Role of hydration layer on rheology of nano alumina suspensions

Technological implication of reduction in viscosity of nanosize ceramic suspensions with environmentally benign and inexpensive additives is not trivial. This presentation will discuss the flow characteristics of concentrated nano-alumina powder suspensions. Unusually high viscosities observed for suspensions of nanoparticles compared to those of micron size powders cannot be explained by current viscosity models. For a given solids content, as the particle size decreases so does the interparticle distance leading to overlapping interparticle forces. Concomitant with the particle size reduction, increase in surface area of the solids requires higher surfactant concentrations for effective steric stabilization. The rheology of nanosize alumina suspensions and its variation with solids content and with saccharide concentration were explored by rheometry. The mechanism of dramatic viscosity reduction by saccharide addition (primarily fructose) is studied by TGA, DSC, and NMR. The interparticle forces between the nanometric alumina particles in water and in fructose solutions were investigated by AFM. The interactions between the nano-alumina particles in water can be explained by the DLVO theory. However, DLVO theory can not adequately describe the interactions between particles for suspensions containing saccharide. The interaction forces (amplitude and range) between nanometric alumina particles decrease with increasing saccharide concentration. Formation of so-called hydration layer on alumina nanoparticles in water was hypothesized for years, but never observed experimentally. The direct visualization of hydration layer over nanosize alumina particles was realized with the fluid cell transmission electron microscopy in situ. The hydration layer over the particle aggregates was observed and it was shown that these hydrated aggregates constitute new particle assemblies which in turn alter the flow behavior of the suspensions. These nanoclusters alter the effective solids content and the viscosity of nanosize alumina suspensions. Our findings elucidate the source of high viscosity observed for nano particle suspensions and are of direct relevance to many industrial sectors including materials, food, cosmetics, pharmaceutical among others employing colloidal slurries with nanosize particles

Boron modified molybdenum silicide and products

A boron-modified molybdenum silicide material having the composition comprising about 80 to about 90 weight % Mo, about 10 to about 20 weight % Si, and about 0.1 to about 2 weight % B and a multiphase microstructure including Mo.sub.5 Si.sub.3 phase as at least one microstructural component effective to impart good high temperature creep resistance. The boron-modified molybdenum silicide material is fabricated into such products as electrical components, such as resistors and interconnects, that exhibit oxidation resistance to withstand high temperatures in service in air as a result of electrical power dissipation, electrical resistance heating elements that can withstand high temperatures in service in air and other oxygen-bearing atmospheres and can span greater distances than MoSi.sub.2 heating elements due to improved creep resistance, and high temperature structural members and other fabricated components that can withstand high temperatures in service in air or other oxygen-bearing atmospheres while retaining creep resistance associated with Mo.sub.5 Si.sub.3 for structural integrity

Carbon or boron modified titanium silicide

A titanium silicide material based on Ti.sub.5 Si.sub.3 intermetallic compound exhibits substantially improved oxidative stability at elevated temperatures. In particular, carbon is added to a Ti.sub.5 Si.sub.3 base material in an amount (e.g. about 0.3 to about 3.6 weight % C) effective to impart substantially improved oxidative stability at elevated temperatures, such as about 1000 Si.sub.3 base material in an amount (e.g. about 0.3 to about 3.3 weight % B) to this same end

Effect of W substitutions on the phase stability and oxidation behaviour of Mo-Si-B alloys

Mo-Si-B alloys are potential candidate materials for extreme environments, especially at temperature regimes beyond the operating limits of superalloys. Metal rich compositions show excellent creep resistance and fracture toughness, but the oxidation resistance is poor due to low Si and B content. Intermetallic rich compositions show excellent oxidation resistance, but poor fracture toughness. Therefore, the inherent challenge in this system is designing an alloy with excellent prime reliance on its intrinsic oxide scale, with excellent creep and adequate fracture toughness. We address this conundrum by attempting to destabilize the brittle phase A15 Mo3Si in the intermetallic phase field. In the current work, ab-initio calculations were used to evaluate the thermodynamic phase stability of the A15 phase. Experiments indicated that this phase becomes unstable beyond a critical W content (~ 10 atom%). Single crystal studies revealed the site occupancies with W addition to be in accordance with the thermodynamic models. Following studies on phase stability, a series of oxidation experiments were carried out at different temperatures and time intervals with sintered as well as cast alloys. The microstructural length scales and morphologies changed significantly with processing conditions. Transient oxidation studies reveal a strong microstructure dependence of oxidation in this alloy. In addition to transient oxidation studies at 1100 and 1400°C, we shall also present isothermal oxidation behavior of this alloy in the 1100 – 1500°C range. Tungsten additions modify the pesting range of this material, due to the higher volatilization temperature of (WO3)3. The lower vapor pressure of (WO3)3, in comparison to (MoO3)3 also results in a lower initial metal recession, especially in alloys with finer microstructures. The oxidized cross-sections revealed the formation of a continuous borosilicate scale that covers the alloy surface completely resulting in excellent high temperature oxidation resistance

Designing oxidation resistant ultra-high temperature ceramics through the development of an adherent native thermal barrier

We present a design concept for developing ZrB2-SiC-AlN composites with enhanced oxidative stability at ultra-high temperatures (∼2000 °C) and low pressures (100 Torr). The oxidative stability of these materials arises from a protective silica based scale. However, active oxidation of SiC above 1700 °C presents a challenge, which we circumvent through the in-situ growth of a zirconia layer that serves as a thermal barrier, ensuring that the effective temperature at the zirconia/Si rich subscale is less than the active oxidation temperature. The design concept is validated by a series of ultra-high temperature oxidation experiments under static as well as cyclic conditions

Synthesis and characterization of ionic block copolymer templated calcium phosphate nanocomposites

Self-assembling thermo-reversibly gelling anionic and zwitterionic pentablock copolymers were used as templates for precipitation of calcium phosphate nanostructures, controlling their size and ordered structural arrangement. Calcium and phosphate ions were dissolved in a block-copolymer micellar dispersion at low temperatures. Aging at ambient temperature produced inorganic nanoparticles, presumably nucleated by ionic interactions. The self-assembled nanocomposites were characterized by small-angle X-ray and neutron scattering (SAXS/SANS), nuclear magnetic resonance (NMR), thermogravimetric analysis (TGA), and transmission electron microscopy (TEM). 1H-31P NMR with 1H spin diffusion from polymer to phosphate proved the formation of nanocomposites, with inorganic particle sizes from ∼2 nm, characterized by 1H-31P dipolar couplings, to \u3e 100 nm. TEM analysis showed polymer micelles surrounded by calcium phosphate. SAXS attested that a significant fraction of the calcium phosphate was templated by the polymer micelles. SANS data indicated that the order of the polymer was enhanced by the inorganic phase. The nanocomposite gels exhibited higher moduli than the neat polymer gels. The calcium phosphate was characterized by TGA, X-ray diffraction, high-resolution TEM, and various NMR techniques. An unusual crystalline phase with \u3e2 chemically and \u3e3 magnetically inequivalent HPO4 2- ions was observed with the zwitterionic copolymer, highlighting the influence of the polymer on the calcium phosphate crystallization. The inorganic fraction of the nanocomposite was around 30 wt % of the dried hydrogel. Thus, a significant fraction of calcium phosphate has been templated by the tailored self-assembling ionic block copolymers, providing a bottom-up approach to nanocomposite synthesis

Block copolymer/ferroelectric nanoparticle nanocomposites

Nanocomposites composed of diblock copolymer/ferroelectric nanoparticles were formed by selectively constraining ferroelectric nanoparticles (NPs) within diblock copolymer nanodomains via judicious surface modification of ferroelectric NPs. Ferroelectric barium titanate (BaTiO3) NPs with different sizes that are permanently capped with polystyrene chains (i.e., PS-functionalized BaTiO3NPs) were first synthesized by exploiting amphiphilic unimolecular star-like poly(acrylic acid)-block-polystyrene (PAA-b- PS) diblock copolymers as nanoreactors. Subsequently, PS-functionalized BaTiO3 NPs were preferentially sequestered within PS nanocylinders in the linear cylinder-forming polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) diblock copolymer upon mixing the BaTiO3 NPs with PS-b-PMMA. The use of PS-b-PMMA diblock copolymers, rather than traditional homopolymers, offers the opportunity for controlling the spatial organization of PS-functionalized BaTiO3 NPs in the PS-b-PMMA/BaTiO3 NP nanocomposites. Selective solvent vapor annealing was utilized to control the nanodomain orientation in the nanocomposites. Vertically oriented PS nanocylinders containing PS-functionalized BaTiO3 NPs were yielded after exposing the PS-b-PMMA/BaTiO3 NP nanocomposite thin film to acetone vapor, which is a selective solvent for PMMA block. The dielectric properties of nanocomposites in the microwave frequency range were investigated. The molecular weight of PS-b-PMMA and the size of BaTiO3 NPs were found to exert an apparent influence on the dielectric properties of the resulting nanocomposites