Electric heating – Metal heating – By arc
Reexamination Certificate
2001-03-12
2001-12-18
Walberg, Teresa (Department: 3742)
Electric heating
Metal heating
By arc
C219S121470, C219S121510, C427S446000
Reexamination Certificate
active
06331689
ABSTRACT:
BACKGROUND
1. Field of the Invention
The invention relates to a method for producing a powder aerosol, which can be fed in metered form via a gas stream to a plasma burner, at a constant mass flow rate, without any aggregates. In addition, the invention also relates to an apparatus for producing a powder aerosol having a container which is connected via an ultrasound horn to an ultrasound transmitter and which has a feed tube and an outlet tube.
2. Related Art
A high-power plasma burner, with radio-frequency excitation, can be used to completely vaporize substances which are in the form of powder, in a short time, and then deposit, from the vapor phase, a coating with a predetermined composition onto a substrate, with a high growth rate. A first application for this method, which is known as plasma flash evaporation, is the evaporation of fine-grained powder composed of yttrium-barium-copper oxide, with subsequent deposition of a high-temperature superconductor layer (HTSL). Another major application is the coating of components, for example the application of a protective coating to a turbine blade.
In order to propel the powder into the plasma, the powder is first introduced into a propellant gas, so that an aerosol is produced which can be passed into the plasma burner. The aerosol must be free of aggregates of powder particles which cannot be vaporized, that is to say aggregates formed from individual particles. Preferably, the powder feed process is reproducible and uniform, that is to say free of variations with time.
Various methods, associated with the general plasma spraying technique, are known for producing the aerosol required for plasma flash evaporation. For example, a disk feed device operates in such a way that an amount of powder which is defined by the dimensions of a metering groove is deposited continuously, by means of the metering groove, on a slowly rotating disk. The powder, which revolves with the disk, is moved by a wiping apparatus to the edge of the disk where it falls into a retaining apparatus, where it is retained within a flowing gas stream. In the case of powders whose grain size is very fine, for example in a range equal to or less than 20 &mgr;m, the dispersion of the powder is highly restricted by the forces which act between the particles. This leads to aggregates with dimensions in the millimeter range entering the gas stream at certain time intervals rather than a continuous flow of discrete, individual powder grains, which are separated from one another, as required. Even if the aggregates are broken down within the gas stream during transportation to the plasma burner, the lumpy structure of the introduced powder results in fluctuations in the mass flow rate which are not acceptable, that is to say a variation in the amount of powder introduced into the plasma over time. Screw-type or worm-type feed devices, which are also known from the prior art, and in which the powder is fed by a worm gear to a mixing area where it enters the propellant gas stream, have the same disadvantage.
In the case of a brush metering device, the powder, which is formed into a plug in the form of a cylindrical body, is pushed forward through a tube to a rotating metal brush which removes from the plug body an amount of powder proportional to the feed rate. A gas stream passing through the brush area picks up the powder and transports it to the point at which it is required. Experience has shown that the process of rubbing the powder off the plug body results in aggregates which cannot be vaporized.
WO 96/03216 A1 discloses a powder feed device which operates on the ultrasound principle. In this case, the powder to be fed has ultrasound applied to it in a vessel in which it is also stored, is swirled, its accumulations are broken down, and it is picked up by a gas flow passing through the vessel. Experiments have shown that the feed process is reproducible and is free of fluctuations. This method is limited by the fact that only a limited amount of powder can be stored and fed. In particular, the feed rate, that is to say the mass fed per unit time, can in this case not be adjusted independently of the gas flow rate, since the gas flow also contributes to the swirling of the powder, in addition to the effect of the ultrasound.
Against this background, the object of the invention is to improve the preparation of the aerosol and to provide an associated apparatus and to specify a suitable application.
SUMMARY OF THE INVENTION
In accordance with the invention, mechanical means are used to convert a powder to an aerosol. For the purposes of the invention, a gas stream is fed with the powder aerosol to a plasma burner through a storage or buffer means to which ultrasound can be advantageously applied.
The invention assures the powder aerosol is present in the gas stream with a uniform, fine particle distribution. In the process, the powder aerosol, which has been treated in this way, uniformly has its aggregates broken down, and any initial fluctuation in the flow rate of the powder is stabilized.
In a preferred embodiment apparatus for carrying out the invention employs a feed tube connected to a mechanical powder feed device that dispenses the powder within a gas stream flowing within the feed tube. The feed tube is connected at a downstream end to a storage and/or buffer vessel to which ultrasound is applied A vessel outlet conduit conveys the gas stream to the plasma burner. The powder aerosol enters the volume formed by the ultrasound vessel, is made more uniform by the effect of the ultrasound, and leaves the ultrasound vessel as a more uniform and continuous aerosol flow and is fed to a plasma burner. In this case, the vessel is preferably closed at the top by a cover. In particular, the vessel may at the same time contain the ultrasound horn.
The invention thus makes it possible for a very fine powder to be introduced into a plasma in any desired amount continuously, without aggregates, reproducibly and without any variation in the mass flow rate of the powder. This is achieved by the combination of a disk, worm or brush feed device using the ultrasound principle on which WO 96/03216 A1 is based.
The invention is used for the coating of components and is particularly suitable for applying a protective coating to turbine blades. In this case, it is particularly advantageous that suitable coating thicknesses can be deposited in reasonable times due to the slight relative movement which is possible between the apparatus according to the invention and the turbine blades.
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Branston David-Walter
Lins Guenter
Verieger Jobst
Eckert Seamans Cherin & Mellott , LLC
Siemens Aktiengesellschaft
Van Quang
Walberg Teresa
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