Structural Design of Polymer‐Derived SiOC Ceramic Aerogels for High‐Rate Li Ion Storage Applications

SiOC ceramic aerogels with different porosity, pore size, and specific surface area have been synthesized through the polymer‐derived ceramic route by modifying the synthesis parameters and the pyrolysis steps. Preceramic aerogels are prepared by cross‐linking a linear polysiloxane with divinylbenzene (DVB) via hydrosilylation reaction in the presence of a Pt catalyst under highly diluted conditions. Acetone and cyclohexane are used as solvent in our study. Wet gels are subsequently supercritically dried with CO₂ to get the final preceramic aerogels. The SiOC ceramic aerogels are obtained after a pyrolysis treatment at 900°C in two different atmospheres: pure Ar and H₂ (3%)/Ar mixtures. The nature of the solvent has a profound influence of the aerogel microstructure in terms of porosity, pore size, and specific surface area. Synthesized SiOC ceramic aerogels have similar chemical compositions irrespective of processing conditions with ~40 wt% of free carbon distributed within remaining mixed SiOC matrix. The BET surface areas range from 215 m²/g for acetone samples to 80 m²/g for samples derived from cyclohexane solvent. The electrochemical characterization reveals a high specific reversible capacity of more than 900 mAh/g at a charging rate of C (360 mA/g) along with a good cycling stability. Samples pyrolyzed in H₂/Ar atmosphere show a high reversible capacity of 200 mAh/g even at a high charging/discharging rate of 20 C. Initial capacities were recovered after whole cycling procedure indicating their structural stabilities resisting any kind of exfoliations.     

Particle Motion in Simple Shear Flow with Gravity

The motion of aerosol particles in simple shear flow, subject to gravity, is analyzed. The combination of gravity and shear-induced lift is shown to give rise to particle drift. It is shown that in shear flow near a wall, when gravity points in the direction of flow, particles drift towards the wall, while for gravity pointing against the flow the drift is away from the wall. These results are also demonstrated experimentally, with fair qualitative agreement between analysis and experiments.     

Shear Modulus and Yield Stress Measurements of Attractive Alumina Particle Networks in Aqueous Slurries

The shear modulus and yield stress of attractive alumina particle networks in aqueous slurries was determined as a function of volume fraction (0.1 to 0.5), pH (2, 4, 5, 6, and 9), and salt (NH₄l) concentration (0.25M to 2.34) using both vane and couette rheological tools. Consistent with previous observations concerning the relative strength of attractive particle networks, the shear modulus increased to a plateau value with salt concentration. In this work we have shown that the salt concentration at which this plateau value is achieved is a function of the pH, and thus, the surface charge density. The values of the shear modulus [G'], yield stress [τ_y], and yield strain [γ_y] of the attractive networks can be described with power law functions for particle volume fraction [φ] (G'∝φ^4.75, τ_y∝φ^3.6, and γ_y∝φ^−1.1) and salt concentration [c] (G'∝ [c]^2.0, τ, ∝ [c]^1.15, and γ_y∝ [c]^−0.85).     

Matrix‐Based Encapsulation of Construction Chemicals Using High‐Shear Agglomeration

Encapsulation of dry superplasticizers in matrix‐based encapsulation systems was investigated. As basic material, commercially available fly ash was granulated by high‐shear agglomeration. Due to a high variability of factors affecting the encapsulation process and later release of admixtures, the design‐of‐experiments method was applied to reduce the quantity of experiments. Statistical evaluation indicates that the particle characteristics of the agglomerates were mostly influenced by the binder viscosity during the investigations. The delayed admixture release was enhanced by high binder viscosity and low energy input during the agglomeration process due to a coating of the bigger superplasticizer particles by the smaller fly ash. These results will help to develop encapsulated construction chemicals with controlled admixture delivery for the future application in a wide range of different building materials.     

Silicon Carbide Whisker‐Reinforced Ceramic Tape for High‐Power Components

An attempt to enhance both mechanical strength and thermal conductivity of glass‐based tapes is described. Flexural strength of ~420 MPa and thermal conductivity of ~10.3 W/m/K have been achieved in fully densified tape comprising calcium aluminoborosilicate glass, aluminum nitride, and silicon carbide whiskers. Silicon carbide whiskers aligned parallel to the casting direction contributed significantly to the reinforcement of the microstructure with accompanying extensive densification over a broad temperature range. These results are compared with the more typical alumina filler substituted for the aluminum nitride.     

Low‐Temperature Co‐Fired Ceramic Substrates for High‐Performance Strain Gauges

Recent advances in the development of high gauge factor thin films for strain gauges prompt the research on advanced substrate materials. A glass ceramic composite has been developed in consideration of a high coefficient of thermal expansion (9.4 ppm/K) and a low modulus of elasticity (82 GPa) for the application as support material for thin‐film sensors. In the first part, constantan foil strain gauges were fabricated from this material by tape casting, pressure‐assisted sintering, and subsequent lamination of the metal foil on the planar ceramic substrates. The accuracy of the assembled load cells corresponds to accuracy class C6. That qualifies the load cells for the use in automatic packaging units and confirms the applicability of the low‐temperature co‐fired ceramic (LTCC) substrates for fabrication of accurate strain gauges. In the second part, to facilitate the deposition of thin‐film sensor structures to the LTCC substrates, pressure‐assisted sintering step is modified using smooth setters instead of release tapes, which resulted in fabrication of substrates with low average surface roughness of 50 nm. Titanium thin films deposited on these substrates as test coatings exhibited low surface resistances of 850 Ω comparable to thin films on commercial alumina thin‐film substrates with 920 Ω. The presented material design and advances in manufacturing technology are important to promote the development of high‐performance thin‐film strain gauges.     

High‐Temperature Shear Strength and Bonding Mechanism of the Mullite Ceramic/Fiber Brick Component Joined by Phosphate Adhesive

The mullite ceramic/fiber brick system was bonded by two kinds of phosphate adhesives. The specimens were treated from 200 to 1400°C. The mechanical properties were tested at room temperature and at high temperature, and the relevant bonding mechanism was also discussed. The results show that the addition of silicon can greatly improve the adhesive's mechanical properties. The room‐temperature shear strength of the component bonded by adhesive with the silicon calcined at 800°C can reach 6.58 MPa. The shear strength of the adhesive with silicon tested at 800°C can reach 0.42 MPa.     

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.     

High‐Speed Camera‐Based Analysis of the Lithium Ceramic Pebble Fabrication Process

Lithium ceramic pebbles are a main component of the tritium breeding blanket in a future thermonuclear fusion plant. The melt‐based fabrication of the pebbles using the Plateau–Rayleigh instability emerged to be a promising approach in past investigations. However, the influence of process parameter variations on the pebble fabrication process is not yet well understood. Here, a new high‐speed camera‐based method to analyze the melt jet and the droplets directly in their formation phase is presented. Via special image processing algorithms, various individual properties such as dynamic changes, velocities, or agglomerations of the droplets are extracted. This new high‐speed camera‐based method provides additional information on the process state during the fabrication process. It is therefore an essential part in adapting and improving process parameters with respect to an overall enhanced product quality.     

Surface Modification in Polycrystalline Y2O3 Under High‐Pressure/High‐Temperature Processing Conditions

A pressure‐induced phase transformation is used to refine the grain size of polycrystalline Y₂O₃, by a factor of 3000. A surface modification effect accompanies the observed grain refinement, which becomes more apparent with increasing holding time under high pressure. The surface‐modified layer exhibits lower hardness and lower oxygen content relative to the underlying material. Moreover, it possesses columnar‐grained structure with cubic symmetry, whereas the interior has a monoclinic structure.     

Heat and Mass Transfer in Ceramic Lattices During High‐Temperature Oxidation

This work reports on the heat and mass transfer evolution of ceramic lattices during their oxidation at 1400°C and 1600°C in air. Si–SiC and Si–SiC–ZrB₂ systems were employed as skeleton material because they, previously produced as monolithic bars, showed promising oxidation behavior at high temperatures. Regular arrays of tetrakaidecahedra were first designed by CAD, then 3D printed and finally converted into ceramic by replica technique followed by reactive silicon infiltration. The surface area of each sample was calculated and specific weight variations were evaluated as a function of time. During oxidation, effective thermal conductivity and pressure drop of each sample were measured. Finally, results were correlated with the phenomena occurring during high‐temperature oxidation.     

Fiber Formation in Medium and Ultra‐High‐Molecular‐Weight Polyhydroxybutyrate Blends under Shear Flow

Summary: Polyhydroxybutyrate (PHB) is an ideal bioplastic, however, this polymer undergoes a severe embrittlement process because of its spherulitic structure, rendering the material brittle. Using a series of in‐situ rheo techniques, we have previously observed only the rapid initial stage of shish formation, we term a partial shish, which existed at high shears in medium‐molecular‐weight PHB, = 360 000. The shish kebab morphology is anticipated to remove or severely lessen this embrittlement process whilst providing new properties and applications. For medium and ultra high‐molecular‐weight (MMWT, = 360 000/UHMWT, = 5 × 10⁶) PHB 99/1 and 99.5/0.5 blends only a partial shish is identified. However, the initial shish formation stage and subsequent stages were observed at 98/2 and 97/3 blend ratios resulting in a complete shish, we term the full shish, and fiber formation was evident. We believe this fiber morphology achieved by high molecular weights is crucial to sustaining the shish kebab structure for an excessive period.

Left: In‐situ rheo‐light scattering micrograph; 97/3 MMWT/UHMWT PHB at 100 s^?1 for 1 s shear shish held at 75 s. Right: In situ rheo‐optical micrograph; PHB fiber morphology observed at 50 s^?1 for 2 s shear 98/2 MMWT/UHMWT PHB after 1 min.     

A Lead‐Free and High‐Energy Density Ceramic for Energy Storage Applications

In this work, we demonstrate a very high‐energy density and high‐temperature stability capacitor based on SrTiO₃‐substituted BiFeO₃ thin films. An energy density of 18.6 J/cm³ at 972 kV/cm is reported. The temperature coefficient of capacitance (TCC) was below 11% from room temperature up to 200°C. These results are of practical importance, because it puts forward a promising novel and environmentally friendly, lead‐free material, for high‐temperature applications in power electronics up to 200°C. Applications include capacitors for low carbon vehicles, renewable energy technologies, integrated circuits, and for the high‐temperature aerospace sector.     

Performance Evaluation of Two High‐Shear Impellers in an Unbaffled Stirred Tank

High‐shear impellers (HSIs) are mixers used in industrial stirred tanks to incorporate powders into liquids and break down particle agglomerates. A detailed numerical study of two commercial ring‐style HSIs of laboratory scale was carried out and their performance was compared with the Rushton turbine (RT). It was found that power and pumping numbers or their ratio cannot be simply connected for properly selecting an impeller in applications where highly localized viscous dissipation is desirable. The ratio of the average viscous dissipation in the impeller swept volume to the mean in the entire volume at two constant values of power input turned out to be lower for HSIs compared to that evaluated for RT. However, at higher power input, the dimensionless average viscous dissipation in the blade swept volume was found to be similar for the HSI of two rings and the RT, corroborating the high local viscous dissipation of this HSI when operated at higher speeds.     

Mixing Agglomeration in a High‐Shear Mixer with a Stirred Mixing Vessel

This paper deals with the agglomeration process in a high‐shear mixer. High‐shear mixers rotate with a very high mixing tool speed such that not only a mixing effect, but also a grinding effect is achieved. The parameter study reported here was carried out to determine the parameters influencing mixing agglomeration. The results will help the user to decide which parameters have to be considered for an optimum mixing agglomeration. This article will highlight some of the findings obtained from the comprehensive parameter study.     

分散剂及粉体粒径对光固化氧化铝陶瓷浆料粘度及制件性能的影响

针对光固化氧化铝陶瓷3D打印过程中的浆料粘度及制件性能,通过旋转粘度计测量得到不同分散剂及氧化铝粉体级配条件下的陶瓷浆料的粘度,优化了分散剂的选择及氧化铝粉体级配;通过对光固化3D打印、脱脂和烧结氧化铝陶瓷样件的弯曲强度和收缩率、致密度测试,得到了粉体级配前后不同固相含量氧化铝的抗弯曲性能、收缩率及致密度.研究结果表明,光固化氧化铝陶瓷3D打印浆料制备过程,选择PMA25作为其分散剂,选择10μm(60wt％)+5μm(10wt％)+2μm(30wt％)的粉体级配的氧化铝粉体,可以有效降低浆料粘度.同时,通过选择不同粒径的氧化铝陶瓷粉体,可以减小粉体之间的间隙,增加了粉体之间的有效粘接面积,使得氧化铝粉体之间的粘接更加牢固,陶瓷制件的抗弯曲性能更好、致密度更高.     

Fine‐Scaled Piezoelectric Ceramic/Polymer 2‐2 Composites for High‐Frequency Transducer

A novel technique was utilized to fabricate fine‐scaled piezoelectric ceramic/polymer 2‐2 composites for high‐frequency ultrasonic transducers. Lead zirconate titanate (PZT) was used as raw material. Tape‐casted acetylene black tapes were used to define kerfs after sintering. A one‐directional supporter was utilized to avoid distortion of PZT elements. PZT elements with 20 ± 2 μm width exhibited good consistency in longitudinal direction. A resonant method was utilized to evaluate the piezoelectric and dielectric properties of the composites. A 72‐μm‐thick composite with an aspect ratio of ~3.6 exhibited a k_t of 0.61 with satisfied piezoelectric and dielectric properties. A prototype high‐frequency ultrasonic transducer was fabricated and evaluated by an underwater pulse‐echo test. The center frequency was found to be 23.75 MHz, with ?6 dB bandwidth of 5.5 MHz.     

Investigation of Energetic Particle Distribution from High‐Order Detonations of Munitions

Military training with munitions containing explosives will result in the deposition of energetic materials on ranges. These residues contain compounds that may result in human health impacts when off‐range migration occurs. Models exist that predict the spatial and mass distribution of particles, but they have proven to be difficult to apply to detonating munitions. We have conducted a series of tests to determine if modelling results can be directly applied to simple detonation scenarios. We also command detonated several rounds to obtain an initial indication of high‐order detonation particle distributional heterogeneity. The detonation tests indicate that particle distributions will be quite heterogeneous and that the model used did not adequately describe the distribution of detonation residues. This research will need to be expanded to build an empirical database sufficient to enable the refinement of existing models and improve their predictions. Research on low‐order detonations should be conducted as low‐order detonations will result in higher mass deposition than high‐order detonations. Distribution models verified with empirical data may then be incorporated into range management models.     

Effect of Shear Rate Distribution on Particle Aggregation in a Stirred Vessel

The modeling of particle aggregation under a simple shear flow and the extension of the model to a stirred vessel is described. The model quantitatively demonstrates the change of the number of aggregates with time for each shear rate. This number increased with higher shear rate and, conversely, the aggregate size became small when raising the shear rate. This was because aggregates were broken by the stronger shear force. The number of aggregates for different impellers was determined. The shear rate was back‐calculated from the experimentally obtained aggregate size and the model equation. This shear rate was different from that estimated from the Metzner‐Otto method, consequently, some revisions of the Metzner‐Otto equation might be necessary for its application to particle aggregation.