全文获取类型
收费全文 | 1284篇 |
免费 | 125篇 |
国内免费 | 98篇 |
专业分类
化学 | 832篇 |
晶体学 | 33篇 |
力学 | 58篇 |
综合类 | 4篇 |
数学 | 181篇 |
物理学 | 399篇 |
出版年
2023年 | 17篇 |
2022年 | 30篇 |
2021年 | 36篇 |
2020年 | 28篇 |
2019年 | 34篇 |
2018年 | 37篇 |
2017年 | 30篇 |
2016年 | 39篇 |
2015年 | 49篇 |
2014年 | 71篇 |
2013年 | 94篇 |
2012年 | 92篇 |
2011年 | 104篇 |
2010年 | 69篇 |
2009年 | 46篇 |
2008年 | 60篇 |
2007年 | 67篇 |
2006年 | 65篇 |
2005年 | 51篇 |
2004年 | 61篇 |
2003年 | 35篇 |
2002年 | 28篇 |
2001年 | 25篇 |
2000年 | 29篇 |
1999年 | 20篇 |
1998年 | 14篇 |
1997年 | 20篇 |
1996年 | 16篇 |
1995年 | 18篇 |
1994年 | 11篇 |
1993年 | 7篇 |
1992年 | 12篇 |
1991年 | 12篇 |
1990年 | 13篇 |
1989年 | 10篇 |
1988年 | 7篇 |
1987年 | 8篇 |
1986年 | 6篇 |
1985年 | 9篇 |
1984年 | 12篇 |
1982年 | 8篇 |
1981年 | 10篇 |
1980年 | 22篇 |
1979年 | 9篇 |
1978年 | 5篇 |
1977年 | 11篇 |
1976年 | 7篇 |
1974年 | 7篇 |
1973年 | 7篇 |
1969年 | 4篇 |
排序方式: 共有1507条查询结果,搜索用时 984 毫秒
41.
Advances in microchannel electrophoretic separation systems for DNA analyses have had important impacts on biological and biomedical sciences, as exemplified by the successes of the Human Genome Project (HGP). As we enter a new era in genomic science, further technological innovations promise to provide other far-reaching benefits, many of which will require continual increases in sequencing and genotyping efficiency and throughput, as well as major decreases in the cost per analysis. Since the high-resolution size- and/or conformation-based electrophoretic separation of DNA is the most critical step in many genetic analyses, continual advances in the development of materials and methods for microchannel electrophoretic separations will be needed to meet the massive demand for high-quality, low-cost genomic data. In particular, the development (and commercialization) of miniaturized genotyping platforms is needed to support and enable the future breakthroughs of biomedical science. In this review, we briefly discuss the major sequencing and genotyping techniques in which high-throughput and high-resolution electrophoretic separations of DNA play a significant role. We review recent advances in the development of technology for capillary electrophoresis (CE), including capillary array electrophoresis (CAE) systems. Most of these CE/CAE innovations are equally applicable to implementation on microfabricated electrophoresis chips. Major effort is devoted to discussing various key elements needed for the development of integrated and practical microfluidic sequencing and genotyping platforms, including chip substrate selection, microchannel design and fabrication, microchannel surface modification, sample preparation, analyte detection, DNA sieving matrices, and device integration. Finally, we identify some of the remaining challenges, and some of the possible routes to further advances in high-throughput DNA sequencing and genotyping technologies. 相似文献
42.
Martinez-Diaz MV Rodriguez-Morgade MS Feiters MC van Kan PJ Nolte RJ Stoddart JF Torres T 《Organic letters》2000,2(8):1057-1060
[formula: see text] New unsymmetrically substituted DB24C8-phthalocyanines, which are able to form complexes with suitable dialkylammonium cations, have been prepared. These complexes most probably have a pseudorotaxane geometry. 相似文献
43.
In an earlier study we showed that a blend of thermoresponsive and nonthermoresponsive hydroxyalkylcelluloses could be used to create a thermally tunable polymer network for double-stranded (ds) DNA separation. Here, we show the generality of this approach using a family of polymers suited to a wider range of DNA separations: a blended mixture of N,N-dialkylacrylamide copolymers with different thermoresponsive behaviors. A mixture of 47% w/w N,N-diethylacrylamide (DEA)/53% w/w N,N-dimethylacrylamide (DMA) (DEA47; thermoresponsive, transition temperature = 55 degrees C in water) and 30% w/w DEA/70% w/w DMA (DEA30; nonthermoresponsive, transition temperature > 85 degrees C in water) copolymers in the ratio of 1:5 w/w DEA47:DEA30 was used to separate a dsDNA restriction digest (PhiX174-HaeIII). We investigated the effects of changing mesh size on dsDNA separation, as controlled by temperature. We observed good DNA separation performance with the copolymer blend at temperatures ranging from 25 degrees C to 48 degrees C. The separation selectivity was evaluated quantitatively for certain DNA fragment pairs as a function of temperature. The results were compared with those obtained with a control matrix consisting only of the nonthermoresponsive DEA30. Different DNA fragment pairs of various sizes show distinct temperature-dependent selectivities. Over the same temperature range, no significant temperature dependence of selectivity is observed for these DNA fragment pairs in the nonthermoresponsive control matrix. Overall, the results show similar trends in the temperature dependency of separation selectivity to what was previously observed in hydroxyalkylcellulose blends, for the same DNA fragment pairs. Finally, we showed that a ramped temperature scheme enables improved separation in the blended copolymer matrix for both small and large DNA fragments, simultaneously in a single capillary electrophoresis (CE) run. 相似文献
44.
In this paper, a micellar electrokinetic chromatographic (MEKC) method using ionic liquid as modifier for the quantification of the active components of lignans found in the medicinal herbs Schisandra species was developed for the first time. Preliminary investigations employing sodium dodecyl sulfate (SDS) as surfactant did not lead to the necessary resolution of the studied compounds, the addition of ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM-BF4) to the SDS micellar system resulted in the complete separation of all the compounds. The effects on the separation by several parameters such as BMIM-BF4 and SDS concentration, applied voltage, background electrolyte pH and concentration, were evaluated. Under the optimal conditions (5 mM borate-5 mM phosphate buffer in the presence of 20 mM SDS and 10 mM BMIM-BF4, pH 9.2, applied voltage 25 kV and detection at 254 nm), the method successfully applied to the determination of lignans in extracts of Schisandra chinensis (Turcz.) Baill. and Schisandra henryi C.B. Clarke in less than 13 min. The separation mechanism was also discussed. 相似文献
45.
An atomic-absorption spectrophotometric method has been developed for the indirect determination of phosphorus. The calibration graph is linear over the range 0.025-0.2 mug of phosphorus. The relative standard deviation is 1.3% for 0.2 mug of phosphorus (6 replicates). The method has been applied to the determination of phosphate in natural water samples. The enhancement effect of acetone on the determination is discussed. 相似文献
46.
Sayaka Taji Takeshi Yamada Yasuko In Shun‐ichi Wada Yoshihide Usami Kazuo Sakuma Reiko Tanaka 《Helvetica chimica acta》2007,90(11):2047-2057
Three new lanostane‐type triterpenoids, inonotsulides A, B, and C ( 1 – 3 , resp.) were isolated from the sclerotia of Inonotus obliquus (Pers .: Fr.) (Japanese name: Kabanoanatake; Russian name: Chaga). Their structures were determined to be (20R,24S)‐3β,25‐dihydroxylanost‐8‐en‐20,24‐olide ( 1 ), (20R,24R)‐3β,25‐dihydroxylanost‐8‐en‐20,24‐olide ( 2 ), and (20R,24S)‐3β,25‐dihydroxylanosta‐7,9(11)‐dien‐20,24‐olide ( 3 ) on the basis of chemical transformation, NMR spectroscopy including 1D and 2D (1H,1H‐COSY, NOESY, HMQC, HMBC), EI‐MS, and single‐crystal X‐ray analysis. 相似文献
47.
Konno T Usami M Hirotsu M Yoshimura T Kawamoto T 《Chemical communications (Cambridge, England)》2004,(20):2296-2297
Treatment of [Ni(L)][L =((-)SCH(2)CH(2)NH[double bond, length as m-dash]C(CH(3))-)(2)] with Ag(+) in water gave a pinwheel-like S-bridged Ni(II)(3)Ag(I)(2) structure in [Ag(2)[Ni(L)](3)](2+), which further reacted with [Ni(L)] to produce a Ni(II)(4)Ag(I)(2) structure in [Ag(2)[Ni(L)](4)](2+) and a Ni(II)(7)Ag(I)(4) structure in [Ag(4)[Ni(L)](7)](4+). 相似文献
48.
纳秒强激光场中苯电离产生高价离子的研究 总被引:2,自引:0,他引:2
用25 ns脉冲Nd-YAG 532 nm的激光,在1010~1011 W•cm-2的光场强度下,利用飞行时间质谱对He、 N2、Ar载气条件下苯的激光电离过程进行了研究.发现当利用氩作为载气时,除观察到C2+、C2H2+、C3H3+、C6H6+离子外,还观察到很强的Cq+(q=1~3)高价离子.这些离子都有很高的平动能, C2+的最可几平动能为12.9 eV, C3+为37.5 eV.通过改变载气种类和压力及在不同光场强度条件下的实验,可以认为这些高价离子来源于含苯团簇的库仑爆炸过程. 相似文献
49.
本文应用碱性气体吸附的红外光谱和TPD技术研究了FeZSM-5(F_4)的酸性,酸性顺序为:H-F_4>Fe_2O_3/F_4>F_4原粉>水热处理的K-F_4>Li-F_4>NaF_4>K-F_4>NaOH/F_4,比较了酸强度和积炭对乙苯氧化脱氢性能的影响,以及催化剂酸碱性质与积炭的关系.结果表明,乙苯氧化脱氢反应是在酸碱协同作用下进行的,积炭也参与了氧化脱氢反应,催化剂表面上L酸中心产生的具有一定C、H、O比的积炭具有高的氧化脱氢活性.碱金属阳离子改性的具有L酸中心的FeZSM-5是乙苯氧化脱氢的高活性催化剂。 相似文献
50.
Andy Hay James Aguayo Walter Stark Chung-Ho Chen Ugur Camli Lou-Sing Kan 《中国化学会会志》1989,36(2):169-176
Intact human Senses incubated at 5.5 mM (normal) and 35.5 mM glucose were examined by phosphorus-31 nuclear magnetic resonance (31P NMR) spectroscopy. Lense in 35.5 mM glucose showed an altered metabolic steady-state characterized by a lowered adenosine triphosphate/inorganic orthophosphate ratio. 31P NMR spectroscopy can be used to measure metabolic changes in the lens. This model offers an important means to study dynamic metabolism in the human lens in the setting of diabetic cataractogenesis. 相似文献