Silver-substituted (Ag1-xCux)2ZnSnS4 solar cells from aprotic molecular inks

To battle the high open-circuit voltage deficit (V_OC,def) in kesterite (Cu₂ZnSnS₄ or CZTS) solar cells, a current field of research relates to point defect engineering by cation substitution. For example, by partly replacing Cu with an element of a larger ionic radius, such as Ag, the degree of Cu/Zn disorder decreases, and likewise does the associated band tailing. In this paper, solution-processed (Ag_1-xCu_x)₂ZnSnS₄ (ACZTS) samples are prepared through the aprotic molecular ink approach using DMSO as the solvent. The successful incorporation of silver into the CZTS lattice is demonstrated with relatively high silver concentrations, namely Ag/(Ag+Cu) ratios of 13% and 26%. The best device was made with 13% Ag/(Ag+Cu) and had an efficiency of 4.9%. The samples are compared to the pure CZTS sample in terms of microstructure, phase distribution, photoluminescence, and device performance. In the XRD patterns, a decrease in the lattice parameter c/a ratio is observed for ACZTS, as well as significant peak splitting with Ag addition for several of the characteristic kesterite XRD reflections. In addition to the improvement in efficiency, other advantageous effects of Ag-incorporation include enhanced grain growth and an increased band gap. A too high concentration of Ag leads to the formation of secondary phases such as SnS and Ag₂S as detected by XRD.     

Liquid phase sintering and characterization of SiC ceramics

The present study focuses on the sintering of silicon carbide-based ceramics (SiC) by liquid phase sintering (LPS) followed by characterization of the produced ceramics. AlN/Re₂O₃ mixtures were used as additives in the LPS process. In the first step, the LPS-SiC materials were produced in a graphite resistance furnace in the form of discs at different temperatures. The conditions with the best results regarding real density and relative density were taken as reference for sintering in the form of prismatic bars. In the second step, these samples were evaluated regarding fracture toughness (K_IC), by the Single Edge V Notch Beam – SEVNB – method, and flexural strength. K_IC behavior was evaluated according to the depth and curvature radius of the notches. Reliable K_IC values were presented when the ceramic displayed a small curvature radius at the notch tip. When the radius was large, it did not maintain the square root singularity of the notch tip. Tests were carried out to determine K_IC values in atmospheric air and water. K_IC results were lower in water than air, with a decrease ranging between 2.56% and 11.26%. The observations indicated a direct grain size correlation between K_IC values and fracture strength of the SiC ceramics.     

Microstructural and compositional aspects of ZnO-based varistor ceramics prepared by direct mixing of the constituent phases and high-energy milling

ZnO-based varistor samples were prepared by the direct mixing of the constituent phases (DMCP) and sintering at 1100 °C for 2 h. The influence of the starting powder mixture's composition – the amounts of the pre-reacted varistor compounds and their composition – and its preparation, either with or without mechano-chemical activation (MCA), on the microstructure, phase composition and electrical characteristics of the varistor samples was studied. It showed that MCA improved the density and microstructural homogeneity of the varistor samples. MCA strongly affected the grain growth: it enhanced the nucleation of inversion boundaries (IBs) in the ZnO grains and the IBs-induced grain-growth mechanism resulted in uniform grain growth and hence a microstructure with smaller ZnO grains and a narrower grain size distribution. The final phase composition of the samples prepared by the DMCP method mainly depended on the presence of varistor dopants that can prevent the formation of the pyrochlore phase, especially Cr₂O₃, while MCA can affect it mostly by providing a homogeneous distribution of those dopants. The DMCP varistor samples prepared with MCA had much better current–voltage characteristics than the samples of the same composition prepared from unactivated powders.     

Phase composition and properties of solid solutions of GdFeO3–GdInO3 bulks

Solid solutions of the GdFeO₃–GdInO₃ system were prepared at 1550 °C by ceramic powder processing. The formulated composition was Gd(Fe_1−xIn_x)O₃ (GFI) with the indium contents at x = 0, 0.25, 0.5, 0.75, and 1.0. A stable phase of Gd(Fe_1/3In_2/3)O₃ in our system was identified by X-ray diffraction and phase composition analysis. Multi-phase morphologies were observed for GFI bulks with x = 0.5 and 0.75. Dielectric and electrical properties of the GFI bulks were investigated. The addition of 25% In³⁺ in GdFeO₃ had an obvious enhancement in polarization and led to an elevated resonance frequency. Dielectric properties of GFI bulks except GdInO₃ were strongly dependent upon the test frequency, which corresponded to the response of polarization mechanism. GdInO₃ displayed as a stable dielectric, which was frequency- and temperature-insensitive. GdInO₃ was thermally activated and became leaky until above 600 °C.     

Cellular glass obtained from non-powder preforms by foaming with steam

Cellular glass, or foamed glass, has been obtained as a result of the heating (to 700–800 °C) of heavy and strong preforms formed due to the binding properties of the silicate additives. Durability of the preforms reached 6 MPa at the density of 1.8 g/cm³. The main expanding agent in the composition is steam, which can also be a carbon oxidizer and increase the amount of the evolved gases and decrease the density of the foamed glass obtained. As a result of changing the initial composition structure, the density of the obtained foamed glass varied from 0.14 to 0.6 g/cm³, its breaking strength - from 0.6 to 5.0 MPa. and heat conductivity – from 0.045 to 0.15 W/(m·К), respectively. The speed of expansion of the preforms had an extreme character with the induction period typical for topochemical reactions. The obtained cellular materials possessed a distinct crystalline structure. The experiments showed the possibility of obtaining cellular materials with acceptable properties from different types of glass for the solution of environmental tasks. Various technological methods of obtaining cellular material blocks from preforms of various forms were tested to use them for thermal insulation and facing materials.     

Facile size-controlled synthesis of well-dispersed spherical amorphous alumina nanoparticles via homogeneous precipitation

Well-dispersed spherical amorphous alumina nanoparticles with a narrow size distribution were obtained by facile homogeneous precipitation and subsequent calcination. In the synthesis, formamide was used as the precipitant, and mixtures of aluminum sulfate and aluminum nitrate with different molar ratios were used as the aluminum sources. The average size of the amorphous alumina nanoparticles was successfully controlled by adjusting the amount of formamide and the sulfate/nitrate molar ratio. The particle size decreased with increasing amount of formamide and decreasing sulfate/nitrate molar ratio. Dispersed spherical amorphous alumina nanoparticles with average sizes of 23, 34, 45, and 57 nm were prepared using 100 mL formamide at sulfate/nitrate molar ratios of 1:9, 2:8, 3:7, and 4:6, respectively.     

Synthesis and phase stability of precursor derived HfO2/Si-C-N-O nanocomposites

Hafnium alkoxide modified polysilazane was synthesized by the drop-wise addition of hafnium tetra(n-butoxide) to polysilazane. The solid state thermolysis (SST) temperature and the ceramic yield for both the polysilazane and modified polysilazane were determined by performing thermogravimetry. Fourier transform infrared spectroscopy was performed to understand the polymer to ceramic conversion as well as the bonding characteristics of the ceramics. The modified polymer after crosslinking was subjected to SST at 800 °C at a constant heating rate of 5 °C/min for a dwell time of 2 h in atmospheric ambience. From the X-ray diffractograms, the as-thermolysed ceramics were observed to remain X-ray amorphous and on heat-treatment resulted in the crystallization of tetragonal hafnia. However, on heat-treatment at 1500 °C, reverse phase transformation from tetragonal to monoclinic hafnia was observed. Raman spectroscopy and transmission electron microscopy were employed to further understand the phase evolution. The thermal stability and the influence of amorphous matrix on the coarsening of HfO₂ were also evaluated.     

Effect of Nb2O5 content and sintering temperature on the microstructure and bending strength of porous Ni-YSZ cermets

Porous Ni-YSZ cermets are prepared by reducing NiO-YSZ composites upon exposure to (Ar+6% H₂) gas. The porous cermets are prepared by the addition of carbon black (0.123 mol) to mixed NiO-YSZ powders and the conversion of NiO to Ni in the NiO-YSZ composites. The microstructure and bending strength of porous Ni-YSZ cermets as functions of sintering temperature and Nb₂O₅ content are discussed. The Ni-YSZ cermets consist of uniformly distributed Ni and YSZ grains as well as pores. Both higher sintering temperature and higher Nb₂O₅ content yield lower porosity, thus increasing the bending strength. The bending strength of 0.00470 mol% Nb₂O₅–containing Ni-YSZ cermets sintered at 1400 °C (111 MPa) is about two times higher than that of Nb₂O₅–free Ni-YSZ cermets sintered at 1400 °C (59 MPa).     

Novel magnetodielectric cobalt ferrite–titania–silica ceramic composites with tunable dielectric properties

The cobalt ferrite (CF)–titania (TiO₂)–silica (SiO₂) system has been studied to produce new ceramic composites by conventional solid state reaction. The microstructure of the sintered CF–TiO₂–SiO₂ mixture has been related to compositional modifications in terms of SiO₂/TiO₂ weight ratio keeping constant the CF weight percentage. Microstructural characterization of the sintered bodies was performed in order to understand microstructure evolution, and to quantify the phases volume fraction. The final compositions after sintering differ significantly from the starting ones as a consequence of the reaction of titania with the ferrite, and the formation of the ilmenite-type CoTiO₃. Four different distributed phases are present, depending on the starting SiO₂/TiO₂ weight ratio. The complex permittivity dispersion of ceramic composites was investigated and correlated to their microstructure. Lastly, CF-SiO₂ magneto-dielectric composites are suggested as possible candidates for high frequency applications as miniaturized antennas.     

Mechanical activation-assisted autoclave processing and sintering of HfB2-HfO2 ceramic powders

This study reports on the synthesis and consolidation of HfB₂-HfO₂ ceramic powders via mechanical activation-assisted autoclave processing followed by pressureless sintering (PS) or spark plasma sintering (SPS). HfCl₄, B₂O₃ and Mg starting powders were mechanically activated for 5 min to obtain homogeneously blended precursors with active particle surfaces. Autoclave synthesis was carried out at a relatively low temperature at 500 °C for 6 or 12 h. As-synthesized powders were purified from reaction by-products such as MgO and MgCl₂ by washing and acid leaching treatments. The characterization investigations of the as-synthesized and purified powders were performed by using an X-ray diffractometer (XRD), stereomicroscope (SM), scanning electron microscope (SEM) and particle size analyzer (PSA). The purified powders with an average particle size of about 190 nm comprised the HfB₂ phase with an amount of 79.6 wt% in addition to the HfO₂ phase and a very small amount of Mg₂Hf₅O₁₂ phase after mechanical activation for 5 min and autoclave processing for 12 h. They were consolidated at 1700 °C both by PS for 6 h and SPS for 15 min. The Mg₂Hf₅O₁₂ phase decomposed during sintering and bulk samples only had the HfB₂ and HfO₂ phases. The bulk properties of the sintered samples were characterized in terms of microstructure, density, microhardness and wear characteristics. The HfB₂-HfO₂ ceramics consolidated by PS exhibited poor densification rates. A considerable improvement was obtained in the relative density (~91%), microhardness (~16 GPa) and relative wear resistance (2.5) values of the HfB₂-HfO₂ ceramics consolidated by SPS.