High Power Density Electronics Research Articles

Multi-entropy electrocaloric film induced by three-dimensional thermal conductive ceramic network for synergistic thermal management

Imparting high thermal conductivity to electrocaloric refrigeration polymers is a promising solution for efficiently synergizing and accurately managing the vast heat generated by high-power density electronics. We employed a sacrificial template method to construct a three-dimensional thermally conductive SiOC ceramic network and embedded it in the electrocaloric polymer. The electrocaloric composite film with a 1 wt. % SiOC achieved a synergistic improvement in heat transfer and electrocaloric cooling, while the thermal diffusivity of the 10 wt. % composites increased sixfold. The three-dimensional ceramic network not only acted as a heat transfer pathway but also induced the transition of the electrocaloric polymer chain to a high-entropy state, increasing the number of dipole entities that could be driven by the electric field. The system reached a refrigeration temperature change of 2 K at a field strength of as low as 30 MV m−1. The high-entropy and high-crystallized electrocaloric composite film induced by the SiOC thermal conductive interface provides a tool for synergistic thermal management.

A Thermally Enhanced Film Adhesive for Assembling High Power Density Electronic Devices

Assembly adhesives play a crucial role in packaging and thermal management design for microelectronics devices. The main function of the adhesives is attaching multiple components together to deliver designed functionalities and ensure integrity and reliability of the device. As requirements for performance and functionality increase, heat generated from these devices increases exponentially. For example, local heat flux can easily exceed 50 W/cm2 for IGBT power modules while it may reach several hundred W/cm2 at active areas for Gallium Nitride (GaN) power amplifiers. However, intrinsic thermal conductivity of polymer-based adhesives is around 0.2 W/mK, which increasingly becomes a thermal bottleneck in high power density electronics devices, especially those used in defense and aerospace systems. Effectively and safely dissipating the high heat and ensuring device reliability have become one of the critical tasks in the development of modern aerospace and defense systems. This paper presents an adhesive film that uses non-electrically conductive particles with high thermal conductivity as fillers in the adhesive to enhance thermal transport. The adhesive film offers an effective thermal conductivity around 0.7 W/mK, which is more than a 3X improvement over conventional assembly adhesives. In addition, the film adhesive has a long work life at ambient conditions with significantly improved processability and controllability, such as bondline thickness control and uniformity, no voids/air pockets, no or minimum resin bleed, etc. They can also be easily converted into preforms with complicated shapes/geometries with typical tolerances less than 10 mil to facilitate assembly processes and ensure a clean and waste free environment.

Point-Contact Bonding of Integrated Three-Dimensional Manifold Microchannel Cooling Within Direct Bonded Copper Platform

Abstract A microchannel heat sink integrated with a three-dimensional manifold using direct bonded copper (DBC) is promising for high power density electronics due to the combination of low thermal resistance and reduced pressure drop. However, this requires much progress on the fabrication and high-quality point-contact bonding processes of the microchannel substrate and three-dimensional manifold DBCs. In this study, we have developed processing techniques for surface preparations and high-quality point-contact solder bonding between the two DBC substrates. We utilized chemical polishing followed by electroless plating to prevent excess solder from blocking the microchannels. We performed a parametric study to investigate the impact of bonding time and surface roughness on the tensile strength of the bonding interface. The bonding strength increased from 1.8 MPa to 2.3 MPa as the bonding time increased from 10 to 30 min while reducing the surface roughness from Rz = 0.21 to 0.05 μm, resulting in increasing the bonding strength from 0.16 MPa to 2.07 MPa. We successfully tested the microcooler up to the inlet pressure of 70 kPa and pressure drop of 30 kPa, which translates to the tensile strength at the bonding point contacts, which remains well below the 2.30 MPa. We achieved the junction-to-coolant thermal resistance of 0.2 cm2 K/W at chip heat flux of 590 W/cm2. Thus, our study provides an important proof-of-concept demonstration toward enabling high power density modules for power conversion applications.

Aligning graphene nanoplates coplanar in polyvinyl alcohol by using a rotating magnetic field to fabricate thermal interface materials with high through-plane thermal conductivity

With the continuous advancement of electronic devices, there is an urgent need for advanced thermal interface materials (TIMs) to prevent high-power density electronics from overheating. The orderly arrangement of thermally conductive fillers in TIMs plays a crucial role in enhancing thermal conduction along preferred directions. However, it is challenging to control the orientation of the fillers, especially for two-dimensional fillers. In this study, graphene nanoplates (GNPs) were co-planarly arranged in polyvinyl alcohol (PVA) using a rotating magnetic field, which significantly increased the thermal conductivity of the composites. The coplanar vertically aligned GNPs/PVA (CVGNPs/PVA) exhibited a through-plane thermal conductivity of 11.78 W m−1 K−1, which is about 10 times higher than that of the composites with disorderly distributed GNPs (1.14 W m−1 K−1). The rotating magnetic field facilitated the alignment of GNPs and increased face-to-face contact between adjacent GNPs, which significantly boosted the through-plane thermal conductivity of the composite. The compressive modulus of the CVGNPs/PVA composites was only 1.06 MPa, and it helped to reduce the thermal interface resistance to 49 mm2 K W−1. These results offer a novel approach for preparing excellent TIMs that could be used in various applications.

Enhance flow boiling in Tesla-type microchannels by inhibiting two-phase backflow

Improving CHF and HTC of microchannel flow boiling is highly desirable for high-power density electronics cooling. However, two-phase flow instabilities strongly influence the enhancement of flow boiling in confined domains. Although extensive surface modifications and advanced functional structures have significant effects on two-phase heat transfer, gradual suppression of vapor backflow along in the entire channel length still lacks. A novel tesla-type copper microchannel configuration was explored with aim to inhibit vapor backflow and promote two-phase transport simultaneously in the forward direction in this work. Also, the effect of this configuration on two-phase mixing is investigated in the backward direction. Experiments are conducted on DI-water with mass velocity varying from 60 to 360 kgm−2s−1. Flow boiling and two-phase regimes are comprehensively investigated and compared to plain-wall microchannel. Overall HTC and CHF are significantly increased owing to effective suppression of vapor backflow in forward direction and enhanced two-phase mixing in backward direction. For example, at G = 360 kgm−2s−1, CHF and HTC are up to 274 Wcm−2 and 144 kWm−2K−1, respectively, accompanying with enhancements of 88.4% and 86.8% in the forward direction. Interestingly, the thermal performances in the forward direction outperform that in backward direction as the further increase of mass velocities, indicating the successful regulation of backflow. Upon the management of vapor backflow, two-phase flow instability regarding pressure fluctuation is dramatically reduced up to 7-fold.

A Hybrid Pressureless Silver Sintering Technology for High-Power Density Electronics

Aerospace and defense applications present unique challenges for material suppliers. As increasing adoption of advanced semiconductor materials and Diverse Accessible Heterogeneous Integration technologies, power density of defense and aerospace devices increases rapidly. Traditional die-attaching technology is becoming an increasingly limiting factor in microelectronics packaging for the next generation aerospace and defense systems. Metal particle sintering creates a porous metal foam, which can significantly enhance heat transfer within the sintered material. However, the traditional silver sintering requires very high sintering temperatures that cannot be tolerated by typical microelectronics devices. In addition, the sintered metal foam contains open-cell pores that can absorb/entrap moisture and dusts, which poses a reliability risk. This article introduces an advanced hybrid silver sintering material, which incorporates highperformance silver sintering with high reliability and process-friendly of epoxy-based die-attaching technology. This hybrid sintering paste can be processed without applying any pressures in temperature ranges that are normal in microelectronics packaging processes. Preliminary experimental studies, including Scanning Electron Microscopic study, volume resistivity tests, die shear strength tests, and thermal resistance tests, were performed for developing a low temperature sintering schedule that is compatible with normal assembly processes of high-power density electronics devices. The results indicated that the hybrid material could achieve silver sintering at 150 degrees C and offer 36 enhancement on thermal performance comparing with a widely used die-attach material.

A Hybrid Pressure-less Silver Sintering Technology for High-power Density Electronics

Integration of smaller, higher-functioning devices and use of advanced high thermal chip structures present thermal management challenges in the aerospace sector where reliability is the top priority and fail-safe processes/materials are the standard. As power density increases rapidly, traditional die attaching technology is becoming an increasingly limiting factor in microelectronics packaging for the next generation aerospace and defense systems. This paper introduces an advanced hybrid silver sintering technology, which incorporates ultra-high thermal and electrical performance of silver sintering with high reliability and process friendly of epoxy-based die attach technology. Unlike traditional silver sintering that requires high temperature and pressure, this hybrid sintering paste can be processed without applying any pressures in temperature ranges that are normal in microelectronics packaging processes. This paper presents results of an application study aimed at developing this unique technology in the field of high-power density devices for aerospace applications.

Drop impact phenomena and spray cooling on hot nanotextured surfaces of various architectures and dynamic wettability

Spray cooling has been used to quench metal slabs during casting, cool nuclear reactors, suppress accidental fires, and remove heat from high-power density electronics. In particular, the miniaturization of electronic devices inevitably results in an increased power density or heat flux on the microelectronics surfaces and poses a threat of a thermal shutdown of such devices when cooling is insufficient. Surface nanotexturing effectively augments additional liquid-to-substrate surface area, thereby increasing cooling capability, as well as an effective heat transfer coefficient. In spray cooling, surface dynamic wettability also affects drop impact dynamics and subsequent coolant evaporation on a hot surface. Herein, we introduced various nanotextured surfaces and affected dynamic wettability using the so-called thorny-devil nanofibers, nickel nanocones, Teflon and titania nanoparticles, and zinc nanowires. The effect of these different nanoscale architectures on drop impact phenomena and subsequent evaporative cooling was investigated. These nanotextured surfaces were fabricated using various deposition methods, including electrospinning, electroplating, supersonic spraying, aerosol deposition, and chemical bath deposition. We found that the surface with greater dynamic wettability related to the hydrodynamic focusing considerably improved the heat removal capability by furthering the Leidenfrost limit and facilitating drop spreading. In particular, the thorny-devil nanofiber surface yielded the highest heat flux at all ranges of the Reynolds and Weber numbers. Spray cooling on a model electronic kit also confirmed that the thorny-devil nanofibers were most effective in cooling the surface of the model kit during multiple cycles of water spraying.

Secondary flow through lateral passages to improve hydrothermal performance of liquid-cooled microchannel heat sinks

This study proposes a simple cost-effective concept to improve hydrothermal performance of liquid-cooled microchannel heat sinks utilized in high power density electronics. It consists of parallel microchannels comprising interdigitated cavities on sidewalls interconnected using lateral passages. With suitable misalignment of cavities in adjacent microchannels, secondary flow forms in lateral passages which can improve the heat sink performance. Different cavity shapes are chosen and hydrothermal performances are compared with that of a simple straight-channel heat sink. 3D numerical simulations are performed on a model consisting of a Gallium Nitride substrate as the heat source, a Silicon heat sink, and liquid deionized water as the coolant considering temperature-dependent thermophysical properties of Silicon and water. Compared to the base case, heat sink with prismatic cavities and lateral channels has the largest shrinkage of hotspot area and reduces hotspot temperature by 1.6 °C at Reynolds number of 160. This configuration enhances the average Nusselt number by 20.2% at the same Re. Moreover, implementing backward cylindrical cavities with lateral channels improves the Coefficient of Performance and Performance Enhancement Factor by 31.7% (at Re=40) and 22.7% at (Re=160), respectively. At the end, performance improvements proposed in the previous works are provided within an itemized review table.

Parametric investigation on the close-loop R410A flash spray system for high power electronics cooling under low temperature

For high power density electronics, flash spray cooling is one of the most effective ways of dissipating heat at safe working temperature. Spray parameters exert significant influences on the heat transfer capacity. This paper conducted an experimental investigation to explore the influences of spray chamber pressure, mass flow rate and subcooling degree on the cooling capacity on a flat heated surface in a spray cooling cycle using R410A as the working fluid. The results indicated that the initial increases in spray chamber pressure, mass flow rate and subcooling degree all improved the heat transfer performance. The best cooling performance was achieved once these parameters reached their optimal values; those were 0.57 MPa, 4.9 g/s and 5 °C respectively. The subsequent increase of them deteriorated spray cooling heat transfer, decreasing critical heat flux and heat transfer coefficient while increasing surface temperature. The main reasons responsible for the heat transfer were related to the atomization performance of flash spray affected by the spray parameters. The critical heat flux and maximum heat transfer coefficient yielded on a simple flat copper surface achieved 300 W/cm2 and 550 kW/(K m2) while surface temperature was maintained below 30 °C under the optimal spray parameters.

Recent Advances in Thermal Interface Materials for Thermal Management of High-Power Electronics

With the increased level of integration and miniaturization of modern electronics, high-power density electronics require efficient heat dissipation per unit area. To improve the heat dissipation capability of high-power electronic systems, advanced thermal interface materials (TIMs) with high thermal conductivity and low interfacial thermal resistance are urgently needed in the structural design of advanced electronics. Metal-, carbon- and polymer-based TIMs can reach high thermal conductivity and are promising for heat dissipation in high-power electronics. This review article introduces the heat dissipation models, classification, performances and fabrication methods of advanced TIMs, and provides a summary of the recent research status and developing trends of micro- and nanoscale TIMs used for heat dissipation in high-power electronics.

Improving through-plane thermal conductivity of PDMS-based composites using highly oriented carbon fibers bridged by Al2O3 particles

Efficient thermal interface materials (TIMs) are urgently needed for heat dissipation of high-power density electronics. In this study, vinyl polydimethylsiloxane (PDMS) composites with the spatial alignment of carbon fibers (CFs) bridged by Al2O3 particles were fabricated by the flow field. The through-plane thermal conductivity (TPTC) of the composites with 24 vol% CFs and 47 vol% Al2O3 loading reached 38.0 W m−1 K−1. The oriented CFs bridged by Al2O3 acted as the efficient through-plane thermal conductive network. Furthermore, the effects of shape factor (b/a), spatial angle (γ) of CFs, and CF loading (Vf) on the TPTC were quantitatively discussed by steady-state finite element simulation combined with micro-computed tomography and machine learning. The positive contribution of the increased Vf to TPTC was in competition with the negative contribution of b/a and γ, both of which increased with the increase of Vf. Moreover, b/a exerted more negative effects than γ. The PDMS composites demonstrated excellent thermal stability (Td = 407.5 °C, CTE = −55.3 × 10−6 K−1), low compress modulus (1.71 MPa), and hardness (47 (Shore C)), which made them potential candidates for TIMs. This work offers a feasible method to prepare TIMs on large scale and refreshes the thermal conduction mechanism of TIMs by introducing the influencing factors (b/a and γ).

Additively Manufactured Flow Manifolds Enable High Performance Direct Cooling for High Density Power Electronics

Additively Manufactured Flow Manifolds Enable High Performance Direct Cooling for High Density Power Electronics

Channel Design Optimization for 1.2-kV 4H-SiC MOSFET Achieving Inherent Unipolar Diode 3rd Quadrant Operation

1.2 kV 4H-SiC MOSFETs operating with a low knee voltage and forward voltage drop under third quadrant mode are proposed and experimentally demonstrated by using optimized accumulation mode channels to eliminate external SiC SBDs in power converter applications. By removing epitaxial regrowth process step, the overall fabrication process for the proposed MOSFET is more concise and manageable. Channel design parameters, such as channel doping concentration, channel length, and different gate oxides were investigated to achieve low knee voltage and forward voltage drop under third quadrant operation mode ensuring high breakdown voltages. To understand the impact of channel potential (barrier) on the current conduction behavior as well as leakage current during the blocking mode, 2D simulations were conducted. In addition, the reverse recovery characteristics and switching characteristics are discussed. Based on the optimized channel design, the promising devices for high density power electronics are proposed.

Highly conductive thermal interface materials with vertically aligned graphite-nanoplatelet filler towards: High power density electronic device cooling

Thermal interface materials (TIMs) play a crucial role in enhancing the reliability and sustainable utilization of next-generation electronics and thus can help meet the increasing demand for multifunctional devices with higher performance. Herein, we introduce a method for creating a TIM with high cross-plane thermal conductivity based on graphite nanoplatelet (GNP)/polyurethane (PU) films. The graphite nanoplatelets ensured the heat transfer properties of the TIM. Moreover, the hot-pressing procedure improved the thermal conductivity to 26.3 W (m K)−1 with the improved orientation of the GNPs in the PU matrix, as confirmed by microscopy investigation. Under a thermal dissipation power of 10–20 W, a drastic reduction in the chip temperature (17.5–42.3 °C) was achieved using our oriented GNP/PU TIM compared to a commercial silicone TIM (5.0 W (m K)−1). In addition, as-prepared pads can be mass produced at an acceptable cost, indicating that our work provides a promising new approach to fabricating TIMs for application in the next-generation thermal management of high power density electronics.

Ferroelectric behavior in paracrystalline poly(vinyl trifluoroacetate)

Abstract Dielectric behavior in paracrystalline poly(vinyl trifluoroacetate) was investigated from the viewpoint of ferroelectricity. This polymer has a large CF3 dipole moment (2.3 Debye) and structural defects due to the atactic sequence in its chain conformation. It is possible to rotate the dipoles in paracrystals with defects under high electric field. The dielectric behavior was measured from 20 to 200 °C. A large dielectric constant and dielectric relaxation strength (Δε = 17 at 110 °C) were observed in the α-relaxation region. Corona poling on the samples was carried out at DC field 80 MV/m and 80 °C. Ferroelectric D–E hysteresis loop was observed under high electric field, and the remanent polarization and coercive field at 40 °C were 15 mC/m2 and 155 MV/m, respectively. Pyroelectric response and thermally stimulated current were measured from the current through the electrode irradiated by a pulsed semiconductor laser. A pyroelectric constant of about 6 μC/m2K was observed, which was stable up to near the poling temperature. The ferroelectricity in poly(vinyltrifluoroacetate) stems from the rotation of molecular chains in its paracrystals and orientation of the CF2 dipoles. Poly(vinyltrifluoroacetate) dielectrics can be used for capacitors with high power density, artificial skins, muscles and other flexible electronics.

Extraordinarily enhanced evaporation of water droplets on graphene-nanostructured coated surfaces

Functionalized graphene coating possessing the unique property of ultrafast water permeation manifests itself in enhancing the evaporation heat transfer. The intermolecular interactions between the intercalated water molecules with the hydrophobic carbon atoms and the oxygenated functional groups in the graphene nanocapillaries facilitate the magnificent spreadability and permeability of water on a heated surface. The fast water permeation property of graphene can be controlled through a thermal curing process at different curing temperatures. Here we experimentally investigate the application of graphene nanoplatelets (GNPs) coatings for the enhancement of evaporation of small sessile water droplets at sub-boiling temperatures. The ultrafast water permeation within the functionalized nanostructured GNPs facilitates the fast absorption of latent heat of evaporation, leading to the anomalously effective filmwise evaporation. Ultimately, the cooling effectiveness of a water droplet on the GNPs-coated surface is extraordinarily enhanced by an unprecedented maximum of 4866% at a surface temperature of 75 °C compared against the bare surface. These results envision the great potential of graphene-based coatings to address the issues of increased heat generated from the high power density electronics, leading to enhanced performance and extended component lifespans.

Grain growth-induced thermal property enhancement of NiTi shape memory alloys for elastocaloric refrigeration and thermal energy storage systems

We interrogate the extent to which grain size plays a role in augmenting the thermal conductivity and thermal energy storage capacity of a NiTi shape memory alloy (SMA) using the optical pump-probe technique frequency-domain thermoreflectance and advanced calorimetry techniques, respectively. To alter grain size, we apply a solution anneal process to a nanograined commercial NiTi sample and show that grain growth (from ~ 40 nm to ~ 60 µm) significantly increases thermal conductivity in both phases and improves the thermal storage capacity of the material by up to a factor of ~ 50. These results are contrary to the performance metrics achieved using conventional mechanisms that improve PCM thermal conductivity, and provide a viable route to develop elastocaloric refrigeration systems with a high coefficient of performance (COP) as well as large figure-of-merit passive thermal regulation methods for cooling high power density electronics.

Systematic Investigation of Spontaneous Emission Quantum Efficiency Drop up to 800 K for Future Power Electronics Applications

Future high-density power electronics applications may require high-temperature optoelectronic devices for gate drive. Thus, a systematic study of optoelectronic material from 10 to 800 K has been performed to understand the potential of the high-temperature operation of optoelectronic devices. The temperature dependence of the photoluminescence (PL) of indium gallium nitride/gallium nitride multiple quantum wells was studied. The integrated PL intensity dropped by an order of magnitude at 800 K compared to 10 K. The spontaneous emission quantum efficiency was calculated from the power-law relation linking the integrated PL signal and the excitation pump power. The validation of the traditional ABC model for solid-state lighting is extended to 800 K. This paper demonstrates the feasibility of developing high-temperature optoelectronic devices, which have operating temperatures over 500 K.

Enhancement of diamond seeding on aluminum nitride dielectric by electrostatic adsorption for GaN-on-diamond preparation

Abstract