Fabrication and characterization of potassium-sodium niobate piezoelectric ceramics by spark-plasma-sintering method

High density (Na_1−xK_x)NbO₃ (x = 0.5, 0.6, and 0.7) ceramics were successfully prepared by spark-plasma-sintering (SPS) method. The dielectric and piezoelectric properties of the SPS samples were investigated and compared to that of hot-pressed samples. It is found that, the SPS-sintered (Na_1−xK_x)NbO₃ samples show higher room temperature dielectric constant, higher coercive fields, lower remnant polarizations and lower electromechanical coefficients than that of the hot-pressed (Na_1−xK_x)NbO₃ samples. The dielectric and piezoelectric property differences between the SPS-sintered and hot-pressed samples have been attributed to grain size effects.     

A numerical study of the heterogeneous porosity effect on self-propagating high-temperature synthesis (SHS)

Self-propagating high-temperature synthesis (SHS), or the so-called micropyretic/combustion synthesis, is a technique whereby a material is synthesized by the propagation of a combustion front across a powder. Heterogeneous distributions of porosities are common during self-propagating high-temperature synthesis when powders are pressed and the conventional modeling treatments thus far have only considered uniform systems. Heterogeneities in the porosity are thought to result in local variations of such thermophysical/chemical parameters for the reactants as density and thermal conductivity further changing the combustion temperature, the propagation velocity, and the propagation pattern of a combustion front. This study investigates the impact of porosity variations during self-propagating high-temperature synthesis with Ti + 2B. In addition, the simulations for the propagation of combustion fronts across a non-uniform compact where the porosity is monotonically decreased or increased along the specimens due to die wall friction are also carried out.     

Synthesis of bismuth titanate by citrate method

A simple citrate gel route was adopted for the preparation of bismuth titanate (Bi₄Ti₃O₁₂) powders. Stoichiometric quantities of BiCl₃ and TiOCl₂ were mixed with required amount of citric acid, and the mixture was heated on a water bath. This leads to formation of gel, which was decomposed at 673 K. Formation of bismuth titanate was observed on calcining powders at 973 K by X-ray diffraction studies. The average particle size is found to be 20 nm by microscopy. The room temperature dielectric constant is found to be 175 at 1 kHz. The hysteresis loop parameters were also obtained by home built Sawer-Tower circuit.     

Ceramic fibrous monolithic structures

High-strength ceramic fibers and composite structures that contain them are generally expensive. In a lower-cost approach for fabricating fibrous composites, reinforcing fiber-like cells that are distinct from a continuous matrix phase called the cell boundary can be formed in situ from powders. Structures can be constructed by assembly and consolidation of filaments that consist of the cell phase and its surrounding cell boundary.

Fabrication of ceramic fibrous monoliths (FMs) is reviewed and mechanical properties of the most widely studied FMs are discussed. Those based on Si₃N₄ cells within a BN cell boundary have achieved the best overall properties and uniformity of manufacture, but degrade severely at high temperatures in oxidizing environments. Those based on oxides are more stable, but are substantially weaker. Assessment of the future of FMs is offered, including cost reduction, fabrication practice, property improvement, and formation of complex structures.     

Influence of the common varistor dopants (CoO, Cr2O3 and Nb2O5) on the structural properties of SnO2 ceramics

The influence of dopants commonly used in SnO₂ varistor ceramics, such as CoO, Cr₂O₃ or Nb₂O₅, on the structural properties of SnO₂ was investigated. Several SnO₂-based ceramics containing only one of the dopants were prepared and characterized. Spectroscopic investigations [visible, near infrared (IR) and IR region] were performed to obtain information about dopants valence states inside the ceramics, as well as about their influence on electronic structure of the material. Structural properties were investigated by X-ray diffraction analysis and mechanisms of dopant incorporation were proposed. Obtained results were confirmed with results of the electrical measurements. Microstructural changes in doped ceramics were investigated by scanning electron microscopy (SEM) analysis that showed great differences in densities, grain size, and morphology of the SnO₂ ceramics depending on type of dopants and their distribution.     

Synthesis of BaTiO<Subscript>3</Subscript> powder from barium titanyl oxalate (BTO) precursor employing microwave heating technique

Cubic barium titanate (BaTiO₃) powder was synthesized by heating barium titanyl oxalate hydrate, BaTiO(C₂O₄)₂·4H₂O (BTO) precursor in microwave heating system in air at 500°C. Heating BTO in micro-wave above 600°C yielded tetragonal form of BaTiO₃. Experiments repeated in silicon carbide furnace showed that BaTiO₃ was formed only above 700°C. The product obtained was cubic.     

工程陶瓷的新加工方法

工程陶瓷材料具有耐高温、耐腐蚀、抗磨损及抗热冲击等优点。但由于硬度高、脆性大、大多数陶瓷材料不导电，传统的机械加工方法难以加工，使用范围受到限制。近年来，高压磨料水射流这种新兴的加工工程陶瓷的方法越来越受到人们重视。它具有效率高、成本低等优点，并可以加工出形状复杂的产品。     

Correlating phase and microstructure development versus dielectric properties in La and Er co-doped Bi4Ti3O12 ferroelectric ceramics

Bi_3.25La_0.75−xEr_xTi₃O₁₂ and Bi_3.25La_0.75Ti_3−xEr_xO_12−δ ceramics were prepared and studied in this work in terms of dopant-induced phase and microstructure development as well as dielectric response. The results show that introduction of Er³⁺ tends to reduce the materials’ sintering temperature and average grain size. Moreover, it was noted that in these systems the substitution site of this dopant is controlled by valence state and ionic radii mismatch effects. In particular, even when a nominal substitution of Ti⁴⁺ is conceived, here it is found that Er³⁺ also incorporates at the (Bi,La)³⁺ sites. These and other interesting concluding remarks from this work, including Er³⁺ tolerance, were possible only after comparing, especially, the X-ray diffraction results and the intrinsic ferroelectric characteristics extracted from the dielectric measurements.     

Research and prospect of ceramics for automotive disc-brakes

This article mainly discusses the research status and development trends of ceramics for automotive disc-brakes. According to the ceramic disc-brakes, various properties and characteristics, including fracture toughness, strength, compactness, corrosion resistance, wear resistance, micro-morphology, and thermal stabilities are analyzed. In the field of disc-brakes research, the research directions of the ceramics, including high-temperature performance, bionic structure, layered structure, porous structure, eutectic performance, superhard structure, and machinability are analyzed. The analysis of ceramics is expected to find disc-brake materials in line with the future development trends.     

From bulk to cellular structures: A review on ceramic/graphene filler composites

A revision on ceramic/graphene composites is presented. The more representative results for a wide number of bulk composites are compared, making special emphasis on their mechanical (fracture toughness, strength) and elastic properties, along with wear and friction topics. The electrical functionality boosted by the contacted graphene network is critically assessed for conducting and dielectric ceramic matrices. Regarding thermal transport, the enhancement or depletion of thermal conductivity is reviewed for different materials and specific orientations. Furthermore, new developments on layered materials and coatings, as well as on three-dimensional cellular composites, which certainly widen the scope of applications for this remarkable group of ceramic materials, are looked over.