Thermal conductivity Aluminium nitride

MUL 95] MULLOT J., LECOMPTE J.P., JARRIGE J., High thermal conductivity aluminium nitride substrates with Y2O3 or YF3 additives . Fourth Euro Ceramics, Part II, Ed. C. Galassi, p. 235-241,1995. 

A number of nonmetallic materials are called high thermal conductivity materials. The most notable of these is diamond, with a thermal conductivity of 2000 Wm K. All of the others have a dia-mond-Uke stracture, and include boron nitride, BN, and aluminium nitride, AIN (Table 15.2). 

Their choice fell on aluminium nitride, a relatively untned material, which showed thermal conductivity 5 times that of alumina, very close to that of beryllia, whilst avoiding the toxicity of that material. What is more, the thermal expansion of AIN is very close to that of silicon, so that AIN seemed an ideal material for advanced electronic packages. 

Thermal conductivity studies have been conducted on a wide range of filled polymers and composites including carbon-fibres [2,37-43], aluminium powder [40], aluminium nitride [41], magnetite, barite, talc, copper, strontium, ferrite [42], glass fibre filled PP [44] and manganese and/or iron filled polyaniline [42]. 

Yu and co-workers have reported results of thermal conductivity measurements on PS-aluminium nitride composites [37]. 

Conductive heat transfer has a phononic nature, which means that the heat is transferred due to the oscillation of the atoms in the crystal lattice. Crystals with a simple lattice, such as sihcon carbide or carbon, have a lower dissipation of heat waves and a higher thermal conductivity compared to crystals with a more complex lattice. For example, the conductive thermal conductivity of aluminium nitride or silicon carbide (binary compounds with approximately equal atomic weights) is higher than alumina, magnesia, and zirconia. And the conductive thermal conductivity of said alumina, magnesia, and zirconia is higher than that of spinel, mullite, and zircon. 

Figure 19 Comparison of the thermal conductivity data published in the literature (a) adhesives filled with boron nitride [37,40] (b) aluminium (c) diamond [37-39] (d) aluminium nitride [37-42] (e) crystalline silica.

Curves of Figure 19 compare the data published for (a) boron nitride [37,40] (b) aluminium (c) diamond-[37-39] (d) aluminium nitride [37-42] (e) crystalline silica. It can be seen that, at 45 vol.%, the maximum thermal conductivity achieved with diamond powder is 1.5 W m K, while crystalline boron nitride at 35 vol.% affords 2.0Wm K. The thermal conductivity of silver-filled adhesives was studied by using silicon test chips attached to copper and molybdenum substrates [43]. The authors outline the importance of the shape factor A, related to the aspect ratio of the particles, to achieve the highest level of thermal conductivity. Another study reports the variation of the effective thermal resistance, between a test chip and the chip carrier, in relation to the volume fraction of silver and the thickness of the bond layer [44]. The ultimate value of bulk thermal conductivity is 2 W m at 25 vol.% silver. However, the effective thermal conductivity, calculated from the thermal resistance measurements, is only one-fifth of the bulk value when the silicon chip is bonded to a copper substrate. 

The conclusion that can be drawn from these experiments is that the use of highly priced fillers such as diamond powder does not improve the thermal conductivity better than less expensive materials such as aluminium nitride, boron nitride, boron carbide, or sdicon carbide. Within certain limits, the higher the X value of the filler particles, the higher the thermal conductivity of the adhesives with respect to the X/Xp ratio that exhibits a favourable optimized value at about 100. This means that fillers with a thermal conductivity in the range... 

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