Compositions: ceramic – Ceramic compositions – Glass compositions – compositions containing glass other than...
Reexamination Certificate
2002-03-21
2003-12-09
Sample, David (Department: 1755)
Compositions: ceramic
Ceramic compositions
Glass compositions, compositions containing glass other than...
C423S335000
Reexamination Certificate
active
06660671
ABSTRACT:
The present invention relates to a method of producing a composite material having a high SiO
2
content, wherein quartz glass grains are embedded in an SiO
2
-containing matrix, the method comprising the following steps: preparing a suspension from a particle mixture consisting of finely divided SiO
2
powder having at least two different particle fractions and of said quartz glass grains, forming the suspension into a green compact, and sintering the green compact.
Furthermore, the present invention relates to a composite material comprising an SiO
2
-containing matrix in which glass grains are embedded.
Furthermore, the present invention relates to a permanent mold made of the composite material according to the invention.
Structural members consisting of a composite material having a high SiO
2
content of at least 99% by wt. are characterized by a small coefficient of expansion and by high chemical resistance. Such structural members are e.g. used in metallurgy, e.g. in the form of crucibles for melting metals, nozzles, protective tubes or pouring channels.
A composite material according to the above-mentioned type and a method for producing a refractory sintered article of silica glass is known from DE 693 06 169 (T2). This document describes a method in which two SiO
2
powders with different particle sizes are used as the starting materials having a binder phase for a further SiO
2
-containing component in the form of coarse SiO
2
grains with a grain size between 40 &mgr;m and 1000 &mgr;m. The two more fine-grained SiO
2
powders are present, on the one hand, as quartz dust which is formed from substantially spherical particles, or as fine-grained SiO
2
particles of a particle size below 40 &mgr;m. These components are premixed in a dry-milling process, and a slip is then made therefrom under addition of a stabilizer. The weight percent of the individual components are, in the order of their above indication, 54% (coarse SiO
2
grains), 33% (fine-grained SiO
2
particles) and 13% (quartz dust). The slip is degassed under vacuum and cast into a plaster mold. The green compact produced thereby is dried and sintered in a furnace at 1050° C. to obtain the composite structural member. Coarse quartz glass grains which are embedded in a relatively continuous matrix of fine particles and of spherical particles of quartz dust are typical of the microstructure of the structural member. The structural member has an open porosity of 13%, and its density is at 1.91 g/cm
3
. The crystallographic analysis shows a cristobalite content of less than 2%.
On account of its open, i.e. continuous, porosity, the known composite material cannot be used in unlimited form for structural members in the case of which density or high purity are of importance. Metallic melts can penetrate into the wall of the structural member through the pores and result in leakage. In principle, a higher density and a lower porosity could be achieved through a higher sintering temperature or a longer sintering period, but an increased cristobalite formation would have to be accepted. The formation of cristobalite is caused by impurities of the starting components or by possible additives such as stabilizers and sintering adjuvants, and rapidly progresses at elevated temperatures. This, however, would entail a reduced resistance to temperature changes and a lower strength of the composite material.
It is therefore the object of the present invention to indicate a method which permits an inexpensive production of a composite material having a high SiO
2
content of at least 99% by wt. which is characterized by a high resistance to temperature changes together with a high density, and to provide a composite material which is suited for applications where temperature resistance, density and high purity are of importance, and to indicate a suitable use thereof.
As for the method, this object starting from the above-mentioned method is achieved according to the invention in that the matrix has an SiO
2
content of at least 99% by wt. and is formed from at least a first and a second particle fraction, each of said particle fractions being present as granules of nanoscale, amorphous, synthetically produced SiO
2
primary particles having a mean primary particle size of less than 100 nm.
For the preparation of the composite material, exclusive use is made of amorphous starting substances in the method according to the invention and also in the above-described known method, so that in the composite material the (vitreous) quartz glass grains are embedded in a matrix (binding phase) which also consists essentially of quartz glass. Due to the fact that the two essential components of the composite material, namely “matrix” and “quartz glass grains”, consist of amorphous SiO
2
, stresses caused by different coefficients of expansion are avoided. The matrix is formed from at least two different particle fractions which, in contrast to the known method, are each formed as granules of nanoscale, amorphous, synthetically produced SiO
2
primary particles.
The quartz glass grains serve as a filler. They are devoid of any open porosity and restrict shrinkage of the green compact during sintering. A closed porosity of the quartz glass grains is not detrimental to the method according to the invention and may be necessary, for instance for setting the desired opacity of the material.
The porosity of the composite material is essentially defined by the matrix. In the method according to the invention, the matrix is substantially formed by sinter-active components, as will be explained in more detail in the following. The use of at least two different particle fractions for forming the matrix permits a higher packing density in the green compact together with an optimization of density and strength of the composite material.
Said optimized condition is brought about by the particle fractions differing from one another, either in the size of the granules, their density or with respect to their sinterability; density and sinterability of the respective particle fraction are here substantially set by thermal pretreatment.
Due to the fact that at least the first particle fraction and the second particle fraction consist of granules formed from nanoscale, amorphous SiO
2
primary particles having a mean primary particle size of less than 100 nm, compaction and consolidation that are of advantage to the later sintering process are already observed in the green compact stage. This is due to a certain solubility and movability of the individual SiO
2
primary particles in the suspension, which contributes to the so-called “neck formation” between neighboring granules in the green compact. During drying of the SiO
2
-enriched liquid phase in the area of the “necks”, said necks consolidate, resulting in a firm connection of the individual granular particles and in a compaction and consolidation of the green compact which facilitate the subsequent sintering process and thus yield a relatively high density of the composite material, i.e. already at a low sintering temperature. The solubility of individual primary particles and of the granules formed therefrom in the suspension is the more pronounced the greater the specific surface of the granule is. These effects based on the nanoscale amorphous SiO
2
primary particles have a stabilizing influence on both the green compact and the composite material. Therefore, the method according to the invention makes it possible to form the green compact without the help of binders and stabilizers and permits a sintering of the composite material without the addition of sintering adjuvants. Thus, the impurities in the composite material that accompany the use of such an additive are avoided.
Hence, due to their high sintering activity, the nanoscale, amorphous SiO
2
primary particles contribute to a high density, mechanical strength and purity of the composite material.
This effect of the amorphous SiO
2
primary particles which is conducive to mechanical strength and density is intensified in that the m
Gertig Udo
Koeppler Rainer
Leist Johann
Werdecker Waltraud
Bolden Elizabeth A
Heraeus Quarzglas GmbH & Co. KG.
Sample David
Tiajoloff Andrew L.
Tiajoloff & Kelly
LandOfFree
Method of producing a composite material having a high SiO2... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method of producing a composite material having a high SiO2..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method of producing a composite material having a high SiO2... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3177571