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Bonding between polymer filaments deposited during Fused Filament Fabrication (FFF) is a critical process that determines the quality of the printed part. This process is governed by the temperature history of the deposited filame...
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Bonding between polymer filaments deposited during Fused Filament Fabrication (FFF) is a critical process that determines the quality of the printed part. This process is governed by the temperature history of the deposited filament. In general, the longer the filament stays above glass transition temperature, the greater is the quality of bonding. This paper presents a technique to enhance FFF quality by localized dispensing of hot air from nozzles integrated with the main polymer-dispensing nozzle, thereby providing a hot micro-environment around the filament. The temperature field during this process is measured using infrared thermography. It is shown that under the correct process conditions, this approach results in significantly reduced heat transfer from the filament, thereby increasing the cool down time and improving the quality of bonding with the adjacent filaments. The improved thermal history of the filament due to hot air dispensing is shown to translate into increased neck size, leading to 35% increase in thermal conductivity, 19% increase in tensile strength and 145% increase in tensile toughness. Compared to other thermal techniques for improving the FFF process proposed in the past, the present approach provides a highly localized, in situ thermal enhancement of the local environment around the deposited filament, and integrates seamlessly with the filament-dispensing nozzle. It is expected that the technique described here may help improve the quality of FFF process and enable the printing of parts with improved thermal and mechanical properties.
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For traditional data centers, one-third of the total energy consumed is directed toward cooling information technology equipment. High demand for new data centers, vast amount of energy consumption, and their impact on the climate...
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For traditional data centers, one-third of the total energy consumed is directed toward cooling information technology equipment. High demand for new data centers, vast amount of energy consumption, and their impact on the climate requires the data center industry to make them energy efficient and opt for immersion cooling technologies. From a thermal energy management perspective, immersion cooling is better than traditional air-cooling technology. However, detailed study of material compatibility of the various electronics packaging materials for immersion cooling is essential to understand their failure modes and reliability. The stiffness and thermal expansion are critical material properties for the mechanical design of electronics. Printed circuit board/substrate is a critical component of electronic package and heavily influences failure mechanism and reliability of electronics both at the package and board level. This study mainly focuses on two major challenges. The first part of the study focuses on the impact of thermal aging in dielectric fluid for single-phase immersion cooling on the low-loss material printed circuit board's (PCB's) thermomechanical properties. The PCB sample weight is compared to quantify absorption of the dielectric fluid into PCBs or leaching of the plasticizers into the fluid. The second part of the study is the impact of thermal aging on thermomechanical properties of low-loss PCBs in the air. The low-loss PCBs, Megtron6 are aged in mineral oil, and air for 720 hr at four different temperatures: 22, 50, 75, and 105 degrees C. The complex modulus and coefficient of thermal expansion are characterized before and after aging for both parts and compared.
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While additive manufacturing offers significant advantages compared to traditional manufacturing technologies, deterioration in thermal and mechanical properties compared to properties of the underlying materials is a serious conc...
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While additive manufacturing offers significant advantages compared to traditional manufacturing technologies, deterioration in thermal and mechanical properties compared to properties of the underlying materials is a serious concern. In the context of polymer extrusion based additive manufacturing, post-process approaches, such as thermal annealing have been reported for improving mechanical properties based on reptation of polymer chains and enhanced filament-to-filament adhesion. However, there is a lack of similar work for improving thermal properties such as thermal conductivity. This paper reports significant enhancement in build-direction thermal conductivity of polymer extrusion based parts as a result of thermal annealing. Over 150% improvement is observed when annealed at 135 degrees C for 96 h. The effect of annealing temperature and time on thermal conductivity enhancement is investigated through experiments. A theoretical model based on Arrhenius kinetics for neck growth and a heat transfer model for the consequent impact on inter-layer thermal contact resistance is developed. Predicted thermal conductivity enhancement is found to be in good agreement with experimental data for a wide range of annealing temperature and time. The theoretical model may play a key role in developing practical thermal annealing strategies that account for the multiple constraints involved in annealing of polymer parts. This work may facilitate the use of polymer extrusion additive manufacturing for producing enhanced thermal conductivity parts capable of withstanding thermal loads.
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The adhesion and merging of adjacent filaments in polymer extrusion based additive manufacturing (AM) plays a key role in determining the thermal and mechanical properties of the built part. It is well known that maintaining the d...
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The adhesion and merging of adjacent filaments in polymer extrusion based additive manufacturing (AM) plays a key role in determining the thermal and mechanical properties of the built part. It is well known that maintaining the deposited filaments at a high temperature aids in the process of adhesion and merging. While external mechanisms such as laser and infrared heating have been used in the past to heat up deposited filaments, this paper presents a simpler, less invasive and in situ mechanism for heating of previously deposited layers using a hot metal block integrated with and rastering together with the filament-dispensing nozzle. Infrared thermography based quantitative measurement of temperature field along the raster line is carried out for two configurations - a preheater and a postheater traveling ahead of or behind the nozzle respectively. In each case, significant temperature rise in the deposited filaments is shown. A configuration comprising both preheater and postheater is shown to result in additional thermal benefits. The measured temperature rise is shown to be a function of process parameters such as raster speed and heater-to-base gap. Experimental measurements are shown to agree well with theoretical and simulation models. Cross-section imaging of samples printed without and with the in situ heating clearly show significant improvement in neck growth and filament-to-filament merging compared to the baseline case. Improvement in thermal and structural performance of printed samples is also demonstrated. Compared to other techniques proposed in the past, the heating approach presented in this work is passive and requires minimal additional costs or complexity. The improved temperature field and consequently enhanced filament adhesion reported here may help design and build parts with superior thermal and mechanical properties using polymer AM.
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Water droplet dispensing in microfluidic parallel-plate electrowetting-on-dielectric (EWOD) devices with various reservoir designs has been numerically studied. The Navier-Stokes equations are solved using a finite-volume formulat...
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Water droplet dispensing in microfluidic parallel-plate electrowetting-on-dielectric (EWOD) devices with various reservoir designs has been numerically studied. The Navier-Stokes equations are solved using a finite-volume formulation with a two-step projection method on a fixed grid. The free surface of the liquid is tracked by a coupled level set and volume-of-fluid method with the surface tension force determined by the continuum surface force model. Contact angle hysteresis which is an indispensible element in EWOD modeling has been implemented. A simplified model is adopted for the viscous stresses exerted by the parallel plates at the solid-liquid interface. Good agreement has been achieved between the numerical results and the corresponding experimental data. The dispensing mechanism has been carefully examined, and droplet volume inconsistency for each design has been investigated. It has been discovered that the pressure distribution on the cutting electrode at the beginning of the cutting stage is of considerable significance for the inconsistency of droplet volumes. Several key elements which directly affect the pressure distribution and volume inconsistency have been identified.
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Phosphorylation has been hypothesized to alter the ability of tau protein to bind with microtubules (MT), and pathological level of phosphorylation can incorporate formation of Paired Helical Filaments (PHF) in affected tau. Study...
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Phosphorylation has been hypothesized to alter the ability of tau protein to bind with microtubules (MT), and pathological level of phosphorylation can incorporate formation of Paired Helical Filaments (PHF) in affected tau. Study of the effect of phosphorylation on different domains of tau (projection domain, microtubule binding sites and N-terminus tail) is important to obtain insight about tau neuropathology. In an earlier study, we have already obtained the mechanical properties and behavior of single tau and dimerized tau and observed tau-MT interaction for normal level of phosphorylation. This study attempts to obtain insights on the effect of phosphorylation on different domains of tau, using molecular dynamics (MD) simulation with the aid of CHARMM force field under high strain rate. It also determines the effect of residue focused phosphorylation on tau-MT interaction and tau accumulation tendency. The results show that for single tau protein, unfolding stiffness does not differ significantly due to phosphorylation, but stretching stiffness can be much higher than the normally phosphorylated protein. For dimerized tau protein, the stretching required to separate the protein forming the dimer is similar for phosphorylation in individual domains but is significantly less in case of phosphorylation in all domains. For tau-MT interaction simulations, it is found that for normal phosphorylation, the tau separation from MT occurs at higher strain for phosphorylation in projection domain and N-terminus tail, and earlier for phosphorylation in all domains altogether than the normal phosphorylation state. The residue focused phosphorylation study also shows that tau separates earlier from MT and shows stronger accumulation tendency at the phosphorylated state, while preserving the highly stretchable and flexible characteristic of tau. This study provides important insight on mechanochemical phenomena relevant to traumatic brain injury (TBI) scenario, where the result of mechanical loading and posttranslational modification as well as conformation decides the mechanical behavior.
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Heat generating bodies are typically immersed in surrounding fluid that acts as a coolant, either though forced or natural convection. For safe operation, the heat generated should be transferred to the surrounding fluid at a rate...
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Heat generating bodies are typically immersed in surrounding fluid that acts as a coolant, either though forced or natural convection. For safe operation, the heat generated should be transferred to the surrounding fluid at a rate that is required to maintain the internal temperature below a critical value. In cases such as nuclear fuel rods and lithium-ion batteries, it is very difficult to install conventional measuring devices like thermocouples and pyrometers inside the heated equipment to monitor the internal operating temperature and trigger warnings when it reaches the critical point. It is however relatively easy to measure the temperature of the surrounding coolant in which they are immersed. From these measurements, the temperature distribution inside the equipment can be estimated using inverse techniques. Inverse analysis can generate data that provides insight into thermal behavior of equipment and help detect overheating events so that corrective action may be taken. A generic and novel framework for conducting inverse analysis to predict internal temperature of cylindrical heat generating bodies has been developed and validated with experimental setup. Simulation-based inverse analysis techniques require large computation effort for complex geometries and high-fidelity models, which make them impractical for real time applications. To circumvent this, a neural network-based model was utilized for predicting temperature inside the system in lieu of a high-fidelity simulation model. Training data was generated through high fidelity simulation using OPENFOAM with an axi-symmetric model. The approach was applied to an experimental setup using a cartridge heater as a representation of a heat generating rod. The coolant height and temperature measured in fluid region was given as input to a trained neural network model to predict surface temperature and inside temperature of the heat generating rod. Inversely predicted temperature was in good agreement with experiment data over wide range of power input. In addition, temperature obtained from this model was used to display in real time on an augmented reality device, which could allow field workers to easily monitor in the field. Novelty of this framework is that the measured coolant temperature in which the heat generating body is immersed is used as input for the rapid and accurate inverse prediction of internal temperature.
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This work presents a methodology based on mesh morphing techniques for transfer of high-fidelity X-ray computed tomography (CT) data, including manufacturing defects information, into finite element (FE) models for failure prognos...
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This work presents a methodology based on mesh morphing techniques for transfer of high-fidelity X-ray computed tomography (CT) data, including manufacturing defects information, into finite element (FE) models for failure prognosis in composite structures. An IM7/8552 carbon epoxy hat-section element with complex layup is considered. Failure initiates at the location of seeded defects, including in-plane and out-of-plane fiber waviness, representative of irregularities that could occur during manufacturing of composite parts with complex layup and geometry. The ability of the method to efficiently transition high fidelity CT data into structural models for assessment of the effects of manufacturing defects is essential for condition-based structural substantiation. Stress-based failure criteria are considered for prediction of matrix-dominated failure under quasi-static loading. FE results are compared with experimental data, including correlation with CT-based post-failure damage information. (C) 2016 Elsevier Ltd. All rights reserved.
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While Li-ion cells offer excellent energy conversion and storage capabilities for multiple applications, including electric vehicles, heat removal from a Li-ion cell remains a serious technological challenge that directly limits p...
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While Li-ion cells offer excellent energy conversion and storage capabilities for multiple applications, including electric vehicles, heat removal from a Li-ion cell remains a serious technological challenge that directly limits performance, and poses serious safety concerns. Due to poor thermal conductivity of Li-ion cells, traditional cooling methods like air cooling on the cell surface do not effectively access and cool the core. This may lead to overheating of the cell core. This paper investigates the cooling of Li-ion cells using an annular channel through the axis of the cell. Air flow through this channel and heat pipe insertion are both shown to result in effective cooling. A temperature reduction of 18-20 degrees C in the cell core is observed in heat pipe experiments, depending on heat pipe size, for 1.62 W heat dissipation. Similar effect is observed when a thin metal rod is used instead of a heat pipe. Experimental measurements are close to finite-element simulation results. Experiments demonstrate that a heat pipe successfully prevents overheating in case of sudden increase in heat generation due to malfunction such as cell shorting. This paper illustrates fundamental thermal-electrochemical trade-offs, and facilitates the development of novel and effective cooling techniques for Li-ion cells. (C) 2016 Elsevier Ltd. All rights reserved.
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Heat transfer plays a key role in polymer extrusion based additive manufacturing (AM) processes. Measurement and modeling of how temperature distribution on the platform bed changes with time during the dispensing of a polymer fil...
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Heat transfer plays a key role in polymer extrusion based additive manufacturing (AM) processes. Measurement and modeling of how temperature distribution on the platform bed changes with time during the dispensing of a polymer filament is of particular interest, as it determines the effectiveness of merging with neighboring filaments, and therefore, the properties of the built part. This paper reports infrared thermography based measurement of temperature field on the platform bed as a function of time during filament dispense and comparison with an analytical model based on moving heat source theory. Measurements identify two key heat transfer processes that influence temperature distribution on the bed. Data show that diffusion of thermal energy deposited on the bed with the filament and heat transfer from the hot nozzle tip both influence the temperature distribution on the bed. The relative contributions of these two sources of temperature rise on the bed are measured for different values of key process parameters. Measurement of temperature rise due to diffusion of thermal energy deposited on the bed is found to be in excellent agreement with an analytical model based on moving heat source theory. Evolution of the temperature field after completion of rastering of a line is measured. These data play a critical role in determining how effectively a filament merges with neighboring filaments. By identifying the key heat transfer processes that determine the temperature field on the platform bed, this work contributes towards thermal optimization of polymer-based additive manufacturing.
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