Influence of crystallographic orientation on hydration of MgO single crystals

This study has been performed in order to find out the influence of crystallographic orientation on hydration of MgO single crystal substrates with (1 0 0)-, (1 1 0)-, and (1 1 1)-orientations. The samples were left in a hydration chamber with an 88% relative humidity for 18 h at room temperature. The effect of humidity on the samples was examined by scanning probe microscope (SPM) and scanning electron microscope (SEM) which showed that the degree of hydration was noticeably influenced by the crystallographic orientation. It was found that the MgO with (1 1 1)-orientation has the highest tendency to hydrate than the other orientations. Second most affected sample was (1 1 0) crystal. Loss of MgO on the surface by hydration is most severe when the crystal is oriented in (1 1 1) plane with the maximum hydrate layer thickness of 174 nm after 18 h of exposure.     

Experimental phase relations between MgO-saturated magnesium-aluminate spinel (MgAl2O4) and CuOx-rich liquid

Experimental phase equilibrium data for the Cu-O-Al₂O₃-MgO system is required to improve the performance of MgAl₂O₄-containing refractories and slagging in non-ferrous smelting. In this work, the phase relations of MgAl₂O₄ in the Cu-O-Al₂O₃-MgO system were studied experimentally in air within a temperature range of 1100–1400 °C using the equilibration and quenching method. The chemical compositions of the phases in the quenched samples were determined using electron probe microanalysis (EPMA). Less than 1 wt% of Al₂O₃ or MgO were found in the oxide liquid phase, whereas the solid MgAl₂O₄ and MgO phases contained up to 23 wt% and 30 wt% of ‘Cu₂O’, respectively. Discrepancies between these results and the corresponding calculated values generated by the MTDATA 6.0 software and Mtox database Version 8.2 ranged from 3 wt% to 19 wt%. The results of this work indicate that the MgAl₂O₄ spinel is chemically stable in the presence of a CuO_x-rich liquid under the conditions studied.     

Synthesis and densification of magnesium aluminate spinel: effect of MgO reactivity

Stoichiometric magnesium aluminate spinel was synthesized by reaction sintering of alumina with caustic and sintered magnesia. The volume expansion of 5–7% during MgAl₂O₄ formation was utilized to identify the starting temperature of spinel formation and densification by high temperature dilatometry. The magnesia reactivity was determined by measurement of crystallite size and specific surface area. Caustic magnesia and sintered magnesia behave differently vis-à-vis phase formation and densification of spinel. Densification of stoichiometric Mag-Al spinel was carried out between 1650 and 1750 °C. Attempts were made to correlate the MgO reactivity with microstructure and densification of spinel.     

Degradation of MgO–C refractories by MnO-rich stainless steel slags

In order to identify the influence of MnO on the wear rate of MgO–C bricks for the production of high Mn stainless steel, MgO–C refractory samples were exposed to slags containing different MnO levels (up to 26 wt.%). Although the investigations of the worn brick microstructures revealed the presence of numerous Mn-rich metal particles and the formation of a (Mg,Mn)O solid solution at the slag/refractory interface, no clear evidence of wear rate enhancement was observed due to high MnO concentration in the slags. With respect to refractory wear, the MgO content in the slag is the dominant factor. The degradation processes are discussed by combining experimental results and thermodynamic calculations.     

A microstructural study of surface hydration on a magnesia refractory

Hydration of the surface of a periclase refractory was studied in a controlled humidity atmosphere (92% relative humidity) at 26 °C for up to 92 h. The effect of humidity on the specimens was examined using a scanning probe microscope (SPM) and a scanning electron microscope (SEM), which showed that the degree of hydration was noticeably less in the intergranular bond phase compared to the periclase crystallites. The SPM study indicated significant growth of hydrates on the refractory grain surface. Growth rate of the hydrate layer was least on (0 0 1) orientation, higher for the (1 0 3) orientation, and highest for the (1 0 1) oriented grain. The rate of loss of refractory phases by hydration on a (1 0 1) oriented grain was 4.53 nm/h. This material loss may be insignificant for a bulk brick, but is a serious threat for a magnesia castable consisting of fine magnesia powder due to large surface area exposed to humidity or water itself.     

The effect of nano-size additives on the electrical conductivity of matrix suspension and properties of self-flowing low-cement high alumina refractory castables

Influence of two nano-size additives on electrical properties of suspension matrix of self-flowing low-cement high alumina refractory castable is investigated. For this purpose, castament FS 10 and FS 20 on the basis of polycarboxylate ether were considered. The self-flow value, workability and mechanical strength of the castable are evaluated and their relations with electrical conductivity are determined. Using these relations, the type and optimum amount of proper additive for these refractory castables are determined. It was shown that if the electrical conductivity of matrix suspension is less than 0.71 mS/cm, high alumina low-cement self-flowing refractory castable can be obtained. The best self-flow, sufficient working time and adequate mechanical strength in the castables are obtained with 0.08 wt.% FS 20.     

Synthesis, characterization and optical properties of Mg(OH)2 micro-/nanostructure and its conversion to MgO

Magnesium hydroxide (Mg(OH)₂) micro- and nanostructures have been synthesized by a single step hydrothermal route. Surface morphology analysis reveals the formation of micro- and nanostructures with varying shape and size at different synthesis conditions. Structural investigations by X-ray diffraction (XRD) and transmission electron microscopy (TEM) confirm that the synthesized material is Mg(OH)₂ with hexagonal crystal structure. An optical band gap of 5.7 eV is determined for Mg(OH)₂ nanodisks from the UV–vis absorption spectrum. A broad emission band with maximum intensity at around 400 nm is observed in the photoluminescence (PL) spectra of Mg(OH)₂ nanodisks at room temperature depicting the violet emission, which can be attributed to the ionized oxygen vacancies in the material. Furthermore, Mg(OH)₂ has been converted to MgO by calcination at 450 °C. Optical studies of the MgO nanodisks have shown an optical band gap of 3.43 eV and a broadband PL emission in the UV region. Mg(OH)₂ and MgO nanostructures with wide-band gap and short-wavelength luminescence emission can serve as a better luminescent material for photonic applications.     

Combination of Brazilian test and digital image correlation for mechanical characterization of refractory materials

Flexibility of refractories is an interesting property to improve thermal shock resistance of refractories. It can often be obtained by promoting a nonlinear mechanical behaviour which reduces brittleness. Among industrial refractory materials developed in this aim, magnesia hercynite is of a particular interest for cement industry. As linings for rotary kilns, magnesia hercynite can be submitted to tensile and compressive stresses. Since tensile strength is usually much lower than compressive one for brittle materials, the mechanical characterization is, in such case, more significant in tension than in compression. To overcome difficulties involved by direct tensile test, an indirect tensile test (Brazilian test) has been applied here and combined to digital image correlation in order to measure kinematic fields on the surface of the sample during loading. This combination has allowed to accurately measure initial elastic properties (Young’s modulus and Poisson ratio), to detect crack initiation and to analyze fracture process.     

Thermal expansion and heat capacities of holmium and erbium orthotantalates ceramics

Monoclinic M-orthotantalates of holmium and erbium ceramics were obtained by co-precipitation and annealing at 1773 K and characterized by X-ray powder diffraction, scanning electron microscopy, and chemical analysis. Using the high-temperature X-ray diffraction method, the temperature dependence of the monoclinic lattice parameters of both ceramics were determined at range 298-1273 K. The molar heat capacity of M-HoTaO₄ and M-ErTaO₄ were measured by adiabatic (7-350 K) and differential scanning (320-1350 K) calorimetry. Standard thermodynamic function (entropy, enthalpy change, and reduced Gibbs energy) were calculated on the smoothed values of molar capacity, without taking into account contribution of low-temperature interval: 0-7 K for M-HoTaO₄ and 0-8 K for M-ErTaO₄. The general shape of the anomalous Schottky contribution to the molar heat capacity was determined.     

Acidic and basic binders for magnesite based aggregate in plaster of tundish

Binders are generally inorganic, organic or organomineral and have an important influence on the performance and corrosion resistance of slag line and deskulling. Since silicate and phosphate binders have some side effects, in this work sulphate binders such as sulphamic acid, H₂NSO₃H; aluminum sulphate, Al₂(SO₄)₃; ammonium sulphate, (NH₄)₂SO₄; magnesium sulphate, MgSO₄; calcium sulphate, CaSO₄; sodium sulphate, Na₂SO₄; and potassium sulphate, K₂SO₄, are investigated. Cold crushing strength at different heat treatments of room temperature, 110 °C, 1100 °C, 1400 °C is measured. Apparent porosity of samples without pulp and bulk density together with pH of the binder solution is evaluated and XRD and SEM studies are performed. Among these sulphate binders MgSO₄ was found to be the best. It is acidic in nature and develops strong bonds to the basic aggregate, MgO, at low temperatures. At high temperatures it dissociates from MgO(s) and SO₃(g) and the remained portion of MgO is the same as host oxide, with no corrosion and easy deskulling. Basic binders such as calcium sulphate, sodium sulphate and potassium sulphate could not strongly bond the MgO aggregates.     

Castable matrix,additives and their role on hydraulic binder hydration

The influence of alumina-based castable matrices, dispersants and Li₂CO₃ on the hydration process of different hydraulic binders was evaluated. The experiments were followed by temperature measurements with the time, the oscillatory rheometry technique and the normal force during the penetration of a blade through the material’s surface. By doing these tests, the chemical features of binder hydration, working and demolding time of suspensions could be assessed. The experiments have shown that the alumina-based matrix presented an accelerating effect which is related to its sodium content. Citric acid and ammonium citrate were the dispersants with the highest delaying effect and could be effectively associated with the accelerator (Li₂CO₃) in order to adjust the setting time of the suspensions. Nonetheless, their accelerating and retarding effect varied according to the hydraulic binder.     

Beta-sialon phosphor deposits fabricated by electrophoretic deposition (EPD) process in a magnetic field

Phosphor deposits of β-sialon:Eu²⁺ were prepared by electrophoretic deposition (EPD) process within a magnetic field. Under the action of the magnetic force, which was parallel to the direction of the electric field of the EPD, the β-sialon:Eu²⁺ crystals were aligned along the c-axis of the hexagonal cell unit to form an oriented deposit via the EPD fabrication. Higher orientation degree was obtained at longer depositing time (300 s) and stronger applied magnetic field (12 T). The oriented deposit aligned along the c-axis obtained higher relative deposit density than the randomly fabricated deposit. Due to the improved relative density, the oriented deposit prepared within the magnetic field possessed an enhanced external quantum efficiency (η_ex). Also, because of different relative densities of the deposits prepared within and without the magnetic field, they presented different chromaticity coordinates.     

Regeneration and reuse of magnesium oxide (MgO) nanocrystallites

The study focuses on regeneration and reuse of in-house synthesized MgO nanocrystallites used as an adsorbent for the removal of Acid Orange 7 dye from wastewater. The dye-adsorbed MgO were regenerated by two major steps: drying and combustion. The used MgO were dried at room temperature of 35°C ± 5°C, at an optimized time of 8–10 h time and the dried MgO were combusted at an optimized temperature of 500°C in muffle furnace for 25 min. The regeneration and reuse of MgO could be effectively carried out for five adsorption-combustion cycles. In every cycle of regeneration, 8–10% by weight of MgO was lost. The X-ray diffraction analysis showed the phase purity and the calculated crystallite size of synthesized, first, second, third, fourth and fifth regenerated MgO were noted as 21.67, 21.67, 21.67, 24.74, 24.74 and 24.74 nm, respectively. The scanning electron microscopy analysis showed that agglomeration of regenerated MgO particles increased with the increase in number of adsorption-combustion cycles.     

Factors controlling the mechanical behavior of alumina–magnesia–carbon refractories in air

This work analyzes the mechanical behavior of alumina-magnesia-carbon (AMC) refractories in air up to 1260 °C. AMC refractory bricks are used on sidewalls and bottoms working linings of steel-making ladles. In plant, AMC bricks are exposed to air atmosphere during the periods of time when the ladle is empty, such as preheating. Stress–strain relationships were determined in compression, and the following parameters were calculated from these curves: strength, apparent Young's modulus, fracture strain and yield strength. Young's modulus at room temperature was also determined by the impulse excitation technique. To identify the main determining factors, the tested specimens were analyzed by apparent porosity measurements, X-ray diffraction and scanning electron microscopy coupled with X-ray dispersive energy. Thermodynamic simulations of the AMC refractories were also performed using FactSage software, so as to understand the mineralogical changes that occur in the refractories as temperature increases.     

Characterization of alumina-magnesia-carbon refractory bricks containing aluminium and silicon

The present paper is a continuation of previous ones focused on the characterization of alumina-magnesia-carbon refractories (AMC) bricks. Unlike those characterised materials which only have aluminium as an antioxidant, a brick also containing silicon is comparatively analysed in this paper. Along with metallic aluminium, silicon is among the most commonly used antioxidant additives in oxide-C refractories, but the studies on the incorporation of this metal into AMC bricks are rather limited.In this work, several complementary techniques were used in combination to achieve a detailed characterization in relation to the physical and chemical characteristics and thermal evolution of the Si-containing AMC refractory: X-ray fluorescence, plasma emission spectroscopy, gravimetry, granulometry, X-ray diffraction, differential thermal and thermogravimetric analyses, reflection optical and scanning electron microscopies, mercury intrusion porosimetry, and density, porosity, permanent linear change and permeability measurements. In addition, the characterization was completed by evaluating the chemical resistance to air and to a basic slag and calculating the equilibrium composition of the brick when it is subjected to high temperatures (up to 1400?°C). A comparison with the results obtained for previously characterised AMC bricks with similar MgO contents and metallic aluminium as an antioxidant was also made. The presence of silicon was related to finer open porosity and higher oxidation resistance at high temperature (1400?°C).     

High temperature gaseous corrosion resistance of MgO-containing refractories – A comparative study

In the work the results of investigations into corrosive resistance to the attack of sulphur oxides of six commercially available basic refractories containing magnesium oxide have been presented. The research was conducted for materials applied in glass furnace regenerators: three types of magnesia-zirconia-forsterite products, one magnesia-forsterite and one magnesia product as well as a magnesia-chromite product to be applied in copper convertors. Investigations were carried out in a semi-flow chemical reactor. Investigations into corrosive resistance were conducted at 600 °C, 800 °C and 1000 °C. The real and equilibrium composition of gaseous phase in the reactor versus temperature was determined. It was assumed that the relative increment of the tested products' mass after a certain time of their staying in the reactor would be and indicator of corrosive resistance. Based on investigations into the phase composition of the products of reaction, it was found that at 600 °C and 800 °C the product's corrosion resulted from the formation of MgSO₄, as the main product of reaction, whereas at 1000 °C – it was the formation of CaMg₂(SO₄)₃. In products with an addition of ZrO₂ destabilisation of the regular structure of c-ZrO₂ was observed as well as its change into the monoclinic variety - m-ZrO₂. The examined refractory materials were ordered according to their corrosive resistance to sulphur oxides at particular temperatures applied in the test. The conducted investigations qualitatively describe the behaviour of refractories exposed to the corrosive attack of sulphur oxides and can be useful in the selection of refractory materials for particular industrial applications.     

Preparation of high solid loading,low viscosity ZrB2–SiC aqueous suspensions using PEI as dispersant

PEI was used as dispersant for ZrB₂ and SiC powders in water. The dispersion behavior of ZrB₂ and SiC in water was studied by zeta potential measurements, particle size distribution measurements and interparticle interaction calculations. Well-dispersed ZrB₂ and SiC aqueous suspensions were obtained using 0.6 wt% PEI at pH 6. The rheological behavior of ZrB₂–SiC aqueous suspensions was also investigated. Finally, a high solid loading (52 vol%), low viscosity (980 mPa s at 100 s⁻¹) ZrB₂–SiC aqueous suspension was successfully prepared.     

Effect of sintering temperature on mechanical properties of magnesia partially stabilized zirconia refractory

The optimized sintering conditions for a 3.5 wt% magnesia partially stabilized zirconia (Mg-PSZ) refractory were proposed in our recent research. The influence of the sintering temperature on the development of phase composition, microstructure, densification, thermal expansion and mechanical strength was studied in detail by X-ray diffraction (XRD), scanning electron microscope (SEM), He-pycnometer, high temperature dilatometry and three-point bending test. The samples sintered at 1670 °C had the highest bend strength, the maximum densification, the lowest thermal expansion coefficient (CTE), a homogeneous microstructure and a linear change in thermal expansion.     

Influence of the acid–basic character of oxide surfaces in dispersants effectiveness

Physical–chemical surface character has been described as a determinant factor in adsorbing polyelectrolytes onto oxide surfaces. Since basically composed by hydroxyl groups, the acidity or basicity induced by the bulk lattice in such groups seems to be of great relevance in the adsorption process. The influence of such properties in the adsorption of polyacrylic acid derivated polymers onto SnO₂ and Al₂O₃ surfaces is, therefore, studied in this paper. Polyacrylate acid molecules are observed to hardly adsorb onto the SnO₂ surface but strongly adsorb, as reported previously, onto the alumina surface. This behavior is explained based on the pronounced difference in relation to the acidity of both surfaces and, in this sphere of thought, a basic polymer (Chitosan; CS) is discussed as a functional dispersant for powders with acid surface character, such as SnO₂. Zeta potential measurements showed that this polymer raises the SnO₂ potential to +30 mV without pH variations.