Priscila Bueno
Development of inverse photonic crystals using mullite or alumina/mullite coatings
Thesis presented to the Graduate Program in Materials Science and Engineering of the Federal University of Santa Catarina, as a requirement for obtaining the PhD title in Materials Science and Engineering
Advisors:
Dachamir Hotza/ UFSC Kaline P. Furlan/ TUHH Rolf Janssen/ TUHH
Priscila Bueno
Development of Inverse Photonic Crystals Using Mullite or Alumina/Mullite Coatings
This thesis was presented to obtain the Title of PhD in Materials Science and Engineering and approved in its final version by the Graduate Program in Materials Science and Engineering (PGMAT) of the Federal University of Santa Catarina (UFSC).
Florianópolis, 2018.
Prof. Guilherme Mariz de Oliveira Barra, Dr. Coordinator/PGMAT
Examination Board:
Prof. Dr. Dachamir Hotza Advisor/UFSC
Dr. Rolf Janssen Advisor/TUHH
Dr. Kaline P. Furlan Advisor/TUHH
Prof. Dr. Guilherme Barra UFSC
Prof. Dr. Sergio Gomez UFSC
Prof. Dr. Célia Malfatti UFRGS
Acknowledgements
First of all, I would like to thank God for giving me strength, especially during past 3 and half years, along all my journey to
conquer my PhD.
I would like to thank my advisors: Dachamir Hotza, Kaline P. Furlan and Rolf Janssen, for the confidence and for all support and knowledge shared with me. To the colleagues and researchers of TUHH (Technische Universität Hamburg Harburg), especially to:
Kaline P. Furlan for all help provided during my internship in Germany (sandwich doctorate).
To all my colleagues of INTELAB (Laboratory of Integrated Technologies) for standing by me during all my PhD research. To the Brazilian funding agencies CAPES and CNPq and to the Graduate Program on Materials Science and Engineering (PGMAT).
I would also like to thank my friends for all their support during my year abroad, especially Larissa Bezerra, Cece Wang, Fitore
Muharremi and Yasin Kantar.
Most important, I would like to thank my family, my parents and brother, for always being by my side and always believing in me.
Nothing in life is to be feared, it is only to be understood. Now is the time to understand more, so that we may fear less.”
RESUMO
Materiais macroporosos tridimensionalmente (3DOM) ordenados para aplicações em temperaturas elevadas foram desenvolvidos por infiltração de cristais fotônicos diretos seguida por termólise e calcinação. Mulita foi escolhida para servir de exemplo para a síntese de outros óxidos ternários. Cristais fotônicos diretos foram preparados a partir de esferas de poliestireno por automontagem vertical convectiva ou drop casting, posteriormente infiltrados por spin coating com gel de mulita. Os sol-géis de mulita foram preparados com duas composições diferentes variando a razão dos precursores iniciais. Embora os resultados indiquem uma formação de mulita difásica, e os cristais fotônicos inversos de mulita calcinados não apresentem propriedades ópticas, uma estabilidade estrutural avançada foi observada até 1500 °C para mulita de composição rica em alumina, o que permite a aplicação desses materiais 3DOM a altas temperaturas. Um estudo do desenvolvimento de um 3DOM de um material combinado (alumina e mulita) usando técnicas alternativas de revestimento, atomic layer deposition (ALD e infiltração sol-gel, também foi relatado. Um cristal fotônico inverso de alumina revestido com mulita foi preparado com esferas de poliestireno por automontagem e drop casting, antes do revestimento de alumina por ALD, com subsequente infiltração horizontal de sol-gel de mulita com duas diferentes composições e inversão de estrutura por calcinação. Os cristais fotônicos inversos calcinados, neste caso, mostraram não apenas estabilidade estrutural de 900 até 1500 °C, mas também propriedades ópticas, o que permite o uso do material como revestimento fotônico. Palavras-Chave: Sol-gel de Mulita; Infiltração Horizontal; Cristal Fotônico Inverso; ALD; Alumina.
ABSTRACT
Three-dimensionally ordered macroporous (3DOM) materials for applications at elevated temperatures have been developed by infiltration of direct photonic crystals followed by burnout and calcination. Mullite was chosen to serve as example for the synthesis of other ternary oxides. Direct photonic crystals were prepared from polystyrene spheres templates by vertical convective self-assembly or drop casting, later infiltrated by spin coating with mullite sol-gels. Mullite sol-gels with two different compositions were prepared by varying the initial precursors ratio. Although the results indicate a diphasic mullite formation, and the annealed mullite inverse photonic crystals showed no optical properties, an advanced structural stability was observed up to 1500 °C for an alumina rich mullite, which enables the application of these 3DOM materials at high temperatures. A study of the development of a 3DOM combined material (alumina and mullite) using alternative coating techniques, atomic layer deposition (ALD) and sol-gel infiltration, was also reported. A mullite-coated alumina inverse photonic crystal was prepared with polystyrene spheres templates by self-assembly and drop casting, prior to alumina deposition by ALD, followed by horizontal infiltration of two different mullite compositions and inversion of structure by calcination. The annealed inverse photonic crystals in this case showed not only structure stability from 900 to 1500 °C, but also optical properties, which enable the use of the material as a photonic material.
Keywords: Mullite Sol-gel; Horizontal Infiltration; Inverse Photonic Crystal; ALD, Alumina.
LIST OF TABLES
Table 2.1- Chemical routes for sol-gel process. ... 40 Table 3.1 - Microstructure analysis of PS direct photonic crystals and mullite inverse photonic crystals before and after heat treatment by digital image processing. Ø = roundness (ideal sphere = 1); s* = minimum distance (interstitial space) between PS microspheres; As* = percentage area of space between PS microspheres; s** = minimum strut thickness after filling the interstitial space with mullite gel; As** = percentage area of struts after filling with mullite sol-gel; dp* = diameter of PS microspheres; Ap* = percentage area occupied by PS microspheres; dp**= diameter of pores after thermal treatment; Ap**= percentage area occupied by pores after thermal treatment. ... 63 Table 4.1 - Microstructure analysis of PS direct photonic crystals and mullite inverse photonic crystals before and after heat treatment by digital image processing. Ø = roundness (ideal sphere = 1); s* = minimum distance (interstitial space) between PS microspheres; As* = percentage area of space between PS microspheres; s** = minimum strut thickness after filling the interstitial space with mullite gel; As** = percentage area of struts after filling with mullite sol-gel; dp* = diameter of PS microspheres; Ap* = percentage area occupied by PS microspheres; dp**= diameter of pores after thermal treatment; Ap**= percentage area occupied by pores after thermal treatment. ... 88
LIST OF FIGURES
Figure 2.1- Inverse photonic crystals found in nature. ... 29
Figure 2.2 - Types of structures with different spatial variation of the periodic network. ... 30
Figure 2.3 - Example of application of thermal barrier coatings. ... 31
Figure 2.4 - Image of PS direct photonic crystals. ... 32
Figure 2.5 - Schematics of direct photonic crystals obtained by vertical convective self-assembly. ... 33
Figure 2.6 -Schematics of direct photonic crystals obtained by drop casting. . 33
Figure 2.7- Schematics of coating by ALD. ... 35
Figure 2.8 - Schematics of coating by sol-gel infiltration. ... 37
Figure 2.9 – Mullite phase diagram. ... 38
Figure 2.10 – Schematics of sol-gel process. ... 40
Figure 2.11–Thermal analyses: (a) TGA and (b) DTA curves of mullite sol-gel obtained by different methods [41]. ... 42
Figure 2.12 – XRD patterns of mullite single-phase gel by sol-gel [66]... 43
Figure 2.13 – SEM micrograph of inverse photonic crystal of: a) titania; b) yttria-stabilized zirconia; and c) alumina [68, 69, 70]. ... 44
Figure 2.14 – Reflectance spectra of an YSZ inverse photonic crystal [69]. ... 44
Figure 3.1 - Schematics for production of mullite inverse photonic crystals. .. 56
Figure 3.2 - Example of binary images of SEM micrographs of: a) PS direct photonic crystals obtained by self-assembly, b) PS direct photonic crystals obtained by drop casting, c) mullite inverse photonic crystals infiltrated with low alumina sol-gel annealed at 1200 °C and d) mullite inverse photonic crystals infiltrated with high alumina sol-gel annealed at 1500 °C. Image analysis by Image J. ... 58
Figure 3.3 - TGA and DTA curve of Mullite sol-gel: a) (74 wt.% Al2O3, 26 wt.% SiO2) b) (80 wt.% Al2O3, 20 wt.% SiO2) recorded at 5°C min-1in flowing air. 59 Figure 3.4 - XRD patterns of mullite sol-gel after calcination in air at different temperatures: a) 74 wt.% Al2O3, 26 wt.% SiO2; b) 80 wt.% Al2O3, 20 wt.% SiO2. Mullite peaks (JCPDS card no. 15-776), (JCPDS card no. 84-1205) are represented by θ; δ-alumina (JCPDS card no. 4 6-1131) by δ; cristobalite (JCPDS card no. 27-605) by β and α-alumina (JCPDS card nr. 88-0826) by α. ... 60
Figure 3.5- SEM micrograph of PS direct photonic crystals obtained through: a) self-assembly; b) drop casting. ... 61 Figure 3.6 – SEM images of mullite inverse photonic crystals in: a) sapphire substrate infiltrated with sol-gel (74 wt.% Al2O3, 26 wt.% SiO2) obtained through self-assembly; and b) sapphire substrate infiltrated with sol-gel (80 wt.% Al2O3, 20 wt.% SiO2) obtained through drop casting. ... 62 Figure 3.7 - Reflectance spectra of PS direct and mullite inverse photonic crystal with respectively image of the direct photonic crystal: a) obtained through self-assembly and coated with sol-gel (74 wt.% Al2O3, 26 wt.% SiO2); and b) obtained through drop casting and coated with sol-gel (80 wt.% Al2O3, 20 wt.% SiO2).64 Figure 3.8- XRD (GI) patterns of: a) mullite inverse photonic crystal (74 wt.% of Al2O3, 26 wt.% SiO2); b) mullite inverse photonic crystal (80 wt.% Al2O3, 20 wt.% SiO2) after calcination in air at 900, 1000, 1200 °C for 1 h and at 1400 °C for 4 h, 1500 °C for 8 h. Mullite peaks (JCPDS card no. 15-776), (JCPDS card no. 84-1205) are represented by θ; and cristobalite (JCPDS card no. 27-605) by β; α-alumina peaks (JCPDS card nr. 88-0826) by α; δ-alumina (JCPDS card no. 46-1131) by δ and aluminum sample holder by γ. ... 66 Figure 3.9- SEM micrographs of mullite inverse photonic crystals: a) - d) sapphire substrate infiltrated with sol-gel (74 wt.% Al2O3 , 26 wt.% SiO2)obtained through self-assembly and annealed in air at 900, 1000, 1200 °C for 1h and at 1400 °C for 4h, respectively. ... 67 Figure 3.10 - SEM micrographs of mullite inverse photonic crystals: a) - e) sapphire substrate infiltrated with sol-gel (80 wt.% Al2O3, 20 wt.% SiO2) obtained through drop casting annealed in air at 900, 1000, 1200 °C for 1 h and at 1400 °C for 4 h, 1500 °C for 8 h, respectively. ... 68 Figure 4.1 - Schematics of synthesis of mullite-coated alumina inverse photonic crystals. ... 80 Figure 4.2 - Example of binary images of SEM micrographs of: a) PS direct photonic crystals obtained by self-assembly, b) PS direct photonic crystals obtained by drop casting, c) mullite-coated alumina inverse photonic crystals infiltrated with low alumina sol-gel annealed at 1500 °C and d) mullite-coated alumina inverse photonic crystals infiltrated with high alumina sol-gel annealed at 1500 °C. Image analysis by Image J. ... 82 Figure 4.3- Schematics of heat treatment of mullite-coated alumina inverse photonic crystals. ... 83 Figure 4.4 - TGA and DTA curve of mullite sol-gel: a) 74 wt.% Al2O3,26 wt.% SiO2; and b) 80 wt.% Al2O3, 20 wt.% SiO2. Recorded at 5°C min-1 in flowing air. ... 84 Figure 4.5 - XRD patterns of mullite sol-gel: a) 74 wt.% Al2O3, 26 wt.% SiO2; b)
80 wt.% Al2O3, 20 wt.% SiO2 after calcination in air at different temperatures. Mullite peaks (JCPDS no. 15-776), (JCPDS card no. 84-1205) are represented by θ; delta alumina (JCPDS no.46-1131) by δ; cristobalite (JCPDS no. 27-605) by β and α-alumina (JCPDS card nr. 88-0826) by α. ... 85 Figure 4.6 - SEM micrograph of: a) PS direct photonic crystals obtained through Self-Assembly; b) PS direct photonic crystals obtained through drop casting. 86 Figure 4.7 - SEM images of mullite-coated alumina inverse photonic crystals in: a) sapphire substrate infiltrated with sol-gel (74 wt.% Al2O3, 26 wt.% SiO2) obtained through self-assembly; and b) sapphire substrate infiltrated with sol-gel (80 wt.% Al2O3, 20 wt.% SiO2) obtained through drop casting. ... 87 Figure 4.8 - Reflectance spectra of PS direct, alumina direct and mullite-coated alumina inverse photonic: a) obtained through self-assembly and coated with sol-gel (74 wt.% Al2O3, 26 wt.% SiO2); and b) obtained through drop casting and coated with sol-gel (80 wt.% Al2O3, 20 wt.% SiO2). ... 89 Figure 4.9- XRD (GI) patterns of mullite-coated alumina inverse photonic crystals after calcination in air at 900, 1000, 1200 °C for 1 h and at 1400 °C for 4h, 1500 °C for 8 h: a) 74 wt.% Al2O3, 26 wt.% SiO2 obtained through self-assembly; b) 80 wt.% Al2O3, 20 wt.% SiO2 obtained through drop casting. Mullite peaks (JCPDS card no. 15-776), (JCPDS card no. 84-1205) are represented by θ; and cristobalite (JCPDS card no. 27-605) by β; α-alumina peaks (JCPDS card nr. 88-0826) by α; δ-alumina (JCPDS card no. 46-1131) by δ and aluminum sample holder by γ. ... 91 Figure 4.10 - SEM micrograph of mullite-coated alumina inverse photonic crystals: a) - e) obtained through self-assembly (74 wt.% Al2O3, 26 wt.% SiO2) and annealed in air at 900, 1000, 1200 °C for 1 h and at 1400 °C for 4 h, 1500 °C for 8h, respectively. ... 92 Figure 4.11 - SEM micrograph of mullite-coated alumina inverse photonic crystals: a) - e) obtained through drop casting (80 wt.% Al2O3, 20 wt.% SiO2)and annealed in air at 900, 1000, 1200 °C for 1 h and at 1400 °C for 4 h, 1500 °C for 8 h, respectively . ... 93 Figure 4.12 - Reflectance spectra of mullite-coated alumina inverse photonic crystals annealed in air at 900, 1000, 1200 °C for 1 h and at 1400 °C for 4 h, 1500 °C for 8 h respectively a) obtained through self-assembly (74 wt.% Al2O3, 26 wt.% SiO2); b) obtained through drop casting (80 wt.% Al2O3, 20 wt.% SiO2).94
LIST OF ABBREVIATIONS
ALD = Atomic Layer Deposition
ANN = Aluminum Nitrate Nonahydrate
CVD = Chemical Vapor Deposition
DLS = Dynamic Light Scattering
DOM = Three-Dimensional Macroporous
DTA = Differential thermal analysis
GI = Grazing Incidence
JCPDS = Joint Committee on Powder
Diffraction
PBG = Photonic Bandgap
PMMA = Polymethylmethacrylate
PS = Polystyrene
PSZ = Partially Stabilized Zirconia
SEM = Scanning Electron Microscopy
SOFC = Solid Oxide Fuel Cells
TBC = Thermal Barrier Coatings
TEOS = Tetraethyl Orthosilicate
TGA = Thermal Gravimetric Analysis
TMA = Trimethylaluminium
UV = Ultraviolet
XRD = X-Ray Diffraction
CONTENTS 1 INTRODUCTION ... 25 1.1 OBJECTIVES ... 26 1.1.1 MAIN OBJECTIVE ... 26 1.1.2 SPECIFIC OBJECTIVES ... 26 1.2 THESIS STRUCTURE ... 26 1.3 REFERENCES ... 27 2 LITERATURE REVIEW ... 29 2.1 PHOTONIC CRYSTALS ... 29 2.1.1 CONCEPT ... 29
2.1.2 APPLICATION AS THERMAL BARRIER COATINGS ... 30
2.2 PREPARATION OF DIRECT PHOTONIC CRYSTALS ... 31
2.2.1 DIRECT PHOTONIC CRYSTALS BY SELF-ASSEMBLY FROM COLLOIDAL PARTICLES ... 31
2.3 INVERSE PHOTONIC CRYSTALS ... 34
2.3.1 COATING METHODS ... 34
2.3.1.1 Chemical Vapor Deposition (CVD) ... 34
2.3.1.2 Atomic Layer Deposition ... 35
2.3.1.3 Sol-gel ... 35
2.4 MULLITE SOL-GEL ... 37
2.4.1 MULLITE ... 37
2.4.2 MULLITE SYNTHESIS ... 38
2.5 CHARACTERIZATION TECHNIQUES ... 41
2.5.1 PARTICLE SIZE DISTRIBUTION ... 41
2.5.2 THERMAL ANALYSIS... 41
2.5.3 PHASE IDENTIFICATION ... 42
2.5.4 MICROSTRUCTURAL ANALYSIS ... 43
2.5.5 PHOTONIC BAND GAP ... 44
2.6 REFERENCES ... 44
3 HIGH-TEMPERATURE STABLE INVERSE PHOTONIC CRYSTALS VIA MULLITE SOL-GEL INFILTRATION OF ORGANIC DIRECT PHOTONIC CRYSTALS ... 51
3.2 INTRODUCTION ... 52 3.3 MATERIALS AND METHODS ... 53 3.3.1 PREPARATION OF MULLITE GELS ... 53
3.3.2 TEMPLATE PREPARATION OF PHOTONIC CRYSTALS ... 54 3.3.2.1 Materials ... 54 3.3.2.2 Vertical Convective Self-assembly ... 54 3.3.2.3 Drop Casting ... 55 3.3.3 PREPARATION OF INVERSE PHOTONIC CRYSTALS BY HORIZONTAL
INFILTRATION ... 55
3.3.4 CHARACTERIZATION ... 56 3.3.4.1 Mullite Sol-gel ... 56 3.3.4.2 Photonic crystals ... 57
3.4 RESULTS AND DISCUSSION ... 58
3.4.1 MULLITE SOL-GEL ... 58 3.4.2 PHOTONIC CRYSTALS ... 61
3.5 CONCLUSION ... 69
3.6 REFERENCES ... 69
4 THERMAL STABILIZATION OF PHOTONIC CRYSTALS:
STRUCTURAL BEHAVIOR AND PHASE TRANSFORMATION OF MULLITE-COATED ALUMINA INVERSE PHOTONIC CRYSTAL AT HIGH TEMPERATURES ... 75 4.1 ABSTRACT ... 75 4.2 INTRODUCTION ... 76
4.3 EXPERIMENTAL SECTION ... 77
4.3.1 PREPARATION OF MULLITE GELS ... 77 4.3.2 PREPARATION OF ALUMINA DIRECT PHOTONIC CRYSTALS ... 78 4.3.2.1 Materials ... 78 4.3.2.2 Vertical Convective Self-assembly ... 78 4.3.2.3 Drop Casting ... 78 4.3.3 PREPARATION OF INVERSE PHOTONIC CRYSTALS ... 79
4.3.3.1 Atomic Layer Deposition ... 79 4.3.3.2 Horizontal Infiltration by Mullite Sol-gel ... 79 4.3.4 CHARACTERIZATION ... 80 4.3.4.1 Mullite Sol-gel ... 80 4.3.4.2 Photonic crystals ... 81
4.4.1 MULLITE SOL-GEL ... 83 4.4.2 PHOTONIC CRYSTALS ... 86 4.5 CONCLUSIONS ... 94 4.6 REFERENCES ... 95 5 FINAL CONSIDERATIONS ... 101 5.1 SPECIFIC CONSIDERATIONS ...101 5.2 GENERAL CONSIDERATIONS ...102 5.3 FUTURE PERSPECTIVES ...102
1 INTRODUCTION
The development of three-dimensional ordered macroporous (3DOM) ceramic materials (inverse photonic crystals ) have been highly investigated in the past years due to the wide range of application of this type of material, especially as photonic material.[1] Photonic structures require a high reflectivity in a wide spectral range; 3DOM structures present certain properties, such as ordered arrangement of the pores, which can lead to photonic bandgap (PGB), a reflection of the incident electromagnetic radiation in certain wavelengths (Visible and Infrared). Besides their optical properties (PGB), [2] those materials can also show thermal stability to operate at high temperatures. Those properties combined can qualify the materials for photonic applications.
The inverse photonic crystals are produced by direct coating of photonic crystals. The direct photonic crystals can be manufactured by vertical convective self-assembly, spin coating or drop casting of monodispersed colloidal particles of polymers or silica spheres [3]. The choice of coating materials and techniques depends on the properties required for the final product, since photonic bandgaps (PBGs) are dependent on the refractive index of the materials. For photonic applications, several oxides are already reported as coating materials, such as titania [4], alumina [5], and yttria-stabilized zirconia [6]. Mixed oxide materials can also be applied as coating, such as mullite a ceramic material well known for its stability at high temperature, which can be obtained by the sol-gel process [7].
Sol-gel is characterized by the formation of different types of gel, depending on the precursors chosen. The control of several parameters may lead to a single-phase gel (type I), allowing for the material to present a low crystallization temperature [8].
Another advantage of the sol-gel process is that this is a low-cost technique when compared with other deposition methods such as chemical vapor deposition (CVD) and atomic layer deposition (ALD). Coating of the direct photonic crystals can also be achieved by combining different materials and methods to enhance certain properties [6,7].
Facing all properties stated of a mullite obtained by sol-gel process to develop photonic crystals, this work focus on the development of an inverse photonic crystal using this ceramic material as main coating, obtained by sol-gel, and on selecting the most suitable techniques to produce a material with enhanced properties to be applied as a refractory or photonic material.
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1.1 Objectives 1.1.1 Main Objective
To develop a photonic material, a refractory structure with optical properties, through the obtention of inverse photonic crystals (three dimensionally organized materials)), using mullite via sol-gel process as main coating material to produce the photonic crystals, since is not reported yet.
1.1.2 Specific Objectives
Mullite inverse photonic crystals and mullite-coated alumina photonic crystals.
Mullite by sol-gel with further investigation of type of gel obtained (I, II or III).
Template preparation (PS and alumina direct photonic crystals) by self-assembly and drop casting, with subsequent alumina deposition by ALD for the alumina templates.
Coating of polystyrene (PS) direct photonic crystals and alumina direct photonic crystals, respectively by horizontal infiltration of mullite sol-gel, with further study of the thermal stability of both annealed materials and possible application as a photonic material
1.2 Thesis Structure
This document is presented in the format of papers. The content of each chapter is described below:
Chapter 1: thesis objectives and structure.
Chapter 2: review of the state of the art of the development of inverse photonic crystals with emphasis on coating materials and techniques, including thermal stability of those materials and possible applications.
Chapter 3: experimental procedure and results of the development of an inverse photonic crystal focusing on mullite coating by sol-gel infiltration.
Chapter 4: experimental procedure and results of the development of an inverse photonic crystal focusing on mullite-coated alumina.
Chapter 5: final considerations and suggestions for future work.
1.3 References
1. V. Shklover, L. Braginsky, G. Witz, M. Mishrikey, and C. Hafner, High-Temperature Photonic Structures. Thermal Barrier Coatings, Infrared Sources and Other Applications, J. Comput. Theor. Nanosci., 5 (5)862–93 (2008).
2. D.K. Hwang, H. Noh, H. Cao, and R.P.H. Chang, Photonic Bandgap Engineering with Inverse Opal Multistacks of Different Refractive Index Contrasts,” Appl. Phys. Lett., 95 [9] 91101–3 (2009).
3. A. Lashtabeg, J. Drennan, R. Knibbe, J. L. Bradley, and G.Q. Lu, Synthesis and Characterisation of Macroporous Yttria Stabilised Zirconia (YSZ) Using Polystyrene Spheres as Templates, Micropor. Mesopor. Mater. 117 [1–2] 395–401 (2009).
4. Y. Li, F. Piret, T. Leonard, and B.-L. Su, Rutile TiO2 Inverse Opal with Photonic Bandgap in the UV-Visible Range, J. Colloid Interface Sci., 348 [1]43–8 (2010).
5. S. Sokolov, D. Bell, A. Stein, Preparation and characterization of macroporous alfa-alumina, J. Am. Ceram. Soc 86 (9) (2003) 1481–1486.
6. J.J. do Rosário, P.N Dyachenko, R. Kubrin, R.M. Pasquarelli, A.Y. Petrov, M. Eich, et al. Facile deposition of YSZ- inverse photonic glass films. ACS Appl.Mater. Interfaces. 15(6)(2014) 12335–12345.
7. K.P. Furlan, T. Krekeler, M. Ritter, R. Blick, G.A. Schneider, K. Nielsch, R. Zierold, R. Janssen, Low-temperature mullite formation in ternary oxide coatings deposited by ALD for high-temperature applications, ACS Adv. Mater. Interfaces 4 (23) (2017) 1–8.
8. K.J.D. MacKenzie, R.H. Meinhold, J.E. Patterson, H. Schneider, M. Schmucker, D. Voll, Structural Evolution in Gel-Derived Mullite Precursors, J. Eur. Ceram. Soc. 16(12) (1996) 1299– 1308.
2 LITERATURE REVIEW 2.1 Photonic Crystals 2.1.1 Concept
For the last three decades, there has been a progressive advance in the development of devices capable of controlling the propagation of photons [1] in micro and nanometric scales. Photonic materials make it possible to manipulate light emission, due to the fact those materials present structures with periodic variation of a refractive index and network constant comparable to wavelengths of visible spectral range, which affects the way that photons propagate [2]. This means that the crystalline network leads to the inhibition of spontaneous light emission in certain spectral bands, also known as photonic band gaps (PBGs) [3].
An example of photonic crystal is the inverse photonic crystals that can be found in nature. Opals are minerals composed of silica crystals, which are responsible for its iridescence (Figure 2.1 a)). Another great example of those structures in nature is found on the wings of certain butterflies (Figure 2.1 b)), whose coloration comes from the diffraction of light by a periodic porous network [4].
Figure 2.1- Inverse photonic crystals found in nature.
Photonic crystals can be classified into three different types, according to the spatial variation of the periodic network. In the case where this variation occurs only along a single direction, the crystal is called unidimensional (1D). If periodicity occurs along a plane, the crystal is called two-dimensional (2D), and if the periodic lattice extends across all space directions, the photonic crystal is said to be three-dimensional (3D). Figure 2.2 shows examples of one, two and three-dimensional crystalline networks [5].
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Figure 2.2 - Types of structures with different spatial variation of the periodic network.
Advances in manufacturing techniques have considerably increased the scientific interest in the implementation of photonic crystals. This made it possible to investigate the optical properties of various crystal lattice configurations, ranging from 1D to 3D photonic crystals. The latter is applied to the investigation of the optical properties of a crystal formed by latex opals organized in a face-centered cubic (FCC) arrangement. [6] Several works highlight the photonic bands of colloidal crystals formed by artificial polystyrene opals [7,8], showing how the refractive index contrast can be used to control the width of the PBG in crystals formed by inverse opals, that is, air spheres immersed in a dielectric material with refractive index greater than 1.0. However, the major disadvantage of these crystals is the difficulty of introducing controlled defects for optical guidance and switching applications [9].
2.1.2 Application as Thermal Barrier Coatings
Efforts have been made for developing and producing ceramic coatings used as thermal barrier, since the materials that were used to that application have reached their maximum in terms of temperature they are able to withstand. Thermal barrier coating (TBCs) are systems that use advanced materials and that are normally applied to metal surfaces in areas where the temperatures involved are extremely high. These coatings are for materials that are subject to high temperatures during significant periods, thus protecting them from thermal exposure, which may lead to degradation [10].
TBCs are used in transition parts, combustion and in areas where high temperature is applied, such as in turbines (Figure 2.3). Generally, ceramic coatings are used on metallic parts, and due to the difference of thermal conductivities between the two types of material, it is possible to reduce the temperature of the metal substrates leading to a consequent increase in their life span, as well as increase the operation temperature
of turbines, for example [11].
TBC systems usually consist of two layers, a bond coat layer, and a ceramic layer, the insulating part, called a top coat. The bond coat layer, normally a metal, has the function of protect the substrate from corrosion and oxidation [12]. To be used as a top coat for TBC, a ceramic material needs to present certain properties, such as low thermal conductivity, coefficient of expansion similar to the material used as bond coat to avoid thermal tensions, high melting point and good mechanical properties at high temperatures. The first ceramic used as TBC was a zirconia-based coating [13]. During the 1980s, materials for TBCs significantly improved due to the use of zirconia partially stabilized with yttria (PSZ). This ceramic has proven to have exceptional properties, such as high fracture toughness, and has been the reference material for this type of applications. In the case of PSZ, its relevant properties are limited to a temperature of 1200 °C [14].
Other ceramic materials to be applied as top coats need to present properties sufficiently attractive above these temperatures. Aircraft turbines are subject to mechanical, thermal and chemical stresses, so that the material used must have a high resistance to those requirements. This is where ceramic materials [15] show better results when compared to metals, due to their resistance to oxidation and corrosion, and because they are very good thermal insulators.
Figure 2.3 - Example of application of thermal barrier coatings.
2.2 Preparation of Direct Photonic Crystals
2.2.1 Direct Photonic Crystals by Self-Assembly from Colloidal Particles
By using polymer spheres with sp2-hybridized carbon atoms to form self-assembled nanosphere monolayer templates (direct photonic crystals) [16], it is possible to achieve a highly crystalline inverse photonic structure. However, the difficulty in producing a low-defect and large-area nanosphere monolayer using this method is also reported [17].
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High colloidal crystallinity can be confirmed by the presence of a photonic bandgap in a certain wavelength range (Visible and Infrared) [18]. The spheres produced can be further infiltrated using a metal or ceramic to produce materials with enhanced properties [19,20]. Figure 2.4 shows direct photonic crystals formed by PS monodispersed colloidal particles.
Figure 2.4 - Image of PS direct photonic crystals.
The infiltration occurs into the interstitial spaces (voids between colloidal spheres) of the PS colloidal assembly to form a metal or ceramic/PS 3-dimensional ordered macroporous (3DOM) hybrid. After removal of the PS spheres (burn out) a highly ordered macroporous material (an inverse opal structure) results [21].
Self-assembly can be achieved by several techniques, such as vertical convective method [22], where the film is formed by capillary force as can be seen in Figure 2.5, that self-assemble monodispersed, spherical colloids into complex aggregates with well-controlled sizes and shapes. Spin-coating [23,24] has also being used, especially for scaling up to an industrial scale, since techniques like vertical convective self-assembly, are suitable only for small volumes or laboratory scale. With spin coating it is possible to produce films in a mass fabrication, but unfortunately it is hard to control uniformity and thickness of the film, since is necessary to adjust spin speed and time to produce a material with similarity when compared with vertical convective self-assembly [25].
Figure 2.5 - Schematics of direct photonic crystals obtained by vertical convective self-assembly.
Another approach to produce films trough self-assembly of monodispersed colloidal particles is drop casting [26]. This is not only a simpler and faster technique (it can be achieved in minutes) when compared with vertical convective self-assembly (which could take days), but also can produce films with larger areas and higher thickness. The main disadvantage is that, such as spin coating, it can present anon-uniform crystalline thickness. Figure 2.5 shows the formation of a polymer film formed by drop casting, where the suspension with the used polymer is dropped slowly with the aid of a micropipette on the top of the substrate, placed in a hot plate until the growing film is completely dryied.
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2.3 Inverse Photonic Crystals 2.3.1 Coating Methods
2.3.1.1 Chemical Vapor Deposition (CVD)
The Chemical Vapor Deposition (CVD) process consists in depositing solid material from a gas phase. The process is similar to Physical Vapor Deposition (PVD), but in that case the coating material (precursor) is originally solid. In the CVD process, the substrate is placed in a reactor that receives gas feed. The process principle is a chemical reaction between gases. The product of this reaction is a solid material that condenses on all surfaces within the reactor, forming the desired coating film on the substrate. [27]
CVD is one of the preferred methods of thin film deposition and coatings. There are several applications of CVD coated thin films, from semiconductors to optoelectronics, refractory ceramic materials, energy conversion devices, or diffusion barriers [28].
The two most important CVD technologies are Low Pressure CVD (LPCVD), which produces layers with thickness uniformity quality material. However, this is performed with a very high deposition temperatures (above 600 °C) and at low deposition rate. Alternatively, Plasma Assisted CVD (PECVD) is a process that can operate at lower temperatures (around 300 °C) due to the energy supplied to the molecules by the plasma of the reactor. However, the quality of the film is lower than that of processes conducted at higher temperatures. Most PECVD systems deposit the coating film on one side of the substrate, while the LPCVD systems deposit the film on both [29].
The films produced by CVD are highly uniform and have good reproducibility. The technique allows the use of several chemical precursors, producing a variety of coatings, such as single layer, multilayer, nanostructured, or composite, with well-controlled dimension and specific structure at low temperatures. One disadvantage of the technique is the difficulty to deposit multicomponent materials by controlling the stoichiometry with several different precursors, because the adhesion of those precursors will occur at different vaporization rates [30].
2.3.1.2 Atomic Layer Deposition
Atomic layer deposition (ALD) is a technique for the deposition of homogeneous thin films. Due to its simplicity, reproducibility of films, it appears as a promising deposition technique. The technique is based on a reaction between precursor materials. In ALD, the reactants are purged into the chamber one by one, and between the reactant purging periods the excess reactants and reaction by-products are purged and evacuated from the chamber by the inertia gas [31], as can be seen in Figure 2.7. The remaining precursors, except those chemisorbed, are removed.
ALD is a process that enables atomic layer control (angstrom or monolayer level) and insulating deposition. It is possible to obtain good quality materials at low processing temperatures [32].
With ALD is also possible to process multiple substrates. Since the ALD precursors are gas phase molecules, the entire space will be filled no matter what the substrate geometry is. One limitation of the technique is the time of the process; the monolayer deposition occurs only a fraction by cycle [33]. The materials for coatings by ALD are numerous; the most reported ALD material is alumina [34].
Figure 2.7- Schematics of coating by ALD.
2.3.1.3 Sol-gel
An alternative to the aforementioned methods that can be used to fabricate ordered porous films, CVD [27] or ALD [31], is the sol-gel infiltration.
36
exhibits good homogeneity, easy thickness control, and relatively low cost when compared CVD and ALD. Sol-gel is a chemical process used for the synthesis of a colloidal suspension of solid particles in a liquid, sol, and subsequently the formation of a double phase material of a solid network occupied with a moist solvent, the gel [35].
Precursors, usually metal alkoxides, undergo two chemical reactions in the sol: hydrolysis and condensation or polymerization. The sol-gel transition, also known as gelation, begins with a system that consists of dispersed colloidal particles (sol), which result from the polymerization of the monomer. These particles bind to small three-dimensional branched chains and microgel regions. The system presents an elastic behavior when the viscosity tends to infinity and the sol reaches the gel point [36].
In order to obtain the layers of the films in the sol-gel process in specific substrates, several techniques are used, such as dip-coating, spin coating and spray coating, being the first two the most used.
Dip-coating [37] consists of five stages: immersion, emersion, deposition, drainage and evaporation. The substrate is attached to the gripper of the apparatus and then immersed and removed from the solution under controlled temperature and velocity. Evaporation of the solvent occurs simultaneously to deposition and drainage. Hydrolysis occurs as the film is exposed to air, where controlled air humidity is harnessed. The film layer obtained is a wet gel.
The elimination of the organic components and higher adhesion to the substrate is accomplished through sintering (densification) of the system (substrate + film) at temperatures that can vary from 100 °C to 500 °C. Temperatures higher than these allow to control the porosity of the film and to obtain crystalline materials.
Dip coating allows the deposition of more than one deposition layer. The process may be repeated to increase the film thickness, either in the same solution or in other solutions to deposit a different material. This technique also allows the deposition of layers of films on both sides of the substrate in addition to being easy and low financial cost [38].
The spin coating differs from dip coating because the deposition of the film is performed by centrifugal scattering of the sol-gel on the substrate. The process is divided into four stages: deposition, spin-up, spin-off and evaporation, although this occurs simultaneously with the other steps [39].
The substrate is rotated at a high angular velocity, causing the excess liquid to flow radially outwards due to centrifugal force. All the excess of liquid flowing out of the substrate in the form of drops, is referred to as
the spin-off step. After this stage, the thin layer remaining on the substrate is further reduced by evaporation of the solvent. Evaporation is taken as the primary mechanism of decreasing film thickness [40].
Generally, sol-gel process results in a highly pure homogenous product with good adhesion and low temperature processing [41]. The coating using sol-gel is described in Figure 2.8.
Figure 2.8 - Schematics of coating by sol-gel infiltration.
2.4 Mullite sol-gel 2.4.1 Mullite
Mullite is the only stable crystalline phase in the phase equilibrium of the Al2O3.SiO2 system under atmospheric pressure [42,43]. The first
phase balance diagram of the system Al2O3SiO2 was published in 1909
(Figure 2.9) [44]. The stable binary referred to the silimanite phase, whose formula is Al2O3SiO2. Later it was showed that the stable aluminosilicate
present in the Al2O3SiO2 has the composition 3Al2O32SiO2 instead of
Al2O3SiO2 [45]. The stoichiometric mullite (3Al2O32SiO2) of
orthorhombic structure is formed by octahedral chains of AlO6 that share
their vertices among themselves and are interconnected by tetrahedral of AlO4 or SiO4 forming double chains parallel to the c axis [46].
38
Figure 2.9 – Mullite phase diagram.
The stoichiometric mullite is obtained by replacing the Si+4 ions with
ions Al+3 of the tetrahedral sites and for compensation in the positive
charge is created an oxygen gap. There have been considerable interest applications of mullite, due to its properties, among them: low thermal expansion, high thermal stability, low density, low thermal conductivity, good resistance mechanics and creep resistance, good stability in severe chemical environments, among others [47,48].
Moreover, the raw materials for its production (ex: alumina, silica, aluminum silicates, clays, among others) are widely found in nature. Mullite is thus widely used in high temperature applications such as gas filters heat exchangers, gas turbines, internal combustion processes, in ceramic composites in aircraft, in the manufacture of bricks and crucibles, and as optical windows in a wavelength range in the infrared [49, 50]. These applications at high temperatures are due to their high creep resistance, thermal shock and its intrinsic thermal stability in oxidation conditions [51]. The properties of mullite ceramics depend, mainly of the composition, purity of the reagents, the homogeneity of the mixture of substances that will react to form the mullite and the synthetic process used. In that sense, many methods of synthesis have been studied in order to achieve better structural and morphological properties and enable its application in all examples listed before.
2.4.2 Mullite Synthesis
co-precipitation, hydrothermal processes, chemical vapor deposition, and sol-gel [27,42,52].
Mechanisms of reactions for mullite formation can vary considerably according to the precursors and the methods employed. Precursors chemically synthesized, become mullite in a temperature range between 850 and 1350 °C. Mullite mechanism formation and its crystallization temperature are dependent on the level of homogeneity of precursor in the process [53,54]. When precursors show a high degree of homogeneity, the start temperature of mullite formation is low. However, when there is heterogeneity or segregation, the formation temperature of mullite is considerably increased at above 1400 °C [55,56].
Mullite formation occurs by nucleation and growth. At high temperatures, alumina ions diffuse into the silica particles, forming a silico-alumina liquid. Increasing even more the temperature, diffusion of the ions proceeds, and the liquid is gradually enriched in aluminum ions up to quantities of silicon and aluminum ions reach the stoichiometric phase. The nucleation of mullite is thus initiated, followed by its growth that occurs by diffusion and precipitation [57].
Interest in sol-gel processing began in mid-1900s on silica gels. [58] In the 1950s, researchers [59] recognized the potential to achieve very high levels of chemical homogeneity in colloidal gels using sol-gel and synthesized a large number of new ceramics with oxide compositions, involving Al, Si, Ti, Zr, which could not be done using other methods. Since then, interest in the sol-gel method has grown, with greatest motivation for such interest being the fact that this technique provides powders with high purity and chemical homogeneity and also at low processing temperatures [60].
The sol-gel method includes a variety of techniques that allow obtaining high purity compositions with homogeneity at the molecular level. There are essentially two different types of sol-gel technologies: colloidal sol-gel, and polymeric sol-gel (Table 2.1) [61].
40
Chemical Routes Advantages Disadvantages Polymeric Sol-gel Control of the speed
of hydrolysis, single-phase gel. It requires improvement of the solubility of the precursor to complete the polycondensation reaction.
Colloidal Sol-gel Simple technique, it can yield a product with high
homogeneity when is in a nanoscale level.
It is hard to control the hydrolysis rate, which can lead to an increase of crystallization temperature, hence phase segregation. Table 2.1- Chemical routes for sol-gel process.
The first objective in all sol-gel processes is preparation of a homogeneous precursor solution from which a semi-rigid gel can be isolated with a level of homogeneity atomic [35]. Figure 2.10 shows a basic representation of the sol-gel process.
2.5 Characterization Techniques 2.5.1 Particle Size Distribution
The size of the nanoparticles is a highly important factor not only for characterization of the synthesized powder but also in the definition of their application.
There are several techniques to estimate the size of nanoparticles; among them is the dynamic light scattering (DLS) [62]. The great advantage of this technique is the required equipment is easy to operate and time for data acquisition is short (less than 5 min). It is necessary to prepare the samples to avoid contamination with dust and correct parameters settings to prevent false data.
The technique is based on the principle that when the light hits small particles, it disperses in all directions without loss or energy gain. If you use a laser as the light source, it is possible to observe a fluctuation in the intensity of scattering those changes with time. These fluctuations are the results of the fact that nanoparticles do not remain static, but in constant random (Brownian) motion and so the distance between the scatters in the solution is also changing with time. Through mathematical methods, it is possible then to associate the variation of scattered light intensity with time with the size of the particles that are dispersed in the suspension [63]. 2.5.2 Thermal Analysis
The technique of thermogravimetric analysis (TGA) quantifies and qualifies the behavior of the material as a thermal transformation that result in gain or loss of weight. This type of analysis measures the weight variation of materials obtained in relation to temperature and time when samples are subjected to a controlled temperature program. This method is usually applied simultaneously with differential scanning calorimetry (DSC) or differential thermal analysis (DTA). Figure 2.11 shows a typical TGA and DTA curves for mullite [41].
42
Figure 2.11–Thermal analyses: (a) TGA and (b) DTA curves of mullite sol-gel obtained by different methods [41].
2.5.3 Phase Identification
X-Ray Diffractometry (XRD) is based on the periodicity of the atomic arrangement, which is also the major limitation of the technique. It does not apply to the fully amorphous solids (shapeless) such as glass or polymers (no crystal netshape).
The sample to be analyzed is bombarded by X-rays of a certain wavelength, λ, at a certain incident angle, θ. Depending on the incident angle, the x-ray can interact with certain atoms in the lattice. The interaction makes the X-rays to be scattered at every atomic location. A constructive interference will occur for some angles and a destructive
interference for others. The constructive interference amplifies the signal and the destructive cancels each other. Large peaks on XRD pattern is a representation of constructive interference. This phenomenon is governed by the Bragg’s law [64].
Powdered sol-gel can be analyzed by XRD, after material annealing at several temperatures. With the analysis of the width and the profile of diffraction peaks, it is possible to identify material phases, hence the type of gel [65].
In Figure 2.12, it is possible to analyze the mullite phase forming through XRD spectra of the mullite gel using calcination process as heat treatment; the material reported has three very specifics intensity peaks in the 2θ range [66].
Figure 2.12 – XRD patterns of mullite single-phase gel by sol-gel [66].
2.5.4 Microstructural Analysis
Electron microscope is a tool that uses a beam of electrons to form an image of a sample. The advantage of the electron microscopes in comparison with optical microscopes is the much shorter wavelength of the electron (e.g., λ = 0.005 nm) when compared to the wavelengths of visible light (λ = 400 nm to 700 nm). The Scanning Electron Microscopy (SEM) generates an image with the help of secondary electrons that gives the viewer the impression of three dimensions. Depending on the instrument used, samples can be magnified roughly between 10 and 100,000 times in SEM. With the scanning electron microscope, it is possible to analyze the structure of the material, being suitable to analyze structure stability of samples when high temperature is applied [67]. Figure 2.13 shows a SEM micrograph of several inverse photonic crystals.
44
Figure 2.13 – SEM micrograph of inverse photonic crystal of: a) titania; b) yttria-stabilized zirconia; and c) alumina [68, 69, 70].
2.5.5 Photonic Band Gap
Photonic crystals may or may not present optical properties, such as photonic band gap (PGB). The presence of the PGB can be measured using a UV-Vis spectrophotometer, since the reflectance of the photonic crystals in a specific radiation wavelength will occurs due to the arrangement of the pores in this high ordered structure. The working range is dependent of the material refractive index [2]. Figure 2.14 shows typical reflectance spectra of a photonic crystal.
Figure 2.14 – Reflectance spectra of an YSZ inverse photonic crystal [69].
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3 HIGH-TEMPERATURE STABLE INVERSE PHOTONIC CRYSTALS VIA MULLITE SOL-GEL INFILTRATION OF ORGANIC DIRECT PHOTONIC CRYSTALS
3.1 Abstract
Three-dimensionally ordered macroporous materials for applications at elevated temperatures have been developed by infiltration of direct photonic crystals followed by burnout and calcination. Mullite was chosen to serve as example for the synthesis of other ternary oxides. Direct photonic crystals were prepared from polystyrene spheres templates by vertical convective self-assembly or drop casting, later infiltrated by spin coating with mullite sol-gels. Mullite sol-gels with two different compositions were prepared by varying the initial precursors ratio. Although the results indicate a diphasic mullite formation, and the annealed mullite inverse photonic crystals showed no optical properties (PGB), an advanced structural stability was observed up to 1500 °C for an alumina rich mullite, which enables the application of these 3D ordered macroporous materials at high temperatures.
Keywords: Mullite Sol-gel; Infiltration; Photonic Crystal; Stability; Macroporous.
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3.2 Introduction
The development of new high-temperature-stable material for 3D photonic crystals is currently a challenge, driven by its possible use in different systems, such as reflectors in solar cells [1], SOFCs (solid oxide fuel cells), photocatalysts [2] and photonics thermal barrier coatings (TBCs) [3].
The use of this kind of structure as a photonic material requires certain properties, namely the thermal stability to operate at high temperatures. The capability of photonic crystals to reflect radiation in a specific wavelength occurs due a photonic bandgap (PBG) [4] because of the 3D arrangement of the pores (inverse photonic crystals) or spheres (direct photonic crystals) in a highly ordered manner and the refraction index differences in the system. Furthermore, the stability of this materials when operating at high temperatures is dependent of the structural changes that may or may not occur, i.e. how stable is the 3D ordered structure [5].
Photonic crystals, also known as opaline structures, can exist in the form of direct phonic crystal [6] (colloidal particles thin films) and inverse photonic crystals (direct photonic crystals coated with materials that can enhance its properties). The direct photonic crystals are usually manufactured by vertical convective self-assembly [7] of monodisperse colloidal particles [8,9] such as PS, PMMA or silica spheres. The 3D structures can also be achieved by spin coating [10,11] and drop casting [12].
Although the vertical convective self-assembly usually presents more homogeneous films, it is still a very slow process (72-120 h depending on the conditions) when compared with other techniques presented (drop casting process can be performed in less than 30 min). [13] The infiltration of direct photonic crystals in order to obtain inverse photonic crystals can be performed by deposition methods such as atomic layer deposition (ALD) [14] or chemical vapor deposition (CVD), [15] and also by vertical and horizontal infiltration of solutions or sol-gels, [16]. The infiltration methods are, in comparison to ALD or CVD, less time and energy consuming methods. Moreover, the infiltration of sol-gel can lead to a high performance material when precursors with high purity are employed.
By means of sol-gel infiltration, a variety of ceramic oxide materials can be realized such as alumina, titania, zirconia, and even oxide mixtures like mullite. Mullite is mixture of Al2O3 and SiO2 that exist in two
process is well reported and can lead to a mullite phase with controlled properties and high purity [18]. Unfortunately, a very strict control of the sol-gel and high purity precursors are necessary to produce monophasic mullite. Therefore, mullite sol-gel routes usually deal with the possibility of formation of a diphasic gel [19], consisting of a mixture of mullite and alpha alumina spinel, with higher crystallization temperature (over 1200 ºC) [20].
Several studies have been performed to depict the parameters required to obtain a mullite monophasic gel. The main parameters are the effect of the precursors [21], which usually consists in a salt/alkoxides mixture and controlling the hydrolysis rate of the process, since a fast hydrolysis rate can generate diphasic gels [22]. Most common is a solution of TEOS (tetraethyl orthosilicate) and ANN (aluminum nitrate nonahydrate) in ethanol stirring at 60 °C for several days. A hydrolysis polymerization reaction between the precursors will occur, leading to the gel forming. A slow hydrolysis is more likely to happen because of the water of crystallization (9H2O) from the aluminum nitrate [23].
Furthermore, the phase transformation temperature to mullite is only substantially reduced for sol-gels that are homogeneous in atomic scale (type I), whereas for diphasic gels (type II) this temperature is still above 1200 °C up to 1500 °C [24]and for hybrid gels (type III) in between. The type of gel obtained during synthesis depends on a large variety of factors such as type of starting precursors, pH, organic and inorganic additive additions as well as solvent type and amount [25].
After infiltration, a heat treatment is performed to burnout the photonic crystals colloidal polymer particles [26]. Even though the production of photonic crystals [27,28] is already well studied, the production of an mullite inverse photonic crystal by sol-gel infiltration has not yet been accomplished.
In this work, we demonstrate the synthesis of sol-gel-based mullite inverse photonic crystals by sol-gel horizontal infiltration via spin coating, as a low-cost and versatile technique to produce high-temperature stable inverse photonic crystals [29].
3.3 Materials and Methods 3.3.1 Preparation of mullite gels
Mullite gels with 2 compositions, nearly stoichiometric low-alumina 3Al2O32SiO2 (74 wt.% Al2O3, 26 wt.% SiO2) and high-alumina content
5Al2O32SiO2 (80 wt.% Al2O3, 20 wt.% SiO2), were synthesized by
