Patent Publication Number: US-2013251834-A1

Title: Field emission device and nanofiber manufacturing device

Description:
CROSS REFERENCE RELATED APPLICATION 
     This application claims foreign priority of Japanese Patent Application No. 2010-272071, filed on Dec. 6, 2010 and Korean Patent Application No. 10-2011-0016676, filed on Feb. 24, 2011, which are incorporated by reference in their entirety into this application. 
     TECHNICAL FIELD 
     The present invention relates to a field emission device and a nanofiber manufacturing device. 
     BACKGROUND ART 
     A field emission device which conducts field emission in such a way as to apply high voltage to a collector while a nozzle block is grounded was proposed in Japanese Patent laid-open Publication No. 2008-506864 (hereinafter, referred to as “Patent document 1”). 
       FIG. 11  is a view illustrating a field emission device  900  disclosed in Patent document 1. As shown in  FIG. 11 , the field emission device  900  proposed in Patent document 1 includes: a material tank  901  which stores a polymer solution therein; a nozzle block  902  which includes a solution discharge nozzle  904  that discharges a polymer solution, and a gas nozzle which forms the flow of gas; a collector  920  which is made of a conductive element; and a feed roll  924  and a winding roll  926  which are used to transfer a long sheet  918 . In  FIG. 11 , reference numeral  912  denotes a suction blower,  914  denotes a gas collection pipe, and  922  denotes a support. 
     In the field emission device  900  disclosed in Patent document 1, negative high voltage is applied to the collector  920  and, simultaneously, the solution discharge nozzle  904  discharges a polymer solution while the nozzle block  902  is grounded, thus forming nanofibers on the long sheet, which is being transferred, through field emission. 
     According to the field emission device  900  disclosed in Patent document 1, all of the nozzle block  902 , the ┌polymer solution before being discharged from the solution discharge nozzle  904 ┘, ┌the material tank  901  for storing the polymer solution┘ and ┌a polymer solution transfer unit (for example, a pipe and a pump) for transferring the polymer solution from the material tank  901  to the nozzle block  902 ┘ become ground potentials. Therefore, it is unnecessary for the material tank  901  or the polymer solution transfer unit to have high resistance against voltage. As a result, a problem of the structure of the field emission device being complicated, which may occur if the material tank  901  or the polymer solution transfer unit is required to have a high voltage-resistance structure, can be fundamentally prevented. 
     Furthermore, in the field emission device  900  disclosed in Patent document 1, high voltage is applied to the collector  920  capable of having a comparatively simple shape and structure, and the nozzle block  902 , which has a comparatively complex shape and structure, is grounded. Under these conditions, field emission is conducted. Therefore, undesirable voltage discharge or drop can be prevented from occurring, whereby the field emission can be conducted continuously under stable conditions. 
     DISCLOSURE 
     Technical Problem 
     However, in the field emission device  900  disclosed in Patent document 1, comparatively large electrostatic attractive force is generated between the collector and the long sheet due to high voltage applied to the collector. Thereby, the long sheet is pulled towards the collector, whereby the transfer of the long sheet may be impeded. Therefore, uniform conditions cannot be maintained for a long period of time during the process of forming nanofibers through field emission. In a severe case, the operation of the field emission device may have to be interrupted. As a result, it becomes very difficult to mass-produce, with high productivity, nanofibers having uniform quality (for example, related to an average diameter of nanofibers, diameter distribution of nanofibers, the amount of deposition of nanofibers, the meshes of a nanofiber layer, the thickness of the nanofiber layer, the gas permeability of the nanofiber layer, etc.). 
     Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide a field emission device and a nanofiber manufacturing device which make it possible to reliably mass-produce nanofibers having satisfactory quality, even if field emission is conducted in such a way that high voltage is applied to a collector while a nozzle block is grounded. 
     Technical Solution 
     In order to accomplish the above object, in an aspect, the present invention provides a field emission device, including a collector, a nozzle block disposed facing the collector, the nozzle block being provided with a plurality of nozzles discharging a polymer solution, and a power supply applying high voltage between the collector and the nozzle block, wherein one of a positive electrode and a negative electrode of the power supply is connected to the collector, while a remaining one of the positive electrode and the negative electrode of the power supply is connected to the nozzle block and a potential of the remaining one drops to a ground potential. The field emission device forms, through field emission, nanofibers on a long sheet that is being transferred at a predetermined transfer speed. The field emission device further includes an auxiliary belt device, having: an auxiliary belt comprising an insulating and porous endless belt rotatably provided in such a way as to encircle the collector; and an auxiliary belt drive unit rotating the auxiliary belt at the transfer speed of the long sheet. 
     In the field emission device according to the present invention, preferably, the auxiliary belt is made of a polymer substrate that has a thickness ranging from 0.7 mm to 10.0 mm. The polymer substrates may be made of polyamide such as polyethylene, polyacetylene, polyurethane, polypropylene, nylon, etc., polyacetal, polycarbonate, modified polyphenylene ether, polybutyleneterephtalate, polyethylene terephthalate, amorphous polyallylate, polysulfone, polyethersulfone, polyphenylene sulfide, polyether ether keton, polyimid, poly ethyl imide, fluorine resin, liquid crystal polymer, and so on. A plurality of openings, each of which has an area ranging from 0.001 mm 2  to 1.0 mm 2 , are preferably formed in the auxiliary belt. 
     In the field emission device according to the present invention, the auxiliary belt drive unit may include a plurality of auxiliary belt rollers around which the auxiliary belt is wrapped so that the auxiliary belt is rotated by the auxiliary belt rollers, and a drive motor rotating at least one of the auxiliary belt rollers. 
     In the field emission device according to the present invention, the auxiliary belt drive unit may further include an auxiliary belt position control device controlling a position of an end of one of auxiliary belt rollers with respect to with respect to a direction from an inside of the auxiliary belt towards an outside thereof, thus controlling a position of the auxiliary belt with respect to a longitudinal direction of the auxiliary belt roller. 
     In the field emission device according to the present invention, the auxiliary belt position control device may include: a pair of air springs disposed on opposite sides of a support shaft, the support shaft rotatably supporting the end of the auxiliary belt roller; and an expansion-rate control unit independently controlling expansion rates of the air springs. 
     In the field emission device according to the present invention, the auxiliary belt drive unit may further include an auxiliary belt position sensor measuring the position of the auxiliary belt with respect to the longitudinal direction of the auxiliary belt roller and control the position of the auxiliary belt with respect to the longitudinal direction of the auxiliary belt roller based on a result of measurement of the auxiliary belt position sensor. 
     In the field emission device according to the present invention, the auxiliary belt drive unit may further include a tension control device controlling a position of at least one of the auxiliary belt rollers with respect to the direction from the inside of the auxiliary belt towards the outside thereof, thus controlling tension applied to the auxiliary belt. 
     In the field emission device according to the present invention, the tension control device may control a position of the auxiliary belt roller with respect to the direction from the inside of the auxiliary belt towards the outside thereof in such a way that the auxiliary belt roller is moved to a position at which the auxiliary belt can be easily wrapped around the auxiliary belt rollers or can be easily removed therefrom. 
     In the field emission device according to the present invention, a width of the auxiliary belt may be greater than a width of the collector that corresponds to a width of the long sheet. 
     In the field emission device according to the present invention, an opening ratio of the auxiliary belt may range from 1% to 40%. 
     In the field emission device according to the present invention, the auxiliary belt may include a net-shaped substrate formed by weaving threads having diameters ranging from 0.1 mm to 2.0 mm. 
     In the field emission device according to the present invention, the net-shaped substrate may comprise a plurality of net-shaped substrates, and the auxiliary belt may be configured such that the net-shaped substrates are stacked on top of one another. 
     In another aspect, the present invention provides a nanofiber manufacturing device, including: a transfer device transferring a long sheet at a predetermined transfer speed; and the field emission device depositing nanofibers on the long sheet that is being transferred by the transfer device. 
     In the nanofiber manufacturing device according to the present invention, the field emission device may comprise a plurality of field emission devices arranged in series in a direction in which the long sheet is transferred. 
     In the nanofiber manufacturing device according to the present invention, the auxiliary belt device may include: one or a plurality of auxiliary belts provided in such a way as to encircle the collectors of two through all of the field emission devices; and an auxiliary belt drive unit rotating the one or the plurality of auxiliary belts at a speed corresponding to the transfer speed of the long sheet. 
     Advantageous Effects 
     A field emission device according to the present invention is provided with a power supply, one electrode of which is connected to a collector while the other electrode is connected to a nozzle block in such a way that the potential of the electrode connected to the nozzle block drops to the ground potential. Thus, all of the nozzle block, ┌polymer solution before being discharged from nozzles┘, ┌a material tank for storing the polymer solution┘ and ┌a polymer solution transfer unit (for example, a pipe and a pump) for transferring the polymer solution from the material tank to the nozzles┘ become ground potentials. Therefore, in the same manner as the case of the field emission device disclosed in Patent document 1, it is unnecessary for the material tank or the polymer solution transfer unit to have high resistance against voltage. As a result, the present invention can prevent a problem of the structure of the field emission device being complicated, which may occur if the material tank or the polymer solution transfer unit is configured to have high resistance against voltage. 
     Because the field emission device according to the present invention is provided with the power supply, one electrode of which is connected to the collector while the other electrode is connected to the nozzle block  110  in such a way that the potential of the electrode connected to the nozzle block  110  drops to the ground potential, field emission is conducted under conditions, in which high voltage is applied to the collector capable of having a comparatively simple shape and structure, and the nozzle block, which has a comparatively complex shape and structure, is grounded, in the same manner as the case of the field emission device disclosed in Patent document 1. Therefore, undesirable voltage discharge or drop can be prevented from occurring, whereby the field emission can be conducted continuously under stable conditions. 
     In addition, the field emission device according to the present invention is also provided with an auxiliary belt device, which includes an auxiliary belt which is rotatably provided in such a way as to encircle the collector and is made of an insulating and porous endless belt, and an auxiliary belt drive unit which rotates the auxiliary belt at a speed corresponding to the speed at which the long sheet is transferred. Accordingly, even if comparatively large electrostatic attractive force occurs between the collector and the long sheet, because the auxiliary belt is present between the long sheet and the collector, the long sheet W can be reliably prevented from being pulled towards the collector, or the long sheet can be smoothly transferred without being impeded. As a result, it is possible that nanofibers are formed under uniform conditions through field emission, and the field emission device can be continuously operated without being interrupted. Thereby, nanofibers having uniform quality (for example, an average diameter of nanofibers, diameter distribution of nanofibers, the amount of deposition of nanofibers, the thickness of nanofiber nonwoven fabric, gas permeability of nanofiber nonwoven fabric, etc.) can be mass-produced with high productivity. 
     Furthermore, in the field emission device according to the present invention, since the auxiliary belt rotates at a speed corresponding to the transfer speed of the long sheet, there is no problem of the auxiliary belt of having a bad influence on the transfer of the long sheet. Moreover, in the field emission device of the present invention, the auxiliary belt is an insulating and porous endless belt, thus not affecting electric field distribution between the collector and the nozzle block. 
     Moreover, in the field emission device according to the present invention, the auxiliary belt drive unit includes a plurality of auxiliary belt rollers which rotate the auxiliary belt wrapped around them, and a drive motor which rotates at least one of the auxiliary belt rollers. Therefore, the auxiliary belt can be reliably rotated at a speed corresponding to the transfer speed of the long sheet. 
     Meanwhile, in the field emission device according to the present invention, the auxiliary belt drive unit further includes an auxiliary belt position control device which controls the position of an end of one of the auxiliary belt rollers with respect to a direction from the inside of the auxiliary belt towards the outside thereof so that the position of the auxiliary belt with respect to the longitudinal direction of the auxiliary belt roller can be controlled. As such, the inclination of the auxiliary belt roller can be controlled, and this makes it possible to maintain the position of the auxiliary belt with respect to the longitudinal direction of the auxiliary belt roller within an appropriate range over a long period of time. 
     In the field emission device according to the present invention, the auxiliary belt position control device includes a pair of air springs which are disposed on opposite sides of a support shaft that is coupled to one end of the corresponding auxiliary belt roller and rotatably supports it, and an expansion-rate control unit which independently controls the expansion rate of each air spring. Therefore, the auxiliary belt position control device can smoothly control the position of the auxiliary belt with respect to the longitudinal direction of the auxiliary belt roller. 
     Furthermore, in the field emission device according to the present invention, the auxiliary belt drive unit includes an auxiliary belt position sensor which measures the position of the auxiliary belt with respect to the longitudinal direction of the auxiliary belt roller. Based on the result of the measurement of the auxiliary belt position sensor, the position of the auxiliary belt with respect to the longitudinal direction of the auxiliary belt roller is controlled. Therefore, the position of the auxiliary belt with respect to the longitudinal direction of the auxiliary belt roller can be more precisely and reliably controlled. 
     Meanwhile, in the field emission device according to the present invention, the auxiliary belt drive unit further includes a tension control device which adjusts the position of the auxiliary belt roller with respect to the direction from the inside of the auxiliary belt towards the outside thereof and thus controls the tension applied to the auxiliary belt. Hence, even if comparatively large electrostatic attractive force occurs between the collector and the auxiliary belt, because it is possible for the tension of the auxiliary belt to be adjusted to a degree to overcome the electrostatic attractive force, not only can the long sheet be prevented from being pulled towards the collector, but the transfer of the long sheet can also be reliably prevented from being impeded. 
     Moreover, in the field emission device according to the present invention, the tension control device can adjust the position of the corresponding auxiliary belt roller with respect to the direction from the inside of the auxiliary belt towards the outside thereof in such a way that the auxiliary belt roller is moved to a position at which the auxiliary belt can be easily wrapped around the auxiliary belt rollers or can be easily removed therefrom. Therefore, the operation of wrapping the auxiliary belt around the auxiliary belt rollers or removing it therefrom can be facilitated. 
     In the field emission device according to the present invention, the width of the auxiliary belt is greater than that of the collector that corresponds to the width of the long sheet. Thus, the auxiliary belt is reliably disposed between the long sheet and the collector, whereby the long sheet can be prevented from being pulled towards the collector, or the long sheet can be smoothly transferred without being impeded. 
     In addition, in the field emission device of the present invention, the opening ratio of the auxiliary belt ranges from 1% to 40%. The reason for this is due to the fact that, when the opening ratio of the auxiliary belt is 1% or more, it does not greatly influence electro field distribution formed between the collector and the nozzle block, and when the opening ratio of the auxiliary belt is 40% or less, it does not greatly reduce the mechanical strength of the auxiliary belt. 
     In the field emission device according to the present invention, because the auxiliary belt is made of net-shaped substrates which are formed by weaving threads of 0.1 mm to 2.0 mm in diameter, the auxiliary belt has both high mechanical strength and flexibility. Therefore, the auxiliary belt can be smoothly rotated with strong tension applied thereto. 
     Furthermore, in the field emission device according to the present invention, because the auxiliary belt is configured in such a way that net-shaped substrates are stacked on top of one another, the auxiliary belt has sufficient mechanical strength, thus making it possible to rotate the auxiliary belt with increased tension. 
     In this case, it is preferable that the net-shaped substrates which are stacked on top of one another to form the auxiliary belt are made of different kinds of materials. Thereby, the mechanical strength and durability of the auxiliary belt can be further enhanced. For instance, a substrate formed on a surface of the auxiliary belt that faces the long sheet may be made of soft material, and a substrate formed on a surface of the auxiliary belt that faces the collector may be made of material having high mechanical strength. The stacked structure of the net-shaped substrates may be a two-, three-, four- or more-layer structure, or it may be a 2.5-layer structure. The term ┌2.5-layer structure┘ refers to a structure such that the thread density of one layer of the two-layer structure is higher than that of the other layer so that the surface planarity and the abrasion resistance of the auxiliary belt can be enhanced without increasing the weight thereof. 
     Meanwhile, a nanofiber manufacturing device according to the present invention uses the field emission device having the above-mentioned construction. Therefore, even if field emission is conducted in such a way that high voltage is applied to the collector while the nozzle block is grounded, the nanofiber manufacturing device can mass-produce nanofibers having uniform quality with high productivity. 
     Furthermore, the nanofiber manufacturing device according to the present invention includes several field emission devices. Because the several field emission devices are used to manufacture nanofibers, the nanofiber manufacturing device can mass-produce nanofibers with further enhanced productivity. 
     Moreover, the nanofiber manufacturing device according to the present invention may be configured such that one or more auxiliary belt devices are provided, and one or each auxiliary belt device includes an auxiliary belt which is wrapped around two or more collectors of the field emission devices, and an auxiliary belt drive unit which rotates the auxiliary belt at a speed corresponding to the transfer speed of the long sheet. In this case, the number of the auxiliary belt drive units can be reduced. For example, in the case of the nanofiber manufacturing device that is provided with eight field emission devices, a single auxiliary belt device may be installed for every two field emission devices. Alternatively, a single auxiliary belt device may be installed for every four field emission devices. As a further alternative, a single auxiliary belt device may be installed for all of eight field emission devices. 
     The field emission device or the nanofiber manufacturing device according to the present invention can manufacture different kinds of nanofilbers for various purposes of use, for example, medical supplies such as highly functional and highly sensitive textiles, health and beauty related products such as health or skin care products, industrial material such as cloths, filters, etc., electronic-mechanical material such as separators for secondary batteries, separators for condensers, carriers for different kinds of catalysts, material for a variety of sensors, and medical material such as regenerative medical material, bio-medical material, medical MEMS material, bio-sensor material, etc. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a view illustrating a nanofiber manufacturing device according to a first embodiment of the present invention. 
         FIG. 2  illustrates a critical portion of a field emission device. 
         FIG. 3  is a view illustrating an auxiliary belt. 
         FIG. 4  is a view showing the operation of an auxiliary belt position control device. 
         FIG. 5  illustrates the operation of the auxiliary belt position control device. 
         FIG. 6  illustrates the operation of a tension control device. 
         FIG. 7  illustrates a modification of the auxiliary belt. 
         FIG. 8  is a view showing a modification of the auxiliary belt position control device. 
         FIG. 9  is a view showing a modification of an auxiliary belt position sensor. 
         FIG. 10  is a front view showing a critical portion of the field emission device. 
         FIG. 11  is a view illustrating a field emission device proposed in Patent document 1. 
     
    
    
     BEST MODE 
     Hereinafter, a field emission device and a nanofiber manufacturing device according to an embodiment of the present invention will be described with reference to the attached drawings. 
     Embodiment  
     1. Construction of a Nanofiber Manufacturing Device  1  According to an Embodiment 
       FIG. 1  is a view illustrating the nanofiber manufacturing device  1  according to the embodiment of the present invention.  FIG. 1   a  is a front view of the nanofiber manufacturing device  1 , and  FIG. 1   b  is a plan view of the nanofiber manufacturing device  1 .  FIG. 2  illustrates a critical portion of a field emission device.  FIG. 2   a  is a front view of the critical portion of the field emission device.  FIG. 2   b  is a view showing an auxiliary belt device from a direction in which a long sheet is transferred.  FIG. 3  is a view illustrating an auxiliary belt.  FIG. 3   a  is an enlarged plan view of the auxiliary belt, and  FIG. 3   b  is an enlarged sectional view of the auxiliary belt. In  FIGS. 1 and 2 , a polymer solution supply unit and a polymer solution collection unit are not designated. Also,  FIG. 1   a  illustrates some parts as sectional views. In  FIG. 3 , warp threads and weft threads are indicated by hatching. 
     As shown in  FIG. 1 , the nanofiber manufacturing device  1  according to the embodiment of the present invention includes a transfer device  10  which transfers a long sheet W at a predetermined transfer speed V, a plurality of field emission devices  20  which are arranged in series with respect to a transfer direction A in which the long sheet W is transferred by the transfer device  10 , a gas permeability measurement device  40 , and a main controller  60 . The main controller  60  controls the transfer device  10 , the field emission device  20 , a heating device  30  which will be explained later herein, the gas permeability measurement device  40 , a VOC treatment device  70  which will be explained later herein, an inert gas supply device (not shown), a polymer supply device (not shown) and a polymer collection device (not shown). 
     In the nanofiber manufacturing device  1  according to this embodiment, four field emission devices  20  are arranged in series in the transfer direction A in which the long sheet W is transferred. 
     The nanofiber manufacturing device  1  according to this embodiment further includes the heating device  30  which is disposed between the field emission device  20  and the gas permeability measurement device  40  and heats the long sheet W on which nanofibers are deposited, the VOC treatment device  70  which burns volatile components generated when nanofibers are deposited on the long sheet W and eliminates them, and the inert gas supply device which receives a signal from the main controller  60  and supplies inert gas into a field emission chamber  102  of a field emission device  20  that is detected to be abnormal. 
     As shown in  FIG. 1 , the transfer device  10  includes a feed roller  11  which supplies the long sheet W, a winding roller  12  around which the long sheet W is wound, and auxiliary rollers  13  and  18  and drive rollers  14 ,  15 ,  16  and which are disposed between the feed roller  11  and the winding roller  12 . The feed roller  11 , the winding roller  12  and the drive roller  14 ,  15 ,  16  and  17  are configured to be rotated by a drive motor which is not shown in the drawings. 
     As shown in  FIG. 2 , the field emission device  20  includes a casing  100  which is conductive, a collector  150  which is attached to the casing  100  with an insulator  152  interposed therebetween, a nozzle block  110  which is disposed facing the collector  150  and is provided with a plurality of nozzles  112  to discharge polymer solution, a power supply  160  which applies high voltage (e.g., 10 kV to 80 kV) between the collector  150  and the nozzle block  110 , a field emission chamber  102  which is a space that covers the collector  150  and the nozzle block  110 , and an auxiliary belt device  170  which assists in transferring the long sheet W. 
     As shown in  FIG. 2 , the nozzle block  110  includes, as the nozzles  112 , a plurality of upward nozzles  112  which discharges a polymer solution from outlets thereof upwards. The nanofiber manufacturing device  1  is configured such that the polymer solution is discharged from the outlets of the upward nozzles  112  in such a way as to overflow from the outlets of the upward nozzles  112 , thus forming nanofibers through field-emission and, simultaneously, the polymer solution that has overflowed from the outlets of the upward nozzles  112  is collected so that it can be reused as material for nanofibers. The upward nozzles  112  are arranged at intervals, for example, of 1.5 cm to 6.0 cm. The number of upward nozzles  112  is, for example, 36 (6 by 6, when the same in length and width) ˜21904 (148 by 148). The nozzle block  110  may be directly grounded or, alternatively, it may be grounded through the casing  100 . In the field emission device according to the present invention, various sizes and shapes of nozzle blocks can be used, and the nozzle block  110  preferably has a rectangular shape (including a square shape), one side of which ranges from 0.5 m to 3 m, when viewed as the plan view. 
     The power supply  160  includes a current supply unit  164 , a current measurement unit  166  which measures current supplied from the current supply unit  164 , and a control unit  162  which controls the operation of the current supply unit  164  and processes the result of current measurement of current measurement unit  166 . Furthermore, the power supply  160  applies high voltage between the collector  150  and the nozzles  112 , measures current supplied from the power supply  160 , and transmits a measured value to the main controller  60 . When the power supply  160  receives a current interruption signal from the main controller  60 , power supply is interrupted. 
     As shown in  FIG. 2 , the auxiliary belt device  170  includes an auxiliary belt  172  which is rotatably provided in such a way as to encircle the collector  150  and is made of an insulating and porous endless belt, and an auxiliary belt drive unit  171  which rotates the auxiliary belt  172  at a speed corresponding to the speed at which the long sheet W is transferred. 
     As shown in  FIG. 2  or  3 , the auxiliary belt  172  is configured in such a way that net-shaped substrates, each of which is fabricated by weaving threads having diameters of 0.1 mm to 2.0 mm, are stacked on top of one another. The auxiliary belt  172  has an opening ratio ranging from 1% to 40%, and the width thereof is greater than that of the collector  150  that corresponds to the width of the long sheet W. In detail, for example, a first layer net, in which polyethylene threads, the diameters of which are 0.27 mm and 0.18 mm, are alternately arranged, and a second layer net, in which polyacetylene threads of 0.35 mm in diameter and polyethylene threads of 0.35 mm in diameter are alternately arranged, are weaved with polyethylene warp threads of 0.25 mm in diameter, thus forming a net-shaped substrate (refer to  FIG. 3   a ). Net-shaped substrates are stacked on top of one another, thus forming a belt having a 2.5-layer structure (refer to  FIG. 3   b ). The width of the auxiliary belt  172  is 2 m, and a plurality of holes are formed in the auxiliary belt  172  such that the opening ratio thereof is 4%. The peripheral length of the auxiliary belt is preferably 6 m, although it is changed depending on the length of the collector  150 . 
     As shown in  FIG. 2 , the auxiliary belt drive unit  171  includes five auxiliary belt rollers  173   a,    173   b,    173   c,    173   d  and  173   e  (hereinafter, also denoted by  173 ) around which the auxiliary belt is wrapped so that the auxiliary belt is rotated by the auxiliary belt rollers, a drive motor (not shown) which rotates the auxiliary belt rollers  173 , an auxiliary belt position control device  174  which controls the position of the auxiliary belt  172  with respect to the longitudinal direction of the auxiliary belt roller  173   a,  an auxiliary belt position sensor  173  which measures the position of the auxiliary belt  172  with respect to the longitudinal direction of the auxiliary belt roller  173   a,  and a tension control device  178  which controls the tension applied to the auxiliary belt  172 . 
     Each of the auxiliary belt rollers  173  which rotate the auxiliary belt  172  has a length of 2.2 m with respect to the transverse direction of the auxiliary belt  172 . The auxiliary belt roller  173   a  receives a signal from the auxiliary belt position control device  174  so that one end of the auxiliary belt roller  173   a  is controlled in position with respect to the direction from the inside of the auxiliary belt  172  towards the outside thereof. The auxiliary belt roller  173   c  receives a signal from the tension control device  178  and is controlled in position with respect to the direction from the inside of the auxiliary belt  172  towards the outside thereof. The auxiliary belt roller  173   d  is rotated by the drive motor (not shown). 
     In other words, the auxiliary belt roller  173   d  is a drive roller, and the other auxiliary belt rollers are driven rollers. The field emission device of the present invention may have two or more drive rollers. 
     The auxiliary belt position control device  174  includes a pair of air springs  176  and an expansion-rate control unit  177 . The air springs  176  are disposed on opposite upper and lower sides of a support shaft, which is coupled to one end of the auxiliary belt roller  173   a  and rotatably supports it. The expansion-rate control unit  177  independently controls the expansion rate of each air spring  176 . 
     Based on the result of measurement of the auxiliary belt position sensor  175 , the expansion-rate control unit  177  transmits a signal to control the expansion-rates of the air springs  176  so that one end of the auxiliary belt roller  173   a  moves upwards or downwards whereby the auxiliary belt roller  173   a  is tilted and the auxiliary belt is moved to a predetermined position in the transverse direction of the long sheet W. 
     The air springs  176  function to move the one end of the auxiliary belt roller  173   a  upwards or downwards and tilt the auxiliary belt roller  173   a.  For example, if the lower air spring is expanded while the upper air spring is contracted, the one end of the auxiliary belt roller  173   a  is moved upwards so that the auxiliary belt roller  173   a  is tilted in a corresponding direction. If the lower air spring is contracted while the upper air spring is expanded, the support shaft side of the auxiliary belt roller  173   a  is moved downwards so that the auxiliary belt roller  173   a  is tilted in the other direction. A typical air spring can be used as each air spring  176 . 
     The auxiliary belt position sensor  175  is a position sensor which measures the position of the auxiliary belt with respect to the transverse of the auxiliary belt  172 . The auxiliary belt position sensor  175  transmits the result of measurement to the auxiliary belt position control device  174 . A camera can be used as the position sensor. 
     The tension control device  178  controls the tension applied to the auxiliary belt  172  in such a way that the tension applied to the auxiliary belt is measured and, when necessary, the auxiliary belt roller  173   c  is moved by pneumatic pressure. Furthermore, the tension control device  178  is used to facilitate work of wrapping the auxiliary belt  172  around the auxiliary belt rollers  173  or removing it therefrom. 
     The field emission device  20  is installed in a room set at an ambient temperature ranging from 20° C. to 40° C. and an ambient humidity ranging from 20% to 60%. 
     The heating device  30  is disposed between the field emission device  20  and the gas permeability measurement device  40  and functions to heat the long sheet W on which nanofibers are deposited. Although the temperature to which the long sheet W is heated can be changed depending on the kind of long sheet W or nanofiber, it is preferable that the long sheet W is heated to a temperature ranging from 50° C. to 300° C. 
     The gas permeability measurement unit  40  measures the gas permeability of the long sheet W on which nanofibers are deposited by the field emission devices  20 . 
     The main controller  60  controls the transfer device  10 , the field emission devices  20 , the heating device  30 , the gas permeability measurement device  40 , the VOC treatment device  70 , the inert gas supply device, the polymer supply device and the polymer collection device. 
     The VOC treatment device  70  functions to burn volatile components generated when nanofibers are deposited on the long sheet, thus eliminating them. 
     2. A Method of Manufacturing Nanofibers Using the Nanofiber Manufacturing Device According to an Embodiment 
     Hereinafter, a method of manufacturing nanofiber nonwoven fabric using the nanofiber manufacturing device  1  having the above-mentioned construction will be described. 
       FIG. 4  is a view illustrating an auxiliary belt position control device  174 .  FIG. 5  illustrates the operation of the auxiliary belt position control device  174 .  FIG. 5   a  through  5   e  are process views.  FIG. 6  is a view illustrating a tension control device  178 .  FIG. 6   a  is a view illustrating the operation of the tension control device  178  when operation of controlling the tension applied to the auxiliary belt  172  is conducted.  FIG. 6   b  is a view illustrating the operation of the tension control device  178  when operation of removing the auxiliary belt  172  from the auxiliary belt rollers is conducted. In  FIGS. 4 through 6 , to simplify the drawings, illustration of elements, other than the auxiliary belt  172  and the auxiliary belt rollers  173 , is omitted. 
     First, the long sheet W is set on the transfer device  10 . Thereafter, while the long sheet W is transferred from the feed roller  11  to the winding roller  12  at a predetermined transfer speed V, the field emission devices  20  successively deposit nanofibers on the long sheet W. Subsequently, the heating device  30  heats the long sheet W on which nanofibers have been deposited. In this way, nanofiber nonwoven fabric including the long sheet on which nanofibers are deposited is manufactured. 
     During this process, the auxiliary belt device  170  rotates the auxiliary belt  172  at a speed corresponding to a speed, at which the long sheet W is transferred, uses the auxiliary belt position sensor  175  to measure the position of the auxiliary belt  172  with respect to the longitudinal direction of the auxiliary belt roller  173   a,  and uses the tension control device  178  to measure the tension applied to the auxiliary belt  172 . 
     If the auxiliary belt position sensor  175  detects that the auxiliary belt  172  has been dislocated from a correct position with respect to the longitudinal direction of the auxiliary belt roller  173   a,  the auxiliary belt position control device  174  moves the one end of the auxiliary belt roller  173   a  upwards or downwards, as shown in  FIG. 4 , thus controlling the position of the auxiliary belt  172  with respect to the longitudinal direction of the auxiliary belt roller  173   a  such that it is located at the correct position. 
     For example, if the auxiliary belt is moved in the left-right direction and displaced from its original position (refer to  FIGS. 5   a  and  5   b ), the auxiliary belt position sensor  175  transmits the result of position measurement to the auxiliary belt position control device  174 . Then, the auxiliary belt position control device  174  calculates a displacement to move the end (in this embodiment, the right end) of the auxiliary belt roller  173   a  downwards and controls the air springs  176  based on the calculated displacement in such a way as to adjust the expansion rates of the air springs  176  (refer to  FIG. 5   c ). As a result, the auxiliary belt  172  is moved to the right with respect to the ground ( FIG. 5   d ) and is eventually returned to the correct position ( FIG. 5   e ). 
     Meanwhile, if the tension control device  179  detects that the tension applied to the auxiliary belt  172  has been reduced, loosening the auxiliary belt  172 , as shown in  FIG. 6   a , the auxiliary belt roller  173   c  is moved outwards with respect to the direction from the inside of the auxiliary belt  172  towards the outside thereof, thus returning the tension of the auxiliary belt to a predetermined value. 
     As shown in  FIG. 6   b , to wrap the auxiliary belt  172  around the auxiliary belt rollers  173  before field emission begins or remove the auxiliary belt  172  from the auxiliary belt roller  173  after the field emission has finished, the auxiliary belt roller  173   c  is moved inwards (in the direction from the inside of the auxiliary belt  172  towards the outside thereof) to a position at which the auxiliary belt wrapping or removal operation can be facilitated. 
     The main controller  60  controls the transfer speed V, at which the transfer device  10  transfers the long sheet W, based on a gas permeability measured by the gas permeability measurement device  40  or an average gas permeability. For example, if field emission conditions are changed in such a way that the measured gas permeability becomes greater than a predetermined gas permeability, the main controller  60  reduces the transfer speed V such that the amount of cumulative nanofibers deposited on the long sheet per a unit area is increased, thus reducing the gas permeability. Furthermore, the main controller  60  also controls the auxiliary belt drive unit  171  so that the auxiliary belt  172  rotates at a speed corresponding to the controlled transfer speed of the long sheet W. 
     During the manufacturing process, when each field emission device carries out field emission while voltage, e.g., of 35 kV, is applied between the collector  150  and the nozzle block  110 , if it is detected that current supplied from one of the power supplies  160  or more than one power supply  160  is larger than 0.24 mA, the main controller  60  transmits a current interruption signal to the corresponding one or more power supplies  160 . 
     Furthermore, when it is detected that current of less than 0.18 mA is supplied from one ore more power supplies  160 , the main controller  60  generates an alarm (a warning sound or warning sign) to notify that the one or more power supplies  160  are abnormal. 
     Moreover, when a current interruption signal is transmitted to an associated one or more power supplies  160 , the main controller  60  also transmits a transfer speed reduction signal to the transfer device  10  so as to maintain the amount of cumulative nanofibers deposited on the long sheet W per a unit area within a predetermined range. 
     Here, when the number of power supplies  160  that have supplied current for a first period before the transfer speed is reduced is ┌n┘ and the number of power supplies  160  that supply power for a second period after the transfer speed has been reduced is ┌m┘, the main controller  60  controls the transfer device such that the transfer speed for the second period is ┌m/n┘ times the transfer speed for the first period. Subsequently, the main controller  60  more finely controls the transfer speed based on the gas permeability measured by the gas permeability measurement device  40 . Also, the main controller  60  controls the auxiliary belt drive unit  171  such that the auxiliary belt  172  is rotated at a speed corresponding to the transfer speed at which the long sheet W is transferred. 
     The control of the transfer speed V can be embodied by controlling the rpm of the drive rollers  14 ,  15 ,  16  and  17 . 
     Hereinafter, field emission conditions in the nanofiber manufacturing method according to the present invention will be described by example. 
     Nonwoven fabric, cloth, knitted fabric, a film etc., which are made of different kinds of materials, can be used as the long sheet. Preferably, the thickness of the long sheet ranges from 5 μm to 500 μm, and the length thereof ranges from 10 m to 10 km. 
     Polylactic acid (PLA), polypropylene (PP), polyvinyl acetate (PVAc), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyethylene naphthalate (PEN), polyamide PA, polyurethane (PUR), polyvinyl alcohol (PVA), polyacrylonitrile (PAN), polyether lmide (PEI), polycaprolactone (PCL), polylactic acid glycolic acid (PLGA), silk, cellulose, chitosan, etc. can be used as material of the polymer for nanofibers. 
     Dichloromethane, dimethylformaide, dimethyl sulfoxide, methyl ethyl ketone, chloroform, acetone, water, formic acid, acetic acid, cyclohexane, THF, etc. can be used as a solvent which is used for the polymer solution. A mixture of different kinds of solvents may be used, and an addition agent such as a conductivity improver may be added to the polymer solution. 
     The gas permeability P of nanofiber nonwoven fabric ranges from 0.15 m/cm 2 /s to 200 cm 3 /cm 2 /s. The transfer speed V is preferably set to a speed of 0.2 m/minute to 100 m/minute. The voltage which is applied to the nozzles, the collector  150  and the nozzle block  110  can be set to a voltage of 10 kV to 80 kV (in the field emission device  20  according to this embodiment, it is preferable for the voltage to be set to about 50 kV). 
     The temperature in the emission area is preferably set to 25° C., and the humidity in the emission area is set to 30%. 
     3. The Effects of the Field Emission Device and the Manufacturing Device According to the Embodiment 
     The field emission device  20  according to the embodiment is provided with the power supply  160 , one electrode of which is connected to the collector  150  while the other electrode is connected to the nozzle block  110  and the potential thereof drops to the ground potential. All of the nozzle block  110 , ┌polymer solution before being discharged from the nozzles  112 ┘, ┌a material tank for storing the polymer solution┘ and ┌a polymer solution transfer unit (for example, a pipe and a pump) for transferring the polymer solution from the material tank to the nozzles  112 ┘ become ground potentials. Therefore, in the same manner as the case of the field emission device disclosed in Patent document 1, it is unnecessary for the material tank or the polymer solution transfer unit to have high resistance against voltage. As a result, the present invention can prevent a problem of the structure of the field emission device being complicated, which may occur if the material tank or the polymer solution transfer unit is configured to have high resistance against voltage. 
     Furthermore, because the field emission device  20  according to the embodiment is provided with the power supply  160 , one electrode of which is connected to the collector  150  while the other electrode is connected to the nozzle block  110  and the potential thereof drops to the ground potential, field emission is conducted under conditions, in which high voltage is applied to the collector  150  capable of having a comparatively simple shape and structure, and the nozzle block  110 , which has a comparatively complex shape and structure, is grounded, in the same manner as the case of the field emission device disclosed in Patent document 1. Therefore, undesirable voltage discharge or drop can be prevented from occurring, whereby the field emission can be conducted continuously under stable conditions. 
     In addition, the field emission device  20  according to the embodiment is also provided with the auxiliary belt device  170 , which includes the auxiliary belt  172  which is rotatably provided in such a way as to encircle the collector  150  and is made of an insulating and porous endless belt, and an auxiliary belt drive unit  171  which rotates the auxiliary belt  172  at a speed corresponding to the speed at which the long sheet W is transferred. Accordingly, even if comparatively large electrostatic attractive force occurs between the collector  150  and the long sheet W, because the auxiliary belt  172  is present between the long sheet W and the collector  150 , the long sheet W can be reliably prevented from being pulled towards the collector  150 , or the long sheet W can be smoothly transferred without being impeded. As a result, it is possible that nanofibers are formed under uniform conditions through field emission, and the field emission device  1  can be continuously operated without being interrupted. Thereby, nanofibers having uniform quality (for example, an average diameter of nanofibers, diameter distribution of nanofibers, the amount of deposition of nanofibers, the thickness of nanofiber nonwoven fabric, gas permeability of nanofiber nonwoven fabric, etc.) can be mass-produced with high productivity. 
     Further, in the field emission device  20  according to the embodiment, since the auxiliary belt  172  rotates at a speed corresponding to the transfer speed of the long sheet W, there is no problem of the auxiliary belt  172  of having a bad influence on the transfer of the long sheet W. Moreover, in the field emission device  20  of the present invention, the auxiliary belt  172  is an insulating and porous endless belt, thus not affecting electric field distribution between the collector  150  and the nozzle block  110 . 
     Also, in the field emission device  20  according to the embodiment of the present invention, the auxiliary belt drive unit  171  includes the several auxiliary belt rollers  173  which rotate the auxiliary belt  172  wrapped around them, and the drive motor  176  which rotates the auxiliary belt roller  173   c.  Therefore, the auxiliary belt  172  can be reliably rotated at a speed that corresponds to the transfer speed of the long sheet W. 
     Furthermore, in the field emission device  20  according to the embodiment of the present invention, the auxiliary belt drive unit  171  includes an auxiliary belt position control device  174  which controls the position of one end of the auxiliary belt roller  173   a  with respect to the direction from the inside of the auxiliary belt  172  towards the outside thereof so that the position of the auxiliary belt  172  with respect to the longitudinal direction of the auxiliary belt roller  173   a  can be controlled. As such, the inclination of the auxiliary belt roller  173   a  can be controlled, and this makes it possible to maintain the position of the auxiliary belt  172  with respect to the longitudinal direction of the auxiliary belt roller  173   a  within an appropriate range over a long period of time. 
     Particularly, in the field emission device  20  according to the embodiment, the auxiliary belt position control device  174  includes the two air springs  176  which are disposed on opposite sides of the support shaft that is coupled to one end of the auxiliary belt roller  173   a  and rotatably supports it, and the expansion-rate control unit  177  which independently controls the expansion rate of each air spring  176 . Therefore, the auxiliary belt position control device  174  can smoothly control the position of the auxiliary belt with respect to the longitudinal direction of the auxiliary belt roller. 
     Furthermore, in the field emission device  20  according to the embodiment, the auxiliary belt drive unit  171  includes the auxiliary belt position sensor  175  which measures the position of the auxiliary belt  172  with respect to the longitudinal direction of the auxiliary belt roller  173   a.  Based on the result of the measurement of the auxiliary belt position sensor  175 , the operation of controlling the position of the auxiliary belt  172  with respect to the longitudinal direction of the auxiliary belt roller  173   a  is conducted. Therefore, the position of the auxiliary belt with respect to the longitudinal direction of the auxiliary belt roller can be more precisely and reliably controlled. 
     Also, in the field emission device  20  according to the embodiment, the auxiliary belt drive unit  171  further includes the tension control device  178  which adjusts the position of the auxiliary belt roller  173   c  with respect to the direction from the inside of the auxiliary belt  172  towards the outside thereof and thus controls the tension applied to the auxiliary belt  172 . Hence, even if comparatively large electrostatic attractive force occurs between the collector  150  and the auxiliary belt  172 , because it is possible for the tension of the auxiliary belt  172  to be adjusted to a degree to overcome the electrostatic attractive force, not only can the long sheet W be prevented from being pulled towards the collector  150 , but the long sheet W can be smoothly transferred without being impeded. 
     Moreover, in the field emission device  20  according to the embodiment, the tension control device  178  can adjust the position of the auxiliary belt roller  173   c  with respect to the direction from the inside of the auxiliary belt  172  towards the outside thereof in such a way that the auxiliary belt roller  173   c  is moved to a position at which the auxiliary belt  172  can be easily wrapped around the auxiliary belt rollers  173  or can be easily removed therefrom. Therefore, the operation of wrapping the auxiliary belt  172  around the auxiliary belt rollers  173  or removing it therefrom can be facilitated. 
     Furthermore, in the field emission device  20  according to the embodiment, the width of the auxiliary belt  172  is greater than that of the collector  150  that corresponds to the width of the long sheet W. Thus, the auxiliary belt  172  is reliably disposed between the long sheet W and the collector  150 , whereby the long sheet W can be prevented from being pulled towards the collector  150 , or the transfer of the long sheet W can be prevented from being impeded. 
     In addition, according to the field emission device  20  of the embodiment, the opening ratio of the auxiliary belt  172  ranges from 1% to 40%. The reason why the opening ratio of the auxiliary belt  172  ranges from 1% to 40% is due to the fact that, when the opening ratio of the auxiliary belt  172  is 1% or more, the opening ratio can be regarded to be a level that does not greatly influence electro field distribution formed between the collector  150  and the nozzle block  110 , and when the opening ratio of the auxiliary belt  172  is 40% or less, it does not greatly reduce the mechanical strength of the auxiliary belt  172 . 
     Further, in the field emission device  20  according to the embodiment, because the auxiliary belt  172  is made of net-shaped substrates which are formed by weaving threads of 0.1 mm to 2.0 mm in diameter, the auxiliary belt  172  has both high mechanical strength and flexibility. Therefore, the auxiliary belt  172  can be smoothly rotated with strong tension applied thereto. 
     Also, in the field emission device  20  according to the embodiment of the present invention, the auxiliary belt  172  is configured in such a way that net-shaped substrates are stacked on top of one another. As a result, the auxiliary belt  172  has sufficient mechanical strength, thus making it possible to rotate the auxiliary belt  172  with increased tension. More preferably, the net-shaped substrates of the auxiliary belt  172  are made of different kinds of materials. In this case, the mechanical strength and durability of the auxiliary belt  172  can be further enhanced. For instance, a substrate formed on a surface of the auxiliary belt  172  that faces the long sheet W may be made of soft material, and a substrate formed a surface of the auxiliary belt  172  that faces the collector  150  may be made of material having high mechanical strength. The stacked structure of the net-shaped substrates may be a two-, three-, four- or more-layer structure, or it may be a 2.5-layer structure. The term ┌2.5-layer structure┘ refers to a structure such that the thread density of one layer of the two-layer structure is higher than that of the other layer so that the surface planarity and the abrasion resistance of the auxiliary belt can be enhanced without increasing the weight thereof. 
     Meanwhile, the nanofiber manufacturing device  1  according to the embodiment of the present invention uses the field emission device having the above-mentioned construction. Therefore, even if field emission is conducted in such a way that high voltage is applied to the collector while the nozzle block is grounded, the nanofiber manufacturing device  1  can mass-produce nanofibers having uniform quality with high productivity. 
     Furthermore, the nanofiber manufacturing device  1  according to the embodiment includes several field emission devices  20  which are arranged in series in a transfer direction A in which the long sheet W is transferred. Therefore, because the several field emission devices are used to manufacture nanofibers, the nanofiber manufacturing device can mass-produce nanofibers with further enhanced productivity. 
     Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, the present invention is not limited to the embodiments. Those skilled in the art will appreciate that various modifications are possible, without departing from the scope and spirit of the invention. For instance, the following modifications are also possible. 
     (1) in the above-mentioned embodiment, although the nanofiber manufacturing device of the present invention has been illustrated as including four field emission devices, the present invention is not limited to this construction. For example, the present invention may be applied to a nanofiber manufacturing device which includes two, three, five or more field emission devices. 
     (2) in the above-mentioned embodiments, although the nanofiber manufacturing device has been illustrated as including four field emission devices and each field emission device has been illustrated as having a single auxiliary belt device (refer to  FIG. 7   a ), the present invention is not limited to this construction.  FIG. 7  illustrates modifications of the auxiliary belt device  170  according to the embodiment of the present invention. As shown in  FIG. 7 , the nanofiber manufacturing device according to the present invention may be configured such that one or more auxiliary belt devices  170  are provided, and one or each auxiliary belt device  170  includes an auxiliary belt  172  which is wrapped around two or more collectors  150  of the field emission devices  20 , and an auxiliary belt drive unit  171  which rotates the auxiliary belt  172  at a speed corresponding to the transfer speed of the long sheet W. In this case, the number of the auxiliary belt drive units  171  can be reduced. For example, in the nanofiber manufacturing device that is provided with four field emission devices  20 , a single auxiliary belt device  170  may be installed for every two field emission devices (refer to  FIG. 7   b  or  7   c ), or a single auxiliary belt device  170  may be installed for all of the four field emission devices  20  (refer to  FIG. 7   d  or  7   e ). 
     (3) in the above-mentioned embodiment, each field emission device of the nanofiber manufacturing device of the present invention has been illustrated as being a bottom-up type field emission device provided with upward nozzles, the present invention is not limited to this. For instance, the present invention may be applied to a nanofiber manufacturing device which includes a top-down type field emission device provided with downward nozzles or a side type field emission device provided with side nozzles. 
     (4) in the above-mentioned embodiment, the field emission device of the nanofiber manufacturing device has been illustrated as being configured such that the positive electrode of the power supply  160  is connected to the collector  150  while the negative electrode of the power supply  160  is connected to the nozzle block  110  and the casing  100 , the present invention is not limited to this. For example, the present invention may be applied to a nanofiber manufacturing device which includes a field emission device that is configured such that the negative electrode of the power supply is connected to the collector  150  while the positive electrode of the power supply is connected to the nozzle block  110  and the casing  100 . 
     (5) in the above-mentioned embodiment, although the two air spring  181  have been illustrated as being disposed on opposite upper and lower sides of the support, the present invention is not limited to this construction.  FIG. 8  is a view showing a modification of the auxiliary belt position control device. For example, as shown in  FIG. 8 , two air springs  176  may be configured such that they are disposed on opposite left and right sides of the support shaft. In this case, the position of the auxiliary belt  172  with respect to the longitudinal direction of the auxiliary belt roller  173   a  can be controlled by horizontally moving the end of the auxiliary belt roller  173   a.    
     (6) in the above-mentioned embodiment, the camera is used as the position sensor, but the present invent is not limited to this.  FIG. 9  illustrates a modification of the auxiliary belt position sensor. As shown in  FIG. 9 , for example, a disk-shaped position measurement sensor may be used as the position sensor. Also, a typical laser distance measurement device or other typical position measurement sensors may be used as the position sensor. 
     (7) in the above-mentioned embodiment, although the nanofiber manufacturing device of the present invention has been illustrated as being configured such that a single nozzle block is installed in each field emission device, the present invention is not limited to this construction.  FIG. 10  is a front view illustrating a critical portion of a field emission device  20   a.  As shown in  FIG. 10 , two nozzle blocks  110   a   1  and  110   a   2  may be installed in each field emission device  20   a.  In addition, the present invention may be applied to a nanofiber manufacturing device in which two or more nozzle blocks are provided in each field emission device. 
     In this case, all of the nozzle blocks may have the same nozzle arrangement interval. Alternatively, the nozzle blocks may be configured such that the nozzle arrangement intervals thereof differ from each other. Furthermore, all of the nozzle blocks may have the same height or, alternatively, the heights of the nozzle blocks may differ from each other. 
     (8) the nanofiber manufacturing device of the present invention may further include a reciprocating unit which reciprocates the nozzle block at a predetermined period in the transverse direction of the long sheet. In the case where the field emission operation is performed while the nozzle block is reciprocated at a predetermined period by the reciprocating unit, the amount of cumulative nanofibers deposited on the long sheet can be uniform with respect to the transverse direction of the long sheet. In this case, preferably, each field emission device or each nozzle block is independently controlled with regard to the period or distance of the reciprocating motion of the nozzle block. Therefore, all of the nozzle blocks may be configured to be reciprocated on the same cycle. The nozzle blocks may be reciprocated on different cycles. Furthermore, all of the nozzle blocks may be configured such that the distances of the reciprocating motion of the nozzle blocks are the same as each other. Alternatively, the distances of the reciprocating motion of the nozzle blocks may be different from each other.