Electroconductive Magnetic Carrier, Developer Using the Same and Image Formation Method

1. Field of the Invention

The present invention relates to electroconductive magnetic carrier particles, a developer comprising the carrier particles and an electrophotographic image formation method using such a developer for use with a printer, facsimile apparatus and digital copying apparatus.

2. Discussion of Background

According to the electrophotographic image formation method based on the Carlson process, which is now widely employed, image formation is basically carried out in such a manner that the surface of a photoconductor is uniformly charged to a predetermined polarity and the photoconductor thus charged is selectively exposed to the original light images to form latent electrostatic images on the photoconductor. Then, the latent electrostatic images are developed with a developer, so that visible toner images can be obtained on the photoconductor. The visible toner images are then transferred to a sheet of an image-receiving medium and fixed thereon.

On the other hand, many proposals on the image formation method not using the Carlson process, but using the rear side exposure system have been reported, for example, in The Journal of the Institute of Image Electronics Engineers of Japan vol. 16, (5), 306 (1987); and Japanese Laid-Open Patent Applications 61-149968, 63-10071 and 63-214781, by which rear side exposure system the image formation apparatus can be made compact and the image formation process can be made simple.

In the rear side exposure system, the surface of the photoconductor is provided with a developer to form a developer resident portion, through which the photoconductor is subjected to a cleaning operation, and the photoconductor is uniformly charged. The light images are applied to the photoconductor from the rear side thereof and the latent images formed on the surface of the photoconductor are simultaneously developed into toner images with the developer.

However, there are too many difficult problems in the rear side exposure system to put it into practice. More specifically, the requirements for each function in the rear side exposure system are made extremely severe because it is necessary to inject the electric charge in a sufficient amount for the development into the photoconductor through the developer accumulated in the developer resident portion and to form sharp and stable toner images on the photoconductor by development at a relatively small developer resident portion.

In addition, it is necessary to impart the electroconductivity to the developer since the electric charge is injected into the photoconductor through the developer. Therefore, when a developer to be employed is a one-component type developer, an electroconductive magnetic toner is essentially required. The toner image thus formed on the photoconductor cannot be transferred to a sheet of plain paper by the electrostatic image transfer method such as corona transfer or bias roller transfer. As a result, only a sheet of paper with high resistivity can be used in this system.

The method of forming a multi-color recording image on a sheet of plain paper by the rear side exposure system is disclosed in Japanese Patent Publication 60-59592. In this method, however, since a photoconductor is prepared by overlaying an insulating layer on a photoconductive layer, the photoconductor cannot stand the repetition of formation of multi-color images thereon. To solve this problem, it is proposed that the residual latent image formed on the photoconductor be erased by application thereto of a transfer electrical field. This proposal is still insufficient in practice for obtaining clear images over an extended period of time.

As in the Journal of the Institute of Electrophotography Engineers of Japan vol. 27, No. 3, p.442 (1988) and Japanese Laid-Open Patent Application 61-46961, the image formation can be achieved by the rear side exposure and the simultaneous development system, with the application of a charging bias and a development bias to a photoconductor, having counter polarities, using a two-component type developer comprising iron carrier particles with a resistivity of 10.sup.4 to 10.sup.8 .OMEGA..cm and magnetic toner particles with insulating properties.

However, when the above-mentioned image formation method is applied to the practically-used copying apparatus, it is difficult to control the image formation system for obtaining a clear image over an extended period of time, and in addition, the structure of the apparatus necessarily becomes complicated.

Metallic particles such as copper particles are conventionally known as the electroconductive carrier in addition to the above-mentioned iron particles. However, when the particle size of such metallic particles is less than a definite value, the metallic particles conspicuously tend to cause aggregation, and at the same time, the surface of the metallic particles is oxidized, so that the resistivity thereof is increased. Therefore, the metallic carrier particles which constantly show low resistivity cannot be obtained, and it is difficult to produce metallic carrier particles with a particle diameter of as small as about 20 .mu.m. Furthermore, since the density of the metallic carrier particles is too large, the toner concentration in a developer cannot be increased, thereby lowering the image density.

In addition to the iron particles with a density of 7.8 g/cm.sup.3, magnetite particles with a density of 5.2 g/cm.sup.3 and ferrite particles with a density of 5.6 g/cm.sup.3 are conventionally known as the magnetic carrier particles. Any of the above-mentioned conventional magnetic carrier has extremely large density, so that a sufficient toner concentration cannot be obtained.

There is proposed in National Technical Report, Vol. 28, No. 4, Aug., 1982, an image formation system employing a resin carrier in which magnetic particles are dispersed in a binder resin. In this system, the resin carrier is electrically charged to a polarity of opposite to that of a toner to be employed. To produce images with an image density of 1.0 or more, it is required to set the surface potential of the employed photoconductive drum at 500 V or more. However, when the image formation is carried out with the surface potential of the photoconductive drum being increased to a high level as previously stated, the carrier is also attracted and transferred to the photoconductive drum together with the toner (hereinafter referred to as "carrier attraction phenomenon"). It is necessary to increase the particle size of the carrier to prevent the occurrence of the above-mentioned carrier attraction phenomenon. However, there are many problems in extremely increasing the particle size of the carrier from the viewpoints of guarantee of image quality, injection of electric charge into the photoconductive drum and transporting characteristics of the carrier.

Furthermore, the image formation methods by use of a magnetic carrier prepared by dispersing a magnetic material in a binder resin have been proposed. For example, a developer comprising the above-mentioned magnetic carrier and an electrically insulating non-magnetic toner is employed in Japanese Laid-Open Patent Applications 53-33152 and 55-41450; and a developer comprising the above-mentioned magnetic carrier and an electrically insulating magnetic toner is employed in Japanese Laid-Open Patent Applications 53-33152, 53-33633 and 53-35546. Furthermore, it has also been reported that a charge controlling agent such as nigrosine dye, resin particles, and titanium dioxide particles are fixed on the surface of a magnetic resin carrier particle prepared by dispersing a magnetic material in a binder resin. In these disclosures, however, the carrier component in a developer has insulating properties and the development is carried out by the conventional Carlson process.

In addition, in the case where the electroconductive magnetic carrier is used for development, it is necessary to impart magnetism to the toner to some extent to prevent the scattering of the toner in the image formation apparatus. However, it is difficult to produce a color image with sufficient transparency using the magnetic toner provided with the magnetic material. This involves many problems in coping with the formation of clear color images.

Accordingly, a first object of the present invention is to provide a carrier with high electroconductivity, great magnetic force and small density, capable of retaining a toner and efficiently transporting it to the surface of a photoconductor without the carrier attraction phenomenon and the scattering of the toner in an image formation apparatus.

A second object of the present invention is to provide a developer with a high toner concentration and proper electroconductivity, capable of yielding an image with high image density, and particularly yielding a color image without any difficulty.

A third object of the present invention is to provide an image formation method using the rear side exposure system, by which method the electric charge can be readily injected into a photoconductor, a latent electrostatic image can be satisfactorily developed with a developer, and the obtained toner image can be easily transferred to a sheet of an image-receiving medium.

The first object of the invention can be achieved by an electroconductive magnetic carrier for use with a developer comprising carrier particles with an average particle diameter of 5 to 100 .mu.m, each of the carrier particles comprising a basic particle which comprises a binder resin and magnetic particles with an average particle diameter of 0.05 to 0.1 .mu.m dispersed in the binder resin, and electroconductive finely-divided particles with an average particle diameter of 0.5 .mu.m or less which are fixed on the surface of the basic particle.

The second object of the invention can be achieved by a developer comprising (a) an electroconductive magnetic carrier comprising carrier particles with an average particle diameter of 5 to 100 .mu.m, each of the carrier particles comprising a basic particle which comprises a binder resin and magnetic particles with an average particle diameter of 0.05 to 0.1 .mu.m dispersed in the binder resin, and electroconductive finely-divided particles with an average particle diameter of 0.5 .mu.m or less which are fixed on the surface of the basic particle, and (b) an electrically insulating toner. The developer thus obtained has electroconductivity and is capable of producing a toner image with insulating properties, thereby enabling the toner image to be transferred to a sheet of plain paper by electrostatic transfer without any difficulty.

The third object of the present invention can be achieved by an image formation method of forming a toner image corresponding to a light image on a photoconductor by use of (a) a photoconductor which comprises a light-transmitting support, and at least a light-transmitting electroconductive layer and a photoconductive layer which are successively overlaid on the light-transmitting support, (b) a developer comprising an electroconductive magnetic carrier comprising carrier particles with an average particle diameter of 5 to 100 .mu.m, each of the carrier particles comprising a basic particle which comprises a binder resin and magnetic particles with an average particle diameter of 0.05 to 0.1 .mu.m dispersed in the binder resin, and electroconductive finely-divided particles with an average particle diameter of 0.5 .mu.m or less which are fixed on the surface of the basic particle, and an electrically insulating toner, (c) a development means which is disposed on the side of the photoconductive layer of the photoconductor and supplies the developer onto the surface of the photoconductor, (d) a voltage application means for applying a voltage across the light-transmitting electroconductive layer of the photoconductor and the development means, and (e) an exposure means which is disposed on the side of the light-transmitting support of the photoconductor in such a configuration as to be directed toward the development means, comprising the steps of: bringing the developer into contact with the surface of the photoconductor; and applying a light image to the photoconductive layer located near a position where the light-transmitting support and the development means are mutually directed, from the side of the light-transmitting support, under the application of a voltage across the light-transmitting electroconductive layer and the development means.

A more complete appreciation of the present invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

FIG. 1 is a schematic cross-sectional view of one embodiment of a carrier particle according to the present invention;

FIG. 2A is a schematic cross-sectional view of another embodiment of a carrier particle according to the present invention;

FIG. 2B is a partial development of FIG. 2A;

FIG. 3 is a diagram of an image formation apparatus in which the image formation method of the present invention is carried out:

FIG. 4 is a schematic cross-sectional view of a photoconductor for use with the image formation method of the present invention;

FIG. 5 is a partially sectional view of a photoconductor and a development means for use in one embodiment of the image formation method according to the present invention; and

FIG. 6 is a partially sectional view of a photoconductor and a development means for use in another embodiment of the image formation method according to the present invention.

FIG. 1 is a schematic cross-sectional view of one embodiment of a carrier particle according to the present invention.

In FIG. 1, a carrier particle 11 comprises (i) a basic particle 13 comprising a binder resin and magnetic particles 15 dispersed in the above-mentioned binder resin, and (ii) electroconductive finely-divided particles 17 fixed on the basic particle 13.

In the carrier particle as shown in FIG. 1, it is not always necessary to coat the overall surface of the basic particle 13 with the electroconductive finely-divided particles 17. Namely, only an electroconductive part may be formed on the surface of the basic particle 13 so as to impart the sufficient electroconductivity to the carrier particle. As shown in FIG. 1, therefore, the surface of the basic particle 13 may be partially exposed without the electroconductive finely-divided particles 17. In addition, part of the magnetic particles 15 may protrude over the surface of the basic particle 13.

Examples of the binder resin contained in the basic particle 13 of the carrier particle 11 according to the present invention are vinyl resins such as polystyrene resin, polyester resins, nylon resins and polyolefin resins.

As the magnetic particle 15 for use in the basic particle 13 of the carrier particle, a spinel ferrite such as magnetite or gamma-iron-oxide; a spinel ferrite comprising at least one metal, except iron, such as Mn, Ni, Mg or Cu; a magnetoplumbite-type ferrite such as barium ferrite; and finely-divided particles of iron or alloys thereof having a surface oxidized layer can be employed in the present invention. The shape of the magnetic particle 15 may be a granule, a sphere or a needle.

In the case where the carrier particle of the present invention is required to be highly magnetized, finely-divided particles of a strongly magnetic substance such as iron may be employed. It is preferable that finely-divided particles of the strongly magnetic substance, that is, the above-mentioned spinel ferrite such as magnetite or gamma-iron-oxide, and magnetoplumbite-type ferrite such as barium ferrite be used as the magnetic particle 15 for use in the basic particle 13, with the chemical stability taken into consideration.

The basic particle 13 having a desired magnetic force can be obtained by appropriately selecting the kind of finely-divided particles of the strongly magnetic substance serving as the magnetic particle 15 and the content thereof in the basic particle 13. It is proper that the amount of the magnetic particle 15 be 70 to 90 wt. % of the total weight of the basic particle 13 in the carrier particle 11 of the present invention.

Examples of the electroconductive finely-divided particles 17 fixed on the basic particle 13 include particles of carbon black, tin oxide, electroconductive titanium oxide which is surface-treated to be electroconductive by coating an electroconductive material on titanium oxide, and silicon carbide. It is desirable that the electroconductive materials not losing its electroconductivity by oxidation in the air be used as the electroconductive finely-divided particles 17.

To deposit the electroconductive finely-divided particles 17 to the surface of the basic particle 13, for example, the basic particles 13 and the electroconductive finely-divided particles 17 are uniformly mixed in such a fashion that the electroconductive finely-divided particles 17 may adhere to the surface of the basic particle 13. Subsequently, these electroconductive particles 17 are fixed to the basic particle 13 with the application of mechanical or thermal shock thereto, so as not to completely embed the electroconductive particles 17 into the basic particle 13, but to allow part of the electroconductive particles 17 to protrude over the basic particle 13.

In the present invention, since the electroconductive finely-divided particles 17 are attached to the basic particle 13, as shown in FIG. 1, high electroconductivity can be efficiently imparted to the carrier particle 11. Furthermore, in the present invention, it is not necessary to contain the electroconductive finely-divided particles 17 in the basic particle 13, so that large quantities of the magnetic particles 15 can be contained in the basic particle 13 to increase the magnetic force of the carrier particle 11.

The apparatus for depositing the electroconductive finely-divided particles 17 to the basic particle 13 is commercially available as a surface-modification apparatus or surface-modification system.

For example:

"Mechanomill" (Trademark), made by Okada Seiko Co., Ltd.

"Mechanofusion System" (Trademark), made by Hosokawa Micron Corporation

"Hybridization System" (Trademark), made by Nara Machinery Co., Ltd.

"Kryptron" (Trademark), made by Kawasaki Heavy Industries, Ltd.

"Dispercoat" (Trademark), made by Nisshin Flour Milling Co., Ltd.

"Coatmizer" (Trademark), made by Freund Industrial Co., Ltd.

"Surfusing" (Trademark), made by Nippon Pneumatic Mfg. Co., Ltd.

"Spray dry" (Trademark), made by Ohgawara Kakouki Co., Ltd.

To impart the predetermined magnetic force to the carrier 11, and at the same time to prevent the carrier attraction phenomenon, the average particle size of the carrier particle 11 of the present invention is in the range of 5 to 100 .mu.m, preferably in the range of 10 to 100 .mu.m, and more preferably in the range of 10 to 50 .mu.m.

When the particle size of the carrier particle 11 exceeds 100 .mu.m, the surface of the photoconductor cannot be uniformly charged and the concentration of toner (the amount ratio of a toner component to a carrier component) cannot be increased. On the other hand, when the particle size of the carrier particle 11 is less than 5 .mu.m, the previously mentioned carrier attraction phenomenon is caused. In addition, the transporting performance of the obtained developer on a development sleeve becomes poor and it is difficult to charge the photoconductor to a predetermined potential. In the present invention, the particle diameter of the carrier particle 11 may be substantially the same as the diameter of the basic particle 13 because the electroconductive finely-divided particle 17 fixed on the basic particle 13 is remarkably small as compared with the basic particle 13, so that the diameter of the electroconductive finely-divided particle 17 can be ignored.

In the carrier of the present invention, the average particle diameter of the magnetic particle 15 dispersed in the binder resin in the basic particle 13 is in the range of 0.05 to 1.0 .mu.m, preferably in the range of 0.1 to 0.5 .mu.m.

When the average particle diameter of the magnetic particle 15 is less than 0.05 .mu.m, this magnetic particle 15 becomes chemically unstable, so that the magnetic particle 15 tend to aggregate and they are not uniformly dispersed in the binder resin. As a result, the carrier cannot be uniformly provided with the magnetic properties. On the other hand, when the average particle diameter of the magnetic particle 15 exceeds 1.0 .mu.m, the magnetic particles 15 easily slip from the surface of the basic particle 13 in the step of pulverization or classification of the manufacturing process of the basic particles 13. In addition, the magnetic particle 15 falls off the carrier particle 11 due to the stress applied thereto in the course of development operation, thereby shortening the life span of the carrier.

The average particle diameter of the electroconductive finely-divided particle 17 attached to the surface of the basic particle 13 is 0.5 .mu.m or less, and more preferably in the range of 0.01 to 0.2 .mu.m.

When the average particle diameter of the electroconductive finely-divided particle 17 exceeds 0.5 .mu.m, it is difficult to steadily fix the electroconductive finely-divided particles 17 on the surface of the basic particle 13, so that an electroconductive part cannot be uniformly formed on the surface of the carrier particle 11.

For the same reason as above, the relationship between the size of the carrier particle 11 and that of the electroconductive finely-divided particle 17 is of great importance. It is preferable that the ratio of the average particle diameter of the electroconductive finely-divided particle 17 to that of the carrier particle 11 be 1/100 or less, more preferably 1/500 or less.

It is desirable that the true density of the carrier 11 be in the range of 3.0 to 4.5 g/cm.sup.3. The bulk density of the carrier 11 is preferably 2.5 g/cm.sup.3 or less, more preferably 2.0 g/cm.sup.3 or less, and further preferably 1.5 g/cm.sup.3 or less.

In the developer, the maximum amount of the toner attached to the carrier in the form of a layer can be calculated when the particle size and the density of the toner are measured. This maximum amount of the toner means the maximum toner concentration in the developer. The above-mentioned maximum toner concentration in the developer can be increased by decreasing the particle size and density of the carrier. In this case, however, the carrier attraction phenomenon readily occurs.

In the present invention, to prevent the carrier attraction phenomenon, and at the same time, to increase the toner concentration of the developer, it is preferable that the maximum magnetization (.sigma..sub.1) of the carrier be 40 emu/g or more, and more preferably 45 to 60 emu/g in a magnetic field of 1 kOe. Furthermore, it is preferable that the ratio of the maximum magnetization (.sigma..sub.1) to the density (m) of the carrier be 13 or more, and more preferably 14 or more. In this relationship, the larger the value of .sigma..sub.1 /m, the stronger the magnetic force per unit density of the carrier.

The maximum magnetization (.sigma..sub.1) of the carrier can be measured by a vibrating magnetometer with the application of a magnetic field of 1 kOe. The density of the carrier can be theoretically calculated as well as measured with a specific gravity bottle.

FIG. 2A is a schematic cross-sectional view of another embodiment of the carrier particle according to the present invention; and FIG. 2B is a partial development obtained by developing the outer surface of the carrier particle shown in FIG. 2A into a plane. In a carrier particle 11, as shown in FIG. 2A, a basic particle 13 comprises a binder resin and magnetic particles 15 which are uniformly dispersed in the binder resin, and electroconductive finely-divided particles 17 and chargeable finely-divided particles 19 are attached to the surface of the basic carrier particle 13.

The above-mentioned electroconductive finely-divided particles 17 fixed onto the basic particle 13 form an electroconductive part, by which the electroconductivity required for the carrier particle 11 is imparted to the carrier particle 11. The chargeable finely-divided particles 19 fixed onto the basic particle 13 form a chargeable part which performs a function in electrostatically attracting the toner thereto and allowing it to stay thereon. To impart the required electroconductivity to the carrier particle 11, it is desirable that the electroconductive part be formed on the outer surface of the carrier particle 11 by bringing adjacent electroconductive finely-divided particles 17 into contact with each other or in close vicinity to each other, as shown in FIG. 2B.

Specific examples of the electroconductive finely-divided particles 17 for use in the present invention are previously mentioned.

Organic or inorganic insulating materials can be used for the chargeable finely-divided particles 19. Specific examples of the chargeable finely-divided particles 19 of the organic material are finely-divided particles of polystyrene, styrene-based copolymers, acrylic resin, acryl-based copolymers, nylon, polyethylene, polypropylene and fluoroplastics, and crosslinked materials thereof. The desired chargeable level and polarity of the carrier can be determined by selecting the above-mentioned materials and polymerization catalysts, and performing the surface treatment of the above materials.

Specific examples of the chargeable finely-divided particles 19 of the inorganic material are finely-divided particles of negatively-chargeable inorganic insulating materials such as silica and titanium dioxide and positively-chargeable inorganic insulating materials such as alumina.

It is preferable that the average particle diameter of the above-mentioned chargeable finely-divided particles 19 be in the range of 0.01 to 0.2 .mu.m.

The amount of the electroconductive finely-divided particles 17 attached to the surface of the basic particle 13 is preferably 50 wt. % or more, and more preferably, 70 wt. % or more, of the total weight of the electroconductive finely-divided particles 17 and the chargeable finely-divided particles 19.

The chargeable finely-divided particles 19 can be fixed on the surface of the basic particle 13 likewise the electroconductive finely-divided particles 17 after uniformly mixing the basic particles 13, the electroconductive finely-divided particles 17 and the chargeable finely-divided particles 19.

A chargeable part can be partially formed on the carrier particle 11 by fixing the chargeable finely-divided particles 19 on the surface of the basic particle 13. This makes it possible to electrostatically attract the insulating toner particles to the chargeable part formed on the carrier particle 11 and support them thereon. The developer thus obtained can prevent the scattering of toner particles in the image formation apparatus without addition of the magnetic material to the toner. This facilitates the preparation of an excellent color developer.

It is preferable that the volume resistivity of the carrier 11 of the present invention be 10.sup.5 .OMEGA..cm or less, more preferably 10.sup.4 .OMEGA..cm or less, and further preferably in the range of 10.sup.2 to 10.sup.4 .OMEGA..cm. When the volume resistivity of the carrier 11 according to the present invention is within the above range, the electroconductivity of the carrier is not impaired and the electric charge can be readily injected into the photoconductor in such a degree that the charge quantity of the photoconductor reaches a level sufficient for the rear side exposure system. The electroconductivity of the carrier 11 is mainly determined by the electroconductive finely-divided particle 17.

To measure the volume resistivity of the carrier 11, 1.5 g of carrier particles 11 are placed in a Teflon-made cylinder with an inner diameter of 20 mm, having an electrode at the bottom thereof, and the volume resistivity of the carrier 11 is measured when a counter electrode with an outer diameter of 20 mm is put on the carrier particles, with a load of 1 kg being applied to the top portion of the carrier particles.

The developer according to the present invention comprises the above-mentioned carrier and a toner component.

As the toner for use in the developer of the present invention, the conventional electrically insulating toner particles with a volume resistivity of 10.sup.14 .OMEGA..cm or more, preferably 10.sup.15 .OMEGA..cm or more can be employed. The volume resistivity of the toner can be measured by the same method as in the case of the carrier.

The toner for use in the present invention may comprise a binder resin, a coloring agent, a charge controlling agent and an off-set preventing agent. In addition, a magnetic toner can be prepared by using a magnetic material, which is effective for preventing the scattering of toner particles in the image formation apparatus.

In the toner, vinyl resins represented by a polystyrene resin, such as styrene - acrylic acid derivative copolymer and polyester resins are usable as the binder resin.

As the coloring agent for use in the toner, a variety of dyes and pigments such as carbon black can be used.

Examples of the charge controlling agent for use in the toner are quaternary ammonium compounds, nigrosine, bases of nigrosine, crystal violet and triphenylmethane compounds.

As the off-set preventing agent or image-fixing promoting assistant, olefin waxes such as low molecular weight polypropylene, low molecular weight polyethylene and modified materials of the above compounds can be employed in the present invention.

For preparing the magnetic toner, magnetite and ferrite can be used as the magnetic material.

In the developer of the present invention, it is preferable that the ratio of the average diameter of a carrier particle to that of a toner particle be 1 to 5, and more preferably 1 to 3. When the ratio of the average diameter of the carrier particle to that of the toner particle is within the above range, the surface area of the carrier particle which is coated with the toner particles can be controlled to increase the toner concentration in the developer. Thus, a decrease in image density can be prevented. The average diameter of the toner particle for use in the present invention is generally 20 .mu.m or less, and preferably 15 .mu.m or less.

The charging characteristics of the toner can be adjusted by depositing the chargeable finely-divided particles to the basic toner particle just like the case of the carrier.

When the magnetic toner is used in the present invention, it is desirable to control the balance of the magnetic force, the average particle diameter and the volume resistivity between the toner and the carrier, as shown below.

It is preferable that the maximum magnetization (magnetic flux density) of the carrier in a magnetic field of 1 kOe be in the range of 40 to 60 emu/g or more, more preferably 50 to 60 emu/g. When the magnetic force of the carrier is within the above range, the decrease in transporting performance of the obtained developer can be avoided, and at the same time, the carrier attraction phenomenon can be prevented.

It is preferable that the maximum magnetization (magnetic flux density) of the toner in a magnetic field of 1 kOe be in the range of 4 to 20 emu/g or more, more preferably 10 to 20 emu/g. When the magnetic force of the toner is within the above range, the scattering of toner particles in the image formation apparatus can be effectively prevented and development can readily be performed.

For the same reasons as shown above, the balance between the magnetic force of the carrier and that of the toner plays an important part. It is preferable that the ratio of the maximum magnetization of toner to that of carrier in a magnetic field of 1 kOe be in the range of 1/3 to 1/10, and more preferably in the range of 1/2 to 1/5. The above specified balance between the magnetic forces of the carrier and the toner is significant for the development which is carried out at a low development bias voltage, so that it is suitable for the rear side exposure system to be described later.

The average particle diameter of the carrier of the present invention is in the range of 5 to 100 .mu.m, and more preferably in the range of 10 to 50 .mu.m. When the particle diameter of the carrier exceeds 100 .mu.m, it becomes difficult to uniformly charge the photoconductor and to increase the toner concentration (T/C) in the developer. Furthermore, the quality of obtained images is decreased because the images are influenced by the trace of a magnetic brush constituted by the carrier. On the other hand, when the particle diameter of the carrier is less than 5 .mu.m, the transporting performance of the obtained developer on a development sleeve is decreased, and it becomes difficult to impart the predetermined potential to the photoconductor.

It is preferable that the average particle diameter of the toner for use in the present invention be 5 to 20 .mu.m, and more preferably 7 to 15 .mu.m. When the average particle diameter of the toner is within the above range, the scattering of toner particles in the image formation apparatus can be effectively prevented, and at the same time, the toner concentration in the developer can be sufficiently increased. In addition, the sharpness of obtained images is decreased.

In the developer according to the present invention, the ratio of the average particle diameter of the carrier to that of the toner is of importance. In the present invention, it is preferable that the ratio of the average particle diameter of the carrier to that of the toner be 1 to 10, and more preferably 1 to 5. In this case, the surface area of the carrier to be covered with the toner is not considerably increased, so that the toner concentration of the obtained developer can be increased. Consequently, a decrease in the image density can be prevented when the developer is applied to the image formation method of the present invention to be described later.

The volume resistivity of the carrier according to the present invention is preferably 10.sup.5 .OMEGA..cm or less, more preferably in the range of 10.sup.2 to 10.sup.4 .OMEGA..cm. When the volume resistivity of the carrier is within the above range, the characteristics of the electroconductive carrier are not impaired. As a result, the electric charge can be readily injected into the photoconductor, so that the photoconductor can be charged to a sufficient level when the developer is used in the rear side exposure system.

The volume resistivity of the toner for use in the present invention, which can be measured in the same manner as in the case of the carrier, is preferably 10.sup.14 .OMEGA..cm or more, more preferably 10.sup.15 .OMEGA..cm or more. When the volume resistivity of the toner is within the above range, there is no difficulty in transferring the toner image formed on the photoconductor to a sheet of plain paper in accordance with the electrostatic transferring method.

The volume resistivity of the developer according to the present invention, which can be measured by the same method as in the case of the carrier, is preferably 10.sup.6 .OMEGA..cm or less, more preferably 10.sup.5 .OMEGA..cm or less, further preferably in the range of 10.sup.3 to 10.sup.5 .OMEGA..cm. When the volume resistivity of the developer is within the above range, the photoconductor can be charged to a sufficient level.

It is preferable that the toner concentration in the developer according to the present invention, that is, the amount of toner, be 10 wt. % or more, more preferably 20 wt. % or more, further preferably in the range of 20 to 50 wt. % of the weight of the carrier. When the toner concentration in the developer is within the above range, the sufficient image density can be obtained when the developer is applied to the image formation method according to the present invention. In addition, the satisfactory charge quantity of the photoconductor can be obtained. According to the image formation method of the present invention, the obtained image density is kept almost constant in a wide range of the toner concentration in the developer, so that it is not necessary to control the toner concentration in practice or the control thereof can be drastically omitted.

FIG. 3 is a diagram of an image formation apparatus in which the image formation method of the present invention is carried out.

In FIG. 3, a drum photoconductor 21 comprises a hollow cylindrical light-transmitting support 23, for example, made of glass, a light-transmitting electroconductive layer 25 formed on the support 23, and a photoconductive layer 27 formed on the electroconductive layer 25. Instead of the drum photoconductor as shown in FIG. 3, a belt-shaped (sheet-shaped) photoconductor is available in the present invention.

Examples of the material for the photoconductive layer 27 include amorphous silicon (.alpha.-silicon), Se-alloys and organic materials. The materials of which sensitivity is high and in which the mobility of the electric charge carrier is high are preferred. With the above points taken into consideration, the amorphous-silicon based photoconductive layer is preferably employed. In particular, a photoconductor prepared by forming at least a light-transmitting electroconductive layer, an amorphous-silicon based photoconductive layer and a carrier-injection preventing top layer successively on a light-transmitting support is preferable.

FIG. 4 is a schematic cross-sectional view of the amorphous silicon based photoconductor 21 shown in FIG. 3.

The photoconductor 21 shown in FIG. 4 comprises a light-transmitting support 23, a light-transmitting electroconductive layer 25 formed on the support 23, and a photoconductive layer 27 formed on the electroconductive layer 25. The photoconductive layer 27 consists of a first carrier-injection preventing layer 26, an amorphous-silicon based photoconductive layer 29 and a second carrier-injection preventing layer 28.

The first carrier-injection preventing layer 26 can prevent the carrier with a polarity opposite to that of a development bias from injecting into the amorphous-silicon based photoconductive layer 29 from the light-transmitting electroconductive layer 25 when the surface of the photoconductor 21 is brought into contact with a developer 71 to which the development bias voltage is applied. As a result, the noise disturbing the image formation can be eliminated and the electrostatic contrast between a light-exposed portion and a non-exposed portion on the surface of the photoconductor can be increased, thereby improving the image quality of the obtained images and decreasing the fogging on the background of the photoconductor in the course of development.

The first carrier-injection preventing layer 26 is required to have the above-mentioned electrical characteristics for preventing the carrier from injecting from the light-transmitting electroconductive layer 25, and at the same time, to have a high light transmittance, that is, to have a large optical band gap or high light transmission, in order not to absorb the light for image-exposure which is applied from the inside of the light-transmitting support 23. Furthermore, the first carrier-injection preventing layer 26 is also required to have good adhesion to the light-transmitting electroconductive layer 25 and the photoconductive layer 29, and have good stability not to undergo the great change when the photoconductive layer 29 is formed thereon by the application of heat thereto.

Examples of the material for the first carrier-injection preventing layer 26 for use in the present invention include amorphous silicon such as amorphous silicon carbide (.alpha.-SiC.sub.x), amorphous silicon oxide (.alpha.-SiO.sub.x) amorphous silicon nitride (.alpha.-SiN.sub.x), .alpha.-SiC.O, .alpha.-SiC.N, .alpha.-SiO.N and .alpha.-SiC.O.N; and other organic materials such as polyethylene terephthalate, Parylene, polytetrafluoroethylene, polyimide, polyfluoroethylenepropylene, urethane resin, epoxy resin, polyester resin, polycarbonate resin and cellulose acetate.

In the amorphous-silicon based carrier-injection preventing layer, the content of an element such as C, N or O can be changed in the thickness direction of the layer.

In addition, the p-type or n-type semiconductive amorphous based silicon layer can be used as the first carrier-injection preventing layer 26. In this case, the element of C, 0 or N may be contained in the first carrier-injection preventing layer 26 in order to increase the optical band gap and the adhesion to the light-transmitting electroconductive layer 25 and the photoconductive layer 29, and a doping agent may be contained therein to prevent the carrier from injecting from the light-transmitting electroconductive layer 25.

It is recommendable that an element belonging to the group IIIa in the periodic table (hereinafter referred to as the group IIIa element) be contained in the first carrier-injection preventing layer 26 at a concentration of 1 to 10,000 ppm, more preferably 100 to 5,000 ppm in order to prevent the negative charge carrier from injecting therein. To prevent the positive charge carrier from injecting in the first carrier-injection preventing layer 26, on the other hand, an element belonging to the group Va in the periodic table (hereinafter referred to as the group Va element) may be contained in the first carrier-injection preventing layer 26 at a concentration of 5,000 ppm or less, more preferably in the range from 300 to 3,000 ppm.

When the above-mentioned element is contained in the first carrier-injection preventing layer 26, the content of the element may be distributed with some gradation in the thickness direction of the layer 26. In such a case, the average concentration of the element in the layer 26 may be within the above range.

As previously mentioned, when the group IIIa element is contained in the first carrier-injection preventing layer 26, a development bias with a positive polarity is employed; and when the group Va element is contained in the first carrier-injection preventing layer 26, a bias with a negative polarity is employed for development.

As the preferable examples of the IIIa group element and the Va group element, boron (B) and phosphorus (P) can be respectively used because these elements are so excellent in the covalent bonding performance with silicon (Si) in the layer 26 that the semiconductive characteristics of the layer 26 can be sensitively changeable, and the capability of preventing the carrier from injecting therein is remarkably improved.

It is preferable that the thickness of the first carrier-injection preventing layer 26 be 0.05 to 5 .mu.m, more preferably 0.1 to 3 .mu.m. When the thickness of the first carrier-injection preventing layer 26 is within the above range, the injection of the carrier can be readily prevented and the light for exposure is not absorbed, thereby allowing the .alpha.-silicon based photoconductive layer 29 to efficiently generate the photocarriers. In addition, the increase in the residual potential can be prevented.

The .alpha.-silicon based photoconductive layer 29 can be formed on the first carrier-injection preventing layer 26, for example, by the glow discharge method, sputtering method, ECR method or vacuum-deposition method.

In forming the .alpha.-silicon based photoconductive layer 29, it is preferable that hydrogen (H) or a halogen be contained in an amount of 1 to 40 atom. % for treatment of the terminal of the dangling bond. Further, to obtain the predetermined electrical characteristics such as the electroconductivities in the dark and the light and the desired optical band gap, it is recommendable that the group IIIa element, the group Va element, and the element of C, N or O be contained in the .alpha.-silicon based photoconductive layer 29. As the preferable examples of the IIIa group element and the Va group element, boron (B) and phosphorus (P) are respectively preferable because these elements are so excellent in the covalent bonding performance with silicon (Si) in the photoconductive layer 29 that the semiconductive characteristics of the layer 29 can be sensitively changeable, and the improved photosensitivity can be obtained.

In particular, when the photoconductive layer 29 comprises .alpha.-Si.C, it is preferable to satisfy the Si.sub.1-x C.sub.x, relationship of 0<x.ltoreq.0.5 in the composition of more preferably, the relationship of 0.05.ltoreq.x.ltoreq.0.45. This is because the resistivity of the thus obtained .alpha.-Si.C photoconductive layer 29 becomes high and the mobility of the carrier is further improved when compared with the .alpha.-Si photoconductive layer 29.

Furthermore in the present invention, a function-separating .alpha.-Si based photoconductive layer 29 consisting of a light-excitation layer capable of mainly generating the photocarriers and a carrier-transport layer capable of transporting the generated carriers is employed, the photosensitivity, the electrostatic contrast and the dielectric strength can be improved.

In forming the light-excitation layer of the above-mentioned function-separating type photoconductive layer 29, it is recommended that the following points be taken into consideration to permit the light-excitation layer to efficiently generate the photocarriers:

(1) to decrease the film-forming speed;

(2) to increase the dilution ratio by use of H.sub.2 and He; and

(3) to increase the content of elements which are doped in the light-excitation layer as compared with that in the carrier-transport layer.

The carrier-transport layer mainly serves to increase the dielectric strength of the photoconductor 21 and to smoothly transport the carriers which are injected from the light-excitation layer to the surface of the photoconductor 21. In this carrier-transport layer, the carriers are also generated from the light transmitted through the light-excitation layer. Thus, the carrier-transport layer can also contribute to increase the photosensitivity of the photoconductor 21.

When the above-mentioned function-separating .alpha.-Si based photoconductive layer 29 consisting of the light-excitation layer and the carrier-transport layer is employed, it is desirable that the thickness of the light-excitation layer be almost equal to the thickness of a light-absorption layer which can be obtained from the light absorption coefficient with respect to the wavelength of light used for the image-exposure.

The second carrier-injection preventing layer 28 may comprise an organic material or an inorganic material.

Examples of the inorganic material for use in the second carrier-injection preventing layer 28 are .alpha.-SiC, .alpha.-SiN, .alpha.-SiO, .alpha.-SiC.O and .alpha.-SiN.O. Using such materials, the second carrier-injection preventing layer 28 may be formed on the photoconductive layer by the conventional thin film forming methods.

In particular, when the second carrier-injection preventing layer 28 comprises .alpha.-Si.C, it is preferable to satisfy the relationship of 0.3<x<1.0 in the composition of Si.sub.1-x C.sub.x, more preferably the relationship of 0.5.ltoreq.x.ltoreq.0.95, further preferably the relationship of 0.6.ltoreq.x.ltoreq.0.95.

The thickness of the second carrier-injection preventing layer 28 is generally in the range of 0.05 to 5 .mu.m, preferably 0.1 to 3 .mu.m, and more preferably 0.1 to 2 .mu.m.

When the thickness of the second carrier-injection preventing layer 28 is within the above range, the image density can be sufficiently increased and the dielectric strength of the photoconductor can be improved, and further, the wear of the photoconductor caused by the repeated operations can be decreased. In addition, even when the delicate pattern of the electric charge is formed on the photoconductor 21, the electrical field, that is, the electric line of force does not spread in the plane direction of the second carrier-injection preventing layer 28, thereby obtaining the images with sufficient resolution. In addition to this, the residual potential of the photoconductor does not elevate because the quantity of the residual electric charge on the surface of the photoconductor does not increase, so that there is no decrease in the image density and fogging on the background, and particularly in the repeated operations, the image density is stabilized and a ghost image is not caused.

The entire thickness of the photoconductive layer 27 thus obtained, which varies depending on the structure of the above-mentioned layers, is approximately 1 to 20 .mu.m, and preferably 1 to 15 .mu.m, and more preferably 3 to 10 .mu.m in the case where the LED or EL (electroluminescence) is used as a light source for exposure. When the thickness of the photoconductive layer 27 is within the above range, the light for exposure can be sufficiently absorbed by the photoconductive layer 27 to show excellent photosensitivity. In addition, the sufficient dielectric strength of the photoconductor can be ensured, so that satisfactory images can be obtained even by application of a low bias voltage to the photoconductor.

As shown in FIG. 3, an LED array 41, serving as an exposure means (image signal exposing apparatus) is disposed inside the light-transmitting support 23 of the photoconductor 21 in such a configuration as to be directed toward a development unit 31, thereby conducting the rear side exposure through an optical transmitter 43 (Selfoc lens array). Instead of the LED array serving as the exposure means, an EL light emitting element array, a plasma light emitting element array, a fluorescent dot array, a shutter array obtained by combining a light source with liquid crystal or PLZT (lead (plomb) lanthanum zirconate titanate), and an optical fiber array can be employed in the present invention.

Around the photoconductor 21, there are situated the development unit 31, an image-transfer unit 51 and an image-fixing unit 61.

The development unit 31, which is disposed with facing the photoconductive layer 27 of the photoconductor 21, serves to supply the surface of the photoconductor 21 with a developer 71. An electroconductive sleeve 35 in the development unit 31 is connected to a development bias source 39 capable of applying a voltage across the light-transmitting electroconductive layer 25 of the photoconductor 21 and the development unit 31. In the development unit 31, a magnetic roller 33 having a plurality of magnetic poles (the N and S poles) is included in the electroconductive sleeve 35. The magnetic roller 33 may be fixed to the inside of the sleeve 35 or designed to be freely rotated therein.

The thickness of the developer 71 on the sleeve 35 is adjusted by a doctor blade 37. In the preferred embodiment of the present invention, the photoconductor 21 and the electroconductive sleeve 35 are respectively rotated in the directions of arrows P and S, and thus the developer 71 is transported to the surface of the photoconductor 21.

When the photoconductor 21 and the electroconductive sleeve 35 of the development unit 31 are driven to rotate in the reverse direction, as previously mentioned, a developer is accumulated at a developer resident portion 73 in a gap between the photoconductor 21 and the sleeve 35, which developer resident portion 73 is located lower than the position where the sleeve 35 of the development unit 31 and the photoconductor 21 are mutually directed (a point "A" illustrated in FIG. 5) in the rotational direction of the photoconductor 21.

The developer resident portion 73 is formed when the developer 71 on the sleeve 35 overflows the predetermined thickness. The transporting speed of the developer 71 from the sleeve 35 to the photoconductor 21, the thickness of the developer 71 formed on the sleeve 35 and the gap width between the sleeve 35 and the photoconductor 21 may be appropriately determined according to the rotational speed of the photoconductor 21 and the required size of the developer resident portion 73.

The developer 71 for use in the present invention comprises the previously mentioned electrically insulating toner and carrier which is prepared by fixing electroconductive finely-divided particles on the surface of a basic particle comprising a binder resin and magnetic particles dispersed in the binder resin.

The electroconductive magnetic carrier particles of the present invention constitute a magnetic brush, to which toner particles are attached. When the magnetic toner particles are employed, the toner particles are caused to attach to the carrier particle mainly by means of the magnetic force. In the case of the non-magnetic toner particles, they are caused to attach to the carrier particle by means of the electrostatic charge.

In the image formation procedure, as shown in FIG. 3, the developer 71 is transported from the sleeve 35 to the photoconductor 21 and accumulated at the developer resident portion 73, and the development bias voltage with a positive polarity is applied from the development bias source 39 to the electroconductive sleeve 35. The positively chargeable toner is used in this embodiment, but the chargeability of the toner and the polarity of the development bias voltage may be determined in accordance with the characteristics of the photoconductor 21.

When the photoconductive layer 27 of the photoconductor 21 is brought into contact with the developer 71, the electric charge from the development bias source 39 is injected into the photoconductive layer 27 through the magnetic brush composed of the carrier particles contained in the developer 71. Thus, the residual electric charge remaining on the photoconductor 21 caused by the previous image formation process can be erased and the surface of the photoconductor 21 can be uniformly charged. At the same time, the residual toner particles on the photoconductor 21, which have failed to be transferred to an image-receiving sheet 81 in the image-transfer unit 51, can be removed from the photoconductor 21 by the above-mentioned magnetic brush.

According to the image formation method of the present invention, a light signal corresponding to the original image is applied to the photoconductor, located near a position where the light-transmitting support 23 and the development means 31 are mutually directed, by use of the exposure means such as the LED array 41 which is disposed on the side of the light-transmitting support 23 of the photoconductor 21 in such a configuration as to be directed toward the development unit 31 via the photoconductor 21.

When the uniformly charged photoconductor 21 is selectively exposed to the light signal by use of the LED array, the potential at the light-exposed portions of the photoconductive layer 27 is rapidly decreased, thereby generating the potential difference on the photoconductive layer 27. Depending on the potential difference on the photoconductive layer 27, the toner particles attached to the magnetic brush are freed from the magnetic force or the electrostatic charge exerted thereon by the magnetic brush, separated therefrom, and then deposited to the surface of the photoconductive layer 27.

Even after the photoconductive layer 27 of the photoconductor 21 is separated from the developer in the developer resident portion 73 while the photoconductor 21 is rotated in the direction of the arrow P and the sleeve 35 is rotated in the direction of the arrow S, the above-mentioned toner particles attached to the photoconductive layer 27 remains as they are, so that a toner image 75 can be formed on the surface of the photoconductor 21. In such a development process, since the magnetic brush composed of the magnetic carrier particles is stable, the quantity of the developer in the developer resident portion 73 can be maintained. As a result, sharp and stable images can be obtained.

In the exposure process by use of the LED array, as previously mentioned, the light signal corresponding to the original image is applied to the photoconductor 21, not at the point A illustrated in FIG. 5 where the sleeve 35 of the development unit 31 comes to the position nearest the photoconductor 21, but at the area adjacent to the developer resident portion 73, lower than the point A in the rotational direction of the photoconductor 21. In particular, it is most preferable that the light signal be applied to the photoconductor 21 at the end point (a point B illustrated in FIG. 5) where the developer 71 is separated from the photoconductor 21.

Since the exposure of the photoconductor 21 to the light signal is conducted at the above-mentioned point in the present invention, the application of the development bias voltage to the photoconductor 21 can be sufficiently stabilized by the time when the exposure process is started. Consequently, the surface of the photoconductor 21 can be uniformly charged regardless of the influence of the hysteresis exerted thereon, and the residual toner remaining on the surface of the photoconductor 21 can be satisfactorily recovered. In addition, since the photoconductor 21 is exposed to the light signal to generate the photocarriers after the application of the development bias voltage to the photoconductor 21 is sufficiently stabilized, excellent toner images 75 can be formed on the photoconductor 21. Since the photoconductor 21 is speedily separated from the developer resident portion 73 after the formation of the toner images 75, the toner images 75 on the photoconductor 21 are not impaired by the mechanical shock such as the collision or friction between the toner images 75 and the developer 71. Thus, toner images 75 with excellent resolution can be obtained.

Furthermore, the distance between the surface of the photoconductor 21 and the surface of the magnetic roller 33 of the development unit 31 is longer at the point B of the photoconductor 21 as illustrated in FIG. 5, adjacent to the developer resident portion 73 as compared with the point A as illustrated in FIG. 5, where the sleeve 35 of the development unit 31 comes to the position nearest the photoconductor 21. Therefore, the magnetic force by which the developer 71 is attracted to the magnetic roller 33 is weaker at the point B of the photoconductor 21, so that part of the toner images 75 which have already been formed on the surface of the photoconductor 21 can be prevented from returning to the development unit 31 by the magnetic force. This can prevent the decrease in image density and resolution of obtained images.

In the image formation method of the present invention, in which the charging, exposure and development are simultaneously carried out, as previously mentioned, it is preferable that the development bias voltage be as low as 250 V or less, more preferably 10 to 200 V, and further preferably 30 to 150 V. The above-mentioned development bias voltage is sufficient to uniformly charge the photoconductor and develop the latent image into the toner image with the developer. Furthermore, in general, when the size of the carrier particle contained in the developer is small, the carrier particles are easily attracted to the photoconductor together with the toner particles in the development procedure. However, the above-mentioned bias voltage is not so high that the carrier particles can be prevented from being attracted to the photoconductor.

In addition, the previously mentioned amorphous silicon based photoconductor is appropriate for the development under application thereto of a low bias voltage.

The dynamic resistivity of the developer 71 can be observed by measuring the electric current flowing to the surface of the photoconductor 21 at the developer resident portion 73. In the present invention, it is preferable that the dynamic resistivity of the developer be 10.sup.7 .OMEGA. or less, more preferably 10.sup.6 .OMEGA. or less, and further preferably in the range of 10.sup.4 to 10.sup.6 .OMEGA..

In FIG. 3, the toner image 75 formed on the photoconductor 21 is attracted by a transfer roller 53 to which a transfer bias voltage with a negative polarity is applied by a transfer bias source 55 and transferred to the image-receiving sheet 81 in the transfer unit 51.

The toner for use in the present invention has the insulating properties, so that the toner image can be steadily transferred to the image-receiving sheet at high transfer efficiency even though the employed image-receiving sheet is a sheet of plain paper.

Then, in the image-fixing unit 61, the image-receiving sheet 81 carrying the toner image thereon is caused to pass through the gap between a heat-application roller 63 and a pressure-application roller 65 to fix the toner image to the image-receiving sheet 81.

After the image-transfer operation, the residual toner particles on the photoconductor 21 are removed therefrom in such a manner that the toner particles remaining on the photoconductor 21 are attracted to the magnetic brush composed of the carrier particles when the photoconductor 21 reaches the position where the photoconductor 21 is directed toward the development unit 31 and brought into contact with the developer 71. This mechanism necessitates no cleaning member. As a matter of course, a cleaning unit may be provided for the step prior to development in the development unit 31 in the present invention.

In addition, a quenching means, for example, a quenching light, capable of erasing the residual charge on the photoconductive layer 27 of the photoconductor 21 may be provided between the transfer unit 51 and the development unit 31.

FIG. 6 is a partially sectional view of a photoconductor and a development unit for explaining the another embodiment of the image formation method according to the present invention.

In this embodiment as shown in FIG. 6, a voltage controllable electrode 32 and its attendant members are added to the first embodiment shown in FIGS. 3 to 5.

As shown in FIG. 6, the voltage controllable electrode 32 is provided at the position where the development unit 31 is directed toward the photoconductor 21. Preferably, the voltage controllable electrode 32 is situated upstream in the rotational direction of the photoconductor 21 with respect to the position where the exposure is conducted by use of the LED. In FIG. 6, the voltage controllable electrode 32 is located between the photoconductor 21 and the sleeve 35, where the sleeve 35 of the development unit 31 comes nearest to the photoconductor 21 (at a point A illustrated in FIG. 6).

The voltage controllable electrode 32 is insulated against the sleeve 35 by interposing an insulator 34 between the sleeve 35 and the electrode 32. The voltage controllable electrode 32 and the insulator 34 are not attached to the sleeve 35, but independently disposed between the sleeve 35 and the photoconductor 21, with constantly maintaining their position (the point A as illustrated in FIG. 6) regardless of the rotation of the sleeve 35 and the photoconductor 21.

Independently of the development bias source 39, the voltage is applied to the photoconductor 21 and the developer 71 by a voltage controllable electrode source 36. In addition, to uniformly apply the electric field to the photoconductor 21 and the developer 71, it is desirable that the voltage controllable electrode 32 be a belt-shaped one extending along the length direction of the sleeve 35.

The potential of the voltage controllable electrode 32 can be determined, independently of the potential of the sleeve 35, by controlling the voltage output from the voltage controllable electrode source 36. For example, the potential of the voltage controllable electrode 32, which is grounded, may be equal to that of the light-transmitting electroconductive layer 25 of the photoconductor 21. Alternatively, the potential of the voltage controllable electrode 32 may be higher or lower than that of the sleeve 35.

As shown in FIG. 6, when the voltage controllable electrode 32 capable of applying the electric potential to the photoconductor 21 and the developer 71 is provided independently of the application of the voltage by the sleeve 35, the surface potential of the photoconductor 21 can be controlled by eliminating the hysteresis of the previous image formation processes exerted on the photoconductor 21. Consequently, the development can be steadily carried out in the case where the photoconductor 21 is caused to rotate several times to obtain images on one image-receiving sheet 81. Thus, excellent images can be obtained.

By controlling the potential of the voltage controllable electrode 32, the image density of the obtained images can be adjusted and the optimal image formation conditions without causing the fogging on the background can be obtained.

According to the image evaluation test which has been conducted by the applicants of the present application, the reversal development in which the toner particles are deposited to the non-exposed portions, and not deposited to the exposed portions on the photoconductor can be accomplished by the image formation method of the present invention by increasing the potential of the voltage controllable electrode 32 and decreasing the potential of the sleeve 35.

As previously explained, the carrier particles and the developer according to the present invention can be adapted to the rear side exposure system. The carrier particles according to the present invention can also be applied to various kinds of image formation methods which require a developer with high electroconductivity and magnetic properties. Therefore, the developer according to the present invention can also be applied to various kinds of image formation methods.

According to the present invention, high electroconductivity and strong magnetic properties can be imparted to the carrier particle. Then, since the thus obtained carrier particle is used together with electrically insulating toner to prepare a developer in the present invention, the developer according to the present invention has electroconductivity, and at the same time, the toner image with insulating properties can be produced by using such a developer. Therefore, the toner image obtained by the developer of the present invention can be transferred to a sheet of plain paper by the electrostatic transfer without any difficulty.

According to the image formation method of the present invention, the photoconductor can be uniformly charged and the latent electrostatic images formed on the photoconductor can be steadily developed by using the developer composed of the carrier which comprises a basic particle prepared by dispersing magnetic particles in a binder resin and electroconductive finely-divided particles fixed on the basic particle, and the toner with electrically insulating properties. Thus, the toner image thus developed by the developer of the present invention can be transferred to an image-receiving sheet by the electrostatic transfer and fixed thereon with high image density.

Other features of this invention will become apparent in the course of the following description of exemplary embodiments which are given for illustration of the invention and are not intended to be limiting thereof.

A mixture of the following components was kneaded and pulverized in a jet-mill, and then classified to obtain a basic particle with an average particle diameter of 20 .mu.m for use in a carrier particle.

100 parts by weight of the above obtained basic particles and 2 parts by weight of electroconductive carbon black particles with an average particle diameter of 0.03 .mu.m were thoroughly mixed in a Henschel mixer, so that electroconductive carbon black particles were uniformly attached to the surface of each basic particle.

Then, the carbon black particles were fixed to the surface of the basic particle by the application of the mechanical shock thereto using a commercially available surface modification apparatus "Hybridization System" (Trademark), made by Nara Machinery Co., Ltd. Thus, the carrier particle according to the present invention was prepared.

The characteristics of the above-prepared carrier particle were as follows:

Average particle diameter: 20 .mu.m

Volume resistivity: 2.times.10.sup.3 .OMEGA..cm

Maximum magnetization (in a magnetic field of 1 kOe): 55 emu/g

100 parts by weight of the above-prepared carrier particles and 20 parts by weight of positively-chargeable toner particles with an average particle diameter of 10 .mu.m were uniformly mixed to prepare a developer with a toner concentration (T/C) of 20 wt. % according to the present invention. The volume resistivity of the obtained developer was 5.times.10.sup.4 .OMEGA..cm.

The above-prepared developer was supplied to an image formation apparatus, as shown in FIGS. 3 and 5, to carry out the image formation.

In the image formation apparatus, a photoconductor comprising a cylindrical glass support with an outer diameter of 30 mm, a light-transmitting electroconductive layer made of ITO formed on the support, and an .alpha.-silicon based photoconductive layer formed on the electroconductive layer was employed. The thickness of the photoconductive layer was 6 .mu.m.

The voltage of +50 V was applied to a sleeve of a development unit by a development bias source. With the application of a transfer bias voltage of -200 V to a transfer roller, the toner images were transferred to a sheet of commercially available plain paper in a transfer unit. As a result, images with an image density of 1.4 were steadily obtained.

The procedure for preparation of the basic particles for use in the carrier in Example 1 was repeated except that the average particle diameter of the carrier particles obtained after classification was changed from 20 to 25 .mu.m.

The characteristics of the thus obtained carrier particle were as follows:

Average particle diameter: 25 .mu.m

True density (m): 3.7 g/cm.sup.3 (.sigma..sub.1 /m=15.1)

Volume resistivity: 2.times.10.sup.3 .OMEGA..cm

Maximum magnetization (in a magnetic field of 1 kOe): 55 emu/g

100 parts by weight of the above-prepared carrier particles and 30 parts by weight of positively-chargeable toner particles were uniformly mixed to prepare a developer with a toner concentration (T/C) of 30 wt. % according to the present invention. The volume resistivity of the obtained developer was 5.times.10.sup.4 .OMEGA..cm.

Using the above obtained developer, image formation was carried out in the same manner as in Example 1. As a result, excellent images were steadily generated without the carrier attraction phenomenon.

A mixture of the following components was kneaded and pulverized in a jet-mill, and then classified to obtain a basic particle with an average particle diameter of 25 .mu.m for use in a carrier particle.

100 parts by weight of the above obtained basic particles, 2 parts by weight of electroconductive carbon black particles with an average particle diameter of 0.03 .mu.m and one part by weight of negatively-chargeable resin beads with an average particle diameter of 0.1 .mu.m were thoroughly mixed in a Henschel mixer, so that the electroconductive carbon black particles and the negatively-charageable resin beads were uniformly attached to the surface of each basic particle.

Then, the carbon black particles and negatively-chargeable resin beads were fixed to the surface of the basic particle by the application of the mechanical shock thereto using a commercially available surface modification apparatus "Hybridization System" (Trademark), made by Nara Machinery Co., Ltd. Thus, the carrier particle according to the present invention was prepared.

The characteristics of the above-prepared carrier particle were as follows:

Volume resistivity: 8.times.10.sup.3 .OMEGA..cm

Maximum magnetization (in a magnetic field of 1 kOe): 53 emu/g

100 parts by weight of the above-prepared carrier particles and 20 parts by weight of positively-chargeable quinacridone-containing magenta toner particles with an average particle diameter of 10 .mu.m were uniformly mixed to prepare a developer with a toner concentration (T/C) of 20 wt. % according to the present invention. The volume resistivity of the obtained developer was 3.times.10.sup.4 .OMEGA..cm.

Using the above obtained developer, image formation was carried out in the same manner as in Example 1. As a result, clear magenta images were steadily generated without scattering of the toner particles in the image formation apparatus.

Thereafter, image formation was carried out using a cyan toner containing phthalocyanine blue and a yellow toner containing disazo yellow as the toner instead of the above-mentioned magenta toner. As a result, clear cyan images and clear yellow images were respectively obtained without scattering of the toner particles in the image formation apparatus.

The procedure for preparation of the carrier particles in Example 2 was repeated.

A mixture of the following components was kneaded and pulverized in a jet-mill, and then classified to obtain a toner particle with an average particle diameter of 7 .mu.m.

The characteristics of the above-prepared toner particle were as follows:

Volume resistivity: 7.times.10.sup.15 .OMEGA..cm

Maximum magnetization (in a magnetic field of 1 kOe): 15 emu/g

Using the above obtained developer, image formation was carried out in the same manner as in Example 1. As a result, excellent images with an image density of 1.4 were steadily generated without scattering of the toner particles in the image formation apparatus.