3:30 PM - 3:45 PM
▼ [21p-W611-1] Thermal Conductivity of SiNCs/Polymer Nanocomposite for Super Thermal Insulating Material
Keywords:silicon nanocrsytal,thermal conductivity,nanocomposite
Introduction: Silicon nanocrystals (SiNCs) with grain size less than 10 nm has been widely researched for its size-dependent optical and electronic properties [1]. Moreover, research on SiNCs showed promising result of adjustable thermal conductivity properties of silicon, by controlling the particle grain size [2]. It is particularly important for thermoelectric devices.
In this work, SiNCs with mean size of 6 nm were produced. If NCs are dispersed into the polymer matrix uniformly, overall thermal conductivity of nanocomposite is expected to decrease by three mechanisms. First, phonons are trapped within individual NCs and phonon transportation is suppressed significantly. Second, crystal size is much smaller than the phonon mean-free-path (MFP); therefore phonons in SiNCs experience significant number of scattering at the crystal boundary, which is known as ballistic phonon transportation [2]. Finally, polymer forms nanostructured network and phonons, which is transported through polymer network, experience significant number of scattering at the NC/polymer interfaces. Although individual contribution of above mechanism is hard to evaluate, overall thermal conductivity of the nanocomposite material is expected to decrease significantly.
Experiment: SiNCs were synthesized by non-thermal plasma CVD [3]. The samples then were treated by hydrofluoric (HF) acid vapor etching so that SiNCs are fully terminated by hydrogen. Subsequently, SiNCs were dispersed in benzonitrile with known concentration. The mixed solution of SiNCs and polystyrene (PS) was stirred for 24 hours to obtain a well-dispersed stable solution. The resulting solution was deposited on the substrate by spin casting to produce nanocomposite thin films (4 - 12 um).
For thermal conductivity measurement, TWA (Temperature Wave Analysis) was applied. The spin-casted sample film was attached by a thin ITO layer that works as a heating element on the rear surface, and a thin gold layer was sputtered directly on the front surface of the sample as a sensor. Thin gold layers were also additionally deposited for the electrodes that connect with the wiring system. Temperature wave is generated by a. c. Joule heating on the ITO heater element directly attached to the sample. The temperature wave propagates in the through-thickness direction of the sample film with phase shift, which is used for the measurement of the thermal diffusivity [4,5]. Because the thermal conductivity of a SiNC is not known, porous SiNCs (3 nm, porosity 55%, 1.08 W/m.K) was used for the estimation of thermal conductivity with experimentally obtained thermal diffusivity [6]. Published thermal conductivity of PS (0.1549 W/m.K) was used for this modeling [7].
Result and discussion: The experimental data was compared with five fundamental structural thermal conductivity models for two phase material [8]. Figure 1 shows the comparison of all five models and experimental result of our work. Five models showed that the thermal conductivity of nanocomposite increases with increasing mass fraction of silicon simply because thermal conductivity of silicon is greater than that of PS. Nevertheless, our result shows the negative slope in terms of SiNCs fraction: The decrease of thermal conductivity indicates the significant increase of phonon scattering in the nanocomposite material. Heat managing material due to quantum size effect is successfully demonstrated. Detail of fabrication and measurement will be presented in the conference.
In this work, SiNCs with mean size of 6 nm were produced. If NCs are dispersed into the polymer matrix uniformly, overall thermal conductivity of nanocomposite is expected to decrease by three mechanisms. First, phonons are trapped within individual NCs and phonon transportation is suppressed significantly. Second, crystal size is much smaller than the phonon mean-free-path (MFP); therefore phonons in SiNCs experience significant number of scattering at the crystal boundary, which is known as ballistic phonon transportation [2]. Finally, polymer forms nanostructured network and phonons, which is transported through polymer network, experience significant number of scattering at the NC/polymer interfaces. Although individual contribution of above mechanism is hard to evaluate, overall thermal conductivity of the nanocomposite material is expected to decrease significantly.
Experiment: SiNCs were synthesized by non-thermal plasma CVD [3]. The samples then were treated by hydrofluoric (HF) acid vapor etching so that SiNCs are fully terminated by hydrogen. Subsequently, SiNCs were dispersed in benzonitrile with known concentration. The mixed solution of SiNCs and polystyrene (PS) was stirred for 24 hours to obtain a well-dispersed stable solution. The resulting solution was deposited on the substrate by spin casting to produce nanocomposite thin films (4 - 12 um).
For thermal conductivity measurement, TWA (Temperature Wave Analysis) was applied. The spin-casted sample film was attached by a thin ITO layer that works as a heating element on the rear surface, and a thin gold layer was sputtered directly on the front surface of the sample as a sensor. Thin gold layers were also additionally deposited for the electrodes that connect with the wiring system. Temperature wave is generated by a. c. Joule heating on the ITO heater element directly attached to the sample. The temperature wave propagates in the through-thickness direction of the sample film with phase shift, which is used for the measurement of the thermal diffusivity [4,5]. Because the thermal conductivity of a SiNC is not known, porous SiNCs (3 nm, porosity 55%, 1.08 W/m.K) was used for the estimation of thermal conductivity with experimentally obtained thermal diffusivity [6]. Published thermal conductivity of PS (0.1549 W/m.K) was used for this modeling [7].
Result and discussion: The experimental data was compared with five fundamental structural thermal conductivity models for two phase material [8]. Figure 1 shows the comparison of all five models and experimental result of our work. Five models showed that the thermal conductivity of nanocomposite increases with increasing mass fraction of silicon simply because thermal conductivity of silicon is greater than that of PS. Nevertheless, our result shows the negative slope in terms of SiNCs fraction: The decrease of thermal conductivity indicates the significant increase of phonon scattering in the nanocomposite material. Heat managing material due to quantum size effect is successfully demonstrated. Detail of fabrication and measurement will be presented in the conference.