Liquid-like Motion Research Articles

Hidden structures: a driving factor to achieve low thermal conductivity and high thermoelectric performance.

The long-range periodic atomic arrangement or the lack thereof in solids typically dictates the magnitude and temperature dependence of their lattice thermal conductivity (κlat). Compared to crystalline materials, glasses exhibit a much-suppressed κlat across all temperatures as the phonon mean free path reaches parity with the interatomic distances therein. While the occurrence of such glass-like thermal transport in crystalline solids captivates the scientific community with its fundamental inquiry, it also holds the potential for profoundly impacting the field of thermoelectric energy conversion. Therefore, efficient manipulation of thermal transport and comprehension of the microscopic mechanisms dictating phonon scattering in crystalline solids are paramount. As quantized lattice vibrations (i.e., phonons) drive κlat, atomistic insights into the chemical bonding characteristics are crucial to have informed knowledge about their origins. Recently, it has been observed that within the highly symmetric 'averaged' crystal structures, often there are hidden locally asymmetric atomic motifs (within a few Å), which exert far-reaching influence on phonon transport. Phenomena such as local atomic off-centering, atomic rattling or tunneling, liquid-like atomic motion, site splitting, local ordering, etc., which arise within a few Å scales, are generally found to drastically disrupt the passage of heat carrying phonons. Despite their profound implication(s) for phonon dynamics, they are often overlooked by traditional crystallographic techniques. In this review, we provide a brief overview of the fundamental aspects of heat transport and explore the status quo of innately low thermally conductive crystalline solids, wherein the phonon dynamics is majorly governed by local structural phenomena. We also discuss advanced techniques capable of characterizing the crystal structure at the sub-atomic level. Subsequently, we delve into the emergent new ideas with examples linked to local crystal structure and lattice dynamics. While discussing the implications of the local structure for thermal conductivity, we provide the state-of-the-art examples of high-performance thermoelectric materials. Finally, we offer our viewpoint on the experimental and theoretical challenges, potential new paths, and the integration of novel strategies with material synthesis to achieve low κlat and realize high thermoelectric performance in crystalline solids via local structure designing.

Labile graphitic monolayers as a partition phase for containment of organic chemicals by graphene-based nanomaterials

Graphene-based nanomaterials (GBNMs) (e.g., graphene oxides and carbon nanotubes) display superior electronic and thermal conductivities and varying abilities to contain organic substances. This study sheds light to the idea that GBNMs behave as a dual sorbent, rather than a sole adsorbent, to extract nonionic organic solutes from water by both (competitive) adsorption and (noncompetitive) partition because of the solute interactions with various GBNM nanostructures formed by atomically-thin graphitic monolayers. Essential solute-sorption data with three model GBNMs from this research and similar data from the literature lead to a coherent view that labile graphitic monolayers in GBNMs undergo a liquid-like motion at room temperature to retain nonionic organic solutes by partition while structurally rigid graphitic clusters behave as adsorbents. Because the partition is noncompetitive, the GBNMs possessing high levels of labile graphene layers, as reflected by high BET surface areas, are capable of sequestering vastly higher levels of multiple organic solutes (especially, those of liquids) than conventional adsorbents, e.g., activated carbon (AC). Moreover, the postulated dual functionality of GBNMs makes sense of many otherwise puzzling phenomena, such as the highly concentration-dependent solute competitive effect with certain GBNMs and highly variable “adsorbed capacities” per unit surface area for different organic solutes with a GBNM versus those by a conventional adsorbent (e.g., graphite or AC).Graphical

Light-induced vacuum micromotors based on an antimony telluride microplate

Manipulating motion of microobjects with light is indispensable in various technologies. On solid interfaces, its realizations, however, are hampered by surface friction. To resolve this difficulty, light-induced elastic waves have been recently proposed to drive microobjects against friction. Despite its expected applicability for arbitrary optical-absorptive objects, the new principle has only been tested with microsized gold plates. Herein, we validate this principle using a new material and report directional and continuous movements of a two-dimensional topological insulator (Sb2Te3) plate on an untreated microfiber surface driven by nanosecond laser pulses. The motion performance of the Sb2Te3 plate is characterized by a scanning electron microscope. We observe that the motion velocity can be controlled by tuning the average power of laser pulses. Further, by intentionally increasing the pulse repetition rate and exploiting the low thermal conductivity of Sb2Te3, we examine the thermal effects on actuation and reveal the motion instability induced by formations of microbumps on Sb2Te3 surfaces due to the Marangoni effects. Moreover, as the formed microbumps are heated to viscoelasticity states, liquid-like motion featuring asymmetry in contact angles is observed and characterized, which expands the scope of light-induced actuation of microobjects.

Heat Conductor-Insulator Transition in Electrochemically Controlled Hybrid Superlattices.

Designing materials with ultralow thermal conductivity has broad technological impact, from thermal protection to energy harvesting. Low thermal conductivity is commonly observed in anharmonic and strongly disordered materials, yet a microscopic understanding of the correlation to atomic motion is often lacking. Here we report that molecular insertion into an existing two-dimensional layered lattice structure creates a hybrid superlattice with extremely low thermal conductivity. Vibrational characterization and ab initio molecular dynamics simulations reveal strong damping of transverse acoustic waves and significant softening of longitudinal vibrations. Together with spectral correlation analysis, we demonstrate that the molecular insertion creates liquid-like atomic motion in the existing lattice framework, causing a large suppression of heat conduction. The hybrid materials can be transformed into solution-processable coatings and used for thermal protection in wearable electronics. Our work provides a generic mechanism for the design of heat insulators and may further facilitate the engineering of heat conduction based on understanding atomic correlations.

Peristalsis-like migration of carbon-metabolizing catalytic nanoparticles

Translational and rotational motion of solid matter is normally driven by external physical forces. Here we report that under oxygen atmosphere inside an environmental transmission electron microscope, catalytic palladium nanoparticles underwent a self-propelled, peristalsis-like locomotion on a supporting substrate at a relatively low temperature. Surprisingly, the particles maintained crystalline interior and even conserved their initial crystal orientations during the dramatic liquid-like motion. Such “peristaltic” crystal migration with shape oscillation is found to be mediated by profuse surface diffusion, under chemical driving forces from palladium-catalyzed gaseous oxidation of a carbonaceous layer. These findings open a new avenue of efficient heterogeneous catalysis, and reveal emergent behavior that can arise out of an energy-metabolizing nano-system.

Effect of annealing on crystallization behavior in Cu15Zr85 amorphous film

The crystallization in Cu15Zr85 amorphous film was studied by using molecular dynamics simulation in a wide annealing temperature region. The effect of annealing temperature on crystallization behavior was evaluated. The degree of crystallization increases firstly and then decreases with the increasing of annealing temperature in Cu15Zr85 amorphous film. A comparison of interior and Surface-A components shows that Surface-A component has higher crystallization rate at low annealing temperature and lower degree of crystallization at high annealing temperature because of faster atomic diffusion. Structural evolution of surface monolayer is unique during annealing. No discernable crystal-like structure is found after annealed in the surface monolayer by directly observing the atomic structure. Surface monolayer with hexagon cluster has low degree of order at high annealing temperature because of the fast liquid-like diffusive motion and improved glass-forming ability.

Molecular dynamics approach observes rigid-body and liquid-like motion in protein crystals

Focusing on covariance of alpha-carbon position displacements provides clues for obtaining more accurate protein crystal structures.

Diffusion mechanism of non-interacting Brownian particles through a deformed substrate

We study the diffusion mechanism of non-interacting Brownian particles through a deformed substrate. The study is done at low temperature for different values of the friction. The deformed substrate is represented by a periodic Remoissenet–Peyrard potential with deformability parameter s. In this potential, the particles (impurity, adatoms…) can diffuse. We ignore the interactions between these mobile particles consider them merely as non-interacting Brownian particles and this system is described by a Fokker–Planck equation. We solve this equation numerically using the matrix continued fraction method to calculate the dynamic structure factor S(q,ω). From S(q,ω) some relevant correlation functions are also calculated. In particular, we determine the half-width line λ(q) of the peak of the quasi-elastic dynamic structure factor S(q,ω) and the diffusion coefficient D.Our numerical results show that the diffusion mechanism is described, depending on the structure of the potential, either by a simple jump diffusion process with jump length close to the lattice constant a or by a combination of a jump diffusion model with jump length close to lattice constant a and a liquid-like motion inside the unit cell. It shows also that, for different friction regimes and various potential shapes, the friction attenuates the diffusion mechanism. It is found that, in the high friction regime, the diffusion process is more important through a deformed substrate than through a non-deformed one.

Protective liquid motions

Solar Cells Hybrid organic-inorganic perovskites used in solar cells exhibit long carrier lifetimes compared with all-inorganic perovskites. This is in spite of the higher amount of disorder in the former that would normally increase unproductive carrier recombination. Zhu et al. used time-resolved photoluminescence and optical Kerr effect spectroscopies to compare the hybrid organic-inorganic perovskites CH3NH3PbBr3 and CH(NH2)2PbBr3 and the all-inorganic perovskite CsPbBr3 (see the Perspective by Ziffer and Ginger). The hybrid materials exhibited hot fluorescence indicative of liquid-like organic cation motion, which may protect charge carriers through solvation or formation of large polarons. Science , this issue p. [1409][1]; see also p. [1365][2] [1]: /lookup/doi/10.1126/science.aaf9570 [2]: /lookup/doi/10.1126/science.aai7682

Behavior of Channel Water and CF2H Side-Chain Terminal Groups in Swollen Nafion Polymer Electrolyte Membranes after Thermal Treatment.

Polymer electrolyte membranes (PEMs) are representative systems for the study of the proton conduction mechanism and water dynamics in nanopores/channels. Our 1H nuclear magnetic resonance data for Nafion PEMs, which are subjected to thermal degradation and then swollen in water, indicate that (1) water is present next to the side chains even after the removal of the SO3H groups, (2) longer heat-treatment depletes more SO3H groups and produces more CF2H groups, (3) the water near the side chains allows for the liquid-like motion of the CF2H groups, and (4) the motion correlates well with the content and dynamics of water in the channels. As the thermal degradation progresses, the Nafion membranes lose their ionic and hydrophilic nature due to the conversion of CF2SO3H groups to CF2H groups. In addition, our results demonstrate that increasing channel hydrophobicity leads to increased water dynamics in the channels.

Brownian diffusion in a triple-well potential

ABSTRACTIn this work we present a general theory for diffusion mechanism of Brownian particle submitted to a symmetric periodic triple-well potential. The kinetics description is done by the Fokker-Planck equation, which is resolved numerically using the Matrix Continued Fraction Method, in order to calculate some important correlation functions. The half-width λ(q) at half maximum of the quasi-elastic peak of dynamic structure factor S(q, ω) and the diffusion coefficient D are studied in the high friction regime and low temperature for different form of triple-well potential. Our numerical results of half-width λ(q), show that the diffusion process in triple-well potential can be described by a superposition of both simples hopping and liquid-like motion when the ratio Δ of two potential barriers V1 and V2 is less than one (Δ < 1) and by the longs jumps when Δ tends towards one. For some values of ratio of potential barriers, the diffusion coefficient results show that the intermediates potential barriers accelerate the diffusion process.

Dynamics of asymmetric non-polymeric binary glass formers-A nuclear magnetic resonance and dielectric spectroscopy study.

We study a dynamically asymmetric binary glass former with the low-Tg component m-tri-cresyl phosphate (m-TCP: Tg = 206 K) and a spirobichroman derivative as a non-polymeric high-Tg component (Tg = 382 K) by means of (1)H nuclear magnetic resonance (NMR), (31)P NMR, and dielectric spectroscopy which allow component-selectively probing the dynamics. The entire concentration range is covered, and two main relaxation processes with two Tg are identified, Tg 1 and Tg 2. The slower one is attributed to the high-Tg component (α1-process), and the faster one is related to the m-TCP molecules (α2-process). Yet, there are indications that a small fraction of m-TCP is associated also with the α1-process. While the α1-relaxation only weakly broadens upon adding m-TCP, the α2-relaxation becomes extremely stretched leading to quasi-logarithmic correlation functions at low m-TCP concentrations-as probed by (31)P NMR stimulated echo experiments. Frequency-temperature superposition does not apply for the α2-process and it reflects an isotropic, liquid-like motion which is observed even below Tg 1, i.e., in the matrix of the arrested high-Tg molecules. As proven by 2D (31)P NMR, the corresponding dynamic heterogeneities are of transient nature, i.e., exchange occurs within the distribution G(lnτα 2). At Tg 1 a crossover is found for the temperature dependence of (mean) τα 2(T) from non-Arrhenius above to Arrhenius below Tg 1 which is attributed to intrinsic confinement effects. This "fragile-to-strong" transition also leads to a re-decrease of Tg 2(cm - TCP) at low concentration cm - TCP, i.e., a maximum is observed in Tg 2(cm - TCP) while Tg 1(cm - TCP) displays the well-known plasticizer effect. Although only non-polymeric components are involved, we re-discover essentially all features previously reported for polymer-plasticizer systems.

Muoniated spin probes in the discotic liquid crystal HHTT: Rapid electron spin relaxation in the hexagonal columnar and isotropic phases

Avoided level crossing muon spin resonance (ALC-μSR) spectroscopy was used to study radicals produced by the addition of the light hydrogen isotope muonium (Mu) to the discotic liquid crystal (DLC) 2,3,6,7,10,11-hexahexylthiotriphenylene (HHTT). Mu adds to the secondary carbon atoms of HHTT to produce a substituted cyclohexadienyl radical, whose identity was confirmed by comparing the measured hyperfine coupling constants with values obtained from DFT calculations. ALC-μSR spectra were obtained in the isotropic (I), hexagonal columnar (Col(h)), helical (H), and crystalline (Cr) phases. In the I and Col(h) phases the radicals, which are incorporated within the stacks of HHTT molecules as isolated paramagnetic defects, undergo extremely rapid electron spin relaxation, on the order of a hundredfold faster than in the H or Cr phases. The electron spin relaxation rate increases significantly with increasing temperature and appears to be caused by the liquidlike motion within the columns, which modulates the overlap between the π system of the radical and the π systems of the neighboring HHTT molecules, and hence, the hyperfine coupling constants. Rapid electron spin relaxation should occur for any π radical incorporated within the columns of a DLC, which may limit the utility of DLCs for future spin-based technologies.

Coverage-Dependent Collective Diffusion of a Dense Pb Wetting Layer on Si(111)

The dynamics of a dense Pb wetting layer on the Si(111) surface is studied in the framework of a generalized Frenkel-Kontorova model. Instead of the typical diffusion by random hopping processes, a liquidlike collective motion of the Pb atoms within the dense wetting layer is revealed to give rise to ultrafast kinetics of the wetting layer even at low temperatures. A kinetic Monte Carlo simulation including this collective spreading mechanism of the dense wetting layer quantitatively reproduces the experimental observations.

Micro-channel development and hydrogen adsorption properties in templated microporous carbons containing platinum nanoparticles

Ordered microporous carbons containing dispersed platinum nanoparticles were fabricated and chosen as suitable models to investigate micro-structure development and hydrogen transport properties of zeolite-templated carbons. X-ray photoelectron spectroscopy analysis revealed that the enhanced heat of adsorption is related to the narrow micro-channels templated from the zeolite and the presence of certain C O groups on the carbon. The lack of a well-defined and intense rotational transition line and the persistent broad H 2 recoil spectrum in neutron scattering results suggests a distribution of binding sites. Most interestingly, hydrogen diffusion occurs on two time scales, consisting of a fast liquid-like jump diffusion on the timescale of picoseconds along with an even faster bulk-like diffusion. The liquid-like motion is characterized by a diffusion constant of (2.1 ± 0.3) × 10 −8 m 2/s with an activation energy of ca. 77 K; both values indicate somewhat lower mobility than similar dynamics of H 2 on nanotubes, activated carbon XC-72, or Grafoil, yet greater mobility than that of bulk liquid. These unusual characteristics for hydrogen in carbons are believed to arise from the network of narrow pores in this zeolite-templated image of the zeolite. In fact, the diffusion constants of the templated carbons are extremely similar to those measured for zeolite 13X.

Diffusion and viscous flow in bulk glass forming alloys

We review radiotracer diffusion and isotope measurements in bulk glass forming alloys from the glassy state to the equilibrium melt and compare diffusion and viscous flow. In the glassy as well as in the deeply supercooled state below the critical temperature T c, where the mode coupling theory predicts a freezing-in of liquid-like motion, very small isotope effects indicate a highly collective hopping mechanism. Not only in the glassy state but also in the supercooled state below T c the temperature dependence of diffusion is Arrhenius-like with an effective activation enthalpy. A clear decoupling takes place between the diffusivities of the individual components of the alloys and between time scales related to diffusive transport and viscous flow. While the component decoupling is small for the smaller components a vast decoupling of more than 4 orders of magnitude is observed in Pd–Cu–Ni–P alloys between the diffusivity of the large majority component Pd and of the smaller components at the glass transition temperature T g. The diffusivities of all components merge close to the critical temperature T c of mode coupling theory. Above T c, the onset of liquid-like motion is directly evidenced by a gradual drop of the effective activation energy. This strongly supports the mode coupling scenario. The isotope effect measurements show atomic transport up to the equilibrium melt to be far away from the regime of uncorrelated binary collisions. For Pd, in contrast to the behavior of single component molecular glass formers, the Stokes–Einstein equation even holds in the entire temperature range below T c over at least 14 orders of magnitude. Apparently, the majority component Pd forms a slow subsystem in which the other elements move fast. Rearrangement of the Pd atoms thus determines the viscous flow behavior. The decoupling of atomic mobility seems to arise from a complex interplay between chemical short order and atomic size effects that gets more pronounced on approaching the glass transition temperature. The ability of the bulk glass forming alloys to form a slow subsystem in the liquid state appears to be a key to the understanding of their excellent glass forming properties.

Movement mechanisms of solid-like and liquid-like motion states in a vibrating granular bed

In this study we analyze the granular motion and the variation in energy on the surface of a vibrating bed as well as the flow regime inside the bed. The variation in the granular kinetic energy at the free surface of the bed under different vibration conditions produces the following motion states: solid-like state, transition region and liquid-like state. The granular kinetic energy is measured and found to increase slowly until the vibration acceleration reaches the first specific dimensionless vibration acceleration Γ I. The granular kinetic energy increases sharply as the vibration acceleration continues to increase after exceeding Γ I until reaching the second specific dimensionless vibration acceleration Γ I I. The granular kinetic energy increases with the vibration acceleration afterwards. The movement mechanism inside the granular bed is transformed from particle reorganization to surface convection as the bed state transfers from a solid-like state to a liquid-like state.

Diffusion in Bulk Glass Forming Alloys– from the Glass to the Equilibrium Melt

Since the discovery of bulk metallic glasses there has been considerable research effort on these systems, in particular with respect to mass transport. Now the undercooled melt between the melting temperature and the caloric glass transition temperature, which has not been accessible before due to the rapid onset of crystallization, can be investigated and theories can be tested. Here we report on radiotracer diffusion measurements in metallic bulk-glass-forming Pd-Cu-Ni-P alloys. Serial sectioning was performed by grinding and ion-beam sputtering. The time, temperature as well as the mass dependence, expressed in terms of the isotope effect E, of Co-diffusion were investigated. The Co isotope effect measurements, which have never been carried out near Tc in any material, show atomic transport up to the equilibrium melt to be far away from the hydrodynamic regime of uncorrelated binary collisions. In the glassy state as well as in the deeply supercooled state below the critical temperature Tc, where the mode coupling theory predicts a freezing-in of liquid-like motion, the experimentally determined very small isotope effects indicate a highly collective hopping mechanism involving some ten atoms. Below Tc the temperature dependence shows Arrhenius-type behavior with an effective activation enthalpy of 3.2 eV. Above Tc the onset of liquid-like motion is evidenced by a gradual drop of the effective activation energy and by the validity of the Stokes-Einstein equation, which is found to break down below Tc. Although having strong covalent bonding tendencies, Phosphorous diffusion is only slightly slower than Co diffusion, indicating that it does not determine the overall viscosity below Tc. The Stokes-Einstein equation is presently tested for other constituents of the alloy.

Diffusion in bulk-metallic glass-forming Pd–Cu–Ni–P alloys: From the glass to the equilibrium melt

Since the discovery of bulk-metallic glasses there has been considerable research effort on these systems, in particular with respect to mass transport. Now the undercooled melt between the melting temperature and the caloric glass transition temperature, which has not been accessible before due to the rapid onset of crystallization, can be investigated and theories can be tested. Here we report on radiotracer diffusion measurements in metallic bulk-glass-forming Pd–Cu–Ni–P alloys. Serial sectioning was performed by grinding and ion-beam sputtering. The time, temperature as well as the mass dependence, expressed in terms of the isotope effect E, of Co-diffusion were investigated. In the glassy state as well as in the deeply supercooled state below the critical temperature Tc, where the mode coupling theory predicts a freezing-in of liquid-like motion, the experimentally determined very small isotope effects indicate a highly collective hopping mechanism involving some ten atoms. Below Tc the temperature dependence shows Arrhenius-type behavior with an effective activation enthalpy of 3.2eV. Above Tc the onset of liquid-like motion is evidenced by a gradual drop of the effective activation energy and by the validity of the Stokes–Einstein equation, which is found to break down below Tc. This strongly supports the mode coupling scenario. The Stokes–Einstein equation is presently tested for other constituents of the alloy. The Co isotope effect measurements, which have never been carried out near Tc in any material, show atomic transport up to the equilibrium melt to be far away from the hydrodynamic regime of uncorrelated binary collisions.

1H and 2H NMR relaxation study on the phase transitions of(NH4)3H(SO4)2 and (ND4)3D(SO4)2 single crystals

T1,T1ρ andT2 forthe 1H and 2H nuclei in (NH4)3H(SO4)2 and (ND4)3D(SO4)2 single crystals grown using the slow evaporation method were measured for phases I, II, III, IV and V.The 1H T1,T1ρ,and T2 values were found to exhibit different trends in phases II and III:T1,T1ρ and T2 for 1H do not change significantly near the phase transition at 265 K, whereasnear 413 K they change discontinuously. We conclude that theNH4+ and H(SO4)2− ions do not play an important role in the III–II phase transition, but do playimportant roles in the II–I phase transition. The liquid-like nature of the1H T1ρ andT2 above413 K is indicative of the destruction and reconstruction of hydrogen bonds. Moreover, the phase transitionsof the (NH4)3H(SO4)2 crystal are accompanied by changes in the molecular motion of the(NH4)+ ions. The variations with temperature of the2H T1 and T2 of (ND4)3D(SO4)2 crystals are not similar to those observed for the1H T1 andT2. Our comparison ofthe results for (NH4)3H(SO4)2 and(ND4)3D(SO4)2 crystals indicatesthe following: the 1H T1ρ andT2 of the(NH4)+ andH(SO4)2− ionsabove TC1 are characteristic of fast, liquid-like motion, which is not the case for(ND4)3D(SO4)2; andthe 2H T1 ofD(SO4)2− in(ND4)3D(SO4)2 is longerthan the 2H T1 of(ND4)+ in contrast tothe results for (NH4)3H(SO4)2 crystals.