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Characterisation of porous glasses prepared from Cathode Ray Tube (CRT)
Powder Technology 162 (2006) 59 – 63 www.elsevier.com/locate/powtec
Characterisation of porous glasses prepared from Cathode Ray Tube (CRT) Franc¸ois Mear a, Pascal Yot a,*, Martine Cambon a, Renaud Caplain b, Michel Ribes a a
Equipe de Recherche Technologique 3 ‘‘Caracte´risation des Mate´riaux’’ Laboratoire de Physicochimie de la Matie`re Condense´e, UMR CNRS 5617, Universite´ Montpellier II, CC 003, Place Euge`ne Bataillon, 34095 Montpellier, Cedex 5, France b Laboratoire des Mate´riaux Mine´raux, CNAM, 292, rue St Martin — 75141 Paris cedex 03, France Received 15 May 2005; received in revised form 4 October 2005
Abstract Display tubes such as those used in TV receivers and computer monitors have an evacuated glass envelope, which consists mainly of a screen (front component) and a funnel (back component hidden inside the TV set or monitor). These two components have different compositions: the screen is composed of lead-free glass with strontium and barium oxides, whereas the funnel is composed of glass with lead oxides. In order to comply with future government measures, a method is required for the recycling or re-utilisation of CRT (Cathode Ray Tube) glasses in end-of-life electronic goods. One open-loop recycling method is to create foam glasses from CRTs using a reducing agent. The results for the chemical compositions of these glasses and their physical properties showed that foam glasses can be prepared from glasses from various CRT glassmakers. In this paper, we use several methods to determine the structures of these foam glasses. We use helium pycnometry, Hg porosimetry, specific surface area measurements and scanning electron microscopy as direct methods for determining foam glass structure. These methods provide information about the morphologies and reactivities of these porous materials. Densities, porosities and pore size distributions were measured, which enable us to suggest some potential applications for the fabricated foam glasses. D 2005 Elsevier B.V. All rights reserved. Keywords: Specific surface area; Porosity; Pore size distribution; Microstructure; Foam-glass
1. Introduction Screen and funnel glasses are chemically very different [1– 3] and their chemical compositions can differ from one glassmaker to another. This represents a problem for recycling methods that require very tight material specifications [4,5]. Results for goods collected at dismantling sites show that the great majority of screen and funnel glass is made in Asia [6]. To determine a satisfactory recycling method, the compositions and properties of CRT glasses, which are the raw materials for an ‘‘open loop’’ solution, must be investigated. To determine the glass compositions of a wide range of CRT samples, a Castaing microprobe analysis was performed [7]. Although experimental results confirm that the compositions of colour CRTs differ significantly from those of black and white
* Corresponding author. Tel.: +33 4 67 14 32 94, +33 4 67 14 33 89; fax: +33 4 67 14 42 90. E-mail address: [email protected] (P. Yot). 0032-5910/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2005.12.003
CRTs [2,3], the oxide compositions of colour CRTs are very similar for different glassmakers. Two parameters are very important in glass processing: the vitreous transition temperature and the thermal expansion coefficient [8]. Both of these parameters depend on the oxide composition [9]; surprisingly, however, the values of these parameters do not vary significantly among glass samples from different monitor types, including both colour and black and white monitors. A summary of these properties [3] is given in Table 1. These results indicate that foam glass samples can be prepared without taking the origins of each CRT into account [10,11]. The structures of porous materials [12] are not very well understood, and so an appropriate analysis of foam glass Table 1 Properties of bulk glasses
q [gIcm 3] a [150 – 350 -C] [I10 T g [-C]
6
K
1
]
Screen colour glass
Funnel colour glass
Black and white glass
2.77 10.5 519
3.00 10.5 483
2.68 10.5 472
F. Mear et al. / Powder Technology 162 (2006) 59 – 63
Table 2 Sample compositions and oxide / reducing agent ratios Sample
Glass
Reducing agent
Oxide agent
%ox. / %red.
S1 S2 S3 T1 T2 T3
1/3 – 2/3 1/3 – 2/3
3.34 5.00 6.68 2.66 4.00 5.32
2.00 3.00 4.00 2.00 3.00 4.00
0.60 0.60 0.60 0.75 0.75 0.75
funnel screen funnel screen
wt.% wt.% wt.% wt.% wt.% wt.%
SiC SiC SiC TiN TiN TiN
wt.% wt.% wt.% wt.% wt.% wt.%
MgO MgO MgO MgO MgO MgO
samples should include an analysis of pore size distribution, pore morphology, total porosity and total accessible surface area. The importance of these characteristics will vary depending on the applications of the materials. Mercury intrusion porosimetry is frequently used to estimate the pore size distributions of foam glasses. Their morphologies can be examined using scanning electron microscopy in order to interpret the observed pore size distributions. In order to determine the specific surface areas of these porous materials, the gas adsorption method was used for the determination of adsorption isotherms of krypton at 77 K. A comparison of the BET and Langmuir surface areas shows that the theory of Langmuir applies to our samples: better values of the linear correlation coefficient are obtained when the Kr isotherms are fitted with the Langmuir model. 2. Experimental Various mixtures of glass powders, consisting of funnel or screen or a 2 : 1 ratio of screen to funnel glass by weight, were prepared with reducing agents. Powders with defined granulometry were mixed with a binder, and prepared by uniaxial dry-pressing into disc sharps. Expanded products in pebble form were obtained by heating at about 800 -C for 60 min. To determine the influence on the expansion process of parameters such as the kind and quantity of the reducing agent [13], samples elaborated using several different processing conditions were characterised. Only structural and morphological aspects of these materials were considered in this analysis. The samples were characterised by measuring density, porosity, and pore size with an Hg porosimeter (Micromeritics Autopore II 9220). The microstructures were examined with scanning electron microscopy (HITACHI S-4500 I). In addition, specific surface area measurements were made using Table 3 Bulk density results obtained with helium pycnometry, and skeletal density and open porosity results obtained with mercury porosimetry Sample
Bulk density
Skeletal density
Open porosity (%)
S1 S2 S3 T1 T2 T3
0.76 0.99 0.48 0.70 0.53 0.51
2.60 2.71 2.55 2.65 2.53 2.53
70.7 63.6 81.0 73.7 79.2 79.7
Table 4 Closed porosity results (determined as the differences between the total and open porosities) Sample
Total porosity (%)
Open porosity (%)
Closed porosity (%)
S1 S2 S3 T1 T2 T3
73.4 65.1 82.5 74.7 80.5 80.6
70.7 63.6 81.0 73.7 79.2 79.7
2.7 1.5 1.5 1.0 1.3 0.9
BET equipment (Micromeritics ASAP 2000) to further elucidate the structures of the foam glass samples. 3. Results and discussion In foam glass processing, the CRT glass powder is mixed with a reducing agent (nitride or carbide) and a stabilising oxide [14 – 16]. Two different reducing agents were used, TiN or SiC, in amounts of 4 and 5 wt.%, respectively; the stabilising oxide, magnesium oxide, reacts with the reducing agent and produces the gas that results in the cellular structure [16]. To control the foam glass processing and consequently the sample densities are studied whereas the quantities of each component varied (Table 2). To determine the amount of oxide necessary to produce the reaction responsible for the expansion phenomena, the SiC / MgO ratio was maintained at 0.6, and the TiN / MgO ratio was maintained at 0.75. 3.1. Porosity and pore size distributions Mercury porosimetry and helium pycnometry are convenient methods for measuring the porosities of porous materials. The results for the true densities, skeletal densities and open porosities are given in Table 3. The low values of the true densities can be explained by the chemical reactions that occur during the expansion phase. The nitride or carbide introduced into the sample at the beginning of the process reacts with the glass mixture, reducing some of the oxides with nitrogen or 1.4 log differential intrusion (ml/g)
60
1.2
S1 S2 S3
1.0 0.8 0.6 0.4 0.2 0.0 500
100
10 1 Pore diameter (µm)
0.1
0.01
Fig. 1. Pore size distributions obtained for the samples elaborated with SiC reducing agent.
F. Mear et al. / Powder Technology 162 (2006) 59 – 63
61
log differential intrusion (ml/g)
1.6 1.4
T1 T2 T3
1.2 1.0 0.8 0.6 0.4 0.2 0.0 500
100
10 1 Pore diameter (µm)
0.1
0.01
Fig. 2. Pore size distributions obtained for the samples elaborated with TiN reducing agent.
carbon dioxide emission. The emitted gas creates pores in the expanded sample. Open porosity (also called Effective porosity) refers to the fraction of the total volume in which fluid flow is effectively taking place; this excludes dead-end pores or non-connected cavities, also called Closed porosity. The results for the total and open porosities determined with the two techniques are presented in Table 4. In each case (i.e., the use of SiC or TiN reducing agent), the total porosity values tend towards 80%. The closed porosity values calculated from the total and open porosities are rather low in both cases: 0.9– 2.7%. The pore size distributions of the disc-shaped samples are shown in Figs. 1 and 2. The properties of the samples reduced with SiC reducing agent were found to be vary significantly. The S1 and S2 porous microstructures are composed of pores with an average size of 0.5 Am. In the S2 sample, the size distribution seems to be narrow: a homogeneous region of the sample is displayed. On the other hand, the S3 porous material has three pore size distributions: one about 100 Am, another about 3 Am, and the last one about 0.3 Am. A high amount of SiC tends to produce a heterogeneous microstructure which is evidenced in Fig. 1 by
Fig. 4. SEM micrograph of a foam glass elaborated with TiN, magnification of 130.
the increase of the number of the pore size distribution and the pore size diameter. In the samples reduced with TiN reducing agent, variation in the range of the smaller pore diameters with the amount of TiN was observed: a smaller amount of TiN (T1) results in a larger average pore size (0.3 Am). Note also that the nature of the emitted gas seems to influence the pore diameter distribution. According to the IUPAC classification [17], all our samples are macroporous materials (pore diameter > 50 nm). 3.2. Microstructural characterisation The SEM visual examination enabled the description of the samples in terms of their microstructural parameters [18] such as pore distributions. Figs. 3 and 4 for the S1 and S2 samples show that increases in the amount of SiC result in slight increases in the pore sizes. The high value of the total porosity and the presence of two pore size distributions (as indicated by the mercury porosimetry results) are confirmed by the morphological features of the SEM micrographs. A micropore field (appearing as dark zones) seems to be superposed on the macropores. The dark 0.09
3
-1
Adsorbed volume (cm .g )
0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
-1
Fig. 3. SEM micrograph of a foam glass elaborated with SiC, magnification of 130.
Relative pressure (P.P0 )
Fig. 5. Kr adsorption isotherm (at T = 77.3 K) of a SiC foam glass sample.
62
F. Mear et al. / Powder Technology 162 (2006) 59 – 63
the isotherms can be classified as type II according to the IUPAC classification [23]. This kind of isotherm arises when at relatively low pressures a monolayer of krypton is formed and at relatively high pressures a krypton multilayer is formed. This isotherm is typical of macroporous materials. To estimate the surface areas of our samples, the BET and Langmuir models were applied to the P / Po ranges of the Kr isotherms [24]: the calculated S L and S BET are presented in Table 5. From the calculated values of the linear correlation coefficient (Table 5), it can be seen that the Kr isotherms are better fitted with the Langmuir model. The theory of Langmuir is based on a kinetic model of the adsorption process in which it is considered that adsorption is restricted to a single monolayer, which is more suitable for our macroporous materials.
0.08
3
-1
Adsorbed volume (cm .g )
0.09
0.07 0.06 0.05 0.04 0.03 0.02 0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
-1
Relative pressure (P.P0 )
Fig. 6. Kr adsorption isotherm (at T = 77.3 K) of a TiN foam glass sample.
areas in fact correspond to connecting ‘‘windows’’ between the connecting cells [19 – 21]. They are represented in Figs. 1 and 2 by a rather narrow distribution of pore sizes. The pore sizes measured with mercury porosimetry are not the true sizes of the pores but of their interconnections [19 –21]. Moreover, the average pore sizes are significantly smaller than those of the pores observed in the micrographs. The SEM micrographs show the real sizes of the cavities created by the expansion process. The interconnections between the pores are produced by the bursting of ‘‘bubbles’’ created by the emitted gas (N2 or CO2). In the T2 sample (Fig. 4) image, this phenomenon is even more important, as is evident in the pronounced double pore size distribution. The T2 sample contains large pores corresponding to cavities formed by the rapid growth of gas ‘‘bubbles’’ that block the growth of pores with smaller sizes at their interfaces. The use of TiN reducing agent also results in the generation of interconnecting pores. The SEM study indicates the presence of closed pores.
4. Conclusions The results of the present study show that the structures of foam glasses elaborated from CRTs with a reducing agent can be characterised by using several different and independent methods. Some of these techniques can be used to elucidate surface structures, whereas others provide information about the bulk organisation of the porous material. The porosity and pore size distributions were found to vary with the reducing agent and its amount. The SEM observations showed that the narrow pore size distributions measured for each sample in fact describe interconnecting ‘‘channels’’ between larger pores. Macroporous structures were found for samples produced with high amounts of TiN reducing agent. Finally, the Kr adsorption isotherms we obtained showed that the Langmuir model is compatible with the heterogeneity of our macroporous materials. Determinations of the surface areas of the samples were obtained from these results with the Langmuir equation. These techniques were all found to be very useful for characterising the expanded products. Results such as those presented here given information on the principal features of foam glasses, and hence can be used to tailor the foam glass process to obtain a foam glass suitable for a given purpose.
3.3. Surface area determination The BET-nitrogen method for determining specific areas has become established as the standard procedure for porous materials such as micro- or meso-porous materials [22]. However, for a low specific area (< 1 m2/g), the usual methods are insufficiently sensitive. The inert gas krypton, which has a low pressure of saturated vapour ( P 0 = 270 Pa) at 77 K, was used in order to detect the pressure variation due to adsorption. Figs. 5 and 6 present the isotherms of krypton adsorption obtained for the S1 and T2 samples, respectively. Although it was difficult to undertake measurements at high P / Po (> 0.4),
Acknowledgements The work presented in this paper was performed by a technological research team created in 1999 by the Ministry for Research and Technology (ERT), ERT 3 ‘‘Caracte´risation des Mate´riaux.’’ Our industrial partners are IBM France and APF Industries (Montpellier of France).
Table 5 Specific surface areas values obtained with the BET and Langmuir models Sample
BET model Correlation coefficient
S1 T2
0.99718 0.99644
Langmuir model Specific surface area 2
0.31 T 0.01 m Ig 0.39 T 0.01 m2Ig
1 1
Correlation coefficient
Specific surface area
0.99874 0.99877
0.62 T 0.01 m2Ig 0.93 T 0.01 m2Ig
1 1
F. Mear et al. / Powder Technology 162 (2006) 59 – 63
References [1] M. Valache, B. Lafranchi, F. Weber, C. Guy, Proceeding of R’97 Recovery Recycling Re-Integration Congress, Geneva, Switzerland, 1997, pp. 321 – 327. [2] F. Me´ar, P. Yot, M. Cambon, B. Liautard, Verre Review 9 (1) (2003) 33 – 41. [3] F. Me´ar, P. Yot, M. Cambon, M. Ribes, Waste management (in press). [4] N. Menad, Resources, Conservation and Recycling 26 (1999) 143 – 154. [5] P. Yot, M. Assadinik, M. Cambon, B. Liautard, M. Ribes, CNR, IUT, Roanne, 2001, p. 313. [6] L. Ching-Hwa, C. Ssu-Li, W. King-Min, W. Lih-Chyi, Journal of Hazardous Materials A73 (2000) 209. [7] P. Duncumb, Journal of Analytical Atomic Spectrometry 14 (3) (1999) 357. [8] J. Piers, J. Peelen, Glass for has cone of display tubes, display tubes envelope comprising has cone of such has glass and method of manufacturing has cone from such has glass, U.S. Philips Corporation, 1998, US Patent 5725627. [9] H. Scholze, Le verre: nature, structure et proprie´te´s, 2nd edR, Institut du verre, Paris, France, 1980. [10] F. Me´ar, P. Yot, M. Cambon, B. Liautard, Verre Review 9 (2) (2003) 72 – 77. [11] F. Me´ar, P. Yot, M. Cambon, M. Ribes, Advances in Applied Ceramics 104 (3) (2005) 123 – 130. [12] L. Palacio, P. Pradanos, J.I. Calvo, A. Hernandez, Thin Solid Films 348 (1999) 22.
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[13] J.R. Jones, P. Sepulveda, L.L. Hench, Key Engineering Materials 218 – 220 (2002) 299. [14] C. Garnier, P. Roger, Process for making expanded glass products, Isover Saint-Gobain SA, 1981, Patent EP 041003. [15] C. Garnier, P.Verdier, J. Razafindrafindrakoto, Y. Laurent, Manufacturing process of expanded material in the form of granules starting from glass, Cernix SA, 1998, Patent WO 9831639. [16] F. Me´ar, PhD, Thesis, Montpellier University (France), 2004. [17] L. Cot, A. Ayral, J. Durand, C. Guizard, N. Hovnanian, J. Julbe, A. Larbot, Solid State Sciences 2 (2000) 313 – 334. [18] B. Krause, M.E. Boerrigter, N.F.A. Vander Vegt, H. Strathmann, M. Wessling, Journal of Membrane Science 187 (2001) 181. [19] A. Desforges, M. Arpontet, H. Deleuze, O. Mondain-Monval, Reactive and functional Polymers 53 (2002) 183 – 192. [20] A.F. Lemos, J.M.F. Ferreira, Materials Science and Engineering C11 (2000) 35 – 40. [21] A. Mercier, H. Deleuze, O. Mondain-Monval, Reactive and Functional Polymers 46 (2000) 67 – 79. [22] K. Sing, Colloı¨ds and Surfaces, A: Physicochemical and Engineering Aspects 187 – 188 (2001) 3. [23] F. Rouquerol, J. Rouquerol, K. Sing, Adsorption by powders and porous solids, Academic Press, 1999. [24] D. David, R. Caplain, Me´thodes usuelles de caracte´risation des surfaces, Eyrolles, SFM, 1981, p. 290.
Characterisation of porous glasses prepared from Cathode Ray Tube (CRT) Franc¸ois Mear a, Pascal Yot a,*, Martine Cambon a, Renaud Caplain b, Michel Ribes a a
Equipe de Recherche Technologique 3 ‘‘Caracte´risation des Mate´riaux’’ Laboratoire de Physicochimie de la Matie`re Condense´e, UMR CNRS 5617, Universite´ Montpellier II, CC 003, Place Euge`ne Bataillon, 34095 Montpellier, Cedex 5, France b Laboratoire des Mate´riaux Mine´raux, CNAM, 292, rue St Martin — 75141 Paris cedex 03, France Received 15 May 2005; received in revised form 4 October 2005
Abstract Display tubes such as those used in TV receivers and computer monitors have an evacuated glass envelope, which consists mainly of a screen (front component) and a funnel (back component hidden inside the TV set or monitor). These two components have different compositions: the screen is composed of lead-free glass with strontium and barium oxides, whereas the funnel is composed of glass with lead oxides. In order to comply with future government measures, a method is required for the recycling or re-utilisation of CRT (Cathode Ray Tube) glasses in end-of-life electronic goods. One open-loop recycling method is to create foam glasses from CRTs using a reducing agent. The results for the chemical compositions of these glasses and their physical properties showed that foam glasses can be prepared from glasses from various CRT glassmakers. In this paper, we use several methods to determine the structures of these foam glasses. We use helium pycnometry, Hg porosimetry, specific surface area measurements and scanning electron microscopy as direct methods for determining foam glass structure. These methods provide information about the morphologies and reactivities of these porous materials. Densities, porosities and pore size distributions were measured, which enable us to suggest some potential applications for the fabricated foam glasses. D 2005 Elsevier B.V. All rights reserved. Keywords: Specific surface area; Porosity; Pore size distribution; Microstructure; Foam-glass
1. Introduction Screen and funnel glasses are chemically very different [1– 3] and their chemical compositions can differ from one glassmaker to another. This represents a problem for recycling methods that require very tight material specifications [4,5]. Results for goods collected at dismantling sites show that the great majority of screen and funnel glass is made in Asia [6]. To determine a satisfactory recycling method, the compositions and properties of CRT glasses, which are the raw materials for an ‘‘open loop’’ solution, must be investigated. To determine the glass compositions of a wide range of CRT samples, a Castaing microprobe analysis was performed [7]. Although experimental results confirm that the compositions of colour CRTs differ significantly from those of black and white
* Corresponding author. Tel.: +33 4 67 14 32 94, +33 4 67 14 33 89; fax: +33 4 67 14 42 90. E-mail address: [email protected] (P. Yot). 0032-5910/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2005.12.003
CRTs [2,3], the oxide compositions of colour CRTs are very similar for different glassmakers. Two parameters are very important in glass processing: the vitreous transition temperature and the thermal expansion coefficient [8]. Both of these parameters depend on the oxide composition [9]; surprisingly, however, the values of these parameters do not vary significantly among glass samples from different monitor types, including both colour and black and white monitors. A summary of these properties [3] is given in Table 1. These results indicate that foam glass samples can be prepared without taking the origins of each CRT into account [10,11]. The structures of porous materials [12] are not very well understood, and so an appropriate analysis of foam glass Table 1 Properties of bulk glasses
q [gIcm 3] a [150 – 350 -C] [I10 T g [-C]
6
K
1
]
Screen colour glass
Funnel colour glass
Black and white glass
2.77 10.5 519
3.00 10.5 483
2.68 10.5 472
F. Mear et al. / Powder Technology 162 (2006) 59 – 63
Table 2 Sample compositions and oxide / reducing agent ratios Sample
Glass
Reducing agent
Oxide agent
%ox. / %red.
S1 S2 S3 T1 T2 T3
1/3 – 2/3 1/3 – 2/3
3.34 5.00 6.68 2.66 4.00 5.32
2.00 3.00 4.00 2.00 3.00 4.00
0.60 0.60 0.60 0.75 0.75 0.75
funnel screen funnel screen
wt.% wt.% wt.% wt.% wt.% wt.%
SiC SiC SiC TiN TiN TiN
wt.% wt.% wt.% wt.% wt.% wt.%
MgO MgO MgO MgO MgO MgO
samples should include an analysis of pore size distribution, pore morphology, total porosity and total accessible surface area. The importance of these characteristics will vary depending on the applications of the materials. Mercury intrusion porosimetry is frequently used to estimate the pore size distributions of foam glasses. Their morphologies can be examined using scanning electron microscopy in order to interpret the observed pore size distributions. In order to determine the specific surface areas of these porous materials, the gas adsorption method was used for the determination of adsorption isotherms of krypton at 77 K. A comparison of the BET and Langmuir surface areas shows that the theory of Langmuir applies to our samples: better values of the linear correlation coefficient are obtained when the Kr isotherms are fitted with the Langmuir model. 2. Experimental Various mixtures of glass powders, consisting of funnel or screen or a 2 : 1 ratio of screen to funnel glass by weight, were prepared with reducing agents. Powders with defined granulometry were mixed with a binder, and prepared by uniaxial dry-pressing into disc sharps. Expanded products in pebble form were obtained by heating at about 800 -C for 60 min. To determine the influence on the expansion process of parameters such as the kind and quantity of the reducing agent [13], samples elaborated using several different processing conditions were characterised. Only structural and morphological aspects of these materials were considered in this analysis. The samples were characterised by measuring density, porosity, and pore size with an Hg porosimeter (Micromeritics Autopore II 9220). The microstructures were examined with scanning electron microscopy (HITACHI S-4500 I). In addition, specific surface area measurements were made using Table 3 Bulk density results obtained with helium pycnometry, and skeletal density and open porosity results obtained with mercury porosimetry Sample
Bulk density
Skeletal density
Open porosity (%)
S1 S2 S3 T1 T2 T3
0.76 0.99 0.48 0.70 0.53 0.51
2.60 2.71 2.55 2.65 2.53 2.53
70.7 63.6 81.0 73.7 79.2 79.7
Table 4 Closed porosity results (determined as the differences between the total and open porosities) Sample
Total porosity (%)
Open porosity (%)
Closed porosity (%)
S1 S2 S3 T1 T2 T3
73.4 65.1 82.5 74.7 80.5 80.6
70.7 63.6 81.0 73.7 79.2 79.7
2.7 1.5 1.5 1.0 1.3 0.9
BET equipment (Micromeritics ASAP 2000) to further elucidate the structures of the foam glass samples. 3. Results and discussion In foam glass processing, the CRT glass powder is mixed with a reducing agent (nitride or carbide) and a stabilising oxide [14 – 16]. Two different reducing agents were used, TiN or SiC, in amounts of 4 and 5 wt.%, respectively; the stabilising oxide, magnesium oxide, reacts with the reducing agent and produces the gas that results in the cellular structure [16]. To control the foam glass processing and consequently the sample densities are studied whereas the quantities of each component varied (Table 2). To determine the amount of oxide necessary to produce the reaction responsible for the expansion phenomena, the SiC / MgO ratio was maintained at 0.6, and the TiN / MgO ratio was maintained at 0.75. 3.1. Porosity and pore size distributions Mercury porosimetry and helium pycnometry are convenient methods for measuring the porosities of porous materials. The results for the true densities, skeletal densities and open porosities are given in Table 3. The low values of the true densities can be explained by the chemical reactions that occur during the expansion phase. The nitride or carbide introduced into the sample at the beginning of the process reacts with the glass mixture, reducing some of the oxides with nitrogen or 1.4 log differential intrusion (ml/g)
60
1.2
S1 S2 S3
1.0 0.8 0.6 0.4 0.2 0.0 500
100
10 1 Pore diameter (µm)
0.1
0.01
Fig. 1. Pore size distributions obtained for the samples elaborated with SiC reducing agent.
F. Mear et al. / Powder Technology 162 (2006) 59 – 63
61
log differential intrusion (ml/g)
1.6 1.4
T1 T2 T3
1.2 1.0 0.8 0.6 0.4 0.2 0.0 500
100
10 1 Pore diameter (µm)
0.1
0.01
Fig. 2. Pore size distributions obtained for the samples elaborated with TiN reducing agent.
carbon dioxide emission. The emitted gas creates pores in the expanded sample. Open porosity (also called Effective porosity) refers to the fraction of the total volume in which fluid flow is effectively taking place; this excludes dead-end pores or non-connected cavities, also called Closed porosity. The results for the total and open porosities determined with the two techniques are presented in Table 4. In each case (i.e., the use of SiC or TiN reducing agent), the total porosity values tend towards 80%. The closed porosity values calculated from the total and open porosities are rather low in both cases: 0.9– 2.7%. The pore size distributions of the disc-shaped samples are shown in Figs. 1 and 2. The properties of the samples reduced with SiC reducing agent were found to be vary significantly. The S1 and S2 porous microstructures are composed of pores with an average size of 0.5 Am. In the S2 sample, the size distribution seems to be narrow: a homogeneous region of the sample is displayed. On the other hand, the S3 porous material has three pore size distributions: one about 100 Am, another about 3 Am, and the last one about 0.3 Am. A high amount of SiC tends to produce a heterogeneous microstructure which is evidenced in Fig. 1 by
Fig. 4. SEM micrograph of a foam glass elaborated with TiN, magnification of 130.
the increase of the number of the pore size distribution and the pore size diameter. In the samples reduced with TiN reducing agent, variation in the range of the smaller pore diameters with the amount of TiN was observed: a smaller amount of TiN (T1) results in a larger average pore size (0.3 Am). Note also that the nature of the emitted gas seems to influence the pore diameter distribution. According to the IUPAC classification [17], all our samples are macroporous materials (pore diameter > 50 nm). 3.2. Microstructural characterisation The SEM visual examination enabled the description of the samples in terms of their microstructural parameters [18] such as pore distributions. Figs. 3 and 4 for the S1 and S2 samples show that increases in the amount of SiC result in slight increases in the pore sizes. The high value of the total porosity and the presence of two pore size distributions (as indicated by the mercury porosimetry results) are confirmed by the morphological features of the SEM micrographs. A micropore field (appearing as dark zones) seems to be superposed on the macropores. The dark 0.09
3
-1
Adsorbed volume (cm .g )
0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
-1
Fig. 3. SEM micrograph of a foam glass elaborated with SiC, magnification of 130.
Relative pressure (P.P0 )
Fig. 5. Kr adsorption isotherm (at T = 77.3 K) of a SiC foam glass sample.
62
F. Mear et al. / Powder Technology 162 (2006) 59 – 63
the isotherms can be classified as type II according to the IUPAC classification [23]. This kind of isotherm arises when at relatively low pressures a monolayer of krypton is formed and at relatively high pressures a krypton multilayer is formed. This isotherm is typical of macroporous materials. To estimate the surface areas of our samples, the BET and Langmuir models were applied to the P / Po ranges of the Kr isotherms [24]: the calculated S L and S BET are presented in Table 5. From the calculated values of the linear correlation coefficient (Table 5), it can be seen that the Kr isotherms are better fitted with the Langmuir model. The theory of Langmuir is based on a kinetic model of the adsorption process in which it is considered that adsorption is restricted to a single monolayer, which is more suitable for our macroporous materials.
0.08
3
-1
Adsorbed volume (cm .g )
0.09
0.07 0.06 0.05 0.04 0.03 0.02 0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
-1
Relative pressure (P.P0 )
Fig. 6. Kr adsorption isotherm (at T = 77.3 K) of a TiN foam glass sample.
areas in fact correspond to connecting ‘‘windows’’ between the connecting cells [19 – 21]. They are represented in Figs. 1 and 2 by a rather narrow distribution of pore sizes. The pore sizes measured with mercury porosimetry are not the true sizes of the pores but of their interconnections [19 –21]. Moreover, the average pore sizes are significantly smaller than those of the pores observed in the micrographs. The SEM micrographs show the real sizes of the cavities created by the expansion process. The interconnections between the pores are produced by the bursting of ‘‘bubbles’’ created by the emitted gas (N2 or CO2). In the T2 sample (Fig. 4) image, this phenomenon is even more important, as is evident in the pronounced double pore size distribution. The T2 sample contains large pores corresponding to cavities formed by the rapid growth of gas ‘‘bubbles’’ that block the growth of pores with smaller sizes at their interfaces. The use of TiN reducing agent also results in the generation of interconnecting pores. The SEM study indicates the presence of closed pores.
4. Conclusions The results of the present study show that the structures of foam glasses elaborated from CRTs with a reducing agent can be characterised by using several different and independent methods. Some of these techniques can be used to elucidate surface structures, whereas others provide information about the bulk organisation of the porous material. The porosity and pore size distributions were found to vary with the reducing agent and its amount. The SEM observations showed that the narrow pore size distributions measured for each sample in fact describe interconnecting ‘‘channels’’ between larger pores. Macroporous structures were found for samples produced with high amounts of TiN reducing agent. Finally, the Kr adsorption isotherms we obtained showed that the Langmuir model is compatible with the heterogeneity of our macroporous materials. Determinations of the surface areas of the samples were obtained from these results with the Langmuir equation. These techniques were all found to be very useful for characterising the expanded products. Results such as those presented here given information on the principal features of foam glasses, and hence can be used to tailor the foam glass process to obtain a foam glass suitable for a given purpose.
3.3. Surface area determination The BET-nitrogen method for determining specific areas has become established as the standard procedure for porous materials such as micro- or meso-porous materials [22]. However, for a low specific area (< 1 m2/g), the usual methods are insufficiently sensitive. The inert gas krypton, which has a low pressure of saturated vapour ( P 0 = 270 Pa) at 77 K, was used in order to detect the pressure variation due to adsorption. Figs. 5 and 6 present the isotherms of krypton adsorption obtained for the S1 and T2 samples, respectively. Although it was difficult to undertake measurements at high P / Po (> 0.4),
Acknowledgements The work presented in this paper was performed by a technological research team created in 1999 by the Ministry for Research and Technology (ERT), ERT 3 ‘‘Caracte´risation des Mate´riaux.’’ Our industrial partners are IBM France and APF Industries (Montpellier of France).
Table 5 Specific surface areas values obtained with the BET and Langmuir models Sample
BET model Correlation coefficient
S1 T2
0.99718 0.99644
Langmuir model Specific surface area 2
0.31 T 0.01 m Ig 0.39 T 0.01 m2Ig
1 1
Correlation coefficient
Specific surface area
0.99874 0.99877
0.62 T 0.01 m2Ig 0.93 T 0.01 m2Ig
1 1
F. Mear et al. / Powder Technology 162 (2006) 59 – 63
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