Note: Descriptions are shown in the official language in which they were submitted.
Brief Background of Invention
1. Field
This invention relates to a negative corona discharge system for
uniformly charging an insulating layer and, more particularly, to an
~`~ improved corona discharge electrode for neyatively charging an electro-
photographic imaging surface.
2. Description of the Prior Art
In well known electrostatic printing devices, a corona generating
device including a corona discharge electrode is commonly used to place
positive or negative charges onto a photoconductive surface prior to
exposing the photoconductive surface to a pattern of light. The light
pattern then discharges the photoconductive surface creating an electro-
static image of the light pattern thereon which is subsequently developed
with electrostatic developer material thereby mak;ng the electrostatic
image visible.
~
When a positive corona generated from a metallic filament electrode
is used in the charging process9 the resultant positive charge on the
,
photoconductive surface is relatively uniform due to the uniformity of
the positive corona electrode emission. However, when a negative corona
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1 generated from a metallic filament eleçtrode is ut~lize~ to negatiYely
charge the photoconductive surface, the photocond~ctiye sur~ace obtains
a charge which varies in density from point to point due to the non-uniform
negative corona electrode emission. This non-uniforr~ity in charge is
readily observed in the developed image since areas contalning a higher
charge will attract more electrostatic developer material thereto thereby
- creating a streaked image appearance.
Various prior art negative corona devices have been constructed in
order to produce a uniform negative charge on the substrate being charged.
` 10 One prior approach has been to move the metallic corona wire electrode and
.
the surface being charged simultaneously in orthogonal directions in an
attempt to average out the non-uniform charge. Such a system is necessar-
ily bulky and expensive.
Another prior art approach has been to place an alternating current
in series with the high voltage direct current across the corona wire
electrode. Such approaches have învolved costly equipment which must be -
used to develop the high frequency required for good uniformity. Another
prior art approach has been to raise the potential of the negative corona ~ ~1
wire electrode thereby causing the emission points to move closer together
on the corona wire. While some improvement in uniformity is noted, the
resultant charge is still non-uniform and can be noticed when the charged
substrate is utilized in an electrophotographic process since the non-
uniform charge produces streaked copies. Further, the higher voltages -
require more expensive supplies and also result in increased unwanted
Ozone production. -`-
. j ~ . -, .
Another method used in improving the uniformity of the charge
created by a negative corona electrode is to separate the substrate being
~`1 charged from the corona electrode by an adequate distance so that the ~;
groups of electrons emitted from the nodal points on the electrode have
1, 30 adequate distance to spread out and overlap before being deposited on
! the surface of the substrate. The resulting charge is always more uni-
form with the separation, but there is a great loss of efficiency created
LE9-72-027 - 2 -
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l by the separation distance. That is, the potential on the corona
electrode must be increase~ dramatically as the distance lncrease$
between the corona electrode and the substrate be1n~ charged.
Another prior art approach has been to heat the corona electrode
thereby achieving uniform charging of the substrate by thermionic emis-
sion. The corona electrode must be heated to a su~ficiently high tempera-
ture to assure good emission. However, many of the materials which are
good thermionic emitters also react easily with impurities in the atmosphere.
,
Further, such an approach would require costly electrodes and equipment
for power and therma1 insulation and equipment to compensate for thermal
~`~ expans;on. Accordingly, such a device cannot be utilized readily or
economically in a machine environment.
A still further prior approach has been to utilize an alternating
electric and/or magnetic field in conjunction with the negative corona
,',!, electrode. The utilization of such a field makes requisite an alternating
~ield generator at some considerable expense. Further, while uniformity t -~
'~ of emission is somewhat improved, there nevertheless are numbers of widely
' spaced sites of emission.
Summa~ -
In order to overcome the above noted sho~tcomings o~ the prior art
and to provide a negative DC corona discharge system which provides
uniform emission from the corona discharge electrode, the corona elec~
: ~ .
trode of the present invention is formed of a conductive member having
a thin inorganic dielectric outer layer bonded thereto. The dielectric
layer acts as a suppressor of large and widely spaced nodes thereby
:
eliminating the noding problem common to all negatively biased metal
~ corona electrodes. Since the negatively biased corona electrode of the
`~ present invention has an emission pattern associated therewith much like
that exhibited by a positively biased metallic corona electrode, the
negative electrode may be placed in close proximity to the substrate
which it is charging. The proximity of the corona electrode to the sub-
strate and its lower emission density reduces the pow~r requirements for
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1 a negative corona discharge system thereby greatly increasing the system
efficiency. When such a corona system is ut~11zed in an electrophoto-
graphic copying apparatus, the uniformity of the electron emission from
the negatively biased corona electrode is sufficiently uniform so that no
noticeable streaking or copy quality problems are noted.
It is, therefore, an object of this invention to provide a uniform
negative corona emission from a corona electrode.
It is another object of this invention to provide a simple and
economical negative DC corona source for use in an electrophotographic
reproduction apparatus.
A still further object of this invention is to provide a negative
uniform corona source for uniformly charging an electrophotographic
plate.
The foregoing objects, features, and advantages of the invention
will be apparent from the following more particular description of the
preferred embodiments of the invention as illustrated in the accompanying
drawings.
In the Drawin~s
Figure 1 is a pictorial perspective illustration partially in section 20 of a corona discharge electrode constructed ;n accordance with the present
invention.
Figures 2a-2e are schematic illustrations of typical charging systems
; incorporating the corona discharge electrode of the present invention -~ -~
~ positioned to charge a photoconductive insulator surface.
; Referring now to the drawings and more particularly to Figure 1-~ thereof, an illustration of a corona discharge electrode constructed in
accordance with the present invention is depicted. The corona discharge
electrode 11 comprises a core 13 and a thin outer layer 15 intimately
bonded thereto. Any suitable synthetic or natural filament like material
may be employed as the core of the corona discharge electrode, it being
necessary that the outer-most portion or layer of the core be of a con-
ductive material. Typical conductiv~ filament materials include metallic
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l material~ such as stainl~ss steel, molybdenum, tungsten, a1uminum, gold,
- copper, and ~he like. Typical synthctic ~ilament materjals lnclude
cotton yarn, silk yarn, rayon yarn, and the like. If such synthetic
-filaments are utilized, it is necessary that they be coated with an
outer conductive layer.
Any suitable inorganic dielectric material or combination o~ such
materials may be employed as the thin outer layer 15. It ls, however,
necessary that the dielectric material exhibit a resistiyity of 106 ohm
. ~., - .
centimeters or greater. Typical inorganic dielectric materials include
metal oxides such as oxides of aluminum, zinc, magnesium, titanium,
. :.~ . .
barium, beryllium, calcium, cerium, strontium, zirconium, thorium, and
hafnium. Typical inorganic dielectric materials further include ceram;c
i materials such as silicon nitride, silica, silicon, boron nitride, zir~
., ~, ,
conium silicate, titanates such as lead, barium and calcium, ferrites
:~ such as zinc, aluminum and magnesium and glasses such as phosphosilicate ~-
`~ glasses (PxSiz02 where x typically varies from 0 to 33% and z typically
varies from 66% to 100%), borosilicate glasses (BxSiOz) and metallic
I~ oxide additions thereto (MyPXSizO2).
A corona discharge electrode ll of approximately 0.0025 inches in
diameter is preferred for maximum strength and optimum corona discharge
, ~, .
characteristics. However, smaller diameter electrodes can be used at
lower voltage potentials and similarly larger diameter electrodes can be
,' utilized at higher voltage potentials, the corona potential necessary to
produce the required corona current increasing with an increase in fila-
ment diameter.
l~ The thin outer layer 15 may be applied to the core 13 by utilizing `
;~ ~ various techniques such as chemical vapor deposition, sputtering, plating,
jet coating, dipping and various other well known application techniques
dependjng upon the materials selected. Additionally, the core 13 and the
thin outer layer 15 may be formed from the same material, a metal, which ;
oxidizes.
It is to be further noted that while the corona discharge electrode
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1 11 is depicted as a ~-lament electrode, variolJs other well known sh~ped
electrodes such as knife edge, wedge, or strlp electrodes may bq utilized
in accordance with the configurat~on desired. When such an electrode ls
utilized, it is necessary that the conductive portion thereof haYe a thin
inorganic dielectric layer bonded thereto covering the emitting radius.
As with a ~ilament electrode, the potential required to produce the
necessary corona current increases with an increase in electrode radius.
~ The corona discharge electrode 11 may thereafter be placed in various
I corona discharge systems including a corona system depicted in Figure 2a,
a corotron system depicted in Figure 2b, a scorona system depicted in ~ -
Figure 2c, a scorotron system depicted in Figure 2d, or a "bird cage"
system depicted in Figure 2e, and connected to a negative bias source
17 as in Figure 2a. The corona discharge electrode so constructed will
emit uniformly when biased to a proper negative voltage and when placed
in proximity to a conductive element.
~! Referring to Figure 2a of the drawings, a three wire corona dis-charge system located adjacent an electrophotographic plate is schematlcally -`i deplcted. The electrophotographic plate 18 comprises a conductive sub-
~, strate 19 and an insulating layer 21. A suitable photoconductive insu-
lator material is disclosed in U.S. Patent 3,484,237 issued December 16,
1969. The conductive substrate can comprise an aluminum layer sprayed -~
onto an insulating surface. The uniform emission effected by the nega-
tiYely biased corona discharge electrode 11 facilitates the placement of
i the electrode 11 much closer to the plate 18 than that heretofore
achievable thereby greatly redocing the potential of supply 17 and fur-
ther lowering the emitted current over that required b~ prior deYices.
,:.
three wire corotron discharge system is depicted in Figure 2b
adjacent an electrophotographic plate 18. The utilization of the layered
discharge electrodes 11 of the present inVention ~lso greatly reduces
the power requirements of this device. In a similar manner, the power
requirements of the discharge electrodes 11 o~ the scorona dlscharge
system of Figure 2c, the scorotron discharge system of Figure 2d and
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1 the "bird cage" discharge system of Figure 2e are greatly reduced when
the inorganic dielectric layer of the present 7nvention is incorporaked
over the metallic core of the electrode. Further, the currents associated
with the grids 25 are correspondingly reduced.
; The mechanism by which such uniform emission is obtained from the
. ;, .
corona electrode of the present invention has not be elucidated. However,
several theories may be conjectured.
" , .
Firstly, it is noted that a negatively biased metallic filament
emits a corona discharge from nodal points which may be readily observed.
The non-uniform emission is attributed to work function variat10ns in
;~ the surface layer of the metal which may be occasioned by non-unitorm
oxide formations, grain boundary variations and/or adsorbed gases, and to
non-uniform electric field variations caused by surface asparities. Once ~-
~ ~ .
- ; electrons are emitted from a localized area of the metallic surface, that
area is bombarded with positive ions thereby creating secondary emission
~` at the localized sites.
;~ This nodal emission phenomena can be visually observed with any
metallic corona filament electrode which is negatively biased including
1l those of the most nobel metals such as gold.
il - 20 When such a metal filament has a dielectric inorganic material layer
~ such as a metal ox;de or ceramic material bonded thereto, the distinct
. .
emission sites are no longer readily observable.
Thus, the addition of a dielectric material layer may result in
the obtainment of a more uni~orm work function at the surface of the
corond dlscharge electrode. The uniformity of the emission may be
attributed to the very high secondary electron emission yield of the -~
I dielectric material.
Further, the uniform corona discharge could be attributed to a
resistive shunting effect created by the dielectric layer. That isg
a voltage drop is known to exist across the dielectric material as well
.,
as across the boundary between the corona emission electrode and the
requisite adjacent conductive surface such as the surface 19 of
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1 Figure 2a. Assuming that the dielectric material has substantially uni-
form resistivity, those surFace locations emitting electrons create a
higher voltage drop across the dielectric layer due to the relatively
high current thereto. The higher voltage drop across the material at a
locali~ed point limits the current emitted from that point. Therefore,
many such emission sites are necessary to attain the total current equiva-
lent to that obtained with widely spaced highly emitting nodes of a metal-
lic filament. This effect may thus be thought of as a current limiting
effect or a resistive shunting efFect.
The mechanism of uniform emission can also be explained by an
enhanced field effect theory. That is, a high relatively uniform elec-
; tric field is created across the dielectric layer by the buildup of posi-
tive ions on the outer surface of the dielectric materials. Electrons are -
,'i ' ' .:
then injected from the metal of the core materials into the dielectric
~` layer. The injected electrons then tunnel across the dielectric layer -
:~ and/or avalanches are precipated in the dielectric layer ~Malter effect).
The electrons then emit from the surface of the dielectric layer.
' While the mechanism by which the inorganic dielectric layer promotes
uniform emission from a negative biased metallic electrode is not fully
understood, it is noted that various organic dielectric materials have
been utilized solely or in combination with inorganic dielectric materials
as a coating for the metallic electrode. None of the discharge electrodes
coated with such organic materials has exhibited more than a minimal
improvement in emission uniformity and most of the organic coatings -
rapidly break down due to material fai1ures. It is presumed that these
organic materials are chemically unstable in a negative corona environ- ~
ment and/or they will not support a sufficiently high electric field ~,
thereby preventing their use in obtaining uniform negative corona emis-
sion.
The following are examples of the present invention in detail. The
examples are included merely to aid in the understanding of the invention `~;
and variations may be made by one skilled in the art without departing
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1 from the spirit and scope of this ~nVention.
EX~PLE 1
A coating of silicon nitride was deposited by che~ical ~apor deposi-
tion onto the surface of a 0.0025 inch diameter tungsten wire. This coat-
ing was examined by scanning electron microscopy and found to be uniform
- in topography at a thickness o~ approximately 500A. The silicon nitride
coated wire was then assembled in a scorona charging device along with a
. control 0.0025 inch diameter tungsten wire coated with a 100 microinch
coating of gold.
A voltage of approximately -7,000 volts was then impressed on each
wire at a current level of approximately S0 microamperes per inch. Each
` wire emits a corona discharge. When examined visually, the gold coated
. tungsten wire has an emission pattern of moving beads spaced between 1/16
to 1/8 inch apart. The silicon nitride coated wire initially appears to
have a non-uniform emission, but within ten minutes of turn-on, the
emission becomes a uniform glow along the length of the wire with no
readily observable emission nodes.
After uniform emission is obtained on the silicon nitride coated
tungsten wire, no degradation is thereafter noted when the potential
impressed on the corona electrode is removed and thereafter turned-on
again.
Current density scans of the emission ~rom the wires reveal that
the sillcon nitride coated wire has a substantially more uniform
emission pattern than the gold coated wire, there being a 7:1 improve-
ment in~the relative peak to peak values. Further, the peak to peak
I value of the current density scan of the silicon nitride wire is
: approximately the same as that previously recorded by scanning a gold
coated wire which was positively biased to emit at approximately the same ~ -
, current density as the negative case. -
,~ 30 The silicon nitride coated wire and the gold coated wire were
j then allowed to continually emit with the emission patterns bein~ period-
ically checked both visually and by current density scans. After 1,00Q
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l hours of operation, the silicon nitride coqted wire W~s still e~jttln~
uniformly and the gold coated write had de~raded ~o emttting from wldcly
spaced stationary nodes.
The silicon nitride coaked wire was then removed ~rom the test
apparatus and placed with two other similar silicon nitrjde coated wires
in a three wire scorona charging system and incorporated as the charge
,
corona of an IBM Copier II office copying machine. ~ potential of
approximately -7,000 vol~s with a current of approximately 50 micro- -~, .
amperes per inch (a total system electrode current of 1.5 mill7amperes)
was applied to the wires. Thus, the typical operating point of the corona -
,
: electrodes was 10.5 volt amperes. The wires were located approximately
,,
0.3 inches from the scorona grid which was located approximately 0.05
inches from the surface of the moving photoconductor drum of the machine.
Continuous toned copies from the copying machine showed no evidence of
`~ uneven charging. This test was repeated at 83 F. and at a relative
humidity (R.H.) of 80% without noticeable degradation of copy quality.
It is noted that each hour of corona on time is approximately equal
~; to 1,500 copies produced by an IBM Copier II running continuously. Thus,
l the silicon nitride coated wire having an on time duration in excess
-l 20 of l,000 hours is equivalent to a corona in a machine environment being on
.~ ,
l for a period of time to produce 1,500,000 copies.
..
j The charge corona of an IBM Copier II copying machine comprises a
"bird cage" charging system utilizing three gold plated tungsten wires
located 0.5 inches from the grid wires which are located approximately
-; :, .,
0.05 inches from the photoconductor. A -14,000 Yolt supply supplies
I approximately 80 microamperes per inch of current to the electrodes.
The typical operating point is at -12.5 K volts or 31.25 Yoltamperes.
! No uneven charging is noted. Therefore, power consumption is approxi~
: ' ,,
~ mately reduced by a factor of three by utilizing the dielectr~c coated
; i 30 wires in a scorona configuration. A gold coated tun~sten wire cannot
be utilized in a scorona device because of noding unless high potentials
" ,
and current densities are utilized with separation.
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1 EXAMPLE II
.
A coating of silicon was deposited by chemlcal vapor deposition from
-~- SiH~(Silane) onto the surfaces of several 0.0025 inch diameter tungsten
wires. The thickness of the coatings so deposited varied from wire to
wire from .0001 inch to .0003 inches. The wires were then assembled in
the scorona charging device of Example I and a potential of approximately
, .
-7,000 volts was impressed on the wires at a current level of approxi-
mately 50 microamperes per inch. Initial emission from the wires was very
non-uniform. After a "burn-in" time varying from 5 minutes to 2 hours,
uniform emission was achieved. Continuous improvement in uniformity of
emission occurred during the burn-in time. Once uniform emission was
achieved, the wires continued to uniformly emit regardless of interrup-
tions of the applied voltage. Uniform emission continued in excess of ~ -
700 hours of continuous operation.
~i When subjected to environmental extremes, it was noted that the -
, . ;j ,
- emission became nodal at high temperatures and high relative humidities
. .,~ .
-~ (83F - 80% R.H. and 75 F - 60% R.H.). The nodal emission at such high
humidities is attributed to the adsorption of water by the silicon oxide
surface. Such water adsorption causes the resistivity of the silicon
; 20 oxide surface to markedly decrease creating low resistance paths to nodal
emission sites.
EXAMPLE III
A coating of silica (SiO2) was vapor deposited over one half of the
, ,
` length of a gold coated tungsten wire. The coating had a thickness of
approximately lOOA. The wire was placed in the scorona charging unit
described in Example I and a -7,000 volt potential at a current level of
approximately 50 microamperes per inch was applied thereto. Very uniform ;~
~` emission was noted over the coated half length of the wire while the non~
.-~
; coated half length had discre~e nodes of emission associated therewith.
The uniform emission continued from the coated portion for approximately
1 hour at which time the surface degraded due to wire vibration causing
the brittle coating to crack and spall.
LE9-72=-27
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A phosphosilicate glass coating (P 03S~ 972) was applled to seYera
tungsten wires 010025 inches in diameter The coat~ngs were unlform and
;~ varied in thickness from 2,000A to 6,000A and were noted to be quite duckile.
The wires were then placed in a corotron charging unit a potential of
approximately -79000 volts was applied khereto at a current leyel of 50
~ microamperes per inch. The emission from the wires was noted to be uniform
- with no initial burn-in required. The wires haYe continued to emit uni-
formly in excess of 600 hours. The wires operate to provide uniform lO emission throughout a wide range of environments including high humidity
and temperature.
EXAMPLE V
A wire o~ 6061 aluminum 0.003 inches in diameter is assembled in a
coratron charging device. An electrical potential of approximately
-7,000 volts at a current level of approximately 50 microamperes per inch
is applied to the wire. The initial corona emission ls observed to be
nodal, however~ after approximately 30 minutes of emission, the emission
becomes a uniform glow along the length of the wire. This wire is then
'`i
~; placed in the scorona charging unit and placed in the IBM Copier II copying
machine as described in Example I. Continuous toned copies produced by the
machine show no evidence of non-uniform charging after testing up to 83 F
- 80% R.H. A similar wire is then placed in a corotron charging device
, i, : :
and allowed to emit continuo~sly. Uniform emission is noted for a period
of 18 hours at which point the wire failed by necking down and fracture.
EXAMPLE VI
An aluminum ribbon of 99.84% aluminum was assembled in a scorona
charging device. The ribbon was l/16 inch by 0.0025 inch. An electrical
potential of approximately -8,000 volts at a current level of approximately
50 microamperes per inch is applied to the ribbon. The initial corona
~0 emjssion is observed to be nodali howeYer~ a~ter approximatély 20 minutesof emission, the emission becomes a un~form glow along the top of the
ribbon. The surface uniformly emitted for a period of flve hours after
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1 which nodal emission occurred.
EXAMPL~ VII
A 0.0025 inch diameter gold coated tungsten wire was dip coated with
aluminum by drawing the wire through a molten bath of llO0 aluminum. The
~; resultant aluminum coating was approximately 0.0005 inches in thickness
with uniform coverage of the wire surface. The coated wire was placed in
a scorona charging device and potential of approximately -7,000 volts at a
current level of approximately 50 microamperes per inch was impressed
thereon. The wire initially emitted non-uniformly, but after approximately
;~ 10 30 minutes, uniform emission was achieved. The wire was allowed to emit
continuously for a period of approximately 8 hours at which time nodal
- emission appeared. Examination of the wire surface revealed that thealuminum layer had completely oxidized and that failure occurred at points
were the aluminum oxide had cracked or spalled from the wire surface.
EXAMPLE VIII
A coating of zinc was deposited by chemical vapor deposition onto a
0.0025 inch diameter tungsten wire. The coating had a thickness in the
, O O
range between 500A to l,OOOA. The wire was placed in the scorona testing
device described in Example I and a potential of -7,000 volts at an
approximate current level of approximately 50 microamperes per inch was
applied to the wire. Non-uniform emission was noted for approximately
10 minutes. Thereafter, uniform emission occurred for approximately 140
hours. Failure occurred at about 142 hours, the emission from the corona
occurring from stat;onary nodal points at that time.
EXAMPLE IX
A coating of magnesium was deposited by ion plating magnesium onto
the surface of a 0.0025 inch diameter tungsten wire. The coating was
approximately 500A thick. The wire was placed in a corotron charging
device and a potential of approximately -7,000 Yolts at a current level
of approximately 50 microamperes was applied to the wjre, Non-unifcrm
emission was noted for approximately 5 minutes~ Thereafter, the wire
emitted uniformly in excess of 100 hours of operation.
LE9-72-027 - 13 -
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1 The ~ollowing example~ describe variou5 organic dielectrlc mater-
ials which were coated onto a ~letallic wi`re in an attempt ~o achieYe
uniform negative corona emission.
EXAMPLE_X
A coating oF a silicone rubber (RT~-60 suppl~ed by the General
Electric Company~ filled with 47% by weight ferric oxide particles
(Fe203) and silica (sjo2~ was applied to a 0.0025 inch diameter tungsten
wire. The coating was approximately 0.0001 inch thick and uni~ormly
.
covered the wire surface. When this wire was placed in the scorona
charging device of Example I and a negative potential of 7,000 volts at
a current level of 50 microamperes per inch was applied, the emission
pattern was nodal and no improvement occurred a~ter an emission period
of 1 hour. Examination thereafter of the wire surface revealed the
coating to have been partially removed during the 1 hour emission cycle.
EXAMPLE XI
. . _
Submicron alumina powder was mixed with a silicone resin (SR420
supplied by the General Electric Company). This mixture was subsequently
applied to a gold coated tungsten wire by dip coating to achieve a final
coating thickness of approximately 0.0001 inches. The coated wire was
placed in the scorona charging device of Example I and a negative poten~
tial of approximately 7,000 volts at a current level of 50 microamperes
per inch was impressed thereon. The emission was nodal at the initial
turn-on and remained poor for a period of approximately 1 hour. At this
time the wire was removed and examination revealed that the coating had
been completely removed.
EXAMPLE XII
A coating of 85% by volume solution grade polyurathane (Estane 5740
produced by B. F. Goodrich) mixed with 15% by volume graphite pigment
,
dip coated onto a 0.0025 inch diameter gold coated tungsten wire. The
coating Was approximately O.QOa2 inches thick and uniformly covered the
wire surface. The wire was placed jn the scorona charging devlce of ~;
Example I and a potential of -7,000 volts at a current level o~ 50
.
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1 microamperes per inch Was applied thereto. Nodal emission was obseryed
~or apprQximately the first hour of operation. The nQ~es Were more
closely spaced than those produced by ~he wires of E~amples X and XI, but
were readily observable. During the second hour of operation, extremely
non-uniform emission occurred.
EXAMPLE XIII
A composition of an acrilic resin and a volatile carrier (Krylon
supplied by eorden, Inc.) was spray coated onto a 0.0025 inch diameter
gold coated tungsten wire. The coating was approximately 0.0001 inch
thick. Aluminum oxide was dusted onto the surface of the coating material.
The wire was thereafter placed in the scorona charging device of Example I
and a -7,000 volt potentia1 at a current level of 50 microamperes per inch
was applied thereto. Nodal emission was noted for approximately one hour.
The wire was then observed and the coating was found to be removed.
' From the foregoing, it is readily observed that a corona electrode
~ cornprising a conductive substrate coated with an inorganic dielectric
; material such as a ceramic material or a metal oxide provides uniform elec-
tron emission when negatively biased to a corona discharge potential. ~
l Further, this phenomenon is observed only when inorganic materials are ~ -
;, 20 utilized, the utili~ation of organic materials failing to produce thedesired uniform corona emission. It is further noted that although some
of the inorganic materials require a "burn-in" time (attributable to the
formation of oxides, etc.~, all of such materials exhibit uniform emis-
sion for a considerable time duration. It is further noted that some of
the dielectric inorganic materials exhibit uniform emission over a wide
range of temperatures and humidities to which a device such as an office
copying machine could be subjected.
Whjle the invention has been particularly shown and described with
reference to preferred embodiments thereof, it will be understood by those
skilled in the art that the foregoin~ and other changes in form and details
may be made therein without departing from the spirl~ and scope of the
invention.
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