J Nondestruct Eval (2011) 30:81–90
DOI 10.1007/s10921-011-0093-9
A Dielectric Sensing Approach for Controlling Matrix
Composition During Oxide-Oxide Ceramic Composite Processing
D.D. Hass · H.N.G. Wadley
Published online: 24 February 2011
© Springer Science+Business Media, LLC 2011
Abstract Continuous fiber reinforced ceramic matrix composites (CMC’s) made from aluminum oxide fibers and matrices are usually fabricated using a tape casting process. In
this process, ceramic slurry consisting of the oxide powder,
a polymeric binder and a solvent is infiltrated into a woven ceramic fiber mat. After evaporation of some of the solvent, the resulting flexible tapes can be stacked and sintered
to create a composite component. Because the fraction of
ceramic powder in the slurry can vary during processing,
in-situ compositional sensors are required for on-line feedback control to limit property variations in the composite
material. Since the dielectric properties of the slurry components are distinctly different, the effective permittivity of
the slurry depends upon its composition. Here, a non-contact
capacitance probe has been used to explore the possibility of
capacitance sensing for compositional control. Results indicate that the removal of solvent during a precision drying
step may be monitored by this approach. The feasibility of
monitoring changes in the slurry’s composition during infiltration of the fiber mat is also discussed.
Keywords Ceramic-matrix composites (CMCs) · Slip
casting · Process sensing · Non-destructive testing
1 Introduction
Many ceramic structures and devices are fabricated from ceramic powders using slurry-based methods such as tape casting [1]. In this approach, the ceramic powder is dispersed
D.D. Hass · H.N.G. Wadley ()
Department of Materials Science and Engineering, School of
Engineering and Applied Science, University of Virginia,
Charlottesville, VA 22903, USA
e-mail: [email protected]
in a liquid solvent/binder mixture, formed into a thin tape,
dried, stacked and finally sintered at high temperature. Examples of materials made this way include piezoelectric ceramics [2], multilayer capacitors [3], solid oxide fuel cells
[4], current collectors in batteries [5], and ceramic matrix
composites (CMC’s) [6]. The study described here is motivated by a continuing interest in the development of ceramics for high temperature applications in the hot sections of
gas turbine engines [7].
While monolithic ceramic materials based upon alumina,
silicon carbide, or silicon nitride have a lower density, higher
stiffness, lower creep rate, and (in the case of oxide-based
ceramics) greatly reduced susceptibility to oxidation compared with conventional superalloys [8], their low toughness has restricted widespread application [9]. Ceramic matrix composites (CMC), in which strong continuous ceramic
fibers are embedded in a ceramic matrix can result in a more
damage tolerant mechanical behavior due to crack deflection, fiber pull-out, crack bridging and de-bonding mechanisms [10, 11]. The emergence of oxide fibers such the
alumina based Nextel 440, 610 and alumina with mullite
(2SiO2 3Al2 O3 ) Nextel 720 fibers and alumina based matrices has led to the development of oxide-oxide CMC’s [12]
and interest in their use for a growing number of high temperature applications [13–15]. However, reliable processing
methods are needed if the potential of these materials is to
be fully realized.
The tape casting approach is widely used for making
oxide-oxide composites since equipment costs are low and
production rates are potentially high [16]. The process,
shown schematically in Fig. 1, involves the infiltration of
a woven ceramic fiber mat by a slurry containing ceramic
powder, a polymeric resin and organic solvent of a controlled composition and viscosity. The infiltrated mat is then
precision dried so that a controlled fraction of the solvent is
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J Nondestruct Eval (2011) 30:81–90
Fig. 1 (A) Schematic
representation of the infiltration
step of the tape casting
processing route for the
production of ceramic matrix
composites. A woven fiber mat
is infiltrated with ceramic slurry
by passing the mat into and out
of a slurry container. This is
followed by the partial removal
of the slurry solvent using a
drying oven. (B) A schematic
representation of a potential
capacitance sensor is shown.
The electrodes are placed on
either side of the moving tape at
two locations to monitor the
removal of the solvent during
the drying step
(A)
(B)
evaporated, resulting in the formation of a flexible, slightly
“sticky” tape [17]. This tape consists of ceramic fiber surrounded by a precise amount of ceramic powder held together by the polymeric resin. These mono-tapes can be cut,
stacked and cured using a low temperature/pressure cycle,
and machined to produce the desired component shape. The
resulting green component is then heated at 200–600◦ C to
drive off the residual organic constituents and finally sintered (at typically 1000–1100◦ C) to sinter the matrix [7].
A typical tape cast slurry is composed of finely divided
(<1 µm diameter) ceramic powder which eventually becomes the composite matrix, a polymeric resin that binds
the powder in the green (pre-consolidation) state, an organic
solvent that dissolves and distributes the organic material in
the slurry, and an organic dispersant that inhibits powder agglomeration thus reducing slurry viscosity and allowing for
adequate infiltration of the fiber mat at high powder loadings
[7]. Additional components such as a second solvent, plasticizers, or polymeric precursors are sometimes added when
special properties (such as a specific solvent partial pressure
or increased tape flexibility) are desired [7, 18]. The slurries
are prepared by first mixing the ceramic powder with the solvent and dispersant in a ball mill. During milling, agglom-
erates are dispersed and reagglomeration is prevented as the
dispersant becomes progressively adsorbed on the powder
surface [19, 20]. A resin is added after the initial milling has
adequately dispersed the powder [21]. A secondary milling
step is then used to homogenize the slurry [22]. The composition of the slurry establishes the fraction of matrix present
in the composite. It is critical that this be controlled if the
mechanical performance of the composite is to be reproducible.
The composition of the slurries can be difficult to control
during fiber pre-form infiltration due to wicking and evaporation of the solvent, settling of the powder, and selective
component pick-up by the fiber mat. Changes in the composition of the slurry prior to infiltration also affects the viscosity of the slurry, the extent of slurry infiltration into the
mat, and finally the fraction of ceramic matrix in the monotape. Variation in the solvent evaporation during drying can
also lead to a loss of the desired green tape flexibility and its
tackiness [22]. These effects can all lead to variability in the
final composites properties. An in-situ sensing approach to
monitor compositional changes as the mat emerges from the
slurry and to monitor changes in the tape composition during
J Nondestruct Eval (2011) 30:81–90
83
drying would enable implementation of more sophisticated
controls aimed at reducing property variability.
All of the constituents of the slurry are relatively good
electrical insulators but the electric permittivity’s of the constituents are quite different. As a result, the overall dielectric
properties of the ceramic slurries are anticipated to depend
on slurry composition [23–25]. Dielectric sensing has been
used to infer the porosity in porous materials [26] and an
in-situ sensor which monitors the dielectric properties of the
ceramic slurry or infiltrated fiber pre-form during processing may be useful in controlling compositional changes to
the system.
The magnitude of the dielectric properties of a material is
a measure of the ability of a material to polarize in an electric field. Most polarization mechanisms are time dependant,
so their dielectric properties are often frequency dependant.
Losses also accompany the polarization process and so a
materials permittivity is usually expressed in the complex
form [24, 25, 27] as
ε = ε + iε (1)
where ε is the dielectric constant and ε is the dielectric
loss. The dielectric constant, ε measures a materials ability
to store electrical charge while ε is a measure of the relative
“lossiness” of the material. The ratio of the energy lost to the
energy stored by the material in an alternating electric field
is known as the loss tangent [16],
tan(δ) =
ε ε
(2)
Since the real and imaginary components of the permittivity of a material are frequency dependent, multifrequency
dielectric techniques are necessary to fully characterize the
compositional effects in these systems and to determine appropriate test frequencies for sensing applications.
The simplest measurement approach is based on the use
of parallel plate capacitor. If a material fills the space between the plates of a parallel plate capacitor, changes to the
permittivity of a material will result in changes to the capacitance:
ε=
C
Co
(3)
where Co is the capacitance of the capacitor in vacuum and
C is the capacitance of the capacitor with a dielectric material present. A capacitance monitoring approach based on a
parallel plate capacitor may be used to measure changes in
the permittivity of an infiltrated fiber mat (IFM) during the
slurry based CMC processing approach described above.
For this purpose a parallel plate capacitor having a dielectric material with an air gap on each side needs be considered. This can be described by a stack of dielectric plates
arranged parallel to the electrodes. The effective permittivity
is then given by the relation:
ν1 ν2 ν3
1
=
+ + ε ε1
ε2 ε3
(4)
where vn are the volume fractions and εn the permittivity of
the each of the three layers [16]. Changes to the dielectric
constant of the middle layer results in a change in the capacitance based on (3). This capacitance change is related to
the change in the permittivity of the IFM and the area of the
capacitor electrodes:
C =
Aε
d
(5)
where A is the electrode area and d is the electrode spacing
[27, 28]. Thus, even when a relatively large air-gap (∼5 mm)
is present between the electrodes and the IFM, the device
can be designed to be sensitive to the expected capacitance
variations by altering the electrode area. A potential sensor
design is given in Fig. 1(b). In this case, a large area parallel plate capacitor could be aligned with each plate of the
capacitor on opposite sides of the IFM placed on the thin,
polymeric film. Using this approach, along with sensor designs in which the capacitor area is maximized and the electrode spacing is minimized, a non-invasive measurement approach for monitoring changes in the permittivity of an IFM
at different stages of the process appears feasible.
To investigate this sensing concept, experimental test
cells were developed which could: (i) measure the frequency dependant dielectric properties of ceramic slurries
and (ii) simultaneously monitor the capacitance change in
a non-contact parallel plate capacitor and the mass of a
IFM. Using these test cells, the dielectric constant of ceramic based slurries were found to be strongly composition dependent and the capacitance of infiltrated fiber mats
were found to be related to solvent content, alumina content and the volume of slurry adsorbed during removal from
the slurry. These results suggest that a dielectric sensing approach may be effective for monitoring the precision drying
step in the tape casting process, changes in the fiber mat infiltration during its removal from the slurry and changes in
the composition of the slurry prior to infiltration of the fiber
mat.
2 Experimental
To investigate the viability of the permittivity sensing approach required measurements of both liquid and semi-solid
material permittivity’s, and a test cell that facilitated simultaneous measurement of the weight and the permittivity of a
semi-solid sample during tape drying experiments. As a result, both a liquid test cell for slurry composition measurements and a drying test cell for monitoring the evaporation
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J Nondestruct Eval (2011) 30:81–90
Fig. 2 (A) Schematic
illustration showing the
geometry of the test cell used to
measure the dielectric properties
of a ceramic slurry.
(B) Schematic illustration
showing the geometry of the test
cell used to simultaneously
measure the dielectric properties
and mass of an infiltrated fiber
mat. The infiltrated mat was
placed onto a U-shaped holder
which was then placed between
two electrodes while aligned on
a scale
(A)
(B)
of solvent from an infiltrated fiber mat were designed and
are shown in Fig. 2.
The liquid test cell, consisted of a polyethylene container
with copper electrodes. The guard ring and the low elec-
trode were 3.81 cm in diameter and the high electrode was
2.54 cm in diameter. The electrode spacing was controlled
by a micrometer and the electrodes were aligned using two
chemically resistant Teflon guides. The cell temperature
J Nondestruct Eval (2011) 30:81–90
85
Table 1 Permittivity values for tape casting components
Material
ε
Isopropyl alcohol
20.0
Poly (vinyl butryal)
Alumina
Fig. 3 Schematic illustration showing a microstructure of a ceramic
slurry investigate in this work. The shape and size distribution of the
alumina powder was based on SEM images
was monitored using a resistance temperature device (RTD).
A second test cell was designed for simultaneous measurement of the capacitance and mass of an IFM. This test cell
used a non-contacting electrode method [28] to monitor the
capacitance change in the IFM. It consisted of two platinum
plated niobium mesh electrodes and a stainless steel guard
electrode. The mesh electrodes alleviated condensation of
liquid onto the electrode surface during evaporation. The
electrode area, A, was 1935 mm2 . The electrode spacing
was controlled by a micrometer to be 5.0 mm in all cases.
The drying sample was mounted on a thin (1.5 mm) glass
slide and mounted onto a U-shaped sample holder, Fig. 3.
The holder was placed on a precision scale in-between the
electrodes of the capacitor so that the mass and capacitance
change in the IFM could be monitored simultaneously. In
this case, (3) is replaced by (6)
ε =
1
1
1− 1− C
C2 ×
tg
ta
(6)
where C1 and C2 are the measured capacitances without and
with the sample present respectively and tg and ta are the
gaps between the two electrodes and the sample thickness
respectively.
The capacitance was measured on both cells using a
HP-4194A Impedance Analyzer. Results at a frequency of
1 MHz are reported in all cases. A three-terminal arrangement was used to prevent erroneous results caused by fringing effects [29]. The test cells were interfaced to the impedance analyzer using coaxial wires and a HP-16047C test
fixture. The guard electrode was connected to ground.
Ceramic slurries were produced from materials commonly used in tape casting techniques [16–22, 30]. They
2.8
8.8
Phosphate ester
11.29
Nextel 440 fiber
5.9
consisted of ∼1.0 µm Al2 O3 powder (Mager Scientific),
a polyvinyl butryal (PVB) resin (Butvar B98—Monsanto
Co.), a phosphate ester (PE) dispersant (Emphos-21A—
Witco Co.), and isopropyl alcohol (IPA) solvent. The microstructure of the ceramic slurry is shown schematically
in Fig. 3. Woven Nextel 440 fiber mats (3M) were used.
These mats consisted of fiber bundles (containing ∼400
fibers) of Nextel 440, a 10 µm diameter alumina-silicate
fiber. The fiber composition was 70.0% Al2 O3 – 28.0% –
SiO2 – 2.0% B2 O3 . The bundles were plain weave woven in
a 0◦ /90◦ array. The permittivity of these materials is given
in Table 1. Slurries were produced by mixing alumina powder, isopropyl alcohol, and the phosphate ester dispersant
and milling for 24 hours to fully disperse the powder. The
resin was subsequently added followed by another 24 hour
milling operation to homogenize the slurry.
To investigate the effect of the volume fraction of slurry
components on the slurry permittivity two experiments were
performed. In the first, PVB resin was systematically added
(from 0 to 10.0 vol.% PVB) to an IPA solvent and the resulting permittivity was monitored. In the second, alumina
powder was systematically added (from 10 to 22.5 vol.%)
to a base mixture of IPA and 5.0 vol.% PVB. Experiments
were then conducted to determine the feasibility of monitoring compositional changes in the slurry during the infiltration step. The baseline slurry for these experiments was IPA
/ 20.0 vol.% Al2 O3 /5.0 vol.% PVB / 1.0 vol.% PE. Compositional changes during infiltration were investigated by systematically varying the alumina fraction of the slurry (from
12.0 to 24.0 vol.%). The capacitance of the IFM was then
measured at room temperature one minute after the infiltration of a Nextel™ 440 mat (2.54 × 3.81 cm) into slurries
having the appropriate alumina content. The fiber mat orientation was held constant during the dielectric measurements
with the fibers aligned 0◦ /90◦ to the top surface of the scale.
Variations in the infiltration and retention of the slurry in the
IFM due to variations in the removal rate of the fiber mat
during infiltration were also investigated by altering the IFM
removal rate from the baseline slurry. A motor controlled
tape removal apparatus was devised for this purpose which
could alter the IFM removal rate from 15 to 150 cm/min.)
To investigate the sensitivity of this technique to compositional changes which occur in the IFM during the solvent
86
Fig. 4 Plot showing the change in the dielectric constant of a ceramic
slurry with a composition of IPA—20.0 vol.% Al2 O3 —5.0 vol.%
PVB—1.0 vol.% PE from a measurement frequency of 100 Hz to
1 MHz
removal step, the capacitance change of an IFM was monitored while solvent evaporation occurred. In this case, the
sample mat was infiltrated by dipping a sample of Nextel™
440 mat (2.54×3.81 cm) into the baseline slurry and removing the mat at a rate of 30 cm/min. The mat was then placed
into the test cell and its capacitance and mass changes were
measured while the solvent evaporated. Measurements were
made every 60 seconds at room temperature.
3 Results and Discussion
3.1 Frequency Dependence
In general, the dielectric properties of complex materials are
dependent on the measurement frequency. This is a result of
the multiple polarization mechanisms often present which
can include space-charge, orientation, atomic and electronic
polarization [24]. In Fig. 4 the frequency dependent permittivity of the baseline slurry composition (IPA / 20.0 vol.%
Al2 O3 / 5.0 vol.% PVB / 1.0 vol.% PE) is shown over the
range of 100 Hz to 1 MHz. Dispersion was observed at low
frequency (ω < 104 ), as the permittivity sharply increased
with decreasing frequency and a peak in tan (δ) was observed. At higher frequency (away from the dispersion) the
permittivity approached predicted values (see mixture equations) and became increasingly frequency independent.
The observed low frequency dispersion could not be attributed to the intrinsic properties of the individual components present in the slurry [23–25] and was therefore assumed to be due to polarization mechanisms resulting from
the presence of electrical interfaces in these mixtures. These
J Nondestruct Eval (2011) 30:81–90
include the dielectric relaxation of an electrical double layer
i.e. the counter-ion effect [31–33] and/or contributions from
interfacial charging occurring in electrically heterogeneous
materials (i.e. Maxwell-Wagner effect [34–36]). Both of
these effects result in low frequency dispersions when exposed to an alternating field. In the case of the counterion effect this is caused by the presence of a double layer
containing essentially free charge. This layer is easily polarized by an applied field resulting in a large apparent dielectric constant which increases with decreasing frequency.
The Maxwell-Wagner effects result from electrical heterogeneities which result in charge accumulation at interfaces
when a low frequency electric field is applied.
In the tape cast slurries studied here, interfacial polarizations may be present in several situations. Maxwell-Wagner
polarizations can exist at the electrode/slurry interface (i.e.
electrode polarization) [37], the powder/solvent interface
and the resin and/or dispersant/solvent interface. In addition,
an electrical double layer is certainly present at the Al2 O3
surface as this layer is used to disperse the ceramic powder
in the slurry [38].
The magnitude of the low frequency dispersion was
highly sensitive to the arrangement of the measurement apparatus and potentially processing steps which effected the
powder dispersion (such as the milling time and the dispersant content). Investigation of the effect of alumina content on the frequency dependent dielectric properties of at
low frequencies (i.e. <1 MHz) resulted in non-systematic
and non-reproducible variations in the dielectric constant.
Higher frequency measurement (above 1 MHz) resulted
in more systematic and reproducible compositional effects.
Thus, a measurement frequency of 1 MHz was chosen to
assess the process monitoring applications of interest here.
Additional studies at frequencies of less than 105 Hz can be
found in reference [39].
3.2 Binary Mixture Relationships
When measured at 1 MHz, compositional changes to the
slurry resulted in systematic changes in the permittivity. The
change in the permittivity of IPA for a binary mixtures with
PVB and ternary mixtures with a constant amount of PVB
(5.0 vol.%) and a varying amount of Al2 O3 (from 10 to
22.5 vol.%) are given in Fig. 5. A large reduction was observed for both the PVB and Al2 O3 . The observed data at
1 MHz were compared well to theoretically derived mixture
relationships. The PVB-IPA mixture fit well with a rule-ofmixtures relationship (using values of 3.0 and 18.5 respectively for PVB and IPA). The Al2 O3 additions decreased
the permittivity of the mixture considerably more than a
rule-of-mixtures relationship. It was found that these results
were comparable to the predictions of Bruggeman [40] for
the case of a mixture of dispersed ellipsoids in a dielectric
J Nondestruct Eval (2011) 30:81–90
Fig. 5 Plots showing the change in dielectric constant of (A) an
IPA solvent with PVB additions from 0 to 10.0 vol.% and (B) an
IPA—5.0 vol.% PVB mixture with alumina additions from 10 to
22.5 vol.% at 1 MHz. The PVB–IPA mixture was found to follow a
rule of mixtures equation (e1 = 20.0 and e2 = 2.8 as given in Table 1).
The Al2 O3 –IPA/PVB mixtures followed a Bruggeman mixture equation (e1 = 19.1 as given in (A) and e2 = 8.8 as given in Table 1). Note
the large sensitivity of the dielectric constant on the composition
medium when values of 19.1 and 8.8 for ε1 and ε2 respectively were used. This relationship accounted for geometrical particle shape effects which the effect dielectric properties of heterogeneous mixtures. The alumina particles used
in this case can best be described by an ellipse. The value of
19.1 for ε1 was taken for the measured dielectric constant
for a IPA-5.0 vol.% PVB mixture. The value for ε2 was
taken from literature values for alumina [24]. It should be
noted that the alumina powder used in the slurry could not
be directly measured using the liquid test cell and that the
dielectric constant of alumina is sensitive to small changes
in impurity level. Thus, the comparison to the Bruggeman
prediction is only an approximation.
4 Process Monitoring Applications
The above results indicate that systematic compositional
changes to the slurry can be observed by monitoring permittivity changes. Thus, in-situ monitoring of changes in the
composition during CMC processing appears feasible. For
the process discussed earlier, compositional changes may
potentially occur at two points in the process; (i) in the slurry
prior to or during the infiltration step and (ii) during the precision drying step. The implementation of a dielectric based
87
Fig. 6 Plot showing the change in the capacitance of three separate
infiltrated fiber mats during the removal of the slurry solvent. The infiltrated slurry had a composition of IPA—20.0 vol.% Al2 O3 —5.0 vol.%
PVB—1.0 vol.% PE Solvent removal is represented as a mass change
in the sample
sensing approach for these process steps are discussed in
further detail below.
4.1 Compositional Variation During Fiber Mat Infiltration
The infiltration of the fiber mat is a critical processing step
since it controls the distribution and volume fraction of the
ceramic matrix material. However, changes to the viscosity of the slurry (due to compositional variations) and the
removal rate of the fiber mat can lead to variations in the
infiltration of the slurry into the fiber mat and to changes
in the retention of excess slurry on the exterior of the mat.
To monitor these variations both a contact (prior to slurry
infiltration) and non-contact probes (post infiltration) are of
use.
A non-contact probe could be based on a parallel plate capacitor with the IFM between the electrodes and air gaps on
each side. The capacitance of this cell would depend upon
two variables, (i) the composition of the slurry and (ii) the
efficiency of slurry infiltration and retention on the fiber mat
surface (IRE). The composition is critical as it determines
the permittivity of the infiltrated material. The IRE is a measure of the volume of material adsorbed by a given area of
the fiber mat and thus affects the volume fraction of the IFM
(ν2 in (5)). Both variables have been experimentally investigated, Figs. 6 and 7.
The slurry composition effect was observed by measuring the change in the capacitance of an IFM infiltrated with
88
J Nondestruct Eval (2011) 30:81–90
results could be used in concert with other compositionally
dependent monitoring devices (i.e. viscosity or density monitors) to derive quantitative information about the composition of slurries.
4.2 Solvent Content of an Infiltrated Mat
Fig. 7 Plot showing the effect of alumina volume fraction on the capacitance of an infiltrated fiber mat. The infiltrated slurry had a composition of IPA—X vol.% Al2 O3 —5.0 vol.% PVB—1.0 vol.% PE where
X was systematically altered from 10.0 to 22.5 vol.%. The measured
capacitance was normalized by the volume of the slurry picked up (as
measured using a graduated cylinder) by the fiber mat during infiltration as viscosity changes in the slurry due to the change in the alumina
content were anticipated to vary the volume picked up by the mat and
in turn the initial capacitance
slurries having a variation in Al2 O3 content, Fig. 4. In this
case, increasing the Al2 O3 volume fraction also continually
increased the viscosity of the slurry [29]. This affected the
IRE and thus, the relationship between the slurry composition and the initial capacitance. As a result, the change in
the cell capacitance was normalized by the volume of slurry
removed by the fiber mat during infiltration. The addition
of Al2 O3 powder to the slurry then resulted in a decrease
in the normalized cell capacitance. This was a result of the
decrease in the slurry permittivity from the powder addition
(as shown in Fig. 3).
Changes in the efficiency of slurry infiltration and retention (IRE) were observed by altering the removal rate of the
IFM, Fig. 7. Increasing the removal rate resulted in an increased IRE. By measuring the resulting mass of the IFM,
changes in the slurry IRE could be monitored without varying the slurry composition. Increasing the IRE led to an increase in the cell capacitance as the volume fraction of slurry
increased. Thus, variations in the IRE may be monitored insitu if the composition of the slurry is known.
For the contact probe approach, a parallel plate capacitor, placed directly in the slurry container, could monitor
changes in the permittivity of the slurry. These measurements could then be related to slurry composition from the
high frequency mixture relationships determined above. For
multicomponent slurries (four or more components) these
After infiltration of the slurry into the IFM, a precise amount
of the solvent must be removed to facilitate the final consolidation of the tapes into structural components. Monitoring
the drying of the IFM is well suited to a dielectric approach
since solvent removal occurs at low temperatures, it effects
only one component of the multicomponent slurry and, for
measurements using radio frequencies, the solvent is the
largest contributor to the dielectric constant of the slurry. As
a result, evaporation of small volumes of solvent is anticipated to result in large charges in the cell capacitance. By
simultaneously monitoring the cell capacitance and the IFM
mass, the effect of solvent evaporation on the permittivity of
IFM was investigated. Plots showing the change in capacitance of an IFM during solvent removal are given, Fig. 8.
These plots indicate that the measured capacitance is linearly related to the sample weight (and therefore the amount
of solvent present). Note that, for the same drying conditions, the results are very reproducible with only small deviations observed between the evaporation slope(s). The capacitance change is due to the reduction in the permittivity
of the mixture and the reduction in the volume of the IFM
as the solvent is removed.
To monitor the solvent removal step, the capacitance
change of an IFM would be measured before and after solvent evaporation (or just after evaporation if the infiltration
process is adequately controlled). Such a set-up is shown
in Fig. 1 where a two capacitor configuration is shown in
which sensor #1 and sensor #2 are identical parallel plate
capacitors. Both capacitors are identical with the same electrode spacing, coaxial wire length (from the electrodes to
the analyzer), electrode alignment and temperature. The two
capacitors allow for the measurement of an initial and final
capacitance. Since the capacitance is linearly related to the
weight of the IFM and thus the solvent content, the change
in the volume fraction of solvent in the IFM is monitored.
5 Summary
The manufacture of ceramic matrix composites by a tape
casting technique presents several opportunities for a permittivity based process sensor since the composition of the
slurries can change in several of the processing steps and the
dielectric response of these slurries is strongly composition
dependent. In these systems, solvent wicking and evaporation, selective component pick-up by the fiber mat, powder
J Nondestruct Eval (2011) 30:81–90
Fig. 8 Plot showing the change in initial capacitance on the volume
of slurry picked up by the fiber mat during infiltration. The infiltrated slurry had a composition of IPA—20.0 vol. % Al2 O3 —5.0 vol.%
PVB—1.0 vol.% PE. The picked up volume was altered by systematically varying the pull rate of the fiber mat from the slurry from 10 to
150 mm/minute
settling during the casting step and solvent removal during
drying of infiltrated fiber mats all effect the composition of
either the slurry or the infiltrated fiber mat. The most promising opportunity for sensing appears to be for monitoring the
drying of infiltrated fiber mats since solvent removal occurs
at low temperatures, it effects only one component of the
multicomponent slurry and, for measurements using radio
frequencies, the solvent is the largest contributor to the dielectric constant of the slurry. As a result, its removal leads
to a large change in the permittivity. Monitoring approaches
designed to control compositional variations in the slurry
prior to infiltration and variations in the infiltration process
step also appear feasible.
Acknowledgements We are grateful to Hexcel Inc. for assistance
in this work. This research was supported by GE Aviation under the
program direction of Dan Backman.
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