Seebeck coefficient and electrical conductivity of mesoscopic nanocrystalline SrTiO3

In the present study, we investigate the effect of the grain boundaries on both the electrical transport and the thermoelectric properties. For this purpose, the Seebeck coefficient and the electrical conductivity of a model material, such as nominally pure SrTiO₃ (single crystal, microcrystalline, and nanocrystalline), is measured under oxidizing conditions. The impedance spectroscopy measurements reveal a strong change of the conduction properties of the nanocrystalline sample compared with the unperturbed bulk properties, namely a reduction of the p-type conductivity by two orders of magnitude at high oxygen partial pressure. Similarly, the Seebeck coefficient values of the nanocrystalline sample exhibit remarkable deviations from the single crystal ones: Under oxidizing conditions, values up to 2160 μV K^?1 (at 575 °C) are detected. More importantly, in the nanocrystalline sample, the dependence of the Seebeck coefficient on the concentration of the charge carriers is found to be four times larger than in the single crystal.     

Effect of nanocrystallization on the structural and electrical conductivity enhancement of vanadium-based glasses

A new glass–ceramic nanocomposites material was prepared by a thermal nanocrystallization of V₂O₅–Bi₂O₃–P₂O₅ system with different V₂O₅ content. The amorphous state of glassy materials is confirmed by X-ray diffraction. It was shown by XRD and SEM studies that by suitable heat-treatment glasses can be turned into glass–ceramic nanocomposites consisting of crystallites smaller than 80 nm inserted in the glassy matrix. Also, it was shown that thermal nanocrystallization of as-prepared glassy samples leads to creation of nanocrystalline grains of V₂O₅, Bi₂O₃, and BiVO₄ phases. The glass–ceramic nanocomposites obtained show giant enhancement of electrical conductivity than the as-prepared glasses. The conductivity enhancement was recognized to interfacial regions adjacent crystalline grains. The conduction of the present glasses and their glass–ceramic nanocomposites was confirmed to be due to primarily non-adiabatic hopping of small polaron between vanadium ions.     

Electrochemical characteristics of nano and microcrystalline Fe–Cr alloys

The electrochemical behavior of nano and microcrystalline Fe–10Cr and Fe–20Cr alloys was determined using potentiodynamic polarization in 0.5 M H₂SO₄. Disks of the alloys were prepared by high-energy ball milling followed by compaction and sintering. In the current study, nanocrystalline Fe–Cr alloys reveal significantly different electrochemical characteristics, typified by lower anodic current densities and more negative passivation potentials, compared with their microcrystalline counterparts. In addition to the differences in grain boundary density, compositional characterization of corrosion films carried out by X-ray photoelectron spectroscopy indicates a higher Cr content in the film developed upon nanocrystalline Fe–Cr alloys. Mechanisms for observed enhancement in the corrosion performance of the nanocrystalline Fe–Cr alloys are discussed.     

Local electrical and dielectric properties of nanocrystalline yttria-stabilized zirconia

Grain core and grain boundary electrical and dielectric properties of nanocrystalline yttria-stabilized zirconia (YSZ) were analyzed using a novel nano-Grain Composite Model (n-GCM). Partially sintered pellets with average grain sizes ranging from 10 to 73 nm were analyzed over a range of temperatures using AC impedance spectroscopy (AC-IS). Local grain core and grain boundary conductivities, grain boundary dielectric constants, and effective grain boundary space charge widths were determined from the fitted circuit parameters. Required grain core dielectric constant data were provided by AC-IS measurements of single crystal YSZ over a range of temperatures. The local grain core conductivity of all the nanocrystalline samples was slightly decreased with respect to that of microcrystalline YSZ. Conversely, the local grain boundary conductivity was enhanced up to an order of magnitude compared to microcrystalline YSZ. At the nanoscale, there was a noticeable increase in local grain boundary dielectric constant versus single crystal values at the same temperature.     

Nanocrystalline Oxide Ceramics Prepared by High-Energy Ball Milling

Studies of grain size effects in nanocrystalline materials require a preparation technique which allows adjustment of the grain size. We prepared various nanocrystalline ceramics by high-energy ball milling. The investigated systems are the oxide ceramics Li₂O, LiNbO₃, LiBO₂, B₂O₃, TiO₂ as monophase materials and the composite material Li₂O : B₂O₃. The average grain size was adjusted by variation of the milling time. It was determined via line broadening of X-ray diffraction patterns (XRD) and directly with transmission electron microscopy (TEM). Thermal stability and thermally induced grain growth of the samples can be observed with differential thermal analysis and X-ray analysis. Further information concerning the structure of these heterogeneously disordered materials was extracted from nuclear magnetic resonance (NMR) and infrared spectroscopy. Li diffusion in the lithium-containing compounds is studied with ac conductivity measurements, as well as [⁷Li] NMR relaxation spectroscopy. The TiO₂ is interesting for research on catalytic activity. Ball milling not only causes particle size reduction, but may also lead to phase transitions and chemical reactions. This was verified with XRD.     

Wetting of microcrystalline and nanocrystalline Ni substrates by molten Sn-3.5Ag-0.7Cu alloy

Wetting behavior of molten Sn-3.5Ag-0.7Cu drops on Ni substrates with micrometer and nanometer grains was investigated using a modified sessile drop method. The wettability was poorer while the interfacial reactivity and atomic diffusivity were stronger for the nanocrystalline substrates compared with those for the microcrystalline substrates. The enhanced diffusion of the Ni atoms from the nanocrystallites greatly promoted the nucleation of the intermetallic compounds (IMCs), leading to a much thicker reaction layer and a more distinct regional distribution of the IMCs. On the other hand, the immediately roughened interface greatly decelerated the spreading of the triple line.     

A novel nanocoralloids CrFeO3 nanoparticle: synthesis, electrical property and wettability

Nanocrystalline CrFeO₃ with particle size of about 83 nm was directly synthesized by sol–gel auto-combustion method at room temperature. The overall process involves three steps: formation of homogeneous sol; formation of dried gel; and combustion of the dried gel. Experiments revealed that CrFeO₃ dried gel derived from glycine and nitrate sol exhibits self-propagating combustion at room temperature once it is ignited in air. After auto-combustion, the desired nanocrystalline CrFeO₃ was acquired and no further calcination was needed. The auto-combustion was considered as a heat-induced exothermic oxidation–reduction reaction between nitrate ions and carboxyl group. The synthesized powder was characterized by XRD, TG/DTA, IR spectroscopy, SEM and TEM. Nanocrystalline CrFeO₃ was p type semiconducting material. Shapes of the particles are nanocoralloids in nature. The superhydrophilicity of the mixed oxide investigated by wetting experiments, by the sessile drop technique were carried out room temperature in air to determine the surface and interfacial interactions. The I–V characteristic and electrical properties of the nanocrystalline CrFeO₃ were studied.     

Thermal Conductivity during Phase Transitions

Thermal conductivity is a very basic property that determines how fast a material conducts heat, which plays an important and sometimes a dominant role in many fields. However, because materials with phase transitions have been widely used recently, understanding and measuring temperature‐dependent thermal conductivity during phase transitions are important and sometimes even questionable. Here, the thermal transport equation is corrected by including heat absorption due to phase transitions to reveal how a phase transition affects the measured thermal conductivity. In addition to the enhanced heat capacity that is well known, it is found that thermal diffusivity can be abnormally lowered from the true value, which is also dependent on the speed of phase transitions. The extraction of the true thermal conductivity requires removing the contributions from both altered heat capacity and thermal diffusivity during phase transitions, which is well demonstrated in four selected kinds of phase transition materials (Cu₂Se, Cu₂S, Ag₂S, and Ag₂Se) in experiment. This study also explains the lowered abnormal thermal diffusivity during phase transitions in other materials and thus provides a novel strategy to engineer thermal conductivity for various applications.     

Effect of argon and substrate bias on diamond thin film surface morphology

Nanocrystalline materials are of high interest, because mechanical and physical properties of such materials are different from those or coarse-grained type. Continuous and smooth nanocrystalline diamond (NCD) thin films were successfully grown on mirror polished silicon substrates, using double bias plasma-enhanced hot filament chemical vapour deposition technique. A gas mixture of Ar:CH₄:H₂ and CH₄:H₂ was used as the precursor gas. The effect of the gas composition, flow rate and substrate bias during deposition on diamond crystallite size was investigated. Changing the growth parameters facilitates control of grain size of polycrystalline diamond thin films from microcrystalline to nanocrystalline. The structure of fine-grained NCD films has been studied with scanning electron microscopy and Raman spectroscopy.     

Simultaneous measurements of thermal conductivity and diffusivity of Se80Te20-xInx (x = 2, 4, 6 and 10) chalcogenide glasses at room temperature

Measurements of thermal conductivity and thermal diffusivity of twin pellets of Se₈₀Te_20-xIn_x (x = 2, 4, 6 and 10) glasses, prepared under a load of 5 tons were carried out at room temperature using transient plane source (TPS) technique. The measured values of both thermal conductivity and diffusivity were used to determine the specific heat per unit volume of the said materials in the composition range of investigation. Results indicated that both the values of thermal conductivity and thermal diffusivity increased with the addition of indium at the cost of tellurium whereas the specific heat remained almost constant. This compositional dependence behaviour of the thermal conductivity and diffusivity has been explained in terms of the iono-covalent type of bond which In makes with Se as it is incorporated in the Se-Te glass.     

Thermal plasma synthesized nano-powders of (LaCe)B6 starting from oxide-based precursors and its field electron emission performance

High temperature synthesis process of microcrystalline and nanocrystalline (LaCe)B₆ using Self-propagating High temperature Synthesis (SHS) and arc plasma gas phase condensation methods, respectively have been investigated. These methods are rapid and economically viable processes used for the synthesis of technologically important refractory hexaborides. Microcrystalline powders of (LaCe)B₆ were synthesized using the SHS process, starting from oxide precursors of lanthanum, cerium, and boron. The powders obtained using SHS process were used as a precursor to getting nanocrystalline (LaCe)B₆ using the thermal plasma route. The thermal plasma synthesis was carried out using nitrogen and argon plasmas, respectively. In-situ plasma diagnostics were used to identify evaporated species and determine plasma temperature during the formation of nanocrystalline (LaCe)B₆ using optical emission spectroscopy. Further, an effect of plasma input parameters on the structural and optical properties of as synthesized nanocrystalline (LaCe)₆ were investigated using XRD analysis, Raman spectroscopy, and X-ray photoelectron spectroscopy. A thorough investigation of morphological and structural properties of synthesized nanocrystalline (LaCe)₆ was carried out using transmission electron microscopy. Finally, field effect electron emission properties of the microcrystalline and nanocrystalline product were investigated and current density, turn on field, stability of emission performance were determined.     

Morphology and conductivity studies of a new solid polymer electrolyte: (PEG)<Subscript>x</Subscript>LiClO<Subscript>4</Subscript>

A new solid polymer electrolyte, (PEG)_xLiClO₄, consisting of poly(ethylene)glycol of molecular weight 2000 and LiClO₄ was prepared and characterized using XRD, IR, SEM, DSC, NMR and impedance spectroscopy techniques. XRD and IR results show the formation of the polymer-salt complex. The samples with higher salt concentration are softer, less opaque and less smooth compared to the low salt concentration samples. DSC studies show an increase in the glass transition temperature and a decrease in the degree of crystallinity with increase in the salt concentration. Melting temperature of SPEs is lower than the pure PEG 2000. Room temperature¹H and⁷Li NMR studies were also carried out for the (PEG)_xLiClO₄ system. The¹H linewidth decreases as salt concentration increases in a similar way to the decrease in the crystalline fraction and reaches a minimum at aroundx = 46 and then increases.⁷Li linewidth was found to decrease first and then to slightly increase after reaching a minimum atx = 46 signifying the highest mobility of Li ions for this composition. Room temperature conductivity first increases with salt concentration and reaches a maximum value (σ = 7.3 × 10⁻⁷ S/cm) atx = 46 and subsequently decreases. The temperature dependence of the conductivity can be fitted to the Arrhenius and the VTF equations in different temperature ranges. The ionic conductivity reaches a high value of ∼10 ⁻⁴S/cm close to the melting temperature.     

The influence of alizarin and fluorescein on the photoactivity of Ni,Pt and Ru-doped TiO2 layers

The doping with different metal ions and sensitization with organic compounds are two well known methods used to improve the photoactivity of TiO₂. In this respect, the metallic ions-doped TiO₂ samples were prepared by embedding Ni, Pt and Ru ions into TiO₂ crystalline network and then, each sample was sensitized with alizarin and fluorescein dyes. The qualitative evaluation of prepared TiO₂-based materials was made by: UV–vis spectroscopy, spectrofluorimetry, FT/IR spectroscopy and microscopy, X-ray diffraction and N₂ physisorption measurements. The optoelectronic properties investigated by UV–vis spectroscopy show that the optical response of Ni-doped TiO₂ layer shifts to visible. The X-ray spectra do not show peaks of nickel, platinum and ruthenium oxide crystals or pure metals. The FT/IR spectra proved the presence of dye molecules adsorbed on titania nanoparticles surface. These results demonstrated that the studied dopants and dyes have potential to promote modified TiO₂-based materials as good candidates to be used in photolectrocatalytic processes.     

Enhancement of thermal conductivity of hydrogenated silicon film by microcrystalline structure growth

Hydrogenated silicon film is fabricated by plasma enhanced chemical vapor deposition method, and the enhancement of thermal conductivity of hydrogenated silicon film by microcrystalline structure growth is investigated. The thermal conductivity of films is measured based on Fourier thermal transmitting law by using platinum electrode. Raman spectroscopy characterization reveals the crystalline volume fraction (X _c) of microcrystalline silicon (μc-Si:H) and demonstrates it is embedded with nanocrystals. Spectroscopic ellipsometry with Forouhi–Bloomer model is used to obtain the thickness of films. The measurement results show that the thermal conductivity of μc-Si:H is much higher than amorphous silicon (a-Si:H).     

The interfacial structure of nanocrystalline Fe73.5Cu1Mo13Si13.5B9 studied by Mössbauer spectroscopy

Nanocrystalline Fe_73.5Cu₁Mo₃Si_13.5B₉ prepared by crystallization of the amorphous alloy was investigated by using Mössbauer spectroscopy and X-ray diffractometry. The present study focuses on the interfacial composition and the important role of the interfacial component in the development of the nanocrystalline structure. On the basis of a newly developed fitting program, the amorphous phase is denoted by a low field component and a high field one. Upon crystallization, the former, which corresponds to the boundary regions adjacent to the grains become more significant. The different stages associated with the crystallization are discussed.     

Synthesis and ionic conductivity of silver magnesium zirconium molybdates

The silver magnesium zirconium molybdate Ag₄Mg₂Zr(MoO₄), undoped and aliovalently doped with yttrium and aluminum, has been characterized by impedance spectroscopy, X-ray diffraction, and IR spectroscopy. The materials studied possess relatively high ionic conductivity at temperatures above 550 K. Partial yttrium substitution on the zirconium site ensures an increase in ionic conductivity almost throughout the temperature range studied.     

Microstructure and ion transport in Li1 + x Ti2 − x M x (PO4)3 (M = Cr,Fe, Al) NASICON-type materials

Li_{1 + x} Ti_{2 ? x} M _x (PO₄)₃ (M = Cr, Fe, Al) NASICON-type materials have been prepared by the Pechini process and solid-state reactions and characterized by X-ray diffraction, scanning electron microscopy, and impedance spectroscopy. We have identified the factors that determine the rate of ion transport in nanocrystalline and bulk samples at low and high temperatures. The effects of the preparation procedure and heterovalent doping on the ionic conductivity of the materials have been assessed. Heterovalent doping is shown to have a considerably stronger effect on the ionic conductivity in comparison with the microstructure of the materials.     

The ionic conductivity and dielectric properties of Ba1−xSnxF2 solid solutions prepared by mechanochemical milling

Solid solutions of Ba_1−xSn_xF₂ fluoride ion conductors, with x = 0.1–0.4, have been synthesized by the mechanochemical milling technique for the first time. All of the prepared materials crystallize in the cubic fluorite-type structure, which indicates that the solid solution can be synthesized in the studied composition range by the mechanochemical milling technique at ambient temperature and pressure. The ionic conduction of the investigated materials has been studied by impedance spectroscopy. The ionic conductivity increased considerably, by up to six orders of magnitude compared to pure un-milled BaF₂, with increasing SnF₂ content. From the analysis of the conductivity spectra of the investigated materials it is found that the concentration of mobile fluoride ions is independent of temperature with almost the same values for the investigated materials. The present results suggest that the enhanced mobility of mobile ions is the origin of the higher ionic conductivity. The dielectric properties and the associated relaxation phenomena of the current materials are also described.     

Influence of synthesis methodology on the ionic transport properties of BaSnF4

The high performance fluoride ion conductor BaSnF₄ has been prepared by a simple precipitation in addition to both solid state reaction and mechanochemical synthesis techniques. XRD results indicate that the material BaSnF₄ obtained by all the methods crystallizes with the same tetragonal structure (P₄/nmm). The crystallite size and morphology of the BaSnF₄ particles determined by XRD and FE-SEM show a variation with respect to different methods of preparation. The transport properties of BaSnF₄ have been investigated by impedance spectroscopy and the results show that the conductivity values are closely related to the crystallite size and micro-strain. The transport number measurement and NMR studies further confirm that the increase in conductivity is due to fluoride ions. The scaling result of complex impedance plots shows that the dynamical process occurring at various frequencies are independent of temperature.     

Synthesis of a polytitanocarbosilane and its conversion into inorganic compounds

The novel polytitanocarbosilane, formed by the cross-linking of polycarbosilane with titanium tetra-alkoxide, was synthesized to examine the process of converting a multielement organometallic polymer into an inorganic compound. The chemical structure of this polymer was investigated by the techniques of infra-red spectroscopy (IR), gel permeation chromatography (GPC), number average molecular weight measurements and²⁹Si nuclear magnetic resonance (NMR) measurements. The pyrolysis products in N₂ gas at 1400° C and 1700° C were the microcrystalline and crystalline states of silicon carbide and titanium carbide, respectively.