In situ study of phase transformations and phase equilibria in the tantala and hafnia binary system

Tantala (Ta2O5) has useful dielectric, optical, catalytic and chemical properties while hafnia (HfO2) has found applications as a high temperature structural ceramic. Tantala and hafnia ceramics have yet to be studied in detailed. During this study, the Ta2O5-HfO2 binary system was investigated using high temperature x-ray diffraction from room temperature to 1650˚C in air using synchrotron radiation. The crystal structures of Ta2O5, HfO2 and Hf6Ta2O17 were examined with their corresponding phase transformations and 3-D thermal expansions

Recent developments in geopolymer composites and their potential applications

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In situ synchrotron studies of oxide ceramics to 3,000°C

A quadrupole halogen lamp furnace (QLF) capable of heating to 2,000°C in air has been developed in our laboratory, in collaboration with Dr. Julius Schneider at the Ludwig Maximillian University in München, Germany. A conical nozzle levitator (CNL) developed by Dr. Richard Weber at Materials Modification in Chicago, Illinois is capable of in situ XRD measurements of oxides to 3,000°C in air. These two instruments were used at the Advanced Photon Sources (APS) at the Argonne National Laboratory, and the QLF was used at the National Synchrotron Light Source II (NSLSII) at Brookhaven National Laboratory to carry out the following experiments: (i) Thermal expansion measurements in 3-D (ii) Solid state phase transformations (iii) Solid state chemical reactions (iv) In situ determination of phase diagrams A variety of ceramic and mineral examples are provided to illustrate the seven crystal systems (cubic, tetragonal, orthorhombic, rhombohedral, hexagonal, monoclinic and triclinic). Computer software (Program CTEAS) has been developed to visualize the thermal evolution in 3D for individual {hkl} planes, principle strain directions and whether they are increasing or decreasing. When a crystal undergoes a phase transformation upon heating, the 3D crystal structural, lattice correspondence between the parent and product phases can be identified from the continuity of thermal expansion for planes in the parent phase which approximately “become” planes in the product phase. The example is given of a peritectic reaction in the binary HfO2-Ta2O5 system where Hf6Ta2O17 decomposes on heating into liquid HfO2-Ta2O5 solid solution plus HfO2 at 2242 ± 16 °C. A Z = 4, pseudo-subcell is identified which is common to the parent and product phases, which, coupled with vector analysis identifies a lattice correspondence between them, and hence possible orientation relationship. The oxidation of SiC dispersed into ZrB2 was studied as an example of a solid state reaction where the kinetics and chemical mechanisms were elucidated. Intermediate crystalline phases that were formed during oxidation of ZrB2, could be identified and quantified in real time. The oxidation of ZrB2 phase could be followed independently of concurrent phases, whether amorphous or crystalline, or simultaneous reactions. Increasing the SiC content in the ZrB2-SiC composites retarded the oxidation of ZrB2. A novel approach to estimate the thickness of an oxidation layer formed during oxidation of ZrB2 and ZrB2-SiC composites, in-situ at high temperatures was proposed, based on fractional conversion of ZrB2 to ZrO2 A systematic approach to the rapid production of the high temperature, ternary HfO2-Ta2O5-TiO2 phase diagrams is presented. This study highlights the combined use of: (i) in-situ high temperature X-ray diffraction on heating to 2,000°C in the QLF, as well as on cooling of liquidi from 3000 ˚C in air in the CNL, and (ii) extraction of common atomic motifs with associated material symmetry analysis. The HfO2-Ta2O5-TiO2 ternary phase diagram has 4 congruently melting compounds: HfO2, Ta2O5, TiO2 and TiTa2O7 and 2 incongruently melting compounds: Hf6Ta2O17 and HfTiO4. There are no ternary congruently melting compounds. Symmetry relations between Hf6Ta2O17 and HfTiO4 have been identified. Symmetry decomposition shows that these two structures are simply related to each other via polyhedral rotations. Finally, 10 invariant reactions were identified in this phase space. There is sufficient in-situ high temperature X-ray diffraction data to analyze the ternary between the lowest melting point isotherm and the room temperature isotherm

Thermal expansion and phase transformation behavior in the rare-earth titanate system

The thermal expansion and phase transformation behavior in the rare-earth titanate system has not been well understood. Much of the previous work in this material system was performed ex situ and reveals little information about the thermal expansion, mechanisms behind this expansion, and the relationship between the orthorhombic and hexagonal phases with temperature. In this paper, a unique method using a thermal image furnace and synchrotron radiation was employed to monitor crystallographic changes in this material system in situ to determine the thermal expansion tensor and describe the mechanism behind such behavior. In addition, this information was paired with insights into the volume expansion, structural elements, and geometric units between the orthorhombic and hexagonal phases to describe a potential pathway between two crystallographic cells which have no group-subgroup relationship. The novel pairing of information to describe a reconstructive transformation in this manner is unique and may be a new method to describe such transformations where few tools currently exist today. Additionally, a new experimental technique to study the phase transformation kinetics in situ, which can avoid traditional pitfalls of such investigations, was used to examine the kinetic parameters describing the orthorhombic to hexagonal phase transformation. While the technique still needs development, it nonetheless describes the transformation kinetics in a sufficient manner for an initial experiment and understanding

Low cost synthesis of silicon-based ceramic powders from Na, K and Cs Geopolymer

Geopolymer carbon precursors (1:1:4.5:12+18C) for carbothermal reduction and nitridation were prepared by the following route. NaGP, KGP and CsGP resins were prepared by mixing metakaolin (Al2O3.2SiO2) in NaOH, KOH, or CsOH alkaline solutions, respectively. The viscous and homogenous slurries was poured into a Teflon mold to obtain bar samples at ambient temperature and cured in a constant 50 °C temperature/humidity oven for 24 h. GP bars were powdered by following additional drying in an open air furnace at 300 °C for 1 h. To obtain GP18C precursors, the powdered pure GPs were ball milled with 18 moles of carbon at room temperature using cylindrical zirconia balls in a plastic grinding jar for 5 h, with a ball to powder wt % ratio of 5:1, respectively. The carbothermal reduction and nitridation process of the GP18C precursors were carried out in an atmosphere controlled, tube furnace at temperatures of 1400°, 1500° and 1600 °C for 2 hours using high purity nitrogen (99.99%) under dynamic conditions. XRD, Rietveld refinement and SEM-EDS analyses were made to determine transformation and morphology of all the products after carbothermal reduction and nitridation. Different types of nitride analogues of GPs were examined in the products depending on the alkaline conditions of the GPs. Sialon-type ceramic powders were synthesized from geopolymer carbon precursors (1:1:4.5:12+9C) prepared different alkaline conditions by carbothermal reduction and nitridation. NaOH, KOH, or CsOH alkaline solutions were used to make NaGP, KGP and CsGP resins by mixing metakaolin (Al2O3•2SiO2), with water glass, respectively. NaGP9C, KGP9C and CsGP9C resins were prepared by mixing 9 moles of carbon nano-powder with GP resins. The viscous and homogenous slurries was poured into a Teflon mold to obtain bar samples at ambient temperature and cured in a humidity controlled, constant temperature oven at 50 °C for 24 h. The GP9C compacts were powdered by following additional drying in an open air furnace at 300 °C for 1 h. Subsequently, the GP9C powders were planetary milled with a ball to powder wt % ratio of 5:1 for 10 min to increase their reactivity. As a result, the GP9C precursors were carbothermally reacted in an atmosphere-controlled tube furnace at temperatures of 1400°, 1500° and 1600 °C for 2 hours with high purity nitrogen (99.99%) under dynamic conditions. XRD, Rietveld refinement and SEM-EDS analyses were made to determine transformation and morphology of all the products after carbothermal reduction and nitridation. Depend on the alkaline conditions of the GPs, phase pure or multiple-type sialon compounds were examined

Highly porous mullite ceramics from engineered alkali activated suspensions

Air may be easily incorporated by vigorous mechanical stirring, with the help of surfactants, of activated geopolymer-yielding suspensions. The cellular structure is stabilized by the viscosity increase caused by curing reactions, configuring an inorganic gel casting. The present paper is aimed at extending this approach to mullite foams, obtained by the thermal treatment of engineered alkali activated suspensions. Green foams were first obtained by gel casting of a suspension for Na-geopolymer enriched with reactive -Al2O3 powders. Sodium was later extracted by ionic exchange with ammonium salts. In particular, the removal of Na+ ions was achieved by immersion in ammonium nitrate solution overnight, with retention of the cellular structure. Finally, the ion-exchanged foams were successfully converted into pure mullite foams by application of a firing treatment at 1300 degrees C, for 1hour. Preliminary results concerning the extension of the concept to mullite three-dimensional scaffolds are presented as well

Reactive metal/graphene oxide doping to fabricate porous geopolymers for arsenic removal

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Acid attacks on metakaolin based-geopolymers with recycled corundum: A study focused on the role of anions by NMR characterization

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