Low temperature reaction of point defects in ion irradiated 4H–SiC

The low temperature evolution of point defects induced in SiC by ion irradiation was investigated by deep level transient spectroscopy. The defects were introduced by irradiation with a 7.0 MeV beam of C⁺ ions at a fluence of 6 × 10⁹ cm^? 2. Annealing was then performed in the temperature range of 330–400 K in order to study the change in point defect structure with temperature. The low temperature annealing performed was observed to induce a change in the produced defects. The deep levels related to the S_x (E_C ? 0.6 eV) and S₂ defects (E_C ? 0.7 eV) recovered with annealing while, simultaneously, a new level, S₁ (E_C ? 0.4 eV), was formed. The activation energy of the S₁ defect is 0.94 eV, while the annealing of both the S_x and S₂ levels occurred with activation energy of 0.65 eV.     

The decrease of depolarization temperature and the improvement of pyroelectric properties by doping Ta in lead-free 0.94Na0.5Bi0.5TiO3-0.06BaTiO3 ceramics

Ta-doped lead-free 0.94NBT-0.06BT-xTa (x=0.0–1.0%) ceramics were synthesized by a conventional solid-state route. XRD shows that the compositions are at a morphotropic phase boundary where rhombohedral and tetragonal phases coexist. The depolarization temperature (T_d) shifted to lower temperature with the increase of Ta content. The pyroelectric coefficient (p) of doped ceramics greatly enhanced compared with undoped material and reached a maximum of 7.14×10⁻⁴ C m⁻² °C⁻¹ at room temperature (RT) and 146.1×10⁻⁴ C m⁻² °C⁻¹ at T_d at x=0.2%. The figure of merits, F_i and F_v, also showed a great improvement from 1.12×10⁻¹⁰ m v⁻¹ and 0.021 m² C⁻¹ at x=0.0 to 2.55×10⁻¹⁰ m v⁻¹ and 0.033 m² C⁻¹ at x=0.2% at RT. Furthermore, F_i and F_v show the huge improvement to 52.2×10⁻¹⁰ m v⁻¹ and 0.48×10⁻¹⁰ m v⁻¹ respectively at T_d at x=0.2%. F_C shows a value between 2.26 and 2.42 ×10⁻⁹ C cm⁻² °C⁻¹ at RT at x=0.2%. The improved pyroelectric properties make NBT-0.06BT-0.002Ta ceramics a promising infrared detector material.     

First-principles study of structural,mechanical, and thermodynamic properties of cubic Y2O3 under high pressure

The structural, mechanical, and thermodynamic properties of cubic Y₂O₃ crystals at different hydrostatic pressures and temperatures are systematically investigated based on density functional theory within the generalized gradient approximation. The calculated ground state properties, such as equilibrium lattice parameter a₀, the bulk modulus B₀, and its pressure derivative B₀′ are in favorable agreement with the experimental and available theoretical values. The pressure dependence of a/a₀ and V/V₀ are also investigated. Furthermore, the elastic constants C_ij, bulk modulus B, shear modulus G, Young's modulus E, the ductile or brittle (B/G), Vickers hardness H_v, isotropic wave velocities and sound velocities are calculated in detail in a pressure range from 0 to 14 GPa. It was found that the Debye temperature decreases monotonically with an increase in pressure, the calculated elastic anisotropic factors indicate that Y₂O₃ has low anisotropy at zero pressure, and that its elastic anisotropy increases as the pressure increases. Finally, the thermodynamic properties of Y₂O₃, such as the dependence of the heat capacities C_V and C_P, the thermal expansion coefficient α, the isothermal bulk modulus, and the Grüneisen parameter γ on temperature and pressure, are discussed from 0 to 2000 K and from 0 to 14 GPa, respectively, applying the non-empirical Debye model in the quasi-harmonic approximation.     

Fabrication of YBa2Cu3O7−x (YBCO) superconductor bulk structures by extrusion freeforming

Practical applications of high temperature superconductors may require them to be processed into complex geometries. In this work, slurry-based extrusion freeforming coupled with high temperature treatment was attempted for the fabrication of bulk YBa₂Cu₃O_7−x (YBCO) superconducting structures. YBCO parts with approximately 93% of the theoretical density were successfully fabricated after sintering at 940 °C for 60 h, with the obtained constituent phases strongly dependent on the heat treatment temperature and duration. A high critical transition temperature (T_C=92 K) and good magnetic levitation ability could be obtained after optimization of the heat treatment conditions. Overall, the experimental results demonstrate that extrusion freeforming is a feasible and effective technique for fabricating YBCO superconductors that have desirable configurations and good superconductivity properties.     

Superconducting properties of homoepitaxial CVD diamond

Superconductivity was achieved above 10 K in heavily boron-doped diamond thin films deposited by the microwave plasma-assisted chemical vapor deposition (CVD) method. Advantages of the CVD method are the controllability of boron concentration in a wide range, and a high boron concentration, compared to those obtained using the high-pressure high-temperature method. The superconducting transition temperatures of homoepitaxial (111) films are determined to be 11.4 K for T_C onset and 8.4 K for zero resistance from transport measurements. In contrast, the superconducting transition temperatures of (100) films T_C onset = 6.3 K and T_C zero = 3.2 K were significantly suppressed.     

Structure and magnetic properties of Co3O4/SiO2 nanocomposite synthesized using combustion assisted sol-gel method

This paper reports on novel cobalt oxide nanoparticles (NPs) embedded in an amorphous silica (SiO₂) matrix, synthesized using a modified sol-gel method. SEM and TEM images show as-synthesized particles to aggregate in the shape of spheres and less than 5 nm in size, while XRD and SAED analysis both point to well crystallized cubic spinel cobalt oxide phase with an average crystallite size of about 4.6 nm. Raman analysis confirms the formation of cobalt (III) oxide (Co₃O₄) NPs. As-synthesized Co₃O₄ single-nanocrystallite has magnetic properties that correlate with finite size effects and uncompensated surface spins. Temperature dependence of ZFC-FC magnetization curves reveals a sharp peak around 10 K which corresponds to the blocking temperature. A Curie-Weiss behavior of magnetization above 25 K shows lower Néel temperature of the sample compared with its bulk counterpart T_N=40 K (possibly due to crystal defects and nano-dimensionality of the particles). The magnetic measurements exhibit high magnetization at low temperatures (M_S=54.3 emu/g) which can be associated with random canting of the particles’ surface spins and uncompensated spins in the core which tends to interact ferromagnetically at low temperatures. The initial magnetization curve falls out from the hysteresis loop at 5 K, which could be also the effect of surface spins.     

Giant strain response and structure evolution in (Bi0.5Na0.5)0.945−x(Bi0.2Sr0.7□0.1)xBa0.055TiO3 ceramics

The microstructure, electric-field-induced strain, polarization, and dielectric permittivity in (Bi_0.5Na_0.5)_0.945−x(Bi_0.2Sr_0.7□_0.1)_xBa_0.055TiO₃ (BNBT–xBST) (0 ≤ x ≤ 0.08) electroceramics are investigated. An irreversible transition from rhombohedral and monoclinic coexistence phase to single rhombohedral phase is indicated with the remnant strain S_r = 0.330% at x = 0. As the BST content increases, the ferroelectric order is disrupted resulting in a degradation of the remnant polarization, coercive field, and the ferroelectric-to-relaxor transition temperature (T_F–R). The coexistence of ferroelectric relaxor and ferroelectric phase is observed for the optimum composition x = 0.02 at ambient temperature with a large strain of 0.428% at 60 kV/cm (normalized strain S_max/E_max = 713 pm/V). The large strain is contributed by both ferroelectric domain reorientation behavior and the reversible relaxor to ferroelectric phase transition.     

Enhanced piezoelectric properties of (Na0.53K0.47)(Nb1−xTax)O3 ceramics by Ta substitution

The effects of Ta substitution for B-site Nb in (Na_0.53K_0.47)(Nb_1?xTa_x)O₃ (NKNT) ceramics were investigated in the range of x = 0–0.6. It was found that polymorphic phase transitions (PPT) were significantly influenced by Ta substitution. Transitions among orthorhombic, tetragonal, and cubic phases in sequence with temperature, T_O-T and T_C, respectively, decreased linearly with x. At x = 0.45, T_O-T was reduced to room temperature from 182 °C at x = 0, and subsequently piezoelectric coefficient (d₃₃) at room temperature was enhanced up to 284 pC/N from 120 pC/N at x = 0 due to the coexistence of ferroelectric orthorhombic and tetragonal NKNT phases. With x further increasing beyond x = 0.45, d₃₃ decreased due to there being no orthorhombic but only a tetragonal NKNT phase at room temperature with T_O-T below room temperature.     

Selective low temperature synthesis of carbon nanospheres via the catalytic decomposition of trichloroethylene

The catalytic growth of structured carbon from a C₂H₄ and C₂HCl₃ feed promoted by Ni/SiO₂ in the presence of H₂ over the temperature range 673 K ≤ T ≤ 1023 K has been examined. The supported Ni phase exhibited an exclusive cubic symmetry (XRD analysis) with a range of Ni particle sizes (TEM analysis) and a net shift in the distribution to larger particles with increasing reduction temperature (from 20 to 36 nm), accompanied by a decrease in H₂ chemisorption. Conversion of C₂H₄ generated hydrogenation (C₂H₆), hydrogenolysis (CH₄) and decomposition (C + H₂) products. Ethane formation was favoured at lower temperatures with C formation increasingly preferred at higher temperatures so that C₂H₄ decomposition was the predominant process at T > 723 K; significant CH₄ production was only observed at T > 900 K. Carbon yield from C₂H₄ passed through a maximum at 773 K and took the form of high aspect ratio graphitic nanofibres with a central hollow core and diameters in the range 5–180 nm. The carbonaceous product has been characterized by a combination of TEM-EDX, SEM, XRD, BET area and temperature programmed oxidation (TPO). Carbon formation from C₂HCl₃ exceeded (by a factor of up to an order of magnitude) that generated via the decomposition of C₂H₄ at the same inlet C:Ni ratio to deliver essentially a carbon yield invariance (9.1 ± 0.3 g_C g_Ni^?1) where 898 K ≤ T ≤ 1023 K, which represents a carbon efficiency (fraction of carbon in the inlet feed that is converted to a solid carbon product) in excess of 96%. Ni/SiO₂ promoted a composite dehydrochlorination/decomposition of C₂HCl₃ to HCl + C. The nature of the carbon product generated from C₂HCl₃ is strongly temperature dependent with a shift from a pseudo-fibrous product at 773 K to a predominant nanosphere formation at 923 K. These nanospheres exhibit a wide diameter range (40–700 nm), a significant Cl content (1.1–2.6%, w/w) and a conglomeration or clustering to give a less ordered carbonaceous product than that generated at the lower temperature (773 K). A tentative carbon growth rationale is presented to account for the observed dependence of carbon structure on carbon-containing precursor and reaction temperature.     

The effects of the post-annealing time on the growth mechanism of Bi2Sr2Ca1Cu2O8+∂ thin films produced on MgO (100) single crystal substrates by pulsed laser deposition (PLD)

Bi₂Sr₂Ca₁Cu₂O_8+∂ thin films were deposited on MgO (100) substrates by pulsed laser deposition (PLD). The effects of post-annealing time on the phase formation, the structural and superconducting properties of the films have been investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), temperature dependent resistivity (R–T), atomic force microscopy (AFM), and DC magnetization measurements. The films deposited at 600 °C were post-annealed in an atmosphere of a gas mixture of Ar (93%) and O₂ (7%), at 860 °C for 10, 30, and 60 min. All films have demonstrated a mainly single phase of 2212 with a high crystallinity (FWHM≈0.159°) and c-axis oriented. The critical temperature, T_C, of the films annealed for 10, 30, and 60 min were obtained as 77, 78, and 78 K, respectively. The highest critical current density, J_C, was calculated as 3.34×10⁷ A/cm² for the film annealed at 860 °C for 30 min at 10 K.     

Phase transitions and microwave dielectric properties of Bi3NbO7 ceramics with Bi4B2O9 addition

The effects of Bi₄B₂O₉ on the phase transitions, sinterability and microwave dielectric properties of Bi₃NbO₇ ceramics were investigated. Densities around 96% theoretical could be achieved at 900 °C for samples with up to 20 wt% Bi₄B₂O₉ addition. Phase transitions of cubic→tetragonal→cubic with the increase of sintering temperature were observed for the samples with Bi₄B₂O₉ addition. Moreover, the Bi₄B₂O₉ addition effectively accelerated the phase transition from cubic Bi₃NbO₇ to tetragonal Bi₃NbO₇. Bi₄B₂O₉ addition and the sintering temperature significantly affected the microwave dielectric properties mainly due to the phase transitions. When 20 wt% Bi₄B₂O₉ was added, a dense ceramic could be sintered at 900 °C with relative permittivity ε_r=79, microwave quality factor Qf₀=1010 GHz, and temperature coefficient of resonance frequency τ_f=+8 ppm/°C, which makes it a promising candidate for LTCC applications.     

Large magnetocaloric and magnetoresistance effects in metamagnetic Sm0.55(Sr0.5Ca0.5)0.45MnO3 manganite

The metamagnetic transition, magnetocaloric and magnetoresistance effects are investigated in polycrystalline Sm_0.55(Sr_0.5Ca_0.5)_0.45MnO₃ (SSCMO) manganite. A sharp magnetization jump at Curie temperature (T_C) 73.5 K with large thermal hysteresis is observed. Magnetic measurements and Arrott plots analysis indicate that the transition is first order in nature. Under a low magnetic field change of 1 T, the magnetic entropy change exhibits a peak value of $-\Delta S_M^{\max }(T)$=4.01 J/kg K with relative cooling power (RCP) value of 44.1 J/kg in the vicinity of T_C. The mechanism of charge conduction in insulator phase is polaron transport. Large negative magnetoresistance ratio with value of ~99% is obtained within a broad temperature range below metal insulator transition temperature under 1 T magnetic field. These results indicate the potential applications of SSCMO in magnetic refrigeration and spintronics devices.     

Carbon nanotubes growth in AlPO4-5 zeolites: Evidence for density dependent field emission characteristics

We report on the correlation between the concentration of Fe-catalyst, doped in the aluminum phosphate (AlPO₄-5) zeolite and the resulting density of carbon nanotubes (CNTs) to obtain the optimum electron field emission conditions from the CNTs. Initially, AlPO₄-5 crystallites were impregnated, for a period of ∼ 10–60 min, in the Fe-catalyst solution and subjected to Electron Spectroscopy for Chemical Analysis (E.S.C.A.). The analysis revealed that the concentration of Fe-catalyst, C_Fe, was increased from ∼ 1.7% to ∼ 8.6%, respectively, with increase in impregnation time, I_T. The HRTEM results showed that Fe nano-clusters, with diameter ∼ 7–10 nm, were formed in the surface region of the crystallites. These crystallites were sprayed on the conducting substrates, under identical spraying conditions. SEM study revealed that the coverage of the crystallites on the substrates was ∼ 10³–10⁴ crystallites/cm². These substrates were subjected to direct current plasma enhanced chemical vapor deposition (dc-PECVD) process, to grow CNTs. The SEM micrographs were recorded for the CNT-grown substrates and the average areal density of CNTs, (σ_T)_av, on the crystallites (t/cm²) was estimated. The analysis indicated that (σ_T)_av increased from ∼ 6.24 ± 0.19 × 10¹⁰ to 2.04 ± 0.61 × 10¹¹ t/cm² with gradual increase in C_Fe. The field emission study of the samples revealed that the optimum values of the turn-on electric field, ∼ 3.69 V/μm and the field emission current density, ρ_d, ∼ 1.78 × 10³ μA/cm² were achieved for (σ_T)_av, ∼ 6.24 ± 0.19 × 10¹⁰ t/cm², at a concentration of Fe, C_Fe, ∼ 3.0%, encapsulated in the AlPO₄-5 crystallites.     

Multilayer 0.9Pb(Mg1/3Nb2/3)O3–0.1PbTiO3 elements for electrocaloric cooling

Electrocaloric (EC) cooling elements in the form of multilayers (MLs) were prepared. The elements consist of five layers of the relaxor-ferroelectric 0.9Pb(Mg_1/3Nb_2/3)O₃–0.1PbTiO₃, about 60 μm thick, with internal platinum electrodes and exhibiting a dense, uniform microstructure with a grain size of 1.7 μm. The largest temperature change ΔT_EC of 2.26 K was achieved at an electric field (E) of 100 kV cm⁻¹ and at 105 °C, measured by a high-resolution calorimeter. These results agree well with the indirect measurements. The EC coefficient, ΔT_EC/ΔE, obtained for the MLs, is similar to the value obtained for bulk ceramics of the same composition. The ΔT_EC values above 2 K over a broad temperature range from 75 to 105 °C make the ML elements suitable candidates for EC cooling devices at significantly lower voltages than bulk ceramic plates with comparable dimensions and mass.     

Experimental investigation and mathematical modeling of oxygen permeation through dense Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) perovskite-type ceramic membranes

Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3?δ (BSCF) perovskite powder was synthesized via EDTA/citrate complexation method. BSCF membranes were formed by pressing powder at 400 MPa and sintering at 1100 °C for 10 h. XRD patterns showed that a high pure powder with cubic structure was obtained. SEM micrographs revealed that the membranes are dense with large grains. Effects of temperature, feed and permeate side oxygen partial pressures, flow rates and membrane thickness on oxygen permeation flux were studied experimentally. A Nernst–Planck based mathematical model, including surface exchange kinetics and bulk diffusion, was developed to predict oxygen permeation flux. Considering non-elementary surface reactions and introducing system hydrodynamics into the model resulted in an excellent agreement (RMSD = 0.0617, AAD = 0.0487 and R² = 0.985) between predicted and measured fluxes. The results showed that oxygen permeation flux increases with temperature, feed side oxygen partial pressure and flow rates, however decreases with permeate side oxygen partial pressure and membrane thickness. Contribution of feed side surface exchange reactions, bulk diffusion and permeate side surface exchange reactions resistances in the total resistance are in the range of 8–32%, 10–81% and 11–59%, respectively. Permeation rate-limiting step was determined using the membrane dimensionless characteristic thickness.     

Electrical properties of [Li0.04(K0.5Na0.5)0.96−xAgx](Nb1−ySby)O3 ceramics

Dependence of electrical properties on the structural characteristics of Li_0.04(K_0.5Na_0.5)_0.96(Nb_1?ySb_y)O₃ (LKNNS (x = 0, 0.00 ≤ y ≤ 0.10)) and [Li_0.04(K_0.5Na_0.5)_0.96?xAg_x](Nb_0.925Sb_0.075)O₃ (LKNANS (0.01 ≤ x ≤ 0.05, y = 0.075)) were investigated. The oxygen octahedral distortion was dependent on Ag⁺ and/or Sb⁵⁺ content which affected to the phase transition temperature of LKNNS and LKNANS ceramics. The orthorhombic–tetragonal and tetragonal–cubic phase transition temperatures (T_O–T, T_C) of the specimens were decreased with increasing of average octahedral distortion. With increasing of Sb⁵⁺ content, the electromechanical coupling factor (k_p), piezoelectric constant (d₃₃) and dielectric constant (?_r) of the sintered specimens were increased up to y = 0.075, and then decreased. These results could be attributed to the shift of T_O–T to near room temperature for Li_0.04(K_0.5Na_0.5)_0.96(Nb_0.0925Sb_0.075)O₃.     

An essential work of fracture study of the toughness of thermoset polyester coatings

The fracture toughness of a range of thermoset polyester paints with different cross-link densities has been studied, using the essential work of fracture (EWF) method. The glass transition temperature, T_g, of each of the materials was measured using differential scanning calorimetry, and found to lie between 8 and 46 °C. EWF tests were performed on the paint films at a range of temperatures around the measured glass transition temperature of each material. The essential work of fracture, w_e, at T_g was found to decrease with increasing cross-link density from around 20 kJ/m² at a cross-link density of 0.4 × 10⁻³ mol/cm³ to around 5 kJ/m² for cross-link densities of approximately 1 × 10⁻³ mol/cm³ or higher. A maximum in the essential work of fracture was observed at around T_g when w_e was plotted versus temperature, which could be attributed to the effect of an α-relaxation at a molecular level. The polyesters were found to be visco-elastic, and the applicability of the EWF test to the study of these visco-elastic thermoset materials is discussed.     

Enhancement of pyroelectric properties of lead-free 0.94Na0.5Bi0.5TiO3-0.06BaTiO3 ceramics by La doping

Lead-free 0.94NBT-0.06BT-xLa ceramics at x = 0.0–1.0 (%) were synthesized by a conventional solid-state route. XRD shows that the compositions are at a morphotropic phase boundary where rhombohedral and tetragonal phases coexist. With increasing La³⁺ content pyroelectric coefficient (p) and figures of merits greatly increase; however, the depolarization temperature (T_d) decreases. p is 7.24 × 10⁻⁴C m⁻² °C⁻¹ at RT at x = 0.5% and 105.4 × 10⁻⁴C.m^−2 °C⁻¹ at T_d at x = 0.2%. F_i and F_v show improvements at RT from 1.12 (x = 0%) to 2.65 (x10 ⁻¹⁰ m v⁻¹) (x = 0.5%) and from 0.021 to 0.048 (m².C⁻¹) respectively. F_i and F_v show a huge increase to 37.6 × 10⁻¹⁰ m v⁻¹ and 0.56 m² C⁻¹ respectively at T_d at x = 0.2%. F_C shows values of 2.10, 2.89, and 2.98 (x10⁻⁹C cm⁻² °C⁻¹) at RT at 33, 100 and 1000 (Hz) respectively. Giant pyroelectric properties make NBT-0.06BT-xLa at x = 0.2% and 0.5% promising materials for many pyroelectric applications.     

Structure,magnetocaloric and critical properties of layered La2Sm0.4Sr0.6Mn2O7 perovskite

We report the structure, magnetocaloric effect, and critical phase transition in the manganite La₂Sm_0.4Sr_0.6Mn₂O₇ (LSSMO) synthesized by a sol-gel method. X-ray diffraction together with Rietveld refinement show that the sample crystallizes in a Sr₃Ti₂O₇-type tetragonal structure with a space group of I4/mmm. This compound undergoes a second-order ferromagnetic (FM) to paramagnetic phase transition at T_C=348 K and shows strong FM properties below the T_C. Based on the data of isothermal magnetization measured around the T_C and Maxwell's relation, we calculated the maximum magnetic entropy change (-ΔS_M^max) to be 4.69 J kg⁻¹ K⁻¹ and the relative cooling power to be 233.9 J kg⁻¹ for a μ₀ΔH=5 T magnetic field variation. These results indicate that LSSMO can be considered as a potential candidate material for application in magnetic refrigeration above room temperature. The critical behavior near the T_C was studied through the analysis of the magnetic field dependence of the magnetic entropy change and Widom's scaling relation. The exponent values estimated in this work are fairly close to those theoretically predicted by mean field theory (β=0.5, γ=1.0, and δ=3.0), revealing that long-range FM ordering exists in LSSMO. Scaling law theory also confirms the validity of the deduced critical exponents.     

First-principles supercell studies of the substitutional carbon in c-BN

The effects of hydrostatic pressure on the formation enthalpy and electronic level positions of C_N and C_B in c-BN are investigated by means of ab initio plane-wave pseudopotential method using a supercell approach in the range of 0–60 GPa. The formation enthalpy decreases with pressure for C_B, suggesting that C_B becomes much more stable and has a larger concentration with pressure increase. In contrast to C_B, the C_N impurity exhibits positive dependence of the formation enthalpy on pressure, so the concentration of C_N reduces with pressure. With pressure increase, the defect energy levels of C_B^+ 1 and C_N^– 1 move into the conduction band and the valence band, respectively, while the defect energy levels of C_B and C_N in other charge states all move toward the center of band gap. These results suggest that pressure may make electron transfer between the impurity band and the conduction band or the valence band difficult and weaken its conductivity when a C atom substitutes at a B site or at a N site in c-BN.