In a closed reactor (Supplementary Fig. S3), a slight
excess of Mg (Mg : SiO2 molar ratio 5 2.5 5 1) was added, so that
there would neither be a lack ofMg owing toMg vapor escaping nor a
large excess of Mg to convert Si to Mg2Si. We found that the temperature
ramp rate significantly affects the morphology of the product.
Macroporous Si was obtained with a 40uC min21 rate (sample Si-
BJH (Barrett-Joyner-Halenda) analysis
further revealed that the Si-RH-5 sample has significant pores
less than 10 nm in size, while the Si-RH-40 sample has negligible
porosity with size less than 20 nm. The different characteristics of Si
produced at different temperature ramp rates can be understood by
local heat accumulation. Due to the large negative enthalpies associated
with magnesiothermic reduction, a large amount of heat is
released in local areas. With a fast ramp rate, little time is allowed
for heat radiation or conduction to occur, so local temperature
increases accelerate nearby reactions, which further increases the
local temperature. These high temperatures cause the fusion of Si
products and the disappearance of small pores.