全文获取类型
收费全文 | 4274篇 |
免费 | 680篇 |
国内免费 | 348篇 |
专业分类
化学 | 1633篇 |
晶体学 | 32篇 |
力学 | 515篇 |
综合类 | 24篇 |
数学 | 429篇 |
物理学 | 2669篇 |
出版年
2024年 | 5篇 |
2023年 | 35篇 |
2022年 | 76篇 |
2021年 | 83篇 |
2020年 | 96篇 |
2019年 | 109篇 |
2018年 | 95篇 |
2017年 | 108篇 |
2016年 | 148篇 |
2015年 | 138篇 |
2014年 | 260篇 |
2013年 | 340篇 |
2012年 | 264篇 |
2011年 | 311篇 |
2010年 | 225篇 |
2009年 | 281篇 |
2008年 | 306篇 |
2007年 | 297篇 |
2006年 | 245篇 |
2005年 | 238篇 |
2004年 | 256篇 |
2003年 | 201篇 |
2002年 | 158篇 |
2001年 | 143篇 |
2000年 | 118篇 |
1999年 | 107篇 |
1998年 | 118篇 |
1997年 | 96篇 |
1996年 | 66篇 |
1995年 | 66篇 |
1994年 | 49篇 |
1993年 | 58篇 |
1992年 | 29篇 |
1991年 | 27篇 |
1990年 | 13篇 |
1989年 | 14篇 |
1988年 | 22篇 |
1987年 | 16篇 |
1986年 | 11篇 |
1985年 | 11篇 |
1984年 | 17篇 |
1983年 | 9篇 |
1982年 | 9篇 |
1981年 | 2篇 |
1980年 | 8篇 |
1979年 | 3篇 |
1977年 | 6篇 |
1976年 | 2篇 |
1973年 | 3篇 |
1957年 | 1篇 |
排序方式: 共有5302条查询结果,搜索用时 31 毫秒
91.
XRD,DTA,比表面测定等结果表明,制备方法对MoO3/ZrO2结构有决定性影响.仲钼酸铵浸渍仅经干燥的Zr(OH)4再经高温焙烧所得团体超强酸MoO3/ZrO2(Ⅰ)与浸渍晶态ZrO2所得部分氧化催化剂MoO3/ZrO2(Ⅱ)的载体形态有明显不同.(1)MoO3/ZrO2(Ⅰ)的比表面数倍于MoO3/ZrO2(Ⅱ);(2)MoO3含量增加时,MoO3/ZrO2(Ⅰ)的比表面逐步增大至一极大值再缓缓下降,而MoO3/ZrO2(Ⅱ)的比表面随MoO3含量增加而单调下降;(3)在MoO3/ZrO2(Ⅰ)中,ZrO2以介稳四方相存在,而在MoO3/ZrO2(Ⅱ)中则是稳定的单斜相.在此基础上研究了事先引入的活性组分在载体织构形成过程中的作用,包括延迟ZrO2晶化,阻碍晶粒长大及与之相关的相变等,并讨论了MoO3/ZrO2(Ⅰ)中MoO3最佳含量的科学含义. 相似文献
92.
A relationship is derived to enable the comparison of the dispersion heights of normal and reverse flow injection analysis (FIA). A single channel flow system is employed in the absence of a chemical reaction. The stopped-flow injection method is used to probe the influence of molecular diffusion on the overall dispersion of normal and reverse FIA, which appeared to demonstrate fundamentally different diffusion behaviors. Small discrepancies are observed between the dispersion heights, which are enhanced by the stopped-flow period, especially when unmatched matrix ionic compositions of the indicator and counter solutions were involved. For these conditions, the diffusion flux rate is enhanced considerably, displaying a peak, in addition to the transient, for both methods. The influence of diffusion on the dispersion characteristics of normal and reverse FIA is discussed theoretically. Diffusion in the proposed model is postulated to oppose dispersion by convection. The latter initiates concentration gradients in the injection zone and propagates it with flow time over the dispersion zone profile. The diffusion flux then reacts in order to confine the indicator dispersion for normal FIA and to enhance it for reverse FIA. This model is consistent with the experimental results and accounts for most of the phenomena encountered. Probably owing to the influence of secondary flow phenomena, the use of coiled tubes has suppressed the effects of diffusion on the overall dispersion behavior.Part of the experimental work was performed at IMI Institute for Research and Development, Haifa, Israel. 相似文献
93.
A family of multiderivative methods with minimal phase-lag are introduced in this paper, for the numerical solution of the Schrödinger equation. The methods are called multiderivative since uses derivatives of order two, four or six. Numerical application of the new obtained methods to the Schrödinger equation shows their efficiency compared with other similar well known methods of the literature.Active Member of the European Academy of Sciences and Arts 相似文献
94.
Diffusion coefficients of tetradecyltrimethylammonium bromide in aqueous solutions have been determined at 25, 95 and 135 °C using the Taylor dispersion technique. The diffusion coefficient exhibits a minimum at a surfactant concentration above the critical micelle concentration (CMC). The results are interpreted in terms of electrostatic coupling and rapid exchange between micelle, surfactant monomer and counterions. 相似文献
95.
The role of rigid unit modes in negative thermal expansion 总被引:1,自引:0,他引:1
J.Z Tao 《Journal of solid state chemistry》2003,173(2):442-448
The rigid unit mode (RUM) model provides a valuable computational method to investigate correlations of transverse thermal motions of atoms important in negative thermal expansion (NTE) materials. We report here detailed RUM calculations of ten framework oxide structures that have been studied for their negative (or ultra low) thermal expansion properties. The results negate any simple and direct correlation between presence or absence of RUMs in a structure and its NTE property. All the structures considered can be viewed as networks of polyhedral connected by corners only. All evidence supports the importance the transverse motion of the atoms at the corners, but NTE does not correlate well with the presence RUMs for the polyhedra. 相似文献
96.
Kaster Jeffrey A. Michelsen Donald L. Velander William H. 《Applied biochemistry and biotechnology》1990,(1):469-484
A microbubble dispersion (MBD) was used to supply oxygen for aerobic fermentations in a standard 2 L stirred tank fermenter.
The microbubble dispersion was formed using only surfactants produced naturally. Growth rates ofSaccharomyces cerevisiae cultures were found to be equal or greater with MBD sparging than with gas sparging. The oxygen transfer coefficent with
MBD sparging was found to be 190/h and independent of impeller speed from 100–580 rpm. The oxygen transfer coefficient with
air sparging rose from 55 to 132/h over the same range of impeller speeds. Power requirements for the fermenter systems were
estimated. 相似文献
97.
用晶内形成法制备了Ni_xB/沸石催化剂,用饱和氢化学吸附法结合TEM方法考察了负载金属的分散性,并研究了其己环烷脱氢反应性。结果表明,Ni_xB/沸石样品上负载金属的分散性高,金属粒径分布也相对集中;金属负载量、载体结构和性质以及Ni_xB的制备条件对负载金属的分散性有明显影响。Ni_xB/沸石样品呈现较好的环己烷脱氢反应性。 相似文献
98.
Kai Li Harald D. H. Stver 《Journal of polymer science. Part A, Polymer chemistry》1993,31(10):2473-2479
Narrow disperse microparticles are formed by dispersion polymerization of commercial divinylbenzene in acetonitrile or ethanol solution in the presence of 2,2′-azobis(2-methylpropionitrile) initiator and polyvinylpyrrolidone stabilizer. The particles have average diameters between 1 and 9 μm depending on monomer concentration, solvent, and temperature. While the smaller particles are relatively smooth, surface texture increases with diameter to give popcorn shapes at 9 μm diameter. High crosslinker concentration is shown to be essential for particle formation. © 1993 John Wiley & Sons, Inc. 相似文献
99.
R. De
Lisi G. Lazzara S. Milioto N. Muratore 《Journal of Thermal Analysis and Calorimetry》2007,87(1):61-67
Macromolecule/laponite nanomaterials were
studied by DSC and X-ray diffraction techniques. The matrices are poly(ethylene)
glycols at various molecular masses and poly(ethylene oxides)-poly(propylene
oxides)-poly(ethylene oxides) tri-block copolymers. The latter were tuned
by modulating the molecular masses, at constant hydrophilic/hydrophobic ratio,
and the hydrophilicity. For all the investigated systems, the enthalpy of
melting (ΔH
m)
is nearly constant up to a given composition thereafter it increases monotonically
reaching the value of the pure macromolecule. We proposed a model to interpret
the DSC data.
Briefly, it was invoked a mechanism of interaction
following which some segments of the adsorbed macromolecule are anchored to
the laponite (RD) particles and the remaining segments are radiating away
from the surface. The portion of the macromolecule in contact with RD does
not contribute to ΔH
m
whereas that radiating away from the clay does. Once that the RD surface is
saturated, the excess of the macromolecule behaves like the pure one. The
proposed model allowed to compute successfully the ΔH
m
values. The X-ray diffraction experiments ruled out the polymer intercalation
between the silicate sheets. 相似文献
100.