WO2005000743A2

WO2005000743A2 - Molybdenum doped alumina garnets

Info

Publication number: WO2005000743A2
Application number: PCT/GB2004/002745
Authority: WO
Inventors: John Marius Rodenburg; Heming Wang
Original assignee: Sheffield Hallam University
Priority date: 2003-06-27
Filing date: 2004-06-25
Publication date: 2005-01-06
Also published as: WO2005000743A3; GB2403214A; GB0315036D0

Abstract

A ceramic material is identified having particular utility as a thermal barrier or thermal insulating coating exhibiting anti-corrosion properties. The ceramic materials comprise alumina garnet structures involving addition of a lanthanide or transition metal being accompanied by a doping agent in the form of molybdenum. Specifically, Mo doped YAG and Mo doped YbAG are provided exhibiting significantly reduced calcinating temperatures required for garnet structure crystallisation. A synthetic route is also provided involving sol-gel techniques utilising of metal inorganic and organic precursors.

Description

MOLYBDENUM DOPED ALUMINA GARNETS

Field of the Invention The present invention relates to molybdenum doped alumina garnets, and in particular although not exclusively to molybdenum doped alumina garnets prepared by sol-gel techniques.

Background to the Invention Rare earth alumina garnet ceramics are hard, have a high melting point, low thermal conductivity and are phase stable from room temperature up to their melting point. Such properties have made these compounds attractive candidates for use in thermal barrier coatings being resistant to oxidation, chemical corrosion and physical erosion. Additional uses of the alumina garnets include use as anti-corrosion coatings, optical coatings and decorative coatings, N. P. Padture, P. G. Kiemens, J. Am. Ceram. Soc, 80 (1997), 1018.; T.Yasushi, European Patent 1013623, (2000).; Patel FD, Honea EC et al., IEEE J.Quantum Elect., 37 (2001 ), 135.; M.Shimokozono, N. Sugimoto, et al, Appl. Phys. Lett., 68 (1996), 2177. Alumina garnets have complicated crystallographic structures. Typically, for lanthanide alumina garnets and certain transition metal alumina garnets, such as yttrium alumina garnet (Y₃AI₅0₁₂) two Al atoms exist in the oxygen tetrahedral sites, three Al atoms occupy the oxygen of octahedral sites and the rare earth or transition metals occupy the oxygen dodecahedron sites, resulting in 160 atoms in one cell.

Conventionally, rare earth and transition metal alumina garnets were prepared by high-temperature processing of crushed powders derived from glasses and crystalline ceramics. Typically, ceramics prepared by the crushed powder route involved high sintering temperatures up to 1600°C. More recently rare earth and transition metal alumina garnets have been prepared by sol-gel techniques. In particular, Lo and Tseng (J.R. Lo, T.Y. Tseng, Phase development and activation energy of the Y₂O₃-AL₂O₃ system by a modified sol- gel process, Mater. Chem. Phys. 56 (1998) 56-62.) studied phase development based on yttrium oxide and aluminum oxide systems. Moreover, the reported lowest sintering temperature for producing fully crystallised single phase yttrium aluminum garnet (YAG) by sol-gel techniques is currently about 900°C (Y Lui, Z.F. Zang, B. King, J. Halloran, R. M. Laine, J. Am. Ceram. Soc. 79 (1996) 385). Whilst the sol-gel processing route offers a considerably lower sintering temperature than that associated with conventional ceramic processing techniques, a significant problem still exists.

The use of rare earth alumina garnets as thermal barrier coatings commonly involves application of the coating to a substrate, and in particular a thermally sensitive substrate for example nickel super alloys. Due to the requirement for processing the alumina garnet above 900°C many of these substrates suffer irreversible and detrimental changes in their properties. US 5,866,271 discloses a method of bonding a ceramic thermal barrier coating to a nickel or cobalt based super alloy substrate. Additionally, US 6,015,630 and US 2001007719 disclose methods for coating substrates with ceramic thermal barriers.

Suitable ceramics, for use as thermal barrier coatings, are also selected based on their ability to restrict oxygen diffusion through to the substrate. Presently, thermal barrier coatings comprise a ceramic-metal composite having a metal substrate, a metal bond coat disposed on the metal substrate and a protective ceramic coating disposed on the metal bond coat. During heat treatment, the metallic bond coat forms a thin, alumina (Al₂0₃) film between the substrate and the protective ceramic coating. Such protective ceramic coatings typically comprise polycrystalline zirconia (ZrO₂)-based ceramics, including yttria- stabilised zirconia alloy ceramics.

A number of problems are associated with typical zirconia based barrier coatings, one of the more fundamental is oxidation of the substrate. Oxidation generally results from an environmental oxygen source whereby oxygen is readily transported to the substrate via microscopic defects, such as micro cracks and pores in the zirconia-based ceramic coating. Moreover, such defects are intentionally manufactured into the coating so as to provide thermal insulation by interrupting heat flow and in order to eleviate thermal-expansion strain between the metal substrate (super alloy) and the ceramic coating. In particular, oxygen diffusion is assisted by creating oxygen Voids' within the crystalline structure resulting in high diffusion of oxygen to the substrate in turn providing significant corrosion disadvantages.

Therefore, the high concentration of oxygen deficiencies positively contributes to the low thermal conductivity of the barrier coating however such vacancies are detrimental to the substrate with regard to oxygen induced corrosion stemming from oxygen diffusion through the thermal barrier coating.

The inventors, having identified the above problems, have realised a need for a thermal barrier coating having a low sintering temperature so as to prevent irreversible damage to the underlying substrate whilst obviating the problems associated with oxygen diffusion through the thermal barrier coating resulting in observed substrate corrosion and weakening.

Summary of the Invention Accordingly, the inventors provide a low temperature synthesis technique for lanthanide and transition metal alumina garnets being facilitated by the addition of a molybdenum based doping agent included within a sol-gel processing technique. The technique offers a route for coating metal alloys, and in particular nickel based super alloys, at relatively low temperatures in order to produce high-temperature thermal insulation and oxidation resistant ceramic materials.

According to specific implementations of the present invention, there is provided molybdenum doped lanthanide and transition metal alumina garnets, including Mo doped YbAG, YAG and LaAG. In particular, YbAG sintering temperatures as low as 700°C are possible where YbAG is doped with Mo, such doping including trace amounts of Mo up to and including 35% mole fraction of Mo to Yb. According to the present invention alumina garnet may be doped with 5 Mo above 35% mole fraction if required.

Molybdenum is included within the alumina garnet structure in a six valence state, or 6+ oxidation state, such that replacement of the lanthanide (Yb) or transition metal (Y) occurs in a 1:2 mole ratio replacement. That is, for every0 molybdenum ion included within the garnet structure two lanthanide metal ions or two transition metal ions are replaced whereby cationic voids are created within the garnet structure. The creation of ionic vacancies within the garnet structure facilitates the low thermal conductivity properties observed for the resulting molybdenum doped alumina garnet. Moreover, thermal barrier coatings formed5 from the molybdenum doped alumina garnet, in particular Mo-YbAG and Mo- YAG, do not promote oxygen diffusion due to the omitance of oxygen vacancies present within prior art zirconia based thermal barrier coatings. According to specific implementations of the present invention therefore a thermal barrier coating is provided exhibiting low thermal conductivity and anti-corrosion coating o properties.

According to specific implementations of the present invention, the molybdenum doped alumina garnet is prepared by sol-gel techniques involving formation of an aqueous molybdenum-lanthanide-aluminium sol or molybdenum- 5 transition metal-aluminium sol, gelation and subsequent spinning, dipping or spreading application of the sol-gel onto a suitable substrate prior to heat treating involving sintering temperatures as low as 700°C in the case of Mo-YbAG.

The mechanism by which the addition of molybdenum lowers the o crystalisation temperature of the resultant alumina garnet may centre around the difference in valence states of the molybdenum and lanthanide/transition metal. The inventors postulate that the molybdenum acts as a gel stabilising agent so as to provide a control and ordering of the gelation process due to the increased coordination available by addition of molybdenum within the growing clusters. In particular, the molybdenum may help to suppress random growth of micella-like structures which may ultimately cause defects in the final ceramic.

Accordingly, a decrease in the observed sintering temperature, of the order of at least 100°C for YbAG, is possible for rare earth and transition metal alumina garnets doped with molybdenum. According to a specific implementation of the present invention, molybdenum doped alumina garnets, having a range of coating applications, are provided, including garnets of the rare earth elements and transition metals. In particular, due to the close chemical resemblance of yttrium to the rare earth elements the results presented herein for Mo-YbAG are considered analogous to those of Mo-YAG.

According to the first aspect of the present invention there is provided a molybdenum doped alumina garnet manufactured by a sol-gel process. Preferably, said garnet is a lanthanide alumina garnet.

Preferably, said lanthanide comprises ytterbium or lanthanum.

Preferably, said alumina garnet is a transition metal alumina garnet.

Preferably, said transition metal comprises yttrium.

Preferably, said alumina garnet is doped with molybdenum in the range trace amounts of molybdenum up to and including 35% mole ratio of molybdenum to lanthanide. Preferably, said alumina garnet is doped with molybdenum in the range trace amounts of molybdenum up to and including 35% mole ratio of molybdenum to transition metal. Preferably, said molybdenum comprises an oxidation state of 6+.

Preferably, said lanthanide or said transition metal comprises an oxidation state of 3+. Preferably, 1 mole of molybdenum replaces 2 moles of lanthanide or transition metal within the alumina garnet structure.

According to a second aspect of the present invention there is provided a molybdenum doped alumina garnet being represented by general formula (1 ):

Mo_xD₍₃-₂x₎Al₅θ₁₂ (1) where D is a lanthanide or transition metal and x is trace amounts up to and including 0.61.

Preferably, D is Yb, Y or La.

According to a third aspect of the present invention there is provided an alumina garnet based sensor comprising an alumina garnet according to the present invention configurable for sensing any one or a combination of of the following set of:

• hydrogen; • carbon oxides. According to a fourth aspect of the present invention there is provided a luminescent material comprising an alumina garnet according to the present invention.

5 According to a fifth aspect of the present invention there is provided a coating comprising an alumina garnet according to the present invention. Preferably, said coating is a thermal barrier coating, an anti-corrosion coating, a decorative coating, an optical coating for glass or a vacuum chamber internal surface coating.0 Preferably, said alumina garnet coating is configured for coating a metal alloy substrate and may comprise a low thermal conductivity.

Preferably, said anti-corrosion coating contains substantially no oxygen5 vacancies within the garnet structure.

According to a sixth aspect of the present invention there is provided a method of synthesising a molybdenum doped lanthanide or transitional metal alumina garnet by a sol-gel process comprising mixing an alumina sol with a o molybdenum oxide sol and a lanthanide oxide or transition metal oxide sol.

Preferably, the method further comprises forming a gel from a mixture of said alumina sol, said molybdenum oxide sol and said lanthanide oxide or transition metal oxide sol.5 Preferably, the method further comprises heat-treating the sol, gel or sol-gel to form said molybdenum doped lanthanide alumina garnet or said molybdenum doped transition metal alumina garnet.

o Preferably, said gel is heat-treated above 600°C. According to a seventh aspect of the present invention there is provided a method of synthesising a molybdenum doped lanthanide or transition metal alumina garnet by a sol-gel process comprising: (i) forming an alumina sol using an aluminum based salt or aluminum-organic precursor and water; (ii) adding a lanthanide or transition metal in the form of a salt or lanthanide-organic or transition metal-organic precursor; and (iii) doping the sol with molybdenum by adding molybdenum in the form of a molybdenum sol or molybdenum-organic precursor. Preferably, said organic precursor comprises and alkoxide.

Preferably, the method further comprises forming a gel from a mixture of said alumina sol containing said lanthanide or transition metal, and said molybdenum.

A method of making a molybdenum doped lanthanide or transition metal alumina garnet coating by a sol-gel process comprising: (i) forming a sol of alumina oxide, molybdenum oxide and lanthanide or transition metal oxide, said sol being configured to form a gel; (ii) applying said sol, said gel and/or said sol- gel to a substrate; and (iii) heat-treating said sol, said gel and/or a sol-gel on said substrate to form a monolithic molybdenum doped lanthanide or transition metal alumina garnet.

Preferably, said step of applying said sol to said substrate comprises spinning, dipping or spraying said sol onto said substrate.

Brief Description of the Drawings For a better understanding of the invention and to show how the same may be carried into effect, there will now be described by way of example only, specific embodiments, methods and processes according to the present invention with reference to the accompanying drawings in which: Figure 1 illustrates schematically a flow diagram for a preparation of a molybdenum doped alumina garnet ceramic according to a specific implementation of the present invention;

5 Figure 2 is an X-ray detraction pattern (XRD) for pure YbAG garnet at 850°C;

Figure 3 is an (XRD) patterns for pure YbAG garnet at 875°C;

o Figure 4 is an (XRD) pattern for pure YbAG garnet at 900°C;

Figure 5 is an (XRD) pattern for pure YbAG garnet at 950°C;

Figure 6 shows XRD patterns for 10% Mo doped YbAG garnets at 700°C;5 Figure 7 shows XRD patterns for 10% Mo doped YbAG garnets at 750°C; Figure 8 shows XRD patterns for 10% Mo doped YbAG garnets at 775°C;

o Figure 9 shows XRD patterns for 10% Mo doped YbAG garnets at 800°C;

Figure 10 shows XRD patterns for 10% Mo doped YbAG garnets at 850°C;

Figure 11 shows a further XRD pattern for 10% Mo doped YbAG garnets at 5 850°C;

Figure 12 shows XRD patterns for 30% Mo doped YbAG garnets at 750°C;

Figure 13 shows a further XRD pattern for 30% Mo doped YbAG garnets at 0 750°C;

Figure 14 shows XRD patterns for 30% Mo doped YbAG garnets at 775°C; Figure 15 shows XRD patterns for 30% Mo doped YbAG garnets at 800°C;

Figure 16 shows XRD patterns for 30% Mo doped YbAG garnets at 850°C;

Figure 17 shows structural changes in pure YbAG powders using infrared spectroscopy at various temperatures;

Figure 18 shows infrared spectra of pure YbAG powders sintered at 850°C for 30 minutes;

Figure 19 shows structural changes in Mo doped YbAG powders using infrared spectroscopy at various temperatures; Figure 20 shows infrared spectra of Mo doped YbAG powders sintered at

850°C for 30 minutes;

Figure 21 shows Raman spectra for pure YbAG powders; Figure 22 shows Raman spectra for Mo doped YbAG powders.

Detailed Description There will now be described by way of example a specific mode contemplated by the inventors. In the following description numerous specific details are set forth in order to provide a thorough understanding. It will be apparent however, to one skilled in the art, that the present invention may be practiced without limitation to these specific details. In other instances, well known methods and structures have not been described in detail so as not to unnecessarily obscure the description.

Referring to Figure 1 herein there is illustrated a flow diagram detailing the alumina garnet sol-gel preparation technique. According to the specific implementation of the present invention alumina sols may be prepared 100 followed by addition of the ytterbium or yttrium precursors 101. Molybdenum is subsequently added to the sol at stage 102 so as to form a molybdenum doped garnet sol 103. Following processing of coatings 105 or nanopowders 106 garnet ceramics 107 are formed involving heat treatment 104.

Through specific doping of the lanthanide/transition metal alumina sol prior to gelation, the resulting required sintering temperature may be considerably reduced when compared with prior art garnet ceramic synthetic routes. Additionally, the creation of metal ion vacancies within the garnet structure whilst reducing the thermal conductivity of the ceramic does not promote oxygen diffusion through the ceramic, when for example the alumina oxide is applied as a coating, such that both effective thermal barrier coatings and anti-corrosion coatings may be prepared as a single garnet ceramic.

The inventors have found doping of the sol prior to gelation both assists in gelation formation in addition to the above identified beneficial thermal/mechanical ceramic properties. Additionally, resulting from the relatively low sintering temperatures associated with the Mo doped alumina garnet the present invention may be utilised within a variety of coatings and applications including application as decorative coatings in which the photoluminescent properties of Mo doped YAG or YbAG powders may be exploited to provide a luminescent coating. Specific implementations of the present invention may also be employed as optical coatings and as coatings for the interior surfaces of a vacuum chamber blocks to prolong working life and prevent contamination by sputtering. However, the Mo doped alumina garnet according to the present invention is specifically advantages when employed as a thermal barrier coating for temperature sensitive alloys and in particular nickel super alloy substrates. As sintering of the garnet sol is achieved from temperatures as low as 700°C damage and degradation of the temperature sensitive substrate may be avoided. As a further specific implementation of the present invention Mo doped YbAG or YAG may be employed as a sensor for the detection of, at least hydrogen and/or carbon oxides.

5 Preparation of sol An alumina sol was prepared from an aluminum chloride (AICl₃) precursor. The aqueous solution was prepared first by dissolving the (AICI₃) in distilled water followed by dropwise addition of ammonia solution into the AICI₃. The mixture o was stirred followed by a filtering of the precipitate and subsequent washing using distilled water. The precipitate was dried overnight. An organic solvent (e.g. ethanol, methanol and/or isopropanol and water) were then mixed with the precipitate. The mixed suspension was then stirred for 10 minutes followed by dropwise addition of acetic acid at 80°C followed by continuous stirring for several5 hours until the resulting suspension became an alumina sol. The prepared alumina sol was then sealed in a glass bottle and kept aging for several days.

Rare earth/transition metal sols or organic precursors may be added by dissolving the suitable precursors directly into the pre-prepared alumina sol in 0 addition to introduction of the molybdenum in the form of a molybdenum salt or molybdenum-organic precursor. The molybdenum is added in a 6+ oxidation state.

Preparation of coatings5 Molybdenum doped YAG and YbAG may be prepared by directly dropping the sol onto the substrate (glass, Si-substrates, steel, metal alloys, super metal alloys and Ni-super alloys). Following deposit of the sol onto the substrate the temperature was maintained at 50°C following which the sol dried quickly to form a thin layer. The coated substrate may then be heat treated at the required o sintering temperature (as low as 700°C). Additionally, the garnet sol may be applied to the substrate by spinning, or spraying. Subsequent dipping, spinning or spraying applications and calcination at the required temperature provides coatings with a variable range of thicknesses.

The inventors have realised that thick coatings may be achieved by adding Mo doped YAG or YbAG alumina garnet powder into the sol as the sol is applied to the substrate with subsequent heat-treatment above 600°C to achieve crystallisation and the single-phase Mo doped YAG and YbAG.

Thermal analysis and characterisation XRD patterns were collected on a Philips PW3710 defractometer using Cu Kα radiation. The tube current and voltage were 35 mA and 40 kV, respectively. Figures 2, 3, 4 and 5 herein illustrate XRD patterns for sample of pure

YbAG garnets sintered at various sintering temperatures being 850°C; 875°C; 900°C and 950°C, respectively. Referring to Figure 2 herein XRD patterns a to e represent sintering over the time periods 0.5 hours; 1 hour; 3 hours; 12 hours and 30 hours respectively. XRD patterns a to e of Figure 3 herein correspond to sintering temperatures over the time periods 1 hour; 3 hours; 8 hours; 16 hours and 32 hours, respectively. XRD patterns a to f of Figure 4 herein represent sintering over the time periods 5 minutes; 30 minutes; 1 hour; 3 hours; 8 hours and 16 hours, respectively. XRD patterns a to e of Figure 5 herein illustrates sintering over the time periods 5 minutes; 1 hour; 3 hours; 8 hours and 16 hours, respectively.

Referring to the XRD patterns of Figures 2 to 16 herein each peak of the respective XRD pattern is indicative of the alumina garnet morphology, in this case, where the sample is a polycrystal. Referring to Figure 2 herein the alumina garnet structure becomes increasingly defined with increased sintering time, in this case being well-defined after 30 hours of sintering at 850°C. The onset and establishment of the YbAG garnet structure is evident in Figures 3 to 5 herein as indicated by emergence of the corresponding alumina garnet peaks at the respective sintering temperatures ranging from 850°C to 950°C.

By way of example, the Miller Indices for XRD patterns of Figures 2 to 16 correspond to the following (from peaks left to right): 211; 220; 321; 400; 420; 422; 521 ; 611 ; 444; 640; 721 and 642.

As is apparent from Figures 2 to 5 herein, and in particular XRD pattern b of Figure 2 herein the YbAG garnet crystalline structure begins to form, with the structure becoming established around pattern d involving some 12 hours of sintering at 850°C. Moreover, Figure 5 herein illustrates the formation of the garnet structure around XRD pattern c involving sintering at 950°C for approximately 3 hours. Example one Figures 6, 7, 8, 9, 10 and 11 herein show XRD patterns for 10% Mo doped YbAG garnets at sintering temperatures 700°C; 750°C; 775°C; 800°C and 850°C.

The XRD pattern of Figure 6 herein confirms the formation of the alumina garnet structure at a sintering temperature of 700°C for 10% Mo doped YbAG.

XRD patterns a to d of Figure 7 herein correspond to sintering over the time periods 0.5 hours; 3 hours; 8 hours and 16 hours, respectively. XRD patterns a to d of Figure 8 herein correspond to sintering over 1 hour; 2 hours; 5 hours and 8 hours, respectively. XRD patterns a to d of Figure 9 herein correspond to sintering over 0.5 hours; 1 hour; 3 hours and 5 hours, respectively. XRD patterns a to e of Figure 10 herein corresponds to sintering over 5 minutes; 0.5 hours; 1 hour; 3 hours and 5 hours, respectively. Figure 11 herein details an XRD pattern for a 10% Mo doped YbAG sintered at 850°C.

Being evident from Figure 7 herein, XRD pattern b, the Mo doped YbAG garnet begins to form after only 3 hours of sintering at 750°C. According to .1 ζ. further examples of the present invention the inventors have determined crystalline formation of Mo doped YbAG as low as 700°C involving relatively short to moderate sintering periods as detailed with reference to Figure 6 herein. Figure 6 to 11 herein confirm the formation of Mo doped YbAG garnets at relatively low sintering temperature over relatively short time periods to those illustrated in Figures 2 to 5 herein being concerned with pure YbAG garnets. For example, XRD pattern e of Figure 10 involving a sintering temperature of 850°C over 5 hours illustrates formation of the garnet structure in contrast to XRD pattern f of Figure 4 herein involving a sintering temperature of 900°C over a 16 hour period.

Example two Figures 12, 13, 14, 15 and 16 herein show XRD patterns for 30% Mo doped YbAG garnets at sintering temperatures 750°C; 775°C; 800°C and 850°C.

XRD patterns a to d of Figure 12 herein correspond to sintering over the time periods 1 hour; 2 hours; 5 hours and 8 hours, respectively. XRD patterns a to d of Figure 14 herein correspond to sintering over 1 hour; 2 hours; 5 hours and 0 8 hours, respectively. XRD patterns of a to d of Figure 15 herein corresponds to sintering over 5 minutes; 15 minutes; 1 hour and 3 hours, respectively. XRD patterns a to e of Figure 16 herein corresponds to sintering over 5 minutes; 15 minutes; 1 hour and 3 hours, respectively. Figure 13 herein corresponds to an XRD pattern for a 30% Mo doped YbAG coating sintered at 750°C.5 Further characterisation Figures 17 and 18 herein correspond to infrared spectra of pure YbAG powders providing structural change analysis during heat treatment from the wet gel up to 850°C. The peaks in the region of below 900 cm^"1 provide evidence for o the formation of the monolithic alumina garnet structure. Figures 19 and 20 herein correspond to infrared spectra for 10% Mo doped YbAG powders including structural change analysis through sintering temperatures ranging from the wet gel up to 750°C. The peaks of Figure 20 herein, as indicated with reference to Figure 19 herein illustrate formation of the monolithic garnet crystal at temperatures between 600 to 750°C. Figure 20 in isolation provides evidence for the relatively low calcinating temperatures required to obtain the garnet structure (i.e. between 600 and 750°C).

Figure 21 herein corresponds to infrared Raman spectra for pure YbAG powders, indicating structural changes for two sintering temperatures of 600°C and 850°C over a 30 minute time period. Figure 22 herein corresponds to infrared Raman spectra for Mo doped YbAG powders at sintering temperatures 600°C and 750°C over 30 minute time periods. Figure 22 herein provides evidence for the onset of garnet crystalline formation involving formation of an Mo - O bond appearing at 893 cm^"1 which does not appear in the pure YbAG spectra, the remaining vibrational peaks being attributed to the alumina garnet structure. As will be appreciated by those skilled in the art, the Raman spectra at 600°C provides evidence for the formation of Mo - O bonds within the forming crystalline structure.

Claims

Claims: 1. A molybdenum doped alumina garnet manufactured by a sol-gel process.

2. The alumina garnet as claimed in claim 1 wherein said garnet is a lanthanide alumina garnet.

3. The alumina garnet as claimed in claim 2 wherein said lanthanide comprises ytterbium or lanthanum.

4. The alumina garnet as claimed in claim 1 wherein said alumina garnet is a transition metal alumina garnet.

5. The alumina garnet as claimed in claim 4 wherein said transition metal comprises yttrium.

6. The alumina garnet as claimed in any one of claims 2 to 3 wherein said alumina garnet is doped with molybdenum in the range trace amounts of molybdenum up to and including 35% mole ratio of molybdenum to lanthanide.0

7. The alumina garnet as claimed in any one of claims 4 or 5 wherein said alumina garnet is doped with molybdenum in the range trace amounts of molybdenum up to and including 35% mole ratio of molybdenum to transition metal.5

8. The alumina garnet as claimed in any one of claims 1 to 7 wherein said molybdenum comprises an oxidation state of 6+.

9. The alumina garnet as claimed in any one of claims 2 to 5 wherein o said lanthanide or said transition metal comprises an oxidation state of 3+.

10. The alumina garnet as claimed in any one of claims 2 to 9 wherein 1 mole of molybdenum replaces 2 moles of lanthanide or transition metal within the alumina garnet structure.

11. A molybdenum doped alumina garnet being represented by general formula (1 ):

where D is a lanthanide or transition metal and x is trace amounts up to and including 0.61.

12. The alumina garnet as claimed in claim 11 wherein D is Yb, Y or La.

13. An alumina garnet based sensor comprising an alumina garnet according to any one of claims 1 to 12 configurable for sensing any one or a combination of any one of the following set of:

0 • hydrogen; • carbon oxides.

14. A luminescent material comprising an alumina garnet as claimed in any one of claims 1 to 12.5

15. A coating comprising an alumina garnet according to any one of claims 1 to 12.

16. The coating as claimed in claim 15 wherein said coating is a o thermal barrier coating.

17. The coating as claimed in claim 15 wherein said coating is an anti- corrosion coating.

18. The coating as claimed in claim 15 wherein said coating is a decorative coating.

19. The coating as claimed in claim 15 wherein said coating is an optical coating for glass. 0

20. The coating as claimed in claim 15 wherein said coating is a vacuum chamber internal surface coating.

21. The coating as claimed in any one of claims 15 to 17 wherein said alumina garnet coating is configured for coating a metal alloy substrate.5

22. The coating as claimed in claims 15 or 16 wherein said alumina garnet coating comprises a low thermal conductivity.

23. The alumina garnet coating as claimed in claim 17 wherein said 0 anti-corrosion coating contains substantially no oxygen vacancies within the garnet structure.

24. A method of synthesising a molybdenum doped lanthanide or transitional metal alumina garnet by a sol-gel process comprising mixing an 5 alumina sol with a molybdenum oxide sol and a lanthanide oxide or transition metal oxide sol.

25. The method as claimed in claim 24 further comprising:

o forming a gel from a mixture of said alumina sol, said molybdenum oxide sol and said lanthanide oxide or transition metal oxide sol.

26. The method as claimed in claim 25 further comprising: heat-treating said gel to form said molybdenum doped lanthanide alumina garnet or said molybdenum doped transition metal alumina garnet.

27. The method as claimed in claim 26 wherein said gel is heat-treated above 600°C.

28. The method as claimed in any one of claims 24 to 27 wherein said0 lanthanide comprises lanthanum or ytterbium or said transition metal comprises yttrium.

29. A method of synthesising a molybdenum doped lanthanide or transition metal alumina garnet by a sol-gel process comprising:5 (i) forming an alumina sol using an aluminum based salt or aluminum- organic precursor and water;

(ii) adding a lanthanide or transition metal in the form of a salt or o lanthanide-organic or transition metal-organic precursor; and

(iii) doping the sol with molybdenum by adding molybdenum in the form of a molybdenum sol or molybdenum-organic precursor. 5

30. The method as claimed in claim 29 wherein said lanthanide comprises ytterbium or lanthanum or said transition metal comprises yttrium.

31. The method as claimed in claims 29 or 30 wherein said organic precursor comprises an alkoxide.0

32. The method as claimed in any one of claims 29 to 31 further comprising forming a gel from a mixture of said alumina sol containing said lanthanide or transition metal, and said molybdenum.

33. The method as claimed in claim 32 further comprising heat-treating said gel.

34. A method of making a molybdenum doped lanthanide or transition metal alumina garnet coating by a sol-gel process comprising:

(i) forming a sol of alumina oxide, molybdenum oxide and lanthanide or transition metal oxide, said sol being configured to form a gel;

(ii) applying said sol, said gel and/or a sol-gel to a substrate; and

(iii) heat-treating said sol, said gel and/or said sol-gel on said substrate to form a monolithic molybdenum doped lanthanide or transition metal alumina garnet.

35. The method as claimed in claim 34 wherein said lanthanide comprises ytterbium or lanthanum or said transition metal comprises yttrium.

36. The method as claimed in claims 34 or 35 wherein said step of applying said sol to said substrate comprises spinning, dipping or spraying said sol, said gel and/or said sol-gel onto said substrate.

37. A method of making a thermal barrier coating according to the method of claim 36.

38. A method of making an anti-corrosion coating using the method according to claim 36.

39. A method of making a decorative coating according to the method of claim 36.

40. A method of making an optical coating using the method according to claim 36.

41. A method of making a coating for an internal surface of a vacuum chamber according to the method of claim 36.