A New Type of In/Cu/In Seal for Vacuum Glazing
Jun Fu Zhao, Philip C. Eames, Trevor Hyde, Yueping Fang, and Jinlei Wang
Centre for Sustainable Technologies, School of the Built Environment, University of Ulster,
Newtownabbey, Co. Antrim BT37 0QB, Northern Ireland, UK
Abstract
Vacuum glazing provides good thermal insulation and has a significant potential for energy
saving. To achieve a high level of a vacuum inside the glazing, the edge seal formation is a
critical step in the manufacturing process. A new type of indium-copper-indium (In/Cu/In) seal
was developed recently at the University of Ulster. Introducing a solid copper wire into the
indium seal improves the quality of the seal reducing the quantity of indium required and
reducing outgassing from the seal and permeation of atmospheric gases through any weakly
bonded areas after the vacuum glazing is formed. Adding copper can significantly reduce the
manufacture cost of vacuum glazing.
The experimentally characterized glazing produced by an In/Cu/In seal has demonstrated
excellent performance. The lowest U-value successfully achieved for a 400mm by 400mm
vacuum glazing sealed by In/Cu/In is 1.00± 0.04 Wm-2K-1.
1. Introduction
Vacuum glazing can provide good thermal
insulation while maintaining high levels of
visual transmittance and thus provides
significant potential for energy saving. To
achieve theoretical thermal performance, a
high internal vacuum (<0.1 Pa) with a leakfree seal around the edge between the glass
sheets has to be met. Several sealing
methods have been reported and patented
since the first vacuum glazing was detailed
in a German patent [1]. Among those,
Collins et al have fabricated vacuum glazing
using solder glass to form a contiguous edge
seal at a temperature of around 450C [2-4].
An attempt to form vacuum glazing using a
high temperature laser technique was not
successful [5]. Techniques used for the edge
seal at high temperature either using solder
glass or applying a laser restrict the use of
glass sheets to those only with hard low
emittance coatings. Soft low emittance
coatings can not survive such temperatures
without excessive degradation. In order to
avoid thermal degradation of the coatings
during the high temperature process, the
University of Ulster has developed a low
temperature technique to cerate a novel edge
seal, comprised of an inner vacuum seal
formed from indium and an outer adhesive
seal [6-7].
Indium is an extremely malleable metal with
excellent fatigue resistance. Due to its
ductility, flexibility, low melting point, and
unique creep characteristics, indium has
been widely used in various areas including
cold welding, static seals in cryogenics and
high vacuum environments, solder alloys,
and forming bonds in a variety of non-metal
applications [8-9]. Indium is very applicable
when large plastic strain of the joint is
required to release stresses induced by
thermal expansion mismatch. It is because
of these characteristics, indium was selected
by the University of Ulster in 1998 and used
to make a leak-tight seal for vacuum glazing
at a low process temperature [6]. Since then,
more research has been undertaken to
improve the indium sealing technique. In
this work, an indium-copper-indium
(In/Cu/In) multilayer for the effective edge
seal of vacuum glazing was developed.
Introducing a solid copper wire into the
indium seal improves the quality of the seal
reducing outgassing from the seals and
permeation of atmospheric gases through
any weakly bonded areas after the glazing
is formed. A significant reduction in the
manufacture cost of vacuum glazing is also
possible.
To assess the thermal performances of the
vacuum glazing samples sealed by In/Cu/In,
the samples were characterised using a
guarded hotbox calorimeter and theoretically
analysed using a finite volume model. The
experimentally determined U-values for the
vacuum glazing were also compared with
those predicted theoretically. An additional
investigation of temperature-induced stress
further gives an estimation on the quality
and durability of vacuum glazing fabricated
using this novel seal.
2. Seal design consideration
To join two glass panes together and to have
a leak-free hermetic seal, requires a surface
that is free of oxides and contaminants.
Usually, indium readily forms oxides on its
surface. Thus, to bond indium to a nonmetal substrate such as glass, the surface
oxides have to be removed or destroyed by
compression or plastic deformation during
the bonding process. However, compression
of the seal is prohibited in the vacuum
glazing manufacturing process due to the
fragile nature of glass. In addition to the
oxide on the indium surface, glass used for
making vacuum glazing is normally coated
with a low emittance layer, this will also act
as a solid barrier isolating indium from the
glass surface to be bonded. In order to
remove the oxide surface layer and create a
fresh surface for the formation of bonds
between the indium and the glass, an
ultrasonic
soldering
technique
was
introduced into the manufacturing process to
allow the two glass panes to be joined. The
reason for choosing the ultrasonic soldering
technique is because it can remove surface
oxides and it promotes indium to wet and
bond to the glass surface without requiring a
flux to remove oxides but which will lead to
outgassing afterwards. In addition, as the
ultrasonic soldering process is localized and
bonding time is short, thermal degradation
of the low emittance coatings caused by
high temperature processes can be
efficiently prevented.
Ultrasonic soldering is based on the same
principles as ultrasonic cleaning. If applying
a high-frequency vibration (typically 20kHz)
into the molten solder, the vibration energy
produces cavitation at the interface between
the solder liquid and the solid surface, and
consequently a strong erosion effect on the
solid surface in proximity to the cavitation
[10]. This cavitation breaks up, disperses the
base surface oxides, and allows the molten
solder to wet and bond to a nascent solid
surface [11]. Similar phenomena are
expected in the soldering process for the
indium and the glass surface. However, due
to the ultrasonic high-frequency vibration,
the cavitation produced can also trap air or
gases while increasing capillary action in the
indium liquid phase. This drawback leads to
the formation of defects in the seal such as
micro pinholes or voids with trapped air
inside. One of main reasons leading to
degradation of the vacuum inside the glazing
is because of outgassing from trapped air
from the voids in the indium and permeation
of atmospheric gases through these weakly
bonded areas during the glazings service life.
The indium bond between the glass sheets is
relatively weak and can be damaged by
stresses caused by applied environmental
conditions; any defects appearing in the seal
will weaken its strength, leading to potential
degradation of the vacuum quality and
reduction in the service life of the vacuum
glazing. To reinforce the seal strength and
avoid vacuum degradation within the
glazing, the previous method of forming an
indium seal used in the low temperature
process developed at the University of
Ulster has been improved.
Figure 1 shows several seal design
considerations for the fabrication of vacuum
glazing. In most cases, as shown in Fig.1 (a),
the energy generated by an ultrasonic
soldering iron is effective in promoting
wetting of the glass surface, which enables
the glazing to have tight hermetic seals
around the edges when the sample is formed.
As mentioned above, if any defects
produced by the ultrasonic soldering process
exist in the seal (see Fig.1 (b)), it inevitably
weakens the seal. To minimize the amount
of such defects formed during the ultrasonic
soldering process, a solid material such as
copper wire or ribbon can be embedded into
the indium layer during pre-coating to form
a sandwich seal. This type of seal will be
expected to consolidate the seal strength and
to obtain a high vacuum inside the glazing
after evacuation (see Fig.1(c) - (d)). As pure
copper is oxygen free, soft and easily
bonded with indium, and also low cost
compared to indium, it was selected as a
suitable material and used to form an
In/Cu/In sandwich seal.
Good vacuum inside
Indium seal
without defects
(a)
installed facing each other inside the
vacuum gap. Prior to fabrication, a 3mmdiameter hole was predrilled into one of the
glass panes to provide evacuation access that
is sealed by an indium precoated glass cover
disc when the sample is formed. The glass
panes were initially cleaned using acetone
and isopropyl alcohol, and then baked at
240ºC inside an oven. The edge seal was
applied with a soft copper wire embedded in
indium. The support pillars manufactured
from stainless steel have a diameter of 0.30.4mm and a height of 0.15mm to give a
vacuum glazing with a very narrow space in
which conductive and convective heat
transfer across the space is eliminated.
Pillars
Glass
Poor vacuum inside
(b)
Indium seal with
voids or pinholes
Glass disc seal
In/Cu/In seal
Glass
Support pillars
Low-e coating
Vacuum space
Evacuation
hole
Perfect vacuum
inside
In/Cu/In seal
without defects
(c)
Figure 2 Schematic diagram of a vacuum
glazing sealed by In/Cu/In evacuated using a
modified pump-out technique
Glass
Good vacuum inside
In/Cu/In seal with
a few defects
(d)
Glass
Figure 1 Different seal designs for vacuum
glazing: (a) single indium seal without defects;
(b) single indium seal with voids or pinholes; (c)
In/Cu/In seal without defects; (d) In/Cu/In seal
with a few defects.
3. Fabrication process
Figure 2 shows a schematic diagram of a
vacuum glazing. The size of the 4mm thick
glass panes chosen for this work was
400mm by 400mm; low emittance coatings
are present on one side of each glazing face
The manufacture process used for producing
vacuum glazing includes the following steps:
glass preparation, indium soldering and
copper embedding, support pillar placing,
edge seal formation within the vacuum
chamber at low temperature, evacuation
through a modified pump-out system and
sealing the access hole of the vacuum
glazing in a bakeout oven. In brief, when the
glass preparation was complete, the cleaned
glass was transferred to a hot plate for
indium edge seal coating. A 0.05mm thick
indium layer of 6mm width was
ultrasonically applied along the edges of the
glass sheet. A 0.15mm diameter copper wire
(oxygen free) was then embedded into the
indium coating on one of the glass sheets.
The excellent bonds between the indium and
the glass, the indium and the copper were
formed by ultrasonic soldering. Once the
indium coating and embedding of copper
around the edges is completed, the support
pillars were positioned onto one of glass
sheets in a regular 25mm x 25mm array
using a vacuum picking and placing tool.
The upper glass sheet was then located to
align with the lower glass sheet. The initial
outgassing from the internal surface was
carried out within a vacuum chamber at a
pressure of around 10-5 Pa. The two glass
panes can then be soldered together with the
leak-tight edge seal of the glazing formed
through indium reflow at a temperature
below 200ºC. With an air tight edge seal, the
glazing sample can be efficiently evacuated
using a modified pump-out system, which
has been described in detail elsewhere [12].
To minimize outgassing, the evacuation was
carried out during the baking process in the
bakeout oven. When the internal pressure
between the two glass panes was down to
below 10-5 Pa, the access hole was sealed.
4. Thermal performance
To assess the quality and the thermal
performances of vacuum glazings sealed by
In/Cu/In, two samples A and B were
characterized using a guarded hotbox
calorimeter. The guarded hotbox allows
measurement of heat flow through the
vacuum glazing for given air temperatures
within its warm and cold chambers. Heat
transfer across vacuum glazing including
contributions from pillar conduction,
radiation, and edge seal conduction were
calculated using a finite volume model
described in detail elsewhere [13], and
compared with experimental measurements
of heat transport through the vacuum glazing
sample.
Table 1 summarizes the experimental
thermal data collected from two vacuum
glazing samples, including test ambient
temperatures and temperature differences
between warm and cold sides of the
chamber and the vacuum glazing. The
temperature differences between warm and
cold sides of the vacuum glazing were
obtained after the temperatures had
stabilized. It means that the data was taken
only after possible internal surface
outgassing, which is known to degrade
thermal insulation performance significantly
and results in a decrease in the temperature
difference from a maximum value to a
stabilized value. For example, both samples
produced using 0.15mm copper embedded
indium seal reached a
maximum
temperature difference of 14ºC between the
warm and cold sides, which is the highest
value predicted theoretically for a glazing
sample of 400mm by 400mm under the
given test conditions, but the stabilized
temperature difference was around 11-12ºC.
Copper
T (ºC)
No.
 (mm)
Ambient
Temp.
T (ºC)
Chamber
Glazing
A
0.15
10.38
21.40
12.18±1.66
B
0.15
20.14
18.03
11.09±2.08
Table 1 Temperature differences measured
between the warm and cold chambers and the
warm and cold sides of the vacuum glazing
samples prepared using an In/Cu/In seal.
In order to further assess the quality and
durability of the vacuum glazing samples
fabricated using an In/Cu/In seal, an
additional investigation of the temperatureinduced stress was also carried out on
sample A. The thermal performance of this
sample was repeatedly characterized after
stress tests. Figure 3 shows the measured
and predicted exposed glass surface
temperature profiles along the centre lines
for sample A before and after the stress test.
Air temperatures measured in the warm and
cold chambers were 27.2C and 10.3C for
Fig. 3(a), 26.3C and 4.9C for Fig. 3(b),
respectively. It can be seen that measured
temperatures on the glass surfaces agree
with the predictions to within an
experimental error of 4%. Comparing with
the results obtained for the indium sealed
vacuum glazings in the previous paper [12],
the high temperature differences between
the warm and cold sides of the vacuum
glazing (before and after the stress test)
clearly indicate that the thermal insulating
property of sample A is significantly better.
value for a 400mm by 400mm sample. The
experimentally determined overall heat
transfer coefficients are in good agreement
with the predictions made with the model to
within experimental error.
28
Temperature
(ºC)
(a)
24
20
Without residual gas conduction
16
12
Stress
8
Predicted U-values
Experimentally Measured
(W m-2 K-1)
U-values (W m-2 K-1)
Test
Warm side predicted
Warm side measured
4
Cold side predicted
Cold side measured
Ucentre
Uw
Ucentre
Uw
Before
1.00
1.19
1.00 ± 0.04
1.10 ± 0.10
After
1.00
1.19
1.06 ± 0.08
1.19 ± 0.10
0
0
30
60
90
120
Distance from sightline (mm)
150
180
28
Table 2 Overall heat transfer coefficients of
central glass and total window areas of the
vacuum glazing sample A sealed by In/Cu/In
before and after the stress test.
(b)
(ºC)
20
Temperature
24
16
Vacuum level < 1.0 Pa
12
8
4
Warm side predicted
Warm side Measured
Cold side Predicted
Cold side Measured
0
0
30
60
90
120
150
180
Distance from sightline (mm)
Figure 3 Measured and predicted glass surface
temperature profiles along the centre lines of the
vacuum glazing sample A: (a) before the stress
test, the air temperatures in the warm and cold
chambers were 27.2C and 10.3C; and (b) after
the stress test, the air temperatures in the warm
and cold chambers were 26.3C and 4.9C.
Table 2 gives the experimentally determined
and predicted overall heat transfer
coefficients of the total window areas and
the central glass regions. From Table 2, it
can be noticed that the overall heat transfer
coefficient in the central glass region
determined by experiment for sample A is
1.00 ± 0.04 Wm-2K-1 before the stress test,
which is the lowest theoretical predicted
value, and increases to 1.06 ± 0.08 Wm-2K-1
after the stress test. The thermal
performance of sample A is still excellent
and gives a temperature difference of about
12°C between the warm and cold sides of
the glazing. This result clearly shows that
vacuum glazing using an In/Cu/In seal can
achieve the lowest theoretically predicted U-
From the results obtained from the stress test
given elsewhere [14], it was found that with
an In/Cu/In seal, vacuum glazing samples
can not only withstand the stresses under
standard test condition, but also can survive
without degradation and cracking when
subject to test temperature differences from
0°C to 35°C between the warm and cold
glazing surfaces. This again supports the
theory that copper wire embedded in an
indium seal plays an important role in
enhancing the seal strength leading to a
potential longer service life for indium
sealed vacuum glazing. In addition, it was
found that the heat transfer due to residual
gas in the vacuum space of sample A has
been essentially eliminated. This further
confirms that a pressure of less than 0.1Pa
has been achieved by using this novel
sealing technique, which can efficiently
avoid or stop migration of atmospheric gases
through the edge seal and decrease the
possibility of degradation of the vacuum due
to outgassing from air trapped in the edge
seal after the manufacture of the sample.
5. Conclusions
Vacuum glazing samples have been
successfully fabricated using a In/Cu/In
multilayer seal. Adding a copper wire into
the indium seal for vacuum glazing has
several advantages compared to previous
indium seals. One, applying In/Cu/In seal is
a low temperature process and can avoid
thermal degradation of the coatings during
high temperature processes. Two, the solid
copper wire can act as barrier wall reducing
outgassing from inner indium seal and
migration of atmospheric gases through any
weakly bonded or defect areas of the seal.
Three, due to the ductility of indium, it can
enable the copper to expand and contract
under various ambient temperatures without
cracking the seals over a long service period.
In particular, embedding copper into the
indium seal can enhance the overall strength
and durability of the seal and allow it to
withstand any stresses induced by extreme
conditions. Four, adding copper can greatly
reduce the manufacture cost of vacuum
glazing.
The experimental characterization of a
vacuum glazing fabricated by an In/Cu/In
seal demonstrates excellent
thermal
performance with an overall heat transfer
coefficient in the central glass region of 1.00
Wm-2K-1, which matches the lowest U-value
predicted by theoretical calculations. With
such an edge seal, the glazing samples can
withstand temperature-induced stress with
no degradation and cracking under standard
or even extreme test conditions. The thermal
performance of the vacuum glazings
determined experimentally is in good
agreement with theoretical predictions.
Acknowledgement
This project is supported by the UK
Engineering and Physical Sciences Research
Council,
Swindon
UK
through
GR/S08251/01.
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