Significant Decrease In Thermal Conductivity Research Articles

Heat conduction in extended X-junctions of single-walled carbon nanotubes

Non-equilibrium molecular dynamics (NEMD) simulations are employed to investigate the longitudinal thermal conductivity of non-orthogonal extended X-junction (EX-junction) of single-walled carbon nanotubes (SWCNTs). Different from standard junctions of SWCNTs, two distinct jumps in the temperature profile around the EX-junction are observed, which are responsible for the larger temperature gradient and reduction in thermal conductivity when compared to standard X-junction. Quantum corrected results show that the longitudinal thermal resistance of the X-junction and EX-junction decreases monotonically with increasing temperature which makes the longitudinal thermal conductivity of the tube with junction less sensitive to temperature above 400K comparing with the individual pristine tube. The origin of the significant decrease of thermal conductivity of EX-junction is discussed through phonon spectra analysis.

Several properties of mineral admixtured lightweight mortars at elevated temperatures

SUMMARYThe improvement of thermal and mechanical properties of mortars including expanded perlite aggregate (EPA) containing either clinoptilolite, a type of natural zeolite (NZ), waste glass powder (GP) or blast furnace slag (BFS) cured at elevated temperature was analyzed using thermal conductivity, compressive strength, flexure strength and dry unit weight. EPA mortar specimens were prepared by replacing a varying part of the portland cement with the above minerals. All mortar samples were prepared and cured at 23±1°C lime saturated water for 28 days. The maximum thermal conductivity of 1.3511W/mK was determined with the control samples containing plain cement. GP has shown 1 and 4% decrease for both 10, 20% GP and 25% EPA, respectively. Both BFS and NZ have a decreasing effect on thermal conductivity. The experiments were carried out, in which the samples were subjected to temperature of 300, 500 and 800°C for 2 h, then cooled in air. The results indicated that all the mortars exposed to temperature of 500 and 800°C shown a significant decrease in thermal conductivity, compressive strength and flexure strength. However, compared with the mortars including 25% EPA, adding the other admixtures at all level replacement decreased thermal conductivity, compressive strength, flexure strength and dry unit weight as a function of replacement percent. Copyright © 2010 John Wiley & Sons, Ltd.

Enhanced thermoelectric properties of bulk TiNiSn via formation of a TiNi2Sn second phase

The effect of phase-segregated Heusler TiNi2Sn on high temperature thermoelectric properties of bulk half-Heusler TiNiSn has been studied. In samples expressed by the composition TiNi1+xSn, a significant decrease in thermal conductivity (between 10% and 30%) is observed for two-phase TiNi1.15Sn, despite the second-phase particles existing at the micrometer scale; a 50% increase in the electrical conductivity is also measured. These result in a maximum figure of merit, ZT, of 0.44 at 800 K, which is 25% greater than is observed for the x = 0 sample. Density functional calculations of TiNiSn and TiNi2Sn suggest that the latter should deplete carriers at 0 K.

Comparative Evaluation of Thermal Conductivity of Zirconia Solid and Honeycomb Structures

Porous zirconia ceramic monoliths have been extensively used in thermo-structural applications due to their inherent low thermal conductivity in combination with their adaptability to form complicated shapes through advanced ceramic processing techniques. However, extruded cellular honeycomb structures made from these materials have been less explored for thermal management applications. There exist large potential applications due to their unique configurations, resulting in better heat-management mechanisms. Some of the studies carried out on zirconia honeycombs are safeguarded through patents due to its technical importance, or the information is not in the public domain. In the present study, for the sake of comparison, honeycomb specimens with varying wall thicknesses and unit cell lengths maintaining almost same bulk density of around 90% theoretical and relative density of 0.34–0.37 were prepared and subjected to thermal conductivity evaluation along with the solid samples with relative density of 1.0 using monotonic heating regime methodology. In addition, the effect of channel shape was also evaluated using square and triangular channeled honeycombs with the same relative densities. The results obtained from these specimens were correlated with their configurations to bring out the advantages accrued by using the honeycomb with these configurations. It was observed that a significant decrease in thermal conductivity was achieved in honeycombs, which can be attributed to the behavior of various heat transfer mechanisms that are operative at high temperatures in combination with the considerable reduction in thermal mass and the consequent conduction through the solids.

Thermoelectric properties of Ba8Ga16Ge30 with TiO2 nanoinclusions

The effects on thermal and electrical properties of adding small amounts of TiO2 nanoinclusions to bulk Ba8Ga16Ge30 clathrate have been investigated. The thermal properties were analysed using the transient plane source technique and the analysis showed a significant decrease in thermal conductivity as the volume fraction of TiO2 increased from 0 vol. % to 1.2 vol. %. The introduction of TiO2 nanoparticles caused a shift in the peak value of the Seebeck coefficient towards lower temperatures. The maximum value of the Seebeck coefficient was, however, only little affected. The introduction of TiO2 nanoparticles into the bulk Ba8Ga16Ge30 resulted in an increased electrical resistivity of the sample, thus simultaneously reducing the charge carrier contribution to the thermal conductivity, partly explaining the decrease in total thermal conductivity. Due to the large increase in resistivity of the samples, ZT was only somewhat improved for the material with 0.4 vol. % TiO2 while the ZT values of the other materials were lower than for the reference Ba8Ga16Ge30 material without TiO2 nanoparticles. The combined results are consistent with a scenario where the nanoparticle introduction causes a light doping of the semiconductor matrix and an increased concentration of phonon scattering centres.

Thermal conductivity of (Er1−xYx)2Ti2O7pyrochlore oxide solid solutions

The thermal conductivities of pyrochlore oxide solid solutions of general formula (Er${}_{1\ensuremath{-}x}$Y${}_{x}$)${}_{2}$Ti${}_{2}$O${}_{7}$ with 0 \ensuremath{\leqslant} $x$ \ensuremath{\leqslant} 1 have been determined for high-quality single crystals aligned along the [110] direction, over the temperature range from 3 to 300 K. Er${}_{2}$Ti${}_{2}$O${}_{7}$ and Y${}_{2}$Ti${}_{2}$O${}_{7}$ are isostructural and Er${}^{3+}$ and Y${}^{3+}$ are within 1$%$ in size, but differ by a factor of about 2 in mass. Therefore, this system allows a clear test of the influence of mass of dopant on thermal conductivity, while controlling for other factors such as dopant size and sample purity. Although Y${}_{2}$Ti${}_{2}$O${}_{7}$ has a higher thermal conductivity than Er${}_{2}$Ti${}_{2}$O${}_{7}$ at $T$ $=$ 300 K, from 3 to 200 K their relative thermal conductivities reverse. Furthermore, we observe significant decrease in thermal conductivity upon doping Er${}_{2}$Ti${}_{2}$O${}_{7}$ with Y${}^{3+}$ ions, showing definitively that, in the temperature range from about 3 to 300 K, the impurity scattering effect of the lighter Y${}^{3+}$ ions is the predominant limiter of the thermal conductivity. This conclusion is supported by the finding that the phonon mean free path of the doped compounds decreases with increased dopant concentrations, increasing again as pure Y${}_{2}$Ti${}_{2}$O${}_{7}$ is approached.

Enhancement of geothermal borehole heat exchangers performances by improvement of bentonite grouts conductivity

Abstract Conventional boreholes used as vertical ground heat exchangers are significantly limited in thermal performances by the bentonite grouts thermal conductivity. Bentonite-graphite composites have been elaborated using Graphite Flakes, Expanded Natural Graphite (ENG) and ground Compressed Expanded Natural Graphite CENGg particles. Using graphite flakes, the grouts thermal conductivity enhancements are similar to those already published in previous studies while using CENGg particles, the expected effective thermal conductivity of 5 W m −1 K −1 was obtained at reduced graphite content (5% wt). Partial drying of the grouts induces significant decrease in thermal conductivity in the range of 1 W m −1 K −1 for 10% wt of water content reduction. This inhibiting effect has to be taken into account for geothermal heat storage using borehole based systems.

Enhanced Thermoelectric Figure of Merit of p-Type Half-Heuslers

Half-Heuslers would be important thermoelectric materials due to their high temperature stability and abundance if their dimensionless thermoelectric figure of merit (ZT) could be made high enough. The highest peak ZT of a p-type half-Heusler has been so far reported about 0.5 due to the high thermal conductivity. Through a nanocomposite approach using ball milling and hot pressing, we have achieved a peak ZT of 0.8 at 700 °C, which is about 60% higher than the best reported 0.5 and might be good enough for consideration for waste heat recovery in car exhaust systems. The improvement comes from a simultaneous increase in Seebeck coefficient and a significant decrease in thermal conductivity due to nanostructures. The samples were made by first forming alloyed ingots using arc melting and then creating nanopowders by ball milling the ingots and finally obtaining dense bulk by hot pressing. Further improvement in ZT is expected when average grain sizes are made smaller than 100 nm.

Thermal conductivity of PVD TiAlN films using pulsed photothermal reflectance technique

In the present work, we have measured thermal-conductivity of industrial thin film TiAlN with a thickness of around 3 μm. These films are used in machining industry for cutting tools in order to increase their service life. A series of TiAlN coating with a different Al/Ti atomic ratio were deposited on Fe-304 stainless steel (AISI304) substrate by a lateral rotating cathode arc process. The samples were then coated with a 0.8 μm gold layer on top by magnetron sputtering. We present the thermal-conductivity measurement of these samples using pulsed photothermal reflectance (PPR) technique at room temperature. The thermal conductivity of the pure TiN coating is about 11.9 W/mK. A significant decrease in thermal conductivity was found with increasing Al/Ti atomic ratio. A minimum thermal conductivity of about 4.63 W/mK was obtained at the Al/Ti atomic ratio of around 0.72.

Thermal Conductivity of Nickel Oxide Nanoparticles Synthesized by Combustion Method

AbstractMonodispersed nickel oxide nanoparticles have been synthesized using solution combustion synthesis method. Size of the nanoparticles was controlled by varying different reaction parameters such as reaction temperature and reaction time. Structure and morphology of the nanoparticles were investigated using X-ray diffraction and transmission electron microscopy. BET surface area of 99.7 m2/g was obtained for the nanoparticles synthesized at 300 °C. A decrease in surface area was observed with increase in reaction temperature. The nanoparticles were compacted using spark plasma sintering technique at 950 °C and thermal conductivity was studied on compacted sample. Significant decrease in thermal conductivity was observed for nanoparticles in compared to their bulk counter-part.

Detection and analysis of subsurface cracks in ground ceramics using thermal wave method

During a grinding of ceramics, its superficial layer undergoes structural changes. It results in significant decrease of thermal conductivity of surface. In this paper, a method of quality assessment of ceramics grinding based on analysis of depth profile of thermal properties using thermal wave methods is presented. The method proposed permits to determine a thickness as well as thermal conductivity of damaged layer. It enables indirect optimisation of machining of ceramics' surface.

Thermal properties of neutron-irradiated SiC; effects of boron doping

The temperature dependence (25°C to 1000°C) of thermal conductivity for siliconized (reaction bonded) SiC and alpha phase (sintered) SiC irradiated to neutron fluences of 4 to 8 × 10 24 n/ m 2 ( E>1 MeV) were studied utilizing the heat pulse technique. The fluences are equivalent to 0.8 and 1.6 dpa and the sample temperature during irradiation was ~ 140°C. Silicon carbide exhibits a significant decrease in thermal conductivity after irradiation, specifically a factor of ~ 5 decrease is observed for siliconized SiC. Comparisons were made with SiC samples doped with 10B, 11B, and natural boron to investigate the effects of impurity doping. It was found that the presence of natural boron and 11B have no significant effect on the thermal conductivity of irradiated SiC, whereas SiC doped with 10B exhibits a slightly larger decrease in thermal conductivity due to the enhanced radiation damage (e.g., helium production) through the 10B (n, α) 7Li reaction. The lowering of the thermal conductivity after irradiation can explain the decreased resistance against thermal shcok of irradiated SiC. The decrease in thermal conductivity is due to enhanced phonon scattering by radiation-induced vacancies and dislocations. Results on annealing effects and comparison with mechanical properties are presented.