Mark Kenneth Hoffmeyer

Analysis of Melt Undercooling and Crystallization Kinetics

While melt undercooling is often observed during solidification, the study of nucleation is challenging due to the numerous possible heterogeneous sites present in even high-purity melts. Identification of active nucleation sites requires developing well-planned experimentation. In samples with well-defined and controlled undercooling the identification can be established for the active sites. The successful identification of nucleation sites reveals that there are a number of possible sites and mechanisms that can act to catalyze nucleation. The sites and mechanisms that have been identified involve primary phases developed during cooling of alloy melts, liquid-added particle interfaces being modified (e.g., by adsorption or reaction) creating a particle type-independent nucleation potency, dissolved impurities precipitating out of the melt at high undercoolings to catalyze nucleation, and nucleation sites resulting from residual solid preserved in cavities in inclusions or surface coatings.

Novel graphite-based TIM for high performance computing

A new class of compressible graphite thermal interface material (TIM) was installed, tested, and qualified for use in a new series of IBM high performance computing (HPC) systems. Each system incorporated several high power graphics processing unit (300W GPU) assemblies in which a graphite TIM provided direct contact between bare die GPU devices and either air cooled heatsinks or water cooled cold plates. GPU hardware was removed from the system and exposed to a battery of thermal and mechanical stress tests, and reinstalled for in-system power age and power cycle tests, to quantify TIM reliability within a 3-year service life. After the GPUs were subjected to thermal-mechanical tests, which spanned accelerated thermal cycling (ATC), deep thermal cycling (DTC), thermal chip shock, temperature/humidity exposures, and system shock/vibration tests, the components were periodically reinstalled into systems to monitor power stability and to assess thermal reliability. For in-system tests, continuous power and thermal monitors were incorporated for all power cycle/power age regimens. Control groups of GPUs mounted with conventional grease-based TIMs were exposed to the same battery of thermal-mechanical and in-system server tests. All GPU hardware used for testing shared a common mounting design for TIM and cooling hardware attachments that provided a constant spring force clamping mechanism over the GPU bare die device area while enabling module flexure under load throughout stress test and system operating temperature ranges. Thermal effectiveness was measured by periodically monitoring the power draw of each GPU module and an internal device (junction) temperature over the course of the simulated life cycle. At the end of each evaluation, all GPU assemblies were then disassembled and assessed for TIM condition, which was then correlated with the final thermal resistance and power measurements. Based on comparison of both initial build performance and final test results, an optimized mounting construction was developed that incorporates the compressible graphite TIM.