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Development of Thermoelectric Boron Compounds by Powder

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Development of Thermoelectric Boron Compounds by Powder Impact Objectives • Explore the powder metallurgy of ceramics and intermetallic compounds materials • Improve the manufacturing process for compounds with high melting temperatures by using a liquid phase to lower the sintering temperature • Control the microstructures formed within the compounds High-quality, low cost boron carbide Dr Satofumi Maruyama outlines his research concerning the enhancement of the boron carbide sintering process Your current research Can you talk about some of the current mixing and sintering. In my case, eutectic focuses around boron barriers limiting the industrial application (e.g. of materials that are neither boron carbide. What type of of boron carbide? How do you hope your nor carbon) reactions between boron and properties does this research will help overcome these? metals are introduced into the reaction to compound possess? generate a liquid phase. This improves the The main limitation lies in the fabrication of densification of the sintering process. In Boron carbide has boron carbide materials in bulk. It currently addition to this, metal boride has important several attractive properties notably high takes a lot of energy to produce due to roles for the control of the properties. hardness and high chemical stability. Such the need to heat the raw materials to high attractive properties can be applied in temperature and then hold it there for a Briefly, what type of research is the many areas. Examples include: mechanical prolonged period. By using liquid phase Department of Mechanical Engineering, materials, such as for cutting and polishing sintering technology and spark plasma Tokyo City University involved in? tools; protective materials, such as bullet- sintering techniques, we aim to lower the proof vests and in tanks; and electronics, sintering temperatures required as well In our department, there are a lot of like thermoelectric generation. In addition to as reduce the holding time. Naturally, this different kinds of research activities these, as boron is a good neutron absorber, will save energy, but it also comes with related to engineering machinery. boron carbide materials are used for the the benefit of more readily controlling the Mobility (automobiles), engines and containment of radiation in nuclear power microstructures formed within the material. engine related topics (tribology, etc.), plants. Controlling these microstructures is an autonomous vehicles, material mechanics, important point of the research as some of fluid mechanics, surface engineering and What are some of boron carbide’s physical properties are closely tied to their engineering materials are all studied in limitations? proper formation. our department. In my Laboratory there are three staff members and around 30 One of the limitations of boron carbide use Can you outline the methods and tools you students (about 10 master course students is the poor sinterability of the compound. are using in your investigations? and approximately 20 undergraduate sintering is the key manufacturing students). Together we cover three main process for compounds with high melting I try to sinter poor sinterability materials research topics. We examine the fatigue temperatures such as boron carbide. using spark plasma sintering apparatus and and destruction/fracture of metals, I’m trying to improve the manufacturing liquid phase sintering methods. Obviously, especially in the case of casting iron. We process by using a liquid phase to lower I’m focused on boron carbide, however, also look at non-crystalline materials for the sintering temperature. This is useful I think that it can be applied to other engineering usage (e.g. functional glass). in itself, but it can also improve the covalent materials. Therefore, the research Our final focus, and my area of expertise, mechanical properties, and its electronic method is very simple because it aims is the powder metallurgy of ceramics and properties. to be widely-available. I follow a powder intermetallic compounds such as boron metallurgy method to fabricate boride bulk carbide. l materials. Powder metallurgy is simply 18 www.impact.pub Sintering boron carbide Researchers at Tokyo City University are working to make boron carbide synthesis easier and cheaper so the use of this incredibly strong compound can be expanded The manufacture of high-quality materials third hardest substance ever characterised.’ the substance to be sintered. ‘Powders are with specific properties occurs through The strength of boron carbide is such that it fine particulates that range in size from less a myriad of routes. Chemical syntheses, is the key component in bullet-proof vests, than one micrometre to millimetre scales,’ melting and melding, and even the use of in tank armour and car brake linings, as well explains Maruyama. ‘It is preferable that the electricity, are all methods through which as in cutting and polishing tools, amongst particle sizes of the powders are as small as different materials can be made. Sintering many others. The properties of boron mean possible, but there are problems associated is one key process through which many that boron carbide is an essential compound with the reduction of the particle sizes. As a important products are made. Dr Satofumi for absorbing neutrons in nuclear reactors result, we must consider the balance between Maruyama, based at the Department of and preventing any dangerous escapes. sinterability and the functionality of the bulk Mechanical Engineering, Tokyo City University Finally, boron carbide has potential use as materials.’ This is because sintering relies on in Japan, is a materials expert. ‘Sintering is an a superconductor and in thermoelectrics. the surface area and the surface energy this extremely flexible process where micro-scale ‘Boron carbide can be produced through carries. The release of this energy is what particles are bonded together through heat a sintering process; however, it does not makes the transformation of a compound and pressure,’ he explains. ‘It is often used sinter as readily as many other ceramic consisting of many fine particulates into a to work elements that are compounds with materials,’ points out Maruyama. His work is solid block energetically favourable. ‘This unworkably high melting temperatures.’ now focused on improving the sinterability occurs when the atoms in one particulate of boron carbide. To do this, he is testing a are able to diffuse across the edge of that The technology has been used in basic range of different sintering techniques aiming particulate into the next one,’ he confirms. form for thousands of years through its to produce high-quality boron carbide at a fundamental role in the production of lower cost than is currently available. A FINE BALANCE ceramics and the strengthening of bricks. It The higher the surface area, the higher is the process that allows ceramics to harden SINTERING PROCESS the surface energy and therefore the more into a range of different useful structures in In its most basic form, sintering is the energetically favourable the sintering the kiln oven. During the 19th century, our process of forcing microstructures of process becomes. ‘Such a calculation understanding of the mechanics of sintering compounds together and causing them would imply that the finer the powder, the vastly expanded through visionaries such as to bond together. ‘The conditions for this more easily sintered the compound is,’ William D Coolidge. He used the principles of vary greatly and depend on the compounds observes Maruyama. ‘This is true; however, sintering to create flexible tungsten elements involved,’ says Maruyama. ‘Ice sinters easily the properties of sintered compounds are for incandescent light bulb. From there, under commonly found conditions.’ For dependent on particular microstructures methods for sintering all sorts of materials example, if ice cubes are left together in a such as pore sizes that are formed during were honed. glass of water, the temperature difference the process.’ Without these structures, the between the water and the ice will cause the useful physical properties disappear and BORON CARBIDE cubes to sinter together. Indeed, it is this compounds such as boron carbide would not Boron carbide is a ceramic compound sintering process that causes the creation of be fit for their intended purpose. In addition containing a particular mix of boron and glaciers in nature. to the size of the particulate, other factors carbon. ‘In the right proportions, these two are also important. The temperature plays a elements form a durable, extremely hard and However, in industrial production sintering key role. ‘Sintering is often used in the place resistant material,’ notes Maruyama. ‘It is the is classically conducted utilising powders of of classical melting-based forging because sintering typically occurs at www.impact.pub 19 around half the melting temperature,’ he microscopy.’ This allows them to examine As Maruyama uncovers the ideal conditions continues. ‘However, there can be quite a the structure directly for its consistency and for low-temperature, SPS boron carbide range in this value and sintering at lower presence of key microstructures. Another sintering there is a high possibility that the temperatures is always more desirable.’ route that Maruyama has explored to cost of manufacturing the compound can Additional factors include the use of additive improve boron carbide sintering is the type be significantly reduced. Obviously, this will Through
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