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Materials Science and Engineering A 454–455 (2007) 293–299

A relation between a metallic film covering on diamond and diamondnucleation and growth at high-temperature–high-pressure

in a Ni–Mn–C system

Bin Xu a,∗, Mu-sen Li b, Li Li b, Ai-min Xu a, Yu-chao Niu a

a School of Materials Science and Engineering, Shandong Janzhu University, Fengming Road, Lingang Development Zone, Jinan,Shandong 250101, People’s Republic of China

b School of Materials Science and Engineering, Shandong University, 73 Jing Shi Road, Jinan, Shandong 250061,People’s Republic of China

Received 10 July 2006; received in revised form 4 November 2006; accepted 6 November 2006

bstract

There exists a thin metallic film covering on a growing diamond during diamond synthesis at high-pressure and high-temperature (HPHT).hrough the film the graphite carbon is catalyzed into diamond structure and transferred to the surface of growing diamond. Study on the chemicalomposition and structure of the metallic film is of great importance to study the diamond nucleation and growth mechanism and to prepareigh-quality single-crystalline diamond. In this paper, a thin metallic film covering on diamond crystals grown from a Ni–Mn–C system wasnvestigated by scanning Raman spectroscopy, scanning auger microprobe, electron probe microanalysis and transmission electron microscopy. Itas found that diamond structure was not found in the film, the state of carbon atoms does not significantly change and the fine structures of carbon

toms almost do not transform within the whole metallic film. However, diamond structure was observed on the surface of the film contacting

irectly with the as-grown diamond, while crystalline graphite and amorphous carbon were not found on the surface. Based on the systemic studyf microstructures on the metallic film, it could be reasonably suggested that graphite could not be directly catalyzed into diamond structure in theolten film at HPHT, and the diamond formation should be finished on the diamond–film interface. 2006 Elsevier B.V. All rights reserved.

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eywords: High-temperature and high-pressure; Diamond; Graphite; Thin met

. Introduction

Synthetic diamond can be made by a variety of methods,ut the static pressure via which the graphite discs are placedlternatively with catalyst discs in a cell assembly is mostlysed in industry. The main feature is that diamond nucle-tes on graphite–catalyst interface at high-temperature andigh-pressure (HPHT), and then grows towards graphite discs.imultaneously a thin molten metal film forms and coversn the growing diamond and isolates the diamond from theraphite. It is undoubtedly believed that the structure of graphite

s broken and makes a transition to diamond structure under theffect of the thin film during carbon diffusion towards growingiamond crystal [1–3]. However, it is not quite clear whether

∗ Corresponding author. Tel.: +86 531 86361831; fax: +86 531 86367282.E-mail address: xubin@sdzju.edu.cn (B. Xu).

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921-5093/$ – see front matter © 2006 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2006.11.014

covering on diamond

he carbon atoms for diamond growth are directly from graphiter a transition phase in the film, and whether the diamondormation is finished within the film or at the diamond–filmnterface. Therefore, the study on the thin film may be of greatignificance to reveal the diamond nucleation and growth.espite extensive studies on diamond growth and imperfections

n diamond [4–7], there have been few reports on the thinlm so far. The reason may be due to the extremely difficultyf the film specimen preparation for observation, because ofts very small size and difficulty to separate it from diamondrystals.

It is almost impossible to in situ study the molten film atPHT, but much information of diamond growth at HPHT

an be remained in the film at room temperature and ambient

ressure after the cell assembly is cooling rapidly after finishinghe synthetic process. The present work reports about the

icrostructures obtained from the different regions of the thinetallic film and the film surface contacting directly with

2 Engineering A 454–455 (2007) 293–299

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Fig. 1. SEM image of a pit left by an as-grown diamond particle, showing thecm

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94 B. Xu et al. / Materials Science and

s-grown diamond. The relationship between the diamondrowth and fine structures of the film was analyzed briefly. Thistudy would be helpful to obtain high-quality diamond singlerystals.

. Experimental

Diamond single crystals were synthesized in a LMD-8000Dype cubic anvil apparatus using artificial graphite as a carbonource and Ni75Mn25 alloy as a catalyst. The graphite discsere placed alternatively with the alloy discs, forming a cell

ssembly. After keeping the cell assembly at a temperaturef 1350 ◦C and a high-pressure of 5.5 GPa for 13 min, theemperature was decreased rapidly by turning off the electricower, and then the pressure was unloaded. After this, the cellssembly was taken out from the cubic anvil apparatus foruenching.

The cross-sections of the thin films were examined usingJSM-6380LA type scanning electron microscope (SEM), aerinishaw RM2000 type Raman (Raman), a PHI-610 type

canning auger microprobe (SAM) and a JXA-8800R type elec-ron probe microanalysis (EPMA), respectively. After this, theurface contacting with the as-grown diamond was in turn exam-ned by Raman and SAM. Finally, a film flake was carefully

aken out from the cell assembly under observation of an optical

icroscope, and it was ground by argon ion-milling, until it wasuitable for observation using a Philips CM-30 type transmissionlectron microscopy (TEM).

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Fig. 2. Carbon, nickel and manganese distributions in a cross-section of the thi

ross-sectional fracture of the thin Ni–Mn–C alloy film (1, 2 and 3 indicate threeicro-regions in the film, respectively) and graphite matrix HPHT-synthesized.

. Results and Discussion

.1. Cross-sectional fracture and composition of the film

Fig. 1 shows a pit left by an as-grown diamond single crystal.he fractured cross-sectional morphology of the thin film and theraphite matrix can be clearly observed, respectively. In Fig. 1,

oint 1, 2 and 3 on the cross-section of the film indicate threeifferent micro-regions of the film. Fig. 2 illustrates elementace scan of Ni, Mn and C through the whole film via EPMA

n Ni–Mn–C alloy film and a back-scattered electron pattern (the up-left).

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B. Xu et al. / Materials Science an

ethod. From Fig. 2, it is clear that carbon elements homoge-eously distribute in the whole film, and it is very important toote that there exists carbon-rich region on the diamond–filmnterface, and nickel-poor and manganese-rich regions occur inhe middle region of the film. Meanwhile, un-solvent graphiteakes were also found in the region near graphite, and someranched morphologies can be observed from the back-scatteredlectron pattern, but no fluctuation in chemical composition forhe branches.

.2. Fine microstructures of the film

Fig. 3 shows Raman spectra of the three micro-regions shownn Fig. 1. According to the literature [8,9], the Raman bandsor diamond, graphite crystal and amorphous carbon can beenoted as 1332 cm−1, 1581 cm−1 and 1355 cm−1, respectively.he peaks of 1582 cm−1 from region 1, 1582.9 cm−1 from mid-le region 2, and 1582.9 cm−1 from region 3 near diamond inhe Raman spectra (in Fig. 3) correspond only to graphite struc-ure in the film. Diamond crystal and amorphous carbon wereot observed in the film.

Auger spectra can be used to define the different finetructures of carbon atoms. If there is oxygen or other impu-

ities in a sample, the transition energy of Auger electrons ofarbon atoms in the sample should be changed, but their Augerpectrum shapes could not be evidently changed, especiallyheir representative spectra [10]. Therefore, Auger spectrum

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Fig. 3. Raman spectra of the thin film (a, b and c indicate m

ineering A 454–455 (2007) 293–299 295

hapes and peaks of carbon atoms in the thin film can beompared with that of the as-grown diamond and graphite.ig. 4 gives Auger differential coefficient spectra of carbon.ig. 4a and e are characteristic Auger spectra for diamond251.0 eV and 272.0 eV) and graphite (261.5 eV and 282.5 eV),espectively. The Auger spectra are different from that ofiamond and pure graphite crystals [11]. This may be becausef the electrical conductivity of the sample (the cell assembly).ig. 4b–d show Auger spectra from three micro-regions in thelm. The representative Auger spectra in three regions of thelm are evidently different from that of diamond and graphite.urthermore, there is only a small shift among the representativeuger shapes and peaks in three regions, which suggests thatne structures of carbon atoms in the film almost have not been

ransformed.

.3. Fine structures on the surface of the film contactings-grown diamond

Fig. 5 shows a Raman spectrum on the surface of the filmontacting directly with as-grown diamond. The Raman band1331.9 cm−1) corresponding to diamond is found, although itseak intensity is not too high, and there are no graphite crystal

nd amorphous carbon on the surface. It should be noted thathe representative transitions of Auger electrons and spectrumhape of carbon on the surface in Auger spectra of Fig. 6 is veryimilar to that of diamond, compared with that for the three

icro-region 1, 2 and 3 shown by Fig. 1, respectively).

296 B. Xu et al. / Materials Science and Engineering A 454–455 (2007) 293–299

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ig. 4. Auger differential coefficient spectra of carbon atoms for an as-grown dig. 1, respectively) and for graphite HPHT-synthesized (e).

icro-regions in the film (Fig. 4b–d). In particular, the spectrumhape designated by a black arrow in Fig. 6 is almost approxi-ate with that in diamond (Fig. 4a). The above fine structures on

he surface of the film contacting as-grown diamond suggest thatnly on the surface, fine structures of carbon atoms bear a closeelationship with that of the diamond. Phase analysis by TEMhows that except for striped hexagonal Ni3C single crystalsn the surface of film, nano-scale diamond particle distributing

n �-(Ni,Mn) matrix could also be observed, as depicted inigs. 7 and 8. Graphite and amorphous carbon were not found on

he surface by TEM, which is in agreement with the Raman resultFig. 5).

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nd (a), for the thin film (b, c and d indicate micro-region 1, 2 and 3 shown by

.4. Brief analyses for the catalysis of the thin film foriamond formation and growth

It should be particularly elucidated that because the syn-hetic temperature and pressure for super quality diamond is onlyithin a narrow V range [3,12,13], there exist inevitably solid

tructures in short-range order in the molten film within the range14]. During diamond growth at HPHT, graphite is continuously

issolved in the molten film, forming a gradient phase region ofarbon atoms, and a part of the graphite may be in short-rangerder in the film. After that, it diffuses from the external regionear graphite to the inner region near growing diamond, and then

B. Xu et al. / Materials Science and Eng

Fig. 5. Raman spectra on the surface of the thin film contacting as-grown dia-mond.

Fig. 6. Auger differential coefficient spectrum of carbon atoms on the surfaceof the thin film contacting as-grown diamond.

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Fig. 7. (a) TEM image of a hexagonal diamond particle and �-(Ni,Mn) matrix on the supattern, in which (1 0 1) and (1 0 0) diffraction rings correspond to diamond, while [1

ineering A 454–455 (2007) 293–299 297

t may take a transition gradually in the diamond–film interface.f course, some carbon atoms from the other part of graphite dis-

olved in the molten film may be combined with nickel or man-anese elements to form the carbon-rich carbides in short-rangerder, and then the carbides diffuse from the external region tohe inner region. Apparently, the most of graphite structures inhe film may have been broken at HTHP, as given in Figs. 2, 3nd 4b–d. But the carbon is not directly catalyzed into diamondtructure in the whole film under graphite diffusion. Or less,rails of diamond crystals could be discovered in the three filmegions.

Because diamond and graphite crystals were not simulta-eously found on the surface of the film contacting diamondFigs. 5 and 6), the carbon source for diamond growth may berom carbon atom groups decomposed of carbon-rich phasesn the surface layer of the film. Although previous publicationsoncerning diamond growth reported that the carbide Ni3C isbsent in Ni–C or Ni–Mn–C system at high-pressure (>2.6 GPa),i3C may appear as a metastable phase under a temperatureigher than 1500 ◦C and a pressure of 5–14 GPa [2,12], andecreasing temperature further would result in a decompositionf Ni3C into graphite spherulites and flakes distributing in aickel-rich solid solution. However, neither graphite spherulitesor graphite flakes were found in the region of the film contact-ng diamond besides Ni3C. So it can be suggested that either allf Ni3C formed at HPHT would not decompose or they woulde retained at room temperature and ambient pressure, or a partf them decomposed into diamond particles and nickel-richolid solution �-(Ni,Mn) under this experiment, as illustrated

y Fig. 7. Further research need to be carried out in the future.imilarly, the evidence concerning Fe3C decomposition underiamond nucleation and growth was found in the diamond–film

rface of the thin film contacting as-grown diamond and (b) corresponding SAD1 0] zone axis (from 1̄ 1 1, 0 0 2, 1 1̄ 1) indicates the presence of �-(Ni,Mn).

298 B. Xu et al. / Materials Science and Engineering A 454–455 (2007) 293–299

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ig. 8. TEM image of striped hexagonal Ni3C single crystal on the surface of1 0 1] zone axis of Ni3C.

nterface region in Fe–Ni–C system [15], indicating that graphitetructure is not directly catalyzed into diamond structure in theetallic film at HPHT. We have sufficient reasons to believe,

eing the same as that in Fe–Ni–C system, that diamond growthrocess should be closely related to the diamond–film interface.revious studies by atom force microscopy (AFM) concerning

he diamond–film interface morphologies have shown the finearticles on the (1 0 0) face and terraced structure on (1 1 1)ace of diamond single crystal resulting from carbon atomroups’ recombination and successive step growth, respectively16,17]. It is shown that the catalytic transition from carbide toiamond is related to the diamond–film interface. Therefore,he interface should play a crucial role in the course of diamondrowth.

. Conclusions

We have successfully investigated the microstructures ofhe thin metallic film covering on the HPHT-grown diamondn the presence of Ni-Mn catalyst. It was shown that thearbon element distributes homogeneously in the whole film.rom Auger spectra, it was indicated that the crystalline- andonding-state of carbon do not show any obvious changes atifferent regions of the film, and diamond structure was notound in the film. However, on the surface of the film contactingirectly with diamond, only diamond structure was found, noraphite and amorphous carbon were found. It is believed thatraphite could not be directly catalyzed into diamond structure

n the metallic thin film. It is reasonably proposed that diamondormation should be finished at the diamond–metallic filmnterface. Taken together, the metallic film covering diamondlays crucial roles in the diamond nucleation and growth, such

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in film contacting as-grown diamond and (b) corresponding SAD pattern from

s catalysis and transportation. The carbon was transferredhrough the film in the form of carbide and was catalyzed toiamond structure at the interface of film-diamond, and finallyccumulates on the surface of growing diamond. This studyhould be great help to prepare high-quality diamond andould make a general guide to design new catalyst for diamondynthesis.

cknowledgements

This work was supported by the Natural Science Foundationf China, under Grant Nos. 50371048 and 50372035.

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