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Orthoboric acid as a densifying agent for graphitized carbon black
Materials Research Bulletin 38 (2003) 125±131
Orthoboric acid as a densifying agent for graphitized carbon black Jurgis Barkauskas*, Aivaras Kareiva Department of General and Inorganic Chemistry, Vilnius University, Naugarduko 24, LT-2006 Vilnius, Lithuania Received 16 January 2002; received in revised form 15 May 2002; accepted 4 October 2002
Abstract An effect of lubricant on densifying the form of graphitized carbon black powder was investigated, using orthoboric acid as a densifying agent. The carbon black was synthesized from CO in Boudouard's reaction with Fe catalyst. The iron contamination was removed by means of hot extraction before pressing the carbon black powder into a pressed form. The density of the form was studied by means of densimetric and conductometric measurements. On the basis of these results a friction and lubrication model was used to explain the process of densi®cation. The orthoboric acid due to its lubrication properties effectively favors the displacement of pores from the carbon black form. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Composites; Electrical properties; Mechanical properties; Microstructure
1. Introduction Carbonaceous materials have an extensive application in modern technologies due to the exclusive properties in the range of carbon structures. The vast majority of engineering carbons have sp2 type bonding and are related in some way to the structure of graphite. In the c-direction the bonding of graphite is of van der Waals character with the result that graphite is highly anisotropic in its properties and is probably unique in showing both the highest and the lowest bond strengths in different directions in the same crystal. The microstructures/macrostructures created in the early stages of the formation process are ``locked'' into the material and, since the formation involves decomposition with the evolution of volatile fragments, the products in most cases are porous. In the ®eld of engineering of carbonaceous materials controlling and exploiting of the microporous structure, the microcrystal orientation and the size distribution are among the top priority tasks. In some composite materials (e.g. ®brous carbon±carbon composites) Young's moduli are approaching the theoretical value of basal plane (1020 GPa), whereas estimates of the Young's moduli of pore-free graphites with randomly oriented * Corresponding author. Tel.: 370-2-336214; fax: 370-2-330987. E-mail address: [email protected] (J. Barkauskas).
0025-5408/02/$ ± see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 5 - 5 4 0 8 ( 0 2 ) 0 0 9 8 6 - 8
126
J. Barkauskas, A. Kareiva / Materials Research Bulletin 38 (2003) 125±131
crystallites give values of about 30 GPa. Porous carbonaceous composite materials display strengths in the region of 10±30 MPa [1]. Therefore, the problem of pore elimination is of high importance in the ®eld of engineering for the carbonaceous materials. Two kinds of procedures are the most commonly used for the preparation of a carbon matrix: carbonization (a slow coking of carbonaceous material isolated from the oxidizing environment) and pyrolysis (a deposition of carbon from the gaseous phase). During the carbonization the thermal decomposition takes place at 250±500 8C. This process is accompanied by the mass loss, shrinkage and formation of the great amount of pores in the bulk volume. The temperature gradient during carbonization process should be extremely small, especially in the region of decomposition. Quality of the carbonaceous material after a pyrogenetic transformation depends on the sort of the raw material for carbonization as well. In most cases for this purpose resins are used (such as polyacrilonitrile, polybenzimidazole, phenol formaldehyde, epoxy phenol) and mixtures consisting of graphite, coke ®ring, carbon black and/or other carbonaceous material. The best results are obtained using a pitch with high content of carbon (92±95%) [1]. Formation of the porous structure could be prevented by heating and with high pressure. The process of carbonization should be performed several times to avoid the porous structure [2]. Rate of a carbon deposition during the vaporphase pyrolysis is a slow process. In that case big pores in the volume usually are ®lled up incompletely. The thickness of a specimen in this method is limited [3]. Both the carbonization and the pyrolysis are time consuming processes and employ complex equipment as well. An alternative way to make the densi®cation more ef®cient is by applying a lubricating additive [4]. There is evidence in the literature that orthoboric acid is suitable for this purpose [5]. Due to the layer structure of graphite and H3BO3 and the different nature of chemical bond in those two substances, there should be minor interaction between the surfaces of graphite and H3BO3. Therefore, the orthoboric acid is acting as an ef®cient lubricating additive [6]. The aim of this work was evaluation of the impact of H3BO3 addition on the process of the pore displacement from the carbon black form. 2. Experimental The graphitized carbon black was synthesized from CO by Boudouard's reaction with a Fe catalyst. Synthesis was executed in a quartz tube at a temperature of 500 8C [7]. Carbon monoxide was produced by decomposition of formic acid with H3PO4. The CO stream (2 ml/s) was washed blowing it through the bubbles with KOH solution and dried using a silica-gel. The yield of carbon black at those conditions reached 60%. The synthesized product was deeply contaminated (up to 30%) with iron. Boiling it with diluted nitric acid and successive hot extraction with hydrochloric acid vapor in the Soxhlet's apparatus was used for puri®cation of the graphitized carbon black. After extraction the level of contamination by Fe dropped down to 0.3%. The obtained productÐgraphitized carbon blackÐ was a ®ne-grained black electroconductive powder with an apparent bulk density 0.2336 g/cm3. It was constituted of ®ne crystals of graphite [7]. This powder was ground with 0.5% of polyvinyl alcohol, 3% of water and an amount of H3BO3, and pressed into pellets (1 cm in diameter) using a hydraulic press (Mestra P3) with a stainless steel press-mold. The amount of the polyvinyl alcohol was constant for each sample; function of this additive was to ensure an essential mechanical strength for the pellets. Pressure between the plates reached 1:3 106 Pa and molding time lasted 15 min. The porosity of pellets was estimated from the difference between an apparent density and a pycnometric density. The pycnometric density was determined using benzene as an immersion ¯uid [8]. The ®lling degree of
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127
pores was improved by means of vacuum. Particle size distribution of the graphitized carbon black was determined using sedimentation analysis in benzene [9]. Conductometric measurements were carried out by means of Wheatstone bridge [10]. The cell was assembled using Pt electrodes in the quartz tube; during the measurements it was ®lled up with nitrogen. The temperature was registered by means of platinum±platinorhodium thermocouple in the range of 20±900 8C. 3. Results and discussion Results of densimetric measurements are shown in Fig. 1. Small quantities of the orthoboric acid in the mixture (up to 5 mol%) are able effectively to reduce the amount of pores in the pressed form. Larger quantities of H3BO3 (above 10 mol%) are acting less effectively. Graphitized carbon black as well as the graphite itself is known as an effective lubricant. Nevertheless, it is impossible to obtain the dense porousless specimens of this material by means of pressure without using a special additive [6]. The reason for this is both the random arrangement of the carbon black crystals in the molding form and the nonuniform size of these crystals [11]. Measurements of the carbon black synthesized in our laboratory show that the particle size distribution is varying in the range of 0.05±1.0 mm (Fig. 2). We applied a lubrication model for explanation of the effect of H3BO3 addition. Orthoboric acid has a layered structure similar to that of graphite [12]. Nevertheless, different from graphite, orthoboric acid has dominant hydrogen bonding between molecules and layers. During the process of compression the microcrystals in the load are acted upon by friction. The state of
Fig. 1. Speci®c volume of the pressed carbon black pellets: 1, calculated from the apparent density; 2, from the pycnometric measurements; 3, speci®c volume of the pores in the pressed carbon black pellets.
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Fig. 2. Particle size distribution of the graphitized carbon black powder.
the surfaces during the friction depends on the conditions of sliding. After increasing the contact area during the process of friction, the stick-slip effect is observed in the system. That is the main reason for the existence of the residual pore volume after the molding of a load. In this case the friction coef®cient is a half-empirical function: Fo A=Ao 2 11=2 ; (1) W where Fo is a force necessary to move a slider at initial moment, W is a slider's load, Ao is the initial area of contact and A is the same area at the time of measurement. There we can see the friction coef®cient increasing ad in®nitum with increasing of the contact area A. This situation leads to the stick-slip effect between surfaces; consequently, an extremely high load for the formation of pore-free structure is required [13]. Another type of friction occurs between the lubricated surfaces. Extremely low values of the friction coef®cient are observed in the case of the boundary lubrication. According to the model proposed by Bowden and Tabor, the frictional force is determined as: m
F A1 s1 A2 s2 ;
(2)
where A1 is the contact area between two surfaces, A2 is the contact area between two lubricating ®lms, s1 is the resistance for shearing strain of material and s2 is the resistance for shearing strain of lubricant. This model treats the lubricating ®lm as a narrow area that surrounds A1. Under the Adamson's model [13]: tsm ; (3) m p where sm is the modi®ed resistance for shearing strain in condensed lubricant ®lm, t is the thickness of condensed ®lm, p is the surface pressure of the ®lm: a contact area between two surfaces is extremely
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129
small as compared with the same area between two condensed ®lms of the lubricant. An increase in external pressure, primarily effects the condensed ®lm of the lubricant. Under those conditions the resistance for shearing strain of the condensed ®lm is less than that of uncompressed lubricant. The lubricant particles forming a condensed ®lm are lying parallel to the surface. Therefore, the friction coef®cient in case of boundary lubrication is extremely low [13]. Since the H3BO3 molecules are of a tabular shape with prevailing intramolecular hydrogen bonding, they can play the role of a lubricant for the microcrystals of graphitized carbon black. The densimetric measurements allow an explanation of the Bowden±Tabor's model of lubrication as applied for the system at low concentration of orthoboric acid (approximately up to 5 mol%). In this case a very thin ®lm of orthoboric acid on the surface of graphitized carbon black particles is formed. In the process of compression a permanent disruption of such a ®lm is taking place. This leads to the stickslip effect and the de®nite amount of pores left in the volume of carbon black pellets. Since graphite is a soft material, the formation of orthoboric acid inclusions in the process of molding is possible. The mechanism of friction proposed by Adamson could be expected in the case when the amount of orthoboric acid exceeds 5 mol%. A disruption of orthoboric acid ®lm during the rotation of particles is less probable, and as a result, a more dense structure forms. After the amount of the orthoboric acid exceeds 15 mol%, the behavior of the system becomes comparable with that of a concentrated emulsion. The effect of a phase inversion is observed for those compositions (i.e. H3BO3, content >15%) during the molding process. Powders containing 15 mol% and more H3BO3 prepared by grinding are insulators; the molding converts them into conductors. The real contact area between the neighboring graphite particles was determined from conductometric measurements [13]. Two isotherms of the conductivity dependence on amount of H3BO3 are shown in Fig. 3. For all samples the rise of conductivity with elevation of the temperature
Fig. 3. Speci®c conductivity of the pressed carbon black pellets: 1, at 20 8C; 2, at 900 8C.
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was irreversible. At higher temperatures the orthoboric acid looses the water molecules transmuting into boron oxide B2O3 [12]. Those reactions gradually lead to the formation of metaboric acid HBO2: H3 BO3 200 C ! HBO2 500 C ! B2 O3 : Both metaboric acid and boron oxide obtained in this way are forming a glassy structure, which is quite different from the layered structure of H3BO3. The rise of conductivity re¯ects an increasing of the contact area between the neighboring carbon black particles during the thermal treatment. A model of the more detailed microstructure was proposed in accordance with the densimetric measurements, particle size distribution and conductometric measurements (Fig. 4). Conclusions about the contact area between carbon black particles formed in the process of molding were established from conductivity measurements. The hidden area in Adamson's model was determined from the rise of conductivity with elevation of the temperature. This hidden area is conceived to be near each carbon black particle capable of making contact with another under the temperature treatment. For instance, at high temperatures some part of othoboric acid in condensed ®lm could be displaced from the area that seals up graphite. Models are scaled on the basis of the carbon black particle size distribution. In Fig. 4 there are depicted several characteristic situations. For the Bowden±Tabor's model, which is suitable for the low concentrations of the orthoboric acid, the hidden area is decreasing with addition of H3BO3. For the Adamson's model the hidden area is growing with addition of that substance. The real contact area reaches a maximum when the system is in the state of transition between those two models.
Fig. 4. Models of the contact and nearby areas in the sample pellets. Amount of H3BO3 (in mol%): 1, 0.0; 2, 1.3; 3, 2.0; 4, 5.4; 5, 21.5; 6, 33.3.
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4. Conclusions Orthoboric acid is acting as an agent favoring better packing in the process of molding of the carbon black powder. Addition of 5 mol% of H3BO3 bene®ts reducing the pore volume 2.5 times. The role of orthoboric acid as a lubricating additive to the carbon black composition is discussed. For that purpose two models are applied. The Bowden±Tabor's model describes the system containing up to 5 mol% H3BO3. The Adamson's model is suitable for the system with the higher content of H3BO3 (from 5 to 15 mol% of H3BO3). For the amount of the orthoboric acid exceeding 15 mol%, the behavior of the system becomes comparable with the concentrated emulsion. The models, depicting a contact area between the carbon black particles, are proposed in accordance with the data from densimetric, conductometric and particle size distribution measurements. Within those models an existence of the particular hidden contact area is proposed, formed as a result of sliding between the carbon black particles. The orthoboric acid could have a potential use as an additive in the manufacturing of carbon± carbon composites as well as the other products with a dense structure (bars, bearings, electrodes, etc.). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
V.V. Vasilyev, et al. (Eds.), Composite Materials, Mashynostroenye, Moscow, 1990. L.M. Manocha, O.P. Bahl, Carbon 13 (1988) 1. S.-M. Oh, J.-Y. Lee, J. Mater. Sci. Lett. 11 (1987) 1291. Y. Kawano, T. Fukuda, T. Kawarada, I. Mochida, Y. Korai, Carbon 37 (1999) 555. S. Ichida, K. Tsubouchi, J. Jpn. Soc. Tribol. 46 (2001) 469. J.L. Lauer, Tribol. Lett. 7 (1999) 129. G. Brauer (Herausg.), Handbuch der praÈparativen anorganishen Chemie B. 2, Ferdinand Enke Verlag, Stuttgart, 1978. Encyclopedia of Chemical Technology, vol. 4, Willey, New York, 1979. Y.S. Rybakova, Lab Manual for Physical and Colloid Chemistry, Vysshaya Shkola, Moscow, 1989. P. Profos (Herausg.), Handbuch der industriellen Messtechnik, Vulkan Verlag, Essen, 1984. R. Wolf, Design and Control of Structure of Advanced Carbon Materials for Enhanced Performance, Kluwer Academic Publishers/NATO Scientific Affairs Division, Dordrecht, 2001, p. 217. [12] A.F. Wells, Structural Inorganic Chemistry, Clarendon Press, Oxford, 1986. [13] A.W. Adamson, Physical Chemistry of Surfaces, Willey, New York, 1976.
Orthoboric acid as a densifying agent for graphitized carbon black Jurgis Barkauskas*, Aivaras Kareiva Department of General and Inorganic Chemistry, Vilnius University, Naugarduko 24, LT-2006 Vilnius, Lithuania Received 16 January 2002; received in revised form 15 May 2002; accepted 4 October 2002
Abstract An effect of lubricant on densifying the form of graphitized carbon black powder was investigated, using orthoboric acid as a densifying agent. The carbon black was synthesized from CO in Boudouard's reaction with Fe catalyst. The iron contamination was removed by means of hot extraction before pressing the carbon black powder into a pressed form. The density of the form was studied by means of densimetric and conductometric measurements. On the basis of these results a friction and lubrication model was used to explain the process of densi®cation. The orthoboric acid due to its lubrication properties effectively favors the displacement of pores from the carbon black form. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Composites; Electrical properties; Mechanical properties; Microstructure
1. Introduction Carbonaceous materials have an extensive application in modern technologies due to the exclusive properties in the range of carbon structures. The vast majority of engineering carbons have sp2 type bonding and are related in some way to the structure of graphite. In the c-direction the bonding of graphite is of van der Waals character with the result that graphite is highly anisotropic in its properties and is probably unique in showing both the highest and the lowest bond strengths in different directions in the same crystal. The microstructures/macrostructures created in the early stages of the formation process are ``locked'' into the material and, since the formation involves decomposition with the evolution of volatile fragments, the products in most cases are porous. In the ®eld of engineering of carbonaceous materials controlling and exploiting of the microporous structure, the microcrystal orientation and the size distribution are among the top priority tasks. In some composite materials (e.g. ®brous carbon±carbon composites) Young's moduli are approaching the theoretical value of basal plane (1020 GPa), whereas estimates of the Young's moduli of pore-free graphites with randomly oriented * Corresponding author. Tel.: 370-2-336214; fax: 370-2-330987. E-mail address: [email protected] (J. Barkauskas).
0025-5408/02/$ ± see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 5 - 5 4 0 8 ( 0 2 ) 0 0 9 8 6 - 8
126
J. Barkauskas, A. Kareiva / Materials Research Bulletin 38 (2003) 125±131
crystallites give values of about 30 GPa. Porous carbonaceous composite materials display strengths in the region of 10±30 MPa [1]. Therefore, the problem of pore elimination is of high importance in the ®eld of engineering for the carbonaceous materials. Two kinds of procedures are the most commonly used for the preparation of a carbon matrix: carbonization (a slow coking of carbonaceous material isolated from the oxidizing environment) and pyrolysis (a deposition of carbon from the gaseous phase). During the carbonization the thermal decomposition takes place at 250±500 8C. This process is accompanied by the mass loss, shrinkage and formation of the great amount of pores in the bulk volume. The temperature gradient during carbonization process should be extremely small, especially in the region of decomposition. Quality of the carbonaceous material after a pyrogenetic transformation depends on the sort of the raw material for carbonization as well. In most cases for this purpose resins are used (such as polyacrilonitrile, polybenzimidazole, phenol formaldehyde, epoxy phenol) and mixtures consisting of graphite, coke ®ring, carbon black and/or other carbonaceous material. The best results are obtained using a pitch with high content of carbon (92±95%) [1]. Formation of the porous structure could be prevented by heating and with high pressure. The process of carbonization should be performed several times to avoid the porous structure [2]. Rate of a carbon deposition during the vaporphase pyrolysis is a slow process. In that case big pores in the volume usually are ®lled up incompletely. The thickness of a specimen in this method is limited [3]. Both the carbonization and the pyrolysis are time consuming processes and employ complex equipment as well. An alternative way to make the densi®cation more ef®cient is by applying a lubricating additive [4]. There is evidence in the literature that orthoboric acid is suitable for this purpose [5]. Due to the layer structure of graphite and H3BO3 and the different nature of chemical bond in those two substances, there should be minor interaction between the surfaces of graphite and H3BO3. Therefore, the orthoboric acid is acting as an ef®cient lubricating additive [6]. The aim of this work was evaluation of the impact of H3BO3 addition on the process of the pore displacement from the carbon black form. 2. Experimental The graphitized carbon black was synthesized from CO by Boudouard's reaction with a Fe catalyst. Synthesis was executed in a quartz tube at a temperature of 500 8C [7]. Carbon monoxide was produced by decomposition of formic acid with H3PO4. The CO stream (2 ml/s) was washed blowing it through the bubbles with KOH solution and dried using a silica-gel. The yield of carbon black at those conditions reached 60%. The synthesized product was deeply contaminated (up to 30%) with iron. Boiling it with diluted nitric acid and successive hot extraction with hydrochloric acid vapor in the Soxhlet's apparatus was used for puri®cation of the graphitized carbon black. After extraction the level of contamination by Fe dropped down to 0.3%. The obtained productÐgraphitized carbon blackÐ was a ®ne-grained black electroconductive powder with an apparent bulk density 0.2336 g/cm3. It was constituted of ®ne crystals of graphite [7]. This powder was ground with 0.5% of polyvinyl alcohol, 3% of water and an amount of H3BO3, and pressed into pellets (1 cm in diameter) using a hydraulic press (Mestra P3) with a stainless steel press-mold. The amount of the polyvinyl alcohol was constant for each sample; function of this additive was to ensure an essential mechanical strength for the pellets. Pressure between the plates reached 1:3 106 Pa and molding time lasted 15 min. The porosity of pellets was estimated from the difference between an apparent density and a pycnometric density. The pycnometric density was determined using benzene as an immersion ¯uid [8]. The ®lling degree of
J. Barkauskas, A. Kareiva / Materials Research Bulletin 38 (2003) 125±131
127
pores was improved by means of vacuum. Particle size distribution of the graphitized carbon black was determined using sedimentation analysis in benzene [9]. Conductometric measurements were carried out by means of Wheatstone bridge [10]. The cell was assembled using Pt electrodes in the quartz tube; during the measurements it was ®lled up with nitrogen. The temperature was registered by means of platinum±platinorhodium thermocouple in the range of 20±900 8C. 3. Results and discussion Results of densimetric measurements are shown in Fig. 1. Small quantities of the orthoboric acid in the mixture (up to 5 mol%) are able effectively to reduce the amount of pores in the pressed form. Larger quantities of H3BO3 (above 10 mol%) are acting less effectively. Graphitized carbon black as well as the graphite itself is known as an effective lubricant. Nevertheless, it is impossible to obtain the dense porousless specimens of this material by means of pressure without using a special additive [6]. The reason for this is both the random arrangement of the carbon black crystals in the molding form and the nonuniform size of these crystals [11]. Measurements of the carbon black synthesized in our laboratory show that the particle size distribution is varying in the range of 0.05±1.0 mm (Fig. 2). We applied a lubrication model for explanation of the effect of H3BO3 addition. Orthoboric acid has a layered structure similar to that of graphite [12]. Nevertheless, different from graphite, orthoboric acid has dominant hydrogen bonding between molecules and layers. During the process of compression the microcrystals in the load are acted upon by friction. The state of
Fig. 1. Speci®c volume of the pressed carbon black pellets: 1, calculated from the apparent density; 2, from the pycnometric measurements; 3, speci®c volume of the pores in the pressed carbon black pellets.
128
J. Barkauskas, A. Kareiva / Materials Research Bulletin 38 (2003) 125±131
Fig. 2. Particle size distribution of the graphitized carbon black powder.
the surfaces during the friction depends on the conditions of sliding. After increasing the contact area during the process of friction, the stick-slip effect is observed in the system. That is the main reason for the existence of the residual pore volume after the molding of a load. In this case the friction coef®cient is a half-empirical function: Fo A=Ao 2 11=2 ; (1) W where Fo is a force necessary to move a slider at initial moment, W is a slider's load, Ao is the initial area of contact and A is the same area at the time of measurement. There we can see the friction coef®cient increasing ad in®nitum with increasing of the contact area A. This situation leads to the stick-slip effect between surfaces; consequently, an extremely high load for the formation of pore-free structure is required [13]. Another type of friction occurs between the lubricated surfaces. Extremely low values of the friction coef®cient are observed in the case of the boundary lubrication. According to the model proposed by Bowden and Tabor, the frictional force is determined as: m
F A1 s1 A2 s2 ;
(2)
where A1 is the contact area between two surfaces, A2 is the contact area between two lubricating ®lms, s1 is the resistance for shearing strain of material and s2 is the resistance for shearing strain of lubricant. This model treats the lubricating ®lm as a narrow area that surrounds A1. Under the Adamson's model [13]: tsm ; (3) m p where sm is the modi®ed resistance for shearing strain in condensed lubricant ®lm, t is the thickness of condensed ®lm, p is the surface pressure of the ®lm: a contact area between two surfaces is extremely
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129
small as compared with the same area between two condensed ®lms of the lubricant. An increase in external pressure, primarily effects the condensed ®lm of the lubricant. Under those conditions the resistance for shearing strain of the condensed ®lm is less than that of uncompressed lubricant. The lubricant particles forming a condensed ®lm are lying parallel to the surface. Therefore, the friction coef®cient in case of boundary lubrication is extremely low [13]. Since the H3BO3 molecules are of a tabular shape with prevailing intramolecular hydrogen bonding, they can play the role of a lubricant for the microcrystals of graphitized carbon black. The densimetric measurements allow an explanation of the Bowden±Tabor's model of lubrication as applied for the system at low concentration of orthoboric acid (approximately up to 5 mol%). In this case a very thin ®lm of orthoboric acid on the surface of graphitized carbon black particles is formed. In the process of compression a permanent disruption of such a ®lm is taking place. This leads to the stickslip effect and the de®nite amount of pores left in the volume of carbon black pellets. Since graphite is a soft material, the formation of orthoboric acid inclusions in the process of molding is possible. The mechanism of friction proposed by Adamson could be expected in the case when the amount of orthoboric acid exceeds 5 mol%. A disruption of orthoboric acid ®lm during the rotation of particles is less probable, and as a result, a more dense structure forms. After the amount of the orthoboric acid exceeds 15 mol%, the behavior of the system becomes comparable with that of a concentrated emulsion. The effect of a phase inversion is observed for those compositions (i.e. H3BO3, content >15%) during the molding process. Powders containing 15 mol% and more H3BO3 prepared by grinding are insulators; the molding converts them into conductors. The real contact area between the neighboring graphite particles was determined from conductometric measurements [13]. Two isotherms of the conductivity dependence on amount of H3BO3 are shown in Fig. 3. For all samples the rise of conductivity with elevation of the temperature
Fig. 3. Speci®c conductivity of the pressed carbon black pellets: 1, at 20 8C; 2, at 900 8C.
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J. Barkauskas, A. Kareiva / Materials Research Bulletin 38 (2003) 125±131
was irreversible. At higher temperatures the orthoboric acid looses the water molecules transmuting into boron oxide B2O3 [12]. Those reactions gradually lead to the formation of metaboric acid HBO2: H3 BO3 200 C ! HBO2 500 C ! B2 O3 : Both metaboric acid and boron oxide obtained in this way are forming a glassy structure, which is quite different from the layered structure of H3BO3. The rise of conductivity re¯ects an increasing of the contact area between the neighboring carbon black particles during the thermal treatment. A model of the more detailed microstructure was proposed in accordance with the densimetric measurements, particle size distribution and conductometric measurements (Fig. 4). Conclusions about the contact area between carbon black particles formed in the process of molding were established from conductivity measurements. The hidden area in Adamson's model was determined from the rise of conductivity with elevation of the temperature. This hidden area is conceived to be near each carbon black particle capable of making contact with another under the temperature treatment. For instance, at high temperatures some part of othoboric acid in condensed ®lm could be displaced from the area that seals up graphite. Models are scaled on the basis of the carbon black particle size distribution. In Fig. 4 there are depicted several characteristic situations. For the Bowden±Tabor's model, which is suitable for the low concentrations of the orthoboric acid, the hidden area is decreasing with addition of H3BO3. For the Adamson's model the hidden area is growing with addition of that substance. The real contact area reaches a maximum when the system is in the state of transition between those two models.
Fig. 4. Models of the contact and nearby areas in the sample pellets. Amount of H3BO3 (in mol%): 1, 0.0; 2, 1.3; 3, 2.0; 4, 5.4; 5, 21.5; 6, 33.3.
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131
4. Conclusions Orthoboric acid is acting as an agent favoring better packing in the process of molding of the carbon black powder. Addition of 5 mol% of H3BO3 bene®ts reducing the pore volume 2.5 times. The role of orthoboric acid as a lubricating additive to the carbon black composition is discussed. For that purpose two models are applied. The Bowden±Tabor's model describes the system containing up to 5 mol% H3BO3. The Adamson's model is suitable for the system with the higher content of H3BO3 (from 5 to 15 mol% of H3BO3). For the amount of the orthoboric acid exceeding 15 mol%, the behavior of the system becomes comparable with the concentrated emulsion. The models, depicting a contact area between the carbon black particles, are proposed in accordance with the data from densimetric, conductometric and particle size distribution measurements. Within those models an existence of the particular hidden contact area is proposed, formed as a result of sliding between the carbon black particles. The orthoboric acid could have a potential use as an additive in the manufacturing of carbon± carbon composites as well as the other products with a dense structure (bars, bearings, electrodes, etc.). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
V.V. Vasilyev, et al. (Eds.), Composite Materials, Mashynostroenye, Moscow, 1990. L.M. Manocha, O.P. Bahl, Carbon 13 (1988) 1. S.-M. Oh, J.-Y. Lee, J. Mater. Sci. Lett. 11 (1987) 1291. Y. Kawano, T. Fukuda, T. Kawarada, I. Mochida, Y. Korai, Carbon 37 (1999) 555. S. Ichida, K. Tsubouchi, J. Jpn. Soc. Tribol. 46 (2001) 469. J.L. Lauer, Tribol. Lett. 7 (1999) 129. G. Brauer (Herausg.), Handbuch der praÈparativen anorganishen Chemie B. 2, Ferdinand Enke Verlag, Stuttgart, 1978. Encyclopedia of Chemical Technology, vol. 4, Willey, New York, 1979. Y.S. Rybakova, Lab Manual for Physical and Colloid Chemistry, Vysshaya Shkola, Moscow, 1989. P. Profos (Herausg.), Handbuch der industriellen Messtechnik, Vulkan Verlag, Essen, 1984. R. Wolf, Design and Control of Structure of Advanced Carbon Materials for Enhanced Performance, Kluwer Academic Publishers/NATO Scientific Affairs Division, Dordrecht, 2001, p. 217. [12] A.F. Wells, Structural Inorganic Chemistry, Clarendon Press, Oxford, 1986. [13] A.W. Adamson, Physical Chemistry of Surfaces, Willey, New York, 1976.