نشریه علم و مهندسی سرامیک
سال سوم شماره 3 (پیاپی 10، پاییز 1393)

In present work a tested enamel coat for Ni-Cr wire-wound resistor is studied for its electrical and chemical properties. In the present work it is tried to find appropriate coating by testing different compounds. Results showed that the optimum enamel is the compound with 1 wt% cobalt oxide، 3 wt% chromium oxide and firing temperature 840˚C. Variation of the electrical conduction of the enamel with temperature is measured by an instrument designed by Rasch-Hinrichsen et al. Results indicate that with increasing the temperature to 275˚C electrical conduction reaches to 10-7. 60 Ω-1. cm-1. Dielectric strength test shows 100 KV/cm for breakdown voltage. Weight loss in different liquids with various pH shows that the most chemical resistance is yielded in water and for neutral pH.

Vanadium oxide (VOx) thin films prepared in a direct current magnetron sputtering system. In order to get different vanadium oxide phases the films anneled under different temperature and atmospheres. At first vanadium oxide (VOx) thin films annealed at two temperatures of 450 and 500oC. Mixed structure of VO2 and V2O5 phases formed in both samples while VO2 phase was the main phase at 450oC and changed to V2O5 phase at annealing temperature of 500oC. In the next step vanadium oxide (VOx) thin films prepared under two sputtering pressures of 2 and 5 torrand then annealed at 450oC. The results revealed that sputtering pressure doesnot have a big impact on crystal structure of vanadium oxide thin films. However، vanadion oxide thin films that prepared at lower sputtering pressure shows smaller grain size and higher hardness and Young،s modulus. Then the effects of annealing atmosphere (air and oxygen atmospheres) on microstructure and properties of vanadium oxide thin films that prepared at sputtering pressure of 2 torr and anneled at 450oC was investigated. It was shown that annealing atmospheredoesnot affect crystal structure of the films while mixed structure of VO2 and V2O5 in both samples. There is a possibility that some phases have been formd in amorphous. Vanadium oxide (VOx) thin film which anneled under oxygen، showed smaller grain size and better mechanical properties. The results of current study showed that deposition and annealing conditions have a big impact on microstructure and mechanical properties of vanadium oxide (VOx) thin film.

The most extensively used de-fluoridation process is the adsorption process. Hydroxyapatite is a high efficiency adsorbent and its adsorption properties attract attention recently. In this research، nanostructured calcium hydroxyapatite (HAp) were synthesized by co-precipitation method and characterized using TEM، XRD، FTIR methods. The analysis confirms the formation of pure nanorods hydroxyapatite with the diameter of less than 10 nm and length of about 20 nm. The adsorption capacity of the nano- HAp was investigated for the de-fluoridation of fluoride contaminated water using a batch equilibrium procedure. The various parameters such as pH، contact time، initial fluoride concentration and temperature were varied as part of the study to investigate their effects on the fluoride adsorption capability of the nano-HAp material. Residual fluoride level was measured using ion-chromatography analysis. The Freundlich model was obtained as describing isotherm model. Investigation of three kinetics model of pseudo first، pseudo two and two proved that the adsorption of fluoride ions on hydroxyapatite follows the second order model. The enthalpy، entropy and Gibbs free energy of the process were calculated by thermodynamic equations.

SiC-B4C nanocomposite with 1:1 molar ratio synthesized in situ by mechanically activated SHS method. Raw materials include Si، B2O3، Mg and graphite. Weighed and milled raw materials pressed by uniaxial cold press to prepare samples. Green compacts samples synthesized in tube furnace with argon gas flow at 900 °C. MgO is one of products of B2O3 reduction by Mg. Synthesized MgO eliminated by hydrochloric acid solution leaching. XRD analysis from synthesized and leached products indicated successful of acid leaching to purify the reaction products. Average crystallite size of leached products calculated less than 15 nm by XRD pattern analysis of leached products. SEM and TEM micrographs indicated the formation of synthesized composite from nanometer grains and crystallites. 91. 5% of synthesized nanocomposite particles were less than 500 nm and 42. 6% were less than 100 nm according to particle size analysis of acid leached products. SiC-B4C nanocomposite powder condensed by spark plasma sintering (SPS) with argon gas atmosphere at 1700 °C. Bulk and apparatus density of obtained part were 73. 26% and 97. 83% respectively compared to the composite powder density. Microhardness and flexural strength of sintered sample، determined 5 GPa and 260 MPa، respectively. Results of microhardness and flexural strength analysis indicated a greater effect of porosities on decrease of the hardness than the flexural strength. XRD pattern of sintered part indicated stability of synthesized SiC-B4C composite at sintering temperature and proper purity.

Pollution of water with heavy metals as a result of industrial activities is a serious problem in the new century. Various methods such as ion exchange and adsorption with different material such as active carbon، zeolites، agricultural waste، industrial by-product and other natural or synthesized compounds have been used for the removal of heavy metals from waste waters. In this study، various compounds in CaO: Al2O3: SiO2system were obtained from chemical route and after calcination at 600-1000 ℃were used for uptake of Ni2+ cation from aqueous solution. From different synthesized compounds، calcined sample at 600℃ with chemical analysis CaO=40. 9%، Al2O3=37. 2% and SiO2=21. 9% wt. (C2AS-6 sample) were selected for removal of Ni2+ pollutant. Uptake experiments were performed with batch method at room temperature and at a solid: solution ratio of 0. 002g/mlwith Ni2+cation concentrations 30-70 mg/l at constant time 20 min to 24hr. It was found that the Langmuir and DKR models best fit with results of this research. Maximum adsorption capacity equal 21. 1 mg/g and mean energy of adsorption equal 11. 2 KJ/mol were obtained in this study. These results represent that monolayer coverage of Ni2+ on C2AS sample formed during contact of sample with solution and exchange cation mechanism responsible for proper removal of Ni2+ polluant from aqueous solution.

Dense siliconcarbide bodies can be prepared by liquid phase sintering at temperatures higher than 1800C. many systems are innovated to achieve such bodies in which the combination of earth alkaline oxides and alumina are interested more. The liquid of calcium aluminates wet the silicon carbide surface very well and can result in dense product. In this survey dense silicon carbide body is prepared using less than 7% calcia at 1680C at nitrogen atmosphere. Results show that nitride phases are produced along with some gaseous phases and hence there is a considerable change in phase constituents of samples. Samples with a zero porosity and strength up to 430 MPa are attained.

Antenna protection against external factors (e. g. wind، rain، and objects colliding) is done by radome. In addition to acceptable mechanical properties، radome should satisfy suitable electrical properties such as dielectric constant and loss tangent to prevent negative effect on antenna performance. In order to build radome to be utilizing at high temperatures (over 1000 °C)، for flying objects with hypersonic speed (5 to 10 Mach)، engineering ceramics are used. Porosity affects the electrical and mechanical properties of articles. It simultaneously reduces the dielectric constant and flexural strength. In this study porous ceramic articles of beta-Sialon were fabricated by ball milling beta-Sialon powder، monomers and water for 10 hours and under 20 MPa uniaxial pressure. Average porosity size of this sample is 200 nm with 30% of its porosities are below 100 nm. Dielectric constant and loss tangent of this sample with 20% porosity in 8-12 GHz is 5. 3 and 0. 04، respectively.