Douglas Gouvêa

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.

Energetics of CO2 and H2O Adsorption on Zinc Oxide

Adsorption of H2O and CO2 on zinc oxide surfaces was studied by gas adsorption calorimetry on nanocrystalline samples prepared by laser evaporation in oxygen to minimize surface impurities and degassed at 450 °C. Differential enthalpies of H2O and CO2 chemisorption are in the range -150 ±10 kJ/mol and -110 ±10 kJ/mol up to a coverage of 2 molecules per nm(2). Integral enthalpy of chemisorption for H2O is -96.8 ±2.5 kJ/mol at 5.6 H2O/nm(2) when enthalpy of water condensation is reached, and for CO2 is -96.6 ±2.5 kJ/mol at 2.6 CO2/nm(2) when adsorption ceases. These values are consistent with those reported for ZnO prepared by other methods after similar degas conditions. The similar energetics suggests possible competition of CO2 and H2O for binding to ZnO surfaces. Exposure of bulk and nanocrystalline ZnO with preadsorbed CO2 to water vapor results in partial displacement of CO2 by H2O. In contrast, temperature-programmed desorption (TPD) indicates that a small fraction of CO2 is retained on ZnO surfaces up to 800 °C, under conditions where all H2O is desorbed, with adsorption energies near -200 kJ/mol. Although molecular mechanisms of adsorption were not studied, the thermodynamic data are consistent with dissociative adsorption of H2O at low coverage and with several different modes of CO2 binding.

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.

Modification of surface properties of alumina by plasma treatment

Tetraethylorthosilicate (TEOS), hexamethyldisilazane (HMDS) and n-hexane, plasma deposited on alumina pellets, result in hydrophobic and chemically resistant films, while TEOS treated alumina powder showed significant changes in the zeta potential as a function of pH.

Transport properties of La0.6Y0.1Ca0.3MnO3 compounds with different interfaces

Abstract The electrical resistivity ρ( T ), magnetoresistivity ρ( T , H ), and impedance spectroscopy Z (ω, T ) of polycrystalline La 0.6 Y 0.1 Ca 0.3 MnO 3 compounds with different intergranular surfaces were investigated. The interfaces were modified through controlled addition of pores in samples prepared by the sol-gel technique. X-ray diffraction, scanning electron microscopy, and mercury porosimetry analysis showed that all specimens are single phase having similar microstructures and differing only by the volume fraction of pore. ρ ( T , H ) and Z ( ω , T ) data revealed that all samples undergo a metal-insulator transition at T ∼175 K. In addition, the Z ( ω , T ) data indicate two well defined contributions to the electrical resistivity. The ρ ( T , H ) data exhibit features of colossal magnetoresistivity CMR phenomenon but were found to be independent of porosity. The combined results suggest that the CMR effect is mainly associated with percolation of metallic regions and weakly dependent on sample morphology.

Polymeric precursor synthesis of alumina containing manganese oxide

Ceramic materials have been successfully prepared by using organic precursors. Synthesized compounds include, for example, tin dioxide, zinc oxide, niobates, ferrites and silicates. Among all the additives that can promote alumina densification, manganese oxide is especially effective in the absence of liquid phase. Addition of 7 mol % of manganese during aluminum oxide preparation by organic precursors synthesis promotes α-Al 2 O 3 crystallization at 900°C. The resulting powder presents a high specific surface area. Manganese oxide has not been detected by X-ray diffraction analysis even when high manganese concentration was used, suggesting that segregation occurs to the interfaces, similarly to the Mn doped-SnO 2 powder prepared by polymeric method.

Influência das características ácido-básicas da superfície dos óxidos na estabilidade de suspensões cerâmicas de Al2O3 ou SnO2 com dispersantes comerciais

A stable ceramic dispersion can be obtained by mixing a ceramic powder, a solvent and a dispersant. Polymers adsorbed to ceramic particles may stabilize the dispersions by creating a repulsion charge among then.The stabilization has an electrostatic or steric origin, or even the sum of both processes. The adsorption depends on the characteristics of the powder surface. Isoelectric points (IEP) of the SnO2 and alumina are 4 and 8, respectively. The basic/acid characteristic of the surface originates a competition between the adsorption of polymers derivated from polyacrilic acids and hidroxiles due to acidic characteristics of the SnO2 surface. In the other hand, aluminas basic surface allows polymers to adsorb and to increase the zeta potential without any competition between the dispersants and OH- ions. This study has been carried out by simultaneously measuring the pH, zeta potential temperature and conductivity of the system. ESA (Eletrokinetic Sonic Amplitude) technique was used.

A simple flash sintering setup under applied mechanical stress and controlled atmosphere.

Graphical abstract

Estudo da adsorção de dispersante à base de poliacrilato em um sistema varistor comercial: caracterização físico-química

One of the greatest challenges of ceramic processing is to obtain a stable and homogeneous dispersion of particles when the components present different physical and chemical characteristics, such as density, specific surface area, size distribution and isoelectric point (iep). The ZnO varistor composition contains 7 different oxides (additives: Bi2O3, Sb2O3, Co3O4, Mn2O3, NiO, and Cr2O3) and the stability of the aqueous dispersion strongly depends on the specific surface area, the iep and the dispersant choice. In this work we evaluated the ammonium polyacrylate (PAA-NH4) as a dispersant to determine the influence of the different oxides on the processing. The oxides have been characterized by electrophoretic mobility, particle size distribution and specific surface area. Some of varistor oxides additives present isoelectric point (iep) distinct from the ZnO (iep = 9,4). Among them, Sb2O3 (iep = 3.4) and Cr2O3 (iep = 4.4) have been considered. This difference between the ZnO and these additives promotes a change in the pH dispersion that could compromise the dispersion stability.