Formation of ultrafine and dense α-Al2O3 nanoparticles via kinetic phase change in a dynamic process

Ultrafine (5 nm) Al₂O₃ nanoparticles having a predominant α-type structure and with an internal compressive stress up to ca. 15 GPa were synthesized by pulsed laser ablation on Al target under a very high power density (1.8 × 10¹² W/cm²) with oxygen flow in vacuum. The ultrafine α-Al₂O₃ was alternatively formed from the minor γ-Al₂O₃ nanocondensates upon electron irradiation. In such a case, the polymorphs follow a special crystallographic relationship [110]_γ//[2 $ \bar{1}$ $ \bar{1} $ 0]_α; ( $ \bar{1} $ 1 $ \bar{1} $ )_γ//(0 $ \bar{1} $ 14)_α with a mixed mismatch strain yet nonparallel close-packed planes indicating a reconstructive-type transformation. The formation of metastable α-Al₂O₃ in the dynamic processes can be rationalized by the kinetic phase change from the amorphous lamellar and/or γ-Al₂O₃ depending on their free energy versus cell volume curves. The dense and ultrafine-sized Al₂O₃ polymorphs with a rather low minimum band gap of 3.7 eV shed light on their natural occurrence in dynamic settings and abrasive as well as catalytic/optoelectronic applications.     

Strain analysis of misfit dislocations in α-Fe2O3/α-Al2O3 heterostructure interface by geometric phase analysis

The α-Fe₂O₃/α-Al₂O₃ heterostructure interfaces have been studied using transmission electron microscopy (TEM). The interface exhibited coherent regions separated by equally spaced misfit dislocations. The misfit dislocations were demonstrated to be edge dislocations with dislocation spacing of ∼4 nm. The strain fields around the misfit dislocation core were mapped using a combination of geometric phase analysis and high-resolution transmission electron microscopy images. The strain measurement results were compared with the Peierls–Nabarro dislocation model and the Foreman dislocation model. These comparisons show that the Foreman model (a = 2) is the most appropriate theoretical model to describe the strain fields of the dislocation core.     

Possible cage motion of interstitial Fe in α-Al 2 O 3

In addition to spectral components due to Fe^2?+? and Fe^3?+?, a single line is observed in emission Mössbauer spectra following low fluence (<10¹⁵ cm^???2) implantation of ⁵⁷Fe*, ⁵⁷Mn and ⁵⁷Co in α-Al₂O₃. For the ⁵⁷Co and ⁵⁷Mn implantations, the intensity of the single line is found to depend on the emission angle relative to the crystal symmetry axis. This angular dependence can be explained by a non-isotropic f-factor and/or motion of the Fe ion between sites in an interstitial cage. It is argued that interstitial cage motion is a more likely explanation, as this can account for the lack of quadrupole splitting of the line.     

Defect-annealing in neutron-damaged β-Ga2O3

A study is made of the effect of fast neutrons on the electrical conductivity and thermoelectric power and also the neutron-induced defects in β-Ga₂O₃. It is found that the conductivity decreases while the thermoelectric power increases after an irradiation of about 10¹⁷ nvt. This is explained by assuming that the defects introduced into the Ga₂O₃ lattices act as trapping centres for electrons. The defect anneal by thermally activated processes at temperatures up to about 1000 °K with an activation energy of about 2 eV. The defects are the most likely vacancies of the galium atoms.     

Atom location in complex lattices: Pb in α-Al2O3

The channeling technique has been used to locate Pb implanted into α-Al₂O₃ single crystal. The main purpose has been to learn how to obtain information about the location of an impurity in a complex structure. A Pb location is proposed according to data of full angular scans through different axial and planar channeling dips.     

Synthesis and characterization of dentate-shaped β -Ga2O3 nano/microbelts via a simple method

Dentate-shaped β-Ga₂O₃ nano/microbelts were synthesized successfully via chemical vapor deposition without any other reaction or catalyzer material. The morphology and crystal structure were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) analytic technologies, respectively. The growth mechanism of the products was discussed assisted by the SEM and TEM images. The photoluminescence properties of the β-Ga₂O₃ nano/microbelts have been investigated with a He–Cd laser (325 nm) at room temperature. PACS 81.05.Hd; 81.07.Bc; 81.10.Bk; 81.15.Gh; 81.70.Fy     

Improvement in dose threshold of α-Al2O3:C using thermally assisted OSL

α-Al₂O₃:C phosphor was studied for improvement of its dose threshold using thermally assisted optically stimulated luminescence (TA-OSL) phenomenon. The dose threshold of phosphor depends on the standard deviation of the background signal which affects the signal-to-noise ratio of the instrument. In case of OSL measurement, the background signal is due to the scattering of the stimulation light intensity from the planchet. On reducing the stimulation light intensity, the scattering component and thus the standard deviation of the background signal reduces considerably. The reduction in stimulation light intensity increases the readout time due to the dependence of decay constant of OSL signal on the former. The decay constant depends on the photo-ionization cross-section of the OSL active traps in the phosphor participating in OSL phenomenon and thus on the readout temperature due to the temperature dependence of photo-ionization cross-section. In order to achieve the same decay constant for two sets of measurements for α-Al₂O₃:C i.e. to take the OSL measurement in the same time at lower stimulation light intensity as that taken for higher light intensity, the temperature of measurement for α-Al₂O₃:C for the former was raised. Moreover, the increase of readout temperature does not affect the standard deviation in the background signal of OSL readout. The optimized elevated temperature for α-Al₂O₃:C was found to be 85 °C as the main dosimetric peak starts giving signal due to depletion of its filled traps, at temperatures higher than 90 °C. As a result of lowering the stimulation intensity at higher temperature (85 °C), the standard deviation in the background signal and thus the overall dose threshold of α-Al₂O₃:C was found to improve by 1.8 times.     

Thermoluminescence of α-Al 2O 3 by neutron irradiation at low temperature

Thermoluminescence (TL) mechanisms of neutron-irradiated α-Al₂O₃ at 20 K is reported. The TL glow curves of neutron-irradiated and γ-ray-irradiated α-Al₂O₃ were observed. The TL emission bands near 340, 430, 530 and 694 nm were observed in the neutron-irradiated α-Al₂O₃. The γ-ray-irradiated α-Al₂O₃ only showed the TL emission line nearly at 694 nm, corresponding to the R lines of α-Al₂O₃:Cr³⁺. Therefore, the first three emission bands are related to the atomic displacement defects as F-type centers caused by neutron irradiation.     

Orientation relationship between α-Fe precipitate and α-Al2O3 matrix in iron-implanted sapphire

Fe ions were implanted into α-Al₂O₃ single crystals (sapphire) at room temperature and annealed in a reducing atmosphere. The orientation relationships (ORs) between α-Fe particles and sapphire matrix were investigated using transmission electron microscopy (TEM). All the α-Fe particles have the orientation relationship (OR) of (1 1 1)_α-Fe || (0 0 0 1)_sapphire and ${[1\overline{1}0]}_{\alpha \text{-Fe}}\left|\right|{[11\overline{2}0]}_{\text{sapphire}}$ with sapphire. This OR is predicted precisely by the coincidence of reciprocal lattice points (CRLP) method. The other OR of (1 1 0)_α-Fe || (0 0 0 1)_sapphire and ${[111]}_{\alpha \text{-Fe}}\left|\right|{[5\overline{1}\overline{4}0]}_{\text{sapphire}}$ reported before is confirmed by the same method to be one of the secondary preferred orientation relationships in the α-Fe/sapphire system.     

Nature of the anomalies of the thermolumenescence properties of dosimetric α-Al2O3 crystals

The results of investigations into the anomalies of the thermoluminescence properties of dosimetric corundum crystals are presented. The decisive role of deep-lying traps in the quenching of luminescence in anion-defect Al₂O₃ monocrystals is shown. The existence of deep-lying traps is demonstrated by the method of direct observations of thermoluminescence (TL) peaks associated with them. Experimental evidence for the influence of the degree of occupation of deep-lying traps on the main features of the TL dosimetric peak at 450 K is given. The results obtained are interpreted for a model of the interactive system of traps, which differs radically from the models described in the literature by a consideration of the temperature dependence of the probability of trapping of charge carriers on deep-lying traps. We believe that the heat quenching of luminescence is due to the thermal ionization of excited F-center states. Ural State Technical University. Translated from Izvestiya Vysshikh Uchebhykh Zavedenii, Fizika, No. 3, pp. 55–65, March, 2000.     

Ionoluminescence and formation of color centers in α-Al2O3 single crystals under proton irradiation

This paper describes results of experimental studies on radiation defects in nominally pure single crystals of corundum in two initial states: α-Al₂O₃ with an unperturbed lattice and α-Al₂O₃:C with a high concentration of anion vacancies. Defects were identified from optical absorption spectra, ionoluminescence, pulsed cathodoluminescence and photoluminescence spectra. It is shown that mostly color centers of the F- and F⁺-types are formed in the α-Al₂O₃ lattice under irradiation with 5,7 MeV protons.     

The effect of annealing on the proton conductivity of Mg-doped α-Al2O3

In order to evaluate the effect of annealing treatment on the proton conductivity of ??-alumina, the electrical conductivity of Mg-doped polycrystalline ??-alumina kept at 1,873?K under various conditions of the surrounding atmosphere and then cooled in the furnace was measured in the temperature range 1,173?C1,473?K. The H⁺/D⁺ isotope effect on the electrical conductivity was also examined under a hydrogen atmosphere at 1,273?K. The protonic conductivity measured at 1,273?K increased with the increase in the activity of oxygen and water vapor in the annealing atmosphere at 1,873?K. It is considered that the solubility limit of magnesium ions in ??-alumina in equilibrium with the small amount of the spinel phase increased with the increase in the activity of oxygen and water vapor at 1,873?K. This enhanced amount of magnesium ions is frozen in a non-equilibrium state to 1,273?K and works as the enriched acceptor dopant for the incorporation of protons.     

Kinetic analysis of high temperature secondary thermoluminescence glow peaks in α-Al2O3:C

The kinetic analysis of secondary glow peaks in carbon-doped aluminium oxide is reported. A glow curve measured at 0.4 °C s⁻¹ after beta irradiation to 3 Gy revealed at least five peaks as a result of various techniques of glow curve resolution; the dominant peak at 156 °C (peak II) and two weaker-intensity secondary peaks one at 36 °C (peak I) and the other at 264 °C (peak III). Peaks IIA and IV at 170 and 422 °C respectively only became apparent after removal of preceding more prominent peaks. The secondary peaks are particularly weak in intensity and are as usual dominated by the main dosimetry peak. The analysis in this report focusses on peak III, usually seen adjacent to the main dosimetry peak but whose presence is masked by the extreme sensitivity of the latter. Complementary analyses of the weaker intensity peaks I, IIA and IV are included. Peaks I, IIA and III are subject to first-order kinetics while for peaks II and IV the issue is less conclusive. The activation energy increases from 0.72 eV for peak I to about 1.3 eV for peak IV with values for peak II and IIA similar at ∼1 eV. In general, the frequency factor corresponding to the lower temperature peaks (I, II, and IIA) have values (10¹⁰–10¹² s⁻¹) that are an order of magnitude or so greater than for peaks III and IV (10⁹–10¹¹ s⁻¹). Except for peak I, peak II and all other secondary peaks are affected by thermal quenching whose activation energy was determined as ΔE = 0.95 ± 0.04 eV using peak IIA and as ΔE = 1.48 ± 0.10 eV using peak III. The overall conclusion is that all peaks correspond to electron traps and are associated with the same recombination centre.     

Point defects related to 260 K thermostimulated luminescence in α-Al2O3

Abstract

Thermo- and photostimulated processes are studied in reduced hydrogen containing α-Al₂O₃ excited by UV light. It is found that UV excitation in F absorption band at 90 K results in a ionization of the F-centers and capture of released electrons at defects thus producing an anisotropy absorption band at 4.2 eV and the dominant thermoluminescence (TL) peak at 260 K. The 260 K TSL peak is accompanied by complete bleaching of the 4.2 eV absorption band and vice versa—by light stimulation in the region of the 4.2 eV band the 260 K TSL peak disappear and released electrons recombine with F⁺-centers. Both the effect of the preliminary high-temperature thermal treatment of samples on formation of 4.2 eV-centers and the observed dichroism characteristics allows to conclude that corresponding complex defect contains hydrogen and can involve vacancy pair.     

H2-induced changes in electrical conductance of β-Ga2O3 thin-film systems

H₂-induced changes of electrical conductivity in polycrystalline, undoped -Ga₂O₃ thin films in the temperature range of 400–650° C are described. The sheet conductance of these films depends reversibly, according to a power law p ^1/3, on the partial pressure of hydrogen in the ambient atmosphere of the Ga₂O₃ film. A bulk vacancy mechanism is excluded by experiments and it is shown that the interaction is based on a surface effect. Changes in conductance are discussed to result from the formation of an accumulation layer due to chemisorption on the grain surfaces. Typical coverages are determined to be approximately 10^–4 ML for pH₂=0.05 bar and T=600° C. A possible explanation of the p ^1/3 power law is provided.     

The fluorescence and thermoluminescence characteristics of α-Al2O3:C ceramics

This paper reports that polycrystalline α-Al₂O₃:C ceramics are fabricated by conventional ceramics process. The excitation, emission spectra, thermoluminescence (TL) and optical stimulated luminescence (OSL) of α-Al₂O₃:C polycrystalline ceramics are investigated. There exists 410nm maximum emission peak which is caused by recombination of an electron with F⁺ centre to produce an excited F centre. The samples show three clear TL glow peaks at 405, 493 and 610K. The attenuation constant of exponentially decaying fast component (τ₁) and slow component(τ₂) of OSL curve are 8.43s and 41.73s, respectively. Those fluorescence and thermoluminescence characteristics are similar with α-Al₂O₃:C crystal.