全文获取类型
收费全文 | 1312篇 |
免费 | 36篇 |
国内免费 | 24篇 |
专业分类
化学 | 896篇 |
晶体学 | 17篇 |
力学 | 17篇 |
数学 | 289篇 |
物理学 | 153篇 |
出版年
2022年 | 18篇 |
2021年 | 30篇 |
2020年 | 12篇 |
2019年 | 18篇 |
2018年 | 23篇 |
2017年 | 20篇 |
2016年 | 38篇 |
2015年 | 31篇 |
2014年 | 44篇 |
2013年 | 103篇 |
2012年 | 71篇 |
2011年 | 76篇 |
2010年 | 49篇 |
2009年 | 36篇 |
2008年 | 61篇 |
2007年 | 85篇 |
2006年 | 73篇 |
2005年 | 60篇 |
2004年 | 68篇 |
2003年 | 45篇 |
2002年 | 51篇 |
2001年 | 33篇 |
2000年 | 22篇 |
1999年 | 16篇 |
1998年 | 18篇 |
1997年 | 15篇 |
1996年 | 23篇 |
1995年 | 20篇 |
1994年 | 11篇 |
1993年 | 19篇 |
1992年 | 17篇 |
1991年 | 10篇 |
1990年 | 13篇 |
1989年 | 10篇 |
1988年 | 5篇 |
1987年 | 12篇 |
1986年 | 5篇 |
1985年 | 14篇 |
1984年 | 15篇 |
1983年 | 8篇 |
1982年 | 12篇 |
1981年 | 18篇 |
1980年 | 5篇 |
1979年 | 3篇 |
1978年 | 8篇 |
1977年 | 5篇 |
1974年 | 3篇 |
1973年 | 6篇 |
1972年 | 5篇 |
1970年 | 2篇 |
排序方式: 共有1372条查询结果,搜索用时 15 毫秒
61.
The analysis of samples contaminated by organic compounds is an important aspect of environmental monitoring. Because of the complex nature of these samples, isolating target organic compounds from their matrices is a major challenge. A new isolation technique, solid phase microextraction, or SPME, has recently been developed in our laboratory. This technique combines the extraction and concentration processes into one step; a fused silica fiber coated with a polymer is used to extract analytes and transfer them into a GC injector for thermal desorption and analysis. It is simple, rapid, inexpensive, completely solvent-free, and easily automated. To minimize matrix interferences in environmental samples, SPME can be used to extract analytes from the headspace above the sample. The combination of headspace sampling with SPME separates volatile and semi-volatile analytes from non-volatile compounds, thus greatly reducing the interferences from non-target compounds. This paper reports the use of headspace SPME to isolate volatile organic compounds from various matrices such as water, sand, clay, and sludge. By use of the technique, benzene, toluene, ethyl-benzene, and xylene isomers (commonly known as BTEX), and volatile chlorinated compounds can be efficiently isolated from various matrices with good precision and low limits of detection. This study has found that the sensitivity of the method can be greatly improved by the addition of salt to water samples, water to soil samples, or by heating. Headspace SPME can also be used to sample semi-volatile compounds, such as PAHs, from complex matrices. 相似文献
62.
We have calculated certain dynamic polarizabilities (for both real and imaginary frequencies) for H, He, and H2 and the dispersion-energy coefficients for long-range interactions between them. We have done so in a sum-over-states formalism with explicitly electron-correlated wave functions to describe the states. To be precise, we have determined the dipole (α1), quadrupole (α2), and octupole (α3) polarizabilities of H and He for real frequencies (ω) in a range between zero and the first electronic-transition frequency and for imaginary frequencies (iω) on a 32-point Gauss-Legendre grid running from zero to ?ω = 20 Eh, and for H2, we have found the dipole (α), quadrupole (C), and dipole–octupole (E) polarizability tensors for the same real and imaginary frequencies. The dispersion-energy coefficients, obtained by combining the sum-over-states for-malism for the polarizabilities with analytic integration over ω, gave values of C6, C8, and C10 for the atom–atom systems; C, C, C, C, and C for the atom–diatom systems; and C6, C and C for the H2? H2 system. Nearly all the results are considered to be more reliable than those hitherto published and some have been obtained for the first time, e.g., C(iω), E(ω), and E(iω) for H2 and C, C, and C for the H? H2 system. © 1993 John Wiley & Sons, Inc. 相似文献
63.
The transitional isoelectric focusing (IEF) process (the course of pH gradient formation by carrier ampholytes (CAs) and the correlation of the focusing time with CA concentration) were investigated using a whole-column detection capillary isoelectric focusing (CIEF) system. The transitional double-peak phenomenon in IEF was explained as a result of migration of protons from the anodic end and hydroxyl ions from the cathodic end into the separation channel and the higher electric field at both acidic and basic sides of the separation channel. It was observed that focusing times increase logarithmically with CA concentration under a constant applied voltage. The correlation of focusing time with CA concentration was explained by the dependence of the charge-transfer rate on the amount of charged CAs within the separation channel during focusing. 相似文献
64.
Alicja Boryło Bogdan Skwarzec Grzegorz Romańczyk Janusz Siebert 《Journal of Radioanalytical and Nuclear Chemistry》2013,298(3):1685-1691
The determination of polonium 210Po in human blood samples is presented and discussed in this paper. The human blood samples were collected from patients of Medical University of Gdańsk with ischaemic heart disease (morbus ischaemicus cordis, MIC). The polonium concentrations in analyzed human blood samples are very differentiated. 210Po is of particular interest in public health and although is present in the environment in extremely low amounts, it is easily bioaccumulated to the human body. The study shows that the amount of 210Po that is incorporated into the human body depends on the food habits and some difference in its levels could be observed between smokers and non-smokers. 相似文献
65.
Ignacy Kaliszewski Janusz MiroforidisDmitry Podkopaev 《European Journal of Operational Research》2012,216(1):188-199
We present an approach to interactive Multiple Criteria Decision Making based on preference driven Evolutionary Multiobjective Optimization with controllable accuracy.The approach relies on formulae for lower and upper bounds on coordinates of the outcome of an arbitrary efficient variant corresponding to preference information expressed by the Decision Maker. In contrast to earlier works on that subject, here lower and upper bounds can be calculated and their accuracy controlled entirely within evolutionary computation framework. This is made possible by exploration of not only the region of feasible variants - a standard within evolutionary optimization, but also the region of infeasible variants, the latter to our best knowledge being a novel approach within Evolutionary Multiobjective Optimization.To illustrate how this concept can be applied to interactive Multiple Criteria Decision Making, two algorithms employing evolutionary computations are proposed and their usefulness demonstrated by a numerical example. 相似文献
66.
A food web model with a closed nutrient cycle is presented and analyzed via Monte Carlo simulations. The model consists of three trophic levels, each of which is populated by animals of one distinct species. While the species at the intermediate level feeds on the basal species, and is eaten by the predators living at the highest level, the basal species itself uses the detritus of animals from higher levels as the food resource. The individual organisms remain localized, but the species can invade new lattice areas via proliferation. The impact of different proliferation strategies on the viability of the system is investigated. From the phase diagrams generated in the simulations it follows that in general a strategy with the intermediate level species searching for food is the best for the survival of the system. The results indicate that both the intermediate and top level species play a critical role in maintaining the structure of the system. 相似文献
67.
Janusz Brzdęk Magdalena Piszczek 《Journal of Fixed Point Theory and Applications》2017,19(4):2441-2448
We prove a fixed point theorem for nonlinear operators, acting on some function spaces (of set-valued maps), which satisfy suitable inclusions. We also show some applications of it in the Ulam type stability. 相似文献
68.
Szymon Godlewski Jakub S. Prauzner-Bechcicki Janusz Budzioch Lukasz Walczak Irena G. Stará Ivo Starý Petr Sehnal Marek Szymonski 《Surface science》2012,606(21-22):1600-1607
Helicenes are carbon-rich materials with an ordered structure that possess interesting and remarkable electronic, chiroptical, mechanical and magnetic properties either in bulk or as single molecule devices. We report on the first attempt to anchor and organize [11]anthrahelicene ([11]AH) molecules onto a metal oxide surface. Molecular structures obtained on the (110) and (011) faces of rutile titania are characterized using scanning probe microscopy. On the (110) surface, [11]AH molecules form islands that are comprised of small domains of two types that are mirror reflections of each other and have an ordered quasi-hexagonal lattice. In contrast, molecules form unordered clusters on the (011) surface. Single molecules on both faces are imaged with sub-molecular resolution. 相似文献
69.
Three new chiral urea-type anion receptors were synthesized from aromatic diamines and 1-amino-1-deoxyglucose. The anion binding properties of these receptors were studied using chiral carboxylates derived from mandelic acid and three α-amino acids. We found that the size of the anion binding pocket played an important role in chiral recognition processes. The best results were obtained for 1,8-diaminoanthracene and α-amino acid anions. 相似文献
70.
2,5- and 2,6-dideuterobenzoquinone-1,4 were prepared by a direct catalytic (NaBD4/PdCl2) exchange of bromine in the corresponding dibromo derivatives and the subsequent oxidation with Ag2O. The structure of the products was unambiguously established by proton-carbon and deuteron-carbon splitting pattern of their NMR spectra. 相似文献