Indentation hardness of nano-polycrystalline diamond prepared from graphite by direct conversion

Indentation hardness of nano-polycrystalline diamonds (consisting of fine particles of 10–30 nm size) prepared directly from graphite under high pressure and high temperature conditions were investigated. It was found that a measurable indentation with no cracking can only be formed using the Knoop indenter in a limited loading condition of 2–6 N, and a reliable and accurate measurement is obtained at a load around 4.9 N. The Knoop hardness measurement at the applied load of 4.9 N revealed that some of the nano-polycrystalline diamonds obtained at P≧15 GPa and T≧2300 °C have extremely high hardness (120–145 GPa), which is equivalent to that in the (001)〈100〉 of the synthetic high-purity (type IIa) diamond crystal (116–130 GPa).     

Mechanical properties of different types of diamond

The mechanical properties of different types of diamond (synthetic diamonds with different nitrogen impurity concentrations 0.3 and 200 ppm) have been investigated by sclerometry hardness and wear resistance measurements. Diamond (111) and (100) faces in the 〈100〉 and 〈110〉 directions were tested. It was found the synthetic diamond with nitrogen impurity concentration of 0.3 ppm exceeds other diamond types with respect to hardness and wear resistance, and reveals anisotropy of the mechanical properties, different from other diamond types. The hardness measured on the (111) face for synthetic diamond was 175±5 GPa for 0.3 ppm of nitrogen impurities and 151±5 GPa for 200 ppm of nitrogen impurities. The hardness measurements were performed using an ultrahard fullerite indenter exceeding diamond in hardness and the diamond faces were deformed plastically under scratching conditions.     

Determination of metallic impurities in high-purity type IIa diamond grown by high-pressure and high-temperature synthesis using neutron activation analysis

Metallic impurities in high-purity type IIa and conventional type Ib diamond single crystals grown by high-pressure and high-temperature (HPHT) synthesis were determined by neutron activation analysis using thermal neutrons. Metallic impurities of Fe, Co, Cr and other minor elements were detected in the high-purity type IIa diamond crystal. The typical quantities of these metallic impurities were a few ppb. The influence of these metallic impurities on the electrical properties of the type IIa diamond crystal was practically negligible compared with nitrogen and boron impurities behaving as a donor and an acceptor, respectively. In addition to the impurities detected in the type IIa diamond crystal, Ni impurities of several hundreds of ppb were detected in conventional type Ib diamond crystals. A difference in molten metal solvents used in the synthesis of each diamond crystal resulted in the difference in metallic impurities.     

褐黄色IaA＋Ib型合成金刚石的光谱测试分析（上）

通过对褐黄色合成金刚石样品进行常规宝石学、红外光谱、紫外一可见光分光光度计,拉曼光谱等测试,结果表明：金刚石结构中的氮作为金刚石中的一种主要杂质,是金刚石呈现颜色的重要原因,并可用能带理论来解释,结构中的氮在金刚石呈色机制研究上有重要的意义。文章重点研究褐黄色合成金刚石的颜色成因,指出金刚石的呈色机制是一个很复杂的问题,它与金刚石中的杂质成分、色心、缺陷均有关系。     

褐黄色IaA＋Ib型合成金刚石的光谱测试分析（下）

通过对褐黄色合成金刚石样品进行常规宝石学、红外光谱、紫外—可见光分光光度计,拉曼光谱等测试,结果表明：金刚石结构中的氮作为金刚石中的一种主要杂质,是金刚石呈现颜色的重要原因,并可用能带理论来解释,结构中的氮在金刚石呈色机制研究上有重要的意义。文章重点研究褐黄色合成金刚石的颜色成因,指出金刚石的呈色机制是一个很复杂的问题,它与金刚石中的杂质成分、色心、缺陷均有关系。     

On the interaction of molecular hydrogen with diamonds: An experimental study using nuclear probes and thermal desorption

Hydrogen behavior in monocrystalline diamonds with different concentrations and types of nitrogen defects was studied using Nuclear Reaction and micro-Elastic Recoil Detection Analyses and Thermal Desorption Spectroscopy. Diamonds were studied in as-received state, after HPHT treatment and after hydrogenation in molecular hydrogen. A considerable amount of hydrogen was found to be bonded to diamond surfaces. Kinetics of surface hydrogen desorption is similar to what was reported for plasma-hydrogenated diamonds. The solubility of hydrogen in type IIa diamonds is very low. The efficiency of hydrogen traps in diamond bulk varies with the dominant type of nitrogen-related defects. The cross-section of traps decreases in row Ib > IaA > IaB diamonds, though binding energy in type IaB crystals may be higher, than in type IaA. Dislocations may promote hydrogen diffusion. A marked dependency of the hydrogen content and diffusivity between diamond growth sectors were observed for some samples. The total hydrogen content is higher in octahedral sectors.     

Characterization of ballas diamond depositions

Unfaceted, polycrystalline spherically grown diamond deposits having a radial structure have been observed since the early days of low pressure CVD diamond synthesis. Because the structure is quite similar to natural ballas stones, unfaceted CVD diamond is called ballas. So far, the general trend in diamond deposition has focused on well-faceted diamond layers, so CVD ballas deposits have not been systematically investigated.Low pressure growth of ballas always occur under conditions that are “non-optimal”, i.e. at least one parameter exceeds the range for a diamond growth leading to well-faceted diamond crystals. CVD ballas can consist of more than 99% of pure diamond; its microstructure reveals high amounts of micro-twins. Several morphological ballas structures have been observed by varying the deposition conditions, i.e. ballases having faceted areas, flat ballases, ballases with graphitic inclusions etc. Various deposits were characterized by Raman spectroscopy and impurities were measured by SIMS.Low pressure ballas diamond layers have a hardness quite similar to pure diamond. Of particular interest is the fact that cleavage and crack propagation along crystallographic planes can — due to the presence of micro-twins — be expected to be much lower in ballas than in single-crystalline diamonds. Thus, ballas structures are of particular interest for wear applications.Ballas type diamonds containing fine graphite particles could also be of interest for flat panel displays, as the graphite permits high electron emissions.     

Positron annihilation investigation of vacancies in as-grown and electron-irradiated diamonds

Vacancy-type defects in the four main types of diamond (Ia, Ib, IIa and IIb) were investigated using positron lifetime, Doppler broadening and optical absorption spectroscopies. In unirradiated samples vacancy clusters were found in all types, synthetic as well as natural. These clusters are situated in highly defected regions, rather than homogeneously distributed, and their concentration varies significantly from sample to sample. For synthetic Ib diamonds vacancy clusters were investigated as a function of nitrogen content. The bulk lifetime for diamond is calculated to be 98±2 ps and the bulk Doppler S parameter is estimated to be 25% lower than that for silicon. Electron irradiation (2.3 MeV) produced neutral monovacancies in IIa diamond and the positron data correlated well, as a function of dose, with the GR1 optical zero-phonon line; the introduction rate was estimated to be 0.5±0.2 cm⁻¹. In Ib diamond, monovacancies were found to be negatively charged. The positron lifetime for monovacancies was (40±6)% larger than the bulk lifetime and the Doppler S parameter increased by (8±1)%. At-temperature Doppler measurements between 30 and 770 K indicated that irradiation-produced neutral monovacancies can convert to the negatively charged state above 400 K but this was dependent on diamond type. Isochronal annealing of irradiated Ib diamonds showed that the complex of a substitutional nitrogen and a vacancy, formed upon annealing close to 600°C, undergoes two detectable modifications between 600 and 870°C reaching a configuration stable to 1170°C. Key conclusions based on positron and optical data are in mutual accord.     

Diamonds of new alkaline carbonate–graphite HP syntheses: SEM morphology,CCL-SEM and CL spectroscopy studies

Colorless octahedral diamonds up to 150 μm in size were spontaneously crystallized from carbon solutions in alkaline–carbonate melts in the Na₂Mg(CO₃)₂–graphite and NaKMg(CO₃)₂–graphite systems at pressures of 8–10 GPa and temperatures of 1700–1800 °C. Seeded growth of carbonate–carbon (CC) diamond layers was realized on both octahedral {111} and cubic {100} faces of natural and synthetic “metal–carbon” (MC) diamond single crystals 0.5–0.7 mm in size. Scanning electron microscopy (SEM) morphology studies clearly demonstrate that a preferable mechanism of diamond growth from alkaline CC melts is the deposition of newly formed layers in parallel with octahedral faces, in much the same way as in the case of natural diamonds. A color cathodoluminescence (CL) SEM study shows that the specific feature of the CC diamonds is the lack of surface color CL as for natural diamonds of type II with lower nitrogen concentration. The CL spectra of the CC diamonds consist of three-band system H3, 575 nm, and a weak blue A-band. The structure of the H3 band closely resembles that of natural diamonds of type IIa.     

Photoluminescence studies of type IIa and nitrogen doped CVD diamond

Photoluminescence (PL) studies were carried out on CVD, type IIa and high purity HPHT diamond samples irradiated with electrons of energies between 150 and 300 KeV; near threshold energies for carbon displacement. The majority of PL spectra are obtained using a 488-nm lasing line, with samples cooled to approximately 7 K. Of particular interest is the behaviour of the self-interstitial related centre, 3H, at 503.5 nm. The centre is particularly sensitive; its formation varies significantly with dose and dose rate and is severely quenched with incident laser power in excess of 10 mW. 3H is the dominant centre in highly doped (50–100 ppm) nitrogen samples, for doses between (10¹⁹–10²⁰) el/cm², but reduces with higher doses. In lower nitrogen (few ppm) samples, the centre is considerably weaker after equivalent doses, comparable to the Raman line. In type IIa crystals, creation of 3H varies considerably from sample to sample. Upon annealing, 3H is at an optimum between 310 and 330 °C for type IIa diamonds and vanishes by 400 °C. Indications show these temperatures increase slightly as nitrogen content is increased. Migration of the centre well outside the irradiated area is frequent, tens of microns after irradiation and hundreds of microns post annealing. Other centres of interest include GR1, the neutral diamond vacancy, which is found to be created linearly with dose and be rate independent. Using 325 and 457.9 nm lines the TR12 centre was studied. It has a strong dose rate dependence, growing as dose rate raised to a power of approximately 2 and is unaffected by annealing up to 700 °C. A 244-nm line was used to study the 5RL centre and contrary to some reports was observed in samples containing approximately 0.1 ppm of nitrogen. PL provides an extremely sensitive way of measuring the nitrogen concentration in diamond, to levels of less than 0.1 ppm. The problem remains how to obtain an accurate measurement.     

Characterization of top-quality type IIa synthetic diamonds for new X-ray optics

We have performed a complex analysis of top-quality synthetic diamond HPHT type IIa single crystals in view of their potential application as X-ray optics elements, namely as free electron laser mirrors. We have employed X-ray topography and high-resolution diffractometry along with optical spectroscopy techniques for characterization of our synthetic diamonds.     

Multiple twinning and nitrogen defect center in chemical vapor deposited homoepitaxial diamond

Homoepitaxial diamond films were grown on polished {100} faces of single crystal type IIa diamond substrates using microwave plasma assisted chemical vapor deposition system. 14 homoepitaxial diamond films were grown under a variety of substrate temperatures (1000–2000°C), methane concentration (1–6% in hydrogen gas) and processing pressure (60–200 Torr). Electron paramagnetic resonance (EPR) studies demonstrate that nitrogen is incorporated as a singly substitutional impurity (P1-defect center) and the nitrogen concentration is in the range 10–100 parts per million (ppm). The substitutional nitrogen concentration in homoepitaxial diamond was observed to decrease with increasing substrate temperature. Multitwin percentages of all grown diamonds derived from EPR spectra are correlated with the growth parameter α, which is simply the growth velocity along the 〈100〉 direction divided by the growth velocity along the 〈111〉 direction. With the aid of multitwin morphology and multitwin percentages derived from EPR, we describe conditions under which a twin-free and low defect single crystal diamond can be grown from the vapor phase on the {100} oriented substrates.     

A study of radiation dosimeters based on synthetic HPHT diamond

We investigated the applicability of synthetic diamond crystals routinely produced by the HPHT (High Pressure High Temperature) technique for the detection of ionising radiation. Split-sphere-type multi-anvil equipment was used to produce these HPHT diamonds. Both as-grown type Ib and HPHT treated type IaA diamonds were examined at the Centre of Oncology in Kraków using a therapeutic Co-60 γ-ray beam. Detectors fabricated from type IaA diamonds showed, on average, better performance than those based on type Ib diamonds. IaA- type synthetic diamonds containing up to 150-200 ppm nitrogen showed sensitivity up to 160 nC/(Gy mm³), charge collection distance up to 180 μm and close to linear dose-rate response. Considerable variations in the detecting characteristics were observed for the devices containing similar concentrations of nitrogen. From our preliminary qualitative analysis we conclude that the observed response variations may be associated with varying concentration of the nickel impurity present in these diamond crystals.     

Micro-/nanostructural investigation of laser-cut surfaces of single- and polycrystalline diamonds

Micrometer- to nanometer-scale structures of the cut surfaces of single- and polycrystalline diamonds by a pulsed ultraviolet laser have been thoroughly investigated by scanning and transmission electron microscopy. Within the laser-cut grooves, the processed diamond surfaces are extensively covered with laser-modified debris which consists of complex layered units of graphite with various crystallinities. The units consist of 1) highly oriented graphite, 2) corrugated graphite, and 3) nanocrystalline graphite, which are sequentially located from the surface of the underlying diamond substrate to the center of the grooves. Detailed textural examinations revealed that the highly oriented graphite unit is a product of the initial graphitization of diamond by a solid-state diffusion process, whereas the latter two units are deposition products from the liquid and/or vapor phases of carbon in the later stage. The present study demonstrates that the laser-cutting of diamonds proceeds in a two-step process: 1) extensive graphitization of laser-scanning path and 2) subsequent sublimation of the pre-formed graphite. These processes are basically identical among the three different types of diamonds (single crystal type Ib, single crystal type IIa and nano-polycrystalline aggregate) tested in this study.     

Thermally activated deformation under Knoop indentations in super-hard directions of high-quality synthetic type-IIa diamond crystals

The deformation resistance at room temperature against a Knoop indentation in (001)<110> (the<110> direction on the (001) plane) of high-quality synthetic type-IIa diamond is known to be extremely high. The behavior of deformation in the hard direction activated thermally by heating was investigated, using super-hard Knoop indenters prepared from high-quality diamond crystals by taking the tip orientation to (001)<110>. Indentation tests in (001)<110> with a load of 4.9 N revealed that the formation of normal Knoop impressions arises suddenly at a threshold temperature of 200–240 °C, whereas no impressions are observed up to 200 °C. The hardness values derived from the impressions in (001)<110> formed above the threshold temperatures are as low as 50–60% those in (001)<100> at the same temperatures. The anisotropy in the Knoop hardness at such high temperatures is consistent with the nature of anisotropy predicted by an effective resolved shear stress model for a {111}<110> slip deformation.     

The vacancy as a probe of the strain in type IIa diamonds

The width of the vacancy related photoluminescence from diamond is used to qualitatively measure the strain in type IIa diamonds of a range of brown colours and to measure the effect on the strain of high pressure high temperature annealing of the brown diamonds. The results indicate that for untreated type IIa diamonds the strain in colourless diamonds is generally less than that observed in brown ones. Annealing to remove the brown colour brings about a measurable reduction in the strain as assessed via the luminescence line width; but a dependence on the depth of the original brown colour is retained with the value remaining in most cases above 1.70 meV, a width below which 92% of the untreated colourless diamonds lie.     

Effect of substrate facets on homoepitaxial growth of diamonds during plasma-assisted hot filament chemical vapor deposition

In this report, the effect of substrate facets has been investigated during homoepitaxial growths of diamonds on polyhedral diamond grains in a plasma-assisted hot filament chemical vapor deposition (HFCVD) system. Homoepitaxial diamonds grown on the (100) planes present smooth surfaces, whereas textured surfaces form on the (111) facets, which is attributed to the different growth modes corresponding to the single-crystal substrate facets. With the accretion of the methane concentration in the gas supply, a few crystallites appear on the smooth growing surfaces of the (100) facets, and a change from (111) to (100) textured surface takes place on the (111) facets, showing that the variation of plasma vapor chemistry further significantly adjusts the homoepitaxy of diamonds. Photoluminescence spectroscopy investigations reveal that the 575-nm N–V peaks of the homoepitaxial grown layers on the (100) facets are much weaker than those of the (111) facets, demonstrating that there are less vacancies in the diamonds homoepitaxially grown from the (100) facets than the (111) ones.     

HTHP合成钻石在首饰中的鉴别特征（上）

采用宝石显微镜、红外光谱仪、X荧光光谱仪、紫外-可见光分光光谱仪、紫外荧光灯、Diamond View等对HPHT合成钻石做了较详细的测试与分析。结果表明：这些HTHP合成钻石具有较为一致的黄色,放大检查可见合成钻石内部含有大量棒状、柱状、细小微粒状的铁镍合金包裹体,这些合成钻石几乎都有磁性甚至有些磁性较强。HTHP合成钻石的红外反射光谱非常特征均具有明显的1131cm-1的吸收峰,为Ib型钻石（此类钻石在天然钻石中极少见到）。HTHP合成钻石在X荧光光谱仪下有强烈的铁峰和镍峰,并且在短波紫外线下多数具有绿黄色荧光。HTHP合成钻石在Diamond View下具有不同程度的黄绿色荧光,个别具有黑十字现象。     

HTHP合成钻石在首饰中的鉴别特征（下）

采用宝石显微镜、红外光谱仪、X荧光光谱仪、紫外-可见光分光光谱仪、紫外荧光灯、Diamond View等对HPHT合成钻石做了较详细的测试与分析。结果表明：这些HTHP合成钻石具有较为一致的黄色,放大检查可见合成钻石内部含有大量棒状、柱状、细小微粒状的铁镍合金包裹体,这些合成钻石几乎都有磁性甚至有些磁性较强。HTHP合成钻石的红外反射光谱非常特征均具有明显的1131cm^-1的吸收峰,为Ib型钻石（此类钻石在天然钻石中极少见到）。HTHP合成钻石在X荧光光谱仪下有强烈的铁峰和镍峰,并且在短波紫外线下多数具有绿黄色荧光。HTHP合成钻石在Diamond View下具有不同程度的黄绿色荧光,个别具有黑十字现象。     

Small-angle X-ray scattering in type Ia diamonds

The results of a small-angle X-ray scattering (SAXS) investigation of natural and synthetic diamonds, with different types of nitrogen-related defects, are presented. They reveal a presence of impurity-rich spherical clusters with a diameter of approximately 90 Å in an IaB diamond. SAXS scattering in an IaA diamond has a much lower intensity, and originates from larger clusters with a considerable variation in size. It is likely that the dominant impurity in these clusters is nitrogen. Present data suggest the possibility that some of the nitrogen-related point defects (A- and B-centers) are not randomly distributed in the diamond lattice, but instead are concentrated in clusters. It is suggested that, although optical manifestations of the A-nitrogen are similar in different diamonds, IaA diamonds can be divided into two sub-groups. The IaA₁ group includes annealed, synthetic, and some natural diamonds. They contain randomly distributed nitrogen pairs, which do not influence mechanical properties and do not produce X-ray scattering. In the IaA₂ group, the diamond aggregation process went further and the formation of nitrogen-containing clusters began. The process of nitrogen aggregation is described in terms of the decay of a supersaturated solid solution of nitrogen in diamond. The driving force for the creation of impurity clusters is the lowering of the free energy of nitrogen dissolution in the diamond lattice, and the decrease of strain.