Thermal fatigue and hypothermal atomic oxygen exposure behavior of carbon nanotube wire

In low earth orbit (LEO), components of space systems are exposed to damaging hypothermal atomic oxygen and thermal fatigue. Carbon nanotube (CNT) wires are candidate materials for different applications in space systems. Thirty-yarn CNT wire’s behavior was evaluated when exposed to hypothermal atomic oxygen and thermal fatigue. CNT wire specimens were exposed to a nominal fluence of hypothermal atomic oxygen of 2 × 10²⁰ atoms/cm². The erosion rate due to hypothermal collision between atomic oxygen and CNT wires was calculated to be 2.64 × 10⁻²⁵ cm³/atom, which is comparable to highly ordered pyrolytic graphite. The tensile strength of CNT wire was not affected by this exposure, and a minor reduction of electrical conductivity (2.5%) was found. Scanning electron microscopy (SEM) and Energy Dispersive X-ray spectroscopy analysis showed erosion of surface layer with depleted carbon and increased oxygen. Thermal fatigue excursion of 5000 cycles from 70 to −50 °C at a rate of 55 °C/min showed no loss in tensile strength; however a large decrease in conductivity (18%) was seen. SEM analysis showed that the thermal fatigue created buckling of yarn and fracture of individual CNTs bundles. These reduced the effective area and electrical conductivity of CNT wire.     

A near edge X-ray absorption fine structure study of oxidized single crystal and polycrystalline diamond surfaces

The influence of oxidizing environments on single crystal diamond and polycrystalline chemical vapor deposited CVD diamond films was studied using the near edge X-ray absorption fine structure (NEXAFS) pre-edge region in both bulk and surface sensitive modes. The NEXAFS of (100) oriented single crystal diamond was measured following (i) exposure to a microwave (MW) hydrogen plasma, (ii) annealing to 1000 °C, (iii) exposure of the as annealed surface to H₂O, and (iv) exposure of the as annealed surface to O₂. From these measurements particular surface bonding configurations have been assigned to features in the pre-edge structure. The NEXAFS of microcrystalline CVD diamond films was studied following different oxidative treatments using (i) a thermal atomic oxygen (AO) environment, (ii) a hyperthermal (5 eV) AO source, and (iii) an RF oxygen plasma exposure. The nature of the surface layer was found to be different for differently oxidized surfaces. These treatments were carried out as part of a study of CVD diamond durability in the low Earth orbit space environment.     

Polyhedral oligomeric silsesquioxane polyimide nanocomposites for color filters and flexible conductive films

Color filters and conductive films are widely used in spacecraft, while the lack of lightweight, flexibility, and atomic oxygen (AO) durability confine their potential applications in low earth orbit. In this study, a clear poly(amic acid) with an empirical 20 wt% polyhedral oligomeric silsesquioxane (POSS) solid content is designed for transparency and AO durability. Red, green, blue, and yellow dyes are reinforced with small amounts of 1–5 wt% in POSS polyimides for color filters. A silver nanowire network film is infiltrated onto the POSS polyimide for conductive film. Erosion depth upon hyperthermal AO exposure, surface morphology, surface chemistry, optical transparency, and conductivity have been systematically investigated. The erosion yields of all 20 wt% POSS polyimides decrease by an order of magnitude when subjected to 2.32 ± 0.05 and 2.39 ± 0.06 × 10²⁰ atoms cm⁻² AO fluences, as passivating SiO_x networks are formed on film surface. The small-amount dye additives into polyimides do not introduce obvious changes in AO durability and surface chemistry. The silver nanowire infiltrated POSS polyimide film shows a 65.7% transmittance at 550 nm and a sheet resistance of 8.50 ± 0.36 Ohm square⁻¹. This study reveals promising attempts of POSS-polyimide-based color filters and flexible conductive films for potential space applications.     

Effect of oxygen plasma treatment on non-thrombogenicity of diamond-like carbon films

The non-thrombogenicity of oxygen-plasma-treated DLC films was investigated as surface coatings for medical devices. DLC films were deposited on polycarbonate substrates by a radio frequency plasma enhanced chemical vapor deposition method using acetylene gas. The deposited DLC films were then treated with plasma of oxygen gas at powers of 15 W, 50 W, and 200 W. Wettability was evaluated by water contact angle measurements and the changes in surface chemistry and roughness were examined by X-ray photoelectron spectroscopy and atomic force microscope analysis, respectively. Each oxygen-plasma-treated DLC film exhibited a hydrophilic nature with water contact angles of 11.1°, 17.7° and 36.8°. The non-thrombogenicity of the samples was evaluated through the incubation with platelet-rich plasma isolated from human whole blood. Non-thrombogenic properties dramatically improved for both 15 W- and 50 W-oxygen-plasma-treated DLC films. These results demonstrate that the oxygen plasma treatment at lower powers promotes the non-thrombogenicity of DLC films with highly hydrophilic surfaces.     

Studies of physical and chemical properties of styrene-based plasma polymer films deposited by radiofrequency Ar/styrene glow discharge

Styrene-based plasma polymer (SPP) films having thickness in the range of 900–1800 nm are deposited from radiofrequency (RF) Ar/styrene glow discharge. Depositions of the SPP films are carried out at working pressure of 1.2 × 10^?1 mbar and in the RF power range of 40–130 W. The physical and chemical properties of the SPP films are investigated as a function of RF power. Optical emission spectroscopy (OES) studies on Ar/styrene glow discharge reveal that the relative concentrations of active plasma species are strongly dependent on the variation of RF power. Fourier transform infrared (FT-IR) and X-ray photoelectron spectroscopy (XPS) are used to analyze the internal chemical structures of the films. It is revealed that the SPP film with highest carbon content exhibits enhanced scratch and corrosion resistance behavior along with stable thermal properties. The thermogravimetric (TGA) and gel permeation chromatography (GPC) results suggest the presence of both aliphatic and aromatic units in the SPP films. Attempts are made to correlate the results obtained from OES, FT-IR and XPS analyses with the deposited films properties. The possibility of using SPP films as protective coatings is also explored.     

Halogen plasma erosion resistance of rare earth oxide films deposited on plasma sprayed alumina coating by aerosol deposition

Dense yttria, erbia and dysprosia films with thicknesses of 44.8 ± 1.7, 51.6 ± 0.9 and 36.8 ± 3.1 μm, respectively, were deposited on plasma sprayed alumina coating by aerosol deposition (AD). The rare earth oxide films remarkably enhanced the erosion resistance of the plasma sprayed alumina coating upon exposure to the halogen gas plasma. The enhanced plasma erosion resistances were related to their low surface porosity as well as the low vapor pressures of the rare earth fluorides and chlorides compared with those of corresponding aluminum halogenides. Electrical breakdown voltages of the samples with yttria, erbia and dysprosia films on the plasma sprayed alumina coating were 5.5, 6.2 and 5.3 kV, respectively, at room temperature and 4.0, 4.2 and 3.9 kV, respectively, at 573 K. The breakdown voltages at RT and 573 K were more than double that of the plasma sprayed alumina coating without the AD films.     

Erosive wear resistance of NCD coatings produced by pulsed microwave discharges

Erosion tests on nanocrystalline diamond (NCD) films are relevant not only for the evaluation of the erosive wear resistance, anticipating applications where coated materials are exposed to particle impacts, but also as a way to evaluate their adhesion to the substrates. NCD films were grown on Si₃N₄ ceramic by microwave plasma assisted deposition in continuous (CW) and pulsed (PW-50 Hz and PW-500 Hz) discharge modes in argon-rich gas mixture. The films grown in PW modes presented lower crystallite size and lower surface roughness than those grown in CW one, while the use of CF₄ plasma pre-treatment of the substrate lead to further film homogeneity. The erosive wear resistance of NCD was evaluated by solid particle impact using SiC (45–250 μm size) as erodent material, with selected parameters accordingly to Hertzian stress field calculations. Film weight loss was undetectable until delamination took place. When tested with 150 μm SiC particles, the CF₄ plasma pre-treated substrates yield a three-fold increase (15 min) in delamination time comparing to untreated specimens, while samples coated under PW-50 Hz conditions presented a six times lower erosion rate compared to CW ones. It is believed that the improved nucleation behaviour by the use of PW mode and its higher homogeneity on the CF₄ plasma pre-treated samples decrease the flaw population on the diamond/substrate interface, leading to improved adhesion levels.     

Hydrogen plasma etching of diamond films deposited on graphite

Poly- and nanocrystalline diamond films have been deposited using microwave plasma enhanced CVD with gas mixtures of x%CH₄/15%H₂/Ar (x = 0.5, 1, 3, and 5). After deposition the resulting films were exposed to a hydrogen plasma etching for 30 min. The hydrogen plasma produced preferential etching of non-diamond carbon on the surface of the samples and the development of steps and pits. Raman spectroscopy and X-ray photoelectron spectroscopy analyses on the etched films showed increased sp³/sp² ratio and decreased surface oxygen. The etch mechanism proposed is regression of pre-existing steps and step flow.     

Effect of the type of plasma on the polydimethylsiloxane/collagen composites adhesive properties

Polydimethylsiloxane (PDMS) films were treated with either oxygen (O₂), nitrogen (N₂) or argon (Ar) plasma between 40 W and 120 W for 5–15 min and their surface properties studied by contact angle measurements, infrared spectroscopy (FTIR), scanning electron microscopy (SEM) and atomic force microscopy (AFM). Lower contact angles and increases in surface roughness, assessed by SEM and AFM, were observed for all used gases when plasma power and time increased, with argon treatment being the one that showed the most significant change in roughness.PDMS/collagen type I composites obtained after treating PDMS with oxygen at 80 W for 13 min or nitrogen and argon at 80 W for 14 min showed a peel strength of 0.1N/mm (oxygen plasma), 0.08 N/mm (nitrogen plasma) and 0.09 N/mm (argon plasma). In all cases, peel strength was higher than that measured for the untreated bilayer composite. An increase in adhesion strength, after oxygen and nitrogen plasma, was mostly attributed to chemical interaction between functional groups introduced on the PDMS surface and the functional groups on collagen as detected by FTIR. In contrast, the high peel strength observed on PDMS treated with argon plasma was attributed to its increased roughness which in turn increased mechanical interlocking. The properties of these composites render them suitable for adhesive free skin substitutes.     

Characterization of Cu(In,Ga)Se2 thin films prepared by RF magnetron sputtering using a single target without selenization

Cu(In_1?xGa_x)Se₂ (CIGS) thin films were prepared using a single quaternary target by RF magnetron sputtering. The effects of deposition parameters on the structural, compositional and electrical properties of the films were examined in order to develop the deposition process without post-deposition selenization. From X-ray diffraction analysis, as the substrate temperature and Ar pressure increased and RF power decreased, the crystallinity of the films improved. The scanning electron microscopy revealed that the grains became uniform and circular shape with columnar structure with increasing the substrate temperature and Ar pressure, and decreasing the RF power. The carrier concentration of CIGS films deposited at the substrate temperature of 500 °C was 2.1 × 10¹⁷ cm^?3 and the resistivity was 27 Ω cm. At the substrate temperature above 500 °C, In and Se contents in CIGS films decreased due to the evaporation and it led to the deterioration of crystallinity. It was confirmed that CIGS thin films deposited at optimal condition had similar atomic ratio to the target value even without post-deposition selenization process.     

Effect of O2 plasma pretreatment on structural and optical properties of ZnO films on PES substrate by atomic layer deposition

ZnO films were deposited on the O₂ plasma treated polyethersulfone (PES) substrates by atomic layer deposition. X-ray diffraction (XRD) measurements reveals that the grains in ZnO films show strongly (0 0 2) preferential orientation, when the duration of plasma pretreatment increases. The decreased grain size and improved crystallinity results in the decreased surface roughness of ZnO films. In contrast, when the duration of plasma pretreatment increases to 60 min, the surface roughness increases again due to the increased grain size and worse crystallinity. In photoluminescence measurement, slight blue shift of near-band-edge emission occurs with increasing duration of plasma pretreatment up to 30 min.     

Influence of nitrogen plasma post-treatment on diamond-like carbon films synthesized by RF plasma enhanced chemical vapor deposition

DLC films were synthesized by RF plasma enhanced chemical vapor deposition and the effects of nitrogen plasma post-treatment at different pressures on the structure and properties of DLC films were investigated. Higher roughness was obtained after plasma post-treatment at higher pressures (0.3 and 0.9 torr) and plasma post-treatment at a lower pressure (0.15 torr) resulted in lower roughness than that of original films. The hardness of DLC films decreased with the decrease of post-treatment pressure, which is consistent with the Raman results of I_D/I_G ratio and G peak position. Compared to the original DLC film, the residual stress after plasma post-treatment decreased slightly due to the relatively thin region involved in the plasma post-treatment.     

Composite diamond films for short-mm wave and THz traveling wave tube windows

With an increase in frequency, the diamond thickness of the microwave windows for short-mm wave and THz traveling wave tubes (TWTs) approaches 100 μm or even tens of μm. This poses problems of mechanical strength and air tightness to the polycrystalline diamond (PCD) window. To overcome these problems, we have studied a composite diamond film that consists of PCD and ultra-nanocrystalline diamond (UNCD). First, SEM was used to examine the early growing process of UNCD on PCD. The 5 μm thick UNCD grown on 40 μm PCD exhibited a hillock structure with densely packed ≤ 20 nm granules, in contrast to the PCD layer showing randomly packed, micrometer sized grains. Then, the effect of UNCD thickness on fracture strength and thermal conductivity was studied using the test samples with thin layers of UNCD having thicknesses of 1, 2.5, 5, and 10 μm on 100 μm thick PCD films, respectively. The fracture strengths of all the films are 2–3 times higher than that of the PCD films, which is 350 ± 150 MPa. As expected, the thermal conductivity of the samples measured at ~ 20 °C decreases with an increase in UNCD thickness, particularly in the range of 0 to 2.5 μm. At a thickness of 10 μm, the thermal conductivity was found to be ~ 10 W/cm 1 K. Finally, a 100 μm sandwich-like structure with a total UNCD thickness of 10 μm was fabricated and two 180 GHz TWT windows were assembled. RF tests show that for the operating frequency range of 175 to 185 GHz, the transmission loss (S₂₁) was found to be ≤ 1.22 and ≤ 1.71 dB, respectively, indicating an excellent RF performance. Mechanical strength and air tightness of the windows were also found improved and able to meet the requirement of the device. This work provides a novel approach for fabricating relatively thin diamond films for RF applications, such as TWT windows.     

p-type conductive NiOx: Cu thin films with high carrier mobility deposited by ion beam assisted deposition

Transparent conductive NiO thin films with 18 at% Cu dopant were fabricated by ion beam assisted deposition (IBAD). Their structural and optoelectronic properties were compared with undoped NiO films and NiO films doped with 12 at% Cu, and also compared with NiO:Cu (18 at%) films deposited by RF sputtering as reported in our previous work. The results show that the crystallinity of NiO thin films deposited through IBAD technology is much better than that of the films deposited by RF sputtering. Thanks to this reason, the highest carrier mobility above 45 cm²V^?1s^?1 for NiO:Cu (18 at%) film can be realized here. Meanwhile, the films’ resistivity remains an acceptable value, varying from 2.05 to 0.064 Ω cm with oxygen ion beam current changing from 0.2 to 0.8 A. This feature is imperative for p-type transparent conductive oxides (TCOs) applied in various domains. In addition, with oxygen ion beam current increase, the increase of the Ni³⁺/Ni²⁺ ratio leads to more Ni²⁺ vacancies be introduced into NiO films, which is beneficial to generate holes and improve carrier concentration. In this work, the optimal carrier mobility of NiO film doped with 18 at% Cu is obtained when the oxygen ion beam current is 0.2 A. Its carrier concentration and electrical resistivity are 7.26 $\times$10¹⁶ cm^?3 and 2.05 Ω cm, respectively.     

The changes in structural and optical properties of (ZnO:AlN) thin films fabricated at different RF powers of ZnO target

AlN doped ZnO thin films were prepared on glass and Si (100) substrates by RF sputtering. The ratio of nitrogen (N₂) to Argon (Ar) used to prepare the films was 80:20. The films were deposited at different RF powers of 150 W, 175 W, 200 W, 225 W and 250 W for ZnO target and 200 W for AlN target. XRD results revealed the existence of (002) ZnO phase for RF power of ZnO target above 175 W. However, at the RF power of 150 W, the film exhibited amorphous properties. The prepared films showed transmission values above 70% in the visible range. The average calculated value of energy band gap and the refractive index were 3.43 eV and 2.29 respectively. The green and UV emission peaks were observed from PL spectra. Raman Peaks at 275.49 cm^?1 and 580.17 cm^?1 corresponding to ZnO:N and ZnO:AlN were also observed.     

Induction of ferroelectricity in nanoscale ZrO2 thin films on Pt electrode without post-annealing

Large stable ferroelectricity in nanoscale undoped zirconia (ZrO₂) thin films prepared without post-annealing has been demonstrated for the first time. Remanent polarizations up to 12 μC cm⁻² were obtained in the as-deposited ZrO₂ thin films prepared by remote plasma atomic layer deposition at 300 °C substrate temperature on the Pt electrode. Ferroelectric crystallization of the films was achieved without post-annealing, which is highly beneficial to the application of the films in non-volatile memories and ultralow-power nanoelectronics. The existence of the ferroelectric orthorhombic phase with noncentrosymmetric space group Pbc2₁ in the as-deposited ZrO₂ thin films was confirmed by high-resolution transmission electron microscopy.     

Deposition of nanocomposite CNx/SiO2 films in inductively coupled r.f. discharge

Hard films were deposited in an inductively coupled r.f. discharge at a frequency of 3.5 MHz by the chemical transport of carbon from a graphite target in a nitrogen atmosphere combined with evaporation from a quartz tube. The nitrogen flow was varied from 1.0 to 4.0 sccm. The r.f. power supplied was in the range 2.5–3.5 kW. Silicon substrates were placed on the graphite holder whose temperature was 700–800°C. In the emission spectra the CN, N₂ and C₂ molecular bands and the silicon, carbon, oxygen and nitrogen atomic lines were observed. The ratio of nitrogen to carbon in the films ranged from 0.1 to 0.55 while the ratio of oxygen to silicon was about 2 for all the films studied. The films had a maximum hardness of 35 GPa and they showed a high elasticity up to 88%, good fracture toughness and adhesion to the substrate. Unlike carbon nitride films the CN_x/SiO₂ films were almost non-absorbing in the visible range.     

Changes in chemical bonding of diamond-like carbon films by atomic-hydrogen exposure

We have deposited unhydrogenated diamond-like carbon (DLC) films on Si substrate by pulsed laser deposition using KrF excimer laser, and investigated the effects of atomic-hydrogen exposure on the structure and chemical bonding of the DLC films by photoelectron spectroscopy (PES) using synchrotron radiation and Raman spectroscopy. The fraction of sp³ bonds at the film surface, as evaluated from C1s spectra, increased at a substrate temperature of 400 °C by atomic-hydrogen exposure, whereas the sp³ fraction decreased at 700 °C with increasing exposure time. It was found that the sp³ fraction was higher at the surfaces than the subsurfaces of the films exposed to atomic hydrogen at both the temperatures. The Raman spectrum of the film exposed to atomic hydrogen at 400 °C showed that the clustering of sp² carbon atoms progressed inside the film near the surface even at such a low temperature as 400 °C.     

X-ray reflectivity analysis on initial stage of diamond-like carbon film deposition on Si substrate by RF plasma CVD and on removal of the sub-surface layer by oxygen plasma etching

The multi-layered structure of thin diamond-like carbon (DLC) films was investigated by X-ray reflectivity (XRR) analysis. Thin DLC films were deposited on Si substrate by RF plasma chemical vapor deposition (CVD) from acetylene source gas with short duration of plasma operation from 0.08 to 4.99 s. It was confirmed from XRR analysis that the thin DLC film on Si substrate had 3 layers consisting of a subsurface layer on the grown surface, a mixing layer at the interface to Si substrate, and a bulk-DLC layer sandwiched between the 2 layers. The 3 layers had been formed in 0.08 s at beginning of deposition with distinctive bulk-DLC layer of 1.7 nm thick already appeared due to extremely higher deposition rate only at the initial stage of CVD. The thickness of bulk-DLC layer increased with increasing CVD duration while both the mixing layer of higher density and the sub-surface layer of extremely low density continuously existed. By oxygen plasma etching, it was confirmed by XRR analysis that the sub-surface layer was clearly removed and another layer of lower density than the bulk DLC appeared.     

Hard silicon carbonitride films obtained by RF-plasma-enhanced chemical vapour deposition using the single-source precursor bis(trimethylsilyl)carbodiimide

Amorphous silicon carbon nitride (Si/C/N) coatings were prepared on steel substrates by RF plasma-enhanced chemical vapour deposition (RF-PECVD) from the single-source precursor bis(trimethylsilyl)carbodiimide (BTSC). The films were characterised by X-ray diffraction (XRD), ellipsometry, FTIR, glow discharge optical emission spectroscopy (GDOES), optical microscopy, AFM, hardness measurements, scratch-, tribological- and corrosion-tests. The results of these studies show that the coatings obtained on the RF-powered electrode (cathode) were black, thick (>20 μm) and hard (21–29 GPa), while those grown on the grounded electrode (anode) were yellow, thin (<4 μm) and soft (∼5 GPa). Coatings on the anode contained around 19 at.% oxygen and exhibited silicon predominantly bonded to oxygen. In contrast, the oxygen content of the films deposited on the cathode was below 2 at.%. Silicon atoms in these coatings are co-ordinated predominantly to nitrogen and carbon. The surface of all coatings was very smooth with a maximum rms roughness between 2 nm and 5 nm for an area of 5 μm × 5 μm. Scratch and tribological tests reveal a brittle nature of the cathode-coatings and rather weak adhesion to the metal substrates. Salt-spray tests indicate an excellent corrosion resistance of the material.