全文获取类型
收费全文 | 4625篇 |
免费 | 622篇 |
国内免费 | 167篇 |
专业分类
化学 | 4436篇 |
晶体学 | 3篇 |
力学 | 27篇 |
综合类 | 17篇 |
数学 | 668篇 |
物理学 | 263篇 |
出版年
2023年 | 52篇 |
2022年 | 230篇 |
2021年 | 198篇 |
2020年 | 209篇 |
2019年 | 151篇 |
2018年 | 170篇 |
2017年 | 130篇 |
2016年 | 213篇 |
2015年 | 240篇 |
2014年 | 278篇 |
2013年 | 388篇 |
2012年 | 247篇 |
2011年 | 316篇 |
2010年 | 245篇 |
2009年 | 299篇 |
2008年 | 264篇 |
2007年 | 284篇 |
2006年 | 238篇 |
2005年 | 208篇 |
2004年 | 166篇 |
2003年 | 153篇 |
2002年 | 202篇 |
2001年 | 59篇 |
2000年 | 65篇 |
1999年 | 63篇 |
1998年 | 55篇 |
1997年 | 43篇 |
1996年 | 38篇 |
1995年 | 27篇 |
1994年 | 29篇 |
1993年 | 23篇 |
1992年 | 9篇 |
1991年 | 12篇 |
1990年 | 8篇 |
1989年 | 8篇 |
1988年 | 9篇 |
1987年 | 12篇 |
1986年 | 5篇 |
1985年 | 8篇 |
1984年 | 5篇 |
1983年 | 8篇 |
1982年 | 8篇 |
1981年 | 5篇 |
1980年 | 8篇 |
1979年 | 6篇 |
1976年 | 2篇 |
1975年 | 2篇 |
1973年 | 3篇 |
1969年 | 2篇 |
1968年 | 2篇 |
排序方式: 共有5414条查询结果,搜索用时 15 毫秒
991.
《Ultrasonics sonochemistry》2014,21(2):606-611
The present work deals with the mapping of an ultrasonic bath for the maximum extraction of mangiferin from Mangifera indica leaves. I3− liberation experiments (chemical transformations) and extraction (physical transformations) were carried out at different locations in an ultrasonic bath and compared. The experimental findings indicated a similar trend in variation in an ultrasonic bath by both these methods. Various parameters such as position and depth of vessel in an ultrasonic bath, diameter and shape of a vessel, frequency and input power which affect the extraction yield have been studied in detail. Maximum yield of mangiferin obtained was approximately 31 mg/g at optimized parameters: distance of 2.54 cm above the bottom of the bath, 7 cm diameter of vessel, flat bottom vessel, 6.35 cm liquid height, 122 W input power and 25 kHz frequency. The present work indicates that the position and depth of vessel in an ultrasonic bath, diameter and shape of a vessel, frequency and input power have significant effect on the extraction yield. This work can be used as a base for all ultrasonic baths to obtain maximum efficiency for ultrasound assisted extraction. 相似文献
992.
993.
Bassanese DN Crawford-Clark S Bowen BJ Henderson LC Barnett NW Conlan XA 《Journal of separation science》2012,35(9):1110-1117
The need to monitor biogenic amines levels is essential for many areas of the food industry for two main reasons: the caustic nature and potential toxicity of these amines, and the potential to use amine levels as markers for freshness and quality in foodstuffs. Optimised analysis conditions used for the determination of biogenic amines derivatised with 2-napthyloxycarbonyl chloride has been applied to different pet food samples to assess the effectiveness of this method for complex sample matrices. Further to this, the use of high-resolution mass spectrometry has enabled the previously unconfirmed derivatised form of seven biogenic amines to be established. The derivatised forms identified include as mono substituted (tryptamine and histamine), bisubstituted (putrescine, cadaverine and tyramine), trisubstituted (spermidine) and tetrasubstituted (spermine). The methodology of biogenic amine determination was performed successfully to a range of pet food products highlighting the applicability to a variety of complex sample matrices. 相似文献
994.
Eliseo Herrero‐Hernández Eva Pose‐Juan Alba Álvarez‐Martín María Soledad Andrades María Sonia Rodríguez‐Cruz María J. Sánchez‐Martín 《Journal of separation science》2012,35(24):3492-3500
A reliable multiresidue method based on solid phase extraction was developed using GC–MS to determine and quantify 34 pesticides, including herbicides, fungicides, insecticides, and some of their degradation products, in groundwater in a vineyard region of La Rioja (northern Spain). Different parameters were optimized and good recoveries (65–108% range) and precisions (12–19% range) were achieved with spiked water samples for a concentration of 0.1 μg/L. The experimental results showed an excellent linearity (r2 > 0.99) over the 0.1–1.5 μg/L range. The detection limits of the proposed method were 1–37 ng/L for most of the compounds studied. The methodology has been successfully applied to the analysis of groundwater samples from vineyard areas in La Rioja and the presence of pesticides, especially fungicides and herbicides, at several concentration levels was revealed. Terbuthylazine, its metabolite desethyl‐terbuthylazine, and fluometuron were the pesticides most frequently detected in higher concentrations. Overall and taking into consideration the European Union maximum residue limit of pesticides in groundwater, 16 of the 34 compounds included in this study were detected in concentrations over that limit in at least one of the samples analyzed. 相似文献
995.
Yong Li Zhi‐Xing Chen Dr. Qing Xiao Qin‐Da Ye Tian‐Wen Sun Fan‐Ke Meng Dr. Wei‐Wu Ren Lin You Ling‐Min Xu Yue‐Fan Wang Dr. Jia‐Hua Chen Dr. Zhen Yang 《化学:亚洲杂志》2012,7(10):2334-2340
The successful synthesis of the highly complex model compound ( 2 ) of the CEFGH ring system of schindilactone A ( 1 ) is described. Several synthetic methodologies were developed and applied to achieve this goal, including ring‐closing metathesis (RCM) and Co–thiourea‐catalyzed Pauson–Khand reactions. Furthermore, two independent approaches were developed for the construction of the GH ring of model compound 2 , the key steps of which included Pd–thiourea‐catalyzed carbonylative annulation, methylation, and sequential RCM/oxa‐Michael‐addition reactions. The chemistry developed herein has provided a greater understanding of the synthesis of schindilactone A ( 1 ) and its analogous compounds of the same family. 相似文献
996.
Dr. Hayato Fukuda Yuko Nishiyama Shiina Nakamura Yutaro Ohno Prof. Dr. Tadashi Eguchi Prof. Dr. Yoshiharu Iwabuchi Prof. Dr. Takeo Usui Prof. Dr. Naoki Kanoh 《化学:亚洲杂志》2012,7(12):2872-2881
We have developed two syntheses of vicenistatin and its analogues. Our first‐generation strategy included the rapid and sequential assembly of the macrocyclic lactam by using an intermolecular Horner–Wadsworth–Emmons reaction between the C3–C13 fragment and the C1–C2, C14–C19 fragment, followed by an intramolecular Stille coupling reaction. The second‐generation strategy utilized a ring‐closing metathesis of a hexaene intermediate to generate the desired 20‐membered macrolactam. This second‐generation strategy made it possible to prepare synthetic analogues of vicenistatin, including the C20‐ and/or C23‐demethyl analogues. Evaluation of the cytotoxic effect of these analogues indicated the importance of the fixed conformation of aglycon for determining the biological activity of the vicenistatins. 相似文献
997.
998.
999.
1000.