Technical Paper: PIM International, Vol.4 No.3 September 2010, pages 64-70, 3513 words

Authors: Valmikanathan P. Onbattuvelli[1], Srikar Vallury[1], Timothy McCabe[2],  Seong Jin Park[3] and Sundar V. Atre[1]

[1] Oregon Nanoscience and Microtechnologies Institute, Oregon State University, OR 97331, USA
[2] Kinetics Inc., Wilsonville, OR, USA
[3] Pohang University of Science & Technology, Republic of Korea

  

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Abstract

Aluminium nitride (AlN) and silicon carbide (SiC) were chosen for the fabrication of thermal management devices due to their high thermal conductivity and coefficient of thermal expansion values equivalent to that of silicon and silicon carbide dies. Additionally, powder injection moulding (PIM) was selected for net-shaping these ceramics in order to harness its production and geometric advantages. This paper presents an in-depth study on the effect of nanoparticles addition on the thermal and rheological properties of the SiC and AlN feedstocks. Specifically, bimodal mixtures of nanoscale and sub-microscale particles addition were found to significantly improve the powder content (solids loading) in the powder-polymer mixtures (feedstocks) for injection moulding. The implications of the variation in feedstock properties with nanoparticle additions are discussed in the context of the mould filling behaviour of these material systems.

Keywords: SiC, AlN, powder injection moulding, thermal management, nanoparticles

Introduction

Silicon carbide (SiC) and aluminium nitride (AlN) exhibit a combination of thermal and mechanical properties (Table 1) that are relevant to applications in electronics, aerospace, defence and automotive industries [1-4]. However, the successful translation of these properties into final applications lies in the net-shaping of these ceramics into fully dense microstructures. Extensive research has been done on net-shaping SiC and AlN via slip casting [5-6], tape casting [7-8], hot pressing [9-10], hot isostatic pressing [11-12] and cold isostatic pressing [13-14]. Additional methods including pyrolysis of pre-ceramic polymers were also explored but are in their infant stages at the present time [15-16]. Unfortunately, little research involving powder injection moulding (PIM) of SiC and AlN has been reported to date [17-19]. It is evident from a comparative study presented in the Table 2 that PIM has many advantages over competing net-shaping techniques to mass produce complex geometric parts for thermal management with closer dimensional tolerances.

Irrespective of the fabrication techniques, dependant on the thermal and mechanical properties, the densities of the fabricated ceramics are aimed towards their theoretical limits [20-21]. Prior reports investigated this issue by exploring the material and process parameters. For example, immense effort was taken in the past to understand the effect of nature and content of the sintering additives on the densification of SiC and AlN [22-23]. Similarly, process parameters including sintering temperature, hold time and pressure were varied to study the pattern of densification and/or grain growth [24-26]........

Further sections of this paper include:

Experimental Methods
- Materials
- Feedstock Formulation and Scale-Up
- Instrumentation
Results and Discussions
- Thermal Properties
- Rheological Properties
- PVT Measurements
- Injection Moulding Simulations
Conclusions
Acknowledgment
References

Figures and Tables:

Fig. 1 DSC curves confirming the melt peaks of the monomodal and bimodal SiC (a) and AlN(b) feedstocks along with their specific heat.
Fig. 2 TGA confirming the composition of the monomodal and bimodal SiC (a) and AlN (b) feedstocks along with their thermal profiles.
Fig. 3 Viscosity of the extruded monomodal (a) and bimodal (b) SiC feedstocks at different shear rate and temperature combinations, confirming its pseudo-plastic behaviour
Fig. 4 Viscosity of the extruded monomodal (a) and bimodal (b) AlN feedstocks at different shear rate and temperature combinations, confirming its pseudo-plastic behaviour
Fig. 5 Comparison of powder packing behaviour in monomodal (a) and bimodal (b) SiC feedstocks
Fig.6 Comparison of powder packing behaviour in monomodal (a) and bimodal (b) AlN feedstocks
Fig. 7 PVT relationships for monomodal (a) and bimodal (b) SiC feedstocks
Fig. 8 PVT relationships for monomodal (a) and bimodal (b) AlN feedstocks
Fig. 9 Progressive filling pattern during the injection moulding of monomodal SiC feedstock at 170°C, (a) 25% fill, (b) 50% fill, (c): 75% fill and (d) 100% fill
Fig. 10 Fill time vs injection temperature for all feedstocks, revealing that the nanoparticle addition slows the mould filling process
Fig. 11 Melt velocity vs injection temperature for all feedstocks indicating the nanoparticle addition slows down the melt’s fluidity thereby increasing the mould fill time.
Fig. 12 Shear stress at the wall vs injection temperature for all feedstocks, indicating that nanoparticle addition increases the shear stress on the walls
Fig. 13 Multi-slotted parts injection moulded with the monomodal SiC feedstock

Table 1 Properties of sintered SiC and AlN ceramics
Table 2 Comparison of the net-shaping processes for SiC and AlN
Table 3 Twin screw extrusion conditions used for compounding SiC and AlN feedstocks
Table 4 Values of different rheological constants for SiC and AlN feedstocks
Table 5 PVT coefficients for SiC and AlN feedstocks