Sintering is the process of forming and compacting a material by pressure and heat. Sintering is a major step in powder metallurgy and ceramics processing. The driving force for sintering is the reduction in total particle surface area. Surface energies are larger in magnitude than grain boundary energies.
Sintering is usually carried out below the melting temperature, so a liquid phase is normally not present. Sintering is often chosen as the shaping process for materials with extremely high melting points, such as tungsten, molybdenum, or uranium dioxide ceramics. For example, tungsten carbide (WC), which is used extensively in mining in top hammer rock drill bits, downhole hammers, and many more applications, is made by powder metallurgy, thus made by sintering in the final stage of production. A familiar example of sintering is forming a hard snowball by pressing loose snow together.
During sintering, compacted metal powders are bonded or sintered by heating in a furnace to a temperature usually below the melting point of the major constituent. The sintering of powder metals is a process in which particles under pressure chemically bond to themselves to form a coherent shape when exposed to a high temperature. This process is known as solid-state sintering. If the temperature is above the melting point of a component in the powder metal part, the liquid of the melted particles fills the pores. This type of sintering is known as liquid-state sintering.
Sintering time and temperature are the most significant factors from a practical perspective, with the temperature being the most important variable. During this process, the number of characteristics increases, including the material’s strength, ductility, toughness, and electrical and thermal conductivity. If different elemental powders are compact and sintered, the material will form into alloys and intermetallic phases.
Most PWRs use uranium fuel, which is in the form of uranium dioxide. Uranium dioxide is a black semiconducting solid with very low thermal conductivity. On the other hand, uranium dioxide has a very high melting point and has well-known behavior. The UO2 is fed into dies and pressed biaxially into cylindrical pellet form using a load of several hundred MPa – this is done in pressing machines operating at high speed. These ‘green’ pellets are then sintered into the solid cylinder (with a height and diameter of about 1 centimeter, the height being greater than the diameter) by heating in a furnace at about 1750°C under a precisely controlled reducing atmosphere (usually argon-hydrogen) to consolidate them. This also affects pellets densification. The dimensions of the fuel pellets and other components of the fuel assembly are precisely controlled to ensure consistency in the characteristics of the fuel. These pellets are then loaded and encapsulated within a fuel rod (a metallic cladding tube) made of zirconium alloys.