Key Engineering Materials Vol. 655

Abstract: Alumina ceramics are the most widely used structural ceramics, especially in aviation, spaceflight and war industry area. However, alumina is of covalent bond, with low fracture toughness and poor thermal shock resistance, which baffle the application of the alumina ceramics in engineering. In this paper, mullite fiber was used to improve its toughness and thermal shock resistance. Alumina-mullite composite ceramics were prepared by hot press sintering. The effect of mullite fiber on thermal shock resistance of ceramics was investigated through indention quenching method. Meanwhile, the mechanism of its reinforcing and toughening and the relationship between mechanical properties and addition of mullite fiber were also discussed.

Abstract: Due to its excellent chemical stability, high temperature resistance and mechanical properties, alumina porous ceramic was the first choice of the production of microfiltration membrane support body material. In this paper, alumina porous ceramic was prepared by alumina as main materials and adding proper amount of adhesives and sintering additives. Effects of the kinds and addition of pore formers, such as starch, graphite and carbon-black, on properties of alumina porous ceramic was studied. The porosity of the fired samples was evaluated by Archimedes law. The fired samples were characterized by scanning electron microscopy (SEM), compressive strength and mercury porosimeter measurements. Alumina porous ceramic composition was prepared by mixing 100% alumina with different particle size and 8 % graphite pore former. The sample was pressured at 10 MPa and fired at 1650 °C for 2 h, and endowed with 6-8 μm of the average pore diameter, 37.65 % of the porosity and 40.33 MPa of the compressive strength, respectively. The influence law of the kinds and addition of graphite pore former on the performance of alumina porous ceramic was studied for application in micro-filtration mmembrane supports.

Abstract: Alumina ceramic microspheres were prepared via internal gelation process. By dropping sol from a cold broth into hot silicone oil gelled microspheres would solidify within a few seconds by decomposition of hexamethylenetetramine (HMTA). The effect of the broth component on the performance of the broth and gelled microspheres obtained has been investigated. The Al₂O₃ ceramic microspheres can be obtained after aging, washing, drying and sintering. The Al₂O₃ microspheres prepared with optimized formulation have uniform size and good sphericity, with an average diameter of 690μm and crush strength of greater than 12N/sphere.

Abstract: Porous Al₂O₃-SiO₂-ZrO₂ ceramic composite membranes were fabricated by the sol-gel method. The effects of constitution of the composite sol, dip-coating parameters and calcination parameters on the morphology, phase composition, pore size distribution and BET surface area of the membranes were investigated extensively. The thermogravimetric analysis (TGA)/differential scanning calorimeters (TG/DSC), scanning Electron Microscopy (SEM), XRD analysis and N₂ adsorption-desorption analysis was employed for the membranes characterization. TG/DSC curve shows that the suitable calcination temperature of Al₂O₃-SiO₂-ZrO₂ membranes is 1000°C. The SEM morphology displays a smooth and free-crack layer of Al₂O₃-SiO₂-ZrO₂ composite membranes on the surface of the support after calcination at 1000°C. Furthermore, the membranes were also successfully coated with a good adhesion to the support. The optimal composite membranes were obtained when the moral ratio of Al₂O₃-SiO₂-ZrO₂ was 10:2.3:1, the dip-coating parameters were four times dip-coating with every time of 60s respectively.

Abstract: In the present paper, low cost porous mullite ceramics with high porosity, high strength and low thermal conductivity were fabricated using ceramic waste powder and clay as raw materials, and sawdust as porogen. The correlation of phase composition and physicochemical properties of such porous mullite ceramics were researched by varying sintering temperature and the extra addition of sawdust. The results show that: The ceramics show the best comprehensive properties when the sintering temperature is 1400 oC and the addition of sawdust is 30wt%. The results of SEM and EDS analysis confirm that new mullite phase is generated in the ceramics, which can strengthen the ceramics. And the best value of the bulk density, apparent porosity, cold crushing strength and the thermal conductivity of the prepared ceramics is 0.96g.cm-3,60.7%,5.3MPa and 0.212W·m-1·K-1, respectively.

Abstract: The purpose of this article is to contrast the relative translucency of four ultra-transparent dental zirconia materials after veneered, including Cercon zirconia, Zenostar zirconia, Lava zirconia and Upcera zirconia. All zirconia ceramics were prepared into samples of diameter 10mm and thickness 0.5±0.01mm.The relative translucency of samples were measured with color difference meter and statistically analyzed with SPSS 17.0.This experiment confirms that the existence of statistically significant differences in the relative translucency between different zirconia ceramics. According to the numerical level, the relative translucency of zirconia materials from high to low is Zenostar ZR Translucent>Lava Plus High Translucency zirconia> Cerconht full contour zirconia > Upcera zirconia. Zenostar ZR Translucent and Lava Plus High Translucency zirconia can be used to restore adjacent teeth that are higher translucent, while Cercon ht full contour zirconia and Upcera zirconia is better for molar teeth with poor relative translucency.

Abstract: This study aimed to investigate the color value of four kinds of highly transparent ceramics (Zenostar Zirconia, Lava Zirconia, Cercon Zirconia and Upcera Zirconia)after veneered on three different post core materials including noble metal alloy, Co-Cr alloy and composite resin. Highly transparent ceramics after veneered were made into specimens with 10mm in diameter and 1.0mm in thickness. The color of core ceramics after veneered on different post core materials were measured with Color Analyzes. The results showed that their color value were significantly different from each other. Zenostar ZR Translucent and Lava Plus High Translucency Zirconia can be easily affected by the background color because of its high translucency. Therefore, resin post core materials should be selected without cause prosthesis mismatch of colors in clinical work. When making Cerconht full contour zirconia restorations, you could try to use a resin material as a post core material. And selecting precious metals, you had better make it opaque. Upcera zirconia owned a poor relative translucency, when making clinical restorations; you could use any kind of post core materials.

Abstract: Magnesium-zinc (Mg-Zn) alloys are strong candidates as medical implant materials due to their good biocompatibility and relatively high strengths. To manipulate the degradation of Mg-Zn alloy in the human body, a plasma electrolytic oxidation (PEO) treatment was applied to Mg-10%wt. Zn in this study because it produces a coating that is non-harmful to the human body and the process is inexpensive and environmentally friendly. Potentiodynamic polarization corrosion tests, performed in a simulated body fluid (Hanks’ Balanced Salt Solution) were applied to the coated and uncoated Mg-Zn samples. The results of the testing showed that the coated Mg-Zn exhibited higher corrosion resistance than the substrate. With the PEO coating thickness of 7.2 microns, the corrosion current density was reduced by 1.00 μA/cm² from the uncoated Mg-Zn respectively, indicating a significant reduction in the degradation rate between pure Mg-Zn and coated Mg-Zn from 7.0 to 3.7 kg/year. A pin-on-disc tribometer was employed to measure the coefficient of friction (COF) for the coated and uncoated Mg-Zn samples, lubricated with and without Hanks’ solution. The measured COF of the coated sample was very low (averaging to be about 0.22 under the lubricated condition) and comparable to that of the substrate which exhibited an averaged COF of 0.13 under the lubricated condition.

Abstract: In an electrowetting-based liquid microactuator, two types of insulating materials are used for device operation: an dielectric material to provide capacitance between the liquid and conductor, and a hydrophobic coating at the interface of liquid and insulator. This paper investigates important physical properties for the insulating materials. Several alternative dielectric materials are compared for application in EW chips, breakdown phenomenon of insulating layers with different thicknesses is tested before and after droplet actuation. It appears that the used EW devices have lower breakdown voltages than unused devices, probably due to the locally trapped charges in the insulating layer.

Abstract: Lead lanthanum zirconate titanate (PLZT) transparent ceramics have attracted more and more attentions for their unique electric-induced light scattering effects. The PLZT ceramics of 7.45 mole% lanthanum with different Zr/Ti ratios (Zr/Ti=68/32, 69/31, 70/30, 71/29, 72/28) were prepared by the hot-press sintering method. The X-ray diffraction patterns confirm that all the PLZT samples are perovskite structure without detected secondary phases. The SEM images of the fracture surfaces of the samples show that all the PLZT materials tend to fracture transgranularly, and exhibit fully dense and uniform microstructures. All the samples with different Zr/Ti ratios present typical relaxation ferroelectric phase transition characteristics with square shaped hysteresis loops. Noticeably, all the original PLZT samples show an excellent transparency in the wavelength range from visible to near-infrared, and they will become opaque after an appropriate electric field experienced. The opaque sample can also recover to the transparent state when an inverse compatible electric field acts on it. The contrast ratio of the sample increases firstly and then decreases when the Zr/Ti ratio increases from 68/32 to 72/28 (λ=632.8nm). The PLZT (7.45/70/30) material shows a maximum contrast ratio, which makes it promising to apply in optical modulators.