DE2810859A1 - PHOTOCONDUCTIVE MATERIAL AND METHOD FOR MANUFACTURING IT - Google Patents

Description

PATENT LAWYERS

DR. CLAUS REINLÄNDER DIPL.-ING. KLAUS BERNHARDT

Orthstrasse 12 D-8000 Munich 60 Telephone 832024/5 Telex 5212744 Telegrams Interpatent

• 6-

Vl P465 D

13. M-J- 1978

Varian Associates, Inc. Palo Alto, CaI., USA

Photoconductive material and "method for its manufacture

Priority: July 11, 1977 - USA - Ser. 814.512

summary

A photoconductor of the cadmium sulfide / cadmium carbonate type is described which contains cadmium oxide in the range from 0.05 to 1.5 mol % . The calcination of this photoconductor is performed in the relatively high temperature range of about 250 C to 3IO C in an inert atmosphere or in air, whereby preferably between 265 ° C and 290 ⁰ C. The resulting photoconductor, which can be used in both toner transfer and charge transfer electrophotography, has higher photographic sensitivity, faster discharge of residual charge, and less deterioration in electrophotographic properties, particularly charge acceptance at high humidities.

809883/0590 " ^{m / 2}

Background of the invention

The invention relates to an improved photoconductor of the cadmium sulfide / cadmium carbonate type and a method its manufacture. The photoconductor is typically fixed in an organic binder and is both in toner transfer as well as charge transfer electrophotography useful.

Many patents and other publications describe methods of making photoconductors of the cadmium sulfide / cadmium carbonate type dispersed in an organic binder (U.S. Patent 3,494-789; 3,506,595; 3,541,028; 3,589,928; 3,615,401; 3 704 123 and 3 867 139).

In the earlier US patent application 501 566 it is taught that it is important to keep ionic impurities out of the end product and processes are described remove these ionic contaminants and ensure that they remain removed. It is described in detail that

1. Instead of cadmium sulphide one cadmium selenide or a mixture of cadmium sulphide and Can use cadmium selenide;

2. A useful range for the amount of cadmium carbonate between 0 and 4 moles of cadmium carbonate per mole of cadmium sulfide, cadmium selenide, or a mixture thereof;

3 · the ionic impurities from which the resulting end product is free include sodium, chlorine, bromine, potassium and nitrate; and

... / 3 809883/0690

4. The photoconductor is activated with a dopant which belongs to the group copper chloride, silver chloride, Copper sulfate, silver sulfate, copper nitrate, Silver nitrate, copper ammonia complex, silver ammonia complex, dissolved in ammonia Copper carbonate and silver carbonate dissolved in ammonia is selected.

U.S. Patent Application 715,804 teaches that the Calcination in an inert atmosphere, e.g. nitrogen gas instead of air, is a good protective measure, in order to prevent the introduction of undesired ionic contamination, particularly the sulphation.

However, electrophotographic plates made according to the teachings of the prior art suffer certain restrictions. These include the following:

1. The photographic sensitivities are not high enough for some applications, for example in copiers where a high throughput rate is desired and / or the amount the light available for exposure is limited.

2. Together with low sensitivities, one often encounters a "memory effect". For example some copier machines that use a cylinder coated with a photoconductive layer can be used is manufactured according to the prior art, more than one complete cylinder revolution to manufacture each require a copy, and an unwanted "ghost" image can add to the desired image be superimposed. With more sensitive photoconductors

... A 809833 / 0S90

2810853

the remaining charge is discharged more fully using an erasing light, and ghosting don't appear so easy.

3. At high humidities there will be a slight but noticeable deterioration in the electrophotographic Properties observed, especially the charge acceptance. It is possible that under the calcining conditions and methods for the photoconductive powder used in the prior art are taught that a small residue of ionic substances can be present even if this is the case is not easily ascertained by analysis.

These and other disadvantages of the prior art are overcome by the invention, which includes the following Tasks are based on:

The main object of the invention is to provide a photoconductor of the cadmium sulfide / cadmium carbonate type with high photographic properties Sensitivity coupled with sufficient charge acceptance for most copier applications.

Furthermore, the memory or storage effect should be eliminated by the invention, which is less sensitive photoconductors is associated.

Another aim of the invention is to produce a photoconductor with a more reliable operating behavior in high humidity environments.

The invention is also intended to enable the production of a photoconductor which is particularly suitable for sequential charge transfer electrophotography.

281085S

Furthermore, a photoconductor is to be formed by the invention, which has a high stability of the sensitivity In relation to time.

The invention is also intended to make a photoconductor available, the spectral characteristic curve of which for various Use cases is tailored.

Summary of the invention

In short, according to the above objects, the invention relates to an electrophotographic layer, in which is the cadmium sulfide / cadmium carbonate type pigment, and a method for producing such an electrophotographic layer. Instead of cadmium sulfide you could Use cadmium selenide or a mixture of cadmium sulfide and cadmium selenide. The final calcination of the electrophotographic powder is carried out at a high temperature, for example 275 ° C For hours, preferably in an inert atmosphere, to improve performance in high humidity and to achieve high photographic sensitivity.

The powder is characterized by the "presence of a small but measurable amount of cadmium oxide, for example 0.05 to 1.5 mole percent and preferably 0.2 to 1 mole percent CdO. In fact, it should be referred to as a cadmium sulfide / cadmium carbonate / cadmium oxide photoconductor Controlling the amount of CdO for optimal performance is critical.

Photoconductors made from such powders show an almost panchromatic spectral characteristic. This characteristic is further developed for any special applications by controlling the doping optimized. 809S £ 3 / Ö590 ,, -

Binder layers made from such powders have a memory or memory effect that is derived from Device designers can be easily handled.

These and other detailed and specific objects and features of the invention will be apparent from the following Description in connection with the drawing; show it:

1 shows a flow diagram of the method for preparation the photoconductor according to the invention;

2 schematically shows the hydrogen sulfide conversion step the method for producing the photoconductor layer according to the invention; and

Fig. 3 schematically shows the furnace used in the calcining step of the production of photoconductive Layers according to the invention is used.

Details of the preparation of the photoconductor are given in the following examples, with reference to the drawings, particularly Fig. 1, reference is made.

Example 1:

The following materials are placed in a ceramic grinding jar (Norton size 00, capacity 1.13 liters), the 900 g grinding cylinder (Norton Burundum, 1/2 "χ 1/2") contains:

Cadmium carbonate reagent

(Allied Chemical Code 14-83, lot E069) 140 g

CuCl ₂ + 2H ₂ 0 (reagent), 1% solution (dopant) 56 ml

Deionized water 5 ^ -0 ml

803883/0690 '

2810855

Grind

The jar is placed on rollers for 23 hours and the mixture is milled at a speed of 76 revolutions per minute. It is then placed in a 1250 ml beaker 20 equipped with a mechanical or magnetic stirrer 30 (see Fig. 2). 70 ml of concentrated ammonium hydroxide reagent (28-30 % NH,) is added and stirring is continued for 5 to minutes before beginning the sulfide conversion step.

Sulfide conversion

During the sulfide conversion, a controlled amount of hydrogen sulfide (hydrogen sulfide) reacts with CdCO ,, to give a CdS-X CdCO -, - material. As an alternative, hydrogen selenide or a mixture of hydrogen sulfide and hydrogen selenide are used to react with the cadmium carbonate. This step is illustrated in more detail in FIG. A Tylan flow regulator 50 is set at 48 standard cc / min. Check valves 41, 42 and 43 are one-way valves that are inserted into inlet lines 72 and 74 and gas line 70, respectively, to insure that gas can only flow in the forward direction through the system. A gas filter 10 is inserted between check valves 41 and 42 and flow regulator 50 to protect the latter. Solenoid valves 91 and 92 inserted in inlet lines 72 and 74 upstream of check valves 41 and 42, respectively, are on-off valves that control the flow of gas flowing through inlet lines 72 and 74, respectively, and that are input by signals - and switched off, which come from a timer 95 which can be set to control the time in which N _{2 flows} through the inlet line 72 and

... / 8 809883 / 0S90

• A2-

HpS through, the inlet line 74- flows. The timer is via electrical with the solenoid valves 91 and 92 Lines 96 and 97 in connection. A glass frit as Dispersion bubbler 60 (porosity C) is connected to gas line 70 and into the carbonate slurry used just above the stirrer 30. The system is first purged with nitrogen gas, which is in enters the system through inlet line 72. The current is then switched to HpS entering through inlet line 74- for three hours, and the color of the suspension gradually changes from white to yellow-orange. Thickening of the slurry also occurs, and it may be desirable to add more deionized water either before or during the conversion. To three hours, the solenoid valve 92 is closed and the valve 91 is opened so that the gas becomes nitrogen is switched back and the remaining HpS is flushed out. The bubbler 60 is then removed from the cup 20, a wash bottle of deionized water is used to remove most of the precipitate, which sticks to the glass frit, and the product is ready to be filtered.

Filtration and washing

A centrifugal filter (Damon / IEC chemical centrifuge Division, model CH), which is equipped with a perforated stainless steel basket, is used to remove the liquid and wash the product. The basket comes with a strip of filter paper (e.g. Carl Schlecher and Schuell Co., No. 576) lined and placed in the centrifuge; the paper is wetted to ensure a proper fit and before filtration with deionized Hurled water. The slurry is then added relatively slowly to avoid overflowing over the top

809 * 83/0590 "* ^{/ 9}

-4- ./li ».

To prevent lip of the basket. The filtrate is collected first, because something overflow or an imperfect Filtration can occur on the first pass, if that is the case the liquid is returned, until it passes through with some degree of clarity. At this point the filter cake is filled with deionized water washed while spinning for about 60 minutes. The water is then turned off and the cake centrifuged dry until a network of cracks is observed, about another 15 minutes are required. The centrifugation speeds are about 2,700 rpm during the filtration3.3 «700 rpm during the washing and 4-200 rpm during spin dry.

dry

The basket containing the cake and filter paper is placed in a drying oven at about 110 for about 15 hours brought to 120 C to dry the material and remove some of the volatile salts. as Alternatively, the cake can be removed after two hours and transferred to a bowl for further drying will.

Breaking the cake

The dry filter cake consists of lumps that must be pulverized before calcination. That Filter paper is peeled off and the lumps either through a mortar and pestle and then through a sieve with 710 micron sieve opening (25 mesh per inch) or preferably through a laboratory mill Crusher (e.g. Quaker City Mill, manufacturer A.W. Sträub). This operation should take place under a Hood, and it is also desirable to cover the grinder with a sack to prevent dust formation to limit. The broken powder will

809883/0590 ' ^m ' ^{/ 10}

281085S

.AS-

in an aluminum trough 110 (30 cm χ 6.4- cm χ 3 cm high) brought and smoothed out. It is now ready to be calcined under a stream of dry nitrogen to become.

Calcining

Temperature uniformity and control in the calcination step is very important and the calciner (Fig. 3) is designed to provide a long zone of uniform temperature. The calcination may be carried out at temperatures in the range of 25O ⁰ C to 310 ⁰ C, with 275 ° C are preferred in the event that the firing is carried out in nitrogen and 29o ⁰ C are preferred when the firing is carried out in air. The furnace tube 120 is divided into two parts, a furnace body 122 and a furnace head I50. The two parts of the furnace are connected to one another by means of a flange 152. The two halves of flange 152 surround an O-ring 154- forming a vacuum seal and are gripped with one or more clamps I56 that hold body 122 and head 150 tightly together. Two circular heat shields 14-0 and 14-5 fit within the cylindrical interior cavity of the furnace body 122 and serve to keep as much heat as possible within a "hot zone" of the furnace body 122. Each heat shield is pierced with a small opening 14-1 or 14-6, which are used to allow a gas flow from the “hot zone” to the remaining parts of the furnace body 122 and vice versa. A nitrogen gas supply is arranged so that nitrogen gas is supplied to furnace head I50 through nitrogen supply line 216. At one point of the nitrogen supply line 216, a nitrogen flow meter 210 is arranged, which is provided with a regulating valve 212, which continuously controls the nitrogen flow rate through the supply line 216.

809883/0590 " ^{m / 11}

281085S

The heat shield 140 is at least one part that is virtually permanently attached to the furnace body 122. The trough HO ₁ containing the broken powder is inserted into the furnace tube 120, followed by a measuring thermocouple and the heat shield 14-5 leads to an indicator pyrometer 155 which measures the temperature in the "hot zone" of the furnace, that is, the zone which lies between the heat shields 14-0 and 14-5. An additional hole 14-7 may be drilled in the heat shield 14-5 to allow external connection to the thermocouple 130. The furnace head 150 is then clamped to the furnace body 122. To effectively remove air from the powder, the following sequence of events is performed:

1. The nitrogen supply valve 160 and the tube exit valve 170 are closed. These valves are on-off valves that fit into the nitrogen supply line 216 and the outlet line 175 are inserted to the gas flow in the furnace tube 120 and out of this check.

2. The vacuum pump 180 is started and the vacuum valve 190 in the furnace head is opened. This The valve is an on-off valve that, when open, allows the pump 180 to vent air out of the Remove tube 120.

3. Continue pumping until the pressure gauge shows 30 "of mercury (762 mm of mercury). This should take about 5 minutes.

4-, The vacuum valve 190 is closed, the nitrogen regulating valve 212 torn open and that

... / 12

809833/0690

Nitrogen supply valve 160 is opened. the The flow rate should be dampened so that powder is not blown all over the furnace: it is advisable to keep it below a standard liter / minute.

5. When the pressure is close to atmospheric, will the nitrogen supply valve 160 closed, so that the nitrogen flow from the nitrogen supply 215 is stopped in the pipe 120.

6. The vacuum valve 190 is opened again and the Oven pumped down a second time to remove gas from stovepipe 120 (which is on the second round degassing faster).

7. Steps 3 and 4- are repeated »

8. When the pressure has reached atmospheric pressure again, the vacuum pump 180 is switched off and that Vacuum valve 190 is opened briefly to relieve the pump. The vacuum valve 19Ο is closed again.

9. If the pressure slightly exceeds atmospheric pressure, the outlet valve 170 is opened and the nitrogen flow is increased to 500 cc / min by means of of the nitrogen regulating valve 212 is set.

10. The timer 220 is set to turn on the oven power for a desired length of time e.g. 6 hours plus 45 minutes of warm-up time. The timer that works with an AC power source 225, supplies a temperature controller 230. The temperature controller 230 is set to use the oven

8098Ö3 / 0S90 "° ^{/ 13}

281085S

holds at 275 ° C. The heating element 24-5, which is the stovepipe 120 is connected to the temperature controller 230 via an electrical line 240. A Regulation thermocouple 250 measures the temperature within the hot zone of the furnace and forwards this information via line 255 to the controller 230 Further.

11. The calcined powder is ^{cooled to 150 °} C. or lower under nitrogen, after which it can be removed from the furnace and stored.

The calcined powder has a CdS: CdCO molar ratio close to 1.

During the calcination, any remaining salts that were not previously removed by the washing process are volatilized. A Part of the chloride introduced as cupric chloride can also volatilize as ammonium chloride. In this However, no copper is lost in step.

Mix with binder and solvents

The photoconductor powder can be coated with an electrically insulating Binders and solvents can be added to substrates by any of the known methods to be applied, for example spraying on, dipping or bringing by knife-on. A typical one Mixture for spraying is prepared as follows:

Calcined photoconductor powder 140 g Acryloid AT-50 (a thermosetting

Acrylic resin from Rohm & Haas Co.) 130 g

SC-IOO (an aromatic hydrocarbon solvent with
a boiling range of 161-177 C, Central Solvents & Chemical Co.) 205 ml

... / 14 809883/0590

-Vf-

n-butanol 14 ml

2-ethoxyethyl acetate 22 ml

1% solution of DC-200

(Silicone oil, Dow Corning Corp.)

in xylene 2.5 ml

Grind

This preparation is ground in a ceramic vessel (Norton size 00), the 900 g 1/2 "χ 1/2" Burundum Grinder cylinder (Norton Company) contains 23 hours at 76 revolutions of the vessel per minute. The viscosity the mixture is then adjusted by adding a solvent of 92 parts by volume SC-100/8 parts by volume n-butanol is added to a value of 25 seconds below Using a Zahn No. G2 Cup Viscometer (General Electric Co.). The paint is sifted through a 4A micron (325 mesh per inch) screen and is then ready to be sprayed on. Because of the high specific gravity of the photoconductive pigment, it is It is advisable to equip the spraying system with a system for circulating the paint. One not immediately The mixture used is placed in a jar and rolled without grinding media.

Coating a substrate

The mixture is applied to a conductive substrate or "plate", usually in a Final thickness from 20 to 35 microns. The term "plate", As used here, in addition to a flat plate, it can also refer to a cylinder or a flexible belt. Suitable substrate materials are, for example, aluminum, aluminized plastic film, zinc oxide-coated film Glass, etc.

... / 15

809803/0690

■ 3ο-

Drying and hardening

The sprayed panel is first dried, cured, with or without support by mild heating, then for one hour in an air circulating oven set at 200 ⁰ C. It is then cooled and is ready for use in an electrophotography machine.

Example 2 :

Instead of procuring a cadmium carbonate, the material will prepared by starting from pure compounds containing cadmium and carbonate. Because The importance of avoiding ionic contamination in the photoconductor must limit the choice of such chemicals that any undesirable residues of such reactions are removed during one of the processing steps (e.g. washing, calcining). The cadmium salt could be nitrate or acetate, the carbonate compound could be ammonium carbonate or bicarbonate, or COp itself. The carbonate precipitate is caused by the fact that solutions be mixed or COp by a cadmium salt solution is bubbled, as is known in the art. The copper chloride doping solution can with the Cadmium solution can be mixed before precipitation or can be added after precipitation, however before hydrogen sulfide is bubbled through. From the addition of ammonia and conversion onwards the steps described in Example 1 carried out.

example 3-

This is identical to Example 1, except that the calcination is carried out in an open-air furnace is open. This has the disadvantage that some un-

809833/0590 ... / 16

• 34-

desired sulfate by air oxidation of cadmium sulfide is generated, which degrades the long-term stability of photoconductors made of such a material are made. Nevertheless, the higher calcination temperature again results in a sensitive photoconductor, the is clearly useful in use cases where long-term stability is not a serious problem. It was found that for comparable photoconductor sensitivities the raw material, which is calcined in air, is fired at a slightly higher temperature should be used as raw material to be calcined in nitrogen, e.g. 290 C compared to 275 C.

The higher photoconductor sensitivities achieved by the higher calcining temperatures were completely unexpected for both the air calcining and nitrogen calcining examples. There are good reasons to believe that the higher sensitivity is due to the presence of a small amount of cadmium oxide in the photoconductor, which is calcined at higher temperatures. Above 25O ⁰ C, the decomposition rate of cadmium carbonate according to the reaction CdCO, ♦ £. CdO + COp noticeable and of course the higher the temperature, the greater the extent of decomposition. For this reason, tight control of the calcining conditions is necessary in order to achieve reproducible photoconductor properties.

The usable range of the cadmium oxide concentration in the Photoconductor is 0.05 to 1.5 mol%, and the preferred one Range is 0.2 to 1.0 mole percent. When the amount of CdO becomes too large, the charge absorption capacity of the begins Ladder to fall off.

... / 17 809883/0590

- is? -

• 38-

The amount of CdO present is below the detection limit of a routine X-ray diffraction analysis. However, it can be determined by burning a sample of the photoconductor in a stream of carbon dioxide and measuring the weight gain. The following procedure is used: A clean boat (for example, molten silica, alumina) is tared, filled with a sample powder and calcined photoconductor for several hours in an oven at 75 - 100 ⁰ C maintained. It is then cooled in a dryer and weighed. The boat is then placed in the calcining furnace (Fig. 3) described above, which in the present case is connected to a source of carbon dioxide instead of nitrogen. Before heating, steps 1 to 9 described above for the calcining procedure are carried out. The firing under a COP current is carried out at about 22O ⁰ C for three to four hours, and then the sample is cooled under CO _2. The boat is then in the oven at 75 - returned 100 ⁰ C, cooled in the dryer, and reweighed. The weight gain is converted into an equivalent amount of cadmium oxide according to the equation CdO + CO ₂ - ^ CdCO.

Spectral behavior and doping

The photoconductive layer described here is over most of the spectrum of visible light: in the possibility of copying different colors, it is close to a panchromatic characteristic curve. The hardest color to copy is yellow, and red colors are more difficult to copy than the cyan parts of the spectrum, this is especially true for Copiers that are equipped with an incandescent light source for exposure that produce high levels of red light supplies.

... / 18

809883/0 590

The spectral behavior of the photoconductor is influenced somewhat by the degree of doping. This is especially true for the red end of the spectrum. As mentioned in the examples, cupric chloride is a representative dopant. A useful doping range is between 0.05 and 2.0 mol % copper, based on the total number of moles in the photoconductor powder, that is, the total number of moles containing either cadmium or copper, with a preferred range of 0.1 to 1, 5 mole percent. At lower doping levels, the ability to copy red image areas is improved, but photoconductor memory is somewhat higher and the constancy of print density suffers somewhat. The reverse applies at higher doping levels. The doping level in the examples (about 0.4% ) gives a generally useful photoconductor. The selection of a doping concentration is determined by the specific application of the photoconductor.

809883/0590

e one side

Claims (24)

Vl F4-65 Claims Photoconductive.es material consisting of an electrically insulating binding material and small particles of a photoconductive powder which is substantially uniformly dispersed in the binding material, characterized in that the photoconductive powder consists of cadmium sulfide, cadmium selenide or a mixture thereof, between 0 and about 4 moles of cadmium carbonate per mole of cadmium sulfide, cadmium selenide, or mixtures thereof, and between about 0.05 and about 1.5 mole percent cadmium oxide. 2. Photoconductive material according to claim 1, characterized in that that the material is essentially free of ionic contaminants, except where appropriate of an activator dopant, is » 3. Photoconductive material according to claim 2, characterized in that that the ionic impurities are sodium, chlorine, sulfate, bromine, potassium and / or nitrate. 4 ·. Photoconductive material according to Claim 2, characterized in that that the ionic components are sodium, chlorine, bromine, potassium and / or nitrate. 5. Photoconductive material according to claim 2, characterized in that that the activator dopant copper chloride, silver chloride, copper sulfate, silver sulfate, copper nitrate, Silver nitrate, copper-ammonia complex, silver-ammonia complex, is copper carbonate dissolved in ammonia and / or silver carbonate dissolved in ammonia. O ../ A2 80 983 3/059 0 .1 6. Photoconductive material according to claim 2, characterized in that that the dopant is copper chloride. 7. Photoconductive material according to claim 6, characterized in that between 0.05 and 2.0 % of the totality of the moles of the photoconductor powder are moles of copper. 8. Photoconductive material according to claim 6, characterized in that that between 0.1 and 1.5 ^ of the total of the moles of the photoconductor powder are moles of copper. 9. A method for producing an electrophotoconductive plate, characterized by the following steps: Hydrogen sulfide gas, hydrogen selenide gas or a gas, comprising a mixture of hydrogen sulfide and hydrogen selenide is added to a cadmium carbonate suspension; the resulting product is dried and broken into a fine powder; the powder is between about 250 and about 310 ° C calcined to produce a photoconductor powder containing cadmium oxide; the photoconductor powder is mixed with binder and solvents to form a paste; and the paste is applied to a relatively more conductive substrate to create an electro-photoconductive plate. 10. The method according to claim 9, characterized in that the calcining at a
an inert gas takes place. calcining at a temperature of about 275 ° C in 11. The method according to claim 10, characterized in that the inert G-as is nitrogen. 12. The method according to claim 11, characterized in that the calcining is carried out for about 6 hours. ORIGINAL 13 · The method according to claim 9i, characterized in that the calcining in a '
is carried out in air. calcining at a temperature of about 290 ° C 14. The method according to claim 9, characterized in that the cadmium carbonate suspension is prepared by that cadmium carbonate reagent, an activator dopant and deionized water are mixed and ground. 15. The method according to claim 14, characterized in that the dopant copper chloride, silver chloride, copper sulfate, silver sulfate, copper nitrate, silver nitrate, copper-ammonia complex, Silver ammonia complex, in ammonia dissolved copper carbonate and / or silver carbonate dissolved in ammonia. 16. The method according to claim 14, characterized in that the dopant is copper chloride. 17. The method according to claim 16, characterized in that the amount of copper is between 0.05 and 2.0% of the total number of moles of photoconductor powder. 18. The method according to claim 9 »characterized in that the photoconductor powder consists of cadmium sulfide, cadmium carbonate and between 0.05 and 1.5 mol % of cadmium oxide. 19. The method according to claim 9, characterized in that the cadmium carbonate is prepared in that a Cadmium salt is reacted with a carbonate compound. 20. The method according to claim 9, characterized in that the photoconductive material is substantially free of ionic impurities, optionally excluding an activator dopant. 809883/0690 " 2810853 ■ k. 21. The method according to claim 20, characterized in that the ionic impurities sodium, chlorine, sulfate, Are bromine, potassium and / or nitrate. 22. The method according to claim 20, characterized in that the ionic impurities sodium, chlorine, bromine, Are potassium and / or nitrate. 23. Photoconductive material, consisting of an electrically insulating binder material and small particles of photoconductor powder, which are substantially uniformly dispersed through the binder material, characterized in that the photoconductor powder consists of cadmium sulfide, between 0 and 4 moles of cadmium carbonate per mole of cadmium sulfide, and between 0, 2 and 1.0 mole% cadmium oxide consists in that the material consists essentially of substance which is essentially free of ionic contamination, with the exception of between 0.1 and 1.5 mole % copper as an activator dopant. 24. A method of making a photoconductive material characterized by the steps of: bubbling hydrogen sulfide gas through a cadmium carbonate suspension containing copper chloride so that the resulting product contains between about 0.1 and 1.5 mole percent copper as an activator dopant; the resulting product is filtered, washed, dried and broken into a fine powder; the powder is calcined at about 275 ° C for about 6 hours in nitrogen gas to produce a photoconductor powder containing cadmium sulfide, between 0 and 4 moles of cadmium carbonate per mole of cadmium sulfide, and between 0.2 and 1.0 mole percent cadmium oxide; the photoconductor powder is mixed with an electrically insulating binder and solvents to form a paste produce; ... / A5 8098S3 / 0S90 the paste is applied to an electrically conductive substrate piled up; and the coated substrate is dried and cured to produce a photoconductive material; wherein the photoconductive material consists essentially of material which is essentially free of ionic Contamination is except that the material contains the noted amount of copper chloride dopant. 809833/0590