全文获取类型
收费全文 | 3887篇 |
免费 | 111篇 |
国内免费 | 20篇 |
专业分类
化学 | 2276篇 |
晶体学 | 22篇 |
力学 | 235篇 |
数学 | 766篇 |
物理学 | 719篇 |
出版年
2023年 | 24篇 |
2022年 | 64篇 |
2021年 | 65篇 |
2020年 | 59篇 |
2019年 | 66篇 |
2018年 | 48篇 |
2017年 | 56篇 |
2016年 | 126篇 |
2015年 | 105篇 |
2014年 | 110篇 |
2013年 | 231篇 |
2012年 | 241篇 |
2011年 | 231篇 |
2010年 | 156篇 |
2009年 | 139篇 |
2008年 | 223篇 |
2007年 | 224篇 |
2006年 | 190篇 |
2005年 | 172篇 |
2004年 | 197篇 |
2003年 | 149篇 |
2002年 | 151篇 |
2001年 | 52篇 |
2000年 | 37篇 |
1999年 | 43篇 |
1998年 | 46篇 |
1997年 | 48篇 |
1996年 | 57篇 |
1995年 | 44篇 |
1994年 | 48篇 |
1993年 | 32篇 |
1992年 | 38篇 |
1991年 | 31篇 |
1990年 | 28篇 |
1989年 | 21篇 |
1988年 | 36篇 |
1987年 | 31篇 |
1986年 | 22篇 |
1985年 | 45篇 |
1984年 | 35篇 |
1983年 | 37篇 |
1982年 | 36篇 |
1981年 | 32篇 |
1980年 | 28篇 |
1979年 | 30篇 |
1978年 | 20篇 |
1977年 | 17篇 |
1976年 | 15篇 |
1975年 | 15篇 |
1974年 | 18篇 |
排序方式: 共有4018条查询结果,搜索用时 15 毫秒
71.
The use of the simplex procedure as a non-linear least-squares curve-fitting method is proposed for evaluation of equivalence points in sigmoidal and segmented titration curves. The application of this procedure to theoretical curves affected by different amounts of random noise indicates its effectiveness and accuracy for locating the correct end-point in titrations characterized by very low reaction constants. The relevant results are compared with those obtained by other regression methods as well as by the first-derivative and Gran approaches. 相似文献
72.
Giorgini MG Arcioni A Polizzi C Musso M Ottaviani P 《The Journal of chemical physics》2004,120(10):4969-4979
We have investigated the Raman profiles of the nu(C[Triple Bond]N) and nu(C=O) vibrational modes of the nematic liquid crystal ME6N (4-cyanophenyl-4(')-hexylbenzoate) in the isotropic phase at different temperatures and used them as probes of the dynamics and structural organization of this liquid. The vibrational time correlation functions of the nu(C[Triple Bond]N) mode, rather adequately interpreted within the assumption of exponential modulation function (the Kubo-Rothschild theory), indicate that the system experiences an intermediate dynamical regime that gets only slightly faster with increasing temperature. However, this theory fails in predicting the non-exponential behavior that the time correlation functions manifest in the long time range (t>3 ps). For this reason we have additionally approached the interpretation of vibrational correlation functions in terms of the theory formulated by Rothschild and co-workers for locally structured liquids. The application of this theory reveals that the molecular dynamics in this liquid crystal in the isotropic phase is that deriving from a distribution of differently sized clusters, which narrows as the temperature increases. Even at the highest temperature reached in this study (87 degrees C above the nematic-isotropic transition), the liquid has not yet achieved the structure of the simple liquid and the dynamics has not reached the limit of the single channel process. The vibrational and orientational relaxations occur in very different time scales. The temperature independence of the orientational dynamics in the whole range from 55 degrees C to 135 degrees C has been referred to the nonhydrodynamic behavior of the system, arising when local pseudonematic structures persist for times longer than the orientational relaxation. The occurrence of the process of resonant vibrational energy transfer between the C=O groups of adjacent molecules has been revealed in the isotropic phase by a slightly positive Raman noncoincidence effect in the band associated with the nu(C=O) mode. A qualitative interpretation is tentatively given in terms of partial cancellation of contributions deriving from structures having opposite orientations of their C=O groups. 相似文献
73.
In designing an experimentin which non-linear regression analysis is used to evaluate the physicochemical parameters that chracterize the behavior of the system being studied, it would be useful to be able to predict how the number of data points affects th reliabilities of the values obtained. The relation between these depends on hos the points are distributed as well as on the nature of the equation to which thd data are fitted. Weighted non-linear regression analysis has been applied to six common equations involving a total of fourteen parameters, using a number of n of points that was varied from 4 to 相似文献
74.
Atmospheric bulk deposition of major and trace elements was measured at Venice from November 1995 to October 1997. Collection was carried out using polyethylene bulk passive samplers, samples being collected bi-weekly. In order to highlight the contribution of the atmosphere to water chemistry and particle budgets in the Lagoon of Venice, the geochemical composition (Si, Al, Ca, Mg, K, Na, Mn, Cr, Zn, Pb, Cd, Cu, As) of dissolved and insoluble bulk fractions was determined by AAS + ICP mass spectrometry. Great sample variability was found, with almost two orders of magnitude between maximum and minimum values for several metals. All fluxes in 1995/96 were 30% lower than in 1996/97, ranging from -3% (Ca) to -57% (Li), except for Zn, Cd and As. On the contrary, the solubility of all elements decreased during 1996/97. Partitioning between soluble and insoluble phases shows that Al, Cr, Fe and Si are mainly in the insoluble form, whereas for As, Ca, Cu, Mg, Na, Ni, K, Pb and Zn the dissolved fraction represents 50-90% of total input. The amount of particle load affects partitioning between dissolved and particulate, especially for Al and Pb. Seasonal variability was evident. The lowest pH values (approximately 5.2) were recorded in winter, causing an increase of solubility for all metals except for As, which showed the highest solubility in summer. 相似文献
75.
The 1,3-dipolar cycloadditions of C-glycosyl nitrile oxides and acetylenes to an alkyne and an azide, respectively, bearing a masked glycinyl moiety furnished disubstituted isoxazoles and triazoles. Unveiling the glycinyl group in these cycloadducts afforded C-glycosyl alpha-amino acids in which the two bioactive entities were tethered through rigid five-membered heterocycles. Optimized entries to the same compounds involved the use of unmasked but protected alkyne- and azide-containing amino acids as the partners of 1,3-dipolar cycloadditions. 相似文献
76.
[reaction: see text] A new mild method for protecting alcohols as t-butyl ethers is reported. The reaction proceeds with Mg(ClO4)2 and Boc2O and shows general applicability. The deprotection of t-butyl ethers has also been revisited. Preliminary results indicate the CeCl3 x 7H2O/NaI system is a very suitable catalyst for their removal. 相似文献
77.
The rotational dynamics of CO single molecules solvated in small He clusters (CO @ HeN) has been studied using reptation quantum Monte Carlo simulations for cluster sizes up to N = 30. Our results are in good agreement with the rotovibrational features of the infrared spectrum recently determined for this system and provide a deep insight into the relation between the structure of the cluster and its dynamics. Simulations for large N also provide a prediction of the effective moment of inertia of CO in the He nanodroplet regime, which has not been measured so far. 相似文献
78.
The reaction of some primary amines with the methyl 2-[(α-oxobenzyl)-(α-bromo)methyl]phenylacetate ( 5 ) afforded the isoquinolinones 7a-d , which in turn were hydrogenated to 10a-d . The synthesis of these compounds was designed on the basis of a potential depressant and antiinflammatory activity found in other structurally related compounds. 相似文献
79.
Antonia Sacchi Paolo De Caprariis Luciano Mayol Giovanni De Martino 《Journal of heterocyclic chemistry》1995,32(3):1067-1069
4-Hydroxy-3-phenylprolines were synthesized via 1-acetyl-2,2-diethoxycarbonyl-2,3-dihydro-3-phenyl-1H-pyrrole. Reversed phase hplc resulted in the isolation of the products which were characterized by 1H and 13C nmr spectroscopy. 相似文献
80.
Paolo Lisca 《Inventiones Mathematicae》1995,119(1):347-359
Summary We compute the Donaldson SU(2)-invariants of the double cover of 2 branched over a smooth algebraic curve of degree eight. From this we deduce a formula for the relative invariants of the blow-up of the Gompf nucleusN
2, and we show how this gives a blow-up formula for a class of 4-manifolds which includes essentially all the simply connected 4-manifolds known to have big diffeomorphism group. We apply the result on the nucleus also to prove a formula for the invariants of minimal simply connected elliptic surfaces which reduces the computation to the case of geometric genus one. In particular, we compute all the Donaldson invariants of minimal simply connected elliptic surfaces without multiple fibers. Our main tool is Donaldson-Floer theory.Oblatum IX-1993 & 26-IV-1994 相似文献