Effect of phonon focusing on Knudsen flow of phonon gas in single-crystal nanowires made of spintronics materials

Effect of anisotropy of elastic energy on the phonon propagation in single-crystal nanowires made of Fe, Cu, MgO, InSb, and GaAs materials that are used to fabricate spintronics devices in the regime of the Knudsen flow of phonon gas has been studied. A new method of analyzing the focusing of quasi-transverse modes has been suggested, which made it possible to determine the average values of the densities of phonon states in the regions of focusing and defocusing slow and fast quasi-transverse modes. The effect of phonon focusing on the anisotropy of heat conductivity and lengths of the phonon free paths has been analyzed for all acoustic modes that exist in spintronics nanostructures. It has been shown that for all the nanowires investigated the angular dependences of the free paths of fast and slow transverse modes in the {100} and {110} planes correlate with the angular dependences of the densities of phonon states for these modes. Directions of the heat flux that ensure the maximum and minimum phonon heat conductivity in the nanowires have been determined.     

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.     

Models of thermal conductivity of multilayer wear resistant coatings

The thermal conductivity of non-metallic nano- and microstructured materials is a key parameter when considering wear resistant coatings. Here, different models of phonon transport and estimation of thermal conductivity are analyzed. The hopping mechanism of phonon transport was found to be applicable for small-grain-size materials in understanding thermal conductivity, whereas large grain size materials can be studied using the correlation function approach. The Chapman–Enskog approach to the Boltzmann transport equation assuming that the transport of phonons is controlled by scattering on the grain boundaries has been analyzed. The Fourier law of thermal conductivity is obtained with the thermal conductivity inversely proportional to the specific surface of the boundaries.     

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。     

晶界几何结构对纳晶ZnO材料导热过程的影响

为了探究晶界几何结构对纳晶ZnO材料导热性能的影响,将晶界表面抽象出几种典型几何形状,深入讨论了晶界表面粗糙度的计算以及声子入射角对镜面反射率的影响,改进了晶界镜面反射率的计算模型.采用PhonTS软件,用迭代法求解玻尔兹曼输运方程模拟计算得到了纳晶ZnO晶格热导率.基于分子动力学理论计算了ZnO完美材料的热导率,分析...     

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.     

Optical and acoustic phonon modes confined in gallium phosphide nanoparticles

The main aim of this paper is to discuss the confinement effects on the optical and acoustic phonon vibrational modes in gallium phosphide(GaP) nanoparticles(cylindric grain).The Raman scattering from the GaP nanoparticles was investigated.It was found that the red-shifts of the longitudinal optical(LO) mode and transverse optical(TO) mode were 15 cm?1 and 13.8 cm?1,respectively.It is generally accepted that the red-shifts of the optical phonon modes are due to the presence of smaller nanosized particles(～1.2 nm) acting as the nanoclustered building blocks of the GaP nanoparticles.In the low frequency Raman spectrum,a set of Stokes lines with almost the same spacing was clearly observed.The scattering feature originates from the discrete phonon density of states of the nanoclustered building blocks.According to Lamb's vibrational theory,the Raman shift wavenumbers of the spheroidal mode and torsional mode of the lowest energy surface modes for the nanoclustered building blocks were calculated.Good agreement can be achieved between the calculated results and the observed scattering peaks.These results indicate that the corresponding Raman peaks are due to scattering from the localized acoustic phonons in the nanoclustered building blocks in the GaP nanoparticles.     

Structural, morphological, luminescent and electronic properties of sprayed aluminium incorporated iron oxide thin films

Thin films of aluminium incorporated Fe₂O₃, synthesized by simple chemical spray pyrolysis on to glass substrates using aqueous solutions of analytical reagent grade ferric trichloride and aluminium nitrate as precursors. The influence of aluminium doping on to morphological properties, contact angle, X-ray photoelectron spectroscopy, photoluminescence and thermal conductivity properties have been investigated. The preparative parameters have been optimized to obtain good quality thin films which are uniform and well adherent to the substrate. The FE-SEM and AFM micrographs depict the films are compact and homogeneous (spindle-shaped hematite nanostructures) with varying grain sizes (average grain size ~ 20-60 nm). Contact angle measurement show the films are hydrophobic in nature. The chemical composition and valence states of constituent elements in Fe₂O₃ are analyzed by X-ray photoelectron spectroscopy. The excitonic strong violet emission has been observed in photoluminescence. The specific heat and thermal conductivity study shows the phonon conduction behavior is dominant in these polycrystalline films. We studied interparticle interactions like grains, grain boundary effects using complex impedance spectroscopy.     

Effects of Sn-doping on the electrical and thermal transport properties of p-type Cerium filled skutterudites

p-type Sn-doped CoSb₃-based skutterudite compounds have been prepared using melting-quenching-annealing method and spark plasma sintering technique. Sn atoms in our samples are completely soluted on Sb-site with a fixed charge state and non-magnetic feature, providing a better choice to ascertain the effect of element doping at the [Co₄Sb₁₂] framework on the electrical and thermal transport properties in p-type skutterudites. Doping Sn at the framework introduces additional ionized impurity scattering to affect the electron transport greatly. Similar electrical transport properties between Ce_0.2Co₄Sb_11.2Sn_0.8 and Co₄Sb₁₁Sn_0.6Te_0.4 suggest that Ce fillers contribute little to the valence band edge. Filling Ce into the voids and doping Sn at the framework introduce additional phonon resonant and point defect scattering mechanisms, thereby reducing lattice thermal conductivity remarkably. Moreover, our data suggest that combining these two effects is more effective to suppress lattice thermal conductivity through scattering broad range of phonons with different frequencies.     

应变影响下单晶Ge薄膜热导率分析

利用非平衡态分子动力学模拟方法研究了应变对Ge薄膜热导率的影响。结果表明系统应变对单晶Ge薄膜热导率产生明显影响,热导率随着拉伸应变的增大而减小,随着压缩应变的增大而增大,得出声子速率降低以及薄膜表面重构是产生该模拟结果的内在原因。同时,采用修正的Callaway模型对NEMD结果进行理论验证,两种方法得到的结果吻合得较好。理论结果表明应变弛豫时间对Ge单晶薄膜的热导率产生了重要影响。     

Nonlinear rearrangement of the structure of domain walls in a thin magnetic film with a uniaxial in-plane anisotropy

The structures of domain walls have been investigated by the method of three-dimensional computer simulation in films with uniaxial in-plane anisotropy at different film thicknesses. For the calculations, the parameters characteristic of permalloy films have been taken. It has been established that, depending on the film thickness, the walls that prove to be stable are one-dimensional Néel walls, cross-tie walls, and C- and S-shaped walls. Film thicknesses at which the transition between the different types of walls occurs have been determined. The structure of Bloch lines and Bloch points in C- and S-shaped walls has been investigated. The values of topological charges for different micromagnetic structures have been calculated.     

Correlation between the chemical composition and the conduction mechanism of barium strontium titanate thin films

Sol-gel barium strontium titanate thin films with different barium-to-strontium (Ba:Sr) values have been fabricated as MFM configurations. The Perovskite phase for the films is confirmed via XRD. In order to correlate the effect of the chemical composition of the films with the conduction mechanism, different AC electrical parameters have been addressed. The results show that the impedance and dielectric constant decrease as Ba content in the film increases, whereas the conductivity shows the opposite variation; this is attributed to the grain size and dipole dynamics. Complex impedance (Z*) and electric modulus (M*) planes show three overlapping regions as the response for the bulk, the grain boundaries and the film/electrode interface mechanisms. These mechanisms have been represented by an equivalent circuit. The imaginary component of electric modulus (M″) versus frequency plots, which reveal relaxation peaks that are not observed in the dielectric loss (?″) plots, and it is found that these peaks are of a non-Debye-type. Furthermore, the frequency dependent conductivity plot shows three regions of conduction processes.     

The effect of low-temperature conditions on the electrochemical polymerization of polypyrrole films with high density,high electrical conductivity and high stability

High-quality polypyrrole-hexafluorophosphate (PPy-PF₆) films with high density (∼1.4 g/cm³), high conductivity (>300 S/cm for unstretched film) and high electrochemical stability are obtained reproducibly by galvanostatic polymerization at low-temperature conditions. The optimal polymerization current density of J_p=0.02–0.05 mA/cm² was obtained at the polymerization temperature of T_p=−40°C. The surface morphology of the film sensitively varies depending upon the properties of electrode and its surface conditions. The transport measurements characterize the high-density PPy-PF₆ film as a disordered metal close to the boundary of disorder induced metal–insulator (M–I) transition. The X-ray diffraction measurements suggest that partially crystalline structure of PPy-PF₆ film is related to the transport properties. The uniaxial stretching induces an increase of the conductivity up to ∼930 S/cm in a direction parallel to stretching as well as the anisotropy of conductivity. The comparative studies of thermogravimetric analysis (TGA), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) for PPy-PF₆ films prepared at room-temperature and low-temperature conditions show that the latter exhibit better thermal stability as well as electrochemical stability under long oxidative polarization.     

Effects of deposition parameters on microstructure and thermal conductivity of diamond films deposited by DC arc plasma jet chemical vapor deposition

The uniform diamond films with 60 mm in diameter were deposited by improved DC arc plasma jet chemical vapor deposition technique. The structure of the film was characterized by scanning electronic microcopy(SEM) and laser Raman spectrometry. The thermal conductivity was measured by a photo thermal deflection technique. The effects of main deposition parameters on microstructure and thermal conductivity of the films were investigated. The results show that high thermal conductivity, 10.0 W/(K-cm), can be obtained at a CH4 concentration of 1.5% (volume fraction) and the substrate temperatures of 880-920 ℃ due to the high density and high purity of the film. A low pressure difference between nozzle and vacuum chamber is also beneficial to the high thermal conductivity.     

Role of mechanical strain on thermal conductivity of nanoscale aluminum films

Thin film components of conventional and flexible solid-state devices experience mechanical strain during fabrication and operation. At the bulk scale, small values of strain do not affect thermal conductivity, but this may not true for grain sizes comparable with the electron and phonon mean free paths and for higher volume fraction of grain boundaries. To investigate this hypothesis, thermal and electrical conductivity of nominally 125-nm-thick aluminum films (average grain size 50 nm) were measured as functions of tensile thermo-mechanical strain, using a modified version of the 3-ω technique. Experimental results show pronounced strain–thermal conductivity coupling, with ～50% reduction in thermal conductivity at ～0.25% strain. The analysis shows that mechanical strain decreases the mean free path of the thermal conduction electrons, primarily through enhanced scattering at the moving grain boundaries. This conclusion is supported by similar effects of mechanical loading observed on the electrical conduction in the nanoscale aluminum specimens.     

Pinning structure of dimensional phonon resonance in quantum wires

The dimensional phonon resonances in dimensionally-quantum semiconductor wires are considered. It is supposed that the absorption of light is caused by electron transitions between dimensionally-quantum subbands in potential wells with the participation of long-wavelength phonons. The influence of resonance electron-phonon interaction on a spectrum of one-dimensional electrons in quantum wires is investigated. It is shown that the spectrum contains two branches and also the splitting of the peak of the dimensional phonon resonance into two components; the distance between them is determined by the resonance electron-phonon interaction.     

A study on the thermal conductivity measurements of Cu thin films utilizing temperature distribution

Thin film type materials are widely used in high-tech industries including electronics, photonics and even machine tools. Often, knowledge of the thermal properties of thin films is needed to assess reliability through thermal stress analysis when the thin film type materials are applied to functional electronic parts. Only a few methods have been developed for thermal conductivity measurement of a thin film on a substrate. In this study Cu thin films were processed on the borosilicate glass substrate of prismatic bar shape using sputtering. Two Cu coated surfaces of specimens were brought into contact to maintain the insulated boundary conditions. The temperature distributions were measured from the back surface of the substrate using radiation thermometry. The thermal conductivities of the Cu thin films were measured and found to be much lower than those of bulk materials. The measured thermal conductivities were found to be closely related to the microstructures of the Cu thin films.     

Ferromagnetic resonance in a two-layer exchange-coupled ferromagnetic film with a combined uniaxial and cubic anisotropy in the layers

Dynamic properties of ferromagnetic two-layer exchange-coupled (100) films with a combined cubic and uniaxial magnetic anisotropy of layers have been studied numerously upon the magnetization along the [100], [010], and [011] directions. The allowance for cubic anisotropy substantially affects the dependence of the frequencies of the ferromagnetic resonance on the field strength. Repeated changes in the localization of the ferromagnetic-resonance modes between the layers of the film have been found to occur with an increase in the strength of the magnetic field. At a certain relationship between the constants of the combined anisotropy for the directions [010] and [011], an increase in the field leads to a shift of the maximum of the dynamic-susceptibility distribution toward the interlayer boundary without a change in the localization of the modes.     

Localized vibrations around a soliton in polyacetylene

Localized vibrations and optical and acoustic phonons are calculated for finite length C_nD_n+2 containing a charged soliton. The molecular orbital method and normal coordinate analysis have been used in this calculation. The localized translational mode couples with the CH bending mode when the chain length is short (n ≈17). The coupling explains well the two broad peaks in the i.r. spectrum. It is also found that dopant pinning has effects similar to chain shortening; i.e. raising the frequency of the T mode and causing coupling of the T and TH modes. Approximate phonon dispersions are calculated for finite trans-polyacetylene with a charged soliton. Branch modes of the previously known localized modes are discussed in connection with the corresponding optical phonons.     

单晶锗薄膜热导率的分子动力学模拟

采用非平衡分子动力学(NEMD)方法研究平均温度为400 K,厚度d=2.8288～11.315 nm的单晶锗薄膜法向的热导率.模拟结果表明,单晶锗薄膜热导率随薄膜厚度的增加以接近线性的规律增加,其数值明显低于同等温度下体态锗的试验值.当薄膜厚度一定时,单晶锗薄膜的热导率随温度增加变化幅度很小,与同体态锗热导率随温度的变化规律相比表现出明显的尺寸效应.