Showing papers in "Journal of the American Ceramic Society in 2015"
TL;DR: Additive manufacturing (AM) is a technology which has the potential not only to change the way of conventional industrial manufacturing processes, adding material instead of subtracting, but also to create entirely new production and business strategies as discussed by the authors.
Abstract: Additive manufacturing (AM) is a technology which has the potential not only to change the way of conventional industrial manufacturing processes, adding material instead of subtracting, but also to create entirely new production and business strategies. Since about three decades, AM technologies have been used to fabricate prototypes or models mostly from polymeric or metallic materials. Recently, products have been introduced into the market that cannot be produced in another way than additively. Ceramic materials are, however, not easy to process by AM technologies, as their processing requirements (in terms of feedstock and/or sintering) are very challenging. On the other hand, it can be expected that AM technologies, once successful, will have an extraordinary impact on the industrial production of ceramic components and, moreover, will open for ceramics new uses and new markets.
691 citations
TL;DR: In this paper, the authors showed that the BMN addition could lower the sintering temperature of BT-based ceramics, and they also revealed a pure perovskite structure for all studied samples.
Abstract: Perovskite solid solution ceramics of (1 − x)BaTiO3–xBi(Mg2/3Nb1/3)O3 (BT–BMN) (x = 0.05–0.2) were synthesized by solid-state reaction technique. The results show that the BMN addition could lower the sintering temperature of BT-based ceramics. X-ray diffraction results reveal a pure perovskite structure for all studied samples. Dielectric measurements exhibit a relaxor-like characteristic for the BT–BMN ceramics, where broadened phase transition peaks change to a temperature-stable permittivity plateau (from −50°C to 300°C) with increasing the BMN content (x = 0.2), and slim polarization–electric field hysteresis loops were observed in samples with x ≥ 0.1. The dielectric breakdown strength and electrical resistivity of BT–BMN ceramics show their maxima of 287.7 kV/cm and 1.53 × 1013 Ω cm at x = 0.15, and an energy density of about 1.13 J/cm3 is achieved in the sample of x = 0.1.
423 citations
TL;DR: In this article, a lead-free relaxor ferroelectric ceramic of (0.67−x)BiFeO3-0.33BT-xBMN, x = 0−0.1] was prepared by a solid-state reaction method.
Abstract: A novel lead-free relaxor ferroelectric ceramic of (0.67−x)BiFeO3–0.33BaTiO3–xBa(Mg1/3Nb2/3)O3 [(0.67−x)BF–0.33BT–xBMN, x = 0–0.1] was prepared by a solid-state reaction method. A relatively high maximum polarization Pmax of 38 μC/cm2 and a low remanent polarization Pr of 5.7 μC/cm2 were attained under 12.5 kV/mm in the x = 0.06 sample, leading to an excellent energy-storage density of W ~1.56 J/cm3 and a moderate energy-storage efficiency of η ~75%. Moreover, a good temperature stability of the energy storage was obtained in the x = 0.06 sample from 25°C to 190°C. The achievement of these characteristics was basically attributed to an electric field induced reversible ergodic to ferroelectric phase transition owing to similar free energies near a critical freezing temperature. The results indicate that the (0.67−x)BF–0.33BT–xBMN lead-free realxor ferroelectric ceramic could be a promising dielectric material for energy-storage capacitors.
234 citations
TL;DR: In this article, a comprehensive review on the oxide electrolytes with high-lithium-ion conductivity, with special emphasis on the aspect of materials selection and design for applications as solid electrolytes in lithium batteries, is presented.
Abstract: As the most promising candidate of the solid electrolyte materials for future lithium batteries, oxide electrolytes with high–lithium-ion conductivity have experienced a rapid development in the past few decades. Existing oxide electrolytes are divided into two groups, i.e., crystalline group including NASICON, perovskite, garnet, and some newly developing structures, and amorphous/glass group including Li2O–MOx (M = Si, B, P, etc.) and LiPON-related materials. After a historical perspective on the general development of oxide electrolytes, we try to give a comprehensive review on the oxide electrolytes with high–lithium-ion conductivity, with special emphasis on the aspect of materials selection and design for applications as solid electrolytes in lithium batteries. Some successful examples and meaningful attempts on the incorporation of oxide electrolytes in lithium batteries are also presented. In the conclusion part, an outlook for the future direction of oxide electrolytes development is given.
210 citations
TL;DR: In this article, a review of the state of the art on single crystal mullite is presented, focusing on the crystal structure of mullite and its properties, such as elasticity, compressibility, strength, toughness, creep and thermal properties.
Abstract: Mullite is certainly one of the most important oxide materials for both conventional and advanced ceramics. Mullite belongs to the compositional series of orthorhombic aluminosilicates with the general composition Al2(Al2+2xSi2-2x)O10-x. Main members are sillimanite (x = 0), stoichiometric 3/2-mullite (x = 0.25), 2/1-mullite (x = 0.40), and the SiO2-free phase ι-alumina (x = 1, crystal structure not known). This study gives an overview on the present state of research regarding single crystal mullite. Following a short introduction, the second part of the review focuses on the crystal structure of mullite. In particular, the characteristic mullite-type structural backbone of parallel chains consisting of edge-sharing MO6 octahedra and their specific cross-linkage by TO4 tetrahedra is explained in detail, the role of cation disorder and structural oxygen vacancies is addressed, and the possibility of cation substitution on different sites is discussed. The third part of the study deals with physical properties being relevant for technical applications of mullite and includes mechanical properties (e.g., elasticity, compressibility, strength, toughness, creep), thermal properties (e.g., thermal expansion, heat capacity, atomic diffusion, thermal conductivity), electrical conductivity, and optical properties. Special emphasis is put on structure–property relationships which allow for interpretation of corresponding experimental data and offer in turn the possibility to tailor new mullite materials with improved properties. Finally, the reported anomalies and discontinuities in the evolution of certain physical properties with temperature are summarized and critically discussed.
185 citations
TL;DR: In this article, the structural development of a calcium (sodium) aluminosilicate hydrate (C-(N-)A-S-H) gel system, obtained through the reaction of sodium metasilicate and ground granulated blast furnace slag, is assessed by high-resolution 29Si and 27Al MAS NMR spectroscopy during the first 2 yr after mixing.
Abstract: The structural development of a calcium (sodium) aluminosilicate hydrate (C-(N-)A-S-H) gel system, obtained through the reaction of sodium metasilicate and ground granulated blast furnace slag, is assessed by high-resolution 29Si and 27Al MAS NMR spectroscopy during the first 2 yr after mixing. The cements formed primarily consist of C-(N-)A-S-H gels, with hydrotalcite and disordered alkali aluminosilicate gels also identified in the solid product assemblages. Deconvolution of the 27Al MAS NMR spectra enables the identification of three distinct tetrahedral Al sites, consistent with the 29Si MAS NMR data, where Q3(1Al), Q4(3Al), and Q4(4Al) silicate sites are identified. These results suggest significant levels of cross-linking in the C-(N-)A-S-H gel and the presence of an additional highly polymerized aluminosilicate product. The mean chain length, extent of cross-linking, and Al/Si ratio of the C-(N-)A-S-H gel decrease slightly over time. The de-cross-linking effect is explained by the key role of Al in mixed cross-linked/non-cross-linked C-(N-)A-S-H gels, because the cross-linked components have much lower Al-binding capacities than the noncross-linked components. These results show that the aluminosilicate chain lengths and chemical compositions of the fundamental structural components in C-(N-)A-S-H gels vary in a way that is not immediately evident from the overall bulk chemistry.
182 citations
TL;DR: In this article, one-part "just add water" geopolymer binders are synthesized through the alkali-thermal activation of the red mud which is relatively rich in both alumina and calcium.
Abstract: In this study, one-part “just add water” geopolymer binders are synthesized through the alkali-thermal activation of the red mud which is relatively rich in both alumina and calcium. Calcination of the red mud with sodium hydroxide pellets at 800°C leads to decomposition of the original silicate and aluminosilicate phases present in the red mud, which promotes the formation of new compounds with hydraulic character, including a partially ordered peralkaline aluminosilicate phase and the calcium-rich phases C3A and α-C2S. The hydration of the “one-part geopolymer” leads to the formation of zeolites and a disordered binder gel as the main reaction products, and the consequent development of compressive strengths of up to 10 MPa after 7 d of curing. These results demonstrate that red mud is an effective precursor to produce one-part geopolymer binders, via thermal and alkali-activation processes.
181 citations
TL;DR: In this paper, the microstructure and electrical properties of PBLZST antiferroelectric (AFE) ceramics have been investigated using hot-press sintering method and conventional solid-state reaction process.
Abstract: (Pb0.87Ba0.1La0.02)(Zr0.68Sn0.24Ti0.08)O3 (PBLZST) antiferroelectric (AFE) ceramics have been prepared by hot-press sintering method and conventional solid-state reaction process, and the dependence of microstructure and energy storage properties of the ceramics on sintering approaches has been studied. The results reveal that not only the microstructure, but also the electrical properties of the PBLZST AFE ceramics are significantly improved by using the hot-press sintering method. Samples resulting from the hot-press sintering process have high breakdown strength of 180 kV/cm which results from the increase of density. Coupled with large polarization, the hot-pressed AFE ceramics are shown to have a high recoverable energy density of 3.2 J/cm3. The recoverable energy density of the hot-pressed PBLZST AFE ceramics is 100% greater than the conventional sintered specimens with recoverable energy density of 1.6 J/cm3.
163 citations
TL;DR: In this article, a novel ZnO photocatalyst was synthesized by a green method using lemon juice and zinc acetate as precursors, and then the effect of sucrose addition on the initial precursor was investigated.
Abstract: A novel ZnO photocatalyst was synthesized by a green method using lemon juice and zinc acetate as precursors, and then the effect of sucrose addition on the initial precursors was investigated. The samples were characterized by field-emission scanning electron microscopy, X-ray diffraction, Fourier transform infrared, UV-visible, and photoluminescence analysis. The result showed that the as-obtained products with the mix of lemon juice (30 mL) and sucrose had a spherical morphology with the mean particle size of about 21.5 nm. The photocatalytic activity of this sample was tested for the degradation of methyl orange, methyl red and methylene blue solutions. The results also revealed the good photocatalytic activity for the degradation of these three organic dyes. Furthermore, as-synthesized sample was used in decolorization processes and the treatment of textile dyes (reactive blue 21).
155 citations
TL;DR: In this paper, an air plasma spray process has been used to apply a model tri-layer Yb2SiO5/Al6Si2O13/Si environmental barrier coating system on SiC test coupons.
Abstract: An air plasma spray process has been used to apply a model tri-layer Yb2SiO5/Al6Si2O13/Si environmental barrier coating system on SiC test coupons. Significant differences in the thermal expansion of the component layers resulted in periodically spaced mud cracks in the Yb2SiO5 and Al6Si2O13 layers. Upon thermal cycling between 1316°C and 110°C in a 90% H2O/10% O2 environment flowing at 4.4 cm/s, it was found that partial delamination occurred with the fracture plane located within a thermally grown oxide (TGO) at the Al6Si2O13–Si interface. Delamination initiated at test coupon edges where the gaseous environment preferentially oxidized the exposed Si bond coat to form β-cristobalite. Simultaneous ingress of the gaseous environment through mud cracks initiated local formation of β-cristobalite (SiO2), the thickness of which was greatest directly below mud cracks. Upon cooling, cristobalite transformed from the β to α phase with a large, constrained volume contraction that resulted in severe microfracture of the TGO. Continued thermal cycling eventually propagated delamination cracks and caused partial spallation of the coatings. Formation of the cristobalite TGO appears to be the delamination life-determining factor in protective coating systems utilizing a Si bond coat.
146 citations
TL;DR: In this article, a class of Yb3+/Er3+ co-doped NaY(MoO4)2 upconversion (UC) phosphors have been successfully synthesized by a facile hydrothermal route with further calcination.
Abstract: A class of Yb3+/Er3+ co-doped NaY(MoO4)2 upconversion (UC) phosphors have been successfully synthesized by a facile hydrothermal route with further calcination. The structural properties and the phase composition of the samples were characterized by X-ray diffraction (XRD). The UC luminescence properties of Yb3+/Er3+ co-doped NaY(MoO4)2 were investigated in detail. Concentration-dependent studies revealed that the optimal composition was realized for a 2% Er3+ and 10% Yb3+-doping concentration. Two-photon excitation UC mechanism further illustrated that the green enhancement arised from a novel energy-transfer (ET) pathway which entailed a strong ground-state absorption of Yb3+ ions and the excited state absorption of Yb3+–MoO42− dimers, followed by an effective energy transfer to the high-energy state of Er3+ ions. We have also studied the thermal properties of UC emissions between 303 and 523 K for the optical thermometry behavior under a 980 nm laser diode excitation for the first time. The higher sensitivity for temperature measurement could be obtained compared to the previous reported rare-earth ions fluorescence based optical temperature sensors. These results indicated that the present sample was a promising candidate for optical temperature sensors with high sensitivity.
TL;DR: In this paper, a lead-free piezoceramics was fabricated using a conventional solid-state reaction method, which exhibited an electric-field-induced irreversible structural transition from pseudocubic to ferroelectric rhombohedral phase at room temperature.
Abstract: Ternary solid solutions of (1 − x)(0.8Bi0.5Na0.5TiO3–0.2Bi0.5K0.5TiO3)– xNaNbO3 (BNKT–xNN) lead-free piezoceramics were fabricated using a conventional solid-state reaction method. Pure BNKT composition exhibited an electric-field-induced irreversible structural transition from pseudocubic to ferroelectric rhombohedral phase at room temperature. Accompanied with the ferroelectric-to-relaxor temperature TF-R shifted down below room temperature as the substitution of NN, a compositionally induced nonergodic-to-ergodic relaxor transition was presented, which featured the pinched-shape polarization and sprout-shape strain hysteresis loops. A strain value of ~0.445% (under a driving field of 55 kV/cm) with large normalized strain of ~810 pm/V was obtained for the composition of BNKT–0.04NN, and the large strain was attributed to the reversible electric-field-induced transition between ergodic relaxor and ferroelectric phase.
TL;DR: In this paper, the aqueous chemical coating method was utilized to modify the BaTiO3 particles, and the fine-crystalline BaNiO3 ceramics with an average grain size below 200nm without abnormal grain growth by co-coating Al2O3 and SiO2 was prepared.
Abstract: The multilayer structure of capacitor demands for fine grain size of dielectric ceramics in devices, because the thinner layer which needs ceramics with fine grain size is helpful in enlarging the capacitance. In this paper, the aqueous chemical coating method was utilized to modify the BaTiO3 particles. The fine-crystalline BaTiO3 ceramics with an average grain size below 200 nm without abnormal grain growth by co-coating Al2O3 and SiO2 has been prepared. The phase composition, microstructures of coated particles and ceramics, and dielectric properties were investigated. For samples containing 3 wt% of Al2O3 and 1 wt% of SiO2, the energy storage density is 0.725 J/cm3 and the efficiency of the ceramic samples can keep above 80%. The breakdown strength was improved to about 190 kV/cm.
TL;DR: In this article, the authors introduce the recently developed principle of microstructural evolution to explain the basis of the microstructure control in polycrystals for solid-state conversion, and report recent technical developments in fabricating single crystals by the solidstate single crystal growth (SSCG) method and their physical properties.
Abstract: Solid-state conversion of single crystals from polycrystalline materials has the advantages of cost-effectiveness, chemical homogeneity, and versatility over the conventional melt growth and solution growth methods, particularly for systems with high melting points, incongruent melting, high reactivity (volatility), and phase transformations at high temperature. Nevertheless, for commercial production, this technique has only been successful in a few limited systems, in particular ferroelectric systems. This is mostly because of the difficulty in controlling the microstructure, particularly suppressing grain growth in the polycrystal during its conversion. This article describes the principle and the current status of the solid-state conversion of single crystals. We first introduce the recently developed principle of microstructural evolution to explain the basis of the microstructure control in polycrystals for solid-state conversion. We then report recent technical developments in fabricating single crystals by the solid-state single crystal growth (SSCG) method and their physical properties. The SSCG method is expected to be studied and utilized more widely in fabricating single crystals with complex compositions as a strong alternative to the melt growth and solution growth methods.
TL;DR: In this paper, a 10-layered stack-type multilayer actuator was fabricated from the optimal BNKT-ST lattice and showed a large Smax/Emax of 600pm/V at a relatively low driving field of 4.5
Abstract: Lead-free piezoelectric ceramics, 0.96[{Bi0.5 (Na0.84K0.16)0.5}1−xLix(Ti1−yNby)O3]–0.04SrTiO3 (BNKLiTN–ST) with x, y = 0–0.030, were synthesized by solid-state reaction method. X-ray diffraction patterns indicated that Li and Nb successfully diffused into the BNKT–ST lattice and formed a pure perovskite structure with x, y ≤ 0.025. Increasing the Li and Nb contents (x, y = 0.020) induced a phase transformation from the coexistent rhombohedral–tetragonal phases for pure BNKT–ST ceramics to a pseudocubic phase, resulting in degradation of the remnant polarization and coercive field. However, the field-induced strain was markedly enhanced at x, y = 0.020, giving rise to a giant dynamic piezoelectric constant (d33* = Smax/Emax = 800 pm/V). Furthermore, the temperature dependence of the field-induced strain response showed temperature-insensitivity up to 120°C. To explore its potential for device applications, a 10-layered stack-type multilayer actuator was fabricated from the optimal composition (x, y = 0.020). This actuator showed a large Smax/Emax of 600 pm/V at a relatively low driving field of 4.5 kV/mm suggesting highly promising results in lead-free BNT-based ceramics.
TL;DR: In this paper, the temperature dependence of piezoelectric properties (direct and converse) of lead-free piezoceramics have been compared, and it is found that the electromechanical coupling coefficient has a better temperature stability than the piezolectric coefficient in the studied system due to a large temperature-dependent compliance change.
Abstract: The temperature dependence of piezoelectric properties (direct piezoelectric coefficient d33, converse piezoelectric coefficient d33(E = 0), strain S and electromechanical coupling coefficient kp) for two niobate-based lead-free piezoceramics have been contrasted. 0.92(Na0.5K0.5)NbO3–0.02(Bi1/2Li1/2)TiO3–0.06BaZrO3 (6BZ/2BLT/92NKN) has a morphotropic phase boundary (MPB) between rhombohedral and tetragonal at room temperature and 0.92(Na0.5K0.5)NbO3–0.03(Bi1/2Li1/2)TiO3–0.05BaZrO3 (5BZ/3BLT/92NKN) features an MPB engineered to be located below room temperature. At 30°C, d33, d33(E = 0), S (at 2 kV/mm), and kp are 252 pC/N, 230 pm/V, 0.069%, 0.51 for 5BZ/3BLT/92NKN; and 348 pC/N, 380 pm/V, 0.106%, 0.57 for 6BZ/2BLT/92NKN, respectively. With increasing temperature, the piezoelectric properties decrease. At 200°C, d33, d33(E = 0), S (at 2 kV/mm), and kp are 170 pC/N, 160 pm/V, 0.059%, 0.36 for 5BZ/3BLT/92NKN; and 181 pC/N, 190 pm/V, 0.061%, 0.39 for 6BZ/2BLT/92NKN. It is found that the electromechanical coupling coefficient has a better temperature stability than the piezoelectric coefficient in the studied system due to a large temperature-dependent compliance change. The results demonstrate that engineering an MPB is highly effective in tailoring temperature stability of piezoceramics.
TL;DR: In this article, a model of a core-shell functional ceramics was introduced to discuss structure-property relationships and the evolution of the core and shell with field and temperature.
Abstract: The design of core–shell materials affords additional degrees of freedom to tailor functional properties as compared to solid solution counterparts. Although to date most of the work in core–shell materials has focused on dielectrics, piezoelectric core–shell ceramics may gain similar interest. Generalities of core–shell functional ceramics features are addressed in this work. A model system, Bi1/2Na1/2TiO3–SrTiO3, is introduced to discuss structure–property relationships. We demonstrate that this system features a core–shell microstructure for the composition corresponding to 25 at.% Sr. The material is studied by means of macroscopic functional properties and in situ structural characterization techniques at different length scales, such as X-ray diffraction, transmission electron microscopy, and Raman spectroscopy. The evolution of the core–shell with field and temperature determines its functional properties. The high strain of the system, ~0.3% at 4 kV/mm, is due to an electric-field-induced phase transition of the core and shell. Upon field removal the core remains in a poled state, whereas the shell is characterized by a reversible transformation. The reversibility of the phase transition of shells and associated switching are key features in the observed giant strain. Dielectric anomalies are found to be related to changes in oxygen octahedral tilting angles within the core and shell.
TL;DR: In this article, the effects of aging time and temperature on the structure and in vitro bioactivity of gel-derived 45S5 glass-ceramic and amorphous45S5 Bioglass® were investigated.
Abstract: Porous bioactive glass-ceramics based on the 45S5 Bioglass® composition were fabricated by an acid-catalyzed sol–gel method. The effects of aging time and temperature on the structure and in vitro bioactivity were investigated. Fourier-transform infrared spectroscopy (FTIR) was carried out on the samples to understand the structure and to monitor the formation of hydroxyapatite (HA) after immersion in simulated body fluid (SBF). The bioactivity of gel-derived 45S5 glass-ceramic and amorphous 45S5 Bioglass® was compared. The results showed that an increase in both aging time and temperature can enhance crystallization, whereas bioactivity is reduced with increasing aging time but not significantly influenced by aging temperature. Compared with amorphous 45S5 Bioglass®, gel-derived glass-ceramic aged for 3 d at 60°C exhibited a more rapid rate of HA formation after immersion for less than 7 d. Amorphous 45S5 Bioglass® showed higher HA formation rate after immersion in SBF for more than 7 d, whereas the quantity of formed HA on gel-derived 45S5 glass-ceramic was still comparable to that of amorphous 45S5 Bioglass® after immersion for 14 d. It is suggested that the lower bioactivity of 45S5 glass-ceramics could be outweighed by the higher surface area and higher content of Si–NBO groups in gel-derived glass-ceramics. The results thus confirm that gel-derived 45S5 glass-ceramic exhibiting bioactivity comparable to that of amorphous 45S5 Bioglass® can be fabricated by sol–gel method under suitable aging conditions.
TL;DR: In this paper, the effect of hot-pressing temperature on the microstructure and Li-ion transport of Al-doped, cubic Li7La3Zr2O12 (LLZO) was investigated.
Abstract: The effect of hot-pressing temperature on the microstructure and Li-ion transport of Al-doped, cubic Li7La3Zr2O12 (LLZO) was investigated. At fixed pressure (62 MPa), the relative density was 86%, 97%, and 99% when hot-pressing at 900°C, 1000°C, and 1100°C, respectively. Electrochemical impedance spectroscopy showed that the percent grain-boundary resistance decreased with increasing hot-pressing temperature. Hot pressing at 1100°C resulted in a total conductivity of 0.37 mS/cm at room temperature where the grain boundaries contributed to 8% of the total resistance; one of the lowest grain-boundary resistances reported. We believe hot pressing is an appealing technique to minimize grain-boundary resistance and enable correlations between LLZO composition and bulk ionic conductivity.
TL;DR: In this article, the influence of processing on GZO microstructure and lifetime of double-layer TBCs was investigated using atmospheric plasma spraying (APS) experiments and particle diagnostics were utilized for the optimization of the process parameters with respect to different microstructures and stoichiometry.
Abstract: During the past decade, gadolinium zirconate (Gd2Zr2O7, GZO) has attracted interest as an alternative material to partially yttria-stabilized zirconia (YSZ) for thermal barrier coatings (TBCs). Despite the well-known benefits of GZO, such as lower thermal conductivity and superior temperature capability compared to YSZ, processing of GZO via atmospheric plasma spraying (APS) still remains a challenge. Here, we report on APS experiments which were performed to investigate the influence of processing on GZO microstructure and lifetime of GZO/YSZ double-layer TBCs. Different microstructures of GZO were produced and characterized in terms of porosity, stoichiometry, Young′s modulus, and their effects on the lifetime of YSZ/GZO double-layer TBCs were discussed. Particle diagnostics were utilized for the optimization of the process parameters with respect to different microstructures of GZO and stoichiometry. It was found that both cumulative porosity of GZO and pore size distribution, which alter the Young′s modulus significantly, govern the lifetime of double layers. In addition, it was shown that the deviation in GZO stoichiometry due to gadolinia evaporation in the investigated range does not display any critical effect on lifetime.
TL;DR: In this paper, a series of ReVO4 (Re, La, Ce) microwave dielectric ceramics fabricated by a standard solid-state reaction method are reported, where phase purity, sintering behavior, and microstructure are explored.
Abstract: We report a series of ReVO4 (Re = La, Ce) microwave dielectric ceramics fabricated by a standard solid-state reaction method. X-ray diffraction and scanning electron microscopy measurements were performed to explore the phase purity, sintering behavior, and microstructure. The analysis revealed that pure and dense monoclinic LaVO4 ceramics with a monazite structure and tetragonal CeVO4 ceramics with a zircon structure could be obtained in their respective sintering temperature range. Furthermore, LaVO4 and CeVO4 ceramics sintered at 850°C and 950°C for 4 h possessed out-bound microwave dielectric properties: er = 14.2, Q × f = 48197 GHz, τf = −37.9 ppm/°C, and er = 12.3, Q × f = 41 460 GHz, τf = −34.4 ppm/°C, respectively. The overall results suggest that the ReVO4 ceramics could be promising materials for low-temperature-cofired ceramic technology.
TL;DR: In this paper, the dielectric properties of high-purity (4N degree) rutile TiO2 ceramics were investigated over a wide temperature (100 −1073 K) and frequency (20 −10 −MHz) ranges.
Abstract: Dielectric properties of high-purity (4N degree) rutile TiO2 ceramics were investigated over a wide temperature (100–1073 K) and frequency (20 Hz–10 MHz) ranges. X-ray photoemission spectroscopy measurement revealed the sample possesses mixed-valent states of Ti3+/Ti4+. Four thermally activated relaxations were observed. The lowest temperature relaxation (R1) features two Arrhenius segments with activation energy of 30 and 80 meV for the low- and high-temperature segments, respectively. This relaxation was argued to be a polaron relaxation due to electrons hopping between Ti3+ and Ti4+ ions. The second relaxation (R2) appears around room temperature showing activation energy of 0.68 eV is believed to be a Maxwell-Wagner relaxation. The high-temperature relaxations R3 and R4 with activation energy of 0.84 and 1.26 eV were ascribed to the conduction process due to the hopping motions of singly and doubly charged oxygen vacancies, respectively.
TL;DR: In this paper, multilayer ytterbium-hafnate/silicate coatings deposited by directed vapor deposition and designed to protect SiC-based ceramic matrix composites were assessed to determine their thermochemical stability and resistance to attack by molten silicate deposits.
Abstract: Multilayer ytterbium-hafnate/silicate coatings deposited by directed vapor deposition and designed to protect SiC-based ceramic matrix composites were assessed to determine their thermochemical stability and resistance to attack by molten silicate deposits (CMAS). The study revealed that reactions occurring at the interface between Yb2Si2O7 and Yb4Hf3O12 layers promote coating delamination following isothermal annealing for 100 h/1500°C while coating architectures involving Yb2SiO5 in contact with Yb4Hf3O12 do not experience similar degradation. The outer Yb4Hf3O12 layers, segmented for compliance, were only moderately effective in mitigating CMAS infiltration at 1300°C and 1500°C. The results indicate that the reaction between the melt and coating forms large volumes of a silicate garnet phase at 1300°C, or a cuspidine-type aluminosilicate at 1500°C, in addition to the apatite and reprecipitated fluorite phases observed in related systems.
TL;DR: In this article, the Eu2+-doped α form Ca3(PO4)2 (α-TCP:Eu2+) was used as a new kind of bright cyan-emitting phosphor.
Abstract: A case of phosphor is reported where the cooling rate parameter significantly influences the luminescence property. By quenching the sample after the high-temperature solid-state reaction at 1250°C, we successfully prepared the Eu2+-doped α form Ca3(PO4)2 (α-TCP:Eu2+) as a new kind of bright cyan-emitting phosphor. The unusual emission color variation (from cyan to blue) depends on the cooling rate after sintering and Eu2+ doping level as it was observed in the TCP-based phosphors. By the Rietveld analysis, it is revealed that the cyan- and blue-emitting phosphors are two different TCP forms crystallizing in the monoclinic (space group P21/a, α-TCP) and the rhombohedral structure (space group R3c, β-TCP), respectively. Upon 365 nm UV light excitation, α-TCP:Eu2+ exhibits an asymmetric broad-band cyan emission peaking at 480 nm, while β-TCP:Eu2+ displays a relatively narrow-band blue emission peaking at 416 nm. The Eu2+-doping in Ca3(PO4)2 shifts the upper temperature limit of the stable structural range of β form from 1125°C to ≥1250°C. Moreover, the crystal structures of α/β-TCP:Eu2+ were compared in the aspects of compactness and cation site sets. The emission thermal stability of α/β-TCP:Eu2+ was comparatively characterized and the difference was related to the specific host structural features.
TL;DR: In this article, several multilayer architectures, based on the YSZ-GDZ system, have been developed and tested for durability under furnace thermal cycling conditions, and the results point to new strategies in the design and manufacturing of optimal multi-layer coatings.
Abstract: Thermal barrier coatings (TBCs) are increasingly playing a vital role in enhancing efficiency and performance of gas turbine engines. As engine operating temperatures rise, yttria-stabilized zirconia (YSZ), the currently principal TBC material, reaches its operational limits. Gadolinium Zirconate (GDZ)-based pyrochlore oxides are now emerging contenders, not only due to their lower thermal conductivity, but also their ability to resist attack by silicate deposits. However, GDZ cannot be directly substituted for YSZ due to its incompatibility with the thermally grown alumina layer, therefore requiring to be a component of multilayer system. Although industry has already adopted these materials in various applications, a number of fundamental issues arise with respect to layered-coating design, their properties and processing dependence. In this study several multilayer architectures, based on the YSZ–GDZ system, have been developed and tested for durability under furnace thermal cycling conditions. Coating designs considered optimization of microstructure and properties of individual layers based on their location within the top-coat thickness to address competing interests of thermal conductivity, compliance, and resistance to silicates. A large variance in durability was observed in coatings made with different multilayer designs. The results and the associated failure mechanisms are rationalized through preliminary evaluation of elastic energies at the failure locations and corresponding energy release rates. The results point to new strategies in the design and manufacturing of optimal multilayer coatings.
TL;DR: In this article, the crystal structure, microstructure, and microwave dielectric properties of forsterite-based (Mg1-xNix)2SiO4 (x = 0.02-0.20) ceramics were systematically investigated.
Abstract: The crystal structure, microstructure, and microwave dielectric properties of forsterite-based (Mg1–xNix)2SiO4 (x = 0.02–0.20) ceramics were systematically investigated. All samples present a single forsterite phase of an orthorhombic structure with a space group Pbnm except for a little MgSiO3 secondary phase as x > 0.08. Lattice parameters in all axes decrease linearly with increasing Ni content due to the smaller ionic radius of Ni2+ compared to Mg2+. The substitution of an appropriate amount of Ni2+ could greatly improve the sintering behavior and produce a uniform and closely packed microstructure of the Mg2SiO4 ceramics such that a superior Q × f value (152 300 GHz) can be achieved as x = 0.05. The τf value was found to increase with increasing A-site ionic bond valences. In addition, various additives were used as sintering aids to lower the sintering temperature from 1500°C to the middle sintering temperature range. Excellent microwave dielectric properties of er~6.9, Q × f~99800 GHz and τf~−50 ppm/°C can be obtained for 12 wt% Li2CO3-V2O5-doped (Mg0.95Ni0.05)2SiO4 ceramics sintered at 1150°C for 4 h.
TL;DR: In this paper, the authors examined the expansion and hydration characteristics of various ordinary portland cement (OPC)-CSA blends and found that the increase in CSA cement content increased the extent of expansion.
Abstract: Calcium sulfoaluminate (CSA) cements have lower carbon footprint than that of portland cement, which makes them a suitable alternative as a sustainable cementitious binder. Early-age expansion of CSA cements can be exploited to induce compressive stress in restrained concrete which can later counteract tensile stress developed during drying shrinkage, thus enhancing the resistance against shrinkage cracking. However, a proper understanding of the expansion behavior is critical to eliminate any risk related to expansion-induced cracking. This study examines the expansion and hydration characteristics of various ordinary portland cement (OPC)-CSA blends. Early-age expansion of paste samples was monitored. The increase in CSA cement content increased the extent of expansion. Samples having the highest CSA content (30% by mass) exhibited excessive expansion which led to their cracking. Quantitative X-ray diffraction, pore solution extraction, porosity, tensile strength, and dynamic modulus tests were performed to monitor the physico-chemical changes in OPC-CSA blends. It was shown that the ettringite supersaturation in the investigated systems gave rise to the crystallization stress, responsible for the expansion. Thermodynamic models enabled a reasonable prediction of tensile failure, particularly in the blends with the higher CSA content.
TL;DR: In this article, a linear ln(E2/Ton4) versus Ton−1 relation was predicted for flash sintering under a constant heating rate, which determined the activation energy and the relative ranking of the electrical conductivity of the tested materials.
Abstract: When the sample temperature is warm enough so that the Joule heating takes over the environment's radiation heating as the dominant heating means, thermal runaway follows and flash sintering is triggered. This condition accurately predicts the reported onset temperature Ton of all the constant-field (E) experiments on flash sintering, performed under a constant heating rate. The predicted linear ln(E2/Ton4) versus Ton−1 relation determines the activation energy and the relative ranking of the electrical conductivity of the tested materials.
TL;DR: In this article, the microstructure, phase structure, ferroelectric, and dielectric properties of (1−x)Bi0.5Na 0.5TiO3-xNaNbO3]-Ceramics were investigated as a candidate for capacitor dielectrics with wide temperature stability.
Abstract: The microstructure, phase structure, ferroelectric, and dielectric properties of (1−x)Bi0.5Na0.5TiO3-xNaNbO3 [(1−x)BNT-xNN] ceramics conventionally sintered in the temperature range of 1080°C–1120°C were investigated as a candidate for capacitor dielectrics with wide temperature stability. Perovskite phase with no secondary impurity was observed by XRD measurement. With increasing NN content, (1−x)BNT-xNN was found to gradually transform from ferroelectric (x = 0–0.05) to relaxor (x = 0.10–0.20) and then to paraelectric state (x = 0.25–0.35) at room temperature, indicated by P–I–E loops analysis, associated with greatly enhanced dielectric temperature stability. For the samples with x = 0.25–0.35, the temperature coefficient of capacitance (TCC) was found <11% in an ultra-wide temperature range of −60°C–400°C with moderate dielectric constant and low dielectric loss, promising for temperature stable capacitor applications.
TL;DR: In this article, annealing was employed to fabricate dense (Ba0.4Sr0.6)TiO3 (BST) ceramics with high dielectric loss at room temperature, induced by the formation of oxygen defects during the MWS under vacuum atmosphere (−60kPa), the as-sintered samples were thermally annealed in air to reduce tanδ and recover the insulating performance.
Abstract: Microwave-sintering (MWS) technique was employed to fabricate dense (Ba0.4Sr0.6)TiO3 (BST) ceramics. With respect to the high dielectric loss at room temperature, induced by the formation of oxygen defects during the MWS under vacuum atmosphere (−60 kPa), the as-sintered samples were thermally annealed in air to reduce tanδ and recover the insulating performance. Accompanied by the decreased tanδ, the energy storage properties for annealed MWS BST were optimized, with increasing energy density (γ) from 0.77 to 1.15 J/cm3 and energy efficiency (η) from 60% to 82%. The lower oxygen vacancy concentrations were believed to account for the enhanced insulating characteristics of grain boundaries and contribute to the improved properties after annealing. Electrical characterization of grain and grain boundary by impedance spectroscopy demonstrated that the annealing preferentially modified the grain boundary. In addition, resistances extracted from the high temperature impedance analysis were found to be inadequate for evaluating the electrical characteristics of materials affected by extrinsic mechanisms, such as the interfacial polarization. For comparison, annealing effect on energy storage properties were also discussed for conventionally sintered BST.