全文获取类型
收费全文 | 3198篇 |
免费 | 165篇 |
国内免费 | 85篇 |
专业分类
化学 | 326篇 |
晶体学 | 11篇 |
力学 | 1178篇 |
综合类 | 2篇 |
数学 | 1203篇 |
物理学 | 728篇 |
出版年
2024年 | 1篇 |
2023年 | 37篇 |
2022年 | 28篇 |
2021年 | 40篇 |
2020年 | 66篇 |
2019年 | 75篇 |
2018年 | 94篇 |
2017年 | 75篇 |
2016年 | 84篇 |
2015年 | 80篇 |
2014年 | 106篇 |
2013年 | 263篇 |
2012年 | 140篇 |
2011年 | 185篇 |
2010年 | 161篇 |
2009年 | 208篇 |
2008年 | 183篇 |
2007年 | 203篇 |
2006年 | 156篇 |
2005年 | 155篇 |
2004年 | 135篇 |
2003年 | 133篇 |
2002年 | 97篇 |
2001年 | 74篇 |
2000年 | 88篇 |
1999年 | 79篇 |
1998年 | 74篇 |
1997年 | 51篇 |
1996年 | 56篇 |
1995年 | 32篇 |
1994年 | 49篇 |
1993年 | 49篇 |
1992年 | 34篇 |
1991年 | 37篇 |
1990年 | 15篇 |
1989年 | 22篇 |
1988年 | 18篇 |
1987年 | 8篇 |
1986年 | 12篇 |
1985年 | 9篇 |
1984年 | 11篇 |
1983年 | 4篇 |
1982年 | 8篇 |
1981年 | 6篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1977年 | 2篇 |
1975年 | 1篇 |
1969年 | 1篇 |
排序方式: 共有3448条查询结果,搜索用时 15 毫秒
51.
We propose a new approach for high-throughput continuous titration based on a flow ratiometry. The method was applied to potentiometric titrations of acids and bases. A base solution, the flow rate FB of which was varied in response to controller output voltage Vc, was merged with an acid solution under constant total (acid + base) flow rate. Downstream, the pH of the mixed solution was measured with a flow-through glass electrode. Initially, Vc, and thus FB was increased linearly. At the instant the equivalence point (EP) was sensed, the ramp direction of Vc was reversed from upward to downward. The direction was reversed to upward when EP was sensed again. Such the feedback-based operation gives a triangular waveform of Vc, because there is a delay corresponding mainly to the transit time of merged solutions to reach the sensor. The value of Vc that gives EP composition, VE, was estimated by averaging the most recent maximum and minimum values of Vc. Next, fixed triangular waves of Vc was used to control FB. The amplitude and the scan rate of the waves were fixed narrower and faster, respectively, than those in the feedback-based operation in order to improve the throughput rate. The EP can be located as long as the scan range covers VE. These automated processes limited the titration to just the narrow range around EP, and thus realized extremely high throughput rate of maximally 17.6 titrations per minute (=3.4 s per titration) at R.S.D. = 0.35%. 相似文献
52.
Glycosylations are notoriously difficult reactions that require extensive optimization regarding the type of anomeric leaving group, solvent, reaction temperature, and reaction time. Described is the use of a silicon‐based microreactor to screen reaction conditions and to scale‐up synthetic procedures. For the first time, glycosyl phosphates were employed in a microreactor. The optimized reaction conditions were successfully transferred to a batch process. 相似文献
53.
Moreno-Cid A Yebra MC Cancela S Cespón RM 《Analytical and bioanalytical chemistry》2003,377(4):730-734
Iron was extracted on-line from solid meat samples by a simple and rapid continuous ultrasound-assisted extraction system (CUES). The CUES is connected to a flow injection manifold, which allows the on-line flame atomic absorption spectrometric determination of iron. A Plakett–Burman design was used for the optimisation of the CUES. The method achieved a total sampling frequency of 11 samples per hour with a relative standard deviation for the complete procedure of 0.4%. The detection limit was 0.6 g g–1 (dry mass) for a sample amount of 30 mg. Accurate results were obtained by measuring the certified reference materials BCR-186 (pig kidney) and BCR-184 (bovine muscle). The analytical procedure was applied to different real meat samples with satisfactory results. 相似文献
54.
Some polymer melts (of high viscosity ) can wet completely the surface of a non miscible, simple liquid. We discuss here the laws of spreading for a macroscopic droplet of this type, when the internal friction of the droplet dominates. We predict a droplet radius increasing liket
1/4 wheret is the spreading time, or equivalently a droplet curvature decreasing liket
–1. The droplet should be surrounded by a precursor film, which is not discussed in the present note. 相似文献
55.
本文研究了Mn~(2+)和Co~(2+)对7-(8-羟基-3,6-二磺基萘偶氮)-8-羟基喹啉-5-磺酸-硼砂反应体系的混合荧光增敏作用。实验条件下,荧光增敏强度满足线性加和关系的范围是:Mn~(2+)浓度0~2.9×10~(-7)mol/L;Co~(2+)浓度0~8.8×10~(-7)mol/L;总浓度不超过1.0×10~(-6)mol/L。检出限量为Mn~(2+)4.5×10~(-9)mol/L和Co~(2+)1.4×10~(-8)mol/L。实现了Mn~(2+)和Co~(2+)的连续测定。 相似文献
56.
在一个石英反应室内,让激光解离的Ni^+与连续喷入的酯类化合物分子束流反应,生成的产物离子经飞行时间质谱仪检测,研究了Ni^+与乙酸乙酯、乙酸丙烯酯、乙酸乙烯酯、乙酸丁酯的气相反应。根据如下机理满意的解释了发生的反应:(1)Ni^+与酯形成激发态络合物;(2)β-H迁移至烃氧基的氧上形成Ni^+的双配体化合物离子;(3)失去一中性配体分子得Ni^+的单配体化合物离子。由酯的ΔH离和D(Ni^+-A 相似文献
57.
This paper describes the screening for metabolites of butoprozine, a new anti-anginal drug, in dog bile by means of reverse-phase HPLC. Although it does involve a simple clean-up step to remove a substantial amount of endogenous bile compounds, this screening method nevertheless avoids extraction of metabolites and thus allows all metabolites to be introduced into the chromatographic system. A single run of 100 minutes from 100% water to 100% methanol in a linear gradient effects adequate separation of the great majority of metabolites without interference from remaining endogenous compounds. Two methods of differentiating between metabolite peaks and endogenous peaks have been worked out. The first one makes use of 14C-labeled butoprozine by measuring the amount of radioactivity in the column effluent while simultaneously recording the UV absorbance. The second method compares continuous gradient chromatograms of bile recorded before and after butoprozine administration under very similar conditions. The latter method can be applied to both radioactive and non-radioactive materials. 相似文献
58.
59.
Rafael Lucena 《Analytica chimica acta》2004,509(1):47-54
A multipurpose chamber capable of implementing a variety of continuous separation techniques is reported. The proposed module can be used in conjunction with flow manifolds coupled on-line to optical detectors in order to develop simple autoanalyzers for full sample pretreatment. The analytical applicability of such a device is discussed with specific regard to gas diffusion, dialysis, solid-phase extraction (SPE), and precipitation/dissolution techniques. Its versatility and simplicity are among the main advantages. Four different automated analyzers based on the use of the multipurpose chamber have been developed for monitoring of different quality control parameters of liquid food samples, namely, carbon dioxide in beer, ascorbic acid in fruit juice, caffeine in soft drinks and lactose, fat and protein in milk. The results obtained were compared with those provided by the existing alternatives for the same applications in terms of sensitivity and precision. 相似文献
60.
《Electroanalysis》2005,17(8):668-673
A self‐sampling‐and‐flow biosensor was fabricated by sandwiching a nitrocellulose strip on the working electrode side of the double‐sided microporous gold electrodes and a wicking pad on the counter electrode side. The double‐sided microporous electrodes were formed by plasma sputtering of gold on a porous nylon substrate. Sample was taken up to the enzyme‐immobilized working electrode by the capillary action of the front nitrocellulose strip dipped into the sample solution, analyzed electrochemically at the enzyme‐immobilized electrode, and diffuses out to the backside wicking pad through the micropores of the electrodes, constituting a complete flow cell device with no mechanical liquid‐transporting device. Biosensor was formed by co‐immobilizing the glucose oxidase and electron transfer mediator (ferrocene acetic acid) on the thioctic acid self‐assembled monolayer‐modified working electrode. A typical response time of the biosensor was about 5 min with the sensitivity of 2.98 nA/mM glucose, providing linear response up to 22.5 mM. To demonstrate the use of self‐sampling‐and‐flow biosensor, the consumption rate of glucose in the presence of yeast was monitored for five days. 相似文献