I have a material

I need to determine its thermal conductivity

Contact surface $115$mm$^2,28$mm thick.

The material is placed between a hot and a cold source.

Question how to determine thermal conductivity?

Data:

Cold water is initially at $12\backslash$deg C

Every $30$sec we note the water temperature and the temperature difference between $2$ zones of the material

Cold water temperature :

$-30$sec: $12\mathrm{.}2\backslash$deg C

$-$ : $12,3$

$-12,4$

$-12,4$

$-12,5$

$-12,5$

$-12,5$

$-12,5$

$-12,6$

$-12,7$

Temperature difference t$0$ : $4\mathrm{.}6$

$-30$sec : $4,6\backslash$deg C

$-$: $4,6$

$-4,8$

$-5,2$

$-5,3$

$-5,5$

$-5,6$

$-6,9$

$-6,1$

$-6,2$

$-6,1$

Cold water mass: $450$g

Hot source at $95\mathrm{.}6\backslash$deg C$,$ constant throughout the test