Papers by Keyword: Microstructure

Abstract: BaZr_xTi_1-xO₃-based (BZT) ceramics with Al₂O₃ and MgO addtives were prepared by the conventional solid state method with BaCO₃, ZrO₂, TiO₂, Al₂O₃ and MgO as raw materials and B₂O₃ and Li₂CO₃ as sintering additive. The morphologies were analysized by scanning electron microscopy (SEM). The dielectric constant and dielectric loss of ceramics were measured by LCR meter. The temperature dependences of dielectric constant were measured by high-low temperature incubator tank and LCR meter at 1 MHz and a temperature range-55 to 125 °C. The tunabilities were tested by C-T-V converter and LCR meter at 1 MHz at room temperature. The results show that with the increase of Zr/Ti, BZT ceramic dielectric constant increases, the loss increases, the Curie temperature moves to a lower temperature, and dielectric bias field coordination is relatively lower. The SEM images show that the grain size reaches about 1-2 μm when the sintering temperature is 1100 °C, and the addition of Al₂O₃ and MgO promote the grain growth and densification of the composite ceramics. The Curie peaks are broadened and depressed with the addition of Al₂O₃ and MgO. The tunability is improved to 9.59% under a DC electric field of 7.0 kV/cm after the addition of Al₂O₃. The dielectric constant and dielectric loss of BaZr_0.25Ti_0.75O₃-30wt%Al₂O₃ and BaZr_0.25Ti_0.75O₃-30wt%MgO are 586, 0.011 and 486, 0.003, respectively. The optimistic dielectric properties make it a promising candidate for the application of tunable capacitors and phase shifters.

Abstract: ZrO₂ wet gel was prepared using zirconium n-butoxide as precursor and 1,2-propylene oxide (PO) as a gelation agent in sol-gel process. Then, tetraethoxysilane (TEOS) was used to strengthen the wet gel in aging process. Finally, the monolithic silica/zirconia aerogel was obtained by supercritical fluid drying. N₂ adsorption-desorption method, scanning electron microscope (SEM), and Fourier transform infrared spectroscopy (FTIR) were used to characterize microstructure of the aerogel. The influence of pH of the aging solution on microstructure of silica/zirconia aerogel was studied. The results indicate that the particle size distribution of SiO₂/ZrO₂ aerogel was more uniform and less aggregate when aging in acid solution. However, the average pore diameter and particle size of SiO₂/ZrO₂ aerogel was increased by aging in alkaline solution. Aging in alkaline condition was useful for largening the pore size of SiO₂/ZrO₂ aerogel, and the maximum specific surface area and pore volume were obtained at pH=9. Meanwhile, different reactions were produced between the acid/alkaline solution and the hydroxyl of SiO₂/ZrO₂ aerogel, and the ester group/amino group was obtained relatively which can modify the network of aerogel and decrease the shrinkage of aerogel during the supercritical fluid drying.

Abstract: Tin doped indium oxide (ITO) thin films were prepared on IR glass substrates at different oxygen flow rate by ion-assisted electron beam evaporation method, high purity ITO particles (In₂O₃: SnO₂ = 9:1 Wt%) were used as source material. Properties such as microstructure, morphology and mechanical properties were investigated by X-ray diffractometer, SEM and scratch tester, respectively. Meanwhile, lattice constant a, crystal grain size and residual stress situation of films as-deposited were calculated and discussed in detail. The results indicated that all of the films as-deposited were polycrystalline and represented [111] preferential orientation. With the increasing of the oxygen flow rate, grain size and surface roughness of films as-deposited decreased, and inner stress remained in film increased. There were two types of failure mode occurred in ITO films according to different stress situation. Relative high level of residual stress improved the mechanical properties of ITO films in a certain extent.

Abstract: In this project, coal fly ash was transformed into ceramic materials by adding a certain amount of fusing agent. Ash samples were compacted and sintered with the addition of potassium carbonate (K₂CO₃·1/2H₂O) under a suitable sintering temperature range. Mineralogy and microstructure of the obtained products were characterized by means of X-ray diffraction and field emission scanning electronic microscope techniques respectively. The results indicate that K₂CO₃·1/2H₂O facilitates the transformation of mullite and quartz phases above 800°C, and the mineralogy phases of the product is leucite (KAlSi₂O₆) and potassium aluminum silicate (KAlSiO₄). In the process, K⁺ interacts with oxygen atom and destroys the original lattice. The regular morphology of the sintered samples was confirmed by the observation under SEM, which reveals a uniform dense ceramic is formed at 900°C with the 40wt% addition of K₂CO₃·1/2H₂O.

Abstract: Wetting behaviors of pure nickel and nickel-based alloys on sintered silicon carbide ceramics and their interfacial microstructures were investigated in this presentation. The nickel-based alloys were mainly the commercial nickel-molybdenum-chromium products. The wetting and spreading properties were observed by a real-time thermal optical measurement system under flowing argon-5%hydrogen atmosphere. As temperature increased, the pure nickel cylinder sample had few changes before 1356°C except for the thermal expansion in size and changed to liquid drop-shape when the temperature reached 1366°C. The contact angle was about 90^o. And nickel could not contact sintered silicon carbide ceramics well. However, the introduction of molybdenum and/or chromium in the pure nickel was good for the wetting properties of pure nickel on sintered silicon carbide ceramics. The contact angles of nickel-based alloys (B-1, C-1 and C-2) on sintered silicon carbide ceramics after holding 15min at 1400°C were 25^o, 12.5^o, 11.5^o, respectively. And they hardly reduced as temperature increased. The SEM images indicated that the interfacial microstructures of B-1 and C-1 on sintered silicon carbide ceramic substrates were uniform and the dissolved interface near silicon carbide ceramics indicated that they had a good bonding. At the same time, the reciprocal diffusion was clear in the intermediate layer. Besides that, the introduction of chromium was supposed to reduce the diffusion rate of molybdenum from the alloy to the ceramic substrate.

Abstract: C_f/SiC-ZrB₂ composites were fabricated by chemical vapor infiltration and precursor infiltration pyrolysis. Specimens were then tested for mechanical properties , thermo-physical properties and microstructure. Results show flexural strength, tensile strength and compressive strength reach 430MPa,120 MPa and 365 Mpa, respectively. The mechanical properties at high temperature were tested. And the fractograph and microstructure of composites were analysized.

Abstract: C_f/SiC composites were fabricated by chemical vapor infiltration with CH₃SiCl₃ as precursor and H₂ as carrier gas. The carbon preform was noodled by continuous carbon fiber and network tires layer. The screws were processed with C_f/SiC composites by diamond tool. The mechanical properties of screws were studied in this paper. Results show that the tensile and double shear strength reach separately 128.15 Mpa and 58Mpa. The microstructure of fracture surfaces from tensile and shear test were analysized.

Abstract: Barium strontium titanate glass-ceramics with different La₂O₃ concentrations have been prepared by a melting-annealing technique. The effect of La₂O₃ concentration on phase evolution, microstructure and electrical properties of (Ba,Sr)TiO₃-Al₂O₃-SiO₂ glass-ceramics has been investigated. The X-ray diffraction results showed the volume fraction of the major perovskite (Ba,Sr)TiO₃ crystalline phase increased with increasing La₂O₃ concentration. In addition, the scanning electron microscopy observations revealed that small amounts of La₂O₃ could accelerate the growth of crystals. When the La₂O₃ concentration was increased to 1.5 mol%, the microstructure of these samples displayed nonuniform distribution of the crystals. The dc conductivity measurements showed an increasing temperature dependence of conductivity with La₂O₃ doping at high temperature range above 400 °C. Impedance spectroscopy analysis indicated that the additive of La₂O₃ changed the major polarization mechanism to the relaxation polarization and the relaxation time for the La₂O₃ doped samples was much shorter than the undoped ones. The frequency dependence of dissipation factor curve displayed a peak at the range of 1 to 10 kHz due to the La₂O₃ addition. And the peak value decreased with increasing La₂O₃ concentration.

Abstract: This study reports the synthesis of bismuth ferrite ceramics (BiFeO₃, BFO) by the sol-gel method and the effect of different sintering temperatures (800 °C, 820 °C, 850 °C, 880 °C, 900 °C) on the crystal structure, morphology, dielectric properties and ferroelectric properties of the bismuth ferrite ceramics. It is observed that the bismuth ferrite phase are formed when the molar ratio of bismuth and ferric is 1.025 : 1, and the bismuth ferrite ceramics have less impurity phase at all the five sintering temperature. The morphology studies revealed that the grain of bismuth ferrite ceramics is spherical and the grain size increases with the increasing of the sintering temperature. When the bismuth ferrite ceramics with excessive bismuth sintered at 820 °C, it exhibits the higher dielectric constant and the lower dielectric loss, the remnant polarization and the coercive field are 0.35 μC/cm² and 4.03kV/cm, respectively. The observed ferroelectricity in bismuth ferrite ceramics owes to the point defects associated with oxygen vacancy defects as well as the smaller size effect.

Abstract: Cu matrix composites with various contents of Ti₂AlN were fabricated by powder metallurgy using spark plasma sintering (SPS) method. Ti₂AlN ceramic particles were pre-treated by electroless copper plating, the result showed that Ti₂AlN reinforcement and Cu matrix were strongly bonded. The effects of Ti₂AlN content on microstructure, electrical resistivity and mechanical properties were systematically investigated. With the addition of low fraction of Ti₂AlN, the hardness and tensile strength of matrix were improved without losing the fracture toughness too much. Tensile strength of Ti₂AlN/Cu composites were about 380 MPa with the content of 7 wt. % reinforcing phase, and the conductivity of the composites remained about 61.5 % IACS. Moreover, the wear tests illustrated that the loads were effectively born by the Ti₂AlN reinforcement, the main tribological mechanism changed from adhesive wear to abrasive wear compared with Cu, thus the friction and wear resistance was also obviously improved.