Mean Square Displacements Of Atoms Research Articles

Effect of water activity on the mechanical glass transition and dynamical transition of bacteria-solute systems

This study examined the mechanical glass transition and dynamical transition of bacteria (Cronobacter sakazakii)-solute (glycerol and glucose) systems. From the effect of water activity (aw) on the mechanical relaxation, the aw at which mechanical glass transition occurs at 25 °C (a typical ambient temperature) was determined as 0.225 for the glycerol-added sample and 0.540 for the glucose-added sample. These values were lower than that (0.667) reported previously for non-added sample. The effect of temperature on the mean square displacement of atoms within the samples was investigated by incoherent elastic neutron scattering. The aw at which dynamical transition occurs at 25 °C was determined as 0.156 for the glycerol-added sample and 0.475 for the glucose-added sample. These values were lower than that (0.675) reported previously for non-added sample. According to the Couchman-Karasz model, the Tg for the bacteria plasticized by the additive and that for the bulk additive were obtained.

High-efficiency TOPCon solar cell with superior P + and P++ layer via one-step processing

The boron diffusion process in the front field of N-type tunnel oxide passivated contact (TOPCon) solar cells is crucial for PN junction formation and the creation of a selective emitter. This study presents a theoretical model of boron diffusion in silicon using molecular dynamics. The research examines the mean square displacement and diffusion coefficient of boron atoms at varying temperatures, confirming their diffusion behavior. The simulations indicate predominant boron diffusion in the z-direction within the silicon matrix, with the diffusion depth being temperature dependent. The optimal temperature range for boron diffusion in silicon is identified as 950 °C to 1050 °C. Using boron-doped silicon paste and boron trichloride as dopants, thermal diffusion experiments were conducted to fabricate the front-field PN junction (p+ layer) and selective emitter (p++ layer) by one step. Subsequent processing and performance evaluation were performed on a production line. Experimental findings reveal a decrease in boron diffusion at higher temperatures, reduced sheet resistance, increased doping concentration, and deeper junction formation. The ideal boron concentration in the p+ layer is 8.68 × 1018 atom/cm3 with a depth of 0.53 μm, while the p++ layer is 2.35 × 1019 atom/cm3 and 0.82 μm. The efficiency of the optimized TOPCon + cell production line reaches up to 25.17 %, marking an improvement of 0.23 % over the standard cell production line. This research contributes to elucidating the mechanism of boron diffusion and offers insights for enhancing the efficiency of TOPCon solar cells.

Atomic-Scale Dislocation Structure Evolution and Crystal Ordering Analysis of Melting and Crystallization Microprocesses in Laser Powder Bed Melting of γ-TiAl Alloys

Laser Powder Bed Fusion (LPBF) technology exhibits significant advantages in the manufacturing of components with high dimensional accuracy and intricate internal cavities. However, due to the inherent room-temperature brittleness and high-temperature gradient induced by the laser forming process, the LPBF fabrication of γ-TiAl alloy is often accompanied by the initiation and propagation of defects. The aim of this study is to investigate the forming process of γ-TiAl alloy by the LPBF method through molecular dynamics simulation, and to explain the microparticle arrangement and displacement evolution of the melting and crystallization processes, thus elucidating the link between the variations in the laser process parameters and defect generation during microscopic laser heating. The results show that during the melting process, the peaks of the radial distribution function (RDF) decrease rapidly or even disappear due to laser heating, and the atomic disorder is increased. Although subsequent cooling crystallization reorders the atomic arrangement, the peak value of the RDF after crystallization is still 19.3% lower than that of the original structure. By setting different laser powers (200–800 eV/ps) and scanning speeds (0.2–0.8 Å/ps), the effects of various process parameters on microforming and defect evolution are clarified. When the laser power increases from 200 to 400 eV/ps, the stable value of atomic displacement rises from 6.66 to 320.87, while it rises from 300.54 to 550.14 when the scanning speed is attenuated from 0.8 to 0.4 Å/ps, which indicates that, compared with the scanning speed, the atomic mean-square displacements are relatively more sensitive to the fluctuation of laser power. Dislocation analysis reveals that a higher laser power significantly increases the cooling rate during the forming process, which further aggravates the generation and expansion of dislocation defects.

Effects of Pd atom vibration in the Sr–Te octahedral interstitial space in Zintl compound SrPdTe on lattice anharmonicity and thermoelectric properties

Based on first-principles calculations, the current study deeply explores the thermoelectric properties of the Zintl compound SrPdTe. We found that the anharmonic vibration of Pd atoms plays an important role in the quartic anharmonic effect and the temperature dependence of the thermal conductivity. In the crystalline structure, Sr atoms form octahedra with eight surrounding Te atoms, while Pd atoms are located in the gaps between the octahedra. This structure makes the strong atomic mean square displacement of Pd atoms the main factor leading to the ultralow thermal conductivity. The study also reveals the effects of phonon frequency renormalization and four-phonon scattering on heat transfer performance. Even considering the spin–orbit coupling effect, multiple secondary valence band tops maintain the power factor of the material at high temperatures, providing a potential opportunity for achieving excellent thermoelectric performance.

Sc diffusion in HCP high entropy alloys

Sc diffusion in hexagonal close packed Al15Hf25Sc10Ti25Zr25 and Al5Hf25Sc20Ti25Zr25 high-entropy alloys is measured using the CERN facility for implantation of the 46Sc isotope. The Sc diffusivity is found to be slower than that of previously measured Ti in both alloys. Ab initio-informed calculations are used to quantify the interplay of chemical complexity and local atomic strains. The mean-square atomic displacements are shown to contribute to the observed “anti-sluggish” diffusion behavior in these high-entropy alloys.

The effect of composite curing agent ratio on the photooxidative aging behavior of epoxy asphalt based on molecular dynamics simulation

In order to explore the effect of composite curing agent ratio on the photooxidative aging behavior of epoxy asphalt with an acidic composite curing system, three curing agent ratios of epoxy asphalt were prepared to obtain different cross-linking network densities. The prepared epoxy asphalt was subjected to photooxidation aging for 240 h at 60 ℃. Fourier transform infrared (FTIR) and elemental analysis tests were conducted to determine the evolution of chemical components during photooxidative aging. Based on the chemical components before and after aging, molecular models of photooxidation aged epoxy asphalt with different curing agent ratios were constructed. The mean square displacement (MSD) of oxygen atoms and the oxygen uptake of each component were calculated to study the diffusion behavior of oxygen atoms and the oxidation degree of each component under various cross-linking networks. Glass transition temperature (Tg) and interaction energy were determined to characterize the thermodynamic properties and interaction between asphalt phase and epoxy resin phase of epoxy asphalt before and after aging. The radial distribution function (RDF) was determined to simulate the distribution of asphaltene and epoxy resin crosslinked networks in different cross-linking networks. The results show that the formation of carbonyl groups, alcohol hydroxyl groups, and the fracture of methylene groups was observed during the photooxidative aging process of epoxy asphalt. The rise in the proportion of anhydride curing agents improved the stability of the chemical components in epoxy asphalt under photooxidation aging. High cross-linking density and methylene fracture weakened the diffusion ability of oxygen atoms in epoxy asphalt due to a reduction in free volume. In addition, photooxidation aging had a negative impact on the low-temperature performance and interaction between asphalt phase and epoxy resin phase of epoxy asphalt. However, a high content of acid anhydride alleviated the damage of photooxidation aging to low-temperature performance. Photooxidation aging developed the polar functional groups of epoxy asphalt, promoting the stacking of asphaltene under a high content of dicarboxylic acid curing agent, while breaking the stacking of asphaltene under a high content of acid anhydride curing agent.

Depicting Defects in Metallic Glasses by Atomic Vibrational Entropy.

Due to the chaotic structure of amorphous materials, it is challenging to identify defects in metallic glasses. Here we tackle this problem from a thermodynamic point of view using atomic vibrational entropy, which represents the inhomogeneity of atomic contributions to vibrational modes. We find that the atomic vibrational entropy is correlated to the vibrational mean-square displacement and polyhedral volume of atoms, revealing the critical role of vibrational entropy in bridging dynamics, thermodynamics, and structure. On this method, the local vibrational entropy obtained by coarse-graining the atomic vibrational entropy in space can distinguish more effectively between liquid-like and solid-like atoms in metallic glasses and establish the correlation between the local vibrational entropy and the structure of metallic glasses, offering a route to predict the plastic events from local vibrational entropy. The local vibration entropy is a good indicator of thermally activated and stress-driven plastic events, and its predictive ability is better than that of the structural indicators.

Temperature effects on thermodynamic properties of rare-earth ytterbium

Thermodynamic quantities (including Debye temperature and frequency, atomic mean-square displacement, specific heats at constant volume and constant pressure, linear thermal expansion coefficient, and total entropy) of face-centered cubic ytterbium metal have been studied by means of the statistical moment method in statistical mechanics. This theoretical approach allows us to calculate these thermodynamic quantities taking into account the anharmonicity of lattice vibrations. Numerical calculations for ytterbium have been conducted in temperature range from 0 K to 1100 K using Lennard-Jones potential to describe the interaction between ytterbium atoms. Our derived results are compared with those of mean-field potential approach as well as previous experimental data showing the good agreement. We have shown that the atomic mean-square displacement contains the zero-point vibration (quantum effect) at low temperature. At high temperature it changes linearly with respect to temperature. Furthermore, our theoretical calculations have also pointed out the anharmonic contribution to specific heat at constant volume of ytterbium metal is negative with temperature. This research presents an effective statistical approach to study temperature effects on thermodynamic quantities of materials.

Rattling-induced suppression of thermal transport in cubic In2O3 with Sn-Ga diatomic defect

We present the first principles study of cubic In2O3 with a diatomic defect composed of a Sn atom substituting the In atom at the b-site and a Ga atom embedded in the nearest c-site (structural vacancy) with lattice positions according to the Wyckoff notations. Structural, electronic, phononic and thermal properties were investigated within density functional theory formalism. The lattice anharmonicity effects were taken into account for all possible three-phonon scattering processes. The phonon transport was considered within the Peierls-Boltzmann transport equation with relaxation time approximation. In the relaxed lattice, a strong rearrangement of the initial positions of the atoms in the defect vicinity was revealed, which primarily manifests itself in the displacement of the Sn atom toward another interstitial site. Thus, a cage is formed around the defect by 12 O and 12 In atoms. The calculations of elastic constants and mean square displacements of cage region atoms showed the rattling-like behavior of the Sn atom. Bader charge analysis and electron localization function allowed a deeper understanding and explanation of such behavior. Phonon energy spectra as compared to In2O3 and In2O3:(Sn) demonstrated flattening of phonon branches with spatial localization of phonon modes. They also revealed a decrease in average group velocities of phonons, including those of acoustic type, the presence of avoided-crossing features in the low energy range, and an increase of available phase space for three-phonon scattering. Accounting for all these vibrational features due to defect atoms resulted in a thermal conductivity drop at room temperature by more than seven times compared to In2O3.

Temperature-dependent yield stress of single crystals of non-equiatomic Cr-Mn-Fe-Co-Ni high-entropy alloys in the temperature range 10-1173 K

The temperature-dependent yield strengths of [1¯23]-oriented single crystals of four quinary non-equiatomic Cr-Mn-Fe-Co-Ni high-entropy alloys (HEAs) were investigated at temperatures of 10-1173 K. They exhibit similar trends with a rapid decrease of the critical resolved shear stress (CRSS) from 10 K to 300 K, followed by a much slower rate of decrease with increasing temperature and a plateau above 873 K. The HEAs with the highest and lowest Cr contents exhibit the highest and lowest yield strengths respectively at 10 K, but almost identical yield strengths at 300 K. Thus, at cryogenic temperatures, the magnitude of the temperature dependence of yield strength is positively correlated with Cr content. All the non-equiatomic HEAs also exhibit similar dislocation structures with planar arrays of smoothly-curved dislocations without any preferred orientations. Their stacking fault energy (SFE) decreases with increasing Cr content. An empirical expression is proposed to describe the composition dependence of SFE in the Cr-Mn-Fe-Co-Ni quinary system. A new route for optimizing the alloy design of non-equiatomic Cr-Mn-Fe-Co-Ni alloys in terms of strength and ductility through the combination of mean-square atomic displacement (MSAD) and SFE is discussed based on the results obtained.

Atomic mean square displacement study of the bond breaking mechanism of energetic materials before explosive initiation.

Understanding and predicting the bond breaking mechanism of energetic materials before explosion initiation is one of the huge challenges in explosion science. By means of the mean square displacement of the atom from the equilibrium position and theoretical bond breaking tensile change of the chemical bond, we establish a new criterion to judge whether the chemical bond is broken. Further, α-RDX is used as the verification object to verify the accuracy of this model. We obtained an initial decomposition temperature of 434-513 K for α-RDX at 0 GPa, and the initial bond breaking type was N-NO2. Finally, based on this model, we discussed in detail the breaking of chemical bonds of solid nitromethane near the detonation pressure. We think that the high temperature and high pressure caused by the shock wave may break all the chemical bonds of nitromethane near the detonation pressure.

Thermal stability of equiatomic FePt nanoclusters of different habitus with ordered and disordered structure

In this work, the thermal stability of FePt nanoclusters of different habitus is considered using the molecular dynamics method with the multiparticle interaction potential MEAM-2NN. Three variants of determining the temperatures of structural disorder/melting of these nanoclusters are presented: by the normalised per atom total energy of clusters from temperature, with approximation of curves by linear sections, the intersection points of which were used to determine the disorder temperatures; the second by the mean-square displacement of atoms; the third by visualisation of cross sections of atomic planes perpendicular to the face <100>. It is shown that the melting of the cluster occurs layer by layer from the surface to the volume. The temperatures of structural disorder of nanoclusters determined by different methods are in good agreement with each other.

Multi-methodological study of temperature trends in Mössbauer effect in Sn

We have performed a multi-methodological theoretical study of impact of thermal vibrations on the Mössbauer effect in the tetragonal β-phase of tin. We have seamlessly combined (i) atomic-scale numerical data in the form of mean square displacements of Sn atoms determined by quantum-mechanical calculations, (ii) continuum-level thermodynamic modeling based on the quasi-harmonic approximation and (iii) theoretical analysis of Mössbauer effect resulting in the prediction of temperature dependence of Mössbauer factor. The computed results were compared with our Mössbauer and X-ray experimental data. We show that classical theoretical approaches based on simplistic Debye model of thermal vibrations of solids can be nowadays replaced by exact ab initio calculations of individual thermal vibrations. While both Debye and our approach slightly deviate from the experimental data, our suggested methodology bears promises for future improvement and a better agreement with measurements, i.e. the prospect that the over-simplified Debye model may not offer.

Revisiting the melting curves of vanadium and niobium metals under pressure

The Lindemann criterion is adjusted to revisit melting curves of vanadium and niobium metals under pressure thanks to the statistical moment method (SMM) in statistical mechanics. The equation-of-state and atomic mean-square displacement in SMM approach are applied to implement numerical calculations for vanadium and niobium metals up to 250 GPa and 150 GPa, respectively. The obtained theoretical melting curves are compared with results of previous experiments and molecular dynamics (MD) simulations where possible to check the theory. We expose that our theoretical melting line of vanadium is overall in good agreement with previous results determined by in situ X-ray diffraction (XRD) and diffuse scattering signals using synchrotron XRD together with shock experiments up to pressure 250GPa. For niobium, our melting curve follows very well with those of recent experimental XRD data as well as ab initio Z method up to 124 GPa. However, we also observe the difference of classical MD simulation results from SMM calculations at pressure below 2.5 GPa. This research enriches the high-pressure melting point database as well as provides a relatively simple approach with low computational cost to estimate high-pressure melting points of materials.

Effect of water activity on the mechanical glass transition and dynamical transition of bacteria

The purpose of this study was to clarify the glass-transition behavior of bacteria (Cronobacter sakazakii) as a function of water activity (aw). From the water sorption isotherm (298 K) for C. sakazakii, monolayer water content and monolayer aw were determined to be 0.0724 g/g-dry matter and 0.252, respectively. Mechanical relaxation was investigated at 298 K. In a higher aw range of over 0.529, the degree of mechanical relaxation increased with an increase in aw. From the effect of aw on the degree of mechanical relaxation, the mechanical awc (aw at which mechanical glass transition occurs at 298 K) was determined to be 0.667. Mean-square displacement of atoms in the bacteria was investigated by incoherent elastic neutron scattering. The mean-square displacement increased gradually with an increase in temperature depending on the aw of samples. From the linear fitting, two or three dynamical transition temperatures (low, middle, and high Tds) were determined at each aw. The low-Td values (142–158 K) were almost independent from aw. There was a minor effect of aw on the middle Td (214–234 K) except for the anhydrous sample (261 K). The high Td (252–322 K) largely increased with the decrease in aw. From the aw dependence of the high Td, the dynamical awc was determined to be 0.675, which was almost equivalent to the mechanical awc. The high Td was assumed to be the glass-transition temperature (Tg), and anhydrous Tg was estimated to be 409 K. In addition, molecular relaxation time (τ) of the bacteria was calculated as a function of aw. From the result, it is suggested that the progress of metabolism in the bacterial system requires a lower τ than approximately 6 × 10−5 s.

Structural and mass transport properties of liquid ytterbium in the temperature range 1123 K–1473 K

We have studied the structural and atomic transport properties of liquid f-shell Yb in the temperature range 1123 K–1473 K. Pair interactions between atoms are derived using a local pseudopotential. The potential parameters are fitted to the phonon dispersion curve at room temperature. The local pseudopotential used in the present study is computationally more efficient with only three parameters, and it is found to be transferable to the liquid phase without changing the parameters. Since the various computed properties agree with reported theoretical and experimental findings, the adopted fitting scheme is justified. As a significant outcome of the study, we find that (i) the melting in Yb is governed by the Lindemann’s law, (ii) the mass transport mechanism obeys the Arrhenius law, (iii) the role of the three-particle correlation function in deriving the velocity autocorrelation function is small, (iv) the mean-square atomic displacement is more sensitive to the choice of interaction potential than the other bulk properties, and (v) liquid Yb does not show liquid–liquid phase transition within the studied temperature range. Further, due to the good description of the structural and mass transport properties, we propose that Yb remains divalent at reduced density.

Debye temperature of iron nanoparticles: Finite-size effects in the scaling regime

The Debye temperature ${\mathrm{\ensuremath{\Theta}}}_{\mathrm{D}}$ characterizes the vibrations of a solid and marks the transition between quantized and classical behaviors of nuclear motion. While thermodynamical theories suggest that for nanoparticles ${\mathrm{\ensuremath{\Theta}}}_{\mathrm{D}}$ should be lower than the bulk limit and increase with increasing nanoparticle size, various experimental measurements have reported intriguing variations with size, including values above the bulk limit and possibly decreasing with size. In this paper we have theoretically determined the Debye temperature of iron nanoparticles of up to about 7 nm diameter at the atomistic level of detail, using complementary approaches based on the equilibrium heat capacity or the mean-square atomic displacement. Both methods consistently indicate steady finite-size effects in the scaling regime, with no evidence for values higher than the bulk or varying in opposite ways, but they also produce marked quantitative differences. Further comparison with the melting temperature ${T}_{m}$ indicates that the two quantities correlate with each other through ${T}_{m}\ensuremath{\propto}{\mathrm{\ensuremath{\Theta}}}_{\mathrm{D}}^{\ensuremath{\alpha}}$, but with an exponent $\ensuremath{\alpha}$ close to 3 that deviates from the expected value of 2 that classical thermodynamical theories predict based on pure bulk ingredients.

Large negative thermal expansion in GdFe(CN)6 driven by unusual low-frequency modes

Understanding the negative thermal expansion (NTE) mechanism is of great importance. In this work, we consider the new NTE compound GdFe(CN)6 (αv = −34.2×10-6 K-1) as a case study to investigate the NTE mechanism from the perspective of the lattice vibrational dynamics. The atomic mean-square displacements suggest that the NTE of GdFe(CN)6 comes from the strong tension effect induced by the transverse vibrations of the atomic –Fe–CN–Gd– linkages, with the largest contribution given by N atoms. Lattice dynamics calculations show that three low-frequency optical modes at about 50 cm-1 show the largest negative Grüneisen parameters thus providing the largest contribution to the NTE. The existence of these unusual low-frequency vibrational modes can be ascribed to the presence of GdN6 trigonal prisms in the framework structure of GdFe(CN)6.

Development of the temperature-dependent interatomic potential for molecular dynamics simulation of metal irradiated with an ultrashort pulse laser

Laser ablation is often explained by a two-temperature model (TTM) with different electron and lattice temperatures. To realize a classical molecular dynamics simulation of the TTM, we propose an extension of the embedded atom method to construct an interatomic potential that is dependent on the electron temperature. This method is applied to copper, and its validity is demonstrated by comparison of several physical properties, such as the energy–volume curve, phonon dispersion, electronic heat capacity, ablation threshold, and mean square displacement of atoms, with those of finite-temperature density functional theory.

Static and dynamic components of Debye-Waller coefficients in the novel cubic polymorph of low-temperature disordered Cu2ZnSnS4.

Cu2ZnSnS4 (CZTS) is an attractive material for sustainable photovoltaics and thermoelectrics, and several properties originate from its marked polymorphism. High-energy mechanical alloying is found to lead to a disordered phase that possesses a sphalerite-like cubic structure. This is investigated in detail with the aid of laboratory and synchrotron radiation X-ray diffraction, Raman spectroscopy, electron microscopy and ab initio molecular dynamics. The disordered cubic polymorph is preserved below 663 K. With thermal treatments above 663 K, the tetragonal kesterite phase forms, used here as a reference for structural and microstructural features. Particular attention is paid to the stacking arrangement: a significant fraction of twin faults was found in the disordered cubic samples, which then progressively annealed with domain growth and with the transition to the ordered tetragonal phase. This study also focuses on Debye-Waller coefficients, which were found to be considerably larger for the disordered cubic than the tetragonal sample. Indeed, disorder leads to an ∼1 Å2 upward shift through the temperature range 100-700 K, a feature confirmed by ab initio calculations, which points to a particularly high contribution from disordered Sn cations. This supports the general understanding that structural disorder introduces a temperature-independent static contribution to the atomic mean-square displacement. Debye-Waller coefficients are found to be a good measure of this disorder, known to have a critical effect on transport properties.