Heat Flow Path Research Articles

Exfoliated boron nitride nanosheet/MWCNT hybrid composite for thermal conductive material via epoxy wetting

Abstract Herein, we describe the fabrication of thermally conductive composites based on an epoxy matrix with a hybrid filler of hexagonal boron nitride (h-BN) and multi-walled carbon nanotubes (MWCNTs) via the wetting method. The h-BN particles were exfoliated to boron nitride nanosheets (BNNSs) by heating in a tubular furnace with steam and sonication. The h-BN particles in the composite were densely packed and oriented almost perpendicular to the bottom. On the other hand, the BNNSs were more loosely packed and randomly oriented, and exhibited higher through-plane thermal conductivity despite the low filler content. After the incorporation of MWCNTs, the filler thermal conductivities of both the composites significantly increased. In particular, 9 wt% of MWCNTs was sufficient to increase the thermal conductivity of the BNNS composite from 3.12 to 4.25 W m−1 K−1 because of the intercalation of the nanotubes between the BNNSs, which provided a three-dimensional heat flow path. Moreover, the MWCNTs interrupted the dense particle packing and the filler concentration was reduced from 56.7 to 43.8 wt%. In the case of h-BN composite, this effect was relatively weak because the nanotubes were agglomerated between the micron-sized BN particles.

Measuring structure functions of power devices in inverters

This paper proposes the measuring of structure function from power devices on-board induction motor drives and multilevel converters. It puts forward the issues and methodology related to on-board measurement of the cooling curve and derivation of the structure function during idle times in induction motor drives for maintenance purposes. The structure function uses the thermal resistances and capacitances in the Cauer form to identify changes in the device structure. The advantage of the structure function is that it does not only reveal the value but also the location of the thermal resistance and capacitance in the heat flow path. The novelty in this work is the methodology used to achieve the measurement of the cooling curve and obtaining the structure function despite issues related to freewheeling current due to energy stored as a result of motor inductance. A detailed description of the measurement circuit is presented. The possibility of applying this technique to multilevel converters in different application is also elaborated.

Development of a cooling-load calculator for the Mexican conditions of climate, construction and occupancy

The objective of this document is to develop a cooling-load calculator program in function of the indoor air temperature suitable for the climate and occupancy conditions of Mexico as well as the main characteristics of its building designs. The cooling load calculator is developed from the building heat balance. Thus, the cooling demand is determined depending on the zone volume and the comfort temperature set-point for each case study. As the indoor temperature was determined by using EnergyPlus the proposed forward step is to develop a program in MatLab that calculates the temperature with the correspondent gains and losses due to the various heat flow paths that occur in the building heat balance such as through the envelope, internal heat gains and heat exchange between the indoor and the outdoor air.

Analysing Heat Flows Through Building Zones in Aspect of their Orientation and Glazing Proportion, under Varying Conditions

One of the most important objectives in building design is to achieve acceptable indoor comfort levels with minimum energy requirements. To accomplish this, firstly a serious consideration must be given to various construction and design parameters, such as the building's envelope structure and shape, its orientation, as well as the climatic data of the region. Secondly, as the envelope heat transmission contributes to the heating and cooling loads, the incorporation of an adequate heating, ventilating and air-conditioning system (HVAC) must be considered. In this study, the effect of the thermophysical properties of a brickwork layer and the thickness of a thermal insulation layer, on the heat flows of a building zone with a varying orientation, is determined. This building zone, having a specific outline geometry, is exposed to winter conditions that correspond to the northern region of Greece (Thessaloniki). The heating loads are assessed for different indoor temperature settings within the building enclosure, while the operation of the system is assumed to be continuous. Although some other aspects could be considered, this study also stresses to the importance of incoming solar radiation by glazing surfaces on the internal surfaces of the studied building zone; the above solar heat gain analysis relies on the sunlit-pattern approach. The foregoing presentation is concerned with varying proportions of glazing surfaces to the total wall surface. As it is shown, the consideration of the above issues is influential to maintain an acceptable indoor environment, while decreasing the heat flow exchanges through building envelopes. For the purposes of this study, a lumped thermal-network model incorporating several heat flow paths has been employed (transient thermal analysis).

Melt-processable aggregated boron nitride particle via polysilazane coating for thermal conductive composite

In this study, thermally conductive composite was fabricated using hexagonal and spherically aggregated boron nitride filler based on epoxy and polyphenylene sulfide (PPS) matrixes. The spherically aggregated BN (A-BN) particles were shown the outstanding thermal conductivity compared hexagonal BN (H-BN) in the epoxy matrix. However, A-BN is very weak to external forces and broken during PPS based melt processing, causing it to lose the advantage of its particulate shape. Therefore, we adopted a particle coating method for the A-BN to enhance its durability, using pre-ceramic polysilazane (PSZ) in solution. With the PSZ coating of A-BN, the spherical particle shape was retained after melt mixing. Moreover, the interfacial affinity between BN and PPS was enhanced by secondary interactions between the polar functional groups comprising PSZ, thereby increasing the through-plane thermal conductivity from 1.82 to 3.9Wm−1K−1 at 70wt% of A-BN filler, because the spherical filler particles formed a three-dimensional heat flow path.

Thermal Real-Time Monitoring of a Gearbox Integrated IPM Machine for Hybrid-Electric Traction

Aiming to monitor the temperatures of critical components of gearbox integrated interior permanent magnet synchronous machines for hybrid-electric vehicles, we present the development of a real-time lumped parameter thermal network model. To this aim, sensitivity and heat flow path analyses are first carried out to identify the most relevant thermal paths. Next, the influence of the gearbox integration on the temperatures within the electric machine and its consideration in the modeling are investigated. The models are then verified experimentally, showing an accuracy of ±5 K. Finally, challenges and uncertainties of the real-time on-board modeling context are ultimately discussed.

Core-shell structured BN/PPS composite film for high thermal conductivity with low filler concentration

BN/PPS core-shell structured pellet and its composite film were primarily fabricated via a particle coating method using an adhesive layer. A thermally conductive composite was fabricated using BN-coated PPS by hot pressing. The core-shell structure did not collapse after hot pressing and formed a well-connected particle chain. Moreover, the BN/PPS composite prepared using the proposed method has a higher thermal conductivity compared to the conventional composite fabrication method, melt mixing and powder mixing, at the same filler concentration because the connected particle chain acts as a continuous heat flow channel. Moreover, the thickness of the particle layer can be controlled by controlling the viscosity of the adhesive epoxy layer using acetone as the solvent. We fabricated BN/PPS pellets with various BN layer thickness. A 1:0.3 wt ratio of epoxy and acetone has the highest thermal conductivity enhancement ratio compared to the melt mixed composite. We fabricated a composite using the hybrid method; the polymer core was prepared via melt mixing and the BN particles were coated on this core. The BN particle in the core and the outer layer has a synergetic effect, forming a 3-dimensional heat flow path, leading to outstanding thermal conductivity with a only small amount of filler.

Experimental analysis on passive residual heat removal in molten salt reactor using single cooling thimble test system

An experimental setup has been designed and fabricated for the analysis of the passive residual heat removal system as applied to molten salt reactor. The main characteristic of this test facility is the presence of two natural circulation loops and a cooling thimble with double-wall barrier. The objective is to investigate the thermal performance of the drain tank cooling system, as well as further understand the heat transfer process. It is observed that the passive cooling system with single cooling thimble is capable of removing 2175 W at the desired temperature of 699 °C. Due to the large temperature difference, radiation heat transfer plays a significant role between the thimble and the bayonet tube. The contributions of several constituent thermal resistances in the heat flow path, together with the overall thermal resistance, have been obtained at various boundary conditions. It is also found that the operating mode is quite unstable with periodical oscillations at relative low temperature. From this study, valuable reference data and useful information are provided for practical applications.

Thermo-hydraulic design of single and multi-pass helical baffle heat exchangers

A shortcut sizing approach for single and multipass heat exchangers with helical baffles is presented. An alternative approach for the determination of the correction factor of the logarithmic mean temperature difference as a function of the number of heat transfer units is introduced. Multipass arrangements give rise to complex internal heat flow paths; in this work, such flow paths are modelled using a simplified methodology that breaks up the complex flow and represents it as a network of simple arrangements such as counter flow and parallel flow. The network is solved to determine the outlet temperatures from where the correction factor is determined. The methodology is demonstrated on case studies from the open literature.

Generating optimal topologies for heat conduction by heat flow paths identification

Knowing how heat “flows” inside a structure is significant for designers to make sure that the structure will perform its intended heat conduction function properly. This paper describes the principle and the process of defining an optimal topology for heat conduction by interpreting the heat flow paths obtained from a standard finite element analysis. First, a simple equilibrium equation is introduced to characterize heat flow paths along which the heat transfer rate remains constant as it traverses the conduction domain. Then, mathematical properties of the path plots are discussed in detail, providing designers with an insight of what kind of path distribution is the best for heat conduction. A brand new optimizer is developed to sketch the most effective topology by homogenizing the spacing of heat flow paths across the conduction domain. Finally, the optimization procedure is illustrated in a directive and descriptive way, where the well-known dichotomic constructal tree is selected as the initial layout to be tested. Unlike the conventional methods, the new approach needs neither the sensitivity analysis nor the modification of the existing finite element programs, it is very straightforward to use in practice.

Quantification of cracked area in thermal path of high-power multi-chip modules using transient thermal impedance measurement

Transient thermal impedance measurement is commonly used to characterize the dynamic behaviour of the heat flow path in power semiconductor packages. This can be used to derive a “structure function” which is a graphical representation of the internal structure of the thermal stack. Changes in the structure function can thus be used as a non-destructive testing tool for detecting and locating defects in the thermal path. This paper evaluates the use of the structure function for testing the integrity of the thermal path in high power multi-chip modules. A 1.2kV/200A IGBT module is subjected to power cycling with a constant current. The structure function is used to estimate the level of disruption at the interface between the substrate and the baseplate/case. Comparison with estimations of cracked area obtained by scanning acoustic microscopy (SAM) imaging shows excellent agreement, demonstrating that the structure function can be used as a quantitative tool for estimating the level of degradation. Metallurgical cross-sectioning confirms that the degradation is due to fatigue cracking of the substrate mount-down solder.

Vertical particle alignment of boron nitride and silicon carbide binary filler system for thermal conductivity enhancement

Thermally conductive BN/SiC binary filler and epoxy composite materials were fabricated via magnetic alignment. The magnetic iron oxide particles on the surface of the filler allowed particle re-orientation under the external magnetic field. Owing to its anisotropy, the vertically aligned BN composite had a high thermal conductivity and generated a vertical heat flow path. When the SiC nanoparticles were added to the binary filler, they hindered BN-particle aggregation and led to the formation of a three-dimensional heat conduction path, thereby resulting in increased thermal conductivity. The maximum thermal conductivity (5.77 W/mK) was obtained with an addition of SiC filler, and was 3.08-fold and 1.1-fold higher than that of randomly mixed BN and vertically aligned BN composites, respectively. The additional SiC–Fe3O4 particles resulted in significant aggregation of the filler, which in turn led to a decrease in the thermal conductivity. The measured storage modulus of the BN–Fe3O4/SiC binary filler composite was also higher than those of the BN–Fe3O4 and BN–Fe3O4/SiC–Fe3O4 composites, owing to the aggregation of particles.

Thermal Characteristics Analysis of Die Attach Layer Based on Time-Constant Spectrum for High-Power LED

The performance and reliability of the high-power light-emitting diodes (HP LEDs) are closely related to the quality of the die attach adhesive (DAA) layer, because voids or delaminations in the layer may cause higher junction temperature, and even result in the break of the chip. In this paper, a dynamic compact thermal model is constructed for an independent HP LED without being attached on a cold plate. The time-constant spectrum method based on the thermal model is applied to characterize the transient thermal behavior of the DAA layer and other layers along the heat flow path. Thermal transient experiments are carried on HP LED samples with different DAA dosages. The theoretical and experimental results demonstrate that the time-constant spectrum for LEDs tested without cold plate can reflect the thermal characteristics of the DAA layer as well as that with cold plate. Differences in the HP LED samples with different DAA layers can be obviously distinguished in the time-constant spectrums as well. Compared with the traditional testing method with cold plate, direct test in air for HP LEDs has the advantages of easy operation, time saving, and no pollution to samples.

Dynamic compact thermal model of high power light emitting diode

Dynamic compact thermal model plays important roles in predicting the junction temperature and characterizing the transient thermal behavior of an electronic device. In this paper, analytical dynamic compact thermal model for high power LEDs is established based on the three-directional heat flow paths under junction-to-ambient definition. Comparative results with finite element method model indicate that the presented model is accurate and efficient. It is further reduced by analyzing the influence of different components of the LED package. Good agreement with experimental results confirms that the reduced analytical model can predict the junction temperature and characterize the thermal behavior effectively.

Nanothermal Interface Materials: Technology Review and Recent Results

Thermal interface materials (TIMs) play a critical role in conventionally packaged electronic systems and often represent the highest thermal resistance and/or least reliable element in the heat flow path from the chip to the external ambient. In defense applications, the need to accommodate large differences in the coefficients of thermal expansion (CTE) among the packaging materials, provide for in-field reworkability, and assure physical integrity as well as long-term reliability further exacerbates this situation. Epoxy-based thermoplastic TIMs are compliant and reworkable at low temperature, but their low thermal conductivities pose a significant barrier to the thermal packaging of high-power devices. Alternatively, while solder TIMs offer low thermal interface resistances, their mechanical stiffness and high melting points make them inappropriate for many of these applications. Consequently, Defense Advanced Research Projects Agency (DARPA) initiated a series of studies exploring the potential of nanomaterials and nanostructures to create TIMs with solderlike thermal resistance and thermoplasticlike compliance and reworkability. This paper describes the nano-TIM approaches taken and results obtained by four teams responding to the DARPA challenge of pursuing the development of low thermal resistance of 1 mm2 K/W and high compliance and reliability TIMs. These approaches include the use of metal nanosprings (GE), laminated solder and flexible graphite films (Teledyne), multiwalled carbon nanotubes (CNTs) with layered metallic bonding materials (Raytheon), and open-ended CNTs (Georgia Tech (GT)). Following a detailed description of the specific nano-TIM approaches taken and of the metrology developed and used to measure the very low thermal resistivities, the thermal performance achieved by these nano-TIMs, with constant thermal load, as well as under temperature cycling and in extended life testing (aging), will be presented. It has been found that the nano-TIMs developed by all four teams can provide thermal interface resistivities well below 10 mm2 K/W and that GE's copper nanospring TIMs can consistently achieve thermal interface resistances in the range of 1 mm2 K/W. This paper also introduces efforts undertaken for next generation TIMs to reach thermal interface resistance of just 0.1 mm2 K/W.

Altering crystal growth and annealing in ice-templated scaffolds.

The potential applications of ice-templating porous materials are constantly expanding, especially as scaffolds for tissue engineering. Ice-templating, a process utilizing ice nucleation and growth within an aqueous solution, consists of a cooling stage (before ice nucleation) and a freezing stage (during ice formation). While heat release during cooling can change scaffold isotropy, the freezing stage, where ice crystals grow and anneal, determines the final size of scaffold features. To investigate the path of heat flow within collagen slurries during solidification, a series of ice-templating molds were designed with varying the contact area with the heat sink, in the form of the freeze drier shelf. Contact with the heat sink was found to be critical in determining the efficiency of the release of latent heat within the perspex molds. Isotropic collagen scaffolds were produced with pores which ranged from 90 μm up to 180 μm as the contact area decreased. In addition, low-temperature ice annealing was observed within the structures. After 20 h at −30 °C, conditions which mimic storage prior to lyophilization, scaffold architecture was observed to coarsen significantly. In future, ice-templating molds should consider not only heat conduction during the cooling phase of solidification, but the effects of heat flow during ice growth and annealing.

Thermal time-constant spectrum extraction method in AlGaN/GaN HEMTs

The transient temperature rise in the active region in AlGaN/GaN high electron mobility transistors (HEMTs) is measured using an electrical method. The original data are smoothed and denoised by a nonparametric fitting algorithm, called locally weighted scatterplot smoothing (LOWESS). The thermal time-constant spectrum is extracted to analyze the physical structure of the heat-conduction path in AlGaN/GaN HEMTs. The thermal time-constant spectra extracted using the LOWESS algorithm are richer and the RC network obtained is greater compared with those with the traditional denoising method (multi-exponential fitting). Thus, the analysis of the heat-flow path is more precise. The results show that the LOWESS nonparametric fitting algorithm can remove noise from measured data better than other methods and can retain the subtle variation tendency of the original discrete data. The thermal time-constant spectra extracted using this method can describe the subtle temperature variations in the AlGaN/GaN HEMT active region. This will help researchers to precisely analyze the layer composition of the heat-flow path.

Enhanced series-parallel model for estimating the time-dependent thermal conductivity of fly ash soil mixtures

An enhanced series-parallel (S-II) model of one-dimensional conductive heat flow through a unit cell was developed to estimate the time-dependent thermal conductivity of Class C fly ash stabilized soils. The unit cell is composed of three separate heat flow paths: a continuous path through solid contacts (soil contacts, fly ash contacts, and soil fly ash contacts) in parallel arrangement, a series-parallel path of solids in a series arrangement with a parallel arranged path of solid-miniscule pores (miniscule portion of solid water and miniscule portion of solid air), and a continuous path through water and air in parallel arrangement. The solid-miniscule pores volume fraction and the fluid/air volume fraction changes during pozzolanic reaction were modeled by estimating the interparticle pore water consumption rate. In addition, the time-dependent thermal conductivity characteristics of the fly ash soil mixtures were investigated by conducting experimental tests. Based on the test observations and parametric analysis, interparticle voids, fly ash percentage and degree of cementation, volume change of the interparticle liquids, and curing time were all found to play critical roles on the effective thermal conductivity of fly ash soil mixtures. The predicted thermal conductivity using the enhanced S-II model was compared with the experimental test data with good agreement, suggesting that the enhanced S-II model is a robust tool for estimating the global effective thermal conductivity of Class C fly ash stabilized soils.

Evaluation of contribution of thermal radiation in transferring decay heat to the moderator, in case of 18 rod bundle facility, simulating advanced nuclear reactors

In case of heavy water moderated nuclear reactors, the decay heat of a fuel bundle can be transferred to the moderator available outside calandria vessel. Radiative heat transfer accounts for significant amount of total energy exchange from the fuel bundle during LOCA (loss of coolant accident) situation. In order to simulate one channel of an advanced reactor comprising 54 nuclear rods of 3.5 m length, scaling analysis has been used to scale it down to an 18 rod bundle facility of 1 m test length. The experimental analysis, in order to examine the contribution of energy transfer by thermal radiation and natural convection in removing decay heat of fuel/heater rods using moderator as a heat sink, is presented. It focuses mainly on the role of moderator in removal of decay heat, for situation with air alone in the annular gap between pressure tube and calandria tube coupled with flowing and standstill/boiling moderator situations. Four experimental tests have been presented in this paper. It is seen that, 80% of heat energy is transferred from the heater bundle of rods to the enclosing pressure tube by means of radiation mode, out of this 20–45% of energy is carried away by the moderator. It depends mainly on the resistances involved in the heat flow path and the test set up conditions.

A thermal circuit model consistent with integral energy balance for internal forced convection in a circular tube

The equivalent thermal resistance circuit based on integral energy balance of thermally developing forced convection in a circular tube is derived describing the heat flow path between the heated tube wall and inlet fluid. The axial heat conduction of the flow is assumed to be negligibly small. To validate the thermal circuits developed, numerical simulations have been performed for laminar thermally developing forced convection of water in a circular tube with an imposed constant heat flux or constant wall temperature. The overall thermal resistances for the convective circuit separating the heated tube wall and inlet fluid are simply the convection thermal resistances due to the local/average heat transfer coefficients, respectively, defined based on the difference between local/average tube wall temperature and inlet fluid temperature. Applications of the convection thermal circuit derived have been demonstrated for direct or indirect evaluation of the pertinent heat transfer parameters of technical interest characterizing the forced convection heat transfer of thermally developing fluid flow in the iso-flux or iso-thermally heated tube.