SAMPE Journal - May/June 2021 - 44

FEATURE / SOL AR RADIATION
against nIR, allowing heated box to emit nIR for
temperature reduction. This again proved that
good dispersion of NPs is much more important
than simply increase the concentration of NPs.
Mechanical Properties of Spectrum Selective
Films
The stress-strain curves of the films are presented
in Figure 7. The ultimate strains of the ZnO/LDPE
films decreased from 330 to 15% with the increase
of the NP concentration from 0 to 6 % although their
yield strains are all around 10 - 15%. On the other
hand, the tensile strength increased with increased
NP concentration as reported in the literature30,
31
. It seems that the mechanical properties of the
films changed from being those of a typical ductile
material to those of a relatively brittle material due
to failure mechanism change from localized plastic
deformation to more defect-dominant failure.
CONCLUSIONS
Composite passive cooling films are made of LDPE
doped with three different concentrations of ZnO
NPs, namely 4, 5 and 6 wt%. The dispersion of the
NPs in the composite films was reasonably uniform.

The crystallinities of the films were not influenced
by the change of the NP concentration within the
range of the study. The transmissivities of the films
were decreased monotonically in UV and high
energy part of Vis light as the NP concentration
increased. In the lower energy part of Vis and nIR
range, increased NP concentration suppressed the
transmissivity of solar irradiation. However, the 5%
film had a lower transmissivity than the 6% film
due to existence of agglomeration of the NPs. The
absorption of the films was negligible as compared
with reflection. All three composite films showed
strong the reflectance to solar irradiation from 19%
- 37%. The reflectance of the 6% film was lower than
that of the 5% film, making it a better passive cooling
material because of much better NP dispersion.
The temperature reduction of the 4% and 5% films
were 19 and 21% in the testing temperature range
while that of the 6% film reached 35%. This indicate
that the NP concentration and their dispersion are
both critical for optimization of the passive cooling
performance of the composite films. Mechanical
properties of the films were reasonable for the kind
of applications.

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SAMPE JOURNAL

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SAMPE Journal - May/June 2021

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