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and oxygen gases. Splitting of water can also be achieved by directly using sunlight in the presence of a catalyst. Right now, titanium dioxide-based ceramic photocatalysts are being developed for this purpose. The idea of hydrogen as the medium of distributing energy for transportation and power generation has fascinated many researchers for several decades. The product of oxidation of hydrogen is water, which is environment-friendly. The most important energy conversion device for power generation from hydrogen is the solid oxide fuel cell (SOFC). This device is almost entirely made from ceramics. Stabilised zirconia ceramics can conduct electricity through the diffusion of oxygen ions at temperatures above ~600°C. This property is exploited for building SOFCs. The chemical energy of the fuel-oxygen reaction is released directly as electricity. While hydrogen is the most common fuel, hydrocarbon fuels like methane and combustion by-product gases like producer gas can also be used as fuels in an SOFC. This is not possible in other types of fuel cells based on polymers and expensive platinum. Such fuel flexibility of SOFCs is perhaps its strongest advantage. Apart from stabilised zirconia, SOFCs make use of ceramics such as strontium doped lanthanum manganite (LSM) in cathodes, lanthanum chromite in interconnects and glass-ceramic sealants. The only metallic material required is nickel catalyst in the anode. Research efforts are in progress to develop better materials for SOFCs. Nuclear energy Nuclear energy is a major alternative to solar energy. Current nuclear power plants are of fission types, i.e. large atoms emit radiation and become smaller atoms. These power plants also have steam turbines and heat exchangers that are coated with ceramics. However, the biggest contribution of ceramics is in the form of radioactive waste immobilisers. Glass ceramics and crystalline ceramics with the ability to hold radioactive atoms within their structural cages are being developed, so that the waste can be prevented from entering the ecosystems for centuries to come. Only ceramic materials offer this possibility of managing radioactive waste. Another type of nuclear power plant uses a fusion reactor. Such reactors are still
A well-established application of ceramic coatings is that of porcelain enamels on steel components of heat exchangers. Specially developed compositions are coated using frits in the form of slurries. Enamelling is completed in furnaces at temperatures of ~425°C and above.
engines. These ‘thermal barrier coatings’ (TBC) represent one of the most critical applications of ceramic coatings. They allow the gas turbine to be maintained at high temperatures, which enhances efficiency. Moreover, the lifespan of metallic components is increased as TBCs keep them below their maximum permitted temperature. Air plasma spray or electron beam physical vapour deposition is used for depositing TBCs with thickness in the range of about 150 to 250 micrometre. Usually, a temperature difference of ~100°C is maintained across the TBC. Another application of ceramics in gas turbines is in the form of abradable coatings, which maintain a tight seal between the rotating turbine and its casing. This is not all. The efficiency of internal combustion engines, especially diesel engines can be augmented by using ceramics. For instance, a thick coating of a zirconia ceramic on the piston head can increase performance or lifespan. Similarly, ceramic turbochargers can operate at high temperatures and increase the power delivered by the diesel engine for the same amount of fuel consumed. Many automobile components are coated with wear-resistant ceramics such as nitrides to enhance wear resistance.
ROLE OF CERAMICS IN ALTERNATIVE ENERGY Ceramic materials hold huge potential in alternative energy technologies, be it solar or nuclear. Solar energy Focussing on solar energy harvesting, the main photovoltaic (PV) material continues to be silicon. Ceramic materials, particularly silicate glasses, are used as substrates on which the various layers of a photovoltaic cell or a solar cell are deposited. Also, transparent but electrically conducting ceramics such as oxides of indium, tin, zinc and their combinations act as contacts in the PV cell. As mentioned earlier, the electrical energy generated in a solar cell can be stored in rechargeable batteries, or it can be used for dissociating water into hydrogen
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in the experimental stage. In a fusion reactor, small atoms like deuterium and tritium, which are isotopes of hydrogen, are made to collide with each other to produce larger atoms like helium. The primary challenge in fusion reactors is to develop the materials that can face plasma and withstand bombardment of neutrons. Ceramic materials like SiC/SiC composites are candidate materials for confining plasma. In magnetically confined fusion reactors like Tokamaks, superconducting ceramics are used for generating strong magnetic fields.
BECOMING GLOBAL LEADERS IN ENERGY TECHNOLOGY In the Indian context, and in terms of manufacturing technologies, there are ample opportunities in energy conversion technologies. Surface engineering of power plant components is well-established in India. Various coating deposition techniques like plasma spraying, high velocity oxyfuel (HVOF), detonation gun, cold spray, and chemical vapour deposition can be carried out for depositing ceramic coatings for wear resistance, corrosion and oxidation resistance, and thermal insulation (TBCs). However, the raw materials for these deposition techniques are almost always imported. Indian manufacturing enterprises have the opportunity to not only carry out the deposition jobs, but also develop the raw materials and further improve the coating techniques. Alternative energy technologies have tremendous opportunities for getting ahead of the competition. In the area of solar cells, cost reduction is the greatest challenge in the Indian context, and indeed for the whole world. Innovative manufacturing techniques can achieve this goal. Ceramics processing technologies are put to the test when developing solid oxide fuel cells. Any advancement in this area will prove crucial for the profitability of this technology. There are several exciting opportunities in the area of ceramic materials for energy technology for Indian entrepreneurs. Academic institutions and government research laboratories are putting in a large amount of research efforts in this domain. Now the time has come for the entrepreneurs to grab these opportunities for becoming global leaders in energy technology. Dr Ashutosh S Gandhi, Department of Metallurgical & Materials Engineering, IIT, Madras Email: a.s.gandhi@iitm.ac.in