Nitride formulations

As noted, the oxidation resistance of silicon nitride ceramics depends on the type and concentration of the sintering aids. In materials designed for high temperature appHcations the specific weight gain resulting from oxidation upon a 500-h air exposure at 1200°C and 1350°C is about 1—2 g/m and 2—4 g/m, respectively. The kinetics of the oxidation process have been iavestigated (63,64) as has the corrosion resistance (65). Corrosion resistance is also dependent on material formulation and density. 

Salt-like nitrides are exemplified by Li3N (mp 548°C, decomp) and M3N2 (M = Be, Mg, Ca, Sr, Ba). It is possible to write ionic formulations of these compounds using the species N though charge separation is... 

Using silicon nitride powder in a polypropylene/microcrystalline wax/stearic acid binder formulation, the effect of filler volume fraction (V) (over the range 50 to 70%) on relative viscosity (rjj.) was predicted from Eq. 5 ... 

Considerable recent activity in the area of ceramic processing is aimed toward the formulation of materials with high strengths, comparable to the room temperature strength of metal alloys, at high temperatures (of the order of 2000 K). The impetus comes from the significant gains made in the last 20 years with materials formed from submicron powders of silicon nitride and silicon carbide and the promise of similar improvements in the near future. 

Nitrogen reacts directly with some electropositive metals to form nitrides such as Li3N and Ca3N2. Although these can be formulated with nitride ion N3- the bonding may be partially covalent. Other compounds with metals are amides and imides (containing NH2- and NH2-, respectively) and azides containing N3-. Metal azides are thermodynamically unstable and often explosive. 

Other Metal Nitrides Many other metal nitrides are used in ceramic formulations. These include AIN, TiN, VN, and BN. These metal nitride powders are produced by carbothermal reduction of the relevant metal oxide in a nitrogen-containing atmosphere or reaction of the relevant metal with a nitrogen-containing reducing atmosphere. These metal nitrides are used as abrasives and in hi -temperature wear applications. 

These points are more readily appreciated if we formulate nitrides with 4 N atoms, for if all the metal atoms are 4-coordinated and all N atoms have the same coordination number this c.n. is equal to the total number of metal atoms in the... 

The conversion of the molecular precursors into boron nitride requires a ceramisation under an ammonia flow. Ammonia is used as a reductive atmosphere, and its ability to replace halo or amino groups on the borazine framework is well known [12,13]. Ammonia is also a curing reagent in the borazine polymerisation [25]. So, for the preparation of BN from molecular precursors, one usually needs to perform a low temperature ceramisation, up to 600°C at least, under an ammonia flow. The conversion of each precursor into BN has been studied using TGA up to 1000°C in order to optimise the ceramisation conditions and the properties of the obtained ceramic. The measurements were realised in a pure ammonia flow up to 650°C and then under nitrogen. The three tested compounds presented original behaviours related to their formulation and their reactivity towards ammonia. 

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