Fabrication of Functionally Graded SiAlON Ceramics by Tape Casting

Functionally graded SiAlON ceramics (FG-SiAlONs) were successfully prepared by the tape casting and lamination approach. Non-aqueous SiAlON slurries with five different α to β-SiAlON ratios (85α:15β, 70α:30β, 55α:45β, 40α:60β, and 25α:75β) were prepared by a 66 methyl ethyl ketone/34 ethanol (vol%) mixture. Phase and microstructure analyses incorporation with hardness measurements clearly show that the FG-SiAlONs, prepared by tape casting, exhibit a continuous and gradual change in composition and hardness. Thus, the tape casting approach is a viable method to produce FG-SiAlONs with a precisely controlled composition, and subsequently properties, as a function of position.     

Prevention of Cracks in Functionally Graded SiO2‐Mo Materials Fabricated by Uniaxial Compression Casting and Pressureless Sintering

In industrial high‐intensity discharge lamps, cracks and delaminations occasionally develop at the interface between SiO₂ and the Mo foil in the seal. Here, functionally graded SiO₂‐Mo materials for use in these lamps were fabricated by uniaxial compression casting and pressureless sintering. Consequently, vertical cracks developed across the sintered body layers, and interfacial cracks developed between the 100 wt% SiO₂ and 90 wt% SiO₂‐Mo layers. Therefore, the effects of residual stress, difference in the coefficient of thermal expansion (CTE), and difference in the volume shrinkage on these cracks were investigated. Vertical cracks were suppressed when residual stress was relaxed by annealing near the annealing point of silica glass during the cooling step in the sintering process. Interfacial cracks were suppressed when the difference in the CTE of the interface between the 100 wt% SiO₂ and 90 wt% SiO₂‐Mo layers was relaxed by inserting layers of 95 wt% SiO₂‐Mo between them. Furthermore, the suppression effect became stronger when the difference in the volume shrinkage of the layers was relaxed by sintering to join the separately sintered monolayers. Thus, the development of these cracks was influenced by the residual stress, CTE, and volume shrinkage. Therefore, these cracks can be prevented by optimizing these factors.     

Numerical Modeling of the Side Flow in Tape Casting of a Non‐Newtonian Fluid

One of the most common ways used to produce multilayer ceramics (MLC) is tape casting. In this process, the dried tape thickness is of great interest to control the desired products and applications. One of the parameters that influences the final tape thickness is the side flow factor (α) which is mostly measured at the end of the process by a volumetric comparison of the tape which flowed outside the casting width to the tape within the casting width. This phenomenon has not been predicted theoretically yet in the literature. In this study, the flow of (La_0.85Sr_0.15)_0.9MnO₃ (LSM) slurry in the tape casting process is modeled numerically with ANSYS FLUENT in combination with an Ostwald‐de Waele power law constitutive equation. Based on rheometer experiments, the constants in the Ostwald‐de Waele power law are identified for the considered LSM material and applied in the numerical modeling. This model is then used for different values of substrate velocity, initial doctor blade height and material load in the reservoir, to investigate their effect on α. It is found that this factor mostly ranges between 0.8 and 0.9. Results of the modeling are compared with experimental findings and good agreement is found.     

Fabrication of Functionally Graded SiO2‐Mo Material by Centrifugation and Floc‐Casting of Highly Concentrated Slurry

To fabricate functionally graded materials, a highly concentrated slurry of SiO₂‐Mo system was prepared and centrifugal force was applied in an attempt to achieve a graded composition. Subsequently, we formed a homogeneous green body with compositional gradation by floc‐casting at 80°C, which was then fired at 1750°C for 10 min in Ar. The sintered body had compositional ratios of SiO₂ and Mo as well as electrical conductivities that changed gradually along the direction of centrifugal force. The results demonstrate that centrifugation and control of slurry characteristics such as flocculation are effective in fabricating functionally graded SiO₂‐Mo materials.     

Translucent Cerium Fluoride Ceramics Fabricated by Hot‐Pressing for Scintillation Application

In this study, translucent CeF₃ ceramics was firstly fabricated by hot‐pressing, aiming to develop an economical, radiation hard scintillation material. After sintering at 950°C and pressure exertion of 300MPa for 2 h, a transmittance of 10–25% was achieved in the visible region, through a thickness of 0.75 mm. Microstructure investigations indicated the average grain size was around 42.7 μm, with no obvious pores observed on the fracture surface. The X‐ray luminescence spectra, decay time, and thermoluminescence spectra were measured, and the results of the ceramics were similar to its corresponding single crystals, which proved it to be a promising scintillation material.     

Porous yttria‐Stabilized Zirconia Ceramics Fabricated by Nonaqueous‐Based Gelcasting Process with PMMA Microsphere as Pore‐Forming Agent

Porous yttria‐stabilized zirconia (YSZ) ceramics were fabricated using tert‐butyl alcohol (TBA)‐based gelcasting with monodisperse polymethylmethacrylate (PMMA) microspheres as both pore‐forming agent and lubricant agent. The TBA‐based slurry of 50 vol% solid loading with excellent rheological properties appropriate for casting was successfully prepared by using a commercial polymer dispersant DISPERBYK‐163 as both dispersant and stabilizer. The distribution of the spherical pores made from PMMA microspheres was very homogeneous. Their average diameter decreased from 16.9 to 15.7 μm when the sintering temperature was increased from 1350°C to 1550°C. The compressive strength increased from 14.57 to 142.29 MPa and the thermal conductivity changed from 0.17 to 0.65 W/m·K when the porosity decreased from 71.6% to 45.1%. The results show that this preparation technology can make all the main factors controllable, such as the porosity, the size and shape of pores, the distribution of pores, and the thickness and density of pore walls. This is significant for fabricating porous ceramics with both high compressive strength and low thermal conductivity.     

Low‐Temperature Magnetic and Dielectric Anomalies in Rare‐Earth‐Substituted BiFeO3 Ceramics

In this work, we report on the magnetic and dielectric anomalies observed in dense Bi_1–xRE_xFeO₃ ceramics (RE = Dy, Tb; 0 ≤ x ≤ 0.3) at cryogenic temperatures. For compositions with a high content of rare‐earth ions, thermomagnetic experiments revealed a distinct anomaly in the magnetization curves at temperatures below 200 K. The temperature of the magnetic anomaly along with a thermal hysteresis was found to be dependent on the rare‐earth concentration and magnetic field strength. Low‐temperature dielectric measurements showed an anomalous relaxor‐like behavior of the relative permittivity and dielectric loss in highly doped ceramic samples. The anomalies in low‐temperature magnetization and dielectric response are suggested to result from the presence of GdFeO₃‐like orthoferrite phase and/or bismuth rare‐earth‐mixed iron garnet impurities.     

Ceramic tubular MOF hybrid membrane fabricated through in situ layer‐by‐layer self‐assembly for nanofiltration

Nanofiltration has been playing an important role in water purification, in which the developments of novel membrane materials and modules are among significant. Herein, a metal‐organic framework (MOFs) hybrid membrane, ZIF‐8/PSS was fabricated on a tubular alumina substrate through a layer‐by‐layer self‐assembly technique. ZIF‐8 particles in situ grow into PSS layers to improve their compatibility and dispersion, thereby getting high quality membrane, which was loaded into a steel tubular module for nano‐filtrating dyes from water. Under optimized conditions, it shows outstanding nanofiltration properties toward methyl blue, with the flux of 210 Lm^?2 h^?1 MPa^?1 and the rejection of 98.6%. Furthermore, the good pressure resistance ability and running stability of the membrane were revealed, which can be attributed to use the ceramic substrate and the inherent stability of ZIF‐8. This work thus illustrates a simple approach for fabricating MOFs hybrid membranes on tubular ceramic substrates, having great potential for industrial applications. © 2015 American Institute of Chemical Engineers AIChE J, 62: 538–546, 2016     

Room-Temperature Freeze Casting for Ceramics with Nonaqueous Sublimable Vehicles in the Naphthalene–Camphor Eutectic System

Freeze casting for Al₂O₃ was accomplished at room temperature with nonaqueous sublimable vehicles in the naphthalene–camphor eutectic system with a eutectic temperature of 31°C. A fully dense sintered body (>99.5% of theoretical density (T.D.)) was obtained with a eutectic composition vehicle, whereas at most 90% T.D. was obtained with an off-eutectic (i.e., hypo- or hypereutectic) composition vehicle due to formation of large uniquely shaped voids. Microstructural observation suggested that growing pro-eutectic crystals rejected the suspended Al₂O₃ particles to form large voids during the solidification process. At the eutectic composition, formation of fine lamellar microstructure in a solidified vehicle is considered to inhibit particle rejection resulting in large voids.     

Interaction of BN‐ZrO2‐SiC Ceramics with Inclusions in Si‐killed Steel

Boron nitride (BN)‐based ceramics are often used as containment refractory in thin‐strip steel production. The interaction of this material (BN‐ZrO₂‐SiC:‐MYCROSINT^® SO43) was studied using a steel‐inclusion system in a laboratory setup. Isothermal interaction tests were performed to evaluate the interaction of inclusions with this refractory. It was determined that corrosion of the BN refractory occurs through a dissolution mechanism and it was more severe in contact with steel compared to slag. A thermodynamic model was developed using FactSage to study the change in liquid steel/slag composition with refractory interaction. Thermodynamic predictions were in agreement with the experimental data obtained through isothermal interaction tests.     

Three‐dimensional Ceramic/Camphene‐based Coextrusion for Unidirectionally Macrochanneled Alumina Ceramics with Controlled Porous Walls

We report the utility of three‐dimensional ceramic/camphene‐based coextrusion, newly developed in this study, for the production of unidirectionally macrochanneled alumina ceramics with three‐dimensionally interconnected porous alumina walls. In this technique, a continuous ceramic/camphene filament with a diameter of 1 mm, comprised of a pure camphene core and a frozen alumina/camphene shell, was produced by the coextrusion process and then deposited in a layer‐by‐layer sequence using a computer‐controlled 3‐axis moving table. Unidirectionally aligned macrochannels (~400 μm in diameter) and three‐dimensionally interconnected pores (several tens of micrometers in size) in the alumina walls were created by removing the camphene core and the camphene dendrites formed in the alumina/camphene region, respectively. The sample showed much higher compressive strength in the macrochannel direction than in the perpendicular direction. In addition, the compressive strength of the sample could increase with an increase in initial alumina content owing to a decrease in the total porosity.     

Ultra High Energy‐Storage Density in the Barium Potassium Niobate‐Based Glass‐Ceramics for Energy‐Storage Applications

The barium potassium niobate‐based glass‐ceramics with high energy‐storage density, high discharge efficiency, and fast discharge speed have been prepared. It was found that dielectric breakdown strength decreases when the crystallization temperature increases. Glass‐ceramics have high energy‐storage density up to 14.58 ± 1.14 J/cm³ with high breakdown strength of 2382 ± 92 kV/cm. Discharge energy density and discharge efficiency of glass‐ceramic capacitor were achieved through a pulse charge–discharge circuit. The reduction of discharge efficiency with the increase of crystallization temperature is mainly caused by interfacial polarization.     

Evaluating Pore Space in Macroporous Ceramics with Water‐Based Porosimetry

We show that water‐based porosimetry (WBP), a facile, simple, and nondestructive porosimetry technique, accurately evaluates both the pore size distribution and throat size distribution of sacrificially templated macroporous alumina. The pore size distribution and throat size distribution derived from the WBP evaluation in uptake (imbibition) and release (drainage) mode, respectively, were corroborated by mercury porosimetry and X‐ray micro‐computed tomography (μ‐CT). In contrast with mercury porosimetry, the WBP also provided information on the presence of “dead‐end pores” in the macroporous alumina.     

Layer Structure of Textured CaBi4Ti4O15 Ceramics Fabricated by Slip Casting in High Magnetic Field

Textured CaBi₄Ti₄O₁₅ (CBT) ceramics were fabricated by slip casting in a high magnetic field of 12 T, and a layer structure was obtained by co-firing with the textured CBT ceramics and a metal internal electrode. The crystalline a ( b )-axis of CBT was aligned parallel to the magnetic field, and highly a ( b )-axis textured ceramics were obtained. The orientation degree of the textured ceramics was 0.82, and the electromechanical coupling coefficient of thickness extensional (TE) vibration was about 1.5 times larger than that of the randomly oriented ceramics. The second harmonic TE vibration was successfully generated in the layer structure specimens, which contained two layers of the textured ceramics and an internal electrode. Therefore, it was recognized that the layer structure was satisfactorily fabricated.     

Recrystallization of Er3+:CaF2 in Transparent Fluorophosphate Glass‐Ceramics with the Co‐Firing Method

Transparent glass‐ceramics containing Er³⁺:CaF₂ crystallites were prepared with the co‐firing method. The formation process of the glass‐ceramics was investigated by means of SEM, XRD, and DSC. The results reveal that the Er³⁺:CaF₂ nanocrystals do not dissolve into the fluorophosphates (FP) glassmelt until the co‐firing temperature increase higher than about 920°C. Below this temperature, Er³⁺:CaF₂ survives the co‐firing process and the nanocrystals just grow to spherical crystals of micrometers in size. Co‐firing temperature higher than this temperature leads to the dissolution of Er³⁺:CaF₂ and the dissolved Er³⁺:CaF₂ recrystallized during quenching process and takes the shape of dendrite.     

Extension of Rapid Sizing Algorithm for Shell‐and‐Tube Heat Exchangers with Tube‐Side Pressure Drop Constraint and Multipasses (Part B)

There are occasions in which process engineers need approximate estimates of the exchanger size rather than full detailed designs. These situations are often accompanied by a need for speed. This paper provides a simple procedure for exchanger sizing which is rapid and with reasonable accuracy. The methodology involved is described in detail here and it is shown that the procedure can be applied to pure counterflow and also for multipass exchangers.     

Creating a Protective Shell for Reactive MoSi2 Particles in High‐Temperature Ceramics

Alumina encapsulated molybdenum silicide (MoSi₂) intermetallic particles were synthesized using a simple precipitation method followed by calcining at temperatures of 800°C–1000°C, to prevent the premature oxidation of MoSi₂ at high temperatures. The shell composition and the influence of the calcining temperature on microcapsule integrity were investigated by means of X‐ray photoelectron spectroscopy, X‐ray diffraction, scanning electron microscopy, and thermogravimetric analysis. The results demonstrate that the composition and the mechanical stability of the alumina shell can be tuned by the annealing temperature. After calcining at 800°C and 850°C the alumina shell remains intact. Calcining at higher temperature promotes the formation of mullite, which leads to cracking of the shell. However, when annealed at 1000°C for 24 h these cracks were filled with mullite and preserved the molybdenum silicide particles. Furthermore, the mechanical stability of the shell was improved by applying an intermediate calcining treatment at 450°C prior to the annealing process at 1000°C.     

Influence of Aggregates in α‐Al2O3 Slurry on Orientation Degree of Powder Compact Fabricated by Magnetic Forming Method

The crystallographic orientation of α‐Al₂O₃ powder compact under a magnetic field is affected by the presence of aggregates. In this work, the influence of aggregates on the orientation degree was studied from the experimental and theoretical viewpoints. The optical parameter of birefringence was used as an evaluation index for the crystallographic orientation of α‐Al₂O₃ compact samples. The effect of aggregates on the orientation degree was estimated from the simulation of the distribution of the relative angles between the crystal axes of primary particles for any constituting number of aggregates. The orientation of an aggregate depends on the apparent crystal axes calculated from the vector summation of the crystal axes for all constituting particles. The measured degree of orientation of the powder compact was consistent with the theoretical values for all powder compact samples containing various concentrations and constituting numbers of aggregates. The elimination of aggregates is the key for good particle alignment under a magnetic field.     

Synthesis and Electronic Spectra of Substituted p‐Distyrylbenzenes for the Use in Light‐Emitting Diodes

The influence of substitution on the absorption and Luminescence spectra of oligo(phenylenevinylene)s has been studied using distyrylbenzene (DSB) as a model compound. The degree, character, and pattern of substitution was varied systematically, altering the electronic properties of the DSB, the wavelength of the emitted light could be tuned over a range of 100 nm. The syntheses of 6b—h were performed by twofold Wittig Horner‐olefinations of bisphoshonates 1a, b with substituted benzaldehydes 2a—i, 6i via Heck‐reaction of the dibromosulfonylbenzene 3, 6k by Siegrist‐reaction of 4 with N‐phenylbenzaldimine and the Knoevenagel‐reaction of benzyl cyanide with 5 led to 6l .     

Kinetics of Corrosion of BN–ZrO2–SiC Ceramics in Contact with Si‐Killed Steel

The mechanism and kinetics of corrosion of BN–ZrO₂–SiC (MYCROSINT^®SO43) by molten Si‐killed steel was studied. Isothermal corrosion tests were performed for duration between 2 and 8 h. Refractory and steel composition and morphology changes were investigated. A kinetic model using process simulation software METSIM and thermochemical software FactSage was developed to understand refractory–steel interactions. The corrosion process showed a deviation from parabolic kinetics and was fitted by a combination of linear and parabolic terms. It was determined that corrosion of the BN–ZrO₂–SiC refractory was governed by dissolution of SiC and BN and removal of ZrO₂ as the other phases were eliminated.