JPH07101752A - High weather-resistant thin film - Google Patents

Abstract

PURPOSE:To provide a high weather-resistant thin film capable of widening the range of usable limit of a substrate. CONSTITUTION:This thin film is applied on the substrate by sol-gel method and is expressed by a general formula, xSiO2.yA (where, each of x and y is 60<x<99 and 1<y<40 by mol% and A is at least one kind selected from B2O3 and Li2O3). And this thin film is applied on the substrate by solgel method and is expressed by a general formula, xSiO2.yA.zB (where, each of x, y and z is 50<x<95, 1<y<40 and 1<z<40 by mol%, B is at least one kind selected from TiO2, ZrO2 and Al2O3).

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to coating on the surface of a glass substrate, and more particularly to a highly weather resistant thin film suitable for surface coating of optical lenses, eyeglass lenses, various filters and the like.

[0002]

2. Description of the Related Art Generally, a high weather resistance thin film of this type is coated on a transparent substrate and has transparency itself. Recently, high weather resistance thin films are required to have so-called weather resistance whose characteristics do not change depending on temperature and humidity.

As a protective film applied on a glass substrate for the purpose of improving weather resistance, (a) an organic-containing inorganic thin film (JP-A-62-17043, JP-A-1-308847).
(See Japanese Patent Publication No.), (b) SiO that is generally commercially available
₂ Thin film (J. Non-Cryst. Solid 147 & 148 (1992) 437-431,
J. Ceram. Soc. Japan 90 (1982) 328-333, etc.), TiO ₂ —SiO ₂ (J. Ceram. Soc. Japan 100
(9) (1992) 1094-1097, etc.) is used. In any case, it is desirable that the substrate coated with these highly weather resistant thin films does not deteriorate with time or its transmittance does not change with time.

[0004]

However, the thin film of (a) above is fired at a low temperature in order to utilize its characteristics, that is, to leave an organic functional group, but it has a drawback that it is porous. The thin film (b) does not become non-porous unless it is heat-treated at 700 to 800 ° C. or higher, but when used in a glass having a low glass transition temperature (Tg), it is porous when fired at Tg or lower. Become. Therefore, the conventional thin film may not be sufficiently effective as a protective film for glass having relatively poor weather resistance.

Therefore, a technical object of the present invention is to provide a highly weather-resistant thin film capable of widening the restricted range of usable substrates.

[0006]

In the present invention, in order to widen the restricted range of usable substrates, that is, in order to make a thin film non-porous or a state corresponding thereto at a lower firing temperature,
At least one of B ₂ O ₃ and Li ₂ O components is added to SiO ₂ . Furthermore, the addition of B ₂ O _3, Li ₂ O component, since the direction of worsening weather resistance, in order to prevent this, TiO ₂ to improve the weather resistance SiO _2,
ZrO ₂ and Al (ell) ₂ O ₃ components are added.

According to the present invention, a highly weather-resistant thin film coated on a substrate by a sol-gel method, having the general formula x
SiO ₂ · yA (where x and y are mol%, 60 <x <
99, 1 <y <40, A is at least one selected from B ₂ O ₃ and Li ₂ O).

Here, in the present invention, the reason for limiting the glass component as described above is as follows. SiO ₂
The reason for limiting the content to 60 to 99 mol% is that if it is less than 60 mol%, a transparent sol liquid and a transparent film cannot be obtained, and if it exceeds 99 mol%, the temperature at the time of non-pore formation is lowered. Because you cannot do it. On the other hand, the reason why the component A is limited to 1 to 40 mol% is that if the amount is less than 1 mol%, the thin film will not become non-porous or a state equivalent thereto at a lower firing temperature, and if it exceeds 40 mol%. At the time of making holes (when firing)
In, the phase is separated or crystallized and becomes opaque.

Further, according to the present invention, a highly weather-resistant thin film coated on a substrate by a sol-gel method, having the general formula xSiO ₂ .yA.zB (where x, y, z are mol%) And 50 <x <95, 1 <y <40, 1 <z <40,
A is at least one selected from B ₂ O ₃ and Li ₂ O, B is TiO ₂ , ZrO ₂ , and Al (ell) ₂ O
A highly weather-resistant thin film is obtained, which is characterized by being represented by at least one selected from among ₃ ).

Here, in the present invention, the reason for limiting the glass components as described above is as follows. SiO ₂
Is limited to 50 to 95 mol% because if it is less than 50 mol%, a transparent sol liquid and a transparent film cannot be obtained, and if it exceeds 95 mol%, the temperature at the time of non-pore formation is lowered. Because you cannot do it. On the other hand, the reason why the component A is limited to 1 to 40 mol% is that if the amount is less than 1 mol%, the thin film will not become non-porous or a state equivalent thereto at a lower firing temperature, and if it exceeds 40 mol%. At the time of making holes (when firing)
In, the phase is separated or crystallized and becomes opaque.
Further, the reason why the component B is limited to 1 to 40 mol% is that if it is less than 1 mol%, high weather resistance cannot be obtained.
This is because if it exceeds%, the material becomes phase-separated or crystallized and becomes opaque when it is made non-porous (during firing).

Components other than the above may be added as long as the object of the present invention is not impaired.

Further, the present invention will be specifically described.

The glass thin film of the present invention uses a sol-like alkoxide solution prepared by a sol-gel method using an alkoxide corresponding to a predetermined oxide as a starting material, and the alkoxide solution is oxidized with a solvent such as alcohol. Using a diluted solution of 1 to 15 wt% as a coating liquid, a thin film is coated on a substrate such as glass by a method such as a normal dipping method, a spin method, or a spray method, and the temperature is from room temperature to 120 ° C. Dry for 30 minutes, 450
Bake at ~ 550 ° C for 10-60 minutes. By this series of operations, the highly weather-resistant thin film of the present invention can be obtained.

In the present invention, the alkoxide may be used as it is in the alkoxide solution, but the alkoxide may be hydrolyzed. The hydrolysis is usually performed by stirring the alkoxide and water, but the silicon alkoxide having a slow hydrolysis rate may be first hydrolyzed and then another alkoxide may be mixed. The amount of water used for this hydrolysis depends on the amount of alkoxide,
Although it may be about twice the molar amount of the alkoxide, the hydrolysis time can be shortened by using more water than this. Also, during hydrolysis, hydrochloric acid as a catalyst,
By using an acid such as nitric acid, the reaction time can be shortened and the peptizing action can be performed. The amount of catalyst may be about 1 × 10 ⁻³ to 1 × 10 ⁻² times the molar amount of alkoxide.

In the present invention, a chelating agent or an organic polymer may be added to the alkoxide solution in order to control the reaction rate and the film thickness.

The alkoxide used in the present invention includes Si (OR) ₄ , Al (ell) (OR) ₃ ,
Ti (OR) ₄ , Zr (OR) ₄ , B (OR) ₃ , (where R is CH ₃ −, C ₂ H ₅ −, C ₃ H ₇ −, C ₄ H
_{9 -, C 5 H 11 -} , C 6 H 13 -, can use at least one selected from at least one) of the.

Further, as an alternative to the above alkoxide,
(1) Silicon alkoxide oligomers, alkoxide derivatives such as titanium chelates and aluminum chelates, (2) Inorganic acid salts such as nitrates, sulfates, carbonates, chlorates, hypochlorites, chlorides, (3) Acetic acid Salt, oxalate,
Organic acid salts such as acetylacetonato salt can also be used as a raw material.

[0018]

EXAMPLES Examples of the present invention will be described below with reference to comparative examples.

(Example 1) Silicon ethoxide {Si
741 g of (OC ₂ H ₅ ) ₄ } is added to 64 g of a 0.15 M molar nitric acid aqueous solution and isopropanol (i-C ₃ H ₇ O).
H) was mixed with 1069 g with stirring to hydrolyze. Titanium-n-butoxide {Ti (OC ₄ H ₉ ) ₄ } 139 g was added to the hydrolyzed solution of Si (OC ₂ H ₅ ) _4.
And lithium nitrate (LiNO ₃ ) 17g in methanol (C
H ₃ OH) (150 g) was added and the solution was stirred.

Further, 239 g of water (H ₂ O), 25 g of 61% nitric acid solution (HNO ₃ ) and 7556 g of i-C ₃ H ₇ OH were added to the above solution, and hydrolysis and dilution were performed to obtain a coating solution.

A glass substrate was coated with the obtained coating solution by dipping. For the glass substrate, we chose a commercially available UV transparent glass that does not have very good weather resistance. After coating, it was dried at 120 ° C. for 10 to 30 minutes and baked at 500 ° C. for 10 to 60 minutes to obtain a substrate having a weather resistant thin film. As shown in Table 1 below, the fired weather-resistant thin film was composed of SiO ₂ 87%, TiO ₂ 10%, and Li ₂ O.
It had a composition of 3%.

The fired substrate was held in a thermo-hygrostat at 65 ° C. and 90% RH for 500 hours to perform a humidity resistance test.
The moisture resistance test was evaluated by comparing the transmittance (T) of light having a wavelength of 300 nm before and after the test. Here, the light with a wavelength of 300 nm was selected because the increase or decrease in the transmittance of the ultraviolet transmitting glass before and after the test was most suitable for the comparison.

In Table 1 below, a is the substrate, b is the coated substrate, c is the transmittance of the coated substrate after the humidity resistance test, and c-a is the change in the transmittance of the coating itself due to the humidity resistance test. c-b shows changes in the transmittance of the coated substrate before and after the humidity resistance test. From Table 1 below, in the coated substrate according to Example 1, c-a is -2.5%, c
-B was 1.2%, which was found to be extremely small in the change of the transmittance of the coat itself and the substrate with the coat. This means that the transparency of the weather-resistant thin film hardly changes in the humidity resistance test.

Example 2 Similar to Example 1, silicon ethoxide {Si (OC ₂ H ₅ ) ₄ } was added to a 0.15 M aqueous nitric acid solution and isopropanol (i-C ₃ H ₇ OH).
And mix with stirring to hydrolyze. Titanium-n-butoxide {Ti (OC
₄ H ₉ ) ₄ } and lithium nitrate (LiNO ₃ ) dissolved in methanol (CH ₃ OH) was added and stirred, and then water (H ₂ O), nitric acid (HNO ₃ ), i- C ₃
H ₇ OH was added to the above solution for hydrolysis and dilution to obtain a coating solution.

A glass substrate was coated with the obtained coating solution by dipping. Using the same glass substrate as in Example 1, drying and firing were performed in the same manner as in Example 1, and as shown in Table 1 below, SiO ₂ 81 was formed on the substrate.
%, TiO ₂ 9%, and Li ₂ O 10% were obtained. For the obtained coated substrate,
A moisture resistance test was conducted in the same manner as in Example 1, and the moisture resistance was evaluated by the transmittance to ultraviolet rays. As a result, as shown in Table 1 below, in the coated substrate according to Example 2, c-
It was found that the changes in the transmittance of the coat itself and the coated substrate were extremely small, such as a-7.7% and c-b-3.5%. This means that the transparency of the weather-resistant thin film hardly changes in the humidity resistance test.

(Example 3) As in Example 1, silicon ethoxide {Si (OC ₂ H ₅ ) ₄ } was added at a molar concentration of 0.1.
5M aqueous nitric acid and isopropanol (i-C ₃ H ₇ O
H) with stirring and hydrolyzed. Boron butoxide {B (OC ₄ H ₉ ) was added to this hydrolyzed solution.
₃ } was added and refluxed. After reflux, lithium nitrate (LiN
O ₃ ) in methanol (CH ₃ OH) was added and stirred, and water (H ₂ O), nitric acid (HNO ₃ ), i-
C ₃ H ₇ OH was added for hydrolysis and dilution to obtain a coating solution. Using the obtained coating solution,
A glass substrate similar to that in Example 1 was coated by dipping, dried and fired in the same manner as in Example 1 to obtain a substrate having a weather resistant thin film. As shown in Table 1, SiO ₂ 8
A substrate having a weathering thin film having a composition of 3%, B ₂ O ₃ 10%, and Li ₂ O 7% was obtained. Using this board,
A moisture resistance test was conducted in the same manner as in Example 1. The results are shown in Table 1.
As shown in FIG.
-A shows -11.3%, c-b shows -11.6%, and in the change of the transmittance of the coat itself and the coated substrate,
It turned out to be extremely small. This means that the transparency of the weather-resistant thin film hardly changes in the humidity resistance test.

(Example 4) As in Example 1, silicon ethoxide {Si (OC ₂ H ₅ ) ₄ } was added at a molar concentration of 0.1.
5M aqueous nitric acid and isopropanol (i-C ₃ H ₇ O
H) with stirring to hydrolyze. In this embodiment, after the hydrolysis, zirconium -n- propoxide _{{Zr (OC 3 H 7)} 4} and lithium nitrate (LiNO
₃ ) and methanol (CH ₃ OH) are dissolved in the solution and stirred, and further water (H ₂ O), nitric acid (HN
O _3), was added i-C ₃ H ₇ OH, to give a coating solution subjected to dilution and hydrolysis. Using the obtained coating solution, a glass substrate similar to that in Example 1 was coated by dipping, dried and baked in the same manner as in Example 1, and as shown in Table 1, SiO ₂ 86%, ZrO ₂ 5 %, And a substrate having a weather resistant thin film having a composition of 9% Li ₂ O was obtained. A moisture resistance test was performed on this substrate in the same manner as in Example 1.

After performing the moisture resistance test, the moisture resistance was evaluated by the transmittance to ultraviolet rays as in Example 1. As shown in Table 1 below, in the coated substrate according to Example 4, c-a was -10.1% and c-b was -10.5%, and the transmittance of the coat itself and the coated substrate was The changes proved to be extremely small. This means that the transparency of the weather-resistant thin film hardly changes in the humidity resistance test.

Example 5 As in Example 1, 731 g of silicon ethoxide {Si (OC ₂ H ₅ ) ₄ } was added to 63 g of an aqueous nitric acid solution having a molar concentration of 0.15 M and isopropanol (i-C ₃ H ₇ OH). It was hydrolyzed by mixing with 1054 g with stirring. The Si (OC ₂ H _{5) 4} of hydrolyzed solution in aluminum -sec- butoxide (Al _{(El) (OC 4 H 9) 3} ) was added and 142g, and refluxed.
After the reflux, a solution of lithium nitrate (LiNO ₃ ) 46 dissolved in 400 g of methanol (CH ₃ OH) was added and stirred. Further, 279 g of water (H ₂ O), 67 g of nitric acid (HNO ₃ ) and 7218 g of i-C ₃ H ₇ OH were added to this solution, and hydrolysis and dilution were performed to obtain a coating solution.
A glass substrate similar to that used in Example 1 was coated with the obtained coating solution by dipping. It was dried and calcined in the same manner as in Example 1 to obtain SiO _{2 as} shown in Table 1.
85% was obtained Al ₂ O ₃ 7% and a substrate with a weather-resistant film having a composition of Li ₂ O8%. The substrate thus obtained was subjected to a moisture resistance test in the same manner as in Example 1. As a result, as shown in Table 1, in the coated substrate according to the fifth embodiment, c-a
Indicates -8.0%, and c-b indicates -8.3%, which proved to be extremely small in the change in the transmittance of the coat itself and the coated substrate. This means that in the humidity resistance test,
This means that the transparency of the weather resistant thin film hardly changes.

Example 6 As in Example 1, 528 g of silicon ethoxide {Si (OC ₂ H ₅ ) ₄ } was added to 46 g of an aqueous nitric acid solution having a molar concentration of 0.15 M and isopropanol (i-C ₃ H ₇ OH). It was hydrolyzed by mixing with 762 g with stirring. The hydrolyzed solution of Si (OC ₂ H ₅ ) ₄ was added with zirconium-n-propoxide {Zr (OC ₃
H _{7) 4}} 134g boronic -n- butoxide {B (O
C ₄ H _{9) 3}} were added 75g reflux. After the reflux, 113 g of lithium nitrate (LiNO ₃ ) was added to methanol (CH ₃
OH) dissolved in 1000 g was added and stirred.
Furthermore, 332 g of water (H ₂ O) and nitric acid (HNO 3) were added to this solution.
₃ ) 168 g and 6842 g of i-C ₃ H ₇ OH were added, and hydrolysis and dilution were performed to obtain a coating solution. A glass substrate similar to that in Example 1 was coated with the obtained coating solution by dipping. Next, in the same manner as in Example 1, it was dried and baked, and as shown in Table 1, S
iO ₂ 62%, ZrO ₂ 10%, B ₂ O ₃ 8%, and L
A substrate provided with a weather resistant thin film having a composition of 20% i ₂ O was obtained. A moisture resistance test was conducted on this substrate in the same manner as in Example 1. As a result, as shown in Table 1, in the substrate according to Example 6, c-a shows -6.9%, c-b shows -7.4%, and the transmittance of the coating itself and the coated substrate is shown. It was found that the change was extremely small. This means that the transparency of the weather-resistant thin film hardly changes in the humidity resistance test.

Example 7 As in Example 1, silicon ethoxide {Si (OC ₂ H ₅ ) ₄ } was added at a molar concentration of 0.1.
5M aqueous nitric acid and isopropanol (i-C ₃ H ₇ O
H) with stirring and mixed to hydrolyze. After hydrolysis, zirconium-n-propoxide {Zr (OC
₃ H ₇ ) ₄ } and lithium nitrate (LiNO ₃ ) dissolved in methanol (CH ₃ OH) is added and stirred, and water (H ₂ O), nitric acid (HNO ₃ ), i-C ₃ H ₇ is added. OH was added, and hydrolysis and dilution were performed to obtain a coating solution.

A glass substrate similar to that of Example 1 was coated with the obtained coating solution by dipping. Next, this substrate was dried and baked in the same manner as in Example 1 to obtain SiO ₂ 65% and ZrO ₂ 3 as shown in Table 1.
A substrate provided with a weather resistant thin film having a composition of 0% and Li ₂ O 5% was obtained. Coated on a glass substrate. Using a glass substrate similar to that in Example 1, drying was performed in the same manner as in Example 1,
It was baked and subjected to a moisture resistance test. As a result, as shown in Table 1, in the coated substrate according to Example 7, c−a is −
6.0% and c-b showed -6.2%, which was found to be extremely small in the change of the transmittance of the coated substrate and the coated substrate. This means that the transparency of the weather-resistant thin film hardly changes in the humidity resistance test.

Example 8 As in Example 1, silicon ethoxide {Si (OC ₂ H ₅ ) ₄ } was mixed with 0.15M aqueous nitric acid solution and isopropanol (i-C ₃ H ₇ OH) while stirring. And hydrolyzed. This Si (OC ₂ H
₅ ) A solution prepared by dissolving lithium nitrate (LiNO ₃ ) in methanol (CH ₃ OH) was added to the hydrolysis solution of ₄ and stirred.

In addition, water (H ₂ O), nitric acid (H 2
NO _3), was added i-C ₃ H ₇ OH, to give a coating solution subjected to dilution and hydrolysis. A glass substrate similar to that in Example 1 was coated with the obtained coating solution by dipping. Next, in the same manner as in Example 1, the substrate was dried and baked to obtain a substrate having a weather resistant thin film having a composition of 90% SiO _{2 and} 10% Li ₂ O as shown in Table 1. A moisture resistance test was conducted on this substrate in the same manner as in Example 1. As a result, as shown in Table 1, in the coated substrate according to Example 8, c-a was -13.5% and c-b was -13.9%. It was found that the change in transmittance was extremely small.
This means that the transparency of the weather-resistant thin film hardly changes in the humidity resistance test.

Comparative Example 1 Silicon ethoxide {Si
(OC ₂ H ₅ ) ₄ } 867 g was added with 75 g of a 0.15 M concentration nitric acid aqueous solution and isopropanol (i-C ₃ H ₇ OH) 1
It was hydrolyzed by mixing with 069 g with stirring. To this Si (OC ₂ H ₅ ) ₄ hydrolyzed solution, 225 g of a 0.15 M concentration nitric acid aqueous solution and i-C ₃ H ₇ OH2583 g were added, and hydrolysis and dilution were performed to obtain a coating solution. A glass substrate was coated with the obtained coating solution by dipping, and a thin film was formed under the same conditions as in Examples 1-8. The glass substrate used, the drying and firing conditions, and the evaluation method are the same as in Example 1. The results are shown in Table 1. In Comparative Example 1, as is clear from Table 1 below, the transmittance was reduced by 40% or more in the moisture resistance test, and Comparative Example 1 is not practical.

Comparative Example 2 As in Example 1, silicon ethoxide (Si (OC ₂ H ₅ ) ₄ ) was added in an amount of 71 g of 0.15 M nitric acid aqueous solution and isopropanol (i-C ₃ H ₇ OH).
Was hydrolyzed by mixing with stirring. This (Si
Titanium-n- was added to the hydrolyzed solution of (OC ₂ H ₅ ) ₄ ).
Butoxide {Ti (OC ₄ H ₉ ) ₄ } in methanol (C
H ₃ OH) was added and the solution was stirred. Furthermore,
Water (H ₂ O), nitric acid (HNO ₃ ), and i-C ₃ H ₇ OH were added, and hydrolysis and dilution were performed to obtain a coating solution. A glass substrate was coated with the obtained coating solution by dipping, and a thin film was formed under the same conditions as in Examples 1-8. The glass substrate used, the drying and firing conditions, and the evaluation method are the same as in Example 1. The results are shown in Table 1. In Comparative Example 2, as is clear from Table 1,
In the humidity resistance test, the transmittance has dropped by more than 20%,
Similar to Comparative Example 1, Comparative Example 2 is not practical.

(Comparative Example 3) The same moisture resistance test as in Examples 1 to 8 and Comparative Examples 1 and 2 was performed with the substrate used in Example 1 as it was. The results are shown in Table 1. As in Comparative Example 3,
When the weather-resistant thin film is not applied to the substrate, the moisture resistance test shows that the transmittance is significantly reduced.

[0038]

[Table 1]

As shown in Table 1 above, in Example 1, the transmittance at 300 nm after the humidity resistance test was reduced by 2.5% from that of the substrate before the test, while in Comparative Example, the moisture resistance was decreased. According to the test, the decrease in transmittance is 20% to 45%
On the other hand, in the examples of the present invention, the decrease in transmittance is suppressed to less than 20%, and it can be seen that the examples of the present invention are superior in weather resistance to the comparative examples. .

Although the above examples and comparative examples have been described, the respective raw materials for the respective oxides can be changed to the following raw materials.

For example, with respect to the SiO ₂ component, oligomers of silicon methoxide {Si (OCH ₃ ) ₄ }, silicon methoxide and silicon ethoxide, etc. can be used. Also, regarding the B ₂ O ₃ component, H ₃ BO
₃ etc. can be used. Regarding the Li ₂ O component, lithium methylate (LiOCH ₃ ), lithium ethylate (LiOC ₂ H ₅ ), lithium acetate (CH ₃
COOLi · 2H ₂ O), lithium chloride (LiCl), or the like can be used. Furthermore, regarding the TiO ₂ component, titanium-n-propoxide {Ti (OC
₃ H ₇ ) ₄ }, titanium-iso-propoxide {T
i (OC ₃ H ₇ ) ₄ }, titanium-n-propoxide, titanium-iso-propoxide and oligomers of titanium-n-ptoxide, di-n-propoxy.
Bis (acetylacetonato) titanium (Ti [OCH (C
_{H 3) 2] 2 [OC} (CH 3) CHCOCH 3] 2).
As for the ZrO ₂ component, zirconium-n-butoxide {Zr (OC ₄ H ₉ ) ₄ } or the like can be used. Furthermore, regarding the Al (ell) ₂ O ₃ component, aluminum-iso-propoxide (Al (ell) (O)
C ₃ H _{7) 3,} aluminum diisopropoxide ethylacetoacetate chelate (Al (El) C ₁₂ H
₁₃ O ₅ ) or the like can be used.

[0042]

As described above, according to the present invention, a highly weather resistant thin film is used for a substrate in a high temperature and high humidity environment, especially for a glass having a low glass transition temperature (Tg) having relatively poor weather resistance. It has the effect of preventing a decrease in transmittance.

Claims (2)

[Claims] 1. A highly weather-resistant thin film coated on a substrate by a sol-gel method, which has the general formula xSiO ₂ .yA
(However, x and y are mol%, 60 <x <99, 1 <y <
40, A is represented by at least one selected from B ₂ O ₃ and Li ₂ O), which is a high weather-resistant thin film. 2. A highly weather-resistant thin film coated on a substrate by a sol-gel method, wherein the thin film has the general formula xSiO ₂ .yA.
zB (where x, y, and z are mol%, 50 <x <95,
1 <y <40, 1 <z <40, A is B ₂ O ₃ , Li ₂ O
At least one selected from the group, B is TiO ₂ , Z
A high weather resistance thin film represented by rO ₂ and at least one selected from Al (ell) ₂ O ₃ .