William D.A. Rickard

Fly ash based geopolymer thin coatings on metal substrates and its thermal evaluation

Class F fly ash based Na-geopolymer formulations have been applied as fire resistant coatings on steel. The main variables for the coating formulations were Si: Al molar and water: cement weight ratios. We have determined that the adhesive strength of the coatings strongly depend on geopolymer composition. The ease with which geopolymer can be applied onto metal surfaces and the resultant thickness depend on the water content of the formulation. Adhesive strengths of greater than 3.5 MPa have been achieved on mild steel surfaces for compositions with Si:Al of 3.5. Microstructure evolution and thermal properties of the optimised coating formulations show that they have very promising fire resistant characteristics.

Nanogeochronology of discordant zircon measured by atom probe microscopy of Pb-enriched dislocation loops

Atom probe yields geologically meaningful ages from nanoscale Pb-enriched dislocation loops in discordant zircon. Isotopic discordance is a common feature in zircon that can lead to an erroneous age determination, and it is attributed to the mobilization and escape of radiogenic Pb during its post-crystallization geological evolution. The degree of isotopic discordance measured at analytical scales of ~10 μm often differs among adjacent analysis locations, indicating heterogeneous distributions of Pb at shorter length scales. We use atom probe microscopy to establish the nature of these sites and the mechanisms by which they form. We show that the nanoscale distribution of Pb in a ~2.1 billion year old discordant zircon that was metamorphosed c. 150 million years ago is defined by two distinct Pb reservoirs. Despite overall Pb loss during peak metamorphic conditions, the atom probe data indicate that a component of radiogenic Pb was trapped in 10-nm dislocation loops that formed during the annealing of radiation damage associated with the metamorphic event. A second Pb component, found outside the dislocation loops, represents homogeneous accumulation of radiogenic Pb in the zircon matrix after metamorphism. The 207Pb/206Pb ratios measured from eight dislocation loops are equivalent within uncertainty and yield an age consistent with the original crystallization age of the zircon, as determined by laser ablation spot analysis. Our results provide a specific mechanism for the trapping and retention of radiogenic Pb during metamorphism and confirm that isotopic discordance in this zircon is characterized by discrete nanoscale reservoirs of Pb that record different isotopic compositions and yield age data consistent with distinct geological events. These data may provide a framework for interpreting discordance in zircon as the heterogeneous distribution of discrete radiogenic Pb populations, each yielding geologically meaningful ages.

Precambrian reidite discovered in shocked zircon from the Stac Fada impactite, Scotland

Terrestrial impact events have had a profound influence on Earth’s geological, geochemical, and biological evolution. However, the record of Precambrian impacts is poorly constrained due to the dynamic nature of plate tectonics, erosion, and deposition of younger rocks that may destroy or cover the evidence. Here we report the first Precambrian occurrence of the rare mineral reidite (ZrSiO4) within grains of shocked zircon in the ca. 1.18 Ga Stac Fada Member (Stoer Group), northwestern Scotland. The reidite, preserved as <2-µm-wide lamellae, is unambiguous evidence of shock pressures in excess of ∼30 GPa and confirms the impact origin for the Stac Fada deposit. The reidite lamellae are locally deformed, and sites of deformation record its decomposition to baddeleyite (ZrO2) and amorphous silica, the first natural example of this transformation. The findings demonstrate that reidite and baddeleyite may form and be transported in high-energy ejecta without physical or chemical breakdown and are stable during sedimentary diagenesis and low-grade metamorphism. Thus, reidite may be preserved over time scales exceeding 1 b.y., establishing the use of reidite within detrital shocked zircon from Precambrian strata as a viable and valuable means of recognizing and characterizing ancient terrestrial impact events.

Nanoscale gold clusters in arsenopyrite controlled by growth rate not concentration: Evidence from atom probe microscopy

Abstract Auriferous sulfides, most notably pyrite (FeS2) and arsenopyrite (FeAsS), are among the most important economic minerals on Earth because they can host large quantities of gold in many of the world’s major gold deposits. Here we present the first atom probe study of gold distribution in arsenopyrite to characterize the three-dimensional (3D) distribution of gold at the nanoscale and provide data to discriminate among competing models for gold incorporation in refractory ores. In contrast to models that link gold distribution to gold concentration, gold incorporation in arsenopyrite is shown to be controlled by the rate of crystal growth, with slow growth rate promoting the formation of gold clusters and rapid growth rate leading to homogeneous gold distribution. This study yields new information on the controls of gold distribution and incorporation in sulfides that has important implications for ore deposit formation. More broadly this study reveals new information about crystal-fluid interface dynamics that determine trace element incorporation into growing mineral phases.

Direct application of cobaltite-based perovskite cathodes on the yttria-stabilized zirconia electrolyte for intermediate temperature solid oxide fuel cells

In this communication, cobaltite-based perovskite (CBP) cathodes are directly applied on the yttria-stabilized zirconia (YSZ) electrolyte via an in situ assembly process without the addition of a doped ceria interlayer and pre-sintering at high temperatures. The results demonstrate for the first time that a CBP electrode/YSZ electrolyte interface can be formed in situ under cathodic polarization at a solid oxide fuel cell (SOFC) operating temperature of 750 °C. Nevertheless, the performance of cells with Sr-containing CBP cathodes deteriorates due to the surface segregation of Sr species and formation of a Sr-rich reaction layer at the interface. However, the stability and power density of cells with in situ assembled CBP cathodes can be further enhanced by B-site doping or by using a Sr-free CBP. The direct application of CBPs on the YSZ electrolyte revolutionizes the design of intermediate temperature SOFCs.

Thermal Properties of Geopolymers

The physics and testing methodology of thermal properties are introduced prior to a review of OPC and geopolymer thermal properties. The amorphous inorganic structure of the geopolymers lends itself to good thermal resistance which leads to potential applications such as thermal insulation. Thermal expansion can generate destructive internal stresses when structural parts are heated and restrained from moving. The thermal expansion measuring techniques commonly utilised are dilatometry, interferometry and thermomechanical analysis (TMA). Thermal expansion measurements of metakaolin and fly ash based geopolymers show several distinct regions as the temperature increases. The extent of these regions varies from system to system and the changes are attributed to dehydration, dehydroxylation, densification and crystallisation. Fillers and aggregates can be added to geopolymers to reduce the thermal expansion of the composite and extend the usable temperature range. Thermal conductivity determination is required to assess geopolymers’ suitability for potential applications in thermal barriers and construction structural members. The two approaches to measuring thermal conductivity: steady state and transient (non-steady state) techniques are compared. The microstructure of geopolymer profoundly influences thermal conductivity; particularly porosity which if increased leads to a reduction in thermal conductivity. The addition of aggregate influences the thermal conductivity of OPC and geopolymer concrete and at the same time decreases thermal durability due to mismatch of thermal conductivity between aggregate and matrix. Some geopolymers with low initial strength have been shown to gain strength after exposure to high temperatures. It has been hypothesised that unreacted precursor levels can convert to geopolymer at high temperature and increase the strength. Once again the importance of the geopolymer microstructure is highlighted.

Highly Stable Sr-Free Cobaltite-Based Perovskite Cathodes Directly Assembled on a Barrier-Layer-Free Y2O3-ZrO2Electrolyte of Solid Oxide Fuel Cells

Direct assembly is a newly developed technique in which a cobaltite-based perovskite (CBP) cathode can be directly applied to a barrier-layer-free Y2 O3 -ZrO2 (YSZ) electrolyte with no high-temperature pre-sintering steps. Solid oxide fuel cells (SOFCs) based on directly assembled CBPs such as La0.6 Sr0.4 Co0.2 Fe0.8 O3-δ show high performance initially but degrade rapidly under SOFC operation conditions at 750 °C owing to Sr segregation and accumulation at the electrode/electrolyte interface. Herein, the performance and interface of Sr-free CBPs such as LaCoO3-δ (LC) and Sm0.95 CoO3-δ (SmC) and their composite cathodes directly assembled on YSZ electrolyte was studied systematically. The LC electrode underwent performance degradation, most likely owing to cation demixing and accumulation of La on the YSZ electrolyte under polarization at 500 mA cm-2 and 750 °C. However, the performance and stability of LC electrodes could be substantially enhanced by the formation of LC-gadolinium-doped ceria (GDC) composite cathodes. Replacement of La by Sm increased the cell stability, and doping of 5 % Pd to form Sm0.95 Co0.95 Pd0.05 O3-δ (SmCPd) significantly improved the electrode activity. An anode-supported YSZ-electrolyte cell with a directly assembled SmCPd-GDC composite electrode exhibited a peak power density of 1.4 W cm-2 at 750 °C, and an excellent stability at 750 °C for over 240 h. The higher stability of SmC as compared to that of LC is most likely a result of the lower reactivity of SmC with YSZ. This study demonstrates the new opportunities in the design and development of intermediate-temperature SOFCs based on the directly assembled high-performance and durable Sr-free CBP cathodes.

Smart utilization of cobaltite-based double perovskite cathodes on barrier-layer-free zirconia electrolyte of solid oxide fuel cells

Cobaltite-based double perovskite oxides with high electrocatalytic activity and conductivity have been developed as high-performance cathode alternatives for solid oxide fuel cells (SOFCs). However, the use of cobaltite-based double perovskites on Y2O3 stabilized ZrO2 (YSZ)-based SOFCs requires the application of a doped ceria barrier layer. This is due to their poor chemical and physical compatibility with the YSZ electrolyte during high-temperature sintering and fabrication processes. Here we report a viable approach to in operando assemble double perovskites such as PrBa0.5Sr0.5Co1.5Fe0.5O5+δ (PBSCF), on YSZ electrolyte and thus effectively form an electrode/electrolyte interface without high-temperature processing. The electrochemical performance of the in situ assembled PBSCF cathode is comparable to that of the cathode prepared by conventional methods. A single cell with an in situ assembled PBSCF–GDC (Gd-doped ceria) cathode achieved a peak power density (PPD) of 1.37 W cm−2 at 750 °C and exhibited a high stability at 500 mA cm−2 and 750 °C for 100 h. Surface and cross-sectional microstructure analysis offer solid evidence that the PBSCF–GDC cathode/YSZ electrolyte interface was formed by electrochemical polarization. This work offers new opportunities to effectively and effortlessly use high-performance double perovskite cathodes in commercial SOFCs.

Other Potential Applications for Alkali-Activated Materials

The focus of this chapter is the discussion of a variety of niche applications (other than as a large-scale civil infrastructure material) in which alkali-activated binders and concretes have shown potential for commercial-scale development. The majority of these applications have not yet seen large-scale AAM utilisation, except as noted in the various sections of the chapter. However, there have been at least pilot-scale or demonstration projects in each of the areas listed, and each provides scope for future development and potentially profitable advances in science and technology. In addition to the applications specifically discussed in this chapter, there are also commercial and academic developments in alkali-activation for specific applications including a commercial product which is being marketed as a domestic tiling grout showing some self-cleaning properties [1], as well as alkali-activated metakaolin binders as a vehicle for controlled-release drug delivery [2, 3]. Although undoubtedly promising and of commercial interest, these are rather specialised applications, and so the focus of this chapter is instead on broader categories of research and development rather than in providing detailed analysis of specific products. The areas to be discussed will include lightweight materials, well cements, fire-resistant materials, and fibre-reinforced composites.

Fire-Resistant Geopolymers

Overview.- History of Geopolymers.- Portland Cement (OPC) and Concrete.- Geopolymer Applications.- Precursors and Additives for Geopolymer Synthesis.- Geopolymer Chemistry.- Fibres: Technical Benefits.- Thermal Properties of Geopolymers.- Fire Resistance of OPC and geopolymer.- Conclusion.