looking through walls

8/11/2019 Looking Through Walls

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Looking through walls - Coatings on glass for buildings

by Wolfgang Theiss

M. Theiss Hard- and Software for Optial Spetrosopy! "r.-#ernhard-$lein-Str. %%&! "-'(&)* +ahen! ,erany

Abstract

Modern buildings are euipped with windows ha/ing theral insulating

properties like solid walls. +d/aned thin fil oatings not only effiiently

suppress the eission of heat radiation but pro/ide a large /ariety of possible

olors for arhitetural design. Optial spetrosopy fro the ultra/iolet to theinfrared plays an iportant role in the de/elopent of these produts. The

relation between deposition onditions and optial properties of the produed

aterials is studied! and the interation of the indi/idual layers with their

neighbours in the stak is in/estigated. The optial design of optii0ed

ultilayers and spetrosopi prodution ontrol is disussed.

Moderne Gebude sind mit Fenstern ausgestattet, die thermisch so gutisolieren wie massive Wnde. Die dabei eingesetzten Dnnschichtsysteme

unterdrcken eizient die Wrmeabstrahlung des Glases und geben

gleichzeitig dem !rchitekten eine grosse Gestaltungsm"glichkeit bezglich der

Farbe der #erglasung. $%tische &%ektrosko%ie vom ultravioletten bis zum

inraroten &%ektralbereich s%ielt eine gro'e (olle bei der )ntwicklung dieser

*rodukte. Der +usammenhang zwischen den De%ositionsbedingungen und den

erzielten o%tischen )igenschaten der %roduzierten Materialien sowie dieWechselwirkung der einzelnen &chichten mit ihren achbarn im

&chichtsystem werden untersucht. Das o%tische Design der

#ielachschichtsysteme und die s%ektrosko%ische *roduktionskontrolle werden

diskutiert.

Introduction

The way we build houses and large buildings has hanged draatially during the last

entury. + oparison of two odern offie buildings 1see figs.% and (2 shows that

solid walls! gi/ing shelter fro wind and rain as well as low and high outsideteperatures! are ore and ore replaed by transparent windows.

This hange in arhiteture is based on two industrial de/elopents! the large sale

prodution of alost perfetly flat glass and the large area oating tehnology. The

ass prodution of glass panes is possible using the float proess where a ontinuous

strea of glass o/es 1on a bath of liuid tin2 fro the hot liuid state to the solid

state. 3iees of typially 4 5 6.(& are ut at the end of the line. The pane

prodution is followed by a oating step7 The theral properties of unoated glasswould lead to a treendous heat loss if the teperature inside an offie building like

the one shown in fig.( is signifiantly higher than outside. On sunny days in the

suer! on the other hand! buildings euipped with unoated glass would strongly

heat up and produe signifiant osts for air-onditioning. Thin fil systes on glassa/oiding heat losses are alled low-e oatings! whereas so-alled 8solar ontrol


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oatings8 a/oid to soe e9tent

the building8s heating up by

sunlight. The funtion of bothoating types is disussed in the

ne9t setions.

The ost-effeti/e prodution oflarge area oatings is a

peranent struggle for

hoogeneity7 The deposition of

layers with thikness /ariationsof less than % n on a 4 5 6.(&

substrate would be diffiult

enough :%;. aent layers during andafter the deposition is

disussed.

The design of new oating

produts ust take into

aount the wanted target

properties of the oating! thea/ailable range of optialonstants and fil

thiknesses in the prodution

line! possible hoogeneity

probles and osts.

The last setion is about the

optial inspetion of theprodution whih is an

iportant tool to ontrol the

deposition. =nforation about

thiknesses and optial onstants of the layers produed at present is used to take

appropriate deposition ontrol ations to obtain wanted and stable oating properties.

+ll oputations shown in this artile ha/e been done with the CO"? software :(;.

Low emission coatings

@eplaing a solid wall by a piee of glass does not only inrease the throughput of

light but also that of heat. The transfer of heat through a wall is usually speified by

the so-alled


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at (F& and 6&& $.

=n order to suppress the

radiation e9hange it wouldbe nie to be able to swith

off the 8war8 light soure.

This an be ahie/ed by thedeposition of a etalli layer

on the pane ounted on the

interior side of the building.

The layer inreases therefletane! dereases the

absorption and this way

diinishes the eission of

infrared radiation. Sil/er

turned out to be the best

aterial for this purpose. Eig.

D shows the strong effet of the sil/er thikness on the


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Eig. * shows a oparison of the transittane spetra of typial low-e and solarontrol oatings. The latter is uh ore 8seleti/e8! i.e. there is a sharp derease of

the transission fro the /isible to the infrared. This ideally step-like feature annot

be obtained with a single sil/er layer. =n ost ases! two sil/er layers being ebedded

in and separated by o9ide layers are applied to ahie/e high seleti/ity. Solar ontrol

oatings are deposited on the inner side of the e9terior pane in double gla0ing

windows. Their perforane is e9pressed in two uantities7 The 8Light transittane8

1a/erage transittane in the /isible! weighted with our eyes8 sensiti/ity2 :'; indiateshow well we an look through the window. The 8Total solar energy transittane8 g

:'; gi/es the o/erall throughput of solar radiation! inluding the diret solar

transittane and seondary energy streas by absorption and re-eission. + good

solar ontrol oating has a high light transittane and a low g-/alue. + ratio of (between the two uantities is onsidered to be alost ideal.

Fig. / =olor variation o low:e coatings with the same 6:value o -.2 W>m?2 91

Fig. @ 5ransmission s%ectra o double glazing windows without coating, with low:e and solar controlcoatings. 5he intensity distribution o incident solar radiation the so:called !M -.< radiationdistribution is taken rom A4B1 is shown as well.

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Thin films produced by large area coaters

The layer staks disussed abo/e ust be deposited on large glass panes with a /ery

high degree of hoogeneity and at reasonable osts. +t present the prodution shee

skethed in fig.F is the ethod of hoie :4;. The unoated panes are o/ed into a

large e/auated /olue. Linear sputtering de/ies 1usually operated with +r gas2

optii0ed for hoogeneity perpendiular to the otion of the glass deposit etals or

1with additional reati/e gas inlets2 o9ides or nitrides. "epending on the speed of the

substrates! the reuired layer thikness and the a/ailable sputtering rates se/eral

athodes are grouped together to produe one of the oating8s funtional layers.

To aintain the /auu and to properly separate the 8o9ide areas8 fro the 8etalareas8 a treendous puping euipent is reuired. The sputtering de/ies onsue a

lot of eletrial power! too.

Hene the goal is to finish

as any panes as possible

without interruption

typially e/ery inute a

oated pane lea/es theoater. =f e/erything works

fine about ('&&& ( glass

an be oated on one day.

Howe/er! reati/e

sputtering of o9ides is a

stable proess only in theso-alled o9idi ode

where the partial pressure

of o9ygen is /ery high.


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Eor the proper design of oatings it is /ery iportant to in/estigate the dependene of

the obtained optial onstants on the deposition onditions! e.g. the applied eletrial

power and o9ygen 1or nitrogen2 pressure. The best way to do this is to produe singlelayers of all rele/ant aterials and analy0e optial spetra 1like refletane and

transittane2 to deterine the ople9 refrati/e inde9 n K i k of the sputtered

aterials. With appropriate ad>ustable dispersion odels 1like the OL odel foraorphous aterials :*;2 one an desribe the optial properties of all aterials used

in glass oatings with a few key paraeters 1see fig.%%2. The relation of these

paraeters to the deposition onditions gi/es an e9ellent basis for oating design and

prodution ontrol. This inreased knowledge >ustifies to use the e9pensi/eprodution line for se/eral hours to produe single layers under /arious prodution

onditions.

Fig. -- !nalysis o three measured s%ectra the red s%ectra on the right1 by itting a dis%ersion modeland the thickness o an amor%hous oCide layer on glass. 5he simulated s%ectra are drawn blue. 5he

gra%h o the obtained reractive indeC model let side1 shows the real %art n in blue and the imaginary%art k in red.

$nowing single layer properties is not suffiient to desribe spetra of ople9

oating produts with high uality. Layers in a layer stak an ha/e different

properties than single layers on glass. The growth of a thin fil depends on the type!teperature and roughness of the underlying aterial. =n addition! high rate sputtering

is not a /ery gentle ethod! and the deposition of a layer ay influene 1daage2 thefils underneath. Slow diffusion proesses ay lead to hanges of the oatings e/en a

long tie after finishing the deposition.

The sil/er layers whih are /ery iportant for the funtion of low-e and solar ontrol

oatings are /ery sensiti/e to their neighbourhood in the layer stak. The high

refletane of thin sil/er fils depends ritially on the obility of the eletrons.Their free otion an be disturbed by ipurities and defets in the bulk! but also by

surfae roughness. =n a etalli fil of a few nanoeters thikness there are freuent

boundary ollisions of the eletrons sooth interfaes lead to irror-like refletions

of the eletron wa/es with no derease of the obility whereas rough surfaes ause

diffuse eletron sattering with signifiant onseuenes for the onduti/ity. The

daping onstant of the siple "rude odel for the eletrons :F; in sil/er is a goodparaeter to harateri0e the uality of the layer fig.%( shows the influene of the

@ 1oating side2

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O9ide analysis 1single layer2

+ir

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sil/er uality on the


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are used as optii0ation goals. usted siultaneously. Eor paraeter fine-

tuning one an use optii0ation ethods that o/e fro the starting design to the

ne9t loal iniu of the de/iation to the target /alues. This is a atter of seonds

or inutes and an be i9ed with anual user interations. Methods that searh for

the global de/iation iniu are uh slower and should not reuire any userations in order to be applied in o/ernight optii0ation runs.

Optical production control

+s disussed abo/e! thin fil deposition in large area oating lines ust be perantly

obser/ed and orreted. "ue to the high operational osts! one ust detet de/iations

fro the target properties of the oating as early as possible.

Eig. %D shows the priniple of an optial prodution ontrol syste :4;. +t e/eryiportant position along the prodution line appropriate optial easureents are

done. +n 8optial network8 is responsible for the oordination of the data auisition

and analysis of the easured data. =t pro/ides status inforation about the presentondition of the prodution line. The operator an use this inforation in order to

deide if prodution paraeters should be hanged and whih ations are reuired.

=t would be ad/antageous to reord se/eral spetra at eah position7 @efletane frothe oating and the glass side as well as transittane! if possible in a large spetral

range. These spetra would pro/ide enough inforation to safely deterine the

thikness and the optial onstants of eah ain layer of the oating. =deally one

would like to easure at se/eral spots 1at least 62 perpendiular to the diretion of the

glass otion in order to hek the lateral hoogeneity of the deposition.

There are soe liitations! howe/er. @eording 6 spetra at ' positions along theprodution line with 6 spots perpendiular to the line would ean to buy! install and

operate D' spetroeters. ?/en if low-ost array spetroeters are used! this is >usttoo e9pensi/e at the oent. +lso the analysis and the handling of the obtained data

Fig. -4 )Cam%le or successul uantitative modelling o a solar control coating with -3 layers. 5hemeasured red1 relectance rom the coating side (1, the transmission 51 and the relectance withbackside illumination are re%roduced very well by the model simulated s%ectra in blue1. 5here are nomeasured data or the rontside relectance at an angle o incidence o /0.

@

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Gauu

Material %

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Solar ontrol oating7 uantitati/e odelling


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would reuire se/eral oputers and a sophistiated software whih would also be

e9pensi/e.

+ good urrent oproise is to reord transittane spetra in the oating line!

only in the enter of the panes. The final produt is inspeted outside the /auu

haber with a o/ing syste of ( or 6 spetroeters 1@! T! e/entually @ fro the

bakside2 whih reord spetra at se/eral spots perpendiular to the glass otion 1see

fig. %'2. This way about %&

spetroeters are reuired.+nother restrition onerns the

a/ailable spetral range. Sinethere is not too uh tie for

spetru reording! array

detetors ha/e to be used. These

are a/ailable at low ost 1


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looking through walls

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