CS253693B1 - A method for etching a solid containing aluminum ions - Google Patents

Abstract

A method of etching a solid substance, insoluble in water, containing aluminum ions, by a process that eliminates the formation of insoluble etching products adhering to the surface of the substance, by which these surfaces can be easily cleaned, whereby by choosing the temperature and concentration of the etching solution, either high chemical purity and/or optical transmittance of the etched surface can be achieved, suitable for example for cleaning products made of corundum single crystals or phosphate photoluminescent glasses, by etching the surface in a solution prepared from 60 to 99 wt.% phosphoric acid H3PO4 and/or sulfuric acid H28O4 and 1 to 40 hypot. % water with such addition of boron and lithium compounds that the mass ratio of aluminum, boron and lithium ions in the solution during etching is Al3+ : B3+ 3 and Al3+ : Li112, or without this preparation, when the composition of the etched substance corresponds to the above mass ratio of aluminum, boron and lithium ions.

Description

The invention relates to a method of etching a solid substance, insoluble in water, containing aluminum ions, by a process that eliminates the formation of insoluble etching products adhering to the surface of the substance.

Alumina is used as a starting material for a number of products, such as single crystals, ceramics, glasses, and the like.

During the actual preparation or processing of these materials, their surfaces become contaminated, when the impurities are so strongly bound that the use of simple physical processes for thorough cleaning of surfaces is not enough. This not only worsens the useful properties of the respective products, but in many cases, the dimensionally or otherwise unusable parts cannot even be used as raw material for the preparation of a new batch. For this reason, it is necessary to etch the surface of water-insoluble substances containing aluminum ions. Sulfuric acid can be used as an etching bath, and in the case of chemically particularly resistant substances, phosphoric acid, especially at elevated temperatures. However, if the etching bath and/or the etched substance contains phosphate ions in higher concentrations, firmly adhering islands or thin but continuous coatings of aluminum phosphate usually form on the surface of the etched substance, which is detrimental both in terms of achieving a clean transparent surface and in terms of chemical purity, because the resulting reaction product prevents the etching of the surface layer.

The above-mentioned shortcomings are eliminated by the method of etching solids containing aluminum ions according to the invention, the essence of which is that these surfaces are etched in a solution consisting of 60 to 99 wt.% phosphoric acid H^PO^ and/or sulfur253 693

and 1 to 40 wt.# of water and such an amount of boron and lithium compounds that the mass ratio of ions in the solution during etching is AI ♦ B^ ⁴ 3 & Al^ ⁴ with Li ⁺ 12.

jl 1

The ions B and Li have an ionic radius smaller than the ions Al^ ⁴ , so they easily enter the lattice of aluminum phosphate, which then contains defects, which in practice is manifested by its slight adhesion to the surface of the etched substance 'against pure aluminum phosphate. The admixture of ions B^ ⁴ ” for example in the form of boric acid and ions Li ^{4 ,} for example in the form of lithium carbonate, does not need to be added to the etching solution when boron or lithium is present in the etched substance in such an amount that the mass ratios of Al^ ⁴ with B^ ⁴ 3 and Al^ ^{4 with} £i ⁺

12, applies to her as well.

Etching then releases Al^ ^4- , B^ ⁴ and Li ions into the solution in equal proportions, _and the B*^ ions, respectively borates and Li ^+, prevent the formation of integral aluminum phosphate on the surface of the etched substance, and etching can then proceed at high speeds.

The method according to the invention can be used to simply clean the surfaces of substances containing aluminum ions, while by choosing the temperature and concentration of the etching solution, it is possible to achieve a high chemical purity and/or optical transmittance of the etched surface.

Example 1

Single crystals grown by the Verneuil method, made of sapphire, had an opaque surface, which was caused by the adhesion of particles of the starting powdered aluminum oxide to the crystals during growth. For this reason, it was not possible to determine the content of internal defects in the single crystals, even using immersion. Therefore, the single crystals were cleaned by etching in a mixture consisting of 10 wt.# sulfuric acid HgSO^ 98# and 90 wt.# phosphoric acid H^PO^ 85#. Etching was carried out by heating the single crystals in the above-mentioned acid mixture to a temperature of 250 °C, which was reached in 1 h and the temperature was maintained at this temperature for 1/4 h. After cooling, the single crystals were rinsed with water and their surface was mostly transparent, but showed numerous firmly adhering islands of a foreign phase, the X-ray analysis of which showed that it was aluminum phosphate A1P0/ *

-3253 693

The etching mixture used was practically clear. However, if, according to the invention, 8 g of boric acid 1.4 g of B^ ⁺ and 2 g of lithium carbonate, which corresponds to 0.375 g of Li , were added to 1000 ml of the acid mixture before etching, the surface of the single crystals was completely clean after etching. Analysis of the etching mixture after etching showed that it contained 4.2 g of Al^ ⁺ in 1000 ml, which corresponds to the mass ratio Al^ ⁺ : B^” = 3 and Al^ ⁺ : Li ⁺ = 11.2. Only with a higher content of aluminum ions did deposits appear on the surface of the single crystal.

Example 2

Photoluminescent glass with a composition of 50 wt.% aluminum phosphate Al(PO^)^, 25 wt.% potassium phosphate KPO^, wt.% barium phosphate BaCPO^), and 5 wt.% silver phosphate AgPO^ was surface cleaned of abrasive residues after cutting and grinding by etching in 60% phosphoric acid at a temperature of 75 to 85 °C for 60 min. To prevent individual pieces of glass from sticking together, they were placed separately on a Teflon grid. Boric acid H ₃ BO ₄ was added to the phosphoric acid in an amount of 10g/1000 ml, i.e. 1.75 g B ³⁺ /1000 ml, and lithium carbonate in an amount of 3.2 g/1000 ml, i.e. 0.6g Li ⁺ /1000 ml. Etching proceeded without problems even when the same etching bath was used repeatedly, up to a concentration of aluminum ions of approximately

5.3 g Al^/lOOO ml, which corresponds to the mass ratio Al^ ⁺ : B^ ⁺ = 3 and Al^' : Li « 8.8. When the concentration of aluminum ions increased above the specified limit, insoluble coatings consisting mainly of the aluminum phosphate phase AlPO^ began to form on the glass surface already during its first use, even if the aluminum phosphate content in the glass was up to 1/3 lower. In contrast, the glass, where the potassium phosphate content KPO^ was reduced by 10% and this reduction was replaced by 10% of the total amount with boric oxide and 5% with lithium phosphate, had after etching a completely clean surface without any foreign phase particles, with respect to the content of boric and lithium ions dissolving together with aluminum ions, because the weight ratio of Al^ s and Al^ ⁺ : Li ⁺ was 1.63 : 1 and 5.57 : 1, respectively. The same results were also achieved with the glass, where boric oxide was used, replacing barium phosphate and lithium phosphate was used, replacing potassium phosphate.

Instead of the above-mentioned short-term heating to a temperature of up to 85 °C, 24 to 36 h of etching at a temperature of 20 °C could be used with similar results.

Claims (1)

A process for etching a solid containing aluminum ions, characterized in that its surface is etched in a solution containing 60 to 99 wt% phosphoric acid H 2 PO 4 and / or sulfuric H 2 SO 4 and 1 to 40 wt% water and such an amount of compounds of boron and lithium so that the weight ratio of aluminum, boron and lithium ions is in the grate during the etching process « Al ³ *: B ³⁺ 3 and Al ³⁺ Li Li ⁺ 12.