Dissipation Capability Research Articles

Demonstration of near‐size independent EQE for 368 nm UV micro‐LEDs

UV‐ranged micro‐LEDs are being explored for numerous applications due to their high stability and power efficiency. However, previous reports have shown reduced EQE and increased leakage current due to the increase in surface‐to‐volume ratio with a decrease in the micro‐LED size. In this work, we studied the size‐related performance for UV‐A micro‐LEDs, ranging from 8 × 8 µm2 to 100 × 100 µm2. These devices exhibited reduced leakage current with the implementation of ALD based sidewall passivation. A systematic EQE comparison was performed with minimal leakage current and obtained a size‐independent on‐wafer EQE of around 5.5%. Smaller sized devices experimentally showed enhanced EQE at high current density due to their improved heat dissipation capabilities. To the best of authors’ knowledge, this is the highest reported on‐wafer EQE demonstrated in < 10 µm dimensioned 368 nm UV LEDs.This article is protected by copyright. All rights reserved.

Significantly Enhanced Thermal Conductivity of hBN/PTFE Composites: A Comprehensive Study of Filler Size and Dispersion.

High-temperature polymers are attractive for applications in extreme temperatures, where they maintain their mechanical flexibility and electrical insulating properties. However, their heat dissipation capability is limited due to their intrinsically low thermal conductivities. Hexagonal boron nitride (hBN) is a chemically inert, thermally stable, and electrically insulative compound with a high thermal conductivity, making it an ideal candidate as a filler within a high-temperature polymer matrix to increase the thermal conductivity. This study evaluates the effect of filler size and dispersion on thermal conductivity by producing homogeneous composite samples using a combination of solvent mixing and resonant acoustic mixing (RAM). We carefully characterized our samples, including the spread of the size distribution, and observed that the smaller sized hBN centered around 5 μm was able to integrate more seamlessly into the polytetrafluoroethylene (PTFE) matrix with particle size in the 15 μm range and hence outperformed 30 μm, in contrast to the conventional wisdom, which asserts that larger fillers universally perform better than smaller ones. Our thermal conductivity of hBN/PTFE composites at 30 wt % is 2× higher than the literature values. Notably, we reached the record-high value of 3.5 W/m K at 40 wt % with an onset of percolation at 20 wt %, attributed to optimized hBN dispersion that facilitates the formation of thermal percolation. Our findings provide general guidelines to enhance the thermal conductivity of polymer composites for thermal management, ranging from power transmission to microelectronics cooling.

Seismic performance of resilient self-centring bridge piers equipped with SMA bars

This study examines the seismic behaviour of precast post-tensioned segmental (PPS) bridge piers that are equipped with shape memory alloy (SMA) bars. PPS piers are designed to minimise overall and localised damage to bridges by incorporating natural hinges and rocking mechanisms. The introduction of super-elastic SMA bars has the potential to enhance the use of PPS piers in highly seismic regions. A parametric study is conducted to identify piers with the highest energy dissipation capacity, which are then subjected to dynamic analysis. The aim is to assess the energy dissipation capability of these systems by subjecting the selected piers, both with and without SMA bars, to far-field ground motions using a finite-element framework. The seismic performance of the piers is evaluated using incremental dynamic analysis (IDA) curves, which are obtained from analysing the piers under 44 far-fault ground motions. The IDA results indicate a significant reduction in the drift responses of the piers when SMA bars are utilised.

Finite element analysis of precast concrete beam‐column joint based on bolt connection

SummaryA novel proposal for an assembled bolted joint between a precast reinforced concrete beam and column is presented in this paper. In order to evaluate the behavior of the joint, a comprehensive experiment was carried out on a full‐scale precast concrete beam‐column joint based on bolted connection subjected to pseudo‐static loading. The experiment aims to investigate the failure mode, load carrying capacity, and energy dissipation characteristics of the joint. To further understand the performance of the joint, a finite element model was developed using ABAQUS. The difference in failure mode and ultimate bearing capacity between the bolted joints and cast‐in‐situ joints were analyzed in conjunction with the experimental results. Moreover, the impact of various parameters on the performance of the bolted joints was quantified. The characteristics of the bolted joints, such as the span‐depth ratio, reinforcement ratio of the beam, the stirrup ratio at the beam‐column junction, T‐end length, and the bolt strength grade, were meticulously analyzed through an examination of bearing capacity curve, stiffness degradation patterns, and energy dissipation capability. The results show that, in comparison with the traditional cast‐in‐situ joints, the damage failure extent exhibited by the bolted joints is notably smaller, thereby rendering it more advantageous for seismic restoration and replacement. Furthermore, the span‐depth ratio, reinforcement ratio of the beam, and the stirrup ratio have a greater influence on the seismic performance of bolted joints than the T‐end length and bolt strength grade.

Experimental study and multi‐scale refinement model of high damping acrylic polymer matrix VEDs for civil structural seismic retrofit

AbstractThe viscoelastic dampers (VEDs), which can provide both stiffness and damping, have been recently introduced into the field of structural vibration control for seismic enhancement of civil engineering structures. In this study, a kind of high damping acrylic polymer matrix VEDs (HDPVED) is developed independently, and this innovative HDPVED can solve the significant problem of service performance under medium‐high temperature environments, as well as low‐frequency vibration control under earthquake actions for civil engineering structures. To systematically investigate the influencing rules of frequency, temperature, and displacement amplitude on the mechanical properties and damping dissipation performance of HDPVEDs, a series of dynamic mechanical performance tests of the developed HDPVEDs are carried out at different frequencies, temperatures, and displacement amplitudes. The research results show that HDPVED exhibits excellent damping dissipation capability and adaptability under medium‐high temperature environments and low‐frequency excitations. The mechanical properties and energy dissipation performance present a strong correlation with frequency, temperature and displacement amplitude, and there is an obvious coupling effect between the three influencing factors. Based on the macroscopic mechanical property research of HDPVED, the microscopic damping mechanism and microscopic mechanical properties of HDPVED are then investigated. High‐order fractional derivative fraction Voigt and Maxwell model in parallel (FVMP) models are preferred to characterize the combined hyper‐elasticity and viscoelasticity owned by networked molecular chains and free molecular chains. The breaking and reconstruction theory of microphysical bonds is used to assess the effect of packing particles, and the time‐temperature equivalence principle is introduced to assess the effect of temperature. The multi‐scale refinement model is proposed, and the validity and accuracy of this model are verified by testing data of HDPVED. The study results show that the proposed model can accurately describe the effects of frequency, temperature, displacement amplitude, and microstructure on the multi‐scale mechanical properties of HDPVED. It provides a theoretical basis for the multi‐scale design and development of high damping VEDs.

Cyclic and uniaxial tensile properties of superelastic niti shape memory alloy cables

This study investigates the impact of various factors, including annealing duration, strain amplitude, cyclic loading, loading rate, and pre-training, on the mechanical properties of Nickel–Titanium shape memory alloy (SMA) cable. The primary focus is on evaluating their recovery ability and energy dissipation capabilities. The tested SMA cable has an outer diameter of 9 mm and a 7 × 7 configuration. The variation of strength, stiffness, residual strain, hysteretic energy, and equivalent viscous damping ratio of SMA cable with the loading cycle is analyzed. Furthermore, the impact of various annealing durations on the tensile strength and elongation of both SMA cables and wires was examined through monotonic tensile tests. The results indicate that the annealing duration considerably affects the superelastic behavior of SMA cables by shifting the stress-strain loops down and widening them. The recovery ability of SMA cable degrades more progressively with increasing loading amplitude and the number of loading cycles. The mechanical properties gradually stabilized after 20 times of constant strain amplitude loading and unloading training. The strain selection for cyclic training should not make the SMA cable in the martensite hardening stage. The recovery ability and peak stress of SMA cable can be significantly improved by pre-training. With the increase of annealing duration, the tensile strength of the SMA cable decreases gradually. Compared with SMA wire, SMA cable has better ductility and robustness and provides sufficient restoring force under large deformation.

Experimental study on GFRP-steel double-skin rubber concrete composite piers under mixed loading

To address the issues related to the insufficient compressive and shear strength of rubberized concrete and to enhance the deformation capacity and longevity of composite piers, a novel approach involving the confinement of rubberized concrete with glass fiber-reinforced polymer (GFRP) tube and steel tube has been proposed. This method aims to create a GFRP-steel double-skin rubberized concrete composite pier. Pseudo-dynamic and quasi-static tests were conducted on the composite pier to assess its seismic response and residual seismic performance. The study investigated the failure mode, hysteretic characteristics, ductility, energy dissipation, stiffness, and self-centering capability of composite piers. Additionally, a correction coefficient for the equivalent plastic hinge length was proposed. The results indicate that the specimen maintains its elastic state when subjected to frequent seismic events. However, when subjected to infrequent seismic events, the glass fiber layer may fracture, leading to the development of white stripes, while the steel tube experiences yielding, transitioning into a plastic state. The shock absorption and toughness of rubberized concrete were effectively utilized when subjected to horizontal low cyclic loading, thereby mitigating the appearance of more pronounced damage in seismic-damaged specimens. The hysteretic curve was complete, demonstrating a significant energy dissipation capacity. This confirmation indicates that the composite pier can be a viable solution for sea-crossing bridges and complex environments.

Structure Design and Heat Transfer Performance Analysis of a Novel Composite Phase Change Active Cooling Channel Wall for Hypersonic Aircraft.

Efficient and stable heat dissipation structure is crucial for improving the convective heat transfer performance of thermal protection systems (TPSs) for hypersonic aircraft. However, the heat dissipation wall of the current TPS is limited by a single material and structure, inefficiently dissipating the large amount of accumulated heat generated during the high-speed maneuvering flight of hypersonic aircraft. Here, a convection cooling channel structure of TPS is proposed, which is an innovative multi-level structure inspired by the natural honeycomb. An active cooling channel (PCM-HC) is designed by using a variable-density topology optimization method and filled with phase change material (PCM). Numerical simulations are used to investigate the thermal performance of the PCM-HC wall, focusing on the influence of PCM properties, structural geometric parameters, and PCM types on heat transfer characteristics. The results demonstrate that the honeycomb-like convection cooling channel wall, combined with PCM latent heat of phase change, exhibits superior heat dissipation capability. With a heat flux input of 50 kW/m2, the maximum temperature on the inner wall of PCM-HC is reduced by 12 K to 20 K. Different PCMs have opposing effects on heat transfer performance due to their distinct thermophysical properties. This work can provide a theoretical basis for the design of high-efficiency cooling channel, improving the heat dissipation performance in the TPS of hypersonic aircraft.

Seismic performance of precast steel fiber reinforced lightweight aggregate concrete shear walls assembled with a socket horizontal joint

In the context of constructing a precast shear wall structure, the connection of precast units plays a crucial role in ensuring structural safety and facilitating efficient construction, while the use of conventional concrete for heavy precast units leads to low lifting efficiency. It promotes the study of this paper, which proposed a novel type of socket horizontal joint for precast shear walls made of steel fiber reinforced lightweight aggregate concrete (SFRLAC). Three specimens were tested using the low-cyclic loading test: two reinforced SFRLAC shear walls assembled with a socket horizontal joint and a cast-in-place reinforced SFRLAC shear wall as a reference. The failure patterns and process of the shear walls were observed, while the load-displacement curves were obtained. Additionally, finite element (FE) numerical models were built to conduct parametric analysis on different influencing factors. The bearing capacity, energy dissipation capability, deformation ability, and stiffness degradation of the shear walls were studied. Results indicate that when the precast shear walls failed in bending-shear, the socket horizontal joint remained intact. The three shear walls exhibited fewer differences in seismic behaviors with relatively full hysteretic curves. The seismic mechanisms of the shear walls assembled with a socket horizontal joint are elucidated by analyzing the numerical distribution of stress in both SFRLAC and steel bars at different stress levels. Meanwhile, the study assesses the mechanical properties of precast shear walls by analyzing the impact of the shear to span ratio and the axial compression ratio.

A numerical investigation of energy dissipation mechanism of visco-hyperelastic bimaterials subjected to low-velocity impact

Impact protection equipment typically relies on the material failure mechanism and is frequently discarded after the impact. A promising alternative emerges utilizing a foam-infilled lattice bimaterial with shape recovery trait and customizable impact protection capability. This study employs a numerical approach to elucidate the energy dissipation mechanism exhibited by the bimaterial under low-velocity impact condition. The experiment-based inverse method is used to characterize visco-hyperelastic models for two base materials. The investigation reveals that the interaction between the two base materials significantly amplifies the foam’s energy dissipation capability, resulting in an overall higher energy dissipation for the bimaterial. Besides, the study establishes a close correlation between the bimaterial’s effective viscoelastic properties and the enhancement in energy dissipation. Furthermore, the numerical design of experiments (DOE) analysis of base material properties indicates that there is an optimal region for selecting the critical foam material constants for a significant improvement of bimaterial energy dissipation. The results reveal that the combination effect of material interaction and material viscoelastic properties constitutes the fundamental elements of bimaterial energy dissipation mechanism. The insight derived from this study can inform the future design and material selection of a more complicated bimaterial across a wide range of impact protection applications.

Low temperature effect on cyclic behavior of shape memory alloy U-shaped dampers

Although Nickel-Titanium (NiTi) shape memory alloys (SMAs) have been widely studied as seismic response mitigation devices in civil structures, their temperature sensitivity due to the coupled thermo-mechanical behavior prevents their practical implementation in cold temperature environments. This paper investigated the effects of varying ambient temperatures (particularly low ambient temperature) on the hysteretic behavior and self-centering (SC) ability of shape memory alloy U-shaped dampers (SMAUDs) that were made of NiTi showing superelasticity (SE) at room temperature. Thermo-mechanical properties of SMAUDs were identified through differential scanning calorimetry (DSC) tests. Cyclic loading experiments on SMAUDs were conducted at a wide ambient temperature range from −40 °C to 20 °C. The variations in the hysteretic characteristics of the SMAUDs, including partial superelasticity (SE), strength, stiffness, SC, and energy dissipation capabilities, at different ambient temperatures were investigated. In general, the decreasing ambient temperature leads to the degradation of the SC ability. At ambient temperatures below the phase transformation temperature, the SMAUDs lose the SE but still maintain certain levels of strength and energy dissipation, which is different from common axial-type SMA elements. Meanwhile, the SMAUDs can restore their SC ability and strength after the recovery from low temperatures to room temperature, making them suitable for use in a cold environment.

Seismic performance of recentering energy dissipation bracing with pendulum in prefabricated steel frame structure systems

This research introduces the recentering energy dissipation bracing with a pendulum in prefabricated steel frame (REBP-PSF) structures. The primary objective of this study is to mitigate potential severe damage to steel frames during seismic events and to expedite postearthquake rehabilitation. This innovative system harnesses energy dissipation through friction between structural members and facilitates recentering through the cable pretensioning force. Consequently, this may make postearthquake repairs minimal or even unnecessary. Three REBP-PSF structures and a prefabricated steel frame with cantilever beam splices (PSF-CBS) were meticulously modeled using the ABAQUS finite element method to evaluate the seismic performance of this novel system. The study investigated the effects of pretensioning parameters and varied structural morphologies on seismic performance metrics, including failure modes, hysteresis performance, energy dissipation capacity, and residual displacements. This paper introduced a simplified method to simulate the restoring force model of REBP using the connection element method. Subsequently, a model for the REBP-RSF overall structure was established, enabling nonlinear dynamic time-history analysis, which was then compared with the rigid steel frame (RSF) structure. The results showed that the REBP-PSF structure outperformed the PSF-CBS in terms of initial stiffness, yield load, ultimate load, and energy dissipation capability. The bearing and energy dissipation capacities of the REBP-PSF structure were enhanced by augmenting the cable pretensioning force within this structure. The proposed connection element method could accurately simulate the hysteretic behavior of REBP. Compared with the RSF structure, the REBP-RSF demonstrated superior seismic performance, characterized by small story drift and residual displacements during seismic activities. Furthermore, a notable augmentation was observed in the lateral stiffness and torsional rigidity of the proposed structure.

Seismic performance of self-centering steel column base with buckling-restrained bars

Self-centering structures, emerging as a vital innovation in earthquake-resilient design, offer notable advantages in structural integrity, rapid functional recovery post-earthquake, and economic efficiency over the building's life cycle. This paper introduces a self-centering steel column base, employing buckling-restrained bars for energy dissipation. Through an analysis of its force mechanism, it conducts low-cycle reciprocating load test on four set of specimens with varying buckling-restrained bars and axial compression ratios. The results demonstrate the column base's superior seismic performance, characterized by its ability to rapidly restore seismic functionality after unloading and replacing the buckling-restrained bars. The force-displacement hysteresis curves exhibit a distinctive double-flag shape, with a minimal residual story drift of 0.001 rad, underscoring the structure's exceptional self-centering capability. The displacement ductility coefficients and equivalent viscous damping coefficients confirm the column base's excellent deformation and energy dissipation capabilities. Furthermore, a refined ABAQUS finite element model is developed and validated against the load test results, further analyzing the seismic performance relative to key structural parameters.

Experimental and numerical study of locking steel plate damper for seismic resistance

In this paper, a new metal energy dissipator named locking steel plate damper (LSPD) is proposed for structural seismic applications. The damper is easy to fabricate and convenient to install and replace. The LSPD consists of two locking steel plates with energy dissipating steel rings and an outer shell plate for peripheral restraint and steel ring reset. Experiments were conducted using a proposed static cyclic displacement loading regime, which loaded the two specimens. The results of experiment showed that the damper possessed excellent energy dissipation capability. Thirteen comparison models were designed by varying the radius, thickness and width of the ring and FE simulations were carried out. By comparing the simulation results of the 13 models, the effects of the steel ring design parameters on the energy dissipation capacity of the damper are summarized. Finally, the suggested formulas for the yield displacement and the practical formulas for the initial stiffness of the LSPD are presented through theoretical derivation and parametric analysis. The average error between theoretical and simulated values for the 13 models was 2.11%, with a maximum error of 7.33%, which provide reliable suggestions for the design and fabrication of the damper.

A novel tree -like bionic structure for liquid-cooled lithium-ion battery plates

To maintain the optimal operating temperature range for the power batteries of new energy vehicles, this paper proposes a novel tree-structured channel cold plate. An orthogonal experimental approach was employed to investigate the effects of inlet flow rate m, inlet channel quantity n, channel width d, and hierarchy ratio α on battery temperatures Tmax, standard temperature difference Tσ, and pressure drop ΔP. Non-dominated sorting genetic algorithm II (NSGA-II) was utilized to obtain an optimized solution set. The results reveal that the tree-like cold plate configuration including m = 15 g/s, n = 7 and 8, d = 3 mm, and α = 1:2:1 demonstrates superior performance. Thus, the developed cold plate was further compared with the conventional straight-through cold plate design. The results indicate that the proposed tree-like cold plate exhibits the maximum decreases of 13.94% in Tmax, 52.94% in Tσ, and 61.5% in ΔP. The most significant improvement in the composite indicator PEC was 89%. In conclusion, the tree-like channel cold plate developed in this study provides excellent heat dissipation capability for power batteries, offering valuable insights for the thermal design of battery liquid cooling systems.

GRAPHITE FOAM STRUCTURES AS AN EFFECTIVE MEANS TO COOL HIGH-PERFORMANCE ELECTRONICS

Due to their unique heat transfer features, graphite foams are used in the current analysis to form heat sinks effective enough to dissipate extreme heat generated within high-performance electronics. The heat sinks proposed are formed from foamed-baffles arranged either in parallel or perpendicular to the coolant paths through the staggered slots in between to alleviate the penalty of pressure drop while maintaining high heat dissipation capability. Two different sorts of dielectric coolants namely, air and the FC-3283 electronic liquid developed by 3MTM, have been utilized to directly dissipate the heat generated. The feasibility of the currently proposed heat sinks has been examined numerically based on the volume averaging concept of porous media employing the local thermal non-equilibrium model to account for interstitial heat exchange between the foam solid matrix and the fluid particles flowing across. A wide range of design parameters has been tested including the heat sink configuration along with structural characteristics of the graphite foam used. It has been found that foam baffles oriented perpendicular to the path of coolant flow can dissipate heat by about 50% better than those parallel to it, but with higher pressure losses. It has also been found that heat dissipation capability, for a certain orientation of baffles, can be improved by up to 100% when the foam pore size is doubled with outstanding saving in pressure losses by up to 300%. The impact of operating conditions, including the coolant flowrate and the heat flux applied, has also been inspected. The currently proposed heat sinks have been found efficient to meet the thermal demands of high-performance electronics and sweep away the extreme heat generated there with reasonable cost of pressure drop, where the proper selection of design parameters in light of the operating conditions applied can prevent the emergence of hot spots entirely. Extreme operating conditions, i.e. with heat density of up to 10W/cm2 for air-cooled heat sinks and 100W/cm2 for those cooled with FC-3283, can be well managed when a heat sink is configured from baffles that are oriented perpendicularly to the coolant flow path and formed of graphite foam having low porosity (∅=0.8) and larger pore size

Experiment Studies of a DC Inductive Superconducting Fault Current Limiter With Energy Dissipation Capability

Experiment Studies of a DC Inductive Superconducting Fault Current Limiter With Energy Dissipation Capability

Thermally conductive, electrically insulating, and radiative cooling PVDF/BNNS/Al2O3/cordierite nanocomposites

For now, the introduction of fillers with highly intrinsic thermal conductivity (TC) into polymers is the most effective way to prepare composites with excellent heat dissipation capability. However, reducing the filler loading without lowering the TC is the most challenging. In this article, patterned electrospinning and hot pressing were employed to prepare thermally conductive and electrically insulating polyvinylidene fluoride (PVDF)-based composites containing 20 wt% boron nitride nanosheets (BNNSs) as the dominant filler and 5 wt% Al2O3 nanopowders as the auxiliary filler. We have proven “1 + 1>2″ in terms of enhancing the TC of composites at nanoscale by the synergistic effect of nanosheet and nanopowder. The results showed that the in-plane TC (8.55 W/(m·K)), volume resistivity (1.56 × 1015 Ω cm), and breakdown strength (406.2 kV/mm) of the composites were improved by 4759.1 %, 649.0 %, and 34.1 % compared with the neat polymer, respectively. Furthermore, the potential application of such composites in contact heat dissipation and radiative cooling was demonstrated by the experiment of integrating cordierite particles with high infrared radiation onto the films.

Research on high power and heat flux aerospace electronic products of integrated thermal conduction technology

In response to the increasing demand for enhanced computational capabilities within satellite systems, there has been a simultaneous rise in the heat flux density of onboard electronic components. The thermal output of these components has significantly increased compared to traditional satellite processing electronics, leading to a concurrent elevation in the heat flux density related to processing chips and the surfaces accommodating equipment installations. This paper establishes a model for conduction heat dissipation in aviation electronic products. The outlined calculation methodology facilitates a preliminary assessment of the system’s heat dissipation capacity. The article further examines the inherent heat dissipation capabilities in two commonly employed structural configurations. Additionally, it introduces an integrated design approach for structural heat dissipation, including experimental considerations and validation outcomes. The assessment methodologies and design principles detailed in this article are particularly relevant and applicable to the thermal design imperatives associated with high-power electronic products deployed for the extended operational durations typical of spacecraft.

Comparative seismic performance evaluation of friction, self-centering and hybrid self-centering dampers

This study explores comparative seismic performance of a shape memory alloy (SMA)-only device, a friction damper, and a hybrid self-centering device. The self-centering device considered in this study, named as superelastic friction dampers (SFDs), is a hybrid damper operating on a mechanism where SMA cables and frictional components are arranged in parallel. A detailed description of the device and an experimental characterization of mechanical behavior are presented first. Then, an 8-story archetype steel frame building is designed and modeled with each seismic protection device (SMA-only, hybrid self-centering, and friction only). To enable comparative performance assessment, each device is designed to have similar initial stiffness and the same maximum force at design displacement. Nonlinear time history analyses are conducted using a total of 44 ground motion records. The results are evaluated to compare performance of each frame design at design-basis and maximum considered earthquake hazard levels. Next, a risk-based seismic hazard demand analysis framework that involves comprehensive incremental dynamic analyses is employed for further assessing the performance of each system. The experimental results validate the parallel working mechanism of SMA cables and frictional components in the SFD, with 90% of the damper deformations recovered. Comparative analysis of seismic responses demonstrates the excellent performance of the SFDs in reducing interstory drift ratio, residual interstory drift ratio, and absolute acceleration responses. Furthermore, fragility analysis and risk-based seismic hazard assessment further reveal that the SFDs provide a well-balanced combination of energy dissipation capability and self-centering ability, thereby enhancing the seismic resilience of structures.