bluuebunny/crossref_metadata_2025_split huggingface dataset:
A Highly Efficient Approach for Bi-Level Programming Problems Based on Dominance Determination
Authors: Wang Guan , Zou Qiang , Zhao Chuke , Liu Yusheng , Ye Xiaoping 
Published: November 2021

Abstract
Bi-level programming, where one objective is nested within the other, is widely used in engineering design, e.g., structural optimization and electronic system design. One major issue of current solvers for these bi-level problems is their low computational efficiency, especially for complex nonlinear problems. The existing methods usually applied time-consuming nested computational structure, which requires an amount of function evaluations (FEs) since a lower-level (LL) optimization needs to be constructed for each upper-level (UL) solution. To solve this issue, a new method based on bi-level grey wolf optimizer (BLGWO) is proposed in this paper. The basic idea is to drop the conventional nested computational structure and instead use a simultaneous computational structure. The simultaneous structure is built on top of a dominance determination process for the grey wolf optimizer, so that the upper-level and lower-level problems can be optimized simultaneously, which greatly improves the efficiency of solving the bi-level problems. The effectiveness of this new method has been validated with ten benchmark functions and two engineering design examples, as well as comparisons with three important existing methods in the bi-level programming domain.
Comparison of Under-Actuated and Fully Actuated Serial Robotic Arms: A Case Study
Authors: Bottin Matteo , Rosati Giulio 
Published: November 2021

Abstract
Under-actuated robots are very interesting in terms of cost and weight since they can result in a state-controllable system with a number of actuators lower than the number of joints. In this paper, a comparison between an under-actuated planar three-degrees-of-freedom (DOF) robot and a comparable fully actuated two-degrees-of-freedom robot is presented, mainly focusing on the performances in terms of trajectories, actuator torques, and vibrations. The under-actuated system is composed of two active rotational joints followed by a passive rotational joint equipped with a torsional spring. The fully actuated robot is inertial equivalent to the under-actuated manipulator: the last link is equal to the sum of the last two links of the under-actuated system. Due to the conditions on the inertia distribution and spring placement, in a simple point-to-point movement the last passive joint starts and ends in a zero-value configuration, so the three DOF robot is equivalent, in terms of initial and final configuration, to the two DOF fully actuated robot, thus they can be compared. Results show how while the fully actuated robot is better in terms of reliable trajectory and actuator torques, the under-actuated robot wins in flexibility and in vibrations at a given configuration.
Numerical Investigation of a Phase Change Material Building Integrating Solar Thermal Collector PCM-BST
Authors: Benkaddour Ayman , Faraji Mustapha 
Published: November 2021

Abstract
Sensible thermal energy storage (TES) systems can reduce energy environmental fluctuation dependency with the nocturnal energy needs usage in maintaining the building's comfort levels. In the present paper, phase change material (PCM) is introduced to improve the thermal energy storage capacity of a solar collector integrating a novel composite PCM/concrete wall. A mathematical model based upon the conservation and heat transfer equations has been developed using the enthalpy method. The equations that govern the problem were discretized with the control volume scheme and solved iteratively using the tridiagonal matrix algorithm (TDMA). The numerical investigation has been implemented into a personal fortran code. Many series of simulation runs were executed. The position of the PCM layer within the wall and the PCM melting temperature are varied in the range 0 cm ≤ xm ≤ 7.5 cm and 15 °C ≤ Tm ≤ 35 °C, respectively. The objective is to let inner temperature Tin swing close to a comfort threshold. The position of PCM close to the absorber improves the efficiency of the room heating with good nocturnal use of latent heat stored during the day. PCM melting temperature affects deeply the composite PCM/concrete wall/solar collector behavior. Lastly, PCM gained the system an important benefit which is the solar collector high-temperature isolation as to not reach the room and disturb the inside comfort zone by melting and solidifying. Those parameters can be considered as the primary pointers for PCM/wall integrated solar collector design. Also, a daily heating potential, Qh, and thermal load leveling, TLL, are introduced to evaluate the system performance.
Thermal Design of Variable Frequency Drives for Hybrid and Electric Transport Applications
Authors: Lakhkar Nikhil R. 
Published: November 2021

Abstract
The Electrification of commercial vehicles is happening at a rapid pace. Most of the major automotive corporations are pursuing this opportunity to include electric vehicles in their portfolios. The commercial trucking industry has also been exploring the use of electric vehicles for goods transport including perishables. The transition to electrically driven vehicles has led to the need for electrically driven HVAC systems. To support this evolving commercial market, we developed an electrically driven variable speed compressor platform comprising of compressor and variable frequency drive (VFD). The product platform addresses two categories of vehicles: 1) Hybrid vehicle – the vehicle in this category uses conventional IC engine and has traditional batteries that output 48VDC. 2) Electric vehicle – the vehicle in this category is electrically driven using battery bank or traction drive that gives 650VDC as output. Hence, 650V DC is input to VFD. Both these applications were addressed with two drive designs.
In this paper, we discuss the thermal design aspects of both 48V and 650V variable frequency drives. In this publication, the product development process is described from product conception, to final product. The mechanical / environmental design considerations while designing these drives were, (1) The drive was expected to be mounted under the vehicle bed and hence should be strong enough to withstand shock and vibration, (2) the drive was decided to be air cooled (4) the drive was designed to be IP67 so that it can withstand harsh road conditions, (5) the desired operating temperature range was between −40°C to 85°C for 48V and −40°C to 65°C for 650V and (6) the estimated time of service was expected to be 10 years. We were able to achieve an operating margin of −40°C to 70°C at full load for hybrid vehicle drive (48VDC) and −40°C to 65°C for electric vehicle drive (650VDC) using air cooling.
Thermal Performance of Different Carbonaceous Nanoparticles as Additives to Thermal Paste as an Interface Material
Authors: Bharadwaj Bharath , Singh Prashant , Mahajan Roop L. 
Published: November 2021

Abstract
With increased focus on miniature high power density electronic packages, there is a need for the development of new interface materials with lower thermal resistance. To this end, high conductivity thermal paste or similar thermal interface materials (TIMs), reinforced with superior thermal conductivity materials such as multi-walled carbon nanotubes (MWCNTs), graphene nanoplatelets (GNPs), graphite-derived multilayer graphene (g-MLG) offer an effective strategy to provide efficient paths for heat dissipation from heat source to heat sink. In an earlier paper, we had demonstrated that multilayer graphene derived from coal (coal-MLG) synthesized using our in-house developed one-pot process, has increased presence of phenolic groups on its surfaces, which translates into better dispersion of coal-MLG in silicone thermal paste. In this paper, we first compare the thermal conductance of a high conductivity thermal paste (k = 8.9 W/mK) using coal-MLG as an additive with that realized with other nano additives — MWCNTs, GNPs, and g-MLG. The data shows that coal-MLG as an additive outperforms all the other investigated nano additives in enhancing the thermal performance of the paste. With the coal-MLG as an additive, ∼70% increase in thermal performance was observed as compared to the base thermal paste used. This increase is about 2.5 times higher than that obtained using g-MLG as an additive. We also measured the thermal performance of coal-MLG-based TIM with its different wt.% fractions. The data confirmed our hypothesis that the optimum level of the loading fraction of the additive that can be dispersed in the matrix (paste in this case) before the onset of agglomeration is higher for the coal-MLG (3%) than for the other additives (2%). The implication is further improvement thermal performance with coal-MLG. The data shows the additional thermal enhancement to ∼2X. Finally, since coal-MLG produced by our in-house process is relatively cheaper and more environmentally friendly, we believe that these results would pave the path for enhanced thermal performance with non-silicone thermal pastes at a significantly lower cost. We also expect similar benefits for the silicone-based thermal pastes.
Epoxy Die Attach Combined With Face-Up Die Bonding for Improved XYZ Placement Accuracy
Authors: Avanessian Tadeh , Clatterbaugh Jim , Zinsmaster Robin L. , Hashemi Leyla 
Published: November 2021

Abstract
Epoxy die attach is widely used in microcircuit assembly and enjoys advantages such as ease of deposition, fast curing, reworkability, and non-toxicity. These qualities also make it suitable for automated mass production. However, this method falls short when high placement accuracy is desired as the die can shift on uncured epoxy leading to die displacement from its original location. Gold to gold face-up bonding is another method utilized in microelectronics packaging given its proven bonding reliability and high placement accuracy for small devices. Nevertheless, it is difficult to achieve a reliable bond using this method for relatively larger devices. The nonplanarity of the bonding collet or the variation in the height of the gold bumps results in a tilted die attach and/or a weak bond between the die and the substrate. Moreover, CTE (Coefficient of Thermal Expansion) mismatch between the die, the gold bumps, and/or the substrate leads to bond failure due to temperature fatigue. This paper discusses a hybrid method to take advantage of the strengths of both methods mentioned above, culminating in a reliable process with high XYZ placement accuracy. To apply this method, epoxy is first dispensed on a gold-plated substrate. Using a flip chip machine, samples with plated gold bumps on their ground side are then placed on the substrate. The gold bumps are mainly used as targets and stand-offs to improve the placement accuracy and to control epoxy glue-line thickness. The force applied on the die, the time the force is applied, and the substrate temperature are controlled for optimum die attach. Moreover, along with the force applied by the vacuum tip, epoxy is partially cured on the flip chip machine heated stage before it is moved to an oven to complete the cure process. Die shear test results before and after temperature conditioning are compared with standard epoxy die attach and gold to gold face-up bonding for identical samples and the advantages are discussed.
Module-Level TIM Degradation in HALT
Authors: Tompkins Joshua , Huitink David 
Published: November 2021

Abstract
In this study, TIM degradation is driven through HALT using temperature cycling and random vibration for two commercially available materials providing thermal conductivities of 6.5 and 8.0 W/m-K. HALT specimen were prepared by applying TIM through a 4-mil stencil over AlSiC baseplates in the shape of those used in Wolfspeed CAS325M12HM2 power electronics modules. Baseplates were mounted onto aluminum carrier blocks with embedded thermocouples to characterize the thermal resistance across the baseplate and TIM layer. Thermal dissipation into the top of the baseplates was provided by a custom heating block, which mimics the size and placement of the die junctions in CAS325 modules, applying power loads of 200, 300, and 400W. After initial characterization, samples were transferred to the HALT chamber with one set of samples exposed to temperature cycling only (TCO) and the other temperature cycling and vibration (TCV). Both sample sets were cycled between temperature extremes of −40 and 180 °C with random vibrations applied at a peak acceleration of 3.21 Grms. After hundreds of cycles, samples were reevaluated to assess changes in thermal resistance to provide an accelerated measure of TIM degradation. This will allow for reliability calculations of useful lifetime, provide a basis for developing accelerated testing method to related temperature cycling to faster methods of degradation, and additionally provide a means by which to develop a maintenance schedule for servicing the power modules which will enhance cooling and lifetime operation.
Mechanical Behavior of Heat Activated Film Adhesives
Authors: Janamanchi Prabhat , Dasgupta Abhijit , Singh Narendra 
Published: November 2021

Abstract
Heat activated film (HAF) adhesives enable the bonding of materials that are difficult to bond with conventional adhesives where one of the substrates is flexible. Although they are traditionally found in the context of industrial and structural applications, they are increasingly finding use in in electronic products.
This paper provides a preliminary evaluation of the mechanical behavior of HAFs. Two commercial polyurethane HAF formulations (designated as HAF-II and HAF-III, for the purpose of this paper) were evaluated to assess the dependence of the mechanical properties on phase transitions and on the thermal exposure history of the material. Both factors were found to play a significant role in determining the overall usability of the materials. The melting transition occurs under 50°C for both materials, after which their stiffness drops significantly. Among the two, HAF-III is preferable as it undergoes a delayed melting transition, effectively extending the usable operating temperature range.
As with any adhesive, there are several other factors that can influence the performance of adhesives such as post-curing age, storage, and curing conditions to name a few. The impact of these factors on the material’s strength is also discussed in brief and a recommendation for favorable conditions is provided.
Design and Application of Innovative 3DVC in AI Server System
Authors: Tan Xianguang , He Yongzhan , Liu Bin , Yu Jiang , Nishi Ahuja , Tian Wenbin 
Published: November 2021

Abstract
With the accelerated application of cloud computing and artificial intelligence, the computing power and power consumption of chips are greatly enhanced, which brings severe challenges to heat dissipation. Based on this, Baidu has adopted advanced phase change cooling technology and successfully developed an innovative 3dvc air cooling scheme for AI server system. This paper introduces the design, test and verification of the innovative scheme in detail. The results show that the scheme can reduce the GPU temperature by more than 5 °C compared with the traditional heat pipe cooling scheme, save 30%+ of the fan power consumption, and achieve good cooling and energy saving effect.
Electromigration Analysis of Solder Joints for Power Modules Using an Electrical-Thermal-Stress-Atomic Coupled Model
Authors: Kato Mitsuaki , Omori Takahiro , Goryu Akihiro , Fumikura Tomoya , Hirohata Kenji 
Published: November 2021

Abstract
Numerical analysis of electromigration in solder joints has mainly examined ball grid arrays (BGAs) in flip-chip packages, and few numerical study has been reported on solder joints in power modules. This report describes an electromigration analysis of solder joints for power modules with a Si-based power device, which are still widely used today, using an electrical-thermal-stress-atomic coupled analysis. To evaluate electromigration, a solder joint with a power device and a substrate as used in power modules was simulated. Due to current crowding, the current density at the edge of the solder joint exceeded the electromigration threshold even in Si-based power modules. Unlike general electromigration phenomena, the vacancy concentration increased at the center and decreased at the edges of the solder joint, regardless of whether it was on the cathode side or anode side. The vacancy concentration clearly increased with increasing current density and size ratio. Creep strain increased significantly with increasing current density, temperature, and size ratio. The largest change in vacancy concentration and creep strain was at the anode edge where current crowding occurred. In addition, we modeled the two-dimensional behavior of metal atoms passing through the interface of the solder joint. The expansion of intermetallic compound was accelerated by increasing the temperature and current density.
Thermal Conductivity Prediction Model for Composite Thermal Interface Materials Using Copper Metal Foam
Authors: Kawakita Shinya , Ishizaka Yuki , Fushinobu Kazuyoshi 
Published: November 2021

Abstract
In the previous research, we prototyped the TIC in which a conventional TIM composed of silicone resin and filler was filled in pores of copper foam, and measured its thermal conductivity by a steady-state method. In addition, the effective thermal conductivity of TIC was predicted by Bhattacharya’s equation and Boomsma’s equation. As a result, it was reported that the experimental value and the predicted value match within 0.7 W/(m·K) by modifying the thermal conductivity of copper to 120 W/(m·K) in the Boomsma’s equation. The issue of that was to investigate the cause of the decrease in thermal conductivity of copper to 120 W/(m·K). In this paper, the effective thermal conductivity of TIC was predicted using the WP structure instead of the Kelvin structure, which is the basis of the Bhattacharya’s equation and Boomsma’s equation. As the result, it was clarified that the effective thermal conductivity predicted by the three-dimensional thermal conductivity calculation model based on the WP structure is more accurate than that predicted by the Kelvin model. And it was found that the experimental value and the predicted value match in the range of 0.4 W/(m·K) by considering the TIC surface structure without modifying the thermal conductivity of copper.
Machine Learning-Based Model for Optimal Operating Conditions of Thermosyphons for Electronic Cooling Applications
Authors: Kim John , Amalfi Raffaele L. 
Published: November 2021

Abstract
Two-phase cooling systems based on the thermosyphon operating principle exhibit excellent heat transfer performance, reliability, and flexibility, therefore can be applied to overcome thermal challenges in a wide range of electronic cooling applications and deployment scenarios. However, extremely complex nature of two-phase flow physics involving flow patterns and phase transitions has been the major challenge for technology adoption in industry. This paper demonstrates a machine learning (ML) based model for evaluating the thermal performance and refrigerant mass flow rate, of a thermosyphon cooling system for telecom equipment. Unlike conventional laboratory approach that requires numerous sensors attached to a cooling system to capture their thermal performance, the new model requires a minimum number of sensors to monitor the health of a thermal management solution. Using the proposed model, a system control module can be further developed which could identify optimal operating parameters in real-time under dynamically changing heat load conditions and actively maintain safety and thermal requirements.
A Novel Concept for Air Removal in Two-Phase Immersion Cooling Systems
Authors: Peterson Eric , Morris Seth , Alissa Husam , Keehn Nicholas , Ramakrishnan Bharath , Oruganti Vaidehi , Manousakis Ioannis , Mckay Noah 
Published: November 2021

Abstract
A 10 kW scale model of a decoupled immersion cooling rig is constructed in order to serve as a testbed for immersion cooling, using 3M FC3284 dielectric cooling fluid. A species separator is constructed and demonstrates an ability to remove air from the flowfield before the condensable gases enter the condenser vessel, verified with Schlieren photography. The condenser underperformed significantly compared to initial sizing calculations using the NTU method, and film thickness of FC3284 liquid on the surface of the condenser was determined to be the cause due to low thermal conductivity of the liquid. The average film thickness on the surface of the condenser is calculated. In addition to the performance detriment of the film, air is also shown to reduce the condenser’s performance. The height of a transient stratification line is measured and compared against condenser power. Condenser efficacy losses are large and variable based on the concentration of air in the condenser vessel. A low vs high-mounted boiler is investigated. The mounting of the boiler has an effect on how much vapor is lost during a maintenance event. Finally, a comparison of the test rig’s overall cooling efficiency is made with various air-cooled datacenters by tracking energy consumption to cool a given IT load. This also translates to a reduction in carbon emissions.
Rack-Level Thermosyphon Cooling and Vapor-Compression Driven Heat Recovery: Evaporator Model
Authors: Khalid Rehan , Amalfi Raffaele Luca , Wemhoff Aaron P. 
Published: November 2021

Abstract
An in-rack cooling system connected to an external vapor recompression loop can be an economical solution to harness waste heat recovery in data centers. Validated subsystem-level models of the thermosyphon cooling and recompression loops (evaporator, heat exchangers, compressor, etc.) are needed to predict overall system performance and to perform design optimization based on the operating conditions. This paper specifically focuses on the model of the evaporator, which is a finned-tube heat exchanger incorporated in a thermosyphon cooling loop. The fin-pack is divided into individual segments to analyze the refrigerant and air side heat transfer characteristics. Refrigerant flow in the tubes is modeled as 1-D flow scheme with transport equations solved on a staggered grid. The air side is modeled using differential equations to represent the air temperature and humidity ratio and to predict if moisture removal will occur, in which case the airside heat transfer coefficient is suitably reduced. The louver fins are modeled as individual hexagons and are treated in conjunction with the tube walls. A segment-by-segment approach is utilized for each tube and the heat exchanger geometry is subsequently evaluated from one end to the other, with air property changes considered for each subsequent row of tubes. Model predictions of stream outlet temperature and pressure, refrigerant outlet vapor quality and heat exchanger duty show good agreement when compared against a commercial software.