全文获取类型
收费全文 | 54639篇 |
免费 | 3614篇 |
国内免费 | 3939篇 |
专业分类
化学 | 20418篇 |
晶体学 | 399篇 |
力学 | 4289篇 |
综合类 | 587篇 |
数学 | 15656篇 |
物理学 | 20843篇 |
出版年
2024年 | 80篇 |
2023年 | 491篇 |
2022年 | 1173篇 |
2021年 | 1188篇 |
2020年 | 1254篇 |
2019年 | 1276篇 |
2018年 | 1105篇 |
2017年 | 1277篇 |
2016年 | 1433篇 |
2015年 | 1099篇 |
2014年 | 1796篇 |
2013年 | 3568篇 |
2012年 | 2092篇 |
2011年 | 2412篇 |
2010年 | 1978篇 |
2009年 | 3127篇 |
2008年 | 3403篇 |
2007年 | 3803篇 |
2006年 | 3199篇 |
2005年 | 2546篇 |
2004年 | 2114篇 |
2003年 | 2460篇 |
2002年 | 2824篇 |
2001年 | 2137篇 |
2000年 | 2039篇 |
1999年 | 1728篇 |
1998年 | 1695篇 |
1997年 | 1036篇 |
1996年 | 951篇 |
1995年 | 797篇 |
1994年 | 827篇 |
1993年 | 614篇 |
1992年 | 673篇 |
1991年 | 458篇 |
1990年 | 437篇 |
1989年 | 346篇 |
1988年 | 311篇 |
1987年 | 308篇 |
1986年 | 263篇 |
1985年 | 254篇 |
1984年 | 249篇 |
1983年 | 151篇 |
1982年 | 207篇 |
1981年 | 191篇 |
1980年 | 128篇 |
1979年 | 156篇 |
1978年 | 134篇 |
1977年 | 123篇 |
1976年 | 82篇 |
1973年 | 62篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
This article introduces the relationship between chemistry and fish farming, fish selling, and fish food. The nitrogen cycle process in fish ponds is introduced to explain the fish farming. "Yufuling" is used as the entry point to fish selling to connect food safety with chemistry. Fishy smell is explained from a chemical perspective for fish food. This paper aims at making a scientific explanation of life phenomena, deepening the consciousness of chemical life, helping people establish a correct view of chemical science, and inspiring youth to learn to use the knowledge that they have learned to explain life phenomena. This paper also tries to show the youth that life cannot be separated from chemistry, and chemistry cannot be separated from life. 相似文献
992.
Band structure calculations at the level of LMTO-ASA provide insight into the electronic structure of BaV10O15 and the origin of the structural phase transition. A crystal orbital Hamiltonian population/integrated crystal orbital Hamiltonian population analysis provides evidence that the crystallographic phase transition is driven by V-V bond formation. As well, the energy bands near the Fermi level are very narrow, <1 eV, consistent with the fact that the observed insulating behavior can be due to electron localization via either Mott-Hubbard correlation and/or Anderson disorder. The partial solid solution, BaV10−xTixO15, was examined to study the effect of Ti-doping at the V sites on the structure and electronic transport properties. In spite of the non-existence of “BaTi10O15”, the limiting x=8, as indicated by a monotonic increase in the cell volume and systematic changes in properties. This limit may be due to the difficulty of stabilizing Ti2+ in this structure. For x=0.5 both the first order structural phase transition and the magnetic transition at 40 K are quenched. The samples obey the Curie-Weiss law to x=3 with nearly spin only effective moments along with θ values which range from −1090 K (x=0.5) to −1629 K (x=3). For x>3 a very large, ∼2×10−3 emu/mol, temperature independent (TIP) contribution dominates. Conductivity measurements on sintered, polycrystalline samples show semiconducting behavior for all compositions. Activation energies for Mott hopping derived from high temperature data range from ∼0.1 eV for x=0-1 and fall to a plateau of 0.06 eV for x=3-7. Low temperature data for x=3, 5 and 7 show evidence for Mott variable range hoping (VRH) with a T1/4 law and in one case between 5 and 17 K, a Efros-Shklovskii correlated hopping, T1/2 law, was seen, in sharp contrast to BaV10O15 where only the E-S law was observed up to 75 K. Seebeck coefficients are small (<35 μV/K), positive, roughly TIP and increase with increasing x up to x=5. This may point to a Heikes hopping of holes but a simple single carrier model is impossible. The compositions for x>3 are remarkable in that local moment behavior is lost, yet a metallic state is not reached. The failure of this system to be driven metallic even at such high doping levels is not fully understood but it seems clear that disorder induced carrier localization plays a major role. 相似文献
993.
一种低磁矩、大功率复合稀土石榴石铁氧体的工艺和性能研究 总被引:4,自引:0,他引:4
为满足低场区几百兆赫的超大功率器件的性能要求,以钇铁石榴石铁氧体为基础,制备了一种低磁矩复合稀土石榴石铁氧体YGd CaVInIG,研究了少量Mn3+替代Fe3+和预烧温度、烧结温度对铁氧体性能和结构的影响。实验表明,以少量Mn3+替代Fe3+可以提高铁氧体电阻率,降低磁损耗和介电损耗。Mn3+掺入量以x为0.04~0.06比较合适,铁氧体最佳予烧温度为1050℃,最佳烧结温度1350~1380℃,保温5h,氧气中烧结,其性能为:4πMs=500±10%kA·m-1,ΔH=5.25~5.55kA·m-1,TC>180℃,tgδe≤0.5×10-5,这种材料适合微波低频段器件性能要求。X射线衍射分析指出,掺Mn3+的YGdCaVInIG铁氧体相变完全,呈单相复合石榴石铁氧体,空间群为I230,点阵常数1.25057~1.25101nm,单胞分子数n为7.75~7.80。 相似文献
994.
Keshtov M. L. Rusanov A. L. Keshtova S. V. Petrovskii P. V. Shchegolikhin A. A. 《Russian Chemical Bulletin》2002,51(1):117-123
A series of previously unknown unsymmetrical difluoroaromatic compounds, viz., p-fluorobenzoylphenyl(p-fluorophenyl)-substituted imidazoles, pyrazines, and quinoxalines, were synthesized according to multistep procedures with the use of chloral as the key compound. The reactivities of the resulting difluoroaromatic compounds were estimated based on 19F and 13C NMR spectral data and the results of quantum-chemical calculations. The calculated charge densities on the Cipso atoms correlate linearly with the experimental chemical shifts in the 19F and 13C NMR spectra. Difluoroaromatic compounds, which are characterized by F > –110 and C > 163 and by the charge density on the Cipso atom higher than 0.08 e, are sufficiently activated to be used for the preparation of high-molecular-weight polyethers. 相似文献
995.
New activated bis- and tetrafluoroaromatic compounds containing the bis-phenylquinoxaline fragments, viz., methylene-bis-6(7),6"(7")-{2-(p-fluoro)phenyl-3[4-(p-fluorobenzoyl)phenyl]quinoxalines}, were prepared according to multistep procedures with the use of chloral as the starting compound. The presence of the activating carbonyl groups in the para positions with respect to two fluorine atoms opens up the possibility of the use of these monomers for the synthesis of high-molecular-weight aromatic quinoxaline-containing polyether ketones. 相似文献
996.
Syntheses and spectral characteristics of cadmium(II) compounds (CdSeO4, CdSeO3, and Cd(NCSe)2(nia)2) containing selenium in oxidation states (VI), (IV), and (-II) are described. In Cd(NCSe)2(nia)2, nicotinamide (nia) and selenocyanate anions are bonded to Cd atom as N-donor monodentate ligands. Nicotinamide is coordinated
through the ring nitrogen atom. The effects of these selenium compounds as well as Cd(NCS)2(nia)2 on the growth and Cd accumulation in roots and shoots of hydroponically cultivated chamomile plants (cultivar Lutea) were studied. In the applied concentration range (12–60 μmol dm−3) Cd(NCS)2(nia)2 affected neither the length nor the dry mass of roots and shoots. Other compounds applied at 24 μmol dm−3 and 60 μmol dm−3 significantly reduced dry mass of roots and shoots. Selenium oxidation state in the cadmium compounds affected Cd accumulation
in plant organs as well as Cd translocation within the plants, which was reflected in the values of bioaccumulation (BAF)
and translocation factors (S/R). Cd amount accumulated by shoots was lower than that in the roots. The highest BAF values
determined for Cd accumulation in shoots were obtained with CdSeO4. Substitution of S with Se in the Cd(NCX)2(nia)2 (X = Se or S) caused an increase of Cd translocation into the shoots.
Presented at the XVIIIth Slovak Spectroscopic Conference, Spišská Nová Ves, 15–18 October 2006. 相似文献
997.
František Kvasnička Jaroslav Dobiáš Kamila Klaudisová-Chudáčková 《Central European Journal of Chemistry》2003,1(1):91-97
A simple, rapid and reproducible capillary isotachophoretic on-line coupled with capillary zone electrophoresis (CITP-CZE)
method for the determination of IMz in food packaging extracts and its residues in apples is described. A good separation
of the IMZ from other sample constituents was achieved within 15 minutes without any sample clean up. Method characteristics
(linearity, accuracy, intra-assay and detection limit) were determined. Less amount of time involved, sufficient sensitivity
and low running cost are the important attributes of CITP-CZE method. 相似文献
998.
Synthesis and binding studies of new bis-calix[4]arenes containing aromatic and heteroaromatic units
A series of new bis-calix[4]arenes containing different aromatic and heteroaromatic moieties have been synthesized. The complexing behavior of these bis-calix[4]arenes have been studied towards different metal ions and it has been found that these bis-calix[4]arenes bind silver ions selectively over other metal ions. The complexation has been studied by liquid-liquid extraction and by NMR and IR spectroscopy. 相似文献
999.
V. Slavinska Dz. Sile G. Rozenthal G. Maurops J. Popelis M. Katkevich V. Stonkus E. Lukevics 《Chemistry of Heterocyclic Compounds》2006,42(5):570-573
The hydrogenation of ethyl 4-R-2,4-dioxobutyrates (R = phenyl, 2-furyl) at 5% Pt/Al2O3 catalyst, modified with cinchonidine, and at palladium black was investigated. The former had low activity under the conditions
we tested. The main products during the hydrogenation of these compounds at palladium black are ethyl 4-R-2-hydroxy-4-oxobutyrates.
The yield of the phenyl derivative amounts to 68.5%, while the yield of the corresponding 2-furyl derivative amounts to 97%.
In the last case ethyl 2-hydroxy-4-oxo-4-(2-tetrahydrofuryl)butyrate was detected as impurity. The optimum conditions for
the formation of ethyl 2-hydroxy-4-phenylbutyrate (yield 88.2%) were determined.
__________
Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 655–659, May, 2006. 相似文献
1000.
F. Buiarelli F. Coccioli R. Jasionowska M. Merolle A. Terracciano 《Chromatographia》2006,64(7-8):475-481
Resveratrol (3,5,4′-trihydroxystilbene) is a phytoalexin that belongs to the group of stilbenes, known to occur in grapes and consequently in grape products. Its presence in wine is an important qualitative parameter because of the several beneficial effects on human health. The aim of this work is the development of a high-performance liquid chromatographic (HPLC) method for the determination of trans resveratrol in wines, and comparisons between the results obtained by different detection techniques: UV-vis spectroscopy, fluorescence spectroscopy and mass spectrometry. Resveratrol is analysed on a C-18 column using gradient elution. The method permits direct injection of sample, revealing to be time-saving, overcoming the need of sample pre-treatment steps. Detection limits were 154.8 ng mL−1 by HPLC-UV, 118.0 ng mL−1 by HPLC-FL and 48.0 ng mL−1 by HPLC-MS. Trans resveratrol has been then quantified in a group of 52 wines derived from different Italian regions, cultivars and winemaking technologies by HPLC-UV. 相似文献