DE102015009423B4 - Dimensionally stable silicate glass ceramic - Google Patents

Abstract

Silicate glass ceramic made of SiO2 with a content of Al2O3 as a dopant, which has a dimensional stability up to a temperature of 1600°C, wherein the ceramic has a cross-section which is formed between one surface and the opposite surface, characterized in that the content of the dopant Al2O3 has a concentration gradient of at least 0.1 Ma% to a maximum of 6 Ma% over the cross-section, which runs from one side to the opposite side.

Description

The invention relates to a silicate glass or silicate glass ceramic, namely a doped silicate glass ceramic, which is dimensionally stable and resistant to temperature changes up to at least 1600°C, and to its use.

Silica or silicate glasses are used for many industrial processes. They are used permanently in the semiconductor industry, the solar industry, and also for the production of various raw materials up to temperatures of 1100°C. For these applications, both extreme purity of the material and very good thermal shock resistance (TSR) are necessary. At temperatures above 1100°C, however, such silicate or silica glasses usually no longer show sufficient dimensional stability. For this reason, such silicate glass products are dimensionally stabilized. This is usually achieved by providing such materials, particularly pipes, with a stabilizing coating after their final dimensioning. Typical coatings are applied externally and include suspensions of amorphous SiO ₂ or dopants that act as nucleating agents. Such dopants are, for example, Al ₂ O ₃ , BaO, ZrO ₂ , TiO ₂ or CaO. Combinations of the aforementioned agents are also often used. When applying such suspensions, the usual concentration of the dopant based on the solids content of the suspension is usually 0.1 to 6 wt.%. Such an amorphous SiO ₂ suspension or slip containing dopants is then applied to the silica glass, typically in layer thicknesses of 40 to 150 µm. Application is usually carried out using a spraying or brushing process. The applied layer is then fused to the silica glass body, whereby a diffusion layer is formed between the applied layer and the silica glass body, which is largely homogeneously doped with the dopants and is completely connected to the silica glass tube. By means of such a coating, in particular the diffusion layer created in the process, a thin region is created in temperature ranges above 1200°C, which consists entirely of high cristobalite. A high cristobalite layer formed in this way is transparent and very compact. It is crystalline, in particular cubic. The thickness of a high cristobalite layer formed in this way is usually approximately 50 to 70% of the thickness of the suspension or slip layer originally applied. Using high cristobalite formed in this way, such pipes can also be used at temperatures of up to 1400 to 1500°C.

For example, in the US 3,927,697 A a quartz glass element is described which serves as a diffusion tube for the production of semiconductor elements. Such a tube consists of a quartz glass with an outer layer which contains SiO ₂ and 4 - 800 ppm of a crystallization-promoting agent. Such agents or crystallization nuclei are in particular such elements as boron, aluminum, phosphorus, antimony, zinc, magnesium, calcium, gallium and mixtures thereof. To produce them, the coated tubes are subjected to a crystallization process by tempering, which is carried out at 900 to 1550°C.

In DE 100 41 582 A1 a quartz glass crucible is described in which a doped SiO ₂ intermediate layer is arranged in the wall. This intermediate layer is intended to prevent impurities from diffusing into the interior of the crucible during later use of the crucible, ie during the production of semiconductor crystals. The SiO ₂ crucible with an intermediate layer that has not yet crystallized into cristobalite is produced as a solid blank, whereby the blank thus obtained is mixed with the semiconductor material to be produced during the first use and only during the first use, ie when the interior of the crucible is melted, does the previously formed intermediate layer crystallize into cristobalite.

The EN 101 56 137 A1 discloses the production of such a quartz glass crucible for drawing using the Czochralski process. This is based on the problem that cracks are formed in the crucible due to the β- to α-cristobalite conversion known from the literature, which renders the crucible unusable. Thus, a porous amorphous silica glass green body is first produced, into which a substance is infiltrated that promotes crystallization. The crystallization-promoting substance can be added before or after the crucible is formed, with the compound being added in liquid and/or solid form, which is done by spraying, dipping, soaking or externally applying.

Furthermore, the DE 10327496 A1 a process for producing a quartz glass component with high thermal stability. A hydrolyzable surface layer is applied to a base body using a sol-gel process, from which a layer of high cristobalite is then formed by heating.

Furthermore, the EN 10 2004 001 176 A1 the use of a glass ceramic, especially for halogen lamps and high-pressure/low-pressure discharge lamps and for flat screens. Such a transparent ceramic contains, in addition to other glass-forming the elements 35 to 70% SiO ₂ and 14 to 40% Al ₂ O ₃ .

However, the coated silicate glasses previously described for use in high-temperature processes have the disadvantage that at the activation temperature during tempering, despite the incubation or tempering time being greatly reduced by the dopants, a deformation of the glass shape can occur due to the viscosity of the silicate glass.

Another disadvantage is that such coated pipes, once the shape stabilization layer has been activated, can no longer be cooled below 300°C, as the cooling process destroys or splinters the applied and formed high cristobalite layer. Tensions between pure silica glass and the high cristobalite layer lead to large-scale cracking during cooling. In addition, the high form of the high cristobalite is transformed into the low form in the range between 280 and 180°C, which then leads to the formation of a crackle-like or net-like crack structure and the complete destruction of the crystalline layer and thus of the entire silicate glass pipe.

The invention aims to overcome the problems described above.

It has now been found that the problems described above can be solved by the features defined in the claims.

Surprisingly, it has been shown that a silicate glass ceramic with high dimensional stability up to at least 1600°C can be produced if this silicate glass ceramic contains Al ² O ₃ as a dopant and the content or concentration of the dopant in the ceramic is not constant, but has a gradient which runs in particular between two opposite sides or glass ceramic surfaces.

It was also found that a silicate glass ceramic with the features defined in the claims can be obtained which not only has the desired dimensional stability up to 1600°C, but can also be cooled back to room temperature without any problems. The silicate glass ceramic obtained using the process described below also has good thermal shock resistance.

The silicate ceramic according to the invention preferably shows a high dimensional stability even at high operating temperatures of up to 1600°C, in particular 1580°C or at operating temperatures between 1400°C and 1600°C. In addition, such a glass ceramic shows good resistance to temperature changes when subjected to a thermal shock test based on DIN EN 820-3.

The silicate glass ceramic according to the invention consists mainly of small crystals of high cristobalite, mullite and spinel. The proportion of high cristobalite is preferably at least 90%, in particular 95%, with at least 98 or at least 99% high cristobalite being particularly preferred. In a typical embodiment, the silicate glass ceramic also contains proportions of spinel and mullite.

In a preferred embodiment, the crystals of the silicate ceramic according to the invention have a minimum size of 200 nm, in particular at least 300 nm, with at least 400 nm or 500 nm being preferred. The maximum size of the crystals is a maximum of 60 µm, with a maximum of 50 µm, in particular a maximum of 40 µm being preferred.

The concentration of the dopant Al ₂ O ₃ at the start of the gradient is greater than 0 mass%. It is at least 0.1 mass%, in particular at least 0.2 mass%, with at least 0.5 mass% being preferred. Mass percentages of at least 0.8 mass%, in particular 1.0 mass%, are particularly preferred, with a minimum content of 1.5 mass% having proven particularly preferred. Maximum contents of dopant Al ₂ O ₃ at the end of the gradient are a maximum of 6 mass% in the gradient. Preferred amounts and maximum concentrations in the gradient are a maximum of 4 mass%, in particular a maximum of 3 mass%, with a maximum of 2.5 mass% having proven to be useful.

In a further preferred embodiment, the difference between the maximum and minimum content of dopant Al ₂ O ₃ in the gradient is at least 40%, with at least 50% having proven to be particularly useful. A difference of at least 60%, in particular at least 70%, is particularly preferred, with a difference of at least 90% being particularly preferred. The gradient required according to the invention can be continuous and/or stepped, although the gradient typically runs more or less linearly. If the gradient is created by stepwise layering, the steps can develop in the form of a wave-like increase in concentration when the raw mass is melted.

The ceramic according to the invention consists essentially exclusively of SiO ₂ and the dopant Al ₂ O ₃ . If necessary, small amounts of In particular, other glass-ceramic forming components may be included as impurities, provided that these do not interfere with the formation of high cristobalite.

In a further embodiment, the ceramic has tiny cracks that are so small that they are not noticeable, but can stop the crack propagation and thus prevent the silicate glass ceramic from breaking.

The density of the silicate glass ceramic is at least 2.3 and a maximum of 2.4 g/cm ³ , with a maximum of 2.35, in particular 2.33 g/cm ³ being preferred. The ceramic is usually opaque and, unless colorants are added, has a white appearance.

A method for producing the ceramic according to the invention is also described below. A base body made of SiO ₂ doped with Al ₂ O ₃ is produced in such a way that it has a gradient of dopant Al ₂ O ₃ as defined above, which runs from one side or surface of the base body to the opposite side or surface. Such a base body can be produced, for example, in such a way that grains or particles, in particular fine particles of SiO ₂ and Al ₂ O ₃ , are applied. This can be achieved, for example, by mixing Al ₂ O ₃ with SiO ₂ in ever higher concentrations. In principle, it is also possible to apply different layers of doped SiO ₂ on top of one another, in which the dopant increases or decreases from layer to layer, depending on which concentration was applied first. In addition to granular particles, fine, expediently powdered SiO ₂ and/or Al ₂ O ₃ particles are particularly suitable for producing such base mixtures. In principle, it is also possible to first melt SiO ₂ and Al ₂ O ₃ mixtures in different concentrations of dopants, then grind them into particles and build up a doping gradient by layering such particles with different doping contents.

In a further embodiment, the base body is produced by means of a conventional sol-gel process in which the content of dopant in the sol is continuously increased or decreased during application. After application, the sol-gel is solidified in a generally known manner and then, as described later, melted and tempered.

A further possibility is to produce the gradient according to the invention by means of a layer of extremely thin silicate glasses stacked on top of one another, with each layer having an increasing or decreasing dopant content.

After the base body has been formed as described above, the resulting mold is melted. Melting preferably takes place at a temperature of at least 1800°C, preferably up to about 2200°C. After melting, the resulting silicate glass is cooled at least until the glass mass solidifies.

In a further preferred embodiment, after the melting process, relaxation cooling takes place at a temperature between 950°C and 1250°C, preferably between 1000°C and 1200°C.

Another way of creating a gradient is to melt such a mass using an arc. Melting in the arc is preferably carried out in an atmosphere of an inert gas or a mixture thereof. Suitable inert gases are, for example, argon, nitrogen or helium. A mass melted in this way in the arc is then cooled at least until it solidifies, but preferably to ambient temperature. The high heat in the arc causes the gradient to adjust itself, as the Al ₂ O ₃ is partially removed.

Mixtures used in this procedure for the base mass for producing the silicate glass body according to the invention contain 90 to 99.9% by weight of SiO ₂ and 0.1 to 10% by weight of Al ₂ O ₃ . The base mass preferably contains at least 95% SiO ₂ , with at least 98 or 99% being preferred. A preferred maximum amount of Al ₂ O ₃ is 5% by weight, with a maximum of 2% by weight, in particular a maximum of 1% by weight being particularly preferred.

The silica glass ceramic obtained by this procedure has an Al ₂ O ₃ gradient that decreases from the side on which the heat acts during melting towards the side on which the heat acts for tempering or ceramization.

The aluminum oxide content on the melting side of the heat supply (e.g. arc) is >0, at least 0.1, preferably 0.5 wt.%, with at least 1 and 1.5 being preferred. The maximum content on the tempering side of the glass body or ceramic is a maximum of 6 wt.%, with a maximum of 4, in particular a maximum of 2.5 wt.% being particularly preferred. The Al ₂ O ₃ gradient from the outside to the inside is usually at least 50%.

In general, the production of the ceramic can also be carried out in two stages, whereby the silicate glass raw body obtained by melting the starting mass is cooled to room temperature and only then further treated. However, it has also proven to be useful to produce the glass ceramic directly in a one-stage process. After the molten silicate glass has solidified, it is immediately tempered to crystallize.

Typically, tempering is carried out at a temperature between at least 1370°C and at most 1520°C or 1480°C. Preferred tempering temperatures are at least 1390°C and at most 1550°C, with at least 1410°C being particularly preferred. A minimum temperature of 1415°C is particularly preferred. Preferred maximum temperatures for the crystallization region are 1440°C or 1430°C, with 1425°C being particularly preferred.

It has been shown that for heating to tempering temperature, the heat supply is expediently carried out at a heating rate of at least 10 K/min, with at least 20 K/min being preferred. Heating rates of 30 K/min, in particular 45 K/min, are very particularly preferred.

The tempering is carried out until at least 90% of the doped silicate or fused silica glass has been converted to high cristobalite and a corresponding glass ceramic has been formed.

Furthermore, the heat supply for melting the base mass can be carried out using any heat source.

In a preferred embodiment, cooling is carried out at a cooling rate of 2-7 K/min, 3-6 K/min, preferably 4.5 K/min, in particular ± 0.5 K/min between 1420°C and 400°C and at 0.5 to 0.1 K/min between 400°C and room temperature.

The tempering or crystallization temperature is usually maintained until at least 90% of the doped silicate glass has been converted to high cristobalite. The conversion to high cristobalite is preferably at least 95%, with a high cristobalite content of at least 97% or 98%, in particular 99%, being particularly useful. High cristobalite contents of at least 99.5%, in particular at least 99.8% by weight, are particularly preferred, with 99.9% being particularly preferred. Typically, the duration of the holding temperature for crystal conversion is at least 1 and a maximum of 100 hours, depending on the thickness of the tempered silicate glass body, with minimum durations of 10, in particular 15 or even 20 hours being typical. Typical upper limits for the formation of the high cristobalite structure are 48, in particular 45 hours, with a maximum of 35 hours often being sufficient.

After the end of the crystallization phase, the glass ceramic obtained in this way is cooled at a maximum rate of 270 K/h or K/min, in particular 200 K/h or K/min., with cooling rates in the temperature range greater than 500°C between 200 K/h and 270 K/h, in particular those between 220 K/h and 250 K/h, being preferred. At temperatures from 500°C to 200°C, cooling is preferably carried out at a cooling rate of <60 K/h, in particular <30 K/h. From temperatures of approx. 300°C, cooling takes place individually depending on the ambient temperature, and is usually dependent on the characteristics of the tempering furnace or tempering device used.

If the tempering process is carried out in a two-stage process, no further significant temperature increase occurs preferably at a temperature of between 850°C and 1050°C for a period of 1 to 10, in particular 3 to 8 hours. Then, as described above, it is heated to the final temperature or holding temperature for crystallization, which is then carried out for a period as described above. The heating and cooling gradients or rates are also the same in this two-stage process as previously described for the single-stage tempering process.

After cooling, the previously melted and possibly tempered doped silicate glass blank or ceramic blank is removed from the furnace vessel and is usually freed of the external adhering particles of the starting mixture or deposits and unevenness, which is conveniently done by scraping or grinding. If necessary, the blanks, especially pipes, are also further smoothed, in particular further ground or polished, so that a more or less smooth surface is created.

Blanks obtained in this way can be conveniently cut into smaller segments. These segments are subjected to a drawing process for later use before tempering, i.e. they are drawn to a desired length or wall thickness. Drawing is usually carried out with appropriate heating to a temperature of 1800°C to 2100°C. It is also possible to process the tubes and segments in front of the flame using glassblowing to create complex device elements and apparatus.

In a particularly preferred embodiment, the method described is suitable for producing doped silicate ceramic tubes. The previously described mixture of SiO ₂ and Al ₂ O ₃ is introduced into a preferably metal furnace vessel, e.g. a hollow cylinder. The furnace vessel rotates about its longitudinal axis so that the introduced particles are distributed more or less uniformly on the inner surface of the furnace vessel. An arc is then ignited inside the furnace vessel, which melts the particles starting from the inside of the furnace vessel. After melting and subsequent cooling, the hollow cylindrical ingot thus obtained is removed from the furnace vessel. In a practical embodiment, the outer particles adhering to the metal tube are not melted, so that the finished silicate glass blank simply slips or slides out of the metal tube. The unmelted particles act in a similar way to a separating layer or a roller bearing.

In a particular embodiment, the silicate glass body according to the invention has the shape of a tube or a flat plate. The glass ceramic body itself is opaque. The tube or plate according to the invention has the previously described dimensional stability at the previously defined temperatures as well as the resistance to thermal shock.

In particular, the tubes or plates according to the invention are characterized by a gradient of the dopant Al ₂ O ₃ .

The invention is explained in more detail using the following examples:

example 1

A high-purity quartz powder is dry-doped with a very pure Al ₂ O ₃ powder. In this case, the dopant content is 2.0 mass% Al ₂ O ₃ . The two powders are mixed very intensively in a Y-mixer so that the Al ₂ O ₃ is evenly distributed in the quartz powder. The powder is filled into a cylindrical rotating furnace vessel. The rotation of the furnace vessel accelerates the homogeneous powder mixture towards the furnace wall, where it forms a material ring of defined thickness in the form of a pre-compacted powder heap. This powder heap is melted from the inside out by igniting an arc inside the powder heap and stabilizing it for approx. 2 hours. The resulting silica glass has the shape of a hollow cylinder with the dimensions of an outer diameter of 230 mm, an inner diameter of 190 mm and a length of 1800 mm. The melting process creates an exponential concentration gradient across the wall thickness (from the outside to the inside of the cylinder), which averages 0.016 Ma% Al ₂ O ₃ /mm.

The cooled cylinder is cooled in a subsequent process and then drawn into tubes with various diameters and wall thicknesses of up to 5 mm using a tool-free tube drawing system. The forming temperatures are between 1900 - 2100°C.

The tubes produced in this way are heat treated in a crystallization furnace at a temperature of 1420°C for 24 hours under an air atmosphere. The heating rate from RT to 1420°C is 20 K/min. The cooling rate was not explicitly measured but was adjusted freely according to the furnace characteristics.

After reaching room temperature, the crack-free crystallized tubes are removed from the furnace and further cold processed according to the application.

Example 2

A silicate glass ceramic tube manufactured as described, which has a high cristobalite content of 97.9 - 98.5 wt.% and a dopant or aluminum content that has a gradient from the inside of the tube to the outside of the tube of 1.5 to 2.1 wt.%, was heated to 900°C and then quenched by immersion in water. It was shown that it withstood many thermal shocks without any problems. The maximum thermal shock resistance (TSR) was found at an aluminum content of 1.5 to 2.1 wt.%. The individual results are shown in the 1 shown.

Claims (7)

Silicate glass ceramic made of SiO ₂ with a content of Al ₂ O ₃ as a dopant, which has a dimensional stability up to a temperature of 1600 ° C, wherein the ceramic has a cross-section which is formed between one surface and the opposite surface, characterized in that the content of the dopant Al ₂ O ₃ over the cross-section has a concentration gradient of at least 0.1 Ma% to a maximum of 6 Ma%, which runs from one side to the opposite side. Silicate glass ceramic according to one of the preceding claims, characterized in that the difference in the content of the dopant from the beginning to the end of the gradient is at least 50% of the maximum dopant content. Silicate glass ceramic according to one of the preceding claims, characterized in that the gradient is continuous or stepped. Silicate glass ceramic according to one of the preceding claims, characterized in that it consists of at least 90% high cristobalite, mullite and spinel. Silicate glass ceramic according to one of the preceding claims, characterized in that the crystals have a size of at least 200 nm. Silicate glass ceramic according to one of the preceding claims, characterized in that the crystals have a maximum size of 60 µm. Use of a silicate glass ceramic according to one of the Claims 1 until 6 at temperatures above 1350°C to 1600°C for the production of semiconductor elements, solar elements and refractory applications such as furnace tubes, furnace linings, process chambers, internal structures of furnaces, supports for heating coils, kiln furniture and other furnace equipment which is exposed to high temperatures and temperature changes.

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