Hot-filament Technique Research Articles

Polyimide removal, cleaving, and fusion splicing of cylindrical and square fused silica capillaries for new separation and detection layouts in capillary electrophoresis and chromatography.

Fusion splicing is an already mature technology used to join the ends of optical fibers in the telecommunication industry. However, there is a lack of reliable and well-described protocols to join the ends of fused silica microtubes or capillaries using fusion splicing. In this work, a step-by-step procedure is proposed and demonstrated to find the optimal operation conditions to splice the ends of fused silica capillaries by fusion. The two most appropriate and known technologies were tested and optimized: the microfurnace technique (or hot filament technique) and the electric arc technique. It is shown that both produce good splices when the capillaries are well cleaved. For this reason, the removal of the external coating, which is important for good cleavage, was also revised. The cleaving techniques were also compared among themselves. As a result, fusion splices of cylindric-to-cylindric, cylindric-to-square, and square-to-square capillaries are demonstrated. They have potential applications in new detection schemes and separation layouts in capillary electrophoresis, electrophoresis-based hydrodynamic concentrators, chromatography, cell sorters, and microfluidics in general.

Enhanced Deposition and Reflective Properties of Thin Aluminium Films by Substrate Vibration

The influence of substrate's vibration during vacuum deposition of aluminium thin films on copper substrates was examined. Aluminium metal was evaporated in specially designed vacuum chamber using the hot-filament technique. Copper substrates were subjected to a vibration of 7.6 kHz during deposition. The Al coatings were identified using X-ray diffraction spectroscopy and scanning electron microscopy was used to examine the resulting microstructures deposited on the substrates. Coatings deposited under substrate vibration had fewer particles, spherical in shape and deposited over uniformly over the entire surface. This was not the case for the non-vibrated substrates, which tended to have much more densely packed granular shaped particles. The reflectivity experiments revealed that vibrated substrates were superior to the non-vibrated substrates by 28 %, while the difference in the thermal response was around 14 %.

Interfaces in Nano-/Microcrystalline Multigrade CVD Diamond Coatings

The interfaces of multilayered CVD diamond films grown by the hot-filament technique were characterized with high detail using HRTEM, STEM-EDX, and EELS. The results show that at the transition from micro- (MCD) to nanocrystalline diamond (NCD), a thin precursor graphitic film is formed, irrespectively of the NCD gas chemistry used (with or without argon). On the contrary, the transition of the NCD to MCD grade is free of carbon structures other than diamond, the result of a higher substrate temperature and more abundant atomic H in the gas chemistry. At those transitions WC nanoparticles could be found due to contamination from the filament, being also present at the first interface of the MCD layer with the silicon nitride substrate.

Characterization of CVD Diamond Thin Film Electrodes in Terms of their Semiconductivity

A series of boron-doped diamond films were grown using hot filament technique, over wide doping level ranges. Their semiconductor characteristics (the uncompensated acceptor concentration and flat band po- tential) were determined from Mott-Schottky plots mea- sured by electrochemical impedance spectroscopy. These parameters can be used for the films' characterization. The applicability of the electrochemical impedance spectros- copyapproachin the semiconductor diamond characterization is discussed.

X-ray photoelectron spectroscopy study on Fe and Co catalysts during the first stages of ethanol chemical vapor deposition for single-walled carbon nanotube growth

Optimized chemical vapor deposition processes for single-walled carbon nanotube (SWCNT) can lead to the growth of dense, vertically aligned, mm-long forests of SWCNTs. Precise control of the growth process is however still difficult, mainly because of poor understanding of the interplay between catalyst, substrate and reaction gas. In this paper we use x-ray photoelectron spectroscopy (XPS) to study the interplay between Fe or Co catalysts, SiO2 and Al2O3 substrates and ethanol during the first stages of SWCNT forest growth. With XPS we observe that ethanol oxidizes Fe catalysts at carbon nanotube (CNT) growth temperatures, which leads to reduced carbon nanotube growth. Ethanol needs to be decomposed by a hot filament or other technique to create a reducing atmosphere and reactive carbon species in order to grow vertically aligned single-walled carbon nanotubes from Fe catalysts. Furthermore, we show that Al2O3, unlike SiO2, plays an active role in CNT growth using ethanol CVD. From our study we conclude that metallic Fe on Al2O3 is the most optimal catalyst/substrate combination for high-yield SWCNT forest growth, using hot filament CVD with ethanol as the carbon containing gas.

Formation of oxides on tungsten conductors heated by electric current

High-temperature oxidation of tungsten is studied experimentally with the hot-filament technique. Steady-state oxidation of tungsten conductors (d = 300 μm, L=0.1 m) heated electrically is examined using optical pyrometry and electrothermography. Temperature profiles along the conductor and its average steady-state temperatures are measured for different current intensities. It is found out that tungsten trioxide filiform crystals show up on surface at 800 K. They grow up and gradually transform into acicular and lamellar crystals. Their size, morphology, and surface density depend on oxidation temperature and duration and on gas-phase pressure and composition. After the tungsten conductor is oxidized in air at atmospheric pressure (T = 1400 K) for 30 minutes, the mean lamellar crystals are 60 μm in diameter, the coarsest crystals reaching 320 μm.

The role of surface activation prior to seeding on CVD diamond adhesion

A special surface pre-treatment protocol consisting of four steps is proposed to provide high adhesion levels for CVD diamond by hot-filament technique. In step 1, the silicon nitride ceramic substrates are surface ground with 15 µm diamond slurry and mirror-like polished with colloidal silica (0.05 µm). In step 2, plasma etching with CF 4 (PE), mechanical anchoring is promoted due to the surface roughness increment. Surface activation (SA) is the following step, where the substrates are subjected to CVD diamond growth conditions for a short time (30 min), leading to the deposition of an amorphous carbon layer responsible for enhancing the nucleation density and the further growth of diamond during early stages of deposition. The final step 4 is 0.5–1 µm diamond powder seeding in an ultrasonic bath. The results show that the Si 3 N 4 ceramic surface modified by the surface pre-treatment protocol allowed a high nucleation density promoting a continuous and homogeneous film of equally sized diamond grains, at a growth rate of ap. 1.5 µm h − 1 . Brale tip indentation tests confirmed that highly adherent films are thus produced (no delamination under 900 N).

Enhanced tribological performances of nanocrystalline diamond film

Diamond films are well known for their outstanding properties such as high hardness, possible low coefficient of friction, high thermal conductivity, excellent biocompatibility and electrical insulation. Diamond films with nanocrystalline grains (grain sizes between 3 and 15 nm) offer further advantages of low compressive stress, low surface roughness, and high amount of surface atoms in relation to volume leading to enhanced surface properties. In view of these, the present investigation is undertaken to explore the possibility of using nanocrystalline diamond (NCD) films in advanced automotive equipment. Accordingly NCD-films have been deposited using a modified hot-filament technique. Tribological behaviour of these films has been evaluated by means of a reciprocating model tribometer with different lubricant qualities. The worn surfaces were examined using scanning electron microscopy (SEM) and 3D white light confocal microscopy. The results show the influences of coating qualities and test conditions on the tribological response. Comparable friction coefficient can be found with high treated and low treated lubricants. These films exhibited negligible wear for the range of load tested.

Characterization of UV irradiated nanocrystalline diamond

The exposure of H-terminated nanocrystalline diamond (NCD) to ultraviolet (UV) light in air and at room temperature modifies the features of the diamond surface, in terms of wettability, electrical conductivity and chemical reactivity. This allows the development of a soft, dry and non invasive method to tailor the surface properties for the development of new chem- or bio-sensors. In this work, we report about the analysis of hydrogen terminated nanocrystalline diamond films grown by hot-filament technique and of their surface modification following UV light exposure. This UV treatment induces an hydrophobic to hydrophilic transformation and an outstanding increase of the electrical resistivity (> 10 5). X-ray (XPS) and ultraviolet (UPS) photoelectron spectroscopy provide insight into the role of chemisorbed oxygen on the modification of the valence band and on the transformation of the electron affinity (from negative to positive) at the diamond surface.

Reactivity and sintering kinetics of Au/TiO2(110) model catalysts: particle size effects

We review here our studies of the reactivity and sintering kinetics of model catalysts consisting of gold nanoparticles dispersed on TiO2(110). First, the nucleation and growth of vapor-deposited gold on this surface was experimentally examined using x-ray photoelectron spectroscopy and low energy ion scattering. Gold initially grows as two-dimensional islands up to a critical coverage, θ cr, after which 3D gold nanoparticles grow. The results at different temperatures are fitted well with a kinetic model, which includes various energetic parameters for Au adatom migration. Oxygen was dosed onto the resulting gold nanoparticles using a hot filament technique. The desorption energy of Oa was examined using temperature programmed desorption (TPD). The Oa is bonded ~40% more strongly to smaller (thinner) Au islands. Gaseous CO reacts rapidly with this Oa to make CO2, probably via adsorbed CO. The reactivity of Oa with CO increases with increasing particle size, as expected based on Bronsted relations. Propene adsorption leads to TPD peaks for three different molecularly adsorbed states on Au/TiO2(110), corresponding to propene adsorbed on gold islands, to Ti sites on the substrate, and to the perimeter of gold islands, with adsorption energies of 40, 52 and 73 kJ/mol, respectively. Thermal sintering of the gold nanoparticles was explored using temperature-programmed low-energy ion scattering. These sintering rates for a range of Au loadings at temperatures from 200 to 700 K were well fitted by a theoretical model which takes into consideration the dramatic effect of particle size on metal chemical potential using a modified bond additivity model. When extrapolated to simulate isothermal sintering at 700 K for 1 year, the resulting particle size distribution becomes very narrow. These results question claims that the shape of particle size distributions reveal their sintering mechanisms. They also suggest why the growth of colloidal nanoparticles in liquid solutions can result in very narrow particle size distributions.

Residual Stress Distribution, Intermolecular Force, And Frictional Coefficient Maps In Diamond Films: Processing-Structure-Mechanical Property Relationship

AbstractCarbon in its various forms, specifically nanocrystalline diamond, may become a key material for the manufacturing of micro- and nano-electromechanical (M/NEMS) devices in the 21st Century. In order to utilize effectively these materials for M/NEMS applications, understanding of their microscopic structure and physical (mechanical properties, in particular) become indispensable. The micro- and nanocrystalline diamond films were grown using hot-filament and microwave chemical vapor deposition techniques involving novel CH4 / [TMB for boron doping and H2S for sulfur incorporation] in high hydrogen dilution chemistry. To investigate residual stress distribution and intermolecular forces at nanoscale, the films were characterized using Raman spectroscopy and atomic force microscopy in terms of topography, force curves and force volume imaging. Traditional force curve measures the force felt by the tip as it approaches and retracts from a point on the sample surface, while force volume is an array of force curves over an extended range of sample area. Moreover, detailed microscale structural studies are able to demonstrate that the carbon bonding configuration (sp2 versus sp3 hybridization) and surface chemical termination in both the un-doped and doped diamond have a strong effect on nanoscale intermolecular forces. The preliminary information in the force volume measurement was decoupled from topographic data to offer new insights into the materials's

Sp3's experience using hot filament CVD reactors to grow diamond for an expanding set of applications

The present status of sp3 in the area of diamond growth using hot filament CVD is given. After a brief description of the deposition systems being used by sp3 and a discussion of selected applications of sp3's products, the paper focuses on why the hot filament technique was selected as the deposition technique of choice. Topics discussed include ease of scaling, deposition system costs, film growth rate and film uniformity, safety issues, and temperature and process control. Finally, the economics of hot filament reactors compared to other deposition types are discussed.

The X-ray photoelectron spectroscopy C 1s diamond peak of chemical vapour deposition diamond from a sharp interfacial structure

While the X-ray photoelectron spectroscopy is an excellent technique for the characterisation of chemical vapour deposition diamond thin films, previous analyses of the diamond C 1s peak have yielded a range of binding energies. The presence of an intermediate layer, the incorporation of materials other than diamond such as Si, and defect clusters in the diamond matrix could indeed shift the diamond C 1s position. In this work, diamond is deposited using the standard hot filament technique on α-(0001) sapphire substrates. In addition to the absence of an intermediate layer, there is virtually no sp 2 carbon material in the formation of diamond as evidenced from Raman spectroscopy and Auger electron spectroscopy. The C 1s diamond peak is observed at 283.3 eV.

A comparative machining study of diamond-coated tools made by plasma torch, microwave, and hot filament techniques

An effective metal-cutting tool is usually a combination of a hard coating and a tough substrate. The successful deposition of diamond outside its thermodynamic stability range has stimulated the development of a new class of cutting tools: those with diamond-coated inserts of any desired style and edge geometry. The successful implementation of diamond coatings also expedited similar research in the deposition of cubic boron nitride. This paper presents superhard coating tools, with emphasis on diamond-coated WC-Co tools, the corresponding deposition of technologies and the foreseen metal-cutting applications.

Tribological properties of diamond-like carbon films produced by different deposition techniques

DLC films were deposited on Si wafers using RF plasma CVD, hot filament (HF) plasma CVD and ECR plasma CVD with plasma-based ion implantation (PBII). Tribological properties of the films were tested using a ball-on-plate friction tester sliding against a steel ball under wide range of conditions. DLC films produced by the three techniques were classified into two groups. DLC films prepared by ECR had good tribological properties over the wide range of conditions tested. DLC films prepared by RF and hot filament techniques showed low friction coefficient and high wear resistance, except for the lower load condition. The wear debris and the worn surfaces were analyzed by a Micro-laser Raman instrument. Micro-structural analyses of the wear debris reveals that ECR-DLC film is different from that of the other two, while the debris of HF-DLC and RF-DLC shows almost the same structure. The change in the structure appears to be correlated with the differences in the tribological behavior of each film.

Analysis of residual stress in diamond films by x-ray diffraction and micro-Raman spectroscopy

We investigate the residual stress in diamond films grown on (001) silicon substrates as a function of film thickness. The diamond films were deposited at 1070 K by the conventional hot filament technique using a gas mixture of methane (1.0% vol) and hydrogen (99.0% vol). The film thickness, obtained from cross section scanning electron micrographs, varied from 3.0 to 42 μm as the growth time increased from 1 to 10 h. These images evidenced that the columnar growth is already established for films thicker than 10 μm. Top view micrographs revealed predominantly faceted pyramidal grains for the films at all growth stages. The grain size, obtained from these images, was found to vary linearly with film thickness. Using a high resolution x-ray diffractometer, the residual stress was determined by measuring, for each sample, the (331) diamond Bragg diffraction peak for Ψ values ranging from −60° to +60°, and applying the sin2 ψ method. For the micro-Raman spectroscopy, we used the summation method, which consists in recording and adding a large number of spectra in different places of a selected area of the sample. All Raman spectra were fitted with Lorentzian lines to separate the contribution of the pure diamond and the other nondiamond (graphite) phases. This spectral analysis performed in each sample allowed the determination of the residual stress, from the diamond Raman peak shifts, and also the diamond purity, which increases from 70% to 90% as the thickness goes from 3 to 42 μm. The type and magnitude of the residual stress obtained from x-ray and micro-Raman measurements agreed well for films thicker than 10 μm. For films thinner than this value, an opposite behavior between both results was observed. We attributed this discrepancy to the domain size characteristic of each technique.

Phase diagram study for the alkali metal-oxychloride system

Oxychloride systems are very important in the disposal of ashes generated from municipal wastes. The alkali metal-oxychloride system is a basic system of secondary fly ash generated in the process to reduce the volume of bottom ash and fly ash from incineration processes. Toxic materials such as compounds of heavy metals and dioxin may be absorbed by or contained in the secondary fly ash. In this study, the phase stability of three binary systems was examined using a chemical equilibrium technique and a new hot filament technique, and a phase diagram for the NaCl-Na2CO3 system and liquidus for the NaCl-Na2SiO3 and LiCl-Li2SiO3 systems were obtained.

Phase diagram study on the Al2O3-CaO-TiO2 system

For the continuous casting of aluminum containing stainless steel, there should be a minimal amount of silica in the mold powder. It oxidizes aluminum in molten steel, allowing chemical composition to vary, and undesired inclusions to form. In mold powder, silica is important component to lower the melting point and maintain adequate viscosity. A substitute for SiO2 might be TiO2, because of its nature as an acidic compound. Although the liquidus and solidus of the system is minimum knowledge for the melting point, they have not been well established.The phase relations of the Al2O3-CaO-TiO2 system were investigated by the hot filament technique under various oxygen partial pressures. The effect of B2O3 addition to the Al2O3-CaO-TiO2 system on the liquidus and solidus was also investigated.

Manufacture of gem quality diamonds: a review

Gem quality diamonds have been grown using high temperature, high pressure processes like the solvent catalyst method and the temperature gradient method. This review with 64 references focuses on the thermodynamics, kinetics of the growth processes and the apparatus used to grow diamonds. Gem quality diamonds can be synthesized by the high pressure, high temperature process, either by the solvent catalyst method or the reconstitution technique. The Hall belt apparatus and the toroid anvil are the commonly used equipment to generate high pressures. In the high pressure, high temperature processes a catalyst is essential for synthesis. The commonly used catalysts are Fe, Co and Ni whereas recently hydroxides and carbonates have also been used to synthesize diamond. Surface chemistry plays an important role in determining the quality of the crystal. If the carbon flux to the nucleating diamond exceeds a certain limit, graphite nucleates instead of diamond. Temperature, pressure and impurities like nitrogen and boron also affect the quality and growth rates of the synthesized diamond. High growth rates have also been observed if substantial amount of paramagnetic nitrogen is dispersed in the reaction bath. Recent developments of growing diamond by chemical vapour deposition techniques like microwave plasma and hot filament technique have been reviewed. Non-destructive, optical methods to characterize diamonds have been briefly described. ©

Infrared Emission Spectra of CaF2-CaO-SiO2 Melt.

The infrared emission spectra of CaF2–CaO–SiO2 melt were investigated by a method combining infrared emission spectroscopy and the hot-filament technique. Emission for spectra related to Si–O bond can be observed by this in situ technique. The influence of SiO2 and fluoride content on the spectrum was investigated, and the emission attributable to bonds corresponding to siliconbridging oxygen and silicon-non bridg-ingoxygen was discussed. Results suggest that fluoride ion dominantly substitutes for a non-bridging bond in low SiO2 concentration region. The structure of silicate was compared with calculated infrared emission of Si–F–O.