A comparative study of the influence of trivalent and tetravalent nano-sized oxides on the performance and microstructure of dolomite refractories
Abstract Contrary to the many benefits of dolomite refractories, their usage has been limited due to poor hydration resistance. For this reason, the aim of this research work is a comparative study of the influence of trivalent and tetravalent nano-sized oxides [iron (Fe 2 O 3 ), alumina (Al 2 O 3 ), zirconia (ZrO 2 ), and titania (TiO 2 )] on the performance and microstructure of dolomite refractories with an affirmation their hydration resistance improvement. After preparation and firing the specimen's, physical properties such as bulk density, apparent porosity, and hydration resistance tests were examined. Furthermore, XRD and SEM/EDX analysis were used to evaluate the ceramic phases and their microstructure, respectively. Results revealed that specimens with trivalent and tetravalent nano-sized oxides have improved densification and hydration resistance of the specimens through the liquid phase mechanism and solid state firing mechanism, respectively. Also, the specimens with tetravalent nano-sized oxides have higher hydration resistance than the specimens with trivalent nano-sized oxides. Generally, the hydration resistance improvement trend of specimens with various nano-sized oxides presence is Al 2 O 3 2 O 3 2 2 .
Effect of Nano-Sized Fe2O3 on Microstructure and Hydration Resistance of MgO-CaO Refractories
In this work the effect of Nano-sized Fe2O3 on microstructure and hydration resistance of MgO-CaO refractories with 35 wt% CaO was investigated. MgO-CaO refractories with  35 wt% CaO content was prepared by using dolomite and magnesite calcined as starting materials and Nano-sized Fe2O3 (2,4, 6 and 8 wt%) as an additive. Samples were pressed into briquettes at a pressure of 90 MPa. Briquettes after drying at 110 oC for 24 hours were sintered at 1650 oC for 3 hours. Hydration resistance was measured at 25 oC in 95% relative humidity through the weight gain after 72 hours. According to the results, it is observed that when the amount of Nano-sizedFe2O3 added increased to 8 wt% the bulk density increased to 3.14 gr/cm3 and apparent porosity decreased to 8.20%. For the sample without addition, the mass gain after 72 hours was 1.76, which sharply decreased to 1.27 with increasing the amount of Nano-sized Fe2O3. With the addition of Nano-sized Fe2O3, bulk density and hydration resistance of the samples increased while apparent porosity decreased. Densification of MgO-CaO refractories was promoted to increase of Nano-sized Fe2O3 content. Nano-sized Fe2O3 addition led to formation of low melting phases such as C2F (2CaO.Fe2O3), CF (CaO.Fe2O3) and C3A (3CaO.Al2O3). The formation of this low melting point phases surrounding the CaO and MgO grain and grain boundaries and promoted densification of MgO-CaO refractories. The nature of Nano-sized Fe2O3 promoting densification is increasing the liquid phase sintering.
Dispersity and phase composition of some ultradisperse ZrO2-bearing powders
Results of an electron-microscopic study (including microdiflraction) and a x-ray laser microanalyzer study of the dispersity of powders in the ZrO2-A12O3 system containing from 94.9 to 29.6% ZrO2 and obtained by plasmachemical synthesis are presented. The features of the crystalline and morphological structure of the powder particles are considered. It is inferred that the crystal structure and the morphology of the powder particles affect the properties of the powders in the manufacturing process.