剑桥大学Salje教授讲学通知
时间:2017-10-30 14:52:42   阅读:   标签: 理学院

 专家简介: Salje教授,剑桥大学地球科学系教授,地球科学系前系主任,克莱尔学院前院长;英国皇家科学院院士、德国科学院院士以及西班牙皇家科学院和艺术院院士,国际著名的材料科学家及矿物学家。主要研究领域为铁性材料相变行为与畴界工程。在Nature Mater, Phys Rev Lett, Nano Lett, Adv Mater等国际期刊上发表论文500余篇,SCI他引1万余次,H因子65。专著《Phase transitions in ferroelastic and co-elastic crystals》已被国际同行引用1000余次。Salje教授获得很多重要的国际奖,例如,1994年获得矿物学家最高荣誉奖Abraham-Gottlob-Werner(全世界每年仅1) 1998年获得矿物学家的最高荣誉之一Schlumberger奖; 2002年获得汉堡大学金奖(用于奖励提升汉堡大学水平的人员)2004年获得由法国总统颁发的Chevlier dans I'ORDER des Palmes Academiques 奖; 2006年获得应用矿物领域的最高奖之一Agricola 奖;同年获得德国功勋奖章一等奖(奖励对德国作出杰出贡献的人员)

讲学题目: Ultrafast Switching in Avalanche-Driven Ferroelectrics by Supersonic Kink Movements

时间: 2017111号上午 930-1100

地点: 新活动中心331

内容简介: Devices operating at GHz frequencies can be based on ferroelectric kink-domains moving at supersonic speed. The kinks are located inside ferroelastic twin boundaries and are extremely mobile. Computer simulation shows that strong forcing generates velocities well above the speed of sound. Kinks are accelerated from v = 0 continuously with Döring masses in the order of skyrmion masses under constant strain rates. Moving kinks emit phonons at all velocities, and the emission cones coincide with the Mach cones at supersonic speed. Kinks form avalanches with the emission of secondary kinks via a mother–daughter nucleation mechanism and may be observable in acoustic emission experiments. Supersonic kinks define a new type of material; while mobile domains are the key for ferroelastic and ferroelectric device applications at low frequencies, it is expected that fast kink movements replace such domain movements for materials applications at high frequencies.

讲学题目: Acoustic emission: a powerful indicator for changing nano-structures

时间: 2017112号上午 930-1100

地点: 新活动中心331

内容: It was first observed that mining accidents and earth quakes are sometimes preceded by high levels of crackling noise, and universal rules for the collapse. Further studies in ferroelectric and ferroelastics showed that similar ‘crackling noise’ also occurs during domain switching. On a coarser scale, such crackling noise is similar to Barkhausen noise in magnets. Great advances in the detection of such acoustic noise has made it possible to follow structural changes over 7 to 8 orders of energy during switching and hence allows for a very detailed analysis of the moving domain structures.

    I will focus on the universal character of this new technique. Archetypal test cases are the collapse of cavities in sandstone and coal. Their collapse mechanism is almost identical to earthquakes: the crackling noise in large, porous samples follows a power law (Gutenberg-Richter) distribution P similar to E-epsilon with energy exponents epsilon for near critical stresses of epsilon = 1.55 for dry and wet sandstone, and epsilon = 1.32 for coal. The exponents of early stages are slightly increased, 1.7 (sandstone) and 1.5 (coal), and appear to represent the collapse of isolated, uncorrelated cavities. A significant increase of the acoustic emission, AE, activity was observed close to the final failure event, which acts as " warning signal" for the impending major collapse. Waiting times between events also follow power law distributions with exponents 2+xi between 2 and 2.4. Aftershocks occur with probabilities described by Omori coefficients p between 0.84 (sandstone) and 1 (coal). The "Bath's law" predicts that the ratio between the magnitude of the main event and the largest aftershock is 1.2. 

    I will then describe most recent results on the field induced switching in BaTiO3 and some martensitc transitions. In each case, we find close statistical similarities with cavity collapses although the collapse mode consists of moving domain walls and their interaction. A detailed computer simulation study identifies the kinetic wall energies.

 

 

 

发布:任哲 |  审核:杨彬 |  来源: 理学院