Coating processes – Spray coating utilizing flame or plasma heat – Silicon containing coating
Reexamination Certificate
2001-11-07
2004-12-14
Meeks, Timothy (Department: 1762)
Coating processes
Spray coating utilizing flame or plasma heat
Silicon containing coating
C427S585000, C427S255370, C427S255500, C427S163200, C065S421000, C065S425000, C065S441000
Reexamination Certificate
active
06830781
ABSTRACT:
The present invention relates to a method for producing an SiO
2
blank by forming SiO
2
particles in a burner flame assigned to a deposition burner and by depositing said particles under the effect of an electrical field on a carrier rotating about its longitudinal axis, the at least one deposition burner being reciprocated during a predetermined sequence of movement along the developing blank between turn-around points.
Furthermore, the present invention relates to an apparatus for producing an SiO
2
blank, the apparatus comprising a carrier which is rotatable about its longitudinal axis, at least one deposition burner for producing SiO
2
particles in a burner flame assigned to the deposition burner, a drive device by means of which the deposition burner can be reciprocated along the carrier during a predetermined sequence of movement along the developing blank over a path of movement between turn-around points, 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.
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 turn-around 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 by the body being heated twice in quick succession upon reversal of the direction of movement in the area of the turn-around points of the burner movement. This thermal effect is particularly noticed during use of a burner block, for local overheating and thus axial density variations in the blank may take place over the entire blank surface due to the many turn-around points of the burner. Density variations 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.
To avoid such local density variations, it is suggested in EP-A1 476 218 that both the right and the left turn-around points should be displaced by a few millimeters after each burner passage. Although the local density variations created at the turn-around points are distributed more uniformly in the preform, they are not avoided altogether. Moreover, the local distribution of the turn-around points requires great efforts with respect to apparatus and technical control.
Therefore, DE-A1 196 28 958 suggests another method in which an overheating of the preform in the areas around the turn-around points is prevented or reduced by increasing the circumferential speed of the developing preform in said areas, by lowering the flame temperature of the deposition burners or by increasing the distance of the deposition burners from the preform surface. An increase in temperature of the preform surface in the area of the turn-around points can be compensated entirely or in part by each of said measures or by a combination of the measures, so that the preform is subjected over its whole length to a heating power that is as high as possible in time and space. Axial density gradients in the preform can thereby be avoided to a substantial degree. These measures, however, require a time change in gas flows, distances or relative speeds in the area of the turn-around points, which might entail undesired changes in the deposition rate.
It is therefore the object of the present invention to indicate a method by means of which the blank can be produced with a predetermined, in particular axially homogeneous density and mass distribution, and to provide a simple apparatus which is suited for carrying out the method.
As for the method, this object starting from the above-mentioned method is achieved according to the invention in that the geometrical shape of the burner flame is varied by the electrical field in dependence upon the position of the deposition burner during the sequence of movement.
When looking at the surface temperature of the blank in the area of the point of impact of the burner flame during movement of the deposition burner from the one turn-around point to the other one (here designated as “sequence of movement”), one will notice that despite a constant temperature of the burner flame the surface temperature is normally not kept constant, but is locally different and, moreover, depends on the direction of movement of the deposition burner. Each position of the deposition burner during its movement towards one of the turn-around points and each position during its movement towards the other turn-around point can have assigned thereto corresponding surface temperatures which, when put together one after the other, result in a temperature profile typical of the deposition burner and the specific deposition parameters. This temperature profile depends, inter alia, on the temperature and geometry of the burner flame.
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 according to the invention for modifying the above-mentioned temperature profile typical of a deposition burner by varying the geometrical shape of the burner flame during the sequence of movement of the deposition burner. For instance, local temperature peaks or valleys of a temperature profile can be compensated or avoided by decreasing or increasing the width of the burner flame by the effect of the electrical field accordingly.
The electrical field is adjusted either in dependence upon the position of the deposition burner or in dependence upon a measured variable which can be correlated with the sequence of movement of the deposition burner, e.g. the surface temperature, the volume, the mass or the diameter of the developing blank. When the electrical field is adjusted in d
Heraeus Quarzglas GmbH & Co. KG
Meeks Timothy
Tiajoloff & Kelly
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