Athermal Regime Research Articles

Velocity and forced excitation effects on atomic friction force with deformable substrate

We have investigated the effect of scan velocity on atomic friction force in a Prandtl–Tomlinson model at finite temperature. We use a nonsinusoidal substrate potential with variable shape to capture surface features. The velocity dependence of atomic friction force is classified into two regimes: thermal and athermal regimes. We numerically find turning velocity between these two regimes, and its shape parameter dependence is determined. Furthermore, we have studied the torsional oscillation effect on a cantilever. We show that friction force can be reduced by controlling the frequency and the amplitude of the superimposed torsional oscillation at low scan velocity. The numerical calculations have shown that the substrate shape strongly affects the occurrence of resonances, so the torsional resonance frequency dependence of the shape parameter is established.

On the role of thermal heterogeneities on the rheology of MgO under conditions of the Earth’s lower mantle

The Earth’s mantle is characterised by large thermal heterogeneities associated with hot rising plumes and cold downwelling slabs. These lateral temperature variations in excess of 1000K may have a crucial influence on the rheology of mantle rocks. Here we use a numerical multiscale model that allows us to make predictions from first principles with no adjustable parameters on the deformation of MgO under the extreme conditions of mantle pressure, temperature and strain rate, in order to investigate the sensitivity of mantle viscosity to the temperature heterogeneities inferred from a global high resolution mantle circulation model. Our results show that under the very low strain rates of the mantle, MgO deforms mostly at low stresses (few tens of MPa) in an athermal regime, where the deformation is insensitive to both temperature and strain rate, leading to a very weak phase throughout much of the upper half of the lower mantle. In its lower half, the weak phase gives way to high material strength with thermally activated viscosities in the cold downwelling slabs, while much of the hot upwelling flow remains in the athermal regime, resulting in large lateral variations in the inferred material strength of MgO. Our results suggest the presence of large lateral viscosity variations in the deepest parts of the lower mantle, associated in particular with the graveyard of old subducted oceanic lithosphere.

Thermal and athermal crackling noise in ferroelastic nanostructures

The evolution of ferroelastic microstructures under external shear is determined by large-scale molecular dynamics simulations in two and three dimensions. Ferroelastic pattern formation was found to be almost identical in two and three dimensions, with only the ferroelastic transition temperature changing. The twin patterns generated by shear deformation depend strongly on temperature, with high wall densities nucleating under optimized temperature conditions. The dynamical tweed and mobile kink movement inside the twin walls is continuous and thermally activated at high temperatures, and becomes jerky and athermal at low temperatures. With decreasing temperature, the statistical distributions of dynamical tweed and kinks vary from a Vogel–Fulcher law to an athermal power-law distribution . During the yield event, the nucleation of needles and kinks is always jerky, and the energy of the jerks is power-law distributed. Low-temperature yield proceeds via one large avalanche. With increasing temperature, the large avalanche is thermally broken up into a multitude of small segments. The power-law exponents reflect the changes in temperature, even in the athermal regime.

Universal and non-universal features in coarse-grained models of flow in disordered solids.

We study the two-dimensional (2D) shear flow of amorphous solids within variants of an elastoplastic model, paying particular attention to spatial correlations and time fluctuations of, e.g., local stresses. The model is based on the local alternation between an elastic regime and plastic events during which the local stress is redistributed. The importance of a fully tensorial description of the stress and of the inclusion of (coarse-grained) convection in the model is investigated; scalar and tensorial models yield similar results, while convection enhances fluctuations and breaks the spurious symmetry between the flow and velocity gradient directions, for instance when shear localisation is observed. Besides, correlation lengths measured with diverse protocols are discussed. One class of such correlation lengths simply scale with the spacing between homogeneously distributed, simultaneous plastic events. This leads to a scaling of the correlation length with the shear rate as γ̇(-1/2) in 2D in the athermal regime, regardless of the details of the model. The radius of the cooperative disk, defined as the near-field region in which plastic events induce a stress redistribution that is not amenable to a mean-field treatment, notably follows this scaling. On the other hand, the cooperative volume measured from the four-point stress susceptibility and its dependence on the system size and the shear rate are model-dependent.

Twinning in Strained Ferroelastics: Microstructure and Statistics

The generation of functional interfaces such as superconducting and ferroelectric twin boundaries requires new ways to nucleate as many interfaces as possible in bulk materials and thin films. Materials with high densities of twin boundaries are often ferroelastics and martensites. In this review, we show that the nucleation and propagation of twin boundaries depend sensitively on temperature and system size. Sudden changes of the domain pattern manifest themselves as avalanches or “jerks” in the potential energy of the sample. At high temperatures, the change of the twin pattern is thermally activated; the probability P to find sudden energy changes of jerks E follows the Vogel–Fulcher statistics P(E) ~ exp (E/(T − T VF)), whereas the athermal regime at low temperatures corresponds to power-law statistics P(E) ~ E −e . We find that the complexity of the pattern is well characterized by the number of junctions between twin boundaries. Materials with soft bulk moduli have much higher junction densities than those with hard bulk moduli. Soft materials also show an increase in the junction density with diminishing sample size. The change of the complexity and the number density of twin boundaries represents an important step forward in the development of “domain boundary engineering,” where the functionality of the materials is directly linked to the domain pattern.

Ultrafast Laser Inscription in Soft Glasses: A Comparative Study of Athermal and Thermal Processing Regimes for Guided Wave Optics

The combination of ultrafast laser inscription and engineered soft glasses is enabling a new class of photonic devices offering long wavelength transparency, high nonlinearity, and optical gain. However, this field of research also possesses its own unique set of fabrication challenges, which range from the predictable, such as self‐focusing effects, material stress, and damage to the unexpected, such as photo‐induced index changes of different sign. In this article, we review many of the fabrication challenges surrounding ultrafast laser‐written soft‐glass photonics and highlight these by comparing and contrasting laser processing of common soft glasses in both the athermal and thermal writing regimes.

Athermal Martensites, Temperature-Time-Transformation Diagrams and Thermal Hysteresis: Monte Carlo Simulations of Strain Pseudospins

We explore the kinetics of a three-state strain pseudospin model for a square/rectangle ferroelastic transition, described by a temperature dependent hamiltonian without quenched disorder, using temperature quench Monte Carlo simulations. The model hamiltonian includes power law anisotropic long range interactions, which lock the domain walls in a symmetry breaking diagonal direction. In athermal parameter regime, there are fast conversions at the athermal transition temperature, but with delay tails above it, as in experiment. The conversion delay tails have a Vogel-Fulcher divergence at transition to austenite. The incubation delays and their insensitivity to elastic energy scales are attributed to entropy barriers. Temperature cycling shows hysteretic behavior in physical quantities.

Modelling the rheology of MgO under Earth’s mantle pressure, temperature and strain rates

Plate tectonics, which shapes the surface of Earth, is the result of solid-state convection in Earth's mantle over billions of years. Simply driven by buoyancy forces, mantle convection is complicated by the nature of the convecting materials, which are not fluids but polycrystalline rocks. Crystalline materials can flow as the result of the motion of defects--point defects, dislocations, grain boundaries and so on. Reproducing in the laboratory the extreme deformation conditions of the mantle is extremely challenging. In particular, experimental strain rates are at least six orders of magnitude larger than in nature. Here we show that the rheology of MgO at the pressure, temperature and strain rates of the mantle is accessible by multiscale numerical modelling starting from first principles and with no adjustable parameters. Our results demonstrate that extremely low strain rates counteract the influence of pressure. In the mantle, MgO deforms in the athermal regime and this leads to a very weak phase. It is only in the lowermost lower mantle that the pressure effect could dominate and that, under the influence of lattice friction, a viscosity of the order of 10(21)-10(22) pascal seconds can be defined for MgO.

Multiscale modeling of binary polymer mixtures: Scale bridging in the athermal and thermal regime

Obtaining a rigorous and reliable method for linking computer simulations of polymer blends and composites at different length scales of interest is a highly desirable goal in soft matter physics. In this paper a multiscale modeling procedure is presented for the efficient calculation of the static structural properties of binary homopolymer blends. The procedure combines computer simulations of polymer chains on two different length scales, using a united atom representation for the finer structure and a highly coarse-grained approach on the mesoscale, where chains are represented as soft colloidal particles interacting through an effective potential. A method for combining the structural information by inverse mapping is discussed, allowing for the efficient calculation of partial correlation functions, which are compared with results from full united atom simulations. The structure of several polymer mixtures is obtained in an efficient manner for several mixtures in the homogeneous region of the phase diagram. The method is then extended to incorporate thermal fluctuations through an effective χ parameter. Since the approach is analytical, it is fully transferable to numerous systems.

Effective Soft-Core Potentials and Mesoscopic Simulations of Binary Polymer Mixtures

Mesoscopic molecular dynamics simulations are used to determine the large scale structure of several binary polymer mixtures of various chemical architecture, concentration, and thermodynamic conditions. By implementing an analytical formalism, which is based on the solution to the Ornstein-Zernike equation, each polymer chain is mapped onto the level of a single soft colloid. From the appropriate closure relation, the effective, soft-core potential between coarse-grained units is obtained and used as input to our mesoscale simulations. The potential derived in this manner is analytical and explicitly parameter dependent, making it general and transferable to numerous systems of interest. From computer simulations performed under various thermodynamic conditions the structure of the polymer mixture, through pair correlation functions, is determined over the entire miscible region of the phase diagram. In the athermal regime mesoscale simulations exhibit quantitative agreement with united atom simulations. Furthermore, they also provide information at larger scales than can be attained by united atom simulations and in the thermal regime approaching the phase transition.

Spectral dependence of the ultrafast optical response of nonspherical gold nanoparticles

The extension of the discrete dipole approximation (DDA) model to the calculation of the ultrafast transient optical response of metal nanoparticles, whatever their shape, is presented. For this, the DDA is combined with the resolution of the Boltzmann equation for the conduction electrons within the low-perturbation limit and the Rosei model for the metal band structure. This approach thus takes into account the athermal regime for the electron gas. It is then illustrated in the cases of gold nanospheres and nanopeanuts. The spectral dependence of the ultrafast response is shown to be very sensitive to the particle morphology, the surface plasmon resonance enhancing the transient modification of the interband transitions. These results are used to interpret spectrally resolved pump-probe measurements performed on a dense nanoparticle assembly, demonstrating selective-excitation and selective-detection processes.

Influence of laser pulse characteristics on the hot electron contribution to the third-order nonlinear optical response of gold nanoparticles

The third-order nonlinear optical response of noble-metal nanoparticles embedded in a dielectric matrix depends on the particle intrinsic third-order susceptibility ${\ensuremath{\chi}}_{m}^{(3)}$. We propose a model which allows one to calculate the hot electron contribution ${\ensuremath{\chi}}_{\text{he}}^{(3)}$ to ${\ensuremath{\chi}}_{m}^{(3)}$ in the case of gold. This phenomenon stems from the modification of the conduction-electron distribution induced by an optical excitation, and is significant when picosecond or subpicosecond laser pulses are considered. We show, in the case of a weak perturbation, the importance of the athermal regime for pulse widths lower than about 1 ps. Applying this model to two different samples, we then highlight the strong influence of the linear optical properties of the material on the spectral dispersion of ${\ensuremath{\chi}}_{\text{he}}^{(3)}$. Finally, the variation in ${\ensuremath{\chi}}_{\text{he}}^{(3)}$ with intensity in the high excitation regime is discussed for picosecond pulses.

Theoretical coarse-graining approach to bridge length scales in diblock copolymer liquids

A microscopic theory for coarse graining diblock copolymers into dumbbells of interacting soft colloidal particles has been developed based on the solution of liquid-state integral equations. The Ornstein-Zernike equation is solved to provide a mesoscopic description of the diblock copolymer system at the level of block centers of mass, and at the level of polymer centers of mass. Analytical forms of the total correlation functions for block-block, block-monomer, and center-of-mass pairs are obtained for a liquid of structurally symmetric diblock copolymers as a function of temperature, density, chain length, and chain composition. The theory correctly predicts thermodynamically driven segregation of diblocks into microdomains as a function of temperature (chi parameter). The coarse-grained description contains contributions from density and concentration fluctuations, with the latter becoming dominant as temperature decreases. Numerical calculations for the block coarse-grained total correlation functions, as a function of the proximity of the system to its phase transition, are presented. Comparison with united atom molecular dynamics simulations are carried out in the athermal regime, where simulations and theory quantitatively agree with no need of adjustable parameters.

Grain Growth in Nanocrystalline Metal Thin Films under In Situ Ion-Beam Irradiation

Abstract In-situ observations in a transmission electron microscope (TEM) were used to study the microstructure evolution in metal Zr, Pt, Cu, and Au nanocrystalline thin films under ion-beam irradiation. Free-standing films were prepared by sputter deposition. Samples were irradiated in-situ at the Intermediate Voltage Electron Microscope (IVEM) at Argonne National Laboratory with Ar and Kr ions to fluences in excess of 1016 ion/cm2. As a result of irradiation, grain growth was observed in all samples using Bright Field (BF) imaging in the TEM. The average grain size increased monotonically with ion fluence until it reached a saturation value. Similarly to thermal grain growth, the ion-irradiation induced grain growth curves could be best fitted with curves of the type: Dn-D0n=KΦ. The irradiations were done at temperatures ranging from 20 to 773 K. The results suggest the existence of three regimes with respect to irradiating temperature: (i) a purely thermal regime, which appears to start above the bulk coarse-grained recrystallization temperature, (ii) a thermally assisted regime where thermal diffusion and irradiation effects combine to increase the rate of grain growth relative to that resulting from either of these mechanisms alone, and (iii) an athermal regime (low-temperature regime) where irradiation can by itself cause grain growth. The transition temperature between the athermal regime and the thermally assisted regime depends on the material, but is in the range 0.14–0.22 times the melting point. The influence of the ion type was also investigated on Zr-Fe irradiated with 600 keV Kr ions versus 600 keV Ar ions.

An efficient model for the analysis of fission gas release

This paper presents the fission gas release (FGR) model that has been developed at Framatome ANP and incorporated into its fuel rod performance code COPERNIC in order to accurately predict FGR into pressurized water reactor fuel rods under normal and off-normal operating conditions including UO 2, gadolinia and MOX fuels. The model is analytical, thus enabling fast and robust fuel rod calculations, a must within an industrial framework where safety evaluations may require the analyses of a full core and of a very large number of transients. Although the model is simple, it includes the most important FGR features: athermal, thermal, steady-state, and transient regimes, burst effect, rim formation, and MOX-type microstructure. The validation of the model covers 400 irradiated rods that include high burnups, high powers, short to long transients, and shows the quality of the prediction of the model in all types of conditions. As temperature is a key parameter that affects FGR, the COPERNIC thermal model is briefly described and its impact on fission gas released uncertainty is discussed.

Low temperature yield properties of two 7–9Cr ferritic/martensitic steels

Tensile properties of two 7–9Cr ferritic/martensitic steels have been investigated over the temperature range 77–293 K. Along with the temperature dependence, the strain rate sensitivity of the yield stress has been studied by performing strain rate jump tests. Activation volume and activation energy have been determined and used to identify the rate controlling mechanism of dislocation motion. It has been observed that the temperature and stress dependence of the activation parameters are both consistent with a Peierls mechanism below 190 K. Between 190 and 293 K, other mechanisms begin to operate, replacing the Peierls mechanism. At static strain rates, the athermal regime is reached slightly above room temperature.

The temperature-variant hardness response of duplex TBCs

The Vickers microhardness response of electron-beam physical vapour deposited (EB-PVD) and thermally sprayed duplex thermal barrier coatings has been investigated as a function of temperature. Hardness-temperature plots reveal a transition between a strongly temperature sensitive regime and an athermal regime, and this has been analysed using an activation energy approach. The behavioural transition was found to be ∼200 °C, with hardness falling rapidly above this temperature to ∼100 kgf mm −2 at 800 °C in each case. Post-annealing hardness-temperature response for the two coatings was very similar despite significant differences in as-received hardness and microstructure. For the temperature-sensitive regime, activation energies for plastic deformation have been calculated as 61 kJ mol −1 for EB-PVD and 79 kJ mol −1 for the sprayed coating. Possible deformation mechanisms are discussed, as is the relevance of our data to possible in-service performance.

Deformation behavior of a Ni−30Al−20Fe−0.05Zr intermetallic alloy in the temperature range 300 to 1300 K

The deformation behavior of an extruded Ni-30 (at. pct) Al−20Fe−0.05Zr intermetallic alloy was studied in the temperature range of 300 to 1300 K under initial tensile strain rates varying between about 10−6 and 2×10−3 s−1 and in constant load compression creep between 1073 and 1300 K. The deformation microstructures of the fractured specimens were characterized by transmission electron microscopy (TEM). Three deformation regimes were observed: Region I consisted of an athermal regime of low tensile ductility (less than 0.3 pct) occurring between 400 and 673 K, where the substructure consisted of slip bands in a few grains. Exponential creep was dominant in region II between 673 and 1073 K, where the substructure changed from a mixture of dislocation tangles, loops, and dipoles at 673 K to a microstructure consisting of subgrains and dislocation tangles at 973 K. The tensile ductility was generally about 2.0 to 2.5 pct below 980 K in this region. A significant improvement in tensile ductility was observed in region III, which occurred between 1073 and 1300 K. An analysis of the data suggests that viscous glide creep with a stress exponent,n, of about 3 and high-temperature dislocation climb withn≈4.5 where the two dominant creep mechanisms in this region depending on stress and temperature. The average activation energy for deformation in this region was about 310±30 kJ mol−1 for both processes. In this case, a transition from viscous glide creep to dislocation climb occurred when σ/E<1.7×10−4, where σ is the applied stress andE is the Young’s modulus.

Ion mixing of III-V compound semiconductor layered structures

Compositional disordering of III-V compound superlattice structures has received considerable attention recently due to its potential application for photonic devices. The conventional method to induce compositional disorder in a layered structure is to implant a moderate dose of impurity ions (∼1015/cm2) into the structure at room temperature, followed by a high-temperature annealing step (this process is referred to as IA here). Ion irradiation at room temperature alone does not cause any significant intermixing of layers. The subsequent high-temperature annealing step tends to restrict device processing flexibility. Ion mixing (IM) is capable of enhancing compositional disordering of layers at a rate which increases exponentially with the ion irradiation temperature. As a processing technique to planarize devices, ion mixing appears to be an attractive technology. In this work, we investigate compositional disordering in the AlGaAs/GaAs and the InGaAs/InP systems using ion mixing. We found that the ion mixing behavior of these two systems shows a thermally activated regime as well as an athermal regime, similar to that observed for metal-metal and metal-semiconductor systems. Ion mixing is observed to induce compositional disordering at significantly lower temperatures than that for the IA process. We have compared the two processes in terms of five parameters: (1) irradiation temperature, (2) dose dependence, (3) dose rate dependence, (4) annealing, and (5) ion dependence (including electrical effects and mass dependence). We found that the IM process is more efficient in utilizing the defects generated by ion irradiation to cause disordering. Both the physical mechanism of ion mixing and possible device implications will be discussed.

Ion mixing in Si/Ge layered structures

Ion mixing of semiconductor layered structures has been found to exhibit a thermally activated regime and an athermal regime, similar to that observed for metal-semiconductor and metal-metal systems. The kinetics of ion mixing in the Si/Ge system were investigated in this work; the emphasis was placed on the effects of impurities and the mismatch strain on the athermal mixing rate. It was found that the presence of H, Au, W, and O does not affect the ion mixing kinetics between Si and Ge. The athermal mixing rate, however, appears to be significantly affected by the mismatch strain in the layered structure. These results indicate that strain energy plays an important role in biasing the random walk process of mixing.