Relative Thermal Conductivity Research Articles

Influences of Dispersed Lanthanum Oxide Additive on the Properties of Tungsten-Based Plasma-Facing Material

To investigate the influences of dispersed lanthanum oxide (La2O3) additive on the properties of a tungsten (W)-based plasma-facing material, ultrafine-grained W-1% La2O3 composite has been successfully fabricated using the resistance sintering under ultrahigh pressure method, which can suppress W grain growth during sintering processes. Its relative density, Vickers microhardness, microstructure, and thermal conductivity have been analyzed and compared with those of pure W. Moreover, its behaviors under fusion-related conditions, i.e., edge plasma loading in the HT-7 tokamak and transient heat flux simulated by a high-intensity pulsed ion beam, have been evaluated. It is shown that without the fine-grain strengthening effect of dispersed particles, the La2O3 additive as second-phase particles being dispersed in W-based plasma-facing material degrades the material resistance ability under plasma heat loading.

Preparation and Thermal Conductivity of Spark Plasma Sintered Aluminum Matrix Composites Reinforced with Titanium‐Coated Graphite Fibers

Short graphite fiber/Al composites are fabricated by spark plasma sintering technique. A titanium coating synthesized on the graphite fiber surface through vacuum microdeposition is proposed to improve the interfacial bonding between graphite fibers and aluminum matrix. The influences of surface modification, sintering temperature, and graphite fiber volume fraction on relative density and thermal conductivity (TC) of the composites are systematically investigated. The results indicate that compared to uncoated graphite fiber/Al composites, the densification, interfacial bonding, and TC of titanium‐coated composites are greatly enhanced. The in‐plane TC of 50 vol% titanium‐coated graphite fiber/Al composites sintered at 610 °C is 238 W m−1 K−1, nearly twice as high as that of uncoated composites. From the calculation based on the experimental TC by Maxwell–Garnett effective medium approach, the interfacial thermal resistance is evidently decreased by above 1 order of magnitude with the introduction of titanium nanolayer.

Wetting and Evaporative Aggregation of Nanofluid Droplets on CVD-Synthesized Hydrophobic Graphene Surfaces

The wetting and evaporative aggregation of alumina nanofluids (Al2O3) are examined for CVD-synthesized graphene-coated (GC) surfaces that are known as strongly hydrophobic (θcontact ≈ 90°). Our findings are compared to those associated with a hydrophilic cover glass (CG) substrate (θcontact ≈ 45°). The nanofluidic self-assemblies on the GC substrate are elaborately characterized in terms of the droplet wetting/crack formation, the particle migration time over the evaporative time (CR), the Derjaguin-Landau-Verwey-Overbeek forces (FDLVO), and the relative thermal conductivity (KR). The GC substrate forms relatively thicker and larger cracks and requires a longer evaporation time. Both the GC and CG substrates share approximately the same time constant CR, which suggests the formation of coffee-ring patterns for both substrates. The GC shows negative FDLVO, which implies a repulsive force between the nanoparticles and the substrate, and the CG shows a positive FDLVO of attraction. Furthermore, a more than 3 order of magnitude larger thermal conductivity of GC compared to that of CG drives significantly different particle/fluid motions near the drop edge areas between the two substrates.

Numerical Analysis of Spatial Unsteady Regimes of Conjugate Convective-Radiative Heat Transfer in a Closed Volume with an Energy Source

Unsteady regimes of convective-radiative heat transfer in a cubic enclosure with finitely thick heat-conducting walls in the presence of a constant-temperature energy source have been modeled mathematically under the conditions of convective heat exchange with the environment. A mathematical model has been formulated in dimensionless variables “vector potential–vorticity vector–temperature;” the model was realized numerically by the finite-difference method. An analysis of radiative heat transfer has been made on the basis of the surface-radiation approximation with the balance method in Polyak’s version. Three-dimensional temperature and velocity fields and dependences for the average Nusselt number have been obtained; they reflect the influence of the reduced emissivity factor of interior surfaces of enclosing walls, of the relative thermal conductivity, and of the unsteadiness factor on the flow regimes and heat transfer.

Development of butyl rubber–rutile composites for flexible microwave substrate applications

Butyl rubber–micron rutile (BR/MRT) and butyl rubber–nano rutile (BR/NRT) composites were prepared by sigma mixing. The effect of micron rutile and nano rutile content on dielectric properties at 1MHz and 5GHz were investigated. For 0.30 volume fraction (vf) of micron rutile loading, the composites have attained a relative permittivity (εr) of 8.59 and loss tangent (tanδ) of 0.0024 at 5GHz, and for the same volume fraction of nano rutile content, the composite showed εr of 7.62 and tanδ of 0.0031 at 5GHz. The thermal properties such as thermal conductivity (TC) and coefficient of thermal expansion (CTE) of both composites were studied as a function of filler loading. It is found that the nano rutile added butyl rubber composites have better thermal properties than those micron composites. The stress–strain curves of both composites show the mechanical flexibility of the composites. The experimental relative permittivity and thermal conductivity of both composites were compared with theoretical models.

Heat transfer enhancement by layering of two immiscible co-flows

Enhancement of heat transfer in minichannels due to co-flowing of two immiscible fluids in a direct contact is investigated in this work. Different fluid combinations are analyzed. The momentum and energy equations for both flows are solved analytically and numerically. The numerical and analytical solutions are found to be in good agreement. A parametric study including the influence of fluids relative viscosity, thermal conductivity, thermal capacity and height ratios is conducted for various Peclet numbers. Different ranges of the parameters that augment the heat transfer are obtained, and different physical aspects of the problem are discussed. For practical fluid combinations with small Peclet numbers, the enhancement factor can increase up to 2.6 folds. However, that increase is about 1.2 folds when the Peclet number is increased by two orders of magnitude. This work establishes the mechanisms for heat transfer enhancement utilizing two immiscible co-flows.

Fabrication of W–Cu functionally graded material by spark plasma sintering method

Three-layered (W–25Cu/W–50Cu/W–75Cu, volume fraction) W/Cu functionally graded material (FGM) was synthesized by spark plasma sintering (SPS) at different temperatures for 5min under a load of 40MPa. The influences of different sintering processes on relative density, hardness, thermal conductivity and microstructure at various layers of sintered samples were investigated. The experimental results indicated that the graded structure of the composite could be well densified after the SPS process. The relative density increased with the increment of sintering temperature and it was up to 96.53% as sintered at 1050°C. In addition, the thermal conductivity reached 140W/m·K at room temperature and 151W/m·K at 800°C, which could be ascribed to the specific structure that W particles enwrapped by net-like Cu. And the Vickers hardness was converted from 4.11 to 4.68GPa.

Effect of SiC particle size on the physical and mechanical properties of extruded Al matrix nanocomposites

In this work, the effect of SiC particle size and its amount on both physical and mechanical properties of Al matrix composite were investigated. SiC of particle size 70nm, 10μm and 40μm, and Al powder of particle size 60μm were used. Composites of Al with 5 and 10wt.% SiC were fabricated by powder metallurgy technique followed by hot extrusion. Phase composition and microstructure were characterized. Relative density, thermal conductivity, hardness and compression strength were studied. The results showed that the X-ray diffraction (XRD) analysis indicated that the dominant components were Al and SiC. Densification and thermal conductivity of the composites decreased with increasing the amount of SiC and increased with increasing SiC particle size. Scanning electron microscope (SEM) studies showed that the distribution of the reinforced particle was uniform. Increasing the amount of SiC leads to higher hardness and consequently improves the compressive strength of Al–SiC composite. Moreover, as the SiC particle size decreases, hardness and compressive strength increase. The use of fine SiC particles has a similar effect on both hardness and compressive strength.

Fabrication and thermal conductivity of short graphite fiber/Al composites by vacuum pressure infiltration

Short graphite fiber/Al composites were fabricated by a modified two-step vacuum pressure infiltration technique. Copper-coated graphite fibers preform was infiltrated with liquid aluminum at 800℃ under infiltration pressure of 1 MPa and solidification pressure of 30 MPa for 30 min. The effects of surface modification and the processing parameters of vacuum pressure infiltration on relative density and thermal conductivity of the composites were systematically studied. The results show that short graphite fiber/Al composite with relatively high density of 99.1% and thermal conductivity of 208 W·m−1·K−1 was successfully fabricated. Through the application of copper coating onto the graphite fibers, the in-plane thermal conductivity of the composite was effectively enhanced from 117 W·m−1·K−1 to 208 W·m−1·K−1 as a result of improved interfacial bonding. The obtained short graphite fiber/Al composites are promising materials for electronic packing applications.

Gelcasting of aluminum nitride ceramics using hydantion epoxy resin as gelling agent

Abstract A gelling system based on the polymerization of Hydantion epoxy resin and 3,3′-Diaminodipropylamine (DPTA) was developed for gelcasting aluminum nitride (AlN) ceramics. The effect of the dispersant concentration and solid loading on the rheological behavior of the slurries was investigated. The highest solid loading of 58 vol% AlN ceramic slurry and the highest green relative density of 64.5% were obtained with the addition of 15 wt% Hydantion epoxy resin and 0.3 wt% dispersant Polyethyleneimine (PEI). With the addition of 20 wt% Hydantion epoxy resin, the flexural strength of AlN green body reached as high as 34.5 MPa. For sintered AlN ceramics, the relative density, flexural strength and thermal conductivity reached 99.6%, 360 MPa and 185 W/m K, respectively.

Thermal conductivity of SiC ceramic fabricated by liquid infiltrating molten Si into mesocarbon microbeads-based preform

Abstract The thermal conductivity of SiC ceramics fabricated by liquid infiltrating molten Si into mesocarbon microbeads-based preform was investigated. The thermal conductivity of SiC ceramics decreased from 103 to 36 W·m− 1·K− 1 at room temperature when the volume fractions of SiC, residual Si and unreacted C varied from 76.7 to 51.2 vol.%, 11.2 to 0 vol.% and 10.0 to 45.3 vol.%, respectively. The microstructural investigation and Maxwell's model analysis revealed that the difference in thermal conductivity of the samples was mainly affected by the unreacted C and transition layer, which surrounding around the unreacted C, was composed of inhomogeneous and sparser SiC nanocrystalline. Since the volume of the transition layer was mainly determined by the outer area of the carbon particles, the unreacted C and transition layer, with relative low thermal conductivity, can make the thermal conductivity sharply decrease with the unreacted C content increasing.

Axisymmetric thermocapillary migration of a fluid sphere in a spherical cavity

The quasi-steady thermocapillary migration of a spherical fluid drop situated at an arbitrary position in a second fluid within a spherical cavity is studied theoretically in the limit of negligible Marangoni and Reynolds numbers. The imposed temperature gradient is constant and along the line connecting the centers of the drop and cavity. To solve the thermal and hydrodynamic governing equations, the general solutions are constructed from the fundamental solutions in the two spherical coordinate systems based on the drop and cavity. The boundary conditions at the drop surface and cavity wall are satisfied by a collocation technique. Numerical results for the thermocapillary migration velocity of the drop normalized by its value in an unbounded medium are presented for various values of the relative viscosity and thermal conductivity of the drop, the relative conductivity of the cavity phase, the drop-to-cavity radius ratio, and the relative distance between the drop and cavity centers. In the particular case of the migration of a spherical drop in a concentric cavity, these results agree excellently with the exact solution derived analytically. The normalized thermocapillary migration velocity of the confined drop decreases monotonically with an increase in the drop-to-cavity radius ratio or its relative distance from the cavity center and vanishes in the touching limit of the drop and cavity surfaces. On the other hand, this velocity increases with an increase in the relative thermal conductivity of the cavity phase, but can increase or decrease with an increase in the relative viscosity or thermal conductivity of the drop for a given configuration. The boundary effect on thermocapillary migration can be significant, but is weaker than that on sedimentation.

Influence of Ln3+ and B3+ Ions Co‐Substitution on Thermophysical Properties of LnMB11O19‐type Magnetoplumbite LaMgAl11O19 for Advanced Thermal Barrier Coatings

Lanthanum hexaaluminate is a promising competitor to establish yttria partially stabilized zirconia as a thermal barrier coating material for Ni‐based superalloy due to its relative low intrinsic thermal conductivity and low sinterability at temperatures exceeding 1100°C. Sr2+ and Ti4+ were selected as two dopants to partially substitute the La3+ and Al3+ in LaMgAl11O19, respectively. The variation in thermal conductivity with Sr2+ and Ti4+ fractions was analyzed based on structure information provided by X‐ray diffraction and Raman spectroscopy. The average crystal size of LaMgAl11O19 sintered at 1600°C for 10 min by spark plasma sintering is in nanoscale. The fully dense La1−xSrxMgAl11−xTixO19 solid solution showed a minimum thermal conductivity value (λ = 1.12 W/(m K)−1,T = 1273 K) at the composition of La0.5Sr0.5MgAl10.5Ti0.5O19,which possibly reduces from the enhanced phonon scattering due to mass and strain fluctuations at the Ln3+ and B3+ sites.

Editorial

Prof. Bo-Young Hur Dr. Seung-Eun Kim Prof. John Banhart Dr. Francisco García Moreno Numerous types of cellular metals have been developed in the past decades and have become an independent class of new engineering materials by now. They possess a spectrum of unique properties such as high impact energy absorption capability, air, and water permeability, unusual acoustic properties, and low relative thermal conductivity. Their applications include shock and impact absorbers, dust and fluid filters, engine exhaust mufflers, porous electrodes, heat exchangers, sensors, and catalyst substrates. Porous and cellular metals and metallic foams should be now used to tackle engineering problems that are awaiting a solution. To enable this, it is crucial to bring together scientists and application engineers so that the ones developing and producing such materials know of the ones who could apply them and vice versa. This was the motivation for holding the International Conference on Porous Metals and Metal Foams (MetFoam). The past six international conferences of the MetFoam series have already contributed to the success of those materials. Previous conferences were held at Bremen (1999, 2001) and Berlin (2003), both in Germany, Kyoto (2005) in Japan, Montreal (2007) in Canada and Bratislava (2009) in Slovakia. The 7th International Conference (MetFoam 2011) was held in Busan, Korea in September 2011. Recent achievements in porous metals and metallic foams were extensively discussed there. This special issue of AEM contains a selection of the papers presented at MetFoam 2011. The principal purpose of the conference was to provide a state-of-the-art review of porous metals and metallic foams. This was achieved by encompassing papers dealing with basic fundamentals, fabrication, morphological, and microstructural characterization, property profiles, secondary operations, and various applications of porous metals and metallic foams. More than 200 papers were presented by a broad group of researchers and scientists representing universities, federal laboratories, and industries. The complete collection has been published in a book.1 It is our hope that this selection of more easily accessible papers provides a representative insight into the developments presented at MetFoam 2011. Gyeongsang, Changwon and Berlin, March 2013 Prof. Bo-Young Hur Dr. Seung-Eun Kim Prof. John Banhart Dr. Francisco García Moreno

Viscosity and thermal conductivity of dispersions of gum arabic capped MWCNT in water: Influence of MWCNT concentration and temperature

Experiments were carried out on the measurement of viscosity and thermal conductivity of multi-walled carbon nanotubes (MWCNT)-water nanofluids prepared using gum arabic as dispersant. The influence of temperature (28–60°C), MWCNT concentration (0.14–0.24vol%) on viscosity and effective thermal conductivity has been investigated. Our results on the effect of MWCNT concentration on relative viscosity and thermal conductivity ratio are in qualitative agreement with those reported in literature. Thermal conductivity ratios increase strongly with temperature, especially above 45°C. Relative viscosities too increased with temperature above 45°C at relatively higher MWCNT concentration indicating the potential role of Brownian motion.

Mathematical modeling of the laminar regime of conjugate convective heat transfer in an enclosure with an energy source under surface-radiation conditions

A numerical analysis of the laminar regime of heat transfer in a square enclosure with finite-thickness heatconducting walls in the presence of a constant-temperature heat-releasing element under the conditions of radiative heat exchange has been made in a surface-radiation approximation. A mathematical model has been formulated in dimensionless variables “stream function–vorticity–temperature,” which was realized numerically by the finite-difference method. Temperature and streamline distributions reflecting the influence of the reduced emissivity factor of interior surfaces of enclosing walls, of the relative thermal-conductivity coefficient, and of the factor of unsteadiness on the flow regimes and heat transfer have been obtained.

Preparation and characterization of sub-micron dispersions of sand in ethylene glycol-water mixture

Experiments were carried out on the preparation and characterization of dispersions of sand in ethylene glycol-water (50-50%) mixture. The dispersions were prepared by stirred bead milling of 20-30 μm sand (in water) followed by dilution with water and ethylene glycol. The influence of temperature (31-45 °C), particle concentration (< 2 vol %) and ultrasonication on the viscosity of sand-ethylene glycol-water dispersions was studied. The thermal conductivity of dispersions as a function of particle concentration and ultrasonication has also been investigated. Correlations were developed for the prediction of relative viscosity and thermal conductivity ratio of the dispersions. Our results indicate that the sand-ethylene glycol-water dispersions, prepared by stirred bead milling and ultrasonication, have potential as coolants.

Porous alumina and zirconia ceramics with tailored thermal conductivity

The thermal conductivity of porous ceramics can be tailored by slip casting and uniaxial dry pressing, using either fugitive pore formers (saccharides) or partial sintering. Porous alumina and zirconia ceramics have been prepared using appropriate powder types (ungranulated for casting, granulated for pressing) and identical firing regimes (but different maximum temperatures in the case of partial sintering). Thermal diffusivities have been measured by the laser- and xenon-flash method and transformed into relative thermal conductivities, which enable a temperature-independent comparison between different materials. While the porosity can be controlled in a similar way for both materials when using pore formers, partial sintering exhibits characteristic differences between alumina and zirconia (for alumina porosities below 45 %, full density above 1600 °C, for zirconia porosities below 60 %, full density above 1300 °C). The different compaction behavior of alumina and zirconia (porosity after pressing 0.465 and 0.597, respectively) is reflected in the fact that for alumina the relative conductivity data of partially sintered materials are below the exponential prediction, while for zirconia they coincide with the latter. Notwithstanding these characteristic differences, for both alumina and zirconia it is possible to tailor the thermal conductivity from 100 % down to approx. 15 % of the solid phase value.

Viscosity and thermal conductivity of dispersions of sub-micron TiO2 particles in water prepared by stirred bead milling and ultrasonication

Experiments were carried out on the preparation of dispersions of sub-micron TiO2 particles in water by stirred bead milling, for potential use as coolants. The prepared dispersions were characterized through the measurement of particle size distribution, zeta potential, viscosity and thermal conductivity. The effects of particle concentration (0.27–1.39vol%), ultrasonication time (0–7h) on viscosity and thermal conductivity have been studied. The effect of temperature (29–55°C) on viscosity has also been investigated. The results indicate that the ultrasonication can be utilized to tailor the transport properties of the sub-micron dispersions produced by stirred bead milling. The entire particle size distribution data has been utilized to develop correlations for prediction of relative viscosity and thermal conductivity ratio of these dispersions. These dispersions possess higher thermal conductivity than water and can also be utilized as coolants.

An Investigation of Treating Adiabatic Wall Temperature as the Driving Temperature in Film Cooling Studies

In film cooling heat transfer analysis, one of the core concepts is to deem film cooled adiabatic wall temperature (Taw) as the driving potential for the actual heat flux over the film-cooled surface. Theoretically, the concept of treating Taw as the driving temperature potential is drawn from compressible flow theory when viscous dissipation becomes the heat source near the wall and creates higher wall temperature than in the flowing gas. But in conditions where viscous dissipation is negligible, which is common in experiments under laboratory conditions, the heat source is not from near the wall but from the main hot gas stream; therefore, the concept of treating the adiabatic wall temperature as the driving potential is subjected to examination. To help investigate the role that Taw plays, a series of computational simulations are conducted under typical film cooling conditions over a conjugate wall with internal flow cooling. The result and analysis support the validity of this concept to be used in the film cooling by showing that Taw is indeed the driving temperature potential on the hypothetical zero wall thickness condition, i.e., Taw is always higher than Tw with underneath (or internal) cooling and the adiabatic film heat transfer coefficient (haf) is always positive. However, in the conjugate wall cases, Taw is not always higher than wall temperature (Tw), and therefore, Taw does not always play the role as the driving potential. Reversed heat transfer through the airfoil wall from downstream to upstream is possible, and this reversed heat flow will make Tw &gt; Taw in the near injection hole region. Yet evidence supports that Taw can be used to correctly predict the heat flux direction and always result in a positive adiabatic heat transfer coefficient (haf). The results further suggest that two different test walls are recommended for conducting film cooling experiments: a low thermal conductivity material should be used for obtaining accurate Taw and a relative high thermal conductivity material be used for conjugate cooling experiment. Insulating a high-conductivity wall will result in Taw distribution that will not provide correct heat flux or haf values near the injection hole.