Thermal conductivity of electron-doped CaMnO3 perovskites: Local lattice distortions and optical phonon thermal excitation

The thermal transport properties of a series of electron-doped CaMnO₃ perovskites have been investigated. Throughout the temperature range 5–300 K, phonon thermal conductivity is dominant, and both electron and spin wave contributions are negligible. The short phonon mean free paths in this system result in the relatively low thermal conductivities. The strong phonon scatterings stem from the A-site mismatch and bond-length fluctuations induced by local distortions of MnO₆ octahedra. The thermal conductivity in the magnetically ordered state is enhanced as a result of the decrease in spin–phonon scattering. The results also indicate that above the magnetic ordering temperature, observable thermal excitation of optical phonons occurs. The contribution of optical phonons to thermal conductivity becomes non-negligible and is proposed to play an important role in the glass-like thermal transport behavior (i.e. positive temperature dependence of the thermal conductivity) in the paramagnetic state. These features can be understood in terms of an expression of thermal conductivity that includes both acoustic and optical phonon terms.     

Nano-sized hydroxyapatite powders prepared by flame spray pyrolysis

Pure and doped CaMnO₃ were synthesized by applying modified glycine/nitrate procedure. The detailed structural characterization of prepared nanometric size powders, their properties, as well as the results on studying sintering and microstructure of the sintered bodies are given. Since the data on additive influence on densification of CaMnO₃ during sintering are difficult to find in the literature, their role in densification is discussed. It was found that increasing Y concentration enhanced densification during sintering, and in addition, suppressed grain growth process. The results of the electrical conductivity measurements as a function of dopant concentration are also discussed.     

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。     

(Ca0.96D0.04)MnO3(D=Ca, Sr, Rb, Sm)氧化物的制备和热电性能研究

利用溶胶凝胶法制备出(Ca_0.96D_0.04)MnO₃(D=Ca, Sr, Rb, Sm)氧化物粉末后,先分别采用氩气气氛的放电等离子烧结和空气气氛的常压烧结制备出CaMnO₃(CMO)块体,并对其相组成进行分析,选择出更为优异的CaMnO₃块体制备方法。再进一步制备出(Ca_0.96D_0.04)MnO₃(D=Sr, Rb, Sm)氧化物块体,最后对(Ca_0.96D_0.04)MnO₃(D=Ca, Sr, Rb, Sm)块体的物相组成、显微组织和热电性能进行测试分析。实验结果表明：氩气气氛放电等离子烧结制备的CaMnO₃块体发生物相分解,原因是放电等离子烧结的烧结环境贫氧;空气气氛的常压烧结可以得到物相较纯净的(Ca_0.96D_0.04)MnO₃(D=Ca, Sr, Rb, Sm)块体;在整个测试温度范围内,(Ca_0.96D_0.04)MnO₃(D= Sr, Rb, Sm)的ZT值在873K时达到最大,分别为0.11、0.08和0.07,相比于未掺杂试样提高了约1.3~2.2倍。     

Optical properties of Si-N doped BaMgAl10O17:Eu, Mn phosphors for plasma display panels

The photoluminescence properties of Si-N doped BaMgAl₁₀O₁₇:Eu²⁺, Mn²⁺ phosphors were studied. Photoluminescence spectrum, powder X-ray diffraction and decay curves were used. The electronic structure of un-doped BaMgAl₁₀O₁₇ was investigated by using the density functional theory. It reveals that an ideal hexagonal shape and particle size in 3-5 μm are obtained by Si-N doping. Additionally, its photoluminescence and thermal stability are both improved. The energy transfer from Eu²⁺ to Mn²⁺ also enhanced by suitable Si-N doping. These are expected to be potentially applicable to industrial production of the phosphor in plasma display panels.     

First principle investigation of electronic structure of CaMnO3 thermoelectric compound oxide

First principle calculations are employed to investigate the anti-ferromagnetic CaMnO₃ with regard to its geometry, ground state electronic structure and charge distributions. The G-type anti-ferromagnetic CaMnO₃ is found to be more stable via total energy minimization calculations; the calculated energy band structure reveals its band gap of 0.7 eV. There are combinations of light carriers in conduction bands and heavy carriers in valence bands that should favor high thermoelectric properties. The Mnd and Op orbitals are responsible for energy bands near Fermi level and they contribute to electronic property. There is strong hybridization between Mnd and Op orbitals and, the hybridization between Mn and O1 orbitals is stronger, it is indicated that the charge carriers are apt to transport along Mn-O1.     

Thermoelectric properties of Zn4Sb3 intermetallic compound doped with Aluminum and Silver

The β-phase Zn₄Sb₃ has attracted much attention because of its high thermoelectric performance in the intermediate temperature range thanks to disorder in the Zn lattice site. In this work are presented structural, thermal, electric and thermoelectric characterization of Zn₄Sb₃ pure and Ag, Al doped, prepared by a simple synthesis. Structural and microstructural analyses reveal homogeneous one-phases having compositions in agreement with the nominal ones. After thermoelectric characterization, Ag doping results mostly effective in lowering the resistivity and Seebeck coefficient value, by introducing holes in the system. On the other hand, the Al substitution yields a very small decrease of the Seebeck coefficient but, at the same time, a significant decrease of the thermal conductivity mainly due to the depressed phonon contribution. The thermal conductivity behavior is the main responsible for the good thermoelectric performances of (Zn_0.99Al_0.01)₄Sb₃, whose thermoelectric figure of merit reaches the encouraging value of 0.23 at 260 K.     

Lattice dynamics of magnesium fluoride from a semiempirical two-body potential model

A range of lattice dynamical properties of MgF₂ in sellaite phase with rutile structure is calculated using a two-body potential model. The potential energy of the structure is minimized, and the phonon frequencies are calculated for the structural parameters corresponding to the energy minimum. The computed phonon frequencies are in good agreement with the experimental data, and the phonon dispersion curves are reproduced just as well. The frequencies calculated throughout the Brillouin zone are used to construct the one-phonon density of states, and the harmonic contributions to the thermodynamic functions, including the heat capacity, are calculated up to 800 K. The thermal expansion coefficient is calculated in a perturbative approximation. The results calculated for the heat capacity and the thermal expansion coefficient are in good agreement with the experimental results, at least below 200 K.     

Mechanical properties of La2O3 doped diamond-like carbon films

Twelve La₂O₃ doped diamond-like carbon (DLC) nanofilms were deposited using unbalanced dual-magnetron sputtering. AFM, XRD, Raman spectroscopy, AES, XPS, TEM, contact surface profiler and nanoindenter were employed to investigate the structure and tribological properties of deposited films. The results show that the La₂O₃ doped DLC films are amorphous. La₂O₃ doping obviously decreases internal stress, and effectively increases the elastic modulus. This results from the dissolving and dissolution of La₂O₃ within the amorphous DLC matrix. Furthermore, the friction coefficient of the doped DLC films decreases, and adhesion strength increases. These are attributed to the lubrication function of La₂O₃ and the formation of transition layer at interface, respectively.     

First-principles study on the electronic and optical properties of F- and Nb-doped anatase TiO2

We present a comparative study on the electronic and optical properties of Nb- and F- doped anatase TiO₂ by first-principles calculations based on the density functional theory (DFT). Although both TiO₂:Nb and TiO₂:F have a similar band structure, the effective mass of TiO₂:F is larger than that of TiO₂:Nb, and the carriers density in the former is lower than that in the latter, indicating that TiO₂:Nb has better electronic conductivity than TiO₂:F. The interaction between photons and electrons in TiO₂:F is much stronger than that in TiO₂:Nb, resulting in increased photon absorption and reduced transmittance, especially in the visible and near-infrared regions. The results demonstrate that TiO₂:F is not suitable for transparent conductive oxide applications.     

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.     

Graphene micro-substrate-induced π gap expansion in MgB2

The lattice effect induced by tensile strain on the superconductivity of graphene–MgB₂ composites was studied systematically to deduce the electron–phonon coupling (EPC) and the multiple superconducting gap behavior. Compared with nano-carbon doped MgB₂, graphene–MgB₂ composites show larger lattice parameters and higher critical superconducting transition temperatures (T_c). The EPC strength of MgB₂ with ∼2 wt.% graphene addition is even higher than that of the pure reference sample, as estimated from the Sommerfeld constant. The π gap was found to be expanded by graphene addition through the analysis of heat capacity data, and it is responsible for both the enhanced EPC strength and the weak dependence of T_c on the graphene content.     

Thermoelectric properties of Indium doped Cu2GeSe3

Cu₂Ge_1−xIn_xSe₃ (x = 0, 0.05, 0.1, 0.15) compounds were prepared by a solid state synthesis. The powder X-ray diffraction pattern of the undoped sample revealed an orthorhombic phase. The increase in doping content led to the appearance of additional peaks related to cubic and tetragonal phases along with the orthorhombic phase. This may be due to the substitutional disorder created by Indium doping. Scanning Electron Microscopy micrographs showed a continuous large grain growth with low porosity, which confirms the compaction of the samples after hot pressing. Elemental composition was measured by Electron Probe Micro Analyzer and confirmed that all the samples are in the stoichiometric ratio. The electrical resistivity (ρ) systematically decreased with an increase in doping content, but increased with the temperature indicating a heavily doped semiconductor behavior. A positive Seebeck coefficient (S) of all samples in the entire temperature range reveal holes as predominant charge carriers. Positive Hall coefficient data for the compounds Cu₂In_xGe_1−xSe₃ (x = 0, 0.1) at room temperature (RT) confirm the sign of Seebeck coefficient. The trend of ρ as a function of doping content for the samples Cu₂In_xGe_1−xSe₃ with x = 0 and 0.1 agrees with the measured charge carrier density calculated from Hall data. The total thermal conductivity increased with rising doping content, attributed to an increase in carrier thermal conductivity. The thermal conductivity revealed 1/T dependence, which indicates the dominance of Umklapp phonon scattering at elevated temperatures. The maximum thermoelectric figure of merit (ZT) = 0.23 at 723 K was obtained for Cu₂In_0.1Ge_0.9Se₃.     

The effects of temperature and composition on the thermal conductivities of [(ZrO2)1−x(CeO2)x]0.92(Y2O3)0.08 (0 ? x ? 1) solid solutions

The thermal conductivities of [(ZrO₂)_1−x(CeO₂)_x]_0.92(Y₂O₃)_0.08 (0 ? x ? 1) solid solutions are studied in this paper. The incorporation of ZrO₂ and CeO₂ in the solid solution decreases the thermal conductivity compared with their end members (YSZ and YDC). The thermal conductivities of the solid solutions show clearly different temperature dependences in the ZrO₂-rich (0 ? x ? 0.5) region and in the CeO₂-rich region (0.5 ? x ? 1). The composition and the temperature dependence of the thermal conductivities are discussed based on established phonon scattering theories. We have concluded that the composition dependence of the thermal conductivity of this system is mainly controlled by the mass difference between Zr⁴⁺ and Ce⁴⁺, while the thermal conductivity-temperature relationship is dominated by the randomness of the defect distribution.     

基于第一性原理的Mo2CoB2三元过渡金属硼化物掺杂效应研究(英文)

利用第一性原理计算研究了掺杂第四周期过渡金属元素对Mo₂CoB₂的结构、力学和热力学性质的影响。通过计算结合能和形成焓以及与Born-Huang标准比较，发现所有模型都满足力学与热力学稳定条件。采用点缺陷理论确定了掺杂元素在Mo₂CoB₂晶体中的占位以及占位偏好。结果表明，Sc和Ti对Mo点位表现出强烈的占位偏好，V对Mo点位仅有较弱的占位偏好。同时，Cr、Mn、Fe、Cu和Zn对Co点位具有较弱的占位偏好，而Ni对Co点位有较强的占位偏好。通过对比计算得到德拜温度，发现除Mo₇TiCo₄B8、Mo₇VCo₄B₈和Mo₇CrCo₄B₈外，其他掺杂模型的德拜温度都低于未掺杂模型，这说明实验中应尽量避免在Mo₂CoB₂硬质相中大量添加除Ti、V和Cr以外的过渡金属元素。最后，除Cr掺杂模...     

Mechanical Properties and Thermal Shock Resistance of Al2O3-TiC-Co Composites

Ductile cobalt was introduced into Al₂O₃-TiC (AT) composites by using a chemical deposition method to improve toughness and resistance to thermal shock. The mixture of Co-coated Al₂O₃ and TiC powders was hot-pressed into an Al₂O₃-TiC-Co (ATC) composite. The flexure strength and fracture toughness of the ATC composites have been improved considerably, compared with AT and Al₂O₃. The fracture surface of ATC shows a large proportion of transgranular cracks with some intergranular type, unlike the intergranular fracture modes of AT and Al₂O₃. The thermal shock properties of the composites were evaluated by water quenching technique and compared with the traditional AT and Al₂O₃. The composites containing only 3.96 vol.% cobalt exhibited higher critical temperature difference and retained flexure strength. The SEM examination of the fracture surfaces of the ATC composites after single thermal cycle showed that voids increased in number and size, and most isolated voids coalesced with increasing temperature difference, which caused the density and strength to decrease. The ATC composite is less sensitive to repeated thermal shock than the AT composite.     

Thermodynamic functions from lattice dynamic of KMgH3 for hydrogen storage applications

The dynamic and the thermodynamic properties of KMgH₃ have been investigated by density functional theory (DFT). We have found that the calculated lattice parameters differ from the experimental data by less than 0.6% and the electronic density of states (DOS) reveals that the KMgH₃ is an insulator. The formation energy of KMgH₃ from binary hydrides (MgH₂ and KH) has been calculated. Using density-functional perturbation theory, we have calculated the phonon dispersion curves, the phonon density of states, the Born effective charge tensors, the dielectric permittivity tensors and the phonon frequencies at the center of the Brillouin zone of KMgH₃. Also we have assigned the calculated phonon frequencies at the gamma point for Infrared-active and Raman-active modes. For the first time, the thermodynamic functions are computed using the phonon density of states.     

Theoretical study of structural, electronic, lattice dynamical and dielectric properties of SrAl2O4

We investigate the structural, electronic, lattice dynamical, and dielectric properties of SrAl₂O₄ within density-function theory. The crystal structure is fully relaxed, and the structural parameters are found to be well consistent with the experimental data. The first pressure derivatives of the bulk modulus are predicted to be 2.5 and 4.3 for local density approximation (LDA) and generalized gradient approximation (GGA), respectively. The electronic band structure shows that the valence band maximum is comprised of O 2p states and a small amount of Al 3s and 3p states, and the conduction band minimum is comprised of Sr 5s and a small amount of O 2p, Al 3s and Al 3p states. The phonon frequencies at the center of the Brillouin zone and the dielectric permittivity tensors are calculated using density-function perturbation theory. The electronic (?_∞) and static (?₀) dielectric permittivity tensors are theoretically predicted by the calculations with both LDA and GGA formalisms. The results show that the electronic dielectric permittivity is isotropic, while the static dielectric permittivity exhibits to be somewhat anisotropic due to the dominant ionic contributions in static dielectric permittivity.     

Lattice dynamics,thermodynamics and elastic properties of monoclinic Li2CO3 from density functional theory

Monoclinic Li₂CO₃ has been identified as a critical component of the solid electrolyte interphase (SEI), a passivating film that forms on Li-ion battery anode surfaces. Here, lattice dynamics, finite temperature thermodynamics and the elastic properties of monoclinic Li₂CO₃ are examined with density functional theory (DFT) and various exchange–correlation functionals. To account for LO-TO splittings in phonon dispersion relations of Li₂CO₃, which is a polar compound, a mixed-space phonon approach is employed. Bond strengths between atoms are quantitatively explored with phonon force constants. Temperature variations of the entropy, enthalpy, isobaric heat capacity and linear (average) thermal expansion are computed using the quasiharmonic approach. The single-crystal elasticity tensor components along with polycrystalline bulk, shear and Young’s moduli are computed with a least-squares approach based upon the stress tensor computed from DFT. Computed thermodynamic properties as well as structural and elastic properties of the monoclinic Li₂CO₃ are in close accord with available theoretical and experimental data. In contrast to a recent DFT study, however, computed vibrational spectra suggest that neither the monoclinic Li₂CO₃ nor its high-temperature hexagonal phase exhibits either elastic or vibrational instabilities.     

Filler-reduced phonon conductivity of thermoelectric skutterudites: Ab initio calculations and molecular dynamics simulations

The phonon conductivities of CoSb₃ and its Ba-filled structure Ba_x(CoSb₃)₄ are investigated using first-principle calculations and molecular dynamics (MD) simulations, along with the Green–Kubo theory. The effects of fillers on the reduction of the phonon conductivity of filled skutterudites are then explored. It is found that the coupling between filler and host is strong, with minor anharmonicity. The phonon density of states and its dispersion are significantly influenced by filler-induced softening of the host bonds (especially the short Sb–Sb bonds). Lattice dynamics and MD simulations show that, without a change in the host interatomic potentials, the filler–host bonding alone cannot lead to significant alteration of acoustic phonons or lowering of phonon conductivity. The observed smaller phonon conductivity of partially filled skutterudites is explained by treating it as a solid solution of the empty and fully filled structures.