nitrides, assignement di gruppo ceramic materials

NITRIDES

Introduction

Refractory nitrides are interesting materials for both industrial applications and scientific research. They have been known for over one hundred years, but their industrial importance is growing and not only thanks to their well-known properties, that are high strength and refractory nature, but also due to their electrical properties, recently investigated. We will describe nitrides, their properties, the processes through which they are formed and their relevant applications.

Classification and properties

It is defined as nitride a compound that nitrogen forms with elements of lower or about equal electronegativity. Nitrides can be classified in five categories according to their electronic structure and bonding characteristic: interstitial, covalent, intermediate, salt-like and volatile nitrides. Most used and investigated nitrides are refractory nitrides, which are inorganic materials characterized by high resistance to decomposition by heat, pressure, or chemical.

attack. In other² words, refractory nitrides have high chemical stability, melting point, hardness and wear resistance.

Considering the five categories mentioned above, only some nitrides accomplish the refractory requirements: nitrides of boron, silicon and aluminum for what concern covalent nitrides; nitrides² of titanium, vanadium, zirconium, niobium, hafnium and tantalum for what concern interstitial nitrides.

There are three factors which influence nitrides properties from the atomic point of view:

• Size of the respective atoms: nitrogen has one of the smallest radius on the periodic table, so it forms interstitial nitrides (i.e., TiN) in presence of higher radius elements and covalent nitrides (i.e., Si N) in presence of elements with a similar radius.^{3 4}
• Difference in electronegativity: nitrogen has very high electronegativity so this difference is small in case of covalent nitrides, high in case of interstitial nitrides.
• Electronic bond nature: nitrides

Bonds can be covalent, as in covalent nitrides, or a combination of metallic, covalent and ionic behavior, as in interstitial nitrides.

Interstitial nitrides are defined as crystalline inorganic compounds in which a metal, with atomic radius higher than a minimum value (maximum radii ratio N-Metal=0.59), hosts nitrogen atoms in its octahedral interstitial sites.

Within the different compositions of interstitial nitrides, the most common and important one is the mononitride (MN, M=metal) with face-centered cubic close-packed structure (FCC) and x-1 octahedral interstitial sites in which nitrogen atom fits.

The melting point of these nitrides lowers with the group: indeed, group VI nitrides (Cr, Mo, W) are not refractory since they have not sufficiently high melting temperature and chemical stability. We will just consider refractory nitrides properties.

Knowing that interstitial nitrides have complex bonds with a high metallic component, high electrical and thermal

conductivities are observed (see Table 3). Though, values of thermal conductivity are still low if compared to best conductors, such as covalent nitrides. Finally, we report mechanical properties of these refractory nitrides, such as hardness and Young's modulus. Covalent nitrides Within all covalent nitrides only boron, silicon and aluminum nitrides are refractory. Common features are low density, covalent electronic bonding (simpler to consider than interstitial bonding) and low atomic weight. Atomic structure and electronic configuration highly affect covalent nitrides properties: Boron nitride.- Boron can be considered a non-metal and its nitride has two main allotropes: the first has a hexagonal structure, similarly to graphite, which leads to an anisotropic, soft and lubricious nitrogen; the second has cubic structure (zincblende), similarly to diamond, which leads to extreme hardness, excellent chemical resistance and isotropy. Aluminum nitride- : Aluminum is more metallic than boron,but it has still ionic and covalent characteristics. It has a hexagonal close-packed structure of wurtzite type, which is similar to that of c-BN and leads to isotropy and high hardness.

Silicon nitride- : Silicon is essentially non-metallic and has two hexagonal crystallographic forms that slightly differ but behave exactly as c-BN. These nitrides are non-metallic compounds, so good electrical insulators. Moreover, single crystals of c-BN and AlN have extremely high thermal conductivity, not far from those of best conductors such as silver (420 W/mK) and copper (385 W/mK).

Covalent nitrides have variable mechanical properties according to the fabrication process and the crystalline structure: for example, as shown in the table below, HCP-BN has low hardness and Young modulus if compared to cubic BN. A remarkable behavior for a ceramic material is that of silicon nitride, with very high toughness.

Other nitrides Intermediate nitrides usually decompose readily and are not stable from the chemical

Point of view, as in the case of manganese, iron, and nickel nitrides. Salt-like nitrides are essentially characterized by an ionic bond between elements and have similar properties to salts. For example, they are very sensitive from a chemical point of view, so they cannot be defined as refractory. Despite that, salt-like nitrides are industrially used as sintering additives for the production of silicon and aluminum nitrides. Recently, a very interesting class of more complex nitrides, known as MAX phases, have been investigated. These are nitrides formed by an early transition metal (M), an element of group 13 or 14 (A), and carbon or nitrogen (X).