The experiments however did not prove so high thermal conductivity improvements, but found significant increase of the critical heat flux of the coolants. Some significant improvements are achievable; e.g. silver nanorods of 55±12 nm diameter and 12.8 µm average length at 0.5 vol.% increased the thermal conductivity of water by 68%, and 0.5 vol.% of silver nanorods increased thermal conductivity of ethylene glycol based coolant by 98%. Alumina nanoparticles at 0.1% can increase the critical heat flux of water by as much as 70%; the particles form rough porous surface on the cooled object, which encourages formation of new bubbles, and their hydrophilic nature then helps pushing them away, hindering the formation of the steam layer. Nanofluid with the concentration more than 5% acts like non-Newtonian fluids. Solids In some applications, solid materials are used as coolants. The materials require high energy to vaporize; this energy is then carried away by the vaporized gases. This approach is common in spaceflight, for ablative atmospheric reentry shields and for cooling of rocket engine nozzles. The same approach is also used for fire protection of structures, where ablative coating is applied. Dry ice and water ice can be also used as coolants, when in direct contact with the structure being cooled. Sometimes an additional heat transfer fluid is used; water with ice and dry ice in acetone are two popular pairings. Sublimation of water ice was used for cooling the space suit for Project Apollo. References External links CO2 as a natural coolant — FAQs Engine cooling systems Cooling technology