全文获取类型
收费全文 | 161648篇 |
免费 | 8556篇 |
国内免费 | 6169篇 |
专业分类
化学 | 95553篇 |
晶体学 | 2602篇 |
力学 | 7312篇 |
综合类 | 400篇 |
数学 | 16429篇 |
物理学 | 54077篇 |
出版年
2022年 | 1717篇 |
2021年 | 1793篇 |
2020年 | 2131篇 |
2019年 | 2151篇 |
2018年 | 1969篇 |
2017年 | 1958篇 |
2016年 | 3120篇 |
2015年 | 2986篇 |
2014年 | 3742篇 |
2013年 | 7835篇 |
2012年 | 6869篇 |
2011年 | 7907篇 |
2010年 | 5362篇 |
2009年 | 5205篇 |
2008年 | 6829篇 |
2007年 | 6415篇 |
2006年 | 6276篇 |
2005年 | 5757篇 |
2004年 | 4792篇 |
2003年 | 4273篇 |
2002年 | 4202篇 |
2001年 | 4502篇 |
2000年 | 3425篇 |
1999年 | 2817篇 |
1998年 | 2330篇 |
1997年 | 2275篇 |
1996年 | 2160篇 |
1995年 | 2075篇 |
1994年 | 1836篇 |
1993年 | 1761篇 |
1992年 | 1925篇 |
1991年 | 1899篇 |
1990年 | 1826篇 |
1989年 | 1776篇 |
1988年 | 1770篇 |
1987年 | 1746篇 |
1986年 | 1670篇 |
1985年 | 2194篇 |
1984年 | 2293篇 |
1983年 | 1924篇 |
1982年 | 2233篇 |
1981年 | 2054篇 |
1980年 | 2101篇 |
1979年 | 2089篇 |
1978年 | 2205篇 |
1977年 | 2116篇 |
1976年 | 2124篇 |
1975年 | 2069篇 |
1974年 | 1909篇 |
1973年 | 2115篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
A molecular dynamics method has been used to simulate the argon ion-assisted deposition of Cu/Co/Cu multilayers and to explore ion beam assistance strategies that can be used during or after the growth of each layer to control interfacial structures. A low-argon ion energy of 5–10 eV was found to minimize a combination of interfacial roughness and interlayer mixing (alloying) during the ion-assisted deposition of multilayers. However, complete flattening with simultaneous ion assistance could not be achieved without some mixing between the layers when a constant ion energy approach was used. It was found that multilayers with lower interfacial roughness and intermixing could be grown either by modulating the ion energy during the growth of each metal layer or by utilizing ion assistance only after the completion of each layers deposition. In these latter approaches, relatively high-energy ions could be used since the interface is buried and less susceptible to intermixing. The interlayer mixing dependence upon the thickness of the over layer has been determined as a function of ion energy. 相似文献
992.
A. N. W. Hone 《Transactions of the American Mathematical Society》2007,359(10):5019-5034
The Somos 5 sequences are a family of sequences defined by a fifth order bilinear recurrence relation with constant coefficients. For particular choices of coefficients and initial data, integer sequences arise. By making the connection with a second order nonlinear mapping with a first integral, we prove that the two subsequences of odd/even index terms each satisfy a Somos 4 (fourth order) recurrence. This leads directly to the explicit solution of the initial value problem for the Somos 5 sequences in terms of the Weierstrass sigma function for an associated elliptic curve.
993.
Amanda J. Ziegler Craig C. McLauchlan Cyndi R. Sosnowski Albert W. Herlinger 《Acta Crystallographica. Section C, Structural Chemistry》2007,63(2):o132-o134
The title compound (H2DTMSP[EBP]), C14H36O6P2Si2, was crystallized by the slow evaporation of a solution in a 20:1 mixture of pentane and acetone. The H2DTMSP[EBP] molecule lies about an inversion center. In the solid state, the molecule exists in an anti configuration, with the molecular backbone C—C bond located on an inversion center. The compound exists in the solid state as hydrogen‐bonded infinite sheets in the ab plane, unlike the methylene analogue, which exists as hydrogen‐bonded infinite chains, demonstrating an `even–odd' effect of the length of the backbone alkyl chain. 相似文献
994.
针对90t RH精炼装置,以水力学模拟研究了吹气孔直径对真空循环精炼过程中钢液流动和混合特性的影响.模型和原型的相似比为15.模型吹气孔直径分别为0.8 mm和1.2 mm.结果表明,1.2 mm孔径下环流量和主要工艺参数间的关系为Ql Q80.26Du0.69Dd0.80;在给定的吹气量和插入管内径下,增大吹气孔直径并不显著改变精炼过程中钢液流动特征,环流量随吹气孔直径的增大而略有增大,其关系式为Ql=2.40Q 0.23gD0.72u D0.88d d0.13in;相应地,混合时间稍有缩短;0.8和1.2 mm孔径下,τm与搅拌能密度的关系分别为τm ε5和τm ε0.49. 相似文献
995.
Two series of size‐fractionated combusted residues, a bag‐house ash and a scrubber residue, from a municipal solid waste incinerator (MSWI) were prepared and extracted by means of sequential chemical extraction (SCE) procedures proposed by Tessier. Concentrations of water‐soluble phase, exchangeable phase, carbonated phase, Fe/Mn oxide phase, organic matter phase, and residual phase bound amphoteric metals (Pb and Zn) were obtained, and have been compared with those of single batch extraction of toxic characteristic leaching procedure (TCLP). In each size fraction of baghouse ash or scrubber residue, lead or zinc shows a common phenomena that the TCLP value is always lower than the content of the water‐soluble fraction. For lead, baghouse ash has a significant size‐dependent distribution on the water‐soluble phase to perform a size‐dependent test of lead in the TCLP test. The zinc TCLP data having less size dependence might be due to that neither the baghouse ash nor the scrubber residue has a size‐dependent distribution on the water‐soluble zinc phase. 相似文献
996.
投资组合保险CPPI策略研究 总被引:4,自引:0,他引:4
随着期权理论应用的发展,投资组合保险在国外已成为一种盛行的资产配置策略, 常数比例投资组合保险策略(CPPI)以其模型简单、参数的设置又能充分反映投资人不同的风险偏好、而且易于实施,成为大型安全型基金的基金经理首选的投资策略.本文研究并推广了CPPI策略,找出CPPI与期权的关系,讨论了借贷限制对(CPPI策略的影响,最后对CPPI策略在中国市场的可投资性进行了评测. 相似文献
997.
Muon spin relaxation in zero field and longitudinal field was measured in single crystal samples of KH2PO4 (KDP) and KD2PO4 (DKDP) over a temperature range of 5 K to 300 K. At low temperatures, diamagnetic muons and muon substituted radicals with nuclear hyperfine coupling can be observed. For both KDP and DKDP, a minor change was observed in the dynamics of the muon below 140 K. Above 140 K, the mobility of the muon appears to increase and the diffusion rate becomes faster with increasing temperature. Only a small increase in the relaxation rate is observed in KDP due to the presence of theH
+, suggesting that the relaxation effects probably originate from the31P. 相似文献
998.
999.
A textured pellet of YBa2Cu3O7?δ was prepared and doped with57Co. After subsequent heat treatments the cobalt was found at the two copper sites. The degree of alignment of individual crystallites (about 40%) was determined from the Mössbauer spectrum by using a method developed by the authors [1]. The same method showed that the most characteristic doublet component of such spectra (δ=0.08 mm/s; |ΔE Q|=1.96 mm/s) is associated with a site, most probably a Cu(1) site, where the EFG is parallel to thec direction of the unit cell. The sign ofV zz turned out to be negative, giving a negative quadrupole splitting (ΔE Q=?1.96 mm/s) for that component. 相似文献
1000.