全文获取类型
收费全文 | 3292篇 |
免费 | 222篇 |
国内免费 | 564篇 |
专业分类
化学 | 3435篇 |
晶体学 | 79篇 |
力学 | 50篇 |
综合类 | 46篇 |
数学 | 5篇 |
物理学 | 463篇 |
出版年
2024年 | 9篇 |
2023年 | 44篇 |
2022年 | 92篇 |
2021年 | 103篇 |
2020年 | 122篇 |
2019年 | 105篇 |
2018年 | 98篇 |
2017年 | 124篇 |
2016年 | 122篇 |
2015年 | 123篇 |
2014年 | 133篇 |
2013年 | 375篇 |
2012年 | 191篇 |
2011年 | 159篇 |
2010年 | 154篇 |
2009年 | 172篇 |
2008年 | 153篇 |
2007年 | 175篇 |
2006年 | 167篇 |
2005年 | 169篇 |
2004年 | 155篇 |
2003年 | 132篇 |
2002年 | 127篇 |
2001年 | 90篇 |
2000年 | 66篇 |
1999年 | 87篇 |
1998年 | 85篇 |
1997年 | 78篇 |
1996年 | 74篇 |
1995年 | 58篇 |
1994年 | 52篇 |
1993年 | 33篇 |
1992年 | 41篇 |
1991年 | 22篇 |
1990年 | 29篇 |
1989年 | 22篇 |
1988年 | 23篇 |
1987年 | 13篇 |
1986年 | 20篇 |
1985年 | 14篇 |
1984年 | 6篇 |
1982年 | 13篇 |
1981年 | 5篇 |
1980年 | 4篇 |
1978年 | 6篇 |
1977年 | 4篇 |
1976年 | 7篇 |
1975年 | 4篇 |
1973年 | 7篇 |
1972年 | 3篇 |
排序方式: 共有4078条查询结果,搜索用时 15 毫秒
51.
光度法同时测定铂和钯 总被引:1,自引:0,他引:1
水溶性试剂N-间甲苯基-N’-(对氨基苯磺酸钠)硫脲(MMPT)在HAc-NaAc缓冲介质和CTMAB(溴化十六烷基三甲胺)存在下,与铂(Ⅳ)和钯(Ⅱ)反应形成绿色和褐色水溶性络合物,其最大吸收波长分别为754.4和304.6 nm。符合比尔定律的浓度范围分别为0~32 μg·(25 mL)-1, 0~25 μg·25(mL)-1,摩尔吸光系数为εPt754.4=8.6×104 L·mol-1·cm-1和εPd304.6=7.4×104 L·mol-1·cm-1。在754.4和304.6 nm波长测定铂,钯混合液的吸光度,换算出铂、钯的含量。50种共存离子中, 仅有Cu2+和Co2+ 对钯的测定有干扰。该法选择性好,用于合成样品、矿石和催化剂样品中铂、钯含量的测定, 其相对标准偏差RSD小于2.0%;回收实验回收率是96%~104%。由于试剂铂、钯形成的络合物都易溶于水、不需予先分离同时测定,操作简便,快速,且环境友好。 相似文献
52.
Energies and wavefunctions of low-lying states and Rydberg states for the sodium atom in uniform magnetic fields varying from 0 to 10^5T are calculated using a pseudospectral approach with a model potential in spherical coordinates. The energies are comparable with experimental results as well as those obtained by other calculations. The spectra of oscillator strength are worked out. The evolution of them with the magnetic field is shown. 相似文献
53.
对肝素钠、多硫酸软骨素标准品进行了1H NMR测试并对谱图进行了全归属,指出: 肝素钠乙酰甲基特征峰在δ 2.04处,多硫酸软骨素的乙酰甲基特征峰在δ 2.15处. 同时考察了精细测试的实验条件,结果表明:当样品浓度为25 mg/0.6 mL、定标物TSP含量为0.020%(W/V)、样品旋转为12 Hz、处理参数(LB)为1.0、仪器频率为500 MHz时,能快速并准确的检测出肝素钠中的污染物(多硫酸软骨素). 相似文献
54.
将激光频率锁定于合适的参考频率,可以有效地抑制激光器的频率起伏。本文采用铷原子D2线超精细跃迁线的饱和吸收光谱和偏振光谱分别获得鉴频曲线,通过电子伺服系统将频率校正信号负反馈到780 nm光栅外腔反馈半导体激光器外腔的压电陶瓷上的方法对激光器进行稳频。介绍了两种方法的基本原理和实验方案。与激光器自由运转300s时激光器典型的频率起伏约6.6 MHz相比,采用饱和吸收光谱和偏振光谱进行稳频,运转300 s时激光器典型的残余频率起伏分别约为1.5 MHz和0.6 MHz。分析表明,饱和吸收光谱稳频采用了相敏检波技术,需要对激光器进行频率调制,带来了额外的频率噪声,而偏振光谱稳频则是一种完全无频率调制的稳频方案。 相似文献
55.
56.
57.
几种常见光源特性的研究 总被引:3,自引:1,他引:3
本通过大学物理实验中常用光源特性的分析,说明了实验中光源使用的合理性。 相似文献
58.
The aim of the study was to investigate the impact of sodium alginate (ALG) pretreated by ultrasound on the enzyme activity, structure, conformation and molecular weight and distribution of papain. ALG solutions were pretreated with ultrasound at varying power (0.05, 0.15, 0.25, 0.35, 0.45 W/cm2), 135 kHz, 50 °C for 20 min. The maximum relative activity of papain increased by 10.53% when mixed with ALG pretreated by ultrasound at 0.25 W/cm2, compared with the untreated ALG. The influence of ultrasound pretreated ALG on the conformation and secondary structure of papain were assessed by fluorescence spectroscopy and circular dichroism spectroscopy. The fluorescence spectra revealed that ultrasound pretreated ALG increased the number of tryptophan on papain surface, especially at 0.25 W/cm2. It indicated that ultrasound pretreatment induced molecular unfolding, causing the exposure of more hydrophobic groups and regions from inside to the outside of the papain molecules. Furthermore, ultrasound pretreated ALG resulted in minor changes in the secondary structure of the papain. The content of α-helix was slightly increased after ultrasound pretreatment and no significant change was observed at different ultrasound powers. ALG pretreated by ultrasound enhanced the stability of the secondary structure of papain, especially at 0.25 W/cm2. The free sulfhydryl (SH) content of papain was slightly increased and then decreased with the increase of ultrasonic power. The maximum content of free SH was observed at 0.25 W/cm2, under which the content of the free SH increased by 6.36% compared with the untreated ALG. Dynamic light scattering showed that the effect of ultrasound treatment was mainly the homogenization of the ALG particles in the mixed dispersion. The gel permeation chromatography coupled with the multi-angle laser light scattering photometer analysis showed that the molecular weight (Mw) of papain/ALG was decreased and then increased with the ultrasonic pretreatment. Results demonstrated that the activity of immobilized papain improved by ultrasonic pretreatment was mainly caused by the variation of the conformation of papain and the effect of interactions between papain and ALG. This study is important to explain the intermolecular interactions of biopolymers and the mechanism of enzyme immobilization treated by ultrasound in improving the enzymatic activity. As expected, ALG pretreated by appropriate ultrasound is promising as a bioactive compound carrier in the field of immobilized enzyme. 相似文献
59.
60.