全文获取类型
收费全文 | 1208篇 |
免费 | 29篇 |
国内免费 | 2篇 |
专业分类
化学 | 816篇 |
晶体学 | 15篇 |
力学 | 38篇 |
数学 | 165篇 |
物理学 | 205篇 |
出版年
2023年 | 8篇 |
2022年 | 50篇 |
2021年 | 41篇 |
2020年 | 26篇 |
2019年 | 38篇 |
2018年 | 21篇 |
2017年 | 26篇 |
2016年 | 51篇 |
2015年 | 38篇 |
2014年 | 44篇 |
2013年 | 96篇 |
2012年 | 87篇 |
2011年 | 92篇 |
2010年 | 62篇 |
2009年 | 63篇 |
2008年 | 73篇 |
2007年 | 79篇 |
2006年 | 44篇 |
2005年 | 34篇 |
2004年 | 35篇 |
2003年 | 27篇 |
2002年 | 28篇 |
2001年 | 11篇 |
2000年 | 12篇 |
1999年 | 12篇 |
1998年 | 6篇 |
1997年 | 11篇 |
1996年 | 9篇 |
1995年 | 6篇 |
1994年 | 10篇 |
1993年 | 4篇 |
1992年 | 7篇 |
1991年 | 3篇 |
1990年 | 5篇 |
1989年 | 6篇 |
1988年 | 3篇 |
1987年 | 5篇 |
1986年 | 6篇 |
1985年 | 4篇 |
1984年 | 3篇 |
1983年 | 4篇 |
1981年 | 4篇 |
1979年 | 6篇 |
1976年 | 7篇 |
1975年 | 4篇 |
1974年 | 4篇 |
1972年 | 6篇 |
1971年 | 6篇 |
1970年 | 2篇 |
1962年 | 2篇 |
排序方式: 共有1239条查询结果,搜索用时 31 毫秒
941.
Tânia Cecília Trevisan Evandro Luiz Dall’Oglio Eudes da Silva Velozo Paulo Teixeira de Sousa Jr. 《Tetrahedron letters》2008,49(44):6289-6292
The phytochemical investigation of the methanol crude extract obtained from Acosmium dasycarpum (Vog.) Yakovlev root bark led to the isolation of the quinolizidine alkaloids lupanine, acosmine, acosminine and lupanacosmine, as well as the diaza-adamantane alkaloids panacosmine and dasycarpumine. Lupanacosmine (4) and dasycarpumine (6) have been described for the first time herein. 相似文献
942.
Maria A. Martins Rogrio M.B. Moreno Colleen M. McMahan Jenny L. Brichta Paulo de S. Gonalves Luiz H.C. Mattoso 《Thermochimica Acta》2008,474(1-2):62-66
The improvement of rubber tree species is of key importance due to the worldwide market demand of this renewable material essential for several types of industries. The thermal performance of natural rubber produced from new clones of IAC 300 series, and the Malaysian RRIM 600 clone (used as control) have been evaluated using DSC and TGA/DTG techniques in air and nitrogen atmospheres. The results have shown that in oxidative atmosphere, the thermal stability of the Brazilian clones was slightly better than that of the Malaysian clone RRIM 600, and that, in an inert atmosphere, there are no significant differences in the thermal behavior among them. The natural rubbers from IAC clones have shown good thermal stability up to 300 °C in inert and oxidative atmospheres, which is important for several industrial applications. The decomposition changes from a one step process to a three step process, when moving from an inert to an oxidative atmosphere. The index of thermal stability has shown that the natural rubbers have higher thermal stability in inert atmosphere. 相似文献
943.
Antonio Luiz Pereira de Siqueira Campos 《International Journal of Infrared and Millimeter Waves》2008,29(7):684-692
In this paper, a theoretical analysis of scattering from a doubly frequency selective surfaces (FSS) with dielectric and metallic
losses in the millimeter wave range is presented. The theoretical analysis involves the solution of the electric field integral
equation relating the induced current in the FSS in the presence of anisotropic dielectric layers. The method of moment is
employed to obtain numerical results. The frequency selective surface structure considered is composed by conducting patch
elements sandwiched between two anisotropic dielectric layers. Three different anisotropic materials are considered: Epsilam-10,
Alumina, and Sapphire. The sapphire presents low losses when compared with the other two dielectric materials. The most common
metals, as copper and aluminum, are considered in this analysis. None significant metallic losses were observed for any considered
metal. Numerical results are presented for the scattering characteristics and for dielectric and metallic losses parameters. 相似文献
944.
Rmulo A. Ando Rodrigo L. O. R. Cunha Luiz Juliano Antonio C. Borin Paulo S. Santos 《Journal of Raman spectroscopy : JRS》2008,39(4):453-459
The electronic (UV‐vis) and resonance Raman (RR) spectra of a series of para‐substituted trans‐β‐nitrostyrenes were investigated to determine the influence of the electron donating properties of the substituent (X = H, NO2, COOH, Cl, OCH3, OH, N(CH3)2, and O−) on the extent of the charge transfer to the electron‐withdrawing NO2 group directly linked to the ethylenic (C = C) unit. The Raman spectra and quantum chemical calculations show clearly the correlation of the electron donating power of the X group with the wavenumbers of the νs(NO2) and ν (C = C)sty normal modes. In conditions of resonance with the lowest excited electronic state, one observes for X = OH and N(CH3)2 that the symmetric stretching of the NO2, νs(NO2), is the most substantially enhanced mode, whereas for X = O−, the chromophore is extended over the whole molecule, with substantial enhancement of several carbon backbone modes. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
945.
Bone healing process in critical-sized defects by rhBMP-2 using poloxamer gel and collagen sponge as carriers 总被引:2,自引:0,他引:2
Issa JP do Nascimento C Iyomasa MM Siéssere S Regalo SC Defino HL Sebald W 《Micron (Oxford, England : 1993)》2008,39(1):17-24
The bone morphogenetic protein type 2, is an osteoinduction protein, but new carriers to improve its actions are being studied in recent researches. The objective of this study was to evaluate the poloxamer gel as potential carrier of the rhBMP-2 during the bone healing process and verify if the collagen sponge can improve this process. Fifty-six male Wistar rats were used, which were divided into two groups, considering two periods of time. Thus, each one of these groups was divided in four groups with seven animals each, according to the treatment and the defect filled by 4 microg rhBMP-2 in aqueous solution, 4 microg rhBMP-2 in aqueous solution+collagen sponge, 4 microg rhBMP-2 combined with poloxamer gel, and 4 microg rhBMP-2 combined with poloxamer gel+collagen sponge. After 2 and 4 weeks, respectively, animals were perfused and the hemi-mandibles removed for histological analysis. Statistical analysis of the data showed significant difference between all analyzed groups (p<0.01), and between the periods of time (p=0.0118). There was no interaction between the applied treatment and considered periods of time (p=0.642). Results showed that the rhBMP-2 used in this study was able to induce bone regeneration and had its action optimized when combined to the used carriers, being the bone repair time-dependent. 相似文献
946.
Luiz G. Guimarães Edson E.S. Sampaio 《Journal of Quantitative Spectroscopy & Radiative Transfer》2008,109(5):811-821
Investigation of the wave problem related to multiple reflections and attenuation in marine controlled source electromagnetic is important for geophysical sounding of the sea substratum. An approach towards this goal is to study the scattering of obliquely incident plane waves by a four-layered earth: sea, sea bottom sedimentary rock, hydrocarbon reservoir, and lower sedimentary rock. The approach employs Debye theory in the frequency domain and assumes that each layer is a planar lossy medium. Explicit expressions show that the secondary waves related to the sea substratum layers multireflections carry useful information about their features. 相似文献
947.
In a recent publication, we have discussed the effects of boundary conditions in finite quantum systems and their connection with symmetries. Focusing on the one-dimensional Hubbard Hamiltonian under twisted boundary conditions, we have shown that properties, such as the ground-state and gap energies, converge faster to the thermodynamical limit (\(L \rightarrow \infty \)) if a special torsion Θ? is adjusted to ensure particle-hole symmetry. Complementary to the previous research, the present paper extends our analysis to a key quantity for understanding correlations in many-body systems: the entanglement. Specifically, we investigate the average single-site entanglement 〈Sj〉 as a function of the coupling U/t in Hubbard chains with up to L =?8 sites and further examine the dependence of the per-site ground-state ??0 on the torsion Θ in different coupling regimes. We discuss the scaling of ??0 and 〈Sj〉 under Θ? and analyze their convergence to Bethe Ansatz solution of the infinite Hubbard Hamiltonian. Additionally, we describe the exact diagonalization procedure used in our numerical calculations and show analytical calculations for the case study of a trimer. 相似文献
948.
Evaluation of metal release from battery and electronic components in soil using SR‐TXRF and EDXRF 下载免费PDF全文
Felipe Rodrigues dos Santos Eduardo de Almeida Pedro Daniel da Cunha Kemerich Fábio Luiz Melquiades 《X射线光谱测定》2017,46(6):512-521
Potentially toxic elements may be leached contaminating the soil, surface, and ground water due to the improper disposal of batteries and electronic devices. The objective of this study was to evaluate metal release from batteries and electronic components deposited in specific receptacles filled with soil in which acid rain was simulated. The leachate solution and the soil were analyzed by synchrotron radiation total reflection x‐ray fluorescence and benchtop energy dispersive x‐ray fluorescence, respectively. Results indicate that batteries released K, Mn, Fe, Cu, Zn, and Pb and electronic component released Ti, Mn, Fe, Cu, Zn, and Pb. For batteries' leachate test samples, higher amounts of Fe, Cu, Mn, Zn, and Pb have been released compared with electronic component ones under same the experimental conditions. The Fe, Cu, and Pb concentrations in battery leachate test samples were above their National Environment Council maximum permitted values (MPV) and in the electronic waste leachate ones, only the Pb concentrations was above MPV. For soil sample containing batteries K, Mn and Zn presented higher concentrations, mainly at the 10‐cm topsoil, ranging from 0.16 to 0.50, 0.27 to 8.67, and 0.03 to 1.26, in % (% w/w), respectively. The Zn–C battery soil samples present similar behavior to the alkaline ones. The impact due to the Pb release was higher in the soil test samples with electronic components, in which their concentrations ranged from 51 to 394 mg/kg, above its MPV up to 28‐cm soil layer. The X‐ray fluorescence techniques employed were suitable for water and soil environmental evaluation. 相似文献
949.
Cleidjane Gomes Faustino Fernando Antnio de Medeiros Allan Kardec Ribeiro Galardo Alex Bruno Lobato Rodrigues Anderson Luiz Pena da Costa Rosany Lopes Martins Lethicia Barreto Brando Lizandra Lima Santos Marcos Antnio Alves de Medeiros Patrick de Castro Canturia Ana Luzia Ferreira Farias Jader Santos Cruz Sheylla Susan Moreira da Silva de Almeida 《Molecules (Basel, Switzerland)》2021,26(21)
This work aimed to prepare a nanoemulsion containing the essential oil of the Protium heptaphyllum resin and evaluate its biocidal activities against the different stages of development of the Aedes aegypti mosquito. Ovicide, pupicide, adulticide and repellency assays were performed. The main constituents were p-cymene (27.70%) and α-pinene (22.31%). The developed nanoemulsion showed kinetic stability and monomodal distribution at a hydrophilic–lipophilic balance of 14 with a droplet size of 115.56 ± 1.68 nn and a zeta potential of −29.63 ± 3.46 mV. The nanoemulsion showed insecticidal action with LC50 0.404 µg·mL−1 for the ovicidal effect. In the pupicidal test, at the concentration of 160 µg·mL−1, 100% mortality was reached after 24 h. For adulticidal activity, a diagnostic concentration of 200 µg·mL−1 (120 min) was determined. In the repellency test, a concentration of 200 µg·mL−1 during the 180 min of the test showed a protection index of 77.67%. In conclusion, the nanobiotechnological product derived from the essential oil of P. heptaphyllum resin can be considered as a promising colloid that can be used to control infectious disease vectors through a wide range of possible modes of applications, probably as this bioactive delivery system may allow the optimal effect of the P. heptaphyllum terpenes in aqueous media and may also induce satisfactory delivery to air interfaces. 相似文献
950.
Silvnia Vaz de Melo Mattos Luiz Fernando Cappa de Oliveira Andria A. M. Nascimento Cynthia Peres Demicheli Rubn Dario Sinisterra 《应用有机金属化学》2000,14(9):507-513
Methylmercury (MeHg) is one of the most toxic forms of mercury in the environment. It can be accumulated in fish through the food chain; after, consumption the fish is then dangerous to fetuses and younger children, causing abnormal brain development and nervous system disorders. Cyclodextrins (CDs) are cyclic oligosaccharides consisting of six, seven or eight units of glucose. In accord with the dimensions and hydrophilic–lipophilic properties, one can obtain inclusion of hydrophobic guests in a CD cavity. In the present work we used this characteristic of CD to obtain an inclusion compound between MeHgCl and the α‐cyclodextrin, looking for a new method to reduce MeHgCl toxicity and pre‐concentration. The inclusion compound was characterized through IR, 1H, 13C NMR and Raman spectroscopy. X‐ray diffraction and thermal analysis (TG, DTG and DSC) methods were also used. Finally, biological tests were carried out and the minimum inhibitory concentrations (MICs) for MeHgCl, α‐cyclodextrin, the MeHgCl–CD complex and a physical mixture were determined. This host–guest strategy using cyclodextrins offers an alternative and powerful method for mercury remediation. Copyright © 2000 John Wiley & Sons, Ltd. 相似文献