Laser ablation of diamond particles suspended in ethanol: Effective formation of long polyynes

Polyynes were synthesized by laser ablation of diamond particles with various sizes suspended in ethanol. Chain length distributions and total yields of polyynes produced were compared with those produced from graphitized diamond particles and graphite particles. The relative amounts of long polyynes such as C₁₄H₂ and C₁₆H₂ produced from diamond particles were found to be larger than those from graphitized diamond particles and graphite particles. From the change of the chain length distribution with the laser irradiation time, it is concluded that the long polyynes are produced directly from diamond particles at the initial stage of ablation. Furthermore, the total yield of polyynes was found to increase with the size of diamond particles and decrease as their graphitization proceeds. Possible mechanisms of these results are discussed.     

Interfacial zone surrounding the diamond in reaction bonded diamond/SiC composites: Interphase structure and formation mechanism

Diamond/SiC composites have attracted considerable research interests due to their outstanding properties sought for a wide range of applications. Among a few techniques used for the fabrication of diamond/SiC composites, molten Si infiltration is an approach highly favored due to its cost-effectiveness and process flexibility. This study critically evaluated the interfacial zone surrounding the diamond in a reaction bonded (RB) diamond/SiC composite. XRD suggests that the composite consists of diamond, α-SiC, β-SiC, Si, and graphite. TEM reveals that a thin layer of graphite surrounds the diamond grain and it appears to form through a process of diamond graphitization and amorphous carbon transformation during the fabrication. In addition, a carbon dissolution and saturation process is proposed as a predominant mechanism for the formation of nano-crystalline SiC near the interface as well as the defects inside the SiC grits. A minor Al₄C₃ phase is occasionally detected near the interface region.     

Submicron laser writing on diamond

A synthesized HPHT IIb diamond plate is irradiated with a KrF excimer laser (wavelength λ=248 nm, pulse duration τ =20 ns). The beam is focused onto the surface with an ultrafluar objective [numerical aperture (NA)=1]. The quality of the optical imaging system is first investigated by performing atomic force microscopy (AFM) measurements on holes ablated into Perspex (PMMA). On diamond, holes are drilled at various pulse energies down to values slightly above the ablation threshold. The hole depth and width are measured by AFM after annealing in air at 600°C during 3 h. Holes with submicrometre diameters are obtained, and suggestions on how to produce even smaller holes are given.     

Conversion of polycyclic aromatic hydrocarbons to graphite and diamond at high pressures

The polycyclic aromatic hydrocarbons (PAH): naphthalene, anthracene, pentacene, perylene, and coronene were submitted to temperatures up to 1500 °C at 8 GPa. To avoid catalytic action of metals on thermal conversion, graphite was used as container material. Moreover, graphite is very permeable to the gaseous products of thermal decomposition of PAH. The resulting thermal transformations and their evolution were studied by X-ray diffraction, Raman spectroscopy and scanning electron microscopy as a function of temperature for 60-s treatments. The nature of the initial compounds clearly affects the products of the different stages of carbonization and the first steps of graphitization. This becomes hardly discernible in the final stages of graphitization above 1000 °C. Above 1200 °C, graphite with high crystallinity forms in all cases. The temperature of the beginning of diamond formation does not seem to be influenced by the nature of the initial PAH and is equal to ∼1280 °C for all investigated compounds. Diamonds formed from the PAH are high-quality 5-40 μm single crystals. The p,T values of diamond formation here obtained are significantly lower than those previously known for direct graphite-diamond transformation.     

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.     

Femtosecond laser microstructuring in the bulk of diamond

In this paper we report the fabrication of graphitic microstructures in the bulk of diamond using 120-fs-laser pulses at 800 nm wavelength. Polished plates of single crystal diamond and optical quality polycrystalline CVD diamond were used as samples for 3D microstructuring. Under low fluence conditions and focusing a laser beam into the bulk of diamond plates, multipulse irradiation was found to result in the appearance and continuous growth of a laser-modified (graphitized) region from the focal plane towards the laser beam. Controlling the laser fluence and sample translation velocity (scanning beam velocity) allowed high-aspect-ratio ‘graphitic wires’ – microstructures of a few microns in diameter and several hundred micrometers in length – to be fabricated in the bulk of diamond. Physical processes responsible for the continuous growth of microscopic graphitic regions towards a laser beam are discussed. Results of comparative investigations of graphitic microstructures produced by laser pulses of different durations (120 fs and 300 ps) are presented to show the advantages of ultrashort laser pulses in 3D microstructuring of diamond.     

Amorphization and graphitization of single-crystal diamond — A transmission electron microscopy study

The amorphization and graphitization of single-crystal diamond by ion implantation were explored using transmission electron microscopy (TEM). The effect of ion implantation and annealing on the microstructure was studied in (100) diamond substrates Si⁺ implanted at 1 MeV. At a dose of 1 × 10¹⁵ cm^− 2, implants done at 77 K showed a damage layer that evolves into amorphous pockets upon annealing at 1350 °C for 24 h whereas room temperature implants (303 K) recovered to the original defect free state upon annealing. Increasing the dose to 7 × 10¹⁵ Si⁺/cm² at 303 K created an amorphous-carbon layer 570 ± 20 nm thick. Using a buried marker layer, it was possible to determine that the swelling associated with the amorphization process was 150 nm. From this it was calculated that the layer while obviously less dense than crystalline diamond was still 15% more dense than graphite. Electron diffraction is consistent with the as-implanted structure consisting of amorphous carbon. Upon annealing, further swelling occurs, and full graphitization is achieved between 1 and 24 h at 1350 °C as determined by both the density and electron diffraction analysis. No solid phase epitaxial recrystallization of diamond is observed. The graphite showed a preferred crystal orientation with the (002)g//(022)d. Comparison with Monte Carlo simulations suggests the critical displacement threshold for amorphization of diamond is approximately 6 ± 2 × 10²² vacancies/cm³.     

Theoretical aspects of diamond films and laser action

The thresholds for laser action at 309 nm wavelength from highly n-doped homogeneous diamond crystals (as opposed to the F-centre diamond laser) at excitation by electron beam irradiation have been evaluated by analogy with calculations which predicted the laser thresholds of electron beam irradiated GaAs lasers. With the condition that the material has to be a direct band semiconductor, the results of Picket and Mehl (SPIE, 877 (1988) 64) are used for calculations at very high stresses, allowing the laser wavelength to be shifted even further into the UV. In the present case, this stress is produced by crystal defects, and is similar to the stress produced in silicon after strong ion implantation. By comparison with other e-beam excited lasers, the UV diamond laser should achieve a high efficiency of up to 35%.     

Research on maximizing the diamond content of diamond/SiC composite

Diamond content is a key factor affecting diamond/SiC composite performance, especially thermal and mechanical properties, but the composite with high diamond content manufacturing is still challenging issues. Hot mold pressing combined with liquid silicon infiltration to make diamond/SiC composites with high diamond content and relative density has been proposed in this paper. In addition, the effect of diamond particle size on the maximization of diamond content as well as properties of the composites were evaluated. The experiment shows that the content of diamond in the composites increases with the increase of the diamond particle size. When the particle size of diamond is 400 µm, the volume fraction of diamond reaches 59.08%. The highest thermal conductivity (d_dia= 300 µm) and highest bending strength (d_dia= 50 µm) are 616.77 W/m K (It is the maximum TC of diamond/SiC prepared by pressureless infiltration at present) and 380 MPa, respectively. This work provides a novel and efficient preparation method for further improving the thermal conductivity of diamond/SiC composites.     

Diamond growth by carbon ion implantation of diamond

Medium energy (5–25 keV) ¹³C⁺ ion implantation into diamond (100) to a fluence ranging from 10¹⁶ cm⁻² to 10¹⁸ cm⁻² was performed for the study of diamond growth via the approach of ion beam implantation. The samples were characterized with Rutherford backscattering/channelling spectroscopy, Raman spectroscopy, X-ray photoemission spectroscopy and Auger electron spectroscopy. Extended defects are formed in the cascade collision volume during bombardment at high temperatures. Carbon incorporation indeed induces a volume growth but the diamond (100) samples receiving a fluence of 4 × 10¹⁷ to 2 × 10¹⁸ at. cm⁻² (with a dose rate of 5 × 10¹⁵ at. cm⁻² s⁻¹ at 5 to 25 keV and 800 °C) showed no He-ion channelling. Common to these samples is that the top surface layer of a few nanometers has a substantial amount of graphite which can be removed by chemical etching. The rest of the grown layer is polycrystalline diamond with a very high density of extended defects.     

Graphitization of small diamond cluster — Molecular dynamics simulation

Molecular dynamics simulation was used to study graphitization process of a small diamond cluster at 1200, 1500, and 1800 K. The cluster was in the shape of a sphere of about 3 nm in diameter, and interaction between carbon atoms was described by the reactive bond order potential. Results obtained for 1500 K showed transformation of diamond nanoparticle into a carbon onion with diamond-like core and graphite layers in its outer shell. At 1800 K the process was faster and graphitization more effective. The whole final cluster was basically comprised of the onion structure, but it was irregular and separation between layers ranged from 0.2 to 0.3 nm.     

Transparent diamond ceramics from diamond powder

Diamond possesses a unique combination of excellent optical, thermal, and mechanical properties, and is therefore an ideal transparent ceramic material for harsh and extreme environments. Due to its important applications in technology, transparent diamond ceramic (TDC) has been explored and prepared by chemical vapor deposition (CVD) or direct conversions of non-diamond carbon precursors at high pressure and high temperature (HPHT), but the preparation of large-size TDC with high mechanical strength remains a challenge. Here, we report for the first time, a transparent polycrystalline diamond ceramic from diamond powder with a transmittance of ～60 % at wavelengths of 400–1600 nm. The analyses of phase composition, residual stress and microstructure evolution of the sintered samples with different sintering conditions indicate that compression at high temperatures (>2000 ?C) facilitates the deformation of diamond grains, allowing for densification and diamond-diamond bonding formation. The sintering pressure of the diamond powders with an optimized particle size distribution was dramatically reduced from 16 GPa to 10 GPa. Our results, based on successfully preparing centimeter-sized TDC, set the standard and the precedent for the large-scale preparation of larger TDC in proximity to industrial conditions.     

Superlattice structures from diamond

Diamond superlattices were fabricated by producing multilayer structures of isotopically pure carbon-12 (¹²C) and carbon-13 (¹³C), which confine charge carriers through a difference in band-gap energy. ¹²C/¹³C homojunctions of a diamond layer in which isotopic composition modulation occurred were grown via a microwave plasma-assisted chemical vapor deposition on diamond substrates. Secondary ion mass spectrometry (SIMS) and imaging measurements were employed to characterize the isotopic composition of the diamond superlattices. Layers between 2 and 30 nm thick were designed and fabricated.     

Structural design and mechanical properties of porous structured diamond abrasive tool by selective laser melting

Additive manufacturing is an important and promising way to realize the structural-functional integration of diamond abrasive tools. In the presented study, the porous diamond grinding heads with different pore structure and porosity were designed and fabricated by selective laser melting (SLM). By analyzing the stress distribution of overall structures and pore units, the 50 %-porosity square-pore structure with lowest stress concentration degree was optimized. The porous composite samples had good SLM formability, including good integrity and connectivity of pore units, and the diamond abrasives were evenly distributed and exhibited good retention and protrusion height. The high retention was attributed to the multiple interfacial system composed of carbide layer and solid solution strengthening layer. Compared with other porous samples, the 50 %-porosity square-pore structured sample with frame supporting unit and uniform stress distribution showed high deformation resistance of 430 MPa in yield strength and energy absorption capacity of 56.4 MJ/m³, which well verified the simulation results. The wear and grinding tests showed that the sharpness and self-sharpening ability of porous samples were significantly superior to the full-dense sample, and the grinding ratio increased with the increasing of the porosity.     

Fast neutron diamond spectrometer

This work is devoted to the development of fast neutron diamond spectrometer intended for the use in diagnostic systems of ITER project. Mathematical code including full model of diamond detector response under neutron irradiation and the problem of the reconstruction of the original neutron spectrum is being developed. Diamond detector response under ²⁴¹Am-Be neutron irradiation has been measured as well as calculated. Acceptable agreement of the original and the reconstructed neutron spectra has been achieved.     

TEM and Raman characterisation of diamond micro- and nanostructures in carbon spherules from upper soils

Carbonaceous spherules of millimeter size diameter and found in the upper soils throughout Europe are investigated by TEM, including SAED, HRTEM and EELS, and Raman spectroscopy. The spherules consist primarily of carbon and have an open cell-like internal structure. Most of the carbon appears in an amorphous state, but different morphologies of nano- and microdiamond particles have also been discovered including flake shapes. The latter observation, together with the original findings of some of these spherules in crater-like structures in the landscape and including severely deformed rocks with some spherules being embedded in the fused crust of excavated rocks, points towards unique conditions of origin for these spherules and particles, possibly of exogenic origin.     

Effects of hydrogen on electronic properties of doped diamond

We have investigated the effects of hydrogen on the electronic structure of diamond doped boron and sulfur using cluster model method within the frame of ab initio density functional theory (DFT). The results show that the presence of hydrogen results in a deep donor level with no change of conductivity type in sulfur-doped diamond samples and the formation of the multiple hydrogen-boron complexes may cause a conductivity type transition in the hydrogen-rich boron-doped diamond samples.     

Large cross-section edge-coupled diamond waveguides

A new approach to fabricate large cross-section edge-coupled waveguides on free-standing thin diamond substrates is reported. Combining inkjet printing of photoresist multilayers with photolithographic patterning, both edge and ‘coffee stain’ effects were successfully eliminated, allowing the fabrication of well-defined, millimetre-scale uniform photoresist micro-stripes which extend to the very edge of the diamond substrate. Subsequent transfer of these micro-stripe structures into diamond by inductively coupled plasma (ICP) etching allowed long edge-coupled waveguides in diamond to be made. Guided wave propagation in these diamond waveguides was also confirmed.     

Grinding performance improvement of laser micro-structured silicon nitride ceramics by laser macro-structured diamond wheels

Silicon nitride ceramics are widely used in various industrial fields because of their excellent characteristics: high hardness, high elastic modulus, abrasion resistance, and high heat resistance. Diamond wheel grinding is the predominant and most productive method to machine silicon nitride ceramics. However, a lot of heat is generated due to high friction between a diamond grinding wheel and extremely rigid silicon nitride during grinding. This causes surface/subsurface damage, wheel wear, etc., which impairs the surface quality of silicon nitride. This impairment can restrict the use of silicon nitride ceramic components. To improve the surface quality and service life of grinding wheels, a laser macro-micro combination structured grinding (LMMCSG) method was presented. The results indicated that the grinding force ratio and surface roughness when using LMMCSG were respectively 31% and 40% lower than the grinding force ratio and surface roughness when using conventional grinding. Moreover, the LMMCSG method effectively reduced the wheel wear and workpiece subsurface damage.     

Synthesis and properties of single phase diamond ceramics

We report on the synthesis of single phase diamond ceramics and its microstructural and physical characterization. The most relevant physical properties are listed and are compared to natural diamond. The ceramic solid has been fabricated from chemically treated micro crystalline diamond powder, where oxy-functional groups have been attached to the surface. The special surface treatment is considered essential to achieve direct atomic bonding between adjacent grains.The hot isostatic pressing method (HIP) has been applied for materials processing that pertains to the stability region of the related carbon phase diagram. No further additives have been used for preparation. Transmission and scanning electron microscopy, energy dispersive X-ray analysis, X-ray diffraction and Raman spectroscopy have been used for the micro structural analysis. The achievable density is close to that of natural diamond, revealing porosity values of <3%. The micro structural analyses indicated the presence of small amounts of isolated diamond micro crystals, embedded into a matrix of polycrystalline diamond with a very small grain size. The grains are much smaller than the originally used micro crystalline source material, indicating crystal break-up and atomic rebonding during the sintering process. Also traces of sp²-hybridized carbon have been identified, located primarily at grain boundaries. Fracture of the material appears mostly transgranular. Relevant physical properties as thermal and electrical conductivity, hardness and Young's modulus approach those of natural diamond.