Nanocrystalline composites based on boron carbide
Nanocrystalline composites based on boron carbide with enhanced strength and wear-resistance
Tech Area / Field
- FIR-MAT/Materials/Fission Reactors
3 Approved without Funding
Tbilisi State University, Georgia, Tbilisi
- Institute of Metallurgy, Georgia, Tbilisi
- Eduard-Zintl Instituteof Inorganic and Physical Chemistry, Germany, Darmstadt\nBoston University / College of Engineering, USA, MA, Boston\nStockholm University, Sweden, Stockholm
Краткое описание проектаBoron carbide (B4C) and the boron carbide-based alloys (B4C-TiB2 and B4C-ZrB2) are featured with an enormously wide complex of chemical, physical and mechanical properties. That is why these materials are widely used in modern industry and technique (nuclear power plants, defense, medicine, etc.). In addition to their dual purpose (military and civil), these materials are closest to those capable to solve the problems of possibly complete "localization" of the nuclear chain reaction. This, on the one hand, means to ensure efficient burning of nuclear fuel and, on the other hand, minimizing its negative consequences for the environment. Boron carbide can successfully accomplish both of these functions, especially when having an opportunity to use the types such as B4C (of natural composition) as well as enriched in appropriate boron isotopes 10B4С (neutrons absorber) and 11B4С (neutrons reflector). The latter, for example, provides return back of "fleeing" from the combustion process fast neutrons without causing any side effects, being the most secure way for "utilizing" the actinides accumulated in nuclear fuel (which, due to the problems related to radioactive wastes, is very actual and important). According to the ITER project where everything was thoroughly calculated, for one junction it was needed the material capable to reduce neutrons flow by 107, considering that the material weight would not exceed 45 tons. The practice, against the project implementers’ desire, showed that such type of a material could be only boron carbide. If not applying more examples but only the cause - consequential principles, it is not difficult to show that using of "secure" materials for creating the 4th generation nuclear power plants, besides diminishing the risks, will provide much better protection of the environment from radioactive wastes and radiation. Security, environmental safety, stability – those are Alfa and Omega of the nuclear technology. Above, we have briefly (on the abstract level) shown that those three requirements are greatly dependent on boron carbide and similar materials, especially when the nanostructured versions of boron carbide and made on its base composites are considered. It should also be noted that the problems discussed above are important not only for stationary (dissolution or synthesis) reactors, but also for the space technology.
Boron carbide and boron carbide-based nanocrystalline composites will also be indispensable materials for the closed (fuel)-cycle reactors working on fast neutrons; these reactors are considered the future of nuclear energy and a sample of complete localization of the phenomena connected to nuclear transitions. Implementation of this idea can make nuclear energy recyclable and solve the problem of accumulating utilized nuclear fuel and radioactive wastes. For example, one of the most significant disadvantages of the reactors of an open (fuel) cycle is the need for a permanent armed guard of graves (the possibility of theft of fissile nuclides from burial places by terrorists also seems real).
Unique nuclear features are not the only reason for a great interest caused by boron carbide and boron carbide-based composites. Unique features of these materials are also: high hardness, low specific weight (among the high temperature nard materials used up to these days, boron carbide has the highest hardness/density relative ratio), high corrosion-resistance, high melting point, thermal expansion coefficient, etc. However, high brittleness and, appropriately, low crack-resistance (which according to the literature data ranges within 1.8-4.3 MPa ½), are retarding factors for widespread application of boron carbide to industry. For instance, due to the unique radiation resistance (no swelling!), it seems real to create constructive nanocrystalline ceramics based on 11B4S-Zr11B2 working in the active zone of a reactor (e.g. shaft, slide bearings, etc.). That is why boron carbide studies are mostly aimed at improving the strength (impact strength, crack-resistance, σ-bend) characteristics.
First attempts for eradicating the above disadvantages were connected to alloying processes i.e. creating B4C-based heterophase compounds (B4С–TiB2 and B4С–ZrB2). These efforts increased the crack-resistance, although the increase was insignificant. Subsequent experiments and analyses showed that the desirable effect of improving mechanical characteristics could not be achieved by only alloying or even by submicron level variations of the powder dispersion. It was clear that the solution of this task required a complex approach and using of other efficient methods together with alloying.
It is well known that the ceramic materials with fine-grained structure are characterized by high density, high hardness and high strength, because the strength of a material is inversely proportional to grain size (Hol–Pet’s effect). In addition, the processes of cracks nucleation and their propagation through the material with finely-dispersed structure are inhibited because of the small size of structural fragments and a large number of borders. Recent data prove that the strength in the compact materials with stable nanosized structure can be increased up to the theoretical limit.
Main tasks of the project are:
• Development and manufacture of nanocrystalline powders and appropriate nanostructured composites B4C–TiB2, B4C–ZrB2, 11B4С–Ti11B2 and 11B4С–Zr11B2 with substantially increased strength and wear-resistant properties;
• Fundamental research of physical-mechanical and thermal-physical properties, also optical properties of the obtained composites under ordinary and extreme conditions;
• Theoretical modeling of the fabrication processes of boron carbide-based nanocomposite materials on the basis of quasi-classical method of calculating substance atomic and electronic structures;
• Within the framework of the project, a) on the basis of MMI, the organization of a pilot-industrial site for the production of the product (cutters, dies, molds, etc.) with enhanced operational properties, as well as the product operating in the active zone of a reactor (plain bearings, shafts, couplings, etc.); b) fabrication and certification of the pilot samples of armor plates made of B4C-TiB2, B4C-ZrB2, 11B4С-Ti11B2, 11B4C-Zr11B2 composites with nanocrystalline structure.
As seen from the list of main tasks, the obtained results will be of a great importance for the scientists, working in the area of both, fundamental and applied research. The produced material will be used for the fabrication of product which will work in extreme conditions (including the active zone of the reactor – 11B4С–Ti11B2, 11B4С–Zr11B2).
The Project, in the optimal version, considers identification of the foreign firms and enterprises that may be interested in these developments and further cooperation.
Solution of the problems to be implemented within the scope of the Project to a large degree will be ensured by high skills of the Project participants as well as by the availability of sufficiently equipped experimental bases at the E. Andronikashvili Institute of Physics of the I. Javakhishvili Tbilisi State University (TSU) and F. Tavadze Metallurgy and Materials Science Institute (MMI).
The Andronikashvili Institute almost for two decades was a basic institute at the Academy of Sciences of the former SU, leading the field of the radiation physics of solid states. V. Kvatchadze, Doctor of Sciences in Physics and Mathematics is a Chief Researcher of the E. Andronikashvili Institute of Physics, an expert in the area of radiation physics of crystals and ceramics: MgO, MgAl2O4, 11B4C, 11B4C–Zr11B2. Z. Mestvirishvili is one of the young leaders of the Technology National Center of Georgia, in particular, Head of the Production Department. I. Bairamashvili, who is a leading expert on boron and boron containing materials, is a Consultant of the Project.
The MMI team for last fifty years is involved in technological developments of metallurgical and machine-building plants, taking part in successful scientific researches in the fields of materials science, powder metallurgy, nanocrystalline and composite materials. L. Chkhartishvili, Doctor of Sciences in Physics and Mathematics, Professor. His research interests are: condensed matter physics, nanophysics, materials science, (published 270 scientific papers). The modeling processes offered in the project will be based on the theories suggested by him in recent works. L. Chkhartishvili actively participates in the experimental works; he is co-author of the papers which create ideology of the project and which were fulfilled in the MMI. He and his co-workers not once participated in the regular International Symposia on Boron, Borides and Related Materials (ISBB) which was held twice in Tbilisi