Hollow‐Structured Materials for Thermal Insulation

Abstract

Heating and cooling represent a significant portion of overall energy consumption of our society. Due to the diffusive nature of thermal energy, thermal insulation is critical for energy management to reduce energy waste and improve energy efficiency. Thermal insulation relies on the reduction of thermal conductivity of appropriate materials that are engineerable in compositions and structures. Hollow‐structured materials (HSMs) show a great promise in thermal insulation since the existence of high‐density gaseous voids breaks the continuity of heat‐transport pathways in the HSMs to lower their thermal conductivities efficiently. Herein, a timely overview of the recent progress in developing HSMs for thermal insulation is presented, with the focus on summarizing the strategies for creating gaseous voids in solid materials and thus synthesizing various HSMs. Systematic analysis of the documented results reveals the relationship of thermal conductivities of the HSMs and the size and density of voids, i.e., reducing the void size below ≈350 nm is more favorable to decrease the thermal conductivity of the HSMs because of the possible confinement effect originated from the nanometer‐sized voids. The challenges and promises of the HSMs faced in future research are also discussed.

Thermal Transport in 3D Nanostructures

“…According to the Grashof number (Gr), convection of air is enabled when (Gr ≥ 1000), which corresponds to the void diameter larger than 10 mm. Thus, gas convection in the nanocellular foams makes no contribution to the heat transfer . Meanwhile, the radiation contribution can be calculated from .…”

“…Here, n i and ρ are the mean index of refraction and the apparent density of the specimen, respectively; T is the temperature; and σ is the Stefan–Boltzmann constant. e ( T ) denotes the specific Rossland mean extinction coefficient, which dependents on the constituent materials and affects the overall heat transfer of the nanocellular foams . The solid phase conduction depends on the phonon transport, which can be described by the kinetic theory of gases, i.e., , where C p and v g are the specific heat capacity and the mean group velocity of phonons; and Λ is the phonon mean free path (MFP) 1a.…”

“…Thus, gas convection in the nanocellular foams makes no contribution to the heat transfer. [52] Meanwhile, the radiation contribution can be calculated from 16 3 ( ) r i 2 3…”

“…e(T) denotes the specific Rossland mean extinction coefficient, which dependents on the constituent materials and affects the overall heat transfer of the nanocellular foams. [52] The solid phase conduction depends on the phonon transport, which can be described by the kinetic theory of gases, i.e.,…”

Preparation and heat-insulating properties of Al2O3–ZrO2(Y2O3) hollow fibers derived from cogon using an orthogonal experimental design

“…It is obvious that cogon bers have a length of 2-3 mm and an internal diameter of about 20 mm. Given Hu's theory about hollow-structured materials (HSMs), 27 it can be feasible to decrease the thermal conductivity of Al 2 O 3 -ZrO 2 (Y 2 O 3 ) hollow bers using a hollow structure.…”