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Showing papers in "Journal of the American Ceramic Society in 2014"


Journal ArticleDOI
TL;DR: In this paper, the impact factors on the hysteresis loops are discussed based on recent developments in ferroelectric and related materials, including the effect of materials (grain size and grain boundary, phase and phase boundary, doping, anisotropy, thickness), aging, and measurement conditions (applied field amplitude, fatigue, frequency, temperature, stress), which can affect the hysteretic behaviors of the ferroelectrics.
Abstract: Due to the nature of domains, ferroics, including ferromagnetic, ferroelectric, and ferroelastic materials, exhibit hysteresis phenomena with respect to external driving fields (magnetic field, electric field, or stress). In principle, every ferroic material has its own hysteresis loop, like a fingerprint, which contains information related to its properties and structures. For ferroelectrics, many characteristic parameters, such as coercive field, spontaneous, and remnant polarizations can be directly extracted from the hysteresis loops. Furthermore, many impact factors, including the effect of materials (grain size and grain boundary, phase and phase boundary, doping, anisotropy, thickness), aging (with and without poling), and measurement conditions (applied field amplitude, fatigue, frequency, temperature, stress), can affect the hysteretic behaviors of the ferroelectrics. In this feature article, we will first give the background of the ferroic materials and multiferroics, with an emphasis on ferroelectrics. Then it is followed by an introduction of the characterizing techniques for the loops, including the polarization–electric field loops and strain–electric field curves. A caution is made to avoid misinterpretation of the loops due to the existence of conductivity. Based on their morphologic features, the hysteresis loops are categorized to four groups and the corresponding material usages are introduced. The impact factors on the hysteresis loops are discussed based on recent developments in ferroelectric and related materials. It is suggested that decoding the fingerprint of loops in ferroelectrics is feasible and the comprehension of the material properties and structures through the hysteresis loops is established.

869 citations


Journal ArticleDOI
TL;DR: In this paper, the filler effect is attributed to the surface of the filler providing nucleation sites for C-S-H as there is a clear dependence on the surface provided by the filler particles.
Abstract: The filler effect, due to the physical presence of mineral additions in cement, is mainly known to accelerate the hydration of the clinker component. Previously, this was attributed to the surface of the filler providing nucleation sites for C-S-H as there is a clear dependence on the surface provided by the filler particles. Our results reveal that the increase in nucleation is quite low compared to the area provided. Based on the isothermal calorimetry experiments and SEM images, we demonstrate that the most important parameter is the interparticle distance. We propose that this is mainly the result of the shearing conditions rather than extra surface available for C-S-H as formerly assumed. Quantitatively slag and fly ash behave very similarly to quartz. Limestone, on the other hand, seems also to stimulate C-S-H nucleation giving it a higher efficiency in accelerating clinker hydration.

463 citations


Journal ArticleDOI
TL;DR: In this paper, the thermodynamic and kinetic origins of the formation and stabilization of the frequently observed secondary, nonperovskite phases, such as Bi25FeO39 and Bi2Fe4O9, are discussed.
Abstract: Bismuth ferrite (BiFeO3), a perovskite material, rich in properties and with wide functionality, has had a marked impact on the field of multiferroics, as evidenced by the hundreds of articles published annually over the past 10 years. Studies from the very early stages and particularly those on polycrystalline BiFeO3 ceramics have been faced with difficulties in the preparation of the perovskite free of secondary phases. In this review, we begin by summarizing the major processing issues and clarifying the thermodynamic and kinetic origins of the formation and stabilization of the frequently observed secondary, nonperovskite phases, such as Bi25FeO39 and Bi2Fe4O9. The second part then focuses on the electrical and electromechanical properties of BiFeO3, including the electrical conductivity, dielectric permittivity, high-field polarization, and strain response, as well as the weak-field piezoelectric properties. We attempt to establish a link between these properties and address, in particular, the macroscopic response of the ceramics under an external field in terms of the dynamic interaction between the pinning centers (e.g., charged defects) and the ferroelectric/ferroelastic domain walls.

355 citations


Journal ArticleDOI
TL;DR: In this article, two classes of down conversion materials: phosphors and semiconductor quantum dots (QDs) are discussed and the challenges in the development of down converters that satisfy all these criteria.
Abstract: The wavelength down conversion approach to solid-state lighting (SSL) uses down conversion materials to produce visible light when excited by near-UV or blue emission from InGaN LEDs. This review discusses two classes of down conversion materials: phosphors and semiconductor quantum dots (QDs). Strong absorption of the excitation wavelength; high luminous efficacy of radiation, which enables white light with a high color rendering index and a low correlated color temperature; high quantum efficiency; and thermal and chemical stability are some of the criteria for down converters used in SSL. This review addresses the challenges in the development of down converters that satisfy all these criteria. We will discuss the advantages and disadvantages of several phosphor compositions for blue and near-UV LEDs. The use of core/shell architectures to improve the photoluminescence and moisture resistance of phosphors is presented. QDs are another class of down conversion materials for near-UV and blue LEDs. Strategies to improve the photostability and reduce the thermal quenching of QDs include strain-graded core/shell interfaces and alloying. We discuss Cd-containing II–VI QDs, and Cd-free III–V and I–III–VI QDs and their potential for SSL applications. Finally, a description of different methods to integrate the phosphors and QDs with the LED is given.

344 citations


Journal ArticleDOI
TL;DR: In this article, the authors present an overview of the results obtained when AAMs are exposed to aggressive testing conditions such as elevated concentrations of CO2, sulfates or chlorides.
Abstract: In the last decade, there has been rapid growth in interest in alternative binders, as part of the toolkit of cement technologies needed to mitigate the carbon footprint associated with the construction industry. Alkali-activated materials (AAMs), including geopolymer binders and other related systems, have been identified as a key component of this move to lower CO2 cements and concretes. These are clinker-free cements which can exhibit comparable performance to conventional portland/blended cements, when they are adequately formulated and cured. However, AAMs have a somewhat limited record of durability in service, and this is one of the main limitations facing their commercial adoption at present. To provide the best possible answers to the question of long-term durability within an experimentally accessible timeframe, standardized accelerated degradation testing methods have been widely adopted, in an attempt to simulate natural processes. It has been identified that the interactions between material and environment, which take place on microstructural and nanostructural levels, have a very significant influence on the outcomes of the durability tests. Here, we present an overview of the results obtained when AAMs are exposed to aggressive testing conditions such as elevated concentrations of CO2, sulfates or chlorides. The key outcome of this article is a broader synthesis of the available data regarding the interactions between these new materials and their surrounding environment, which is then available to be used in the design, development, and implementation of environmentally sustainable, high-performance cements and concretes for the 21st century.

307 citations



Journal ArticleDOI
TL;DR: The Calvet calorimeter for high-temperature oxide melt solution calorimetry has been commercially available for the past eighteen years and an extensive table of enthalpies of drop solution (sample at room temperature dropped into molten oxide solvent and dissolved) is provided for the user community as mentioned in this paper.
Abstract: To assess progress in high-temperature calorimetry over the past eighteen years, this feature article discusses both technical developments and new areas of application, with primary emphasis on high-temperature oxide melt solution calorimetry. The Calvet calorimeter for such measurements is now commercially available, and an extensive table of enthalpies of drop solution (sample at room temperature dropped into molten oxide solvent in calorimeter and dissolved) is provided for the user community. New developments in methodology are described and applications to nanomaterials, to nitrides and other monoxide materials, and to lanthanides and actinides are given.

170 citations


Journal ArticleDOI
TL;DR: In this paper, the effect of DC electric field on sintering, and on the electrical conductivity of undoped rutile, TiO2 (99.99%), has been investigated at fields ranging from 0 V to 1000 V/cm.
Abstract: The effect of DC electric field on sintering, and on the electrical conductivity of undoped rutile, TiO2 (99.99%), has been investigated at fields ranging from 0 V to 1000 V/cm. The experiments were carried out at a constant heating rate of 10°C/min with the furnace temperatures reaching up to 1150°C. The sintering behavior falls into two regimes: at lower fields, up to 150 V/cm, sintering is enhanced, but densification occurs gradually with time (Type A or FAST sintering). At higher fields sintering occurs abruptly, and is accompanied by a highly nonlinear increase in conductivity, which has been called flash sintering (Type B or FLASH sintering). Arrhenius plots of conductivity yield an activation energy of 1.6 eV in Type A and 0.6 eV in Type B behavior; the first is explained as ionic and the second as electronic conductivity. The evolution of grain size under both types of sintering behavior are reported. These results highlight that the dominant mechanism of field-assisted sintering can change with the field strength and temperature. We are in the very early stages of identifying these mechanisms and mapping them in the field, frequency, and temperature space.

154 citations


Journal ArticleDOI
TL;DR: In this paper, a composite geopolymeric material was synthesized from Bayer red mud combined with granulated blast-furnace slag, and thermal pretreatment was applied to improve the solubility of red mud in alkaline solution to promote geopolymerization.
Abstract: A composite geopolymeric material was synthesized from Bayer red mud combined with granulated blast-furnace slag. Thermal pretreatment was applied to improve the solubility of red mud in alkaline solution to promote geopolymerization. The dissolution efficiencies of alumina and silica reached a maximum when red mud was calcined at 800°C, resulting in the highest compressive strength of binders. It was demonstrated that a higher solubility of calcined red mud led to a higher strength of the composite binders. The characteristic microstructures of hydration products were studied to illustrate the geopolymerization process by XRD, FTIR and SEM. The results showed that aluminosilicates were dissolved in the alkaline solution to form nanostructural particulates during the early dissolution process, and then accumulated to form highly dense geopolymeric matrices through solidification reaction. The coexistence of geopolymer and C–(A)–S–H is suggested to contribute to the good performance of the composite binders.

150 citations


Journal ArticleDOI
TL;DR: In this paper, the BNT-BT−0.3SBT has a very high electrostrictive strain S = 0.152% with hysteresis-free behavior, much more than the reported S in other ferroelectrics.
Abstract: Relaxor ferroelectrics (0.94 − x)(Bi0.5Na0.5)TiO3–0.06BaTiO3−x(Sr0.7Bi0.2□0.1)TiO3 (BNT–BT–xSBT) (0 ≤ x ≤ 0.5), were prepared by a solid-state reaction process, and their structures were characterized by the transmission electron microscopy and Raman spectroscopy. The BNT–BT–0.3SBT has a very high electrostrictive strain S = 0.152% with hysteresis-free behavior, much more than the reported S in other ferroelectrics. S–P2 profiles perfectly follow the quadratic relation, which indicates a purely electrostrictive effect with a high electrostrictive coefficient (Q11) of 0.0297 m4/C2. Even, its Q11 keeps at a high level in the temperature range from ambient temperature to 180°C. The field-induced large electrostrictive strain of BNT–BT–0.3SBT was attributed to the existence of ferroelectric nanodomains.

145 citations


Journal ArticleDOI
TL;DR: In this article, the dielectric and electromechanical properties of 0.75Bi1/2Na 1/2TiO3 (25ST) as a function of temperature and frequency were studied.
Abstract: The dielectric and electromechanical properties of 0.75Bi1/2Na1/2TiO3–0.25 SrTiO3 (25ST) as a function of temperature and frequency were studied. It is shown that the 25ST is a relaxor ferroelectric as evidenced by the temperature-dependent dielectric relaxations with an incipient piezoelectricity featured by the presence of a reversible electric-field-induced phase transformation at room temperature. The transition occurs on a broad electric field strength range depending on field amplitude and frequency. It is also accompanied by a huge strain that is attributed to repetitive poling and depoling originating due to the reversibility of the phase transition. The 25ST makes an attractive lead-free candidate for stack actuators as it presents a high normalized d33* of ~600 pm/V at a low electric field of 4 kV/mm for frequencies ranging from 0.1 up to 100 Hz.

Journal ArticleDOI
Hui Dong1, Guan-Jun Yang1, Cheng-Xin Li1, Xiao-Tao Luo1, Chang-Jiu Li1 
TL;DR: In this paper, a gradient thermal cycling test was performed on atmospheric plasma-sprayed thermal barrier coatings (TBCs) with different thermally grown oxide (TGO) thicknesses.
Abstract: Gradient thermal cycling test was performed on atmospheric plasma-sprayed (APS) thermal barrier coatings (TBCs) with different thermally grown oxide (TGO) thicknesses. The TBCs with a thickness of TGO from 1.3 μm to 7.7 μm were prepared by controlling isothermal oxidation time of cold-sprayed MCrAlY bond coat. The gradient thermal cycling test was performed at a peak surface temperature of 1150°C with 150°C difference across 250 μm thick YSZ with a duration of 240 s for each cycle. Results indicate that the thermal cyclic lifetime of APS TBCs is significantly influenced by TGO thickness. When initial TGO thickness increases from 1.3 μm to 7.7 μm, the thermal cyclic lifetime decreases following a power functions by a factor of about 20. It was revealed that there exists a critical TGO thickness over which the thermal cyclic lifetime is reduced more significantly with the increase in TGO thickness. Moreover, two typical failure modes were observed. The failure mode changes from the cracking within APS YSZ at a TGO thickness less than the critical value to through YSZ/TGO interface at TGO thickness range higher than the critical value.

Journal ArticleDOI
TL;DR: In this article, the authors used particle temperature measurements at different current levels to detect potential degrees of evaporation of Gd2Zr2O7 by atmospheric plasma spraying.
Abstract: Processing of Gd2Zr2O7 by atmospheric plasma spraying (APS) is challenging due to the difference in vapor pressure between gadolinia and zirconia. Gadolinia is volatilized to a greater extent than zirconia and the coating composition unfavorably deviates from the initial stoichiometry. Aiming at stoichiometric coatings, APS experiments were performed with a TriplexPro™ plasma torch at different current levels. Particle diagnostics proved to be an effective tool for the detection of potential degrees of evaporation via particle temperature measurements at these varied current levels. Optimized spray parameters for Gd2Zr2O7 in terms of porosity and stoichiometry were used to produce double-layer TBCs with an underlying yttria-stabilized zirconia (7YSZ) layer. For comparison, double layers were also deposited with relatively high torch currents during Gd2Zr2O7 deposition, which led to a considerable amount of evaporation and relatively low porosities. These coatings were tested in thermal cycling rigs at 1400°C surface temperature. Double layers manufactured with optimized Gd2Zr2O7 spray parameters revealed very good thermal cycling performance in comparison to standard 7YSZ coatings, whereas the others showed early failures. Furthermore, different failure modes were observed; coatings with long lifetime failed due to TGO growth, while the coatings displaying early failures spalled through crack propagation in the upper part of the 7YSZ layer.

Journal ArticleDOI
TL;DR: In this paper, the sintering behavior of nanocrystalline zinc oxide under external AC electric field between 0 and 160 V/cm was analyzed, where high current densities flow through the sample at high electric fields, entailing a sudden increment of the temperature estimated to several hundreds of K and an exaggerated grain growth.
Abstract: We report the sintering behavior of nanocrystalline zinc oxide under external AC electric field between 0 and 160 V/cm. In situ acquisition of density by means of laser dilatometry, evaluation of specimen temperature, real-time measurement of electric field and current help analyze this peculiar behavior. Field strength and blocking electrodes significantly affect densification and microstructure, which was evaluated in the vicinity of the flash event and for the fully sintered material. High current densities flow through the sample at high electric fields, entailing a sudden increment of the temperature estimated to several hundreds of K and an exaggerated grain growth. In contrast, low current density flows through the sample at lower electric fields, which guarantees normal grain growth and highest final density. Macroscopic photoluminescence measurements give insights into the development of the defect structure. Electric fields are expected to enhance defect mobility, explaining the high densification rates observed during the sintering process.

Journal ArticleDOI
TL;DR: In this article, the electrical and dielectric properties of (1 − x) ceramics were investigated by impedance spectroscopy over a wide temperature range, and the presence of a highly polarizable phase for all compositions was revealed by electric modulus (M″) spectra.
Abstract: The electrical and dielectric properties of (1 − x)(0.94Bi1/2Na1/2TiO3–0.06BaTiO3)–x(K0.5Na0.5NbO3) with x = 0, 0.03, 0.09, 0.18 have been investigated by impedance spectroscopy over a wide temperature range. The dc conductivity of the ceramics follows the Arrhenius law with an activation energy ranging from ~1.20 to 1.50 eV. Measurements under different atmospheres show the materials exhibit n-type semiconducting behavior at elevated temperatures. The presence of a highly polarizable phase for all compositions is revealed by electric modulus (M″) spectra. The Burns temperature decreases with increasing KNN content. The change in temperature-dependent permittivity with composition is explained by the difference in thermal evolution of polar nanoregions induced by the addition of KNN.

Journal ArticleDOI
TL;DR: In this article, pure ZrB2 powder was flash sintered in an SPS furnace (FSPSP) and the samples were densified up to 95.0% in 35 seconds under an applied pressure of 16MPa.
Abstract: Pure ZrB2 powder was Flash sintered in an SPS furnace (FSPS). The samples were densified up to 95.0% in 35 s under an applied pressure of 16 MPa. Compared to Conventional SPS (CSPS), the newly developed FSPS technique resulted in an unprecedented energy and time savings of about 95% and 98% respectively. ZrB2 monoliths obtained by CSPS and FSPS were compared with respect to microstructures, densification behavior, and grain growth. The developed methodology might find application to a wide range of highly conductive ceramics as such refractory borides and carbides.

Journal ArticleDOI
TL;DR: In this article, a systematic study of the influence of coating microstructure on the fracture toughness of atmospheric plasma sprayed (APS) TBCs has been carried out, and the results indicated significant variance in fracture toughness among coatings with different microstructures including changes induced by thermal aging.
Abstract: Fracture toughness has become one of the dominant design parameters that dictates the selection of materials and their microstructure to obtain durable thermal barrier coatings (TBCs). Much progress has been made in characterizing the fracture toughness of relevant TBC compositions in bulk form, and it has become apparent that this property is significantly affected by process-induced microstructural defects. In this investigation, a systematic study of the influence of coating microstructure on the fracture toughness of atmospheric plasma sprayed (APS) TBCs has been carried out. Yttria partially stabilized zirconia (YSZ) coatings were fabricated under different spray process conditions inducing different levels of porosity and interfacial defects. Fracture toughness was measured on free standing coatings in as-processed and thermally aged conditions using the double torsion technique. Results indicate significant variance in fracture toughness among coatings with different microstructures including changes induced by thermal aging. Comparative studies were also conducted on an alternative TBC composition, Gd2Zr2O7 (GDZ), which as anticipated shows significantly lower fracture toughness compared to YSZ. Furthermore, the results from these studies not only point towards a need for process and microstructure optimization for enhanced TBC performance but also a framework for establishing performance metrics for promising new TBC compositions.

Journal ArticleDOI
TL;DR: The composition and structure of the calcium-silicate-hydrate (C-S-H) phases formed by hydration of white portland cement-metakaolin (MK) blends have been investigated using 27Al and 29Si MAS NMR as discussed by the authors.
Abstract: The composition and structure of the calcium-silicate-hydrate (C–S–H) phases formed by hydration of white portland cement–metakaolin (MK) blends have been investigated using 27Al and 29Si MAS NMR. This includes blends with 0, 5, 10, 15, 20, 25, 30 wt% MK, following their hydration from 1 d to 1 yr. 29Si MAS NMR reveals that the average Al/Si ratio for the C–S–H phases, formed by hydration of the portland cement–MK blends, increases almost linearly with the MK content but is invariant with the hydration time for a given MK content. Correspondingly, the average aluminosilicate chain lengths of the C–S–H increase with increasing MK content, reflecting the formation of a C–S–H with a lower Ca/Si ratio. The increase in Al/Si ratio with increasing MK content is supported by 27Al MAS NMR which also allows detection of stratlingite and fivefold coordinated aluminum, assigned to AlO5 sites in the interlayer of the C–S–H structure. Stratlingite is observed after prolonged hydration for MK substitution levels above 10 wt% MK. This is at a somewhat lower replacement level than expected from thermodynamic considerations which predict the formation of stratlingite for MK contents above 15 wt% after prolonged hydration for the actual portland cement–MK blends. The increase in fivefold coordinated Al with increasing MK content suggests that these sites may contribute to the charge balance of the charge deficit associated with the incorporation of Al3+ ions in the silicate chains of the C–S–H structure.

Journal ArticleDOI
TL;DR: In this paper, the phase composition, morphology, and electromagnetic properties of spinel ferrite were analyzed, and the results showed that all the as-prepared CoxZn(1−x)Fe2O4 ferrites exhibited the homogeneous nanofibrous shape.
Abstract: To solve the heavy mass problem of the traditional spinel ferrite using as the microwave absorber, the CoxZn(1−x)Fe2O4 (x = 0.2, 0.4, 0.6, 0.8) ferrite nanofibres were synthesized by electrospinning method. The phase composition, morphology, and electromagnetic properties were analyzed. The results showed that all the as-prepared CoxZn(1−x)Fe2O4 ferrites exhibited the homogeneous nanofibrous shape. The saturation magnetization and coercivity were enhanced by tuning the Co2+ content. The electromagnetic loss analysis indicated that the Co0.6Zn0.4Fe2O4 ferrite nanofiber performed the strongest microwave attenuation ability. The microwave absorbing coating containing 15 wt% of Co0.6Zn0.4Fe2O4 ferrite nanofiber showed the reflection loss less than −10 dB in the whole X-band and 80% of the Ku-band frequencies. Meanwhile, the surface density was only 2.4 Kg/m2.

Journal ArticleDOI
TL;DR: In this article, Li2MoO4 disks were fabricated by moistening water-soluble Li 2 MoO4 powder with deionized water and compressing it under a pressure of 130 MPa.
Abstract: Lithium molybdate disks were fabricated by moistening water-soluble Li2MoO4 powder with deionized water and compressing it under a pressure of 130 MPa. Disks were postprocessed at room temperature, at 120°C, and at 540°C, which is a common sintering temperature for Li2MoO4. Regardless of the postprocessing temperature, densities as high as 87%–93% of the theoretical value were achieved. The X-ray diffraction patterns of processed disks were all the same with no signs of hydrates or constitutional water. The samples also exhibited very similar microstructures and microwave dielectric properties with a relative permittivity of 4.6–5.2 and a Q × f value of 10 200–18 500 at 9.6 GHz, depending on the postprocessing temperature.

Journal ArticleDOI
TL;DR: In this article, two MAX compounds, (Cr2/3Ti1/3)(3)AlC2 and (Cr5/8Ti3/8)(4) AlC3, were successfully synthesized by hot-pressing elemental powders at 1500 degrees C for 1 h under 30 MPa in a flowing argon atmosphere.
Abstract: Two new MAX compounds, (Cr2/3Ti1/3)(3)AlC2 and (Cr5/8Ti3/8)(4) AlC3, were successfully synthesized by hot-pressing elemental powders at 1500 degrees C for 1 h under 30 MPa in a flowing argon atmosphere. Their crystal structures were indentified and characterized by X-ray diffraction and transmission electron microscopy analysis. (Cr2/3Ti1/3)(3)AlC2 and (Cr5/8Ti3/8)(4)AlC3 have the same crystal structures with the well-characterized Ti3AlC2 and Ti4AlN3, respectively.

Journal ArticleDOI
TL;DR: In this paper, a thermodynamic analysis for diffusionless phase diagrams of ferroelectric solid solutions that display a morphotropic phase boundary separating adjacent tetragonal and rhombohedral phases is presented.
Abstract: A thermodynamic analysis is presented for the diffusionless phase diagrams of ferroelectric solid solutions that display a morphotropic phase boundary (MPB) separating adjacent tetragonal and rhombohedral phases. Equations are developed for the shape of the MPB, the locations of triple and tricritical points, and for the line along which the anisotropy of polarization vanishes. The appearance of lower symmetry orthorhombic and monoclinic phases is considered and the topologies of energy surfaces in the region of the phase diagram where these phases may stabilize are illustrated. The theory is applied to the solid solution of lead zirconate with lead titanate (PZT) and relationships between polar anisotropy and the transformation strain, dielectric susceptibility and piezoelectric properties, are discussed. The analysis is used to reproduce phase boundary lines for solid solutions of lead titanate with lead magnesium niobate (PMN-PT) and lead zinc niobate (PZN-PT) and composition–temperature diagrams along isopleths in the ternary system PMN-PZT are estimated. The anisotropies of polarization in solid solutions based on lead titanate and barium titanate are contrasted. The results provide a thermodynamic framework useful for guiding experimental investigations of ferroelectric solid solutions and for generating energy functions used in constitutive modeling and phase field simulations of microstructure and properties.

Journal ArticleDOI
TL;DR: In this article, a review of the literature on lead-free piezoelectric ceramics is presented, which shows that fatigue under cyclic electrical loading is prevalent in many lead-fertile piezo-lectric ceramic compositions.
Abstract: A considerable body of knowledge now exists from studies involving the development of lead-free piezoelectric ceramics and a number of high potential alternatives to current lead-based materials have been identified. Stability under cyclic electric fields is an important property of piezoelectric materials. Here, we review the research to date which shows that fatigue under cyclic electrical loading is prevalent in many lead-free piezoelectric ceramic compositions. However, the variety of compositions and mechanisms for piezoelectric behavior in these materials corresponds to significant variances in the nature of fatigue degradation and the likely mechanisms thereof, which do not directly parallel those of well-studied lead-based materials. In particular, the use of field-induced phase changes as an actuation mechanism provides distinctive fatigue behaviors. Particular attention is given to fatigue of ferroelectric and relaxor (ergodic and nonergodic) structures and their dependence upon temperature and electric field and the potential design of materials with high fatigue resistance.

Journal ArticleDOI
TL;DR: In this paper, the thermal expansion tensors for monoclinic and tetragonal phases of ZrO2 and HfO2 have been measured in air, by high-resolution, high-temperature X-ray diffraction.
Abstract: The thermal expansion of a low symmetry crystal can be much more interesting than the lattice parameter expansion would suggest. Here, the complete thermal expansion tensors for monoclinic and tetragonal phases of ZrO2 and HfO2 have been measured in air, by high-resolution, high-temperature X-ray diffraction. These results reveal the highly anisotropic nature of thermal expansion in the monoclinic phase as well as a cooperative movement of ions and the existence of a zero thermal expansion plane.

Journal ArticleDOI
TL;DR: In this paper, the authors used the concept of optical basicity (OB) to provide a quantitative chemical basis for the screening of CMAS-resistant TBC compositions, which could also be extended to environmental barrier coatings (EBCs).
Abstract: The higher operating temperatures in gas-turbine engines enabled by thermal barrier coatings (TBCs) engender new materials issues, viz silicate particles (sand, volcanic ash, fly ash) ingested by the engine melt on the hot TBC surfaces and form calcium–magnesium–alumino–silicate (CMAS) glass deposits. The molten CMAS glass degrades TBCs, leading to their premature failure. In this context, we have used the concept of optical basicity (OB) to provide a quantitative chemical basis for the screening of CMAS-resistant TBC compositions, which could also be extended to environmental barrier coatings (EBCs). By applying OB difference considerations to various major TBC compositions and two types of important CMASs—desert sand and fly ash—the 2ZrO2·Y2O3 solid solution (ss) TBC composition, with the potential for high CMAS-resistance, is chosen for this study. Here, we also demonstrate the feasibility of processing of 2ZrO2·Y2O3(ss) air-plasma sprayed (APS) TBC using commercially developed powders. The resulting TBCs with typical APS microstructures are found to be single-phase cubic fluorite, having a thermal conductivity <0.9 W·(m·K)−1 at elevated temperatures. The accompanying Part II paper presents results from experiments and analyses of high-temperature interactions between 2ZrO2·Y2O3(ss) APS TBC and the two types of CMASs.

Journal ArticleDOI
TL;DR: In this article, a bilayer approach was proposed to improve the furnace cycle durability of TBCs by providing dense, high toughness coating at regions prone to delamination failure (near interface), while allowing for the majority of the coating to contain high porosity, resulting in reduced overall modulus.
Abstract: The durability of plasma sprayed thermal barrier coatings (TBCs) has been of significant interest ever since their introduction in gas turbine engine components. Of particular importance is the role of coating processing, microstructure and ensuing properties on their thermal cycle life. Among the coating properties of the ceramic top coat that have shown strong correlations with durability include the elastic modulus (i.e., compliance) and the fracture toughness, both of which are influenced by processing as well as thermal aging during service. In this article, we have systematically investigated furnace cycle durability of plasma sprayed TBCs produced from controlled processing conditions, yielding differences in both modulus and toughness. Following performance assessment and mechanistic insights obtained from single layer ceramic coatings, novel bilayer architectures have been proposed and fabricated, in an effort to improve furnace cycle durability. The bilayer approach targets coating properties based on location, by providing dense, high toughness coating at regions prone to delamination failure (near-interface), while allowing for the majority of the coating to contain high porosity, resulting in reduced overall modulus. Such improved bilayers simultaneously display both high durability and low thermal conductivity enabling a promising approach for functionally optimized coatings. The plasma spray process together with its ability to dynamically change process parameters enables the fabrication of these novel architectures.

Journal ArticleDOI
TL;DR: In this article, a point defect model was applied to describe the lattice strain associated with hydration and its likely impact on ceramic fuel cells/hydrogen separation membranes utilizing a proton-conducting electrolyte.
Abstract: The crystal structures of proton-conducting BaZr1-xYxO3-x/2 (BZY05-BZY20) and BaCe0.8Y0.2O2.9 (BCY20) during hydration/dehydration has been studied by in situ high-temperature X-ray diffraction and thermal analysis. A contraction/expansion of the crystal lattice associated with dehydration/hydration was observed for all materials at elevated temperatures and the polymorphic phase transition temperatures of BaCe0.8Y0.2O2.9 were depressed by lowering the vapor pressure of water. A thermodynamic formalism is introduced to describe the chemical expansion associated with the hydration of oxygen vacancies in acceptor-doped oxides. A conventional point defect model was applied to describe the lattice strain associated with the hydration. The chemical expansion is discussed with respect to the available volumetric data on the hydration of proton-conducting oxide materials and its likely impact on ceramic fuel cells/hydrogen separation membranes utilizing a proton-conducting electrolyte.

Journal ArticleDOI
TL;DR: In this paper, the structural and optical properties of Eu, Tb-, and Dy-doped oxyfluoride silicate glasses for LEDs were studied by means of X-ray diffraction, photoluminescence spectra, Commission Internationale de L'Eclairage (CIE) chromaticity coordinates, and correlated color temperatures (CCTs).
Abstract: Luminescence glass is a potential candidate for the light-emitting diodes (LEDs) applications. Here, we study the structural and optical properties of the Eu-, Tb-, and Dy-doped oxyfluoride silicate glasses for LEDs by means of X-ray diffraction, photoluminescence spectra, Commission Internationale de L’Eclairage (CIE) chromaticity coordinates, and correlated color temperatures (CCTs). The results show that the white light emission can be achieved in Eu/Tb/Dy codoped oxyfluoride silicate glasses under excitation by near-ultraviolet light due to the simultaneous generation of blue, green, yellow, and red-light wavelengths from Tb, Dy, and Eu ions. The optical performances can be tuned by varying the glass composition and excitation wavelength. Furthermore, we observed a remarkable emission spectral change for the Tb 3+ single-doped oxyfluoride silicate glasses. The 5 D3 emission of Tb 3+ can be suppressed by introducing B2O3 into the glass. The conversion of Eu 3+ to Eu 2+ takes place in Eu single-doped oxyfluoride aluminosilicate glasses. The creation of CaF2 crystals enhances the conversion efficiency. In addition, energy transfers from Dy 3+ to Tb 3+ and Tb 3+ to Eu 3+ ions occurred in Eu/Tb/Dy codoped glasses, which can be confirmed by analyzing fluorescence spectra and energy level diagrams.

Journal ArticleDOI
TL;DR: In this paper, a fully dense SiC ceramic with high thermal conductivity was obtained by conventional hot pressing, with 1 ǫvol% Y2O3-Sc 2O3 additives.
Abstract: A fully dense SiC ceramic with high thermal conductivity was obtained by conventional hot pressing, with 1 vol% Y2O3–Sc2O3 additives. The ceramic had a bimodal microstructure consisting of large and small equiaxed SiC grains. Observation with high-resolution transmission electron microscopy (HRTEM) showed two kinds of homophase (SiC/SiC) boundaries, that is crystallized and clean boundaries, and a fully crystallized junction phase. The thermal conductivity of the SiC ceramic was 234 W (m·K)−1 at room temperature. The high thermal conductivity was attributed to a clean SiC lattice and good contiguity between SiC grains.

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TL;DR: In this paper, the phase transition and high-temperature properties of rare-earth niobates (LnNbO4) were studied in situ at high temperatures using powder X-ray diffraction and thermal analysis methods.
Abstract: Phase transition and high-temperature properties of rare-earth niobates (LnNbO4, where Ln = La, Dy and Y) were studied in situ at high temperatures using powder X-ray diffraction and thermal analysis methods. These materials undergo a reversible, pure ferroelastic phase transition from a monoclinic (S.G. I2/a) phase at low temperatures to a tetragonal (S.G. I41/a) phase at high temperatures. While the size of the rare-earth cation is identified as the key parameter, which determines the transition temperature in these materials, it is the niobium cation which defines the mechanism. Based on detailed crystallographic analysis, it was concluded that only distortion of the NbO4 tetrahedra is associated with the ferroelastic transition in the rare-earth niobates, and no change in coordination of Nb5+ cation. The distorted NbO4 tetrahedron, it is proposed, is energetically more stable than a regular tetrahedron (in tetragonal symmetry) due to decrease in the average Nb–O bond distance. The distortion is affected by the movement of Nb5+ cation along the monoclinic b-axis (tetragonal c-axis before transition), and is in opposite directions in alternate layers parallel to the (010). The net effect on transition is a shear parallel to the monoclinic [100] and a contraction along the monoclinic b-axis. In addition, anisotropic thermal expansion properties and specific heat capacity changes accompanying the transition in the studied rare-earth niobate systems are also discussed.