Metal halide perovskite quantum dots (PQDs), with excellent optical properties and spectacular characteristics of direct and tunable bandgaps, strong light-absorption coefficients, high defect tolerance, and low nonradiative recombination rates, are highly attractive for modern optoelectronic devices. However, the stability issue of PQDs remains a critical challenge of this newly emerged material despite the recent rapid progress. Here, the encapsulation strategies to improve the stability of PQDs are comprehensively reviewed. A special emphasis is put on the effects of encapsulation, ranging from the improvement of chemical stability, to the inhibition of light-induced decomposition, to the enhancement of thermal stability. Particular attention is devoted to summarizing the encapsulation approaches, including the sol-gel method, the template method, physical blending, and microencapsulation. The selection principles of encapsulation materials, including the rigid lattice or porous structure of inorganic compounds, the low penetration rate of oxygen or water, as well as the swelling-deswelling process of polymers, are addressed systematically. Special interest is put on the applications of the encapsulated PQDs with improved stability in white light-emitting diodes, lasers, and biological applications. Finally, the main challenges in encapsulating PQDs and further investigation directions are discussed for future research to promote the development of stable metal halide perovskite materials.

Improving the Stability of Metal Halide Perovskite Quantum Dots by Encapsulation

The fate and role of in situ formed carbon in polymer-derived ceramics

Silicon-based polymers are outstanding materials for coating applications. These compounds have excellent properties, such as strong adhesion to most substrates, and high chemical, thermal and UV resistance. Additionally, they can be converted into ceramic materials (polymer-derived ceramics) by a heat treatment and, in some cases, by chemical reactions or radiation. Hence, ceramic coatings can be obtained after deposition of the polymers by simple lacquer techniques. The properties and composition of polymeric and ceramic coatings can be changed by tailoring the chemical structure of the precursors or by the addition of fillers. This enables the preparation of coatings with a great variety of properties for different applications. In this review paper, the main aspects of the use of silicon polymers for coatings are elucidated. The advantages and disadvantages of these materials, and the processing methods developed are discussed. Finally, a summary of the applications and the prospects for future research are presented.

Polymeric and ceramic silicon-based coatings – a review

Silicon oxycarbides can be considered as being carbon‐containing silicates consisting of glass networks in which oxygen and carbon share bonds with silicon. The carbon‐for‐oxygen substitution in silicate glass networks has been shown to induce significant changes in the network connectivity and consequently strong improvements in the properties of the silicate glass network. For instance, SiOC glasses exhibit Young's moduli, hardness values, glass transition, and crystallization temperatures which are superior to those of vitreous silica. Moreover, the silicon oxycarbide glass network exhibits unique structural features such as reduced mass fractal dimension and nano‐heterogeneity, which significantly affect and/or dictate its properties and behavior. In the present Review, a consideration of the current state of the art concerning the synthesis, processing, and various structural and functional properties of silicon‐oxycarbide‐based glasses and glass‐ceramics is done. Thus, the synthesis of silicon oxycarbides starting from macromolecular precursors such as polysiloxanes or alkoxysilanes‐based sol‐gel systems as well as current advances related to their processing will be critically reviewed. In addition, various structural and functional properties of silicon oxycarbides are presented. Specific emphasis will be put on the intimate correlation between the molecular architecture of the precursors and the structural features and properties of the resulting silicon oxycarbides.

Silicon oxycarbide glasses and glass-ceramics: “All-Rounder” materials for advanced structural and functional applications

Porous ceramics, in particular cellular ceramics with open and closed porosity, are key enabling components in different demanding applications because of their favorable set of properties. The production of porous ceramics from preceramic polymers offers advantages in terms of simple processing methodology, low processing cost, and ease of control over porosity and other properties arising from the composition of the resultant ceramics. Therefore, a vast amount of research has been conducted, in the past decade, towards the fabrication and characterization of porous ceramics produced from different polymeric precursors, such as polysiloxane, polycarbosilane, polysilazane and borazine. The potential of polymer-derived-ceramics can only be fully achieved if the type of fabrication method used is tailored taking into account the specific application of interest. For brevity, this review deals specifically with the different processing strategies employed to obtain, from polymer precursors, ceramic foams, membranes, aerogels or porous ceramics for which porosity higher than 50 vol% has been deliberately introduced, including also components with high specific surface area or hierarchical porosity. The different processing strategies, such as replica, sacrificial template, direct foaming, etching, aerogel forming, fiber spinning and automated manufacturing, are explained and the published literature has been reviewed with particular concern for the critical issues that affect the pore size, the amount of porosity and strength of the ceramic components.

Porous polymer derived ceramics

An effective and low-cost method for applying polysilazane-based barrier coatings on stainless steel is presented. Two different precursors in the system SiCN (ABSE) and SiN (PHPS) have been used for applications of polymeric and ceramic-like coating materials. With corresponding precursor solutions in toluene or ether, steel substrates were coated by means of a simple dip coating technique. The influence of different annealing atmospheres such as ambient air and nitrogen on the resulting coatings was determined. Therefore precursor-based powders and coatings were characterised by TGA, REM, TEM, GDOES and chemical composition measurements. Oxidation tests on coated samples were done in air at temperatures of 800–1000 °C. By measuring the weight gain and the oxidation depth, parabolic oxidation kinetics were determined. The precursor-based coatings protect stainless steel from oxidation up to 1000 °C.

Advanced Coatings on the Basis of Si(C)N Precursors for Protection of Steel against Oxidation

A novel environmental barrier coating system for steel consisting of a perhydropolysilazane (PHPS) bond coat and a polysilazane-based glass/ceramic composite top coat has been developed. After stabilising the coating slurries, double layers were applied on mild and stainless steel substrates by the dip-coating technique. Parameters like pre-treatment of the steel substrates, filler systems, particle size of the fillers or coating thickness were varied to optimize the coatings. The thermal treatment was performed in air at temperatures up to 800 °C. Microstructural analysis by SEM and XRD revealed the formation of a coating system consisting of a SiNO bond coat and a ZrO2-filled glass/ceramic top coat. A uniform, well adherent, dense and crack-free coating system with a noteworthy thickness up to 100 μm was achieved. Even after cyclic oxidation tests on coated samples at 700 °C the coating system was still undamaged and no oxidation occurred on the mild steel substrates.

High performance environmental barrier coatings, Part I: Passive filler loaded SiCN system for steel

Polymer-derived ceramics exhibit a convenient route for the processing of low-dimensional ceramics like coatings or fibres. In previous investigations unfilled and composite coatings have been developed using ammonolysed bis(dichloromethylsilyl)ethane (ABSE) or perhydropolysilazane (PHPS) as precursors and BN, ZrO2 or glass particles as filler materials. The coating systems provide excellent corrosion and oxidation resistance to underlying metals. This paper reports on the effect of the precursor system and the pyrolysis parameters on the conversion behaviour, shrinkage and mechanical properties, including hardness and Young's modulus, of ABSE- and PHPS-based coatings. Therefore the crosslinking and pyrolysis behaviour as well as the mechanical properties of the coatings were investigated up to pyrolysis temperatures of 1000 °C in nitrogen and in air by ATR-IR, SEM, profilometry and nanoindentation measurements. The coatings pyrolysed at 1000 °C in nitrogen, have hardness values of 13 GPa and Young's moduli up to 155 GPa.

Conversion behaviour and resulting mechanical properties of polysilazane-based coatings

Polymer derived ceramic (PDC) matrix composite coatings are a promising candidate to be used as alternative environmental barrier coatings, such as in oxidation protection. This paper reports the processing of three PDC coating systems on a superalloy substrate, Inconel 617, using a simple dip coating method. The performance of SiON coating, particle filled SiOC coating, and their combined coating system is evaluated by long time static oxidation testing at 800 °C for 100–200 h. Two key parameters have been analyzed: the weight gain of the coating samples and the thickness of the thermally grown oxide (TGO) layer at the ceramic–metal interface. Results show that all three coating systems are able to significantly reduce the weight gain of metal substrates due to oxidation. However, the SiON bond coat + particle filled SiOC top coat double-layer coating system is most effective in maintaining the integrity of the substrate in the investigated 200 h. Based on the kinetics of oxidation, likely rate controlling steps are identified.

High performance environmental barrier coatings, Part II: Active filler loaded SiOC system for superalloys

Precursor-based composite coatings were developed as barrier coatings on steel against oxidation and corrosion using a silazane in the system SiN as a polymer material and BN particles as a passive filler. After thermal treatment in air up to 800°C, dense and very well adhered ceramic composite SiNO/BN coatings with a thickness of 12 μm were achieved. These were investigated by means of scanning electron microscopy, energy-dispersive spectrum, glow discharge optical emission spectroscopy, and adhesion measurements. Static oxidation tests on coated mild steel substrates up to 700°C displayed parabolic oxidation kinetics and a reduced weight gain by two orders in magnitude compared with uncoated substrates.

Martin Günthner

Papers

Advanced Coatings on the Basis of Si(C)N Precursors for Protection of Steel against Oxidation

High performance environmental barrier coatings, Part I: Passive filler loaded SiCN system for steel

Conversion behaviour and resulting mechanical properties of polysilazane-based coatings

High performance environmental barrier coatings, Part II: Active filler loaded SiOC system for superalloys

Particle-Filled PHPS Silazane-Based Coatings on Steel