Properties Of Dielectric Coatings Research Articles

Low dielectric loss and infrared emissivity coating with enhanced mechanical property by grafting modification of EPDM

Composite coating materials or structures with low infrared emissivity and high radar transmittance are in urgent need in practical applications, among which binder is one of the important factors affecting the mechanical properties, infrared and dielectric properties of coatings. Acrylic acid (AA) and maleic anhydride (MAH) were used as graft monomers to modify ethylene propylene diene monomer (EPDM) binder. Composite coating with low infrared emissivity and low dielectric properties was prepared by using modified EPDM as binder, floating aluminum (Al) powder and antistatic carbonaceous fiber (CF) as filler. The effects of the type and amount of modified monomer on the mechanical properties, infrared emissivity and dielectric properties of the binder and the composite coating were systematically studied. The results show that the tensile strength (σb) and elongation at break (e) of the composite coating can be increased by 32% and 18%, respectively, by appropriate grafting modification of the binder. The infrared emissivity and the dielectric properties of the composite coating has a slight change with the increase of graft monomer content. Grafting modification of the binder improves the compatibility, wettability and interfacial bonding between filler and binder, which remarkably enhanced the mechanical and comprehensive properties of the composite coating. The effect mechanism of grafting modification of AA and MAH is disclosed from the aspects of polarity, branching and crosslinking. This work increases the selection range of binders needed to prepare low infrared emissivity materials.

Dielectric Properties of Plasma-Sprayed Fully Natural Garnets

Various kinds of natural garnets belonging to the almandine type (3FeO·Al2O3·3SiO2) were sprayed by plasma spray technique to build coatings on metallic substrates. The experimental garnet powders came from different mines in the Czech Republic and Mongolia. After coating and cooling the substrates were removed. In this way, self-supporting plates were obtained and further studied with microscopy, X-ray diffraction, and dielectric spectroscopy. Mechanical properties were in our focus as well. Microhardness was measured on cross sections dedicated to microstructure observation. Wear resistance in wet conditions was tested in a slurry. Reflectance was measured applying visible and infrared (VIS-NIR) radiation. Dielectric properties of coatings were studied at low voltage capacitance, loss tangent and also under direct current (DC) resistance. The results show that garnet minerals are interesting candidates for various optical and electronic applications; they have similar dielectric behavior as, for example, aluminum oxide or similar high-purity synthetic oxides, and, simultaneously, they have extraordinarily low reflectance in VIS-NIR radiation. The differences between natural powders and resulting coatings are discussed in connection with their chemical and phase compositions.

Evaluation of Technological Capability to from Dielectric Coatings on AK6 Alloy, Using a Method of Microarc Oxidation

The dielectric properties of coatings on AK6 alloy formed with a microarc oxidation method in two electric modes in alkaline-silicate electrolytes are estimated. It is shown that both modes, a galvanostatic mode and an arbitrarily falling power mode in alternating current circuits, make it possible to obtain coatings with a thickness of 30–60 μm, with sufficiently high electrophysical parameters: bulk specific resistance ρv = 3–9 × 109 Ω m and dielectric strength E = 9–14 V/μm. It is established that higher values of ρv and E in the both modes can be achieved in the solutions of 1 g/L KOH + 6 g/L of liquid glass (LG) and 12 g/L of LG. In terms of absolute value, the parameters of the coatings formed in the mode of arbitrarily falling power exceed the same characteristics of the oxide layers formed under conditions of galvanostatic mode by a factor of 1.5–2.5.

Influence of the Modes of Plasma-Electrolytic Oxidation on the Dielectric Properties of Coatings on D16T Alloy

We study the influence of plasma-electrolytic oxidation (PEO) of deformed D16Т alloy on the dielectric properties of PEO coatings in alkaline-silicate electrolytes. It is shown that the increase in the duration of oxidation from 1 h to 2 h under the conditions of arbitrarily falling power (AFP) and alternating current leads to worsening of the parameter ρv by a factor of 2–2.5 and a decrease in the electric strength E by 30–40%. In the galvanostatic mode, under the conditions of identical thicknesses of the coatings, the electrical parameters insignificantly decrease as the anode current increases within the range 5–15 A/dm2 . At the same time, their dependence on the composition of electrolyte is more pronounced. It is known that the best characteristics of volume resistance ρv = 3.3–4.3·109 Ω·m and E = 10–13 V/μm in both modes are attained for the alkaline-silicate (1:6) and silicate (0:12) solutions of electrolytes. The absolute values of characteristics of the coatings formed under the conditions of AFP are 1.2–1.9 times higher than the similar characteristics of oxide layers formed in the galvanostatic mode.

Control of the dielectric properties of a grain oriented electrical steel coating

The Franklin tester is the only standardized device that is used for the determination of the dielectric properties of electric-insulation coatings of electrical steels. It is used to measure the current value on ten electrodes with equal surface areas that contact the surface of a material; the current is then recalculated into the electric-resistance coefficient of the polarizing coating, which acts as the main characteristic of dielectric properties. It is shown that the values that are measured by a Franklin tester do not characterize the particular dielectric properties of the coating, but only indicate the presence of areas (defects) on which the coating is absent.

Dielectric and microwave absorbing properties of low power plasma sprayed Al2O3/Nb composite coatings

► Al 2 O 3 /Nb coatings have been fabricated by low power atmospheric plasma spraying. ► The coatings show a uniform dispersion of metal particles in alumina matrix. ► Both ɛ ′ and ɛ ″ increase with increasing Nb contents in the X-band (8.2–12.4 GHz). ► Variations of ɛ ′ and ɛ ″ are due to space charge polarization and conductance loss. ► Reflection loss values exceeding −10 dB are achieved between 10.0 to 11.8 GHz. Al 2 O 3 /Nb composite coatings were sprayed on graphite substrates by low power atmospheric plasma spraying (LPAPS) with an internally fed powder torch. The composite particles were agglomerated with different mass fractions by spray-drying technology. The microstructure and dielectric properties of coatings were investigated. The microstructure of composite coatings shows a uniform dispersion of metal particles in the composite coatings. Both the real ( ɛ ′) and imaginary ( ɛ ″) parts of the complex permittivity increase with increasing Nb content over the frequency range of 8.2–12.4 GHz, which is ascribed to space charge polarization and conductance loss, respectively. By calculating the microwave-absorption as a single-layer absorber, reflection loss values exceeding −10 dB can be obtained in the frequency range of 10.0–11.8 GHz with 10 wt% Nb content coating when the coating thickness is 1.5 mm.

Theoretical analysis of optical properties of dielectric coatings dependence on substrate subsurface defects

Abstract A model for refractive index of stratified dielectric substrate was put forward according to theories of inhomogeneous coatings. The substrate was divided into surface layer, subsurface layer and bulk layer along the normal direction of its surface. Both the surface layer (separated into N1 sublayers of uniform thickness) and subsurface layer (separated into N2 sublayers of uniform thickness), whose refractive indices have different statistical distributions, are equivalent to inhomogeneous coatings, respectively. And theoretical deduction was carried out by employing characteristic matrix method of optical coatings. An example of mathematical calculation for optical properties of dielectric coatings had been presented. The computing results indicate that substrate subsurface defects can bring about additional bulk scattering and change propagation characteristic in thin film and substrate. Therefore, reflectance, reflective phase shift and phase difference of an assembly of coatings and substrate deviate from ideal conditions. The model will provide some beneficial theory directions for improving optical properties of dielectric coatings via substrate surface modification.

Improved mechanical properties of optical coatings via an enhanced sputtering process

Magnetron sputtering is the process of choice for the deposition of a wide range of industrially important coatings. However, current processes have inherent limitations in terms of controlling, independently, the flux and energy of particles incident at the substrate at each stage of the deposition process. Consequently, film properties may not be optimal. In order to overcome this limitation, studies are being made of an enhanced sputtering system, which incorporates a grid-less linear ion source suitable for use in a reactive gas environment. The impact of this source on the structure and properties of dielectric coatings, in comparison with films grown in a standard system, is reported here. Coatings have been characterised in terms of their structures, optical properties and mechanical properties using a range of techniques including atomic force microscopy, X-ray diffraction, spectrophotometry, critical load scratch testing and wear testing. Results to date demonstrate that the enhanced system improves the flexibility of control of the growth parameters in a magnetron sputtering system.