Porosity has a significant influ-
ence on the thermal conductivity. Between the two aluminosilicate shells (#4 and #6), #6 with higher total porosity (37.65%) exhibited lower thermal conductivity values throughout the measured tempera- ture range compared to shell #4 hav- ing lower total porosity (33.52%). Another good finding is the
weak temperature dependence of conductivity in the alumina-based shell (#5). Since the photon radia- tion in alumina is not significant until 1,832F (1,000C), this radiation compensates phonon scattering in alumina and the porosity effects and consequently the thermal conduc- tivity didn’t change much over the elevated temperature range. Te thermal conductivity and specific heat capacity values mea- sured from laser flash for the shells studied are listed in Fig. 3. Shell #7 (rapid shelling technique) was highly porous and broke apart when being surface ground during laser flash sample preparation. Effective density calculated from sample surface topography was used to calculate these values. It was found that laser flash showed a similar trend to the inverse method on both thermal con- ductivity and heat capacity values.
Using the Data When putting thermal property
Fig. 4. Graphs show the comparison of Cp and K values determined by the inverse and improved laser flash methods: (a) shell #3; (b) shell # 4; (c) shell #5; and (d) shell #6.
46 | MODERN CASTING January 2016
data from the inverse method and laser flash method together, as shown in Fig. 4., the thermal conductiv- ity values were fairly close, yet the inverse method presented higher specific heat capacity values than the laser flash method. Because many thermal reactions among the shell components and phase transforma- tion within the a morphous silica take place at high temperature, the amount and rate of these reactions will significantly affect the specific heat capacity values used in mod- eling. In the inverse method, the shell is heated rapidly when metal is poured and cooled down at a relatively slower cooling rate as the metal solidifies. Tese processes associate with more instantaneous measurement of a property which
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