Effect of contact surfaces on the thermal and photoxidation of dehydrated castor oil

The effect of stainless steel, glass, zirconium and titanium enamel surfaces on the thermal and photooxidative toughening mechanism of dehydrated castor oil films deposited on these surfaces was investigated using different analytical and spectroscopic methods. The conjugated and non-conjugated double bonds were identified and quantified using both Raman spectroscopy and 1D and 2D NMR spectroscopy. The disappearance of the double bonds in thermally oxidised oil-on-surface films was shown to be concomitant with the formation of hydroperoxides (determined by iodometric titration). The type of the surface had a major effect on the rate of thermal oxidation of the oil, but all of the surfaces examined had resulted in a significantly higher rate of oxidation compared to that of the neat oil. The highest effect was exhibited by the stainless steel surface followed by zirconium enamel, titanium enamel and glass. The rate of thermal oxidation of the oil-on-steel surface (at 100 °C, based on peroxide values) was more than five times faster than that of oil-on-glass and more than 21 times faster than the neat oil when compared under similar thermal oxidative conditions. The rate of photooxidation at 60 °C of oil-on-steel films was found to be about one and half times faster than their rate of thermal oxidation at the same temperature. Results from absorbance reflectance infrared microscopy with line scans taken across the depth of thermally oxidised oil-on-steel films suggest that the thermal oxidative toughening mechanism of the oil occurs by two different reaction pathways with the film outermost layers, i.e. furthest away from the steel surface, oxidising through a traditional free radical oxidation process involving the formation of various oxygenated products formed from the decomposition of allylic hydroperoxides, whereas, in the deeper layers closer to the steel surface, crosslinking reactions predominate.

Graphene layers produced from carbon nanotubes by frication

Graphene layers have been produced from multi-walled carbon nanotube (MWCNT) bulk materials by friction when polished on ground-glass, offering a novel and effective method to produce graphene layers, which, more importantly, could be transferred to other substrates by rubbing. Field emission scanning electron microscopy, Raman spectroscopy, atomic force microscopy, transmission electron microscopy and selected area electron diffraction confirmed the formation of graphene layers. They were thought to be peeled away from the MWCNT walls due to friction. The reflection spectra showed that absorption of as-produced graphene layers decreased with wavelength in the range of 250–400 nm, compared to the MWCNT bulk material having strong absorption at 350 nm. Nanoscratch test was used to determine the mechanical properties of graphene films, suggesting the tolerance of as-produced graphene film to flaws introduced by scratch.

Biomorphous SiC ceramics prepared from cork oak as precursor

Porous ceramic materials of SiC were synthesized from carbon matrices obtained via pyrolysis of natural cork as precursor. We propose a method for the fabrication of complex-shaped porous ceramic hardware consisting of separate parts prepared from natural cork. It is demonstrated that the thickness of the carbon-matrix walls can be increased through their impregnation with Bakelite phenolic glue solution followed by pyrolysis. This decreases the material's porosity and can be used as a way to modify its mechanical and thermal characteristics. Both the carbon matrices (resulted from the pyrolysis step) and the resultant SiC ceramics are shown to be pseudomorphous to the structure of initial cork. Depending on the synthesis temperature, 3C-SiC, 6H-SiC, or a mixture of these polytypes, could be obtained. By varying the mass ratio of initial carbon and silicon components, stoichiometric SiC or SiC:C:Si, SiC:C, and SiC:Si ceramics could be produced. The structure, as well as chemical and phase composition of the prepared materials were studied by means of Raman spectroscopy and scanning electron microscopy.

Dielectric transition of polyacrylonitrile derived carbon nanofibers

The dielectric behavior of polyacrylonitrile derived carbon nanofibers formed at different carbonization temperatures was investigated using impedance spectroscopy. The impedance data are presented in the form of Cole-Cole plots and four equivalent electrical circuits are derived. It is found that by increasing carbonization temperature from 500 to 800 °C, a strong capacitive element in the parallel equivalent circuit is transformed into an inductive element, while the contact resistance and parallel resistance are significantly decreased. Along with the morphological and chemical structural evolution, respectively witnessed by scanning electron microscopy and Raman spectroscopy, the dielectric transition deduced from the transformation of electrical circuits can be correlated to the proposed microstructural changes of polyacrylonitrile derived carbon nanofibers and the interaction/interference among them.

Femtosecond laser-induced microstructures on diamond for microfluidic sensing devices applications

This paper reported a three-dimensional microfluidic channel structure, which was fabricated by Yb:YAG 1026?nm femtosecond laser irradiation on a single-crystalline diamond substrate. The femtosecond laser irradiation energy level was optimized at 100?kHz repetition rate with a sub-500 femtosecond pulse duration. The morphology and topography of the microfluidic channel were characterized by a scanning electron microscope and an atomic force microscope. Raman spectroscopy indicated that the irradiated area was covered by graphitic materials. By comparing the cross-sectional profiles before/after removing the graphitic materials, it could be deduced that the microfluidic channel has an average depth of ~410?nm with periodical ripples perpendicular to the irradiation direction. This work proves the feasibility of using ultra-fast laser inscription technology to fabricate microfluidic channels on biocompatible diamond substrates, which offers a great potential for biomedical sensing applications.

Bulk and surface switching in Mn-Fe-based Prussian blue analogues

Many Prussian Blue Analogues are known to show a thermally induced phase transition close to room temperature and a reversible, photo-induced phase transition at low temperatures. This work reports on magnetic measurements, X-ray photoemission and Raman spectroscopy on a particular class of these molecular heterobimetallic systems, specifically on Rb0.81Mn[Fe(CN)6]0.95_1.24H2O, Rb0.97Mn[Fe(CN)6]0.98_1.03H2O and Rb0.70Cu0.22Mn0.78[Fe(CN)6]0.86_2.05H2O, to investigate these transition phenomena both in the bulk of the material and at the sample surface. Results indicate a high degree of charge transfer in the bulk, while a substantially reduced conversion is found at the sample surface, even in case of a near perfect (Rb:Mn:Fe=1:1:1) stoichiometry. Thus, the intrinsic incompleteness of the charge transfer transition in these materials is found to be primarily due to surface reconstruction. Substitution of a large fraction of charge transfer active Mn ions by charge transfer inactive Cu ions leads to a proportional conversion reduction with respect to the maximum conversion that is still stoichiometrically possible and shows the charge transfer capability of metal centers to be quite robust upon inclusion of a neighboring impurity. Additionally, a 532 nm photo-induced metastable state, reminiscent of the high temperature Fe(III)Mn(II) ground state, is found at temperatures 50-100 K. The efficiency of photo-excitation to the metastable state is found to be maximized around 90 K. The photo-induced state is observed to relax to the low temperature Fe(II)Mn(III) ground state at a temperature of approximately 123 K.

Femtosecond laser-induced microstructures on diamond for microfluidic sensing device applications

This paper reported a three-dimensional microfluidic channel structure, which was fabricated by Yb:YAG 1026?nm femtosecond laser irradiation on a single-crystalline diamond substrate. The femtosecond laser irradiation energy level was optimized at 100?kHz repetition rate with a sub-500 femtosecond pulse duration. The morphology and topography of the microfluidic channel were characterized by a scanning electron microscope and an atomic force microscope. Raman spectroscopy indicated that the irradiated area was covered by graphitic materials. By comparing the cross-sectional profiles before/after removing the graphitic materials, it could be deduced that the microfluidic channel has an average depth of ~410?nm with periodical ripples perpendicular to the irradiation direction. This work proves the feasibility of using ultra-fast laser inscription technology to fabricate microfluidic channels on biocompatible diamond substrates, which offers a great potential for biomedical sensing applications.

High rate growth of nanocrystalline diamond films using high microwave power and pure nitrogen/methane/hydrogen plasma

In this work, we investigate the impact of minute amounts of pure nitrogen addition into conventional methane/hydrogen mixtures on the growth characteristics of nanocrystalline diamond (NCD) films by microwave plasma assisted chemical vapour deposition (MPCVD), under high power conditions. The NCD films were produced from a gas mixture of 4% CH4/H2 with two different concentrations of N2 additive and microwave power ranging from 3.0 kW to 4.0 kW, while keeping all the other operating parameters constant. The morphology, grain size, microstructure and texture of the resulting NCD films were characterized by using scanning electron microscope (SEM), micro-Raman spectroscopy and X-ray diffraction (XRD) techniques. N2 addition was found to be the main parameter responsible for the formation and for the key change in the growth characteristics of NCD films under the employed conditions. Growth rates ranging from 5.4 μm/h up to 9.6 μm/h were achieved for the NCD films, much higher than those usually reported in the literature. The enhancing factor of nitrogen addition on NCD growth rate was obtained by comparing with the growth rate of large-grained microcrystalline diamond films grown without nitrogen and discussed by comparing with that of single crystal diamond through theoretical work in the literature. This achievement on NCD growth rate makes the technology interesting for industrial applications where fast coating of large substrates is highly desirable.

Femtosecond laser-induced microstructures on diamond for microfluidic sensing devices applications

This paper reported a three-dimensional microfluidic channel structure, which was fabricated by Yb:YAG 1026?nm femtosecond laser irradiation on a single-crystalline diamond substrate. The femtosecond laser irradiation energy level was optimized at 100?kHz repetition rate with a sub-500 femtosecond pulse duration. The morphology and topography of the microfluidic channel were characterized by a scanning electron microscope and an atomic force microscope. Raman spectroscopy indicated that the irradiated area was covered by graphitic materials. By comparing the cross-sectional profiles before/after removing the graphitic materials, it could be deduced that the microfluidic channel has an average depth of ~410?nm with periodical ripples perpendicular to the irradiation direction. This work proves the feasibility of using ultra-fast laser inscription technology to fabricate microfluidic channels on biocompatible diamond substrates, which offers a great potential for biomedical sensing applications.

SE(R)RS devices fabricated by a laser electrodispersion method

A novel laser electrodispersion (LE) technique was employed to deposit gold nanoparticles onto Si and SiOx surfaces. The LE technique combines laser ablation with cascade fission of liquid metal micro-drops, which results in the formation of nanoparticles upon rapid cooling. The shape and the size distribution of the Au nanoparticles prepared by LE depend on the nature of the support. Gold nanoparticles were also deposited in the channels of microreactors fabricated by wet etching of Si and used as SE(R)RS sensors. The influence of the nanoparticle surface density as well as of the nature of the substrate on the Raman response was studied. At an appropriate surface density of the deposited nanoparticles a significant enhancement of Raman signal was observed showing the possibility to create efficient SERS substrates. Application of microfluidic devices in surface enhanced Raman spectroscopy (SERS) in continuous-flow mode with sensor regeneration is described. © 2011 The Royal Society of Chemistry.

Analysis of femtosecond laser surface patterning on bulk single-crystalline diamond

Preliminary work is reported on 2-D and 3-D microstructures written directly with a Yb:YAG 1026 nm femtosecond (fs) laser on bulk chemical vapour deposition (CVD) single-crystalline diamond. Smooth graphitic lines and other structures were written on the surface of a CVD diamond sample with a thickness of 0.7mm under low laser fluences. This capability opens up the opportunity for making electronic devices and micro-electromechanical structures on diamond substrates. The fabrication process was optimised through testing a range of laser energies at a 100 kHz repetition rate with sub-500fs pulses. These graphitic lines and structures have been characterised using optical microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and atomic force microscopy. Using these analysis techniques, the formation of sp2 and sp3 bonds is explored and the ratio between sp2 and sp3 bonds after fs laser patterning is quantified. We present the early findings from this study and characterise the relationship between the graphitic line formation and the different fs laser exposure conditions. © 2012 Taylor & Francis.

Structure and reactivity of sol-gel sulphonic acid silicas

A range of mesoporous solid sulphonic acid catalysts have been prepared from a mercaptopropyl-trimethoxysilane (MPTS) precursor by sol-gel synthesis. The creation of surface sulphonic acid functionality via thiol oxidation has been followed by XPS and Raman spectroscopy. It is possible to continuously vary the sulphonic acid loading from 1 to 12wt.% while maintaining pore volume and mesostructure. The resulting materials exhibit high thermal stability and acid strength across the composition range and show good activity and selectivity in esterification and condensation reactions. © 2002 Elsevier Science B.V. All rights reserved.

Growth of ultrananocrystalline diamond film by DC Arcjet plasma enhanced chemical vapor deposition

Self-standing diamond films were grown by DC Arcjet plasma enhanced chemical vapor deposition (CVD). The feed gasses were Ar/H 2/CH 4, in which the flow ratio of CH 4 to H 2 (FCH4/FH2) was varied from 5% to 20%. Two distinct morphologies were observed by scanning electron microscope (SEM), i.e. the pineapple-like morphology and the cauliflower-like morphology. It was found that the morphologies of the as-grown films are strongly dependent on the flow ratio of CH 4 to H 2 in the feed gasses. High resolution transmission electron microscope (HRTEM) survey results revealed that there were nanocrystalline grains within the pineapple-like films whilst there were ultrananocrystalline grains within cauliflower-like films. X-ray diffraction (XRD) results suggested that (110) crystalline plane was the dominant surface in the cauliflower-like films whilst (100) crystalline plane was the dominant surface in the pineapple-like films. Raman spectroscopy revealed that nanostructured carbon features could be observed in both types of films. Plasma diagnosis was carried out in order to understand the morphology dependent growth mechanism. It could be concluded that the film morphology was strongly influenced by the density of gas phases. The gradient of C2 radical was found to be different along the growth direction under the different growth conditions. © 2012 Elsevier B.V. All rights reserved.

Effect of methane concentration in hydrogen plasma on hydrogen impurity incorporation in thick large-grained polycrystalline diamond films

We investigate the impact of methane concentration in hydrogen plasma on the growth of large-grained polycrystalline diamond (PCD) films and its hydrogen impurity incorporation. The diamond samples were produced using high CH4 concentration in H2 plasma and high power up to 4350 W and high pressure (either 105 or 110 Torr) in a microwave plasma chemical vapor deposition (MPCVD) system. The thickness of the free-standing diamond films varies from 165 µm to 430 µm. Scanning electron microscopy (SEM), micro-Raman spectroscopy and Fourier-transform infrared (FTIR) spectroscopy were used to characterize the morphology, crystalline and optical quality of the diamond samples, and bonded hydrogen impurity in the diamond films, respectively. Under the conditions employed here, when methane concentration in the gas phase increases from 3.75% to 7.5%, the growth rate of the PCD films rises from around 3.0 µm/h up to 8.5 µm/h, and the optical active bonded hydrogen impurity content also increases more than one times, especially the two CVD diamond specific H related infrared absorption peaks at 2818 and 2828 cm−1 rise strongly; while the crystalline and optical quality of the MCD films decreases significantly, namely structural defects and non-diamond carbon phase content also increases a lot with increasing of methane concentration. Based on the results, the relationship between methane concentration and diamond growth rate and hydrogen impurity incorporation including the form of bonded infrared active hydrogen impurity in CVD diamonds was analyzed and discussed. The effect of substrate temperature on diamond growth was also briefly discussed. The experimental findings indicate that bonded hydrogen impurity in CVD diamond films mainly comes from methane rather than hydrogen in the gas source, and thus can provide experimental evidence for the theoretical study of the standard methyl species dominated growth mechanism of CVD diamonds grown with methane/hydrogen mixtures.

Carbon nanowalls grown by microwave plasma enhanced chemical vapor deposition during the carbonization of polyacrylonitrile fibers

We used microwave plasma enhanced chemical vapor deposition (MPECVD) to carbonize an electrospun polyacrylonitrile (PAN) precursor to form carbon fibers. Scanning electron microscopy, Raman spectroscopy, and Fourier transform infrared spectroscopy were used to characterize the fibers at different evolution stages. It was found that MPECVD-carbonized PAN fibers do not exhibit any significant change in the fiber diameter, whilst conventionally carbonized PAN fibers show a 33% reduction in the fiber diameter. An additional coating of carbon nanowalls (CNWs) was formed on the surface of the carbonized PAN fibers during the MPECVD process without the assistance of any metallic catalysts. The result presented here may have a potential to develop a novel, economical, and straightforward approach towards the mass production of carbon fibrous materials containing CNWs. © 2013 American Institute of Physics.