Journal of Nanoparticle Research 4: 157–165, 2002.© 2002 Kluwer Academic Publishers. Printed in the Netherlands.

Technology and applications

Nanoparticle-doped polymer foils for use in solar control glazing

G.B. Smith1, C.A. Deller1, P.D. Swift1, A. Gentle1, P.D. Garrett2 and W.K. Fisher2,∗1Department of Applied Physics, University of Technology, Sydney, P.O. Box 123 Broadway, NSW, 2007 Australia(Tel.: +61 2 9514 2224; Fax: +61 2 9514 2219; E-mail: g.smith@uts.edu.au); 2Solutia Inc., 730 Worcester St,Springfield MA 91151, MA, USA (Tel.: +1 413 730 2572; Fax: +1 413 730 3610; E-mail: PDGARR@solutia.com);∗Author for correspondence (Tel.: +1 413 730 2107; Fax: +1 413 730 3610; E-mail: wkfish@solutia.com)

Received 8 August 2001; accepted in revised form 19 November 2001

Key words: effective medium theory, laminated windows, metal oxides, nanoparticles, NIR absorption, solarcontrol glazing

Abstract

Since nanoparticles can provide spectrally selective absorption without scattering they can be used to dope polymersfor use in windows, to provide a clear view while strongly attenuating both solar heat gain and UV, at lower cost thanalternative technologies. The underlying physics and how it influences the choice and concentration of nanoparticlematerials is outlined. Spectral data, visible and solar transmittance, and solar heat gain coefficient are measured forclear polymers and some laminated glass, in which the polymer layer is doped with conducting oxide nanoparticles.Simple models are shown to apply making general optical design straightforward. Use with clear glass and tintedglass is considered and performance shown to match existing solar control alternatives. A potential for widespreadadoption in buildings and cars is clearly demonstrated, and scopes for further improvements are identified, so thatultimately both cost and performance are superior.

Introduction

The growing commercial availability of nanoparticleshas opened up a new low cost option for controlling thelight and energy from the sun entering a window. Solarheat and light may enter directly or be reflected fromthe surroundings and it is important in any modificationto maintain the central function of most windows, pro-vision of a clear view. The easiest place to incorporatethe nanoparticles is in the laminating polymer layer insafety glass, which is essential in cars and trucks, andis used in large commercial buildings in some parts ofthe world. Cars and trucks have a major solar thermalproblem in many geographic zones but existing solu-tions are expensive and only available in luxury cars.‘Window film’ polymer layers fixed to the surface ofthe glass, or all polymer glazing, could also be usedas particle carriers. The potential configurations of thepolymers containing the nanoparticles thus fall into

Glass layer

nanoparticle

doped polymer layer

(a) (b) (c)

Figure 1. Configurations in which nanoparticle-doped polymerfilms can be used in glazing systems.

three basic classes as shown in Figure 1: (i) a polymerlayer laminated between sheets of glass with thepolymer typically PVB – polyvinyl butyral; (ii)a polymer foil, typically PET glued to the glasssheet; and (iii) a clear ‘stand alone’ polymer glazingpanel typically PMMA – polymethyl methacrylate or

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Nanoparticles absorb

Micrometreparticles (or clusters) scatter

(a) Transparent (b) Translucent

Nanoparticles absorb

Micrometresized particles (or clusters) scatter

(a) Transparent (b) Translucent

Figure 2. Schematic of haze reduction for transmitted lightwhen incident on (a) nanoparticle-doped polymer as comparedto (b) micron-size pigment doped polymer or polymer containingclusters of nanoparticles.

PC – polycarbonate. The first class is the one widelyused in automobiles and trucks, and for safety archi-tectural glass. Why have nanoparticles opened up thisoption? The key aspect is in Figure 2.

Classical inorganic pigment particles such as tita-nium oxide, barium sulphate or zinc sulphide added toa clear polymer sheet or foil during processing, tendto make it translucent so any transmitted daylight isscattered as shown in Figure 2(b). Thus while theycan provide daylight they cannot be used in a win-dow required to provide a clear outside view. Theyare thus traditionally used when we wish to spreadtransmitted light to produce even light and reduceglare, as in a polymer skylight or some roof glazings.These pigment particles are typically micron sized.However if the added particles are made smaller andkept apart, transparency can eventually be restoredas in Figure 2(a). The sizes required for low haze aretypically well under 100 nm, and depend also on theparticle refractive index relative to that of the hostpolymer. The need to keep them separate dictates thatconcentrations cannot be large and special attention,mechanically and/or chemically, has to be paid todispersion in production. It is fortunate, as we shallsee, that the types of materials needed and the specialintrinsic absorbing properties of nanoparticles, allowvery low concentrations to achieve the desired control.

These separated low concentrations of non-scattering nanoparticles do not cease to have an opti-cal influence, but can modify transmitted radiation byabsorption, and thus if selected carefully according totheir absorptive properties, can be used for spectrallyselective control of transmitted solar radiation. It canalso be used for providing high visible transmittancewhile blocking the near infra red (NIR) component of

solar radiation. UV blocking is a useful bonus in thematerials we discuss. Materials for use in sunscreenpastes and gels for UV blocking such as ZnO, whichwere traditionally white because of high scattering, arenow available in less obvious, clear sunscreens for thesame reason, they are now nanoparticles. It was notthe ‘whiteness’ reflecting solar radiation that preventedsunburn, but the spectral absorptive properties of ZnO.

There has been little published in the general scien-tific literature on nanoparticles in polymer glazing sys-tems so the underlying science has not as yet receiveda systematic treatment. However, a number of patentstouching on this topic have been published includingKase and Aklyama (1997), Shouji et al. (1996) andKondo (1998).

Parameters of interest in glazing andtheir measurement

Transmittance T (λ) and reflectance R(λ) as a func-tion of wavelength and angle of incidence can be mea-sured using a spectrophotometer, while absorptanceA(λ) = 1 − R − T . Calculated properties used forassessment of the solar control capabilities of windowsare the visible transmittance (Tvis) for solar radiation,total solar transmittance (Tsol), solar absorptance (Asol),and the solar heat gain coefficient (SHGC), plus CIEcolor coordinates of transmitted radiation, and haze.The latter is an important check of dispersion qualityin the nanoparticle systems.

The total solar transmittance is the measured trans-mittance of a window integrated over the solar wave-length range (300–2500 nm) weighted by the solarspectrum. Air Mass 1.5 (AM 1.5), which is equivalentto the spectrum of solar radiation after passing through1.5 times the perpendicular atmospheric thickness wasthe standard solar spectrum used for all calculationspresented in this paper and is currently the most widelyaccepted standard. The visible transmittance is the solartransmittance, weighted by the photoptic response ofthe human eye, which peaks at 555 nm.

For normal incidence

Xsol =∫

dλX(λ)S(λ)∫dλS(λ)

, (1)

where X(λ) represents T , A or R as measured, andS(λ) is the AM 1.5 solar spectrum.

For visible transmittance of daylight we use

Tvis =∫

dλT (λ)S(λ)Y (λ)∫dλS(λ)T (λ)

, (2)

159

0.0

0.5

1.0

1.5

2.0

300 500 700 900 1100 1300 1500 1700 1900 2100 2300 2500

Wavelength (nm)

Sp

ectr

al

Den

sity

(W

/m2/n

m)

AM 1.5 Solar Spectrum

Photopic Response of the Human Eye

Figure 3. Air mass 1.5 spectrum of solar radiation at the earth’ssurface and the photoptic sensitivity spectrum of the human eye.

where Y (λ) is the human eye spectral response, whichis shown in Figure 3, along with S(λ). Xsol, Tsol, R,T and A should also be defined at relevant anglesof incidence but it is common to use normal inci-dence for simple comparisons as data is often not avail-able at non-normal incidence. Note however that othertechnologies, which we compare to nanoparticle-dopedpolymers below, have quite different variations withangle of incidence that are important to account forin practice. It is in fact much easier to describe andengineer the nanosystems in this aspect. Often onlyTsol and not SHGC is reported, but with an absorbingmechanism from the nanoparticles, there is a significantrise from Tsol to SHGC with much of the heat enter-ing indirectly. This difference is particularly noticeablewhen both the glass and the nanoparticle layer absorbstrongly.

It is worth noting for the purpose of choosing whichnanoparticles to use, that around 50–55% of the incom-ing solar energy is at NIR wavelengths and the bulkof this is between 750 and 1100 nm, although S(λ) inFigure 3 extends to 2500 nm. The SHGC is a mea-sure of the fraction of incident solar heat transferredthrough a window into a room or space. This consistsof [Tsol × (incident energy)] which is direct, plus theheat re-emitted in a forward direction after absorptionby the window, which is indirect. A relationship givingSHGC is thus

SHGC = Tsol + AsolU

h0

. (3)

U is the thermal heat transfer coefficient as definedby ASHRAE,

U =[

1

h0

+ 1

hi

+ L

kt

]−1

, (4)

h0 and hi are the thermal conductivities of the outerand inner surfaces of the window, and are dependenton factors such as temperature and wind speed. L is thethickness of the window and kt the thermal conductiv-ity of the window material. Standard summer values asdefined in ASHRAE (1997) handbooks are used for allcalculations in this work since we are primarily inter-ested in reducing heat gain. Special calculation shouldbe done for cars but for simplicity we use only a verticalwindow.

The CIE Colorimetric System (the CIE 1931 2◦

Standard Observer) was used for color analysis. Colorof radiation is specified by three tristimulus values.Color specifications for observers with normal visionare given in terms of chromaticity coordinates, whichare a measure of the color quality of transmitted orreflected radiation relative to an observer. The rel-evant equations can be found in Kaye and Laby(1973).

Competitive technologies: a comparison tothe new nanosystems

Granqvist (1991) gives an introduction to advancedglazings for solar control. Before a detailed exam-ination of nanoparticle-doped polymer systems it isworth noting the competitive technologies for solarNIR control in windows both from a technical and acost perspective. Some of these have been availablefor a number of years and others are quite recent. Upuntil a few years ago it has been exclusively donewith multilayer thin films, on the exterior of the glass,or on one surface of a multi-laminate layer betweenglass (as produced by Viracon), or on a polymer gluedto the glass. These multilayers are composed of thinmetallic layers dispersed between dielectric layers ofhigh refractive index (such as SnO2, In2O3, TiO2, ZnOand ZnS). The dielectric layers act as both protectorsand anti-reflectors for the metal layers, which havelow durability if exposed at the surface and are soft,with silver the most commonly used. Spectral selectiv-ity can be tuned by altering the thickness of the lay-ers (Wilson, 1999). They control mainly by reflectionof the NIR with some absorption, whereas the mech-anism we present is absorptive. Other simpler coat-ings used for reflectance include refractory metals suchas TiN or stainless steel. These cannot provide highTvis while blocking the NIR, so lack the spectral con-trol of the nanosystems. A reflection mechanism doeslead to less heating of the glass and hence less dif-ferential between the solar transmittance Tsol and the

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SHGC. However, comparable SHGC values to the filmscan be achieved with the polymer foils doped withnanoparticles whose main advantage is ease of massproduction.

A quite new mechanism has been recently demon-strated for metal-free reflecting in the NIR which usesonly polymer and can be used in laminates (Boettcher,2001; Shrenk et al., 1991, 1992). It involves a stack oflarge number of microlayers, in effect a stack of bilay-ers, where the alternate layers are polymers of differentindex. Each layer must be precise in thickness, indexand uniformity for such a system to achieve its desiredreflecting properties and should not distort internallyon bending.

It should be noted that limited NIR absorption isalready widely used in most car windscreens in warmercountries, using glass with higher iron content. Butthere are strong limits to the amount of NIR that canbe blocked by tinting in this way because the visibil-ity properties also get affected. Glasses commerciallyavailable for many years, such as Solex by PPG, usethe absorption of the iron silicates. There has beenan advance in the 1990s in this area to yield whatare termed, ‘high performance tinting’ glass, whichcan achieve the performance of the nanoparticles, butrequires an expensive electric arc process to achieve thespecial oxide additives within the glass. PPG Solextrautilises high-performance tinting.

Other competitive possibilities are reported mainlyin patents. They include the use of IR absorbing organicdyes in polymer, which are generally quite unstablein bright sunlight, but progress is occurring in dyephotostability; another use of nanoparticles is produc-tion of thin films, closely packed layers of nanoparti-cles directly onto a glass sheet. Sumitomo, ShowCoatand others can supply the coating materials to makesuch layers. These are easy to apply, but their long-term retention on the surface is an issue, with frequentre-coating possibly required unless other measures aretaken.

Spectral control with hybrid systems of nanoparti-cles in polymer along with another absorption mecha-nism is also quite attractive, the aim being to have thecombined absorption spectra giving an optimised per-formance across the entire solar spectrum for a givenapplication or building situation. The most obviouschoice is to combine the nano-tinted polymer with alow-cost tinted glass to keep the cost attractive. Thetinted glass should thus absorb radiation where thenanoparticles used have weaker absorption.

Nanoparticles for NIR spectrallyselective absorption

What materials are best as dopant nanoparticles forabsorbing the NIR and providing little or moder-ate visible absorption? The classical pigments notedabove have little use as nanoparticle solar absorbers,as they will transmit uniformly well across the solarspectrum. Materials containing moderate densities ofmobile electrons are useful for two reasons, they absorbin this range but most importantly they have over allor part of this wavelength range a negative dielectricconstant ε (strictly, the real part of ε = ε1). A smallspherical particle for example, with a negative value ofε1 has an absorption resonance (Bohren & Huffman,1983; Van der Hulst, 1981) when

εp1 = −2εh (5)

with subscript p denoting particles and h the host poly-mer matrix. Provided the particle density is low and theparticle size is small we can make a quasi-static dipoleapproximation in which each particle’s polarizabilityP is given as in Eq. (6) using basic electrostatics oreffective medium theory (Niklasson, 1991) where thereal part of the denominator clearly vanishes at the res-onance defined by Eq. (5). This creates a maximum inthe absorption coefficient, which is proportional to theimaginary part of P (Meeten, 1989). For a sphere ofvolume V , P is given by

P = 3Vεp − εh

εp + 2εh

(6)

For non-spherical particles, equations similar to (5)and (6) can be found (Skryabin et al., 1997). Theabsorption strength and bandwidth depends only on theimaginary part of εp = εp2, if εh is real, though manyof the polymers of interest also have weak absorptionbands in the NIR. The spectral location of the reso-nance is the key issue, along with its bandwidth andstrength. The enhancement in absorption strength dueto the small particle resonance in Eq. (6), makes thesenanoparticles much more efficient absorbers per atomor per unit of surface area than the bulk material fromwhich they are made. This efficiency is such that wehave found in a 0.7 mm thick layer that to achieve thedesired response we typically require less than 0.5%by weight of nanoparticles in the polymer. Tuning theresonance location means that we have to find a mate-rial that satisfies Eq. (5) at a wavelength that is well

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into the NIR. The resonance peak cannot be in the vis-ible or even just into the NIR if we want no impacton the visible transmittance, because the absorptionresonance width may spill excessive absorption overinto the visible, if the absorption peak is below around900 nm. If, however, we wish to also have color tintingand reduced glare, then a lower wavelength peak maybe acceptable. Note, however, that such an absorptionis likely to be much larger at red wavelengths with ram-ifications for quite distinct colors of transmitted light.Color balancing with other pigments may be needed.

Two simple pieces of physics can guide us inselecting materials, namely that as the mobile carrierdensity decreases the onset of a negative dielectric con-stant shifts to longer wavelengths and that at wave-lengths longer than this onset the dielectric constantgets increasingly negative. Thus as carrier densitydecreases the resonance shifts to longer wavelengthssince the host dielectric constant is approximatelyfixed. The candidates are thus metals or conductingoxides. For metals such as silver and gold it will occurin the visible but the strength of absorption in theseparticles is such that quite a small quantity of metalnanoparticles may absorb too strongly. Such a windowcould be almost black or slightly red in appearance.This phenomenon in small metal particles is behindthe performance of many solar selective absorbersand their strong absorption across the full solar spec-trum (Granqvist, 1989; Smith, 1979). A material witha lower density of free carriers is thus needed forthe desired spectral selectivity, with an absorptionpeak in the NIR and a high Tvis. Oxide conductorsare the ideal candidates (though some other materi-als can be considered). Thus, we will focus on these,in particular indium tin oxide (ITO) and antimony tinoxide (ATO).

The systems studied

We have studied both nanoparticle-doped polymers bythemselves where the host is either PMMA sheet orPC thin foil, and laminated glazings, where the host isPVB which is thin and flexible by itself, and typicallycontains a number of other ingredients apart from PVBitself. As well as measuring the spectral, lighting andsolar control properties of these systems we have alsodeveloped and tested a complete and accurate mod-elling capability for radiation from any angle. We candetermine starting with a given doped polymer the

full performance of a window system of any design,with any number and configuration of glass sheetsof specified spectral properties, and any variation innanoparticle concentration, provided the particles arefully dispersed. That is, having data on one polymer foilsample or window containing a doped polymer of anytype with known particle concentration, we can use theresults on that system to model any desired variation.The first step is in effect simply to determine the atten-uation coefficient per nanoparticle or equivalently theshift in the imaginary part of the refractive index of thecomposite (k) per nanoparticle. We have proven thisto work accurately in numerous practical cases, one ofwhich appears below.

We can if needed, go one step further and do firstprinciple physics modelling of the spectral transmit-tance of the nanoparticle-doped polymer. To do this,either of the optical constants of the nanoparticle mate-rial must be known. Data in Palik (1999) or Stjernaet al. (1994) can be used if bulk constants suffice, orthey can be modelled using simple modified Drudefree electron theory. Sometimes an extra inter-bandoscillator may be needed as well as the free oscilla-tor (Drude) charge system. In ATO a simple Drudemodel suffices in the NIR in thin films (Stjerna et al.,1994), and nanoparticles, with a shift in the constantterm in ε from 1 (due to residual effects of higher fre-quency oscillators). This works because the onset ofabsorption is well into the NIR. If particle optical con-stants are not available or only known approximately,we can, in principle, work back, to determine themfrom T and R measurements on materials of knownparticle concentration, and hence find carrier concen-trations. This is easy for the attenuation part as notedabove, but at the low concentrations we use, it is dif-ficult to experimentally find the real part of refractiveindex for the nanoparticles. Optical constant data alsoallows a check on such issues as the effect of parti-cle size on electron relaxation time and if any particleshape asymmetries are present. Further details on themodelling and theory will be presented elsewhere withthe emphasis here on data and straightforward windowmodels, but we note that while in nano-metal systemselectron transport relaxation times fall from the bulkvalues, in the conducting oxides needed for NIR workthe bulk relaxation time is already short enough not tobe strongly influenced by size. Thus, we can use bulkrefractive indices for ITO and ATO as a good startingguide, although especially for ATO some variations inthe particle’s occur in carrier concentration, depending

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on details of the manufacturing process and relativeamounts of tin and antimony oxides.

When clustering is not a problem, simple effectivemedium models, at least for ITO and ATO nano-spheres, based on Eq. (6) at optical and NIRwavelengths appear to give a quite accurate opticaldescription of these systems if the particles are welldispersed and scattering is weak or negligible. This isvery handy from a design point of view when assess-ing what concentrations to use for a particular system.Sometimes scattering is evident in the results only atshort, blue wavelengths while the simple models workwell in the NIR.

PC samples were prepared by solvent casting toproduce thin layers 0.13 mm for microscopic stud-ies, and optical studies to extract attenuation coeffi-cients for modelling thicker samples. Higher concen-trations by factor above 4 were used, compared tosheets used in practice, which will be at least 0.7 mmas a window film layer or 4–6 mm if stand alone. Inpractice PC will be extruded so the nanoparticles willbe dispersed within the molding beads. Our PMMAwas cast.

PVB layers containing nanoparticles were extrudedusing a Killion extruder. Extrusion temperature was190◦C. The sheet was extruded through a slit die, thetake up speed adjusted to give a sheet thickness of0.076 cm. The laminates were made by hand and sub-jected to an autoclave cycle of 143◦C and 12.7 atm. foran hour. Some excellent low scattering samples wereproduced in the PVB but care in initial dispersion dur-ing formulation of the PVB was still needed.

For other polymers, particles were added and wellmixed in the relevant solutions at concentrations rang-ing from 0.2% to 2% by weight. Clustering is moreof a problem at the higher concentrations as needed inthin PC. In this PC study simple dispersion techniquesin the solvent plus dissolved polymer were used. Weshow in a PC samples how short wavelength scatteringeffects can occur but the simple quasi-static models stillwork in the NIR, while in the good samples the simplemodels work at all visible and NIR wavelengths.

Micrographs from a field effect SEM were usedto study dispersion and check nanoparticle size.Well-dispersed nanoparticles were usually found, espe-cially at concentrations under 0.5% with diameters inthe 30–40 nm range. Note that the particle volume frac-tions are needed for use with Eq. (6) which one uses tofind the total dipole moment per unit volume, so if thereare N particles per unit volume we need Nα which isproportional to NV = f , with f the volume fraction.

The volume fraction f is calculated from particle massfractions, and particle and host densities. The densityof individual nanoparticles was taken to be 6.8 g cm−3

for ATO and 7.1 g cm−3 for ITO, and the density of thefinal polymer was measured.

Example results

A CAREY UV-Vis-NIR spectrophotometer was usedto obtain results for specular transmittance andreflectance, at select concentrations. Universal mod-elling parameters were obtained from some of thisdata for each class of nanoparticle (defined by mate-rial and shape) in a given matrix when scattering isabsent and tested against a range of other data. Theprimary parameter of interest is absorption coefficientαn(λ) per unit volume fraction of nanoparticles whereI = I0e−αx defines total attenuation with depth x, sothat α(λ) = f αn(λ)+(1−f )αh(α), with f the volumefraction of particles and αh(α) the attenuation coeffi-cient of the undoped host polymer at wavelengthλ. Thissimple linear approach is inaccurate at f values above afew per cent, and its refinement will be discussed else-where, but it works well at concentrations below 1%which applies to most windows in this study. A univer-sal attenuation factor at each wavelength per volumeper cent of nanoparticle material in the polymer canthus be obtained in two ways, from any concentrationsample in the same polymer or from a first principlesDrude model of the ATO nanoparticles plus Eq. (6).Either approach allows a study of the full range of pos-sibilities both for high visible transmittance as neededin car windscreens (Tvis > but near 70%) and for appli-cations where a clear view is needed but higher solarcontrol and glare control is also desired (Tvis near 0.38 isone example). Here, we only consider normal incidenceand concentrate our results on high Tvis systems, asthese are the most demanding in terms of spectral selec-tivity. The key performance parameters derived frommodelled and measured spectral data appear in Table 1for Tvis windows suitable for cars (Tvis > 70%). The twowith an asterisk denotes samples that have been mod-elled without matching experimental data. The ATOlaminate case is based on first principles models whichalso work with thin PC plus ATO, as in Figure 4, andthe 4 mm thick PC model is based on attenuation coef-ficients obtained from a thin sample at different con-centration. This case is interesting because it showsone can get comparable performance to existing highiron glasses in a clear polymer window with a very low

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Table 1. High Tvis options. Impact of ITO and ATO nanoparticledoping on laminate systems and clear polymer, and comparisonto some existing solar control, high visible transmittance glazings

Laminate/glazing system Tvis Tsol Asol SHGC

Clear glass laminate 0.88 0.73 0.19 0.78Clear glass laminate 0.87 0.62 0.34 0.68

(ITO 2000 ppm)Clear glass laminate 0.83 0.61 0.32 0.70

(ATO 2000 ppm)Green glass laminate 0.76 0.51 0.43 0.61Green laminate 0.74 0.43 0.50 0.56

(ITO 2000 ppm)4 mm PC (ITO 350 ppm) 0.70 0.50 0.44 0.595 mm Pilkington Evergreen 0.72 0.42 0.50 0.574 mm PPG Solex laminate 0.80 0.58 0.36 0.694 mm PPG Solextra laminate 0.70 0.39 0.56 0.52

0.0

0.2

0.4

0.6

0.8

1.0

300 500 700 900 1100 1300 1500 1700 1900 2100 2300 2500

Wavelength (nm)

Tra

nsm

itta

nce

Experimental

Modelled

Figure 4. Measured and modelled specular transmittancespectrum for a thin layer of PC in which nanoparticles are partlyclustering. The model assumes specularity and no clustering.

concentration of ITO but the total ITO used per unitarea will be equivalent to that in the 0.7 mm thick PVBcase. Solar transmittance of polymer apertures such askylights has been a longstanding problem. Some datafrom existing competing window technologies for solarcontrol, which have close to the nanoparticle perfor-mance also appears in Table 1. Cost and durability arethen the discriminating issues.

In Figure 4 data appears on a hazy, incompletelydispersed sample of 2% ATO in PC where scatteringloss is obvious below 1 µm. The degree of hazinessdepends also on the actual angular spread of scatteredlight, which here is not large as measured in our pho-togoniometer, and thus this sample still gave a reason-ably clear, only slightly hazy view. It is enough to dropthe signal a lot in the specular beam instrument, asshown, and still shows good NIR blocking that can be

0.0

0.2

0.4

0.6

0.8

1.0

300 500 700 900 1100 1300 1500 1700 1900 2100 2300 2500

Wavelength (nm)

Tra

nsm

itta

nce

Figure 5. Modelled specular transmittance spectrum for alaminated window with two clear glass panels in which the PVBcontains 0.2% by weight of ATO.

0.0

0.2

0.4

0.6

0.8

1.0

300 500 700 900 1100 1300 1500 1700 1900 2100 2300 2500

Wavelength (nm)

Tra

nsm

itta

nce

PPG Clear Laminate

PPG Clear | PVB + 0.2% ITO| PPG Clear (Measured)

PPG Clear | PVB + 0.2%ITO| PPG Clear (Modeled)

Figure 6. Specular transmittance comparison of a laminated win-dow with two clear glass panels and undoped PVB, with one inwhich the PVB contains 0.2% by weight of ITO. Model curvebased on a universal attenuation parameter for ITO sphericalnanoparticles in PVB.

accurately modelled. The extension of the model curveshows what should occur if it was fully dispersed at visi-ble wavelengths. Figure 5 shows modelled results usingEq. (6), for 0.2% ATO nanoparticles in 0.7 mm of PVBlaminated between two clear PPG glass sheets. Theparticle refractive indices are the same as used to suc-cessfully fit NIR PC data. This curve yielded one set ofresults in Table 1. Figure 6 shows transmittance spectrafor a window with two clear PPG sheets around 0.7 mmthick PVB with 0.2% ITO nanoparticles, along withresults for the two clear glass laminates with no additionto the PVB. The extra absorption of the nanoparticlesin the NIR is obvious in this figure. A model has alsobeen carried out using a separately determined univer-sal attenuation factor for ITO in PVB per cent of ITO,

164

0.0

0.2

0.4

0.6

0.8

1.0

300 500 700 900 1100 1300 1500 1700 1900 2100 2300 2500

Wavelength (nm)

Tra

nsm

itta

nce

T (Green glass | PVB | Green glass)

T (Green glass| PVB + ITO 0.2%| Green glass)

Figure 7. Specular transmittance comparison of a laminated win-dow using two low-cost standard car windshield glass panels andundoped PVB, with one in which the PVB contains 0.2% of ITO.

which indicates we can use these factors comfortablyto estimate performance at any concentration below afew per cent in any laminate structure and with variousglass combinations.

Finally in Figure 7 we show the traditional tinted‘green’ or high iron glass laminated window as usedin cars, such as PPG Solex, without any addition tothe PVB. This is compared to what happens with theaddition of 0.2% of ITO to the PVB and both solarperformances are summarized in Table 1. Both typesof model and experimental data agree well in thissample. The combined glass + laminate layer NIRattenuation is quite strong, but this figure shows thereis still plenty of opportunity for further cuts in NIRthroughput, using either different nanoparticles in addi-tion, or various combined approaches to absorption inthe range 750–1100 nm, where NIR solar energy (seeFigure 3) is strongest. To do this and not affect Tvis

and transmitted color is not easy. Hence much lowersolar input should be obtainable for architectural win-dows with moderate Tvis using nanoparticles combinedwith other absorption mechanisms in the glasses orthe PVB. Combinations of nanoparticles, and color-balancing pigments as necessary, may provide one low-cost solution to additional solar control and will besubject to future analysis. To see how far we can gowith the two-nanoparticle types by themselves, as usedin this study, we show models for thicker panels of PCpolymer in Table 2 and compare two commercial mul-tilayer alternatives. Concentrations of nanoparticle atthese thicknesses are still low but if we want to addthem to PVB in a laminate concentrations would beundesirably high and the best option then is to combine

Table 2. Solar control in ITO and ATO nanoparticle-doped PCsheet compared to existing multilayer thin film laminate systemswith similar Tvis

Laminate Tvis Tsol Asol SHGC

12.8 mm Viracon solar 0.38 0.25 0.64 0.42screen laminate

Viracon multilaminate 0.38 0.15 0.64 0.326 mm PC (5000 ppm ATO) 0.39 0.24 0.71 0.386 mm PC (1000 ppm ITO) 0.37 0.20 0.74 0.34

with spectrally complementary nanoparticles, or otherabsorbing mechanisms, including tinted glass.

The fact ITO clearly gives better solar blocking per-formance than ATO is seen in all of the data presented,especially in Table 2 where it outperforms ATO despitehaving one-fifth its concentration. This is due to itsonset of absorptance being at shorter wavelengths thanATO (compare Figures 5–7). Unfortunately, ITO costsmuch more than ATO with indium being scarce, butthe lower concentrations reduce the cost differential.For the glare blocking architectural windows it can beseen in Table 2 that SHGC with the nanoparticle solu-tion, should be able to match or improve on existingapproaches and in a hybrid absorber should be muchlower in cost.

We note that both types of nanoparticle-doped sheetsat these concentrations almost completely block theUVB component of incoming solar energy since theparticle’s interband absorption is strong in the UV. Thisis a useful addition to the protective value of these win-dows. Chromaticity coordinates for transmitted day-light when the particles are fully disperse, are closer toneutral in ATO than ITO because it has a longer wave-length onset of absorption, but ITO doped sheet alsogives a reasonable neutral colored light. For examplethe CIE color coordinates are x = 0.312, y = 0.343with the clear glass ITO laminate in Figure 6.

Conclusions

Nanoparticle doped polymers can provide laminatedwindows and polymer glazings for good solar con-trol with high visual and daylighting performance. Theability of nanoparticles made of certain conductingmaterial to resonantly absorb, especially in the NIR, isthe key to the appeal of this technology. They can matchthe performance of the available multilayer coatingson glass and especially on high-performance glasses.ATO nanoparticles are less able to block solar heat thanITO particles, but may have applications with other

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complementary absorbing processes, having the bene-fit of much lower cost. There is considerable scope forfurther improvements even when Tvis > 70%, wherean SHGC approaching 0.4 may be achievable with theaid of nanoparticles. The potential benefits to passen-gers in cars and occupants in buildings, and hence toenergy savings and safety of this technology in warmclimates is very high.

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