Heat Dissipation Model Research Articles

Multiscale Electrothermal Modeling of Nanostructured Devices

In this study, we develop an electro-thermal model to investigate heating and heat dissipation in nanostructured devices. The heating is computed by a drift-diffusion simulation, which accounts for the dissipative transport of charge carriers. The heat dissipation model is based on the phonon Boltzmann transport equation (PBTE). Application of the electrothermal model to a truncated-pyramid-shaped GaN dot embedded in an AlGaN nanocolumn reveals the existence of mesoscopic effects such as a hotspot across the quantum dot and thermal boundary resistances. We enhance the computational efficiency of the thermal model by implementing a coupled PBTE/Fourier model. This method, based on the domain partitioning, provides the same maximum temperature as that computed by the simple PBTE model, resulting, therefore, a powerful scheme for capture local heating effect with relatively low computational effort. Details about the numerical implementation are also provided.

Thermal effects in quantum cascade lasers at λ∼4.6 μm under pulsed and continuous-wave modes

The thermal effects in InGaAs/InAlAs quantum cascade lasers (QCLs) emitting at λ∼4.6 μm under pulsed and continuous-wave (CW) modes using a three-dimensional (3D) heat dissipation model were investigated. Based on the experimentally measured results, the thermal characteristics were theoretically analyzed for various device and heatsinking structures. Also, the heat accumulation effects and dissipation processes were studied in detail under pulsed operation. High cooling efficiencies were achieved by a relatively fast heat diffusion rate from the active core region for the epilayer-down bonded single ridge waveguide buried heterostructure (BH) with a thick electroplated Au around the laser ridge. A further improvement was made by the use of InP embedding layer. In CW mode, the thermal conductance (G th) value of 445 W/(K cm2) at 298 K was obtained for the epilayer-down bonded double-channel ridge waveguide QCL with AlN submount, which indicates a reasonable consistency with the available experimental data. By optimizing the device and heatsinking structures, the G th was improved to a high value of 673 W/(K cm2) at 298 K for the epilayer-down bonded single ridge waveguide BH QCL with InP embedding layer on diamond submount in CW mode.

Optoelectronic and thermal characteristics of GaN-based monolithic light emitting diode arrays

We investigated the optoelectronic and thermal characteristics of InGaN/GaN monolithic light emitting diode (LED) arrays operating at λ∼ 470 nm. The optical output power (Pout) and forward voltage (VF) were almost linearly increased with the number of LEDs for series arrays. In the case of parallel LED arrays, the maximum operating current was increased by increasing the number of devices, but the VF was kept almost constant. Around 235 mA, the maximum Pout of 55.6 mW was obtained for the 1 × 3 series LED array fabricated with a separation distance of 1000 µm, while the Pout was 20.4 mW for the 1 × 3 parallel LED array. From the measured light–current–voltage data, the maximum internal temperature (Tmax), i.e. a maximum value of internal temperature rise within the devices under operation, was theoretically determined using a three-dimensional steady-state thermal heat dissipation model based on the finite element method. Also, the temperature profiles were obtained for various separation distances and array sizes. At injection current of 220 mA, the Tmax was theoretically calculated as 325.6 K and 306.6 K for 1 × 3 series and parallel LED arrays with a 1000 µm separation distance, respectively. The Tmax was increased and decreased on decreasing the separation distance and substrate thickness, respectively, and its dependence on Tmax became more significant at higher injection current.

Heat for Nothing or Activity for Free? Evidence and Implications of Activity-Thermoregulatory Heat Substitution

If heat generated through activity can substitute for heat required for thermoregulation, then activity in cold environments may be energetically free for endotherms. Although the possibility of activity-thermoregulatory heat substitution has been long recognized, its empirical generality and ecological implications remain unclear. We combine a review of the literature and a model of heat exchange to explore the generality of activity-thermoregulatory heat substitution, to assess the extent to which substitution is likely to vary with body size and ambient temperature, and to examine some potential macroecological implications. A majority of the 51 studies we located showed evidence of activity-thermoregulatory heat substitution (35 of 51 studies), with 28 of 32 species examined characterized by substitution in one or more study. Among studies that did detect substitution, the average magnitude of substitution was 57%, but its occurrence and extent varied taxonomically, allometrically, and with ambient temperature. Modeling of heat production and dissipation suggests that large birds and mammals, engaged in intense activity and exposed to relatively warm conditions, have more scope for substitution than do smaller endotherms engaged in less intense activity and experiencing cooler conditions. However, ambient temperature has to be less than the lower critical temperature (the lower bound of the thermal neutral zone) for activity-thermoregulatory heat substitution to occur and this threshold is lower in large endotherms than in small endotherms. Thus, in nature, substitution is most likely to be observed in intermediate-sized birds and mammals experiencing intermediate ambient temperatures. Activity-thermoregulatory heat substitution may be an important determinant of the activity patterns and metabolic ecology of endotherms. For example, a pattern of widely varying field metabolic rates (FMR) at low latitudes that converges to higher and less variable FMR at high latitudes has been interpreted as suggesting that warm environments at low latitudes allow a greater variety of feasible metabolic niches than do cool, high-latitude environments. However, activity-thermoregulatory heat substitution will generate this pattern of latitudinal FMR variation even if endotherms from cold and warm climates are metabolically and behaviorally identical, because the metabolic rates of resting and active animals are more similar in cold than in warm environments. Activity-thermoregulatory heat substitution is an understudied aspect of endotherm thermal biology that is apt to be a major influence on the physiological, behavioral and ecological responses of free-ranging endotherms to variation in temperature.

Thermal analysis of oxide-confined VCSEL arrays

A thermo-electric 3-D analysis of 980nm vertical cavity surface emitting laser (VCSEL) arrays based on the finite element method (FEM) is presented in this paper. High performance VCSEL array structures with square mesas are modeled by applying a steady-state 3-D heat dissipation model. Several oxide aperture diameters (Da), substrate thicknesses, current densities, array sizes, heat flux, and temperature profiles are considered. The analysis shows that the maximum internal temperature of a VCSEL array ranges from 306.5K for a 20μm Da, 100μm substrate thickness, 666A/cm2 current density, and a 1×1 array size to 412K for a 5μm Da, 300μm substrate thickness, 1200A/cm2 current density, and a 4×4 array size.

Research on thermal contact conductance between crystal rod and heat sink in LD end-pumped solid-state laser

In LD end-pumped solid-state laser, the crystal rod is held and cooled by the metal heat sink. The force applied to the side surface of the crystal is non-axisymmetric. Under such circumstances, three kinds of thermal contact conductance (TCC) models are established, including not using thermal interface material, using thermal interface material with its thickness equal to the average thickness of the gaps and using thermal interface material with its thickness much greater than the thickness of the gaps. Regarding to the first two models, the influences of the assembly force and the equivalent root-mean-square (RMS) roughness on thermal contact conductance are discussed based on the Truncated-Gaussian model and the plastic-deformation model. The contact heat dissipation model of the crystal rod and the heat sink is established. For the Gaussian heat consumption, the spatial distributions of temperature inside the crystal with and without thermal interface material are obtained by the finite element method. The results show that without thermal interface material, the thermal contact conductance between the crystal rod and the heat sink changes significantly in the circumferential direction, which reaches a maximum on the bottom of the heat sink groove and a minimum on the contact area of the heat sink couple. With the assembly force increasing and the equivalent root-mean-square roughness decreasing, the thermal contact conductance gets larger and more nonuniform, and the temperature of the whole crystal rod reduces. When the indium foil is used as thermal interface material, the thermal contact conductance gets larger and more uniform, the temperature of the whole crystal rod reduces as well and its distribution is axisymmetric.

Numerical Analysis for Thermal Performance of High Power Multi-Chip LED Packaging

Heat dissipation and cost are the key issues for light-emitting diode (LED) packaging. In this paper, based on the thermal resistance network model of LED packaging, three-dimensional heat dissipation model of high power multi-chip LED packaging is developed and analyzed with the application of finite element method. Temperature distributions of the current multi-chip LED packaging model are investigated systematically under the different materials of the chip substrate, die attach, and/or different structures of the heat sink and fin. The results show that the junction temperature can be decreased effectively by increasing the height of the heat sink, the width of the fin, and the thermal conductivity of the chip substrate and die attach materials. The lower cost and higher reliability for LED source can be obtained through reasonable selection of materials and structure parameters of the LED lighting system.

Thermal analysis and characterization of the effect of substrate thinning on the peformances of GaN‐based light emitting diodes

AbstractWe report the effect of substrate thickness on the optical and thermal characteristics of InGaN/GaN light emitting diodes (LEDs), operating at λ ∼ 450 nm, with different mesa sizes. For various mesa sizes with different substrate thicknesses, the junction temperature (Tj) is measured as a function of injection current by the forward voltage method and the characteristic temperature is also investigated by measuring the temperature‐dependent electroluminescence spectrum. Based on the experimentally determined heat source density, the junction temperature, heat flux, and thermal resistance (Rth) are calculated theoretically by using a three‐dimensional anisotropic heat dissipation model. The use of a thin substrate thickness of 150 µm effectively improves the heat extraction capability due to the shorter heat transfer path length. For 150 µm (443 µm) thick substrate, the Tj value of LEDs is experimentally estimated as 307.7 K (322.6 K), 311.6 K (329.5 K), and 323 K (355.9 K) under an injection current of 80 mA for 450 × 450, 400 × 400, and 300 × 300 µm2, respectively. At a high injection current of 400 mA, the Tj of 450 × 450 µm2 LED with 150 µm thick substrate is reduced by ∼25.7% compared to that of LED with 443 µm substrate thickness, indicating a value of Tj = 384.5 K. The Rth value is decreased for larger mesa size and thinner substrate thickness, resulting in Rth < 30 K/W for 450 × 450 µm2 LED with 150 µm thick substrate. The theoretically calculated Rth values indicate a good agreement with the experimentally measured data.

Thermal Analysis of InGaN/GaN Multiple Quantum Well Light Emitting Diodes with Different Mesa Sizes

We analyze experimentally and theoretically the thermal characteristics of 470 nm InGaN/GaN light emitting diodes (LEDs) with different mesa sizes for efficient heat dissipation. For various mesa sizes, the junction temperature is measured as a function of injection current by the diode forward voltage method. The junction temperature (Tj) increases linearly with injection current and it rises rapidly with the reduction in mesa size. The thermal parameters, such as the junction temperature, temperature profile, and thermal resistance (Rth), are obtained from thermal simulations with the heat source density, determined from the measured light–current–voltage data, using a three dimensional heat dissipation model. At 200 mA, the Tj values of LEDs are theoretically calculated as 392.7 and 578 K for 450 ×450 and 250 ×250 µm2, respectively. The thermal resistance is kept nearly constant due to a linear relationship between junction temperature and injection current and it is reduced as the mesa size increases, indicating a low Rth of < 75 K/W for 450 ×450 µm2 LED. The Rth values obtained by the thermal simulation are fairly consistent with the experimental data. The use of the substrate with a relatively high thermal conductivity can also improve significantly the Rth of LEDs.

Validation Of The Total Resistance Heat Dissipation Model For Heat Transmission Through Annular Fins With Variable Heat Transfer Coefficient

The present paper includes analytical investigation of the validity of total resistance heat dissipation model using variable heat transfer coefficient for annular fins constant thickness. The model is given as: Furthermore, a finite difference method using SOR technique is devised to serve the two purposes of verifying the two dimensional heat transfer model and to cover a wide range of fin parameters and heat transfer coefficient models. The results were in agreement and proved the validity of the suggestion model under the both the constant and variable heat transfer coefficient assumptions. Key words: Total resistance, Variable heat transfer coefficient, Heat transfer, Annular fins

Thermal analysis of asymmetric intracavity-contacted oxide-aperture VCSELs for efficient heat dissipation

The asymmetric intracavity-contacted oxide-aperture vertical-cavity surface-emitting lasers (VCSELs), operating at λ ∼ 980 nm, with different oxide aperture diameters were fabricated and their thermal analysis was theoretically performed using a three-dimensional cylindrical heat dissipation model. The heat flux, temperature profile, and thermal resistance ( R th) of the devices were investigated by incorporating heat source values, obtained from experimentally measured results, into the thermal simulation. For the fabricated VCSELs with benzocyclobutene passivation layer, the R th decreased from 4612 K/W to 1130 K/W as the oxide aperture diameter ( D a) increased from 8 μm to 16 μm and it increased significantly below 8 μm. The use of the thin substrate and the passivation layer with a high conductivity enhances the heat dissipation, allowing for a low R th. Furthermore, thick Au layers on contact pads and top DBR in intracavity-contacted VCSEL structures help increase heat removal from the active region. For D a = 8 μm and 16 μm, the VCSELs with SiN x passivation layer, 5 μm thick extra Au layer, and 100 μm thick substrate indicate R th = 3050 K/W and 778 K/W, respectively, leading to an improvement by >30% compared to the fabricated devices.

Theoretical analysis of the heat dissipation mechanism in high power photonic crystal fiber lasers

Starting from the special structure of photonic crystal fiber (PCF), the heat dissipation model of a PCF laser is constructed. Based on the heat dissipation model, the temperature distributions along the radial and axial directions of the PCF (DC-Yb-17040) for forward pump of 200 W and two-end pump of 100 W each side are calculated numerically by using the finite element method (FEM). The results show that the temperature distribution for two-end pump mode is more even than that for forward pump mode and the maximum temperature in the fiber decreases by 178.16 °C. With the thermal power in fiber core being assumed to be fixed, the effects of the core radius, outer cladding radius, and air-clad width on the temperature distribution along the fiber are analyzed numerically. The results show that the changing of core radius only affects the temperature in core region slightly and the decreasing of air-clad width decreases the temperature in inner cladding and core regions effectively. Meanwhile, the temperature of the whole fiber can be decreased by increasing the cladding radius.

Bias-dependent MOS transistor thermal resistance and non-uniform self-heating temperature

The relationship of the biasing condition and the self-heating effect of a multi-channel metallic oxide semiconductor field effect transistor (MOSFET) device was examined using various channel currents ranging from triode to saturation regimes. When temperature non-uniformity is generated due to transistor self-heating, the local thermal resistance is conventionally defined through the difference between the hottest local temperature and the backside temperature. It was found that the thermal resistance independence of the input power is only valid if we take into consideration the size and shape change of the heating body. We show in this paper the evolution of MOSFET self-heating through both direct thermoreflectance imaging observation and a three-dimensional analytical heat dissipation model of the device. The device thermal resistance obtained can change by a factor larger than 5 for different drain–source voltages. Even in the saturation regime, the thermal resistance of the transistor can change by 50% as a function of the bias.

A model of heat dissipation for MR based brake

In contrast to conventional brakes actuators based on magnetorheological fluids (MRF) offer an advantage in short term, peak load decelerating. The dissipation of a high amount of energy in a short period of time results in a thermal destruction of conventional brakes. Due to the volume based energy dissipation of MR actuators, instead of the surface based energy dissipation of conventional brakes, the rise of temperature and the distribution of energy shows significant advantages. In this paper a design rule for special peak load MR actuators is derived. Furthermore the simplified model, which is the basis of the design rule, is compared to several simulation models, with different levels of detail.

Thermal analysis of InP-based quantum cascade lasers for efficient heat dissipation

We have theoretically investigated the thermal characteristics of double-channel ridge–waveguide InGaAs/InAlAs/InP quantum cascade lasers (QCLs) using a two-dimensional heat dissipation model. The temperature distribution, heat flow, and thermal conductance (Gth) of QCLs were obtained through the thermal simulation. A thick electroplated Au around the laser ridges helps to improve the heat dissipation from devices, being good enough to substitute the buried heterostructure (BH) by InP regrowth for epilayer-up bonded lasers. The effects of the device geometry (i.e., ridge width and cavity length) on the Gth of QCLs were investigated. With 5 μm thick electroplated Au, the Gth is increased with the decrease of ridge width, indicating an improvement from Gth=177 W/K⋅cm2 at W=40 μm to Gth=301 W/K⋅cm2 at W=9 μm for 2 mm long lasers. For the 9 μm×2 mm epilayer-down bonded laser with 5 μm thick electroplated Au, the use of InP contact layer leads to a further improvement of 13% in Gth, and it was totally raised by 45% corresponding to 436 W/K⋅cm2 compared to the epilayer-up bonded laser with InGaAs contact layer. It is found that the epilayer-down bonded 9 μm wide BH laser with InP contact layer leads to the highest Gth=449 W/K⋅cm2. The theoretical results were also compared with available obtained experimentally data.

Theoretical analysis of the heat dissipation mechanism in Yb3+-doped double-clad fiber lasers

Based on the special structure of high-power Yb3+-doped double-clad fiber lasers, a heat dissipation model and the corresponding steady-state heat equations with consideration of both convective and radiative heat transfer are constructed. The temperature distributions along the radial and axial directions of the fiber for forward pump of 1000 W and two-end pump of 500 W each side are calculated numerically by using the steady-state rate equations and the heat equations. The results show that the temperature distribution for two-end pump mode is more even than that for forward pump mode and the maximum temperature in the fiber decreases by 139.8°C. Finally, a comparison between the temperature distributions with and without consideration of radiative heat transfer is performed, the results of which indicate that radiative heat transfer plays an important role in heat dissipation of high-power fiber lasers. Therefore, radiative heat transfer has to be considered in the thermal, stress, and thermo-optic effects of any type of high-power fiber lasers.

Experimental and theoretical study on designing of cooling device for the kilowatt-level double cladding fiber laser

Though the double cladding fiber (DCF) laser’s ability of heat dissipation is better than the traditional solid-state laser’s, the heat generation in double-cladding fiber laser is the bottleneck in the development of high power lasers. The distribution of heat deposition in the practical 400 W DCF laser is quantitatively analyzed for the first time. Based on the heat dissipation model developed in this work, in order to ensure the safe and stable operation of the kilowatt-level DCF laser, the convective coefficient near the pumping ends should be greater than 2.8×10-2W·cm-2K-1. The cooling device for high power DCF laser’s pumping ends was designed and successfully used in a laser system, and the output power of 1kW was obtained.

Thermal Analysis of the PediaFlow Pediatric Ventricular Assist Device

Accurate modeling of heat dissipation in pediatric intracorporeal devices is crucial in avoiding tissue and blood thermotrauma. Thermal models of new Maglev ventricular assist device (VAD) concepts for the PediaFlow VAD are developed by incorporating empirical heat transfer equations with thermal finite element analysis (FEA). The models assume three main sources of waste heat generation: copper motor windings, active magnetic thrust bearing windings, and eddy currents generated within the titanium housing due to the two-pole motor. Waste heat leaves the pump by convection into blood passing through the pump and conduction through surrounding tissue. Coefficients of convection are calculated and assigned locally along fluid path surfaces of the three-dimensional pump housing model. FEA thermal analysis yields a three-dimensional temperature distribution for each of the three candidate pump models. Thermal impedances from the motor and thrust bearing windings to tissue and blood contacting surfaces are estimated based on maximum temperature rise at respective surfaces. A new updated model for the chosen pump topology is created incorporating computational fluid dynamics with empirical fluid and heat transfer equations. This model represents the final geometry of the first generation prototype, incorporates eddy current heating, and has 60 discrete convection regions. Thermal analysis is performed at nominal and maximum flow rates, and temperature distributions are plotted. Results suggest that the pump will not exceed a temperature rise of 2 degrees C during normal operation.

Computational model for operation of 2 μm co-doped Tm,Ho solid state lasers

A computational model for operation of co-doped Tm,Ho solid-state lasers is developed coupling (i) 8-level rate equations with (ii) TEM00 laser beam distribution, and (iii) complex heat dissipation model. Simulations done for Q-switched approximately 0.1 J giant pulse generation by Tm,Ho:YLF laser show that approximately 43% of the 785 nm light diode side-pumped energy is directly transformed into the heat inside the crystal, whereas approximately 45% is the spontaneously emitted radiation from (3)F(4), (5)I(7) , (3)H(4) and (3)H(5) levels. In water-cooled operation this radiation is absorbed inside the thermal boundary layer where the heat transfer is dominated by heat conduction. In high-power operation the resulting temperature increase is shown to lead to (i) significant decrease in giant pulse energy and (ii) thermal lensing.

Mode II fracture behavior of a Zr-based bulk metallic glass

Shear band formation and fracture are characterized during mode II loading of a Zr-based bulk metallic glass. The measured mode II fracture toughness, K IIc=75±4 MPa√m, exceeds the reported mode I fracture toughness by ∼4 times, suggesting that normal or mean stresses play a significant role in the deformation process at the crack tip. This effect is explained in light of a mean stress modified free volume model for shear localization in metallic glasses. Thermal imaging of deformation at the mode II crack tip further reveals that shear bands initiate, arrest, and reactivate along the same path, indicating that flow in the shear band leads to permanent changes in the glass structure that retain a memory of the shear band path. The measured temperature increase within the shear band is a fraction of a degree. However, heat dissipation models indicate that the temperature could have exceeded the glass transition temperature for less than 1 ms immediately after the shear band formed. It is shown that this time scale is sufficient for mechanical relaxation slightly above the glass transition temperature.