WO2005010225A1

WO2005010225A1 - Solar selective surface coatings, materials for use therein and a method of producing same

Info

Publication number: WO2005010225A1
Application number: PCT/NZ2004/000159
Authority: WO
Inventors: Antoine Bittar
Original assignee: Antoine Bittar
Priority date: 2003-07-25
Filing date: 2004-07-23
Publication date: 2005-02-03
Also published as: AU2003903853A0

Abstract

The present invention relates to solar selective surface coatings, materials for use therein and a method for the production of the solar selective surface coatings. The coatings are stable in air at high temperatures and are made from materials comprising a composite of a ceramic material and a ceramic compound.

Description

SOLAR SELECTIVE SURFACE COATINGS. MATERIALS FOR USE THEREIN AND A METHOD OF PRODUCING SAME.

Field of the Invention:

The present invention relates to solar selective surface coatings, materials for use therein and a method for the production of the solar selective surface coatings. The coatings may be used in air at high temperatures.

Background to the Invention:

There has been a long standing interest in both scientific and solar energy communities in the production of air stable solar selective surface coatings on a variety of substrates, able to operate at temperatures of 300°C and above without sizeable degradation of their optical performance. For solar selectivity, the desirable optical parameters include high solar absorptance and low thermal emittance. Several such coatings have been developed, so far on a small number of substrates, which have proved stable at these temperatures but only under vacuum or inert atmosphere conditions. Air stable coatings on industrially viable substrates are still required. These have very wide applications as stable selectively absorbing surfaces in solar energy systems designed to concentrate the sun's energy and thus achieve operating temperatures above 250°C.

Difficulties encountered so far in producing such high performance coatings have included degradation, in air and at temperature, of the materials making up the coating; degradation of their structure (often graded or layered in composition) leading to degradation of performance; or interaction of the coating with the substrate material at these temperatures and in air, again leading to degradation of their useful properties; or any combination of the above.

The general structure of these coatings includes, on top of the solar receiver substrate, a barrier layer, a solar coating and an antireflection capping layer. Various materials have been tried for the various layers, requiring separate and multiple processes for the production of a full coating on an appropriate substrate. No successful coating is presently available.

Numerous selective surfaces for solar applications have been proposed, produced and used in commercial applications. Methods of production of these surfaces have ranged from electrodeposition, vacuum deposition, chemical vapour deposition, anodic oxidation and more recently sol-gel methods.

Materials used in these surfaces have also included semiconductors, metal/insulator composites (often called CERMET) as well as dielectric multilayer stacks and metal- dielectric multilayer stacks each used separately to achieve the solar absorptivity and thermal emissivity required for operation.

The most common, and most successful selective surfaces have been those that use composite metal-ceramic materials. Examples of these include chrome black and nickel black where the metal is chromium or nickel and the dielectric (or Ceramic component) has been chromium oxide or nickel oxide. Other CERMET based selective surfaces have been studied and produced. The metal component of these CERMET surfaces has included Au, Pt, Ag, Al, Fe as well as numerous other metals. The ceramic component of these CERMET surfaces has also included a large number of ceramic materials eg SiO₂, SiO_x, TiO₂, Al₂O_3; A1N, MgF₂ or other ceramic or dielectric material. The composite material has therefore been, in all cases, a mixture of a pure metal and a ceramic or dielectric material in appropriate proportions (often indicated by the metal fill fraction). The requirements for both a metal and a dielectric component relates to the optical properties that need to be achieved- high absorption across the solar spectrum and high reflectance across the thermal spectrum - that lead to selectivity. This has, to some extent mitigated the physical and high temperature properties achievable with the CERMET surface, as the physical properties of metals and dielectrics are quite different, their resistance to oxidation, their diffusion properties as well as their high temperature stabilities are quite different. Often, outstanding solar selectivity can be achieved initially, but deterioration of the optical properties of the CERMET material occurs during operation or at high temperatures; this deterioration often occurs through changes in the metal component of the cermet, at least at temperatures below 400°C and may be caused by either oxidation or diffusion of the metal component or both, in the CERMET. A solution to this difficulty, already proposed, has been to select the metal component such that diffusion and oxidation do not occur, at least at the operating temperatures required. Very few such surfaces have been developed, the only one widely known being based on Platinum metal and Aluminium Oxide Ceramic components. This has however not been successfully commercialised.

Object of the Invention:

It is an object of this invention to provide means of overcoming, or at least ameliorating some or all of the above mentioned disadvantages without substantially degrading the optical performance of the coatings.

It is an object of this invention to propose materials to produce the appropriate solar selective coatings and the antireflection top coating required.

It is a further object of this invention to detail a barrier layer that effectively isolates the coating from the substrate and eliminates or delays the deleterious interactions between them.

It is also an object of this invention to provide a method for producing the full coating using only two elemental components and a single process.

It is also an object of this invention to indicate the process whereby such coatings may be produced, may or may not incorporate one or more barrier layers, and may be produced on industrially viable substrates such as steel, stainless steel, tool steel, industrial high temperature alloys and others.

Description of the Invention:

The invention provides a material for use in a surface coating on the base substrate of a solar receiver in a solar thermal energy system, said material comprising a composite of a ceramic material and a ceramic compound.

In a preferred embodiment the ceramic compound is selected from the group comprising: SiO₂; SiO_xN_y; SiOx (x < 2) SiN TiO₂ TiO_x (x ≤ 2) Al₂O₃ A1N AlO_xN_y MgF₂

In a preferred embodiment the ceramic material is selected from the group comprising: titanium nitride; titanium oxynitride; titanium aluminium nitride; titanium aluminium oxynitride; titanium silicon nitride; titanium silicon oxynitride; titanium boron nitride; titanium boron oxynitride; zirconium nitride; . zirconium oxynitride; zirconium aluminium nitride; zirconium aluminium oxynitride; zirconium silicon nitride; zirconium silicon oxynitride; zirconium boron nitride; zirconium boron oxynitride.

The invention also provides a surface coating for use on the base substrate of a solar receiver in a solar thermal energy system, said surface coating incorporating a material comprising a composite of a ceramic material and a ceramic compound.

Description of the Drawings:

In further describing the invention, reference will be made to the accompanying drawings in which:

Figure 1 depicts the spectral reflectances of several samples of TiN thin films of various thicknesses;

Figure 2 depicts the spectral reflectance of a full coating using TiAIN and A1N materials; and Figure 3 depicts the spectral reflectance of two TiN/Ti/TiN barrier stacks on a thick titanium layer and on a thick Aluminium layer respectively. Note the horizontal scale is in nm to clarify the spectral reflectance in the wavelength range 0.2 to 3.0 urn. The reflectance is shown as a percentage, so that 100 indicates 100% or 1.0 reflectance value.

Detailed description of the Invention.

Throughout this specification and claims:

CERMET is defined as a mixture of a ceramic material and a metal, forming a composite material.

Reflectance is defined as ratio of the amount of electromagnetic radiation reflected from a surface to the amount originally striking the surface. Spectral reflectance is this ratio at a specific wavelength of the incident radiation.

Thermal emittance is defined as the ratio of radiation emitted by a surface at a given temperature to that emitted by a black body at the same temperature.

Barrier layer is defined as one or more layers of material that inhibits or retards the movement of material across it. In this application, a barrier layer inhibits or retards the movement of material from and/or to the substrate that it is applied to.

Solar selective surface is defined as a surface having high absorption of electro magnetic radiation at the Solar spectral wavelengths and low thermal emittance in the thermal infrared wavelength range defined by its temperature of operation.

Antireflection layer is defined as layer of material that reduces or eliminates the reflectance of a surface.

Metallic behavior implies the existence, in a general material, of charge carriers free to move beyond atomic distances, leading to electrical conduction and high reflectance in the infra-red wavelength range. The present invention concerns the use of a new set of materials which are inherently ceramic materials but show sufficient "metallic behaviour" to be acceptable for use in place of the metal component in a CERMET composite. These materials are mostly compounds, as opposed to the elemental metals mentioned above, and exhibit metallic electronic properties, but ceramic mechanical and thermal properties.

Examples of materials which exhibit mostly ceramic physical and chemical properties as well as metallic electronic properties are well known in a variety of material science fields, including the recently discovered high temperature superconductors. However they have not so far been used to replace the metal component of a so called CERMET composite in solar selective surfaces. This invention uses such materials to replace the metal component in a CERMET composite material in order to benefit from their ceramic like durability and other properties, while enabling the tailoring of the optical properties of the composite as required for solar selectivity.

These materials, examples of which include titanium nitride, titanium oxynitride, titanium aluminium nitride, titanium aluminium oxynitride, and other titanium nitride and oxynitride compounds, with silicon and boron (for example), have been produced recently as hard ceramic protective coatings for high speed cutting tools and other steel substrates. In this application, it is exclusively their ceramic properties that are exploited.

The present invention uses titanium nitride, titanium oxynitride, titanium aluminium nitride, titanium aluminium oxynitride, titanium nitride and oxynitride compounds with silicon and boron as well as the analogous zirconium nitride, zirconium oxynitride, zirconium aluminium nitride, zirconium aluminium oxynitride, zirconium nitride and oxynitride compounds with silicon and boron as replacements for the metal component in CERMET solar selective surfaces in order to capitalise on both their ceramic like mechanical and thermal stability and on their metal like optical properties. This enables the achievement of desirable optical properties (solar selectivity and thermal emissivity) in the selective composite at the same time as achieving stability and durability at high operating temperatures. These properties are then achieved using an "all ceramic" composite layer. For an illustration of this aspect of this invention, Figure 1 presents the spectral reflectance of three titanium nitride (TiN) sample materials produced under similar conditions to those described above. The hardness of the ceramic material has been confirmed by a simple diamond tip scratch test. The spectral reflectance confirms the typical yellow colouration, and more importantly for solar applications, the relatively high spectral reflectance (>75-85%) in the infrared region, expected from a metal like material. The limitation of the infrared spectral reflectance to values around 0.85 and below shows that the ceramic material's "metallic like behaviour" approximates a relatively mediocre metal, especially when compared to that of known metals such as copper, silver or gold.

Similar results obtain for ceramic titanium aluminium nitride alloys (Ti_xAl_yN), with values of x and y each spanning the range 0 to 1. However, in the case of these materials, the infrared spectral reflectance can reach values of 0.90 and above in the infrared wavelength region.

This dual set of properties displayed by these materials makes them eminently suited to replace the metal component in a CERMET composite for stable high temperature solar selective surfaces, (other uses , as infrared reflection layers for example, also obtain and are proposed below).

The ceramic component of such a composite surface may be selected from the dielectric materials noted above, but would preferably be aluminium nitride (A1N) as described below in order to restrict the number of elemental materials used in the production of a complete selective surface.

The materials may be selected, one from each of a these two sets of materials, to produce the composite material.

As a general example, using TiN or Ti_xAl_yN (or alternatively, TiN or Ti_xSi_yN or other) materials for replacement of the metal component, along with A1N (or SiN as appropriate) as the dielectric component in composite layers, selective solar coatings comprising a substrate, the composite layer and an antireflection layer can be produced which achieve theoretical solar absorptances of 0.94 and above, and thermal emittances of around 0J2 at operating temperatures of 400°C. The spectral reflectance curve for such a complete coating is shown in Figure 2 as an example of the high spectral selectivity achievable with a single layer of TiN/AIN based "all ceramic" composite material. The solar absorptance of this coating is 0.94, thermal emittance at nominal 400°C operation is 0J08, the thermal performance is 8.7 and thermal efficiency is 0.88. Further optimisation of the layer thicknesses, the material selected or other component or parameter, allow improved values of these optical parameters and performance to be achieved.

It is understood that the solar selective coating itself may be produced from combination of two or more of these composites as layers, as compounds or as a nanocomposite where one material is embedded as nanoparticles in a matrix of the other.

As a further aspect of this invention, these same materials are also used in barrier layers between substrate and selective coating, taking advantage of their ceramic like high temperature stability and adhesion to a variety of substrates, thus forming barriers to substrate diffusion into the selective coating and also a barrier to oxygen diffusion.

TiN and Ti_xAl_yN, (as well as other alloys of Ti and N with Si or B outlined above for examples) have been used extensively as hard coating materials on steel, high speed steels and on tools steels. Their protective properties have been investigated, proven and reported on extensively in the literature. In these applications, their ceramic properties, such as high temperature stability and hardness have been paramount. They have also shown resistance to oxygen diffusion and hence resistance to oxidation at 500°C and over 700°C respectively, possibly through development of a thin oxide layer that inhibits oxygen diffusion. Because of this and other properties, they have also been used as oxygen and metal diffusion barrier layers in both the electronic and ferroelectric industries.

As described below, these materials may be produced either as uniform films or as micro- structured films or as sequential multi sub-layered films making up a single barrier layer with the desirable properties. In this aspect of this invention there is provided a process for producing a barrier layer that isolates the selective coating from the substrate while still maintaining desirable infrared properties.

The barrier layer may comprise one or more of the. following sublayers: Aluminium (Al), Titanium Nitride (TiN), Titanium, titanium nitride in the sequence -Al/TiN/Ti/TiN - with TiN and Ti sublayer thicknesses of 50 nm or less.

Or alternatively, the barrier layer may comprise one or more of the following sublayers Ti_xAl_yN/TiN/Ti_xAl_yN, with sublayer thicknesses 50 nm or less and values of x and y between 0 and 1 inclusive, with or without an underlying Al sublayer .

Or alternatively Aluminium may be selectively replaced by other materials such as silicon, producing Silicon Nitride (SiN) and Ti_xSi_yN composites and compounds in similar sublayer structures eg Ti_xSi_yN/TiN/Ti_xSi_yN with or without an underlying Al sublayer.

The barrier sublayers may be produced by among other means, physical vapour deposition of Al and Ti in a vacuum or in a Nitrogen atmosphere, by sputtering of Al and Ti in an appropriate atmosphere, by cathodic arc evaporation or by other means. The morphology of the sublayers may be controlled to obtain epitaxy or a nanocomposite structure of the nitrides of Ti and Al or their compounds.

This invention also provides a method of producing barrier layers from single or multiple layers of Ti, Al, TiN, Ti_xAl_yN, Si, Ti_xSi_yN B, TiON, Ti_xAl_yON, Si, Ti_xSi_yON OR Zr analogues etc or any combination of these in order to achieve both barrier layer properties and appropriate high spectral reflectivity infrared properties needed for their application in solar selective coatings as diffusion barriers between the substrate and any selective film deposited onto it.

In Figure 3 is shown the spectral reflectance of two such barrier layers produced according to the above method; the first is made of a thick Ti layer followed by a TiN/Ti/TiN multilayer stack, the second is made of an Aluminium layer covered with a similar TiN/Ti/TiN multilayer stack. The layer thicknesses in the stacks are preferably in the range from 2nm to 5000 nm each, all materials being produced according to the method proposed. This stack acts favourably for temperatures below 500°C. In the latter case of an aluminium layer covered by the stack, an infrared reflectance of over 90% is achieved, a value necessary to the good optical performance of a selective coating.

The multilayer stack preferably comprise layers such as Ti_xAl_yN/TiN/ Ti_xAl_yN, enabling the diffusion barrier to operate effectively at temperatures above 500°C. The layer thicknesses in the stacks are preferably in the range from 2 nm to 5000 nm each, all materials being produced according to the method proposed.

The choice for the antireflection capping layer is preferably A1N of thickness lOOOnm or less.

In a totally similar manner to all the above, Al may be replaced by other appropriate material such as Silicon (Si) or other, to produce SiN, Ti_xSi_yN, etc for the various sublayers or nanocomposites as well as SiN or SiO₂ antireflection layer.

En a totally similar manner to all the above, Ti may be replaced by other appropriate material such as Zirconium (Zr) or other, to produce ZrN, Zr_xAl_yN, etc for the various sublayers or nanocomposites as well as A1N or SiO₂ antireflection layer.

According to this invention, the barrier layer, coating layer and antireflection layer may be deposited on a variety of substrates including steel, stainless steel, tool steel and high temperature alloys by reactive sputtering, thermal evaporation, cathodic arc evaporation, Ion assisted deposition or other technique that enables the achievement of the appropriate microstructures and appropriate properties.

En a further aspect a method is provided to produce the whole coating, including barrier layer, selective coating and antireflective coating in one single deposition process such as sputtering or cathodic arc deposition, using only two materials (for example, Al and Ti or Si and Ti) and nitrogen as the reactive gas. Existing solar selective coatings often require several processes each aimed at producing each of the (at least) three appropriate layers, namely the barrier layer, the solar selective layer and the dielectric antireflection layer. The use of the above described ceramic materials, singly or in combination, leads to a simplified, single process for production of the complete solar selective coating. Also the simplified process has the added advantage of needing only two elemental materials, namely Titanium and Aluminium (or Titanium and Silicon for further example); making it eminently suitable for cost effective production.

According to the invention, the deposition of each of the components of the coating may be accomplished by, among other means, reactive sputtering of two metallic targets in a mixed Nitrogen Argon atmosphere.

Preferably the process will, in this case, include the following steps: 1 - The provision of a vacuum chamber 2- Provision of solid Al and Ti targets 3 - Provision of substrates 4- Provision of nitrogen and argon gases

The deposition of each of the layers is carried out substantially under the conditions of:

Vacuum system base pressure < 3 X 10^"5 Torr Deposition pressure lxlO^"2 - 1 X 10^"5 Ton- Nitrogen to argon gas flow ratios variable from 0:15 to 1:1 And argon gas flow variable from 0 to 20 seem

Deposition rates for Ti and Al variable from 0.01 to 5A/sec or more as required. And wherein the power to the targets may be controlled to produce the desired morphology or nanostructure for the Ti, TiN or Ti_xAl_yN sublayer (or composite of these materials) and the effective deposition rates determine the stoichiometry of the alloy or nanocomposite materials produced.

In this example, the completed coating then preferably comprises a selected barrier layer, a selected selective coating and an antireflection layer all based on A1N, TiN and their compounds or nanocomposite mixtures. The production of the full coating is, in the proposed case, carried out in a single process, resulting in a simplified method of production using only two metal targets for all layers.

By way of an example and in the case of Al and Ti being selected as elemental materials, the process for production of such a coating may be carried out using any of the above deposition techniques, following the steps described below:

1- Production of a barrier layer: preferably made of (a) A single Ti_xAl_yN layer with x and y in the range 0 to 1 and layer thickness preferably between lOnm and 5000nm or (b) An Al layer followed by a TiN/Ti/TiN multilayer stack with stack layer thicknesses between 2nm and 50nm or more or (c) An Al layer followed by a Ti_xAl_yN/TiN/ Ti_xAl_yN multilayer stack with stack layer thicknesses between 2nm and 50nm or more. or a combination of (a), (b), and (c) above.

2- Production of a solar selective layer preferably made of (a) an "all ceramic" composite layer preferably TiN/AIN composite with TiN volume fraction between 0J and 0.7 and total thickness between lOnm and lOOOnm. or (b) an "all ceramic" composite layer preferably Ti_xAl_yN/AlN with x and y and fill fraction between 0J and 0.7

3- Production of an antireflection layer preferably made of A1N and preferably between lOnm and lOOOnm total thickness.

All steps 1 to 3 to be carried out in one single process necessitating only two solid sources of solid material and appropriate gases. As a further example an entirely similar coating may be realised using Ti and Silicon or Zr and Al as the starting materials and appropriate gases

It is understood that the deposition of each of the components of the coating may be accomplished by other means also, by chemical vapour deposition, or other physical vapour deposition such as Ion Assisted Deposition (IAD), cathodic arc evaporation, thermal evaporation or electron beam evaporation under appropriate reactive or non reactive atmospheres using only two elemental targets or appropriate gas streams.

Although the invention has been described by way of examples and with reference to particular embodiments, it is to be understood that modifications and/or improvements may be made without departing from the scope or spirit of the invention.

Claims

Claims:

1. A material for use in a surface coating on the base substrate of a solar receiver in a solar thermal energy system, said material comprising a composite of a ceramic material and a ceramic compound.

2. A material as claimed in claim 1 wherein the ceramic compound is selected from the group comprising: SiO₂; SiO_xN_y; SiOx (x < 2) SiN TiO₂ TiO_x (x < 2) Al₂O₃ A1N AlO_xN_y MgF₂

3. A material according to claim 1 or 2 wherein the ceramic material is selected from the group comprising: titanium nitride; titanium oxynitride; titanium aluminium nitride; titanium aluminium oxynitride; titanium silicon nitride; titanium silicon oxynitride; titanium boron nitride; titanium boron oxynitride; zirconium nitride; zirconium oxynitride; zirconium aluminium nitride; zirconium aluminium oxynitride; zirconium silicon nitride; zirconium silicon oxynitride; zirconium boron nitride; zirconium boron oxynitride.

4. A surface coating for use on the base substrate of a solar receiver in a solar thermal energy system, said surface coating incorporating a material comprising a composite of a ceramic material and a ceramic compound.

5. A surface coating according to claim 4 wherein the ceramic compound is selected from the group comprising: SiO₂; SiO_xN_y; SiOx (x <2) SiN TiO₂ TiO_x (x < 2) Al₂O₃ A1N AlO_xN_y MgF₂

6. A surface coating according to claim 4 or 5 wherein the ceramic material is selected from the group comprising: titanium nitride; titanium oxynitride; titanium aluminium nitride; titanium aluminium oxynitride; titanium silicon nitride; titanium silicon oxynitride; titanium boron nitride; titanium boron oxynitride; zirconium nitride; zirconium oxynitride; zirconium aluminium nitride; zirconium aluminium oxynitride; zirconium silicon nitride; zirconium silicon oxynitride; zirconium boron nitride; zirconium boron oxynitride.

7. A surface coating according to any one of claims 4 to 6 wherein said coating is of a layered configuration.

8. A surface coating according to claim 7 wherein the coating further comprises one or more of a barrier layer on the surface of the base substrate, a solar selective layer and an antireflection capping layer.

9. A surface coating according to claim 8 wherein said barrier layer is further characterised by being of a single or multiple layered configuration.

10. A surface coating according to claim 8 or 9 wherein the barrier layer is selected from the group comprising: Ti; Al; Zr; Si; TiN; Ti_xAl_yN; Ti_xSi_yN; Ti_xB_yN; TiO_xN_y, Ti_xAl_yO_zN_w, Ti_xSi_yO_zN_w; Ti_xB_yO_zN_w; ZrN; Zr_xAl_yN; Zr_xSi_yN; Zr_xB_yN; ZrO_xN_y; Zr_xAl_yO_zN_w; Zr_xSi_yO_zN_w; Zr_xB_yO_zN_w or any combination of these.

11. A surface coating according to claim 7 wherein the solar selective layer is a composite; with the ceramic material selected from the group comprising: titanium nitride; titanium oxynitride; titanium aluminium nitride; titanium aluminium oxynitride; titanium silicon nitride; titanium silicon oxynitride; titanium boron nitride; titanium boron oxynitride; zirconium nitride; zirconium oxynitride; zirconium aluminium nitride; zirconium aluminium oxynitride; zirconium silicon nitride; zirconium silicon oxynitride; zirconium boron nitride; zirconium boron oxynitride; and the ceramic compound is selected from the group comprising: SiO₂; SiO_xN_y; SiOx (x < 2) SiN TiO₂ TiO_x (x < 2) Al₂O₃ A1N AlO_xN_y MgF₂

12. A surface coating according to claim 7 wherein the antireflection layer is selected from the group comprising: SiO₂; SiO_xN_y; SiOx (x < 2) SiN TiO₂ TiO_x (x < 2) Al₂O₃ A1N AlO_xN_y MgF₂

13. A method of forming a surface coating wherein material is deposited on a substrate by a vapour deposition process.

14. A method according to claims 13 wherein the vapour deposition process is selected from: reactive sputtering; thermal evaporation; cathodic arc evaporation; ion assisted deposition; chemical vapour deposition or electron beam evaporation.

15. A method of forming a surface coating on a substrate by vapour deposition of two elemental targets in a mixed nitrogen/argon atmosphere.

16. A method according to claim 15 wherein the elemental targets are respectively aluminium and titanium.

17. A method according to claim 15 wherein the elemental targets are respectively titanium and silicon.

18. A method according to claim 15 wherein the elemental targets are respectively Zirconium and Aluminium.

19. A method according to claim 15 wherein the elemental targets are respectively Zirconium and Silicon.

0. A solar receiver incorporating a surface coating according to any one of claims 4 19.