مجله مواد نوین
پیاپی 3 (بهار 1390)

Silica as the most ubiquitous material has been used profoundly in ceramic industry for many years. One of the recent sources for containing silica materials is colloidal silica nanoparticles which have been surprisingly used in electronic substrates, thin film, emulsifier, stabilizer, catalysis, binder, chemical and mechanical polishing. The quality of these products directly depends on the both size uniformity and distribution of silica nanoparticles. The size, size distribution and the yield of silica were controlled by the concentration of reactants, ammonia content as the famous basic catalyst, temperature, pH and mixing mode. In this study, the synthesis of nanometer silica particles via hydrolysis and condensation of tetraethylorthosilicate (TEOS) is described in different ammonia content and pH. Monodispersed spherical colloidal silica nanoparticles with uniform size and narrow size distribution were obtained by this technique. The increase in ammonia concentration resulted in bigger silica nanoparticles (from 100nm to 300nm) and higher yield of silica. In acidic pH range, only silica layers containing many cracks were obtained; however, nanosilica particles agglomerations were synthesized in basic range. The Fourier transform infrared spectroscopy (FTIR) shows the both asymmetric and symmetric stretch vibration of silioxan bridge (Si-O-Si) and silianol groups (Si-OH) on the silica nanoparticles surface.

For long years, Austempered Ductile Iron (ADI) has found widespread application in different fields such as transportation, agriculture and other heavy industries due to its high strength and hardness coupled with reasonably good ductility. However, it’s low machinability and hence high machining costs limits further exploration of this material in certain areas. In the present work, a new engineering material having desirable properties and higher machinability, called machinable austempered ductile iron has been designed and studied. This new material has a specific composition with low alloying elements and the heat treatment cycle consisted of austenitizing at 850°C for one hour followed by austempering at 390°C for an hour, Subsequently the material has been evaluated for hardness, tensile and fatigue properties. A hardness of 235 HB has been obtained in the material with yield strength of 705 N/mm2 and a tensile strength of 900 N/mm2. The corresponding percent elongation in the material was %15 with fatigue strength of 370 N/mm2. The results indicate significant improvement in machinability and endurance limit compared to regular austempered ductile iron. Thus, this new machinable austempered ductile iron can become a substitute for regular ADI in many applications to improve production and/or performance efficiency.

Al-B4C composites have attracted attention of a large number of researchers and industrial societies dealing with military, nuclear, transport, and computer fields. In this research, the structure of these composite materials produced by a method based on accumulative roll bonding (ARB) was considered. The results showed that applying a specific draft percentage of reduction of %66 during inserting the reinforcement layers and then that of %50, a desirable cold weld between the sheets is obtained. X-ray diffraction and transmission electron microscopy analyses revealed that after 7 ARB passes, nanostructures elongated in the rolling direction are developed. Furthermore, a suitable distribution of the reinforcing boron carbide particles in the nanostructured aluminum matrix was achieved after 7 ARB passes, suggesting the pleasant efficiency of the employed route.]

Mixtures of Likak celestite concentrate (98% SrSO4) and activated carbon (99% carbon) were prepared and reduced in high pure argon atmosphere (99.99%). The products after carbothermic reduction were studied by a combination of XRD and TGA/DTA experiments. The thermogravimetric analyses and XRD results showed that the formation temperature of strontium sulfide (SrS) appeared at about 720 0C. The carbothermic reduction progressed gradually and finished at about 1100 0C. The XRD results and thermodynamic analyses revealed that the only solid product was strontium sulfide and no intermediate compound was observed during carbothermic reduction.

A plasma electrolytic nitrocarburizing process was carried out on austenitic stainless steel in order to enhance surface properties in medical implants applications. An urea-based solution with 10 percent NH4Cl and 3 percent Na2CO3 was used as the electrolyte. The samples were coated using the least possible voltage (150 volts). Cross-section and surface morphology of the coated samples were studied using a scanning electron microscope. Microstructure and composition of the samples were investigated using X-Ray Diffraction analysis and Energy Dispersive X-Ray analysis. The microstructure of the outer layer of the coating consisted of iron and chromium oxides. In order to study the bio-compatibility of nitrocarburized samples and to compare the results with those of untreated samples, a group of each type of samples was placed inside rabbit's tribia bones. After six weeks, the pins were removed from the body and were studied using X-Ray and EDX analyses, to obtain images of the sample's surfaces, both post and prior to the implantation. Studying these images revealed the formation of a layer of tissue on the surface. EDX analysis also showed the presence of a significant amount of phosphorous and calcium with a non-homogenous distribution. These results indicated tissue growth on the surface due to the presence of a porous structure.

One of the most important issues in three sheet resistance spot welding is the insufficient growth of the weld nugget in the sheet/sheet interface, where it is most needed. This reduces the mechanical properties of the joint and increases the tendency to fail in the interfacial failure mode. In this paper the effect of sheet thickness on the pattern of weld nugget development during resistance spot welding of three low carbon steel sheets of equal thickness is studied. Results showed that there is a critical sheet thickness at which the fusion zone size at the sheet/sheet interface is nearly equal to its size at the geometrical center of the joint. When sheet thickness is lower than the critical value, an elliptical nugget is formed in which the weld nugget growth in the geometrical center of the joint is higher than the sheet/sheet interface. While, increasing the sheet thickness above the critical size causes the formation of a non-elliptical nugget in which the weld nugget growth mostly occurs in the sheet/sheet interfaces.

Silica particles occupy noticeable position among metal oxide particle, because of their simple preparation and wide uses such as catalysts, electronic and thin film substrates electrical and thermal in insulators, humidity sensors, emulsifiers, stabilizers, etc. Silica Nano particles use as filler to reinforce the rubber, polymer and also concrete. Recently in experimental studies of preparation of silica particle, it's often desirable to produce particle, which are spherical and uniform size. Such monodispersed particles yield results which are not influenced by shape and size distribution effects. The process of nanosilica formation in the reaction of precipitation from solution of sodium silicate and sulfuric acid has been studied. Sodium dodecyl sulfate (SDS) and Polyvinylpyrrolidone (PVP) were used as surfactant. Optimum parameters of the precipitation were undertaken. Principal physicochemical properties of the silica were tested, including particle shape, particle size and morphology. X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) have been used for the Characterizations of the produced nano silica. The result shows that using SDS and Polyvinylpyrrolidone (PVP) in process have changed the properties of precipitated silica.

Since in Iranian sanitary ware industries, the traditional slurry casting method is done, the plaster moulds have a major role in final product quality. Therefore plaster moulds in this industry in terms of lifetime, water absorption and finished surface are all important which optimization of these parameters depends on a series of factors including strength of plaster, grain size, water absorption, permeability, fluidity and setting time of plaster molding and some conditions such as rounds of the mixer, mixing time, water temperature, water hardness and the way of drying of mould. In this investigation, first the own properties of plaster and its effect on the quality of mould has been studied and after selecting the appropriate plaster, cast effects during making examined and then with consideration of the possibility of changing these parameters, the plaster mould was made with suitable characteristics for production of sanitary ware products.