Electron field emission from filtered arc deposited diamond-like carbon films using Au and Ti layers

In this work, tetrahedral diamond-like carbon (DLC) films are deposited on Si, Ti/Si and Au/Si substrates by a new plasma deposition technique — filtered arc deposition (FAD). Their electron field emission characteristics and fluorescent displays of the films are tested using a diode structure. It is shown that the substrate can markedly influence the emission behavior of DLC films. An emission current of 0.1 μA is detected at electric field E_DLC/Si=5.6 V/μm, E_DLC/Au/Si=14.3 V/μm, and E_DLC/Ti/Si=5.2 V/μm, respectively. At 14.3 V/μm, an emission current density J_DLC/Si=15.2 μA/cm², J_DLC/Au/Si=0.4 μA/cm², and J_DLC/Ti/Si=175 μA/cm² is achieved, respectively. It is believed that a thin TiC transition layer exists in the interface between the DLC film and Ti/Si substrate.     

Photoconductivity of DLC film deposited by pulsed discharge plasma CVD

DLC films were deposited by a new pulsed DC discharge plasma chemical vapour deposition (CVD) using hydrogen and methane gas mixture. When methane concentration (Cm) i.e. CH₄/(H₂ + CH₄) was increased from 3 to 40%, the graphitization of the carbon film increases as evident from Raman study. When Cm was increased to 30%, DLC film shows photoconducting property. The white light photoconductivity (S = I_l/I_d, where I_l is light current and I_d is dark current) measured with solar simulator under AM 1.5 condition was approximately 20 at room temperature. The photoconductivity was not clear when Cm was lower than 20%. ESR measurements also show that the electron spin density was slightly decreased with decreasing concentration of methane. Thus we can conclude here that at higher concentrations of methane at 30%, Sp² content of the film increases and the DLC film becomes photoconducting.     

Deposition of hard amorphous hydrogenated carbon films by radiofrequency parallel-plate hollow-cathode plasmas

Hard amorphous hydrogenated carbon (a-C:H) films were deposited by plasma decomposition of CH₄ gas in a RF parallel-plate hollow-cathode system. The deposition system was built by placing a metallic plate in parallel to and in electrical contact with an usual RF-PECVD planar cathode. Self-bias versus RF power curves were used to make an initial characterization of plasma discharges in nitrogen gas atmospheres, for pressures between 10 and 100 mTorr. The strongly increased power consumption to obtain the same self-bias in the hollow-cathode system evidenced an increase in plasma density. The a-C:H films were deposited onto Si single crystalline substrates, in the − 50 to − 500 V self-bias range, at 5, 10 and 50 mTorr deposition pressures. The film deposition rate was found to be about four times than that usually observed for single-cathode RF-PECVD-deposited films, under methane atmosphere, at similar pressure and self-bias conditions. Characterization of film structure was carried out by Raman spectroscopy on films deposited at 10 and 50 mTorr pressures. Gaussian deconvolution of the Raman spectra in its D and G bands shows a continuous increase in the I_D/I_G integrated band intensity ratio upon self-bias increase, obeying the expected increasing behavior of the sp² carbon atom fraction. The peak position of the G band was found to increase up to − 300 V self-bias, showing a nearly constant behavior for higher self-bias absolute values. On the other hand, the G band width showed a nearly constant behavior within the entire self-bias range. Nanohardness measurements have shown that films deposited with self-bias greater than 300 V are as hard as films obtained by the usual PECVD techniques, showing a maximum hardness of about 18 GPa. Films were also found to develop high internal compressive stress. The stress dependence on self-bias showed a strong maximum at about − 200 V self-bias, with a maximum stress value of about 5 GPa.     

The effect of refractive index on the friction coefficient of DLC coated polymer substrates

This paper describes the enhanced mechanical performance that can be achieved by the application of diamond-like carbon (DLC) coatings to polymer substrates. The polymers coated are silicone and polyethylene, and the effect on the friction coefficient is studied. Film adhesion is found to depend on the DLC film refractive index (n), whereas the friction is largely independent of n in the range studied. Films were deposited from a He/C₂H₂ mixture at 20 Pa (0.15 Torr) on to the polymer substrates placed on a 10-cm-diameter electrode driven at 13.56 MHz. Film growth was monitored by in-situ ellipsometry (at 675 nm), which was performed on a glass slide placed near the polymer substrate. Friction measurements were obtained using a pin-on-disk tribometer, and measurements were carried out using a stainless-steel pin at a linear speed of 6 cm s⁻¹. Film adhesion was evaluated using a pull-adhesion tester. It was found that DLC coatings adhere well to the polymer substrates and can significantly reduce the friction coefficient of polymers such as silicone. Higher refractive index films (which are harder and have a higher mechanical strength) were found to have a poorer adhesion and provide a slightly increased friction on the polymer surface when compared to lower-index films. This study indicates that DLC may be used to enhance the tribological properties of polymers with potential applications in the biomaterials and light-engineering industries.     

Protective properties of cataphoretic epoxy coating on aluminium alloy AA6060 modified with electrodeposited Ce-based coatings: Effect of post-treatment

Ce-based conversion coatings (CeCCs) are a promising alternative to toxic chromate coatings on the metal substrates. In this work the CeCCs were electrodeposited on aluminium alloy AA6060 from aqueous solution of Ce(NO₃)₃ at different potentials (−0.95 V, −1.2 V and −1.4 V). Effect of deposition potential and post-treatment in the phosphate solution on morphology and protective properties of CeCCs with top cataphoretic epoxy coating was studied. To assess the differences between the protective systems, originating from the different CeCCs pre-treatments, electrochemical impedance spectroscopy (EIS), polarization measurements, AFM and SEM/EDS analysis were used. The EIS study was undertaken to follow the evolution of corrosion behaviour of epoxy coating/CeCCs protective systems over prolonged time of exposure to the chloride environment (3 wt.% NaCl). Results suggest significantly improved corrosion stability of epoxy coating on AA6060 with as-deposited CeCCs sub-layers with respect to the same epoxy coatings with phosphate post-treated CeCCs. The far best protective properties, i.e., the greatest value of pore resistance and the lowest value of corrosion current density were provided by the epoxy coating/CeCC protective system with CeCC deposited at −1.2 V and without post-treatment.     

A study of microstructural and electrochemical properties of ultra-thin DLC coatings on altic substrates deposited using the ion beam technique

Microstructural and electrochemical characterization of diamond like carbon (DLC) ion beam-deposited on AlTiC (70 wt% Al₂O₃+30 wt% TiC) substrate has been carried out. Tapping mode atomic force microscopy (AFM) imaging showed that the island-like topography of DLC-coated substrates is similar to the un-coated one, indicating the uniform coverage of DLC without visible pinholes. Confocal micro-Raman analysis demonstrated that the total Raman intensity, as well as the I_D/I_G ratio, increases with the coating thickness. Electrochemical impedance spectra showed that with the increasing DLC coating thickness, a transition from one-time constant response to two-time constant response occurred when the coating thickness equals 5 nm (IS2), indicating the existence of micro-defects in the coatings which are invisible for AFM. More detailed analysis using the equivalent circuit model revealed that the charge transfer resistance (R_ct) at electrolyte/substrate interface and the resistance (R_p) related to DLC coatings increase significantly with the coating thickness, while the double-layer capacitance (C_dl) and the capacitance (C_co) of DLC coatings decrease dramatically. All these phenomena can be interpreted in terms of the evolution of the subsurface diamond-like phase (sp³-bond) and the reduction of micro-defects in the DLC coatings with the growing film. As a result, an increase in the corrosion potential (E_c) with the DLC coating thickness was also detected using the Tafel technique. In consequence, the DLC coatings can improve significantly the anti-corrosion properties of AlTiC substrates when the coating thickness is more than a few tens of nanometres.     

Effect of bias voltage on microstructure and phase evolution of Cr–Mo–N coatings by an arc bonded sputter system

Cr–Mo–N hard coatings were deposited on SKD11 and silicon wafer substrates at various substrate bias voltages by hybrid PVD consisting of arc ion plating and unbalanced magnetron sputtering. The results showed that the microstructure, phase evolution, and mechanical properties of the coatings were significantly altered at the different substrate bias voltage ranging from 0 to −400 V. The X-ray diffraction analysis results revealed that most of the diffraction peaks originated from the Cr–N phase. These peaks were observed at lower positions with no substrate bias and were shifted to higher positions with increasing substrate bias power. The preferred orientation of the (200) plane became dominant accompanying the (220) plane as the bias voltage was increased. Maximum hardness of approximately 30 GPa was obtained at a bias voltage of −200 V. Additionally, wear test results reveal that the lowest coefficient of friction, between 0.4 and 0.5, was obtained from the Cr–Mo–N film formed at a bias voltage of −200 V.     

The microstructure and properties of energetically deposited carbon nitride films

The intrinsic stress, film density and nitrogen content of carbon nitride (CN_x) films deposited from a filtered cathodic vacuum arc were determined as a function of substrate bias, substrate temperature and nitrogen process pressure. Contour plots of the measurements show the deposition conditions required to produce the main structural forms of CN_x including N-doped tetrahedral amorphous carbon (ta-C:N) and a variety of nitrogen containing graphitic carbons. The film with maximum nitrogen content (~ 30%) was deposited at room temperature with 1.0 mTorr N₂ pressure and using an intermediate bias of − 400 V. Higher nitrogen pressure, higher bias and/or higher temperature promoted layering with substitutional nitrogen bonded into graphite-like sheets. As the deposition temperature exceeded 500 °C, the nitrogen content diminished regardless of nitrogen pressure, showing the meta-stability of the carbon–nitrogen bonding in the films. Hardness and ductility measurements revealed a diverse range of mechanical properties in the films, varying from hard ta-C:N (~ 50 GPa) to softer and highly ductile CN_x which contained tangled graphite-like sheets. Through-film current–voltage characteristics showed that the conductance of the carbon nitride films increased with nitrogen content and substrate bias, consistent with the transition to more graphite-like films.     

Microstructure and corrosion properties of Ni-TiN nanocoatings prepared by jet pulse electrodeposition

Ni–TiN nanocoatings were successfully prefabricated by jet pulse electrodeposition. The effect of jet rate on cross-sectional composition, microstructure, microhardness, and corrosion properties of nanocoatings was examined by X-ray photoelectron spectroscopy, high-resolution transmission electron microscope, atomic force microscopy, microhardness tester and electrochemical workstation. Results illustrated that Ni–TiN nanocoatings deposited at jet rate of 3 m/s exhibited high concentration of Ni and Ti with average concentrations of Ni and Ti of 54.5 at% and 19.8 at%, respectively. Average diameters of Ni grains and TiN nanoparticles in Ni–TiN nanocoatings prepared at 3 m/s were 47.8 nm and 30.5 nm, respectively. Nanocoatings deposited at 1 m/s, 3 m/s and 5 m/s showed surface root-mean-square roughness value of 95.431, 30.091 and 58.454 nm, respectively, and presented maximum microhardness of 789.5, 876.2, and 849.9 HV, respectively. Ni–TiN nanocoating obtained at 3 m/s demonstrated minimum I_corr and E_corr values of 1.02 × 10⁻³ mA/cm² and − 0.551 V, respectively, signifying to offer the best corrosion resistance.     

The influence of the surface texture of hydrogen-free tetrahedral amorphous carbon films on their wear performance

Hydrogen-free and predominantly tetrahedrally bonded amorphous carbon thin films (ta-C) are excellent coatings to protect surfaces from wear due to their low coefficient of friction and high hardness. Since these coatings may be several times harder than common engineering materials counterpart wear can be significant. Therefore the surface texture of the ta-C coating is critical to wear applications. While the surface roughness is an important factor, the paper shows that other surface texture parameters have to be considered as well to predict the wear performance of the coating. Wear data are compared of as deposited, polished and brushed ta-C coatings. The results show that typically referenced average values for the surface roughness such as R_a and R_z may prove insufficient to reliably predict the wear behavior of the coating. Additional parameters describing the surface texture such as the “Skewness” (R_sk) and “Kurtosis” (R_ku) can provide relevant information. For example, a brushed ta-C surface with an average roughness of R_a = 31 nm showed a tenfold improved wear performance over a polished ta-C surface with an average roughness of R_a = 10 nm. This phenomenon is explained by analyzing the R_sk and R_ku data, which prove to more closely capture the post-treatment specific changes to the surface texture of the coatings.     

Application of Box–Behnken design and response surface methodology for modeling of some Turkish coals

The aim of our research was to apply Box–Behnken experimental design and response surface methodology for modeling of some Turkish coals. As a base for this study, standard Bond grindability tests were initially done and Bond work indexes (W_i) values were calculated for three Turkish coals. The Box–Behnken experimental design was used to provide data for modeling and the variables of model were Bond work index, grinding time and ball diameter of mill. Coal grinding tests were performed changing these three variables for three size fractions of coals (−3350 + 1700 μm, −1700 + 710 μm and −710 μm).Using these sets of experimental data obtained by mathematical software package (MATLAB 7.1), mathematical models were then developed to show the effect of each parameter and their interactions on product 80% passing size (d₈₀). Predicted values of d₈₀ obtained using model equations were in good agreement with the experimental values of d₈₀ (R² value of 0.96 for −3350 + 1700 μm, R² value of 0.98 for −1700 + 710 μm and R² value of 0.94 for −710 μm). This study proved that Box–Behnken design and response surface methodology could efficiently be applied for modeling of grinding of some Turkish coals.     

Electrochemical properties of lithium nickel oxide synthesized by the combustion method in an O2 stream

An appropriate mole ratio of urea/nitrate for preheating to synthesize LiNiO₂ was examined by varying the ratio from 1.2 to 9.6. The chemical equation of the combustion reaction was deduced from the XRD analysis result of the mixture after preheating. The XRD pattern of the LiNiO₂ sample calcined at 800 °C for 24 h, after preheating at the mole ratio of urea/nitrate of 3.6 at 400 °C, shows clear split of the 1 0 8 and 1 1 0 peaks, and the largest value of I₀₀₃/I₁₀₄. The sample calcined at 800 °C for 24 h has a relatively high first discharge capacity (164.2 mAh g^?1) and a good cycling performance. Derivative ?dx/|dV| vs. V curve of the LiNiO₂ sample at the voltage range of 2.7–4.4 V for the first cycle exhibits four peaks for charging and discharging, showing that this sample goes through four phase transitions.     

The correlation between the structure and the dielectric properties of KxBa1−xGa2−xGe2+xO8 ceramics

A study of K_xBa_1−xGa_2−xGe_2+xO₈ ceramics revealed that these solid solutions undergo a monoclinic-to-monoclinic P2₁/a ⇔ C2/m phase transition. The temperature of this phase transition decreases with an increase in x, in a similar way to the sintering temperature, which decreases from 1100 °C (x = 0) to 970 °C (x = 1). The temperature of the P2₁/a ⇔ C2/m phase transition is below the sintering temperatures of K_xBa_1−xGa_2−xGe_2+xO₈ (0.67 ≤ x ≤ 1) solid solutions, whereas the compositions at lower x (x = 0.4 and 0) remain in the P2₁/a modification over a wide temperature range above the sintering temperature. Compared to the C2/m modification of K_xBa_1−xGa_2−xGe_2+xO₈ (0.67 ≤ x ≤ 1), with a permittivity of 6.2–6.9, the P2₁/a modifications exhibit permittivities of 5.9–7.0 and three-to-eight times higher Q × f values of ∼100,000 GHz (at ∼12 GHz). The temperature coefficient of the resonant frequency is ∼−25 ppm/K, regardless of the composition.     

Effects of Ta-substitution on the dielectric properties of Ba6Ti2(Nb1−xTax)8O30 thin films

Tunable Ba₆Ti₂(Nb_1−xTa_x)₈O₃₀ (BTN-xTa; x = 0, 0.25, 0.4) thin films with a tetragonal tungsten bronze structure (TTB) were deposited on platinized Si substrates using the pulsed laser deposition (PLD) technique and their properties were investigated from the viewpoint of orientation and ferroelectric phase transition. Crystal structures and dielectric properties were characterized using an X-ray diffractometer and an impedance analyzer. Pure BTN (BTN-0Ta) thin films showed tunability as high as 60% and the tunability decreased as the amounts of Ta-substitution increased at 150 kV/cm and at 1 MHz. The dielectric constants also decreased from 436 to 88 at 1 MHz through the Ta-substitution. The low tunability and dielectric constants of Ta-substituted thin films were mainly ascribed to the lowered ferroelectric transition temperature (T_c). Ferroelectric BTN (BTN-0Ta) thin films may have been changed into a paraelectric state through the Ta-substitution since the T_c of BTN thin films were shifted to temperatures far below room temperatures (approximately −60 °C).     

Atmospheric plasma deposition of diamond-like carbon coatings

The atmospheric pressure plasma-enhanced chemical vapor deposition of diamond-like carbon (DLC) has been investigated. The DLC coatings were grown with a mixture of acetylene, hydrogen and helium that was fed through a linear plasma source. The plasma was driven with radio frequency power at 27.12 MHz. Deposition rates exceeded 0.10 µm/min at substrate temperatures between 155 and 200 °C. Solid-state carbon-13 nuclear magnetic resonance revealed that the coatings contained approximately 43% sp²-bonded carbon and 57% sp³-bonded carbon. Coefficient of friction values for the coatings were found to be 0.24 ± 0.02, which is within the range observed for vacuum deposited DLC.     

Crystal and geometry-optimized structure,Hirshfeld surface analysis and spectroscopic studies of tetrachlorocuprate and nitrate salts of 1-(2-fluorophenyl)piperazine cations, (C10H15FN2)[CuCl4] (I) and (C10H14FN2)[NO3] (II)

Two new organic-inorganic hybrid materials, 1-(2-fluorophenyl)piperazine-1,4-diium tetrachlorocuprate, (C₁₀H₁₅FN₂)[CuCl₄] (I) and 1-(2-fluorophenyl)piperazin-4-ium nitrate, (C₁₀H₁₄FN₂)[NO₃] (II), have been synthesized by an acid/base reaction at room temperature in the presence of 1-(2-fluorophenyl)piperazine as an organic-structure directing agent and their structures were determined by single crystal X-ray diffraction. Compound (I), (C₁₀H₁₅FN₂)[CuCl₄], crystallizes in the monoclinic system and P2₁/c space group with a = 7.5253 (2), b = 20.6070 (7), c = 9.7281 (3) Å, β = 103.6730 (17)°, V = 1465.82 (8) Å³ with Z = 4. Full-matrix least-squares refinement converged at R = 0.037 and wR(F²) = 0.088. Compound (II), (C₁₀H₁₄FN₂)[NO₃], belongs to the monoclinic system, space group P2₁/c with the following parameters: a = 10.8034 (2), b = 7.5775 (1), c = 14.4670 (3) Å, β = 111.761 (2)°, V = 1099.91 (4) ų and Z = 4. The structure was refined to R = 0.044, wR(F²) = 0.136.In the structures of (I) and (II), the anionic and cationic entities are interconnected by means of set of hydrogen bonding contacts forming three-dimensional networks. Intermolecular interactions were investigated by Hirshfeld surfaces and the contacts of the four different chloride atoms were notably compared. The results of the optimized molecular structure are presented and compared with the experimental one. The Molecular Electrostatic Potential (MEP) maps and the HOMO and LUMO energy gap of both compounds were computed. The vibrational absorption bands were identified by infrared spectroscopy. Theory (DFT) calculations of normal mode frequencies are compared with experimental ones.     

Novel carbon diamond-like phases LA5, LA7 and LA8

In this work, the structure and properties of seven diamond-like carbon phases obtained by linking the graphene layers were calculated using DFT and PM3 methods. The LA5 (Cmca), LA7 (Cmcm), and LA8 (I4₁/amd) diamond-like phases were predicted and studied in this work for the first time. Values of the unit cell parameters of the predicted phases are: a = 4.337 Å, b = 5.024 Å, and c = 4.349 Å for LA5 phase; a = 4.942 Å, b = 4.808 Å, and c = 4.390 Å for LA7 phase; and a = 4.906 Å and c = 4.960 Å for LA8 phase. For these LA5, LA7, and LA8 phases, various structure characteristics, densities, cohesive energies, bulk moduli, electronic densities of states and X-ray patterns were calculated. The comparative analysis showed that the diamond-like phase properties depend on the extent of their structure deformation relative to the cubic diamond structure.     

Synthesis and mechanical properties of carbon nanotube/diamond-like carbon composite films

Diamond-like carbon (DLC) coatings were successfully deposited on carbon nanotube (CNT) films with CNT densities of 1 × 10⁹/cm², 3 × 10⁹/cm², and 7 × 10⁹/cm² by a radio frequency plasma-enhanced chemical vapor deposition (CVD). The new composite films consisting of CNT/DLC were synthesized to improve the mechanical properties of DLC coatings especially for toughness. To compare those of the CNT/DLC composite films, the deposition of a DLC coating on a silicon oxide substrate was also carried out. A dynamic ultra micro hardness tester and a ball-on-disk type friction tester were used to investigate the mechanical properties of the CNT/DLC composite films. A scanning electron microscopic (SEM) image of the indentation region of the CNT/DLC composite film showed a triangle shape of the indenter, however, chippings of the DLC coating were observed in the indentation region. This result suggests the improvement of the toughness of the CNT/DLC composite films. The elastic modulus and dynamic hardness of the CNT/DLC composite films decreased linearly with the increase of their CNT density. Friction coefficients of all the CNT/DLC composite films were close to that of the DLC coating.     

Piezoelectric and ferroelectric properties of lead-free [Bi1−y(Na1−x−yLix)]0.5BayTiO3 ceramics

Lead-free [Bi_1−y(Na_1−x−yLi_x)]_0.5Ba_yTiO₃ (BNLB-x/y) piezoelectric ceramics were prepared by sintering the constituent oxides, and their piezoelectric and ferroelectric properties studied. The results of X-ray diffraction (XRD) suggest that Li⁺ and Ba²⁺ diffuse into the Bi_0.5Na_0.5TiO₃ (BNT) lattices to form a solid solution with a single-phase perovskite structure. The ceramics can be well sintered at 1100–1150 °C. The introduction of Li⁺ and Ba²⁺ into Bi_0.5Na_0.5TiO₃ significantly decreases the coercive field, E_c but maintains the large remanent polarization, P_r of the materials. The ceramics exhibit relatively good piezoelectric properties and very strong ferroelectricity: piezoelectric constant, d₃₃ = 208 pC/N, planar electromechanical coupling factor, k_p = 37.0%, remanent polarization, P_r = 38.5 μC/cm², coercive field, E_c = 3.27 kV/mm. The depolarization temperature, T_d of BNLB-0.075/0.04 ceramics is about 190 °C.     

Nanostructure,mechanical and tribological properties of reactive magnetron sputtered TiCx coatings

This study describes the correlation between microstructure, mechanical and tribological properties of TiC_x coatings (with x being in the range of 0–1.4), deposited by reactive magnetron sputtering from a Ti target in Ar/C₂H₂ mixtures at ~ 200 °C. The mechanical and tribological properties were found to strongly depend on the chemical composition and the microstructure present. Very dense structures and high hardness, combined with low wear rates and friction coefficients, were observed for coatings with chemical composition close to TiC. X-ray diffraction and X-ray photoelectron spectroscopy analysis, used to evaluate coating microstructure, composition and relative phase fraction, showed that low carbon contents in the coatings lead to sub-stoichiometric nanocrystalline TiC_x coatings being deposited, whilst higher carbon contents gave rise to dual phase nanocomposite coatings consisting of stoichiometric TiC nanocrystallites and free amorphous carbon. Optimum performance was observed for nanocomposite TiC_1.1 coatings, comprised of nanocrystalline nc-TiC (with an average grain size of ~ 15 nm) separated by 2–3 monolayers of an amorphous a-DLC matrix phase.