Remarkable Reduction In Thermal Conductivity Research Articles

Insights and theoretical model of thermal conductivity of thermally damaged hybrid steel-fine polypropylene fiber-reinforced concrete

The use of hybrid steel-fine polypropylene fiber-reinforced concrete (HFRC) may be an effective strategy for avoiding concrete thermal spalling under fire loading. The thermal conductivity of HFRC is one key parameter for the fire resistance assessing. In this research, the thermal conduction behavior of HFRC is experimentally and numerically studied to obtain a thorough understanding on its thermal degradations and their mechanisms after high-temperature exposure. New mechanisms are discovered to explain the remarkable reduction of thermal conductivity. In addition to the effect of moisture loss and porosity increase, the combined effect of heat bridge and interfacial thermal resistance (ITR) is found to be the dominant mechanisms. Moreover, the evaporation of polypropylene fibers adhering to the coarse aggregates enhances the ITR effect. Based on these mechanisms, a multi-scale homogenization method is also proposed to evaluate the thermal conductivity of thermally damaged HFRC, whose accuracy are demonstrated by the excellent agreements between the experimental data and the numerical results.

Enhanced Electron Correlation and Significantly Suppressed Thermal Conductivity in Dirac Nodal-Line Metal Nanowires by Chemical Doping.

Enhancing electron correlation in a weakly interacting topological system has great potential to promote correlated topological states of matter with extraordinary quantum properties. Here, the enhancement of electron correlation in a prototypical topological metal, namely iridium dioxide (IrO2 ), via doping with 3d transition metal vanadium is demonstrated. Single-crystalline vanadium-doped IrO2 nanowires are synthesized through chemical vapor deposition where the nanowire yield and morphology are improved by creating rough surfaces on substrates. Vanadium doping leads to a dramatic decrease in Raman intensity without notable peak broadening, signifying the enhancement of electron correlation. The enhanced electron correlation is further evidenced by transport studies where the electrical resistivity is greatly increased and follows an unusual dependence on the temperature (T). The lattice thermal conductivity is suppressed by an order of magnitude via doping even at room temperature where phonon-impurity scattering becomes less important. Density functional theory calculations suggest that the remarkable reduction of thermal conductivity arises from the complex phonon dispersion and reduced energy gap between phonon branches, which greatly enhances phase space for phonon-phonon Umklapp scattering. This work demonstrates a unique system combining 3d and 5d transition metals in isostructural materials to enrich the system with various types of interactions.

Dispersion Methodology for Technical Lignin into Polyester Polyol for High-Performance Polyurethane Insulation Foam

The incorporation of lignin into rigid polyurethane foam (RPUF) has been explored for the last two decades for replacing petrochemical polyols and producing sustainable high-performance insulation materials. However, to date, the issues associated with the dispersion of technical lignin in the commonly used polyols for RPUF have highly limited the improvement in mechanical and thermal insulation performance. This study reports the enhanced dispersion of kraft lignin (KL) up to 75 wt % in the glycerol-substituted aromatic polyester polyol blend. The influence of significantly well-dispersed KL on RPUF in terms of loading levels, the viscosity of the polyol, the microstructure, and the thermal and mechanical properties of RPUF is discussed. The KL incorporated (0.5–6.0 wt %) in polyol afforded a remarkable reduction in thermal conductivity (32%–34%) of the resultant RPUF with minimal variation in density and insignificant change in compressive strength. The scale of this improvement, to the best of our knowledge, has not been reported to date in lignin-incorporated RPUF systems. Furthermore, the presence of the KL in the RPUF also resulted in a mild improvement in the flame retardance performance. This study provides insights into producing KL-incorporated RPUF for thermal insulation application.

High thermal conductivity in covalently bonded bi-layer honeycomb boron arsenide

Semiconducting materials with high thermal conductivity are valuable for thermal management in highly integrated systems, in which heat must be dissipated efficiently. Cubic boron arsenide (BAs) is recently attracting numerous research attention due to its comparable thermal conductivity with diamond. However, thermal conductivity of monolayer honeycomb BAs is much lower than the bulk counterpart. Herein, based on Boltzmann transport equation and non-equilibrium Green’s function calculation, we suggest a new unexplored covalently bonded bi-layer BAs to exhibit high thermal conductivity, which is about two times of that of bulk silicon. Unlike the remarkable reduction in thermal conductivity from monolayer graphene to bi-layer graphene, thermal conductivity of bi-layer BAs is about 70% higher than its monolayer counterpart. Central to this novel behavior was the largely suppressed phonon scattering phase space and anharmonicity. The anomalously high thermal conductivity of the covalently bonded bi-layer BAs and the emergent underlying mechanism discussed here open up space for further manipulation of thermal properties of 2D materials, and facilitate their applications to important problems in nanoscale electronic and optoelectronic devices.

Synergetic modulation of power factor and thermal conductivity for Cu3SbSe4-based system

Many methods have been focused on improving the thermoelectric performance of Cu3SbSe4 based materials. However, no effective way can simultaneously achieve the significant improvement of power factor and the remarkable reduction of thermal conductivity. In this work, we introduce AgSb0.98Sn0.02Se2 particles into Cu3Sb0.96Sn0.04Se4 matrix forming Cu3Sb0.96Sn0.04Se4/x wt%AgSb0.98Sn0.02Se2 (x = 0, 1, 3, 5, 7) nanocomposites by milling and spark plasma sintering technique. Due to the reduced thermal conductivity and enhanced carrier mobility, a record-high ZT value of 1.17 is obtained for the sample Cu3Sb0.96Sn0.04Se4/5wt%AgSb0.98Sn0.02Se2. The reduced thermal conductivity is attributed to the enhanced phonon scattering caused by the second phase AgSb0.98Sn0.02Se2 with multiscale size and the enhanced power factor originated from increased carrier mobility due to the reduced carrier concentration, which is caused by the new acceptor energy level related to the interfacial defects. Present results demonstrate that excellent thermoelectric performance can be realized in Cu3Sb0.96Sn0.04Se4 matrix with AgSb0.98Sn0.02Se2 particles embedded.

Ultra-low thermal conductivity of two-dimensional phononic crystals in the incoherent regime

Two-dimensional silicon phononic crystals have attracted extensive research interest for thermoelectric applications due to their reproducible low thermal conductivity and sufficiently good electrical properties. For thermoelectric devices in high-temperature environment, the coherent phonon interference is strongly suppressed; therefore phonon transport in the incoherent regime is critically important for manipulating their thermal conductivity. On the basis of perturbation theory, we present herein a novel phonon scattering process from the perspective of bond order imperfections in the surface skin of nanostructures. We incorporate this strongly frequency-dependent scattering rate into the phonon Boltzmann transport equation and reproduce the ultra low thermal conductivity of holey silicon nanostructures. We reveal that the remarkable reduction of thermal conductivity originates not only from the impediment of low-frequency phonons by normal boundary scattering, but also from the severe suppression of high-frequency phonons by surface bond order imperfections scattering. Our theory not only reveals the role of the holey surface on the phonon transport, but also provide a computation tool for thermal conductivity modification in nanostructures through surface engineering.

Remarkable reduction of thermal conductivity in graphyne nanotubes by local resonance

By using equilibrium molecular dynamics simulations and lattice dynamic calculations, we investigate the thermal transport properties of graphyne nanotubes (GNTs). The results demonstrate that the GNTs exhibit relatively low thermal conductivity, which is approximately 10–25% of that in a carbon nanotube. Detailed lattice dynamic analyses indicate that the presence of sp carbon atoms can induce local resonance modes in the low frequency region, which results in the remarkable decrease of thermal conductivity. Meanwhile, the high-frequency phonon lifetimes also drop because of the vibrational mismatch between sp and sp2 carbon atoms (enhanced phonon scattering). Our results show the local resonance of phonons is an important mechanism for the reduction of thermal conductivity in low-dimensional nanostructures.

Experimental and modeling investigation of the thermal conductivity of fiber-reinforced soil subjected to freeze-thaw cycles

The thermal conductivity of fine-grained soil, both unreinforced and reinforced with randomly oriented basalt, glass, and steel fibers, was tested by means of the transient hot-wire method with a Quickline-30 Thermal Properties Analyzer. The thermal conductivities of specimens were determined as a function of fiber volume fractions, freeze-thaw cycles, and temperature through laboratory studies. Thermal conductivity of the fiber-reinforced soil decreased for all freeze-thaw cycles and temperature values. The most remarkable reduction of thermal conductivity was measured on all ratios of the steel fiber-reinforced soil and 1% basalt fiber-reinforced soil. Moreover, the statistical-physical model proposed by Usowicz was applied to evaluate the thermal conductivity of fiber-reinforced soil by considering soil-fiber composites and environmental factors. The results showed a close match between the values estimated by the statistical-physical model and the experimental values for various fiber-reinforced soils in a wide range of fiber ratios, temperatures, water contents, and freeze-thaw cycles.

A novel chemical process of Bi2Te2.7Se0.3 nanocompound for effective adjustment in transport properties resulting in remarkable n-type thermoelectric performance

Chemical reaction processes for thermoelectric nanomaterials have received a lot of attention because of subsequent small and uniform nanostructures, causing a remarkable reduction in thermal conductivity; however, they typically showed low electrical properties possibly due to contaminations by chemicals resulting in low ZT. We devised a new chemical process for synthesizing an n-type Bi2Te2.7Se0.3 nanocompound without the chemicals causing potential contaminations. The product exhibited a predominant electrical conductivity while retaining the advantage of chemical reaction routes; thus, synergistic effect of the thermoelectric transport properties was greatly induced, resulting in the highest ZT among n-type Bi2Te3 materials in bulk phase.

Remarkable reduction of thermal conductivity in phosphorene phononic crystal

Phosphorene has received much attention due to its interesting physical and chemical properties, and its potential applications such as thermoelectricity. In thermoelectric applications, low thermal conductivity is essential for achieving a high figure of merit. In this work, we propose to reduce the thermal conductivity of phosphorene by adopting the phononic crystal structure, phosphorene nanomesh. With equilibrium molecular dynamics simulations, we find that the thermal conductivity is remarkably reduced in the phononic crystal. Our analysis shows that the reduction is due to the depressed phonon group velocities induced by Brillouin zone folding, and the reduced phonon lifetimes in the phononic crystal. Interestingly, it is found that the anisotropy ratio of thermal conductivity could be tuned by the ‘non-square’ pores in the phononic crystal, as the phonon group velocities in the direction with larger projection of pores is more severely suppressed, leading to greater reduction of thermal conductivity in this direction. Our work provides deep insight into thermal transport in phononic crystals and proposes a new strategy to reduce the thermal conductivity of monolayer phosphorene.

Effects of Lattice Defects and Niobium Doping on Thermoelectric Properties of Calcium Manganate Compounds for Energy Harvesting Applications

We have investigated the thermoelectric (TE) properties of Ruddlesden–Popper (RP) CaO(CaMnO3)mn-type compounds, to be applied for TE waste heat recovery at elevated temperatures. We prepared several Nb-doped and undoped CaO(CaMnO3)m compounds having different CaO planar densities by controlling the Ca content via solid-state reaction, and characterized the resulting microstructures by x-ray diffraction analysis and high-resolution scanning electron microscopy. The thermal conductivity, electrical conductivity, and TE thermopower of the different compounds were measured in the range from 300 K through 1000 K. We observed a remarkable reduction in thermal conductivity as a result of increasing the CaO planar density for the Nb-doped RP compounds, from a value of 2.9 W m−1 K−1 for m = ∞ down to 1.3 W m−1 K−1 for m = 1 at 1000 K. This trend was, however, accompanied by a corresponding reduction in electrical conductivity from 76 Ω−1 cm−1 to 2.9 Ω−1 cm−1, which is associated with electron scattering. Finally, we propose an approach that enables optimization of the TE performance of these RP compounds.

Reduced thermal conductivity in niobium-doped calcium-manganate compounds for thermoelectric applications

Reduction of thermal conductivity is essential for obtaining high energy conversion efficiency in thermoelectric materials. We report on significant reduction of thermal conductivity in niobium-doped CaO(CaMnO3)m compounds for thermoelectric energy harvesting due to introduction of extra CaO-planes in the CaMnO3-base material. We measure the thermal conductivities of the different compounds applying the laser flash analysis at temperatures between 300 and 1000 K, and observe a remarkable reduction in thermal conductivity with increasing CaO-planar density, from a value of 3.7 W·m−1K−1 for m = ∞ down to 1.5 W·m−1K−1 for m = 1 at 400 K. This apparent correlation between thermal conductivity and CaO-planar density is elucidated in terms of boundary phonon scattering, providing us with a practical way to manipulate lattice thermal conductivity via microstructural modifications.

Reduce thermal conductivity by forming a nano-phononic crystal on a Si slab

Due to the strong interaction with phonons, nano-phononic crystals (NPC) have attracted much consideration in the study of novel thermoelectric materials. Using a long wavelength approximation, we demonstrate that the dispersions of phonons in NPC can be extracted from the corresponding elastic wave dispersion. A modified plane-wave expansion method is utilized to calculate the dispersion of phonons in Si-based NPC with mesoscopic holes for nearly the entire spectrum. In addition, the Callaway-Holland model is used to analyze the thermal transportation properties of such Si-based NPC. We show more than fifty times reduction of thermal conductivity in NPCs compared with that in bulk silicon, which results in a considerable improvement in the thermoelectrical efficiency. We attribute the remarkable reduction in thermal conductivity to the participation of both coherent and incoherent scatterings of phonons.

Low thermal conductivity in ultrathin carbon nanotube (2, 1).

Molecular dynamic simulations reveal that the ultrathin carbon nanotube (CNT) (2, 1) with a reconstructed structure exhibits a surprisingly low thermal conductivity, which is only ~16–30% of those in regular CNTs, e.g. CNT (2, 2) and (5, 5). Detailed lattice dynamic calculations suggest that the acoustic phonon modes greatly soften in CNT (2, 1) as compared to regular CNTs. Moreover, both phonon group velocities and phonon lifetimes strikingly decrease in CNT (2, 1), which result in the remarkable reduction of thermal conductivity. Besides, isotope doping and chemical functionalization enable the further reduction of thermal conductivity in CNT (2, 1).

Extraordinary electron and phonon transport through metal-semiconductor hybrid nanocomposite: decoupling electrical and thermal conductivities for thermoelectric application

We report a simple method involving mechanical attrition followed by hot press sintering to fabricate tablet samples consisting of silver nanoparticles (Ag NPs) dispersed in luminescent silicon nanocrystal (Si NC) matrix. A dramatic increase in current by almost six orders of magnitude is observed for this type of Si NC-Ag NP hybrid nanocomposite compared with bare Si NC-based tablets. Collective charge transport through such hybrid system exhibits unexpected and fascinating characteristics. On the other hand, a remarkable reduction in thermal conductivity by two orders of magnitude is observed in the Si NC-Ag NP nanocomposite compared with the bulk counterpart. This is due to the combined effect of reduction in thermal conductivity of individual NPs and enhanced phonon scattering at the interface. We have, therefore, successfully decoupled electrical and thermal conductivity, which is not possible in any bulk material. The huge increase in electrical conductivity with substantial reduction in thermal conductivity makes this hybrid system amenable for designing efficient thermoelectric with high thermoelectric figure of merit, which has remained a significant challenge.

Remarkable thermal conductivity reduction in metal-semiconductor nanocomposites

Remarkable reduction in thermal conductivity, by ∼2 orders of magnitude compared to the bulk counterpart, is observed in a metal-semiconductor nanocomposite consisting of silver (Ag) and silicon (Si) nanostructures. The variation of thermal conductivity with temperature and with volume fraction of metallic inclusion exhibits counter-intuitive behavior. Contrary to bulk composites, thermal conductivity decreases with the increase in the volume fraction of Ag nanocrystals (at least till 0.067 experimented) and increases with temperature over the range of 303-473 K. This remarkable reduction in the thermal conductivity of the nanocomposite is due to the interplay of size-dependent reduction in thermal conductivity of the individual nanostructures, increased contribution of phonon scattering at the interfaces between nanoparticles, and electron-phonon coupling inside metallic nanocrystals and across metal-semiconductor interface. Such hybrid metal-semiconductor nanostructures with reduced thermal conductivity offer immense potential for developing high efficiency thermoelectric materials.

Effects of nano-TiO2 dispersion on thermoelectric properties of Co4Sb11.7Te0.3 composites

Nano-TiO2/Co4Sb11.7Te0.3 composites were prepared by mechanical alloying (MA) and cold isostatic pressing (CIP) process. The phase composition, microstructure, and thermoelectric properties were characterized. The diffraction spectra of all samples well corresponds to CoSb3 skutterudite diffraction plane. TiO2 agglomerates into irregular clusters. They locate at the grain boundaries or some are distributed on the surface of Co4Sb11.7Te0.3 particles. For composites with high TiO2 content (0.6% and 1.0% TiO2), the phonon scattering by TiO2 particle, pores, and small size grains can result in a remarkable reduction in thermal conductivity. The maximum value of ZT is 0.79 for sample with 0.6 wt.% TiO2 at 700 K, which is 11% higher than that of non-dispersed sample.

Preparation and thermoelectric properties of AgPb 18SbTe 20− xSe x ( x = 1, 2, 4) materials

AgPb 18SbTe 20− x Se x ( x = 1, 2, 4) bulk materials were prepared by combining hydrothermal synthesis and melting. Thermoelectric properties were measured from room temperature up to 773K. The materials showed n-type conduction and exhibited degenerate semiconductor behavior. The power factors of the materials varied greatly with increase of Se content ( x). Partial substitution of Se for Te in AgPb 18SbTe 20 resulted in remarkable reduction of thermal conductivity in the whole temperature range and increase of power factor at lower temperatures; therefore, the dimensionless figure of merit, ZT, was enhanced below 600K. A maximum ZT value of ∼0.82 is obtained at 523K for the AgPb 18SbTe 18Se 2 sample.

Remarkable Reduction of Thermal Conductivity in Silicon Nanotubes

We propose to reduce the thermal conductivity of silicon nanowires (SiNWs) by introducing a small hole at the center, i.e., construct a silicon nanotube (SiNT) structure. Our numerical results demonstrate that a very small hole (only 1% reduction in cross section area) can induce a 35% reduction in room temperature thermal conductivity. Moreover, with the same cross section area, thermal conductivity of SiNT is only about 33% of that of SiNW at room temperature. The spatial distribution of vibrational energy reveals that localization modes are concentrated on the inner and outer surfaces of SiNTs. The enhanced surface-to-volume ratio in SiNTs reduces the percentage of delocalized modes, which is believed to be responsible for the reduction of thermal conductivity. Our study suggests SiNT is a promising thermoelectric material with low thermal conductivity.