Nanotechnology

A range of savings for raw materials

Worldwide raw material consumption explodes. At the same time as economies are being imposed, in particular in the area of fossil fuels for which reserves are limited and the environmental impact disastrous, “nanos" are opening up many new, industrially viable, possibilities.

The risks of climate change due to an increase in the greenhouse effect are forcing the introduction of compulsory methods for reduction of fossil fuel usage. In the developed world, it is going to be necessary to reduce emissions by a factor of four, between now and 2050. This implies reducing consumption of fossil fuels - oil and gas - or replacing them as soon as possible. Through its research in nanotechnology and nanomaterials, the CEA is able to offer innovative, industrially viable, solutions that are applicable in the areas of building, transport and the environment.

Transport: zero hydrocarbon!

Filtration membranes composed of a carpet of carbon nanotubes will, one day, be used desalinate sea-water (read more on page 13). However, one of the the most eagerly awaited applications is the replacement of hydrocarbons in transport. One solution which is being investigated with that goal in mind is aimed at attaining acceptable performance from electric vehicles. This requires the efficiency of the batteries used for energy storage to be improved. Today, the best available batteries provide a range of 200 kilometres with a recharge time of between 10 and 12 hours. However, it should be possible to improve their storage capacity by a factor of 10 by replacing the graphite, used for the negative electrode, with silicon. “Silicon is in fact capable of absorbing 10 times more Li ions than graphite", explained Hélène Burlet, the scientific director at LITEN. However, the insertion and removal of Li ions during the batteries' charging and discharging cycles, subjects the silicon to very significant changes in volume, on the order of 300%, strongly increasing the risks of fracture.” To relieve this problem, researchers at LITEN plan to use silicon in the shape of nanowires. The empty space created by this nano-structuring of the material provides the possibility for the lithium to wrap itself around the wires, and hence to absorb the variations in the silicon volume without the matrix being subjected to the consequences.
A mature technology, using nano-structured electrodes, already makes possible a gain in available power, through reduction of the distance for diffusion and for transfer of ions and electrons. This technology was developed by applying the Nanosafe approach for making processes safe (read more on page 12).

70% platinum or less

Low temperature fuel cells (PAC) are promising candidates to replace the combustion engine. However, they are not expected before 2020 because of their high cost, which is largely due to the quantity of platinum required for their operation. This metal, which catalyses the electricity producing reaction between hydrogen and oxygen, is in fact more expensive than gold. Moreover, the entire global reserves would not be sufficient to equip all of the automobiles in France. Currently, a hybrid vehicle with a 20 kW fuel cell requires nearly 20 grams of platinum: “It is going to be necessary to reduce the amount of precious metal required by a factor of 10, in order to arrive at quantities which are equivalent to those currently used in catalytic converters. To achieve this we will need to make use of each platinum atom.” At the present time, manufacturers use inks made from carbon, recovered from nano-particles of platinum between 2 and 3 nanometres in diameter. This ink is then deposited in a very thin layer by MOCVD (Metal Organic Chemical Vapour Deposition) onto the anode and cathode, exactly where the metal is required. Key advantage: a reduction in the quantity of metal required by a factor of three.
To progress further, the technology needs to be developed to create bimetallic nano-structures by adding other, less expensive, metals, such as cobalt or nickel, to the platinum. The metals would either be mixed in a homogeneous way, or arranged in a core-shell structure, with a layer of platinum on a core of cobalt or nickel. In the long term, LITEN will reconsider the architecture of all of the electrodes on the nanometre scale using nanotubes, nano-wires and other nano-structures.

Energy economy at source

In parallel, the CEA is working on development of bio inspired catalysts which do not require noble metals. However, these still require some fundamental research means their deployment is likely to be in the longer term. While waiting for a total replacement for hydrocarbons, it is possible, in the meantime, to reduce their consumption in traditional combustion engines by using nanotechnology. “Total fuel consumption can be reduced by between 10 and 12 %, simply by recovering the heat from the exhaust and converting it to electric current”, continued Hélène Burlet.
Thermoelectricity is the reversible conversion of a temperature gradient to an electric current in certain special materials: thermoelements. In this case, use of super-networks and nanopowders has tripled the energy conversion performance of these materials, which had seen no progress over the last 40 years: “In general, the thermal and electrical conductivities of a material will move in the same direction. Hence it is difficult to reduce the thermal conductivity of a material, which would increase the probability of a current appearing, without reducing its electrical conductivity at the same time. However, nano-structuring of materials has made it possible to decouple these two physical properties through quantum confinement effects.” Researchers are working on a thermoelectric generator recovering the heat from the exhaust-pipe of vehicles in order to supply power to the instrument panel, or even to replace the alternator.

Building: testing solar cells

The building sector is the second largest consumer of fossil fuels. If solar energy is to be an interesting alternative then the efficiency of solar cells must be improved. Currently efficiency is around 15 to 17 % for silicon cells depending on the technology used, which is low. These cells, known as silicon homojunctions, only create electricity by conversion of energy from the part of the solar spectrum corresponding to the silicon band gap: photons with too high an energy are converted into heat and those with too low an energy are not absorbed. To improve this situation, the CEA is working on multi-junction cells made up of several materials presenting different value band gaps. This combination increases the energy conversion efficiency through improved use of the solar spectrum. “Materials with adjustable band gaps are obtained by playing with their nano-structure, and particularly with the size of the constituent nano-crystals", explains Hélène Burlet. Hence, cells composed of stacks of junctions with different band gaps should allow conversion efficiencies of 20% to be attained” Another approach to the investigation consists of replacing bulk silicon with silicon nanowires to take advantage of the quantum confinement linked to the morphology of the nanowires.

The infinite potential of the infinitely small

Researchers need to control the properties of the nanomaterials and nano-devices which they develop. To do this, a simulation platform has been set up within the CEA, combining resources and expertise in simulation on the atomic scale from several institutes; INAC, LITEN, LETI and IRAMIS. Mission: predictive simulation of the structural and transport properties of nanomaterials through a description of the relevant physical mechanisms. In particular, the researchers are refining tools to predict new effects linked to nanometre dimensions.
In parallel, in order to control finished products and the properties of nano-objects on a smaller and smaller scale, the CEA's characterisation resources have been brought together on the same site: the Nanocarac platform, installed at Minatec (Grenoble). This platform offers a great diversity of microscopes which makes possible study of the morphological, electrical, crystalline, chemical and magnetic properties of components on the nanometre scale. “No one technique would be sufficient", insists Hélène Burlet. "Only through use of all the platform's microscopes is it possible to obtain the required information.”