Highly Porous Zirconia Ceramic Foams with Low Thermal Conductivity from Particle‐Stabilized Foams

Zirconia ceramic foams with ultra‐high porosity of 96%–98% have been fabricated using sodium dodecyl sulfate (SDS) as the particle stabilizer of zirconia particles for the first time. The wet foams stabilized by zirconia particles are ultra‐stable due to partially hydrophobic zirconia particles modified by SDS. Zirconia foams exhibit close cells with thin cell wall and small grain size. Increasing SDS concentration favors the foamability of the suspension, and further increases the porosity of ceramic foams. Zirconia ceramic foams with porosity of 98.1% have compressive strength of 0.26 ± 0.05 MPa. Decreasing solid loading leads to the porosity of ceramic foams. The compressive strength could be improved significantly by increasing the sintering temperature. Zirconia ceramic foams with porosity of 97.9% has low thermal conductivity of 0.027 ± 0.004 W·(m·K)^?1, which could be used as thermal insulation and refractory material.     

Producing Ceramic Foams with Hollow Spheres

Ceramic foams have a large potential for many applications and can be produced in several ways. In the past, many have attempted to manufacture ceramic foams that combined sufficient strength and controlled microstructure. This study proposes a new two-step processing route to fulfill these requirements. First, sacrificial cores are coated with ceramic powder slurry and packed in a die. Second, the cores are connected together using another ceramic slurry coating. After they are burned out and sintered, ceramic foam structures can be obtained that have a density <10% of theoretical density. By varying the size and shape of the initial cores, ceramic foams with tailor-made microstructures can be easily produced.     

Ultralight Silicon Nitride Ceramic Foams from Foams Stabilized by Partially Hydrophobic Particles

Ultralight Si₃N₄ ceramic foams have been successfully prepared through particle‐stabilized foams method, which is based on the adsorption of in situ hydrophobized Si₃N₄ particles to the liquid/air interface of the foams. Here, we firstly used a long‐chain surfactant cetyltrimethylammonium chloride to render the Si₃N₄ particles partially hydrophobic. By tailoring the surfactant concentration and pH values of the suspensions, the wet foams were stabilized to avoid coarsening and coalescence. SEM results show that the Si₃N₄ ceramic foams possess single strut walls with elongated β‐Si₃N₄ grains interlocking with each other, and their pores are uniform with an average pore size of 95 μm. The obtained ceramic foams maintain compressive strength of 1.34 ± 0.13 MPa with porosity of 92.0%, when the suspension contains 3 mmol/L surfactant at the pH of 11.0.     

Processing,Characterization, and Modeling of Room‐Temperature‐Vulcanized Silicone‐Derived Ceramic Foams

A novel method was developed to produce ceramic foams from a silicone precursor which was foamed and vulcanized at room temperature. Silicone foams were prepared by platinum‐catalyzed cross‐linking and dehydrogenation of reactive polysiloxanes. Silicone foams were converted to ceramic foams after being pyrolyzed at 1200°C in argon. Near‐net‐shape polymer‐to‐ceramic conversion was achieved when SiC particles were added to the polymer as a solid filler. A simple physical model was created to describe the rising and pyrolysis of the silicone foam, and was validated by experimental data. Foam density was largely dependent on the content of ethanol, which was used as a chemical blowing agent. Up to 1.8 wt% ethanol was effective in driving foam rising without leading to foam collapse. SiC filler helped reduce weight loss and volumetric shrinkage during pyrolysis, and slightly increased foam density. Scanning electron microscopy indicated that although incorporating a solid filler helps to reduce the bulk shrinkage, it cannot prevent local microcracking and residual porosity.     

Processing and Properties of Porous PZT Ceramics from Particle‐Stabilized Foams via Gel Casting

In the present work, porous lead zirconate titanate (PZT) ceramics with porosity ranging from 27.8% to 72.4% were fabricated by the gel‐casting process of particle‐stabilized wet foams with the initial solid loading of 10–30 vol%. The phase, the microstructure, the dielectric property, and the piezoelectric property were characterized. The relative permittivity (ε_r) and longitudinal piezoelectric strain coefficient (d₃₃) of the investigated samples decreased with increasing porosity. Both the values of hydrostatic strain coefficient (d_h) and hydrostatic voltage coefficient (g_h) increased moderately with the increase in porosity, which was beneficial for enhancing the value of hydrostatic figure of merit (HFOM). As a result, the prepared sample possessed a maximal HFOM value of 15236 × 10^?15 Pa^?1 with the porosity of 72.4%. The acoustic impedance (Z) of specimens decreased linearly with increasing porosity, and had the lowest value of 1.95 MRayls, making them promising candidates for application of medical ultrasonic imaging or underwater sonar detectors.     

Silica‐Supported Dendrimer‐Palladium Complex‐Catalyzed Selective Hydrogenation of Dienes to Monoolefins

The selective hydrogenation of cyclic and acyclic dienes to monoolefins occurs under very mild conditions, in the presence of silica‐supported PAMAM‐Pd complexes. The activity and selectivity of this reaction is sensitive to the dendrimer structure. These dendritic complexes display excellent recycle properties, retaining activity for up to eight recycles.     

Observation of Particle Motion in High‐Concentration Ceramic Slurries Under Low Shear Rate

The dispersed state of particles in high‐concentration slurries has a significant effect on the development of the particle packing structure of the powder compact. We observed individual particle motion directly in highly concentrated ceramic slurries under slow flow through confocal laser scanning fluorescent microscopy using transparent slurries. In particular, we focused on particle motion soon after the application of a constant low shear rate. Measurements of the shear stresses indicated that the stress of a 30 vol% slurry gradually increased from 0.018 Pa over time, whereas the stress of a 40 vol% slurry soon reached 0.03 Pa and remained constant. For direct observation, shear stress was applied to the slurry, which was set between two glass plates, by moving one unilateral glass plate at 0.8 μm/s, to create a shear rate of 0.01 s^?1. The motion of the particles could be observed continuously. In the early stage, particles in the 30 vol% slurry near the moving plate were united as one body and did not alternate positions. Then, a speed incline developed gradually over time. For the 40 vol% case, the particles moved at the same time and together. Increases in the shear stress were caused by increased interaction when the particles were close together.     

Rhodium‐BisbenzodioxanPhos Complex‐Catalyzed Homogeneous Enantioselective Pauson–Khand‐Type Cyclization in Alcoholic Solvents

The chiral atropisomeric diphosphane ligand (S)‐BisbenzodioxanPhos was found to be highly effective in the co‐operative processeses of aldehyde decarbonylation and cascaded enantioselective Pauson–Khand‐type reactions. Various 1,6‐enynes were transformed to the corresponding bicyclic cyclopentenones in good yields and enantiomeric excesses (up to 96% ee). The attractive feature of this new Rh‐catalyzed homogeneous dual catalysis system is that the reaction can be performed in alcoholic solution.     

Electro‐Sintering of Yttria‐Stabilized Cubic Zirconia

Electro‐sintering, i.e., electrically enhanced densification without the assistance of Joule heating, has been observed in 70% dense 8 mol% Y₂O₃‐stabilized ZrO₂ ceramics at temperatures well below those for conventional sintering. Remarkably, full density can be obtained without grain growth under a wide range of conditions, including those standard for solid oxide fuel cell (SOFS) and solid oxide electrolysis cell (SOEC), such as 840°C with 0.15 A/cm². Microstructure evidence and scaling analysis suggest that electro‐sintering is aided by electro‐migration of pores, made possible by surface flow of cations across the pore meeting lattice/grain‐boundary counter flow of O^2?. This allows pore removal from the anode/air interface and densification at unprecedentedly low temperatures. Shrinkage cracking caused by electro‐sintering of residual pores is envisioned as a potential damage mechanism in SOFC/SOEC 8YSZ membranes.     

Flexural Behaviour of Fibre‐Reinforced Syntactic Foams

Summary: Syntactic foams containing 0.9, 1.76, 2.54, 3.54 and 4.5 vol.‐% of E‐glass fibres in the form of chopped strands were processed and subjected to three‐point bending tests. The results showed that introduction of chopped strand fibres into the syntactic foam system increased the flexural strength. The values increased with the amount of fibres in the foam system except for the one containing 3.5 vol.‐% of fibres, which showed a lower value than other fibre‐reinforced systems, thereby deviating from the trend. This exception was attributed to the difference in processing route adopted for this particular fibre‐bearing foam. However, in general, the incorporation of chopped strand fibres improved the flexural behaviour of the syntactic foam system without much variation in density, thus making reinforced syntactic foams to act as better and improved core materials for sandwich applications.

Fibre‐debonding and protuberance, and river pattern in a failed sample.     

Low‐Density Nanocellular Foams Produced by High‐Pressure Carbon Dioxide

A new type of nanocellular polymeric material based on PMMA and a MAM triblock copolymer is presented. The production avoids the use of physical additives and leads to completely homogeneous nanostructured polymers with a large number of CO₂ nucleation sites. The foamed materials show average cell sizes <200 nm and relative foam densities of 0.4, presenting a homogeneous cell structure. A physical effect not measured before in nanocellular materials is demonstrated, which leads to an increase of the glass transition temperature due to the confining effect of PMMA chains in the cell walls of the nanocellular foam. The effects of changing saturation pressure and MAM content in the cellular structure are described, together with three‐point bending Young's modulus measured using DMA.

     

2H‐Chromenes Generated by an Iron(III) Complex‐Catalyzed Allylic Cyclization

A straightforward method based on an iron(III) complex‐catalyzed cyclization of 2‐(1‐hydroxyallyl)phenols is reported to access a large variety of 2H‐chromenes. This method was applied to the total synthesis of a natural product, tephrowatsin B.

     

Desert Rose‐Shaped Zircon Synthesized by Low‐Temperature Hydrothermolysis

This study has presented synthesis, characterization, and formation mechanism of a kind of novel porous zircon with desert rose‐shaped morphology, which was hydrothermally fabricated by a two‐step quasi‐in situ reaction in a system of silica hydrogel and well‐dispersed ZrO₂ precursor microspheres. The micro–mesoporous zircon product possesses a sole crystalline phase of hydroxyl‐fluorinated ZrSiO₄ without unreacted ZrO₂ or SiO₂ in the final resultant. It has a specific surface area exceeding 120 m²/g and can maintain more than 50% micro‐mesoporic pores after calcination at 800°C due to its magnificent thermal stability. A four‐stage formation mechanism has been proposed to elucidate the process of quasi‐in situ crystallization and growth for the rose‐shaped zircon.     

Experimental Analysis of Fine–Particle Migration during Ceramic Filtration Processes

Slip casting and filter pressing are ceramic forming processes which can be affected by the migration of fine particles during filtration. Fine particles can be carried along with the filtrate and deposited within the ceramic compact and /or filter medium, thereby clogging and reducing the permeabilities of the porous media. This in turn affects the growth rate as well as the porosity of the compact. Evidence of clogging of the ceramic compact and filter medium was obtained by (1) SEM analysis of the compact and filter media, (2) surface area measurements of cross sections of compacts, and (3) measurements of the thickness of compacts as a function of casting time.     

Storable and Air‐Stable Zirconium Complex‐Catalyzed Highly Enantioselective Darzens Reaction of Diazoacetamide with Aldehydes

An asymmetric Darzens reaction of aldehydes with diazo‐N,N‐dimethylacetamide ( 3 ) catalyzd by an air‐stable and storable chiral zirconium Lewis acid catalyst, which is formed from 3,3′‐diiodobinaphthol and tetrabutoxyzirconium, gives solely the cis‐glycidic amides in high yields with excellent enantioselectivity (up to 97% yield, >99% ee).     

Efficient Hydrogenation of Nitrogen Heterocycles Catalyzed by Carbon‐Metal Covalent Bonds‐Stabilized Palladium Nanoparticles: Synergistic Effects of Particle Size and Water

We reveal here the first hydrogenation of nitrogen heterocycles catalyzed by carbon–metal covalent bonds‐stabilized palladium nanoparticles in water under mild conditions. Using a one‐phase reduction method, smaller metal–carbon covalent bond‐stabilized Pd nanoparticles were prepared with a size distribution of 2.5±0.5 nm, which showed extraordinary synergistic effects with water in the catalytic hydrogenation of nitrogen heterocycles. Water was supposed to accelerate substrate absorption and synergistic activation of molecular hydrogen on the Pd nanoparticles surface. The nanosized Pd catalyst could be easily recovered and reused for 5 runs.

     

CO2‐Induced Mechanical Reinforcement of Polyolefin‐Based Nanocellular Foams

The effect of CO₂‐induced crystallization on the mechanical properties, in particular the yield and the ultimate stresses, of polyolefins is studied. PP and SEBS copolymer blends are used as examples and foamed after sorption of CO₂ at temperatures below T_m. CO₂ sorption thickens the crystalline lamellae and consequently increases T_m from 160 to 178 °C for both pure PP and PP/SEBS blend systems. Foams with an average cell size smaller than 250 nm retain the ultimate stress at the level of the polymer before foaming, even without the effect of CO₂‐induced crystallization. Including CO₂‐induced crystallization, the yield and the ultimate stresses of the foam can be improved by 30 and 50% over solid PP and by 22 and 40%, for solid PP/SEBS blends, respectively.

     

Reactive‐Injecting Follow‐Through Shaped Charges from Sequent‐Material Conical Liners

Shaped charges using reactive‐metal liners have the potential for beyond‐penetration effects, e.g. thermodynamic events that increase pressure in a volume adjacent to the penetration. Shaped charge liners made from a copper penetrating‐ and an aluminum reactive‐component in a sequent‐material configuration were compared in tests to baseline homogeneous copper liners. Copper lined shaped charges had greater mild steel penetration performance, but lacked beyond‐penetration pressurization effects exhibited by the sequent‐material lined units. Jet capture experiments, beyond‐penetration constant‐volume tests, and thermochemical equilibrium calculations provide evidence supporting the aluminum slug comminution into unoxidized reactive fuel, augmenting beyond‐penetration effects.     

Increased Emulsion Stability for Reverse Y‐Shaped Sugar‐Based Surfactants

A series of reverse Y‐shaped surfactants containing aromatic and aliphatic linkers to combine two short hydrocarbon chains and one carbohydrate head group was prepared. Liquid crystalline behavior, air–water interfacial properties, and efficiency as an emulsifier was investigated for each reverse Y‐shaped surfactant. All reverse Y‐shaped surfactants mediated higher emulsion stabilities for water‐in‐oil compared to common typical reference surfactants, reflecting an improved ability to cope with a curvature towards water. The introduction of a benzene ring into the linker substantially increased the affinity of the surfactant for hydrophobic media, resulting in improved emulsion stability for both water‐in‐oil and oil‐in‐water.