The heat generation of a lithium-ion cell is predominantly comprised of irreversible and reversible heating, with the latter term dictated by the entropic heat coefficient which describes the variation of the cell's open circuit voltage with respect to temperature. The entropic heat coefficient is also dependent upon the cell's state of charge, and the correct description of its behaviour is required to implement numerical and analytical models which accurately describe the heat generation rate of lithium-ion cells. In this study, the entropic heat coefficient of a single 26650 LiFePO4 cylindrical lithium-ion cell is determined through a novel liquid immersion experimental set-up, offering greater thermal uniformity and control in comparison to the environmental chambers typically utilised. The arrangement is more cost effective and easily implemented in a laboratory setting, with a thermal non-uniformity of 0.2 degrees C and setpoint stability of f 0.1 degrees C achieved. The entropic heat coefficient is determined for six setpoint temperatures in the range of 15 degrees C to 40 degrees C across the cell's entire state of charge, which is adjusted in increments of 10 %. This work also details the influence of open circuit voltage instability arising from cell self-discharge and relaxation, and a method to adjust for its effect is proposed and implemented. The entropic heat coefficient is subsequently utilised to determine the heat generation rate of the cell under twophase liquid immersion cooling conditions.

A fully 3D-printed dielectric resonator antenna (DRA) with right-hand circular polarization, fed by a cross-shaped aperture is presented. The DRA utilizes parasitic conductive elements to improve the circular polarization. The antenna operates in the S-band (2-3GHz). The effects of the conductive element parameters on the axial ratio and gain are examined, as well as the effect of using a 3D-printed conductive material versus copper for fabrication. The two prototypes are then fabricated. The microstrip line, dielectric substrate, ground plane with the aperture slot, and the conductive walls are all 3D-printed in a single process using fused deposition modeling technology.

The thermal performance of the electrode terminals or tabs of a 26650 LiFePO4 cylindrical lithium-ion battery under direct contact liquid immersion cooling conditions is experimentally investigated during charging and discharging, highlighting their contribution to the overall heat transfer from the battery which has not been examined previously. High rates of heat transfer occur from the terminal surfaces for complete immersion in Novec 7000 due to the battery’s anisotropic thermophysical properties, coupled with additional heating from the electrical connections. The establishment of two-phase conditions for initial bulk fluid temperatures of 33 °C ± 0.5 °C further augments the heat transfer, providing greater thermal uniformity across the entire battery as nucleate boiling is induced on the terminal surfaces. Vigorous vapour bubble growth and departure limits the temperature difference between the terminals and the bulk fluid, indicative of the heat transfer intensity, with values two to three times lower than those observed under natural convection liquid immersion conditions. For the discharge rate of 10C, the phase change restricts the temperature difference between the positive and negative terminals and the bulk fluid to a maximum of 3.5 °C and 5 °C respectively. A corresponding cell thermal inhomogeneity of 2.2 °C is maintained, minimising accelerated electrochemical material degradation. Similar performance is exhibited during charging at the rate of 4C, restricting the temperature difference between the positive and negative terminals and the bulk fluid to a maximum of 1.4 °C and 2.2 °C respectively, and the cell thermal inhomogeneity to 1 °C.

The thermal management of a lithium -ion battery module subjected to direct contact liquid immersion cooling conditions is experimentally investigated in this study. Four 2.5 Ah 26650 LiFePO 4 cylindrical cells in a square arrangement and connected electrically in parallel are completely immersed in the dielectric fluid Novec 7000. The thermal and electrical behaviour of the module is assessed at charging and discharging rates of 1C to 4C. Experiments are conducted with initially ambient temperature liquid, resulting in single phase natural convection cooling, as well as preheated liquid temperatures of 33 degrees C +/- 0.5 degrees C to study the influence of the phase change process under pool boiling conditions. Superior performance is observed when two-phase immersion cooling conditions are established for discharge rates of 2C and above, limiting the average cell temperature rise to 1.9 degrees C at the end of 4C discharge, corresponding to a maximum temperature of 34.7 degrees C. For the most onerous charging rate of 4C, considered fast charging, this maximum temperature rise is limited to 1.3 degrees C, corresponding to a maximum temperature of 35 degrees C. Vigorous boiling is observed from the cells ' electrodes, leading to more effective heat transfer from the locations of high heat flux. Excellent module thermal homogeneity is exhibited, maintaining a maximum temperature difference of 1.2 degrees C for all cases investigated. The axial temperature gradients of the module ' s individual cells are also greatly reduced under two-phase conditions. The influence of cell spacing within the module is also investigated for inter -cell spacings of 0.25 D and 1 D , where D is the cell diameter. Marginally improved heat transfer performance is observed for the more closely spaced cell arrangements, reducing the maximum cell temperatures and thermal inhomogeneity within the module.

The peak forces generated in friction stir welding impose substantial constraints on the process window of the technology, affecting factors such as machine scale, design, and the range of applicable materials. In this study, a combination of tool force monitoring and post-weld evaluations is employed to investigate the impact of auxiliary assistive heating on various parameters including the maximum forces and torques and the effects preheating has on the microstructure and mechanical behaviours during friction stir spot welding of AA2024-T3 plates in a lap joint configuration. The methodology involves preheating the workpieces to a range of different temperatures, and the forces were measured using a dynamometer. Most notably, preheating the workpiece before the plunge stage yields a substantial reduction in peak forces. A decrease of 69 % and 76 % in vertical force and torque, respectively, was observed as the preheating temperature increased from 20 °C to 400 °C. Furthermore, the addition of auxiliary heating to the workpieces exhibited no significant statistical effect on the shear tensile performance of the resulting welds. These reductions in the process forces offer the potential for extended tool life, decreased machinery expenses, and broader material applicability, all while maintaining original joint integrity.

Abstract This study details an experimental investigation of forced convective liquid immersion cooling of lithium-ion batteries. Twelve Samsung INR 18650 20S cylindrical cells, each with a nominal voltage of 4.2 V and nominal capacity of 2 Ah, are placed in a 4-in-series, 3-in-parallel arrangement inside a polycarbonate chamber. The cells are immersed in the dielectric fluid SF-33 and are discharged at C rates in the range of 1C to 4C. Temperature sensors placed along the body of each cell near the electrodes monitor their thermal behaviour during experiments. Results show that liquid immersion cooling is an effective method to maintain all cell temperatures within the desired temperature range of 15°C to 35°C for all cases tested, maintaining temperature differences within individual cells to ≤1.5°C, and those between cells to ≤3°C. This research contributes to the understanding of thermal management strategies for lithium-ion batteries, particularly in scenarios involving high discharge rate applications.

Gradient index lens antennas find their application in modern wireless communication including radar systems, satellite communication, etc. They can ensure highly directional beamforming, support multibeam operations and beam steering capabilities within a wide angle of view. The development of 3D printing technologies enables the implementation of complex gradient refractive index distribution with high accuracy as well as obtaining a significant reduction in the production time and cost. This paper presents the state of the art in gradient refractive index lens antenna design using different types of additive manufacturing processes. It shows the applicability of 3D printing to a wide range of operational frequency bands from microwave to sub-THz and THz.

The thermal and electrical performance of lithium-ion batteries subjected to liquid immersion cooling conditions in a dielectric fluid has been experimentally investigated in this study. A single 26650 LiFePO4 cylindrical cell is completely immersed in Novec 7000 and charged and discharged at onerous maximum rates of up to 4C and 10C, respectively, where C can be defined as the measure of the rate at which a cell is charged or discharged relative to its rated capacity. Immersion cooling offers high rates of heat transfer from the cell's surface, in particular when the saturation temperature of the fluid is exceeded, and two-phase conditions are established. At a preheated liquid pool temperature of 33 & PLUSMN; 0.5 & DEG;C for discharge rates & GE; 2C, subcooled boiling conditions develop, with the cell's temperature rise limited to 3.6 & DEG;C at the end of 10C discharge. Furthermore, for 4C charging under the same preheating conditions, the cell's temperature rise does not exceed 1 & DEG;C. Superior performance is observed under two-phase immersion cooling conditions in comparison to both single phase liquid immersion and natural convection air cooling for the same charge and discharge rates. Excellent thermal homogenisation across the cell is also determined, with a maximum axial temperature difference of 0.25 & DEG;C and 1 & DEG;C for 4C charging and 10C discharging respectively.

Direct contact liquid immersion cooling is receiving increased attention as a potential battery thermal management method. This method offers greater cell thermal homogenization and increased pack performance through enhanced rates of heat transfer. However, limited published literature exists on this thermal management method, particularly on module arrangements. This study presents an experimental investigation of a battery module consisting of four LiFePO 4 cylindrical cells connected electrically in parallel and completely immersed in the dielectric fluid Novec 7000. The module is subjected to increasing constant current discharge rates to a maximum of 3C, examining the thermal performance of the proposed method and its capability to minimize cell-to-cell temperature difference, particularly under two-phase conditions.

Stimuli-responsive hydrogels have attracted much attention owing to the versatility of their programmed response in offering intelligent solutions for biomimicry applications, such as soft robotics, tissue engineering, and drug delivery. To achieve the complexity of biomimetic structures, two photon polymerization (2PP) has provided a means of fabricating intricate 3D structures from stimuli-responsive hydrogels. Rapid swelling hydrogel microstructures are advantageous for osmotically driven stimuli-response, where actuation speed, that is reliant on the diffusion of analytes or bioanalytes, can be optimized. Herein, the flexibility of 2PP is exploited to showcase a novel sugar-responsive, phenylboronic acid-based photoresist. This offers a remarkable solution for achieving fast response hydrogel systems that have been often hindered by the volume-dependent diffusion times of analytes to receptor sites. A phenylboronic acid-based photoresist compatible with 2PP is presented to fabricate stimuli-responsive microstructures with accelerated response times. Moreover, microstructures with programmable actuation (i.e., bending and opening) are fabricated using the same photoresist within a one-step fabrication process. By combining the flexibility of 2PP with an easily adaptable photoresist, an accessible fabrication method is showcased for sophisticated and chemo-responsive 3D hydrogel actuators.

This paper investigates the 3D printing of a hemispherical dielectric resonator antenna (DRA) on a ground plane made from a 3D printed conductive material. The DRA is designed to operate in the C-band (3700 – 4200 MHz) and is intended for satellite communication (SATCOM) applications. The proposed antenna prototype achieved a -10 dB bandwidth of 12.2% with an average and peak gain of 4.69 dBi and peak gain of 5.39 dBi respectively.

As-built, SLM-printed Ti-6Al-4V parts suffer from non-equilibrium, brittle microstructures due to the formation of metastable α’-phase martensite as a result of the printing process. Post-processing heat treatments are required to alleviate residual stress and decompose the martensite into equilibrium phases. Using unconventional ‘cyclical’ heat treatment profiles, it is possible to achieve a bimodal microstructure as opposed to a more typical lamellar microstructure achieved using conventional heat treatment profiles. In the present research, the effect of cyclical heat treatment parameters (maximum temperature, minimum temperature, number of cycles and cooling regime) on the grain morphology, phase composition and mechanical performance (static and dynamic) has been evaluated. Furthermore, a comparison to a typical lamellar microstructure has been completed as a reference.

This work aims to apply the Theory of Critical Distances (TCD) to the fatigue assessment of additively manufactured (AM) Ti-6Al-4V material produced via the selective laser melting (SLM) process. Modified alternatives to traditional TCD methods are considered. In this sense, it is sought to develop a fatigue prediction model that is better suited to assessing the impact of multiple stress-rising features which are located in close proximity to each other. Hereby, consideration has been given to modelling process-inherent surface roughness in combination with an internally positioned artificial defect, shaped as a feature that is reminiscent of a pore. Simultaneously, the research also seeks to circumnavigate a potential issue with respect to the current TCD methodology. This concerns the matter of applying TCD practices to components whereby the area of interest for conducting stress-distance analytics is on a size scale that is smaller than that of the critical distance length parameter itself. Several different strategies were attempted as a way to try and achieve meaningful modifications to the TCD process. Results show that it is possible to overcome such challenges that can often present themselves during the fatigue appraisal of AM metal parts. In this sense, the optimal novel strategy that was experimented with returned average error margins of 13.7% or better. It is anticipated that such models may assist in further optimising the accuracy of service life evaluation for metallic AM components that are intended for industry.

An approach to study the effect of geometric parameters on the Poisson’s ratio of a three-dimensional re-entrant Cobalt Chrome Auxetic structure has been presented. The Auxetic mechanical structure has been realized with the help of Additive Manufacturing. The mechanical properties and Poisson’s ratio of the physical samples were determined using compression testing and Digital Image Correlation (DIC). A Finite Element model is used to estimate the Poisson’s ratio numerically. A Design of Experiments (DOE) and Finite Element Analysis (FEA) based optimization study is further performed in order to investigate the effects of the different geometric parameters on the Poisson’s ratio of the Auxetic samples. The DOE model is used to determine the values of the parameters for an optimal performance of the samples in terms of the observed Auxetic behaviour.

The main priority for additive manufacturing research must be an improvement in process repeatability and reproducibility. This paper proposes a modified energy density equation as a basis for location dependent control of process parameters. This equation was used to adapt the process parameters in laser powder bed fusion of cobalt chrome alloy to achieve a more uniform energy density input across the build. Using this modified equation, the standard deviation of the mechanical properties was reduced by 40%. This improvement in consistency was verified using additional testing using 316L. The reduction in the variation in local mechanical property values was verified to be statistically significant.

Thermal cycling is known to adversely affect the performance and lifespan of thermoelectric generators (TEGs) yet has received limited attention to date. The current study experimentally investigates the effect of thermal cycling on the performance of twelve nominally identical TEG modules. Six samples were subjected to the same thermal cycle profile with an average heating time of 154 s to examine the variation in their outputs. The maximum cycling temperature was varied between 170 °C and 190 °C for a further six samples to investigate the effect of maximum set-point temperature on performance. Degradation in performance was exhibited by all modules, with maximum power outputs between 28 ZT of 21

Selective Laser Melting is an additive manufacturing practice that permits the production of metal alloy-based parts. While facilitating the design of complex geometry, SLM leads to the fabrication of a unique material structure that showcases distinct behavioural characteristics relative to their traditional methods of material manufacture. Defects that are innate to SLM inspire the presence of a compositional outlook that is inhomogeneous in nature and only serves to hinder part efficiency. Thus, the Theory of Critical Distances offers a refreshed proposal to evaluating notched Ti-6Al-4V material produced by additive manufacturing processes. Key principles of the theory’s working mechanisms are outlined. Subsequently, symmetrical notches of contrasting size are assessed. Findings reveal that the Theory of Critical Distances is adequately compatible with accurate fatigue prediction of SLM Ti-6Al-4V in its as-built state. Additionally, fracture surface analysis reveals that crack initiation is predominantly a surface-based phenomenon. Hereby, increased focus must be given to the quality of processed material that is located at the externalities of additively manufactured components, in order to enhance their service life capabilities. This will induce an increasingly uniform material structure that will allow for more predictable behavioural characteristics.

Cobalt-chromium (CoCr) alloys are widely known for their biomedical applications due to their numerous favorable characteristics. The fabrication process for these alloys is crucial and plays an important role in the mechanical and metallurgical properties. Powder bed fusion (PBF) is a type of additive manufacturing technique, which includes selective laser melting (SLM) process as one of the 3D printing techniques that is used for CoCr fabrication. There are different SLM process parameters that affect the mechanical properties of the built part. The present work is the review of different parameters that affect the component manufactured using SLM. It was found that the laser scanning speed was the most influential factor for the density and hardness of the SLM formed CoCr alloys. It was also found that the mechanical properties of the SLM manufactured CoCr alloys are higher as compared to the cast alloys due to the dense, compact, homogeneous, and finer microstructure formed during the SLM process.

Due to the many advantages associated with metal additive manufacturing (AM) processes, research into these technologies has grown significantly over the last number of years. However, repeatability and reproducibility are critical issues in AM when compared to traditional manufacturing processes. This work reviews the current state of repeatability/reproducibility in metal additive manufacturing, specifically in powder bed fusion technologies such as selective laser melting (SLM) and electron beam melting (EBM). The repeatability issues present in these AM processes is shown to be pervasive across the available literature. The additive manufacturing process is separated into pre-process, para-process and post-process stages and a critical review of the causes and impact of repeatability found at these stages is conducted. Key parameters of these stages: powder properties, laser characteristics and post-processing are studied in depth as sources of variation that can diminish repeatability. An understanding of the limitations of analysing repeatability from the current literature is developed and areas for further research towards the improvement of repeatability are identified.