My flask needs to be warmed with boiling water first before you add the hot coffee. This is because the inner metal walls would be colder than the coffee. If there is a temperature difference then there will be conduction into the walls. There is actually a double wall with a vacuum in between. There are no particles in a vacuum so it cannot conduct. The walls are shiny silver because silver is a very bad emitter of infra-red heat radiation. Heat radiation would be the only way that thermal energy could be transferred across the vacuum. Now look carefully at the top picture. You can see hot water vapour coming out of the top of the flask. This is a problem for two reasons. The first is that evaporation cools a liquid by removing the particles with most energy. The second is convection. The air above the liquid is warmed. It expands, becomes less dense and floats. There needs to be a hole in the top to let liquid be poured in and out, so it is blocked with a thick plastic stopper. Plastic is an insulator, so even if the convection heats its underside, the thermal energy will conduct up through it slowly. My coffee was still hot at lunchtime!
