全文获取类型
收费全文 | 400篇 |
免费 | 8篇 |
国内免费 | 16篇 |
专业分类
化学 | 222篇 |
晶体学 | 2篇 |
力学 | 32篇 |
综合类 | 1篇 |
数学 | 5篇 |
物理学 | 162篇 |
出版年
2024年 | 1篇 |
2023年 | 6篇 |
2022年 | 11篇 |
2021年 | 1篇 |
2020年 | 13篇 |
2019年 | 5篇 |
2018年 | 5篇 |
2017年 | 12篇 |
2016年 | 7篇 |
2015年 | 6篇 |
2014年 | 22篇 |
2013年 | 21篇 |
2012年 | 21篇 |
2011年 | 17篇 |
2010年 | 26篇 |
2009年 | 33篇 |
2008年 | 29篇 |
2007年 | 28篇 |
2006年 | 28篇 |
2005年 | 26篇 |
2004年 | 14篇 |
2003年 | 21篇 |
2002年 | 9篇 |
2001年 | 12篇 |
2000年 | 9篇 |
1999年 | 7篇 |
1998年 | 4篇 |
1997年 | 6篇 |
1996年 | 6篇 |
1994年 | 3篇 |
1993年 | 3篇 |
1992年 | 3篇 |
1991年 | 2篇 |
1990年 | 2篇 |
1989年 | 2篇 |
1988年 | 2篇 |
1981年 | 1篇 |
排序方式: 共有424条查询结果,搜索用时 15 毫秒
11.
本文报道了采用自行设计组装的光声光谱(PAS)测量系统,测定(Zn,Cd)(S,Se)粉末固溶体的带隙与组分的关系;观测ZnS:Mn粉末发光材料的光声谱,确定出Mn~(2+)五个激发态能级的位置;利用光声谱的深度剖面分析,对Cu_xS-ZnS:Mn粉末材料进行分析,获得满意的结果。表明光声谱法对凝聚态物质的研究和监测能提供多方面的信息,是一种极有用的新型光谱测量技术。 相似文献
12.
13.
In this paper, we present a rheological study of a compact layer of highly swollen gel beads. The flow regimes of these systems are modelled by an elastic sinusoid moving in a Newtonian fluid. This model predicts a yield stress varying with the square root of the elastic modulus of the gel bead. The slope at the origin (at zero velocity) of the flow curve is expected to depend only on geometrical parameters and on the solvent viscosity. The experimental data are in qualitative agreement with the theoretical predictions. 相似文献
14.
A partial least squares (PLS) Fourier transform Raman spectrometry procedure based on the measurement of solid samples contained inside standard glass vials, has been developed for direct and reagent-free determination of sodium saccharin and sodium cyclamate in table top sweeteners. A classical 22 design for standards was used for calibration, but this system provides accuracy errors higher than 13% w/w for the analysis of samples containing glucose monohydrate. So, an extended model incorporating glucose monohydrate (23 standards) was assayed for the determination of sodium saccharin and sodium cyclamate in all the samples. Mean centering spectra data pre-treatment has been employed to eliminate common spectral information and root mean square error of calibration (RMSEC) of 0.0064 and 0.0596 was obtained for sodium saccharin and sodium cyclamate, respectively. A mean accuracy error of the order of 1.1 and 1.9% w/w was achieved for sodium saccharin and sodium cyclamate, in the validation of the method using actual table top samples, being lower than those obtained using an external monoparametric calibration. FT-Raman provides a fast alternative to the chromatographic method for the determination of the sweeteners with a three times higher sampling throughput than that obtained in HPLC. On the other hand, FT-Raman offers an environmentally friendly methodology which eliminates the use of solvents. Furthermore, the stability of samples and standards into chromatographic standard glass vials allows their storage for future analysis thus avoiding completely the waste generation. 相似文献
15.
Anhydrous disodium phosphonate, Na2HPO3, was prepared by dehydration of its pentahydrate. The crystal structure of Na2HPO3 was solved from high resolution X‐ray powder diffraction data (P21/n; Z = 4; a = 9.6987(1), b = 6.9795(1), c = 5.0561(1) Å, β = 92.37(1)°; V = 341.97(1) Å3). The crystal structure consists of two types of sodium‐oxygen polyhedra, which are connected via common edges and vertices forming layers perpendicular to [100]. These Na(1)‐ and Na(2)‐layers are interlinked via common edges, forming in a 3D‐framework. The resulting topology is providing oxygen arrangements that please the coordinative requirement of phosphorus(III). 相似文献
16.
Micrometric BN powders used as catalyst support: influence of the precursor on the properties of the BN ceramic 总被引:1,自引:0,他引:1
Thin powders and foams of boron nitride have been prepared from molecular precursors for use as noble metal supports in the catalytic conversion of methane. Different precursors originating from borazines have been tested. The best results were obtained using a precursor derived from trichloroborazine (TCB) which, after reacting with ammonia at room temperature and then thermolyzing up to 1800°C, led to BN powders with a specific area of more than 300 m2 g−1 and a micrometric spherical texture. Comparable results were obtained using polyborazylene under similar conditions. Aminoborazine-derived precursors did not yield such high specific area ceramics but the BN microstructure resembled a foam with a crystallized skin and amorphous internal part. These differences were related to the chemical mechanism of the conversion of the precursor into BN. Polyhaloborazines and polyborazines yielded BN through gas-solid reactions whereas aminoborazine polymers could be kept waxy up to high temperatures, which favored the glassy foam. Catalysts composed of BN support and platinum have been prepared using two routes: from a mixture of precursor or by impregnation of a BN powder leading to very different catalysts. 相似文献
17.
Ruslan P. Liferovich 《Journal of solid state chemistry》2004,177(6):2188-2197
Ternary lanthanide scandates (Ln=La, Pr, Nd, Sm, Eu, Gd, Tb, Dy, and Ho) have been synthesized at ambient pressure. Their structure has been investigated at room temperature by Rietveld analysis of powder X-ray diffraction data. The Ln-scandates are orthorhombic perovskites, adopting space group Pbnm (? 62), a≈b≈√2ap, c≈2ap, Z=4. Heavy lanthanides (Er-Lu), and Y do not form perovskites at ambient conditions. Compositionally driven phase transitions were not observed. The unit-cell parameters decrease with increasing ScO6 octahedron rotation and atomic number of the Ln cation. In common with lanthanide orthoferrites, the uniform structural evolution is interrupted at the middle-heavy part of the lanthanide sequence. This is probably due to an interplay between: (i) enlargement of the ScO6 octahedra relative to BO6 in other perovskites (e.g., FeO6 in GdFeO3); (ii) reduction in size of the first coordination sphere of Ln3+ coincident with the lanthanide contraction; (iii) coincident expansion of the second coordination sphere due to screening effects of OI1 on OI2, and entry of Sc to the lanthanide coordination sphere; (iv) complex mixing between oxygen and lanthanide lanthanide f- and scandium d-orbitals. In the series studied, Ln3+ are in eight-fold coordination (tetragonal antiprism), and are considerably displaced from the center of the LnO8 polyhedron along [001]. Evolution of the crystallochemical characteristics through the Ln orthoscandate series is complex due to both the antipathetic distortions of A- and B-site coordination polyhedra and interaction of the orbitals of oxygen, Ln and Sc. Empirically obtained limits of Goldschmidt and observed viiito tolerance factors for ternary LnBO3 compounds adopting the Pbnm structure are 0.795 and 0.841, respectively. 相似文献
18.
A number of novel routes to the alkali metal compounds of hydroquinone M2[p‐C6H4O2] (M = Li, Na, K, Rb, Cs) and M[p‐C6H4O(OH)] (M = K, Rb, Cs) have been synthetically explored. The selective synthesis of the alkali 4‐hydroxyphenolates and 1, 4‐phenylenediolates is based on optimized reaction conditions (solvents, temperatures). All compounds were structurally characterized by means of powder X‐ray diffraction using Rietveld profile refinement including C—C and C—O bond distance restraints. The atomic arrangement of M2[p‐C6H4O2](M = Na, K) (tetragonal, space group: P42/ncm) is characterized by infinite pillars of [M2[3]O2[3]]‐units along the c axis connected by [p‐C6H4O2]2—‐anions with stacking direction along c. The coordinatively unsaturated alkali metals, surrounded by three oxygen atoms, exhibit symmetrical (K) as well as asymmetrical (Na) interactions with the phenylene rings. M[p‐C6H4O(OH] (M = K, Rb) (tetragonal, space group: P4/n) forms hydrogen‐bridged linear chains of [p‐C6H4O(OH)]—‐anions along the c direction. The phenylene planes of neighboring chains have an almost orthogonal arrangement while the interchain planes are parallel. K and Rb are fourfold coordinated by two different oxygen coordination spheres. 相似文献
19.
Felicia Dragan I. Bratu Gh. Borodi Mihaela Toma A. Hernanz S. Simon Gh. Cristea R. Peschar 《Journal of inclusion phenomena and macrocyclic chemistry》2007,59(1-2):125-130
Aqueous-solution complexes of β-cyclodextrin (β-CD) with metoprolol tartrate (MET) have been analysed with 1H NMR and UV–vis spectroscopy. With 1H NMR a [1:1] stoichiometry could be established for the β-CD-MET complex while its stability constant was determined with
UV–vis spectroscopy. Powder diffraction data of a polycrystalline sample of the β-CD-MET complex show that a novel product
has been formed, likely to be a β-CD-MET [1:1] inclusion complex. Also Hyperchem MM+ molecular-dynamics results suggest an
inclusion complex and from 1H NMR data it is inferred that probably the MET is docked in the CD with the formers methoxyethyl-benzene moiety in front.
Mihaela Toma is Socrates/Erasmus student at UNED Madrid 相似文献
20.
《Particuology》2016
The mixing performance of a multi-bladed baffle inserted into a traditional Gallay tote blender is explored by graphic processing unit-based discrete element method software. The mixing patterns and rates are investigated for a binary mixture, represented by two different colors, under several loading profiles. The baffle effectively enhances the convective mixing both in the axial and radial directions, because of the disturbance it causes to the initial flowing layer and solid-body zone, compared with a blender without a baffle. The axial mixing rate is affected by the gap between the baffle and the wall on the left and right sides, and an optimal blade length corresponds to the maximum mixing rate. However, the radial mixing rate increases with the blade length almost monotonically. 相似文献