全文获取类型
收费全文 | 204281篇 |
免费 | 1977篇 |
国内免费 | 592篇 |
专业分类
化学 | 102535篇 |
晶体学 | 3341篇 |
力学 | 9294篇 |
综合类 | 5篇 |
数学 | 22567篇 |
物理学 | 69108篇 |
出版年
2021年 | 1687篇 |
2020年 | 1810篇 |
2019年 | 2029篇 |
2018年 | 2724篇 |
2017年 | 2616篇 |
2016年 | 3790篇 |
2015年 | 2317篇 |
2014年 | 3642篇 |
2013年 | 9115篇 |
2012年 | 7265篇 |
2011年 | 8758篇 |
2010年 | 6058篇 |
2009年 | 5920篇 |
2008年 | 7926篇 |
2007年 | 8090篇 |
2006年 | 7730篇 |
2005年 | 6896篇 |
2004年 | 6427篇 |
2003年 | 5652篇 |
2002年 | 5493篇 |
2001年 | 6426篇 |
2000年 | 4932篇 |
1999年 | 3796篇 |
1998年 | 3039篇 |
1997年 | 3045篇 |
1996年 | 2792篇 |
1995年 | 2613篇 |
1994年 | 2462篇 |
1993年 | 2337篇 |
1992年 | 2692篇 |
1991年 | 2671篇 |
1990年 | 2600篇 |
1989年 | 2397篇 |
1988年 | 2449篇 |
1987年 | 2448篇 |
1986年 | 2279篇 |
1985年 | 2972篇 |
1984年 | 3225篇 |
1983年 | 2641篇 |
1982年 | 2805篇 |
1981年 | 2777篇 |
1980年 | 2640篇 |
1979年 | 2728篇 |
1978年 | 2826篇 |
1977年 | 2820篇 |
1976年 | 2886篇 |
1975年 | 2676篇 |
1974年 | 2645篇 |
1973年 | 2776篇 |
1972年 | 1764篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
271.
Benjamin S. Hsiao Rong-Ming Ho Stephen Z. D. Cheng 《Journal of Polymer Science.Polymer Physics》1995,33(17):2439-2447
Unique crystallization and melting behavior in poly(aryl ether ketone ketone) containing alternated terephthalic and isophthalic moieties were studied by time-resolved synchrotron x-ray methods. Recently, this material has been shown to exhibit three polymorphs (forms I, II, and III). In this work, we further investigated their distinctive thermal properties and found that form I is the dominating and the most thermally stable phase while form II is favored by fast nucleation conditions and is the least stable phase. On the other hand, form III represents a minor intermediate phase that usually coexists with form I and can be transferred from form II and to form I. Structural and morphological changes in form I have been followed by simultaneous wide-angle x-ray diffraction (WAXD)/small-angle x-ray scattering (SAXS) measurements during cold- or melt-crystallization and subsequent melting. In all cases, a larger dimensional change was found in the crystallographic a-axis than the b-axis during heating and cooling. This may be due to the greater lateral stress variation with respect to temperature along the a direction of the primary lamellae which is induced by either the formation of secondary lamellae or the preferential chain-folding direction in poly(aryl ether ketone ketone)s. During the phase transitions of form II ← III in the cold-crystallized specimen and form III ← I in the melt-crystallized samples, lamellar variables (long period, lamellar thickness, and invariant) obtained from SAXS remain almost constant. This indicates that the density distribution in the long spacing is independent of the melting in form II or III. For melt-crystallization, the corresponding changes in unit-cell dimensions and lamellar morphology during the annealing-induced low endotherm are most consistent with the argument that these changes are due to the melting of thin lamellar population. © 1995 John Wiley & Sons, Inc. 相似文献
272.
Microencapsulation of DNA Within alginate microspheres and crosslinked chitosan membranes for in vivo application 总被引:4,自引:0,他引:4
T. Alexakis D. K. Boadi D. Quong A. Groboillot I. O’Neill D. Poncelet R. J. Neufeld 《Applied biochemistry and biotechnology》1995,50(1):93-106
Calf thymus DNA was microencapsulated within crosslinked chitosan membranes, or immobilized within chitosan-coated alginate
microspheres. Microcapsules were prepared by interfacial polymerization of chitosan, and alginate microspheres formed by emulsification/
internal gelation. Diameters ranged from 20 to 500 Μm, depending on the formulation conditions. Encapsulated DNA was quantifiedin situ by direct spectrophotometry (260 nm) and ethidium bromide fluorimetry, and compared to DNA measurements on the fractions
following disruption and dissolution of the microspheres. Approximately 84% of the DNA was released upon core dissolution
and membrane disruption, with 12% membrane bound. The yield of encapsulation was 96%. Leakage of DNA from intact microspheres/capsules
was not observed. DNA microcapsules and microspheres were recovered intact from rat feces following gavage and gastrointestinal
transit. Higher recoveries (60%) and reduced shrinkage during transit were obtained with the alginate microspheres. DNA was
recovered and purified from the microcapsules and microspheres by chromatography and differential precipitation with ethanol.
This is the first report of microcapsules or microspheres containing biologically active material (DNA) being passed through
the gastrointestinal tract, with the potential for substantial recovery. 相似文献
273.
We study the conjugate boundary-value problem with boundary conditions whose orders can be as large as the order of the differential equation or larger. We construct the Green's function of the problem over the entire space when the separating surface is a hyperplane, and we describe its properties. Bibliography: 6 titles.Translated fromMatematicheskie Metody i Fiziko-Mekhanicheskie Polya, Issue 30, 1989, pp. 20–24. 相似文献
274.
275.
ALEPH Collaboration D. Buskulic D. Casper I. De Bonis D. Decamp P. Ghez C. Goy J. -P. Lees A. Lucotte M. -N. Minard et al. 《Zeitschrift fur Physik C Particles and Fields》1995,69(1):379-392
A data sample of about 3.0 million hadronicZ decays collected by the ALEPH experiment at LEP in the years 1991 through 1994 is used to make an inclusive selection of
B hadron events. In this event sample 4227±140±252B* mesons in the decayB*→Bγ and 1944±108±161B**
u,d
mesons decaying into a B meson and a charged pion are reconstructed. Here and in the followingB**
u,d
denotes the eightL=1(bū) and (bd) states and their charge conjugate.
For the well establishedB* meson the following quantities are obtained:ΔM=M
B*−MB=(45.30±0.35±0.87) MeV/c2 andN
B*/(N
B+N
B*)=(77.1±2.6±7.0)%. The angular distribution of the photons in theB* rest frame is used to measure the relative contribution of longitudinalB* polarization states to beσ
L/(σ
L+σ
T)=(33±6±5)%. 相似文献
276.
Published in Teoreticheskaya i Matematicheskaya Fizika, Vol. 95, No. 2, pp. 183–186, May, 1993. 相似文献
277.
Modeling and numerical simulations of the convective flows induced by the vibration of the monocrystal during crystal growth have been performed for two configurations simulating the Cz and FZ methods. This permitted to emphasize the role of different vibrational mechanisms in the formation of the average flows. It is shown that an appropriate combination of these mechanisms can be used to counteract the usual convective flows (buoyancy- and/or thermocapillary-driven) inherent to crystal growth processes from the liquid phase. While vibrational convection is rather complex due to these identified mechanisms, the new modeling used in the present paper opens up very promising perspectives to efficiently control heat and mass transfer during real industrial applications of crystal growth from the liquid phase. 相似文献
278.
279.
D. I. Zhukhovitskii 《Journal of Experimental and Theoretical Physics》2002,94(2):336-344
Numerical experiments showed that the number-of-bonds distribution of particles that form a fairly large molten argon-like cluster was bimodal. This result was interpreted as a consequence of the formation of two “phases, ” namely, particles inside the cluster and a monolayer of particles lying above the others. Particle chains were shown to be formed near the surface of the cluster. Splitting off of separate particles from them was the most probable mechanism of vaporization. Model concepts that described the dependences observed in numerical experiments were developed. 相似文献
280.
A self-consistent model of the development of dendrites and partial discharges in a dielectric solid under a variable voltage
is suggested. Dendrites originate at sites with the enhanced local field strength and also where the dielectric is broken
down under the action of partial discharges. The numerical simulation is used to quantitatively describe the space-time dynamics
of the dendrites and partial discharges in epoxy resin for the tip-plane electrode configuration. The simulated data are compared
with electrical measurements of partial discharges and with optical images of dendrite growth under the same conditions. 相似文献