Thermal stability of in situ synthesized (TiB + La2O3)/Ti composite

Thermal stability of in situ synthesized (TiB + La₂O₃)/Ti composite is investigated. The phase analysis is identified by X-ray diffraction. Microstructure of the melted and forged titanium matrix composites (TMCs) after heat treatment is characterized by means of optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The room temperature tensile properties after an additional thermal exposure at 873 K, 923 K or 973 K for 100 h are tested. After the thermal exposure, the strength of specimen increases and ductility decreases. This is attributed to precipitation of ordered α₂ phase (Ti₃Al) and S₁ (silicide) in the titanium matrix composites after the thermal exposure.     

Effect of annealing on microstructure and mechanical properties of bulk nanocrystalline Fe3Al alloy with 5 wt.% Cu prepared by aluminothermic reaction

Microstructure and mechanical properties of bulk nanocrystalline Fe₃Al based alloy with 5 wt.% Cu prepared by aluminothermic reaction before and after annealed at 873, 1073 and 1273 K for 8 h were investigated. Microstructures of the alloy before and after the annealing consisted of a Fe-Al-Cu matrix, a little Al₂O₃ sphere and Fe₃AlC_x fiber phases. The matrix of the alloy before the annealing was composed a nanocrystalline phase with disordered bcc crystal structure and a little amorphous phase. The amorphous phase disappeared after the annealing and Fe₃Al phase with ordered DO₃ structure appeared in the alloy after annealed at 1073 and 1273 K in the matrix of the alloy. Size of the Fe₃AlC_x fiber phase increased with the annealing temperature. The alloy after the annealing had better plasticity, higher yield strength than that of the alloy before the annealing, and the alloy after annealed at 1273 K had the highest yield strength.     

Fluorite-like phases in the BaF2-BiF3-Bi2O3 system—synthesis, conductivity and defect clustering

A fluorite-like solid solution Ba_1 − xBi_xO_zF_{2 + x − 2z} on the basis of cubic BaF₂ was synthesised in the BaF₂-Bi₂O₃-BiF₃ system and the homogeneity range at 873 K was determined. The samples were studied by X-ray powder diffraction and electron diffraction, and their transport properties were measured by the complex impedance method at 300-623 K. Tendencies of variation of lattice parameters and transport properties were determined. These tendencies are discussed on the basis of a defect clustering hypothesis. Thermal treatment at 573 K of the solid solution, quenched from 873 K results in the formation of a new ordered tetragonal fluorite-like phase with lattice parameters a = 9.5355(4) Å, c = 18.151(1) Å.     

Ba2.1Bi0.9(O, F)6.8−δ: A new ordered anion-excess fluorite

The anion-excess ordered fluorite-related phase Ba_2.1Bi_0.9(O, F)_6.8−δ has been synthesized by a solid state reaction of BaF₂, BiF₃ and Bi₂O₃ at 873 K with subsequent short annealing at 573 K. The crystal structure of the new phase has been solved using electron diffraction and X-ray powder diffraction (a = 9.5372(1) Å, c = 18.1623(3) Å, space group I4/m, R_I = 0.025, R_P = 0.029). Interstitial anions in the fluorite-based structure are considered to form isolated cuboctahedral 8:12:0 clusters. The structural relationship between the oxyfluoride phase Ba_2.1Bi_0.9(O, F)_6.8−δ and similar rare-earth-based fluorides is discussed.     

Synthesis of multiwalled carbon nanotubes by high-temperature vacuum annealing of amorphous carbon

Vacuum annealing of a mixture of amorphous carbon and cobalt nanoparticles supported on microporous zeolite at high-temperature results in the formation of multiwalled carbon nanotubes which are essentially filled with metal nanoparticles or nanowires as observed by transmission electron microscopy. The electronic properties of nanotubes by variable temperature ESR techniques shows that g values show little change with temperature from 77 to 327 K but the line width (ΔH_pp) of the ESR signal for nanotubes synthesized from amorphous carbon increases from 7.9 G at 77 K to 9.5 G at 327 K.     

Preparation and dielectric properties of polycrystalline films with dense nano-structured BaTiO3 by chemical vapor deposition using inductively coupled plasma

To clarify the dielectric properties of BaTiO₃ with nanometer size region, it is necessary to fabricate the dense structure composed of BaTiO₃ nanoparticles. In the present study, BaTiO₃ nanoparticles were directly deposited on Pt/Al₂O₃/SiO₂/Si substrate by introducing Ba(DPM)₂ and Ti(OiPr)₄ into an inductively coupled plasma (ICP). The optimal condition for preparing dense structure of BaTiO₃ nanoparticles was investigated by changing the substrate temperature. Single phase BaTiO₃ of perovskite structure was obtained at the substrate temperatures between 773 and 1173 K. The dense structure of BaTiO₃ nanoparticles with particle sizes of about 30 nm was successfully obtained at the substrate temperature of 773 K. At the substrate temperature>873 K, the deposited nanoparticles sintered to be the columnar structure. The ε_r and tan δ of the BaTiO₃ nanoparticles were estimated to be 285 and 6.6%, respectively (1 kHz and 100 mV). The phase of the BaTiO₃ nanoparticles were found to be paraelectric by the measurement of C-V curves. The breakdown field of the dense structure of BaTiO₃ nanoparticles was estimated to be 649 kV/cm according to I-V curves. These features are favorable for applying the structure to the dielectric layer of multilayer capacitors.     

Direct patterning of SnO2 composite films prepared with various contents of Pt nanoparticles by photochemical metal-organic deposition

The optical and electrical characteristics of SnO₂ composite films with various contents (0, 0.05, 0.1, 0.2, and 0.3 at.%) of Pt nanoparticles were evaluated. The Pt nanoparticles were synthesized by a methanol reduction method and their average size was controlled to 3 nm using poly(N-vinyl-2-pyrrolidone) as a protecting agent. The lowest resistivity of 2.031 × 10^− 2 Ω cm was obtained in the SnO₂ film containing 0.2 at.% Pt nanoparticles after annealing at 700 °C while its average transmittance in the visible region was 85.24%. The enhanced electrical properties were attributed to the increase of the carrier concentration and crystallinity of the films due to donation from Pt nanoparticles as well as the increased annealing temperature. Meanwhile, the slight degradation of the transmittance was due to scattering from the introduction of Pt nanoparticles and the increased crystallite size due to the increase of the annealing temperature to 700 °C. Well-defined 20-μm wide direct-patterned composite SnO₂ films containing Pt nanoparticles were formed by a simple photochemical metal-organic deposition process involving a photosensitive starting precursor, UV exposure, and removal of the unpatterned area by rinsing with solvent. Based on the results of this study, we suggest that direct-patternable SnO₂ films with Pt nanoparticles can be easily applied to transparent electrodes in electrical devices without requiring an expensive and toxic process such as dry etching.     

Tribological behavior of radio-frequency sputtering WS2 thin films with vacuum annealing

Thin films of tungsten disulfide (WS₂) were deposited on 3Cr13 martensitic stain less steel substrate by radio-frequency (RF) sputtering. The as-deposited films were annealed at 473, 673 and 873 K respectively for 2 h in 5 × 10^− 4 Pa vacuum. Composition of the films was inspected by energy dispersive spectroscopy. Surface morphology and structure properties were studied by scanning electron microscopy and X-ray diffraction techniques. Tribological behavior was also examined using tribometer. At 473 K, the films exhibited low crystallization structure and no significant improvement in the tribological performance. At 673 K, the tribological performance was improved and a transition from non-crystalline to hexagonal structure took in place. When the annealing temperature rose up to 873 K, the films cracked and fell off from the substrate. The results suggested that with suitable technical parameters vacuum annealing could promote crystallization and improve tribological performance of RF sputtering WS₂ films.     

Ultra-thin zinc oxide film on Mo(100)

Zinc oxide films on a single crystal Mo(100) substrate were fabricated by annealing the pre-deposited metal Zn films in 10^− 5-10^− 4 Pa O₂ ambience at 300-525 K, and were characterized by in situ Auger electron spectroscopy, electron energy loss spectroscopy, low energy electron diffraction and high-resolution electron energy loss spectroscopy. The results show that the atomic ratio of oxygen to zinc in zinc oxide film is significantly dependent on sample annealing temperature and O₂ pressure. A stoichiometric zinc oxide film has been obtained under ∼10^− 4 Pa O₂ at about 400 K. A redshift of Fuchs-Kliewer phonon energy correlated with surface oxygen deficiency is observed.     

The Morin transition in nanostructured pseudocubic hematite: Effect of the intercrystallite magnetic exchange

The Morin transition in α-Fe₂O₃ particles with homogeneous pseudocubic morphology of about 1.8 μm side has been studied by different techniques. The particles are made up of nanometric crystallites with a narrow size distribution. Samples annealed at 673, 773 and 873 K, yielded Morin transition temperatures, T_M, of about 230, 241, and 245 K, respectively, which are lower than the bulk value, T_M ≈ 263 K. In addition, the temperature range ΔT_M through which these samples undergo the transition is very narrow with respect to the values reported in the literature. Ranges of 20, 10 and 8 K, were obtained for the samples annealed at 673, 773 and 873 K. In this work, through the estimation of the exchange magnetic energy, we demonstrate that the existence of the Morin transition, and its shift and breadth are related to the presence of intercrystallite interactions within the pseudocubic particles. These intercrystallite interactions are modified by the annealing treatments, which help to remove non-stoichiometric hydroxyl groups and incorporated water molecules.     

Preparation and electrical properties of Ga-doped ZnO nanoparticles by a polymer pyrolysis method

In this article, preparation of Ga-doped zinc oxide (GZO) nanoparticles by a polymer pyrolysis method is reported. The pyrolysis behaviors of the polymer precursors prepared via the in situ polymerization of metal salts and acrylic acid are analyzed using thermalgravity-differential scanning calorimetry (TG-DSC) techniques. Then, the structural characteristics of the products are studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM). It is revealed by the results that the GZO nanoparticles calcined at 600 °C show good crystallinity with the wurtzite structure. GZO nanoparticles doped with 4 mol% Ga have a mean particle size of 26 nm with spherical-like morphology. Electrical resistivity measurement shows that, before and after high temperature annealing under H₂ atmosphere, the resistivity of the GZO nanoparticles is decreased by one and four orders in magnitude, respectively, compared with the pure ZnO nanoparticles. In addition, due to its versatility, this in situ polymer pyrolysis method can be easily extended to synthesis of other n-type doped semiconductor, such as In and Al doped ZnO or Sb doped SnO₂.     

Microstructure, martensitic transformation and superelasticity of Ti49.6Ni45.1Cu5Cr0.3 shape memory alloy

The aim of this study is to investigate the microstructure, martensitic transformation behavior, shape memory effect and superelastic property of Ti_49.6Ni_45.1Cu₅Cr_0.3 alloy, with Cu and Cr substituting for Ni. After annealing, the alloy showed single step A-M/M-A transformations within the whole annealing temperature range of 623 K to 1273 K even in the presence and Ti₂(Ni, Cu) precipitates. With the increase of the annealing temperature, the transformation temperatures exhibited three stages: increasing from 623 K to 873 K, decreasing from 873 K to 1023 K and unchanging from 1023 K to 1273 K. Meanwhile, the critical stress for stress induced martensitic (SIM) transformation decreased to a minimum value and increased after that, exhibiting a V shape curve. The alloy annealed at 623, 773 and 923 K exhibited shape recovery ratio more than 90% when the deformation strain was below 20%.     

High-temperature properties of extruded titanium composites fabricated from carbon nanotubes coated titanium powder by spark plasma sintering and hot extrusion

Pure titanium matrix composite reinforced with carbon nanotubes (CNTs) was prepared by spark plasma sintering and hot extrusion via powder metallurgy process. Titanium (Ti) powders were coated with CNTs via a wet process using a zwitterionic surfactant solution containing 1.0, 2.0 and 3.0 wt.% of CNTs. In situ TiC formation via reaction of CNTs with titanium occurred during sintering, and TiC particles were uniformly dispersed in the matrix. As-extruded Ti/TiCs composite rods were annealed at 473 K for 3.6 ks to reduce the residual stress during processing. After annealing process, the tensile properties of the composites were evaluated at room temperature, 473, 573 and 673 K, respectively. Hardness test was also performed at room temperature up to 573 K with a step of 50 K. The mechanical properties of extruded Ti/CNTs composites at elevated temperature were remarkably improved by adding a small amount of CNTs, compared to extruded Ti matrix. These were due to the TiC dispersoids originated from CNTs effectively stabilized the microstructure of extruded Ti composites by their pinning effect. Moreover, the coarsening and growth of Ti grain never occurred even though they were annealed at 573, 673 K for 36 ks and 673 K for 360 ks, respectively.     

Effects of annealing time on the structural and magnetic properties of L10 FePt thin films

Thermal annealing of [Fe 1.65 nm/Pt 1.84 nm]₅₀ multilayers at 673 K for various annealing times between 60 and 12000 s leads to the direct formation of the fully ordered L1₀ FePt phase with (111) texture. The average grain sizes, determined from X-ray diffraction size-strain analysis, are smaller than the critical size for multi-domain FePt particles, suggesting the presence of single-domain (SD) grains. The coercivity increases with annealing time and increasing grain size and reaches values of about 955 kA/m. The remanence values are typical for randomly oriented weakly-interacting particles. A decrease of the remanence with annealing time suggests a decrease of the intergrain exchange interactions with annealing time. Analysis of minor loops and the initial magnetization curves shows the presence of a broad distribution of critical fields, which the individual SD particles have to overcome for the magnetization reversal.     

Microstructural, mechanical and magnetic properties of shape memory alloy Ni55Mn23Ga22 thin films deposited by radio-frequency magnetron sputtering

The near-stoichiometric Ni₂MnGa ferromagnetic alloys are one of the smart materials, that are of a great interest when they are deposited as a thin film by r.f. sputtering. These thin films of shape memory alloys are prospective materials for micro and nanosystem applications. However, the properties of the shape memory polycrystalline thin films depend strongly on their structure and internal stress, which develop during the sputtering process as well as during the post-deposition annealing treatment. In this study, about 1 μm Ni₅₅Mn₂₃Ga₂₂ thin films were deposited in the range 0,45 to 1,2 Pa of Ar pressure and P = 40 to 120 W. Their composition, crystallographic structure, internal stress and stress gradient, indentation modulus, hardness, deflection induced by magnetic field and magnetic properties were systematically studied as a function of the temperature of the silicon substrate ranging from 298 to 873 K and the vacuum annealing treatment at 873 K for 21,6 ks and 36 ks. A silicon wafer having a native amorphous thin SiO_x buffer layer was used as a substrate. This substrate influences the microstructure of the films and blocks the diffusion process during the heat treatment.The crystal structure of the martensitic phase in each film was changed systematically from bct or 10 M or 14 M. In addition, the evolution of the mechanical properties such as mean stress, stress gradient, roughness, hardness and indentation modulus with the temperature (of substrate or of heat treatment) were measured and correlated to crystal structure and morphology changes.Moreover, it has been shown that it is necessary to associate a high temperature (873 K) annealing during a long time (21 ks and 36 ks) to obtain good ferromagnetic properties. Thus, for the well annealed films (36 ks at 873 K) the magnetostrain is about - 170 ppm for a magnetic field of 1 MA m^- 1 applied along the beams.As a conclusion, the response of free-standing magnetic shape memory films to a magnetic field of 0,2 MA m^- 1 depends strongly on the martensitic structure, internal mechanical stress (mean and gradient) and magnetic properties. The free-standing annealed film at 873 K for 36 ks points out a considerable magnetic actuation associated with bct or 10 M or 14 M martensitic structures.     

Effect of room-temperature aging on shape memory characteristics of Ti–10Nb–10Zr–11Ta (at.%) alloy

In this study, the effect of room-temperature (RT) aging on the shape memory characteristics of the Ti–10Nb–10Zr–11Ta (at.%) alloy was investigated by tensile tests. Ingots were prepared using the arc melting method and then cold-rolled at a reduction of up to 95%. After cold-rolling, the plates were solution-treated at 1173 K for 1.8 ks, followed by aging at RT and temperatures up to ∼573 K for various periods of times. Superior superelasticity was observed at RT in the solution-treated specimen. The critical stress for inducing martensitic transformation (σ_SIM), tensile strength, and critical stress for slip (σ_S) of specimens aged at RT increased with increasing aging time up to 60 days, showing no noticeable changes with further increases in the aging time. On the other hand, in the specimens aged at 373 K, 423 K, and 473 K for 3.6 ks, the values of σ_SIM and the tensile strength increased with increasing aging temperature, while the specimen aged at 573 K exhibited mature fractures. There were little change in σ_SIM and σ_S of the specimen that was solution-treated followed by aging at 373 K for 3.6 ks during RT aging. This result indicated that aging at 373 K resulted in good resistance against the effect of RT aging.     

Magnetization studies on ?-Fe2.4Co0.6N nanoparticles

?-Fe_2.4Co_0.6N nanoparticles are prepared by nitridation of ultrafine iron-cobalt oxide in NH₃ gas atmosphere at 873 K for 3 h. Their magnetic properties are studied using ac and dc magnetization techniques. Two peaks at 69 and 151 K are found in ac susceptibility study. The former peak is assigned to superparamagnetic blocking temperature (T_B ∼ 69 K), whereas the origin of the latter one could not be ascertained since it is observed only from imaginary part of ac susceptibility study. The positions of peaks are found to be dependent on the applied fields and frequencies. Also, dc magnetization studies corroborate T_B. Anisotropy constant (K_a) is found to be 2.3 × 10⁵ erg/cm³. The peaks disappear at high-applied field due to the overcome of applied filed over anisotropic energy. Ferromagnetic domains rotate incoherently. At low temperature, Bloch law cannot be applied, but Curie law can be applied above T_B. Interestingly, spin relaxation is faster at 10 K as compared to that at 70 K indicating the presence of frustration of spins at lower temperature.     

Microstructure and magnetic behavior of electrodeposited CoPt thick films upon annealing

CoPt films with a thickness about 5-μm consisting of a disordered face-centered cubic phase have been prepared by using the electrodeposition technique. X-ray diffraction measurements and transmission electron microscopy studies reveal that the annealed CoPt films possess a nanocomposite microstructure of A1 + L1₀ CoPt and the size and volume fraction of the L1₀ ordered phase increase with annealing time. A strong magnetic exchange coupling between the A1 phase and the L1₀ ordered phase is observed after annealing for 2.5 h, which yields a large coercivity H_c = 14.2 kOe and a high remanence ratio M_r/M_s = 0.88 for the nanocomposite thick films of A1 + L1₀ CoPt.     

Low-resistance p-type ohmic contacts for high-power InGaAs/GaAs-980 nm CW semiconductor lasers

In this paper, the optimization of ohmic contacts for semiconductor lasers based on InGaAs/GaAs/GaAlAs layers is reported. Transmission electron microscopy (TEM) and electrical methods were used to study extensively the Pt/Ti/Pt/Au metallization system. The contact fabrication technology was optimized towards achieving the lowest electrical resistance. The technological control and optimization concerned the contact annealing temperature and thickness of metallic layers that form the contact. The average specific contact resistance was below 5×10⁻⁶ Ω cm² (with the record value of 8×10⁻⁷ Ω cm²) for the 10 nm Pt/20 nm Ti/30 nm Pt/150 nm Au system. The presented system was used in fabrication of continuous wave (CW) operated laser diodes. The chips mounted on passively cooled copper block achieved optical powers over 1 W, threshold current density values of 140-160 A/cm² and differential efficiencies above 1 W/A. The value of the characteristic temperature T₀ for discussed lasers varied in the range of 180-200 K.     

Polycrystalline SnO2 nanowires coated with amorphous carbon nanotube as anode material for lithium ion batteries

One-dimensional (1D) SnO₂ nanowires, coated by in situ formed amorphous carbon nanotubes (a-CNTs) with a mean diameter of ca. 60 nm, were synthesized by annealing the anodic alumina oxide (AAO) filled with a sol of SnO₂. X-ray diffraction (XRD) and selected area electron diffraction (SAED) patterns revealed that the prepared SnO₂ nanowires exist in polycrystalline rutile structure. The coating of carbon nanotubes has some defects on the wall after the internal SnO₂ nanoparticles were removed. The 1D SnO₂ nanowires present a reversible capacity of 441 mAh/g and an excellent cycling performance as an anode material for lithium ion batteries. This suggests that 1D nanostructured materials have great promise for practical application.