Unidirectional Freeze Casting of Ceramic
Suspensions
Old Dominion University
Mechanical & Aerospace Engineering Department
Project Design & Management II – MAE 435 Midterm Report
Fall 2015
Team Members: John Simmonds, Mike Beachy, Joe Hastings
Mike Harty, Mike Smith, Nikhil Dhavale
1. Abstract:
This research is to prove specific ceramic suspensions produced in a unidirectional freeze casting process
will produce a strong and tough material, similar to the naturally occurring substance, nacre. To accomplish this, an
apparatus was designed and constructed to simultaneously produce the ceramic material accurately and precisely,
and to measure the freezing velocity of each slurry sample. The ceramic product was produced by formulating a
particular ceramic slurry, unidirectionally freezing it at a specific freeze front velocity, then freeze dried and
sintered. This method of ceramic development produces a “brick and mortar” type structure upon review by
microscope. This same construction occurs naturally in nacre, and is the reason how a ceramic can be strong and
have a high modulus of elasticity. The synthetically produced ceramic material has great promise within the
medical community in creating bone and joint replacements.
2. Introduction:
Unidirectional freeze casting is a relatively new process capable of producing ceramic material that is both
stronger and tougher than other traditional ceramics. Substrate, solvent and binding agents are combined to form
the ceramic slurry which is then exposed to liquid nitrogen. This induces freezing in the slurry in an axial direction
and results in the separation of solvent and substrate into micro-structures similar to columns. Further processing
with freeze drying and sintering produces a ceramic material with a similar microstructure to naturally occurring
nacre [1], offering incredible increases of strength and toughness. Nacre, otherwise known as mother of pearl, is a
naturally formed material produced by most gastropods, often studied in parallel to freeze cast ceramics [2].
Ceramics are a material containing very small particles that have been compacted together. They are
generally strong, but brittle. In this study, ceramics are produced using unidirectional freeze casting. This method
of freezing arranges the particles in a staggered platelet microstructure, held together by a binding agent. This
gives the newly formed ceramic composite not only increased strength but toughness as well [3]. Comparative
testing was conducted on both unidirectional frozen as and traditional ceramics with results demonstrating the
differences of the two materials’ properties. Various testing procedures have been used to determine the
processes and substrate/solvent/binder mixtures that produce ceramic materials that demonstrate more desirable
properties [4]. Variations in the method of unidirectional freeze casting, sintering as well as the composition and
mixture ratios of the slurry have shown to greatly affect the material properties of the sample [5]. Although the
process of unidirectional freeze casting was proven successful in small scale testing, it has not progressed to larger
or commercially viable scale.
During testing in compression, materials produced with unidirectional freeze casting display increases in
strength and ductility when compared to the base material of the sample. Materials produced in this manner show
potential for use in multiple fields that require strong, durable ceramics, particularly filtration, impact protection
and synthetic bone replacement material due to its strength in compression [6, 7]. In this study, the freeze front
velocities, cooling rates, and solution percentages will be varied when producing a ceramic in a unidirectional
manner and the resulting sample materials will then be tested using compression, ballistic impact and hardness
testing, in order to verify established data.
3. Methods:
3.1 Completed Procedures
3.1.1 Mold Fabrication:
To create the molds for the freeze casting, a two part polymer (polydimethylsiloxane) is used. First a
stainless steel master form cylinder measuring 13mm diameter and 300mm in height, is placed centrally into a
plastic cylindrical exterior form.
Both the master form and the mold form are then coated with a releasing agent. The polymer was then
manually mixed in a disposable cup to a 10:1 ratio by weight mixture of PDMS-Sylgard Silicone Elastomer 184 and
Sylgard Curing Agent 184 (Dow Corning Group) until a uniform consistency had been achieved. The mixture then is
poured into the mold forms which were then placed into a variable vacuum chamber (Bel-art Model 420100000
vacuum desiccator, Bel-art, Wayne, NJ). For approximately 30 minutes the pressure inside the vacuum chamber is
varied between atmospheric pressure and -30 psi to remove air bubbles present in the polymer mixture.
Once the air bubbles were removed, the molds were then moved to an oven, where the PDMS was cured
at 400°C for 24 hours. The rigid polymer was then removed from the mold form and the master form was removed
from the polymer. PDMS polymer was chosen due to the transparency of the substance which aides in the
estimation of the freeze front velocity through visual observation of the freezing process.
These PDMS molds were used initially to establish freeze front velocities by careful monitoring and used
to create small ceramic suspension samples. Currently, Teflon© tubes, 19mm dia. x 80mm length are being utilized
to make larger samples of the ceramic material. These tubes allow for larger and longer samples to be produced,
as the samples only produce approximately 60% viable and usable material.
3.1.2 Device construction:
For the freeze casting operation, the freezing apparatus controls the freeze front velocity by holding a
ceramic sample a specific distance from the surface of the liquid nitrogen [8]. The device that is used for this
experiment was custom built and consists of a thermally insulated Dewar (where liquid Nitrogen is contained), and
a movable main arm mounted on a threaded rod. On the main arm, a removable test plate was used to hold the
sample away from the liquid nitrogen. Also, a high density foam ring was made to fit around the circular test plate,
and contour snuggly against the Dewar, to focus the cooling rate of the test plate. To control the height of the
sample from the liquid nitrogen surface, a threaded ring is manually turned, which raises and lowers the sample.
Additionally, a liquid level indicating / fill tube was utilized in order to ensure the sample was kept at a constant
height above the liquid nitrogen. A K-type thermocouple was run through the main arm and subsequently placed
in contact with the test plate within 1 mm of the sample. A varied range of cooling rates, has been achieved using
the threaded ring to adjust the distance between the cold finger (test plate) and the liquid nitrogen. Freeze front
velocities were established during the operational testing of the freezing device using deionized water only in the
mold. By using differing levels of liquid nitrogen from 2.125ltr to 1.75ltr, water samples were frozen and timed to
establish the freeze front velocity values. This method of establishing a freeze front velocity failed to produce
viable samples as the temperature gradients were never changed. Then experimentation was then tried by using
constant volume of liquid nitrogen and changing the sample height above liquid. This proved to be the key to
establishing varying freeze-front velocities. The cooling rate was established by taking the difference of the
temperatures with respect to the difference in time for the temperature to fall from 0deg C to -60deg C. The
freezing front velocity was then determined by taking the length of the sample and dividing it by the time it took
for ice to start forming, to a completed frozen cylindrical sample.
3.2 Methods:
3.2.1 Porous Ceramic Freeze Casting Procedures:
3.2.1.1 Slurry Preparation:
For second part of the experiment, a colloidal suspension of calculated amounts of water, dispersant,
binder and ceramic powder was prepared in a typical batch size of 100 mL. An amount of deionized water was
weighed and poured into a clean container. Then the calculated amount of dispersing agent was added to the
water. The container is then sealed and the solution mixed with a magnetic stirrer for approximately 5 minutes.
The appropriate amount of ceramic powder was slowly added to the mixture while stirring continued. To prevent
clumping and to achieve a more uniform mixture, this part of the preparation is constantly varied according to the
conditions of the mixture (i.e. particle size). The resultant ceramic slurry is then ball-milled for approximately 24
hours to obtain a better suspension of all of the components as well as mixing. The final steps of the preparation
involved the addition of the required amounts of binding agents to the suspension, which is then ball milled again
for about 20 minutes. The ceramic slurry is then placed in a vacuum chamber to remove any air introduced into
the slurry, before finally being ready for freeze casting.
3.2.1.2 Freeze-Casting, De-molding, and Sublimation:
With slurry preparations complete, the PDMS molds were placed on the cooling plates of the apparatus
and the foam ring was securely placed within the Dewar. The ceramic mixture is then poured into the mold and
the sample was placed at the desired height from the liquid nitrogen. After freezing, the frozen samples were
quickly removed from the molds by hand to prevent damage to the sample. Finished samples were placed into a
freeze dryer for 24 hours to sublimate the ice from the sample. Once the ice was fully sublimated from the sample,
or “green body,” the green bodies were ready for sintering.
3.2.1.3 Sintering:
Sintering was carried out in a furnace at normal atmospheric conditions. The green bodies were heated to
3250 C at a constant ramp speed of 600 C/h in order to burn out the binding agent. Once the green bodies reached
3250 C, they were held at temperature for 60 minutes, at which time they were brought up to 5000 C at the same
ramp speed. The sintering cycle was completed by cooling the sample to room temperature at a rate of 3000 C/h
[9, 10, 11, 12, 13].
4. Results:
After completion of the initial test runs, the cooling rates and freezing front velocities were obtained for
four levels of liquid nitrogen for deionized water samples. The sample was held at a constant height of 1mm above
the liquid nitrogen to achieve the baseline values. Trials of freeze casting ceramic slurries were then conducted
from several different nitrogen levels as well as gap distances between the test plate and the liquid nitrogen
surface. It was discovered that when the sample was held farther from the nitrogen surface, and was frozen at a
slower rate, the ice and ceramic columns formed were larger and more prevalent. Samples were frozen at gaps of
20mm, 25mm, 30mm, 35mm, 40mm, 45mm, 50mm, 60mm, and 65mm. The resulting freeze-front velocities were
linearly dependent on the height and were seen to decrease as height increases. The fastest time was ~22μm/s at
20mm while a ~6 μm/s velocity was seen at 65mm.
5. Discussion:
The Nacre-like ceramic is a relatively new technology and is being pursued by scientists and engineers for
its strong, yet durable material properties [14]. With the development of these ceramics on an industrial scale,
medicine would have access to recreating bone, teeth, and more [15, 16]. The purpose of this study is to develop a
process that produces nacre-like ceramics by unidirectional freezing methods and to test the product’s material
properties [17, 18]. Thus far, an apparatus for freeze-casting has been developed, initial testing of the system was
done using de-ionized water samples, and many samples of different mixtures of ceramic slurry suspensions have
been made. Freeze front velocities of the samples that were slower and took longer to cast have shown more
promising results in achieving our desired material. The finalized experimental system was able to achieve a
repeatable, step by step procedure that generated thoroughly frozen samples.
Currently, plans for having a conduction cooling control instead of convection cooling are being discussed. Having
the sample freeze by conduction might centralize and focus the freezing rates of the samples to a more unified
formation of ice and ceramic columns. Also, a new temperature measuring system is being purchased for more
accurate and more precise results. More consistent thermal data images will give a better picture into the cooling
rates and freeze front velocities of each sample. This is to be accomplished by purchasing a T-type thermocouple
and a commercially viable data logger. Once the new thermocouple system is installed and several more samples
are created, the samples will be tested and examined by microscope.
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