Review of physical vapor deposited (PVD) spectrally selective coatings for mid- and high-temperature solar thermal applications

Plasma enhanced chemical vapor deposition(PECVD) is being increasingly used for the fabrication of transparent dielectric optical films and coatings. This involves single-layer, multilayer, graded index, and nanocomposite optical thin filmsystems for applications such as optical filters, antireflective coatings, optical waveguides, and others. Beside their basic optical properties (refractive index, extinction coefficient, optical loss), these systems very frequently offer other desirable “functional” characteristics. These include hardness, scratch, abrasion, and erosion resistance, improved adhesion to various technologically important substrate materials such as polymers, hydrophobicity or hydrophilicity, long-term chemical, thermal, and environmental stability, gas and vapor impermeability, and others. In the present article, we critically review the advances in the development of plasma processes and plasmasystems for the synthesis of thin film high and low index optical materials, and in the control of plasma–surface interactions leading to desired film microstructures. We particularly underline those specificities of PECVD, which distinguish it from other conventional techniques for producing optical films (mainly physical vapor deposition), such as fabrication of graded index (inhomogeneous) layers, control of interfaces, high deposition rate at low temperature, enhanced mechanical and other functional characteristics, and industrial scaleup. Advances in this field are illustrated by selected examples of PECVD of antireflective coatings, rugate filters, integrated optical devices, and others.

Plasma deposition of optical films and coatings: A review

Spectrally selective solar absorbers harvest solar energy in the form of heat. Solar absorbers using cermet-based coatings demonstrate a high absorptance of the solar spectrum and a low emittance in the infrared (IR) regime. Extensive work has been done to optimize cermet-based solar absorbers to achieve high performance by exploring different cermet (ceramic–metal composite) materials and film configurations through different preparation techniques such as electrodeposition, sputtering, pulsed laser deposition, and solution-based methods. In this article, we review the progress of cermet-based spectrally selective absorbers with high solar absorptance and low thermal emittance, such as Cr2O3, Al2O3, AlN, SiO2, and ZrO2 based cermets as absorption layers. We also present an outlook for cermet-based spectrally selective absorbers with high thermal stability and high conversion efficiency from sunlight to heat.

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A review of cermet-based spectrally selective solar absorbers

Titanium nitride (TiN) is one of the most well-established engineering materials nowadays. TiN can overcome most of the drawbacks of palsmonic metals due to its high electron conductivity and mobility, high melting point and due to the compatibility of its growth with Complementary Metal Oxide Semiconductor (CMOS) technology. In this work, we review the dielectric function spectra of TiN and we evaluate the plasmonic performance of TiN by calculating (i) the Surface Plasmon Polariton (SPP) dispersion relations and (ii) the Localized Surface Plasmon Resonance (LSPR) band of TiN nanoparticles, and we demonstrate a significant plasmonic performance of TiN.

/pdf/optical-properties-and-plasmonic-performance-of-titanium-50m8avvjt1.pdf

Optical Properties and Plasmonic Performance of Titanium Nitride

Immobilizing a protein, that is fully compatible with the patient, on the surface of a biomedical device should make it possible to avoid adverse responses such as inflammation, rejection, or excessive fibrosis. A surface that strongly binds and does not denature the compatible protein is required. Hydrophilic surfaces do not induce denaturation of immobilized protein but exhibit a low binding affinity for protein. Here, we describe an energetic ion-assisted plasma process that can make any surface hydrophilic and at the same time enable it to covalently immobilize functional biological molecules. We show that the modification creates free radicals that migrate to the surface from a reservoir beneath. When they reach the surface, the radicals form covalent bonds with biomolecules. The kinetics and number densities of protein molecules in solution and free radicals in the reservoir control the time required to form a full protein monolayer that is covalently bound. The shelf life of the covalent binding capability is governed by the initial density of free radicals and the depth of the reservoir. We show that the high reactivity of the radicals renders the binding universal across all biological macromolecules. Because the free radical reservoir can be created on any solid material, this approach can be used in medical applications ranging from cardiovascular stents to heart-lung machines.

https://www.pnas.org/content/pnas/108/35/14405.full.pdf

Free radical functionalization of surfaces to prevent adverse responses to biomedical devices

High efficiency MoAl2O3 cermet selective surfaces for high-temperature application

Elastic strain energy under some conditions provides the major contribution to the total energy of a film growing on a substrate from condensing vapour. Polycrystalline films grown with intrinsic stress induced by energetic bombardment are expected to show orientations which minimize total energy. Even for a cubic crystal in a non-hydrostatic stress field the energy is a function of the relative orientation of the stress field and the crystallographic axes. The Gibbs free energy is minimized under constant stress and temperature conditions at thermal equilibrium. In this paper we derive expressions for the Gibbs free energy of a cubic crystal in uniaxial and biaxial stress fields and find the conditions under which it is a minimum. The sign of the expression is the quantity which determines the behaviour of a cubic crystal and if negative, predicts that the [111] direction of the crystal will align with the principal stress of a uniaxial stress field and will lie normal to the plane of principal stresses in a biaxial stress field. Experimental evidence is presented which shows that titanium nitride, TiN, which has a negative value of , obeys these predictions. If is positive, then the [100] direction of the crystal obeys the above rules rather than the [111] direction.

https://iopscience.iop.org/article/10.1088/0953-8984/8/32/008/pdf

The orientation dependence of elastic strain energy in cubic crystals and its application to the preferred orientation in titanium nitride thin films

Direct current reactive sputtering Cr-Cr2O3 cermet solar selective surfaces for solar hot water applications

Thermal oxidation properties of titanium nitride and titanium-aluminum nitride materials — A perspective for high temperature air-stable solar selective absorber applications

Stoichiometric titanium nitride (TiN) films were deposited at less than 130 °C in a new configuration of the helicon activated reactive evaporation system. An in situ ellipsometer was used to monitor the optical properties of the films during growth. The film stress, lattice parameter, surface roughness, crystallite size, and preferred orientation were investigated as a function of substrate bias and hence ion energy of the incident species. The intrinsic stress dependence on ion energy shows the expected result also shown by a variety of materials and is in good agreement with the theoretical model of Davis [Thin Solid Films 226, 30 (1993)]. The high stress level at −50 V bias is confirmed by measurement of lattice parameter. X‐ray‐diffraction measurements show that the (111)‐preferred orientation normal to the substrate surface correlates strongly with the stress level of the films. By correlating our results with those from a number of other reported studies at higher substrate temperature we conclude ...

Y. Yin

Papers

High efficiency MoAl2O3 cermet selective surfaces for high-temperature application

The orientation dependence of elastic strain energy in cubic crystals and its application to the preferred orientation in titanium nitride thin films

Direct current reactive sputtering Cr-Cr2O3 cermet solar selective surfaces for solar hot water applications

Thermal oxidation properties of titanium nitride and titanium-aluminum nitride materials — A perspective for high temperature air-stable solar selective absorber applications

Properties of TiN films deposited at low temperature in a new plasma‐based deposition system