全文获取类型
收费全文 | 4339篇 |
免费 | 198篇 |
国内免费 | 44篇 |
专业分类
化学 | 2871篇 |
晶体学 | 42篇 |
力学 | 132篇 |
数学 | 447篇 |
物理学 | 1089篇 |
出版年
2024年 | 19篇 |
2023年 | 44篇 |
2022年 | 236篇 |
2021年 | 226篇 |
2020年 | 172篇 |
2019年 | 172篇 |
2018年 | 176篇 |
2017年 | 141篇 |
2016年 | 216篇 |
2015年 | 147篇 |
2014年 | 215篇 |
2013年 | 382篇 |
2012年 | 311篇 |
2011年 | 305篇 |
2010年 | 240篇 |
2009年 | 192篇 |
2008年 | 212篇 |
2007年 | 180篇 |
2006年 | 138篇 |
2005年 | 74篇 |
2004年 | 100篇 |
2003年 | 55篇 |
2002年 | 56篇 |
2001年 | 33篇 |
2000年 | 28篇 |
1999年 | 31篇 |
1998年 | 20篇 |
1997年 | 29篇 |
1996年 | 26篇 |
1995年 | 12篇 |
1994年 | 26篇 |
1993年 | 20篇 |
1992年 | 20篇 |
1991年 | 11篇 |
1990年 | 19篇 |
1989年 | 13篇 |
1988年 | 15篇 |
1987年 | 16篇 |
1985年 | 23篇 |
1984年 | 23篇 |
1983年 | 15篇 |
1982年 | 20篇 |
1981年 | 14篇 |
1980年 | 38篇 |
1979年 | 11篇 |
1977年 | 15篇 |
1976年 | 11篇 |
1973年 | 11篇 |
1969年 | 9篇 |
1968年 | 8篇 |
排序方式: 共有4581条查询结果,搜索用时 15 毫秒
961.
S. Waheed J. H. Zaidi S. Ahmad M. Saleem 《Journal of Radioanalytical and Nuclear Chemistry》2002,254(3):597-605
Twenty-three individual food items commonly consumed in a high altitude region (Kashmir) have been investigated using instrumental neutron activation analysis. The concentration of Ca, Cl, Co, Cr, Cu, Fe, K, Mg, Mn, Mo, Na, Se, Sn, and Zn is reported for these food articles. For quality assurance of our data, IAEA and NIST standard reference materials have also been studied. The dietary intake of these elements was calculated in accordance with the weekly food selection habits of the residents of this area. The dietary contribution of most of these elements was found to be in agreement to the RDA/ESadi levels. The purpose was to design a database of essential elements for this area and to study the adequacy levels of these elements in different dietary items. Fairly adequate intake of Ca, Co, Cr, Cu, Fe, Mg, Mn, Mo, Se, Sn, and Zn through these foodstuffs is reported, whereas, lower intakes through these items were found for Cl, K, and Na. 相似文献
962.
S. Ahmad A. Thorén G. Skarnemark 《Journal of Radioanalytical and Nuclear Chemistry》1984,86(4):247-253
A solvent extraction procedure for rapid separation of uranium from complex nuclear reaction product mixtures is suggested. The procedure has been tested in batch experiments with tracer amounts of representative elements. It has also been tested with fission products and uranium tracer using the continuous chemical separation system SISAK at the Mainz TRIGA reactor. 相似文献
963.
A rapid, sensitive and highly selective spectrophotometric method for the determination of manganese oxide content of Portland cement and cement raw meal is developed. The method is based on the reaction of manganese(II) with 1,2,4 trihydroxyanthraquinone (purpurin, PURP) in 50% v/v ethanol-water solution at pH 8.5. The solution equilibria of manganese chelates are demonstrated and characterized for delineating optimal conditions of the complexation reaction and analytical aspect of the Mn-PURP system. The analysis of cement materials of variable manganese content is feasible over the concentration range 1.67-8.13 microg ml(-1) Mn, the limit of detection (at the 95% confidence level) of the method is 68 ng ml(-1) for manganese. Under optimum conditions, the use of first derivative spectrophotometry has the advantage of high sensitivity than normal spectrophotometric method and allows the determination of 0.5 microg ml(-1) of manganese. 相似文献
964.
Lauricella R Allouch A Roubaud V Bouteiller JC Tuccio B 《Organic & biomolecular chemistry》2004,2(9):1304-1309
A new kinetic approach to the evaluation of rate constants for the spin trapping of superoxide/hydroperoxyl radical by nitrones in buffered media is described. This method is based on a competition between the superoxide trapping by the nitrone and the spontaneous dismutation of this radical in aqueous media. EPR spectra are recorded as a function of time at various nitrone concentrations, and kinetic curves are obtained after treatment of these spectra using both singular value decomposition and pseudo-inverse deconvolution methods. Modelling these curves permits the determination of the rate constants k(T) and k(D) for the superoxide trapping and the adduct decay reactions, respectively. Kinetics parameters thus obtained with six nitrones, namely the 2-ethoxycarbonyl-2-methyl-3,4-dihydro-2H-pyrrole N-oxide (EMPO), the 5-diethoxyphosphoryl-5-methyl-3,4-dihydro-5H-pyrrole N-oxide (DEPMPO), the 5,5-dimethyl-3,4-dihydro-5H-pyrrole N-oxide (DMPO), the 1,3,5-tri[(N-(1-diethylphosphono)-1-methylethyl)-N-oxy-aldimine]benzene (TN), the N-benzylidene-1-ethoxycarbonyl-1-methylethylamine N-oxide (EPPN), and the N-[(1-oxidopyridin-1-ium-4-yl)methylidene]-1-ethoxycarbonyl-1-methylethylamine N-oxide (EPPyON), indicate that cyclic nitrones trapped superoxide faster than the linear ones. However, the low k(T) values obtained for compounds show that there is still a need for new molecules with better spin trapping capacities. 相似文献
965.
This paper is concerned with asymptotic stability of a system of reaction-diffusion equations which is expressed in terms of Volterra integrals, under homogeneous Dirichlet boundary conditions. The effect of diffusion and delays on the stability of the system are analyzed, and sufficient conditions are given for the existence of positive solutions of the corresponding steady-state problem. Their global attraction with respect to nonnegative solutions of the time-dependent system is discussed. The main tools are Lyapunov functionals, positive definite kernels, and Laplace transforms. 相似文献
966.
Ahmad AL-SALMAN 《Frontiers of Mathematics in China》2021,16(1):13-28
We introduce a class of singular integral operators on product domains along twisted surfaces.We prove that the operators are bounded on Lp provided that the kernels satisfy weak conditions. 相似文献
967.
Ahmad H. Alghamdi 《Journal of Saudi Chemical Society》2010,14(1):1-7
The adsorptive collection of zinc(II) complex with alizarin ligand, coupled with the square-wave voltammetric technique at the hanging mercury drop electrode, yields a very sensitive electroanalytical procedure for the determination of zinc. The optimized experimental conditions include: supporting electrolyte (carbonate buffer), pH (11), alizarin concentration (1 × 10?6 mol l?1), accumulation time (60 s), accumulation potential (?0.1 V), scan rate (700 mV s?1), pulse amplitude (0.06 V) and SW frequency (80 Hz). The monitored stripping voltammetric current was linear over the range of 5 × 10?8 – 4 × 10?7 mol l?1 and the detection limit was 1 × 10?8 mol l?1. The relative standard deviation was calculated as 1.3% (n = 10) for 1 × 10?8 mol l?1 Zn(II) and the obtained electrochemical signal was stabile for up to 60 min. Possible interferences by either co-existing metal ions or other chelating agents were also investigated. The applicability of the proposed SW-AdSV method to the analysis of foodstuff was assessed by the determination of zinc content in instant coffee samples. The accuracy of the obtained voltammetric analytical results was validated by comparing with that obtained by atomic absorption spectrometric method and conducting the necessary statistical evaluation. 相似文献
968.
Glued-diaphragm fibre optic pressure sensors that utilize standard telecommunications components which are based on Fabry–Perot
interferometry are appealing in a number of respects. Principally, they have high spatial and temporal resolution and are
low in cost. These features potentially make them well suited to operation in extreme environments produced in short-duration
high-enthalpy wind tunnel facilities where spatial and temporal resolution are essential, but attrition rates for sensors
are typically very high. The sensors we consider utilize a zirconia ferrule substrate and a thin copper foil which are bonded
together using an adhesive. The sensors show a fast response and can measure fluctuations with a frequency up to 250 kHz.
The sensors also have a high spatial resolution on the order of 0.1 mm. However, with the interrogation and calibration processes
adopted in this work, apparent errors of up to 30% of the maximum pressure have been observed. Such errors are primarily caused
by mechanical hysteresis and adhesive viscoelasticity. If a dynamic calibration is adopted, the maximum measurement error
can be limited to about 10% of the maximum pressure. However, a better approach is to eliminate the adhesive from the construction
process or design the diaphragm and substrate in a way that does not require the adhesive to carry a significant fraction
of the mechanical loading.
相似文献
969.
Madeeha Aslam Fozia Fozia Anadil Gul Ijaz Ahmad Riaz Ullah Ahmed Bari Ramzi A. Mothana Hidayat Hussain 《Molecules (Basel, Switzerland)》2021,26(20)
Green synthesis of silver nanoparticles (AgNPs) employing an aqueous plant extract has emerged as a viable eco-friendly method. The aim of the study was to synthesize AgNPs by using plant extract of Sanvitalia procumbens (creeping zinnia) in which the phytochemicals present in plant extract act as a stabilizing and reducing agent. For the stability of the synthesized AgNPs, different parameters like AgNO3 concentration, volume ratios of AgNO3, temperature, pH, and contact time were studied. Further, AgNPs were characterized by UV–visible spectroscopy, FT-IR (Fourier Transform Infrared Spectroscopy), XRD (X-ray Diffraction), SEM (Scanning Electron Microscopy), and EDX (Energy Dispersive X-ray Spectrometer) analysis. FT-IR analysis showed that the plant extract contained essential functional groups like O–H stretching of carboxylic acid, N–H stretching of secondary amides, and C–N stretching of aromatic amines, and C–O indicates the vibration of alcohol, ester, and carboxylic acid that facilitated in the green synthesis of AgNPs. The crystalline nature of synthesized AgNPs was confirmed by XRD, while the elemental composition of AgNPs was detected by energy dispersive X-ray analysis (EDX). SEM studies showed the mean particle diameter of silver nanoparticles. The synthesized AgNPs were used for photocatalytic degradation of Orange G and Direct blue-15 (OG and DB-15), which were analyzed by UV-visible spectroscopy. Maximum degradation percentage of OG and DB-15 azo dyes was observed, without any significant silver leaching, thereby signifying notable photocatalytic properties of AgNPs. 相似文献
970.
Gabriela Schneider-Rauber Mihails Arhangelskis Wei-Pin Goh James Cattle Nicole Hondow Rik Drummond-Brydson Mojtaba Ghadiri Kushal Sinha Raimundo Ho Nandkishor K. Nere Shailendra Bordawekar Ahmad Y. Sheikh William Jones 《Chemical science》2021,12(42):14270
Crystal engineering has advanced the strategies for design and synthesis of organic solids with the main focus being on customising the properties of the materials. Research in this area has a significant impact on large-scale manufacturing, as industrial processes may lead to the deterioration of such properties due to stress-induced transformations and breakage. In this work, we investigate the mechanical properties of structurally related labile multicomponent solids of carbamazepine (CBZ), namely the dihydrate (CBZ·2H2O), a cocrystal of CBZ with 1,4-benzoquinone (2CBZ·BZQ) and the solvates with formamide and 1,4-dioxane (CBZ·FORM and 2CBZ·DIOX, respectively). The effect of factors that are external (e.g. impact stressing) and/or internal (e.g. phase transformations and thermal motion) to the crystals are evaluated. In comparison to the other CBZ multicomponent crystal forms, CBZ·2H2O crystals tolerate less stress and are more susceptible to breakage. It is shown that this poor resistance to fracture may be a consequence of the packing of CBZ molecules and the orientation of the principal molecular axes in the structure relative to the cleavage plane. It is concluded, however, that the CBZ lattice alone is not accountable for the formation of cracks in the crystals of CBZ·2H2O. The strength and the temperature-dependence of electrostatic interactions, such as hydrogen bonds between CBZ and coformer, appear to influence the levels of stress to which the crystals are subjected that lead to fracture. Our findings show that the appropriate selection of coformer in multicomponent crystal forms, targetting superior mechanical properties, needs to account for the intrinsic stress generated by molecular vibrations and not solely by crystal anisotropy. Structural defects within the crystal lattice, although highly influenced by the crystallisation conditions and which are especially difficult to control in organic solids, may also affect breakage.Crystal engineering has advanced the strategies of design and synthesis of organic solids with the main focus being on customising the properties of the materials. 相似文献