Thermal Conductivity Resistance Research Articles

Systematic Investigations on the Thermal Management Performance of Graphite Thin Films for High-Electron-Mobility Transistors

Self-heating is a severe problem for high-electron-mobility transistors (HEMTs). Graphene and thin graphite films are attractive candidates for heat spreader materials at the transistor level due to their ultrahigh in-plane thermal conductivity. However, a realistic and systematic simulation study of how they can improve the thermal management of HEMTs has not been carried out yet and is needed to push the method into practical use and realize industrialization. This study systematically investigated the effect of graphene and thin graphite films, and the most practical boundary condition in the simulations was primarily selected with comparisons. Then, the thermal properties consisting of thermal conductivity and interfacial thermal resistance (ITR) between graphite and other materials were fully investigated, which may bottleneck the actual improvement of the thermal management in applications. Our steady-state results indicated that the ITR between graphite and SiO2 determined the whole improvement in thermal management performance through the added graphite heat bridge. Subsequently, the transient thermal analysis was conducted on the HEMTs to explore the effect of graphene and thin graphite films under repeated joule heating pulses on a microsecond time scale, and we obtained the effect of graphene and thin graphite films on each heating and cooling process. Finally, based on all the results and analysis mentioned above, we can confirm that the graphite-based heat-spreader structures can still offer efficient heat dissipation under near-real conditions, thereby indicating ways of the optimization of HEMTs heat dissipation.

Thermal conductivity of epoxy composites modified by microspheric molybdenum disulfide

In order to improve the thermal conductivity of epoxy resins (EP) without damaging their dielectric properties, a kind of molybdenum disulfide with microspheres structure (S-MoS2) was used as modifiers to add into EP matrix, which were prepared via the surfactant promoting hydrothermal process, and a new kind of S-MoS2/EP composites was finally obtained. The morphology of the prepared S-MoS2 was observed by X-ray diffraction (XRD) scanning electron microscopy (SEM), while the influence of different S-MoS2 loading on the dielectric properties, thermal conductivity and thermal resistance of S-MoS2/EP composites was also researched. The results suggested that the reasonable content of S-MoS2 can highly improve the thermal conductivity of S-MoS2/EP, which can be attributed to the excellent thermal resistant and uniform dispersion of S-MoS2 in EP matrix.

Tuning thermal transport across monolayer MoS2/Si heterostructure via substrate nanogrooving

Substrate-supported two-dimensional (2D) material heterostructures have been widely applied in electronic and photonic devices. However, the substrate reduces the original high thermal conductivity of 2D materials and limits the heat dissipation due to the interface thermal resistance. Here, the effects of the substrate surface topography on the thermal transport properties of the MoS2-Si heterostructure are investigated via molecular dynamics simulations. The decreased in-plane thermal conductivity of the monolayer MoS2 and surprisingly enhanced interface thermal transport of the MoS2-Si heterostructure are found by introducing the shallow nanogroove on the substrate surface. The results are ascribed to the morphology change of the supported MoS2, which bends to fit the substrate surface topography due to the van der Waals force at the small groove depth. In addition, the force weakens and the supported MoS2 restores to flat with the increase of groove depth, resulting in higher in-plane thermal conductivity and thermal resistance compared to those without grooves, which is due to the disappearance of the substrate effect in the nanogroove area. This work elucidates the fundamental understanding of heat transfer in heterostructures. It provides new insights to enhance the heat dissipation in electronic devices by introducing nanoscale roughness.

Research Progress of Self-Healing Thermal Barrier Coatings: A Review

Reliability and durability are two important performance indicators for thermal barrier coatings (TBCs). The reliability of TBCs usually includes high adhesive strength, low thermal conductivity and high thermal shock resistance. The high reliability of TBCs ensures basic usage requirements. Durability demands TBCs have a long service lifetime before their eventual failure. The lifetimes of TBCs under actual service conditions are strongly dependent on crack initiation and propagation. Controlling and delaying the dynamic process of crack initiation and propagation is a direct approach to prolonging the service lifetime of TBCs. Self-healing TBCs usually have the specific function of inhibiting crack propagation, and thus promote the self-healing process of TBCs. The research progress of self-healing TBCs was reviewed. Firstly, the concept of self-healing or self-healing materials is clarified. Secondly, the research progress about some self-healing ceramic materials as well as self-healing TBCs is reviewed. Based on the review, the micro-structure design, propagation patterns of the crack and self-healing mechanism are discussed systematically. Additionally, the future development trend of the self-healing TBCs is also overviewed in this paper.

Prediction of Equivalent Thermal Conduction Resistance of Printed Circuit Heat Exchangers

Prediction of Equivalent Thermal Conduction Resistance of Printed Circuit Heat Exchangers

Impact of Various Thermistors on the Properties of Resistive Microbolometers Fabricated by CMOS Process.

Microbolometers based on the CMOS process has the important advantage of being automatically merged with circuits in the fabrication of larger arrays, but they typically suffer from low detectivity due to the difficulty in realizing high-sensitivity thermistors in the CMOS process. In this paper, two resistive microbolometers based on polysilicon and metal Al thermistors, respectively, are designed and fabricated by the standard CMOS process. Experimental results show that the detectivity of the two resistive microbolometers can reach a maximum of 1.78 ´ 109 cmHz1/2/W at 25 μA and a maximum of 6.2 ´ 108 cmHz1/2/W at 267 μA. The polysilicon microbolometer exhibits better detectivity at lower bias current due to its lower effective thermal conductivity and larger resistance. Even though the thermal time constant of the polysilicon thermistor is three times slower than that of the metal Al thermistor, the former is more suitable for designing a thermal imaging system with sensitive and low power consumption.

Investigation of the thermal shrink mechanism, thermal conductivity and compressive resistance of TaTiP3O12 ceramics

Dense TaTiP3O12 ceramics were synthesized by the solid-state method and spark plasma sintering (SPS) with 6 wt% V2O5 as a sintering aid, and their phase, microstructure, thermal conductivity, hardness, compressive strength, and expansion property and mechanism were investigated. Results show that the pure phase can be achieved by the two methods. In particular, the sample prepared by SPS possesses a relative density of 97.62% and a porosity of 3.07%, and has better properties than that prepared by the solid-state method. The SPS sample has a thermal conductivity at room temperature of 2.03 w/(m· °C), a Vickers hardness of 4.34 GPa and a compressive strength of 175.98 MPa, which are 0.95, 1.49 and 1.59 times greater than those of the sample prepared by the solid-state method, respectively. In addition, the TaTiP3O12 ceramic prepared by SPS exhibits a linear ultralow negative thermal expansion property with a coefficient of thermal expansion of −0.74 × 10−6 °C −1 (-100–400 °C). The negative thermal expansion in TaTiP3O12 is induced by the coupling effect of [Ta(Ti)O6] octahedron and [PO4] tetrahedron caused by the transverse vibration of bridging oxygen atoms.

Life cycle assessment of post-demolition autoclaved aerated concrete (AAC) recycling options

Autoclaved aerated concrete (AAC) is a widely used building material for masonry units, prefabricated reinforced components, and lightweight mineral insulation boards. Its low thermal conductivity and good fire resistance increase its popularity in residential buildings. Thus, post-demolition wastes are expected to increase in the future. However, post-demolition AAC (pd-AAC) is mainly disposed in landfills while landfill capacities decrease and legal framework conditions in Europe are tightening. This study performed life cycle assessments (LCA) of different pd-AAC recycling options and compared them to each other and to current landfilling to identify the best end-of-life handling of pd-AAC from an ecological perspective. The functional unit was 1 kg pd-AAC, and the system boundaries included pd-AAC at the demolition site, transports, pd-AAC treatment, and secondary production processes. Final products of the recycling process gained environmental credits/rewards for avoiding primary production using system expansion. Providing primary resources, primary production, and use phase were not in the scope of this study. Results show that especially closed-loop recycling of pd-AAC in AAC production has a high potential of improving environmental impacts. In the best recycling option (high substitution in AAC-0.35), potential savings per kg pd-AAC compared to landfilling reach up to 0.5 kg CO2-Eq, 7 MJ fossil resources, 0.005 mol H+-Eq (acidification), 0.17 CTU (freshwater ecotoxicity), 0.2 g P-Eq (freshwater eutrophication), 5.2 × 10-9 CTUh (carcinogenic effects), 4.4 × 10-8 CTUh (non-carcinogenic effects), 2.5 × 10-5 g CFC-11-Eq (ozone layer depletion), and 1.6 g NMVOC-Eq (photochemical ozone creation). Despite data uncertainties, recycling of pd-AAC is advantageous for several recycling options, including the production of AAC, light mortar, lightweight aggregate concrete, and shuttering blocks made from concrete without fine fractions (no-fines concrete). In Germany, up to 280,000 t CO2-Eq could have been saved in 2022 by pd-AAC recycling using different recycling options instead of landfilling.

Analysis of pure nanofluid (GO/engine oil) and hybrid nanofluid (GO–Fe3O4/engine oil): Novel thermal and magnetic features

Abstract Hybrid nanofluids can provide better physical strength, thermal conductivity, and mechanical resistance in many thermodynamic systems than pure nanofluids. To establish the novel results, using superior types of hybrid nanoparticles like graphene oxide (GO) and iron oxide (Fe3O4) is the main focus of recent work. This study investigates the innovative thermal and magnetic features of both pure nanofluid GO/engine oil (EO) and hybrid nanofluid GO–Fe3O4 /EO under the simultaneous effects of induced as well as applied magnetic field. The chemical reaction phenomenon together with activation energy has also been taken into account. A novel algorithm based on order reduction and finite difference discretization is developed in order to numerically treat the problem. The efficiency of the code is appraised by a numerical comparison which is found to be in a good correlation with the existing results. From the consequences of this study, it is deduced that the reduction in induced magnetic field and fluid’s velocity (in case of either pure or hybrid nanofluid) is associated with the enlarging values of magnetic Prandtl number and induced magnetic field parameter. Further, activation energy is responsible for enhancement in concentration. The hybrid nano-composition of GO–Fe3O4/EO can provide the thermal stability, prevent the corrosion and make the liquid to stay in high temperature.

Integrating Diamond for Cooling Electronics

III-Nitrides promise to offer high-frequency power amplifiers at higher power densities. This is attractive for realizing device platforms for 5G and beyond. It also enables integrating Si-based electronics to complement functionalities. However, to obtain the full potential of III-Nitrides one must operate these transistors at higher power densities, for which thermal management is crucial.Due to the continuous need for higher performance in various applications ranging from DC to W-band frequencies, semiconductor devices are pushed to higher power densities. This results in a higher junction temperature owing to the Joule heating in the channel resulting in device performance degradation and premature failure. Using devices based on wide-bandgap (WBG) semiconductors such as Gallium Nitride (GaN) and Gallium Oxide (GaOx) with a larger critical electric field and high-temperature tolerance, one could increase the power density. However, in order to increase the output power to the levels that is promised by these wide bandgap materials, junction temperature reduction through device-level cooling is critical. We are actively researching thermal management at device, circuit and packaging level using synthetic polycrystalline (PC) diamond. PC-diamond, at an appropriate grain size offers a TC is higher than any competing thermal vias with metals without interfering the electrical performance. Over the past 5 years we have demonstrated the potential of integrating diamond with GaN and recently achieving a record low diamond/GaN thermal boundary resistance (TBR) along with a relatively high diamond thermal conductivity (TC). This remarkable thermal performance was be achieved by maintaining the electrical performance, as demonstrated by the unharmed channel charge and mobility results.There are two key requirements for thermal management using heat spreading from the channel: first, a low thermal boundary resistance (TBR) between the hot spot in the channel and the heat spreader is required (in this study between GaN channel and diamond). Second, a low thermal resistance path to the heat-sink is crucial from the diamond layer which is directly proportional to diamond’s thermal conductivity (TC). In collaboration with Univ of Bristol, we have reported a TBR of ~3 m2K/GW, to date, known to be the lowest reported TBR for GaN HEMT technology. Diamond-GaN TBR depends strongly on the interface smoothness and the thickness of interfacial Si3N4 layer. A thinner Si3N4 resulted in lower TBR and allowed superior phonon coupling from the channel to the spreader. We have also measured a remarkably high TC for a 2 μm-thick diamond layer yielding over 650 W/m.K. The diamond grains are near isotropic in shape that allows excellent in-plane and cross-plane thermal conductivity. Achieving high TC within a thin film of diamond, grown heterogeneously, underscores the importance of our technique, since it reduces the residual stress when integrating with different materials that involves many thin layers like GaN/AlGaN HEMT epi-layers.β-Ga2O3 is an emerging ultra-wide bandgap material showing promises for both power and RF devices. However, the material’s low thermal conductivity poses to be a challenge for efficient power delivery (at any frequencies). PC diamond was successfully grown on (‾¯201)β-Ga2O3 using proper nucleation technique and thermal characterizations were conducted. A thermal conductivity (diamond + nucleation) and thermal boundary resistance at the diamond/β-Ga2O3 interface of 110 ± 33 W/mK and 30.2 ± 1.8 m2K/GW, respectively were measured.Our current results demonstrates a very promising roadmap for wide bandgap semiconductors via diamond integration.

Analysis and evaluation of the thermal performance of combinations of suits and life jackets in water

Life jackets are essential life-saving devices, and also play an important role in the thermal protection of drowning victims in cold water. To better understand the thermal performance of the combinations of suits and life jackets in water, a thermal manikin named “Walter” in a water tank was used to simulate the environment surrounding the drowning person. Three sets of suits and three sets of life jackets form a total of 12 test combinations to simulate the clothes worn by drowning people. The thermal resistance of 12 combinations in water were measured according to ISO 15831 and heat transfer coefficients for the chest, back, upper arm, arm, thigh and leg of the 12 test combinations were calculated and compared. Results showed that the combinations of the clothes and life jacket efficiently lowered conduction and convective heat transfer coefficients in the water. The total thermal resistance of the 12 test combinations ranged from 0.011 to 0.054°C · m2/W. The post-hoc analysis revealed that the conductive thermal resistances of the clothing combinations with life jackets were all elevated compared to those of the clothing combinations without life jackets ( p < 0.001), showing that life jackets provide better thermal insulation. The linear regression indicated that the conductive thermal resistance of the test combination had a considerable impact on the convective heat transfer coefficient. This study helps to establish the systematic study of the thermal protection performance of life jackets.

Interfacial thermal transport properties and its effect on thermal conductivity of functionalized BNNS/epoxy composites

• The thermal conductivity of functionalized BNNS and its interfacial thermal resistance with epoxy were calculated by molecular dynamics. • The mechanism of functionalization on reducing the interfacial thermal resistance of BNNS-epoxy was studied according to the phonon vibration density of state. • The influence of the BNNS content, size and functionalization rate on the thermal conductivity of composites was analyzed. • Under the synergistic effect of reduced BNNS thermal conductivity and enhanced BNNS-epoxy interfacial thermal conductance, there is a critical size for the effect of functionalized BNNS on the thermal conductivity of composite. The miniaturization and high-performance development of integrated circuits poses a major thermal challenge to microelectronic packaging, and there is an urgent need to study high thermal conductivity composites to improve the thermal management performance of high-power devices. In this paper, aiming at the thermal conductivity of BNNS/epoxy composites, the reverse non-equilibrium molecular dynamics method is used to study the effect of covalent and non-covalent functionalization on BNNS thermal conductivity and BNNS-epoxy interfacial thermal resistance. The strengthening mechanism of functionalization on interfacial thermal conductance is discussed according to phonon vibration density of states. And the effect of BNNS content, size, and functionalization rate on the thermal conductivity of composites is studied. The results found that the intrinsic thermal conductivity of BNNS reduced while the interfacial thermal conductance of BNNS-epoxy enhanced through functionalization. Under the synergistic effect of the two, there is a critical size for the effect of functionalized BNNS on the thermal conductivity of composite. When the BNNS is smaller than the critical size, functionalization can enhancing the thermal conductivity of the composite. The results can provide important theoretical guidance for the preparation of high thermal conductivity BNNS/epoxy composites.

Recent Advances in Thermal Interface Materials for Thermal Management of High-Power Electronics

With the increased level of integration and miniaturization of modern electronics, high-power density electronics require efficient heat dissipation per unit area. To improve the heat dissipation capability of high-power electronic systems, advanced thermal interface materials (TIMs) with high thermal conductivity and low interfacial thermal resistance are urgently needed in the structural design of advanced electronics. Metal-, carbon- and polymer-based TIMs can reach high thermal conductivity and are promising for heat dissipation in high-power electronics. This review article introduces the heat dissipation models, classification, performances and fabrication methods of advanced TIMs, and provides a summary of the recent research status and developing trends of micro- and nanoscale TIMs used for heat dissipation in high-power electronics.

Fabrication of Multifunctional Composites with Hydrophobicity, High Thermal Conductivity and Wear Resistance Based on Carbon Fiber/Epoxy Resin Composites

The design and preparation of hydrophobic, wear-resistant, and thermally conductive multifunctional composites is an important direction of scientific research and application. In this study, A-CF/EP/FEP composites were prepared by incorporating APDMS-modified carbon felt (A-CF) into an epoxy resin (EP) and fluorinated ethylene propylene resin (FEP) mixed resin. The low surface energy of FEP, good adhesion of EP, and the supporting of carbon felt framework endow the A-CF/EP/FEP composites with hydrophobicity, wear resistance, and thermal conductivity at the same time. Firstly, the water contact angle (WCA) of A-CF/EP/FEP composites with 20 wt% FEP reaches 109.9 ± 2.6°, and the WCAs of all composites with different FEP contents (10, 20, 30, 40, and 50 wt%) is greater than 90°, indicating the composites have a hydrophobic surface. Secondly, the A-CF/EP/FEP composites have high wear resistance and maintain long-term hydrophobicity after tribological tests, because the residual debris and nanoparticles generated by external loading adhere to the friction interface, regenerating the microstructure of the hydrophobic surface. Finally, the A-CF/EP/FEP composites have high thermal conductivity up to 0.38 W/(m·K), which is 1.81 and 2.0 times that of pure EP and EP/FEP composites, respectively. This is because a relatively complete heat conduction network is formed after the addition of A-CF to the composites. The synergy among epoxy resin, FEP, and the A-CF filler plays a particularly important role in constructing hydrophobic surfaces and improving wear resistance and thermal conductivity. The EP enhances adhesion, the FEP supplies low surface energy, and the A-CF framework improves the wear resistance of A-CF/EP/FEP composites. This work provides ideas for the design and preparation of multifunctional composites and will underlie the application of high-performance epoxy resin and its composites.

Synergistic Enhanced Thermal Conductivity and Crack Resistance of Reactor Epoxy Insulation with Boron Nitride Nanosheets and Multiwalled Carbon Nanotubes.

Epoxy composites with high thermal conductivity, excellent dielectric, and mechanical properties are very promising for solving epoxy cracking faults in reactors and for extending their service life. In this work, we report on epoxy composites enhanced by ternary fillers of boron nitride nanosheets (BNNSs), multiwalled carbon nanotubes (MWCNTs), and silica (SiO2) nanoparticles. The obtained BNNSs/MWCNTs/SiO2/epoxy composites exhibit a high thermal conductivity of 0.9327 W m−1 K−1, which is more than 4-fold higher than that of pure epoxy. In addition, the resultant composites present an improved mechanical strength (from 2.7% of epoxy to 3.47% of composites), low dielectric constant (4.6), and low dielectric loss (0.02). It is believed that the integration of multifunctional properties into epoxy composites provides guidance for optimizing the design of high-performance materials.

Integrated construction of silkworm cocoon-inspired 3D scaffold for improving cell manufacture and cryopreservation

Although cellular therapy holds enormous promise in treating intractable diseases, its application potential has been significantly hampered due to the scarcity of reliable and consistent cell sources. Therefore, a high-efficiency strategy that improves cell production and storage is desperately needed. Herein, we develop a versatile 3D bioinspired scaffold (Cryosilk) for improving scalable cell manufacture and cryopreservation. A bottom-up fabrication technique integrating electrospinning, in situ surface functionalization and freeze-shaping was explored to construct Cryosilk with biomimetic features and functions of silkworm cocoons. Cryosilk is composed of a core-shell heterostructure with silk fibroin/poly alanine fiber core and silk sericin shell, generating a 3D cocoon-mimicking fibrous structure. Importantly, Cryosilk possesses improved thermal conductivity and ice crystal resistance capability, thus enabling to cryopreserve biological samples with minimal cryodamage. Furthermore, Cryosilk not only promotes cell adhesion and growth, but achieves rapid and uniform rewarming process, which provides high cryopreservation efficacy for immune cells and stem cells. Particularly, Cryosilk can maintain cell viability and biofunctions of stem cell-scaffold constructs after freeze-thawing, which can be directly implanted to promote wound healing. Thus, Cryosilk offers unprecedented efficacy in cell manufacture and cryopreservation, which provides sufficient and high-quality precious cells and tissue engineered scaffolds for cellular therapy.

Comparison of different post-demolition autoclaved aerated concrete (AAC) recycling options

Autoclaved aerated concrete (AAC) is used as masonry blocks and prefabricated reinforced elements preferably in residential buildings. Due to its porous structure and mineral composition, it combines low thermal conductivity and fire resistance properties. Consequently, the popularity of AAC increases. However, due to significant AAC production volumes in many European countries since the 1960s and 1970s and given building lifetimes, strongly increasing post-demolition AAC waste volumes can be expected in the following decades. Recycling these post-demolition AAC wastes could protect primary resources and landfill capacities and reduce greenhouse gas emissions. But, recycling of post-demolition AAC is not yet established. The majority of the waste is landfilled even though landfill capacities have decreased and the legal framework conditions in Europe regarding a circular economy are becoming stricter. Therefore, new recycling options are needed. Current research approaches propose different open-loop recycling routes for post-demolition AAC, e.g. lightweight aggregate concrete, lightweight mortar, no-fines concrete, floor screed, animal bedding, oil- and chemical binders, and insulating fills for voids and interstitial spaces. Additionally, closed-loop recycling is possible and under research. Finely ground post-demolition AAC powder can be directly used in AAC production or can be chemically converted to belite (C2S) clinker to substitute primary cement in AAC production. These promising recycling options are compared regarding environmental and economic aspects. We find that the resource consumption is lower in all recycling options since post-demolition AAC helps to save primary resources. Furthermore, greenhouse gas emissions associated with the substituted primary resources are saved - especially when substituting primary cement in closed-loop recycling. In economic terms, increasing landfill costs could be avoided, which leaves a considerable margin for the cost of pre-processing, transport and recycling. The results can help decision-makers to implement circular management for AAC by fostering post-demolition AAC recycling and reducing its landfilling.

Improved Thermal Properties and Erosion Resistance of Silicone Composites With Hexagonal Boron Nitride

In this article, silicone rubber (SiR) is filled with industry preferred micro-silica and specialty hexagonal boron nitride (h-BN) nanosheets, by varying the ratio between the two fillers, while keeping the filler to matrix ratio the same in all three composites. The fillers are added to SiR using an electrostatic disperser which aids in homogenizing the filler-polymer mixture by means of shearing force along with elongation. The performance of synthesized silicone composites are evaluated using thermal, erosion resistance, morphological, mechanical, and high-temperature dielectric properties. Addition of as low as 5 wt% h-BN fillers significantly improved the thermal properties; 6%–20% improvement in thermal conductivity and in less than half the weight loss compared to only silica filled composites with laser erosion. The morphological and mechanical analyses showed improved filler dispersion and filler to silicone matrix interaction owing to good filler dispersion achieved through the electrostatic dispersing method. The dielectric properties measured in wide frequency range, between 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−4</sup> and 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> Hz and at elevated temperatures (up to 175 °C) showed a negligible effect of increase in h-BN content. Thus, this article implies that low amounts of h-BN in silica-silicone composites improve thermal conductivity and erosion resistance significantly and not altering the dielectric responses of these hybrid composites even at elevated temperatures. Hence these composites can find applications both in the high voltage industry and as electronic packaging materials.

Experimental investigation of aluminum fins on relative thermal performance of sintered copper wicked and grooved heat pipes

Heat pipes are one of the most modern thermal devices used for high heat flux applications like fuel engines and thermal boilers to hydrogen storage tanks etc. Various kinds of thermal performance enhancement techniques have been employed to increase heat transfer through these pipes. For this purpose, manufacturing of a finned heat pipe is one of the famous techniques which maximizes heat transfer and increases operating range of heat pipes too. This study compiles the experimental results; 1) of installing Aluminum fins on the condenser section of heat pipes, 2) and a comparison between finned and finless distilled water based sintered wicked as well as grooved heat pipes, by keeping all other parameters same. The distribution of surface temperatures, thermal conductivity and thermal resistance is considered for comparison purposes. It is clearly observed that power surface temperature has an inverse relationship with the evaporator thermal resistance that is; if the power surface temperatures increases then the evaporator thermal resistance decreases. Also, installation of fins is more significant in sintered heat pipes as compared to grooved heat pipes. Due to installation of fins, there is an increase in the operating range which was observed 47% for sintered heat pipes and 30.1% for grooved heat pipes, when compared at 20 W. It has also been noted that the thermal conductivity of sintered heat pipes increased up to 73.2% due to the installation of fins and unexpectedly decreased up to 58.8% in case of grooved heat pipes. Thermal resistance of finned sintered heat pipes was 42% lower than finless type, and surprisingly it has increased up to 1.43 times in grooved heat pipes. Finally, a through comparison of finless sintered and grooved heat pipes results that finless grooved heat pipe shows the best performance and up to a 4.7% lower evaporator surface temperature was observed.

Effect of Dy2O3 doping on phase compositions, microstructures, thermophysical properties and fracture toughness of YSZ coatings

Yttria-stabilized zirconia (YSZ) coatings with different Dy2O3 doping contents were prepared via plasma spraying. The effects of phase compositions, microstructures and lattice defects on the thermal conductivity, fracture toughness and thermal shock resistance were analyzed. The result indicates that the Dy2O3 doping significantly affects thermophysical properties of YSZ coatings. The Dy2O3 doping can improve the insulation performance of the YSZ coating and the minimum thermal conductivity of the coating has reached 0.89 W/K·m. A decrease in thermal conductivity with increasing Dy2O3 doping content can be attributed to the lattice distortion, ions substitution, oxygen vacancies and reducing grain size. Meanwhile, the Dy2O3 doping causes the fracture toughness of YSZ coatings to decrease. Consequently, the Dy2O3-doped YSZ coatings exhibit weaker thermal shock resistance because the Dy2O3 doping promotes the formation of the cubic YSZ phase. The tetragonal phase in plasma-sprayed coatings is beneficial for service lifetime owing to its phase transformation and ferroelasticity-induced toughening under stress. Although the Dy2O3-doped YSZ coatings undergo more severe sintering owing to a decrease in the activation energy of grain growth caused by lattice distortion, they exhibit better phase stability than the YSZ coating under high temperatures because the Dy2O3 doping inhibits the diffusion of Y. Finally, a Dy2O3-doped ZrO2 coating composed mainly of the tetragonal ZrO2 phase was designed, which exhibited a better thermal shock resistance.