Papers by Keyword: Pressureless Sintering

Abstract: Potassium sodium niobate ((K_0.5Na_0.5)_1-xLi_xNb_1-yTa_yO₃) powers were prepared by solid phase synthesis and (K_0.5Na_0.5)_1-xNb_1-yTa_yO₃ lead-free piezoelectric ceramics were fabricated by the pressuerless sintering. The relationship among the powder phase composition and electric properties was studied. The results show that when x=0.5, the sintering temperature is 1050°C,and the polarization voltage is 3 kV/mm, the (K_0.5Na_0.5)_1-xLi_xNb_1-yTa_yO₃ piezoelectric ceramics with the better piezoelectric properties are prepared. (The relative density is 94%, the piezoelectric constant d₃₃=105pC/N, the planar electro-mechanical coupling coefficient k_p=0.39, the medium loss tanδ=0.29, the mechanical quality factor Q_m=45, and the dielectric constant ε_r=720). As the increase of the polarization votage, the ceramic piezoelectric properties are increased. When the polarization voltage E=3kV/mm, the polarization tends to be saturation on the whole.

Abstract: A high purity of Ti₂AlC powder has been synthesized by pressureless sintering a mixture of Ti-Al-TiC-Sn (Sn as an additive) powders. Four recipes with different mole ratios of Ti-Al-TiC-Sn were examined at sintering temperature from 1400°C to 1480°C. A high purity of Ti₂AlC powder can be obtained by sintering all these four recipes at temperature 1450°C for 10 min in an Ar atmosphere. The synthesis of Ti₂AlC on this large mole ratio scale of starting materials is associated with the evaporation of Al at high temperature and the structure stability of Ti₂AlC. From the X-ray diffraction analysis, a reaction path for the Ti₂AlC formation is proposed. Scanning electron microscopy was also used to characterize the samples.

Abstract: The ZrB₂-SiC composite ceramic was prepared using Al₂O₃ and Y₂O₃ as additives through pressureless sintering under 1800°C. The electrical and mechanical properties were studied with the different mass ratio of ZrB₂ and SiC. The microstructure and phase transformation of the ZrB₂-SiC composites ceramic were characterized by scanning electron microscopy. Results show that the densities of the samples are decreased with the decrease of the mass ratio of ZrB₂ and SiC while the resistivities are increased. When the mass ratio of ZrB₂ and SiC is 4 the resistivity reaches 14μΩ•mm while the density is 4.85g/cm³. When the mass ratio of ZrB₂ and SiC is13:7, the flexural strength reaches the max value and toughness reaches the minimum value. The flexural strength reaches 233Mpa and the toughness reaches 4.51 Mpa• m ^1/2. The changes of the properties of the ceramic were analyzed through the microstructure and the conduction mechanism was studied.

Abstract: In this paper, silicon nitride porous ceramics with high porosity and bimodal pore structure were prepared using pressureless sintering at 900~1100°C. In these porous ceramics, zirconium phosphate (ZrP2O7) was used as a binder and starch and naphthalene powders were used as pore forming agents. The obtained results showed that the porosity could be controlled in the range of 34 % to 70 % by changing the content of pore forming agents. The pores were formed by the continuous reaction of ZrP2O7 at ~250 °C and burnout of starch at ~550 °C (when starch was used as a pore forming agent), or sublimation of naphthalene at 80°C (when naphthalene was used as a pore forming agent). The bimodal pore structure was produced with pore size of less than 0.5m and ~10 m when using starch as a pore forming agent and the pore size of less than 0.5m and ~30μm when using naphthalene as a pore forming agent.

Abstract: In this paper, titanium silicon carbide (Ti₃SiC₂) powders were synthesized from TiH₂ as Ti source by pressureless sintering in flowing argon atmosphere without preliminary dehydrogenation. Starting materials are powder mixtures with the mole ratio of 3TiH₂/Si/2C or 3TiH₂/SiC/C. Both kinds of starting materials were sintered in a tube furnace at the temperature range from 1300°C to 1500°C for 10~180min in flowing argon atmosphere. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize the phase compositions and morphology of the products after different treatments. It was showed that almost single phase Ti₃SiC₂ powder (94.7 wt.%) can be synthesized by pressureless sintering from 3TiH₂/Si/2C powders at 1400~1425°C for about 180min or from 3TiH₂/SiC/C powders at 1425~1500°C for about 180min. From SEM micrographs, as-synthesized samples were porous. Most plate-like grains were about 5~10 μm in diameter and 1~2 μm in thickness. The speed of temperature increasing is an important factor to affect the purity of as-synthesized Ti₃SiC₂.

Abstract: SiC-TiB2 composite was prepared at 2298 K by using pressureless sintering. The microstructures and mechanical properties of the SiC-TiB2 composite were investigated. The SiC-TiB2 composite with 5 mass % B4C-C showed the optimized comprehensive properties. The volume density, hardness, flexural strength and fracture toughness of that were 3.43 g/cm3, 18.5 GPa, 353 MPa and 4.67 MPa•m1/2, respectively. The excessive B4C-C content might decrease the mechanical properties of the composite.

Abstract: Ternary titanium aluminum carbide powder was prepared by pressureless sintering at different temperature using elemental powder mixture of Ti, Al, and active carbon whose molar ratios was 3Ti/1.1Al/1.8C and effects of doping Si were discussed from 3.0Ti/1.0Al/0.1Si/1.8C and 3.0Ti/0.9Al/0.2S/1.8C. X-ray diffractions (XRD) patterns were used to detect the phase composition and scanning electron microscope (SEM) patterns were observed to investigate the microstructure of samples respectively. Results showed that Ti₂AlC and Ti₃AlC₂ which had obvious layered structure were synthesized by pressureless sintering of 3Ti/1.1Al/1.8C at 1300°C-1500°C. However, the samples doping Si were made of Ti₂AlC, Ti₃AlC₂ and Ti₃SiC₂. When the amount of Si-doped increased and Al/Si decreased, the amount of Ti₃AlC₂ and Ti₃SiC₂ went up with Ti₂AlC going down. Interestingly, the character of layered structure became unconspicuous at same temperature. Moreover, the mechanism that doping Si was not helpful to form Ti₂AlC but Ti₃AlC₂ was explained.

Abstract: The low-cost production and performance of 6061-aluminium alloy matrices reinforced with coated silicon carbide particulate has been studied. The micro-wave vacuum sintering was adopted to prepare the composite in order that the loss ignition is very small. The effect of copper coated silicon carbide has proved beneficial to interfacial bonding and improved the mechanical properties. Differences in the fracture characteristics of specimens containing coated and non-coated particles were observed. The particulates size has much influence on mechanical properties.

Abstract: In the present study, hydroxyapatite was synthesized from local gypsum by microwave-hydrothermal method. Different percentage amounts of zirconia (0, 20, 30 and 40 wt.%) and poly-methyl methacrylate (40, 50 and 60 wt.%) mixed with hydroxyapatite (HA) for six hours. These powder mixture were deposited using deposition machine to produce specimens. These specimens were sintered at a temperature of 140°C with holding time for 1 hour into the green parts. These green parts were sintered at temperature of 1450°C with holding time for 2 hours. This process produces porous hydroxyapatite-zirconia composites with porosity between 62.76-73.92 percent. These composites were examined by XRD, XRF, SEM-EDX, BET analysis and compressive strength testing. Compressive strength of porous hidroxyapatite-zirconia composite decreased from 3.706 to 0.039 MPa when percentage amounts of zirconia increased up to 40 wt.%. This caused by several factors i.e. increased porosity, grain zirconia cracked, zirconia reacted with HA to produce CaZrO₃, β-TCP and α-TCP, HA matrix cracks because of the phase change of tetragonal-zirconia into monoclinic-zirconia.

Abstract: In this work, porous TiB₂ ceramics were consolidated by pressureless sintering method using metallic Ti and Fe as additives in order to perform sintering at temperatures lower than 1700°C. It was shown that processing parameters including milling time of the starting mixture had a considerable effect on final properties of sintered specimens and their behavior in molten aluminum. Microstructural studies were carried out using optical microscope, SEM and EPMA. It was found that specimens with uniform and crack-free microstructure could be produced using the pre-mixed powders milled for as low as 30 min prior to compaction and sintering. Sessile drop test was performed on the specimens milled for 30 and 240 minutes. Their interaction with molten aluminum was also studied. It was found that 30 min milling time resulted in better electrical conductivity, wettability and stability in liquid aluminum.