全文获取类型
收费全文 | 418566篇 |
免费 | 5494篇 |
国内免费 | 2896篇 |
专业分类
化学 | 214465篇 |
晶体学 | 6088篇 |
力学 | 21279篇 |
综合类 | 77篇 |
数学 | 66008篇 |
物理学 | 119039篇 |
出版年
2019年 | 2566篇 |
2018年 | 12529篇 |
2017年 | 12267篇 |
2016年 | 10496篇 |
2015年 | 4612篇 |
2014年 | 5357篇 |
2013年 | 15482篇 |
2012年 | 14836篇 |
2011年 | 23855篇 |
2010年 | 14320篇 |
2009年 | 14748篇 |
2008年 | 18850篇 |
2007年 | 20910篇 |
2006年 | 12312篇 |
2005年 | 12356篇 |
2004年 | 11529篇 |
2003年 | 10692篇 |
2002年 | 9613篇 |
2001年 | 10874篇 |
2000年 | 8463篇 |
1999年 | 6662篇 |
1998年 | 5175篇 |
1997年 | 5130篇 |
1996年 | 5145篇 |
1995年 | 4626篇 |
1994年 | 4363篇 |
1993年 | 4159篇 |
1992年 | 4832篇 |
1991年 | 4693篇 |
1990年 | 4357篇 |
1989年 | 4398篇 |
1988年 | 4331篇 |
1987年 | 4277篇 |
1986年 | 3996篇 |
1985年 | 5637篇 |
1984年 | 5623篇 |
1983年 | 4616篇 |
1982年 | 4950篇 |
1981年 | 4780篇 |
1980年 | 4654篇 |
1979年 | 4756篇 |
1978年 | 5015篇 |
1977年 | 4819篇 |
1976年 | 4698篇 |
1975年 | 4555篇 |
1974年 | 4374篇 |
1973年 | 4532篇 |
1972年 | 2811篇 |
1968年 | 2506篇 |
1967年 | 2741篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
A multilayer structure consisting of undoped GaAs and AlAs layers was incorporated into a buffer layer on GaAs substrates. This reduces the spreading of impurities from the substrate (e.g. tellurium) into layers deposited on top of the multilayer system as observed by SIMS and C/V profiling. Incorporation coefficients for tellurium in GaAs, AlAs and multilayers were calculated. 相似文献
952.
MacLatchy C.S. Smith H.C.L. 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》1991,19(6):1254-1258
The electron current to a Langmuir probe in a moving high-pressure plasma has been measured and analyzed. The plasma is an atmospheric-pressure propane flame, and the probe's speed relative to the plasma was varied from 10 ms-1 to 30 ms-1. The current is linearly dependent on the speed and appears to be comprised of two components-one driven by diffusion and the other by convection. A model is devised which can be used to measure the electron density. By comparing the density computed from this model to that measured from the ion current, it has been possible to measure the electron mobility in the flame gases. The value obtained by the authors is in good agreement with theoretical calculations found in the literature 相似文献
953.
A Triolo J Bertini C Mannucci A Perico V Pestellini 《Journal of chromatography. A》1991,568(2):281-289
A method for the analysis of platelet-activating factor in platelets employing gas chromatography and selected-ion monitoring mass spectrometry with low-energy electron impact and stable isotope dilution was developed. The procedure involved Bligh and Dyer extraction of the sample followed by thin-layer chromatographic purification. Platelet-activating factor is successively hydrolysed to the corresponding 2-acetyl-1-O-alkylglycerol by digestion with phospholipase C, and the product is allowed to isomerize to the more thermodynamically stable 3-acetyl-1-O-alkylglycerol before column purification and derivatization of the free OH with tert.-butyldimethylchlorosaline-imidazole. This reagent is of common use in platelet-activating factor derivatization, but is made to react with 2-acetyl instead of 3-acetyl isomer. The advantages of using the latter for the final derivatization are discussed and this method is compared with others currently available for gas chromatographic-mass spectrometric analysis of platelet-activating factor. 相似文献
954.
Electron energy distribution functions (eedf) in CO2 laser discharge (He—CO2—N2—CO mixture) have been calculated by solving the Boltzmann equation in the presence of given concentrations of excited (vibrational and electronic) states. The results show a well structured eedf as a result of second kind collisions coming from metastable electronic states of N2 and He as well as a strong dependence of rate coefficients for CO2 dissociation and for the ionization of the different species. 相似文献
955.
Ulrich Mayerhofer T. von Egidy J. Jolie H. G. Börner G. Colvin S. Judge B. Krusche S. J. Robinson K. Schreckenbach S. Brant V. Paar 《Zeitschrift für Physik A Hadrons and Nuclei》1991,341(1):1-8
Gamma rays of199Au obtained after double neutron capture in197Au were measured at the ILL high flux reactor. A level scheme up to 1770 keV excitation energy is established. The result is compared with IBFM and Boson-Fermion-Symmetry calculations. 相似文献
956.
The DNA complexes of triostin A, echinomycin, and the monoquinoline (1QN) and bisquinoline (2QN) biosynthesized derivatives of echinomycin were investigated by optical detection of triplet-state magnetic resonance (ODMR) spectroscopy, with the quinoxaline and quinoline moieties of the DNA-binding peptides used as intrinsic probes. Plots of zero-field splitting (zfs)D parameter versus monitored wavelength revealed heterogeneity in the phosphorescence emission of echinomycin, triostin A, and 2QN ascribed to the occurrence of major and minor forms of the peptides in aqueous solution. ODMR results, in conjunction with findings from phosphorescence studies, indicate that the quinoxaline and quinoline chromophores of the major forms of the peptides are involved in aromatic stacking interactions in complexes with the natural DNAs fromMicrococcus lysodeikticus, Escherichia coli, and calf thymus as evidenced by red shifts in the phosphorescence 0,0 bands of the drugs, reductions in the phosphorescence lifetimes and zfsD andE parameters, and polarity reversal of the ODMR slow passage signals upon drug complexation. The reversal in ODMR signal polarity of echinomycin and 2QN is a consequence of changes in the triplet-state sublevel decay constants upon peptide binding to the natural DNAs. The extent of reduction of theD parameter for the major form of echinomycin, 2QN, and the quinoline moiety of 1QN upon complexation with polymeric DNAs was found to correlate with the binding affinities measured for these targets [1], but no correlation was found for the quinoxaline moiety of 1QN. Preliminary studies of triostin A-DNA complexes also revealed no correlation between the reduction in zfsD-value upon complexation and binding affinity, although the largest reductions inD-value among the peptides investigated in this report were exhibited by the poly(dG-dC)·poly(dG-dC) and natural DNA complexes of triostin A. 相似文献
957.
H. Postma J. D. Bowman C. D. Bowman J. E. Bush P. P. J. Delheij C. M. Frankle C. R. Gould D. G. Haase J. Knudson G. E. Mitchell S. Penttilä N. R. Roberson S. J. Seestrom J. J. Szymansky S. H. Yoo V. W. Yuan X. Zhu 《Hyperfine Interactions》1992,75(1-4):153-163
Parity violation effects have been studied at 40 neutron p-wave resonances of the even-even nuclei238U and232Th. Of these 11 show parity violation effects larger than 2 standard deviations, making parity violation a rather common phenomenon.
Parity mixing up to 10% has been found. The root-mean squared matrix elements for parity violation derived from these resonances
are M=0.58 (+0.50/-0.25) meV for238U, respectively 1.39 (+0.35/-0.38) meV for232Th. 相似文献
958.
H. Niehus 《Applied Physics A: Materials Science & Processing》1991,53(5):388-402
Low-energy ion backscattering and scanning tunneling microscopy (STM) have been used in combination to get better insight into the field of surface crystallography. The synergic effectiveness resulting from the complementing character of the two methods has been exemplified at clean NiAl(111) and for oxygen and nitrogen adsorption on Cu(110). The position of the atom cores is accessible by the low-energy noble gas impact collision ion scattering spectroscopy with neutral detection (NICISS). As a technique averaging over a macroscopic area of the sample, NICISS is better suited to supply information on features of completely developed phases, either on clean or adsorbate saturated surfaces. Additional information, on the other hand, can be gained by scanning tunneling microscopy (STM), which as a powerful local probe may be used to image surfaces with atomic resolution and to monitor defects, steps and the growth kinetics of e.g. adsorption-induced phase changes. 相似文献
959.
Periodic Nodal Surfaces (PNS) of Fourier series are derived and classified as fundamental invariants of structured matter. Relationships to periodic minimal surfaces PMS and to periodic zero potential surfaces (POPS) are given. A basic set of cubic PNS is represented in arithmetic form. The special importance of the invariance of the zeros to the type of the potential is stressed. 相似文献
960.