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Nanostructure of Aerogels and their applications in thermal energy insulation Mina Noroozi, Mahyar Panahi-Sarmad, Mahbod Abrisham, Arian Amirkiai, Narges Asghari, Hooman Golbaten-Mofrad, Navid Karimpour-Motlagh, Vahabodin Goodarzi, Ahmad Reza Bahramian, and Beniamin Zahiri ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b01157 • Publication Date (Web): 16 Jul 2019 Downloaded from pubs.acs.org on July 17, 2019
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ACS Applied Energy Materials
Nanostructure of Aerogels and their applications in thermal energy insulation Mina Noroozi a, Mahyar Panahi-Sarmad *, a, Mahbod Abrisham b, Arian Amirkiai b, Narges Asghari b, Hooman Golbaten-Mofrad c, Navid Karimpour-Motlagh d, Vahabodin Goodarzi *, e, Ahmad Reza Bahramian *, a, Beniamin Zahiri *, f
a Polymer
Engineering Department, Faculty of Chemical Engineering, Tarbiat Modares University, P. O.
Box: 14115-114, Tehran, I.R. Iran b Department
of Polymer Engineering, Amirkabir University of Technology, P.O. Box:15875-4413,
Tehran, I.R. Iran c
School of Chemical Engineering, College of Engineering, University of Tehran, P.O. Box 11155‐4563,
Tehran, Iran d Department e Applied
of Processing, Iran Polymer and Petrochemical Institute, P.O. Box 14965/115, Tehran, Iran
Biotechnology Research Center, Baqiyatallah University of Medical Science, P.O. Box 19945-
546, Tehran, Iran f
Clean Energy Research Centre, The University of British Columbia, 6250 Applied Science Lane,
Vancouver, BC V6T 1Z4, Canada
Corresponding Authors, Email: Mahyar Panahi-sarmad,
[email protected] Vahabodin Goodarzi,
[email protected] &
[email protected] Ahmad Reza Bahramian,
[email protected] Beniamin Zahiri,
[email protected] 1 ACS Paragon Plus Environment
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Abstract The recently global energetic context is recognized by a requirement to reduce our energy consumption, to prolong the fossil fuel shortage, and to decelerate greenhouse gas transpiration. In order to reduce energy consumption, using insulator and decrease its thermal conductivity recognized as the most effective way over the last few years. Aerogels as super insulating materials permit reducing the heat exchange between two environments while producing via the facile solgel and divers drying routes. Aerogels have intrigued scientists and engineers due to their unique nano characteristics, such as low density, fine internal void spaces and open-pore geometry, which originate from sol particles 3D random network. Noteworthy, aerogel-based materials have a supreme potential as thermal insulation owing to their very low thermal conductivity based on trapped air in the meso/nano-porous structure. Indeed, aerogels have great appeal in term of its thermal efficiency, produce simplicity and performance reliability as compared with a traditional insulator. In this paper, we will review the main milestones along with the concept of aerogels, and then discuss some new trends, strategies and opportunities in employing various morphological and nano-structural control methods to improve the performance of aerogels, especially enhancing insulation efficiency or decreasing thermal conductivity. The focus will be on (I) tailoring porous-structure of carbon-based aerogel such as graphene-oxide and reducedgraphene-oxide to accommodate high thermal behavior, and (II) design strategies to achieve intrinsically super insulating materials in synthesized polymer and bio-based materials, with/without embedding additional component. Keywords: Organic Aerogels, Carbon-based Aerogels, Thermal Insulation, Porous Structure, Preparation Strategy, Thermal Conductivity
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ACS Applied Energy Materials
Graphical Abstract
ρ density, Kg/m3
Nomenclature Cv or Cp volume specific heat, J/m3.K
σB Boltzmann constant
dp aerogel particle diameter, m
σel electrical conductivity
D mean pore size of aerogel, m
Subscripts
Es/ρs specific extinction coefficient of aerogel, m2/Kg 0 aerogel backbone L Lorenz number
bulk bulk parameters
n refractive index
c coupling heat transfer
S specific surface area, m2/Kg
el electronic
T temperature, K
g gaseous
υ sound velocity, m/s
g-s gas-solid
Vpore pore volume, m3/Kg
p particle ph phononic
Greek symbols β parameter
r radiative
λ thermal conductivity, W/m.K
s solid
Λ mean free path, m
total total heat flux
1. Introduction 3 ACS Paragon Plus Environment
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Aerogels are the open cell materials with unique properties such as low density, high specific surface area, high porosity, low dielectric constant as well as super low thermal conductivity and so on 1,2. A silica wet-gel was dried via supercritical drying without the eradication of gel structure at first time in 1932 by Kistler
3–5,
it is called ‘‘aerogel’’. Additionally, Kistler
6
synthesized
alumina, tungstic, ferric, stannic oxide, nickel tartrate, cellulose, nitrocellulose, gelatin, agar, and egg albumin aerogels. In the 1980s, a resorcinol-formaldehyde aerogel is produced by Pekala via polycondensation method, and a carbon aerogel was discovered with pyrolyzing the products in the 1990s 7,8. In the recent century, the fascinating development occurred in aerogel’s world, such as CNT aerogel, graphene aerogel, gradient aerogel and so forth 9–14. According to statistical data of Scopus (figure 1), the publications on thermal insulation aerogels have increased year by year.
Figure1. Progress trend of the aerogel applications as a thermal insulator in the last three decades.
A wet-gel has been prepared via sol-gel process at low temperature and then dried via different ways to make diverse types that called aerogel, xerogel and cryogel, as shown in figure 2. When a wet gel heats up to reach the critical temperature (Tc) and critical pressure (Pc) in a locked container, the trapped liquids have escaped from the structure; thus, highly porous aerogels can be obtained. The evaporation of solvent at ambient pressure makes xerogels and also sublimation of freezing solvent from a gel gives cryogels (freeze-drying). Although, the the term "aerogel" nowadays serves for materials with low density and high surface area that most of its volume is formed by pores (>90%)
15,16.
The most prevalent solvents which used in the drying step are
areacetone/ethanol and water for xerogel/aerogel and cryogel, respectively. Eventually, modern aerogels are the promising material for exploiting at diverse applications, such as acoustic/thermal 4 ACS Paragon Plus Environment
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insulators
17–19,
absorption/desorption filter
20–22,
energy storage devices
23,24,
sensors
25,
optics,
electronics, aerospace 26, automobile, catalyst 27 and biomaterials 28,29, due to the unique properties and facile construction process.
Figure 2. The sol-gel process and different type of drying.
Inorganic, organic and carbon are the three classifications of aerogels. Inorganic materials including silica, clay and some metals such as Cr, Fe, Al 30 and Zr can be applied to fabricate an aerogel. Silica aerogels containing SiO2 units are the relevant category of inorganic classification due to consisting the nano pore size (average pore diameter about 10-100 nm), high porosity (8599.8%), high surface area (600-1000 m2/g) and low bulk density (0.003-0.35 g/cm3) 4. Generally, two steps seem necessary for the construction of aerogels: I. Gel formation and II. Drying. While the third step, that is known as aging, is considered for reinforcing the gel after the gelation step in silica aerogels 31. Nonetheless, the prominent physical, thermal, optical and acoustical properties of silica aerogels are undeniable, but these aerogels are intrinsically hydrophilic in nature and tremendously brittle/fragile, which lead to have a weak mechanical property and make its application limited
32.
On the other hand, the organic aerogels such as polymer-based aerogels
(including polyimide, polyvinyl alcohol, polyurea 33, resorcinol-formaldehyde 34,35 and melamineformaldehyde 36,37) and bio-based aerogels (including cellulose, starch and pectin 28,38) have been produced to overcome the mentioned drawbacks of silica-based aerogels. Therefore, proper properties of organic-based aerogels, such as appreciate mechanical behavior and low thermal conductivity, can candidate them to apply in diverse applications 39. As a comparison view, the 5 ACS Paragon Plus Environment
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thermal conductivity of silica-based aerogels is ~0.016 W/m.K and its value for resorcinolformaldehyde (RF) aerogels with low bulk density (0.04-0.1 g/cm3) and high specific surface area (~800 m2/g) is ~0.012 W/m.K 40–42. Moreover, the eco-friendly cellulose aerogels as a bio-based material with very low bulk density (0.0005–0.35 g/cm3) and high specific surface area (100-400 m2/g) have greater compressive strength (5.2 kPa–16.67 MPa) than that of silica aerogels (
