Al2O3 Crystals Research Articles

DPSS Nd:YAG Laser Precission Machining of Monocrystalline Sapphire

The paper presents some aspects concerning use of pulse laser source for precise machining of Al2O3 crystals for medical applications. The scheme of laser beam manipulation and the test stand has been presented. Application of the mathematical model enabled determination of values of basic sapphire machining parameters. Presented results gave rough information about dynamics of the process and were used for the test stand designing, development as well as determination of machining precision.

Atomic rearrangements in amorphous Al2O3 under electron-beam irradiation

The electron-irradiation-induced crystallization of amorphous Al2O3 (a-Al2O3) was investigated by in-situ transmission electron microscopy under the wide electron-energy region of 25–300 keV. The formation of γ-Al2O3 nanocrystallites was induced by irradiating the a-Al2O3 thin film along with the formation of nanovoids in the crystalline grains regardless of the acceleration voltage. The crystallization became more pronounced with decreasing the electron energy, indicating that electronic excitation processes play a dominant role in the formation of γ-Al2O3. Radial distribution analyses suggested that a-Al2O3 transforms to γ-phase via the “excited” (“stimulated”) amorphous state, in which the breaking and rearrangement of unstable short-range Al-O bonds, i.e., fivefold-coordinated Al-O (AlO5) basic units, occur.

Thermoluminescence of Al2O3 crystals grown by temperature gradient techniques

Color center concentrations of the Al2O3 crystal grown by temperature gradient techniques were calculated from Gaussian fits to absorption spectra. The concentrations for F- and F +-centers at 204, 232, and 255 nm were determined to be 1.361×1017, 0.098×1017, and 0.325×1017cm−3, respectively. Studies have shown that the thermoluminescence (TL) glow curve exhibits a prominent 450 K peak that matches well the first-order fitting curve. The thermal activation energy E and frequency factor s of the trap were determined to be 0.94456±0.00545 eV and 5.8703×1011 s−1. With this theoretical analysis, a simple one-trap/one-center TL model is presented to provide a theoretical explanation of the TL process.

Degradation of the Transmissive Optics for a Laser-Driven IFE Power Plant under Electron and X-Ray Irradiation

Several facilities are used to test trasmissive optics of the IFE target chamber and KrF laser driver in regard of transient and residual radiation-induced absorption in the range of 150–1200nm. Comparative study of radiation darkening of various fused silica glasses, high-purity CaF2 ,M gF2, and Al2O3 crystals under irradiation by 300keV electrons, soft (hν = 6–20keV) and hard (hν ∼ 400keV) X-rays is done and better radiation-proof optical materials are selected; the opposite effect of optics bleaching under UV laser light is observed. c

Structural investigation of epitaxial 2H-SiC–α-Al2O3 1-D heterostructures

2H-SiC–α-Al2O3 epitaxial heterostructures in a 1-D nanosystem were achieved via a simple CVD (chemical vapor deposition) process. Detailed structural characterizations and elemental analyses were carried out using methods such as elemental mapping, EELS (electron energy loss spectroscopy), XPS (X-ray photoelectron spectroscopy) and Raman spectroscopy, which identified the amounts of Al atoms that occupied the Si sites in the SiC lattice and that a small quantity of oxygen atoms were found on the SiC side. The carbon atoms dissolve in the Al2O3 crystal, substituting the oxygen elements, and there is only a small amount of Si in the Al2O3 part. These results supply a comprehensive understanding about the interface and information about the atoms that diffuse between 2H-SiC and α-Al2O3. We hope that this work contributes to the design of SiC–Al2O3 based epitaxial heterostructures.

Comparative study of the luminescence of Al2O3:Ti and Al2O3 crystals under VUV synchrotron radiation excitation

The comparative study of the luminescent properties of Al2O3:Ti crystal in comparison with those for undoped Al2O3 crystal counterpart is performed under synchrotron radiation excitation with an energy of 3.7–25eV. Apart from the main emission band peaked at 725nm related to the 2E→2T2 radiative transitions of Ti3+ ions, the luminescence of excitons localized around Ti ions in the band peaked at 290nm and the luminescence of F+–Ti and F–Ti centers in the bands peaked at 325 and 434nm are also found in the emission spectra of Al2O3:Ti crystal. We show also that the luminescence of Ti3+ ions in Al2O3:Ti crystal can be effectively excited by the luminescence of excitons localized around Ti dopant as well as by the luminescence of F–Ti centers.

Molecular Dynamics Study of Interaction between Acrylamide Copolymers and Alumina Crystal

Four acrylamide polymer flocculants, anionic polyacrylamide P(AA-co-AM), cationic polyacrylamide P(DMB-co-AM), nonionic polyacrylamide P(AM), and hydrophobical polyacrylamide P(OA-co-AM) have been prepared by copolymerizing with acrylic acid, cationic monomer dimethylethyl (acryloxyethyl) ammonium bromide (DMB) and hydrophobical monomer octadecyl acrylate with acrylamide. The interactions between the flocculants with the (012) surface of alumina crystal (Al2O3) have been simulated by molecular dynamics method. All the polymers can bind tightly with Al2O3 crystal, the interaction between the O of polymers and Al of the (012) surface of Al2O3 is significantly strong. The order of binding energy is as follows: P(DMB-co-AM)>P(OA-co-AM)>P(AA-co-AM)>P(AM), implying a better flocculation performance of P(DMB-co-AM) than the others. Analysis indicates that binding energy is mainly determined by Coulomb interaction. Bonds are found between the O atoms of the polymers and the Al atoms of Al2O3. The polymers' structures deform when they combine with Al2O3 crystal, but the deformation energies are low and far less than non-bonding energies. Flocculation experiments in suspension medium of 1%Kaolin show a transmittancy of 90.8% for 6 mg/L P(DMB-co-AM) and 73.0% for P(AM). The sequence of flocculation performance of four polymers is P(DMB-co-AM)>P(OA-co-AM)>P(AA-co-AM)>P(AM), which is in excellent agreement with the simulation results of binding energy.

TEM characterization of the reaction products formed in Al–Cu/SiO2 couples due to high temperature interaction

Classical sessile drop experiment was performed for intensive microstructure and phase composition studies of the reaction product region (RPR) formed because of high temperature interaction between the aluminum–copper alloy (Al—16.7 at.% Cu) and SiO2 (amorphous) substrate. The experiment was performed in vacuum at 1173 K for 2 hour contact time. Scanning and transmission electron microscopy techniques were applied to reveal the details of the complex microstructure of reactively formed product zone at the drop/substrate interface. Three regions of different structure and phase composition were well distinguished in the RPR: the first layer was composed of large-faced Al2O3 crystals of alpha type surrounded by the Al2Cu metallic channels, the second one had the same phase composition but different morphology of the alpha alumina and with silicon as an extra component (dissolved within these phases and as precipitates). The third area revealed very fine-grained (100–200 nm) microstructure, in which Al2Cu and Si grains were embedded in orthorhombic δ-Al2O3 matrix. Moreover, the presence of the deformation twins in silicon, twinned on (1–11) plane was related to large strains present in the area close to the SiO2 substrate and coming from the volumetric mismatch of SiO2 and the freshly formed Al2O3.

Luminescent vacuum ultraviolet spectroscopy of Cr3+ ions in nanostructured aluminum oxide

Impurity Cr3+ centers in submicron and nanostructured Al2O3 crystals of different phase compositions at temperatures of 300 and 7.5K were studied by a luminescent vacuum ultraviolet (VUV) spectroscopy method. Photoluminescence (PL) spectra and the energies of 2E, 4T2, and 4T1 excited states of Cr3+ ion depend on the type of crystalline samples phase. The PL excitation spectrum of R-line in α-Al2O3 nanoscale crystals is formed by intracenter transitions (2.5–5.5eV region), by charge transfer band (6.9eV) and by effective formation of impurity-bound excitons (9.0eV region). Such impurity-bound excitons correspond to O2p→Al3s electron transition in surroundings of an impurity Cr3+ center. The efficiency of impurity-bound excitons formation decreases with the increase of the grain size above 100nm. The size dependence is noticeably shown in PL excitation spectra in VUV region. Excitons bound to impurity centers do not appear in nanostructured δ+θ-Al2O3 crystals. The effect of the electron excitation multiplication is observed distinctly in nanostrucured α-Al2O3 at an excitation energy above 19eV (more than 2Eg).

Investigations of g factors for the 2E state of V2+ ions in Al2O3 crystals

The exchange charge model of crystal field was used to calculate the crystal filed parameters (CFPs) acting upon the V2+ electrons in the Al2O3 crystals. With those CFPs, the crystal field Hamiltonian was diagonalized in the complete space spanned by all wave functions of the 3d3 electron configuration of divalent vanadium. The obtained wave functions of the calculated energy levels were used to estimate the zero filed splitting (ZFS) of the ground state and g factors of ground and the first excited states for V2+ ion in Al2O3 crystals. The theoretical results are in good agreement with the corresponding experimental data. Numerical estimations of the contributions to the ZFS and g factors arising from the nearest ligands and ions from further coordination spheres were performed. It has been shown that consideration of a large cluster around an impurity ion leads to improvement of the calculated results.

Creation and clustering of Frenkel defects at high density of electronic excitations in wide-gap materials

A complex nature of the dependence of the intensity of intrinsic or impurity emission on the excitation density by single electron pulses is determined by the existence or absence of self-trapped holes and/or excitons in ZnS, BaF2, MgO, BeO and Al2O3. A powerful electron (300keV) or ion (Au, U, ∼2GeV) irradiation of pure and doped LiF, MgO and Al2O3 crystals induces the optical absorption, certain high-temperature annealing stages of which appear only under high LET conditions. Swift-ion-irradiation causes drastic changes in the spectrum of fundamental reflection of LiF, especially in the region of the exciton resonance. The irradiation providing high density of electronic excitations (LET>20keV/nm) leads not only to the creation of stable Frenkel defects but also to the excitation of a whole group of crystal ions, thus, causing the creation of bivacancies, lithium and fluorine interstitials as well as their associations/clusters.

Influence of Al2O3 crystallization on band offsets at interfaces with Si and TiNx

Experiments on internal photoemission of electrons from Si and TiNx into thin Al2O3 layers suggest that crystallization of amorphous Al2O3 to the cubic γ-Al2O3 is accompanied by only a ≈ 0.5 eV upshift of the conduction band bottom edge. As it is known that in transforming from the amorphous to the γ-phase, the bandgap of Al2O3 increases from 6.2 eV to 8.7 eV, more than 80% of this gap widening must occur at the VB side of the gap. The latter suggests that interaction between electron states of O anions plays the dominant role in the oxide gap widening.

Properties of Al Doped ZnO Thin films by DC Reaction Magnetron Sputtering

The high quality ZAO thin films were successfully produced by DC reaction magnetron sputtering technology. The XRD，electrical and optical properties of films are particular investigated. The results show that ZAO films are polycrystalline hexagonal wurtzite structure，and Al2O3 crystal phase are not found. At the same time，the high quality ZAO films with the minimum resistivity of 4.5x10-4Ω•㎝, the transmittance in visible region above 80% and the reflectivity in IR region above 70% are gained.

Change in the surface morphology and chemical composition of some oxide crystals under UV laser irradiation

The effect of the 248-nm KrF and 355-nm YAG:Nd3+ laser radiation on the surface morphology and chemical composition of SrTiO3, Sr2RuO4, PbMoO4, LiNbO3, Y3Al5O12, and Al2O3 crystals has been studied. A relationship between the laser energy density on the sample surface and the surface roughness caused by the irradiation is determined. A technique for determining exactly the geometric surface characteristics is proposed. The effect of the surface roughness on the results of energy-dispersive X-ray (EDX) microanalysis has been investigated. A method for correcting the EDX data for samples with a rough surface has been developed. It is shown that the small variation in the composition of PbMoO4, LiNbO3, Y3Al5O12, and Al2O3 samples after laser irradiation can be explained by the measurement error, related to the change in the surface roughness. At the same time, the irradiation of SrTiO3 and Sr2RuO4 crystals by a YAG:Nd laser changes the chemical composition of their surface layers.

The Hardness and Strength Properties of WC-Co Composites.

The industrially-important WC-Co composite materials provide a useful, albeit complicated materials system for understanding the combined influences on hardness and strength properties of the constituent WC particle strengths, the particle sizes, their contiguities, and of Co binder hardness and mean free paths, and in total, the volume fraction of constituents. A connection is made here between the composite material properties, especially including the material fracture toughness, and the several materials-type considerations of: (1) related hardness stress-strain behaviors; (2) dislocation (viscoplastic) thermal activation characterizations; (3) Hall-Petch type reciprocal square root of particle or grain size dependencies; and (4) indentation and conventional fracture mechanics results. Related behaviors of MgO and Al2O3 crystal and polycrystal materials are also described for the purpose of making comparisons.

Investigation of THz birefringence measurement and calculation in Al_2O_3 and LiNbO_3

Based on the polarization-sensitive terahertz time-domain spectroscopy, we measured the birefringence for Al₂O₃ and LiNbO₃ single crystals, which correspond to trigonal structures that have an uniaxial birefringence, in the THz frequency range of 0.25 to 1.4 THz. For more comprehensive understanding of the THz birefringence, the measured birefringence is compared with the results of ab initio calculations. The measured birefringence shows good agreement with the calculated value.

Spin-Spin and Spin-Other-Orbit Interactions of Spin-Hamiltonian Parameters for 3d2(8)Ions in Al2O3Crystals

By considering weaker magnetic interactions (including spin–spin and spin–other-orbit interactions) in Hamiltonian, the spin-Hamiltonian parameters, including the zero-field splitting parameter D and g factors (g‖, g⊥) for 3d ions in Al2O3 crystals have been investigated. The results show that the contributions to D, g‖ and g⊥ from the spin–orbit interaction are dominant. The contributions from weaker magnetic interactions to D decrease (increase) with increasing charge of impurity ions for 3d (3d) ions.

Analysis of ZAO Thin Films by DC Reaction Magnetron Sputtereing

The high quality ZAO thin films were produced by DC reaction magnetron sputtering technology. The XRD, electrical and optical properties of ZAO thin films were particularly investigated. The results show that ZAO films are polycrystalline hexagonal wurtzite structure，and Al2O3 crystal phase are not found. At the same time，the high quality ZAO films with the minimum resistivity of 4.5x×10-4Ω·㎝, the transmittance in visible region above 80% and the reflectivity in IR region above 70% are gained.

Thermal, structural and crystallization kinetics of SiO2–BaO–ZnO–B2O3–Al2O3 glass samples as a sealant for SOFC

The present work describes the development of a glass-ceramic in SiO2–BaO–ZnO–B2O3–Al2O3 system. The prepared glass samples were found to have good compatibility to act as a sealant in planar solid oxide fuel cell (SOFC) in terms of coefficient of thermal expansion (TEC) and glass transition temperature (Tg). The crystallization kinetics of present glass samples were investigated by various characterization techniques such as differential thermal analyzer (DTA), Dilatometery, X-Ray diffraction (XRD) and scanning electron microscopy (SEM). The crystallization behavior of the chosen glass samples was influenced by replacing B2O3 with Al2O3. With the addition of Al2O3 there is increase in glass transition temperature (Tg) and glass crystallization temperature (Tc). Also with the addition of Al2O3 crystallization phenomenon is hindered. XRD and SEM study was done at various temperatures for different time durations. The detail of the above discussed study is done in the present paper.

Microstructure characterization of a pressureless infiltrating oxidized SiCp preform by Al-8Mg

The microstructure of a pressureless infiltrating 55vol% oxidized SiC preform by Al-8Mg alloy was characterized by transmission electron microscopy (TEM), high resolution TEM (HRTEM), field emission scanning electron microscopy (FE-SEM), and X-ray diffraction. The TEM image of the interface between Al and SiC shows that the surface of SiC is covered by a rough nanocrystal layer of MgAl2O4, Al2O3, and Si, produced by the interfacial reaction of Al, Mg, and SiO2 on the surface of SiC. The Al-SiC interface is also examined by HRTEM to be better understood how MgAl2O4 and Al2O3 are produced. Dendritic Al2O3 crystals are embedded in the pores of the composite generated from the mutual bonding of SiO2 on the surface of SiC. Columnar AlN crystals of about 250 nm in length are bunched vertically on the SiC particle surface.