Showing papers in "Journal of the American Ceramic Society in 2011"

Boron Carbide: Structure, Properties, and Stability under Stress

Abstract: Boron carbide is characterized by a unique combination of properties that make it a material of choice for a wide range of engineering applications. Boron carbide is used in refractory applications due to its high melting point and thermal stability; it is used as abrasive powders and coatings due to its extreme abrasion resistance; it excels in ballistic performance due to its high hardness and low density; and it is commonly used in nuclear applications as neutron radiation absorbent. In addition, boron carbide is a high temperature semiconductor that can potentially be used for novel electronic applications. This paper provides a comprehensive review of the recent advances in understanding of structural and chemical variations in boron carbide and their influence on electronic, optical, vibrational, mechanical, and ballistic properties. Structural instability of boron carbide under high stresses associated with external loading and the nature of the resulting disordered phase are also discussed.

Piezoelectric Materials for High Temperature Sensors

Abstract: Piezoelectric materials that can function at high temperatures without failure are desired for structural health monitoring and/or nondestructive evaluation of the next generation turbines, more efficient jet engines, steam, and nuclear/electrical power plants. The operational temperature range of smart transducers is limited by the sensing capability of the piezoelectric material at elevated temperatures, increased conductivity and mechanical attenuation, variation of the piezoelectric properties with temperature. This article discusses properties relevant to sensor applications, including piezoelectric materials that are commercially available and those that are under development. Compared to ferroelectric polycrystalline materials, piezoelectric single crystals avoid domain-related aging behavior, while possessing high electrical resistivities and low losses, with excellent thermal property stability. Of particular interest is oxyborate [ReCa4O (BO3)3] single crystals for ultrahigh temperature applications (>1000°C). These crystals offer piezoelectric coefficients deff, and electromechanical coupling factors keff, on the order of 3–16 pC/N and 6%–31%, respectively, significantly higher than those values of α-quartz piezocrystals (~2 pC/N and 8%). Furthermore, the absence of phase transitions prior to their melting points ~1500°C, together with ultrahigh electrical resistivities (>106 Ω·cm at 1000°C) and thermal stability of piezoelectric properties (< 20% variations in the range of room temperature ~1000°C), allow potential operation at extreme temperature and harsh environments.

Electric Current Activation of Sintering: A Review of the Pulsed Electric Current Sintering Process

Abstract: The phenomenal increase during the past decade in research utilizing pulsed electric current to activate sintering is attributed generally to the intrinsic advantages of the method relative to conventional sintering methods and to the observations of the enhanced properties of materials consolidated by this method. This review focuses on the fundamental aspects of the process, discussing the reported observations and simulation studies in terms of the basic aspects of the process and identifying the intrinsic benefits of the use of the parameters of current (and pulsing), pressure, and heating rate.

The Antiferroelectric ↔ Ferroelectric Phase Transition in Lead-Containing and Lead-Free Perovskite Ceramics

Abstract: A comprehensive review on the latest development of the antiferroelectric ↔ ferroelectric phase transition is presented. The abrupt volume expansion and sudden development of polarization at the phase transition has been extensively investigated in PbZrO3-based perovskite ceramics. New research developments in these compositions, including the incommensurate domain structure, the auxetic behavior under electric fields in the induced ferroelectric phase, the ferroelastic behavior of the multicell cubic phase, the impact of radial compression, the unexpected electric field-induced ferroelectric-to-antiferroelectric transition, and the phase transition mechanical toughening effect have been summarized. Due to their significance to leadfree piezoelectric ceramics, compounds with antiferroelectric phases, including NaNbO3, AgNbO3, and (Bi1/2Na1/2)TiO3, are also critically reviewed. Focus has been placed on the (Bi1/2Na1/2)TiO3–BaTiO3 solid solution where the electric field-induced ferroelectric phase remains even after the applied field is removed at room temperature. Therefore, the electric field-induced antiferroelectric-to-ferroelectric phase transition is a key to the poling process to develop piezoelectricity in morphotropic phase boundary (MPB) compositions. The competing phase transition and domain switching processes in 0.93 (Bi1/2Na1/2)TiO3–0.07BaTiO3 are directly imaged with nanometer resolution using the unique in situ transmission electron microscopy (TEM) technique.

Origins of Electro-Mechanical Coupling in Polycrystalline Ferroelectrics During Subcoercive Electrical Loading

Abstract: The electromechanical coupling in ferroelectric materials is controlled by several coexisting structural phenomena which can include piezoelectric lattice strain, 180 degrees and non-180 degrees domain wall motion, and interphase boundary motion. The structural mechanisms that contribute to electromechanical coupling have not been readily measured in the past, particularly under the low-to-medium driving electric field amplitudes at which many piezoelectric materials are used. In this feature, results from in situ, high-energy, and time-resolved X-ray diffraction experiments are interpreted together with macroscopic piezoelectric coefficient measurements in order to better understand the contribution of these mechanisms to the electromechanical coupling of polycrystalline ferroelectric materials. The compositions investigated include 2 mol% La-doped PbZr0.60Ti0.40O3, 2 mol% La-doped PbZr0.52Ti0.48O3, 2 mol% La-doped PbZr0.40Ti0.60O3, undoped PbZr0.52Ti0.48O3, and 2 mol% Fe-doped PbZr0.47Ti0.53O3. In all compositions, a strong correlation is found between the field-amplitude-dependence of the macroscopic piezoelectric coefficient and the contribution of non-180 degrees domain wall motion determined from the diffraction data. The results show directly that the Rayleigh-like behavior of d(33) piezoelectric coefficient is predominantly due to a Rayleigh-like behavior of non-180 degrees domain wall motion. Furthermore, after separating contributions from lattice (atomic level) and domain wall motion (nanoscale level) to the measured macroscopic piezoelectric properties, we show that previously ignored intergranular interactions (microscopic level) account for a surprisingly large portion of the electromechanical coupling. These results demonstrate that electromechanical coupling in polycrystalline aggregates is substantially different from that observed in single crystalline materials. The construct of emergence is used to describe how averaged macrolevel phenomena are different from the material response observed in an isolated subcomponent of the material. Consequently, and due to its size-scale complexity, the description of grain-to-grain interactions is presently inaccessible in most ab initio and phenomenological approaches. Results presented here demonstrate the need to account for these interactions in order to completely describe macroscopic electromechanical properties of polycrystalline materials.

Large Plastic Deformation in High-Capacity Lithium-Ion Batteries Caused by Charge and Discharge

Abstract: Evidence has accumulated recently that a high-capacity electrode of a lithium-ion battery may not recover its initial shape after a cycle of charge and discharge. Such a plastic behavior is studied here by formulating a theory that couples large amounts of lithiation and deformation. The homogeneous lithiation and deformation in a small element of an electrode under stresses is analyzed within nonequilibrium thermodynamics, permitting a discussion of equilibrium with respect to some processes, but not others. The element is assumed to undergo plastic deformation when the stresses reach a yield condition. The theory is combined with a diffusion equation to analyze a spherical particle of an electrode being charged and discharged at a constant rate. When the charging rate is low, the distribution of lithium in the particle is nearly homogeneous, the stress in the particle is low, and no plastic deformation occurs. When the charging rate is high, the distribution of lithium in the particle is inhomogeneous, and the stress in the particle is high, possibly leading to fracture and cavitation.

Influence of Externally Imposed and Internally Generated Electrical Fields on Grain Growth, Diffusional Creep, Sintering and Related Phenomena in Ceramics

Abstract: Microwaves and spark plasma sintering (SPS) enhance sinterability. Simple electrical fields, applied by means of a pair of electrodes to bare specimens, have been shown to accelerate the rate of superplastic deformation, reduce the time and temperature for sintering, and to retard the rate of grain growth. By inference, the influence of electrical and electromagnetic fields on grain boundary energetics and kinetics is unmistakable. Often, in ceramics, grain boundaries are themselves endowed with space charge that can couple with externally applied fields. The frequency dependence of this coupling ranging from zero frequency to microwave frequencies is discussed. The classical approach for modeling grain growth, creep, and sintering, considers chemical diffusion (self-diffusion) under a thermodynamic driving force, underpinned by a physical mechanism that visualizes the flow of mass transport in a way that reproduces the phenomenological observations. In all instances, the final analytical result can be separated into a product of three functions: one of the grain size, the second related to the thermodynamic driving force, and the third to the kinetics of mass transport. The influence of an electrical field on each of these functions is addressed.The fundamental mechanisms of these electrical interactions are discussed in the following ways: (i) dielectric loss and Joule heating in the crystal and at the grain boundary, (ii) the coupling between mechanical stress and the electrochemical potential of charged species, (iii) the interaction between applied electrical fields and the intrinsic fields that exist within the space charge layers, (iv) and the possibility of nucleating defect avalanches under electrical fields. We limit ourselves to ceramics that have at least some degree of ionic character. In these experiments the electrical fields range from several volts to several hundred volts per centimeter, and the power dissipation from Joule heating is of the order of several watts per cubic centimeter of the specimen. Metals, where very high current densities are obtained at relatively low applied electric fields, leading to phenomenon such as electromigration, are not considered.

Energy‐Storage Properties of 0.89Bi0.5Na0.5TiO3–0.06BaTiO3–0.05K0.5Na0.5NbO3 Lead‐Free Anti‐ferroelectric Ceramics

Abstract: Energy-storage properties of 0.89Bi0.5Na0.5TiO3–0.06BaTiO3–0.05K0.5Na0.5NbO3 (0.89BNT–0.06BT–0.05KNN) lead-free ceramics were first investigated. Measurements of dielectric properties together with switching current curves indicate a rather diffuse ferroelectric (FE) to anti-ferroelectric (AFE) phase transition when heating from 20°C to 90°C. The energy density (W), which was calculated from P–E hysteresis loops, increases linearly with the external electric field when E exceeds AFE-FE transition field. W is independent of temperature and frequency, and maintains around 0.59 J/cm3 under 5.6 kV/mm in the stable AFE phase region. These properties indicate that 0.89BNT–0.06BT–0.05KNN ceramics might be a promising lead-free AFE material for energy-storage capacitor application.

HF-Based Etching Processes for Improving Laser Damage Resistance of Fused Silica Optical Surfaces

Abstract: The effect of various HF-based etching processes on the laser damage resistance of scratched fused silica surfaces has been investigated. Conventionally polished and subsequently scratched fused silica plates were treated by submerging in various HF-based etchants (HF or NH4F:HF at various ratios and concentrations) under different process conditions (e.g., agitation frequencies, etch times, rinse conditions, and environmental cleanliness). Subsequently, the laser damage resistance (at 351 or 355 nm) of the treated surface was measured. The laser damage resistance was found to be strongly process dependent and scaled inversely with scratch width. The etching process was optimized to remove or prevent the presence of identified precursors (chemical impurities, fracture surfaces, and silica-based redeposit) known to lead to laser damage initiation. The redeposit precursor was reduced (and hence the damage threshold was increased) by: (1) increasing the SiF62− solubility through reduction in the NH4F concentration and impurity cation impurities, and (2) improving the mass transport of reaction product (SiF62−) (using high-frequency ultrasonic agitation and excessive spray rinsing) away from the etched surface. A 2D finite element crack-etching and rinsing mass transport model (incorporating diffusion and advection) was used to predict reaction product concentration. The predictions are consistent with the experimentally observed process trends. The laser damage thresholds also increased with etched amount (up to ∼30 μm), which has been attributed to: (1) etching through lateral cracks where there is poor acid penetration, and (2) increasing the crack opening resulting in increased mass transport rates. With the optimized etch process, laser damage resistance increased dramatically; the average threshold fluence for damage initiation for 30 μm wide scratches increased from 7 to 41 J/cm2, and the statistical probability of damage initiation at 12 J/cm2 of an ensemble of scratches decreased from ∼100 mm−1 of scratch length to ∼0.001 mm−1.

Probing Ferroelectrics Using Optical Second Harmonic Generation

Abstract: Nonlinear optics is an essential component of modern laser systems and optoelectronic devices. It has also emerged as an important tool in probing the electronic, vibrational, magnetic, and crystallographic structure of materials ranging from oxides and metals, to polymers and biological samples. This review focuses on the specific technique of optical second harmonic generation (SHG), and its application in probing ferroelectric complex oxide crystals and thin films. As the dominant SHG interaction mechanism exists only in materials that lack inversion symmetry, SHG is a sensitive probe of broken inversion symmetry, and thus also of bulk polar phenomena in materials. By performing in-situSHG polarimetry experiments in different experimental conditions such as sample orientation, applied electric field, and temperature, one can probe ferroelectric hysteresis loops and phase transitions. Careful modeling of the polarimetry data allows for the determination of the point group symmetry of the crystal. In epitaxial thin films with a two-dimensional arrangement of well-defined domain orientations, one can extract information about intrinsic material properties such as nonlinear coefficients, as well as microstructural information such as the local statistics of the different domain variants being probed. This review presents several detailed examples of ferroelectric systems where such measurements and modeling are performed. The use of SHG microscopic imaging is discussed, and its ability to reveal domain structures and phases not normally visible with linear optics is illustrated.

Flash‐Sintering of Cubic Yttria‐Stabilized Zirconia at 750°C for Possible Use in SOFC Manufacturing

Abstract: We show that cubic 8 mol% yttria (8YSZ) can be sintered at 750°C with the application of DC electrical fields; in comparison the lowest sintering temperature for 3YSZ was 850°C. Furthermore, cubic zirconia exhibits the onset of flash sintering at 30 V/ cm, whereas 3YSZ begins flash sintering at 60 V/cm. However, the volume specific power dissipation for the onset of flash sintering remains similar at ∼10 mW/mm 3 . The easier sintering of 8YSZ is ascribed to its higher ionic conductivity.

Rhombohedral–Tetragonal Phase Coexistence and Piezoelectric Properties of (NaK)(NbSb)O3–LiTaO3–BaZrO3 Lead-Free Ceramics

Abstract: The phase transitional behavior and composition-dependent piezoelectric properties of (NaK)(NbSb)O3–LiTaO3–BaZrO3 (NKNS–LT–BZ) pseudo-ternary system were investigated. A composition–temperature phase diagram was generalized within a certain range of BZ content in which a rhombohedral–tetragonal (R–T ) ferroelectric phase boundary connecting orthorhombic and cubic phase zones is formed near room temperature between two trifurcate points. Piezoelectric and electromechanical properties of NKNS–LT–BZ ceramics exhibit optimum values of d33=365 pC/N and kp=45% in the vicinity of the R–T phase coexistence zone. The dielectric and ferroelectric properties and the phase transition of NKNS–LT–BZ ceramics were discussed from a crystallographic point of view.

Microwave Dielectric Properties of Li2WO4 Ceramic with Ultra-Low Sintering Temperature

Abstract: A new ultra-low-temperature firing microwave dielectric ceramic, Li2WO4 with the phenacite structure, was prepared via solid-state reaction method. The Li2WO4 ceramic can be well sintered at 640°–660°C, with a microwave relative permittivity ∼5.5, a Q×f value about 62 000 GHz, and a negative temperature coefficient of −146 ppm/°C at 15.7 GHz. From an X-ray diffraction analysis, the Li2WO4 ceramic was found to be chemically compatible with both silver and aluminum powders at 640°C. All the results indicate that the Li2WO4 ceramic is a promising candidate for ultra-low temperature cofired ceramic technology, especially for dielectric substrate application.

In situ Transmission Electron Microscopy Study on the Phase Transitionsin Lead-Free (1−x)(Bi1/2Na1/2)TiO3–xBaTiO3 Ceramics

Abstract: The phase transitions in unpoled lead-free (1−x)(Bi1/2Na1/2)TiO3–xBaTiO3 (x = 0.06 and 0.11) ceramics are investigated using hot-stage transmission electron microscopy (TEM). It is found that large ferroelectric domains in both ceramics start to disappear around Td, the depolarization temperature. After the transition, both compositions exhibit the P4bm tetragonal symmetry in the form of nanodomains. The structural transition observed by the in situTEM experiments seems to be gradual and occurs within a temperature range of several tens of degrees, in contrast to the sharp anomaly at Td revealed by the dielectric characterization. With further increasing temperature, no structural change was observed for both compositions across TRE, where the dielectric frequency dispersion vanishes, and Tm, where the dielectric permittivity reaches maximum. The tetragonal-to-cubic transition is diffuse and takes place in a broad temperature window well above both TRE and Tm. These results of structural phase transitions are summarized in a phase diagram with its composition range covering the morphotropic phase boundary (MPB).

Transparent Tetragonal Yttria-Stabilized Zirconia Ceramics: Influence of Scattering Caused by Birefringence

Abstract: The correlation between grain size, optical birefringence, and transparency is discussed for tetragonal zirconia (ZrO2) ceramics using the Mie, Rayleigh, and Rayleigh–Gans–Debye scattering models. Our results demonstrate that at the degree of mean birefringence in the range (0.03–0.04) expected for tetragonal ZrO2, only the Mie theory provides reasonable results. At small particle size (o50 nm) the more straightforward Rayleigh approximation correlates with the Mie model. A real in-line transmission of B50% at visible light and 1 mm thickness is expected at a mean grain size o40 nm and B70% at a mean grain size o20 nm. At an infrared (IR) wavelength of 5 lm there should not be any scattering caused by birefringence for grain sizes o200 nm. Our simulations were validated with experimental data for tetragonal ZrO2 (3 mol% Y2O3) ceramics made from a powder with an initial particle size of B10 nm by sintering in air and using hot-isostatic pressing. The maximum in-line transmission of about 77% was observed at IR wavelengths of 3–5 lm.

Phase Stability of t′-Zirconia-Based Thermal Barrier Coatings: Mechanistic Insights

Abstract: The temperature capability of yttria-stabilized zirconia thermal barrier coatings (TBCs) is ultimately tied to the rate of evolution of the “nontransformable” t′ phase into a depleted tetragonal form predisposed to the monoclinic transformation on cooling. The t′ phase, however, has been shown to decompose in a small fraction of the time necessary to form the monoclinic phase. Instead, a modulated microstructure consisting of a coherent array of Y-rich and Y-lean lamellar phases develops early in the process, with mechanistic features suggestive of spinodal decomposition. Coarsening of this microstructure leads to loss of coherency and ultimately transformation into the monoclinic form, making the kinetics of this process, and not the initial decomposition, the critical factor in determining the phase stability of TBCs. Transmission electron microscopy is shown to be essential not only for characterizing the microstructure but also for proper interpretation of X-ray diffraction analysis.

Carboaluminate Phases Formation During the Hydration of Calcite-Containing Portland Cement

Abstract: It has been an established fact that finely ground calcium carbonate represents to a certain extent an active component during the hydration of Portland cement and the formation of calcium monocarboaluminate has been confirmed many times. Additionally, the formation of calcium hemicarboaluminate, as another possible compound, has been mentioned. There is, however, lack of specific information regarding these two compounds: there has been no experimental data on their hydration time-dependent formation, their interrelations, and their amount in a hardened cement paste. This paper describes the time-related formation of hemicarboaluminate and monocarboaluminate and reports for the first time in the literature the conversion of hemicarboaluminate into monocarboaluminate on the basis of X-ray diffraction by following the content of the latter using the Rietveld method. Hemicarboaluminate appears at early hydration times in calcite-containing Portland cement, even in the presence of large amounts of calcium carbonate. As the hydration progresses, a gradual conversion of hemicarboaluminate into monocarboaluminate occurs. The only detectable AFm-type compound present in well-hydrated cement is calcium monocarboaluminate.

Structure, Dielectric Properties and Temperature Stability of BaTiO3–Bi(Mg1/2Ti1/2)O3 Perovskite Solid Solutions

Abstract: (1−x)BaTiO3–xBi(Mg1/2Ti1/2)O3 [(1−x)BT–xBMT] polycrystalline ceramics were obtained via solid-state processing techniques. The solubility limit for (1−x)BT–xBMT was determined to be about x=0.07. A systematic structural change from the ferroelectric tetragonal phase to pseudocubic phase was observed at about x≥0.05 at room temperature. Dielectric measurements revealed a gradual change from normal ferroelectric of pure BaTiO3 to highly dispersive relaxor-like characteristics in the solid solution with 30–60 mol% Bi(Mg1/2Ti1/2)O3, showing low-temperature coefficients of capacitance over a wide temperature range. The properties of Nb2O5-doped 0.85BT–0.15BMT ceramics were investigated to better understand the formation mechanism of core-shell structure, for further improving the temperature stability of the dielectric behavior.

Alumina Scale Formation: A New Perspective

Abstract: Oxidation of Al2O3 scale-forming alloys is of immense technological significance and has been a subject of much scientific inquiry for decades. The oxidation reaction is remarkably complex, involving issues of alloy composition, kinetics, thermodynamics, microstructure, mechanics and mechanical properties, crystallography, etc. A brief overview of the formation of passivating, thermally grown oxide Al2O3 scales will be given in the light of recent findings on the defect structure and associated transport behavior of α-Al2O3. It is inferred that the electronic structure of Al2O3 is of direct relevance to understand the Al2O3 scale growth. We also discuss the effect of the so-called “reactive” elements (REs)—Y, Zr, and Hf—on reducing the rate of Al2O3 scale thickening by reducing the outward flux of aluminum. An important aspect of the “new perspective” is the suggestion that the REs change the electronic structure of Al2O3—the relevant near-band-edge defect (grain boundary) states that are crucial to vacancy creation both at the scale/gas and scale/metal interfaces.

Alkali–Silica Reaction: the Influence of Calcium on Silica Dissolution and the Formation of Reaction Products

Abstract: In a model system for alkali―silica reaction consisting of microsilica, portlandite (0-40 mass%), and 1M alkaline solutions (NaOH, KOH), the influence of calcium on silica dissolution and on the formation of reaction products is investigated. The reaction and its products are characterized using calorimetry, X-ray diffraction, thermogravimetric analysis, nuclear magnetic resonance, desorption experiments, and pore solution analysis in combination with thermodynamic modeling. Silica dissolution proceeds until portlandite is consumed due to the formation of C-S-H, and subsequently, saturation of dissolved silica in the alkaline solution is reached. As a result, the amount of dissolved silica increases with the increasing portlandite content. Depending on the amount of portlandite added, the reaction products show differences in the relative amounts of Q 1 , Q 2 , and Q 3 sites formed and in their average Ca/Si ratio. The ability of the reactions products to chemically bind water decreases with the decreasing relative amount of Q 3 sites and with the increasing Ca/Si ratio. However, the amount of physically bound water in the reaction products reaches a maximum value at a Ca/Si ratio between 0.20 and 0.30.

A Raman Study of Iron–Phosphate Crystalline Compounds and Glasses

Abstract: Ferrous and ferric phosphate crystalline compounds and glasses were studied using Raman spectroscopy. A comparison of the spectra from crystalline and glassy ortho-, pyro-, and metaphosphates indicates that similar phosphate anions constitute the structures of the respective materials, and some information about the compositional dependence of the phosphate-site distributions in the glasses can be gleaned from relative peak intensities. A correlation exists between the average P–O bond distance and the Raman peak frequencies in the crystalline compounds, and this correlation is used to provide information about the structures of the iron phosphate glasses. For example, the average P–O bond distance is estimated to decrease from about 1.57 A for iron metaphosphate glasses (O/P∼3.0) to 1.54 A for iron orthophosphate glasses (O/P∼4.0). These bond distances are in good agreement with those reported from diffraction studies of similar glasses.

Structural and Dielectric Properties of Bi (Mg1/2Ti1/2)O3–BaTiO3 Lead-Free Ceramics

Abstract: By the conventional mixed oxide method (1−x)Bi(Mg1/2Ti1/2)O3–xBaTiO3 ((1−x)BMT–xBT, x = 0.2−1.0) ceramics were prepared. For x ≤ 0.4, a bismuth-rich phase was observed beside the perovskite phase. For 0.5 ≤ x ≤ 0.92 ceramics, all the compositions belong to the pseudocubic phase, while the tetragonal phases were formed when x ≥ 0.94. With the increase of BT content, a change from a relaxor-like behavior to a normal ferroelectric behavior was observed, while the change of Tmax is in the form of “U” curve. Furthermore, stable dielectric permittivity (1500–3000) and low losses (tan δ < 2%) were obtained in the 0.4 ≤ x ≤ 0.6 ceramics in the temperature range 200°C–400°C, indicating a potential for high-temperature applications.

Effect of SiO2 on Densification and Microstructure Development in Nd:YAG Transparent Ceramics

Abstract: This paper examines the influence of SiO2 doping on densification and microstructure evolution in Nd3xY3−3xAl5O12 (Nd:YAG) ceramics. Nd:YAG powders were doped with 0.035–0.28 wt% SiO2 and vacuum sintered between 1484° and 1750°C. 29Si magic-angle spinning nuclear magnetic resonance showed that Si4+ substitutes onto tetrahedrally coordinated Al3+ sites. High-resolution transmission electron microscopy showed no grain boundary second phases for all silica levels in samples sintered at 1600°–1750°C. Coarsening was limited by a solute drag mechanism as suggested by cubic grain growth kinetics and transmission electron microscopy energy-dispersive X-ray spectroscopy observations of increased Nd3+ concentration near grain boundaries. Increasing SiO2 content increased both densification and grain growth rate and led to increasingly coarsening-dominated sintering trajectories. Fine-grained ( 82% real in-line transmission) ceramics were produced by sintering 0.035 wt% SiO2-doped ceramics at 1750°C for 8 h. Coarse-grained (18 μm), transparent samples were obtained with 0.28 wt% SiO2-doped Nd:YAG when sintered at 1600°C for 8 h.

Protocols for the Optimal Design of Multi-Functional Cellular Structures: From Hypersonics to Micro-Architected Materials

Abstract: Cellular materials with periodic architectures have been extensively investigated over the past decade for their potential to provide multifunctional solutions for a variety of applications, including lightweight thermo-structural panels, blast resistant structures, and high-authority morphing components. Stiffer and stronger than stochastic foams, periodic cellular materials lend themselves well to geometry optimization, enabling a high degree of tailorability and superior performance benefits. This article reviews a commonly established optimal design protocol, extensively adopted at the macro-scale for both single and multifunctional structures. Two prototypical examples are discussed: the design of strong and lightweight sandwich beams subject to mechanical loads and the combined material/geometry optimization of actively cooled combustors for hypersonic vehicles. With this body of literature in mind, we present a motivation for the development of micro-architected materials, namely periodic multiscale cellular materials with overall macroscopic dimensions yet with features (such as the unit cell or subunit cell constituents) at the micro- or nano-scale. We review a suite of viable manufacturing approaches and discuss the need for advanced experimental tools, numerical models, and optimization strategies. In analyzing challenges and opportunities, we conclude that the technology is approaching maturity for the development of micro-architected materials with unprecedented combinations of properties (e.g., specific stiffness and strength), with tremendous potential impact on a number of fields.

Hydration Degree of Alkali‐Activated Slags: A 29Si NMR Study

Abstract: A commercial blast furnace slag was activated using either sodium hydroxide or hydrous sodium metasilicate, and the degree of hydration was determined by 29Si magic angle spinning nuclear magnetic resonance (NMR). The results are compared with measurements made using scanning electron microscopy image analysis (SEM-IA). The results from both 29Si NMR and the SEM-IA measurements indicated a fast initial reaction with the alkali, and similar degrees of hydration throughout the reaction. The 29Si NMR results were analyzed using two different methods for fitting the residual slag in the decomposition of the 29Si NMR spectra: the first method used the spectrum of the anhydrous slag, whereas the second method used the spectrum of the dissolution residue of the hydrated sample. Only the first method provided a satisfactory simulation. The degree of hydration and the Al/Si atomic ratio within the C–S–H, deduced by 29Si NMR were in agreement with SEM-IA and EDX analyses.

Remarkably Strong Piezoelectricity of Lead-Free (K0.45Na0.55)0.98Li0.02(Nb0.77Ta0.18Sb0.05)O3 Ceramic

Abstract: Various lead-free (KxNa1−x)0.98Li0.02(Nb0.82−yTa0.18Sby)O3 ceramics with x=0.50, y=0.00–0.07 or x=0.40–0.60, y=0.05 were prepared by the conventional solid-state reaction method. Systematic investigation on the microstructures, crystalline structures, and dielectric and piezoelectric properties was carried out. Remarkably strong piezoelectricity has been achieved in (K0.45Na0.55)0.98Li0.02(Nb0.77Ta0.18Sb0.05)O3 ceramic, which shows the excellent piezoelectric properties of d33=413 pC/N, d31=−153 pC/N, kp=0.50, and k33=0.62. It is considered that the observed strong piezoelectricity should be ascribed to several combined decisive factors, such as the phase coexistence due to an orthorhombic–tetragonal polymorphic phase transition near room temperature, the high electronegativity of Sb5+ ions as compared with those of Nb5+ ions and Ta5+ ions, and the relatively ideal microstructure with high density, large average grain size and narrow grain-size distribution.

Dielectric, Ferromagnetic Characteristics, and Room-Temperature Magnetodielectric Effects in Double Perovskite La2CoMnO6 Ceramics

Abstract: The magnetic, dielectric, and magnetodielectric properties of La2CoMnO6 ceramics were evaluated together with the polarization-electric field hysteresis loops. The phase segregation consisting of ordered and disordered regions was determined in the present ceramics. The ordering of Co2+ and Mn4+ gave rise to ferromagnetic transition temperature as high as 210 K, while the disordering of Co3+ and Mn3+ resulted in low ferromagnetic transition temperature of 80 and 150 K. The relaxor-like behavior combined with a giant dielectric constant (∼105) was determined in La2CoMnO6 ceramics, which was attributed to the charge ordering of Co2+ and Mn4+. Owing to the mutual origin of magnetism and dielectric relaxation, La2CoMnO6 ceramics showed considerable magnetodielectric effects (∼0.8% at 10 kOe) at room temperature.

Relaxor Characteristics of Morphotropic Phase Boundary (Bi1/2Na1/2)TiO3–(Bi1/2K1/2)TiO3 Modified with Bi(Zn1/2Ti1/2)O3

Abstract: Morphotropic phase boundary (Bi1/2Na1/2)TiO3–(Bi1/2K1/2)TiO3 (BNT–BKT), was modified with increasing additions of Bi(Zn1/2Ti1/2)O3 (BZT). Microstructure, electric-field-induced strain and polarization, dielectric permittivity, and temperature-dependent piezoelectric coefficient were investigated and compared with crystal structure measured in situ as a function of applied electric field. Furthermore, permittivity and piezoelectric coefficient were characterized as a function of electric field. For small additions of BZT, an applied electric field leads to an irreversible phase transition into a ferroelectric phase with remanent polarization and a reduced relative permittivity. Increasing the content of BZT increased the threshold field for the transition. For additions of more than 2 mol% BZT, the piezoelectric coefficient dropped, permittivity remained almost constant, and a high normalized strain of up to 500 pm/V was observed. However, no field-dependent structural change was evidenced by the in situ X-ray experiment.

Sintering Temperature Dependence of Energy‐Storage Properties in (Ba,Sr)TiO3 Glass–Ceramics

Abstract: Sintering temperature effects on the energy-storage properties in barium strontium titanate glass–ceramics have been studied by polarization hysteresis measurements. In phase development and microstructure evolution tests, it was found that with the increase of sintering temperature, the crystallinity degree of primary ferroelectric phase increases. Dielectric measurements revealed a rapid increase over the sintering temperature range from 800° to 830°C. This effect is believed to be due to the emergence of ferroelectric phase. The variation in dielectric constant with sintering temperature is attributed to the change in crystallization mechanism between surface and interior of glass–ceramics. Moreover, the charged and discharged energy densities for the glass–ceramic samples sintered at different temperatures were measured by the use of the Sawyer–Tower circuit under unipolar field. It has been shown that the low released energy density in glass–ceramics is mainly caused by interfacial polarization.

High-Temperature Corrosion of EB-PVD Yttria Partially Stabilized Zirconia Thermal Barrier Coatings with an Artificial Volcanic Ash Overlay

Abstract: High-temperature interaction of sol–gel-derived artificial volcanic ash (AVA) matching the bulk composition of the April 15, 2010 Eyjafjallajokull (Iceland) volcanic eruption with a standard 4 mol% (7 wt%) Y2O3-stabilized ZrO2 (YSZ) electron-beam physical vapor deposition (EB-PVD) thermal barrier coating and a corresponding YSZ powder is investigated in order to access possible implications of similar volcanic ashes on the performance of coated turbine engine airfoils. Up to 900°C, AVA deposits and EB-PVD YSZ do not show significant interaction. Viscous flow above the glass transition of AVA (Tg∼930°C) yields proceeding wetting of EB-PVD YSZ coatings. At 1100°C, the YSZ surface is covered by a dense glaze-like AVA overlay. At 1200°C, AVA is mostly infiltrating the coating, leaving a crystalline plagioclase- and hematite-type residue at the interface. Moreover, some ZrSiO4 is formed at the expense of YSZ. The overall thermochemical effects on short-term exposure of the EB-PVD YSZ coating to a small AVA load were moderate, in particular before complete infiltration. On the other hand, AVA acts as a solvent for the stabilizing Y2O3 beyond 1000°C and a progressive depletion of Y2O3 in the YSZ is observed at the AVA/YSZ interface. Detrimental effects on YSZ phase stability and hence coating lifetime cannot be ruled out for long-term exposure and higher AVA loads.