Silicon Nitride Ceramics from Silicon Wastes
Development of a Technology for Producing Silicon Nitride Ceramics from Silicon Industrial Wastes from Kyrgyz Republic
Tech Area / Field
- MAT-SYN/Materials Synthesis and Processing/Materials
8 Project completed
Senior Project Manager
Mitina L M
Kyrgyz-Russian Slavonic University, Kyrgyzstan, Bishkek
- VNIIEF, Russia, N. Novgorod reg., Sarov\nRussian Academy of Sciences / Institute of Applied Physics, Russia, N. Novgorod reg., N. Novgorod
- Korea Institute of Science and Technology, Korea, Seoul\nForschungszentrum Karlsruhe Technik und Umwelt / Institut für Hochleistungsimpuls und Mikrowellentechnik, Germany, Karlsruhe\nLoughborough University, UK, Loughborough
Project summaryThe objective of this project is to develop a technology for fabricating ceramic products from reaction-bonded silicon nitride via silicon powder nitridation and subsequent sintering using microwave heating. The project also pursues the goal of creating a process that would utilize silicon waste from the industry of Kyrgyz Republic.
The present stage of technology development requires materials capable of operation under extreme conditions, often combining high temperature, mechanical stresses and aggressive environment. It is known that ceramic and composite nitride and carbide materials are among the most promising for these applications due to their high mechanical strength at temperatures above 1,000 °C and chemical stability. The most efficient in the technical, economical, and ecological respects is the reaction-bonded silicon nitride (RBSN) technology which involves preparation of compacts from silicon powder and sintering aids, silicon nitridation (i.e., chemical conversion to silicon nitride) and subsequent sintering of silicon nitride. However, as long as conventional methods of thermal treatment are used, development of RBSN products is restrained by long duration of the nitridation process (as a rule, completion of a nitridation process takes several days) and a two-step process needed to obtain the Si3N4 ceramics (nitridation at 1,150–1,450 °C and subsequent sintering at 1,750–1,800 °C). Development of novel methods that would be free from these drawbacks appears to be a highly important task, especially for regions possessing large reserves of silicon waste which can be successfully utilized as a starting material for RBSN fabrication.
The results of recent investigations carried out mostly in the U. S. A., Japan, and Russia, suggest that a highly promising option is the use of microwave heating in the processes of silicon nitridation and RBSN sintering. It is known that the main feature of the use of microwave energy in technological processes is volumetric heating of materials by electromagnetic radiation. This feature plays a special role in the processes with participation of solid porous materials and a gaseous phase. Silicon nitridation and subsequent RBSN sintering fall into this category of processes. When using conventional methods of heating from surface the densification that accompanies nitridation leads to partial closure of pores near the surface and prevents gaseous nitrogen from penetration into the bulk. The microwave-based process is free from this drawback since the nitridation reaction front propagates from the inside (where the temperature is the largest in that case) to the surface. This feature of the process provides considerable reduction in the duration of the nitridation process and facilitates obtaining final materials of higher density. Both conclusions are supported by the results of the experiments on microwave nitridation of silicon performed both in the U. S. A. and (by the co-authors of this Proposal) in Russia. Furthermore, it has been demonstrated experimentally that using microwave heating it is possible to successfully implement a one-step process of fabricating silicon nitride products that combines the nitridation and high-temperature RBSN sintering steps. Despite an order-of-magnitude reduction in the process duration, the materials obtained via microwave heating even exceed the conventionally fabricated ones in fracture toughness, being not inferior to them in the overall mechanical properties.
In the course of implementation of ISTC Project # 364-96 the co-authors of this Proposal have demonstrated that the use of the millimeter-wave radiation (of frequencies 24 GHz and above) has a number of important advantages that are especially important from the viewpoint of practical implementation of microwave technologies for producing and sintering RBSN:
– With transition to the 30 GHz frequency, the duration of the nitridation process reduces down to 4–4.5 hours (the minimum duration reported for 2.45 GHz was 12–15 hours).
– Due to an increase of materials’ absorptivity with the radiation frequency, at frequencies of about 30 GHz efficient microwave heating not using any additional radiation succeptors becomes possible. The presence of carbon-containing radiation succeptor materials in the reaction chamber for obtaining or sintering Si3N4 ceramics is highly undesirable since it deteriorates its physical and mechanical properties.
– In the supermultimode cavities fed by millimeter-wave radiation a much higher degree of uniformity in the microwave energy distribution can be achieved. This opens opportunities for implementation of high productivity processes in which many workpieces are loaded simultaneously into the reactor.
The scientists participating in this Proposal possess a many-year experience both in materials science, including creation of novel materials, and in the development of equipment for materials processing with millimeter-wave radiation. The many-year activity of the Institute of Applied Physics (IAP) team has resulted in the development of world’s first gyrotron systems for millimeter-wave processing of materials based on volumetric heating. The team has created a unique gyrotron system facility that includes two 30 GHz – 20 kW and a 83 GHz – 20 kW continuous operation gyrotron systems for materials processing. The researchers participating in this Proposal have accomplished a broad range of experimental studies in the field of physics of millimeter-wave interaction with materials and developed a technology for fabricating refractory parts from silicon nitride ceramics based on single crystalline silicon waste.
The following results are expected from implementation of the proposed project:
– knowledge will be gained that is needed for developing a technology for obtaining reaction-bonded silicon nitride (RBSN) and its sintering using millimeter-wave electromagnetic radiation;
– upgrade and adaptation of the millimeter-wave processing equipment to allow implementation of the silicon nitridation and RBSN sintering processes will be accomplished;
– usability of the silicon industrial waste available in Kyrgyz Republic for the purpose of obtaining silicon nitride-based ceramics will be assessed;
– test specimens of RBSN parts will be produced and their performance parameters will be determined.
The use of microwave energy in high temperature processing entails such benefits as energy and labor saving, increase in productivity, reductions in waste and environmental impact. For instance, specific energy consumption for sintering ceramics is reduced by approximately an order of magnitude compared to conventional methods. The reduction in the energy consumption upon implementation of the microwave silicon nitridation process – which is tens of times faster than conventional – should be even more dramatic. A significant economical and ecological effect should be expected from the use of silicon industry waste as the starting material for obtaining ceramic products. All this should result in a significant cost reduction of ceramic products and their wide-scale introduction into modern mass production industry.
More than a half of the full number of the project personnel will be scientists and specialists previously involved in weapon development. The facilities available at VNIIEF, in particular, the comprehensive analytical equipment, and personnel expertise in materials science will be directed to the development of a novel technology aimed to produce materials of great demand at the civil market. The international nature of the proposed collaboration, publication of the scientific results and their presentation at international conferences will promote integration of the scientists of CIS states into the international scientific community. Implementation of the proposed project will provide the solution of a technical problem of an international importance. The planned commercialization of the technology resulting from the project will contribute to the development of a market-based economy responsive to civil needs in the country.
The proposed activities will be based on the experience accumulated during implementation of an ISTC Project devoted to the use of millimeter-wave radiation for creation of materials with novel properties. The verified methodology will be further developed with due regard to the specific tasks of this project. Special attention will be given to the following aspects of the technical approach:
– comprehensive attestation of the starting raw materials;
– development of methods for silicon purification, milling, and preparation of compacts for nitridation and sintering;
– comparative studies of the microwave and conventional thermal treatment methods;
– wide use of computer simulation methods for process optimization;
– exhaustive characterization of the obtained ceramic samples using a wide spectrum of analytical techniques, including those provided by foreign collaborators;
– finding relationships between the regimes and conditions of microwave treatment and parameters of final materials, in particular, their microstructure and mechanical properties;
– use of laboratory results for development of an avant-project of process equipment for the fabrication of ceramic parts from silicon waste.
An important role in the proposed project will be played by foreign collaborators from Japan, United Kingdom, and Germany. Among them are world-renowned leaders in the development of high-temperature silicon nitride-based ceramic materials and application of millimeter-wave radiation in technology. The scope of cooperation with the foreign collaborators will include information exchange in the course of project implementation, shared use of sample materials, cross checks of results obtained in the course of project implementation, and conduction of joint seminars and workshops.