Hierarchical Structure of Porous Silicon Nitride Ceramics with Aligned Pore Channels Prepared by Ice‐Templating and Nitridation of Silicon Powder

A unique hierarchical porous structure of silicon nitride ceramic with 76.5% porosity is fabricated by combining an ice‐templating method and nitridation for a silicon powder. The porous silicon nitride ceramics were composed of a lamellar structure with aligned pore channels and ceramic walls filled with fibrous whiskers. This study is focused on the influences of freezing rate on the microstructures and properties of the silicon nitride ceramics. The properties were characterized by compressive strength and gas permeability, which were shown to vary with controlled microstructure. The compressive strength and the permeability reached up to 32.2 MPa and 0.035^?12 m², respectively.     

Thermoelectric Ceramics for Energy Harvesting

Metal oxides (Ca₃Co₄O₉, CaMnO₃, SrTiO₃, In₂O₃), Ti sulfides, and Mn silicides are promising thermoelectric (TE) material candidates for cascade‐type modules that are usable in a temperature range of 300–1200 K in air. In this paper, we review previous studies in the field of TE materials development and make recommendations for each material regarding future research. Furthermore, the R&D of TE modules composed of metal oxide materials and the prospect of their commercialization for energy harvesting is demonstrated.     

Fabrication of Porous Ceramics with Unidirectionally Aligned Continuous Pores

Porous alumina ceramics with unidirectionally aligned continuous pores were fabricated via the slurry coating of fugitive fiber. Cotton thread was coated with ceramic slurry by pulling it through the slurry, and specimens were produced by spooling the coated thread. The obtained porous alumina ceramics had an average pore diameter of 165 μm, 35% open porosity, and a bending strength of 160 MPa. It was suggested that the pore size and the porosity could be adjusted using the diameter of the cotton thread and the solids concentration of the slurry, respectively.     

Effect of Polystyrene Addition on Freeze Casting of Ceramic/Camphene Slurry for Ultra-High Porosity Ceramics with Aligned Pore Channels

We investigated the effect of polystyrene (PS) addition on the freezing behavior of a very dilute alumina/camphene slurry with an initial solid loading of 5 vol% for the fabrication of ultra-high porosity ceramics with aligned pore channels. To accomplish this, slurries with various PS contents (10, 20, and 30 vol% in relation to the alumina powders) were prepared by ball milling at 60°C and then cast into molds at a constant temperature of 20°C. After removing the frozen camphene, the samples were heat treated to burn out the organic phases and sinter the alumina walls. The addition of the PS binder remarkably enhanced the green strength of the sample, making it possible to handle it without difficulty. All of the sintered samples showed ultra-high porosities of >88% without the collapse of the porous structure, wherein the pore channels were completely interconnected. Three distinctive zones having different pore structures were observed, viz. the outer shell with elongated pores, the inner zone with long aligned pore channels, and the center with equiaxed pores. The content of the PS polymer significantly affected the pore morphologies in the three distinctive zones. In addition, it was found that the addition of the PS polymer was highly beneficial to the alignment of the pore channels.     

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。     

能量收集用压电陶瓷材料研究进展

压电能量收集器件发展迅速,发电性能不断提高。然而,压电能量收集器受压电陶瓷材料本身低的机电转换性能限制,机电转换效率较低。制备具有高机电转换性能压电陶瓷材料具有重要的应用价值。评述了以获得高机电转换性能为目标所进行的压电陶瓷材料性能调控方面的研究成果,介绍了非谐振状态和谐振状态下,能量收集器件对压电陶瓷材料的性能要求,比较了能量收集用铅基、无铅压电陶瓷材料的机电转换性能,讨论了具有高机电转换性能压电陶瓷材料设计方法,为进一步开展能量收集用压电陶瓷材料的研究提供思路。总结了该领域存在的问题及对未来的展望。     

Ferroelectric Energy Conversion with PZT Ceramics Under Shock Loading

The behavior of various PZT compositions under shock loading shows that obtaining high energy densities requires ceramics having a high remanent polarization and a low permittivity, which release their energy by a ferroelectric → antiferroelectric phase transition. Few "hard" PZT ceramics have these properties. The results are presented and discussed.     

Synthesis of Porous Silicon Nitride with Unidirectionally Aligned Channels Using Freeze-Drying Process

Porous silicon nitride with macroscopically aligned channels was synthesized using a freeze-drying process. Freezing of a water-based slurry of silicon nitride was done while unidirectionally controlling the growth direction of the ice. Pores were generated subsequently by sublimation of the columnar ice during freeze-drying. By sintering this green body, a porous silicon nitride with high porosity (over 50%) was obtained and its porosity was controllable by the slurry concentration. The porous Si₃N₄ had a unique microstructure, where macroscopically aligned open pores contained fibrous grains protruding from the internal walls of the Si₃N₄ matrix. It is hypothesized that vapor/solid phase reactions were important to the formation mechanism of the fibrous grains.     

Synthesis of Porous Ceramics with Complex Pore Structure by Freeze-Dry Processing

Porous ceramics with complex pore structure were synthesized by a freeze-dry process. Freezing-in of a water-based ceramics slurry was done while controlling the growth direction of the ice. Sublimation marks of the ice were generated by drying under reduced pressure. Porous ceramics having a complex pore structure were obtained by sintering the green body: aligned macroscopic open pores contained micropores in their internal walls. The pore structure was substantially affected by the starting slurry concentration and sintering temperature. The pore formation mechanism is discussed in relation to these effects.     

Fracture Energy of an Aligned Porous Silicon Nitride

The fracture energy of a porous silicon nitride with aligned fibrous grains was investigated, using a chevron-notched-beam technique. A crack was constrained to propagate normal to the grain alignment. The obtained fracture energy was ∼500 J/m², which was ∼7 times larger than that of a dense silicon nitride with randomly oriented fibrous grains. The large fracture energy was attributable primarily to the sliding resistance associated with interlocking grains.     

烧结温度对多孔陶瓷压电性能的影响

研究了烧结温度对海藻酸钠离子凝胶法制备3-1型多孔PZT压电陶瓷和凝胶注模法制备3-3型多孔PZT压电陶瓷性能的影响.结果表明:当烧结温度从1150℃升至1250℃,多孔PZT陶瓷的孔隙率降低,晶粒尺寸、介电常数、压电系数、厚度机电耦合系数和抗压强度增大,静水压压电系数与静水压品质因数随之降低.3-1型PZT陶瓷具有定...     

Highly Aligned Porous Silicon Carbide Ceramics by Freezing Polycarbosilane/Camphene Solution

We fabricated highly aligned porous silicon carbide (SiC) ceramics with well-defined pore structures by freezing a polycarbosilane (PCS)/camphene solution. In this method, the solution prepared at 60°C was cast into a mold at temperatures ranging from 20° to −196°C, which resulted in a bicontinuous structure, in which each phase (camphene or PCS) was interconnected in a regular pattern. After the removal of the frozen camphene network, the samples showed highly porous structures, in which long straight and short elongated pore channels were formed parallel and normal to the direction of freezing, respectively. Thereafter, porous SiC ceramics were produced by the pyrolysis of the porous PCS objects at 1400°C for 1 h in a flowing Ar atmosphere, while preserving their mother pore structures having aligned pore channels.     

Porous Ceramic Bodies with Interconnected Pore Channels by a Novel Freeze Casting Technique

Porous ceramic bodies with interconnected pore channels were fabricated by a novel freeze casting technique using camphene-based slurries. The pore channels are surrounded by almost fully dense walls and have nearly circular cross-sections. The pore volume fraction and the channel size were controllable by the solid content in the slurry. The channels are replicas of entangled dendrites of frozen camphene, which sublimed during the freeze-drying process. This porous structure with entangled pore channels is considered potentially useful in many applications such as implantable bone scaffolds.     

多孔陶瓷孔径及其分布测定方法研究进展

孔径及其分布决定了多孔陶瓷的性能及应用,因此对其测定和定量表征非常重要。本文综述了多孔陶瓷孔径及孔径分布的常见测定方法,包括气泡法、压汞法、气体透过法、气体吸附法、气体渗透法、液-液法、悬浮液过滤法、X射线小角度散射法、核磁共振成像法、X射线断层扫描法以及电子显微镜图像分析法。比较了各种测试方法的优缺点,认为电子显微镜图像分析法是最直接有效的测定方法,并对多孔陶瓷的测试表征方法提出了展望。     

Cyclic Electrical Energy Harvesting Using Mechanical Confinement in Ferroelectric Ceramics

Present study introduces a novel energy conversion cycle for giant electro‐mechanical energy conversion using ferroelectric materials. The proposed cycle is used to perform indirect measurements for harnessable energy densities of two well‐known configurations ((0.5PbZrO₃‐0.5Pb(Ni_1/3Nb_2/3)O₃: (PZ‐PNN) and 0.9Pb(Zr_1/2Ti_1/2)O₃‐0.1Pb(Zn_1/3Nb_2/3)O₃: (0.9PZT‐0.1PZN)). PZ‐PNN is depicted to illustrate an energy density of 70 kJ/m³ under the conditions of 0–106 MPa applied stress and 1–15 kV/cm electric field. On the other hand, a maximum energy density of 50 kJ/m³ is obtained for 0.9PZT‐0.1PZN under ambient conditions of 0–173 MPa compressive stress and 1 to 18 kV/cm electric field.     

孔结构不同的两种多孔羟基磷灰石陶瓷的性能比较研究

采用有机泡沫浸渍法和注浆成型法分别制备了网孔和球孔结构的羟基磷灰石多孔陶瓷;观察比较了2种多孔体的孔结构,系统地研究了2种孔结构的羟基磷灰石多孔体孔隙率和压缩强度的关系,并进行了体外模拟试验。结果表明:球形孔结构的HA多孔体有较好的抗压缩强度,其中,孔径为200 μm,孔隙率为65％的支架其压缩强度可达 8 MPa左右。体外模拟实验表明2种孔结构羟基磷灰石(HA)支架均有利于细胞的生长。     

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.     

Dispersion and Setting of Powder Suspensions in Concentrated Aqueous Urea Solutions for the Preparation of Porous Alumina Ceramics with Aligned Pores

Highly concentrated alumina powder suspensions have been prepared in aqueous urea solutions of concentrations in the range 200–360 g/100 mL using an ammonium poly(acrylate) dispersant at 80°C. The dispersant concentration for the suspension viscosity minimum in the urea solutions is higher than that in water due to the higher processing temperature. The urea solutions having higher dielectric constant than that of water offer higher interparticle potential that resulted in better dispersion of the powder as evidenced from the lower viscosity and yield stress of the suspensions. The decrease in temperature increased the suspension viscosity and the suspension formed a strong gel when cooled to room temperature due to the crystallization of urea. The minimum urea solution concentration for a 55 vol% alumina suspension to form a dimensionally stable gel is 240 g/100 mL. The compressive strength and Young's modulus of the gels increased with the increase in urea solution concentration. The alumina ceramics prepared by the urea removal followed by sintering at 1500°C had porosity in the range 28–36 vol% with the rectangular rod‐shaped aligned pores.     

发泡剂对多孔氧化铝陶瓷孔结构和抗弯强度的影响

以十二烷基苯磺酸钠(SDBS)作为发泡剂,凝胶注模结合发泡法制备多孔氧化铝陶瓷.研究了发泡剂SDBS的添加量对多孔氧化铝陶瓷的气孔率、孔径尺寸及气孔分布和抗弯强度的影响.研究结果表明,在一定范围内随发泡剂SDBS添加量的增加,总气孔率和闭气孔率会有明显的上升趋势,孔径尺寸差异逐渐变小,气孔分布的均匀性逐渐变好.当SDBS的添加量超过1.0wt％ 后,气孔率虽然无明显变化,但是样品孔径尺寸及气孔分布的均匀性均变差,样品的抗弯强度随着SDBS用量的增加有明显降低的趋势.当SDBS的添加量为1.0wt％ 时,可以制备出闭气孔率为49％,抗弯强度为35 MPa,孔径尺寸及气孔分布均匀的多孔氧化铝陶瓷.