TEM and DFT Study of Basal-plane Inversion Boundaries in SnO2-doped ZnO

In our recent study (Ribie et al. 2020) we reported the structure of inversion boundaries (IBs) in Sb2O3 -doped ZnO. Here, we focus on IBs that form in SnO2-doped ZnO. Using atomic resolution scanning transmission electron microscopy (STEM) methods we confirm that in SnO2-doped ZnO the IBs form in head-to-head configuration, where ZnO4 tetrahedra in both ZnO domains point towards the IB plane composed of a close packed layer of octahedrally coordinated Sn and Zn atoms. The in-plane composition is driven by the local charge balance, following Pauling's principle of electroneutrality for ionic crystals, according to which the average oxidation state of cations is 3+. To satisfy this condition, the cation ratio in the IB-layer is Sn4+ : Zn2+ =1:1. This was confirmed by concentric electron probe analysis employing energy dispersive spectroscopy (EDS) showing that Sn atoms occupy 0.504 +/- 0.039 of the IB layer, while the rest of the octahedral sites are occupied by Zn. IBs in SnO2-doped ZnO occur in the lowest energy, IB3 translation state with the cation sublattice expansion of Delta IB(zn-zn) of +91 pm with corresponding O-sublattice contraction Delta IB(O-O) of -46 pm. Based on quantitative HRTEM and HAADF-STEM analysis of in-plane ordering of Sn and Zn atoms, we identified two types of short-range distributions, (i) zigzag and (ii) stripe. Our density functional theory (DFT) calculations showed that the energy difference between the two arrangements is small (similar to 6 meV) giving rise to their alternation within the octahedral IB layer. As a result, cation ordering intermittently changes its type and the direction to maximize intrinsic entropy of the IB layer driven by the in-plane electroneutrality and 6-fold symmetry restrictions. A long-range in-plane disorder, as shown by our work would enhance quantum well effect to phonon scattering, while Zn2+ located in the IB octahedral sites, would modify the the bandgap, and enhance the in-plane conductivity and concentration of carriers

The influence of spark plasma sintering temperature on the properties of Sb-doped barium stannate ceramics

Barium-stannate (BaSnO3, BSO) is a member of the perovskite-type alkaline earth stannates ASnO3 (A = Ca, Sr, Ba) with an ideal cubic crystal structure (space group: ). Doping with antimony (Sb5+) can change this wide band-gap semiconductor (Eg = 3.1-3.4 eV) into an n-type semiconductor with high electrical conductivity at room temperature. The major drawbacks in the BSO-based ceramics synthesis are phase composition and low density of final ceramic materials. These problems could be solved using spark plasma sintering (SPS), a current and pressure-assisted technique, which enables the preparation of dense ceramics at significantly lower temperatures and for a shorter time. To investigate the influence of spark plasma sintering temperature on the structural, microstructural and electrical properties of BaSn1-xSbxO3 (BSSO, x = 0.00; 0,04; 0.06; 0.08; and 0.10) ceramics samples, BSSO powders were spark plasma sintered at 1100 °C, 1200 °C and 1250 °C for 5 min. X-ray diffraction (XRD) analysis confirmed that all ceramic samples sintered at 1100 °C crystallized in a single-phased cubic BSO structure. Their relative densities were in the range of 72–82% ρt. Sintering at 1200 °C increased the samples’ relative densities to 79–96% ρt, but also induced the formation of a barium-rich secondary phase, Ba2SnO4. Rising the sintering temperature further to 1250 °C induced the melting of all samples except BaSn0.92Sb0.08O3. Field emission scanning electron microscopy (FE-SEM) revealed that doping with antimony decreased the grain sizes in BSSO samples sintered at 1100 °C and 1200 °C up to the concentration x = 0.08. Electrical measurements revealed the typical semiconductor behavior of the undoped samples, showing nonlinear current-voltage characteristic and the existence of one semicircle in their impedance spectra, characteristic for materials with double Schottky barrier at the grain boundaries. However, samples with higher dopant concentrations (x = 0.08 and 0.10) showed significantly lower electrical resistivity and linear current-voltage characteristic. The lowest and almost constant value of electrical resistivity in the temperature range of 25–150 °C, and complete loss of the semicircle in its impedance spectrum revealed the metallic-like behavior of sample BaSn0.92Sb0.08O3 sintered at 1200 °C

Effect of the sintering technique on the properties of Sb-doped BaSnO3 ceramics

Barium stannate, BaSnO3 (BSO), a cubic perovskite-type oxide with its interesting structural, optical and electrical properties has wide application as an electrode material, thermally stable capacitor, transparent conductive oxide, photocatalyst, humidity and gas sensor material [1]. Partial substitution of Sn by Sb in BSO leads to drastic changes primarily in its electrical properties, resulting in metallic-like conductivity of doped ceramics. Major problems concerning the synthesis of Sb doped BSO (BSSO) are connected to the phase composition and density of final ceramic material. This study covers the comprehensive investigation of structural, microstructural and electrical properties of the Sb-doped BaSn1-xSbxO3 (x = 0.00, 0.04 and 0.08, BSSO) ceramic materials obtained by two different sintering techniques: conventional sintering (CS) and Spark Plasma sintering (SPS). The relative densities of the BSSO-CS ceramic samples sintered at 1600 °C for 3 h was in the range of 79– 96 %. On the other side, the relative densities of BSSO-SPS ceramic samples in the range of 86–96 % were obtained at 1200 °C, with sintering time of only 5 minutes. The XRD analysis confirmed that cubic BaSnO3 is a major phase in all BSSO samples. The presence of tetragonal Ba2SnO4 as a secondary phase was detected in BSSO-SPS ceramic samples, with its content decreasing upon Sb-doping. As expected, the grain size of the samples sintered at higher temperatures (BSSO-CS) is larger in comparison with BSSO-SPS ceramic samples, which was confirmed by Scanning Electron Microscopy (SEM). SEM analysis also revealed the layered structure within the grains of BaSn0.92Sb0.08O3-CS sample while HRTEM analysis confirmed the existence of the low angle grain boundaries (LAGBs) in the SPSed sample with the same composition. The electrical resistivity decreased upon Sb doping, and all doped BSSO samples showed the linear I-U characteristic in the temperature range of 25–150 °C. The semiconductor behavior of all BSSO-CS and BSSO-SPS (x = 0.00 and 0.04) ceramic samples was confirmed through the existence of semicircles in their impedance spectra. On the other hand, the BaSn0.92Sb0.08O3 sample showed the metallic-like behavior resulting from the loss of the electrostatic barriers at LAGBs, which is manifested through the absence of the semicircle in its impedance spectra

Correlation between the microstructure and electrical properties of Sb-doped BaSnO3 ceramics

The non-magnetic, non-inductive electroconductive materials with linear current-voltage characteristic and low and almost constant electrical resistivity in the wide temperature range could be used in conditions unfavorable for metals and alloys. Particular emphasis is placed on the performance and endurance of these materials in conditions at constant high voltage, current, and energy, as well as operating in acidic and humid environmental conditions. The aim of this work was to investigate the influence of antimony concentration and sintering parameters on the structure, microstructure, and electrical properties of antimony-doped barium stannate, BaSn1-xSbxO3 (BSSO, x = 0,00; 0,04; 0,06; 0,08 and 0,10) to obtain conductive electroceramic samples with linear current-voltage (I- U) characteristics and low electrical resistivity. For this purpose three different sintering techniques were used: conventional, spark plasma and cold sintering. According to the X-ray diffraction (XRD) analysis, single-phase ceramic mater- ials with cubic BaSnO3 structure were obtained by conventional sintering at 1600 °C for 3 h and spark plasma sintering at 1100 °C for 5 min. Raising the spark plasma sintering temperature to 1200 °C induced the formation of Ba-rich secondary phase, Ba2SnO4. XRD analysis confirmed the presence of unreacted SnO2 and BaCO3 in cold sintered BaSn0.92Sb0.08O3 sample (310 °C for 5 min, 20 wt.% 1 M acetic acid). Scanning electron microscopy (SEM) indicates a significant decrease in grain size upon doping, regardless of the sintering technique. High-resolution transmission electron microscopy (HRTEM) revealed the presence of low angle grain boundaries (LAGBs) in conventionally and spark plasma sintered (1200 °C for 5 min) samples with x = 0.08. The results of electrical measurements confirmed the semiconducting properties of all BSSO, except the spark plasma sintered BaSn0.92Sb0.08O3 (1200 °C for 5 min) sample. This sample showed linear current-voltage characteristic, the lowest and almost constant electrical resistivity in the temperature range of 25–150 °C resulting from the loss of potential barriers at grain boundaries due to the large fraction of LAGBs present in BaSn0.92Sb0.08O3 ceramic sample

The structural, electrical and optical properties of spark plasma sintered BaSn1-xSbxO3 ceramics

Antimony doped barium-stannate dense ceramic materials were synthesized using spark plasma sintering technique out of mechanically activated precursor powders. The influence of various Sb concentrations (x =0.00 – 0.10) on properties of BaSn1-xSbxO3 ceramics was investigated. Relative densities of prepared samples were in the range of (79–96) %. TEM analysis revealed the presence of many dislocations in undoped BaSnO3, and their significant reduction upon doping with Sb. All samples except BaSn0.92Sb0.08O3 exhibit non-linear I-U characteristic, typical for semiconductors with potential barrier at grain boundaries. Low angle grain boundaries found only in BaSn0.92Sb0.08O3 caused the loss of potential barrier at grain boundaries which was confirmed by AC impedance spectroscopy measurements. Consequently, BaSn0.92Sb0.08O3 showed the lowest electrical resistivity and linear I-U characteristic. UV–vis analysis confirmed the increasing of band gap (Burstein–Moss shift) values in all doped samples.This is the peer-reviewed version of the manuscript: Vukasinovic J, Pocuca-Nesic M, Golic DL, Ribic V, Brankovic Z, Savic SM, Dapcevic A, Bernik S, Podlogar M, Kocen M, Rapljenovic Z, Ivek T, Lazovic V, Dojcinovic B, Brankovic G, The structural, electrical and optical properties of spark plasma sintered BaSn1-xSbxO3 ceramics, Journal of the European Ceramic Society (2020), 40, 15, 5566-5575, doi: [https://doi.org/10.1016/j.jeurceramsoc.2020.06.062]The published version: [http://cer.ihtm.bg.ac.rs/handle/123456789/3628

Ultraviolet Protection Factor of Gray-state Plain Cotton Knitted Fabrics

The protection provided by clothing against ultraviolet (UV) radiation has been the subject of considerable recent research. However, a lack of investigations concerned with the influence of yarn properties on UV protection capabilities of fabrics seems to be present. This study investigated the influence of yarn twist and surface geometry on these properties of fabrics. The gray-state plain cotton knitted fabrics were produced from yarn differing in twist level under controlled conditions, so as to obtain as similar as possible construction of the fabrics. These plain knitted (single jersey) fabrics were spectrophotometrically assessed and UV protection factor was calculated. The results obtained indicated that yarn twist to a great extent influenced the UV protection properties of the knitted fabrics through the influence on yam compactness and surface properties, which in turn influenced the open porosity of the fabric. The results were also interpreted as the consequences of minimal differences between knitted fabrics' construction (different stitch density), which could not be avoided because of the yam twist differences

Novel engineering approach to optimization of thermal comfort properties of hemp containing textiles

The aim of this research was to investigate the possibilities of producing comfort hemp containing textile fabrics by assembling a pure hemp yarn with other-fibre containing yarn. The plain knitted fabrics were produced from two-assembled hemp and three variants of cotton yarns which differed in twist level, all having the same linear density. The transport properties (air permeability, water vapour permeability and thermal resistance) of the hemp-based knitted fabrics were quantitatively analysed. The results obtained demonstrated that the introduction of cotton into hemp-based textiles reduces air and water vapour permeability with the downward trend in thermal resistance. The extent to which the transport properties varied among the hemp/cotton knitted fabrics was dependent on the twist intensity of the cotton yarns. Therefore, the yarn assembling technique is an effective way not only to combine different fibre properties but to take advantage of intrinsic properties of component yarns

UV protection afforded by textile fabrics made of natural and regenerated cellulose fibres

In the last decades the media have highlighted the ozone depletion as major environmental problem resulting in an increase in ultraviolet radiation (UVR) reaching the earth's surface. Besides the beneficial effects of human exposure to UVR, this radiation is capable of causing damage to human population. The healthy lifestyle is becoming widely accepted by the public, and the UV protection provided by clothing becomes the significant subject of interest of the producers and consumers of textile fabrics. Natural cellulose fibres are commonly used in summer clothing due to their excellent comfort properties. However, these fibres have very poor UV protection ability. In this project, the UV protection property of textile fabrics made of natural and regenerated cellulose fibres have been compared and analysed in order to highlight the potential of hemp fibre for the development of more sustainable and healthy functionalized (UV protective) textile products. A group of homogeneous and blended cellulose textile fabrics were manufactured in a knitwear factory, evaluated in terms of the structure, and spectrophotometrically assessed to indicate their UV protection ability. The knitted fabrics merited sun protection ratings of "good" for pure hemp, through "very good" for viscose containing fabrics to "excellent" UV protection category for cotton based fabrics. Relatively high values of the Ultraviolet Protection Factor (UPF) of the cellulose. materials resulted from the interaction of fibre type, yarn geometry, fabric properties and common processing techniques. The increased UVR transparency of the pure hemp fabric, which resulted from hemp elasticity limitations, overcame by blending with other softer and more elastic cellulose fibres (cotton, viscose). The engineering approach proposed in this study was confirmed as an effective way to create more sustainable (more sustainable resource, pollution prevention, energy and cost savings) textile products with high level of UV protection at the knitting production stage avoiding the use of any additional mechanical and chemical treatments. These results revealed that the future application of hemp fibres in textile products with high added-value are promising. A co-ordinated effort of different subjects of the agro-and textile-industry production chain need to continue so as to overcome the limitations associated with hemp production and fibres properties

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 degrees 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

Thermal Design Method for Optimization of Dry Heat Transfer through Hemp-Based Knitted Fabrics

In the present research, folding of yarns was applied as a method for designing thermal transport characteristics of hemp-based knits. The rib-knitted fabrics were made by combining one of two cotton two-folded yarns (differing in folding twist) with one of the two complex hemp yarns. The parameters describing the thermal properties in the steady-state and transient regimes of heat transfer were determined. The results indicated that the packing density of the fibers in the yarn plays a crucial role in the heat conduction through the fabric. The two-assembled hemp component appeared to be the main heat conductor in the knitted fabrics containing it. Between the knits composed of the two-folded hemp and two-folded cotton yarns, that having two-folded more twisted cotton yarn as a component was characterized by higher thermal conductivity. The higher thermal conductivity resulted in higher thermal absorptivity, but not necessarily in higher thermal diffusivity, as the increased volumetric heat capacity may limit the speed of heat spreading through the fabric. Therefore, the knits containing two-assembled hemp yarn differed in thermal diffusivity despite their similar thermal conductivity. Both knits with the two-folded more twisted cotton yarn as a component had similar thermal diffusivity despite their different thermal conductivity