Note: Descriptions are shown in the official language in which they were submitted.
~L~34~4
This invention relates to glass structures bearing a thin, func-
tionalJ inorganic coating te.g. a coating of tin oxide forming means to promote
reflectivity of infra-red light~ which structures have improved appearance as
a consequence of reduced iridescence historically associated with said thin
coatings, and methods for achieving the aforesaid structures.
Our co-pending application is divided out of this parent applica-
tion and relates to an apparatus suitable for making such glass structures.
Glass and other ~ransparent materials can be coated with trans-
parent semiconductor films such as tin oxide, indium oxide or cadmium stannate,
in order to reflect infra-red radiation. Such materials are useful in provid-
ing windows with enhanced insulating value (lower heat transport), e.g. for
use in ovens, architectural windows, etc. Coatings of these same materials
also conduct electricity, and are employed as resistance heaters to heat
windows in vehicles in order to remove fog or ice.
One objectionable fea~ure of these coated windows is that they
show interference colors ~iridescence) in reflected light,
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and, to a lesser extentJ in transmitted light. This iridescence
has been a serious barrier to widespread use of these coated
windows (see, for example, American Institute of Physics
Conference Proceeding No. 25~ New York, 1975~ Page 288).
In some circumstances, i.e. when the glass is quite
dark in tone (say, having a light transmittance of less than
about 25o) this iridescence is muted and can be tolerated.
However, in most architectural wall and window applications,
the iridescent effect normally associated with coatings of less
than about 0.75 microns is aesthetically unacceptable to many
people (See, for example, United States Patent 3~710~074 to
Stewart).
Iridescent colors are quite a general phenomenon in
transparent films in the thickness range of about 0.1 to 1
micron, especially at thicknesses below about 0.85 micron.
Unfortunately, it is precisely this range of thickness which is
of practical importance in most commercial applications.
Semiconductor coatings thinner than about 0.1 micron do not show
interference colors, but such thin coatings have a markedly
inferior reflectance of infra-red light, and a markedly reduced
capacity to conduct electricity.
Coatings thicker than about 1 micron also do not show
visible iridescence in daylight illumination, but such thick
coatings are much more expensive to make, since larger amounts
- of coating materials are required, and the time necessary to
deposit the coating is correspondingly longer. Furthermore,
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, -
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films thicker than l micron have a tendency to show haze, which arises from
light scattering from surface irregularities, which are larger on such films.
Also, such films show a greater tendency to crack, under thermal stress,
because of differential thermal expansion.
As a result of these technical and economic constraints, almost
all present commercial production of such coated glass articles comprise
films in the thic~ness range of about 0.1 to 0.3 microns, which display
pronounced iridescent colors. Almost no architectural use of this coated
glass is made at present, despite the fact that it would be cost-effective
in conserving energy to do so. For example, heat loss by infra-red radiation
through the glass areas of a heated building can approximate about one-half
of the heat loss through uncoated windows. The presence of iridescent colors
on these coated glass products is a major reason for the failure to employ
these coatings.
Co-pending application, serial number 313,843, filed on October 20,
1978, discloses means to reduce this iridescence to unobservably small values,
by means of an additional layer or layers placed in register with the main
coating~ including a gradient-type coating. The present disclosure is directed
primarily toward improved means for forming such a gradient-type anti-iride-
~0 scent layer.
Summary of the Invention
It is one object of the present invention to provide means
~3~%~
to eliminate the visible iridescence from semi-conducting thin
film coatings on glass, while maintaining their desirable
properties of visible transparency, infra-red reflectivity, and
electrical conductivity.
Another object of the present invention is to achieve the
above goals without increasing the cost of production
significantly over the cost of using ordinary iridescent films.
Another object of the present invention is to achieve the
above aims with a process which is continuous and fully
compatible with modern manufacturing processes in the glass
industry.
A further object of the present invention is to achieve
all of the above goals with products which are highly durable and
stable to light, chemicals and mechanical abrasion.
Another object is to achieve all of the above goals using
materials which are sufficiently abundant and readily availahle
to permit widespread use.
A further object of the invention is to provide means to
reduce the total amount of light reflected from the coated
~a surface of glass and thereby increase the total transmission of
light by the glass.
Another object of the invention is to provide a glass
structure comprising a compound coating wherein an outer coating
is formed of an infra-red reflecting surface of about 0~7 micron
or less and wherein an inner coating forms means for (a)
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2~
reducing haze on the coated glass and, simultaneously and independently (b)
reducing the iridescence of the glass structure.
A further object of the invention is to provide a glass structure
having the non-iridescent characteristics referred to above which structure
is characterized by a gradual change in coating composition between glass and
said outer coating.
Other objects of this inven~ion and that of our divisional appli-
cation are to provide novel apparatus and processes which are suitable for
making the above identified novel products and, indeed, which are suitable
for use in making coatings of gradually changing compositions and from gaseous
reactants whether or not such coating be on glass or some other substrate
and whether or not such coatings comprise a maximum amount of one component
within or at an extremity of the depth of the coating structure.
Other objects of the invention will be obvious to those skilled in
the art on reading the instant invention.
According to one aspect of the present invention there is provided
3 process for continuous coating of a substrate by a film formed of reactive
components o a gas mixture in which properties of the coating vary continu-
ously through the thickness of said film, said process comprising the steps
~0 o~
~ a) flowing said gas mixture through a reaction zone defined by
a flow path for said reactive components contiguous to, parallel with and
bound by, a surface of the substrate
~ b) depositing, preferentially, a reaction product derived from
a more reactive component of said mixture on a portion of said surface exposed
earlier to said gas mixture,
(c) depositing, preferentially, a reaction product derived from a
less reactive component of said mlxture on a portion of said surface exposed
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~3~4
later to said gas mixture, and
~ d) moving said substrate surface through said reaction zone in a
direction parallel to said flow path to obtain a change in the composition of
said coating throughout the thickness of said coating, as said substrate emerges
from said reac*ion zone.
According to a further aspect of the present invention there is
provided a process for forming on a substrate, a thin, coating, which has a
changing composition from a predominantly first coating composition nearest the
substrate to a predominan*ly second coating composition more remote from the
substrate, said process comprising the steps of
~ 1) introducing into a first end of a reaction chamber and out the
other end of said chamber a mixture of a first reactant gas from which said
first coating compound is formed, a second reactant gas from which said second
coating compound is formed, and a third gas which forms means to react with each
of said reactant gases to form said coating compounds, wherein said first re-
actant gas reacts at a substantially different rate with said third gas, than
does said second reactant gas, the different rate of reaction with said third
gas forming means to provide a difference in relative concentration of said
reactant gases from one end of said chamber to the other and to provide differ-
~0 ent quantities of said coating compo~mds from one end to another; and
(2) continuously passing a substrate to be coated through said
reaction chamber from said first end of the chamber to said other end of the
chamber; and
~ 3) coating said substrate with a progressively changing composition
as it moves through said chamber, said composition formed by depositing said
coating compounds said compositions indicative of the relative reactivity and
concentration of said reactant gases along said chamber.
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According to another aspect of the present invention there is pro-
vided a transparent glass product substantially free of iridescent appearance,
having a glass substrate bearing a coating which is substantially uniform
across the surface area thereof, said coating consisting of
~ a) a lower coating zone comprising a material formed of at least
ttio components which is characterized by a gradual change from a first composi-
tion pro~imate to said substrate having a relatively high proportion of a
irst component to a second composition more remote from said substrate having
a relatively large proportion of said second component; and
~b) an upper coating zone having a refractive index substantially
the same as that of said second component.
According to the invention of the divisional application there
is provided in an apparatus for forming a continuous coating of progressively
changing composition on heated glass over the length of a processing path
through which said glass passes said apparatus comprising, in addition to
means to support said glass and means to move it continuously along said pro-
cessing path, the improvement wherein there is provided a reaction zone the
interior surfaces of which are formed primarily by said moving glass and a
heat-transfer-medium moderated, temperature-controlled wall opposed to said
2~ glass said zone having, at one end thereof, a port means at a first station
to introduce and distribute a gaseous reaction mixture comprising reactants
which form reaction products that deposit on said glass at different rates,
a gas flow path within said reaction zone wherein said gaseous reaction mix-
ture flows along said glass surface, and said temperature controlled wall to
port means at a second station at the opposite end of said zone to remove
residual gaseous reaction mixture, wherein said second station being relatively
positioned along said reaction zone with respect to said first station that
said heated glass wall of said
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reaction zone forms means to provide sufficient energy to said gaseous reaction
mixture to achieve a substantial difference in composition deposited on said
glass between the reaction gas mixture composition, as it passes between said
first station and second station and wherein said temperature-controlled wall
is maintained at a temperature low enough to avoid deposits of reaction procucts
thereon.
The invention utilizes the formation of layers of transparent mater-
ial between the glass and the semiconductor film. These layers have refractive
indices intermediate between those of the g1ass and the semi-conductor film.
With suitable choices of thickness and refractive index values of these inter-
mediate layers, it has been discovered that the iridescent colors can be made
too faint for most human observers to de*ect, and certainly too faint to inter-
ere with widespread commercial
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use even in architectural applications. Suitable materials for these inter-
mediate layers are also disclosed herein, as well as processes for the forma-
tion of these layers.
In the preferred form of the invention, these intermediate layers
blend together continuously to form a graded layer in which the refractive
index varies, preferably in a smooth transition, as one moves through the
layer away from the glass toward the semiconductor coating, from a value at
the glass surface matching the index of the glass, to a refractive index
value matching that of the overlying semiconductor film, at a point proximate
1~ to that overlying film.
A coating with refractive index varying through its thickness
may be produced by a novel method disclosed herein, in which a gas mixture
with components of different reactivities, flows along the surface of a
moving glass substrate.
In the Drawings
Figure 1 is a graph illustrating the variation of calculated color
intensity of various colors with semiconductor film thickness.
Figure 2 illustrates, schematically and in section, a non-iride-
scen~ coated glass constructed according to the invention, with an anti-iride-
scent interlayer of continuously-varying composition according to the invention.
~ igure 3 is a graph indicative of a typical gradient of refractive
indices, i`dealized, and representing the gradual transition from 100% SiO2
to 100% SnO2.
Figure 4 is a section, somewhat simplified to facilitate the
description thereof of, a novel apparatus according to the divisional appli-
cation of the type convenient for use in the process of the present invention.
Figure 5 illustrates the experimental measurement of the gradient
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~39~i2~4
in chemical composition of a silica-tin oxide gradient zone prepared according
to the invention.
Figure 6 shows an observed variation of the refractive index of
the initial deposit of SiO2-SnO2 at the glass surface, as a function of gas
composition.
Methods and As~sumptions
It is believed desirable, because of the subjective nature of color
perception, to provide a discussion of the methods and assumptions which have
been used to evaluate this invention and that of the divisional invention.
It should be realized that the application of much of the theory discussed
below is retrospective in nature because the information necessarily is being
provided in hindsight, i.e. by one having a knowlè~ge of the subject matter
di~sclosed herein.
In order to make a suitable quantitative evaluation of
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various possible con~structions which suppress iridescent colors,
the intensities of such colors were calculated using optical
data and color perception data. In this discussion, film
layersare a~ssumed to be planar, with uniform thickness and
uniform refractive index within each layer. The refractive
index changes are taken to be abrupt at the planar interfaces
between adjacent film layer~s. A continuously varying refractive
index may be modelled as a sequence of a very large number of
very thin layers with closely spaced refractive indices. Real
refractive indices are used, corresponding to negligible
absorption losses within the layers. The reflectinn
coefficients are evaluated for normally incident plane waves of
unpnlarized light.
Using the above assumptions, the amplitudes for reflection
and transmission from each interface are calculated from
Fresnel`s formulae. Then the~se amplitudes are summed, taking
into acc~unt the phase differences produced by propagation
through the relevant layers. These results have been found to
be equivalent to the Airy formulae (See, for example, Optics of
Thin Films, by F. Knittl, Wiley and Son~s, New York, 1976) for
multiple reflection and interference in thin films, when those
formulae were appled to the same cases.
The calculated intensity of reflected light has been
observed to vary with wavelength, and thus is enhanced in
certain colors more than in others. To calculate the reflected
- : : . .. : . ... . :, . ~ :. - . ::
~l3~ 4
color seen by an observer, it is desirable first to specify the
spectral distribut-ion of the incident light. For the purpose,
one may use the International Commission on Illumin~tion
Standard Illuminant C, which approximates normal daylight
illumination. The spectral distribution of the reflected light
is the product of the calculated reflection coefficient and the
spectrum of Illuminant C. The color hue and color saturation as
~seen in reflecion by a human observer, are then calculated from
this reflected spectrum, using the uniform color scales such as
those known to the art. One useful scale is that disclosed by
Hunt~r in Food Technology, Vol. 21, pages 100-105, 1967. This
~scale has been used in deriving the relationship now to be
di.sclosed.
The result.s of calculation~s, for each combination of
refractive indices and thicknesses of the layers, are a pair of
numbers, i.e. "a" and "b". "a" represents red (if positive) or
green tif negatlve) color hue, while "b" describes a yellow ~if
po~sitive) or blue (if negative) hue. These color hue results
are useful in checking the calculations against the observable
colors of samples including those of the invention. A ~single
number, "c", represents the "color satu-ration": c = (a2+b2)1/2.
Thi~s color saturation index, "c", is directly related to the
ability of the eye to detect the troublesome iridescent color
hues. When the saturation index is below a certain value, one
is not able to see any color in the re1ected light. The
g
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LZl~
numerical value of thi.s threshold saturation of observability
depends on the particular uniform color .scale used, and on the
viewing condition.s and level of illumination (see,-or example,
R.S. Hunter, The Mea.surement of Appearance, Wiley and Sons, New
York, 1975, for a review of numerical color scales.)
! In order to establish a basis fnr comparison of structures
a first series of calculations was carried out to simulate a
~single semiconductor layer on glass. The refractive index of
the semiconductor layer was taken at 2.0, which is a value
approximating tin oxide, indium oxide, or cadmium stannate
film~s. The value 1.52 was used for the glass substrate; this is
a value typical of commercial window glass. The calculated color
saturation values are plotted-in Figure 1 as a function of the
semiconductor film thickness. The color saturation is found to
be high for reflections from films in the thickness range 0.1 to
0.5 miceons~ For films thickner than 0.5 micron, the color
saturation decreases with increasing thickness. These results
are in accord with qualitative obs~rvations of actual films.
The pronounced oscillation~s are due to the varying sensitivity
o~ the eye to different spectral wavelengths. Each of the peaks
corresponds to a particular color, as marked on the curve
(R=red, Y=yellow, G=green, B=blue).
Using these results, the minimum observable value of color
saturation was e~stablished by the following experiment: Tin
oxide films with continuously varying thickness, up to about 1.5
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~3~2~
microns, were depo.sited on glass plates, by the oxidation of
tetramethyltin vapor. The thickness profile was e.stabli.shed by
a temperature variation from about 450C to 500C across the
gla~ss surface. The thickness profile was then mea~sured by
observing the interference fringes under monochromatic light.
When observed under diffused daylight, the films .showed
interference colors at the correct po.sitions shown in Figure 1.
The portion.s of the films with thicknes.ses greater than 0.85
micron showed no observable interference colors in diffused
daylight. The green peak calculated to lie at a thickness of
0.88 micron could not been seen. Therefore, the threshold of
observability is above 8 of these color units. Likewise, the
calculated blue peak at 0.03 micron could not been seen, so the
threshold is above 11 color units, the calculated value for this
peak. However, a faint red peak at 0.81 micron could be seen
under good viewing conditions, e.g. using a black velvet
background and no colored objects in the field of view being
reflected, so the threshold is below the 13 color units
calculated for this color. We conclude from these ~studies that
the threshold for nb.servation of reflected color is between 11
and 13 color units on this scale, and therefore we have adopted
a value of 12 unit.s to represent the threshold for observability
of reflected color under daylight viewing conditions. In other
words, a color saturation of more than 12 units appears as a
vi.sibly colored iride.scence, while a color saturati~n of less
11
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~L~3~Z~
than 12 units is ~seen as neutral.
It is believed that there will be little objection to
commercialization of products having color saturation values of
13 or below. However, it is much preferred that the value be 12
or below and, a~s will appear in more detail hereinater, there
app~ar~s to be no practical rea~son why the most advantageouS
products according to the invention, e.g. those characterized by
wholly color-free surEaces, i.e. below about 8, cannot be made
economically.
A value of 12 or less is indicative of a reflection which
does not di~stort the color of a reflected image in an observable
way. This threshold value of 12 units is taken to be a
quantitative ~standard with which one can evaluate the success or
failure of various multilayer designs, in suppressing the
iridescence colors.
Coatings with a thickness of 0.85 micron or greater have
c~lor ~saturation values less than this thre~shold of 12, as may
be ~se~n in Figure 1. Expèriments conirm that these thicker
coatings do not show objectionable iridesc~nce colors in
daylight illuminatinn.
Use of an Interlayer of Graduated Refractive Ind_
It has been discovered that a film intermediate betwen the
glass substrate and a semiconductor layer can be built up of a
graded com~osition, e.g. gradually changing from a silica film
to a tin oxide film. Such a film may be pictured as one
.:i, ~ ., .,: . . . .
~3~
comprising a very large number of intermediate layers.
Calculation~s have been made of reflected color saturation for a
variety of refractive index profiles between glass of refractive
index n=1.52 and ~semiconductor cnatings of refractive index
n=2Ø For transition layers thicker than about 0.15 micron,
the calculated color saturation index is usually below 12, i.e.
neutral to the eye, and, for, transitions more than about 0.3
micron~s the color i~s always undetectable. The exact shape of
the refractive index profile has very little effect on these
result~s, provided only that the change is gradual through the
graded layer.
What Materials Can be Used
A wide range of tran~sparent materials are among those which
can be selected to make products meeting the aforesaid criteria
by forming anti-iridescent undercoat layers. Various metal
~xides and nitrides, and their mixtures have the correct optical
pr~perties of transparency and refractive index. Table A lists
some mixtures which have the correct refractive index range
between glass and a tin oxide or indium oxide film. The weight
percent~s necessary can be taken from measured refractive index
versus composition curves, or calculated from the usual Lorentz-
Lorenz law for refractive indices of mixtures (z. Knittl,
~f Thin Films, Wiley and S~ns, New Y~rk, 976, page 473), using
-
measured refractive indices for the pure films. This mixing law
generally gives sufficiently accurate interpolations for optical
- , ~ . ' ' 1 . : .
~ 4'~
work, although the calculated refractive indices are .sometimes
slightly lower than the measured values. Film refractive
indices also vary somewhat with deposition method and conditions
employed.
Figure 3 gives a typical curve of refractive index versus
compo~sition for the important case of .silicon dioxide-tin
dioxide mixtures.
Table A. Some combinations of compounds yielding
transparent mixtures whose refractive indices fipan the range
from 1.5 to 2.0
S i2 SnO2
SiO2 Si3N4
SiO2 TiO2'
sio2 In203
SiO2 Cd2SnO4
Proces~s for Forming Films
Films can be formed by simultaneous vacuum evaporation of
the appropriate materials of an appropriate mixture. For
c~ating nf large areas, .such a window glass, chemical vapor
depn~sition (CVD) at normal atmo.spheric pressure is more
convenient and less expen~sive. However, the CVD method requires
suitable volatile compounds for forming each material. Silicon
dioxide can be deposited by CVD from gases such as silane, SiH
dimethyl~silane (CH3)2SiH2~ etc, Liquids which are sufficiently
volatile at room temperature are almost as convenient as gases;
. .. . . ., , ,, . . . ., .: : .
. , :. :: , . .
,: .,.. ,, ., ::, .. , ........ ~ . ,:
~3~ 4
tetramethyltin is ~such a source for CVD of tin compounds, while
d S Cl are volatile liquid sources for silicon.
(C2H5)2slH2 an l 4
A continunusly graded layer of mixed .silicon-tin oxide may
be built up during a continuous CVD coating process on a
continuou.s ribbon of glass by the following novel procedure. A
gas mixture i.s cau~sed to flow in a direction parallel to the
glass flow, under (or over) the ribbon nf hot glass, as shown,
~nr example, in Figure 4. The gas mixture contains an
oxidizàble silicon compound, an oxidizable tin compound, and
oxygen or other oxidizing gas. The compounds are chosen so that
the silicon compound is somewhat more quickly oxidized than i.s
the tin compound, so that the oxide deposited on the glass where
the gas mixture fir.st .strikes the hot glass surface, is mainly
c~mposed of silicon dioxide, with only a small percentage of tin
dioxide. The proportions of silicon and tin compounds in the
vapor phase are adjusted so that this initially deposited
material ha~s a refractive index wich closely matches that of the
glas~s itself. Then, as the gas continues in contact with the
glass ~surface, the proportion of tin oxide in the deposited
~ilmincrease.s, until at the exhaust end of the deposition
region, the silicon compound has been nearly completely depleted
in the gas mixture, and the depo.sit formed there is nearly pure
tin oxide. Since the glass is also continually advancing from
the relatively silicon-rich (initial) deposition region to a
relatively tin-rich (final) region, the glass receives a coating
.
~34;~4
with a graded refractive index varying continuously through the
coating thickness, starting at the glass ~surface with a value
matching that of glass, and ending at its outer sur~ace, with a
value matching that of tin oxide. Subsequent deposition
regions, indicated in Figure 3, can then be used to build up
Eurther layers of pure tin oxide, or layers of tin oxide doped,
for example, with fluorine.
A suitable gas mixture for this purpose, preferably
include~s the oxidizable silicon compounds, 1,1,2,2,
tetramethyldisilane (H~le2sisiMe2H); 1,1,2,trimethyldi~silane
H2MeSiSiMe2H, and/or 1,2, dimethyldisilane (H2MeSiSiMeH2)
along with tetramethyltin (Me4Sn). It has been found that the
initially depo~sited film i~s silicon-rich, and has a refractive
index close to that of gla~ss, while the later part of the
depo.sit is almost pure tin oxide.
The Si-H bonds in the above-disclosed ~silicon compounds are
highly useful in the proce~ss, ~since compounds without Si.H
bonds, such as tetramethylsil~ne M~4Si, or hexamethyldisilane
Me3SiSi~e3~ are ~xidiz~d more sl~wly than is tetramethyltin,
and the initial deposit is mainly tin oxide, and the latter part
of the deposit is mainly silicon dioxide. In such a case, i.e.
when one is using compound~s such as Me4Si, one may flow the gas
and glass in opposite directions in order to achieve the desired
gradation of refractive index, provided the gas flow is faster
than the glass flow. However, the preferred embodiment is to
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16
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.. .. ...... . . .
:; ,: , ,
~3~Z~
use the more easily oxidizable .silicon compound.s, and concurrent
gas and glass flow directinns.
It is also desirable, in forming coating.s wherein the
composition varies monntonically with distance from the
sub~strate, that the silicon compounds have a Si-Si bond as well
as the Si-H bnnd. For example, a compound containing Si-H but
not SiSi bond-s. dimethYlSilan~ Me2SiH2, along with
tetramethyltin, produces an initial deposit of nearly pure tin
oxide, which then becomes silicnn-rich at an intermediate time
and finally becomes tin-rich fitill later in the deposition.
Although Applicant does not wish to be bound by the theory, it
is believed that the Si-Si-H arrangement facilitates rapid
oxidation by an initial therma~lly induced decomposition in which
the hydrogen migrates to the neighboring silicon
~ 2H~M~2siH2+Me2si~ The reactive
dimethylsilylene Me2Si species is then rapidly oxidized,
releasing free radicals such as hydroxyl (OH~, which then
rapidly abstract hydrogen from the Si-H bonds, thu.s creating
more reactive silylene radicals, forming a chain reaction. The
tetramethyltin is less reactive to these radicals, and thus
mainly enters into the later stages of the oxidation. The
SiH2 lacks the rapid initial decomposition step, and
thus, cannot begin oxidation until after some tetramethyltin has
deC~mp~Sed to form radicals (CH3r OH, O, etc.) which then
preferentially attack the ~1e2SiH2, at int~rmediate times, until
,, -: , ; :
,~ , . .
~ ~ 3 .~A 2 ~ ~
the Me2SiH2 is consumed, after which stage the oxidation of tetramethyltin
becomes dominant again.
It is preferred to have at least two methyl groups in the disilane
compound, since the disilanes with one or no methyl substituents are spontane-
ously flammable in air, and thus must be pre-mixed with an inert gas such as
nitrogen.
Other hydrocarbon radicals, such as ethyl, propyl, etc., may
replace methyl in the above compounds, but the methyl ones are more volatile
and are preferred.
Higher partially alkylated polysilanes, such as polyalkyl-substi-
tuted trisilanes or tetrasilanes, function in a way similar to the disilanes.
However, the higher polysilanes are harder to synthesize, and less volatile
than the disilanes, which are therefore preferred.
l~hen the initial deposition of the silica-tin oxide films contain
less than about 40% of tin oxide, there will be little or no haze created at
the interface of the glass substrate and the coating thereover. If it, for
some reason, is desired to start the gradient above about 30% of tin oxide,
it is preferable to have the glass coated with a haze-inhibitlng layer, i.e.
silicon dioxide. ~uch a haze-inhibiting layer may be very thln, e.g. in the
nature of 25 to 100 angstoms.
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. ................... .. ,.~ ,. . .. ...... .
: : : : : . :: :: : :
Figure 4 illustrates a section of a lehr in a float glass line.
The structure of the lehr itself is not shown for purposes of clarity. The
hot glass 10, e~g. about 500-600C, is carried on rollers 12, 14, and 16
through the lehr. Between rollers 12 and 14 is positioned gas duct assembly
18 which comprises a gas inlet duct 20 and a gas outlet duct 22. Between
ducts 22 and 20 and separated therefrom by heat exchanging wall members 24 is
a duct 25 forming means to carry a heat exchange fluid, which, in turn forms
means to cool gas exhaust from duct 22 and to heat gas flowing through duct 20.
The temperature of the heat exchange fluid is maintained at a sufficiently
low temperature so that coating does not take place on the surface of the
inlet duct.
Gas entering inlet 20 travels through a slit-like opening 28,
thence along a reaction zone formed by the top surface 30 of duct assembly 18
and the lower surface of glass sheet 10. Upon reachinga second slit-like
opening 32, the remaining gas is exhausted through duct 22. It is during the
passage of the gas along the lower surface of glass sheet 10 that a gradient
-19-
~a , ,,
: . . . . .
: ~
. . .
~3~2~
coating is formed by the selective depletion of one of the reactants at
different points along the length of the deposition zone between rollers 12
and 14.
In the apparatus of Figure 4 a second gas duct assembly 38 is
used to complete ~he deposition of a coating, e.g. by adding a fluoride-doped
tin oxide coating to the pre-deposited gradient coating. Again, it is con-
venient to have gas enter the upstream port 28a and leave the downstream
port 32a.
The ducting is suitably formed of corrosion resistant steel
alloys and comprises a jacket 50 of thermal insulation.
Illustrative Examples of the Invention
In this application and accompanying drawings there is shown
and described a preferred embodiment of this invention and that of the
divisional application and suggested various alternatives and modifications
thereof, but it
-20-
C
~.~3~
is to be understood that the.se are not intended to be exhaustive
and that other changes and modiEications can be made within the
scope of the invention. These suggestions herein are selected
and included for purposes of illustration in order that others
skilled in the art will more fully understand the invention and
the principles thereof and will be able to modify it and embody
it in a variety of forms, each as may be best suited in the
condition of a particular case.
Example 1
Glas.s heated to about 580C is moved at a rate of 10 cm/sec
acro.ss the apparatus shown in Figure 4. The temperature of the
gas inlet duct is maintained at a temperature of about 300C, by
blowing appropriately heated or cooled air through the
temperature control duct. The first deposition region reached
by the glas~s is supplied with a gas mixture of the following
composition (in mole percent):
1,1,2,2 tetramethyldisilane 0.7%
tetramethyltin 1.4%
bromotrifluorom.ethane 2.0~
dry air balance
The second deposition region is supplied with a gas mixture of
the following composition (ln mole percent):
tetramethyltin 1.6%
bromotrifluoromethane 3.0~
dry air balance
.
~ 21
t
~3~
The flow rates of these gas mixtures are adjusted so that the
average duration of contact between a given element of the gas
mixture and the glass surface is about 0.2 seconds.~
The resulting coated gla.ss is color-neutral in appearance,
in reflected daylight. It has a visible reflectivity of 15%,
and no vi.sible haze. The infrared reflectivity is 90~ at a 10
micron wavelength. The electrical resi.stance is measured to be
5 nhms per ~square. The coating is about 0.5 micron~s thick.
Example 2
The deposition described in Example 1 is repeated, the only
difference being the composition of the gas mixture supplied to
th~ first deposition region:
1,2 dimethyldi.silane 0.4%
1,1,2 trimethyldi.silane 0,3%
1,1,2,2 tetramethyldisilane about 0.02%
tetramethyltin 1.5%
! bromotrifluoromethane 2.0%
dry air balance
The properties of the resulting product are indistinguishable
Erom tho~se of Example 1.
~ Samples of these coated glasses have been subject to Auger
j ch~mical analysis of the coating compo~sition along with ion
I ~sputter-etching to reveal their chemical composition versus
thickness. Figure 5 shows the resulting chemial composition
profile of the deposit over the region in which it varies. Near
22
.
~3~
the glass .surface the deposit is mainly silicon dioxide, with
abnut one .silicon atom out of eight being replaced by tin. A.s
the di.stance away from the glass surface increa.se.s,.the tin
concentration increases and the .silicon concentratian decreases,
so that by distances greater than 0.18 micron from the gla.s.s
~surface, the deposit becomes tin oxide, with about 1.5 percent
oE the oxygen replaced by the fluorine. Using Figure 3, the
silicon-tin composition profile is converted to a refractive
index versus distance profile, which is also plotted in Figure
5. These results confirm the ability of the disclosed process
to produce the desired variation of refractive index through the
thickness of the depo.sited film.
Example 3
A tin oxide coating is placed on a glass substrate at
difEerent thicknesses (the glass substrate is first coated with
an ultra-thin film of silicon dioxide to provide an amorphous,
ha2e-inhibiting surface.)
23
Thickne.s~s of_Tin Oxide Iridescence Vi.sibility
0.3 micron strong
0.6 micron distinct, but weaker .-
0.9 micron barely detectable except in
fluorescent light
1.3 micron weak, even in fluorescent light
The latter twn materials are not ae.sthetically objectionable for
architectural use, confirming the visual cnlor saturation scale
used to evaluate the designs.
In order to provide the most effective suppression of
iridescent color, it is desirable that the refractive index of
the initial depnsit match clnsely that of the glass substrate,
preferably to within +.04, or more preferably to within +.02
refractive index units. In order to achieve this match, one
varies the parameters of the depositinn, particularly the ratio
~f tin to .silicon atoms in the inlet gas. As an example of such
variation, Figure 6 shows the variatinn of refractive index in
the initial depnsit from tetramethyltin plus 1,1,2,2
tetramethyldisilane gas mixtures, as a function of gas
cnmpnsitinn. The other parameters for these depo.sitions were
fixed as in Example 1. Figure 6 shows, for example, that an
initial depnsit of refractive index 1.52 (appropriate to match
usual windnw glas.s refractive indices) is produced by a gas
composition of equal numbers of silicon and tin atoms. Matching
tn 1.52 +.02 is achieved when the gas compnsition is kept
.
24
between 47 and 52 a-tomic per cent of tin. While these exact
numbers may differ .somewhat in other conditions of depo.sition
such as other temperatures or other compounds, it i.s a matter of
rnutine experimentation to establish calibration curves such as
Figure 6, in order to produce a suitable match of refractive
indice~s between the substrate and the initially deposited
coating compo.sition.
It is to be noted that the reflection of light from the
~surface of the coated products of Example 3 is about 16 to 17~,
i.e. about 10~ higher than that from the coated gla.ss in
Example.s 1 and 2 which do have a graded undercoat according to
th~ invention.
It is also to be understood that the following claims are
intended to cover all of`the generic and .specific features of
the invention herein described and all statements of the scope
oE the invention which might be .said to fall therebetween.
.: :., - . . ~ : ^.-
