Conjugated polymer chains have many degrees of conformational freedom and interact weakly with each other, resulting in complex microstructures in the solid state. Understanding charge transport in such systems, which have amorphous and ordered phases exhibiting varying degrees of order, has proved difficult owing to the contribution of electronic processes at various length scales. The growing technological appeal of these semiconductors makes such fundamental knowledge extremely important for materials and process design. We propose a unified model of how charge carriers travel in conjugated polymer films. We show that in high-molecular-weight semiconducting polymers the limiting charge transport step is trapping caused by lattice disorder, and that short-range intermolecular aggregation is sufficient for efficient long-range charge transport. This generalization explains the seemingly contradicting high performance of recently reported, poorly ordered polymers and suggests molecular design strategies to further improve the performance of future generations of organic electronic materials.

http://absimage.aps.org/image/MAR14/MWS_MAR14-2013-020288.pdf

A general relationship between disorder, aggregation and charge transport in conjugated polymers

Conventional solid–solid phase-change materials (SSPCMs) exhibit good thermal energy storage (TES) ability and shape stability, but they cannot be recycled and re-shaped once fabricated due to the chemical cross-links. Hence, endowing SSPCMs with recyclability and malleable properties is advantageous for numerous applications, and contributes to environmental protection. Herein, a novel class of dynamic cross-linked SSPCMs (DC-PCMs) was facilely fabricated by bonding polyethylene glycol (PEG) to a polymeric skeleton through the thermally reversible Diels–Alder (DA) reaction for sustainable TES. PEG segments function as energy-storage units, while the polymeric skeleton affords shape stability and robust mechanical properties. The crystalline structure of DC-PCMs was investigated by X-ray diffraction (XRD), polarized optical microscopy (POM) and differential scanning calorimetry (DSC). DSC characterization also confirms the reversible energy-capture and release ability of DC-PCMs. The results suggest that SSPCMs show typical solid–solid phase transitions within the temperature range from 40 °C to 70 °C and can achieve a high latent heat storage capacity of about 107.2 J g−1. Meanwhile, the thermally reversible DA reaction endows the DC-PCMs with good thermal recyclability and solid-state plasticity. After multiple thermal cycles, DC-PCMs can retain the original mechanical properties and TES ability. Moreover, the DC-PCMs are highly scalable for practical applications because of their uncomplicated and cost-effective fabrication.

Thermally reliable, recyclable and malleable solid–solid phase-change materials through the classical Diels–Alder reaction for sustainable thermal energy storage

Numerical investigation of the thermal performance enhancement of latent heat thermal energy storage using longitudinal rectangular fins and flat micro-heat pipe arrays

The high adsorption capacity of the phase change mediums in porous supports is a key requirement for the shape-stabilized phase change materials (ss-PCMs) with high latent heat. Here, ship-shaped diatom (Pennales) frustule-based composite ss-PCMs with high polyethylene glycol (PEG) absorption capacity and high phase change enthalpy was prepared by a solution-assisted vacuum impregnation method for high-performance thermal energy storage. To improve the diatom frustules’ specific surface area and form a multi-level pore structure, the effects of calcination temperature on the microstructure of diatom frustules were studied. It was found that diatom frustules calcined at 400 °C (400CDF) had a relatively high specific surface area (~155.9 m2/g) with a well-maintained skeleton, which was a suitable PEG supporter. The devised PEG/400CDF composites with 72.7% loading of PEG4000 that had a latent heat value of 128.9 J/g for melting and 136.7 J/g for freezing, and the relative enthalpy efficiency reached up to 97.7%. The composite ss-PCMs exhibited thermal and chemical stability even after 200 thermal cycles. The current work demonstrated that ss-PCMs from biomass-based artificially cultured diatoms could slow the spread of heat by absorbing thermal energy. Moreover, the phase change mechanisms of the PEG/CDF composites under the nanoconfinement in the diatom frustules framework were also explored to explain the obtained high adsorption capacity.

Biomass-based shape-stabilized phase change materials from artificially cultured ship-shaped diatom frustules with high enthalpy for thermal energy storage

Preparation, characterization and performance of paraffin/sepiolite composites as novel shape-stabilized phase change materials for thermal energy storage

Experimental investigation of Phase Change Materials for thermal management of handheld devices

Limited heat dissipation and increasing power consumption in processors has led to a utilization wall. Specifically due to high transistor density, not all processors can be used continuously without exceeding safe operating temperatures. This is more significant in mobile electronic devices which, despite relatively large chip area, are limited by poor heat dissipation — primarily natural convection from the exposed surfaces. In the past, solid-to-liquid phase change materials (PCMs) have been employed for passive thermal control — absorbing energy during the phase change process while maintaining a relatively fixed temperature. However, the lower thermal conductivity of the liquid phase after melting often limits the heat dissipation from the PCM, and in the liquid state, the material can flow away from the desired location. Here we focus on characterization of thermal performance of PCMs with the goal of evaluating dry (gel-to-solid/amorphous-to-crystalline) phase change materials which are intended to mitigate the pumpout issue. Critical thermophysical properties include the thermal conductivity, heat capacity, and latent heat of the phase/state change. The thermal resistance throughout the phase change process is measured by in-house rig which miniaturizes the reference bar method for use with infrared temperature sensing.Copyright © 2015 by ASME

https://engineering.purdue.edu/MTEC/ConferencePapers/passive-thermal-management-using-phase-change-materials-experimental-evaluation-of-thermal-resistances/Ganatra,%20Marconnet%20-%202015%20-%20Passive%20Thermal%20Management%20Using%20Phase%20Change%20Materials%20Experimental%20Evaluation%20of%20Thermal%20Resistances.pdf

Passive Thermal Management Using Phase Change Materials: Experimental Evaluation of Thermal Resistances

Passive thermal management with phase change materials (PCMs) has become the one of the most promising methods to cool cell phone processors due to the relatively simple implementation and profound impact on processor temperatures. Enhancing the thermal properties of conventional PCMs, mainly thermal conductivity and latent heat storage, allows for an overall improved thermal management system. This study aims to improve the thermal conductivity of paraffin wax (a typical commercial PCM) by the introduction of an expanded graphite (EG) filler to form a paraffin wax composite, and then infiltration of the EG/paraffin composite into an aluminum foam matrix. The thermal conductivity of the EG/paraffin composites increases respectively to the volume percent of expanded graphite. While the thermal conductivity increased, there is some negative impact on latent heat storage compared to pure paraffin wax. The pore size of the aluminum foam matrixes also has a profound impact on both thermal conductivity and latent heat storage of the overall system. These results will allow for improvements in cooling techniques incorporated within cell phones and other mobile devices, allowing for future development of their processors (higher computational power), prolonged reliability, and longer anticipated life cycles.

/pdf/investigation-of-aluminum-foams-and-graphite-fillers-for-293sbqe9jx.pdf

Investigation of aluminum foams and graphite fillers for improving the thermal conductivity of paraffin wax-based phase change materials

Ganatra,Yash Yogesh M.S.M.E, Purdue University, December 2016. Passive Thermal Management using Phase Change Materials. Major Professor: Amy Marconnet, School of Mechanical Engineering. The trend of enhanced functionality and reducing thickness of mobile devices has led to a rapid increase in power density and a potential thermal bottleneck since thermal limits of components remain unchanged. Active cooling mechanisms are not feasible due to size, weight and cost constraints. This work explores the feasibility of a passive cooling system based on Phase Change Materials (PCMs) for thermal management of mobile devices. PCMs stabilize temperatures due to the latent heat of phase change thus increasing the operating time of the device before threshold temperatures are exceeded. The primary contribution of this work is the identification of key parameters which influence the design of a PCM based thermal management system from both the experiments and the numerical models. This work first identifies strategies for integrating PCMs in an electronic device. A detailed review of past research, including experimental techniques and computational models, yields key material properties and metrics to evaluate the performance of PCMs. Subsequently, a miniaturized version of a conventional thermal conductivity measurement technique is developed to characterize thermal resistance of PCMs. Further, latent heat and transition temperatures are also characterized for a wide range of PCMs. In-situ measurements with PCMs placed on the processor indicate that some PCMs can extend the operating time of the device by as much as a factor of 2.48 relative to baseline tests (with no PCMs). This increase in operating time is investigated by computational thermal models that explore various integration locations, both at the package and device level.

/pdf/passive-thermal-management-using-phase-change-materials-i4czpb3bsz.pdf

Yash Ganatra

Papers

Experimental investigation of Phase Change Materials for thermal management of handheld devices

Passive Thermal Management Using Phase Change Materials: Experimental Evaluation of Thermal Resistances

Investigation of aluminum foams and graphite fillers for improving the thermal conductivity of paraffin wax-based phase change materials

Passive thermal management using phase change materials