Room Temperature Thermal Conductivity Research Articles

Thermoelectric Properties of Ge-doped Cu3SbSe4

ABSTRACTTernary variations of the II-VI zincblende semiconductors have received little attention for thermoelectric applications. Here we present the first systematic doping study on Cu3SbSe4, a zincblende-like ternary semiconductor with a unit cell four times larger than the parent II-VI compounds. The large unit cell of Cu3SbSe4results in a low room temperature thermal conductivity (~3.0 W/m*K) and its large hole effective mass produces a Seebeck coefficient approaching 500 μV/K in the undoped compound. Our results show that Ge is an effective p-type dopant in Cu3SbSe4, and the power factor reaches nearly 16 μW/cm*K2at 630K when 3% Ge is added, rivaling that of state-of-the-art thermoelectric materials at this temperature.

Investigation into the thermoelectric properties of GaSb/InAs superlattice structures

ABSTRACTWe report on our investigation into the use of III-V superlattice structures for thermoelectric (TE) applications. Preliminary review of III-V materials trends indicate that the GaSb/InAs superlattice system should offer one of the best potentials for high thermoelectric performance in the 500K-800K range. MOCVD growth of GaSb/InAs superlattice structures was carried out, and relevant structural, thermal, and electrical characterization has been performed. TEM and XRD results demonstrate a well-ordered superlattice structure. Thermal conductivity measurements reveal a reduction in the room-temperature thermal conductivity of GaSb/InAs superlattices (4.4-10.0 W/m-K), relative to either binary GaSb (32 W/m-K) or InAs (27 W/m-K). Additionally, we have worked to optimize the thermoelectric power factor (α2σ), studying both Se- and Te-doping of the superlattice structures, in an effort to demonstrate optimal thermoelectric performance. Our results demonstrate a maximum ZT of 0.36 at 400K for optimally doped n-type GaSb/InAs superlattice structures.

Effect of single-walled carbon nanotubes on thermal and electrical properties of silicon nitride processed using spark plasma sintering

Si 3N 4 nanocomposites reinforced with 1-, 2-, and 6-vol% single-walled carbon nanotubes (SWNTs) were processed using spark plasma sintering (SPS) in order to control the thermal and electrical properties of the ceramic. Only 2-vol% SWNTs additions were used to decrease the room temperature thermal conductivity by 62% over the monolith and 6-vol% SWNTs was used to transform the insulating ceramic into a metallic electrical conductor (92 S m −1). We found that densification of the nanocomposites was inhibited with increasing SWNT concentration however, the phase transformation from α- to β-Si 3N 4 was not. After SPS, we found evidence of SWNT survival in addition to sintering induced defects detected by monitoring SWNT peak intensity ratios using Raman spectroscopy. Our results show that SWNTs can be used to effectively increase electrical conductivity and lower thermal conductivity of Si 3N 4 due to electrical transport enhancement and thermal scattering of phonons by SWNTs using SPS.

Thermal conductivity and Seebeck coefficients of icosahedral boron arsenide films on silicon carbide

The thermal conductivity of icosahedral boron arsenide (B12As2) films grown on (0001) 6H–SiC substrates by chemical vapor deposition was studied by the 3ω technique. The room temperature thermal conductivity decreased from 27.0 to 15.3 W/m K as the growth temperature was decreased from 1450 to 1275 °C. This is mainly attributed to the differences in the impurity concentration and microstructure, determined from secondary ion mass spectrometry and high resolution transmission electron microscopy, respectively. Callaway’s theory was applied to calculate the temperature-dependent thermal conductivity, and the results are in good agreement with the experimental data. Seebeck coefficients were determined as 107 μV/K and 136 μV/K for samples grown at 1350 °C with AsH3/B2H6 flow ratio equals to 1:1 and 3:5, respectively.

Preparation and laser performance of Nd-doped yttrium lanthanum oxide transparent ceramic

1.5 at.% Nd:Y 1.8La 0.2O 3 transparent ceramic was fabricated by a solid-state reaction method and sintered at 1650–1700 °C for 40–50 h under H 2 atmosphere. The spectroscopic properties were investigated at room temperature. The transparent ceramic has excellent spectroscopic properties, with the absorption cross section of 1.50 × 10 −20 cm 2 and broad full width at half maximum (FWHM) of about 8 nm at LD wavelength 806 nm, the emission cross section of 2.03 × 10 −20 cm 2 at 1079 nm, and the decay lifetime of 200 μs. Laser performance was carried out using an uncoated Nd:Y 1.8La 0.2O 3 ceramic plate under laser diode end-pumping without any water cooling device. The room temperature thermal conductivity of this ceramic is 6.20 W/mK. For Nd:Y 1.8La 0.2O 3 ceramic laser, a maximum output power of 62 mW was obtained at 1079 nm under a 808 nm diode pump.

Fabrication of Microdevices with Integrated Nanowires for Investigating Low-Dimensional Phonon Transport

Phonons in low-dimensional structures with feature sizes on the order of the phonon wavelength may be coherently scattered by the boundary. This may give rise to a new regime of heat conduction, which can impact thermal energy transport and conversion. Traditional methods used to investigate phonon transport in one-dimensional structures suffer from uncertainty due to contact resistance, defects, and limited control over sample dimensions. We have developed a new batch-fabrication technique for suspended microdevices with integrated silicon nanowires from silicon-on-insulator (SOI) wafers. The nanowires are defect-free and have extremely high aspect ratios (length/critical dimension >2000). The nanowire dimensions (length and critical dimension) can be precisely controlled during fabrication. With these novel devices, phonon transport in silicon nanowires is systematically investigated. The room temperature thermal conductivity of nanowires with critical width around 80 nm is about 20 W/(m K) and much lower than that in smooth VLS wires. This suggests that the surface morphology of the structures has a significant effect on the thermal conductivity, but this phenomenon is not currently understood. This fabrication technique can also be used for thermal transport investigation in a wide-range of low-dimensional structures.

Embedded Ag-rich nanodots in PbTe: Enhancement of thermoelectric properties through energy filtering of the carriers

The concept of energy filtering of the carriers to control the thermoelectric properties of PbTe is experimentally applied in this present work. The energy barriers at the grain interfaces of the nanocomposites and the embedded Ag-rich nanodots within the grains are supposed to control the energy dependency of carrier scattering: that is what we mean by energy filtering of carriers. As a case study, vertical Bridgman grown bulk PbTe:undoped, PbTe:Ag crystals and nanocomposites of PbTe:Ag are used as samples. Thermoelectric properties of all the samples have been evaluated through temperature dependent electrical conductivity, Seebeck coefficient and room temperature Hall and thermal conductivity measurements. It is found that the PbTe:Ag nanocomposites has the highest power factor of 18.78×10−4 W m−1 K−2 with a room temperature thermal conductivity of 1.69 W m−1 K−1. The crystal structures of these samples show the effective potential barrier at the grain boundaries and embedded nanodots within the grains to facilitate the energy filtering of the carriers.

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.

Control of pore channel size during freeze casting of porous YSZ ceramics with unidirectionally aligned channels using different freezing temperatures

Porous yttria-stabilized zirconia (YSZ) ceramics with unidirectionally aligned pore channels were prepared by freezing YSZ/tert-butyl alcohol (TBA) slurry under different freezing temperatures of −30, −78 and −196 °C, respectively. After removing the frozen TBA via freeze-drying in vacuum at −50 °C, the green samples were sintered at 1450 °C for 2 h in air. The results showed that the freezing temperature significantly influenced microstructure and properties of the porous YSZ ceramics. Both microstructure observation and pore size distribution indicated that the pore channel size decreased significantly with decreasing freezing temperature, regardless of microstructure variations in the individual sample. Both porosity and room-temperature thermal conductivity of the porous YSZ ceramics varied under different freezing temperatures. Regardless of microstructure variations in the samples under different freezing temperatures, all samples had unidirectional pore channels with increasing pore channel size along the freezing direction. The fabricated samples had remarkably low thermal conductivities both in directions perpendicular and parallel to the channel direction, thus rendering them suitable for applications in thermal insulations.

Porous yttria-stabilized zirconia ceramics with ultra-low thermal conductivity. Part II: temperature dependence of thermophysical properties

This study describes the experimental results of thermal diffusivity, specific heat at constant pressure, and thermal conductivity of porous 8 mol% yttria-stabilized zirconia (YSZ) ceramics in a temperature range from room temperature to 1,400 °C. It is a follow-up study of the earlier report titled by “Porous YSZ ceramics with ultra-low thermal conductivity”, which focused on the room-temperature thermal conductivity. The thermal diffusivity of porous YSZ ceramics decreased with the increase of the measurement temperature up to 600–1,000 °C, followed by an increasing trend with increasing temperature. The specific heat did not exhibit any significant dependence on sintering temperature and agreed with literature data. The thermal conductivity of the porous YSZ ceramics showed an insensitive tendency of change with measurement temperature. The thermal conductivity fell in groups by the sintering temperature level. This investigation also discussed an appropriate sintering temperature of porous YSZ ceramics, which had both low thermal conductivity and high strength required by the practical service.

Lattice thermal conductivity of freestanding gallium nitride nanowires

We report detailed calculations of the lattice thermal conductivity of freestanding gallium nitride (GaN) nanowires with diameters ranging from 20 to 140 nm. Results are compared with experimental data on GaN nanowires grown by thermal chemical vapor deposition (CVD). Calculations are based on the Boltzmann transport equation and take into account the change in the nonequilibrium phonon distribution in the case of diffuse scattering at the surfaces. Phonon dispersion relation is obtained in the elastic continuum approximation for each given nanowire. For valid comparisons with the experimental data, simulations are performed with a dopant concentration and impurity profile characteristic of thermal CVD GaN nanowires. Our results show that the room-temperature thermal conductivity of the nanowires has very low values, ranging from 6.74 W/m K at 20 nm to 16.4 W/m K at 140 nm. The obtained results are in excellent agreement with the experimental data. We have also demonstrated that in addition to impurity scattering, boundary scattering, and phonon confinement, the change in the nonequilibrium phonon distribution leads to a further reduction in the thermal conductivity of the nanowires and has to be taken into account in the calculations. Our conclusion is different from that of an earlier study which attributed the very low thermal conductivity to the unusually large mass-difference scattering in the nanowires.

Theory of the lattice thermal conductivity in bulk and films of GaN

We report on a systematic theoretical investigation of the lattice thermal conductivity of several GaN samples (bulk and films) over a wide range of temperature, by applying Callaway's relaxation-time theory in its full form and Srivastava's scheme for anharmonic three-phonon scattering processes. The role of the usually neglected three-phonon normal-drift term has been quantified. We have attempted to quantify the role of phonon scattering by various defects and imperfections, present in the film samples, in controlling the temperature dependence of thermal conductivity. We find that except for the purest sample, the phonon-impurity scattering plays a significant role in controlling the thermal conductivity of GaN not only around the thermal-conductivity peak region but also over a very large range of temperature. It can also be predicted from our numerical study, and with available experimental results, that the highest possible thermal conductivity of bulk GaN can only be realized when point impurities such as oxygen and silicon are in small concentration ($\ensuremath{\simeq}{10}^{16}\text{ }{\text{cm}}^{\ensuremath{-}3}$ or less) and other defects are either absent (from experimental study) or present in very small concentration (our numerical study). The highest value of the room-temperature thermal conductivity is achieved for samples grown by the high-temperature and high-pressure growth technique.

Dimensional crossover of thermal transport in few-layer graphene

Graphene, in addition to its unique electronic and optical properties, revealed unusually high thermal conductivity. The fact that thermal conductivity of large enough graphene sheets should be higher than that of basal planes of bulk graphite was predicted theoretically by Klemens. However, the exact mechanisms behind drastic alteration of material's intrinsic ability to conduct heat as its dimensionality changes from 2-D to 3-D remain elusive. Recent availability of high-quality few-layer graphene materials allowed us to study dimensional crossover experimentally. Here we show that the room-temperature thermal conductivity changes from K~3000 W/mK to 1500 W/mK as the number of atomic plains in few-layer graphene increases from 2 to 4. We explained the observed evolution from 2-D to bulk by the cross-plane coupling of the low-energy phonons and corresponding changes in the phonon Umklapp scattering. The obtained results shed light on heat conduction in low-dimensional materials and may open up few-layer graphene applications in thermal management of nanoelectronics.

Thermal Transport in Suspended and Supported Monolayer Graphene Grown by Chemical Vapor Deposition

Graphene monolayer has been grown by chemical vapor deposition on copper and then suspended over a hole. By measuring the laser heating and monitoring the Raman G peak, we obtain room-temperature thermal conductivity and interface conductance of (370 + 650/-320) W/m K and (28 + 16/-9.2) MW/m(2) K for the supported graphene. The thermal conductivity of the suspended graphene exceeds (2500 + 1100/-1050) W/m K near 350 K and becomes (1400 + 500/-480) W/m K at about 500 K.

Temperature dependent thermal conductivity of polycrystalline ZnO films

We report on measurements of the thermal conductivity of atomic layer deposited polycrystalline zinc oxide (ZnO) films between 30 and 300 K. The room temperature thermal conductivity of the films is found to be twofold less than that of single-crystal ZnO. The maximum thermal conductivities of the polycrystalline samples occur at about 60 K and their values are almost an order of magnitude lower than bulk ZnO. The effect of the grain size and the transparency of the intergrain boundary on the thermal conductivity are analyzed within the framework of the phonon-hopping transport model.

Low temperature liquid-phase-assisted sintering of Si3N4 ceramics as an inert matrix for confinement of minor actinides

Silicon nitride ceramics is a candidate for inert matrix, since it is very tolerant for neutron irradiation and it has relatively high thermal conductivity. For these reasons, Si3N4 ceramics containing large amounts of CeO2, as a simulant of transuranium elements, were manufactured by sintering. The low-temperature sintering behavior and chemical and thermal properties of the obtained ceramics are reported. CeO2 (16 or 30 mass%), SiO2 (3 mass%) and MgO (5 mass%) were mixed with fine Si3N4 powder. The mixed powder was uniaxially pressed into a pellet under 60 MPa. These pellets were sintered at 1400~1750°C for 2~4 h in flowing N2 (2 L/min). The pellet density rapidly increased after the sintering between 1400 and 1430°C in the case of 4 h keeping and bulk density of 3.34 g/cm3 was attained at 1450°C (Relative density ~95%), in the case of 16mass% CeO2. Room temperature thermal conductivity increased with sintering temperature from ~10 to ~40W/mK after sintering at 1450°C and 1650°C, respectively. High temperature thermal conductivity of sintered ceramics at 1650°C was approximately four times as high as that of UO2 up to 800°C. The dissolution rate of the sintered pellets for 3mol/L HNO3 solution at 80°C was measured. After 200 h, almost 25% of grain boundary phase was eluted from the pellet containing 16mass% CeO2.

Preparation and thermoelectric properties of sintered type-I clathrates K8GaxSn46−x

Polycrystalline clathrate samples of nominal K(8)Ga(x)Sn(46-x) were prepared by the spark plasma sintering method to investigate their crystal structures, mobilities and thermoelectric properties. The samples almost had a single-phase type-I clathrate structure, and their relative densities reached as high as 98%. The room-temperature mobility of the K(8)Ga(8)Sn(38) sample was 25 cm(2) V(-1) s(-1), which substantially exceeded the reported mobilities of Rb- and Cs-containing Sn clathrates. Moreover, the mobility was larger than those of the type-I Ba(8)Ga(16)Sn(30), which had more Ga substituting atoms in unit cell. A higher mobility might accordingly be achieved in thermoelectric clathrates with a smaller number of substituting atoms. Their electrical conductivities and Seebeck coefficients in the temperature range 300-450 K were typical of n-type doped semiconductors in the extrinsic region while their room-temperature lattice thermal conductivities were as low as approximately 11 mW cm(-1) K(-1). The maximum dimensionless figure of merit ZT was estimated to be 0.27 at 490 K from the Seebeck coefficient of -262 microV K(-1) and the electrical conductivity of 96 S cm(-1) for the K(8)Ga(8)Sn(38) sample with a carrier concentration of 2.9 x 10(19) cm(-3).

Thermal conductivity of Ni, Co, and Fe-doped La 5Ca 9Cu 24O 41 thin films measured by the 3ω method

The effect of Ni-, Co- and Fe-substitution for Cu on the thermal conductivity of La 5Ca 9Cu 24O 41 thin films is investigated. Highly b-axis oriented polycrystalline films were grown onto MgO (100) substrates using the pulsed laser deposition technique. Thermal conductivity measurements were made between 90 K and 300 K using the dynamic 3ω method. The room-temperature thermal conductivity of 1% Ni:La 5Ca 9Cu 24O 41 and 1% Co:La 5Ca 9Cu 24O 41 is approximately 15% smaller than the thermal conductivity of pristine La 5Ca 9Cu 24O 41 thin films of similar thickness. On the other hand, the thermal conductivity of 1% Fe:La 5Ca 9Cu 24O 41 is approximately the same as the thermal conductivity of pristine La 5Ca 9Cu 24O 41 thin films of similar thickness. These results enable us to indirectly conclude that Ni 2+ and Co 2+ ions substitute Cu 2+ ions in the crystal structure while Fe ions are not incorporated in the lattice.

Thermal Properties of Ultrathin Hafnium Oxide Gate Dielectric Films

Thin-film HfO2 is a promising gate dielectric material that will influence thermal conduction in modern transistors. This letter reports the temperature dependence of the intrinsic thermal conductivity and interface resistances of 56-200-Aring-thick HfO2 films. A picosecond pump-probe thermoreflectance technique yields room-temperature intrinsic thermal conductivity values between 0.49 and 0.95 W/(mmiddotK). The intrinsic thermal conductivity and interface resistance depend strongly on the film-thickness-dependent microstructure.

Synthesis, mechanical and physical properties of bulk Zr 2Al 4C 5 ceramic

Bulk Zr 2Al 4C 5 ceramics was synthesized by an in situ reactive hot-pressing process using ZrC, Al and carbon black as staring materials. The reaction path of synthesis was discussed based on differential thermal analysis, Raman spectra and X-ray diffraction results. Compared with the reported Zr 2Al 3C 4, Zr 2Al 4C 5 has a relatively high hardness (Vickers hardness of 12.2 GPa), superior strength (flexural strength of 420 MPa) and lower electrical resistivity (1.05 μΩ m). In addition, the room-temperature thermal conductivity value, 14.6 W (m K) −1 for Zr 2Al 4C 5, was smaller than the value for Zr 2A 3C 4 ceramics (15.5 W (m K) −1), and the heat capacity of Zr 2Al 4C 5 is much higher than that of Zr 2A 3C 4 in the overall temperature range. The results here indicated that Zr 2Al 4C 5 is a potential high-temperature structural ceramic.