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The effects of CaTiO3 (CT) and BaZrO3 (BZ) modification upon the crystal structure and electromechanical properties of lead-free Bi0.5Na0.5TiO3–SrTiO3 piezoelectric ceramics were compared within a doping range of 0–4 mol%. The different effects of CT and BZ modification upon the phase transition are clearly observed in the polarization and strain hysteresis loops. The CT-modified specimens maintain strong ferroelectricity without any abnormal enhancement in the electric field-induced strain. However, the addition of as little as 1 mol% BZ induces a transition from a nonergodic relaxor phase to an ergodic relaxor phase, thus resulting in disruption of the ferroelectric order and the generation of a high field-induced strain. The present authors believe that the substitution of large ions (such as Zr4+) into the B-sites, rather than the A-sites, of the Bi0.5Na0.5TiO3-based ceramics plays a significant role in the phase transition behavior.
Potassium and cerium co-doped Bi4Ti2.86W0.14O12 ceramics with a formula of (K0.5Ce0.5)xBi4−xTi2.86W0.14O12 (abbreviated as KC100x-BITW, x = 0, 0.02, 0.04, 0.06, 0.08, 0.1) were prepared by a conventional solid-state reaction method. The effect of (K0.5Ce0.5) complex doping amount on the structure, dielectric, and piezoelectric properties of the KC100x-BITW ceramics was investigated. X-ray diffraction results indicated that the KC100x-BITW ceramics are Aurivillius-type phase with the bismuth layer structure. (K0.5Ce0.5) complex addition first increases and then decreases the grain size which can be observed by scanning electron microscopy. With the increase of (K0.5Ce0.5) complex doping amount, the Curie temperature (TC) was slightly decreased from 632 to 608 oC. The dielectric and piezoelectric properties were optimized in KC100x-BITW ceramics with x = 0.08 as follows: d33 = 24 pC/N, kp = 8.2%, Qm = 6766, εr = 135 (@100 kHz), tanδ = 0.28% (@100 kHz), Tc = 611 oC, and resistivity ρ = 2.9 × 106 Ω cm at 500 oC, indicating that the KC100x-BITW ceramics are suitable for high-temperature piezoelectric sensing applications.
The past decade has witnessed an intensification of research into the use of pottery by hunter-gatherers. Long viewed by Western scholars as a marginal practice among these groups, pottery production is now known to have been widespread among prehistoric hunter-gatherers, many of whom practised no other activities associated with agriculture. In emphasising the centrality of ceramics to these communities, however, we risk marginalising those who did not adopt pottery. Here, the authors critically examine a series of different models proposed for hunter-gatherer pottery innovation and adoption within the context of the aceramic communities who inhabited Britain and Ireland during the fifth millennium cal BC.
The mitigation of CMAS (calcium–magnesium–aluminum–silicon oxide) infiltration is a major requirement for the stability of thermal barrier coatings. In this study, yttria-stabilized zirconia (YSZ)–Al2O3–SiC, YSZ–Al2O3–Ta2O5, and YSZ–Al2O3–Nb2O5 self-healing composites produced by uniaxially pressing powders were investigated as an alternative to YSZ. CMAS infiltration in these materials was tested at 1250 °C for 10 h. Comparing the depth of CMAS infiltration using scanning electron microscope (SEM) in tandem with electron-dispersive X-ray spectroscopy (EDS), all self-healing materials were found to perform better than the reference materials. While standard YSZ shows massive CMAS infiltration, SEM micrographs and EDS maps revealed a 33-fold improvement in CMAS resistance for the YSZ–Al2O3–Nb2O5 system, which exhibited the best performance among the selected self-repairing materials. X-ray diffraction and high-resolution SEM micrographs taken 10 μm below the surface revealed that CMAS only infiltrated pores in the topmost region of the samples. Both YSZ–Al2O3–Ta2O5 and YSZ–Al2O3–Nb2O5 systems showed no signs of chemical reaction with CMAS.
While the basic sequence of innovations that characterise ceramic production in southern Britain during the first centuries b.c. and a.d. is well-established, our understanding of resistance to these innovations remains in its infancy. Led by the theoretical principles of social constructionism, this paper presents a detailed technological characterisation of Silchester ware, a hand-built ceramic type common in late Iron Age and early Roman Berkshire and northern Hampshire, and a conspicuous example of technological and stylistic anachronism when compared to contemporary wheel-made pottery. Multi-period analyses using radiography, petrography and typology indicate that Silchester ware was not merely a technological ‘hangover’, but a traditional form of material culture with its own role in changing socio-economic structures. Contextualisation of the findings within the local archaeological background further suggests that Silchester ware may have been instrumental in the maintenance of local community and identity at a time when these aspects of social life were under threat. Supplementary material available online (https://doi.org/10.1017/S0068113X20000355) comprises a characterisation of the chaînes opératoires of Silchester ware and its middle Iron Age antecedents, and a summarised version of the data, interpretations and the original radiographs.
Calcium–magnesium–alumino-silicate (CMAS) reaction and infiltration behavior were studied in phase pure and mixed phase ytterbium silicate environmental-barrier coating (EBC) materials at 1300 °C. Phase pure Yb2Si2O7 (YbDS) was infiltrated by CMAS via grain boundaries/pores, resulting in loss of its structural integrity. Phase pure Yb2SiO5 (YbMS) reacted with CMAS to form either apatite (Ca2Yb8(SiO4)6O2) or YbDS, depending on the initial glass composition. Both reactions in YbMS slowed infiltration kinetics considerably compared to YbDS. Samples having a YbDS matrix with controlled amounts and dispersions of YbMS were also investigated as a model for air plasma spray coatings. Samples containing ≥20 vol% coarse YbMS showed dramatically improved infiltration behavior compared to phase pure YbDS. YbDS samples containing a fine dispersion of YbMS displayed a new mode of CMAS attack in which glass spread on the sample surfaces. The results of this study suggest that EBC phase compositions and microstructures may be tailored for optimized CMAS resistance.
Cerium-doped lanthanum magnesium bulk aluminate (La1–xCexMgAl11O19, x = 0.03–0.50; abbreviated as LMA) was prepared via the Pechini sol–gel method after heating at 1200 °C for 2 h. The resulting single-phase ceramics was studied in terms of its structure using X-ray diffraction and optical properties using photoluminescence, its decay time, and radioluminescence spectroscopy. The diffraction and electron microscopy demonstrated LMA's plate-shaped nanocrystals with structure anisotropy and relatively broad particle size distribution. The optical measurements fully manifested the complexity of the LMA crystal structure. The radioluminescence study of cerium-doped LMA is here presented for the first time and, thus, contributes to the basic knowledge of Ce-doped materials. Additionally, the magnetic susceptibility exhibiting paramagnetic behavior of Ce3+ ions is presented. The magnetic data were interpreted in terms of local atomic Hamiltonian involving the crystal field and the Zeeman effect applied on the ground state J = 5/2 multiplet.
High-temperature (1500 °C) interactions of promising environmental-barrier coating (EBC) ceramics in the rare-earth (RE) pyrosilicate system, Yb(2-x)YxSi2O7 (x = 0, 0.2, 1, or 2), with three different calcia–magnesia–aluminosiliate (CMAS) glass compositions, are explored. Only the Ca/Si ratio is varied in the CMAS: 0.76, 0.44, or 0.10. Interaction between the highest Ca/Si CMAS and the EBC ceramic with the lowest x (=0, Yb2Si2O7) promotes no reaction but the formation of “blister” cracks. In contrast, the highest x (=2, Y2Si2O7) promotes the formation of an apatite reaction product, but no “blister” cracks. Observationally, it is found that a decrease in the CMAS Ca/Si ratio (0.76–0.10) and a decrease in Y-content decreases the propensity for reaction crystallization (apatite formation) and “blister” cracks. These results are rationalized based on the relative affinities between Ca2+ in the CMAS and Y3+ or Yb3+ in the EBC ceramics, suggesting a way to tune the CMAS interactions in RE pyrosilicate solid solutions.
The present study deploys a continuum mechanics approach called peridynamics to investigate the damage behaviour of a 2D microstructure, which was taken from a plasma sprayed ceramic coating used in solid oxide fuel cell (SOFC) sealing systems. At the beginning, two benchmark cases, namely, plate with a hole as well as plate with a single edge notch, are considered. The results are compared to an analytical solution and a very good agreement is obtained. Based on these findings, a microstructural model from a plasma sprayed ceramic coating of SOFC sealing systems is investigated. These micromechanical simulations show that structural defects influence the crack initiation as well as the crack propagation during interconnecting the defects. Typical crack mechanisms, such as crack deflection, crack shielding or multiple cracking, are observed. Additionally, an anisotropy of the effective mechanical properties is observed in this heterogeneous material, which is well known for plasma sprayed materials.
Here, we develop and characterize high thermal conductivity/high thermal shock-resistant bulk Ce-doped Al2O3 and propose it as a new phosphor converting capping layer for high-powered/high-brightness solid-state white lighting (SSWL). The bulk, dense Ce:Al2O3 ceramics have a 0.5 at.% Ce:Al concentration (significantly higher than the equilibrium solubility limit) and were produced using a simultaneous solid-state reactive current activated pressure-assisted densification (CAPAD) approach. Ce:Al2O3 exhibits a broadband emission from 400 to 600 nm, which encompasses the entire blue and green portions of the visible spectrum when pumped with ultraviolet (UV) light that is now commercially available in UV light–emitting devices and laser diodes (LD). These broadband phosphors can be used in the commonly used scheme of mixing with other UV-converting capping layers that emit red light to produce white light. Alternatively, they can be used in a novel composite down-converter approach that ensures improved thermal–mechanical properties of the converting phosphor capping layer. In this configuration, Ce:Al2O3 is used with proven phosphor conversion materials such as Ce:YAG as an active encapsulant or as a capping layer to produce SSWL with an improved bandwidth in the blue portion of the visible spectrum. To study the effect of crystallinity on the Ce photoluminescent (PL) emission, we synthesize Ce:YAG ceramics using high-pressure CAPAD at moderate temperatures to obtain varying crystallinity (amorphous through fully crystalline). We investigate the PL characteristics of Ce:Al2O3 and Ce:YAG from 295 to 4 K, revealing unique crystal field effects from the matrix on the Ce dopants. The unique PL properties in conjunction with the superior thermal–mechanical properties of Ce:Al2O3 can be used in high-powered/high-brightness–integrated devices based on high-efficiency UV-LD that do not suffer efficiency droop at high drive currents to pump the solid-state capping phosphors.
During the Epiclassic period (AD 600–900), the northern frontier of Mesoamerica consisted of a regional network of polities focused on large, hilltop centers, including the site of La Quemada in the Malpaso Valley of Zacatecas, Mexico. Although extensive archaeological research has been conducted at the site, the refinement of its chronology is essential for two reasons: (1) to establish the chronological control necessary to characterize social processes diachronically and (2) to ensure that the occupational history of La Quemada is accurately integrated into the regional chronology of the northern frontier. A combination of frequency seriation, correspondence analysis, and discriminant function analysis results in the recognition of three occupational phases across the areas excavated by the La Quemada-Malpaso Valley Archaeological Project (LQ-MVAP). Our three-phase chronology independently confirms both the intra-context ordering of analytic units and the previously proposed growth trajectory of the site: beginning in the monumental core, expanding into the western flank, and later retracting back into the core. The separation of the LQ-MVAP material record into chronological phases means it is now possible to track changes in the social processes that may have contributed to the formation, maintenance, and decline of La Quemada and other northern frontier polities.
The Gaspereau Lake Reservoir Site Complex in Nova Scotia, Canada, yielded a large ceramic assemblage that permitted the first fine-grained analysis of ceramic change in the region at the Middle–Late Woodland Transition from ca. 1550 BP to ca. 1150 BP. The aim of this study was to refine the standard regional chronology first proposed by researchers J B Petersen and D Sanger. To do this, ceramics were directly dated using accelerator mass spectrometry (AMS), and the assemblage was categorized and analyzed to identify clusters of attributes. Ten AMS dates were acquired on carbonized food residue on the interiors of pottery and yielded the largest continuous ceramic sequence in the Maritime Provinces of Canada. This sequence was used to infer a change in manufacturing practices between the Middle (2150–1300 BP) and Late (1300–500 BP) Woodland periods and to propose five new subperiods between 1650 BP and 950 BP. Increasing incidence of coil breaks and temper percentage from the Middle to the Late Woodland were found to be chronologically sensitive. The analysis showed that, at Gaspereau Lake, a gradual shift from finely decorated and manufactured pottery to expediently made pottery suggests that pottery was made in larger numbers to support large-scale gatherings.
Nuclear fuel debris generated at the Fukushima Daiichi nuclear power plant during the loss of coolant accident in 2011, still resides within the reactor units, constantly cooled by water. Until it is retrieved, the fuel debris will corrode, releasing radioactive elements into the coolant water and the ground surrounding the reactors. To predict the corrosion behaviour of these materials, and to establish parameters for experiments with U-containing and real fuel debris, the corrosion of two surrogate fuel debris materials, with a composition of Ce(1-x)ZrxO2 (x = 0.2 and 0.4), was investigated. Materials were synthesised by a wet chemistry route and pellets were sintered at 1700°C in air atmosphere. Due to the slow corrosion kinetics, aggressive conditions were applied, and corrosion experiments were performed in 9 mol.L-1 HNO3 under static conditions. The incorporation of Zr into the structure of Ce reduced the normalised dissolution rate; from (3.75 ± 0.15) × 10-6 g.m-2.d-1 to (4.96 ± 0.28) × 10-6 g.m-2.d-1 for RL(Ce) of Ce0.8Zr0.2O2 and Ce0.6Zr0.4O2, respectively.
Computational modeling is an important aspect of the research on nuclear waste materials. In particular, atomistic simulations, when used complementary to experimental efforts, contribute to the scientific basis of safety case for nuclear waste repositories. Here we discuss the state-of-the-art and perspectives of atomistic modeling for nuclear waste management on a few cases of successful synergy of atomistic simulations and experiments. In particular, we discuss here: (1) the potential of atomistic simulations to investigate the uranium oxidation state in mixed-valence uranium oxides and (2) the ability of cementitious barrier materials to retain radionuclides such as 226Ra and 90Sr, and of studtite/metastudtite secondary peroxide phases to incorporate actinides such as Np and Am. The new contribution we make here is the computation of the incorporation of Sr by C-S-H (calcium silicate hydrate) phases.
Immobilisation of Pu in a zirconolite matrix (CaZrTi2O7) is a viable pathway to disposition. A-site substitution, in which Pu4+ is accommodated into the Ca2+ site in zirconolite, coupled with sufficient trivalent M3+/Ti4+ substitution (where M3+ = Fe, Al, Cr), has been systematically evaluated using Ce4+ as a structural analogue for Pu4+. A broadly similar phase assemblage of zirconolite-2M and minor perovskite was observed when targeting low levels of Ce incorporation. As the targeted Ce fraction was elevated, secondary phase formation was influenced by choice of M3+ species. Co-incorporation of Ce/Fe resulted in the stabilisation of a minor Ce-containing perovskite phase at high wasteloading, whereas considerable phase segregation was observed for Cr3+ incorporation. The most favourable substitution approach appeared to be achieved with the use of Al3+, as no perovskite or free CeO2 was observed. However, high temperature treatments of Al containing specimens resulted in the formation of a secondary Ce-containing hibonite phase.
Improved budgeting of heat loads arising from radiogenic heating in high level wastes (HLW) could allow enhanced usage of geological disposal facility space. Separation of high heat generating nuclides from HLW, such as Cs, would simplify management of heat loads. A potential host matrix for Cs-disposal is hollandite. The incorporation of Cs into the hollandite phase is aided by substitution of cations on the B-site of the structure; these ions may include Ni and Zn. Two series of hollandites, Ni-substituted and Zn-substituted, were synthesised via an alkoxide-nitrate route and consolidated by cold uniaxial pressing and sintering or by hot isostatic pressing. Characterisation of the resultant material by X-ray diffraction and scanning electron microscopy found that hollandite was formed for all levels of substitution. Materials produced via HIP were found to be denser indicating lower Cs loss. HIPed Ni hollandites were found to contain fewer secondary phases and it was concluded that they were the most suitable candidates
Residual stress can considerably weaken systems with ceramics-to-metal joints. Herein, we investigate the dependence of bonding strength and residual stress variation of a ceramics-to-metal joint system on the interface wedge angle and bonding temperature condition. First, disparity between large-scale displacement models with varying work-hardening parameters was confirmed using thermal elastoplastic Finite Element Method (FEM) analysis. Each interface wedge shape was set to a plane surface to compare FEM results to experimental results related to the effect of the interface wedge angle on the practical bonding strength. The experimental results were specifically for a system consisting of Si3N4-WC/TiC/TaC bonded to Ni plate. The effects of the wedge angle of the metal side on residual stress near the interface edge were numerically predicted using FEM models. The interface wedge angles for this model, φ1 and φ2, were defined using the configuration angle between the interface and free surfaces of both materials. The numerical results showed that the stress σr on the free surface of the ceramic side was concentrated near the interface edge at which discontinuity in the stress state is generated. Dependence of the residual stress variation on both the wedge angle and temperature conditions can be predicted. It was confirmed that the bonding strength improves with decreasing residual stress in geometrical conditions. Therefore, residual stress appears to be a predominant factor affecting bonding strength. The observed fracture pattern showed that the fracture originated near the interface edges, after which small cracks propagated on the ceramic side. The residual stress is presumed to dominate bonding strength as the fracture occurred near the interface edge of the ceramic side. Results showed that the maximum bonding strength appears at the geometrical condition where the fracture pattern changes to φ2 lower than 90° of joint bonded at 980 °C. Therefore, the optimum interface wedge angle depends on a combination of materials and bonding temperature conditions, because the weak point of the bonded joint system will affect the stiffness balance of both materials and the adhesion power of the bonded interface.
Millimetre UO2 single crystals were cut and oriented at JRC Karlsruhe. The orientation of each face of the parallelepiped single crystals was determined with Laue diffraction and the corresponding surface area by geometric measurements. Then, the (111), (100), (110) faces of each single crystal were polished to optical grade and characterized by XRD in order to confirm the surface orientation. The dissolution of the three single crystals was achieved in nitric acid media under dynamic conditions, at room temperature. Two dissolution regimes were observed for all samples. The normalized dissolution rate measured in the first step was not influenced by the crystallographic orientation of the faces. However, during the second step, (110) oriented faces were found to dissolve 4 times faster than the (100) faces. One explanation could involve the atomic composition of each oriented surface in the fluorite-type structure
This work reports on the use of diatomaceous earth (DE) waste and organic binder derived from Corchorus olitorius, locally known as “Mrenda” in the design of an efficient water filtration membranes. Charcoal powder was incorporated to enhance the porosity of the membrane. The firing was done at temperatures varying from 700.0 °C to 1150.0 °C. The DE waste samples comprised 79.0% silica (by mass) and 11.0% total flux content compared to porter’s clay that had 50.0% silica, 28.8% AL2O3 and 7.0% total flux content. On the other hand, the “Mrenda” binder contained 6.5% total organic matter. The use of the plant-derived binder enhanced the mechanical strength of the greenware by 52.7% and the fired membranes by 152.2%. The fabricated DE waste-based membranes were 15.0% stronger than clay-based ceramic membranes prepared under similar conditions. A sintering temperature of 900.0 °C was optimal in producing porous membranes for filtering of 4.1 liters of water per hour. The pore diameter of the membranes fabricated from DE waste only ranged between 2.0 nm – 99.0 nm. On micro-organisms filtering efficacy, the DE waste-based membranes and those fabricated with 5.0% charcoal were 99.9% and 88.4% effective in the removal of E. coli and Rotavirus respectively.
A zirconolite glass-ceramic material is a candidate wasteform for immobilisation of chlorine contaminated plutonium residues, in which plutonium and chlorine are partitioned to the zirconolite and aluminosilicate glass phase, respectively. A preliminary investigation of chlorine speciation was undertaken by analysis of Cl K-edge X-ray Absorption Near Edge Spectroscopy (XANES), to understand the incorporation mechanism. Cl was found to be speciated as the Cl- anion within the glass phase, according to the characteristic chemical shift of the X-ray absorption edge. By comparison with Cl K-edge XANES data acquired from reference compounds, the local environment of the Cl- anion is most closely approximated by the mineral marialite, in which Cl is co-ordinate to 4 x Na and/or Ca atoms.