Investigations of diffusion behaviour in Al-doped zinc oxide and zinc stannate coatings

Multi-layer dielectric/silver/dielectric coating systems have excellent proprieties as heat insulators and for solar energy reflection and electrical conductivity. The largest market is dominated by low-emissivity (low-E) coatings, which are applied to large area architectural glazing to reduce heat losses from buildings. They combine high visible transparency with high reflectance in the far-infrared region, where the thin (~ 10 nm) silver layer reflects long wavelength IR back into the building and the dielectric layers both protect the silver and act as anti reflectance layers.In this study, a range of dielectric coatings has been deposited onto soda-lime glass substrates by reactive sputtering from metallic targets. The magnetrons were driven in DC mode and also in mid-frequency pulsed DC and AC modes. Process variables investigated include operating pressure, oxygen flow rate and magnetron configuration. Selected coatings were annealed at 650 °C and analysed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscope (AFM).The oxide samples were then over-coated with silver and annealed for a second time. These coatings were analysed by secondary ion mass spectroscopy (SIMS) to determine the diffusion rates of silver and sodium (from the substrate) through the oxide coatings.The results to date, presented here, show the diffusion of silver and sodium atoms through zinc oxide and zinc stannate thin films deposited under a vast range of conditions. Preliminary attempts have been made to estimate diffusion coefficients for these coating systems and to relate these values to processing conditions and the structural variations observed.     

Self-assembled monolayer of 3-aminopropyltrimethoxysilane for improved adhesion between aluminum alloy substrate and polyurethane coating

A good adhesion between a polymer coating and a metal or metal alloy substrate such as Al 2024-T3 plays a critical role in corrosion protection of metal substrates. In our study, a self-assembled monolayer film of 3-aminopropyltrimethoxysilane was formed on Al 2024-T3 substrate by covalent bonding. The adhesion property of a self-priming polyurethane coating was evaluated by pull-off adhesion test, wet tape test and thermal cycling test. All the testing results indicate that both dry and wet adhesion properties of the polyurethane coating were improved significantly after APS treatment of Al 2024-T3 in polar solvents such as methanol and acetone. In nonpolar solvents such as hexane, the APS treatment led to inconsistent improvement or sometime decreased adhesion of polyurethane coating. X-ray photoelectron spectroscopic study revealed that while a monolayer film was formed on the aluminum alloy surface after treating the substrate with APS in methanol and acetone, a multilayer film was formed on the substrate surface when the treatment was conducted in hexane. The APS monolayer film served as a covalent bond linkage between polymer coating and aluminum alloy substrates, which led to the increased adhesion property of polymer coating and corrosion resistance of the metal alloy substrate.     

Control of stress in protected silver mirrors prepared by plasma beam sputtering

Plasma beam sputtering was used to deposit dielectric-protected silver mirrors that exhibited excellent durability and controlled stress. The durability of the mirrors was strongly dependent upon the presence of a very thin chromium adhesion layer between the silver layer and the dielectric overcoat. The stress of the five-layer mirror was balanced by controlling the compressive stress in the top dielectric layer, offsetting the net tensile stress of the combination of layers below.     

Site-selective electroless nickel plating on patterned thin films of macromolecular metal complexes

We demonstrate a simple route to depositing nickel layer patterns using photocross-linked polymer thin films containing palladium catalysts, which can be used as adhesive interlayers for fabrication of nickel patterns on glass and plastic substrates. Electroless nickel patterns can be obtained in three steps: (i) the pattern formation of partially quaterized poly(vinyl pyridine) by UV irradiation, (ii) the formation of macromolecular metal complex with palladium, and (iii) the nickel metallization using electroless plating bath. Metallization is site-selective and allows for a high resolution. And the resulting nickel layered structure shows good adhesion with glass and plastic substrates. The direct patterning of metallic layers onto insulating substrates indicates a great potential for fabricating micro/nano devices.     

Electro-fragmentation analysis of dielectric thin films on flexible polymer substrates

An electro-fragmentation method was developed as a fast alternative to the time consuming fragmentation test carried out in situ in a microscope, to investigate the failure of dielectric inorganic coatings on polymer substrates. An ultrathin conductive layer was used to probe the onset of tensile failure in the dielectric coating through changes of its electrical resistance. A careful selection of the conductive layer has been carried out to avoid artifacts resulting for instance from a change of the cohesive properties (e.g. internal stress state) of the investigated structures. Au layers were found to be too ductile, contrary to Al-Ti layers that were too brittle, which invalidated the use of both materials to probe the failure of the dielectric coatings. In contrast, for structures on high-temperature polymer substrates, a 10 nm thick amorphous graphite (a-G) layer was found to accurately reproduce the cracking of the coating. The Young's modulus and coefficient of thermal expansion of the a-G layer are low enough not to impact the internal strain, hence the crack onset strain of the dielectric coating. The a-G layer is also sufficiently brittle, and its cohesive failure and resulting increase of electrical resistance is triggered by the failure of the dielectric coating. The a-G electro-fragmentation method is presently limited to polymers substrates with a glass-transition temperature higher than 100 °C.     

Anti-reflective optical coatings incorporating nanoparticles

This paper presents a simple approach for forming anti-reflective film stacks on plastic substrates employing aqueous colloidal dispersions of metal oxide nanoparticles. Results demonstrate that it is possible to fabricate a polymeric thin film of continuously tunable refractive index over a wide range by loading the film with varying concentrations of metal oxide nanoparticles. Specifically, the refractive index for the polymer film was tuned from 1.46 to 1.54 using silica nanoparticle loadings from 50 to 0?wt% and from 1.54 to 1.95 using ceria nanoparticle loadings from 0 to 90?wt%, respectively. The low and high refractive index layers are then combined to create an anti-reflective coating which exhibits a reflectance spectrum, abrasion resistance, haze and transmission values that compare well with those produced using state-of-the-art vacuum based techniques. Furthermore, the results show that it is possible to begin with aqueous dispersions and then dilute them with organic solvents for use in a spin coating method to prepare the polymer-metal oxide nanoparticle composite films.     

The effect of electron beam sterilization on the physical properties of the bioresorbable polymer coatings on the titanium 6-aluminum 4-vanadium substrate

The aim of the work is to determine the physical properties of titanium 6-aluminum 4-vanadium alloy with poly (glycolide-ϵ-caprolactone) coating after electron beam sterilization. First, the metal substrate is machined with grade 120 and 320 grinding papers. Some of the samples are subjected to anodic oxidation. Then, the samples are coated with a biodegradable polymer layer of poly (glycolide-ϵ-caprolactone). Samples with polymer coatings are subjected to electron beam sterilization. To evaluate the effect of sterilization on physical properties of modified titanium alloy the scanning electron microscopy and atomic force microscopy, adhesion studies of the polymer coating to the metal substrate and wettability tests are applied. On the basis of the obtained results, an increase of the contact angle value is found both after applying the polymer coating to the surface of the tested titanium 6-aluminium 4-vanadium alloy as well as after electron beam sterilization. In addition, a slight increase of the adhesion in sterilized samples comparted to non-sterilized is observed. In scanning electron microscopic observations, traces of machining on the surface of the metal substrate and the continuity of the polymer coatings before and after sterilization are found. In the atomic force microscopic studies in relation to the initial state, a very good mapping of the surface topography of the samples with a homogeneous coating is found.     

Metallic Adhesive Layers for Ag‐Based First Surface Mirrors

Metallic thin films have been evaluated as adhesive layers for first surface mirrors. Thin films (0.5–4 nm thick) of Cr, Cu, Ge, Sn, and Ni, capped with an over layer of 100 nm of Ag have been deposited by means of ion assisted electron beam evaporation onto glass substrates. The specular reflectance, conductivity, morphology, adhesion, and abrasion resistance of the mirrors have been assessed and compared. This work demonstrates the superior Ni and Ge adhesion and abrasion resistance to Ag thin films, while Cr and again Ni are the best candidates in terms of reflectance enhancement. In particular, 4 nm of Ni increases the Ag reflectance from 95.7% to 96.7%, exhibits the strongest adhesion as determined by the cross hatch tape test and the lowest loss of material after the different abrasion tests is just 50% in comparison to 100% for Ag‐coated glass substrates.

     

Reactivities of aluminium and aluminium-magnesium alloy powders in polymeric composites

Polymeric compositions containing Al-Mg alloys show higher reactivities, in comparison with similar compositions containing aluminium. This is observed irrespective of the amount of oxidizer, type of oxidizer used, type of polymeric binder, and over a range of the particle sizes of the metal additive. This is evident from the higher calorimetric values obtained for compositions containing the alloy, in comparison to samples containing aluminium. Analysis of the combustion residue shows the increase in calorimetric value to be due to the greater extent of oxidation of the alloy. The interaction between the polymeric binder and the alloy was studied by coating the metal particles with the polymer by a coacervation technique. On ageing in the presence of ammonium perchlorate, cracking of the polymer coating on the alloy was noticed. This was deduced from differential thermal analysis experiments, and confirmed by scanning electron microscopic observations. The increase in stiffness of the coating, leading to cracking, has been traced to the cross-linking of the polymer by magnesium.     

Tolerances For Layer Thicknesses in Dielectric Multilayer Coatings and Interference Filters

A theory is developed for dielectric multilayer coatings in which the layers depart from calculated thickness. The theory is applied to alternating systems of quarter wave layers of ZnS and MgF₂. The effects of thickness errors are: (1) A shift of the wavelength at which maximum reflectance occurs; and (2) a change in phase shift upon reflection. The magnitude of these effects, and also their dependence on various parameters, are determined. Statistical tolerances for layer thicknesses are computed for given tolerances on the multilayer performance. The accuracy required for producing dielectric interference filters is up to about 40 times higher than the accuracy sufficient for the production of dielectric mirrors and beam splitters. Various techniques of experimentally controlling film thicknesses, and their accuracies, are discussed. The production of mirrors and beam splitters deviating from theoretical maximum reflectance by only 1 percent seems to be possible with Dufour’s simple single photocell method of monitoring film thicknesses. With more precise methods, such as those developed by Giacomo and Jacquinot, or Traub, the production of interference filters appears to be possible to within plus or minus one half their half widths.     

Materials for solar-transmitting heat-reflecting coatings

A coating for solar energy applications which combines heat reflection with transparency to solar radiation may be of four different types: a metallic film which is sufficiently thin to be transparent; a metal-based multilayer coating; a wide band gap heavily doped semiconductor such as SnO₂ or In₂O₃; a conducting microgrid. We prepared such coatings on glass by evaporating thin films of silver, copper, gold, aluminium, cobalt, iron, chromium and nickel of various thicknesses and by spraying SnO₂ films. The spectral variations in the transmittance, and the front side and back side reflectances were measured in the wavelength range 0.4–15 μm. The properties of a three-layer coating of the dielectric/metal/dielectric type were calculated with a multilayer program using known bulk optical constants. The effect of these films when coated onto a domestic window was demonstrated with a heat transfer calculation using an equivalent thermal net. When a large transmittance over a broad range of the solar spectrum is required, gold is an equally good, or a slightly better, choice than silver as the metal in a three-layer coating. In general, an SnO₂ film exhibits a higher solar transmittance as well as a higher emittance than a coating containing metals. This implies that the oxide is to be preferred as a coating on a window when the maximum passive solar heating is sought. However, a metal-based coating could be better when a very low U_L value is the most important requirement.     

Mechanical properties and adhesion strength of TiN and Al coatings on HSS, steel, aluminium and copper characterized by four testing Methods

TiN and Al coatings on substrates of high-speed steel, steel, aluminium and copper have been used to study mechanical properties of coating systems, especially the adhesion of the coating. The quantities measured are internal stress of the coating, determined by X-ray diffraction, the critical load of the scratch test, the microhardness obtained by the indenter technique, and the interface fracture energy, determined by a three-point bend test developed recently by the authors. The fracture energy, G_c, is a measure for the adhesion strength of a coating system. The effect of bias voltage, sputter cleaning and contamination of the substrates on the adhesion strength and other mechanical properties are investigated with the four methods mentioned. Each of the testing methods reveal only specific aspects of the behaviour of the coating systems. The data obtained depend on bulk properties of the film and the substrate material and on properties of the interface. Variation of the bias voltage can change them in quite different ways. In addition, the inter-relations between the adhesion strength of the coating and the failure behaviour of the three-point bend test samples are discussed.     

Haftfeste Direktmetallisierung von Kunststoffen durch Beschichtung mit Ionen

Direct metallization of plastics by high powerimpulse magnetron sputtering Even if polymers are today industrially used for decades the direct metallization of plastics is still a hot topic for research and development. Especially in light of the ban of hexavalent chromium an increasing demand for well‐adhering plastic metallization arises. High power impulse magnetron sputtering HIPIMS is a recent technology that can offer a high potential for successfully solving this challenge. This article focuses on the direct metallization of plastics by HIPIMS. In the frame of the investigations any pretreatment was ignored and the different polymers were directly under vacuum metallized. Compared to conventional mid‐frequency sputtering a significant improvement in adhesion was shown for different polymers. In detail the metallization of Plexiglas (PMMA) was investigated, since this polymer is highly challenging with respect to plasma processes. Due to the UV radiation damage of PMMA during plasma deposition direct metallization is usually not possible. Using ionized deposition it was possible to directly metallize the substrates with excellent adhering films without any interface layers or special pretreatments. The characterization of the substrate‐coating interface showed that for the well adhering films a cohesive fracture, i.e. a fracture within the polymer occurred.     

Influence of unsaturated carbonic acids on hemocompatibility and cytotoxicity of poly-vinylacetate based co-polymers

The aim of this study was to investigate hemocompatibility and cytotoxicity properties of synthetic polymer coatings containing various unsaturated carbonic acids with vinylacetate. Co-polymers of vinylacetate and crotonic acid (CA), maleic acid (MA), and itaconic acid (IA) were made. The materials were characterized in terms of their adhesion to metal supports (titanium and stainless steel) as well as hemocompatibility (% hemolysis, wettability, erythrocyte aggregation, hemoglobin content, thrombocyte count and lipid peroxidation levels) and cytotoxicity (human endothelial cell activity in vitro and chromosome aberrations, bone marrow proliferation and cell ploidy in rats). Co-polymers of unsaturated carbonic acids with vinylacetate exhibited good hemocompatibility properties, as opposed to vinylacetate homopolymer for which substantial levels of hemolysis were observed. By coating the metal supports with co-polymers the cytotoxic effects associated with the bare metal samples were markedly reduced. MA samples showed excellent hemocompatibility and no cytotoxicity, yet they lacked good adhesion properties to metal substrate, probably due to high water content. CA samples, having the highest density of carboxylic groups among the samples under investigation, showed increased bone marrow proliferation activity and cell ploidy in rats, as compared to controls. The most promising results in the present study were obtained for the samples with IA, which showed good adhesion to metal substrates, good hemocompatibility and low cytotoxicity. Thus, co-polymers of vinylacetate and IA rich in carboxylic groups are promising materials for the design of novel drug-eluting stents.     

Microwave irradiation-assisted method for the deposition of adherent oxide films on semiconducting and dielectric substrates

We report a method for the deposition of thin films and thick coatings of metal oxides through the liquid medium, involving the microwave irradiation of a solution of a metal-organic complex in a suitable dielectric solvent. The process is a combination of sol-gel and dip-coating methods, wherein coatings can be obtained on nonconducting and semiconducting substrates, within a few minutes. Thin films of nanostructured ZnO (würtzite) have been obtained on Si(100), glass and polymer substrates, the nanostructure determined by process parameters. The coatings are strongly adherent and uniform over 15 mm × 15 mm, the growth rate ∼ 0.25 μm/min. Coatings of nanocrystalline Fe₂O₃ and Ga₂O₃ have also been obtained. The method is scalable to larger substrates, and is promising as a low temperature technique for coating dielectric substrates, including flexible polymers.     

Reflection of light in the vicinity of the Brewster angle from a multilayer system of ultrathin inhomogeneous dielectric films

An analysis of the influence of nanoscopically stratified dielectric overlayer on the reflection parameters of linearly polarized light from transparent substrates is carried out. The second-order formulas for characteristic Brewster angles are derived and their accuracy is estimated by using exact numerical methods for the solution of the inhomogeneous reflection problem. The possibilities are discussed for determining the parameters of nanometre-scale dielectric layers by means of characteristic reflection angles. A novel scanning angle differential reflectance method in the vicinity of the classical Brewster angle, whose sensitivity is in principle the same as that of ellipsometry, is developed.     

Influence of the metal film thickness on the sensitivity of surface plasmon resonance biosensors

The influence of the metal film thickness (i.e., the chromium adhesion promoting film and the gold film) on the sensitivity of surface plasmon resonance (SPR) signals (i.e., resonance angle shift and reflectance change) towards the thickness variation of the nonabsorbing dielectric film is investigated. The sensitivity of reflectance change decreases when a thick chromium film or a thin gold film is employed. Its linear range becomes narrower as the thickness of the metal films increases. The sensitivity and linear range of the resonance angle shift are not affected by the thickness variation of the metal films. The phenomena were theoretically explained based on the attenuated total reflection (ATR) generated evanescent field at the prism/metal interface and the SPR-generated evanescent field at the metal/dielectric interface.     

1D-photonic crystals prepared from the amorphous chalcogenide films

We describe the preparation and optical properties of the 15-layer chalcogenide dielectric mirrors with the first order stop bands in near infrared range. The high refractive index Sb–Se and low refractive index Ge–S layers were deposited on silicon and glass substrates using thermal evaporation method. To centre the stop bands of the prepared chalcogenide mirrors at 1.55 μm, the layer thicknesses, d(Sb–Se) = 117 and d(Ge–S) = 183 nm, were calculated from the quarter wave stack condition. The optical reflectivity measurements revealed the total reflection from the 15-layer chalcogenide mirrors in the range of 1,400–1,600 nm for the unpolarized light with normal incidence. The effect of annealing on the optical properties of the prepared chalcogenide mirrors was studied as well. Using spectral ellipsometry, we examined the angular dependence of the multilayers reflectivity for the light with s- and p-polarization. The preparation of the dielectric mirrors for near infrared region from chalcogenide films seems to be possible exploiting good optical quality of chalcogenide films and their simple deposition.     

Reflection intensity optical fiber sensors for the mid-infrared

Two kinds of reflection intensity sensor made of chalcogenide glass fiber for the mid-IR region are demonstrated. One is a double-fiber reflection sensor based on two tied fibers with a gold-coated hollow metal waveguide connected to the far end of the fibers. The other is a single-fiber reflection sensor based on contact couplers. These reflectance sensors were coupled to a Fourier-transform IR spectrometer by a unique accessory based on nonimaging concentrators. This setup was built to measure absorption spectra of a polymer coating of an aluminum can and a sheet of drafting paper. A theoretical model treating the ratio between the signal from the target and the background is introduced. This model was helpful in deriving the sensitivity characteristics of the sensors from experimental absorption peak heights. Hence, the absorption peaks heights that we obtained using a single-fiber reflection sensor with a symmetric coupler were nearly 50% relative to those obtained with a double-fiber reflection sensor.