Skutterudite compounds were studied as new potential candidates for thermoelectric applications.CoSb 3 showed good electrical properties and doping this material with nickel and tellurium would improve the thermoelectric properties.In this work the mechanisms for the electrodeposition of cobalt, nickel, antimony and telurium in citrates solutions on a nanostructured gold template was studied.Comprehensive deposition-stripping experiments have been performed in order to understand the mechanism of the co-deposition of cobalt and antimony, as well as of the Ni and Te dopants.The results showed that there were two mechanisms operating in this system, i.e. the deposition of Co and Ni is controlled by the electrochemical reaction at the Au electrode, while Sb and Te depositions are controlled by diffusion.Although the experiments were performed in an electrolyte that contained citrates to form Sb and Te complexes and to increase the solubility of Sb 2 O 3 and TeO 2 , the deposition of Sb and Te is still controlled by diffusion due to their low solubility.

Skutterudite compounds form a new class of potential candidates for thermoelectric applications. Cobalt triantimonide (CoSb3) shows good thermoelectric properties at medium and high temperatures. Doping this system with substitution elements, for either Co or Sb or both, may result in an increase of the thermoelectric figure of merit (ZT). This work focused on the electrochemical doping and characterization of films and nanowires of Co-Sb system in citrate solutions using gold-coated PCTE templates. The electrodeposition was performed on gold surface that was pre-treated electrochemically to ensure reproducible results. The electrochemical treatment acted as an annealing process for the surface, which resulted in an increase in Au(111) as demonstrated by XRD. Detailed electrochemical studies including deposition-stripping experiments was performed in order to develop a better understanding of the co-deposition kinetics and a better control over the composition of doped Co-Sb system. Scanning electron microscopy (SEM/EDS) helped study the morphology and the composition of the doped and undoped Co-Sb system. Co-deposition of Co-Sb showed that the amount of Co is higher in nanowires than in film or mushroom caps due to the slow Sb deposition rate dictated by slow Sb(III) complex diffusion. Doped nanowires have been also obtained. Both Ni and Te electrochemical doping of the Co-Sb system affected the composition of the deposit but there was no effect on nanowire morphology.

Bulk nanocrystalline Mg2Si thermoelectric materials were synthesized and consolidated in a one-step process through a solid-state reaction between magnesium hydride and silicon, using the spark plasma sintering (SPS) method. The hydrogen produced in the process alleviates the problem of the oxidation of Mg. The samples were reactively sintered at temperatures in the range 723-823 K and under a uniaxial pressure in the range of 71-164 MPa in 5 min. Powder X-ray diffraction (XRD) analysis showed the products to be pure Mg2Si. The grain size of the consolidated samples was less than 500 nm, as determined by transmission electron spectroscopy (TEM). Residual nano-pores were observed by scanning electron microscopy at grain boundaries; their presence is believed to be the consequence of hydrogen evolution during the reactive sintering. The effect of synthesis temperature and pressure on crystallite size, density, and transport properties was determined. The results showed that use of MgH2 instead of Mg in the one-step method prevents the formation of MgO. The addition of 1 at.% Bi as a dopant improved the power factor significantly. Samples with 1 at.% Bi had a ZT of 0.6 at 775 K. (C) 2014 Elsevier B. V. All rights reserved.

Microcalorimetry was used to study the adsorption of water molecules on the surface of ZnAl_2O_4 nanoparticles ranging from the anhydrous to the fully hydrated states. Water adsorption of ZnAl_2O_4 showed similar behavior to the isostructural γ-Al_2O_3 and revealed possible existence of hydrophobic sites on the surfaces. At the lowest measured coverage (0.49 H_2O per nm^2), the enthalpy of adsorption is −155.46 kJ/mol. This value decays with increasing coverage and at around 13 H_2O per nm^2, the heat of adsorption levels at −44 kJ/mol, suggesting further adsorbed water has liquid-like features. The anhydrous surface energy for ZnAl_2O_4 was calculated to be 1.36 ± 0.08 J/m^2 using water adsorption microcalorimetry data. High-temperature oxide melt solution calorimetry was also used to assess the surface energy, which was 1.29 ± 0.33 J/m^2. Surface energies at different hydration states are reported and showed decrease with increasing coverage, suggesting that low humidity conditions allow higher driving forces for coarsening.

The goal of consolidating powders to achieve high densities at lower temperatures and with a small grain size has motivated considerable efforts in the search for methods to activate the sintering process. Enhancement of the consolidation process has been attempted through various approaches including mechanical activation of the powders, the addition of sintering aids, and the use of electromagnetic fields. The latter approach has received considerable attention in recent years, largely due to the widespread use of devices utilizing current and pressure to consolidate powders. The Spark Plasma Sintering method (also known by other names) has seen a remarkable increase in its utilization over the past two decades. This was largely due to the many significant, and in some cases, unique accomplishments. In this chapter we will focus then on the role of the electric field in sintering with emphasis on recent observations, particularly those pertaining to the consolidation of nanostructured materials.

The role of an applied electric field during the densification of ionic ceramics is still being debated. Here we describe how the polarization of a dielectric material contributes to the field strengths at particle surfaces and interfaces during the initial stage of sintering. Using numerical models, it is shown that significant increases in local field strengths can be expected during initial neck formation and continue, with decreasing contribution, through approximately half of the first stage of sintering. The field strengths achievable in common commercial and custom lab-scale electric field assisted sintering systems are found to be comparable to those at which electric fields have been shown to enhance the densification behavior of ionic ceramics.

In order to establish the relative contributions of thermal and athermal mechanisms to densification in the absence of an extrinsic sintering pressure, nanometric powder compacts were sintered with and without applied fields using varied heating rates from 50°C/min up to 800°C/min. The relative contribution of the thermal and athermal mechanistic contributions to the densification behavior of two model dielectric ceramics, hydroxyapatite and zinc oxide, is evaluated in the context of the current leading theories of field-assisted sintering mechanisms. The effects of elevated heating rates in nanometric, dielectric ceramics are found to be minimal in the absence of a field. However, in the presence of an applied field there appears to be a synergistic effect with heating rate.

Recent investigations regarding the role of applied fields on the grain growth and densification behavior of ionic ceramics are providing strong insights into the efficacy of Field Assisted Sintering Technique (FAST), aka Spark Plasma Sintering (SPS). Explanations of the observed behaviors, such as grain growth suppression and densification enhancement, are based upon the conjectured presence of a Joule heating driven temperature differential between grain interfaces and grain cores. These differentials were thought to be responsible for providing increased densification rates and lower densification temperatures through grain growth suppression and/or increased local kinetics at the forming necks. In this paper, we analyze the energetic, thermal, and practical details of this process in the context of the commonly accepted stages of sintering.

The electrochemical behavior of the Co–Sb system on Au substrate during cyclic voltammetry and potentiostatic deposition was investigated. Electrochemical behavior of Co and Sb was studied and compared to the Co–Sb system. At a negative potential (−0.9 V vs. Ag/AgCl) the electrochemical behavior of this binary system was similar to that of individual Co and Sb combined. For more negative vertex potentials (e.g., −1.2 V vs. Ag/AgCl), results from cyclic voltammetry have shown the presence of a new compound different from Co and Sb which could only be detected at slow sweep rate. The deposition performed at constant potentials between −1.0 and −1.2 V have resulted in films that were made of CoSb3 and Sb as indicated by XRD. Surface film studied by SEM and EDS has shown morphological and compositional non-uniformities caused by hydrogen evolution.

A straight-forward set of experiments using differential scanning calorimetry was used to obtain the average grain boundary enthalpy at high temperatures for 10 mol. % yttria-stabilized zirconia (10YSZ) by exploiting the heat of grain growth on nanograined dense samples consolidated by spark plasma sintering. The heat of grain growth was measured and correlated with the quantified microstructure evolution during the process. The average grain boundary enthalpy of 10YSZ was found to be 1.00 ± 0.29 J m−2 for the temperature range 900–1300 °C. Comparing this result with room temperature data in the literature, small temperature dependence of the grain boundary enthalpy could be found outside the experimental uncertainties in both experiments.

We report here the on-command cargo controlled delivery using an alternating magnetic field (AMF) from magnetic silica mesoporous supports capped with a lipid bilayer.

Surface energies have postulated importance to catalysis, crystal growth,. sintering, polymorphism, and many other fields. importance is even more critical when dealing with nanostructured materials, where the surface-to-volume ratio is considerably higher and the surface term. accounts for a much larger fraction of the total free energy. Here we present a novel approach to experimentally assess the average anhydrous and hydrated surface energies of oxides, and used the method to determine, the surface energy of gamma alumina The method uses a water adsorption setup combined with a microcalorimeter where the heat of adsorption can be monitored as a function of the adsorbed amount Maintaining a closed system, the approach enables the correlation of the molecular configuration of absorbed water with the thermodynamic data, and hence the definition of the point where a liquid water configuration exists (at high relative humidity). This information allows the calculation of the surface energy at room temperature for any coverage state by using the adsorption calorimetric data For gamma alumina, a close relationship between the water adsorption behavior and the surface energy was observed, evidencing that higher surface energies are associated with highly energetic dissociative behaviors of water and a continuous surface energy decrease upon water adsorption. Three adsorption stages were clearly observed from the combination of adsorption isotherm and microcalorimetric data, consistently with presented models.

Field-assisted sintering technique (FAST) and spark plasma sintering (SPS) apparatuses are based on the same technical principles, but differ in design. A current flows directly through the graphite pressing tool, which acts as heating element in both machines. The main differences are the pressing tool geometry and material, current pulsing and the position of the temperature measurements. Tests under identical conditions (same heating schedule, applied load, and pressing tool, as well as similar pulse patterns) were conducted on both types of set-ups. Three different oxide materials were tested: alumina, 8 mol% yttria-stabilized zirconia (8YSZ) and zinc oxide.

In this work, zinc sulfide (ZnS) nanoparticles were formed by nucleation and growth in ultrathin films of polydiallyldi-methylammonium chloride (PDDA)–polystyrenesulfonate sodium salt (PSS) film produced by the Layer-by-Layer (LbL) deposition technique. Multilayer thin film assemblies, fabricated by sequential adsorption of polyelectrolytes on a quartz substrate, were used as a supramolecular reaction template to study the in-situ nucleation and growth of ZnS nanoparticles. ZnS nanoparticles were nucleated within the polymeric supramolecular structure through cyclic expo-sure to the solutions of Zn(NO3)2 and thiourea. The growth and nucleation of nanoparticles were accomplished by a cyclic repetition of reductive hydrolysis reactions. The growth of a thin film on a flat substrate via LbL was monitored by ultraviolet-visible (UV-Vis) spectroscopy. Analysis of the UV-visible absorption spectra of the films revealed that the nanoparticles grew with increasing number of cycles. The presence of ZnS nanoparticles were verified by transmission electron microscopy (TEM). Selected area electron diffraction (SAED) showed that the ZnS has a cubic spheralite structure.

The compositional effects in ZrB2–SiC–ZrC ultra high temperature composites with four different compositions were investigated via Spark Plasma Sintering (SPS) at a maximum temperature of 1800°C. Density, Rockwell hardness, and thermal conductivity were measured, along with structural X-ray diffraction (XRD) and microstructural characterization. The relative amounts of SiC and ZrC had an influence on the composites’ density, mechanical and thermal properties.

Preparation of fine grained, hard and ductile pure tungsten for future fusion reactor applications was tested using the bottom-up approach via powder consolidation by spark plasma sintering (SPS) at different temperature (1300–1800 °C) and pressure (90–266 MPa) conditions. Pure tungsten powders with an average particle size of about 1 μm were sintered to high density (about 94%) with almost no grain growth at a temperature below 1400 °C and an applied pressure up to 266 MPa. These samples had a multi-modal grain size distribution (resembling the size distribution of the initial powder) and a very high Vickers hardness (up to 530 kg/mm2). Above 1500 °C fast grain growth occurred and resulted in a drop in hardness. XRD on the surface of bulk samples showed a small amount of tungsten oxides; however, XPS and EDS indicated that these oxides were only surface contaminants and suggested a high purity for the bulk samples. The results demonstrate that SPS can lead to ultrafine and nanocrystalline tungsten if used to consolidate pure nano tungsten powders.

The phenomenal increase during the past decade in research utilizing pulsed electric current to activate sintering is attributed generally to the intrinsic advantages of the method relative to conventional sintering methods and to the observations of the enhanced properties of materials consolidated by this method. This review focuses on the fundamental aspects of the process, discussing the reported observations and simulation studies in terms of the basic aspects of the process and identifying the intrinsic benefits of the use of the parameters of current (and pulsing), pressure, and heating rate.