Papers by Keyword: Ceramic

Abstract: Yttrium Aluminium Garnet (YAG) has a extensive applications in many fields because of its excellent properties. In this study the products synthesized by co-precipitation method is regular, and the reaction is complete, the YAG ceramic obtained is white, its density is 2.42g/cm³ and shrinkage is 7.78%. The YAG ceramic tablets were investigated by Field emission scanning electron microscopy (FE-SEM), Electron probe microanalysis (EPMA) and X-ray diffractometer (XRD). The results show that the structure of YAG ceramic is loose with spherical grain size of 0.2-0.3μm. The main mineral phase of YAG ceramic is composed of polycrystalline Al₅Y₃O₁₂ with good crystallinity, and containing very small amount of unreacted alumina and other impurities.

Abstract: High-porosity silicon nitride ceramics with excellent mechanical properties were fabricated by the carbothermal reduction of SiO₂. The influences of sintering conditions on microstructure and mechanical properties were studied. The results showed that microstructure and mechanical properties of porous silicon nitride ceramics were dependent mostly on the sintering conditions. The sintered porous silicon nitride ceramics exhibited the formation of fibrous microstructure with submicrometer-sized, high-aspect ratio b-Si₃N₄ grains, and uniform pore structure. Porous Si₃N₄ ceramics with a porosity of about 70%, and a flexural strength of about 70 MPa were obtained by sintering at 1750°C, with lower rate of temperature rise and no retaining time. The high strength was attributed to fine, high-aspect ratio b-Si₃N₄ grains and uniform pores between grains.

Abstract: The wear performances of three types of ceramics (ZrO₂, Al₂O₃, Si₃N₄)/1Cr18Ni9Ti stainless steel rubbing pairs were analyzed under 0% (pure water), 10%, 30%, 50%, 70% and 90% hydrogen peroxide (H₂O₂) solution. The results showed that, under different concentrations of H₂O₂ solution, the worn surface roughness of the 1Cr18Ni9Ti stainless steel was bigger than that under pure water when the counterpart was ZrO₂ ceramic ball, while the worn surface roughness of the 1Cr18Ni9Ti stainless steel was smaller than that under pure water when the counterpart was Al₂O₃ ceramic ball. For the ZrO₂, Al₂O₃, Si₃N4 ceramics ball, the average worn surface roughness of Al₂O₃ ceramic ball was the smallest, and the average worn surface roughness of Si₃N₄ ceramic ball was the biggest. When the concentration of H₂O₂ solution was 10%, the metal particles from the ZrO₂/1Cr18Ni9Ti and Al₂O₃/1Cr18Ni9Ti stainless steel rubbing pairs had the features with large size and bright surface. The surfaces of the metal particles were oxidized clearly when the concentrations of H₂O₂ solution were 70% and 90%.

Abstract: Ceramics will be used for power generating systems in the next generation. When they are used in this system, damage due to foreign object is inevitable. However, few systematic and comprehensive investigations have been reported on this subject. Various ceramics including fiber-reinforced mortars were investigated to understand their behavior when impacted by a spherical projectile. The volume of the cone cracks was large in ceramics which underwent transgranular fracture, while it was small in which underwent an intergranular one. Even though the energy consuming ability by the formation of surfaces was low up to 3.5% of the kinetic energy of a projectile, this ability increased with the ratio of the intergranular fracture to the transgranular one. Boron carbide showed a lower pressure as compared to the other ceramics. Fiber reinforcing increased the ballistic limits, but no clear advantage was suggested when absorbing the kinetic energy of a projectile far over its limits.

Abstract: Micromachining of advanced ceramics has been growing tremendously especially in the MEMs industry. All the time, researchers and industrial engineers have strived to achieve the lowest production cost at possible highest quality in micromachining operations. In this paper, the micromachining operation by means of chemical etching of ceramics is discussed. Machinable glass-ceramic (MGC) is used as the substrate and the influence of various input factors of the etching process is analyzed. These factors include etching temperature, etching period and, etching solution. The etching rate is then analyzed by calculating the weight loss per minutes. In order to establish the relationship between these factors, central composite design (CCD) and artificial neural network are used. Additionally, a prediction model that can be used with a high level of confidence in the industry is created at the end of the analysis.

Abstract: Lead-free ceramics with the composition of Mn-doped Ba(Zr_0.06Ti_0.94)O₃ – xmol%MnO₂ (BZT6-xMn, x=0, 1 and 2) have been synthesized by a mixed oxide process. Structure and micrograph characterization were examined using X-ray diffraction (XRD) and scanning electron microscopy (SEM), which revealed that all samples have the orthorhombic phase with dense and the uniform microstructure. The grain size of Mn-doped BZT6 ceramics decreased with increasing MnO₂ doping. The effect of MnO₂ doping on the dielectric and ferroelectric properties was also investigated. The maximum dielectric permittivity decreased dramatically with increasing MnO₂ doping in the BZT6 ceramics. A peculiar double-hysteresis-like loop was observed. Mn ions as acceptor doping occupy the B site of ABO₃ in the form of Mn²⁺ and Mn³⁺, which brought an increase in the oxygen vacancies concentration. The defect dipole model was explained the pinning mechanism.

Abstract: Pure and Hf-doped BaZr_0.2Ti_0.8O₃ (short for BZT) ceramics are prepared by a conventional solid state reaction method. The crystal structure and dielectric properties of Hf-doped BZT ceramics have been investigated. The results indicate that Hf4+ ions have entered the unit cell maintaining the perovskite structure of solid solution and the pure and Hf-doped BZT ceramics are cubic phase. Addition of hafnium leads to the fall of the phase transition temperature and can decrease the dielectric loss of BZT ceramics at room temperature. When Hf content is more than 0.5 wt.%, the diffuseness of the phase transition enhances with the increasing of Hf content and when Hf content is 3 wt.%, the diffuseness of Hf-doped BZT ceramics is more than that of the pure BZT ceramics.

Abstract: 100(1-y)wt%Ba_0.50Sr_0.50TiO₃—100ywt%Mg₂TiO₄ (BST-MT, y=0.76, 0.8, 0.82, marked as S1, S2, S3) composite ceramics with low permittivity (ε<60) were prepared by a traditional solid reaction method. The microstructure and dielectric properties of BST-MT composite ceramics were investigated. With the content of MT increasing, the dielectric peaks of BST-MT were depressed and broadened, thus the thermal stability was improved. However, the tunability declined with y increasing. In order to offset the temperature sensitivity, we obtained an Electrical field-Temperature (E-T) relationship by changing the dc bias field within a wide temperature range so that we could keep the tunability at a fixed value (3%). Such result explored a new route to compensate the temperature sensitivity of BST system in designing tunable microwave devices

Abstract: CaCu₃Ti₄O₁₂ ceramics doped with 0-2.0 wt% Li₂CO₃ were prepared by the solid-state reaction, and their electric and dielectric properties were investigated. It is found that these ceramics had the properties of high dielectric constant and comparatively low dielectric loss. At the doping amount of 0.5 wt%, the dielectric constant is kept to be 10⁵ with weak frequency dependence below 10⁵ Hz, and its loss tangent (tan δ) is suppressed below 0.1 between 300 Hz-5 kHz (with the minimum value of 0.06 at 1 kHz from 218 K to 338 K). The impedance spectroscopy analysis confirms that the decrease of dielectric loss is mainly due to the increase of resistance in the grain boundary, which may be related to the influence of Ti₄O₇ secondary phase. Our result indicates that doping Li₂CO₃ is an efficient method to optimize the dielectric properties of CaCu₃Ti₄O₁₂.

Abstract: Using tetraethoxysilane, aluminum nitrate and aluminum fluoride as raw materials, the precursor of mullite was prepared by sol-gel process. When the precursor sintered at 1200°C, mullite ceramic was obtained. Differential thermal analysis, X-ray powder diffraction and scanning electron microscope were used to characterize the dried mullite gel and ceramic blocks. The results suggest mullite is synthesized by solid-phase reaction mechanism. X-ray powder diffraction indicates mullite is the main crystals phase in the ceramic specimen. SEM micrograph shows the mullite grains in the shape of short rod with length of 20 um when sintered at 1200 °C for 2 h and the grains grown up to acicular with length of more than 50 um when the treating time under 1200 °C achieved to 4 h.