Phonon Density Of States Curves Research Articles

Differential study on the thermal–physical properties of metal and its oxide nanoparticle-formed nanofluids: Molecular dynamics simulation investigation of argon-based nanofluids

Abstract The influence of nanoparticle shape, volume fraction, and temperature on the thermal properties of nanofluids plays a pivotal role in engineering applications. However, there remains a considerable lack of systematic research comprehensively considering these factors to study the similarities and differences in the thermal properties of nanofluids composed of metals and their oxides and to conduct in-depth analyses of their internal mechanisms and characteristics. In this study, molecular dynamics simulations were conducted, employing reversing perturbation non-equilibrium molecular dynamics and non-equilibrium molecular dynamics methods. The thermal conductivity and viscosity of Al–Ar and Al2O3–Ar nanofluids were thoroughly investigated under the various influencing factors. Results reveal that under identical conditions, the thermal conductivity of Al–Ar nanofluid surpasses that of Al2O3–Ar nanofluid, exemplified by values such as 0.1832 W/m K (Al–Ar, 1.5%, cylinder, 86 K) versus 0.17745 W/m K (Al2O3–Ar, 1.5%, cylinder, 86 K). Furthermore, the viscosity of Al–Ar nanofluid is lower than that of Al2O3–Ar nanofluid, demonstrated by values such as 0.0004882 Pa S (Al–Ar nanofluid, 86 K, 2.5%, platelets) compared to 0.008975 Pa S (Al2O3–Ar nanofluid, 86 K, 2.5%, platelets). Subsequently, this study analyzed the difference in thermal conductivity between the two nanofluids from the perspective of microscale interface heat conduction by comparing the phonon density of states curves of Al, Ar, and Al2O3 in the two nanofluids for overlap. Subsequently, through radial distribution function analysis, the viscosity difference between Al–Ar and Al2O3–Ar nanofluids is explained based on nanofluid–solid interface and microstructural considerations. This research addresses the comprehensive lack of comparative studies on the thermal properties of nanofluids formed by metals and their oxides. The internal mechanisms underlying the thermal property differences of nanofluids formed by metals and their oxides were revealed from a microscopic perspective, which holds significant implications for the engineering applications of nanofluids.

Investigation of nanocomposite PNIPAm‐g‐graphene with pre‐lower critical solution temperature rapid thermal response function for chip adaptive cooling: A molecular dynamics and computational fluid dynamics simulations

AbstractTemperature‐sensitive hydrogels have been widely used for rapid adaptive cooling in electronic device thermal management with promising applications. However, existing temperature‐sensitive hydrogels can only regulate the flow in the chip cooling system after the ambient temperature reaches their lower critical solution temperature (LCST). Before reaching LCST, effective rapid heat dissipation for electronic chips is not achievable. This study aims to develop a temperature‐sensitive hydrogel that can provide assisted adaptive cooling for electronic chips before reaching its LCST. This requires the hydrogel to have a thermal conductivity far surpassing existing hydrogel materials. Using the temperature‐sensitive hydrogel PNIPAm and graphene molecules as base materials, this research utilized molecular dynamics simulations to graft graphene molecules onto PNIPAm molecules in different ways, resulting in the temperature‐sensitive hydrogel material PNIPAm‐g‐graphene. Non‐equilibrium molecular dynamics (NEMD) was employed to calculate the thermal conductivity of this material under different temperature conditions. The results indicate that the thermal conductivity of PNIPAm‐g‐graphene can reach up to 1.95474 W/m K (graphene grafted at CH3 functional group, temperature at 280 K). Compared to the thermal conductivity of PNIPAm under the same conditions (0.45 W/m K), the increase in thermal conductivity is significant, demonstrating excellent thermal conductivity compared to PNIPAm. Subsequently, this study analyzed the underlying mechanisms of different thermal conductivities in materials obtained by grafting graphene molecules at different points using the method of overlap in Phonon Density of States Curves (PDOS) from the perspective of interfacial thermal conduction. Finally, through computational fluid dynamics (CFD) simulations, this study investigates the chip's adaptive cooling performance with PNIPAm‐g‐graphene material. The results show that, compared to traditional temperature‐sensitive hydrogels, PNIPAm‐g‐graphene can achieve efficient adaptive cooling of chip hotspots before the cooling fluid temperature reaches its LCST value. This finding is significant for the field of chip cooling. The study proposes a new method for rapid, adaptive cooling of chip hotspots and explores its feasibility from the perspectives of molecular dynamics and CFD simulation. It holds importance in the thermal management of electronic devices and the rapid adaptive cooling of electronic chips.

Vibrational dynamics in the undercooled liquid of ultra-fragile metallic glasses

The lattice vibrational dynamics in ultra-fragile Pt57Cu23P20 and Pt60Cu20P20 glasses were investigated using in situ inelastic neutron scattering to measure the evolution of the phonon density of states (DOS) through the glass transition, into the undercooled liquid phase, and through crystallization. The phonon DOS curves are remarkably similar for the undercooled liquids and the amorphous solids. The vibrational entropy was obtained with these phonon DOS curves, and changes in vibrational entropy caused by the glass transition are less than 0.05kB/atom. For these highly fragile metal/metalloid glasses, the excess vibrational entropy of the undercooled liquid is about 0.5–1.5 J mol−1 K−1, compared to 0.1 J mol−1 K−1 reported earlier for stronger Cu–Zr glasses. Our work suggests that, consistent with our previous report for less fragile metallic glasses, the fraction of the total excess entropy that is vibrational is small, and the discontinuity in excess entropy at the glass transition is mainly due to configurational entropy.

First-principles calculations to investigate structural, electronic and phonon properties of sodium bromide (NaBr) and sodium iodide (NaI) crystals

In this paper, the first-principles study is performed to calculate the structural, electronic and phonon properties of alkali halides NaBr and NaI. The calculations for structural and electronic properties have been carried out using the full-potential linearized augmented plane-wave (FP-LAPW) method that is employed in the WIEN2k code, whereas the phonon properties are studied using the Quantum Espresso program based on density-functional theory within plane-wave fundamental sets and pseudo-potentials approaches. The calculation of structural, electronic band properties were carried out using generalized gradient approximation (GGA), local-density approximation (LDA) and a combination of modified Becke-Johnson exchange potential with LDA and GGA (TB-mBJ-GGA/LDA) for exchange-correlation potential. The calculated structural parameters are in good agreement with experimental results. In our electronic calculations, the obtained bandgaps are underestimated by LDA and GGA functionals compared to empirically derived results, which is an expected result. However, the calculated bandgaps by the modified Becke-Johnson exchange potential are in perfect agreement with the experiment. Finally, the dynamic properties of NaBr and NaI compounds were investigated. Both phonon dispersion and phonon density of states curves were calculated for compounds along with the high symmetry directions in the Brillouin region. The dynamical properties of NaBr and NaI crystals are analyzed and discussed.

Variations in the thermal conductivity of La2Zr2O7 and Gd2Zr2O7 with variable La/Gd concentrations

The phonon density of states and thermal conductivity of La2Zr2O7 and Gd2Zr2O7 are tuned to get materials with improved thermal insulation properties. In this study, pure and doped models of La2Zr2O7 and Gd2Zr2O7 were developed and simulated based on density functional theory. Doping Gd at La sites in La2Zr2O7 was observed to modify the band structure. The density of states analysis showed that Gd d states were introduced in the band gap. With increasing Gd concentration, the band gap of La2Zr2O7 was initially populated and then, a clear band gap was observed at high Gd/La ratios. Smoothness in the phonon density of states curves was observed with an increase in Gd concentration. However, irregular phonon density of states curves was obtained for the Gd2Zr2O7 system. Among the simulated models, the La16-5Gd5Zr16O56 system with Gd/La ratio of 0.454 provided minimum thermal conductivity.

Phase transition, elasticity, phonon spectra, and superconductive properties of equiatomic TiZr, TiHf, and ZrHf alloys at high pressure: Ab initio calculations

The pressure-induced behaviors of structural transformation, elasticity, phonon, and superconductivity of TiZr, TiHf, and ZrHf alloys are investigated by employing ab initio method based on the density-functional theory. Our obtained equilibrium structural parameters and the pressure-included phase transition sequence (α→ω→β) accord well with available experimental and previous theoretical results. Based upon our calculated elastic constants, we have indicated that the α and ω phases at 0 K and 0 GPa are mechanically stable while the β phase is unstable. The phonon and phonon density of states curves results for these three alloys show that the α and ω phases are dynamically stable at ambient conditions while the β phases can be stabilized through compressing or heating, in accordance with what have been found in Zr and Hf elements. The superconducting transition temperatures and the related parameters of TiZr, TiHf, and ZrHf alloys are also calculated in wide range of pressure.

Understanding the thermodynamic properties of 20% CeO2 stabilized ZrO2 coatings with atomistic modeling and simulations

To understand the improved stability and reduced thermal conductivity of 20% CeO2 stabilized ZrO2 coatings compared to pure zirconia, different Ce doped ZrO2 systems are modeled. The Ce insertion in ZrO2 lattice elongates the lattice parameters and the elongation is proportional to the Ce doping concentration. Moreover, the cell volume linearly increases with the Ce doping level. X-ray diffraction analysis confirmed the Zr0.84Ce0.16O2 phase, describing the substitutional Ce doped at Zr sites. Ce doping modified the electronic band structure of ZrO2 without creating any isolated states in the band gap. The Ce4Zr12O32 modeled system with Ce doping concentration of 8.33% provided the stable phonon density of states curves compared to the pure and doped systems. Ce doping reduced the thermal conductivity of ZrO2. Increasing Ce doping level further reduced the thermal conductivity reaching the maximum reduction at 8.33% doping concentration. Further increase in the Ce level has no effective reduction in the thermal conductivity values. The reduced thermal conductivity is attributed to the enhanced phonon scattering due to the substitutional Ce4+ dopant and optimal doping concentration. The experimental observation could be satisfactorily explained by the calculations results.

Lattice dynamics of Ru2FeX (X = Si, Ge) Full Heusler alloys

In present work, the lattice dynamics of Ru2FeX (X = Si, Ge) full Heusler alloys are investigated using density functional theory (DFT) within generalized gradient approximation (GGA) in a plane wave basis, with norm-conserving pseudopotentials. Phonon dispersion curves and phonon density of states are obtained using first-principles linear response approach of density functional perturbation theory (DFPT) as implemented in Quantum ESPRESSO code. Phonon dispersion curves indicates for both Heusler alloys that there is no imaginary phonon in whole Brillouin zone, confirming dynamical stability of these alloys in L21 type structure. There is a considerable overlapping between acoustic and optical phonon modes predicting no phonon band gap exists in dispersion curves of alloys. The same result is shown by phonon density of states curves for both Heusler alloys. Reststrahlen band for Ru2FeSi is found smaller than Ru2FeGe.

Electronic structure and vibrational entropies of fcc Au-Fe alloys

Phonon density of states (DOS) curves were measured on alloys of face-centered-cubic (fcc) Au-Fe using nuclear resonant inelastic x-ray scattering (NRIXS) and inelastic neutron scattering (INS). The NRIXS and INS results were combined to obtain the total phonon DOS and the partial phonon DOS curves of Au and Fe atoms from which vibrational entropies were calculated. The main effect on the vibrational entropy of alloying comes from a stiffening of the Au partial phonon DOS with Fe concentration. Force constants were calculated from first principles for several compositions and show a local stiffening of Au-Au bonds close to Fe atoms. The calculated phonon DOS curves reproduce the experimental trend. The stiffening is attributed to two main effects comparable in magnitude: (i) an increase in electron density in the free-electron-like states and (ii) stronger sd hybridization.

Effect of anharmonicity on the phonon density of states and specific heat of a monoatomic, one-dimensional crystal lattice

We analyze the lattice equation of motion involving terms up to third order in lattice displacement. The phenomenological arguments suggest that the force constant ${D}_{1}$ of the quadratic term must always be positive and the force constant ${B}_{1}$ of the cubic term may take either positive or negative value. The criterion for stability of the lattice provides constraint on the relative magnitudes of the three force constants. We solve the equation of motion using root mean-square spatial fluctuation approximation and obtain the seminonperturbative dispersion relation both for positive and negative ${B}_{1}$. The nature of phonon density of states curves for positive ${B}_{1}$ show some close resemblance with the experimental observations. At very low temperature, the specific heat of this system to leading order in large positive ${B}_{1}$ varies as square root of temperature and it obeys Debye's $T$ law in one dimension for small negative ${B}_{1}$. At very high temperature, the specific heat may fall below or above its classical value depending on the relative magnitudes of ${B}_{1}$ and ${D}_{1}$ for ${B}_{1}>0$ and it always falls above its classical value for ${B}_{1}<0$. The lattice model with positive ${B}_{1}$ emerges as a good candidate for description of a monoatomic crystal.

Vibrational entropies of phases of Co3V measured by inelastic neutron scattering and cryogenic calorimetry

Abstract The difference in heat capacity of the LI2 and hP24 phases of Co3V was measured by calorimetry at temperatures from 80 to 320 K. Inelastic neutron scattering measurements were performed on Co3V with the LI2, hP24 and fcc phases at elevated temperatures, and the neutron-energy-loss spectra were converted into phonon density-of-states (DOS) curves. Heat capacities calculated from the LI2 and hP24 DOS curves were approximately consistent with the calorimetry data. The phonon DOS curves showed a significant dependence on temperature, although not the temperature dependence of the simple Gruneisen approximation. The thermal expansion of the hP24 phase was measured and was interpreted adequately from the temperature dependence of the phonon DOS. By interpreting our cryogenic calorimetry data with a harmonic model, we estimate the difference in vibrational entropy of the LI2 and hP24 phases to be (0.12±0.03)k B atom−1 at low temperatures, with the LI2 phase having the larger vibrational entropy. From th...

Phonons in superconducting Nb-Zr alloys: Experiment and theory

Measurements of the phonon density of states curves F(ν) for Nb, Nb75Zr25 and Nb20Zr80 are presented, obtained from inelastic neutron scattering experiments. For comparison theoretical F(ν) curves have been calculated microscopically. A significant softening with increasing Zr content is observed.