纳米结构碲化铋合金的制备及电热输运特性

采用惰性气体保护蒸发-冷凝法制备了纳米Bi及Te粉末, 结合机械合金化和放电等离子烧结技术, 在不同烧结温度下制备出了单一物相且具有纳米层状结构及孪晶亚结构的n型Bi₂Te₃块体材料, 并系统研究了块体材料的晶粒尺度、微结构及其对电热传输特性的影响. SEM, TEM分析结果表明, 以纳米粉末为原料, 通过有效控制工艺条件, 可以制备出具有纳米层状结构Bi₂Te₃合金块体材料, 同时纳米层状结构中存在孪晶亚结构; 热电性能测试结果表明, 具有纳米层状结构及孪晶亚结构的块体试样与粗晶材料相比, 热导率大幅度降低, 在423 K附近, 热导率由粗晶材料的1.80 W/mK降至1.19 W/mK, 晶格热导率从1.16 W/mK降至0.61 W/mK, 表明纳米层状结构与孪晶亚结构共存, 有利于进一步提高声子散射, 降低晶格热导率. 其中在693 K放电等离子烧结后的试样于423K附近取得最大值的无量纲热电优值(ZT), 达到0.74.     

Hydrothermal synthesis and microstructure investigation of nanostructured bismuth telluride powder

Nanostructured thermoelectric Bi₂Te₃ powders with various morphologies were hydrothermally synthesized using different precursors and routes to give an experimental comprehension on the formation of the nanopowders. It was found that the polyhedral Bi₂Te₃ particles are formed by surface nucleation in a continuous nucleation process, the hexagonal Bi₂Te₃ thin sheets are formed in a nucleus saturation process due to the anisotropic growth of the crystals, and the mono-atom reaction model leads to irregular bent thin Bi₂Te₃ sheets. Some quasi one-dimensional nanorods and nanotubes were also found in the synthesized Bi₂Te₃ powders. PACS 81.05.Hd; 81.20.Ka; 61.46.+w     

Mobility restriction of charge carriers in bismuth films due to film block structure

Electric properties of bismuth thin films on mica substrates depending on their structure and concentration of defects have been studied by atomic force microscopy. The obtained results allow optimization of the production of bismuth films with a structure close to single-crystal by thermal evaporation in vacuum.     

Fabrication of bismuth telluride nanotubes via a simple solvothermal process

The unique Bi₂Te₃ tubes were obtained via a simple solvothermal reaction in the presence of ethylenediaminetetraacetic acid disodium salt. The product was characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Bi₂Te₃ nanosheets are vertically grown off the tube wall to form Bi₂Te₃ tubes. A possible formation mechanism is proposed.     

Drop deposition of thin nanostructured coatings of lead telluride

The current-voltage relationships of deposited structures are measured for cluster structures consisting of nanoparticles of lead telluride. Variation in the value of the tunnel current is shown. Optimum conditions for the possible emergence of quantum-hopping conductivity due to carrier tunneling (the characteristic sizes of the nanoclusters and the distances between them) are determined.     

Energy harvesting based on piezoelectric Ericsson cycles in a piezoceramic material

The possibility of recycling ambient energies with electric generators instead of using batteries with limited life spans has stimulated important research efforts over the past years. The integration of such generators into mainly autonomous low-power systems, for various industrial or domestic applications is envisioned. In particular, the present work deals with energy harvesting from mechanical vibrations. It is shown here that direct piezoelectric energy harvesting (short circuiting on an adapted resistance, for example) leads to relatively weak energy levels that are insufficient for an industrial development. By coupling an electric field and mechanical excitation on Ericsson-based cycles, the amplitude of the harvested energy can be highly increased, and can reach a maximum close to 100 times its initial value. To obtain such a gain, one needs to employ high electrical field levels (high amplitude, high frequency), which induce a non-linearity through the piezoceramic. A special dynamic hysteresis model has been developed to correctly take into account the material properties, and to provide a real estimation of the harvested energy. A large number of theoretical predictions and experimental results have been compared and are discussed herein, in order to validate the proposed solution.     

Fabrication of a nanostructured gold-polymer composite material

A facile synthesis route is described for the preparation of a poly-(o-aminophenol)-gold nanoparticle composite material by polymerization of o-aminophenol (AP) monomer using HAuCl₄ as the oxidant. The synthesis was carried out in a methanol medium so that it could serve a dual solvent role, a solvent for both the AP and the water solution of HAuCl₄. It was found that oxidative polymerization of AP leads to the formation of poly-AP with a diameter of 50±10nm, while the reduction of AuCl₄ ^- results in the formation of gold nanoparticles (∼ 2nm). The gold nanoparticles were uniformly dispersed and highly stabilized throughout the macromolecular chain that formed a uniform metal-polymer composite material. The resultant composite material was characterized by means of different techniques, such as UV-vis, IR and Raman spectroscopy, which offered the information about the chemical structure of polymer, whereas electron microscopy images provided information regarding the morphology of the composite material and the distribution of the metal particles in the composite material.     

A nanostructured hybrid material based on polymer infiltrated porous silicon layer

In this work, we report on the fabrication of an hybrid material based on a porous silicon (PSi) layer infiltrated by an amino functionalized poly(ε-caprolattone) (PCL-NH₂). The organic–inorganic structure has been characterized by variable angle spectroscopic ellipsometry, optical reflectometry and water contact angle measurements. A polymer coated PSi layer, 3 μm thick, shows improved wettability and chemical stability against long lasting, up to 18 min, exposure to an alkaline environment. Even after basic treatment, the structure still retains a very good quality optical response, so that it may be proposed as a high performance platform for biochemical sensing applications.     

Band gap and type of optical transitions at the interband absorption edge in solid solutions based on bismuth telluride

The spectra of optical absorption in multicomponent n- and p-type solid solutions based on bismuth and antimony chalcogenides with substitutions in both sublattices of Bi₂Te₃ have been investigated. It has been found that, in all the solid solutions studied, just as in the parent compound Bi₂Te₃, direct allowed transitions occur at the interband absorption edge at T = 300 K. The band gap E _g in the n-Bi_{2 ? x} Sb _x Te_{3 ? y ? z} Se _y S _z solid solutions weakly increases with increasing number of substituted atoms in the Bi and Te sublattices. These atomic substitutions do not leads to an increase in E _g as compared to that of the n-Bi₂Te_2.7Se_0.3 composition. An analysis of the optical absorption spectra suggests that the solid solutions under consideration are weakly degenerate, a conclusion supported by the earlier studies of the thermoelectric and galvanomagnetic properties. It has been established that, in the conduction band of the Bi_1.8Sb_0.2Te_2.7Se_0.15S_0.15 solid solution, there is an additional extremum lying above the main extremum at a distance no more than 0.1 eV.     

Energy and momentum relaxation of charge carriers in Ge and Si under uniaxial stress

Measurements of the piezoresistance and the energy relaxation time T_?, in Ge and Si under uniaxial stress up to 4 kb are reported in the temperature range 30 K < T < 300 K. The measurements of t_? have been performed in the warm carrier range using the harmonic mixing technique. The experimental results for the piezoresistance and energy relaxation time in n-type material can roughly be understood in terms of carrier redistribution in the conduction band valleys whose degeneracy is lifted by the stress. Information is obtained from these measurements about the relative strength of ? and g type intervalley scattering in n-Si; we find nearly equal coupling strength for both scattering types. For the p-type material the experiments show convincingly that the main effect of stress on transport quantities is also caused by the lifted degeneracy of the heavy and light hole bands, as predicted already by Adams, and by Pikus and Bir. The nonlinear dependence of the piezoresistance and τ_? on the stress can very well be explained by approximating the heavy and light hole bands as spherical and parabolical and using the deformation potential constant b as a parameter. The value of ≈2eV obtained for |b| is in good agreement with earlier results for p-Ge.     

On the possibilities of implementing a stochastic model for the distribution of nonequilibrium minority charge carriers in a semiconductor material

Some possibilities of implementing a procedure for estimating statistical characteristics (expectation and autocorrelation function) of the distribution of minority charge carriers (MCCs) generated in a homogeneous semiconductor material are studied. The developed procedure is based on the projection method and the matrix operator technique. It is assumed that the electrophysical parameters of the material (lifetime, diffusion coefficient, and surface recombination rate of MCCs) are random quantities (variables) and obey the Gaussian distribution law. The effect of the variance of these quantities on the depth distribution of MCCs is considered. Some possibilities of this method are illustrated for the case of MCC excitation by a broad beam of electrons with moderate energies.     

Fused silica as a composite nanostructured material

A method for calculating the refractive index of optical fused silica by applying the model of effective permittivity of composite homogeneous media is proposed and realized. The calculation was performed using the tabular data of the refractive index of crystalline α quartz and the ratio of the quartz glass and α quartz densities. It was suggested that fused silica contains nanosized pores with a glass filling number q immersed in a matrix with a density differing from the α quartz density by a factor of κ, where κ is slightly less than unity. It was established that the Maxwell-Garnett model makes it possible to calculate the refractive index of quartz glass and its dispersion in the transparency range (404 nm ≤ λ ≤ 671 nm) with a deviation less than 0.0002 from the tabular values. The calculated and experimental values coincide at q = 0.155 and κ = 0.986.     

Injection of charge carriers from a point contact

Electron injection has been carried out in KCl, KBr, mixed KCl and KBr crystals under constant electric field and at different temperatures. The activation energy connected to ionic zone has been obtained and found to be characteristic for alkalihalides.     

Photogeneration of charge carriers in amorphous selenium

A model is proposed to explain the outstanding features of the free-carrier photogeneration process in amorphous selenium. The excitation of an electron-hole pair and the subsequent separation or recombination are considered as a function of photon energy, applied electric field and temperature. The results of the model are compared in detail with new experimental data and also with published results from a number of sources.     

Mobility of charge carriers in disordered dielectrics

We compare experimental data on the mobility of holes (the majority charge carriers) in polyepoxy-propylcarbazole, measured using the time-of-flight technique (drift mobility) and the nonsteady-state radiation-induced electrical conductivity method (effective mobility). We show that these two quantities are quite different in the dispersive transport regime; and while the second quantity is a characteristic of the material, the first quantity depends in a complicated fashion on the ratio of the electric field strength to the sample thickness. The untreated data on drift mobility measurements using the time-of-flight technique do not have direct physical meaning and cannot be compared with the conclusions of modern microscopic theories of the mobility of charge carriers in disordered matrices.High-Voltage Scientific-Research Institute at Tomsk Polytechnical University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 82–88, July, 1994.     

Lifetime of charge carriers in multiwalled nanotubes

The nature of low-energy excitations in multiwalled nanotubes (MWNTs) is investigated by means of two-color time-resolved photoemission. A careful analysis of the ballistic transport, secondary excitations, and band structure effects was employed in order to extract single electron lifetimes from the observed relaxation trend. It is demonstrated that in the vicinity of the Fermi level the energy dependence of e-e scattering times is inversely proportional to approximately the square of the excitation energy. This result provides strong evidence that electron transport in MWNTs exhibits a Fermi-liquid behavior, indicating that long-range e-e interaction along the tube vanishes due to screening.     

Thermoelectric properties of nano-bulk bismuth telluride prepared with spark plasma sintered nano-plates

The pursuit for a high-performance thermoelectric n-type bismuth telluride-based material is significant because n-type materials are inferior to their corresponding p-type materials in highly efficient thermoelectric modules. Herein, to improve the thermoelectric performance of an n-type Bi₂Te₃, we prepared Bi₂Te₃ nano-plates with a homogeneous sub-micron size distribution and thickness range of about a few tens of nanometers. This was achieved using a typical nano-chemical synthetic method, and the prepared materials were then spark plasma sintered to fabricate n-type nano-bulk Bi₂Te₃ samples. We observed a significant enhancement of the anisotropic electrical transport properties for the nano-bulk sample with a higher power factor along the in-plane direction (24.3?μW?cm^?1?K^?2 at 300?K) than that along the out-of-plane direction (8.1?μW?cm^?1?K^?2 at 300?K). However, thermal transport properties were insensitive along the measured direction for the nano-bulk sample. We used a dimensionless figure of merit ZT to calculate the thermoelectric performance. The results showed that the maximum ZT value of 0.69 was achieved along the in-plane direction at 440?K for the nano-bulk n-type Bi₂Te₃ sample, which was however smaller than that of the previously reported n-type samples (ZT of 1.1). We believe that a further enhancement of the ZT value in the fabricated nano-bulk sample could be accomplished by effectively removing the surface organic ligand of the Bi₂Te₃ nano-plate particles and optimizing the spark plasma sintering conditions, maintaining the nano-plate morphology intact.     

Micro‐Raman investigation of tin dioxide nanostructured material based on annealing effect

Rutile‐structured nanocrystalline tin dioxide (SnO₂) powder was synthesized by the chemical precipitation method using the precursor SnCl₂• 5H₂O. The SnO₂ powder was annealed at different temperatures, namely, 600, 800 and 1000 °C. Micro‐Raman spectra were recorded for both the as‐grown and annealed SnO₂ nanocrystalline samples. Micro‐Raman spectral measurements on the SnO₂ nanoparticle show the first‐order Raman modes A_1g (633 cm⁻¹), E_1g (475 cm⁻¹) and B_2g (775 cm⁻¹), indicating that the grown SnO₂ belongs to the rutile structure. The first‐order A_1g mode is observed as an intense band, whereas the other two modes show low intensity. The full width at half‐maximum and band area of the Raman lines of SnO₂ nanoparticle annealed at various temperatures were calculated. The effect of high‐temperature annealing on the vibrational modes of SnO₂ was studied. The optical image of SnO₂ nanocrystalline material was used to understand the surface morphology effect. Copyright © 2011 John Wiley & Sons, Ltd.     

The mobility of charge carriers in a size-quantized coated semiconductor wire

Within the framework of a staircase infinitely deep potential well model, the mobility of charge carriers is calculated for scattering on impurity centers located on the axis of a size-quantized semiconducting coated wire. Calculations are done for the dielectric constant mismatch of the wire, coating and surrounding environment, taking into account the difference of the effective masses in the wire and coating. The effect of a longitudinal magnetic field on mobility is also considered. Numerical results are presented for the GaAs–Ga_1−xAl_xAs system at different values of the wire and coating radii, the alloy concentration x, and magnetic field.