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Refractory Metals
Published in Anshuman Patra, Oxide Dispersion Strengthened Refractory Alloys, 2022
Refractory metals are defined as metals with a high melting point and, in general, their lower limit of melting point is 2000°C. Refractory metals have varied applications, such as in aerospace, nuclear fields, defense systems, chemicals, and electronics [1]. In the following sections, the structure, the properties, the applications, and the limitations of the refractory metals such as Tungsten (W), Molybdenum (Mo), Niobium (Nb), Rhenium (Re), Tantalum (Ta), Iridium (Ir), Ruthenium (Ru), Osmium (Os), and Hafnium (Hf) will be discussed.
Powder Metallurgy
Published in Sherif D. El Wakil, Processes and Design for Manufacturing, 2019
The word refractory means “difficult to fuse.” Therefore, metals with high melting points are considered refractory metals. These basically include four metals: tungsten, molybdenum, tantalum, and niobium. Some other metals can also be considered to belong to this group. Examples are platinum, zirconium, thorium, and titanium. Refractory metals, as well as their alloys, are best fabricated by powder metallurgy. The technique used usually involves pressing and sintering, followed by working at high temperatures. The applications are not limited to incandescent lamp filaments and heating elements; they also include space technology materials, the heavy metal used in radioactive shielding, and cores for armor-piercing projectiles. Titanium is gaining an expanding role in the aerospace industry because of its excellent strength-to-specific-weight ratio and its good fatigue and corrosion resistance.
Properties and Applications of Molybdenum
Published in C. K. Gupta, Extractive Metallurgy of Molybdenum, 2017
In its pure state, molybdenum is a lustrous gray malleable metal, capable of being filed and polished. It can also be turned and milled without difficulty. Molybdenum is an important refractory metal with a very high melting point (~2610°C). Only carbon, tungsten, rhenium, tantalum, and osmium possess still higher melting points. Its boiling point is 5560°C and its density is 10.22 g/cm3 at 20°C. The coefficient of thermal expansion is about one third to one half that of most steels. At elevated temperatures, this low expansion provides dimensional stability and minimizes the danger of cracking. Its thermal conductivity is several times that of many high temperature alloys and approximately half that of copper. Good thermal conductivity together with low specific heat allows molybdenum to be rapidly heated or cooled, with lower resultant thermal stresses than most other metals.
Multiscale Simulations of Thermal Transport in W-UO2 CERMET Fuel for Nuclear Thermal Propulsion
Published in Nuclear Technology, 2021
Marina Sessim, Michael R. Tonks
One fuel type being considered for NTP reactors is a ceramic-metal composite (CERMET). The CERMET fuel is composed of ceramic fuel particles embedded in a metal matrix. Uranium dioxide (UO2), uranium carbide (UC), and uranium nitride (UN) are possible uranium-bearing materials with a high enough melting temperature to withstand the conditions during the propulsion cycles. The metal matrix must also have a high enough melting temperature to withstand the propulsion cycles and a higher thermal conductivity than the fuel. Thus, refractory metals such as tungsten (W) or molybdenum (Mo) are the primary candidates. In addition, the neutron absorption cross section of the metal should be as low as possible, as any neutron absorption in the metal matrix will lower the efficiency of the CERMET fuel. Early NTP CERMET fuel designs used highly enriched UO2 as the fuel particle and W as the metal matrix material.7 However, there is now a move to low enrichment, so fuel materials with a higher uranium density than UO2, such as UN, are being considered.9 Molybdenum or a W-Mo alloy is being considered for the matrix, as Mo has a lower neutron absorption cross section than W. Many fuel element designs also include cladding of the fuel element outer surface and the subchannel surfaces. This cladding is often composed of the same material as the metal matrix.
IR laser ablation of high boiling elements (C, Mo, Ta, W and Re)
Published in Radiation Effects and Defects in Solids, 2021
Elements with high boiling points, such as C, and the refractory metals, Mo, Ta, W and Re, represent a class of materials extraordinarily resistant to heat and wear and tear (1). Except carbon, the refractory metals are inert and have high density and hardness (2). Some of their applications include tools to work metals at high temperatures, wire filaments, casting molds, chemical reaction vessels for high-temperature treatments. Special attention is given to the realization of alloys and thin films with peculiar properties, which are useful for many applications in different areas; from solid state physics, to chemistry, from biomedicine to microelectronics, from engineering to nuclear physics. Mo, Ta and W, for example, can be employed as first walls in Tokamak to withstand the high temperatures and ion sputtering effects of hot and high-density plasmas (3).