Low‐Temperature Mullite Formation in Ternary Oxide Coatings Deposited by ALD for High‐Temperature Applications

Abstract

coatings on geometrically complex substrates, [2][3][4] such as anodic aluminum oxide membranes, aerogels, photonic crystals, and nanorods, with a film thickness control on the Å-scale. Additionally, by combining two or more binary ALD processes in a super cycle ALD process, multicomponent materials can be achieved, such as nanolaminates, doped thin films, and ternary or quaternary oxides. [5] The ability of compositional control of thin films at an atomic level provides the possibility for development of tailor-made atomically mixed systems, pointing to novel materials functionalities.Direct and inverse opals are examples of photonic crystals, which can be manufactured by colloidal self-assembly [6,7] and ALD. [8,9] Its high ordered porosity of interconnects is attractive for a variety of technological applications, such as porous membranes, catalysts, solid oxide fuel cells, and photonics applications. [10][11][12] The 3D periodic arrangement of pores provides a photonic bandgap to these artificial materials, which prohibits the transmission and hence leads to a reflection of electromagnetic radiation in spectral range which is determined by the materials' properties. Specifically, the radiation wavelength, which will be reflected, is influenced by the material microstructure assemble, mono-or multistacked, and also the macropore size. [8,9,[13][14][15] This selective behavior in transmission and reflection might pave the way to effective solar thermo photovoltaic energy conversion devices [16,17] and also next-generation thermal barrier coatings.However, retention of the 3D structure for high-temperature applications remains a significant challenge. [10,18] As the length scales in these porous materials are decreased, as higher is the surface area and specific activity. When exposed to high temperatures, the material's structure may suffer from detrimental morphological changes, such as extensive grain growth, distortion, and eventually destruction of the total periodically organized structure. [9,10] Therefore, this material needs to not only interact with electromagnetic radiation but also have structural stability and keep its function up to high temperatures (usually higher than 1000 °C). The latter fact could be addressed by coating the substrate material with a ceramic oxide that is stable at high temperatures, either by chemical vapor deposition (CVD) [19] or ALD [20] processes.Herein, mullite has been chosen: A ceramic material, which is well known for its stability at high-temperature environments, Atomic layer deposition (ALD) process presents thickness control in an Ång-strom scale due to its inherent surface self-limited reactions. In this work, an ALD super cycle approach, where a superposition of nanolaminates of SiO 2 and Al 2 O 3 is generated by cycling the precursors APTES-H 2 O-O 3 and TMA-H 2 O, is used to deposit thin films with varying ratio of Al 2 O 3 :SiO 2 into silicon wafers and into inverse photonic crystals. The resulting ternary oxide films deposited at low temperature (150 °C) ar...

Influence of Alumina Addition on the Optical Properties and the Thermal Stability of Titania Thin Films and Inverse Opals Produced by Atomic Layer Deposition

“…The refractive indexes after ALD for the full titania and full alumina sample amounted to n = 2.187 ± 0.012 and 1.635 ± 0.002 (@632.8 nm), respectively, being in agreement with earlier reports [ 18 , 19 , 20 , 21 , 22 , 23 , 47 , 48 , 49 , 50 , 51 , 52 ]. All the other super-cycle samples fell within this interval ( Figure 2 ), indicating that the concept of tailor-made mixing of the oxides by variation of the sub-cycle ratio can be applied successfully.…”

“…Atomic layer deposition is a process based on self-limiting surface-controlled chemical reactions between vaporized precursors and substrate, capable of producing a wide variety of materials comprising sulfides, selenides, tellurides, metals, nitrides, and oxides [ 13 ]. ALD has already been demonstrated for the production of inverse opal photonic crystals (iPhC) made from gallium phosphide [ 14 ], zinc sulfide [ 15 ], tungsten [ 8 ], zinc oxide [ 16 , 17 ], aluminum oxide [ 17 , 18 , 19 ], aluminosilicates [ 20 , 21 ], and titanium oxide [ 17 , 22 , 23 , 24 ]. In particular, the latter one is of interest for optical applications since titania features a relatively high refractive index, often higher than 2.3 at 632.8 nm.…”

High‐temperature stable inverse opal photonic crystals via mullite‐sol‐gel infiltration of direct photonic crystals

“…Mullite gels with two compositions, nearly stoichiometric low‐alumina 3Al 2 O 3 ·2SiO 2 (74 wt.% Al 2 O 3 , 26 wt.% SiO 2 ) and high‐alumina content 5Al 2 O 3 ·2SiO 2 (80 wt.% Al 2 O 3 , 20 wt.% SiO 2 ), were synthesized by sol‐gel, based in a procedure reported elsewhere . For the low‐alumina gel, a 50 mL sol was prepared by mixing 1 mol/L aluminum nitrate nonahydrate (ANN, 99.997% trace metals basis, Sigma‐Aldrich) prepared in a 50/50 vol.% water/ethanol with 50 vol.% tetraethyl orthosilicate (TEOS, ≥99.0%, Sigma‐Aldrich) aqueous solution.…”

“…Nevertheless, specific mullite studies are not so widespread when comparing to titania or zirconia. Synthesis of mullite by sol‐gel is well reported and can be tailored in a way that mullite forms already at temperatures as low as 1000°C . However, a very strict control of the sol‐gel and high‐purity precursors are necessary to produce a sol‐gel with low crystallization temperature for mullite (≤1000°C).…”

Modulation of terahertz properties of 3D ceramic photonic crystals via post‐creation non‐metal anion doping treatment

“…Among them, ceramics are indispensable materials because they could be used in demanding environment due to their characteristic strong chemical stability and high temperature resistance. For example, Furlan reported that an Al 2 O 3 inverse opal coated with mullite could retain its 3D structure even at 1400°C. Although several kinds of ceramic TPCs had been reported in literatures, their THz properties were fixed after their creation.…”