Historical PerspectiveZirconia aerogels for thermal management: Review of synthesis, processing, and properties information architecture
Graphical abstract
Introduction
Aerogels have unique material properties, such as high surface areas and low thermal conductivities. They are useful for a broad range of applications and have gained attention over the last several decades. Zirconia aerogels are used in an expansive variety of applications, including catalyst supports, sorption media, and electrodes in solid oxide fuel cells [1,2]. Due to the extremely low thermal conductivity and high porosity of zirconia aerogels, these materials are often used in thermal management applications, such as high-temperature thermal insulation, especially in aeronautics and aerospace systems [3]. In comparison with silica aerogels, which display sintering and loss of mesoporous structure at high temperatures, zirconia aerogels are promising candidates for applications taking place at temperatures above 1000°C due to their high melting point of ZrO2 at 2715°C, low thermal conductivity, and presence of both acid and base active centers. The high thermal stability of zirconia aerogels can be increased further by optimizing the aerogel formulation; for example, zirconia metal oxide aerogels stabilized by yttria rare earth dopants are especially thermally stable. This is due to the low thermal conductivity of yttria-stabilized zirconia (0.5 – 2.36 W/m·K), which is made even lower when used as an aerogel system (0.168 – 0.212 W/m·K) [4]. Since the formulation of zirconia aerogels can directly influence the thermal stability of these materials, an understanding of the effects of synthesis and processing on the zirconia aerogel system is valuable.
This review begins with a look at the usefulness of aerogels in general applications over the last decade and a comparison of zirconia aerogels to aerogels made with other various precursors, such as silica, alumina, and carbon. This review then seeks to further understand the synthetic pathways of zirconia aerogels as the synthesis methods, processing conditions, and characterization of zirconia aerogels are investigated. Depictions of the general synthesis and processing pathways of zirconia aerogels, which are similar to various other aerogel systems, can be found in Scheme 1. Prior to formation of the gel, the aerogel’s sol must be formulated using a variety of zirconium precursors, rare earth dopant stabilizers, solvent systems, templating agents, and gelation agents. Following formulation of the sol, the sol-gel method is most commonly used to initiate gelation; however, hydrothermal, sonochemical, electrolysis, solution heating, chemical precipitation, and microwave irradiation methods can also be used. After gelation, the gel can be dried using a variety of drying methods, such as freeze drying, ambient pressure drying, oven drying, atmospheric drying, and most frequently for aerogels, supercritical drying. Dried aerogels are often heat-treated, causing sintering and densification of the pore structure, at a variety of temperatures and times to determine behavior of the system upon high-temperature exposure. As-dried and heat-treated aerogel samples are then characterized using a variety of characterization techniques, including scanning electron microscopy, nitrogen physisorption, x-ray diffraction, and thermogravimetric analysis, to name a few. The entire experimental process from initial sol formulation to characterized product can take on average up to two weeks depending on chosen methods.
There are an extensive number of synthetic variables and processing conditions for zirconia aerogels. The number of synthetic pathways can be determined using the relationship mn where n is the number of synthetic variables considered and m is the number of distinct options possible for each variable. Looking at the sol formation alone, there are at least five distinct synthetic variables to be considered: zirconium precursor, rare earth dopant stabilizer, solvent system, templating agent, and gelation agent. If there were only two distinct options for each of these five variables (m = 2, n = 5), there would be 32 synthetic pathways; if there were three distinct options (m = 3, n = 5), 243 synthetic pathways would be feasible. Typically, m is much greater than two or three, as there are many more potential options for each synthetic variable; for example, m is greater than 17 for the gelation agent variable. The concentrations of each of the five synthetic variables added to the sol also need to be considered, further increasing the value of m. Following the formulation of the sol, variables included in the gelation method, drying method, and sintering method need to also be examined, increasing the value of n.
The large number of synthetic variables shows an exponential increase as each processing step is evaluated. Due to the lengthy experimental time between initial formulation and final characterized product, as well as cost of materials and methods, arbitrary investigation of the synthetic pathways of zirconia aerogels should be avoided. An optimized synthetic pathway, creating zirconia aerogels with high surface area and mesoporous structure, should be determined prior to experimentation. Yet, the sheer number of synthetic and processing variables of zirconia aerogels can make the optimization of these materials difficult and can hinder potential development.
This review uses an architected information structure to begin to mitigate the volume of synthetic variables, which will aid in the understanding and enhancement of these materials. The synthesis, processing, and characterization of 600 zirconia aerogel materials reported since 2014 is the focus of the information structure [1,[3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40]]. This knowledge graph contains nearly 3,800 nodes with over 10,000 relationships that visually and dynamically display the connections between aerogel properties and the synthesis and processing methods used to produce them. It is anticipated that this information structure can lead to a greater knowledge of zirconia aerogels and a future optimized synthetic pathway of these materials. In a broader aspect, this review provides an example for ways in which data science can be used to enhance colloidal chemistry, providing more efficient experimentation and development of materials, specifically those with high experimental dimensionality.
Section snippets
Aerogel-based technologies
Aerogels have high specific surface area, high porosity, and low density, making them the lightest synthetic solid in existence [41]. Aerogels also have low thermal conductivity and some can have high mechanical strength. Due to these extreme material properties, aerogels can be used for many different applications. In this review, the benefits of using aerogels in general, what properties make aerogels useful for certain applications, and zirconia aerogel use in specific application examples
Aerogel properties and applications: ZrO2, SiO2, Al2O3, Al2O3/SiO2, and carbon
The focus of this review is the synthesis and processing optimization of zirconia (ZrO2) aerogels. Following an introduction to zirconia aerogels, selected non-zirconia types of aerogel systems - silica, alumina, aluminosilicate, and carbon - are briefly covered, as these aerogel systems have also been considered for high-temperature applications. This discussion includes synthesis methods, properties, and applications, which are summarized in Table 1, specific to each aerogel system. Scheme 2
Synthetic routes to zirconia aerogels
Prior to drying and processing to synthesize zirconia aerogels, zirconia gels must first be prepared. This review considers several different routes for zirconium colloidal dispersion and gel formation, along with a variety of zirconium precursors, dopants, gelation agents, modifying agents, and surfactant templates that are added to the system prior to gelation. The technique chosen to synthesize zirconia sols, gels, and aerogels influences the properties of as-dried and heat-treated aerogels.
Post-synthesis aerogel processing
Following synthesis, zirconia gels are further processed to form zirconia aerogels. Here, different drying techniques, sintering parameters, and characterization methods that are used in the processing of zirconia aerogels are discussed. It should be noted that, while applicable to zirconia aerogels, the majority of these processing techniques are generally used for a wide range of aerogel systems.
Information architecture for zirconia aerogels
There are an extensive number of compositional and process variables that must be considered in the synthesis and processing of zirconia aerogels. Each of these synthetic pathways can influence the final properties of the zirconia aerogels, specifically the surface area, mesoporous structure, density, thermal conductivity, and crystalline phase. In order to organize the variables and resulting pathways to be considered, an information structure was architected.
The information structure was made
Conclusions and perspectives
Zirconia aerogels, unique mesoporous materials with high specific surface areas and low thermal conductivities, can be useful in a wide range of thermal management systems, including high-temperature applications in aerospace and aeronautics. However, it is important to optimize the synthesis methods and processing conditions of these materials so that the mesoporous structure is retained when the aerogel is exposed to temperatures above 1000°C. This review investigates an expansive number of
Funding
The authors would like to acknowledge NASA Fellowship 80NSSC18K1697 and NASA 80NSSC18K0453 for financial support of this work.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors would like to thank Adam Luxon and Quang Le for assistance in the construction of the aerogel knowledge graph.
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