Periclase Phase Research Articles

Magnesite Effect to the Alumina Sintering for Heat Sink Application

In the modern world with an extensive improvement of science and technologies, alumina (Al2O3) ceramics have been identified as one of the usable materials in production of electronic components. This project attempts to investigate the magnesite (MgCO3) effect to Al2O3 sintering for heat sink application. The effect of MgO percentage varying from 0% to 50% has been in investigated, emphasizing on the flexural strength test, thermal conductivity test, elastic modulus test, shrinkage percentage and density measurement. The result revealed that the MgO give an effect to the final thermal conductivity and mechanical properties of Al2O3 material at high sintering temperature (i.e. at temperature of 1600°C). Throughout an X-ray diffraction (XRD) analysis of Al2O3 and MgO compositions, it was revealed that spinel, periclase and corundum phases exist for all sintered temperature which contributes to strengthen grain boundaries and improved mechanical properties.

Surfactant-assisted synthesis and catalytic activity for SOx abatement of high-surface-area CuMgAlCe mixed oxides

CuMgAlCe mixed oxides were prepared by a modified coprecipitation–calcination method using CTAB as surfactant template. All the precursors showed hydrotalcite-like layered structure and mixed oxides with mainly periclase phase were obtained after calcination. Catalytic activity for SO2 removal of mixed oxides was examined through adsorption–reduction cycles under the conditions similar to those of FCC units. The results showed that incorporation of both Ce and Cu could improve SOx oxidative chemisorption. CTAB/metal molar ratio during synthesis had a significant influence on the structural properties of mixed oxides. Sample CuMgAlCe-0.1 prepared by CTAB/metal molar ratio of 0.1 had the highest specific area 142.2m2/g and also presented the best SO2 adsorption rate and capacity. This behavior is mainly due to its exposed more adsorption sites provided by high specific surface area, facilitating SO2 diffusion and contact with active components. It still possessed excellent cyclic stability that is beneficial for industrial application.

Effect of composition on thermal conductivity of MgO–Nd2Zr2O7 composites for inert matrix materials

Inert matrix fuels based on magnesium oxide (MgO) as the inert phase have been of historic interest due to its high thermal conductivity. Minor actinide-bearing phases possessing an A2B2O7 pyrochlore structure are also believed to contain a range of favorable attributes, suggesting a possible pairing with MgO to yield a high performance inert matrix fuel. The thermal diffusivity, heat capacity, and thermal expansion of MgO–Nd2Zr2O7 composites were measured from room temperature to 1273K, where the MgO phase content was varied from 40 to 70 volume percent. The thermal conductivity of each composition was calculated using these results and then compared to widely employed methods to approximate the thermal conductivity of composite materials based upon the properties of the constituent phases. Results suggest that use of either a rule of mixtures or geometric mean approximation for the thermal conductivity of composite systems such as this one would be subject to significant uncertainties when the constituent properties widely differ. A sigmoidal average of the upper and lower Hashin–Shtrikman bounds was found to be in good agreement with the thermal conductivity of the composites as determined experimentally.

Effect of calcination temperature for magnesite on interaction of MgO-rich phases with boric acid

Abstract Magnesia (MgO), which can be obtained by calcination of natural magnesite, is one of the most effective known sorbents for borate in aqueous solutions. Here we examine the effect of calcination temperature on sorption of borate for MgO-rich phases produced by calcination of magnesite at 873–1373 K. Calcination at or above 1273 K produced a single magnesium oxide phase, whereas basic magnesium carbonates ( m MgCO 3 · n Mg(OH) 2 · x H 2 O) formed in association with magnesium oxide at or below 1073 K. Calcination temperature directly affected the efficiency of decarbonation of magnesium carbonate, and the solubility and basicity of magnesium oxide in the resultant calcined products. These factors (along with the boron concentration) essentially control the immobilization mechanism of borate on the calcined MgO-rich phases. After sorption of borate on products calcined at lower temperatures, different types of basic magnesium carbonates were formed that are less effective at immobilizing borate. At low borate concentrations, under saturation for magnesium borate hydrate (Mg 7 B 4 O 13 ·7H 2 O), co-precipitation of borate with Mg(OH) 2 predominates. However, as magnesium borate hydrate becomes supersaturated, both precipitation of Mg 7 B 4 O 13 ·7H 2 O and co-precipitation with Mg(OH) 2 contribute significantly to borate immobilization. Calcination temperature is a key practical factor affecting the borate sorption efficiency by changing the immobilization mechanism.

Preparation and Characterization of Mg‐Zr Mixed Oxide Aerogels and Their Application as Aldol Condensation Catalysts

A series of Mg-Zr mixed oxides with different nominal Mg/(Mg+Zr) atomic ratios, namely 0, 0.1, 0.2, 0.4, 0.85, and 1, is prepared by alcogel methodology and fundamental insights into the phases obtained and resulting active sites are studied. Characterization is performed by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, N(2) adsorption-desorption isotherms, and thermal and chemical analysis. Cubic Mg(x)Zr(1-x)O(2-x) solid solution, which results from the dissolution of Mg(2+) cations within the cubic ZrO(2) structure, is the main phase detected for the solids with theoretical Mg/(Mg+Zr) atomic ratio ≤0.4. In contrast, the cubic periclase (c-MgO) phase derived from hydroxynitrates or hydroxy precursors predominates in the solid with Mg/(Mg+Zr)=0.85. c-MgO is also incipiently detected in samples with Mg/(Mg+Zr)=0.2 and 0.4, but in these solids the c-MgO phase mostly arises from the segregation of Mg atoms out of the alcogel-derived c-Mg(x)Zr(1-x)O(2-x) phase during the calcination process, and therefore the species c-MgO and c-Mg(x)Zr(1-x)O(2-x) are in close contact. Regarding the intrinsic activity in furfural-acetone aldol condensation in the aqueous phase, these Mg-O-Zr sites located at the interface between c-Mg(x)Zr(1-x)O(2-x) and segregated c-MgO display a much larger intrinsic activity than the other noninterface sites that are present in these catalysts: Mg-O-Mg sites on c-MgO and Mg-O-Zr sites on c-Mg(x)Zr(1-x)O(2-x). The very active Mg-O-Zr sites rapidly deactivate in the furfural-acetone condensation due to the leaching of active phases, deposition of heavy hydrocarbonaceous compounds, and hydration of the c-MgO phase. Nonetheless, these Mg-Zr materials with very high specific surface areas would be suitable solid catalysts for other relevant reactions catalyzed by strong basic sites in nonaqueous environments.

Nanocomposite stability in Fe-, Co-, and Mn-based perovskite/spinel systems

Abstract

Dissolution behavior of plutonium containing zirconia–magnesia ceramics

This study explores the dissolution properties of zirconia–magnesia ceramics containing plutonium as the basis of an inert matrix nuclear fuel. The magnesium oxide phase remains pure MgO, while the zirconia incorporates a small amount of magnesium oxide along with all of the plutonium oxide and erbium oxide. The performance of the material under reactor and repository environments was examined. Reactor conditions are examined using a pressure vessel to expose the material to 300°C water. To assess the performance of the material as a waste form it was submerged in 90°C water for 1000h. In both aqueous dissolution studies there was minimal release of less than 0.8wt.% of plutonium from the material. To examine the potential for recycling, the dissolution behavior of the fuel matrix was examined in acidic solutions: pure nitric acid and a nitric acid–hydrofluoric acid–peroxide solution. Both acidic media exhibit potential for dissolving plutonium from the zirconia matrix. The experiments performed in this study are meant to lay a foundation for the chemical performance of zirconia–magnesia inert matrix fuel containing fissile material and burnable poison.

Mg–Al hydrotalcites as efficient catalysts for aza-Michael addition reaction: A green protocol

Mg–Al hydrotalcite was synthesized by a co-precipitation method. We have studied the effect of calcination temperature and hydration of the calcined phases on their catalytic activity for the synthesis of pyrazolo[1,5- a]pyrimidine derivatives (aza-Michael addition product). The structure of the as-synthesized sample and the presence of the anions in the interlayer galleries of hydrotalcites, have been determined by X-ray diffraction and FTIR spectroscopy. On calcining the material at 450 °C, it was amorphous periclase phase. Re-hydration of the calcined phase resulted in the formation of hydrotalcite-like phase. Such treatment to the as-synthesized hydrotalcite significantly changed the pore structure and the BET-surface area as determined from N 2 physisorption at 77 K. The as-synthesized Mg–Al-hydrotalcite catalyst was found to be the most efficient for the aza-Michael reaction relative to the activated solid catalysts tested. The high performance of this catalyst was attributed to the co-operative contribution of its acidic and basic sites. We have shown that this microwave assisted reaction provides an eco-friendly alternative to the conventional syntheses where soluble bases are used. Furthermore, the reaction was performed over a considerably shorter time scale and generated significantly higher yields than traditional methods.

Acidic dissolution behavior of U containing ZrO 2–MgO ceramics

This study explores the possibility of dissolving zirconia–magnesia inert matrix fuel containing uranium oxide as a fissile material and plutonium homolog and erbium oxide as a burnable poison with nitric and sulfuric acid as a potential first step in a reprocessing scheme. The progress of the dissolution is followed by monitoring the amount of material in solution by inductively coupled plasma-atomic emission spectroscopy, assessing the speciation of the material by time resolved laser fluorescence spectroscopy, and determining and quantifying the crystalline phases present in the remaining residue by X-ray diffraction. This study has shown a linear incongruent dissolution of the cubic zirconia phase in concentrated nitric acid under certain chemical compositions, while the magnesium oxide phase is completely soluble. In sulfuric acid uranium, erbium, and magnesium are soluble to different extents while zirconium forms a colloidal suspension that conglomerates and settles out of solution. The feasibility of the dissolution of zirconia–magnesia inert matrix fuel with nitric and sulfuric acid for reprocessing is discussed.

Effect of reaction heat on microstructure of composite fabricated by combustion reaction and reactive infiltration between SiO2 and Mg

Magnesium matrix composites were fabricated with the help of a reaction between magnesium and silicon oxide (SiO2) powders. The specimens were prepared by two processing routes (a powder processing route and an infiltration route). The basic reaction behavior was investigated by heating compacted SiO2 and magnesium powder blends. The effects of aluminum oxide (Al2O3) powder addition on the microstructures were investigated by the infiltration route. Magnesium oxide (MgO) and magnesium silicide (Mg2Si) were synthesized by the reaction between magnesium and SiO2. By the powder processing route, an explosive reaction took place when the volume blending ratios of SiO2/Mg were in between 20/80 and 30/70, and the specimens were highly porous. This explosive reaction became remarkable by decreasing the SiO2 powder size. The thermodynamic calculation revealed that the adiabatic temperature (Tad) of the specimen exceeded the boiling point of magnesium when SiO2/Mg were in between 20/80 and 30/70. Molten magnesium infiltrated into the SiO2 powder phase spontaneously under the heating condition of 800°C for 3.6 ks. The microstructure after the infiltration consisted of MgO, Mg2Si and magnesium. The course MgO agglomerates, which might be formed by the high temperature at aninfiltration front, were observed in the microstructure after the infiltration. By blending an Al2O3 powder (30 vol%, 50 vol%) in the SiO2 powder, the MgO phase became finer to several microns. The pores were hardly visible in the microstructure of the specimens fabricated by the infiltration route. The micro-Vickers hardness data showed that the hardness of the specimen was improved by blending the Al2O3 powder.

High CO2 Storage Capacity in Alkali‐Promoted Hydrotalcite‐Based Material: In Situ Detection of Reversible Formation of Magnesium Carbonate

Alkali-promoted hydrotalcite-based materials showed very high CO(2) storage capacity, exceeding 15 mmol g(-1) when the carbonation reaction was carried out at relatively high temperature (300-500 °C) and high partial pressure of steam and CO(2). In situ XRD experiments have allowed correlation of high CO(2) capacity to the transformation of magnesium oxide centres into magnesium carbonate in alkali-promoted hydrotalcite-based material. Moreover, it has been clearly shown that crystalline magnesium carbonate may be reversibly formed at temperatures above 300 °C in the presence of sufficient partial pressure of steam in the gas phase, conditions that are prevalent in pre-combustion CO(2) capture. The role of steam appears to be of utmost importance for the formation of the bulk carbonate phase and for its reversibility. It is proposed that a high partial pressure of steam keeps the magnesium oxide periclase phase sufficiently hydroxylated to allow magnesium carbonate formation if a relatively high partial pressure CO(2) is present in the gas phase.

High stability of Ce-promoted Ni/Mg–Al catalysts derived from hydrotalcites in dry reforming of methane

Ce-promoted Ni/Mg–Al catalysts were synthesized by means of a methodology that involves the doping of Ni–Mg–Al mixed oxides derived from hydrotalcites with [Ce(EDTA)] − and subsequent thermal decomposition. The effect of the nominal load of Ce in the catalytic performance of the materials was studied. The solids were characterized by means of XRD, BET area, TPR-H 2, TPD-CO 2, chemical analysis by ICPs, TGA, SEM and TEM and were evaluated in CO 2 reforming of methane at 700 °C. The results indicate the partial reconstruction of the periclase phase during the doping with [Ce(EDTA)] − and the formation of a mixture of crystalline periclase and fluorite phases after the calcination. Catalysts with particle sizes of Ni 0 between 5 and 9 nm were obtained. Ce presents a promote effect in the degree of reduction of Ni and the amount and strength of the basic sites. It was evident a beneficial effect of cerium in the catalytic activity and selectivity of the doped materials. The increase of the nominal Ce load between 1 and 10% causes no considerable effect in the catalytic activity and selectivity or in the size of crystallite in these materials but in the inhibition of the coke formation. The catalysts show excellent catalytic performance under drastic conditions of reaction and long operation times. The Ce-doped Ni/Mg–Al catalyst is stable up to 100 h of reaction using a feed mixture of CH 4/CO 2/He 10/10/80 at 24 L g −1 h −1, up to 20 h of reaction using CO 2/CH 4 20/20 at 48 L g −1 h −1 and up to 15 h of reaction using CO 2/CH 4 40/40 at 96 L g −1 h −1. The filamentous coke formation is demonstrated on the surface of the catalyst when gas of dilution in the reactants is not used. The developed method of synthesis becomes an interesting methodology for obtaining catalysts for CO 2 reforming of methane.

The Role of Acetic Acid and Glycerol in the Synthesis of Amorphous MgO Aerogels

Although sol–gel‐derived magnesium‐oxide (MgO) aerogels have a wide variety of applications, mostly thin films and powders of brucite, i.e. Mg(OH)2 aerogels, have been reported. Typically, the brucite structure is converted to the periclase form of MgO upon heat‐treatment. We report the role of acetic acid (HAc) and glycerol in the synthesis of amorphous MgO aerogel monoliths through the alkoxide sol–gel method. The two ligands work in concert to retard the nucleophilic substitution with water and decrease the rate of gelation, allowing for Mg–O bond formation while reducing the number of hydroxyl bonds. The amorphous material crystallizes to the periclase (MgO) phase at 400°C without going through the brucite transition, resulting in a novel amorphous MgO aerogel. Specifically, we discuss the modification of the Pechini process to an Mg‐alkoxide sol–gel process. The roles of the ligands are investigated in the MgO gelation process through Fourier transform infrared spectroscopy, X‐ray diffractometry, and thermogravimetric analysis/DSC. The physical properties of the aerogels are also reported.

Methods for improving volume stability of steel slag as fine aggregate

Suitable methods for enhancing the volume stability of steel slag utilized as fine aggregate were determined. The effects of steam treatment at 100 °C and autoclave treatment under 2.0 MPa on the soundness of steel slag sand were investigated by means of powder ratio, linear expansion, compressive and flexural strength. DTA, EDX, XRD and ethylene glycol methods were employed to analyze both the treated slags and susceptible expansion grains. Experimental results indicate that powder ratio, content of free lime and rate of linear expansion can express the improvement in volume stability of different treated methods. Steam treatment process cannot ultimately prevent specimens from cracking and decrease of strength, but mortar made from autoclave treated slag keeps integration subjected to hot water of 80 °C until 28 d and its strength do not show significant decrement. The hydration of over-burn free lime and periclase phase are the main cause for the disintegration or crack of untreated and steam treated steel slag’s specimens. Autoclave treatment process is more effective than steam treatment process on enhancement of volume stability of steel slag.

Synthesis of Magnesium Oxide Nanoparticles by Sol-Gel Process

Cubic shaped Magnesium oxide nanoparticles were successfully synthesized by sol-gel method using magnesium nitrate and sodium hydroxide at room temperature. Hydrated Magnesium oxide nanoparticles were annealed in air at 300 and 500°C. X-ray diffraction patterns indicate that the obtain nanoparticles are in good crystallinity, pure magnesium oxide periclase phase with (200) orientation. Morphological investigation by FESEM reveals that the typical sizes of the grown nanoparticles are in the range of 50-70nm. Powder composition was analyzed by the FTIR spectroscopy and the results confirms that the conversion of brucite phase magnesium hydroxide in to magnesium oxide periclase phase was achieved at 300°C.The Thermo-gravimetric analysis showed the phase transition of the synthesized magnesium oxide nanoparticles occurs at 280-300°C.

Using confocal scanning laser microscopy for the in situ study of high-temperature behaviour of complex ceramic materials

The confocal scanning laser microscopy (CSLM) technique has been successful in many metallurgical fields. This paper assesses its applicability to the in situ investigation of the high-temperature behaviour of complex ceramic materials. Magnesia–chromite refractory is selected as a ceramic test material. At room temperature, CSLM images correspond well to typical light optical microscopy (LOM) and backscattered electron (BSE) micrographs. In fact, because of the high axial resolution (short focal depth) and the confocal optics of the CSLM technique, the porosity level of the mirror polished ceramic specimens is more truthfully assessed by the CSLM (2-D) images than by the BSE micrographs (long focal depth). However, at high-temperatures (1550–1650 °C) the observed CSLM (2-D) image quality is slightly poorer. The principal explanation is the in situ roughening of the specimen surface during heating. The roughening has two causes: differential thermal expansion of the two primary phases in the ceramic test material and, to a lesser extent, thermal grooving. Nevertheless, it is shown that the CSLM image quality suffices for an in situ study of the high-temperature behaviour of ceramic materials. This is illustrated for the magnesia–chromite system by examining the dissolution mechanism of secondary (magnesiochromite) spinel into the periclase phase.

In Situ Observation of Aluminothermic Reduction of MgO with High Temperature Optical Microscope

Desulfurization and deoxidation of molten iron with magnesium vapor produced in-situ by aluminothermic reduction of magnesium oxide has been developed. In the present study, the mechanism of aluminothermic reduction of magnesium oxide by use of pellets composed of magnesium oxide and aluminum powders was discussed. SEM observation of the pellets at different reduction stages for various temperatures showed that fracture and disappearance of the alumina films on the aluminum particles were stimulated above 1 473 K, and thus the reduction of magnesium oxide was greatly accelerated.From in-situ observation of the aluminothermic reduction of magnesium oxide using a high temperature optical microscope (HTOM), it was found that the reduction proceeded after the penetration of molten aluminum into the magnesium oxide phase. The penetration began at 1 273 K and it was accelerated above 1 373 K. The reduction took place violently at 1 473 K. SEM observation and EDS analysis of the cooled sample revealed that the penetration took place only through cracks of the alumina film.In-situ observation of the melting process of aluminum particles together with SEM observation of the cooled sample showed that the thermal stress and the stress formed during phase transformation could break up the alumina film and the outflow of molten aluminum did take place at the elevated temperature.

Influence of reducibility of vanadium–magnesium mixed oxides on the oxidative dehydrogenation of propane

A series of vanadium–magnesium mixed oxides used for the propane oxidative dehydrogenation were synthesized by combining the impregnation and precipitation methods. Several techniques such as, FTIR, XRD and TPR-H 2 were applied to characterize physicochemical properties. XRD analysis confirms that the V–Mg–O oxides consist of Mg 3V 2O 8 structure with VO 4 tetrahedral units together with periclase phase. Magnesium incorporation with vanadium inhibits the formation of V 2O 5 containing exposed V O bonds. TPR-H 2 results show two peaks located between 500 and 750 °C, which, respectively, correspond to reduction of V 5+ species in distorted VO 4 units and V 5+ species belonging to bulk Mg orthovanadate. The highest activity and selectivity were obtained on catalysts in which isolated VO 4 tetrahedral are present. A relationship between reducibility degree of V 5+ distorted species (into unities VO 4), and catalytic activity was observed. In order to get a higher activity a maximum content of 45% as vanadium oxide.

Synthesis, characterization and evaluation of CeO 2/Mg,Al-mixed oxides as catalysts for SO x removal

Hydrotalcite-derived Mg,Al-mixed oxides (MO) with variable Mg/Al ratios (3, 1, 1/3) were impregnated with 17 wt.% of CeO 2 and evaluated for SO x removal under conditions similar to those of FCC units. The catalyst with Mg/Al = 1, for which both an Mg(Al)O periclase-type and an MgAl 2O 4-spinel were present before sulfation, showed the best performance for both SO x uptake and catalyst regeneration. The results from TPR/MS and XRD along reduction indicate the existence of different species in the sulfated samples. The reduction of these sulfate species leads to the evolution of SO 2 or H 2S, Mg(Al)O being the solid product in any case. Our results do not support the assertion that the reduction of S 6+ to S 2− is a consecutive reaction. The evolution of SO 2 upon reduction decreases as the relative importance of the spinel-phase in the mixed oxides increases. The sulfates formed from the spinel-phase are more easily reduced than those from the periclase phase. The growth of the sulfate phase in the early stages of sulfation destroys the small mesopores of the mixed oxide. Upon regeneration, the sulfate phase is consumed and larger mesopores are formed.

Physicochemical and textural characterization of vanadium–magnesium mixed oxides

A series of vanadium–magnesium mixed oxides (V–Mg–O) with a mesoporous structure used for the propane oxidative dehydrogenation was synthesized by combining the impregnation and precipitation methods. Several techniques such as N 2 physisorption, X-ray diffraction (XRD), FTIR and temperature-programmed reduction (TPR) were applied to characterize the textural properties, phase composition and surface oxygen reduction features of the oxides. Textural properties of the calcined V–Mg–O oxides were found very sensitive to chemical composition: bimodal or multimodal pore size distributions was developed, which progressively varied with vanadium content. XRD analysis and infrared (IR) characterization confirm that the V–Mg–O oxides consist of a Mg 3V 2O 8 structure with VO 4 tetrahedral units together with periclase phase. Magnesium incorporation with vanadium completely inhibits the formation of V 2O 5 containing exposed VO bonds. TPR results showed two peaks located between 500 and 750 °C, corresponding to the reduction of V 5+ species in distorted VO 4 units and to V 5+ species belonging to bulk Mg orthovanadate, respectively.