Page 400 - Book Hosokawa Nanoparticle Technology Handbook

FUNDAMENTALS CH. 6 EVALUATION METHODS FOR PROPERTIES OF NANOSTRUCTURED BODY
UV rays as compared to theoretical calculations [8].
Their result indicates an actual shielding ability that is Exit window
close to the computational result of Stamatakis et al.
They have also reported that the shielding ability of
titanium oxide against UV rays is attributable to the Diffuse reflector
combination of scattering power and absorbing power, Entrance window
and that absorbing and scattering powers are dominant
for UV rays of short and long wavelengths, respec-
tively. That is, uniform particle dispersion and optical
absorption based on the band structure must be exam-
ined in control of transmittance and shielding ability, as
well as a simple primary particle diameter. Regarding
absorption, photochromic powder, whose reflectivity to Figure 6.7.3
visible light decreases under strong light and recovers The configuration of an integrating sphere.
to the initial level so that the original powder color is
restored in darkness, has been put to practical use for wavelength range, such as magnesium oxide or barium
cosmetics [9,10]. In addition, transparency is affected sulfate, is applied to the inner surface of an integrating
by the particle shape. Flat and tabular particulates sphere. This transforms the incident light into an
reflect light to a constant direction to produce a glossy almost perfect diffusion state. A slight residual specu-
appearance, whereas spherical particles scatter light lar reflection light component can be removed by an
into many directions to produce a matte appearance. arrangement such that an entrance window and an exit
The shape of a transparent matrix is also an important window are positioned normal to each other. If the
factor. A diffusion film for LCD backlights implements sample powder is fluorescent, the fluorescent compo-
light diffusion and collection performance by means of nent should be removed using a cut-off filter for the
scattering by internal particles and scattering and col- excitation wavelength region, or by synchronizing the
lection by the outline of the film. detection side using a monochromatic light source.
Optical properties of powder particles as a cluster Light emitted from one particle interacts again with
are also important. Practical embodiments include another particle when light penetrates into a powder
fluorescent materials applied to display panels or layer. Newly diffracted or scattered light occurs sub-
luminescent lamps. Unlike the case of nanoparticles sequently; this phenomenon will be repeated further.
in transparent matrices like solution or thin films, This process cannot be solved exactly mathematically.
light scattering cannot be ignored in the measurement Therefore, a certain approximation is necessary to
of reflection and absorption spectra of powder mate- describe it. Typical analysis methods include:
rials. Measurements of optical properties require us to
observe diffuse reflection light, which penetrates 1. the Scuster–Kubelka–Munk method, which
inside powder particles, repeats transmission and
reflection and comes out on the surface again. regards a powder layer as a continuum [11–13],
Because diffuse reflected light has penetrated into 2. the Johnson method, which assumes a stack of
powder particles repeatedly, weak absorption bands thin films [14,15] and
tend to be emphasized. Therefore, the measured result 3. the Monte Carlo method, which computes scat-
must be comparable with the transmission spectrum tered light stochastically [16–18].
and to undergo quantitative treatment with the
Kubelka–Munk function by Kubelka and Munk Various nanoparticles used in pigments and fluores-
[11–13]. Light from a powder layer comprises a spec- cent materials have various absorption and lumines-
ular reflection light component that has been directly cent properties and shape. Almost no available reports
reflected on the surface, and a direct light component describe whether the above-mentioned procedures are
that has passed through the gap of powder particles. It applicable to such nanoparticles. This will be a sub-
is necessary to remove these to the greatest degree ject for our future study.
possible. For smaller particle sizes, the absorbed
component by the powder layer is smaller, as the rel-
ative reflecting surface increases. That is, light cannot References
penetrate deeply into the powder layer. Moreover, par- [1] G. Mie: Ann. Phys., 25, 377 (1908).
ticles with a diameter smaller than the wavelength of
light would engender wavelength dependence in [2] H.C. Van De Hulst, H.C. Hulst: Light Scattering by
scattered light. An integrating sphere is effective for Small Particles (Structure of Matter Series.), Dover
measurement of reflection and absorption spectra to Publications, New York (1982).
collect accurate data. The configuration of an inte- [3] J.W.S. Rayleigh: The Collected Optics Papers of Lord
grating sphere is shown in Fig. 6.7.3. Powder with Rayleigh, Optical Society of America, Washington
uniform diffuse reflection properties in a wide (1994).

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