全文获取类型
收费全文 | 12001篇 |
免费 | 1265篇 |
国内免费 | 532篇 |
专业分类
化学 | 6420篇 |
晶体学 | 562篇 |
力学 | 449篇 |
综合类 | 89篇 |
数学 | 757篇 |
物理学 | 5521篇 |
出版年
2024年 | 8篇 |
2023年 | 76篇 |
2022年 | 164篇 |
2021年 | 179篇 |
2020年 | 222篇 |
2019年 | 233篇 |
2018年 | 217篇 |
2017年 | 244篇 |
2016年 | 428篇 |
2015年 | 389篇 |
2014年 | 531篇 |
2013年 | 974篇 |
2012年 | 592篇 |
2011年 | 836篇 |
2010年 | 699篇 |
2009年 | 825篇 |
2008年 | 820篇 |
2007年 | 904篇 |
2006年 | 819篇 |
2005年 | 628篇 |
2004年 | 630篇 |
2003年 | 557篇 |
2002年 | 445篇 |
2001年 | 355篇 |
2000年 | 327篇 |
1999年 | 270篇 |
1998年 | 248篇 |
1997年 | 173篇 |
1996年 | 162篇 |
1995年 | 159篇 |
1994年 | 144篇 |
1993年 | 97篇 |
1992年 | 85篇 |
1991年 | 56篇 |
1990年 | 56篇 |
1989年 | 45篇 |
1988年 | 54篇 |
1987年 | 27篇 |
1986年 | 22篇 |
1985年 | 14篇 |
1984年 | 16篇 |
1983年 | 11篇 |
1982年 | 16篇 |
1981年 | 8篇 |
1980年 | 4篇 |
1979年 | 5篇 |
1977年 | 6篇 |
1976年 | 4篇 |
1975年 | 3篇 |
1973年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
Carolyn Pratt Brock 《Helvetica chimica acta》2023,106(2):e202200170
A survey has been made of the well determined (R≤0.050) organic crystal structures that have only a single glide plane (e. g., are in space group Pc) and that have more than one crystallographically independent formula unit (Z′>1); the goal was to discover what fraction have additional approximate symmetry. Of the 377 unique structures examined 8 % should almost certainly been refined in a higher-symmetry unit cell; 86 % of the remaining 347 have approximate symmetry that is periodic in at least one dimension. While these percentages are similar to those found for P1 structures (C. P. Brock, Acta Crystallogr., Sect. B 2022 , 78, 576–588), the types of approximate symmetry differ because 89 % of the P1, Z′>1 structures were composed of enantiopure material. The 347 reliable Pc structures include 118 that are slightly distorted or mimicked Cc and P21/c structures, 15 of which were reported as having been determined at room temperature. The distortions in another 72 are so large that the approximate symmetry must be seen as periodic in two dimensions only. These results suggest that symmetry lowering may accompany transformation of a crystal nucleus to a macroscopic crystal. 相似文献
992.
Martin Sahlberg Premysl Beran Thomas Kollin Nielsen Yngve Cerenius Krisztina Kads Marko P.J. Punkkinen Levente Vitos Olle Eriksson Torben R. Jensen Yvonne Andersson 《Journal of solid state chemistry》2009,182(11):3113-3117
A novel aluminium rich alloy for hydrogen storage has been discovered, ScAl0.8Mg0.2, which has very promising properties regarding hydrogen storage capacity, kinetics and stability towards air oxidation in comparison to hydrogen absorption in state-of-the-art intermetallic compounds. The absorption of hydrogen was found to be very fast, even without adding any catalyst, and reversible. The discovered alloy crystallizes in a CsCl-type structure, but decomposes to ScH2 and Al(Mg) during hydrogen absorption. Detailed analysis of the hydrogen absorption in ScAl0.8Mg0.2 has been performed using in situ synchrotron radiation powder X-ray diffraction, neutron powder diffraction and quantum mechanical calculations. The results from theory and experiments are in good agreement with each other. 相似文献
993.
We report a novel approach for fabrication of cubic ZrWMoO8 applying a melt quenching method. The stoichiometric mixture of ZrO2, WO3 and MoO3 in a 1:1:1 M ratio was melted and quenched at room temperature by pouring the melts between two metal plates. The quenched sample was characterized by X-ray diffraction (XRD), differential thermal analysis (DTA) and scanning electron microscopy (SEM) analysis. As quenched material is polyphase contains above 70% ZrWMoO8. The obtained composite is thermal stable up to 930 °C. The ZrWMoO8 was indexed as cubic with a lattice parameter a = 9.1263 (3) Å calculated by the “PowderCell” program. By SEM observation it was established that the microstructure of bulk composite material is built up of one-direction orientated agglomerates with cuboid crystal morphology. 相似文献
994.
Richard Nader Michel Kazan Elie Moussaed Thomas Stauden Merten Niebelschütz Pierre Masri Jörg Pezoldt 《Surface and interface analysis : SIA》2008,40(9):1310-1317
The influence of Ge deposition prior to carbon interaction with 3° off‐axis Si(111) substrates on the structural and morphological properties of the formed silicon carbide (SiC) layer is studied. In situ reflection high‐energy electron diffraction (RHEED) and X‐ray diffraction (XRD) revealed the formation of the cubic silicon carbide (3C‐SiC) modification. In situ spectroscopic ellipsometry measurements revealed a decreasing 3C‐SiC thickness with increasing Ge predeposition. Atomic force microscopy (AFM) studies revealed that the surface overlayer morphology is mainly formed by periodic step arrangements whose relevant geometric parameters, i.e. lateral separation, height and terrace width, depend on the Ge content. Besides the changes of the step morphology, the surface roughness and the grain size and the strain of the formed 3C‐SiC decreases with increasing germanium precoverage. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
995.
Madhav Prasad Neupane Yu Kyoung Kim Il Song Park Kyoung A Kim Min Ho Lee Tae Sung Bae 《Surface and interface analysis : SIA》2009,41(3):259-263
The size and shape of nanocrystals have a strong effect on the optical, electrical and catalytic properties. Therefore, controlling the size, shape and structure of nanocrystals is technically important. The controlled synthesis of CuO nanostructures was achieved using a hydrothermal process by simply controlling the precipitation reaction temperature between copper nitrate trihydrate and sodium hydroxide. The Scanning Electron Microscopy (SEM), EDS, XRD, and FTIR analysis revealed that the synthesized product at 200 °C is of pure copper oxide particles. From Scherrer formula, the prepared CuO particles varied approximately 3–7 nm in size simply by varying the reaction temperature. The synthesized particles exhibited a regular flake like morphology and had a uniform size distribution. The morphology and size depend on the reaction conditions. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
996.
Synthesis and Crystal Structure of Mercury(II) Metal Crown Ether with N‐Heterocyclic Carbene Linkage
The mercury(II) metal crown ether ( 2a ) was obtained in high yield by reaction of the carbene precursor 1,2‐bis[N‐(1‐naphthylmethylene)imidazoliumethoxy]benzene dihexafluorophosphate ( 1 ) and Hg(OAc)2. Addition of NaI to the acetone solution of 2a resulted in precipitation of pale yellow solid 2b . The structures of 2a and 2b were determined by single‐crystal X‐ray diffractometry. Both molecules display a helical conformation with a torsional cycle. The mercury atom in complex 2a is tricoordinated by two intramolecular carbene carbon atoms and an acetate oxygen atom. The mercury atom in complex 2b is tetracoordinated by two intramolecular carbene carbon atoms and two cis‐iodine atoms. 相似文献
997.
998.
Summary Triacetylcellulose coated on silica is a novel HPLC sorbent introduced for the separation of enantiomers. A comparison of
enantioselectivities α, and resolutions R5 of this material with known pure triacetylcellulose I shows considerable differences. From X-ray wideangle diffraction it
is suggested that the novel sorbent also consists of the triacetylcellulose modification I, but its degree of crystallinity
is probably lower than that of the non-dilute sorbent. The advantages of the two stationary phases are discussed.
Liquid Chromatography on Triacetylcellulose, Part 13. For Part 12, see ref. [1]. 相似文献
999.
The potential of rapid thermal processing (RTP) for the preparation of thin films of niobium oxynitrides was investigated. The 200 and 500 nm niobium films were deposited via sputtering on oxidized silicon(1 0 0)- and on sapphire(1 −1 0 2)-substrates. At first, oxidation of niobium films in molecular oxygen and then nitridation in ammonia using an RTP-system was performed. The films were characterized before and after the oxidation and nitridation processes by X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM) and secondary ion mass spectrometry (SIMS). The influence of the two different substrates, amorphous SiO2 and single crystalline sapphire on the reactivity of the niobium films was studied in dependence of temperature, time of reaction and film thickness. The existence of niobium oxynitride formation was verified for some of the films. In some of the experiments, crack formation in the films or even delamination of the Nb-films from the substrates was observed. 相似文献
1000.
Konstantin A. Kozhanov Michael P. Bubnov Nina N. Vavilina Larisa Yu. Efremova Georgy K. Fukin Vladimir K. Cherkasov Gleb A. Abakumov 《Polyhedron》2009,28(13):2555-2558
Structural study of dialkyl (PCP)-pincer o-semiquinonato nickel complexes indicated sloped relative positions of bi- and tri-dentate ligands. It became bright confirmation of propositions made on the base of their EPR spectra followed by molecular modelling. 相似文献