In this work, we focused on the chemical and (micro)structural changes that occur during curing at different temperatures of geopolymers (GP) prepared with a mullite-based precursor obtained by ...sol-gel synthesis. Several factors, such as the Si-to-Al ratio in the precursors and the curing temperatures, affect geopolymerisation. While potassium, sodium and even lithium are the most commonly studied cations, for this study, we examined geopolymers with sodium cations only. The H2O:Na2O ratio was set high at 22.80 to allow good workability of the paste having in mind the intended thin film application, i.e. the water-to-geopolymer binder ratio was set at 0.64. We performed extensive structural (diffraction and vibrational spectroscopy), morphological (electron microscopy) and electrical (impedance spectroscopy) characterisation, to facilitate an understanding of the conditions behind the optimal geopolymerisation. Particular attention was paid to the study of the evolution of the GP system as a function of curing temperature using solid-state magic angle spinning nuclear magnetic resonance. This allowed us to better understand the influence of the chemical composition of the constituents and their homogeneity, on the evolution of the organizational domains and electrical properties of the GP samples. We have explored the feasibility of moving from a relatively porous bulk to a thin film configuration, highlighting the applicability of geopolymers as constituents in photovoltaic facade systems.
In this paper, a relationship between the structure and the electrical properties of a nanocrystalline composite ceramics xNa2O·(100 − x)V2O5 with ‘x’ of 5, 15, 25, 35, and 45 mol%, abbreviated as ...xNV, was investigated by X-ray diffractometry (XRD), X-ray absorption spectroscopy (XAS), Cyclic Voltammetry (CV), Electrochemical impedance spectroscopy (EIS), and cathode active performance in Na-ion battery (SIB). For the expected sodium vanadium bronzes (NaxV2O5) precipitation, the preparation of xNV was performed by keeping the system in the molten state at 1200 °C for one hour, followed by a temperature decrease in the electric furnace to room temperature at a cooling rate of 10 °C min−1. XRD patterns of the 15NV ceramic exhibited the formation of Na0.33V2O5 and NaV3O8 crystalline phases. Moreover, the V K-edge XANES showed that the absorption edge energy of ceramics 15NV recorded at 5479 eV is smaller than that of V2O5 at 5481 eV, evidently indicating a partial reduction from V5+ to V4+ due to the precipitation of Na0.33V2O5. In the cyclic voltammetry, reduction peaks of 15NV were observed at 1.12, 1.78 V, and 2.69 V, while the oxidation peak showed up only at 2.36 V. The values of the reduction peaks were related to the NaV3O8 crystalline phase. Moreover, the diffusion coefficient of Na+ (DNa+) gradually decreased from 8.28 × 10−11 cm2 s−1 to 1.23 × 10−12 cm2 s−1 with increasing Na2O content (x) from 5 to 45 mol%. In the evaluation of the active cathode performance of xNV in SIB, ceramics 15NV showed the highest discharge capacity 203 mAh g−1 at a current rate of 50 mA g−1. In the wider voltage range from 0.8 to 3.6 V, the capacity retention was maintained at 50% after 30 cycles, while it was significantly improved to 90% in the narrower voltage range from 1.8 to 4.0 V, although the initial capacity decreased to 56 mAh g−1. It is concluded that the precipitation of the Na0.33V2O5 phase improved the structural and electrical properties of 15NV, which provides a high capacity for the Na-ion battery when incorporated as a cathode active material.
•Sol-gel synthesized and heat-treated 40FexAlSi samples with ‘x’ of 10, 15, and 20 resulted in the precipitation of fayalite, hematite and maghemite NPs with photocatalytic properties.•The ...corresponding degradation rate constant k, following the first-order reaction kinetics, was 14.89 × 10−2 min−1 for 40Fe2O3-15Al2O3-45SiO2 sample.•The presence of various crystalline phases that contain iron ions in mixed-valence states favors the formation of continuous pathways for uninterrupted polaronic transport which enhances an increase in DC conductivity.•Electrical conductivity and photocatalytic activity of the composites increase with the alumina content.
The glass samples with the composition of 40Fe2O3-xAl2O3-(60−x)SiO2 were prepared by sol-gel method. The samples were characterized by XRD, 80 K 57Fe Mössbauer spectroscopy, TEM, FTIR and impedance spectroscopy. In the glass samples with less than 10% alumina, fayalite and hematite were crystallized by addition of hercynite. Whereas nanoparticles of hematite (α-Fe2O3) and maghemite (γ-Fe2O3) doped with Al were confirmed with the increase of alumina contents (15% and 20%). The glass samples exhibit the electronic conduction which is explained by the polaronic conduction mechanism. It is noticed that the catalysts containing a mixture of hematite and maghemite nanoparticles showed higher photocatalytic degradation of Rhodamine B dye than those containing hematite alone. It is suggested from the characterizations of newly developed iron-containing aluminosilicate glass samples that the materials with several bandgaps and without dissolving itself should be developed in the future for water purification under visible light irradiation.
Phosphate glasses containing transition metal oxides such as MoO3 and WO3 are well-known for their semiconducting nature with polaronic conduction mechanism. These glasses can also accommodate a ...relatively high amount of alkali and silver oxides which give rise to ionic conductivity. Such a large compositional and preparation variability enables tuning of the types and mechanisms of the electrical conduction and makes these materials attractive for application in modern electrochemical devices. In this contribution, we discuss various factors that influence electrical transport in these glasses, from simple binary WO3/MoO3-P2O5 systems to complex ones containing variable amounts of alkali/silver oxides. Interestingly, WO3 and MoO3 can have very different roles in the electrical conduction in these materials. While MoO3 contributes weakly via polaronic transport in phosphate glasses, it strongly increases the mobility of alkali/silver ions by forming the mixed phosphate-molybdate network. On the other hand, the role of WO3 depends on the glass composition; it can contribute either directly, by introducing significant polaronic conductivity, or indirectly, by facilitating the transport of alkali/silver ions. The former behaviour is related to the formation of clusters of tungsten units in the glass network which enable fast transport of polarons whereas the latter one originates from the facilitating effect of mixed phosphate-tungstate units on the dynamics of alkali ions.
•WO3 and MoO3 can exhibit different influence on structural and transport properties of phosphate glasses.•Clustering of tungsten units in glass network facilitates polaronic conductivity.•Formation of mixed phosphate-molybdate and phosphate-tungstate units increases mobility of alkali ions.
For the transparent conductive films, the upgrading of their inherent properties has received much attention in their extensive areas of use, such as for opto-electronic devices. In comparison to the ...widely used indium tin oxide (ITO), aluminium-doped zinc oxide (AZO) films represent a low-cost alternative that can be easily fabricated by a variety of chemical and physical methods of deposition. In order to find the optimal conditions for obtaining AZO films that can be used as transparent conductive oxide layers (TCO), we investigated AZO films prepared by magnetron sputtering (MS) on glass substrates by using two targets: ZnO and Al. We individually varied power at targets during the deposition and subsequently thermally treated the prepared films. All films showed either a hexagonal wurtzite zincite structure or no diffraction pattern without clear relation to deposition parameters. Morphologic parameters were observed by scanning electron and atomic force microscopies and scattering experiments. Primarily Al loading and the extent of the thermal treatment were held responsible for influencing the course of the development of the structure, morphology and electrical functionality. From the functional point of view, we focussed on frequency-dependent electrical measurements (solid-state impedance spectroscopy) to show and describe electrical conductivity behaviour. In conclusion, it was evident that there are several strategies available for the tailoring of the electrical and optical properties of the MS-derived AZO films, one of which is thermal post-processing.
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In this paper, we studied the influence of polyvinylpyrrolidone (PVP) as a stabilization additive on optical and electrical properties of perovskite formamidinium lead iodide (FAPI) polycrystalline ...thin films on ZnO nanorods (ZNR). FAPI (as an active layer) was deposited from a single solution on ZNR (low temperature processed electron transport layer) using a one-step method with the inclusion of an anti-solvent. The role of PVP in the formation of the active layer was investigated by scanning electron microscopy and contact angle measurements to observe the effect on morphology, while X-ray diffraction was used as a method to study the stability of the film in an ambient environment. The effect of the PVP additive on the optical and electrical properties of the perovskite thin films was studied via photoluminescence, UV-Vis measurements, and electrical impedance spectroscopy. We have demonstrated that PVP inclusion in solution-processed perovskite FAPI thin films prevents the degradation of the film in an ambient atmosphere after aging for 2 months. The inclusion of the PVP also improves the infiltration of FAPI perovskite into ZnO nanostructures, increases electrical conductivity and radiative recombination of the photo-generated charge carriers. These results show promising information for promoting PVP stabilized FAPI perovskites for the new generation of photovoltaic devices.
•Structural and transport properties of potassium iron phosphate glasses are studied.•Predominantly pyrophosphate structure is observed for all glasses.•The DC conductivity changes in a non-monotonic ...fashion exhibiting a broad maximum.•Potassium ions are weakly mobile in iron pyrophosphate glass network.•Dominant conduction mechanism in all glasses is polaronic.
Structural and electrical properties of glasses of the molar composition xK2O−(40−x/2)Fe2O3−(60−x/2)P2O5, x = 0–36 mol%, are investigated by Raman, Mössbauer and Impedance spectroscopy. All glasses show pyrophosphate structure with a growing tendency for depolymerization with increasing K2O content. The DC conductivity changes in a non-monotonic fashion, exhibiting a broad flat maximum as K2O gradually substitutes Fe2O3 and P2O5. The observed conductivity trend could be related to the changes in the number density of polarons which depends on Fe2O3 and fraction of ferrous ions, indicating dominant polaronic conduction mechanism in all glasses. Interestingly, a relatively small addition of K2O, up to 12 mol%, has a positive effect on the polaronic transport due to slight modifications of glass network. Potassium ions are weakly mobile in iron pyrophosphate glass network and have a negligible contribution to the electrical transport even when their molar fraction is as high as 36 mol%.
BackgroundThe dental ceramic materials are constantly being developed due to their continuous clinical application in the field of esthetic dentistry. Glass ceramics (GC) materials are also of ...special interest for dental application due to their specific properties; and thus, they can be applied as crowns, veneers and small bridges. PurposeHowever, due to a variety of different GC materials, it is of keen interest to inspect their morphology and ion-diffusion, which also governs aging properties. Material and methodsIn this study, two different GC materials were processed, i.e., lithium silicate (LS-10) and lithium disilicate (LS-20). The aforementioned properties can be inspected by using impedance spectroscopy (IS) and scanning electron microscopy (SEM). ResultsSEM study suggested that LS-10 material is harder to mechanically process by computer-aided design/computer-aided manufacturing (CAD/CAM) technology. Furthermore, IS measurements showed that LS-20 (vs. LS-10) has more pronounced resistance properties. ConclusionAccording to IS data, it was concluded that LS-20 (vs. LS-10) has more pronounced resistance properties that point out to hindered ion-diffusion and to better aging properties.
Transformation of electrical transport from ionic to polaronic in glasses, which are a potential class of new cathode materials, has been investigated in four series containing WO3/MoO3 and Li+/Na+ ...ions, namely: xWO3–(30−0.5x)Li2O–(30−0.5x)ZnO–40P2O5, xWO3–(30−0.5x)Na2O–(30.5x)ZnO–40P2O5, xMoO3–(30−0.5x)Li2O–(30−0.5x)ZnO–40P2O5, and xMoO3–(30−0.5x)Na2O–(30−0.5x)ZnO–40P2O5, 0 ≤ x ≤ 60, (mol%). This study reports a detailed analysis of the role of structural modifications and its implications on the origin of electrical transport in these mixed ionic‐polaron glasses. Raman spectra show the clustering of WO6 units by the formation of W–O–W bonds in glasses with high WO3 content while the coexistence of MoO4 and MoO6 units is evidenced in glasses containing MoO3 with no clustering of MoO6 octahedra. Consequently, DC conductivity of tungstate glasses with either Li+ or Na+ exhibits a transition from ionic to polaronic showing a minimum at about 20‐30 mol% of WO3 as a result of ion‐polaron interactions followed by a sharp increase for six orders of magnitude as WO3 content increases. The formation of WO6 clusters involved in W‐O‐W linkages for tungsten glasses plays a key role in significant increase in DC conductivity. On the other hand, DC conductivity is almost constant for glasses containing MoO3 suggesting an independent ionic and polaronic transport pathways for glasses containing 10‐50 mol% of MoO3.
Transparent conducting oxides (TCO) with high electrical conductivity and at the same time high transparency in the visible spectrum are an important class of materials widely used in many devices ...requiring a transparent contact such as light-emitting diodes, solar cells and display screens. Since the improvement of electrical conductivity usually leads to degradation of optical transparency, a fine-tuning sample preparation process and a better understanding of the correlation between structural and transport properties is necessary for optimizing the properties of TCO for use in such devices. Here we report a structural and magnetotransport study of tin oxide (SnO2), a well-known and commonly used TCO, prepared by a simple and relatively cheap Atmospheric Pressure Chemical Vapour Deposition (APCVD) method in the form of thin films deposited on soda-lime glass substrates. The thin films were deposited at two different temperatures (which were previously found to be close to optimum for our setup), 590 °C and 610 °C, and with (doped) or without (undoped) the addition of fluorine dopants. Scanning Electron Microscopy (SEM) and Grazing Incidence X-ray Diffraction (GIXRD) revealed the presence of inhomogeneity in the samples, on a bigger scale in form of grains (80–200 nm), and on a smaller scale in form of crystallites (10–25 nm). Charge carrier density and mobility extracted from DC resistivity and Hall effect measurements were in the ranges 1–3 × 1020 cm−3 and 10–20 cm2/Vs, which are typical values for SnO2 films, and show a negligible temperature dependence from room temperature down to −269 °C. Such behaviour is ascribed to grain boundary scattering, with the interior of the grains degenerately doped (i.e., the Fermi level is situated well above the conduction band minimum) and with negligible electrostatic barriers at the grain boundaries (due to high dopant concentration). The observed difference for factor 2 in mobility among the thin-film SnO2 samples most likely arises due to the difference in the preferred orientation of crystallites (texture coefficient).
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