KIA SEALANTS
KIA has glasses available that were developed to seal to a variety of materials, ranging
from quartz (thermal expansion of 5 X 10-7/C) to copper (thermal expansion of 170 X
10-7/oC). Glass-glass, glass-ceramic, glass-metal, metal-metal,
ceramic-metal and ceramic-ceramic seals can all be made by using the proper sealant. We have sealant glasses that seal at both high and low temperatures to meet varying application needs. Refer to
Table 1 for a list of those glasses developed for sealant application.
KIA's sealants fall into two general categories:
1.) Vitreous Type
2.) Devitrifying Type
The vitreous type sealants are non-crystallizing glasses which can be reworked repeatedly and still maintain the same characteristics and properties.
The devitrifying type sealants crystallize during the curing cycle and form a two phase
glass-ceramic material which is usually translucent or opaque. Upon reheating, (even to temperatures above the initial curing temperature), these glasses remain stable,
A special type of devitrifying glass is conductive solder glass. Conductive solder glasses are recommended for use as a seal for a low electrical resistivity path.
USES AND METHODS OF APPLICATIONS
Vitreous Type Sealant Glasses
Vitreous glasses are considerably easier to apply than the devitrifying type and require a shorter firing time to effect complete sealing. They may be used in small seal areas where there is not much stress in the seal at higher temperature. The lower temperature vitreous sealants can be used in applications where it may be necessary to readily separate and reseal the component parts. Normally, the vitreous type sealants do not offer the strength of the devitrifying types. They also have somewhat higher electrical resistivity and a lower dielectric constant than the devitrifying type.
There are two general methods of application for vitreous type sealant glasses:
(1 .) Hot Method of Application
This "hot dip" method is generally more suitable for the lower temperature glasses. The glass is heated in a platinum crucible by either gas flame or electrical radiant heating elements to a temperature above the working point, where it is very fluid. The component parts are preheated (for glass parts preheat to about 200C above the annealing point). Each part is dipped into the molten sealant glass for
10 - 20 seconds, withdrawn and allowed to "set" in air for about 5 seconds. The parts are then immediately placed in an oven set at the annealing point of the sealant material. They are held at this annealing temperature for
15 - 20 minutes, then cooled at a rate not to exceed 50C per minute.
The cooled parts can be sealed together by aligning their sealing edges and placing them in an oven, under load. They are sealed according to the recommended cycle, heating at
10C per minute and cooling to room temperature at a rate not greater than 50C per minute. (For large glass parts with thick sections, reducing the cooling rate to as low as
1OC per minute is recommended).
After dipping, the parts can also be sealed together immediately, while the sealant glass on the sealing edges is still soft, by properly aligning and pressing the parts together. The sealed assembly then is placed in a hot annealing oven, annealed and cooled to room temperature.
Satisfactory seals can be made by having just one of the component parts pretreated with sealant, but it has been found that better seals can be obtained by coating both edges prior to sealing. Generally, seals made by the "hot dip" method show a higher seal strength than that of the "cold paste" method.
(2 .) Cold Method of Application
In the "cold paste" method, the finely ground glass powder is mixed with a vehicle to form a paste. The vehicle must readily decompose below the softening point of the glass so that it does not cause contaminated or seedy seals. It is suggested that the mixture be thoroughly blended just prior to application. The desired viscosity of the paste will be controlled by the type and amount of vehicle used. A thick paste would be suitable for spatula or extrusion application. The thinner pastes are applied by spray, roller coat or screening. The parts can also be dipped into a very thin paste. Satisfactory seals are usually obtained when the paste is applied to only one surface. However, both surfaces can be coated to insure good seals. Depending upon the vehicle used, the coated parts may have to be dried to evaporate volatile organics before firing.
For sealing, the parts are then placed in an oven, exercising care that the sealant glass coating is not damaged. A maximum heating rate of
10C per minute is recommended. Parts can be held at the recommended sealing temperature for just a few minutes, or up to one hour. The maximum cooling rate should be 5°C per minute. In some cases it may be necessary to apply additional loading to the parts being sealed to produce proper sealant flow and proper seal line thickness. Maximum seal strength is obtained at thickness of 3 to 10 mils.
Devitrifying Type Sealant Glasses
Devitrifying type glasses are very stable crystalline materials. They may be compared to
thermo-plastic's, in that the glass after maturing has properties which enable it to be reheated to a reasonably high temperature without showing any glass flow. The Initial firing or curing cycle for these glasses
is very important to achieve proper crystal growth and the strongest seal.
Method of Application
For the devitrifying type sealants, the accepted method of application is to use a finely ground glass in paste form and to make either a "cold dip" seal or to apply the paste in the same manner of application as for vitreous glasses.
When the ground form of the sealant glass is heated, it first melts to a vitreous
state. In this state, the glass flows relatively well and can wet the material being sealed, thus contributing to a strong bond. On heating still further, the glass begins to devitrify and the temperature to which it is heated determines the type and size of crystals which will be formed. This size and type of crystal
in turn controls the thermal expansion of the fully matured glass, so that some variation of thermal expansion properties will occur with variation
in heating conditions. Sufficient time must be allowed to completely mature the sealant glass at the temperature desired. Otherwise, on reheating, stress changes will take place which may break the seal. Seals can be made at temperatures higher than the recommended temperature, but the strength decreases considerably.
It is also necessary to observe some degree of care, in heating and cooling the sealed article, because the glass seals tend to go through a region of relatively high temporary stresses at temperatures between their maturing
temperature and room temperature. (See temporary stresses in seals.)
Conductive Sealant Glasses
Conductive glasses are formed by combining devitrifying type glasses with selected additives. These materials can be applied and sealed in the same method as a regular devitrifying type glass.
However, they are extremely temperature sensitive. Overheating will reduce or destroy the electrical properties, while
under heating will produce seals that may leak. The vitreous range is not as great as for other devitrifying types of sealants; therefore some pressure is usually necessary to cause the glass to flow and wet the base material. A seal with our commercial conductive sealants is best made at a temperature of
425C plus or minus 50C for one hour. Reheating of the seals can be accomplished if the temperatures do not exceed the recommended maximum of
450C. Protracted heating, or many
reheat cycles at 450C should be avoided.
Special Processing for Conductive Sealants
In most applications involving conductive glass, electrical leads must be attached to the glass. The
following three methods have been used successfully:
1 .) Metal leads made from 18% chrome-iron alloy (#430 type) ribbon may be imbedded in the glass during application. After firing, standard connections can be made to the metal leads. Care should be taken to avoid flexing of the metal at the
glass-metal interface to avoid seal fractures.
2 .) A silver paint (such as Dupont #6704) may be applied over the dry surface of the glass before firing. After firing, a
glass-bonded surface will result to which soft soldered connections can be made.
3 .) Air drying silver paint (such as General Cement Company #212), may be painted on after firing.
Contact-type leads can then be applied to this paint by pressure.
FORMS AVAILABLE
KIA can supply glasses in different forms to meet varying needs. The forms generally used are detailed below:
Ground Material .
The most common form available is as a powdered or ground material. The glass is ground to a specified particle size distribution. It can be ground dry, or as a slurry in water or alcohol. Standard grinds are available, or we can supply a grind to meet a customer's particular application need. The glass is classified according to the percent of material that is passed through or retained on US Sieve sizes of standard mesh screens. For example, a glass that is identified as minus 100 mesh, plus 400 mesh
(100m+400m) means that all the glass will pass through a 100 mesh screen and be retained on the 400 mesh screen.
The table below lists the U.S. Sieve Series that KIA uses and the mesh opening sizes for them.
Opening Size
Sieve Size Inches
Microns
Millimeters
No.
40
.0165 420
0.42
80
.0070 177
0.177
100
.0059 149
0.149
120
.0049 125
0.125
140
.0041 105
0.105
170
.0035 88
0.088
200
.0029 74
0.074
230
.0024 62
0.062
270
.0021 53
0.053
325
.0017 44
0.044
400
.0015 37
0.037
Our standard grinds are all 100% - 100m with the following percent of material passing through the 400 mesh screen:
"A" Grind 67 + 5% -400m
"B" Grind 87 + 5% -400m
"C" Grind 99.5% -400m
Micro-Rods
Another form available is the micro-rod or fiber. These fibers are redrawn from a larger rod to diameters ranging from .005" to .250". The shapes are usually round, square or rectangular. Upon request,
KIA can furnish information on special shapes or sizes.
Miscellaneous Forms - KIA will also furnish some of our glasses as frit or flakes or as a solid slab or rod.
TEMPORARY STRESSES IN SEALS
Vitreous Glasses
The stresses that develop in a glass seal during the heating and cooling associated with annealing or further processing are an Important factor to consider when selecting a glass to seal to metals or other materials.
Stresses develop in a seal due to the difference In the contraction (or expansion) curves of the materials used in the seal. A vitreous glass will normally
'set-up' at about 10 to 150C below the annealing point arid the final (room temperature) stress can be estimated by comparing the curves at that temperature. If two glasses are being sealed together, the stress calculation should be made at the setting temperature of the lower glass.
(To calculate the amount of stress In a seal, read the elongation values on both curves and multiply the difference of these by 50. This gives an approximate seal stress in PSI. The material with the higher value will be in tension.)
The seal may also undergo temporary stresses that build up during heating and cooling due to the different shapes of curves. These temporary stresses may be much higher than the final stress and may be in a different direction (tension or compression). A seal that goes through changes in direction (tension to compression to tension) may be more apt to fail much as a wire will break if bent back and forth. In addition to the magnitude and direction of the stress, the temperature at which the highest stress occurs Is also important in order to avoid holding the sealed part at this temperature for prolonged periods of time.
NOTE: A temperature difference will exist between the metal and glass parts of a seal during heating and cooling. The metal "leads" the glass due to Its higher thermal conductivity. which means that the metal is warmer during heating and cooler during cooling. This temperature difference will appear as additional tension In the temporary stress during heating and additional compression during cooling. seducing heating and cooling rates will reduce these stresses.
Crystalline Materials
The devil devitrifying-type materials 'set-up' at the firing temperature and the final stress can be approximated by comparing the curves at this temperature.
(Refer to Thermal Contraction Curves)
SEE TABLE HERE
NOTES:
1. For crystallizing glasses, the contraction coefficient is measured from the
recommended sealing temperature to 25° C.
2. The annealing point and the softening point apply only to the vitreous glasses.
3. These temperatures can be tolerated for up to ten (10) minutes.
4. ASTM Metal Designations:
F-15 29% nickel, 17% cobalt, 53% iron (eg. KOVAR® , RODAR® ,
THERLO® )
F-29 Copper clad - 42% nickel, iron leg. Dumet)
F-30 Nickel-iron alloys: 42% nickel, 44% nickel, 46% nickel 49% nickel, 52% nickel
F-31 42% nickel, 5.5-6% chromium, balance iron (eg. Sylvania #4, Carpenter #426)
F-256 18% chromium, 82% iron (eg. #430 alloy)
F-257 28% chromium, 729/o iron (eg. #446 alloy)
