Thermal insulator transition induced by interface scattering

Abstract

We develop an effective medium model of thermal conductivity that accounts for both percolation and interface scattering. This model accurately explains the measured increase and decrease of thermal conductivity with loading in composites dominated by percolation and interface scattering, respectively. Our model further predicts that strong interface scattering leads to a sharp decrease in thermal conductivity, or an insulator transition, at high loadings when conduction through the matrix is restricted and heat is forced to diffuse through particles with large interface resistance. The accuracy of our model and its ability to predict transitions between insulating and conducting states suggest it can be a useful tool for designing materials with low or high thermal conductivity for a variety of applications.The ability to obtain high or low thermal conductivity makes composite materials of great technological interest. Materials with low thermal conductivity are needed for insulating thermoelectrics (TE), 1,2 while materials with high thermal conductivity are needed for thermal interface materials (TIM).3,4 Since the efficiency of a TE device increases with the figure of merit ZT , 5 which is proportional to temperature, improving the insulation of a TE device leads to less lateral heat loss, larger operating temperatures, and thus higher efficiency.2 On the other hand, TIMs are needed to transfer heat from semiconductor devices such as transistors to a heat sink or heat spreader.3,4,6 Improving the thermal conductivity of TIMs reduces the thermal resistance of the heat transfer system and lowers the operating temperature of the underlying device, improving lifetime and reliability.The current approaches for achieving high or low thermal conductivity have considerable limitations. For example, the porous silica aerogels 1,2 used to insulate TE devices are mechanically brittle, leading to short lifetimes and high maintenance costs, while the thermal conductivity of TIMs consisting of ceramic particles in a polymer matrix is limited by thermal resistance associated with the mismatch in elastic properties at the interfaces. 3,4To improve the performance of insulators and TIMs, it is necessary to develop models that account for all of the physical phenomena that affect thermal conductivity. For composite materials, this includes two competing effects: percolation and interface scattering.4,7-9 Percolation, which occurs at high loading when a continuous path is formed through the conductive phase, increases the effective thermal conductivity. Interface scattering, which arises from the difference in the phonon density of states between the particle and matrix, 10 reduces the effective thermal conductivity, and is particularly important for nanoparticles with large surface-to-volume ratio.The most accurate models for calculating thermal conductivity of composite materials are based on molecular dynamics simulations or the Boltzmann transport equation.6,11 While accurate, these models are computationally expensive, pa...

From amorphous to nanocrystalline: the effect of nanograins in an amorphous matrix on the thermal conductivity of hot-wire chemical-vapor deposited silicon films

“…Through a systematic study of thermal conductivity of a-Si and nc-Si thin films prepared by HWCVD with a hydrogen dilution ratio varying from R = 1 to 10, we find that thermal conductivity increases by about 50-70% as the crystalline volume fraction increases from 0 to 65% and average grain size increases from 2 to 9 nm. The percentage increase of κ is however slightly larger near room temperature than at lower T. The increase of κ with R can be best explained by the GBSM [36]. The percolation of nanocrystalline conduction paths in an amorphous matrix helps improve heat conduction while scattering of heat carriers from nanograins keeps κ from increasing beyond those of their corresponding intragrain conductivities.…”

“…Incorporating nanocrystals into a-Si matrix causes two opposing effects in κ: percolation and interface scattering. Recently, an analytical form of generalized Bruggeman symmetric model (GBSM) has been introduced by Slovick and Krishnamurthy [36], which predicts the effective thermal conductivity κ eff of composite materials by taking into account both effects. The authors found that this model can provide a good description of thermal conductivity in epoxy containing 7 μm-AlN particles and in zinc sulfide containing 100 nm-diamond particles.…”

“…On one hand, embedding nanoparticles generally helps to reduce κ. On the other hand, at high loading of nanoparticles, a percolation path may form to open up an additional heat conduction channel, which enhances the effective thermal conduction [36], and a 100% loading returns to the nanocrystalline material discussed earlier. It is thus important to understand the interplay between grain boundary scattering and percolation of additional phonon conduction paths.…”

A 0D Lead‐Free Hybrid Crystal with Ultralow Thermal Conductivity

“…The lower limit of thermal conductivity of amorphous solids is well explained by the Einstein limit where the mean free path is assumed to be the same as half of the wavelength of phonons . However, the major bottleneck of employing amorphous low thermal conductivity materials such as silica aerogels is their brittleness, which leads to short lifetimes and higher maintenance costs . As a result, there have been efforts to search for low thermal conductivity materials through nanostructure engineering, i.e., using thin films, superlattices, nanostructured samples, and disordered crystals .…”

Thermal conductivity reduction by acoustic Mie resonance in nanoparticles

“…Materials with low thermal conductivity are useful for many applications [1,2]. Transparent materials with low thermal conductivity are desired for window insulation [3,4], while those with low thermal conductivity and high electrical conductivity are useful for thermoelectric (TE) applications [5]. Efficient TE devices are desired for applications such as energy harvesting of waste heat and solid-state refrigeration [6,7].…”