Crystalline Reaction Products Research Articles

Microstructure Refinement of EB-PVD Gadolinium Zirconate Thermal Barrier Coatings to Improve Their CMAS Resistance

Rare-earth zirconates are proven to be very effective in restricting the CMAS attack against thermal barrier coatings (TBCs) by forming quick crystalline reaction products that seal the porosity against infiltration. The microstructural effects on the efficacy of Electron Beam-Physical Vapor Deposition gadolinium zirconate (EB-PVD GZO) against CMAS attack are explored in this study. Four distinct GZO microstructures were manufactured and the response of two selected GZO variants to different CMAS and volcanic ash melts was studied for annealing times between 10 min and 50 h at 1250 °C. A significant variation in the microstructural characteristics was achieved by altering substrate temperature and rotation speed. A refined microstructure with smaller intercolumnar gaps and long feather arms lowered the CMAS infiltration by 56%–72%. Garnet phase, which formed as a continuous layer on top of apatite and fluorite, is identified as a beneficial reaction product that improves the CMAS resistance.

Shear deformations in crystalline alkali-silica reaction products at the molecular scale: anisotropy and role of specific ion effects

Alkali-silica reaction (ASR) products exhibit viscoelastic behavior. Creep of nanolayered adsorbing materials is associated with shear deformations at the molecular scale. Crystalline ASR products are layered silicates with potassium and/or sodium as a charge balance ion in the interlayer space. These products are formed only in small quantities, exhibit long-range disorder, and are often mixed with other phases present in hydrated cement systems. In this context, experimental assessment of these products is a challenge and atomistic simulations arise as a valuable alternative. In this work, the shear response of crystalline ASR products is studied using classical molecular simulations. Understanding shear deformations are important to fully characterize the behavior of ASR products and may constitute a first step towards the comprehension of the viscoelastic behavior of these products. Further, the charge balance cation affects the shear modulus, von Mises yield stress, and residual strain, showing that the chemistry plays a crucial role in the mechanical response of these layered materials.

Experimental study on the use of RoadCem blended with by-product cementitious materials for stabilisation of clay soils

This work presents an experimental study on the physical, mechanical and microstructural characteristics of two clay soils treated with by-product materials (GGBS and PFA) blended with a nano technology-based additive called RC (RC). The soils were initially treated with 8% of cement in the first phase of mixing, and in the other phases of mixing, the cement content was reduced by 50%, 60% and 70% and substituted with GGBS or different combinations of GGBS and 1%RC or PFA and 1%RC. Further, the paper discusses the shear strength, stabilisation mechanism, microstructural characteristics, and swell of the treated soils based on results of series of strength, scanning electron microscope and swell test. The results show that the inclusion of 1%RC increases undrained shear strength and reduces swell of the treated soils due to encapsulation effects associated with the formation of the crystalline reaction product in the hydration process and the resulting modification of cementitious product to bind very heavy clays together. The soil treated with 1%RC combined with 4%Cem and 3%GGBS, produces the best performance in terms of undrained shear strength and microstructural characteristics and the stabilisation mechanism of cement, RC and GGBS shows that it is due to interlocking of particles and wrapping effect.

Reactivity Assessment of Modified Ferro Silicate Slag by R3 Method

Traditional methods to track the reactivity of supplementary cementitious materials (SCMs) and their contribution to the hydration mechanism mostly use Portland Cement (PC) as an activator. Alternatively, a novel method to assess the reactivity of SCMs called R3 was recently presented. This novel method uses lab grade chemicals such as portlandite (CH), K2SO4, KOH, and CaCO3 to activate the SCM by resembling the pH of the alkaline pore solution created by PC. By using this method, the reactivity of the SCM can be easily quantified from measured heat release, bound water content, and CH consumption. The primary objective of the current study is to apply the novel methodology to analyze the reactivity of Modified Ferro Silicate (MFS) Cu slag benchmarked against siliceous fly ash (FA), ground granulated blast-furnace slag (GGBFS), and inert quartz filler. GGBFS showed the highest cumulative heat release and bound water content due to its latent hydraulic behavior. Determination with XRD analysis of the major phase of the R3 model MFS slag paste showed the participation of Fe in the hydration mechanism by forming Fe-AFm. R3 paste with GGBFS showed the presence of hydrotalcite/Al-AFm, whereas FA showed the presence of ettringite (AFt) as their crystalline reaction products. The experiments also indicate that the MFS slag acts as a reactive pozzolanic material with an acceptable performance in heat release, bound water content, and CH consumption, and can be used as SCM to make concrete. With the possibility of using MFS slag as SCM to replace part of PC, sustainability and circular economy can be fairly well achieved.

Recovery and concentration of ammonia from return liquor to promote enhanced CO2 absorption and simultaneous ammonium bicarbonate crystallisation during biogas upgrading in a hollow fibre membrane contactor

In this study, thermal desorption was developed to separate and concentrate ammonia from return liquor, for use as a chemical absorbent in biogas upgrading, providing process intensification and the production of crystalline ammonium bicarbonate as the final reaction product. Applying modest temperature (50 °C) in thermal desorption suppressed water vapour pressure and increased selective transport for ammonia from return liquor (0.11MNH3) yielding a concentrated condensate (up to 1.7MNH3). Rectification was modelled through second-stage thermal processing, where higher initial ammonia concentration from the first stage increased mass transfer and delivered a saturated ammonia solution (6.4MNH3), which was sufficient to provide chemically enhanced CO2 separation and the simultaneous initiation of ammonium bicarbonate crystallisation, in a hollow fibre membrane contactor. Condensate recovered from return liquor exhibited a reduction in surface tension. We propose this is due to the stratification of surface active agents at the air-liquid interface during primary-stage thermal desorption which carried over into the condensate, ‘salting’ out CO2 and lowering the kinetic trajectory of absorption. However, crystal induction (the onset of nucleation) was comparable in both synthetic and thermally recovered condensates, indicating the thermodynamics of crystallisation to be unaffected by the recovered condensate. The membrane was evidenced to promote heterogeneous primary nucleation, and the reduction in the recovered condensate surface tension was shown to exacerbate nucleation rate, due to the reduction in activation energy. X-ray diffraction of the crystals formed, showed the product to be ammonium bicarbonate, demonstrating that thermal desorption eliminates cation competition (e.g. Ca2+) to guarantee the formation of the preferred crystalline reaction product. This study identifies an important synergy between thermal desorption and membrane contactor technology that delivers biogas upgrading, ammonia removal from wastewater and resource recovery in a complimentary process.

Effects of crystal structure and cation size on molten silicate reactivity with environmental barrier coating materials

AbstractRare earth (RE) disilicates are utilized in environmental barrier coatings to protect Si‐based engine components from destructive reactions with water vapor and other combustion species. These coating materials, however, degrade when exposed to molten silicate deposits in the engine. Four RE‐disilicates (RE2Si2O7, RE = Er, Dy, Gd, Nd) are analyzed herein in thermochemical interactions with glassy calcium‐magnesium‐aluminosilicate (CMAS) compositions at 1400°C. Crystalline reaction products included RE2Si2O7, SiO2, and a Ca2+yRE8+x(SiO4)6O2+3x/2+y apatite‐type silicate. RE2Si2O7 formation was favored in interactions with CMAS having low CaO:SiO2 ratios. Increased reactivity was observed for higher CaO:SiO2 ratios in CMAS combined with larger RE3+ cation size, resulting in apatite formation of varying stoichiometry and changes in lattice parameters. The crystallization of SiO2 was dependent on both thermodynamic equilibrium at low CaO:SiO2 ratios and sequestration of silicate modifiers at higher CaO:SiO2 ratios, although residual amorphous content after CMAS exposure in both cases was still substantial.

Dissolution Behavior and Phase Evolution During Aluminum Oxide Dissolution in BOF Slag

Basic oxygen furnace (BOF) slag is a byproduct in the steel making process and is produced in considerable amounts. Thus, its valorization is important. Additives such as Al2O3 can be utilized to modify the composition and mineralogy of the slag provided that a fast and effective dissolution of these additives in molten slags can be ensured. With this in mind, the dissolution of Al2O3 particles in molten BOF slag has been investigated between 1500 and 1600 °C. In all cases, a dissolution zone forms around the dissolving particle which limits the dissolution rate in the melt under natural convection conditions. The dissolution in the current system is found to be an indirect process, wherein crystalline reaction products like CaAl4O7 (CA2) and CaAl2O4 (CA) form on the surface of the particles. CA forms at 1500 °C and CA2 at 1600 °C. These reaction products form a continuous layer on the surface of the dissolving particles but the thickness of this reaction layer remains limited. The experimental data have been utilized to evaluate the dissolution paths. Near the Al2O3/slag interface, the inter-diffusion of Al2O3 and CaO occurs due to which the dissolution path shifts towards the Al2O3-CaO binary system. A phenomenological model is presented to explain the evolution of the dissolution process in the current system, wherein the kinetics of the process is used to explain the reaction products formed at 1500 °C and 1600 °C.

Modifications of basic-oxygen-furnace slag microstructure and their effect on the rheology and the strength of alkali-activated binders

Microstructure tailoring of metallurgical slags allows the production of alternative construction binders with customized properties. In this study, the variations of rheology and strength of alkali-activated basic-oxygen-furnace (BOF) slags are quantified. Two modifications of BOF slag were created adding amounts of alumina and silica at high temperature (>1250 °C). The additions, defined by thermodynamic modeling, lowered the liquidus temperature facilitating the generation of amorphous when the slag was fast cooled. The first modification (SAT1) incorporated 5 wt% silica and 11 wt% alumina, while the second (SAT2) included around 13 wt% alumina. Both modifications generated a hybrid microstructure composed of cementitious and non-cementitious crystalline phases and an amorphous fraction. During alkali activation using NaOH solutions of 0.25 M, rheological measurements on fresh paste using SAT2 registered plastic viscosity values 2.3 times higher than those of SAT1. The compressive strength after 28 days for the binder developed from SAT2 slag was 10–30% stronger than the one from SAT1. These binders showed similar crystalline reaction products but compositional differences in the amorphous gel correlated to the initial slag modification. The detected differences in the binder properties are significant enough to justify BOF-slag engineering as a way to deliver customized precursors for alkali activation.

Pore Structure Characterization of Sodium Hydroxide Activated Slag Using Mercury Intrusion Porosimetry, Nitrogen Adsorption, and Image Analysis.

The pore structure of alkali-activated slag has a significant influence on its performance. However, the literature shows insufficient studies regarding the suitability of different techniques for characterizing the pore structure and the influences of Na2O and curing age on pore structure development. In pursuit of a better understanding, the pore structure of sodium hydroxide activated slag paste was characterized by multiple techniques, e.g., mercury intrusion porosimetry (MIP), nitrogen (N2) adsorption, and scanning electron microscopy (SEM) image analysis. The sodium hydroxide activated slag pastes were prepared with three different contents of Na2O (Na2O/slag = 4, 6, and 8%) and cured for different times up to 360 days. The microstructure observation reveals that outer C–(N–)A–S–H and inner C–(N–)A–S–H grow successively around the reacting slag grains, along with crystalline reaction products which are formed in the empty coarse pore space. The increase of Na2O content and curing age lead to a finer pore structure. The MIP measurements show that the total porosity drops about 70% within the first day, and that one peak at most, corresponding to gel pores, was identified in the differential curves of all the investigated samples from 1 to 360 days. On the contrary, only one peak, corresponding to capillary pores, was identified by SEM-image analysis. The differential curves derived from N2 adsorption generally reveal two peaks, and the trend that the pore diameters of those two peaks vary with curing age depends on the content of Na2O. Compared to Portland cement, sodium hydroxide activated slag has a higher pore space filling capacity (χ, Vproducts/Vslag-reacted), while the capacity decreases with increasing Na2O content and curing age.

Toward an understanding of surface layer formation, growth, and transformation at the glass–fluid interface

Silicate glass is a metastable and durable solid that has application to a number of energy and environmental challenges (e.g., microelectronics, fiber optics, and nuclear waste storage). If allowed to react with water over time silicate glass develops an altered layer at the solid-fluid interface. In this study, we used borosilicate glass (LAWB45) as a model material to develop a robust understanding of altered layerformation (i.e., amorphous hydrated surface layer and crystalline reaction products). Experiments were conducted at high surface area-to-volume ratio (∼200,000 m−1) and 90 °C in the pressurized unsaturated flow (PUF) apparatus for 1.5-years to facilitate the formation of thick altered layers and allow for the effluent solution chemistry to be monitored continuously. A variety of microscopy techniques were used to characterize reacted grains and suggest the average altered layer thickness is 13.2 ± 8.3 μm with the hydrated and clay layer representing 74.8% and 25.2% of the total altered layer, respectively. The estimate of hydrated layer thickness is within the experimental error of the value estimated from the B release rate data (∼10 ± 1 μm/yr) over the 1.5-year duration. PeakForce® quantitative nanomechanical mapping results suggest the hydrated layer has a modulus that ranges between ∼20 and 40 GPa, which is in the range of porous silica that contains from ∼20 to ∼50% porosity, yet significantly lower than dense silica (∼70–80 GPa). Scanning transmission electron microscopy (STEM) images confirm the presence of pores and an analysis of a higher resolution image provides a qualitative estimate of ≥22% porosity in the hydrated layer with variations in void volume with increasing distance from the unaltered glass. Chemical composition analyses, based on a combination of time-of-flight secondary-ion mass spectrometry (ToF-SIMS), scanning electron microscopy with X-ray energy dispersive spectroscopy (EDS), and STEM-EDS, clearly show that the altered layer is mainly composed of Al, H, Si, and O with the clay layer being enriched in Li, Zn, Fe, and Mg. The amorphous hydrated layer is enriched in Ca, H, and Zr with a minor amount of K. Furthermore, ToF-SIMS results also suggest the B profile is anti-correlated with the H profile in the hydrated layer. Our selected-area electron diffraction results suggest the structure of the hydrated layer closely resembles opal-AG (amorphous gel-like) with an average crystallite size of ∼0.7 nm which is smaller than the critical nucleus for silica nanoparticles (i.e., 1.4–3 nm). These results suggest the hydrated layer is more consistent with a polymeric gel rather than a colloidal gel and is comprised of molecular units (<1 nm in size) that result from the difficult to hydrolyze bonds, such as SiOZr units, during the glass corrosion process. The size of individual particles or molecular units is a function of formation conditions (e.g., pH, ionic strength, nano-confinement, solute composition) in the hydrated layer.

Organic-inorganic hybrid materials from divalent metal cations and expanded N,N′-donor linkers

Abstract The rod-shaped linker (E,E)-N,N′-(3,3′-dimethyl-4,4′-biphenyldiyl)bis[1-(3-pyridinyl)methanimine] (L) is exploited for the first time in the synthesis of extended structures. Four new coordination polymers of composition {[ZnL(OAc)2]·EtOH}n (1), {[CdL(OAc)2]·MeOH}n (2), {[Cu2L(OAc)4]·CH2Cl2}n (3) and [MnL(N3)2]n (4) have been structurally characterized. The metal cations and the anionic ancillary ligands play pivotal roles for the topology of these compounds. In the crystalline reaction products of Zn(II), Cd(II) and Cu(II) acetate with the organic linker, the acetate anions connects two neighboring cations to dinuclear [M2(OAc)4] subunits. These secondary building units are further crosslinked by the N,N′-donor ligand, either perpendicular to the acetato bridges, leading to a ladder-like ribbon for 1 and 2, or in the direction of the metal···metal separation, resulting in a simple chain in the case of 3. Instead of dinuclear secondary building units, a different topology results from reaction of the N,N′ linker with Mn(ClO4)2 in the presence of azide anions: 1,3 bridging by the N3 − groups leads to infinite chains. These are crosslinked by L in perpendicular direction, and the layer structure 4 is obtained. Natural bond orbital (NBO) analyses revealed information on the basis of orbital interactions about the coordination environments of the metal ions. Thermogravimetric measurements indicate the highest thermal stability for 2. Strong antiferromagnetic coupling within the dinuclear subunits of 3 is observed as a consequence of superexchange via the acetato bridges.

Interconversion of One-Dimensional Thiogallates Cs2[Ga2(S2)2–xS2+x] (x = 0, 1, 2) by Using High-Temperature Decomposition and Polysulfide-Flux Reactions

The potential of cesium polysulfide-flux reactions for the synthesis of chalcogenogallates was investigated by using X-ray diffraction and Raman spectroscopy. An investigation of possible factors influencing the product formation revealed that only the polysulfide content x in the Cs2Sx melts has an influence on the crystalline reaction product. From sulfur-rich melts (x > 7), CsGaS3 is formed, whereas sulfur-poor melts (x < 7) lead to the formation of Cs2Ga2S5. In situ investigations using high-temperature Raman spectroscopy revealed that the crystallization of these solids takes place upon cooling of the melts. Upon heating, CsGaS3 and Cs2Ga2S5 release gaseous sulfur due to the degradation of S22– units. This decomposition of CsGaS3 to Cs2Ga2S5 and finally to CsGaS2-mC16 was further studied in situ by using high-temperature X-ray powder diffraction. A combination of the polysulfide reaction route and the high-temperature decomposition leads to the possibility of the directed interconversion of these thi...

Investigation of amorphous and crystalline phosphates in magnesium phosphate ceramics with solid-state 1H and 31P NMR spectroscopy

Magnesium phosphate ceramics are chemically-bonded ceramics for bioengineering and civil engineering applications. Information about the nature of the reaction products, especially the amorphous fraction, and their role in the setting reaction, essential for linking reaction mechanisms to microstructure and performance of ceramics, is lacking. By exploiting 1H and 31P magic-angle spinning nuclear magnetic resonance spectroscopy, in addition to the crystalline reaction product (MKP), two amorphous phases, characterized by two distinct proton and phosphorous environments, have been identified. The results pointed to amorphous hydrated orthophosphate compounds which are the precursors of MKP. They show different 1H spin-lattice relaxation dynamics, and, higher water mobility with respect to MKP. Although these amorphous precursors should not be crypto-structural variants of MKP, they likely host similar structural units. Conversion into MKP is thought to occur through relatively minor rearrangements, as in Ca phosphate hydrates. The new information provided in this work allows us to propose a model for the setting reaction based on an existing theory involving a densification process, analogue to sol-gel processing of ceramics, coherent with results from small angle neutron scattering and mechanisms predicted by reaction kinetics analysis.

Bond Durability of Carbon-Microfiber-Reinforced Alkali-Activated High-Temperature Cement Adhering to Carbon Steel

The study aims at evaluating the bond durability of a carbon microfiber (CMF)-reinforced alkali-activating calcium aluminate cement (CAC)/fly ash F (FAF) blend cementitious material adhering to carbon steel (CS) under stresses induced by a 350℃ heat-25℃ water cooling cycle. This cementitious material/CS joint sample was originally prepared in an autoclave at 300℃ under a pressure of 8.3 MPa. For comparison, two reference geothermal well cements, Class G modified with silica (G) and calciumaluminum phosphate (CaP), were employed as well reinforced with CMF. In the CAC/FAF blending cement systems, the CAC-derived cementitious reaction products preferentially adhered to CS surfaces, rather than that of FAF-related reaction products. CMF played a pivotal role in creating tough interfacial bond structure of cement layer adhering to CS. The bond toughness also was supported by the crystalline cementitious reaction products including sodalite, brownmillerite, and hedenbergite as major phases, and aragonite, boehmite, and garronite as minor ones. The brownmillerite as an interfacial reaction product between cement and CS promoted the chemical bonding of the cement to CS, while the other phases served in providing the attractive bonding of the cement to CS. The post-stress-test joint samples revealed the formation of additional brown-millerite, aragonite, and garronite, in particular brownmillerite as the major one. The combination of chemical bonding and self-advancing adherence behavior of the cement was essential for creating a better interfacial bond structure. A similar interfacial bond structure was observed with CaP. The crystalline phase composition of the autoclaved cement revealed apatite, zeolite, and ferrowyllieite as major reaction products, and aragonite and al-katoite as the minor ones. Ferrowyllieite was identified as cement/CS interfacial reaction product contributing to the chemical bond of cement, while the other phases aided in providing the attractive bond of cement. After a stress test, two phases, ferrowyllieite and aragonite, promoted the self-advancing adherence of cement to CS. However, the effectiveness of these phases in improving adherence performance of cement was less than that of CAC/FAF blend cement, reflecting the fact that the bond durability of CAC/FAF blend cement was far better than that of the CaP. In contrast, the autoclaved silica-modified G cement consisting of xonotlite, and 0.9 nm-to-bermorite and riversideite, with calcite as the crystalline reaction products, had no significant effect on improving the shear bond strength and the bond’s toughness. No interaction product with CS was found in the cement adhering to CS. After a stress test, the calcite phase acted only to promote the self-advancing adherence of cement, but its extent was minimal compared with that of the other cements, thereby resulting in poor bond durability.

Types of alkali–aggregate reactions and the products formed

The alkali–aggregate reaction comprises the alkali–silica reaction (ASR) and the alkali–carbonate reaction (ACR). Reaction kinetics of the ASR depends on the grain size and crystalline structure of the reactive silicon dioxide. The reaction starts in the aggregates along the particle periphery and progresses inward. After cracking of the particle, larger amounts of reaction products are formed and are eventually extruded in the cement paste, where they fill cracks and voids. ASR products within aggregates predominantly consist of (hydrated) silicon, alkalis and calcium with characteristic atomic ratios of (Na + K)/Si ∼ 0·25 and Ca/Si ∼ 0·25. They take up additional calcium while releasing alkalis when extruded. Amorphous and crystalline reaction products occur and coexist. Expansion is the result of water adsorption by the reaction products. In ACR, harmless dedolomitisation is distinguished from deleterious reaction of fine-grained silica disseminated throughout the carbonate matrix. Further research is needed to gain more in-depth knowledge about the thermodynamics and kinetics of ASR products and the mechanisms of expansion. This should allow the establishment of a better link between concrete structures and both accelerated testing and models.

The influence of LiH on the rehydrogenation behavior of halide free rare earth (RE) borohydrides (RE = Pr, Er).

Rare earth (RE) metal borohydrides are receiving immense consideration as possible hydrogen storage materials and solid-state Li-ion conductors. In this study, halide free Er(BH4)3 and Pr(BH4)3 have been successfully synthesized for the first time by the combination of mechanochemical milling and/or wet chemistry. Rietveld refinement of Er(BH4)3 confirmed the formation of two different Er(BH4)3 polymorphs: α-Er(BH4)3 with space group Pa3[combining macron], a = 10.76796(5) Å, and β-Er(BH4)3 in Pm3[combining macron]m with a = 5.4664(1) Å. A variety of Pr(BH4)3 phases were found after extraction with diethyl ether: α-Pr(BH4)3 in Pa3[combining macron] with a = 11.2465(1) Å, β-Pr(BH4)3 in Pm3[combining macron]m with a = 5.716(2) Å and LiPr(BH4)3Cl in I4[combining macron]3m, a = 11.5468(3) Å. Almost phase pure α-Pr(BH4)3 in Pa3[combining macron] with a = 11.2473(2) Å was also synthesized. The thermal decomposition of Er(BH4)3 and Pr(BH4)3 proceeded without the formation of crystalline products. Rehydrogenation, as such, was not successful. However, addition of LiH promoted the rehydrogenation of RE hydride phases and LiBH4 from the decomposed RE(BH4)3 samples.

Impacts of organic ligands on forsterite reactivity in supercritical CO2 fluids.

Subsurface injection of CO2 for enhanced hydrocarbon recovery, hydraulic fracturing of unconventional reservoirs, and geologic carbon sequestration produces a complex geochemical setting in which CO2-dominated fluids containing dissolved water and organic compounds interact with rocks and minerals. The details of these reactions are relatively unknown and benefit from additional experimentally derived data. In this study, we utilized an in situ X-ray diffraction technique to examine the carbonation reactions of forsterite (Mg2SiO4) during exposure to supercritical CO2 (scCO2) that had been equilibrated with aqueous solutions of acetate, oxalate, malonate, or citrate at 50 °C and 90 bar. The organics affected the relative abundances of the crystalline reaction products, nesquehonite (MgCO3 · 3H2O) and magnesite (MgCO3), likely due to enhanced dehydration of the Mg(2+) cations by the organic ligands. These results also indicate that the scCO2 solvated and transported the organic ligands to the forsterite surface. This phenomenon has profound implications for mineral transformations and mass transfer in the upper crust.

Effects of cation substitution and temperature on the interaction between thermal barrier oxides and molten CMAS

Abstract The effectiveness of candidate rare earth (RE) bearing oxides to mitigate degradation of thermal barrier coatings by molten silicates is determined by the constitution and crystallization kinetics of the reaction products. The relationships between the oxide composition, test temperature, and reaction product constitution were investigated using sintered pellets of hafnates or zirconates containing YbO1.5, GdO1.5, or LaO1.5. The results suggest that the composition of the reprecipitated ZrO2- or HfO2-based phase, typically fluorite, is a useful indicator of the effectiveness of the silicate crystallization reactions. Meanwhile, the composition of the primary crystalline reaction product, typically a RE oxy-apatite, is relatively insensitive to the experimental variables considered herein. The observed trends indicate that the larger RE cations are more potent in rapidly crystallizing the silicate melt and that their relative effectiveness increases with the REO1.5 concentration in the coating material.

Modeling Interfacial Glass‐Water Reactions: Recent Advances and Current Limitations

Describing the reactions that occur at the glass–water interface and control the development of the altered layer constitutes one of the main scientific challenges impeding existing models from providing accurate radionuclide release estimates. Radionuclide release estimates are a critical component of the safety basis for geologic repositories. The altered layer (i.e., amorphous hydrated surface layer and crystalline reaction products) represents a complex region, both physically and chemically, sandwiched between two distinct boundaries—pristine glass surface at the inner most interface and aqueous solution at the outer most interface. Computational models, spanning different length and timescales, are currently being developed to improve our understanding of this complex and dynamic process with the goal of accurately describing the mesoscale changes that occur as the system evolves. These modeling approaches include geochemical simulations (i.e., classical reaction path simulations and glass reactivity in allowance for alteration layer simulations), Monte Carlo simulations, and molecular dynamics methods. Discussed in this manuscript are the advances and limitations of each modeling approach placed in the context of the glass–water reaction and how collectively these approaches provide insights into the mechanisms that control the formation and evolution of altered layers. New results are presented as examples of each approach.

Degradation of La2Zr2O7 and other novel EB-PVD thermal barrier coatings by CMAS (CaO–MgO–Al2O3–SiO2) and volcanic ash deposits

Corrosive attack of CMAS deposits is studied for a series of advanced EB-PVD thermal barrier coatings including 14YSZ, HfSZ, 29DySZ, CeSZ, La2Zr2O7, and Gd2Zr2O7 upon a 2hour anneal at 1260°C in air. Furthermore, the effects of volcanic ash deposits on La2Zr2O7, and Gd2Zr2O7 are studied. A 7YSZ standard coating is employed for benchmarking the resulting microstructures and the coating performances in terms of infiltration depth and chemical reactivity. While all investigated zirconia-based coatings become fully infiltrated by the deposits, the zirconates form crystalline reaction products that prevent complete infiltration. The La2Zr2O7 system is treated as a case study with detailed insight on the newly formed phases via analytical TEM.