نشریه علم و مهندسی سرامیک
سال دوم شماره 1 (پیاپی 4، بهار 1392)

As a high temperature structural ceramic، cermets exhibit less brittle than the ceramic-ceramic composites or the monolithic ceramics in presence of the ductile metal second phase. On the other hand، they have higher refractory، higher creep and thermo-mechanical resistance and higher abrasive resistance in comparable of metals. In the present study، Alumina-Chromium cermets with different proportions of Al2O3، Cr2O3 and Al، different ball-milling time and different heat treatment temperatures have been fabricated. In order to evaluate the optimum manufacturing conditions، specimens with different composition of ball-milled starting materials and for different times were subjected to DTA and TGA analysis. Obtained materials were characterized by microstrutural observations (SEM)، phase analyses (XRD)، mechanical properties measurements (toughness and strength- three point bending test) and by hardness testing (Vickers). The results of DTA show that for ball-milling more than two hours، there is a decreasing of intensity and temperature of exothermic peak relating to thermit reaction. Higher proportion of x in starting materials، 3xAl، xCr2O3، (1-x) Al2O3، cause increasing the density and strength until x=0. 5 and increasing toughness of sintered samples until x=0. 7. The hardness however، constantly decreases on increasing of the x value. In addition، the mechanical properties of sample with x=0. 5 is improved by increasing of the sintering temperature to 1550 °C. he difference between thermal expansion coefficient of the metallic and ceramic phase، oxidation of the chromium phase، low wetting of alumina by chromium phase and a coarser structure are the principle responsible for decreasing of properties in higher value of x and high sintering temperature.

In this article، the effect of type and content additive on sintering behavior and properties of translucent alumina ceramics were investigated. α-Al2O3 nanopowder with various content of additives (MgO، La2O3)، was compacted under a pressure of 300MPa and was sintered at temperatures of 1700 and 1875 ºC. The microstructure of sintered samples were studied by scanning electron microscope (SEM) and Optical microscope. The results indicate that with increasing content of MgO increase formation of Spinel secondary phase within grains that these flaws act as points of scattering and reduce the transparency of the sample. With increase La2O3 alumina grains with almost no pores trapped in، become fine and compact. Alumina codoped with 0. 12%wt MgO and 0. 2%wt La2O3 sintered at 1750 °C for 1 h show maximum transmittance 60%، hardness 1840 VHN and relative density 99. 95%.

In this study، mechanical milling was used to synthesize nanostructured Al2024 and Al2024-B4C nanocomposite powders in attrition mill under argon atmosphere up to 50 h. The mechanically milled powders were extruded at an extrusion ratio of 10:1. X-ray diffraction (XRD)، tensile and hardness tests were used to characterize the powders and hot extruded samples. The morphology of the mechanically milled powders was also investigated by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Results showed that the addition of B4C particles accelerate the milling process، leading to a faster work hardening rate and fracture of aluminum matrix. Furthermore، the results revealed a lower elongation، higher strength and hardness for Al2024-B4C nanocomposite in contrast to the coarse grained Al2024 and nanostructured Al2024 matrix.

In this study، mesoporous alumina powder was synthesized by evaporation-induced self-assembly (EISA) method in the presence of two different solvent types، including ethanol and tetrahydrofuran. The alumina powder was prepared using aluminum isopropoxide and aluminum nitrate as precursors and Pluronic P123 as a polymer template for the self-assembly. As-prepared powders were characterized by thermal analysis (DTA/TG) and fourier transform infrared spectroscopy (FTIR). After calcination at 400 ºC for 8 h، the samples were evaluated using nitrogen adsorption- desorption isotherms، small and wide angle X-ray diffraction، scanning probe microscopy and high resolution transmission electron microscopy. The results of FTIR and DTA/TG confirmed elimination of the surfactant after calcination. The nitrogen adsorption-desorption isotherms showed type IV، which is a characteristic of mesoporous materials. The prepared sample in ethanol and tetrahydrofuran showed H2 and H3 hysteresis loops characteristic، indicating wormlike and slit-like pores، respectively. The surface area and pore volume for the prepared samples in ethanol were 100 m2/g and 0. 35 cm3/g and those for the prepared samples in tetrahydrofuran were 120 m2/g and 0. 4 cm3/g، respectively. The BJH results showed a narrower pore size distribution when tetrahydrofuran was used as solvent.

Spontaneous opal glasses were synthesized by addition of ZrO2، P2O5 and TiO2 to a base glass composition of the system ZnO-B2O3-SiO2. The opacity of the samples was measured by UV-Visible spectrophotometry and their chemical resistance was determinedina harsh alkaline environment، according to ISO 695. Based on the microstructural analysis، while addition of P2O5 led to a liquid-liquid phase separation، addition of ZrO2 and TiO2 led topartially crystallized glasses. Furthermore، The ZrO2bearing glass، which opacified through crystallization of zirconia، willemite and gahnite، showed the best chemical resistance.

Lead free Bi0. 5 (Na1-xKx) 0. 5TiO3 piezoceramics (BNKTx) with various amount of potassium (x=0. 18، 0. 20، 0. 22 and 0. 24) around morphotropic phase boundary (MPB) were prepared via a conventional solid state reaction. The crystal structure، electrical properties and strain were significantly influenced by the amount of K+ substitution. The XRD patterns revealed that by increasing K+ amount the tetragonal phase increased as well as more K+ content resulted in the enhanced piezoelectric properties، a large piezoelectric constant d33=195 pC/N was obtained at x=0. 20، however depolarization temperature (Td) reached to 100 oC. The BNKTx ceramics exhibited high remnant polarization Pr= 37. 50 µC/cm2 of the MPB composition (x=0. 20). The increase of K+ content destabilized the ferroelectric order of the BNKTx ceramics، leading to degradation of the remnant polarization، reduction of coercive field and changing the shape of hysteresis loop towards antiferroelectric phase. An outstanding large electric field induced strain (S=0. 13%) under 1 kV/mm and normalized strain d_33^*= 1320 pm/V were obtained at the MPB for x=0. 20

Iron oxide nanoparticles were synthesized by coprecipitation method using NaOH as precipitation agent. Magnetic properties of nanoparticles were measured by VSM. The effect of concentration of reactants on the size and magnetic properties of nanoparticles was investigated. The size of particles was calculated by Sherrer formula and magnetic data، and then compared with the size of nanoparticles obtained by TEM. The size of produced nanoparticles ranged from 8 to 12 nanometer and their saturation magnetizations were between 65 to 84 emu/gr. It was found that decrease of concentration ratio of NaOH to iron ions increases the saturation magnetization of nanoparticles significantly، but not their size.