全文获取类型
收费全文 | 3663篇 |
免费 | 159篇 |
国内免费 | 14篇 |
专业分类
化学 | 2987篇 |
晶体学 | 33篇 |
力学 | 70篇 |
数学 | 257篇 |
物理学 | 489篇 |
出版年
2023年 | 25篇 |
2022年 | 10篇 |
2021年 | 38篇 |
2020年 | 78篇 |
2019年 | 77篇 |
2018年 | 48篇 |
2017年 | 51篇 |
2016年 | 103篇 |
2015年 | 79篇 |
2014年 | 95篇 |
2013年 | 168篇 |
2012年 | 287篇 |
2011年 | 336篇 |
2010年 | 134篇 |
2009年 | 113篇 |
2008年 | 260篇 |
2007年 | 279篇 |
2006年 | 275篇 |
2005年 | 241篇 |
2004年 | 225篇 |
2003年 | 176篇 |
2002年 | 149篇 |
2001年 | 23篇 |
2000年 | 18篇 |
1999年 | 16篇 |
1998年 | 26篇 |
1997年 | 39篇 |
1996年 | 40篇 |
1995年 | 23篇 |
1994年 | 37篇 |
1993年 | 30篇 |
1992年 | 24篇 |
1991年 | 21篇 |
1990年 | 22篇 |
1989年 | 20篇 |
1988年 | 19篇 |
1987年 | 10篇 |
1986年 | 19篇 |
1985年 | 27篇 |
1984年 | 26篇 |
1983年 | 11篇 |
1982年 | 25篇 |
1981年 | 22篇 |
1980年 | 17篇 |
1979年 | 13篇 |
1978年 | 12篇 |
1977年 | 10篇 |
1976年 | 8篇 |
1974年 | 8篇 |
1973年 | 8篇 |
排序方式: 共有3836条查询结果,搜索用时 15 毫秒
1.
2.
Single-phase permeability k has intensively been investigated over the past several decades by means of experiments, theories and simulations. Although the effect of surface roughness on fluid flow and permeability in single pores and fractures as well as networks of fractures was studied previously, its influence on permeability in a random mass fractal porous medium constructed of pores of different sizes remained as an open question. In this study, we, therefore, address the effect of pore–solid interface roughness on single-phase flow in random fractal porous media. For this purpose, we apply a mass fractal model to construct porous media with a priori known mass fractal dimensions \(2.579 \le D_{\mathrm{m}} \le 2.893\) characterizing both solid matrix and pore space. The pore–solid interface of the media is accordingly roughened using the Weierstrass–Mandelbrot approach and two parameters, i.e., surface fractal dimension \(D_{\mathrm{s}}\) and root-mean-square (rms) roughness height. A single-relaxation-time lattice Boltzmann method is applied to simulate single-phase permeability in the corresponding porous media. Results indicate that permeability decreases sharply with increasing \(D_{\mathrm{s}}\) from 1 to 1.1 regardless of \(D_{\mathrm{m}}\) value, while k may slightly increase or decrease, depending on \(D_{\mathrm{m}}\), as \(D_{\mathrm{s}}\) increases from 1.1 to 1.6. 相似文献
3.
Mathematische Zeitschrift - Let G be a connected algebraic group. An unrefinable chain of G is a chain of subgroups $$G = G_0> G_1> \cdots > G_t = 1$$ , where each $$G_i$$ is a... 相似文献
4.
Timothy F. L. McKenna 《大分子反应工程》2019,13(2)
This review gives an overview of the evolution of the technology of condensed mode cooling, primarily for the case of ethylene polymerization on supported catalysts in fluidized bed reactors. It is well known that this mode of heat removal is quite effective in allowing polyolefin manufacturers to increase significantly production rates. What is perhaps less well understood are all of the issues that, in addition to the effect of the latent heat of vaporization of injected liquid components, also have an impact on the rate of production and behavior of the reactor. However, the liquid components injected into the reactor can vaporize rapidly under full‐scale conditions, leaving behind several heavy components (with respect to ethylene) that have numerous effects on how the particles behave, on the reaction rate, and on fluidization, fouling, and other parameters related to reactor and process performance. 相似文献
5.
The development of improved syntheses of PPARγ-sparing,insulin sensitizing thiazolidinedione-ketones
Steven P. Tanis Jerry R. Colca Timothy T. Parker Gerald D. Artman Scott D. Larsen Robert C. Gadwood James R. Zeller 《Tetrahedron letters》2019,60(33):150931
Ketones 2 (MSDC-0160) and 3 (MSDC-0602) had been selected for clinical development, however their initial syntheses were considered suboptimal for application deep into clinical trials. Difficulties ranging from the nature of the starting material, alcohol oxidation problems, epoxide opening regioisomeric issues, and endgame ketone redox problems had been encountered. Direct ketone introduction/maintenance was desired for maximum efficiency and convergence was found to be critically dependent upon the acidity of the nucleophilic species (13, 18) and the use of pre- or post-alkylative oximino-ether/oxime protection (vide infra). Improvements in overall yield for the syntheses of 2 (MSDC-0160) and 3 (MSDC-0602) from 20% (2) and 31% (3) respectively, to 44% (2) and 59% (3) were realized. 相似文献
6.
7.
Roly J. Armstrong Wasim M. Akhtar Tom A. Young Fernanda Duarte Timothy J. Donohoe 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(36):12688-12692
Hydrogen borrowing catalysis serves as a powerful alternative to enolate alkylation, enabling the direct coupling of ketones with unactivated alcohols. However, to date, methods that enable control over the absolute stereochemical outcome of such a process have remained elusive. Here we report a catalytic asymmetric method for the synthesis of enantioenriched cyclohexanes from 1,5‐diols via hydrogen borrowing catalysis. This reaction is mediated by the addition of a chiral iridium(I) complex, which is able to impart high levels of enantioselectivity upon the process. A series of enantioenriched cyclohexanes have been prepared and the mode of enantioinduction has been probed by a combination of experimental and DFT studies. 相似文献
8.
Journal of Chemical Crystallography - A new Group 2 bimetallic nitride fluoride phase of approximate composition SrCaNF has been prepared and characterized via high resolution single crystal X-ray... 相似文献
9.
Crossed Beam Study on the F+D2→DF+D Reaction at Hyperthermal Collision Energy of 23.84 kJ/mol 下载免费PDF全文
Hei-long Wang Shu Su Sheng-rui Yu Li Che Guo-rong Wu Kai-jun Yuan Xue-ming Yang Timothy K. Minton 《化学物理学报(中文版)》2019,32(1):151-156
We presented an experimental apparatus combining the H-atom Rydberg tagging time-of-flight technique and the laser detonation source for studying crossed beam reactions athyperthermal collision energies. The preliminary study of the F+D2→DF+D reaction at hyperthermal collision energy of 23.84 kJ/mol was performed. Two beam sources were used in this study: one is the hyperthermal F beam source produced by a laser detonation process, and the other is D2 beam source generated by liquid-N2 cooled pulsed valve. Vibrational state-resolved di erential cross sections (DCSs) of product for the title reaction were determined. From the product vibrational state-resolved DCS, it can be concluded that products DF(v'=0, 1, 2, 3) are predominantly distributed in the sideway and backward scattering directions at this collision energy. However, the highest vibrational excited product DF(v'=4), is clearly peaked in the forward direction. The probable dynamical origins for these forward scattering products were analyzed and discussed. 相似文献
10.
Dr. David Wifling Dr. Christopher Pfleger Jonas Kaindl Passainte Ibrahim Dr. Ralf C. Kling Prof. Dr. Armin Buschauer Prof. Dr. Holger Gohlke Prof. Dr. Timothy Clark 《Chemistry (Weinheim an der Bergstrasse, Germany)》2019,25(64):14613-14624
Histamine H4 receptor (H4R) orthologues are G-protein-coupled receptors (GPCRs) that exhibit species-dependent basal activity. In contrast to the basally inactive mouse H4R (mH4R), human H4R (hH4R) shows a high degree of basal activity. We have performed long-timescale molecular dynamics simulations and rigidity analyses on wild-type hH4R, the experimentally characterized hH4R variants S179M, F169V, F169V+S179M, F168A, and on mH4R to investigate the molecular nature of the differential basal activity. H4R variant-dependent differences between essential motifs of GPCR activation and structural stabilities correlate with experimentally determined basal activities and provide a molecular explanation for the differences in basal activation. Strikingly, during the MD simulations, F16945.55 dips into the orthosteric binding pocket only in the case of hH4R, thus adopting the role of an agonist and contributing to the stabilization of the active state. The results shed new light on the molecular mechanism of basal H4R activation that are of importance for other GPCRs. 相似文献