Fabrication and Characterization of Near‐Net‐Shape In Situ Reaction‐Bonded Porous Cordierite/SiC Ceramics

Porous cordierite/SiC ceramics were fabricated by in situ reaction bonding using α‐SiC, α‐Al₂O_3, and MgO powders as the starting materials. During sintering, part SiC is oxidized to SiO₂ and then the latter reacts with Al₂O₃ and MgO to form cordierite. As a result, porous cordierite/SiC ceramics were obtained, and the ceramics are strengthened by the residual SiC. Due to the large volume expansion introduced by the oxidation of SiC, the ceramics exhibit small sintering‐induced dimension variations. In addition, a fine‐grained microstructure and good thermal and mechanical properties were obtained for the porous cordierite/SiC ceramics.     

Near‐Net‐Shape Fabrication of Ti3SiC2‐based Ceramics by Three‐Dimensional Printing

This paper focuses on the preparation of near‐net‐shaped dense Ti₃SiC₂‐based materials via an indirect three‐dimensional printing (3D printing) and postreactive melt infiltration (RMI) processes. TiC preforms with bimodal pore size distribution were fabricated through 3D printing, followed by the infiltration of Si melt and Al–Si alloy (Al₄₀Si₆₀ and Al₇₀Si₃₀). Dense composites with density of ~4.1 g/cm³ were obtained after the infiltration. No volume shrinkage was obtained after the reactive infiltration with Al–Si alloy. The participation of Al during the infiltration process promoted the formation of Ti₃SiC₂. The as‐fabricated Ti₃SiC₂‐based materials showed enhanced mechanical and electromagnetic interference shielding properties.     

Near‐Net‐Shaping Methods for Ceramic Elements of (Body) Armor Systems

Shape‐forming techniques which may be useful in producing components for body armor are reviewed. The techniques are classified in three general categories, dry, wet, and plastic. The different shaping techniques are compared based on key parameters including shape limitations, rate of production, cost, and safety. The techniques are evaluated as to their suitability to be used to produce different body armor components such as breast plates, deltoid, shin and knee protection, and helmets. Dry‐pressing is the current standard for producing “relatively flat” components such as breast plates, but performance is limited by the inherent problem associated with dry‐pressing, namely, the difficulty in producing homogeneous green bodies because of agglomerates in the powder. Plastic processing has the potential to be useful to produce more reliable “flat” components with improved performance due to high shear mixing breaking up agglomerates. Wet (colloidal) processing techniques such as gelcasting and freeze casting may be useful to produce components with high curvature and more complex shape such as helmets. Tiles or segments may be combined to produce shaped components with increased flexibility.     

Thermal Properties of Transparent Yb‐Doped YAG Ceramics at Elevated Temperatures

This work presents the thermal properties of ytterbium‐doped yttrium aluminium garnet (Yb:YAG) transparent ceramics at elevated temperatures in dependence on the dopant concentration and on temperature. Transparent polycrystalline Yb:YAG ceramics were prepared by solid‐state reaction of oxide powders sintered under high vacuum. The dopant amount varied from 0 to 20 at.% of Yb. Thermal diffusivity of the sintered samples was measured by the laser and xenon flash methods at temperatures ranging from room temperature to 900°C. Both the thermal diffusivity and thermal conductivity values decreased with increasing dopant content, and until 500°C a decrease was observed also with increasing temperature. When available, the measured values were compared to data published in literature, and were found to be in good agreement. Based on the measured values, empirical relations in the form of shifted power laws are proposed for the temperature dependence of thermal diffusivity.     

Infrared Glass–Ceramics With Fine Porous Surfaces for Optical Sensor Applications

GeS₂–Sb₂S₃–CsCl glass–ceramics with fine porous surfaces were synthesized and tested as optical elements. The porosity is obtained through a two-step process, including controlled nucleation of CsCl nuclei in the glass matrix followed by selective etching of the nuclei with an acid solution. The porous surface is several hundred nanometers thick and results in a surface area increase of almost four orders of magnitude. The pores size is approximately 150 nm and can be tailored by controlling the nucleation process and the etching time. It is shown that the creation of the porous surface does not critically affect the optical transmission of these infrared (IR) transparent glass–ceramics. These materials can therefore be used for the design of optical elements and an attenuated total reflexion plate with porous surface was fabricated and tested as an optical IR sensor. The porous element shows higher detection sensitivity in initial experiments with an analyte sprayed at the plate surface and a coating of silane molecules.     

High Yttria–Zirconia and Yttria Ceramics for Petrochemical Reverse‐Flow Reactor Applications

Reverse‐flow reactors achieve the desired hydropyrolysis reaction of natural gas and other hydrocarbon feeds at very high temperatures of up to 2000°C, which enables the production of many high‐value chemicals. To identify refractory ceramic materials suitable for constructing key components of the reactor, the full range of solid solutions between zirconia and yttria having 18 to 100 mol% yttria have been tested in a laboratory reactor. Conventional yttria‐stabilized zirconia (YSZ) materials having 8 mol% Y₂O₃ appear to accommodate reactor thermal severity, but are prone to a new form of corrosion termed ceramic dusting that is observed when pyrolysis and oxidation cycles are alternated under reverse‐flow conditions. Yttria and high yttria–zirconia ceramics having ~80 mol% or more yttria suppress ceramic dusting corrosion in steam‐free pyrolysis environments. The addition of low levels of steam of ~5% to the pyrolysis gas stream increases the stability of YSZ materials substantially, so that the stability threshold is closer to 40 mol% Y₂O₃ in the yttria–zirconia system. The two approaches can be combined to optimize reactor performance. Key experimental results are presented and discussed taking into account the thermodynamic phase stability of the different phases.     

Dielectric Loss to Detect Liquid Phase in Ceramics at High Temperatures

Rapidly increasing dielectric loss values, coinci nt with the formation, with increasing temperature, of small amounts of liquid phase, have been seen in an alumina doped with titania and soda (Al₂O₃-l mol^c TiO₂-0.5 mol% NaO₁₂). The method appears to have the sensitivity to detect liquids in ceramics down to 0.1% in favorable cases.     

Ultrasonic Absorption in Fused Silica at Low Temperatures and High Frequencies

The absorption of high-frequency (60 kc. to 20 me. per second) sound waves in fused silica shows a large peak at low temperatures (30° to 50°K.). This absorption appears to result from some property of the glassy state since the absorption does not occur in crystalline silica. Data are presented showing the variation of rigidity and internal friction with temperature and frequency. It is shown that the source of the internal friction may be a structural relaxation. Some speculations are made on the nature of the structural relaxation and its relation to the glassy state.     

Corrosion Behavior of Alumina Ceramics in Caustic Alkaline Solutions at High Temperatures

The corrosion rate and changes in the microstructure and fracture strength of alumina ceramics (93.0% Al₂O₃ and 99.5% Al₂O₃) were studied in 0.1 m to 25 m NaOH solutions at 150°C to 200°C, where m = mol/(kg of H₂O). The attack of the caustic alkaline solution started at the grain boundaries. Consequently, the corrosion resistance increased with decreasing SiO₂ content in Al₂O₃ ceramics, and the corrosion resistance of 99.5% pure Al₂O₃ was similar to that of Si₃N₄ ceramics. Since large pits are formed by corrosion, the surface area increased first and the apparent corrosion rate increased with time in the initial stage of the corrosion. The corrosion rate of Al₂O₃ increased linearly with increasing NaOH concentration, and the activation energy was 102 kJ/mol. The fracture strength of corroded Al₂O₃ decreased monotonically as the degree of dissolution of alumina increased.     

Nontoxic Processing of Reliable Macro‐Porous Ceramics

A direct‐foaming technique to produce macro‐porous ceramics is presented considering three key aspects: (i) a processing route without toxic additives, (ii) optimizing particle packing and dispersion of ceramic suspensions to obtain denser struts, and (iii) processing reliability for large‐scale production. To achieve the latter goal, specific foaming equipment was designed. Ceramics with large dimensions (250 × 60 × 110 mm³), high porosity level (>70%), homogeneous and narrower pore‐size distribution, high Weibull modulus (8 to 11), improved mechanical strength (3 to 15 MPa) for different compositions and low thermal conductivity at 1200°C (0.3 to 0.9 W/mK) were attained.     

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.     

Measurement of Elastic Moduli from the Impact Sound of Engineering Ceramics and Composites at Elevated Temperatures

A simple instrumentation system is developed for determining the elastic moduli of engineering ceramics and their composites from the sound produced by the impact of a sphere on a specimen which is suspended by thin wires or ceramic threads in a furnace. The natural frequencies of the flexural and torsional vibration modes are measured from the impact noise by using a fast Fourier transform analyzer. Elastic moduli are then computed from those frequencies by using Timoshenko's beam theory and also Saint-Venant's torsion theory. The validity of the measurementmethod is demonstrated by room-temperature tests on ceramic and metallic specimens of known elastic moduli. SiAION and Al-Al₂O₃ composites are tested at elevated temperatures up to 900°C. It is shown that the Young's and shear moduli of the materials tested decrease with an increase in temperature, while Poisson's ratio remains almost constant.     

Barium/Lead-Rich High Permittivity Glass–Ceramics for Capacitor Applications

Dielectric properties of glass–ceramics containing barium/lead-based sodium niobates and barium titanate-based silicates were evaluated for capacitor applications. The glasses were formed by melt-rolling the respective constituents which were then crystallized by reheating them at higher temperatures. Crystallization schedules were formulated based on differential thermal analysis results. X-ray diffractometer patterns indicated that the samples were highly crystallized. Microstructure and microchemistry of samples were studied by transmission electron micropscopy. Perovskite, tungsten–bronze and fresnoite phases developed during crystallization have a strong effect on the resulted dielectric properties with permittivities ranging from 20 to 700. Resistivity measurements were done to study conduction mechanisms in samples and the resistivity values for glass–ceramics were found to be between 10¹¹ and 10¹³Ω·cm at 150°C. MnO₂ additions were made to improve the electrical resistivity of glass–ceramics.     

Deflagration of HMX‐Based Explosives at High Temperatures and Pressures

We measure the deflagration behavior of energetic materials at extreme conditions (up to 520 K and 1 GPa) in the LLNL High Pressure Strand Burner, thereby obtaining reaction rate data for prediction of violence of thermal explosions. The apparatus provides both temporal pressure history and flame time‐of‐arrival information during deflagration, allowing direct calculation of deflagration rate as a function of pressure. Samples may be heated before testing. Here we report the deflagration behavior of several HMX‐based explosives at pressures of 10–600 MPa and temperatures of 300–460 K. We find that formulation details are very important to overall deflagration behavior. Formulations with high binder content (≥15 wt%) deflagrate smoothly over the entire pressure range regardless of particle size, with a larger particle size distribution leading to a slower reaction. The deflagration follows a power law function with the pressure exponent being unity. Formulations with lower binder content (≤10 wt% or less) show physical deconsolidation at pressures over 100–200 MPA, with transition to a rapid erratic deflagration 10–100 times faster. High temperatures have a relatively minor effect on the deflagration rate until the HMX β→δ phase transition occurs, after which the deflagration rate increases by more than a factor of 10.     

Porous PZT Ceramics with Aligned Pore Channels for Energy Harvesting Applications

In this study, aligned porous lead zirconate titanate (PZT) ceramics with high pyroelectric figures‐of‐merit were successfully manufactured by freeze casting using water‐based suspensions. The introduction of aligned pores was demonstrated to have a strong influence on the resultant porous ceramics, in terms of mechanical, dielectric, and pyroelectric properties. As the level of porosity was increased, the relative permittivity decreased, whereas the Curie temperature and dielectric loss increased. The aligned porous structure exhibited improvement in the compressive strength ranging from 19 to 35 MPa, leading to easier handling, better processability and wider applications for such type of porous material. Both types of pyroelectric harvesting figures‐of‐merit (F_E and F′_E) of the PZT ceramics with a porosity level of 25–45 vol% increased in all porous ceramics, for example, from 11.41 to 12.43 pJ/m³/K² and 1.94 to 6.57 pm³/J, respectively, at 25°C, which were shown to be higher than the dense PZT counterpart.     

Thermal Conductivity of Very Porous Kaolin‐Based Ceramics

A clay‐based material exhibiting high pore volume fraction and low thermal conductivity suitable for thermal insulation is described. Starting with a commercial clay containing >75% kaolinite, foams were made by mixing in water and methyl cellulose as a surfactant then beating. After drying at 70°C, the pore volume fraction >94% remains almost constant for treatments up to 1150°C. In contrast, the phases constituting the solid skeleton evolve strongly with removal of surfactant, dehydroxylation of kaolinite, and formation of mullite. The latter leads to greater mechanical strength but also an increase in thermal conductivity. Thermal treatment of the kaolin foam at 1100°C yields a suitable compromise between low thermal conductivity of 0.054 W.(m.K)^?1 at room temperature with a compressive yield stress of 0.04 MPa. The radiation component in the effective thermal conductivity is <10% at 20°C increasing to >50% at 500°C.     

Oxidation‐Induced Sintering: An Innovative Method for Manufacturing Porous Ceramics

We report for the first time a novel method of manufacturing macroporous ceramics by oxidation‐induced sintering (OIS) of porous compacts composed of Ti powders and fugitive poreformers. Using this novel manufacturing method, novel porous TiO₂ ceramics can be fabricated over a wide range of porosity at a relatively faster heating rate (4°C/min) and shorter sintering periods (≤4 h) at 1450°C. The porous ceramics were subsequently characterized by SEM, XRD, and compressive strength measurements. The detailed microstructure study showed that the solid network in these solids is interconnected.     

大功率铅基压电陶瓷材料的研究进展

本文综述了大功率压电陶瓷材料的研究进展,介绍了其体系结构、应用和制备方法,最后指出掺杂改性、探索新的材料体系和制备工艺是改进其制备的有效途径。     

Wavelength Tailorability of Broadband Near‐Infrared Luminescence in Cr4+‐Activated Transparent Glass‐Ceramics

Four Cr⁴⁺‐activated transparent glass‐ceramics containing different species of silicate nano‐crystals (Zn₂SiO₄, Mg₂SiO₄, Li₂ZnSiO₄, and Li₂MgSiO₄) were successfully prepared. Absorption spectra, photoluminescence spectra, lifetime decay curves, and quantum yield of these transparent glass‐ceramics were measured. According to the crystal field strength of Cr⁴⁺‐incorporated tetrahedral sites, the broadband near‐infrared (NIR) luminescence of Cr⁴⁺ can be tailored from 1130 to 1350 nm and the lifetime of Cr⁴⁺ luminescence can be prolonged from 6 to 100 μs. Quantum yield in the transparent glass‐ceramics containing Li₂ZnSiO₄ nano‐crystals reached at 17%, which is the highest value of NIR luminescence in transition‐metal‐activated glass materials.