A correlation between structural and optical properties of cobalt oxide nanoparticles for various annealing conditions

Two stable phases of cobalt oxide nanoparticles of controlled sizes have been synthesized using environmentally friendly inorganic precursor. Structural characterization using X-ray diffraction (XRD) shows a single-phase spinal Co₃O₄ structure up to annealing temperature of 800 °C and a mixed phase of Co₃O₄ and CoO particles for T>900 °C. Single-phase CoO nanoparticles are also obtained by annealing the particles at a temperature >900 °C and cooling in inert atmosphere. Average macro- and micro-strain were estimated using XRD data. Macrostrain was found to be the minimum for particles annealed at 600 °C, whereas microstrain was found to decrease with increasing annealing temperature up to 900 °C. A correlation between the density of localized states (DOS) in the band gap and strain is expected because the origin of both strain and DOS are defects and bond length distortions. Sub-gap absorption measurement and model calculations have been used for the determination of DOS. For cobalt oxide nanoparticle samples we find a correlation between estimated strain and density of states in the band gap.     

Derived thermodynamic design data for rankine power cycle systems operating on R22

Theoretical Rankine power cycle efficiencies ν_R and the pressure (PR) have been presented for rankine power cycles operating on R22. These values are listed in tabular form for temperature drops of 5–75°C and for boiler temperatures 40–85°C in 5°C increments. Composite graph showing the relationship between ν_R, T_BO, (PR) and temperature drop (T_BO - T_CO) illustrates the feasible operating range for R22 power cycle systems. The derived thermodynamic data can be used for the rapid preliminary design of the Rankine power cycle systems operating on R22.     

Derived thermodynamic design data for rankine power cycle systems operating on R502

Theoretical Rankine power cycle efficiencies ν_R and the pressure (PR) have been presented for Rankine power cycles operating on R502. These values are listed in tabular form for temperature drops of 5–75°C and for boiler temperatures 35–80°C in 5°C increments. A composite graph showing the relationship between ν_R, T_BO, (PR) and temperature drop (T_BO - T_CO illustrates the feasible operating range for R502 power cycle systems. The derived thermodynamic data can be used for the rapid preliminary design of Rankine power cycle systems operating on R502.     

Derived thermodynamic design data for rankine power cycle systems operating on R142b

Theoretical Rankine power cycle efficiencies ν_R and the (PR) have been presented for Rankine power cycles operating on R142b. These values are listed in tabular form for temperature drops of 5–75°C and for boiler temperatures 25–125°C in 5°C increments. A composite graph showing the relationship between ν_R, T_BO, (PR) and temperature drop (T_BO − T_CO) illustrates the feasible operating range for R142b power cycle systems. The derived thermodynamic data can be used for the rapid preliminary design of the Rankine power cycle systems operating on R142b.     

Self-organization of the structure in the Cu60Fe40 composite during deformation-thermal treatment

The deformation-thermal stability of a clusterized amorphous-crystalline structure prepared from a Cu₆₀Fe₄₀ powder mixture at a logarithmic strain e = 4.6 and subjected to isochronous (40 min) annealings at T _a = 200–800°C has been investigated. Periodic changes (ΔT = 300°C) in the order and disorder with a maximum ordering at T _a = 300 and 600°C and a maximum disordering at T _a = 400 and 700°C have been observed. The periodicity of the dominant crystallographic order with a period ΔT = 400°C in the annealing temperature has been revealed for face-centered cubic copper phase planes separated by a singular point at T = 500°C characterized by the dominant body-centered cubic iron phase ordering. It has been shown that the sawtooth shape of the size distribution of strain clusters formed within the crystal structure of deformed samples slowly changes with increasing annealing temperature from exponential (T _a = 200–700°C) to linear (T _a = 800°C). This indicates a high density of internal local distortions in structural units.     

Investigation of the optimal annealing temperature for the enhanced thermoelectric properties of MOCVD-grown ZnO films

In this study, we demonstrate the optimization of the annealing temperature for enhanced thermoelectric properties of ZnO. Thin films of ZnO are grown on a sapphire substrate using the metal organic chemical Vapor Deposition (MOCVD) technique. The grown films are annealed in an oxygen environment at 600–1000°C, with a step of 100°C for one hour. Seebeck measurements at room temperature revealed that the Seebeck coefficient of the sample that was not annealed was 152 μV/K, having a carrier concentration of N _D ~ 1.46 × 10¹⁸ cm^–3. The Seebeck coefficient of the annealed films increased from 212 to 415 μV/K up to 900°C and then decreased at 1000°C. The power factor is calculated and found to have an increasing trend with the annealing temperature. This observation is explained by the theory of Johnson and Lark–Horovitz that thermoelectric properties are enhanced by improving the structure of ZnO thin films. The Hall measurements and PL data strongly justify the proposed argument.     

Characteristics of molecular beam epitaxy-grown GaFeAs

Diluted magnetic semiconductors GaFeAs were grown by molecular beam epitaxy and characterized. Ga_1−xFe_xAs ternary alloys were obtained at the growth temperature T_s=200 °C ranging from x=0.005 to 0.03. The effects of thermal treatment on behaviors of defects, affecting to the magnetic properties of GaFeAs layer were particularly elucidated. As-grown samples were annealed at temperatures varying from 400 to 600 °C. From the measurement of double crystal X-ray diffraction, we observed Fe-related peak which shifted to a higher diffraction angle as the Fe content increased, indicating that the lattice constant decreases with increasing Fe content. In contrast, above the annealing temperature 500 °C, the lattice constant becomes smaller than that of GaAs. Using the deep level transient spectroscopy, various defects in GaFeAs layer were observed and identified in conjunction with magnetic properties.     

Superconductivity of niobium nitride single crystals after implantation of carbon and nitrogen

Single crystals of δ-NbN_0.85 with a superconducting transition temperature T_c of 14.3 K were implanted with nitrogen and carbon ions at room temperature and subsequently annealed at high temperatures. Implantation was also performed at high substrate temperatures. After implantation at about 920°C maximum T_c-values of 16.5 and 17.8 K were obtained with N- and C-ions respectively. Disorder observed after room temperature implantation consisted of displaced Nb-atoms which could not be completely annealed in an isochromous annealing process up to 1000°C. For annealing temperatures above 1100°C nitrogen diffusion out of the implanted layers resulted in a reduction of T_c.     

Modification of the CVD-graphene resistivity by post-processing sample annealing

In this paper, we present the results of an additional annealing effect on the temperature dependences of the resistivity for CVD-graphene samples of a large area. We found that an annealing in a Ar/H₂ mixture at different temperatures modifies both the value of the resistivity and the slope of its temperature dependence. The annealing effect on the resultant sample quality depends on the type of the ρ(T) dependence for the initial sample. For samples with a metallic-like ρ(T) dependence, a low-temperature annealing (at T = 250 °C) results in a slight decrease in the resistivity value and an increase of the ρ(T) curve slope. Increasing the annealing temperature up to T = 400 °C leads to a stronger increase in the ρ(T) curve slope but to an increase in the resistivity value. For samples with a semiconductor-like ρ(T) dependence, increasing the annealing temperature up to T = 750 °C results in a gradual suppression of the activation character of the resistivity behavior at low temperatures. The additional annealing is concluded to be accompanied by two processes: a cleaning of the graphene surface from adsorbed contaminations and an additional defect formation in the graphene structure. A relative role of these processes in dependence on the annealing temperature and the type of the ρ(T) dependence for the initial sample is discussed.     

Metal atom-induced diffuse scattering from short-range order among deuterium atoms in single crystal α-phase V2D

X-ray scattering has been studied above the critical ordering temperature, T_c (133°C), in b.c.c. α-phase V₂D. At 140 and 200°C quite weak diffuse intensity was observed centered on the reciprocal lattice points of the ordered superlattice. This intensity is due to short-range order among deuterium atoms which induces a local or short-range modulation of the parent vanadium lattice and suggests the possibility of local octahedral D—D fluctuations building up well above T_c.     

Evolution of the spectral characteristics upon annealing of lithium borate glasses containing europium and aluminum

The spectral and structural characteristics of lithium borate glasses containing europium and aluminum have been investigated upon annealing at different temperatures. It has been found that the spectral characteristics of the studied system change nonmonotonically with an increase in the annealing temperature. After annealing at a temperature of 600°C, the luminescence spectra of the glasses exhibit broad structureless bands that are specific for the amorphous phase containing Eu³⁺ ions. Then, after annealing at T = 700°C, narrow lines appear in the wavelength ranges 585–595 and 610–620 nm, which correspond to the luminescence of the Eu(BO₂)₃ and EuAl₃(BO₃)₄ borates. A further increase in the annealing temperature (T = 800–900°C) leads to the disappearance of europium aluminum borate. In the luminescence spectra of these samples, there are narrow bands in the wavelength range λ = 585–595 nm, which are typical of europium metaborate. Finally, at a temperature of 1050°C, these bands disappear and narrow lines appear again in the wavelength range 610–620 nm, which are characteristic of the EuAl₃(BO₃)₄ borate. Thus, the temperature annealing makes it possible to purposely change the spectral characteristics of the studied system in the wavelength range 590–615 nm.     

Thermoelectric power of magnetic metals: II. Anisotropic metals

We present a theory of the thermoelectric power tensor of anisotropic ferromagnetic metals with localized magnetic moments starting from the Boltzmann equation and incorporating anisotropy effects due to the lattice structure through a parameter measuring the anisotropy in the sound velocity. Elastic and inelastic phonon and spin scattering contributions are taken into account through a linear superposition of scattering cross sections. A mean field approximation is used to describe the ordered magnetic phase. Spin wave and impurity scattering, phonon and magnon drag are not included. In a range encompassing the Curie temperature, i.e. at “moderate temperatures”, the theory quantitatively reproduces observed features except for specific details (e.g. rounding near T_c) needing other physical input. We compare our theory to data on single crystals of Gd and Tb₇₅Gd₂₅. The c-axis thermoelectric power is well recovered for very reasonable values of anisotropy and scattering strength parameters. A conjecture is given to explain the basal plane thermoelectric power positive slope at high temperature.     

First observation of a wetting phase transition in low-angle grain boundaries

The wetting phase transition at low-angle intercrystallite grain boundaries has been experimentally observed. In contrast to the high-angle grain boundaries with the misorientation angels θ > 15°, the low-angle grain boundaries (θ < 15°) are not continuous two-dimensional defects, but constitute a discrete wall (network) of lattice dislocations (edge and/or helical). The theory predicts that, depending on θ, either a continuous layer of the liquid phase or a wall (network) of microscopic liquid tubes on wetted dislocation nuclei is formed at completely wetted low-angle grain boundaries. It has been shown that the continuous liquid layers at low-angle grain boundaries in the Cu-Ag alloys appear at the temperature T _wminL = 970°C, which is 180°C higher than the onset temperature T _wmin = 790°C and 50°C lower than the finish temperature of the wetting phase transition at high-angle grain boundaries, T _wmax = 1020°C.     

Intrinsic defects, impurities and doping in ZnO nanorods grown at low temperature

We report on the defect properties of single-crystalline ZnO nanorods grown from solutions at temperatures below 90 °C. The nanorods can easily be doped by providing impurity precursors during growth. In the as-grown state the nanorods exhibit considerable lattice strain and distortions which compromise their electrical and optical properties. Upon annealing at moderate temperatures of <400 °C the lattice strain is converted into dislocation-type defects, and the dopant impurities become optically active. In the annealed state the near-bandgap photoluminescence quantum efficiency is improved more than 5 times and reaches ～16 % at room temperature. Thus with moderate annealing, interesting device applications become feasible for nanorods grown at T<90 ^°C.     

Amorphous-crystalline transition in Ge films by photoemission

Photoelectron second-derivative, energy distribution spectra reveal subtle changes in the density-of-states of vapour-deposited, amorphous Ge films, arising from microstructural ordering processes. Evidence is presented for ‘ideal’ amorphous Ge films, prepared at a deposition temperature T_s = 120 ± 20°C, which undergo an abrupt amorphous-crystalline transition upon annealing to T_A ? 230 ± 10°C. These films are thought to approach closely the random-network Polk-model. High densities of localized states in the pseudogap are observed in imperfect films.     

Structure of (Ba0.7Sr0.3)TiO3 films prepared by chemical solution deposition on sapphire substrates

Barium strontium titanate (BST) films on single-crystal sapphire substrates are prepared by chemical solution deposition upon annealing at temperatures T = 700, 850, and 1000°C. The structure of the BST films is investigated using transmission electron microscopy, high-resolution electron microscopy, and x-ray diffraction. It is established that, upon annealing at T = 700 and 850°C, the film crystallizes in the tetragonal phase of the (Ba_0.7Sr_0.3)TiO₃ perovskite without texture and transition layers. The mean grain sizes are 17 and 37 nm, respectively. However, an increase in the annealing temperature to 1000°C brings about a decrease in the mean grain size to 25 nm and the appearance of additional phases due to the interaction at the film-substrate interface.     

Untersuchungen zur Supraleitung des Legierungssystems Zr-Rh

The transition temperatures of Zr-Rh alloys with small amounts of Rh are essentially higher than the transition temperature of pure Zr. Rhodium is not dissolved in the Zr lattice. In the unannealed specimens Rh stabilizes the body centered cubic phase which has aT _c of 6.4°K. After annealing an intermetallic compound is formed with aT _c of about 12°K. This compound is also formed in the unannealed specimens at higher Rh content.     

Lattice thermal expansion of cesium plumbo chloride

The temperature variation of the lattice parameter of CsPbCl₃ in the cubic phase has been studied by x-ray method, from a determination of the precision lattice parameter at various temperatures, ranging from 50°C to 400°C. The coefficient of thermal expansion of CsPbCl₃ can be expressed by the quadratic equation,α _T = 21.6 × 10⁻⁶ + 2.44 × 10⁻⁹ T + 5.90 × 10⁻¹¹ T ².     

Thermolumineszenz von KCl und NaCl nach Röntgen- und UV-Bestrahlung bei Heliumtemperatur

Glow curves of luminescence are recorded in the range from 10° K to 300° K. One gets characteristic changes by prior annealing the single crystals in O₂ or HCl. Also an increase of the lattice disorder causes new glow bands. In KCl a strong glow band always appears at 40° K after irradiating with X-rays or ultraviolet light in the range of the exciton bands. It is ascribed to trapped excitons, which become mobile at that temperature. For X-ray irradiated KCl the glow curve of luminescence is compared with the electrical glow bands and with the concentration change of known defects. The half widthΔT of all glow bands is found proportional to the temperature of the maximum:ΔT=(0.08±0.02)T _m .     

Transport properties of CeCu6 single crystals

The electrical resistivity ? and the thermoelectric power of CeCu₆ single crystals are strongly anisotropic. The inverse of the temperature of the Kondo resistivity maximum (T_max) roughly scales the linear temperature coefficient B of ? as well as the residual value (?₀ ÷ B ÷ 1/T_max). Along the [1 0 0] direction ? follows a T² Fermi-liquid law between 30 and 90 mK. The thermoelectric power is positive over the investigated temperature range (1–300 K) and shows two contributions.