Extraterrestrial Thermal Insulation
Developer:
Dr. Jeff Smith and Todd Sander
Project for Grade Level:
High School
Discipline:
Materials, Physics, Chemistry
Topic Area:
Density, Volume, Binders
Time Required:
Goals:
Understand how common dense materials can be formed into
lightweight objects and what properties affect thermal
conductivity.
Objectives:
Fabricate a high temperature resistant, ultra lightweight silica fiber
insulation tile.
Introduce the students to fabrication processes.
Understand how colloidal solutions behave as binders.
Materials:
High purity silica fiber slurry
Sodium Silicate/Colloidal Silica solution
Mold AssemblyCylinder, plunger, 60M screen, basin
Filter paper 5.5cm
Balance
Fiber Furnace
Refractory setter plate
University of Missouri-Rolla – Ceramic Engineering Department
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533559890
Safety Precautions:
Follow the instructions. Wear proper safety equipment such as goggles
and vinyl gloves. Wear temperature resistant gloves during the use of the
furnace.
Procedure:
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Assembly the mold system as shown in the diagram.
Calculate the amount of fiber slurry required to produce a fiber compact
with a density of 0.12g/cm3. See Calculation Sheet.
Pour the fiber slurry into the top of the cylinder.
Insert the plunger and slowly compress the slurry until the slurry height is
1cm.
Gently remove the plunger and allow the compact to drain thoroughly.
Calculate the amount of silicate solution required. See Calculation Sheet.
Dilute the required amount of silicate solution with deionized water to a
volume that is 2 times the volume of the compact.
Pour the silicate solution into the cylinder.
Place a filter paper disk over the opening then insert the plunger and apply
pressure to aid in the filtration.
Remove the plunger when the compact has been compressed to 1 cm.
Allow the compact to fully drain.
Carefully remove the cylinder and allow the compact to dry in the ambient
air.
When the compact is dry enough to handle without damaging it, move it to
the setter plate.
Weigh and measure the diameter and height of the compact.
To dry the compact, place it in a dryer or oven, drying at 50°C (122°F) for
12h then 110°C (220°F) for 12h.
Transfer the fiber compact on the setter plate to the kiln.
To sinter the compact, set the kiln controller on 2 for the first hour, then 4
for the second hour and 5 for the final hour. When the kiln has reached
900°C turn the controller to 0 and unplug the kiln. Allow the compact to
cool without opening the chamber.
Weigh and measure the diameter and height of the tile.
Observations and Questions:
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Calculate the green (unfired) and fired densities of the compact.
Calculate the weight loss and shrinkage.
How would you create tiles of different densities?
Would tiles with different densities have different thermal properties?
What behaviors of the silicate solution make it a good binder for a fiber
compact?
University of Missouri-Rolla – Ceramic Engineering Department
http://campus.umr.edu/ceramics
533559890
Teacher’s Note:
Supplemental MaterialRicherson, David W., The Magic of Ceramics, The American Ceramic Society, 2000.
ISBN 1-57498-050-5, p.232-236 “Aerospace Uses for Ceramics”.
Vocabulary:
Ceramic-
any class of inorganic, nonmetallic products including metallic oxides,
borides, carbides or nitrides and mixtures or compounds of such materials
Colloidal-
a substance in the form of submicroscopic particles which do not settle out
when in solution or suspension
Flux-
any substance which promotes the fusion and flow of a ceramic or glass
mixture when subjected to heat
Insulation-
thermal property, any material which will prevent or retard the transfer of
heat
Polymerizationa chemical reaction in which two or more small molecules
combine to form larger molecules
Refractory-
the property of being resistant to high temperatures
Sodium silicate- aqueous solutions produced by fusing varying proportions of sand
(SiO2) and soda ash (Na2CO3); these solutions have physical and
chemical properties that are useful in bonding and coating applications.
Silica-
SiO2, the most common mineral in the majority of sands
Sintering-
the bonding of powdered materials by solid-state reactions at temperatures
lower than those required for the formation of a liquid phase
Slurry-
a mixture or suspension of ground glass, clay or other ceramic materials in
water or other liquid
Sol-
a fluid colloidal system
University of Missouri-Rolla – Ceramic Engineering Department
http://campus.umr.edu/ceramics
533559890
Background Information:
The NASA’s space shuttle must withstand extremes in temperature during a mission.
The highest temperatures are seen when the shuttle is entering the earth’s atmosphere on
its way home. Most of the insulating material used is based upon ceramic materials.
In areas where ceramic tiles are used, temperature can reach around 1250C. The basic
material used to make these tiles is a very fine glassy fiber made from silica (SiO2). The
silica used is common sand, the same stuff you find under your feet on a sandy beach—
the only difference being that this silica is nearly 100% pure. The silica is melted, spun
into fibers as it cools, purified by cleaning it with acid, then finally chopped to lengths of
less than a millimeter. The fibers that are about 100 times smaller in diameter than a
human hair are formed into blocks that can be machined into nearly any shape. Under a
microscope, the fibers that make up the tiles appear stuck together at their ends forming a
web of fibers with large open spaces. It’s the large open spaces that make these materials
good heat insulators.
To manufacture the tiles, chopped fibers and a colloidal silica binder are added to water
to form slurry that is poured into a mold and cured, forming a soft, porous compact.
Additional colloidal silica binder is soaked into the green tiles and allowed to dry.
In this experiment, the same silica fibers will be used but a sodium silicate solution will
be used in the place of colloidal silica so that the tiles can be processed at less than
1000°C. Slurry of high purity silica fiber will be placed in a cylinder mold and filtered.
The plunger will assist in liquid removal and allow us to compress the fiber matt to a
desired thickness. The resulting soft porous fiber matt can now be infiltrated with a
silicate binder solution to give it the necessary cohesion and strength. The wet compact
will consist of fiber, a silicate coating and water. Drying reduces the free moisture
content and allows the silicate coating to gel. During firing, the silicate gel will undergo
polymerization, releasing additional moisture. As the temperature is increased further,
the fiber-fiber contacts undergo sintering and densification.
The resulting tile will be light weight, refractory with a low thermal mass and thermal
conductivity.
Colloidal Silica Binder Chemistry
The particles in a colloidal silica sol are discrete uniform spheres of silica (nanometer
size) which have no detectable crystals. They are opalescent or milky white liquids. Most
are dispersed in an alkaline liquid which reacts with the silica to surface to form a
negative charge. The particles repel one another forming a stable dispersion. The sols can
be gelled by removing water, changing the pH or adding a salt to the solution. Higher
temperatures increase the evaporation of water but also increase the movement of the
colloidal particles in suspension. Changes in pH and the additional of salts alter the
University of Missouri-Rolla – Ceramic Engineering Department
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533559890
surface charge or reduce the repelling forces. In all cases, the particles are allowed or
forced to move closer together. This particle interact results in gelation. If the gelled
silica is dried further, hydroxyl groups on the surface of the particles are removed and SiO-Si bonds are formed resulting in polymerization. When these silica particles are mixed
with other granular or fiberous materials, they are effective binders for those materials.
Dried colloidal silica is heat resistant and chemically inert. If the colloidal silica is further
heat treated at an elevated temperature, the silica particles sinter together and with the
other granular of fiberous materials co-mixed with them.
Material Data Sheet-
Fiber Specifications
Chemistry:
Diameter:
Length:
Specific Gravity:
100% Amorphous Silica
1.4 m
3.0 mm
2.19
Sodium Silicate N Specifications
Specific Gravity:
1.38
Yield on Ignition:
37.6 wt.% Na2O/SiO2
Mold Specifications
Diameter:
5.2 cm
Slurry Specifications
Fiber Concentration: 3.0 wt.%
Dried Tile Target Specifications
Specific Gravity:
0.15
Diameter:
5.2 cm
Thickness:
1 cm
University of Missouri-Rolla – Ceramic Engineering Department
http://campus.umr.edu/ceramics
533559890
Calculation SheetCalculate the volume of the fiber compact if the tube diameter is 5.2cm and the compact
1cm thick.
If the density of the compact fiber matt is to be 0.12g/cm3, calculate the number of grams
of fiber needed for a 1 cm thick compact.
If the fiber slurry contains 3wt% fiber, how many grams of fiber slurry solution are
needed for a 1 cm thick compact?
If the final density of the compact is to be 0.15g/cm3, how many grams of silica are
needed?
To obtain the gram of silica needed, how many grams of colloidal silica solution are
needed?
University of Missouri-Rolla – Ceramic Engineering Department
http://campus.umr.edu/ceramics
533559890
Calculation Sheet- Solutions
Calculate the volume of the fiber compact if the tube diameter is 5.2cm and the compact
1cm thick.
Volume= hπr2 = (1cm)(3.14)(2.6cm)2 = 21.2cm3
If the density of the compact fiber matt is to be 0.12g/cm3, calculate the number of grams
of fiber needed for a 1 cm thick compact.
(0.12g fiber/cm3 )(21.2cm3) = 2.55g fiber
If the fiber slurry contains 3wt% fiber, how many grams of fiber slurry solution are
needed for a 1 cm thick compact?
(2.55g fiber)(100g slurry/3.0g fiber) = 85g slurry
If the final density of the compact is to be 0.15g/cm3, how many grams of silica are
needed?
Final density 0.15g/cm3
Fiber density 0.12g/cm3
Need (0.15g/cm3 – 0.12g/cm3) = 0.03g/cm3 silicate solids
(0.03g silicate/cm3)(21.2cm3) = 0.64g silicate
To obtain the gram of silica needed, how many grams of colloidal silica solution are
needed?
(0.64g silicate)(100g silicate solution/37.6g silicate) = 1.7g silicate solution
Dilute to 2x volume of compact - 2 x 21.2cm3 = 42.4cm3
(1.7g silicate soln)(1 cm3 silicate soln/1.38g silicate soln) = 1.23cm3 silicate soln
1.2cm3 silicate soln + 41.2cm3 water = 42.4cm3 diluted soln
University of Missouri-Rolla – Ceramic Engineering Department
http://campus.umr.edu/ceramics
533559890