Aqueous and Nonaqueous Colloidal Processing of Difficult‐to‐Densify Ceramics: Suspension Rheology and Particle Packing

Aqueous and nonaqueous colloidal processing of zirconium diboride (ZrB₂) and boron carbide (B₄C) has been investigated. The aqueous and nonaqueous ZrB₂ and B₄C suspension formulations have been optimized. The suspensions were cast into green bodies using slip casting. The correlation between the state of dispersion with the rheological properties of the suspensions and the resulting packing density was observed in both aqueous and nonaqueous processing. The attractive interactions between powder particles in water were difficult to overcome with electrical double layer or electrosteric repulsion. Reasonably low viscosity aqueous ZrB₂ suspensions up to 45 vol% solids could be prepared. It was not possible to produce low viscosity (viscosity below 1 Pa·s at shear rate of 100 s^?1) aqueous B₄C suspensions with solid content above 30 vol%. Slip casting of the weakly aggregated ZrB₂ suspensions resulted in low packing densities (~55% relative density) of the green bodies. On the other hand, dispersion of powder particles in nonaqueous media (cyclohexane and dodecane) enabled suspensions with lower viscosities and a higher maximum solid concentration (up to 50 vol%) to be prepared. The well‐dispersed nonaqueous suspensions promoted an efficient particle packing, resulting in higher green densities (64% and 62% relative density for ZrB₂ and B₄C, respectively) compared to aqueous processing. The significantly high green densities are promising to allow densification of the materials at lower sintering temperature.     

Colloidal Processing of Zirconium Diboride Ultra‐High Temperature Ceramics

Colloidal processing of the Ultra‐High Temperature Ceramic (UHTC) zirconium diboride (ZrB₂) to develop near?net‐shaping techniques has been investigated. The use of the colloidal processing technique produces higher particle packing that ultimately enables achieving greater densification at lower temperatures and pressures, even pressureless sintering. ZrB₂ suspension formulations have been optimized in terms of rheological behavior. Suspensions were shaped into green bodies (63% relative density) using slip casting. The densification was carried out at 1900°C, 2000°C, and 2100°C, using both hot pressing at 40 MPa and pressureless sintering. The colloidally processed materials were compared with materials prepared by a conventional dry processing route (cold pressed at 50 MPa) and subjected to the same densification procedures. Sintered densities for samples produced by the colloidal route are higher than produced by the dry route (up to 99.5% relative density by hot pressing), even when pressureless sintering is performed (more than 90% relative density). The promising results are considered as a starting point for the fabrication of complex‐shaped components that can be densified at lower sintering temperatures without pressure.     

Pressureless Sintering of ZrB2 Prepared by Colloidal Processing: Particle Packing,Sintering Conditions,and Additives

Nonaqueous colloidal processing in cyclohexane and slip casting allowed the preparation of green ZrB₂ compacts with 67.3% TD (theoretical density), which upon pressureless sintering at 2100°C densified to a final density of 92.4% TD. The effect of controllable sintering parameters (temperature, time, and heating rate) on the sintered properties of ZrB₂ compacts prepared using the nonaqueous colloidal processing method was also studied, to maximize the sintered density and tailor the desired microstructure (i.e., dense final object with small grain size). The competition between pore elimination and grain growth mainly controls the final density achievable. ZrB₂ compacts with maximum sintered densities of 93% TD could be achieved by controlling the sintering conditions. Finally, the use of carbon and B₄C additives to improve the sintered density and microstructure of ZrB₂ compacts was also investigated. The additives, compatible with colloidal processing, allowed compacts with greater sintered densities (96–98% TD) and finer grain sizes to be achieved by pressureless sintering, even at a lower temperature of 1900°C.     

Ice templating of ZrB2 porous architectures

For the first time, freeze casting of aqueous ZrB₂ based suspension was performed in order to produce porous architectures with main unidirectional anisotropic pores interconnected by diffuse globular-isotropic pores. The porosity characterizing the green bodies after the sublimation process is the replica of the ice crystals. The effects of solid content, suspension stabilization and cold transmission depending on mold type were studied since micro and macrostructure and, hence, the properties of the sintered body are determined by controlling the growth direction of the ice crystals during freezing. Improved mechanical performances are obtained by changing the above mentioned parameters, in virtue of the formation of thinner and more homogenously distributed ceramic lamellae. Possible applications of these materials include high temperature porous volumetric absorbers for concentrating solar power systems.     

Microstructure evolution of a W‐doped ZrB2 ceramic upon high‐temperature oxidation

This basic research deals with the microstructure evolution of a W‐doped ZrB₂ ceramic, as‐sintered and upon oxidation at 1650°C. Transmission electron microscopy enabled to disclose microstructural features occurred during oxidation never observed before. In the pristine material, (Zr,W)B₂ solid solutions surround the original ZrB₂ nuclei, whereas refractory W‐compounds at triple junctions and clean grain boundaries are distinctive of this ceramic. After oxidation, the microstructure is typified by intragranular nanostructures, in which nanosized W inclusions remained trapped within ZrO₂ grains, or decorate their surfaces. The understanding of the oxidation reactions occurring in the system as a function of the oxygen partial pressure was fundamental to conclude that W‐based compounds do not notably suppress or retard the oxidation of ZrB₂ ceramics.     

Producing Large Complex‐Shaped Ceramic Particle Stabilized Foams

Highly porous ceramic foams can be produced by combining particle stabilized foams and gelcasting concepts. Sulfonate‐type surfactants are selected to weakly hydrophobize the alumina surface and stabilize air bubbles in suspensions containing gelcasting additives, polyvinyl alcohol (PVA), and 2,5‐dimethoxy‐2,5‐dihydrofurane (DHF). The aim of this work was to prepare large complex‐shaped ceramic foam objects with homogeneous microstructure and high porosity. A key to avoiding drying cracks is to strengthen the wet green body via gelcasting. The influence of the amount of gelcasting additives on the mechanical strength of the ceramic foam green bodies is investigated as well as the effect of using cross‐linking agent versus the addition of just a binder. The presence of a cross‐linked polymeric network within the green body increases its mechanical strength and minimizes crack formation during drying.     

Zirconium Diboride with High Thermal Conductivity

Thermal properties were characterized for zirconium diboride produced by reactive hot pressing and compared to ZrB₂ ceramics that were hot pressed from commercial powders. No sintering additives were used in either process. Thermal conductivity was calculated from measured values of heat capacity, thermal diffusivity, and density for temperatures ranging from 298 to 2273 K. ZrB₂ produced by reactive hot pressing achieved near full density, but had a small volume fraction of ZrO₂, whereas hot‐pressed ZrB₂ contained porosity and carbon inclusions. Reactive hot pressing produced a ceramic with higher thermal diffusivity and heat capacity, resulting in thermal conductivities of 127 W·(m·K)^?1 at 298 K and 80 W·(m·K)^?1 at 2273 K, which were up to ~30% higher than typically reported for hot‐pressed ZrB₂.     

Finite element modeling on the effect of intra‐granular porosity on the dielectric properties of BaTiO3 MLCCs

The effect of porosity on the electrical properties of BaTiO₃‐based Multilayer Ceramic Capacitors (MLCCs) is studied. A dense ceramic prepared via powder from a solid‐state processing route is compared against a ceramic that contains intra‐granular pores from powder prepared via hydrothermal processing. Finite element models are created to contain intra‐granular pores, solved and analyzed to show an increase in the electric field and current density surrounding the pores. For single‐pore and two intra‐pore arrangements, the electric field is enhanced by a factor of ~1.5 and 2.5, respectively, when compared to a fully dense (pore‐free) material. For ceramics with equivalent density, the number of pores dramatically alters the electrical response. For a system containing 100 pores, the electric field can increase at least fourfold, therefore facilitating a possible starting route for electrical breakdown of the grain. These results are compared to the Gerson‐Marshall model, typically used in the literature for the calculation of the breakdown strength due to porosity. The results highlight the need to include the effect of adjacent pore interactions. Although studied here for BaTiO₃‐based MLCC's the results are applicable to other devices based on ceramics containing porosity.     

Preparation and freezing behavior of TiO2 nanoparticle suspensions

Freeze casting can prepare porous materials with high porosity, directional pores, and complex shapes. However, due to the difficulty of obtaining nanoparticle suspensions with high solids loading, the formed bodies usually experience large shrinkage and have low strength in the process of vacuum drying and heat treatment. To address these problems, we studied the zeta potential, agglomeration, and rheological property of commercial P25 TiO₂ nanoparticle suspensions by adjusting the pH value and the sodium hexametaphosphate additive amount of the suspensions. Suspensions with up to 30 vol% solids loading were prepared. The water freezing process and the directional arrangement of the pores are influenced by freezing temperature gradient, and porous TiO₂ samples with directional laminar structures are obtained.     

Joining of Cf-SiC composites and ZrB2-SiC based composites for ultra high temperature applications

Gas tungsten arc welding (GTAW) of Cf–SiC composites to themselves and to ZrB₂-SiC based composites have been carried out with a filler material of (ZrB₂-SiC-B₄C-Y₂O₃-Al₂O₃) composite. The weld interfaces of joints of composites were clean and free from porosity and cracks. Penetration of filler material into voids and pores existing in the Cf-SiC composites was observed. An average shear strength of 25.7?MPa was achieved. The ZrB₂-SiC based composite joined to Cf-SiC (CVD) composite was exposed for 300?s to the oxy-propane flame at 2300?°C. The joint and interfaces between the filler material and parent composites were found to be unaffected by thermal cycling and oxidation during the exposure to the oxy-propane flame.     

Processing of Hierarchical and Anisotropic Porosity LSM‐YSZ Composites

Aqueous dispersions of lanthanum strontium manganite (LSM) and yttria‐stabilized zirconia (YSZ) particles were controllably freeze‐cast and then partially sintered resulting in anisotropic, hierarchically porous ceramics for Solid Oxide Fuel Cell (SOFC) cathodes. The resulting microstructures have aligned pores with a characteristic spacing (λ) between pore centers. The effect of freezing rate, slurry viscosity, and solid loading on solidification velocity and resultant microstructures was explored. Varying these parameters resulted in samples with a range of independently controllable and reproducible microstructures. Homogenous dispersion of LSM and YSZ in the freeze‐cast structures was confirmed through elemental mapping. Freezing rate was found to have a significant effect on λ while solid loading affected overall porosity and ceramic wall‐ to‐pore size ratio but had only a small influence on λ. Viscosity was found to have a complex albeit small impact on λ but a significant effect on particle dispersion and colloid stability.     

Hybrid Microwave Sintering of Infrared Transparent Nano‐Y3Al5O12 Synthesized by a Modified Combustion Technique

Infrared transparent ceramics found to have numerous civilian and defense applications. In the present work, Y₃Al₅O₁₂ nanoparticles were synthesized by an auto‐igniting modified single‐step combustion method. The structure and morphology of the as‐prepared powder revealed the phase purity and ultrafine nature of the powder having an average crystallite size of 16 nm and well‐defined lattice planes. Coupling of the resistive and microwave heating at precise proportion leads to a sintered density of the powder with 99.3% of the theoretical density at a temperature as low as 1470°C for a soaking duration of just 20 min. Marked reduction in grain size and the porosity was also observed for the hybrid sintered pellets. An average grain size of 167 nm was measured for the sintered pellets, which also showed a high transmittance of 80% in the UV–vis region and 82.5% in the mid‐IR region.     

New Sintered Li2O–Al2O3–SiO2 Ultra‐Low Expansion Glass‐Ceramic

We developed a new Li₂O–Al₂O₃–SiO₂ (LAS) ultra‐low expansion glass‐ceramic by nonisothermal sintering with concurrent crystallization. The optimum sintering conditions were 30°C/min with a maximum temperature of 1000°C. The best sintered material reached 98% of the theoretical density of the parent glass and has an extremely low linear thermal expansion coefficient (0.02 × 10^?6/°C) in the temperature range of 40°C–500°C, which is even lower than that of the commercial glass‐ceramic Ceran^® that is produced by the traditional ceramization method. The sintered glass‐ceramic presents a four‐point bending strength of 92 ± 15 MPa, which is similar to that of Ceran^® (98 ± 6 MPa), in spite of the 2% porosity. It is white opaque and does not have significant infrared transmission. The maximum use temperature is 600°C. It could thus be used on modern inductively heated cooktops.     

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.     

Ammonium hydrogen carbonate provided viscous slurry foaming—A novel technology for the preparation of porous ceramics

In current research a novel technology for the preparation of porous ceramics was developed. The ammonium hydrogen carbonate was used as foaming agent for the generation of pores in the glycerol-based viscous ceramic slurry. Total and open porosity of obtained ceramics depends on the amount and granulometric distribution of NH₄HCO₃ as well as particle size of HAp powder used for the preparation of viscous slurry. Varying amount of NH₄HCO₃ in the range from 0 to 2.75 wt.%, open and total porosity increased from 25 to 69% and from 32 to 73% respectively. The formation of well-connected open porosity with irregularly shaped pores was observed for sintered ceramics.     

Microstructure and indentation damage resistance of ZrB2‐20 vol.%SiC ipo‐eutectic composites

Zirconium diboride with 20 vol.% silicon carbide bulk composites were fabricated using directionally solidification (DS) and also by spark plasma sintering (SPS) of crushed DS ingots. During the DS the cooling front aligned the c‐axis of ZrB₂ grains and its Lotgering factor of f_(00l) was high as 0.98. The Vickers hardness was anisotropic and it was high as 17.6 GPa along the c‐axis and 15.3 GPa when measured in an orthogonal direction. On both surfaces, even when using 100 N indentation load, no cracks were observed, suggesting a very high resistance to crack propagation. Such anomalous behavior was attributed to the hierarchical structure of DS sample where the ZrB₂ phase was under strong compression and the SiC phase was in tension. In the SPSed sample, the microstructure was isotropic respect to the direction of applied pressure. Indentation cracks appeared around the indent corners but not emanated from the diagonals, confirming high damage resistance.     

Foam glass obtained through high‐pressure sintering

Foam glasses are usually prepared through a chemical approach, that is, by mixing glass powder with foaming agents, and heating the mixture to a temperature above the softening point (10^6.6 Pa s) of the glass. The foaming agents release gas, enabling expansion of the sintered glass. Here, we use a physical foaming approach to prepare foam glass. First, closed pores filled with inert gases (He, Ar, or N₂) are physically introduced into a glass body by sintering cathode ray tube (CRT) panel glass powder at high gas pressure (5‐25 MPa) at 640°C and, then cooled to room temperature. The sintered bodies are subjected to a second heat treatment above the glass transition temperature at atmospheric pressure. This heat treatment causes expansion of the pores due to high internal gas pressure. We found that the foaming ability strongly depends on the gas pressure applied during sintering, and on the kinetic diameters of the gases. The pressure for attaining maximum expansion, that is, lowest density and highest porosity, is found to be around 20 MPa.     

Mechanical Properties and Reliability of Li2O‐Stabilized β″‐Alumina Ceramics: Effect of Synthesizing Method

Li₂O‐stabilized β″‐alumina powder has been synthesized by the solid‐state and double‐zeta processes. It was shown that by the double‐zeta process, the β″‐alumina fraction of sintered samples was 10% higher, showing around 99% of β″‐alumina fraction. Using β″‐alumina powder produced by the double‐zeta process, the sintered density improved, microstructure was more uniform and leads to improvement in hardness, strength and Weibull modulus due to more uniform microstructure and absence of abnormal grain growth. The higher fracture toughness of the solid‐state‐processed samples could be due to crack deflection mechanism.     

Physical and mechanical properties of the fully interconnected chitosan ice‐templated scaffolds

Porous chitosan scaffolds were prepared with a freeze‐casting technique with different concentrations, 1.5 and 3 wt %, and also different cooling rates, 1 and 4°C/min. The pore morphology, porosity, pore size, mechanical properties, and water absorption characteristics of the scaffolds were studied. Scanning electron microscopy images showed that the freeze‐cast scaffolds were fully interconnected because of the existence of pores on the chitosan walls in addition to many unidirectionally elongated pores. Increases in the chitosan concentration and freezing rate led to elevations in the thickness of the chitosan walls and reductions in the pores size, respectively. These two results led to the enhancement of the compressive strength from 34 to 110 kPa for the scaffolds that had 96–98% porosity. Also, augmentation of the chitosan concentration and decreases in the freezing rate led to the reduction of the number of pores on the chitosan walls. Furthermore, the volume of water absorption increased with a reduction in the chitosan concentration and cooling rate from 690 to 1020%. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41476.     

One‐ or two‐dimensional channel structures and properties of piezoelectric composites via freeze‐casting

By employing carefully tailored tert‐butyl alcohol (TBA)‐based freeze‐casting parameters, a large amount of porosity (>70 vol%) and one or two‐dimensional pore channels created were produced into alkali niobate‐based (NKN) ceramics. The relationship between processing factors and microstructures has here been studied, in terms of (i) porosities controlled by adjusting the solid loading in the initial slurry and (ii) strategically attempted freezing direction to make varied pore channels, in which two freezing directions from the bottom or side of mold can produce unidirectional elongated and radially centrosymmetric microstructures, respectively. In addition to that, NKN/epoxy composites with 3‐1 or 3‐2 type polymer channels in the NKN matrix have been fabricated by infiltration of the polymer into the porous NKN hosts. The effect of the channel directions on the mechanical and piezoelectric properties of the composites was investigated for varied volume fractions of the active ceramic phase, mechanical loading, and the poling direction, leading to very high‐piezoelectric g₃₃ coefficients at >60 mV·m/N in the composites with unique channel structures.