90° domain dynamics and relaxation in thin ferroelectric/ferroelastic films

We investigated the dynamics and relaxation of 90° domains in 60-nm-thick lead-zirconium titanate (PbZr0.3 T0.7 O3) films, with enhanced piezoresponse force microscopy. We show that under opposite electric fie ld, ferroelectric domains are reversibly switched while ferroelastic domains reorganize in a nonreversible way. Moreover, we show that the relaxation-time constant of 90° domains is two orders of magnitude shorter than for the previously reported 180° domains relaxation. Furthermore, we demonstrate the influence of geometry and scale on the relaxation process. Finally, we propose a relaxation mechanism for ferroelastic-ferroelectric systems, with implications for devices based on these materials. © 2010 The American Physical Society.

Estimating material elasticity by spherical indentation load-relaxation tests on viscoelastic samples of finite thickness

A two-step viscoelastic spherical indentation method is proposed to compensate for 1) material relaxation and 2) sample thickness. In the first step, the indenter is moved at a constant speed and the reaction force is measured. In the second step, the indenter is held at a constant position and the relaxation response of the material is measured. Then the relaxation response is fit with a multi-exponential function which corresponds to a three-branch general Maxwell model. The relaxation modulus is derived by correcting the finite ramp time introduced in the first step. The proposed model takes into account the sample thickness, which is important for applications in which the sample thickness is less than ten times the indenter radius. The model is validated numerically by finite element simulations. Experiments are carried out on a 10% gelatin phantom and a chicken breast sample with the proposed method. The results for both the gelatin phantom and the chicken breast sample agree with the results obtained from a surface wave method. Both the finite element simulations and experimental results show improved elasticity estimations by incorporating the sample thickness into the model. The measured shear elasticities of the 10% gelatin sample are 6.79 and 6.93 kPa by the proposed finite indentation method at sample thickness of 40 and 20 mm, respectively. The elasticity of the same sample is estimated to be 6.53 kPa by the surface wave method. For the chicken breast sample, the shear elasticity is measured to be 4.51 and 5.17 kPa by the proposed indentation method at sample thickness of 40 and 20 mm, respectively. Its elasticity is measured by the surface wave method to be 4.14 kPa. © 2011 IEEE.

Breathers and thermal relaxation as a temporal process: A possible way to detect breathers in experimental situations

Breather stability and longevity in thermally relaxing nonlinear arrays is investigated under the scrutiny of the analysis and tools employed for time series and state reconstruction of a dynamical system. We briefly review the methods used in the analysis and characterize a breather in terms of the results obtained with such methods. Our present work focuses on spontaneously appearing breathers in thermal Fermi-Pasta-Ulam arrays but we believe that the conclusions are general enough to describe many other related situations; the particular case described in detail is presented as another example of systems where three incommensurable frequencies dominate their chaotic dynamics (reminiscent of the Ruelle-Takens scenario for the appearance of chaotic behavior in nonlinear systems). This characterization may also be of great help for the discovery of breathers in experimental situations where the temporal evolution of a local variable (like the site energy) is the only available/measured data. © 2005 American Institute of Physics.

Relaxation and fracture characteristics in Co-based amorphous alloys

In this article a study of the fracture characteristics of Co66Fe4Mo2Si16B12 amorphous ribbon in the as-quenched state and after relaxation is presented. In the as-quenched state, the morphology of the crack surface shows a 'vein pattern' structure that corresponds to a large amount of plastic flow. After relaxation the surface morphology of the crack shows that when the temperature of the thermal annealing increases the plastic flow involved in the crack decreases. In the as-quenched state dynamic fracture characteristics (crack branching and stress wave induced crack) have been observed. These dynamic characteristics have not been observed in the relaxed samples but in the samples annealed at 250 °C for 20 min apart from the main crack, a crack along the width of the ribbon has been observed. © 2006 Elsevier B.V. All rights reserved.

Metastability mechanisms in thin film transistors quantitatively resolved using post-stress relaxation of threshold voltage

A new approach is presented to resolve bias-induced metastability mechanisms in hydrogenated amorphous silicon (a-Si:H) thin film transistors (TFTs). The post stress relaxation of threshold voltage (V(T)) was employed to quantitatively distinguish between the charge trapping process in gate dielectric and defect state creation in active layer of transistor. The kinetics of the charge de-trapping from the SiN traps is analytically modeled and a Gaussian distribution of gap states is extracted for the SiN. Indeed, the relaxation in V(T) is in good agreement with the theory underlying the kinetics of charge de-trapping from gate dielectric. For the TFTs used in this work, the charge trapping in the SiN gate dielectric is shown to be the dominant metastability mechanism even at bias stress levels as low as 10 V.

Local stress relaxation and shear banding in a dry foam under shear.

We have developed a realistic simulation of 2D dry foams under quasistatic shear. After a short transient, a shear-banding instability is observed. These results are compared with measurements obtained on real 2D (confined) foams. The numerical model allows us to exhibit the mechanical response of the material to a single plastication event. From the analysis of this elastic propagator, we propose a scenario for the onset and stability of the flow localization process in foams, which should remain valid for most athermal amorphous systems under creep flow.