Note: Descriptions are shown in the official language in which they were submitted.
- ~075068
BACKGROUND OF THE INVENTION
This invention relates to xerography, and in particular
to a system utilizing a photoreceptor having a stable middle
layer. The art of xerography involves the use of a photosensi-
tive element containing a photoconductive insulating layer which
is first uniformly electrostatically charged in order to sensi-
tize its surface. The plate is then exposed to an image of
activating electromagnetic radiation such as light, x-ray, or
the like, which selectively dissipates the charge in the
irradiated areas of the photoconductive insulator while leaving
behind an electrostatic latent image in the non-irradiated
areas. The latent electrostatic image may then be developed
and made visible by depositing finely divided electroscopic
marking particles on the surface of the photoconductive insu-
lating layer. This concept was originally disclosed by Carlson, ~ -
in U.S. Patent 2,297,691, and is further amplified and described
by many related patents in the field.
The use of vitreous selenium as described by Bixby in
U.S. Patent 2,970,906, remains the most widely used photocon-
ductor in commercial xerography in that it is capable of holdingand retaining an electrostatic charge for relatively long periods
of time when not exposed to light, and because it is relatively
- sensitive to light as compared to other photoconductive
materials.
Ullrich, U.S. Patent 2,803,542 and Mayer et al, U.S.
Patent 2,822,300, both teach the concept of improving the pro-
perties of vitreous selenium by the addition of elemental
arsenic in amounts of up to about 50 percent by weight. Arsenic
concentrations greater than about 10 percent by weight exhibits
increased spectral response in the yellow-red band of electro-
magnetic spectral.
~k
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~0750~8
Paris, U.S. Patent 2,803,541, discloses two-layered
photoreceptor structures having layers of selenium and selenium-
arsenic alloys. Improved photosensitivity is attributed by
using a top layer of vitreous selenium-tellurium over a layer of
selenium. This structure, however, does not provide adequate
abrasion resistance for automatic xerographic machine operation
and also exhibits high dark discharge. Protective organic and
inorganic overcoatings have been developed to provide improved
abrasion resistance but a major problem with these overcoatings
is their inability to function properly through a wide range of
environmental conditions.
Sechak, U.S. Patent 3,655,377, discloses a three-
layered structure, i.e., a tri-layer photoreceptor, comprising
a top layer of arsenic-selenium alloy for abrasion resistance,
temperature stability and improved dark decay; a second layer of
selenium-tellurium alloy to yield panchromatic light response;
and a bottom bulk layer of xerographic grade selenium or halogen
doped arsenic-selenium. Sechak does not disclose the member of
the instant invention, i.e., a ternary alloy, as the middle
layer. Sechak instead discloses a binary alloy as the middle
layer. Neither does Sechak appreciate the problem associated
with the loss of electrical properties of a tri-layer photo-
receptor upon aging at elevated temperatures, i.e., 115F and
above.
Chiou, U.S. Patent 3,861,913, discloses a xerographic
plate comprising a conductive substrate, a charge transport
layer and a charge generation layer consisting essentially of
from 5 to 35 percent by weight tellurium and from 0.5 to 20 per-
cent by weight arsenic, with the substantial balance being
vitreous selenium. This patent does not disclose the tri-layer
10750~8
member of the instant invention nor does this patent appreciate
the problem associated with the loss of electrical properties of
a tri-layer photoreceptor upon aging at elevated temperatures,
i.e., 115F. and above.
It has been discovered that when using a tri-layer photo-
receptor comprising a top layer of arsenic-selenium alloy; a
second or middle layer of selenium-tellurium alloy and a third
layer of vitreous selenium or arsenic-selenium, that upon accel-
erated heat aging or over an extended period of time at elevated - ---
temperatures, the electrical properties of the photoreceptor
greatly deteriorates. It is believed that a portion of the
tellurium in the middle layer migrates from this middle layer -- -
into the adjacent top or bottom layers resulting in the above-
mentioned degradation of electrical properties. Therefore, it
has been discovered that the addition of arsenic to the middle
layer forming an arsenic-tellurium-selenium alloy for the middle
layer in the tri-layer photoreceptor prevents degradation of
these electrical properties, upon accelerated heat aging or over
an extended period of time at above ambient temperatures. It is
believed that the addition of arsenic prevents migration of the
tellurium from this middle layer greatly extending the maximum
` capability of the tri-layer photoreceptor.
OBJECTS OF THE INVENTION
It is, therefore, an object of this invention to provide
a system for utilizing a composite three-layered photoreceptor
which overcomes the above noted disadvantages.
It is another object of this invention to provide a three-
layered photoreceptor having greatly improved stability of
electrical properties over an extended period of time at elevated
temperatures.
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`` iO75~t;8
It is yet another object of this invention to provide
a method of imaglng a novel photoreceptor.
It is another object of this invention to provide a
novel photoreceptor.
SUMMARY ~F THE INVENTION
The foregoing objects and others are accomplished in
accordance with this invention by providing a composite three-
layered photoreceptor. The three-layered photoreceptor comprises
a top layer of an arsenic-selenium alloy; a second or middle
layer of an arsenic-tellurium-selenium alloy which prevents
degradation of the electrical properties of the photoreceptor
upon accelerated heat aging or over an extended period of time
when exposed to above ambient temperatures; and a bottom layer
of vitreous selenium or an arsenic-selenium alloy or a halogen
doped arsenic-seLenium alloy. This multi-layered photoreceptor
may be coated or evaporated onto a standard xerographic base
by conventional techni~ues known to the art.
T~hus, in accordance with the present teachings, a
composite photoreceptor member is provided which comprises a
first layer of vitreous selenium, a second layer which comprises
a vitreous arsenic-tellurium-selenium alloy which overlies the
fLrst seleniam layer and a third layer which comprises a vitreous
arsenic-selenium alloy which overlies the second layer of
arsenic-tellurium-selenium alloy.
In accordance with a further embodiment of the
present teachings, a photoconductive member is provided which
comprises a support, a first layer of vitreous selenium which
overlies the support, a second layer of vitreous arsenic-
tellurium-selenium alloy which overlies the first selenium
layer and a third layer of vitreous arsenic-selenium alloy
overlying the second arsenic-tellurium-selenium layer.
More specifically in accordance with the present
B ~ s
` 10750f~
teachings, a photoconductive member is provided which comprises
a conductive support, a 40 to 100 microns thick first layer of
vitreous arsenic-selenium containing from about 0.1 to 2.0 percent
by weight arsenic, a 0.1 to 1.0 micron thick second layer of - -
vitreous arsenic-tellurium-selenium alloy which contains from
about 15.0 to 25.0 percent by weight of tellurium and from about
3.0 to 5.0 percent by weight of arsenic which overlies the first
layer of arsenic selenium, and a 0.1 to 5.0 micron thick third
layer of vitreous arsenic-selenium alloy which contains from
about 0.1 to 3.0 percent by weight arsenic which overlies the
second layer of arsenic-tellurium-selenium alloy.
By yet a further embodiment of the present teachings,
- an imaging method is provided which comprises providing a
photoconductive member comprising a conductive substrate coated
with a first layer of vitreous selenium, about 40 to 100 microns
thick, a second layer of vitreous arsenic-tellurium-selenium - -
alloy about 0.1 to 1.0-microns thick which overlies the first
layer of selçnium and a third layer of vitreous arsenic-selenium
alloy about 0.1 to 5.0 microns thick overlying the arsenic-
tellurium-selenium layer. A latent electrostatic image is
formed on the member and the image is subsequently developed.
BRIEF DESCRIPTION OF THE DRAWINGS
The advantages of this improved photoreceptor will
become apparent upon consideration of the following disclosure
of the invention, especially when taken in conjunction with the
accompanying drawings wherein:
Fig. 1 is a schematic illustration of a cross-
` sectional view of one of the embodiments of a multi-layered
~ xerographic photoreceptor as contemplated by this invention;
- 30 Figs. 2, 3 and 4 represent a plot of sensitivity,
initially and after accelerated heat aging, of a multi-layered
photoreceptor which contains a top layer of arsenic-selenium, a
,B ~ -5a-
`~` 1075068
middle layer of a binary alloy of tellurium-selenium and a
bottom layer of an arsenic-selenium alloy doped with a halogen.
The middle layer does not contain arsenic,
:
B -5b-
10750~;~
Figs. 5, 6, 7 and 8 represent a plot of the sensitivity
of the multi-layered device of the instant invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to Fig. 1 which shows a schematic drawing
of one of the embodiments of the instant invention which com-
prises a multi-layered xerographic photoreceptor. The multi- -
layered, i.e., three-layered structure, is referred to as a "tri-
layer" photoreceptor. Photoreceptor 4 may be self-supporting or
it may have a conventional support member 5 which may be either
electrically insulating or electrically conductive. Preferably,
support member 5 is a conventional electrical support such as -
brass, aluminum, nickel, steel or any other suitable material.
The support member 5 may be of any conventional thickness, rigid
or flexible and may be in any desired form such as a sheet, web,
plate, cylinder, drum or any other suitable shape. It may also
comprise any other materials such as metallized paper, plastic -
sheets coated with a thin layer of metal such as aluminum or
copper oxide or glass coated with a thin layer of chromium or tin
oxide. If desired, the photoreceptor may also be formed on an
electrically insulating support and electrically charged by
xerographic processes well known to the art of xerography for
photoreceptors having insulating backings. As mentioned above,
member 5 may, in some cases, be dispensed with entirely.
Layer 1 may comprise a first layer of vitreous selenium
or a vitreous arsenic-selenium alloy. Preferably, layer 1 com-
prises a vitreous arsenic-selenium alloy containing up to about
3.0 percent by weight arsenic. Layer 1 may contain from about
0.1 to about 2.0 percent by weight arsenic. More preferably,
layer 1 contains up to about 1.0 percent by weight arsenic and
most preferred, layer 1 contains up to about 0.5 percent by
weight arsenic with the balance selenium. When layer 1 is either
'
10750~
vitreous selenium or an arsenic-selenium alloy, layer 1 may also
contain a halogen dopant. The preferred halogen dopant consists
essentially of chlorine, bromine and iodine. The halogen dopant
is preferably present in concentrations of from about 5 parts
per million to about 10,000 parts per million. Layer 1 may be
from about 10 to about 300 microns thick, preferably from about
40 to 100 microns thick, more preferably from about 52.0 to 68.0
microns thick and most preferred 60 microns thick.
Layer 2, which is a second or middle layer, may comprise
a second layer of vitreous arsenic-tellurium-selenium alloy
which overlies the first layer 1. Middle layer 2 comprising a
second layer of vitreous arsenic-tellurium-selenium alloy may
contain from about 0.1 to about 40.0 percent by weight arsenic
and from about 1.0 to about 50.0 percent by weight tellurium and
from about 50.0 to 90.0 percent by weight selenium. More pre-
ferably, the arsenic-tellurium-selenium alloy layer 2 may contain
from about 3.0 to about 5.0 percent by weight arsenic and from
about 15.0 to about 25.0 percent by weight tellurium with the
balance selenium. Arsenic in the range of about 3.0 to 5.0 per-
cent by weight has been found to be particularly desirable inthe alloy of layer 2. The particularly desired composition of
layer 2 is about 4.0 percent by weight arsenic, about 20.5 per-
cent by weight tellurium and about 75.5 percent by weight
selenium. It is critical that layer 2 contain arsenic since the
presence of arsenic results in the photoreceptor retaining its
electrical properties upon accelerated heat aging or over an
extended period of time at elevated temperatures. i.e., above
115F
Middle layer 2 may preferably also comprise a vitreous
arsenic-tellurium-selenium alloy containing a halogen dopant.
The halogen dopant may be of the same material and concentration
as disclosed for layer 1.
-7-
':
-' 10750~;~
Layer 2 may be from about 0.1 to about 2.0 microns thick,
preferably from about 0.1 to 1.0 microns thick, more preferably
from 0.1 to 0.5 microns thick, and is most preferred at a thick-
ness of 0.3 microns.
Layer 3, the top or third layer, comprises a layer of
vitreous arsenic-selenium alloy which overlies layer 2. Layer 3
preferably comprises an arsenic-selenium alloy containing from
about 0.1 to about 40.0 percent by weight arsenic and preferably
from about 0.1 to about 5.0 percent by weight arsenic, and more
10 preferably from about 0.1 to about 3.0 percent by weight arsenic
with the balance selenium. Layer 3 may be doped with a halogen.
The halogen dopant may be the same material and in the same
concentration as disclosed for layers 1 and 2.
Layer 3 may be from about 0.1 to about 5.0 microns thick,
preferably from about 1.0 to 4.0 microns thick and is most pre-
ferred at a thickness of about 3.0 microns.
Member 4 may comprise a photoconductive member. The
member may comprise a support 5 and a first layer 1 of vitreous
selenium overlying support 5. Overlying layer 1 may be a second
20 layer 2 of a vitreous arsenic-tellurium-selenium alloy. Over-
lying layer 2 may be a third layer of a vitreous arsenic-selenium
alloy. The first layer 1 may be from about 40 to 100 microns
thick and contain up to about 1.0 percent by weight arsenic, pre-
ferably up to about 0.5 percent by weight arsenic with the
balance selenium. The second layer 2 of arsenic-tellurium- -
selenium may be from about 0.1 to about 1.0 microns thick and may
A contain up to about 40.0 percent by weight arsenic and up to
about 50.0 percent by weight tellurium, preferably up to about
5.0 percent by weight arsenic and up to about 25.0 percent by
- 30 weight tellurium with the balance selenium. The third layer 3 of
arsenic-selenium alloy may be from about 0.1 to about 5.0 microns
-8-
1075068
thick and may contain up to about 40.0 percent by weight arsenic
preferably up to about 5.0 percent by weight arsenic with the
balance selenium. All three layers, i.e., layers 1, 2 and 3,
may be doped with a halogen. Preferably, a halogen selected
from the group consisting of chlorine, bromine and iodine in a
preferred concentration of from about 5 parts per million to
about 10,000 parts per million.
In conventional xerography when substrate 5 is electri-
cally conductive, it is generally grounded during a charging
step to facilitate the deposit of a uniform layer of charge upon
layer 3. Charging of the photoreceptor 4 is readily accomplished
in a variety of ways, for example, by rubbing layer 4 with a
soft brush or fur or more preferably, by utilizing corona
charging techniques and devices, for example, as described in
Vyverberg, U.S. Patent 2,836,725 and Walkup, U.S. Patent
2,777,957. Charging is usually accomplished in the absence of
actinic radiation, i.e., that radiation which makes the photo-
receptor 4 relatively more electrically conductive in radiation
struck portions. After charging, the next conventional xero-
graphic process step may be to expose the photoreceptor to apattern of electromagnetic actinic radiation thereby discharging
light struck areas of photoreceptor 4 relative to non-light
struck areas thereby forming a latent electrostatic image upon
or in the surface of the photoreceptor.
Other methods of forming a latent image on photoreceptor
4 are known in the art and include first forming such a charge
pattern on a separate photoconductive insulating layer according
~ to conventional xerographic reproduction techniques and then
- transferring this charge pattern to layer 3 of photoreceptor 4
by bringing the two layers into very close proximity and utilizing
breakdown techniques as described, for example, in U.S. Patents
~: .
~0750~i~
2,982,647 to Carlson and 2,825,814 and 2,937,943 to Walkup. In
addition, charge patterns conforming to selected, shaped elec-
trodes or combinations of electrodes may be formed on layer 3
by the "TESI" discharge technique as more fully described in U.S.
Patents 3,023,731 and 2,919,967 to Schwertz or by techniques
described in U.S. Patents 3,001,848 and 3,001,849 to Walkup as
well as by the preferred electron beam recording techniques
known in the art and as illustratively described in Glenn, U.S.
Patent 3,113,179.
Thereafter, the latent image is then generally rendered
visible, i.e., developed by contacting the latent image with
finely divided marking material generally electrostatically
charged to a polarity opposite to the polarity of the latent
electrostatic image, by bringing such material into surface con-
tact with layer 3 causing the material to be held thereon in a
pattern corresponding to the latent image.
Any suitable developing system may be used to develop
latent images on the photoreceptor of this invention and many
such systems exist in the art.
For example, the system of cascade development has found
extensive commerical acceptance and generally consists of gravi-
tationally flowing developer material consisting of a two com-
ponent material of the type disclosed in Walkup et al, U.S.
Patent 2,638,416, over the photoreceptor bearing the latent image.
The two components consist of an electroscopic powder called
"toner" and a granular material called "carrier" and which by
mixing acquire triboelectric charges of opposite polarity. In
development, the toner component, usually oppositely charged to
the latent image is deposited on the latent electrostatic image
to render that image visible.
--10--
- : , . .
1075~68
Other typical developing systems include magnetic brush
development, for example, see Giamo, U.S. Patent 2,930,351;
Simmons et al, U.S. Patent 2,791,949 and Hall et al, U.S. Patent
3,015,305; fluid development, for example, see Carlson, U.S.
Patents 2,221,776; 2,551,582; 2,690,394; 2,761,416 and 2,928,575;
Thompson, U.S. Patent 3,064,622; Gundlach, U.S. Patents 3,068,115
and 3,084,043 and Metcalfe, U.S. Patents 2,907,674; 3,001,888;
3,032,432 and 3,078,231; skid development, for example, Mayo,
U.S. Patent 2,895,847, and others.
Once developed, the loosely adhering powder image may be
transferred to another support surface to which it may be affixed
by solvent vapors, heat or other suitable means to render the
image indefinitely usable or the loose powder image may be
affixed directly on the photoreceptor either as a result of
developing or a separate step thereafter.
Referring now to Fig. 2 which represents a sensitivity
plot of a tri-layer photoreceptor member with a top layer about
3.0 microns thick, of an alloy of about 0.5 percent by weight
arsenic and 99.5 percent by weight selenium, a middle layer,
about 0.3 microns thick, of an alloy of about 25 percent by
weight tellurium and 75 percent by weight selenium and a bottom
layer about 60.0 microns of an alloy of about 0.33 percent by
weight arsenic and 99.66 percent by weight selenium doped with
20 parts per million of chlorine. As mentioned above, this
photoreceptor does not have arsenic in the middle or second
layer. This member is exposed to a helium neon beam laser which
emits activating radiation at 6328 angstroms. Up the left side
of the graph is shown surface potential in volts. Across the
bottom of the graph is relative exposure in microwatts of laser
power. The above described member was initially charged to a
surface potential of 800 volts and stepwise discharged by pro-
` ' 1075068
gressively exposing the photoreceptor to larger amounts of acti-
vating radiation measured in microwatts using a helium neon
laser emitting activating radiation at 6328 angstroms. After a
number of exposures, the member was discharged significantly,
i.e., to approximately 110 volts. The initial sample was then
subjected to accelerated heat aging for 7 days at 125F. The
member was again recharged to a surface potential of 800 volts
and stepwise exposed to activating radiation under the same con-
ditions as initially. After the same number of exposures and
; 10 under the same conditions as used initially, the member dis-
charged to about 700 volts. This is represented by the top line
in Fig. 2.
Referring now to Fig. 3 which also represents sensitivity
curves using a tri-layer member similar to the one described in
conjunction with Fig. 2, which also has no arsenic in the middle
- layer. The member was exposed initially, after accelerated heat
aging for 3 days at 125F. and then after accelerated heat aging
for 7 days at 125F.
Referring now to Fig. 4 which illustrates using a similar
20 member as in Fig. 3 under the same conditions except that the
member was exposed initially, and after accelerated heat aging
? for 7 days at 125F. --
- Referring now to Fig. 5 which represents standard sensi-
tivity curves of a member of the instant invention taken initially
and after 7 days accelerated heat aging at 125F. The member of
Fig. 5 represents a member containing a top layer, about 3.0
microns thick, of an alloy of about 0.5 percent by weight arsenic
and 99.5 percent by weight selenium and a middle layer, a ternary
alloy, about 0.65 microns thick, of about 3.5 percent by weight
30 arsenic and 20.5 percent by weight tellurium with the balance
being selenium and a bottom layer, about 60 microns thick, of an
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.. . . .
.
.' : . ' - --:
107506~
alloy of about 0.33 percent by weight arsenic and 99.66 percent
by weight selenium which is doped with 20 parts per million of
chlorine. The member was initially charged sufficiently to a
surface potential of 800 volts then discharged by exposing to
activating radiation, i.e., microwatts, from a helium neon laser
emitting activating radiation at 6328 angstroms. The member
initially discharged to a surface potential of about 75 volts.
After accelerated heat aging for 7 days at 125F. the same mem-
ber also discharged to a surface potential of about 75 volts.
Referring now to Figs. 6, 7 and 8 where the member was, -
in both Fig. 6 and Fig. 8, accelerated heat aged and tested
initially, after 3 days at 125F. and after 7 days at 125F. As
the Figs. 6, 7 and 8 indicate, none of the members substantially
lose their sensitivity or electrical properties after accelerated
heat aging. The same results were observed as shown in Fig. 7
except that the member in Fig. 7 was tested initially and aft~r
accelerated heat aging for 7 days at 125F.
It has been discovered that when using a tri-layer photo-
receptor comprising a top layer of arsenic-selenium alloy, a
second or middle layer of selenium-tellurium alloy and a third
or bottom layer of vitreous selenium or arsenic-selenium that
upon accelerated heat aging or over an extended period of time
at elevated temperatures, i.e., above 115F., the electrical
properties of this tri-layer photoreceptor greatly deteriorates.
It is believed that a portion of the tellurium in the middle
layer of this tri-layer photoreceptor migrates from the middle
layer into the adjacent top or bottom layers of the photoreceptor
resulting in the degradation of the electrical properties of the
photoreceptor. Therefore, it has been discovered that the addi-
tion of arsenic to the middle layer of the tri-layer photoreceptor
forming a ternary alloy of arsenic-tellurium-selenium in the
-13-
` 10750~8
middle layer, prevents degradation of these electrical properties
upon accelerated heat aging or over an extended period of time at
above ambient temperatures, i.e., above 115F. It is believed
that the addition of the arsenic prevents migration of the tel-
lurium from the middle layer thereby greatly extending the maxi-
mum capability of the tri-layer photoreceptor.
The tri-layer photoreceptor of this invention may be
prepared by any suitable technique. A typical technique includes
vacuum evaporation wherein each photoconductive layer is sequen-
tially evaporated onto its corresponding base material. In thistechnique, the bottom or first layer of selenium or arsenic-
selenium alloy, the middle or second layer of arsenic-tellurium
selenium alloy and the top or third layer of arsenic-selenium
alloy layers are each evaporated by separate steps, under vacuum
conditions varying from about 10 5 to 10 7 torr. In another
embodiment of this technique, the three photoreceptor layers are
continuously vacuum evaporated, one right after another, in the --
same vacuum chamber without breaking the vacuum, by sequentially
activating three separate sources of selenium or selenium-arsenic
20 for the bottom or first layer, and arsenic-tellurium-selenium - --
for the middle or second layer and arsenic-selenium for the top
or third layer.
Another typical technique includes co-evaporation, where-
in the appropriate amount of material for each of the alloy layers
is placed in separate heated crucibles maintained under vacuum
conditions, with a source temperature of each alloy constituent
being controlled so as to yield the appropriate percentage of
the alloy desired. This technique is illustrated in U.S. Patent
3,940,903.
Another typical method of evaporation includes flash
evaporation under vacuum conditions similar to those defined in
-14-
750~i~
co-evaporation, wherein a powder mixture such as arsenic, tellur-
ium and selenium, are selectively dropped into a heated crucible
maintained at a temperature of about 400C. to 600C. The
vapors formed by the heated mixture are evaporated upward onto
a substrate supported above the crucible.
In all the above methods, it is desired that the substrate
onto which the photoconductive material is evaporated, is main-
tained at a temperature of from about 50C. to about 80C. If
desired, a water cooled platen or other suitable cooling means
may be used in order to maintain a constant substrate tempera-
ture. In general, a selenium base or bulk layer thickness of
about 60 microns is obtained when evaporation is continued for
about 1 hour at a vacuum of about 5 x 10 5 torr at a crucible
temperature of about 280C.
Ullrich, U.S. Patent 2,803,542; Mayer et al, U.S. Patent
2,822,300; Dessauer et al, U.S. Patent 2,901,348, Schaffert, U.S.
Patent 2,963,376; Bixby, U.S. Patent 2,970,906 and Blakney et al,
U.S. Patent 3,077,386 all illustrate vacuum evaporation techniques
which are suitable for the formation of alloy layers of the
instant invention. The crucibles which are used for evaporation
for the photoreceptor layers may be of any inert material such as
quartz, molybdenum, stainless steel vacuum coated with silicon
monoxide, or any other equivalent materials. The selenium or
selenium alloy being evaporated is maintained at a temperature
between above its melting and boiling points.
The photoreceptor of the instant invention exhibits the
ability to maintain its electrical properties upon accelerated
heat aging or over an extended period of time at above ambient
temperatures, i.e., temperatures above 115F. The photoreceptor
of the instant invention also has improved dark decay properties.
Furthermore, sensitivity curves and the increase in the Mottling
-15-
10750~;8
index, i.e., step-by-step charging of the photoreceptor, indicates
that the photoreceptor of the instant invention does not sub-
stantially lose its electrical properties upon accelerated heat
aging or over an extended period of time at above ambient temper-
atures, i.e., above 115F. Mottling indexes clearly show that
the photoreceptor of the instant invention after accelerated heat
aging retains substantially the same sensitivity, i.e., electrical
properties, as when initially tested.
When a photoreceptor does not have the ability to be
uniformly charged and uniformly maintain a surface potential, it
is not a desirable photoreceptor for cyclic use. The instant
photoreceptor has excellent charge uniformity properties and
exhibits the ability to uniformly maintain a surface potential.
; Tri-layer photoreceptors without arsenic in the middle or second
layer, i.e., a middle layer of arsenic-tellurium-selenium, lose
their ability to be uniformly charged and to maintain a uniform
surface potential upon accelerated heat aging or over an extended
period of time at elevated temperatures, i.e., temperatures
above 115F.
The following examples further specifically define the
present invention with respect to the method of making a tri-
layer photoreceptor member of the instant invention. The per-
; centages in the disclosure, examples and claims are by weight
unless otherwise indicated. The examples below are intended to
illustrate various preferred embodiments of making a tri-layer
photoreceptor. The photoreceptor alloy elements described in
the specification and examples; arsenic, tellurium and selenium,
are all high purity xerographic grade materials (arsenic and
- selenium 99.999 percent by weight pure and tellurium 99.99 percent
by weight pure) available from Canadian Copper Refiners.
-16-
~ 1075C~
EXAMPLE I
An oxidized aluminum substrate, i.e., an aluminum drum
with a thin aluminum oxide coating thereon, approximately 3 3/8
inches in diameter and 10 3/8 inches long, is placed in a vacuum
chamber. A starting alloy for the first or bottom layer of the
tri-layer photoreceptor is prepared by placing elemental selenium
shots about 1/16 to 3/16 inches in diameter in an SiO coated
stainless steel evaporation boat. The evaporation boat is posi-
tioned above 6 inches below the surface of the drum.
A starting alloy for the second or middle layer of the
tri-layer photoreceptor is prepared by weighing pellets about
1/16 to 3/16 inches in diameter of elemental arsenic, tellurium
and selenium in a ratio of 20.5 percent by weight of tellurium;
4.0 percent by weight arsenic and 75.5 percent by weight selenium
in a second SiO coated stainless steel boat which is placed
adjacent to the first SiO coated stainless steel boat.
A starting alloy for the third or top layer of the tri-
layer photoreceptor i5 prepared by weighing pellets about 3/8 to
1/2 inches in diameter of elemental arsenic and selenium in a
; 20 ratio of 0.5 percent by weight arsenic and 99.5 percent by weight
selenium in a CrO, chromic oxide, boat which is placed adjacent
to the first SiO coated stainless steel boat. Each boat is con-
nected directly to a source of electrical power adaptable to
control the temperature of the respective boats. The chamber is
then evacuated to a vacuum of about 10 5 torr while the drum is
rotated about 10 RPM. The first boat containing the selenium is
heated to a temperature of about 310C. over a period of about
35 minutes to form a layer of vitreous selenium about 60 microns
thick on the aluminum drum. The electrical power to the first
boat containing the selenium is still maintained in order to pre-
vent any buildup on the boat of the other materials being evapo-
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750~;~
rated. Without brea~ing the vacuum, the drum speed is increased
to about 30 RPM. The temperature of the second boat containing
the arsenic-tellurium-selenium alloy is gradually increased to
450C. over a period of about 8.5 minutes and maintained at that
temperature for about 1 minute to form an arsenic-tellurium-
selenium alloy coating about 0.3 microns thick on the first or
bottom selenium layer. The power to the second boat is main-
tained. The drum speed is maintained at 30 RPM.
The temperature of the third boat containing the arsenic-
selenium alloy is gradually increased to 350C. over a period of
about 10 minutes and maintained at that temperature for about 7
minutes to form a 3.0 micron thick top layer of arsenic-selenium
alloy on the middle layer of the arsenic-tellurium-selenium
alloy. At the end of this time the vacuum chamber is cooled to
room temperature, the vacuum broken, and the drum containing the
tri-layer photoconductor is removed from the chamber.
EXAMPLE II
The drum containing the tri-layer photoreceptor of
Example I is then charged to a positive potential by the method
described by Carlson in U.S. Patent 2,588,699. The corona
charging unit is maintained at about 7,500 volts thereby charging
the drum to an acceptance potential of about 850 - 900 volts.
An analog signal obtained from a written document is put into a
helium-neon laser which emits activating radiation at about 6328
angstroms, modulating the laser's intensity in accordance with - -
the video information onto the drum. As a result, the drum is
imagewise exposed forming a latent electrostatic image thereon.
This method is described by Mason in U.S. Patent 3,870,816. The
drum bearing the latent electrostatic image is then developed by
cascading an electroscopic marking material over the photocon-
ductive surface of the drum. The developed image is transferred
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' 10750f~
to a sheet of paper and heat fused to make it permanent. The
final image exhibits good resolution, high density and is an
excellent copy of the original.
EXAMPLE I I I
A drum as prepared in Example I is placed in a Xerox
X200 telecopier and a line copy of an original image of black on
white on conventional printing paper is made. This image also
exhibits good resolution and high density.
EXAMPLE IV
A drum as prepared in Example I and as charged in
Example I is exposed to a black on white line copy original
image contained on conventional printing paper. A 15 watt
General Electric cool white fluorescent lamp No. F15T8CW is
used as the exposure source to form a latent electrostatic
image. The drum bearing the latent electrostatic image is then
developed by cascading an electroscopic marking material over
the photoconductive surface of the drum. The developed image
is transferred to a sheet of paper and heat fused to make it
permanent. The final image exhibits good resolution and high
density and is an excellent copy of the original.
.
~ 30
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