⇦Chinese Physics Letters, 2020, Vol. 37, No. 5, Article code 058101 Synthesis of Polycrystalline Diamond Compact with Selenium: Discovery of a New Se–C Compound * Wen-Dan Wang (王文丹)1**, Ao Li (黎傲)1, Guo-Heng Xu (徐国恒)2, Pei Wang (王培)3, Yue-Gao Liu (刘月高)4, Li-Ping Wang (王李平)3** Affiliations 1School of Physical Science and Technology, Southwest Jiaotong University, Chengdu 610031 2Materials Research Center, Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000 3Academy for Advanced Interdisciplinary Studies, Southern University of Science and Technology (SUSTech), Shenzhen 518055 4CAS Key Laboratory for Experimental Study under Deep-Sea Extreme Conditions, Institute of Deep-Sea Science and Engineering, Chinese Academy of Sciences, Sanya 572000 Received 20 February 2020, online 25 April 2020 *Supported by the National Natural Science Foundation of China (Grant No. 51402245), the Fundamental Research Funds for the Central Universities of China (Grant No. 2682016CX062), the China Scholarship Council (Grant No. 201707005071), the Shenzhen Peacock Plan (Grant No. KQTD2016053019134356), and the Guangdong Innovative & Entrepreneurial Research Team Program (Grant No. 2016ZT06C279). HPCAT operations are supported by US DOE/NNSA under Award No. DE-NA0001974 and DOE-BES under Award No. DE-FG02-99ER45775, with partial instrumentation funding by NSF. APS is supported by DOE-BES under Contract No. DE-AC02-06CH11357.
**Corresponding author. Email: wendan.wang@yahoo.com; wanglp3@sustech.edu.cn Citation Text: Wang W D, Li A, Xu G H, Wang P and Liu Y G et al 2020 Chin. Phys. Lett. 37 058101    Abstract Sintering of polycrystalline diamond with selenium was investigated under pressure of 6.5–10.5 GPa at a constant temperature of 1850 $^{\circ}\!$C. A new carbon-selenium compound with a most plausible chemical formula of SeC and a WC-type hexagonal structure (space group $P\bar{6}m2$) has been discovered in the recovered samples sintered at 10.5 GPa and 1850 $^{\circ}\!$C. Refined lattice parameters are as follows: $a = 2.9277(4)$ Å, $c = 2.8620(4)$ Å, $V = 21.245(4)$ Å$^{3}$. The diamond compacts hot-pressed at 10.5 GPa have excellent mechanical properties with a Vickers hardness of about 68 GPa at a loading force of 19.6 N. Diamond intergrowths observed in these samples may have benefited from the catalytic effects of Se/SeC on the nucleation and crystal growth of diamond. DOI:10.1088/0256-307X/37/5/058101 PACS:81.05.Uw, 81.20.Ev, 61.66.Fn © 2020 Chinese Physics Society Article Text Diamond is the hardest substance known and it has attracted research interests for decades because of its extraordinary properties. Since GE researchers first reported successful diamond synthesis in laboratory in 1955,[1] nearly 70 years have passed and a variety of devices and methods for producing diamonds have been developed.[2] At present, chemical vapor deposition[3] and high-pressure high-temperature (HPHT) growth[1,4] are mainly used in industry to produce manmade diamond. In the conventional HPHT method, the pressure and temperature conditions were generally within the thermodynamically stable region of diamond and temperature was high just enough so that the metal catalysts, usually saturated with carbon, are molten at high pressure.[4] For these catalysts, which are mostly transition metals, especially Fe, Ni, Co and their alloys,[4,5] the diamond-growth region is bounded by the "melting line" and the diamond-graphite equilibrium line.[6,7] In the 1990s, with the discovery of some new metals that have catalytic effects at temperatures much higher than their respective melting points, a new model based on the "reaction line" was proposed to explain the behavior of these new catalysts during diamond growth.[7] Over the next two decades, more materials were discovered as new catalysts for diamond synthesis, most of which are nonmetallic, such as silicates,[8] germanium,[9,10] phosphorus,[11,12] sulfur,[13] nitrides,[14] sulfates,[15] alkali halides,[16] carbonate,[17,18] carbonate-silicate,[19] and supercritical fluids of C–O–H systems.[20–22] Recently, a new mechanism has been proposed to explain the graphite-to-diamond transformation under compression combined with shear.[23] However, unlike a few materials such as ferromagnesian carbonate,[24] magnesium[25–28] and NiS,[29] most nonmetallic catalysts have not been studied in detail for their catalytic mechanisms. Significant catalytic effects of selenium on diamond synthesis were reported in 2009.[30] However, the nucleation and growth of diamond in the system are still not well understood. To better understand the catalytic mechanism of selenium in the Se–C system, a series of sintering experiments on polycrystalline diamond with the presence of selenium at 6.5–10.5 GPa and 1850 $^{\circ}\!$C have been carried out in this work, and the reaction products have been characterized by multiple analytical techniques. A new compound in the Se–C system has been discovered in the recovered samples, and its crystal structures have been determined with constraints from XRD and SEM data. Well-sintered diamond composites suitable for industrial applications have been obtained. The reaction mechanism shown in this study may help to produce diamond compacts with tailored properties in future. Commercial diamond powder (99.99%, grain size about 5 µm, SF Diamond Co., China) and selenium powder (99.999%, grain size about 70 µm, Aladdin) were used as precursors in the current study. Selenium powder was first ball-milled until its grain size reached about 5 µm, then 3.404 g of diamond powder and 0.143 g of processed selenium powder (3vol% Se) were mixed in ethanol in an ultrasonic bath. About 50 mg of dry mixture was sealed in an h-BN capsule (0.3 mm in wall thickness, 3 mm in inner diameter, and 3 mm in depth) and then treated under HPHT conditions for 60 min. The HPHT experiments were carried out in a slide-type six-to-eight multi-anvil high pressure apparatus developed at Southwest Jiaotong University.[30,31] The edge length of the corner truncations of tungsten carbide cubes is 8 mm, and the edge length of the octahedral pyrophyllite pressure medium is 14 mm. The octahedral pyrophyllite was also served as the sample chamber. Pressure in the cell has been calibrated against well-known phase transitions in Bi, ZnTe and ZnS at room temperature, and temperature against a W3% Re-W25% Re thermocouple (Type D). The calibration curves, pressure vs load and temperature vs power, were fairly reproducible during all experimental runs, and the estimated uncertainties were about $\pm$0.5 GPa for pressure and about $\pm 50\,^{\circ}\!$C for temperature. Powder x-ray diffraction (XRD) of the starting mixture was performed on a Philips $X^{\prime}$Pert MRD diffractometer (Cu $K_\alpha$ 1 radiation, $\lambda = 1.54059$ Å) at room temperature with a 2$\theta$ range of 10–100$^{\circ}$, a step size of 0.02$^{\circ}$, and a duration of 2 s. XRD analyses on recovered samples were carried out at the Beamline 16-BMD of the High Pressure Collaborative Access Team (HPCAT), Argonne National Laboratory (ANL), using a monochromatic x-ray beam ($\lambda = 0.4133$ Å) and MAR345 area detector. The synthetic diamond composites were broken into fragments and those from the central area of each sample were selected for synchrotron XRD analysis. The Rietveld refinements were performed on the obtained XRD data with GSAS software.[32] The morphology and the microstructure of the HPHT-treated samples were investigated using a field-emission scanning electron microscope (SEM) (JSM7800F PRIME, JEOL Ltd). Hardness of samples was estimated using a Vickers hardness tester (FV-700, Future-Tech. Japan) with an indentation time of 15 s at 0.3, 0.5, 1 and 2 kg of applied load. In this work, powder mixtures of diamond and selenium were sintered at 1850 $^{\circ}\!$C and various pressures from 6.5 to 10.5 GPa. Typical synchrotron XRD patterns for the recovered samples are shown in Fig. 1. Trials with the sintering pressure below 9.6 GPa, the value at which the catalytic effect of selenium on the nucleation and growth of diamond in the Se–C system was first observed,[30] were carried out in search of the possibility of sintering diamond powder with selenium at a lower pressure. Unfortunately, the mechanical properties of diamond compacts sintered at low pressures are very poor, and some of them cannot even be polished well enough for hardness test. These observations are consistent with the results from the XRD analysis. As shown in Fig. 1, the sample hot-treated at 6.5 GPa and 1850 $^\circ\!$C is a mixture of selenium, diamond and graphite, which was converted from diamond at high temperature. The sample sintered at 9 GPa and 1850 $^\circ\!$C is still a mixture of selenium and diamond; neither the phase transformation of diamond to graphite nor the reaction between diamond and selenium is observed. These results strongly suggest that the phase equilibrium boundary of graphite–diamond is likely between 6.5 and 9 GPa at 1850 $^\circ\!$C. In stark contrast to those treated at lower pressures, the sample hot-pressed at 10.5 GPa and 1850 $^\circ\!$C contained no starting selenium phase as the diffraction peaks of selenium completely disappeared (Fig. 1). In the meantime, a set of new peaks appeared, indicating additional new phase(s) formed during the processes of HPHT treatment.
cpl-37-5-058101-fig1.png
Fig. 1. Typical XRD patterns of recovered samples treated under different sintering conditions. D: diamond, Se: selenium, 2H: graphite.
SEM analyses show that besides diamond, there appears in the composite treated at 10.5 GPa and 1850 $^\circ\!$C only one additional solid that is rather uniform, and that the second solid is composed of carbon and selenium (see later section, Fig. 3). Consequently, it can be reasoned with high certainty that all new diffraction peaks shown in Fig. 1 for the sample belong to a single compound in the Se–C binary system. This is consistent with the fact that an extensive search failed to yield a match among all elemental crystalline phases (including the high-pressure polymorphs) reported so far for the carbon or selenium system. Carbon diselenide (CSe$_{2}$),[33] the only known compound in the Se–C system, exists as a liquid under ambient conditions. This fact alone, however, does not exclude some recoverable high-pressure polymorphs of CSe$_{2}$ (if there is any) as candidates for the unknown solid in the diamond composites. Alternatively, the unknown solid is a new compound in Se–C with rather different chemical composition. To further constrain the structure and composition of the unknown crystalline solid, a synchrotron XRD pattern with the highest intensities of new peaks was selected for the Rietveld analysis. Various structural models were employed for the refinement, but satisfactory results were obtained only when assuming a chemical formula of SeC and a WC-type structure (space group $P\bar{6}m2$, No. 187) for the unknown phase. The observed synchrotron XRD data for the sample synthesized at 10.5 GPa and 1850 $^\circ\!$C and a diffraction pattern calculated on the basis of the Rietveld analysis are shown in Fig. 2. The overall results of the Rietveld refinement and refined structural parameters for SeC at room temperature and pressure are summarized in Table 1. Close examination of Fig. 2 revealed that the diffraction peaks of SeC are much broader than those of diamond, and the differences between the observed data and calculated pattern are mostly due to the misfit to the observed peak widths of SeC. The peak broadening in SeC could result from the small grain size, or the residual strain after thermal quenching and decompression, or the combination of both. The new compound SeC discovered in this study likely formed as a result of the reaction between selenium and diamond at simultaneous high pressure and temperature. However, the crystal structure is determined from the data obtained on a mixture. Samples of the new compound with pure phase are needed to confirm the crystal structure and precise chemical composition. Many questions remain regarding its physical properties and phase stabilities. For instance, SeC upon its formation at HPHT may or may not have the same WC-type structure as the sample recovered under ambient conditions. Further studies are needed to address these uncertainties.
cpl-37-5-058101-fig2.png
Fig. 2. X-ray diffraction pattern of SeC$+$diamond under ambient conditions ($\lambda = 0.4133$ Å). Symbols: experimental data (black dots), calculated diffraction pattern (red solid line), residuals of the refinement (blue solid line), and peak positions of SeC (green vertical bar) and diamond (orange diamond).
Table 1. SeC at room temperature and pressure. Space group: $P\bar{6}m2$, $Z = 1$. Lattice parameters (hexagonal): $a = 2.9277(4)$ Å, $c = 2.8620(4)$ Å, $V = 21.245(4)$ Å$^{3}$, $R_{\rm wp} = 6.56{\%}$, $R_{\rm p} = 4.66{\%}$, and $\chi^{2} = 0.56$.
Atom Site $x$ $y$ $z$ $g$ Uiso(Å$^{2}$)
Se $1a$ 0.0000 0.0000 0.0000 1 0.0053
C $1d$ 0.3333 0.6667 0.5000 1 0.0888
The polished surfaces of the recovered samples were coated with a thin layer of gold to increase the conductivity of the sample, which is necessary for SEM investigation. Figure 3 shows the back-scattered-electron (BSE) images (Figs. 3(a) and 3(b)) and the energy-dispersive spectroscopy (EDS) spectra (Figs. 3(c) and 3(d)) for the sample sintered at 10.5 GPa and 1850 $^\circ\!$C. It is clear that SeC (light gray areas) is evenly distributed among diamond particles (dark gray areas with clearly shaped outlines, marked 1 in Fig. 3(b)), mainly at grain boundaries and triple junctions (Fig. 3(a)). This observation is consistent with the reported phase diagram of selenium showing that the melting curve comes to a maximum at $P = 8.5 \pm 0.5$ GPa, $T = 712 \pm 10\,^\circ\!$C,[34] which means that selenium was liquid at 10.5 GPa and 1850 $^\circ\!$C and therefore it could evenly distribute along the boundaries. No secondary phases are present within the light gray areas. The completely and partially disappeared grain boundaries between diamond particles (marked 2 in Fig. 3(b)) seem to suggest diamond intergrowths, indicating the catalytic effect of Se/SeC. EDS spectra show Se and C in the sample (Au is from the surface coating) (Fig. 3(c)), and the light gray areas (marked 3 in Fig. 3(b)) contains Se and C only (Fig. 3(d)). A few white spots are rich in Se; they are likely the residual selenium due to incomplete reaction.
cpl-37-5-058101-fig3.png
Fig. 3. Microstructure of the sample synthesized at 10.5 GPa, 1850 $^\circ\!$C. (a) An SEM image of the sample; (b) an SEM image with higher magnification; numbers 1, 2, 3 marked for diamond grains, diamond intergrowth and SeC, respectively; (c) typical EDS spectrum of a larger area (20 µm $\times 20\,µ$m) of the sample; (d) typical EDS spectrum of the light grey area of the sample.
Polishing for hardness tests was attempted on all recovered samples, but it is not possible to obtain a mirror surface for most samples, except the one sintered at 10.5 GPa and 1850 $^\circ\!$C. The sample was polished using diamond paste down to 0.5 µm. Two to six measurements were carried out to obtain an average hardness at each loading force (Fig. 4). The sintered diamond compact with WC-type SeC binder has an average Vickers hardness of about 68 GPa at a loading force of 19.6 N and 15 s indentation time. This value is lower than those of the (111) plane of natural diamond (92 GPa)[35] and synthetic diamond (80 GPa),[36] but apparently higher than those of the cBN single crystal (49 GPa)[36] and polycrystalline diamond compact (PCD) sintered with c-Si$_{3}$N$_{4}$ (42 GPa).[37] It is comparable to those of the commercially available diamond compacts sintered with magnesium carbonate (70 GPa)[25] and metal binders Ni–Zr alloy (63 GPa).[38] Hence the diamond composites synthesized in this study may find industrial applications in the future.
cpl-37-5-058101-fig4.png
Fig. 4. Vickers hardness of the sintered sample and common superhard materials, including nature diamond, synthetic diamond, cBN single crystal and polycrystalline diamond compacts (PCD) sintered with magnesium carbonate, Ni–Zr alloy and c-Si$_{3}$N$_{4}$. Empty symbols ($\lozenge$) show individual measurements in this study, whereas filled symbols ($\blacklozenge$) represent the average value at each loading force.
The effect of selenium on the sintering of polycrystalline diamond has been investigated at pressures of 6.5–10.5 GPa and a constant temperature of 1850 $^{\circ}\!$C. While experiments at lower pressures yielded less satisfactory results, well-sintered diamond compacts have been synthesized at 10.5 GPa and 1850 $^{\circ}\!$C. These diamond compacts have excellent mechanical properties with a Vickers hardness of about 68 GPa. A new compound in the Se–C system, most likely SeC with a WC-type hexagonal structure, was discovered in the recovered composites in which SeC acts as the binder. The grain intergrowth of diamond observed in the compacts may have been aided by Se/SeC. The synchrotron x-ray diffraction was performed at HPCAT (Sector 16), Advanced Photon Source (APS), Argonne National Laboratory (ANL). We thank Dr. Shiming Hong for his guidance at the beginning of the project, and Dr. Minghua Ren, Department of Geosciences, University of Nevada Las Vegas (UNLV), for his help in SEM analysis. References Man-Made DiamondsPreparation of DiamondNew Catalysts for Synthesis of DiamondDiamond‐Graphite Equilibrium Line from Growth and Graphitization of DiamondNew catalysts for diamond growth under high pressure and high temperatureGrowth of diamond in hydrous silicate meltsGermanium: a new catalyst for diamond synthesis and a new optically active impurity in diamondHigh-Pressure Synthesis and Characterization of Ge-Doped Single Crystal DiamondPhosphorus: An Elemental Catalyst for Diamond Synthesis and GrowthDiamond Growth from a Phosphorus–Carbon System at High Pressure High Temperature ConditionsSulfur: a new solvent-catalyst for diamond synthesis under high-pressure and high-temperature conditionsHPHT synthesis of diamond with high nitrogen content from an Fe3N–C systemHigh Pressure Synthesis of Diamond in the Systems of Grahpite-Sulfate and Graphite-HydroxideGrowth of HPHT diamonds in alkali halides: Possible effects of oxygen contaminationDiamond formation from mantle carbonate fluidsCrystallization of diamond from a silicate melt of kimberlite composition in high-pressure and high-temperature experimentsCrystallization of diamond from C–O–H fluids under high-pressure and high-temperature conditionsDiamond and graphite crystallization from C–O–H fluids under high pressure and high temperature conditionsFormation of diamond from CaCO3 in a reduced C–O–H fluid at HP–HTDecompression-Induced Diamond Formation from Graphite Sheared under PressureNatural diamond formation by self-redox of ferromagnesian carbonateSynthesis of polycrystalline diamond compact with magnesium carbonate and its physical propertiesProperties of diamonds seed-grown in the magnesium-carbon systemHigh-pressure crystallization and properties of diamond from magnesium-based catalystsDiamond crystallization from an Mg–C system under high pressure, high temperature conditionsSynthesis and Characteristics of Type Ib Diamond Doped with NiS as an AdditiveSelenium and tellurium: Elemental catalysts for conversion of graphite to diamond under high pressure and temperatureA slide-type multianvil ultrahigh pressure apparatus and calibrations of its pressure and temperatureEXPGUI , a graphical user interface for GSASStructure of solid carbon diselenide (CSe2) at 17.5, 50 and 200KPressure–temperature phase diagram of molten elements: selenium, sulfur and iodineUltrahardness: Measurement and EnhancementVickers Hardness of Diamond and cBN Single Crystals: AFM ApproachSuperhard composites of cubic silicon nitride and diamondSintering of ultrafine diamond particles under high temperature and high pressure
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