Thermal Properties Specific Heat Capacity
Specific heat capacity relates to the amount of energy required to heat and cool a substance. There is a common misconception that adding minerals to plastics helps reduce specific heat capacity. This misunderstanding is even supported by some textbooks (Xanthos 2010) and stems from the choice of units. Specific heat capacity is often expressed as kJ kg^{-1} K^{-1}, and, on that basis, mineral fillers do have a
Fig. 2 SEM image of MagniF 10 high-purity natural magnetite courtesy of LKAB minerals
Table 3 Magnetite has higher volumetric heat capacity than other solids
Material |
Density (g ^{cm-3)} |
Mass-specific heat capacity (kJkg^{-1}K^{-1}) |
Volume-specific heat capacity (kJ L^{-1} K^{-1}) |
BN (hexagonal) |
2.25 |
0.79 |
1.8 |
Quartz |
2.65 |
0.8 |
2.1 |
Silver |
10.5 |
0.19 |
2.0 |
Talc |
2.7 |
0.82 |
2.3 |
Tungsten |
19.4 |
0.09 |
1.7 |
Glass fiber |
2.6 |
0.83 |
2.2 |
Magnetite |
5.2 |
0.73 |
3.8 |
numerically smaller heat capacity than polymers. However, a more fitting choice of units is kJ L^{-1} K^{-1}, i.e., the volumetric heat capacity, the reason being that the filler is added to displace a certain volume of plastic where the total volume of the final part is maintained at a constant value. When one looks at volumetric heat capacity (equal to mass-based specific heat capacity x density), it turns out that almost all solids have very similar values with an average around 2.2 kJ L^{-1} K^{-1}. There are a few exceptions including diamond and magnetite, the latter having a value some 70% higher than other solids (Robertson 1988). An application of this property is in storage heaters where low-cost nighttime electricity is used to heat a heat sink with high thermal mass made of magnetite-filled ceramic bricks. The heat is then released in the daytime when electricity is significantly more costly. An additional advantage of magnetite in concrete is an increase in thermal shock resistance (Chan 2013) so the product does not crack even after multiple heat cycles (Table 3).