Room Temperature Thermal Conductivity Research Articles

Thermal Conductivity Measurements of Nylon 11-Carbon Nanofiber Nanocomposites

Carbon nanofibers (CNFs) were incorporated into nylon 11 to form nylon 11-carbon nanofiber nanocomposites via twin screw extrusion. Injection molding has been employed to fabricate specimens that possess enhanced mechanical strength and fire retardancy. The thermal conductivity of these polymer nanocomposites was measured using a guarded hot plate method. The measurement results show that the room temperature thermal conductivity increases with the CNF loading from 0.24±0.01 W/m K for pure Nylon 11 to 0.30±0.02 W/m K at 7.5 wt % CNF loading. The effective medium theory has been used to determine the interface thermal resistance between the CNFs and the matrix to be in the range of 2.5–5.0×10−6 m2 K/W from the measured thermal conductivity of the nanocomposite.

Synthesis, Structure and Charge Transport Properties of Yb5Al2Sb6: A Zintl Phase with Incomplete Electron Transfer

We report the synthesis, structure, spectroscopic properties, charge and thermal transport, and electronic structure of a new member of the Zintl family, Yb(5)Al(2)Sb(6). The compound crystallizes in the Ba(5)Al(2)Bi(6) structure type and requires the addition of Ge or Si in the synthesis, which appears to act as a catalyst. Yb(5)Al(2)Sb(6) has an anisotropic structure with infinite anionic double chains cross-linked by Yb(2+) ions. Polycrystalline ingots of Yb(5)Al(2)Sb(6) prepared in the presence of 0.5 mol equiv of Ge showed room-temperature conductivity, thermopower, and thermal conductivity of approximately 1100 S/cm, approximately 20 microV/K, and approximately 3.8 W/m.K, respectively. Investigations of other solid solutions of Yb(5)Al(2)Sb(6), doping effects, and chemical modifications are discussed. Sr only partially replaces Yb in the structure leading to Sr(0.85)Yb(4.15)Al(2)Sb(6). Electronic structure calculations performed using a highly precise full-potential linearized augmented plane wave method within the density functional theory scheme show the presence of a negative band gap and suggest incomplete electron transfer and a metallic character to the compound.

Phonon thermal conduction in graphene: Role of Umklapp and edge roughness scattering

We investigated theoretically the phonon thermal conductivity of single-layer graphene. The phonon dispersion for all polarizations and crystallographic directions in graphene lattice was obtained using the valence-force field method. The three-phonon Umklapp processes were treated exactly using an accurate phonon dispersion and Brillouin zone, and accounting for all phonon relaxation channels allowed by the momentum and energy conservation laws. The uniqueness of graphene was reflected in the two-dimensional phonon density of states and restrictions on the phonon Umklapp scattering phase-space. The phonon scattering on defects and graphene edges has also been included in the model. The calculations were performed for the Gruneisen parameter, which was determined from the ab initio theory as a function of the phonon wave vector and polarization branch, and for a range of values from experiments. It was found that the near room-temperature thermal conductivity of single-layer graphene, calculated with a realistic Gruneisen parameter, is in the range $\ensuremath{\sim}2000--5000\text{ }\text{W}/\text{mK}$ depending on the flake width, defect concentration and roughness of the edges. Owing to the long phonon mean free path the graphene edges produce strong effect on thermal conductivity even at room temperature. The obtained results are in good agreement with the recent measurements of the thermal conductivity of suspended graphene.

Determination of the room temperature thermal conductivity of RuO2 by the photothermal deflection technique

A thermal conductivity study of RuO2 single crystals has been carried out via the photothermal deflection technique in transverse configuration at room temperature. The thermal diffusivity and conductivity of RuO2 crystals are determined to be 0.17 cm2 s−1 and 0.50 W cm−1 K−1, respectively. Furthermore, it was found that the thermal conductivity has inversely proportional temperature dependence in the region between 50 and 300 K.

Magnetic, transport, and thermodynamic properties ofCaMn2O4single crystals

Physical properties including magnetic susceptibility, room-temperature electrical resistivity, thermal conductivity, heat capacity, and thermal expansion are reported for high quality single-crystal samples of marokite CaMn2O4. We determined that CaMn2O4 is highly electrically insulating and exhibits long-range antiferromagnetic order below TN=217.5 0.6 K with easy axis along a. Anisotropic thermal expansion, similar to that of crystallographically layered materials, is observed, suggesting that the crystal structure of CaMn2O4 is also assembled from previously undescribed layers. An extensive thermodynamic study of the antiferromagnetic transition was undertaken resulting in a heat-capacity critical exponent =0.082 0.007 and calculated pressure derivative dTN /dP=5.154 0.174 K /GPa.

Influence of Mo addition on dielectric properties of AlN ceramic matrix composites

AlN-Mo composite ceramics were prepared by spark plasma sintering (SPS) with CaF2 as sintering aids. Effect of Mo addition on the thermal conductivity and dielectric properties of the composite ceramics had been studied. The results show that the room temperature thermal conductivity increases with increasing the content of Mo, and the value begins to decrease slightly when the Mo concentration exceeds 20 vol. %. Analyses indicate that the key factors to dielectric properties are the metal phase concentration and the microstructure of Mo particles. 1 vol. % Ni has been added into the composite ceramics to change the distribution of the Mo phase. The elongated shape particles which link with each other have a tendency to acquire rounded forms which are thermodynamically more stable. Consequently, the dielectric constant and loss of the composite ceramics could be adjusted and the material becomes an electrical conductor in the case of Mo volume fraction of more than 23%. Furthermore, the dielectric properties could be improved to a large extent by transforming the microstructure of the metal particles when the concentration of Mo is fixed.

Solid-State Self-Assembly of Nanostructured Oxide as a Candidate High-Performance Thermoelectric Material

We have focused on the recently reported nanostructured bulk ZnMn2−x Ga x O4 to evaluate whether this type of nanostructured oxide can effectively reduce thermal conductivity. Firstly, powdered samples of ZnMn2−x Ga x O4 (x = 0 to 2) were prepared and the effect of heat treatment on the obtained phases was examined. Secondly, we have picked out the composition of ZnMnGaO4, in which two distinct types of rectangular nanorods with different compositions spontaneously interlace to form a cross-sectional checkerboard pattern. To confirm the effect of nanostructure on thermal transport properties, the room-temperature thermal conductivity of this nanostructured oxide was evaluated.

Thermal properties of the pyrochlore, Y 2Ti 2O 7

The thermal conductivity and heat capacity of high-purity single crystals of yttrium titanate, Y 2Ti 2O 7, have been determined over the temperature range 2 K⩽ T⩽300 K. The experimental heat capacity is in very good agreement with an analysis based on three acoustic modes per unit cell (with the Debye characteristic temperature, θ D, of ca. 970 K) and an assignment of the remaining 63 optic modes, as well as a correction for C p − C v . From the integrated heat capacity data, the enthalpy and entropy relative to absolute zero, are, respectively, H( T=298.15 K)− H 0=34.69 kJ mol −1 and S( T=298.15 K)− S 0=211.2 J K −1 mol −1. The thermal conductivity shows a peak at ca. θ D/50, characteristic of a highly purified crystal in which the phonon mean free path is about 10 μm in the defect/boundary low-temperature limit. The room-temperature thermal conductivity of Y 2Ti 2O 7 is 2.8 W m −1 K −1, close to the calculated theoretical thermal conductivity, κ min, for fully coupled phonons at high temperatures.

Low thermal conductivity and improved figure of merit in fine-grained binary PbTe thermoelectric alloys

Fine-grained binary PbTe thermoelectric materials have been fabricated by a combination of a reflux chemical synthesis and hot pressing. The grain sizes of the hot pressed bulk samples varied from 200 to 400 nm, which significantly contributed to the thermal conductivity reduction due to the enhanced phonon scattering at grain boundaries. Low room temperature thermal conductivity of 0.75 W m−1 K−1 has been obtained after porosity correction for the sample with a grain size of ∼200 nm. The highest figure of merit ZT of the fine-grained binary PbTe bulk sample reaches 0.8 at 580 K although the composition was unoptimized. The value is comparable to that of the best PbTe alloys.

Electronic and thermal properties of Ti3Al(C0.5,N0.5)2, Ti2Al(C0.5,N0.5) and Ti2AlN

In this paper we report on the electronic, magnetotransport, thermoelectric, and thermal properties of Ti3Al(C0.5,N0.5)2, Ti2Al(C0.5,N0.5), and Ti2AlN. The electrical conductivities, Hall coefficients, and magnetoresistances are analyzed within a two-band framework and compared with the end members, Ti2AlC and Ti3AlC2. The analysis shows that all compounds are compensated conductors with hole and electron carrier densities of about 1.5×1027 m−3. The room temperature thermal conductivities of the carbonitrides are both over ≈50 W/mK, with the phonon contribution of Ti3Al(C0.5,N0.5)2 particularly large at over 35 W/mK. The low-temperature specific heat of Ti3Al(C0.5,N0.5)2 and Ti2Al(C0.5,N0.5) yield Debye temperatures of 685 and 724 K, respectively, comparable to those of the end members.

Breakdown of Fourier’s Law in Nanotube Thermal Conductors

We present experimental evidence that the room temperature thermal conductivity (kappa) of individual multiwalled carbon and boron-nitride nanotubes does not obey Fourier's empirical law of thermal conduction. Because of isotopic disorder, kappa's of carbon nanotubes and boron-nitride nanotubes show different length dependence behavior. Moreover, for these systems we find that Fourier's law is violated even when the phonon mean free path is much shorter than the sample length.

Electrical, thermal, and elastic properties of theMAX-phase Ti2SC

We report on the electronic, thermal, and elastic properties of the layered ternary, Ti2SC. Resistivity, Hall effect, and magnetoresistance were measured as a function of temperature between 2 and 300 K and at fields up to 9 T. The Hall coefficient is negative and roughly temperature independent. The transport results were analyzed within a two-band framework, with electrons as the dominant charge carrier. The room-temperature thermal conductivity (≈60 W/m K) is the highest of any MAX phase measured to date, with a substantial phonon contribution. The specific heat was measured from 2 to 300 K, yielding a Debye temperature of 765 K and in agreement with the Debye temperature of 745 K found from ultrasonic time-of-flight measurements. Young’s, shear, and bulk moduli from the latter measurements were 290, 125, and 145 GPa, respectively. The calculated values of the lattice parameters (a=3.2051 Å and c=11.2636 Å), and Young’s, shear, and bulk moduli (329, 138, and 179 GPa, respectively), based on the results of density functional theoretical simulations, compare favorably with measurements.

Thermal transport in a semiconductor heterostructure measured by time-resolved x-ray diffraction

We report studies of thermal transport across the interface of a semiconductor heterostructure using x-ray diffraction to measure the time-dependent lattice expansion after ultrafast laser excitation. Femtosecond laser pulses are used to rapidly and locally heat the substrate at the buried interface of an ${\text{Al}}_{0.3}{\text{Ga}}_{0.7}\text{As}/\text{GaAs}$ heterostructure grown by molecular-beam epitaxy. High-resolution time-resolved x-ray diffraction is used to study the heating and cooling of the film and substrate independently. The data are compared with a simple model for the thermal transport incorporated into dynamical diffraction calculations allowing us to extract the room-temperature cross-plane film thermal conductivity. The value is 40% lower than that extrapolated from prior results on liquid-phase epitaxy grown samples of varying concentrations.

Processing and thermal properties of an Mo 5Si 3C–SiC ceramic

Composite ceramics containing ∼25 vol% of the Mo–Si–C ternary compound and SiC (referred to as MS) were reaction hot pressed up to 96% relative density. The electrical resistivity of the composite processed at 2150 °C decreased from ∼4.60 Ω cm at room temperature to 4.15 Ω cm at 700 °C. Thermal conductivity of the MS ceramics processed at 2150 °C was above 100 W/m K at room temperature, decreasing to between 62 and 68 W/m K at 500 °C. Using the Eucken model for thermal conductivity, the interconnected SiC phase in the MS materials was calculated to have a room temperature thermal conductivity between 160 and 170 W/m K. As compared to a baseline SiC composition, the continuous SiC in the MS materials had an average increase in thermal conductivity of ∼50% over the temperature range of room temperature to 500 °C. This increase in thermal conductivity was attributed to the accommodation of impurities that would typically be present in SiC grains and grain boundaries (e.g., N and O) into the ternary phase.

Thermal conductivity of nitrogenated ultrananocrystalline diamond films on silicon

The authors report on the experimental investigation of the thermal conductivity of nitrogenated ultrananocrystalline diamond (UNCD) films on silicon. For better accuracy, the thermal conductivity was measured by using two different approaches: the 3ω method and transient “hot disk” technique. The temperature dependence of the thermal conductivity of the nitrogenated UNCD films was compared to that of undoped UNCD films and microcrystalline diamond (MCD) films on silicon. It was shown that the temperature dependence of the thermal conductivity of UNCD films, which is substantially different from that for MCD films, can be adequately described by the phonon-hopping model. The room-temperature thermal conductivity of UNCD is 8.6–16.6 W/m K and decreases with the addition of nitrogen. The obtained results shed light on the nature of thermal conduction in partially disordered nanostructured materials and can be used for estimating the thermal resistance of doped UNCD films.

Electro-Mechanical and Thermal Properties of Multiwalled Carbon Nanotube Reinforced Alumina Composites

Multi-walled carbon nanotube (MWNT) reinforced alumina composites with different MWNT contents (5 and 10 vol %) were fabricated by spark plasma sintering. The room temperature dc electrical conductivity, thermal conductivity, and mechanical properties were investigated. Results showed that the electrical conductivity has improved around twelve orders of magnitude by addition of 5 vol% of MWNT. The fracture toughness changed from 3.2 to 4.4 MPa m1/2 with 39% improvement over monolithic Al2O3. The thermal conductivity decreased with increase of MWNT contents. The low values of thermal conductivity suggest that interfacial thermal barrier play an important role in determining these properties. MWNT can be used to improve concurrently electrical, mechanical properties of Al2O3 but with lower values of thermal properties.

Thermal conductivity across nanostructured porous silicon films

This paper models the effect of pore size, pore arrangement and porosity on room temperature thermal conductivity across low porosity meso-porous silicon (meso-PS) films known as n+ and p+ type PS. Whereas the meso-PS films are simulated using a three-dimensional (3D) pore network generator, the heat conduction is modeled using the Monte Carlo method (MCM) recently developed for simulating steady-state phonon transport in submicron structures. The simulations show that (i) the thermal conductivity across meso-porous films decreases when the pore size decreases or the porosity increases, in accordance with past studies. (ii) The thermal conductivity of n+ type pore network is always greater than that of p+ type due to the difference in pore morphology. Finally, the simulations corresponding to p+ type pore network are shown in good agreement with existing experimental data for low porosity p+ type PS. Such comparison confirms the validity of the current 3D modeling.

Drop and recovery of thermal conductivity of AlN upon UV irradiation

We have performed calculations of the room-temperature thermal conductivity of oxygen contaminated aluminium nitride (AlN) by employing the Callaway model with a detailed account of three-phonon scattering processes. The role of Al vacancy and O substitution of N has been examined in the form of extended defects (clusters) and point defects. Our work provides support for the theoretical model proposed by Harris et al. [Phys. Rev. B. 47, 5428 (1993)] to explain the experimentally observed drop in the conductivity upon UV irradiation and its recovery upon UV removal and subsequent illumination of the sample with visible light at room temperature. With the reported oxygen concentration in the sample, the scattering of phonons from oxygen-related extended defects is found to be ineffective. Within the picture presented by Harris et al., the point impurity scattering parameter increases by around 17% upon UV irradiation of the sample at room temperature.

Lattice thermal conductivity of group-IV and III–V semiconductor alloys

The room-temperature thermal conductivity of semiconductor alloys is analyzed using a simplified model of the alloy-disorder scattering. Good agreement is achieved between the present model and published experimental data on various group-IV and III−V semiconductor alloys. A complete set of alloy-disorder parameters are estimated, which makes it possible to calculate the lattice thermal conductivity for optional composition of III−V semiconductor alloys, including III−N alloys. An ordering effect is also examined for the explanation of some intermetallic and semiconductor compounds like CuAu and SiC.

Spinodal Decomposition and Nucleation and Growth as a Means to Bulk Nanostructured Thermoelectrics: Enhanced Performance in Pb1-xSnxTe−PbS

The solid-state transformation phenomena of spinodal decomposition and nucleation and growth are presented as tools to create nanostructured thermoelectric materials with very low thermal conductivity and greatly enhanced figure of merit. The systems (PbTe)(1-x)(PbS)(x) and (Pb(0.95)Sn(0.05)Te)(1-x)(PbS)(x) are not solid solutions but phase separate into PbTe-rich and PbS-rich regions to produce coherent nanoscale heterogeneities that severely depress the lattice thermal conductivity. For x > approximately 0.03 the materials are ordered on three submicrometer length scales. Transmission electron microscopy reveals both spinodal decomposition and nucleation and growth phenomena the relative magnitude of which varies with x. We show that the (Pb(0.95)Sn(0.05)Te)(1-x)(PbS)(x) system, despite its nanostructured nature, maintains a high electron mobility (>100 cm(2)/V x s at 700 K). At x approximately 0.08 the material achieves a very low room-temperature lattice thermal conductivity of approximately 0.4 W/m x K. This value is only 28% of the PbTe lattice thermal conductivity at room temperature. The inhibition of heat flow in this system is caused by nanostructure-induced acoustic impedance mismatch between the PbTe-rich and PbS-rich regions. As a result the thermoelectric properties of (Pb(0.95)Sn(0.05)Te)(1-x)(PbS)(x) at x = 0.04, 0.08, and 0.16 were found to be superior to those of PbTe by almost a factor of 2. The relative importance of the two observed modes of nanostructuring, spinodal decomposition and nucleation and growth, in suppressing the thermal conductivity was assessed in this work, and we can conclude that the latter mode seems more effective in doing so. The promise of such a system for high efficiency is highlighted by a ZT approximately 1.50 at 642 K for x approximately 0.08.