首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   260篇
  免费   4篇
化学   129篇
晶体学   1篇
力学   5篇
数学   11篇
物理学   118篇
  2020年   4篇
  2016年   3篇
  2013年   16篇
  2012年   6篇
  2011年   10篇
  2010年   10篇
  2009年   5篇
  2008年   6篇
  2007年   12篇
  2006年   7篇
  2005年   4篇
  2004年   7篇
  2003年   3篇
  2002年   7篇
  2001年   7篇
  2000年   8篇
  1999年   3篇
  1998年   3篇
  1996年   5篇
  1995年   14篇
  1994年   6篇
  1993年   3篇
  1992年   6篇
  1990年   8篇
  1989年   5篇
  1988年   5篇
  1987年   3篇
  1986年   5篇
  1984年   3篇
  1983年   2篇
  1982年   2篇
  1981年   4篇
  1980年   2篇
  1979年   2篇
  1978年   4篇
  1977年   3篇
  1976年   4篇
  1975年   4篇
  1974年   7篇
  1973年   7篇
  1968年   2篇
  1939年   3篇
  1933年   2篇
  1930年   2篇
  1914年   2篇
  1905年   2篇
  1892年   2篇
  1888年   2篇
  1883年   2篇
  1876年   2篇
排序方式: 共有264条查询结果,搜索用时 46 毫秒
71.
Raman spectroscopy is used to study the complexes of 4,4′-dinitrobiphenyl with biphenyl, 4-hydroxybiphenyl, 4-bromobiphenyl and p-terphenyl, which crystallize in a highly unusual geometry. Their phonon spectra at 125 K and 18 K are compared and the effect of isotopic substitution of biphenyl on the phonon spectra of its complex is examined. Internal vibrations of the components in the crystalline complex are compared with those observed in the pure crystals of the components. The results from both phonon and intramolecular vibration studies show that these complexes form in fixed stoichiometries, are governed by geometrical factors, and are stabilized primarily by van der Waals interaction, although other kinds of interactions may provide additional stabilization. The 4,4′-dinitrobiphenyl molecule as well as biphenyl and p-terphenyl are centrosymmetric and remain so when the complexes are cooled from room temperature to 18 K. For biphenyl complex, this conclusion is supported by the observed IR spectra which show mutual exclusion between IR-active and Raman active vibrations. Crystal splitting is observed on the 410 cm?1 vibration of 4,4′-dinitrobiphenyl. This splitting is attributed to the presence of more than one 4,4′-dinitrobiphenyl molecules in the complex unit.  相似文献   
72.
In this paper, an a posteriori error estimator suitable for use in geotechnical engineering has been developed. This error criterion is based on the superconvergent patch recovery method with equilibrium and boundary conditions method, taking into account Biot's coupled consolidation theory. After testing on several problems, the introduction of pore pressures and other parameters into the error estimator was found to improve results significantly, providing a convergence rate of approximately two orders higher than the global rate.  相似文献   
73.
We have demonstrated efficient frequency doubling of high-energy fundamental Nd:YAG laser pulse energy of the multi-joule (J) level at a high repetition rate using high optical-quality top-seeded solution growth CsB3O5(TSSG-CBO) crystal for the first time. Second-harmonic (532 nm) generation (SHG) output energy of 1.2 J at 10 Hz is obtained with a conversion efficiency of 60%. This result has been obtained at the multi-J level by the growth of high optical-quality TSSG-CBO crystal with the large effective nonlinear coefficient and high damage threshold. These results indicate that TSSG-CBO is a good candidate material for high-energy SHG of Nd-doped lasers at the several J level or more with high repetition rate.  相似文献   
74.
A new model is proposed for the encapsulation of catalyst metal particles by graphite layers that are obtained, for example, in low-temperature chemical vapor deposition production of carbon nanotubes (CNTs). In this model graphite layers are primarily formed from the dissolved carbon atoms in the metal-carbide particle when the particle cools. This mechanism is in good agreement with molecular dynamics simulations (which show that precipitated carbon atoms preferentially form graphite sheets instead of CNTs at low temperatures) and experimental results (e.g., encapsulated metal particles are found in low-temperature zones and CNTs in high-temperature regions of production apparatus, very small catalyst particles are generally not encapsulated, and the ratio of the number of graphitic layers to the diameter of the catalyst particle is typically 0.25 nm(-1)).  相似文献   
75.
Multireference, scan-based near-field acoustical holography is a useful measurement tool that can be applied when an insufficient number of microphones is available to make measurements on a complete hologram surface simultaneously. The scan-based procedure can be used to construct a complete hologram by joining together subholograms captured using a relatively small, roving scan array and a fixed reference array. For the procedure to be successful, the source levels must remain stationary for the time taken to record the complete hologram; that is unlikely to be the case in practice, however. Usually, the reference signal levels measured during each scan differ from each other with the result that spatial noise is added to the hologram. A procedure to suppress the effects of source level, and hence reference level, variations is proposed here. The procedure is based on a formulation that explicitly features the acoustical transfer functions between the sources and both the reference and scanning, field microphones. When it is assumed that source level changes do not affect the sources' directivity, a nonstationarity compensation procedure can be derived that is based on measured transfer functions between the reference and field microphones. It has been verified both experimentally and in numerical simulations that the proposed procedure can help suppress spatially distributed noise caused by the type of source level nonstationarity that is characteristic of realistic sources.  相似文献   
76.
77.
Imino(triphenyl)phosphorane, Ph3P=NH 1a, is metallated by ethylmagnesium chloride to give the N-magnesioiminophosphorane complex [Ph3P=NMgCl·O=P(NMe2)3]2 4, whose X-ray structure has been determined.  相似文献   
78.
Abstract— 5-Methylphenazinium (MP+) cation salts are known to stimulate photophosphorylation in photosynthetic systems. The mechanism is thought to involve the translocation of protons across the thylakoid membrane although few details are known concerning the mechanism and the identity of the proton carrier. As there is some indication that the semireduced species, the 5-methyl-10-hydro-phenazinium (MPH+) cation radical, may be the proton carrier, we have studied the interaction of MPH+ with micelles as a model system for aqueouslipid interactions in a membrane. In this study we have used the techniques of NMR and EPR. In the presence of sodium dodecyl sulfate (NaLS) micelles the EPR spectrum of MPH + is broadened indicating specific binding of MPH+ to the micelle. A binding constant of -105M-1 was obtained. The 220 MHz NMR spectrum of a micellar solution with added MPH! shows specific broadening of the alkyl protons. The use of model paramagnetic adducts has allowed us to infer from the NMR results that the MPH+ ion penetrates the hydrocarbon part of the micelle to a considerable extent. This would indicate to us that MPH+ can indeed function as a proton carrier as it should be able to penetrate deeply into the lipid layer of the membrane. From these results, we conclude that:
  • 1 Both MP+ and MPH+ interact strongly with NaLS micelles. MPH+ interacts more weakly with neutral Triton X-100 micelles while in this case, no evidence of interaction is seen for MP+. No binding of either MP+ or MPH+ is observed with cationic micelles from CTAB.
  • 2 MP+ is adsorbed to NaLS micelles in a very hydrophilic region, probably In the Stern layer. MPH+, however, is much less hydrophilic than MP+ and appears to reside in the Palisade layer a few angstroms below the head groups.
On the basis of these results, we make the following proposals:
  • 1 MPH+ is sufficiently hydrophobic to be a candidate for the reduced MP+ species which actually carries the proton across the thylakoid membrane.
  • 2 The site at which MP+ is reduced by X must be anionic in nature in order for MP+ to be bound strongly.
  相似文献   
79.
We present a search for the standard model Higgs boson (H) in pp collisions at sqrt(s)=1.96 TeV in events containing a charged lepton (?), missing transverse energy, and at least two jets, using 5.4 fb(-1) of integrated luminosity recorded with the D0 detector at the Fermilab Tevatron Collider. This analysis is sensitive primarily to Higgs bosons produced through the fusion of two gluons or two electroweak bosons, with subsequent decay H→WW→?νq'q, where ? is an electron or muon. The search is also sensitive to contributions from other production channels, such as WH→?νbb. In the absence of a signal, we set limits at the 95% C.L. on the cross section for H production σ(pp→H+X) in these final states. For a mass of M(H)=160 GeV, the limit is a factor of 3.9 larger than the cross section in the standard model and consistent with an a priori expected sensitivity of 5.0.  相似文献   
80.
The synthesis of ketone 1 from the ethyl ester of 4-amino-2-methyl-5,6,7,8-tetrahydrobenzo[b]thieno[2,3-b]pyridine-3-carboxylic acid using β-ketosulfone methodology is described.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号