In situ investigation of the formation of Cu(In,Ga)Se2 from selenised metallic precursors by X-ray diffraction—The impact of Gallium, Sodium and Selenium excess

The chemical reactions during rapid thermal processing of stacked elemental layers were investigated by angle-dispersive in situ X-ray diffraction. With a time resolution of 5 diffractograms per minute four different solid state reactions resulting in ternary chalcopyrites were identified: (A) CuSe+InSe→CuInSe₂, (B) Cu₂Se+2InSe+Se→2CuInSe₂, (C) Cu₂Se+In2Se₃→2CuInSe₂, (D) Cu₂Se+Ga2Se₃→2CuGaSe₂. All these reactions form pure tenary chalcopyrites. The reaction resulting in the mixed crystal Cu(In,Ga)Se₂ starts not before (B) has begun. The reaction speed of (A) and the fraction of CuInSe₂ formed by (B) depend on Na-doping and Se-pressure, (C) takes place only, if the reaction paths (A) and (B) are suppressed. Reaction (D) is observed only, if 25% In is replaced by Ga in the precursor. The diffractograms were evaluated by Rietveld refinement to give the phase contents of the samples as a function of reaction time.     

Investigations on co-evaporated Cu2SnSe3 and Cu2SnSe3-ZnSe thin films

Cu₂SnSe₃ is an important precursor material for the growth of Cu₂ZnSnSe₄, an emerging solar cell absorber layer via solid state reaction of Cu₂SnSe₃ and ZnSe. In this study, we have grown Cu₂SnSe₃ (CTSe) and Cu₂SnSe₃-ZnSe (20%) films onto soda-lime glass substrates held at 573 K by co-evaporation technique. The effect of annealing of these films at 723 K for an hour in selenium atmosphere is also investigated. XRD studies of as-deposited Cu₂SnSe₃ and Cu₂SnSe₃-ZnSe films indicated SnSe as secondary phase which disappeared on annealing. The direct optical band gap of annealed Cu₂SnSe₃ and Cu₂SnSe₃-ZnSe films were found to be 0.90 eV and 0.94 eV respectively. Raman spectroscopy studies were used to understand the effect of ZnSe on the properties of Cu₂SnSe₃.     

Cu2ZnSnSe4 films by selenization of Sn–Zn–Cu sequential films

This paper deals with the formation of Cu₂ZnSnSe₄ (CZTS) in the process of selenization of metal precursor layers in elemental selenium vapour. Metallic precursors were sequentially evaported from Sn, Zn and Cu sources. Precursor Sn–Zn–Cu films have a “mesa-like” structure and consist mainly of Cu₅Zn₈ and Cu₆Sn₅ phases. It was confirmed that the formation of different binary copper selenides is the dominating process of selenization in elemental Se vapour at temperatures up to 300 °C. The formation of kesterite CZTS films begins at 300 °C and dominates at higher temperatures, always resulting in multiphase films that consist of high-quality Cu₂ZnSnSe₄ crystals and of a separate phase of ZnSe.     

Synthesis of earth-abundant Cu2SnS3 powder using solid state reaction

Cu₂SnS₃ (CTS) powder has been synthesized at 200 °C by solid state reaction of pastes consisting of Cu and Sn salts and different sulphur compounds in air. The compositions of the products is elucidated from XRD and only thiourea is found to yield CTS without any unwanted CuS_x or SnS_y. Rietveld analysis of Cu₂SnS₃ is carried out to determine the structure parameters. XPS shows that Cu and Sn are in oxidation states +1 and +4, respectively. Morphology of powder as revealed by SEM shows the powder to be polycrystalline with porous structure. The band gap of CTS powder is found to be 1.1 eV from diffuse reflectance spectroscopy. Cu₂SnS₃ pellets are p-type with electrical conductivity of 10⁻² S/cm. The thermal degradation and metal–ligand coordination in CTS precursor are studied with TGA/DSC and FT-IR, respectively, and a probable mechanism of formation of CTS has been suggested.     

Structural and electrical properties of high-energy ball-milled NASICON type Li1.3Ti1.7Al0.3(PO4)2.9(VO4)0.1 ceramics

Nano-crystallites of Li_1.3Ti_1.7Al_0.3(PO₄)_2.9(VO₄)_0.1 NASICON type material are prepared by means of solid-state reaction of a stoichiometric mixture after milling it for 22 and 55 h. The milling reduces the average crystallite size of the ceramic to 80 and 60 nm, respectively. Mechanical milling changes structural parameters and the strain induced at the grain-boundaries plays a major role in improving electrical conductivity. An order of magnitude increase in electrical conductivity is observed in the material milled for 55 h compared to the unmilled material, which is also reflected in permittivity loss. Modulus and permittivity representations substantiate the constriction effect of grain-boundaries observed in the complex impedance representation.     

Valence band photoemission study of the copper chalcogenide compounds, Cu2S, Cu2Se and Cu2Te

The electronic structures of the copper chalcogenide compounds, Cu₂S, Cu₂Se and Cu₂Te have been investigated by taking photoemission data with synchrotron photon sources. The band calculations are done using the full-potential linear-muffin-tin-orbital method. Since the crystal structures are not clarified well, several simplified structure models are used. The calculated densities of states are compared with the observed spectra. The analysis shows that a sharp peak at −3.5 eV is due to the Cu 3d states, and that the tails at the high and low energy sides of the Cu 3d peak are due to the chalcogen p states.     

One-step fastest method of nanocrystalline CuAlS<Subscript>2</Subscript> chalcopyrite synthesis,and its nanostructure characterization

Nanocrystalline CuAlS₂ chalcopyrite has been mechanosynthesized at room temperature for the first time by ball milling the stoichiometric mixture of elemental of Cu, Al, and S powders under argon atmosphere. Initially, the CuAlS₂ phase is formed by solid state reaction of elemental powders within 15 min of milling and in the course of milling crystallite size decreases slowly to ~5 nm within 10 h of milling. Microstructure characterization and phase transformation kinetics of the elemental powders toward the chalcopyrite phase formation has been made by employing the Rietveld analysis using X-ray diffraction data of unmilled and ball-milled samples.     

The effect of sulfide substitution in the mixed conductor Cu7PSe6

Cu₇PSe₆ is a mixed conductor exhibiting structural phase transitions above and below room temperature that are accompanied by step-like changes in electrical conductivity. The substitution of S²⁻ for Se²⁻ in Cu₇PSe₆ significantly enhances electrical conductivity at room temperature compared to that observed for the pure compound. In the case of Cu₇P(Se_0.80S_0.20)₆, a nearly temperature-independent electrical conductivity exceeds 1 S/cm with no evidence of any phase transitions throughout the temperature interval 200-400 K. However, the ionic contribution accounts for just 2% of the total electrical conductivity in this solid solution at room temperature.     

Characteristics of spherical-shaped Li4Ti5O12 anode powders prepared by spray pyrolysis

Spherical-shaped Li₄Ti₅O₁₂ anode powders with a mean size of 1.5 μm were prepared by spray pyrolysis. The precursor powders obtained by spray pyrolysis had no peaks of crystal structure of Li₄Ti₅O₁₂. The powders post-treated at temperatures of 800 and 900 °C had the single phase of spinel Li₄Ti₅O₁₂. The powders post-treated at a temperature of 1000 °C had main peaks of the Li₄Ti₅O₁₂ phase and small impurity peaks of Li₂Ti₃O₇. The spherical shape of the precursor powders was maintained after post-treatment at temperatures below 800 °C. The Brunauer-Emmett-Teller (BET) surface areas of the Li₄Ti₅O₁₂ anode powders post-treated at temperatures of 700, 800 and 900 °C were 4.9, 1.6 and 1.5 m²/g, respectively. The initial discharge capacities of Li₄Ti₅O₁₂ powders were changed from 108 to 175 mAh/g when the post-treatment temperatures were changed from 700 to 1000 °C. The maximum initial discharge capacity of the Li₄Ti₅O₁₂ powders was obtained at a post-treatment temperature of 800 °C, which had good cycle properties below current densities of 0.7 C.     

Gas-sensing properties of perovskite La0.875Ba0.125FeO3 nanocrystalline powders

La_0.875Ba_0.125FeO₃ nanocrystalline powders have been prepared by a sol-gel method. The structure, conductance and gas-sensing properties were investigated. La_0.875Ba_0.125FeO₃ crystallizes as a perovskite phase with the orthorhombic structure. The La_0.875Ba_0.125FeO₃ based sensor shows good sensitivity and selectivity to alcohol gas. The highest sensitivity to 500 ppm alcohol gas reached was 58 at 170 °C. The adsorption of O₂ on the La_0.875Ba_0.125FeO₃ (0 1 0) surface was studied with the first-principles calculation based on the density functional theory. The results show that the surface states are near the Fermi energy level and that the Fe ion plays an important role in the process of oxygen adsorption, which affects the gas-sensing properties.     

Nanoparticles of the giant dielectric material, CaCu3Ti4O12 from a precursor route

A method of preparing the nanoparticles of CaCu₃Ti₄O₁₂ (CCTO) with the crystallite size varying from 30 to 200 nm is optimized at a temperature as low as 680 °C from the exothermic thermal decomposition of an oxalate precursor, CaCu₃(TiO)₄(C₂O₄)₈·9H₂O. The phase singularity of the complex oxalate precursor is confirmed by the wet chemical analyses, X-ray diffraction, FT-IR and TGA/DTA analyses. The UV-vis reflectance and ESR spectra of CCTO powders indicate that the Cu(II) coordination changes from distorted octahedra to nearly flattened tetrahedra (squashed) to square-planar geometry with increasing annealing temperature. The HRTEM images have revealed that the evolution of the microstructure in nanoscale is related to the change in Cu(II) coordination around the surface regions for the chemically prepared powder specimens. The nearly flattened tetrahedral geometry prevails for CuO₄ in the near surface regions of the particles, whereas square-planar CuO₄ groups are dominant in the interior regions of the nanoparticles. The powders derived from the oxalate precursor have excellent sinterability, resulting in high-density ceramics which exhibited giant dielectric constants upto 40,000 (1 kHz) at 25 °C, accompanied by low dielectric loss <0.07.     

Rapid exothermic synthesis of chalcopyrite-type CuInSe2

Chalcopyrite-type CuInSe₂ (CIS) was synthesized from Cu, In and Se powders by a mechanochemical process (MCP) without any additional heating. When the transparent reactor bottle was strongly shaken, the elemental powders underwent an explosive reaction. The reaction generated a large amount of heat accompanied by simultaneous strong light emission. The product was confirmed to be chalcopyrite-type CIS by X-ray powder diffraction analysis. From the results, we categorized that preparation of CIS by MCP is a form of ‘self-propagating high-temperature synthesis’ (SHS) or ‘gasless combustion synthesis’. In ordinary SHS, a reaction is initiated from a sample surface by a heat flux such as a heated wire, electric spark, or laser beam. On the other hand, in the present reaction system (Cu+In+2Se), was naturally ignited only by mechanical stimulation. Following initiation by an external stimulus, the reaction was self continuing via the exothermic heat generated. The reaction mechanism of the preparation of CIS by the MCP is discussed on the basis of present reaction observations and thermochemical data.     

Structural evolutions of the mechanically alloyed Al<Subscript>70</Subscript>Cu<Subscript>20</Subscript>Fe<Subscript>10</Subscript> powders

Elemental mixtures of Al, Cu, Fe powders with the nominal composition of Al₇₀Cu₂₀Fe₁₀ were mechanically alloyed in a planetary ball mill for 80 h. Subsequent annealing of the as-milled powders were performed at 600–800°C temperature range for 4 h. Structural characteristics of the mechanically alloyed Al₇₀Cu₂₀Fe₁₀ powders with the milling time and the heat treatment were investigated by X-ray diffraction (XRD), differential scanning calorimeter (DSC) and differential thermal analysis (DTA). Mechanical alloying of the Al₇₀Cu₂₀Fe₁₀ did not result in the formation of icosahedral quasicrystalline phase (i-phase) and a long time milling resulted in the formation of β-Al(Cu,Fe) solid solution phase (β-phase). The i-phase was observed only for short-time milled powders after heat treatment above 600°C. The β-phase was one of the major phases in the Al₇₀Cu₂₀Fe₁₀ alloy. The w-Al₇Cu₂Fe₁ phase (w-phase) was obtained only after heat treatment of the short-time milled and unmilled samples. The present investigation indicated that a suitable technique to obtain a large amount of quasicrystalline powders is to use a combination of short-time milling and subsequent annealing.     

Microwave-assisted synthesis and optical property of CdMoO4 nanoparticles

Microwave-assisted synthesis is a novel method used to synthesize CdMoO₄ nanoparticles in propylene glycol. The effects of reaction time and microwave power on phase, morphologies, and optical properties of CdMoO₄ nanoparticles were studied, using X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), and UV-visible spectroscopy. The present analyses proved that these crystalline powders were scheelite-type tetragonal structured CdMoO₄, with the crystallite size of 14-20 nm, and 4.51-4.73 eV band gaps, controlled by the synthetic conditions.     

Observation of iron impurity diffusion in silicon under bending stress by Mössbauer spectroscopy

In the present work, the formation of the Al₇₀Cu₂₀Fe₁₀ icosahedral phase by mechanical alloying the elemental powders in a high-energy planetary mill was investigated by X-ray diffraction and Mössbauer spectroscopy. It was verified that the sample milled for 80 h produces an icosahedral phase besides Al(Cu, Fe) solid solution (β-phase) and Al₂Cu intermetallic phase. The Mössbauer spectrum for this sample was fitted with a distribution of quadrupole splitting, a doublet and a sextet, revealing the presence of the icosahedral phase, β-phase and α-Fe, respectively. This compound is not a good hydrogen storage. The results of the X-ray diffraction and Mössbauer spectroscopy of the sample milled for 40 h and annealed at 623°C for 16 h shows essentially single i-phase and tetragonal Al₇Cu₂ Fe phase.     

Internal stress dependent structural transition in ferromagnetic Ni-Mn-Ga nanoparticles prepared by ball milling

Ni-Mn-Ga nanoparticles were prepared by ball milling technique. X-ray diffraction pattern of the milled powders has a broad peak near the location of the prominent peak for the Heusler phase of Ni₂MnGa, indicating very disordered structures with small particle sizes. Structural properties of milled Ni-Mn-Ga particles recover to those of the bulk state after appropriate annealing temperature. It is worth noting that particles with size above 50 nm exhibit martensitic transformation. The average internal stress was calculated to be 2.83-1.13 MPa stored in the distorted lattice. Saturation magnetization of the milled sample increases with annealing temperature due to re-crystallization and grain growth.     

Structure and magnetic properties of nanocrystalline Fe75Si25 powders prepared by mechanical alloying

Nanocrystalline Fe₇₅Si₂₅ powders were prepared by mechanical alloying in a planetary ball mill. The evolution of the microstructure and magnetic properties during the milling process were studied by X-ray diffraction, scanning electron microscope and vibrating sample magnetometer measurements. The evolution of non-equilibrium solid solution Fe (Si) during milling was accompanied by refinement of crystallite size down to 10 nm and the introduction of high density of dislocations of the order of 10¹⁷ m⁻². During the milling process, Fe sites get substituted by Si. This structural change and the resulting disorder are reflected in the lattice parameters and average magnetic moment of the powders milled for various time periods. A progressive increase of coercivity was also observed with increasing milling time. The increase of coercivity could be attributed to the introduction of dislocations and reduction of powder particle size as a function of milling time.     

Influence of annealing temperature on structural, optical and magnetic properties of Zn0.95Cu0.02Cr0.03O powders

Zn_0.95Cu_0.02Cr_0.03O powders have been synthesized by the sol-gel method and sintered in argon atmosphere under different temperatures. The structural, optical and magnetic properties of the powders were investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) and vibrating sample magnetometer (VSM). The XRD results demonstrated that Cr and Cu ions are incorporated into ZnO successfully when annealing temperatures were 600 and 700 °C. But when the samples were annealed at 500 °C, the crystallinity of the samples was not very good. However, when the annealing temperature was increased to 800 °C, the secondary phase of Cu and ZnCr₂O₄ appeared in the samples. The PL spectra revealed that the position of the ultraviolet (UV) emission peak of the samples showed a blue shift and the green emission peak enhanced significantly with the annealing temperature increasing from 600 to 700 °C. Magnetic measurements indicated that the room temperature ferromagnetism of Zn_0.95Cu_0.02Cr_0.03O was intrinsic in nature. In addition, the saturation magnetization (Ms) increased from 0.0078 to 0.0088 emu/g with the annealing temperature increased from 600 to 700 °C.     

In-situ XAS study on the Cu and Ce local structural changes in a CuO-CeO2/Al2O3 catalyst under propane reduction and re-oxidation

The redox behaviour of a CuO-CeO₂/Al₂O₃ catalyst is studied under propane reduction and re-oxidation. The evolution of the local Cu and Ce structure is studied with in-situ transmission X-ray absorption spectroscopy (XAS) at the Cu K and Ce L₃ absorption edges.CuO and CeO₂ structures are present in the catalyst as such. No structural effect on the local Cu structure is observed upon heating in He up to 873 K or after pre-oxidation at 423 K.Exposure to propane at reaction temperature (600-763 K) fully reduces the Cu²⁺ cations towards metallic Cu⁰. Quick EXAFS spectra taken during reduction show a small amount of intermediate Cu¹⁺ species. Parallel to the CuO reduction, CeO₂ is also reduced in the same temperature range. About 25% of the Ce⁴⁺ reduces rapidly to Ce³⁺ in the 610-640 K temperature interval, while beyond 640 K a further slower reduction of Ce⁴⁺ to Ce³⁺ occurs. At 763 K, Ce reduction is still incomplete with 32% of Ce³⁺.Re-oxidation of Cu and Ce is fast and brings back the original oxides.The propane reduction of the CuO-CeO₂/Al₂O₃ catalyst involves both CuO and CeO₂ reduction at similar temperatures, which is ascribed to an interaction between the two compounds.     

Phase evolution and microwave dielectric properties of Bi3SbO7 ceramic

The Bi₃SbO₇ ceramic was prepared by the solid state reaction method and its phase evolution at different temperatures was studied. Low temperature phase α-Bi₃SbO₇ was formed at about 890 °C and it started to transform to high temperature phase β-Bi₃SbO₇ at about 960 °C. Microwave dielectric constants of α-Bi₃SbO₇ ceramic and β-Bi₃SbO₇ ceramic were 43.2 and 37.6, Qf value were 2080 and 5080 GHz, respectively. TCF of α-Bi₃SbO₇ ceramic was near zero and TCF of β-Bi₃SbO₇ ceramic was about −120 ppm/°C. The Bi₃SbO₇ ceramic is a promising candidate for low temperature co-fired ceramic (LTCC) technology due to its large dielectric constant, low dielectric loss at microwave region, low sintering temperature and simple composition.