全文获取类型
收费全文 | 245838篇 |
免费 | 2331篇 |
国内免费 | 1089篇 |
专业分类
化学 | 114825篇 |
晶体学 | 3202篇 |
力学 | 14154篇 |
综合类 | 7篇 |
数学 | 48633篇 |
物理学 | 68437篇 |
出版年
2021年 | 991篇 |
2020年 | 1112篇 |
2019年 | 1127篇 |
2018年 | 11333篇 |
2017年 | 11135篇 |
2016年 | 8204篇 |
2015年 | 2707篇 |
2014年 | 2744篇 |
2013年 | 7949篇 |
2012年 | 9884篇 |
2011年 | 18014篇 |
2010年 | 10303篇 |
2009年 | 10855篇 |
2008年 | 13605篇 |
2007年 | 15659篇 |
2006年 | 7171篇 |
2005年 | 7761篇 |
2004年 | 7303篇 |
2003年 | 6919篇 |
2002年 | 5901篇 |
2001年 | 6233篇 |
2000年 | 4801篇 |
1999年 | 3597篇 |
1998年 | 2781篇 |
1997年 | 2733篇 |
1996年 | 2755篇 |
1995年 | 2388篇 |
1994年 | 2233篇 |
1993年 | 2116篇 |
1992年 | 2393篇 |
1991年 | 2370篇 |
1990年 | 2066篇 |
1989年 | 2063篇 |
1988年 | 2082篇 |
1987年 | 2015篇 |
1986年 | 1929篇 |
1985年 | 2819篇 |
1984年 | 2792篇 |
1983年 | 2242篇 |
1982年 | 2431篇 |
1981年 | 2308篇 |
1980年 | 2269篇 |
1979年 | 2215篇 |
1978年 | 2277篇 |
1977年 | 2217篇 |
1976年 | 2144篇 |
1975年 | 2134篇 |
1974年 | 2032篇 |
1973年 | 2128篇 |
1972年 | 1215篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
On the stokes problem in Lipschitz domains 总被引:3,自引:0,他引:3
972.
C. Basu 《Il Nuovo Cimento D》1992,14(9):947-953
Summary A simple method has been suggested to estimate the acoustic characteristics of porous structure from a hybrid model—a hybridisation
of Biot's phenomenological model and the microscopic multiple-scattering theory which introduces the idea of an effective
tortuosity. Without using any adjustable parameter this model may be used to provide rough estimates of the tortuosity, the
fast, the shear and the slow sound speeds. The predictions are compared with observation on water-saturated glass bead samples.
The author of this paper has agreed to not receive the proofs for correction. 相似文献
973.
In 1987 the Canadian Department of National Defence enunciated the Total Force concept. The Total Force is comprised of Regular and Reserve components. The intention is to make maximum use of the Reserve to reduce defence expenditures and at the same time to ensure that military capability remains adequate to support national policy objectives. This paper discusses some of the governing parameters that affect the modelling of the composition of the Total Force and analyses the mix of regular and reserve forces. The interplay between the key factors and their marginal costs will be stressed. The models are employed to study two units in the Canadian Forces, a maintenance support unit and a tactical unit with high operational activity cost. The lessons drawn from these studies are highlighted. 相似文献
974.
Cheng-kung Liu John A. Cuculo Thomas C. Allen A. Willem Degroot 《Journal of Polymer Science.Polymer Physics》1991,29(2):181-196
Fiber formation via the cellulose/ammonia/ammonium thiocyanate system by wet spinning has been investigated. This report presents a characterization of the structure and tensile properties of fibers spun under various coagulation conditions. Microscopic observations showed that the molecular size of coagulant was the dominant factor governing the crosssectional shape of the fibers. Density, birefringence, and crystallinity data indicated that a higher cellulose concentration and lower coagulation temperature favored development of a fiber with a denser and more oriented structure. Under optimum conditions, a welldefined fibrillar structure was obtained. Fiber tensile property measurements suggested the existence of a linear relationship between the fiber breaking tenacity and the product of the square of the Hermans' orientation factor and the infrared crystallinity index. 相似文献
975.
A new porphyrin, peroxyacetic acid uroporphyrin I, has been isolated from the urine of patients with congenital erythropoietic porphyria by reversed phase high performance liquid chromatography. The porphyrin was characterized by high resolution mass spectrometry and by typical chemical reactions of a peroxyacid. 相似文献
976.
Muon irradiation of pure liquid 3‐chloropropene, CH2=CH-CH2Cl, yields a primary radical, \dot\mboxCH2-CHMu-CH2Cl, and a secondary radical, MuCH2-\rm\dot\mboxCH-CH2Cl. 2‐methyl‐3‐chloropropene yields only the tertiary radical, MuCH2-\rm\dot\mboxC(CH3)-CH2Cl. These three chloroalkyl radicals have been characterized by μSR and μLCR, and the hyperfine coupling constants (hfcs)
have been determined over a range of temperatures, either in the pure liquid precursor or in concentrated solution. The temperature
variation of the hfcs has been analyzed to obtain estimates of the barrier to internal rotation about the C_\alpha-C_\beta
axis for various alkyl groups, and also their minimum energy conformations, i.e. their orientations with respect to the axis
of the 2p_z orbital of the unpaired electron. The tertiary radical is particularly interesting because all three methyl‐like
groups, -CH3,-CH2Cl and -CH2Mu, are represented. The results can be compared to electron spin resonance data for analogous radicals, to provide information
on the effects of Mu substitution for H.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
977.
E. C. Aschenauer V. E. Markushin 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1997,41(2):111-115
The use of a generalized exponential function r v?1 exp(?ζr μ ) as a radial basis function in atomic calculations is studied with our special interest in the variationally optimum value of the parameter μ, since special cases of μ = 1 and μ = 2 correspond respectively to the radial parts of commonly-used Slater-type and Gaussian-type functions. Roothaan-Hartree-Fock calculations are performed for ground-state neutral atoms with atomic number Z = 2–54, singly-charged cations with Z = 3–55, and anions with Z = 1–53 within the single-zeta (or minimal basis) framework. For all the species examined, the optimtum μ values are found to be smaller than unity and increase towards unity as the atomic number increases. The present results support the use of Slater-type functions when μ is restricted to be an integer, but suggest from the variational point of view that even the exponential decay of Slater-type functions is too “strong” within the single-zeta approximation. 相似文献
978.
A. Paesano Jr. S. C. Zanatta S. N. De Medeiros L. F. CÓtica J. B. M. Da Cunha 《Hyperfine Interactions》2005,161(1-4):211-220
We have investigated the mechanosynthesis of gadolinium and yttrium iron garnets by high-energy ball-milling of α-Fe2O3 and Gd2O3 or α-Fe2O3 and Y2O3, respectively, followed by short thermal annealings conducted at moderate temperatures. The samples were characterized by X-ray diffraction and Mössbauer spectroscopy, in order to determine the influence of the milling time and annealing conditions on the final products. For as-milled samples of each rare-earth system, the results revealed the formation of perovskite phases, in relative amounts that depend on the milling time. The formation of garnet phases was observed in as-annealed samples treated at 1000°C for 2 h or 1100°C for 3 h, i.e., at very modest annealing requirements when compared with ordinary solid-state-reaction processes performed without previous high-energy milling. Also, the occurrence was verified of a milling time for which the relative amount of garnet phases formed by annealing was maximized. This time depends on the rare-earth composing the garnet phase and on the annealing temperature. 相似文献
979.
Hua Ning LIU Wen Peng ZHANG 《数学学报(英文版)》2007,23(5):915-924
The main purpose of this paper is to use the generalized Bernoulli numbers, Gauss sums and the mean value theorems of Dirichlet L-functions between a quadratic residue and its inverse modulo p value formula. to study the asymptotic property of the difference (a prime), and to give an interesting hybrid mean 相似文献
980.
Summary We have recently reported on a new scaling theory of the rupture of colloid aggregates. The scaling theory is compared with
a simulation study of the transient response of sheared 2d aggregates. In this paper we discuss the origins of deviations from the predicted scaling behaviour. We also report some
of the steady-state configurations formed post rupture.
Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994. 相似文献