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Effects of carbonyl nickel powders on {100}-oriented single diamond under high pressure and high temperature
Int. Journal of Refractory Metals and Hard Materials 34 (2012) 27–31
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Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Effects of carbonyl nickel powders on {100}-oriented single diamond under high pressure and high temperature Yong Li ⁎, Xiaopeng Jia, Meihua Hu, Bingmin Yan, Zhenxiang Zhou, Chao Fang, Zhuangfei Zhang, Hongan Ma State Key Lab of Superhard Materials, Jilin University, Changchun 130012, PR China
a r t i c l e
i n f o
Article history: Received 22 December 2011 Accepted 9 March 2012 Keywords: High pressure and high temperature Diamond Nitrogen concentration Morphology
a b s t r a c t In this paper, perfect {100}-oriented single diamonds with higher nitrogen concentration are successfully synthesized at a pressure of 5.5 GPa and temperature ranging from 1285 to 1294 °C by the adjustment of catalyst (Fe64Ni36) components. The content of carbonyl nickel powders has significant effect on the nitrogen concentration and the growth rate of the diamond crystals. Fourier transform infrared (FTIR) measurements reveal that the value of nitrogen concentration for the synthesized diamond is up to approximately 1020 ppm. Furthermore, basing on the diamond surface configuration, the structure of growth steps arranged layer by layer on {111} face is noticed using scanning electron microscopy (SEM), whereas the occurrence is not observed on {100} face. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction Diamond is a candidate for a number of applications due to its excellent combination of properties. These applications range from hard coatings, optical windows, Micro-Electro-Mechanical Systems (MEMS), surface acoustic wave devices (SAW) to electrochemical electrodes and electron emitters. It is well known that nitrogen is the dominant impurity in diamond and most of physical properties essentially depend on the form and concentration of nitrogen. In diamond lattice, nitrogen may be present in the form of single substitutional atoms (C centers) or aggregation of substitutional atoms (A and B centers). Diamonds containing nitrogen in the single substitutional form are classified as type Ib and those containing aggregated nitrogen forms are termed as type Ia. Generally speaking, most of synthetic diamonds obtained by conventional high pressure and high temperature (HPHT) techniques correspond to type Ib. An overwhelming majority of natural diamonds corresponds to Ia type with maximum nitrogen concentrations up to 3000–5000 ppm [1]. Particularly, nitrogen concentration up to 11,000 ppm was reported for Kokchetav diamonds [2]. However, synthetic diamonds obtained in metal–carbon systems typically contain 200–400 ppm of nitrogen approximately. A large number of researches are devoted to investigating the synthesis of diamonds with higher nitrogen concentration. Probably, they can be divided into two categories: the first route is to choose nonmetallic solvents to obtain diamonds with higher nitrogen concentration and another route concerns on the addition of nitrogencontaining compounds to the metal catalysts. For example, diamonds
⁎ Corresponding author. E-mail address: [email protected] (Y. Li). 0263-4368/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2012.03.005
produced using sulfur [3,4], sulfides [5], and carbonates [6] as solvents or in complex carbonate–oxide–sulfide [7] systems were found to contain 1000–1500 ppm of nitrogen. The maximum nitrogen concentration measured for diamonds synthesized in the Fe3N–C system was approximately 3300 ppm, which was the highest value reported so far for synthetic diamonds and close to the nitrogen concentrations found in natural diamonds [8]. In addition, diamonds with the nitrogen content ranging from 1000 to 2400 ppm were synthesized in metal–carbon systems with the additions of NaN3 and Ba(N3)2 [9–12,13]. Nevertheless, the high-nitrogen diamonds previously obtained always performed as twinned crystals, multicrystals or skeleton crystals. Up to now, even high-quality diamonds synthesized with higher nitrogen concentration prevailingly exhibit {111} orientation [10,12,13]. Considering the important role of nitrogen in the crystallization processes of natural and synthetic diamonds, as well as its determining influence upon diamond properties, we should pay more attention to investigating the synthesis of {100}-oriented single diamonds with higher nitrogen concentration. In order to seek the breakthrough, we attempt to adjust the components of metal solvent (Fe64Ni36) by the addition of carbonyl nickel powders. The synthesized diamonds were characterized by Fourier transform infrared (FTIR) spectrometry and scanning electron microscopy (SEM). 2. Experimental details Experiments on diamond crystal growth via the temperature gradient growth (TGG) were carried out using a china-type large volume cubic high-pressure apparatus (CHPA) (SPD-61×200). The schematic diagram of growth cell was presented in Fig. 1. The synthetic
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Y. Li et al. / Int. Journal of Refractory Metals and Hard Materials 34 (2012) 27–31
Fig. 1. The schematic diagram of growth cell: (1) pyrophyllite (2), (5) ceramic cylinder and cover (3) graphite heater (4) seed (6) steel ring (7) insulator (8) carbon source (9) alloy solvent and (10) metal plate.
temperature was calibrated using a Pt6%Rh–Pt30%RH thermocouple. The synthetic pressure was measured at room temperature by the change in resistance of standard substances and at temperatures by the graphite–diamond equilibrium. High-quality seed crystals with {100} facets of about 0.5 × 0.5 mm 2 were selected as the growth facet. High-purity graphite powders (99.9% purity) were employed as carbon source and single diamonds were grown in the FeNi–C system. We chose carbonyl nickel powders (the special nickel with 99.5 wt.% in purity and size in 3–5 μm) as reagent. The carbonyl nickel powders were obtained by decomposing Ni(CO)5 with a purity of 99.9 wt.% in a stream of ammonia (NH3) in a running reactor at 300 °C and 0.1 MPa conditions. Then the carbonyl nickel powders were deoxidized under hydrogen for 1 h. As a result, the major impurities of the nickel powders were nitrogen, oxygen and a little hydrogen. After that, carbonyl nickel powders were directly added into the catalyst (Fe64Ni36) to adjust the components of the catalyst. After synthesis experiments, crystallization samples were firstly dissolved in hot acids to remove the graphite and metal catalyst, which remained on the crystal surfaces. Then, the infrared spectra were obtained on a Perkin-Elmer 2000 Fourier-transform infrared spectrometer in the spectral range between 400 and 4000 cm − 1 with a spectral resolution of 2 cm − 1 in the transmittance mode. Furthermore, morphology and structural properties of the synthesized samples were characterized by scanning electron microscopy. 3. Results and discussion 3.1. The synthesis of {100}-oriented single diamond with higher nitrogen concentration The results of experiments performed in the FeNi–C system at fixed pressure 5.5 GPa and temperature ranging from 1285 to 1294 °C are summarized in Table 1. Fig. 2 displays the optical morphology of the single diamonds synthesized from the designed FeNi–C system. The referential experiment (R-1) performed without the addition of carbonyl nickel powders yields the diamond crystal with typical yellow color for type Ib diamond and weighing 30.1 mg. Furthermore, all the diamond crystals display cubic habits
Table 1 Experimental results of diamond synthesis in FeNi–C system at HPHT. Runs
Carbonyl nickel powders (wt.%)
T (°C)
Growth rate (mg/h)
Nitrogen content (ppm)
R-1 R-2 R-3 R-4 R-5
0.0 2.5 5.0 7.5 7.5
1285 1290 1292 1294 1294
2.01 0.97 0.48 0.23 0.92
260 180 850 1020 1005
with dominant {100} and minor {111} faces. To all appearances, the colors of the diamond crystals change from yellowish to dark green with the content of carbonyl nickel powders in Fe64Ni36 catalyst increasing, shown in Fig. 2. It is found from our experiments that the growth rate of the synthesized diamond decreases from 2.01 to 0.23 mg/h. The experimental results obviously indicate that the growth rates of the diamonds are significantly affected due to the addition of carbonyl nickel powders. The features of diamond crystallization found in our experiments seem not to be surprising, which can be considered as two possibilities. Firstly, it is well-known that the growth rate of diamond is mainly controlled by the rate of carbon diffusion and crystals grow in diffusion-limited regime in metal melts, which has been proved in previous literatures [14–17]. Secondly, from the practice and theory of crystal growth, it is well known that the decrease of the growth rate results from the slowing down of the surface processes [18]. The features of diamond crystallization found in the experiments result from the inhibition by the surface-active impurity. Herein, nitrogen incorporated in diamond structure is considered as the dominant impurity playing an important role on the growth rate of diamond [19]. Shown in Fig. 2(e), high-quality diamond with a higher nitrogen content size of 3.3 mm at a high growth rate of 0.92 mg/h can be obtained by using a 1 × 1 mm 2 seed crystal, which can effectively increase the growth rate. The morphology of diamond crystals in Fe64Ni36–C system is mainly affected by temperature and pressure conditions in the growth process and exhibits cubic (region A), cubic-octahedral (region B) and octahedral (region C) shapes, marked in Fig. 3. Under constant pressure condition, when the temperature condition is higher than the valid region A, the cubic-octahedral or octahedral shape will be produced. Correspondingly, well-formed diamonds cannot be obtained at lower temperature, because of the formation of skeleton crystals. Therefore, the section of the diamond displaying cubic shape is narrow. For typical Ib-type diamond, the region of the allowable temperature at 5.5 GPa is about 20 °C. As for diamond with higher nitrogen value, the region of the appropriate temperature decreases to approximately 11 °C, depicted in region D. The region becomes considerably narrower and the similar tendency is found under different pressure conditions (6 and 6.5 GPa). The results combined allow us to conclude that the growth region of cubic diamond containing high nitrogen concentration becomes narrower than that for typical Ib-type diamond. Hence, the high-precision temperature control technique is essential to grow high-quality diamond crystals containing higher nitrogen concentration. 3.2. The nitrogen concentration of the synthesized diamond The concentration of nitrogen was investigated by the typical FTIR spectra of diamonds, which were recorded and shown in Fig. 4. Under these conditions, nitrogen impurities predominantly enter into the diamond structure in the form of single substitutional atoms (Ccenters), which is known to considerably distort the diamond lattice [20]. Previously, it has reported that nitrogen impurity affects the integral perfection of diamond lattice, which gives to the defectintroduced broadening of the diamond Raman line [21]. Absorptions at 1130 and 1344 cm − 1 are attributed to defect center with a single substituting nitrogen atoms and typical for HPHT-treated diamond. Herein, it should be noted that the FTIR spectra are measured basing on {100} growth faces. It is well known that the nitrogen concentration is proportional to absorption intensity in one-phonon region between 1000 and 1350 cm − 1 [22]. In ideal spectrum, the concentration of nitrogen in Ib-type diamond is proportional to the intensity of absorption at 1130 cm − 1 and the proportion is 25 [23]. According to Davies [22], the absorption coefficient at 2000 cm − 1, where a dip is present, is 1.23 mm − 1 for diamond at room temperature. Then the absorption intensity at 2000 cm − 1 can be used to normalize the
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Fig. 2. Optical images of the diamond crystals with different carbonyl nickel powders: (a) 0.0 wt.% (b) 2.5 wt.% (c) 5.0 wt.% and (d) 7.5 wt.%.
Fig. 3. Schematic P–T diagrams of and graphite crystallization in FeNi–C system. The regions of A, B, C, and D demarcate different sections of diamond crystallization: (A) cubic shape (B) cubic-octahedron shape (C) octahedron shape (D) cubic shape with higher nitrogen.
Fig. 4. FTIR spectra of diamonds synthesized with different contents of carbonyl nickel powders: (a) 0.0 wt.% (b) 2.5 wt.% (c) 5.0 wt.% and (d) 7.5 wt.%.
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intensity of one-phonon region related to nitrogen. Whereas the baseline is much curve ranging from 700 cm − 1 to 2600 cm − 1, the dip at 2120 cm − 1 is used instead of the intensity at 2000 cm − 1 to reduce the error caused by the curve of the baseline. Compared with the intensity of 2000 cm − 1, it is achieved that the depth of the dip corresponds to 5.5 cm − 1. So the nitrogen impurity concentration can be evaluated according to the absorption coefficient of FTIR spectra, determined by the following [24]: . −1 −1 Nitrogen concentration ðppmÞ ¼ μ 1130 cm μ 2120 cm 5:5 25; where μ(1130 cm − 1) and μ(2120 cm − 1) represent absorption intensities of 1130 cm − 1 and the dip at 2120 cm − 1, respectively. Basing on the intensities of the peaks in Fig. 4, the average nitrogen concentration of the obtained diamonds is evaluated as 260–1020 ppm according to the above formula. As the ratio of carbonyl nickel is up to 7.5 wt.% in the catalyst, the highest nitrogen concentration reaches approximately 1020 ppm. It should be noted that the difference of nitrogen concentration between samples (d) and (e) is associated with the growth rate in spite of the same content of carbonyl nickel powders in the chamber. The work by Kiflawi et al. indicates that a faster growth rate causes an increase in the concentration of Ni impurities, which leads to the nitrogen concentration decreasing. Recent literature has inferred that nitrogen can be considered as an impurity stabilizing octahedral growth form of diamond. Namely, the growth faces of diamond crystals are following the sequence: {111} > {100} or even to {111} ≫ {100} [25]. But, our experimental fact well demonstrates that diamond with high nitrogen content exhibiting cubic shape can be obtained under HPHT conditions. Maybe, the difference of the behaviors is probably due to the different growth capsule components. The high value of nitrogen concentration in synthesized diamond could be explained by the following. Firstly, a certain amount of nitrogen impurity remained in carbonyl nickel powders will affect the content nitrogen impurity incorporated into diamond structure and make nitrogen abundant in the final crystal. Moreover, during the growth process of diamond from catalyst–C system, the amount of free nitrogen available for incorporation into diamond can be significantly affected by the nitrogen dissolubility of the catalyst. The dissolubility of nitrogen in liquid metal is related to the electronic structure of metal atoms. A higher deficiency in d-shell electrons, which is equivalent to a lower atomic number in the same of row of the periodic table, implies a greater potential for nitrogen retention in metal solution in the process of diamond growth. As we all know, the d-shell of Fe, Ni and Co atoms own 6, 7 and 8 electrons, respectively. Thus, Ni will have the lowest nitrogen solution than Fe and Co. In that way, the nitrogen levels in synthesized diamonds tend to increase with the increase the content of carbonyl nickel powders during diamond growth process.
3.3. Morphology of the diamond crystal The SEM is used to carefully analyze the surface configuration of the synthesized diamond. Fig. 5(a) shows that the obtained diamond is predominantly composed of the primary {100} and minor {111} faces and the faces are all smooth and flat. Generally speaking, it was usually thought that the formed diamond layer had a similar height to the diamond lattice (a = 0.35667 nm). However, we can find from Fig. 5(b) that the enlarged SEM image clearly demonstrates a well defined stepped structure with homogeneous average step height about several tens of nanometers on the {111} plane. But, the structure of growth steps arranged layer by layer on {100} face is not observed. Besides, big pits with the shape of down pyramid found on {100} face of B-doped diamond are not present in Fig. 5(c) [26]. It is interestingly seen from Fig. 5(c) that series of arteries and veins similar to dendrites are proverbially distributed on the {100} face. According to Kanda et al., only {111} faces grow by a layer or spiral growth mechanism, whereas the {100} faces are rough in diamonds [27]. The results obtained by SEM in this paper are in accordance with previous report. Based on our experimental results, we could conclude that the growth for the {111} faces under HPHT conditions belongs to twodimensional growth, which is similar to the growth mechanism of the CVD diamond film discussed [28]. The growth of diamond contains two simultaneous growth process on the {111} faces that are the deposition of the growth steps and extending of the diamond layers. The diamond morphology is determined by relative difference between the rate of the deposition of the growth steps and that of the extension of the diamond layers on {111} faces. As the rate of deposition is far greater than that of the extension, many new steps deposit on the {111} faces before the former ones covering the original faces. As a result, the structures arranged layer by layer are found on the {111} faces. As for {100} faces, the diamond layers may be formed by the expansion of the arteries and veins. During the growth process, it is probable that the arteries and veins can expand freely anT1d branch off for the release of the latent heat in the system. Eventually, the arteries and veins will connect with the others and spread out to cover the face of the diamond and generate a new integrated layer.
4. Conclusion In this paper, all experiments are carried out at 5.5 GPa and temperatures ranging from 1285 to 1294 °C. We have shown that {100}-oriented diamond crystal with higher nitrogen concentration is realized in FeNi–C system by adjusting the catalyst components. The growth rate of diamond is significantly influenced by the incorporation of nitrogen impurity in diamond structure during the growth process. FTIR measurements made on a number of synthesized diamonds reveal that the highest nitrogen concentration measured is approximately up to 1020 ppm. Furthermore, the different defects on {100} and {111} growth faces are detected by the SEM.
Fig. 5. SEM images of the diamond crystal grown with 5.0 wt.% carbonyl nickel powders: (a) the panorama image, (b) and (c) the enlarged images for the points of b and c, respectively.
Y. Li et al. / Int. Journal of Refractory Metals and Hard Materials 34 (2012) 27–31
Acknowledgments The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China under Grant 51172089 and New Century Excellent Talents in University. References [1] Woods GS. Infrared absorption studies of the annealing of irradiated diamonds. Philos Mag B 1984;50:673–88. [2] Cartigny P, De Corte K, Shatsky VS, Ader M, De Paepe P, Sobolev NV, et al. The origin and formation of metamorphic microdiamonds from the Kokchetav massif, Kazakhstan: a nitrogen and carbon isotopic study. Chem Geol 2001;176:265–81. [3] Pal'yanov YN, Borzdov Y, Kupriyanov I, Gusev V, Khokhryakov A, Sokol A. Highpressure synthesis and characterization of diamond from a sulfur–carbon system. Diamond Relat Mater 2001;10:2145–52. [4] Pal'yanov YN, Kupriyanov IN, Borzdov YM, Sokol AG, Khokhryakov AF. Diamond crystallization from a sulfur−carbon system at HPHT conditions. Cryst Growth Des 2009;9:2922–6. [5] Pal'yanov YN, Borzdov YM, Khokhryakov AF, Kupriyanov IN, Sobolev NV. Sulfide melts–graphite interaction at HPHT conditions: implications for diamond genesis. Earth Planet Sci Lett 2006;250:269–80. [6] Pal'yanov YN, Borzdov YM, Kupriyanov IN, Evans DJF, Quinn SJ, Lawson SC. Diamond conference 2002; Oxford: abstract no. 6. 1; 2002. [7] Pal'yanov YN, Borzdov YM, Bataleva YV, Sokol AG, Palyanova GA, Kupriyanov IN. Reducing role of sulfides and diamond formation in the Earth's mantle. Earth Planet Sci Lett 2007;260:242–56. [8] Borzdov Y, Pal'yanov Y, Kupriyanov I, Gusev V, Khokhryakov A, Sokol A, et al. HPHT synthesis of diamond with high nitrogen content from an Fe3N–C system. Diamond Relat Mater 2002;11:1863–70. [9] Liang ZZ, Kanda H, Jia XP, Ma HA, Zhu PW, Guan QF, et al. Synthesis of diamond with high nitrogen concentration from powder catalyst–C-additive NaN3 by HPHT. Carbon 2006;44:913–7. [10] Zhang YF, Zang CY, Ma HA, Liang ZZ, Zhou L, Li SS, et al. HPHT synthesis of large single crystal diamond doped with high nitrogen concentration. Diamond Relat Mater 2008;17:209–11. [11] Yu RZ, Ma HA, Liang ZZ, Liu WQ, Zheng YJ, Jia XP. HPHT synthesis of diamond with high concentration nitrogen using powder catalyst with additive Ba(N3)2. Diamond Relat Mater 2008;17:180–4.
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[12] Huang GF, Jia XP, Li Y, Hu MH, Li ZC, Yan BM, et al. Growth of gem-grade diamond crystals heavily doped with the additive of Ba(N3)2. Chin Phys B 2011;20:78103. [13] Huang GF, Jia XP, Li SS, Zhang YF, Li Y, Zhao M, et al. Synthesis of large diamond crystals containing high-nitrogen at high pressure and high temperature using Ni-based solvent by temperature gradient method. Chin Phys B 2010;19:118101. [14] Strong HM, Hanneman RH. Crystallization of diamond and graphite. J Chem Phys 1967;46:3668–76. [15] Strong HM, Chrenko RM. Further studies on diamond growth rates and physical properties of laboratory-made diamond. J Phys Chem 1971;75:1838–43. [16] Pal,yanov YN, Khokhryakov AF, Borzdov YM, Sokol AG, Gusev VA, Rylov GM, et al. Growth conditions and real structure of synthetic diamond crystals. Geol Geofiz 1997;38:882–906. [17] Sumiya H, Toda N, Satoh S. Development of high-quality large-size synthetic diamond crystals. SEI Tech Rev 2005;60:10–5. [18] Sangwal K. Additives and crystallization processes: from fundamental to applications. England: John Wiley; 2007. p. 451. [19] Kiflawi I, Kanda H, Lawson SC. The effect of the growth rate on the concentration of nitrogen and transition metal impurities in HPHT synthetic diamonds. Diamond Relat Mater 2002;11:204–11. [20] Lang AR, Moore M, Makepeace APW, Wierzchowski W, Welbourn CM. Philos Trans R Soc Lond A 1991;337:497–520. [21] Surovtsev NV, Kupriyanov IN, Malinovsky VK, Gusev VA, Pal'yanov YN. Effect of nitrogen impurities on the Raman line width in diamonds. J Phys Condens Matter 1999;11:4767–74. [22] Davies G, Walker PL, Thrower PA. Chemistry and physics of carbon, 13. New York: Dekker; 1977. p. 1–43. [23] Kiflawi I, Mayer AE, Spear PM, Van Wyk JA, Woods GS. Infrared absorption by the single nitrogen and A defect centres in diamond. Philos Mag B 1994;9:1141–7. [24] Liang ZZ, Jia XP, Ma HA, Zang CY, Zhu PW, Guan QF, et al. Synthesis of HPHT diamond containing high concentrations of nitrogen impurities using NaN3 as dopant in metal–carbon system. Diamond Relat Mater 2005;14:1932–5. [25] Pal'yanov YN, Borzdov YM, Khokhryakov AF, Kupriyanov IN, Sokol AG. Effect of nitrogen on diamond crystal growth process. Cryst Growth Des 2010;10:3169–75. [26] Zhang JQ, Ma HA, Jiang YP, Liang ZZ, Tian Y, Jia X. Effects of the additive boron on diamond crystals synthesized in the system of Fe-based alloy and carbon at HPHT. Diamond Relat Mater 2007;16:283–7. [27] Kanda H, Kaishi MA, Setaka N. Morphology of synthetic diamonds grown from Na2CO3 solvent-catalyst. J Cryst Growth 1990;106:471–5. [28] Hirmke J, Schwarz S, Rottmair C, Rosiwal SM, Singer RF. Diamond single crystal growth in hot filament CVD. Diamond Relat Mater 2006;15:536–41.
Contents lists available at SciVerse ScienceDirect
Int. Journal of Refractory Metals and Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Effects of carbonyl nickel powders on {100}-oriented single diamond under high pressure and high temperature Yong Li ⁎, Xiaopeng Jia, Meihua Hu, Bingmin Yan, Zhenxiang Zhou, Chao Fang, Zhuangfei Zhang, Hongan Ma State Key Lab of Superhard Materials, Jilin University, Changchun 130012, PR China
a r t i c l e
i n f o
Article history: Received 22 December 2011 Accepted 9 March 2012 Keywords: High pressure and high temperature Diamond Nitrogen concentration Morphology
a b s t r a c t In this paper, perfect {100}-oriented single diamonds with higher nitrogen concentration are successfully synthesized at a pressure of 5.5 GPa and temperature ranging from 1285 to 1294 °C by the adjustment of catalyst (Fe64Ni36) components. The content of carbonyl nickel powders has significant effect on the nitrogen concentration and the growth rate of the diamond crystals. Fourier transform infrared (FTIR) measurements reveal that the value of nitrogen concentration for the synthesized diamond is up to approximately 1020 ppm. Furthermore, basing on the diamond surface configuration, the structure of growth steps arranged layer by layer on {111} face is noticed using scanning electron microscopy (SEM), whereas the occurrence is not observed on {100} face. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction Diamond is a candidate for a number of applications due to its excellent combination of properties. These applications range from hard coatings, optical windows, Micro-Electro-Mechanical Systems (MEMS), surface acoustic wave devices (SAW) to electrochemical electrodes and electron emitters. It is well known that nitrogen is the dominant impurity in diamond and most of physical properties essentially depend on the form and concentration of nitrogen. In diamond lattice, nitrogen may be present in the form of single substitutional atoms (C centers) or aggregation of substitutional atoms (A and B centers). Diamonds containing nitrogen in the single substitutional form are classified as type Ib and those containing aggregated nitrogen forms are termed as type Ia. Generally speaking, most of synthetic diamonds obtained by conventional high pressure and high temperature (HPHT) techniques correspond to type Ib. An overwhelming majority of natural diamonds corresponds to Ia type with maximum nitrogen concentrations up to 3000–5000 ppm [1]. Particularly, nitrogen concentration up to 11,000 ppm was reported for Kokchetav diamonds [2]. However, synthetic diamonds obtained in metal–carbon systems typically contain 200–400 ppm of nitrogen approximately. A large number of researches are devoted to investigating the synthesis of diamonds with higher nitrogen concentration. Probably, they can be divided into two categories: the first route is to choose nonmetallic solvents to obtain diamonds with higher nitrogen concentration and another route concerns on the addition of nitrogencontaining compounds to the metal catalysts. For example, diamonds
⁎ Corresponding author. E-mail address: [email protected] (Y. Li). 0263-4368/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2012.03.005
produced using sulfur [3,4], sulfides [5], and carbonates [6] as solvents or in complex carbonate–oxide–sulfide [7] systems were found to contain 1000–1500 ppm of nitrogen. The maximum nitrogen concentration measured for diamonds synthesized in the Fe3N–C system was approximately 3300 ppm, which was the highest value reported so far for synthetic diamonds and close to the nitrogen concentrations found in natural diamonds [8]. In addition, diamonds with the nitrogen content ranging from 1000 to 2400 ppm were synthesized in metal–carbon systems with the additions of NaN3 and Ba(N3)2 [9–12,13]. Nevertheless, the high-nitrogen diamonds previously obtained always performed as twinned crystals, multicrystals or skeleton crystals. Up to now, even high-quality diamonds synthesized with higher nitrogen concentration prevailingly exhibit {111} orientation [10,12,13]. Considering the important role of nitrogen in the crystallization processes of natural and synthetic diamonds, as well as its determining influence upon diamond properties, we should pay more attention to investigating the synthesis of {100}-oriented single diamonds with higher nitrogen concentration. In order to seek the breakthrough, we attempt to adjust the components of metal solvent (Fe64Ni36) by the addition of carbonyl nickel powders. The synthesized diamonds were characterized by Fourier transform infrared (FTIR) spectrometry and scanning electron microscopy (SEM). 2. Experimental details Experiments on diamond crystal growth via the temperature gradient growth (TGG) were carried out using a china-type large volume cubic high-pressure apparatus (CHPA) (SPD-61×200). The schematic diagram of growth cell was presented in Fig. 1. The synthetic
28
Y. Li et al. / Int. Journal of Refractory Metals and Hard Materials 34 (2012) 27–31
Fig. 1. The schematic diagram of growth cell: (1) pyrophyllite (2), (5) ceramic cylinder and cover (3) graphite heater (4) seed (6) steel ring (7) insulator (8) carbon source (9) alloy solvent and (10) metal plate.
temperature was calibrated using a Pt6%Rh–Pt30%RH thermocouple. The synthetic pressure was measured at room temperature by the change in resistance of standard substances and at temperatures by the graphite–diamond equilibrium. High-quality seed crystals with {100} facets of about 0.5 × 0.5 mm 2 were selected as the growth facet. High-purity graphite powders (99.9% purity) were employed as carbon source and single diamonds were grown in the FeNi–C system. We chose carbonyl nickel powders (the special nickel with 99.5 wt.% in purity and size in 3–5 μm) as reagent. The carbonyl nickel powders were obtained by decomposing Ni(CO)5 with a purity of 99.9 wt.% in a stream of ammonia (NH3) in a running reactor at 300 °C and 0.1 MPa conditions. Then the carbonyl nickel powders were deoxidized under hydrogen for 1 h. As a result, the major impurities of the nickel powders were nitrogen, oxygen and a little hydrogen. After that, carbonyl nickel powders were directly added into the catalyst (Fe64Ni36) to adjust the components of the catalyst. After synthesis experiments, crystallization samples were firstly dissolved in hot acids to remove the graphite and metal catalyst, which remained on the crystal surfaces. Then, the infrared spectra were obtained on a Perkin-Elmer 2000 Fourier-transform infrared spectrometer in the spectral range between 400 and 4000 cm − 1 with a spectral resolution of 2 cm − 1 in the transmittance mode. Furthermore, morphology and structural properties of the synthesized samples were characterized by scanning electron microscopy. 3. Results and discussion 3.1. The synthesis of {100}-oriented single diamond with higher nitrogen concentration The results of experiments performed in the FeNi–C system at fixed pressure 5.5 GPa and temperature ranging from 1285 to 1294 °C are summarized in Table 1. Fig. 2 displays the optical morphology of the single diamonds synthesized from the designed FeNi–C system. The referential experiment (R-1) performed without the addition of carbonyl nickel powders yields the diamond crystal with typical yellow color for type Ib diamond and weighing 30.1 mg. Furthermore, all the diamond crystals display cubic habits
Table 1 Experimental results of diamond synthesis in FeNi–C system at HPHT. Runs
Carbonyl nickel powders (wt.%)
T (°C)
Growth rate (mg/h)
Nitrogen content (ppm)
R-1 R-2 R-3 R-4 R-5
0.0 2.5 5.0 7.5 7.5
1285 1290 1292 1294 1294
2.01 0.97 0.48 0.23 0.92
260 180 850 1020 1005
with dominant {100} and minor {111} faces. To all appearances, the colors of the diamond crystals change from yellowish to dark green with the content of carbonyl nickel powders in Fe64Ni36 catalyst increasing, shown in Fig. 2. It is found from our experiments that the growth rate of the synthesized diamond decreases from 2.01 to 0.23 mg/h. The experimental results obviously indicate that the growth rates of the diamonds are significantly affected due to the addition of carbonyl nickel powders. The features of diamond crystallization found in our experiments seem not to be surprising, which can be considered as two possibilities. Firstly, it is well-known that the growth rate of diamond is mainly controlled by the rate of carbon diffusion and crystals grow in diffusion-limited regime in metal melts, which has been proved in previous literatures [14–17]. Secondly, from the practice and theory of crystal growth, it is well known that the decrease of the growth rate results from the slowing down of the surface processes [18]. The features of diamond crystallization found in the experiments result from the inhibition by the surface-active impurity. Herein, nitrogen incorporated in diamond structure is considered as the dominant impurity playing an important role on the growth rate of diamond [19]. Shown in Fig. 2(e), high-quality diamond with a higher nitrogen content size of 3.3 mm at a high growth rate of 0.92 mg/h can be obtained by using a 1 × 1 mm 2 seed crystal, which can effectively increase the growth rate. The morphology of diamond crystals in Fe64Ni36–C system is mainly affected by temperature and pressure conditions in the growth process and exhibits cubic (region A), cubic-octahedral (region B) and octahedral (region C) shapes, marked in Fig. 3. Under constant pressure condition, when the temperature condition is higher than the valid region A, the cubic-octahedral or octahedral shape will be produced. Correspondingly, well-formed diamonds cannot be obtained at lower temperature, because of the formation of skeleton crystals. Therefore, the section of the diamond displaying cubic shape is narrow. For typical Ib-type diamond, the region of the allowable temperature at 5.5 GPa is about 20 °C. As for diamond with higher nitrogen value, the region of the appropriate temperature decreases to approximately 11 °C, depicted in region D. The region becomes considerably narrower and the similar tendency is found under different pressure conditions (6 and 6.5 GPa). The results combined allow us to conclude that the growth region of cubic diamond containing high nitrogen concentration becomes narrower than that for typical Ib-type diamond. Hence, the high-precision temperature control technique is essential to grow high-quality diamond crystals containing higher nitrogen concentration. 3.2. The nitrogen concentration of the synthesized diamond The concentration of nitrogen was investigated by the typical FTIR spectra of diamonds, which were recorded and shown in Fig. 4. Under these conditions, nitrogen impurities predominantly enter into the diamond structure in the form of single substitutional atoms (Ccenters), which is known to considerably distort the diamond lattice [20]. Previously, it has reported that nitrogen impurity affects the integral perfection of diamond lattice, which gives to the defectintroduced broadening of the diamond Raman line [21]. Absorptions at 1130 and 1344 cm − 1 are attributed to defect center with a single substituting nitrogen atoms and typical for HPHT-treated diamond. Herein, it should be noted that the FTIR spectra are measured basing on {100} growth faces. It is well known that the nitrogen concentration is proportional to absorption intensity in one-phonon region between 1000 and 1350 cm − 1 [22]. In ideal spectrum, the concentration of nitrogen in Ib-type diamond is proportional to the intensity of absorption at 1130 cm − 1 and the proportion is 25 [23]. According to Davies [22], the absorption coefficient at 2000 cm − 1, where a dip is present, is 1.23 mm − 1 for diamond at room temperature. Then the absorption intensity at 2000 cm − 1 can be used to normalize the
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Fig. 2. Optical images of the diamond crystals with different carbonyl nickel powders: (a) 0.0 wt.% (b) 2.5 wt.% (c) 5.0 wt.% and (d) 7.5 wt.%.
Fig. 3. Schematic P–T diagrams of and graphite crystallization in FeNi–C system. The regions of A, B, C, and D demarcate different sections of diamond crystallization: (A) cubic shape (B) cubic-octahedron shape (C) octahedron shape (D) cubic shape with higher nitrogen.
Fig. 4. FTIR spectra of diamonds synthesized with different contents of carbonyl nickel powders: (a) 0.0 wt.% (b) 2.5 wt.% (c) 5.0 wt.% and (d) 7.5 wt.%.
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intensity of one-phonon region related to nitrogen. Whereas the baseline is much curve ranging from 700 cm − 1 to 2600 cm − 1, the dip at 2120 cm − 1 is used instead of the intensity at 2000 cm − 1 to reduce the error caused by the curve of the baseline. Compared with the intensity of 2000 cm − 1, it is achieved that the depth of the dip corresponds to 5.5 cm − 1. So the nitrogen impurity concentration can be evaluated according to the absorption coefficient of FTIR spectra, determined by the following [24]: . −1 −1 Nitrogen concentration ðppmÞ ¼ μ 1130 cm μ 2120 cm 5:5 25; where μ(1130 cm − 1) and μ(2120 cm − 1) represent absorption intensities of 1130 cm − 1 and the dip at 2120 cm − 1, respectively. Basing on the intensities of the peaks in Fig. 4, the average nitrogen concentration of the obtained diamonds is evaluated as 260–1020 ppm according to the above formula. As the ratio of carbonyl nickel is up to 7.5 wt.% in the catalyst, the highest nitrogen concentration reaches approximately 1020 ppm. It should be noted that the difference of nitrogen concentration between samples (d) and (e) is associated with the growth rate in spite of the same content of carbonyl nickel powders in the chamber. The work by Kiflawi et al. indicates that a faster growth rate causes an increase in the concentration of Ni impurities, which leads to the nitrogen concentration decreasing. Recent literature has inferred that nitrogen can be considered as an impurity stabilizing octahedral growth form of diamond. Namely, the growth faces of diamond crystals are following the sequence: {111} > {100} or even to {111} ≫ {100} [25]. But, our experimental fact well demonstrates that diamond with high nitrogen content exhibiting cubic shape can be obtained under HPHT conditions. Maybe, the difference of the behaviors is probably due to the different growth capsule components. The high value of nitrogen concentration in synthesized diamond could be explained by the following. Firstly, a certain amount of nitrogen impurity remained in carbonyl nickel powders will affect the content nitrogen impurity incorporated into diamond structure and make nitrogen abundant in the final crystal. Moreover, during the growth process of diamond from catalyst–C system, the amount of free nitrogen available for incorporation into diamond can be significantly affected by the nitrogen dissolubility of the catalyst. The dissolubility of nitrogen in liquid metal is related to the electronic structure of metal atoms. A higher deficiency in d-shell electrons, which is equivalent to a lower atomic number in the same of row of the periodic table, implies a greater potential for nitrogen retention in metal solution in the process of diamond growth. As we all know, the d-shell of Fe, Ni and Co atoms own 6, 7 and 8 electrons, respectively. Thus, Ni will have the lowest nitrogen solution than Fe and Co. In that way, the nitrogen levels in synthesized diamonds tend to increase with the increase the content of carbonyl nickel powders during diamond growth process.
3.3. Morphology of the diamond crystal The SEM is used to carefully analyze the surface configuration of the synthesized diamond. Fig. 5(a) shows that the obtained diamond is predominantly composed of the primary {100} and minor {111} faces and the faces are all smooth and flat. Generally speaking, it was usually thought that the formed diamond layer had a similar height to the diamond lattice (a = 0.35667 nm). However, we can find from Fig. 5(b) that the enlarged SEM image clearly demonstrates a well defined stepped structure with homogeneous average step height about several tens of nanometers on the {111} plane. But, the structure of growth steps arranged layer by layer on {100} face is not observed. Besides, big pits with the shape of down pyramid found on {100} face of B-doped diamond are not present in Fig. 5(c) [26]. It is interestingly seen from Fig. 5(c) that series of arteries and veins similar to dendrites are proverbially distributed on the {100} face. According to Kanda et al., only {111} faces grow by a layer or spiral growth mechanism, whereas the {100} faces are rough in diamonds [27]. The results obtained by SEM in this paper are in accordance with previous report. Based on our experimental results, we could conclude that the growth for the {111} faces under HPHT conditions belongs to twodimensional growth, which is similar to the growth mechanism of the CVD diamond film discussed [28]. The growth of diamond contains two simultaneous growth process on the {111} faces that are the deposition of the growth steps and extending of the diamond layers. The diamond morphology is determined by relative difference between the rate of the deposition of the growth steps and that of the extension of the diamond layers on {111} faces. As the rate of deposition is far greater than that of the extension, many new steps deposit on the {111} faces before the former ones covering the original faces. As a result, the structures arranged layer by layer are found on the {111} faces. As for {100} faces, the diamond layers may be formed by the expansion of the arteries and veins. During the growth process, it is probable that the arteries and veins can expand freely anT1d branch off for the release of the latent heat in the system. Eventually, the arteries and veins will connect with the others and spread out to cover the face of the diamond and generate a new integrated layer.
4. Conclusion In this paper, all experiments are carried out at 5.5 GPa and temperatures ranging from 1285 to 1294 °C. We have shown that {100}-oriented diamond crystal with higher nitrogen concentration is realized in FeNi–C system by adjusting the catalyst components. The growth rate of diamond is significantly influenced by the incorporation of nitrogen impurity in diamond structure during the growth process. FTIR measurements made on a number of synthesized diamonds reveal that the highest nitrogen concentration measured is approximately up to 1020 ppm. Furthermore, the different defects on {100} and {111} growth faces are detected by the SEM.
Fig. 5. SEM images of the diamond crystal grown with 5.0 wt.% carbonyl nickel powders: (a) the panorama image, (b) and (c) the enlarged images for the points of b and c, respectively.
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