فصلنامه سرامیک ایران
سال هفدهم شماره 4 (پیاپی 68، زمستان 1400)

High-temperature adhesives are considered for high-temperature applications such as the refractory industry. In this research, inorganic high-temperature phosphate-based adhesive was prepared using alumina and micro silica as the main raw materials and monoaluminum phosphate as a binder. The effect of firing temperature on the properties of the adhesive including phases, microstructure, compressive strength and shear strength was investigated. For this purpose, adhesive samples were fired at three temperatures of 1200, 1300, and 1430 °C for 3 hours. At a firing temperature of 1200 °C, corundum and cristobalite phases were detected and by increasing the firing temperature, the mullite phase was detected in the adhesive. The compressive strength and shear strength of the adhesive improved significantly with increasing firing temperature so that with increasing temperature from 1200 ° C to 1430 °C, the compressive strength increased from 56 to 239 MPa and the shear strength increased from 1.8 to 6.2 MPa. The increase in the mechanical properties of the adhesive at higher firing temperatures is attributed to the presence of the mullite phase.

In this study, we investigate calcium aluminate phases that were prepared by three different solid phase synthesis methods and a sol-gel method (with and without EDTA). The purpose of this study is to compare these three methods in terms of achieving higher purity, optimal phase and complexity in the production of the final product. Comparing the results, we found that the sol-gel method is more problematic than the solid-state synthesis. The phases prepared are: (CA), (C12A7), (C3A) and (CA2). In the sol-gel method, we have explored two different innovative regimens with EDTA. The results on the purity of the resulting phases and the effect of adding EDTA showed that, in general, the addition of EDTA leads to better purity. Quantitative x-ray diffraction analysis was used to determine the percentage of prepared calcium aluminate phases. The CA2 phase achieved 100% purity in the first preparation method in all three ways.

a-Al2O3 is the most important raw material for synthesizing modern ceramics and high alumina refractories. Active Alumina (also known as Hydratable Alumina and r-Al2O3) is a kind of transition alumina which is produced by flash-calcining of Gibbsite at 500-600 oC. In this research, the effect of Ammonium bi-fluoride additive (NH4HF2), on the formation of a-Al2O3 from active alumina was investigated. Suspensions containing active alumina and different amounts of fluoride additive (0-3 wt%) were stirred for 1 hour. The water of each suspension was removed by filter paper and the resultant powders were dried at 110 oC for 24 hrs. Powders were calcined up to 1400 oC for DSC and XRD analysis. The DSC test results showed that by having 1 wt% fluoride additive, a sharp peak appeared at 1100 oC indicating the formation of a-Al2O3. Increasing the additive amount to 3 wt%, shifted the transformation peak to as low temperatures as 925 oC. FESEM studies showed that in samples containing fluoride additive, the particles possessed hexagonal morphology. However, the ordering of the platelets and their thickness was a function of additive content. The higher the fluoride additive, the more ordered hexagonal shape as well as more thickness of the platelets.

Flash sintering is one of the latest sintering processes of ceramics, which has recently attracted the attention of researchers due to its high sintering speed and significant energy reduction. An essential feature of successful flash is negative temperature coefficient (NTC) semiconductor behavior. In this research, the flash sinter process of ceramic varistor based on ZnO-Bi2O3-Sb2O3 (ZBS) has been investigated and the effect of changing the molar ratio of the compound and adding cobalt oxide (Co3O4) as an increasing factor of electrical conductivity has been studied. The optimal conditions of the flash process were determined by applying different values ​​of electric field and current density, and finally, the electric field was 300 V/cm and the current density was 100 mA/mm2. The structural investigation and identification of the formed phases have been done by scanning electron microscope and X-ray diffraction, respectively. The electrical characteristics of the varistor (non-linear coefficient, breakdown voltage, and leakage current) were measured under the DC electric field. The density of the sample with cobalt oxide additive was 97% of the theoretical density. SEM results indicate the formation of varistor microstructure with an average grain size of 3 microns. By determining the electrical characteristics, the behavior of the varistor was observed, in which the nonlinear coefficient was 22, the threshold voltage was 4500 V/cm and the leakage current density was 50 μA/mm2.

Investment casting is an effective way to produce very thin and most complex components with very high-dimensional accuracy and surface finish. One of the steps in the production of components by investment casting is the production of ceramic molds, which are often made of colloidal silica adhesive due to the strength of the molds fired molds. However, this type of adhesive has low strength in green molds, which sometimes causes the molds to fail; hence additives such as fiber or polymer are used to avoid mold failure. Turbine blades are also produced by investment casting due to their sensitivity and high dimensional accuracy. The present study aims to achieve the desired properties in ceramic molds by adding SBR latex to the slurry. For this purpose, layers of slurry with the following two compositions are applied to the wax patterns: a) refractory powder and colloidal silica, b) refractory powder, colloidal silica and latex SBR. Based on this, two ceramic molds from the blade model were layered with the two mentioned compositions and different thicknesses. According to the results, the addition of SBR latex increased Sag Resistance and flexural strength of the molds in the green state and at high temperatures. Examination of the 3D scan results showed that the dimensional accuracy of the cast blades improved at 1400 ° C.

One of the ways to improve the performance of ceramic insulators in polluted climates is to use polymer coatings reinforced with ceramic particles. The purpose of this study is to compare the effect of adding Titania nanoparticles and Titania micro-nanoparticles on hydrophobic properties of PU. In this method, stearic acid, which is a cheap and environmentally friendly material, was used for surface modification of TiO2. FTIR analysis was conducted to ensure that Titania was modified. nanocomposite and micro-nanocomposite (hybrid) coatings on glass and free films of polyurethane-titania containing 1%wt and 3%wt  titania were prepared using a simple and inexpensive method based on magnetic stirring and ultrasonication. AFM analysis and surface topography FESEM images showed higher increase in surface roughness in hybrid composites compared to the nanocomposite samples. The contact angles of the hybrid composites were also higher than those of the nanocomposites with the same Titania content. The hybrid sample containing 0.5%wt nanoparticles and 0.5%wt microparticles had the maximum contact angle. The contact angle of this sample increased by 22◦.