Field emission of polycrystalline diamond films grown by microwave-plasma chemical vapor deposition. I. Effects of surface morphology of diamond

The effects of surface morphology on the field emission of non-doped polycrystalline diamond films with thicknesses ranging from 5 to 55 μm were studied. Diamond films grown by a microwave-plasma chemical vapor deposition technique had both the diamond and non-diamond components with pyramidal and angular crystalline structures. Although the average crystallite size increased with increasing the film thickness (d), the volume fraction of the non-diamond components in the films was insensitive to the film thickness. However, the turn-on electric field, F_T, (defined as the low-end electric field to emit electrons) showed a U-shape dependence on the film thickness. This U-shape dependence was explained by a model in which the emission current was controlled by Fowler–Norheim tunneling of electrons at surface of the pyramids when d was thinner than 20 μm and by carrier transport in the polycrystalline diamond film when d was thicker than 20 μm. The lowest field of 4 V/μm was obtained in the film with 20 μm thick.     

Field emission of polycrystalline diamond films grown by microwave plasma chemical vapor deposition. II. Effect of p-type doping in diamond

The effects of boron (B) doping on the field emission (FE) of diamond films grown by a microwave plasma chemical vapor deposition technique were studied. Raman scattering spectroscopic analysis revealed that B-doping significantly suppressed formation of non-diamond components in the diamond film. The B-doped p-type diamond films had low resistivity, ranging from 0.07 to 20 Ω cm, and various volume fractions of non-diamond components in the diamond films. The turn-on electric field, F_T, was independent of the resistivity, the film thickness, and the volume fraction of the non-diamond components. The lowest F_T value of 8 V μm⁻¹ and the highest emission current of 3×10⁻² A cm⁻² were obtained in the B-doped diamond films. The high efficiency of the electron emission in the B-doped diamond films was believed to be due to the increase in volume fraction of the conductive regions in the film and the high density of emission sites on the film surface.     

The quadratic electrooptic effect and estimation of antipolarization in ADP

The quadratic electrooptic coefficients g₁₁₁₁ and g₂₂₁₁ have been measured interferometrically in ADP at room temperature. In addition, ‖g₁₁₁₁-g₂₂₁₁‖ was determined by a dynamic polarimetric technique at the same temperature. Both experiments yield values for g₁₁₁₁-g₂₂₁₁ at 21 C and λ = 0.633 μm with an average of (-5.9±0.5)×10^-20 m²/V². An estimate is made, using this value, of the spontaneous antipolarization appearing in the antiferroelectric phase of ADP. This antipolarization is found to be comparable in magnitude with the spontaneous polarizations observed in ferroelectrics belonging to the KDP family of crystals.     

The ratio of the extinction coefficient to the mass of atmospheric aerosol particles as a function of the relative humidity

The ratio R_K of the extinction coefficient of aerosol particles in cm ¹, at the wavelength of light λ = 0·55 μm, and the mass of atmospheric aerosol particles in gem³ has been computed as function of the relative humidity for six types of continental and maritime aerosols. With the mean value R_K = 10⁴ cm²/g for all aerosol types being assumed to be independent of relative humidity, only the order of magnitude of the aerosol mass can be determined from visibility observation or measurement of the extinction coefficient of the aerosol particles.     

Excimer Laser Induced Removal of Particles from Hydrophilic Silicon Surfaces

A pulsed KrF excimer laser was used to remove several types of submicron-sized particles from silicon surfaces. Polystyrene latex particles, 0.1 μm and larger, were removed from silicon surfaces by dry laser cleaning (no water layer condensed on the surface) but SiO₂ particles could not be so removed. However, during steam laser cleaning, in which a thin film of water is deposited on the surface as both an energy transfer medium and an adhesion force reduction agent, these 0.1-0.2 μm SiO₂ particles were almost entirely removed. Calculations of the various forces contributing to adhesion indicate that hydrogen bonds are the major contributor to the adhesion of inorganic particles to substrate surfaces. Photoacoustic detection, using piezoelectric transducers, monitored the surface vibrations induced by the laser pulses.     

WETTABILITY AND SURFACE ENERGETICS OF ROUGH FLUOROPOLYMER SURFACES

Hydrophobic solid surfaces with controlled roughness were prepared by coating glass slides with an amorphous fluoropolymer (Teflon® AF1600, DuPont) containing varying amounts of silica spheres (diameter 48 μm). Quasi-static advancing, θ_A, and receding, θ_R, contact angles were measured with the Wilhelmy technique. The contact angle hysteresis was significant but could be eliminated by subjecting the system to acoustic vibrations. Surface roughness affects all contact angles, but only the vibrated ones, θ_V, agree with the Wenzel equation. The contact angle obtained by averaging the cosines of θ_A and θ_R is a good approximation for θ_V, provided that roughness is not too large or the angles too small. Zisman's approach was employed to obtain the critical surface tension of wetting (CST) of the solid surfaces. The CST increases with roughness in accordance with Wenzel equation. Advancing, receding, and vibrated angles yield different results. The θ_A is known to be characteristic of the main hydrophobic component (the fluoropolymer). The θ_V is a better representation of the average wettability of the surface (including the presence of defects).     

Pretreatment of substrate surface for improved adhesion of diamond films on hard metal cutting tools

The d.c. plasma jet CVD process is one of the most promising coating processes used for the production of polycrystalline diamond films. In comparison with other CVD processes, its obtainable linear growth rates, in the range of 100 μm/h, are much higher than growth rates of microwave or hot filament CVD (1–10 μm/h).

One interesting application is the diamond coating of cutting tools. The main problem here is the poor adhesion of the films. Therefore, a mechanical or chemical pretreatment or intermediate layers are used to improve the adhesion.

In these investigations the influence of mechanical pretreatment by grinding and polishing with diamond powder of different grain sizes as well as chemical etching are examined on WC-Co hardmetals. Sputtered metallic interlayers of different thicknesses and arc-ion plated amorphous carbon films are deposited on these substrates, and diamond films were synthesized on these pretreated cutting tools by d.c. plasma jet CVD.

Adhesion and wear resistance of the diamond films have been examined by dry turning tests on very abrasive metal-matrix composites. Distinct improvement in adhesion of diamond coatings on hard metal substrates was achieved by two methods of substrate surface pretreatment: etching with Murakami's solution and following ultrasonically seeding with diamond particles or using an amorphous carbon film as intermediate layer.     

A new elegant technique for polishing CVD diamond films

It is well known that the columnar growth nature of CVD diamond results in a very rough growth surface and the surface roughness steeply increases with film thickness, especially for thick CVD diamond films. In this paper, we report the successful implementation of a new elegant technique for polishing thick polycrystalline CVD diamond films at high polishing rate of up to 10 μm/h. This technique involves polishing the as-grown polycrystalline diamond films with another thick as-grown polycrystalline diamond film, which acts as a polishing abrasive. Two types of diamond films were prepared using microwave plasma CVD and then polished for 2 h using the new polishing technique. A stylus profilometer, scanning electron microscopy and Fourier transform infrared spectroscopy were used to measure the surface roughness, characterize morphology and optical transmission of the samples before and after polishing, respectively. By polishing, thickness of 20–30 μm was removed from the top surface, and the mean surface roughness R_a of the films reduced significantly, e.g. for one film R_a reduced initially from 5.2 to 1.35 μm and the other from 3.2 to 0.55 μm. The principal advantages of this new polishing technique are simplicity, flexibility and time saving. This simple method can serve as ‘rough chipping’ to quickly remove the rough top surface and then combine with conventional polishing methods for precision machining to further reduce the surface roughness to a specific desired degree.     

Numerical and experimental simulations of diffusion charging of non-spherical aerosol particles by dense bipolar ions

Numerical and experimental simulations have been conducted for the time history of the diffusion charging process on the surface of aerosol particles by dense bipolar ions under continuum conditions. The range of conditions treated in the numerical simulations include positive-negative ion diffusion coefficient ratio from 0 to 1, aerosol particle radius from 0.1 to 10 μm, Debye ratio R_p/λD from 0 to 1 (equivalent to maximum charge density up to N₁ = 10¹² cm⁻³ for an ion temperature of 300 K), the major-to minor axis ratios of prolate spheroids, L, from 1 to 100. The experimental simulation was conducted by using a conductive dummy particle suspended by a thin shielded wire, and the charged particle deposition current flux was measured and the bipolar environments. Then the effect of particle surface charges was simulated by imposing an electric potential on the dummy particles. The results show that, (1) for small ion density (R_{p/λD 10⁻²}); the present results are in good agreement with model of Chang et al. (1978, 1983). (2) the aeroso particle charging speed and charging limit increase with increasing Debye ratio; (3) for larger Debye ratio, bipolar charging is faster than unipolar charging; (4) the effect of particle shape L is observed to be significantly influenced by Debye ratios: (5) the charging limit of the aerosol particle increases with L.     

Preadhesion laser surface treatment of polycarbonate and polyetherimide

The effects of carbon dioxide (10.6 μm) and argon fluoride excimer (193 nm) lasers as preadhesion surface treatments for polycarbonate and polyetherimide were studied. Single lap shear specimens bonded with a flexible polyurethane adhesive were utilized to investigate the effect of the various laser treatments on final shear properties and mode of failure. Experimental results based on mechanical properties, scanning electron microscopy and Fourier transform infra-red have indicated that the excimer laser is more effective than the carbon dioxide laser with respect to shear properties and the level of adhesion obtained with the thermoplastic adherends and polyurethane adhesive studied. Polycarbonate with its lower glass transition temperature T_g was affected by the carbon dioxide laser to a higher extent than the higher T_g polyetherimide. Furthermore, polycarbonate exhibited enhanced shear properties and a higher level of chemical modification following excimer laser irradiation compared to polyetherimide.     

Application of laser ablation technique for removal of chemically inert organically modified silicate coatings

Removal of organically modified silicate (Ormosil) coatings from 2024-T3 aluminum alloy substrates has been investigated using a laser ablation technique utilizing a 308 nm excimer laser. Incorporation of UV-absorbing dye molecules containing λ_max in the vicinity of the laser wavelength (butyl-PBD, Furan 2, Morin, HQSA) into the Ormosil thin film was found to facilitate coating removal. Ormosil thin films containing 0.1–0.5 mol% UV-absorbing dye molecules were subjected to laser treatment at various fluences ranging from 0.2 to 0.6 J/cm². The presence of the UV-absorbing dye molecules in the Ormosil thin film was found to facilitate coating removal at a lower fluence as compared to dye-free coatings as determined by scanning electron microscopy. The effectiveness of coating removal was found to depend on several parameters including laser fluence, number of pulses per spot, and dye concentration.     

Deformation and toughness of polymeric systems: 3. Influence of crosslink density

In part 1 of this series the phenomenon of a critical ligament thickness (ID_c) below which brittle polymers become ductile was investigated for polystyrene (PS). Using the thermoplastic polystyrene-poly(2,6-dimethyl-1,4-phenylene ether) (PS-PPE) model system, it was demonstrated in part 2 of this series that the absolute value of ID_c as well as the maximum toughness (i.e. maximum strain to break) was dependent on the network density of the polymer used. In this study the toughness and ID_c of crosslinked thermosetting polymers were investigated using epoxides based on the diglycidyl ether of bisphenol A as a model system. The crosslink density (v_c) is varied between values comparable with (v_c = 9 × 10²⁵ chains m⁻³), up to values much higher than (v_c = 235 × 10²⁵ chains m⁻³), the entanglement density in the thermoplastic PS-PPE system. The maximum macroscopic toughness proportional to the strain to break (λ_macr) or given by the slow-speed fracture toughness (G_Ic) and the notched high-speed tensile toughness (G_h) of core-shell rubber-modified epoxides uniquely increases with an increasing molecular weight between crosslinks (M_c). Only by using extreme testing conditions (notched high-speed impact testing), could the ID_c of a limited range of epoxides be determined: 0.21 μm (v_c = 9 × 10²⁵ chains m⁻³) ≤ ID_c ≤ 0.29 μm (v_c = 14 × 10²⁵ chains m⁻³). Both the experimentally determined values of ID_c and the toughness of the epoxides compare well with the values determined for the entangled thermoplastic PS-PPE model system in the same range of network densities, elucidating the principal similarity of the influence of entanglements and crosslinks on the deformation processes. Good agreement was observed between the experimentally determined values of ID_c of the epoxides and the values predicted by the simple model introduced in part 2 of this series.     

Pulsed laser surface modifications of diamond thin films

In view of practical applications requiring diamond films, plates and membranes with very smooth surfaces, ArF excimer laser polishing treatments were applied to thin (30 μm) diamond films grown by CVD on silicon substrates. The as-prepared diamond surfaces and the laser-treated parts of the samples were characterised by SEM analysis, Raman and micro-Raman spectroscopy. The presence on the laser-treated surface of a thin amorphous carbon layer responsible for the higher surface electrical conductivity and for the different optical reflectivity properties was evidenced. Using confocal micro-Raman spectroscopy a comparative depth profile analysis of the phase quality, below the surface in different regions of the films, was carried out. After short (10 min) treatment by H₂ plasma etching in the CVD chamber the graphitic top layer was completely removed from the samples.     

Diamond membranes with controlled porosity

Porous diamond membranes were fabricated using a method based on growing diamond on a patterned silicon surface. Protrusions on the silicon surface acted as molds for forming the pores on the diamond film. The membrane described here was 10 μm thick, 15 mm in diameter, had a pore size of about 60 μm in diameter and a nominal pore density of 1.56 mm⁻². The method is highly versatile and a wide variety of pore surface configurations can be formed; the technique used allows a wide parameter range. These kinds of membranes could have application as heat exchangers and as filtering devices.     

Tribological properties of partly polished diamond coatings

Extremely low friction coefficient was achieved with “partly polished diamond coatings”. Diamond coatings were deposited onto Si substrates by MWCVD with the mixture of CH₄ and H₂. Deposited films were characterized by X-ray diffraction (XRD), Raman spectroscopy and Electron Spectroscopy for Chemical Analysis (ESCA). Sharp peak derived from polycrystalline diamond was observed by XRD. Whereas Raman profile of partly polished diamond coatings was close to that of ta-C. This result suggests that small diamond grains were surrounded by amorphous carbon structure in the diamond coatings. Deposited diamond coating was polished with each other. Surface roughness R_a was reduced to 0.3, 0.2 and 0.08 μm, respectively. The hardness of the polished diamond coatings investigated by Nanoindentation technique was approximately 40.8 GPa, which was relatively lower value compared with conventional as-deposited CVD diamond coatings. For the tribological properties, we examined the effect of surface roughness using flat-ended pin-on-disk apparatus and ball-on-disk apparatus with bearing ball (SUJ2) and stainless steel (SUS304). Diamond coatings were deposited onto flat-ended pin and disk, and they were polished to R_a = 0.3, 0.2 and 0.08 μm. After the 6000 cycle process extremely low friction coefficient, μ = 0.05, was achieved with the pair of R_a (flat-ended pin, disk) = R_a (0.08, 0.3) in flat-ended pin-on-disk apparatus. In order to clarify the effect of surface roughness, ball-on-disk was carried out with different surface roughness, R_a = 1.7, 0.3, 0.2 and 0.08 μm. Here as-deposited diamond coating, R_a = 1.7 μm, was used as a reference point. Friction coefficient of μ = 0.09 was obtained for both balls. After the tribological tests balls were analyzed by scanning electron microscope (SEM) and energy dispersed X-ray spectrometer (EDX).     

Ultraviolet-irradiated precision polishing of diamond and its related materials

In this study, a novel ultraprecision polishing process for single-crystal diamond substrates was developed utilizing ultraviolet (UV) irradiation. This polishing is basically a mechanochemical polishing (MCP) process combined with a UV-induced photochemical reaction. Carbon atoms on the topmost surface of diamond are oxidized by active species such as hydroxyl radicals (OH radicals) and oxygen radicals at localized high temperature and finally removed as CO and CO₂. This polishing process was applied to diamond substrates, chemical vapor deposition (CVD) diamond-coated films and polycrystalline diamond (PCD). The results showed that the surface roughness of the entire substrate reached 0.2 nmRa within 1 – 3 h at a comparatively high removal rate. The characteristics and removal mechanism of UV polishing were also discussed.     

Surface engineering of diamond tips for improved field electron emission

Results are reported on the study of surface engineering of diamond microtips for improved field electron emission. Two-dimensional (2D) arrays of diamond pyramids were prepared using a molding technique. As-grown diamond pyramids of 3 and 9 μm in size with sharp apexes were insulating and showed a relatively poor field electron emission. Several ways are examined to provide an electrical conductivity of diamond in order to supply electrons for the emission: diamond boron doping, partial graphitization/amorphization by nitrogen ion implantation and/or high temperature annealing, formation of built-in conductive metal layers. A perceptible improvement of surface electrical conductivity and reduction of field electron emission thresholds down to 10 V/μm was observed. For explanation of the results it was enough to take into account only the geometric field enhancement on pyramids apexes.     

Walden product, ion-solvent interactions and solvent structure in water-rich mixtures ionic conductances in water-sulfolane, water-acetonitrile and water-dimethylsulfoxide

Conductance measurements are reported for several salts in binary aqueous mixtures containing up to 60 mole % sulfolane, 20 mole % acetonitrile and 20 mole % dimethylsulfoxide. The variations of R = (λ^±₀η₀)_s/(λ^±₀η₀)_w with solvent composition have been compared with those observed in other water-rich mixtures. Alkali cations show R values greater than one with maxima in all the solvent mixtures. This behaviour has been discussed in terms of “sorting”, “averaging” and “steric” effects. Contrary to what happens to alkali cations, halide ions show R values greater or lesser than one according to whether the organic solvent respectively increases or decreases water structure. On these bases we suggest that conductometric behaviour of the halide ions may be indicative of the effect of the cosolvent on the water structure in water-rich mixtures and that DMSO is a water structure breaker.     

Internal burning of pulverized semi-anthracite: the relation between particle structure and reactivity

Measurements have been made of structural properties of particles of pulverized semi-anthracite at various stages of combustion at temperatures in the range 1400–2200 K. Four size-graded fractions of the particles (mass-median sizes 78, 49, 22 and 6 μm) were burned at oxygen partial pressures of about 0.1 and 0.2 atm. The structural properties determined included specific surface areas, helium densities, X-ray diffraction patterns, and pore-volume distributions. The relation of these properties to the temperatures prevailing during reaction and to the degree of burn-off which took place showed that macropores (pores above 0.02 μm width) developed during reaction and that the micropore volume was reduced. The opening up of the macropores results from the combustion process, but the reduction in micropore volume is due mainly to the effect of the higher temperatures on the structure of the solid. The structural data are combined with kinetic data relating to the reaction of oxygen with the particles to determine values of the intrinsic reactivity coefficient R_s (the rate of carbon consumption per unit of total internal area per unit pressure of oxygen) under various conditions. R_s is independent of particle size and is given by the equation R_s(g/cm² s atm O₂) = 55 [−40 000/(RT_p)] where T_p is the temperature of the particles and R is in cal/mol K^*. Calculations of the intrinsic reactivity via two routes, treating the pores as having either a unimodal or bimodal distribution of sizes respectively, gave similar values of the reactivity. Values of R_s are used to predict, within a factor of two, the results of other workers' measurements of semi-anthracite combustion rates.