全文获取类型
收费全文 | 13633篇 |
免费 | 799篇 |
国内免费 | 824篇 |
专业分类
化学 | 9190篇 |
晶体学 | 11篇 |
力学 | 40篇 |
综合类 | 89篇 |
数学 | 85篇 |
物理学 | 1073篇 |
综合类 | 4768篇 |
出版年
2024年 | 34篇 |
2023年 | 172篇 |
2022年 | 528篇 |
2021年 | 398篇 |
2020年 | 354篇 |
2019年 | 372篇 |
2018年 | 363篇 |
2017年 | 516篇 |
2016年 | 587篇 |
2015年 | 586篇 |
2014年 | 677篇 |
2013年 | 822篇 |
2012年 | 1000篇 |
2011年 | 880篇 |
2010年 | 708篇 |
2009年 | 825篇 |
2008年 | 654篇 |
2007年 | 877篇 |
2006年 | 694篇 |
2005年 | 645篇 |
2004年 | 559篇 |
2003年 | 408篇 |
2002年 | 319篇 |
2001年 | 283篇 |
2000年 | 227篇 |
1999年 | 220篇 |
1998年 | 188篇 |
1997年 | 175篇 |
1996年 | 173篇 |
1995年 | 146篇 |
1994年 | 138篇 |
1993年 | 115篇 |
1992年 | 106篇 |
1991年 | 103篇 |
1990年 | 108篇 |
1989年 | 71篇 |
1988年 | 46篇 |
1987年 | 53篇 |
1986年 | 24篇 |
1985年 | 16篇 |
1984年 | 15篇 |
1983年 | 10篇 |
1982年 | 15篇 |
1981年 | 6篇 |
1980年 | 6篇 |
1979年 | 8篇 |
1977年 | 4篇 |
1976年 | 6篇 |
1974年 | 4篇 |
1973年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
Yasuyuki Takeda Chihiro Takagi 《Journal of inclusion phenomena and macrocyclic chemistry》1994,17(1):93-97
The theory is derived to determine the dimer-formation constant,K
2, of a crown ethermetal salt 111 complex in organic solvents of low dielectric constant by solvent extraction. The theoretical predictions are verified experimentally by extraction of sodium picrate (NaA) with 15-crown-5 (15C5) into carbon tetrachloride. All the experiments were conducted at 25°C. The logK
2 value of the Na(15C5)A complex in carbon tetrachloride has been determined to be 4.05±0.11. Moreover, the partition constant of the complex is calculated. 相似文献
112.
The acetone-butanol production by simultaneous saccharification and extractive fermentation (SSEF) was investigated. In the
SSEF employing cellulase enzymes andClostridium acetobutylicum, both glucan and xylan fractions of pretreated aspen are concurrently converted into acetone and butanol. Continuous removal
of the fermentation products from the bioreactor by extraction was an important factor that allowed long-term fed-batch operation.
The use of membrane extraction prevented the problems of phase separation and extractant loss. Increase in substrate feeding
as well as reduction of nutrient supply was found to be beneficial in suppressing the acid production, thereby improving the
solvent yield. Because of prolonged low growth conditions prevalent in the fed-batch operation, the butanol-to-acetone ratio
in the product was significantly higher at 2.6–2.8 compared to the typical value of two. 相似文献
113.
114.
人参中总皂甙的超声波快速提取方法研究 总被引:7,自引:0,他引:7
建立了人参中总皂甙超声波半微量快速提取方法,与索氏提取法相比,此法提取时间由索氏法的1-2个工作日缩短到30min,节省大量试剂,综合提取效率提高数十倍,与索氏提取法对照分析表明,此法可完全满足人参中总皂甙的快速测定要求。 相似文献
115.
《Arabian Journal of Chemistry》2022,15(10):104116
The misuses of veterinary drugs can result in the accumulation of residues in food of animal origin that can make its way to the final consumer. Herein we describe a simple method for the accurate determination of beta-lactams, quinolones, sulphonamides, and tetracyclines in fish, poultry, and red meat. No extraction cartridges were used; instead, the extraction process consisted of the addition of an organic solvents, shaking, centrifugation, and dilution. An extensive validation process demonstrated an excellent linearity (R2 ≥ 0.99) for 23-drug residues. The recovery of drugs in different matrices at two concentration levels (n = 6) was in the range of 82–119%. The method was proved to be repeatable and reproducible with intra/inter-day measurements (RSDs lower than 20%). The quantification limits of drug residues were in the range of 0.8 to 45.3 ug/kg, which is well below the maximum residue limits set by most regulatory authorities. This method was successfully applied to the routine analysis of 20 fish, poultry, and red meat samples (n = 60). 相似文献
116.
Sequential extraction of phosphorus in soil and sediment using a continuous-flow system 总被引:1,自引:0,他引:1
Distribution and mobility of phosphorus in soil and sediment are usually studied by sequential extraction. In the extraction procedure, a sample is treated with a series of reagents to distinguish the phases to which phosphorus is associated such as carbonate and iron or manganese oxides, etc. There have been a number of extraction schemes presented for phosphorus. At present, all of the existing schemes are carried out batchwise. Phosphorus contents derived from all sequences are operationally defined and depend on experimental conditions.An extraction procedure, which is a continuous-flow-based technique, was recently proposed by our group for metals in soils and sediments. The extraction is carried out in a closed chamber through which extractants are passed sequentially. In this paper, the system was investigated using the extraction scheme of Hieltjes and Lijklema to study distribution of phosphorus in three certified reference materials (CRMs). A number of fractions were collected for each reagent for subsequent colorimetric determination. The results are compared with those obtained from a batch extraction. The summation of phosphorus contents of all phases were compared with the certified values and with the values obtained from total digestion. These results have demonstrated that the continuous extraction system developed is also applicable for fractionation of phosphorus. Advantage and disadvantage are discussed. 相似文献
117.
Three acyclic polythioethers containing 2-thienyl units at both ends were synthesized and the effect of substituent on the extraction of copper(II) was studied. The methyl groups in the terminal thiophene ring have imparted an appreciable degree of increase in the percent extraction of copper(II), while the introduction of chlorine atoms into the 2-thienyl unit resulted in the reverse effect. Among the counter dye anions examined, tetrabromophenolphthalein ethylester was the best one for copper(II) extraction. The composition of extracted species was evaluated to be 122 (Cu(II)/polythioether/dye anion). Quantitative extraction of copper(I) was attained as complexes with various triphenylmethane dyes, i.e., bromocresol green, bromothymol blue, bromo-phenol blue and pyrogallol red. Copper(I) in organic phase was completely back-extracted with 2 mole/l sulfuric acid containing 10% hydrogen peroxide. 相似文献
118.
吴镇扬 《东南大学学报(自然科学版)》1995,(Z1)
一些科学工作者对人类以及动物听觉的自由声场至耳膜的传递函数进行了测量,他们发现在宽带信号激励的情况下,不同频率分量之间的幅度差异是与声源的方向有关的,这一发现已被用于展现“真实”的三维听觉,称作虚拟听觉空间仿真。 相似文献
119.
Summary A new method for the determination of microcystins PR and LR in cyanobacterium was developed using cyano-type prepacked cartridges. The microcystins were extracted with 10% acetic acid and the extract applied to a Baker 10 cyano cartridge. After elution from the cartridge with 0.5M acetic acid-acetonitrile (19) the microcystins were determined by HPLC. Better recoveries and chromatograms were observed than with ODS cartridges. 相似文献
120.
现有的双谱特征提取方法可以满足信号分类识别,但是出现了交叉项、平凡双谱以及特征维数过高等一些问题。针对以上问题,提出一种双谱二次特征提取方法,将双谱转化为灰度图像,以灰度值表示双谱幅度;再利用图像处理技术提取双谱二次特征,提取出能够表征辐射源信号双谱图像纹理信息的灰度共生矩阵特征集;将该特征集与Hu-不变矩特征集进行对比实验。仿真结果表明:该方法具有更好的分类识别性能,对于CW、LFM和NLFM信号的平均识别率均达90%以上。 相似文献