Thermal Diffusivity of SPS Pressed Silicon Powders and the Potential for Using Bottom–Up Silicon Quantum Dots as a Starting Material

Abstract

The production of nanostructured bulk materials from silicon powders has been well documented as being one way of bringing down the thermal conductivity of silicon while still maintaining its high power factor. This reduction of thermal conductivity is predicted to lead to significant increases in its figure-of-merit, ZT. The size of the starting particles has a major effect on the nanostructuring and grain size of the final silicon-based materials. Using particles of differing size and distribution, pellets were produced using spark plasma sintering. The results show a significant lowering in the thermal diffusivity as the particle size in the powders is decreased. As the starting particle size deceases from 1 lm to 60 nm, we see a tenfold decrease in the thermal diffusivity at 300 K, from 20 mm 2 S À1 to 2 mm 2 S À1 . Both these show a significant decrease from the thermal diffusivity of 88 mm 2 S À1 observed from bulk silicon. A further decrease to 1 mm 2 S À1 is observed when the particle size of the starting material is decreased from 60 nm to sub-10 nm. The results also highlight the potential of using particles from solution approaches as a potential starting point for the prediction of nanostructured bulk materials.

Photothermal Effects and Heat Conduction in Nanogranular Silicon Films

“…For example, n-type thick porous Si samples have multi-porous (nano, meso, micro) morphologies, while p+, p-type samples have single porous morphologies [30]. In general, measured k = 0.19-0.53 W/mK of our Si nanogranular films are in the range of the lowest k values of nanostructurely voided Si films [29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48], Si nanowire films [53][54][55][56], amorphous porous Si films [49][50][51][52], crystalline porous Si membranes [57,58] and sin-Si NP tablets [24][25][26][27][28]. Additionally, when the size of a nanostructure ≤ l MFP , phonons collide with intergranular boundaries much more often than in single crystals.…”

“…For example, n-type thick porous Si samples have multi-porous (nano, meso, micro) morphologies, while p+, p-type samples have single porous morphologies [ 30 ]. In general, measured = 0.19–0.53 W/mK of our Si nanogranular films are in the range of the lowest values of nanostructurely voided Si films [ 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 ], Si nanowire films [ 53 , 54 , 55 , 56 ], amorphous porous Si films [ 49 , 50 , 51 , 52 ], crystalline porous Si membranes [ 57 , 58 ] and sin-Si NP tablets [ 24 , 25 , 26 , 27 , 28 ].…”

“…To the best of our knowledge, the thermal transport in drop-casted substrate-supported randomly packed Si nanogranular films with defined NP sizes and spherical shapes, has never been studied before compared to other various types of nanostructurally voided Si films ranging from low porosity pressure sintered nanostructured bulk Si (sint-Si) [ 24 , 25 , 26 , 27 , 28 ], crystalline porous Si (c-por-Si) [ 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 ], amorphous porous Si (a-por-Si) [ 49 , 50 , 51 , 52 ], crystalline porous Si nanowire (c-por-Si NW) films [ 53 , 54 , 55 , 56 ] to c-por-Si membranes [ 57 , 58 ]. As opposed to mainly top-down fabrication methods [ 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , …”

Photothermal Effects and Heat Conduction in Nanogranular Silicon Films

“…For example, n-type thick porous Si samples have multi-porous (nano, meso, micro) and p+, p-type samples have single porous morphologies [30]. In general, measured thermal conductivities (0.53 -0.19 W/mK ) of our Si nanogranular films are in the range of the lowest thermal conductivity values of nanostructurely voided Si films [29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48], Si nanowire films [53][54][55][56], amorphous porous Si films [49][50][51][52], crystalline porous Si membranes [57,58] and sin-Si NP tablets [24][25][26][27][28]. Additionally, when the size of a nanostructure becomes comparable or smaller than , phonons collide with intergranular boundaries much more often than in single crystalline bulk materials.…”

“…To the best of our knowledge, the thermal transport in drop-casted substrate-supported randomly packed Si nanogranular films with defined NP sizes and spherical shapes, have never been studied before compared to other various types of nanostructurally voided Si films ranging from low porosity pressure sintered nanostructured bulk Si (sint-Si) [24][25][26][27][28], crystalline porous Si (c-por-Si) [29][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47][48], amorphous porous Si (a-por-Si) [49][50][51][52], crystalline porous Si nanowire (c-por-Si NW) films [53][54][55][56] to c-por-Si membranes [57,58]. As opposed to mainly top-down fabrication methods [29][30][31][32][33][34][35][36][37][38][39]…”