Patent Publication Number: US-2013251838-A1

Title: Field emission device and nanofiber manufacturing device

Description:
CROSS REFERENCE RELATED APPLICATION 
     This application claims foreign priority of Japanese Patent Application No. 2010-272070, filed on Dec. 6, 2010 and Korean Patent Application No. 10-2011-0016675, 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 is grounded was proposed in Japanese Patent laid-open Publication No. 2008-506864 (hereinafter, referred to as “Patent document 1”). 
     FIG. 9 is a view illustrating a field emission device 900 disclosed in Patent document 1. As shown in FIG. 9, 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 collection sheet 918. In FIG. 9, 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 collection 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, according to a study conducted by the inventor of the present invention, even in a field emission device, such as that disclosed in Patent document 1, in which high voltage is applied to the collector and field emission is performed while the nozzle block is grounded, insulation between the collector and the casing or the other elements is substantially likely to be insufficient. If, in field emission, very high voltage (e.g., 35 kV) is applied between the nozzle block and the collector so as to manufacture nanofibers having high performance, insulation breakdown between the collector and the casing or the other elements may be induced, or leak current may be increased to an undesirable level, even if insulation breakdown is not induced. In this case, interruption of the operation of the field emission device is required. As a result, it is difficult to continuously operate the field emission device for a long time, and it is also difficult to mass-produce nanofibers having satisfactory performance. 
     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 performance. 
     Technical Solution 
     In order to accomplish the above object, in an aspect, the present invention provides a field emission device, including: a conductive casing; a collector attached to the casing by an insulator; 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 a high voltage between the nozzle block and the collector, 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 the casing, wherein when the collector is viewed from the nozzle block, a periphery of the insulator is closer to an outside of the field emission device than a periphery of the collector, and when a thickness of the insulator is ┌a┘ and a distance between the periphery of the insulator and the periphery of the collector is ┌b┘, both ┌a≧6 mm┘ and ┌a+b≧50 mm┘ are satisfied. 
     In the field emission device of the present invention, preferably, ┌a≧8 mm┘ may be satisfied. 
     Furthermore, in the field emission device of the present invention, ┌a+b≧80 mm┘ may be satisfied. 
     In the field emission device of the present invention, the insulator may be made of polyamide, 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, polypropylene, high-density polyethylene or polyethylene. 
     Preferably, the field emission device may be 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%. 
     In the field emission device of the present invention, when the distance between the collector and upper ends of the nozzles is ┌c┘, ┌c≧60 mm┘ may be satisfied. 
     Furthermore, in the field emission device of the present invention, the nozzles of the nozzle block may comprise a plurality of upward nozzles discharging the polymer solution from outlets thereof upwards. The field emission device may be configured such that the polymer solution is discharged from the outlets of the upward nozzles in such a way as to overflow from the outlets of the upward nozzles, thus forming nanofibers through field-emission and, simultaneously, polymer solution that has overflowed from the outlets of the upward nozzles is collected and reused as material for nanofibers. 
     In another aspect, the present invention provides a nanofiber manufacturing device, including a feed unit, a winding unit, a transfer device transferring a long sheet, 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. 
     Advantageous Effects 
     In a field emission device according to the present invention, because a casing or a nozzle block is grounded, all of the nozzle block, a ┌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 nozzle block┘ become ground potentials. 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. Therefore, 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. 
     Also, in the field emission device according to the present invention, when the collector is viewed from the nozzle block, the periphery of an insulator is closer to an outside of the device than the periphery of the collector. When the thickness of the insulator is ┌a┘ and the distance between the periphery of the insulator and the periphery of the collector is ┌b┘, not only ┌a≧6 mm┘ but also ┌a+b≧50 mm┘ are satisfied. Consequently, sufficient insulation between the collector and the casing or the other elements can be ensured. As can be clearly understood from experimental examples which will be described later herein, even when 35 kV is applied between the nozzle block and the collector to conduct field emission, breakdown of insulation between the collector and the casing or the other elements can be prevented. Moreover, leak current can be controlled to be within a predetermined range. Consequently, it is possible that the frequency of interruption of the field emission device is reduced to a very low level. Therefore, the field emission device can be continuously operated for a long time, thus reliably making mass-production of nanofibers having satisfactory performance possible. 
     Further, because the field emission device according to the present invention is configured such that leak current can be controlled within a predetermined range, it is possible to detect an abnormality of the field emission device early by always monitoring the current that is supplied from the power supply. 
     Furthermore, the field emission device according to the present invention is configured such that ┌a≧8 mm┘ is satisfied. Therefore, even if a voltage of 40 kV is applied between the collector and the nozzle block to conduct the field emission, a problem of insulation breakdown between the collector and the casing or the other elements is prevented, and leak current can be controlled to be within a predetermined range. 
     Moreover, the field emission device according to the present invention is configured such that ┌a+b≧80 mm┘ is satisfied. Thereby, even if a voltage of 40 kV is applied between the collector and the nozzle block to conduct the field emission, breakdown of insulation between the collector and the casing or the other elements is not induced, and leak current can be reliably controlled to be within a predetermined range. 
     Furthermore, in the field emission device according to the present invention, the insulator is made of polyamide, 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, polypropylene, high-density polyethylene or polyethylene. These materials have satisfactory insulation performance and high mechanical strength and high machinability and thus can be appropriately used as the insulator of the field emission device. 
     The field emission device according to the present invention is installed in the room set at an ambient temperature ranging from 20° C. to 40° C. and an ambient humidity ranging from 20% to 60%. Therefore, it is possible that leak current is stably maintained at a low level. 
     Furthermore, the field emission device according to the present invention is configured such that, when the distance between the collector and the upper ends of the nozzles is ┌c┘, ┌c≧60 mm┘ is satisfied, thus making it possible to manufacture superfine nanofibers. 
     Moreover, the field emission device according to the present invention can manufacture nanofibers through field emission in such a way as to discharge polymer solution from the outlets of the upward nozzles. Therefore, unlike the conventional field emission device disclosed in Patent document 1 in which the downward nozzle is used, a droplet phenomenon can be prevented, whereby the quality of manufactured nanofibers can be markedly enhanced. 
     In addition, the field emission device can manufacture nanofibers through field emission in such a way as to overflow polymer solution from the outlets of the upward nozzles. Therefore, a sufficient amount of polymer solution can be supplied to the upward nozzles so that the quality of manufactured nanofibers can be uniformly maintained. 
     Moreover, polymer solution which has overflowed from the outlets of the upward nozzles can be collected and reused as material for nanofibers. Thereby, the amount of raw material used can be reduced, thus making it possible to reduce the production cost of nanofiber. This satisfies the recent trend towards resource saving. 
     Although the nozzle block provided with the upward nozzles requires a mechanical unit for collecting the polymer solution, this mechanical unit does not make the entire device complex, because the mechanical unit does not need to have a high voltage resistance structure. 
     Meanwhile, the nanofiber manufacturing device according to the present invention uses the field emission device, thus reliably making mass-production of nanofibers having satisfactory performance possible. 
     Furthermore, the nanofiber manufacturing device according to the present invention includes a plurality of field emission devices. Therefore, because the field emission devices are used to manufacture nanofibers, the nanofiber manufacturing device can mass-produce nanofibers with high productivity. 
     Here, in the case of the nanofiber manufacturing device provided with the several field emission devices, if a problem occurs in any one of the several field emission devices (for example, in any one of the several field emission devices, breakdown of insulation between the collector and the casing or the other elements is induced or a leak current is increased to an undesirable level without insulation breakdown being induced), the operation of the corresponding field emission device is required to be interrupted. Given this, it may be considered that it is difficult to continuously operate the nanofiber manufacturing device for a long time. However, in the nanofiber manufacturing device of the present invention, because it is provided with the field emission devices which can be continuously operated for a long time compared to that of the conventional technique, the nanofiber manufacturing device can also be continuously operated for a long time. Moreover, the present invention makes it possible to reliably mass-produce nanofibers having satisfactory performance. 
     The field emission device or the nanofiber manufacturing device according to the present invention can manufacture different kinds of nanofibers 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 an embodiment of the present invention. 
         FIG. 2  is a view illustrating a field emission device according to an embodiment of the present invention. 
         FIG. 3  is an enlarged view of a critical portion of the field emission device according to the embodiment of the present invention. 
         FIG. 4  is a view illustrating the operation of a main controller according to an embodiment of the present invention. 
         FIG. 5  is a block diagram showing the operation of the main controller according to the embodiment of the present invention. 
         FIG. 6  is a view showing the result of Experimental example 1. 
         FIG. 7  is a view showing the result of Experimental example 2. 
         FIG. 8  is a view illustrating a filed emission device. 
         FIG. 9  is a view illustrating a conventional field emission device. 
     
    
    
     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. 
     1. A Field Emission Device and a Nanofiber Manufacturing Device According to an Embodiment 
       FIG. 1  is a view illustrating a nanofiber manufacturing device according to an embodiment of the present invention.  FIG. 1   a  is a front view of the nanofiber manufacturing device, and  FIG. 1   b  is a plan view of the nanofiber manufacturing device.  FIG. 2  is a view illustrating a field emission device according to an embodiment of the present invention.  FIG. 3  is an enlarged view of a critical portion of the field emission device according to the embodiment of the present invention.  FIG. 3   a  is an enlarged sectional view of the critical portion of the field emission device, and  FIG. 3   b  is an enlarged plan view of the field emission device.  FIGS. 4 and 5  are views illustrating the operation of a main controller according to the embodiment of the present invention. In  FIGS. 1 and 2 , a polymer solution supply unit and a polymer solution collection unit are not designated. Furthermore,  FIG. 1   a  illustrates some parts using sectional views. 
     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  which 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  190  which will be explained later herein, a polymer supply device and a polymer collection device. 
     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 heating device  30  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  burns volatile components generated when nanofibers are deposited on the long sheet W and eliminates them. The nanofiber manufacturing device  1  according to this embodiment further includes an inert gas supply device ( 190 , refer to  FIG. 4 ) which receives a signal from the main controller  60  and supplies inert gas into a filed 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 50 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 unit  170  which assists transferring the long sheet W. 
     As shown in  FIGS. 2 and 3 , 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 from the plan view. 
     As stated above, the collector  150  is attached to the conductive casing  100  by the insulator  152 . A positive electrode of the power supply  160  is connected to the connector  150 , and a negative electrode of the power supply  160  is connected to the nozzle block  110  and the casing  100 . As shown in  FIG. 3 , when the collector  150  is viewed from the nozzle block  110 , a periphery of the insulator  152  is closer to the outside of the device than a periphery of the collector  150 . When the thickness of the insulator  152  is ┌a┘ and a distance between the periphery of the insulator  152  and the periphery of the collector  150  is ┌b┘, both ┌a≧6 mm┘ and ┌a+b≧50 mm┘ are satisfied. 
     For instance, the insulator  152  is made of polyamide, 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, polypropylene, high-density polyethylene or polyethylene. 
     When the distance between the collector  150  and upper ends of the nozzles  112  is ┌c┘, ┌c≧60 mm┘ is satisfied. 
     As shown in  FIGS. 4 and 5 , 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 a current measurement of the current measurement unit  166 . Furthermore, the power supply  160  applies a 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 unit  170  includes an auxiliary belt  172  which is rotated in synchronous with a speed at which the long sheet W is transferred, and five auxiliary belt rollers  174  which assists the rotation of the auxiliary belt  172 . One of the five auxiliary belt rollers  174  or more than one of the five auxiliary belt rollers  174  are drive rollers, and the rest is a driven roller. Because the auxiliary belt  172  is disposed between the collector  150  and the long sheet W, the long sheet W can be smoothly transferred without being attracted to the collector  150  to which positive high voltage is applied. 
     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 to be 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. 
     As shown in  FIG. 5 , the gas permeability measurement device  40  includes a gas permeability measurement unit  42  which measures gas permeability P of the long sheet W on which nanofibers are deposited, a drive unit  43  which reciprocates the gas permeability measurement unit  42  in a transverse direction of the long sheet W at a predetermined cycle T, and a control unit  44  which controls the operation of the drive unit  43  and the gas permeability measurement unit  42  and, simultaneously, receives the result of measurement from the permeability measurement unit  42  and processes the result. The drive unit  43  and the control unit  44  are installed in a main body  41 . A typical known gas permeability measurement device can be used as the gas permeability measurement device  40 . 
     As shown in  FIG. 4 , the inert gas supply device  190  includes an inert gas cylinder  192  which supplies inert gas, an inert gas supply line  194  which supplies inert gas to the field emission chambers  102 , and on-off valves  196  which control supply of inert gas to the respective field emission chambers  102  based on a signal transmitted from the main controller  60 . 
     The main controller  60  controls the transfer device  10 , the field emission device  20 , the heating device  30 , the gas permeability measurement device  40 , the VOC treatment device  70 , an inert gas control device  192 , 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 W, 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. 
     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 deposits 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 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 the current interruption signal is transmitted to the 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 . 
     The control of the transfer speed V can be embodied by controlling the rpm of the drive rollers  14 ,  15 ,  16  and  17 . 
     Furthermore, when it is detected that current of more than 0.24 mA is supplied from one or more power supplies  160 , the main controller  60  transmits a signal to the inert gas supply device  190  to supply inert gas into the field emission chambers  102  of the field emission devices  20  associated with the one or more power supplies  160 . 
     Meanwhile, 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. 
     Hereinafter, field emission conditions in the nanofiber manufacturing method according to the present invention will be described by example. 
     Nonwoven fabric, cloth, knitted fabric, 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 imide (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. 
     Preferably, the gas permeability P of nanofiber nonwoven fabric ranges from 0.15 cm 3 /cm 2 /s to 200 m/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. 
     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 
     In the field emission device  20  according to the embodiment, the casing  100  or the nozzle block  110  is grounded. 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 nozzle block  110 ┘ 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. Thus, 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, in the field emission device  20  according to the embodiment, when the collector  150  is viewed from the nozzle block  110 , the periphery of the insulator  152  is closer to the outside of the device than the periphery of the collector  150 . When the thickness of the insulator  152  is ┌a┘ and the distance between the periphery of the insulator  152  and the periphery of the collector  150  is ┌b┘, not only ┌a≧6 mm┘ but also ┌a+b≧50 mm┘ are satisfied. Consequently, sufficient insulation between the collector  150  and the casing  100  or the other elements can be ensured. As can be clearly understood from experimental examples which will be described later herein, even when 35 kV is applied between the nozzle block  110  and the collector  150  to conduct field emission, breakdown of insulation between the collector  150  and the casing  110  or the other elements can be prevented. Moreover, leak current can be controlled to be within a predetermined range. Consequently, it is possible that the frequency of interruption of the field emission device  20  is reduced to a very low level. Therefore, the field emission device can be continuously operated for a long time, thus reliably making mass-production of nanofibers having satisfactory performance possible. 
     Further, because the field emission device  20  according to the embodiment is configured such that leak current can be controlled within a predetermined range, it is possible to detect an abnormality of the field emission device early by always monitoring current supplied from the power supply  160 . 
     In addition, the field emission device  20  according to the embodiment is installed in the room set at an ambient temperature ranging from 20° C. to 40° C. and an ambient humidity ranging from 20% to 60%. Therefore, it is possible that leak current is stably maintained at a low level. 
     Furthermore, the field emission device  20  according to the embodiment is configured such that, when the distance between the collector  150  and the upper ends of the nozzles  112  is ┌c┘, ┌c≧60 mm┘ is satisfied, thus making it possible to manufacture superfine nanofibers. 
     Moreover, the field emission device  20  according to the embodiment can manufacture nanofibers through field emission in such a way as to discharge a polymer solution from the outlets of the upward nozzles  112 . Therefore, unlike the conventional field emission device disclosed in Patent document 1 in which the downward nozzle is used, a droplet phenomenon can be prevented, and the quality of manufactured nanofibers can be markedly enhanced. 
     In addition, the field emission device  20  according to the embodiment can manufacture nanofibers through field emission in such a way as to overflow the polymer solution from the outlets of the upward nozzles  112 . Therefore, a sufficient amount of the polymer solution can be supplied to the upward nozzles so that the quality of manufactured nanofibers can be uniformly maintained. 
     Moreover, the field emission device  20  according to the embodiment is configured such that the polymer solution which has overflowed from the outlets of the upward nozzles  112  can be collected and reused as material for nanofibers. Thereby, the amount of raw material used can be reduced, thus making it possible to reduce the production cost of nanofiber. 
     This satisfies the recent trend towards resource saving. 
     Although the nozzle block  112  provided with the upward nozzles requires a mechanical unit for collecting the polymer solution, this mechanical unit does not make the entire device complex, because the mechanical unit does not need to have a high voltage resistance structure. 
     The nanofiber manufacturing device  1  according to the embodiment of the present invention uses the field emission device  20  having the above-mentioned construction, thus reliably making mass-production of nanofibers having satisfactory performance possible. 
     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 in which the long sheet is transferred. Therefore, because the several field emission devices are used to manufacture nanofibers, the nanofiber manufacturing device can mass-produce nanofibers with high productivity. 
     In addition, the nanofiber manufacturing device  1  according to the embodiment of the present invention uses the field emission devices which can be continuously operated for a long time, compared to the conventional technique. Thus, the nanofiber manufacturing device can also be continuously operated for a long time. Moreover, the nanofiber manufacturing device can mass-produce nanofibers having satisfactory performance. 
     Further, the nanofiber manufacturing device  1  according to the embodiment includes the main controller  60 . When the emission devices  20  are continuously operated for a long time, even if an abnormality occurs in only one of the field emission devices  20  (for example, when it is detected that a larger amount of current than a predetermined first preset current is supplied from one or more power supplies), the abnormality can be instantaneously detected, whereby the reliability of the nanofiber manufacturing device can be markedly enhanced. 
     Also, because the nanofiber manufacturing device  1  according to the embodiment includes the main controller  60  having the above-mentioned construction, even if an abnormality occurs in only one of the field emission devices while the field emission devices are being continuously operated for a long time, the only the operation of the corresponding field emission device that has the abnormality need be interrupted, while the remaining field emission devices do not need to be interrupted. Therefore, the process of manufacturing nanofibers can be continuously conducted without being interrupted. 
     As a result, in the nanofiber manufacturing device according to the embodiment of the present invention, a range of criteria in determining whether a state of a field emission device is abnormal or not can be reduced. Thereby, the nanofiber manufacturing device can mass-produce nanofibers with high productivity without reducing the reliability of products. 
     Furthermore, in the nanofiber manufacturing device  1  provided with the main controller  60 , even if the operation of a field emission device in which an abnormality occurs is interrupted, the main controller  60  controls the device such that the transfer speed is reduced, thus making it possible to fall the amount of cumulative nanofibers deposited on the long sheet per a unit area within a predetermined range. As a result, the gas permeability and the thickness of manufactured nanofiber nonwoven fabric can be maintained uniform during the mass-production process. 
     In addition, in the nanofiber manufacturing device  1  according to the present invention, the main controller  60  controls the transfer device such that the transfer speed V 2  for the second period T 2  is reduced to ┌m/n┘ times the transfer speed V 1  for the first period T 1 , thus making it possible to maintain the amount of cumulative nanofibers deposited on the long sheet per a unit area within a predetermined range. 
     Moreover, in the nanofiber manufacturing device  1 , the transfer speed can be controlled based on the gas permeability measured by the gas permeability measurement device  40 . Therefore, even if there is a little difference between the amounts of nanofibers deposited per a unit area in the field emission devices, nanofiber nonwoven fabric with uniform gas permeability can be mass-produced. 
     Experimental Example 1 
     Experimental example 1 is directed to determining the thickness ┌a┘ of the insulator  152  that is required to control leak current within a predetermined range. Table 1 shows experiment results for Experimental example 1.  FIG. 6  is a graph showing the experiment results for Experimental example 1. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 a = 5 mm 
                 a = 6 mm 
                 a = 8 mm 
                 a = 10 mm 
                 a = 12 mm 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Cur. 
                 Volt. 
                 Cur. 
                 Volt. 
                 Cur. 
                 Volt. 
                 Cur. 
                 Volt. 
                 Cur. 
                 Volt. 
               
               
                 (mA) 
                 (kV) 
                 (mA) 
                 (kV) 
                 (mA)) 
                 (kV) 
                 (mA) 
                 (kV) 
                 (mA) 
                 (kV) 
               
               
                   
               
               
                 0.01 
                 28.0 
                 0.01 
                 37.2 
                 0.01 
                 41.0 
                 0.01 
                 43.0 
                 0.01 
                 46.3 
               
               
                 0.02 
                 34.4 
                 0.02 
                 44.3 
                 0.02 
                 46.6 
                 0.02 
                 49.0 
                 0.02 
                 54.0 
               
               
                 0.03 
                 — 
                 0.03 
                 49.1 
                 0.03 
                 54.8 
                 0.03 
                 56.0 
                 0.03 
                 59.0 
               
               
                 0.04 
                 — 
                 0.04 
                 50.3 
                 0.04 
                 59.7 
                 0.04 
                 — 
                 0.04 
                 — 
               
               
                 Ins. 
                 38.0 
                 Ins. 
                 53.4 
                 Ins. 
                 — 
                 Ins. 
                 — 
                 Ins. 
                 — 
               
               
                 break 
                   
                 break 
                   
                 break 
                   
                 break 
                   
                 break 
               
               
                   
               
               
                 * b is fixed to 30 mm 
               
            
           
         
       
     
     In Table 1, the term ‘Ins. break’ refers to insulation breakdown. 
     In Experimental example 1, the field emission device  20  according to the embodiment was used (the thickness ┌a┘ was 5 mm, 6 mm, 8 mm, 10 mm or 12 mm, and the distance ┌b┘ was fixed to 30 mm). In the state in which no polymer solution was supplied to the nozzle block  110 , voltage was applied between the collector  150  and the nozzle block  110  such that current (in this case, leak current) supplied from the power supply  160  becomes a predetermined current, for example, 0.01 mA, 0.02 mA, 0.03 mA, 0.04 mA or 0.05 mA. Leak current and applied voltage in each case were recorded in a graph. As can be understood from Table 1 and  FIG. 6 , when the thickness ┌a┘ of the insulator  152  is 6 mm or more, even if voltage of 35 kV is applied between the collector  150  and the nozzle block  110 , it is possible that leak current is restricted to be about 0.01 mA. Furthermore, when the thickness ┌a┘ of the insulator  152  is 8 mm or more, even if voltage of 40 kV is applied between the collector  150  and the nozzle block  110 , it is possible that leak current is restricted to be about 0.01 mA. In addition, when the thickness ┌a┘ of the insulator  152  is 12 mm or more, even if voltage of 45 kV is applied between the collector  150  and the nozzle block  110 , it is possible that leak current is restricted to be about 0.01 mA. 
     Experimental Example 2 
     Experimental example 2 is directed to determining the distance ┌a+b┘, defined by the insulator  152  and the collector  150  on the surface of the insulator  152 , that is required to maintain leak current within a predetermined range. Table 2 shows experiment results for Experimental example 2.  FIG. 7  is a graph showing the experiment results for Experimental example 2. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 a + b = 45 mm 
                 a + b = 50 mm 
                 a + b = 60 mm 
                 a + b = 80 mm 
                 a + b = 100 mm 
                 a + b = 120 mm 
                 a + b = 140 mm 
                 a + b = 160 mm 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Cur. 
                 Volt. 
                 Cur. 
                 Volt. 
                 Cur. 
                 Volt. 
                 Cur. 
                 Volt. 
                 Cur. 
                 Volt. 
                 Cur. 
                 Volt. 
                 Cur. 
                 Volt. 
                 Cur. 
                 Volt. 
               
               
                 (mA) 
                 (kV) 
                 (mA) 
                 (kV) 
                 (mA) 
                 (kV) 
                 (mA) 
                 (kV) 
                 (mA) 
                 (kV) 
                 (mA) 
                 (kV) 
                 (mA) 
                 (kV) 
                 (mA) 
                 (kV) 
               
               
                   
               
               
                 0.01 
                 32.2 
                 0.01 
                 37.0 
                 0.01 
                 39.2 
                 0.01 
                 40.3 
                 0.01 
                 41.2 
                 0.01 
                 41.7 
                 0.01 
                 41.8 
                 0.01 
                 42.8 
               
               
                 0.02 
                 34.8 
                 0.02 
                 37.3 
                 0.02 
                 50.2 
                 0.02 
                 50.7 
                 0.02 
                 50.9 
                 0.02 
                 52.7 
                 0.02 
                 55.3 
                 0.02 
                 57.1 
               
               
                 0.03 
                 36.3 
                 0.03 
                 40.8 
                 0.03 
                 — 
                 0.03 
                 53.0 
                 0.03 
                 54.8 
                 0.03 
                 60.3 
                 0.03 
                 60.5 
                 0.03 
                 60.8 
               
               
                 0.04 
                 37.0 
                 0.04 
                 42.6 
                 0.04 
                 — 
                 0.04 
                 56.5 
                 0.04 
                 57.3 
                 0.04 
                 — 
                 0.04 
                 — 
                 0.04 
                 — 
               
               
                 0.05 
                 — 
                 0.05 
                 — 
                 0.05 
                 — 
                 0.05 
                 — 
                 0.05 
                 60.7 
                 0.05 
                 — 
                 0.05 
                 — 
                 0.05 
                 — 
               
               
                 Ins. 
                 — 
                 Ins. 
                 46.0 
                 Ins. 
                 54.0 
                 Ins. 
                 58.8 
                 Ins. 
                 — 
                 Ins. 
                 — 
                 Ins. 
                 — 
                 Ins. 
                 — 
               
               
                 break 
                   
                 break 
                   
                 break 
                   
                 break 
                   
                 break 
                   
                 break 
                   
                 break 
                   
                 break 
               
               
                   
               
               
                 * a is fixed to 40 nun 
               
            
           
         
       
     
     In Table 2, the term ‘Ins. break’ refers to insulation breakdown. 
     In Experimental example 2, the field emission device  20  according to the embodiment was used (the distance ┌a+b┘ was 45 mm, 50 mm, 60 mm, 80 mm, 100 mm, 120 mm, 140 mm or 160 mm, and the thickness ┌a┘ was fixed to 40 mm). In the state in which no polymer solution was supplied to the nozzle block  110 , voltage was applied between the collector  150  and the nozzle block  110  such that current (in this case, leak current) supplied from the power supply  160  becomes a predetermined current, for example, 0.01 mA, 0.02 mA, 0.03 mA, 0.04 mA or 0.05 mA. Leak current and applied voltage in each case were recorded in a graph. 
     As can be understood from Table 2 and  FIG. 7 , when the distance ┌a+b┘ is 50 mm or more, even if voltage of 35 kV is applied between the collector  150  and the upward nozzles  112 , it is possible that leak current is restricted to be about 0.01 mA. Furthermore, when the distance ┌a+b┘ is 80 mm or more, even if a voltage of 40 kV is applied between the collector  150  and the upward nozzles  112 , it is possible that current leakage is restricted to be about 0.01 mA. 
     As shown in Experimental examples 1 and 2, when the thickness of the insulator  152  is ┌a┘ and the distance between the periphery of the insulator  152  and the periphery of the collector  150  is ┌b┘, if both ┌a≧6 mm┘ and ┌a+b≧50 mm┘ are satisfied, it is possible that current leakage which does not contribute to field emission is restricted to be a very low value. Due to this, ┌a difference between current in normal operation and a first preset current that is the upper limit threshold current where the filed emission device is determined to be abnormal┘ or ┌a difference between current in normal operation and a second preset current that is the lower limit threshold current where the field emission device is determined to be abnormal┘ can be very small. As a result, an abnormality can be detected early, and the reliability of the nanofiber manufacturing device can be markedly enhanced. 
     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 one to three or five or more field emission devices. 
     (2) 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. 
     (3) 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 . 
     (4) 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. 8  is a view illustrating a field emission device  20   a . As shown in  FIG. 8 , two nozzle blocks  110   a   1  and  110   a   2  may be installed in each field emission device  23   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. 
     (5) 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.