J. Am. Ceram. Soc.

, 90 [7] 2024–2031 (2007)

DOI: 10.1111/j.1551-2916.2007.01720.x
r 2007 The American Ceramic Society

Journal
Fabrication of Functionally Graded and Aligned Porosity in Thin
Ceramic Substrates With the Novel Freeze–Tape-Casting Process
Stephen W. Sofiew
Mechanical and Industrial Engineering, Montana State University, Bozeman, Montana 59717

Functionally graded and continuously aligned pore structures eliminating porosity through high solids loading.8–10 It was fur-
have been fabricated by a modified tape-casting process for use ther discovered that the use of freezing additives in aqueous
as solid oxide fuel cell electrodes, catalysts, sensors, and filtra- systems such as glycerol, glycols, antifreeze proteins, and alco-
tion/separation devices. Pore gradients from o5 to 100 lm and hols was necessary to achieve high green density and particle
aligned pore tubules have been directly fabricated in various ce- packing after solidification.8 The undesirable porosity that was
ramic materials with thin substrate sections approximately 500– first recognized from these freezing processes focused on fabri-
1500 lm utilizing both low-toxicity aqueous-based slips and or- cating dense ceramics, however, showed pore morphologies that
ganic solvents. This process allows for the generation of pores have driven the technology toward the fabrication of porous
without the use of thermally fugitive pore formers in a single ceramics with unique microstructures not achievable through
processing step with no need for tape lamination. The incorpo- other routes.11–16 Recently, freeze-casting processing has wit-
ration of tape casting, unidirectional solidification, and the nessed several new stages of development, including new room-
freeze-drying process results in uniformly acicular pores aligned temperature solidifying solvent systems such as camphene.17–19
with the direction of the moving carrier film. Processing and This new focus on pore forming is very promising; however,
microstructure variability will be discussed as it pertains to the aside from fabricating complex/bulk shapes with intricate dies
effects of solids loading, freezing temperatures, and solvent type. and ethanol/dry ice baths, the commercial viability of freeze
Applications for this ceramic processing technology will also be casting large area porous membranes using dies and cryogenic
discussed. baths is poor at best. Further, freeze casting relatively thick
cross sections in dies or cavities can result in erratic/noncontin-
uous long-range pore alignment that is inherent in the tumultu-
I. Introduction ous process of solvent solidification.9,12,13,16
A combination of the tape-casting process and freeze-casting
T APE casting is a commercial processing technology used
around the world for the manufacture of electronic and
structural ceramics with thicknesses typically ranging from 25 to
process has resulted in a new freeze–tape-casting process that has
been developed as a direct means of forming and controlling
1000 mm. More recently, continuously processed porous tape- complex pore structures in large area green tapes through com-
cast ceramics have been used globally in the manufacture of mercially viable routes. The freeze–tape-casting process not only
catalyst support structures, solid oxide fuel cells, and filtration allows tailoring of continuously graded pores through the entire
membranes. Current pore-forming techniques in the fabrication cross section but also allows for long-range alignment of acicular
of porous tape-cast ceramics utilize thermally fugitive com- pores from the surface that is not readily seen in freeze casting
pounds (polystyrene, carbon/graphite, etc.) to generate a vari- through the use of small dies. The freeze–tape-casting process
ety of pore structures that are dependent upon the morphology starts with the traditional tape-casting process, where an aqueous
and packing of these compounds.1,2 Graded pores structures can ceramic slip (ceramic particles suspended in water) is cast onto a
then be based on constant pore volume (fewer, but larger pores Mylart or Teflont carrier film via a doctor blade apparatus. A
on one side of the structure), constant pore size (more pores of standard tape caster is utilized that has been modified only with a
identical size on one side of the structure), or a combination of thermally isolated freezing bed to allow for unidirectional solid-
both. Given the traditional techniques, the ability to engineer ification of the slurry after casting. As with traditional tape cast-
pore structures is limited to manipulation of thermal fugitive ing, the slip contains sufficient organic binders that make the
particle orientation and stability during the slurry process. Fur- tape strong and flexible after solvent evaporation for handling
ther, the generation of graded pore structures is accomplished and cutting. The additional benefit of freezing tapes has several
through multiple lamination steps that lead to increased costs in distinct advantages for specific applications requiring porous
industrial environments and additional modes of failure that substrates. The freezing of the tape, typically solidified in just
arise from processing-related defects. Traditional tape casting several minutes, eliminates particles settling out of the suspension
presents another limitation, the drying process, which restricts as opposed to the traditional process of allowing the solvent to
tape thickness due to settling and drying stresses and is one of evaporate slowly, which can ultimately result in compositional
the most crucial and difficult steps in the traditional process. and physical changes in the uniformity of the product, particu-
The use of freeze casting to process ceramics is one technique larly when mixed inorganic powders of varying size and density
that may be useful to tailor pore structures in complex-shaped are used. After the solidification process, the tape is subsequently
advanced ceramics.3–7 Freeze-casting concepts involving the so- cut and freeze dried under a vacuum for quick solvent removal
lidification of a solvent and subsequent freeze drying were ini- through sublimation, where the frozen liquid transforms from a
tially developed with the focus to create dense ceramics by solid to a gas without an intermediate liquid phase. The subli-
mation process eliminates capillary forces due to liquid–vapor
transitions, thus negating the drying stresses that can lead to
J. Halloran—contributing editor
warping and density gradients. This feature makes the freeze–
tape-casting system very effective in the fabrication of relatively
thick tapes, exceeding several millimeters or more. As a result,
Manuscript No. 22466. Received November 9, 2006; approved February 13, 2007. freeze–tape-cast green tapes can be processed without the exten-
This work was conducted at the NASA Glenn Research Center and supported in part by sive optimization of processing additives that is driven predom-
the LEAP (Low Emission, Alternate Power) program.
w
Author to whom correspondence should be addressed. e-mail: ssofie@me.montana.edu inantly by complexities of the drying process.
2024
July 2007 Fabrication of Porous Thin Ceramic Substrates 2025

Furthermore, in lieu of using water, several organic solvents process while allowing control of the freezing platen tempera-
with high melting points, including tertiary butyl alcohol (TBA) ture to modify the tape properties. The freeze–tape-casting flow-
discussed later in this paper, and the camphene system can form chart can be seen in Fig. 2, illustrating the processing steps
pore channels or other unique pore structures when solidified utilized in this technique. Aqueous and TBA-based slurries were
from a single direction. These solvent systems are also compat- prepared by traditional ball milling with the addition of dispers-
ible with the freeze–tape-casting process and offer the ability to ants and binders to achieve the appropriate uniformity, flexibil-
cast ceramics that are reactive with water, with melting temper- ity, and strength for postcast processing and characterization.
atures that exceed room temperature. Approximately 8 in.  15 in. tapes were cast using an 8-in. doc-
The work presented in this study illustrates the level of mi- tor blade assembly. The freezing bed was maintained at a con-
crostructural control, including pore gradient and alignment stant temperature during casting, monitored with a k-type
that can be achieved using water and TBA, while allowing the thermocouple on the platen, to ensure a consistent solidifica-
scale-up potential and cost-effectiveness for industrialized use. tion rate for the entire tape. A pulling rate of o10 mm/s was
The relationships between solids loading, freezing platen tem- utilized to ensure a continuous and directional solidification
perature, and density have been examined in the preliminary front over the length of the tape, which is essential in the for-
study of this processing technology to enable integration of these mation of long-range pore ordering.
materials into emerging fuel cell and filtration technologies. Aqueous YSZ slurries were prepared at 10, 20, 30, 40, and 50
However, the freeze–tape-casting process, aside from minor vol% solids loading and frozen at 351C to characterize the
variables, is a material-independent process, allowing the fabri- effects of solids loading on the density and pore morphology.
cation of many ceramic materials, and nanopowder ceramics YSZ slurries at 30 vol% solids were then frozen at 101, 201,
with tape thicknesses exceeding the outer bounds of traditional 301, 401, 501, and 601C to characterize the effects of
tape casting. freezing temperature/rate on the density and pore morphology.
TBA slurries were prepared at 10, 20, 30, 40, and 50 vol% solids
loading and frozen at 101C. While Mylart (DuPont Teijin
II. Experimental Procedure Films, Hopewell, VA) was utilized in this initial study as a stan-
dard carrier film, solidification can also be performed on an
(1) Materials and Procedure aluminum foil and Grafoilt carriers (Advanced Energy Tech-
Yttria-stabilized zirconium oxide (YSZ) was selected as the pre- nology, Inc., Lakewood, OH) to improve heat transfer through
liminary material for use in this study, given its heavy use in the carrier, which can dramatically affect the tape microstruc-
catalyst supports, biological membranes, and fuel cell applica- tures. The samples were frozen in a laboratory-scale vacuum
tions. Yttria-stabilized zirconia (8YSZ; Zirconia Sales (America) freeze dryer (FreeZone 12; Labconco Corp., Kansas City, MO)
Inc., Kennesaw, GA) with a nominal particle size of 0.55 mm at various temperatures under a 4 Pa vacuum for 12 h before
and a surface area of 8.3 m2/g was utilized as received. Water- punching 2.54 cm circular disks. Disks were sintered at a con-
based suspensions were prepared with an ammonium polyacry- stant heating rate of 51C/min to 14001C for 2 h before density
late dispersant (Darvan C-N, R.T. Vanderbilt Co., Inc., Nor- and microstructural characterization. The porosity present in
walk, CT) and an acrylic latex emulsion binder system the green state improves the manner in which polymer decom-
(Duramax, Rohm & Haas, Philadelphia, PA) in which the pow- position gases are allowed to escape the green body. This, com-
der/binder ratio was kept constant for all tapes. For near-room- bined with a process designed to make thin ceramic substrates,
temperature processing, TBA (Fisher Scientific Inc., Fair Lawn, negates the need for a special binder burnout process.
NJ) with a melting point of 251C and room-temperature viscos-
ity of 4.0 mPa
s was utilized with the addition of a fish oil dis-
persant (Z-3; Richard Mistler Inc., Yardley, PA) to obtain stable (2) Characterization
slurries. A standard polyvinyl butyral binder system (B-98; Circular YSZ disks 2.54 cm in diameter were cut from approx-
Richard Mistler Inc.) was used with a nonaqueous solvent. imately 500–1500-mm thick tapes and characterized before and
The freeze–tape-casting apparatus is shown in Fig. 1 and after sintering via geometric density measurement. Densities
consists of a standard research tape caster (TTL-1200; Richard were reported as relative densities based on the theoretical den-
Mistler Inc.) in which the drying bed has been modified with the sity of 8YSZ, 5.90 g/cm3.
integration of cooling lines and a low-temperature recirculating The microstructural development of sintered parts was ob-
chiller (Neslab ULT-80; Thermo Electron Corp., Waltham, served on fracture and polished surfaces utilizing a scanning
MA). The freezing bed is thermally isolated from the rest of
the unit to introduce a sharp temperature transition. This design
allows the cast slurry to flow and level before the solidification

Fig. 1. Modified freeze–tape-casting apparatus. Fig. 2. Freeze–tape-casting processing flowchart.

2026 Journal of the American Ceramic Society—Sofie Vol. 90, No. 7

pears to be a loss of pore alignment and continuity. The porosity

of samples solidified at constant solids loading, 30 vol%, using
various freezing platen temperatures, is shown in Fig. 5 to es-
tablish the effects of freezing rate. Further, the microstructure of
sintered specimens can be seen in Fig. 6 for samples frozen at
various temperatures. At very cold temperatures, Fig. 6(A), the
ice crystals form very columnar pores with minimal departure
from perpendicular growth; however, as the freezing rate is re-
duced, significant dendritic growth can be seen in Fig. 6(B). As
the freezing rate is lowered close to the Tm, sporadic ice growth
results in noncolumnar pores; however, pore grading is still ev-
ident through the cross section in Fig. 6(C). A surface micro-
graph of the freeze–tape-cast microstructure reveals the aligned,
acicular morphology of the ice crystals/pores, shown in
Fig. 6(D).

Fig. 3. Aqueous freeze-cast density of yttria-stabilized zirconium oxide

as a function of solids loading at 351C. (2) Nonaqueous Freeze–Tape Casting
Figure 7 shows the green and sintered porosity of TBA-pro-
electron microscope (SEM; Model 840; JEOL Ltd., Peabody, cessed YSZ slurries at varying solids loading utilizing a constant
MA). Specimens were vacuum infiltrated with epoxy before pol- freezing platen temperature of 101C. The representative micro-
ishing. structure of sintered freeze–tape-cast YSZ can be seen in Fig. 8.
Figure 8(B) illustrates the formation of a dense/porous bi-layer
as a result of TBA skin formation before freezing.
III. Results
(1) Aqueous Freeze–Tape Casting
IV. Discussion
Freeze casting aqueous solutions requires green densities in ex-
cess of 50% green density and solids loading in excess of 55 (1) Aqueous Solidification Behavior
vol% solids to achieve dense ceramic bodies5,8–10; therefore, sol- While water is seen very favorably from an environmental and
ids loading from 10 to 50 vol% was prepared for freeze–tape economic viewpoint, the disadvantages of water are vast from a
casting in order to create significant porosity on the order of ceramic processing perspective. These disadvantages include:
40%–90%. Figure 3 shows the green and sintered porosity of high surface tension (poor wetting), low vapor pressure (slow
aqueous zirconia slurries at varying solids loading. Very high drying), hydrogen bonding characteristics (less stable, higher
porosity is obtained in samples processed at 10 vol% solids; viscosity dispersions), and organics packages more dramatically
however, specimens with porosity exceeding 90% do not offer affected by residual humidity (shorter shelf-life and green tape
the level of structural support needed for traditional porous ce- stability). A strong advantage of water beyond the traditional
ramics. Microstructures of the sintered bodies can be seen in processing perspective is the solidification behavior and its rel-
Fig. 4 for samples of varying solids loading in which columnar evance to freeze casting. It has been established that soluble
pores, growing perpendicular to the freezing platen, are evident polymeric additives such as glycerol and polyethylene glycol can
from 10 to 35 vol%. Beyond 35 vol% solids loading, there ap- affect the freezing behavior of water-based systems in addition

Fig. 4. Representative pore morphologies of yttria-stabilized zirconium oxide aqueous freeze–tape-cast substrates frozen at 351C for solids loading
ranges of: (A) 10–35 vol% solids; (B) 35–40 vol% solids; (C) 40–45 vol% solids; (D) 445 vol% solids (bar 5 25 mm).
July 2007 Fabrication of Porous Thin Ceramic Substrates 2027

During solidification, the aqueous systems not only reject

dissolved binders and dispersants but also push/reject the sus-
pended ceramic particles that results in pure solvent crystals,21,22
depicted in Fig. 4, with a negligible inorganic phase. This rejec-
tion phenomenon is further indicated by the high density of in-
organic struts that form the backbone of the porous ceramic,
even when processed at very low solids loading. Thus, the re-
moval of the essentially pure ice crystals results in the formation
of pores, while yielding relatively high green densities in the
surrounding inorganic structure between pore channels. Particle
rejection appears to be dominant in the transverse direction
(perpendicular to growth) due to the high density of the struts
and the relatively constant pore volume fraction through the
columnar specimens, which has also been established in the
camphene system.20 Further, this transverse rejection is also in-
Fig. 5. Aqueous freeze-cast density of 30 vol% yttria-stabilized zirco- dicated by a large number of pore channels that are effectively
nium oxide substrates as a function of solidification platen temperature. capped off within 100 mm of growth, shown in Figs. 4(a) and
5(a). This bi-modal distribution of pore lengths is evident in the
to polystyrene in the camphene-based system.8,20 Acrylic emul- 100-mm range and the length of the total cross-sectional thick-
sion binders, consisting of insoluble latex acrylic particles sus- ness of the tape (500–1500 mm).
pended in water, were selected for this study, given their minimal These unique pore morphologies essentially create a new class
effect on water solidification behavior, thus allowing the study of pore gradient in which a single pore of expanding cross-sec-
of freezing rate and solids loading effects. The addition of sol- tional area can extend through the entire cross section. Further,
uble polymeric additives may, however, serve as a mechanism to the development of continuously aligned, graded pore channels
further tailor pore morphology. in the green state allows for the incorporation of additional in-
A simplified cross-sectional representation of the freeze–tape- organics before high-temperature sintering and provides the po-
casting process can be seen in Fig. 9, which shows the general tential for means of effective compositional grading of ceramic
solidification behavior of an aqueous system. The unidirectional membranes.
solidification of the doctor-bladed slip yields overlapping, ta-
pered pores that are aligned perpendicular to the solidification
platen. This behavior is due to the exaggerated growth of the (2) Aqueous Density and Microstructure
basal planes in the hexagonal form of ice. As the ice crystals Aqueous freeze–tape-cast substrates were evaluated at varying
nucleate on the surface of the Mylart carrier, they grow solids loading at a fixed freezing temperature, and evaluated at
through the temperature gradient, causing the crystals to varying freezing temperatures at a fixed solids loading. The ef-
diverge, forming a continuous distorted cone, in which some fects of water expansion can be clearly seen in Fig. 3, in which
of these pores span the entire thickness of the tape. The shape of the values of green porosity are substantially greater than that
the acicular cones determined from SEM images is approximat- dictated by solids loading. Examination of the variation in Fig. 3
ed in Fig. 10, which is an average for YSZ tapes cast at 30 vol% shows a nonlinear decrease in sintered porosity as solids loading
solids loading and frozen at temperatures of o301C. is increased under a fixed solidification temperature of 351C.

Fig. 6. Electron micrographs of 30 vol% solids yttria-stabilized zirconium oxide aqueous freeze–tape-cast substrates: (A) frozen at 501C; (B) frozen at
251C; (C) frozen at 51C; and (D) surface morphology of large pores (bar 5 25 mm).
2028 Journal of the American Ceramic Society—Sofie Vol. 90, No. 7

Fig. 7. Tertiary butyl alcohol freeze-cast density of yttria-stabilized zir-

conium oxide as a function of solids loading. Fig. 9. Solidification schematic of aqueous solvent systems.

This trend is attributed to the generation of noncontinuous there is significant variation in density attributed to the distri-
closed porosity at higher solids loading, allowing for greater bution and morphology of pores in the localized areas of sam-
densification of the ceramic matrix. On the other hand, highly ples cut for density measurement. Variations in density during
porous ceramic freeze–tape-cast membranes form separated sol- firing can be readily explained in that solid-state sintering is de-
id pillars that span too great a gap for solid-state diffusion pendent upon particle contact and thus the formation of necks
mechanisms to bridge, resulting in a o4% change in porosity at within the sample as a high chemical potential sink for atomic
10 vol% solids. The shrinkage behavior observed in Fig. 3 can transport and hence densification. Changes in the morphology
thus be utilized to optimize cosintering processes. The micro- of the pores can affect the interparticle contact within the spec-
structures shown in Fig. 4 illustrate the effects that solids load- imen, altering the degree of shrinkage, thus yielding density
ing has on the solidification process. While the freezing platen variations. The ratio of green and sintered porosities, however,
temperature is kept constant for the solids loading study, it is remains nearly constant in Fig. 5, indicating that while there is
anticipated that changes in solids loading may affect the heat some local variation in microstructure, the degree of densificat-
transfer characteristics of the slurry, thus altering the freezing ion is relatively unaffected. The tailoring of pore morphologies
rate. Faster freezing rates, combined with higher solids loading, by altering the freezing temperature is shown in Fig. 6. Very cold
may further limit pore continuity as evidenced in Fig. 4. The temperatures (fast freezing rates) result in accelerated ice crystal
effects of freezing rate and solids loading could not be readily growth, in which little to no deviation from columnar growth is
separated; however, given the low thermal conductivity of YSZ, noted in Fig. 6(A). Pores grown in this low-temperature regime
it is expected that the solids loading effect is dominant. A sig- show excellent ordering with uniform size and shape. A slowing
nificant range of solids loading from 10 to 35 vol% yields the of ice crystal growth rate is shown to introduce some dendritic
highly columnar ice crystal growth. While continuous pore growth of the ice crystals as shown in Fig. 6(B); however, this is
channels can be fabricated at 35 vol% solids, higher solids load- also accompanied by a discontinuity of porosity through the
ing introduces heterogeneous nucleation and/or incomplete ce- cross section. A strong bimodal distribution of pores, as dis-
ramic particle rejection, resulting in graded, but noncontinuous, cussed previously, is also evident through the cross section of
pore morphologies. The ability to tailor pore structures beyond Figs. 6(A) and (B), in which the ice crystals appear to diverge
45 vol% solids via solidification processes is no longer possible rapidly until a specific surface/volume energy equilibrium crys-
due to excessive particle packing and diminished water content. tallite size is achieved at which point the ice crystal grows at a
The maximum solids loading limit of attaining columnar-graded significantly reduced rate of divergence. Further decrease of the
pore structures in water is approximately in the range of 35 solidification temperature close to the melting point, shown in
vol% solids, which is ideal for creating low-viscosity slips with Fig. 6(C), results in slow and tortuous growth patterns that de-
submicrometer and potentially nano-sized ceramic powders. viate strongly from columnar, but still maintain overall pore
While solids loading plays a leading role in determining the grading through the entire tape thickness.
final microstructure of freeze–tape-cast substrates, the freezing While porous graded structures tend to show dramatic dis-
platen temperature (i.e., freezing rate) strongly effects the mor- tortion upon sintering due to differential densification across the
phology of the ice. Figure 5 shows the change in porosity as a cross-sectional thickness, the freeze–tape-cast structures remain
function of freezing rate, and while it is expected that the den- predominantly flat during sintering without the use of weight or
sities will remain unchanged due to constant solids content, creep flattening steps. While the freeze–tape-cast structures have

Fig. 8. Microstructure of 25 vol% tertiary butanol freeze–tape-cast yttria-stabilized zirconium oxide ((A) bar 5 100 mm, (B) bar 5 10 mm).
July 2007 Fabrication of Porous Thin Ceramic Substrates 2029

Fig. 10. Morphology of water-based freeze-cast pores.

minimal long-range density gradients, the basis of warping dur-

ing sintering, densification can also be affected by pore size, in
which sufficiently large pores limit atomic transport.8 In freeze–
tape-cast samples 43 in. in diameter, the development of a
slight concave curvature on the small pore side has been ob- Fig. 12. Electron micrograph of a solid oxide fuel cell fabricated with a
served. This general lack of warping is also attributed to the NiO/YSZ freeze–tape-cast anode with performance that approached 1
W/cm2 (bar 5 100 mm). YSZ, yttria-stabilized zirconium oxide.
discrete columnar structure of the freeze–tape-cast substrates, in
which stress concentrations between columnar ceramic scaffolds
result in local fracture during sintering, evident in Fig. 4(A), thus include cyclohexane, TBA, as well as the new camphene-based
impeding atomic transport and thereby mitigating the effects of system. In addition to desirable surface tensions and minimal
differential shrinkage across the thickness. hydrogen bonding characteristics, several of these solvents have
Figure 6(D) reveals the acicular nature of the pores on the solidification temperatures greater than that of room tempera-
upper surface of the tape, illustrating the large aspect ratio of the ture (201–251C). The physical properties of solvents suitable for
pores. Of particular importance is the alignment of the pores’ freeze–tape casting are shown in Table I. TBA was selected as
long axis with respect to the casting direction of the mylar tape the most promising organic solvent system for initial evaluation
carrier, another method that may used for additional micro- due to its near-room-temperature melting point, thus requiring
structural control. The pores maintain this alignment over the the minimal addition of heat during mixing to prevent freezing
entire surface of the cast tape for all samples prepared in this and minimal refrigeration for solidification. Further, the most
study. This characteristic of control is attributed to the tape- important aspect in choosing TBA lies in its compatibility with
casting aspect of the freeze–tape-casting process. Traditional common organic solvent additives used in tape casting, which
freeze-cast specimens are cast in molds, where converging sol- requires dispersants, binders, and plasticizers to fabricate large-
vent crystals can create local inhomogeneities in the pore struc- area green tapes suitable for handling and shaping. The use of
ture. The freeze–tape-cast process allows for not only TBA allows greater flexibility in processing where several dif-
unidirectional solidification through the thickness but also ferent choices of processing additives are readily available,
through the length of the tape by visual observation of the which may not be the case with cyclohexane- or camphene-
propagating solidification front and adjustment of carrier speed. based suspensions.

(3) TBA Solidification Behavior

(4) TBA Density and Microstructure
Water yields a 9% volumetric expansion upon freezing and is
not compatible with some inorganic powders such as magne- The evidence of TBA undergoing a volumetric contraction dur-
sium oxide due to the formation of surface hydrates, thus mak- ing solidification is clearly evident in Fig. 7. The change in green
ing water a poor choice in some specific processes. There are, density as a function of solids loading and solvent contraction
however, several organic solvents suitable for freeze–tape cast- on solidification is nearly as significant as the expansion in the
ing that show negligible and even net volumetric contraction, water-based system. Therefore, there exists the tendency for
thus increasing the green density of the cast tape. These solvents TBA-processed tapes to sinter to much higher densities, which is
advantageous in the preparation of low-viscosity TBA slips,
given the higher viscosity of the solvent and even greater in-
crease with the addition of processing additives. While aqueous-
based YSZ slips show ideal freezing behavior and porosity at 30
vol% solids, the TBA-based systems generate the same level of
porosity at 10 vol% solids. Unlike the water-based casting,
TBA-cast tapes do not form the same continuous or graded
pore morphologies of the aqueous system as depicted in Fig. 6.
A more tortuous porosity is generated in the TBA system in
which substantial interconnected porosity is generated at o30
vol% solids; however, the solidification kinetics of TBA appear
to limit the size of the solidified crystals to those depicted in
Fig. 8(B) for all solids loadings and freezing temperatures eval-
uated. This may be further exasperated by the less pronounced,
but still present solute rejection phenomenon in the TBA system.
While it was previously considered that only water exhibited the
rejection phenomenon, this behavior has been extended to
nonaqueous systems including TBA in this study as well as
camphene-based systems.23 While continuous line-of-sight pore
channels through the entire thickness of the tape are not readi-
Fig. 11. Cross-sectional micrograph of a prototype solid-state water/air ly evident in the TBA system, pore ordering is still feasible as
separation device (bar 5 100 mm). evidenced at the top of the micrograph in Fig. 8(B) and
2030 Journal of the American Ceramic Society—Sofie Vol. 90, No. 7

Table I. Physical Properties of Freeze-Casting Solvents

Viscosity (mPa
s at 201C) Melting point (1C) Surface tension (mN/m at 201C) Vapor pressure (kPa at 201C)

Water 1.0 0 73 2.3

Cyclohexane 1.0 6.5 25 10.3
Tertiary butanol 3.3 at 301C 26 21 4.0
Camphene 1.6 at 551C 47 40 at 551C 2.0 at 551C

pores are aligned nearly perpendicular to the solidification cessing technology as discussed elsewhere.26 These applications
platen. demonstrate the versatility and material-independent nature of
Figure 7 shows the green and sintered density of TBA-pro- this processing technology.
cessed zirconia slurries at varying solids loading utilizing a con-
stant freezing platen temperature. Given the solidification
behavior of TBA and the negligible dimensional change during V. Conclusions
freezing, the densities follow a linear trend with solids loading. Porous ceramic substrates were successfully fabricated using the
While TBA is effective at creating open porosity, this solvent new freeze–tape-casting process utilizing both aqueous and TBA
system does not show as dramatic or continuous a pore growth solvent systems. Graded and aligned porosity was achieved in
like the aqueous process. The vapor pressure of TBA is condu- thin, large-area YSZ tapes, and the capability to tailor pore
cive to skinning before solidification, and Fig. 8 shows the for- structures was demonstrated by means of altering solids loading
mation of a dense/porous bi-layer fabricated in a single and freezing bed temperatures. Unique columnar line-of-sight
processing step using TBA-based tapes as also evidenced in channels can be fabricated through the entire cross section of
the camphene system.24 The formation of a dense layer is clearly YSZ substrates exceeding 1-mm thickness in a single processing
seen in slurries 420 vol% solids. Given the high vapor pressures step. Beyond cross-sectional microstructure control, the aque-
of the organic solvents, the formation of skins may be beneficial, ous freeze–tape-casting technique allows the ability to order
but may also be difficult to avoid for fabricating porosity pore morphology on the surface of the tape, a concept that may
through the entire tape thickness. Preparation of YSZ bi-layers be further exploited for gas/liquid transport applications. The
in effect creates a porous layer near the Mylar-casting surface, ceramic particle rejection phenomenon allows the fabrication of
and a dense layer at the top of the tape shown in Fig. 8, just the pores without the use of traditional thermal fugitives and eases
opposite of the aqueous system. the thermolysis process with the ability to generate pores in the
green state. The TBA system offers an alternative to water-based
(5) Freeze–Tape-Casting Applications processing where room-temperature solidification is desired.
The functional pore grading generated by this process is highly Further, the high vapor pressure of the TBA-based systems al-
suited to numerous applications requiring both liquid and gas lows direct fabrication of porous/dense bi-layers in a single pro-
transport. In the case of liquid transport, the capillary pressure, cessing step, thus creating a means of preparing both the catalyst
DP, is defined in Eq. (1), where g is the interfacial energy, cos y is and support in a single step. This processing technology has
the wetting angle, and r is the pore radius. shown to be effective in fabricating fuel cells and separation
membranes.
DP ¼ 2g cos y=r (1)

Acknowledgments
Given that interfacial energy and wetting angle are constant
for a substrate comprised solely of YSZ, the capillary pressure The author would like to thank Dr. Thomas Cable and Mr. John Setlock for
their invaluable input on fuel cell applications and ceramic processing.
on the small pore surface can exceed  100 that of the large pore
surface. In effect, the pore grading creates a substantial differ-
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