全文获取类型
收费全文 | 1767篇 |
免费 | 41篇 |
国内免费 | 4篇 |
专业分类
化学 | 1455篇 |
晶体学 | 20篇 |
力学 | 19篇 |
数学 | 21篇 |
物理学 | 297篇 |
出版年
2020年 | 12篇 |
2019年 | 17篇 |
2018年 | 15篇 |
2017年 | 11篇 |
2016年 | 21篇 |
2015年 | 34篇 |
2014年 | 40篇 |
2013年 | 96篇 |
2012年 | 67篇 |
2011年 | 87篇 |
2010年 | 62篇 |
2009年 | 42篇 |
2008年 | 86篇 |
2007年 | 84篇 |
2006年 | 114篇 |
2005年 | 115篇 |
2004年 | 101篇 |
2003年 | 100篇 |
2002年 | 83篇 |
2001年 | 33篇 |
2000年 | 34篇 |
1999年 | 27篇 |
1998年 | 18篇 |
1997年 | 22篇 |
1996年 | 34篇 |
1995年 | 13篇 |
1994年 | 18篇 |
1993年 | 14篇 |
1992年 | 19篇 |
1990年 | 17篇 |
1989年 | 22篇 |
1988年 | 20篇 |
1987年 | 13篇 |
1986年 | 17篇 |
1985年 | 17篇 |
1984年 | 27篇 |
1983年 | 12篇 |
1982年 | 25篇 |
1981年 | 28篇 |
1980年 | 36篇 |
1979年 | 15篇 |
1978年 | 13篇 |
1977年 | 8篇 |
1976年 | 15篇 |
1975年 | 9篇 |
1974年 | 13篇 |
1973年 | 12篇 |
1969年 | 8篇 |
1968年 | 12篇 |
1967年 | 12篇 |
排序方式: 共有1812条查询结果,搜索用时 140 毫秒
991.
Primordial black holes (PBHs) accumulate weakly interacting massive particles (WIMPs) around them and form ultracompact minihalos (UCMHs), if the WIMP is a dominant component of the dark matter (DM). In this Letter, we discuss that the UCMHs seeded by the PBHs with sub-earth mass enhance the WIMP annihilation in the present Universe and can successfully explain the positron and/or electron excess in cosmic ray observed by PAMELA/Fermi experiments. The signal is very similar to that from a decaying dark matter, which can explain the PAMELA and/or Fermi anomaly without conflict with any constraints as long as the decay mode is proper. In this scenario, the boost factor can be as large as 105. In addition, we discuss testability of our scenario by gamma-ray point source and gravitational-wave experiments. 相似文献
992.
Jason L. EvansBrian Feldstein William Klemm Hitoshi Murayama Tsutomu T. Yanagida 《Physics letters. [Part B]》2011,703(5):599-605
The fundamental constraint on two Higgs doublet models comes from the requirement of sufficiently suppressing flavor-changing neutral currents. There are various standard approaches for dealing with this problem, but they all tend to share a common feature; all of the Higgs doublets couple very weakly to the first generation quarks. Here we consider a simple two Higgs doublet model which is able to have large couplings to the first generation, while also being safe from flavor constraints. We assume only that there is an SUf(3) flavor symmetry which is respected by the couplings of one of the Higgs doublets, and which is broken by Hermitian Yukawa couplings of the second doublet. As a result of the large permitted couplings to the first generation quarks, this scenario may be used to address the excess in W+dijet events recently observed by CDF at the Tevatron. Moreover, Hermitian Yukawa coupling matrices arise naturally in a broad class of solutions to the strong CP problem, providing a compelling context for the model. 相似文献
993.
By analyzing a huge amount of point-of-sale data collected from Japanese supermarkets, we find power law relationships between price and sales numbers. The estimated values of the exponents of these power laws depend on the category of products; however, they are independent of the stores, thereby implying the existence of universal human purchase behavior. The rate of sales numbers around these power laws are generally approximated by log-normal distributions implying that there are hidden random parameters, which might proportionally affect the purchase activity. 相似文献
994.
Furuta T Sakai M Isobe T Matsushita S Nakajima A 《Langmuir : the ACS journal of surfaces and colloids》2011,27(11):7307-7313
Four patterned surfaces with hydrophilic areas of different sizes were prepared using photolithography with a smooth octadecyltrimethoxysilane (ODS) hydrophobic coating. The hydrophilic area in the surfaces was aligned hexagonally with a constant area fraction. The sliding angle and contact angle hysteresis of the water droplets increased concomitantly with increasing pattern size. The increase of the contact line distortion between defects at the receding side plays an important role in this trend. The droplet sliding velocity also increased concomitantly with increasing pattern size. This trend was simulated by a simple flow model. The contribution of the interface between the ODS region and the hydrophilic area was deduced from this trend. This study demonstrated the different size dependency of the chemical surface defects for sliding behavior between the critical moment at which a droplet slides down and the period when a droplet is sliding. 相似文献
995.
Aki Tsuruhara Hirohiko Kaneko So Kanazawa Yumiko Otsuka Nobu Shirai Masami K. Yamaguchi 《Optical Review》2013,20(3):277-281
Both horizontal and vertical binocular disparities produce depth perception in adults. In developmental studies, infants aged around 4 to 6 months were shown to perceive depth from horizontal disparity. However, infants’ sensitivity to vertical disparity has not been shown clearly. To examine the sensitivity in infants, this study measured preferential looking behavior of infants aged 20 to 27 weeks. Results showed a significant preference for the stimulus with vertical disparity, providing the first evidence of infants’ sensitivity to vertical disparity. The infants in the same age group did not show preference for stimulus with horizontal disparity when the stimulus were comparable to the stimulus with vertical disparity, while their sensitivity to horizontal disparity was confirmed with the stimuli used in a previous study. Our results would suggest that properties in processing horizontal and vertical disparities are different in infancy, and that the sensitivity to horizontal disparity are still premature in 27 weeks after birth. 相似文献
996.
A temperature-dependent gas-surface kinetic model for methane oxidation over palladium is proposed. Thermodynamic data for the surface species (O, H, OH, H2O, and CO) are derived from statistical mechanic analysis using literature heats of desorption and vibrational frequencies. The rate parameters in the model also satisfy thermo-kinetic constraints. The hydrogen oxidation submodel is validated against literature stagnation flow reactor experiments at 1300 K and 13 Pa. The current model is further tested against catalytic methane ignition in a laminar flow reactor at atmospheric pressure, and with time-resolved measurements of the size distribution of palladium nanoparticles generated in situ from an aerosol containing palladium acetate. The improved gas-surface model predicts closely the experimental data. The role of palladium nanoparticles in enhancing methane ignition is attributed to heat release due to catalytic methane oxidation over distributed nanoparticle surfaces, leading to a temperature rise and thus an accelerated gas-phase chain-branching process. 相似文献
997.
Yu Tokita Kotaro Sugawara Riho Awayama Yoshihiro Ohta Tsutomu Yokozawa 《Journal of polymer science. Part A, Polymer chemistry》2019,57(24):2498-2504
We investigated the synthesis of polyfluorene with a pinacol boronate (PinB) moiety at one end and with controlled molecular weight by means of Suzuki–Miyaura coupling polymerization of pinacol (7‐bromo‐9,9‐dioctyl‐9H‐fluoren‐2‐yl)boronate ( 1 ) with a palladium(0) precatalyst in the presence of pinacol 4‐trifluoromethylphenylboronate ( 2 ) as a chain terminator and CsF/18‐crown‐6 as a base. When we used AmPhos Pd G2, which has a propensity for intramolecular catalyst transfer on a π‐electron face, polyfluorene with the PinB moiety at one end and PhCF3 (derived from 2 ) at the other end was obtained, and the molecular weight increased in proportion to the feed ratio of [ 1 ]0/[catalyst]0, though the molecular weight distribution was broad. Since the molecular weight also linearly increased with respect to the conversion of 1 until the middle stage of polymerization, the polymerization appears to involve chain‐growth polymerization through intramolecular catalyst transfer from the Pd catalyst inserted into the C? Br bond of 1 . The broad molecular weight distribution might be mainly due to slow initiation and slow termination with 2 , rather than polymer–polymer coupling. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 2498–2504 相似文献
998.
Shinichi Yamabe Tsutomu Minato Teruyuki Watanabe Takahisa Machiguchi 《Theoretical chemistry accounts》2011,130(4-6):981-990
The title reactions were investigated experimentally and theoretically. The benzyne--tropone pair has been known to give a Diels?CAlder [4+2] cycloadduct. However, in the present experiment using two sources of benzyne, many unexpected products were obtained. In particular, a zwitterion with a C?CCl bond is strikingly a major product, the structure of which was determined by the X-ray analysis. The bond is formed by quenching the Cl atom from the solvent CH2Cl2, which demonstrates that the reactions proceed via radical processes. DFT calculations revealed the elementary processes of the reactions. The presence of the novel zwitterion in the solid state has been interpreted in terms of the permanent dipole?Cdipole attractions/interactions/stabilization. 相似文献
999.
1000.
Takayoshi Tajima Yuhsuke Akagi Takuya Kumamoto Noriyuki Suzuki Tsutomu Ishikawa 《Tetrahedron letters》2012,53(4):383-387
Jadomycin A and related jadomycin M and W aglycons were synthesized. Easy re-construction of 4-(hydroxymethyl)-1,3-oxazolidin-5-one system into an isomeric 1,3-oxazolidine-4-carboxylic acid system during the synthetic trial of jadomycin S aglycon requests us to more precisely re-examine the structures proposed for jadomycins S and T, derived from hydroxyl-containing amino acids of l-serine and l-threonine, respectively. 相似文献