نشریه فرآیندهای نوین در مهندسی مواد
پیاپی 26 (پاییز 1392)

Initially mine raw material unwashed kaolin Zenouz (ZK1) was calcined for 5 hours at 800oC to obtain Meta kaolin. After that by using Meta kaolin، sodium silicate activation solution and sodium hydroxide geopolymer samples were prepared with ratios of 0. 9 Na2O/SiO2 = 0. 6 and molar ratio Na2O/Al2O3 = 1. Curing time and temperature of 3، 7 and 28 days and 25، 50 and 75 °C was selected respectively. Compressive strength was taken from prepared samples that the results showed high Compressive strength of some geopolymeric samples compared with Portland cement paste samples. For further study analysis strength، FTIR and SEM samples was used. FTIR results indicate that the formation of the geopolymer structure than Meta kaolin changes that result from their condensation polymerization of the monomers. SEM results showed that geopolymeric paste is composed of two parts; gel and Quartz remaining particles. Also was seen with increasing curing time remaining particle size reduced.

Deep cryogenic heat treatment is vastly used in recent years due to its effect in the hardness and wear resistance improvement of tools and is commonly performed before tempering and after quenching. In this study the samples were austenized at 920 and 850 for 20 and 80 minutes followed by the deep cryogenic heat treatment at -195 for 24 hours. The samples were then analyzed via optical microscope (OM) and scanning electron microscope (SEM) to study the microstructure and wear and hardness testing machine to investigate the effect of austenizing temperature and time on the deep cryogenic behavior of 100Cr6 steel samples. Results show that deep cryogenic heat treatment improves the hardness and wear behavior of samples due to retained austenite elimination as well as increasing the precipitated carbides. The highest value of hardness and wear resistance belongs to the samples austenized at 920 for 20 minutes. The wear mechanism was a combination of abrasive and tribo-chemical wear.

In this study, cold roll bonding (CRB) process was performed on the commercial purity aluminum (AA1050) strips, in the presence and absence of TiO2 nano-particles. The bond strength was evaluated by the peeling test. In order to evaluate the fractured surface after peeling test, scanning electron microscopy (SEM) was used. The effect of process parameters on the bond strength was investigated. The results indicated that by increasing the particles content, the bond strength of aluminum strips after CRB process was decreased. When the reduction in thickness was increased, the undulation of average peeling force values was increased at constant particles content. Also, by increasing the particles content, the threshold deformation was increased.

In the present study، Ni-P-CNT composite coatingswith different contents of carbon nanotube were successfully deposited on copper surface via electroless plating. The effect of CNTs concentration in the bath on its content in the composite coatings and corrosion behaviour of the coatings has been studied. Scanning electron microscopy (SEM) and EDS analysis were used to characterize the coatings. The SEM results showed that the thickness of the coatings was about 30µm and the deposited coatings had nodular features with a typical cauliflower-like structure. The corrosion behaviour of the coatings with different contents of CNTs was evaluated using Tafel polarization and electrochemical impedance spectroscopy (EIS) methods in 3. 5 wt. % NaCl aqueous solution. According to Tafel extrapolation results، the incorporation of CNTs into the Ni-P coatings is led to the formation of a better passive film. It was also concluded that with increasing the CNTs concentration in the bath، corrosion rate decreased at first and then increased. Furthermore، the EIS results indicated that the real impedance of the Ni-P coating is much smaller than that of the Ni-P-CNT coatings. In this context، as the concentration of CNTs in the bath increased، charge transfer resistance values were initially increased and then decreased. This behavior could be related to the role of the CNTs in the formation of low ionic conductivity surface passive film.

In this study taguchi design of experiments has been used for optimizing the parameters of the micro arc oxidation process on AZ31 magnesium alloys. As a consequence by evaluation the analyze of variance of the experimental data''s (ANOVA)، the effectiveness of different factors such as current density، Na2SiO3 and NaF concentration and time on two different responses namely resistance polarization and hydrophilisity (contact angle) of coating was investigated. The statistical results showed that for gaining the greatest polarization resistance and the lowest water contact angle، two factors of Na2SiO3 concentration and time had the most effect respectively. The effect of Na2SiO3 concentration on polarization resistance can be attributed to variation of Mg2SiO4 and MgSiO3 phases in the coating. Moreover factor of time can affect on hydrophilisity of specimens by altering the roughness and Mg2SiO4 containing of the coating، so that by increasing the time، contact angle will be decreased first and then it will be increased.

Microstructural and mechanical properties of dissimilar bonding of INCONEL 625 and 316L austenitic stainless steel using AWS BNi-2 has been evaluated. The bonding process was done at 1020◦C for 30،60 and 120 minutes، respectively with Argon protection followed by homogenization at the temperature of 1080◦C for 4 hour. Chemical analysis from the joint clearance microstructure and mechanical data indicated that the higher the bonding time from 30 to 120 minutes، the lower fraction of precipitates at the joint centerline. Given this، the ultimate shear tensile strength raised from 265 MPa to 360 MPa. Post-bonding homogenization led to more uniform microstructure along with mechanical properties. Moreover، the ultimate shear tensile strength increased to 520 MPa، too.

Thick films of Ce-YSZ and Ni-YSZ were prepared by screen printing method and YSZ sensor sensitivity caused the addition of 10 mol% ceria and 1. 5 mol% nickel oxide (using the precursor of cerium nitrate and nickel nitrate) as dopants has been studied to ethanol gas. The electrical sensor was constructed with electrical connection by using silver adhesive and electric heater. Change in electrical conductivity was recorded in the presence of air and then the sensitivity of the samples examined in the presence of different amounts of gas and air. Regarding to the sensing results، cerium oxide dopant causes a better sensitivity than nickel oxide. Other advantages of using ceria as a dopant are the lower working temperature and less temperature sensitivity.

In this report، various compositions were selected based on wollastonite in CaO-SiO2-MgO system. XRD analysis revealed that wollastonite was the main crystalline phase in the sintered glass-ceramics. Sinterability and crystallization of glass ceramics were analyzed at various temperatures. 830 oC، was selected as the optimum sinterability temperature. Finally dielectric properties of glass-ceramic samples at 830 oC، in 12-8 GHz frequency were reported. Dielectric constant and dielectric loss of glass ceramics are 6. 7-7. 5 and 0. 0028-0. 0034 respectively.

In this research، HP heat-resistant cast steel was welded by Inconel Weld Metal ER-NiCr3using pulsed current gas tungsten arc welding)PCGTAW) and continuous current gas tungsten arc welding)CCGTAW) and the effect of these welding processes on the microstructure of the steel was studied and compared. In as-cast condition when continuous current gas tungsten arc welding was used، due to incomplete mechanical mixing، an unmixed zone was observed at interface of HP heat-resistant steel and Inconel Weld Metal ER-NiCr3. This region was eliminated using pulsed current gas tungsten arc welding with peak current of 180 A، base current of 60 A، frequency of 10-12 Hz and percentage on time 40 and also grains of weld metal became finer.

During the recent half century، using of inorganic zinc-rich coatings has been one of the efficient methods to prevent corrosion of steel structures in corrosive atmospheres. Micaceous iron oxide (MIO) is one of the most widely used pigments for anticorrosive barrier coatings. In the present study waterborne zinc-rich silicate coatings were formulated based on nanosilica-modified potassium silicate and different contents of zinc and micaceous iron oxide (MIO). After being applied on steel panels، the coatings were subjected to atomic force microscopy (AFM)، electrochemical impedance spectroscopy (EIS)، corrosion potential measurements and salt spray test. Replacement of zinc by MIO in the formulation of zinc-rich paints was revealed to shorten the duration of cathodic protection stage while on the other hand gave satisfactory results in salt spray test. It was concluded that MIO pigments can be used to control the reactivity of zinc-rich silicate coatings.

This study aimed at preparing and characterizing fluor-hydroxyapatite (FHA) nanocrystals using the sol-gel process. Phosphorus pentoxide، calcium nitrate، and hexafluoro acid were used as the sources of phosphorus، calcium، and fluorine to achieve the stochiometric ratio of 5 for calcium to fluorine، and 1. 67 for calcium to phosphorus، respectively. In order to characterize and evaluate the properties of the particles، methods of elemental analysis (XRF)، phase analysis (XRD)، fourier transform infrared spectroscopy (FTIR)، scanning electron microscope (SEM)، and transmission electron microscope (TEM) were applied. The elemental analysis showed that the required oxides and elements deos exist in the compound in desired amounts. The results of XRD confirmed the structure of fluorapatite. The sizes of the crystallites estimated from XRD patterns using the Scherrer equation was about 40 nm. FTIR analysis proved the partial substitution of ion F with OH and the formation of fluorapatite. SEM and TEM images showed that the fluorapatite nanoparticles less than 50 nm were formed having spherical shapes. The results indicated that it is possible to prepare fluor-hydroxy apatite nanocrystals with desired composition and structure through sol-gel method.

In this research a vertical ZnO nanowire array films synthesized by a low temperature aqueous method. Sensitivity of synthesized films to CO2 and O2 gases in room temperature was considered. XRD and SEM studies show that the film is one dimensional، having harmonized and perpendicular structure. High surface area / volume ratio of vertical ZnO nanowire array film reveals high sensitivity even in room temperature.