全文获取类型
收费全文 | 3225篇 |
免费 | 97篇 |
国内免费 | 17篇 |
专业分类
化学 | 2104篇 |
晶体学 | 19篇 |
力学 | 59篇 |
数学 | 505篇 |
物理学 | 652篇 |
出版年
2021年 | 33篇 |
2020年 | 31篇 |
2019年 | 42篇 |
2018年 | 28篇 |
2017年 | 30篇 |
2016年 | 55篇 |
2015年 | 43篇 |
2014年 | 56篇 |
2013年 | 133篇 |
2012年 | 122篇 |
2011年 | 192篇 |
2010年 | 108篇 |
2009年 | 91篇 |
2008年 | 156篇 |
2007年 | 133篇 |
2006年 | 152篇 |
2005年 | 156篇 |
2004年 | 153篇 |
2003年 | 129篇 |
2002年 | 112篇 |
2001年 | 49篇 |
2000年 | 58篇 |
1999年 | 36篇 |
1997年 | 28篇 |
1996年 | 58篇 |
1995年 | 23篇 |
1994年 | 42篇 |
1993年 | 38篇 |
1992年 | 43篇 |
1991年 | 29篇 |
1990年 | 33篇 |
1989年 | 27篇 |
1988年 | 26篇 |
1986年 | 28篇 |
1985年 | 46篇 |
1984年 | 41篇 |
1983年 | 36篇 |
1982年 | 46篇 |
1981年 | 38篇 |
1980年 | 47篇 |
1979年 | 42篇 |
1978年 | 53篇 |
1977年 | 40篇 |
1976年 | 29篇 |
1975年 | 38篇 |
1974年 | 46篇 |
1973年 | 42篇 |
1972年 | 28篇 |
1970年 | 25篇 |
1968年 | 25篇 |
排序方式: 共有3339条查询结果,搜索用时 15 毫秒
91.
O. Nilsson 《Journal of separation science》1979,2(12):720-722
Kaiser's “abt” concept rests on the experimental establishment of a linear relation between band width at half height, b0.5, and capacity ratio, k, according to His column specification quantity, hreal, is dependent on the square of the slope a, obtained from a linear regression analysis based on this relation. It is shown in the present paper that an improvement of the experimental conditions leading to a lower extra-column contribution to band broadening, b, introduces a tendency towards a negative curvature in the plot at low k values. The mean slope increases, and an uncritical linear regression would yield an hreal-value which is higher than before and which would be in the opposite direction to the requirements of a reliable column specification. Generally the linear regression analysis has to be moved to higher k regions in cases of curvature, especially as it is also shown that hreal is an estimation of the traditional HETP value, H∞, that would be obtained when k approaches infinity. This theoretically expected relation is a strong motivation for the use of hreal as a column specification, since the b0.5-value associated with H∞ is expected to be practically independent of b. 相似文献
92.
2-Pyridylcopper/dibutyl sulfide, 2-pyridylcopper/tributylphosphine, lithium di(2-pyridyl)cuprate and lithium (2-pyridyl)(phenyl)cuprate all add the 2-pyridyl group to 4-phenyl-3-buten-2-one in good yields. The cuprates also add the 2-pyridyl group to ethyl 3-phenylpropenoate. 相似文献
93.
Olle Nilsson 《Journal of separation science》1982,5(1):38-44
The total length-based second moment contribution from longitudinal sample diffusion in both phases on a column, σD , is derived by adding individual partial differential contributions to a partial differential equation accounting for the longitudinal diffusion processes only. Although each diffusion-dispersed sample part is equilibrated between two phases, the resulting σ,D (= 2D mt m + 2D st s) can be interpreted as the sum of two independent contributions in accordance with the variance addition rule. (D m and D s are the mean diffusion coefficients and t mand t s the mean residence times of the sample in the mobile and stationary phases, respectively.) The same σD expression is derived from the random walk model of Giddings by treating the diffusional process in each phase as statistically independent of the other processes. Under these conditions the broadening contribution from longitudinal diffusion in the mobile phase is shown to be independent of the velocity profile. 相似文献
94.
Daniel Ammann Ren Bissig Marc Güggi Ern Pretsch Wilhelm Simon Irving J. Borowitz Louis Weiss 《Helvetica chimica acta》1975,58(6):1535-1548
The preparation of a series of non-cyclic, uncharged ligands able to selectively complex alkali and alkaline earth metal cations is described. These molecules are designed to be used as carriers for cations through membranes. Some of the compounds show high Ca2+ and Na+ selectivity, respectively, in liquid membrane electrodes. 相似文献
95.
Escherichia coli single-stranded DNA-binding protein,a molecular tool for improved sequence quality in pyrosequencing 总被引:2,自引:0,他引:2
Pyrosequencing is a four-enzyme bioluminometric DNA sequencing technique based on a DNA sequencing by synthesis principle. Currently, the technique is limited to analysis of short DNA sequences exemplified by single-nucleotide polymorphism analysis. In order to expand the field for pyrosequencing, the read length needs to be improved and efforts have been made to purify reaction components as well as add single-stranded DNA-binding protein (SSB) to the pyrosequencing reaction. In this study, we have performed a systematic effort to analyze the effects of SSB by comparing the pyrosequencing result of 103 independent complementary DNA (cDNA) clones. More detailed information about the cause of low quality sequences on templates with different characteristics was achieved by thorough analysis of the pyrograms. Also, real-time biosensor analysis was performed on individual cDNA clones for investigation of primer annealing and SSB binding on these templates. Results from these studies indicate that templates with high performance in pyrosequencing without SSB possess efficient primer annealing and low SSB affinity. Alternative strategies to improve the performance in pyrosequencing by increasing the primer-annealing efficiency have also been evaluated. 相似文献
96.
Staudinger ligation: a peptide from a thioester and azide 总被引:1,自引:0,他引:1
[reaction: see text] The technique of native chemical ligation enables the total chemical synthesis of proteins. This method is limited, however, by an absolute requirement for a cysteine residue at the ligation juncture. Here, this restriction is overcome with a new chemical ligation method in which a phosphinobenzenethiol is used to link a thioester and azide. The product is an amide with no residual atoms. 相似文献
97.
A. Bergier H. Gennow S. Nilsson J. Norrby W. Evans J. R. Fry P. Mason H. Muirhead H. Fenker J. Marraffino S. Reucroft C. Roos M. S. Webster 《Zeitschrift fur Physik C Particles and Fields》1980,5(4):265-274
We present data on \(\bar pn\) and π? n collisions obtained from an exposure of the 30′' FNAL deuterium filled bubble chamber to a mixed \({{\bar p} \mathord{\left/ {\vphantom {{\bar p} {\pi ^ - }}} \right. \kern-0em} {\pi ^ - }}\) beam with a momentum of 100 GeV/c. We find that in 17±2% of the collisions with the antiproton there is an interaction on the spectator while for the collisions with π? mesons the corresponding number is 15±2%. The \(\bar pn\) and π? n multiplicity distributions have average charged multiplicities of 6.46±0.07 and 6.53±0.08 respectively. The average multiplicities for both types of interactions are slightly smaller than those for the corresponding reactions on hydrogen by an amount that is the same as observed at other energies. As an estimate of \(\bar pn\) annihilation we have calculated the difference \(\sigma _n (\bar pn) - \sigma _n (pn)\) for each prong numbern. We find an average multiplicity of 9±1, a value close to that for \(\bar pp\) annihilation at the same energy. combining our data with lower energy \(\bar pn\) annihilation data, we observe that the average negative multiplicity is systematically larger than that for \(\bar pp\) annihilation similar to the difference between neutron and proton target data with other beam projectiles. 相似文献
98.
Björklund J Isetun S Nilsson U 《Rapid communications in mass spectrometry : RCM》2004,18(24):3079-3083
Gas chromatography/ion trap mass spectrometry with in-source ionization and dissociation was used in positive-ion chemical ionization (PICI) mode for the determination of organophosphate triesters in indoor air. These compounds are widely used as additive flame retardants and plasticizers in different types of materials and have become ubiquitous pollutants in indoor environments. When using collision-induced dissociation in PICI mode the fragmentation of the organophosphate triesters can be performed in a more controllable way than in electron ionization (EI) mode. The developed selected-reaction monitoring method provided high selectivity for the investigated compounds. For 8-h air measurements (corresponding to 1.5 m3 of sampled air) the limit of detection of the method was determined to be in the range 0.1-1.4 ng m(-3), which is comparable with nitrogen-phosphorus detection and about 50-fold lower than when using EI in selected-ion monitoring mode. The presented method was applied to samples from three common indoor environments, in which a number of organophosphate triesters were identified and quantified. The dominating compound was found to be tris(2-chloropropyl) phosphate, which occurred at levels up to 0.8 microg m(-3). 相似文献
99.
Reactions of HgCl2 with η5-C5H5Fe(CO)2R (R CH2CHCH2 and CH2C(CH3)CH2) in THF at 25°C rapidly afford adducts of the two reactants. These adducts were converted to the corresponding PF6? salts, [η5-C5H5Fe(CO)2(η2-CH2C(R)CH2HgCl)]+ PF6? (R H and CH3), for characterization. Slower reactions with cleavage of the ironcarbon σ bond and elimination of the R group from η5-C5H5Fe(CO)2R occur for R CH2CHC(CH3)2, CH2CHCHC6H5, and CH2CCC6H5. Both elimination and adduct formation are observed when R CH2CHCHCH3. The kinetics of the cleavage reactions are presented and possible mechanisms for both cleavage and adduct formation are discussed. 相似文献
100.
We report three highly stereoselective pericyclic reactions occurring in cascade leading to the synthesis of Decalins skeletons possessing two contiguous quaternary centers. The tandem reaction is triggered by an oxy-Cope rearrangement to create in situ a 10-membered ring enol ether macrocyle 6, which immediately rearranges via a Claisen [3,3] shift to the corresponding E-cyclodec-6-en-1-one 7. The latter spontaneously cyclizes via a transannular ene reaction to produce Decalin 5. Analysis of the mechanism with respect to the origin of the high diastereoselectivity of the tandem oxy-Cope/Claisen/ene reaction is presented. 相似文献