Thin Cermet Research Articles

Performance Evaluation of Thin Cermet Coatings Produced by HVAF Spray: A New Approach for Hard Chrome Replacement

Performance Evaluation of Thin Cermet Coatings Produced by HVAF Spray: A New Approach for Hard Chrome Replacement

Stability and Performance Investigation of Co-Sputtered Ni Cermet Thin Films at Reduced Temperature

Thin film solid oxide fuel cells (thin film SOFCs) were introduced to reduce the high operating temperature of traditional SOFCs. By employing thin film techniques such as sputtering, atomic layer deposition, and pulsed laser deposition, less than 1 μm-thick electrolytes have been successfully fabricated and functioned properly. Through these thin film electrolytes, the low ionic conductivity of the electrolyte materials at reduced temperature is compensated by the shortened travel length of the oxygen ions. However, low temperature also leads the poor electrode activity, and this lowered activity has not been fully overcome.In this study, we investigated various types of Ni-based cermet films to gain better electrode activity. The cermet films were fabricated by magnetron sputtering. For ion-conducting materials, yttria-stabilized zirconia (YSZ) and scandia-stabilized zirconia (ScSZ) were used. These ion-conducting materials were co-sputtered with Ni. The sputtering power of each material was varied for changing Ni contents of the films. To obtain the Ni contents, we utilized X-ray photoelectron spectroscopy (XPS). Four-point probe measurement was also conducted to verify the degree of Ni percolation of the films. In addition, the surface and cross-sectional images of the films were observed to investigate the microstructure and porosity.For analyzing the electrochemical performance of the co-sputtered cermet films at reduced temperature, we fabricated several experimental SOFCs. On the one side of the solid electrolyte pellet, the thin cermet anode was deposited, while thin Pt cathode was deposited on the other side of the pellet. Then, the experimental SOFCs were operated at below 500 oC. During the operation, the j-V characteristics and electrochemical impedance spectroscopy (EIS) of the cells were studied. The thermal stability of the cells was also characterized by potentiostatic measurement and the measured data were compared to the cell with pure Ni anode.

Microstructure and temperature coefficient of resistance of thin cermet resistor films deposited from CrSi2–Cr–SiC targets by S-gun magnetron

Technological solutions for producing nanoscale cermet resistor films with sheet resistances above 1000Ω∕◻ and low temperature coefficients of resistance (TCR) have been investigated. 2–40nm thick cermet films were sputter deposited from CrSi2–Cr–SiC targets by a dual cathode dc S-gun magnetron. In addition to studying film resistance versus temperature, the nanofilm structural features and composition were analyzed using scanning electron microscopy, atomic force microscopy, high-resolution transmission electron microscopy, energy-dispersive x-ray spectroscopy, and electron energy loss spectroscopy. This study has revealed that all cermet resistor films deposited at ambient and elevated temperatures were amorphous. The atomic ratio of Si to Cr in these films was about 2 to 1. The film TCR displayed a significant increase when the deposited film thickness was reduced below 2.5nm. An optimized sputter process consisting of wafer degassing, cermet film deposition at elevated temperature with rf substrate bias, and a double annealing in vacuum, consisting of in situ annealing following the film sputtering and an additional annealing following the exposure of the wafers to air, has been found to be very effective for the film thermal stabilization and for fine tuning the film TCR. Cermet films with thicknesses in the range of 2.5–4nm deposited using this technique had sheet resistances ranging from 1800to1200Ω∕◻ and TCR values from −50ppm∕°C to near zero, respectively. A possible mechanism responsible for the high efficiency of annealing the cermet films in vacuum (after preliminary exposure to air), resulting in resistance stabilization and TCR reduction, is also discussed.

In-situ Electrochemical Characterization of SOFC cell Degradation: The Cases of Mechanical Damage and Carbon Deposition

A methodology has been proposed to analyze the electrochemical degradation of the ASC configuration. This methodology, based on the measurement of the cell response by impedance spectroscopy, has been applied to study the cell performances degradation, (i) when the anode is re-oxidized under an ionic current, and (ii) upon direct methane reforming. In the first case, a thin cermet layer growing from the anode/electrolyte interface has been oxidized and then reduced. This 'redox' cycle has been repeated until the complete degradation of the cell. It has been found that the impedance diagram's evolution is consistent with a delamination between the ACC/AFL interface. In the second case, the cell has been operated under methane conditions known to favour the coking. The analysis of the impedance diagrams has shown a degradation of the current collection. This result has been found to be consistent with the location of carbon deposit inside the cell.

Manufacturing of nanocomposite structural ceramic materials and coatings

So far research on ceramic nanocomposites was focused on materials science aspects neglecting the development of suitable manufacturing technologies. The high performance properties of nanocomposites make final machining unaffordable. Near-net-shape manufacturing is mandatory to accomplish cost targets. Feedstocks with tailored micro-nano architecture can fulfil these requirements, using homogeneously dispersed nanosized powders in a matrix of sub-micron sized grains. Nanoceramic coatings deposited by thermokinetic deposition processes open new prospects and applications in surface technology. The novel HVSFS-process permits direct spraying of liquid nanodispersions in a HVOF torch yielding ultradense and thin oxide and cermet coatings for automotive, aerospace and mechanical engineering.

NOISE MEASUREMENTS ON NbN THIN FILMS WITH A NEGATIVE TEMPERATURE RESISTANCE COEFFICIENT DEPOSITED ON SAPPHIRE AND ON SiO2

We characterize granular NbN x thin cermet films deposited on either sapphire substrate or on SiO 2 and compare the 1/f noise at 300 K and 80 K. The films were characterized with an impedance analyzer from 20 Hz to 1 MHz and analyzed as a resistor R in parallel with a capacitor C. The calculated noise voltage spectral density SvTh of the sample is in agreement with the experimentally observed noise for unbiased samples. The noise measurements on biased samples show 1/f noise. We checked that contact noise does not contribute to our measurements. The 1/f noise was compared for the films deposited on the silicon substrate and on the sapphire. Finally a model is proposed to explain the observed trends in the temperature resistance coefficient (TRC < 0 for all samples) and the relative 1/f noise at 300 K and 80 K with the composition x = N/Nb of the layers.

Conduction mechanisms in Cu–GeO2thin cermet films

Conduction mechanisms in Cu–GeO₂thin cermet films

Film thickness and deposition rate dependence of thermoelectric power of Cu-GeO2thin cermet films

Film thickness and deposition rate dependence of thermoelectric power of Cu-GeO₂thin cermet films

Contact fatigue failure modes in hot isostatically pressed WC-12%Co coatings

The objective of this experimental study was to investigate the influence of the post-treatment, Hot Isostatic Pressing (HIPing), on the Rolling Contact Fatigue (RCF) performance of thermal spray (WC-12%Co) coatings. Thermal spray coatings were deposited using a JP5000 High Velocity Oxy Fuel (HVOF) system in three different thicknesses on the surface of 440-C steel substrate cones to vary the depth of the shear stress within the Hertzian stress field. The furnace temperature during the HIPing process was varied at 850 °C and 1200 °C. RCF tests were conducted using a modified four ball machine under identical tribological conditions of contact stress, configuration and lubrication. Surface observations were made using Scanning Electron Microscopy (SEM) and Light Microscopy. Results of this preliminary study, which is the first of its kind in published literature to evaluate the RCF of HIPed cermet coatings, indicate that variation in HIPing temperature can have a significant influence on a coating's delamination resistance. These results are discussed to comprehend the performance and ascertain the fatigue failure modes in HIPed HVOF coated rolling elements. Apart from comparing the failure modes between HIPed and As-Sprayed coatings, results indicate that by increasing the HIPing temperature to 1200 °C, and maintaining full film lubrication, it is possible to achieve a fatigue life in excess of 70 million stress cycles without failure in relatively thin (50 μm) cermet coatings. Coating failure was attributed to maximum shear stress occurring either at the coating/substrate interface or within the coating microstructure, resulting in delamination due to cyclic loading.

Optical behavior of RF sputtered Au-TiO2thin cermet films, influence of the gold particles size and concentration

We study the optical behavior of Au-TiO 2 thin cermet films, the gold volume fraction of which is varied over the range 10 −3 to 8 × 10 −2 . In this low concentration range, it is mainly governed by the metallic particles size, which very much depends on the deposition substrate temperature and the annealing time and temperature. The samples structure and morphology are characterized by several complementary techniques. Spectrophotometric measurements at quasi-normal incidence have been performed in visible and near infrared regions. Using a short pulse Nd:YAG laser, we also look for nonlinear optical behavior due to the enhancement of the electromagnetic field close to the surface plasmon resonance of the metallic grains.

Surface second-harmonic generation on thin Au–TiO 2 cermet films

We study the second-harmonic generation (SHG) in Au–TiO 2 thin cermet films. The samples are prepared by RF-sputtering technique. The gold volumic fraction in the cermets is varied in the 3–100% (continuous gold layer) range. X-Ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) are used to characterize the samples. The Nd : YAG laser used works in a picosecond pulse regime and the SH signal is detected in reflection configuration at 532 nm. Experiments are performed by varying the angle of incidence and the polarization of the ω and 2 ω beams.

First AFM observation of thin cermet films close to the percolation threshold using a conducting tip

Optical, electrical and magnetic properties of cermet thin films (metallic nanograins embedded in an insulating matrix) are strongly dependent on their morphology. The size, shape and distribution of metallic grains are the main parameters as well as roughness of the surface of the samples. Usual techniques such as transmission electron microscopy, scanning electron microscopy, atomic force microscopy, while very useful to characterize some of them, are inefficient for others and have to be used in a complementary way. Until now, it was impossible to point out metallic nanograins from the matrix using a commercial setup. We have developed a new technique of local contact resistance measurements with a conducting tip AFM called “Resiscope”. In this paper, we show the first electrical mapping obtained with our set-up on gold-alumina and nickel-alumina cermet, with different filling factors, very close to percolation threshold and we briefly correlate these observations to magnetic properties.

AC electrical conductivity of electron beam evaporated Cu-GeO2 thin cermet films

AC electrical properties of 410 nm think 30 at.wt% Cu-70 at.wt% GeO2 thin films are reported for the frequency range 104 to 106 Hz and temperature range 150 to 425 K. The loss tangent (tan δ) and the dielectric loss (∈′′/∈0) are found to show striking minima around a cut-off frequency ∼105 Hz. In the lower frequency range (≤105 Hz), σ1(ω) ∝ ωsTn is obeyed with s (0 to 0.51) increasing as a function of temperature and n (0.10 to 0.14) showing a very weak temperature dependence. In the higher frequency region (≥105 Hz), σ1(ω) and ∈′′/∈0 increase sharply leading to the quadratic behavior of σ1(ω) with s equal to 2. These processes are discussed by analyzing an equivalent circuit which shows that at lower frequencies, the effects of series resistance in leads and contacts can be neglected, while at higher frequencies such effect give rise to spurious ω2 dependance for the conductance. A weakly activated AC conductivity and a frequency exponent s that increases with increasing temperature suggest that the low frequency behavior originates from carrier migration by tunneling process.

Effect of composition on DC conductivity of co-evaporated Cu-GeO2 thin cermet films

Thin films containing 0 to 100 vol%Cu was prepared by thermal co-evaporation. The samples were 200 nm thick, deposited at 600 K at a rate of 0.8 nm/s. DC conductivity measurements were carried out using the van der Pauw four probe technique for 0 to 100 vol%Cu films in the temperature range 150 to 600 K. Samples containing ≥60 vol%Cu exhibited a ‘metallic-like’ behaviour with positive TCR whereas lower concentrations exhibited an activated conduction mechanism with negative TCR.

High performance Al–N cermet solar coatings deposited by a cylindrical direct current magnetron sputter coater

High efficiency Al–N cermet solar selective coatings have been designed using a numerical computer model and deposited experimentally. In numerical modeling calculations, Bruggeman approximations of dielectric function for composite materials were employed. Aluminum is used as a metallic component and AlON as a ceramic component in cermet. Numerical calculation results show that the double cermet layer film structure has the highest photothermal conversion efficiency for the Al–AlON cermet solar selective coatings. The optimized film has a solar absorptance of 0.958 and a hemispherical emittance of 0.035 at 80 °C. The optimized film yields two cermet layers with metal volume fractions of 0.143 and 0.275, and with layer thicknesses of 43 and 80 nm, going from the antireflection coating to the infrared reflector layer. In the optimized double cermet layer film the solar radiation is efficiently absorbed internally and by phase interference. Thermal loss is also efficiently reduced by using thin cermet layers with such low metal volume fractions, which are substantially transparent in the main wavelength region of blackbody radiation within the temperature range of interest, 50–200 °C. The predicted film structure has been used as a guide for the experimental deposition. The Al–N cermet solar selective coatings with double cermet layer film structure have been deposited onto batches of solar collector tubes using a commercial-scale cylindrical dc magnetron sputter coater. Two Al–N cermet solar absorber layers are deposited by dc reactive sputtering in a gas mixture of argon and nitrogen. The different metal volume fractions in the cermet layers are achieved by changing the reactive nitrogen gas flow rate, while the sputtering current is fixed. A solar absorption of 0.96 and a normal emittance of 0.08 at 80 °C have been achieved for deposited Al–N cermet solar coatings. The baking process, for 1 h at 400 °C in vacuum, hardly affects solar absorptance, however the emittance reduces about 0.01 at 80 °C. These high-performance low-cost Al–N cermet solar collector tubes may be a less-polluting replacement for black chromium solar collectors produced by electrochemical methods.

DC electrical conduction in electron beam evaporated Cu-GeO2 thin cermet films

Dc conductivity, Hall effect, and optical absorption measurements were performed on Cu-GeO 2 thin films to elucidate the dc conduction mechanism. The dc activation energy is temperature dependent, increasing significantly above a transition temperature T t . Hall mobility exhibited a sign reversal around T t , above which it increased to a maximum value at a critical temperature T c . The almost identical transition temperatures in conductivity and Hall mobility suggest that the conductivity results from a combination of two processes. Above T t , the conductivity is best described by small polaron phonon-assisted hopping model; hopping appears to be non-adiabatic in nature. Below T t , the conductivity is weakly activated, magnetoresistance is negative, and the Hall mobility shows a significant rise with decreasing temperature. These are features which suggest non-polaronic thermally activated hopping through a spectrum of localized states. Various parameters associated with polaron formation are calculated.

Optical properties of co-evaporated Cu-GeO2 thin cermet films

Optical properties of co-evaporated Cu-GeO2 thin cermet films

Electrical conductivity and Hall effect in Cu-GeO2thin cermet films

The DC conductivity and Hall effect in electron-beam evaporated Cu-GeO2 thin films have been investigated in the temperature range 307 to 413 K. The conductivity and Hall mobility curves show two distinct regions. In the lower temperature range, the conductivity is weakly activated but mobility shows a rapid increase with increasing temperature, indicating that mobility is associated with the hopping of carriers in localized states. In the higher temperature range, the Hall mobility decreases and the DC conductivity increases sharply with increasing temperature, indicating that carrier concentration may be the principal factor governing the conductivity via extended states.

Film-thickness effects on the optical and electrical properties of Cu-GeO2 thin cermet films

The effect of film thickness on the optical and electrical properties of Cu-30 wt % GeO2-70 wt % thin cermet films prepared by electron-beam deposition at about 10−3 Pa and at a substrate temperature of 300 K is reported. The ultraviolet, visible and direct current (d.c.) conductivity results are analysed with the aim of determining the optical band gap,E opt, the width of the band tails,E e, and the d.c. thermal activation energy,E a. It was found that the optical energy gap increases with increasing thickness and that the absorption was due to indirect transitions ink-space. The general feature of the absorption edge remains similar for both unannealed and annealed films, but annealing has the effect of decreasingE opt. The d.c. conductivity results show thatE a decreases with increasing thickness. From a knowledge ofE opt andE a, a probable model of the electronic band structure in Cu-GeO2 thin films has been suggested.

Characterization of Surface Plasmons on Metal-oxide-semiconductor Structures

Abstract Using the Otto (prism-air gap-sample) configuration p-polarized light of wavelength 632·8 nm has been coupled with greater than 80% efficiency to surface plasmons on the aluminium electrode of silicon-silicon dioxide-aluminium structures. The results show that if the average power per unit area dissipated on the metal film exceeds approximately 1 mW mm−2, then the coupling gap and thus the characteristics of the surface plasmon resonance are noticeably altered. In modelling the optical response of such systems the inclusion of both a non-uniform air coupling gap and a thin cermet layer at the aluminium surface may be necessary.