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Chemistry

Powerful liaisons

Ceramic nanoribbons are candidates for making connections in circuits and transistors

The forecasts for the future development of areas like electrical/electronics, computing or any other industrial segment will not be complete without instruments, parts, or any kind of development that can be measured in nanometers, a measurement comparable to the size of the parts of a human hair divided 100 thousand times. One of the forecasts closest to being implemented in computers or electronic devices in the next 20 years is the use of metal nanowires in the connection between components of a chip or of an integrated circuit board. Many are the studies being carried out all over the world that point to this path in the direction of facilitating even more the miniaturization of circuits and making the processing capacity of electronic equipment more rapid. But even before these nanowires reach the industrial environments, ceramic nanoribbons are emerging and they look promising in the this technology race.

“Nanoribbons have the advantage of not fusing like metal nanowires. They can be given high voltages of electric current without breaking. They support ten times more density of current than a gold nanowire, for example”, explains physicist Marcelo Ornaghi Orlandi, from the team of researchers from the Federal University of São Carlos (UFSCar) that has developed new kinds of ceramic nanoribbons. The group is also part of the Multidisciplinary Center for the Development of Ceramic Materials (CMDMC), one of the Research, Innovation and Diffusion Centers (Cepids) financed by FAPESP.

The nanoribbon developed in São Carlos is the first in the world in this format, according to the researchers. Up until now, there were only fine films of this material, which is produced with a semiconductor, in this case, indium oxide (In2O3) doped with tin (SnO2), another metal. This means that some atoms of the molecule of indium were replaced by others, of tin. The doped material, called ITO, for Indium Tin Oxide, becomes a conductor of electric currents. The films of ITO, for their characteristics of being transparent, are suitable for working as a defogger in car windows. When given a small electric current, they warm up and eliminate the fogging up.  The problem is that these films are still very expensive for this kind of installation.

High charge
The first nanoribbon from the Ceramics Cepid has in its molecule 85% of indium oxide and 15% of tin oxide. With it, it is possible to interconnect components with a good level of passing electric current. “The nanoribbons will be useful where there is a need for a high capacity of electrical power, in the connections of circuits”, says Orlandi. They will be capable of being adapted to the current process for manufacturing integrated circuits, because ceramics resist the corrosive substances used in this process. The nanoribbons will also be useful in constructing and connecting transistors, which are amplifiers of electrical signals. In computers, for example, each chip carries various tiny transistors in its inside, and the nanoribbons will make the connections between them, allowing them to work at higher processing speeds, about ten times higher than the current ones. Accordingly, a computer could run at 30 gigahertz (GHz), instead of the current 3 GHz. The higher the frequency, the greater the processing of the information in the circuit of a cell phone, a computer or a television set.

The ceramic nanoribbons measure from 40 to 800 nanometers in width and from 4 to 100 in thickness and, in this same size, provide for a high density of electrical current. This occurs because the nanoribbons have excellent crystalline characteristics, with a low concentration of imperfections. Accordingly, they make it possible to conduct electrons with very low level of scattering, which makes it easier to get high performance transistors. The researchers are now testing, successfully, ceramic nanoribbons with 1 ampere of electrical current. This is equivalent to a density of roughly 15 million amperes per square centimeter, sufficient current to melt a copper wire with a diameter of 0.025 millimeters, while the nanoribbon, with 0.00008, does not break.

Patent guaranteed
The results of the nanoribbon were shown in October 2004, at a congress of the Materials Research Society (MRS) in Boston, in the United States, and attracted the attention of a representative of a Japanese multinational. “But we were preparing the patent for filing with the National Institute of Industrial Property (INPI), and we have not yet given them the more precise information they are asking for”, says Orlandi. “Now, we can already negotiate, because we have one year, according to the worldwide rules, to file the patent in other countries”, says Professor Elson Longo, the coordinator of the CMDMC, who recently left the teaching staff of UFSCar and is now connected with the Chemistry Institute of the São Paulo State University (Unesp) in Araraquara. The two universities, plus the Institute of Nuclear Energy and Research (Ipen), make up the Ceramics Cepid.

Another advantage of the new material is its production process, far cheaper than other ways used by researchers all over the world to produce doped nanomaterials. “We use a controlled chemical method instead of the physical methods that use laser beams, much more expensive”, explains Longo. There are already in the world films of ITO produced in furnaces, and the innovation of the group from São Carlos was to produce this material in the form of nanoribbons. “We did the synthesis of the nanoribbons of ITO at 1,100°C, a temperature regarded as low for the growth of the material and for the control of the doping”, says Orlandi. Doping, which is introducing atoms into a molecule, is done inside a furnace in which the tin and indium oxides are put next to carbon. On burning, the carbon reacts with the oxides, forming gases of tin oxide and indium oxide. Next, they interact with oxygen to form ITO in the cold region of the furnace, with a precise control of temperature and pressure.

The use of nanoribbons is still an industrial project for the medium term, some 20 years. In parallel to the reduction in the size of these devices, it will be necessary to develop nanomanipulation techniques, because the control over the position of where to put the conducting nanoribbons or nanowires in an electrical circuit is still a very hard task and on an industrial scale currently unthinkable. “Another problem that limits the use of nanowires, which are the object of advanced studies, in electronic devices, is that the joining together of circuits is not so efficient as when using wires of macroscopic sizes. The heat produced by the high value of electric current makes metal nanowires break”, says Orlandi. “With time, these nanowires also undergo a process of oxidation and do not support the great densities of electrical charges”, says Longo. Accordingly, they are betting on studying and on developing ceramics on a nanometric scale.

Smoke sensor
Unlike the nanoribbons, which are a bet for the future, another ceramic compound with a high technological content may arrive on the market more rapidly. It is a sensor of toxic gases developed by the Ceramics Cepid. It has the main function of being connected to an automobile catalytic converter and giving information on its performance. This catalytic converter is a component, also produced with ceramics, that is fitted to the exhaust system of vehicles to transform the gases produced by the engine, like carbon monoxide (CO) and nitrogen oxide (NOx), leaving them in the form of nitrogen (N2), totally inert, and carbon dioxide (CO2), less pollutant than CO. Coordinated by Professors Edson Leite and Elson Longo, the researchers developed the sensor for it to detect CO and NOx gases, when the catalytic converter proves faulty. Installed by the side of the catalytic converter on the exhaust pipe, the sensor detects these two harmful gases and sends an electronic signal to the vehicle’s dashboard. Other forms of use include the production of small devices to be able to be fitted to the tail pipe, both for highway police testing and for workshops. “In the case of the Highway Police, it would be like a breathalyzer for the car”, says Elson Longo. Vehicles emitting gases in excess of the levels accepted by the legislation may fall under the heading of environmental crime, with fines that range from R$ 500.00 to R$ 10 thousand.

The sensor is produced from tin oxide with nanometric particles that measure 8 nanometers and are capable of supporting the high temperatures of an engine, around 400 to 500°C, without modifying their physical characteristics. “We developed a method in which the tin was doped with elements called rare earths, like cerium (Ce), lanthanum (La) and the metal called yttrium (Y), to give stability to the nanoparticles”, says Leite. The sensor works with the action of the gases on its surface, modifying its electrical characteristics and emitting an electrical signal that can be converted into a light or sound signal.

The sensor that is undergoing tests at the São Carlos Engineering School of the University of São Paulo (USP) and is still showing problems of functionality in an extremely dirty environment like the exhaust pipe of an automobile has a chance of interesting four companies that produce catalytic converters in Brazil. In the essentially academic part, the work with the sensor has generated 15 articles in international magazines and three doctorates. The first article, published in the Advanced Materials magazine, in 2000, was regarded as one of the most cited (amongst the 1% most cited) in the area during three years. Since 2000, there have been 51 citations in the area of new materials.

The projects
1. Conducting nanoribbons; Modality Research, Innovation and Diffusion Centers (Cepid); Coordinator Elson Longo – Multidisciplinary Center for the Development of Ceramic Materials (CMDMC); Investment R$ 1,200,000.00 a year for the whole CMDMC
2. Sensors of toxic gases; Modality Research, Innovation and Diffusion Centers (Cepid); Coordinator Elson Longo – Multidisciplinary Center for the Development of Ceramic Materials (CMDMC); Investment R$ 1,200,000.00 a year for the whole CMDMC

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