Synthesis of alumina-bonded polycrystalline diamond by detonation

In this study, nanodiamond, aluminum isopropoxide, and hexogen (RDX) were used as starting materials to synthesize alumina-bonded polycrystalline diamond materials under high-temperature and high-pressure conditions generated by the detonation of the explosive. During detonation, the surface of the nanodiamond is coated with boron, silicon, and chromium through vacuum diffusion. Carbides of boron, silicon, and chromium referred as “bridges” are formed at the diamond/metal interface during the carbonization reaction. The "bridge" formed between diamond and nanoalumina considerably reduced the possibility of oxidation of nanodiamond as well as its graphitization during the detonation reaction. The phase, morphology, microstructure, and elemental composition of the detonation products were characterized by X-ray diffraction, scanning and transmission electron microscopy, and energy-dispersive X-ray spectroscopy. The results revealed that the explosion causes alumina and diamond to bond tightly to form alumina-bonded polycrystalline diamond composites. The thermal stabilities of the nanodiamond particles coated with boron, silicon, and chromium were found to be markedly higher, and the diamond phase remained intact even after heating at elevated temperatures. Thus, boron, silicon, and chromium reduced the wetting angle of diamond and alumina and improved the degree of bonding between them. Furthermore, boron facilitated the bonding between nanodiamond and alumina. In contrast, the bond was weaker in the case of silicon. Chromium also aided the bonding of the nanodiamond and alumina but introduced a large amount of oxygen into the composite.     

Raman scattering,AFM and nanoindentation characterisation of diamond films obtained by hot filament CVD

In this work, structure and mechanical properties of diamond films fabricated by HFCVD on silicon substrates with nanodiamond seeding were investigated. Raman spectroscopy was used to characterise the diamond phase content, crystalline quality and source of stresses in these films. Topography, hardness and Young's modulus were studied by scanning force microscopy (SFM) and nanoindentation methods. It has been ascertained that for the diamond films grown on silicon substrates with nanodiamond seeding hardness and crystalline quality is higher than for films on scratched silicon. The diamond films demonstrate Raman upshift with respect to natural diamond, indicating presence of internal compressive stress. It was shown that various types of impurities and defects induce compressive stresses in the diamond grains.     

Nanodiamond growth on diamond by energetic plasma bombardment

The present work studies the growth of nanodiamond under prolonged DC glow discharge plasma bombardment of 1 μm thick polycrystalline CVD diamond. Using Raman spectroscopy, near edge X-ray absorption fine structure (NEXAFS) and secondary ion mass spectroscopy (SIMS) it is shown that nanodiamond formation on diamond starts directly, skipping the nucleation stage required to form the precursor material with the appropriate density and hydrogen content. On the other hand, the amorphous carbon/nanodiamond composite structure is substantial to the nanodiamond nucleation under energetic plasma bombardment and cannot be eliminated by starting with a micro-crystalline diamond substrate. The results give additional insight to the phenomena of nanocrystalline nucleation and growth under energetic particle bombardment relevant to a variety of systems including bias enhanced nucleation of diamond and cBN deposition by ion assisted methods.     

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.     

Electronic properties of dehydrogenated nanodiamonds: A first-principles study

First-principles calculations using quantum-mechanical density functional theory (DFT) are carried out to study the geometric structure and electronic properties of dehydrogenated nanodiamonds with diameters varying from 0.8 nm to 1.6 nm. The results show that the electronic properties of dehydrogenated nanodiamond are quite different from those of bulk diamond or hydrogenated nanodiamond. Surface atoms play an important role in the electronic structure, especially the states near the Fermi level, for dehydrogenated nanodiamond. In addition, it has been revealed that the size-dependent feature in the electronic properties for dehydrogenated diamonds is also contributed by the surface effect, in addition to the quantum confinement effect.     

The mechanical enhancement of chemical vapor deposited diamond film by plasma low-pressure/high-temperature treatment

Direct-current (DC) arc plasma jet has been utilized to anneal chemical vapor deposited (CVD) polycrystalline diamond produced by the same device. Intense heating pulses around several target temperatures were achieved under constant plasma state and chamber pressure (∼5 kPa), which ensured this plasma low-pressure/high-temperature (LPHT) treatment succeed without diamond graphitization. The treatment significantly improves the fracture strengths of the diamond samples. The enhancement is ideally proportional to the temperature and the largest increase is up to 96.38%. Fractographic analysis has been done by scanning electron microscope (SEM), and the transgranular fracture/intergranular fracture ratio increases as the temperature increases. Raman spectra indicate that there exists huge compressive stress (>1.6 GPa) at the grain boundaries after the treatment so that the boundary strength is greatly enhanced. The interacting relationship between the interface graphitization and the compressive stress might play a crucial role in preventing the interface from further graphitization. This suggests that the plasma LPHT treatment can be improved by using higher temperature or longer time.     

Nanodiamond lateral field emitter devices on thick insulator substrates for reliable high power applications

Nanocrystalline diamond field emitter array devices on a thick insulator substrate are being developed for high power applications. These monolithic lateral emitter diodes in comb array configurations demonstrate potential for high emission current applications. A 640 μm-thick aluminium nitride insulating substrate has been integrated with nanodiamond for device electrode isolation. The fabrication process and preliminary field emission characterization results are discussed. The nanodiamond lateral vacuum device may be a superior way to achieve reliable high-speed and high-power electronics.     

Three-dimensional laser writing in diamond bulk

Laser fabrication of micrometer-scale graphitic structures in the bulk of monocrystalline CVD diamond was investigated. Ultra-short (1 ps) pulses of Ti:sapphire laser were tightly focused inside the sample. Initiation of graphitization in the focal volume and propagation of graphitization front towards the laser beam are considered. Electrical resistivity of the produced graphitic material was measured (3.6-3.9 Ω cm). Problems and strategies of 3D laser writing are discussed paying special attention to graphitization homogeneity, minimization of the wire diameter, control of the structure shape and reduction of the surrounding diamond damage. Laser fabrication of complex microstructures including pillars, plates, hexagonal chain and periodic arrays of straight wires has been demonstrated.     

Detonation synthesized nanodiamond powder for the preparation of porous polycrystalline micron powders

Characteristics and properties of detonation-synthesized nanodiamond (UDD) powder and polycrystalline micron (PDD) powders produced by high pressure–high temperature sintering of detonation-synthesized diamond and a subsequent crushing of the resulting compacts have been described.It has been shown that during the sintering in the diamond stability region the phase composition of the superhard material under study changes and the grain secondary recrystallization takes place. As a result a porous material with the nanostructure, which when ground to the micron powder size exhibit good polishing properties, is produced.     

In situ formation of onion-like carbon from the evaporation of ultra-dispersed nanodiamonds

We report the in situ formation of onion-like carbon (OLC) by evaporation from a nanodiamond source under ultra-high vacuum conditions. The OLC is characterized by transmission electron microscopy (TEM), atomic force microscopy (AFM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) and is found to be highly defective but completely separated. The absence of any signature in XPS, Raman spectra and TEM associated with nanodiamond in the film suggests that the OLC is formed from carbon vapor or by the direct evaporation of only the smallest particles resulting from nanodiamond graphitization. The method thus provides a route to the formation of individually separated OLC nanoparticles.     

Chemically vapor deposited diamond-tipped one-dimensional nanostructures and nanodiamond–silica–nanotube composites

Composite thin films of nanodiamond and silica nanotubes were synthesized by means of microwave plasma assisted chemical vapor deposition (MPCVD) on silica nanotube matrix that was seeded with nanodiamond particles. SEM, Raman spectroscopy, and EDX were used to analyze the composite. Wet chemical etching was applied to selectively remove exposed silica from the composites for further revealing the nanostructure of the composites. Nanodiamond grew around silica nanotubes and filled the space left between silica nanotubes to form a continuous film. When appropriately selected sizes of nanodiamond particles were used as diamond seeds, silica nanotubes capped with CVD-grown diamond crystals were also obtained. Potential applications and implication of composites of nanodiamond and 1-D nanostructures will be discussed.     

Microstructural features and thermal stability of alumina-bonded nano-polycrystalline diamond synthesized by detonation sintering

In this work, alumina-bonded nano-polycrystalline diamond (NPD) was synthesized by detonation sintering in the temperature range of 3000–3500 K and pressure range of 15–25 GPa. The microstructures and thermal stability of the NPD detonation sintered at 3255.05 K and 24.51 GPa were studied, and are described herein. Transmission electron microscopy and electron diffraction revealed that the polycrystalline diamond has a unique formation process and no graphitization. Scanning electron microscopy indicated that the size of polycrystalline particles increased in samples 2, 3, and 4. And thermogravimetric analysis indicated that the thermal stability of the diamond particles was enhanced. The 18% mass loss of specimen corresponded to the oxidation and decomposition of the amorphous carbon and carbon-containing compounds synthesized by detonation. Finally, graphitization calculations showed that the graphitization probability of polycrystalline diamond produced at the temperature of 3255.05 K and pressure of 24.51 GPa was 15.04%.     

Electron transport and electron field emission of nanodiamond synthesized by explosive detonation

Electron transport and electron field emission of nanometer-size diamond powders coated on quartz and n⁺-type Si substrates have been characterized. The nanodiamond powders were synthesized by explosive detonation. The measurement of temperature-dependent conductivity shows that the conduction of the nanodiamond coating is non-Arrhenius leading to an interesting behavior at low temperatures. The material shows a good behavior of electron field emission. In the electric field range from 3 to 5 V/μm, the emission can be approximately described by the Fowler–Nordheim (F–N) equation. A stable emission current density as high as ∼95 mA/cm² was obtained under an applied field of 5 V/μm. It has been suggested that the novel electron transport and the high emission current density of the samples might originate from their non-continuous network structure of the nanodiamond particles.     

Preparation of diamond/SiC composites by the liquid silicon infiltration method and their microstructure and properties

Diamond/SiC composites have long been recognized as advanced materials for thermal management as they exhibit excellent thermal and mechanical properties. The objective was to investigate and understand the phase composition, diamond graphitization behavior, microstructure, and properties of diamond/SiC composites developed following the liquid silicon infiltration process. The results revealed that the incorporation of α-SiC particles increased the degree of uniformity of the microstructure of the diamond/SiC composites. The acoustic mismatch model was used to analyze the samples before and after diamond graphitization to evaluate the interfacial thermal resistance of the composites. The results indicated that the interfacial thermal resistance of the graphitized composites was 11.9 times higher than the interfacial thermal resistance of the un-graphitized composites. Finally, the correlation between the diamond content of the composites and their thermal and mechanical properties was investigated.     

Effects of the hosting nano-environment modifications on NV centres fluorescence emission

The spontaneous emission rate properties of diamond nitrogen-vacancy (NV) centres are largely dependent on the hosting material, especially in nanodiamond. Inside a nanodiamond the fluorescence emission dramatically deteriorates as compared with the case where the defect is embedded in single crystal diamond. In addition the nano-environment, i.e. the hosting nanodiamond, can modify the spontaneous emission properties. It is important to study and identify the mechanisms responsible for these modifications to eventually employ the defect as a sensor of local density of states or to achieve emission enhancement. In this work, we summarize our approach to separate the electromagnetic effects, inducing radiative modification in the spontaneous emission of NV, from other non-radiative effects, these last leading to the quenching of NV emission. We also observe blinking which may be associated with photo-induced charge fluctuations by these additional non-radiative decay channels.     

Structural studies of nanodiamond by high-energy X-ray diffraction

The atomic scale structure of explosive diamond nanoparticles has been studied using high-energy X-ray diffraction. The diffraction data have been converted to the real space representation in the form of the radial distribution function. Spherical and truncated octahedron nanodiamond clusters containing from 729 to 1182 atoms have been computer generated and then relaxed using the molecular dynamics method with the reactive empirical bond order potential for carbon-carbon interaction and the Lennard–Jones potential with parameters for inter-layer interactions. Validity of such constructed models has been verified by comparison of the simulations and the experimental data in both real and reciprocal space. The obtained results show that the structure of the investigated diamond nanoparticles cannot be satisfactorily described in terms of the model based on the perfect diamond lattice. The core-shell model with an average size of 22.5–23.4 Å, consisting of the diamond core and the graphite-like shell, accounts very well for the experimental data.     

Piezo-actuated nanodiamond cantilever technology for high-speed applications

Diamond cantilever actuators show high resonance frequencies but need also high actuation forces, pointing towards piezoelectric actuation by a PZT/diamond unimorph. In this study lead zirconate titanate (Pb(Zr,Ti)O₃, PZT) layers have been deposited onto nanocrystalline diamond films by sol–gel deposition, to realize high-speed MEMS actuators. The fabrication technology is based on self-aligned patterning and on optical lithography. A mechanical resonance frequency of 3.9 MHz has been obtained for 30 µm cantilever length dominated by the nanodiamond Young's modulus of approximately 1000 GPa.     

Thermal analysis for graphitization and ablation depths of diamond films

In the present study, a thick diamond film deposited on a 6″ silicon wafer was planarized by an ArF excimer laser. In order to predict the graphitization thickness formed in the diamond film by a laser beam moving with a sliding velocity as well as a fluence, the general solution of the three-dimensional temperature distribution is successfully developed to be a function of the fluence and pulse duration of the laser beam. A reported model developed for the graphitization probability of diamond film is adopted in the present study to determine the graphitization thickness if the temperature solutions are substituted into this model. The ablation thickness in the graphitization layer can be determined so long as the local temperature in the specimen is higher than the ablation temperature of the graphitization layer. Then, the thickness of graphite residual on the working surface after ablation can be obtained by the subtraction of the ablation thickness from the graphitization thickness. This graphite thickness is proved to be very close to the value obtained from scratching tests. This implies that the temperature solutions predicted by the present model are trustworthy. The effect of the scanning velocity is insignificant to the surface temperature if the pulse duration time is much shorter than the time period between two adjacent laser pulses.     

Effect of vacuum annealing on field emission from heavily phosphorus doped homoepitaxial (111) diamond

Vacuum anneal treatments effects on field emission properties of phosphorus doped diamond are discussed. The C1s core level of diamond is characterized by X-ray photoelectron spectroscopy (XPS). With increasing annealing temperature, firstly oxygen desorption takes place from surface, which induce surface reconstruction followed by graphitization of surface. Field emission properties, therefore, strongly depend on vacuum anneal temperature. The threshold voltage of diamond annealed at 900 °C is the lowest. Fowler–Nordheim plots indicate that diamond annealed at 900 °C has the lowest barrier height, which is in good agreement with electron affinities as measured on carbon reconstructed surface. Further increased annealing temperature induces surface graphitization, which causes a threshold voltage increase of field emission.     

Deaggregation of ultradispersed diamond from explosive detonation by a graphitization–oxidation method and by hydroiodic acid treatment

The deaggregation of ultradispersed diamond (UDD) from explosive detonation has been carried out by a graphitization–oxidation method and by a hydroiodic acid (HI) treatment. The first method is used to break the crystalline bridge between the diamond primary particles, and the second one is employed to break down C–O–C ethereal linkages formed between the diamond crystallites. The samples after the above-mentioned chemical treatments were suspended in water by ultrasonic waves and the resulted suspensions were examined by a dynamic laser scattering method to obtain particle size distributions of samples in water. Experimental results show that after graphitization–oxidation treatment, nearly 35% of the particles, and after further HI treatment 43% of the diamond aggregates were reduced to less than 50 nm, and the particle size of nearly 98% of the aggregates is less than 200 nm in diameter. Thus, these two methods are useful for the deaggregation of UDD from detonation. But it is evident that the results obtained are far from the monodispersion state of UDD. In order to reach an even higher level of deaggregation, further study to improve treating methods is needed.