Ricardo H. R. Castro

Surface segregation of additives on SnO2 based powders and their relationship with macroscopic properties

Abstract Surface properties of ceramic powders frequently play an important role in producing high-quality, high-performance, and reliable ceramic products. These properties are related to the surface bond types and interactions with the surroundings. Oxide surfaces generally contain adsorbed hydroxyl groups and modifications in the chemical composition of the surface may be studied by infrared spectroscopy. In this work, we prepared SnO2 containing Fe or Mg ions by organic chemical route derived from Pechini’s method. The prepared powders were characterized by infrared spectroscopy (FT-IR), X-ray diffraction (XRD), dynamic electrophoretic mobility and surface area determination. Results demonstrated that the studied additives segregate onto the oxide surface and modify the hydroxyl IR bands of the adsorbed hydroxyl groups. These surface modifications change some macroscopic properties of the powder such as the isoelectric point (IEP) in aqueous suspensions and the final specific surface area. The increase of the surface area with additive concentration is supposedly due to the reduction of surface energy of the powders when additives segregate on the powder surface.

Sintering: the role of interface energies

Abstract Sintering has been recognized as a complex evolution of microstructure during thermal treatments. The densification in such process is usually attributed to diffusion parameters, i.e. certain mass transport mechanisms would promote densification and others would not. In this work, a new model demonstrating that the interface energies are determinant factors in the densification is drawn. Considering that the mass transport mechanism of grain growth is the same as that of pore elimination, and supposing some reasonable hypothesis about the geometric evolution during sintering, densification is related to the dihedral angle of the system. Since the dihedral angle is directly proportional to interface energies, the ratio between grain surface and grain boundary energies determines densification. Good agreement of the numerical predictions with the experimental data was obtained.

Radiation Tolerance of Nanocrystalline Ceramics: Insights from Yttria Stabilized Zirconia

Materials for applications in hostile environments, such as nuclear reactors or radioactive waste immobilization, require extremely high resistance to radiation damage, such as resistance to amorphization or volume swelling. Nanocrystalline materials have been reported to present exceptionally high radiation-tolerance to amorphization. In principle, grain boundaries that are prevalent in nanomaterials could act as sinks for point-defects, enhancing defect recombination. In this paper we present evidence for this mechanism in nanograined Yttria Stabilized Zirconia (YSZ), associated with the observation that the concentration of defects after irradiation using heavy ions (Kr+, 400 keV) is inversely proportional to the grain size. HAADF images suggest the short migration distances in nanograined YSZ allow radiation induced interstitials to reach the grain boundaries on the irradiation time scale, leaving behind only vacancy clusters distributed within the grain. Because of the relatively low temperature of the irradiations and the fact that interstitials diffuse thermally more slowly than vacancies, this result indicates that the interstitials must reach the boundaries directly in the collision cascade, consistent with previous simulation results. Concomitant radiation-induced grain growth was observed which, as a consequence of the non-uniform implantation, caused cracking of the nano-samples induced by local stresses at the irradiated/non-irradiated interfaces.

Bioassay-Guided Isolation and HPLC Determination of Bioactive Compound That Relate to the Antiplatelet Activity (Adhesion, Secretion, and Aggregation) from Solanum lycopersicum

In seeking the functionality of foodstuff applicable to medicine, ripe tomato fruits were found to show an antiplatelet activity. Therefore, the bioactive compound was isolated, structurally identified, and studied for an inhibitory effects on platelet adhesion, secretion, and aggregation. The concentration of adenosine in ripe tomato fruits (pulp and skin extracts) and its processing by-products (paste and pomace) was determined by reversed-phase high-performance liquid chromatography (HPLC). According to platelet aggregation inhibition induced by ADP, the total extract residual was fractionated by liquid-liquid separation, obtaining aqueous, ethyl acetate and petroleum ether extracts. The aqueous extract was subjected to repeated permeation over sephadex LH-20 and semipreparative TLC. The isolate finally obtained was identified as adenosine on the basis of ESI-MS, 1H NMR, HPLC, and UV spectra. Adenosine concentration dependently (2.3–457 μM) platelet aggregation inhibited induced by ADP. Also, adenosine present inhibition of platelet secretion and thrombus formation under flow conditions. The quantitative HPLC analysis revealed significant amounts of adenosine in ripe tomato fruits and its processing by-products. From these results, extracts/fractions of ripe tomato fruits and their processing by-products may be referred to as functional food and functional ingredients containing a compound that inhibits platelet function with a potent preventive effect on thrombus formation, as those that occur in stroke.

Direct measurement of grain boundary enthalpy of cubic yttria-stabilized zirconia by differential scanning calorimetry

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.

The influence of the Chitosan adsorption on the stability of SnO2 suspensions

Abstract The influence of the polyssacharide CS on the zeta potential variation of a SnO 2 aqueous suspension has been systematically studied. The maximum potential obtained for the suspension was about 30 mV, which is high enough to stabilize the suspension. No significant effects on the zeta potential variation were identified when the molecular weight of CS was raised. Kinetics experiments have also been carried out and the potential of the suspension was stabilized about 10 s after the addition of the polymer. A new procedure was proposed for obtaining the adsorption isotherm curve, studying its relation with ζ potential variation in increasing CS concentration. The method avoids some inconveniences of experimental procedures and the results obtained were very reasonable. The interaction of the CS with the surface was studied by FTIR and indicates that the most probable interaction is the formation of hydrogen bonds between the polymer and the surface hydroxyl groups. A deflocculating curve of a suspension of SnO 2 as a function of CS concentration is also presented, indicating that at about 1.5 mg of CS/g of SnO 2 the viscosity falls to zero.

Modeling grain growth kinetics of binary substitutional alloys by the thermodynamic extremal principle

The thermodynamics and kinetics fundaments of grain growth in binary substitutional alloys were analyzed using the thermodynamic extremal principle. Applying the regular solution approximation, a new equation for solute segregation at steady-state diffusion is proposed, which suggests reduced solute segregation as the grain boundary (GB) solute concentration increases, differently from previous models [Acta Mater 2009;57(5):1466, Acta Mater 2012;60:4833, Scripta Mater 2010;63:989] that adopt constant segregation enthalpy. Furthermore, a self-consistent consideration has been carried out to account for the coupled changes in GB energy and GB mobility as a result of solute segregation. On this basis, the quantitative relation is evaluated between the thermodynamic and kinetic effects of solute segregation to determine the dominant role in retarding and even suppressing grain growth, by comparison of the dimensionless GB energy (i.e., the GB energy of alloy over that of pure solvent) and the dimensionless effective GB mobility (i.e., the effective GB mobility over that of pure solvent): the kinetic effect prevails if the dimensionless effective GB mobility is smaller than the dimensionless GB energy, and vice versa. The present model is adopted to describe well the experimental results for Fe–P alloys, and nanocrystalline Ni–P and Pd–Zr alloys.

Obtaining highly dense YSZ nanoceramics by pressureless, unassisted sintering

For technological applications, zirconia is commonly blended with other oxides to stabilise the tetragonal and/or cubic phases at low temperature, being yttria the most frequently added dopant. It is generally desirable to obtain highly dense ceramics while maintaining grain sizes in the nanoscale ( < 100 nm). Small grains contribute to stabilise the tetragonal phase and to improve the toughness and flexural strength. Moreover, a higher ionic conductivity for cubic zirconia electrolytes is achieved with smaller grain size and lower thickness of the intergranular regions. The sintering onset temperatures required for nanometric particles are significantly reduced when compared to conventional micrometric powders. However, densification is generally accompanied by an undesirable grain coarsening. A Ramp and Hold Sintering (RHS) is the simplest densification schedule, consisting of heating up to the peak temperature followed by a holding time at that temperature. Another approach, called Two-Step Sintering (TSS) is based on the principle that the activation energy for grain growth is lower than the activation energy of densification. The key elements in this method are heating up to a high temperature to achieve a density >75% Theoretical Density (TD) to render the pores unstable, and then cooling down rapidly to a lower temperature to finish sintering and hinder grain growth. Alternatively, considerable efforts have been made in increasing the heating rate and/or reducing the hold time at the peak temperature during sintering cycles in processes that are generally referred to as ‘Fast Firing’ (FF) or ‘rapid sintering’. This review summarises the attempts in the literature for obtaining dense monolithic nanocrystalline Yttria-Stabilized Zirconia (YSZ) ceramics by pressureless sintering schedules carried out in conventional furnaces. RHS, FF and TSS schedules are reported only from YSZ as starting powders, i.e. without the aid of any additional dopant or grain growth inhibitor. For the sake of comparison, the discussion is focused mainly on YSZ nanoceramics with final densities >99% TD and average grain size  < 100 nm. Powder and shaping effects on microstructure and properties of bulk nanoceramics are discussed. A comparison among sintering approaches is then made taking into account the microstructural development of the nanostructures and some key properties of the products.

Synthesis of stoichiometric nickel aluminate spinel nanoparticles

Abstract Nickel aluminate is a transition metal oxide with spinel structure with potential applications as catalysts and sensors. Both applications benefit from high specific surface areas as well as chemical stoichiometry control. However, a systematic approach to understand synthetic parameters affecting stoichiometry and agglomeration of nickel aluminate nanoparticles is still lacking. In this work, coprecipitation using direct and reverse strikes and polymeric precursor techniques were comparatively studied to address this problem. While the polymeric method could deliver stoichiometric spinel, the samples were highly agglomerated exhibiting low surface area. Both co-precipitation procedures produced smaller sizes and less agglomerated nanoparticles as compared to the polymeric precursor, but for the reverse-strike, Ni2+ preferentially formed a soluble complex with ammonia and led to nickel deficient nanoparticles. Stoichiometric (1 mol NiO:1 mol Al2O3) nanocrystalline nickel aluminate was only achieved when using controlled excess Ni2+. The normal-strike lead to more stoichiometric compositions without need for excess cations, but the obtained nanoparticles were less homogeneous and showed smaller surface areas as compared to the reverse-strike method.

Overview of Conventional Sintering

This introductory chapter addresses an overview of the main processes taking place during the so-called Sintering phenomenon. The driving forces and mechanisms for the competitive processes, coalescence and grain growth, are also briefly discussed. The goal is not to provide and exhaustive review of the field, but to give the reader a concise perspective of the commonly accepted concepts of sintering, pathing the way for the next chapters of this book to address the specifics of nanosintering and field assisted processes.