Microstructure evolution process of Ferro-Aluminum based sandwich composite for electromagnetic shielding

In this paper, sandwich composite (SWC) with Fe–Al soft magnetic alloy sandwiched between pure iron substrates was proposed and fabricated by hot pressing and diffusion treatment. The microstructure evolution process of the composite was investigated. Fe/Fe₂Al₅/Fe diffusion couple was obtained at 700 °C and subsequently kept at 900 °C for further isothermal diffusion. During the diffusion reactive process, we confirmed that major FeAl₂ and minor Fe₄Al₁₃ were produced when Fe₂Al₅ dissolved. After 10 h of diffusion treatment, FeAl and α-Fe(Al) were the only two intermetallic phases left. Except FeAl₂, the thickness of each intermetallic layer held good parabolic relationship with the diffusion annealing time.     

Effect of substrate temperature on few-layer graphene grown on Al2O3 (0 0 0 1) via direct carbon atoms deposition

Few-layer graphene (FLG) was grown on Al₂O₃ (0 0 0 1) substrates at different temperatures via direct carbon atoms deposition by using solid source molecular beam epitaxy (SSMBE) method. The structural properties were characterized by reflection high energy electron diffraction (RHEED), Raman spectroscopy and near-edge X-ray absorption fine-structure (NEXAFS). The results showed that the FLG started to form at the substrate temperature of 700 °C. When the substrate temperature increased to 1300 °C, the quality of the FLG was the best and the layer number was estimated to be less than 5. At higher substrate temperature (1400 °C or above), the crystalline quality of the FLG would be deteriorated. Our experiment results demonstrated that the substrate temperature played an important role on the FLG layer formation on Al₂O₃ (0 0 0 1) substrates and the related growth mechanism was briefly discussed.     

Microstructure of multilayer interface in an Al matrix composite reinforced by TiNi fiber

A multilayer interface was formed in the Al matrix composite which was reinforced by 30% volume fraction of TiNi fiber. The composite was fabricated by pressure infiltration process and the interface between the TiNi fiber and Al matrix was investigated by transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS). When the TiNi fiber was pre-oxidized in the air at 773 K for 1 h, three layers have been found and characterized in the interface: TiNi–B₂ layer near the TiNi fiber, Ti–Al compound layer with Ti and granular TiO₂ near the Al matrix, and Ti–Ni compound layer between TiNi–B₂ and Ti–Al compound layers. The effect of the multilayer interface on the mechanical properties of the composite was also discussed. The result showed that the uniaxial tensile strength of the composite at room temperature was 318 MPa, which was very close to the theoretical calculation value of 326 MPa. Moreover, the composite with good ductility exhibited a typical ductile-fracture pattern.     

Hardness and microstructural variation of Al–Mg–Mn–Sc–Zr alloy

Variations of Vickers hardness were observed in Al–Mg–Mn alloy and Al–Mg–Mn–Sc–Zr alloy at different ageing times, ranging from a peak value of 81.2 HV at 54 ks down to 67.4 HV at 360 ks, below the initial hardness value, 71.8 HV at 0 ks for the case of Al–Mg–Mn–Sc–Zr alloy. Microstructures of samples at each ageing stage were examined carefully by transmission electron microscopes (TEMs) both in two-dimensions and three-dimensions. The presence of different types, densities, and sizes of particles were observed dispersed spherical Al₃Sc_1−xZr_x and also block-shaped Al₃Sc precipitates growing along <1 0 0>_Al with facets {1 0 0} and {1 1 0} of the precipitates. TEM analysis both in two-dimensions and three-dimensions, performed on various samples, confirmed the direct correlation between the hardness and the density of Al₃Sc.     

Synthesis and photoluminescence properties of YVO4:Eu3+, Al3+ phosphor

The Y_0.95?xAl_xVO₄:5%Eu³⁺ (0≤x≤0.1) phosphors were successfully synthesized by solid state reaction at 900 °C for 6 h, and their luminescence properties were investigated under UV and VUV excitation. Monitoring at 619 nm, a strong broad absorption was enhanced by co-doping of Al³⁺ into the YVO₄:Eu³⁺ lattices at 256 nm under UV excitation. The VUV excitation spectra also showed the enhanced excitation bands at about 156 and 200 nm. Under 254 or 147 nm excitation, it was found that Y_0.95?xAl_xVO₄:Eu³⁺(0≤x≤0.1) phosphors showed strong red emission at about 619 nm corresponding to the electric dipole ⁵D₀^–7F₂ transition of Eu³⁺. The improvement of luminescence intensity of YVO₄:Eu³⁺ was also observed after partial substituting Y³⁺ by Al³⁺ and the optimal luminescence intensity appeared with incorporation of 2.5 mol% Al³⁺.     

Electrical conductivity and thermal expansion of the oxy-cuspidine Gd4Al2O9 substituted with Ca and Sr

The series of Gd_4 ? xM_xAl₂O_9 ? x/2 (M = Ca, Sr) with x = 0, 0.01, 0.05, 0.10 and 0.25 was prepared by the citrate complexation method. Both Gd_4 ? xCa_xAl₂O_9 ? x/2 and Gd_4 ? xSr_xAl₂O_9 ? x/2 show the monoclinic cuspidine structure with space group of P2₁/c up to 0.05–0.1 and 0.01–0.05 mol for Ca and Sr, respectively. Beyond the substitution limit of Gd₄Al₂O₉, GdAlO₃ and SrGd₂Al₂O₇ appear as additional phases. The highest electrical conductivity obtained at 900 °C yielded σ = 1.49 × 10^? 4 S/cm for Gd_3.95Ca_0.05Al₂O_8.98. In comparison, the conductivity of pure Gd₄Al₂O₉ was σ = 1.73 × 10^? 5 S/cm. The conductivities determined are in a similar range as those of other cuspidine materials investigated previously. The thermal expansion coefficient of Gd₄Al₂O₉ at 1000 °C was 7.4 × 10^? 6 K^? 1. The phase transition between 1100 and 1200 °C reported earlier changes with increasing substitution of Ca and Sr.     

Surface reactions of TiCl4 and Al(CH3)3 on GaAs(100) during the first half-cycle of atomic layer deposition

GaAs(100) was exposed to pulses of trimethylaluminum (TMA, Al(CH₃)₃) and titanium tetrachloride (TiCl₄) to mimic the first half-cycle of atomic layer deposition (ALD). Both precursors removed the 9.0 ± 1.6 Å-thick mixed oxide consisting primarily of As₂O₃ with a small Ga₂O component that was left on the surface after aqueous HF treatment and vacuum annealing. In its place, TMA deposited an Al₂O₃ layer, but TiCl₄ exposure left Cl atoms adsorbed to an elemental As layer. This suggests that oxygen was removed by the formation of a volatile oxychloride species. A small TiO₂ coverage of approximately 0.04 monolayer remained on the surface for deposition temperatures of 89 °C to 135 °C, but no TiO₂ was present from 170 °C to 230 °C. The adsorbed Cl layer chemically passivated the surface at these temperatures and blocked TiO₂ deposition even after 50 full ALD cycles of TiCl₄ and water vapor. The Cl and As layers desorbed simultaneously at higher temperature producing peaks in the temperature programmed desorption spectrum in the range 237–297 °C. This allowed TiO₂ deposition at 300 °C in single TiCl₄ pulse experiments. On the native oxide-covered surface where there was a higher proportional Ga oxide composition, TiCl₄ exposure deposited TiO₂.     

Photoluminescence of Lu3Al5O12:Bi and Y3Al5O12:Bi single crystalline films

Single crystalline films of Lu₃Al₅O₁₂:Bi and Y₃Al₅O₁₂:Bi have been studied at 4.2–450 K by the time-resolved luminescence spectroscopy method. Their emission spectrum consists of two types of bands with strongly different characteristics. The ultraviolet band consists of two components, arising from the electronic transitions which correspond to the ³P₁ → ¹S₀ and ³P₀ → ¹S₀ transitions in a free Bi³⁺ ion. At T < 80 K, mainly the lower-energy component with the decay time ～10^?3 s is observed, arising from the metastable ³P₀ level. At T > 150 K, the higher-energy component prevails, arising from the thermally populated emitting ³P₁ level. The visible emission spectrum consists of two dominant strongly overlapped broad bands with large Stokes shifts. At 4.2 K, their decay times are ～10^?5 s and ～10^?4 s and decrease with increasing temperature. Both of the visible emission bands are assumed to be of an exciton origin. The lower-energy band is ascribed to an exciton, localized near a single Bi³⁺ ion. The higher-energy band, showing a stronger intensity dependence on the Bi³⁺ content, is assumed to arise from an exciton, localized near a dimer Bi³⁺ center. The structure of the corresponding excited states is considered, and the processes, taking place in these states, are discussed.     

One step ultrafast mechanosynthesis of nanocrystalline cubic Ti0.9Al0.1B and its microstructure evolution

Nanocrystalline single phase cubic Ti_0.9Al_0.1B has been prepared at room temperature in a minimum duration of 4 h by mechanical alloying the stoichiometric mixture of Ti, Al and B powders in a high energy planetary ball mill under argon atmosphere. The Rietveld's structure refinement of X-ray diffraction data reveals that cubic Ti–Al–B phase is initiated just after 1 h of milling and at the same time α-Ti (hcp) phase partially transforms to metastable β-Ti (bcc) phase. In the course of milling, ordered Ti–Al–B lattice gradually transforms to a distorted state and the degree of distortion increases with milling time up to 15 h. The formation of cubic Ti_0.9Al_0.1B is also confirmed from the selected area electron diffraction (SAED) pattern. Microstructure characterization by high resolution transmission electron microscopy (HRTEM) reveals that Ti–Al–B nanoparticles are isotropic in nature with average particle size ~4.5 nm and is in good agreement with the value obtained from the Rietveld analysis of X-ray diffraction data.     

Structural and electrical properties of a high-k SmAlO3 charge trapping flash memory

In this study, we proposed the Al/Al₂O₃/SmAlO₃/SiO₂/Si flash memory devices using high-k SmAlO₃ film as a charge trapping layer and high-k Al₂O₃ film as a blocking layer. The structural and morphological features of these films were explored by X-ray diffraction, X-ray photoelectron spectroscopic and atomic force microscopy. The SmAlO₃ flash memory devices annealed at 800 °C showed excellent electrical properties, such as a large memory window of ～2.61 V (measured at a sweep voltage range of ±5 V) and a small charge loss of ～7.1% (measured time up to 10⁴ s). In addition, the charge trap centroid and charge trap density were extracted by constant current stress method.     

Magnetic properties of rare-earth transition metal aluminides R6T4Al43 with Ho6Mo4Al43-type structure

The magnetic properties of 53 aluminium-rich intermetallic compounds R₆T₄Al₄₃ with R=rare-earth elements and T=Ti, V, Nb, Ta, Cr, Mo, W were investigated using polycrystalline samples and a SQUID magnetometer in the temperature range from 2 to 300 K with magnetic flux densities up to 5.5 T. The yttrium and lutetium compounds are Pauli paramagnetic, indicating that the transition metal atoms do not carry magnetic moments. The samarium compounds show van Vleck behavior and antiferromagnetic order with Néel temperatures of less than 12 K. Of these Sm₆Ti₄Al₄₃ becomes metamagnetic. The ytterbium compounds show a mixed or intermediate valent behavior and no magnetic order down to 2 K. All other compounds obey the Curie–Weiss law above 30 K. Their effective magnetic moments correspond to the theoretical moments of the rare-earth ions. They show ferromagnetic or metamagnetic behavior with ordering temperatures all below 20 K. The magnetization curves of most compounds (recorded up to 5.5 T) reach about 50% of the theoretical magnetization already at 0.5 T. The gadolinium compounds are exceptional in that they reach at 0.5 T only about 10% of their theoretical magnetization. The crystal structures of the isotypic compounds Yb₆V₄Al₄₃ and Yb₆Ta₄Al₄₃ were refined from single-crystal X-ray data.     

Application of laser in seam welding of dissimilar steel to aluminium joints for thick structural components

Laser welding-brazing technique, using a continuous wave (CW) fibre laser with 8000 W of maximum power, was applied in conduction mode to join 2 mm thick steel (XF350) to 6 mm thick aluminium (AA5083-H22), in a lap joint configuration with steel on the top. The steel surface was irradiated by the laser and the heat was conducted through the steel plate to the steel-aluminium interface, where the aluminium melts and wets the steel surface. The welded samples were defect free and the weld micrographs revealed presence of a brittle intermetallic compounds (IMC) layer resulting from reaction of Fe and Al atoms. Energy Dispersive Spectroscopy (EDS) analysis indicated the stoichiometry of the IMC as Fe₂Al₅ and FeAl₃, the former with maximum microhardness measured of 1145 HV 0.025/10. The IMC layer thickness varied between 4 to 21 μm depending upon the laser processing parameters. The IMC layer showed an exponential growth pattern with the applied specific point energy (E_sp) at a constant power density (PD). Higher PD values accelerate the IMC layer growth. The mechanical shear strength showed a narrow band of variation in all the samples (with the maximum value registered at 31.3 kN), with a marginal increase in the applied E_sp. This could be explained by the fact that increasing the E_sp results into an increase in the wetting and thereby the bonded area in the steel-aluminium interface.     

Epitaxial growth of well-ordered ultra-thin Al2O3 film on NiAl (1 1 0) by a single-step oxidation

Well-ordered ultra-thin Al₂O₃ films were grown on NiAl (1 1 0) surface by exposing the sample at various oxygen absorption temperatures ranging from 570 to 1100 K at dose rates 6.6 × 10⁻⁵ and 6.6 × 10⁻⁶ Pa. From the results of low-energy electron diffraction (LEED), Auger electron spectrometer (AES) and X-ray photon spectroscopy (XPS) observations, it was revealed that oxidation mechanism above 770 K is different from well-known two-step process. At high temperature, oxidation and crystallization occurred simultaneously while in two-step process oxidation and crystallization occurred one after another. At high-temperature oxidation well-ordered crystalline oxide can be formed by a single-step without annealing. Well-ordered Al₂O₃ layer with thickness over 1 nm was obtained in oxygen absorption temperature 1070 K and a dose rate 6.6 × 10⁻⁶ Pa at 1200 L oxygen.     

Solid state interactions in nano-sized oxides

Comparative study of reactivity of nano- and micro-sized alumina and nickel oxide, obtained by the electrical explosion of metal wires in oxidizing atmosphere, was carried out for the reactions NiO + MoO₃, NiO + Al₂O₃, and Al₂O₃ + Bi₂O₃ by coupled anneals of ceramics, measurements of the conductivity of individual oxides and raw oxide mixtures, X-ray diffraction and differential thermal analysis. The total conductivity of nano-structured oxides was found lower than that of micro-structured ceramics. Mixing bismuth oxide with nano-structured alumina leads to stabilization of the low temperature polymorph α-Bi₂O₃ up to 780 °C. The diffusion permeability of NiMoO₄ layer grown at the surface of NiO ceramics, having submicron grains, was found 2 times lower if compared to NiMoO₄ grown at micro-sized NiO ceramics. NiO and Al₂O₃ nano-powders preserve the high reactivity even when heated up to 1000 °C. The results are discussed in terms of size effects on the solid state reactivity of oxides.     

Giant magnetic refrigerant capacity in Ho3Al2 compound

Magnetic properties and magnetocaloric effects (MCEs) of the intermetallic Ho₃Al₂ compound are investigated by magnetization and heat capacity measurements. Two successive magnetic transitions, a spin-reorientation (SR) transition at T_SR=31 K followed by a ferromagnetic (FM) to paramagnetic (PM) transition at T_C=40 K, are observed. Both magnetic transitions contribute to the MCE and result in a large magnetic entropy change (ΔS_M) in a wide temperature range. The maximum values of ?ΔS_M and adiabatic temperature change (ΔT_ad) reach 18.7 J/kg K and 4.8 K for the field changes of 0–5 T, respectively. In particular, a giant value of refrigerant capacity (RC) is estimated to be 704 J/kg for a field change of 5 T, which is much higher than those of many potential refrigerant materials with similar transition temperatures.     

Luminescent properties of Eu2+and Dy3+ ions in Ba4Al2O7 phosphor for solid state lighting

In this paper we report the combustion synthesis of rare earth (RE=Eu, Dy) doped Ba₄Al₂O₇ phosphors. Prepared phosphors were characterized by X-ray powder diffraction (XRD), scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR), CIE color co-ordinates and their photoluminescence (PL) properties were also investigated. In case of Ba₄Al₂O₇: Eu²⁺, the emission spectra show unique band centered at 495 nm, which corresponds to the 4f⁶5d¹→4f⁷ transition of Eu²⁺, and PL emission spectra of Dy³⁺ ion under 348 nm excitation give two bands centered at 478 nm (blue) and 575 nm (yellow), which originate from the transitions of ⁴F_9/2→⁶H_15/2 and ⁴F_9/2→⁶H_13/2 of Dy³⁺, respectively. The results indicate that the Eu²⁺ and Dy³⁺ activated Ba₄Al₂O₇ phosphor could find application in solid state lighting.     

Kinetic studies of electronically conducting Yb3Al5O12 garnet

Upon reduction, originally fully transparent and insulating ytterbium alumina garnet single crystals, Yb₃Al₅O₁₂, become deeply colored and electrically conducting with a conductivity of the order of 10^− 3 Ω^− 1 cm^− 1 in the temperature range of 550 °C to 1000 °C. The redox kinetics of the material is studied by means of conductivity relaxation experiments performed at oxidising and reducing conditions. Good agreement is obtained with an optical study into the redox kinetics of Yb₃Al₅O₁₂.     

Investigation of enhancement mode HfO2 insulated N-polarity GaN/InN/GaN/In0.9Al0.1N heterostructure MISHEMT for high-frequency applications

In this paper, we examined normally-OFF N-polar InN-channel Metal insulated semiconductor high-electron mobility transistors (MISHEMTs) device with a relaxed In_0.9Al_0.1N buffer layer. In addition, the enhancement-mode operation of the N-polar structure was investigated. The effect of scaling in N-polar MISHEMT, such as the dielectric and the channel thickness, alter the electrical behavior of the device. We have achieved a maximum drain current of 1.17 A/mm, threshold voltage (V_T) =0.728 V, transconductance (g_m) of 2.9 S mm⁻¹, high I_ON/I_OFF current ratio of 3.23×10³, lowest ON-state resistance (R_ON) of 0.41 Ω mm and an intrinsic delay time (τ) of 1.456 Fs along with high-frequency performance with f_t/ f_max of 90 GHz/109 GHz and 180 GHz/260 GHz for T_CH =0.5 nm at V_ds =0.5 V and 1.0 V. The numerically simulated results of highly confined GaN/InN/GaN/In_0.9Al_0.1N heterostructure MISHEMT exhibits outstanding potential as one of the possibility to replace presently used N-polar MISHEMTs for delivering high power density and frequency at RF/power amplifier applications.     

Characteristics of CeCoIn5/Al/AlOx/Nb and CeCoIn5/Al/AlOx/Al tunnel junctions

We report characteristics of CeCoIn₅/Al/AlO_x/Nb and CeCoIn₅/Al/AlO_x/Al tunnel junctions fabricated on the (0 0 1) surface of CeCoIn₅ crystal platelets. The main result of this work is the observation of a low Josephson current (as compared with that expected from the Ambegaokar–Baratoff formula), which is consistent with idea that the order parameter in the heavy-fermion superconductor CeCoIn₅ has unconventional pairing symmetry.