Electron beam generator

Details Electron beam generator Electron beam generator

2000-11-02

2003-04-15

Patel, Nimeshkumar D. (Department: 2879)

Electric lamp and discharge devices

Fluent material supply or flow directing means

C313S359100, C313S362100, C313S231010, C313S231310, C118S7230EB, C118S7230VE, C118S7230HC, C315S111810, C315S111910

Reexamination Certificate

active

06548946

ABSTRACT:

FIELD OF THE INVENTION
This invention generally relates to electron beam generators, particularly of the type used in electron beam physical vapor deposition apparatuses to deposit ceramic coatings. More particularly, this invention is directed to an electron beam generator that exhibits improved service life at high operating temperatures.
BACKGROUND OF THE INVENTION
Electron beam physical vapor deposition (EBPVD) is a well-known process for producing ceramic coatings, such as thermal barrier coatings (TBC) for the high-temperature components of gas turbine engines. Various ceramic materials have been used as TBC's, particularly zirconia (ZrO
2
) stabilized by yttria (Y
2
O
3
), magnesia (MgO) or other oxides. Advantageously, TBC's can be deposited by EBPVD to have a columnar grain structure that is able to expand with its underlying substrate without causing damaging stresses that lead to spallation, and therefore exhibits enhanced strain tolerance.
Processes for producing TBC by EBPVD generally entail heating a component to be coated to a temperature of about 1000° C. or more within a partially evacuated coating chamber. During coating, the component is supported above an ingot of the ceramic coating material (e.g., YSZ), and an electron beam generated by an electron beam (EB) gun is projected onto the ingot to melt the surface of the ingot and produce a vapor of the coating material. The vapor then travels upward toward the component and condenses on the component surface to form the desired coating. In order to melt ceramic materials such as YSZ, electron beam guns must be operated at a high voltage (e.g., 35 kV) and power level (e.g., 50 to 120 kW). The EB gun component that produces the electron beam is a beam generator.
FIG. 1
represents a generator
110
that is commercially available from ALD Vacuum Technologies, Inc., of East Windsor Conn., USA. The generator
110
has a primary cathode (filament)
140
which produces an electron flux that heats a primary tungsten anode (block)
148
to about 2000° C. The block
148
then serves as a secondary cathode to an external secondary anode (not shown), by which the tungsten block
148
emits an electron beam due to a high voltage between it and the secondary anode. If any connection in the circuit containing the filament
140
becomes resistive due to oxidation, or mechanically opens due to thermal stress, or both, the beam generator
110
ceases to emit, stopping evaporation and terminating the coating process.
The filament circuit contains several bimetallic contacts in close proximity to the hottest section of the generator
110
. The two metals most widely used are copper and molybdenum, the former for its electrical and thermal conductivity and the latter for its high melting point and stability at high temperatures. For example, a conductor rod
112
that delivers current to the filament
140
is most often copper. A molybdenum ion catcher
128
has a first end
130
threaded into a bore
126
of the conductor rod
112
, by which a molybdenum spacer
124
is secured to the rod
112
. A first molybdenum filament tower
138
is then secured and electrically connected to the spacer
124
with a threaded stud
160
and copper nut
162
, which clamps a disk-shaped insulator
166
between the spacer
124
and tower
138
. A second molybdenum filament tower
139
is secured with a second stud and nut assembly
160
/
162
to the insulator
166
, between which is clamped a molybdenum mounting plate
164
. As such, both of the filament towers
138
and
139
are secured in place as a result of the spacer
124
being secured to the rod
112
with the ion catcher
128
. The rod
112
, spacer
124
, ion catcher
128
and filament tower
138
constitute a forward leg of the filament circuit. Because of their high temperature environment, the threaded connections can loosen and oxidize during operation due to differing expansion and heat conduction of the two metals. If the ion catcher
128
becomes loose and releases the spacer
124
, the filament circuit opens and the generator
110
cannot be restarted.
The filament tower
139
, a molybdenum cap
144
, the molybdenum mounting plate
164
, a copper fitting
142
and a copper guide tube
134
constitute the return leg of the filament circuit. The molybdenum cap
144
is threaded onto the copper fitting
142
, which is brazed or otherwise permanently attached to the copper guide tube
134
surrounding the conductor rod
112
. The cap
144
clamps the mounting plate
164
to the fitting
142
to complete the filament circuit between the tower
139
and the guide tube
134
. Consequently, if the cap
144
loosens, the filament circuit is open and the generator
110
ceases operating. The threads of the fitting
142
can distort at the high operating temperatures of the generator
110
. In addition, the threaded portion of the cap
144
may bell and crack during extended operation of the generator
110
. If the clamping action between the cap
144
and fitting
142
is lost, the filament circuit opens, again with the result that the generator
110
shuts down and cannot be restarted.
In view of the above, it can be appreciated that improved service life for an EB gun could be obtained if the reliability of the EB generator
110
mechanical connections could be improved. However, any change in the mechanical design of the generator
110
must be made carefully and tested with caution due to the very high operating voltages, power levels, amperage and operating temperatures involved.
BRIEF SUMMARY OF THE INVENTION
The present invention is an electron beam generator of the type used in an EBPVD apparatus. A feature of the generator is that critical interconnections between individual components are made less prone to the adverse effects of thermal cycling.
The generator of this invention generally includes a conductor rod within a guide tube, generally as done in the prior art. However, the adjacent ends of both the conductor rod and guide tube are configured differently for purposes of the invention. The end of the conductor rod is modified to accept one end of a center conductor member. The opposite end of the center conductor member is formed to have an integrally-formed flange extending radially therefrom. An outer conductor member is secured to the adjacent end of the guide tube. A first tower is secured and electrically connected to the flange of the center conductor member, while a second tower is adjacent the first tower and electrically connected to the outer conductor member. A filament is mounted to and between the first and second towers, and a member is positioned adjacent to the filament for generating an electron beam when a sufficient current is applied to the filament via the conductor rod, the center conductor member, the flange and the first tower, which constitute a forward leg of the filament circuit. A return leg of the filament circuit includes the second tower and the guide tube, interconnected by the outer conductor member.
An important feature of the invention is the elimination of the discrete spacer
124
between the ion catcher
128
and the conductor rod
112
of the prior art generator
110
of FIG.
1
. Instead, the function of the spacer
124
is performed by the integral flange of the center conductor member, which serves as an intermediate connector between the first tower and the conductor rod. By eliminating the need for a discrete spacer and therefore the possibility of it loosening, the center conductor member is able to considerably reduce the possibility of an open circuit occurring between the ion catcher and the conductor rod as compared to the prior art generator
110
.
According to one aspect of the invention, at least one and preferably each of the center conductor member, first and second towers, outer conductor member and cap are formed of stainless steel, instead of the conventional molybdenum. As a result, the differences in coefficient of thermal expansion are less between the stainless steel components and the conventional c

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