Size and surface effects of hexagonal boron nitrides on the physicochemical properties of monolithic phase change materials synthesized via sol–gel route

Abstract

Monolithic shape-stabilized phase change materials (ss-PCMs) are among the most promising materials to store large amounts of solar energy, waste heat and off-peak electricity as heat in energy-saving buildings, battery- and water-heating systems. Generally, graphite-like additives, such as hexagonal boron nitride platelets (BN), are used to increase the thermal conductivity of ss-PCMs. However, it is still unknown which particle size and surface type of graphite-like additives increase the thermal conductivity of ss-PCMs to the highest extent. Here, we present the first experimental study about the influence of BN with different particle sizes and specific surface areas on the physicochemical properties of ss-PCMs synthesized via the sol–gel route. With a BN particle size of 25 µm, the average thermal conductivity of ss-PCMs containing 5–20 wt% BN increased by 57% when compared to the same ss-PCMs prepared with smaller BN particles (3 µm). This effect possibly results from the stronger exfoliation of the larger BN particles during synthesis and a larger amount of BN particles ordered in the same alignment in the silica structure. BN particles larger than 25 µm hinder the formation of mechanically stable silica structures. Smaller BN particles lead to higher viscous reaction mixtures with shorter gelation times and, thus, result in smaller silica macropores with diameters below 1 µm and higher compressive strengths of ss-PCMs up to 2.4 MPa at 10 °C and 1.7 MPa at 30 °C. An increasing specific surface area of BN up to 20 m2/g decreases the thermal conductivity by 0.11 W/mK (30 °C) and the compressive strength by 0.28 MPa (30 °C) of ss-PCMs with 20 wt% BN because of more BN agglomeration. Therefore, ss-PCMs containing 20 wt% BN with a particle size of 25 µm and a specific surface area of 3 m2/g have the highest thermal conductivity of 1.14 W/mK at 10 °C and 0.62 W/mK at 30 °C, but a 0.7–0.3 MPa lower compressive strength (10 °C) than the ss-PCM synthesized using 3 µm sized BN particles and specific surface areas of 12–20 m2/g.