Kiarash Gordiz

Thermal rectification in multi-walled carbon nanotubes: A molecular dynamics study

Using nonequilibrium molecular dynamics simulations, we show if a multi-walled carbon nanotube (MWNT) is heated asymmetrically, an evident thermal rectification can be detected. We attribute the observed rectification to the asymmetric radial thermal transport between constructing layers in MWNT. The underlying physics is explained by calculating temperature distribution of MWNT layers and phonon power spectrum. Thermal rectification in this carbon nanotube based thermal rectifier does not diminish by increasing the system size, and exists in a wide range of temperatures. These results open a door in the applicability of MWNTs in nanoscale engineering of thermal transport devices.

Molecular dynamics simulation of the specific heat capacity of water-Cu nanofluids

This paper presents molecular dynamics (MD) modeling for calculating the specific heat of nanofluids containing copper nanoparticles. The Cu nanoparticles with 2-nm diameter were considered to be dispersed in water as base liquid. The MD modeling procedure presented and implemented to calculate the specific heat of nanofluids with volume fractions of 2 to 10%. Obtained results show that the specific heat capacity of Cu-water nanofluids decreases gradually with increasing volume concentration of nanoparticles. The simulation results are compared with two existing applied models for prediction of the specific heat of the nanofluid. The obtained specific heat results from the MD simulation and the prediction from the thermal equilibrium model for calculating specific heat of nanofluids exhibit good agreement and the other simple mixing model fails to predict the specific heat capacity of Cu-water nanofluids particularly at high volume fractions.

Phonon Transport at Crystalline Si/Ge Interfaces: The Role of Interfacial Modes of Vibration

We studied the modal contributions to heat conduction at crystalline Si and crystalline Ge interfaces and found that more than 15% of the interface conductance arises from less than 0.1% of the modes in the structure. Using the recently developed interface conductance modal analysis (ICMA) method along with a new complimentary methodology, we mapped the correlations between modes, which revealed that a small group of interfacial modes, which exist between 12–13 THz, exhibit extremely strong correlation with other modes in the system. It is found that these interfacial modes (e.g., modes with large eigen vectors for interfacial atoms) are enabled by the degree of anharmonicity near the interface, which is higher than in the bulk, and therefore allows this small group of modes to couple to all others. The analysis sheds light on the nature of localized vibrations at interfaces and can be enlightening for other investigations of localization.

Thermal rectification in pristine-hydrogenated carbon nanotube junction: A molecular dynamics study

Using non-equilibrium molecular dynamics method, we investigate thermal rectification (TR) in hybrid pristine carbon nanotube (PCNT) and hydrogenated carbon nanotube (HCNT) structures. The interface thermal resistance of the junction is dependent on the direction of thermal transport, leading to TR. We show that by selecting nanotubes of smaller diameters, and/or increasing the hydrogen coverage of HCNT, the TR can be amplified. The observed TR does not decrease by increasing the system length, which presents PCNT/HCNT system as a promising thermal rectifier at room temperature.

Ensemble averaging vs. time averaging in molecular dynamics simulations of thermal conductivity

In this report, we compare time averaging and ensemble averaging as two different methods for phase space sampling in molecular dynamics (MD) calculations of thermal conductivity. For the comparison, we calculate thermal conductivities of solid argon and silicon structures, using equilibrium MD. We introduce two different schemes for the ensemble averaging approach and show that both can reduce the total simulation time as compared to time averaging. It is also found that velocity rescaling is an efficient mechanism for phase space exploration. Although our methodology is tested using classical MD, the approaches used for generating independent trajectories may find their greatest utility in computationally expensive simulations such as first principles MD. For such simulations, where each time step is costly, time averaging can require long simulation times because each time step must be evaluated sequentially and therefore phase space averaging is achieved through sequential operations. On the other han...

Phonon transport at interfaces between different phases of silicon and germanium

Current knowledge and understanding of phonon transport at interfaces are wholly based on the phonon gas model (PGM). However, it is difficult to rationalize the usage of the PGM for disordered materials, such as amorphous materials. Thus, there is essentially no intuition regarding interfaces with amorphous materials. Given this gap in understanding, herein we investigated heat conduction at different crystalline and amorphous Si/Ge interfaces using the recently developed interface conductance modal analysis method, which does not rely on the PGM and can therefore treat an interface with a disordered material. The results show that contrary to arguments based on lower mean free paths in amorphous materials, the interface conductances are quite high. The results also show that the interfacial modes of vibration in the frequency region of 12–13 THz are so important that perturbing the natural vibrations with velocity rescaling heat baths (i.e., in non-equilibrium molecular dynamics simulations) affects the...

Examining the Effects of Stiffness and Mass Difference on the Thermal Interface Conductance Between Lennard-Jones Solids

To date, the established methods that describe thermal interface conductance (TIC) and include mode-level dependence have not included anharmonicity. The current intuition is therefore based on the behavior in the harmonic limit, whereby the extent of overlap in the bulk phonon density of states (DoS) (e.g., frequency overlap) dictates the TIC and more frequency overlap leads to higher TIC. Here, we study over 2,000 interfaces described by the Lennard-Jones potential using equilibrium molecular dynamics simulations, whereby we systematically change the mass and stiffness of each side. We show that the trends in TIC do not generally follow that of the bulk phonon DoS overlap, but instead more closely follow the vibrational power spectrum overlap for the interfacial atoms. We then identify the frequency overlap in the interfacial power spectra as an improved descriptor for understanding the qualitative trends in TIC. Although improved, the results show that the basic intuition of frequency overlap is still insufficient to explain all of the features, as the remaining variations are shown to arise from anharmonicity, which is a critical effect to include in interface calculations above cryogenic temperatures.

Interface conductance modal analysis of lattice matched InGaAs/InP

We studied the heat conduction at InGaAs/InP interfaces and found that the total value of interface conductance was quite high ∼830 MW m−2 K−1. The modal contributions to the thermal interface conductance (TIC) were then investigated to determine the mode responsible. Using the recently developed interface conductance modal analysis method, we showed that more than 70% of the TIC arises from extended modes in the system. The lattice dynamics calculations across the interface revealed that, unlike any other interfaces previously studied, the different classes of vibration around the interface of InGaAs/InP naturally segregate into distinct regions with respect to frequency. In addition, interestingly, the entire region of frequency overlap between the sides of the interface is occupied by extended modes, whereby the two materials vibrate together with a single frequency. We also mapped the correlations between modes, which showed that the contribution by extended modes to the TIC primarily arises from coup...

Empirical interatomic potentials optimized for phonon properties

Molecular dynamics simulations have been extensively used to study phonons and gain insight, but direct comparisons to experimental data are often difficult, due to a lack of accurate empirical interatomic potentials for different systems. As a result, this issue has become a major barrier to realizing the promise associated with advanced atomistic-level modeling techniques. Here, we present a general method for specifically optimizing empirical interatomic potentials from ab initio inputs for the study of phonon transport properties, thereby resulting in phonon optimized potentials. The method uses a genetic algorithm to directly fit the empirical parameters of the potential to the key properties that determine whether or not the atomic level dynamics and most notably the phonon transport are described properly.Molecular dynamics: Optimized potentials for studying phononsA framework has been developed that can optimize the potentials needed to more accurately study phonons using molecular dynamics. Molecular dynamics simulations are an indispensable tool for studying how atoms interact. Despite their widespread use, however, it is often difficult to determine the potentials needed to accurately describe the various interactions involved for phonons, which are the excitations that underpin physical properties such as thermal conductivity. An international team of researchers led by professor Asegun Henry from the Georgia Institute of Technology presents an approach, based on a genetic algorithm, that can optimise the empirical interatomic potentials for phonons from first principles inputs, that can be used in classical molecular dynamics simulations. And although they demonstrate this method with semiconducting silicon and germanium, it should be extendable to alloys and disordered systems.

Interconnect patterns for printed organic thermoelectric devices with large fill factors

Organic materials can be printed into thermoelectric (TE) devices for low temperature energy harvesting applications. The output voltage of printed devices is often limited by (i) small temperature differences across the active materials attributed to small leg lengths and (ii) the lower Seebeck coefficient of organic materials compared to their inorganic counterparts. To increase the voltage, a large number of p- and n-type leg pairs is required for organic TEs; this, however, results in an increased interconnect resistance, which then limits the device output power. In this work, we discuss practical concepts to address this problem by positioning TE legs in a hexagonal closed-packed layout. This helps achieve higher fill factors (∼91%) than conventional inorganic devices (∼25%), which ultimately results in higher voltages and power densities due to lower interconnect resistances. In addition, wiring the legs following a Hilbert spacing-filling pattern allows for facile load matching to each application...