U.S. patent application number 11/542268 was filed with the patent office on 2008-04-03 for process for producing a rotary anode and the anode produced by such process.
This patent application is currently assigned to H.C. Starck Inc.. Invention is credited to Leah F. Haywiser, Leonid N. Shekhter.
Application Number | 20080081122 11/542268 |
Document ID | / |
Family ID | 39283721 |
Filed Date | 2008-04-03 |
United States Patent
Application |
20080081122 |
Kind Code |
A1 |
Haywiser; Leah F. ; et
al. |
April 3, 2008 |
Process for producing a rotary anode and the anode produced by such
process
Abstract
The present invention is directed to an improved process for
manufacturing a rotary anode for an x-ray tube, said rotary anode
comprising a molybdenum support member on which a target layer
consisting essentially of tungsten or a tungsten-rhenium alloy is
provided by plasma spraying, the improvement comprising: a)
preheating the support member to a temperature of from 1150.degree.
C. to 1600.degree. C., b) placing the support member in a gaseous
atmosphere containing hydrogen and having a pressure of from 0.5 to
0.9 bars and wherein hydrogen is present in a molar ratio of
hydrogen to tungsten dioxide of from 5:1 to 50:1, and c) plasma
spraying the target layer onto the support layer in said gaseous
atmosphere. The invention is also directed to the anode produced by
the process.
Inventors: |
Haywiser; Leah F.;
(Arlington, MA) ; Shekhter; Leonid N.; (Ashland,
MA) |
Correspondence
Address: |
BAYER MATERIAL SCIENCE LLC
100 BAYER ROAD
PITTSBURGH
PA
15205
US
|
Assignee: |
H.C. Starck Inc.
|
Family ID: |
39283721 |
Appl. No.: |
11/542268 |
Filed: |
October 3, 2006 |
Current U.S.
Class: |
427/455 |
Current CPC
Class: |
H01J 35/108 20130101;
C23C 4/134 20160101; C23C 4/02 20130101 |
Class at
Publication: |
427/455 |
International
Class: |
C23C 4/08 20060101
C23C004/08 |
Claims
1. In a process for manufacturing a rotary anode for an x-ray tube,
said rotary anode comprising a molybdenum support member on which a
target layer consisting essentially of tungsten or a
tungsten-rhenium alloy is provided by plasma spraying, the
improvement comprising: a) preheating the support member to a
temperature of from 1150.degree. C. to 1600.degree. C., b) placing
the support member in a gaseous atmosphere containing hydrogen and
having a pressure of from 0.5 to 0.9 bars and wherein hydrogen is
present in a molar ratio of hydrogen to tungsten dioxide of from
5:1 to 50:1, and c) plasma spraying the target layer onto the
support layer in said gaseous atmosphere.
2. The process of claim 1, wherein the support member is preheated
to a temperature of from 1300.degree. C. to 1500.degree. C.
3. The process of claim 1, wherein said pressure is from 0.7 to 0.9
bars.
4. The process of claim 1, wherein said ratio is from 10:1 to
30:1.
5. The rotary anode produced according to the process of claim 1.
Description
BACKGROUND OF THE INVENTION
[0001] Tungsten is used as a characteristic x-ray radiation
generating material, for x-ray anode assemblies in CT scanners. Due
to tungsten's high density and high atomic number, it is possible
to produce gamma radiation with narrow frequency range. This
explains why tungsten is the primary material of choice for x-ray
generation. However, due to tungsten's density, and the stresses
experienced during high rotational speed, tungsten is not the
choice material of construction for the entire anode. Molybdenum
(which has a high enthalpy, high melting point, lower density, and
is easier to machine) is used as the base material for
construction. The tungsten is then applied as a thin track along
the surface of electron beam incidence to generate the x-radiation.
Normally up to 10% by weight of rhenium is added to the tungsten to
improve its malleability. Rhenium is less brittle than tungsten,
and is the next element in the periodic system after tungsten,
which explains its widespread use as an alloying element for
tungsten.
[0002] There are three conventional methods for the application of
the tungsten-rhenium track. The first method is via powder
metallurgy, where the tungsten-rhenium powder is tape cast, slip
cast, roll compacted, thermally sprayed, or waterfall processed
into a track which is pressed, sintered and forged along with the
molybdenum substrate (see U.S. Pat. No. 6,428,904). The '904 patent
indicates that the process described therein improves the evenness
of the surface and the interface between the track and the x-ray
target substrate.
[0003] A second method describes the use of chemical vapor
deposition (CVD) to apply the tungsten and molybdenum in a
non-oxidizing atmosphere (see U.S. Pat. No. 4,461,020). The '020
patent indicates that this technique creates an improved bond and
is an easily reproducible method.
[0004] The third and most prevalent method is plasma spray coating
of the tungsten-rhenium track (see, e.g., U.S. Pat. Nos. 4,090,103,
4,390,368, 4,534,993 and 4,641,333). The '993 patent indicates that
the plasma spray coating is carried out in a reduced pressure
atmosphere of from 20 to 70 kPa. The '993 patent also indicates
that in order to obtain optimum density, particle sizes of at most
45 .mu.m be used.
[0005] U.S. Pat. No. 6,132,812 describes a plasma spray technique
using inductive vacuum plasma spraying. The '812 patent indicates
that the process described therein allows for increased residence
time, which improves the ability of all particle sizes to melt and
be deposited on the substrate in the molten state. It is also
indicated that the process results in improved fatigue crack
strength, and improved density.
[0006] All of these techniques fail to address causes for porosity,
as well as the impact of oxygen on the density and propensity to
degas.
DESCRIPTION OF THE INVENTION
[0007] The present invention is directed to an improved process for
manufacturing a rotary anode for an x-ray tube, said rotary anode
comprising a molybdenum support member on which a target layer
consisting essentially of tungsten or a tungsten-rhenium alloy is
provided by plasma spraying, the improvement comprising: [0008] a)
preheating the support member to a temperature of from 1150.degree.
C. to 1600.degree. C., [0009] b) placing the support member in a
gaseous atmosphere containing hydrogen and having a pressure of
from 0.5 to 0.9 bars and wherein hydrogen is present in a molar
ratio of hydrogen to tungsten dioxide of from 5:1 to 50:1, and
[0010] c) plasma spraying the target layer onto the support layer
in said gaseous atmosphere.
[0011] The invention is also directed to the anode produced by the
improved process.
[0012] The general method of manufacturing rotary anodes using a
plasma spray coating technique is known in the art and is described
in U.S. Pat. Nos. 4,090,103, 4,390,368, 4,534,993 and 4,641,333,
the disclosures of which are herein incorporated by reference.
[0013] The use of hydrogen and a slightly reduced pressure will
improve the density, adherence, and purity of the tungsten or
tungsten-rhenium track. Hydrogen will serve to remove oxygen from
the surfaces of tungsten, rhenium, and molybdenum. The oxygen
removal will have two effects--first is to "activate" the surface
of both the coating and the substrate improving adherence and
secondly to remove oxygen prior to deposition resulting in an
improved density and purity of the coating. The slightly reduced
pressure will serve to remove the reactant oxygen bearing species.
When referring to "slightly reduced pressure", the closer the
process is too atmospheric the more suppressed is the
volatilization of tungsten oxide bearing species.
[0014] The determination of the amount of oxygen present in the
tungsten or tungsten-rhenium alloy as tungsten dioxide is readily
determined. Techniques for measuring the oxygen content of metal
powders are known and include, for example, total x-ray
fluorescence, secondary ion mass spectroscopy, x-ray photoelectron
spectroscopy and Auger spectroscopy. Suitable analyzers are also
available form LECO Corporation (TC400, TC500 and RO500C series).
Based on the amount of oxygen measured, the actual amount of oxygen
present in the form of the oxide present in the form of tungsten
dioxide can then readily calculated The devices noted report the
oxygen content as a % by weight per one gram sample. The moles of
tungsten dioxide can then be calculated according to the following
formula:
Mole of tungsten dioxide=[OC times WGT]/32
where OC is the % by weight of oxygen per 1 gram sample, WGT is the
total weight of the powder to be sprayed and 32 is the molecular
weight of oxygen.
[0015] The molybdenum support member is preheated to a temperature
of from 1150.degree. C. to 1600.degree. C. (preferably from
1300.degree. C. to 1500.degree. C.) and placed in a gaseous
atmosphere containing hydrogen and having a pressure of from 0.5 to
0.9 bars (preferably from 0.7 to 0.9 bars) and wherein hydrogen is
present in a molar ratio of hydrogen to tungsten dioxide of from
5:1 to 50:1 (preferably in a molar ratio of from 10:1 to 30:1). The
tungsten or tungsten-rhenium alloy is the plasma sprayed onto the
support layer.
[0016] Suitable devices for use in plasma spray coating known in
the art and are commercially available. Such devices are included
DC, arc and inductively coupled plasma devises. Such devices are
commercially available from Progressive Technologies, Inc.
(Michigan), Plasma Processes, Inc. (Alabama) and Tekna Plasma
Systems Inc. (Canada).
[0017] As noted above, a hydrogen excess is required. This improves
the reduction of WO.sub.2. As the hydrogen to WO.sub.2 molar ratio
increases, the temperature at which complete reduction is possible
decreases, and the temperature at which the volatilization becomes
detectable increases. At a hydrogen to WO.sub.2 molar ratio of
50:1, the temperature at which volatility of tungsten oxide species
becomes detectable is almost 3000.degree. C.; however, further
temperature increase leads to increased volatility of elemental
tungsten. At a hydrogen to WO.sub.2 ratio of 75, no further
increase in temperature for onset of volatility is noted, but
tungsten metal evaporation increases.
[0018] In the presently claimed process, hydrogen is intentionally
required to remove oxide species and activate the molybdenum
substrate. The activation of molybdenum substrate and powder
surfaces will also lead to an increase in the adherence of the
tungsten or tungsten-rhenium alloy coating to the molybdenum
substrate. The process effectively removes oxygen from all the
metals and would increase the density of the coating. The process
also will prevent any future degassing that would occur if oxide
species were present in the coating.
[0019] Although illustrated and described herein with reference to
certain specific embodiments, the present invention is not intended
to be limited to the details described. Various modifications may
be made within the scope and range of equivalents of the claims
that follow without departing from the spirit of the invention.
* * * * *