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Plasma production of silica-modified graphite fibers
Materials Science and Engineering, A 172 ( 1993) 181 - 187
181
Plasma production of silica-modified graphite fibers Y. Z h a n g a n d C. A. P i c k l e s Department of Materials and Metallurgical Engineering, Queen ~ University, Kingston, Ont., K7L 3N6 (Canada) (Received May 1, 1992: in revised form April 26, 1993)
Abstract In this work, a method of modifying graphite fibers is reported. These decorations were produced by condensing silicon monoxide gas on the fibers. This gas was produced by the reaction of silica and carbon in a plasma furnace. Several different product morphologies were observed. These included spherical and elliptical nodules, whiskers on nodules, whiskers and thin coatings. The diameter of the silica nodules was about 25-40/am. The whiskers were about 5-20/am in length and 0.1-0.5/am in diameter, and the thin coating was about 1-2/am in thickness. The mechanism of formation of these structures on the fibers is discussed. These modified graphite fibers can increase the shear strength and the surface area of the fiber.
1. Introduction Generally, the mechanical properties of fiberreinforced polymeric composites are determined by the properties and the volume fractions of the fiber and the matrix as indicated by the law of mixtures. However, the strength of the interface between the fiber and the matrix also has an important effect on the mechanical strength of the composite. In order to achieve the maximum interlaminar shear strength (ILSS) of the fiber-reinforced polymeric composite, the surface of the fibers must be treated. The most common method of surface treatment of graphite fibers is by applying a chemical coating on the graphite fiber surface [1]. These chemical coatings can increase the bonding force between the fiber and the polymer matrix. Other methods of surface treatment include the deposition of metals [2], silica [3] and silicon [4] onto the graphite fiber. The deposition of these materials onto the graphite fiber increases the mechanical strength and the oxidation resistance of the fiber. Another method of improving the ILSS of the fibers is by producing whiskers on the surface of the fibers. In 1967, Milewski and Shyne [5] grew Si3N4 whiskers on SiC or graphite filaments. The whiskers were produced by heating the filaments with silica under a hydrogencontaining atmosphere to 1400-1450 °C for 1-6 h. The deposition of whiskers onto the fibers increased the ILSS by 200%-400%. Mehan [6] reported the production of uniaxiaHy aligned SiC crystallites on uniaxially aligned carbon fibers. The fabrication technique consisted of infiltrating carbon fibers or fibrous 0921-5093/93/$6.00
materials with liquid silicon at reduced pressure. The subsequent reaction between carbon and liquid silicon forms predominantly r-SiC crystallites of sizes in the range 1-50 ~m. In the present work, a method for producing silica structures on graphite fibers using a plasma furnace was investigated. The process consisted of reacting silica and carbon in the correct proportions to produce silicon monoxide gas. The effects of experimental parameters on the production of silicon monoxide were studied. The optimum operating conditions for the generation of silicon monoxide are reported. The condensed silicon monoxide formed various structures on the fibers. Nodules, whiskers and thin coatings were observed. The mechanism of the formation of these structures on the fibers, and the effects of temperature and surface tension are discussed.
2. Experimental procedure Silicon monoxide gas was generated by the carbothermic reduction of silica in a plasma furnace. A schematic diagram of the plasma arc furnace is shown in Fig. l(a). The furnace could either be operated in the single-phase mode at a power level of 24 kW (115 V, 210 A) or in the three-phase mode at a power level of 47 kW (100 V, 270 A). The AGSR grade graphite electrodes were 20 mm in diameter and about 50 cm in length. The steel furnace shell was lined with magnesia (98% MgO). A water-cooled stainless steel lance could be inserted through the wall of the furnace so that © 1993 - Elsevier Sequoia. All rights reserved
182
Y.Zhang, C. A. Pickles / Silica-modified graphite fibers 2SiO(g) = Si(s)+ SiO2(s)
A
electrical~ em -
The effect of the silica to carbon molar ratio on the amount of briquette consumed is shown in Fig. 2. It is clear that there is a maximum in the briquette consumption when the silica to carbon molar ratio is equal to unity. At this ratio, reaction ( 1 ) would be expected to occur. For ratios greater than 1:1, the amount of carbon added is insufficient to complete the conversion of silica to silicon monoxide. This reduces the weight loss of the sample. This would become more significant as the silica to carbon molar ratio increases. For ratios between 1:1 and 0.5:1, silicon will form according to the reaction
steel shel~
c r) U C ( a
~
electrode
~'INN% ~f" ~
cooling system
MgO lining
gas
SiO2(s or 1) + C(s)= Si(s)+ CO2(g )
inlet
(b)
(3)
(4)
lid
At higher carbon contents, silicon carbide will be produced as follows:
graphite fibres
SiO2(s or 1)+ 3C(s)= SiC(s)+ 2CO(g)
silica and carbon
Fig. 1. Schematicdiagrams of (a) the plasma arc furnace, and (b) the graphite crucible.
various gases, such as argon, could be injected into the arc atmosphere in order to increase the arc stability. The graphite crucible containing the briquette, which consisted of a mixture of silicon dioxide and carbon powder, is shown in Fig. l(b). This crucible was positioned on a graphite pedestal in the plasma arc zone. The graphite fibers were placed above the mixed powder inside the graphite crucible. A lid, containing a number of holes to allow the escape of reaction gases, was placed over the crucible. The experimental details have been described previously [7]. 3. Results and discussion
3.1. Silicon monoxide generation Theoretically, the generation of silicon monoxide involves reacting silica and carbon in the correct proportions to produce the gas according to the following reaction:
SiO2(s or 1)+ C(s)= SiO(g)+ CO(g)
(1)
Then this silicon monoxide gas condenses to silica according to one of the following possible reactions: SiO(g) + CO2(g)= SiO2(s) + CO(g) (2)
(5)
Both of these reactions reduce the amount of silicon monoxide generated. Thus, the optimum molar ratio of silica to carbon for the production of silicon monoxide occurs at 1:1. The effects of briquette weight and crucible weight on the generation of silicon monoxide are shown in Figs. 3 and 4 respectively. An increase in crucible weight in the range 145-200 g resulted in only a relatively minor decrease in the amount of silicon monoxide generated. The briquette weight had a more significant effect on the generation of silicon monoxide than the crucible weight. As expected, the amount of silicon monoxide generated increased with briquette weight. From Fig. 3 it can be seen that the percentage of the theoretical amount of silicon monoxide which could be generated decreased with increasing briquette weight. Figure 5 shows the effect of briquetting pressure on the amount of silicon monoxide generated under standard conditions. Also included is an experiment in which loose powder was used. Increasing the briquetting pressure resulted in a decrease in the amount of silicon monoxide generated. The effects of charcoal size and silica size on the amount of silicon monoxide produced are shown in Fig. 6. In both cases, the amount of silicon monoxide generated is low when the particle size of the reactants is either fine or coarse. There is an optimum particle size of reactants at about 200 mesh. A decrease in the particle size increases the reaction area of the reactants and increases silicon monoxide generation. However, when the particle size is less than a critical value, such as 275-300 mesh, the permeability of the briquette decreases, and it is more difficult for the silicon monoxide gas to escape. Consequently, the partial
Y. Zhang, C. A. Pickles
/
Silica-modified graphite fibers
183
,[
80 7"
~3
60
o Z 0
4C e~
o~t gh
20 e~
0 SILICA T O CARBON MOLAR RATIO
,d
3
o
14
1
35
BR1QUETHNGPRESSURE(MPa)
Fig. 2. The effect of silica to carbon molar ratio on briquette consumption.
Z
7
Fig. 5. The effect of briquetting pressure on the amount of silicon monoxide generated.
/
1 • carbon o
silica
ot~
2
70 ~ Z ul
C
50~ B R I Q U E T T E W E I G H T (g)
Fig. 3. The effect of briquette weight on silicon monoxide generation: • SiO production, • percentage of theoretical SiO.
46o M E S H SIZE
Fig. 6. The effect of charcoal size and silica size on the amount of silicon monoxide generated.
monoxide gas to produce silicon carbide according to the following reaction: O
L
•
2C(s) + SiO(g)= SiC(s)+ CO(gl o eL
C R U C I B L E W E I G H T (g)
Fig. 4. The effect of crucible weight on the amount of silicon monoxide generated.
pressure of silicon monoxide increases in the briquette and the production of silicon monoxide decreases. Also, as the particles b e c o m e smaller there is increased back-reaction of the carbon particles and the silicon
(6)
This reduces the weight loss of the briquette but does not change the actual amount of silicon monoxide produced. From the above results, it is clear that the permeability of the briquette has a significant effect on the generation of silicon monoxide. Large, highly compacted briquettes containing fine particles have a reduced extent of reaction and thus a decreased generation of silicon monoxide. This would indicate that, in these experiments, the outward diffusion of the product gases is rate controlling when the briquette has a low permeability. These results are in agreement with those of Haas and Khalafalla [8] who studied the effect of physical parameters on the reduction of silica with graphite in a vacuum.
184
Y. Zhang, C. A. Pickles / Silica-modified graphite fibers
3.2. Mechanism of formation of the structures When the reactants in a crucible were heated by the plasma arc, the reaction of the silicon dioxide and carbon powder produced silicon monoxide vapor. This vapor passed from the reaction zone to the condensation zone where the graphite fibers were placed. Since the temperature of the graphite fibers was lower than that of the reaction zone, the ascending silicon monoxide gas condensed on the surface of the fibers forming silica whiskers, as shown in Fig. 7. As the time of the plasma treatment increased, and thus the energy consumption, the whiskers fused to form liquid silica. Owing to the surface tension forces, the liquid silica formed nodular shapes on the fibers, as shown in Figs. 8 and 9. The shape of the nodule was affected by temperature since the surface tension of the liquid is a function of temperature, as shown in eqn. (7) [9]: v(M/p) 2/3 = k( T~ - T - 6)
Figure 10 shows multinodule growth on the surface of the graphite fibers. Figure 11 shows a scanning electron micrograph of the silica whiskers which grew on the nodules. The formation of this structure took place in two stages. Firstly, silica nodules formed at high temperatures, as shown in Fig. 8. Secondly, when the power to the furnace was turned off, the temperature of the crucible decreased and thus the silicon monoxide gas condensed on the surface of the nodules and formed silica whiskers. At extended plasma treatment times and thus very high temperatures, the surface tension of the liquid becomes very low. In this case, the liquid silica spreads
(7)
where 7 is surface tension, and (M/p) is the molar volume. The value of k, which is a proportional constant, is found to be approximately 2.1 erg K - 1 for normal liquids. Tc and T are the critical temperature and temperature of the liquid respectively. Equation (7) shows that the surface tension of a liquid decreases with increasing temperature. At low temperatures the liquid silica formed spherical nodules, as shown in Fig. 8, while at higher temperatures elliptical nodules were produced, as shown in Fig. 9. In a similar manner, the diameter of the nodules was also affected by temperature. The liquid silica tended to form many small nodules with a high surface tension at low temperatures. Also, the number of nodules on the fibers depended on the amount of deposited silica. Large amounts of molten silica produced more nodules.
Fig. 8. A scanning electron micrograph of spherical silica nodules on the graphite fibers.
Fig. 7. A scanning electron micrograph of silica whiskers on the graphite fibers,
Fig. 9. A scanning electron micrograph of elliptical silica nodules on the graphite fibers.
Y. Zhang, C. A. Pickles
/
Fig. 10. A scanning electron micrograph of multinodules of silica on the graphite fibers.
Silica-modifiedgraphitefibers
185
Fig. 12. A scanning electron micrograph of silica coating on the graphite fibers.
180 160 140
o, • ~e ~
'Ysv-'Ysl
o
*~ z
<
80-
Z
40-
O
20-
o - - o - 1 0 0 erg/cm 2 e--e-50 erg/cm l ~ - - ~ 50 erg/cm 2 A--A
100
erg/cm 2
°~o~ i
/
~
/ / ,I, ] 00
I 200
I .300
I 400
500
SURFACE TENSION (erg/cm 2)
Fig. 13. Relationship between the contact angle and the surface tension for silica modified graphite fibers. Fig. 11. A scanning electron micrograph whisker-nodules on the graphite fibers.
of
silica
out to form a thin coating on the surface of the fiber, as shown in Fig. 12. It can be seen that the contact angle of the nodules on the surface of the fiber is variable and the nodules have different shapes and sizes. It is known that the contact angle of a liquid is determined by the liquid-vapor surface tension 7kv, solid-vapor surface tension Y~v, and interracial (liquid-solid) surface tension 7~. The relationship between the contact angle 0 and the surface tension is shown in eqn. (8) [10]: 7~v
cos 0 =
7~,, -
7~]
(8)
Figure 13 shows the relationship between the surface tension and the contact angle of a liquid as the value of 7~,.- 7~1 changes. It can be seen that when the
solid-vapor surface tension is greater than the interfacial (liquid-solid) tension, i.e. 75, - 7]~ > 0, the contact angle increases with the surface tension of the liquid. When the solid-vapor surface tension is lower than the interfacial (liquid-solid) tension, i.e. 7~v-y~l<0, the contact angle decreases with the surface tension. From Fig. 8 it can be shown that the contact angle of the spherical nodules which have a high surface tension is in the range 65o-75 °. Similarly, from Figs. 9 and 12, it can be seen that the contact angle of the elliptical nodules is about 30°-40 ° and the contact angle of the silica coating is 0 ° respectively. From Fig. 10, it can be seen that the contact angle of the multinodule is about 125 °, that is the multinodule does not wet the fiber. It is likely that the surface of the fiber was contaminated during the plasma treatment process. The contamination reduced the compatibility of the silica with the fiber surface and increased the contact angle.
186
Y. Zhang, C. A. Pickles / Silica-modified graphitefibers
3.3. Morphology of the silica structure T h e silica structures on the surface of the fibers produced in this work can be classified into three types: whiskers, nodules and coatings. T h e features of these structures are listed in Table 1. It can be seen that the whisker-nodule and whisker modified fibers have the largest surface areas of over 5000 p m 2 per 2 5 - 4 0 / ~ m length of fiber. From Figs. 8 and 13 it can be seen that whiskers which grew on the surface of either the fibers or the nodules were 0.1-0.5 p m in diameter and about 5 - 2 0 p m in length. T h e high aspect ratio (length to diameter) enables whiskers to have the largest surface area. It can also be seen that the whiskers grew in all directions on the fiber. If these fibers were to be employed as reinforcing agents for composites, then this structure would increase the bonding between the fiber and the matrix, and the shear strength of the fiber. As shown in Figs. 8, 9 and 10, the diameters of the spherical nodules were about 2 5 - 3 0 /~, and the long and short diameters of the elliptical nodules were about 40 p m and 25 p m respectively. T h e surface area of the spherical nodule modified fiber was about 1 9 0 0 - 5 0 0 0 / ~ m 2. This value was larger than that of the elliptical nodule modified fibers, which was about 1 5 0 0 - 3 5 0 0 p m z. T h e effect of these nodules on the strength of whiskers has been discussed in a previous publication [11]. T h e relationship between the whisker and nodule geometries and the strength of the whisker containing nodules was investigated theoretically. T h e results showed that the strength and surface area of the whisker were strongly affected by the nodule radius, the whisker radius, and the distance between two nodules. From Fig. 12, it can be seen that the silica coating is abut 1-2 p m thick. T h e surface of the fiber is completely covered by this coating and thus the mechanical strength of the fibers should be improved. T h e silica coating does not increase the surface area of the graphite fiber. T h e fibers with a silica coating had the
smallest surface area among the several types of structures studied in this work of 4 8 0 - 8 5 0 p m 2 per 2 5 - 4 0 /~m length of fiber. Figure 14 shows an optical micrograph of the crosssection of the interface between the silica nodules and the graphite fiber. It can be seen that the bonding between the silica nodules and the graphite fiber is very strong. Even when the graphite fiber was broken, the silica nodule still remained attached to the graphite fiber.
4. Conclusions A method of decorating graphite fibers with silica using a plasma arc furnace was investigated. T h e silica was generated according to the following reaction: Si02(s or 1) + C(s) = SiO(g) + CO(g)
(1)
Fig. 14. An optical micrograph of the cross-section of the interface between the silica nodules and the graphite fiber.
TABLE 1. A comparison of the properties of different silica structures formed on the graphite fibers Structure Spherical nodule Elliptical nodule Whisker-nodule Multinodule Whisker Coating Graphite fiber
Dimension (/am)
Surface area (/am)2
Contact angle (deg)
D, = 25-40 D~= 40, Ds = 25 l = 5-10, Dw = 0.5, Dn = 25 D, = 10-15 l = 5-20, D = 0.5 t = 1-2 D = 5-6
1900-5000 1500-3500 > 5000, 24-40/am 1250-2800 > 5000, 25-40/am 480-850, 25-40/am 390-750, 25-40/am
75 30-40 30-40 125 -180 --
D n nodule diameter, Dt long diameter of elliptical nodule, Ds short diameter of elliptical nodule, Dw whisker diameter, l length, t thickness.
E Zhang, C. A. Pickles
/
Several different p r o d u c t m o r p h o l o g i e s were p r o duced, such as spherical and elliptical nodules, w h i s k e r - n o d u l e s , whiskers and thin coatings. T h e m o r p h o l o g y of the silica structure was strongly affected by the t e m p e r a t u r e of the p l a s m a treatment, and theref o r e the surface tension of the liquid silica. T h e modification with silica of graphite fibers can increase the surface area and the shear strength of the fiber.
References 1 J. B. Donnett and P. Ehrburger, Carbon fiber in polymer reinforcement, Carbon, 15(1977) 143-152. 2 D. W. Makee and V. J. Mimeault, Surface properties of carbon fibers, Chem. Phys. Carbon, 8(1973) 151-153. 3 J. C. Goan and S. P. Porsen, Interfacial bonding in graphite fiber-resin composites, ASTM STP 452, 1969, (ASTM, Philadelphia, PA)pp. 3-5.
Silica-mod([iedgraphitefibers
187
4 J. W. Herrick, R E. Gruber and F. T. Mansur, Surface treatments for fibrous carbon reinforcement, part I, AFML-TR66-178, United Aircraft Research Laboratories, East Hartford, CT, July 1966. 5 J. V. Milewski and J. J. Shyne, Surface treatment of a material to improve its bonding to another material in the formation of composites, French Patent, 1,586,262 (Cl. BOI]), February 13, 1970; US application, September 26, 1967. 6 R.L. Mehan, J. Mater. Sci., 13(1978) 358-366. 7 C. Lelievre, C. A. Pickles, J. M. Toguri and C. Simpson, Plasma production of silica whiskers. In A. J. Plumpton (ed.), lnt. 3~vmp. on the Production and Processing of Fine Particles, Montreal, Que., C1M, Pergamon, 1988, pp. 589-598. 8 L. A. Haas and S. E. Khalafalla, Effect of physical parameters on the reaction of graphite with silica in vacuum, Rep. Investigations, Ni). 7207, 1968 (US Bureau of Mines). 9 V. Fried, H. F. Hameka and U. Blukis, Physical Chemistry, Macmillan, New York, 1977, pp. 171-173. 10 A. P. Levitt and S. M. Wolf, Whisker Technolog% Wiley, New York, 1970, pp. 249-25(I. 11 Y. Zhang, J. Cameron and C. A. Pickles, The strength of nodule-decorated whiskers, J. Mater. Eng. Perform., 1 (3) (1992) 317-322.
181
Plasma production of silica-modified graphite fibers Y. Z h a n g a n d C. A. P i c k l e s Department of Materials and Metallurgical Engineering, Queen ~ University, Kingston, Ont., K7L 3N6 (Canada) (Received May 1, 1992: in revised form April 26, 1993)
Abstract In this work, a method of modifying graphite fibers is reported. These decorations were produced by condensing silicon monoxide gas on the fibers. This gas was produced by the reaction of silica and carbon in a plasma furnace. Several different product morphologies were observed. These included spherical and elliptical nodules, whiskers on nodules, whiskers and thin coatings. The diameter of the silica nodules was about 25-40/am. The whiskers were about 5-20/am in length and 0.1-0.5/am in diameter, and the thin coating was about 1-2/am in thickness. The mechanism of formation of these structures on the fibers is discussed. These modified graphite fibers can increase the shear strength and the surface area of the fiber.
1. Introduction Generally, the mechanical properties of fiberreinforced polymeric composites are determined by the properties and the volume fractions of the fiber and the matrix as indicated by the law of mixtures. However, the strength of the interface between the fiber and the matrix also has an important effect on the mechanical strength of the composite. In order to achieve the maximum interlaminar shear strength (ILSS) of the fiber-reinforced polymeric composite, the surface of the fibers must be treated. The most common method of surface treatment of graphite fibers is by applying a chemical coating on the graphite fiber surface [1]. These chemical coatings can increase the bonding force between the fiber and the polymer matrix. Other methods of surface treatment include the deposition of metals [2], silica [3] and silicon [4] onto the graphite fiber. The deposition of these materials onto the graphite fiber increases the mechanical strength and the oxidation resistance of the fiber. Another method of improving the ILSS of the fibers is by producing whiskers on the surface of the fibers. In 1967, Milewski and Shyne [5] grew Si3N4 whiskers on SiC or graphite filaments. The whiskers were produced by heating the filaments with silica under a hydrogencontaining atmosphere to 1400-1450 °C for 1-6 h. The deposition of whiskers onto the fibers increased the ILSS by 200%-400%. Mehan [6] reported the production of uniaxiaHy aligned SiC crystallites on uniaxially aligned carbon fibers. The fabrication technique consisted of infiltrating carbon fibers or fibrous 0921-5093/93/$6.00
materials with liquid silicon at reduced pressure. The subsequent reaction between carbon and liquid silicon forms predominantly r-SiC crystallites of sizes in the range 1-50 ~m. In the present work, a method for producing silica structures on graphite fibers using a plasma furnace was investigated. The process consisted of reacting silica and carbon in the correct proportions to produce silicon monoxide gas. The effects of experimental parameters on the production of silicon monoxide were studied. The optimum operating conditions for the generation of silicon monoxide are reported. The condensed silicon monoxide formed various structures on the fibers. Nodules, whiskers and thin coatings were observed. The mechanism of the formation of these structures on the fibers, and the effects of temperature and surface tension are discussed.
2. Experimental procedure Silicon monoxide gas was generated by the carbothermic reduction of silica in a plasma furnace. A schematic diagram of the plasma arc furnace is shown in Fig. l(a). The furnace could either be operated in the single-phase mode at a power level of 24 kW (115 V, 210 A) or in the three-phase mode at a power level of 47 kW (100 V, 270 A). The AGSR grade graphite electrodes were 20 mm in diameter and about 50 cm in length. The steel furnace shell was lined with magnesia (98% MgO). A water-cooled stainless steel lance could be inserted through the wall of the furnace so that © 1993 - Elsevier Sequoia. All rights reserved
182
Y.Zhang, C. A. Pickles / Silica-modified graphite fibers 2SiO(g) = Si(s)+ SiO2(s)
A
electrical~ em -
The effect of the silica to carbon molar ratio on the amount of briquette consumed is shown in Fig. 2. It is clear that there is a maximum in the briquette consumption when the silica to carbon molar ratio is equal to unity. At this ratio, reaction ( 1 ) would be expected to occur. For ratios greater than 1:1, the amount of carbon added is insufficient to complete the conversion of silica to silicon monoxide. This reduces the weight loss of the sample. This would become more significant as the silica to carbon molar ratio increases. For ratios between 1:1 and 0.5:1, silicon will form according to the reaction
steel shel~
c r) U C ( a
~
electrode
~'INN% ~f" ~
cooling system
MgO lining
gas
SiO2(s or 1) + C(s)= Si(s)+ CO2(g )
inlet
(b)
(3)
(4)
lid
At higher carbon contents, silicon carbide will be produced as follows:
graphite fibres
SiO2(s or 1)+ 3C(s)= SiC(s)+ 2CO(g)
silica and carbon
Fig. 1. Schematicdiagrams of (a) the plasma arc furnace, and (b) the graphite crucible.
various gases, such as argon, could be injected into the arc atmosphere in order to increase the arc stability. The graphite crucible containing the briquette, which consisted of a mixture of silicon dioxide and carbon powder, is shown in Fig. l(b). This crucible was positioned on a graphite pedestal in the plasma arc zone. The graphite fibers were placed above the mixed powder inside the graphite crucible. A lid, containing a number of holes to allow the escape of reaction gases, was placed over the crucible. The experimental details have been described previously [7]. 3. Results and discussion
3.1. Silicon monoxide generation Theoretically, the generation of silicon monoxide involves reacting silica and carbon in the correct proportions to produce the gas according to the following reaction:
SiO2(s or 1)+ C(s)= SiO(g)+ CO(g)
(1)
Then this silicon monoxide gas condenses to silica according to one of the following possible reactions: SiO(g) + CO2(g)= SiO2(s) + CO(g) (2)
(5)
Both of these reactions reduce the amount of silicon monoxide generated. Thus, the optimum molar ratio of silica to carbon for the production of silicon monoxide occurs at 1:1. The effects of briquette weight and crucible weight on the generation of silicon monoxide are shown in Figs. 3 and 4 respectively. An increase in crucible weight in the range 145-200 g resulted in only a relatively minor decrease in the amount of silicon monoxide generated. The briquette weight had a more significant effect on the generation of silicon monoxide than the crucible weight. As expected, the amount of silicon monoxide generated increased with briquette weight. From Fig. 3 it can be seen that the percentage of the theoretical amount of silicon monoxide which could be generated decreased with increasing briquette weight. Figure 5 shows the effect of briquetting pressure on the amount of silicon monoxide generated under standard conditions. Also included is an experiment in which loose powder was used. Increasing the briquetting pressure resulted in a decrease in the amount of silicon monoxide generated. The effects of charcoal size and silica size on the amount of silicon monoxide produced are shown in Fig. 6. In both cases, the amount of silicon monoxide generated is low when the particle size of the reactants is either fine or coarse. There is an optimum particle size of reactants at about 200 mesh. A decrease in the particle size increases the reaction area of the reactants and increases silicon monoxide generation. However, when the particle size is less than a critical value, such as 275-300 mesh, the permeability of the briquette decreases, and it is more difficult for the silicon monoxide gas to escape. Consequently, the partial
Y. Zhang, C. A. Pickles
/
Silica-modified graphite fibers
183
,[
80 7"
~3
60
o Z 0
4C e~
o~t gh
20 e~
0 SILICA T O CARBON MOLAR RATIO
,d
3
o
14
1
35
BR1QUETHNGPRESSURE(MPa)
Fig. 2. The effect of silica to carbon molar ratio on briquette consumption.
Z
7
Fig. 5. The effect of briquetting pressure on the amount of silicon monoxide generated.
/
1 • carbon o
silica
ot~
2
70 ~ Z ul
C
50~ B R I Q U E T T E W E I G H T (g)
Fig. 3. The effect of briquette weight on silicon monoxide generation: • SiO production, • percentage of theoretical SiO.
46o M E S H SIZE
Fig. 6. The effect of charcoal size and silica size on the amount of silicon monoxide generated.
monoxide gas to produce silicon carbide according to the following reaction: O
L
•
2C(s) + SiO(g)= SiC(s)+ CO(gl o eL
C R U C I B L E W E I G H T (g)
Fig. 4. The effect of crucible weight on the amount of silicon monoxide generated.
pressure of silicon monoxide increases in the briquette and the production of silicon monoxide decreases. Also, as the particles b e c o m e smaller there is increased back-reaction of the carbon particles and the silicon
(6)
This reduces the weight loss of the briquette but does not change the actual amount of silicon monoxide produced. From the above results, it is clear that the permeability of the briquette has a significant effect on the generation of silicon monoxide. Large, highly compacted briquettes containing fine particles have a reduced extent of reaction and thus a decreased generation of silicon monoxide. This would indicate that, in these experiments, the outward diffusion of the product gases is rate controlling when the briquette has a low permeability. These results are in agreement with those of Haas and Khalafalla [8] who studied the effect of physical parameters on the reduction of silica with graphite in a vacuum.
184
Y. Zhang, C. A. Pickles / Silica-modified graphite fibers
3.2. Mechanism of formation of the structures When the reactants in a crucible were heated by the plasma arc, the reaction of the silicon dioxide and carbon powder produced silicon monoxide vapor. This vapor passed from the reaction zone to the condensation zone where the graphite fibers were placed. Since the temperature of the graphite fibers was lower than that of the reaction zone, the ascending silicon monoxide gas condensed on the surface of the fibers forming silica whiskers, as shown in Fig. 7. As the time of the plasma treatment increased, and thus the energy consumption, the whiskers fused to form liquid silica. Owing to the surface tension forces, the liquid silica formed nodular shapes on the fibers, as shown in Figs. 8 and 9. The shape of the nodule was affected by temperature since the surface tension of the liquid is a function of temperature, as shown in eqn. (7) [9]: v(M/p) 2/3 = k( T~ - T - 6)
Figure 10 shows multinodule growth on the surface of the graphite fibers. Figure 11 shows a scanning electron micrograph of the silica whiskers which grew on the nodules. The formation of this structure took place in two stages. Firstly, silica nodules formed at high temperatures, as shown in Fig. 8. Secondly, when the power to the furnace was turned off, the temperature of the crucible decreased and thus the silicon monoxide gas condensed on the surface of the nodules and formed silica whiskers. At extended plasma treatment times and thus very high temperatures, the surface tension of the liquid becomes very low. In this case, the liquid silica spreads
(7)
where 7 is surface tension, and (M/p) is the molar volume. The value of k, which is a proportional constant, is found to be approximately 2.1 erg K - 1 for normal liquids. Tc and T are the critical temperature and temperature of the liquid respectively. Equation (7) shows that the surface tension of a liquid decreases with increasing temperature. At low temperatures the liquid silica formed spherical nodules, as shown in Fig. 8, while at higher temperatures elliptical nodules were produced, as shown in Fig. 9. In a similar manner, the diameter of the nodules was also affected by temperature. The liquid silica tended to form many small nodules with a high surface tension at low temperatures. Also, the number of nodules on the fibers depended on the amount of deposited silica. Large amounts of molten silica produced more nodules.
Fig. 8. A scanning electron micrograph of spherical silica nodules on the graphite fibers.
Fig. 7. A scanning electron micrograph of silica whiskers on the graphite fibers,
Fig. 9. A scanning electron micrograph of elliptical silica nodules on the graphite fibers.
Y. Zhang, C. A. Pickles
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Fig. 10. A scanning electron micrograph of multinodules of silica on the graphite fibers.
Silica-modifiedgraphitefibers
185
Fig. 12. A scanning electron micrograph of silica coating on the graphite fibers.
180 160 140
o, • ~e ~
'Ysv-'Ysl
o
*~ z
<
80-
Z
40-
O
20-
o - - o - 1 0 0 erg/cm 2 e--e-50 erg/cm l ~ - - ~ 50 erg/cm 2 A--A
100
erg/cm 2
°~o~ i
/
~
/ / ,I, ] 00
I 200
I .300
I 400
500
SURFACE TENSION (erg/cm 2)
Fig. 13. Relationship between the contact angle and the surface tension for silica modified graphite fibers. Fig. 11. A scanning electron micrograph whisker-nodules on the graphite fibers.
of
silica
out to form a thin coating on the surface of the fiber, as shown in Fig. 12. It can be seen that the contact angle of the nodules on the surface of the fiber is variable and the nodules have different shapes and sizes. It is known that the contact angle of a liquid is determined by the liquid-vapor surface tension 7kv, solid-vapor surface tension Y~v, and interracial (liquid-solid) surface tension 7~. The relationship between the contact angle 0 and the surface tension is shown in eqn. (8) [10]: 7~v
cos 0 =
7~,, -
7~]
(8)
Figure 13 shows the relationship between the surface tension and the contact angle of a liquid as the value of 7~,.- 7~1 changes. It can be seen that when the
solid-vapor surface tension is greater than the interfacial (liquid-solid) tension, i.e. 75, - 7]~ > 0, the contact angle increases with the surface tension of the liquid. When the solid-vapor surface tension is lower than the interfacial (liquid-solid) tension, i.e. 7~v-y~l<0, the contact angle decreases with the surface tension. From Fig. 8 it can be shown that the contact angle of the spherical nodules which have a high surface tension is in the range 65o-75 °. Similarly, from Figs. 9 and 12, it can be seen that the contact angle of the elliptical nodules is about 30°-40 ° and the contact angle of the silica coating is 0 ° respectively. From Fig. 10, it can be seen that the contact angle of the multinodule is about 125 °, that is the multinodule does not wet the fiber. It is likely that the surface of the fiber was contaminated during the plasma treatment process. The contamination reduced the compatibility of the silica with the fiber surface and increased the contact angle.
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Y. Zhang, C. A. Pickles / Silica-modified graphitefibers
3.3. Morphology of the silica structure T h e silica structures on the surface of the fibers produced in this work can be classified into three types: whiskers, nodules and coatings. T h e features of these structures are listed in Table 1. It can be seen that the whisker-nodule and whisker modified fibers have the largest surface areas of over 5000 p m 2 per 2 5 - 4 0 / ~ m length of fiber. From Figs. 8 and 13 it can be seen that whiskers which grew on the surface of either the fibers or the nodules were 0.1-0.5 p m in diameter and about 5 - 2 0 p m in length. T h e high aspect ratio (length to diameter) enables whiskers to have the largest surface area. It can also be seen that the whiskers grew in all directions on the fiber. If these fibers were to be employed as reinforcing agents for composites, then this structure would increase the bonding between the fiber and the matrix, and the shear strength of the fiber. As shown in Figs. 8, 9 and 10, the diameters of the spherical nodules were about 2 5 - 3 0 /~, and the long and short diameters of the elliptical nodules were about 40 p m and 25 p m respectively. T h e surface area of the spherical nodule modified fiber was about 1 9 0 0 - 5 0 0 0 / ~ m 2. This value was larger than that of the elliptical nodule modified fibers, which was about 1 5 0 0 - 3 5 0 0 p m z. T h e effect of these nodules on the strength of whiskers has been discussed in a previous publication [11]. T h e relationship between the whisker and nodule geometries and the strength of the whisker containing nodules was investigated theoretically. T h e results showed that the strength and surface area of the whisker were strongly affected by the nodule radius, the whisker radius, and the distance between two nodules. From Fig. 12, it can be seen that the silica coating is abut 1-2 p m thick. T h e surface of the fiber is completely covered by this coating and thus the mechanical strength of the fibers should be improved. T h e silica coating does not increase the surface area of the graphite fiber. T h e fibers with a silica coating had the
smallest surface area among the several types of structures studied in this work of 4 8 0 - 8 5 0 p m 2 per 2 5 - 4 0 /~m length of fiber. Figure 14 shows an optical micrograph of the crosssection of the interface between the silica nodules and the graphite fiber. It can be seen that the bonding between the silica nodules and the graphite fiber is very strong. Even when the graphite fiber was broken, the silica nodule still remained attached to the graphite fiber.
4. Conclusions A method of decorating graphite fibers with silica using a plasma arc furnace was investigated. T h e silica was generated according to the following reaction: Si02(s or 1) + C(s) = SiO(g) + CO(g)
(1)
Fig. 14. An optical micrograph of the cross-section of the interface between the silica nodules and the graphite fiber.
TABLE 1. A comparison of the properties of different silica structures formed on the graphite fibers Structure Spherical nodule Elliptical nodule Whisker-nodule Multinodule Whisker Coating Graphite fiber
Dimension (/am)
Surface area (/am)2
Contact angle (deg)
D, = 25-40 D~= 40, Ds = 25 l = 5-10, Dw = 0.5, Dn = 25 D, = 10-15 l = 5-20, D = 0.5 t = 1-2 D = 5-6
1900-5000 1500-3500 > 5000, 24-40/am 1250-2800 > 5000, 25-40/am 480-850, 25-40/am 390-750, 25-40/am
75 30-40 30-40 125 -180 --
D n nodule diameter, Dt long diameter of elliptical nodule, Ds short diameter of elliptical nodule, Dw whisker diameter, l length, t thickness.
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Several different p r o d u c t m o r p h o l o g i e s were p r o duced, such as spherical and elliptical nodules, w h i s k e r - n o d u l e s , whiskers and thin coatings. T h e m o r p h o l o g y of the silica structure was strongly affected by the t e m p e r a t u r e of the p l a s m a treatment, and theref o r e the surface tension of the liquid silica. T h e modification with silica of graphite fibers can increase the surface area and the shear strength of the fiber.
References 1 J. B. Donnett and P. Ehrburger, Carbon fiber in polymer reinforcement, Carbon, 15(1977) 143-152. 2 D. W. Makee and V. J. Mimeault, Surface properties of carbon fibers, Chem. Phys. Carbon, 8(1973) 151-153. 3 J. C. Goan and S. P. Porsen, Interfacial bonding in graphite fiber-resin composites, ASTM STP 452, 1969, (ASTM, Philadelphia, PA)pp. 3-5.
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4 J. W. Herrick, R E. Gruber and F. T. Mansur, Surface treatments for fibrous carbon reinforcement, part I, AFML-TR66-178, United Aircraft Research Laboratories, East Hartford, CT, July 1966. 5 J. V. Milewski and J. J. Shyne, Surface treatment of a material to improve its bonding to another material in the formation of composites, French Patent, 1,586,262 (Cl. BOI]), February 13, 1970; US application, September 26, 1967. 6 R.L. Mehan, J. Mater. Sci., 13(1978) 358-366. 7 C. Lelievre, C. A. Pickles, J. M. Toguri and C. Simpson, Plasma production of silica whiskers. In A. J. Plumpton (ed.), lnt. 3~vmp. on the Production and Processing of Fine Particles, Montreal, Que., C1M, Pergamon, 1988, pp. 589-598. 8 L. A. Haas and S. E. Khalafalla, Effect of physical parameters on the reaction of graphite with silica in vacuum, Rep. Investigations, Ni). 7207, 1968 (US Bureau of Mines). 9 V. Fried, H. F. Hameka and U. Blukis, Physical Chemistry, Macmillan, New York, 1977, pp. 171-173. 10 A. P. Levitt and S. M. Wolf, Whisker Technolog% Wiley, New York, 1970, pp. 249-25(I. 11 Y. Zhang, J. Cameron and C. A. Pickles, The strength of nodule-decorated whiskers, J. Mater. Eng. Perform., 1 (3) (1992) 317-322.