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Metastable diamond formation from solutions at atmospheric pressure
Diamond and Related Materials, 2 (1993) 505-507
505
Metastable diamond formation from solutions at atmospheric pressure E. P a v e l , G. Bfilul;5. a n d C. G i u r g i u Dacia Synthetic Diamond, PO Box 58-52, Bucharest (Romania)
D. B a r b , V. S a n d u , M. M o r a r i u a n d D. P. Lazfir Institute of Atomic Physics, PO Box 52-06, Bucharest (Romania)
Abstract Recently, attention has been focused upon the growth of diamond on diamond crystals (surface recrystallization) at atmospheric pressure in an N a O H - N i - C system. In this paper, we have investigated the deposits obtained on high pressure, high temperature synthetic diamond crystals and Ni substrates. Examination of the carbon deposits on diamond crystals by scanning electron microscopy shows that the deposition is strongly affected by the synthesis conditions. The deposited layer grows epitaxially on the diamond crystals. The solidified melt has been analysed by IR spectrometry, X-ray diffraction and emission spectroscopy. Specific IR lines were observed. NiO, Na2CO3 and 7-NiO(OH) were involved in the growth process.
1. Introduction
3. Results and discussion
Diamond films have been reported to be synthesized by a recrystallization process occurring in a molten medium under atmospheric pressure [ 1]. This conclusion is based on experimental results which indicated the possibility of the diamond dissolution and surface recrystallization process. In this paper, the diamond coating of high pressure, high temperature synthetic diamOnd crystals at low pressure by molten salts method has been studied. The phases involved in the process were investigated.
As a first stage of investigation, we studied a frozen sample after high temperature treatment. Carbon from high pressure, high temperature synthetic diamond crystals was dissolved in an NaOH melt. Pipkin [2] considered that the etching of diamond crystals is accompanied by the formation of CO. NaOH reacts with CO and CO2 (partly from the atmosphere), leading to Na2CO3. The IR spectrum of the frozen sample (Fig. 1) shows the presence of NazCO 3 [3] (lines at about 700, 880, 1050 and 1460 cm 1). Also, NiO [4] (lines at 402, 430, 490, 530 and 560 cm -1) and 7-NiO(OH) [5] (lines at 560-570 and 620-630 cm -1) were identified in the IR spectrum. After boiling the solidified melt in water, XRD
2. Experimental details
High pressure, high temperature synthetic diamond crystals (about 0.5 mm in size) grown in an F e - N i - C system, Ni foils and commercial grade NaOH were subjected to a high temperature treatment in an Ni crucible, as in ref. 1. Boiling water was used for diamond recovery from the solidified sample. The microstructure of the surfaces of the diamond crystals were studied by scanning electron microscopy (SEM), cathodoluminescence (CL) and energy-dispersive X-ray analysis (EDXA) after treatment in boiling aqua regia for 1 h to eliminate residual NaOH and metallic compounds. The characterization of the deposit on the Ni substrate was performed using SEM, EDXA and X-ray diffraction (XRD), while the solidified melt was investigated by IR spectroscopy and XRD.
0925 9635/93/$6.00
10
%
5 o8
Z <
o
' 1800
1600
'
lZ,' O0
'
12'o 0
'
10
' 800
'
6 0
'
WAVENUM BER, em -1 Fig. 1. IR spectrum of solidified melt (NaOH-Ni-C system).
© 1993-- Elsevier Sequoia. All rights reserved
506
E. Pavel et al. / Metastable diamond formation at atmospheric pressure
Fig. 2. Scanning electron micrograph of the deposit on Ni foil substrate.
and EDXA of the insoluble deposit indicated the presence of 7-NiO(OH). The second stage of the study refers to investigations of the Ni precipitation. The amount of Ni in the melt was measured by means of emission spectroscopy. Results indicated that there was 0.1 wt.% Ni in the melt. These data suggest a ~-NiO(OH) form of the Ni in the melt. After OH elimination, NiO precipitates as large crystals on the Ni foils (Fig. 2). Carbon also precipitates from the melt on the surface of high pressure, high temperature synthetic diamond
(a)
(d)
(b)
(e)
crystals (Fig. 3). Several characteristic patterns were seen on the surfaces of the crystals. The common form of the etch pit observed on the {100} faces (substrate and coating) was a square-based pyramid (Figs. 3(b)-3(d)). The etch pits formed on the coated {111} faces were trigons (Fig. 3(c)). A detailed image of the coating is presented in Fig. 3(e). We conclude that the coating grew epitaxially on the substrate. The pits are associated with surface defects or metal inclusions. As the coating grows, a round cubo-octahedral crystal, nearly a sphere, is obtained. EDXA shows the absence of elements with Z/>11 (Na) in the composition of the coating. CL topographs were taken using the scanning microscope with an attachment for CL. The luminescence from a specimen was detected by a photomultiplier. The CL measurements show that, for high pressure, high temperature synthetic diamond crystals grown in an F e - N i - C system, emission is restricted to the {100} faces [6]. Coated high pressure, high temperature synthetic diamond crystals present weak CL emission. The etching features and EDXA showed that the coating has properties identical to those of the substrate. Growth interruption of the thin layers was observed on the {100} and {111 } faces (Fig. 4). 4. Conclusions
In the present study, the synthesis of diamond using a molten salts method was carried out at low pressure, and the following conclusions were reached:
(c)
Fig. 3. Scanning electron micrographs of coated high pressure, high temperature synthetic diamond crystals.
E. Pavel et al. / Metastable diamond formation at atmospheric pressure
(a)
507
(b)
Fig. 4. Scanning electron micrographs of interrupted growth layer on (a) {100} face and (b) {111} face. (1) The d i a m o n d layer grows epitaxially on the substrate formed by high pressure, high temperature synthetic d i a m o n d crystals. (2) N a z C O a , N i O and 7 - N i O ( O H ) were involved in the process. (3) Thick coatings (about 5 10 Jam) are obtained on high pressure, high t e m p e r a t u r e synthetic d i a m o n d crystals. On some { 100} faces, the deposited layer is detached.
Acknowledgments The authors gratefully acknowledge the assistance given by G. Hriguc, M. Mu~u and A. Zamfir in preparing the paper. Also, we gratefully acknowledge helpful dis-
cussions with Dr. M. Popescu. T h a n k s are expressed to M. Fecioru for his measurements of the IR spectra.
References 1 K. A. Cherian, Proc. First European Conf. on Diamond and Diamondlike Carbon Coatings, Diamond '90, September 17-19, 1990, CransMontana, Surf. Coat. Technol., 47 (1991) 127. 2 N. J. Pipkin, J. Mater. Sci., 15 (1980) 1755. 3 K. Nakamoto, J. Fujita, S. Tanaka and M. Kobayashi, J. Am Chem. Soc., 79 (1957) 4904. 4 S. Mochizuchi, Phys. Status Solidi B, 145 (1988) K 75.
5 I. S. Shamine, O. G. Malandin, S. M. Rakhovskaya and L. A. Vereshchagina, Elektrokhimiya, 10 (1974) 1571. 6 A. T. Collins, H. Kanda and R. C. Burus, Philos. Mag. B, 61 (1990) 797.
505
Metastable diamond formation from solutions at atmospheric pressure E. P a v e l , G. Bfilul;5. a n d C. G i u r g i u Dacia Synthetic Diamond, PO Box 58-52, Bucharest (Romania)
D. B a r b , V. S a n d u , M. M o r a r i u a n d D. P. Lazfir Institute of Atomic Physics, PO Box 52-06, Bucharest (Romania)
Abstract Recently, attention has been focused upon the growth of diamond on diamond crystals (surface recrystallization) at atmospheric pressure in an N a O H - N i - C system. In this paper, we have investigated the deposits obtained on high pressure, high temperature synthetic diamond crystals and Ni substrates. Examination of the carbon deposits on diamond crystals by scanning electron microscopy shows that the deposition is strongly affected by the synthesis conditions. The deposited layer grows epitaxially on the diamond crystals. The solidified melt has been analysed by IR spectrometry, X-ray diffraction and emission spectroscopy. Specific IR lines were observed. NiO, Na2CO3 and 7-NiO(OH) were involved in the growth process.
1. Introduction
3. Results and discussion
Diamond films have been reported to be synthesized by a recrystallization process occurring in a molten medium under atmospheric pressure [ 1]. This conclusion is based on experimental results which indicated the possibility of the diamond dissolution and surface recrystallization process. In this paper, the diamond coating of high pressure, high temperature synthetic diamOnd crystals at low pressure by molten salts method has been studied. The phases involved in the process were investigated.
As a first stage of investigation, we studied a frozen sample after high temperature treatment. Carbon from high pressure, high temperature synthetic diamond crystals was dissolved in an NaOH melt. Pipkin [2] considered that the etching of diamond crystals is accompanied by the formation of CO. NaOH reacts with CO and CO2 (partly from the atmosphere), leading to Na2CO3. The IR spectrum of the frozen sample (Fig. 1) shows the presence of NazCO 3 [3] (lines at about 700, 880, 1050 and 1460 cm 1). Also, NiO [4] (lines at 402, 430, 490, 530 and 560 cm -1) and 7-NiO(OH) [5] (lines at 560-570 and 620-630 cm -1) were identified in the IR spectrum. After boiling the solidified melt in water, XRD
2. Experimental details
High pressure, high temperature synthetic diamond crystals (about 0.5 mm in size) grown in an F e - N i - C system, Ni foils and commercial grade NaOH were subjected to a high temperature treatment in an Ni crucible, as in ref. 1. Boiling water was used for diamond recovery from the solidified sample. The microstructure of the surfaces of the diamond crystals were studied by scanning electron microscopy (SEM), cathodoluminescence (CL) and energy-dispersive X-ray analysis (EDXA) after treatment in boiling aqua regia for 1 h to eliminate residual NaOH and metallic compounds. The characterization of the deposit on the Ni substrate was performed using SEM, EDXA and X-ray diffraction (XRD), while the solidified melt was investigated by IR spectroscopy and XRD.
0925 9635/93/$6.00
10
%
5 o8
Z <
o
' 1800
1600
'
lZ,' O0
'
12'o 0
'
10
' 800
'
6 0
'
WAVENUM BER, em -1 Fig. 1. IR spectrum of solidified melt (NaOH-Ni-C system).
© 1993-- Elsevier Sequoia. All rights reserved
506
E. Pavel et al. / Metastable diamond formation at atmospheric pressure
Fig. 2. Scanning electron micrograph of the deposit on Ni foil substrate.
and EDXA of the insoluble deposit indicated the presence of 7-NiO(OH). The second stage of the study refers to investigations of the Ni precipitation. The amount of Ni in the melt was measured by means of emission spectroscopy. Results indicated that there was 0.1 wt.% Ni in the melt. These data suggest a ~-NiO(OH) form of the Ni in the melt. After OH elimination, NiO precipitates as large crystals on the Ni foils (Fig. 2). Carbon also precipitates from the melt on the surface of high pressure, high temperature synthetic diamond
(a)
(d)
(b)
(e)
crystals (Fig. 3). Several characteristic patterns were seen on the surfaces of the crystals. The common form of the etch pit observed on the {100} faces (substrate and coating) was a square-based pyramid (Figs. 3(b)-3(d)). The etch pits formed on the coated {111} faces were trigons (Fig. 3(c)). A detailed image of the coating is presented in Fig. 3(e). We conclude that the coating grew epitaxially on the substrate. The pits are associated with surface defects or metal inclusions. As the coating grows, a round cubo-octahedral crystal, nearly a sphere, is obtained. EDXA shows the absence of elements with Z/>11 (Na) in the composition of the coating. CL topographs were taken using the scanning microscope with an attachment for CL. The luminescence from a specimen was detected by a photomultiplier. The CL measurements show that, for high pressure, high temperature synthetic diamond crystals grown in an F e - N i - C system, emission is restricted to the {100} faces [6]. Coated high pressure, high temperature synthetic diamond crystals present weak CL emission. The etching features and EDXA showed that the coating has properties identical to those of the substrate. Growth interruption of the thin layers was observed on the {100} and {111 } faces (Fig. 4). 4. Conclusions
In the present study, the synthesis of diamond using a molten salts method was carried out at low pressure, and the following conclusions were reached:
(c)
Fig. 3. Scanning electron micrographs of coated high pressure, high temperature synthetic diamond crystals.
E. Pavel et al. / Metastable diamond formation at atmospheric pressure
(a)
507
(b)
Fig. 4. Scanning electron micrographs of interrupted growth layer on (a) {100} face and (b) {111} face. (1) The d i a m o n d layer grows epitaxially on the substrate formed by high pressure, high temperature synthetic d i a m o n d crystals. (2) N a z C O a , N i O and 7 - N i O ( O H ) were involved in the process. (3) Thick coatings (about 5 10 Jam) are obtained on high pressure, high t e m p e r a t u r e synthetic d i a m o n d crystals. On some { 100} faces, the deposited layer is detached.
Acknowledgments The authors gratefully acknowledge the assistance given by G. Hriguc, M. Mu~u and A. Zamfir in preparing the paper. Also, we gratefully acknowledge helpful dis-
cussions with Dr. M. Popescu. T h a n k s are expressed to M. Fecioru for his measurements of the IR spectra.
References 1 K. A. Cherian, Proc. First European Conf. on Diamond and Diamondlike Carbon Coatings, Diamond '90, September 17-19, 1990, CransMontana, Surf. Coat. Technol., 47 (1991) 127. 2 N. J. Pipkin, J. Mater. Sci., 15 (1980) 1755. 3 K. Nakamoto, J. Fujita, S. Tanaka and M. Kobayashi, J. Am Chem. Soc., 79 (1957) 4904. 4 S. Mochizuchi, Phys. Status Solidi B, 145 (1988) K 75.
5 I. S. Shamine, O. G. Malandin, S. M. Rakhovskaya and L. A. Vereshchagina, Elektrokhimiya, 10 (1974) 1571. 6 A. T. Collins, H. Kanda and R. C. Burus, Philos. Mag. B, 61 (1990) 797.