Magnesium Aluminate Spinel Single Crystals Research Articles

Photoluminescence of high energy xenon ion irradiated magnesium aluminate spinel single crystals

Laser confocal microscopy technique (60 ps laser pulse excitation at 445 nm) and a time-correlated single photon counting (TCSPC) technique have been used to study the photoluminescence (PL) in 152 MeV Xe ion irradiated MgAl2O4 single crystals. It was shown that radiation defects produced by swift xenon ions give rise to broad luminescence band positioned in the 470–800 nm wavelength range. The measurements of the PL decay curves have evidenced that lifetime of defect-related PL signal gradually increases from 6 to 10 ns in 500–620 nm spectral range and decreases with ion fluence increasing. The decrease of the luminescence intensity observed after fluences ∼1013 cm−2 is attributed to overlapping of individual ion track core regions.

Distinctive features of diffusion-controlled radiation defect recombination in stoichiometric magnesium aluminate spinel single crystals and transparent polycrystalline ceramics

MgAl2O4 spinel is important optical material for harsh radiation environment and other important applications. The kinetics of thermal annealing of the basic electron (F, F+) and hole (V) centers in stoichiometric MgAl2O4 spinel irradiated by fast neutrons and protons is analyzed in terms of diffusion-controlled bimolecular reactions. Properties of MgAl2O4 single crystals and optical polycrystalline ceramics are compared. It is demonstrated that both transparent ceramics and single crystals, as well as different types of irradiation show qualitatively similar kinetics, but the effective migration energy Ea and pre-exponent D0 are strongly correlated. Such correlation is discussed in terms of the so-called Meyer-Neldel rule known in chemical kinetics of condensed matter. The results for the irradiated spinel are compared with those for sapphire, MgO and other radiation-resistant materials.

Luminescence characteristics of magnesium aluminate spinel crystals of different stoichiometry

Magnesium aluminate spinel single crystals with different stoichiometry, MgAl2O4 (1:1 spinel) and MgO 2.5Al2O3 (1:2.5) were investigated using different optical methods (cathode-, photo- and thermally stimulated luminescence (TSL), optical absorption, “creation spectra” of TSL peaks and phosphorescence by VUV radiation). Low-temperature charge carrier traps and the position of intrinsic UV emission bands depend on the degree of stoichiometry. Antisite defects (ADs), Mg2+ or Al3+ located in a “wrong” cation site (Mg|A1 or Al|Mg) are the main as-grown structural defects, which serve also as efficient traps for electrons and holes as well as seeds for bound excitons. AD concentration is especially high in 1:2.5 spinel. There are several manifestations of ADs (electronic excitations near ADs) in the spectral region of 7–7.5 eV, slightly below the energy gap.

Growth and characterization of Mn 2+-activated magnesium aluminate spinel single crystals

Several concentrations of Mn 2+-doped MgAl 2O 4 single crystals have been successfully grown using the micro-pulling-down (μ-PD) method. Due to their high melting temperature, the use of special Ir-Re crucibles was necessary. Because of the wide solubility range, the dependence of the behavior of the solid–liquid interface on the growth parameters was carefully studied to establish the growth conditions of stoichiometric MgAl 2O 4 spinel. Rod shaped 〈1 0 0〉 oriented single crystals of undoped and manganese doped MgAl 2O 4 spinel with 3 mm in diameter and a few centimeters in length were obtained in reducing argon atmosphere. The composition of Mn ions along the growth a-axis was controlled by the electron probe micro-analysis (EPMA). The optical transmission, crystallinity and thermal expansion of the grown crystals are investigated as well the annealing effect under oxidizing atmosphere will be detailed.

High-temperature behavior of zirconia and spinel doped with cesium

Chemical and structural modifications occurring in yttria stabilized zirconia and magnesium aluminate spinel single crystals submitted to Cs ion-implantation and thermal treatments are investigated by RBS/C experiments. Changes of the sample composition, damage created in the crystals and behavior of implanted ions are studied as a function of the Cs fluence and annealing temperature. Variations of the stoichiometry, decrease of the lattice disorder and diffusion of implanted species are observed above 700–800°C with marked differences for both materials.

Use of the nuclear microprobe for the characterization of ion-implanted spinel

Stable analogues of a toxic fission product (Cs) were introduced into magnesium aluminate spinel single crystals by room temperature ion implantation. Changes in the atomic concentration of Cs atoms upon annealing at high temperature were monitored by Rutherford backscattering (RBS) experiments using a nuclear microprobe. Results show that Cs atoms are inhomogeneously distributed in the crystal after annealing; they are collected in islands with a size of a few tens of micrometers. The remaining crystal exhibits a very small Cs concentration due to the exfoliation of the implanted layer.

Optical transitions in magnesium aluminate spinel crystals of different compositions exposed to irradiation

Optical absorption of magnesium aluminate spinel single crystals of different compositions MgO · nAl 2O 3 ( n=1.0 and 2.5) were investigated after irradiation with neutrons, X-rays and the UV-light to elucidate existence of specific absorption bands related to lattice defects. The radiation induced absorption envelope is shifted to higher energy and is larger in intensity in non-stoichiometric spinel in comparison with stoichiometric one at the same irradiation conditions. Deconvolution of difference optical spectra of irradiated crystals shows the shift of the F-type centers from 4.75 and 5.3 eV in stoichiometric crystals to 5.08 and 5.63 eV in MgO · 2.5Al 2O 3 for F +- and F-centers, respectively. The absorption bands at 3.1 and 3.78 eV are present in both stoichiometric and non-stoichiometric crystals and are identified as V-type centers. By changing the irradiation time and using isochronal annealing it was revealed that additional bands appear in optical absorption spectra at 4.15 eV for MgO · 1.0Al 2O 3 crystals and at 4.46 eV for MgO · 2.5Al 2O 3 crystals. From the shift of the band energy position and the high intensity of this band in non-stoichiometric spinel crystals, it could be identified with electronic centers related to lattice defects.

Defects and radiation induced electronic processes in magnesium aluminate spinel of different compositions

The time dependence of the formation and decay of irradiation-induced optical absorption centers in magnesium aluminate spinel single crystals of different compositions (MgO - 1.0Al 2 O 3 and MgO - 2.5Al 2 O 3 ) was investigated. The kinetics of accumulation of X-ray irradiation-induced absorption bands is consistent with the mechanism of trap filling with free charge carriers through the conduction band. The model includes Coulomb blocking effects on spatially correlated defects. The observed two-stage decay of absorption bands after termination of X-ray irradiation is explained by electron hole recombination between centers of two different distances and/or different potential barriers. UV-irradiation confirms the existence of charge exchange between complex spatially correlated defects.

Effects of Neutron Irradiation on the Mechanical Properties of Magnesium Aluminate Spinel Single Crystals and Polycrystals

The effect of fast neutron irradiation on the mechanical properties of magnesium aluminate spinel single crystals and polycrystals, such as bending strength, fracture toughness, Young's modulus, and hardness, was summarized based on the reported data. Essentially, the changes in these properties are dependent on the nature and concentration of neutron‐irradiation‐induced defects. Furthermore, the efficiency of damage accumulation is known to be dependent on temperature. Spinel shows superior resistance to the formation of defect aggregates, and the recombination of point defects occurs efficiently under neutron irradiation; therefore, the change in mechanical properties is not critical, up to very high neutron fluences, and the spinel maintains its structural integrity.

Dielectric and Optical Properties of Stoichiometric Magnesium Aluminate Spinel Single Crystals

Stoichiometric magnesium aluminate spinel single crystals grown by the flux, Czochralski, and Verneuil methods have been characterized for their dielectric and optical properties. Dielectric properties were determined from capacitance measurements in the frequency range 102–105 Hz. The dielectric constant and loss tangent of spinel crystals grown by the various methods are comparable. At 1 kc the spinel has a dielectric constant of 8.4 and a loss tangent of 10−4. Optical transmission and reflection measurements were made in the wavelength range uv to IR (0.2–40 µm). Optical characteristics of the crystals are highly dependent on the method of growth. Chromium‐doped spinel single crystals have also been grown by a flux technique. Optical emission and absorption characteristics of Cr+3 in flux‐grown spinel have been studied. The results are compared with those of Cr+3 in natural spinel and synthetic spinel crystals prepared by the Czochralski and Verneuil methods.

Growth and characterization of large stoichiometric magnesium aluminate spinel single crystals

Large stoichiometric magnesium aluminate spinel single crystals with sizes up to one inch on an edge have been grown at 1250 °C by a flux method. Purification of the crystals from microscopic flux inclusions was obtained by a post-growth vacuum annealing process. The crystals were characterized by wet chemical analysis, X-ray and electron diffraction, optical microscopy, emission and mass spectroscopy. Information on composition, homogeneity, impurity content and lattice parameter was obtained. The crystals were examined by Lang X-ray topography. They exhibit dislocation densities as low as 50 lines/cm 2 and are of superior quality when compared to Verneuil and Czochralski melt-grown spinel crystals which were observed to have dislocation densities in excess of 10 5 and 10 3 lines/cm 2 respectively. Stacking faults with fault vectors R = 1/4 〈101〉 and randomly scattered small volumes of precipitates were observed in most of the crystals. Growth striations were found in some crystals. They are probably due to the segregation of impurities at the {111} solid-liquid interfaces during growth.