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The reliability of packaged laser diodes is heavily dependent on the quality of the die attach. Even a small void or delamination may result in a sudden increase in junction temperature, eventually leading to failure of the operation. The contact thermal resistance at the interface between the die attach and the heat sink plays a critical role in thermal management of high-power laser diode packages. This paper focuses on the investigation of interface contact thermal resistance of the die attach using thermal transient analysis. The structure function of the heat flow path in the T3ster thermal resistance testing experiment is utilized. By analyzing the structure function of the transient thermal characteristics, it was determined that interface thermal resistance between the chip and solder was 0.38 K/W, while the resistance between solder and heat sink was 0.36 K/W. The simulation and measurement results showed excellent agreement, indicating that it is possible to accurately predict the interface contact area of the die attach in the F-mount packaged single emitter laser diode. Additionally, the proportion of interface contact thermal resistance in the total package thermal resistance can be used to evaluate the quality of the die attach.

High frequency air transformers for switched-mode power supply (SMPS) are important components of wireless power transfer systems (WPT). This paper describes a compact thermal model of such transformers. This model takes into account self-heating in all components of such a transformer and mutual thermal couplings between these components. Methods of measurement of self- and transfer transient thermal impedances characterizing properties of the considered devices are proposed. The form of the elaborated model is described together with a parameters estimation method. Some results of measurements and calculations illustrating an influence of selected factors on waveforms of self- and transfer transient thermal impedances obtained for different constructions of the tested devices are shown and discussed. Two kinds of constructions of the air transformers are considered—with ferrite plates and without them. Different dimensions of the coils and different distances between them are considered. A good agreement is obtained between the results of measurements and simulations for all the considered constructions of the tested transformers operating in different conditions.

Vertically aligned graphite substrate (VGS)-copper packaging was renowned for improving the robustness against the thermal gradient loading by using micro texturing. The micro-groove array with a line width of 50 μm and a pitch of 100 μm was formed into the VGS by controlling the line depth with the use of fast-rate oxygen plasma etching. Three micro-grooved VGS specimens were wet-plated to fill these microgrooves with copper deposits and to cover the VGS surfaces. The nearly full-deposited VGS-Copper specimens were subjected to a severe thermal transient loading test. The simply Cu-covered package and shallow rib-structured VGS-Cu packages were damaged to delaminate at their interfaces. The VGS-Cu package with the copper rib structure with a height of 50 μm experienced no delamination. This rib-structured VGS-copper package with high rib height had sufficient robustness against the severe thermal transients even with the proof of homogeneous thermal spreading capacity.

Thermal transient problems, essential for modeling applications like welding and additive metal manufacturing, are characterized by a dynamic evolution of temperature. Accurately simulating these phenomena is often computationally expensive, thus limiting their applications, for example for model parameter estimation or online process control. Model order reduction, a solution to preserve the accuracy while reducing the computation time, is explored. This article addresses challenges in developing reduced order models using the proper generalized decomposition (PGD) for transient thermal problems with a specific treatment of the moving heat source within the reduced model. Factors affecting accuracy, convergence, and computational cost, such as discretization methods (finite element and finite difference), a dimensionless formulation, the size of the heat source, and the inclusion of material parameters as additional PGD variables are examined across progressively complex examples. The results demonstrate the influence of these factors on the PGD model's performance and emphasize the importance of their consideration when implementing such models. For thermal example, it is demonstrated that a PGD model with a finite difference discretization in time, a dimensionless representation, a mapping for a moving heat source, and a spatial domain non‐separation yields the best approximation to the full order model. To significantly improve the computational performance of simulations for complex temperature fields, which are often computationally expensive, the proper generalized decomposition method is applied. This article presents and analyses useful tips and tricks to improve the implementation, the convergence, and the overall performance of such a PGD model.

We develop a quasi-steady-state thermal model to analyze transient thermal effects in a Nd :YAG laser rod under quasi-continuous laser-diode (LD) end pumping. Included is a quasi-adiabatic model for pump durations and a quasi-steady-state heat dissipation model for pump-free durations. We give an approximate expression of the transient temperature-difference distribution function of the crystal under quasi-continuous pumping. The method for calculating the transient thermal focal length is also presented. On this basis, the temperature distribution variation in the crystal with time and the variation range of the transient thermal focal length are quantitatively analyzed. For 50 W, 200 μs pulse-width, and 50 Hz pumping, the temperature at the center of the Nd:YAG rod has a range of 304.52 – 306.28 K, and the thermal focal length varied over 1612.8 – 310.43 mm. Both have saw-tooth periodic distributions. This study provides a theoretical reference for the design of laser-diode-pumped thermal Q -switched Nd :YAG lasers.

The main objectives of engine development, given the increased environmental regulations, are performance and efficiency improvement, and emission reduction. Notably, for diesel engines, the significance of fuel consumption and emission reduction is increasing because they emit harmful materials (PM, NOx); however, they are characterized by relatively low CO2 emission in comparison with gasoline engines. Various technologies have been developed to solve these problems of diesel engines; however, majority of the diesel engines are developed in a steady state, and yet their certification and evaluation include the transient state. The transient operation of diesel vehicles leads to additional fuel consumption and emission of NOx; however, the impact of transient operation has not been discussed thus far. Therefore, this study quantitively investigates the fuel consumption and NOx emission during the transient operation of a diesel vehicle. First, the engine and vehicle’s efficiency and NOx emission differences are derived and analyzed in the steady state. Subsequently, the effects of the vehicle’s transient operation are analyzed. A vehicle’s transient operation can be classified into two types: operational and thermal transient effects. Finally, the respective transient effects are quantified and analyzed.

This article presents the results of the investigations concerning the influence of the printed circuit board (PCB) layout design on self and transfer transient thermal impedances characterizing thermal phenomena occurring in the network containing two power MOSFETs. The tested devices have the case D2PAK and are soldered to the PCB using the surface mount technology (SMT). The measurement method is described. The tested transistors are presented with the used PCBs on which they are mounted. The obtained measurement results of the mentioned thermal parameters of the tested transistors operating on all the tested PCBs are shown and discussed. The influence of a cooling area of the tested PCBs on the parameters describing self and transfer transient thermal impedances is analyzed.

To improve the efficiency of existing networks, special mathematical models for assessing the losses and temperature of conductors in real time can be used, with the climatic factors being taken into account. The approximate analytical solution of the nonlinear differential equation of heating and cooling of the insulated conductor with numerical method simulation of heat transfer is proposed comparison in this work. The solution is based on lowering the degree of temperature of the conductor using the least squares method in the integral form. A positive feature of the proposed solution is its universality. It allows the analysis of overhead conductors both with and without insulation. The developed method is almost as accurate as the calculation of the conductor temperature by numerical methods. The reliability of the heat balance equation of overhead power lines at non-stationary thermal mode developed by this method is confirmed by comparison with the results obtained by the finite elements method.

In the field of electronics thermal management (TM), there has already been a lot of work done to create cooling options that guarantee steady-state performance. However, electronic devices (EDs) are progressively utilized in applications that involve time-varying workloads. Therefore, the TM systems could dissipate the heat generated by EDs; however, there seemed to be a necessity for a design that would contain temperature rise within an acceptable range for limiting hot spots and managing thermal transients induced by higher-frequency operating cycles. Heat dissipation issues become more significant when miniaturization in electronics increases. More effective TM often results in enhanced reliability as well as a longer life expectancy for devices. Hence, this paper explicates the TM of EDs, the comparison of cooling methods, the comparison of convections for TM on EDs, the heat source (HS) mounted on the substrate board, and optimization techniques to optimize the size and position of HSs mounted on the substrate board. This paper also analyzes the TM technologies on different EDs from 2014 to 2023 and the comparison of the thermal conductance of EDs with two types of phase change materials (PCMs) and pin-fin heat pipes (HPs).

The coalescence of double neutron star (NS–NS) and black hole (BH)–NS binaries are prime sources of gravitational waves (GW) for Advanced LIGO/Virgo and future ground-based detectors. Neutron-rich matter released from such events undergoes rapid neutron capture ( r -process) nucleosynthesis as it decompresses into space, enriching our universe with rare heavy elements like gold and platinum. Radioactive decay of these unstable nuclei powers a rapidly evolving, approximately isotropic thermal transient known as a “kilonova”, which probes the physical conditions during the merger and its aftermath. Here I review the history and physics of kilonovae, leading to the current paradigm of day-timescale emission at optical wavelengths from lanthanide-free components of the ejecta, followed by week-long emission with a spectral peak in the near-infrared (NIR). These theoretical predictions, as compiled in the original version of this review, were largely confirmed by the transient optical/NIR counterpart discovered to the first NS–NS merger, GW170817, discovered by LIGO/Virgo. Using a simple light curve model to illustrate the essential physical processes and their application to GW170817, I then introduce important variations about the standard picture which may be observable in future mergers. These include ∼ hour-long UV precursor emission, powered by the decay of free neutrons in the outermost ejecta layers or shock-heating of the ejecta by a delayed ultra-relativistic outflow; and enhancement of the luminosity from a long-lived central engine, such as an accreting BH or millisecond magnetar. Joint GW and kilonova observations of GW170817 and future events provide a new avenue to constrain the astrophysical origin of the r -process elements and the equation of state of dense nuclear matter.