I have a TEC-12706 thermoelectric peltier module. I am making a mini refrigerator (simple box refrigerator). I'm using this peltier and heatsink of equal size as compared with the peltier. I have given the picture of the heatsink attached with the hot side of the peltier. I'm facing a very big problem. While I connect this peltier with a DC source, it works properly for some time, i.e., the cold side of the peltier gets cool and the heatsink starts becoming hot. But just after a few time the peltier starts working in a different way. That is, the cold side of the peltier also gets some heated and the heatsink stops or reduces absorbing heat from the hot side of the peltier. I think the heatsink compound (or thermal paste) which I'm using is not proper. What do you think, what may be the problem? Please help me, I'm struck and am not able to make my refrigerator.!See this picture
It is typical of a Peltier device to require an active method of dissipating the heat. You should have a fan on the hot-side capable of moving cooler air over the fins, in order to keep from destroying the device. The same aspect applies to the cold side. Use a fan to move cooled air to the desired location.
Peltier modules are extremely inefficient and therefore need excessive heat dissipation. What you are experiencing is that the peltier module cannot dissipate all of the heat that it is moving (either from a heat load or from itself) and as a result it cannot maintain an appropriate delta-Temp.
I suggest adding a larger heat sink, a fan blowing over the heat sink (good 'ol convective cooling), or a water block.