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.     

Mechanical Properties of Gd‐CeO2 Electrolyte for SOFC Prepared by Aqueous Tape Casting

Mechanical properties of gadolinium‐doped ceria (Ce_0.9Gd_0.1O_1.95, 10GDC) green tape prepared by aqueous‐based tape casting process were characterized by tensile test and shear punch test (SPT). SPT was found to be a useful method for characterizing mechanical properties of green tapes. Microstructures and mechanical properties such as flexural modulus, bending strength, and microhardness of tapes sintered at 1,300–1,500 °C have been evaluated. Indentation fracture toughness was also determined by the method of Palmqvist cracks at different applied loads for tapes sintered at 1,500 °C. Grain size measurements showed that excessive grain growth occurred during sintering despite using 10GDC nanopowders as the starting material. However, mechanical properties of sintered tapes improved by increasing sintering temperature and the results are comparable with those reported for 10GDC in literature.     

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.     

Performance Evaluation of Solid Oxide Fuel Cells with Thin Film Electrolyte Fabricated by Binder‐Assisted Slurry Casting

A gas‐tight yttria‐stabilized zirconia (YSZ) electrolyte film was fabricated on porous NiO–YSZ anode substrates by a binder‐assisted slurry casting technique. The scanning electron microscope (SEM) results showed that the YSZ film was relatively dense with a thickness of 10 μm. La_0.8Sr_0.2MnO₃ (LSM)–YSZ was applied to cathode using a screen‐print technique and the single fuel cells were tested in a temperature range from 600 to 800 °C. An open circuit voltage (OCV) of over 1.0 V was observed. The maximum power densities at 600, 700, and 800 °C were 0.13, 0.44, and 1.1 W cm^–2, respectively.     

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.     

Layer‐by‐layer modification of porous polybenzoxazine with silver nanoparticles for enhanced CO2 storage

Polybenzoxazine (PBZ) xerogels have been synthesized via quasi solventless method and coated with silver nanoparticles using the layer‐by‐layer (LbL) deposition method. After coating, the samples were carbonized at 800 °C to obtain high surface area porous carbon materials to be used for CO₂ storage. Evidences of the successful LbL deposition of the coating was provided by ultraviolet–visible and attenuated total reflection–Fourier transform infrared spectroscopy and the silver nanoparticles top layer was confirmed by scanning electron microscopy–energy‐dispersive X‐ray spectroscopy, transmission electron microscopy, X‐ray diffraction, and X‐ray photoelectron spectroscopy. Results showed that the samples coated with silver nanoparticles displayed an increased CO₂ capacity from 3.02 to 3.39 mmol g^?1 when compared with the plain carbon PBZ. The LbL method for the modification of the pore surface in porous PBZ is simple and allows the facile tuning of the inner PBZ pore's surface chemistry with metallic nanoparticles that could be enhanced CO₂ storage capacity. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134 , 45097.     

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     

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.     

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.     

Layer‐by‐layer self‐assembled multilayer films of single‐walled carbon nanotubes and tin disulfide nanoparticles with chitosan for the fabrication of biosensors

In this study, multilayer films containing chitosan, tin disulfide (SnS₂) nanoparticles, and single‐walled carbon nanotubes were prepared on glassy carbon electrodes with the use of a layer‐by‐layer assembly technique. The resulting films were characterized with cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), scanning electron microscopy, and ultraviolet–visible absorption spectroscopy. The results of CV and EIS indicates that the peak currents and charge‐transfer resistance all had linear responses to the number of assembled layers. The multilayer‐film‐modified electrode showed excellent electrocatalytic properties for some species, such as dopamine hydrochloride (DA), ascorbic acid (AA), and uric acid (UA). The well‐separated voltammetric signals of DA, UA, and AA could be obtained on the assembled multilayer‐film‐modified electrode, and the peak‐to‐peak potential separations were 171, 136, and 307 mV for DA–UA, DA–AA, and UA–AA on CV, respectively. These facts showed that the multilayer‐film‐modified electrode could be used as a new sensor for the simultaneous detection of DA and UA in the presence of AA in a real sample. In addition, the multilayer films were stable, selective, and reproducible. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

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.