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SUMMARY:Laser-based guided ultrasonic waves: from macroscopic properties t
 o nanoscopic models
DTSTART:20130507T131500
DTEND:20130507T141500
DTSTAMP:20260408T034935Z
UID:7e2c125587055c1a936cecbdbbd5fbdfdaa09c22dcc1bc9a86d0a612
CATEGORIES:Conferences - Seminars
DESCRIPTION:Prof. Dr. Peter Hess\nBio : Prof. Dr. Peter Hess is professor 
 (retired) of Physical Chemistry at University of Heidelberg since 1980. Pr
 eviously\, he studied for a diploma in chemistry at the Karlsruhe Institut
 e of Technology (KIT) and for a PhD thesis  on “Physical adsorption pro
 cesses” (1968) and then for the habilitation thesis on “Energy transfe
 r processes in gases” (1972) at Ruprecht-Karls-University  in Heidelber
 g. From 1974 he was research fellow at the Department of Chemistry\, Unive
 rsity of California (Berkely\, USA) and after 1980\, he was regularly visi
 ting scientist at Almaden Research Laboratories\, IBM\, San Jose\, Califor
 nia\, USA for shorter time periods.\nHe has been active in several researc
 h field. (1) Laser-based photoacoustics in gases: chemical relaxation and 
 trace gas analysis. (2) Laser-based surface acoustic waves (SAWs) and wedg
 e waves (WWs): all-optical nondestructive evaluation (NDE)\, linear and no
 nlinear elastic constants and mechanical properties of superhard materials
 \, nonlinear behavior and fracture strength of solids and solitary surface
  waves. (3) Laser-induced desorption\, ablation\, and surface processing: 
 time-of-flight mass spectrometry of specific surface reactions induced by 
 pulsed laser irradiation and measurement of the thermal stability of surfa
 ce end groups and functionalizations. (4) Silicon surface spectroscopy and
  chemistry: in situ and real-time diagnostics of surface reactions (e.g. f
 unctionalization\, oxidation) on silicon with monolayer resolution (FTIR s
 pectroscopy\, IR-UV spectroscopic ellipsometry)\, and atomic force microsc
 opy (AFM).\nHe produced about 300 publications in scientific journals\, on
 e worldwide and one European patent on functionalization and processing of
  silicon surfaces. He was editor or co-editor of 6 books (Springer 1987\, 
 1989\; Elsevier 1995\, 1999\; SPIE 1997\, 2000)\, chairman or co-chairman 
 of ten international conferences.\nAbstract : Current progress in laser ul
 trasonics employing guided elastic waves is reviewed [1]. Excitation and d
 etection of linear and nonlinear ultrasonic waves by a laser pump-probe se
 tup will be described. This includes ultrasound in three-dimensional (3D) 
 bulk waveguides such as rods or rails\, two-dimensional (2D) surface acous
 tic wave (SAW) pulses\, traveling along surfaces and penetrating only abou
 t one wavelength deep into the solid\, and one-dimensional (1D) wedge wave
 s (WWs)\, propagating at the apex of a wedge with the elastic energy remai
 ning at the tip of the edge. The emerging field of laser-based excitation 
 and detection of linear and nonlinear WWs and their potential applications
  will be discussed in detail [2]. The dependence of dispersion and diffrac
 tion of ultrasound on the geometry of the system and dimension of wave pro
 pagation is considered with respect to the degree of nonlinearity that can
  be achieved by pulsed laser excitation of elastic waves. Note that with s
 hort laser pulses of nanosecond to femtosecond duration a localized desint
 egration of solids into electrons and ions (plasma) and\, as a consequence
 \, efficient formation of steep shocks with gigapascal to terapascal press
 ure can be achieved by the resulting confined micro-explosions.\nRecent ap
 plications of linear guided ultrasonic waves (3D to 1D) in nondestructive 
 evaluation (NDE) will be presented. Novel developments in the use of guide
 d bulk waves to monitor flaws in rails\, for example\, as well as the prob
 lems connected with this approach will be discussed. Another important app
 lication is the characterization of real partially closed surface-breaking
  cracks by linear SAW pulses\, since failure usually starts at the surface
 . One-dimensional WWs provide new possibilities for sensitive evaluation o
 f defects or cracks at the apex of wedges\, e.g.\, in cutting tools or tur
 bine blades. On the other hand\, linear WWs recently were also applied in 
 sensor devices and in actuators such as ultrasonic motors or streaming in 
 fluidics.\nStrongly nonlinear SAW and WW pulses developing shock fronts du
 ring propagation due to nonlinearity could be realized experimentally. Sho
 cked SAW pulses were used to measure the fracture strength of single-cryst
 al silicon for selected crystallographic planes and directions. The measur
 ed well-defined critical fracture stresses can be compared directly with a
 b initio calculations if theoretical strengths for these particular config
 urations are available. With the experimental and theoretical information 
 it is possible to describe the tensile bond-breaking process along the wea
 kest Si{111} cleavage plane on the basis of the Griffith approach. This mo
 del introduces a characteristic length scale in the nanometer range. The l
 ength scale is identified with the distance between the (111) planes in th
 e ideal crystal lattice in the case of the theoretical strength and the si
 ze of the largest defect at the surface in the real crystal\, where nuclea
 tion of the surface-breaking crack with lowest critical fracture stress ta
 kes place. On the basis of the normalized model the defect size can be est
 imated. The critical fracture stress measured for shock waves propagating 
 along the Si{111} cleavage plane in the <11-2> direction was 4 GPa\, while
  the corresponding theoretical stress for tensile opening of the perfect l
 attice is 22 GPa. These values point to a defect size of about 9 nm at the
  surface of the silicon specimen that is responsible for impulsive failure
 . Thus\, this method allows the determination of the effective strength of
  real materials and the size of the defect responsible for failure. The la
 tter essentially depends on the manufacturing process (“engineering stre
 ngth”).\n[1] P. Hess\, A. M. Lomonosov\, A. P. Mayer\, Ultrasonics\, to 
 be published\n[2] A. M. Lomonosov\, P. Hess\, A. P. Mayer\, Appl. Phys. Le
 tt. 101\, 031904-1-4 (2012).
LOCATION:ME B3 31 http://plan.epfl.ch/?room=MEB331
STATUS:CONFIRMED
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