Unphysical Heat Transfer by Molecular Dynamics Thomas Prevenslik QED Radiations Discovery Bay, Hong...

Unphysical Heat Transfer by Molecular Dynamics

Thomas PrevenslikQED Radiations

Discovery Bay, Hong Kong

Inter. Conf. Frontiers Mechanical/Materials Engineering (MEME), Hong Kong, July 27-29, 2012

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Molecular Dynamics (MD) is commonly used to simulate heat transfer at the nanoscale in the belief:

Atomistic response using L-J potentials (ab initio) is more accurate than macroscopic finite element (FE) programs, e.g., ANSYS, COMSOL, etc.

In this talk, I argue:

FE gives equivalent heat transfer to MD, but both are invalid at the nanoscale by Quantum Mechanics (QM)

And ask the question:

How to make MD and FE at least consistent with QM

Introduction

2Inter. Conf. Frontiers Mechanical/Materials Engineering (MEME), Hong Kong, July 27-29, 2012

MD and FE Restrictions

MD and FE are restricted by Statistical Mechanics (SM) to atoms having thermal heat capacity


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Validity

Historically, MD simulations of the bulk performed in submicron computation boxes under periodic boundary

conditions (PBC) assume atoms have heat capacity

In the macroscopic bulk being simulated, all atoms do indeed have heat capacity

MD is therefore valid for bulk PBC simulations


4

Today, MD is not made for bulk simulations, but rather for the atomistic response of discrete nanostructures

Problem is MD programs are based on SM and assume the atom has heat capacity that leads to unphysical

results

Conductivity in Thin films depends on thickness

Nanofluids violate mixing rules

Problem


Heat Capacity of the Atom

1

kT

hcexp

hc

E

6

Nanostructures

kT 0.0258 eV

SM, MD and FE (kT > 0)

QM(kT = 0)


In nanostructures, the atom has no heat capacity by QM


Conservation of Energy

Lack of heat capacity by QM precludes EM energy conservation in discrete nanostructures by an increase in

temperature, but how does conservation proceed?

ProposalAbsorbed EM energy is conserved by creating QED photons

inside the nanostructure - by frequency up or down - conversion to the TIR resonance of the nanostructure.

QED = Quantum Electrodynamics

TIR = Total Internal Reflection 7

QED photons create charge or emitted to surroundings

If the refractive index of nanostructure is greater than that of surroundings, the proposed QED photons are confined by TIR

NPs have high surface to volume ratio.

EM energy is absorbed almost totally in the NP surface.

The NP surface is the TIR wave function of the QED photons.

QED photons are created upon EM energy absorption in NPs.

f = c/ = 2nD E = hf

TIR Confinement



QED Heat Transfer

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QED Photons

Phonons

QQED is non-thermal radiation at TIR frequency

Qabs conserved by QQED photons before

thermalization and phonons Charge

QED photons create charge or

are emitted to surroundings

MD - Discrete and PBC

Akimov, et al. “Molecular Dynamics of Surface-Moving Thermally Driven Nanocars,”

J. Chem. Theory Comput. 4, 652 (2008). Sarkar et al., “Molecular dynamics simulation of effective thermal conductivity and study of enhance thermal transport in nanofluids,”

J. Appl. Phys, 102, 074302 (2007).


Pretty Picture v QM Correctness?

MD for Discrete kT = 0, But MD assumes kT > 0

Car distorts but does not move

Macroscopic analogy, FE = MD

Classical Physics

QM differs No increase in car temperature

Charge is produced photoelectric effect Cars move by electrostatic interaction

MD for kT > 0 is valid for PBC because atoms in macroscopic nanofluid

have kT > 0

Thermal Gradients


Q-W Hou, B-Y Cao and Z-Y Guo,“Thermal gradient induced actuation of double-walled carbon nanotubes,”, Nanotechnology, Vol. 20, 495503 , 2009

MD of Concentric CNTs

With MD, no CNT motion found.

Motion by adding a thermophoretic spring, but

then no need for MD

By QM, more QED radiation is produced at hot

than cold endCharge is produced

Outer CNT moves under charge gradient to cold end.

Classical physics does not

produce charge

Sputtering


Vienna U. Technology, www. Research Group Surface & Plasma Technology -.mht

MD 5 keV Ar atoms Impacting Cu

One answer to question: During MD solution, use

Nose-Hoover thermostat to hold temperature constant

as required by QM.

The QED radiation emitted is the net thermostat heat.

Input the QED radiation in FE programs to determine effect on the surroundings..

http://www.matcalc.de/nc/us/white-papers/tribology/

MD heat transfer based on SM assumes atoms have kT energy which is valid only for PBC

MD simulations of discrete nanostructures do not produce charge and are meaningless, except for pretty pictures.

MD and FE provide equivalent heat transfer simulations of discrete nanostructures, but both are invalid by QM and give unphysical results

QM negates SM, thermal conduction, Fourier Theory, and heat current at the nanoscale

RecommendationEstimate the time-history of QED radiation and use in FE simulations to determine the effect on macroscopic surroundings. MD may not even be necessary

Conclusions


Questions & Papers

Email: [email protected]

http://www.nanoqed.org


http://www.nanoqed.org/

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