全文获取类型
收费全文 | 43773篇 |
免费 | 2848篇 |
国内免费 | 2487篇 |
专业分类
化学 | 21419篇 |
晶体学 | 433篇 |
力学 | 2274篇 |
综合类 | 166篇 |
数学 | 10812篇 |
物理学 | 14004篇 |
出版年
2024年 | 41篇 |
2023年 | 321篇 |
2022年 | 577篇 |
2021年 | 638篇 |
2020年 | 668篇 |
2019年 | 732篇 |
2018年 | 1737篇 |
2017年 | 1964篇 |
2016年 | 1463篇 |
2015年 | 1355篇 |
2014年 | 1422篇 |
2013年 | 1804篇 |
2012年 | 4493篇 |
2011年 | 3676篇 |
2010年 | 2773篇 |
2009年 | 2533篇 |
2008年 | 1564篇 |
2007年 | 1451篇 |
2006年 | 1382篇 |
2005年 | 5060篇 |
2004年 | 4435篇 |
2003年 | 2718篇 |
2002年 | 979篇 |
2001年 | 669篇 |
2000年 | 407篇 |
1999年 | 502篇 |
1998年 | 370篇 |
1997年 | 295篇 |
1996年 | 255篇 |
1995年 | 218篇 |
1994年 | 193篇 |
1993年 | 177篇 |
1992年 | 271篇 |
1991年 | 255篇 |
1990年 | 220篇 |
1989年 | 187篇 |
1988年 | 170篇 |
1987年 | 97篇 |
1986年 | 80篇 |
1985年 | 83篇 |
1984年 | 56篇 |
1983年 | 45篇 |
1982年 | 42篇 |
1976年 | 83篇 |
1975年 | 39篇 |
1974年 | 43篇 |
1973年 | 51篇 |
1972年 | 43篇 |
1969年 | 39篇 |
1966年 | 37篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
281.
282.
283.
Classifying wine according to geographical origin via quadrupole-based ICP–mass spectrometry measurements of boron isotope ratios 总被引:1,自引:0,他引:1
The potential of quadrupole-based ICP–MS as a tool for B-isotopic analysis of wines and its usefulness in provenance determinations
were assessed. A precision of 0.1–0.25% RSD (corresponding to a relative standard deviation of the mean of three replicate
measurements of 0.06–0.12%) was sufficient to establish small differences in the B isotope ratios in wines from different
geographical origins. Each sample measurement was bracketed by measurements of a standard and mass bias drift correction made
by interpolation. Sample preparation was kept to a minimum to avoid possible fractionation. Dilution of the wine samples by
a factor of 100 with 0.65% HNO3 was found to reduce matrix-induced mass discrimination substantially. Wines from three wine-producing regions, Stellenbosch,
Robertson, and Swartland, in the Western Cape Province of South Africa, and wines from specific regions in France (Bergerac)
and Italy (Valpolicella) were analyzed by ICP–QMS for their B-isotopic compositions. It was concluded that the 11B/10B ratios can be used to characterize wines from different geographical origins. Average 11B/10B ratios in red wines from South Africa (Stellenbosch), France (Bergerac), and Italy (Valpolicella) were found to differ by
between 0.5 and 1.5%. 相似文献
284.
Carnitine in Pregnancy 总被引:1,自引:0,他引:1
Alfred?LohningerEmail author H.?Karlic S.?Lohninger A.?Tammaa S.?Jinniate Hermann?Mascher Daniel?Mascher H.?Salzer 《Monatshefte für Chemie / Chemical Monthly》2005,136(8):1523-1533
Summary. By the 12th week of gestation, mean whole blood and plasma carnitine levels are already significantly (p<0.01) lower than those of controls, with a further significant (p<0.01) decrease up to parturition. Diminished carnitine levels may cause a downregulation of carnitine palmitoyltransferase1 (CPT1), both the liver isoform (CPT1A) and muscle isoform (CPT1B), carnitine palmitoyltransferase2 (CPT2), and carnitine acetyltransferase (CRAT) in white blood cells of pregnant women, as determined by real time PCR using the LightCyclerSYBR Green technology.L-Carnitine-L-tartrate supplementation of 2 g/d resulted in an up to 10-fold increase of the relative mRNA abundances of CPT1B, CPT2, and OCTN2 and a 5-fold increase of CPT1A, and CRAT.There is a relationship between the relative mRNA levels of CPT1A and CPT1B and the FFA plasma levels. The substitution of 2 g L-carnitine-L-tartrate/d resulted in significant (p<0.001) lower FFA levels compared to untreated controls and the groups substituted with 0.5 and 1 g L-carnitine/d although plasma carnitine levels were not significantly increased. The most substantial effect was the reduced portion of acylcarnitines on total carnitine in those women receiving 2 g L-carnitine-L-tartrate.Carnitine substitution resulted in an enhanced excretion of both, free carnitine and acylcarnitines, whereas acetylcarnitine accounts for 50–65% of total acylcarnitines.The results of the present study provide evidence that L-carnitine supplementation in pregnancy in sufficient doses avoids a striking increase of plasma FFAs, which are thought to be the main cause of insulin resistance and consequently gestational diabetes mellitus (GDM). 相似文献
285.
It is well known that porphyrin derivatives play a key role in the primary process of photo-synthesis[1], in which porphyrins directly absorb the sunlight or indirectly acquire excitation en-ergy from light-harvesting antenna system to reach their excited state, and then donate electrons to quinone acceptors to yield a series of charge-separated species. In general, only first singlet ex-cited state of porphyrins is involved in energy transfer process[2]. However, highly excited state (S2 stat… 相似文献
286.
287.
Background
Kynureninase is a key enzyme on the kynurenine pathway of tryptophan metabolism. One of the end products of the pathway is the neurotoxin quinolinic acid which appears to be responsible for neuronal cell death in a number of important neurological diseases. This makes kynureninase a possible therapeutic target for diseases such as Huntington's, Alzheimer's and AIDS related dementia, and the development of potent inhibitors an important research aim. 相似文献288.
Summary In this paper an application is presented of the median molecule workflow to the de novo design of novel molecular entities with a property profile of interest. Median molecules are structures that are optimised to be similar to a set of existing molecules of interest as an approach for lead exploration and hopping. An overview of this workflow is provided together with an example of an instance using the similarity to camphor and menthol as objectives. The methodology of the experiments is defined and the workflow is applied to designing novel molecules for two physical property datasets: mean molecular polarisability and aqueous solubility. This paper concludes with a discussion of the characteristics of this method. 相似文献
289.
Tseng CM Dyakov YA Huang CL Mebel AM Lin SH Lee YT Ni CK 《Journal of the American Chemical Society》2004,126(28):8760-8768
Photoisomerization and photodissociation of aniline and 4-methylpyridine at 193 nm were studied separately using multimass ion imaging techniques. Photofragment translational energy distributions and dissociation rates were measured. Our results demonstrate that more than 23% of the ground electronic state aniline and 10% of 4-methylpyridine produced from the excitation by 193 nm photons after internal conversion isomerize to seven-membered ring isomers, followed by the H atom migration in the seven-membered ring, and then rearomatize to both methylpyridine and aniline prior to dissociation. The significance of this isomerization is that the carbon, nitrogen, and hydrogen atoms belonging to the alkyl or amino groups are involved in the exchange with those atoms in the aromatic ring during the isomerization. 相似文献
290.
The CHCl3 extract of dried roots of Hemerocallis fulva (L.) L. afforded a novel diterpene named hemerocallal A ( 1 ), which is the second reported naturally occurring diterpene with a trans‐bicyclo[5.1.0]octane system. The BuOH extract afforded a new glycoside named hemerocalloside ( 2 ). Their structures were established on the basis of spectroscopic and chemical studies. 相似文献