One-dimensional self-consistent solution of modulation doped nanocrystalline/crystalline Si heterojunctions

A kind of modulation doped structure of n-type nanocrystalline hydrogenated silicon (nc-Si:H) film with intrinsic nc-Si:H layer with p-type bulk Si 100 substrate was proposed. The numerical self-consistent solutions of one-dimensional Schrödinger and Poisson equations along the direction normal to the heterojunction were performed to calculate the distribution of electron density and profiles of conduction-band as a function of ionized donor concentration in doped film, thickness of intrinsic layer, and other device parameters in the junctions. The calculated results are shown to be in agreement with experimental data. The relation of mobility vs sheet density of two-dimensional electron gases under different scattering mechanisms was analyzed. The obtained consequences may be used to evaluate optimum design for the modulation doped nc-Si:H-based devices.     

Preparation and microwave-absorbing properties of NiFe2O4-polystyrene composites

NiFe₂O₄ nanoparticles were synthesized by the polyacrylamide gel method with acrylamide as the monomer and N,N′-methylenediacrylamide as lattice agent. The average crystallite sizes of the nickel ferrites annealed at 500, 600 and 800 °C are about 10, 30 and 50 nm, respectively. Ferrite-polystyrene composites were made by hot pressing, and microwave-absorbing properties of the composites with different contents of 35, 45, 55 and 65 wt% ferrite were investigated by testing complex permeability and complex permittivity in the X-band (8.2-12.4 GHz) frequency range. All the parameters, ε′, ε″, μ′ and μ″, increase with increasing ferrite content. The reflection losses were calculated based on a model of a single-layered plane wave absorber backed by a perfect conductor. The composite with 65 wt% ferrite content shows a minimum reflection loss of −13 dB at 11.5 GHz with a −10 dB bandwidth over the extended frequency range of 10.3-13 GHz for an absorber thickness of 2 mm.     

Synthesis of terbium doped calcium phosphate nanocrystalline powders by citric acid sol–gel combustion method

Terbium doped calcium phosphate (Tb-doped CaP) nanocrystalline powders were synthesized by the citric acid sol–gel combustion method. The phase composition, morphology and luminescent property of Tb-doped CaP nanocrystalline powders were characterized by powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy, fluorescence spectrophotometer and fluorescence microscopy. At 700 °C, Tb-doped CaP nanocrystalline powders are composed of HAP (main phase) and β-TCP (minor phase) with Tb doping content of 0.5–4%. SEM and TEM observations show that the 4% Tb-doped CaP nanocrystalline powders are about 50–150 nm spherical particles. The 4% Tb-doped CaP nanocrystalline powders exhibit the strongest emission at 548 nm (λ_excitation = 240 nm) and show strong green fluorescence under fluorescence microscopy.     

Indirect evidence of supramolecular changes within cellulose microfibrils of chemical pulp fibers upon drying

Dried and never-dried chemical pulps were subjected to strong sulfuric acid hydrolysis and the dimensions of the resulting cellulose nanocrystals (CNCs) were characterized by AFM image analysis. Although the average length of CNCs was fairly similar in all samples (55–65 nm), the length distribution histograms revealed that a higher number of longer crystals and a lower number of shorter crystals were present in the CNC suspensions prepared from never-dried pulps. The distinction was hypothetically ascribed to tensions building in individual cellulose microfibrils upon drying, resulting in irreversible supramolecular changes in the amorphous regions. The amorphous regions shaped by tensions were deemed as more susceptible to acid hydrolysis.     

An X-ray scattering study of hydrogenated nanocrystalline silicon with varying crystalline fraction

Using X-ray diffraction (XRD) and small angle X-ray scattering (SAXS), we probed the nanostructural features of several PECVD grown nc-Si:H thin films with varying crystalline volume fraction. XRD results of a mixed phase film, 70% a-Si:H and 30% c-Si:H, show these crystallites have a preferred [220] orientation in the growth direction. Another film with approximately 90% c-Si also shows elongated grains, but with a preferred [111] orientation. The SAXS results also show an increase in scattering intensity when compared to the mixed phase material. In the mixed phase material, models show that the electron density fluctuations between the amorphous and crystalline phases are not enough to explain the measured SAXS scattering. Hydrogen clustered at the crystallite boundaries and in void regions of the a-Si phase must be included as well.     

Control of the nanocrystalline zirconia structure through a colloidal sol-gel process

A simple method to synthesize tetragonal zirconia stabilized at ambient temperature is developed and allows the monitoring of the tetragonal-monoclinic transition via a colloidal sol-gel process. By increasing the pH of an aqueous solution consisted of a zirconium precursor and a complexing agent (acetylacetone), a colloidal sol and then a gel can be formed under slightly acidic condition. After a drying step, tetragonal zirconia is easily obtained with an adequate thermal treatment at low temperature. The tetragonal-monoclinic transition occurs when the calcination temperature is increased. The relationship between the crystallite size, the crystallographic structure and the thermal treatment has been investigated by X-Ray Diffraction and the behaviour of the system from the gel state to the final powder has been studied by using Small Angle X-Ray Scattering and thermal analysis techniques. We demonstrate that compared to a chemical precipitation route, this colloidal sol-gel process allows the nanostructure of the material to be controlled due to the formation of primary nanoparticles. The presence of these nanoparticles makes possible the specific determination of the zirconia crystallographic phase through an accurate control of the nanostructure during the thermal treatment.     

Modeling emissivity of low-emissivity coating containing horizontally oriented metallic flake particles

The scattering and absorption cross sections of horizontally oriented metallic flake particles are estimated by extended geometric optics that includes diffraction and edge effects. Emissivity of the coating containing those particles is calculated using Kubelka–Munk theory. The dependence of emissivity of the coating on the radius, thickness, content of metallic flake particles and coating thickness is discussed. Finally, theoretical results are compared with the experimental measurements with Al/acrylic resin coating system and the results show that simulation values are in good agreement with experimental ones.     

温度对CaO-MgO-Al_2O_3-SiO_2系纳米晶玻璃陶瓷结构影响的拉曼光谱研究

采用熔融法制备了CaO-MgO-Al2O3-SiO2系纳米晶玻璃陶瓷材料。在-190~310℃温度范围,利用显微共聚焦拉曼光谱测量分析了该体系纳米晶玻璃陶瓷硅氧四面体结构的变化规律。结果表明,随着温度的降低具有不同非桥氧键的硅氧四面体结构单元变化并不一致,位于三维硅氧四面体结构边缘的具有二个非桥氧键的硅氧四面体(Q2)和具有三个非桥氧键的硅氧四面体(Q1),以及完全独立于三维硅氧网络结构外的具有四个非桥氧键的硅氧四面体(Q0)受温度影响明显,其拉曼光谱均向高波数移动,键的力常数变强,硅氧键长变短。为丰富外环境对纳米晶玻璃陶瓷材料结构与性能影响的研究提供了实验依据,也为控制该体系纳米晶玻璃陶瓷材料的膨胀系数提供了一定的实验依据。     

Thermal stability of grain boundaries in nanocrystalline Zn studied by positron lifetime spectroscopy

Nanocrystalline Zn prepared by compacting nanoparticles with mean grain size about 55 nm at 15 MPa has been studied by positron lifetime spectroscopy. For the bulk Zn sample, the vacancy defect is annealed out at about 350 °C, but for the nanocrystalline Zn sample, the vacancy cluster in grain boundaries is quite difficult to be annealed out even at very high temperature (410 °C). In the grain boundaries of nanocrystalline Zn, the small free volume defect (not larger than divacancy) is dominant according to the high relative intensity for the short positron lifetime (τ₁). The oxide (ZnO) inside the grain boundaries has been found having an effect to hinder the decrease of average positron lifetime (τ_av), which probably indicates that the oxide stabilizes the microstructure of the grain boundaries. This stabilization is very important for the nanocrystalline materials using as radiation resistant materials.     

Effect of nitrogen addition on hydrogen incorporation in diamond nanorod thin films

The effect of nitrogen addition in the feed gas on the finally incorporated amount of hydrogen in the diamond nanorods (DNRs) thin films has been investigated. The Raman spectroscopy measurements helped to understand the structural and quality changes with increasing nitrogen gas flow rate during CVD deposition. The hydrogen concentration was measured with 3.0 MeV He²⁺ beam using elastic recoil detection analysis technique and it was found that with the addition of nitrogen, the hydrogen concentration was increased. The results of non-Rutherford backscattering spectroscopy (NRBS) used to measure the amount of nitrogen in the DNRs thin films have shown that the incorporated nitrogen is below the detection limit of NRBS technique. Our results suggested that the addition of nitrogen has affected the overall quality of diamond films in two ways; increasing the thickness of diamond films by increasing the non-diamond carbon content and increasing the hydrogen impurity incorporation. The role of nitrogen additive on diamond growth and hydrogen incorporation is discussed.