The transition temperature at which all the ferromagnetic materials become paramagnetic materials is termed as the 'Curie temperature'. Its value for cobalt is __________ °C.

ANS:B - 1127

Solid state sintering of powders is a process used to form solid materials from powders by heating them below their melting point. During this process, several mechanisms can be active to facilitate the bonding and densification of the powder particles. Let's explore these mechanisms in detail:

1. Solid State Diffusion Processes: One of the primary mechanisms involved in solid state sintering is solid-state diffusion. At elevated temperatures, atoms on the surface of powder particles diffuse across the particle boundaries and join adjacent particles. This diffusion process is driven by the thermal energy provided by the heat source. As particles come into contact, they bond together, leading to densification of the material.
2. Evaporation and Condensation: In some cases, particularly for materials with low melting points or volatile components, there may be some degree of evaporation and condensation occurring during sintering. At elevated temperatures, volatile components may evaporate from the powder particles, creating a vapor phase. These vapors can then condense onto neighboring particles, contributing to particle bonding and densification.
3. Liquid Phase Sintering: In certain situations, the sintering process may involve the formation of a liquid phase. As the temperature increases, some materials may undergo partial melting, leading to the formation of a liquid phase at the particle interfaces. This liquid phase can facilitate particle rearrangement and enhance the diffusion of atoms across the particle boundaries, promoting densification and the formation of strong bonds between particles.
4. Creation of More Dislocations: Sintering can also involve the creation and movement of dislocations within the material. Dislocations are defects in the crystal structure of a material that can facilitate plastic deformation. During sintering, the application of compressive stress and thermal energy can generate dislocations, which can aid in particle rearrangement and densification.

Overall, the solid-state sintering process is complex and can involve multiple mechanisms working simultaneously or sequentially to achieve the desired densification and bonding of powder particles. The specific mechanisms at play depend on factors such as the material composition, sintering temperature, pressure, and processing conditions. Understanding these mechanisms is crucial for optimizing the sintering process to produce materials with desired properties.