Aluminum nitride (AlN) powder filler plays an important role in producing next-generation, high-performance resins (e.g., those with high thermal conductivity of around 10 W/m-K, as well as electric insulation). These materials are in demand in order to satisfy the strong need for heat dissipation in modern high-power consumption electronic devices, such as electric vehicles (EVs) in the automobile industry, high-power LED lamps, and other thermal sheets/greases for chip packages.

Powder Types and Properties

Today’s most popular commercial AlN powder fillers are produced either via direct nitridation of aluminum metal or by carbothermal reduction-nitridation of alumina.¹ Both processes produce different AlN particle sizes and shapes, and entail different costs. In addition, their application methods greatly differ.

For the direct nitridation process, AlN powder is crushed and ball milled with a top-down approach from several millimeters to micrometers after being synthesized. The irregular shape and predetermined particle size distribution are achieved through mechanical pulverization and screening, respectively. Figure 1 shows a typical AlN SEM image by direct nitridation, while Figure 2² is a cross-sectional TEM micrograph of AlN filler after coating. The impervious silane coating layer is evident.

For the carbothermal reduction-nitridation (CRN) of alumina, the AlN powder basically retains the original shape (i.e., “close” to spherical) and particle size distribution of the alumina raw material. This is normally in about a 1 µm range, with a high AlN purity after the CRN process. SEM images before and after CRN process³ are shown in Figure 3.

In addition to these two powders, sintered AlN powder with a spherical shape is also popular. This powder is made via sintered granulated powders that contain micrometer-sized AlN particles along with some additives. The extra granulating and sintering processes produce AlN granules that are tens of micrometers in size and spherical in shape. Some SEM images of typical AlN granules⁴ are shown in Figure 4.

Table 1 details the powders’ properties. Based on the table, it is evident that spherical sintered AlN powders can satisfy almost of the requirements of highly thermally conductive polymer-matrix composites, but their high cost is the biggest obstacle to their adoption in the market. In addition, direct nitridation-based AlN powder can also achieve relatively high thermal conductivity, but the irregular shape exerts a negative impact on particle packing and filling, due to forming more thermal-resistant junctions.

The smaller CRN powder is restricted for use in thin thermally conductive polymer-matrix sheets (normally less than 100 µm in thickness) or by mixing with some larger particle sizes for filling the interstitial space to give an increased packing density. In fact, the major application of CRN AlN powder is ceramic substrates with high thermal conductivity.

The thermal conductivity of the composites can be increased through several methods:

•  Forming conductive networks through appropriate packing of the filler in the matrix

•  Decreasing the amount of thermally resistant junctions involving a polymer layer between adjacent filler units by using large filler units with little or no defects

•  Decreasing the thermal contact resistance at the filler-matrix interface by minimizing the interfacial flaws⁵

Alternative Approach

An alternative approach exists for a “cost-effective” filler from among these three types of AlN powders, in order to achieve thermal conductivity greater than 10 W/m-K. The concept resulted from tests that were done regarding the improvement of thermal conductivity when replacing a part of the spherical Al₂O₃ powder with AlN powder in the formulation of silicone pads.

For the filling volume fraction of a polymer-matrix, the irregular AlN powder by direct nitridation process couldn’t achieve a high volume fraction (up to 70%) all by itself. (The actual maximum rate is about 63 vol%, and can’t be molded when increased to 67%.) However, with the aid of spherical Al₂O₃, 70 vol% is easily achievable. In addition, increased thermal conductivity can be reached with a higher fraction of AlN powder. The high shear friction caused by the irregular AlN particles during mixing with the polymer can be reduced by introducing the spherical Al₂O₃ particles, thus increasing the filling rate (solid content).

The same methodology can be extended to spherical sintered AlN powders. Few companies are currently able to fabricate a polymer-matrix composite capable of achieving 10 W/m-K by using spherical sintered AlN powders. This is only restricted to a niche market, however, considering the high price of spherical AlN material. In the previously mentioned scenario, if 50% of spherical sintered AlN powder can be replaced with an irregular-shaped AlN powder, the material cost can be reduced by up to 25-33% (assuming a 2-3 times cost difference between them), while retaining a relatively high thermal conductivity value. Regarding the issue of lower thermal conductivity achieved with the mixed irregular and spherical fillers instead of pure spherical filler, data show that large particle sizes of irregular filler can compensate for the thermal conductivity loss (see Table 3).

When undertaking partial replacement of CRN AlN powders with direct-reduction-based powders, it is important to keep in mind that, in a thin sheet polymer-matrix application, small spherical AlN powders ( 5 µm) will have a higher filling rate than irregular AlN powders of the same particle size. This is due to lower shear resistance and viscosity during and after the mixing process. However, the polymer-matrix tends to “flow out” on the edges of the composite, due to the lower surface area of the spherical particles that “holds” less polymer-matrix than the irregular particles. In addition, when the composite is pressed or heated before curing, some polymer-matrix may flow out due to the internal pressure. Replacing a small fraction of the spherical AlN powders with an irregular (direct-nitridation) AlN powder can hold the polymer-matrix on the composite.  

References

1.         Selvaduray, G. and Sheet, L., Materials Science and Technology, 9 [6], 463-473, 1993.

2.         www.thrutek.com.tw for cross-sectional micrograph of coated AlN.

3.         Jung, W.S., Journal of the Ceramic Society of Japan, 119 [12] 968-971, 2011.

4.         Wang, Q., Olhero, S.M., Ferreira, J.M.F., Cui, W., Chen, K., and Xie, Z., J. Am. Ceram. Soc., 96 [5] 1383–1389, 2013.

 5.         Xu, Y., Chung, D.D.L., Mroz, C., Composites Part A: Applied Science and Manufacturing, 32, 1749-1757, 2001.