Method for producing a coated component of quartz glass or fused silica

The present invention has the following steps: (a) A step of preparing a base material which is made of quartz glass or quartz product (Quarzgut) and has a coated surface. (b) SiO on the coated surface₂In the step of applying the granular material layer, the granular material layer borders the coating surface with the inner region having the first granular material-fine content and the free surface of the granular material layer. Tangent, having an outer region with a second granularity-fine content, where the second granularity-fine content is higher than the first granularity-fine content, (c) The granular layer is sintered and SiO₂-The process of forming the surface layer The present invention relates to a method for producing a coated component made of quartz glass or a quartz product, including the above.

Components made of undoped or doped quartz glass or quartz products are used in many applications where high chemical stability and thermal stability or light transmission are important. Hereinafter, the term "quartz glass" is also used as a general term for quartz products. Examples include cladding, flasks, cover plates, mirror substrates for optical equipment, reactors, equipment or crucibles made of quartz glass in chemical process engineering and equipment assembly, carrier trays, bell jars, crucibles or protective shields in semiconductor fabrication. Can be mentioned.

Prefabricated quartz glass-components may be provided with functional layers in whole or in part tailored to a particular intended use for optimization of mechanical, optical or chemical surface properties. Known surface modifications include coating with materials with relatively high softening points to improve temperature or chemical stability or with high purity materials to reduce the risk of contamination from components. Includes coating.

Fused Quartz-For applications where the components are exposed to high thermal loads and chemical attack environments, a smooth, defect-free surface is also often important. This is because the dense, low-bubble surface layer improves etching resistance and corrosion resistance.

So, for example, the walls of a quartz crucible for silicon production or the inner walls of an opaque quartz glass crucible, such as those used to pull a single crystal according to the so-called Czochralski method, are exposed to contact with the silicon melt. Under high mechanical, chemical and thermal loads. Therefore, in order to reduce the corrosive attack of the silicon melt and to minimize the release of impurities from the crucible wall that accompanies it, on an otherwise opaque wall, than on dense and transparent quartz glass. An inner layer consisting of is made.

In the simplest case, the surface is smoothed and compressed by "flame polishing". At that time, the porous wall is locally heated to a high temperature in the range of 1650 ° C to 2200 ° C using a plasma flame or an oxyhydrogen gas flame, so that the opaque porous base material is heated. It turns into clear quartz glass in the area close to the surface. However, it has been found that it can only obtain a very thin transparent layer. This is because the vitrified transparent surface layer acts as a thermal insulator, making it difficult to adequately heat the underlying, still opaque areas. Processing at a higher flame temperature is likely to cause plastic deformation and lead to evaporation of gaseous silicon monoxide (SiO) .

Dense sintering can be mitigated by imparting lower viscosity or higher sintering activity to components-regions close to the surface that should be compressed by heat. This proposal is made by, for example, DE102008030310A1, and from the literature, the types of methods listed at the beginning are also known. At that time, the quartz glass crucible is SiO₂It is composed of multiple layers of granules, the average particle size of which is continuously decreasing. On a crucible-shaped granular layer of mechanically fixed coarse-grained quartz sand, a layer of synthetically manufactured quartz glass granules with a particle size in the range of 50-120 μm is first formed on top of it. Again, a synthetically produced spherical fine SiO with an average particle size of about 15 μm.₂The innermost layer of particles is formed. The granular layer is subsequently sintered from the inside to the outside using an arc, in which the fine-grained quartz glass powder of the innermost granular layer first melts, and then the dense glass. Layers form. Due to its high sintering activity, a sufficiently thick, particularly uniform glassy inner layer can be obtained at relatively low temperatures and short heating durations, without local inhomogeneities and bubbles. become.

However, the production of a transparent inner layer is cumbersome, because it is a variety of SiO with different properties and layer formation properties.₂This is because many coating steps using granules are required. So, for example, the synthetic SiO of the innermost layer₂The high sintering activity of the granules is based on their fineness and BET specific surface area. But on the other hand, these properties make it difficult to handle under normal process conditions. Therefore, for example, fine-grained granules are affected by arc pressure and easily generate bubbles due to gas flow and heat convection, which makes it difficult to produce a granular material layer having a uniform thickness. ..

Avoiding this drawback is, by itself, fine-grained SiO₂It is the production of a layer using a conventional slip casting method that takes the powder into a dispersion and thereby makes it easier to handle.

The DE2004051846A1 thus describes the manufacture of a quartz glass component with a reflector layer, wherein the reflector layer is also made of quartz glass and is manufactured using the slip casting method. At that time, a highly filled, castable, water-based slurry was produced, which was amorphous SiO.₂Contains particles. Amorphous SiO₂The particles are SiO₂It is produced by wet grinding of granules and has an average particle size in the range of 1-50 μm. Fused Quartz-A layer of slurry is applied on the surface of the matrix to be coated by spray coating, electrostatically assisted spray coating, sink coating, centrifugal coating, dipping coating or brush coating. The slurry layer is dried to a green layer (Gruenschicht) and subsequently sintered to be a reflector layer made of opaque quartz glass.

For one variant of this method according to DE102006046619A1, SiO₂The flow behavior of the slurry is SiO₂With the addition of nanoparticles, the orientation is changed to better ductility. SiO in the total solid content₂SiO when the mass ratio of nanoparticles is in the range of 0.2% by mass to 15% by mass₂A rather structural viscous-thixotropy flow behavior of the slurry is obtained, which prevents it from flowing down the arched surface.

WO2011 / 042262A2 has also selected a similar method. Fragmented SiO to reproducibly produce a surface layer of quartz glass on the matrix by simple spraying of the slurry layer₂Granular, amorphous spherical SiO₂Particles, SiO₂Composite-slurries containing nanoparticles and nonionic alkali metal-free surfactants are proposed. The higher the proportion of spherical particles, the higher the solid content density in the slurry layer can be adjusted, which suppresses the generation of stresses during drying and sintering. However, the spray properties still require a relatively low solid content between 74% and 78% by weight. Surfactants reduce interfacial stresses and thus increase the viscosity of the slurry at low shear stresses.

SMOlhero et al. Particle Segregation phenomena occuring during the slip casting process, CERAMICS INTERNATIONAL, vol.28, No.4,1. SiO with mass ratio₂Coarse fraction and SiO₂A description is made of the phenomenon of segregation of slip-cast particles produced using a slurry containing nanoparticles. Segregation is basically perceived as a drawback because it produces a mechanically weak maternal body. As a result, there are two mechanisms: gravity and SiO.₂It has been reported that a tendency of clogging due to nanoparticles called "clogging the cake" is acting. Whether one mechanism or the other mechanism dominates depends on a number of factors, especially the percentage of solids in the slurry-which varies between 40% and 50% by volume-and particle size distribution. Depends on.

DE102006032687A1 describes a slip casting method for producing stamped products made of industrial ceramics. The slurry contains particles made of a ceramic material having an advantageous size in the range of 10-30 μm, which contains the segregating agent in the form of nanoparticles made of the same industrial ceramic. Nanoparticles have an average agglomerate diameter between 50 nm and 300 nm. They are sent to the surface of the part in the die casting process and enriched there. As a result, the density of the surface is increased, so that the water pressurized in the direction of the molded body at the time of removing from the mold cannot penetrate into the surface of the green body. Therefore, damage to the green body is avoided by this water.

However, depending on the coating technique and the surface quality of the coated surface of the constituent members, a porous surface may occur after sintering, especially if the coated surface itself is porous in the first place. High sintering temperatures are required to obtain a sufficiently thick, transparent vitrified layer and a dense layer, which tends to produce rough and bumpy surfaces.

<p num="0016"><patcit num="1"><text>DE102008030310A1</text></patcit><patcit num="2"><text>DE2004051846A1</text></patcit><patcit num="3"><text>DE102006046619A1</text></patcit><patcit num="4"><text>WO2011 / 042262A2</text></patcit><patcit num="5"><text>DE102006032687A1</text></patcit></p>

<p num="0017"><nplcit num="1"><text>Particle Segregation phenomena occuring during the slip casting process , CERAMICS INTERNATIONAL, vol.28, No.4,1.Januar 2002</text></nplcit></p>

<p num="0018"> However, like this₂There is a limit to increasing the sintering activity of the slurry layer produced by further increasing the load of the slurry by the nanoparticles. The flow characteristics of the slurry then change in an unacceptable manner. Besides, SiO₂Higher nanoparticle content results in more and more shrinkage cracks as a result of stronger shrinkage during drying and sintering.</p><p num="0019"> Therefore, it is unlikely that it is not complicated to apply a plurality of slurry layers in order to continuously produce a granular material layer having a sintering activity according to the above-mentioned production method.</p><p num="0020"> On the other hand, the slip casting method, especially the spraying of the slurry layer (Aufspruehen), allows for the relatively low cost production of the layer. Moreover, the sprayed slurry layer is characterized by having a particularly uniform distribution of particle size over the layer thickness. Therefore, it is desirable to minimize its drawbacks when producing dense layers on quartz glass.</p><p num="0021"> Basically, a slurry layer that is as flawless as possible is desired. The criterion here is that the layer does not flow out under the influence of gravity. In practice, this property of the slurry layer is called "Tragen" and is said to be "layer lasting". The slurry to be processed by spraying must be fluid. The layer thickness should be relatively thin so that the liquid slurry layer can still withstand.</p><p num="0022"> Therefore, the issues underlying the present invention are relatively low cost and reproducibly above 100 μm on the coated surface of the quartz glass-component, especially when the coated surface itself is porous. It is to show the method of producing the smooth and dense surface layer which can be produced by the layer thickness.</p>

<p num="0023"> This task starts from the method mentioned at the beginning, and according to the present invention, SiO according to the step (b).₂The application of the granular material layer (I) Dispersion and a coarse fraction with a particle size in the range of 1 μm to 50 μm and SiO with a particle size of less than 100 nm₂Amorphous SiO that forms a fine fraction of nanoparticles₂A step of preparing a dispersion system containing particles, wherein the solid content of the dispersion system is between 70% by mass and 80% by mass, and from 2% by mass to 15% by mass. The range between SiO₂Applies to nanoparticles, (II) A step of applying the dispersion system by injecting (Aufgiessen) or spraying (Aufspritzen) onto the coated surface to form a slurry layer having a layer thickness of at least 0.3 mm. (III) The rate and direction at which the fine fraction is enriched in the outer region of the granular material layer under the action of the discharged dispersion liquid, and at that time, an injection film (Giesshaut) is formed. The step of drying the slurry layer by discharging with It is solved by having.</p><p num="0024"> Hereinafter, the dried slurry layer is also referred to as a green layer. This represents, in the sense of the present invention, the "granular layer" required to form a surface layer with a high density of thermal compression by sintering. Sintering properties are strongly dependent on the composition of the slurry layer in the region close to the surface. The crucial thing here is SiO₂The proportion of nanoparticles. Higher proportions result in higher sintering activity, which makes it possible to heat compress at relatively low temperatures or short sintering durations to obtain glasses with higher densities and lower porosities. In the ideal case, in the region of the slurry layer close to the surface, relatively fine SiO₂Only particles are found.</p><p num="0025"> To approach this, the slurry layer is given ample opportunity for dissociation or segregation throughout the drying process in the case of the method according to the invention, which is hereafter referred to as "Segregation". Segregation occurs inside the produced slurry layer, which borders the coated surface, mainly SiO.₂It provides two parts, a lower region where the coarse content of the particles is seen and an outer region bordering the free surface of the layer, which is rich in fine fractions. SiO₂The fine fraction of the particles is formed by nanoparticles. Nanoparticles typically have thousands of SiO₂Consists of a combination of molecules, usually 50-400 m²It has a BET specific surface area in the range of / g. Unlike the known method mentioned at the beginning, which requires the overlapping of a plurality of layers of different granules, in the case of the method according to the present invention, only one layer coating is sufficient. In this case, the separation of particles of different particle sizes is carried out by particularly striking segregation, resulting in an optically distinguishable injection membrane, as described in more detail below.</p><p num="0026"> The slurry layer is characterized by a non-uniform particle size distribution, where the transition between the lower and upper regions is not uninterrupted and appears discriminatively in the green body when observed under a microscope.</p><p num="0027"> The critical parameters for the success of this particularly striking segregation are:</p><p num="0028"> Liquid content at the beginning of the slurry-this is SiO inside the slurry layer₂In order to ensure sufficient mobility of the nanoparticles, it must be in a sufficiently high amount, i.e. at least 20% by weight. A relatively low solid content can result in relatively high shrinkage and crack formation of the slurry layer and is therefore not desired as such. Therefore, the minimum solid content is 70% by mass. However, the low solid content is SiO in the outer region of the slurry layer.₂It reduces the transfer of nanoparticles, which is therefore advantageous in the method according to the invention. -How to apply the slurry layer-This is not configured continuously, as in the case of spraying the slurry layer, for example, but is injected or sprayed with a minimum thickness of 0.3 mm in the work process. Thereby, on the one hand, SiO₂It can provide a sufficiently large amount of reservoir of nanoparticles, and on the other hand, prevents rapid drying of the layer, which can interfere with sufficient injection membrane-formation. When coating layers by injection or spraying, slurry coating avoids a continuous flow, i.e. not splitting into individual droplets, or the droplet diameter of the slurry decreasing below its equilibrium size. This is done by a very small split shock (Zerteilung simpuls) such as (droplet diameter is at least 1 mm). Therefore, when coating the slurry layer, there is no notable decrease in the liquid content of the slurry. At that time, the slurry layer preferably takes its final shape under the action of a tool such as a blade, a brush, a spray nozzle or a putty knife. The spreading action of the processing tool makes the layer surface slightly fluid, which makes SiO even when the liquid content is fairly low.₂Particle enrichment is reduced. In this connection, spray coatings exhibit additional drawbacks compared to injection or spraying, because liquids below equilibrium size typically in the range 1-500 μm due to mechanical shock during spraying. This is because suspension droplets having a droplet diameter are formed, and in the case of this droplet diameter, notable drying inevitably starts at the flight stage as early as possible. Therefore, the liquid content of the slurry layer formed by spraying is clearly different from that of the starting slurry. The liquid loss cannot be easily compensated by increasing the initial liquid content due to the increasing dissociation tendency at that time. High enough to allow notable enrichment in the outer region of the slurry layer-ie at least 2% by weight (based on the total mass of the slurry)-SiO₂Percentage of nanoparticles. However, at high concentrations, SiO₂Nanoparticles also cause high drying shrinkage, which also occurs especially laterally (in the layer plane), which can lead to layer detachment and crack formation. Therefore, SiO₂The upper limit of the nanoparticle content is 15% by mass. Since the solid content ratio is in the range of 70 to 80% by mass, this is because the ratio between 55% by mass and 78% by mass of the solid content is SiO.₂It means that it is based on particles that are not nanoparticles. How to remove the dispersion-This is done slowly and appropriately in the direction of the free surface, so that the escaping liquid is SiO₂Nanoparticles can be entrained upwards into the outer region.</p><p num="0029"> This results in a relatively high proportion of SiO inside the slurry layer, which is particularly dense and sintering activity.₂A volumetric region close to the surface with nanoparticles is created, where the proportion is SiO in the dispersion layer.₂Higher than the average proportion of nanoparticles.</p><p num="0030"> Finer SiO on the surface of the slurry layer₂Enrichment of particles and especially fine fractions can be visually identified as membrane formation, which is also referred to herein as "injection membrane". The slurry layer visually gives the impression of being covered with a wax layer.</p><p num="0031"> The injection membrane is identifiable (in the green layer) even after drying. At that time, the injection membrane means a surface region of a slurry layer having an average porosity of less than 1 μm and a low porosity of less than 10%. The thickness of this layer is preferably in the range of 3 to 15 μm, particularly preferably in the range of 5 to 10 μm.</p><p num="0032"> In addition, the injection membrane is SiO with a particle size of less than 100 nm.₂It is characterized in that the fine fraction composed of nanoparticles occupies a volume ratio of more than 70% of the injection membrane, preferably more than 80%.</p><p num="0033"> Usually SiO in injection membrane₂The nanoparticles do not exist individually, but in the form of aggregates or agglomerates, which are just a few coarse fractions of SiO.₂Surround the particles to partially or completely embrace them. Since the porosity of the injection membrane is low, the volume ratio is SiO₂It is almost equivalent to the mass ratio of nanoparticles. Accordingly, the volume ratio above 70% in the injection membrane is SiO₂It corresponds to about 10 times more enrichment than the typical starting slurry of the method according to the invention, which has a mass ratio of 7% or less of the nanoparticles.</p><p num="0034"> In contrast to spray layers, which are characterized by a substantially uniform particle size distribution over layer thickness, slurry layers coated according to the method according to the invention show a gradient in particle size distribution over layer thickness and are close to the surface. SiO in the region of the injection, that is, in the injection membrane₂It is non-uniform in that the nanoparticles are enriched.</p><p num="0035"> In ceramic process engineering, such injection membranes are usually regarded as a sign of undesired non-uniformity of the slurry layer and are avoided or removed as much as possible. In contrast, the method according to the invention intends to utilize this injection membrane for layer formation. This enrichment allows the green layer to be more compactly sintered than without it. That is, for dense sintering, a lower sintering temperature and / or a shorter sintering duration is required than in the case of a spray layer having a uniform particle size distribution.</p><p num="0036"> Even moderate sintering (ie, relatively low sintering temperature and / or short sintering duration) produces sufficient compression for the majority of applications that are noteworthy. Up to 10% of closed porosity remains in the area of the injection membrane so far.</p><p num="0037"> It has been found that the slurry layer showing the injection film undisturbed during drying can then be sintered into a dense, transparent quartz glass layer with a fairly low surface roughness at a reproducible and low temperature. Average Roughness of Sintered Injection Membrane R_aIs typically less than 5 μm, especially preferably less than 3 μm. This is surprising in the first place, because as we all know, SiO₂Nanoparticles do show high sintering activity, which is the basis for relatively low sintering temperatures, however, on the other hand, the nanoparticles also cause high drying shrinkage at high concentrations, especially This is because it results in the lateral direction (in the layer plane) and can result in layer detachment and crack formation.</p><p num="0038"> The fact that these effects in the method according to the invention do not usually occur is relatively large SiO₂Good engagement between the particle injection membrane and other green layers can explain this.</p><p num="0039"> The substrate consists of doped or undoped quartz glass. Quartz glass is either synthetically manufactured or made from natural raw materials. It is transparent or opaque (semi-transparent). The coated surface itself is an integral component of the substrate or even forms a coating of the substrate.</p><p num="0040"> In addition to the composition of the slurry layer and how it is applied onto the coated surface, removal of the dispersion is a crucial parameter for forming a dense injection film. In this connection, the treatment is advantageously planned such that the slurry layer dries more slowly than when no treatment is performed.</p><p num="0041"> Slow drying can be achieved in the simplest case by drying occurring in an environment with increased moisture compared to the prior art or at relatively low temperatures. Frequently, the temperature of the substrate is raised for drying, for example above 100 ° C. This temperature rise may be omitted. SiO on the surface₂It has been demonstrated that an initial drying duration of at least 2 minutes, preferably 3 minutes, is effective to ensure sufficient segregation of nanoparticles and to form an injection membrane.</p><p num="0042"> Especially for porous coated surfaces, it is useful to moisten the dispersion prior to application according to step (II) in order to achieve a reduction in drying rate. In this case, pre-wetting also contributes to pre-filling the open or closed pores with liquid, thus reducing their suction effect on subsequent application of the slurry layer. This is because the withdrawal of the dispersion is as completely free as possible, in the direction of the free surface, SiO₂This is because the nanoparticles are allowed to be entrained in this direction by the liquid. When the suction effect by the pores occurs, SiO₂The nanoparticles are supposed to be carried away in the opposite direction. Therefore, a possible alternative, that is, increasing the proportion of liquid in the dispersion, is rather ineffective in adjusting the suction effect, and may result in instability of the dispersion due to the otherwise very high proportion of liquid. There is sex. The liquid for moistening the coated surface is a dispersion or other liquid. The porous coated surface is provided, for example, based on the porosity of the component to be coated or based on the porosity of the surface layer of the component.</p><p num="0043"> It turned out to be preferable when the slurry layer is mechanically compressed.</p><p num="0044"> For mechanical compression, SiO₂The volume of voids present between the particles is reduced, and the dispersion liquid present therein is extruded and collects on the free surface. In this process, SiO₂The nanoparticles can be carried together and transferred to the surface of the slurry layer. This allows SiO₂A liquid film containing nanoparticles forms on the surface, which easily forms an injection film when dried. Moreover, due to mechanical compression, closer SiO₂Contact between the particles also occurs, which results in higher raw strength of the slurry layer after drying and better engagement with the substrate. The porosity remaining after drying the slurry layer is preferably less than 10%.</p><p num="0045"> The compression may be performed directly at the time of coating the slurry layer. Suitable methods, such as spreading or blade treatment, produce compressive or shear forces in the layers.</p><p num="0046"> When making a slurry layer with a layer thickness of up to 3 mm, preferably up to 1.5 mm, and the dispersion system is up to 10% SiO based on the total solid content.₂It has been demonstrated to be effective when it contains a mass percentage of nanoparticles.</p><p num="0047"> For layer thickness greater than 3 mm or SiO greater than 10%₂High mass percentages of nanoparticles increase the risk of shrinkage cracks during drying and sintering. This contributes to the engagement of the injection membrane to avoid crack formation and thus prevents tearing during drying or sintering, which is a coarser SiO.₂It can be explained by the importance of some penetration of the green layer by the particles. Therefore, the advantageous thickness of the slurry layer is SiO₂D of particle size distribution in coarse content of particles₅₀It cannot be said that it is significantly higher than the value.</p><p num="0048"> Advantageously, the coarse fraction is D in the range between 3 μm and 30 μm.₅₀Fragmentous amorphous SiO with a particle size distribution with values₂It is a granular material.</p><p num="0049"> Spliced SiO₂The granules contribute to the bondability of the slurry layer and its engagement with the surface, and improve its adhesion. The effect on engagement and adhesion improvement is D between 3 μm and 30 μm.₅₀Best achieved in the case of a unique particle size distribution with values. D₅₀If is less than 3 μm, a notable increased drying shrinkage of the slurry layer occurs, and D greater than 30 μm.₅₀Granules with a value interfere with the high solids density in the slurry, which also contributes to the increased drying shrinkage. The splinter-like granules are most easily produced by milling, preferably by wet milling.</p><p num="0050"> The dispersion is preferably present on an aqueous basis.</p><p num="0051"> The polarity of the aqueous phase is SiO₂It can affect the interaction of particles.</p><p num="0052"> Particularly preferred are the solid content of the dispersion in the range of 74% to 78% by weight.</p><p num="0053"> This is a relatively low solid content, such as that typical of spray slurries. The spray characteristics are required to have a low viscosity and thus a low solid content. However, the low solid content is SiO₂The nanoparticles are facilitated to move into the outer region of the slurry layer, so that the content does not use the dispersion system as a spray slurry in the method according to the invention, but other coating techniques such as brush coating or blade coating. It is advantageous even in the case of applying the above, which makes it possible to use a dispersion system having a higher solid content.</p><p num="0054"> A "green layer" is obtained after the slurry layer has dried. Sintering of the dried green layer is carried out by heating in a furnace, or by using a combustion flame, by plasma or arc, or by a laser having a predetermined working wavelength. At that time, the green layer may contain a component that absorbs the working wavelength of laser radiation or plasma radiation, thus limiting the heating action during compression in a short time and locally, and for plastic deformation or thermal stress. Occurrence can be substantially avoided. The component that favorably absorbs plasma or laser radiation is SiO₂Additives in the form of particles with a different chemical composition or amorphous SiO₂Doping of particles (Dotierung), or an interface where plasma or laser radiation is diffusely reflected and absorbed by it.</p><p num="0055"> Sintered SiO₂The surface layer is transparent or completely or partially opaque, depending on the application, and is characterized by no cracks in either case and high adhesion strength on the parent quartz glass. Typically, it is finished in the form of a flat layer, or it also has a shape that forms a functional component of the substrate, eg, as a thickening or ridge.</p><p num="0056"> Amorphous SiO₂Particle SiO₂The content is preferably at least 99.99% by mass. This is a splinter of SiO₂Spherical SiO for granules₂It also applies to particles. Such SiO₂The solid content ratio of the slurry produced using particles is at least 99.99% by mass SiO₂Consists of. No binders or such additives are planned. The content of metal impurities is preferably less than 1 mass ppm. There is no risk of contamination or crystallization of this starting material. Dried SiO₂The proportion of cristobalite in the slurry layer (= green layer) is preferably at most 1% by weight, because otherwise crystallization can occur during sintering, which interferes with compression and This is because it can lead to defects in the components.</p><p num="0057"> Preferably, a binder-free dispersion is used.</p><p num="0058"> By not containing the binder, impurities are prevented from entering the slurry. Sintering is preferably carried out without the use of a sintering aid. The layer thus prepared is characterized by high purity. SiO over 99.99%₂The content is achieved. Among them, the alkali metal element, which may be present in the binder additive as a constituent or as an impurity, leads to the formation of cristobalite in the crystalline phase at elevated temperatures. Such depermeability causes crack formation upon temperature changes and also hinders compression in the sintering process.</p><p num="0059"> Next, the present invention will be described in more detail with reference to examples and drawings.</p>

<figref num="1">A photograph of the green layer of the sample shown at the same magnification as in FIGS. 2 to 5.</figref><figref num="2">A photograph of the green layer of the sample shown at the same magnification as in FIGS. 1 and 3-5.</figref><figref num="3">A photograph of the green layer of the sample shown at the same magnification as in FIGS. 1, 2, 4 and 5.</figref><figref num="4">A photograph of the green layer of the sample shown at the same magnification as in FIGS. 1 to 3 and 5.</figref><figref num="5">A photograph of the green layer of the sample shown at the same magnification as in Figures 1 to 4.</figref><figref num="6">The figure which shows the graph which has the measurement result of the average roughness of the surface layer of a different sample.</figref><figref num="7">The figure which shows the high-resolution computer tomography (micro CT tomography) from the side of the green layer produced according to the method of this invention.</figref><figref num="8">The figure which shows the green layer made by spraying and drying a slurry layer by micro CT image from the side.</figref><figref num="9">The figure which showed the fragment edge (Bruchkante) in the green layer according to this invention by the scanning electron microscope.</figref>

<u style="single">SiO</u>_{<u style="single">2</u>}<u style="single">Slurry production</u> Amorphous quartz glass granules made of natural raw materials having a particle size in the range of 250 μm to 650 μm are mixed into the dispersion liquid in a drum mill lined with quartz glass. The quartz glass granules were previously purified by a high temperature chlorine treatment method. Note that the cristobalite content is less than 1% by weight.

The mixture is milled in a silica glass mill ball at 23 rpm for a duration of 3 days in a roller block until a homogeneous slurry is formed. SiO that dissolves in the process of crushing₂Because of this, the pH value drops to about 4.

SiO obtained after crushing quartz glass granules₂Granular particles are splinters and have a D of about 8 μm.₅₀Value and D of about 40 μm₉₀The particle size distribution characterized by the value is shown. SiO having a diameter of about 40 nm is added to this homogeneous slurry.₂Add nanoparticles (pyrolytic silica). After further homogenization, binder-free SiO₂A slurry is obtained.

Different but similar composition SiO₂Using the slurry, coating samples were made on different substrates and using different coating techniques. The composition of each slurry and the coating results obtained using it are shown in Table 1.

<u style="single">Sample 1 (comparative example)</u> SiO₂The slurry has a low viscosity and can be used directly as a spray slurry (Spruehschlicker) by itself. In the first test, this slurry was used to make a coating on a porous sheet. The sheet consists of absorbent opaque quartz glass with open pores.

For coating, a quartz glass sheet is mounted sideways in the spray chamber, and the top surface is sprayed with slurry to withstand about 0.7 mm thick SiO.₂Slurry layers were provided one after another. For this purpose, a spray gun was used to continuously supply the spray slurry.

On the slurry layers applied one after another in this way, a rough and uneven surface layer is formed within 1 minute during the subsequent drying in the air. In any case, this result is partly attributed to the rapid drying of the slurry layer, which makes it impossible to segregate the fine content in the upper region of the slurry layer due to the porous substrate. Therefore, a dense and closed injection membrane could not be formed.

Further drying was then performed slowly by allowing the slurry layer to rest in the air for 8 hours. Complete drying is carried out in air for 4 hours using an IR heater.

An inhomogeneous surface layer consisting of opaque porous quartz glass with coarse crevices is obtained, which has the appearance image shown in FIG.

Subsequently, the dried green layer is sintered in a sintering furnace at a temperature of about 1400 ° C. and is about 1.9 g / cm.³Obtains an opaque surface layer with a density of.

<u style="single">Sample 2 (comparative example)</u> In order to eliminate the effect of the porous substrate on drying, in a further test, a quartz glass sheet having a dense smooth surface was used instead of the porous quartz glass sheet. In this case, since the slurry layer is easy to flow, the solid content was adjusted to be slightly higher than that in the case of sample 1, and the final thickness of the slurry layer was only 0.4 mm in this case. In other respects, it followed the same manufacturing parameters as sample 1.

As a result, after the slurry layer was dried in air, a surface layer whose appearance image was shown in a plan view in FIG. 2 was obtained. This has slightly less roughness and heterogeneity than that of sample 1. However, it is a relatively small improvement.

The very slight improvement is due to the fact that there is no notable difference in the initial drying rate of the slurry layers in Samples 1 and 2 despite the use of a non-absorbable substrate in Sample 2. Can be explained. It can be said that this is due only to the coating technique itself. This is because the spray produces fine droplets of slurry, which lose moisture as early as during the flight phase. Moreover, the slurry layers are configured as a plurality of overlaps one after another. The individual overlaps are thin and dry instantly in the air. Only by using a fluid spray slurry, we succeed in constructing a layer having a sufficient thickness in the first place. But by that, SiO₂A sufficiently large amount of reservoir of nanoparticles cannot be provided due to the segregation of fine content on the surface of the spray layer.

Further drying and sintering is performed as described above based on sample 1.

<u style="single">Sample 3</u> 2 mm thick SiO on a flat sheet of absorbent opaque quartz glass with open pores, as in sample 1.₂A surface layer will be prepared.

Unlike sample 1, the slurry layer is made by blade coating (also called "injection"). In this case, SiO with a thickness of about 4 mm is placed on a quartz glass sheet placed sideways.₂The slurry layer is applied with a blade, and immediately after that, a blade device is used to apply a high pressure to the layer so that the slurry layer is compressed to a thickness of about 0.8 mm.

A thin liquid film is formed on the slurry layer thus applied and compressed, and a homogeneous and closed surface layer is formed upon subsequent drying in air. Microscopic observation shows high dissociated fine content. That is, inside the injection membrane, fine SiO₂Particles, especially SiO₂The proportion of nanoparticles is notably higher than the proportion in the rest of the slurry layer.

The method of full layer thickness coating in the work process, on the one hand, is suitable for segregation on the surface in a sufficiently large amount of SiO.₂If the nanoparticles were used simultaneously and, on the other hand, the drying of the layer in the air was prevented too quickly and the drying was not prevented, it is probable that segregation and injection membrane formation were hindered. Therefore, sample 3 results in a slower drying of about 3-5 minutes, despite initially having slightly lower solids content and otherwise similar process parameters to sample 1. Moreover, the solidification of the slurry layer forms a durable layer that enables the formation of a substantially smooth injection film.

Upon injection, the slurry layer takes the final shape under the action of tools such as blades, brushes, batter knives or outflow nozzles from which a continuous stream of slurry emerges during coating. The spreading action of the processing tool makes the layer surface slightly fluid, which makes SiO even when the liquid content is fairly low.₂The enrichment of nanoparticles is reduced. This result-that is, no notable reduction in the liquid content of the slurry-is also expected for other coating techniques (eg spraying), where the slurry layer is at once in its overall thickness. And, it is produced without dividing into fine droplets of less than 1 mm.

The slurry layer thus prepared is dried within 3 minutes to form a layer that can withstand, and then more slowly, dried by letting it sit in the air for 1 hour. At that time, the injection membrane takes a waxy appearance image. Complete drying is performed in air using an IR heater for 4-8 hours, after which the infusion membrane has the appearance image shown in FIG.

The surface looks smooth in nature. Smooth surface area is SiO₂Fine content, that is, SiO₂It is formed by nanoparticles and their agglomerates or aggregates. Rough surface area is SiO₂Produced by gross content (see also Figure 9 for this). Evaluation of the surface structure by photographic analysis shows that about 85% of the entire surface is smooth. When observing the surface layer as a lateral cut surface-although it appears weakly-a membrane layer (injection membrane) with a thickness of about 9 μm is identifiable (see Figure 9).

SiO enriched in the surface area of the dried slurry layer₂Nanoparticles exhibit high sintering activity and improve layer compression. When the dried green layer is sintered in a sintering furnace at a temperature of approximately 1400 ° C., the layer first becomes dense and then becomes closed porosity. Approximately 2.1 g / cm, crack-free and substantially smooth³A surface layer made of opaque quartz glass with a density of 5% and a porosity of 5% is obtained accordingly.

<u style="single">Sample 4</u> Further tests were carried out as described based on Sample 3, where a quartz glass sheet with a dense and smooth surface was used as the substrate instead of the porous quartz glass sheet. SiO to prevent the slurry layer from flowing out₂The content of nanoparticles and solids was increased overall.

As a result, after drying the slurry layer in air, a surface layer was obtained, although its roughness was much less than that of sample 3, but containing cracks, as shown in FIG. This result, which is surprising in itself, can be explained by the fact that, in this case, it takes longer to dry to obtain a durable layer because of the non-absorbent substrate. This means that SiO in the upper region of the slurry layer₂It leads to a stronger enrichment of nanoparticles, that is, a relatively thick injection membrane in the sense of the present invention. This effect can result in strong drying shrinkage and thus crack formation. When observing the surface layer in the lateral cross-sectional view-although it appears weakly-a membrane layer (injection membrane) with a thickness of about 6 μm can be identified. In this case, coarsely granular SiO₂SiO with particles that are almost completely fine-grained₂Embedded in a compound consisting of, the particles do not penetrate the surface, which is explained by the particularly smooth surface layer of sample 4. Evaluation of the surface texture by photographic analysis shows that in this case, almost 100% of the entire surface is smooth.

However, crack formation has also shown that the initial drying rate is an important parameter for forming an injection membrane of optimum thickness in order to obtain optimal results regarding surface quality. It has been found that the initial drying duration of about 2 minutes to a maximum of 5 minutes to obtain a durable layer is optimal. In connection with this, the absorbency of the substrate is also a crucial parameter.

Drying and sintering are performed as described above based on sample 3. A dense surface was obtained. This indicates that the cracks obtained after drying were limited to the area close to the surface, as much as possible to the injection membrane itself.

<u style="single">Sample 5</u> In further testing, we decided to optimize the results obtained in sample 3. SiO with a thickness of 2 mm on a flat sheet made of absorbent opaque quartz glass with open pores₂A surface layer will be prepared.

Unlike sample 3, the quartz glass sheet was first soaked sideways in an ultrasonic bath for 5 minutes, thus filling the outwardly open pores with substantially water. Subsequently, the quartz glass sheet is lifted in an ultrasonic bath so that the upper surface of the sheet protrudes from the liquid surface of the bath.

With ultrasonic vibration turned on, SiO with a thickness of about 0.8 mm is placed on a quartz glass sheet placed sideways.₂The slurry layer is applied with a blade. At that time, a blade device is used to apply as high a pressure as possible so that the slurry layer is compressed to a thickness of about 0.7 mm.

A thin liquid film is formed on the slurry layer thus applied and compressed, and a homogeneous and closed surface layer is formed upon subsequent drying in air. After further drying as described based on Sample 3, the layer is substantially smooth as shown in FIG. 5 and its appearance is comparable to that of Sample 3. Again, photographic analysis of the surface texture reveals a high smooth proportion in the range of about 75% of the total surface.

When the surface layer is observed under a microscope as a lateral cut surface-although it appears weakly-a membrane layer (injection membrane) with a thickness of about 4 μm appears. Inside the injection membrane, SiO₂The proportion of nanoparticles is notably higher than the proportion in the rest of the green layer, and is clearly above 70% of the volume of the injection membrane.

SiO enriched in the surface area of the dried slurry layer₂The nanoparticles exhibit high sintering activity and improve layer compression during subsequent sintering. Approximately 2.1 g / cm, equivalent to 5% porosity³A crack-free homogeneous layer of opaque quartz glass with a thickness of is obtained.

<u style="single">Sample 6</u> In a further test, we decided to investigate whether the results obtained in Sample 5 could be obtained even with other absorbent substrates. For this purpose, a porous SiO with a thickness of 1.5 mm is first placed on a sheet made of quartz glass.₂A surface layer composed of was prepared. The porous surface layer was prepared according to Sample 2 by spraying the slurry layer and subsequent drying and sintering of the slurry layer. The volume of this surface layer, and thus its water absorption, is slightly lower than that of Sample 5.

On the surface layer thus pretreated, a slurry layer was prepared using the slurry of Sample 5 and the drying and sintering treatment techniques described based on this sample. No notable difference was confirmed in the surface layer thus obtained as compared with the surface layer of Sample 5. The graph in Fig. 6 shows the surface roughness (R) of samples 1 to 5 of the green layer._aValue), specifically, R measured at different measurement points_aThe maximum value, minimum value and average value (average surface roughness) of the values are shown respectively.

As a result, it can be seen that the slurry layer sprayed after drying shows a relatively high average roughness as compared with the surface layer produced by the blade treatment. The lowest surface roughness is obtained with sample 4, which, however, shows cracks. These cracks, however, are R_aThere is no notable effect on the value. The components having this surface sample can be used in applications where a smooth, while non-dense surface is required. Sample 5 has the second lowest surface roughness value.

The micro-CT images shown in FIGS. 7 and 8 show the cross sections of the green layers of samples 1 and 5 at the same magnification (about 50 times) each time. Accordingly, sample 1 (FIG. 8) shows a bumpy and irregular surface, whereas the surface of sample 5 (FIG. 7) is substantially smooth. Close observation reveals some thin areas of the surface, which can be attributed to the injection membrane.

This surface layer is more clearly identifiable when viewed at the fragment edge of sample 5 according to FIG. In this case, the injection membrane 1, which has a particularly fine-grained structure and a thickness d of about 9 μm, clearly stands out from the rest of the layer 2 which has a coarser granular structure. Inside the injection membrane 1, coarsely granular SiO₂The particles are almost completely, especially SiO₂Fine-grained SiO formed from nanoparticles and their agglomerates₂Embedded in a compound consisting of. The volume ratio of this compound in an injection membrane about 9 μm thick is greater than 75% overall. Coarse granular SiO₂It is also identifiable that the particles can barely penetrate the surface, resulting in a substantially smooth and dense surface layer, as shown in Samples 3 and 6.

Table 1 summarizes the characteristic manufacturing parameters and measurement results of Samples 1 to 5.

<tables num="1"><img id="000002" he="109" wi="158" file="JP5940134B2_D0001.tif" img-format="tif" img-content="drawing" /></tables> In the table, DI is deionized water (as part of the dispersion), ET is ethanol (as part of the dispersion), and F is the mass ratio of solids to the total mass of the dispersion, Soot. Is SiO in the solid content of the dispersion system₂Mass ratio of nanoparticles, R_aMeans the average value of the surface roughness of the surface after sintering, and Q means whether a suitable surface was obtained after sintering.

A comparison of samples 1, 3, 5 and 6 on the one hand and a comparison of samples 2 and 4 on the other show that the surface roughness is substantially dependent on the type of substrate, rather decisively. The important thing is the coating technique. At that time, the drying duration of the slurry layer or the effective segregation time until a durable layer is formed is important. The drying transit time is determined by the interaction with the moisture content of the dispersion, the coating technique and thickness of the slurry layer. A long drying duration is preferable in situations where a dense injection film is basically formed and a smooth surface is involved. However, if the drying duration is very long, as Sample 4 shows, correspondingly very dense injection membranes and crack formation during drying can occur.

1 injection membrane, 2 layers, d thickness