Amorphous Silica Layer Research Articles

Minimizing the impact of passivation during allanite-(Ce) decomposition in sulfuric acid media for rare earth recovery

Acid decomposition using concentrated H2SO4 at elevated temperature (>200 °C) is a common process to recover rare earth elements (REE) from refractory minerals where the reactions form REE sulfates, which are dissolved during a subsequent water leach. While the decomposition of REE orthophosphates and fluorocarbonates is well-documented, investigations focusing on REE silicates are more limited. The current study focuses on allanite-(Ce), since, in addition to the REEs, the presence of other sulfate-forming cations in the crystalline structure can help better define the decomposition of complex silicates. After treatment using concentrated H2SO4 at temperatures between 175 °C and 225 °C, important passivation effects from the formation of amorphous silica layers were observed. Decomposition occurs rather during the water leach if its temperature is maintained at near-boiling conditions for 20 h, suggesting that water helps attenuate passivation. Consequently, experiments were performed where water was premixed with H2SO4 during the initial acid treatment to lower concentration down to 55 wt%. The water addition drastically enhanced decomposition rate at acid treatment temperatures down to 100–125 °C, where REE recoveries >90 wt% were achieved after a room-temperature water leach. The extent of passivation may be linked to the solubility of produced sulfates. With concentrated acid, low sulfate solubility induces local saturation at the crystal dissolution front leading to diffusion-limited cation transfer across the growing silica layer. When the water content is increased, the sulfate-forming cations can enter in solution providing efficient transport, hence minimizing passivation. This decreases both acid treatment and water leach temperatures, significantly reducing energy consumption without additional reagent costs.

Electromagnetic properties of FeSiCr alloy powders modified with amorphous SiO2

The effects of amorphous SiO2 coating onto FeSiCr alloy powder on the electric and magnetic properties were investigated. FeSiCr alloy powder with an average particle size of 10 μm was coated with nano-sized silica powder to increase the resistivity and DC bias superposition characteristics. Inexpensive water glass (sodium silicate) was used as the raw material to form the amorphous silica layer onto the FeSiCr alloy powder surface acting as a binding and insulation layer. The amorphous silica layer thickness was about 20 nm to 50 nm. The thickness can be controlled by the amount of sodium silicate addition. The amorphous silica layer can significantly promote anti-oxidation, resistivity and DC bias superposition characteristic of the FeSiCr alloy.

Intrinsic Effect of Alkali Concentration on Oxidation Reactivity and High-Temperature Lubricity of Silicate Melts between Rubbed Steel/Steel Contacts.

The present study investigated oxidation reactivity and hot lubricity of a sodium silicate melt at different Na2O/SiO2 ratios under elevated temperature stimulation. Static oxidation prevention was achieved at 920 °C when the Na2O/SiO2 ratio reached 1:3 (trisilicate) and 1:2 (disilicate), but it started to deteriorate in the case of 1:1 (metasilicate). At a high concentration of sodium (metasilicate), a severe corrosion reaction between the melt and oxide took place that resulted in a composite coating on the steel substrate. This high-temperature reaction accelerated the formation of ionic charges from the steel base and promoted oxidation. However, friction and wear reduction is proportional to an increase in the sodium oxide fraction. Metasilicate (1:1) exhibited excellent lubricity under the hot frictional test at 920 °C compared to other lubricants. It was due to the formation of the sodium-saturated surfaces and an amorphous silica layer, which was associated with the high-temperature reactivity of sodium toward the oxide surface. In addition, the NaFeO2-Fe2O3 composite film, as the reaction product of individual sodium charge and oxide, plays a significant role in maintaining the tribofilm stability for metasilicate, which was not present for disilicate. This study advances the understanding of how sodium-containing compounds perform oxidation prevention and generate lubricity at hot rubbed surfaces.

Red ruby glass from gold nanoparticles obtained by LASiS – a new approach.

Currently, most of the red colored glasses are still produced using cadmium sulfoselenide, despite its high toxicity. Alternative solutions, more environmentally friendly, have been sought, most of them going through the use of gold or copper nanoparticle, the color being obtained by striking.This work presents a new methodology for production of a red colored glass by the incorporation of gold nanoparticles in the melt, trying to take advantage of an amorphous silica layer involving the nanoparticles. Gold nanoparticles (AuNP) were produced by laser ablation synthesis in solution (LASiS), using a solution of tetraethyl orthosilicate (TEOS) and isopropanol (IPA).A xerogel was prepared using the TEOS:IPA:AuNp suspension, mixed with a alkaline-earth aluminosilicate colorless glass and melted at 1350 °C producing a glass with an intense and homogeneous ruby red color without the need of a striking stage.

Preparation and thermoelectric characterization of phosphorus-doped Si nanocrystals/silicon oxide multilayers

Phosphorus-doped silicon nanocrystals (Si-NCs)/silicon oxide (SiOy) multilayers were prepared by plasma enhanced chemical vapor deposition and post-annealing. After annealing, Si-rich amorphous silicon oxide (a-SiOx) and a-SiOy layer were converted into a Si-NCs layer and lowly thermal conductive layer, respectively. The diameter of Si-NCs were changed by varying the thickness of a-SiOx layer from 3 to 10 nm. Electrical and thermal conductivity of Si-NCs/SiOy multilayers were decreased with decreasing the diameter (d) of Si-NCs. Thermal conductivity at d = 3 nm was estimated at 1.48 W m−1 K−1, which is much lower than that of bulk silicon. These results show that phosphorus doped Si-NCs/SiOy multilayers are promising for thermoelectric materials.

Ultrathin Amorphous Silica Membrane Enhances Proton Transfer across Solid‐to‐Solid Interfaces of Stacked Metal Oxide Nanolayers while Blocking Oxygen

AbstractA large jump of proton transfer rates across solid‐to‐solid interfaces by inserting an ultrathin amorphous silica layer into stacked metal oxide nanolayers is discovered using electrochemical impedance spectroscopy and Fourier‐transform infrared reflection absorption spectroscopy (FT‐IRRAS). The triple stacked nanolayers of Co3O4, SiO2, and TiO2 prepared by atomic layer deposition (ALD) enable a proton flux of 2400 ± 60 s−1 nm−2 (pH 4, room temperature), while a single TiO2 (5 nm) layer exhibits a threefold lower flux of 830 s−1 nm−2. Based on FT‐IRRAS measurements, this remarkable enhancement is proposed to originate from the sandwiched silica layer forming interfacial SiOTi and SiOCo linkages to TiO2 and Co3O4 nanolayers, respectively, with the O bridges providing fast H+ hopping pathways across the solid‐to‐solid interfaces. Together with the complete O2 impermeability of a 2 nm ALD‐grown SiO2 layer, the high flux for proton transport across multi‐stack metal oxide layers opens up the integration of incompatible catalytic environments to form functional nanoscale assemblies such as artificial photosystems for CO2 reduction by H2O.

Effect of heating treatment and mixture on optical properties of coating materials used in gravitational-wave detectors

The interferometer mirrors of gravitational-wave detectors (GWDs) are Bragg reflectors made of alternate amorphous silica (SiO2) and titania-doped tantala (TiO2:Ta2O5) layers as low- and high-refractive index materials, respectively. A thermal treatment is usually performed to reduce both mechanical losses and near-infrared (NIR) optical absorptions of the coatings. The authors present a spectroscopic ellipsometry (SE) investigation of the effect of annealing and Ti:Ta mixing on Ta2O5 coatings deposited under conditions similar to those adopted for building up mirrors of GWDs. The broadband analysis covers both the NIR and the fundamental absorption threshold region. The data show an evident annealing-induced reduction of the fundamental optical absorption broadening. Modeling the data through the Cody-Lorentz formula confirms that NIR absorptions are below the SE sensitivity and shows a notable annealing-induced reduction of so-called Urbach tails. Titania-doping of tantala slightly reduces the Urbach energy. After the heating treatment, the resulting Urbach energy of the doped material is lower than that of annealed pure tantala. The observed reduction of Urbach tails is important because it parallels the reduction of the so-called internal friction observed in mechanical measurements so that SE emerges as a convenient tool for an agile diagnostic of both the optical and the mechanical quality of amorphous oxide coatings.

Synthesis of Silica-Coated Silver-Cobalt Ferrite Nanoparticles for Biomedical Applications

In this work, a series of silver-substituted cobalt ferrite (AgxCo1-xFe2O4, 0 ≤ x ≤ 0.1) nanoparticles (NPs) were synthesized, and then a sample with monophase structure and optimum magnetic properties was coated with an amorphous silica layer. First, Ag-substituted Co-ferrite NPs were synthesized via a sol-gel auto combustion method from aqueous metal nitrates solutions. Then, all the powders were characterized by several techniques such as X-ray diffraction (XRD), field emission electron microscopy (FE-SEM), and vibrating sample magnetometry (VSM). These results showed that when the Ag content was between 0 and 0.08, the synthesized powders were the cubic spinel structure, having spherical-shaped particles with an average size of about 20–25 nm. Afterward, the silica coating was applied on the surfaces of the selected sample (i.e., Ag0.08Co0.92Fe2O4 NPs with monophase structure and optimum magnetic properties) by a sol-gel approach based on the Stober process from the tetraethyl orthosilicate (TEOS) as the precursor of silica. Transmission electron microscopy (TEM), Fourier-transform infrared (FTIR) spectroscopy, and zeta potential investigations were used to study the characteristics of the silica-coated Ag-Co-ferrite NPs. The results of FTIR and TEM analyses confirmed the presence of the silica coating on the surfaces of the Ag-Co-ferrite NPs. Although the silica-coated sample showed the saturation magnetization (MS) value slightly lower than that of the uncoated one, however, its magnetic properties are suitable for use in different biomedical applications. Also, a water-based suspension containing the silica-coated Ag-Co-ferrite NPs showed a more negative zeta potential value at the pH of 7 in comparison with a suspension containing uncoated ones. Therefore, it can be concluded that the synthesized silica-coated Ag0.08Co0.92Fe2O4 NPs in this study could be considered as a good candidate for the preparation of biomedical ferrofluids.

Mechanical and environmental properties of carbonated steel slag compacts as a function of mineralogy and CO2 uptake

Accelerated carbonation is a treatment for converting alkaline industrial residues into added-value products and storing CO2 in solid form. This work investigated the influence of reacting phases and CO2 uptake on microstructure development, mechanical properties and the environmental behavior of carbonated compacts produced from Basic Oxygen Furnace (BOF) and Electric Arc Furnace (EAF) slags, characterized by a different mineralogy. The compacts were cured under a 100% CO2 atmosphere at 50 °C and pressure of 1.3 or 10 bar for 15 min to 4 h. The BOF slag reacted very fast in the first 30-60 min due to the complete conversion of portlandite to calcite, amorphous calcium carbonate and aragonite, and continued to react over time due to the presence of slower reacting Ca-silicate phases. For the EAF slag, rich in Ca-silicates, the CO2 uptake was lower, and increased only slightly over time at 1.3 bar and became almost stable after 15 minutes at 10 bar; the EAF slag products however presented a higher compressive strength than the BOF slag ones, because of the different phases involved in the carbonation reaction. For the BOF slag, portlandite dissolution caused the formation of voids, only partially filled up by the reaction products. For the EAF slag, formation of a carbonate and amorphous silica layer around the reacting silicates yielded a denser matrix. pH and Ba leaching decreased for both types of slag, whereas V release increased due to the dissolution of reactive phases such as dicalcium silicates, which initially contained this element.

Direct Observations of the Coupling between Quartz Dissolution and Mg-Silicate Formation

Although quartz is a stable mineral at Earth surface conditions, field samples have shown its rapid dissolution in combination with the precipitation of Mg-silicate phases. Atomic force microscopy (AFM) experiments were performed to investigate the dissolution of quartz and the precipitation of secondary phases in high-pH, Mg-rich solutions both in situ and ex situ. Experiments were conducted at room temperature with varying MgCl 2 or MgSO 4 concentrations (0.1-100 mM), pH (8.9-12) and ionic strength (<1-530 mM). The results suggest that quartz dissolves by the removal of nanoparticles on the time scale of minutes, and that a nanometer-scale gel-like layer of amorphous silica forms on the quartz surface and is thicker at higher pH. During the in situ experiments, soft and poorly attached precipitates form on the surface when the Mg concentration is high (100 mM). After 20 h in a high-pH, Mg-rich solution, solid Mg-rich precipitates can be observed at places on the surface where the gel-like silica layer is present, predominantly near surface edges where dissolution is enhanced. This suggests a coupling between the dissolution of quartz, that resulted in the gel-like layer, and the formation of secondary phases, indicating an interface-coupled dissolution-precipitation mechanism. The precipitates could not be precisely identified but evidence suggests they are likely to be amorphous Mg-silicate phases. Such a coupled reaction may provide a pathway for Mg-Si phase formation suitable as a new environmentally friendly cement.

ITO-Free Silicon Heterojunction Solar Cells With ZnO:Al/SiO2 Front Electrodes Reaching a Conversion Efficiency of 23%

Silicon heterojunction (SHJ) solar cells have been increasingly attracting attention to the photovoltaic community in the last years due to their high efficiency potential and the lean production process. We report on the development of a stable baseline process for SHJ cells with focus on the optical improvement of the solar cells’ front side. An amorphous silicon oxide layer (a-SiO2) was used as an antireflective coating (AR) on the front side the finished SHJ devices. Both optical simulations and experimental results demonstrate a short-circuit current density ( J sc) improvement of 0.4 mA/cm2 when applying the a-SiO2 AR, yielding maximum conversion efficiencies of 23.0%. Full-size cells with 244 cm2 total area have been produced using three front contact stacks: indium tin oxide (ITO) as reference, ZnO:Al, and ZnO:Al/SiO2 showing the J sc improvement with the double AR configuration. Damp-heat tests on those samples demonstrate an enhanced stability of cells with ZnO:Al front TCO when capped with SiO2.

Electronic structure of molybdenum-involved amorphous silica buffer layer in MoOx/n-Si heterojunction

An amorphous silica layer containing Mo component (a-SiOx(Mo) ∼ 3.5–4.0 nm) is naturally formed in the process of evaporating MoO3 power onto n-type silicon, which plays a significant role on the non-equilibrium carriers’ transport of MoOx/n-Si heterojunction photovoltaic device. The electronic structure and the charge transition levels of the amorphous silicon oxide doped with Mo are derived from density functional theory. The density of states show that the five local states are existed in the band gap of amorphous silicon oxide, caused by the hybridization of Mo 4d orbital and O 2p orbital. The charge transition level of ε(+1/0) is located at 3.59 eV above the valance band maximum of a-SiO2, which could be a passageway of holes produced in the opto-electric conversion. Defects level-assisted quantum transport mechanism is put forwarded to explain hole transport phenomenon in the MoOx/SiOx(Mo)/n-Si PV device.

Interpretation on Nanoporous Network Structure in Rice Husk Silica Layer: A Graph Model.

The rice plant produces an amorphous silica layer in the husk covering the brown rice grain as a part of a protective respiration system. The layer shows high permeation molecular flow while the Brunauer–Emmett–Teller isotherm indicates the existence of nanometer-sized pores. Here, we interpret the inner structure of the layer as a porous network consisting of void spheres with a degree of 2–5 and tunnels with a length of 2–7 nm based on the transmission electron microscopy images. In the network, the gas molecules travel through the tunnels and move in random directions after collisions with the walls of the spheres. A tree network was introduced to understand the permeance of the layer and the reflection of the molecule of the root or parent sphere was estimated for a specific case. The tree becomes a graph with cycles in a finite space such as the silica layer and the reflection of the root sphere in the graph converses to that of the tree. On the basis of the properties of the network, the high permeance of the silica layer in the rice husk can be explained. It is suggested that the specific system restricts the movements of the gas molecules and can be applied to reduce the size of gas phase separation and chemical reactor systems providing a new view to understand nanoscaled porous materials.

Effect of Temperature on Thermal Treatment of Silica Coated Magnetic Nanoparticles

In the quest for developing a catalyst with as many desired characteristics, a facile synthetic route was designed for the preparation of mesoporous silica coated magnetic nanoparticles(MSMNP) employing a colloid mill reactor. The composite particles were characterized by the techniques, such as nitrogen adsorption-desorption isotherms, scanning electron microscopy(SEM), transmission electron microscopy(TEM), X-ray diffraction patterns (XRD), thermo-gravimetric analysis(TGA), Fourier transform infrared spectroscopy(FTIR) and vibrating sample magnetometer(VSM), etc. The analysis showed that the resulted MSMNP composites were composed of silica shell layers with open pores connecting channels and NiFe2O4 with spinel structure, so the thermal treatment temperature did not show significant effect on pore textural properties, and its specific surface areas were in the range of 443–474 m2/g, while pore volume of about 0.8 cm3/g with an average pore size of around 9.5 nm. The composites with super paramagnetic nature were encapsulated entirely with amorphous silica layers contributing to optimum porosity and abundant surface hydroxyl groups.

Magnetic Core/Shell Nanocomposites MgFe2O4/SiO2 for Biomedical Application: Synthesis and Properties

Magnetic core/shell (CS) nanocomposites (MNCs) are synthesized using a simple method, in which a magnesium ferrite nanoparticle (MgFe2O4) is a core, and an amorphous silicon dioxide (silica SiO2) layer is a shell. The composition, morphology, and structure of synthesized particles are studied using X-ray diffraction, field emission electron microscopy, transmission electron microscopy (TEM), energy-dispersive spectroscopy (EDS), scattering electrophoretic photometer, thermogravimetric analysis (TGA), and Mossbauer spectroscopy. It is found that the MgFe2O4/SiO2 MNC has the core/shell structure formed by the Fe‒O–Si chemical bond. After coating with silica, the MgFe2O4/SiO2 MNC saturation magnetization significantly decreases in comparison with MgFe2O4 particles without a SiO2 shell. Spherical particles agglomerated from MgFe2O4 nanocrystallites ∼9.6 and ∼11.5 nm in size function as cores coated with SiO2 shells ∼30 and ∼50 nm thick, respectively. The total size of obtained CS MNCs is ∼200 and 300 nm, respectively. Synthesized CS MgFe2O4/SiO2 MNCs are very promising for biomedical applications, due to the biological compatibility of silicon dioxide, its sizes, and the fact that the Curie temperature is in the region required for hyperthermal therapy, 320 K.

Plasma electrolytic oxidation of monocrystalline silicon using silicate electrolyte containing boric acid

The feasibility of preparing oxide layer on a monocrystalline silicon (mc-Si) using the plasma electrolytic oxidation (PEO) process was investigated in this study and the influence of adding boric acid to the silicate based electrolyte on the formation and the properties of the oxide layer was evaluated. The morphology, chemical composition and phase structure of the layers were characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), Fourier transform infrared spectroscopy (FTIR), grazing incidence X-ray diffractometry (GIXRD) and transmission electron microscopy (TEM) techniques. The optical and electrical properties of the grown layers were assessed by ultraviolet–visible spectroscopy (UV–Vis), photoluminescence spectroscopy (PL) and current-voltage measurement methods. The results revealed that an amorphous silicon oxide (SiO2) layer with a crater-like morphology was formed on the mc-Si substrate in the silicate based electrolyte. The addition of boric acid to the electrolyte led to dramatic arcing, stable layer formation conditions, and improved feature quality. The presence of BO33− anions in the electrolyte accelerated the rate of growth, increased the layer surface porosity, and doped the SiO2 structure with boron cations. As a result of these changes, the optical absorbance and photoluminescence emission intensity of the doped oxide layer was enhanced in both ultraviolet and visible regions, as well as the Urbach tail widened, which is declining the band gap energy. In addition, despite the increased surface electrical resistance, the current gain was also increased significantly.

Multi-characterization study of interface passivation quality of amorphous sub-stoichiometric silicon oxide and silicon oxynitride layers for photovoltaic applications

Si solar cells have achieved a world record efficiency of 26.7% as a result of both improvement of Si ingot growth and optimal passivation of surfaces and interfaces. In this framework, a clear understanding of the electronic, optical, structural and passivation properties of innovative Si based layers is mandatory. The present study reports on the characterization of amorphous sub-stoichiometric silicon oxide (a-SiOx) and silicon oxynitride (a-SiOxNy) layers and their surface passivation properties. The layers have been deposited on float zone Si wafers (2 Ω cm, (100)-oriented, 250 µm thick) by plasma enhanced chemical vapour deposition (PECVD) adding increasing fractions of N2O and CO2 to the SiH4 flux during deposition to increase the energy band gap of the layers. Composition, optical properties, light induced electronic transitions and minority carrier lifetimes of Si wafers passivated with these layers have been investigated by Fourier-transform infrared spectroscopy (FTIR), spectral ellipsometry, surface photovoltage (SPV) spectroscopy and photo conductance decay (PCD). The overall characterization of the layers has allowed us to understand the effect of increasing N2O and CO2 flux ratios during deposition on the interface properties. The present study establishes the importance of the approach of using multiple characterization methods in the evaluation of the passivation capability of layers that combine large optical band gap and surface passivation.

Controlling performance of a-Si:H solar cell with SnO2:F front electrode by introducing dual p-layers with p-a-SiOx:H/p-nc-SiOx:H nanostructure

Dual p-type layers consisting of a p-type hydrogenated amorphous silicon oxide (p-a-SiOx:H) layer and a p-type hydrogenated nanocrystalline silicon oxide (p-nc-SiOx:H) layer were used as the window layer in a hydrogenated-amorphous-silicon (a-Si:H) p–i–n thin-film solar cell with a fluorine-doped tin-oxide (FTO, SnO2:F) front electrode. Through this method, high conductivity, which originated from the p-nc-SiOx:H layer with higher optical bandgap, was imparted to the a-Si:H solar cell. The destabilization problems of Sn4+ in FTO were solved by using a high-flux hydrogen plasma during the growth of p-nc-SiOx:H. The properties of the a-Si:H solar cell were controlled by changing the thickness of the p-a-SiOx:H and p-nc-SiOx:H layers. The dual p-type layers in the a-Si:H solar cell exhibited enhanced transmission of holes and decreased recombination current. The p-nc-SiOx:H film in the dual p-type layers increased the built-in potential in the a-Si:H solar cell to obtain a higher open-circuit voltage and a higher short-circuit current density. Finally, the conversion efficiency of the a-Si:H solar cell was enhanced by 12.90% through the adoption of the p-a-SiOx:H(7 nm)/p-nc-SiOx:H(7 nm) nanostructure for the dual p-type layers.

Complex silica composite nanomaterials templated with DNA origami.

Genetically encoded protein scaffolds often serve as templates for the mineralization of biocomposite materials with complex yet highly controlled structural features that span from nanometres to the macroscopic scale1-4. Methods developed to mimic these fabrication capabilities can produce synthetic materials with well defined micro- and macro-sized features, but extending control to the nanoscale remains challenging5,6. DNA nanotechnology can deliver a wide range of customized nanoscale two- and three-dimensional assemblies with controlled sizes and shapes7-11. But although DNA has been used to modulate the morphology of inorganic materials12,13 and DNA nanostructures have served as moulds14,15 and templates16,17, it remains challenging to exploit the potential of DNA nanostructures fully because they require high-ionic-strength solutions to maintain their structure, and this in turn gives rise to surface charging that suppresses the material deposition. Here we report that the Stöber method, widely used for producing silica (silicon dioxide) nanostructures, can be adjusted to overcome this difficulty: when synthesis conditions are such that mineral precursor molecules do not deposit directly but first form clusters, DNA-silica hybrid materials that faithfully replicate the complex geometric information of a wide range of different DNA origami scaffolds are readily obtained. We illustrate this approach using frame-like, curved and porous DNA nanostructures, with one-, two- and three-dimensional complex hierarchical architectures that range in size from 10 to 1,000 nanometres. We also show that after coating with an amorphous silica layer, the thickness of which can be tuned by adjusting the growth time, hybrid structures can be up to ten times tougher than the DNA template while maintaining flexibility. These findings establish our approach as a general method for creating biomimetic silica nanostructures.

Stable Formamidinium‐Based Perovskite Solar Cells via In Situ Grain Encapsulation

AbstractFormamidinium (FA)‐based lead iodide perovskites have emerged as the most promising light‐absorber materials in the prevailing perovskite solar cells (PSCs). However, they suffer from the phase‐instability issue in the ambient atmosphere, which is holding back the realization of the full potential of FA‐based PSCs in the context of high efficiency and stability. Herein, the tetraethylorthosilicate hydrolysis process is integrated with the solution crystallization of FA‐based perovskites, forming a new film structure with individual perovskite grains encapsulated by amorphous silica layers that are in situ formed at the nanoscale. The silica not only protects perovskite grains from the degradation but also enhances the charge‐carrier dynamics of perovskite films. The underlying mechanism is discussed using a joint experiment‐theory approach. Through this in situ grain encapsulation method, PSCs show an efficiency close to 20% with an impressive 97% retention after 1000‐h storage under ambient conditions.