全文获取类型
收费全文 | 508篇 |
免费 | 21篇 |
国内免费 | 2篇 |
专业分类
化学 | 322篇 |
晶体学 | 5篇 |
力学 | 24篇 |
数学 | 54篇 |
物理学 | 126篇 |
出版年
2023年 | 6篇 |
2022年 | 13篇 |
2021年 | 12篇 |
2020年 | 9篇 |
2019年 | 14篇 |
2018年 | 9篇 |
2017年 | 11篇 |
2016年 | 30篇 |
2015年 | 27篇 |
2014年 | 19篇 |
2013年 | 34篇 |
2012年 | 44篇 |
2011年 | 48篇 |
2010年 | 21篇 |
2009年 | 12篇 |
2008年 | 27篇 |
2007年 | 27篇 |
2006年 | 31篇 |
2005年 | 27篇 |
2004年 | 12篇 |
2003年 | 18篇 |
2002年 | 13篇 |
2001年 | 10篇 |
2000年 | 2篇 |
1999年 | 5篇 |
1998年 | 1篇 |
1997年 | 4篇 |
1996年 | 5篇 |
1995年 | 1篇 |
1994年 | 2篇 |
1993年 | 3篇 |
1992年 | 2篇 |
1991年 | 3篇 |
1990年 | 1篇 |
1989年 | 3篇 |
1988年 | 4篇 |
1987年 | 1篇 |
1985年 | 4篇 |
1984年 | 2篇 |
1983年 | 3篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1979年 | 1篇 |
1978年 | 2篇 |
1977年 | 1篇 |
1969年 | 1篇 |
1968年 | 1篇 |
1967年 | 1篇 |
1965年 | 1篇 |
排序方式: 共有531条查询结果,搜索用时 31 毫秒
21.
Eduardo Rosado Paloma Delgado-Fernndez Blanca de las Rivas Rosario Muoz Francisco Javier Moreno Nieves Corzo Cesar Mateo 《Molecules (Basel, Switzerland)》2022,27(4)
The synthesis of β-galactosyl xylitol derivatives using immobilized LacA β-galactosidase from Lactobacillus plantarum WCFS1 is presented. These compounds have the potential to replace traditional sugars by their properties as sweetener and taking the advantages of a low digestibility. The enzyme was immobilized on different supports, obtaining immobilized preparations with different activity and stability. The immobilization on agarose-IDA-Zn-CHO in the presence of galactose allowed for the conserving of 78% of the offered activity. This preparation was 3.8 times more stable than soluble. Since the enzyme has polyhistidine tags, this support allowed the immobilization, purification and stabilization in one step. The immobilized preparation was used in synthesis obtaining two main products and a total of around 68 g/L of β-galactosyl xylitol derivatives and improving the synthesis/hydrolysis ratio by around 30% compared to that of the soluble enzyme. The catalyst was recycled 10 times, preserving an activity higher than 50%. The in vitro intestinal digestibility of the main β-galactosyl xylitol derivatives was lower than that of lactose, being around 6 and 15% for the galacto-xylitol derivatives compared to 55% of lactose after 120 min of digestion. The optimal amount immobilized constitutes a very useful tool to synthetize β-galactosyl xylitol derivatives since it can be used as a catalyst with high yield and being recycled for at least 10 more cycles. 相似文献
22.
Least-squares support vector machines and near infrared spectroscopy for quantification of common adulterants in powdered milk 总被引:5,自引:0,他引:5
This paper proposes the use of the least-squares support vector machine (LS-SVM) as an alternative multivariate calibration method for the simultaneous quantification of some common adulterants (starch, whey or sucrose) found in powdered milk samples, using near-infrared spectroscopy with direct measurements by diffuse reflectance. Due to the spectral differences of the three adulterants a nonlinear behavior is present when all groups of adulterants are in the same data set, making the use of linear methods such as partial least squares regression (PLSR) difficult. Excellent models were built using LS-SVM, with low prediction errors and superior performance in relation to PLSR. These results show it possible to built robust models to quantify some common adulterants in powdered milk using near-infrared spectroscopy and LS-SVM as a nonlinear multivariate calibration procedure. 相似文献
23.
Fabio Cesar Gozzo Marcos N. Eberlin 《Journal of the American Society for Mass Spectrometry》1995,6(7):554-563
Ion-molecule reactions of the mass-selected distonic radical cation +CH2-O-CH 2 · (1) with several heterocyclic compounds have been investigated by multiple stage mass spectro- metric experiments performed in a pentaquadrupole mass spectrometer. Reactions with pyridine, 2-, 3-, and 4-ethyl, 2-methoxy, and 2-n-propyl pyridine occur mainly by transfer of CH 2 +· to the nitrogen, which yields distonic N-methylene-pyridinium radical cations. The MS3 spectra of these products display very characteristic collision-induced dissociation chemistry, which is greatly affected by the position of the substituent in the pyridine ring. Ortho isomers undergo a δ-cleavage cyclization process induced by the free-radical character of the N-methylene group that yields bicyclic pyridinium cations. On the other hand, extensive CH 2 +· transfer followed by rapid hydrogen atom loss, that is, a net CH+ transfer, occurs not to the heteroatoms, but to the aromatic ring of furan, thiophene, pyrrole, and N-methyl pyrrole. The reaction proceeds through five- to six-membered ring expansion, which yields the pyrilium, thiapyrilium, N-protonated, and N-methylated pyridine cations, respectively, as indicated by MS3 scans. Ion 1 fails to transfer CH 2 +· to tetrahydrofuran, whereas a new α-distonic sulfur ion is formed in reactions with tetrahydrothiophene. Unstable N-methylene distonic ions, likely formed by transfer of CH 2 +· to the nitrogen of piperidine and pyrrolidine, undergo rapid fragmentation by loss of the α-NH hydrogen to yield closed-shell immonium cations. The most thermodynamically favorable products are formed in these reactions, as estimated by ab initio calculations at the MP2/6-31G(d,p)//6-31G(d,p) + ZPE level of theory. 相似文献
24.
Cesar Garcías-Morales Selene H. Martínez-Salas Armando Ariza-Castolo 《Tetrahedron letters》2012,53(26):3310-3315
The effect of the nitrogen nonbonding electron pair on the 1JC,H values of 1,3-diazaheterocycles was analyzed and compared to 1,5-diazabiciclo[3.2.1]octanes, which have a restricted conformation. The 1JC,H values were measured by observing the 13C satellites in the 1H NMR spectra and then determining the 1H-coupled 13C NMR spectra. The 1JC,H values are 10 Hz larger when the α-hydrogen is synperiplanar rather than antiperiplanar to the nonbonding electron pair on the nitrogen, which serves as experimental evidence of the orbital nN→σ1C,Hap interactions. In addition, the homoanomeric effect from the interactions of the nitrogen lone pair with the antibonding orbital of the equatorial hydrogen, which was in the β position, was discussed (nN→σ1C(β),Heq). 相似文献
25.
26.
27.
28.
Jørgensen LV Amoretti M Bonomi G Bowe PD Canali C Carraro C Cesar CL Charlton M Doser M Fontana A Fujiwara MC Funakoshi R Genova P Hangst JS Hayano RS Kellerbauer A Lagomarsino V Landua R Lodi Rizzini E Macrì M Madsen N Mitchard D Montagna P Rotondi A Testera G Variola A Venturelli L van der Werf DP Yamazaki Y;ATHENA Collaboration 《Physical review letters》2005,95(2):025002
We have developed a new method, based on the ballistic transfer of preaccumulated plasmas, to obtain large and dense positron plasmas in a cryogenic environment. The method involves transferring plasmas emanating from a region with a low magnetic field (0.14 T) and relatively high pressure (10(-9) mbar) into a 15 K Penning-Malmberg trap immersed in a 3 T magnetic field with a base pressure better than 10(-13) mbar. The achieved positron accumulation rate in the high field cryogenic trap is more than one and a half orders of magnitude higher than the previous most efficient UHV compatible scheme. Subsequent stacking resulted in a plasma containing more than 1.2 x 10(9) positrons, which is a factor 4 higher than previously reported. Using a rotating wall electric field, plasmas containing about 20 x 10(6) positrons were compressed to a density of 2.6 x 10(10) cm(-3). This is a factor of 6 improvement over earlier measurements. 相似文献
29.
30.
The (C,F)-construction from a previous paper of the first authoris applied to produce a number of funny rank one infinite measurepreserving actions of discrete countable Abelian groups G withunusual multiple recurrence properties. In particular,the following are constructed for each p N{}:
- a p-recurrent actionT=(Tg)gG such that (if p) no one transformationTg is (p+1)-recurrentfor every element g of infinite order;
- an action T=(Tg)gGsuch that for every finite sequence g1,...,grGwithout torsionthe transformation Tg1x...x Tgr is ergodic,p-recurrent but(if p) not (p+1)-recurrent;
- a p-polynomially recurrent (C,F)-transformationwhich (if p)is not (p+1)-recurrent.