MgAl2O4 Crystals Research Articles

Luminescence and EPR spectroscopy of neutron-irradiated single crystals of magnesium aluminium spinel

Neutron irradiated single crystals of stoichiometric MgAl2O4 and MgAl2O4:Mn2+ (0.15 wt. %) were studied using the methods of luminescence spectroscopy and versions of electron paramagnetic resonance. In stoichiometric MgAl2O4:Mn2+ single crystals, the transition of a part of manganese impurity ions from tetrahedral to octahedral coordination, caused by irradiation with fast fission neutrons, is detected using photoluminescence spectra. This fact confirms the partial inversion of a normal spinel due to neutron-irradiation. Using pulse EPR technique it is shown that an electron in the field of oxygen vacancy (i.e. from neutron-irradiation induced F+ centres, which are spatially separated from each other) is in hyperfine interaction only with neighbouring 27Al nuclei with nuclear spin 5/2.

Interfacial microstructure and compressive properties of Al–Mg syntactic foam reinforced with glass cenospheres

Glass cenospheres/Al syntactic foams are successfully produced with about 50% volume fraction using pressure infiltration process. The interfacial reactions between glass cenospheres and Al–Mg alloys (5A03 with 3wt% Mg and 5A06 with 6wt% Mg) have been deeply studied. The MgAl2O4 crystals produced by the reaction of 2Al(l)+Mg(l)+2SiO2(s) → 2MgAl2O4(S) + 2Si(S) are the main reaction products in the size of ∼100 nm and the reaction layer is about 800 nm thick. The interfacial reaction region is composed of a large amount of MgAl2O4 uniformly coating on the glass cenospheres due to the spherical structure of cenosphere. Si is believed to diffuse through the grain boundaries between the MgAl2O4 crystals and aggregate on the surface of the reaction zone. With the increase of Mg content in matrix such as 5A06 alloy, Si formed above transforms into Mg2Si by the reaction of 2 Mg(l) + Si(s) → Mg2Si(s). The compressive strength of composites increases with the increase of Mg content which is not only contributed to the improved strength of matrix but the suitable interfacial reaction coating and the forming of Mg2Si in matrix.

Thermodynamic investigation of Ti doping in MgAl2O4 based on the first-principles method

The doping form of Ti in MgAl2O4 crystals was investigated using a theoretical thermodynamic approach, and a quantitative analysis model of the thermodynamic stability and probability was constructed.

Strain relaxation and order–disorder phase transition in irradiated MgAl2O4

MgAl2O4 spinel is widely studied in many fields of material science because of its variety of interesting properties and potential applications. The influence of cation disorder on the physical properties of MgAl2O4 makes understanding of the effects related to this disorder particularly important. It is known that, upon ion irradiation at low temperature, MgAl2O4 undergoes an order-disorder phase transition followed by amorphization. This paper reports a combined high resolution X-ray diffraction and transmission electron microscopy study elucidating the linkage between this phase transition and irradiation-induced elastic strain. Irradiations were carried out on [110] and [111]-oriented single crystals of MgAl2O4 at T = 77 K with 600 keV Xe ions over a wide range of doses. The data suggests that the beginning of the order-disorder phase transition coincides with the beginning of strain relaxation. This result indicates that the volume of the new phase is slightly smaller than that of the unirradiated spinel. The dose at which the phase transformation occurs is found to be dependent on the crystal orientation, which can be attributed to both elastic and crystallographic properties.

Non-rare earth white emission phosphor: Ti-doped MgAl2O4

White emission produced by Ti-doped MgAl2O4 phosphor powder is reported, which is in contrast to blue emission from most Ti-doped single crystals of MgAl2O4. The white emission peak consists of four deconvoluted peaks: 440, 490, 550, and 620 nm, when was excited by 260 nm wavelength. Ti4+ in octahedral sites was found to contribute mostly to greenish blue emissions at 490 and 550 nm. The red emission at 620 nm was produced by abundant Mg2+ and O2− vacancies in the spinel powder.

High-strength glass-ceramics in the system MgO/Al2O3/SiO2/ZrO2/Y2O3 – microstructure and properties

Glasses with the composition 51.9SiO2·21.2MgO·21.2Al2O3·5.7ZrO2 (in mol%) were crystallized in different annealing steps at 950 °C and 1060 °C. Using X-ray diffractometry and high resolution (scanning) transmission electron microscopy with energy-dispersive X-ray spectroscopy, it is found that the annealing-induced devitrification results in glass-ceramics with α-quartz solid solution as the main crystal phase, with a few additional ZrO2 and MgAl2O4 crystals as secondary crystal phases. If 2.5 mol% Y2O3 are added to the base glass, however, an identical heat treatment leads to glass-ceramics with dendritic ZrO2 crystals as the main crystal phase, and subsequently grown MgAl2O4 as the secondary crystal phase. The quartz solid solution was found to be absent in those Y2O3-bearing samples, as Y2O3 is incorporated in the residual glass matrix, thus stabilizing it. It was observed that the bending strength was comparable for both sample sets, thus indicating that the existence of α-quartz is not necessarily an important prerequisite for excellent mechanical properties of glass-ceramic materials based on the MgO/Al2O3/SiO2 system. Strengthening mechanisms for both Y2O3-bearing and Y2O3-free glass-ceramics are discussed.

Fracture modes in micropillar compression of brittle crystals

Abstract

Atomistic structure and energetics of interface between Mn-doped γ-Ga2O3 and MgAl2O4

The interface between an Mn-doped γ-gallium oxide (Ga2O3) thin film and an MgAl2O4 (001) substrate has been investigated using high-resolution transmission electron microscopy (HRTEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), and first-principles calculations. A high-quality Mn-doped γ-Ga2O3 film with a defective spinel structure has been epitaxially grown by pulsed laser deposition. The γ-Ga2O3 crystal shows an uniform tetragonal distortion with a tetragonality of 1.05 throughout the film thickness of 75 nm. HRTEM and HAADF-STEM observations reveal that the γ-Ga2O3 and MgAl2O4 crystals form a coherent interface without any interfacial layers or precipitates. The atomistic structure and energies are theoretically evaluated for the interfaces with two types of termination plane, i.e., Mg- and Al2O4-termination of MgAl2O4. The cation sublattice is found to be continuous for both interfaces despite the defective spinel structure of Mn-doped γ-Ga2O3 with some vacant cation sites. The Al2O4-termination shows a lower interfacial energy than the Mg-termination under most conditions of the chemical potentials. This behavior is attributed to the energetic preference of the Mn–Al2O4 local configuration at the interface.

Low Temperature Synthesis of Magnesium Aluminate Spinel from AlOOH and MgO Powder

Ultrafine MgAl2O4 powders have been successfully synthesized by solid-state reaction from the mixture of AlOOH and MgO at the lower temperature. The samples were investigated by DTA/TG、XRD and SEM analyses. The results showed that AlOOH can be decomposed into γ-Al2O3 from DG/TDA curves, which can improve the activity of Al2O3 and promote the process of solid-state reaction. The XRD analysis showed the single-phase MgAl2O4 was formed at 1400°C. Meanwhile it was detected by ICP-MS that the purity of powders was 99.995%. The SEM images showed that the mineralizer containing fluoride such as AlF3, NH4F can promote the resultant to grow into regular octahedrons and disperse the particles. The XRD analysis indicated the mixture containing 5% AlF3 can transform the integrated MgAl2O4 powders at 1200°C, nearly 200°C lower than general solid-state reaction. Furthermore, The SEM images showed that fluoride can crystal the fine particles into the larger MgAl2O4 crystals, about 0.5～1.5µm.

Structure-Disorder induced Magneto-Resistance intensification on spin-dependent-conduction in magnetite (Fe3O4) thin film produced by RF magnetron sputtering method

The intensification of magneto-resistance (MR) caused by structure-disorder has been observed in magnetite (Fe3O4) thin film (MTF) produced by the rf magnetron sputtering method. The MTFs with the thickness of 20nm, 50nm and 100nm were deposited on the substrates of SiO2-glass, MgAl2O4 (100) and MgO (100) single crystals. We have observed that the MR of the MTF on SiO2-glass substrate changes 3–5 times larger than that of the MTFs on MgAl2O4 and MgO substrates. From the AFM, SEM and XRD measurements, we have found that the MTFs are composed of magnetite nano-particles (MNPs) and the crystal-axis directions of the MNPs in the MTF on SiO2-glass are mutually random and those of the MTFs on MgAl2O4 and MgO are almost aligned along (100) direction. In addition, from the electrical and magnetization measurements, we have found that the MNPs in the MTF on SiO2-glass show the amorphous-like behavior and those of the MTFs on MgAl2O4 and MgO indicate the crystal-like behavior.

A study on the formation of MgAl2O4 and MgO crystals in Al–Mg/quartz composite by differential thermal analysis

The present study has examined the thermodynamics of MgAl2O4 and MgO formations in Al–Mg alloy/quartz (>99% crystalline silica) through differential thermal analysis (DTA). The formation of MgAl2O4 and MgO is detected as exothermic peaks in the heat flow curve and the reaction is confirmed by the Si dissolution peaks observed during the reheating of samples and SEM analysis of the reacted sample. The presence of MgAl2O4 and MgO is confirmed in the XRD analysis of the reacted sample. The study has enabled the production of nano sized MgAl2O4 and MgO crystals at the interface of Al–Mg alloy and quartz. The reaction between them is found to be influenced by the oxidation of Mg, which is reduced by increasing heating rates.

Optical absorption, paramagnetic resonance and depolarisation currents in MgAl2O4 spinel

AbstractOptical Absorption (OA), Electron Paramagnetic Resonance (EPR) and Thermally Stimulated Depolarisation Currents (TSDC) techniques were applied to study the effects of irradiation and thermal treatments in the formation, aggregation and destruction processes of dipole defects in MgAl2O4 spinel. Irradiated MgAl2O4 crystals present two OA bands centred at 3.4 eV and 5.1 eV. The 3.4 eV band increases with the irradiation‐dose, stabilizes its maximum height for doses near 10 kGy and is completely destroyed for thermal treatments above 500 K. This same band decays when the sample is maintained at room temperature reaching a constant value after a few weeks. The EPR spectrum is composed by two superimposed bands at g = 2.011. The gamma dose dependence, the thermal treatments between 370 K and 500 K and the thermal decay of the two bands at room temperature, show that each band behaves in a different way thus indicating that they are associated with two different defects. Gamma dose of 10 kGy produces a TSDC band at 245 K. When the sample is maintained at room temperature, after the gamma irradiation, for several weeks, a displacement in the peak position to 290 K, is observed. Thermal treatments above 500 K destroy the 290 K band. This band is associated to at least two V‐type centres. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Molecular dynamics modelling of radiation damage in normal, partly inverse and inverse spinels

The radiation response of perfect crystals of MgAl2O4, partially inverted MgGa2O4 and fully inverse MgIn2O4 were investigated using molecular dynamics. Dynamical cascades were initiated in these spinels over a range of trajectories with energies of 400eV and 2keV for the primary knock-on event. Collision cascades were set up on each of the cation and anion sublattices and were monitored up to 10ps. Simulations in the normal MgAl2O4 spinel for the 2keV energy regime resulted in similar defect structures as obtained at the post-threshold 400eV energies, with little clustering occurring. The predominant defect configurations were split interstitials and cation antisites. For the inverse spinels, a much wider variety of lattice imperfections was observed. More defects were also produced due to the formation of interstitial–vacancy cation chains and oxygen crowdions.

Intrinsic and impurity luminescence and multiplication of excitations in complex oxides

The emission bands at 5.1 and 5.0eV have been detected at 9K in the cathodoluminescence spectra of pure and doped with Eu2+ ions BaMgAl10O17 and SrMgAl10O17 phosphors, respectively. A comparison of these emissions with the spectra of pure and doped MgO, α-Al2O3 and MgAl2O4 crystals allowed to interpret the VUV emissions of phosphors as the recombination of conduction electrons with the holes localized at oxygen ions near Mg2+ substituting Al3+ in spinel blocks. The creation regions of two or three electron–hole pairs by one photon of 22–32eV that causes the ionization of the 2p6 or 2s2 shells of oxygen in spinel blocks, has been separated in the excitation spectra of blue and UV emissions of BaMgAl10O17 at 9K. Besides thoroughly investigated 4f7→ 4f65d1 electron transitions in Eu2+ centres (absorption at 3.5–6.2eV), the creation region of the electronic excitations connected with the excitation and ionization of oxygen ions nearby Eu2+ or Mg2+ ions (6.6–7.8eV) can be tentatively separated in the excitation spectra of steady luminescence and recombination phosphorescence of SrMgAl10O17:Eu.

Excited state absorption and passive Q-switch performance of Co2+ doped oxide crystals

Excited state absorption spectra (ESA) of the LiGa5O8 (LGO), MgAl2O4 (MALO) and LaMgAl11O19 (LMA) crystals doped with the tetrahedral Co2+ ion were studied under excitation at the 4A2→4T1(4F) transition. The ESA band was observed near 710–740 nm and assigned to the 4T2→4T1(4P) transition of the Co2+ ion. The lifetime of the 4T2 level of Co2+ ion at room temperature was estimated to be 130–350 ns depending on the crystal host. Absorption saturation measurements were performed and ground state and excited state absorption cross sections were estimated at the wavelengths of 1.34 and 1.54 μm. The crystals studied were used to Q-switch flashlamp-pumped 1.34 μm Nd3+:YAlO3 and 1.54 μm Er3+:glass lasers. Passive Q-switching of a diode pumped 1.35 μm Nd:KGd(WO4)2 laser was demonstrated using Co2+:MALO saturable absorber.

The formation and thermostability of MgO and MgAl2O4 nanoparticles in oxidized SiC particle-reinforced Al-Mg composites

Interfacial reactions and their products in oxidized SiC particle-reinforced Al-Mg matrix composites were investigated using X-ray diffraction and Field EmissionScanning Electron Microscopy (FE-SEM). Observation of the interfacial reaction between oxidized SiC particles and aluminum alloys containing Mg showed that nanoparticles of MgO form initially and do not change form when more than 4 wt. % Mg is in the matrix. However, MgO transforms into octahedral MgAl2O4 crystals when less than 2 wt. % Mg is in the matrix .Comparison of the amounts and the sizes of the reaction products MgAl2O4 and MgO between the Al-Mg alloyswith different matrix compositions shows that fewer MgAl2O4 crystals form at the surface of the particles in the 2014Al matrix composite than in the Al-2 wt. % Mg (Al-2Mg) matrix composite. Also, the size of MgAl2O4 in the former composite is greater than that of the latter composite under the same conditions. However, the amount and the size of MgO crystals that form in the Al-4 wt. % Mg (Al-4Mg) matrix composite is almost the same as that of the Al-8 wt. % Mg (Al-8Mg) composite, and the size of MgO changes a little during heat-treatment at elevated temperatures. The amount of the reaction product (either MgO or MgAl2O4) depends on nucleation rates and density of nucleation sites on the oxidized SiC particles at the initial reaction. The more completely the nuclei cover the surface of the oxidized SiC particles, the smaller the resulting size. According to the results, an addition of Mg into the matrix can be used to control the interfacial characteristics in the oxidized SiC/Al composites.

Interfacial microstructure evolution in aluminium matrix composites reinforced with unoxidized and oxidized SiC particles

AbstractInterfacial microstructure evolution of Al matrix composites with non‐oxidized and oxidized SiC particles as reinforcement was investigated using x‐ray diffraction, field emission scanning electron microscopy (FE‐SEM) and transmission electron microscopy (TEM). Hexagonal platelet‐shaped Al4C3 and Si are the two major interfacial reaction products in the non‐oxidized SiC‐particle‐reinforced Al matrix composites (SiCp/Al). In addition, the eutectic θ‐CuAl2 and Si phases precipitate at the SiCp/matrix interface during the last solidification stage. Observation of the interfacial reaction between the oxidized SiC particles and Al alloy containing Mg shows that the SiO2 layer has good bonding with both SiC and Al matrix and that nanoparticles of MgO have formed initially. The MgO particles will change into octahedral MgAl2O4 crystals when there is a low Mg content in the matrix (≤2 wt.%) and will remain as MgO when there is a high Mg content (≥4 wt.%). Copyright © 2001 John Wiley & Sons, Ltd.

Modification of the interface in SiC/Al composites

Methodologies both to avoid the formation of Al4C3 and to tailor the interfacial structures in a SiC/2014 Al composite were demonstrated. Modification of the interfacial structures in the SiC/2014 Al composite was made by forming SiO2 layers on the surfaces of SiC via passive oxidation at elevated temperatures. In the 2014 Al composite reinforced with the oxidized SiC, MgAl2O4 and Si crystals were observed to be present at the interfacial region as a result of the reaction between the SiO2 layer and the matrix. On the other hand, in the case of the 2014 Al composite reinforced with unoxidized SiC, SiC was found to react with the Al matrix to form both Al4C3 and Si. Qualitative measurements of the interfacial bonding strength were carried out on composites having various types of interfaces and thicknesses. Detailed interfacial structures and phase identifications, which were examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM), were presented.

He-ion damage and He-release from spinel MgAl2O4

Spinel is one of the candidate matrices for transmutation of radiotoxic actinides in nuclear reactors. During the transmutation process, large quantities of He are formed by α-decay. The behavior of He in spinel and its release upon heating are therefore of large interest. To obtain relevant information, single crystals of stoichiometric spinel MgAl2O4 were implanted with He-ions of three energies: 100, 260 and 2 MeV. The Rutherford backscattering/channeling (RBS/C) technique was used to measure damage formation as a function of He-fluence, and samples irradiated to different fluences were annealed in a Knudsen cell to measure the release of He. Depending on fluence and heating rate, spalling of the implanted layer, blister formation or release compatible with a diffusion process were observed. The RBS/C measurements showed damage areas at the end of the range of the He, and little damage between the surface and the depth of the range. Spinel is thus structurally stable against He-ions, even at very high concentrations.

Helium irradiation effects in single crystals of MgAl2O4

Magnesium aluminate spinel, (MgAl2O4), is a promising material as a uranium free matrix for the transmutation of americium. Fission products and α-particles are produced during the transmutation. The impact of α-particles is simulated by 30 keV 3He ion implantations at room temperature (RT) with the doses 6.2, 16, 20 and 53×1015cm−2. In another set of experiments a single crystal MgAl2O4 (100) sample is irradiated with α-particles (4.5 MeV) from a 241Am source at RT to a dose of 1.3×1012cm−2.Helium release from the implanted samples was studied by thermal desorption spectrometry (TDS). The numerical analysis of the experimental thermal desorption results of α-implanted samples to a very low helium concentration (0.0288 appm in the irradiation zone of 12.4 μm) show that helium release is dominated by helium interstitial diffusion with an activation energy of 1.8 eV. In the case of high dose implantation to 1.74 at.% in the implantation zone approximately of 100 nm, helium is released from He-vacancy clusters with the activation energy of 2.35 eV.The evolution of the helium concentration profile in the temperature range from RT to 1483 K is monitored by neutron depth profiling (NDP). It confirms that the release of helium is governed by dissociation from vacancy clusters.