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Direct foaming driven synthesis and thermophysical characterization of silica-alumina foams: Applications for thermal insulation
Journal Pre-proof Direct foaming driven synthesis and thermophysical characterization of silica-alumina foams: Applications for thermal insulation P.R. Rao, K. Muralidharan, M. Momayez, Keith A. Runge, D.A. Loy PII:
S0272-8842(20)30043-2
DOI:
https://doi.org/10.1016/j.ceramint.2020.01.042
Reference:
CERI 23974
To appear in:
Ceramics International
Received Date: 28 August 2019 Revised Date:
4 January 2020
Accepted Date: 6 January 2020
Please cite this article as: P.R. Rao, K. Muralidharan, M. Momayez, K.A. Runge, D.A. Loy, Direct foaming driven synthesis and thermophysical characterization of silica-alumina foams: Applications for thermal insulation, Ceramics International (2020), doi: https://doi.org/10.1016/j.ceramint.2020.01.042. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
Direct foaming driven synthesis and thermophysical characterization of silica-alumina foams: Applications for thermal insulation P. R. Rao*, K. Muralidharan, M. Momayez, Keith A. Runge, D. A. Loy Corresponding author details Pratish R Rao 1235 James E Rogers Way, Mines building. Materials Science and Engineering, Mining and Geological Engineering University of Arizona, Tucson USA Email: [email protected]
ABSTRACT
Using earth-abundant materials such as quartz and corundum powders as precursors, we have developed a simple, but effective low energy direct foaming based method for synthesis of thermally insulating porous foams with thermal conductivity as low as 0.08 W/mK. Specifically, the adopted synthesis procedure provides a straightforward avenue for obtaining closed-cell silicaalumina foams, with densities ranging from 1.7 to 0.22 g/cc. The ability to tune the densities is achieved by controlled addition of blowing agents (H2O2) and surfactants (stearic acid) respectively. The thermal conductivity of these foams decreased monotonically with increasing porosity (or decreasing densities), and the density-thermal conductivity relationship is governed by a power law. In addition, morphological and structural characterization was performed via optical and electron microscopy, x-ray diffraction, and nuclear magnetic resonance, and the results from these studies are also discussed in the context of structural evolution of the foams as a function of
porosity. While this work represents a straightforward and simple path towards obtaining silicaalumina foams with very low density, the developed method is directly applicable to obtaining a wider variety of multicomponent ceramic foams.
KEYWORDS ceramic foams, stearic acid, hydrogen peroxide, low density, thermal conductivity, thermal insulation
INTRODUCTION Ceramic foams are systems of great interest, finding applications as catalysts, filtration membranes, bone scaffold materials, as well as thermal insulators [1–6]. While pores in ceramic matrices can result in premature mechanical failure, leading to a reduction in the structural performance of the foams, the ability to tune and control pore sizes as well as their distributions can enhance their functionalities as light-weight material-systems with controllable gas permeability, low dielectric constant, and high thermal and acoustic insulation properties. There have been numerous studies that have examined the optimization of the synthesis and processing process of ceramic foams [1,3,7–11]:. There are four primary techniques involved in the synthesis of porous ceramics i) partial sintering; ii) sacrificial fugitives; iii) replica templates; and iv) direct foaming. In partial sintering, ceramic powders are compacted and heat treated to achieve desire porosity that is primarily determined by the size and morphology of starting powders [12]. In addition, sintering conditions such as temperature and pressure influence the powder-binding as well as the ensuing microstructure and mechanical properties [13]. Sacrificial fugitives such as coal, liquid paraffin, graphite, glass particles and organic fibers have been introduced in ceramic suspensions as pore forming agents. These fillers have to be removed by pyrolysis leading to underlying porosity [14,15]. Replica templates, such as polyurethane sponge, have been introduced in ceramic suspensions to create foams. In particular, the template is burned off at the final stages of setting thus introducing interconnected porosity in the foams. Compared to partial sintering, replica templating, and sacrificial fugitive methods, direct foaming represents the most straight-forward method of obtaining ceramic foams. Direct
foaming avoids complicated processing steps involving high temperature use during sintering and pyrolysis as well as template removal and extraction from the final ceramic structures. In direct foaming, the thermodynamic stability and kinetics of bubble formation determines the underlying foam microstructure. In particular, the drainage, coalescence and disproportionation related to bubble evolution controls porosity, pore size distribution as well as bubble dynamics [2,16–18]. Numerous research groups have worked on the development of porous ceramics by the direct foaming method [19–22]. Blowing agents such as H2O2 (HP), air, CO2, NH3, etc. as well as mechanical agitation can be used to produce bubbles in the ceramic slurry. Of particular significance is HP, which is an inexpensive compound, and serves as an efficient blowing agent as it chemically decomposes evolving oxygen gas and water [23] leading to void formation. In order to control and stabilize these voids, surfactants are also incorporated while foaming. A noteworthy example is stearic acid, which is an 18 carbon saturated fatty acid and a costeffective surfactant that is suitable for tuning porosity [24,25]. In this regard, we present a simple, low-cost, low energy footprint, direct foaming driven method for the synthesis of silica-alumina foams (SAF) derived from silica and alumina precursors, which are earth-abundant, and inexpensive. The developed process includes creating a slurry of Al2O3 and SiO2 powders, which is simply alkali activated by the addition of an aqueous NaOH solution and further, by incorporating appropriate amounts of stearic acid and HP, we demonstrate the ability to control the porosity and the resultant density of the foams. Further, as pointed out earlier, the adopted method in this work which is based on direct blowing, avoids complicated post-processing steps after curing, such as removal of any sacrificial agents or templates. In addition, mechanical agitation and the use of high temperature sintering have
also been avoided. Thus, this technique will be most beneficial and cost-effective for bulk manufacture of these foams. To provide further context for this study, we provide a brief summary of past investigations that have specifically focused on using direct foaming of aluminosilicate foams for thermal applications. Bai et.al. developed foams by direct foaming based on Metakaolin (MK) as raw material which was alkali activated by 11 M KOH[20]. They used HP as a blowing agent to create pores and different oils (vegetable, sunflower, canola, olive) as pore stabilizing agents. They observed that their foams showed interconnected pores with pore sizes ranging from 150 to 400 µm in addition to confirming that stabilizing agents lowered pore sizes as well as maintained homogenity. They reported foams to have a density as low as 0.37 g/cc with a k value of 0.11 W/mK which could potentially be used as fireproof insulation. In another study, Medri et.al. incorporated HP as blowing agent and expanded vermiculite as filler into a ultrafine MK based matrix activated by polysilicate solution[19]. They reported a range of pore sizes ranging from < 1 to 100 µm from Hg intrusion porosimetry and obtained foams with 0.7 g/cc possessing a k of 0.2 W/mK. In another study by Novais et.al. MK with porous FA biomass served as aluminosilicate source and foams were produced by HP aiding as blowing agent. 12 M NaOH was used as an alkaline activator to produce a slurry[21]. They observed an increase in pore area ratio with the solid material with increasing HP content from 4.3 % (at 0.03 wt. % HP) to around 40 % (at 1.2 wt.%). These foams were reported to have density as low as 0.56 g/cc and k of 0.1 W/mK. A. Hajimohmmadi et.al. made MK based foams with HP[26]. They reported pore homogeneity to be a function of SS concentrations. The group was able to obtain foams of 0.55 g/cc corresponding to a k value of 0.16 W/mK.
As compared to previous relevant studies as discussed above, a distinguishing feature of this work is the successful implementation of direct foaming to obtain much lower density closed cell SAF (~0.22 g/cc). Equally importantly, our starting feedstocks were pure alumina and silica powders, while previous studies relied on processed aluminosilicate precursors. Further, in this work we also provide a detailed analysis of the direct foaming process. By carefully tailoring the chemistry of the direct foaming process, a noticeable chemical evolution in terms of change of cation coordination, which underlay the ability of the foams to retain their mechanical and structural integrity, was demonstrated. Equally importantly, the low-density SAF structures demonstrated excellent thermal insulation properties (much lower conductivity than previous studies), making them highly suitable as building blocks for construction and mining applications. Another salient feature of this work was the characterization of the structurethermophysical property relations of the foams using a combination of nuclear magnetic resonance (NMR), optical and scanning electron microscopy (SEM), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), in conjunction with transient plane source methods (for measuring thermal conductivity). Fundamental insights into the thermal conductivity dependence on foam porosity were thus developed. Details regarding the synthesis and characterization of the developed foams are given in the following sections. A brief overview of the synthesis and characterization methods are provided in the Experimental Methods section, with a more detailed discussion provided in the Supplementary Information document. The ensuing structural, chemical, and thermophysical characterization of the foams with a special emphasis on thermal conductivity trends are then presented in the Results and Discussion section.
Experimental Methods The details of sample preparation and processing is discussed in the supplementary information. The block diagram (Fig.1), provides a visual aid for the stepwise synthesis of control and SAF systems. As shown in Fig. 1, the blowing agent and the surfactants are added prior to curing. There was no decomposition of HP observed before the slurry was introduced into the oven. Further it is also useful to note that the use of stearic acid (used extensively in skin care products) as a surfactant is deemed safe by US FDA. Moreover, the overall content of SA in the SA foam is no more than 0.2 % (by mol.). All samples were made from NaOH (10M) activation of pristine quartz and corundum powders. The high pH was mainly responsible for nucleoliphlic substitution leading to the formation of hydroxy species on both Si and Al sites. Furthermore, Na+ ions serve as charge balancing cations to compensate for the residual charge deficiency on the aluminum ion. Preliminary analysis showed that a Si:Al ratio of 3:1 was the optimal ratio that enabled further processing of the starting materials into foams. Further discussion is provided in the Supplementary Information document. The slurry was first homogenized by whisking and further by sonication for at least 6 min (for controls) and 8 min (for foams). To obtain foams, additional steps were introduced such as the addition of 3 wt.% HP (stock solution) and 20 wt.% stearic acid (stock solution) incorporated at different concentrations to obtain SAF structures of different densities. Note that for comparison, control samples were also prepared, for which neither the surfactant nor the blowing agent was incorporated. The exact molar composition for the control and SAF (0.22 g/cc) is provided in Table 1. The respective slurries were then poured onto a Teflon dish and cured in a convection oven at 100 oC for 72 hours (for control) and 12 hours (for SAF’s). A total of 12 different SAF
structures (including control) were prepared, with each characterized by a unique density and composition. The choice of the curing temperature was based on an initial examination of the role of temperature on the resulting microstructures. In our studies, curing at temperatures much higher or lower than 100 oC, resulted in porous structures with inconsistent pore size distribution. This is attributed to an interplay between bubble generation due to decomposition of the blowing agent, bubble stability and the evaporation rate of water, which are all critically dependent on the curing temperature.
RESULTS AND DISCUSSION The synthesis and fabrication of SAF with controllable thermal insulation properties represents the primary objective of this work. As a first step towards this end, the ability to obtain on-demand SAF structures with tunable porosity is discussed and characterized in terms of their thermophysical properties. The steps enumerated in Fig. 1 were undertaken to obtain SAF structures of varying porosity. For comparison, control samples without the blowing agent and surfactant were also prepared. Images of representative samples are provided in Fig.2 and Fig.3. Interestingly, but not unexpectedly, for the chosen Si:Al ratio (=3), the resultant foam density (and pore distribution) critically depended on the H2O2 to stearic acid molar ratio (HP/SA) as shown in Fig. 4. Notably, lowest densities were achieved for higher HP/SA ratio leading to mechanically stable foams with density equaling up to 0.22 g/cc. However, higher HP/SA ratios than that reported in Fig. 4 resulted in loss of homogeneity of pores. By homogeneity, we mean the pore shape, uniformity or pore distribution though out the foam.
Chemical homogeneity was examined in terms of their EDS spectra, and as shown in Fig.5, the elemental constituents (Si, O, Al, Na) were uniformly distributed for all the SAF structures. Porosity of the foams was expressed in terms of the effective pore volume (∅), which was calculated from Equation 1, with the ideal matrix density (
) corresponding to a
silica-alumina matrix obtained from rules of mixtures ∅ % = 1 −
× 100 (1)
Based on Equation (1), it was seen that ∅ of the synthesized structures varied between 40% and 92% respectively. In particular, despite the lack of the blowing agent, the control samples also exhibited porosity ( ~40% characterized by mm sized large pores which is attributed to the coalescence of voids that are generated by water evaporation from the Al-Si slurry during curing at 100oC. Moreover, the control samples exhibited a highly inhomogeneous spatial distribution of pores in addition to a wide pore-size distribution as compared to the more porous samples, for which both HP and stearic acid were added. These observations point to the importance of the surfactant (stearic acid) in tandem with the blowing agent (HP) in the ability to control pore homogeneity and size. Specifically, the decomposition of HP in the basic Si-Al slurry leads to the evolution of O2 and H2O, which drives pore formation. Without stearic acid, the decomposition of HP leads to density reduction as compared to control samples, but nevertheless, demonstrates higher densities than corresponding samples with stearic acid. The addition of stearic acid, enables the spacing and even distribution of pores, in addition to preventing pore coalescence, which is reflected in the observed trends shown in Fig. 4. The ‘sweet-spot’ for obtaining low-density SAF occurs between a HP/SA ratio (by mol.) of 4 and 8; beyond the
upper limit, the relative lack of stearic acid leads once again to void coalescence and unequal and inhomogeneous spatial distribution of pores. SEM images and corresponding pore size distributions for the foams within the sweet spot is given in Fig. 6 and Fig.7 respectively. As seen from SEM and optical micrographs, typical pore-sizes ranged from 300-600
for these foams. It is evident that these pores are of closed cell type with thin
struts acting as walls for each cell. These struts, we hypothesize, are composed of SiO2 and Al2O3 particles as well as their respective aluminosilicate compounds formed by alkali activation. We refer the reader to the latter part of this section, where evidence (based on NMR and XRD) for the presence of both unreacted particles and reacted aluminosilicates are provided. Having discussed the ability to synthesize SAF structures and control the porosity in a systematic fashion, we now turn our attention to characterizing the thermal conductivity of the pores and developing insights into the role of porosity on the ensuing thermophysical characteristics. The variation in thermal conductivity as a function of density is given in Fig. 8. As evident from the figure, there is a monotonic decrease in thermal conductivity as a function of decreasing density, with the interdependence of thermal conductivity and density following a power law fit. Note that due to practical difficulties in terms of ensuring perfect contact between the sensors (of the thermal conductivity measurement unit) and the samples corresponding to the highest porosity/lowest-density foams (~0.22 g/cc), we restricted our measurements to SAF with densities greater than equal to 0.41 g/cc. Nevertheless and importantly, it is clear that adopted synthesis procedure provides a straightforward pathway to obtain highly thermally insulating ceramic foams ( ~ 0.08 #/ % .
To further characterize the observed trends linking the thermal conductivity and underlying porosity, as well as develop fundamental insights into this interplay, we examined the variations in heat capacity, in addition to acoustic and mechanical properties of the foams as a function of density. Specifically, we focused on obtaining the volumetric heat capacity (&'( ) and the longitudinal speed of sound ()* ), of the respective foams. Measuring &'( , and )* in conjunction with the thermal conductivity (
, provides the ability
to obtain an effective phonon mean free path (,) via Eqn.2, and thereby shed light on the role of the pores as scatterers of thermal phonons. In addition, other important properties such as the elastic modulus (i.e. Young’s modulus) can be directly calculated from )* (Eqn. 3), enabling important characterization of the interplay between density and ensuing mechanical modulus. -
= &'( )/ , .
)/ = 0
1
(2)
(3)
Using the differential scanning calorimetry (DSC) technique, the specific heat at constant pressure (&' was measured for the different SAF structures at 300 K. The measured &' of all foams were similar and calculated to be in the range of ~ 770 (±5) 2/%3℃, while &'( , which is calculated from &' and the sample density, was found to decrease monotonically with increasing porosity. The observed invariance in &' as a function of density indicates that chemically and structurally the different SAF structures are in fact similar; more attention will be paid to the evolution in structural and chemical characteristics in the latter part of this manuscript. Variations in &' , &'( are given in Table 2.
As a next step, ultrasonic studies were undertaken to examine )/ dependence on density. Figure 9 presents the variation in sound velocity as a function of density. The observed trends are consistent with available literature on foams [27,28] in terms of a consequent reduction in velocity with increased porosity (reduced density). Interestingly, the calculated Young’s modulus (using Eqn. 3) demonstrates a power law behavior similar to the thermal conductivity trend, and of note is the fact that this trend correlates well with a recent computational investigation on the variation in modulus as a function of density/porosity for closed cell foams [29,30]. In comparison to polymer foams of similar densities such as rigid PU foams ranging between 115 to 794 MPa (density range 0.24 to 0.641 g/cc)[31], the SAF’s demonstrate superior mechanical stability for identical density ranges. Based on the obtained values of &'( and )* , the effective phonon mean free path (, variations were also estimated; it was seen that , was of the order of 1nm for all the SAF structures, which is characteristic of thermal insulators[32,33]. Unlike &'( and )* which decreased with density, there was no apparent variation in , with porosity, pointing to the stronger dependence of
on &'( and
)* . From a structure, composition, and chemical bonding point of view, we carried out XRD, NMR, and TGA characterization respectively, to shed further light into the intimate structurecomposition-processing-property relations of the SAF structures. To ensure clarity, we focus on the either ends of the ‘density-spectrum’ of the SAF structures (i.e. the control sample and the lowest-density foam) to develop valuable insights and highlight essential features, while more information is provided in the Supplementary Information. Figure 10 provides the XRD patterns of the control sample and the lowest density foam. For reference a physically mixed quartz (SiO2) and corundum (Al2O3) sample’s XRD pattern is also
provided and the respective (SiO2, Al2O3) peaks are indexed. From a comparison of the different patterns, it is obvious that both SAF structures (control and low-density foam) demonstrate striking similarities to the physical mixture; however, as shown in the inset of Fig. 10, the control sample exhibits a notable bump as compared to the physical-mixture, which is characteristic of an additional amorphous aluminosilicate gel phase [34,35] that is formed due to the hydration and alkali activation steps involved in the control sample preparation. On an equally relevant note, the XRD pattern of the low-density SAF structure, in addition to the unreacted SiO2 and Al2O3 peaks, demonstrates new features as indicated in Fig 11, which are characteristic of a crystalline sodium aluminate-silicate phase[36]. Thus based on the XRD patterns, it can be inferred that the SAF structures primarily consist of unreacted SiO2, and Al2O3 particles, with a secondary binder phase that is either amorphous (as in control) or crystalline ( as in lower-density foams), depending on the foaming process. Deducing the fundamental mechanisms that underlie the formation of the crystalline phase seen in the the low-density foams is beyond the scope this paper; however, using the Medusa-Hydra chemical equilibrium software[37], an initial assessment of speciation based on the high pH and the relative concentrations of HP, NaOH, Al2O3 and SiO2 shows the stability of the precipitating crystalline NaAlSiO4 phase. 29
The Si,
27
23
Al, and
Na NMR signals of the control sample and the low-density foam are 29
given in Fig. 12. For reference the baseline
27
Si and
Al NMR spectra of the respective
precursors are also provided. Interestingly, there are distinct chemical shifts for the control and the foams. In particular, in addition to the central peak at ~18 ppm corresponding to Al octahedral coordination, the
27
Al NMR spectra also demonstrates a secondary peak
corresponding to tetrahedrally coordinated Al in both the control and the foam samples, which correlates well with the observations of the secondary binder phase in the respective XRD patterns. In the case of the
29
Si spectra, the chemical shifts are very distinct for the
low-density foam versus the control. For the control sample, we see a relative reduction in the Q4 peak and the emergence of another peak, which when deconvoluted points to the presence of a mixture of Q3, Q2, and Q1 type coordination. A comparison of the relative peak-heights suggests that pristine silica feedstock in the control sample was more susceptible to chemical activation and conversion to the secondary aluminosilicate phase. On the other hand, for the low-density foam, there is a bimodal distribution corresponding to Q4 and Q1/Q0 type coordinations. The relatively larger Q1 peak in the low-density foam can be correlated to an increase in the silica network disruption due to silicon atoms that are bonded to multiple non-bridging oxygens, which is also reflected in the decrease in the Young’s modulus. For the
23
Na spectra too, there are distinct differences between the control and the low-
density foam, with a strong secondary peak at around 21 ppm for the foam. This is attributed probably to the formation of Na2O2, due to the reaction of HP (blowing agent) with Na ions in solution. While more work is needed to further interpret the NMR data in terms of the various combinations of coordinations that occur in the SAF structures, it is reasonable to assume that the chemical activation and the introduction of the blowing agent leads to key differences in the chemical environments and structural coordination of the constituent Si, Al, and Na atoms.
TGA analysis, as shown in Fig.13, indicates the volatiles present in the samples. The control sample retained most solvent (~ 8 % water) as seen from the weight loss in the TGA plot. No other solvents were used in the synthesis of control samples. The relative lower amounts of retained water in the case of foams is directly attributed to the presence of surfactants (i.e. stearic acid). Here the surfactant hydrophobic tails hinder the amount of water adsorption. For the higher densities (SAF 1.2 g/cc), the volatiles were mostly water as the amounts of stearic acid was relatively lower. As density reduced (SAF 0.25 g/cc), transitional weight losses were more evident due to remaining volatiles like ethanol and stearic acid being removed from the sample. These losses are evident from intermediate drops at 80 oC and 260 oC corresponding to the boiling points and vaporization temperatures of EtOH and stearic acid respectively. All SAF samples showed a weight loss of approximately 6 % on reaching a final temperature of 900 oC except control which had ~ 8 wt. % water retained within it.
CONCLUSIONS A simple and straightforward direct foaming process has been implemented to obtain closed-cell ceramic foams (up to 0.22 g/cc) that are derived from earth abundant materials such as silica and alumina. The synthesis process that consists of an alkali activation step followed by a low-energy thermal curing step, led to development of highly thermally insulating foams with thermal conductivity as low as 0.08 W/mK. Underlying the synthesis is the interplay between the blowing agent (HP
and surfactant (stearic acid), which
provides the ability to control the density and porosity of the ensuing foams. Moreover, the simplicity of this synthesis technique by direct foaming through in-situ chemical decomposition of HP as well as bubble stabilization by SA, facilitates large scale production
of the foams eliminating post processing steps of sintering as well as pyrolysis. With regards to structure, these foams primarily consist of unreacted silica and alumina particles, and a secondary aluminosilicate binder phase; in the absence of HP, the binder phase is shown to be amorphous, while with increasing HP content in conjunction with stearic acid, the binder phase demonstrates crystalline features. Ultrasonic testing was used for characterizing acoustic properties as well as obtaining moduli of the respective foams, while thermal conductivity was measured using a transient plane source technique. Interestingly, both thermal conductivity and elastic modulus followed a power law as a function of foamdensity. Specific heat of the foams was also measured using the DSC technique, and based on these observations, it was seen that both volumetric heat capacity as well as the speed of sound contributed to the underlying thermal conductivity trends. In the future, the applicability of the developed process to a wider variety of multicomponent ceramic foams will be examined. Further, a more elaborate characterization of the evolution of the chemical bonding and structural characteristics of the foams as a function of the process parameters will be carried out.
ACKNOWLEDGEMENTS We would like to express our sincere gratitude to NIOSH for their continued financial support. The authors also like to thank Dr. Brooke Beam at KECK facility at UA for SEM imaging, Dr. Andrei Eskatchine for XRD analysis and D r . J i x u n D a i the Chemistry department at UA for solid state NMR analysis. Additionally, we also like to thank David Streeter and Dr. Sahila Peranathan for their kind assistance.
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Table 1. Detailed composition of different chemicals in control and SAF (0.22 g/cc). NaOH SiO2
HP
SA
3 wt.%
(20 wt./v%)
Al2O3
(10M) Mol.wt.
40
60
102
34
284
Vol. (or mass)
17 mL
30 g
8.5 g
0.36 g
0.4 g
moles (for control)
0.17
0.5
0.0083
0
0
moles (for SAF- 0.22g/cc)
0.17
0.5
0.0083
0.01
0.0015
Table 2. Volumetric heat capacities and sound velocity for SAF of varying densities. Density g/cm3
Density kg/m3
(MJ/m3C))
(m/s)
0.25
250
0.193
1700
0.4
400
0.3096
2100
1.2
1200
0.9204
3000
Vs
LIST of FIGURES
Figure 1. Block representation of steps involved in preparation of control samples and SAF structures.
Figure 2. Cross-sectional view of SAF structures: (a) control (~ 1.7 g/cc), (b) foam of density = 1 g/cc
Figure 3. Cross-sectional view of SAF sample (0.22 g/cc)
Figure 4. The variation of density of SAF structures with differing ratios of hydrogen peroxide to stearic acid.
Figure 5. EDS mapping of elemental distribution: (a) Core image of SAF (0.22 g/cc); (b) Na (sodium); (c) Al (aluminum); (d) Si (silicon); (e) C (carbon); (f) O (oxygen). The figures show homogenous distribution of elements of interest throughout the foam under observation.
(a)
(b)
(c)
Figure 6. High resolution (a) SEM image, and (b) Optical micrograph of SAF foam (density = 0.22 g/cc in reflection) and (c) Optical micrograph of SAF foam (density 0.22 g/cc in transmission) that clearly shows the closed cell foam structure.
Pore size distribution for control sample (~1.6 g/cc) Population of pores (%)
35 29
30 25
21
20
16 13
15 10
7 4
5 0
0
1
1
0
Pore diameter (µm)
0
1
3
1
0
Figure 7. Histograms showing pore size distribution (% of pores of particular diameter range) for 0.22 g/cc foam (top) and control sample ~1.6 g/cc (bottom).
Figure 8. Thermal conductivity ( ) variation with density. The blue data point is extrapolated value for a 0.22 g/cc foam as obtained from the power law relationship.
Sound velocity (Vs m/s) vs density (ρ in gcm-3) 3000
2800
Vs (m/s)
2600
2400
2200
2000
1800 0.4
0.6
0.8
1
1.2
ρ (gcm-3)
Figure 9. The variation of speed of sound (Vs) with increasing foam density.
1.4
Figure 10. XRD spectra of SiO2-Al2O3 physical mix (in red) and control sample with indexed peaks (green). Also included is the inset showing amorphous bump for control sample representing formation of gel phase.
Figure 11. XRD spectra of SiO2-Al2O3 physical mix (in red) and SAF 0.22 g/cc (in green).
(a)
(b)
(c) Figure 12. 29Si (a) and 27Al (b) and 23Na (c) NMR spectra of different samples including the starting materials.
Fig ure 13. TG A plots of control and SAF’s (3 different densities) showing possible volatiles that were discharged with increase in temperature.
Declaration of interests ☒ 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. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0272-8842(20)30043-2
DOI:
https://doi.org/10.1016/j.ceramint.2020.01.042
Reference:
CERI 23974
To appear in:
Ceramics International
Received Date: 28 August 2019 Revised Date:
4 January 2020
Accepted Date: 6 January 2020
Please cite this article as: P.R. Rao, K. Muralidharan, M. Momayez, K.A. Runge, D.A. Loy, Direct foaming driven synthesis and thermophysical characterization of silica-alumina foams: Applications for thermal insulation, Ceramics International (2020), doi: https://doi.org/10.1016/j.ceramint.2020.01.042. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.
Direct foaming driven synthesis and thermophysical characterization of silica-alumina foams: Applications for thermal insulation P. R. Rao*, K. Muralidharan, M. Momayez, Keith A. Runge, D. A. Loy Corresponding author details Pratish R Rao 1235 James E Rogers Way, Mines building. Materials Science and Engineering, Mining and Geological Engineering University of Arizona, Tucson USA Email: [email protected]
ABSTRACT
Using earth-abundant materials such as quartz and corundum powders as precursors, we have developed a simple, but effective low energy direct foaming based method for synthesis of thermally insulating porous foams with thermal conductivity as low as 0.08 W/mK. Specifically, the adopted synthesis procedure provides a straightforward avenue for obtaining closed-cell silicaalumina foams, with densities ranging from 1.7 to 0.22 g/cc. The ability to tune the densities is achieved by controlled addition of blowing agents (H2O2) and surfactants (stearic acid) respectively. The thermal conductivity of these foams decreased monotonically with increasing porosity (or decreasing densities), and the density-thermal conductivity relationship is governed by a power law. In addition, morphological and structural characterization was performed via optical and electron microscopy, x-ray diffraction, and nuclear magnetic resonance, and the results from these studies are also discussed in the context of structural evolution of the foams as a function of
porosity. While this work represents a straightforward and simple path towards obtaining silicaalumina foams with very low density, the developed method is directly applicable to obtaining a wider variety of multicomponent ceramic foams.
KEYWORDS ceramic foams, stearic acid, hydrogen peroxide, low density, thermal conductivity, thermal insulation
INTRODUCTION Ceramic foams are systems of great interest, finding applications as catalysts, filtration membranes, bone scaffold materials, as well as thermal insulators [1–6]. While pores in ceramic matrices can result in premature mechanical failure, leading to a reduction in the structural performance of the foams, the ability to tune and control pore sizes as well as their distributions can enhance their functionalities as light-weight material-systems with controllable gas permeability, low dielectric constant, and high thermal and acoustic insulation properties. There have been numerous studies that have examined the optimization of the synthesis and processing process of ceramic foams [1,3,7–11]:. There are four primary techniques involved in the synthesis of porous ceramics i) partial sintering; ii) sacrificial fugitives; iii) replica templates; and iv) direct foaming. In partial sintering, ceramic powders are compacted and heat treated to achieve desire porosity that is primarily determined by the size and morphology of starting powders [12]. In addition, sintering conditions such as temperature and pressure influence the powder-binding as well as the ensuing microstructure and mechanical properties [13]. Sacrificial fugitives such as coal, liquid paraffin, graphite, glass particles and organic fibers have been introduced in ceramic suspensions as pore forming agents. These fillers have to be removed by pyrolysis leading to underlying porosity [14,15]. Replica templates, such as polyurethane sponge, have been introduced in ceramic suspensions to create foams. In particular, the template is burned off at the final stages of setting thus introducing interconnected porosity in the foams. Compared to partial sintering, replica templating, and sacrificial fugitive methods, direct foaming represents the most straight-forward method of obtaining ceramic foams. Direct
foaming avoids complicated processing steps involving high temperature use during sintering and pyrolysis as well as template removal and extraction from the final ceramic structures. In direct foaming, the thermodynamic stability and kinetics of bubble formation determines the underlying foam microstructure. In particular, the drainage, coalescence and disproportionation related to bubble evolution controls porosity, pore size distribution as well as bubble dynamics [2,16–18]. Numerous research groups have worked on the development of porous ceramics by the direct foaming method [19–22]. Blowing agents such as H2O2 (HP), air, CO2, NH3, etc. as well as mechanical agitation can be used to produce bubbles in the ceramic slurry. Of particular significance is HP, which is an inexpensive compound, and serves as an efficient blowing agent as it chemically decomposes evolving oxygen gas and water [23] leading to void formation. In order to control and stabilize these voids, surfactants are also incorporated while foaming. A noteworthy example is stearic acid, which is an 18 carbon saturated fatty acid and a costeffective surfactant that is suitable for tuning porosity [24,25]. In this regard, we present a simple, low-cost, low energy footprint, direct foaming driven method for the synthesis of silica-alumina foams (SAF) derived from silica and alumina precursors, which are earth-abundant, and inexpensive. The developed process includes creating a slurry of Al2O3 and SiO2 powders, which is simply alkali activated by the addition of an aqueous NaOH solution and further, by incorporating appropriate amounts of stearic acid and HP, we demonstrate the ability to control the porosity and the resultant density of the foams. Further, as pointed out earlier, the adopted method in this work which is based on direct blowing, avoids complicated post-processing steps after curing, such as removal of any sacrificial agents or templates. In addition, mechanical agitation and the use of high temperature sintering have
also been avoided. Thus, this technique will be most beneficial and cost-effective for bulk manufacture of these foams. To provide further context for this study, we provide a brief summary of past investigations that have specifically focused on using direct foaming of aluminosilicate foams for thermal applications. Bai et.al. developed foams by direct foaming based on Metakaolin (MK) as raw material which was alkali activated by 11 M KOH[20]. They used HP as a blowing agent to create pores and different oils (vegetable, sunflower, canola, olive) as pore stabilizing agents. They observed that their foams showed interconnected pores with pore sizes ranging from 150 to 400 µm in addition to confirming that stabilizing agents lowered pore sizes as well as maintained homogenity. They reported foams to have a density as low as 0.37 g/cc with a k value of 0.11 W/mK which could potentially be used as fireproof insulation. In another study, Medri et.al. incorporated HP as blowing agent and expanded vermiculite as filler into a ultrafine MK based matrix activated by polysilicate solution[19]. They reported a range of pore sizes ranging from < 1 to 100 µm from Hg intrusion porosimetry and obtained foams with 0.7 g/cc possessing a k of 0.2 W/mK. In another study by Novais et.al. MK with porous FA biomass served as aluminosilicate source and foams were produced by HP aiding as blowing agent. 12 M NaOH was used as an alkaline activator to produce a slurry[21]. They observed an increase in pore area ratio with the solid material with increasing HP content from 4.3 % (at 0.03 wt. % HP) to around 40 % (at 1.2 wt.%). These foams were reported to have density as low as 0.56 g/cc and k of 0.1 W/mK. A. Hajimohmmadi et.al. made MK based foams with HP[26]. They reported pore homogeneity to be a function of SS concentrations. The group was able to obtain foams of 0.55 g/cc corresponding to a k value of 0.16 W/mK.
As compared to previous relevant studies as discussed above, a distinguishing feature of this work is the successful implementation of direct foaming to obtain much lower density closed cell SAF (~0.22 g/cc). Equally importantly, our starting feedstocks were pure alumina and silica powders, while previous studies relied on processed aluminosilicate precursors. Further, in this work we also provide a detailed analysis of the direct foaming process. By carefully tailoring the chemistry of the direct foaming process, a noticeable chemical evolution in terms of change of cation coordination, which underlay the ability of the foams to retain their mechanical and structural integrity, was demonstrated. Equally importantly, the low-density SAF structures demonstrated excellent thermal insulation properties (much lower conductivity than previous studies), making them highly suitable as building blocks for construction and mining applications. Another salient feature of this work was the characterization of the structurethermophysical property relations of the foams using a combination of nuclear magnetic resonance (NMR), optical and scanning electron microscopy (SEM), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), in conjunction with transient plane source methods (for measuring thermal conductivity). Fundamental insights into the thermal conductivity dependence on foam porosity were thus developed. Details regarding the synthesis and characterization of the developed foams are given in the following sections. A brief overview of the synthesis and characterization methods are provided in the Experimental Methods section, with a more detailed discussion provided in the Supplementary Information document. The ensuing structural, chemical, and thermophysical characterization of the foams with a special emphasis on thermal conductivity trends are then presented in the Results and Discussion section.
Experimental Methods The details of sample preparation and processing is discussed in the supplementary information. The block diagram (Fig.1), provides a visual aid for the stepwise synthesis of control and SAF systems. As shown in Fig. 1, the blowing agent and the surfactants are added prior to curing. There was no decomposition of HP observed before the slurry was introduced into the oven. Further it is also useful to note that the use of stearic acid (used extensively in skin care products) as a surfactant is deemed safe by US FDA. Moreover, the overall content of SA in the SA foam is no more than 0.2 % (by mol.). All samples were made from NaOH (10M) activation of pristine quartz and corundum powders. The high pH was mainly responsible for nucleoliphlic substitution leading to the formation of hydroxy species on both Si and Al sites. Furthermore, Na+ ions serve as charge balancing cations to compensate for the residual charge deficiency on the aluminum ion. Preliminary analysis showed that a Si:Al ratio of 3:1 was the optimal ratio that enabled further processing of the starting materials into foams. Further discussion is provided in the Supplementary Information document. The slurry was first homogenized by whisking and further by sonication for at least 6 min (for controls) and 8 min (for foams). To obtain foams, additional steps were introduced such as the addition of 3 wt.% HP (stock solution) and 20 wt.% stearic acid (stock solution) incorporated at different concentrations to obtain SAF structures of different densities. Note that for comparison, control samples were also prepared, for which neither the surfactant nor the blowing agent was incorporated. The exact molar composition for the control and SAF (0.22 g/cc) is provided in Table 1. The respective slurries were then poured onto a Teflon dish and cured in a convection oven at 100 oC for 72 hours (for control) and 12 hours (for SAF’s). A total of 12 different SAF
structures (including control) were prepared, with each characterized by a unique density and composition. The choice of the curing temperature was based on an initial examination of the role of temperature on the resulting microstructures. In our studies, curing at temperatures much higher or lower than 100 oC, resulted in porous structures with inconsistent pore size distribution. This is attributed to an interplay between bubble generation due to decomposition of the blowing agent, bubble stability and the evaporation rate of water, which are all critically dependent on the curing temperature.
RESULTS AND DISCUSSION The synthesis and fabrication of SAF with controllable thermal insulation properties represents the primary objective of this work. As a first step towards this end, the ability to obtain on-demand SAF structures with tunable porosity is discussed and characterized in terms of their thermophysical properties. The steps enumerated in Fig. 1 were undertaken to obtain SAF structures of varying porosity. For comparison, control samples without the blowing agent and surfactant were also prepared. Images of representative samples are provided in Fig.2 and Fig.3. Interestingly, but not unexpectedly, for the chosen Si:Al ratio (=3), the resultant foam density (and pore distribution) critically depended on the H2O2 to stearic acid molar ratio (HP/SA) as shown in Fig. 4. Notably, lowest densities were achieved for higher HP/SA ratio leading to mechanically stable foams with density equaling up to 0.22 g/cc. However, higher HP/SA ratios than that reported in Fig. 4 resulted in loss of homogeneity of pores. By homogeneity, we mean the pore shape, uniformity or pore distribution though out the foam.
Chemical homogeneity was examined in terms of their EDS spectra, and as shown in Fig.5, the elemental constituents (Si, O, Al, Na) were uniformly distributed for all the SAF structures. Porosity of the foams was expressed in terms of the effective pore volume (∅), which was calculated from Equation 1, with the ideal matrix density (
) corresponding to a
silica-alumina matrix obtained from rules of mixtures ∅ % = 1 −
× 100 (1)
Based on Equation (1), it was seen that ∅ of the synthesized structures varied between 40% and 92% respectively. In particular, despite the lack of the blowing agent, the control samples also exhibited porosity ( ~40% characterized by mm sized large pores which is attributed to the coalescence of voids that are generated by water evaporation from the Al-Si slurry during curing at 100oC. Moreover, the control samples exhibited a highly inhomogeneous spatial distribution of pores in addition to a wide pore-size distribution as compared to the more porous samples, for which both HP and stearic acid were added. These observations point to the importance of the surfactant (stearic acid) in tandem with the blowing agent (HP) in the ability to control pore homogeneity and size. Specifically, the decomposition of HP in the basic Si-Al slurry leads to the evolution of O2 and H2O, which drives pore formation. Without stearic acid, the decomposition of HP leads to density reduction as compared to control samples, but nevertheless, demonstrates higher densities than corresponding samples with stearic acid. The addition of stearic acid, enables the spacing and even distribution of pores, in addition to preventing pore coalescence, which is reflected in the observed trends shown in Fig. 4. The ‘sweet-spot’ for obtaining low-density SAF occurs between a HP/SA ratio (by mol.) of 4 and 8; beyond the
upper limit, the relative lack of stearic acid leads once again to void coalescence and unequal and inhomogeneous spatial distribution of pores. SEM images and corresponding pore size distributions for the foams within the sweet spot is given in Fig. 6 and Fig.7 respectively. As seen from SEM and optical micrographs, typical pore-sizes ranged from 300-600
for these foams. It is evident that these pores are of closed cell type with thin
struts acting as walls for each cell. These struts, we hypothesize, are composed of SiO2 and Al2O3 particles as well as their respective aluminosilicate compounds formed by alkali activation. We refer the reader to the latter part of this section, where evidence (based on NMR and XRD) for the presence of both unreacted particles and reacted aluminosilicates are provided. Having discussed the ability to synthesize SAF structures and control the porosity in a systematic fashion, we now turn our attention to characterizing the thermal conductivity of the pores and developing insights into the role of porosity on the ensuing thermophysical characteristics. The variation in thermal conductivity as a function of density is given in Fig. 8. As evident from the figure, there is a monotonic decrease in thermal conductivity as a function of decreasing density, with the interdependence of thermal conductivity and density following a power law fit. Note that due to practical difficulties in terms of ensuring perfect contact between the sensors (of the thermal conductivity measurement unit) and the samples corresponding to the highest porosity/lowest-density foams (~0.22 g/cc), we restricted our measurements to SAF with densities greater than equal to 0.41 g/cc. Nevertheless and importantly, it is clear that adopted synthesis procedure provides a straightforward pathway to obtain highly thermally insulating ceramic foams ( ~ 0.08 #/ % .
To further characterize the observed trends linking the thermal conductivity and underlying porosity, as well as develop fundamental insights into this interplay, we examined the variations in heat capacity, in addition to acoustic and mechanical properties of the foams as a function of density. Specifically, we focused on obtaining the volumetric heat capacity (&'( ) and the longitudinal speed of sound ()* ), of the respective foams. Measuring &'( , and )* in conjunction with the thermal conductivity (
, provides the ability
to obtain an effective phonon mean free path (,) via Eqn.2, and thereby shed light on the role of the pores as scatterers of thermal phonons. In addition, other important properties such as the elastic modulus (i.e. Young’s modulus) can be directly calculated from )* (Eqn. 3), enabling important characterization of the interplay between density and ensuing mechanical modulus. -
= &'( )/ , .
)/ = 0
1
(2)
(3)
Using the differential scanning calorimetry (DSC) technique, the specific heat at constant pressure (&' was measured for the different SAF structures at 300 K. The measured &' of all foams were similar and calculated to be in the range of ~ 770 (±5) 2/%3℃, while &'( , which is calculated from &' and the sample density, was found to decrease monotonically with increasing porosity. The observed invariance in &' as a function of density indicates that chemically and structurally the different SAF structures are in fact similar; more attention will be paid to the evolution in structural and chemical characteristics in the latter part of this manuscript. Variations in &' , &'( are given in Table 2.
As a next step, ultrasonic studies were undertaken to examine )/ dependence on density. Figure 9 presents the variation in sound velocity as a function of density. The observed trends are consistent with available literature on foams [27,28] in terms of a consequent reduction in velocity with increased porosity (reduced density). Interestingly, the calculated Young’s modulus (using Eqn. 3) demonstrates a power law behavior similar to the thermal conductivity trend, and of note is the fact that this trend correlates well with a recent computational investigation on the variation in modulus as a function of density/porosity for closed cell foams [29,30]. In comparison to polymer foams of similar densities such as rigid PU foams ranging between 115 to 794 MPa (density range 0.24 to 0.641 g/cc)[31], the SAF’s demonstrate superior mechanical stability for identical density ranges. Based on the obtained values of &'( and )* , the effective phonon mean free path (, variations were also estimated; it was seen that , was of the order of 1nm for all the SAF structures, which is characteristic of thermal insulators[32,33]. Unlike &'( and )* which decreased with density, there was no apparent variation in , with porosity, pointing to the stronger dependence of
on &'( and
)* . From a structure, composition, and chemical bonding point of view, we carried out XRD, NMR, and TGA characterization respectively, to shed further light into the intimate structurecomposition-processing-property relations of the SAF structures. To ensure clarity, we focus on the either ends of the ‘density-spectrum’ of the SAF structures (i.e. the control sample and the lowest-density foam) to develop valuable insights and highlight essential features, while more information is provided in the Supplementary Information. Figure 10 provides the XRD patterns of the control sample and the lowest density foam. For reference a physically mixed quartz (SiO2) and corundum (Al2O3) sample’s XRD pattern is also
provided and the respective (SiO2, Al2O3) peaks are indexed. From a comparison of the different patterns, it is obvious that both SAF structures (control and low-density foam) demonstrate striking similarities to the physical mixture; however, as shown in the inset of Fig. 10, the control sample exhibits a notable bump as compared to the physical-mixture, which is characteristic of an additional amorphous aluminosilicate gel phase [34,35] that is formed due to the hydration and alkali activation steps involved in the control sample preparation. On an equally relevant note, the XRD pattern of the low-density SAF structure, in addition to the unreacted SiO2 and Al2O3 peaks, demonstrates new features as indicated in Fig 11, which are characteristic of a crystalline sodium aluminate-silicate phase[36]. Thus based on the XRD patterns, it can be inferred that the SAF structures primarily consist of unreacted SiO2, and Al2O3 particles, with a secondary binder phase that is either amorphous (as in control) or crystalline ( as in lower-density foams), depending on the foaming process. Deducing the fundamental mechanisms that underlie the formation of the crystalline phase seen in the the low-density foams is beyond the scope this paper; however, using the Medusa-Hydra chemical equilibrium software[37], an initial assessment of speciation based on the high pH and the relative concentrations of HP, NaOH, Al2O3 and SiO2 shows the stability of the precipitating crystalline NaAlSiO4 phase. 29
The Si,
27
23
Al, and
Na NMR signals of the control sample and the low-density foam are 29
given in Fig. 12. For reference the baseline
27
Si and
Al NMR spectra of the respective
precursors are also provided. Interestingly, there are distinct chemical shifts for the control and the foams. In particular, in addition to the central peak at ~18 ppm corresponding to Al octahedral coordination, the
27
Al NMR spectra also demonstrates a secondary peak
corresponding to tetrahedrally coordinated Al in both the control and the foam samples, which correlates well with the observations of the secondary binder phase in the respective XRD patterns. In the case of the
29
Si spectra, the chemical shifts are very distinct for the
low-density foam versus the control. For the control sample, we see a relative reduction in the Q4 peak and the emergence of another peak, which when deconvoluted points to the presence of a mixture of Q3, Q2, and Q1 type coordination. A comparison of the relative peak-heights suggests that pristine silica feedstock in the control sample was more susceptible to chemical activation and conversion to the secondary aluminosilicate phase. On the other hand, for the low-density foam, there is a bimodal distribution corresponding to Q4 and Q1/Q0 type coordinations. The relatively larger Q1 peak in the low-density foam can be correlated to an increase in the silica network disruption due to silicon atoms that are bonded to multiple non-bridging oxygens, which is also reflected in the decrease in the Young’s modulus. For the
23
Na spectra too, there are distinct differences between the control and the low-
density foam, with a strong secondary peak at around 21 ppm for the foam. This is attributed probably to the formation of Na2O2, due to the reaction of HP (blowing agent) with Na ions in solution. While more work is needed to further interpret the NMR data in terms of the various combinations of coordinations that occur in the SAF structures, it is reasonable to assume that the chemical activation and the introduction of the blowing agent leads to key differences in the chemical environments and structural coordination of the constituent Si, Al, and Na atoms.
TGA analysis, as shown in Fig.13, indicates the volatiles present in the samples. The control sample retained most solvent (~ 8 % water) as seen from the weight loss in the TGA plot. No other solvents were used in the synthesis of control samples. The relative lower amounts of retained water in the case of foams is directly attributed to the presence of surfactants (i.e. stearic acid). Here the surfactant hydrophobic tails hinder the amount of water adsorption. For the higher densities (SAF 1.2 g/cc), the volatiles were mostly water as the amounts of stearic acid was relatively lower. As density reduced (SAF 0.25 g/cc), transitional weight losses were more evident due to remaining volatiles like ethanol and stearic acid being removed from the sample. These losses are evident from intermediate drops at 80 oC and 260 oC corresponding to the boiling points and vaporization temperatures of EtOH and stearic acid respectively. All SAF samples showed a weight loss of approximately 6 % on reaching a final temperature of 900 oC except control which had ~ 8 wt. % water retained within it.
CONCLUSIONS A simple and straightforward direct foaming process has been implemented to obtain closed-cell ceramic foams (up to 0.22 g/cc) that are derived from earth abundant materials such as silica and alumina. The synthesis process that consists of an alkali activation step followed by a low-energy thermal curing step, led to development of highly thermally insulating foams with thermal conductivity as low as 0.08 W/mK. Underlying the synthesis is the interplay between the blowing agent (HP
and surfactant (stearic acid), which
provides the ability to control the density and porosity of the ensuing foams. Moreover, the simplicity of this synthesis technique by direct foaming through in-situ chemical decomposition of HP as well as bubble stabilization by SA, facilitates large scale production
of the foams eliminating post processing steps of sintering as well as pyrolysis. With regards to structure, these foams primarily consist of unreacted silica and alumina particles, and a secondary aluminosilicate binder phase; in the absence of HP, the binder phase is shown to be amorphous, while with increasing HP content in conjunction with stearic acid, the binder phase demonstrates crystalline features. Ultrasonic testing was used for characterizing acoustic properties as well as obtaining moduli of the respective foams, while thermal conductivity was measured using a transient plane source technique. Interestingly, both thermal conductivity and elastic modulus followed a power law as a function of foamdensity. Specific heat of the foams was also measured using the DSC technique, and based on these observations, it was seen that both volumetric heat capacity as well as the speed of sound contributed to the underlying thermal conductivity trends. In the future, the applicability of the developed process to a wider variety of multicomponent ceramic foams will be examined. Further, a more elaborate characterization of the evolution of the chemical bonding and structural characteristics of the foams as a function of the process parameters will be carried out.
ACKNOWLEDGEMENTS We would like to express our sincere gratitude to NIOSH for their continued financial support. The authors also like to thank Dr. Brooke Beam at KECK facility at UA for SEM imaging, Dr. Andrei Eskatchine for XRD analysis and D r . J i x u n D a i the Chemistry department at UA for solid state NMR analysis. Additionally, we also like to thank David Streeter and Dr. Sahila Peranathan for their kind assistance.
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Table 1. Detailed composition of different chemicals in control and SAF (0.22 g/cc). NaOH SiO2
HP
SA
3 wt.%
(20 wt./v%)
Al2O3
(10M) Mol.wt.
40
60
102
34
284
Vol. (or mass)
17 mL
30 g
8.5 g
0.36 g
0.4 g
moles (for control)
0.17
0.5
0.0083
0
0
moles (for SAF- 0.22g/cc)
0.17
0.5
0.0083
0.01
0.0015
Table 2. Volumetric heat capacities and sound velocity for SAF of varying densities. Density g/cm3
Density kg/m3
(MJ/m3C))
(m/s)
0.25
250
0.193
1700
0.4
400
0.3096
2100
1.2
1200
0.9204
3000
Vs
LIST of FIGURES
Figure 1. Block representation of steps involved in preparation of control samples and SAF structures.
Figure 2. Cross-sectional view of SAF structures: (a) control (~ 1.7 g/cc), (b) foam of density = 1 g/cc
Figure 3. Cross-sectional view of SAF sample (0.22 g/cc)
Figure 4. The variation of density of SAF structures with differing ratios of hydrogen peroxide to stearic acid.
Figure 5. EDS mapping of elemental distribution: (a) Core image of SAF (0.22 g/cc); (b) Na (sodium); (c) Al (aluminum); (d) Si (silicon); (e) C (carbon); (f) O (oxygen). The figures show homogenous distribution of elements of interest throughout the foam under observation.
(a)
(b)
(c)
Figure 6. High resolution (a) SEM image, and (b) Optical micrograph of SAF foam (density = 0.22 g/cc in reflection) and (c) Optical micrograph of SAF foam (density 0.22 g/cc in transmission) that clearly shows the closed cell foam structure.
Pore size distribution for control sample (~1.6 g/cc) Population of pores (%)
35 29
30 25
21
20
16 13
15 10
7 4
5 0
0
1
1
0
Pore diameter (µm)
0
1
3
1
0
Figure 7. Histograms showing pore size distribution (% of pores of particular diameter range) for 0.22 g/cc foam (top) and control sample ~1.6 g/cc (bottom).
Figure 8. Thermal conductivity ( ) variation with density. The blue data point is extrapolated value for a 0.22 g/cc foam as obtained from the power law relationship.
Sound velocity (Vs m/s) vs density (ρ in gcm-3) 3000
2800
Vs (m/s)
2600
2400
2200
2000
1800 0.4
0.6
0.8
1
1.2
ρ (gcm-3)
Figure 9. The variation of speed of sound (Vs) with increasing foam density.
1.4
Figure 10. XRD spectra of SiO2-Al2O3 physical mix (in red) and control sample with indexed peaks (green). Also included is the inset showing amorphous bump for control sample representing formation of gel phase.
Figure 11. XRD spectra of SiO2-Al2O3 physical mix (in red) and SAF 0.22 g/cc (in green).
(a)
(b)
(c) Figure 12. 29Si (a) and 27Al (b) and 23Na (c) NMR spectra of different samples including the starting materials.
Fig ure 13. TG A plots of control and SAF’s (3 different densities) showing possible volatiles that were discharged with increase in temperature.
Declaration of interests ☒ 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. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: