Silicon nitride
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| Silicon nitride | |
|---|---|
| Identifiers | |
| CAS number | [] |
| Properties | |
| Molecular formula | N4Si3 |
| Molar mass | 140.28 g mol-1 |
| Appearance | grey, odorless powder |
| Density | 3.44 g/cm3, solid |
| Melting point |
1900 °C, 2173 K, 3452 °F (decomposes) |
| Hazards | |
| EU classification | not listed |
| Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references |
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Silicon nitride (Si3N4) is a hard, solid substance, that can be obtained by direct reaction between silicon and nitrogen at high temperatures. Silicon nitride is the main component in silicon nitride ceramics, which have relatively good shock resistance compared to other ceramics.
Rollers made of silicon nitride ceramic are sometimes used in high-end skateboard bearings, due to the material's shock and heat-resistant characteristics. It is also used as an ignition source for domestic gas appliances, hot surface ignition.
In microelectronics, silicon nitride is usually formed using chemical vapor deposition (CVD) method, or one of its variants, such as plasma-enhanced chemical vapor deposition (PECVD). It is usually used either as an insulator layer to electrically isolate different structures or as an etch mask in bulk micromachining. As a passivation layer for microchips, it is superior to silicon dioxide, as it is a significantly better diffusion barrier against water molecules and sodium ions, two major sources of corrosion and instability in microelectronics. It is also used as a dielectric between polysilicon layers in capacitors in analog chips.
Bulk, monolithic silicon nitride is used as a material for cutting tools, due to its hardness, thermal stability, and resistance to wear. It is especially recommended for high speed machining of cast iron. For machining of steel, it is usually coated by titanium nitride (usually by CVD) for increased chemical resistance.
[edit] Crystal Structure
There exist 3 crystallographic structures of silicon nitride (Si3N4), designated as α, β and γ phases. The α and β phases are the most common forms of Si3N4, and can be produced under normal pressure condition. The γ phase can only be synthesized under extremely high pressures and is third hardest material, following diamond and cubic boron nitride (BN).
See crystallographic structure of the α- and β- Si3N4 in [1] and γ phase Si3N4 in [2]
α- and β-Si3N4 have hexagonal structures, which are built up by corner-sharing SiN4 tetrahedra. They can be regarded as consisting of layers of silicon and nitrogen atoms in the sequence ABAB... or ABCDABCD... in β-Si3N4 and α-Si3N4, respectively. The AB layer is the same in the α and β phases, and the CD layer in the α phase is related to AB by a c-glide plane. The Si3N4 tetrahedra in β-Si3N4 are interconnected in such a way that tunnels are formed, running parallel with the c axis of the unit cell. Due to the c-glide plane that relates AB to CD, the α structure contains cavities instead of tunnels. The cubic γ-Si3N4 is often designated as c modification in the literature, in analogy with the cubic modification of boron nitride (c-BN). It has a spinel-type structure in which two silicon atoms each coordinate six nitrogen atoms octahedrally, and one silicon atom coordinates four nitrogen atoms tetrahedrally. [3]
