Journal of Advanced Materials in Engineering (Esteghlal)

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In this study, forsterite nanopowder was prepared by mechanical alloying and post-heat treatment method. Bioactive properties of forsterite nanopowder were studied by immersing the powder in the SBF. Nanostructure forsterite bulk dense form was prepared by the two step sintering method. It was found that pure forsterite nanopowder with 25-60nm particle size was produced. The results of soaking of forsterite nanopowder in the SBF showed that forsterite nanopowder is bioactive. Also, forsterite dense bulk with the optimal hardness of 940 Hv and fracture toughness of 3.61 MPa.m1/2 was produced. These findings suggest that forsterite nanostructure ceramics possess good biocompatibility, bioactivity and mechanical properties and could be suitable for orthopedic and dental implant materials.

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In the present study, a bioceramic-based composite with remarkable mechanical properties and in vitro apatite forming ability was synthesized by sintering compacts made up of mixtures of hydroxyapatite (HA) and sol-gel derived bioactive glass (64SiO2-26CaO-5MgO-5ZnO) (based on mole %). HA was synthesized through co-precipitation method. The stabilization temperature of the bioactive glass was set to be 700 ºC according to simultaneous thermal analysis (STA). Laser Particle Size Analysis (LPSA) was used to compare the particle size distributions of the synthetic powders. HA matrix was mixed with different weight percentages of bioactive glass (5, 10, 15, 20, 25 and 30 wt. %) and compressed by 80 MPa pressure. After sintering the uniaxial compression test of the samples was done and the specimen with the highest compressive strength (20 wt. % bioactive glass) was selected to be immersed in the Simulated Body Fluid (SBF) for 3, 7 and 14 days. The results showed that the compressive strength of the sample decreased after keeping it in the SBF. Also, inductively coupled plasma analysis (ICP) was used to study the ion release behavior of the sample in the SBF. Finally, phase composition, microstructure and functional groups in the composite were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform infra-red spectroscopy (FTIR) techniques, respectively.

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In this study, mullite– irconia composite samples were prepared by reaction sintering of alumina and zircon powder via gel casting process. Gel casting is a new ceramic forming technique. This process is based on the casting of slurry, containing ceramic powder, dispersant and premix monomer solution. To achieve stabilized, high solid loading (80 wt%) and castable slurry, the rheological properties of slurry were optimized. The monomers polymerized the slurry to form gelled specimens. After gelation, the specimens were unmolded, then dried out under controlled condition. Burning out and sintering of the specimens was carried out in the range of 1400-1700°C. Apparent porosity and bulk density of the sintered samples were measured by soaking in water. Crystalline phase evolution and microstructure were determined by XRD and SEM techniques. Results showed that the reaction sintering and mullite formation was completed at 1700°C due to very slow diffusion of Al3+ ions within amorphous silica formed at the decomposition of zircon. The sintered samples at this temperature also showed the lowest apparent porosity (≈ 4%) and the highest bulk density (≈3.40 gr/cm-3).

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In this article, sintering activation methods of Mo powder (chemical, mechanical and surface activation) were studied. For this purpose, the milled/reduced Mo nanopowder was sintered at 900, 1100 and 1400 ºC for 1 hr. For comparison of sintering activation methods (mechanical and chemical activation) and their effects on microstructural characteristics, commercial micropowder Mo as well as Ni additive was used. The samples were compacted under a pressure of 400 MPa and then sintered at 1400 ºC for 1 hr. The microstructure of sintered samples was studied by scanning electron microscope (SEM) along with EDS. Phase analysis was performed using X-ray diffraction (XRD) technique. The sintered densities of samples were measured by Archimedes method. Relative densities of specimens obtained from micro, nano, micro+20% nano and micro+1.5% Ni additive powders were attained as 80%, 93%, 86% and 95%, respectively. It was found that the δ-NiMo intermetallic layer may be formed at the grains' boundaries due to Ni additive, leading to grain boundary microcracks as well as loss of mechanical properties of samples.

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In this study, the effect of different amounts of Y2O3 on the properties of mullite-zirconia composites was investigated. For this purpose, these composites were fabricated by reaction-sintering of alumina and zircon as raw materials. Besides, the slip casting method was used for forming these composites, and sintering process was carried out at 1600 °C. Then, the physical and mechanical properties, phase composition and the microstructure of these composites were investigated. The results showed that yittria addition up to 0.5 wt.% has no effect on the properties of these composites. Besides, addition of more than 0.5 wt.% yittria formed solid solution with zirconia grains and led to stabilization of tetragonal zirconia phase and increasing of its amount. Hence, yittria addition increases the hardness and bending strength of composite by stabilizing tetragonal zirconia phase and then, decreasing the micro-crack formation during zirconia phase transformation. As results show, addition of 0.75 wt.% yittria leads to a considerable increase in the bending strength.

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In this study, ZrB2-HfB2 composite was produced by pressureless sintering method. MoSi2 B4C and SiC particles were used as reinforcement. ZrB2 powder was milled in planetary ball mill apparatus and then reinforcement particles were added to the milled powder. The composite powders were then CIPed and sintered at 2100oC and 2150oC. Scanning electron microscopy (SEM) with an energy dispersive X-ray spectrometer (EDS), flexural test, and resonance frequency method were used to compare the added particle effects on mechanical properties and pressureless sintering behavior of ZrB2-HfB2 composite. The analysis showed that the ZrB2-HfB2-MoSi2-SiCnano composite displays the largest gain in flexural strength. Furthermore, increasing the sintering temperature leads to an increase in flexural strength of samples.

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In this study, the effect of different amounts of Y2O3 on the properties of mullite-zirconia composites was investigated. For this purpose, these composites were fabricated by reaction-sintering of alumina and zircon as raw materials. Besides, the slip casting method was used for forming these composites, and sintering process was carried out at 1600 °C. Then, the physical and mechanical properties, phase composition and the microstructure of these composites were investigated. The results showed that yittria addition up to 0.5 wt.% has no effect on the properties of these composites. Besides, addition of more than 0.5 wt.% yittria formed solid solution with zirconia grains and led to stabilization of tetragonal zirconia phase and increasing of its amount. Hence, yittria addition increases the hardness and bending strength of composite by stabilizing tetragonal zirconia phase and then, decreasing the micro-crack formation during zirconia phase transformation. As results show, addition of 0.75 wt.% yittria leads to a considerable increase in the bending strength

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Simultaneous application of mechanical pressure and electrical charge on powder samples in spark plasma sintering process, has resulted in a sample with a density close to the theory. In the present study, a thermal-electrical-mechanical coupled finite element model of spark plasma sintering system using multi-objective optimization algorithm is proposed to optimize the mold variable. The simulation performed for Si₃N₄-SiO₂ (1:1 mol) specimen has good agreement with the experimental results. Multi-objective genetic algorithms was used for optimization of mold design in order to maximize the temperature of sample core and minimize the mises stress in the mold. The results show that the optimized dimensions cause 8% increase in sample temperature and about 18% decrease in temperature difference between mold surface and sample core. This leads to better uniformity in the porosity distribution of final sample.

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Magnesium-manganese ferrite nanopowders (Mg_xMn_1-xFe₂O₄, x=0.0 up to 1 with step 0.2) were prepared by coprecipitation method. The as-prepared samples were pressed with hydrolic press to form a pellet and were sintered in 900, 1050 and 1250˚C. Scanning Tunneling Microscope (STM) images showed the particle size of powders about 17 nm. The X-ray patterns confirmed the formation of cubic single phase spinel structure in samples sintered at 1250˚C. Substituting Mg²⁺ with Mn²⁺ in these samples, the lattice parameter decreased from 8.49 to 8.35Å and magnetization saturation decreased from 74.7 to 21.2emu/g. Also, coercity (H_C ) increased from 5 to 23Oe and Curie temperature (T_C ) increased from 269 to 392˚C. Samples with x= 0.2, 0.4, 0.6 sintered at 1250 ˚C, because of their magnetic properties, can be recommended for hyperthermia applications and for phase shifters.

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In this paper, the possibility of elephant foot phenomenon in sintered alloys with volatile components has been studied. To do this, Cu-28Zn brass samples were sintered at the range of 890-970°C for 20 min. The in situ images from brass samples were taken at various sintering conditions. It is concluded that although liquid is pulled down by gravity, but elephant phenomenon was not appeared in these compacts. Instead, the samples were swelled. Chemical composition change and pores coarsening due to zinc evaporation could be considered as the main cause of swelling in brass compacts.

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In this study, Spark Plasma Sintering (SPS)  of both slip casted and powder specimens of alumina/ yttria core-shell nanocomposite were utilized for fabricating transparent Yttrium Aluminum Garnet (YAG) ceramics. Phase evolution, optical transmittance and the microstructure of sintered samples were compared. In slip casting process, Dolapix CE64 was used as a dispersant for preparing the stable aqueous slurry of this nanocomposite powder. The effect of Dolapix concentration and pH value on the stability of the suspension was described, and the viscosity diagrams were investigated at different pH value and different weight percents of Dolapix. The rheological behavior of the nanocomposite powder slipped at 60-70 wt% solid loading was studied by measuring their viscosity and shear stress as a function of shear rate of the slurry. The results showed that, the suspension has a minimum viscosity at pH of 10 by addition of 2.5 wt% Dolapix. Also, the slurry with solid loading of 60 wt% showed the Newtonian behavior and this rheological behavior was preserved even above this solid loading values. Slip casting technique caused the uniform size and pores distribution as well as eliminating large pores in the green body. Consequently, transparent YAG ceramic with 60% optical transmittance was achieved after SPS process of slip casted green body which was much higher than that of nanocomposite powder, i.e. about 30% at the same sintering conditions.
 

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