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aluminum nitride

Aluminum nitride, molecular formula AlN, is a chemical compound of aluminum and nitrogen. It crystallizes in the wurtzite structure within the hexagonal space group P6 3 mc. The Al atoms form a dense sphere packing on a hexagonal lattice, the N atoms occupy half of the tetrahedral gaps of this lattice. The lattice constants are a: 3,1114 Å and c: 4,9792 Å. The radiopacity of AlN is 3.26 g / cc. Aluminum and nitrogen are predominantly covalently bonded, the proportion of ionic bond is 45%. The molecular weight M is 40.99 u. In a nitrogen atmosphere, it has a melting point of 2000 ° C and otherwise sublimation point of 2000 ° C and has hardness according to Mohs of. 9, These are the features of aluminum nitride that you should know about.

aluminum nitride

Aluminum nitride ceramic

Aluminum nitride ceramic is usually sintered at temperatures of about 1800 ° C without pressure. With the help of suitable sintering additives, liquid phase sintering occurs. In practice, the doping with calcium and yttrium oxide has become widely accepted as the standard method.

Since AlN ceramic has a very good thermal conductivity of 180 W / mK, it is used primarily as a substrate material in power electronics. Furthermore, the use of AlN ceramics is interesting in places where a lot of heat has to be dissipated, but the materials must not conduct electrical current. Aluminum nitride can also be obtained as a thin film by means of physical deposition processes (PVD), so-called sputtering. AlN is weakly piezoelectric.

Synthesis

Aluminum nitride powder can be prepared from alumina, nitrogen or ammonia and carbon in excess at a temperature> 1600 ° C in a carbothermic reaction:

\ mathrm {2 \ Al_2O_3 + 9 \ C + 4 \ NH_3 \ longrightarrow 4 \ AlN + 3 \ CH_4 + 6 \ CO}

\ mathrm {Al_2O_3 + 3 \ C + N_2 \ longrightarrow 2 \ AlN + 3 \ CO}

Another way is the direct nitriding. In this type of synthesis, metallic aluminum or aluminum oxide powder is reacted at temperatures> 900 ° C. with N 2 or NH 3 to give AlN:

\ mathrm {2 \ Al + N_2 \ longrightarrow 2 \ AlN}

\ mathrm {Al_2O_3 + 2 \ NH_3 \ longrightarrow 2 \ AlN + 3 \ H_2O}

Responsiveness

Aluminum nitride powder has a high sensitivity to hydrolysis. In the water incomplete cleavage of aluminum nitride in aluminum hydroxide and ammonia is observed. Sintered ceramic has no sensitivity to hydrolysis. In soda lye, both aluminum nitride powder and sintered AlN ceramic in ammonia and aluminate solution decompose according to:

\ mathrm {AlN + NaOH + 3 \ H_2O \ longrightarrow NH_3 + Na [Al (OH) _4]}

Further physical properties

The following properties apply to the AlN solid-state phase. The properties of AlN thin films are strongly dependent on the process so that z. B. intrinsic stress and AlN in thin layers characterized by other mechanical and electrical properties.

Breaking strength: 300-400 MPa (4 point bending test)

Modulus of elasticity: 350 GPa

Thermal expansion coefficient: 4.63 · 10 -6 (RT to 1850 ° C)

Specific heat: 0.738 J / gK

Thermal conductivity: 180-220 W / mK

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