Growth rate improvements in the hot-filament CVD deposition of nanocrystalline diamond

The hot-filament CVD, a less used technique for NCD films growth using Ar/H₂/CH₄ gas mixtures, is optimized for the coating of silicon nitride ceramics. Parameters such as gas composition (Ar/H₂ and CH₄/H₂ ratios), total gas pressure, total mass flow and substrate and filament temperatures, are studied to assess their effect on NCD growth kinetics as well as on film quality and morphology. The smallest diamond crystallite sizes (8 nm) were recorded for the slowest growth rate of 0.1 μm h^− 1. A remarkable result is the very high growth rate of 1.6 μm h^− 1 of continuous NCD coatings with 28 nm of crystallite size, obtained in selected deposition conditions.     

Direct oxidation of sodium borohydride on Pt, Ag and alloyed Pt–Ag electrodes in basic media. Part I: Bulk electrodes

We studied the borohydride oxidation reaction (BOR) by voltammetry for BH₄⁻ concentrations between 10⁻³ M and 0.1 M NaBH₄ in 0.1–1 M NaOH for bulk polycrystalline Pt, Ag and alloyed Pt–Ag electrocatalysts. In order to compare the different electrocatalysts, we measured the kinetic parameters and the number of electrons exchanged (faradic efficiency). BOR on bulk Pt is more efficient when the concentration of NaBH₄ increases (3e⁻ in 1 mM and 6e⁻ in 10 mM BH₄⁻/0.1 M NaOH). BOR on Pt can occur both in a direct pathway and in an indirect pathway including hydrogen generation via heterogeneous hydrolysis of BH₄⁻ and subsequent oxidation of its by-products (e.g. BH₃OH⁻ and H₂). BOR on Ag strongly depends on the pH: improved faradic efficiency is monitored for high pH (2e⁻ at pH 12.6 and 6e⁻ at pH 13.9 at 25 °C). The BOR kinetics is faster for Pt than for Ag (i_Pt=0.02 A cm⁻², i_Ag=1.4 10⁻⁷ A cm⁻² at E=−0.65 V vs. NHE in 1 mM NaBH₄/0.1 M NaOH, 25 °C) both as a result from Pt high activity regarding the BH₄⁻ heterogeneous hydrolysis and subsequent HOR, above −0.83 V vs. NHE and following direct oxidation of BH₄⁻ or BH₃OH⁻ below −0.83 V vs. NHE. Both Pt–Ag bulk alloys show unique behaviour: the number of electrons exchanged is rather high whatever the BH₄⁻ concentration and pH, while the kinetic parameters are quite similar to that of platinum, showing possible synergistic alloying effect.     

Thermal and mechanical properties of LaNbO4-based ceramics

Thermal and mechanical properties of polycrystalline La_1−xA_xNbO₄ (x = 0, 0.005, 0.02 and A = Ca, Sr and Ba) are reported. The materials possess a ferroelastic to paraelastic phase transition close to 500 °C, and the linear thermal expansion is significantly lower (8.6 ± 0.5 × 10⁻⁶ °C⁻¹) for the paraelastic phase compared to the ferroelastic phase (15 ± 3 × 10⁻⁶ °C⁻¹). The hardness was significantly higher for acceptor doped materials (6 GPa) compared to pure LaNbO₄ (3 GPa) due to a significantly smaller average grain size. The fracture toughness of La_0.98Sr_0.02NbO₄, measured by single edge V-notched beam method, was 1.7 ± 0.2 MPa m^1/2 independent of temperature up to 600 °C. The ferroelastic properties of the materials were confirmed by non-linear relationships between stress and strain during compression/decompression, a remnant strain after decompression and the presence of ferroelastic domains. The mechanical properties of LaNbO₄-based materials are discussed with focus on ferroelasticity, microcracking due to crystallographic anisotropy and pinning of ferroelastic domain boundaries.     

Carbon nanotubes field emission integrated triode amplifier array

Because the high frequency operation of a field emission triode amplifier is dictated by the cutoff frequency and not the electron transit time, a high ratio of transconductance, g_m to the overlapping interelectrode capacitance, C_g is the desired outcome. Consequently, to achieve high frequency performance of the CNT amplifier array in this study, C_g was reduced by performing a dual-mask photolithography process to minimize the overlapping gate area, and, the insulating layer's thickness was increased. Moreover, wedge-shaped CNT emitter arrays are employed to increase emission sites, resulting in return higher g_m. Both dc and ac performance of the amplifier were characterized. The triode amplifier array exhibited a high current of  0.32 mA (74 mA/cm²), g_m of  63 μS and voltage gain of  18 dB. Frequency response of the triode amplifier up to 20 kHz was also investigated. A theoretical cutoff frequency of > 70 MHz could be achieved with proper shielding of the test setup.     

Near-ambient X-ray photoemission spectroscopy and kinetic approach to the mechanism of carbon monoxide oxidation over lanthanum substituted cobaltites

We have studied the oxidation of carbon monoxide over a lanthanum substituted perovskite (La_0.5Sr_0.5CoO_3−d) catalyst prepared by spray pyrolysis. Under the assumption of a first-order kinetics mechanism for CO, it has been found that the activation energy barrier of the reaction changes from 80 to 40 kJ mol⁻¹ at a threshold temperature of ca. 320 °C. In situ XPS near-ambient pressure (0.2 torr) shows that the gas phase oxygen concentration over the sample decreases sharply at ca. 300 °C. These two observations suggest that the oxidation of CO undergoes a change of mechanism at temperatures higher than 300 °C.     

Structural evolution of aging agar-gelatin co-hydrogels

Agar-gelatin co-hydrogel was investigated over a period of ≈30-days by dynamic light and small angle neutron scattering, and rheology to quantify changes occurring inside the hydrogel. Degree of non-ergodicity was extracted as a heterodyne contribution from the measured dynamic structure factor data. From the analysis of the data, we observed two relaxation modes namely fast and slow modes with relaxation times τ_f and τ_s whose dependence with aging time, t_a fall on a scaling behavior given by power-laws, τ_f  (t_a)^−1/5 and τ_s  (t_a)^3/5 respectively. Further the analysis showed the heterogeneity size (ξ_SANS) obtained from SANS also follows power-law behavior, ξ_SANS  (t_a)^−1/5. The data taken together revealed the “speeding up” of fast and “slowing down” of slow mode relaxation processes.     

Electrically conductive properties of tungsten-containing diamond-like carbon films

Tungsten-containing diamond-like carbon films with different metal concentrations were investigated. The films of several hundred nanometers in thickness were deposited on the silicon wafer using RF-PECVD (radio frequency plasma enhanced chemical vapor deposition) method. During deposition, metal component was co-sputtered using DC magnetron of tungsten target. The six samples with the concentration of 3.8, 6.1, 8.0, 16.3, 24.3 and 41.4 at.% of tungsten were made. The structural analyses were performed by TEM (transmission electron microscope) and Raman spectroscopy. These results indicated that tungsten clusters were well dispersed in amorphous carbon host matrix in the case of tungsten concentration from 3.8 to 24.2 at.%. However, no such a structure can be observed in the sample with 41.4 at.%. The AC electrical resistance was measured in the temperature range of 2–300 K using four-probe method in vacuum condition. The observed temperature dependence of electrical conductivity can be expressed by σ=σ₀exp−2(C₀/kT)^1/2 and tungsten concentration from 3.8 at.% to 24.2 at.%. In addition, the sample with 41.4 at.% showed the resistive superconducting transition at T_c of around  5.5 K.     

Direct oxidation of sodium borohydride on Pt, Ag and alloyed Pt–Ag electrodes in basic media: Part II. Carbon-supported nanoparticles

We studied the borohydride oxidation reaction (BOR) by voltammetry in 0.1 M NaOH/10⁻³ M BH₄⁻ on carbon-supported Pt, Ag and alloyed PtAg nanoparticles (here-after denoted as Pt/C, Ag/C and Pt–Ag/C). In order to compare the different electrocatalysts, we measured the BOR kinetic parameters and the number of electrons exchanged per BH₄⁻ anion (faradaic efficiency). The BOR kinetics is much faster for Pt/C than for Ag/C (i_Pt=0.15, i_Ag=3.1×10⁻⁴ A cm⁻² at E=−0.65 V vs. NHE at 25 °C), but both materials present similar Tafel slope values. The n value involved in the BOR depends on the thickness of the active layer of electrocatalysts. For a “thick layer” (approximately 3 m), n is nearly 8 on Pt/C and 4 on Ag/C, whereas n decreases for thinner Pt/C active layers (n2 for thickness <1 m). These results are in favour of the sequential BH₄⁻ hydrolysis (yielding H₂) followed by hydrogen oxidation reaction (HOR), or direct sequential BOR on Pt/C, whereas Ag/C promotes direct but incomplete BOR (Ag has no activity regarding hydrogen evolution reaction, HER). The n value close to 8 for the thick Pt/C layer displays the sufficient residence time of the molecules formed (H₂ by heterogeneous hydrolysis or BOR intermediates) within the active layer, which favours the complete HOR and/or BOR. Two PtAg/C nanoparticles alloys have been tested (noted APVES-4C and APVES-E1). They show different behavior; the borohydride oxidation reaction kinetics is faster on APVES-E1 than on APVES-4C (b=0.15, and b=0.31 V dec⁻¹,  A cm⁻², respectively, at 25 °C), but the n values are higher on APVES-4C than APVES-E1 (nearly 8 vs. 3, respectively, at 25 °C). These discrepancies probably originate from the heterogeneity of such bimetallic materials, as observed from physicochemical characterizations.     

Good temperature stability of K0.5Na0.5NbO3 based lead-free ceramics and their applications in buzzers

(K_0.5−xLi_x)Na_0.5(Nb_1−ySb_y)O₃ (KLNNSx–y, x = 0–4 mol% and y = 0–8 mol%) lead-free piezoelectric ceramics were prepared by the conventional mixed oxide method. The denser microstructure and better electrical properties of the ceramics were obtained as compared to the pure K_0.5Na_0.5NbO₃ ceramic. The temperature stability of the electrical properties of the ceramics was also investigated. The experimental results show that the KLNNS2.5–5 ceramic exhibits good electrical properties (k_p  49%, k₃₁  30% and , tan δ  0.019), and possesses good temperature stability in the temperature range of −40 to 85 °C. The related mechanisms for improved electrical properties and temperature stability were also discussed. Moreover, buzzers based on the KLNNS2.5–5 ceramic have been fabricated and their characterization is presented. These results show that the KLNNS2.5–5 ceramic is a promising lead-free material for practical application in buzzers.     

Bacterial inactivation and organic oxidation via immobilized photo-Fenton reagent on structured silica surfaces

A woven inorganic silica fabric loaded with Fe-ions (EGF-Fe) was tested under simulated solar light during hydroquinone degradation. The abatement of hydroquinone was observed to attain about 80% within 3 h. The photo-catalyst was also tested to inactive Escherichia coli K12 at “natural” pH and in the presence of a low concentration of H₂O₂. Addition of H₂O₂ (10 mg L⁻¹) did not by itself to bacterial inactivation. Total bacterial inactivation was mediated by EGF-Fe fabrics in the presence of H₂O₂ (10 mg L⁻¹) under solar light irradiation. A sample containing active (culturable) bacteria (10⁵ CFU mL⁻¹) decreased to values <1 CFU mL⁻¹ within 3 h reaction. Fe-mediated homogeneous process decreased the bacterial CFU content by about two orders of magnitude within 4 h. During the degradation of hydroquinone only a small amount of iron ions were found in solution of about 1.2 mg L⁻¹, within 90 min decreasing to values ≤0.5 mg L⁻¹ after 180 min. The leaching of Fe-ions did not affect the photo-catalyst performance since EGF-Fe fabric did not deactivate after five or more cycles. The Fe-ions founds in solution mineralized hydroquinone to levels below 37% of its initial content. The present study presents the first report on the beneficial role of a heterogeneous iron supported catalyst leading to efficient bacterial inactivation in aqueous solution with iron leaching <0.1 mg L⁻¹, the detection limit for Fe-analysis in solution. No bacterial re-growth was observed during a post-irradiation period up to 24 h in the dark.     

The role of particle size on the electrochemical properties at 25 and at 55 °C of the LiCr0.2Ni0.4Mn1.4O4 spinel as 5 V-cathode materials for lithium-ion batteries

The role of the particle size on the electrochemical properties at 25 and at 55 °C of the LiCr_0.2Ni_0.4Mn_1.4O₄ spinel synthesized by combustion method has been determined. Samples with different particle size were obtained by heating the raw spinel from 700 to 1100 °C, for 1 h in air. X-ray diffraction patterns revealed that all the prepared materials are single-phase spinels. The main effect of the thermal treatment is the remarkable increase of the particles size from 60 to 3000 nm as determined by transmission electron microscopy. The electrochemical properties were determined at high discharge currents (1C rate) in two-electrode Li-cells. At 25 and at 55 °C, in spite of the great differences in particle size, the discharge capacity drained by all samples is similar (Q_dch ≈ 135 mAh g⁻¹). Instead, the cycling performances strongly change with the particle size. The spinels with Φ > 500 nm show better cycling stability at 25 and at 55 °C than those with Φ < 500 nm. The samples heated at 1000 and 1100 °C, with high potential (E ≈ 4.7 V), elevate capacity (Q ≈ 135 mAh g⁻¹), and remarkable cycling performances (capacity retention after 250 cycles >96%) are very attractive materials as 5V-cathodes for high-energy Li-ion batteries.     

Nanodiamond lateral comb array field emission diode for high current applications

Nanodiamond comb-shaped lateral field emitter arrays in diode configuration were fabricated and characterized for high current field emission. Nitrogen-incorporated nanocrystalline diamond with grain size of 5–10 nm was micropatterned using RIE to realize interconnected arrays of comb structures equipped with uniformly spaced high aspect ratio lateral emitter fingers. A 9000-fingered nanodiamond lateral comb array diode with an inter-electrode spacing of 8 μm demonstrated a high emission current of  25 mA at an anode voltage of 260 V (electric field  32 V/μm) in 10^− 7 Torr vacuum. The lateral emitter configuration shows potential for higher power with no emission current saturation observed. These vacuum micro/nanoelectronic devices comprised of nanodiamond lateral field emission diodes are attractive for low-voltage operating high current electron sources, high-power and high-speed switches, and other extreme demand/extreme environment electronics.     

Low-temperature thermionic emission from nitrogen-doped nanocrystalline diamond films on n-type Si grown by MPCVD

Temperature-dependent emission current–voltage measurements were carried out for nitrogen (N)-doped nanocrystalline diamond (NCD) films grown on n-type Si substrates by microwave plasma-assisted chemical vapor deposition (MP-CVD). Low threshold temperature (~ 260 °C) and low threshold electric field (~ 5 × 10^− 5 V/µm) were observed. Both the temperature dependence and the electric field dependence have shown that the obtained emission current was based on electron thermionic emission from N-doped NCD films. We have also studied the relation between nitrogen concentration and the saturation emission current. The saturation current obtained was as high as 1.4 mA at 5.6 × 10^− 3 V/µm at 670 °C when the nitrogen concentration was 2.4 × 10²⁰ cm^− 3. Low value of effective work function (1.99 eV) and relatively high value of Richardson constant (~ 70) were estimated by well fitting to Richardson–Dushman equation. The results of smaller φ and larger A′ suggest that N-doped NCD has great possibility of being a highly efficient thermionic emitter material.     

Electrochemical behavior of Ni(II)/Ni in a hydrophobic amide-type room-temperature ionic liquid

Electrochemical reaction of Ni(II)/Ni was investigated in a hydrophobic room-temperature ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA) containing Ni(TFSA)₂ as a Ni source. The UV–vis spectra showed that Ni(II) in BMPTFSA is octahedrally coordinated with TFSA⁻ anions. The average activation energy for the diffusion coefficients of this Ni(II) complex was 26 kJ mol⁻¹, which was close to that for the viscosity. The diffusion coefficient of Ni(II) was estimated to be 9.3 × 10⁻⁸ cm² s⁻¹. Chronoamperometric measurements showed that the electrodeposition of Ni on a platinum substrate involved three-dimensional instantaneous nucleation under diffusion control at room-temperature. The electrodeposits obtained by galvanostatic electrolysis with the current density of −0.046 mA cm⁻² at 70 and 100 °C were identified as metallic Ni by XRD.     

Impact of supercritical drying and heat treatment on physical properties of titania/silica aerogel monolithic and its applications

TiO₂–SiO₂ monolithic aerogels were homogeneously prepared using sol–gel method. Critical point of drying of TiO₂–SiO₂ gels with ethanol was studied for 30, 60, 90 and 120 min. Subsequently, the gels were dried with supercritical ethanol, resulting in amorphous aerogels that crystallized following heat treatment at 550 °C from 1 to 5 h. The TiO₂–SiO₂ aerogels were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) and surface area measurements. The molar ratio of SiO₂:TiO₂ was 6 and the synthetic strategy revealed that TiO₂–SiO₂ aerogel, had a surface area 868 m²/g, particle size 40 nm, density 0.17 g/cm³ and 80% porosity. The finding indicated that from economic point of view, TiO₂–SiO₂ gel should be supercritical dried for 30 min and heat-treated for 5 h. The TiO₂–SiO₂ aerogel monoliths photocatalyst synthesized using sol–gel method provided insight into the characteristics that make a photocatalyst material well-suited for photodegradation of phenol and cyanide in an industrial waste stream containing Cl⁻, S²⁻ and NH₄⁺. Interestingly, after multiple reuse cycles (i.e. ≥7), photodegradation systems with regenerated photocatalyst showed a slightly decreasing of photoactivity 2–4%. The overall kinetics of photodegradation of either phenol or cyanide using TiO₂–SiO₂ aerogel photocatalyst was found to be of first order.     

Low temperature sintering of barium titanate ceramics assisted by addition of lithium fluoride-containing sintering additives

By addition of LiF-containing sintering additives to commercial BaTiO₃ powder, more than 98% of the theoretical density was reached at a sintering temperature of 900 °C both on powder compacts and laminates. Dielectric measurements were performed on ceramic samples in the temperature and frequency ranges from −20 °C to 125 °C and from 10³ to 10⁶ Hz, respectively. High relative permittivity (_r  3160) and low dielectric loss (tan δ  0.014) were measured for tapes of the favoured material. The breakdown strength for tapes with a thickness of about 80 μm is 30 kV/mm. The microprobe analysis showed, that no interfacial reaction between the dielectric layer and the Ag-electrode had occurred.The newly developed barium titanate ceramics completely densifying at 900 °C turned out to be promising for integration of buried capacitors in LTCC multilayers.     

Hierarchical structures in agar hydrogels

Small angle neutron scattering experiments were performed on agar solutions and gels to explore their differential microscopic structures. In solution state, the wave vector, q, dependence of static structure factor, I(q), could be described by . Statistical analysis gave: R_g = 18 nm and α = 0.85 ± 0.07 indicating the existence of rod-like rigid structures of length, R_g ≈ 63 nm. In gels, which had discernible Gaussian, power-law and Kratky–Porod regimes in the low, intermediate and high-q regions. Regression analysis yielded a characteristic length, Ξ = 3.3 − 4 nm for gels with agar concentration, c = 0.1 − 0.3% (w/v). The exponent β = 1.2 ± 0.2 and the cross-sectional radius of cylindrical fibres, R_c = 1.5 ± 0.3 nm remained invariant of agar concentration. This assigned a value 5 nm to the persistence length of the fibres in the solution phase that reduced to 3 nm in the gel phase indicating differential hydration of the fibres.     

Electrochemical promotion of CO oxidation on Pt/YSZ: The effect of catalyst potential on the induction of highly active stationary and oscillatory states

We have investigated the kinetics, rate oscillations and electrochemical promotion of CO oxidation on Pt deposited on YSZ using a standard oxygen reference electrode at temperatures 300–400 °C. We have found that electropromotion is small (ρ < 3) when the catalyst potential U_WR, is below 0.4 V and very pronounced (ρ  9, Λ  1500) when U_WR exceeds 0.4 V. This sharp transition in the electropromotion behavior is accompanied by an abrupt change in reaction kinetics and in catalyst potential. For fixed temperature this transition, which leads to a highly active electropromoted state, occurs at specific ratio and catalyst potential. It is shown via comparison with independent catalyst potential–catalyst work function measurements that the transition corresponds to the onset of extensive O²⁻ spillover from YSZ onto the catalyst surface, and concomitant establishment of an effective double layer at the catalyst–gas interface, which is the cause of the highly active electropromoted state.     

Nanocrystalline diamond coatings on the interior of WC–Co dies for drawing carbon steel tubes: Enhancement of tube properties

Tungsten carbide (WC–Co) dies are commercially used for the tube drawing process. However they wear out progressively and are unable to meet the high demands required by the industry. In this study, the effect of nanocrystalline diamond (NCD) coatings on the interior of WC–Co drawing dies using a hot filament chemical vapour deposition technique is reported. A field trial was conducted on the production line for drawing AISI 1541 steel tubes to investigate the quality of the drawn tubes. The surface roughness of the tubes drawn through the NCD coated die was lower (R_a = 381 nm) when compared to the tubes drawn through a regular carbide die (R_a = 527 nm). The average residual stress of tubes drawn through the NCD coated drawing die was lowered by 25%. A pin-on-disc sliding wear test, carried out to estimate the coefficient of friction, showed that the coefficient of friction in the case of the NCD coated die was almost half that of the regular WC–Co dies. The excellent thermal conductivity and lower friction coefficient of NCD coatings also helped to decrease the working temperature of the tube drawing process, thereby resulting in a superior product.     

A novel “coral” carbon microprobe with and without N2 incorporation

“Coral”-type microstructure carbon films, with and without N₂ incorporation, were grown on sharpened tungsten microprobes by plasma enhanced chemical vapor deposition (PECVD) using H₂/CH₄/N₂ and H₂/CH₄ gas mixtures, respectively. The electrochemical behaviors of the coral-type carbon coated tungsten microprobe, characterized by various concentrations of ferrocyanide in a background of 0.1 M KCl, show excellent structural stability with similar microstructure before and after prolonged analysis without the need of surface pretreatment. The microprobes exhibit quasi-reversible kinetics with high signal-to-noise S/B ratio. The N₂ incorporated microprobe shows a slightly wider potential window, no surface adsorption of the analyte and higher sensitivity as compared to the sample without nitrogen incorporation. Furthermore, the wide potential window of  3 V is very good as compared to boron-doped diamond electrodes which are  3.5 V. This well behaved; broad electrochemical behavior and the simple fabrication method make the “coral” carbon film microprobe an excellent candidate for electrochemical sensing.