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Nuclear Instruments and Methods in Physics Research B 264 (2007) 302–310

NIMBBeam Interactions

with Materials & Atoms

Solar parameters of induced WO3-coated glass

Nilgun Dogan Baydogan a, Esra Ozkan Zayim b,*, A. Beril Tugrul a

a Istanbul Technical University, Institute of Energy, Ayazaga Campus, 34469 Istanbul, Turkeyb Istanbul Technical University, Science and Literature Faculty, Ayazaga Campus, 34469 Istanbul, Turkey

Received 18 June 2007; received in revised form 27 August 2007Available online 15 September 2007

Abstract

WO3 films were prepared by sol–gel deposition process on Corning 2947, microscope slide substrates. The effects of irradiation on thesolar parameters of WO3 films were investigated between�1 and 21 kGy absorbed dose by Co-60 radioisotope. Three characteristic opticaldensity bands explained the causes of color due to the absorption of sunlight at induced color centers of the transition elements such as W, Feand Zr after the gamma irradiation. These bands lead to variations on the solar control in terms of shading coefficient. The absorbed doseplays a key role in the improvement of the shading coefficient dramatically, hence the solar parameters changed considerably depending onthe induced color centers of the transition elements and the variations of the grains and the void sizes of the film. The results of the solarparameters of irradiated WO3 films were compared with the unirradiated WO3 films and uncoated corning in the solar range.� 2007 Elsevier B.V. All rights reserved.

PACS: 61.80.�x; 68.55.�a; 81.15.Aa; 96.60.Tf

Keywords: Co-60; Film deposition; Solar properties; Tungsten oxide; Thin films

1. Introduction

Some transition metal oxides exhibit variable opticalproperties. Thin films of these transition metal oxides arebeing pursued as vital components in switchable devices[1]. Tungsten oxide films are of much interest for applica-tion as working electrochromic layers in electrochromicdevices. Its advantages are high coloration efficiency andrelatively low price [2]. The solar control in vehicles glazingwith different types of films is an important task for severalpurposes and appears to be a growing global trend [3].Besides, adjustable surface emissivity devices are of interestfor smart temperature controlling. The spectral region, inwhich the emissivity of the device has to be controlled,depends on the temperature of the surface. Thermal con-trol of satellites is accomplished by balancing the energydissipated by satellite electrical components against theenergy emitted as IR radiation [4–6]. Coated glass materi-

0168-583X/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.nimb.2007.09.024

* Corresponding author. Fax: +90 212 285 63 86.E-mail address: ozesra@itu.edu.tr (E. Ozkan Zayim).

als can be used broadly in space vehicles depending on theiroptical and solar performances of them. However, thedevices that consist of WO3 films on satellites and spaceshuttles can be affected especially by gamma irradiationas the cumulative dose during their space missions. Underirradiation, the defect centers in these glass materials areformed as a result of charge trapping by radiolytic elec-trons or holes. One of the essential processes occurringduring the irradiation of the samples is the formation ofelectron–hole pairs. These free carriers can move andrecombine so that the photoelectrons are trapped at struc-tural defects or impurities, such as oxygen vacancies andmultivalent impurities, while the holes are self-trapped atbridging or non-bridging oxygens. These new electronicconfigurations give rise to some preferential high absorp-tion levels called color centers [7–9].

Surface radiation emission at room temperature(26.85 �C) occurs in the infrared spectral region and satelliteemittance can be characterized by a blackbody spectrum atroom temperature degree. IR-emittance modulation devicesare of interest for their thermal control in satellites

N.D. Baydogan et al. / Nucl. Instr. and Meth. in Phys. Res. B 264 (2007) 302–310 303

positioned in low Earth orbit, where the satellite tempera-ture is close to 26.85 �C [4,5,10,11]. The maximum of spec-tral distribution of surface emission shifts to shorterwavelengths with increasing temperatures [4]. Vehicles inouter space are exposed to radiations from the Sun. Thetemperature swings of a satellite surface can exhibit largetemperature swings as it moves from sunlight to shade.The temperatures produced can vary from 121.1 �C to�121.1 �C in near-Earth environments. The exact tempera-tures will depend on the type of surface and the conditionsof the solar radiation. The surface can be shaded by systemsof its own, such as solar panels. Besides, some effects canoccur, for example, the reduction of surface electron emis-sions and the development of electrical change buildup.The solar absorbance of the metal oxide coating increasesto about 2.3 times its initial value during the trip of thespacecraft from Earth to Mars [10–13]. The main causesof heating in low Earth orbit are direct sunlight, sunlightreflected from the Earth, and blackbody radiation emittedfrom the Earth. WO3 films are the best choice for reflectancemodulation, especially in the infrared region [6].

The radiation damage leads to variations in the solarproperties of WO3 film. The provenance of changes onthe shading coefficient of irradiated WO3 film on Corning2947 is the absorbed dose in this study. For the investiga-tion of the solar parameters clearly and the evaluation ofthe solar control in terms of shading coefficient, the deter-minations of solar direct transmittance (se), reflectance (qe),absorbance (ae), total solar energy transmittance (g), thesecondary heat transfer factor (qi) and shading coefficient(sc) are required properly [14–16]. This paper presentssome data on the solar changes of induced tungsten oxidefilms on Corning substrate. The properties of solar controlwere evaluated by means of the investigation of shadingcoefficient.

2. Details on solar parameters

For the convenience of EN (European Norm) and CEN(European Committee for Standardization) standards tosolar parameters in EN 410 [14–16], the investigation ofsolar properties of induced WO3 film on Corning is impor-tant to know the shading coefficient in the solar range(300–2100 nm) including infrared terrestrial window region(800–1300 nm). The total solar energy transmittance, g, iscalculated as the sum of the solar direct transmittance se

and the secondary heat transfer factor qi of the glazingtowards inside (Eq. (1)) according to EN (European Norm)at the solar range. Solar control achieved by absorptiononly exhibits optimum effect if it is positioned externally.This becomes clear from the definition of the g value [14]

g ¼ se þ qi: ð1Þ

The effectiveness of solar control mechanism can be madeclear using [17]

se þ qe þ ae ¼ 1: ð2Þ

The absorption term (aeue) is subsequently split into twoparts qiue and qoue, which are energy transferred to the in-side and outside, respectively,

ae ¼ qi þ q0; ð3Þ

where qi is the secondary heat transfer factor of the glazingtowards the outside. The solar direct transmittance (se) ofthe glazing is calculated using

se ¼P2500

300 SksðkÞDkP2500

300 SkDk; ð4Þ

where Sk is the relative spectral distribution of the solarradiation and s(k) is the spectral transmittance of the glaz-ing. It is considered that radiation has the spectral compo-sition of global radiation (direct sun at a height of 90� andcorresponding diffuse radiation from sky). Contrary to thereal situation of glass in the building, it is always assumed,for simplification, that the spectral distribution of the solarradiation is not dependent upon the atmospheric condi-tions (e.g. dust, mist, moisture content) and that the solarradiation strikes the glazing as a collimated beam and atnormal incidence. The resulting errors are very small[15,18]. Therefore, solar control measurements can be ob-tained for solar control mechanisms in satelliteapplications.

The corresponding values SkDk are given in such a waythat

PSkDk = 1. The solar direct reflectance (qe) of the

glazing is calculated using

qe ¼P2500

300 SkqðkÞDkP2500

300 SkDk; ð5Þ

where P(k) is the spectral reflectance of the glazing[15,18]. For the calculation of the secondary heat transferfactor towards the inside qi, the coefficients of the glazingtowards the outside he and towards the inside hi are needed.These values mainly depend on the position of the glazing,wind velocity, inside and outside temperatures and further-more on the temperature of the two internal glazing sur-faces. The purpose of this standard is to provide basicinformation on the performance of glazing. The conven-tional conditions have been stated for simplicity: Positionof glazing – vertical; Outside surface – wind velocity(�4 m/s), corrected emissivity = 0.837; Inside surface –natural convection, emissivity optional, air space is unven-tilated. Under these conventional average conditions, stan-dard values for he and hi are obtained: he = 23 W/(m2 K),hi = 8 W/(m2 K).

Secondary internal heat transfer factor qi of a singleglazing is calculated using [15,18].

qi ¼ aehi=ðhe þ hiÞ: ð6Þ

The shading coefficient is a ratio of the ability of the clearglass to reject solar heat gain [15,18]. This coefficient of aglazing is the ratio between the quantity penetratingthrough the glazing and the energy that was penetratinginto it through a single glass, in the same irradiation of

304 N.D. Baydogan et al. / Nucl. Instr. and Meth. in Phys. Res. B 264 (2007) 302–310

direct radiation alone is considered. It is related with thesolar heat gain that the solar energy transmitted througha glazing plus the portion of solar radiation that is absorbedand either convected or reradiated inwards [16,19]. Theshading coefficient is calculated by dividing the solar factorof the glazing in question by that of the clear glass. Thevalue of it standardized by the American Society ofHeating, Refrigerating and Air Conditioning Engineers(ASHRAE) in the USA is 0.88 [14]. It enables air-condition-ing engineers to adapt all the tables and formula for a singleclear glass to the case of special glazing [20].

3. Experimental details

Soda-lime glass gives good results in obtaining adherentfilm with the tungsten element. Sodium, which diffuses outof soda-lime glass, supports the film and acts as a surf-actant at the film [21]. Corning 2947, microscope slide sub-strate is accepted as a standard soda-lime glass in industry[22]. On the other hand, the optical and solar property ofclear soda-lime-silica glass is standardized with 1 mmthickness at the solar range by CEN in 1992 [15,16,20].Hence, the standard Corning 2947 glass with 1 mm thick-ness was used in this work because it can be more usefuland practicable for the evaluation optical results and solarchanges of samples according to declared standards.

The transmittance and reflectance of all samples at thesolar range between 300 and 2500 nm were detected usinga spectrophotometer Perkin Elmer Lambda 9 UV/VIS/NIS before and after the irradiation process.

The detection of transition elements at WO3 film onCorning 2947 was made using Innov-X XRF Analyzerand results given in Table 1.

3.1. Details on sol–gel process

The optical and structural properties of the thin filmscould be tailored easily by sol–gel deposition methods.The optical properties such as refractive index are smallerthan those of coated by the physical vapor deposition tech-nique due to the porosity of the sol–gel [23]. Porous filmswith a large surface area that lead to improved kinetic per-formance are desirable [24–26]. Hence, it is thought thatthe use of sol–gel procedure for the preparation of WO3

film has some advantages for the investigation of solarparameters in this study. Sol–gel tungsten oxide films weredeposited by a spin-coating technique onto pre-cleanedCorning 2947 substrates. The details of the cleaning of

Table 1XRF Analysis of transition elements

Elements % +/�W 0.23 0.02Fe 0.03 0.01Zr 0.01 0.00

the substrate, coating solution and the process aredescribed elsewhere [8]. The final thicknesses of the filmswere in the range of 100– 150 nm for all the samples.

3.2. Details on irradiation process

The Co-60 radioisotope is an appropriate source toirradiate the glass materials [7]. Therefore, the usage of aCo-60 radioisotope in this study and the cumulative doseis the total dose resulting from the repeated exposures ofionizing radiation to the same portion over a period oftime. When a material is used in either terrestrial or spaceapplications, it is exposed to radiation fields that areuniform or non-uniform. Radiation fields may vary withgeometry or with time [12]. Therefore, cumulative dose ofthe materials is an important parameter as is the absorbeddose of irradiated material at the place of interest duringthe space mission. Samples were placed against the pointsource panoramically at the same geometry condition forthe irradiation. The activity of the source was 5.4 Ci. Testsamples were exposed to five different gamma doses; as 0.9,1.4, 2.6, 4.5, and 21.1 kGy in the same irradiation plane.Irradiation processes were performed in room temperature.

The estimated uncertainty in absorbed dose is accept-able. Co-60 radioisotope has changed the color of WO3

films on substrates at the end of the irradiation process.The color of the films turned to dark brownish color tonesdepending on the applied dose levels. Induced color centersby gamma radiation led to changes in color depending onthe improvement of the shading coefficient of the WO3 film.

4. Results and discussion

The results of transmittance and absorbance are given inFig. 1. The irradiated WO3 films deposited by spin-coatingtechnique are non-absorbing beyond �350 nm and theabsorption edges are similar. The optical transmittance ofirradiated WO3 films varies considerably in the visibleregion depending on the absorbed dose.

The solar properties of the test samples were evaluatedusing the solar parameters according to the standardizationconcept of EN 410 in this study. For the evaluation of theeffectiveness of solar control mechanism of the test sam-ples, the changes of calculated solar direct transmittance(se), reflectance (qe), and absorbance (ae) via the absorbeddose are shown in Fig. 2. There are not many changes insolar direct reflectance when the absorbed dose increases.However, direct absorbance (ae) of the samples increaseswhile direct transmittance (se) decreases exponentially asa function of the applied dose. The change of the solardirect transmittance (se) of irradiated one is �65% less thanthe unirradiated one at 21 kGy. But the changes of thesolar absorbance (ae) of the unirradiated one is �86% lessthan the irradiated one at this dose level. These resultsmade it clear that the effectiveness of the solar propertiesof induced WO3 film on Corning 2947 has more small value(se) and the largest value (ae) after the irradiation process.

0

10

20

30

40

50

60

70

80

90

100

0 2 00 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400

Wavelength, λ (nm)

A (

%)

2 A(%)

1 A(%)

1 - Uncoated Corning

2 - Unirradiated

3 - 0.93 kGy

4 - 1.44 kGy

5 - 2.58 kGy

6 - 4.50 kGy

7 - 21.11 kGy

7 A(%)6 A(%)5 A(%)4 A(%)

3 A(%)

Fig. 1(b). Absorbance A (%) of WO3 films on glass before irradiation and after gamma irradiation depending on the wavelength.

0

10

20

30

40

50

60

70

80

90

100

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400

Wavelength, λ (nm)

T (%

)

2 T (%)3 T(%)

4 T(%)5 T(%)6 T(%)7 T(%)

1 T (%)1 - Uncoated Corning2 - Unirradiated3 - 0.93 kGy4 - 1.44 kGy5 - 2.58 kGy6 - 4.50 kGy7 - 21.11 kGy

Fig. 1(a). Transmittance T (%) of WO3 films on glass before irradiation and after gamma irradiation depending on the wavelength.

N.D. Baydogan et al. / Nucl. Instr. and Meth. in Phys. Res. B 264 (2007) 302–310 305

There are several types of radiations, such as direct and/or indirect ionizing radiation and illumination radiation athigh dose areas, irradiation constructions, the hot cells andthe peeping holes; the secondary heat transfer coefficients ofthe glazing (towards the inside, hi and towards the outside,ho) are of the same value at these special investigation con-ditions. Hence, the secondary heat transfer factors of theglazing towards the inside qi and outside qo are also equalto the same value as qs at these places for several specialexperimental conditions according to Eqs. (3) and (6) [19].

For the description of the transferred proportion ofabsorbed solar energy, secondary heat transfer factor ofinside qi and outside qo at the glazing of the building or spe-cial experimental condition, qs is shown in Fig. 3. Second-ary heat transfer factors rise exponentially when theabsorbed dose increases.

Shading coefficient of induced WO3 film on Corningdecreases exponentially when the absorbed dose increasesas shown in Fig. 4. Since the induced color centers affected

the shading coefficient this coefficient decreased sharply upto �3 kGy absorbed dose level. The color centers increasedrapidly due to the absorbed dose of the samples at thelower dose levels. Since the creation of color centersincreases slowly over 3 kGy, the shading coefficientdecreases gently. When the shading coefficient decreased,the shading ability enhanced depending on an increase oftotal solar heat transmittance and the increase of theabsorbed dose thereby improving the shading coefficient.There is a saturation condition of the shading coefficientat special (scs) and normal conditions (sc) at �20 kGyabsorbed dose. The 21 kGy absorbed dose level fulfils thesaturation condition. This is the main reason to investigatethe saturation condition of shading coefficient for whichthe variation range of shading coefficient is determinedvia absorbed radiation as a cumulative dose of the irradi-ated material in space.

Soda-lime-silicate glass is a type of glass, which consistsmainly of 70% SiO2 and 15% Na2O. The addition of soda

3 5 80 10

Absorbed

10

30

50

0

20

40

Sec

onda

ry H

eat T

rans

fer

Fac

tor,

qi,

qo,q

s

100

Fig. 3. Secondary heat transfer factor of WO3 films on gla

3 5 80 10

Absorbe

0.70

0.90

0.60

0.80

1.00

Shad

ing

Coe

ffici

ent,

sc

0.00

Fig. 4. Shading coefficient of WO3 films on glass fo

3 85 13 18 230 10 15 20 25Absorbed Dose (kGy)

10

30

50

70

90

0

20

40

60

80

100

e(%

),

e(%

),

e(%

)

α ρ

τ

τe

αe

ρe

Fig. 2. Solar direct transmittance T (%), reflectance R (%) and absorbanceA (%) of WO3 films on glass depending on the absorbed dose.

306 N.D. Baydogan et al. / Nucl. Instr. and Meth. in Phys. Res. B 264 (2007) 302–310

(Na2O) to silica lowers the softening point by 800–900 �C.Alumina (Al2O3) is added to improve chemical resistancein soda-lime silicate glass [7,14,27–29]. This glass is usedespecially as sheet (including windows), containers andlight bulbs. As a component of soda-lime glasses, ZrO2

tends to increase viscosity, refractive index, and resistanceto weathering [14]. The transmittance and absorption ofsoda-lime-silicate glass depends on the purity and the stateof oxidation (stoichiometry) of silica [19]. Ionic impurities,color centers and defects depending on the purity and thestate of oxidation (stoichiometry) are important to knowthat optical density is the reason for the change of colorof WO3 film on glass material. The evaluation of opticaldensity was made using Eq. (7) [17,30] in which D is theoptical density; t, the film thickness; a, the absorbanceand q, the reflectance. The evaluation of the reflectancetogether with absorbance is important for the examinationof the color centers [7,8,10,18].

D ¼ atlog10e� log10ð1� qÞ2: ð7Þ

Specifically, the changes of optical density bands due to ab-sorbed dose affect the shading coefficient. Optical density is

13 18 2315 20 25

Dose (kGy)

qs

qi

qo

ss for the building and special experiment conditions.

13 18 2315 20 25

d Dose (kGy)

scs

sc

r the building and special experiment condition.

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400

Wavelength, λ (nm)

Opt

ical

Den

sity

, D

Uncoated Corning

Coated and Unirradiated

4.5 kGy21.11 kGy

2.58 kGy1.44 kGy0.93 kGy

Fig. 5. Optical density of WO3 films on glass before and after gamma irradiation.

N.D. Baydogan et al. / Nucl. Instr. and Meth. in Phys. Res. B 264 (2007) 302–310 307

evaluated depending on the wavelength is depicted in Fig. 4so the evaluation of optical density bands requires anexplanation of the absorption of sunlight by color centersdue to elements. After the irradiation, optical density ofWO3 films on Corning 2947 indicated that the gamma irra-diation induced absorption mainly at three bands as de-picted in Fig. 5. Gamma irradiation is effective for thecreation of color centers both in the Corning 2947 substrateand in the WO3 film, so three bands were observed in allirradiated samples.

Two bands have occurred between �380 and 460 nmand �620 nm at the visible range. But the third band isdetermined between �1000 and 1300 nm at the near infra-red range. It can be thought that first optical density bandbetween �380 and 460 nm is associated with the existenceof ferric iron (Fe3+) and zirconium at the glass structure[7,27,31]. Zirconium shows broad absorption featurebetween �230 nm and 400 nm [32]. Besides, the valancestate of iron affects the color of glass. Fe3+ gives a yellow-ish color hue, while Fe2+ is colorless in the structure. Bylosing one electron the Fe2+ is converted to Fe3+ [33].Impurity atoms such as Fe3+ causes the coloration in glasswith increasing absorbed dose and especially Fe2+ is oxi-dized to Fe3+ as a result of the irradiation [34]. There isthe reduction of W6+ to W4+ at �630 nm in the secondoptical density band as a typical position [7,28]. Colorationproperties of WO2 and WO3 films are different from eachother. The color of the WO2 films is blue or brown; onthe other hand, the color of the WO3 films is lemon yellowunder coloration conditions. It is indicated that WO3 filmhas a high transmission in normal mode and a high opticalmodulation at the electrochromic application [29]. On theother hand, WO3 films take different shades of blue duringcoloration using several devices and techniques [35,36].Monochromatic transmittance of dark blue is at�633 nm at the colored devices [37]. There is an interfer-ence peak around 600 nm for pure WO3 film obtained fromsol–gel method [29]. It can be thought that there is a reduc-tion of W6+ to W4+ while the Fe2+ ions oxidize to Fe3+ and

therefore, there is the emittance of an electron by Fe2+ ionsduring the capture of an electron by W6+ ion during thegamma irradiation at our study.

Third optical density band is determined between �1000and 1300 nm as a result of the coloration of both the filmand the glass structure. It can be thought that there aretwo main reasons for the creation of third optical bandbetween �1000 and 1300 nm after irradiation process.One of them can be explained with the build up of theabsorption that is responsible for the coloration of WO3.The transmittance of the samples is reduced significantlyafter gamma irradiation, especially at �1000–1300 nm.The typical absorption spectrum on the induced WO3 filmoccurs around 1000 nm [38,39]. The absorption due to fer-rous (Fe2+) iron is at �1050 nm in soda-lime-silicate glass.The wavelength position of Fe2+ ions change between 900and 1300 nm depending on the glass composition [7,28]. Asa result, third optical density band is related with both col-ored WO3 film and the existence of Fe2+ ions at the glassstructure. So the changes in the color of the samples fromlight to dark brown with the increase of absorbed dosewere explained clearly by the induced color centers dueto elements.

The surface morphology of the films was examinedusing an atomic force microscope (AFM), SPM-9500 Ser-ies, Shimadzu, in the dynamic mode. It seems that thereare voids between the grains in some areas of surface asdepicted in Fig. 6(a)–(f). The surface of the unirradiatedspecimens is composed of the equaxial grains as depictedin Fig. 6(a) and (d). Their dimensions are �125 nm at 2DAFM images in Fig. 6(a). The dimensions of the voidsare �100 nm as shown in Fig. 6(a).

The grains were induced intensely by gamma irradiationat 0.93 kGy. The dimension of grains at 0.93 kGy is lowerthan the dimension of grains at unirradiated one inFig. 6(b) and (e). On the other hand, the dimension ofthe grains in this absorbed dose is not obvious as muchas the dimension of the grains at the unirradiated one.The dimensions of the grains decrease generally at the

Fig. 6(b). 2D AFM image of irradiated film at 0.93 kGy.

Fig. 6(d). 3D AFM image of unirradiated film.

Fig. 6(e). 3D AFM image of irradiated film at 0.93 kGy.

Fig. 6(a). 2D AFM image of unirradiated film.

Fig. 6(c). 2D AFM image of irradiated film at 21.1 kGy. Fig. 6(f). 3D AFM image of irradiated film at 21.1 kGy.

308 N.D. Baydogan et al. / Nucl. Instr. and Meth. in Phys. Res. B 264 (2007) 302–310

N.D. Baydogan et al. / Nucl. Instr. and Meth. in Phys. Res. B 264 (2007) 302–310 309

whole area of the surface of WO3 film at 0.93 kGy in both2D and 3D AFM images. There is not so much change in thedimension of voids at 0.93 kGy. While the grains separate tomore small grains due to the absorbed dose of the film, theystart to gather and pile up at several places on the surface ofthe film at this dose level as depicted in Fig. 6(b) and (e). Thedimension of grains is �50 nm as shown in Fig. 6(b). Theproperties of the surface started to change obviously at thisdose level. So it is thought that the absorbed dose of�1 kGyis important to investigate the solar properties of WO3 film.The dimension of grains can be observed more clearly over0.93 kGy. It is interesting to note that when the absorbeddose of the specimen reaches 21.1 kGy, the dimension ofthe grains increases again. Moreover, the gathering of thegrains in the induced film takes place more obviously atthe certain places on the surface of the film as shown inFig. 6(c) and (f). The dimension of grains is �125 nm at21.1 kGy in Fig. 6(c). The surfaces of irradiated specimenshave some voids, but it seems that the number of voidsdecreases excessively due to the combination of voids witheach other. The largest dimension of the voids seems to be�150 nm at this highest dose level as shown in Fig. 6(c).On the other hand, the shapes of the voids changed extre-mely and the axial of the voids have not had the equaxialat 2.1 kGy in both 2D and 3D AFM images as depicted inFig. 6(c) and (f). The gathering of the grains and the combi-nation of voids with each other at the surface of the filmincreased when the absorbed dose increased.

The absorbed dose is considered to play a significant keyrole in the shading coefficient in dependence of the inducedcolor centers and the variation of the grain size. Besides,the shading coefficient is improved with the increaseof the void size and with the gathering of the grains, whilethe absorbed dose increases.

5. Conclusions

The experimental details on the shading coefficient ofinduced WO3 film on Corning 2947 were explained usingthe spectrophotometric measurements and the calculationsof solar parameters in standards.

Gamma irradiation generated three main optical densitybands. These bands have addressed the transition elementssuch as Zr, Fe and W as responsible for the coloration ofthe structure. The changes in the bands by the absorbeddose explained the causes of coloration by induced colorcenters depending on the transition elements. The enhance-ment of color by W element in WO3 film and by Fe and Zrelements in the glass substrate can be explained by meansof the created color centers at the certain optical densitybands.

Induced WO3 film on Corning 2947 reduces the inci-dence of solar radiation considerably. Changes at the shad-ing coefficient as the amount of heat passes through theinside have been related with the increase of the inducedcolor centers in both WO3 film and the glass structure.The created color centers have changed the performance

of shading coefficient considerably until �3 kGy. Besides,the improved shading coefficient has addressed the note-worthy gain in solar energy at WO3 film on glass. The bestperformance can be obtained over 5 kGy from an irradi-ated WO3 film on glass with a lower shading coefficientat hot climate, special study conditions or experimentalmedia. Improved shading coefficient by gamma irradiationcan enable effective solar energy saving at the hot climate.As the solar control was described in terms of shading coef-ficient, solar control could be changed by absorbed doseconsiderably at the saturation condition (�21 kGy). Shad-ing coefficient was reduced to approximately 43% to pro-vide solar shading in summer by absorbed dose at�21 kGy. Although solar direct transmittance of WO3 filmis 80 percent of the available solar radiation, irradiated oneat �21 kGy is 50%. Hence, a low shading coefficient can beachieved with minimal visible reflectance to the outside ofthe building (�10.5%) and the high visible transmission.Shading coefficient of less than 0.7 can be achived at�21 kGy for the solar control applications. So WO3 filmmay provide optimum summer shading coefficient.

The number of pores on coating can be changed with theabsorbed dose of the film. Besides, the dimension and theshape of pores can be chosen depending on the applicationpurposes. The gathering of small grains in the induced filmby gamma irradiation can be explained with the change ofthe valance state of the ions of impurity atoms and the for-mation of new electronic order in defect centers as a resultof the new arrangement of ionization in the structure dueto the absorbed dose of film. It is thought that there maybe a relationship between the induced grain size and theincrement of the void size on the surface of irradiatedWO3 film and solar control in terms of shading coefficient.

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