Showing papers in "Journal of the American Ceramic Society in 2012"

Lead-Free Relaxor Ferroelectrics

Abstract: Feature size is a natural determinant of material properties. Its design offers the technological perspectives for material improvement. Grain size, crystallite size, domain width, and structural defects of different nature constitute the classical design elements. Common ferroelectric ceramics contain micrometer grain sizes and submicrometer domain widths. Domain wall mobility is a major contribution to their macroscopic material properties providing approximately half of the macroscopic output in optimized materials. The extension into the dynamic nanoworld is provided by relaxor ferroelectrics. Ionic and nanoscale field disorders form the base to a state with natural nanometer-size polar structures even in bulk materials. These polar structures are highly mobile and can dynamically change over several orders of magnitude in time and space being extremely sensitive to external stimuli. This yields among the largest dielectric and piezoelectric constants known. In this feature article, we want to outline how lead-free relaxors will offer a route to an environmentally safer option in this outstanding material class. Properties of uniaxial, planar, and volumetric relaxor compositions will be discussed. They provide a broader and more interesting scope of physical properties and features than the classical lead-containing relaxor compositions.

Correlation Between the Microstructure and Electrical Properties in High‐Performance (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 Lead‐Free Piezoelectric Ceramics

Abstract: Using three different sintering methods: spark plasma sintering, two-step sintering, and normal sintering (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3 (BCZT) lead-free piezoelectric ceramics with grain sizes in the range of 0.4–32.2 μm were prepared. The effects of grain size on the electrical properties and temperature stability of BCZT ceramics were systematically investigated. Results showed that reducing grain size shifted both the Tc and TT-R to higher temperatures, and tended to enhance the relaxor behavior. A strong dependence of piezoelectric properties on the grain size was observed, and ~10 μm was a critical point for fabricating high-performance BCZT ceramics. For samples with grain sizes >10 μm, excellent piezoelectric properties of kp > 0.48, kt > 0.46, d33 > 470 pC/N and d33* > 950 pm/V were obtained. Moreover, no evident relationship between the grain size and temperature stability existed in this material, and all samples exhibited thermal instability below the Curie temperature. However, increasing grain size was helpful for improving the resistance to thermal depoling. The depolarization was assisted by internal mechanical stresses and the movement of 180° and 90° domain walls, which explained the increased resistance to thermal depoling in coarse-grained samples.

Temperature-Dependent Properties of (Bi1/2Na1/2)TiO3–(Bi1/2K1/2)TiO3–SrTiO3 Lead-Free Piezoceramics

Abstract: Ferroelectric and piezoelectric properties of SrTiO3–modified (0, 3, and 5 mol%) 0.8Bi1/2Na1/2TiO3–0.2Bi1/2K1/2TiO3 lead-free piezoceramics were investigated as a model system in an attempt to lay a guideline for developing lead-free piezoelectric materials with large strains. Two guidelines, one for the choice of base composition and the other for the choice of chemical modifiers, were assumed from our current understanding of the mechanism involved. Dielectric permittivity of both poled and unpoled samples was measured and compared, leading to a conclusion that the frequency-independent anomaly (TF-R) is the temperature at which induced-ferroelectric order converts back to relaxor state. The correlation between TF-R and depolarization temperature (Td) was shown by the comparison with Td determined by thermally stimulated depolarization current measurements, whereas the ferroelectric-relaxor transition temperature TF-R was determined using poled samples. A large unipolar strain of 0.36% (Smax/Emax = 600 pm/V) at a driving field of 6 kV/mm was obtained at room temperature for a SrTiO3 content of 5 mol%. Temperature-dependent measurements of both polarization and strain from room temperature to 200°C revealed that the origin of the large strain is due to a reversible field-induced ergodic relaxor-to-ferroelectric phase transformation.

Large Strain Response in the Ternary Bi0.5Na0.5TiO3–BaTiO3–SrTiO3 Solid Solutions

Abstract: A ternary solid solution (0.935-x)Bi0.5Na0.5TiO3–0.065BaTiO3–xSrTiO3 was designed and fabricated using a conventional fabrication process. Temperature and composition dependence of the ferroelectric, dielectric, and piezoelectric properties were systematically investigated and a schematic phase diagram was established. The SrTiO3 substitution was found to induce a transition from ferroelectric to relaxor pseudocubic phases. Around a critical composition x of 0.22, large strain response of ~0.2% (under a moderate field of 4 kV/mm) with normalized strain of 490 pm/V was obtained. The large unipolar strain response would be of great interest for environmental-friendly “on-off” actuators.

Ambient Temperature Drying Shrinkage and Cracking in Metakaolin-Based Geopolymers

Abstract: Ambient temperature drying shrinkage in metakaolin-based geopolymer pastes exposed to low relative humidity environments has been investigated. The effect of varying the geopolymer composition (water content, Si:Al ratio, Na:Al ratio, and Na + or K + cations) on the sensitivity to ambient temperature drying shrinkage is reported. The definition of “structural” water as being the minimum water content required that prevents contractions in the gel structure, and thus drying shrinkage from occurring, is introduced. From the results presented, it is clear that the ionic charge density of cations, the total quantity of cations, and the relative quantities and stabilities of cation: AlO4 � pairs in the paste are major factors affecting the sensitivity of pastes to ambient temperature drying shrinkage.

Thermal Activation of Albite for the Synthesis of One‐Part Mix Geopolymers

Abstract: Precursors for the preparation of one-part geopolymers are synthesized by thermal activation of albite with sodium hydroxide and sodium carbonate, then cooling and crushing the resulting product. Albite is stable under thermal treatment up to 1000°C, but is able to be converted to depolymerized, disordered, and X-ray amorphous geopolymer precursors in the presence of sodium hydroxide or sodium carbonate at elevated temperatures. The geopolymer precursors react with the addition of water (i.e., form a “one part geopolymer mix”), forming geopolymers with acceptable compressive strength. One-part geopolymers synthesized via thermal activation of albite with NaOH show a higher compressive strength than those produced with Na2CO3 at the same dosage. Some crystalline sodium-aluminosilicate hydrates (zeolites) are also formed in addition to geopolymer gel in the geopolymers synthesized from albite activated by NaOH, compared to predominantly amorphous phases in the samples activated by Na2CO3. The activation of natural aluminosilicates including albite by thermal treatment with alkalis has great potential in the development of novel one-part mix geopolymers.

PZT-Based Piezoelectric MEMS Technology

Abstract: This review article presents recent advancements in the design and fabrication of thin-film (<3 μm) lead zirconate titanate (PZT) microelectromechanical system (MEMS) devices. The article covers techniques for optimizing highly (001)/(100) oriented chemical solution deposited PZT films to achieve improved piezoelectric coefficients. These PZT films combined with surface and bulk micromachining techniques are fabricated into actuators and transducers for radio frequency (RF) switches, nanomechanical logic, resonators, and power transformers for use in communication systems and phased-array radar. In addition, the large relative displacements generated by PZT thin films have been used to demonstrate mechanical mobility in MEMS devices, including insect-inspired flight actuators and ultrasonic traveling wave motors. In conjunction with actuation, PZT films are being developed for feedback sensors for the integrated control of insect-inspired robots.

Defect Structure of Flash‐Sintered Strontium Titanate

Abstract: Flash sintering of strontium titanate (SrTiO3) is studied at different applied fields to understand its effect on density and grain growth. In particular, the defect structure is investigated by optical and structural analysis. SrTiO3 exhibited a trend in densification opposite that of ionically or electronically conductive ceramics: as the applied voltage decreased, the density increased. Abnormal grain growth in conventionally sintered SrTiO3 is arrested by flash sintering. Interestingly, undoped SrTiO3 behaved differently than undoped Al2O3, which did not exhibit any signs of flash sintering. Previous attempts at flash sintering could only be achieved in MgO-doped Al2O3. We believe that non-stoichiometric Ruddlesden-Popper phases in SrTiO3, as indicated by ultrafast optical spectroscopy, X-ray diffraction, conductivity measurements, and transmission electron microscopy, assist flash sintering by increasing local conductivity through enhanced defect content.

Characterizing Three‐Dimensional Textile Ceramic Composites Using Synchrotron X‐Ray Micro‐Computed‐Tomography

Abstract: Three-dimensional (3-D) images of two ceramic-matrix textile composites were captured by X-ray micron-resolution computed tomography (lCT) on a synchrotron beamline. Compared to optical images of sections, CT data reveal comprehensive geometrical information about the fiber tows; information at smaller scales, on matrix voids, individual fibers, and fiber coatings, can also be extracted but image artifacts can compromise interpretation. A statistical analysis of the shape and positioning of the fiber tows in the 3-D woven architecture is performed, based on a decomposition of the spatial variations of any geometrical characteristic of the tows into non-stochastic periodic trends and non-periodic stochastic deviations. The periodic trends are compiled by exploiting the nominal translational invariance of the textile, a process that maximizes the information content of the relatively small specimens that can be imaged at high resolution. The stochastic deviations (or geometrical defects in the textile) are summarized in terms of the standard deviation of any characteristic at a single point along the axis of a tow and correlations between the values of deviations at two different points on the same or different tows. The tow characteristics analyzed consist of the coordinates of the centroids of a tow, together with the area, aspect ratio, and orientation of its cross-section. The tabulated statistics are sufficient to calibrate a probabilistic generator (detailed elsewhere) that can create virtual specimens of any size that are individually distinct but share the statistical characteristics of the small specimens analyzed by X-ray lCT. The data analysis presented herein forms the first step in formulating a virtual test of textile composites, by providing the statistical information required for realistic description of the textile reinforcement.

Enhancement of Photoluminescence and Color Purity of CaTiO3:Eu Phosphor by Li Doping

Abstract: High color purity red phosphors of Ca1−3/2xEuxTiO3 and Ca1−2xEuxLixTiO3 (0 < x ≤ 0.3) are synthesized via a solid-state reaction method. The red emission photoluminescence intensity and color purity are enhanced by the incorporation of Li+ into CaTiO3:Eu3+. The Li+ doping increases the emission probability from 5D0 state, increases photoluminescence intensity by 1.6 times, increases color purity to 92.1%, and shortens the decay time. With increasing Eu3+ and Li+ content, the color coordinates approach the ideal red chromaticity values, coming closer than commercial Y2O2S:Eu3+ red phosphor.

Fundamental Aspects of Spark Plasma Sintering: II. Finite Element Analysis of Scalability

Abstract: A comprehensive three-dimensional fully coupled thermo-electro-mechanical finite element framework is developed for modeling spark plasma sintering (SPS). The finite element model is applied to the simulation of spark plasma processing with four different tooling sizes and various temperature regimes. The comparison of modeling and experimental results shows that the model is reliable for qualitative predictions of the densification behavior and of the grain growth in powder specimens subjected to SPS with a given temperature regime. The conducted modeling indicates the possibility of changing the heating pattern of the specimen (warmer central areas of the specimen's volume and cooler outside areas or vice versa) depending on the size of the tooling. High heating rates and large specimen sizes elevate the temperature and, in turn, material structure gradients during SPS processing. The obtained results suggest that the industrial implementation of SPS techniques should be based on the predictive capability of reliable modeling approaches.

Effects of Water Content and Chemical Composition on Structural Properties of Alkaline Activated Metakaolin‐Based Geopolymers

Abstract: In this study, we evaluate the effects of chemistry and curing and aging conditions on water loss kinetics, porosity, and the structure of geopolymers. Geopolymer samples were prepared by mixing metakaolin precursor with K or Na silicate solutions having SiO2/A2O3 molar ratios from 2.5 to 4 and H2O/ (SiO2 + Al2O3) molar ratios from 2 to 4. The samples were cured in sealed and unsealed molds at 60°C for 24 h and then aged at room temperature in open containers. The weight changes were monitored during curing and aging until a steady-state weight was observed. This study shows conclusively that the amount of water [H2O/(SiO2 + Al2O3)] in the initial geopolymer mixture is the most dominant factor affecting density and open porosity of geopolymers after curing and extended aging. The results also indicate that regardless of the amount of water in the initial mixture and curing conditions (i.e., in sealed or unsealed molds), ~6–10 wt% of all K-activated samples after aging for 21 days is water whereas in the case of all Na-activated samples that amount ranges between ~15 and 20 wt%. Furthermore, the results demonstrate that SiO2/Al2O3 molar ratio does not have a direct effect on density and open porosity of geopolymers.

Strontium-Substituted Calcium Deficient Hydroxyapatite Nanoparticles: Synthesis, Characterization, and Antibacterial Properties

Abstract: Strontium-substituted apatites have provoked increased interest in recent years for their beneficial effects on osteoporotic bone treatment and replacement. In this study, rod- and acicular-shaped, strontium-substituted calcium deficient hydroxyapatite (CDHA) nanoparticles with (Ca + Sr)/P ratio of 1.61 were synthesized via accelerated microwave processing. The X-ray powder diffraction analysis indicates the synthesized nanoparticles as apatite phase with diffraction patterns similar to those of hydroxyapatite. The hydrodynamic diameter of the particles were observed to be ~200–500 nm and found to increase with strontium substitution along with an increase in the negative zeta potential by dynamic light scattering method, suggesting the particles to be agglomerates in water. The morphology of the nanoparticles was studied using transmission electron microscopy (TEM), where, pure CDHA showed globular and strontium substituted CDHAs showed rod and acicular shape for 5% and 10% Sr substitution, respectively. The average size of the particles in TEM was measured to be 33 nm × 5 nm, 40 nm × 6 nm, and 55 nm × 8 nm (L × W) for pure and strontium-substituted CDHAs, respectively. Inductively coupled plasma spectroscopy, energy dispersive X-ray analysis, and Fourier transform infrared spectroscopy further confirm the substitution of strontium and deficiency of calcium in the synthesized nanoparticles. Thermal stability and in vitro solubility of CDHA nanoparticles were observed to increase with strontium substitution. The MTT [3-(4, 5-Dimethylthiazole-2-yl)-2, 5-diphenyl tetrazolium bromide] assay indicate that the substituted nanoparticles are non-toxic to human periodontal ligament fibroblast (HPDLF) cells. Cell uptake study by fluorescence microscopy using rhodamine-123 and actin/DAPI stained HPDLF cells show cellular localization of the nCDHA, nSr5CDHA, nSr10CDHA nanoparticles without any adverse effects. The strontium-substituted CDHAs showed significant antimicrobial activity against Escherichia coli and Staphylococcus aureus bacteria by colony count method. The 10% Sr substituted CDHA show the maximum microbial reduction of around 56% for E. coli and 35% for S. aureus with 1 × 105 cells/mL of respective bacterial culture.

Crystallization Mechanism of the Bioactive Glasses, 45S5 and S53P4

Abstract: The crystallization kinetics of the two commercial bioactive glasses, 45S5 and S53P4, was studied using differential thermal analysis (DTA), optical microscopy, and scanning electron microscopy (SEM). The thermal properties, the activation energy of crystallization, and the Johnson-Mehl-Avrami (JMA) exponent were determined for two glass fractions: fine powder (<45 μm) and coarse powder (300–500 μm). The crystallization behavior of 45S5 was significantly different for the two fractions, whereas the particle size did not affect the crystallization behavior of S53P4. The JMA exponent of S53P4 suggested surface crystallization for both size fractions. However, for 45S5, the JMA exponent suggested that, with increasing particle size, crystallization evolves from predominantly surface crystallization to predominantly bulk crystallization. Surprisingly, SEM imaging did not support this conclusion. A method based on the crystallization rate dα/dt showed that the JMA approach could not be employed for 45S5. The crystallization mechanism of 45S5 appears to be more complex than a simple nucleation and growth process. Nucleation-like curves were measured for both fractions of the two glasses. The maximum nucleation rate occurred at 566 ± 4°C and 608 ± 4°C for the coarse powders of 45S5 and S53P4, respectively. The higher maximum nucleation temperature of S53P4 was attributed to the higher SiO2 content. The nucleation temperature range of these two glasses together with DTA data makes it possible to develop guidelines for tailoring thermal treatment parameters to achieve desired glass-to-crystal ratios.

BaTiO3–Bi(Zn1/2Ti1/2)O3–BiScO3 Ceramics for High-Temperature Capacitor Applications

Abstract: Ceramics based on solid solutions of xBaTiO3–(100−x)(0.5Bi(Zn1/2Ti1/2)O3–0.5BiScO3), where x = 50, 55, and 60 were prepared by solid-state reaction which resulted in a single perovskite phase with pseudocubic symmetry. Dielectric property measurements revealed a high relative permittivity (>1000), which could be modified with the addition of Bi(Zn1/2Ti1/2)O3 (BZT) and BiScO3 (BS) to engineer a temperature-stable dielectric response with a temperature coefficient of permittivity (TCe) as low as −182 ppm/°C. By incorporating 2 mol% Ba vacancies into the stoichiometry, the resistivity increased significantly, especially at high temperatures (>200°C). Vogel–Fulcher analysis of the permittivity data showed that the materials exhibited freezing of polar nanoregions over the range of 100–150 K. An analysis of optical absorption near the band edge for the Ba-deficient compositions suggested that the enhanced resistivity values were linked to a decrease in the concentration of defect states. An activation energy of ~1.4 eV was obtained from DC resistivity measurements suggesting that an intrinsic conduction mechanism played a major role in the high temperature conductivity. Finally, multilayer capacitors based on these compositions were fabricated, which exhibited dielectric properties comparable to the bulk material. Based on these results, this family of materials has great promise for high-temperature capacitor applications.

Fabrication and Properties of 3-D Cf/SiC–ZrC Composites, Using ZrC Precursor and Polycarbosilane

Abstract: Three-dimensional (3-D) needled Cf/ZrC–SiC composites were successfully fabricated by polymer infiltration and pyrolysis combined with ZrC precursor impregnation. The microstructure and mechanical properties of the composites were studied using XRD, SEM and three-point bending test. The composite with PyC interphase between fiber and matrix had a bulk density of 2.20 g/cm3, an open porosity of 13%, and a bending force of 356 N, and exhibited a nonbrittle failure behavior due to propagation and deflection of cracks, and fracture and pullout of fibers. The mass lose and linear recession rate of the 3-D needled Cf/SiC–ZrC composite during oxy-propane torch test were 0.010 g/s and 0.001 mm/s, respectively. The formation of ZrSiO4 melt on the surface of the composite contributed mainly the excellent high-temperature property.

Synthesis and Size Control of Tetragonal Barium Titanate Nanopowders by Facile Solvothermal Method

Abstract: A facile synthetic strategy was implemented to obtain nanosized barium titanate (BaTiO3) powders with tetragonal structure. The nanoparticles were synthesized using solvothermal process employing diethanolamine and triethanolamine to suppress the particle growth and the as-prepared nanopowders were characterized using X-ray diffraction, scanning electron microscopy, and high-resolution dispersive Raman spectroscopy. It was found that the particle size can be easily tuned by adjusting the experimental parameters while retaining the tetragonality. The average diameters of the particles prepared with and without the organic amines were found to be 80 and 100 nm, respectively. All the synthesized BaTiO3 nanopowders exhibit a narrow size distribution with a uniform morphology. Rietveld refinement of the XRD patterns and Raman spectra revealed that the synthesized BaTiO3 nanopowders have tetragonal asymmetry dominant structures. A slight decrease in the tetragonality of the prepared powders with decrease in particle size is attributed to the presence of cubic shell layer and inner defects. The tetragonal-dominant structure was also confirmed by normalizing the peak area of the Raman spectra.

A High-Temperature-Capacitor Dielectric Based on K0.5Na0.5NbO3-Modified Bi1/2Na1/2TiO3–Bi1/2K1/2TiO3

Abstract: A high-temperature dielectric, (1–x)(0.6Bi1/2Na1/2TiO3–0.4Bi1/2K1/2TiO3)–xK0.5Na0.5NbO3, off the morphotropic phase boundary of the parent matrix 0.8Bi1/2Na1/2TiO3–0.2Bi1/2K1/2TiO3, has been developed for application as a high-temperature capacitor. In addition to temperature-dependent permittivity and dielectric loss, DC conductivity and field-dependent permittivity are reported. These properties are correlated with temperature-dependent structure data measured at different length scales using Raman spectroscopy and neutron diffraction. It is suggested that all materials investigated are ergodic relaxors with two types of polar nanoregions providing different relaxation mechanisms. The most attractive properties for application as high-temperature dielectrics are obtained in a material with x = 0.15 at less than 10% variation of relative permittivity of about 2100 between 54°C and 400°C.

Very Low Loss Tangent and High Dielectric Permittivity in Pure‐CaCu3Ti4O12 Ceramics Prepared by a Modified Sol‐Gel Process

Abstract: We present a simple strategy to achieve excellent dielectric and nonlinear current–voltage properties for pure–CaCu3Ti4O12 ceramics prepared by a modified sol–gel method with different calcining conditions. At 1 kHz and room temperature, the best CCTO ceramic can exhibit a high dielectric constant (e′) of 9516 with a very low dielectric loss, tan δ ~0.020. The minimum value of tan δ is 0.018 at 2.5 kHz with e′ ~9433. High breakdown field of 4032 V/cm and nonlinear coefficient of 7.05 were obtained.

Strong ZrB2–SiC–WC Ceramics at 1600°C

Abstract: High temperature flexural strength of ZrB2–20 vol% SiC ceramics (ZS) up to 1600°C in high purity argon atmosphere was significantly improved by adding 5 vol% WC, but degraded when 5 vol% ZrC was added. ZrB2–20SiC–5WC ceramic (ZSW) has a very high strength (mean ± SD) of 675 ± 33 MPa at 1600°C, and also an elastic and transgranular fracture mode was observed. According to the analysis of the fracture modes and crack origins in ZSW ceramics, the improvement in strength above 1000°C was attributed to the removal of the oxide impurities from grain boundaries.

Improved Structure Stability and Multiferroic Characteristics in CaTiO3-Modified BiFeO3 Ceramics

Abstract: BiFeO3 multiferroic ceramics were modified by introducing CaTiO3. The stability of perovskite BiFeO3 was increased by the present modification, and the single-phase (Bi1 − xCax)(Fe1 − xTix)O3 solid solutions were obtained when x ≥ 0.15, where the symmetry changed from R3c (x ≤ 0.2) to Pbnm (x ≥ 0.3). The ferroelectric property of BiFeO3 was enhanced by introducing CaTiO3 because of the reduction of leakage current and the increase of resistivity. The increased dielectric constant, decreased dielectric loss, and enhanced remnant magnetization Mr were also obtained. The best combination of multiferroic characteristics were achieved at x = 0.2.

Giant Dielectric Constant and Good Temperature Stability in Y2/3Cu3Ti4O12 Ceramics

Abstract: Y2/3Cu3Ti4O12 ceramics were successfully prepared by the conventional solid-state reaction method. Effects of sintering conditions on microstructure, phase structure, and the dielectric properties of Y2/3Cu3Ti4O12 ceramics were investigated in detail. Y2/3Cu3Ti4O12 ceramics sintered at 1010°C for 25 h exhibited a giant dielectric constant (1.10 × 104) and a relatively low dielectric loss (0.033) around room temperature. The samples showed good temperature stability (ΔCT/C25∘C=-6.7% – 9.5%) in the temperature range from −60°C to 125°C at 10 kHz. It was also found that a new dielectric relaxation III appeared at higher temperatures (>200°C). The complex impedance spectroscopy analysis suggested that Y2/3Cu3Ti4O12 ceramics were electrically heterogeneous, and they consisted of semiconducting grains and insulating grain boundaries, which could be modeled to a first approximation on an equivalent circuit based on two parallel RC elements connected in series. The Cu2+/Cu3+ and Ti3+/Ti4+ aliovalences were observed in Y2/3Cu3Ti4O12 ceramics. The giant permittivity phenomenon could be explained by internal barrier layer capacitance (IBLC) effect.

Evidence on the Dual Nature of Aluminum in the Calcium-Silicate-Hydrates Based on Atomistic Simulations

Abstract: Hydration of tri-calcium silicate (C3S) and di-calcium silicate (C2S) precipitates calcium-silicate-hydrate (CSH) which is the bonding phase responsible for the strength of cementitious materials. Substitution of part of C3S and C2S with aluminum-containing additives alters the chemical composition of hydration products by precipitating calcium-aluminate-silicate-hydrate (CASH). Incorporation of aluminum in the molecular building blocks of CSH entails structural and chemo-mechanical consequences. These alterations can be measured through solid state nuclear magnetic resonance (NMR) experiments. By conducting a wide spectrum of atomistic simulation methods on thousands of aluminum-containing molecular CASH structures, an overall molecular approach for determination of CASH nanostructure is presented. Through detailed analysis of different order parameters, it is found that aluminum can exhibit a tetra-/penta-/octahedral behavior which is fully consistent with the recent NMR observations. This corresponds to the formation of a class of complex three-dimensional alumino-silicate skeletons with partial healing effect in the CASH nanostructure potentially increasing durability and strength of hydration products. We explored the variation of mechanical observables by increasing aluminum content in CASH structures of varying calcium to silicon ratio. Finally, deformation of CSHs and CASHs of different chemical formula in a multi-scale fashion unravels the effect of chemical composition on the strength and kinematics of deformation in this particular type of composites.

High Energy Density, High Temperature Capacitors Utilizing Mn-Doped 0.8CaTiO3–0.2CaHfO3 Ceramics

Abstract: Single layer air co-fired capacitors with Pt internal electrodes were prototyped for the compositions 0.8CaTiO3–0.2CaHfO3 (CHT) and 0.5 mol% Mn-doped 0.8CaTiO3–0.2CaHfO3 (CHT + Mn) to yield a material with a room-temperature relative permittivity of er ~170, thermal coefficient of capacitance (TCC) of ±15.8% to ±16.4% from −50°C to 150°C, and a band gap of ~4.0 eV. Impedance spectroscopy revealed that doping with Mn reduces both the ionic and electronic conductivity. Undoped CHT single layer capacitors exhibited ambient energy densities as large as 9.0 J/cm3, but showed a drastic decrease in energy density above 100°C. When doped with 0.5 mol% Mn, the temperature dependence of the breakdown strength was minimized, and energy densities similar to ambient values (9.5 J/cm3) were observed up to 200°C. At 300°C, energy densities as large as 6.5 J/cm3 were measured. The design rationale for these dielectrics centered on materials with large band gaps, linear or weakly nonlinear permittivities, and high breakdown strengths. These observations suggest that with further reductions in grain size and dielectric layer thickness, the CaTiO3–CaHfO3 system is a strong candidate for integration into future power electronics applications.

Geometrically Complex Silicon Carbide Structures Fabricated by Robocasting

Abstract: Geometrically complex, three-dimensional (3-D) structures of SiC were produced by a colloidal printing method known as robocasting, followed by low-pressure spark plasma sintering (SPS) to produce dense ceramic bodies. A concentrated, aqueous colloidal ink consisting of SiC, Al2O3, and Y2O3 particles in a dilute polymer solution with a total solids volume fraction of 0.44 was developed to have pseudoplastic behavior with yield stress rheology. Lattice structures consisting of extruded filaments deposited in an overall cylindrical or cuboid shape were printed through nozzles ranging in diameter from 150 to 330 μm. After printing, drying and calcining processes, the structures were sintered at 1700°C in argon by SPS. The final average grain size was 1–2 μm and samples displayed above 97% of theoretical density, showing ~22.8% linear shrinkage from green to sintered state.

Flash-Sinterforging of Nanograin Zirconia: Field Assisted Sintering and Superplasticity

Abstract: We report on the influence of a uniaxial applied stress on flash-sintering and field assisted superplastic behavior of cylindrical powder preforms of 3 mol% tetragonal-stabilized zirconia. The experiments use the sinterforging method, where, in addition to pressure, a dc electrical field is applied by metal electrodes sandwiched between the push-rods and the specimen. The axial and radial strains in the experiment provide simultaneous measurement of the time-dependent densification and shear strains. Large effects of the electric field on sintering and superplasticity are observed. We see flash-sintering which is characterized by a threshold level of temperature and electric field. With higher applied fields, the sample sinters at a lower furnace temperature. Surprisingly, the applied stress further lowers this critical temperature: a sample, which sinters at 915°C under a stress of 1.5 MPa, densifies at only 850°C when the stress is raised to 12 MPa. This stress induced reduction in sintering temperature maybe related to the additional electrical fields generated within the specimen by the electro-chemo-mechanical mechanism described by Pannikkat and Raj [Acta Mater., 47 (1999) 3423]. Remarkably, we also show that the sample deforms in pure shear to 30% strain in just a few seconds at anomalously low temperatures. The specimen temperature was measured with a pyrometer, during the flash sintering, as a check on Joule heating. A reading of 1000°C–1100°C was obtained, up to 200° above the furnace temperature. This temperature is still too low to explain the sintering in just a few seconds. It is suggested that the electric field can nucleate a defect avalanche that enhances diffusion kinetics not by changing the activation energy but by increasing the pre-exponential factor for the diffusion coefficient, noting that the pre-exponential factor depends on concentration of defects, and not upon their mobility.

ZrB2 – SiC Sharp Leading Edges in High Enthalpy Supersonic Flows

Abstract: Aero-thermodynamic tests have been carried out in an arc-jet supersonic plasma wind tunnel using a very sharp wedge made of ultra-high temperature ceramic (UHTC) in the ZrB2–SiC system. The comparison with a lower thermal conductivity ceramic material (Si3N4–MoSi2) with the same sharp shape, pointed out at the performance advantages of the UHTC material. When subjected to heat fluxes in the order of 7 MW/m2, the surface temperature of the UHTC wedge increased up to 2450°C near the leading edge. The present study demonstrated that the high thermally conductive UHTC survived such extreme conditions by re-distributing heat over colder regions downstream of the sharp tip. As a consequence, radiative equilibrium temperatures in the range 1400°C–1650°C were established over 85% of the exposed surface. On the other hand, the less thermally conductive Si3N4–MoSi2 material failed to withstand the same heat flux and underwent partial melting with significant mass loss. The post-test microstructural observations of the UHTC wedge proved to be a fundamental source of information which was input into a Computational Fluid Dynamics (CFD) code and by a thermal simulation software to simulate the experimental tests and correlate the in situ observations of the material evolution during testing.

Tailoring the Magnetic and Optical Characteristics of Nanocrystalline BiFeO3 by Ce Doping

Abstract: The Ce-doped BiFeO3 (BFO) nanoparticles (NPs) were synthesized using a facile solgel route with varying Ce concentrations in the range of 15 mol%. Ferroelectric transition temperature was found to shift from 723 degrees C +/- 5 degrees C for pristine BFO NPs to 534 degrees C +/- 3 degrees C for 5 mol% Ce-doped BFO NPs. UVVis absorption spectra of BFO NPs showed a significant blue shift of similar to 100 nm on Ce doping. The Fourier transformed infrared (FTIR) spectrum centered similar to 550 cm(-1) becomes considerably broadened on Ce doping which is due to additional closely spaced vibrational peaks as revealed by the second derivative FTIR analysis. High-frequency EPR measurements indicated that clustering occurs at high dopant levels, and that Fe is present as Fe(3+)corroborating Mossbauer measurements. The values of saturation and remanent magnetization for 3% Ce-doped BFO NPs are 3.03 and 0.49 emu/g, respectively, which are quite significant at room temperature, making it more suitable for technological applications.

Effects of Crystal Structure on the Microwave Dielectric Properties of ABO4 (A = Ni, Mg, Zn and B = Mo, W) Ceramics

Abstract: The dependence of microwave dielectric properties on the crystal structure, bond character, and electronic characteristics of AMoO4 and AWO4 (A = Ni, Mg, Zn) ceramics was investigated. The dielectric constant (K) of specimens was principally affected by the dielectric polarizabilities, molar volume, and electronic oxide polarizabilities. MgMoO4 and AWO4 (A = Ni, Mg, Zn) display a single phase monoclinic wolframite structure, whereas ZnMoO4 is a single phase triclinic wolframite structure. The quality factor (Qf) of AWO4 was higher than that of AMoO4 (A = Mg, Zn); these results were attributed to the packing fraction due to effective ionic size. The temperature coefficient of the resonant frequency (TCF) of the specimens was dependent on the cations' bond valence between the cation and oxygen ions. This suggests the ability to tailor ABO4 microwave K, Qf, and TCF via ionic design rules.

Modeling Oxidation Kinetics of SiC‐Containing Refractory Diborides

Abstract: Experimental data on the oxidation kinetics of SiC-containing diborides of Zr and Hf in the temperature regime of 1473–2273 K are interpreted using a mechanistic model. The model encompasses counter-current gas diffusion in the internal SiC depleted zone, oxygen permeation through borosilicate glass channels in the oxide scale, and boundary layer evaporation at the surface. The model uses available viscosity, thermodynamic and kinetic data for boria, silica, and borosilicate glasses, and a logarithmic mean approximation for compositional variations. The internal depletion region of SiC is modeled with CO/CO2 counter diffusion as the oxygen transport mechanism. Data reported for pure SiC in air/oxygen, for ZrB2 containing varying volume fractions of SiC, and for SiC–HfB2 ultra-high temperature ceramics (UHTCs) by different investigations were compared with quantitative predictions of the model. The model is found to provide good correspondence with laboratory-furnace-based experimental data for weight gain, scale thicknesses, and depletion layer thicknesses. Experimental data obtained from arc-jet tests at high enthalpies are found to fall well outside the model predictions, whereas lower enthalpy data were closer to model predictions, suggesting a transition in mechanism in the arc-jet environment.