U.S. patent application number 11/184258 was filed with the patent office on 2005-12-01 for radiator and apparatus including the radiator.
This patent application is currently assigned to Matsushita Electric Industrial Co., Ltd.. Invention is credited to Horiuchi, Makoto, Kaneko, Yuriko, Kimoto, Mitsuhiko, Ohkubo, Kazuaki, Sakaue, Mika.
Application Number | 20050263269 11/184258 |
Document ID | / |
Family ID | 34993960 |
Filed Date | 2005-12-01 |
United States Patent
Application |
20050263269 |
Kind Code |
A1 |
Kaneko, Yuriko ; et
al. |
December 1, 2005 |
Radiator and apparatus including the radiator
Abstract
A radiator 1 according to the present invention converts heat
into electromagnetic waves and then radiates the electromagnetic
waves through its surface. A number of microcavities are made in at
least some areas on the surface, and the surface of the
microcavities 2 is covered with a layer including tungsten that is
bonded to carbon.
Inventors: |
Kaneko, Yuriko; (Nara-shi,
JP) ; Horiuchi, Makoto; (Sakurai-shi, JP) ;
Ohkubo, Kazuaki; (Osaka, JP) ; Kimoto, Mitsuhiko;
(Nara-shi, JP) ; Sakaue, Mika; (Osaka,
JP) |
Correspondence
Address: |
AKIN GUMP STRAUSS HAUER & FELD L.L.P.
ONE COMMERCE SQUARE
2005 MARKET STREET, SUITE 2200
PHILADELPHIA
PA
19103
US
|
Assignee: |
Matsushita Electric Industrial Co.,
Ltd.
|
Family ID: |
34993960 |
Appl. No.: |
11/184258 |
Filed: |
July 19, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11184258 |
Jul 19, 2005 |
|
|
|
PCT/JP05/01130 |
Jan 27, 2005 |
|
|
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Current U.S.
Class: |
165/146 |
Current CPC
Class: |
H01K 3/02 20130101; H01K
1/10 20130101; H01K 1/08 20130101 |
Class at
Publication: |
165/146 |
International
Class: |
F28F 013/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 17, 2004 |
JP |
2004-075784 |
Claims
1. A radiator for converting heat into electromagnetic waves and
then radiating the electromagnetic waves through its surface,
wherein a plurality of microcavities are made in at least some
areas of the surface, and wherein the areas have a layer including
tungsten and carbon.
2. The radiator of claim 1, wherein the layer including tungsten
and carbon contains tungsten that is bonded to carbon.
3. The radiator of claim 1, wherein the microcavities make an array
in at least those areas.
4. The radiator of claim 1, wherein each of the microcavities is a
recess with an inside diameter of 1 .mu.m or less and a depth that
is greater than the inside diameter.
5. The radiator of claim 1, wherein the microcavities are arranged
regularly at a pitch of 2 .mu.m or less.
6. The radiator of claim 1, wherein the microcavities are defined
by gaps between a number of columnar members arranged.
7. The radiator of claim 1, wherein the radiator has a body that is
made essentially of tungsten.
8. The radiator of claim 1, wherein the radiator is made
essentially of tungsten carbide.
9. The radiator of claim 1, wherein the radiator operates at a
temperature of 2,000 K or more.
10. An apparatus comprising: the radiator of claim 1; a container
for shutting off the radiator from the air; and energy supply means
for supplying the radiator with energy and making the radiator
emits electromagnetic waves.
11. A thermoelectric converter comprising: the radiator of claim 1;
a container for shutting off the radiator from the air; and a
converter, which receives the electromagnetic waves that has been
emitted from the radiator and converts the electromagnetic waves
into electric energy, wherein the thermoelectric converter supplies
the radiator with energy, thereby making the radiator radiate the
electromagnetic waves.
12. A method of making a radiator that converts heat into
electromagnetic waves and then radiates the electromagnetic waves
through its surface, the method comprising the steps of: providing
a tungsten member; making a plurality of microcavities in at least
some areas on the surface of the tungsten member; and carbonizing
at least some of the areas on the surface of the tungsten
member.
13. A method of making a radiator that converts heat into
electromagnetic waves and then radiates the electromagnetic waves
through its surface, the method comprising the steps of: providing
a member that has a layer including tungsten and carbon in at least
some areas on its surface; and making a plurality of microcavities
in at least those areas on the surface of the member.
14. The method of claim 13, wherein the layer including tungsten
and carbon contains tungsten that is bonded to carbon.
15. The method of claim 12, wherein the step of making a plurality
of microcavities includes making the microcavities by laser
irradiation or sandblasting.
16. A method of making a radiator that converts heat into
electromagnetic waves and then radiates the electromagnetic waves
through its surface, the method comprising the steps of: providing
a number of wires, each having a layer that includes tungsten and
carbon in at least some areas on its surface; and bundling the
wires together, thereby making a plurality of microcavities in gaps
between the wires.
17. The method of claim 16, wherein the layer including tungsten
and carbon contains tungsten that is bonded to carbon.
Description
[0001] This is a continuation of International Application
PCT/JP2005/001130, with an international filing date of Jan. 27,
2005.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a radiator with a
microcavity structure for improving its radiation efficiency in a
particular wavelength range.
[0004] 2. Description of the Related Art
[0005] An incandescent lamp, used extensively today as a common
illumination source, includes a filament functioning as a thermal
radiator. The "thermal radiator" is a radiation source that emits
electromagnetic waves by thermal radiation. And the "thermal
radiation" means radiation (of electromagnetic waves) produced by
applying heat to atoms or molecules of an object. The thermal
radiation energy is determined by the temperature of the object and
has a continuous spectrum. In the following description, the
thermal radiator will be simply referred to herein as a
"radiator".
[0006] An incandescent lamp achieves an excellent color rendering
index and can be lit by a simple appliance. However, as the
incandescent lamp uses a radiation produced by a filament that is
generating heat, the radiation produced by the incandescent lamp in
the visible wavelength range is just 10% of the overall radiations
thereof (in a situation where the operating temperature is 2,600 K,
for example). More specifically, the majority of the other
radiations are infrared radiations, of which the energy density
accounts for as much as about 70% of that of the overall
radiations. Also, the current incandescent lamp causes a heat
conduction due to an enclosed gas or a heat loss of as much as
about 20% due to convection and have a visible radiation efficiency
of only about 15 lm/W. Thus, various techniques of improving the
visible radiation efficiency by cutting down the infrared
radiations, which account for about 70% of the overall
electromagnetic waves emitted from the radiator, have been
researched.
[0007] U.S. Pat. No. 5,079,473 discloses a radiator including an
array of micro-waveguides (which will be referred to herein as
"microcavities") on its surface. This radiator propagates only
electromagnetic waves, of which the wavelengths are shorter than a
predetermined wavelength that is defined by the shape and
dimensions of the cavities, thereby cutting down the infrared
radiations. According to U.S. Pat. No. 5,079,473, the cavities do
not propagate electromagnetic waves if the wavelengths of the rays
are twice or more as long as the inside diameter of the cavities.
Thus, if the cavities have an inside diameter of 350 nm and the
wall portions between the cavities have a thickness of 150 nm, then
photons with wavelengths of 700 nm or more can be radiated only
through those wall portions. However, no infrared electromagnetic
waves with wavelengths of 700 nm or more will be propagated through
the array of cavities.
[0008] When these design parameters are adopted, the sum of the
areas of the array of cavities will account for 50% of the overall
area of a plain surface with no cavities at all. According to U.S.
Pat. No. 5,079,473, the total radiated rays with wavelengths
exceeding 700 nm can be reduced to about one-tenth compared to
tungsten at the same temperature, and the visible radiation
efficiency can be approximately six times as high as the
conventional one at an operating temperature of 2,100 K.
[0009] The spectrum of thermal radiation in thermal equilibrium
state depends on the temperature following the Planck radiation
formula. FIG. 1 is a graph showing the temperature dependence of
blackbody radiation. In FIG. 1, the ordinate represents the
spectral radiance B.sub..lambda..DELTA..lambda.
[W.multidot.cm.sup.-2 str.sup.-1] (where .DELTA..lambda.10 nm) of
blackbody, while the abscissa represents the wavelength [.mu.m] of
radiation. If the operating temperature of an incandescent lamp is
1,600 K, for example, then the spectral radiance distribution of
the light radiated from its filament is represented by a curve with
"1600 K" in this graph. This curve shows that the peak is located
at a wavelength of about 2 .mu.m and that infrared radiations
account for a high percentage.
[0010] As is clear from FIG. 1, if the temperature of the radiator
increases from 1,200 K to 2,000 K, then the radiation in the
visible range increases by three orders of magnitude or more but
the radiation in the infrared range does not change so much. That
is why to obtain visible radiation efficiently, the operating
temperature is preferably set to at least 2,000 K. Particularly
when the radiator is used as an illumination source, the operating
temperature should not be lower than 2,000 K because the resultant
light would be excessively reddish if the operating temperature
fell short of 2,000 K. For that reason, the radiator is preferably
made of a refractory material such as tungsten that can withstand a
high-temperature operation at 2,000 K or more.
[0011] The present inventors actually made such an array of
cavities on the surface of a tungsten radiator and carried out
various experiments on that radiator. As a result, in a tungsten
radiator with an array of microcavities, each having a size of 1
.mu.m or less, that array of cavities collapsed in a short time at
a temperature of about 1,200 K, which is a sort of unusual and
curious phenomenon. As described above, the filament of an
incandescent lamp needs to operate at as high a temperature as
2,000 K or more and the incandescent lamp should have a
sufficiently long life. If that specially designed cavity array
structure, which has been reduced to a sub-micron dimension in
order to minimize the infrared radiations, collapsed so easily,
then such a radiator would no longer be applicable to an
incandescent lamp and other devices that need to operate at
elevated temperatures.
SUMMARY OF THE INVENTION
[0012] In order to overcome the problems described above, an object
of the present invention is to provide a radiator that can operate
with good stability at such elevated temperatures even if the array
of cavities arranged on the surface of the radiator has a very
small size of 1 .mu.m or less.
[0013] Another object of the present invention is to provide an
incandescent lamp that includes such a radiator and that can emit
visible radiation efficiently.
[0014] Yet another object of the present invention is to provide a
non-illumination apparatus including the radiator and a method of
making the radiator.
[0015] A radiator according to the present invention converts heat
into electromagnetic waves and then radiates the electromagnetic
waves through its surface. A plurality of microcavities are made in
at least some areas of the surface, and the areas have a layer
including tungsten and carbon.
[0016] In one preferred embodiment, the layer including tungsten
and carbon contains tungsten that is bonded to carbon.
[0017] In another preferred embodiment, the microcavities make an
array in at least those areas.
[0018] In another preferred embodiment, each of the microcavities
is a recess with an inside diameter of 1 .mu.m or less and a depth
that is greater than the inside diameter.
[0019] In another preferred embodiment, the microcavities are
arranged regularly at a pitch of 2 .mu.m or less.
[0020] In another preferred embodiment, the microcavities are
defined by gaps between a number of columnar members arranged.
[0021] In another preferred embodiment, the radiator has a body
that is made essentially of tungsten.
[0022] In another preferred embodiment, the radiator is made
essentially of tungsten carbide.
[0023] In another preferred embodiment, the radiator has an
operating temperature of 2,000 K or more.
[0024] An apparatus according to the present invention includes a
radiator according to any of the preferred embodiments of the
present invention described above, a container for shutting off the
radiator from the air, and energy supply means for supplying the
radiator with energy and making the radiator emits electromagnetic
waves.
[0025] A thermoelectric converter according to the present
invention includes a radiator according to any of the preferred
embodiments of the present invention described above, a container
for shutting off the radiator from the air and a converter, which
receives electromagnetic waves that has been radiated from the
radiator and converts the electromagnetic waves into electric
energy. The thermoelectric converter supplies the radiator with
energy, thereby making the radiator radiate the electromagnetic
wave.
[0026] A radiator making method according to the present invention
is a method of making a radiator that converts heat into
electromagnetic waves and then radiates the electromagnetic waves
through its surface. The method includes the steps of: providing a
tungsten member; making a plurality of microcavities in at least
some areas on the surface of the tungsten member; and carbonizing
at least some of the areas on the surface of the tungsten
member.
[0027] Another radiator making method according to the present
invention is a method of making a radiator that converts heat into
electromagnetic waves and then radiates the electromagnetic waves
through its surface. The method includes the steps of: providing a
member that has a layer including tungsten and carbon in at least
some areas on its surface; and making a plurality of microcavities
in at least those areas on the surface of the member.
[0028] In one preferred embodiment, the layer including tungsten
and carbon contains tungsten that is bonded to carbon.
[0029] In another preferred embodiment, the step of making a
plurality of microcavities includes making the microcavities by
laser irradiation or sandblasting.
[0030] Yet another radiator making method according to the present
invention is a method of making a radiator that converts heat into
electromagnetic waves and then radiates the electromagnetic waves
through its surface. The method includes the steps of providing a
number of wires, each having a layer that includes tungsten and
carbon in at least some areas on its surface, and bundling the
wires together, thereby making a plurality of microcavities in gaps
between the wires.
[0031] In one preferred embodiment, the layer including tungsten
and carbon contains tungsten that is bonded to carbon.
[0032] According to the present invention, carbon is introduced
into some surface areas of tungsten, thereby increasing the thermal
stability of the microcavity structure. As a result, a radiator
with high radiation efficiency is realized by keeping the
microstructure on the surface intact even at an elevated
temperature and minimizing radiations having wavelengths that are
equal to or greater than a predetermined wavelength. Also, an
incandescent lamp according to the present invention, including
such a radiator, realizes a luminaire that can convert thermal
energy into visible radiation and then radiates the visible
radiation with high efficiency.
[0033] In addition, since radiation efficiency can be increased in
a particular wavelength range, the present invention can also
achieve beneficial effects even when applied to various other types
of apparatuses, not just illumination sources.
BRIEF DESCRIPTION OF THE DRAWINGS
[0034] FIG. 1 is a graph showing the spectral radiances of
blackbody radiation.
[0035] FIG. 2 illustrates a first specific preferred embodiment of
a radiator according to the present invention.
[0036] FIGS. 3(a) through 3(e) are cross-sectional views
schematically illustrating various relationships between a
microcavity and a tungsten compound layer.
[0037] FIG. 4 is an SEM photograph showing the surface of tungsten
that has been subjected to a carburizing process.
[0038] FIG. 5 is a graph showing the results of measurements by
X-ray photoelectron spectroscopy (XPS).
[0039] FIGS. 6(a) and 6(b) are surface SEM photographs of a
radiator as a comparative example that has not yet been heated and
the same radiator that has been heated, respectively.
[0040] FIGS. 6(c) and 6(d) are surface SEM photographs of a
radiator 1 as a preferred embodiment of the present invention that
has not yet been heated and the same radiator that has been heated,
respectively.
[0041] FIG. 7 is a graph showing the concentrations (partial
pressures) of saturated oxygen that contributes to the oxidation
reaction of tungsten.
[0042] FIG. 8 is a graph showing the Gibbs free energy of a
refractory material during the oxidation reaction.
[0043] FIG. 9 is a graph showing the emissivity values of tungsten
(W) and tungsten carbide (WC).
[0044] FIG. 10 schematically shows the microcavity collapse
temperatures and melting points of tungsten and tungsten
carbide.
[0045] FIG. 11 illustrates an exemplary configuration for an
incandescent lamp including a radiator 1 according to a preferred
embodiment of the present invention.
[0046] FIG. 12 illustrates an electrode that has been formed by
going through a carburizing process.
[0047] FIG. 13 schematically illustrates a preferred embodiment of
a thermoelectric converter according to the present invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0048] Hereinafter, preferred embodiments of radiators according to
the present invention will be described with reference to the
accompanying drawings.
Embodiment 1
[0049] First, referring to FIG. 2, shown is a plan view
schematically illustrating the surface of a radiator 1 according to
a first specific preferred embodiment of the present invention. In
FIG. 2, the dashed-line rectangle is a schematic representation
showing a part of the surface of the radiator 1 on a larger
scale.
[0050] The radiator 1 of this preferred embodiment has a ribbon
shape as a whole with a width of 0.1 mm, a length of 10 mm and a
thickness of 0.05 mm, and is made essentially of tungsten. On the
surface of the radiator 1, provided is an array of cavities 2, each
of which has a cylindrical shape with a diameter of 0.7 .mu.m and a
depth of 1.2 .mu.m. These cavities 2 have a size of 1 .mu.m or less
each on a plane that is defined parallel to the radiation plane,
and will be referred to herein as "microcavities".
[0051] In this preferred embodiment, these microcavities 2 are
arranged substantially regularly on the surface of the radiator 1
and the arrangement pitch (i.e., the distance between the center
axes of two adjacent cavities) is set to 1.4 .mu.m.
[0052] Those microcavities 2 can be made by any of various fine
processesing technologies. In this preferred embodiment, the
microcavities 2 are made by irradiating the surface of the radiator
with a pulsed laser beam. Such a method of making very small
recesses on the surface of a workpiece using a pulsed laser beam is
described in Japanese Patent Application Laid-Open Publication No.
2001-314989, for example. More specifically, in this preferred
embodiment, the fine processing is carried out by irradiating the
radiator with a laser beam having a pulse energy of 0.1 mJ and a
pulse width of 100 femtoseconds. The radiator 1 is repeatedly
exposed to these laser pulses several tens to several thousands of
times to make just one microcavity 2.
[0053] The radiator 1 to be processed with the laser beam is put on
an X-Y stage. By irradiating the radiator with the laser beam
synchronously with the movement of the X-Y stage, an array of
microcavities such as that shown in FIG. 2 can be made. And if the
movement of the X-Y stage is controlled with high precision, then
the arrangement pattern of the array can be defined arbitrarily. In
this preferred embodiment, the microcavities 2 are arranged
regularly at a substantially constant pitch. Alternatively, the
density of the microcavities 2 may be set high in one area but low
in another such that the radiator 1 has radiation properties that
change from one position to another. The inside diameter and depth
of the microcavities 2 can be set to arbitrary values by adjusting
the irradiation energy density, beam spot size, number of shots and
other parameters of the laser pulses.
[0054] Optionally, to make a huge number of microcavities at the
same time, the photolithography and etching techniques, which are
used extensively in semiconductor device fabrication and
micro-electro-mechanical systems or MEMS technologies, may also be
adopted.
[0055] The prime feature of the radiator 1 of this preferred
embodiment is that a surface region of the radiator 1 to a depth of
about 2 .mu.m as measured from the radiation plane of the radiator
1 is a layer including tungsten and carbon. As will be described
more fully later, at least a part of tungsten is chemically bonded
to another element (such as carbon) in that layer including
tungsten and carbon. Thus, this layer will be referred to herein as
a "tungsten compound layer".
[0056] In this preferred embodiment, the surface of tungsten is
subjected to a carburizing process to make such a tungsten compound
layer. The carburizing process is a process for carbonizing the
surface of a metal, for example, and may be carried out by any of
various methods that have been developed so far. For example,
according to a plasma carburizing process, using the furnace body
or thermal insulator as an anode and the workpiece as a cathode,
respectively, a high DC voltage is applied between these electrodes
in a rare gas atmosphere containing a hydrocarbon gas such as
methane or propane including argon and hydrogen, thereby generating
glow discharge and eventually plasma. In the plasma, various
electrochemical reactions set in to make ions of the hydrocarbon
gas bombard on the surface of the workpiece and thereby produce
carburizing. This plasma carburizing process is more effective in
activating, cleaning or reducing the surface of the workpiece than
any other type of carburizing process. In a preferred embodiment,
the carburizing process is preferably carried out at a temperature
of 500.degree. C. to 2,000.degree. C. (e.g., at 1,100.degree. C.)
for 4 to 48 hours (e.g., for 8 hours). By modifying the carburizing
process conditions, the thickness of the resultant tungsten
compound layer can be controlled. To improve the thermal stability,
it should be enough to deposit the tungsten compound layer to a
thickness of at least about several nanometers.
[0057] However, the tungsten compound layer does not have to be
made by such a carburizing process but may also be formed by
introducing a constituent element of the compound, such as carbon,
into tungsten by either ion implantation or solid-phase
diffusion.
[0058] In this preferred embodiment, an array of microcavities 2 is
made on the surface of tungsten and then the workpiece is subjected
to the carburizing process. That is why the workpiece has a surface
area that is big enough to carbonize the tungsten efficiently.
Alternatively, tungsten may be subjected to the carburizing process
first, and then processed to make the array of microcavities 2
thereon. In that case, a compound layer may be formed to a
thickness that falls short of the depth of the microcavities 2 to
make. This is because the array structure of the microcavities 2
can be thermally stabilized just by forming such a thin compound
layer on the surface.
[0059] FIGS. 3(a) through 3(e) are cross-sectional views
schematically illustrating various relationships between the
microcavity 2 and the tungsten compound layer 22. In FIG. 3(a), a
tungsten compound layer 22, of which the thickness is smaller than
the depth of the microcavity, has been formed on the surface of
tungsten 21. In the example illustrated in FIG. 3(b), the tungsten
compound layer 22 is even thinner than the counterpart shown in
FIG. 3(a). FIG. 3(c) illustrates a situation where a tungsten
compound layer 22 has been formed on the surface of tungsten 21 and
then a microcavity is made there. In this configuration, there is
no tungsten compound layer 22 on the bottom and side surfaces of
the microcavity 2. Even so, the microcavity structure is also
thermally stabilized. The reasons are as follows. In the prior art,
a microcavity structure that has been defined on the surface of
tungsten easily collapses at a relatively low temperature as
described above. This may be because the tungsten atoms migrate too
actively while tungsten is being heated by electrical currents. To
reduce such migration of atoms, the entire surface of the
microcavity structure is preferably covered with a compound layer
that minimizes the migration. However, only the edge portions,
where the structure loses its stability most easily, may be coated
with that tungsten compound layer.
[0060] FIG. 3(d) illustrates a structure in which only the side
surfaces of a microcavity 2 are coated with a tungsten compound
layer 22. Such a structure can be obtained by subjecting the
structure shown in FIG. 3(b) to a physical etching process such
that a portion of the tungsten compound layer 22, parallel to the
principal surface, is removed thinly. This tungsten compound layer
22 is also present at the edge portions 23 of the microcavity.
Accordingly, even this compound layer, covering just a small area
as a whole, can contribute sufficiently effectively to stabilizing
the structure of the microcavity.
[0061] FIG. 3(e) illustrates an example in which a broad area,
including the microcavity, is entirely covered with the tungsten
compound 22. A structure like this can be obtained by either
subjecting the surface of tungsten to a carburizing process for a
long time or using a tungsten compound such as tungsten carbide,
obtained by a sintering process, for example, as the material of
the radiator 1 as it is. In the latter case, a tungsten carbide
member that has been cut to an appropriate size and shape is
prepared and then patterned to make an array of microcavities
thereon.
[0062] FIG. 4 is a scanning electron microscope (SEM) photograph
showing a cross-sectional structure near the surface of tungsten
that has been subjected to the carburizing process mentioned above.
In FIG. 4, to clarify the cross-sectional layered structure, a
carbon layer (C-deposition) is deposited on the surface of the
sample with a Pt--Pd layer sandwiched between them.
[0063] As can be easily seen from FIG. 4, the layer produced by the
carburizing process does not have as definite a polycrystalline
structure as tungsten and seems to consist of amorphous phases or
microcrystalline phases. In the sample shown in FIG. 4, the layer
produced by the carburizing process has a thickness of about 1.8
.mu.m.
[0064] FIG. 5 is a graph showing the results of measurements by
X-ray photoelectron spectroscopy (XPS). In FIG. 5, the ordinate
represents the intensity (or the count) of photoelectrons (i.e., 4f
electrons of tungsten) that were emitted from the surface of a
sample when exposed to an X-ray in a vacuum, while the abscissa
represents the binding energy. The measurements were done by using
an analyzer ESCA5400HC produced by Physical Electronics USA.
Monochromated-A1K.alpha. (14 KeV at 200 W) was used as an X-ray
anode and the analysis area was a circle with a diameter of 0.6
mm.
[0065] As can be seen from these results of measurements, in the
layer formed by the carburizing process (which will be referred to
herein as a "carburized layer"), the 4f electron binding energy of
tungsten made a chemical shift from the value of tungsten by itself
in its crystals. Based on the results of other measurements, the
present inventors confirmed that carbon in the carburized layer had
a higher concentration than carbon in tungsten. Taking all of these
results into account, at least a portion of tungsten in the
carburized layer would be chemically bonded to another element
(i.e., carbon) to make a compound.
[0066] Based on these results of measurements, the layer formed on
the surface of tungsten by the carburizing process is referred to
herein as a "tungsten compound layer". However, the "tungsten
compound layer" does not necessarily mean that all tungsten atoms
included in that layer are bonded to carbon to make the compound
layer. Rather the "tungsten compound layer" may refer to any layer
in which carbon and at least a portion of a layer containing
tungsten are chemically bonded together.
[0067] A radiator 1 according to this preferred embodiment and a
radiator representing a comparative example, of which the surface
had not been subjected to the carburizing process, were prepared
and heated at 2,000 K for 10 minutes in a vacuum with a pressure of
about 10.sup.-6 Torr. FIGS. 6(a) and 6(b) are surface SEM
photographs of the radiator as a comparative example that had not
been heated yet and the same radiator that had been heated,
respectively. FIGS. 6(c) and 6(d) are surface SEM photographs of a
radiator 1 as a preferred embodiment of the present invention that
had not been heated yet and the same radiator that had been heated,
respectively. The surface of the radiator 1 shown in FIGS. 6(c) and
6(d) is the tungsten compound layer described above.
[0068] As can be seen from FIGS. 6(a) through 6(d), the microcavity
structure of the radiator 1 of this preferred embodiment did not
change at all even after having been subjected to the heating test,
while the microcavity structure of the radiator as the comparative
example collapsed completely after the test with no traces
left.
[0069] The evaporation rate of tungsten depends on the pressure of
the atmospheric gas. The higher the degree of vacuum, the more
easily tungsten evaporates. When the radiator 1 of this preferred
embodiment is actually used as the filament of an incandescent
lamp, the filament may be placed in an inert gas atmosphere with a
pressure of 1 atm, for example. The life expectancy of the filament
in such a situation may be estimated by a diffusion equation.
According to the calculations, if the array of microcavities could
be held stabilized at 2,000 K for 10 minutes within a vacuum of
about 10.sup.-6 Torr, then the same array of microcavities would be
kept stabilized at 2,000 K for approximately 9,700 hours when put
in an inert gas atmosphere of 1 atm. Consequently, an incandescent
lamp including the radiator 1 of this preferred embodiment is
expected to have a life of approximately 10,000 hours, which is ten
times as long as the life of 1,000 hours of a lamp that uses the
conventional tungsten filament.
[0070] The present inventors discovered that when formed on the
surface of a tungsten filament as in the comparative example, the
microcavity structure collapsed and disappeared even at a low
temperature of about 1,200 K. As described in "Metal Data Book"
(Revised 3.sup.rd Edition, edited by the Japan Institute of Metals
and published by Maruzen Co., Ltd.), for example, tungsten has a
melting point of 3653.15 K. Accordingly, it is unthinkable that the
cavity structure of tungsten melted at as low a temperature as
about 1,200 K.
[0071] Thus, the present inventors explored the possibility that
the microcavity array structure on the surface of tungsten might
have been thinly oxidized to decrease the melting point of the
surface layer significantly.
[0072] FIG. 7 is a graph showing the partial pressures of saturated
oxygen that contributes to the oxidation reaction of tungsten. In
FIG. 7, the ordinate represents the partial pressure, while the
abscissa represents the temperature. WO (s) and WO.sub.2 (S), for
example, represent the temperature dependence of the partial
pressure of tungsten oxides in solid state. Meanwhile, WO (g) and
WO.sub.2 (g), for example, represent the temperature dependence of
the partial pressure of tungsten oxides in gas state.
[0073] As can be seen from FIG. 7, at a low temperature around room
temperature, the oxidation reaction of tungsten advances with a
very small amount of oxygen in every tungsten oxide. Accordingly,
even if hydrogen reduction or any other treatment is carried out to
remove oxygen from the surface of tungsten, the surface will get
oxidized easily as soon as exposed to the air again.
[0074] Likewise, a conventional tungsten filament, used in common
incandescent lamps, would also be coated with a thin oxide layer
when exposed to the air at room temperature during the
manufacturing process thereof. However, as soon as the lamp is lit,
such an oxide layer will vaporize away and the underlying tungsten
will surface instead. That is why the characteristic of the
incandescent lamps would not be affected by such an oxide
layer.
[0075] On the other hand, if the array of microcavities is provided
on the surface as in the present invention, it's a quite different
story. Specifically, if the array of microcavities on the surface
of tungsten is exposed to the air at room temperature and oxidized,
then the microcavity structure itself will collapse and disappear
when the lamp is lit.
[0076] FIG. 8 is a graph showing the Gibbs free energy of a
refractory material during the oxidation reaction. Specifically,
FIG. 8 shows the oxidation resistance.
[0077] As is clear from the chemical formulae shown in FIG. 8, to
turn tungsten carbide into tungsten oxide, first, tungsten carbide
needs to be oxidized to decompose the tungsten carbide into
tungsten and CO and then the tungsten needs to be oxidized. And the
reactions in these two stages need to occur continuously. Also, the
reaction of decomposing tungsten carbide into tungsten and CO
through oxidation does not occur so easily as the reaction of
producing tungsten oxide from tungsten as shown in FIG. 8.
[0078] Thus, it can be seen that tungsten carbide is less easily
oxidizable than molybdenum, niobium or tungsten. Tantalum carbide
should also have a similar property.
[0079] Considering that tungsten carbide is less easily oxidizable
than tungsten, the tungsten compound layer formed by the
carburizing process of the present invention is also less easily
oxidizable than tungsten and that property may thermally stabilize
the microcavity structure.
[0080] Nevertheless, the compound layer of the present invention
does not have to have the same property as tungsten carbide in
bulk. The analysis results mentioned above show that tungsten is
chemically bonded to another element in the layer formed by the
carburizing process of the present invention on the surface of
tungsten and that carbon is present in that layer at a higher
concentration than in tungsten. Taking these results into
consideration, it is clear that tungsten and carbon are chemically
bonded together. However, since the present inventors could not
confirm the exact composition, we cannot say this compound layer
must be "tungsten carbide". Nevertheless, we can at least say that
this compound layer has a similar property to that of tungsten
carbide. That is why the chances that tungsten carbide was produced
at least partially are high. Consequently, the tungsten compound
mentioned above is typically tungsten carbide but is not limited to
this particular compound.
[0081] Although less easily oxidizable as described above, tungsten
carbide still has not been used as the material of a filament for
an incandescent lamp. This is partly because the melting point of
tungsten carbide is lower than that of tungsten by as much as
several hundreds K and partly because there is a big difference in
emissivity between tungsten and tungsten carbide.
[0082] Hereinafter, the difference in emissivity will be described
with reference to FIG. 9.
[0083] FIG. 9 is a graph showing the emissivity values of tungsten
and tungsten carbide in the infrared range. As can be seen from
FIG. 9, the emissivity of tungsten carbide (WC) in the infrared
range is higher than that of tungsten (W) in the same range. For
example, at a wavelength of 2.5 .mu.m, tungsten has an emissivity
of 20% while tungsten carbide has an emissivity of 70%. As a
result, visible radiation accounts for a lower percentage of the
overall radiations from tungsten carbide. That is why if a filament
is made of tungsten carbide, the luminous efficacy in the visible
radiation range would decrease so significantly compared with a
tungsten filament that the tungsten carbide filament cannot be used
to make a light bulb.
[0084] A history of development of incandescent lamps teaches us
that light bulbs with a carbon filament having a low melting point
and a high infrared emissivity (which are called "Edison bulbs")
were initially used for some time after the incandescent lamp was
invented. After that, the carbon filament was gradually replaced
with a tungsten filament having a higher melting point. Such a
historical background formed a basis for a common technical
misconception that tungsten carbide, having a lower melting point
and a higher infrared emissivity than tungsten, should not be used
for a radiator such as a filament.
[0085] On the other hand, the radiator of the present invention
dares to use tungsten carbide, which has a relatively low radiation
efficiency in the visible radiation range, but has a microcavity
structure on its surface. That is why the radiator of the present
invention can cut down infrared radiations sufficiently and can
reduce the high infrared emissivity, which should be shown by
tungsten carbide otherwise, to a rather low level.
[0086] In addition, since the radiator of the present invention can
improve the radiation efficiency, the operating temperature can
also be decreased compared with a situation where a tungsten
filament is used.
[0087] Considering that the melting point of tungsten carbide is
far lower than that of tungsten, it would be hard to imagine for
those skilled in the art that the collapse of microcavities at an
elevated temperature can be minimized by subjecting tungsten to the
carburizing process.
[0088] FIG. 10 schematically shows the microcavity collapse
temperatures and melting points of tungsten and tungsten carbide.
As can be seen from FIG. 10, tungsten carbide (WC) has a melting
point of about 3,175 K, which is lower than that of tungsten of
about 3,650 K (according to "Metal Data Book", Revised 3.sup.rd
Edition, edited by the Japan Institute of Metals and published by
Maruzen Co., Ltd.) Nevertheless, the microcavity structure
subjected to the carburizing process collapses at about 2,400 K.
This temperature is much higher than the temperature (of about
1,900 K) at which the microcavities of tungsten collapse and is too
high to predict from the melting point of tungsten carbide.
[0089] Hereinafter, a preferred embodiment of an illumination unit
including a radiator according to a preferred embodiment of the
present invention will be described with reference to FIG. 11,
which illustrates an exemplary configuration for an incandescent
lamp including the radiator 1.
[0090] This incandescent lamp includes a radiator (filament) 1 that
emits light, a translucent bulb 12 for shutting off the radiator 1
from the air, a stem 13 for supporting an electrode that is
connected to the radiator 1, and a base 14, which is electrically
connected to the radiator 1 by way of the electrode to supply
electric power to the radiator 1 from an AC outlet. An argon gas,
for example, is preferably enclosed in the bulb 12 to minimize the
evaporation of the filament.
[0091] In the incandescent lamp illustrated in FIG. 11, the
radiator 1 has the thermally stabilized microcavity structure
described above. Thus, even when operated at a temperature of 2,000
K, the lamp can keep emitting radiations, having a spectral
distribution with small infrared radiations, for a long period of
time.
[0092] A tungsten electrode, of which the surface is partially
covered with a layer that has been formed by a carburizing process
(i.e., a carburized layer), has been known in the art (see Japanese
Patent Application Laid-Open Publications Nos. 9-111387 and
9-111388, for example). FIG. 12 schematically illustrates an
exemplary configuration for such an electrode. The tungsten
electrode 30 shown in FIG. 12 has a shape defined by sharpening one
end of a circular tungsten rod with a length of 0.2 m to 15 m into
a conical shape and then cutting off the sharpened end by 0.2 mm to
0.8 mm. This tungsten electrode 30 includes thorium and emits
electrons through the sharpened end. The entire conical portion of
the tungsten electrode 30, except the sharpened portion through
which electrons are emitted, has been subjected to the carburizing
process. The carburizing process is performed to prevent thorium
atoms from diffusing through the crystal grain boundary of the
tungsten electrode and going out of the electrode. In this case,
the carburized layer formed by the carburizing process would be
made of W2C, of which the melting point is lower than that of
tungsten, and would be located in a region of the tungsten
electrode 30 with a relatively low temperature (i.e., not the
high-temperature end portion through which electrons are
emitted).
[0093] Thus, it has been a technique that is well known in the art
to subject a portion of a tungsten electrode for emitting electrons
or thermo electrons to a carburizing process. However, nobody has
ever reported subjecting a member to be used at an elevated
temperature (such as a filament) to the carburizing process.
[0094] Optionally, to cut down radiations of which the wavelengths
exceed a predetermined value, the surface of the radiator may be
treated to have fine unevenness, defined by a huge number of
recesses, by any method other than that described above such that
each of those small recesses (with an average size of 1 .mu.m or
less) functions as a microcavity. For example, by treating the
surface by sandblasting, a huge number of recesses functioning as
microcavities can be made on the surface of the radiator. Even so,
according to the present invention, the decrease in thermal
stability due to oxidation of the surface of the radiator can be
minimized and heat radiation can be emitted for a long time at an
elevated temperature. Thus, the present invention is effectively
applicable for use in a filament for an incandescent lamp, for
example.
[0095] As described above, according to the present invention, the
microstructure on the surface can be kept stabilized even at an
elevated temperature of 2,000 K or more. However, such effects are
achieved not only when recesses are made on the surface of a
radiator but also when a more complicated microstructure is formed
by a MEMS or any other fine-line patterning process. For example, a
photonic crystal structure may be formed on the radiation plane of
a radiator by arranging and stacking very small grid members and
defining a lattice structure at an interval that is approximately
equal to the wavelength of light. According to the present
invention, the surface portion or all of the member that defines
such a microstructure is made of tungsten carbide. Consequently, a
microstructure that can improve the radiation efficiency in a
selected wavelength range can be operated for a long time even at
an elevated temperature.
[0096] Optionally, the radiator of the present invention is also
applicable for use in a three-dimensional tungsten structure as
disclosed in Pamphlet of PCT International Application Publication
No. WO 03/058676A2. That is to say, the present invention can cope
with the "collapse of a microstructure", which is a big problem
that could happen more and more often when a member that has been
made of a material with a very high melting point such as tungsten
to increase the thermal resistance is further downsized in the near
future.
Embodiment 2
[0097] Hereinafter, a preferred embodiment of a thermoelectric
converter will be described as a non-illumination apparatus that
uses the radiator of the present invention.
[0098] FIG. 13 schematically illustrates a configuration for such a
thermoelectric converter. The apparatus illustrated in FIG. 13
includes a radiator 40 according to a preferred embodiment of the
present invention for absorbing sunray (as electromagnetic waves)
and emitting electromagnetic waves with a particular wavelength, a
container (not shown) for shutting off this radiator 40 from the
air, and a converter (such as a photovoltaic cell) 44 that receives
the electromagnetic waves from the radiator 40 and converts them
into electrical energy. In the example shown in FIG. 13, a filter
42 for filtering out components with unnecessary wavelengths is
arranged as an optional member between the radiator 40 and the
converter 44.
[0099] The radiator 40 includes a body portion, which is made
essentially of tungsten and of which the surface has a
microstructure such as microcavities or a photonic crystal
structure. The surface regions of the radiator 40, on which the
microstructure (such as microcavities) for improving the radiation
efficiency at the particular wavelength is provided, are covered
with a layer including tungsten and carbon as in the first
preferred embodiment described above. In this manner, the
microstructure on the surface of the radiator 40 selectively emits
electromagnetic waves with a particular wavelength, which should
agree with the wavelength at which the converter 44 can absorb the
electromagnetic waves efficiently.
[0100] If the radiator 40 is supplied with energy by exposing the
radiator 40 to solar heat collected, for example, then the radiator
40 that has been heated to an elevated temperature (of 2,000 K or
more) emits electromagnetic waves in a particular wavelength range.
On receiving such electromagnetic waves by way of the filter 42,
the converter 44 can convert the radiation into electrical energy
highly efficiently.
[0101] A sunray usually includes a lot of electromagnetic waves,
which fall within wavelength ranges that would result in low
conversion efficiency by the converter 44. However, by using the
radiator 40 of the present invention (and the filter 42),
electromagnetic waves, falling within wavelength ranges that would
result in high conversion efficiency, can be supplied to the
converter 44. As a result, the overall conversion efficiency of an
opto-thermo-electric energy conversion system can be increased.
Such a thermoelectric converter can also generate electrical energy
even by heating the radiator 40 with non-optical energy, and
therefore, can be used in a generator for a
non-opto-thermo-electric conversion system.
[0102] A thermal electromotive force generator system that uses a
radiator with such wavelength selectivity is also disclosed in
Japanese Patent Publication No. 347283, for example. However, this
publication discloses only a radiator made of a tungsten material
and does not mention at all that a microstructure would collapse
due to heat.
[0103] In the preferred embodiment of the present invention
described above, the thermal stability of the microcavity or
photonic crystal structure on the surface of the radiator 40 is
increased by the layer including tungsten and carbon. Thus, the
reliability of the generator system can be kept high for a long
time and the radiator 40 can be operated at even higher
temperatures. As a result, even a significant increase in the
output of generator systems can also be coped with flexibly.
Consequently, the apparatus of this preferred embodiment can
contribute to protecting the global environment as a generator
system that uses sunrays.
[0104] While the present invention has been described with respect
to preferred embodiments thereof, it will be apparent to those
skilled in the art that the disclosed invention may be modified in
numerous ways and may assume many embodiments other than those
specifically described above. Accordingly, it is intended by the
appended claims to cover all modifications of the invention that
fall within the true spirit and scope of the invention.
[0105] This application is based on Japanese Patent Application No.
2004-075784 filed Mar. 17, 2004, the entire contents of which are
hereby incorporated by reference.
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