Magnetic nanofluids – fluids containing magnetic nanoparticles – have been around for decades, but until recently they have been prone to aggregating and forming deposits, inhibiting their potential in applications. Over the past 5–10 years, progress in producing magnetic nanofluids with greater stability has led to a number of studies into the thermal properties and magnetic fields of these materials, and as is so often the case in technology, with greater understanding has come greater device potential.

“Devices using ferro nanofluids do exist but the nanofluids are not the main thermal transporter,” explains Miguel Dias who worked on this magnetic nanofluid thermal device during his Master's studies in a project led by João Ventura. Yet as he adds, magnetic nanofluids have many advantages for thermal transport, such easy control – even with very small fields due to their low weight – and their adaptable shapes, which can make much better contacts than solids.

Dias and colleagues at the IFIMUP-IN, University of Porto used magnetic nanofluids in an insulating cylinder with thermally conducting ends to allow for a heat source at the bottom and a heat sink at the top. When a magnetic field is applied, the magnetic nanoparticles in the fluid move from the heat source to the heat sink, where their heat is dissipated. When the field is removed they fall back towards the heat source under gravity, giving rise to a thermal switch that can be used to control the temperature of device surfaces. To demonstrate the efficacy of the thermal device the researchers used it to cool an LED by 17 °C.

“In some ways we were surprised at the results, in view of the simplicity of the device – it’s not very complicated to make and yet it achieved very good results,” says Dias. He is also very confident that there is a great deal of room to improve the device further so that it should surpass the performance of existing thermal switches used to control heat dissipation. Further papers and patents for improvements to the prototype are already in the pipeline.

Improving on the prototype

The researchers have already identified a number of factors that affect the device performance, which can be used to improve it. Experimenting with different switching frequencies for the magnetic field, the researchers noticed that the best performance was achieved at high frequencies. At lower frequencies the researchers observed large temperature variations at the onset of switching, but at higher frequencies the lower heat exchange rate per cycle resulted in less variation.

An exception to the high frequency requirement for optimum performance was larger devices. When the researchers used 3 cm long devices instead of 1 cm long ones, the higher frequencies did not allow for the additional time needed for the nanoparticles to move from one end to another. Dias suggests a number of parameters that would improve the performance of their device over a greater range of frequencies, including fluid viscosity, as well as device structure, and the use of magnetic fields instead of gravity during the “off state”.

The IFIMUP-IN, University of Porto team are also looking at ways of reducing the device size. So far they have successfully shrunk the dimensions from centimetres to millimetres. Dias believes that the microscale should also be possible and that further modifications would allow the researchers to tailor the device for different applications.

Full details are reported in Nano Energy.