Synthesis of Ti3SiC2 by infiltration of molten Si

High-purity Ti₃SiC₂ compounds have been fabricated by infiltration of molten Si into a precursor, a partially sintered TiC_x (x = 0.67) preform. The Si source and the TiC_x preform were placed side by side on carbon cloth, and the system was heated to 1550 °C. Molten Si infiltrated the preform through the carbon cloth, and a direct reaction between TiC_x and molten Si immediately occurred at the reaction temperature to yield pure Ti₃SiC₂. We could observe phase formation and the microstructure of the bulk products with time, which were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM) equipped with energy-dispersive spectroscopy (EDS). Pure Ti₃SiC₂ compounds were formed on the exterior of the TiC_x preform at 1550 °C when the sintered TiC_x:Si ingot molar ratio was 3:1.4. At 1550 °C, no other minor phases were detected for any of the sintering time ranges.     

Temperature and field induced strain in polycrystalline Ni50Mn35In15−xSix magnetic shape memory Heusler alloys

Magnetic shape memory properties of polycrystalline Ni₅₀Mn₃₅In_15−xSi_x were investigated. A reversible strain of more than 0.4% was observed for x = 0 at a magnetic field H = 5 T that was found to be associated with a field induced reverse martensitic transformation. The strains were found to increase with the substitution of In by Si and strains larger than 1% were observed for x = 2 at H = 5 T. Both the positive and negative strain changes were observed in the vicinity of martensitic transition temperatures. The strain in Ni₅₀Mn₃₅In_15−xSi_x was found to depend on silicon concentration, and on samples texture.     

Structure and high-temperature oxidation behaviour of Cu-Ni-Fe alloys prepared by high-energy ball milling for application as inert anodes in aluminium electrolysis

Cu_xNi_85−xFe₁₅ (0 ? x ? 85 wt.%) compounds were prepared by mechanical alloying. Monophased face centered cubic (fcc) Cu-Ni-Fe alloys were obtained after 10 h of milling for x varying from 0 to 50, whereas bi-phased compounds fcc Cu-Ni-Fe + body centered cubic (bcc) Fe were formed with richer-Cu compounds. Their oxidation kinetics in air at 750 °C is parabolic for all compositions and increases drastically for x > ∼30. A stable anode for aluminium electrolysis in low-temperature (700 °C) KF-AlF₃ electrolyte was obtained for 65 ? x ? 85. However, a substantial increase of the Cu contamination in produced aluminium was observed for x > 70.     

Structural, optical and transport properties of Al doped BiFeO3 nanopowder synthesized by solution combustion method

Aluminum doped Bismuth ferrite (BFO) nanopowders (grain size 13-20 nm) having composition Bi_1−xAl_2xFe_1−xO₃ (x = 0.00, 0.025, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30) were successfully synthesized by solution combustion method using citric acid as fuel at a temperature as low as 200 °C. As-prepared samples were examined by powder XRD for phase identification and crystallite size determination. The d.c. resistivity as a function of temperature was measured by standard two probe setup which exhibits clear metal to insulator transition for all samples. FTIR analysis was carried out to identify the chemical bonds present in the system. The optical band gap was calculated from the UV-vis absorbance spectra using classical Tauc relation which was found to vary from 2.78 eV to 2.93 eV for different Al³⁺ concentrations. The activation energies calculated from the slopes of ln(ρ) versus 10³/T plots are in the range 0.54-0.73 eV.     

Microstructure and martensitic transformation of cast TiNiSi shape memory alloys

The martensitic transformation behavior, second phases and hardness of Ti₅₁Ni_49−xSi_x shape memory alloys (SMAs) with x = 0, 1 and 2 at.% are investigated. The transformation temperature of one stage martensitic reaction B2 ↔ B19′ is associated with the forward (M_s) and reverse (A_s) martensitic transformations, respectively. All experimental DSC results such as martensitic transformation peaks (M*) and reverse martensitic transformation peaks (A*) are increased and became sharper with increasing Si-content. The microstructure investigation of the studied SMAs (Ti₅₁Ni_49−xSi_x) showed that there are two types of precipitated second phase particles. The first one is Ti₂Ni which mainly located at grain boundaries and intermetallic compound of Ti₂(Ni + Si) phase distributed inside the matrix. The volume fraction of these two phases is increased with Si content. Additionally, a small amount of Si remained in solid solution of the matrix of Ti₅₁Ni_49−xSi_x SMAs. Moreover, hardness of Ti₅₁Ni_49−xSi_x SMAs is increased as the Si-content increases.     

Dependence of magnetic properties on the film thickness and Pt atomic fraction of post-annealed Fe1−xPtx-C granular films on MgO(1 0 0) and SiO2/Si(1 0 0) substrates

The Fe_1−xPt_x-C granular films with different Pt atomic fractions (0.09 ≤ x ≤ 0.52) and film thicknesses (5 nm ≤ t ≤ 100 nm) were deposited on MgO(1 0 0) and SiO₂/Si(1 0 0) substrates by facing-target sputtering and post-annealing. With the increasing x, the ordered L1₀ FePt grains form. All of the films are ferromagnetic, and the easy axis is in the film plane. With the decrease of t, the films turn from hard ferromagnetic to soft ferromagnetic. The maximum coercivity of the 100-nm thick Fe_1−xPt_x-C granular films measured at a 10-kOe field is 3.7 kOe at x = 0.48. The coercivity of the Fe_0.56Pt_0.44-C granular films increases, and the magnetization measured at a 10-kOe field decreases with the increasing t. The reversal mechanism of the 100-nm thick Fe_1−xPt_x-C granular films turns from the domain wall motion to the Stoner-Wohlfarth rotation mode as x increases. However, the reversal mechanism of the Fe₅₆Pt₄₄-C granular films with different t approaches the Stoner-Wohlfarth rotation mode, and is film-thickness independent.     

Structural and magnetic nanoclusters in Cu50Zr50−xGdx (x = 5 at.%) metallic glasses

It is shown that phase-separated metallic glasses on the nanoscale can be prepared by rapid quenching of Cu₅₀Zr_50−xGd_x melts with a low concentration of gadolinium (x = 5 at.%). Gd-enriched clusters of 2 nm size are formed as early stages of decomposition in the deeply undercooled melt. The key physical parameter to obtaining such a nanoclustered microstructure upon quenching is the critical temperature of liquid-liquid phase separation which has to be close to the glass transition temperature. Thus, the thermodynamic properties of the liquid phase even in the metastable deeply undercooled melt essentially determine the structure formation. Analysis of the spatial atomic arrangement by atom probe tomography after annealing in the supercooled liquid state provides direct evidence of the spinodal character of the decomposition by uphill diffusion. The Gd-enriched nanoclusters exhibit ferromagnetic ordering below 50 K and the cluster size regime derived from magnetization measurements is in good agreement with that obtained from atom probe tomography investigations. The first stage of crystallization of Cu₅₀Zr₄₅Gd₅ glass is observed to be Ostwald-type ripening on a nanoscale. The phase-separated glass acts as a precursor for the formation of a metastable nanocrystalline structure.     

Effect of tunneling barrier as spacer on exchange coupling of CoFeB/AlOx/Co trilayer structures

Magnetic tunneling junctions (MTJs) have a sandwiched structure, which comprises a top ferromagnetic (FM1) layer, an insulating tunneling layer (spacer), and a bottom ferromagnetic (FM2) layer. Exchange coupling in MTJs has been extensively widely examined because the effect of spacer thickness on the ferromagnetic spin-coupling can be exploited in read-head sensors, spin-valve structures, and magntoresistance random access memories (MRAMs). In this investigation, MTJs were deposited in the sequence, glass/CoFeB(50 Å)/AlO_x(d)/Co(100 Å), where the thickness of the AlO_x layer d = 12, 17, 22, 26 or 30 Å. Saturation magnetization (M_s) results demonstrate that the exchange coupling strength and coercivity (H_c) can be varied considerably by varying the tunneling barrier AlO_x spacer. The X-ray diffraction patterns (XRD) include a main peak from hexagonal close-packed (HCP) Co with a highly (0 0 2) textured structure at 2θ = 44.7°, and AlO_x and CoFeB are amorphous phases. The full width at half maximum (FWHM) of the Co (0 0 2) peak declines as the AlO_x thickness increases, revealing that the Co layer becomes more crystalline. The magnetic results reveal that the magnetic characteristics are related to the Co crystallinity. The exchange coupling strength increases with AlO_x thickness. The coercivity (H_c) also increases, because the Co crystallinity is eliminated.     

Ferromagnetism in ZnTe:Cr film grown on Si(1 0 0)

Zn_1−xCr_xTe (x = 0.0 and 0.05) films were grown on Si(1 0 0) substrate by using thermal evaporation method. The structure of the films was investigated by X-ray diffraction and it showed the formation of ZnCrTe phase with an amorphous background, which indicated poor crystallinity. Composition analysis by XPS disclosed the presence of antiferromagnetic Cr₂O₃ and Cr precipitates. Magnetic domains were observed by using magnetic force microscopy at ambient temperature and the result showed anisotropic domains with an average size of 3.5 nm. Magnetic field dependence of magnetic moment measurements showed obvious hysteresis loop with a coercive field of 121 Oe at 300 K. Temperature dependence of magnetic moment showed short-range ferromagnetic order. The Curie temperature was estimated to be 354.5 K.     

Composition dependence of magnetic properties of directly quenched Nd-Fe-Ti-Zr-B bulk magnets

Magnetic properties, phase evolution, and microstructure of directly quenched Nd_yFe_97−y−zTi_3−xZr_xB_z (x = 0-3; y = 7-10; z = 14-19) bulk magnets of 0.9 mm in diameter have been investigated. Proper Zr substitution for Ti and appropriate Nd and B contents modify the magnetic phases constitution and refine the grain size from 200-250 nm to 50-100 nm. Consequently, the magnetic properties of the rods are enhanced remarkably from _iH_c = 6.2 kOe and (BH)_max = 5.6 MGOe for Zr-free rods to _iH_c = 6.7-13.5 kOe and (BH)_max = 6.7-8.2 MGOe for Zr-substituted Nd_yFe_97−y−zTi_3−xZr_xB_z rods (x = 0.5-2; y = 8-10; z = 14-16). The optimum magnetic properties of B_r = 6.6 kG, _iH_c = 9.6 kOe and (BH)_max = 8.2 MGOe were achieved for Nd_9.5Fe_72.5Ti_2.5Zr_0.5B₁₅ alloy.     

The effects of vacuum annealing on the structure of VO2 thin films

Noncrystalline VO_x thin films were deposited onto p-doped Si (100) substrates at 400 °C using magnetron sputtering. By vacuum annealing, we obtained polycrystalline VO₂ thin films with two different structures under a variety of annealing conditions. With the annealing temperature increasing and the annealing time developing, structures of the films underwent the following transformation: amorphous structure→metastable VO₂ (B)→VO₂ (B) + VO₂ (M). Vacuum annealing is useful of acquiring VO₂ thin films with high surface quality, but too high annealing temperature (500 °C) and too long time (15 h) are harmful, which make the surface degenerate.     

Luminescent properties of UV excitable blue emitting phosphors MSr4(BO3)3:Ce (M = Li and Na)

A series of Ce³⁺ doped novel borate phosphors MSr₄(BO₃)₃ (M = Li or Na) were successfully synthesized by traditional solid-state reaction. The crystal structures and the phase purities of samples were characterized by powder X-ray diffraction. The optimal concentrations of dopant Ce³⁺ ions in compound MSr₄(BO₃)₃ (M = Li or Na) were determined through the measurements of photoluminescence spectra of phosphors. Ce³⁺ doped phosphors MSr₄(BO₃)₃ (M = Li or Na) show strong broad band absorption in UV spectral region and bright blue emission under the excitation of 345 nm light. In addition, the temperature dependences of emission spectra of M_1+xSr_4−2xCe_x(BO₃)₃ (M = Li or Na) phosphors with optimal composition x = 0.05 for Li and x = 0.09 for Na excited under 355 nm pulse laser were also investigated. The experimental results indicate that the M_1+xSr_4−2xCe_x(BO₃)₃ (M = Li or Na) phosphors are promising blue emitting phosphors pumped by UV light.     

Synthesis of Ti3SiC2 bulks by infiltration method

T₃SiC₂ bulks have been synthesized by infiltrating Si liquid into porous precursor pellets composed of solid TiC and Ti powders. Silicon pellets were placed at the bottom of the precursor pellets as the liquid source. The starting compositions can be represented by the formula 2TiC + Ti + xSi, where x = 1.0, 1.2, 1.5 and 1.8, respectively. The phase formation and microstructure of the bulks were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM) equipped with energy-dispersive spectroscopy (EDS) system. The results demonstrated that the TiC/Ti precursor pellet could only react with Si completely when the x value is 1.8. Impurities SiC, Ti-Si binary compounds and Ti₈C₅ appeared along the silicon diffusion direction. It is found that the compositions of impurities strongly depended on the Si-concentration. Reaction mechanism of this Ti₃SiC₂ infiltration synthesis has also been discussed based on the Si-concentration changes on the diffusion path.     

Quantum confinement effects and band gap engineering of SnO2 nanocrystals in a MgO matrix

Nanocrystal formation of SnO₂ in xSnO₂-(100 − x)MgO (x in mol.%) nanocrystalline composite thin films is reported. SnO₂ and MgO exhibit strong immiscibility behavior below 750 °C, leading to controllable particle size of SnO₂ in the MgO matrix by changing their composition. The particle size of SnO₂ can also be controlled by increasing the annealing temperature. Above 750 °C MgO and SnO₂ react to yield a MgO-Mg₂SnO₄-SnO₂ composite in which the size of the SnO₂ nanophase increases with increasing temperature. By controlled choice of the composition and annealing conditions the band gap of SnO₂ can be continuously increased from 3.89 to 4.5 eV. We discuss this behavior in terms of the quantum confinement effect. The method provides a generic approach to tuning the band gap in nanocomposite systems over a wide energy range.     

Structural, electronic, optical and thermodynamic properties of SrxCa1−xO, BaxSr1−xO and BaxCa1−xO alloys

The structural, electronic, optical and thermodynamic properties of Sr_xCa_1−xO, Ba_xSr_1−xO and Ba_xCa_1−xO ternary alloys in NaCl phase were studied using pseudo-potential plane-wave method within the density functional theory. We modeled the alloys at some selected compositions with ordered structures described in terms of periodically repeated supercells. The dependence of the lattice parameters, band gaps, dielectric constants, refractive indices, Debye temperatures, mixing entropies and heat capacities on the composition x were analyzed for x = 0, 0.25, 0.50, 0.75 and 1. The lattice constant for Sr_xCa_1−xO and Ba_xSr_1−xO exhibits a marginal deviation from the Vegard's law, while the Ba_xCa_1−xO lattice constant exhibits an appreciable upward bowing. A strong deviation of the bulk modulus from linear concentration dependence was observed for the three alloys. The microscopic origins of the gap bowing were detailed and explained. The composition dependence of the dielectric constant and refractive index was studied using different models. The thermodynamic stability of these alloys was investigated by calculating the phase diagram. The thermal effect on some macroscopic properties was investigated using the quasi-harmonic Debye model. There is a good agreement between our results and the available experimental data for the binary compounds which may be a support for the results of the ternary alloys reported here for the first time.     

Optical properties of sputtered hexagonal CdZnO films with band gap energies from 1.8 to 3.3 eV

Cd_xZn_1−xO films in a wide range of Cd contents have been grown by reactive direct-current magnetron sputtering. At x ≤ 0.66, the sputtered Cd_xZn_1−xO films are of single hexagonal phase. The optical band gap energies of the Cd_xZn_1−xO films decrease from ∼3.3 eV at x = 0 to ∼1.8 eV at x = 0.66. Correspondingly, the near-band-edge photoluminescence is tuned in a wide visible region from ∼378 to 678 nm. Moreover, the ultraviolet irradiation enhanced PL from the Cd_xZn_1−xO films has been observed.     

Self-assembling into interconnected nanoribbons in thin films of hairy rod poly(9,9-di(2-ethylhexyl)fluorene): Effects of concentration,substrate and solvent

This study utilizes atomic force microscopy to investigate the self-assembling behaviors from dilute solution into thin film of a well-known conjugated polymer, poly(9,9-di(2-ethylhexyl)fluorene) (PF2/6). We have found that the structures of nanoscale aggregates depend on various experimental parameters including concentration, substrate and solvent. The self-assembling of PF2/6 from 0.05 mg/mL solution in toluene onto SiO_x/Si substrate results in the formation of interconnected nanoribbons with thickness and width of about 20 and 150 nm, respectively. Varying polymer concentration and type of substrate (SiO_x/Si or mica) significantly affects the nanoscale structures. The change of solvent to chlorobenzene, a slightly more polar solvent with slower evaporation rate, causes the growth of ribbon width to micron size with slight increase of the thickness. When the solvents with higher evaporation rate (i.e. chloroform and dichloromethane) are used, densely packed nanoribbons are obtained. Its width also grows to micron size. The measurements of UV/vis absorption and photoluminescence spectra detect some discrepancies in pattern, reflecting the variation of local chain organization within thin films prepared by using different solvents.     

Chemical synthesis, structural and magnetic properties of nano-structured Co-Zn-Fe-Cr ferrite

Nanoparticles of Co_1−xZn_xFe_2−xCr_xO₄ (x = 0.0-0.5) ferrites were prepared by chemical co-precipitation technique using metal sulphates. The structural and magnetic properties were investigated by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), vibrating sample magnetometer (VSM), and AC susceptibility measurements. X-ray diffraction patterns indicate that the samples possess single phase cubic spinel structure. The lattice constant initially increases for x ≤ 0.3 and thereafter for x > 0.3 it decreases with increasing x. The saturation magnetization (Ms), magneton number (n_B) and coercivity (Hc) decreases with increasing Cr-Zn content x. Curie temperature deduced from AC susceptibility data decreases with increasing x.     

Magnetic properties of Sn-Mg substituted strontium hexaferrite nanoparticles synthesized via coprecipitation method

Nanoparticles of Sn-Mg substituted strontium hexaferrite with the composition of SrFe_12−x(Sn_0.5 Mg_0.5)_xO₁₉ (x = 0.0-1.0) were synthesized by chemical coprecipitation method. Deionized water/ethanol (50/50) was used as the solvent. The single phase strontium hexaferrites were obtained at pH 13 and Fe³⁺/Sr²⁺ molar ratio of 9 after calcination at 800 °C. The mean particle size of samples was decreased from 82 to 56 nm with increasing the Sn-Mg content from x = 0.0 to x = 0.8. The effect of Sn-Mg substitution on magnetic properties of hexaferrites was studied using vibrating-sample magnetometer. It was found that increasing the Sn-Mg from x = 0.0 to x = 0.8 reduced the coercivity from 4728.9 to 1455.5 Oe and increased the saturation magnetization from 51.34 to 65.49 emu/g. A vector network analyzer was used to investigate the microwave absorption properties. According to microwave measurements, doped strontium hexaferrite composites had much more effective electromagnetic absorption properties than undoped strontium hexaferrite composite.