Crystal Aluminum Research Articles

Temperature dependence of the kinetics rate of the melting and crystallization of aluminum

Using the molecular dynamics method, heterogeneous melting–crystallization of aluminum is simulated under conditions when the phase front propagates over the overheated-overcooled phase. The dependence of the phase front velocity on the temperature deviation from the equilibrium melting temperature is determined. The dependence obtained from atomistic simulation is used as an approximating function to obtain the temperature dependence of the kinetic rate in the analytical form. The steady-state temperature dependence of the kinetic rate v(Tsl) for extreme values of aluminum overheating-overcooling is constructed for the first time.

First principles calculation of thermal expansion coefficients of pure and Cr doped α-alumina crystals

We have performed theoretical analysis of thermal expansion behavior of alumina crystals under finite temperature based on first principles phonon state calculations. Liner thermal expansion coefficients of a pure α-alumina crystal have been evaluated based on quasi-harmonic approximation including crystalline anisotropy. The Cr doping effect on the alumina crystal has also been examined and found that the doping can cause a noticeable change on the thermal expansion coefficient. The present results demonstrate that the first principles theoretical approach can be helpful for reproducing or predicting thermal expansion behaviors including dopant effects, which may pave a way for possible control of thermal expansion of materials by doping or alloying.

Bulk crystal growth and characterization of 2.5–5.0 µm mid-infrared laser crystals

Mid-infrared waveband lasers have demonstrated important applications in the fields of atmosphere pollution monitoring, gas flow monitoring, ocean detection, laser medicine, military confrontation and basic scientific researches etc. In this paper, we review the research development of bulk crystal growth, physicochemical, spectral and laser properties of 2.5–5.0 μm waveband mid-infrared laser crystals in recent years. A serial of gallate and aluminate crystals with low phonon energy were grown by the Czochralski method, such as Er3+ activated and rare earth ions co-doped Gd3Ga5O12(GGG), SrGdGa3O7, SrLaGa3O7, CaYAlO4 and CaGdAlO4 crystals, Ho3+ activated and rare earth ions co-doped GGG crystals, as well as Dy3+:YAlO3 crystals, and 2.5–3.4 μm waveband emissions were obtained in these crystals. High quality Dy3+:KPb2Cl5, Cr2+:CdSe and Cr2+:Fe2+:CdSe crystals were grown by the Bridgman method, and 2.5–5.0 μm waveband emissions were obtained in these crystals. The physicochemical and spectral properties of the above crystals were studied systematically, with focus on analyses of the concentration effect of activated ions Er3+, the sensitization mechanism of the co-dopant sensitized ions and the impact of the depopulated ions on the inhibition of self-termination effect. The fluorescence dynamic of the activated ions within the mid-infrared waveband, the energy transfer efficiencies between activated ions and co-dopant sensitization ions, as well as between the activated ions and co-dopant depopulation ions were calculated, and the energy transfer mechanism was discussed. Based on the results of spectroscopic investigations, the concentrations of Er3+, the types and the concentrations the co-dopant sensitization ions were optimized in different kinds of crystals. The laser properties of Er3+ activated GGG, CaYAlO4 crystals were presented. Finally, 2.7–2.8 µm multi-wavelength CW or pulsed lasers were achieved in Er:GGG, Er,Pr:GGG and Er:GGG/GGG composite crystals end-pumped by 965 nm LD or in Cr,Er:YSGG crystals by Xenon lamp, furthermore, the single longitudinal mode microchip laser was achieved in Er:GGG crystal for the first time.

Evolution of embedded lithium nanoclusters in lithium implanted alumina

High dose of ion implantation followed by annealing is considered a feasible way to generate thermally stable nanoclusters inside a transparent host matrix. Low energy (50 keV) Li ions have been implanted into single crystals of alumina with different fluence (1 × 1015–1 × 1017 ions/cm2). The samples have been annealed at temperatures ranging from 500 to 1100 °C in air in step of 100 °C. Depth dependent Doppler broadening measurements have been carried out using high purity germanium detector coupled to a variable energy slow positron beam. Fractional area in the central and wing regions of Doppler broadened annihilation radiation spectrum, namely, S- and W- parameters, were evaluated from each spectrum. Any variation in positron annihilation probability with valence and core electrons which occurs on trapping of positrons at a defect site is reflected in these parameters. The effect of ion fluence and annealing temperature on evolution of defects and formation of embedded Li nanoclusters have been studied by indexing the variation in line shape S- (W-) parameter as a function of positron implantation depth. These studies supplemented by theoretical calculations confirm that with annealing up to 700 °C, vacancy clusters are created due to the aggregation of vacancies wherein Li nanoclusters are formed. On annealing at higher temperature, there is evidence for the breakdown of these Li clusters leaving behind vacancy clusters in the samples.

On numerically predicting the onset and mode of instability in atomistic systems

We present an accurate and efficient method based on the Lanczos algorithm for predicting the onset and mode of instability in atomistic systems. Specifically, we develop a framework that is identically applicable to all flavors of atomistic simulations, including ab-initio calculations. Notably, we do not make any apriori assumptions regarding the nature of the instability or its location. We verify the accuracy of the proposed approach by studying defect nucleation during the nanoindentation of a triangular lattice and hydrostatic tension test of an aluminum crystal. We demonstrate that the computational cost in practical calculations scales linearly with system size, and is accompanied by a small prefactor. Overall, the proposed method is attractive because it enables the stability analysis of atomistic systems at the mesoscale.

The effect of heating rate on thermoluminescence of anion-defective alumina after high-dose irradiation

The dependences of the light sum of peaks of thermoluminescence (TL) at 180 and 300°C on the rate of heating of single crystals of anion-defective alumina after high-dose irradiation are investigated. It is found that these dependences for the peak at 180°C are the same for all studied samples and match the known dependence measured after low-dose irradiation. The curves for the TL peak at 300°C vary significantly from one sample to another. The parameters of thermal TL quenching are calculated for both peaks using the Monte Carlo method. Practical suggestions regarding the use of the obtained results in high-dose dosimetry are provided.

Studying condensed phase formed upon interaction between a high-power laser pulse and crystal aluminum oxide

Forms of the condensed phase created under the action of a high-power CO2 laser pulse (power, 4 J/pulse; λ = 10.6 μm; pulse duration, 1.5 μs) on optical sapphire (Al2O3) solid targets are investigated. The ablation products emitted from the laser crater and the particles formed in the space above the surface of the target are collected using witness samples oriented in a predetermined manner with respect to the laser crater. Forms of the condensed phase are studied via electron microscopy and atomic force microscopy. Conclusions are drawn as to the mechanisms of formation of particles with different shapes and structures, including Al2O3 vacuum hollow microspheres (hollow bubbles).

Modeling the concentration profiles of aluminum and indium impurities in crystals of germanium–silicon solid solutions

A one-dimensional problem of the axial distribution of Al and In impurities in uniform crystals of Ge–Si solid solutions grown by double feeding of the melt method has been solved in the Pfann approximation. The mathematical modeling results suggest that the impurity concentration profile in Ge–Si crystals of constant composition can be varied in a wide range through appropriate changes in the relationship between the crystal growth rate and the rates of feeding the melt using germanium and silicon rods. We demonstrate the possibility of growing completely homogeneous Ge–Si kAl,Inl crystals, in terms of both the concentrations of their major components and the impurity concentration profile.

Attachment of Alumina on the Wall of Submerged Entry Nozzle During Continuous Casting of Al-Killed Steel

The mechanisms of the formation of different attachments on the walls of submerged entry nozzle (SEN) were studied for the processes of Al-killed steel (Ca-treated, HSLA) and ultra-low carbon Al-killed steel (ULC). To understand the mechanism, the types of inclusions in the steel taken in tundish and in bloom (or slab) were identified. In the case of ULC, the reoxidation product, micro-alumina particles were found to be the source of attachment on the inner wall of the SEN. To avoid reoxidation of the steel by the top slag, removal of the slag could be considered in order to improve the situation. No attached layer was found on the outer surface of the SEN after casting of the ULC steel. In the case of HSLA steel, an attached layer composed of plate-like alumina crystals was found in some trials. The entrainment of oxygen through the mold powder due to improper operation would be the reason for the formation of this type of attachment. The formation of the plate-like crystals was discussed with the help of CFD calculation.

Luminescence and radiation-induced color centers in anion-defective alumina crystals after high-dose irradiation

A method of UV spectroscopy was used to measure photoluminescence (PL) spectra and photoluminescence excitation (PLE) spectra in anion-defective alumina crystals exposed to high doses of gamma-radiation. An additional emission band in the range of 1.6–2.75 eV appears in the exposed crystals. Aggregate F2-type centers in different charge states are responsible for this band. It was found that growing intensity of PL aggregate centers occurs at doses corresponding to saturation of dose response and is accompanied by a sharp drop in the intensity of F+-band in the PL spectrum resulting from combination of F+-centers into aggregates. Uncharged F2-centers are formed when electrons are trapped by F2+ and F22+-centers. The main role of F+-centers in radiation-induced transformations of color centers under high-dose irradiation of anion-defective alumina crystals was indicated.

Quantum Algorithmic Readout in Multi-Ion Clocks.

Optical clocks based on ensembles of trapped ions promise record frequency accuracy with good short-term stability. Most suitable ion species lack closed transitions, so the clock signal must be read out indirectly by transferring the quantum state of the clock ions to cotrapped logic ions of a different species. Existing methods of quantum logic readout require a linear overhead in either time or the number of logic ions. Here we describe a quantum algorithmic readout whose overhead scales logarithmically with the number of clock ions in both of these respects. The scheme allows a quantum nondemolition readout of the number of excited clock ions using a single multispecies gate operation which can also be used in other areas of ion trap technology such as quantum information processing, quantum simulations, metrology, and precision spectroscopy.

Non-stoichiometry of tris(8-hydroxyquinoline) aluminium: is it possible?

The dependence of the 8-hydroxyquinoline (8-Hq) vapour pressure on temperature, log P8-Hq [Torr] = 8.4249 − 2963.5/T, (386–482 K) was measured by Bourdon manometer technique. Tris(8-hydroxyquinoline) aluminum crystals were grown under controlled P8-Hq [0.1–11.3 Torr] and their cell parameters were measured.

Research of gas release during crystallization of aluminium and its alloys

The problem of gas porosity formation in the manufacture of aluminum structural or cast porous alloys is discussed and some results of our research in this area are shown in the article. The main aim of the research is to investigate the influence of various technological, physical and chemical factors on the process of hydrogen gas release during crystallization of aluminum and its alloys. Using system simulation allows a scientist to investigate fully the flow of a process. In this paper to simulate the process of hydrogen gas release it was considered the simplest and, therefore, the most discussed in the literature option when the crystallization front moves in an unlimited amount of melt. The presented method allows determining what factors and how affecting the process of hydrogen gas release during production of aluminum castings and its alloys. The research results can be applied by experts in the field of metal technology, metallurgy, foundry, production of aluminum castings for mechanical engineering.

Single-crystal synthesis and structure refinement of La2Li0.5Al0.5O4 with K2NiF4-type structure

Single crystals of lanthanum lithium aluminum oxide, La2Li0.5Al0.5O4, were synthesized for the first time by a flux method at 773 K. The La2Li0.5Al0.5O4 single crystal is colorless, has an average size of 60 μm with a square platelet shape, and crystallizes in the tetragonal K2NiF4-type structure, space group I4/mmm with a = 3.7742(9) Å, c = 12.753(4) Å, V = 181.66(10) Å3, and Z = 2. The structure was determined using single-crystal X-ray diffraction data and refined to the conventional values R = 1.25% and wR = 1.28% for 524 observed reflections. The octahedral site was randomly occupied by Li and Al atoms. The corresponding polycrystalline sample was also prepared by a conventional solid-state reaction method at 1123 K.

Studying aluminum hydride by means of thermal analysis

Chemical reactions and physical transformations that occur upon heating aluminum hydride (AlH3, alane), stored for 25 years, in the temperature range of 50–1200°C in an atmosphere of nitrogen, argon, and air are studied by means of thermogravimetric analysis and differential scanning calorimetry. The heat of thermal decomposition and the hydrogen content are determined for the AlH3 samples and are found to be 318 ± 25 J/g and 9.32 ± 0.24 wt %, respectively. It is established that the estimated enthalpy of formation of AlH3 in stoichiometric composition (ΔfH ≈ −10.3 kJ/mol) agrees with the literature data. After the release of hydrogen, the mass of the precipitate increases by 0.5 ± 0.3%, relative to the initial mass of the AlH3 samples; the most likely reason for this effect is the adsorption of nitrogen (argon) in the micropores and mesopores that form. Thermal phenomena associated with the crystallization of the amorphous aluminum that forms after hydrogen is released from the alane particles are analyzed. It is established that the aluminum contained in initial AlH3 samples is almost completely transformed into aluminum nitride and oxide (AlN and Al3O3) upon heating to 1200°C in nitrogen and air, respectively.

A novel approach for preparation of modified-biochar derived from marine macroalgae: Dual purpose electro-modification for improvement of surface area and metal impregnation

In the present study, an aluminum electrode-based electrochemical process was newly adopted as a modification method for fabricating physically and chemically modified biochar derived from marine macroalgae. Specifically, a current density of 93.96mAcm−2 was applied for 5min at pH 3.0. Subsequently, the mixture was stirred continuously for 30min without electric field, and the dried sample was then pyrolyzed at 450°C under a N2 environment for 2h. SEM–EDS and XRD analyses clearly indicated that nano-sized aluminum crystals (beohemite, AlOOH) were uniformly present on the EM-biochar surface. Adsorption equilibrium tests showed that the phosphate adsorption onto EM-biochar agreed well with the Langmuir–Freundlich adsorption isotherm model, with a maximum adsorption capacity of 31.28mg-Pg−1. These findings suggest that this novel and simple electro-modification method is a reasonable and effective option for simultaneously upgrading both the surface area and chemical properties of biochar.

Radiation-induced transformations of luminescence centers in anion-defective alumina crystals under high-dose irradiations

Luminescent spectroscopy is used to show formation of new trapping centers of charge carriers in anion-defective alumina crystals at radiation-induced transformations of F and F+-centers created by oxygen vacancies when exposed to high-dose gamma-radiation. A new wide band in the range 440–700nm was registered in the photoluminescence spectrum at excitations with UV photons. High-dose irradiation of the crystals leads to appearance of F2-type aggregate centers in different charged states. These centers are additional traps of charge carriers. The new traps increase a luminescent yield at high-dose irradiation with gamma-rays and an electron beam.

Identification of defects responsible for optically stimulated luminescence (OSL) from copper-diffused LiAlO2 crystals

A large optically stimulated luminescence (CW-OSL) response has been observed from bulk lithium aluminate (LiAlO2) crystals that have been copper-diffused. After introducing the copper, an optical absorption band peaking at 277nm is present and is assigned to Cu+ (3d10) ions at Li+ sites. Photoluminescence (PL and PLE) from copper-diffused samples shows an emission band near 359nm and an excitation band near 273nm. These PL bands are also assigned to the Cu+ ions. After an irradiation at room temperature with x-rays, the copper-diffused crystals give an intense OSL signal when stimulated with light in the 425–500nm region. The OSL emitted light has a peak near 357nm and thus is directly associated with recombination at a Cu ion. Electron paramagnetic resonance (EPR) shows that the x-rays produce two distinct Cu2+ (3d9) centers, both at Li+ sites. One of the trapped-hole centers is a Cu2+ with no nearby defects and the other is a Cu2+ with a neighboring Li+ vacancy. Monitoring the EPR spectra before and after OSL shows that only the isolated Cu2+ ions (i.e., those with no nearby defect) participate in the OSL process. The electron traps participating in the OSL are shown with EPR to be transition-metal ions that are unintentionally present. Specifically, Fe2+ ions replacing Li+ ions, possibly with a nearby lithium vacancy, convert to Fe+ (3d7) ions when they trap an electron during irradiation. These electrons are optically released by the OSL exciting light and migrate to the holes trapped as isolated Cu2+ ions, where electron–hole recombination gives the characteristic Cu+ emission. By combining OSL, PL, and EPR, we establish the general OSL mechanism in copper-diffused LiAlO2 crystals.

Adsorption of tetracycline onto alumina: experimental research and molecular dynamics simulation

Adsorption characteristics of tetracycline (TC) onto alumina were investigated according to batch adsorption experiments, and the mechanism was dissected at molecular level by molecular dynamics (MD) simulation. Results indicated that the adsorption was influenced by alumina dosage, temperature, and oscillating frequency. The optimal temperature was 25°C. By increasing the alumina dosage from 0.1 to 2.0 g/L, the removal efficiency of TC increased from 53.73 to 86.44%. With the increase in oscillation frequency from 90 to 200 r/min, the removal efficiency of TC increased by 28.78%, indicating that the increase of frequency could enhance the adsorption of TC. The adsorption behavior fitted well with the pseudo-second-order model (R2 > 0.99). MD simulations revealed that TC molecule structure was deformed when it clung to the alumina crystal. The magnitude of deformation energy (99.893 kcal/mol) was far less than that of non-bond energy (319,643.811 kcal/mol). Analysis of radial distribution function showed that TC could be adsorbed effectively by alumina mainly through non-bond interaction.

Aluminum extraction from coal ash by a two-step acid leaching method

To improve aluminum extraction efficiency, a two-step acid leaching process was proposed to dissolve aluminum and aluminum-bearing crystals with sulfuric acid. The first leaching residue (FLR) was obtained through direct leaching of coal ash with sulfuric acid. After roasting a mixture of the FLR and sodium carbonate, the aluminum-bearing crystals in FLR were completely destroyed while sodium silicate was generated. Then the roasted products were leached with water to dissolve the sodium carbonate. X-ray diffraction (XRD) analysis indicated that the water leaching residues (WLR) mainly contained the amorphous phase, which allowed easy aluminum extraction in the second leaching step. In the first leaching test with 10 mol/L sulfuric acid at 120 °C in 2 h and a solid to liquid ratio (S/L) of 1:2, aluminum dissolution from raw coal ash reached 81.72%. In the second leaching step with 5 mol/L sulfuric acid, the aluminum dissolutions from WLR were 87.31% at 20 °C and 99.06% at 100 °. Total aluminum dissolutions were more than 97%. Moreover, the leaching process produced two by-products, sodium silicate solution and amorphous silicon dioxide, which contained nearly all the silicon from the coal ash.