Role of vibrational optical phonons in the heat capacity of K3C60

The reported heat capacity C(T) data of alkali doped fulleride K₃C₆₀ is theoretically investigated in the temperature domain 5 ≤ T ≤ 25 K. Calculations of C(T) have been made within the two component scheme: one is the phonon (optic and acoustic) and the other is electronic contribution. We begin with the intercage interactions between the adjacent C₆₀ cages and expansion of lattice due to the intercalation of alkali atoms based on the spring model to estimate vibrational optic and acoustic phonon frequencies from the dynamical matrix for the intermolecular alkali-C₆₀ phonon mode. Lattice specific heat is well estimated from the Debye and Einstein approximation. Fermionic component as the electronic specific heat coefficient is deduced using the band structure calculations for metallic phase. Comparison of the coefficient of the normal state electron contribution to C with band structure calculations gives an estimate of the electron–phonon coupling strength. It is notice that electron correlations are essential to enhanced density of state over simple Fermi liquid approximation in the metallic phase. The present numerical analysis of specific heat shows dominating role of vibrational optical phonons.     

Low thermal conductivity without oxygen vacancies in equimolar YO1.5 + TaO2.5- and YbO1.5 + TaO2.5-stabilized tetragonal zirconia ceramics

A narrow range of composition exists along both the ZrO₂–YTaO₄ and ZrO₂–YbTaO₄ quasi-binaries over which the tetragonal zirconia phase can be retained on cooling. Unlike other stabilized zirconia materials which have low thermal conductivity as a result of phonon scattering by oxygen vacancies, these compositions do not contain oxygen vacancies and yet an equimolar YO_1.5 + TaO_2.5 composition has been reported to also exhibit low thermal conductivity [1]. We find that zirconia compositions along the quasi-binaries have low and temperature-independent thermal conductivities, and that the thermal conductivities and their temperature dependence are consistent with a defect scattering model that takes into account a minimum phonon mean free path due to the inter-atomic spacing. Furthermore, the conductivities of the Yb and Y trivalent-doped compositions scale in a predictable manner with atomic site disorder effects on the cation sub-lattice associated with the lighter Y³⁺ ions and the heavier Yb³⁺ and Ta⁵⁺ ions. The lowest thermal conductivity measured was ～1.4 W mK^–1 at 900 °C. The low thermal conductivity and phase stability makes these systems promising candidates for low conductivity applications, such as thermal barrier coatings.     

Enhanced thermoelectric performance of dual-element-filled skutterudites BaxCeyCo4Sb12

Single-phase polycrystalline dual-element-filled skutterudites Ba_xCe_yCo₄Sb₁₂ (0 < x < 0.4, 0 < y < 0.1) are synthesized by the melting–quenching–annealing and spark plasma sintering methods. The electrical conductivity, Seebeck coefficient, thermal conductivity and low-temperature Hall data of these compounds are reported. Our results suggest that there is essentially no difference in electrical transport properties between the dual-element-filled Ba_xCe_yCo₄Sb₁₂ and single-element-filled Ba_yCo₄Sb₁₂ systems. The Ba–Ce co-filling is more effective in lattice thermal conductivity reduction than Ba single filling in the temperature range of 300–850 K. Very low lattice thermal conductivity values less than 2.0 W m^?1 K^?1 are obtained at room temperature. Consequently, enhanced thermoelectric figure of merits (ZT) for these dual-element-filled CoSb₃ skutterudites are achieved at elevated temperatures, in particular ZT = 1.26 at 850 K for Ba_0.18Ce_0.05Co₄Sb_12.02.     

An extremely low thermal conduction ceramic: RE9.33(SiO4)6O2 silicate oxyapatite

Complex rare-earth silicate oxyapatite RE_9.33(SiO₄)₆O₂ (RE = La, Nd, Sm, Gd, Dy) ceramics have been synthesized and their thermal conduction characteristics investigated. When evaluated using a steady-state laser heat-flux technique under conditions ranging from room temperature to 1000 °C the materials demonstrated very low thermal conductivities (0.96–1.49 W m^–1 K^–1), especially Gd_9.33(SiO₄)₆O₂, which shows a value of 1.10–1.14 W m^–1 K^–1 in the measured temperature range. Phonon mean free path and Raman spectra were used to investigate the thermal transfer mechanism. The source of low thermal conductivity was determined to be the strong intrinsic scattering in the crystal cell, which is due to the phonon mean free path being on the inter-atomic level. Furthermore, a connection between the full width at half maximum Raman spectra and the thermal conductivity of RE_9.33(SiO₄)₆O₂ ceramics at room temperature was established. The insensitivity of the thermal conduction properties to temperature for RE_9.33(SiO₄)₆O₂ ceramics have allowed it to show great potential in high temperature thermal insulation applications.     

Crystal structure,morphology and physical properties of LaCo1−xTixO3±δ perovskites prepared by a citric acid assisted soft chemistry synthesis

The influence of the partial substitution of Co by Ti in the LaCoO₃ perovskite system is studied by evaluating the electrical conductivity, the Seebeck coefficient and the thermal conductivity of the compounds up to T = 1273 K. The X-ray diffraction patterns of the LaCo_1?xTi_xO_3±δ (0.01 ? x ? 0.5) phases show two structural modifications depending on the Ti content. Compounds with x < 0.3 crystallize in the rhombohedrally distorted perovskite structure while samples with x ? 0.3 possess an orthorhombic unit cell. The oxidation state of the Co ions is studied by X-ray absorption near edge structure (XANES) spectroscopy. A negative thermoelectric power is found in the LaCoO₃ system for low level Ti substitution (x = 0.01). In contrast, samples with higher Ti content show a large positive Seebeck coefficient, indicating positive majority charge carriers in the system. The electrical resistivity of the studied materials reveals a semiconducting-like behaviour. The lattice thermal conductivity was found to be low and nearly temperature-independent. The samples exhibit very small crystallite sizes in the range of few nanometres. Therefore, the low thermal conductivity can be assigned to an enhanced phonon scattering on grain boundaries.     

Glass-like thermal conductivity in ytterbium-doped lanthanum zirconate pyrochlore

Glass-like thermal conductivity is observed in (La_1?xYb_x)₂Zr₂O₇ (1/6 ? x ? 1/3), which exhibits great potential as a high-temperature thermal barrier coating material. In the pyrochlore-type La₂Zr₂O₇, the large 16c-site La³⁺ is weakly bonded by its surrounding 48f-site oxygen ions, and substitution of La³⁺ with smaller and heavier Yb³⁺ gives rise to a large atomic displacement parameter (ADP) of Yb³⁺ which behaves as a “rattler”, as evidenced by the X-ray diffraction refinement. The localized “rattling” of Yb³⁺ in the cation sublattice significantly scatters the heat-carrying phonons and lowers the thermal conductivity close to the amorphous limit in combination with the intrinsic oxygen vacancies in the anion sublattice. In contrast, substituting Yb³⁺ with the larger La³⁺ in Yb₂Zr₂O₇ does not result in as remarkable a decrease in the thermal conductivity as in Yb-doped La₂Zr₂O₇ due to the absence of rattling atoms. In this study, a resonant phonon scattering is proposed as a new approach to reduce the thermal conductivity of oxides which are important in various thermal engineering applications.     

Magnetic,electrical and thermal transport properties of Al–Cr–Fe approximant phases

Two stable approximant phases in the Al–Cr–Fe system – a gamma-brass phase (γ-AlCrFe) and a mixture of two orthorhombic approximants of the decagonal phase (O₁/O₂-AlCrFe) – were investigated using magnetic susceptibility, electrical resistivity and thermal conductivity measurements, combined with structural investigations using X-ray diffraction, light microscopy (LM) and scanning electron microscopy (SEM). The investigated approximants exhibit physical properties that are in many respects between those of regular metals and quasicrystals (QCs); their electrical resistivities show very weak temperature dependences and the resistivity values are higher than for regular metals and lower than for Al-based QCs. The magnetic susceptibility results show the existence of a small fraction (of about 1% for the γ-AlCrFe and about 10 times less for the O₁/O₂-AlCrFe) of localized magnetic moments with Curie-like temperature dependence. Thermal conductivity measurements show that the electronic and lattice contributions are of comparable size at room temperature. While the electronic contribution can be described by the Wiedemann–Franz law, the lattice contribution can be reproduced by the sum of the Debye term (long-wavelength phonons) and the term due to hopping of localized vibrations. At the lowest measured temperature (8 K), scattering of phonons on stacking-fault-like defects limits the heat transport, and this type of defect has also been observed in the LM and SEM structural investigations.     

Extraction of electronic parameters of organic diode fabricated with NIR absorbing functional manganase phthalocyanine organic semiconductor

The semiconducting and metal/organic semiconductor properties of the newly synthesized NIR absorbing α-substituted manganase phthalocyanine bearing functional 2,3-dihydroxypropylthio moieties {M[Pc(S–CH₃CH₂(OH)CH₂(OH))]₄X}(M = Mn^III) have been investigated by electrical conductivity–temperature, optical absorption and current–voltage characteristics methods. The electrical conductivity increases with the temperature, suggesting that the peripheral α-substituted-functional manganase phthalocyanine is an organic semiconductor. The optical band gap and trap energy values were determined and were found to be 2.98 eV and 1.95 eV, respectively. The ITO/MnPc/Al diode shows a rectifying behavior due to the formation of MnPc/Al interface with a rectification ratio of 29.4 at ±2 V. The series resistance R_s and ideality factor n values were found to be 102.6 kΩ and 8.89, respectively. The interface state density for the diode was of order of 2.73 × 10¹¹ eV^?1 cm^?2 with the interface time constant of 1.93 × 10^?5.It is evaluated that newly synthesized α-substituted manganase phthalocyanine bearing functional 2,3-dihydroxypropylthio moieties is an organic semiconductor and can be used in electronic device applications as an organic diode.     

Thermal properties of MoSi2 with minor aluminum substitutions

Mo(Si_1−xAl_x)₂ compositions (x = 0–0.1) have been prepared by a modified SHS route under uniaxial hydrostatic pressure. Oxidation studies carried out by thermal analysis and sheet resistivity indicate an improvement in the low temperature (700–900 K) oxidation resistance with increasing aluminum addition. Dilatometric results show a decrease in the α value up to x = 0.05 substitution. With the aluminum substitution, both thermal expansion coefficient and thermal conductivity show decrease in their values except in the biphasic region. The x = 0.05 composition containing both C11_b and C40 phases is a promising material for high temperature thermal barrier coating as it shows higher oxidation resistance and a similar K/α value as compared to pure MoSi₂.     

Glass-like thermal conductivities in (La1-x1Yx1)2(Zr1-x2Yx2)2O7-x2 (x = x1 + x2, 0 ⩽ x ⩽ 1.0) solid solutions

The evolution of structure and thermal conductivity (k) has been studied for a range of Y–La₂Zr₂O₇ solid solutions. Within the pyrochlore range (x < 0.40) Y³⁺ solely substitutes for La³⁺ below a critical composition factor (x = 0.15), above which it substitutes for both La³⁺ and the Zr⁴⁺. A glass-like k, approaching the amorphous limit, is observed within a certain composition range (0.20 ? x < 0.40). The glass-like k behaviour is attributed to a phonon localization effect that arises from small and weakly bound Y³⁺ cations (rattlers) oscillating locally and independently in oversized anionic cages [(La/Y)O₈]. The ultralow and glassy k makes Y³⁺-doped La₂Zr₂O₇ pyrochlores promising candidate materials for high temperature thermal barrier coating topcoats.     

Transport studies of NH4NO3 doped methyl cellulose electrolyte

This work reports on the transport properties of NH₄NO₃ doped methyl cellulose (MC) polymer electrolyte. The polymer electrolyte films were prepared by the technique of solvent casting. The highest room temperature conductivity of MC doped with 25 wt.% NH₄NO₃ is 2.10 ± 0.37 × 10^?6 S cm^?1. Conductivity–temperature relationship obeys the Vogel–Tamman–Fulcher (VTF) rule from which the glass transition temperature, T_g was evaluated. The mobility, μ and number density of charge carrier, n were calculated using the Rice and Roth model.     

Phase separation and thermoelectric properties of Ag2Te-doped PbTe0.9S0.1

Spinodal decomposition is an ideal mechanism for producing bulk nanostructured materials with promising thermoelectric (TE) performance. In this contribution, the phase separation and TE properties of PbTe-PbS samples are investigated. Phase separation driven by spinodal decomposition is observed in PbTe_0.4S_0.6, PbTe_0.5S_0.5, PbTe_0.6S_0.4 and (PbTe_0.9S_0.1)_1?x(Ag₂Te)_x with x = 0, 0.01 and 0.03. The addition of Ag₂Te leads to a deterioration in electrical transport properties at low temperature but to a significantly enhanced higher-temperature power factor of the Ag₂Te-doped PbTe_0.9S_0.1 sample. The very low thermal conductivity of the Ag₂Te-doped sample is attributed to the doping effect of Ag₂Te, the precipitated Ag₂Te, and the nanoscale phase segregation driven by spinodal decomposition. In particular, the spinodal decomposition produces finely dispersed PbTe-rich and PbS-rich phases with solute atoms, coherent or semicoherent interfaces, lattice bending, and other lattice defects, which contribute to the phonon scattering and minimize the thermal conductivity. The highest TE figure of merit, ZT, is ～1.2 at 773 K for the sample with x = 0.03, and even larger ZT values at higher temperature might be expected based on its tendency to increase with the temperature.     

Phonon stress sensitivity for interface characterization of fibrous composites at various temperatures

The stress sensitivity of two aramid fiber optical phonons at 1611 and 1648 cm⁻¹ has been employed to conduct stress as well as interface measurements in aramid/epoxy resin composites at ambient and elevated temperatures (60–100 °C). It was found that when the stress/interface measurements were conducted via the 1648 cm⁻¹ phonon, which is practically insensitive to thermal effects, there was an upper limit of temperature beyond which the results were scattered and important information on the interface integrity was lost. Alternatively, it has been shown here that at elevated temperatures one can obtain the interfacial shear stress directly from the 1611 cm⁻¹ shift and then the axial stress by integration. Useful results were obtained both on the applicability of both techniques and also on the interface yield stress of these composites as a function of temperature.     

Determination of ionic and pure electronic contributions to the electro-optic coefficient of cadmium telluride and gallium arsenide single crystals

High electro-optic figure of merit n₀³r₄₁ and the absence of natural birefringence make semi-insulating gallium arsenide (GaAs) and cadmium telluride (CdTe) attractive materials for the fabrication of electro-optical devices. In this context we characterized both acoustic and optical phonon contributions to the electro-optic coefficient of CdTe and GaAs. Accurate measurements of n₀³r₄₁ as a function of modulation frequency and temperature were carried out on CdTe:In and GaAs crystal at 1.5 μm. For CdTe, to the best of our knowledge, this is the first time that a contribution to the electro-optic coefficient due to optical phonons has been determined. A positive value for the ionic contribution, due to optical phonons in GaAs, is obtained, in disagreement with that previously inferred from Raman scattering efficiency measurements. The pure electronic contribution was then isolated, and the second-order non-linear optical coefficient was derived. The latter was compared to previously reported data from non-linear wavelength conversion measurements.     

Thermal degradation of the dielectric relaxation of 10–90% (w/w) zeolite-conducting polypyrrole composites

The effect of thermal aging of 10–90 wt% zeolite-conducting polypyrrole composite on its dielectric properties is studied in the frequency range 10⁻² to 2 × 10⁶ Hz from room temperature to liquid nitrogen temperature. A dielectric relaxation mechanism, which appears in the fresh samples, is influenced by the thermal annealing. The frequency f_max where a maximum of a dielectric loss peak is located decays exponentially with the aging time and the intensity of the loss peaks shows a maximum at intermediate aging time. A modified Williams–Landel–Ferry law describes the temperature variation of f_max in all specimens. Increasing activation energy values on increasing the aging duration are obtained. The temperature dependence of f_max and the activation energy (regarded as the height of a potential barrier) are different from those characterizing the macroscopic conductivity, which is described by the charging energy limited tunneling model. The intensity of the dielectric mechanism in thermally treated samples deviates from the linear decrease with inverse temperature occurring in fresh polypyrrole. Although the thermal degradation of the logarithm of the dc conductivity decays proportional with the root of the aging time, the equivalent conductivity obtained from the dielectric data decays exponentially with aging duration. Time constants are obtained in both cases. The model of Barton–Nakajima–Namikawa (BNN) can hardly interconnect the dc conductivity with the relaxation process in fresh sample. The divergence augments with the aging time. The thermal aging law and the inadequacy of the BNN model probably indicates that the dc process is probably irrelevant to the relaxation process.     

Organic conductor: Influence of preparation temperature

The conducting polypyrrole–polyethylene glycol (PPy–PEG) composite films were produced at various polymerization temperature ranging from 5 °C to 60 °C using 1 × 10^?3 M PEG, 0.20 M pyrrole and 0.10 M p-toluene sulfonate at 1.20 V (vs. SCE). The polymerization temperature of 5 °C appeared as the optimum preparation temperature showing the highest electrical conductivity of 70 S/cm and the thermal diffusivity of 8.76 × 10^?7 m² s^?1. The electrical conductivity and thermal diffusivity exhibited a decreasing trend with the increase in polymerization temperature in the pyrrole solution used to prepare the composite films. The XRD results reveal that low temperature (5 °C) typically results in more crystalline films, which are denser, stronger and have higher conductivity. The optical microscopy of PPy–PEG shows the globular surface morphology. The surface of the of the solution side of PPy–PEG film prepared at low temperatures showed a globular morphology.     

Realization of high thermoelectric performance in p-type unfilled ternary skutterudites FeSb2+xTe1−x via band structure modification and significant point defect scattering

FeSb₂Te, a ternary derivative of binary CoSb₃, displays anomalous electrical and thermal transport properties because of considerable modifications in the band structure induced by Fe and significant mixed valence state (namely Fe²⁺ and Fe³⁺) scattering of phonons. The substitution of Te for Sb generates more holes without notably affecting the band structure, while markedly improving the electrical conductivity and retaining a high Seebeck coefficient due to the enhanced density of states, thereby leading to dramatically increased power factors. Furthermore, the heat carrying phonons are strongly scattered with increasing x value because of the formation of solid solutions between two end members: □FeSb₂Te and □FeSb₃ (where □ can be viewed as a vacancy). Consequently, high thermoelectric figures of merit were achieved in the FeSb_2+xTe_1?x compounds, with the largest ZT value reaching ～0.65 for the sample with x = 0.2. This is the highest value among all p-type unfilled skutterudites and is comparable with some filled compositions. Prospects for further improving the performance of p-type FeSb₂Te-based skutterudites are discussed.     

Ablation resistance and thermal conductivity of carbon/carbon composites containing hafnium carbide

Ablation properties and thermal conductivity of carbon/carbon (C/C) composites containing hafnium carbide (HfC) were investigated. The C/C composites containing 6.5 wt.% HfC exhibit the best thermal conductivity and ablation resistance. The improvement of the thermal conductivity is attributed to the increased phonon–defect interaction produced by the thermal motion of CO released from the reaction of carbon and ZrO₂. High thermal conductivity of the composites can slow down the ablation of carbon. When the HfC mass fraction is greater than 6.5 wt.%, cracks generated act as diffusion channels for an oxidizing atmosphere and thus accelerate the ablation of the composites.     

Experimental investigation and modeling of the influence of microstructure on the resistive conductivity of a Cu–Ag–Nb in situ composite

A Cu–8.2 wt% Ag–4 wt% Nb in situ metal matrix composite was manufactured by inductive melting, casting, swaging, and wire drawing. The final wire (η=ln(A₀/A)=10.5, A: wire cross section) had a strength of 1840 MPa and 46% of the conductivity of pure Cu. The electrical resistivity of the composite wires was experimentally investigated as a function of wire strain and temperature. The microstructure was examined by means of optical and electron microscopy. The observed decrease in conductivity with increasing wire strain is interpreted in terms of inelastic electron scattering at internal phase boundaries. The experimental data are in very good accord with the predictions of an analytical size-effect model which takes into account the development of the filament spacing as a function of wire strain and the mean free path of the conduction electrons as a function of temperature. The experimentally obtained and calculated resistivity data are compared to those of the pure constituents.     

CeO2的晶格动力学性质和热输运性质的第一性原理计算

采用基于密度泛函理论的有限位移法和玻尔兹曼方程,计算了CeO2的晶格动力学性质、热力学性质和热输运性质,计算结果和实验结果基本符合。通过分析CeO2所有声子模式的振动频率、Gruneisen系数和散射率,揭示了光学声子对增强晶格振动的非简谐性和声子散射率所起的重要作用。此外,还计算了不同自由程的声子模式对热导率的贡献,发现CeO2的晶格热导率主要由声子自由程在1~10 nm之间的声子所贡献。