An Efficient Network Model for Determining the Effective Thermal Conductivity of Particulate Thermal Interface Materials

Particulate composites are commonly used in Microelectronics applications. One example of such materials is Thermal Interface Materials (TIMs) that are used to reduce the contact resistance between the chip and the heat sink. The existing analytical descriptions of thermal transport in particulate systems do not accurately account for the effect of inter-particle interactions, especially in the intermediate volume fractions of 30-80%. Another crucial drawback in the existing analytical as well as the network models is the inability to model size distributions (typically bimodal) of the filler material particles that are obtained as a result of the material manufacturing process. While full-field simulations (using, for instance, the finite element method) are possible for such systems, they are computationally expensive. In the present paper, we develop an efficient network model that captures the physics of inter-particle interactions and allows for random size distributions. Twenty random microstructural arrangements each of Alumina as well as Silver particles in Silicone and Epoxy matrices were generated using an algorithm implemented using a java language code. The microstructures were evaluated through both full-field simulations as well as the network model. The full-field simulations were carried out using a novel meshless analysis technique developed in the author’s (GS) research [26]. In all cases, it is shown that the random network models are accurate to within 5% of the full field simulations. The random network model simulations were efficient since they required two orders of magnitude smaller computation time to complete in comparison to the full field simulation

Experimental and Numerical Investigations of Thermal Ignition of a Phase Changing Energetic Material

Fortuitous exposure to high temperatures initiates reaction in energetic materials and possibilities of such event are of great concern in terms of the safe and controlled usage of explosive devices. Experimental and numerical investigations on time to explosion and location of ignition of a phase changing polymer bonded explosive material (80 per cent RDX and 20 per cent binder), contained in a metallic confinement subjected to controlled temperature build-up on its surface, are presented. An experimental setup was developed in which the polymer bonded explosive material filled in a cylindrical confinement was provided with a precise control of surface heating rate. Temperature at various radial locations was monitored till ignition. A computational model for solving two dimensional unsteady heat transfer with phase change and heat generation due to multi-step chemical reaction was developed. This model was implemented using a custom field function in the framework of a finite volume method based standard commercial solver. Numerical study could simulate the transient heat conduction, the melting pattern of the explosive within the charge and also the thermal runaway. Computed values of temperature evolution at various radial locations and the time to ignition were closely agreeing with those measured in experiment. Results are helpful both in predicting the possibility of thermal ignition during accidents as well as for the design of safety systems

Experimental Investigation of Foam-Phase Change Material Interactions for Thermal Energy Storage

High-density electronics and avionics such as Insulated gate bipolar transistor (IJBTs) and transistors, as well as vehicles themselves, generate excess heat, which must be dissipated to prevent overheating and failure. Phase change materials (PCMs) both rapidly dissipate heat through melting and can store this useful thermal energy for future use. The effectiveness of PCMs is limited by low thermal conductivity, thus, high conductivity metal foams are often introduced to improve the thermal storage performance. In the first part of this thesis, the melting and solidification behavior of a phase change material in a single millimeter-scale cavity is investigated with high resolution infrared (IR) microscopy and compared to numerical models. In the second part of this thesis, the melting and solidification behavior of PCM embedded in commercially available metal foams is explored with high-resolution infrared microscopy. The primary objective of the first experiment is to experimentally evaluate melting and solidification in small individual pockets of phase change material. In particular, we investigated phase change dynamics in cylindrical cavities of varying diameter. The melt/solidification front was tracked and radial temperature distribution curves were obtained to understand phase change in small pockets. This is a precursor to the study of phase change in foams with multiple pores. In the second part, IR microscopy directly observes the impact of foams on the phase change process. Since this work concentrates on heat recovery, more attention is given to the solidification behavior than to the melting behavior. The interface between the foam and PCM is closely observed to understand the thermal interaction between the solid high thermal conductivity scaffold and the phase change material. The foams significantly reduce the solidification times by approximately a factor of 3 due to localization of phase change within the pores and the high effective thermal conductivity of the composite which aids in efficient heat spreading. Decreasing the pore size (increasing the pores per inch within the available range) has little effect on the phase change time. Ultimately, this work provides new insight into the phase change dynamics assisted by high conductivity metal foams for better heat spreading which ultimately will enable the design of better, more efficient thermal storage systems with improved phase change response

Temperature control of thermal radiation from heterogeneous bodies

We demonstrate that recent advances in nanoscale thermal transport and temperature manipulation can be brought to bear on the problem of tailoring thermal radiation from compact emitters. We show that wavelength-scale composite bodies involving complicated arrangements of phase-change chalcogenide (GST) glasses and metals or semiconductors can exhibit large emissivities and partial directivities at mid-infrared wavelengths, a consequence of temperature localization within the GST. We consider multiple object topologies, including spherical, cylindrical, and mushroom-like composites, and show that partial directivity follows from a complicated interplay between particle shape, material dispersion, and temperature localization. Our calculations exploit a recently developed fluctuating-volume current formulation of electromagnetic fluctuations that rigorously captures radiation phenomena in structures with both temperature and dielectric inhomogeneities.Comment: 17 pages, 7 figuer

Depth estimation of inner wall defects by means of infrared thermography

There two common methods dealing with interpreting data from infrared thermography: qualitatively and quantitatively. On a certain condition, the first method would be sufficient, but for an accurate interpretation, one should undergo the second one. This report proposes a method to estimate the defect depth quantitatively at an inner wall of petrochemical furnace wall. Finite element method (FEM) is used to model multilayer walls and to simulate temperature distribution due to the existence of the defect. Five informative parameters are proposed for depth estimation purpose. These parameters are the maximum temperature over the defect area (Tmax-def), the average temperature at the right edge of the defect (Tavg-right), the average temperature at the left edge of the defect (Tavg-left), the average temperature at the top edge of the defect (Tavg-top), and the average temperature over the sound area (Tavg-so). Artificial Neural Network (ANN) was trained with these parameters for estimating the defect depth. Two ANN architectures, Multi Layer Perceptron (MLP) and Radial Basis Function (RBF) network were trained for various defect depths. ANNs were used to estimate the controlled and testing data. The result shows that 100% accuracy of depth estimation was achieved for the controlled data. For the testing data, the accuracy was above 90% for the MLP network and above 80% for the RBF network. The results showed that the proposed informative parameters are useful for the estimation of defect depth and it is also clear that ANN can be used for quantitative interpretation of thermography data

Comparison of calculated and measured temperature fields in laser-heated thin film systems

Thermal modelling of the laser processing of nanoparticulate ITO films has been carried out with models of varying complexity. The results from a simple semi-analytical 1D model and numerical 1D, 2D and 2D-axisymmetric models are reported for continuous wave HeCd laser and nanosecond pulsed XeCl laser irradiation. These results are compared to thermal camera measurements to determine the validity of the models under the different laser regimes.For continuous wave laser heating, it is shown that heat flow out of the laser irradiated volume significantly affects the predicted peak temperature rise. Models with fewer dimensions overestimate the temperature change, by a factor of over 100 times in the worst cases, due to the lack of lateral heat conduction. Consequently, meaningful temperatures are only calculated with 2D-axisymmetric or 3D models. When considering nanosecond pulsed lasers, the energy absorbed does not have enough time during the pulse to diffuse away from the volume in which it was deposited. Because of this, lateral heat flow is less important during heating and all the numerical models converge to the same predicted peak temperature rise. This allows much less computationally taxing models to be solved whilst obtaining the same result.The optical properties of the film are shown to be significant in determining the rate of laser induced heating and resultant temperature rise. However, for continuous wave irradiation, the models were insensitive to changes in the thermal parameters of the film and the peak temperature is controlled by the thermal parameters of the substrate. The opposite is true for the nanosecond pulsed lasers, with the thermal parameters of the film drastically affecting the temperature rise and the substrate parameters only contributing to the cooling which occurred over longer timescales. The differing sensitivity of the models to these parameters has been attributed to the rates of heating under the different laser regimes