Kenneth J. McClellan

Crystal growth and optical characterization of cerium-doped Lu1.8Y0.2SiO5

Czochralski growth of cerium-doped Lu1.8Y0.2SiO5 (LYSO) from a 90/10 solution of Lu2SiO5 (LSO) and Y2SiO5 (YSO) is demonstrated. The alloyed scintillator retains the favorable growth properties of YSO and the desirable physical and optical scintillator properties of LSO. Radioluminescence, thermally stimulated luminescence, optical absorption, and lifetime measurements confirm the equivalence of LYSO and LSO optical properties. Advantages of LYSO Czochralski growth relative to LSO include reduced melting point, less propensity for formation of crystalline inclusions, lower cost of starting material, and easier incorporation of cerium into the host lattice. This material offers an attractive alternative to LSO for scintillator applications.

Synthesis and properties of Mo5Si3 single crystals

The ultra-high temperature structural intermetallic Mo5Si3 has been studied for alloy processing, physical properties, and mechanical behavior. High purity single crystals of Mo5Si3 have been synthesized by both optical floating zone and Czochralski methods. Structural, thermal, and elastic properties of Mo5Si3 single crystals were measured by X-ray powder diffraction, thermal mechanical analysis, and resonant ultrasound spectroscopy, respectively. Results show that the thermal expansion of Mo5Si3, a tetragonal structure with I4/mcm symmetry, is strongly anisotropic along the a and c directions with αc/αa=2.2. Single crystal elastic moduli of Mo5Si3 indicate that it has less elastic anisotropy and lower shear modulus than most transition metal disilicides. The impacts of these physical properties on alloy processing and mechanical behavior are discussed. Room temperature Vickers indentation tests on the (100) and (001) planes have been performed for different orientations of the indenter diagonal and the corresponding hardness, fracture toughness, and deformation behavior have been obtained as a function of the crystallography. Finally, the physical properties and mechanical behavior of Mo5Si3 are compared with those of other high temperature structural silicides, e.g. MoSi2.

Mechanisms of nonstoichiometry in Y3Al5O12

Currently, Y2O3–Al2O3 phase diagrams do not show the technologically significant yttrium aluminum garnet (Y3Al5O12,YAG) phase as deviating from the stoichiometric ratio, i.e., YAG is always expressed as a line compound. In this paper, we not only report the synthesis of nonstoichiometric YAG, but also the use of atomistic simulation to predict the defect structure associated with the deviation. By comparing the experimental variation in the lattice parameter as a function of deviation from stoichiometry with the defect volume changes predicted by atomistic simulation, we predict that nonstoichiometry in YAG proceeds via cation antisite defects.

Mo5Si3 single crystals : physical properties and mechanical behavior

Abstract The materials processing, physical properties and mechanical behavior of an ultrahigh temperature structural silicide, Mo5Si3, have been studied. High purity single crystals of Mo5Si3 have been synthesized by both optical floating zone and Czochralski methods. The thermal and elastic properties of the Mo5Si3 single crystals were experimentally measured. Results show that Mo5Si3 has significant thermal expansion anisotropy along the a and c directions with αc/αa=2.2. The single crystal elastic moduli of Mo5Si3 indicate that it has less elastic anisotropy and lower shear moduli than transition metal disilicides. Tensile stresses of up to 1.8 GPa can develop at grain boundaries after cooling from the melting point due to the thermal expansion mismatch in Mo5Si3, causing grain boundary cracking during processing of polycrystals. Room temperature Vickers indentation tests on (100) and (001) planes have been performed with different indenter diagonal orientations and the orientation dependence of hardness and fracture toughness of Mo5Si3 single crystals have been obtained. The corresponding deformation and fracture modes have been revealed by microscopy studies. Finally, a comparison of Mo5Si3 with other high temperature structural silicides is discussed.

Intrinsic trapping sites in rare-earth and yttrium oxyorthosilicates

Similarity among the thermally stimulated luminescence glow curves of undoped Lu2SiO5 and Ce3+-doped oxyorthosilicates possessing the monoclinic C2/c structure strongly suggests the luminescence traps are intrinsic in origin. They are most likely associated with the configuration of oxygen ions in the vicinity of not only the Ce3+ ion, as suggested in previous work, but also the host lanthanide ion. The optical absorption spectrum of pristine Lu2SiO5 shows the presence of intrinsic absorption centers that are enhanced upon x irradiation as seen in other oxides containing oxygen related point defects.

Defect behavior in rare earth REAlO3 scintillators

The aluminate perovskites YAlO3 (YAP) and LuAlO3 (LuAP) have been identified as potential scintillator materials due to their high light output and short decay time. However, the performance of these materials is significantly reduced by point defects. In this paper, atomistic simulations provide insight into the types of point defects that are expected under various conditions in YAP and LuAP, as well as other REAlO3 compounds (where RE denotes a rare earth ion ranging from Lu to La or Y). For example, we predict that cation antisites are the dominate intrinsic defect for smaller REAlO3 compounds, with the concentration of Schottky-type defects increasing for compounds with larger RE ions. We also predict that cation vacancies will be present in association with the oxidation of the Ce activator. From these results, we show how defects affect different aspects of the scintillation process. Our aim is to provide information that can be used to aid in the intelligent optimization of this family of scintill...

Room temperature single crystal elastic constants of boron carbide

Single crystals of the high temperature ceramic boron carbide have been synthesized by the optical floating zone method. Room temperature elastic constants of a carbon-deficient boron carbide single crystal have been measured using the resonant ultrasound spectroscopy technique. Based upon density measurements, the single crystal stoichiometry was specified as B5.6C. This crystal has room temperature single crystal elastic constants of c11 = 542.81, c33 = 534.54, c13 = 63.51, c12 = 130.59, and c44 = 164.79 GPa, respectively. Analysis of Cauchys relationships, Poissons ratios, and elastic anisotropic factors for the single crystal elastic constants indicates that it is more strongly anisotropic in elasticity and interatomic bonding than most solids. Room temperature isotropic elastic moduli of boron carbide show that its bulk, shear and Youngs moduli are substantially higher than those of most solids, so that boron carbide belongs to the so-called “strong solids”. Its Poissons ratio is significantly lower than that of most solids.

Characterization of Incipient Spall Damage in Shocked Copper Multicrystals

Correlations between spall damage and local microstructure were investigated in multicrystalline copper samples via impact tests conducted with laser-driven plates at low pressures (2—6 GPa). The copper samples had a large grain size as compared to the thickness, which was either 200 or 1000 μm, to isolate the effects of microstructure on the local response. Velocity interferometry was used to measure the bulk response of the free-surface velocity of the samples to monitor traditional spall tensile failure and to examine heterogeneities on the shock response due to microstructure variability from sample to sample. The shock pressure, dynamic yield strength and spall strength were determined from the measured velocity history via standard hydrodynamic approximations, while the effect of strength was explored via 1D hydrocode calculations. Electron Backscattering Diffraction, both in-plane and through-thickness, was used to relate crystallography to the presence of porosity around microstructural features such as grain boundaries and triple points. It was found that the dynamic yield strength measured from velocity histories in different samples correlated well with the crystallographic dependence reported for the dynamic yield strength in single crystals. Transgranular damage dominated in thin specimens with 230 μm grain size, where porosity appeared close to, but not exactly at, grain boundaries. However, a transition to dominant intergranular damage was observed as the grain size was reduced to 150 μm. Thick specimens (450 μm grain size) showed both modes, with intergranular damage found mostly where grains were smaller than average and the sites for preferred damage nucleation in these samples included grain boundaries and triple points. In particular, twin boundaries, especially tips of terminated twins, showed a large mismatch in surface displacements on the diagnostic surface as compared to the surrounding grains as well as a tendency for damage localization on the through-thickness sections.

Dynamic response of materials on subnanosecond time scales, and beryllium properties for inertial confinement fusion

During the past few years, substantial progress has been made in developing experimental techniques capable of investigating the response of materials to dynamic loading on nanosecond time scales and shorter, with multiple diagnostics probing different aspects of the behavior. These relatively short time scales are scientifically interesting because plastic flow and phase changes in common materials with simple crystal structures—such as iron—may be suppressed, allowing unusual states to be induced and the dynamics of plasticity and polymorphism to be explored. Loading by laser-induced ablation can be particularly convenient: this technique has been used to impart shocks and isentropic compression waves from ∼1to200GPa in a range of elements and alloys, with diagnostics including line imaging surface velocimetry, surface displacement (framed area imaging), x-ray diffraction (single crystal and polycrystal), ellipsometry, and Raman spectroscopy. A major motivation has been the study of the properties of be...

Oscillator strengths, Huang–Rhys parameters, and vibrational quantum energies of cerium-doped gadolinium oxyorthosilicate

Temperature-dependent optical absorption of cerium-doped gadolinium oxyorthosilicate (Gd2SiO5:Ce) has been measured and analyzed for impurity-ion-lattice coupling parameters and oscillator strengths. Although the spectrum consists of overlapping Ce3+ bands and Gd3+ lines, two well-resolved Ce3+ bands with 10 K maxima at 3.32 eV (peak a) and 3.61 eV (peak b) are amenable to spectral analysis. These bands, previously assigned to Ce3+ ions occupying crystallographically inequivalent substitutional sites, are characterized by Gaussian line shapes and temperature-dependent half widths that are well described by the linear model of impurity-ion-lattice coupling. Huang–Rhys [Proc. R. Soc. A 204, 404 (1950)] parameters of peaks a and b are 22.7 and 5.7, respectively, indicating strong ion-lattice coupling, with vibrational frequencies 1.83×1013 s−1 (peak a) and 5.07×1013 s−1 (peak b). Peak b centroid is approximately temperature independent, but peak a centroid shifts to higher energy with increasing temperature....