Iranian Journal of Materials Science and Engineering

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The combined influence of both melt filtration and cooling rate on the microstructure features and mechanical properties of A356 cast alloy was studied. A step casting model with five different thicknesses was used to obtain different cooling rates. The effect of melt filtration was studied by using of 10 and 20 ppi ceramic foam filters in the runner. Results showed that secondary dendrite arm spacing decreased from 80 μm to 34 μm with increasing cooling rate. Use of ceramic foam filters in the runner led to the reduction of melt velocity and surface turbulence, which prevented incorporation of oxide films and air in the melt, and consequently had an overall beneficial effect on the quality of the castings. A matrix index, which is the representative of both SDAS and microporosity content, was defined to consider the simultaneous effect of melt filtration and cooling rates on UTS variations. Also, the fracture surface study of test bars cast using 10 and 20 ppi ceramic foam filters showed features associated with ductile fracture.
 

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An accurate prediction of reaction kinetics of silicon nitridation is of great importance in designing procedure of material production and controlling of reaction. The main purpose of the present study is to investigate the effect of temperature on the kinetics of reaction bonded silicon nitride (RBSN) formation. To achieve this, nitrogen diffusion in the silicon nitride layer is considered as a reaction controlling factor and sharp interface method based on this theory is used to develop the analytical model. In the developed model, the variations in the size of silicon particles are calculated for the whole reaction. In the experimental phase, the extent of nitridation is measured for different reaction temperatures and 4 different reaction times and then, the occurrence of full nitridation is shown by EDS analysis. Furthermore, an analytical approach was established for describing the kinetics of compound formation and the performance of the developed model is evaluated through statistical analysis. There was good agreement between experimental data and predictions of the developed model which demonstrates the accuracy of considered presumptions and reaction mechanisms. An accurate prediction of reaction kinetics of silicon nitridation is of great importance in designing procedure of material production and controlling of reaction. The main purpose of the present study is to investigate the effect of temperature on the kinetics of reaction bonded silicon nitride (RBSN) formation. To achieve this, nitrogen diffusion in the silicon nitride layer is considered as a reaction controlling factor and sharp interface method based on this theory is used to develop the analytical model. In the developed model, the variations in the size of silicon particles are calculated for the whole reaction. In the experimental phase, the extent of nitridation is measured for different reaction temperatures and 4 different reaction times and then, the occurrence of full nitridation is shown by EDS analysis. Furthermore, an analytical approach was established for describing the kinetics of compound formation and the performance of the developed model is evaluated through statistical analysis. There was good agreement between experimental data and predictions of the developed model which demonstrates the accuracy of considered presumptions and reaction mechanisms.

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Spread extrusion is a capable method to produce different samples with a wider cross-section from the smaller billets in a single processing pass. In this study, dish-shaped samples are successfully produced from the as-cast cylindrical AM60 magnesium alloy at 300 °C, the mechanical properties and microstructural changes of the final specimens are precisely evaluated. Due to the high amount of plastic strain, which is applied to the initial billet during the material flow in the expansion process, grain refinement occurred as a result of recrystallization and subsequently good mechanical properties achieved. Therefore, mean grain size reduced from 160 µm to 14 µm and initial equiaxed grains changed to the elongated ones surrounded by fine grains. Also, microhardness measurements indicate that hardness increased from 51 Hv to 70 Hv. Some fluctuations were also observed in the hardness profile of the sample which was mainly related to the bimodal structure of the final microstructure. Good mechanical properties, fine microstructure, and also the ability to produce samples with higher cross-section make the spread extrusion process a promising type of extrusion.

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In this work, lithium meta titanate (Li₂TiO₃) nanocrystallites were synthesized by hydrothermal method and subsequent heat treatment. The shrinkage of the powder compact was measured under constant heating rate in order to study the sintering behavior of the synthesized powders. Densification curves of the synthesized powders were also constructed via the dilatometry analysis and evaluated at several heating rates. Two separate methods of analytical procedure and master curve sintering were employed to determine the activation energy of the initial sintering stage. The activation energy values were estimated based on these two distinct methods as 229±14 and 230 kJ/mol respectively, consistenting with each other. Moreover, surface diffusion was determined as the dominant mechanism of densification on initial sintering of Li₂TiO₃ nanocrystallites.

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In this paper the chemical interaction between catalyst and support has been studied to understand the observed different growth rate of CNTs in our previous paper. Both pure MgO and Mg(NO₃)₂ . 6H₂O as sources of the MgO catalyst support and Fe₂(SO₄)₃ · xH₂O as the source of the Fe catalyst, were employed. A Fe catalyst supported on MgO has been synthesized using the wet impregnation method followed by calcination. To compare the catalyst grain size and its distribution, the sample were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray powder diffraction (XRD) and BET specific surface area (SSA) measurement and X-ray photoelectron spectroscopy (XPS). XPS technique have utilized complementary to demonstrate the existence of chemical interaction between MgO support and Fe catalyst. Results revealed that the type of precursor used to prepare the support has a significant influence on the morphology of the support and the associated distribution of the Fe catalysts. The highest yield of MgFe₂O₄ phase was obtained using a pure MgO precursor which after calcination results in a homogenous distribution of nano-sized Fe particles over the support surface

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The aim of the present study is to assess the hardness, corrosion, and cytotoxicity of a commercially available cobalt-chromium (Co-Cr) alloy before and after simulated heat treatments at porcelain firing temperature. Five Co-Cr samples were fabricated using lost wax casting procedure. Heat treatments were carried out at 650°C, 750°C, 850°C, and 950°C. Vickers hardness was measured for as-cast and heat treated samples. The corrosion test was carried out separately in 0.1 N NaCl, 1% citric acid and artificial saliva at room temperature using potentiodynamic polarization technique. Gingival tissue biopsy of patients was taken and cultured to measure the cell viability by MTT colorimetric assay. Lowest hardness was observed at 650°C. 0.1 N NaCl and 1% citric acid corrosion medium showed a similar trend of corrosion rate. The least corrosion rate was found in artificial saliva. Firing temperature has an impact on the physical, chemical and biological properties of Co-Cr alloy in long-term clinical use.

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In the present study, Zirconium modified aluminide coating on the nickel-base superalloy IN-738LC was first created by high activity high temperature aluminizing based on the out-of-pack cementation method. Then, Zr coatings were applied to simple aluminide coatings by sputtering and heat treatment in order to study the effect of Zr on the coating microstructure and oxide spallation. Microstructural studies were conducted by using scanning electron microscopy (SEM), Energy Dispersive X-ray Spectrometry (EDS), and x-ray diffraction (XRD) microanalysis. The results indicated that zirconium modified aluminide coating, like aluminide coating, has a two-layer structure including a uniform outer layer of NiAl and an interdiffusion layer in which zirconium is in a form of solid solution in the coating. Furthermore, the 300nm Zr-coated NiAl demonstrated an excellent scale adhesion, a slow oxidation rate and lower amounts of some other elements such as Ti and Cr in its oxide layer leading to a pure aluminide oxide layer. 

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In this research, the nanostructured titania-coated silica microsphere (NTCSM) membrane consisting of titania-silica core-shell particles on α–alumina substrate was prepared by dip-coating method. The silica microspheres were synthesized by the Stöber method, and the nanostructured titania shell was obtained from a polymeric sol. Then, the prepared core-shell particles were deposited on alumina substrates. The samples were characterized by DLS, TG-DTA, XRD, FTIR and SEM. The photo-catalytic activity of the NTCSM membranes was evaluated using photo-degradation of methyl orange solution by UV–visible spectrophotometer. Also, physical separation capability was investigated by filtration experiment based on methyl orange removal from aqueous solution using a membrane setup. The mean particle size distribution of silica microspheres was determined to be about 650 nm that by deposition of titania nano-particles increased up to about 800 nm. After 60 min UV-irradiation, the dye removal efficiency was determined to be 80% by the membrane. By coupling separation process with photo-catalytic technique, the removal efficiency was improved up to 97%. Thus, the NTCSM membranes showed simultaneous photo-degradation and separation capabilities for dye removal from water.
 

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Kinetic models are the most important instruments for predicting and evaluating the performance of flotation circuits. To determine the kinetic order and rate of flotation of a gilsonite sample, flotation experiments were carried out in both rougher and cleaner stages. Experiments conducted using the combinations of petroleum-MIBC, gas oil-pine oil, and one test without any collector and frother. Five first order kinetic models were applied to the data obtained from the flotation tests by using the Matrix Laboratory software. Statistical analysis showed that the classic first order model perfectly matched the rougher and cleaner results performed using petroleum-MIBC combination. The kinetic constants (k) were calculated as 0.04 (s^-1) and 0.01 (s^-1) for the rougher and cleaner, respectively. Rougher and cleaner tests without collector and frother also matched with the modified gas/solid adsorption and rectangular models with the k values of 0.05 (s^-1), and 0.01 (s^-1), respectively. The relationship between flotation rate constant, maximum combustible recovery and particle size were also studied. The results showed that the maximum flotation combustible recovery and flotation rate were obtained with an intermediate particle size both in the rougher and cleaner flotation processes. The combustible recovery and flotation rate in the rougher flotation process were higher than that in the cleaner flotation process.

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In the present study, the effect of adding minor amounts of scandium and zirconium elements to the 7075 alloyon the re-crystallization behaviour of one aluminium alloy (7000 series) was investigated. For this purpose, two kinds of Al-Zn-Mg-Cu-Sc-Zr alloys with the same amount of Zr and different amount of Sc were prepared. Homogenization durations and temperatures of alloys after alloying were obtained by DSC analysis and optical microstructure observations. The results showed that the optimum homogenization temperatures for Al-Zn-Mg-Cu-0.05Sc-0.1Zr and Al-Zn-Mg-Cu-0.1Sc-0.1Zr alloys were 500⁰C and 490⁰C respectively, and the optimum duration for both alloys was 12hours. After homogenization of alloys, the re-crystallization behaviour of the alloys was investigated by Brinell hardness test. Obtained results showed that although the starting re-crystallization temperature for both alloys was similar in 2 hours, but it was 130°C for alloys with 30% forming, and 120°C for alloys with 50%forming and recrystallization temperature for Al-Zn-Mg-Cu-0.1Sc-0.1Zr alloy was 350in 2 hours. Despite what was expected, the hardness of Al-Zn-Mg-Cu-0.05Sc-0.1

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Strong glass-ceramic foams with a compressive strength of 20 MPa were prepared by adding various amounts of Fe₂O₃ to a soda lime-based glass composition, and SiC as a foaming agent. The foams were prepared by firing the compacted samples in the range of 750–950°C for different soaking times. The crystallization behavior of the samples was investigated by Simultaneous Thermal Analysis (STA), Scanning Electron Microscope, and X-Ray Diffractometer (XRD). Based on the results, solid solutions of pyroxene groups were crystallized by the surface mechanism, between 730˚C and 900˚C during the firing of the specimens, and their amounts increases with increasing of the added iron oxide. Besides, we found that Fe₂O₃ neither acts as a nucleant for pyroxene nor as an oxidizer for SiC. The results also showed that the compressive strength as well as the crystallization behavior of the foams was influenced by the presence of the SiC particles.

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The graphene oxide -TiO₂ (GO-TiO₂) and pre-reduced graphene oxide -TiO₂ (rGO-TiO₂) nanocomposites were fabricated successfully by hydrothermal method. The microstructure of synthesized nanocomposites was investigated using field emission scanning electron microscopy (FESEM) equipped with energy dispersive spectroscopy (EDS) analysis. Moreover, galvanostatic charge/discharge (GCD), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) methods in three electrode system were applied to evaluate electrochemical properties. The results revealed that nanoparticles distributed more uniformly on graphene sheets, at lower concentrations of TiO₂. The rGO-TiO₂ and GO-TiO₂ nanocomposites showed 224 and 32 F/g specific capacitance at 5 mV s^-1 scan rate in 1 M KOH aqueous electrolyte, respectively. The pre-reduction of graphene oxide is the main reason for the better electrochemical performance of rGO-TiO₂ nanocomposite compared to GO-TiO₂ nanocomposite.

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Flash sintering of 8 mol% yttria-stabilized zirconia (8YSZ) as solid oxide fuel cell (SOFC) electrolyte is studied. The relation between relative density, shrinkage, sample temperature during the flash, and incubation time, with the electric field strength, current density, as well as contact paste, are modeled by response surface methodology (RSM). The electric field strength and current density varied from 50 to 400V.cm^-1 and 50 to 200mA.mm^-2, respectively. Also, platinum (Pt) and lanthanum strontium manganite (LSM) used as contact paste. Results show that using LSM paste lead to higher density and more shrinkage compare with Pt paste. Contrary, the electric field strength has no significant effect on density and shrinkage. However, a minimum electric field strength equal to 80 V.cm^-1 is necessary for flash onset. As the field increases, the incubation time decreases dramatically. Compare with samples with LSM paste, samples with Pt contact paste reach to a higher temperature during the flash. Flash sintered 8YSZ shows the mean grain size of 0.3μm, which is about half of the conventionally sintered 8YSZ. Electrochemical Impedance Spectroscopy reveals despite lower mean grain size, the resistivity of flash sintered 8YSZ is lower than conventionally sintered 8YSZ.

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  As an alternative to conventional fertilizers, e.g. NPK (the Nitrogen-Phosphorous-Potassium containing chemical fertilizers) which release their nutrients in a short period of time, due to high solubility in irrigation water, glass fertilizers are ideal as they release macro- and micronutrients for crops and plant nourishment. Also, despite conventional ones, they have no ground-water pollution. In the present study, glass fertilizers were synthesized via Polymer-Derived Ceramics (PDC) method. Despite the melt-casting procedure, PDC needs lower temperatures in heat treatment. The precursors consist of poly-siloxane and active fillers. Thus, thanks to gaseous release during heat treatment of the present active fillers, i.e. Ca(OH)₂, MgCO₃, and Al(OH)₃, a porous microstructure can be generated. In order to manipulate the pore size and specific surface area, fractions of active fillers were used as calcined. The experiments showed that upon increase of non-calcined active fillers, the specific surface area and the amount of porosity was increased due to more gaseous release during heat treatment. Thus, affected by microstructure, the release rate of macro and micro-elements was higher in the sample containing non-calcined active fillers, in comparison to other samples. Additionally, the porous samples were able to be loaded by extra nutrients containing Nitrogen, like KNO₃.

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The Nano Hydroxyapatite (HAp), HAp/PEG and HAp/PVP powders derived from both Gastropod shell (natural source) and chemical precursor by the precipitation method were characterized through various characterization techniques such as FT-IR, XRD, SEM-EDX, TEM, Antibacterial activity and SBF analysis. Based on the structural, chemical, morphological and biological characteristics, HAp/PVP from natural and chemical precursors have been compared successfully. Calculated structural parameters, crystallinity index, C/P ratio, morphology, antibacterial activity and SBF analysis of the products show that HAp/PVP-S (derived from a natural source) exhibits good mechanical property, rod like morphology, good antibacterial activity and apatite formation ability at 14 days. EDX analysis also shows the presence of carbon and sodium in HAp/PVP-S. Comparative analysis reveals that characteristics of HAp/PVP-S such as high carbonate content, low crystallite size, poor crystalline nature, presence of trace metal, non-stoichiometric elemental composition and rod like crystals which are matched with the characteristics of biological apatite. Thus, the HAp/PVP-S has the ability to form bone apatite.