Spray-dried sol-gel glass-ceramic powders based on the tunable thermal expansion of quartz and keatite solid solutions

Lithium aluminosilicate glass-ceramic powders were synthesized by the heat treatment of spray-dried sol-gel glassy nanobeads, obtaining quartz solid solution (Qss) and keatite solid solution (Kss) crystals. Their composition ranged between 75 mol% SiO2 and pure silica along the spodumene join. The metastable crystals displayed tunable coefficients of thermal expansion ranging from +30 × 10−6 to −2.7 × 10−6 K−1 at room temperature, as obtained from their crystallographic characterization. The solid solution boundaries of Kss could be extended to 85 mol% SiO2. Concurrently, X-ray diffraction measurements performed in situ at high temperature and at cryogenic conditions confirmed the known linear shift of the high-low quartz inversion temperature upon increasing Al+Li doping. The obtained results qualify aerosol synthesis as a very versatile method for the production of glass-ceramic powders in the LAS system

Preparation and characterization of Na2O-Al2O3-B2O3 sol-gel glasses with aluminum lactate and formiate as precursors

In this work aluminum lactate and aluminum formiate have been used as precursors to obtain room temperature stable sols and gels and after annealing homogeneous glasses in the System Na2O-Al2O3-B2O3. The local environments and connectivities of boron and aluminum have been investigated by 11B and 27Al solid-state nuclear magnetic resonance (NMR). It was found that the local atomic structures of the sol-gel glasses markedly depend on the precursors and the preparation routes and are also dissimilar to those of melt quenched glasses of the same compositions. Thus, for example, the fractions of BO3/2 and BO4/2 units differ and it is interesting to note that there are no asymmetric ΒO2/2(O-) units present in the sol-gel materials. The 27Al spectra show AI in four-, five- and sixfold coordination, whose relative abundance in a given glass composition is also dependent on the preparadon route. Rotational echo double resonance (REDOR) 11B {27Al} and 27Al {11B} resuhs indicate that the extent of B-O-Al connectivity is diminished in the gel prepared glasses when compared to the melt cooled glasses. Element distributions are reported on the basis of secondary neutral mass spectrometry (SNMS) data, and the nanostructures of surfaces have been characterized by atomic force microscopy (AFM)

Spray-dried TiO2(B)-containing photocatalytic glass-ceramic nanobeads

Glass-ceramic nanospheres of molar composition 0.83 SiO2 · 0.17 TiO2 are produced by the sol-gel spray-drying method followed by controlled heat treatments up to 1200 °C. TiO 2 (B) and anatase nanocrystals are precipitated in the glassy matrix: the latter phase gradually predominates with increasing ceramization temperature and time, in parallel to an overall increase in crystal sizes. The nanospheres exhibit evident photocatalytic activity under UV-A irradiation, especially at annealing stages involving a comparatively higher amount of TiO 2(B) and smaller crystals. The occurrence of TiO2(B) in this simplified binary glass-ceramic material underlines the key role of this phase in the dynamics of crystallizing TiO 2-bearing silicate melts

Inversion of quartz solid solutions at cryogenic temperatures

Quartz solid solution crystals of six different compositions were obtained from crystallization of glass powders belonging to the Li2O–Al2O3‐SiO2 (LAS) system. They were analyzed in situ by laboratory‐based X‐ray diffraction down to cryogenic temperatures (−190°C). Temperature‐resolved analysis of their lattice parameters allowed determination of the critical inversion temperature Tc in these materials, marking the displacive phase transition from a high‐quartz‐ to a low‐quartz‐like lattice. Integrating available data from other literature sources, an updated phase diagram for the occurrence of high and low quartz solid solution phases is provided for the LAS system; these data are expected to support future development of functional materials relying on these crystalline phases

Interface Crystallization of Ceria in Porous Silica Films for Solar Applications

Antireflective (AR) coatings with photocatalytic activity for solar cover glasses are extensively investigated at present, mostly in multilayer systems including titania. In this study, bifunctional single coats from porous silica in combination with up to 33 mol % ceria were prepared by sol–gel dip-coating on low-iron soda-lime float glass. After heat treatment for one hour at 350 °C, the coated glasses were characterized. Solar transmittance decreased with increasing ceria content, whereas photocatalytic activity increases. Crystallization of cubic ceria was detected by grazing incidence X-ray diffraction. Chemical depth profiling by secondary neutral mass spectrometry revealed the enrichment of cerium at the coating surface as well as at the interface to the glass substrate. Self-assembled ceria crystallization at the interfaces resulted in a three-layered mesostructure of the coating, which was verified by field-emission scanning electron spectroscopy. Cubic ceria crystals at the interface act as a barrier for the sodium diffusion from the substrate, which prevents the poisoning of the photocatalyst, while those crystals at the surface act as an electron donor for photooxidation processes, both enabling adequate photocatalytic activity. The triple-layer architecture with the sequence of high/low/high refractive index materials allows for optical interference sustaining the AR-function

Development of a Broadband Antireflection Coated Transparent Silica Window for a Solar-Hybrid Microturbine System

A promising way of a solar-thermal power plant with high overall efficiency is the coupling of a Braytontype microturbine with a solar receiver. Within the SOLHYCO project, a 100kWe solar-hybrid microturbine demo system is currently under development. The compressed air is preheated up to 800°C with solar radiation using a metallic tube receiver. The tube receiver is located in a cavity with a circular opening Thermal analysis of the receiver showed that the aimed overall receiver efficiency of 80% cannot be reached with an economically sized receiver cavity. Reasons for this are high losses by infrared radiation and convection caused by high temperatures of the tubes of up to 950°C. Covering the cavity opening with a transparent window could reduce these losses. Fused silica has ideal properties to fulfill the technical needs. It is highly transparent in the solar spectrum and it can be used at high operating temperatures. Further a very low thermal expansion coefficient allows high temperature gradients. Although there are a few disadvantages: Glass is a brittle material which can not withstand high tensile stresses. Further a flat plate as a receiver window would lead to additional losses by reflected solar radiation. Within the ARTRANS project an antireflective coated fused silica window was developed. The prototype, consisting of ten pipe segments is held by a frame of insulating material. A broadband antireflective coating was developed to reduce the losses by reflected solar radiation. The segments of the prototype were coated by dip-coating from a silica sol containing solution and tempered at 900°C. Optical measurements of the resulting 110 nm porous silica coating showed that the transmission could be increased as expected from 93% to more then 97% in the solar spectrum. The overall performance of the receiver without and with transparent window was supported by numerical simulations. The simulations showed that a receiver with coating will have a 13.1% points higher receiver efficiency. Using a window without coating, the receiver efficiency will still be 9.5% points higher than without window

Fused silica windows for solar receiver applications

A comprehensive study of optical and mechanical properties of quartz glass (fused silica) with regard to application in high temperature solar receivers is presented. The dependence of rupture strength on different surface conditions as well as high temperature is analyzed, focussing particularly on damage by devitrification and sandblasting. The influence of typical types of contamination in combination with thermal cycling on the optical properties of fused silica is determined. Cleaning methods are compared regarding effectiveness on contamination-induced degradation for samples with and without antireflective coating. The FEM-aided design of different types of receiver windows and their support structure is presented. A large-scale production process has been developed for producing fused silica dome shaped windows (pressurized window) up to a diameter of 816 mm. Prototypes were successfully pressure-tested in a test bench and certified according to the European Pressure Vessel Directive

Oxidation and coordination states assumed by transition metal dopants in an invert ultrabasic silicate glass

An ultrabasic invert silicate glass (46SiO2·11Na2O·21CaO·22BaO, optical basicity index equal to 0.71) was synthesized (O2 atmosphere) and used as host for various transition metal dopants. Optical absorption, emission and electron paramagnetic spectroscopies were used to characterize oxidation and coordination states. Some of the dopants displayed only their maximal oxidation state (Ti4+, V5+, Cr6+, Mo6+ and W6+). Others exhibited mixed valences: (i) Mn3+ was the dominant species, alongside Mn2+ and Mn5+; (ii) stable Fe3+ prevailed, although some Fe2+ was preliminarily suggested by the absorption spectrum; (iii) Co3+ probably accompanied the dominant Co2+ tetrahedral oxide complex; (iv) like in “conventional” silicate glasses, only Ni2+ was detected, though simultaneously located in tetrahedral and octahedral sites (somewhat distorted); (v) Cu+ was surprisingly identified alongside the expected 6-fold coordinated Cu2+. Drastic reduction of the oxygen content in the melting atmosphere led to conversion of Cr6+ to Cr3+, despite the extreme basicity of the host.Alessio Zandonà wishes to acknowledge the Deutsche Forschungsgemeinschaft (DFG) for funding his research through the Walter Benjamin Program, project n. 448961237, ZA 1188/1–1. This project has benefited from the expertise and the facilities of the Platform MACLE-CVL which was co-funded by the European Union and centre-Val de Loire Region (FEDER).Peer reviewe