U.S. patent application number 13/095460 was filed with the patent office on 2011-09-01 for high-field anodizing apparatus.
This patent application is currently assigned to Korea Electrotechnology Research Institute. Invention is credited to Yoon-chul HA, Dae-young JEONG.
Application Number | 20110209990 13/095460 |
Document ID | / |
Family ID | 43826465 |
Filed Date | 2011-09-01 |
United States Patent
Application |
20110209990 |
Kind Code |
A1 |
HA; Yoon-chul ; et
al. |
September 1, 2011 |
High-Field Anodizing Apparatus
Abstract
Disclosed herein is a high-field anodizing apparatus, in which
nanostructures are formed on a surface of a metal by immersing a
metal anode and a counter electrode into an electrolyte charged in
an anodizing cell to oxidize the metal, comprising: a power supply
unit for applying a predetermined pattern of voltage between the
metal anode and counter electrode in the electrolyte; a temperature
control unit for maintaining the temperature of the metal anode,
counter electrode and electrolyte constant; and a reaction rate
control unit for measuring the value of current generated by the
voltage supplied by the power supply unit and controlling the
concentration of the electrolyte using the current value to
maintain the current constant. The high-field anodizing apparatus
is advantageous in that it is possible to prevent the damage of
nanostructures caused by the rapid melting of a metal or the
dielectric breakdown of an oxide film attributable to high-field
anodization and to control the growth rate of nanostructures, thus
greatly improving the productivity of nanostructures.
Inventors: |
HA; Yoon-chul; (Changwon-si,
KR) ; JEONG; Dae-young; (Changwon-si, KR) |
Assignee: |
Korea Electrotechnology Research
Institute
Changwon-si
KR
|
Family ID: |
43826465 |
Appl. No.: |
13/095460 |
Filed: |
April 27, 2011 |
Current U.S.
Class: |
204/196.02 |
Current CPC
Class: |
C25D 11/005 20130101;
C25D 11/02 20130101; C25D 21/02 20130101; C25D 17/12 20130101; C25D
21/12 20130101 |
Class at
Publication: |
204/196.02 |
International
Class: |
C23F 13/00 20060101
C23F013/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 1, 2009 |
KR |
10-2009-0093786 |
Dec 7, 2009 |
KR |
PCT/KR2009/007268 |
Claims
1. A high-field anodizing apparatus, in which nanostructures are
formed on a surface of a metal by immersing a metal anode and a
counter electrode into an electrolyte charged in an anodizing cell
to oxidize the metal, comprising: a power supply unit for applying
a predetermined pattern of voltage between the metal anode and
counter electrode in the electrolyte; a temperature control unit
for maintaining the temperature of the metal anode, counter
electrode and electrolyte constant; and a reaction rate control
unit for measuring the value of current generated by the voltage
supplied by the power supply unit and controlling the concentration
of the electrolyte using the current value to maintain the current
constant.
2. The high-field anodizing apparatus according to claim 1, wherein
the metal anode is made of any one of Al, Ti, Zr, Hf, Ta, Nb, W,
and alloys thereof, and is pretreated by heat treatment,
electrolytic grinding or chemical grinding.
3. The high-field anodizing apparatus according to claim 1, wherein
the counter electrode has a tubular shape.
4. The high-field anodizing apparatus according to claim 3, wherein
the electrolyte is cooled by allowing cooling water to flow into
the tubular counter electrode.
5. The high-field anodizing apparatus according to claim 1, wherein
the power supply unit applies any one of direct voltage,
alternating voltage, pulse voltage, bias voltage and combinations
thereof between the metal anode and the counter electrode, and
controls the voltage depending on the distances among pores of
nanostructures.
6. The high-field anodizing apparatus according to claim 1, wherein
the temperature control unit is provided at the rear side of the
metal anode, and comprises a temperature sensor and a cooling unit,
and, if necessary, further comprises a heating unit.
7. The high-field anodizing apparatus according to claim 6, wherein
the temperature control unit further comprises an electrolyte
cooling unit for lowering the temperature of the electrolyte.
8. The high-field anodizing apparatus according to claim 1, wherein
the reaction rate control unit comprises: a measuring unit for
measuring the current generated between the metal anode and the
counter electrode by the voltage supplied by the power supply unit;
and a high-concentration electrolyte supply unit which is opened
when the value of current measured by the measuring unit is lower
than the predetermined value thereof, and which is closed when the
value of current measured by the measuring unit is higher than the
predetermined value thereof.
Description
REFERENCE TO RELATED APPLICATIONS
[0001] This is a continuation of pending International Patent
Application PCT/KR2009/007268 filed on Dec. 7, 2009, which
designates the United States and claims priority of Korean Patent
Application No. 10-2009-0093786 filed on Oct. 1, 2009, the entire
contents of which are incorporated herein by reference.
FIELD OF THE INVENTION
[0002] The present invention relates to a high-field anodizing
apparatus for forming nanostructures on the surface of metal, and,
more particularly, to a high-field anodizing apparatus which can
prevent the damage of nanostructures and control the growth rate of
nanostructures by controlling the reaction temperature and reaction
rate of anodization.
BACKGROUND OF THE INVENTION
[0003] An anodizing process, which is a metal-surface treatment
technology, has been widely used to prevent the corrosion of metal
by forming an oxide film on the surface of metal or to color the
surface of metal. However, recently, an anodizing process has been
actively used to directly form nanostructures, such as nanodots,
nanowires, nanotubes, nanorods and the like, or to make frames for
forming nanostructures.
[0004] Al, Ti, Zr, Hf, Ta, Nb, W, and the like are known as metals
which can be formed into nanostructures by anodization. Among these
metal anodized films, an aluminum anodized film can be easily
formed, can be relatively safely treated by electrolytes, and its
nanopores and thickness can be easily controlled. Therefore, an
aluminum anodized film has been frequently used to conduct research
into nanotechnologies.
[0005] When aluminum is electrochemically anodized in an
electrolyte solution including sulfuric acid, oxalic acid or
phosphoric acid, a thick anodic oxide film is formed on the surface
thereof. This anodic oxide film includes a porous layer formed by
growing regularly-arranged pores in a direction from the outer
surface of aluminum toward the inside of aluminum and a barrier
layer formed by continuously forming pores by the oxidation of
aluminum and the flowing of an oxide film [J. E. Houser, et al.,
Nat Mater. 8, 415-420 (2009)] at the boundary of aluminum and
aluminum oxide.
[0006] In the anodic oxide film including the porous layer and the
barrier layer, it is known that the distance (D.sub.int) between
pores, the size of pores and the thickness of the barrier layer are
generally irrelevant to the kind of electrolyte or temperature, and
are predominantly determined by applied voltage.
[0007] In the anodization of aluminum, mild anodization having a
film growth rate of several micrometers (.mu.m) per hour at a
relatively low voltage and hard anodization having a film growth
rate of several tens of micrometers (.mu.m) per hour at a
relatively high voltage are known. In the high-field anodization of
the present invention, pores are rapidly grown and arranged at high
voltage compared to the conventional hard anodization in aluminum
surface treatment. In relation to the formation of nanostructrues,
the typical mild anodization and high-field anodization, in which
self-ordering occurs, are shown in Table 1 below.
TABLE-US-00001 TABLE 1 The conditions of mild anodization and
high-field anodization in which self-ordering occurs Mild
anodization High-field anodization interpore interpore Class.
voltage distance voltage distance Electrolyte sulfuric acid 19~25
V.sup.1) 50~65 nm 40~80 V.sup.4), 5) 90~140 nm oxalic acid 40
V.sup.2) 100~110 nm 110~150 V.sup.6), 7) 220~300 nm phosphoric acid
160~195 V.sup.3) 405~500 nm -- Film growth rate 2~6 .mu.m/h 30~70
.mu.m/h Current density 2~5 mA/cm.sup.2 (constant) 30~250
mA/cm.sup.2 (decreased with the passage of time) .sup.1)H. Masuda,
et al., J. Electrochem. Soc. 144, L127-L130 (1997). .sup.2)H.
Masuda, et al., Science 268, 1466-1468 (1995). .sup.3)H. Masuda, et
al.,. Jpn. J. Appl. Phys. 37, L1340-L1342 (1998). .sup.4)S. Chu, et
al., Adv. Mater. 17, 2115-2119 (2005). .sup.5)K Schwirn, et al.,
ACS nano 2, 302-310 (2008). .sup.6)W. Lee, et al., Nat. Mater. 5,
741-747 (2006). .sup.7)W. Lee, et al., European patent application
EP 1884578A1, filed Jul. 31, 2006.
[0008] The interpore distance (D.sub.int), which is the most
important factor in aluminum nanostructures, is known to be about
2.5 nm/V in mild anodization and about 2.0 nm/V in high-field
anodization. In the growth rate of an oxide film related to the
formation rate of nanostructures, in the case of mild anodization,
current density is maintained low (several mA/cm.sup.2), so that
temperature does not rapidly increase at the interface between a
metal and an oxide film, with the result that the dielectric
breakdown of the oxide film can be prevented by only a simple
cooling means such as a dual water jacket. However, in the case of
high-field anodization, initial current density is very high
(several hundreds of mA/cm.sup.2), so that the temperature of
electrodes rapidly increases, with the result that a large
electrolyzer is required in order to cool the electrodes [S. Chu,
et al., Adv. Mater. 17, 2115-2119 (2005)] or an additional cooling
means provided with cooling fins must be used [W. Lee, et al., Nat.
Mater. 5, 741-747 (2006)]. Further, in the case of high-field
anodization, when a high voltage of about 700V is applied, in order
to prevent the dielectric breakdown of the oxide film, it is known
that an electrolyte having still lower concentration than that
(0.1.about.0.5 mol/L) of a commonly-used electrolyte must be used
[C. A. Grims, et al., US Patent Application 20030047505A1, filed
Sep. 13, 2002].
[0009] Generally, in order to improve the pore alignment of an
aluminum anodized film, a two-step anodizing method [H. Masuda, et
al., Science 268, 1466-1468 (1995)] can be used. In the mild
anodization, since an oxide film slowly grows, one or more days is
required to form an easily-treatable membrane by removing oxide
film and then oxidizing it. However, in the high-filed anodization,
since pores are aligned within several tens of minutes due to great
initial current, it is possible to obtain a nanomembrane having
excellent pore alignment.
[0010] In order to form an anodic oxide film into a membrane,
residual aluminum and a barrier layer must be removed. In this
case, in order to remove the residual aluminum and the barrier
layer, an electrochemical method and a chemical method are largely
used. First, in the electrochemical method, the barrier layer is
removed and then the residual aluminum is selectively melted by
electrochemical reduction using voltage reduction and current
voltage, and an oxide film is separated from the aluminum and then
the barrier layer is suitably melted by pulse separation. In the
chemical method, the residual aluminum is selectively melted, and
then the barrier layer is melted. Meanwhile, the pore size of a
membrane can be increased by suitably using a process of chemically
separating a barrier layer, and the pore size thereof can also be
decreased by coating the walls of pores using a chemical or
physical process.
[0011] As such, although nanostructures having nanopores, the
distance therebetween and the size thereof being able to be easily
adjusted and the shapes thereof being uniform, are increasingly put
to practical use, they are mostly used in the fundamental research
of an anodic oxide film at a laboratory level, and it is actually
insufficient to develop a process and apparatus for high-field
anodization for mass-producing nanostructures or rapidly producing
nanostructures. In order to produce nanostructures having excellent
pore alignment using high-field anodization, it is necessary to
maintain the temperature of a reaction interface constant and to
lower the initial concentration of an electrolyte. However, in this
case, there is a problem in that sufficient growth rate cannot be
obtained because reaction rate is extremely slow.
SUMMARY OF THE INVENTION
[0012] Accordingly, the present invention has been made to solve
the above-mentioned problems, and an object of the present
invention is to provide a high-field anodizing apparatus which can
prevent the damage of nanostructures and control the growth rate of
nanostructures by controlling the reaction temperature and reaction
rate of anodization.
[0013] In order to accomplish the above object, the present
invention provides a high-field anodizing apparatus, in which
nanostructures are formed on a surface of a metal by immersing a
metal anode and a counter electrode into an electrolyte to
electrochemically oxidize the metal, comprising: a power supply
unit for applying a predetermined pattern of voltage between the
metal anode 13 and counter electrode in the electrolyte; a
temperature control unit for maintaining the temperature of the
metal anode, counter electrode and electrolyte constant; and a
reaction rate control unit for measuring the value of current
generated by the voltage supplied by the power supply unit and
controlling the concentration of the electrolyte using the current
value to maintain the current constant.
[0014] In the high-field anodizing apparatus, the metal anode may
be made of any one of Al, Ti, Zr, Hf, Ta, Nb, W, and alloys
thereof, and may be pretreated by heat treatment, electrolytic
grinding or chemical grinding.
[0015] Further, the counter electrode may have a tubular shape, and
the electrolyte may be cooled by allowing cooling water to flow
into the tubular counter electrode.
[0016] Further, the power supply unit may apply any one of direct
voltage, alternating voltage, pulse voltage, bias voltage and
combinations thereof between the metal anode and the counter
electrode, and may control the voltage depending on the distances
among pores of nanostructures.
[0017] Further, the temperature control unit may be provided at the
rear side of the metal anode, and may comprise a temperature sensor
and a cooling unit, and, if necessary, may further comprise a
heating unit. In addition, the temperature control unit may further
comprise an electrolyte cooling unit for lowering the temperature
of the electrolyte.
[0018] Further, the reaction rate control unit may comprise: a
measuring unit for measuring the current generated between the
metal anode and the counter electrode by the voltage supplied by
the power supply unit; and a high-concentration electrolyte supply
unit which is opened when the value of current measured by the
measuring unit is lower than the predetermined value thereof, and
which is closed when the value of current measured by the measuring
unit is higher than the predetermined value thereof.
[0019] The high-field anodizing apparatus according to the present
invention is advantageous in that it is possible to prevent the
rapid melting of metal caused by high-field anodization or the
damage of nanostructures caused by dielectric breakdown of an oxide
film, and in that the productivity of nanostructures can be greatly
improved by controlling the growth rate of nanostructures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a schematic view showing a high-field anodizing
apparatus according to the present invention;
[0021] FIG. 2 shows the voltage-current-temperature curves at the
time of electrolytic grinding and the surface of an aluminum
material due to the electrolytic grinding;
[0022] FIG. 3 shows the voltage-current-temperature curves at the
time of primary high-field anodization and the shape of an oxide
film formed by the primary high-field anodization;
[0023] FIG. 4 shows the voltage-current-temperature curves at the
time of secondary high-field anodization and the shape of an oxide
film formed by the secondary high-field anodization using the
reaction rate control unit of the present invention;
[0024] FIG. 5 shows the shape of an oxide film formed by pulse
separation; and
[0025] FIG. 6 shows the final shape of nanostructures with their
boundary layer removed and with their pores expanded.
REFERENCE NUMERALS
[0026] 10: anodizing cell [0027] 11: electrolytic bath [0028] 12:
electrolyte [0029] 13: anode [0030] 14: cathode [0031] 15: cathode
lead wire [0032] 16: metal support [0033] 17: stirring unit [0034]
18: O-ring [0035] 19: cooling bed [0036] 100: power supply unit
[0037] 200: temperature control unit [0038] 210: temperature sensor
[0039] 220: cooling unit [0040] 230: heating unit [0041] 300:
reaction rate control unit [0042] 310: measuring unit [0043] 320:
high-concentration electrolyte supply unit
DETAILED DESCRIPTION OF THE INVENTION
[0044] The present invention provides a high-field anodizing
apparatus for producing metal oxide nanostructures having
regularly-aligned nanopores using high-field anodization. The
high-field anodizing apparatus includes: a power supply unit for
applying a predetermined pattern of voltage between a metal anode
and a counter electrode in an electrolyte; a temperature control
unit for maintaining the temperature of the metal anode, counter
electrode and electrolyte constant; and a reaction rate control
unit for measuring the value of current generated by the voltage
supplied by the power supply unit and controlling the concentration
of the electrolyte using the current value to maintain the current
constant.
[0045] This high-field anodizing apparatus is advantageous in that
it is possible to prevent the damage of nanostructures caused by
the rapid melting of a metal or the dielectric breakdown of an
oxide film attributable to high-field anodization and to control
the growth rate of nanostructures, thus greatly improving the
productivity of nanostructures.
[0046] The metal anode, which is an anode material, is made of any
one of Al, Ti, Zr, Hf, Ta, Nb, W, and alloys thereof, and, if
necessary, may be pretreated by heat treatment, electrolytic
grinding or chemical grinding in order to make its texture uniform
and flatten its surface. The counter electrode, which is a cathode
material, is made of a carbon-based material such as graphite,
carbon nanotubes or carbon black, or a conductive material such as
platinum or stainless steel.
[0047] The electrolyte is selectively used depending on the kind of
the anode material. When aluminum is used as the anode material, an
aqueous sulfuric acid solution, an aqueous phosphoric acid
solution, an aqueous chromic acid solution or a mixture thereof may
be used as the electrolyte. When the temperature of the electrolyte
must be decreased below zero, the electrolyte may be mixed with
ethyleneglycol or the like and then used. Further, when titanium
(Ti) or zirconium (Zr) is used as the anode material, a nonaqueous
organic solution containing fluorine ions may be used as the
electrolyte.
[0048] The power supply unit applies direct voltage, alternating
voltage, pulse voltage and bias voltage between the metal anode and
the counter electrode to form an oxide film on the surface of the
metal anode. In this case, the power supply unit must be able to
apply voltage depending on the distances among pores of
nanostructures. That is, the power supply unit must be able to
apply a direct voltage of 250 V or less or a pulse voltage of 700 V
or less, and must have a current density of 500 mA or more per unit
area (cm.sup.2) of a metal.
[0049] The temperature control unit comes into contact with the
rear side of the metal anode, and includes a temperature sensor and
a cooling unit for preventing the temperature of the metal anode
from being increased above reference temperature, and, if
necessary, may further include a heating unit for maintaining the
temperature constant. Further, if necessary, the temperature
control unit may include an electrolyte cooling unit in order to
lower the electrolyte temperature that can be increased by a
cathode reaction. The electrolyte cooling unit may supply cooling
water into the counter electrode.
[0050] The reaction rate control unit includes: an analog or
digital measuring unit for measuring the current generated between
the metal anode and the counter electrode by the voltage supplied
by the power supply unit; and a high-concentration electrolyte
supply unit which is opened when the value of current measured by
the measuring unit is lower than the predetermined value thereof,
and which is closed when the value of current measured by the
measuring unit is higher than the predetermined value thereof. Due
to the reaction rate control unit, the value of current is
maintained at a predetermined level, thus preventing the rapid
melting of a metal or the dielectric breakdown of an oxide film.
For this purpose, it is preferred that a voltage be applied to an
electrolyte having low concentration in the initial stage.
[0051] In order to form a nanomembrane having an interpore distance
of 280 nm in an oxalic acid solution, a high-field anodizing
apparatus according to an embodiment of the present is configured
as follows.
[0052] FIG. 1 is a view showing a high-field anodizing apparatus
using a vertical-type anodizing cell. As shown in FIG. 1, the
vertical-type anodizing cell 10, which is generally used when a
large amount of gas is generated at electrodes, is configured such
that an anode 13 is disposed on a metal support 16 connected to the
(+) terminal of a power supply unit 100, and a cathode 14 is
connected to the (-) terminal of the power supply unit 100 through
a cathode lead wire 15. Further, the vertical-type anodizing cell
10 is configured such that an O-ring 18 is provided between an
electrolytic bath 11 and the anode 13 not to allow an electrolyte
12 to leak out. Furthermore, the vertical-type anodizing cell 10 is
provided with a stirring unit 17 such as an impeller in order to
stir the electrolyte 12.
[0053] Due to the voltage supplied by the power supply unit 100, an
oxide film formation reaction occurs at the anode 13, and a
reduction reaction (electrolysis of water, etc.) occurs at the
cathode 14. In this case, owing to these reactions, the temperature
of the interface between the electrolyte 12 and each of the
electrodes can be increased. Particularly, when the temperature of
the anode 13 is increased above the predetermined temperature, the
alignment of pores deteriorates. Therefore, in the high-field
anodization of aluminum, the temperature of the anode 13 must be
maintained at 0.degree.. For this purpose, a cooling bed 19 is
provided under the metal support disposed beneath the anode 13. The
cooling bed 19 receives low-temperature liquid (0.degree. or lower)
from a circulator as a cooling unit 200 of a temperature control
unit 200, and cools the lower portion of the metal support 16, thus
absorbing the heat of the anode 13 by thermal conduction. For this
purpose, a copper plate having excellent thermal conductivity may
be used as the cooling bed.
[0054] As in the early stage of high-field anodization, when heat
is excessively emitted from the anode 13, for the purpose of more
precise temperature control, if the temperature of the anode 13 is
maintained at 0.degree. by the combination of cooling and heating
by providing a temperature sensor 210 and a heating unit 230 in the
metal support 16 instead of by decreasing the temperature of the
circulator, rapid cooling can be performed by stopping the
operation of the heating unit 230 at the time of emitting excessive
heat. Such a system is very useful in the temperature control when
the area of nanostructures expands.
[0055] Further, in order to decrease the temperature of the
electrolyte 12, cooling water may flow into the cathode 14 using a
metal tube-type counter electrode instead of a platinum net-type
counter electrode which is generally used as the counter electrode,
or a high-concentration electrolyte supply unit 320 may be used.
That is, cooling water is supplied into the tubular counter
electrode by an electrolyte cooling unit of the temperature control
unit 200
[0056] Therefore, the temperature control unit 200 serves to
decrease the temperature of the electrolyte 12 both by cooling the
metal support 16 and thus absorbing the heat emitted from the anode
13, and by supplying cooling water into the counter electrode using
the electrolyte cooling unit.
[0057] Meanwhile, in order to obtain nanostructures having
excellent pore alignment, a two-step anodizing process of removing
the oxide film formed by initial oxidation and then directly
applying a voltage thereto or an imprinting process of forming
regular pattern on the surface of an oxide film must be used.
However, when secondary anodization is conducted in the
high-concentration electrolyte (for example, 0.3 M of oxalic acid)
used in primary anodization, the nanostructures are damaged due to
the rapid melting of metal or the dielectric breakdown of an oxide
film. This problem can be solved by conducting the secondary
anodization in an electrolyte diluted to 1/100. Even in this case,
since initial current is low and gradually decreased, desired
growth rate can be obtained.
[0058] In order to solve the above problem, the reaction rate
control unit 300 controls current such that the current is
maintained above the current value measured by a measuring unit
310, and the high-concentration supply unit 320 supplies a
high-concentration electrolyte. That is, the high-concentration
electrolyte supply unit 320 is opened when the current value
measured by the measuring unit 310 is lower than the predetermined
current value thereof, and is closed when the current value
measured by the measuring unit 310 is higher than the predetermined
current value thereof. Since the current value is maintained at a
constant level by the reaction rate control unit 300, the rapid
melting of a metal or the dielectric breakdown of an oxide film by
high-field anodization can be prevented. For this purpose, it is
preferred that a voltage be applied to a low-concentration
electrolyte in the early stage of high-field anodization.
[0059] FIG. 2 shows a photograph (FIG. 2A) of the sample obtained
by electrolytically grinding an aluminum disk having a purity of
99.999% in a mixed solution of perchloric acid and ethanol of a
volume ratio of 1:4 for 5 minutes, and shows a graph (FIG. 2B) of
the change of voltage, current and sample temperature in this
case.
[0060] FIG. 3 shows a photograph (FIG. 3A) of the primarily
anodized oxide film obtained by increasing a voltage from 0 V to
140 V with respect to a platinum cathode under the conditions of a
sample temperature of 0.degree. C. and an oxalic acid solution of
0.3 M and then maintaining the voltage for 30 minutes, shows a
graph (FIG. 3B) of the change of voltage, current and sample
temperature in this case, shows a SEM photograph (FIG. 3C) of the
initial pores of the lower portion of the oxide film, shows a SEM
photograph (FIG. 3D, wherein aluminum was selectively removed by a
mixed solution of copper chloride and hydrochloric acid) of the
boundary layer of the lower portion of the oxide film, and shows a
photograph (FIG. 3E, wherein an alumina film was selectively
removed by a mixed solution of chromic acid and phosphoric acid) of
the patterned surface of the remaining aluminum and a SEM
photograph (FIG. 3E) thereof. In FIG. 3B, when an oxide film begins
to be rapidly formed at a voltage range of 80.about.90 V to obtain
a maximum of current value and then the voltage reaches a constant
voltage of 140 V, current is rapidly decreased by the diffusion
control mechanism of an electrolyte. In contrast, when current is
slowly decreased at a voltage of 140 V, the alignment of pores
occurs. It is known that this pore alignment is improved with the
increase of primary anodization time.
[0061] FIG. 4 show a photograph (FIG. 4A) of a secondarily anodized
oxide film formed by directly applying a voltage of 140V in an
oxalic acid solution of 0.003 M under the condition of a sample
temperature of 0.degree. C. to the sample obtained by selectively
removing an alumina film from the primarily anodized oxide film,
wherein current density was set to 15 mA/cm.sup.2 and a
high-concentration electrolyte was supplied when the current
density fell below 15 mA/cm.sup.2. Further, FIG. 4 shows a graph
(FIG. 4B) of the change of voltage, current and sample temperature
in this case. In FIG. 4B, the initial current density is rapidly
decreased from 60 mA/cm.sup.2, so that the initial current density
is decreased at the level of mild anodization when a
high-concentration electrolyte is not supplied. As shown in FIG.
4B, since current increases at the time of supplying an
electrolyte, reaction rate can be controlled, and growth rate can
also be controlled constantly or variably depending on time.
[0062] FIG. 5 show a photograph (FIG. 5A) of an oxide film obtained
by applying a voltage of 150 V to the secondarily anodized oxide
film in a mixed solution of perchloric acid and ethanol of a volume
ratio of 1:1 by pulse separation, shows an obliquely-taken SEM
photograph (FIG. 5B) thereof, and shows an SEM photograph (FIG. 5C)
of the entire section thereof. It can be presumed from FIG. 5 that
pores are grown in accordance with the pattern formed by primary
anodization, thus forming an oxide film having a thickness of about
30 .mu.m per hour.
[0063] FIG. 6 shows a photograph (FIG. 6A) of a final membrane
obtained by removing a boundary layer using a 5% phosphoric acid
solution and enlarging pores, and shows a SEM photograph (FIG. 6B)
thereof.
[0064] As described above, the high-field anodizing apparatus of
the present invention can be used to produce nanotemplate formed
without separating an oxide film from a matrix material in addition
to the nanomembrane. Furthermore, the high-field anodizing
apparatus of the present invention can also be used to produce
nanopores, nanowires, nanotubes, and the like.
INDUSTRIAL APPLICABILITY
[0065] The present invention relates to a high-field anodizing
apparatus for forming nanostructures on the surface of metal, and,
more particularly, to a high-field anodizing apparatus which can
prevent the damage of nanostructures and control the growth rate of
nanostructures by controlling the reaction temperature and reaction
rate of anodization.
* * * * *