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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.
CVD coatings
Published in Kwang Leong Choy, Chemical Vapour Deposition (CVD), 2019
Tungsten has the highest melting temperature of all refractory metals (3422°C). It tends to be difficult to form due to its high hardness, and the most common manufacturing technique is powder metallurgy. However, W can be easily deposited with CVD using the common gaseous precursor tungsten hexafluoride, WF6. CVD-deposited W coatings have high purity, density, and thermal conductivity with a fine, well controlled structure and thickness (µm to mm) required to meet the challenging coating requirements for fusion applications such as abrasion resistance to particle bombardment, high thermal load, high operating temperatures and high thermal conductivity. Other applications of pure W include electronic switches, filaments of incandescent light bulb, and elements to produce X-rays. For X-ray anodes manufacturing, tungsten is mixed with rhenium to improve alloy’s ductility, high temperature resistance, corrosion resistance and tensile strength. Such X-ray anodes are used in medical imaging devices and they are integrated into angiography, mammography, computed tomography, and cardiology equipment, as well as for other non-medical applications such as non-destructive inspection and security checks. In addition, thick metal coatings such as W and Re have been also deposited by CVD onto graphite or composites for fusion applications. Figure 6.13a shows a CVD reactor for the deposition of W-Re coatings and Figure 6.13b shows CVD deposited W layers with Re interlayers to reduce the thermal stress and for crack resistance [44]. Tungsten-rhenium deposits can range from 1 µm to 1 mm. In order to deposit a tungsten-rhenium alloy, two precursors are used: tungsten hexafluoride (WF6) and rhenium hexafluoride (ReF6) with dihydrogen being the carrier gas. The chemical reactions which occur on the substrate surface are as follows:
CVD coatings
Published in Kwang Leong Choy, Chemical Vapour Deposition (CVD), 2019
Tungsten has the highest melting temperature of all refractory metals (3422°C). It tends to be difficult to form due to its high hardness, and the most common manufacturing technique is powder metallurgy. However, W can be easily deposited with CVD using the common gaseous precursor tungsten hexafluoride, WF6. CVD-deposited W coatings have high purity, density, and thermal conductivity with a fine, well controlled structure and thickness (µm to mm) required to meet the challenging coating requirements for fusion applications such as abrasion resistance to particle bombardment, high thermal load, high operating temperatures and high thermal conductivity. Other applications of pure W include electronic switches, filaments of incandescent light bulb, and elements to produce X-rays. For X-ray anodes manufacturing, tungsten is mixed with rhenium to improve alloy’s ductility, high temperature resistance, corrosion resistance and tensile strength. Such X-ray anodes are used in medical imaging devices and they are integrated into angiography, mammography, computed tomography, and cardiology equipment, as well as for other non-medical applications such as non-destructive inspection and security checks. In addition, thick metal coatings such as W and Re have been also deposited by CVD onto graphite or composites for fusion applications. Figure 6.13a shows a CVD reactor for the deposition of W-Re coatings and Figure 6.13b shows CVD deposited W layers with Re interlayers to reduce the thermal stress and for crack resistance [44]. Tungsten-rhenium deposits can range from 1 µm to 1 mm. In order to deposit a tungsten-rhenium alloy, two precursors are used: tungsten hexafluoride (WF6) and rhenium hexafluoride (ReF6) with dihydrogen being the carrier gas. The chemical reactions which occur on the substrate surface are as follows:
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.
Review of the 9th European Pulse Plating Seminar and EAST Forum 2020
Published in Transactions of the IMF, 2020
W. E. G. Hansal, S. M. Zajkoska
Refractory metals such as tantalum and niobium have excellent corrosion, heat and wear resistance properties. Owing to their negative standard reduction potential they cannot be electrodeposited from a standard water-based electrolytes system. Dr. Adriana Ispas (Technical University of Ilmenau, Germany) presented results of a study on Ta and Nb electrodeposition from two different ionic liquid electrolyte systems. The choice of the metal precursor and the ionic liquid has a significant effect on the morphology of the deposition (cracks, smoothness, grain size). By optimising the pulse plating parameters such as deposition potential, pulse frequency and duty cycle the occurrence of cracks could be minimised and uniformity of deposits was increased.
Investigation on ammonium perrhenate behaviour in nitrogen, argon and hydrogen atmosphere as a part of rhenium extraction process
Published in Mineral Processing and Extractive Metallurgy, 2018
Shaya Sharif Javaherian, Hossein Aghajani, Hamed Tavakoli
Rhenium is a refractory metal which melts at 3180°C, exceeded only by tungsten among metals. It is also widely known for its high-elasticity modulus and its withstanding alternative thermal cycles without any damage. It also shows no ductile to brittle transmission and its ductile behaviour remains constant until its melting point. So, rhenium and its alloys are widely used in such areas as aerospace, electronics and petro chemistry (Busby et al. 2007; Lou et al. 2010; Naor et al. 2010). High-melting point of rhenium leads its components to be produced by powder metallurgy techniques. So the preparation of rhenium powder, which has favourable performance in powder metallurgy industry, has to be concerned (Trybus et al. 2002).