全文获取类型
收费全文 | 9198篇 |
免费 | 157篇 |
国内免费 | 37篇 |
专业分类
化学 | 5696篇 |
晶体学 | 126篇 |
力学 | 283篇 |
数学 | 1036篇 |
物理学 | 2251篇 |
出版年
2021年 | 53篇 |
2020年 | 80篇 |
2019年 | 84篇 |
2017年 | 52篇 |
2016年 | 100篇 |
2015年 | 99篇 |
2014年 | 132篇 |
2013年 | 417篇 |
2012年 | 332篇 |
2011年 | 397篇 |
2010年 | 207篇 |
2009年 | 226篇 |
2008年 | 390篇 |
2007年 | 405篇 |
2006年 | 410篇 |
2005年 | 369篇 |
2004年 | 331篇 |
2003年 | 298篇 |
2002年 | 287篇 |
2001年 | 193篇 |
2000年 | 209篇 |
1999年 | 106篇 |
1998年 | 98篇 |
1997年 | 102篇 |
1996年 | 139篇 |
1995年 | 116篇 |
1994年 | 170篇 |
1993年 | 181篇 |
1992年 | 166篇 |
1991年 | 121篇 |
1990年 | 119篇 |
1989年 | 129篇 |
1988年 | 102篇 |
1987年 | 122篇 |
1986年 | 111篇 |
1985年 | 189篇 |
1984年 | 158篇 |
1983年 | 102篇 |
1982年 | 127篇 |
1981年 | 121篇 |
1980年 | 137篇 |
1979年 | 133篇 |
1978年 | 131篇 |
1977年 | 115篇 |
1976年 | 130篇 |
1975年 | 115篇 |
1974年 | 136篇 |
1973年 | 128篇 |
1972年 | 56篇 |
1971年 | 65篇 |
排序方式: 共有9392条查询结果,搜索用时 296 毫秒
991.
Cheng-Kung Liu John A. Cuculo Brent Smith 《Journal of Polymer Science.Polymer Physics》1990,28(4):449-465
An investigation of the diffusion competition between solvent and nonsolvent in a coagulation bath is presented for the formation of a new cellulosic fiber by wet-spinning. The system consisted of the spinnable cellulose solution with a mixture of liquid ammonia/ammonia thiocynate as the solvent and low-molecular-weight alcohols as the nonsolvents. The diffusion competition between solvent and nonsolvent was quantitatively characterized in terms of their mass transfer rate differences. The measurements of this rate difference were performed on the model filament shaped from gelled cellulose solutions. Results revealed that an increase in molecular size of coagulant, bath temperature, and coagulant concentration in the bath enhanced preferential diffusion of solvent from cellulose solution. Fiber spinning experiments showed that a higher value of the initial modulus of the fiber was attained with a coagulation condition providing a lower value of mass transfer rate difference. The importance of mass transfer rate difference was also shown in the influence of the fiber cross-sectional shapes. 相似文献
992.
P. G. Reutens F. S. Merritt M. J. Oreglia P. Auchincloss R. E. Blair C. Haber S. R. Mishra E. Oltman M. Ruiz F. J. Sciulli M. H. Shaevitz W. H. Smith R. Zhu R. Coleman H. E. Fisk B. Jin T. Kondo D. Levinthal W. Marsh P. A. Rapidis H. B. White D. Yovanovitch A. Bodek F. Borcherding N. Giokaris K. Lang I. E. Stockdale 《Zeitschrift fur Physik C Particles and Fields》1990,45(4):539-550
We report a measurement of the electroweak parameters sin2θ w and ? based on the ratios of neutral current to charged current events measured in the Fermilab narrow-band neutrino beam at energies of 30–240 GeV. The data are fully corrected for radiative effects, heavy-quark production, and other effects. The best value for sin2θ w obtained, sin2θ w =0.239±0.011, is consistent with the most recent values fromW andZ production, as well as from other neutrino experiments. 相似文献
993.
LetD be a Dedekind domain. It is well known thatD is then an atomic integral domain (that is to say, a domain in which each nonzero nonunit has a factorization as a product
of irreducible elements). We study factorization properties of elements in Dedekind domains with finite class group. IfD has the property that any factorization of an elementα into irreducibles has the same length, thenD is called a half factorial domain (HFD, see [41]). IfD has the property that any factorization of an elementα into irreducibles has the same length modulor (for somer>1), thenD is called a congruence half factorial domain of orderr. In Section I we consider some general factorization properties of atomic integral domains as well as the interrelationship
of the HFD and CHFD property in the Dedekind setting. In Section II we extend many of the results of [41], [42] and [36] concerning
HFDs when the class group ofD is cyclic. Finally, in Section III we consider the CHFD property in detail and determine some basic properties of Dedekind
CHFDs. IfG is any Abelian group andS any subset ofG−[0], then {G, S} is called a realizable pair if there exists a Dedekind domainD with class groupG such thatS is the set of nonprincipal classes ofG which contain prime ideals. We prove that for a finite abelian groupG there exists a realizable pair {G, S} such that any Dedekind domain associated to {G, S} is CHFD for somer>1 but not HFD if and only ifG is not isomorphic toZ
2,Z
2,Z
2 ⊕Z
2, orZ
3 ⊕Z
3.
The first author received support under the John M. Bennett Fellowship at Trinity University and also gratefully acknowledges
the support of The University of North Carolina at Chapel Hill. 相似文献
994.
995.
Walter T. Smith William F. Kluttz Donna Palmieri D. Allan Butterfield 《Journal of polymer science. Part A, Polymer chemistry》1989,27(2):575-582
Graft polymers of three different molecular weights of polyethyleneimine (PEI) with 9-(vinylsulfonylethyl) (9-VSE)-adenine and 1-(vinylsulfonylethyl) (1-VSE)-thymine were prepared. PEI's having molecular weights of 600, 1200, and 1800 were used and various ratios of VSE-nucleobases to ethyleneimine were employed in the grafting process. As expected, a decrease in the nucleobase substitution in the resulting graft polymer resulted in an increase in its water solubility. The degree of substitution in the graft polymers was calculated from sulfur determinations: in most cases this degree of substitution indicated that not all of the VSE derivative has reacted with the PEI due to the highly branched nature of the latter. The presence of nucleobase stacking in the polymers was studied by UV spectroscopy. Stacking was observed for the adenine polymers in both DMSO and 0.1N HCl while stacking in the thymine polymers was observed in 0.1N HCl but not in DMSO. The absence of stackingwithin the thymine polymers in DMSO was attributed to complexation of primary amino groups of the PEI backbone with the pyrimidine ring. 相似文献
996.
S. Ramasamy D. J. Smith P. Thangadurai K. Ravichandran T. Prakash K. PAdmaprasad V. Sabarinathan 《Pramana》2005,65(5):881-891
The ultra high vacuum chamber was developed in the Department of Nuclear Physics, University of Madras with the funding from
DST, India. This UHV chamber is used to prepare nanocrystalline materials by inert gas condensation technique (IGCT). Nanocrystalline
materials such as PbF2, Mn2+-doped PbF2, Sn-doped In2O3 (ITO), ZnO, Al2O3, Ag2O, CdO, CuO, ZnSe:ZnO etc., were prepared by this technique and characterized. Results of some of these materials will be
presented in this paper. In solid-state207Pb NMR on PbF2 a separate signal due to the presence of grain boundary has been observed. The structural phase transition pressure during
the phase transformation from the cubic phase to orthorhombic phase under high pressure shows an increase with the decrease
in grain size. Presence of electronic centres in nanocrystalline PbF2 is observed from Raman studies and the same has been confirmed by photoluminescence studies. Al2O3 was prepared and56Fe ions were implanted. After implantation segregation of56Fe ions was examined by SEM. The oxidation properties of ITO were studied by HRTEM. As against the expectation of oxide coating
on individual nanograins of In-Sn alloy, ITO nanograins grew into faceted nanograins on heat treatment in air and O2 atmosphere. The growth of ITO under O2 atmosphere showed pentagon symmetry. The PMN was initially prepared by solid-state reaction. Further, this PMN relaxor material
will be used to convert into nanocrystalline PMN by IGCT with sputtering and will be studied 相似文献
997.
998.
J. J. Carroll J. Burnett T. Drummond J. Lepak R. Propri D. Smith S. A. Karamian J. Adam F. Stedile F. J. Agee 《Hyperfine Interactions》2002,143(1-4):37-54
Experiments with the long-lived, high-K isomer 178Hfm2 have been recognized as intriguing tests of multi-quasiparticle state structures and their interactions with external radiation.
A triggered release of the energy stored by this isomer, 2.5 MeV per nucleus or 1.2 GJ/gram, in the form of a gamma-ray burst
might prove valuable for numerous applications. The observation of “accelerated” decay of 178Hfm2 during irradiation by 90-keV bremsstrahlung has already been reported, but with poor statistical accuracy due to the experimental
approach. That approach employed single Ge detectors to seek increases in the areas of peaks at energies corresponding to
transitions in the spontaneous decay of the isomer. The need for better quality data to confirm those results has motivated
the development of improved detection concepts. One such concept was utilized here to perform an initial search for low-energy
(<20 keV) triggered gamma emission from 178Hfm2 using the YSU miniball detector array.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
999.
Nontrivial difference sets in groups of order a power of 2 are part of the family of difference sets called Menon difference sets (or Hadamard), and they have parameters (22d+2, 22d+1±2
d
, 22d
±2
d
). In the abelian case, the group has a difference set if and only if the exponent of the group is less than or equal to 2
d+2. In [14], the authors construct a difference set in a nonabelian group of order 64 and exponent 32. This paper generalizes that result to show that there is a difference set in a nonabelian group of order 22d+2 with exponent 2
d+3. We use representation theory to prove that the group has a difference set, and this shows that representation theory can be used to verify a construction similar to the use of character theory in the abelian case. 相似文献
1000.