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The graphitization process and the synthesis of diamonds from a C–Ni–Mn system
Carbon 42 (2004) 2369–2373 www.elsevier.com/locate/carbon
The graphitization process and the synthesis of diamonds from a C–Ni–Mn system Ana L ucia Diegues Skury *, Guerold Sergueevich Bobrovnitchii, Sergio Neves Monteiro Northern State University of Rio de Janeiro–UENF, Av. Alberto Lamego 2000, 28015-620 Campos, RJ, Brazil Received 23 April 2003; accepted 11 February 2004 Available online 26 June 2004
Abstract The influence of the ‘‘degree of graphitization’’ on the synthesis of diamond obtained from a HP–HT process using an anvil type high pressure device was studied for different types of graphites. The ‘‘degree of graphitization’’ was measured by a new parameter the percentage of hexagonal perfection (PHP). Results have shown that for samples with a PHP below 50% no diamond was formed. This suggested that a minimum ‘‘degree of graphitization’’ must be present for a successful HP–HT for diamond synthesis. Ó 2004 Published by Elsevier Ltd. Keywords: A. Diamond; B. Heat treatment, High pressure; C. X-ray diffraction
1. Introduction One of the main phenomena found during the process of graphite-to-diamond transformation is the rearrangement of the atomic structure. Depending upon the type of initial graphite structure, recrystallization will first take place. During this recrystallization a structural rearrangement occurs and, only later, are diamonds formed [1,2]. The graphite recrystallization process is also known as graphitization. The mechanism that governs this kind of transformation is still not well understood and needs comprehensive investigation. The analysis of the behavior of some types of graphite, subjected to high pressure and high temperature, HP–HT, conditions, showed that the possibility for the nucleation and growth of diamond crystals depends on the capacity of the graphite to undergo graphitization [3]. It was also shown that this graphitization capacity is associated with the degree of perfection of the crystalline structure of the material [3]. In particular, for the case of diamond synthesis, the degree of graphitization poses a significant influence on the characteristics of the crystals formed [2]. The objective of the present work was to evaluate the influence of the degree of graphitization in three types of *
Corresponding author. Tel./fax: +55-510-222-726-1533. E-mail address: [email protected] (A.L.D. Skury).
0008-6223/$ - see front matter Ó 2004 Published by Elsevier Ltd. doi:10.1016/j.carbon.2004.02.014
graphite that were subjected to HP–HT in the region of diamond thermodynamic stability using a 42%Ni– 58%Mn alloy [4] as the solvent/catalyst. 2. Experimental procedure The graphite-to-diamond transformation was investigated through high pressure and high temperature treatment from a reactive mixture composed of alternating layers of graphite powder and Ni42 Mn58 alloy powder. These powders, in equal parts, were compacted inside containers, which were placed inside a high pressure device (HPD). Fig. 1 shows an assemblage of a reactive cell and the corresponding container made of calcite. The diamond synthesis process was carried out in a 630 ton press. The synthesis process parameters are presented in Fig. 2. In this work three graphites of Brazilian origin were used, details of which are given in Table 1. If one considers the equilibrium diagram in Fig. 2, it can be seen that the treatment conditions, 6.0 GPa and 1473–1673 K, occur inside the region of thermodynamic stability for diamond. Consequently, for the treatment conditions selected one should expect the two temperature conditions at 6.0 GPa to yield diamond. The pressure applied to the sample was estimated from the load-pressure relation obtained from separate experiments, employing Bi and PbSe as calibrants. The
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A.L.D. Skury et al. / Carbon 42 (2004) 2369–2373
rearrange its atomic structure towards the hexagonal graphitic structure. One measurement of the degree of the graphitization, c, has been obtained by the following relation [5]: c¼
Fig. 1. Cross-sectional view of a reactive cell.
ð1Þ
where d0 0 2 is the interplanar distance in the direction of the c-axis. Another proposed measurement [5,6], which is less used, associates the degree of graphitization, G, with the relation between the intensities of the X-ray peaks, obtained in the diffraction spectrum, corresponding to directions [1 1 2] and [1 1 0].
6.00
Pressure (GPa)
3:440 d0 0 2 0:084
Bundy G-D equilibrium line 5.00
G¼
4.00 1200
1400
1600
1800
Temperature (K) Fig. 2. Pressure–temperature conditions for the diamond synthesis process. The G–D diamond equilibrium line was plotted according Bundy [11].
high temperature was attained by passing an alternating current through the reactive mixture. The temperature was monitored with a Pt–Pt13%Rh thermocouple inserted at the centre of the cell. No correction was made for the effect of pressure on the electromotive force. The time of each HP–HT treatment was always 30 s. After treatment, the sample was cooled to room temperature. The pressure was then released to retrieve the sample. All recovered samples were diametrically cut to be examined by X-ray diffraction and scanning electron microscopy (SEM).
3. Results and discussion 3.1. Degree of graphitization As is well known, the degree of graphitization corresponds to the ability of a carbonaceous material to
I1 1 2 I1 1 0
ð2Þ
Here I112 and I110 are, respectively, the intensities of the 1 1 2 and 1 1 0 X-ray diffraction peaks. The value of c can vary from 0–1.0 [3] while those of G vary from 0–1.5 [4,5]. In Table 2 the results obtained from both ways of measuring the degree of graphitization are presented. The indices c0 and G0 correspond to the initial degree of graphitization before the HP–HT treatment. According to the data in Table 2, there could be a sensible difference between the two indices. In some cases the relative difference, ðc GÞ=G, exceeds 50%, such as for samples B1, B3 and D2 in the Table 2. One may also note that the c values for graphites E and D are greater than one. However, the maximum value for c should not exceed 1.0 [7]. Therefore, we suppose that c is not an adequate index for the present study. A possible reason is that, under the processing conditions used, the interplanar distance in the [0 0 2] direction is substantially altered. This fact can possibly be explained by the change of turbostractic hollow particles present in the initial structure to recrystallized graphite accompanied by a decrease of interplanar spacing which in turn affects the d0 0 2 values [8]. Moreover one may also consider that the abnormal values of c are due to possible distortions suffered by the crystalline structure caused by the high pressure applied [9]. The hexagonal lattice of the graphite can be distorted and partly converted to the rhombohedral lattice [10]. This fact was already noticed by Naka et al. [9]. Therefore, the transformation of
Table 1 Characteristics of the graphites Code
Graphite
Producer
Apparent density (g cm3 )
d0 0 2 (nm)
La (nm)
Lc (nm)
B E D
Synthetic Natural Synthetic
CETEX Nacional Grafite Unimetal
1.57 1.81 1.71
0.3397 0.3367 0.3354
59.61 61.02 47.00
38.84 42.78 50.38
A.L.D. Skury et al. / Carbon 42 (2004) 2369–2373
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Table 2 Results for the degree of graphitization Graphite codes
Pressure (GPa)
Temperature (°C)
c0
G0
c
G
ðc GÞ=G (%)
B
B1 B2 B3 B4
4.6 6.0 4.6 6.0
1200
0.60
0.51
0.99 0.89 0.90 0.89
0.57 0.81 0.59 0.79
74 10 52 13
E1 E2 E3 E4
4.6 6.0 4.6 6.0
1200
0.87
0.91
1.20 1.20 1.26 1.20
0.93 0.94 0.92 0.92
29 28 37 30
D1 D2 D3 D4
4.6 6.0 4.6 6.0
1200
0.95
1.19
1.17 1.32 1.14 1.21
0.80 0.87 0.91 0.92
46 52 25 32
E
D
1400
1400
1400
graphite to diamond proceeds through a drastic change in the chemical bonding and this way the graphitization degree is altered. This situation was always observed in the X-ray powder diffraction patterns of graphite B as non-treated and thermobaric-treated, and is shown in Fig. 3. By contrast, the values of G stayed within a reasonable range and it was therefore assumed that it gives a better measurement of the degree of graphitization for the present work. Fig. 4 shows the X-ray diffraction spectra for the graphite samples after the high pressure–high temperatures treatment. The 1 0 1 line at 43.393° undergoes a significant decrease, around 45–50%, as compared to the sample without treatment.
1600
D(111)
Intensity (arbitrary units)
1400
1200
B2
1000
r(101) 800
600
B1
h(100)
h(101)
400 Original 200
0
42
44
46
2θCuKα (deg) Fig. 3. X-ray powder diffraction patterns of the original graphite B and themobaric treated samples B1 and B2.
3.2. Graphite to diamond transformation The results for the value of G, which was considered the most representative for the degree of graphitization (Table 2), presented the relevant information. First, for the initial value corresponding to the as-received nontreated samples, graphite D already has a high degree of graphitization, 1.19, corresponding to an almost 80% perfect hexagonal structure. One should remember that the maximum value of G is 1.5 [4,5]. Type E graphite with G ¼ 0:91 is 60% perfect hexagonal structure while type B with G ¼ 0:51 is only 34% perfect. Consequently, it was not surprising that type D graphite gave a higher amount of diamond for all treatments. On the other hand, type B graphite with a less perfect initial structure produced a lower diamond yield. The amount of synthesized diamond was quantitatively estimated from SEM micrographs, as in Fig. 5. In fact, after treatment, only samples B2 and B4 produced diamonds. These two samples had values of G of 0.81 and 0.79, respectively, which were 50% above a perfect hexagonal structure. In samples B1 and B3, both treated at the lowest pressure of 4.6 GPa, no diamond was found. For these samples the perfect hexagonal structure reached values less than 50%, (B1 with 38% and B3 with 39%). This set of results suggests that a minimum hexagonal perfection, i.e. a minimum degree of graphitization, is needed for diamond formation. Thus, it is assumed that 50% of the hexagonal structure or a value of 0.75 for graphitization degree is required. It is also important to mention that the value of G for type D graphite decreases after the thermobaric treatment. This is apparently due to the fact that, with more diamond formation, there will be less hexagonal crystalline structure left after treatment. Following the ideas discussed above, a new way to characterize the tendency towards diamond formation from graphite is proposed. This is the use of the ratio between the actual value of G and its maximum possible
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D(111)
G(100)
D(111)
2000
G(101) D(111) 1200
1000
B4
B3 G(100)
D(111)
600
G(101) 400
B2
D4
1000
G(101) G(100) E4 1000
E3 E2
Intensity (arbitrary units)
800
Intensity (arbritary units)
Intensity (arbitrary units)
1500
800
D3 600
D2
400
500 200
200
E1
B1
D1 0 42
(a)
0
0 44
46
2θ,Cu Kα (deg)
42
43
(b)
44
45
46
2θ,CuKα
(c)
42
44
46
2θ,Cukα (deg)
Fig. 4. Portion of diffraction for the graphite samples after the high pressure–high temperature treatment. (a) Graphite B; (b) Graphite E; (c) Graphite D.
value of 1.5, which should correspond to a perfect hexagonal structure. This ratio will be called ‘‘percentage to perfect hexagonal’’ (PHP). Furthermore it is assumed that below a PHP of 50% it is difficult to form diamonds. In Table 3 are presented the results for the value of PHP calculated for all treatment conditions and for the original graphites. The important point made in this work is that a critical PHP value, probably 50%, has to be reached before diamond can be formed. In their original state graphites D and E (Table 3), already have a PHP value above 50%, and diamond is assured to be formed. On the other hand, if the original graphite before treatment has a PHP value below 50% diamond formation will then depend on whether this critical ‘‘degree of graphitization’’ can be reached during treatment. This was the case of graphite B treated at 5.3 Gpa, as shown in Table 3. For practical purposes, the measurement of PHP for the original graphite can only be partially conclusive. If the value of this ‘‘degree of graphitization’’ is above 50% one could expect diamond to be formed. However, if the PHP original value is below 50%, no immediate conclusion can be drawn. Another parameter that could help the interpretation of ‘‘degree of graphitization’’ as an indicative of diamond formation is the graphite density. According to Prikna [4] the density value to be used in the synthesis process should be in the range 1.7– 1.8 g cm3 . In the current work, graphite E has a density value (Table 1) that falls outside this range. This could also explain the absence of diamond formation for this
type of graphite in the lowest pressure and temperatures conditions.
4. Conclusion The main findings derived from the present work are as follows: (i) The degree of graphitization is better expressed by the index G, which is the ratio of the intensities of the 1 1 2 and 1 1 0 X-ray diffraction peaks. (ii) The index c, which takes into account the interplanar distance in the [0 0 2] direction, can only measure the graphitization degree under atmospheric pressure. Therefore, c is not a convenient index for high pressure– high temperature treatments of graphite. (iii) A new way to characterize the tendency towards diamond formation, based on the comparison between the actual value of G and its maximum possible value, is proposed. This is the percentage to perfect hexagonal structure, PHP. Based on experimental results it will be difficult to form diamond. below a PHP of 50%. (iv) Type D graphite, with a higher initial value of G, and PHP of 81%, undergoes an efficient transformation to diamond. As a consequence, there is a reduction in the value of G after thermobaric treatment due to a proportional decrease in hexagonal crystalline structure. (v) Although type E graphite has a PHP above 50% no diamond was formed at the lowest pressure and temperature conditions. This fact is probably associated with the density of this graphite.
A.L.D. Skury et al. / Carbon 42 (2004) 2369–2373
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Table 3 Graphitization degree (PHP) Graphite codes
G
PHP (%)
B, G ¼ 34%
B1 B2 B3 B4
0.57 0.81 0.59 0.79
38 54 39 52
E, G ¼ 66%
E1 E2 E3 E4
0.93 0.94 0.92 0.92
62 62 61 61
D, G ¼ 81%
D1 D2 D3 D4
0.80 0.87 0.91 0.92
53 58 60 61
Acknowledgements The authors wish to thank the following Brazilian agencies for the support of these investigations: FENORTE, FAPERJ and FINEP. References
Fig. 5. SEM micrograph of the samples recovered after thermobaric treatment. (a) B2 sample; (c) E4 sample and (c) D3 sample.
(vi) Type B graphite, with a lower initial value of G or 34% for the PHP, could not improve this value to above of 50%, after thermobaric treatment, in the case of two samples. Samples B1 and B3 did not form diamond. Samples B2 and B4 had PHP above 50% after thermobaric treatment, and they could form diamond.
[1] Zhaoyin H, Yufei C, Lizue C. Relation between recrystallized graphite and diamond growth. J Cryst Growth 1994:370–2. [2] Choi JY, Eun KY, Kang SJ. Effect of carbon crystallinity on the nucleation and growth of diamond under high pressure and high temperature. Diamond Relat Mater 1998;7:1196–200. [3] Tsuzuki A, Hirano SJ, Naka S. Influencing factors for diamond formation from several starting carbons. J Mater Sci 1985;20:2260–4. [4] Prikna AI, Shulshenko AA, Jitnetsky VI. The influence of the structure of graphite on diamond synthesis. J Super Materials 1980;2:2–3. [5] Inagaki M, Naka S. Crystallite orientation in polycrystalline orientation in polycrystalline graphites made from glass-like carbons. J Mater Sci 1975;10(5):814–8. [6] Kurdyumov AV, Ostrovskaya NF, Pilyankevich AN. Crystallography of transformations in carbon and boron-nitride. Acta Crystallogr A 1978;Sup. 34:306–10. [7] Efremenko MM, Kravchik AE, Osmakov AS. Structure of polycrystalline (graphitized) carbons. Russ J Appl Chem 1993;66(1):90–4. [8] Kamiya K, Inagaki M, Noda T. Microscopic observations of hard carbon heat-treated under pressure. Carbon 1971;9(3):287–92. [9] Naka S, Horri K, Takeda Y, Hanawa T. Direct conversion of graphite to diamond under static pressure. Nature 1976;259:38–9. [10] Pavel EG, Baluta G, Giurgiu C, Ilie DG, Barb V, Sandu DP, et al. Some aspects of diamond synthesis. Diamond Relat Mater 1993;2:500–4. [11] Bundy FP, Bassett WA, Weathers MS, Hemley RJ, Mao HU, Goncharov AF. The pressure–temperature phase and transformation diagram for carbon; Updated through 1994. Carbon 1996;34(2):141–53.
The graphitization process and the synthesis of diamonds from a C–Ni–Mn system Ana L ucia Diegues Skury *, Guerold Sergueevich Bobrovnitchii, Sergio Neves Monteiro Northern State University of Rio de Janeiro–UENF, Av. Alberto Lamego 2000, 28015-620 Campos, RJ, Brazil Received 23 April 2003; accepted 11 February 2004 Available online 26 June 2004
Abstract The influence of the ‘‘degree of graphitization’’ on the synthesis of diamond obtained from a HP–HT process using an anvil type high pressure device was studied for different types of graphites. The ‘‘degree of graphitization’’ was measured by a new parameter the percentage of hexagonal perfection (PHP). Results have shown that for samples with a PHP below 50% no diamond was formed. This suggested that a minimum ‘‘degree of graphitization’’ must be present for a successful HP–HT for diamond synthesis. Ó 2004 Published by Elsevier Ltd. Keywords: A. Diamond; B. Heat treatment, High pressure; C. X-ray diffraction
1. Introduction One of the main phenomena found during the process of graphite-to-diamond transformation is the rearrangement of the atomic structure. Depending upon the type of initial graphite structure, recrystallization will first take place. During this recrystallization a structural rearrangement occurs and, only later, are diamonds formed [1,2]. The graphite recrystallization process is also known as graphitization. The mechanism that governs this kind of transformation is still not well understood and needs comprehensive investigation. The analysis of the behavior of some types of graphite, subjected to high pressure and high temperature, HP–HT, conditions, showed that the possibility for the nucleation and growth of diamond crystals depends on the capacity of the graphite to undergo graphitization [3]. It was also shown that this graphitization capacity is associated with the degree of perfection of the crystalline structure of the material [3]. In particular, for the case of diamond synthesis, the degree of graphitization poses a significant influence on the characteristics of the crystals formed [2]. The objective of the present work was to evaluate the influence of the degree of graphitization in three types of *
Corresponding author. Tel./fax: +55-510-222-726-1533. E-mail address: [email protected] (A.L.D. Skury).
0008-6223/$ - see front matter Ó 2004 Published by Elsevier Ltd. doi:10.1016/j.carbon.2004.02.014
graphite that were subjected to HP–HT in the region of diamond thermodynamic stability using a 42%Ni– 58%Mn alloy [4] as the solvent/catalyst. 2. Experimental procedure The graphite-to-diamond transformation was investigated through high pressure and high temperature treatment from a reactive mixture composed of alternating layers of graphite powder and Ni42 Mn58 alloy powder. These powders, in equal parts, were compacted inside containers, which were placed inside a high pressure device (HPD). Fig. 1 shows an assemblage of a reactive cell and the corresponding container made of calcite. The diamond synthesis process was carried out in a 630 ton press. The synthesis process parameters are presented in Fig. 2. In this work three graphites of Brazilian origin were used, details of which are given in Table 1. If one considers the equilibrium diagram in Fig. 2, it can be seen that the treatment conditions, 6.0 GPa and 1473–1673 K, occur inside the region of thermodynamic stability for diamond. Consequently, for the treatment conditions selected one should expect the two temperature conditions at 6.0 GPa to yield diamond. The pressure applied to the sample was estimated from the load-pressure relation obtained from separate experiments, employing Bi and PbSe as calibrants. The
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A.L.D. Skury et al. / Carbon 42 (2004) 2369–2373
rearrange its atomic structure towards the hexagonal graphitic structure. One measurement of the degree of the graphitization, c, has been obtained by the following relation [5]: c¼
Fig. 1. Cross-sectional view of a reactive cell.
ð1Þ
where d0 0 2 is the interplanar distance in the direction of the c-axis. Another proposed measurement [5,6], which is less used, associates the degree of graphitization, G, with the relation between the intensities of the X-ray peaks, obtained in the diffraction spectrum, corresponding to directions [1 1 2] and [1 1 0].
6.00
Pressure (GPa)
3:440 d0 0 2 0:084
Bundy G-D equilibrium line 5.00
G¼
4.00 1200
1400
1600
1800
Temperature (K) Fig. 2. Pressure–temperature conditions for the diamond synthesis process. The G–D diamond equilibrium line was plotted according Bundy [11].
high temperature was attained by passing an alternating current through the reactive mixture. The temperature was monitored with a Pt–Pt13%Rh thermocouple inserted at the centre of the cell. No correction was made for the effect of pressure on the electromotive force. The time of each HP–HT treatment was always 30 s. After treatment, the sample was cooled to room temperature. The pressure was then released to retrieve the sample. All recovered samples were diametrically cut to be examined by X-ray diffraction and scanning electron microscopy (SEM).
3. Results and discussion 3.1. Degree of graphitization As is well known, the degree of graphitization corresponds to the ability of a carbonaceous material to
I1 1 2 I1 1 0
ð2Þ
Here I112 and I110 are, respectively, the intensities of the 1 1 2 and 1 1 0 X-ray diffraction peaks. The value of c can vary from 0–1.0 [3] while those of G vary from 0–1.5 [4,5]. In Table 2 the results obtained from both ways of measuring the degree of graphitization are presented. The indices c0 and G0 correspond to the initial degree of graphitization before the HP–HT treatment. According to the data in Table 2, there could be a sensible difference between the two indices. In some cases the relative difference, ðc GÞ=G, exceeds 50%, such as for samples B1, B3 and D2 in the Table 2. One may also note that the c values for graphites E and D are greater than one. However, the maximum value for c should not exceed 1.0 [7]. Therefore, we suppose that c is not an adequate index for the present study. A possible reason is that, under the processing conditions used, the interplanar distance in the [0 0 2] direction is substantially altered. This fact can possibly be explained by the change of turbostractic hollow particles present in the initial structure to recrystallized graphite accompanied by a decrease of interplanar spacing which in turn affects the d0 0 2 values [8]. Moreover one may also consider that the abnormal values of c are due to possible distortions suffered by the crystalline structure caused by the high pressure applied [9]. The hexagonal lattice of the graphite can be distorted and partly converted to the rhombohedral lattice [10]. This fact was already noticed by Naka et al. [9]. Therefore, the transformation of
Table 1 Characteristics of the graphites Code
Graphite
Producer
Apparent density (g cm3 )
d0 0 2 (nm)
La (nm)
Lc (nm)
B E D
Synthetic Natural Synthetic
CETEX Nacional Grafite Unimetal
1.57 1.81 1.71
0.3397 0.3367 0.3354
59.61 61.02 47.00
38.84 42.78 50.38
A.L.D. Skury et al. / Carbon 42 (2004) 2369–2373
2371
Table 2 Results for the degree of graphitization Graphite codes
Pressure (GPa)
Temperature (°C)
c0
G0
c
G
ðc GÞ=G (%)
B
B1 B2 B3 B4
4.6 6.0 4.6 6.0
1200
0.60
0.51
0.99 0.89 0.90 0.89
0.57 0.81 0.59 0.79
74 10 52 13
E1 E2 E3 E4
4.6 6.0 4.6 6.0
1200
0.87
0.91
1.20 1.20 1.26 1.20
0.93 0.94 0.92 0.92
29 28 37 30
D1 D2 D3 D4
4.6 6.0 4.6 6.0
1200
0.95
1.19
1.17 1.32 1.14 1.21
0.80 0.87 0.91 0.92
46 52 25 32
E
D
1400
1400
1400
graphite to diamond proceeds through a drastic change in the chemical bonding and this way the graphitization degree is altered. This situation was always observed in the X-ray powder diffraction patterns of graphite B as non-treated and thermobaric-treated, and is shown in Fig. 3. By contrast, the values of G stayed within a reasonable range and it was therefore assumed that it gives a better measurement of the degree of graphitization for the present work. Fig. 4 shows the X-ray diffraction spectra for the graphite samples after the high pressure–high temperatures treatment. The 1 0 1 line at 43.393° undergoes a significant decrease, around 45–50%, as compared to the sample without treatment.
1600
D(111)
Intensity (arbitrary units)
1400
1200
B2
1000
r(101) 800
600
B1
h(100)
h(101)
400 Original 200
0
42
44
46
2θCuKα (deg) Fig. 3. X-ray powder diffraction patterns of the original graphite B and themobaric treated samples B1 and B2.
3.2. Graphite to diamond transformation The results for the value of G, which was considered the most representative for the degree of graphitization (Table 2), presented the relevant information. First, for the initial value corresponding to the as-received nontreated samples, graphite D already has a high degree of graphitization, 1.19, corresponding to an almost 80% perfect hexagonal structure. One should remember that the maximum value of G is 1.5 [4,5]. Type E graphite with G ¼ 0:91 is 60% perfect hexagonal structure while type B with G ¼ 0:51 is only 34% perfect. Consequently, it was not surprising that type D graphite gave a higher amount of diamond for all treatments. On the other hand, type B graphite with a less perfect initial structure produced a lower diamond yield. The amount of synthesized diamond was quantitatively estimated from SEM micrographs, as in Fig. 5. In fact, after treatment, only samples B2 and B4 produced diamonds. These two samples had values of G of 0.81 and 0.79, respectively, which were 50% above a perfect hexagonal structure. In samples B1 and B3, both treated at the lowest pressure of 4.6 GPa, no diamond was found. For these samples the perfect hexagonal structure reached values less than 50%, (B1 with 38% and B3 with 39%). This set of results suggests that a minimum hexagonal perfection, i.e. a minimum degree of graphitization, is needed for diamond formation. Thus, it is assumed that 50% of the hexagonal structure or a value of 0.75 for graphitization degree is required. It is also important to mention that the value of G for type D graphite decreases after the thermobaric treatment. This is apparently due to the fact that, with more diamond formation, there will be less hexagonal crystalline structure left after treatment. Following the ideas discussed above, a new way to characterize the tendency towards diamond formation from graphite is proposed. This is the use of the ratio between the actual value of G and its maximum possible
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D(111)
G(100)
D(111)
2000
G(101) D(111) 1200
1000
B4
B3 G(100)
D(111)
600
G(101) 400
B2
D4
1000
G(101) G(100) E4 1000
E3 E2
Intensity (arbitrary units)
800
Intensity (arbritary units)
Intensity (arbitrary units)
1500
800
D3 600
D2
400
500 200
200
E1
B1
D1 0 42
(a)
0
0 44
46
2θ,Cu Kα (deg)
42
43
(b)
44
45
46
2θ,CuKα
(c)
42
44
46
2θ,Cukα (deg)
Fig. 4. Portion of diffraction for the graphite samples after the high pressure–high temperature treatment. (a) Graphite B; (b) Graphite E; (c) Graphite D.
value of 1.5, which should correspond to a perfect hexagonal structure. This ratio will be called ‘‘percentage to perfect hexagonal’’ (PHP). Furthermore it is assumed that below a PHP of 50% it is difficult to form diamonds. In Table 3 are presented the results for the value of PHP calculated for all treatment conditions and for the original graphites. The important point made in this work is that a critical PHP value, probably 50%, has to be reached before diamond can be formed. In their original state graphites D and E (Table 3), already have a PHP value above 50%, and diamond is assured to be formed. On the other hand, if the original graphite before treatment has a PHP value below 50% diamond formation will then depend on whether this critical ‘‘degree of graphitization’’ can be reached during treatment. This was the case of graphite B treated at 5.3 Gpa, as shown in Table 3. For practical purposes, the measurement of PHP for the original graphite can only be partially conclusive. If the value of this ‘‘degree of graphitization’’ is above 50% one could expect diamond to be formed. However, if the PHP original value is below 50%, no immediate conclusion can be drawn. Another parameter that could help the interpretation of ‘‘degree of graphitization’’ as an indicative of diamond formation is the graphite density. According to Prikna [4] the density value to be used in the synthesis process should be in the range 1.7– 1.8 g cm3 . In the current work, graphite E has a density value (Table 1) that falls outside this range. This could also explain the absence of diamond formation for this
type of graphite in the lowest pressure and temperatures conditions.
4. Conclusion The main findings derived from the present work are as follows: (i) The degree of graphitization is better expressed by the index G, which is the ratio of the intensities of the 1 1 2 and 1 1 0 X-ray diffraction peaks. (ii) The index c, which takes into account the interplanar distance in the [0 0 2] direction, can only measure the graphitization degree under atmospheric pressure. Therefore, c is not a convenient index for high pressure– high temperature treatments of graphite. (iii) A new way to characterize the tendency towards diamond formation, based on the comparison between the actual value of G and its maximum possible value, is proposed. This is the percentage to perfect hexagonal structure, PHP. Based on experimental results it will be difficult to form diamond. below a PHP of 50%. (iv) Type D graphite, with a higher initial value of G, and PHP of 81%, undergoes an efficient transformation to diamond. As a consequence, there is a reduction in the value of G after thermobaric treatment due to a proportional decrease in hexagonal crystalline structure. (v) Although type E graphite has a PHP above 50% no diamond was formed at the lowest pressure and temperature conditions. This fact is probably associated with the density of this graphite.
A.L.D. Skury et al. / Carbon 42 (2004) 2369–2373
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Table 3 Graphitization degree (PHP) Graphite codes
G
PHP (%)
B, G ¼ 34%
B1 B2 B3 B4
0.57 0.81 0.59 0.79
38 54 39 52
E, G ¼ 66%
E1 E2 E3 E4
0.93 0.94 0.92 0.92
62 62 61 61
D, G ¼ 81%
D1 D2 D3 D4
0.80 0.87 0.91 0.92
53 58 60 61
Acknowledgements The authors wish to thank the following Brazilian agencies for the support of these investigations: FENORTE, FAPERJ and FINEP. References
Fig. 5. SEM micrograph of the samples recovered after thermobaric treatment. (a) B2 sample; (c) E4 sample and (c) D3 sample.
(vi) Type B graphite, with a lower initial value of G or 34% for the PHP, could not improve this value to above of 50%, after thermobaric treatment, in the case of two samples. Samples B1 and B3 did not form diamond. Samples B2 and B4 had PHP above 50% after thermobaric treatment, and they could form diamond.
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