**5. Conclusion**

produce up to 5 µW. This is certainly a promising material that could potentially be inte‐

The addition of CNTs to microelectrical mechanical systems (MEMS) typically proceeds by either a bottom-up approach which focuses on the deposition of catalytic nanoparticles to control the location of CNT growth or a top-down which concerns the manipulation of the CNTs to the correct position. A top-down method was use to make a CNT thin film on a microelectrical mechanical system which was then characterized in terms of the thermoelec‐ tric coefficients of the aligned SWCNTs [8]. Using the process of 'super-growth' which incor‐ porates water-assisted chemical vapor deposition, a CNT film was made and patterned by electron beam lithography into the required dimensions. By patterning a formed array of gold–SWCNT thermocouples it was found that under standard room temperature the See‐ beck coefficient of the aligned SWCNT film was between 18 and 20 *μ*V C−1. The Seebeck ef‐ fect of the SWCNT film was documented using thermocouples made of gold–SWCNT (Fig. 10.). Electrodes, a hot end and cold end temperature sensor, and a heater were produced by photolithography, and with a gold lift-off process on top of a silicon substrate that was cov‐ ered by an insulating layer of Si3N4. The SWCNT film was then constructed on the gold sur‐

**Figure 10.** Schematic of a device for measuring the Seebeck effect in a CNT film. Figure Adapted from Dau *et al.*

When the device was used, an output voltage of 54 µV was recorded with a temperature difference of 3.07 ◦C. This gave a Seebeck voltage of 19.38 µV K−1 which on average re‐

grated into many thermal systems and help with waste heat recovery.

428 Syntheses and Applications of Carbon Nanotubes and Their Composites

**4.2. Micro-Thermal Electrics**

face using the process of top-down assembly.

(2010).

CNTs have seldom been just another material for novel composites, but their true potential has yet to be transferred from the nano- to macro-scale. More than a two decades after their discovery, their influence has reached almost every aspect of scientific research from engi‐ neering to medicine. Faced with concerns about sustainably and climate change, the use of CNTs have helped to transform our approach to renewable energy. Advances in hydrogen fuel cells with CNT composite electrodes or membranes are helping to reduce and eliminate the need for rare and expensive catalysts. Safety is also another issue for the hydrogen based economy. Many different types of sensors will be required to promote a safe operational en‐ vironment especially when the ignition concentration of hydrogen can be as low as 4%. The same technology that is used in the catalysis process in hydrogen fuel cells can be used to create hydrogen sensors, and work with interconnected CNTs has provided sensitivity val‐ ues that contend with conventional sensors.

The role of interconnected CNTs in the photovoltaic research field is popular because of the potential to make novel hybrid solar cells, whilst increasing the overall efficiency of the de‐ vice. While the early results look promising, there are still some difficult questions to ad‐ dress, like how does the presence of defects on the CNT surface affect the chemistry and ultimately the efficiency of a DSSC?

The integration of CNTs into thermoelectric devices currently does not have the same level of development as the other alternative energy resources, possibly because the field is more geared towards cost saving on an industrial scale and the development of component sys‐ tems for vehicles rather than consumer gadgets or devices, but research into waste heat re‐ covery is substantial. It is likely that thermoelectric devices will conform more to a silent revolution with an uptake in industries that work with high temperature equipment looking at converting some of the heat produced back to electricity. However, the research into ther‐ moelectric fabrics has shown the potential for consumer products that may find a market in the future.

In summary, we are beginning to see a shift towards alternative fuel sources, with a focus on hybrid technologies like those found in the automotive industries, but we need to address the impact of our current economy as we transition to a more sustainable one.
