全文获取类型
收费全文 | 12873篇 |
免费 | 495篇 |
国内免费 | 821篇 |
专业分类
系统科学 | 712篇 |
丛书文集 | 472篇 |
教育与普及 | 229篇 |
理论与方法论 | 64篇 |
现状及发展 | 93篇 |
研究方法 | 31篇 |
综合类 | 12587篇 |
自然研究 | 1篇 |
出版年
2024年 | 51篇 |
2023年 | 133篇 |
2022年 | 231篇 |
2021年 | 288篇 |
2020年 | 236篇 |
2019年 | 139篇 |
2018年 | 160篇 |
2017年 | 188篇 |
2016年 | 227篇 |
2015年 | 394篇 |
2014年 | 586篇 |
2013年 | 564篇 |
2012年 | 644篇 |
2011年 | 721篇 |
2010年 | 683篇 |
2009年 | 762篇 |
2008年 | 914篇 |
2007年 | 881篇 |
2006年 | 813篇 |
2005年 | 741篇 |
2004年 | 540篇 |
2003年 | 409篇 |
2002年 | 455篇 |
2001年 | 440篇 |
2000年 | 393篇 |
1999年 | 427篇 |
1998年 | 297篇 |
1997年 | 299篇 |
1996年 | 237篇 |
1995年 | 247篇 |
1994年 | 225篇 |
1993年 | 141篇 |
1992年 | 138篇 |
1991年 | 144篇 |
1990年 | 122篇 |
1989年 | 110篇 |
1988年 | 93篇 |
1987年 | 60篇 |
1986年 | 32篇 |
1985年 | 10篇 |
1984年 | 3篇 |
1981年 | 3篇 |
1980年 | 2篇 |
1977年 | 1篇 |
1969年 | 1篇 |
1968年 | 1篇 |
1966年 | 1篇 |
1964年 | 1篇 |
1955年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 400 毫秒
831.
Montgomery KT Lee E Miller A Lau S Shim C Decker J Chiu D Emerling S Sekhon M Kim R Lenz J Han J Ioshikhes I Renault B Marondel I Yoon SJ Song K Murty VV Scherer S Yonescu R Kirsch IR Ried T McPherson J Gibbs R Kucherlapati R 《Nature》2001,409(6822):945-946
Our sequence-tagged site-content map of chromosome 12 is now integrated with the whole-genome fingerprinting effort. It provides accurate and nearly complete bacterial clone coverage of chromosome 12. We propose that this integrated mapping protocol serves as a model for constructing physical maps for entire genomes. 相似文献
832.
High critical current density and enhanced irreversibility field in superconducting MgB2 thin films 总被引:5,自引:0,他引:5
Eom CB Lee MK Choi JH Belenky LJ Song X Cooley LD Naus MT Patnaik S Jiang J Rikel M Polyanskii A Gurevich A Cai XY Bu SD Babcock SE Hellstrom EE Larbalestier DC Rogado N Regan KA Hayward MA He T Slusky JS Inumaru K Haas MK Cava RJ 《Nature》2001,411(6837):558-560
The discovery of superconductivity at 39 K in magnesium diboride offers the possibility of a new class of low-cost, high-performance superconducting materials for magnets and electronic applications. This compound has twice the transition temperature of Nb3Sn and four times that of Nb-Ti alloy, and the vital prerequisite of strongly linked current flow has already been demonstrated. One possible drawback, however, is that the magnetic field at which superconductivity is destroyed is modest. Furthermore, the field which limits the range of practical applications-the irreversibility field H*(T)-is approximately 7 T at liquid helium temperature (4.2 K), significantly lower than about 10 T for Nb-Ti (ref. 6) and approximately 20 T for Nb3Sn (ref. 7). Here we show that MgB2 thin films that are alloyed with oxygen can exhibit a much steeper temperature dependence of H*(T) than is observed in bulk materials, yielding an H* value at 4.2 K greater than 14 T. In addition, very high critical current densities at 4.2 K are achieved: 1 MA cm-2 at 1 T and 105 A cm-2 at 10 T. These results demonstrate that MgB2 has potential for high-field superconducting applications. 相似文献
833.
宋发富 《曲靖师范学院学报》2001,20(4):85-91
I.IntroductionThepurposeoftheessay ,accordingtothewriter,is“todemonstratethatlogic,farfrombeingadry ,pedanticsubject,isaliving ,breathingthing,fullofbeauty,passion ,andtrauma .”ThestoryissetinalawschoolattheuniversityofMinnesota,U .S .A .Theprotagonist,aself -con… 相似文献
834.
High-Q photonic nanocavity in a two-dimensional photonic crystal 总被引:1,自引:0,他引:1
Photonic cavities that strongly confine light are finding applications in many areas of physics and engineering, including coherent electron-photon interactions, ultra-small filters, low-threshold lasers, photonic chips, nonlinear optics and quantum information processing. Critical for these applications is the realization of a cavity with both high quality factor, Q, and small modal volume, V. The ratio Q/V determines the strength of the various cavity interactions, and an ultra-small cavity enables large-scale integration and single-mode operation for a broad range of wavelengths. However, a high-Q cavity of optical wavelength size is difficult to fabricate, as radiation loss increases in inverse proportion to cavity size. With the exception of a few recent theoretical studies, definitive theories and experiments for creating high-Q nanocavities have not been extensively investigated. Here we use a silicon-based two-dimensional photonic-crystal slab to fabricate a nanocavity with Q = 45,000 and V = 7.0 x 10(-14) cm3; the value of Q/V is 10-100 times larger than in previous studies. Underlying this development is the realization that light should be confined gently in order to be confined strongly. Integration with other photonic elements is straightforward, and a large free spectral range of 100 nm has been demonstrated. 相似文献
835.
高丰度硅-28同位素的分离技术 总被引:3,自引:1,他引:2
同位素纯硅片具有特殊的物理性质,在信息技术领域有着非常广阔的应用前景。为获得高丰度28Si,利用国产的高速气体离心机,选择SiF4作为分离的工作介质,对硅同位素进行了离心分离实验。通过单机分离实验,调整气体离心机的有关参数,摸索出适合级联分离的单机参数,同时得到气体离心机分离硅同位素的单位质量数的全分离系数γ0。利用由4台离心机组成的级联,经过6个阶段的分离,得到了28Si丰度达到99.5%的样品。实验结果表明,离心法分离硅同位素是完全可行的。 相似文献
836.
杨树不同杂交组合苗期性状遗传变异 总被引:4,自引:0,他引:4
对T120×S3107等8个杂交组合及各组合内无性系间的苗高、胸径、分枝情况等性状变异进行了研究。结果表明,组合间和无性系间在各性状上存在着显著的变异,苗高、胸径、侧枝的广义遗传力分别为77.75%,52.64%,56.96%。各杂交组合苗高和胸径平均值均超过对照T120和I 69。I 69×S3244,I 69×I 63组合的苗高和胸径性状表现都较好。因此,从这批材料中选出优良无性系是有可能的。 相似文献
837.
中国崛起背景下的中美关系 总被引:1,自引:0,他引:1
丁松泉 《杭州师范学院学报(社会科学版)》2003,(1):112-116
中国自1978年实行改革开放以来,经济持续发展,综合国力急剧上升。这引起了美国朝野上下的关注。根据传统的现实主义理论,美国国内出现了许多遏制中国发展的声音。然而中国的发展只会有利于世界的和平与发展。目前,中美两国有着许多共同关心的问题和利益,本着"君子和而不同"的思想,中美两国可以建立新型的、健康稳定的、建设性的国家关系。 相似文献
838.
本文概要介绍《飞向宇宙》多媒体计算机辅助教学(MCAI)课件的制作过程,经了多媒体课件的通用制作方法,应用多媒体技术发展起来的多媒体CAI课件成为极具吸引力的现代化教学手段,它改变了传统的教育思想、教学模式,加速了教学现代化的进程。 相似文献
839.
Nicastrin modulates presenilin-mediated notch/glp-1 signal transduction and betaAPP processing 总被引:26,自引:0,他引:26
Yu G Nishimura M Arawaka S Levitan D Zhang L Tandon A Song YQ Rogaeva E Chen F Kawarai T Supala A Levesque L Yu H Yang DS Holmes E Milman P Liang Y Zhang DM Xu DH Sato C Rogaev E Smith M Janus C Zhang Y Aebersold R Farrer LS Sorbi S Bruni A Fraser P St George-Hyslop P 《Nature》2000,407(6800):48-54
Nicastrin, a transmembrane glycoprotein, forms high molecular weight complexes with presenilin 1 and presenilin 2. Suppression of nicastrin expression in Caenorhabditis elegans embryos induces a subset of notch/glp-1 phenotypes similar to those induced by simultaneous null mutations in both presenilin homologues of C. elegans (sel-12 and hop-1). Nicastrin also binds carboxy-terminal derivatives of beta-amyloid precursor protein (betaAPP), and modulates the production of the amyloid beta-peptide (A beta) from these derivatives. Missense mutations in a conserved hydrophilic domain of nicastrin increase A beta42 and A beta40 peptide secretion. Deletions in this domain inhibit A beta production. Nicastrin and presenilins are therefore likely to be functional components of a multimeric complex necessary for the intramembranous proteolysis of proteins such as Notch/GLP-1 and betaAPP. 相似文献
840.
"数字地球"给我国地球科学带来的机遇和挑战 总被引:5,自引:0,他引:5
该文介绍了数字地球的概念和应用前景,讨论了地球科学面临的发展机遇及我国地球科学迎接"数字地球"的挑战之对策. 相似文献