昌330047)关键词稀土,膨润土,催化酯化可用稀土、膨润土单独作酯化催化剂,但活性不高[1~3]。本文将稀土用离子交换的方法负载到酸活化膨润土中,寻... 相似文献
92.
醇盐法制备稀土化合物超微粉末 总被引:16,自引:2,他引:16
稀土元素具有广泛的用途,在大多数的应用中其效果都与原料的纯度、粒度、粒经分布范围等因素有关。由于超微粉末的比表面积大、化学活性高,因此可以预料,若将稀土化合物制成超微粉末,会使它的催化性能及在各种材料中的性能有明显的改变,也可使稀土有新的、更高的应用价值。 相似文献
93.
The separation procedure for Ag, B, Cd, Dy, Eu and Sm as impurities in Gd matrix using ICP-AES technique with an extraction chromatographic column has been developed. The spectral interference of the Gd matrix on the elements was eliminated using a chromatography technique with 2-ethylhexyl phosphonic acid mono-2-ethylhexyl ester (PC-88A) as the mobile phase and XAD-16 resin as the stationary phase. Ag+, B4O72−, and Cd2+ were eluted with 0.1 M HNO3, while rare earth ions were not. The best eluent for separating Eu and Sm in the Gd matrix was 0.3 M HNO3. The limit of quantitation for these elements was 0.6-3.0 ng mL−1. The recovery of Ag, B, and Cd was 90-104% using 0.1 M HNO3 as the eluent, while that of Eu, Gd, and Sm ranged from 100 to 102% with 0.3 M HNO3. Dy was recovered quantitatively with 4 M HNO3. The relative standard deviation of the methods for a set of three replicates was between 1.0 and 15.4% for the synthetic and standard Gd solutions. The proposed separation procedure was used to measure Ag, B, Cd, Dy, Eu, and Sm in a standard Gd solution. 相似文献
94.
钇对金属陶瓷力学性能和组织的影响 总被引:6,自引:0,他引:6
研究了添加钇对Ti(C,N)基金属瓷力学性能和组织的影响。结果表明,加入适量的钇能提高Ti(C,N)基金属陶瓷的抗变强度和硬度,其原因是由于钇与杂质硫起反应在化合物,净化了陶瓷相-金属相,陶瓷相-陶瓷相界面提高子界面结合强度 相似文献
95.
铕掺杂BaPbO3的EXAFS研究 总被引:3,自引:0,他引:3
采用扩展X射线吸收精细结构方法,研究了铕掺杂BaPbO3多晶粉末中Eu,Pb两种原子的近邻结构。Eu的掺杂,导致八面体中金属Pg,Eu的配位数降低。 相似文献
96.
本文研究了LiLn(π-C_3H_5)_4·nD(Ln=La、Tb,n=3;Ln=Ce、Pr、Nd、Sm、Gd、Er,n=2;Ln=Y、Dy,n=2.5;D为二氧六环)催化异戊二烯和苯乙烯的均聚合反应.其中,钇配合物的催化活性为最高.聚异戊二烯顺式-1,4链节含量在43-81%.聚合体系内少量给电子体的加入使聚合物中3,4-链节含量高达84%.所得聚苯乙烯为无规结构. 相似文献
97.
采用溶胶-凝胶法制备了系列多晶CMR材料La0.7Ca0.3Mn1-xCuxO3(x=0%,1%,5%,10%,15%,20%)。在77-350 K温度范围内测量了样品的电阻率及磁电阻大小。实验结果发现:系列样品TC随掺杂量增加而递减;系列样品的电阻率峰及磁电阻峰在整个测温范围内随掺杂量的增加均依次被展宽直至消失,当掺杂量为x≥10%时,电阻率随温度的变化在整个测温范围内呈现绝缘相行为;同时还发现,掺杂量为x≥10%时,样品的磁电阻在整个测温区内基本表现为隧穿磁电阻。基于我们采用蒙特卡罗模拟方法提出的逾渗模型及隧穿磁电阻模型,对样品的磁电阻行为进行了很好的解释。对系列样品的ρ-T曲线进行lnρ-T^-(1/4)系拟合,发现掺杂量为x=10%样品在整个温区内的lnρ-T^-(1/4)系基本呈线性。 相似文献
98.
Summary A simplex method for determining the constantsS andk
o from the equation lnk’=lnk
o−S ϕ (was developed and applied to the reversed phase separation of high molar mass polystyrenes using gradients of any curvature.
Experimental retention times described using the equation had a standard deviation of 1.1%. The inclusion of a quadratic term
in the equation was found to be unwarranted. BothS and lnk
o varied linearly with ln molar mass. The logarithm of peak width of an eluting polystyrene peak was found to be a linear function
of the mobile phase composition at elution. The slope was equal toS. 相似文献
99.
Mujie Yang Xiaojun Zhang Jianfu Li Zhiquan Shen 《Journal of polymer science. Part A, Polymer chemistry》1992,30(1):63-69
The study of rare earth coordination catalysts for polymerization of 1-octene has been successfully carried out for the first time. Some features and kinetic behavior of polymerization of 1-octene by Nd(naph)3–AIEt3 catalyst system in tetrachloro-methane are described. The overall polymerization activation energy Ea measured was 74.5 kJ/mol and the rate equation could be expressed as Rp = kp [Nd] [M] (kp = 3.21 × 10?3 L/mol s, at 50°C). The catalytic activity of various rare earth elements in Ln (naph)3 and ligands in NdL3 for the polymerization was compared. A 1-octene oligomer with double bonds was obtained. It is either a white or pale yellow waxy semi-solid. Its number-average molecular weight is about 103 and the molecular weight distribution is less than 2. 相似文献
100.
Alexander V Lebedev Marina V Ivanova Alexander A Timoshin Enno K Ruuge 《Chemphyschem》2007,8(12):1863-1869
The unexpected effects of Ca(2+) on the free-radical chain reactions of dopamine, norepinephrine, isoproterenol, and pyrocatechol oxidation are studied using oxygen consumption measurements, EPR-spectroscopy, UV/VIS spectrophotometry, and by potentiometric titration. It is found that the formation of Ca(2+)-catecholate complexes is accompanied by an increase in the dissociation constants (K(ai) ) of their phenolic hydroxyls. At pH>pK(ai) and in the presence of alkaline-earth metal cations, the rate of catecholate oxidation increases (Ca(2+), Mg(2+)> Sr(2+), Ba(2+)), whereas on addition of Zn ions the rate decreases. The effects of Group II metal cations on catecholate autoxidation are concomitant with a transient increase of the EPR signal for metal-semiquinonate complexes. Therefore, the effects of Ca(2+) and other alkaline-earth metal cations on catecholate autoxidation can be defined as 1) additional deprotonation of catechol OH-groups involved in the formation of M(2+)-catecholate complexes, the latter exceeding catechols in the susceptibility to dioxygen-induced oxidation and 2) formation of relatively stable free-radical intermediates responsible for chain propagation. 相似文献