The effect of opacifier properties on thermal conductivity of vacuum insulation panel with fumed silica

The reduction of radiation heat transfer in VIPs is an important issue. In VIPs, the radiation heat transfer is reduced by the use of opacifier. However, the amount of opacifier also significantly affects the cost of VIP. In this study, the effect of different types and sizes of opacifiers on VIP thermal conductivity was investigated. For this purpose, 6 VIP samples containing different opacifiers (SiC, graphite and calcium aluminate in 3 different grain sizes) were produced. Fumed silica, glass fiber and opacifier were used as fillers in VIP cores at 65%, 20% and 15% (by mass), respectively. VIP samples were vacuumed up to 0.1 Pa, and their thermal conductivity coefficients were measured according to TS EN 12667 standard. In addition, transmittance ratios (%T) of fumed silica, glass fiber, opacifiers and core samples were measured by FTIR spectrophotometer and the mass extinction coefficients of each sample were calculated. The silicon carbide (F1000) as the opacifier (e: 399 m(2) kg(-1)) and the graphite-containing core among the VIP core samples (e: 149 m(2) kg(-1)) exhibited the highest mass absorption coefficient. In addition, the thermal conductivity of the core samples without barrier was measured under 1 Pa vacuum and then the radiation conductivities were calculated from the obtained data. F800 type SiC significantly reduced radiation heat transfer, while the solid conduction of the core was increased due to its relatively coarse grain size. VIP sample with SiC (F1200) provided the lowest thermal conductivity at 0.1 Pa (3.94 mW mK(-1)). As a result, F1200 type SiC was found to be the most efficient type of opacifier among the investigated opacifiers.