Coating processes – Direct application of electrical – magnetic – wave – or... – Electrostatic charge – field – or force utilized
Reexamination Certificate
2001-11-07
2004-01-06
Chen, Bret (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Electrostatic charge, field, or force utilized
C427S481000, C427S189000
Reexamination Certificate
active
06673396
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for producing an SiO
2
blank, wherein SiO
2
particles are formed in a burner flame of a deposition burner and deposited under the effect of an electrical field on a substrate.
Furthermore, the present invention relates to a method for producing an SiO
2
blank, using several deposition burners which are arranged at an equal distance from one another along a substrate rotating about a longitudinal axis and which have each assigned thereto a burner flame in which SiO
2
particles are formed and deposited on the substrate under the effect of an electrical field.
Moreover, the present also relates to an apparatus for producing an SiO
2
blank, the apparatus comprising a substrate, at least one deposition burner for producing SiO
2
particles in a burner flame assigned to the deposition burner, a measuring device for sensing a size (or shape) in the area of a deposition surface of the SiO
2
blank, and a pair of electrodes connected to a source of voltage for producing an electrical field which is operative in the area of the burner flame.
BACKGROUND OF THE INVENTION
A method and an apparatus of the above-mentioned type are known from DE-A1 196 29 170. For producing a porous cylindrical SiO
2
body (hereinafter also designated as a “soot body”), SiO
2
particles are produced in the oxyhydrogen gas flame of a plurality of hydrolysis burners and are deposited layerwise on a horizontally oriented carrier tube rotating about its longitudinal axis. The burners are mounted at an equal distance of about 15 cm relative to one another on a burner block extending in parallel with the longitudinal axis of the carrier tube. The burner block is reciprocated along the developing porous cylindrical preform between a left and a right reversal point by means of a controllable displacement device, with the amplitude of the translatory movement of the burner block being smaller than the length of the preform.
To increase the deposition rate of the SiO
2
particles, an electrical field is applied between the carrier tube and the hydrolysis burners. To this end an electrode is provided in the inner bore of the carrier tube, the second electrode (outer electrode) is formed by an elongated metallic mesh which is either connected to the burner block or arranged between the hydrolysis burners and the carrier tube. A potential difference of a few 10 kV is maintained between the two electrodes by means of an electrical DC source. The electrical field produces an electrostatic charge of the dielectric SiO
2
particles which are thereby accelerated towards the soot body. This results in an improvement of the deposition efficiency in comparison with a conventional method without said electrostatic charge.
In the manufacture of such soot bodies as a starting material for preforms for optical fibers, the homogeneity of the soot body poses problems as a rule. To achieve deposition conditions that are as uniform as possible and to obtain an axially homogeneous soot body, charge points which copy the spatial shape of the soot body are produced in the known method by means of the outer electrode. However, it is not possible to avoid local overheating of the soot body caused, in particular, by the body being heated twice in quick succession upon reversal of the direction of movement in the area of the reversal points. This thermal effect is particularly noticed during use of a burner block, for local overheating and thus axial density and mass variations in the blank may take place over the entire blank surface due to the many reversal points of the burner. Density and mass variations, however, result in areas of different reactivity in the blank; these are particularly noticed in the subsequent chemical reactions during processing into a preform and may e.g. leave inhomogeneities after sintering.
Constructional differences of different deposition burners result in different depositions rates and in different temperatures of the burner flame, which may also be noticed in density and mass changes within the blank. This requires considerable adjusting and correcting efforts after every exchange of a deposition burner.
Moreover, on account of the mass and size of the SiO
2
blank which are increasing in the course of the deposition process, there arise thermal effects which may influence the deposition rate and the temperature during deposition and may thus also lead to radial or axial changes in the density or mass of the blank.
SUMMARY OF THE INVENTION
It is therefore the object of the present invention to indicate a simple and inexpensive method by means of which blanks can be produced with a predetermined, in particular axially homogeneous mass and density distribution, and to provide a simple apparatus which is suited for carrying out the method.
As for the method, this object, starting on the one hand from the above-mentioned method, is achieved according to the invention in that the geometrical shape of said burner flame is adjusted by the effect of said electrical field in dependence upon the geometrical parameter of the deposition surface.
In the method according to the invention an electrical field is produced which acts on the geometrical shape of the burner flame, thereby changing the same either by changing the field strength operative in the area of the burner flame, or by measures which change the direction of the field lines in the area of the burner flame. A change in the field strength acting on the burner flame or a change in the direction of the field lines effects a change in the burner flame geometry.
This effect of the electrical field on the geometry of the burner flame is used for achieving a homogeneous density and mass distribution within the SiO
2
blank by adjusting the geometrical shape of the burner flame in dependence upon a geometrical parameter of a burner flame-assigned deposition surface of the substrate. Geometrical parameter in this sense means a geometrical dimension, for instance a length or a diameter, or a geometrical shape, for instance a curvature. The expression “size or shape of the deposition surface” will also be used hereinafter as a more illustrative synonym for the “geometrical parameter of the deposition surface”.
For instance, local deviations in the shape or size of the deposition surface can be compensated or avoided in that the width of the burner flame is decreased or increased accordingly by the effect of the electrical field in this area. To this end the shape or size of the deposition surface is sensed. Such a sensing operation may be carried out either during or after the deposition process. In the last-mentioned case, the deposition surface of the blank is measured, so that in the case of local deviations in the size or shape of the deposition surface, these can be compensated in the next process by suitable changes in the burner flame in this area of the deposition surface. Size or shape of the deposition surface is sensed by directly measuring the respective dimension, or by determining a characteristic value which can be correlated with the geometrical dimension of the deposition surface, e.g. the deposition duration, the weight or volume of the blank.
Moreover, the method according to the invention permits a continuous adaptation of the geometrical shape of the burner flame to the size or shape of the deposition surface. For instance, it is possible to adjust a small burner flame at the beginning of a deposition process and then to expand the flame continuously or stepwise with an increasing outer diameter of the blank. Such a procedure can be used not only for a homogenization as to the radial and axial mass and density distribution, but also helps to increase the deposition efficiency of the SiO
2
particle deposition.
The adjustment of the electrical field and the local size or shape of the deposition surface are correlated with one another such that a predetermined electrical field acts on the burner flame in dependence upon the size or shape of the deposition surface such that a selec
Chen Bret
Heraeus Quarzglas GmbH & Co. KG
Tiajoloff Andrew L.
Tiajoloff & Kelly
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