全文获取类型
收费全文 | 1368篇 |
免费 | 208篇 |
国内免费 | 74篇 |
专业分类
化学 | 712篇 |
晶体学 | 9篇 |
力学 | 136篇 |
综合类 | 8篇 |
数学 | 403篇 |
物理学 | 382篇 |
出版年
2024年 | 1篇 |
2023年 | 16篇 |
2022年 | 25篇 |
2021年 | 29篇 |
2020年 | 50篇 |
2019年 | 43篇 |
2018年 | 30篇 |
2017年 | 46篇 |
2016年 | 58篇 |
2015年 | 37篇 |
2014年 | 79篇 |
2013年 | 119篇 |
2012年 | 72篇 |
2011年 | 76篇 |
2010年 | 58篇 |
2009年 | 70篇 |
2008年 | 81篇 |
2007年 | 98篇 |
2006年 | 84篇 |
2005年 | 68篇 |
2004年 | 77篇 |
2003年 | 70篇 |
2002年 | 37篇 |
2001年 | 44篇 |
2000年 | 38篇 |
1999年 | 42篇 |
1998年 | 38篇 |
1997年 | 24篇 |
1996年 | 20篇 |
1995年 | 24篇 |
1994年 | 26篇 |
1993年 | 9篇 |
1992年 | 11篇 |
1991年 | 5篇 |
1990年 | 3篇 |
1989年 | 7篇 |
1988年 | 4篇 |
1987年 | 8篇 |
1986年 | 4篇 |
1985年 | 4篇 |
1984年 | 4篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1979年 | 2篇 |
1974年 | 1篇 |
1973年 | 2篇 |
排序方式: 共有1650条查询结果,搜索用时 78 毫秒
21.
Arijit BanerjeeSamir Kumar Maji Michael G.B. DrewDebasish Haldar Arindam Banerjee 《Tetrahedron letters》2003,44(4):699-702
A single-crystal X-ray diffraction study of the terminally protected tetrapeptide Boc-β-Ala-Aib-Leu-Aib-OMe 1 (Aib: α-aminoisobutyric acid; β-Ala: β-Alanine) reveals that it adopts a new type of double turn structure which self-associates to form a unique supramolecular helix through intermolecular hydrogen bonds. Scanning electron microscopic studies show that peptide 1 exhibits amyloid-like fibrillar morphology in the solid state. 相似文献
22.
Micha? K. Cyrański Paul von Ragué Schleyer Tadeusz M. KrygowskiHaijun Jiao Georg Hohlneicher 《Tetrahedron》2003,59(10):1657-1665
The stability of a set of 105 five-membered π-electron systems (involving aromatic, non-aromatic and anti-aromatic species) was evaluated using six isodesmic reactions of which two belong to the subclass of homodesmotic reactions, which are based on cyclic and acyclic reference systems. We demonstrate that the ‘Resonance Energies’ derived from isodesmotic schemes have obvious flaws and do not correct or cancel other contributions to the energy, such as the changes of hybridization, homoconjugation of heterosubstituted cyclopentadienes, conjugative interactions of CC or CX (X=N or P) with a π or pseudo π orbital at Y (Y=O, S, NH, PH), strain, etc. as effectively as possible. Likewise, ‘aromatic stabilization energies (ASE)’ derived from homodesmotic schemes based on the acyclic reference compounds do not give satisfactory results. We strongly recommend that only cyclic reference compounds should be used for ASE and other aromaticity evaluations. The analysis is based on ab initio optimized geometries at B3LYP/6-311+G∗∗. 相似文献
23.
Thermal
degradation of vinylidene chloride/[4-(t-butoxycarbonyloxy)phenyl]methyl
acrylate copolymers
B. A. Howell D. A. Spears P. B. Smith 《Journal of Thermal Analysis and Calorimetry》2006,85(1):115-117
Vinylidene chloride polymers containing comonomer
units capable of consuming evolved hydrogen chloride to expose good radical-scavenging
sites might be expected to display greater thermal stability than similar
polymers containing simple alkyl acrylates as comonomer. Incorporation of
a comonomer containing the phenyl t-butyl
carbonate moiety into a vinylidene chloride polymer has the potential to afford
a polymer with pendant groups which might interact with hydrogen chloride
to expose phenolic groups. Copolymers of vinylidene chloride with [4-(t-butoxycarbonyloxy)phenyl]methyl acrylate have been
prepared, characterized, and subjected to thermal degradation. The degradation
has been characterized by thermal and spectroscopic techniques. The degradation
of vinylidene chloride/[4-(t-butoxycarbonyloxy)phenyl]methyl
acrylate copolymers is much more facile than the same process for similar
copolymers containing either [4-(isobutoxycarbonyloxy)phenyl]methyl acrylate
or methyl acrylate, a simple alkyl acrylate, as comonomer. During copolymer
degradation, [4-(t-butoxycarbonyloxy) phenylmethyl
acrylate units are apparently converted to acrylic acid units by extensive
fragmentation of the sidechain. Thus, the phenyl t-butyl
carbonate moiety does function as a labile acid-sensitive pendant group but
its decomposition in this instance leads to the generation of a phenoxybenzyl
carboxylate capable of further fragmentation. 相似文献
24.
Heather Davis Arshad Khan 《Journal of polymer science. Part A, Polymer chemistry》1994,32(12):2257-2265
A partial hydrolysis of amylose followed by the addition of iodine provides a spectrum almost identical to that of the amylopectin–iodine (API) complex suggesting the involvement of smaller “amylose-like” units in the API complex. Our theoretical studies on different polyiodine and polyiodide species suggest that a nearly linear I4 unit stabilized within the cavity of a small “amylose-like” helix is responsible for the characteristic API spectrum. Since there are 2.75 anhydroglucose residues (AGU) for every iodine atom in the amylose–iodine (AI) complex and a structural similarity exists between the API and the AI (amylose–iodine) complexes, we identify (C6H10O5)11I4 to be the chromophore in the API complex. © 1994 John Wiley & Sons, Inc. 相似文献
25.
Jakubus P. Adamski A. Kurzawa M. Sojka Z. 《Journal of Thermal Analysis and Calorimetry》2003,72(1):299-310
The structure and surface properties of ZrO2 strongly depend on its preparation. In the present work the impact of prolonged aging at basic conditions (pH = 9, T = 100°C, t = 48 h), on the phase composition and textural properties, obtained by calcination of the precipitate, was investigated using
several techniques conjointly (DTA/TG, DSC, XRD, porosimetry). The thermal effects accompanying the ZrO2·xH2O gel formation, the coalescence of the particles and crystallization were evaluated and discussed in terms of the structural
differences between the aged and non-aged samples.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
26.
邻-甲氧基苯酚和α-,β-环糊精包合现象的理论与实验研究 总被引:1,自引:0,他引:1
通过紫外可见光谱法考察了水溶液中邻-甲氧基苯酚(o-Mop)和α-与β-环糊精(CD)的分子间相互作用, 利用Hildebrand-Benesi方程给出了两个包合物的稳定常数(Ks). 采用半经验PM3方法研究了α-,β-CD和o-Mop及其类似物苯酚(Phe)、丁香酚(Eug)之间的包络作用, 阐述了这些主客体包合作用过程中体系能量随主客体相对位置改变而变化的细节, 据此推断出主-客体包合物可能的分子结构, 计算了包合物的稳定化能(ΔEs). 研究结果表明, 本文所选主客体体系而言, 当客体和同一种主体分子作用时, 超分子包合物的ΔEs随着客体分子苯环上取代基团数目的增多而增加. 基于PM3方法优化得到的主-客体包合物在真空中的分子结构和通过实验方法在水溶液中测定的结构一致. 相似文献
27.
The conformational transformation of a 30-residue peptide H(Ala-Gly-Ser-Gly-AIa-Gly)5OH, i.e., (AGSGAG)5, extracted from highly crystalline region of Bombyx mori (B. mori) silk fibroin was described by using the high resolution solid state 13^C NMR, and CD spectroscopies. Based on the conformation-dependent 13^C NMR chemical shifts of the Ala, Gly and Ser residues and the line-shape analysis of the conformation sensitive Ala Cβ resonance, the peptide revealed a strong preference for silk Ⅱ structural form, i,e,, an antiparallel fl-sheet structure (φ= - 140±20°and ψ= 135±20°) in solid state. On the contrary, the CD spectra of this peptide in the two non-native hexafluorinated fibre spinning solvents, hexafluoroisopropanol (HFIP) and hexafluoroacetone (HFA), exhibited the existence of an unusual tightly-folded conformation resembling 310-helix (φ=- 60±20° and ψ=-30±20°), as judged from the R ratio of [θ]222/[θ]203 in HFIP solution, whereas a dynamically averaged unordered structure in HFA, Taken together, the information inclined to hypothesis that the primary structure of the highly crystalline regions of B. mori silk fibroin may be easily accessible to the large conformational changes, which in turn may be critical for facilitating the structural transformation from unprocessed silk fibroin (silk I form) to processed silk fiber (silk Ⅱform). 相似文献
28.
As a consequence
of their excellent barrier properties vinyl chloride/vinylidene chloride copolymers
have long been prominent in the flexible packaging market. While these polymers
possess a number of superior characteristics, they tend to undergo thermally-
induced degradative dehydrochlorination at process temperatures. This degradation
must be controlled to permit processing of the polymers. Three series of N-substituted
maleimides (N-alkyl-, N-aralkyl, and N-aryl) have been synthesized, characterized
spectroscopically, and evaluated as potential stabilizers for a standard vinyl
chloride/vinylidene chloride (85 mass%) copolymer. As surface blends with
the polymer, these compounds are ineffective as stabilizers. However, significant
stabilization may be achieved by pretreatment of the polymer with N-substituted
maleimides. The most effective stabilization of the polymer is afforded by
N-aralkyl- or N-arylmaleimides, most notably, N-benzylmaleimide and N-p-methoxyphenylmaleimide. 相似文献
29.
We are concerned with defining new globalization criteria for solution methods of nonlinear equations. The current criteria used in these methods require a sufficient decrease of a particular merit function at each iteration of the algorithm. As was observed in the field of smooth unconstrained optimization, this descent requirement can considerably slow the rate of convergence of the sequence of points produced and, in some cases, can heavily deteriorate the performance of algorithms. The aim of this paper is to show that the global convergence of most methods proposed in the literature for solving systems of nonlinear equations can be obtained using less restrictive criteria that do not enforce a monotonic decrease of the chosen merit function. In particular, we show that a general stabilization scheme, recently proposed for the unconstrained minimization of continuously differentiable functions, can be extended to methods for the solution of nonlinear (nonsmooth) equations. This scheme includes different kinds of relaxation of the descent requirement and opens up the possibility of describing new classes of algorithms where the old monotone linesearch techniques are replace with more flexible nonmonotone stabilization procedures. As in the case of smooth unconstrained optimization, this should be the basis for defining more efficient algorithms with very good practical rates of convergence.This material is partially based on research supported by the Air Force Office of Scientific Research Grant AFOSR-89-0410, National Science Foundation Grant CCR-91-57632, and Istituto di Analisi dei Sistemi ed Informatica del CNR. 相似文献
30.
Copolymerisation of methyl methacrylate with nine novel monomer compounds, triazinyl benzanthrone dyes, triazinyl benzotriazole stabilisers and their combined structures, was investigated. Chemical bonding of the compounds in the polymers obtained was confirmed spectrophotometrically. The quantity of the chemically bonded monomer compounds was determined to be over 50%, providing a colour and a fluorescence stable to solvents. Their influence on the rate of the copolymerisation was studied. It was found that the compounds under study did not significantly affect the rate of copolymerisation. A stabilizing effect towards photo-degradation of PMMA was achieved at 0.1 wt.% initial concentration of the compounds, which made them suitable for “one-step” colouration and stabilisation of PMMA. 相似文献