U.S. patent number 3,898,722 [Application Number 05/347,277] was granted by the patent office on 1975-08-12 for process for forming an electrode.
This patent grant is currently assigned to Xerox Corporation. Invention is credited to D. Paul Foote.
United States Patent |
3,898,722 |
Foote |
August 12, 1975 |
Process for forming an electrode
Abstract
A process for forming an electrode utilized in apparatus for
minimizing the smear of an image produced by xeroradiography. A
sheet comprising a thin layer of dielectric material sandwiched
between two X-ray transparent electrically conductive surfaces is
cut to the desired outside dimensions. A chemically resistant
material is applied to both conductive surfaces in a predetermined
pattern. The sheet is dipped into an etching bath removing material
from the unmasked portions of the conductive surfaces and then
dipped into a neutralizing solution to neutralize the etching
chemical. The sheet is then rinsed, dried and the resistant
material is removed. An adhesive material is coated on one of the
conductive surfaces whereby the sheet, formed into the desired
electrode, is affixed to the lid of a cassette utilized in the
xeroradiography process BACKGROUND OF THE INVENTION Image smear,
due to ion-caused undercutting, is a nonfaithful representation of
the inner structure of a test object manifested as a loss of detail
in image areas of a xeroradiographic reproduction. Image smear
occurs primarily in those low contrast regions of the image which
receive the largest amount of ray energy. Undercutting occurs at an
edge between regions of high and low X-ray exposure. It is well
known that the longer wavelength X-rays are efficient in ionizing
the air near the surface of a xerographic photo-emitting receptor
during exposure. The amount of air ionization is proportional to
the intensity of the X-rays reaching the photoreceptor surface. The
mechanism of ionization of the air includes the direct
photoelectric action of both X-ray photons on the air and the
secondary photo-injected electrons from the photoreceptor on the
adjacent air molecules. Both positive and negative ions are formed
in the process. If these ions are allowed to diffuse randomly, they
will partially discharge the electrostatic image on the
photoreceptor in a manner which tends to destroy rather than
enhance the latent image of the object being examined. Xerographic
undercutting is caused by the ionization of air above the surface
of the xeroradiographic plate. In particular, during exposure to a
specimen of differing thicknesses, the charged plate is discharged
more completely at the thinner sections, causing discontinuities in
the charge pattern. Across these discontinuities, strong localized
fields curve sharply from one charge concentration to the other.
The ions having a polarity opposite to the charge on the
photoreceptor are attracted by the local fields, and are deposited
on and discharge the more highly charged sides of voltage
discontinuities. As the exposure continues, and the edges become
discharged by the action of the ions, the field pattern moves
inward toward the center of the more highly charged area,
additional ions will follow, destroying more and more of the image.
The technique of minimizing ion undercutting in xeroradiography by
positioning a biased electrode in spaced proximity to the
photoreceptor surface is shown by Vyverberg U.S. Pat. No.
2,802,948. As set forth therein, the electrode is biased in the
range from 700 to 1,800 volts, preferably 1,200 volts, by a D.C.
voltage source. In the xeroradiographic system described in
copending application Ser. No. 874,834, filed on Nov. 7, 1969, now
U.S. Pat. No. 3,650,620, a photoreceptor is automatically charged
and inserted into a cassette unit. The cassette unit, forming a
light-tight environment for the enclosed photoreceptor, may be
transported to an exposure station whereat the object to be
examined is exposed to X-rays. To minimize ion undercutting, the
system disclosed by Vyverberg requires the aforementioned voltage
source to generate an appropriate electric field whereby negative
ions are prevented from discharging the latent electrostatic image.
The portability of the exposure system disclosed in Vyverberg is
therefore severly limited by the weight and bulkiness of the
required voltage source. SUMMARY OF THE INVENTION The present
invention provides a technique for forming an electrode which is
utilized to minimize image smear, due to ion undercutting, in
xeroradiography. A thin layer of dielectric material sandwiched
between two X-ray transparent electrically conducting surfaces is
cut to the desired outside dimensions. A chemically resistant
material is applied to both conductive surfaces in a predetermined
pattern. The sheet is dipped into an etching bath removing material
from the unmasked portions of the conductive surfaces and then
dipped into a neutralizing solution to neutralize the etching
chemical. The sheet is then rinsed, dried and the resistant
material is removed, thereby forming the electrode which functions
as a distributed capacitor. An adhesive material is coated on one
of the conductive surfaces. A charged photoreceptor is enclosed in
a light-tight cassette unit and the formed electrode is affixed to
the lid of the cassette unit by means of the adhesive material. The
photoreceptor, comprising a photoconductor overlying a conductive
substrate, is supported adjacent the cassette bottom. The
distributed capacitor is initially charged by a voltage source
which is then removed. The object to be examined is exposed to
X-rays and a latent electrostatic image is formed on the
photoconductor surface. The distributed capacitor retains the
charge initially applied thereto for a substantial time period,
thereby enabling a plurality of exposures to be made with
minimization of image smear without recharging the capacitor. It is
an object of the present invention to provide a novel method for
forming an electrode for utilization in xeroradiography whereby
image smear, or ion undercutting, is minimized. It is a further
object of the present invention to minimize image smear, or ion
undercutting, in a xeroradiographic system wherein the charged
photoreceptor is enclosed in a light-tight cassette unit. It is
still a further object of the present invention to provide a novel
method for forming an electrode utilized in xeroradiography and
comprising a thin layer of dielectric sandwiched between two X-ray
transparent electrically conductive surfaces, the formed electrode
being affixed to the lid of the cassette. In the operating
position, one of the conducting surfaces is positioned adjacent the
photoreceptor surface and the other conducting surface is coupled
to the photoreceptor substrate, the field produced by the capacitor
being sufficient to minimize image smear and ion undercutting.
Inventors: |
Foote; D. Paul (Sierra Madre,
CA) |
Assignee: |
Xerox Corporation (Stamford,
CT)
|
Family
ID: |
23363058 |
Appl.
No.: |
05/347,277 |
Filed: |
April 2, 1973 |
Current U.S.
Class: |
29/25.42; 378/32;
216/20; 216/52; 216/47 |
Current CPC
Class: |
G03B
42/02 (20130101); G03G 15/758 (20130101); Y10T
29/435 (20150115) |
Current International
Class: |
G03G
15/00 (20060101); G03B 42/02 (20060101); H01g
013/00 () |
Field of
Search: |
;29/25.42,625 ;317/261
;156/3,8,11,16 ;250/315A |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Mall; Carl E.
Attorney, Agent or Firm: Ralabate; James J. Anderson; Terry
J. Keschner; Irving
Claims
What is claimed is:
1. A method for forming an electrode for use in a cassette for
enclosing in a light-tight environment a photoconductor layer
formed on a conductive substrate, said photoconductor layer having
a latent electrostatic charge formed thereon, said cassette
including lid and bottom portions, said photoconductor layer being
supported in the bottom portion and said electrode being affixed to
the lid portion of said cassette, said method comprising the steps
of:
a. cutting a starting material comprising a layer of dielectric
material sandwiched between two electrically conductive surfaces to
a predetermined size such that the starting material may be affixed
to said lid portion,
b. applying an etchant resistant material to selected portions of
both conductive surfaces, said selected portions corresponding to a
desired electrode pattern,
c. subjecting the starting material to an etchant whereby the
non-selected portions of the conductive surfaces are removed from
the starting material,
d. removing the starting material from the etchant,
e. removing the etchant resistant material from said starting
material, and
f. applying an adhesive material to one of the conductive surfaces
and affixing the starting material to said cassette lid.
2. The method as defined in claim 1 further including the steps of
neutralizing the etchant remaining on the starting material
subsequent to step (D), and then rinsing and drying the starting
material.
3. The method as defined in claim 1 wherein said conductive
surfaces are X-ray transparent.
4. The method as defined in claim 1 wherein said dielectric
material comprises Mylar and said conductive surface comprise
aluminum.
Description
DESCRIPTION OF THE DRAWINGS
For a better understanding of the invention as well as other
objects and features thereof, reference is made to the following
description which is to be read in conjunction with the
accompanying drawings wherein:
FIG. 1 illustrates the method steps of the present invention;
FIGS. 2 and 3 are schematical representations illustrating the
electrode formed by the novel method of the present invention;
and
FIGS. 4-8 illustrate the embodiment shown in FIGS. 2 and 2 as
utilized in a cassette which encloses a sensitized
photoreceptor.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to FIG. 1, the series of steps (A) -(H) required to
produce the novel electrode of the present invention is
schematically represented.
In step (A), the starting material, comprising a thin dielectric
sandwiched between two X-ray transparent electrically conducting
surfaces, is cut to the desired outside dimensions. Typically, the
starting material may be Mylar, comprising polyethylene
tetraphthalate, a trademark of E. I Dupont de Nemours, Inc., 0.003
inches thick, aluminized on both sides to a thickness in the range
from about 100A to about 1,500A. Aluminized Mylar may be obtained
from the Hy-Sil Manufacturing Company, Revere, Mass. Other
materials which may be utilized as the conducting surfaces include
copper, brass, gold, etc. An example of another dielectric material
includes Teflon, comprising tetraflouroethylene, also a Dupont
trademark. The starting material, regardless of the material
components utilized, is preferably in the sandwich form described
hereinabove.
In step (B) a chemically resistant material (resistant to the
etching material utilized in step (C) is applied to both sides of
the Mylar in the desired pattern. A typical chemically resistant
material which has been successfully utilized in the present
invention comprises Avery Flex K-1 labels, manufactured by the
Avery Products Corporation, San Marino, Calif.
In step (C), the Mylar sheet is dipped into an etching material,
such as 10% sodium hydroxide solution, at room temperature for
approximately 15 seconds. This procedure removes the aluminum from
the unmasked portions of the sheet, thus producing the desired
electrode pattern.
Other etching materials which may be utilized include potassium
hydroxide and hydrofluoric acid.
Different etchants may have to be utilized if the conducting
surfaces are made from material other than aluminum.
The Mylar sheet is then dipped in a weak acid solution, such as 10%
acetic or tartaric acid, to neutralize the sodium hydroxide as
shown in step (D).
In step (E) the Mylar sheet is rinsed with water and in step (F)
the sheet is dried by any conventional technique. After the sheet
is completely dried, the chemically resistant material is removed
from both conducting surfaces and an adhesive is applied to one
conducting surface, as set forth in steps (G) and (H). An adhesive
material which has been utilized successfully in the present
invention is a transfer tape (Avery Type N-10) manufactured by the
Avery Company. At this point, the formed electrode may be mounted
to the xeroradiographic cassette lid.
FIG. 2 is a plan view of an electrode formed in accordance with the
teachings of the present invention and FIG. 3 is a cross-sectional
view taken along line 3--3 of FIG. 2. Dielectric 18 is shown
sandwiched between conducting surfaces 20 and 22 (shown in dashed
lines). Conducting surfaces 20 and 22 are offset to allow for
electrical connections to both surfaces. Typical dimensions (in
inches) of the electrode are as follows: Length of dielectric 18
(Dimension a) 14.62 Width of dielectric 18 (Dimension b) 9.30
Thickness of dielectric 18 0.003 Length of conducting surfaces 20
and 22 (Dimensions C and C') 14.00 Width of conducting surfaces 20
and 22 (Dimension d) 9.00 Thickness of conducting surfaces 20 and
22 1250.+-. 250A Offset between conducting surfaces 20 and 22
(Dimension e) 0.25
Referring now to FIG. 4, there is shown a schematical
representation of apparatus utilizing the electroded cassette
formed in accordance with the teachings of the present invention.
the xeroradiographic plate or photoreceptor 10 comprises generally
a layer of an insulating photoconductor 12, such as vitreous
selenium, overlying a conductive metal backing plate 14. As is well
known in the xerographic art, layer 12 has the property of
retaining charge in the dark and of discharging or dissipating its
retained charge when exposed to light, X-rays, gamma rays or other
penetrating radiation.
Spaced apart from, for example, within 1 cm, and substantially
parallel to plate 10, is element 16 comprising a thin layer of
dielectric material 18 sandwiched between two X-ray transparent,
electrically conductive surfaces 20 and 22 and formed in the manner
described hereinabove. Other materials may be utilized as the
conductive surfaces and dielectric as set forth previously.
Conducting surface 20 is electrically connected to metal backing
plate 14 via resistor 24. An electric field perpendicular to the
surface of the photoconductor 12 is generated in the region between
the xerographic plate 10 and conducting surface 22 due to the
initial charge which is utilized to sensitize the surface of
photoconductor 12. The electric field in this region is further
enchanced by placing an electrostatic charge on conducting surfaces
20 and 22. Dielectric material 18 and conducting surfaces 20 and
22, the electrode formed in accordance with the teachings of the
present invention, functions as a distributed capacitor which is
charged by connecting voltage source 28 across the conducting
surfaces. Voltage source 28 is illustrated in a manner to indicate
that after it charges conducting surfaces 20 and 22, it is removed
from contacts 25 and 27 prior to the exposure of object 26 by the
X-ray source 29.
As set forth hereinabove, the thickness of dielectric layer 18,
when formed of Mylar, is about .003 inches whereas the conducting
surfaces 20 and 22, when formed of aluminum, are approximately
0.0005 inches thick. The value of resistor 24 is typically 10.sup.8
ohms.
In operation, D.C. voltage source 28 is connected across conducting
plates 20 and 22 through resistor 24 and applies a potential
thereacross in the range from about 400 volts to about 1,600 volts,
depending on the initial charge placed on photoconductive layer 12,
and of a polarity as shown such that a negative charge is formed on
conducting surface 22. It is assumed that the surface of
photoconductor 12 has initially been charged to a positive
potential. Object 26 is then exposed to penetrating radiation
generated by X-ray source 29. The voltage source 28 may then be
disconnected from conducting plates 20 and 22 or, alternatively,
may be disconnected prior to exposure.
The electric field perpendicular to the surface of photoconductor
12 tends to confine the air ions to the immediate region where they
are generated. Ions charged to the same polarity as the polarity of
the electrostatic charge on the photoconductor surface are
attracted to the conducting surface 22 while the ions charged to a
polarity opposite to that of the electrostatic charge on the
surface of the photoconductor tend to further discharge the
photoconductor 12 in the same pattern as the X-rays which expose
the photoconductor 12. Consequently, the image of the object being
examined is improved due to the presence of the electric field
generated by the capacitor 16 which is maintained even though the
plate is discharged by the exposure. The electric field tends to
maintain ions in the position over the plate where they were formed
(unless they have excess lateral energy from their formation).
Negative ions are forced toward the plate contributing
constructively to the exposure while positive ions are forced away
from the plate. The distributed (sheet) capacitor 16 does not need
a permanent charging source and may be charged, as set forth
hereinabove, just before exposure. Experience with modest energy
X-ray (in the range from about 25 KVP to about 45 KVP) exposures
shows that capacitor 16 will maintain its charge for at least 50
exposures.
It should be noted that if photoconductor layer 12 is initially
negatively charged, the charge applied to conducting surfaces 20
and 22 should be reversed by reversing the connections of voltage
source 28, i.e. a positive charge is formed on surface 22. In this
manner, minimization of image smear is also accomplished regardless
of the polarity of the initial charge on the surface of
photoconductor 12.
Since capacitor 16 is chosen to be transparent to X-ray radiation,
the X-ray radiation does not affect the charge thereon.
In practice, it has been found that the sum of the charge on
photoconductive layer 12 and the charge placed on the distributed
capacitor 16 should be in the range of about 400 volts to about
2,000 volts. The plates are normally charged in the range from
about 400 volts to about 1,600 volts. For illustrative purposes, if
the plate is charged to 400 volts, the potential applied to the
capacitor may be 1,600 volts.
Resistor 24 is not necessary for the operation of the present
invention. However, resistor 24 acts to prevent the thin aluminum
surfaces from burning off the electrodes 20 and 22 around the
contact area (illustrated in FIGS. 5-8 hereinbelow) during the
charging of capacitor 16 or if conducting surface 22 is accidently
brought into contact with photoconductive layer 12. In addition,
resistor 24 protects against shock hazard when used, for example,
in the cassette illustrated in FIGS. 4-8. In lieu of resistor 24,
substrate 14 may be connected directly to conducting surface 20 or
via a resistor of smaller resistance value.
FIGS. 5-8 show views of the embodiment shown in FIG. 4 when
utilized in a cassette unit of the type described in copending
application U.S. Ser. No. 208,973 filed Dec. 16, 1971. The details
of the cassette shown in the copending application are not set
forth herein since the inventive concept of the present invention
set forth hereinabove may be utilized in any X-ray imaging
cassette. Therefore, the figures shown are highly schematical views
of the electrode formed in accordance with the teachings of the
present invention when utilized in combination with a cassette.
FIG. 5 is a plan view of a cassette unit 30, in the closed
position, shown in a transparent form to illustrate the capacitor
16 clearly. Conducting surface 20 is connected to a 10.sup.8 ohms
deposited resistor 24 (a lumped resistor may be utilized instead)
via conductor 32. Conductor 32 may comprise a conducting tape
formed of aluminum with a conductive adhesive, a deposited aluminum
film, copper tape, etc.
FIG. 6 is a front elevation of the cassette 30 showing the
substrate of photoreceptor plate 10 connected to conducting surface
20 via conductor spring 34, spring contact 36 and deposited
resistor 24. Also shown are guide rails 35 for supporting the
photoreceptor plate within the cassette.
FIG. 8 is a side elevation of cassette 30 further illustrating the
connection of conducting surface 20 to the substrate 14 of plate 10
via conductor spring 34.
FIG. 7 is a plan view of conducting surface 22 with contact surface
38 for conductor spring 34. Contact 25 is connected to conducting
surface 22 via a strip of conducting tape 29 as shown. Contact 27
is connected to contact surface 38 via a thin conducting strip
31.
The formed electrode, or distributed capacitor 16, is placed inside
the cassette lid and held thereto by a suitable adhesive with the
top conducting surface 20 electrically connected to the metallic
substrate of plate 10 as shown.
The present invention provides an additional advantage to those
discussed hereinabove. If the cassette material is made of plastic,
or other insulating material, a surface electrical charge may be
formed in various patterns on different portions of the cassette.
This will lead to a non-uniform electric field above the surface of
photoreceptor 10 which is usually detrimental to the image to be
formed and reduces the possibility of image repeatability. The
utilization of the distributed capacitor 16 in the manner described
hereinabove substantially eliminates this problem by providing a
uniform electric field of a magnitude which masks the effect of the
non-uniform fields.
While the invention has been described with reference to its
preferred embodiment, it will be understood by those skilled in the
art that various changes may be made and equivalents substituted
for elements thereof without departing from the true spirit and
scope of the invention. In addition, many modifications may be made
to adapt a particular situation or material to the teachings of the
invention without departing from its essential teachings.
* * * * *