Abstract:
A print head element part of an ionographic print head is provided having a mechanism for introducing at one end an ionisable gas or gas mixture into a tubular channel, the print head element further having a planar driver electrode and a planar control electrode both surrounding channel, a dielectric body keeping driver electrode and control electrode separate, a source for applying an alternating voltage for the ionization of the gas or gas mixture in channel, a modulation electrode being separated from control electrode by dielectric material, a source for applying a direct current voltage between modulation electrode and control electrode to either block ions in channel or to allow the flow out of ions from channel.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a print head element, print head and printing apparatus including same for use in ionographic printing. 
       BACKGROUND OF THE INVENTION 
       [0002]    For many years printing proceeds with letterpress, gravure (intaglio) or planographic (lithographic) printing machines wherein a printing ink receptor, usually paper, makes direct contact with an inked printing form [ref. e.g. Printing Technology by J. Michael Adams et al.—Delmar Publishers Inc. (1988)]. 
         [0003]    Nowadays other printing processes, so-called non-impact printing processes have found application, e.g. electrostatic printing, and ink jet printing (ref. e.g. “Principles of Non-Impact Printing” by Jerome L. Johnson (1986)—Palatino Press—Irvine Calif., 92715 U.S.A.). 
         [0004]    In order for these non-impact printing systems to be competitive with classical “impact” or “contact” printing they have to be adapted for high speed printing at long runs and have to possess the capability of printing on both sides (duplex printing) which is common praxis in printing of books and journals, and have to be suited for large format printing. 
         [0005]    Two electrostatic printing systems have gained particular importance. 
         [0006]    A first system called electrophotographic printing is based on the use of an electrostatically chargeable photoconductive member that is image wise discharged through image wise photo-exposure whereupon the residual charge pattern is developed with dry toner particles or toner particles dispersed in an insulating liquid. 
         [0007]    A second system is based on the use of a non-photosensitive dielectric member, i.e. electrostatically chargeable member, that is charged in image configuration by various means such as electron beams, ionographic print heads, contacting electrode wires, electronic stencils or shaped masks, the development of the obtained electrostatic charge pattern being the same as described for the electrophotographic printing process. 
         [0008]    Both said electrostatic printing processes have been described in U.S. Pat. No. 5,740,510 and 5,765,081 in connection with the structure and use of an electrostatographic multicolour printing apparatus for single pass sequential duplex printing on a web-type toner image receptor material. 
         [0009]    While photoconductive imaging members require to be shielded against light of the environment, dielectric members used in image wise electrostatic charging have the advantage that they can hold an electrical charge in the presence of visible light which makes them more practical in commercial use and in the design of the housing of the printing machine. 
         [0010]    At present in electrostatic ionographic printing, an electrostatic charge pattern on a dielectric member is formed by means of pattern wise ion-deposition from an ionographic print head, wherein two types of print heads may be distinguished. 
         [0011]    A first type of ionographic print head operates with a corona wire to produce an air-assisted stream of plasma containing positive ions caused to blow out through a slit formed by a machined surface of a modulation array containing electrode fingers that can deflect said ions and locally eliminate them from the air stream. Such is realized in the commercial ionographic print head known under the “trade name” CORJET. Such device operating with corona discharge for plasma formation and ion generation is described e.g. in U.S. Pat. No. 4,743,925, and published European Patent Application EP 0578882. 
         [0012]    A second type of ionographic print head operates with an ion generator including two electrodes, driver and control electrode, separated by a solid insulator (dielectric). When a high frequency electric field (R.F. field) is applied between said electrodes a pool of plasma containing ions and electrons is generated in an ionisable gas that has been injected into a micro chamber comprising a control electrode having a cavity facing a flat modulation electrode also called screen electrode having an aperture for output of an ion beam. Examples of such print heads producing a single type of ions are described e.g. in published European Patent Application 0 541 841 A1, U.S. Pat. Nos. 4,675,703 and 4,697,196. 
         [0013]    In U.S. Pat. No. 4,538,163 a fluid jet assisted ion projection and printing apparatus is described wherein substantially equal numbers of positive and negative ions are generated simultaneously during a series of RF arc breakdowns which take place within an elongated fluid transport channel passing through the body of the apparatus (see e.g.  FIG. 7  thereof). The rapidly moving fluid, i.e. air steam, passes through said elongated channel being actually a slit, transports the ions which may be allowed to pass out of the body or may be neutralized within the body by ion modulation electrodes. Ions of selected sign are accelerated and deposited image wise upon a relatively moving charge receptor. According to U.S. Pat. No. 4,538,163 (see  FIG. 7 ) said elongated channel (slit) is formed in a dielectric body containing an electrode wire as RF electrode operating in conjunction with a top field electrode for creating the desired ions. Modulation electrodes in the form of conductive stripes follow the wall of said elongated channel. In an improvement the modulation electrodes are protected against RF arc discharge taking place in the channel. Therefore a foil interface electrode, preferably made of platinum, is positioned between the dielectric body and a dielectric interface plate. A disadvantage associated with the use of an ionographic print head operating with an elongated channel for producing and conducting ions instead of a row of tubular channels will be cross-talk in the reproduction of neighbouring image dots. Moreover, it will be difficult to have an even flow through of ionisable gas in an elongated channel when using a single tubular gas introduction conduit as illustrated e.g. in  FIGS. 3 to 7 . 
         [0014]    The use of a wire as RF electrode and upstanding electrode strips in the elongated channel will make it very difficult to produce such print head by planar microelectronic production techniques. 
         [0015]    Further, it has to be stated that equal amounts of ions of opposite sign will not be produced with the ionization print head according to U.S. Pat. No. 4,538,163 when as ionisable gases noble gases such as helium and argon are used. Only plasmas containing positive ions and electrons are formed with noble gases by their chemical inactivity in accepting electrons. 
         [0016]    The use of said noble gases in ionography operating with print head elements comprising three electrodes, viz. control, driver and modulation electrode, is described in published European Patent Application number 0 541 841 A1. As described therein it has been found that if a substantial portion of the air in the discharge region, i.e. micro chamber of the ionographic print head, is replaced with nitrogen, elemental noble gases, mixtures of noble gases, or mixtures of nitrogen with one or more noble gases, uniformity and/or print head life can be significantly enhanced. As illustrated in  FIG. 3  of the last mentioned European Patent Application nitrogen or like gas is introduced under pressure into the discharge region through the insulating spacer that separates a cavitated control electrode and an apertured modulation electrode. The intake of ionisable gas proceeds under pressure diametrically to the output aperture of the modulation electrode which will result in turbulence in the projected ion stream followed later on by toner-development of reduced image sharpness and decrease of image resolution. 
         [0017]    An ion print head and image forming apparatus not relying on the use of driver, control and modulation electrodes as in the previous prior art is described in U.S. Pat. No. 5,406,314 (see its  FIGS. 1 and 2 ). 
         [0018]    Each discharge cell in said ion print head operates with a needle electrode inside a discharge tunnel (tubular channel) perpendicular in direction to the wall of said tunnel pointing toward an opposite inner wall of said tunnel. A wraparound electrode is present at an output portion of the tunnel that is adjacent to a dielectric layer serving as positive ion receiving layer for forming a latent electrostatic image thereon. 
         [0019]    The disadvantage of said ion print head resides in the complex arrangement of a needle electrode in a small ionization tunnel perpendicular to the axis of said tunnel and the presence of said wraparound electrode inside said tunnel which does not allow the production of such discharge cells by planar microelectronic production techniques. 
         [0020]    As is known planar microelectronic production techniques make use of coating of dielectric resin layers, photo-hardening of resin layers for forming etching-resistant portions, vapour deposition of metals, sputtering and etching of metals on slices of dielectric material in order to produce planar layered integrated circuits. 
       OBJECT OF THE INVENTION 
       [0021]    It is the object of the present invention to provide an ionographic print head containing print head elements having a non-complicated structure allowing their simultaneous production in large amounts by planar micro-electronic manufacturing techniques. 
         [0022]    Planar micro-electronic manufacturing techniques make it possible to obtain in said print head elements very tiny ion production channels which is in favour of high image resolution. By planar micro-electronic manufacturing techniques it is possible to group said print head elements in modules that are easy mountable and replaceable in the final print head and to have the micro circuitry for addressing their ion production and modulation functions included in said modules. 
       SUMMARY OF THE INVENTION 
       [0023]    According to the present invention a print head element  200  for an ionographic print head  313  is provided comprising:
       1.a means  216  for introducing an ionisable gas or gas mixture into a channel  201 ,   2.a driver electrode  204 ,   3. a control electrode  205 ,   4.a dielectric body ( 202 ,  203 ) keeping said driver electrode  204  and control electrode  205  separate,   5. means  208  for applying an alternating voltage between said driver electrode  204  and control electrode  205 , said voltage being effective for the ionization of said gas or gas mixture in said channel  201  in an ion generation region between said driver electrode  204  and control electrode  205 ,   6.a modulation electrode  206  being separate from said control electrode  205  by dielectric material  213 ,   7.means  211  for applying a direct current voltage between said modulation electrode  206  and control electrode  205  to either block ions in said channel  201  or to allow the flow out of said ions from said channel,   characterized in that:
           (i) said channel is a tubular channel  201  having at one end, at the side of the driver electrode  204 , an inlet opening for receiving said ionisable gas or gas mixture,   (ii) said driver electrode  204  is a planar electrode surrounding said tubular channel  201 ,   (iii) said driver electrode  204  is embedded in dielectric material of said dielectric body  202  and  203 , whereas said control electrode  205  has a border area defining part of said tubular channel  201  so as to make contact with said ionisable gas or gas mixture, and   (iv) said modulation electrode  206  at the other end of said tubular channel  201  has an aperture  207  allowing said flow out of ions from said tubular channel  201 .   
               
 
         [0036]    Further according to the present invention an ionographic printing apparatus  292  comprises the following elements:
       A) an ionographic imaging station  222  comprising a print head  313  facing an electrostatic charge receiving member  320  comprising an electrically insulating layer  214  for receiving at one side thereof a latent electrostatic charge image, said electrically insulating layer  214  being present in an electric field for attracting ions onto said side, said print head  313  comprising print head elements  200  including:
           1. a means  216  for introducing an ionisable gas or gas mixture into a channel  201  of each print head element  200 ,   2. a driver electrode  204 ,   3. a control electrode  205 ,   4. a dielectric body ( 202 ,  203 ) keeping said driver electrode  204  and control electrode  205  separate,   5.means  208  for applying an alternating voltage between said driver electrode  204  and control electrode  205 , said voltage being effective for the ionization of said gas or gas mixture in said channel  201  in an ion generation region between said driver electrode  204  and control electrode  205 ,   6.a modulation electrode  206  being separate from said control electrode  205  by dielectric material  213 ,   7. means  211  for applying a direct current voltage between said modulation electrode  206  and control electrode  205  to either block said ions in said channel  201  or to allow the flow of said ions out from said channel,   
           B) a development station  314  for developing said charge image by means of toner particles so as to obtain a visible toner image,   C) a transfer zone  328  wherein said toner image can be transferred onto an intermediate toner image receiving member  317  or directly onto a final substrate  231 , and   D) a cleaning station  310  for removing residual toner particles, and   E) a station  312  for removing residual electrostatic charges, and   F) a fixing station  293  for fixing said transferred toner image onto said final substrate  231 , characterized in that:
           (i) said channel  201  is a tubular channel  201  having at one end, at the side of the driver electrode  204 , an inlet opening for receiving said ionisable gas or gas mixture,   (ii) said driver electrode  204  is a planar electrode surrounding said tubular channel  201 ,   (iii) said driver electrode  204  is embedded in dielectric material of said dielectric body  202  and/or  203 , whereas said control electrode  205  has a border area defining part of said tubular channel  201  so as to make contact with said ionisable gas or gas mixture, and   (iv) said modulation electrode  206  at the other end of said tubular channel  201  has an aperture  207  allowing said flow out of ions from said tubular channel  201 .   
               
 
         [0054]    Further the present invention relates to a method for placing electrostatic charges by means of ions produced in an ionographic print head  313  in an image wise pattern on a dielectric substrate  214 , said method including the steps of:
       A) introducing an ionisable gas or gas mixture into each channel  201  of print head elements  200  part of said ionographic print head  313 ,   B) producing said ions in said channels  201  by an alternating voltage applied between a driver electrode  204  and a neighbouring control electrode  205  both said electrodes being separated by a dielectric body ( 202 ,  203 ),   C) modulating a stream of ions by either blocking or allowing said ions to leave said channels  201  using a modulation electrode  206  at the output side of said channels  201 ,   D) electrostatically attracting ions that are allowed to leave said channels  201  towards said dielectric substrate  214 ,   characterized in that:
           (i) said ionisable gas or gas mixture is introduced, at the side of the driver electrode  204 , into a tubular channel  201  of each of said print head elements  200 ,   (ii) an alternating voltage being effective for the ionization of said gas or gas mixture is applied between said driver electrode  204  and said control electrode  205 , the driver electrode  204  surrounding tubular channel  201  and being embedded in said dielectric body  202  and  203 , the control electrode  205  having a border area defining part of tubular channel  201  for contact with said ionisable gas or gas mixture, and   (iii) said ions are accelerated out of said tubular channel  201  through an aperture  207  of its modulation electrode  206  by means of a direct current voltage between said modulation electrode  206 , said control electrode  205 , said driver electrode  204  and an electrode  210  backing said dielectric substrate  214 .   
               
 
         [0063]    Specific features of particular and/or preferred embodiments are set out in the dependent claims. 
         [0064]    An advantage of the use of a print head element according to the present invention comes from the straight flow of ionisable gas from input to output of the tubular channel in said element whereby turbulence of ionisable gas is prevented. Hereby the divergence of the jet of ions from said cells towards the receptor surface is kept very low yielding sharp ion dots necessary for obtaining high resolution non-blurred images. 
         [0065]    A further advantage comes from the embodiment of the present invention wherein very tiny print head elements are present in redundancy in the print head grouped therein in sub-modules in staggered position. Thereby it is made practically impossible to have visible white lines produced in the final toner-developed print. Single toner particles can be held by many small ion charge dots corresponding each with very small ion production channels. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0066]      FIG. 1  is a schematic cross-sectional view of a prior art print head element of published European Pat. Application 0 541 841 A1 ( FIG. 3 ). 
           [0067]      FIG. 2  is a schematic cross-sectional view of a prior art ionographic print head element of U.S. Pat. No. 4,538,163 ( FIG. 7 ). 
           [0068]      FIG. 3  is a schematic cross-sectional view of an ionographic print head element  200  according to the present invention. 
           [0069]      FIG. 4   a  represents a bottom view, i.e. at the ion exit side, of a row of print head elements  200  according to the present invention containing said print head elements  200  arranged in staggered position. The bottom view shows the channel apertures  207  surrounded by modulation electrodes  206  having conductive stripes  252  for their electronic addressing. 
           [0070]      FIG. 4   b  represents a plurality of print head elements  200  arranged linearly into a group called sub module  253 . 
           [0071]      FIG. 4   c  represents a plurality of sub-modules  253  arranged on a support in staggered position to form modules  254  that allow their easy mounting and replacement in the print head  313  that has a curved surface  257  at the ion exit side. 
           [0072]      FIG. 5  is a sectional schematic side view of an ionographic imaging station  222  containing a rotatable printing drum  320  operating with a print head  313  according to the present invention in conjunction with an intermediate toner-image receiving drum  317 . 
           [0073]      FIG. 6  is a sectional schematic side view of a printing apparatus  292  according to the present invention for direct printing one colour, e.g. black, on a web-type support  231  guided by a rotatable drum  318 . 
           [0074]      FIGS. 7   a  and  7   b  represent respectively a schematic side view of an ionographic printing apparatus  292  according to the present invention for printing toner images through an offset belt  319  on a flexible substrate  231 , e.g. paper web or textile, and on a rigid substrate  232 , e.g. wooden panel or metal plate. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0075]    From present  FIG. 1 , being actually the  FIG. 3  of prior art published European Pat. Application EP 0 541 841 A1 can be learned that in the illustrated print head element an ionisable gas, nitrogen or like gas at more than ambient pressure, is introduced through the insulating spacer  130  into the discharge region between the control electrode fingers  125  (second electrode) and the screen electrode  131  (third electrode) that has an opening  132 . Openings or jets  136  in said insulating spacer  130  have a vector generally parallel to the control fingers  125 , and said jets are connected to the plenum  135  (main conduit of a manifold), which itself through conduit  137  is connected to a source  138  supplying nitrogen or like gas under pressure. In order to produce plasma ions in the ionisable gas an alternating high frequency voltage is applied between the driver electrode  124  (first electrode) and the control electrode fingers  125 . 
         [0076]    The driver electrode  124  is shown on a conventional backing insulator  140  which in turn is connected to an aluminium backbone  141 . 
         [0077]    The gas for ionization at the discharge region flows outwardly through the opening  132  in the screen electrode  131 , along with the ions. Since a positive pressure is maintained at the discharge region it is extremely unlikely that conductive toner particles could enter that area and become sucked up into the openings of the print head, called ion cartridge. 
         [0078]    From present  FIG. 2 , being actually  FIG. 7  of prior art U.S. Pat. No. 4,538,163 can be learned that the illustrated print head element contains a straight channel  46  which actually is an elongated channel or slit. Adjacent to the entrance of said channel  46  along the length of said channel a field electrode  52  is present and cooperates with an RF electrode  50  that has the form of a wire buried in dielectric body material  44 . Between said both electrodes an RF arc discharge occurs. RF arc electrode  52  connected to earthed alternating voltage source  48  will create ions directly within a moving transport fluid A introduced into the channel  46  by some suitable means, herein represented by tube  54 . 
         [0079]    A modulation electrode  92  in the form of upright fingers in the wall of channel  46  is present on dielectric body material  94  and faces an opposing modulation plate  88 . Both the modulation electrode  92  and the opposing modulation plate  88  are covered with an intermediate dielectric layer  82 . Switch  96  is closed, for addressing the modulation electrode  92 . A mix of positive and negative ions (see prior art  FIG. 6 ) moves in the transport fluid stream A from the ion generation portion to the space between the modulation electrode  92  and the ion modulation plate  88  under the influence of the transverse electric field between the modulation electrode  92  and its opposing modulation plate  88  connected to the pole  90  on earth potential, whereas the electrode fingers  92  are connected over switch  96  to direct current voltage source  98 . As shown, negative ions are attracted to the positively biased modulation electrode  92  and positive ions are attracted to the oppositely biased modulation plate  88 . In that case the positive ions recombine with electrons from the earthed pole  90  and negative ions are attracted towards the electrically insulating substrate  86  of the recording material (see prior art  FIG. 6 ) and that is backed by a positively biased electrode  84  being linked to the direct current voltage source  100 . An intermediate electrode foil  104  prevents the RF arc discharge from adversely affecting the modulation elements as seen in the improved structure  102  of prior art  FIG. 7 . A reference voltage  106  biased to about 100 volts DC concentrating the electric field is connected to the foil interface electrode  104  and completely arrests erosion of the modulation elements  88  and  92 . 
         [0080]      FIG. 3  is a schematic cross-sectional view of an ionographic print head element  200  of an ionographic print head  313  according to the present invention. 
         [0081]    Each print head element  200  of said print head  313  has a tubular and straight channel  201  in a dielectric, i.e. electrically insulating body  202 ,  203 ,  213 . At one end, i.e. at the inlet opening of said tubular channel  201 , a suitable gas supplying means  216  following the print head array, e.g. a manifold having a plurality of exit openings, supplies an ionisable gas or gas mixture into said tubular channel  201 . Said tubular channel  201  is surrounded by a plane driver electrode  204 , and (preferably plane) control electrode  205 , both said electrodes being arranged substantially diametrically to the longitudinal axis of said tubular channel  201 . The driver electrode  204  is embedded in dielectric material of said dielectric body  202  and/or  203 , and is separated by dielectric material from said control electrode  205 . The control electrode  205  forms part of said tubular channel  201 , i.e. has a border area therewith, and so can make contact with said ionisable gas or gas mixture. 
         [0082]    A modulation electrode  206 , being preferably planar, is present on dielectric body  213  and surrounds the exit opening of said tubular channel  201  forming hereby a circular aperture  207 . 
         [0083]    Between said driver electrode  204  and said control electrode  205  a high frequency voltage source  208  (RF voltage source) supplies the necessary voltage pulses for an ionisation in said channel  201  in the region between driver  204  and control electrode  205  forming therein a plasma containing ions and electrons. 
         [0084]    In closed state (as shown in the drawing) switch  209  brings all the electrodes ( 204 ,  205  and  206 ) at the same direct current (DC) voltage by connecting them to the positive pole of DC source  212 . The negative pole of said source  212  is grounded and through the ground connected to the backing electrode  210  of the electrostatic image receptor layer  214 . 
         [0085]    As a result of said connection the positive ions in the plasma of the tubular channel  201  are attracted and accelerated towards said backing electrode  210  and deposited on the electrically insulating layer  214 . 
         [0086]    In open state switch  209  raises by means of DC voltage source  211  the voltage of modulation electrode  206  to a higher level than present at the electrodes  204  and  205 . As a result thereof the positive ions of the plasma are blocked from reaching the electrically insulating layer  214 . Both the control electrode  205  and modulation electrode  206  being connected to the positive pole of direct current (DC) source  211  serve as a drain for the electrons of the plasma. 
         [0087]    According to a particular embodiment the RF voltage applied to the driver electrode  204  is 200 to 500 volt peak to peak. The frequency of the cyclic voltage change of source  208  is 100 kilohertz (kHz) to 10 Megahertz (MHz). The DC voltage of source  211  when operating with tiny tubular channels  201  can be kept fairly low, viz. between 3 to 10 volt, whereas the DC voltage of source  212  is then in the range of 200 to 500 volt. 
         [0088]    The dielectric body  202 ,  203  and  213  of the print head element  200  according to the present invention can have a single material structure or is composed of a number of different layers of same or different material type with the proviso that they are all dielectric materials, e.g. silicon dioxide, aluminium oxide and silicon nitride. When uniting different inorganic layers of dielectric materials as defined above (photo) hardenable silicones may be used. The thickness of the electrodes does not have to be the same for all electrodes. The control electrode may be the thickest. 
         [0089]    Typical suitable materials for forming the electrodes ( 205 ,  206 ) are metals resistive to corrosion, e.g. molybdenum, tantalum and tungsten. The driver electrode  204  being shielded from the plasma by the dielectric body material can be made of silver, copper or aluminium. 
         [0090]    According to a preferred embodiment for ionographically printing with a print head  313  according to the present invention the aperture  207  of the modulation electrode  206  is circular and has a diameter in the range of 0.4 to 40 micrometer, more preferably 0.4 to 10 micrometer. 
         [0091]    The ratio of aperture diameter to tubular channel length is preferably in the range of 2/1 to 1/5. 
         [0092]    The diameter of the tubular channels  201  is not necessarily constant over their whole length with the proviso however, that the above defined aperture diameter is present. By the applied hole-etching process in the formation of said channels a funnel-like structure may be obtained therein, but the largest tubular channel diameter is preferably not more than 60 micrometer. 
         [0093]    According to a preferred embodiment the present printing head elements  200  are used in conjunction with dry toner particles having an average weight toner size of at least 5 micrometer, more preferably in the range of 5 to 10 micrometer. The ratio of the average weight toner particle size to the diameter size of the exit aperture  207  of the tubular channel  201  is preferably in the range of 10/1 to 30/1. 
         [0094]    According to a particular embodiment in the development of the electrostatic charge images obtained by ionographic printing electrically conductive dry toner particles are used in combination with the present ionographic print elements, such for the advantage that these particles in occasional contact with a charged electrically conductive body such as the modulation electrode are repelled by the law of induction. Said law requires that once an electrically conductive particle has taken the charge of a charged conductive body, e.g. an electrode, it obtains the charge sign thereof and becomes repelled thereby. So, a voltage modulated apertured electrode will not be covered or blocked by electrically conductive toner particles. 
         [0095]    According to a particular embodiment conductive toner particles having a conductive substance in the bulk are used, the bulk conductivity of which is at least 10 11  Siemens per centimeter, an example of which can be found in U.S. Pat. No. 06467871. 
         [0096]    According to a special embodiment conductive dry toner particles are used that have shell-core structure in which the shell composition provides an inductive charge separation in the neighbourhood of an electrostatically isolated charge pattern, i.e. here an ion charge pattern, to become attracted thereto. 
         [0097]    Useful for the purpose of development of ionographic charge images are core-shell toner particles having a conductive shell as described in published European Patent Application 0 441 426 A1. Said toner particles carry on their surface and/or in an edge zone close to the surface fine particles of electrically conductive material consisting of fluorine-doped tin oxide. The fluorine-doped tin oxide particles have a primary particle size less than 0.2 micrometer and a specific electrical resistance of at most 50 Ohm.meter. The core of said particles is made e.g. of thermoplastic polyester and may contain magnetically attractable pigment. 
         [0098]    In order to obtain stable and high ionization results the present print head elements are not operated with air but preferably with a noble gas or mixture thereof with nitrogen. Particularly preferred is the use of pure helium or a mixture of helium and argon gas by means of which a plasma containing positive ions and electrons is produced in an alternating field of sufficient strength. 
         [0099]    When the major part of the ionisable gas is helium intensely charged ion beams can be formed which is particularly advantageous when operating with very tiny ionization channels. Such has to do with the fact that helium ions apart from hydrogen ions have the highest electrostatic charge to mass ratio. Argon is preferably present to some amount because argon has a fairly low ionization energy of 15.8 eV and thus operates as an ionization starter, whereas helium requires 24.6 eV being higher than that of nitrogen (14.5 eV). The restricted presence of argon is for the fact that argon in ionized state has sputtering properties which will affect the materials, e.g. thin electrodes in contact therewith. 
         [0100]    It is general knowledge that physical sputtering is driven by momentum exchange between the ions and atoms in the surrounding material. Argon forming already relatively heavy ions is therefore detrimental to the structure of thin-film deposited active parts of an ionographic print head. 
         [0101]    The helium-argon gas mixture applied according to the present invention contains preferably argon in no more than 20 volume percent with respect to helium. More preferably argon is present in admixture with helium in an amount between 0.5 and 10% by volume. Particularly suited is said gas mixture for use in high image resolution ionographic printing applying micro channel print head elements having very thin tubular ionization channels, whereby very fine ion beam spots at relatively low voltage gradients are produced. 
         [0102]    According to a preferred embodiment for fully preventing blocking of electrode apertures, e.g. with errant toner particles, the mixture of helium and argon is supplied at more than atmospheric pressure.  FIG. 4   a  represents an enlarged bottom view  251 , i.e. at the ion exit side, of a row of print head elements  200  according to the present invention containing said print head elements  200  arranged in staggered position. The bottom view shows the dielectric body  213  having channel apertures  207  surrounded by modulation electrodes  206  having conductive stripes  252  for their electronic addressing. 
         [0103]      FIG. 4   b  represents a plurality of print head elements  200  arranged linearly in a group called sub module  253 , the encircled part corresponds with the enlarged part of  251  of  FIG. 4   a . The items  256  represent electronic connection points for addressing the print head elements  200 . 
         [0104]      FIG. 4   c  represents a perspective view of an arrangement in a module  254  of sub-modules  253  placed in staggered position. Each module  254  forms an easily mountable and replaceable part of the print head  313  having a curved surface  257  at the ion exit side. 
         [0105]      FIG. 5  is a sectional schematic side view of a printing apparatus according to the present invention for printing one colour, e.g. black, containing an electrostatic imaging station  222  in the form of a rotatable drum  320  onto which a toner image is formed for transfer onto an intermediate toner-receiving drum  317  having a resilient electrically insulating toner-receiving layer  319  coated on a cylindric support member  318 . 
         [0106]    A print head  313  according to the present invention makes part of said electrostatic imaging station  222  and faces an electrically insulating layer  214  applied on an electrically conductive cylindric support  210  of drum  320 . The print head  313  projecting an ion image onto said electrically insulating layer  214  is followed in order by a toner development station  314 , a cleaning station  315  for removal of superfluous toner particles i.e. for picking up e.g. by suction, non image-wise deposited toner particles, and a toner image transfer zone  328 . In said zone  328  the toner image is transferred onto the supported resilient layer  319  of the rotatable intermediate toner-image receiving drum  317 . 
         [0107]    The toner image transfer zone  328  is followed by a cleaning station  310  in the form of a brush associated with a cleaning blade  322  and suction device  323  for removing toner particles left after toner image transfer. In succession therewith follows a station  326  for applying a powder material like polytetrafluorethylene having a contact angle with respect to water of at least 87° (ref. The Polymeric Encyclopedia volume 5, ed. in chief Joseph C. Salamone, CRC Press, 1996, ISBN 084932470X, p. 3192) or polyethylene having a contact angle with respect to water of at least 70° (ref. Surface characteristics of fibers and textiles, ed. Christopher M. Pastore and Paul Kiekens, CRC Press, 2000, ISBN 0824700023, p41). 
         [0108]    Said station  326  contains a tube like vessel  325  in which a rotating helix pushes finely divided polytetrafluorethylene powder having an average grain size of 0.2 micron onto a soft brush  311 , wherefrom it is sprinkled onto the electrically insulating layer  214  of drum  320 . A rotating soft brush  329  smoothens the applied powder layer optionally filling up micro pores in the electrically insulating layer  214  that may be made of anodized aluminium. 
         [0109]    According to a preferred embodiment the toner development station  314  is a fluidized bed development apparatus, e.g. of the type described in U.S. Pat. 3,380,437. 
         [0110]      FIG. 6  is a sectional schematic side view of an ionographic printing apparatus  292  wherein the transfer of the toner image proceeds directly onto a paper web  231  as final substrate. A rotatable guiding roller  318  conveys the paper web  231  along different electrostatic toner image-forming stations  222  of the type described in  FIG. 5  but each transferring a different ionographically formed toner image corresponding with a colour separation image of a multicolour original. The paper web  231  passes through the nip formed by said guiding roller  318  and the rotatable drums  320  of the toner image-forming stations  222 . 
         [0111]    Members  236  represent intermediate fixing stations. The final fixing of the toner images on the paper web  231  proceeds in station  293 . The members  294  represent guiding rollers. After passing a cutting station  223  printed paper sheets  224  arrive finally in a receptor tray  225 . 
         [0112]      FIGS. 7   a  and  7   b  represent respectively a schematic side view of an ionographic printing apparatus according to the present invention capable of printing respectively toner images onto a flexible substrate  231 , e.g. paper web and onto a rigid substrate  232 , e.g. wooden panel or metal plate. 
         [0113]    An offset belt  319  serving as receiving member for different toner images is rotatably driven by a rotatable guiding member  318  being a drum operating together with roller(s)  334  and contacts rotatably driven imaging stations  222  containing different coloured toners for multicolour printing. 
         [0114]    Printing paper  231  in web form is fed from a paper supply roller  227  and is passed in the nip formed by said offset belt  319  and a backing roller  229 . Said backing roller  229  is a hot fuser roller kept under pressure towards said toner-receiving offset belt  319 . The belt  319  is preheated in station  226  and cooled after toner-image transfer in station  237  preceded by a cleaning station  230 . The pressure applied in the nip of said backing roller  229  and the offset web  319  makes that the paper is moved in synchronism with the peripheral movement of the guiding drum  318 , which is coupled to a speed controllable electric motor (not shown in the drawing). 
         [0115]    Member  235  is a preheating station for raising the temperature of the substrate  231  or  232  to be printed improving the fixing. 
         [0116]    Following the toner image transfer the offset belt  319  passes a cleaning station  230 . The paper web  231  carrying fixed toner images is cut in a cutting station  223  whereupon printed sheets  224  are received in a tray  225 . 
         [0117]      FIG. 7   b  shows how a rigid panel  232 , e.g. a wooden panel, receives a multicolour print. The panel  232  is conveyed on a roller bed  233  into the nip formed between the already mentioned offset belt  319  and the pressure roller  229 . All the other elements shown in the drawing are the same as in  FIG. 7   a.