Advanced Materials Research Vols.105-106 (2010) pp 765-768 Online available since 2010/Apr/15 at www.scientific.net © (2010) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.105-106.765 Effect of Carbon as Foaming Agent on Pore Structure of Foam Glass Dongsheng Lv a, Xiuhua Li, Lei Wang, Juanjuan Du and Jie Zhang Key Laboratory of Advanced Ceramics and Machining Technology of Ministry of Education, Tianjin University, Tianjin 300072, P. R. China a
[email protected] Keywords: Borosilicate foam glass; Foam structure; Foam growth. Abstract. Insulation effect and mechanical performance of foam glass depend, to a large extent, on foam structure. Hence understanding foam formation is not only a problem of significant fundamental interest but also of tremendous practical impact. In this paper, foam growth was modeled comparing to grain growth theory in sintering. TG-DTG analysis of carbon black indicated that pre-oxidation took place prior to foaming temperature. Furthermore effects of heating rate and particle size of carbon black on foam structure have been taken into account. Several borosilicate foam glasses were fabricated by powder sintering process at different heating rates using carbon black of different particle sizes as foaming agent, respectively. It was found that increasing the heating rate tended to decrease the pre-oxidation of carbon black resulting in inhomogeneous foam distribution. Foam structure of sample heated at a rate of 8ºC/min using carbon black with particle size of 0.15mm was optimal. Introduction Production of foam glass is a promising way to recycle fly ash, plate glass waste, cathode-ray tube (CRT) waste glass, etc [1-5], and the essential section we focused on is foaming process. When the temperature of the mixture exceeds the softening temperature, glass particles start sintering and form a continuous sintered body. After a certain temperature is reached, foaming agents start emitting gases frothing the glass melt. Due to gas emission, pores emerge in all parts of the sintered body where the particles of foaming agent were blocked [5,6]. It is generally accepted that foaming agents are of two types: redox and neutralization agents. The former mainly includes carbon and graphite, the latter consists of some sulfates, organic compounds and carbonates such as calcium carbonate, dolomite [5,7]. Sulfates have been less used due to destructive SO2 emission. Moreover reaction temperature of foaming agents should be close to softening temperature of glass [8]. For the borosilicate foam glass with high content of SiO2 in this study, softening temperature was so high [9] that vesicants with low reaction temperature were excluded, and carbon black was lastly chosen. In the work reported here, we investigated the influence of heating rate and particle size of carbon black on foam structure by varying the heating rate between 6 and 10ºC/min and the particle size between 0.074 and 0.5mm. Accordingly, optimal parameters for production of excellent borosilicate foam glass with homogeneous foam distribution and high porosity were obtained. What’s more important is to explore how these factors influence the foam structure. So foam growth process was evaluated comparing with grain growth in sintering, and reaction of carbon black in the foaming process was investigated through TG analysis. Experimental Sample preparation. Batches containing appropriate amounts [10] of SiO2, Al2O3, R2O, B2O3, Sb2O3 and carbon black were well-mixed by wet milling. Carbon black with particle size of 0.5mm, 0.15mm and 0.074mm were employed. After desiccation and dry grinding, batches were ready to be sintered. Heating rates used were 6ºC/min, 8ºC/min and 10ºC/min, with the foaming temperature fixed at 1400ºC and the foaming time fixed at 30 min, respectively. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 128.97.90.221, UCLA EMS Serials, Los Angeles, United States of America-07/05/14,16:34:27) The obvious weight loss happening at about 500 ~ 900ºC was attributed to oxidation of carbon black with release of CO2 and CO.15] and pressure of inner gas became bigger meanwhile. In addition. Thermogravimetric (TG) analysis of carbon black was carried out on a thermal analyzer (STA449C. viscosity of glass melt became lower according to Arrhenius equation [13. so growth became easier. Effects on foam structure of foaming conditions. (1). The bulk density was measured according to Testing Standard of Cellular Glass [11].) could prompt foaming reaction [10.12]. bulk density %Porosity = 1 . The sample was placed in a measurement cell and heated at a rate of 10°C /min to 1500°C. Fig. was obtained in equilibrium conditions mainly by control of considerable temperature schedule and evenly distributed carbon particles. which agreed with the cell model developed by Arefmanesh and Advani [13.766 Chinese Ceramics Communications Characterization methods. In this paper. 2 TG-DTG analysis of carbon black distributed than Fig. Inset was deconvolution of to explain this phenomenon: 1) Carbon powder of upper DTG curve. MnO2. Germany). b) While pores with close sizes met. The carbon reacting with enough oxygen initially generated CO2. Honeycomb foam structure in Fig.14]. they arrayed parallel like counterwork. the glass melt was squeezed and moved.3a whose foam size above was heated at a rate of 10°C/min using α. thus leading to pre-oxidation prior to foaming temperature [3]. the foam growth is similar to grain growth in sintering [3. It must be noted that superbig pore emerged in region where pore size distribution was broad.1a). Japan) and SEM imagines were recorded on a scanning electron microscope (X-650.1b. which was almost ideal. (b): 1400°C Foam growth process. big pores were prone to grow at expense of small ones [16]. big pore surrounded small pore like package. according to the Eq. What’s more. C+O2=CO2↑ (2) 2C+ O2=2CO↑ (3) The processing temperature for carbon black (500 ~ 900ºC) was far lower than the foaming temperature (1400ºC). The total porosity could be obtained from bulk density and powder density using the following Eq. Once adjacent foams interacted. spherical growth was stopped and foam shapes grew to two possible states: a) While big pore encountered small pore. some additives releasing oxygen at high temperature (Sb2O3.1. Japan).2 displays the TG-DTG curves of carbon black. 17]. Foams of Fig. etc. as results of smaller surface energy of big pore. Netzsch. Bubbles grew spherically on their own when they originally generated (Fig.Al2O3 obviously bigger than below. × 100 (1) powder density Fig. Judging from SEM images in Fig. Two models may be used as reference. Optical microphotographs were viewed using an optical microscope (OLYMPUS DP12. Under the force of foam growth. With increasing temperature.3c were more evenly Fig. we suppose the powder densities of samples are same and use bulk density to represent total porosity. Olympus. batch reacting with enough oxygen in pre-oxidation . and then CO was formed subsequently due to reduction of oxygen. (2) and (3). Hitachi.3b and Fig.1 SEM images of samples Results and Discussion sintered at (a): 1200°C. 4c (appro 1. Conclusions Foam growth process was initiatively discussed drawing lessons from well-studied grain growth theory in sintering. evenly distributed carbon black was indispensable to avoid the formation of super-large pore. However. Viscosity of glass and distribution of carbon black were main influencing factors to foam structure. Theoretically.4a) foamed incompletely which can be inferred from the thick bubble wall. carbon black was pre-oxidized prior to foaming temperature. But this phenomenon did not appear in Fig.5mm. So foaming effect of sample (Fig. In terms of foam distribution. Therefore faster heating rate was beneficial to the formation of better foam structure. However.4a and Fig.4a was smallest (appro 0. The foam size in Fig.4 show microstructure of foam glasses fabricated by carbon black of different diameters. which would be a promising way to study the foaming dynamitic. reported by Fedorov.4c.3a. Consequently. We attributed this opposite to the non-uniform temperature field caused by excessive rapid heating rate. 307kg/m3. prior to foaming temperature favored to become larger pore.3c.4b) was best. we should chose carbon black with appropriate particle size which was 0. Since reaction temperature of carbon black was far lower than foaming temperature. Advanced Materials Research Vols.8mm) and biggest in Fig. 105-106 767 Fig.4 SEM images of samples prepared by carbon black with different size: (a) 0. so we supposed that model (1) was reasonable.15mm and (c) 0.4b both distributed uniformly except Fig.3b had a rather narrow pore size distribution compared to Fig. (b) 0. which disagreed with the above conclusion.3mm).3 Optical microscopes of samples sintered at a heating rate of (a) 6ºC/min. According to foam growth process previously discussed.15mm in this research. (b) 8ºC/min and (c) 10ºC/min.3b and Fig.4b (appro 0. which was bigger in Fig. SEM micrographs of Fig. carbon black with smaller size was favorable to foaming because of higher reactivity. But sample (Fig. which was also proved by the minimum bulk density. especially to foam glass. because it could shorten the time of carbon black exposed to air and decrease pre-oxidation in the temperature-rise period. it was practically hard to be dispersed uniformly.074mm. a remarkable thermal insulating material [19-21].5mm).3c whose foaming temperatures were the same as Fig. 355kg/m3. foams of Fig. Fig. 2) It was in accordance with the model [18] that big pores floated up due to the pull of buoyancy at foaming temperature. Although increasing the heating rate was an efficient method to address this pre-oxidation problem leading to an unwished phenomenon that foam size . Fig. And their corresponding bulk densities were 411kg/m3. which would promote “coalescence” at holding temperature. The growth of bubble and viscous flow of vitreous melt happened along the whole foam growth process. R. P. G. Petit: J. Manufacturing. 302. Tan. 603. Viskanta: Glass Sci. Vol. ZÄ Hringer. p. p. 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