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Materials Engineering

Process mastered

Groups from Ipen and USP succeed in making the heart of the oxygen sensor, a part imported up until now

Found in the most varied forms and areas of application, sensors are everywhere. In the devices used to count people who go in and out of stores, in the chemical evaluation of beverages, and in identifying a few molecules of gas in the midst of a million of other ones that are part of the air. In the steel industry, for example, the task of controlling the steelmaking process is carried out with the help of a disposable sensor. Placed in a car with 200 tons of steel at 1,600° Celsius, the device, before it melts, has just 4 or 5 seconds to assess the level of oxygen in the metal, a measure that is fundamental for determining the final characteristics of the product. Made up of over ten components, only one of them, precisely the most important, made of ceramic material and called the heart of the sensor, is not made in Brazil. But, from the results presented in a study carried out with ceramic oxides by two groups from the University of São Paulo (USP) and one from the Institute of Nuclear Energy and Research (Ipen), this dependence on the imported product may be made good with national technology.

“We have the conditions, not just to develop a material that is the same, but one that is even better, because we are acting precisely at the most problematical point of the sensor, which is the intergranular phenomenon”, says Professor Reginaldo Muccillo, from Ipen, the coordinator of the thematic project on ceramic materials funded by FAPESP. The group from Ipen is part of the Multidisciplinary Center for the Development of Ceramic Materials (CMDMC), one of the Foundation’s Research, Innovation and Diffusion Centers (Cepid), where interaction with industries is encouraged. The intergranular phenomenon is related to the process of making of ceramic materials, which has as its starting point a powder, which can have various components. To transform this powder, whether into a vase or a sensor, the material has to be put in the furnace at high temperatures, in a process called sintering. The final product is a solid material made up of little grains. The contact region that there is between them, called intergranular, is critical for letting oxygen pass and conducting electrons and heat, besides determining the mechanical properties.

“In the thematic project, the emphasis is on studying the electrical properties”, Muccillo reports. This choice is due to the fact that in the majority of sensors an electrical signal has to be used to make the desired measurements. “If the outline of the grain is not well shaped, the material becomes resistant to the passage of the oxygen ions”, explains Professor Douglas Gouvêa, from the Metallurgical and Materials Engineering Department of the Polytechnic School at USP, who prepares the internal structure (microstructure) of the ceramics. “We are studying how to process these materials to get a larger or smaller grain, and minimum or maximum porosity, depending on the application”, he says. The third group that is taking part in the project is headed up by Professor Renato Jardim, from USP’s Physics Institute, a specialist in the phenomena of electrical transport of superconducting materials at low temperatures. “One group complements the other”, says Muccillo.

Wide application
In this project, which started in September 2000 and is forecast to end in August next year, several ceramic oxides are studied, like zirconium, stannic and aluminum oxides, lanthanum manganites and ceramic superconductors. These substances have wide applications in devices, sensors, batteries and fuel cells. As an example of an application of zirconium oxide, he mentions the oxygen sensor, which can be used both in the steel industry in the steel making process and in the automobile industry, to determine in real time the level of oxygen in the engine and in the exhaust pipe of a vehicle. These sensors make it possible to get a better yield and a control over the balance between air and fuel. In hospital boilers, too, it is possible to save fuel.

Muccillo has been studying zirconium oxide since 1979. This dedication has now resulted in a request for a patent covering the ceramic paste preparation based on zirconium oxide and magnesium oxide, to be used in disposable oxygen sensors, which are guaranteed to survive 10 seconds, before disintegrating in the cars of steel. He says that Brazil imports this ceramic component, even though it has the raw material for producing zirconium oxide, and research laboratories capable of developing appropriate materials for sensors. “One kilo of oxide would be enough to make about a thousand sensors. A small plant producing 500 kilos a month could meet the country’s current demand”, the coordinator claims.

Heat treatment
Douglas recalls that one of the ideas discussed within the thematic project is reactivating the production of zirconium oxide in Brazil, abandoned after Ipen deactivated its pilot plant. He points out that the group has an interesting contribution to make in the stages of conforming and firing ceramics, parts of the process fully mastered only by companies that have the technology for manufacturing the heart of the sensor. In the firing stage, for example, his group has managed to bring down the level for sintering zirconium oxide from 1,550º over two hours to 1,350º for ten seconds. “Actually, it is a special situation of manufacturing. But if the industry manages to lower the temperature to 1,350º for half an hour, it will have an enormous gain in energy”, says Muccillo.

The researchers have already attested to the possibility of energy gains in the manufacturing process, by reducing the temperature and the time for sintering. Now they are going to work on the characterization of the material, to be sure that all the properties of the ceramic will be maintained and that it will be able to be used in the industrial manufacture of sensors.

The Project
Study of Intergranular Phenomena in Ceramic Oxides (nº 99/10798-0); Modality Thematic Project; Coordinator Reginaldo Muccillo – Ipen; Investment R$ 309,075.00 and US$ 216,752.00