全文获取类型
收费全文 | 950篇 |
免费 | 33篇 |
国内免费 | 2篇 |
专业分类
化学 | 658篇 |
晶体学 | 13篇 |
力学 | 26篇 |
数学 | 123篇 |
物理学 | 165篇 |
出版年
2023年 | 5篇 |
2022年 | 16篇 |
2021年 | 16篇 |
2020年 | 17篇 |
2019年 | 9篇 |
2018年 | 23篇 |
2017年 | 23篇 |
2016年 | 27篇 |
2015年 | 44篇 |
2014年 | 33篇 |
2013年 | 55篇 |
2012年 | 68篇 |
2011年 | 68篇 |
2010年 | 41篇 |
2009年 | 27篇 |
2008年 | 63篇 |
2007年 | 56篇 |
2006年 | 44篇 |
2005年 | 48篇 |
2004年 | 34篇 |
2003年 | 41篇 |
2002年 | 19篇 |
2001年 | 23篇 |
2000年 | 14篇 |
1999年 | 7篇 |
1998年 | 7篇 |
1997年 | 8篇 |
1996年 | 8篇 |
1995年 | 8篇 |
1994年 | 8篇 |
1993年 | 6篇 |
1992年 | 13篇 |
1991年 | 11篇 |
1990年 | 9篇 |
1989年 | 5篇 |
1988年 | 5篇 |
1987年 | 10篇 |
1985年 | 12篇 |
1984年 | 8篇 |
1982年 | 3篇 |
1981年 | 4篇 |
1980年 | 6篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1976年 | 4篇 |
1975年 | 2篇 |
1974年 | 3篇 |
1973年 | 3篇 |
1972年 | 3篇 |
1968年 | 4篇 |
排序方式: 共有985条查询结果,搜索用时 46 毫秒
891.
892.
Ji-Seon Jeong Hee-Jung Sim Yong-Moon Lee Hye-Ran Yoon Dong Hwan Lee Seon-Pyo Hong 《Journal of chromatography. A》2009,1216(30):5709-5714
We have developed a high-performance anion-exchange chromatography with pulsed amperometric detection method for the detection of phenylalanine (Phe) and diagnosis of phenylketonuria (PKU). Sample pretreatment steps were simplified without derivatization. The analyte was separated within 5 min. The detection limit (S/N = 3) for Phe was 50 pg. Linear dynamic range was 1.23–14.43 mg/dL (r2 = 0.9999) for a dried blood spot. The mean recoveries of Phe for intra- and inter-day assays were found to be 96.87–104.16%. This method clearly differentiated PKU-positive groups from normal groups, and proved to be a practical procedure for rapid screening and follow-up monitoring of PKU. 相似文献
893.
894.
M. Rosário M. Domingues Cláudia Simões João Pinto da Costa Ana Reis Pedro Domingues 《Biomedical chromatography : BMC》2009,23(6):588-601
Phosphatidylethanolamines are a major class of phospholipids found in cellular membranes. Identification of the alterations in these phospholipids, induced by free radicals, could provide new tools for in vivo diagnosis of oxidative stress. In this study, 1‐palmitoyl‐2‐linoleoyl‐phosphatidylethanolamine oxidation products, induced by the hydroxyl radical, were studied using LC‐MS and LC‐MS/MS. Data obtained allowed the identification and separation of isomeric oxidative products with modifications in the sn‐2 acyl chain, attributed to long‐ and short‐chain products. Among long‐chain products keto, keto‐hydroxy, hydroxy, poly‐hydroxy, peroxy and hydroxy–peroxy derivatives were identified. Product ions formed by loss of two H2O molecules vs loss of HOOH, allowed the identification of, respectively, di‐ (or poli‐) hydroxy vs peroxy derivatives. Location of functional groups was determined by the product ions formed by cleavage of C–C bonds, in the vicinity of the oxidation positions, allowing the identification of C9, C12 and C13 as the predominant substituted positions. Short‐chain products identified comprised aldehydes, hydroxy‐aldehydes and carboxylic derivatives, with modified sn‐2 acyl lengths of C7–C9 and C11, C12. Among the short‐chain products identified, C9 products showed higher relative abundance. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
895.
Zoltán May László I. Simándi and Attila Vértes 《Reaction Kinetics and Catalysis Letters》2006,89(2):349-358
Summary The kinetics and mechanisms of the catecholase-type biomimetic activation of O2 by the new dioximatoiron(II) complexes [Fe(Hdmed)]+, [Fe(Hdmpd)]+ and [Fe(H2dmdt)]2+ have been studied in methanol. Kinetic measurements reveal first order behavior with respect to catalyst and O2 and a saturation type dependence on the 3,5-di-tert-butylcatechol (H2dtbc) substrate. Added triethylamine increases the rate by changing the reaction mechanism. An important, hitherto unknown
feature is iron(II)-enhanced base catalysis of H2dtbc oxidation, via coordination of HdtbcO2- to the Fe(II) complex present, resulting in a significant acceleration of oxidation. A mechanism involving free radicals
is suggested on grounds of ESR evidence. The activity pattern of the catalyst complexes correlates with coordination number
and symmetry as revealed by M?ssbauer spectra. 相似文献
896.
This work results from our attempts to solve Boltzmann–Sinai’s hypothesis about the ergodicity of hard ball gases. A crucial
element in the studies of the dynamics of hard balls is the analysis of special hypersurfaces in the phase space consisting
of degenerate trajectories (which lack complete hyperbolicity). We prove that if a flow-invariant hypersurface J in the phase space of a semi-dispersing billiard has a negative Lyapunov function, then the volume of the forward image of
J grows at least linearly in time. Our proof is independent of the solution of the Boltzmann–Sinai hypothesis, and we provide
a complete and self-contained argument here.
Submitted: March 14, 2006. Accepted: August 2, 2006. 相似文献
897.
A. A. Vasiliev A. I. Neishtadt C. Simó D. V. Treschev 《Proceedings of the Steklov Institute of Mathematics》2007,259(1):236-247
We consider a two-degrees-of-freedom Hamiltonian system with one degree of freedom corresponding to fast motion and the other
corresponding to slow motion. The ratio of typical velocities of changes of the slow and fast variables is the small parameter
ɛ of the problem. At frozen values of the slow variables, there is a separatrix on the phase plane of the fast variables, and
there is a region in the phase space (the domain of separatrix crossings) where the projections of phase points onto the plane
of the fast variables repeatedly cross the separatrix in the process of evolution of the slow variables. Under a certain symmetry
condition, we prove the existence of many (of order 1/ɛ) stable periodic trajectories in the domain of separatrix crossings. Each of these trajectories is surrounded by a stability
island whose measure is estimated from below by a value of order ɛ. So, the total measure of the stability islands is estimated from below by a value independent of ɛ. The proof is based on an analysis of asymptotic formulas for the corresponding Poincaré map. 相似文献
898.
Pedro D. Simão Ana M. Girão Coelho Frans S.K. Bijlaard 《International Journal of Non》2012,47(7):806-822
The paper presents a procedure for the analysis of stability and initial post-buckling behaviour of spliced columns in sway and non-sway steel frames. The main assumptions are linear elasticity and geometrically perfect columns that are loaded by a compressive force which retains its direction as the column deflects. An energy-based formulation that includes a polynomial Rayleigh–Ritz approximation into the potential energy function, in combination with the Lagrange’s method of undetermined multipliers, has been found very convenient for this type of problem. The system is thus described by a set of kinematically admissible generalized coordinates and a single loading parameter. First, the critical state is characterized by means of linear eigenvalue analysis. A parametric study is implemented to assess the critical load. The numerical results are used to develop a relatively simple yet reasonably accurate engineering method for predicting the critical behaviour of spliced columns in sway and non-sway steel frames. The energy formulation is then applied to the search of post-buckling branches of bifurcation points. The approach embraces path-following methods based on perturbation schemes built on a Newton type iterative procedure. This is illustrated in the application to post-bifurcation in columns with different splice mechanical characteristics. The findings suggest that the splice tangent stiffness has a major influence on the overall column behaviour. 相似文献
899.
In this paper, we consider a generic inexact subgradient algorithm to solve a nondifferentiable quasi-convex constrained optimization problem. The inexactness stems from computation errors and noise, which come from practical considerations and applications. Assuming that the computational errors and noise are deterministic and bounded, we study the effect of the inexactness on the subgradient method when the constraint set is compact or the objective function has a set of generalized weak sharp minima. In both cases, using the constant and diminishing stepsize rules, we describe convergence results in both objective values and iterates, and finite convergence to approximate optimality. We also investigate efficiency estimates of iterates and apply the inexact subgradient algorithm to solve the Cobb–Douglas production efficiency problem. The numerical results verify our theoretical analysis and show the high efficiency of our proposed algorithm, especially for the large-scale problems. 相似文献
900.
Mee-Yi Ryu E KuokstisC.Q Chen J.W YangG Simin M Asif KhanG.G Sim P.W Yu 《Solid State Communications》2003,126(6):329-332
A comparative combined study of photoluminescence (PL), PL kinetics, stimulated emission (SE) and photoreflectance (PR) properties in InxGa1−xN epilayers is carried out in the composition range 0≤x≤0.19. In-incorporation up to 4% leads to the sufficient longer radiative recombination decay time due to the decrease in non-radiative recombination channels, which are peculiar to GaN, and band-to-band optical transitions predominate the spontaneous PL spectrum. Further In-incorporation (x>4%) leads to the localization of carriers and/or excitons at band-tails in the In-rich areas. Correlation between the position of dominant low-energy PR oscillation due to the main band gap and SE peak position shows that band-to-band transitions are responsible for lasing and dominate the PL spectrum in all highly pumped InxGa1−xN samples. 相似文献