مجله مواد مهندسی
سال دوم شماره 4 (پیاپی 8، 2010)

Titanium and titanium alloys are used in medical and dental applications wildly. In spite of good mechanical properties, they have a poor ability of bone growth on the surface of themselves. Therefore, it is necessary that the titanium must be coated with a bioceramic layer due to the excellent osteoconductive properties. In this research, thermal - electrochemical deposition method (ELD) was used to apply hydroxyapatite coating on titanium substrate. Before applying hydroxyapatite coating, surface treatment process was achieved on titanium samples by polishing, Then the effect of salts concentration on the morphology and nanostructure under constant conditions. After achieving above steps, micro structural, dimension of crystals, and morphology of hydroxyapatite coating were studied by using XRD, FTIR, TEM, SEM, and AFM. Obtained results showed by increasing the concentration of calcium and phosphate ions, hydroxyapatite crystals have preferred growth on (002) planes with nano scale.

Nano-crystalline particles of barium hexaferrite have been prepared by co-precipitation route using solution of iron and barium chlorides. Water and a mixture of water and diethylene glycol with volume ratio of 1:3 were used as solvents in the process and co-precipitated powders were calcined at various temperatures for 1 h. The XRD results of calcined samples showed that the formation of single phase barium hexaferrite occurs at 90 oC for sample synthesized in aqueous solution with a Fe+3/Ba+2 molar ratio of 11 and its formation is resulted from the reaction between mainly crystalline phases، while this temperature decreased to 700oC for sample synthesized in diethylene glycol/water solution with a Fe+3/Ba+2 molar ratio of 12 and the formation of barium hexaferrite consists of reactions between amorphous phases with crystalline phases. Crystallites of single phase barium hexaferrite with mean size of 35 nm were observed in the TEM image of the sample synthesized in diethylene glycol/water solution after calcination at 700oC and in this system the single phase barium hexaferrite could not be formed by using of the Fe+ /Ba+2 molar ratio of 11.

In the present study، an attempt is made to study the cooling performance of a wind tower and a solar chimney in a hot and dry region. Also in this study the effects of parameters including wind tower height، variety of the materials used in the wind tower walls، the amount of vaporized water، the temperature of input and output air، the wind velocity and the relative humidity were investigated. Furthermore، to develop، a natural flow of air، for days without blowing a wind the role of solar chimney was considered. Finally، to investigate and take information about streamlines of airflow in wind tower (Baud-Geer)، velocity، pressure، humidity، temperature and density profile of fluid، Fluent software is applied to evaluate the air flow in the wind tower in differential view for two dimensional and steady state conditions with water spraying at the top of wind tower. The results indicate that the evaporative cooling is very effective in a hot and dry region. The temperature decreases considerably، if the wind towers are equipped with the water vaporization system. This causes the air becomes heavier and a natural motion of air through downside of wind tower to be produced.

In this article the design and development of cordierite mullite plates based on ultra low cement refractory castables (LCC) using for kiln furniture applications were investigated. Cordierite powder was synthesized from talc، kaolin and alumina as starting materials and forming the briquettes by pressing and sintering at 1430 oC. The synthetic cordierite aggregates were used as the main constituent of the samples in addition to the andalusite، secar 71 cement، calcined alumina and microsilica. Raw materials were mixed according to the designed formulations and the samples were shaped by casting into the steel dies by vibration method. After drying and sintering the samples at 1250، 1300، 1350 and 1400 oC، their physical and mechanical properties including porosity، density، thermal linear expansion and cold crushing strength were measured. Phase analyses were performed by X-ray diffraction (XRD) and scanning electron microscopy (SEM) was used for microstructural analysis. Thermal expansion coefficient of the samples sintered at 1300 and 1400 oC was determined by dilatometery and had average values of 3. 95×10−6 and 3. 51×10−6 (1/K)، which indicates an excellent thermal shock resistant.

Bioactive 45S5 glass is a silicate-base material used for the regeneration of hard tissues such as bones and middle ear bone. Up to day، several efforts have been made to provide macro-porosity in the glass structure. The porosity plays an important role in healing process by increasing the rate of tissue reconstruction as well as biodegradation. In this work، macro porosity was introduced into the glass structure via gas foaming technique as a simple and effective method. The glass was produced by melting method، ground to fine powder and mixed with various amount of calcium carbonate as gas-foaming agent. The mixture was pressed in tabular shapes and heat treated at 1150°C. CO2 gas produced from thermal decomposition of CaCO3 passed through the soften glass specimen led to formation of spherical macro-pores. The size and content of the pores depended on the amount of calcium carbonate. In this work، a macro-porous glass specimen with 60% porosity and macro-pore size of 60 μm was achieved using 1. 8% wt of calcium carbonate. The results showed that the Heat treating caused to glass crystallization without any undesirable effect on its bioactivity.

Degradation of underground buried pipelines due to corrosion imposes enormous financial damages on industry and environment. Hence، recognition of soil nature as a non-homogeneous electrolyte in corrosion of pipelines is an important issue، which should be taken into consideration during design and execution of cathodic protection (CP). The most important factors regarding soil composition، which affect the corrosion or protection of a buried pipeline، are: soil resistivity، various dissolvable salts in earth، relative humidity، soil composition and grain size، mineral materials، micro organisms، etc. In this study، a length of 5 kilometers from a long cathodicaly protected coated pipeline near Tehran was selected for laboratory and on-site investigations، which had noticeable lack of protection. Five soil samples were collected from five positions adjacent to the pipeline with one kilometer distance between each collect sample. Prior to sample collection، the selected positions were subjected to resistivity measurement، using Wenner measuring device. The soil samples were transferred immediately to the laboratory for further investigation، whereby the pH، grain size and chemical composition of each sample was calculated using standard measuring methods including XRF and Loss on ignition (LOI) for grain size specification and chemical composition، respectively. Furthermore، the corrosion rate of steel specimens with similar composition to the pipeline exposed to each soil sample was obtained via potentiostatic polarization measurement. Also the relative amount of current required with respect to the area of the steel specimen (i. e. I/S) exposed to each soil sample was obtained for a period of 400 hours in each case. Finally، the results for all selected samples were compared and contrasted against each other and the effective parameters on corrosion of steel samples were specified.

One of the newest technologies of grain refining is semi-solid metal process that noted in different industrials. In cooling slope casting method indicated of semi-solid method has been used in order of microstructure refining and spheroidizing by pouring molten on cooling slope. Some parameters such as cooling slop characteristics die temperature and next heat treatment on microstructure and mechanical properties of produced work piece effect on this method. In this present the effect of die temperature and heat treatment have been investigated on microstructure and A360 aluminum alloy hardness. After sample providing the sample with suitable cooling slop characteristics، investigations recognized that sample has suitable condition by 300°C die temperature. Then some samples with suitable condition reheated in 580°C for 5،10 and 15 min، then immediately quenched in 25°C water. Results indicate that the sample in 580°C has better properties in order of size، and grain shape، by the way that in this condition، the amount of hardness increase of 80. 6 HB to 87. 1HB، because of optimizing eutectic phases from the point of shape، size and monotonous dispense.

In this study، the optimum conditions for producing alumina parts by means of injection molding was investigated. First، different binder systems were prepared and their rheological characteristics were investigated. Then binders containing 1، 2 and 4 weight percent of stearic acid was chose. Three different pastes were prepared with 20 wt% binder (including 1، 2 and 4wt% stearic acid) and 80 wt% alumina. Studying rheological behavior of aforementioned compounds proved that using the binder containing 2 wt% of stearic acid as surfactant shows the lowest viscosity which has a constant alteration against shear rate. In continuous، to attain to the proper composition of powder and binder and achieve high density، different percents of powder and binder was prepared. Rheometric investigations showed that the composition containing 81. 5 wt% of alumina and 18. 5 wt% of binder was proper to choose. After injecting the prepared feed stocks into the mold and producing samples، thermal analysis was carried out to distinguish the proper thermal cycle of debinding process. Finally sintering was carried out at 1600 °C for 3 hours. Bending strength and density of sintered samples was 186 MPa and 3. 68 g/cm3، respectively.

Rice husk is the major by-product of rice milling industry. In the present study، an optimal approach for extraction of amorphous silica from rice husk is demonstrated. The effects of Leaching and rinsing treatments on the purity of extracted silica are investigated. The importance of amorphous phase of obtained silica is the reactivity of amorphous silica. Initially، rice husk was converted to ash by calcination and then the silica was extracted from rice husk ash by a proper alkaline solution. For this purpose، white ash was obtained by Leaching process on rice husk and heating treatment at 700°C and then obtained white ash dissolved in alkaline solution of sodium hydroxide (2M). Subsequently، precipitation by means of hydrochloridric acid resulted in the extraction of active silica with high percentage of purity. The quality and purity of extracted silica was analyzed by X-ray fluorescence and X-ray Diffraction techniques. The advantage of applied method in the present research over the classic methods of silica production is the reduction of energy consumption، the decrease of extraction temperature and the abundance of raw materials needed for silica extraction.