Eric Dalton

Silver nanowire array-polymer composite as thermal interface material

Silver nanowire arrays embedded inside polycarbonate templates are investigated as a viable thermal interface material for electronic cooling applications. The composite shows an average thermal diffusivity value of 1.89×10−5 m2 s−1, which resulted in an intrinsic thermal conductivity of 30.3 W m−1 K−1. The nanowires’ protrusion from the film surface enables it to conform to the surface roughness to make a better thermal contact. This resulted in a 61% reduction in thermal impedance when compared with blank polymer. An ∼30 nm Au film on the top of the composite was found to act as a heat spreader, reducing the thermal impedance further by 35%. A contact impedance model was employed to compare the contact impedance of aligned silver nanowire-polymer composites with that of aligned carbon nanotubes, which showed that the Young’s modulus of the composite is the defining factor in the overall thermal impedance of these composites.

Metal nanowire-polymer nanocomposite as thermal interface material

Thermal properties of silver nanowire-silicone (AgNW-silicone) composites at different weight percentage of AgNW in the polymer were studied. The thermal conductivity of these composites was measured using the heat stack method according to D5470 standard. The silver nanowires were synthesized using a polyol process. The nanocomposites were prepared by solution mix processing. The effective thermal conductivity of the AgNW-silicone nanocomposite increased with the enhancement of AgNW concentration and the thermal conductivity were found higher than that of traditional silicone composites using micron sized silver flakes as fillers with the same concentration.

Effects of tensile strain on the nanostructure of irradiated and thermally stabilised ultra high molecular weight polyethylenes for orthopaedic devices

Ultra high molecular weight polyethylene (UHMWPE), of the types used in orthopaedic implants, has been irradiated in air and in nitrogen to give received doses between zero and ten Mrad and has been subject to tensile testing whilst simultaneously recording the wide angle and small angle X-ray scattering (WAXS and SAXS) diffraction patterns, using the synchrotron facilities at Diamond. Commercial implant grade polymer, which has been subject to irradiation and two different thermal annealing or stabilising processes, to prevent age related embrittlement, has also been examined. With all materials, as the sample elongates lamellae reorganisation processes occur at a strain of around 0.1, as the unit cell converts from orthorhombic to monoclinic, and yield commences. Yield in the irradiated polymers is associated with a pronounced peak in the stress–strain curve, whereas yield in unirradiated, and in thermally annealed polymers, does not involve a peak in the stress curve. SAX plots show that lamellae reorganisation, during yield, occurs much faster in irradiated materials. This lamellae reorganisation is thought to involve a slip and glide initiated unravelling of the folds, accompanied by refolding and reorientation of parts of existing lamellae. It is proposed that in irradiated materials this is facilitated by nucleation sites on the surface of the lamellae, resulting from radiation damage. The total crystallinity increases slightly during the yielding process and it is thought that this crystal growth involves an interfacial all-trans amorphous phase previously identified with these materials. The greatest effects arise when the samples are irradiated in air. This suggests that the stabilising processes, involving annealing of irradiated polymers, probably work through restoring crystal perfection, as well as increasing the network density in the amorphous phase. Taken in conjunction with existing knowledge on free radical quenching, the information generated allows a much greater understanding of the steps necessary to produce a stable implant grade UHMWPE. It is well known that premature failure of hard metal on polyethylene joint systems causes much discomfort and difficult revision surgery. In recent years irradiated and annealed polymers have taken over much of the market, but the mechanism of action of the annealing processes is only partially understood. Free radical quenching is clearly part of the story, but is not in itself sufficient to explain why irradiated and heat treated polymers perform much better than those simply irradiated under nitrogen. This paper proposes that the heat treatment processes are important in restoring crystal perfection after radiation damage. Irradiation inserts nucleation sites for crystal slip and glide, which occurs when the polymer is strained, but annealing at temperatures close to Tm, restores perfection. This is shown through simultaneous SAXS and WAXS analysis whilst the sample is being strained in the X-ray beam.

Exploring the Role of Adsorption and Surface State on the Hydrophobicity of Rare Earth Oxides

Rare earth oxides (REOs) are attracting attention for use as cost-effective, high-performance dropwise condensers because of their favorable thermal properties and robust nature. However, to engineer a suitable surface for industrial applications, the mechanism governing wetting must be first fully elucidated. Recent studies exploring the water-wetting state of REOs have suggested that these oxides are intrinsically hydrophobic owing to the unique electronic structure of the lanthanide series. These claims have been countered with evidence that they are inherently hydrophilic and that adsorption of contaminants from the environment is responsible for the apparent hydrophobic nature of these surfaces. Here, using X-ray photoelectron spectroscopy and dynamic water contact angle measurements, we provide further evidence to show that REOs are intrinsically hydrophilic, with ceria demonstrating advancing water contact angles of ≈6° in a clean surface state and similar surface energies to two transition metal oxides (≳72 mJ/m2). Using two model volatile species, it is shown that an adsorption mechanism is responsible for the apparent hydrophobic property observed in REOs as well as in transition metal oxides and silica. This is correlated with the screening of the polar surface energy contribution of the underlying oxide with apparent surface energies reduced to <40 mJ/m2 for the case of nonane adsorption. Moreover, we show that the degree of surface hydroxylation plays an important role in the observed contact angle hysteresis with the receding contact angle of ceria increasing from ∼10° to 45° following thermal annealing in an inert atmosphere. Our findings suggest that high atomic number metal oxides capable of strongly adsorbing volatile species may represent a viable paradigm toward realizing robust surface coating for industrial condensers if certain challenges can be overcome.

Lamella alignment ratio: a SAXS analysis technique for macromolecules

Polyethylenes with various morphologies were analysed, as a function of strain and radiation doses, using a novel lamella alignment ratio (LAR) technique on data obtained from small-angle X-ray scattering (SAXS) measurements at the Diamond Light Source synchrotron. The authors have used this technique previously to describe the influence of irradiation dose on crystal morphologies of orthopaedic grade polyethylenes. The aim of the current work is to highlight and explain the effectiveness of the LAR technique as an analysis tool for macromolecular crystal reorganization, crystallographic structural changes and crystal deformation during simultaneous strain measurements. Using new data for polyethylenes with varying morphology and irradiation dosage, the effectiveness of the technique is detailed. These findings are particularly useful for polymeric materials used in property critical applications and present a facile method for SAXS analysis. It is proposed that this method could potentially be used as a convenient route for the analysis of conditioning and crosslinking mechanisms on crystallographic relationships in more complex monomeric structures.

Nanowire-Polymer Nanocomposites as Thermal Interface Material

Packaging of semiconductor electronic device is a challenge due to the progressive increase in the power level of operating devices which is associated with the increasing device performance. As semiconductor device feature sizes continue to be reduced, ensuring reliable operation has become a growing challenge. The effective transfer of heat from an integrated circuit (IC) and its heat spreader to a heat sink is a vital step in meeting this challenge. The ITRS projected power density and junction-to-ambient thermal resistance for high-performance chips at the 14 nm generation are >100 Wcm-2 and <0.2 °CW-1, respectively. The main bottlenecks in reducing the junction-to-ambient thermal resistance are the thermal resistances of the thermal interface material (TIM) (Prasher 2006) and the heat sink. The primary goal of this chapter is to review the metallic-nanowire nanocomposites as thermal interface material compared to other types of thermal interface materials. The first section of the chapter will review different types of nanowire-polymer composites as well as carbon nanotube-polymer composites as thermal interface material. In recent years, carbon nanotube (CNT) and nanotube-polymer composites were proposed in many publications as a possible TIM with high thermal conductivity and low thermal impedance. However, the possibility of inadvertently incorporating contaminating impurities, the existence of voids between CNTs, and the growth conditions of CNT arrays greatly affect the effective thermal conductivity of CNTs, typically resulting in a TIM with a large performance uncertainty. On the other hand, nanowire-polymer nanocomposites can be proposed as thermal interface material where due to the inclusion of nanowires, composites should achieve high thermal conductivity. The later sections of this chapter will describe the research and development ongoing in the area of nanowire-polymer nanocomposites. The fabrication routes for the nanowires and the nanowire-polymer composites as well as the characterizations of the nanocomposites will be discussed in detail. The applicability of metallic nanowire-polymer nanocomposites as thermal interface material will be evaluated.

Testing method for measuring corrosion resistance of surface mount chip resistors

Surface mount chip resistors are amongst the simplest and most inexpensive of all components used in electronic circuits and systems. Typically, resistor failure modes include open circuits, resistive shorts or variations in resistance indicating parametric drift or intermittent failure, which in some applications result in overall system failure. Corrosion is currently believed to be the number one failure mechanism for chip resistors deployed in developing markets such as Central and Latin America, Asia, India and Pacific regions where aggressive corrosive conditions are prevalent. The objective of this study is to develop a test to identify or screen out corrosion-susceptible parts. Ten precision chip resistors types representative of resistors in contemporary printed circuit board assemblies are subjected to a well-defined multi-stress screen which comprises thermal cycling and mixed flowing gas exposure. The combination of thermal cycling and corrosive gas exposure is shown to provide an acceptable acceleration test to identify corrosion-susceptible parts by replicating field failures. Currently, no other test method exists which is capable of replicating field failures.

Present and future thermal interface materials for electronic devices

ABSTRACT Packaging electronic devices is a growing challenge as device performance and power levels escalate. As device feature sizes decrease, ensuring reliable operation becomes a challenge. Ensuring effective heat transfer from an integrated circuit and its heat spreader to a heat sink is a vital step in meeting this challenge. The projected power density and junction-to-ambient thermal resistance for high-performance chips at the 14 nm generation are >100 Wcm−2 and <0.2 °CW−1, respectively. The main bottleneck in reducing the net thermal resistance are the thermal resistances of the thermal interface material (TIM). This review evaluates the current state of the art of TIMs. Here, the theory of thermal surface interaction will be addressed and the practicalities of the measurement techniques and the reliability of TIMs will be discussed. Furthermore, the next generation of TIMs will be discussed in terms of potential thermal solutions in the realisation of Internet of Things.

Thermal Properties of Carbon Nanotube-Polymer Composites for Thermal Interface Material Applications

This work presents the thermal property study of single wall and multi wall carbon nanotubes (SWCNT and MWCNT) both in their purified and unpurified forms introduced to silicone elastomer to enhance the thermal diffusivity of this industrial polymer. An increase in thermal diffusivity was observed for incremental loading of both purified and unpurified single wall and multiwall CNT in epoxy at different percentages. An increase of thermal diffusivity as high as 130% was achieved for ∼2 wt% loading of both single wall and multi wall nanotubes. Electrical conductivity measurements showed a percolation threshold for 2% loading of multiwall CNT, below which the nanotube-epoxy composite behaved as an insulator — this is a key property for applications where electrical isolation is required. For single wall CNT-epoxy composite all the samples showed high resistance to the conduction of current. Thermal impedance measurements showed a strong dependency of contact resistance with percentage loading. Finally, the feasibility of deploying carbon nanotube-polymer composites as practical thermal interface materials for electronics thermal management is discussed.Copyright

Nanoporous membrane production via block copolymer lithography for high heat dissipation systems

We demonstrate the first steps towards realizing a highly effective hardmask fabrication technique for producing cheap low defect nanoporous membranes, which can be incorporated as fluidic wicking structures for use in evaporative cooling solutions integrated at device level. Next-generation cooling solutions are becoming necessary to dissipate increasing heat fluxes and maintain acceptable junction temperatures in high-speed electronics. The proposed pumpless two-phase evaporation-based heat sink device relies on a supported nanoporous membrane (SNM) as the driving mechanism for generating the requisite capillarity for pumping low surface tension refrigerants. Molecular self-assembling block copolymers (BCPs), specifically cylindrical forming poly(styrene)-block-poly(4-vinyl-pyridine) (PS-b-P4VP) are ideal as a cost effective hardmask fabrication route for patterning sub 80 nm pores when compared to the high cost of ownership of state of the art immersion photolithography. We report on the pattern formation of the phase segregated BCP with optimization of the annealing parameters. The work addresses defect elimination in the BCP template by developing a custom solvothermal annealing chamber which achieves excellent phase segregation of the BCP, limits processing defects and prevents polymer dewetting on a microscale level. The chamber is capable of processing up to 4 inch wafers and allows for in-situ monitoring of a solvent annealing cycle by monitoring film swelling via optical reflectometry.