A new high-pressure polymorph of Ti2O3: implication for high-pressure phase transition in sesquioxides

The post-corundum phase transition has been investigated in Ti₂O₃ on the basis of synchrotron X-ray diffraction in a diamond anvil cell and transmission electron microscopy. The new polymorph of Ti₂O₃ was found at about 19 GPa and 1850 K, and this phase was stable even at about 40 GPa. A new polymorph of Ti₂O₃ can be indexed on a Pnma orthorhombic cell, and the unit-cell parameters are a=7.6965 (19) Å, b=2.8009 (9) Å, c=7.9300 (23) Å, V=170.95 (15) Å³ at 19 GPa, and a=7.8240 (2) Å, b=2.8502 (1) Å, c=8.1209 (3) Å, V=181.10 (1) Å³ at ambient conditions. The Birch–Murnaghan equation of state yields K ₀=206 (3) GPa and K′₀=4 (fixed) for corundum phase, and K ₀=296 (4) GPa and K′₀=4 (fixed) for the post-corundum phase. The molar volume decreases by 12% across the phase transition at around 20 GPa. The structural identification was carried out on a recovered sample by the Rietveld method, and a new polymorph of Ti₂O₃ can be identified as Th₂S₃-type rather than U₂S₃-type structure. The transition from corundum-type to Th₂S₃-type structure accompanies the drastic change of the form of polyhedron: from TiO₆ octahedron in the corundum-type to TiO₇ polyhedron in the Th₂S₃-type structures.     

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.     

Synthesis and thermoelectric properties of Cd-filled CoSb3

Cd-filled CoSb₃ samples have been synthesized by the high pressure method, and the temperature-dependent thermoelectric properties of Cd_xCo₄Sb₁₂ have been investigated from 300 to 600 K. X-ray diffraction results show that near single-phase CoSb₃ could be obtained by the high pressure method in a short time (20 min). The lattice constant increases with the increase in the Cd content. The power factor is improved and the thermal conductivity is depressed drastically by filling CoSb₃ with Cd. The maximum figure of merit reaches 0.54 at 600 K for the sample of Cd_0.38Co₄Sb₁₂, which is about four times higher than that of unfilled CoSb₃.     

The VUV‐VIS Spectroscopic Properties of Dy3+ Ion in Phosphors M5‐2XDyXNaX(PO4)3F (M=Ca,Sr, Ba) and Their Potential Applications in Mercury‐Free Lamps

The phosphors, fluorapatites M₅(PO₄)₃F (M=Ca, Sr, Ba) activated with Dy³⁺ ions, were prepared by a high‐temperature solid‐state reaction technique. The VUV‐UV excitation spectra and emission spectra under VUV/UV excitation were measured. The phosphors show broad and strong absorption near 172 nm and intensive emission with the chromaticity coordinates entering the white light region. Hence, the phosphors may be considered as suitable candidates for Hg‐free lamps.     

Theoretical investigations of the EPR g factors and the local structures for Ni3+ in LiAlyCo1–yO2 at various Al concentrations

The electron paramagnetic resonance (EPR) g factors and local structures for Ni³⁺ in LiAl_yCo_1–yO₂ as the cathode materials for lithium-ion batteries are theoretically investigated at various Al concentrations y (= 0.1, 0.5, 0.8 and 1.0). The [NiO₆]^9– clusters are subject to the tetragonal elongation distortions along the C₄ axis owing to the Jahn–Teller effect and the different degrees of distortions are characterised by the slightly increasing relative elongation ratio ρ (and also crystal-field strength and covalency) with y. In pure LiAlO₂ (y = 1.0), the [NiO₆]^9– cluster exhibits the largest ρ (≈1.4%) and an additional relative variation ratio τ (≈1.4%) of the planar bond lengths, responsible for the perpendicular g anisotropy δg = (g_x – g_y). The number of impurity Ni³⁺ centres increases from one at y = 0 and 1.0 to two at y = 0.1 and 0.8 and reaches the maximum four at y = 0.5, in accordance with the increasing degree of disorder of the systems. The distinct impurity centres for the same y may be accompanied by the comparable energies of the ground and near excited states, similar to ‘resonant states’ with slightly different local elongation distortions and tetragonal level splittings. The present studies will be helpful to understand the relationships between local structures and performances of LiAl_yCo_1–yO₂ type cathode materials.     

Fiber-coupled radioluminescence dosimetry with saturated Al2O3:C crystals: Characterization in 6 and 18 MV photon beams

Radioluminescence (RL) and optically stimulated luminescence (OSL) from carbon-doped aluminum oxide crystals can be used for medical dosimetry in external beam radiotherapy and remotely afterloaded brachytherapy. The RL/OSL signals are guided from the treatment room to the readout instrumentation using optical fiber cables, and in vivo dosimetry can be carried out in real time while the dosimeter probes are in the patient. The present study proposes a new improved readout protocol based solely on the RL signal from Al₂O₃:C. The key elements in the protocol are that Al₂O₃:C is pre-dosed with ∼20 Gy before each measurement session, and that the crystals are not perturbed by optical stimulation. Using 6 and 18 MV linear accelerator photon beams, the new RL protocol was found to have a linear dose-response from 7 mGy to 14 Gy, and dosimetry in this range could therefore be performed using a single calibration factor (∼6 × 10⁶ counts per Gy for a 2 mg crystal). The reproducibility of the RL dosimetry was 0.3% (one relative standard deviation) for doses larger than 0.1 Gy. The apparent RL sensitivity was found to change with accumulated dose ((−0.45 ± 0.03)% per 100 Gy), crystal temperature ((−0.21 ± 0.01)%/ °C), and dose-delivery rate ((−0.22 ± 0.01)% per 100 MU/min). A temporal gating technique was used for separation of RL and stem signals (i.e. Cerenkov light and fluorescence induced in the optical fiber cable during irradiation). The new readout protocol was a substantial improvement compared with the combined RL/OSL protocol, that required relatively long readout times and where the optical stimulation greatly affected the RL sensitivity. The only significant caveat was the apparent change in RL-response with accelerator dose-delivery rate.     

An experimental investigation on heat transfer enhancement in circular jet impingement on hot surfaces by using Al2O3/water nano-fluids and aqueous high-alcohol surfactant solution

The present article reports the heat transfer characteristics of a vertical stainless steel foil of 0.15 mm thickness (SS304) by circular impinging jets of various fluids such as pure water, nano-fluids (Al₂O₃-water, ф = 0.15%, 0.6%), and aqueous high-alcohol surfactant (HAS, i.e., 2-ethyl-hexanol, 100–400 ppm) studied using an infrared thermal imaging camera (A655sc, FLIR System). The enhancement in the heat transfer rates for Al₂O₃-water nano-fluids with ф = 0.15%, ф = 0.60%, and aqueous surfactant solution (150ppm) is found to be 140%, 207%, and 117% higher compared to pure water results, respectively. The surface characteristics of the foil after jet impingement by various fluids are also studied using scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), and surface wettability.     

Ab initio investigation of tensile strengths of metal(1 1 1)/α-Al2O3(0 0 0 1) interfaces

Ab initio calculations using plane wave pseudopotential method within density funtional theory are applied to investigate mechanical and electronic properties of Al-terminated Me(1 1 1)/Al₂O₃(0 0 0 1) (Me = Al, Ag, Cu, Nb) interfaces. Stress–displacement relationships of separation perpendicular to the interface are calculated. It is shown that obtained results such as work of separation and tensile strength can be understood from electronic structure.     

Active metal brazing of Al2O3 to Kovar® (Fe–29Ni–17Co wt.%) using Copper ABA® (Cu–3.0Si–2.3Ti–2.0Al wt.%)

The application of an active braze alloy (ABA) known as Copper ABA® (Cu–3.0Si–2.3Ti–2.0Al wt.%) to join Al₂O₃ to Kovar® (Fe–29Ni–17Co wt.%) has been investigated. This ABA was selected to increase the operating temperature of the joint beyond the capabilities of typically used ABAs such as Ag–Cu–Ti-based alloys. Silica present as a secondary phase in the Al₂O₃ at a level of ~5 wt.% enabled the ceramic component to bond to the ABA chemically by forming a layer of Si₃Ti₅ at the ABA/Al₂O₃ interface. Appropriate brazing conditions to preserve a near-continuous Si₃Ti₅ layer on the Al₂O₃ and a continuous Fe₃Si layer on the Kovar® were found to be a brazing time of ≤15 min at 1025 °C or ≤2 min at 1050 °C. These conditions produced joints that did not break on handling and could be prepared easily for microscopy. Brazing for longer periods of time, up to 45 min, at these temperatures broke down the Si₃Ti₅ layer on the Al₂O₃, while brazing at ≥1075 °C for 2–45 min broke down the Fe₃Si layer on the Kovar® significantly. Further complications of brazing at ≥1075 °C included leakage of the ABA out of the joint and the formation of a new brittle silicide, Ni₁₆Si₇Ti₆, at the ABA/Al₂O₃ interface. This investigation demonstrates that it is not straightforward to join Al₂O₃ to Kovar® using Copper ABA®, partly because the ranges of suitable values for the brazing temperature and time are quite limited. Other approaches to increase the operating temperature of the joint are discussed.     

First-principles study of structural,elastic, electronic and thermodynamic properties of topological insulator Bi2Se3 under pressure

The structural, elastic, electronic and thermodynamic properties of the rhombohedral topological insulator Bi₂Se₃ are investigated by the generalized gradient approximation (GGA) with the Wu–Cohen (WC) exchange-correlation functional. The calculated lattice constants agree well with the available experimental and other theoretical data. Our GGA calculations indicate that Bi₂Se₃ is a 3D topological insulator with a band gap of 0.287 eV, which are well consistent with the experimental value of 0.3 eV. The pressure dependence of the elastic constants C_ij, bulk modulus B, shear modulus G, Young’s modulus E, and Poisson’s ratio σ of Bi₂Se₃ are also obtained successfully. The bulk modulus obtained from elastic constants is 53.5 GPa, which agrees well with the experimental value of 53 GPa. We also investigate the shear sound velocity V_S, longitudinal sound velocity V_L, and Debye temperature Θ_E from our elastic constants, as well as the thermodynamic properties from quasi-harmonic Debye model. We obtain that the heat capacity C_v and the thermal expansion coefficient α at 0 GPa and 300 K are 120.78 J mol^?1 K^?1 and 4.70 × 10^?5 K^?1, respectively.     

Structural and electrical properties of lithium manganese oxide thin films grown by pulsed laser deposition

LiMn₂O₄ thin films were deposited by reactive pulsed laser deposition technique and studied the microstructural and electrical properties of the films. The LiMn₂O₄ thin films deposited in an oxygen partial pressure of 100 mTorr and at a substrate temperature of 573 K from a lithium rich target were found to be nearly stoichiometric. The films exhibited predominantly (111) orientation representing the cubic spinel structure with Fd3m symmetry. The intensity of (111) peak increased and a slight shift in the peak position was observed with the increase of substrate temperature. The lattice parameter increased from 8.117 to 8.2417 Å with the increase of substrate temperature from 573 to 873 K. The electrical conductivity of the films is observed to be a strong function of temperature. The evaluated activation energy for the films deposited at 873 K is 0.64 eV.     

Surface topography of ultrashort laser-irradiated CaF2

A single-crystal CaF₂ (111) was irradiated with single and multiple laser (Ti:sapphire, 800 nm, 25 fs) shots at fluences ranging from 0.25 to 1.5 J cm^?2. In this fluence regime, a single laser pulse usually leads to typical bump-like features ranging from 200 nm to 1.5 μm in diameter and 10–50 nm in height. These bumps are related to compressive stresses due to a pressure build-up induced by fast laser heating and their subsequent relaxation. When CaF₂ is irradiated with successive (in our case 20) shots at a laser fluence of 1.5 J cm^?2, nanocavities at the top of the microbumps are observed. The formation of these nanocavities is regarded as an explosion and is attributed to the explosive expansion generated by shock waves due to laser-induced plasma after the nonlinear absorption of the laser energy by the material. Such kinds of surface structures at the nanometre scale could be attractive for nanolithography.     

Simultaneous intracavity optical parametric oscillation and stimulated Raman scattering pumped by a doubly passively Q-switched Nd:GGG laser

By using a doubly passively Q-switched Nd:Gd₃Ga₅O₁₂(Nd:GGG) laser with Cr⁴⁺:YAG and GaAs as saturable absorbers as pump laser, simultaneous intracavity optical parametric oscillation and stimulated Raman scattering based on a single X-cut KTiOPO₄ (KTP) crystal have been realized. Under an incident diode pump power of 10.5 W, the output powers at the signal wave near 1,569 nm and the first Stokes emission near 1,094 nm were 218 and 72 mW, corresponding to the optical-to-optical conversion efficiencies of 2.08 and 0.69 %, respectively. The measured shortest pulse duration at the signal wave near 1,569 nm was 580 ps, generating a pulse peak power of 43.7 kW, while the minimum pulse duration at the first Stokes emission near 1,094 nm was 1.61 ns. By adjusting the tilt angle of the KTP crystal, up to the third Stokes scattering was also obtained.     

Amethyst and morion quartz gemstone raw materials from Turkey: color saturation and enhancement by gamma,neutron and beta irradiation

Color-enhancement investigations without using heating treatment from dull or pale to ideal saturation and/or changes to the formation of the rarer attractive colors are widely conducted to revalue abandoned gem material sources in the world. Such an investigation is carried out on pale or dull purple-colored amethyst and smoky-colored morion samples, which are two important gem species of the crystalline quartz (SiO₂) mineral that are currently abandoned in natural deposits in Turkey because of their unattractive coloration. The results of color enhancements observed on these samples, after irradiation with artificial gamma, neutron and beta beams, were examined by comparing with samples with the ideal color saturation and also with colorless samples, using optical absorption (OA) and radioluminescence (RL) spectroscopy. The ICP-AES analyses reveal that the main impurity elements of over 100 ppm in abundance in these quartz species are aluminum, iron and titanium for amethyst, and aluminum, iron, titanium and manganese for morion. The OA spectra indicate that vivid purple coloration of amethyst is due to the transmittance at about 395–420 nm band gap as a result of absorbance peaks at 375, 480 and 530 nm. These absorbances may be related to the unusual oxidized small proportions of certain impurity ions, after being exposed mainly to gamma irradiation, such as Al(IV) from the total aluminum, Ti(V) from the total titanium and Fe(IV) from the total iron, respectively. However, the RL spectroscopy of amethyst samples before and after they were exposed to artificial gamma, neutron and beta radiation beams demonstrates that the ions most affected by irradiation are Fe(IV) first and Al(IV) and Ti(V) second, and these ions represent the RL peaks at 600, 720 and 495 nm, respectively. The OA spectra indicate that dark smoky coloration in morion is due to a lack of transmittance at the visible region as a result of the absorbance peaks at 375, 450–490, 620 and 730 nm. These absorbances also may be related to the unusual oxidized small proportions of certain impurity ions by irradiation, such as Al(IV) from the total aluminum, Ti(V) from the total titanium and Mn(III) from the total manganese, respectively. In addition, the buoyancies of these absorbance peaks in the visible region produce the color hues between light smoky and dark smoky colorations in morion samples. These oxidized ion states are more resistant and stable against environmental destructive conditions in comparison with amethyst. Thus, the dark smoky coloration of morion becomes dull or pale after relatively longer periods. But, the RL spectroscopy of morion before and after being exposed to gamma, neutron and beta irradiation beams demonstrates that the most induced ions from the irradiation are Mn(III) and Al(IV) first and Ti(V) second. These ions represent the RL peaks at about 400, 720 and about 500 nm, respectively.     

Inverse Hall–Petch like behaviour in a mechanically milled nanocrystalline Al5Fe2 intermetallic phase

In the present investigation, the indentation study on the high-energy ball-milled nanocrystalline Al₅Fe₂ intermetallic compound has established the inverse Hall–Petch (IHP) behaviour. The structural characterisation of the milled powder particles by X-ray diffraction (XRD) and transmission electron microscopy has shown the evolution of nanocrystalline phase. Micro-indentation measurements have revealed the increase in hardness with decreasing grain size, reaching to a maximum of 9.0 ± 0.3 GPa up to a grain size of 32 ± 4 nm, followed by a decrease. The decrease in hardness with further refinement, an indication of grain size softening, demonstrates the IHP-like behaviour. The deviation from the Hall–Petch behaviour has been discussed using various models based on the dislocations and grain boundary-mediated processes. From the analysis, it appears that the model based on mesocopic grain boundary sliding phenomena is more appropriate to account for the observed grain size softening.     

The central role of oxygen on H-irradiated Lu2SiO5 luminescence

The behavior of self-trapped defects (STDs) in ion-beam irradiated Lu₂SiO₅ (LSO) crystal has been investigated via temperature-dependent radioluminescence (RL) measurements. Production of oxygen vacancies is the major effect of H⁺ irradiation on luminescencent properties of this phosphor. Luminescence centers for self-trapped exciton (STE) and self-trapped hole emission are assigned to oxygen vacancies and oxygen ions, respectively. Ion-induced structural damage modifies the thermal stability of the STDs and creates perturbed STEs. A striking effect of ion irradiation is the approximate factor-of-two enhancement of STE RL intensity that results from implantation of only a thin (∼250 nm) surface layer of LSO. This enhancement is attributed to ion-beam modification of a surface dead layer.     

One‐Dimensional Magnetic Composite of Polypyrrole‐Containing Carbon Nanotubes/Ni0.75Zn0.25Fe2O4

A novel one‐dimensional electromagnetic nanocomposite of polypyrrole (PPY) containing carbon nanotubes (CNTs)/Ni_0.75Zn_0.25Fe₂O₄ was synthesized by an in‐situ polymerization method. The composite was characterized by x‐ray diffraction (XRD), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM) and Fourier transform infrared. The XRD results confirmed that PPY, CNTs, and Ni_0.75Zn_0.25Fe₂O₄ coexisted in the composite. The TEM and HRTEM results indicated that PPY coated the surface of the CNTs/Ni_0.75Zn_0.25Fe₂O₄ with a thickness of 15–30 nm. The lattice spacings, according to the first main peak of the CNTs, Ni_0.75Zn_0.25Fe₂O₄, and PPY, was about 0.34 nm, 0.25 nm, and 0.42 nm, respectively. The FTIR result also indicated that the PPY formed in the composite. A test of magnetic properties indicated that the composite was ferromagnetic with the saturated magnetization of 12.86 electromagnetic units (emu)/g, and the coercive of 127.18 Oersted (Oe).