Synthesis of nanocrystalline Bi2Te3 via mechanical alloying

Bi₄₀Te₆₀ thermoelectric compound was fabricated via mechanical milling of bismuth and tellurium as starting materials. Effect of the milling time and heat treatment temperatures were investigated. In order to characterize the ball milled powders, the X-ray diffraction (XRD) was used. Thermal behavior of the mechanically alloyed powders was studied by differential thermal analysis (DTA). The morphological evolutions were studied by scanning electron microscopy (SEM). Results showed that the nanocrystalline Bi₂Te₃ compound was formed after 5 h of milling. Further milling (25 h) and heating to 500 °C showed that the synthesized phase was stable during these conditions. Nanocrystalline Bi₂Te₃ with 9–10 nm mean grain size and flaky morphology (lamellar structure) was obtained at the end of milling.     

Magnetic and transport properties of Laves phase Dy1−xMmxCo2 (x = 0–0.5) alloys

The influence of Mm substitution (Mm = mischmetal) on structural, transport and magnetic properties of (Dy_1?xMm_x)Co₂ (x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5) alloys has been investigated by means of X-ray diffraction (XRD), temperature dependent electrical resistivity (ρ(T)), ac susceptibility (χ(T)) and thermopower (S(T)). XRD patterns show the formation of solid solutions crystallizing with cubic Laves (C15) type structure at room temperature. The pronounced discontinuities in the resistivity and thermopower at Curie temperature (T_C) are explained based on the suppression of the spin-fluctuation contribution. The gradual decrease in T_C and sharpness of discontinuities in ρ(T) and S(T) with increasing Mm substitution has been discussed.     

Optical diffraction of second harmonic generation in SrBi2(Nb0.7V0.3)2O9 in the SrO–Bi2O3–0.7Nb2O5–0.3V2O5–Li2B4O7 glass system

Transparent glasses in the system (100 − 3x)(Li₂O–4B₂O₃)–x(SrO–Bi₂O₃–0.7Nb₂O₅–0.3V₂O₅) (where x = 10, 30 and 50, in molar ratio) embedded with nanocrystallites of SrBi₂(Nb_0.7V_0.3)₂O₉ exhibited intense second harmonic signals in transmission mode when exposed to IR laser light at λ = 1064 nm. The second harmonic waves were found to undergo optical diffraction. The origin of optical diffraction in these samples was attributed to the self organised structures of fine crystallites of submicrometer size that were inscribed in-situ by the IR laser radiation. Laser Raman studies confirmed these crystallites to be vanadium doped strontium bismuth niobate.     

Characterization of mechanically alloyed nanocomposites in TiO2–B2O3–Mg–C quaternary system

The influence of the simultaneous presence of magnesium and graphite on mechanosynthesis of various nanocomposite powders in TiO₂–B₂O₃–Mg–C quaternary system was investigated. A mixture of boron oxide and titanium dioxide powders along with different amounts of magnesium and graphite was milled using a high-energy planetary ball mill to provide necessary conditions for the occurrence of a mechanically induced self-sustaining reaction (MSR). In the absence of C (100 wt.% Mg), TiB₂ nanopowder was formed as a result of combustion reaction after 34 min of milling. In the presence of both Mg and C, the mechanochemical reaction was completed after different milling times depending on the weight fraction of the reducing agents in the powder mixture. In the presence of x wt.% Mg–y wt.% C (x = 85 and 90; y = 100 − x), the mechanosynthesized composites contained TiB₂ and TiC as major compounds as well as MgO and Mg₃B₂O₆ as unwanted phases. With further increasing the graphite content to 30 wt.%, no mechanical activation was observed after 90 min of milling. The nanocomposite powders showed a bimodal particle size distribution characterized by the presence of several coarse particles (≈ 250 nm) along with finer particles with a mean size of about 75 nm. Formation mechanism of nanocomposites was explained through the analysis of the relevant sub-reactions.     

Synthesis and characterization of the Mg-based amorphous/nano ZrO2 composite alloy

Mg-based composites are fabricated through mechanical alloying (MA) in the planetary mill, using amorphous Mg₆₅Cu_25−xY₁₀Ag_x (x=0, 5, 10) matrix alloy prepared by melt spun and 1–5 vol.% spherical nano-sized ZrO₂ particles. The melt spun amorphous matrix ribbons are ground into powders and mixed with the ZrO₂ nano particles in the planetary mill, after then formed by hot pressing in Ar atmosphere under different pressures at the temperature 5 K above the glass transition temperature (T_g). The microstructure characterizations of the resulting specimens are conducted by means of XRD, FEG-SEM, and TEM techniques. It is found that the nano-sized ZrO₂ dispersed Mg-based composite alloy powders can reach to a homogeneous size distribution (about 80 nm) after 50 h mechanical alloying. After hot pressing of these composite alloy powders under the pressure of 1100 MPa at 409 K, a 96% dense bulk specimen can be formed. Throughout the MA and hot pressing, the amorphous nature of the Mg₆₅Cu_25−xY₁₀Ag_x matrix is maintained. The hardness of the formed bulk Mg-based composites (with 5 vol% nano-sized ZrO₂ particles) can reach to 360 in H_v scale. In addition, the microstructure near the interface between the matrix and nano particles presents a well-bonded condition.     

Crystal structure and X-ray diffraction pattern of CuSnTi3 intermetallic phase

A new ternary compounds of Cu, Sn, and Ti, CuSnTi₃, was found in this study, by isothermally treating Cu–23 at.%Ti–17 at.%Sn alloy at 900 °C for 10 h. This new ternary intermetallic compound has a hexagonal structure with a=0.4636 nm and c=0.5229 nm. Its crystal is of the same structure as that of Ni₃Sn₂, both of which have a space group of P6₃/mmc (hP6, No.194). In such a crystal structure, Cu and Sn are both arranged in the lattice positions of 2c (x=1/3, y=2/3, z=1/4, occ.=0.5), and Ti is arranged in 2a (x=0, y=0, z=0, occ.=1) and 2d (x=1/3, y=2/3, z=3/4, occ.=0.5).     

Magnetic and structural properties of BaFe12−xGaxO19 nanoparticles

The structural and magnetic properties of barium hexaferrite nanoparticles (BaFe_12?xGa_xO₁₉) with x = 0.0–1.0, prepared by ball milling were investigated using XRD, TEM, and VSM. It was found that the particles and crystallites have similar mean size of ～41 nm for all investigated samples. The saturation magnetization decreased slightly and nonlinearly with increasing x, and this was attributed to different preferential site occupation of Ga at low and high concentration ranges. The coercivity decreased slightly with increasing x for low concentrations of Ga (x ≤ 0.2), and then increased with increasing Ga concentration up to x = 1.0. This behavior of the coercivity was attributed to the change in the exchange coupling, which was confirmed by the variation of SFD, remanence ratio and Curie temperature with Ga concentration in the samples.     

Dislocation interaction of layers in the Ge/Ge-seed/GexSi1−x/Si(0 0 1) (x ∼ 0.3–0.5) system: Trapping of misfit dislocations on the Ge-seed/GeSi interface

High-resolution electron microscopy has been applied to study the dislocation redistribution between Ge and GeSi layers at the atomic scale. Ge_0.3Si_0.7 (30 nm in thickness) and Ge_0.5Si_0.5 (10 nm) buffer layers buried between the Si(0 0 1) substrate and the plastically relaxed Ge layer 0.5 μm thick remain in a metastable (stressed) state during the growth of Ge/Ge-seed/Ge_xSi_1?x/Si(0 0 1) (x ～ 0.3–0.5) heterostructures, though the buffer layer thickness is several times greater than the critical value for insertion of misfit dislocations (MDs). An ordered grid of edge MDs is observed only on the Ge/GeSi interface; the mean distance between the MDs is ～10 nm (which is close to the equilibrium value for the non-stressed Ge/Si system). After 30 min of annealing at 700 °С, the Ge_0.3Si_0.7 buffer layer still remains in a metastable state, and the edge MDs are located only on the Ge/GeSi interface with the same dislocation spacing of ～10 nm. At the same time, approximately one-half of MDs in the structure with the Ge_0.5Si_0.5 buffer layer passes through the Ge/GeSi interface to the GeSi/Si(0 0 1) interface, and the buffer layer plastically relaxes by almost 100%. An assumption is put forward that there exists a barrier for the MD transition from the Ge layer to the GeSi layer, which results in MD trapping on this interface. The magnitude of this barrier depends on the difference in the compositions of the main Ge (x = 1) film and the Ge_xSi_1?x buffer layer, and increases with increasing this difference.     

Crystal structure of τ5–TiNi2−xAl5 (x = 0.48) and isotypic {Zr,Hf}Ni2−xAl5−y

The crystal structure of the compound in the Al-rich region of the Ti–Ni–Al system, τ₅–TiNi_2?xAl₅, x = 0.48, has been derived from X-ray powder and single crystal, neutron powder and electron diffraction (space group I4/mmm, a = 0.3984(2) nm, c = 1.4073(3) nm, R_F2 = 0.0133). Titanium atoms were unambiguously located from neutron powder data. τ₅ is isotypic with the crystal structure of ZrNi₂Al₅. Detailed transmission electron microscopy (TEM) in several crystallographic directions confirmed the lattice parameters and crystal symmetry. Although occupancy of Ni in the 4e site revealed a defect (occ. = 0.76), no significant homogeneity region was observed for this phase at 1020°C. Rietveld analyses of X-ray powder diffraction data for the Zr- and Hf-homologues confirmed for both compounds isotypism and revealed defects in the Ni sites and to a lesser extent also in the Al sites: ZrNi_2?xAl_5?y, x = 0.4, y = 0.4 and HfNi_2?xAl_5?y, x = 0.5, y = 0.2. The crystallographic relations among the structure types of Cu, TiAl₃, ZrNi₂Al₅ and Zr(Ni,Ga)₇ have been defined in terms of a Bärnighausen scheme.     

Development of electroless (Ni-P)/BaNi0.4Ti0.4Fe11.2O19 nanocomposite powder for enhanced microwave absorption

Ni-, Ti-substituted nanocrystalline M-type barium hexaferrite powder (BaNi_xTi_xFe_12?2xO₁₉ (x = 0.4) of size ～10 nm) was coated with Ni-P by electroless (EL) coating technique to form EL Ni-P/BaNi_0.4Ti_0.4Fe_11.2O₁₉, a radar absorbing material (RAM) nanocomposite powder. Under TEM, the particle size of RAM powders before and after Ni-P coating were found to be in the range of 10–15 nm and 15–25 nm respectively. A uniform layer of 5–10 nm thick coating is deposited due to the controlled growth of EL Ni-P nanoglobules onto the powder. A growth mechanism was proposed to understand the deposition of EL Ni-P layer onto the RAM powder. The reflection loss (RL) of the EL (Ni-P)/RAM nanocomposite powder in Ku band (12.4 ?18 GHz) was evidently enhanced to ?28.70 dB, as compared to the EL Ni-P nanoglobules (?16.20 dB) and nanocrystalline RAM powder (?24.20 dB). After annealing at 400 °C for 4 h, the RL and bandwidth of EL (Ni-P)/RAM nanocomposite powder was further improved from ?24.20 to ?35.90 dB and 1.50 to 4.00 GHz respectively. The RL enhancement mechanism was explained on the basis of VSM study (hysteresis loops) and electromagnetic theory.     

Microstructural characteristics and microwave dielectric properties of Ba[Mg1/3(Nbx/4Ta(4−x)/4)2/3]O3 ceramics

Sintering behaviors and microstructural characteristics in solid solutions of Ba[Mg_1/3(Nb_x/4Ta_(4−x)/4)_2/3]O₃ (BMN_xT_4−x, x = 0, 1, 2, 3 and 4) were investigated by X-ray diffraction, SEM and TEM. Microwave dielectric properties, such as the relative permittivity (ɛ_r), quality factor (Q) value and temperature coefficient of resonator frequency (τ_f), were also measured. The excellent microwave dielectric property of Ba(Mg_1/3Ta_2/3)O₃ (BMT) sample imply the necessity to sinter at higher temperature (1650 °C) and to use longer soaking times (9 h), but not for Ba(Mg_1/3Ta_2/3)O₃ (BMN). The 1:2 B-site ordering was maintained at all Nb substitution contents and the 1:2 B-site ordering existed in the grains with antiphase domain boundaries (APBs). The Ba[Mg_1/3(Nb_1/4Ta_3/4)_2/3]O₃ specimen exhibited excellent microwave dielectric properties, ɛ_r = 25.534, Qf = 140 666 GHz, and τ_f = 4.8 (ppm/°C). The excellent microwave dielectric property is due to the improvement of sintering property by appropriate Nb atoms substitution in the BMT matrix and the maintaining of 1:2 ordering in the BMN_xT_4−x series.     

x(Mg0.7Zn0.3)0.95Co0.05TiO3-(1−x)(La0.5Na0.5)TiO3 ceramic at microwave frequency with a near zero temperature coefficient of resonant frequency

Dielectric properties of x(Mg_0.7Zn_0.3)_0.95Co_0.05TiO₃-(1?x)(La_0.5Na_0.5)TiO₃ ceramic were investigated at microwave frequencies. A nearly 0 ppm/°C temperature coefficient of resonant frequency was realized at x = 0.9. A two-phase system was confirmed by XRD analysis. A dielectric material applicable to microwave devices with a Q × f of 20,000–87,000 GHz and a dielectric constant of 21.27–26.2 was obtained at 1100 °C after 4 h of sintering. The microwave dielectric material 0.9(Mg_0.7Zn_0.3)_0.95Co_0.05TiO₃-0.1(La_0.5Na_0.5)TiO₃ sintered at 1150 °C for 4 h has a dielectric constant of 24.56, a Q × f of 68,000 GHz, and a τ_f value of 0 ppm/°C. It is proposed as a candidate dielectric for GPS patch antennas.     

Nanoscaled grain boundaries and pores,microstructure and mechanical properties of translucent Yb:[LuxY(1−x)O3] ceramics

Translucent ceramics of Yb:[Lu_xY_(1?x)O₃] system doped by ZrO₂ was sintered from nanopowder synthesized by laser evaporation. The relative density of the ceramics was 99.97%, residual pores had sizes from 8 nm to 20 nm, Young modulus was 200 GPa at the applied load of 2000 mN, the microhardness was 12.8 GPa. The grains of ceramics had sizes 1–10 μm, but the thickness of grain boundaries was about 1 nm. The transcrystalline type of the crack propagation was detected in the specially broken ceramics. The results indicated high strength of grain bonds and good perfection of grain boundaries in the studied ceramics but an increased content of pores (higher than 10^?3 vol.%) and stoichiometry deviation (Lu:Y:O = 0.21:0.79:3) from the required one (Lu:Y:O = 0.25:0.75:3).     

Large spectral shifts of electronic transitions in MoSI molecular wire dispersions as a function of bundle diameter

A systematic study of optical absorption spectra of Mo₆S_9?xI_x (x = 6) molecular wire dispersions in ethanol, fractionated into different bundle diameter populations shows that electronic transitions shift significantly as a function of bundle diameter. Two electronic transitions show significant shifts: the Mo–S charge transfer peak shifts from 1.8 to 1.5 eV and the next inter-band transition shifts from 2.7 to 2.4 eV with increasing bundle diameter d, in the range 5–100 nm. This empirical observation hugely simplifies characterization of Mo₆S_9?xI_x wire dispersions according to diameter, opening the way to rapid advances in processing of these materials. We discuss the possible origin of the shift, dismissing quantum size effects, impurities and solvatochromism as well as stoichiometric variations between x = 6 and x = 4.5.     

A study on mechanochemical behavior of MoO3–Mg–C to synthesize molybdenum carbide

The influence of Mg value in the MoO₃–Mg–C mixture on the molybdenum carbide formation and the mechanism of reactions during mechanochemical process were investigated. In keeping with this aim, magnesium and carbon contents of the mixture were changed according to the following reaction: 2MoO₃ + (6 − x) Mg + (1 + x) C = (6 − x) MgO + Mo₂C + x CO. The value of x varied from 0 to 6. Differential thermal analysis (DTA) results for sample with stoichiometric ratio (x = 0) revealed that in the early stage, carbon reduced the MoO₃ to MoO₂ and subsequently highly exothermic magnesiothermic MoO₂ reduction occurred after magnesium melting. Also, it was indicated that the exothermic reaction temperature shifted to before magnesium melting in the 11 h-milled sample (x = 0) and all the exothermic reactions happened, simultaneously. According to the experimental findings, molybdenum carbide (Mo₂C) was synthesized in the mixture powder with stoichiometric ratio (x = 0) after 12 h milling process and the type of reactions was mechanically induced self-sustaining reaction (MSR). However, at lower Mg content in the MoO₃–Mg–C mixture (0 < x ≤ 2), the magnesiothermic reduction occurred in MSR mode and activated the carbothermal reaction. Further decrease in Mg value (2 < x ≤ 3) resulted in MSR mode magnesiothermic reaction and gradual carbothermal reduction. In samples with lower magnesium contents, partial molybdenum oxide reduction proceeded through a gradual mode magnesiothermic reaction.     

Phase transition behavior and electrical properties of [(K0.50Na0.50)1−xAgx](Nb1−xTax)O3 lead-free ceramics

The effect of AgTaO₃ on the electrical properties of (K_0.5Na_0.5)NbO₃ lead-free ceramics was systematically investigated, and the phase transition behavior of the ceramics was also studied in terms of high temperature X-ray diffraction. The experimental results show that Ag⁺ and Ta⁵⁺ ions diffuse into the (K_0.5Na_0.5)NbO₃ lattices to form a stable solid solution with orthorhombic structure, and also lead to the decrease in the orthorhombic to the tetragonal phase transition temperature and the Curie temperature. The 0.92(K_0.5Na_0.5)NbO₃–0.08AgTaO₃ ceramics exhibit optimum electrical properties (d₃₃ = 183 pC/N, k_p = 41%, T_c = 356 °C, T_o–t = 158 °C, ?_r ～ 683, and tan δ ～ 3.3%) and good thermal-depoling behavior. The piezoelectric properties of (1 ? x)(K_0.5Na_0.5)NbO₃–xAgTaO₃ ceramics are much superior to pure (K_0.5Na_0.5)NbO₃ ceramics. X-ray diffraction patterns for the 0.92(K_0.5Na_0.5)NbO₃–0.08AgTaO₃ ceramic at different temperatures indicated a pure perovskite phase with an orthorhombic structure at below 160 °C, a tetragonal structure at 160–350 °C, and a cubic structure at above 360 °C. As a result, the (1 ? x)(K_0.5Na_0.5)NbO₃–xAgTaO₃ ceramic is one of the promising candidate materials for lead-free piezoelectric ceramics.     

Magnetic properties of nanocrystalline ferrite TixCo1+xFe2−2xO4 and CoFe2O4 composite thin films

Ti_xCo_1+xFe_2−2xO₄ (0 ≦ x ≦ 0.5) ferrite films and its composite films with CoFe₂O₄ synthesized by a sol–gel method were investigated on crystallographic and magnetic properties. Magnetization decreased with the increase of Ti content while coercive force showed a maximum at x = 0.2 and comparatively high at x = 0.5. Composite films of Ti_0.5Co_1.5FeO₄ and CoFe₂O₄ showed larger H_c and smaller grains.     

Synthesis and hydrogen reduction of nano-sized copper tungstate powders produced by a hydrothermal method

The pure nano-sized copper tungstate (CuWO₄) powders were prepared by hydrothermal method and consequent annealing at 500 °C for 120 min. The thermogravimetric analysis was used to study dehydration processes, and the scanning electron microscopy (SEM) indicated that CuWO₄ particles were mostly spherical in the size range from 60 to 90 nm. Hydrogen reduction at 800 °C for 60 min converted the CuWO₄ to W–Cu composite powders. The hydrogen reduction results showed that nano-sized CuWO₄ particles calcining at 500 °C for 120 min indicated finer microstructure than the other calcination temperatures of 0 °C, 400 °C, 620 °C, 650 °C and 700 °C. W–Cu particles were observed finest and homogeneous in the size range from 90 to 150 nm by SEM images. Homogeneous distribution of W and Cu particles was clearly demonstrated by elemental mapping. Encapsulation of Cu phase by the W phase was observed by EDS and TEM. From FFT and HRTEM images, the orientation relationship of (01-1)_Cu ∥(01-1)_W and a semicoherent interface between W and Cu phases could be observed. A good correlation between the HRTEM image and the calculated lattice misfit (δ) was obtained.     

Investigating mechanochemical behavior of Cr2O3–B2O3–Mg–C quaternary system

Fundamental aspects of reaction behavior and formation path in the Cr₂O₃–B₂O₃–Mg–C quaternary system have been studied to synthesize chromium boride–chromium carbide nanocomposite. In order to find the influence of simultaneous presence of magnesium and carbon on final products, various powder mixtures were chosen according to following reaction: B₂O₃ + Cr₂O₃ + (9 − x) Mg + x C. The value of x varied from 0 to 4. In the absence of carbon (x = 0), CrB₂ was synthesize through mechanically induced self-propagating reaction (MSR). In the presence of 8 mol Mg and 1 mol C (x = 1), the dominant boride phase was CrB while no chromium carbide was detected. By increasing C content (x = 2), the magnesiothermic reduction occurred in MSR mode; whereas, the synthesis of Cr₃C₂ initiated after combustion reaction and completed gradually during milling for 6 h. Further increase in C amount (x = 3) resulted in formation of Mg₃(BO₃)₂ as unwanted phases as well as CrB and Cr₃C₂. In the presence of 6 mol Mg and 4 mol (x = 4), no mechanical reaction was observed even after 8 h of milling. Optimum value of x for the formation of CrB–Cr₃C₂ nanocomposite was 2. Based on the morphological evolutions, it is evident that the mechanosynthesized powder is made up of nanometric particles.