Magnesium Oxide Crystals Research Articles

Probing periodic potential of crystals via strong-field re-scattering

Strong-field ionization and re-scattering phenomena have been used to image angstrom-scale structures of isolated molecules in the gas phase. These methods typically make use of the anisotropic response of the participating molecular orbital. Recently, an anisotropic strong-field response has also been observed in high-order harmonic generation (HHG) from bulk crystals (2016 Nat. Phys. 13 345). In a (100) cut magnesium oxide crystal, extreme ultraviolet high-harmonics are found to depend strongly on the crystal structure and inter-atomic bonding. Here, we extend these measurements to other two important crystal orientations: (111) and (110). We find that HHG from these orientations is also strongly anisotropic. The underlying dynamics is understood using a real-space picture, where high-harmonics are produced via coherent collision of strong-field driven electrons from the atomic sites, including from the nearest neighbor atoms. We find that harmonic efficiency is enhanced when semi-classical electron trajectories connect to the concentrated valence charge distribution regions around the atomic cores. Similarly, the efficiency is suppressed when the trajectories miss the atomic cores. These results further support the real-space picture of HHG with implications for retrieving the periodic potential of the crystal, if not the wavefunctions in three-dimensions.

Gas-phase spectra of MgO molecules: a possible connection from gas-phase molecules to planet formation

A more fine-tuned method for probing planet-forming regions, such as protoplanetary discs, could be rovibrational molecular spectroscopy observation of particular premineral molecules instead of more common but ultimately less related volatile organic compounds. Planets are created when grains aggregate, but how molecules form grains is an ongoing topic of discussion in astrophysics and planetary science. Using the spectroscopic data of molecules specifically involved in mineral formation could help to map regions where planet formation is believed to be occurring in order to examine the interplay between gas and dust. Four atoms are frequently associated with planetary formation: Fe, Si, Mg and O. Magnesium, in particular, has been shown to be in higher relative abundance in planet-hosting stars. Magnesium oxide crystals comprise the mineral periclase making it the chemically simplest magnesium-bearing mineral and a natural choice for analysis. The monomer, dimer and trimer forms of (MgO)n with n = 1–3 are analysed in this work using high-level quantum chemical computations known to produce accurate results. Strong vibrational transitions at 12.5, 15.0 and 16.5 μm are indicative of magnesium oxide monomer, dimer and trimer making these wavelengths of particular interest for the observation of protoplanetary discs and even potentially planet-forming regions around stars. If such transitions are observed in emission from the accretion discs or absorptions from stellar spectra, the beginning stages of mineral and, subsequently, rocky body formation could be indicated.

Physicochemical and adsorptive characteristics of activated carbons from waste polyester textiles utilizing MgO template method.

Activated carbons with high specific surface areas were produced, utilizing waste polyester textiles as carbon precursor by magnesium oxide (MgO) template method. Magnesium chloride (MgCl2), magnesium citrate (MgCi), and MgO were employed as MgO precursors to prepare activated carbons (AC-MgCl2, AC-MgCi, and AC-MgO). Thermogravimetry-differential scanning calorimetry was conducted to investigate the pore-forming mechanism, and N2 adsorption/desorption isotherms, XRD, SEM-EDS, TEM, FTIR and pHpzc were achieved to analyze physicochemical characteristics of the samples. The specific surface areas of AC-MgCl2 (1173m2/g) and AC-MgCi (1336m2/g) were much higher than that of AC-MgO (450m2/g), and the pores sizes of which were micro-mesoporous, mesoporous, and macropores, respectively, due to the formation of MgO crystal with different sizes. All activated carbons had abundant acidic oxygen groups. In addition, batch adsorption experiments were carried out to investigate the adsorptive characteristics of the prepared activated carbons toward Cr(VI). The adsorption kinetics fitted well with the pseudo-second order, and the adsorptive capacity of AC-MgCl2 (42.55mg/g) was higher than those of AC-MgCi (40.93mg/g) and AC-MgO (35.87mg/g).

Bifunctional magnesium oxide crystal successively as adsorbent and matrix modifier for preconcentration and determination of arsenic by graphite furnace atomic absorption spectrometry

A novel method for determination of total inorganic arsenic in water samples, based on magnesium oxide crystals quantitatively preconcentration and graphite furnace atomic absorption spectrometry (GFAAS) detection has been developed. As the MgO crystals could successively act as the adsorbent and matrix modifier, the method presents great potential in practical routine analysis of inorganic arsenic. With optimization of the experimental conditions, 1.5mg of MgO was added into 4.0mL water sample and ultrasonic dispersing 15min for adsorbing the arsenic at first. Then, the mixture was centrifuged 5min for separating the adsorbents and the water solution; the deposits were collected and dissolved with HNO3 for arsenic determination. The enrichment factor of 13, limit of detection (LOD) of 0.087μgL−1, relative standard deviation (RSD) of 4.5% (n=7, c=2.0μgL−1) were achieved under the optimized conditions. The procedure was validated by analysis of certified water sample (GBW08605), which results in good agreement between the certified and the found values. The method was further demonstrated for analysis of lake water, snow water and tap water, and satisfactory spiked recoveries of 98.5% to 103.0% were achieved for these samples. These preliminary results indicated the present method was practical and accurate for analysis of trace-arsenic in natural water samples.

Sol-gel preparation of Ag-doped MgO nanoparticles with high efficiency for bacterial inactivation

Herein, Ag-doped magnesium oxide (MgO) nanoparticles were prepared by a citric acid-assist sol-gel method. It is evidenced that the size of MgO particles decreases after Ag doping and a small amount of Ag is doped into MgO crystal. The bacterial inactivation of as-prepared Ag-doped MgO against Escherichia coli (E. coli) suggests that Ag doping can greatly enhance the antibacterial activity of MgO nanoparticles and 1% Ag-doped MgO inactivates effectively 7-log bacterial cells within 20min. The releases of metal ions (Ag+ and Mg2+) from Ag-doped MgO are at a very low level, which would not play the leading role in bacterial inactivation. The mechanism for the improvement of antibacterial activity of Ag-doped MgO is concluded as three aspects. Firstly, Ag doping can inhibit the grain growth of MgO nanoparticles, resulting in smaller size of MgO particles. Secondly, when Ag+ is doped into MgO matrix, more oxygen vacancies will be generated to keep an overall neutral charge. Thirdly, Ag-doped MgO has a relatively low electron-transfer resistance, which can accelerate the electron transfer within MgO crystal, in favour of the single-electron reduction of adsorbed oxygen. All these would substantially enhance ROS production and the contact interaction between bacterial cells and nanoparticles. Therefore, Ag doping can markedly promote the antibacterial activity of MgO nanoparticles.

Preparation of nano-magnesium oxide from seawater bittern using decarboxylation/precipitation method

ABSTRACTThe nanoparticles of magnesium oxide used mainly for additives were synthesized through precipitation method in the presence of precipitants. The effects of preparation parameters, such as NaOH and Ca(OH)2 concentrations, dilution rate, rotation speed on size and morphology of the nanoparticles were investigated. The magnesium oxide was produced from magnesium hydroxide in the waste-seawater bittern. The properties of the obtained nanoparticles were analyzed by XRD and SEM. The obtained magnesium oxide crystals showed a wide distribution of globular structure including sphere or cubic-like morphology. Also size and crystallinity of magnesium oxide crystals increased from 50 to 180 nm with increasing a rotation speed.

Preferential dissolution of SiO2 from enstatite to H2 fluid under high pressure and temperature

Stability and phase relations of coexisting enstatite and H2 fluid were investigated in the pressure and temperature regions of 3.1–13.9 GPa and 1500–2000 K using laser-heated diamond-anvil cells. XRD measurements showed decomposition of enstatite upon heating to form forsterite, periclase, and coesite/stishovite. In the recovered samples, SiO2 grains were found at the margin of the heating hot spot, suggesting that the SiO2 component dissolved in the H2 fluid during heating, then precipitated when its solubility decreased with decreasing temperature. Raman and infrared spectra of the coexisting fluid phase revealed that SiH4 and H2O molecules formed through the reaction between dissolved SiO2 and H2. In contrast, forsterite and periclase crystals were found within the hot spot, which were assumed to have replaced the initial orthoenstatite crystals without dissolution. Preferential dissolution of SiO2 components of enstatite in H2 fluid, as well as that observed in the forsterite H2 system and the quartz H2 system, implies that H2-rich fluid enhances Mg/Si fractionation between the fluid and solid phases of mantle minerals.

Hydration of periclase at 350 ∘ C to 620 ∘ C and 200 MPa: experimental calibration of reaction rate

The hydration of periclase to brucite was investigated experimentally. Single crystals of periclase machined to millimeter sized cubes with (100) surfaces were reacted with distilled water at temperatures of 350 to 620 ∘C and a pressure of 200 MPa for run durations of 5 to 40 minutes. Hydration produced a layer of brucite covering the surface of periclase. While the shrinking periclase largely retained its cube shape a surface roughness developed on the μm scale and eventually outward pointing spikes bounded by (111) faces emerged on the retreating faces of the periclase due to kinetic selection of less reactive (111) and (110) surfaces. The periclase to brucite conversion followed a linear rate law, where the reaction rate increased from 350 to 530 ∘C and then decreased towards higher temperature and finally vanished at about 630 ∘C, where periclase, brucite, and water are in equilibrium at 200 MPa. The overall kinetics of the hydration reaction is conveniently described in terms of a phenomenological interface mobility. Measuring the velocity of the hydration front relative to the lattice of the reactant periclase, the temperature dependence of its mobility is described by an Arrhenius relation with pre-exponential factor 1.7.10−12 m 4/s.J and activation energy of E A =55 kJ/mol.

Morphology characterization of periclase–hercynite refractories by reaction sintering

A periclase-hercynite brick was prepared via reaction sintering at 1600°C for 6 h in air using magnesia and reaction-sintered hercynite as raw materials. The microstructure development of the periclase-hercynite brick during sintering was investigated using X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscopy in combination with energy-dispersive X-ray spectroscopy. The results show that during sintering, Fe2+, Fe3+ and Al3+ ions in hercynite crystals migrate and react with periclase to form (Mg1-xFex)(Fe2-yAly)O4 spinel with a high Fe/Al ratio. Meanwhile, Mg2+ in periclase crystals migrates into hercynite crystals and occupies the oxygen tetrahedron vacancies. This Mg2+ migration leads to the formation of (Mg1-uFeu)(Fe2-vAlv)O4 spinel with a lower Fe/Al ratio and results in Al3+ remaining in hercynite crystals. Cation diffusion between periclase and hercynite crystals promotes the sintering process and results in the formation of a microporous structure.

Two Challenges to the System of Periclase Quality Evaluation

<p class="1Body">A new method of estimating the fused periclase quality – the revision of international standards. Quality and service life of refractories on the basis of fused periclase depend to a large extent upon the size of periclase crystals and its mineralogical composition. During fusion of periclase it is impossible to obtain completely homogeneous material with identical crystals size. That is why it is generally accepted to evaluate quality of fused periclase by the average crystals size. Measurements of this item are usually done with the help of generally adopted method of chords, which was developed in the last century and has a number of drawbacks. Magnezit Group developed a new objective method of digital analysis during microscopic examination of structural elements of fused periclase. It allows to considerably improve objectivity of obtained data. The main feature of the new method is application of the system of images analysis, which allows to carry out in automatic mode measurements on the preliminary created digital model of the whole area of the polished section. With the help of the digital model it is possible to calculate average size of fused periclase crystals taking into account number of crystals as well as percentage of the area occupied by them.</p> <p class="1Body">Does CaO/SiO<sub>2</sub> ratio influence service life of refractories? New view of old rules. Till today it was considered that one of the characteristics of fused periclase quality is coefficient of basicity – CaO/SiO<sub>2</sub>, ratio, which should be more than 2. Magnezit Group carried out investigations of coarse-crystalline fused periclase with MgO >97.5 % content and with various CaO/SiO<sub>2 </sub>ratios. We present in this report the main study results: if the impurities content is low in the fused periclase with MgO >97.5 %, then coefficient of basicity exerts limited influence onto the service life of periclase-carbon bricks.</p>

Thermo- and photostimulated luminescence and stimulated emission of magnesium oxide crystals irradiated in a nuclear reactor

The luminescence of nominally pure magnesium oxide crystals irradiated in a nuclear reactor is studied. It is shown that, in the entire fluence range (1015–1018 n/cm2), the active luminescence centers are chromium impurity ions, which are initially ( 1017 n/cm2), after the radiation-induced transformation Cr2+ → Cr3+, are in the form of Cr3+ ions. The role played by F centers is also shown. Pulsed stimulated emission is observed in crystals irradiated with a fluence of 1018 n/cm2.

Microstructure and texture evolution during growth of magnesio-aluminate spinel at corundum-periclase interfaces under uniaxial load: The effect of stress concentration on reaction progress

Reaction rims of magnesio-aluminate spinel were grown at the contacts between periclase and corundum at temperatures of 1250 °C to 1350 °C and under uniaxial load of 0.026 and 0.26 kN per 9 mm² of initial contact area. Single crystals of periclase with [100] and of corundum with [0001] perpendicular to the polished reaction interface as well as polycrystalline corundum were used as starting materials. Immediate application of the load before heating resulted in deformation twinning and fracturing of corundum introducing stress concentration and lateral variations in the quality of physical contact at the reaction interface. The tight contacts are characterized by enhanced reaction progress which together with the positive volume change of the reaction and limits on plasticity of the studied phases led to the opening of void spaces along the reaction interface and large lateral variations in rim thickness occur. Spinel shows strong topotactic relations to the reactant phases including full topotaxy between spinel and periclase, partial topotaxy with (111)_spi‖(0001)_cor and {101}_spi‖{10-10}_cor, and axiotaxy with (111)_spi‖(0001)_cor between spinel and corundum. Oriented nucleation and selective growth were the main mechanism of texture formation. Stress concentrations and tight physical contacts across the reaction interface may enhance nucleation of topotactic grains. The respective spinel–periclase and spinel–corundum reaction interfaces are mostly semi-coherent with sets of line dislocations accounting for the lattice misfit. The systematic occurrence of porosity along the semi-coherent and its absence along the incoherent sections of the spinel–periclase interface reflect the different capacities of the (semi)-coherent and incoherent interface sections for annihilating the vacancies that were emitted from the advancing spinel–periclase reaction interface.

Direct atomic-scale observation of layer-by-layer oxide growth during magnesium oxidation

The atomic-scale oxide growth dynamics are directly revealed by in situ high resolution transmission electron microscopy during the oxidation of Mg surface. The oxidation process is characterized by the layer-by-layer growth of magnesium oxide (MgO) nanocrystal via the adatom process. Consistently, the nucleated MgO crystals exhibit faceted surface morphology as enclosed by {200} lattice planes. It is believed that the relatively lower surface energies of {200} lattice planes should play important roles, governing the growth mechanism. These results facilitate the understanding of the nanoscale oxide growth mechanism that will have an important impact on the development of magnesium or magnesium alloys with improved resistance to oxidation.

Influence of H2 fluid on the stability and dissolution of Mg2SiO4 forsterite under high pressure and high temperature

High-pressure and high-temperature experiments were carried out in a Mg2SiO4-H2 system using laser-heated diamond-anvil cells to understand the influence of H2 fluid on the stability of forsterite. In situ X-ray diffraction experiments and Raman spectroscopic measurements showed the decomposition of forsterite, and formation of periclase (MgO) and stishovite/quartz (SiO2) in the presence of H2 after being heated in the range between 2.5 GPa, 1400 K and 15.0 GPa, 1500 K. Transmission electron microscopic observation of the samples recovered from 15.0 GPa and 1500 K showed that the granular to columnar periclase grains maintained the original grain shape of forsterite, indicating that the periclase crystals crystallized under high temperature. On the other hand, euhedral columnar stishovite crystals were found at the boundaries between residual forsterite grains and reacted periclase. This implies that the SiO2 component was dissolved in H2 fluid, and that stishovite was considered to have crystallized when the solubility of the SiO2 component became reduced with decreasing temperature. Additional experiment on a SiO2-H2 system clearly showed the dissolution of quartz in H2 fluid, while those on a MgO-H2 system, periclase was hardly dissolved. These lines of evidence indicate that forsterite was incongruently dissolved in H2 fluid to form periclase crystals in the Mg2SiO4-H2 system, which is different from what was observed in the Mg2SiO4-H2O system. The results indicate that the stability of forsterite is strongly affected by the composition of coexisting C-O-H fluid.

New periclase-magnesium aluminate spinel refractories from sintered high-purity dead burned magnesite and new various presynthesized spinel-based compositions. II. Compositional variation between coexisting spinel, periclase, and Ca-silicate and Ca-aluminate phases in magnesia spinel refractories and in their spinel-based precursors

The chemistry and distribution of phases formed in a set of six laboratory-prepared magnesia-magnesium aluminate spinel ceramics and in three of their precursor spinel-based compositions, sintered at 1600°C and 1760°C respectively, are examined and compared. The evaluation of the results is the basis for the industry design of magnesia spinel refractories for new advanced applications. In each type of the materials, the spinel phases formed under their firing temperatures seem to have a strong preference for the normal structure. Cracked and noncracked domains in periclase crystals from spinel-based compositions is probably facilitated by differences in the thermal expansion coefficient of periclase crystals having domains differing in their Al2O3 contents. Periclase and spinel mixtures at lower temperatures (1600°C) involved reactions leading to the formation of different periclase s.s and spinel s.s, which depart clearly from equilibrium assemblages. Minor amounts of stoichiometric Ca-silicate and Ca-aluminate phases were formed in all materials.

Infrared Pulsed Laser Deposition of Niobium Nitride Thin Films

We have successfully fabricated superconducting niobium nitride thin films on single crystals of magnesium oxide using a pulsed laser deposition technique where 1064 nm (photon energy ~1.16 eV) laser pulses from an Nd:YAG laser were used for ablation. A correlation between the superconducting transition temperature, the nitrogen base pressure during deposition and the lattice parameter of the produced NbN films was observed. Superconductor-insulator-normal metal junctions fabricated using these NbN films as the superconductor revealed nonlinear electrical characteristics at 4.2 K associated with quasiparticle tunneling.

Growth of multilayered polycrystalline reaction rims in the MgO–SiO2 system, part II: modelling

Part I of this contribution (Gardes et al. in Contrib Mineral Petrol, 2010) reported time- and temperature-dependent experimental growth of polycrystalline forsterite-enstatite double layers between single crystals of periclase and quartz, and enstatite single layers between forsterite and quartz. Both double and single layers displayed growth rates decreasing with time and pronounced grain coarsening. Here, a model is presented for the growth of the layers that couples grain boundary diffusion and grain coarsening to interpret the drop of the growth rates. It results that the growth of the layers is such that (Δx)2 ∝ t1−1/n, where Δx is the layer thickness and n the grain coarsening exponent, as experimentally observed. It is shown that component transport occurs mainly by grain boundary diffusion and that the contribution of volume diffusion is negligible. Assuming a value of 1 nm for the effective grain boundary width, the following Arrhenius laws for MgO grain boundary diffusion are derived: log Dgb,0Fo (m2/s) = −2.71 ± 1.03 and EgbFo = 329 ± 30 kJ/mol in forsterite and log Dgb,0En (m2/s) = 0.13 ± 1.31 and EgbEn = 417 ± 38 kJ/mol in enstatite. The different activation energies are responsible for the changes in the enstatite/forsterite thickness ratio with varying temperature. We show that significant biases are introduced if grain boundary diffusion-controlled rim growth is modelled assuming constant bulk diffusivities so that differences in activation energies of more than 100 kJ/mol may arise. It is thus important to consider grain coarsening when modelling layered reaction zones because they are usually polycrystalline and controlled by grain boundary transport.

Enhancement of solid-state reaction rates by non-hydrostatic stress effects on polycrystalline diffusion kinetics

Reaction layer growth between two chemically different solids may be controlled by polycrystalline diffusion kinetics in the growing phase. The kinetics depend on the interplay between volume and grain boundary diffusion. Using spinel formation between single crystals of periclase and sapphire as an example, we quantify the effects of an applied mechanical stress on the bulk-transport properties of the reaction layer. The rate of spinel growth increases fourfold when stress normal to the reaction interface increases from 3 to 30 MPa due to stress-induced changes in grain boundary structure. At low applied stress, low-index (i.e., ∑3) “coincidence site lattice” grain boundaries with slow diffusion coefficients dominate, related to epitactic growth of spinel on sapphire. Increasing stress triggers epitactic growth of spinel on periclase, and causes sapphire-grown spinel grains to rotate out of epitaxy, and grain boundaries with fast diffusion coefficients dominate. This effect outweighs the hitherto emphasized influence of grain size on the bulk transport properties of polycrystals.

Effect of surface charge on laser-induced neutral atom desorption

When an ionic metal oxide crystal is cleaved, inhomogeneous electrical charging of the surface can be a result. Such an effect has been well documented in magnesium oxide (100). For example, recent rigorous AFM studies indicate that nanoscale charged clusters of MgO are created during cleavage, with high concentrations often located at terrace step edges (Barth and Henry in J. Phys. Chem. C 113:247, 2009). In addition, ablation processes of freshly cleaved magnesium oxide crystals may be effected by remnant surface charging and microstructures (Stoneham et al. in Appl. Phys. A 69:S81, 1999). We report here that such surface charging strongly impacts neutral atom desorption, even under conditions of extremely mild excitation of surface terrace features. In our experiments, single-crystal MgO (100) is cleaved in air and placed in an ultra-high vacuum chamber (UHV). We irradiate the crystal at 6.4 eV, photon energy resonant with five-coordinated (5-C) terrace sites and probe desorbing neutral oxygen atoms. It is found that a significant fraction of desorbed neutral oxygen atoms from the charged surface possess kinetic energies in excess of 0.7 eV. This is in contrast to uncharged samples (discharged in vacuo over 24 hours) that display a near-thermal oxygen atom distribution.

Growth of multilayered polycrystalline reaction rims in the MgO–SiO2 system, part I: experiments

Growth of transport-controlled reaction layers between single crystals of periclase and quartz, and forsterite and quartz was investigated experimentally at 1.5 GPa, 1100°C to 1400°C, 5 min to 72 h under dry and melt-free conditions using a piston-cylinder apparatus. Starting assemblies consisting of Per | Qtz | Fo sandwiches produced polycrystalline double layers of forsterite and enstatite between periclase and quartz, and enstatite single layers between forsterite and quartz. The position of inert Pt-markers initially deposited at the interface of the reactants and inspection of mass balance confirmed that both layer-producing reactions are controlled by MgO diffusion, while SiO2 is relatively immobile. BSE and TEM imaging revealed thicknesses from 0.6 μm to 14 μm for double layers and from 0 to 6.8 μm for single layers. Both single and double layers displayed non-parabolic growth together with pronounced grain coarsening. Textural evolution and growth rates for each reaction are directly comparable. Forsterite–enstatite double layers are always wider than enstatite single layers, and the growth of enstatite in the double layer is slower than that in the single layer. In double layers, the enstatite/forsterite layer thickness ratio significantly increases with temperature, reflecting different MgO mobilities as temperature varies. Thus, thickness ratios in multilayered reaction zones may contain a record of temperature, but also that of any physico-chemical parameter that modifies the mobilities of the chemical components between the various layers. This potential is largely unexplored in geologically relevant systems, which calls for further experimental studies of multilayered reaction zones.