Nanostructured ceramics are materials that are employed in a wide range of applications, from small-scale uses such as paints and pigments to complex ones such as sensors and imaging.

These materials have distinct microstructures with extremely small grains and a large number of structurally disordered interfaces.

Due to the presence of interfaces with increased free volume, the final bulk density of the nanostructured ceramics is much lower than the density of a large grained material. Research on new technological applications of nanostructured ceramics is motivated by these unique properties.

A typical solar cell consists of a silicon semiconductor which directly absorbs solar energy and converts it into electricity. However, silicon semiconductors are only sensitive to infrared light, and most of high-energy light waves, including visible light, are wasted in the form of heat. Therefore, the theoretical efficiency of conventional single-junction solar cells is 34%, which is not achieved in practice.

Thermophotovoltaic devices are employed to overcome the limitations of solar cells. Unlike silicon semiconductors that directly transmit sunlight to the solar cells, thermophotovoltaic devices feature intermediate components such as an absorber that absorbs sunlight and an emitter that converts sunlight into electricity. The efficiency of thermophotovoltaic systems has so far been limited to only 8% owing to the poor performance of the intermediate component made up of tungsten.

Recently, scientists at Stanford University developed a novel heat-resistant thermal emitter that has the ability to significantly improve solar cell efficiency.

The emitters employed within thermophotovoltaic devices cannot withstand temperatures of 1000�C. To overcome this limitation, researchers at Stanford University have developed a ceramic nanolayer coating for tungsten emitters. The emitters are coated with a hafnium dioxide nanolayer that adjusts light to shorter wavelengths which are suitable for operating a solar cell. As a result, the theoretical efficiency of the solar cell was increased to 80%.

Researchers said that when their complex, three-dimensional nanostructured thermal emitters were subjected to temperatures of 1000�C, the emitters retained their structure for over 12 hours. Samples remained stable for an hour when they were heated to temperatures of 1400�C.

Research on ceramic nanostructured thermal emitters was funded by the Department of Energy’s Light-Material Interactions in Energy Conversion Center and Stanford’s Global Climate and Energy Project obtained dramatic results.

Although the structure of emitters has been modified, researchers confirmed that these ceramic-coated emitters can still produce infrared light necessary for operating solar cells. They also concluded that this advancement in ceramics could not only improve the efficiency of thermophotovoltaics, but also advance other research areas such as electrochemical energy storage, high-temperature catalysis, and harvesting waste heat energy.

Researchers have created a heat-resistant thermal emitter using tungsten and hafnium both of which are abundant and inexpensive. They believe that these unprecedented results obtained during the experiment will likely motivate the thermophotovoltaics industry to employ more of ceramics and other types of materials. Further research will include testing other types of ceramic materials for use in thermal emitters to deliver infrared light to a solar cell.