全文获取类型
收费全文 | 299篇 |
免费 | 10篇 |
国内免费 | 4篇 |
专业分类
化学 | 214篇 |
力学 | 5篇 |
数学 | 29篇 |
物理学 | 65篇 |
出版年
2023年 | 3篇 |
2022年 | 5篇 |
2021年 | 7篇 |
2020年 | 18篇 |
2019年 | 9篇 |
2018年 | 12篇 |
2017年 | 8篇 |
2016年 | 17篇 |
2015年 | 14篇 |
2014年 | 8篇 |
2013年 | 24篇 |
2012年 | 20篇 |
2011年 | 16篇 |
2010年 | 14篇 |
2009年 | 5篇 |
2008年 | 15篇 |
2007年 | 8篇 |
2006年 | 14篇 |
2005年 | 3篇 |
2004年 | 4篇 |
2003年 | 3篇 |
2002年 | 9篇 |
2001年 | 1篇 |
2000年 | 7篇 |
1999年 | 3篇 |
1998年 | 5篇 |
1997年 | 3篇 |
1996年 | 9篇 |
1995年 | 4篇 |
1994年 | 3篇 |
1993年 | 5篇 |
1992年 | 4篇 |
1991年 | 2篇 |
1990年 | 3篇 |
1987年 | 1篇 |
1986年 | 3篇 |
1985年 | 3篇 |
1984年 | 2篇 |
1983年 | 1篇 |
1982年 | 3篇 |
1981年 | 2篇 |
1980年 | 6篇 |
1979年 | 3篇 |
1978年 | 1篇 |
1975年 | 1篇 |
1968年 | 1篇 |
1961年 | 1篇 |
排序方式: 共有313条查询结果,搜索用时 46 毫秒
1.
MDF materials are chemically bonded ceramic materials free of the macrodefects typical of hydraulic cement-based materials. MDF materials arising through reactions of sulfo-aluminate-ferrite belitic (SAFB) clinkers and/or Portland cements (PC) with two types of water-soluble polymer (hydroxy-propylmethyl cellulose {HPMC}, polyphosphate glass {poly-P}) are discussed. Mixes of low energy SAFB clinkers with Portland cement, HPMC and, especially poly-P comprise promising cross-linked compositions additional to the better known MDF materials formed from high alumina cement with polyvinylalcohol/acetate. The principles of co-ordination of P and C atoms (of the polymer) with Al and Fe atoms (originating from the cement) are highlighted from spectroscopic information on next-nearest-neighbour interactions, along with the effects of second co-ordination spheres. Polymers modify the interface through functional bonding/grafting of polymer chains onto the surfaces of cement grains. Both the cross-linked atomic structure and the interface coincide well with the model of functional polymers and represent a new type of atomic-level structure in polymer-modified cements. Interpretation is based on previous magnetic resonance and thermal analysis studies. The compactness of Al(Fe)-O-P cross-links reduces transport through the interfaces, increasing the interfacial interactions and resisting the unfavourable uptake of moisture and carbonation. 相似文献
2.
The impedance spectra of Pb5Ge3O11 single crystals are measured in the frequency range from 5 Hz to 13 MHz at temperatures of 600 to 800 K in dry air and in
a dry or wet nitrogen gas. It is found that the temperature and the gas composition significantly affect the electrical properties
of the compound. The data obtained are used to discuss the origin of crystal lattice defects and their influence on charge
transfer. It is concluded that the conduction is mixed in character (p-type electronic and ionic due to oxygen ions). The proton conduction is shown experimentally to be feasible. The possible
mechanisms of proton transport in Pb5Ge3O11 are discussed. 相似文献
3.
Axel Fischer Peter G. Jones Ion Neda Reinhard Schmutzler Igor V. Shevchenko 《无机化学与普通化学杂志》1994,620(5):908-913
Synthesis and Molecular Structure of new Ring Systems from 1,1,3,3-Tetrachloro-1,3-diphosphapropane 1,1,3,3-Tetrachloro-1,3-diphosphapropane 1 reacts in two different ways to form new heterocycles. Partial oxidation of 1 with tetrachloroorthobenzoquinone furnishes the methylene-bridged λ3P, λ5P species 3 . Subsequent reactions with di- and triethylamine lead to the condensed ring system 6 with the P?C bonds connected to a central four-membered ring. Compound 6 displays crystallographic inversion symmetry, a short transannular P? P distance and an extremely distorted tetrahedral coordination geometry at the four-membered ring phosphorus atoms. 1 reacts with 7 to give the heterocycle 8 with a central eight-membered ring involving four phosphorus atoms. The eight – membered ring shows a ?bent”? crown conformation, the condensed five – membered rings display envelope conformation. 相似文献
4.
Comparison of tear test methods 总被引:1,自引:0,他引:1
Tear strength is an important property of a rubber vulcanisate, particularly when considering it for applications such as cables, hoses and tyres. In the present study four test methods, as specified for vulcanised rubbers in British Standards (BS 903 Part A3 and BS 6899), have been investigated on five different compositions based on a nitrile natural rubber, an ethylene acrylic rubber, a polyurehane and undisclosed chloroprene elastomer. The objective of the study was to examine the influence of thickness, grain effects and strain rate on the tear strength. It was also intended to establish if the different tear test methods would rank materials in the same order and whether a constant ratio exists between the tear strength values obtained using the different methods for different materials.
Sample thicknesses in the range 0·5–3 mm and strain rates in the range 100–500 mm/min all produced small but significant variations in the tear strength, confirming that standardisation of these variables is very desirable. Grain effects were found to produce even larger variations in the test results, particularly for the trouser test pieces. It was found that the tests could be divided into two groups:
- • (a) trouser and cable tests;
- • (b) angle and crescent tests. The tests in each group ranked materials in the same order and the ratios between tests on different materials within each group was approximately the same.
5.
Gordon Slade 《Communications in Mathematical Physics》1985,102(3):425-462
We study the loop expansion for the effective potential, defined as the Fenchel transform (convex conjugate) of the pressure in an external field, in theP()2 quantum field theory. For values of the classical fielda for which the classical potentialU
0(a)=P(a)+1/2m
2
a
2 equals its convex hull and has nonvanishing curvature we prove that the 1-PI loop expansion is asymptotic as 0. We also give an example of a double well classical potential for which the 1-PI loop expansion fails to be asymptotic, and find the true asymptotics.This paper is a condensed version of the author's Ph.D. thesis for the Department of Mathematics, University of British Columbia, Vancouver, B.C., Canada V6T 1Y4 相似文献
6.
Macrodefect-free (MDF) materials are one example of “hot topics” in the field of cement-based materials and composites exerting
new possibilities of the exploitation of added value. These are formed through cross-linking reactions of atoms at the interfaces
of cement grains and functional polymers, when medium pressure and twin-rolling procedure are applied. The MDF-relevance of
the system of Portland cement + polyphosphate is reported, together with optimal synthesis conditions and limiting rules.
The chemistry knowledge about MDF materials has been shown critical for both procedure design and exploitation. Chemical shifts
in both 27Al and 31P MAS NMR spectra confirm Al(6)—O—P(4) cross-linking in virgin probes and indicate secondary hydrolysis during moisture uptake
in domains free of cross-links. Thermogravimetric identification of the contents of hydrated and cross-linked phases in virgin
and in moisture-attacked MDF probes displays that moisture uptake is accompanied by an increase in content of cementitious
hydrates and CaCO3. The key phenomena governing the moisture sensitivity/resistance are the density and compactness of interfacial Al(6)—O—P(4)
cross-links vs. the access of the moist environment to the unreacted cement residue.
The paper was presented at the 20 ICCBiC (International Conference on Coordination and Bioinorganic Chemistry) held on 5–10
June 2005 in Smolenice, Slovakia. 相似文献
7.
Ion Neda Carsten Melnicky Alexander Vollbrecht Axel Fischer Peter G. Jones Reinhard Schmutzler 《无机化学与普通化学杂志》1996,622(6):1047-1059
The reaction of methylisatoic acid anhydride 1 with benzylamines led to the N-benzyl-N′-methylanthranilamide derivatives 2 – 4 . Their reaction with phosphorus trichloride furnished the 2-chloro-1-halobenzyl/benzyl-3-methyl-4(1 H)-1,3,2-benzodiazaphosphorin-4-ones 5 – 7 which, upon reaction with bis-(2-chloroethyl)ammonium chloride/triethylamine, were converted into the P-bis-(2-chloroethyl)amino-1-halobenzyl/benzyl-3-methyl-4(1 H)-1,3,2-benzodiazaphosphorin-4-ones 8 – 10 and 12 . With 2-chloroethylammonium chloride/triethyl-amine the P? NHCH2CH2Cl-substituted compound 11 was obtained from the PIIICl-species 6 . The reaction of 8 – 10 and 12 with hexafluoroacetone (HFA) took an unusual course: apart from the oxidative addition of HFA and formation of the perfluoropinacolyl ring system, one of the two CH2CH2Cl groups was found to alkylate the CH3N atom with formation of a five-membered (diazaphospholane) ring in the tricyclic phosphoranes 13 – 16 . The reaction of 11 with HFA also produced a spirophosphorane 17 which involved a λ5-oxazaphosphetidine ring system. In the reaction of 8, 10 and 12 with tetrachloro-o-benzoquinone, an oxidative addition reaction with concomitant N-alkylation and formation of the tricyclic phosphoranes 18 – 20 was found to take place. Single crystal X-ray structure determinations are described for the phosphoranes 13, 14 and 16 , and for the precursor compound 9 . The following features are common to the isostructural compounds 13 and 16 and the diethyl ether hemisolvate of 14 : the (λ5)-spiro phosphorus atom lies out of the plane of the other atoms of the rings to which it is common, and the dioxaphospholane rings display a twist conformation. In the λ3P-compound 9 the phosphorus atom also lies out of the plane of the other ring atoms. 相似文献
8.
Sailstad DM Boykin EH Slade R Doerfler DL Selgrade MK 《Photochemistry and photobiology》2000,72(6):766-771
The adverse health effects caused by increased exposure to ultraviolet radiation (UVR) due to deterioration of stratospheric ozone are of major concern. These health effects include sunburn, skin cancer, cataracts and immune suppression. Immune suppression has been associated with the release of cytokines, a defect in antigen presentation, induction of suppressor T cells and suppression of contact hypersensitivity (CH). CH is typically assessed by the mouse ear swelling test (MEST). Previous studies have demonstrated enhanced CH responses with vitamin A acetate (VAA) dietary supplementation assessed by MEST and the local lymph node assay (LLNA). To determine the effect that VAA has on UVR-induced immune suppression, we examined both the induction and elicitation phases of CH using murine models. The MEST was used to evaluate the interaction of UVR and VAA on CH elicitation. However, a positive MEST response requires that the induction phase as well as the elicitation phase of CH be functional. The LLNA was used to evaluate the interaction of UVR and VAA only on CH induction. We tested the hypothesis that mice maintained on a VAA-enriched diet are more resistant to UVR-induced immune suppression (CH) than those maintained on a control diet. Mice were maintained on a VAA-enriched or the control diet for 3 weeks and then exposed to UVR 3 days prior to sensitization with 2,4-dinitrofluorobenzene (DNFB). VAA enhanced the MEST response in both UVR-exposed and non-UVR-exposed mice. The VAA-enriched diet did not significantly alter the LLNA response in either UVR- or non-UVR-exposed mice. However, there was significant suppression in CH by UVR as measured by the LLNA. These results indicate that (1) the VAA-enriched diet does not restore the number of proliferating cells in the CH induction phase of UVR-induced immunosuppression; (2) the immunosuppressive effects of UVR affect the induction phase of CH; and (3) the LLNA should be examined as an alternative to the MEST for measurement of UVR-induced immunosuppression. The data indicate that the VAA-enriched diet enhanced the elicitation response (MEST) but not the earlier induction phase (LLNA). Further studies are necessary to define mechanisms of action, but modulation of cytokines and effects of specific lymphocyte subsets, as well as systemic effects and local modulation at the site of elicitation are possible. Additionally, future studies to evaluate the effect of the VAA-enriched diet when multiple doses of both UVR and DNFB are used would be of interest for both the LLNA and MEST end-points. 相似文献
9.
We use the lace expansion to study the standard self-avoiding walk in thed-dimensional hypercubic lattice, ford5. We prove that the numberc
n ofn-step self-avoiding walks satisfiesc
n
~A
n
, where is the connective constant (i.e. =1), and that the mean square displacement is asymptotically linear in the number of steps (i.e.v=1/2). A bound is obtained forc
n(x), the number ofn-step self-avoiding walks ending atx. The correlation length is shown to diverge asymptotically like (––Z)1/2. The critical two-point function is shown to decay at least as fast as x–2, and its Fourier transform is shown to be asymptotic to a multiple ofk
–2 ask0 (i.e. =0). We also prove that the scaling limit is Gaussian, in the sense of convergence in distribution to Brownian motion. The infinite self-avoiding walk is constructed. In this paper we prove these results assuming convergence of the lace expansion. The convergence of the lace expansion is proved in a companion paper.Supported by the Nishina Memorial Foundation and NSF grant PHY-8896163.Supported by NSERC grant A9351 相似文献
10.
Ali Reza Kazemizadeh Nahid Shajari Reza Shapouri Neda Adibpour Reza Teimuri‐Mofrad Parisa Dinmohammadi 《应用有机金属化学》2016,30(3):148-153
A four‐component reaction between aromatic carboxylic acids, (N‐isocyanimino)triphenylphosphorane, ferrocenecarbaldehyde and dibenzylamine is reported. This approach is an efficient, simple and high‐yield procedure for the synthesis of 1,3,4‐oxadiazole derivatives containing a ferrocene unit. The antimicrobial activities of the products were investigated against Staphylococcus aureus and Pseudomonas aeruginosa in in vitro and in vivo assays. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献