Chips will be able to use their own heat to cool themselves – scientists create unusual solid-state refrigerator

A group of German and Japanese scientists have demonstrated a solid-state system in which the mechanical work required for cooling is generated by the heat generated by the object being cooled itself. The chips in the data center will be able to generate the energy needed to cool themselves.

Image source: Karlsruhe Institute of Technology

Scientists from the University of Tsukuba (Japan) and Karlsruhe Institute of Technology (Germany) describe a system in which the electrical drive required for elastic thermal cooling is replaced by a thermosensitive alloy with a shape memory effect – which initiates the cooling cycle from an external heat source rather than a conventional drive. The prototype consists of two ultra-thin metal films that serve as drivers and coolant. A 22-micron-thick TiNi film shrinks when heated, converting thermal energy into mechanical motion – so a 26.5-micron-thick TiNiFe refrigerant film stretches and causes a reversible phase change, allowing for cooling.

In laboratory tests, the TiNi film exposed as a thermal actuator was heated to 86°C by current and produced a temperature difference of 12.9K on the refrigerant film, resulting in a temperature difference of 4K between the hot and cold parts of the system. When electricity was replaced by direct heating from an external source at a temperature of 130°C, the prototype provided 2.2K of cooling.

Thermoelectric coolers used in electronics typically only reach 10-15% of the theoretical efficiency limit of the reverse Carnot cycle, which is a quarter of the efficiency of modern vapor compression systems. In the case of elastothermal cooling, shape memory alloys change their crystal structure under mechanical stress. Mechanical stress causes a phase change, whereby the material heats up – after the heat has been removed and the load has been removed, the reverse process occurs: the material absorbs the heat and cools down, i.e. the carbide acts as a coolant. The problem is that the material still needs to be stretched and compressed multiple times – usually using various mechanical elements for this, which is unacceptable for a microscopic system, so the scientists force one shape memory alloy to drive another. When heated, the TiNi film returns to its original shape and shrinks, creating a cooling cycle on the TiNiFe film to which it is mechanically connected, thereby stretching it.

The thermal actuator made of TiNi film provides a force-to-displacement ratio of 14.5 N/mm, while the force-to-displacement ratio of the electromechanical component is only 1.1 N/mm. The film also has a high surface area to volume ratio, allowing for rapid heat transfer. In the first electric heating test, the stable temperature range was 4.0 K after 20 cycles at a specific air-conditioning power of 4.43 W/g; through direct heating from an external source, it could reach 3.32 W/g and 2.2 K respectively.

In theory, such a system could be powered by the heat generated by the processor during operation. But in reality, without a temperature difference between the hot and cold sides, the cooling power is still only 2.09 mW, which means it cannot yet be used with modern processors; limiting factors include a relatively slow response and the existing geometry of the system; heating also needs to be cycled. To increase cooling capacity, scientists are trying to connect several films in parallel. Demonstration of fouling of active materials, improvement of heat transfer, operating frequency, system lifetime is required.