Controlling the unit-cell topology of microlattice structures can enable the customization of effective anisotropic material properties. A wide range of properties can be obtained by varying connectivity within the unit cell, which then can be further used to optimize structures specific to applications. A methodology for a systematic generation of microlattice structures is presented that focuses on controlling discrete topology instead of average porosity (as is done in conventional porous media). An algorithm is developed to create valid lattice structures without redundancies from a given set of template nodes. A set of possible permutations of structures from an eight-node cubic octant of a unit cell are generated for evaluation of the degree of anisotropy. Generic models are developed to calculate the effective thermal and mechanical properties as an effect of topology and porosity of the micro-architected structure. The thermal and mechanical anisotropies are investigated for the effective properties of micro-architected materials. A few of the structured materials are fabricated using 3D printing technology and their effective properties characterized. Structures are represented as graphs in the form of adjacency matrices. Effective thermal conductivity is analyzed using a resistance network model, and effective stiffness is evaluated using a self-consistent elastic model, respectively. A total of 160 000 structures are generated and compared to porous-metal foams in which porosity is one of the design variables. The results show that it is possible to obtain a wide range of properties spanning more than an order of magnitude in comparison to porous-metal structures. Structures with a maximum anisotropy ratios of 7.1 and 8.2 are observed for thermal and mechanical properties, respectively. Preliminary experimental results validated the anisotropy ratio for the thermal conductivity and stiffness.

We present a study on the effective mechanical compliance of porous aluminum foams. We develop an experimental setup to characterize the elastic properties as well as evaluate surface deformation with respect to porosity as well as pore size in an effort to correlate the properties to contact resistance of the foams when used as thermal interface materials. There have been multiple studies in the past to evaluate the effective elastic modulus of porous structure as a function of porosity through experimentation, simulation as well as analytic models. This work also serves as a validation for analytic and experimental data published by various researchers in the past. This study is one aspect of a larger study to empirically correlate the area of contact to thermal contact resistance. We evaluate samples with three different porosity and three different PPI (pores per inch) specification. Additionally we analyze effect of presence of filler material in the voids — a phase change material. The filler is used as separate stand-alone TIM in the industry currently.

We present a study on the apparent thermal resistance of metal foams as a thermal interface in electronics cooling applications. Metal foams are considered beneficial for several applications due to its significantly large surface area for a given volume. Porous heat sinks made of aluminum foam have been well studied in the past. It is not only cost effective due to the unique production process but also appealing for the theoretical modeling study to determine the performance. Instead of allowing the refrigerant flow through the open cell porous medium, we instead consider the foam as a thermal conductive network for thermal interfaces. The porous structure of metal foams is moderately compliant providing a good contact and a lower thermal resistance. We consider foam filled with stagnant air. The major heat transport is through the metal struts connecting the two interfaces with high thermally conductive paths. We study the effect of both porosity and pore density on the observed thermal resistance. Lower porosity and lower pore density yield smaller bulk thermal resistance but also make the metal foam stiffer. To understand this tradeoff and find the optimum, we developed analytic models to predict intrinsic thermal resistance as well as the contact thermal resistance based on microdeformation at the contact surfaces. The variants of these geometries are also analyzed to achieve an optimum design corresponding to maximum compliance. Experiments are carried out in accordance with ASTM D5470 standard. A thermal resistance between the range 17 and 5 K cm2/W is observed for a 0.125 in. thick foam sample tested over a pressure range of 1–3 MPa. The results verify the calculation based on the model consisting the intrinsic thermal conductivity and the correlation of constriction resistance to the actual area of contact. The area of contact is evaluated analytically as a function of pore size (5–40 PPI), porosity (0.88–0.95), orientation of struts, and the cut plane location of idealized tetrakaidecahedron (TKDH) structure. The model is developed based on assumptions of elastic deformations and TKDH structures which are applicable in the high porosity range of 0.85–0.95. An optimum value of porosity for minimizing the overall interface thermal resistance was determined with the model and experimentally validated.

We present a modeling study of the effective thermal conductance of metal foams as a thermal interface focusing on the electronics cooling applications. Metal foam material as a porous media has been considered for several applications because of its significantly large surface area for a given volume. In the electronics cooling, aluminum porous heat sinks have been well studied. It is not only cost effective due to the unique production process, but also attractive for the theoretical modeling study to determine the performance. In the past studies, effective thermal conductivity for heat transfer through solid with the tetrahedral geometry, while the pores, assumed to have vacant volumes, has been modeled. Instead of allowing the refrigerant flow through the connected pores in the porous medium, we considered the vacant space filled with stationary air. The major transport of the heat is considered to flow through the solid bridges, which connect the onside to the other side directly. Thus, the porous density for our application shall be relatively lower than the best value for heat sink applications. It must, however, be in a specific range such that it is mechanically compliant to make proper contact to both, the cooling target surface and the heat sink surface. It is obvious that the smaller pore ratio makes the metal foam stiffer. We model the contact thermal performance with considering the mechanical stiffness as a function of pore ratio as well as an intrinsic thermal conductance across the metal foam. Due to the limited literature of the variation of the pore size, we present the first order analysis assuming a fixed and uniform pore size.

Use of microchannel heat sinks for high heat flux applications is substantial for thermal management and it is also critical for scalable power generation. For both applications, the energy efficiency consideration of the pump power is crucial. A number of models have been created that predict the performance as a function of the geometrical parameters, taking into account, the pressure loss over the length and volume constraints. Most of the approaches either involve sophisticated calculations incorporating fluid dynamics in channels, or have an analogy with the pin-fin model, which gives simpler calculations but considers only a single laminar flow regime for optimization. Even with the simplified models available, the geometrical impact on mass and pumping power is nonlinear and not apparent for optimization. We propose an optimization of porous medium heat sinks with respect to the heat transfer rate, mass, and pumping power. These are functions of the simplest geometric parameters, i.e. porosity, pore density, and length of the porous medium. Considering large production, mass (cost of raw material) is nearly proportional to the cost of the heat sink, we consider minimizing the mass for indirectly minimizing the overall cost. The other factor for saving energy considered here is the pumping power. This connects to both the heat transfer rate and the power consumption to drive the fluid through the porous medium. The optimization is performed for a specific value of porosity and length of the heat sink. The model considers the effect of flow through the porous medium and the effective thermal conduction as a function of combined conductivity of the solid ligaments and the fluid in pores. An optimum coefficient of performance (COP) is found at over 90% of porosity for minimum mass, pumping work and maximum heat transfer. This mathematical expression of the model will give a quantifiable figure-of-merit to take into account the impact of the mass and the pumping power on the performance to cost ratio.