Antonio Rinaldi  Pedro Peralta  Cody Friesen  Dhiraj Nahar  Silvia Licoccia  Enrico Traversa  Karl Sieradzki 《Small (Weinheim an der Bergstrasse, Germany)》2010,6(4):528-536

The compressive plastic strength of nanosized single‐crystal metallic pillars is known to depend on their diameter D. Herein, the role of pillar height h is analyzed instead, and the suppression of the generalized crystal plasticity below a critical value h_CR is observed. Novel in situ compression tests on regular pillars as well as nanobuttons, that is, pillars with h < h_CR, show that the latter are much harder, withstanding stresses >2 GPa. A statistical model that holds for both pillars and buttons is formulated. Owing to their superhard nature, the nanobuttons examined here underline with unprecedented resolution the extrinsic effects—often overlooked—that naturally arise during testing when the Saint‐Venant assumption ceases to be accurate. The bias related to such effects is identified in the test data and removed when possible. Finally, continuous hardening is observed to occur under increasing stress level, in analogy to reports on nanoparticles. From a metrological standpoint the results expose some difficulties in nanoscale testing related to current methodology and technology. The implications of the analysis of extrinsic effects go beyond nanobuttons and extend to nano‐/microelectromechanical system design and nanomechanics in general.  相似文献   

    

Shiliang Wang  Yuehui He  Xiaosheng Fang  Jin Zou  Yong Wang  Han Huang  Pedro M. F. J. Costa  Min Song  Baiyun Huang  Chain T. Liu  Peter K. Liaw  Yoshio Bando  Dmitri Golberg 《Advanced materials (Deerfield Beach, Fla.)》2009,21(23):2387-2392

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Bomatí-Miguel O  Tartaj P  Morales MP  Bonville P  Golla-Schindler U  Zhao XQ  Veintemillas-Verdaguer S 《Small (Weinheim an der Bergstrasse, Germany)》2006,2(12):1476-1483

Passivated iron nanoparticles (10-30 nm) have been synthesized by laser pyrolysis of a mixture of iron pentacarbonyl and ethylene vapors followed by controlled oxidation. The nanoparticles show a well-constructed iron-iron oxide core-shell structure, in which the thickness and nature (structure similar to maghemite, gamma-Fe2O3) of the shell is found to be independent of the initial conditions. On the other hand, the composition of the core is found to change with the particle size from the alpha-Fe structure to a highly disordered Fe phase (probably containing C atoms in its structure). The dependence of the magnetic properties on the particle size, iron oxide fraction, and temperature was also investigated. In the case of smaller particles, the magnetic data indicate the existence at low temperature of a large exchange anisotropy field, the magnitude of which increases with decreasing temperature in correspondence with the freezing of magnetic moments in the oxide shell.  相似文献   

    

Pedro M. A. Areias  Ted Belytschko 《International journal for numerical methods in engineering》2005,62(3):384-415

A new formulation and numerical procedures are developed for the analysis of arbitrary crack propagation in shells using the extended finite element method. The method is valid for completely non‐linear problems. Through‐the‐thickness cracks in sandwich shells are considered. An exact shell kinematics is presented, and a new enrichment of the rotation field is proposed which satisfies the director inextensibility condition. To avoid locking, an enhanced strain formulation is proposed for the 4‐node cracked shell element. A finite strain plane stress constitutive model based on the logarithmic corotational rate is employed. A cohesive zone model is introduced which embodies the special characteristics of the shell kinematics. Stress intensity factors are calculated for selected problems and crack propagation problems are solved. Copyright © 2004 John Wiley & Sons, Ltd.  相似文献   

    

Sergei V. Kalinin  Brian J. Rodriguez  Albina Y. Borisevich  Arthur P. Baddorf  Nina Balke  Hye Jung Chang  Long‐Qing Chen  Samrat Choudhury  Stephen Jesse  Peter Maksymovych  Maxim P. Nikiforov  Stephen J. Pennycook 《Advanced materials (Deerfield Beach, Fla.)》2010,22(3):314-322

The plethora of lattice and electronic behaviors in ferroelectric and multiferroic materials and heterostructures opens vistas into novel physical phenomena including magnetoelectric coupling and ferroelectric tunneling. The development of new classes of electronic, energy‐storage, and information‐technology devices depends critically on understanding and controlling field‐induced polarization switching. Polarization reversal is controlled by defects that determine activation energy, critical switching bias, and the selection between thermodynamically equivalent polarization states in multiaxial ferroelectrics. Understanding and controlling defect functionality in ferroelectric materials is as critical to the future of oxide electronics and solid‐state electrochemistry as defects in semiconductors are for semiconductor electronics. Here, recent advances in understanding the defect‐mediated switching mechanisms, enabled by recent advances in electron and scanning probe microscopy, are discussed. The synergy between local probes and structural methods offers a pathway to decipher deterministic polarization switching mechanisms on the level of a single atomically defined defect.  相似文献   

    

Eun Chul Cho  Pedro H. C. Camargo  Younan Xia 《Advanced materials (Deerfield Beach, Fla.)》2010,22(6):744-748

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Myrta Rodriguez  Douglas C. Montgomery  Connie M. Borror 《Quality and Reliability Engineering International》2009,25(8):1045-1065

Efficient estimation of response variables in a process is an important problem that requires experimental designs appropriated for each specific situation. When we have a system involving control and noise variables, we are often interested in the simultaneous optimization of the prediction variance of the mean (PVM) and the prediction variance of the slope (PVS). The goal of this simultaneous optimization is to construct designs that will result in the efficient estimation of important parameters. We construct new computer‐generated designs using a desirability function by transforming PVM and PVS into one desirability value that can be optimized using a genetic algorithm. Fraction of design space (FDS) plots are used to evaluate the new designs and six cases are discussed to illustrate the procedure. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

    

Carlos J. S. Alves  Pedro R. S. Antunes 《International journal for numerical methods in engineering》2009,77(2):177-194

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Sanjay Mathur  Sven Barth  Ulf Werner  Francisco Hernandez‐Ramirez  Albert Romano‐Rodriguez 《Advanced materials (Deerfield Beach, Fla.)》2008,20(8):1550-1554

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E. Solórzano  M. A. Rodriguez‐Perez  J. A. de Saja 《Advanced Engineering Materials》2008,10(6):596-602

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