Crystal Aluminum Research Articles

Optically stimulated luminescence for diagnostic and therapeutic low to medium energy X-ray beams experimental dosimetry

Introduction Optically stimulated luminescence (OSL) using aluminium oxide crystals (Al2O3:C) is a radiation detection technique, which has been validated as a reliable and promising alternative to ion chambers measurements in radiation therapy with significantly smaller active volumes and good tissue equivalence at diagnostic and therapeutic X-rays energies. However the precision achieved at lower energies is directly linked to the ability to characterise, model and correct the energy dependence of these detectors. The aim of this study was to characterize the OSL detectors in terms of energy dependence using monochromatic diagnostic and therapeutic beams in the low-medium energy range (25–90 keV); and to evaluate the potential of this technique for exeprimental dosimetry in radiologic and therapeutic applications at these energies. Materials and methods The OSL/OF (Optical Fibers) system is composed of a multichannel reader and OSL OFS (Optical Fibre Sensors) incorporating dosimetric-grade alumina crystals. Al2O3:C (0.09 mm3). Irradiations have been performed at the European synchrotron radiation facility medical beamline using square irradiation fields at 8 energy points from 25 to 90 keV. Absolute dose measurements have been performed at 80 keV at 2 cm depth in a water equivalent phantom. Once irradiated the detector has been read over 300 s to collect all the light available from the trapped electrons, which is directly linked to the absorbed dose. Results The OSL detector exhibit an energy dependance ranking from 10% to 45% from 25 to 90 keV. The use of the OSL detector for absolute dosimetry in a water equivalent phantom at 80 keV has shown a good reproducibility with a standard deviation lower than 5% for a 4 Gy single dose, using 5 × 5 cm2 field. Conclusion Those measurements demonstrated that the OSL energy dependance decreases with the beam energy. Moreover there is a good reproducibility in the OSL signal if being used in absolute dosimetry. The next step of this study is to use OSLD for beam penumbra caracterisation, where conventional detectors exhibit a severe energy dependance at these low energies. In vivo dosimetry applications in synchrotron stereotactic radiotherapy are also currently being investigated.

Study on kinetics of hydrogen dissolution and hydrogen solubility in oxides using imaging plate technique

Using a tritium imaging plate technique, kinetics of tritium dissolution and its solubility in several oxides were examined. Mirror-polished single crystals of alumina, spinel and zirconia were used as specimens, which were exposed to 133Pa of a tritium(T)–deuterium(D) gas mixture (T/(T+D)∼0.17) at temperatures ranging from 673 to 973K for 1–5h. The T surface activity on the specimens increased with increasing temperature and exposure time, it almost saturated at 873K and reached 2×105Bq/cm2 (1×1014T/cm2), and no clear difference appeared among the types of specimens. The T activity in the oxide bulk also increased with temperature, in which there was a trend for the oxides: spinel≧zirconia≧alumina. In the T dissolution process for all oxides, the concentration gradient due to its diffusion was not observed even for short exposure times: the T density was almost uniform over the specimens in transition states and increased with exposure time up to the saturated value. These experimental results suggested that the rate-controlling process of T dissolution in the temperature region should be not its diffusion in the oxides but dissociation of hydrogen molecules (T–D mixture in this case) into atoms, its adsorption on the surface and/or T penetration from the surface into the bulk.

Дослідження лінійної усадки при безперервному литті алюмінію

Research works of plenty of scientists are dedicated to issues of continuous metals casting and rolling. Nevertheless there are no scientific papers, dealing with research of impact of continuous aluminum crystallization on the length of the casted billet and its velocity after casting.The speed process of continuous aluminum casting is characterized by an uneven heat exchange lengthwise the billet which is crystallized in the mould of the casting wheel in the temperatures range. It results in the linear metal contraction, so the length of the casted billet changes. Implementation of the theory of thermal conditions enabled obtaining mathematical expressions for determination of linear metal contraction and its velocity after casting. It can be calculated before its actual occurrence, which allows making pre-emptive actions in the management of production of aluminum products, to manage the process, aiming at not only the deflection of the billet length, but also at perturbations of the casting wheel

An imaging spectrometer based on high resolution microscopy of fluorescent aluminum oxide crystal detectors

Abstract Fluorescent Nuclear Track Detector (FNTD) technology was tested as an imaging, spectroscopic tool for radionuclide analysis. This investigation intended to distinguish between characteristic α-particles of 239 Pu (5.2 MeV), 234 U (4.8 MeV) and 238 U (4.2 MeV). FNTDs are Al 2 O 3 :C,Mg single crystals with color centers that undergo radiochromic transformation. FNTD readout is non-destructive and is performed with fluorescence laser scanning confocal microscopy. Ionization events register in the detector as bright fluorescent features on a weak fluorescent background. Images were acquired at several incrementing depths in the detector to produce 3D datasets. Spectroscopic information was obtained by measuring α-particle range in the detector after 3D image reconstruction. The resolution of this technique is fundamentally limited by particle range straggling (about 3.8% ( k = 1) at these α-particle energies). The spectroscopic line-width as full width at half maximum (FWHM) was determined to be 0.4 MeV enabling discrimination between the isotopes of interest.

Laser Synthesis of Metal Oxide Crystals with the Use of Carbon Nanotubes

The results of experimental studies in the direction of the laser forming of nanostructured arrays of metal oxide crystals are presented in this paper. Laser synthesis of crystals of titanium oxide and aluminum oxide was investigated. Carbon nanotubes have been used to control the process of crystal growth. Formation of structures was carried out under the action of CW ytterbium fiber laser with intensity on the order 105 W/cm2. Laser irradiation of the initial target occurred with applying an external accelerating electric field. Various forms of metal oxide crystals were obtained. Whiskers with cross dimensions on the order of 100 nm have been found among the formed structures.

Power-Law Creep from Discrete Dislocation Dynamics

We report two-dimensional discrete dislocation dynamics simulations of combined dislocation glide and climb leading to "power-law" creep in a model aluminum crystal. The approach fully accounts for matter transport due to vacancy diffusion and its coupling with dislocation motion. The existence of quasiequilibrium or jammed states under the applied creep stresses enables observations of diffusion and climb over time scales relevant to power-law creep. The predictions for the creep rates and stress exponents fall within experimental ranges, indicating that the underlying physics is well captured.

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.

Particle size influence of starting batches on phosphorescence behaviour of Sr4Al14O25 based bluish green phosphors

Photoluminescent material with long afterglow is a kind of energy storage material that can absorb both ultraviolet (UV) and visible lights from sunlight, and gradually releases the energy in the dark at a certain wavelength. These sorts of materials have great potential for various device applications and have been widely studied by many researchers. In recent years, it has also been reported that 2SrO.3Al2O3/Eu2+ and 4SrO.7Al2O3/Eu2+ phosphors as green and blue emitters have even higher quantum efficiencies. To determine the initial particle size effect on the phosphorescence behaviour, Eu2+/Dy3+ doped Sr4Al14O25 phosphors were synthesised by mixing 4SrO and 7Al2O3 with a flux (H3BO3) through high temperature solid state reaction method under weak reducing atmosphere. Such an influence on the crystalline structure and emission colour of phosphorescent pigments was studied by means of X-ray diffraction, scanning electron microscope (SEM), particle size analysis, excitation and emission spectroscopy. The results showed that the emission wavelength of the phosphorescence pigments shifted from green to blue region due to the decrease in average particle sizes of the phosphor batches, forming different types of strontium aluminate crystals.

Lithium Aluminate Crystals as Scintillator for Thermal Neutron Detection

Undoped and Cu+-doped lithium aluminate LiAlO2 singe crystals have been grown using micro-pulling down (μ -PD) technique. After cutting and polishing, optical properties and scintillation characteristics of the crystals were examined. From the transmittance measurements, Cu+-doped crystal showed absorption band around 270 nm, which is corresponding to the transition between Cu+3d10 and 3d94s states. Ultra violet emission peak appeared around 360 nm under 270 nm excitation. The emission decay time was calculated to be about 17.3 μs. 241Am 5.5 MeV alpha-ray excited radioluminescence spectra exhibited intense luminescence band at 360 nm due to the Cu+3d94s-3d10 transitions. By the pulse height spectra 252Cf neutron source excitation, the scintillation light output of Cu+ -doped LiAlO2 was found to be about 8700 ph/n.

Mechanisms of Quick Zeolite Beta Crystallization

Silicate self-assembly and zeolite beta crystallization in the presence of 4,4′-trimethylenebis(N-methyl, N-benzyl piperidinium) (TMP2+) structure directing agent were studied using a combination of 29Si and 27Al nuclear magnetic resonance (NMR), small angle scattering (SAS), and attenuated total reflectance Fourier transform infrared (ATR/FTIR) spectroscopy. Crystallization of nanosized siliceous TMP2+ zeolite beta proceeds along three populations of particles (i.e., primary, secondary, and tertiary), differing in size and internal structure similar to crystallization of aluminum containing zeolite beta with tetraethylammonium (TEA+) as structure directing agent. Aluminosilicate TMP2+ zeolite beta, however, crystallizes along a very unique pathway, wherein secondary particles are final partially crystalline colloidally stable zeolite beta. TMP2+ zeolite beta crystallizes quickly when compared to TEA+ zeolite beta, which takes nearly a hundred hours to crystallize under similar synthesis conditions.

The chemical characterization and reflectivity of the Al(1.0%wtSi)/Zr periodic multilayer

The reflectivity of Al(1.0%wtSi)/Zr multilayer with 40 periods has been measured in the region of 17–19nm. Experimental peak reflectivity is 41.2% at 5° incidence angle. However, the corresponding theoretical value for an ideal Al(1.0%wtSi)/Zr multilayer is 70.9%. In order to explain the difference between theoretical and experimental reflectivity, the multilayer has been characterized by X-ray diffraction and X-ray photoelectron spectroscopy. Based on this analysis, the four impact factors responsible for the loss of reflectivity are inhomogeneous crystallization of aluminum, contamination of the multilayer, surface oxidized layer and interdiffusion between Al and Zr layers. The effects of different impact factors on the EUV reflectivity of the Al(1.0%wtSi)/Zr multilayer have been introduced independently by means of corresponding simulations.

Time-resolved luminescence of defects and Cr3+ impurity centers in nanosized alumina crystals under vacuum-ultraviolet excitation

The photoluminescence (PL) of intrinsic defects and Cr3+ impurity centers in nanosized alumina (Al2O3) crystals of various phase compositions (δ + θ and α phases) has been studied by the method of time-resolved spectroscopy with vacuum-ultraviolet (VUV) excitation at T = 7.5 and 295 K. It is established that the PL excitation spectrum and its kinetic parameters for α-Al2O3 phase can change depending on the crystal grain size. In α-Al2O3 samples with grain sizes below 100 nm, excitons localized on Cr3+ impurity centers and surface-modified defects of the crystal structure are observed even at T = 295 K. Manifestations of these exciton states in the PL spectra strongly depend on the grain size.

Modelling of ultrasonic consolidation using a dislocation density based finite element framework

A dislocation density based constitutive model has been developed and implemented into a crystal plasticity quasi-static finite element framework. This approach captures the evolution of dislocations and grain fragmentation at the bonding interface when boundary conditions pertaining to the Ultrasonic Consolidation process (UC) are prescribed. The model is initially calibrated using experimental data from published refereed literature for simple shear deformation of a single crystal pure aluminum and uniaxial tension of a polycrystalline Aluminum 3003-H18 alloy. The model has then been extended to predict the results of an Al 3003- H18 alloy undergoing UC. Good agreement between the experimental and simulated results has been observed for the evolution of linear weld density and embrittlement due to grain substructure evolution. For computational time efficiencies, a novel time homogenisation approach has been followed which significantly reduces the computational overhead.

Electron beam induced crystallization of sputter deposited amorphous alumina thin films

Crystallization of amorphous alumina (Al2O3) in a thin film capacitor structure was induced by the electron beam of a transmission electron microscope (TEM). The crystallization was initially observed while collecting selected area diffraction (SAD) patterns after 2min of beam exposure at an accelerating voltage of 200keV and a beam current density of 13.0A/cm2. After 16min of beam exposure, distinct ring patterns associated with crystal growth were evident in the SAD pattern. Bright field and dark field TEM images confirmed that crystallization occurred, with crystals growing up to ~50nm in diameter.

A novel approach to study dislocation density tensors and lattice rotation patterns in atomistic simulations

Crystal plasticity caused by the nucleation and interaction of dislocations is an important aspect in crystal deformation. Recent nanoindentation experiments in single crystals of copper or aluminum revealed large deviations in the lattice rotation and an inhomogeneous distribution of the dislocation density in the plastic zone under the indenter tip. Molecular dynamics simulations offer the possibility to study the origin of these phenomena on an atomistic scale, but require sophisticated analysis routines in order to deal with the massive amount of generated data. Here a new efficient approach to analyze atomistic data on the fly during the simulation is introduced. This approach allows us to identify the dislocation network including Burgers vectors on the timescale of picoseconds and below. This data does not only reveal the evolution of dislocation structures, but it offers the possibility to quantify local dislocation density tensors calculated on an atomic level. The numerical results are compared with experimental data from the literature. The presented approach provides useful insight into the active deformation mechanisms during plastic deformation that will help us to bridge simulations on atomic scales and continuum descriptions.

Reversible nanovalves in inorganic materials

This article describes recent developments in the design, fabrication and investigation of reversible nanovalves prepared in inorganic nanoporous materials such as glass, silicon nitride, mesoporous silica, anodized alumina and silica colloidal crystals. Their surfaces have been modified with pH-, temperature- and light-responsive molecules, as well as molecules that can undergo redox reactions and bind small molecules. This surface modification allows reversibly opening and closing of the nanovalves to the transport of ions based on electrostatic interactions, and to the transport of molecules based on mechanically gating the nanopores.

Synthesis and Coating of Aluminum NanoCrystals

ABSTRACTWe show a low temperature gas-phase synthesis route to produce faceted aluminum crystals in the aerosol phase. Use of triisobutylaluminum whose decomposition temperature is below the melting point of elemental aluminum enabled us to grow nanocrystals from its vapor. Combustion tests show an increase in energy release compared to commercial nanoaluminum. Production of aluminum in an oxygen free environment resulted in a bare aluminum surface that was passivated in separate experiments with nickel and iron by decomposition of their carbonyl precursors.

Single crystal growth from the melt and magnetic properties of hexaferrites–aluminates

Single crystals of aluminum substituted barium hexaferrite were grown by the floating zone method with optical heating. Single crystals were produced from a melt of stoichiometric composition. The process was carried out under a pressure of 50atm of oxygen. In the system BaO–(x)Al2O3–(6−x)Fe2O3 the region of single phase crystal growth from the melt is limited by the value x=3. For higher substitutions single-phase crystallization is not observed. The grown single crystals are cylindrical boules with a diameter of 4–5mm and with lengths up to 50mm. To avert cracking the crystals have been annealed during the process of growth at 1100°C. The content of FeO in the composition of single crystals of barium hexaferrite, grown by zone melting under an oxygen pressure of 50atm, is approximately 0.3wt%. In the system of hexaferrite–aluminates the macroscopic magnetic moment of the material disappears at x=3.

Peculiarities of Self-Propagating High-Temperature Synthesis and Structure Formation of TiB2-Al2O3 and CrB2-Al2O3 Composites

Preparation of TiB2-Al2O3 and CrB2-Al2O3 composites with a broad range of phase composition was conducted by self-propagating high-temperature synthesis (SHS) involving reaction of different types. The formation of fibrous crystals of aluminum oxide with length of about 10-25 microns and with diameter of 200-500 nm at self-propagating high-temperature synthesis in the system 2B2O3-Cr2O3-6A1 was established. Thermite mixtures of Al-TiO2 and Al-TiO2-B2O3 were incorporated with the Ti-B combustion system to produce the composites of TiB2-Al2O3, within which the increase of the thermite mixture for a higher content of Al2O3 decreased the reaction temperature and combustion wave velocity. This implies that the thermite reaction of Al with TiO2 reduces the exothermicity of the overall SHS process. It was found that adoption of B2O3 as one of the thermite reagents improved the product formation effectively. For investigate the combustion wave in 0.75TiO2-0.25Ti-2B-Al system the «quenching» method was used. The XRD analysis shows that the final products containing diborides and aluminium oxide.

Infrared studies of CO oxidation by oxygen and by water over Pt/Al2O3 and Pd/Al2O3 catalysts

CO adsorption has been studied over Pd/Al2O3 and Pt/Al2O3 catalysts using IR spectroscopy in different conditions and after different pretreatments. The oxidation of CO by oxygen and water (water gas shift) has also been studied at 130–673K. The spectra show that the most active species as oxidant for CO are dispersed Ptn+ and Pdn+ cations and PdOx species mainly “decorating” the defects of alumina crystals where they modify completely the behaviour of surface OH's and Lewis acid sites. Some of these species oxidize CO to CO2 already below ∼200K. These highly oxidized species escape detection by IR during reaction at 500–673K, when only reduced sites are apparent due to their very strong interaction with CO. The data suggest that redox mechanisms could occur for both WGS and CO oxidation occurring on highly dispersed cationic species non detectable under reaction conditions.