[Paper Review] Transition of thermal rectification in silicon nanocones

Zhongwei Zhang, Yuanping Chen+2 more|arXiv (Cornell University)|Mar 24, 2016

Thermal properties of materials3 citations

TL;DR

This study challenges the conventional belief that thermal rectification in silicon nanocones increases monotonically with geometric asymmetry. Using nonequilibrium molecular dynamics, the authors discover a sharp reversal in rectification ratio (R) at a vertex angle of ~90°, where R decreases for θ > 90° due to the disappearance of localized phonon modes, indicating a fundamental shift in thermal transport mechanism.

ABSTRACT

Current understanding of thermal rectification asserts that the rectification ratio (R), which measures the relative heat flux between two ends of a nanostructure, is determined by its geometric asymmetry. The higher the asymmetry, the higher the R. However, by using nonequilibrium molecular dynamics method we have calculated thermal transport in Si nanocones as an example, the results show that such an understanding may be incorrect and R may not increase monotonically with geometric asymmetry. Rather, R exhibits a sharp reverse when the vertex angle (θ) of the nanocone is approximately 90°. In other words, when θ > 90°, R decreases, rather than increasing. We show that this abnormal behavior is originated from a change in the thermal transport mechanism. At small θs, phonon transport is dominated by localized modes, especially for transport from tip to bottom. At large θs, however, these localized modes disappear, leading to R decrease.

Motivation & Objective

• To investigate the relationship between geometric asymmetry and thermal rectification in silicon nanocones.
• To challenge the prevailing assumption that higher asymmetry always leads to higher rectification ratio (R).
• To identify the underlying thermal transport mechanism responsible for anomalous rectification behavior.
• To explore the role of localized phonon modes in directional heat transport.
• To determine the critical transition point in thermal rectification as a function of vertex angle.

Proposed method

• Nonequilibrium molecular dynamics (NEMD) simulations were used to compute thermal transport in silicon nanocones.
• The rectification ratio (R) was calculated as the ratio of heat flux in forward versus reverse temperature gradient.
• The vertex angle (θ) of the nanocone was systematically varied to study its effect on R.
• Phonon mode analysis was performed to identify localized modes and their contribution to heat transport.
• The transition in thermal transport mechanism was analyzed by comparing results at small (θ < 90°) and large (θ > 90°) vertex angles.
• Simulations were conducted under steady-state thermal bias to ensure accurate heat flux measurement.

Experimental results

Research questions

• RQ1 Does the thermal rectification ratio (R) increase monotonically with increasing geometric asymmetry in silicon nanocones?
• RQ2 What causes the observed reversal in R at a vertex angle of approximately 90°?
• RQ3 How do localized phonon modes influence heat transport from the tip to the base of the nanocone?
• RQ4 What is the nature of the thermal transport mechanism shift at θ ≈ 90°?
• RQ5 Can the rectification behavior be explained by changes in phonon mode structure?

Key findings

• The rectification ratio (R) does not increase monotonically with geometric asymmetry; instead, it exhibits a sharp decrease when the vertex angle (θ) exceeds 90°.
• At θ < 90°, thermal transport is dominated by localized phonon modes, especially in the tip-to-base direction, enhancing rectification.
• For θ > 90°, these localized modes disappear, leading to a reduction in R and a transition to delocalized phonon transport.
• The transition at θ ≈ 90° marks a fundamental shift in the thermal transport mechanism, disrupting the expected asymmetry-rectification correlation.
• The results indicate that geometric asymmetry alone is insufficient to predict rectification behavior; phonon mode structure is a critical determinant.
• The study reveals that thermal rectification in nanocones is not solely governed by shape asymmetry but by the presence or absence of localized phonon modes.