The Effect of Interstitial Material on Thermal Joint Resistance for Several Metallic Joints: Analytical and Experimental Study

Abstract

In this study it was determined whether or not a reduction was realized for thermal joint resistance as a result of applying a TIM (Thermal Interface Material), in particularly eGraf, at an interface formed between metals of various thermal conductivities. The level of this reduction was measured. In addition, the experimental data was compared against an existing analytical model. Experimental data was obtained in vacuum condition for Al 7075, Steel 1018, and Inconel samples with applied interface pressures ranging from 0.15-2.7 MPa. The experimental data showed that thermal joint resistance was reduced for all metals with application of eGraf TIM. However, the experimental data indicated that eGraf was more effective in reducing the joint resistance of materials with low thermal conductivities, and can reduce this value by as much 95% when compared to bare contacting surfaces. The analytical model predicted well the trend of the experimental data, but not the actual magnitudes. This could be due to the uncertainty associated with the eGraf Young’s modulus measured under compressive loading. The agreement between the model and the experimental data improved at high interface pressures (i.e., above 2,000 kPa). * Undergraduate Research Assistant, Conduction Heat Transfer Laboratory. ψ Associate Research Professor, Conduction Heat Transfer Laboratory. Associate Fellow AIAA Email addresses: arianvista@tamu.edu (A. Vistamehr), emarotta@tamu.edu (E. Marotta). NOMENCLATURE Ep = Polymer Young’s modulus; (kPa). H = Hardness; (kPa). Hp = Polymer hardness; (kPa). hb = Bulk conductance; (W/m .K). hc = Thermal conductance; (W/m .K). k1, k2 = Solid thermal conductivities; (W/m.K). kp =Polymer thermal conductivity; (W/m.K). ks = Harmonic mean thermal conductivity; (W/m.K). 2 1,m m = Mean absolute asperity slopes of surfaces; (rad). P = Apparent contact pressure; (kPa). Rj = Thermal joint resistance; (m .K/W). T = Temperature; (K). to = Initial thickness; (m). t = Final thickness; (m). ∇T = Temperature gradient = ∆T/∆x; (K/m). σ1, σ2 = RMS surface roughness; (μm). σ = Effective surface roughness; (m). ω = Uncertainty Subscript 1 2 − = Surface 1 and 2 at the interface c = Constriction, Contact ∆Tinterface = Temperature drop across the interface = (T1-T2); (K). b = Bulk material