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Silicon nitride |
| Silicon nitride | |
|---|---|
| Identifiers | |
| CAS number | 12033-89-5 |
| 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 references |
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Silicon nitride (Si3N4) is a hard, solid substance. It is the main component in silicon nitride ceramics, which have relatively good shock resistance and other mechanical and thermal properties as compared to other ceramics.
Contents |
Silicon nitride can be obtained by direct reaction between silicon and nitrogen at high temperatures. Electronic-grade silicon nitride is usually formed using chemical vapor deposition (CVD), or one of its variants, such as plasma-enhanced chemical vapor deposition (PECVD). Silicon nitride nanowires can also be produced by sol-gel method.1
Natural existence of silicon nitride is restricted to meteorites, where it very rarely occurs as mineral nierite.
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 high pressures and temperatures and has a hardness of 35 GPa2.
Pictures of crystallographic structure of the α- and β- Si3N4 can be found here3, and properties of the γ phase in this reference.4
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.5
Silicon nitride ceramics have relatively good shock resistance compared to other ceramics. Therefore, ball bearings made of silicon nitride ceramic are used in performance bearings. Silicon nitride ball bearings are harder than metal which reduces contact with the bearing track. This results in less friction, less wasted energy and higher speed. They are also much lighter and more durable than metal bearings under steady loads, the downside being their higher cost. Silicon nitride ball bearings can be found in high end automotive bearings, industrial bearings, wind turbines and even sometimes in high-end skateboards.
Silicon nitride is also used as an ignition source for domestic gas appliances, hot surface ignition.
In microelectronics, silicon nitride 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.
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