Solar control dispersions and coatings with rare-earth hexaboride nanoparticles

Article abstract of DOI:10.1111/j.1551-2916.2008.02512.x

Nanoparticle dispersions of rare-earth hexaborides have been prepared using a media agitation mill and have been examined for optical properties. High visible light transmittance coupled with strong absorption in the near-infrared (NIR) wavelengths suitable for solar control windows are reported for hexaboride nanoparticle dispersions with particle size dependence and the effect of artifacts. Nanoparticulate LaB₆ shows the largest NIR absorption among rare-earth hexaborides. NIR absorption is considered to arise from the free electron plasmon resonance. On decreasing the particle size below 120 nm, both visible light transmittance and NIR absorption are found to increase gradually until the size of 18-26 nm when they reach the maximum, and then decrease again at below 18 nm. Zirconia contamination and formation of lanthanum oxide were found to be involved during the milling process, leading to small additional absorptions around 300 and 650 nm, respectively.

Full text of DOI:10.1111/j.1551-2916.2008.02512.x

J. Am. Ceram. Soc., 91 [9] 2897–2902 (2008)
DOI: 10.1111/j.1551-2916.2008.02512.x
r 2008 The American Ceramic Society
ournal
J
Solar Control Dispersions and Coatings With Rare-Earth Hexaboride
Nanoparticles
Hiromitsu Takeda, Hiroko Kuno, and Kenji Adachiw
Ichikawa Research Laboratories, Sumitomo Metal Mining Co. Ltd., 3-18-5 Nakakokubun, Ichikawa,
Chiba 272 8588, Japan
Nanoparticle dispersions of rare-earth hexaborides have been
prepared using a media agitation mill and have been examined
for optical properties. High visible light transmittance coupled
with strong absorption in the near-infrared (NIR) wavelengths
suitable for solar control windows are reported for hexaboride
nanoparticle dispersions with particle size dependence and the
effect of artifacts. Nanoparticulate LaB6 shows the largest NIR
absorption among rare-earth hexaborides. NIR absorption is
considered to arise from the free electron plasmon resonance.
On decreasing the particle size below 120 nm, both visible light
transmittance and NIR absorption are found to increase grad-
ually until the size of 18–26 nm when they reach the maximum,
and then decrease again at below 18 nm. Zirconia contamination
and formation of lanthanum oxide were found to be involved
during the milling process, leading to small additional absorp-
tions around 300 and 650 nm, respectively.
highly transparent solar filters to absorb infrared rays by pos-
sibly activating surface plasmon polaritons of free electrons, as
typically observed in gold and silver colloids.4,5 On the other
hand, lanthanum hexaboride, a violet–black material as a pow-
der, has also been found6–8 to be one of the commercial species
serving as an NIR filter with high visible light transmittance.
Although LaB6 nanoparticles are finding wide commercial ap-
plications in transparent solar control substances such as PET
films and PVB interlayers9 for automotive and architectural
windows, fundamental material aspects of LaB6 dispersions
have not yet been reported. Therefore, the present paper reports
the preparation and processing, the basic material properties,
and the practical optical properties of lanthanum and other
rare-earth hexaboride nanoparticle dispersions for solar control
applications.
II. Experimental Procedure
I. Introduction
99.9% purity powders of rare-earth oxides X2O3 (X5 La, Ce,
Pr, Nd, Gd) and boron carbides (B4C) are mixed well and
heated in vacuum at 15001C for 3 h to obtain raw rare-earth
hexaboride powder a few micrometers in size. The raw XB6
powder is mixed with a high polymer dispersant in a solvent
toluene and dispersed in a media agitation mill, such as a bead
mill and a paint shaker mill, with 0.3 mm YTZ beads made of
partially stabilized ZrO2 containing 5 wt% Y2O3. Considering
the extremely high microhardness (2770 kg/mm2) of LaB6,10
initial trials were carried out to use a plasma reactor to obtain
nanosized raw primary LaB6 particles, but besides the poor pro-
duction rate, the resulting particles were strongly coagulated
and required the same lengthy pulverization process to obtain
nanodispersions. Attempts were also made to utilize contami-
nation-free milling methods such as a selfcollision-type mill and
a high-energy ultrasonic mill, but neither was found to be effec-
tive in pulverizing and reducing the particle size of hard LaB6.
Thus, we have followed the media agitation mill using YTZ
beads, which showed the highest efficiency in reducing the size of
LaB6 particles down to nanoscale.
The particle size and distribution in dispersions have been
evaluated by a particle analyzer (Microtrack (Nanotrac) model
UPA-150, Nikkiso Inc., Higashimurayama-shi, Tokyo, Japan),
which is based on the dynamic light scattering method with a
semiconductor laser of wavelength 780 nm and can measure
undiluted high-concentration liquids. This apparatus gave al-
most the same average particle size values as those observed by
transmission electron microscopy (TEM) (model HF 2200,
Hitachi High-Technologies Corp., Minato-ku, Tokyo, Japan).
Nanoparticle dispersions were mixed with a UV-curing
organic binder (UV-3701 of Toa Gosei Co., Ltd., Minato-ku,
Tokyo, Japan) and coated onto either PET films or inorganic
glass plates using a bar coating method. UV-3701 contains
functionalized acrylic monomers and oligomers, besides the po-
lymerization initiator, to form an acrylic coating layer upon UV
illumination by a radical polymerization reaction. The coated
films and glass plates were dried at 701C for 1 min and illumi-
nated with a high-pressure mercury lamp at UV intensity 76
HERE has been a growing demand in recent years to filter out
the infrared waves of the solar spectrum from housing and
T
automotive windows,1 in accordance with the worldwide
energy-saving and environmental preservation movement. By
reducing the near-infrared (NIR) wavelengths between 780 and
2500 nm, it would be possible to moderate skin irritation under
the strong summer sun without losing brightness. Additionally,
this would help to reduce the energy for air conditioning and
thereby decrease the emission of carbon oxides from housings
and automotives. The attainable driving distance of electric ve-
hicles would also be directly increased by the NIR filters, which
considerably reduce the consumption of battery energies. A ma-
jor practical means to meet these demands includes conventional
heat-absorbing glasses, heat-reflecting glasses, and sputtered-
coating glasses among others. However, each has its own short-
comings, naturally, and the satisfactory answer to these
demands is yet to be fully obtained. For example, a variety of
custom colors is difficult in heat-absorbing glasses, a high lumi-
nous transparency is sacrificed in metal-coated heat-reflecting
glasses, and low productivity and high cost always persist in the
vacuum process solutions.
The application of nanoparticles, on the other hand, is a
unique approach to reduce solar heat, as it provides a potentially
low-cost and high-productivity solution. Not only does it meet
mass-production requirements but also realizes high optical
selectivity, i.e., a high luminous transparency at a given heat-
shielding effect. A well-known material species to serve the pur-
pose is represented by the nanoparticles of transparent oxide
conductors such as tin-doped indium oxide2 (ITO) and anti-
mony-doped tin oxide3 (ATO). They are known to provide
S. Bhandarkar—contributing editor
Manuscript No. 23953. Received November 8, 2007; approved April 23, 2008.
wAuthor to whom correspondence should be addressed. e-mail: Kenji_adachi@
ni.smm.co.jp
2897

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Journal of the American Ceramic Society—Takeda et al.
Vol. 91, No. 9
mW/cm2 and belt speed 3 m/min, with a total UV energy of 146
mJ/cm2. The dried coating thickness was evaluated using an
Optical Profiler (Model NewView6200, Zygo Corp., Middle-
field, CT) as well as by optical microscopy of cross sections. The
amount of filler particles per unit projected area of the coated
films/glasses (g/m2) was evaluated by precise weight measure-
ments of the coated and uncoated films and glass plates. This
quantity is more convenient than dried thickness in the partic-
ulate system because thickness can be varied easily by changing
the filler-to-resin ratio while maintaining the same filler amount.
The optical response of the coatings was measured using a
spectrophotometer (Model U-4000, Hitachi High-Technologies
Corp., Minato-ku, Tokyo, Japan), giving outputs of transmit-
tance and reflectance profiles in the UV, visible, and infrared
ranges (200–2600 nm), and yielding visible light transmittance
(VLT), solar transmittance (ST), haze, Hunter’s chromatic in-
dex, etc. ST is a total directly transmitted solar energy between
280 and 2500 nm (ISO-9050), given by
sidered to be dumped out in the coating due to multiple reflec-
tions and absorption by the nanoparticles. An appreciable
amount of reflectance 20%–30% could, however, be observed
at 1100–2000 nm, when LaB6 nanoparticles are much more
densely packed in the coating, by eliminating the UV-setting
binders.
The apparent absorption at around 1000 nm in Fig. 1 actually
involves both absorption and scattering at the present particle
size,4,5 and is generally called extinction. Although the precise
value of the scattering component using a large integrating
sphere will be reported in a separate paper,11 we may note
here that the scattering contribution is estimated as a small per-
centage considering the low haze value of o2% of those coating
films.
Using a LaB6 dispersion of known concentration held in a
glass cell 12 mm ꢀ 12 mm ꢀ 44 mm (internally 10 mm ꢀ 10
mm ꢀ 43 mm) in size transmittance profiles similar to those
shown in Fig. 1 have been obtained to calculate the molar ex-
tinction coefficient of LaB6 particles, applying Lambert–Beer’s
law by log (I/I0) 5 ecd, where I0 is the intensity of incident light,
I is the intensity of transmitted light, d (cm) is the optical length
(equals unity in the present case), and c is the concentration of
LaB6 nanoparticles in dispersion. A molar concentration
M 5 mol/L of LaB6 in dispersion is used for c, where the mo-
lar quantity of the crystal lattice is defined as Avogadro’s num-
ber of the LaB6 unit cell.
,
2500 nm
X
2500 nm
X
ST ¼
SðlÞTðlÞDl
SðlÞDl
(1)
l¼280 nm
l¼280 nm
where S(l) is the spectral distribution of the solar radiation, T(l)
is the spectral transmittance, and Dl is an increment of wave-
length. ST can be a measure of the solar control property when
compared at constant VLT.
The result, Fig. 2, shows a remarkable absorption capability
of LaB6. A major absorption peak is observed at 970 nm and a
second largest peak at 300 nm, besides a small shoulder that is
appreciable at around 700 nm in the major peak.
III. Results and Discussion
The origin of the main absorption at 970 nm is presumed to
be an activated surface plasmon polariton of free electrons, be-
cause there are no interband transitions observed in the vicinity
of 970 nm (1.278 eV) in the optical conductivity spectrum of
LaB6 reported by Kimura et al.12 LaB6 is a superior conductor
with high carrier density 0.9ꢀ 1022 cmꢁ3, specific resistivity
edge is measured as 1.97 eV14 or 629 nm, which is close to the
observed absorption peak at 970 nm. The second largest peak
observed at 300 nm in Fig. 2 is considered to correspond to the
interband transition, because Kimura et al.12 detected a peak in
the optical conductivity profile at 3.5 eV (354 nm), which they
attributed to the interband transition from the B 2s, 2p bonding
orbital to the unoccupied state in the conduction band. The
small shoulder at 700 nm will be considered in a later section.
In order to compare the strength of the LaB6 absorption,
optical measurements were conducted for the well-known ITO
(1) Optical Profiles of LaB6-Dispersed Films and Dispersions
Figure 1 shows the transmittance and reflectance profiles of
LaB6-dispersed coatings of different thicknesses formed on a
PET film substrate. A dispersion of 1.85 wt% LaB6 milled to a
dispersion particle size of 18 nm has been mixed with the UV-
setting resin UV-3701 at a weight ratio of 1:1 and bar coated on
PET films. The thickness of the coatings has been varied using
bar rods of different diameters. The corresponding dried thick-
ness varied between 4.8 and 28.2 mm. Measured amounts of
LaB6 in a unit-projected area of the coatings are shown in Fig. 1.
The coated films are tinted green but are highly transparent,
with haze values generally o2%. Strong absorption at around
1000 nm is observed for every film while showing a large selected
transmittance at the visible wavelengths between 380 and 780
nm. This apparent absorption is obviously not related to an in-
terference effect because it shows no thickness dependence. The
reflectance of these coated films is low and varies between 3.7%
and 7.7% throughout the measured wavelengths, where most of
the reflected light on each of the nanoparticle surfaces is con-
1.5 ꢀ 10ꢁ5 O ꢂ cm, and mobility 33.1 cm2 ꢂ (Vꢂ s)
.
ꢁ1 13 The plasma
12000
LaB6
90
10000
8000
6000
4000
2000
Transmittance
80
70
60
50
40
30
20
10
0
0.05 g/m2
0.08
0.12
0.18 g/m2
0.27
0.37
Reflectance
ITO
ATO
0
200
600
1000
1400
1800
2200
2600
300
600
900
1200
1500
1800
2100
Wavelength (nm)
Wavelength (nm)
Fig. 1. Transmittance and reflectance profiles of LaB6 nanoparticle-
dispersed acrylic coatings with different thicknesses on PET films. The
number of LaB6 particles determined in a unit-projected area of the
coatings is indicated.
Fig. 2. Molar extinction coefficients of nanoparticulate dispersions of
LaB6, tin-doped indium oxide (ITO), and antimony-doped tin oxide
(ATO). The disturbance at wavelengths between 1620 and 1790 nm is
due to absorption by toluene.

September 2008
Solar Control Dispersions and Coatings
2899
and ATO nanoparticle dispersions with dispersion particle sizes
18 and 12 nm, respectively. The NIR absorption in continuous
ITO and ATO films has been attributed to the surface plasmon
excitation that was experimentally detected by electron energy
loss spectroscopy.15 The maximum extinction coefficients of
LaB6 and ITO at the respective peaks read 12 142 (M ꢂ cm)ꢁ1
at 975 nm and 1065 (M ꢂ cm)ꢁ1 at 2075 nm, respectively. The
peak height of LaB6 is observed as an order of magnitude larger
than that of ITO, clearly suggesting that LaB6 could absorb
NIR waves more strongly than ITO and ATO. This difference is
considered to be related to the much higher carrier concentra-
tion of 0.9 ꢀ 1022 cmꢁ3 in LaB613 as compared with 1020–21 cmꢁ3
in ITO16 and 1019–20 cmꢁ3 in ATO16. The high strength of the
absorption property of LaB6 should provide significant material
and cost advantages over ITO and ATO.
55
45
35
25
15
LaB6
CeB6
PrB6
NdB6
GdB6
0.41
0.411 0.412 0.413 0.414 0.415 0.416
Lattice constant (nm)
(2) NIR Absorption of Rare-Earth Hexaborides
Trivalent rare-earth elements are known to form hexaborides
that have similar free electron plasmon energies such as at 1.97
eV for LaB6, 1.96 eV for CeB6, 1.89 eV for PrB6, 1.90 eV for
NdB6, and 1.90 eV for GdB6, according to Kimura et al.14 Thus,
nanoparticles of various rare-earth hexaborides were examined
for capability of NIR absorption. Dispersions of LaB6, CeB6,
PrB6, NdB6, and GdB6 were prepared with polymer dispersants
in toluene, and their transmittance profiles are shown in Fig. 3.
This figure clearly indicates that the absorption valley around
1000 nm is a common characteristic of these rare-earth hexabo-
ride nanoparticles. They are all milled to a similar particle size so
as not to affect the optical response significantly. The peak
transmittance around 550 nm has been made equal by adjusting
the filler concentration in order to compare the size of the
absorption valleys around 1000 nm in Fig. 3. The depth of the
valleys is found to decrease in the order of LaB6, CeB6, PrB6,
NdB6, and GdB6.
The ionic radius of La, Ce, Pr, Nd, and Gd and the lattice
constants of their hexaborides show a tendency to decrease in
this order as the number of 4f electrons is increased (lanthanoid
contraction). Roughly proportional to this contraction, the
magnitude of the NIR absorption, as evaluated by the differ-
ence between the peak and bottom transmittances, is found to
decrease as shown in Fig. 4.
In the hexaboride structure of the space group Pm3m, each
rare-earth atom is surrounded by 24 nearest-neighbor boron at-
oms at an equal distance. The nearest-neighbor distance in-
creases as the number of 4f electrons in rare-earth elements
increases, and this is assumed to decrease the overlap of electron
clouds of rare-earth 5d with boron 2p that form the conduction
bands and are considered to reduce the density and mobility
of free electrons. Because the origin of the NIR absorption is
assumed to involve free electron plasmons, the largest NIR
absorption of LaB6 among the rare-earth hexaborides, as
Fig. 4. Near infrared absorption of rare-earth hexaboride nanoparti-
cles with regard to their lattice constants.
observed, is considered to be derived from the largest ionic
radius of La and the corresponding largest overlap of electron
clouds.
This interpretation has been extended to alkaline-earth hex-
aborides such as BaB6, SrB6, and CaB6, with lattice parameters
0.4275, 0.4200, and 0.4145 nm, respectively, which are larger
than or comparable to 0.4154 nm of LaB6. NIR absorption was
observed with BaB6, SrB6, and CaB6 nanodispersions, but not as
strong as with lanthanum hexaboride. This suggests that the
valence two alkaline-earth elements such as Ba, Sr, and Ca do
not form a large overlap of electron clouds with boron 2p as
were formed with the valence three rare-earth elements in the
structure of hexaboride.
Kimura et al.14 found a small additional absorption structure
at 0.6 eV (2066 nm) in the optical conductivity profiles of all the
rare-earth hexaborides except for LaB6. They ascribed this ab-
sorption to the 4f electrons, and have shown that with increasing
number of 4f electrons, the effective electron number related to
this absorption, NIR, increases while that related to the Drude
absorption, ND, decreases. While the present observation did
not detect any absorption peaks at 2066 nm, the 970-nm ab-
sorption is observed to decrease with increasing number of 4f
electrons, and thus appears to reflect only ND among the total
conduction electrons ND1NIR
.
(3) Fine Structure and Diffraction Results
The crystal structure of the as-produced LaB6 powder was con-
firmed both by X-ray diffraction (XRD) and by electron diffrac-
tion to be of the cubic CaB6 type. Each as-produced LaB6
particle was polycrystalline as observed by TEM. A well-dis-
persed LaB6 dispersion in toluene, prepared after 28 h in a paint
shaker mill, was observed both by TEM (Fig. 5) and by XRD
(Fig. 6(a)). Fine particles between a few nanometers and 100 nm
in size can be observed in Fig. 5. Numerous fine particles smaller
than 30 nm are involved, although these smaller particles tend to
lose a high contrast probably due to the presence of a high-
polymer dispersant in the matrix. These LaB6 nanoparticles in
well-dispersed solutions have been found mostly as single crys-
talline particles, as illustrated in Fig. 7 by the lattice fringe image
and the corresponding microbeam electron diffraction pattern.
Particle shape is deviated from a sphere because they are debris
from the collisions with YTZ beads during the milling process.
Fine particles observed in Fig. 5 are found as, actually, a mixture
of LaB6 and ZrO2 particles because zirconia beads are also
crushed on the surface. YTZ beads are initially of a tetragonal
phase of partially stabilized zirconia, but as they are subjected to
strong shearing and tearing forces during the milling process
with an increased surface temperature estimated to be up to
8001–10001C, a part of the tetragonal phase is inferred to trans-
100
80
GdB6
NdB6
60
40
PrB6
20
CeB6
LaB6
0
200
600
1000
1400
1800
2200
2600
Wavelength (nm)
Fig. 3. Transmittance profiles of rare-earth hexaboride nanoparticle
dispersions. The peak transmittance at 580 nm is set equal to compare
the near infrared absorption around 1000 nm.

2900
Journal of the American Ceramic Society—Takeda et al.
Vol. 91, No. 9
(100)
(111)
(011)
5 nm
Fig. 7. High-resolution transmission electron microscopic image of a
single LaB6 nanoparticle in the [011] direction with the corresponding
microbeam diffraction pattern.
100 nm
by inductively coupled plasma spectrometry, increases from 0 to
2.9 in 82 h. This significant incorporation of ZrO2 particles into
the dispersion does not, however, lead to much modification of
the optical response of films, as shown in Fig. 10. In order to
assess the optical effect of ZrO2 contamination, ZrO2 nanopar-
ticulate dispersion was separately prepared by milling the coarse
powder of partially stabilized tetragonal ZrO2 with the dispers-
ant in toluene, down to the same size as the LaB6 dispersion.
ZrO2 dispersion is shown to be almost inert optically in the vis-
ible range. Figure 10 gives a calculated transmittance profile of
pure LaB6 dispersion, obtained by subtracting the ZrO2 contri-
bution from the measured spectrum of LaB6 dispersion con-
taining ZrO2 at a ratio of ZrO2/LaB6 5 1.6. This indicates that
the presence of ZrO2 particles leads to some 5.2%–12.4% un-
derestimate of LaB6 transmittance in the visible range but can be
significant in the UV range, e.g., 46.5% underestimate at 300
nm, although the overall characteristic of the profile remains the
same.
Fig. 5. Transmission electron microscopy bright field image of a LaB6
nanoparticulate dispersion in toluene with a high-polymer dispersant.
form into the monoclinic phase martensitically. Both tetragonal
and monoclinic ZrO2 reflections can be identified in the XRD
pattern in Fig. 6(a). It is possible to distinguish ZrO2 particles by
the dark-field technique using diffraction rings derived from
ZrO2, which shows that the size and shape of LaB6 and ZrO2
particles are similar.
A typical particle size distribution measured by UPA-150 is
shown in Fig. 8 for a LaB6 dispersion dispersed in a paint shaker
mill for 28 h. Most of the particles are between 10 and 30 nm in
size with the peak maximum at 15.2 nm, while many larger par-
ticles form a tail up to 150 nm. This tailing was more or less
a common characteristic observed for the bead-milled nano-
dispersions.
Figure 9 shows how the average LaB6 particle size is de-
creased during the milling process when ZrO2 particles are in-
troduced. The average dispersion particle size shows good
agreement with the Scherrer crystallite size except for the first
measurement, indicating that the well-dispersed nanosized par-
ticles are single crystalline, in agreement with the TEM obser-
vation. The ZrO2 weight ratio with respect to LaB6, determined
(4) Effect of Particle Size on Optical Properties
The absorption property of LaB6 as shown in Fig. 2 is a
response of the electromagnetic wave passing through a thin
two-dimensional layer containing homogeneously distributed
4500
3500
2500
1500
500
(c)
(b)
(a)
20
30
40
50
60
70
80
2Θ
Fig. 6. X-ray diffraction patterns of LaB6 powders with dispersion particle size 18 nm (a), 13 nm (b), and the powder (b), further heated in a N2
atmosphere at 5001C for 1 h (c). LB, ZM, ZT, and LO denote LaB6, monoclinic ZrO2, tetragonal ZrO2, and LaO phases, respectively.

September 2008
Solar Control Dispersions and Coatings
2901
35.00
30.00
25.00
20.00
15.00
10.00
5.00
100
15.2 nm
80
60
40
20
0
0.00
0.001
200 400 600 800 1000 1200 1400 1600 1800 2000
Wavelength (nm)
0.01
0.1
Particle size (µm)
1
10
Fig. 10. Refinement of transmittance profiles by excluding ZrO2 con-
tribution. See text for details.
Fig. 8. Particle size distribution of a LaB6 dispersion of average particle
size 15.6 nm milled by a paint shaker mill for 28 h.
nanosized LaB6 particles. It is pointed out4 that the important
variables to determine the absorption profile and peak wave-
length are basically the particle size and shape, and also the re-
fractive index of the surrounding matrix. Therefore, the effect of
particle size on optical properties has been examined below.
Figure 11 shows the ST of LaB6 dispersions, plotted as a
function of average dispersion particle size. Because ST depends
on VLT, the LaB6 concentration was finely adjusted to have
a constant VLT at 70%. ST is found to show a minimum
at around 20–30 nm in Fig. 11, indicating that the best solar
control property with LaB6 dispersions is given in this limited
particle size range.
Figure 12 shows a variation of transmittance profiles of LaB6
dispersion liquids containing a constant concentration, 0.004
wt%, of LaB6 at various particle sizes between 13 and 120 nm.
The shapes of the profiles are found to be significantly different.
Transmittance generally decreases at particle sizes from 120 to
18 nm, while it tends to increase from 18 to 13 nm. Molar ex-
tinction coefficients have been calculated from these optical data
to show the particle size dependence as shown in Fig. 13. An
initial decrease in the particle size from the coarse 120 nm in-
tensifies the extinction coefficient, where the largest extinction
coefficient is observed as 13500 (M ꢂ cm)ꢁ1 for particle sizes of
18 and 26 nm. Further decrease in particle size then slightly
weakens and shifts the extinction coefficient toward the longer
wavelength while further emphasizing the shoulder at 650 nm.
On decreasing the particle size from 120 to 18 nm, a high-
transmission region in the visible range becomes more widely
transmitting, while an absorbing region in the NIR range
becomes more absorptive. The expansion of the visible range
is considered to be due to the much more rapid decrease in
scattered light at shorter wavelengths than at longer wave-
lengths, following Rayleigh’s formula. The increase in absorp-
tion at 970 nm on decreasing the particle size is interpreted from
the two aspects—a decreasing absorption capability of single
nanoparticles and an increasing number of nanoparticles in-
volved in the total absorption. The absorption cross section
clearly decreases with decreasing particle size, as demonstrated
for silver colloids,17 while the number of particles involved in-
creases by the power of three when a filler weight concentration
is kept constant in liquid. When the radius of the particle is re-
duced from r to r/p at a constant weight concentration, the total
absorption, as it is proportional to the total projected area of the
whole particles, increases from pr2 to p(r/p)2 ꢀ p3 ( 5 ppr2), and
therefore the total absorption is expected to increase by a factor
of p. Assuming that the size dependence of the absorption cross
section is smaller than p, the 970-nm absorption is expected to
increase as observed in Fig. 13. A more quantitative account of
this change based on the Mie theory calculations will be given
elsewhere.11
At particle sizes of 18 nm and below, profiles in the visible
range show little change, in contrast to the NIR profile that
grows a shoulder at around 700 nm into a subpeak in Fig. 12.
This subpeak corresponds to the small absorption peak at 650
nm in Fig. 13. Figure 6 shows that on decreasing particle
size from (a) 18 nm to (b) 13 nm, a subtle formation of LaO
140
120
100
80
8
7
6
5
4
3
2
1
48
Dispersion particle size
Scherrer crystallite size
VLT = 70%
47
ZrO /LaB ratio
2
6
46
45
44
43
42
60
40
20
0
100
0
0
20
40
60
80
0
20
40
60
80
100
120
140
Dispersion time (hr)
Particle Size (nm)
Fig. 9. Changes in dispersion particle size, Scherrer crystallite size, and
ZrO2 contamination with dispersion time in a paint shaker mill with
YTZ media beads.
Fig. 11. Effect of dispersion particle size on solar transmittance prop-
erty at a constant visible light transmittance of 70%.

2902
Journal of the American Ceramic Society—Takeda et al.
IV. Conclusions
Vol. 91, No. 9
0.004 wt% LaB6
13 nm
15 nm
18 nm
80
Rare-earth hexaboride nanoparticle dispersions have been pre-
pared in solvent toluene with organic dispersants using a media
agitation mill with zirconia beads. The nano-LaB6 dispersions as
well as their coatings cured with acrylic binders have been shown
to exhibit a strong NIR absorption with a high VLT. The major
absorption at 970 nm is assumed to be due to the free electron
surface plasmon resonance, and other rare-earth hexaborides
CeB6, PrB6, NdB6, and GdB6 having a similar plasma edge are
confirmed as exhibiting the same effect. The molar extinction
coefficient of LaB6 nanoparticulate dispersion is found to be at
least an order of magnitude larger at the main peak than that of
ITO dispersions, showing a high potential as an economical so-
lar control material. On decreasing the dispersion particle size
from 120 to 18 nm, both VLT and NIR absorption are increased
to enhance the solar control property, while from 18 to 13 nm a
part of LaB6 was found to decompose appreciably into LaO,
which degrades the solar control property and gives rise to an
additional absorption around 650 nm. The ZrO2 contamination
is optically not active and exerts only a minor effect on the
optical properties of rare-earth hexaboride dispersions.
60
40
20
0
120 nm
43 nm
26 nm
300
600
900
1200
1500
Wavelength (nm)
Fig. 12. Transmittance profiles of LaB6 dispersions of constant con-
centration at 0.004 wt% with different particle sizes.
is suggested from the patterns. The LaO peaks (200), (220), and
(311) grew more appreciably in the XRD pattern (c) when the 13
nm LaB6 dispersion was slowly heated to evaporate solvent tol-
uene and further heated at 5001C for 1 h in the nitrogen atmo-
sphere. LaO has been reported to be synthesized at 8001–10001C
under high pressure, and has a metallic property with a positive
TCR and a high conductivity of 2.7 ꢀ 10ꢁ4 O ꢂ cm at room tem-
perature.18 Therefore, it is well speculated that either an inter-
band transition or a plasmon resonance could take place in LaO
to produce a small additional absorption at 650 nm. The pres-
ence of tiny LaO crystallites has actually been observed by a
TEM dark field technique in LaB6 aggregates obtained by evap-
orating toluene and heating in air at 2001C for 1 h.
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Wavelength (nm)
Fig. 13. Molar extinction coefficients of LaB6 dispersions of different
particle sizes between 120 and 13 nm. The disturbance at wavelengths
between 1620 and 1790 nm is due to absorption by toluene.
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&