PHOTOLUMINESCENT VITROCERAMIC NANOCRYSTALS BASED ON SILICA-STABILISED ZIRCONIUM, PRODUCTION METHOD THEREOF AND PRODUCT BASED ON THE NANOCRYSTALS, FOR THERMAL BARRIER COATINGS

Abstract

Photoluminescent vitroceramic nanocrystals based on silica-stabilised zirconium; a method for producing same; and a product based on the nanocrystals, for thermal barrier coatings; a method for obtaining the nanocrystals and a product with photoluminescent properties and high-temperature structural stability, for thermal coatings.

Claims

1. A process for the preparation of photoluminescent glass ceramic nanocrystals based on silica-stabilized zirconia, characterized by the process comprising the following steps: a) preparing a precursor of SiO.sub.2 (tetraethoxysilane TEOS); b) preparing ZrO.sub.2; c) mixing products obtained in steps a) and b) to form SiO.sub.2—ZrO.sub.2; d) adding to step c) a mixture of Erbium to a ratio of 1%; e) agitating the mixture obtained in step d) for a period of 10 to 20 hours; f) depositing the mixture in a platinum crucible-type container for thermal treatment; and g) thermally treating the mixture in the platinum crucible-type container with a heating profile wherein the temperature)(T°) is increased from room temperature (18 to 25° C.) to 900° C. at a rate of 18 to 20° C./minute.

2. The process for the preparation of photoluminescent glass ceramic nanocrystals based on silica-stabilized zirconia according to claim 1, wherein in step a), the preparation of the precursor of SiO.sub.2 (TEOS) is carried out by mixing TEOS with a primary alcohol at a ratio of 0.1:15.5 (TEOS:alcohol) with constant stirring for 1 hour and distilled water is added with a mixture ratio TEOS:Water of 0.1:2.8, for two hours and subsequently, an inorganic acid and water in a mixture ratio TEOS:acid:water of 0.1:0.0005:2.8 with constant stirring for 1 hour.

3. The process for the preparation of photoluminescent glass ceramic nanocrystals based on silica-stabilized zirconia according to claim 2, wherein the primary alcohol is ethanol and the inorganic acid is selected from nitric acid (HNO.sub.3) or hydrochloric acid (HCl) and is carried out at room temperature (18-25° C.) and atmospheric pressure.

4. The process for the preparation of photoluminescent glass ceramic nanocrystals based on silica-stabilized zirconia according to claim 1, wherein in step b) EtOH and an inorganic acid, selected from nitric acid (HNO.sub.3) or hydrochloric acid (HCl), are added to the zirconium solution (Zr.sup.+4) in a ratio of zirconium solution:EtOH:acid of 0.9:24.5:1.8 and stirred at room temperature (18-25° C.) and atmospheric pressure for a period of 1 hour.

5. The process for the preparation of photoluminescent glass ceramic nanocrystals based on silica-stabilized zirconia according to claim 1, wherein in step c) the TEOS solution obtained in step a) is added drop by drop to the ZrO.sub.2 solution prepared in step b), and water is also added with constant stirring for 3 hours at normal conditions of temperature and pressure.

6. The process for the preparation of photoluminescent glass ceramic nanocrystals based on silica-stabilized zirconia according to claim 1, wherein in step g) the heating profile is carried out from the temperature (T°) increasing the temperature from room temperature to 900° C. at a rate of 19.2° C./minute over a period of 0.5 to 0.8 hours, and once the temperature of 900° C. is reached, said temperature is maintained for a period of 1 to 2.5 hours, after which the temperature is decreased from 900° C. to 575° C. at a rate of 4 to 5° C./minute over a period of 0.9 to 1.5 hours and the temperature of 575° C. is then maintained for a period from 0.5 to 1.0 hours and after this time, the temperature is decreased to 500° C. at a rate equal to 4 to 5° C./minute over a period of 0.1 to 0.5 hours, and the temperature at 500° C. is maintained for a period of 2 to 4 hours, after which the temperature is increased to 550° C. at a rate of 3 to 4° C./minute for a period of 0.2 to 0.4 hours, and said temperature is maintained for a period of 1 to 3 hours and the thermal treatment is carried out in a total of 10 to 12 hours.

7. Photoluminescent glass ceramic nanocrystals based on silica-stabilized zirconia obtained through a process that contains the following steps: a) preparing a precursor of SiO.sub.2 (tetraethoxysilane TEOS); b) preparing the ZrO.sub.2; c) mixing the products obtained in steps a) and b) to form SiO.sub.2—ZrO.sub.2; d) adding to step c) a mixture of Erbium at a ratio of 1%; e) agitating the mixture obtained in step d) for a period of 10 to 20 hours; f) depositing the mixture in a platinum crucible type container for thermal treatment; g) Thermally treating the mixture from step f) with a heating profile wherein the temperature)(T°) is increased from room temperature (18 to 25° C.) to 900° C. at a rate of 18 to 20° C./minute; wherein said nanocrystals are characterized in that the X-ray diffraction pattern thereof presents with bands 2θ at 30.1, 34.6, 35.1, 42.9, 50.1, 50.5, 59.3, 60.0, 73.0, 74.3, 82.2 and 84.9 and where the nanocrystals have sizes between 7.5 and 12.5 nanometers.

8. A product with photoluminescent properties and structurally stable under high temperature conditions, characterized in that it comprises photoluminescent glass ceramic nanocrystals based on silica-stabilized zirconia, wherein the crystals present an X-ray diffraction pattern with bands 2θ at 30.1, 34.6, 35.1, 42.9, 50.1, 50.5, 59.3, 60.0, 73.0, 74.3, 82.2 and 84.9 and where the nanocrystals have sizes between 7.5 and 12.5 nanometers.

Description

DESCRIPTION OF THE FIGURES

[0012] FIG. 1 illustrates a diagram of the process according to the present invention where an example of amounts of the reagents and mixing times for the composition 90-10 (ZrO.sub.2—SiO.sub.2) is exhibited.

[0013] FIG. 2 illustrates the thermal scheme implemented on the glass ceramic samples according to the process of the present invention.

[0014] FIG. 3 illustrates an XRD spectrum of the SiO.sub.2—ZrO.sub.2 product without and with erbium (Er) doping obtained according to the present invention.

[0015] FIG. 4 illustrates an XRD spectrum of a ZrO.sub.2-only sample.

[0016] FIG. 5 illustrates the TGA and DSC results of the samples (a) ZrO.sub.2—Er, (b) SiO.sub.2—ZrO.sub.2 and (c) SiO.sub.2—ZrO.sub.2—Er.

[0017] FIG. 6 illustrates a photoluminescence spectrum of the SiO.sub.2—ZrO.sub.2—Er.sup.3+ glass ceramic nanocrystals of the present invention.

[0018] FIG. 6A illustrates a photoluminescence Spectrum of a ZrO.sub.2—Er.sup.3+ sample.

[0019] FIG. 7 illustrates a VIS photoluminescence spectrum of a sample of the SiO.sub.2—ZrO.sub.2—Er.sup.3+ glass ceramic nanocrystals of the present invention.

[0020] FIG. 8 illustrates a possible application of a thermal barrier coating with the product of the present invention that self-indicates delamination, through the application of SiO.sub.2—ZrO.sub.2 as a top-coat and an interlayer of Si nanocrystals of SiO.sub.2—ZrO.sub.2—Er.sup.3+ according to the invention.

[0021] FIG. 9 illustrates a reflectance spectrum of a sample of SiO.sub.2—ZrO.sub.2—Er.sup.3+ glass ceramic nanocrystals of the present invention.

[0022] FIG. 10 illustrates the lifetime patterns of a sample of SiO.sub.2—ZrO.sub.2—Er.sup.3+ glass ceramic nanocrystals of the present invention.

[0023] FIG. 11 illustrates TEM images of three samples, in the first (FIG. 1-a) only ZrO.sub.2 is doped with Er, then SiO.sub.2—ZrO.sub.2 (FIG. 1-b) is without the doping material, finally there are images of SiO.sub.2—ZrO.sub.2 doped with Er (FIG. 11-c).

[0024] FIG. 12 illustrates an analysis of nanocrystal images indicating that the crystal size for nanocrystals according to the present invention is approximately 10 nm.

SUMMARY OF THE INVENTION

[0025] The present invention refers to photoluminescent glass ceramic nanocrystals based on silica-stabilized zirconia, a process for their production and a product based on said nanocrystals for thermal barrier coatings. Also, it refers to the process for obtaining said nanocrystals and the product with photoluminescent properties and thermal coatings which are structurally stable in high temperature conditions.

DETAILED DESCRIPTION OF THE INVENTION

[0026] In a first embodiment, the present invention refers to a product in the form of photoluminescent glass ceramic nanocrystals based on zirconia stabilized with silicon oxide, where the material consists of a ratio of 90% ZrO.sub.2 and 10% SiO.sub.2 obtained from a technique known as the sol-gel method doped with a rare earth element such as Erbium (Er).

[0027] In a second embodiment, the present invention refers to the production process of said nanocrystals, where the process comprises the steps of: [0028] a) Preparation of the precursor of SiO.sub.2 (TEOS). See FIG. 1; [0029] b) Preparation of ZrO.sub.2, See FIG. 1; [0030] c) Mixture of the products obtained in steps a) and b) to form SiO.sub.2—ZrO.sub.2, See FIG. 1; [0031] d) Add a mixture of Erbium at a ratio of 1% to step c) where the erbium is in HNO.sub.3 solution; See FIG. 1; [0032] e) Agitate the mixture obtained in step d) for a period of 10 to 20 hours, preferably 16 hours; [0033] f) Deposit the mixture in a platinum crucible-type container for thermal treatment; and [0034] g) Thermally treat the mixture in the crucible-type container in a furnace or muffle with a heating profile whereat the temperature)(T°) is increased from room temperature (18 to 25° C.) to 900° C. at a rate of 18 to 20° C./minute, preferably at 19.2° C./min for a period of 0.5 to 0.8 hours, preferably for a period of 0.77 hours, once the temperature of 900° C. is reached, said temperature is maintained for a period of 1 to 2.5 hours, preferably for 2.22 hours, after which a decrease in temperature from 900° C. to 575° C. is performed at a rate of 4 to 5° C./min, preferably at a rate of 4.9° C./min over 0.9 to 1.5 hours, preferably over 1.1 hours. The temperature of 575° C. is then maintained for a period of 0.5 to 1.0 hours, preferably 0.9 hours, after this time, a new decrease in temperature is performed to 500° C. at a rate equal to 4 to 5° C./minute over a period 0.1 to 0.5 hours, preferably 0.3 hours. The temperature of 500° C. is maintained for a period of 2 to 4 hours, preferably 3 hours, more preferably 3.6 hours, after which the temperature is raised to 550° C. at a rate of 3 to 4° C./min over 0.2 to 0.4 hours, preferably 0.3 hours and said temperature is maintained for a period of 1 to 3 hours, preferably 1.72 hours. This thermal treatment can be carried out in a total of 10 to 12 hours, preferably over 11 hours. This treatment is represented in FIG. 2.

[0035] In step a), the preparation of the precursor of SiO.sub.2 (TEOS), the mixture of TEOS is made using a primary alcohol, such as, for example, alcohols from C1 to C4, such as methanol (MeOH), ethanol (EtOH), propanol, etc., in a ratio of 0.1:15.5 (TEOS:alcohol) with constant stirring for 1 hour, then distilled water is added with a mixture between the range of TEOS:Water of 0.1:2.8 for two hours. An inorganic acid such as nitric acid (HNO.sub.3) or hydrochloric acid (HCl) and water are then added in a TEOS:acid:water mixture ratio of 0.1:0.0005:2.8 with constant stirring for 1 hour. This stage is carried out at room temperature (18-25° C.) and atmospheric pressure.

[0036] Stage b) consists in the preparation of ZrO.sub.2, alcohol and acid are added to a zirconium solution (Zr.sup.+4), for example EtOH and acid, for example nitric acid (HNO.sub.3) or hydrochloric acid (HCl) in the proportions of 0.9:24.5:1.8 zirconium:alcohol; acid solution and stirred at room temperature (18-25° C.) and at atmospheric pressure for a period of 1 hour. The source of zirconium for the process of the present invention may be zirconium propoxide Zr(C3H.sub.7O).sub.4 and zirconyl chloride ZrOCl.sub.2.

[0037] In step c), the TEOS solution obtained in step a) is added dropwise to the ZrO.sub.2 solution prepared in step b), and water is also added, maintaining constant stirring for 3 hours at normal temperature and pressure conditions.

[0038] The invention also includes photoluminescent glass ceramic nanocrystals based on silica-stabilized zirconia obtained by the process according to the present invention, where the nanocrystals are a product applicable to thermal barrier coatings and are characterized by bands 2θ at 30.1, 34.6, 35.1, 42.9, 50.1, 50.5, 59.3, 60.0, 73.0, 74.3, 82.2, and 84.9. Nanocrystals have sizes between 7.5 and 12.5 nanometers.

[0039] In a third embodiment, the invention refers to a product with photoluminescent properties which is structurally stable at high temperature conditions, wherein the material consists of nanocrystals comprising zirconium oxide (ZrO.sub.2) mixed with silicon oxide (SiO.sub.2) and doped with erbium ions (Er), where the nanocrystals are silica-stabilized zirconia-based photoluminescent glass ceramics obtained by the process according to the present invention, where the crystals are characterized by bands 2θ at 30.1, 34.6, 35.1, 42.9, 50.1, 50.5, 59.3, 60.0, 73.0, 74.3, 82.2 and 84.9. The nanocrystals have sizes between 7.5 and 12.5 nanometers. The inputs and reagents for its production are shown in Table 1.

TABLE-US-00001 TABLE 1 Reagents used for the preparation of materials. Component Distributor TEOS Merck H.sub.2O Amresco HNO.sub.3 Burdick & Jackson ZrOCl.sub.2 Merck Er.sub.2O.sub.3 Sigma-Aldrich

[0040] The compositions used in the working example shown in FIG. 1 for the preparation of the 90-10 concentration mixture are represented in Table 2.

TABLE-US-00002 TABLE 2 Quantities of reagents for the mixture 90-10 ZrO.sub.2—SiO.sub.2 (90-10) Molar Ratio Moles Ml Factor = 65.3 TEOS Si — 0.1 22.16 0.339 ml EtOH/Si 15.5 1.55 90.47 1.385 ml H.sub.2O/Si 2.8 0.28 5.05 0.077 ml H.sub.2O inicial — 4.90 0.075 ml H.sub.2O final — 0.15164 0.00232 ml HNO.sub.3/Si 0.0005 0.00005 0.00209 0.00003 ml Zirconium propoxide Zr 0.9 293.64 4.50 ml EtOH/Zr 27.5 24.75 1444.61 22.12 ml HNO.sub.3/Zr 2 1.8 75.12 1.15 ml Mixture H.sub.2O/Zr 5.2 4.68 84.49 1.29 ml

[0041] The molar ratio for the mixture of the reagents is presented in Table 3:

TABLE-US-00003 TABLE 3 Molar ratio for the mixture of reactants. Specimen (TEOS:ZrClO.sub.2) Molar Relationship EtOH:TEOS:H.sub.2O:HNO.sub.3 (15.5:0.1:2.8:0.0005) ZrClO.sub.2:EtOH (0.9:24.75) Er.sub.2O.sub.3 1%

[0042] The material obtained was characterized from the structural point of view using X-Ray Diffraction (XRD), having as a result that the SiO.sub.2—ZrO.sub.2—Er glass ceramic presents an almost totally tetragonal structure, as shown in FIG. 3. This figure includes the result of the glass ceramic without doping with Er for comparison. The XRD spectra are similar, which indicates that the inclusion of SiO.sub.2 in the proportions of the present invention generates a stabilization of the tetragonal phase at room temperature. FIG. 4 represents the material without SiO.sub.2, that is, only ZrO.sub.2 doped with Er, and a completely monoclinic structure is shown, which confirms the stabilization effect generated by SiO.sub.2 in the ZrO.sub.2 structure. This stabilization of the tetragonal phase is what was sought in numerous investigations of materials for applications in thermal barrier coatings (TBC), so that, when the temperature changes, a volumetric change of ZrO.sub.2 is not generated.

[0043] The XRD information is confirmed by the Differential Scanning calorimetry test and thermogravimetric analysis (DSC and TGA) performed on the samples. FIG. 5 represents the DSC and TGA results where it can be seen that the ZrO.sub.2—Er sample, FIG. 5-a, exhibits the typical tetragonal .Math. monoclinic phase change at 1181° C. With the inclusion of SiO.sub.2, a phase change is not visible, especially in the sample doped with Er (FIG. 5-b), which means that the material does not present the tetragonal .Math. monoclinic transformation, maintaining this phase tetragonal at room temperature.

[0044] Although so far, results have been presented that prove the stabilization of the tetragonal phase of ZrO.sub.2 with the inclusion of SiO.sub.2 and erbium, and the effect generated by this inclusion on the optical properties of the material is presented below. These properties are only visible in those samples wherein the erbium was incorporated. FIGS. 6 and 6A demonstrate the photoluminescence of the samples doped with erbium, ZrO.sub.2 and SiO.sub.2—ZrO.sub.2 respectively, in a wavelength range between 1400 and 1700 nm. This photoluminescence was obtained using a 488 nm laser as the excitation source.

[0045] Another of the characteristics of the product obtained by means of the process according to the present invention is observed in FIG. 6, which represents the most important results for new applications of this developed material, since it allows the samples to have an emission in the near infrared, with a photoluminescence in the infrared at 1533 nm, enabling passage of the emission through the thermal barrier and detection from the outside by spectroscopy equipment. The assertion that the emission can cross the thermal barrier is because absorption by zirconia-based materials is only possible in the ultraviolet range and part of the visible range, in particular, by the most commercial known Zirconia when stabilized with Yttria (Y.sub.2O.sub.3—ZrO.sub.2) (FIG. 6a). This has been one of the problems where much effort has been exerted in the development of materials for application in photoluminescent thermography of thermal barriers, or the incorporation of interlayers in the barrier that allow diagnosing the energy performance or detachment of some part of the ceramic protective layer. By depositing the material of the invention in the form of an internal interlayer, irradiation of the barrier is enabled with a visible emission laser from the outside, and then the detection of the fluorescence generated in the material due to the effect of this radiation-material interaction is enabled from the outside with a near-infrared spectrophotometer.

[0046] FIG. 7 represents the luminescence properties of the nanocrystals according to the present invention at other wavelengths in the visible range.

[0047] FIG. 8 is a schematic representation of a possible application of the nanocrystal product according to the present invention, representing a configuration that includes a top-coat that works at high temperatures and the photoluminescent nanocrystals of SiO.sub.2—ZrO.sub.2—Er according to the invention, deposited between the bond-coat and the top-coat. This converts the application into a system that auto-indicates top-coat delamination, preventing the substrate from being exposed to high temperatures and causing system damage. The emission received from the delaminated areas is more intense than in the non-delaminated areas, which enables monitoring of the system from the outside. Therefore, the present invention includes the application of nanocrystals in a TBC that uses materials, especially ZrO.sub.2, stabilized with any ceramic oxide (Al.sub.2O.sub.3, CeO.sub.2, TiO.sub.2, ThO.sub.2, etc.). As for the bond-coat material, NiCrAlY, Al.sub.2O.sub.3 or any other ceramic material can be used.

[0048] In this sense, in order to confirm that the SiO.sub.2—ZrO.sub.2 (undoped) can be useful for the application presented in FIG. 6, a light reflection test represented in FIG. 9 was performed using an OCEAN OPTICS Flame-S-XR1-ES Spectrometer. The results illustrated in FIG. 9 confirm that the material tends to have an absorption of light in the wavelengths that are required, this means that the light can reach the doped layer and enable acquisition of information on energy stability. Therefore, it is apparent that the photoluminescent glass ceramic nanocrystals according to the present invention enable excitation not only with visible radiation at 588 nm, but also at other wavelengths. Additionally, FIG. 10 illustrates the calculation of the lifetime of the emission of Er (.sup.41.sub.13/2) corresponding to 3400 μs, a value that must be taken into account in the tests. In addition, no phase transitions between the temperatures of 0° C. and 1,400° C. were observed, which means that the photoluminescent glass ceramic nanocrystals according to the present invention exhibit stabilization of the tetragonal phase thereof at room temperature.

[0049] Regarding the appearance of the nanocrystals, they have a particle size of approximately 10 nm as represented in FIG. 11. In this figure, the transmission emission microscopy (TEM) images of three samples are presented wherein one of them (FIG. 1-a) is only ZrO.sub.2 doped with Er, then SiO.sub.2—ZrO.sub.2 (FIG. 1-b) without the doping material, and finally Er-doped SiO.sub.2—ZrO.sub.2 images (FIG. 1-c).

[0050] Finally, the size of the crystallites was calculated from the X-Ray Diffraction results. These results are presented in FIG. 12, which are consistent with the TEM results obtained (FIG. 11).

Advantages and Benefits of the Present Invention

[0051] The advantage of the process and the photoluminescent glass ceramic nanocrystals based on silica-stabilized zirconia doped with erbium is exhibited in the optical properties of this new nanostructured material complemented with good thermal behavior, typical of a Zirconia stabilized at room temperature. The optical properties enhance the material so that it becomes, in addition to a thermal barrier, a sensor enabling sensitivity to thermal variables and morphological characteristics of the coating.

[0052] The present invention may be useful for the diagnosis of operational variables in closed thermal systems, where the use of conventional monitoring techniques is not possible. For example, applications in boilers or interiors of commercial thermal systems which can only be reached with a probe. Another advantage is that the material in the form of photoluminescent glass ceramic nanocrystals according to the present invention can be deposited internally in the thermal system and enable in some way diagnosing variable operations, such as surface temperature, without the use of electrical cables, by merely inserting an optical fiber, which involves a few millimeters in the design of the equipment.