Kirigami-Inspired Organic and Inorganic Film-Based Flexible Thermoelectric Devices with Built-In Heat Sink

Thermoelectric (TE) devices can convert heat to electricity directly, which offers a unique opportunity to realize waste heat recovery. However, conventional TE devices inevitably use heat sinks, which are bulky, rigid and heavy, limiting practical applications. Herein, we propose a fully integrated film-based TE device with intrinsically built-in fins as heat sink in a hexagonal honeycomb device structure, that simultaneously achieves high TE performance and conformability, as confirmed by experiments and modelling. A flexible Kapton substrate with copper electrodes, integrating either carbon nanotube (CNT) veils or bismuth telluride (Bi2Te3) TE ‘legs’, both of n- and p-type, achieved a remarkable specific power of 185.4 nW K−2 for a Bi2Te3-based device and 53.1 nW K−2 for a CNT-based device, thanks to the heat dissipation effect granted by the built-in fins. Besides, the addition of oriented polymer films interconnects, contracting when above their glass transition temperature, allowed a single substrate two-dimensional (2D) TE device to self-fold into a three-dimensional (3D) hexagonal honeycomb structure, with built-in fins, contactlessly and autonomously. The demonstrated shape-programmed kirigami-inspired scalable TE device paves the way for realising self-powered applications comprising hundreds of TE legs with both inorganic (e.g., Bi2Te3) and organic (e.g. CNT veils) TE materials and integrated heat sinks

Development, Fabrication, and Experimental Study of Flat Polymer Micro Heat Pipes

An issue of concern with the recent densification of electrical components in integrated circuits is heat removal to avoid damage to the semiconductor structure. Flat heat pipes have been seriously considered and studied over the past 20 years as a solution for thermal management of these devices. In this, flat polymer based micro heat pipes were designed, fabricated and assessed for thermal performance. Novel fabrication processes was developed that uses liquid crystal polymer (LCP) film with copper filled thermal vias and a micro-scale hybrid liquid wicking structure to construct a flat heat pipe suitable for thermal management of semiconductor devices. LCP was chosen for its high chemical resistance, reliability, flexibility, and its ability to be readily incorporated into current printed circuit board production technologies. The microfabrication techniques of photolithography and reactive ion etching were used to form copper filled thermal vias through the polymer to decrease thermal resistance of the casing. Photolithography, wet etching, and electroplating were used to form a hybrid wicking consisting of 200 µm copper pillar forming 31 µm grooves with a woven copper mesh bonded to the top surface. In addition, a novel method for bonding woven metallic mesh to liquid flow channels has been developed. A 250 Å thick layer of atomic layer deposited (ALD) TiO2 was coated on the hybrid wicking structure to enhance the evaporation and capillary force on the liquid in the device. The thermal resistance of the assembled and water charged thermal ground plane displayed a thermal resistance of 0.5 K/W with a power input of 40 W (63 W/cm2) with both adverse and favorable acceleration fields. The same device displayed an effective thermal conductivity of 1653 W/m*K at 0g and 541 W/m*K at 10g acceleration. This high performance suggests that excess capillary pumping pressure was achieved with the hybrid wick. Additionally, flexible thermal ground planes have been developed using multi-layer sintered wick structures and 130 µm thick PET casing material. These devices displayed a thermal resistance up to 4 times less than an equivalent copper reference sample and at a mass of up to 1/6th that of copper

Heat Exchangers

The demand for energy to satisfy the basic needs and services of the population worldwide is increasing as are the economic costs associated with energy production. As such, it is essential to emphasize energy recovery systems to improve heat transfer in thermal processes. Currently, significant research efforts are being conducted to expose criteria and analysis techniques for the design of heat exchange equipment. This book discusses optimization of heat exchangers, heat transfer in novel working fluids, and the experimental and numerical analysis of heat transfer applications

Thermal Performance of Pulsating Heat Stripes Built With Plastic Materials

A low-cost, flexible pulsating heat pipe (PHP) was built in a composite polypropylene sheet consisting of three layers joint together by selective laser welding, to address the demand of heat transfer devices characterized by low weight, small unit thickness, low cost, and high mechanical flexibility. A thin, flexible, and lightweight heat pipe is advantageous for various aerospace, aircraft, and portable electronic applications where the device weight, and its mechanical flexibility are essential. The concept is to sandwich a serpentine channel, cut out in a polypropylene sheet and containing a self-propelled mixture of a working fluid with its vapor, between two transparent sheets of the same material; this results into a thin, flat enclosure with parallel channels hence the name “pulsating heat stripes” (PHS). The transient and steady-state thermal response of the device was characterized for different heat input levels and different configurations, either straight or bent at different angles. The equivalent thermal resistance was estimated by measuring the wall temperatures at both the evaporator and the condenser, showing a multifold increase of the equivalent thermal conductance with respect to solid polypropylene.</jats:p

Closed-cycle gas dynamic laser design investigation

A conceptual design study was made of a closed cycle gas-dynamic laser to provide definition of the major components in the laser loop. The system potential application is for long range power transmission by way of high power laser beams to provide satellite propulsion energy for orbit changing or station keeping. A parametric cycle optimization was conducted to establish the thermodynamic requirements for the system components. A conceptual design was conducted of the closed cycle system and the individual components to define physical characteristics and establish the system size and weight. Technology confirmation experimental demonstration programs were outlined to develop, evaluate, and demonstrate the technology base needed for this closed cycle GDL system

Small diameter fibres as new wick material for capillary-driven heat pipes

Papers presented to the 11th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 20-23 July 2015.Heat pipes with a wick material consisting of small diameter metal fibres of 12 μm are investigated. The container material is copper and the working fluid is water. The fibre mesh heat pipe is compared with two other wick structures: a screen mesh (145 meshes per inch) and a sintered powder wick. All three heat pipes have an outer diameter of 6 mm, a length of 200 mm. The heat pipes are tested in a vertical orientation, both gravity-opposed and gravity-assisted. In the gravity-opposed orientation the heat pipes are tested for a heat input up to 50 W and an operating temperature of 70°C. In the gravity-assisted orientation the heat pipes are tested up to 160 W and 120°C. The thermal resistance and the temperature difference between evaporator and condenser are used as performance indicators. For the gravity-assisted orientation, the screen mesh wick clearly outperforms the fibre and sintered powder wick, due to its higher permeability and better ability to distribute the working fluid over the circumference of the wick. For the gravity-opposed orientation, the fibre and screen mesh heat pipe perform equally well. Both have a lower thermal resistance than the sintered powder heat pipe, as the small diameter fibres and fine mesh create more and very small capillary channels in comparison with the sintered powder wick.am201

Embedded heat speaders in low temperature cofired ceramic substrates

A new heat spreader that operates on a principle similar to heat pipes has been developed in Low Temperature Cofired Ceramic (LTCC) substrate. The heat spreader use sintered metal powder as the wick structure and water as the working fluid. Key topics related to the fabrication of embedded heat spreaders in LTCC substrate were studied. The conventional LTCC procedure has been improved to suit the requirement of heat spreader. A novel sintered porous silver powder has been developed to provide high capillary pressure and permeability for the wick structure. The maximum mass transport rate of the wick was about 0.692 (g/min) at wick height of 4.5cm. The thermal performance test demonstrated that the prototype heat spreader could work properly at power density of more than 70 W/cm2 without any sign of dry out occur. The successful fabrication of the prototype integrated heat spreader provides concept validation of using advanced two-phase heat management system to greatly improve the effective thermal conductivity of LTCC substrate

Exploration of Metal Composites and Carbon Nanotubes for Thermal Interfaces

Modern microelectronics are perpetually pushing against limitations caused by inadequate heat dissipation. One of the critical bottlenecks is at the interfaces between different materials and components. Thermal interface materials (TIM) are used to improve the heat transfer at these interfaces, and to improve TIMs is one of the critical research areas in order to reduce the total thermal resistance for electronics systems.A TIM requires both high thermal conductivity, ability to conform to mating surfaces, and the ability to absorb stress from thermal expansion mismatch during thermal cycling. Solder based TIMs utilize solder to form a strong connection between the mating surfaces with high thermal conductivity, but their stiffness prevents adequate absorption of thermal expansion mismatch. In this thesis, the solder is combined with a fiber network phase, which modifies the mechanical properties, while maintaining the continuous heat paths within the solder. This solder matrix fiber network composite TIM allows for the tailoring of the mechanical properties of solder based TIM while retaining thermal performance. Another promising TIM candidate is based on arrays of vertically aligned CNTs. CNT arrays can achieve good thermal performance, but the reliability had not previously been investigated experimentally. A thorough investigation of the reliability of CNT array TIM revealed that reliability is not guaranteed, but requires careful matching between CNT array height, bonding method and substrate configuration.Furthermore, we developed a new joule self-heating chemical vapor deposition (CVD) method for the synthesis of double-sided CNT arrays on thin foils, which can be used both as TIM or as supercapacitor electrodes. Double-sided arrays are challenging with conventional CNT array synthesis methods, but the Joule heating CVD method allows for rapid, scalable and uniform synthesis of large area double-sided arrays. Finally, this method was used to study the effect of heat treatment of CNT arrays on graphite. The heat treatment serves to simultaneously improve the CNT crystallinity, eliminate catalyst residues, and form a seamless connection between CNT arrays and graphite