全文获取类型
收费全文 | 4586篇 |
免费 | 532篇 |
国内免费 | 411篇 |
专业分类
化学 | 4634篇 |
晶体学 | 197篇 |
力学 | 66篇 |
综合类 | 14篇 |
数学 | 87篇 |
物理学 | 531篇 |
出版年
2024年 | 7篇 |
2023年 | 63篇 |
2022年 | 100篇 |
2021年 | 159篇 |
2020年 | 253篇 |
2019年 | 199篇 |
2018年 | 155篇 |
2017年 | 189篇 |
2016年 | 257篇 |
2015年 | 263篇 |
2014年 | 276篇 |
2013年 | 372篇 |
2012年 | 253篇 |
2011年 | 223篇 |
2010年 | 197篇 |
2009年 | 249篇 |
2008年 | 277篇 |
2007年 | 288篇 |
2006年 | 245篇 |
2005年 | 222篇 |
2004年 | 199篇 |
2003年 | 176篇 |
2002年 | 117篇 |
2001年 | 83篇 |
2000年 | 101篇 |
1999年 | 85篇 |
1998年 | 73篇 |
1997年 | 66篇 |
1996年 | 57篇 |
1995年 | 59篇 |
1994年 | 52篇 |
1993年 | 32篇 |
1992年 | 29篇 |
1991年 | 25篇 |
1990年 | 14篇 |
1989年 | 18篇 |
1988年 | 16篇 |
1987年 | 11篇 |
1986年 | 14篇 |
1985年 | 7篇 |
1984年 | 7篇 |
1983年 | 3篇 |
1982年 | 5篇 |
1981年 | 5篇 |
1980年 | 5篇 |
1979年 | 4篇 |
1978年 | 3篇 |
1976年 | 5篇 |
1974年 | 2篇 |
1973年 | 7篇 |
排序方式: 共有5529条查询结果,搜索用时 15 毫秒
81.
FANG Fang ZHU Yu ZHAO Qian JIANG Gang WANG Hong-Yan 《结构化学》2007,26(9):1092-1096
Geometric and electronic properties of Pdn–1Pb and Pdn (n≤8) clusters have been studied by using density functional theory with effective core potentials, focusing on the differences between mono- and bimetallic clusters. The average bond length of Pdn–1Pb (n≤8) bimetallic clusters is longer than that of pure palladium clusters except for n = 2 and 3. The most stable structure of Pdn–1Pb (n≤7) is the singlet where there is at least a Pd or Pb atom on its excited state. The energy gaps of Pd–Pb binary clusters are narrower than those of Pdn clusters, and then the chemical activity is strengthened when Pdn clusters are doped with Pb. 相似文献
82.
Vesselin Dimitrov 《Journal of solid state chemistry》2005,178(3):831-846
A classification of binary oxide glasses has been proposed taking into account the values obtained on their refractive index-based oxide ion polarizability αO2−(n0), optical basicity Λ(n0), metallization criterion M(n0), interaction parameter A(n0), and ion's effective charges as well as O1s and metal binding energies determined by XPS. Four groups of oxide glasses have been established: glasses formed by two glass-forming acidic oxides; glasses formed by glass-forming acidic oxide and modifier's basic oxide; glasses formed by glass-forming acidic and conditional glass-forming basic oxide; glasses formed by two basic oxides. The role of electronic ion polarizability in chemical bonding of oxide glasses has been also estimated. Good agreement has been found with the previous results concerning classification of simple oxides. The results obtained probably provide good basis for prediction of type of bonding in oxide glasses on the basis of refractive index as well as for prediction of new nonlinear optical materials. 相似文献
83.
Alison T. Ung Roger Bishop Donald C. Craig Ian G. Dance A. David Rae Marcia L. Scudder 《Journal of inclusion phenomena and macrocyclic chemistry》1993,15(4):385-398
The inclusion compounds of 2,8-dimethyltricyclo[5.3.1.13,9]dodecane-syn-2,syn-8-diol,3, with ferrocene and with squalene have been prepared. The crystal structures of these helical tubulate compounds: (3)3·(ferrocene)0.75 [P3121,a=b=13.7480(6),c=7.0312(5) Å,Z=1,R=0.038] and (3)3·(squalene)0.23 [P3121,a=b=13.677(1),c=7.0533(9) Å,Z=1,R=0.042] are described.
Supplementary Data relating to this article are deposited with the British Library as supplementary publication No. SUP 82151 (16 pages). 相似文献
84.
Balamurugan V Hundal MS Mukherjee R 《Chemistry (Weinheim an der Bergstrasse, Germany)》2004,10(7):1683-1690
Using a group of six neutral M(II)Cl(2)-containing coordination compounds as building blocks, the first systematic investigation of C-H...Cl hydrogen-bonding interactions was performed. Single-crystal X-ray structural analyses of four new compounds (pseudo-tetrahedral Co(II) and Zn(II); distorted trigonal bipyramidal Zn(II)) authenticate the metal coordination geometry. To provide a unified view of the presence of noncovalent interactions in this class of compounds, we have re-examined the packing diagram of two previously reported compounds (a distorted square-pyramidal Cu(II) complex and a trans-octahedral Co(II) complex). The organic ligands of our choice comprise bidentate/tridentate pyrazolylmethylpyridines and an unsymmetrical tridentate pyridylalkylamine. This systematic investigation has allowed us to demonstrate the existence of versatile C-H...Cl(2)M interactions and to report the successful application of such units as inorganic supramolecular synthons. Additional noncovalent interactions such as C-H...O and O-H...Cl hydrogen bonding and pi-pi stacking interactions have also been identified. Formation of novel supramolecular architectures has been revealed: 2D lamellar (p-cyclophane) and 3D lamellar, 3D "stitched staircase" (due to additional hydrogen-bonding interactions by water tetramers, with an average O-O bond length in the tetramer unit of 2.926 A, acting as "molecular clips" between staircases), 3D linked ladder, and single-stranded 1D helix. 相似文献
85.
The superstructure and properties of polymers might be improved and enhanced througl intermolecular complexation. In this paper, the preparation, structure and properties of P(MMA-MAA)/PEO intermolecular complex through hydrogen bonding are studied. By acid hydrolysis of PMMA, a series of P(MMA-MAA) copolymers have been prepared, the MAA content of which depends on the temperature and time of hydrolyzation. In case the MAA content in P(MMA-MAA) is beyond 0.4 mole fraction, the hydrogen bonding between P(MMA-MAA) and PEO is strong enough to form intermolecular complex. The supermolecular structure of the complex is quite different from that of its constituent, which was observed by an X-ray diffraction and transmission electron microscope. The crystallization of PEO is confined owing to complexation. The complex exhibits quite a lot of good properties in thermal stability, gas permeability as well as mechanical strength, and could be used in many fields. 相似文献
86.
以2-氰基-3-(4-(2-氯-3-甲基-1-丁酰氧基)-苯基丙烯酸(A)为质子给体,N-(4-吡啶基亚甲基)-4-烷氧基苯胺(nSSZ)为质子受体,合成了一系列新的氢键复和物,经红外光谱证实了分子间氢键的存在,通过DSC,偏光方法及X射线衍射方法对其液晶行为进行研究,结果表明复合物呈现近晶相行为。 相似文献
87.
The essential features (geometries of the minima and of the saddle points, energy barriers) of the potential energy surface of the four hydrides YXH4 mentioned in the title have been determined with two basis sets, of minimal and DZ quality respectively. The importance of the different extent of the deformation of the XH4 group in the different structures of the four hydrides is brought out and discussed. The aspects of charge distribution and bonding are examined drawing on population analysis, comparison of the electrostatic molecular potentials and decomposition of the interaction energy (this last referred to the Y+ + XH
4
–
YXH4 process). The capability of XH3 in effecting the etherolytic disruption of the Y-H bond is finally brought out. 相似文献
88.
Takeharu Haino 《Tetrahedron letters》2004,45(11):2281-2284
Shape-selective recognition for the dicarboxylates in DMSO can be attained by a new calix[4]arene-based receptor 1 having two urea groups. Biologically active chorismate selectively bound in 1 over its dehydrated derivative. Molecular mechanics calculations gave a plausible explanation for the selective binding. 相似文献
89.
H.?Hussain H.?U.?Rehman Z.?AhmadEmail author 《Journal of Sol-Gel Science and Technology》2005,36(3):239-248
Nano-composites from aramid-silica system have been prepared via sol-gel process. Poly (phenyleneterephthalamide) copolymer
chains were prepared by reacting a mixture of p- and m-phenylenediamines with terephthaloyl chloride in dimethylacetamide used as solvent. The sol-gel process in the polymer
matrix was carried out through hydrolysis and condensation of a mixture of tetraethoxysilane and (3-glycidoxypropyl) trimethoxysilane.
The latter was used to develop linkage, on one hand with silica network structure using alkoxy groups and on the other hand
with aramid chains at its secondary amine groups through glycidal groups of silane. Mutual interaction between the two disparate
phases aramid and silica network was thus created. Thin films of the composites containing different proportions of silica
ranging from 5.0 to 25.0-wt% were cast by the solvent elution technique. The α-relaxation temperature associated with the
glass transition was measured by the dynamic mechanical thermal analysis. The results showed an increase in the glass transition
temperature from 328°C for the pure aramid to 352°C for the hybrid materials containing 25-wt% silica, an indicative of the
increased interfacial interaction between the two phases. Films having relatively low silica content were flexible and transparent,
but those with high silica content were opaque and brittle. These films were tested for their tensile strength, modulus and
toughness. The mechanical strength of the composites as compared to the pure aramid increased initially but with further addition
of silica the strength decreased. The initial increase can be explained due to increased interfacial interaction between the
two phases, however agglomeration of silica particles was responsible for decreasing strength at higher silica contents. 相似文献
90.
Odin C 《Magnetic resonance in chemistry : MRC》2004,42(4):381-388
A variable-temperature high-resolution 13C and 87Rb solid-state NMR study of powder rubidium hydrogencarbonate, RbHCO3, is presented for the first time. At ambient temperature, RbHCO3 is formed by centrosymmetric dimers linked by hydrogen bonds, but almost no information is available on this compound concerning proton disorder and the low-temperature phase. However, potassium hydrogencarbonate, KHCO3, which has an isomorphic structure for the high temperature phase, was well studied: it undergoes a non-ferroic, non-ferroelectric phase transition at Tc = 318 K between two monoclinic structures. The protons are disordered in an asymmetric double-well potential in the low-temperature phase, and the double-well potential becomes symmetric in the high-temperature phase. By comparison with recent solid-state NMR experimental results on KHCO3, we show that RbHCO3 undergoes a phase transition at Tc approximately 245 K, and give evidence that the proton dynamic disorder in both compounds is very similar. 相似文献