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Influence of the glass particle size on the foaming process and physical characteristics of foam glasses
Journal of Non-Crystalline Solids 447 (2016) 190–197
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Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Influence of the glass particle size on the foaming process and physical characteristics of foam glasses Jakob König a,b, Rasmus R. Petersen a, Yuanzheng Yue a,⁎ a b
Department of Chemistry and Bioscience, Aalborg University, DK-9220 Aalborg East, Denmark Advanced Materials Department, Jožef Stefan Institute, SI-1000 Ljubljana, Slovenia
a r t i c l e
i n f o
Article history: Received 23 February 2016 Received in revised form 3 May 2016 Accepted 4 May 2016 Available online 14 June 2016 Keywords: Foam glass Manganese oxide Thermal conductivity Porosity Particle size
a b s t r a c t We have prepared low-density foam glasses from cathode-ray-tube panel glass using carbon and MnO2 as the foaming agents. The effect of the glass particle size on the foaming process, the apparent density and the pore morphology is revealed. The results show that the foaming is mainly caused by the reduction of manganese. Foam glasses with a density of b150 kg m−3 are obtained when the particle size is ≤33 μm (D50). The foams have a homogeneous pore distribution and a major fraction of the pores are smaller than 0.5 mm. Only when using the smallest particles (13 μm) does the pore size increase to 1–3 mm due to a faster coalescence process. However, by quenching the sample from the foaming to the annealing temperature the pore size is reduced by a factor of 5–10. The foams with an apparent density of b200 kg m−3 are predominantly open-porous. The foams exhibit a thermal conductivity as low as 38.1 mW m−1 K−1 at a density of 116 kg m−3. For the investigated foam glasses, there exists a great potential to further decrease their thermal conductivity by increasing the closed porosity and by changing the trapped gas to CO2. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Foam glass is a highly porous (N 85 vol.% pores), lightweight material used for thermal and acoustic insulation [1,2]. Depending on the application, e.g., heat insulation or soundproofing, the foam glass has predominantly closed or open porosity, respectively. Foam glass exhibits several advantages over conventional insulating materials such as organic foams and mineral wool, e.g., water and steam resistance, freeze-thaw tolerance, excellent chemical and thermal stability, high surface area and permeability, and superior mechanical properties. Due to the chemical stability and the high degree of closed porosity, the lifespan of foam glass (N100 years [3]) is much longer than conventional thermal insulation materials. Foam glass is produced from powdered glass admixed with a foaming agent that releases gases at elevated temperatures. In the softened glass, the expanding gas bubbles increase the volume of the sample, thus forming a porous lightweight product. In industrial foam-glass production the majority of the glass originates from waste glass [1]. Some production facilities only make use of waste glass, thus eliminating the energyconsuming melting step. Foam-glass production makes it possible to recycle a great diversity of glassware or other amorphous materials into a high-value-added product. The waste glasses most commonly employed are mixed-color bottle glass [4–7] and window glass [8–12], but other glasses, like lamp glass [13], glasses from cathode ray tubes ⁎ Corresponding author. E-mail address: [email protected] (Y.Z. Yue).
http://dx.doi.org/10.1016/j.jnoncrysol.2016.05.021 0022-3093/© 2016 Elsevier B.V. All rights reserved.
(CRTs) [14], fly ashes from coal production [15] and vitrified solid wastes from municipal waste combustion [16] can also be used. The foaming agent, added as a minor part to the powdered glass, releases gases based on a decomposition reaction or a redox reaction with the glass. The solid residues of the foaming agent or its reaction with the glass have an impact on the glass network and foam morphology. Therefore, foaming agents must be selected according to the composition of the glass. Different foaming agents and/or their reactions with the glass lead to a wide variety of gases being found in the pores. In general, CO2 is the best gas to be trapped in the pores, since it is easily obtainable, has a low thermal conductivity and a low toxicity. Gases like H2O, O2 and N2 are also acceptable, but they have a higher thermal conductivity than CO2. More problematic is the presence of CO, H2 or SO3 due to their high toxicity, flammability and/or reactivity. CO2 gas is readily available from the decomposition of carbonates [8,14,17] and carbon-based foaming agents, like pure carbon materials, carbohydrates and SiC [1, 18–20]. The latter are used by the foam-glass industry [1]. When carbon is used as a foaming agent, the gas released at elevated temperatures is a CO/CO2 mixture, where the ratio depends on the availability of oxygen. During cooling the mixture then transforms to CO2, according to Boudouard's equilibrium. When applying carbohydrates or SiC as a foaming agent, the gas-releasing mechanism becomes more complex [18,21]. For this reason, pure carbon is preferred over carbohydrates and SiC. The foaming starts after the glass has sintered to a state with closed pores, where the volume of the entrapped gases is too small for a successful foaming. Therefore, a sufficient amount of free oxygen has
J. König et al. / Journal of Non-Crystalline Solids 447 (2016) 190–197
to be available from the glass melt. In order to promote the gas-releasing reaction, i.e., the oxidation of carbon, an oxidizing agent can be added, which acts as an oxidation/reduction couple with the carbonaceous foaming agent [10,11,22]. The most suitable oxidizing agents are Fe2O3 [10,16], MnO2 [10,11,18] and sulfates [16,18,19]. However, the solid residues of the foaming agent can act as a nucleation and crystallization agent. This means that the impact of the foaming agent on the glass stability has to be considered. Despite the fact that foam-glass technology has been known since the 1930s, the number of academic publications has only started to increase in the past 15 years. The main motivation for the research seems to be environmental consciousness and the increasing price of landfilling waste. The research on foam glass mainly concentrates on its mechanical properties, only few papers report on the thermal conductivity [9,14,15,21,23–26]. However, a worldwide focus on energy efficiency has put insulating properties higher on the agenda. Therefore, attention has been drawn to modified conventional and contemporary materials with improved insulating properties [27–29]. In order to promote the use of foam glass in thermal insulation, its insulating properties need to be enhanced. The thermal insulation properties of a foam glass can be improved by (i) decreasing the density, (ii) closing the porosity, (iii) entrapping a low conducting gas, and (iv) employing glasses with a low thermal conductivity. To prepare a low-density foam glass (b 150 kg m−3), optimum conditions have to be found, i.e., a foaming agent compatible with the composition of the glass, an optimum concentration and the right heat-treatment conditions. Finely milled powder and homogenous mixtures result in small pores [6,30]. If the pores are closed, then convective heat transfer is prevented. Moreover, the viscosity as well as the surface tension should be in a defined range at the foaming temperature to support low density and small, closed pores [1,19]. When the pores are small and closed, the overall thermal conductivity of the foam glass can be estimated by summing the contributions from the conduction through the solid and the gaseous phases [31,32]. Based on an assumption of linear contributions [31,32] the difference in the thermal conductivity between a foam glass filled with O2 (24 mW m− 1 K−1) and a foam glass filled with CO2 (15 mW m− 1 K− 1) [33] would be 8.6 mW m− 1 K− 1 for a foam glass with 95% porosity. The effect of the particle size on the foam density and homogeneity is, despite being well known, scarcely reported [1,6,25,30,34]. It is common to report the particle size of the glass and the foaming agent before they are mixed. However, since the mixing is frequently carried out using milling equipment, the particle size changes, obscuring any clear relation between the particle size and the foam density. It was shown that when foaming with carbon the density of the foam glass decreases rapidly with a glass particle size below 100 μm [1,30]. On the other hand, the relationship between the particle size and the foam density is not so straightforward when using carbonates as the foaming agent [25]. The aim of the present work is to study the influence of the particle size on the foaming process and to prepare foam glasses with a low thermal conductivity. We foamed waste CRT panel glass with carbon and MnO2 as the foaming agents. CRT panel glass has a high glass stability [25,35,36]. To reveal the influence of the particle size on the foaming process, pre-milled powder sieved to different fractions was mixed with the foaming agents and the mixture was homogenized without changing the particle size. In addition, the effect of the heat-treatment conditions on the characteristics of the foam and its stability are revealed and discussed. We characterized the apparent density, the closed porosity, the compressive strength and the thermal conductivity of the foam glasses. 2. Experimental CTR panel glass was crushed in a jaw crusher (Retch BB51, Haan, Germany) and subsequently ball milled in a planetary ball mill (Retsch
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PM100). The powders were then sieved to obtain five powder fractions with different particle sizes using laboratory sieves (38–250 μm). The chemical composition of the CRT panel glass is reported elsewhere [25]. Carbon (activated charcoal; Bie & Berntsen, Søborg, Denmark) and MnO2 (98%, Bie & Berntsen) were ball milled to pulverize the agglomerates and larger particles. The foaming mixtures were prepared from panel glass, 0.93 wt.% carbon and 6.76 wt.% MnO2, according to previously determined optimum concentrations [37]. The carbon/ MnO2 ratio was equal to one-half of the stoichiometric amount needed for the theoretical carbon oxidation reaction: C þ 2MnO2 →CO2 þ 2MnO
ðReaction 1Þ
The powder mixtures were homogenized on a carousel in a glass bottle with four light plastic balls for 17 h at 30 rpm. The particle size distribution of the powder mixtures was measured with a LS 13320 laser granulometer (Beckman Coulter, Brea, US-CA). The D50 and D90 values of the powder mixtures as well as the foaming agents are given in Table 1. The particle size distribution is shown in Fig. S1. Compared to the previous study [37], the powder processing in the present study was different, and thus the influence of the glass particle size on the foaming process could be revealed. The powder mixtures were uniaxially pressed into disk-shaped pellets with 40 MPa. Small pellets (1 g, Ø = 13 mm) were used to study the influence of the particle size and the foaming conditions on the density and homogeneity of the foams. Larger pellets (Ø = 35 mm) of selected powder mixtures were used to prepare large samples (23–45 g) in a stainless-steel cylinder (6 cm in diameter and 5 cm in height) to study the thermal conductivity and compressive strength. The pellets were placed in an electrical laboratory-chamber furnace and heated at different heating rates (5 or 10 °C/min) to 835 °C and heat-treated for different times (5 to 30 min). Subsequently, the samples were cooled at 4 °C/min to the glass-transition temperature (Tg) of 530 °C [35] and then slowly cooled at 1 °C/min to room temperature. The temperature in the furnace was calibrated using an external thermocouple. Thermogravimetic (TG) analyses were performed using a Jupiter 449 simultaneous thermal analysis (STA) instrument coupled with a 403C Aëoloss mass spectrometer (MS) (Netzch, Selb, Germany). The measurements were performed with a heating rate of 10 °C/min in an air atmosphere. The powder mixture was compressed with 40 MPa and a small piece (30–40 mg) was placed into an alumina crucible. The CO2 peak was integrated to calculate the amount of carbon burnt out at the defined temperature. The apparent density (ρapp ) of the small and the large foam samples was determined using Archimedes' principle in a 10 wt.% polyethylenglycol (PEG 3000 Da, Bie & Berntsen) solution or was calculated from the samples' mass and dimensions, respectively. A PEG solution was used to prevent absorption of the liquid medium during the measurement. The pycnometer density (ρ pyc ) of the foam samples was determined with a helium pycnometer (Ultrapyc 1200e, Quantachrome Instruments, Boynton Beach, US-FL). The powder density (ρpowder) of the crushed foamed-glass sample was measured with the pycnometer. The percentage of the closed porosity, out of the
Table 1 Particle size (D50 and D90) of the powder mixtures, carbon and MnO2. Sample
D50 [μm]
D90 [μm]
#1 #2 #3 #4 #5 carbon MnO2
13 33 53 103 196 31 5.4
32 80 92 163 283 54 29
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total porosity, was calculated from the apparent and pycnometer densities according to: ρapp
!
TP ¼
1
CP ¼
ρapp ρapp ρpyc ρpowder
CP ½% ¼
ρpowder: !
CP 100 TP
The compressive strength was measured on large cylinder-shaped samples using a table-top universal tester (Autograph AGS-J, Shimadzu, Kyoto, Japan) with a crosshead speed of 0.5 mm min− 1. Prior to measurements the top and bottom surfaces of the foam samples were coated with a 1-mm layer of a commercial mineral building glue. The thermal conductivity was measured from the top and bottom sides and the average value is reported in relation to the density of the sample. The thermal conductivity was measured with a surface probe (25 × 25 mm2) at 10 °C using a heat-transfer analyzer (ISOMET Model 2104 Heat Transfer Analyzer, Applied Precision, Bratislava, Slovakia). The accuracy of the equipment in the relevant range was tested by measuring commercial expanded-polystyrene and foam-glass samples with known values. The obtained results were in perfect agreement with the declared values. 3. Results and discussion The combination of a carbon-based foaming agent and an oxidizing agent is an effective way to produce low-density foam glass [10,11,18, 22]. In the present investigation we applied pure carbon and MnO2, which would provide CO2 gas according to Reaction 1. However, since the thermal treatment was performed in an air atmosphere, it is expected that part of the carbon will be oxidized before the pores between the softened glass particles are closed as a result of the sintering process. This was observed for the sample foamed with carbon in a previous investigation [37], according to which foaming occurred only when the particle size (D90) is lower than 27 μm. In the present investigation, coarser particles are used, with the lowest D90 value equal to 32 μm. In this respect, it is expected that all carbon will be burnt by the oxygen from surrounding atmosphere. The release of gases and weight loss were determined by performing a TG-MS analysis.
Fig. 1. Sample weight (W) and CO2 evolution (MS) from powder mixtures with different particle sizes (D50) during heating in an air atmosphere.
The analysis revealed a significant difference in the weight-loss (WL) curves between the finest (D50 = 13 μm) and the coarsest (D50 = 196 μm) powders (Fig. 1). Three steps in the WL can be distinguished in both samples, which were separated into the following temperature ranges: (I) 200–564 °C, (II) 564–730 °C and (III) above 730 °C. In step I, the WL of the sample from the fine powder is larger than that of the sample from the coarse powder (1.05 vs. 0.85 wt.%) and the CO2 peak appears at a higher temperature (527 vs. 470 °C). No O2 peak was detected, which suggests that the oxygen released by the decomposition of MnO2 reacts with the carbon. During step I partial burning of the carbon and partial reduction of the MnO2 occurs. At the end of step I, 14 and 5.8 wt.% of unburned carbon remains in the fine and coarse powder samples, respectively. The larger WL and the more unburned carbon in the fine powder sample indicate that more carbon is burnt out by the oxygen originating from MnO2 reduction than in the sample of coarse powders. In the coarse powder sample more carbon can be burnt by the atmospheric oxygen because the green density is lower, and hence more atmospheric oxygen is present between the particles, enabling a higher gas flow in and out of the sample. Thus oxidation of the carbon by the atmosphere occurs at lower temperatures (CO2 peak at 470 °C). On the other hand, less carbon is burnt by the atmospheric oxygen in the fine powder sample, but it eventually burns when the oxygen is released by the decomposition of MnO2 as the temperature increases, with the CO2 peak at 527 °C. The carbon that is not burnt by the atmosphere shifts the reduction of MnO2 to lower temperatures, and thus the overall WL in the fine powder sample in step I is higher, despite more unburnt carbon staying in the sample at 564 °C. In step II (564–730 °C), the rest of the carbon is burnt out and the MnO2 decomposes to Mn2O3 [26,38]. At around 690 °C the sample densifies and the pores close [25,26]. This is seen as a halt in the WL. The closing of the pores leads to an increase in the oxygen partial pressure in the sample, which inhibits further reduction of the MnO2. Therefore, not all of the MnO2 is decomposed to Mn2O3 at the end of step II. A complete reduction of MnO2 to Mn2O3 and complete burning of the carbon would result in a WL of 1.55 wt.%; however, only 1.33–1.37 wt.% is observed. The same mechanism appears in both samples; however, the WL in step II for the coarse powder sample is larger as less MnO2 was decomposed in step I. In step III (N 730 °C) the WL is caused by a further reduction of manganese. The reduction in the fine powder sample continues when the temperature reaches 790 °C and the WL is accelerated above 820 °C. The total WL at 900 °C (1.74 wt.%) indicates that almost all the manganese is in the form of Mn3O4 (theoretical WL is 1.76 wt.%). When pure MnO2 is heated in air the decomposition from Mn2O3 to Mn3O4 begins at 900 °C [26,38]. The presence of a glass melt favors a higher ratio of Mn2+/Mn3+ and shifts the reduction to lower temperatures. This is accordance with our previous study [26]. We suggested that Mn2O3 first dissolves in the glass and then Mn3+ reduces to Mn2+. In the coarse powder sample there is a small, gradual WL from 730 to 860 °C, which then accelerates above 910 °C. Such behavior can be explained if the agglomeration of small MnO2 particles (5.4 μm, D50) between large glass particles (196 μm, D50) in the mixture is considered. Thereafter, a much smaller contact surface between the agglomerated manganese oxide and the glass is available and the dissolution is slower. Consequently, the decomposition of Mn2O3 to Mn3O4 is observed at the same temperature as in pure manganese oxide [26,38]. Some of the agglomerates located on, or close to, the surface of the pellet gradually decompose (gradual WL in the range 730–860 °C), while the reaction in the interior is decelerated due to the high oxygen partial pressure. The TG-MS results show that the carbon is burnt out before the pores close (b 690 °C). Thus the foaming is caused by the oxygen gas released with the reduction of manganese. A great part of the oxygen released by the decomposition of MnO2 to Mn2O3 is lost to the atmosphere as the reaction occurs before the sample is sintered to closed pores [26]. To
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Fig. 2. Influence of the particle size (D50) on the apparent density (ρapp) of the foam glasses prepared at 835 °C with different heating rates and foaming times (see legend). The inset magnifies the density data for smaller particle sizes.
increase the CO2 content in the foam glass prepared with carbon and manganese oxide in an air atmosphere, we suggest that the powders are finely milled [37] and Mn2O3 is used instead of MnO2. However, even when Mn2O3 is used, partial burning of the carbon by the atmospheric oxygen is expected. To prevent burning of the carbon an oxygen-deficient atmosphere should be used. The influence of the particle size on the apparent density is shown in Fig. 2. The apparent density decreases with a decreasing particle size and a prolonged foaming time. Very limited foaming is observed when 196-μm powder (D50) is used. This is in accordance with the TG results that show little WL in the temperature range 730–890 °C. In order to accelerate the gas release the temperature should be increased to above 890 °C. However, similarly low densities as observed for the finer powders cannot be obtained as the foam stability decreases with the drop in viscosity [36]. The upper limit of the particle size at which an important foaming occurs, (i.e. a density below 500 kg/m3) is found at 163 μm (D90). This is significantly lower than the particle size limit reported in the literature, i.e., 400 μm (D90) [1,6]. On the other hand, the particle size required for the preparation of low-density foams (b150 kg m−3) is reported to be well below 10 μm (D50) [13,30,39,40]. Here, we obtained foam glass with a density of 158 kg m−3 using 53 μm powders (D50). Hence, less energy is needed for the processing of the raw materials. When heating the powders with a slower rate (5 °C/min vs. 10 °C/min) or when prolonging the dwell time (30 min vs. 5 min),
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the exposure of the samples to high temperatures is longer, which is reflected in a lower density. This is to be expected since the TG results (Fig. 1) show the gas-forming reaction starts at 790 °C and then accelerates at 820 °C (curve for the fine powder sample). The main reaction in this temperature range is a reduction of the Mn2O3 dissolved in the glass [26]. A larger contact area when using finer powders leads to a faster dissolution and thus the reaction occurring at a lower temperature. On the other hand, in the coarse powder sample the dissolution of Mn2O3 is slower and the temperature has to be increased to 900 °C (see Fig. 1) before the thermal decomposition of Mn2O3 occurs [38]. All the foam glasses are homogeneous and contain only a few larger pores (Figs. 3 and 4). The photographs and SEM images indicate a narrow pore size distribution. In most foam glasses the pore size is below 0.5 mm, except in the sample prepared from the finest particles foamed for 30 min. In this sample the pore size is 1–3 mm. The foam glass made from the coarsest powders has a pore size in the range of 50–300 μm, while the concentration of pores is much lower compared to the other samples (Fig. 4). In this sample only a small amount of gas is released in the softened glass, as seen from the TG results (Fig. 1). Pore-free regions of 200–300 μm that are visible in the sample most probably originate from large glass particles, as the D90 value of this powder is 283 μm. The amount of closed pores is above 95 vol.% in the samples heated at 10 °C/min (Fig. 5). These samples were exposed to high temperatures for a shorter time. The sample with the lowest density and a high degree of closed pores, i.e., the sample that was prepared from 53-μm powders (D50), heated at 5 °C/min and foamed for 5 min, exhibits a pore size of 50–250 μm (Fig. 4b) and a density of 200 kg m−3. In contrast, open porosity dominates in the samples with a density smaller than 200 kg m−3 (inset in Fig. 4). However, the pore size remains small (e.g., 0.1–0.6 mm in Fig. 3c), except for the sample from the finest powders foamed for 30 min. In general, an open porosity and an inhomogeneous microstructure are characteristic of decomposing foaming agents like carbonates [14, 19,25]. The reason for such an occurrence is the sudden increase in the pressure inside the pores at the temperatures where the main part of the decomposition occurs. The pressure increase is fast enough to tear apart the thin walls, so the connections between the pores are formed. The mechanism in MnO2, which is also considered a decomposing foaming agent [6], is different (this study and ref. [26]). The gas release when foaming with MnO2 is not as fast and detrimental as is the case for carbonates [25,35,37]. The TG analysis for a carbonate sample showed a 0.7% WL over a range of 50 °C (see Fig. 1b in ref. [25]), while in this study the WL for the fine powder sample is one-third of that (0.23%) in the temperature range 835–885 °C. However, with prolonged exposure to high temperatures the pore walls become very thin and connections are formed (Fig. 4c). The sharp edges of the pore walls indicate that they broke during the sample preparation. The thickness of these
Fig. 3. Cross-section images of the foam glasses prepared from powders with different particle sizes (D50). Samples were prepared at 835 °C with 5 °C/min for a) 5 min and b) 30 min.
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Fig. 4. SEM micrographs of foam glasses prepared at 835 °C with a 5 °C/min heating rate using different particle sizes (D50) and treatment times: a) 196 μm, 30 min; b) 53 μm, 5 min; c) 33 μm, 30 min; d) 13 μm, 30 min, and subsequently quenched. Open and full arrows in c) show the connections between the neighboring pores and the thin pore wall, respectively. The inset in c) shows a broken pore wall with a thickness estimated from the rolled edges (white arrows) to b100 nm.
walls is estimated to be below 100 nm (inset in Fig. 4c). This is in accordance with the thickness reported in the literature [41]. The formation of connections is unwanted because when a pore opens to the surrounding atmosphere the driving force for further expansion is lost. Closed porosity also decreases the heat convection through the sample and provides a better thermal insulation when a gas with a low thermal conductivity is captured in the pores. When the majority of the pores are interconnected, the density of the foam can only decrease further with the nucleation and growth of new pores [26]. These usually nucleate in the thicker walls or struts, which results in a bimodal pore size distribution. Initially, the size of an individual pore increases due to the release of gases and the temperature increase. When the gas release ceases, the density of the foam starts to increase, while the pores continue to grow due to coalesce. This is observed for the sample from the finest particle size (13 μm, D50) foamed for 30 min that has a slightly higher density compared to the sample from 33-μm powders (D50), but has much larger pores (Fig. 3). The small pore size and homogeneous microstructure observed in all the samples indicate that the coalescence proceeds evenly throughout the sample. To further investigate the coalescence and growth of the pores, as well as the stability of the foam glass, two experiments with a shorter and longer foaming time were performed. In the first experiment, the treatment time was prolonged by decreasing the cooling rate from 4 °C/min to 1 °C/min for the sample with 33-μm powders (D50) foamed for 30 min. Accordingly, the time during which the sample was heated above the softening temperature (i.e. 710 °C [36]) increased from 30 min to 2 h. The resulting foam glass had a high density (320 kg m−3), a very irregular shape, an inhomogeneous pore size distribution and thick walls (Fig. S2). Such an appearance indicates extensive pore coalescence, which resulted in the partial collapse of the foam. The second experiment with a shortened treatment time was performed on a large sample with the finest powder (13 μm, D50).
This sample was first heated to 835 °C at 5 °C/min, held for 30 min and then quenched from the foaming temperature to the glass-transition temperature (~530 °C) by moving it directly into an annealing furnace. Thus the time the sample spent above the softening temperature was shortened by 30 min. The density of the quenched sample is slightly lower than the density of the slowly cooled sample, i.e., 116 kg m−3 vs. 120 kg m−3, respectively (Table 2). In contrast, the pore size is 5–10times smaller (0.1–0.6 mm) in comparison to the sample cooled at 5 °C/min (1–3 mm large), as shown in Fig. 6a and b. Such a large difference suggests that the coalescence proceeds very quickly, even at temperatures below the foaming temperature, i.e., during cooling.
Fig. 5. Closed porosity (CP) in relation to the particle size (D50) and density (ρapp; inset) of the foam glasses prepared at 835 °C with different heating rates and durations (see legend).
J. König et al. / Journal of Non-Crystalline Solids 447 (2016) 190–197
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Table 2 Comparison of the apparent density (ρapp) and closed porosity (CP) between small and large samples prepared from powders with different particle sizes (D50). Small samples D50
ρapp −3
[μm]
[kg m
13 33 103 13 (quenched)
143 137 282 –
a
Large samples CP
]
ρapp
CP −3 a
[%]
[kg m
10.9 10.5 88.1 –
121 ± 2 151 ± 7 355 ± 25 116
]
[%] 5.4 3.4 67 9.6
The average foam density and the variation in the density between three foam samples.
This shows the importance of a fast cooling rate after the foaming process. The apparent densities and closed porosities of the large and small samples prepared under the same conditions are compared in Table 2. The density of the large sample prepared from the finest powder is smaller compared to the small foam sample, while the opposite is observed for the other two powder sizes. The difference occurs because there are different optimum foaming times for the small and large samples. For the small samples prepared from 13-μm powders (D50), the pore size (much greater in comparison to the coarser powders) and the density (greater than in the large sample) indicate that a 30-min foaming time is too long. The 30-min foaming time is closer to the optimum for the large sample size from the 13-μm powders indicated by the lower density (120 kg m−3). In the case of the 33-μm powders (D50), however, the 30-min foaming time is optimum for the small sample, while it is not long enough for the large sample. The extension of the optimum foaming time with the increase in the sample size can be related to the difference in the heat transfer during the heat treatment. Since foaming takes place inside a metal cylinder and the heat transfer is relatively low, the heating and cooling rates of the large samples are slower than those of the small samples. The fractions of closed pores in the large samples are lower, which is most probably related to slower cooling, during which time coalescence occurs. The cross-sections of the large foam samples (Fig. 6) show that the samples are homogeneous and the pore sizes are comparable to the
Fig. 7. Thermal conductivity (λ) of the foam glasses prepared with 0.93 wt.% carbon and 6.76 wt.% MnO2, heated at 5 °C/min to 835 °C and held for 30 min. The measurements were performed at 10 °C and the precision was ±0.5 mW m−1 K−1 (equal to the marker size). For comparison, data for samples prepared with CRT panel glass, 0.95 wt.% carbon and 5.12 wt.% MnO2 [37] and commercial foam glasses [12] are shown. The lines represent linear fits.
small samples (Fig. 2). Despite the large difference in the pore size between the large samples prepared from 13-μm powders (Fig. 6a and b), both samples exhibit comparable densities and a low percentage of closed porosity (Table 2). Such behavior indicates that the stability of the foam is good and not affected by the coalescence when the sample cools to the softening temperature within a reasonable time (i.e., 30 min). The variation in the density presented in Table 2 is in major part caused by the temperature gradients due to different positions of the samples in the furnace. In comparison with a previous investigation [37] where the same raw materials were used, but the mixture was homogenized by ball milling, the foam samples obtained in this study are more uniform throughout the sample height and the pores are smaller. Another difference is that the mixed powders were
Fig. 6. Cross-sections of the large foam glasses prepared at 835 °C for 30 min with a 5 °C/min heating rate. The foams were prepared from powders with different particle sizes (D50): a) 13 μm (quenched), b) 13 μm, c) 33 μm, d) 103 μm.
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foamed at a higher temperature than the ball-milled powders. This is in accordance with the observations of the present study (Figs. 1 and 2) that the gas-forming reaction is observed at lower temperatures when finer powders are used. The compressive strength was measured for the large samples prepared from 33-μm powders (D50). At a density of 144 kg m−3 the compressive strength is 0.48 MPa (Fig. S3). The compressive strength of a commercial foam glass (ρapp = 115 kg m−3) [12] was found to be 0.4 MPa using the same method. If the difference in the density between the samples is taken into account, the compressive strength of the investigated foams is lower than that of the commercial product. The smaller compressive strength is attributed to a high content of open porosity, which weakens the foam structure [14,31]. The thermal conductivity was measured in samples with a density ranging from 116 to 158 kg m− 3 (Fig. 7). The thermal conductivity decreases with increasing density. The lowest value obtained is 38.1 mW m − 1 K − 1 for the quenched sample with a density of 116 kg m−3. The slope of the thermal conductivity decrease with the density is smaller in comparison to the foams prepared by ball-milling the powder mixture [37] and the commercial foam glass [12]. The reasons for such a difference are not known at this point. In the literature only scarce data on the thermal conductivity of foam glasses are available [15,21,37,42]. Our results indicate (Fig. 7) that foam glasses with smaller pores have a slightly lower thermal conductivity in comparison to samples from a previous report [37]. At a density of 150–160 kg m−3 the difference is around 2–3 mW m−1 K−1, while at lower densities the difference diminishes. However, another study on foam glass prepared under similar conditions, but with a density above 200 kg m−3, showed that there is no influence of the pore size on the thermal conductivity [26]. A detailed analysis of the thermal conductivity of foam glasses with similar densities and different pore sizes will have to be performed to address this discrepancy. In comparison to commercial products [12], the thermal conductivity of the samples prepared in this study is lower when comparing values for the same densities (Fig. 7). The best commercial foam-glass product has a thermal conductivity of 38 mW m−1 K−1 at a density of 100 kg m−3, while in this study the same thermal conductivity is obtained at a density of 116 kg m−3. Moreover, the prepared foam glasses have a high degree of open porosity. This shows that the potential for a further decrease in the thermal conductivity lies in optimizing the closed porosity and the pore-gas composition. The thermal conductivity is often expressed as a linear contribution from the solid and gas conductivities [31,32]. Thereafter, a decrease in the gas conductivity could be significant if the air was to be exchanged with CO2. However, a more detailed analysis of the heat transfer in foam glasses needs to be performed to evaluate the contributions of the solid and gaseous conductivity to the overall thermal conductivity. The most important parameter in order to achieve CO2-filled pores is the use of an oxygen-free atmosphere during the heat treatment in order to prevent the premature oxidation of the carbon. Despite a low thermal conductivity of the prepared foam glass, its use in thermal insulation is limited to indoor applications as the open-pore structure is vulnerable to water penetration and freeze-thaw cycling. However, due to the open pore structure it can be used effectively as an acoustic insulator. 4. Conclusions Foaming of mixtures of CRT panel glass, carbon and MnO2 is possible when a particle size of ≤ 103 μm is used, while the foaming is almost completely halted in 196-μm powders (D50). The major part of the foaming results from the reduction of manganese, while the contribution of the carbon is negligible. The apparent density of the obtained foam glasses is below 150 kg m− 3 when a particle size of ≤ 33 μm is used (D50). All the foams are homogeneous and possess very small pores (the majority smaller than 0.5 mm), except for the foam prepared
from the finest powder (13 μm, D50) foamed at 835 °C for 30 min, for which the pore size is in the range of 1–3 mm. However, the pore size remains in the range 0.1–0.6 mm if the sample is quenched from the foaming to the annealing temperature. The foams with an apparent density of b200 kg m−3 are predominantly open-porous. The coalescence and growth of the pores progresses at similar rates throughout the sample and the connections between the pores are formed after the pore-wall thickness is reduced to below 100 nm. The compressive strength of the foam glass is 0.48 MPa at a density of 144 kg m−3, which can potentially be improved by increasing the closed porosity. The foams exhibit a thermal conductivity as low as 38.1 mW m−1 K−1 at a density of 116 kg m−3. Due to the predominantly open-porous structure the foams could be used as indoor thermal and acoustic insulation. However, a great potential exists to further decrease the thermal conductivity of the investigated foam glasses. Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.jnoncrysol.2016.05.021. Acknowledgement We thank L.S. Trankjær, R.E. Gissel and N.J. Klitmøller for preparing the powders, foam glasses, and measuring He-pycnometer and apparent density. We also thank C. Prinds (Danish Technological Institute) for measuring the thermal conductivity and compressive strength of the foam glasses. We thank the Danish National Advanced Technology Foundation for financial support under grant number 012-2011-3. References [1] G. Scarinci, G. Brusatin, E. Bernardo, Glass foams, in: M. Scheffler, P. Colombo (Eds.), Cellular Ceramics, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 2005, pp. 158–176. [2] Foamglas Industrial Insulation Handbook, Pittsburgh Corning Europe, Waterloo, Belgium, 1992. [3] O.V. Kaz'mina, V.I. Vereshchagin, B.S. Semukhin, Structure and strength of foam-glasscrystalline materials produced from a glass granulate, Glas. Phys. Chem. 37 (2011) 371–377. [4] http://www.glapor.com (accessed 20.4.16). [5] O.V. Kaz'mina, V.I. Vereshchagin, A.N. Abiyaka, Prospects for use of finely disperse quartz sands in production of foam-glass crystalline materials, Glas. Ceram. 65 (2008) 319–321 (English translation of Steklo i Keramika). [6] V. Ducman, M. Kovačević, The foaming of waste glass, Key Eng. Mater. 132–136 (1997) 2264–2267. [7] J.P. Wu, A.R. Boccaccini, P.D. Lee, M.J. Kershaw, R.D. Rawlings, Glass ceramic foams from coal ash and waste glass: production and characterisation, Adv. Appl. Ceram. 105 (2006) 32–39. [8] H.R. Fernandes, D.U. Tulyaganov, J.M.F. Ferreira, Preparation and characterization of foams from sheet glass and fly ash using carbonates as foaming agents, Ceram. Int. 35 (2009) 229–235. [9] C. Vancea, I. Lazău, Glass foam from window panes and bottle glass wastes, Cent. Eur. J. Chem. 12 (2014) 804–811. [10] J. García-Ten, A. Saburit, M.J. Orts, E. Bernardo, P. Colombo, Glass foams from oxidation/ reduction reactions using SiC, Si3N4 and AlN powders, Glass Technol. Eur. J. Glass Sci. Technol. Part A 52 (2011) 103–110. [11] A.S. Llaudis, M.J.O. Tari, F.J.G. Ten, E. Bernardo, P. Colombo, Foaming of flat glass cullet using Si3N4 and MnO2 powders, Ceram. Int. 35 (2009) 1953–1959. [12] http://www.foamglas.com (accessed 20.4.16). [13] V.A. Lotov, E.V. Krivenkova, Kinetics of formation of the porous structure in foam glass, Glas. Ceram. 59 (2002) 89–93 (English translation of Steklo i Keramika). [14] E. Bernardo, F. Albertini, Glass foams from dismantled cathode ray tubes, Ceram. Int. 32 (2006) 603–608. [15] H. Hojaji, Development of foam glass structural insulation derived from fly ash, Proc. MRS Fall Meeting, Boston, US-MA, 136 1988, pp. 185–206. [16] A. Steiner, Foam Glass Production From Vitrifies Municipal Waste Fly AshesPhD Thesis Eindhoven University of Technology, Eindhoven, The Netherlands, 2006. [17] H.R. Fernandes, F. Andreola, L. Barbieri, I. Lancellotti, M.J. Pascual, J.M.F. Ferreira, The use of egg shells to produce Cathode Ray Tube (CRT) glass foams, Ceram. Int. 39 (2013) 9071–9078. [18] G. Bayer, Foaming of borosilicate glasses by chemical reactions in the temperature range 950–1150 °C, J. Non-Cryst. Solids 39 (1980) 855–860. [19] Y.A. Spiridonov, L.A. Orlova, Problems of foam glass production, Glas. Ceram. 60 (2003) 313–314 (English translation of Steklo i Keramika). [20] V.E. Manevich, K.Y. Subbotin, Foam glass and problems of energy conservation, Glas. Ceram. 65 (2008) 105–108 (English translation of Steklo i Keramika). [21] S. Köse, G. Bayer, Schaumbildung im System Altglas-SiC und die Eigenschaften derartiger Schaumgläser, Glastech. Ber. 7 (1982) 151–160. [22] E. Bernardo, R. Cedro, M. Florean, S. Hreglich, Reutilization and stabilization of wastes by the production of glass foams, Ceram. Int. 33 (2007) 963–968.
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Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Influence of the glass particle size on the foaming process and physical characteristics of foam glasses Jakob König a,b, Rasmus R. Petersen a, Yuanzheng Yue a,⁎ a b
Department of Chemistry and Bioscience, Aalborg University, DK-9220 Aalborg East, Denmark Advanced Materials Department, Jožef Stefan Institute, SI-1000 Ljubljana, Slovenia
a r t i c l e
i n f o
Article history: Received 23 February 2016 Received in revised form 3 May 2016 Accepted 4 May 2016 Available online 14 June 2016 Keywords: Foam glass Manganese oxide Thermal conductivity Porosity Particle size
a b s t r a c t We have prepared low-density foam glasses from cathode-ray-tube panel glass using carbon and MnO2 as the foaming agents. The effect of the glass particle size on the foaming process, the apparent density and the pore morphology is revealed. The results show that the foaming is mainly caused by the reduction of manganese. Foam glasses with a density of b150 kg m−3 are obtained when the particle size is ≤33 μm (D50). The foams have a homogeneous pore distribution and a major fraction of the pores are smaller than 0.5 mm. Only when using the smallest particles (13 μm) does the pore size increase to 1–3 mm due to a faster coalescence process. However, by quenching the sample from the foaming to the annealing temperature the pore size is reduced by a factor of 5–10. The foams with an apparent density of b200 kg m−3 are predominantly open-porous. The foams exhibit a thermal conductivity as low as 38.1 mW m−1 K−1 at a density of 116 kg m−3. For the investigated foam glasses, there exists a great potential to further decrease their thermal conductivity by increasing the closed porosity and by changing the trapped gas to CO2. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Foam glass is a highly porous (N 85 vol.% pores), lightweight material used for thermal and acoustic insulation [1,2]. Depending on the application, e.g., heat insulation or soundproofing, the foam glass has predominantly closed or open porosity, respectively. Foam glass exhibits several advantages over conventional insulating materials such as organic foams and mineral wool, e.g., water and steam resistance, freeze-thaw tolerance, excellent chemical and thermal stability, high surface area and permeability, and superior mechanical properties. Due to the chemical stability and the high degree of closed porosity, the lifespan of foam glass (N100 years [3]) is much longer than conventional thermal insulation materials. Foam glass is produced from powdered glass admixed with a foaming agent that releases gases at elevated temperatures. In the softened glass, the expanding gas bubbles increase the volume of the sample, thus forming a porous lightweight product. In industrial foam-glass production the majority of the glass originates from waste glass [1]. Some production facilities only make use of waste glass, thus eliminating the energyconsuming melting step. Foam-glass production makes it possible to recycle a great diversity of glassware or other amorphous materials into a high-value-added product. The waste glasses most commonly employed are mixed-color bottle glass [4–7] and window glass [8–12], but other glasses, like lamp glass [13], glasses from cathode ray tubes ⁎ Corresponding author. E-mail address: [email protected] (Y.Z. Yue).
http://dx.doi.org/10.1016/j.jnoncrysol.2016.05.021 0022-3093/© 2016 Elsevier B.V. All rights reserved.
(CRTs) [14], fly ashes from coal production [15] and vitrified solid wastes from municipal waste combustion [16] can also be used. The foaming agent, added as a minor part to the powdered glass, releases gases based on a decomposition reaction or a redox reaction with the glass. The solid residues of the foaming agent or its reaction with the glass have an impact on the glass network and foam morphology. Therefore, foaming agents must be selected according to the composition of the glass. Different foaming agents and/or their reactions with the glass lead to a wide variety of gases being found in the pores. In general, CO2 is the best gas to be trapped in the pores, since it is easily obtainable, has a low thermal conductivity and a low toxicity. Gases like H2O, O2 and N2 are also acceptable, but they have a higher thermal conductivity than CO2. More problematic is the presence of CO, H2 or SO3 due to their high toxicity, flammability and/or reactivity. CO2 gas is readily available from the decomposition of carbonates [8,14,17] and carbon-based foaming agents, like pure carbon materials, carbohydrates and SiC [1, 18–20]. The latter are used by the foam-glass industry [1]. When carbon is used as a foaming agent, the gas released at elevated temperatures is a CO/CO2 mixture, where the ratio depends on the availability of oxygen. During cooling the mixture then transforms to CO2, according to Boudouard's equilibrium. When applying carbohydrates or SiC as a foaming agent, the gas-releasing mechanism becomes more complex [18,21]. For this reason, pure carbon is preferred over carbohydrates and SiC. The foaming starts after the glass has sintered to a state with closed pores, where the volume of the entrapped gases is too small for a successful foaming. Therefore, a sufficient amount of free oxygen has
J. König et al. / Journal of Non-Crystalline Solids 447 (2016) 190–197
to be available from the glass melt. In order to promote the gas-releasing reaction, i.e., the oxidation of carbon, an oxidizing agent can be added, which acts as an oxidation/reduction couple with the carbonaceous foaming agent [10,11,22]. The most suitable oxidizing agents are Fe2O3 [10,16], MnO2 [10,11,18] and sulfates [16,18,19]. However, the solid residues of the foaming agent can act as a nucleation and crystallization agent. This means that the impact of the foaming agent on the glass stability has to be considered. Despite the fact that foam-glass technology has been known since the 1930s, the number of academic publications has only started to increase in the past 15 years. The main motivation for the research seems to be environmental consciousness and the increasing price of landfilling waste. The research on foam glass mainly concentrates on its mechanical properties, only few papers report on the thermal conductivity [9,14,15,21,23–26]. However, a worldwide focus on energy efficiency has put insulating properties higher on the agenda. Therefore, attention has been drawn to modified conventional and contemporary materials with improved insulating properties [27–29]. In order to promote the use of foam glass in thermal insulation, its insulating properties need to be enhanced. The thermal insulation properties of a foam glass can be improved by (i) decreasing the density, (ii) closing the porosity, (iii) entrapping a low conducting gas, and (iv) employing glasses with a low thermal conductivity. To prepare a low-density foam glass (b 150 kg m−3), optimum conditions have to be found, i.e., a foaming agent compatible with the composition of the glass, an optimum concentration and the right heat-treatment conditions. Finely milled powder and homogenous mixtures result in small pores [6,30]. If the pores are closed, then convective heat transfer is prevented. Moreover, the viscosity as well as the surface tension should be in a defined range at the foaming temperature to support low density and small, closed pores [1,19]. When the pores are small and closed, the overall thermal conductivity of the foam glass can be estimated by summing the contributions from the conduction through the solid and the gaseous phases [31,32]. Based on an assumption of linear contributions [31,32] the difference in the thermal conductivity between a foam glass filled with O2 (24 mW m− 1 K−1) and a foam glass filled with CO2 (15 mW m− 1 K− 1) [33] would be 8.6 mW m− 1 K− 1 for a foam glass with 95% porosity. The effect of the particle size on the foam density and homogeneity is, despite being well known, scarcely reported [1,6,25,30,34]. It is common to report the particle size of the glass and the foaming agent before they are mixed. However, since the mixing is frequently carried out using milling equipment, the particle size changes, obscuring any clear relation between the particle size and the foam density. It was shown that when foaming with carbon the density of the foam glass decreases rapidly with a glass particle size below 100 μm [1,30]. On the other hand, the relationship between the particle size and the foam density is not so straightforward when using carbonates as the foaming agent [25]. The aim of the present work is to study the influence of the particle size on the foaming process and to prepare foam glasses with a low thermal conductivity. We foamed waste CRT panel glass with carbon and MnO2 as the foaming agents. CRT panel glass has a high glass stability [25,35,36]. To reveal the influence of the particle size on the foaming process, pre-milled powder sieved to different fractions was mixed with the foaming agents and the mixture was homogenized without changing the particle size. In addition, the effect of the heat-treatment conditions on the characteristics of the foam and its stability are revealed and discussed. We characterized the apparent density, the closed porosity, the compressive strength and the thermal conductivity of the foam glasses. 2. Experimental CTR panel glass was crushed in a jaw crusher (Retch BB51, Haan, Germany) and subsequently ball milled in a planetary ball mill (Retsch
191
PM100). The powders were then sieved to obtain five powder fractions with different particle sizes using laboratory sieves (38–250 μm). The chemical composition of the CRT panel glass is reported elsewhere [25]. Carbon (activated charcoal; Bie & Berntsen, Søborg, Denmark) and MnO2 (98%, Bie & Berntsen) were ball milled to pulverize the agglomerates and larger particles. The foaming mixtures were prepared from panel glass, 0.93 wt.% carbon and 6.76 wt.% MnO2, according to previously determined optimum concentrations [37]. The carbon/ MnO2 ratio was equal to one-half of the stoichiometric amount needed for the theoretical carbon oxidation reaction: C þ 2MnO2 →CO2 þ 2MnO
ðReaction 1Þ
The powder mixtures were homogenized on a carousel in a glass bottle with four light plastic balls for 17 h at 30 rpm. The particle size distribution of the powder mixtures was measured with a LS 13320 laser granulometer (Beckman Coulter, Brea, US-CA). The D50 and D90 values of the powder mixtures as well as the foaming agents are given in Table 1. The particle size distribution is shown in Fig. S1. Compared to the previous study [37], the powder processing in the present study was different, and thus the influence of the glass particle size on the foaming process could be revealed. The powder mixtures were uniaxially pressed into disk-shaped pellets with 40 MPa. Small pellets (1 g, Ø = 13 mm) were used to study the influence of the particle size and the foaming conditions on the density and homogeneity of the foams. Larger pellets (Ø = 35 mm) of selected powder mixtures were used to prepare large samples (23–45 g) in a stainless-steel cylinder (6 cm in diameter and 5 cm in height) to study the thermal conductivity and compressive strength. The pellets were placed in an electrical laboratory-chamber furnace and heated at different heating rates (5 or 10 °C/min) to 835 °C and heat-treated for different times (5 to 30 min). Subsequently, the samples were cooled at 4 °C/min to the glass-transition temperature (Tg) of 530 °C [35] and then slowly cooled at 1 °C/min to room temperature. The temperature in the furnace was calibrated using an external thermocouple. Thermogravimetic (TG) analyses were performed using a Jupiter 449 simultaneous thermal analysis (STA) instrument coupled with a 403C Aëoloss mass spectrometer (MS) (Netzch, Selb, Germany). The measurements were performed with a heating rate of 10 °C/min in an air atmosphere. The powder mixture was compressed with 40 MPa and a small piece (30–40 mg) was placed into an alumina crucible. The CO2 peak was integrated to calculate the amount of carbon burnt out at the defined temperature. The apparent density (ρapp ) of the small and the large foam samples was determined using Archimedes' principle in a 10 wt.% polyethylenglycol (PEG 3000 Da, Bie & Berntsen) solution or was calculated from the samples' mass and dimensions, respectively. A PEG solution was used to prevent absorption of the liquid medium during the measurement. The pycnometer density (ρ pyc ) of the foam samples was determined with a helium pycnometer (Ultrapyc 1200e, Quantachrome Instruments, Boynton Beach, US-FL). The powder density (ρpowder) of the crushed foamed-glass sample was measured with the pycnometer. The percentage of the closed porosity, out of the
Table 1 Particle size (D50 and D90) of the powder mixtures, carbon and MnO2. Sample
D50 [μm]
D90 [μm]
#1 #2 #3 #4 #5 carbon MnO2
13 33 53 103 196 31 5.4
32 80 92 163 283 54 29
192
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total porosity, was calculated from the apparent and pycnometer densities according to: ρapp
!
TP ¼
1
CP ¼
ρapp ρapp ρpyc ρpowder
CP ½% ¼
ρpowder: !
CP 100 TP
The compressive strength was measured on large cylinder-shaped samples using a table-top universal tester (Autograph AGS-J, Shimadzu, Kyoto, Japan) with a crosshead speed of 0.5 mm min− 1. Prior to measurements the top and bottom surfaces of the foam samples were coated with a 1-mm layer of a commercial mineral building glue. The thermal conductivity was measured from the top and bottom sides and the average value is reported in relation to the density of the sample. The thermal conductivity was measured with a surface probe (25 × 25 mm2) at 10 °C using a heat-transfer analyzer (ISOMET Model 2104 Heat Transfer Analyzer, Applied Precision, Bratislava, Slovakia). The accuracy of the equipment in the relevant range was tested by measuring commercial expanded-polystyrene and foam-glass samples with known values. The obtained results were in perfect agreement with the declared values. 3. Results and discussion The combination of a carbon-based foaming agent and an oxidizing agent is an effective way to produce low-density foam glass [10,11,18, 22]. In the present investigation we applied pure carbon and MnO2, which would provide CO2 gas according to Reaction 1. However, since the thermal treatment was performed in an air atmosphere, it is expected that part of the carbon will be oxidized before the pores between the softened glass particles are closed as a result of the sintering process. This was observed for the sample foamed with carbon in a previous investigation [37], according to which foaming occurred only when the particle size (D90) is lower than 27 μm. In the present investigation, coarser particles are used, with the lowest D90 value equal to 32 μm. In this respect, it is expected that all carbon will be burnt by the oxygen from surrounding atmosphere. The release of gases and weight loss were determined by performing a TG-MS analysis.
Fig. 1. Sample weight (W) and CO2 evolution (MS) from powder mixtures with different particle sizes (D50) during heating in an air atmosphere.
The analysis revealed a significant difference in the weight-loss (WL) curves between the finest (D50 = 13 μm) and the coarsest (D50 = 196 μm) powders (Fig. 1). Three steps in the WL can be distinguished in both samples, which were separated into the following temperature ranges: (I) 200–564 °C, (II) 564–730 °C and (III) above 730 °C. In step I, the WL of the sample from the fine powder is larger than that of the sample from the coarse powder (1.05 vs. 0.85 wt.%) and the CO2 peak appears at a higher temperature (527 vs. 470 °C). No O2 peak was detected, which suggests that the oxygen released by the decomposition of MnO2 reacts with the carbon. During step I partial burning of the carbon and partial reduction of the MnO2 occurs. At the end of step I, 14 and 5.8 wt.% of unburned carbon remains in the fine and coarse powder samples, respectively. The larger WL and the more unburned carbon in the fine powder sample indicate that more carbon is burnt out by the oxygen originating from MnO2 reduction than in the sample of coarse powders. In the coarse powder sample more carbon can be burnt by the atmospheric oxygen because the green density is lower, and hence more atmospheric oxygen is present between the particles, enabling a higher gas flow in and out of the sample. Thus oxidation of the carbon by the atmosphere occurs at lower temperatures (CO2 peak at 470 °C). On the other hand, less carbon is burnt by the atmospheric oxygen in the fine powder sample, but it eventually burns when the oxygen is released by the decomposition of MnO2 as the temperature increases, with the CO2 peak at 527 °C. The carbon that is not burnt by the atmosphere shifts the reduction of MnO2 to lower temperatures, and thus the overall WL in the fine powder sample in step I is higher, despite more unburnt carbon staying in the sample at 564 °C. In step II (564–730 °C), the rest of the carbon is burnt out and the MnO2 decomposes to Mn2O3 [26,38]. At around 690 °C the sample densifies and the pores close [25,26]. This is seen as a halt in the WL. The closing of the pores leads to an increase in the oxygen partial pressure in the sample, which inhibits further reduction of the MnO2. Therefore, not all of the MnO2 is decomposed to Mn2O3 at the end of step II. A complete reduction of MnO2 to Mn2O3 and complete burning of the carbon would result in a WL of 1.55 wt.%; however, only 1.33–1.37 wt.% is observed. The same mechanism appears in both samples; however, the WL in step II for the coarse powder sample is larger as less MnO2 was decomposed in step I. In step III (N 730 °C) the WL is caused by a further reduction of manganese. The reduction in the fine powder sample continues when the temperature reaches 790 °C and the WL is accelerated above 820 °C. The total WL at 900 °C (1.74 wt.%) indicates that almost all the manganese is in the form of Mn3O4 (theoretical WL is 1.76 wt.%). When pure MnO2 is heated in air the decomposition from Mn2O3 to Mn3O4 begins at 900 °C [26,38]. The presence of a glass melt favors a higher ratio of Mn2+/Mn3+ and shifts the reduction to lower temperatures. This is accordance with our previous study [26]. We suggested that Mn2O3 first dissolves in the glass and then Mn3+ reduces to Mn2+. In the coarse powder sample there is a small, gradual WL from 730 to 860 °C, which then accelerates above 910 °C. Such behavior can be explained if the agglomeration of small MnO2 particles (5.4 μm, D50) between large glass particles (196 μm, D50) in the mixture is considered. Thereafter, a much smaller contact surface between the agglomerated manganese oxide and the glass is available and the dissolution is slower. Consequently, the decomposition of Mn2O3 to Mn3O4 is observed at the same temperature as in pure manganese oxide [26,38]. Some of the agglomerates located on, or close to, the surface of the pellet gradually decompose (gradual WL in the range 730–860 °C), while the reaction in the interior is decelerated due to the high oxygen partial pressure. The TG-MS results show that the carbon is burnt out before the pores close (b 690 °C). Thus the foaming is caused by the oxygen gas released with the reduction of manganese. A great part of the oxygen released by the decomposition of MnO2 to Mn2O3 is lost to the atmosphere as the reaction occurs before the sample is sintered to closed pores [26]. To
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Fig. 2. Influence of the particle size (D50) on the apparent density (ρapp) of the foam glasses prepared at 835 °C with different heating rates and foaming times (see legend). The inset magnifies the density data for smaller particle sizes.
increase the CO2 content in the foam glass prepared with carbon and manganese oxide in an air atmosphere, we suggest that the powders are finely milled [37] and Mn2O3 is used instead of MnO2. However, even when Mn2O3 is used, partial burning of the carbon by the atmospheric oxygen is expected. To prevent burning of the carbon an oxygen-deficient atmosphere should be used. The influence of the particle size on the apparent density is shown in Fig. 2. The apparent density decreases with a decreasing particle size and a prolonged foaming time. Very limited foaming is observed when 196-μm powder (D50) is used. This is in accordance with the TG results that show little WL in the temperature range 730–890 °C. In order to accelerate the gas release the temperature should be increased to above 890 °C. However, similarly low densities as observed for the finer powders cannot be obtained as the foam stability decreases with the drop in viscosity [36]. The upper limit of the particle size at which an important foaming occurs, (i.e. a density below 500 kg/m3) is found at 163 μm (D90). This is significantly lower than the particle size limit reported in the literature, i.e., 400 μm (D90) [1,6]. On the other hand, the particle size required for the preparation of low-density foams (b150 kg m−3) is reported to be well below 10 μm (D50) [13,30,39,40]. Here, we obtained foam glass with a density of 158 kg m−3 using 53 μm powders (D50). Hence, less energy is needed for the processing of the raw materials. When heating the powders with a slower rate (5 °C/min vs. 10 °C/min) or when prolonging the dwell time (30 min vs. 5 min),
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the exposure of the samples to high temperatures is longer, which is reflected in a lower density. This is to be expected since the TG results (Fig. 1) show the gas-forming reaction starts at 790 °C and then accelerates at 820 °C (curve for the fine powder sample). The main reaction in this temperature range is a reduction of the Mn2O3 dissolved in the glass [26]. A larger contact area when using finer powders leads to a faster dissolution and thus the reaction occurring at a lower temperature. On the other hand, in the coarse powder sample the dissolution of Mn2O3 is slower and the temperature has to be increased to 900 °C (see Fig. 1) before the thermal decomposition of Mn2O3 occurs [38]. All the foam glasses are homogeneous and contain only a few larger pores (Figs. 3 and 4). The photographs and SEM images indicate a narrow pore size distribution. In most foam glasses the pore size is below 0.5 mm, except in the sample prepared from the finest particles foamed for 30 min. In this sample the pore size is 1–3 mm. The foam glass made from the coarsest powders has a pore size in the range of 50–300 μm, while the concentration of pores is much lower compared to the other samples (Fig. 4). In this sample only a small amount of gas is released in the softened glass, as seen from the TG results (Fig. 1). Pore-free regions of 200–300 μm that are visible in the sample most probably originate from large glass particles, as the D90 value of this powder is 283 μm. The amount of closed pores is above 95 vol.% in the samples heated at 10 °C/min (Fig. 5). These samples were exposed to high temperatures for a shorter time. The sample with the lowest density and a high degree of closed pores, i.e., the sample that was prepared from 53-μm powders (D50), heated at 5 °C/min and foamed for 5 min, exhibits a pore size of 50–250 μm (Fig. 4b) and a density of 200 kg m−3. In contrast, open porosity dominates in the samples with a density smaller than 200 kg m−3 (inset in Fig. 4). However, the pore size remains small (e.g., 0.1–0.6 mm in Fig. 3c), except for the sample from the finest powders foamed for 30 min. In general, an open porosity and an inhomogeneous microstructure are characteristic of decomposing foaming agents like carbonates [14, 19,25]. The reason for such an occurrence is the sudden increase in the pressure inside the pores at the temperatures where the main part of the decomposition occurs. The pressure increase is fast enough to tear apart the thin walls, so the connections between the pores are formed. The mechanism in MnO2, which is also considered a decomposing foaming agent [6], is different (this study and ref. [26]). The gas release when foaming with MnO2 is not as fast and detrimental as is the case for carbonates [25,35,37]. The TG analysis for a carbonate sample showed a 0.7% WL over a range of 50 °C (see Fig. 1b in ref. [25]), while in this study the WL for the fine powder sample is one-third of that (0.23%) in the temperature range 835–885 °C. However, with prolonged exposure to high temperatures the pore walls become very thin and connections are formed (Fig. 4c). The sharp edges of the pore walls indicate that they broke during the sample preparation. The thickness of these
Fig. 3. Cross-section images of the foam glasses prepared from powders with different particle sizes (D50). Samples were prepared at 835 °C with 5 °C/min for a) 5 min and b) 30 min.
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Fig. 4. SEM micrographs of foam glasses prepared at 835 °C with a 5 °C/min heating rate using different particle sizes (D50) and treatment times: a) 196 μm, 30 min; b) 53 μm, 5 min; c) 33 μm, 30 min; d) 13 μm, 30 min, and subsequently quenched. Open and full arrows in c) show the connections between the neighboring pores and the thin pore wall, respectively. The inset in c) shows a broken pore wall with a thickness estimated from the rolled edges (white arrows) to b100 nm.
walls is estimated to be below 100 nm (inset in Fig. 4c). This is in accordance with the thickness reported in the literature [41]. The formation of connections is unwanted because when a pore opens to the surrounding atmosphere the driving force for further expansion is lost. Closed porosity also decreases the heat convection through the sample and provides a better thermal insulation when a gas with a low thermal conductivity is captured in the pores. When the majority of the pores are interconnected, the density of the foam can only decrease further with the nucleation and growth of new pores [26]. These usually nucleate in the thicker walls or struts, which results in a bimodal pore size distribution. Initially, the size of an individual pore increases due to the release of gases and the temperature increase. When the gas release ceases, the density of the foam starts to increase, while the pores continue to grow due to coalesce. This is observed for the sample from the finest particle size (13 μm, D50) foamed for 30 min that has a slightly higher density compared to the sample from 33-μm powders (D50), but has much larger pores (Fig. 3). The small pore size and homogeneous microstructure observed in all the samples indicate that the coalescence proceeds evenly throughout the sample. To further investigate the coalescence and growth of the pores, as well as the stability of the foam glass, two experiments with a shorter and longer foaming time were performed. In the first experiment, the treatment time was prolonged by decreasing the cooling rate from 4 °C/min to 1 °C/min for the sample with 33-μm powders (D50) foamed for 30 min. Accordingly, the time during which the sample was heated above the softening temperature (i.e. 710 °C [36]) increased from 30 min to 2 h. The resulting foam glass had a high density (320 kg m−3), a very irregular shape, an inhomogeneous pore size distribution and thick walls (Fig. S2). Such an appearance indicates extensive pore coalescence, which resulted in the partial collapse of the foam. The second experiment with a shortened treatment time was performed on a large sample with the finest powder (13 μm, D50).
This sample was first heated to 835 °C at 5 °C/min, held for 30 min and then quenched from the foaming temperature to the glass-transition temperature (~530 °C) by moving it directly into an annealing furnace. Thus the time the sample spent above the softening temperature was shortened by 30 min. The density of the quenched sample is slightly lower than the density of the slowly cooled sample, i.e., 116 kg m−3 vs. 120 kg m−3, respectively (Table 2). In contrast, the pore size is 5–10times smaller (0.1–0.6 mm) in comparison to the sample cooled at 5 °C/min (1–3 mm large), as shown in Fig. 6a and b. Such a large difference suggests that the coalescence proceeds very quickly, even at temperatures below the foaming temperature, i.e., during cooling.
Fig. 5. Closed porosity (CP) in relation to the particle size (D50) and density (ρapp; inset) of the foam glasses prepared at 835 °C with different heating rates and durations (see legend).
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Table 2 Comparison of the apparent density (ρapp) and closed porosity (CP) between small and large samples prepared from powders with different particle sizes (D50). Small samples D50
ρapp −3
[μm]
[kg m
13 33 103 13 (quenched)
143 137 282 –
a
Large samples CP
]
ρapp
CP −3 a
[%]
[kg m
10.9 10.5 88.1 –
121 ± 2 151 ± 7 355 ± 25 116
]
[%] 5.4 3.4 67 9.6
The average foam density and the variation in the density between three foam samples.
This shows the importance of a fast cooling rate after the foaming process. The apparent densities and closed porosities of the large and small samples prepared under the same conditions are compared in Table 2. The density of the large sample prepared from the finest powder is smaller compared to the small foam sample, while the opposite is observed for the other two powder sizes. The difference occurs because there are different optimum foaming times for the small and large samples. For the small samples prepared from 13-μm powders (D50), the pore size (much greater in comparison to the coarser powders) and the density (greater than in the large sample) indicate that a 30-min foaming time is too long. The 30-min foaming time is closer to the optimum for the large sample size from the 13-μm powders indicated by the lower density (120 kg m−3). In the case of the 33-μm powders (D50), however, the 30-min foaming time is optimum for the small sample, while it is not long enough for the large sample. The extension of the optimum foaming time with the increase in the sample size can be related to the difference in the heat transfer during the heat treatment. Since foaming takes place inside a metal cylinder and the heat transfer is relatively low, the heating and cooling rates of the large samples are slower than those of the small samples. The fractions of closed pores in the large samples are lower, which is most probably related to slower cooling, during which time coalescence occurs. The cross-sections of the large foam samples (Fig. 6) show that the samples are homogeneous and the pore sizes are comparable to the
Fig. 7. Thermal conductivity (λ) of the foam glasses prepared with 0.93 wt.% carbon and 6.76 wt.% MnO2, heated at 5 °C/min to 835 °C and held for 30 min. The measurements were performed at 10 °C and the precision was ±0.5 mW m−1 K−1 (equal to the marker size). For comparison, data for samples prepared with CRT panel glass, 0.95 wt.% carbon and 5.12 wt.% MnO2 [37] and commercial foam glasses [12] are shown. The lines represent linear fits.
small samples (Fig. 2). Despite the large difference in the pore size between the large samples prepared from 13-μm powders (Fig. 6a and b), both samples exhibit comparable densities and a low percentage of closed porosity (Table 2). Such behavior indicates that the stability of the foam is good and not affected by the coalescence when the sample cools to the softening temperature within a reasonable time (i.e., 30 min). The variation in the density presented in Table 2 is in major part caused by the temperature gradients due to different positions of the samples in the furnace. In comparison with a previous investigation [37] where the same raw materials were used, but the mixture was homogenized by ball milling, the foam samples obtained in this study are more uniform throughout the sample height and the pores are smaller. Another difference is that the mixed powders were
Fig. 6. Cross-sections of the large foam glasses prepared at 835 °C for 30 min with a 5 °C/min heating rate. The foams were prepared from powders with different particle sizes (D50): a) 13 μm (quenched), b) 13 μm, c) 33 μm, d) 103 μm.
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foamed at a higher temperature than the ball-milled powders. This is in accordance with the observations of the present study (Figs. 1 and 2) that the gas-forming reaction is observed at lower temperatures when finer powders are used. The compressive strength was measured for the large samples prepared from 33-μm powders (D50). At a density of 144 kg m−3 the compressive strength is 0.48 MPa (Fig. S3). The compressive strength of a commercial foam glass (ρapp = 115 kg m−3) [12] was found to be 0.4 MPa using the same method. If the difference in the density between the samples is taken into account, the compressive strength of the investigated foams is lower than that of the commercial product. The smaller compressive strength is attributed to a high content of open porosity, which weakens the foam structure [14,31]. The thermal conductivity was measured in samples with a density ranging from 116 to 158 kg m− 3 (Fig. 7). The thermal conductivity decreases with increasing density. The lowest value obtained is 38.1 mW m − 1 K − 1 for the quenched sample with a density of 116 kg m−3. The slope of the thermal conductivity decrease with the density is smaller in comparison to the foams prepared by ball-milling the powder mixture [37] and the commercial foam glass [12]. The reasons for such a difference are not known at this point. In the literature only scarce data on the thermal conductivity of foam glasses are available [15,21,37,42]. Our results indicate (Fig. 7) that foam glasses with smaller pores have a slightly lower thermal conductivity in comparison to samples from a previous report [37]. At a density of 150–160 kg m−3 the difference is around 2–3 mW m−1 K−1, while at lower densities the difference diminishes. However, another study on foam glass prepared under similar conditions, but with a density above 200 kg m−3, showed that there is no influence of the pore size on the thermal conductivity [26]. A detailed analysis of the thermal conductivity of foam glasses with similar densities and different pore sizes will have to be performed to address this discrepancy. In comparison to commercial products [12], the thermal conductivity of the samples prepared in this study is lower when comparing values for the same densities (Fig. 7). The best commercial foam-glass product has a thermal conductivity of 38 mW m−1 K−1 at a density of 100 kg m−3, while in this study the same thermal conductivity is obtained at a density of 116 kg m−3. Moreover, the prepared foam glasses have a high degree of open porosity. This shows that the potential for a further decrease in the thermal conductivity lies in optimizing the closed porosity and the pore-gas composition. The thermal conductivity is often expressed as a linear contribution from the solid and gas conductivities [31,32]. Thereafter, a decrease in the gas conductivity could be significant if the air was to be exchanged with CO2. However, a more detailed analysis of the heat transfer in foam glasses needs to be performed to evaluate the contributions of the solid and gaseous conductivity to the overall thermal conductivity. The most important parameter in order to achieve CO2-filled pores is the use of an oxygen-free atmosphere during the heat treatment in order to prevent the premature oxidation of the carbon. Despite a low thermal conductivity of the prepared foam glass, its use in thermal insulation is limited to indoor applications as the open-pore structure is vulnerable to water penetration and freeze-thaw cycling. However, due to the open pore structure it can be used effectively as an acoustic insulator. 4. Conclusions Foaming of mixtures of CRT panel glass, carbon and MnO2 is possible when a particle size of ≤ 103 μm is used, while the foaming is almost completely halted in 196-μm powders (D50). The major part of the foaming results from the reduction of manganese, while the contribution of the carbon is negligible. The apparent density of the obtained foam glasses is below 150 kg m− 3 when a particle size of ≤ 33 μm is used (D50). All the foams are homogeneous and possess very small pores (the majority smaller than 0.5 mm), except for the foam prepared
from the finest powder (13 μm, D50) foamed at 835 °C for 30 min, for which the pore size is in the range of 1–3 mm. However, the pore size remains in the range 0.1–0.6 mm if the sample is quenched from the foaming to the annealing temperature. The foams with an apparent density of b200 kg m−3 are predominantly open-porous. The coalescence and growth of the pores progresses at similar rates throughout the sample and the connections between the pores are formed after the pore-wall thickness is reduced to below 100 nm. The compressive strength of the foam glass is 0.48 MPa at a density of 144 kg m−3, which can potentially be improved by increasing the closed porosity. The foams exhibit a thermal conductivity as low as 38.1 mW m−1 K−1 at a density of 116 kg m−3. Due to the predominantly open-porous structure the foams could be used as indoor thermal and acoustic insulation. However, a great potential exists to further decrease the thermal conductivity of the investigated foam glasses. Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.jnoncrysol.2016.05.021. Acknowledgement We thank L.S. Trankjær, R.E. Gissel and N.J. Klitmøller for preparing the powders, foam glasses, and measuring He-pycnometer and apparent density. We also thank C. Prinds (Danish Technological Institute) for measuring the thermal conductivity and compressive strength of the foam glasses. We thank the Danish National Advanced Technology Foundation for financial support under grant number 012-2011-3. References [1] G. Scarinci, G. Brusatin, E. Bernardo, Glass foams, in: M. Scheffler, P. Colombo (Eds.), Cellular Ceramics, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 2005, pp. 158–176. [2] Foamglas Industrial Insulation Handbook, Pittsburgh Corning Europe, Waterloo, Belgium, 1992. [3] O.V. Kaz'mina, V.I. Vereshchagin, B.S. Semukhin, Structure and strength of foam-glasscrystalline materials produced from a glass granulate, Glas. Phys. Chem. 37 (2011) 371–377. [4] http://www.glapor.com (accessed 20.4.16). [5] O.V. Kaz'mina, V.I. Vereshchagin, A.N. Abiyaka, Prospects for use of finely disperse quartz sands in production of foam-glass crystalline materials, Glas. Ceram. 65 (2008) 319–321 (English translation of Steklo i Keramika). [6] V. Ducman, M. Kovačević, The foaming of waste glass, Key Eng. Mater. 132–136 (1997) 2264–2267. [7] J.P. Wu, A.R. Boccaccini, P.D. Lee, M.J. Kershaw, R.D. Rawlings, Glass ceramic foams from coal ash and waste glass: production and characterisation, Adv. Appl. Ceram. 105 (2006) 32–39. [8] H.R. Fernandes, D.U. Tulyaganov, J.M.F. Ferreira, Preparation and characterization of foams from sheet glass and fly ash using carbonates as foaming agents, Ceram. Int. 35 (2009) 229–235. [9] C. Vancea, I. Lazău, Glass foam from window panes and bottle glass wastes, Cent. Eur. J. Chem. 12 (2014) 804–811. [10] J. García-Ten, A. Saburit, M.J. Orts, E. Bernardo, P. Colombo, Glass foams from oxidation/ reduction reactions using SiC, Si3N4 and AlN powders, Glass Technol. Eur. J. Glass Sci. Technol. Part A 52 (2011) 103–110. [11] A.S. Llaudis, M.J.O. Tari, F.J.G. Ten, E. Bernardo, P. Colombo, Foaming of flat glass cullet using Si3N4 and MnO2 powders, Ceram. Int. 35 (2009) 1953–1959. [12] http://www.foamglas.com (accessed 20.4.16). [13] V.A. Lotov, E.V. Krivenkova, Kinetics of formation of the porous structure in foam glass, Glas. Ceram. 59 (2002) 89–93 (English translation of Steklo i Keramika). [14] E. Bernardo, F. Albertini, Glass foams from dismantled cathode ray tubes, Ceram. Int. 32 (2006) 603–608. [15] H. Hojaji, Development of foam glass structural insulation derived from fly ash, Proc. MRS Fall Meeting, Boston, US-MA, 136 1988, pp. 185–206. [16] A. Steiner, Foam Glass Production From Vitrifies Municipal Waste Fly AshesPhD Thesis Eindhoven University of Technology, Eindhoven, The Netherlands, 2006. [17] H.R. Fernandes, F. Andreola, L. Barbieri, I. Lancellotti, M.J. Pascual, J.M.F. Ferreira, The use of egg shells to produce Cathode Ray Tube (CRT) glass foams, Ceram. Int. 39 (2013) 9071–9078. [18] G. Bayer, Foaming of borosilicate glasses by chemical reactions in the temperature range 950–1150 °C, J. Non-Cryst. Solids 39 (1980) 855–860. [19] Y.A. Spiridonov, L.A. Orlova, Problems of foam glass production, Glas. Ceram. 60 (2003) 313–314 (English translation of Steklo i Keramika). [20] V.E. Manevich, K.Y. Subbotin, Foam glass and problems of energy conservation, Glas. Ceram. 65 (2008) 105–108 (English translation of Steklo i Keramika). [21] S. Köse, G. Bayer, Schaumbildung im System Altglas-SiC und die Eigenschaften derartiger Schaumgläser, Glastech. Ber. 7 (1982) 151–160. [22] E. Bernardo, R. Cedro, M. Florean, S. Hreglich, Reutilization and stabilization of wastes by the production of glass foams, Ceram. Int. 33 (2007) 963–968.
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