全文获取类型
收费全文 | 201篇 |
免费 | 3篇 |
国内免费 | 4篇 |
专业分类
化学 | 128篇 |
力学 | 10篇 |
数学 | 37篇 |
物理学 | 33篇 |
出版年
2023年 | 1篇 |
2021年 | 5篇 |
2020年 | 2篇 |
2018年 | 1篇 |
2017年 | 6篇 |
2016年 | 8篇 |
2014年 | 2篇 |
2013年 | 9篇 |
2012年 | 10篇 |
2011年 | 10篇 |
2010年 | 10篇 |
2009年 | 16篇 |
2008年 | 23篇 |
2007年 | 13篇 |
2006年 | 14篇 |
2005年 | 7篇 |
2004年 | 5篇 |
2003年 | 9篇 |
2002年 | 10篇 |
2001年 | 6篇 |
2000年 | 7篇 |
1999年 | 3篇 |
1998年 | 3篇 |
1997年 | 4篇 |
1996年 | 2篇 |
1995年 | 4篇 |
1994年 | 1篇 |
1993年 | 2篇 |
1992年 | 1篇 |
1990年 | 1篇 |
1988年 | 1篇 |
1985年 | 2篇 |
1983年 | 1篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1977年 | 1篇 |
1975年 | 1篇 |
1972年 | 3篇 |
排序方式: 共有208条查询结果,搜索用时 712 毫秒
91.
A recently introduced experimental and theoretical procedure is presented in order to calculate the magnitude and anisotropy of interaction between a lanthanide and a 3d-metal ion. The general formula of the hetero-one-dimensional molecular compounds is trans-[M(CN)4(μ-CN)2Ln(H2O)4(bpy)]n·nH2O · 1.5nbpy (M = Fe3+, Cr3+; Ln = Er3+, Yb3+), where bpy = 2,2′-bipyridine, abbreviated as [LnM] from now on. The main parts of this procedure are: (a) the evaluation of the effective g-parameters of the lanthanide ion with the help of EPR measurements; (b) the use of dual-mode EPR spectroscopy to define the anisotropic exchange interactions with the help of an anisotropic Hamiltonian model; (c) use of the same magnetic model to fit magnetization and susceptibility data in order to verify the EPR findings. 相似文献
92.
Maria Ochsenkühn-Petropulu Petros Tarantilis Georgios Parissakis Vassilis Psycharis Elias Moraitakis Dimitris Niarchos 《Mikrochimica acta》1994,113(1-2):19-27
The high-temperature superconductor, YBa2Cu3O7–x (x = 0.1–0.2) [YBCO], was prepared using an optimized calcination and sintering process. Thin layers of a few microns of this material were deposited on a silver substrate by applying a simplified electrophoretic deposition technique in a suspension of the fine, < 10 m, superconductor powder in a non-aqueous liquid. To get a uniform and strongly adherent coating, the deposition process is repeated several times, followed by an appropriate sintering procedure. The initially prepared YBCO powder and the coatings produced were characterized for their superconducting properties by X-ray diffraction analysis (XRD), magnetization measurements with a Superconducting Quantum Interference Device (SQUID) and electrical resistivity measurements. By scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) the grain size of the YBCO film, its thickness and impurity content, respectively, were estimated. 相似文献
93.
Panayotis?E.?NastouEmail author Vassilis?Papadinas Panos?M.?Pardalos Yannis?C.?Stamatiou 《Optimization Letters》2016,10(6):1203-1220
In this paper, we define a new combinatorial function on the edges of complete weighted graphs. This function assigns to each edge of the graph the sum of the weights of all Hamiltonian cycles that contain the edge. Since this function involves the factorial function, whose exact calculation is intractable due to its superexponential asymptotic rate of increase, we also introduce a normalized version of the function that is efficiently computable. From this version, we derive an upper bound to the weight of the minimum weight Hamiltonian cycle of the graph based on the weights of the graph edges. Then we investigate the possible algorithmic applications of this normalized function using the Nearest Neighbor Heuristic and a smallest edge first heuristic. As evidence for its applicability, we show that the use of this function as a criterion for the selection of the next edge, improves the performance of both heuristics for approximating the minimum weight Hamiltonian cycles in Euclidean plane graphs. Moreover, our experimental results show that the use of the function is more suitable with the structure of the smallest edge first heuristic since it provides a solution closer to the best known solution of known hard TSP instances but in \(O(n^3)\) time. 相似文献
94.
Curative radiation therapy of pelvic malignancies, frequently results in dose limiting toxicities such as serous, mucoid, or more rarely, bloody diarrhea. Several studies have evaluated the cytoprotective effects of amifostine in preventing rectal mucositis associated with radiation treatment. We searched Medline for published comparative studies that evaluated the use of amifostine to reduce radiation-induced toxicity associated with pelvic irradiation. In ten studies there was an evidence-based cytoprotection (P less than 0.05)by amifostine. Although results are variable, current evidence suggests that amifostine may have a radioprotective effect in the rectal mucosa, particularly when administered intrarectally. Significant improvements were seen in both symptomatic and objective(rectosigmoidoscopy) end points. There is a need to conduct well-designed clinical trials with sufficient numbers of participants to confirm these findings together with a cost benefit study. Objective measurements using rectosigmoidoscopy are superior to subjective measures such as WHO or RTOG/EORTC toxicity grading scales. 相似文献
95.
Catherine Gkioni Athanassios K. Boudalis Yiannis Sanakis Leondios Leondiadis Vassilis Psycharis Catherine P. Raptopoulou 《Polyhedron》2008
The systematic investigation of the parameter space of the CuCl2/H2mal/phen reaction system in MeOH resulted in the isolation of seven different complexes either as mixtures or in pure form, six of which have been structurally characterized. The molar ratios of the reactants and the crystallization methods have been systematically varied, leading to the isolation of compounds [Cu(H2O)(phen)(mal)] (1), [Cu(MeOH)(phen)(mal)] (2), [Cu2Li2Cl2(phen)2(mal)2(MeOH)4] (3), [Cu2(phen)4(mal)][CuCl(phen)(mal)](OH) (4), [CuCl(phen)2]Cl (5), and [CuCl(phen)(mal)][CuCl(phen)2][Cu(phen)2(Hmal)]Cl (6). The coordination versatility of the malonato ligand has been confirmed by the presence of three different coordination modes and its two deprotonation states in compounds 1–6. Solution studies on methanolic solutions of 2–4 and 6 by mass spectrometry revealed the absence of parent ion peak and the presence of fragment ions of low relative abundance not previously found in their crystal structure, thus indicating decomposition and rearrangement/reorganization of the complexes in solution and confirming the dynamic character of their solutions. Compounds 3 and 4 have been also studied in the solid state by EPR spectroscopy and magnetic measurements. 相似文献
96.
Michael A. Terzidis Julia Stephanidou-Stephanatou Aristides Terzis Vassilis Psycharis 《Tetrahedron》2008,64(51):11611-11617
Chromones were reacted with dimethyl acetonedicarboxylate in the presence of DBU in THF at room temperature to furnish good yields of products, their structure depending on the substituent at 3-position. Unsubstituted chromones lead to methyl 7-hydroxy-6-oxo-6H-benzo[c]chromone-8-carboxylates 2, whereas by using 3-bromochromone, the methyl furoate 3c along with the unexpected furylcyclopropyl-chromene carboxylate 4c was isolated. Finally, from 3-formyl-chromones functionalized benzophenones 5 were isolated, in good yields. Plausible mechanisms are proposed. 相似文献
97.
Boudalis AK Pissas M Raptopoulou CP Psycharis V Abarca B Ballesteros R 《Inorganic chemistry》2008,47(22):10674-10681
Complex [Ni 5{pyCOpyC(O)(OMe)py} 2(O 2CMe) 4(N 3) 4(MeOH) 2].2MeOH.2.6H 2O ( 1.2MeOH.2.6H 2O) was synthesized by the reaction of Ni(O 2CMe) 2.4H 2O with pyCOpyCOpy and NaN 3 in refluxing MeOH. It crystallizes in the monoclinic C2/ c space group and consists of five Ni (II) atoms in a helical arrangement. Direct current magnetic susceptibility studies reveal ferromagnetic interactions between the Ni (II) ( S = 1) ions, stabilizing an S = 5 ground state. Alternating current susceptibility experiments revealed the existence of out-of-phase signals indicative of slow magnetic relaxation. Analysis of the signals showed that they are composite, suggesting more than one relaxation process, while analysis of their magnitudes suggests not all molecules undergo slow magnetic relaxation. Magnetization field-sweep experiments revealed hysteresis at 1.8 K, and magnetization decay experiments clearly verified the appearance of slow magnetic relaxation at that temperature. 相似文献
98.
Boudalis AK Clemente-Juan JM Dahan F Psycharis V Raptopoulou CP Donnadieu B Sanakis Y Tuchagues JP 《Inorganic chemistry》2008,47(23):11314-11323
Complexes [Fe(Hhbi)(2)(NO(3))].2EtOH (1.2EtOH) and [Fe(2)(mu-OH)(2)(Hhbi)(4)].2H(2)O.8EtOH (2.2H(2)O.8EtOH) crystallize in the orthorhombic Fdd2 and P4(2)2(1)2 space groups, respectively (Hhbi(-) = the monoanion of 2-(2'-hydroxyphenyl benzimidazole). Complex 1 exhibits paramagnetic relaxation as evidenced by Mossbauer spectroscopy, and significant axial zero-field splitting (1.5 cm(1) 相似文献
99.
Fitilis I Fakis M Polyzos I Giannetas V Persephonis P Mikroyannidis J 《The journal of physical chemistry. A》2008,112(21):4742-4748
Two photon absorption (TPA) and photophysical properties of three new symmetrical chromophores with electron accepting phthalimide edge substituents have been studied. The three chromophores contain fluorene, alcoxy-substituted divinyl benzene, and carbazole moieties as central cores, respectively. The femtosecond time-resolved fluorescence upconversion spectroscopy and two photon excited fluorescence technique have been carried out. The effect of solvent polarity on TPA and on photophysics has also been determined. Ultrafast fluorescence dynamics, with decay times ranging from 1 to 13 ps, are revealed in polar solvents. This is attributed to the relaxation of the chromophores to the intramolecular charge transfer state. The chromophore bearing fluorene central core, being of the type A-pi-A, is the most efficient concerning TPA. Strong TPA, with a cross section value as high as 2100 GM at an excitation wavelength of 770 nm is found in acetophenone which is a solvent of intermediate polarity. The TPA spectra were also reproduced using a sum over states three-state model. A study of the TPA induced photobleaching of the fluorene molecule, doped in a solid poly(methyl-methacrylate) film, has shown that this material is very promising for efficient TPA optical data storage. 相似文献
100.