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5/08 KIProi/Acc8Pres/ClRC/Dateoueindd

ELECTRICAL IMPEDANCE CHARACTERIZATION OF MICROPOROUS FILMS
AT ELEVATED TEMPERATURES WITH INTERDIGITATED DESIGN

By

Arjun Jayaraman

A THESIS
Submitted to
Michigan State University
In partial fulfillment of the requirements
For the degree of
MASTER OF SCIENCE
Department of Electrical and Computer Engineering

2008

ABSTRACT

ELECTRICAL IMPEDANCE CHARACTERIZATION OF MICROPOROUS FILMS
AT ELEVATED TEMPERATURES WITH INTERDIGITATED DESIGN

By

Arjun Jayaraman
A new high temperature four point electrical measurement system was built in
order to facilitate electrical impedance measurements on zeolite microporous
materials over wide temperature and frequency ranges and particularly in
vacuum and gaseous environments. The latter feature is important in sensing
applications for microporous materials. Electrochemical impedance spectroscopy
(IS) is a measurement technique that treats the sample under measurement as
the dielectric in a capacitor. A central component of the system built in this work
is an interdigitated planar capacitor (IDC) which was designed so that a large
surface area of the material may be exposed to a changing environment. These
IDCs were fabricated by combining a photolithographic ‘liftoff’ technique and
Electron Beam Physical Vapor Deposition. Measurements were taken on the

zeolite ZSM-5 and its hydrogen counterpart H-ZSM-5.

TABLE OF CONTENTS

LIST OF TABLES .................................................................................................. iii
LIST OF FIGURES .............................................................................................. iv
CHAPTER 1

.............................................................................................................................. 1
1.1 Introduction ..................................................................................................... 1
1.2 A Review of Zeolites ....................................................................................... 2
1 .2.1 Structural Composition .................................................................................. 3
1.2.2 ZSM-5 .......................................................................................................... 3
1.2.3 Applications of Zeoltes ................................................................................. 4
1.2.4 Electrochemical Properties .......................................................................... 4
1.3 Impedance Spectroscopy ............................................................................... 5
1.3.1 Theory .......................................................................................................... 8
1.4 Summation .................................................................................................... 13
CHAPTER 2

Room Temp Measurements and Interdigitated Design ....................................... 14
2.1 Introduction ................................................................................................... 14
2.2 Probe Station Data on Dehydrated and Non-Dehydrated ZSM-5 ................. 14
2.2.1 Measurements and Results ....................................................................... 16
2.3 Review of the Interdigitated Capacitor (IDC) ................................................. 19
2.3.1 Theory ........................................................................................................ 21
2.3.2 Experimental Procedure ............................................................................ 25
2.3.3 Zeolite Coatings ......................................................................................... 33
CHAPTER 3

HEISS Design and High Temperature Measurements ........................................ 35
3.1 System Capability ......................................................................................... 35
3.2 System Design .............................................................................................. 36
3.3 Calibration and Results ................................................................................. 43
3.3.1 Room Temp Compensation ....................................................................... 43
3.3.2 High Temperature Compensation and H-ZSM-5 Measurement ................. 49
3.4 Conclusion .................................................................................................... 55
3.5 Recommendations for Future Work .............................................................. 55
3.5.1 Accurate Circuit Model ............................................................................... 55
BIBLIOGRAPHY ................................................................................................. 58

iii

LIST OF TABLES

Table 1:,u = j (0C C, where CC is the capacitance of the empty measurement

cell ........................................................................................................................ 7
Table 2: Dimensions and capacitances of the darkfield lDCs ............................. 25

Table 3: Calculated resistance values for the standard load Pt — heater/resistor
vs. temperature ................................................................................................... 50

iv

LIST OF FIGURES

Figure 1: Framework of H-ZSM-5 viewed along [010] ........................................... 2
Figure 2: Ideal Cole-Cole impedance plot of the inset circuit ................................ 8
Figure 3: Frequency response of electric field on dielectric polarization [8] ........ 11
Figure 4: Janus Cryogenic Probe Station ........................................................... 15
Figure 5: ZSM-5 probe station results ................................................................ 18

Figure 6: Layout of the Interdigitated Capacitor. The width (w) of the strips and
the spacing (s) between them are shown. The length of the strips is denoted (p),
while (q) is the width of the striped area .............................................................. 20

Figure 7: Early Designs of lDCs. A hard mask is shown in (a), while (b) and (c)
are the film mask and an image of the actual IDC used in the probe station
experiments, respectively ................................................................................... 21

Figure 8: An Interdigitated Capacitor can be visualized as: (a) a parallel plate
capacitor whose (b) electrodes open up to provide (0) one sided access to the
material under test. The shaded area represents the sample being measured .22

Figure 9: (a) The Design for the darkfield mask. (b) 500x magnification of the
top right corner of IDC 1. The colors are reversed for clarity. The unlabeled

structures are for an unrelated project ................................................................ 24
Figure 10: lDCs 1-4 on Quartz ............................................................................ 30
Figure 11: IDC 2 at 1.063x magnification ............................................................ 30
Figure 12: IDCZ at 5x magnification .................................................................... 31

Figure 13: IDC 5 at 1.063x magnification ............................................................ 31

Figure 14: Screen Printed H-ZSM-5 on IDC 2 at 1.063x ..................................... 32

Figure 15: Screen Printed H-ZSM-5 on IDC 2 at 5x magnification ...................... 33

Figure 16: Design of “High-Temperature Environmental Impedance Spectroscopy

System’ ............................................................................................................... 36
Figure 17: Circuit schematic for the ceramic heaters (furnace) .......................... 37
Figure 18: Ceramic Heater Furnace .................................................................... 38

Figure 19: (a) Stainless steel sample stage (D) heated section of the sample

stage (0) electrical feedthrough hookup .............................................................. 39
Figure 20: OF Cross ........................................................................................... 40
Figure 21: Cross-sectional view of the quartz tube ............................................. 41
Figure 22: LabView Impedance Analyzer Program (front panel) ....................... 42

Figure 23: BNC cable measurements - short 5H2 -13MHz, to shows the direction

of increasing frequency) ..................................................................................... 44
Figure 24: BNC cable measurements - load 5Hz—13MHz ) ................................ 45
Figure 25: System Calibration Short 170Hz—2.8KHz ........................................... 46

Figure 26: System test — 1090 resistor in parallel with a 30pF capacitor (raw
data) .................................................................................................................... 47

vi

Figure 27: System test — 109 Q resistor in parallel with a 30pF capacitor
(compensated data) ............................................................................................ 48

Figure 28: Test leads in the open circuit configuration. The copper leads are
silver pasted ........................................................................................................ 49

Figure 29: Measured resistance values for the standard load Pt - heater/resistor
vs. temperature ................................................................................................... 51

Figure 30: H-ZSM-5 measurements at 685K and 690K. The 690K measurement
was taken after the HEISS system had gone through one temperature cycle. ...52

Figure 31: H-ZSM-5 measurement at 873K ........................................................ 53

Figure 32 H-ZSM-5 screen printed on IDC 2 at 5x magnification — post run ....... 54

Figure 33 Equivalent circuit diagram of a coated IDC ......................................... 56

vii

Chapter 1 — Background and Motivation

1.1 Introduction

This work evolved from a need to evaluate the electrical and ionic conductivity of
microporous materials namely, zeolites. Impedance Spectroscopy is a relatively
simple and inexpensive technique to characterize a material’s dielectric
polarization and conductivity. Microporous materials lend themselves to sensing
applications because of their environmental sensitivity. Tracking changes in
conductivity of zeolites when exposed to a variety of environments will lead to
improved sensor designs.

The classic impedance spectroscopy measurement [1] sandwiches the
sample under test between two electrodes — in the same configuration as a
parallel plate capacitor. An AC signal is applied to the electrodes and a response
is observed.

With zeolites as the sample under test, there is virtually no conduction
across the electrodes at room temperature because of the high thermal activation
energy. Therefore the objective of this thesis is to develop a high temperature
measurement system that can take IS measurements at temperatures ranging
from 25°C — 900°C. Furthermore, the conventional parallel plate design reduces
the sensitivity of the microporous materials to environmental stimuli. Hence
another important objective of this work is to develop electrodes which have a

planar interdigitated design. The sample under test is used to coat the

electrodes making it possible to expose the sample to environmental stimuli such

as vacuum, gas, and moisture.

1.2 A Review of Zeolites

As stated in the introduction, zeolites are microporous materials (or, more
accurately, a material which has a microporous continuous framework) [2].
Microporous materials have pour sizes of less than 2nm in size. The name
‘zeolite’ was coined in 1756 by Swedish Chemist Axel Fredrik Cronstedt using
the Greek words that mean “boiling stone” when he heated the naturally
occurring mineral Stilbite and observed the mineral ‘jump’ as the water inside its

pores evaporated [2].

 

Figure 1: Framework of H-ZSM-S viewed along [010]

2

1.2.1 Structural Composition

The zeolites are crystalline aluminosilicates (SiOz/A1203 blends) - they

are formed by linking the tetrahedra S104 and Si04 [2,3,4]. This gives a three-

dimensional framework structure in which each oxygen of a given tetrahedron is

shared between five tetrahedra. The negative charge of the A104' is typically

compensated by an exchangeable metal cation. This open framework (Fig.1)
allows for water molecules as well as cations to move easily within the crystals

leading to ion conduction/exchange and reversible sorption of water [2,3,4,5].

1.2.2 ZSM-5

For these experiments, synthetic zeolites ZSM-5 and H-ZSM-5 have been
selected. The reason for this is because of the availability of recent work using
these materials [3,4] and also because there is a wide availability of information
regarding their common structure.

ZSM-5 and H-ZSM-5 are very silicon like orthorhombic crystal structures.
Their pore channels, parallel to [100] together with those parallel to [010], give
rise to a three dimensional channel system that can be pictured, as in Fig.1, as a
series of overlapping tubes. H-ZSM-5 is the hydrogen form of ZSM-5. This
means that the charge compensating cation in H-ZSM-5 is hydrogen as opposed

to an alkali metal [3,4].

1.2.3 Applications of Zeolites

The most common applications for zeolites arise due to their ability to sorb
gases, vapors, and liquids; to catalyze reactions; and to act as ion exchangers.
Their selective pour size and regular pore structure makes it possible for zeolites
to act as “molecular sieves” - selectively sorting molecules based on size.
Specific uses of zeolites include: gas sensing, petrochemical cracking, oxygen
extraction (from air), water purification (softening), solar thermal collection,
adsorption refrigeration (wherein the reversible sorption of water is exploited),

and desiccation [2].

1.2.4 Electrochemical Properties

Most zeolites are ionic conductors and exhibit poor electrical conductivity
with an electronic band gap of approximately 7eV [4]. In an ionic conductor the
majority of charge carriers are charged ions rather than electrons — as is the case
with metals. The metal cations or, in the case of H-form zeolites, protons are
electrostatically bound to the polyanionic host lattice. The motion of
exchangeable cations in the zeolite lattice is a “cooperative diffusion process
over a heterogeneous system of potential barriers.” [3]. Unlike electrical
conductivity, ionic conductivity always involves a chemical reaction.
An expression for ionic conductivity in zeolites is derived from the Arrhenius

relation:

E
1T=A ——4—
11(0) +kT (1)

B

where a is “dc” ionic conductivity and A is a factor depending on the charge and

the number of mobile species, its on-site oscillation frequency and hopping
distance. E A is the thermal activation energy of the chemical reaction, k3 is
Boltzmann’s constant and T is the absolute temperature of the system. The

Nemst-Einstein relation correlates the Diffusion coefficient D with the ionic

conductivity 0 :
11:—"‘12 (2)
D kBT

where n is the number mobile species. Combining equations (1) and (2), Simon
et al. have solved for the expression for temperature dependent conductivity in a

cubic system which is [3,4]:
T-l[2/kTI1T2 —EkT 3
0(- )-3Ze B i( )aovoexp( A/ B ) I)
The charge of the mobile charge carriers is denoted as 28 ; ni(T), the

temperature-dependent charge carrier density; V0 , the oscillation frequency of

the located charge carrier (also the inverse lifetime of the ionic state); a0 , the

hopping distance between an occupied and an unoccupied lattice site.

1.3 Impedance Spectroscopy

The most common method of studying the electrochemical properties of
zeolites is known as complex impedance spectroscopy (IS). The process detects
the dynamic changes of an electrode-sample system. The system is excited by a

time-dependent perturbation (an external electric field or applied voltage) into a

5

non-equilibrium state. The system reacts with a time-dependent physically
detectable response Le. a polarization or current.

The impedance is derived by relating the response to the perturbation.
Impedance theory is governed by linear differential equations. Therefore, the
electrode-sample system under investigation must be a linear system. Most real
world electrode-sample systems exhibit non-linear behavior and IS data is only
valid in the range where the response is linear to the perturbation - meaning that
a monochromatic perturbation signal must not generate any harmonic signals [1].
The electrode-material system may be modeled with passive circuit components
where resistors are generally associated with conductivity paths while capacitors
and inductors most often represent space charge polarization regions and
specific adsorption and electro-crystallization at an electrode.

The basic impedance equation relates time-dependent voltage and current

signals as follows:

Z(w)=-‘l_’(Lw“1:—)=R—jX=Z'—j2" (4)

where R is the “real” resistive component and X is the “imaginary” frequency

capacitive component. There are other derived or measured quantities related
to impedance and collectively they are known as immittance. lS may also be
referred to as immittance spectroscopy. There are three basic immittance

equations in addition to the impedance equation (4). The first is admittance:

Y=Z‘1=G+jB=Y'+jY" (5)

in which G is referred to as the conductance and B is the susceptance. The two

other quantities are the modulus function
M=ijCZ=M'+jM" (6)

and the complex dielectric constant or dielectric permittivity

_ -1_Y
s—M _ /ij€:8,_1.8. (7)

where CC = 80/10 /1 is the capacitance of the empty measuring cell of electrode

area Ac and electrode separation length I.

Table 1: ,u = j wC C , where CC is the capacitance of the empty measurement cell

 

 

 

 

 

 

M Z Y a
M M ,u pYI 8'7
z ,u"M z Y’ p" a"
Y ,uMI Z’ Y ,u s
8 [MI ,u'I Z I ,uJY e

 

The most common way of displaying IS data is in a Cole-Cole plot [1, 3, 6]. In
this plot the two parameters (real and imaginary) of the complex conjugate of are
plotted against each other. In general, it is customary to mark the plots with an
arrow showing increasing frequency. Cole-Cole plots are two dimensional and
does not show a linear distribution of frequency plot points even when they are

logarithmically divided.

 

1.3.1 Theory

-lm(Z)

 

 

 

 

Re(Z)

 

Figure 2: Ideal Cole-Cole impedance plot of the inset circuit

Figure 2 shows a basic ideal impedance arc of an electrode-material system with
a geometrical capacitance C1 and a bulk resistance R1 which plots 2*. The

arrow indicates increasing frequency. There is a single time constant for this

system which is simply,
TD =R1 C1 (8)
and is the dielectric relaxation time of the basic material.

mp1,) =1 - (9)

where 09p is the peak frequency of the semi-circle and by extension is the point

where — Z " is at its maximum.

There are two basic ways an electric field can interact with a solid
material. The applied field can:

1. Transport charge through the material by the translational motion of
charge carriers. This leads to a dc conductivity 0 and,
subsequently, a current density:

i = 0E (10)

2. Lead to a displacement current resulting from the reorientation of

electronic dipole moments (in electrochemistry this is related to

complex defects in the material).

141%” (11)
In which D is the electric displacement:
D=soE+P (12)

and E is the electric field and P is the polarization of the dielectric material.
When an impedance arc is centered on the real axis, the time dependent

polarization P(t) is governed by first-order kinetics resulting in a single relaxation
time 1'.

The analysis of experimental data that produces a full semicircular arc in
the complex plane (as in Figure 2) can provide estimates of R1 and C1 leading to
quantitative results regarding conductivity, reaction rates, and relaxation times.
However, experimental data rarely leads to a full semicircle with its center on the

real axis. Common reasons as to why a semicircle may not be ideal are as such:

. Because the maximum resistance is greater than zero and leads to
the arc not passing through the origin.

. There may be multiple arcs present

. Depressed arcs are frequently displaced below the real axis
because of distributed elements in the system. This is caused by
the relaxation time not being single valued. It is continuously or
discretely distributed across a mean value.

. Arcs can also be distorted by other relaxation times that do not
involve the mean time constant being studied. Often, these
extraneous relaxation times can vary from the mean time constant

by two or three orders of magnitude.

10

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Orientational

 

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Types of . .
Polarization I Frequencues frequenCIes

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Figure 3: Frequency response of electric field on dielectric polarization [8]

A distribution of relaxation times or dielectric material polarizations may
result from topological defects, adsorbed species, or a variation in composition

and stoichiometry. There are four basic polarizations in a dielectric material —

11

electronic, ionic, orientational, and space charge - and are caused by the
application of an external electric field. Electronic polarization arises in an atom
when the negative electron cloud is displaced relative to the positive nucleus.
When ionic bonds are broken, positive and negative ions are separated and ionic
polarization results. Orientational polarization occurs when randomly oriented
polarized molecules align with the applied external field. The movement of free-
conduction electrons may be prevented from moving large distances by barriers
such as grain boundaries. When the external field is applied, the electrons ‘pile-
up’ against the barriers and produce a separation of charge that results in space
charge polarization.

Figure 3 shows that properties of a pure dielectric material change rapidly
at frequencies near a resonant frequency. There are separate resonance
frequencies for each type of polarization. The dominant polarization in a material
under test changes with frequency leading to a distributed relaxation time shown
in its Cole-Cole plot.

These material properties contribute to microscopically small resistances
and capacitances over a specific range of frequencies. When modeling a system
that includes these some or all of these material properties, one can use
transmission line theory or distributed circuit elements. The basic distributed
element is the constant-phase element’ (CPE) [1, 7].
ln Is it is much more common to use distributed circuit elements in modeling

systems. The CPE is given as:
ZCPE: Ajwya (13)

12

where A is a pre-exponential factor based on the distribution of time constants. If
a=0 the CPE is a resistor and if a=1 then it is a capacitor.

The disadvantages of IS are associated with ambiguities in the circuit fitting
process. Many circuit models may fit one material system. This means that all
fitting must be correlated with previously obtained data and knowledge regarding
the material under test.

Accurate circuit fitting and system characterization are both aided by the
use of complex non-linear least squares fitting using the Levenberg-Marquardt

algorithm.

1.4 Summation

Recent studies [3,7] have utilized both zeolites and CPE theory to develop
environmental sensors. The aim of this work is to demonstrate the design of an
impedance spectroscopy measurement apparatus that is able to perform at high
temperatures and thereby overcome the thermal activation energy of ionic
conductive charges in zeolitic materials. The overall design of the apparatus
incorporates a vacuum valve, gas inlet port, and planar interdigitated electrodes

for the future creation of a gas sensor.

13

Chapter 2 - Room Temp Measurements and Interdigitated Design

2.1 Introduction

This chapter describes the initial room temperature measurements on
ZSM-5. A review of Interdigitated Capacitors (IDC) is included to illustrate
methods of solving for the lDC's capacitance. This is followed by demonstrating
the proper circuit model for the electrode-material system and explaining why it

was chosen (based on previous experimental work and theory).

2.2 Probe Station Data on Dehydrated and Non-Dehydrated ZSM-5

The first electrical impedance measurements on zeolite material were
conducted in a micromanipulated ultra high vacuum system probe station (Figure
4). The probe station is connected to a roughing and turbo pump and can attain
a vacuum of (>10‘4 Torr). In the main chamber there is a stage to place samples.
The main stage has a small heater-resistor housed underneath it. The maximum
temperature the heater resistor can go to is 450K. There are six probes in the
chamber that can be adjusted individually in the x, y, 2 directions. Two of the
probes are designed for high frequency microwave measurements. The other
four probes are used in lower frequency measurements. These four probes were
connected to a low frequency (5H2 — 13MHz) impedance analyzer.
The impedance analyzer has four output connections - an oscillating output,
providing a (5Hz - 13 MHz) low voltage (<1V) AC signal designated l+, a low

current output channel that was connected to ground in our setup (I-), and two

14

input voltage terminals (V+ and V-). The impedance analyzer measures the drop
across the voltage terminals and divides this drop by the input current and

displays the resulting impedance.

 

Figure 4: Janus Cryogenlc Probe Station

The end probes were each arranged on the surface of a simple
interdigitated capacitor (IDC). with each pair of digits connected to a contact pad.
The sample used in this experiment was the commercially available product of
Zeolyst Int. CBV 3024E ZSM-5. The sample Si02 /A1203 ratio is 30. Water was
added to roughly 0.59 of the powder sample until a paste-like film is formed. The
film is then coated evenly on top of the IDC digit pattern. The |+ and V+ probes

were placed on one contact pad and the l— and V— were connected to the other.

2.2.1 Measurements and Results

Impedance measurements were conducted across the full spectrum of
frequencies (5H2 — 13MHz) with 100 data points, spaced logarithmically,
collected on each run. Measurements were taken at room temperature under
vacuum (around 6 x10‘4 Torr),

The measured impedance is presented in the form of a Cole-Cole plot
(Figure 5). Dehydration of the ZSM-5 resulting from vacuum, leads to a sharp
increase in the lossy part of the impedance. This proves that the conductivity of
the zeolite material is higher when water is present.

If the objective is to quantify the changes in conductivity with changing
levels of humidity present then it is possible to analyze the data in Figure 5 using
the simple RC circuit presented in Chapter 1. Two assumptions must be made to
facilitate this analysis — First, there can be only one dominant time constant.
Second, because of the first constriction, only the higher frequency arc (right
most arc) will be analyzed.

Recall from the first chapter that in a simple ideal lumped element RC
circuit
TD = RC (14)
and subsequently

L
C = 31:4. (15)

16

where mp is the frequency corresponding to the highest y-data point on the arc.

For the ‘hydrated arc’, mph = 70.12Krad / s. The are has been fitted with a

fourth-order polynomial curve. This curve has been extrapolated to the right so

as to find the x-intercept of the curve. This intercept is the approximation of the
resistance of the RC circuit. For the hydrated arc Rh =435KQ, solving for the
capacitance is simply a matter of plugging these values into equation (14) which

gives C h = 32.27 pF . Analysis of the ‘re-hydrated’ arc yield these values —

mp, = 70.12Krad/s, R, = 455KQ, C, = 30.86pF.

17

Room Temperature Measurements on ZSM-5

500 103

 

400103 '

300103 I

200 103

 

 

 

G Hydrated ZS‘II-E
O Dehydrated ZSM-5
V Rehydraled ZSM-5

100103

 

 

 

010%" .
010° 200103 400103 600103 800103 1106 12106

Figure 5: ZSM-5 Probe Station Results. Cole-Cole Plot — where the x-axis ls Realm and the y-
axls ls -lmag{Z}.
This data analysis illustrates impedance spectroscopy technique and

shows how ionic conduction in this material is facilitated by water. However,

18

additional studies must be made to this circuit to determine where and how the

electrode/sample is most sensitive to environmental changes.

2.3 Review of the Interdigitated Capacitor (IDC)

An IDC is a pair of electrodes that resemble two “comb” like interlocking
structures [9,11]. As shown in the example in Figure 6 where the black strips
represent electrodes, the IDC is a planar device. The electrodes are deposited
on a substrate. Over the past two decades there has been an increasing interest
in IDCs. Applications of lDCs range from photosensitive detectors, Surface
Acoustic Wave (SAW) filters and sensors for chemicals and gasses. Since the
long-tenn goal of this work is to develop an environmental sensor using
microporous materials, the IDC serves as an attractive choice for carrying out the
impedance spectroscopy measurements. The material under test is usually
deposited over the interlocking electrodes. The inherent advantage of the planar
IDC in environmental sensing is apparent - only single-sided access to the test
material is needed for penetrating the material with an electric field. This leaves

the other side of the material open to the environment.

19

 

Figure 6: Layout of the Interdigitated Capacitor. The width (w) of the strips and the
spacing (s) between them are shown. The length of the strips ls denoted (p), while (q) Is
the width of the striped area.

There have been many generations of IDC designs created as part of this
thesis. The first was based on a “hard mask” design that is shown Figure 7(a).
This mask was created by etching out a pattern in stainless steel. The mask was
placed over a substrate material in the physical vapor deposition chamber and
metal was deposited. The metal etching tools that were used in creating mask
could not yield an IDC pattern that had the large surface area and small finger
size needed for impedance spectroscopy.

The next design investigated is pictured in Figure 7(b). It was created by
printing out the desired pattern using a high definition laser printer on transparent
film. The film was used in conjunction with the photolithographic liftoff technique
that is detailed later in this chapter. The substrate was made out of UV fused
silica. A photo of the resulting IDC is-shown in Figure 7(c). This IDC was used in

the probe station experiments. Sharp patterns on the substrate were achieved

20

as shown in Figure 7(c), however there were some challenges with these initial
designs. The transparency film would degrade rapidly during the processing,
and the spacing and width of the lines needed to be adjusted for improved
coupling to the sample [9,10,11,12]. To build a more accurate circuit model, and
understand how to optimize the design, the theory of interdigitated capacitors

was studied.

 

(C)

Figure 7: Early Designs of IDCs. A hard mask is shown in (a), while (b) and (c) are the film
mask and an image of the actual IDC used in the probe statlon experiments, respectively.

2.3.1 Theory

As noted in the introduction, the classic Impedance Spectroscopy
experiment utilizes the theory of a parallel plate capacitor - the material under
test serves as the dielectric material separating two charge plates. A fringing
field planar capacitor has the same principles as the parallel-plate model. Figure

8 shows a gradual transition from a parallel plate capacitor to the fringing field

21

design [11]. In each case, electric field lines penetrate the material under test —
the capacitance and conductance between the two charged electrodes depends
on the dielectric properties of the material between electrodes and also on the

electrode material and geometry.

 

 

   

(a) (b)

Figure 8: An Interdigitated Capacitor can be visualized as: (a) a parallel plate capacitor
whose (b) electrodes open up to provide (c) one sided access to the material under test.
The shaded area represents the sample being measured.

In general, the capacitance between two coplanar electrodes (as shown in
Figure 8(c)) is quite small and therefore necessitates repeating the pattern many
times which increases the overall capacitance of the electrode structure - hence
the ‘comb’ like structure.

The widths chosen for the digits and intervening spaces can affect the
Impedance Spectroscopy properties of the device. If the electrode spacing is
small, the alternating current flowing between them will sample only the region of
the film immediately above them; if the spacing is large, the current will tend to
spread out vertically throughout the film thickness, and sample a larger

proportion of it.

22

 

m :I nus—rm. __._)-.‘.." .1.~.

Otter [10] has described an approximate analytical estimation of the
capacitance of an IDC. His analysis is based on the basic equation of C = g.

In this case, C is capacitance, V is the potential, and Q is the charge. They are

 

 

 

 

given by
4V 1 (2n—1)7zs
V _ 0
0"” 7:32;: —1J°( 2a )x
(16)
sin((212 —1)7Lx]exp[(2n — D7431)
a a
s/2+w s/2+w
0V(x.y)
Q= a(x)dx=—1im240 dx <17)
S/IZO- p y—)0 S/JZ Q}
4 2(zn— 1)7rs as]
C _ —g ——
INT Pq mn=12n1- _1 J0( 2a (18)

where p denotes the length of the striped area and q is the width of the striped

area (Figure 6). The sum of s and w is denoted as 3. J0 is the zeroth order

Bessel function of the first kind. Otter notes that equation (x) is a good
approximation of the capacitance if ya > 0. land reflect a previous work [10] that

requires complicated numerical methods.

23

 

;__!_
II. I

 

 

 

 

 

 

 

 

(b)

Figure 9: (a) The Design for the darkfield mask. (b) 500x magnification of the top right
corner of IDC 1. The colors are reversed for clarity. The unlabeled structures are for an
unrelated project.

24

Table 2: The dimensions and calculated capacitances of the darkfleld Interdigitated

 

 

 

 

 

 

 

 

capacitors
S (m) W (W) W p (mm) <1 (mm) Cm mF)
IDC 1 20 5 0.2 4.175 4.640 42.10
IDC 2 20 20 0.5 4.160 2.920 10.22
IDC 3 10 10 0.5 4.180 2.900 20.31
IDC 4 50 5 0.909 1.475 5.300 10.14
IDC 5 50 50 0.5 1.430 2.900 1.396
IDC 6 500 100 0.167 7.500 17.00 12.38

 

 

 

 

 

 

 

2.3.2 Experimental Procedure

The current mask is a darkfield photomask (Figure 9). There are six lDCs.

Their dimensions and calculated theoretical capacitances, based on equation

(18), are shown in Table 2. The capacitance was calculated using Mathcad and

the first 500 terms were summed. The dimensions of IDC 3 were based on an

IDC designed in [13] which had a measured capacitance of 23pF.

Lithographic Procedure:

Quartz Clean

1. Soak substrate in the De-Metal bath - HCl : H 202 : H 20 — 1:1 :6

at 55°C for 1 minute

NH40HzH202 :H20—1:1:6 at 55°C for1 minute

25

Soak substrate in the De-Grease bath

Sonicate substrate in acetone for 5 minutes

 

4.

Soak substrate in methanol for 45 seconds

5. Soak substrate in de-ionized water for 45 seconds

6.

Bake substrate at 70°C for 25 minutes

Photolithography

1.

8.

9.

Spin Shipley Microposit S1813 Positive Photoresist at 3000 rpm for

30 seconds
Bake substrate at 70°C for 25 minutes

Inspect with Zeiss differential interference contrast microscope

. Align substrate with soda-lime mask using Suss MJB3 Submicron

Mask Aligner

Expose substrate with UV light for 1 minute

Soak substrate in chlorobenzene for 1 minute
Develop in MF-319 Developer for 2.5 minutes
Rinse substrate with De-lonized water for 1 minute

N2 dry substrate

10. Inspect with Zeiss differential interference contrast microscope

Physical Vapor Deposition

1.

2.

Clean inside of the PVD chamber using methanol and clean-room
wipe.

Unscrew the hold down ring from sample stage.

26

3. Place quartz substrate on sample stage and fastened using two
razor blades that are screwed in to sample stage.

4. Start the system by turning main power switch to ‘start’

5. Switch on motor switch on outside of chamber door.

6. Carefully turn the sample stage so that substrate is facing down.
Make sure substrate does not fall.

7. Turn of motor and return sample stage to its original position.
Close chamber door.

8. Turn on the N2 tank that is attached to system

9. Open the e-beam target holder by unscrewing large set screws.

10.0pen the e-beam shutter by turning appropriately titled knob on
main panel.

11.Place individual graphite crucibles, one filled with gold other filled
with tungsten, into two open slots in uncovered target holder.

12. Make sure to remove any excess debris in target holder area.
Close the shutter and return target holder to its original position and
tighten set screws.

13. Make sure the Hl-VAC throttle valve is open and Hl-VAC valve is
closed.

14. Pump down chamber to approximately 5X106 Torr. Monitor
pressure with ion gauge.

15. Once this achieved, turn on the water chiller unit. Set temperature

to 20 deg C and the pressure to 65-70psi

27

16. Rotate stage approximately 45 degrees until stage mirror shows
crucible holder.

17. Open the e-beam shutter and make sure the target metal is
reflected in stage mirror.

18.Turn on the power switch at the bottom system. This is for e-beam
gun.

19. Everything is working properly if all 5 interlock LEDs are turned on.

20. Turn up current very slowly to around 10mA or whenever e-beam
becomes visible.

21 . Position the beam on target metal (titanium) and turn off e-beam.
22. Rotate stage so that it is 180 degrees from its original position and
turn on deposition monitor. Make sure displayed thickness has

been reset to zero.

23.Tum on the e-beam current and slowly adjust so that displayed
deposition rate is 0.5 Angstroms per second.

24.0nce desired thickness has been achieved (10 nm), turn down
current on the e-beam and shut it off. Wait until deposition rate is
at zero before closing e-beam shutter.

25. Repeat steps 16-24 for a gold target (200 nm)

26.After final deposition, wait 45 minutes with chiller and vacuum
running to allow for e-beam gun to cool down.

27.Tum off vacuum

28. Wait for green lights on vacuum control panel to turn off.

28

Liftoff

29.Vent system and turn off chiller.

30. Remove substrate from chamber and turn off system.

1. Dry the substrate with N2

2. Soak and gently agitate gold covered substrate in acetone for 2
minutes

3. Inspect with Zeiss differential interference contrast microscope

4. If the interdigitated pattern does not look clean repeat steps 2-3

5. Rinse substrate with De-lonized water for 1 minute

6. Dry the substrate with N2

7. Place in High Temperature Furnace (outlined in chapter 3). And

anneal in vacuum at 450°C

29

 

IDCs 1-4 on Quartz

Figure 10

 

Ion

: IDC 2 at 1.063x magnificat'

Figure 11

30

 

Figure 13: IDC 5 at 1.063x magnification

31

The results of the process are displayed above. Figure 10 shows two
quartz substrates (2" x 1") which have |DC(s) 1-4 printed on top. The IDCs are
cut into individual pieces using a diamond wheel saw. IDC 2 is shown in Figures
11 and 12 and IDC 5 is shown in Figure 13. Both of these Interdigitated
capacitors produced small DC resistances (approximately 10!!) across
electrically connected contact pads and open circuit conditions (<2MQ) for
contacts that were not connected. However, because of the smaller test area
and capacitance of IDC 5, IDC 2 was chosen for the initial high temperature

measurements.

 

Figure 14: Screen Printed H-ZSM-5 on IDC 2 at 1.063x magnification

32

 

Figure 15: Screen Printed H-ZSM-5 on IDC 2 at 5x magnification

2.3.3 Zeolite Coatings

The new lDCs are coated with a zeolite/water suspension using a t-shirt
silk screen. The suspension is made by measuring out 100ml of ZSM-5 powder
with just enough water to ensure a ‘paint like’ consistency - usually around
20 ml of water. The contact pads of the electrodes are covered with Kapton
tape so that the zeolite only covers the interdigitated ‘fingers.’ The screen is then
placed onto the IDC and the zeolite suspension is spread on top of them. A
squeegee is used to ensure that there is a zeolite layer of uniform thickness
across the substrate. Much of the water is strained out of the pores of the zeolite
by this process. This means that these new films will exhibit lower conductivity

than the previous films.

33

It has been shown [13] that at a temperature of 493K, the ZSM-5 is
completely dehydrated and at 673K the thermal activation of the material allows
for ionic conductivity[3]. The next chapter deals with the design and
implementation of high temperature measurement system to investigate the

immittance parameters of ZSM-5 and H-ZSM-5 at elevated temperatures.

34

Chapter 3 - HEISS Design and High Temperature Measurements

The thermal activation energy required for ionic conduction in dehydrated
zeolites is quite high — for H-ZSM-5 this occurs around 400 °C [3, 4]. In order to
facilitate the observation of this phenomenon it became apparent that the
cryogenic probe station would need to be replaced by a high temperature
measurement apparatus. The design, construction, and characterization of this

system is discussed in detail here and is the hallmark of this work.

3.1 System Capability

The intent of the high temperature measurement apparatus is to
accurately take a four-point electrical measurement at temperatures in excess of
400 °C. Specifically it must be able to house a planar interdigitated capacitor and
raise the ambient temperature precisely - a digitally controlled measurement
oven with a planar stage. This apparatus must contain heat effectively so as not
to adversely influence the surrounding environment in particular the impedance
analyzer. Such an apparatus will allow exploration of ionic conduction in
microporous films. It is hoped that these results will be used to investigate how
changing environmental conditions affect conductivity with the ultimate goal
being the development of a gas sensing application. Therefore, the
measurement apparatus contains a gas inlet valve and a vacuum port so as to

create an isolated environment for electrical impedance measurements.

35

3.2. System Design

Gas Inlet

Ball Seal Electrical Feedthrough

L: 50000

 

 

 

 

 

 

 

1— |] :; '::\
To Roughing Pump

 

   

 

 

 

Stainless Steel Horizontal Measurement Stage
Split Ceramic Fumaoe

Figure 16: Design of ‘HIgh-Temperature Environmental Impedance Spectroscopy System‘

The measurement system, which has been dubbed the ‘High Temperature
Environmental Impedance Spectroscopy System’ (HEISS), is drawn in Figure 16.
The system consists of a 5’ quartz tube that has been inserted into two 1’ long
semi-cylindrical heaters. The quartz tube has a ball seal joint on either end of it
that clamp into stainless steel quick flange (QF40) components. One end of the
tube leads to a roughing pump and the other to a stainless steel quick-flanged
cross that contains a multi-pin copper/nickel electrical feedthrough, a gas inlet

valve, and a stainless steel horizontal stage which extends into the middle of the

36

 

furnace section of the tube.

| 0 Input From TC (On=5V DC OFF=0V DC)

 

 

 

 

4.) e ’\/\/\,————’\/V\,
SSR ”1 ° H2 ’

 

 

 

 

110VAC

E9

 

 

 

I

Figure 17: Circuit schematic for the ceramic heaters (furnace)

The ceramic semi-cylindrical heaters are evenly split sections of a tube
furnace. They are made of ceramic fiber and imbedded with helically wound
iron-chrome-aluminum heater wire and have been purchased from Omega
Engineering. The heaters are connected in series as illustrated in Figure 17 —
where they are represented by resistors H1 and H2. A solid state relay controls
the current flow to the heaters via a temperature controller. The temperature
controller determines the temperature of the furnace through a high temperature
Type K thermocouple inserted between the two heater sections. If the ends of
the heaters are filled with insulation, it is possible to raise the temperature inside

the furnace to 950 °C.

37

ma«.r.yeermma;mmié

. 4‘ _ ' \.
/ _ , . . \ /‘ \ . '\ >

\ \

/

/ /” \\ /' '

\.

\ /' \\ 1' \ .
\4_ // \ / \\ /

 

Figure 18: Ceramic Heater Fumaoe

However, when the quartz tube is inserted (Figure 18) the maximum temperature
is reduced to 650 °C. Quartz has a very low thermal conductivity [15], so the
ends of the system remain near room temperature even when the furnace is set

to its highest temperature.

38

f 334' .-

 

 

 

 

Copper Leads - Connected to Electrical Feedthrough -\

m I < 2&0
\\ 2 Bore -Alurrina Tubes

(a)

 

 

 

 

 

 

 

\ I...
o d >
" V+
1 EV. \
_/O a I | \
Stainless Steel Stage ~———— 3" \L Nm\nsmbes

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

  

 

 

 

 

 

 

 

6)) (DC
BNC Connection (to Inpedanoe Analyzer)
L T Electn'ml Feedthrough
I‘ Copper Wire Leads
- 3: Stainless Steel QF Endcap
J

 

 

 

 

(C)

Figure 19: (a) Stainless steel sample stage (b) heated section of the sample stage (c)
electrical feedthrough hookup

39

t " " ' . ' z; mums .1 mm,
. n Mal Stir
.. .. . ,

girl-“MI“

........... eta-Glam)

 

Figure 20: QF Cross

40

Widest section of

Stainless Steel Stage Quartz tube (Inner ’— )

Smallest section of
Quartz Tube (near ball joint)
(inner r=1.6")

 

2 - Bore
Alumina Tubes

Figure 21: Cross-sectional view of the quartz tube.

Illustrated in Figure 19, this horizontal sample stage has two 2’ 2-bore
alumina tubes extending to the middle furnace section of the quartz tube that are
tied down with 10 mil wire. These tubes contain four 10 mil copper electrical
leads which are connected to the electrical feedthrough at the top at the quick
flanged cross. Figure 21 shows a cross-sectional view of the quartz tube. The
feedthrough has four BNC female adapters soldered to its outer pins; this allows

the leads to be connected to the HP4192A LF impedance analyzer.

41

l> Illl["'lItIIll r» ,“r- r urtnm‘nl‘: vr | 'ur-tl innl
5e Eel wade

BEE—EE-a

 

lnpedmoe Andvzer Proqam

Sew-ow Finn—u ‘ .
g) iamlmiifl'rmiiii‘agis‘fiig " "WM”
Path ,
fizzwmzumm

 

I MINI

1.015442
[0.

1 :r . I b,

 

u fl- »‘
I

 

 

Figure 22: LabView Impedance Analyzer Program (front panel)

The impedance analyzer is controlled through a ‘general purpose interface
bus’ (GPIB) by a LabView program that was initially designed by myself but was
improved upon by Adam Downey. Figure 22 shows the front panel of this
program. The program can set the analyzer measurement method (Impedance,
Resistance, Inductance, or Capacitance). The frequency sweep mode: linear —
where the difference between two adjacent frequency points is held constant
throughout the sweep — or logarithmic — in which the sweep is logarithmically
spaced. In addition, the user is able to select start and stop frequencies

(anywhere from 5H2 to 13MHz), the total amount of points in a sweep, whether

42

or not to clear the analyzer’s internal memory and how long to wait between
setting a frequency in the analyzer and collecting the associated impedance
value. The impedance sweep is collected in a comma delimited text file and

saved to a user-specified location.

3.3 Calibration and Results

The HEISS degrades the accuracy of the impedance analyzer by its
characteristic impedance mismatch to the HP4192A. Error correction is needed
to improve the signal. The HP4192A is equipped with an internal spot frequency
Open/Short compensation function called the Zero-Offset Adjustment. It
measures the residual impedance of a test fixture under short circuit conditions
and stray admittance under open circuit conditions [16]. The HEISS is a custom-
made test fixture with non-standard length cable test leads and so the
Open/Short compensation would not provide adequate error correction. Adding
a standard load to the compensation function improves the accuracy and

repeatability of the measurement [17].

3.3.1 Room Temp Compensation

The calibration for the room temp uses solder to connect the l+ and l-
leads and separately connect the V+ and V- leads for the open calibration. The
first step is to see if the impedance analyzer works with the 2m BNC cables that
are used in the system (Figure 20). The test uses a 47 Q resistor for the

standard load in the cable tests. The reason there is no plot of the open circuit is

43

because the analyzer senses that the resistance is greater than 1.28 MO. The

analyzer displays “UCL” which designates an open load.

Cable Calibration - Short 5Hz-13MHz

 

 
 
 
 
   

 

 

 

 

0_02 _ t "”r” r T , r T 1 r I r T 1 r l r 1 r r I r I Tfifi
_ ‘»SifftilfleQLW-mfifim .
L "‘“J’f -
.
o L :1 4
L 4
I -
L .—
-o.02
-0.04 §— 4
L n .4
'_ 1
-0.06 ._ -
E I '_ ‘
L a 4
5' ~
-0.08 a
L E; 4
a) .
i. E4 4‘
t 7 I
L Li; ‘
3' t3. ‘
_O14 L1 111 I 1 1 1 L11_‘ 1 . 1 1 . 1 ' 11 t I . 1 I 2:11_-.Z
-0.05 -0.04 -0.03 -0.02 -0.01 0 0.01 0.02

Figure 23: Cole-Cole plot - where the x-axls is Real{Z} and the y-axls Is -lmag{Z}. BNC
cable measurements - short 5Hz-13MHz. in shows the direction of increasing frequency.

44

Cable Calibration Standard Load - 470

 

 

 

0.2?TllétIergfrri[Virllrrrrlrfirlr'rr
F -<
l— _
0.1 g .
t 4
l
L .4
L 4
:— 1
0 fr— _.
i
Ir— an
L
i
L
-0 1
’- -4
L 4
L.
L "LIL-4 *
; k\./}—”‘~
-O 2 5" "vi-1‘".
' 5 "(1“.- -.
“My“
2 'v-‘¥).-'\.
>50;
;— I"/‘ W
e ' ‘ I A'
z \. .7 ’ x
'— . .-..
-0 3 "“ ‘vaLlfi/‘l-M‘
‘ f ”43' q
L ‘- ~./(v~{ 3” ,_ [A
' 1.1,) if ,_
’- “”1"3’\ /‘- v4
, 2 1,)
L -1
i- 4
-04 L_.._-_1__1.__1_ ,_ = 1 1 1 5 ;J_1 L 1 x 1 1 1 l 1 = 1 1 l 1 1 1 1 l 1 1 L__L_

 

 

46.2 46.3 46.4 46.5 46.6 46.7 46.8 46.9

Figure 24: Cole-Cole plot - where the x-axis is Real{Z} and the y-axis is -lmag{Z}.BNC
cable measurements - load 5Hz-13MHz

The analyzer locks and does not display any measurements between the
frequency ranges of 5 Hz - 169Hz and 2.9KHz - 33KHz when the leads are
shorted. Also, there is a significant amount of interference for the frequency
range of 34KHz-13MHz. Therefore the data range for the HEISS is confined to

170Hz-2.8KHz. The standard load impedance is a 1000 chip resistor.

45

System Calibration Short (170Hz-2.8kHz)

1.3.5114“ 5.,er 1 1 I I r , Tm.Twp“,.___,L._._T__,T_.,..,...1.r.-....T.N....L_ , I,

 

 

 

{ .
r .
1" I“ ,
L ' 59f) . ;
‘. L
O 7 L_l_;‘.__l___.l. ._ .l__-L- _.1__ _1.___3_-_14 1 1 i 1 1 4 L 1 1 L 1 1 1 1

Figure 25: Cole-Cole plot - where the x-axis is Real{Z} and the y-axls is -lmag{Z}. System
Calibration Short 170Hz-2.8KHz

46

1090 n 30pF (170Hz-2.8kHz) Raw Data

 

 

 

 

12 :r 67 T Y {—T 7 T fir T T 1 Y L I l T T L l 7' 1 T
10 9»

8 i— 1
4 —1
2 '_; 1 1 ¥ 1 1 1 l 1 1 1 1 1 1

106 106.5 107 107.5 108 108.5

Figure 26: Cole-Cole plot - where the x-axls is Real{Z} and the y-axls is -lmag{Z}. System
test - 1090 resistor in parallel with a 30pF capacitor (raw data)

47

Compensated 1099 || 30pF (170Hz-2.8KHz)

O _ 7 ' 1' WT i T I i T i i 1 i T T i I i i i 1 "77717" 7'1""""T""""T7 "7"" '

 

0.6 _ / ' (:15? C- ——

{$1.17,}
0.5 '_ (7:.f‘.“ _4

  
  

‘7 x
0 4 c :22 a
' 1- -~_ ("1.71517 .
QIL> C ‘
r 0.1.1, .A. ‘
l— ‘77} ’7“ 7' ‘77.) —i
‘31?va
i C Kim] ~
0 3 *- <;'ri:2:©<:> o 1
1 .5”) if? '00! (I) 1
» '31-’61» 1
~ C(‘_‘.-"T(111':)’.-'.1‘ C) 4
.- .. m 1113315131331 <21 o .
‘14 r” -. (“v s O (77 1.61.1”:
“1111:: -. ' ‘ ~* JL up" @3331 1:,-
" (“‘1 1‘ .’ C‘ :(J O o O C) .-l _‘
L 900.. ' m? 4
‘1) C’ 00° 6" \ . 1“ O C)
' (‘1 M 4
V x,

 

 

" (@333 “
0 1 1 1 1 1 l 1 1 1 1 11‘} 1 1 a 1 1 - 1 ' ' 1 1 1 1 1

109.6 109.7 109.8 109.9 110 110.1

 

Figure 27: Cole-Cole plot - where the x-axis is Real{Z} and the y-axis is -lmag{Z}. System
test — 109 a resistor in parallel with a 30pF capacitor (compensated data)

Figure 29 displays the raw data of a test load of 109 Q resistor in parallel
with a 30pF capacitor. Figure 30 displays the results of the open/short/load
compensation of the data in the previous figure. The reason this circuit is chosen
is because the test capacitance is on the same order of magnitude as the lDCs’

capacflance.

48

3.3.2 High Temperature Compensation and H-ZSM-5 Measurement

 

Figure 28: Test leads in the open circuit configuration. The copper leads are silver pasted

49

Table 2: Calculated resistance values for the standard load Pt - heater/resistor vs.
temperature

 

Temperature Resistance

 

(Kelvin) (n)
273 ‘ 100.00
295 w 108.57
373 138.51
473 175.85
573 L 21205
573 ' ‘ 247.09
773 280.97
873 313.89

 

50

Standard Load 1090 Pt-HeaterlResistor (170Hz-2.8kHz)

 

 

 

 

 

 

 

 

 

 

 

 

60 h T I Y T j ' fl 1 1 1 ‘fi T ‘ T T Y 'fir i i f T T %
8 295K (Room Temp)

50 2 373K
. V 473K 13.-1‘ 1

“ A 573K 1; g

- <1 673K 1
40 j =1 773K e. 4
, 873K . j

30 :— 2
20 _. 1
10 — f1) j
o .l .
-10 I L 1 L 1 L 1 1 1 1 1 l
100 150 200 250 300 350

Figure 29: Cole-Cole plot — where the x-axls is Real{Z} and the y-axis is -lmag{Z).
Measured resistance values for the standard load Pt - heater/resistor vs. temperature

51

H-ZSM-S (170Hz-2.8KHz)
5105 r I

 

‘‘‘‘‘

 

H 685K
t " a 690K (Second Run)

4105 ~
3105 ~

2105 —

 

 

 

1105 ——

 

 

 

 

011111'11311L;51%31111111'131
5105 6105 7105 8105 9105 1106 1.1106

Figure 30: Cole-Cole plot - where the x-axis is Real{Z} and the y-axis is -lmag{Z}. H-ZSM-5
measurements at 685K and 690K. The 690K measurement was taken after the HEISS
system had gone through one temperature cycle.

52

H-ZSM-5 (170Hz-2.8kHz) 873K

 

 

 

 

1.4104Tt011010111001
1.2104L-
110k .0
r 4
8000* =
1 1
0000— a
1 ~«
4000- 4
zoooiwwzw:H-—
8.5104 9104 9.5104 1 105 105105

Figure 31: Cole-Cole plot - where the x-axis is Real{Z} and the y-axis ls -lmag{Z}. H-ZSM-5
measurements at 873K

53

 

Figure 32: H-ZSM-5 screen printed on IDC 2 at 5x magnification - post run.

Silver paste was used to connect the leads for measurements done at
high temperatures (Figure 31). The standard load was a Pt 1000 (273K)
heater/resistor. The temperature dependent resistance values were calculated
using the Callendar-Van Dusen Equations [18].

H-ZSM-5 was measured on IDC 2 at temperatures ranging from 22K to
873K. The zeolite was measured to be an open circuit until 673K, when a
measurable impedance spectra was observed (Figure 34). The temperature
cycle was repeated and the results were the same. Comparing Figures 34 and
35, it is seen that the resistance of the zeolite decreases with increasing

temperature.

54

3.4 Conclusions

A prototype device has been constructed that can be used to measure
impedance spectra for planar substrates over a frequency range of up to 2.8KHz
at temperatures of up to 900K in a vacuum environment. The results obtained
over this frequency for ZSM-5 compare well with literature data. This was
achieved with the use of interdigitated capacitors designed and built in this work.
The cost of building this unit was under $1,500 in contrast to other high

temperature measurement devices reported in the literature.

3.5 Recommendation for future work

The motivation for this project was to calculate the temperature dependent
ionic conductivity of zeolite and to develop a gas sensor. The critical component
that remains to be developed for this goal is an analytical model for

environmental effects on ionic conduction in planar substrates.

3.5.1 Accurate Circuit Model

The new interdigitated capacitor along with the novel approach to zeolite
coating have made it possible to use more advanced circuit models that utilize
the constant-phase element outlined in Chapter 1. For a constant temperature
impedance Spectroscopy analysis the proposed circuit model shown in Figure 37

is suitable for the development of an accurate humidity sensor.

55

 

 

 

 

j

Uncoated IDC

 

 

’\/\/\/
Cioc
R0 H
o——4vvv——~ R,
«N»
——-CPE

 

 

 

Sensitive Layer

Figure 33: Equivalent circuit diagram of a coated IDC

This circuit model has been outlined in [6, 14]. The top half of circuit

represents the iDC test circuit without a coating. Rb represents the resistance

of the substrate while the series resistance R0 represents the electrode sheet

resistance. The capacitance Cb is the capacitance of the IDC. The ‘Sensitive

Layer’ represents the zeolite coating and its environmental interaction. R, is the

resistance of the coating. The constant phase element (CPE) represents a

distribution of relaxation times due to zeolite interaction with the environment as

well as a slight non-uniformity in the coating thickness. The ‘Sensitive Layer’

impedance is given by
Rf

56

(19)

where w is the distorting parameter of the CPE and satisfies 0 s (a S l. r is the

relaxation time. This modification of the circuit may be used to produce a

gas/chemical/humidity sensor.

57

[1]

[2]

[3]

[4]

[5]

[6]

[7]

[8]

[9]

[10]

BIBLIOGRAPHY

J.R. Macdonald and E. Barsoukov, “Impedance Spectroscopy Theory,
Experiment, and Application,” Second Edition 2005, J. Wiley and Sons.

J.V. Smith, “Topochemistry of Zeolites and Related Materials,” Chemical
Reviews, 1988, Vol. 88, pp.149-182.

U.Simon and ME. Franke, “Electrical properties of nanoscaled host/guest
compounds,” Microporous and Mesoporous Materials, 2000, Vol. 41, pp.1-
36.

U.Simon and ME. Franke, “Characteristic Proton Hopping in Zeolite H-
ZSM-5,” Phys. Stat. Sol. (b), 2000, Vol. 218, 99287-290.

I.M. Kalogeras and A. Vassilikou-Dova, “Electrical Properties of Zeolitic
Catalysts” Defect and Diffusion Forum, 1998, Vol. 218, pp.1-36.

F. Josse and R. Lukas and R. Zhou and S. Schneider and D. Everhart
“AC-Impedance-based Chemical Sensors for Organic Solvent Vapors”
Sensors and Actuators B: Chemical, 1996, Vol. 35-36, pp.363-369.

K. Biswas and S. Sen and PK Dutta, “A constant phase element sensor
for monitoring microbial growth” Sensors and Actuators B, 2006, Vol. 119,
pp. 186-191.

E.C. Jordan and K.G. Balmain, “Electromagnetic Waves and Radiating
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