surface characterization of piezoelectric sensor materials

The Pennsylvania State University

The Graduate School

SURFACE CHARACTERIZATION OF PIEZOELECTRIC SENSOR MATERIALS

FOR POTENTIAL USE IN REACTOR VESSEL SENSORS

A Thesis in

Nuclear Engineering

by

Dazhong Ding

© 2019 Dazhong Ding

Submitted in Partial Fulfillment

of the Requirements

for the Degree of

Master of Science

December 2019

The thesis of Dazhong Ding was reviewed and approved* by the following:

Leigh Winfrey

Associate Professor of Nuclear Engineering

Thesis Advisor

Arthur Thompson Motta

Professor of Nuclear Engineering

Jean Paul Allain

Department Head of Nuclear Engineering

*Signatures are on file in the Graduate School

iii

ABSTRACT

Piezoelectric materials can be used as in mechanical sensors in nuclear reactors.

However, the severe environment will accelerate the failure of these materials and decrease the

lifetime of the sensors. The working mechanism and two potential failure mechanisms are

discussed in the thesis. Five piezoelectric crystals are characterized for composition, surface

topography, crystal phase and mechanical parameters. These data will be used in future radiation

studies.

iv

TABLE OF CONTENTS

LIST OF FIGURES ................................................................................................................. v

LIST OF TABLES ................................................................................................................... ix

ACKNOWLEDGEMENTS ..................................................................................................... x

Chapter 1 Introduction to Piezoelectricity .............................................................................. 1

History of Piezoelectricity................................................................................................ 1 Mechanism of Piezoelectricity ......................................................................................... 2 Piezoelectric Constants .................................................................................................... 3 Piezoelectric Ultrasonic Transducer ................................................................................ 4 Potential Failure Mode of Piezoelectricity in Reactor ..................................................... 5

Thermal Depolarization ............................................................................................ 5 Gamma Radiation ..................................................................................................... 6 Neutron Radiation .................................................................................................... 7

Chapter 2 Material Selection .................................................................................................. 8

Aluminum Nitride ............................................................................................................ 9 Yttrium Calcium Oxoborate ............................................................................................. 9 Lanthanum Gallium Silicate ............................................................................................ 9 Lanthanum Gallium Tantalate.......................................................................................... 10 Goal of the Study ............................................................................................................. 10

Chapter 3 Sample Characterization and Surface Topography ................................................ 11

X-Ray Diffraction ............................................................................................................ 11 Optical Profilometry ........................................................................................................ 17 Nanoindentation ............................................................................................................... 20 Scanning Electron Microscope and Focused Ion Beam ................................................... 22

Chapter 4 Conclusion .............................................................................................................. 34

References ................................................................................................................................ 35

Appendix A Optical Profilometry Diagrams of Piezoelectric Crystals .................................. 38

Appendix B Scanning Electron Microscope Images of Piezoelectric Crystals ...................... 48

v

LIST OF FIGURES

Figure 1-1: Schematic representation of the longitudinal (a) direct piezoelectric effect, in

which charges are accumulated with different signs when a negative or positive

strain applies, (b) converse piezoelectric effect, in which an external electrical

potential results in a negative or positive strain of the piezoelectric crystal and (c)

shear piezoelectric effect, in which the deformation is perpendicular to the electric

dipoles’ direction resulting in charges accumulating on the electrodes ........................... 3

Figure 1-2: Diagram of a Classical Single-Element Transducer ............................................. 5

Figure 3-1: Representative X-Ray Diffraction Pattern of the Aluminum Nitride Crystal ....... 12

Figure 3-2: Representative X-Ray Diffraction Pattern of the First Yttrium Calcium

Oxoborate Crystal ............................................................................................................ 13

Figure 3-3: Representative X-Ray Diffraction Pattern of the Second Yttrium Calcium

Oxoborate Crystal ............................................................................................................ 14

Figure 3-4: Representative X-Ray Diffraction Pattern of the Lanthanum Gallium Silicate

Crystal .............................................................................................................................. 15

Figure 3-5: Representative X-Ray Diffraction Pattern of the Lanthanum Gallium

Tantalate Crystal .............................................................................................................. 16

Figure 3-6: Representative Surface Roughness of the Aluminum Nitride Crystal .................. 17

Figure 3-7: Representative Surface Roughness of the First Yttrium Calcium Oxoborate

Crystal .............................................................................................................................. 18

Figure 3-8: Representative Surface Roughness of the Second Yttrium Calcium Oxoborate

Crystal. ............................................................................................................................. 18

Figure 3-9: Representative Surface Roughness of the Lanthanum Gallium Silicate Crystal .. 19

Figure 3-10: Representative Surface Roughness of the Lanthanum Gallium Tantalate

Crystal .............................................................................................................................. 19

Figure 3-11: Force-Displacement Curve Obtained by Nanoindentation. ................................ 20

Figure 3-12: Contact Depth, Modulus and Hardness from Nanoindentation .......................... 21

Figure 3-13: Representative Image of the Aluminum Nitride Crystal ..................................... 23

Figure 3-14: Representative Energy Spectrum of the Aluminum Nitride Crystal. .................. 24

Figure 3-15: Representative Image of the First Yttrium Calcium Oxoborate Crystal. ............ 25

vi

Figure 3-16: Representative Energy Spectrum of the First Yttrium Calcium Oxoborate

Crystal. ............................................................................................................................. 26

Figure 3-17: Representative Image of the Second Yttrium Calcium Oxoborate Crystal ......... 27

Figure 3-18: Representative Energy Spectrum of the Second Yttrium Calcium Oxoborate

Crystal .............................................................................................................................. 28

Figure 3-19: Representative Image of the Lanthanum Gallium Silicate Crystal ..................... 29

Figure 3-20: Representative Energy Spectrum of the Lanthanum Gallium Silicate Crystal ... 30

Figure 3-21: Representative Image of the Lanthanum Gallium Tantalate Crystal .................. 31

Figure 3-22: Representative Energy Spectrum of the Lanthanum Gallium Tantalate

Crystal .............................................................................................................................. 32

Figure 3-23: Cross-section of Yttrium Calcium Oxoborate ..................................................... 33

Figure A-1: Surface Roughness of the Aluminum Nitride Crystal 1 ....................................... 38




Figure A-5: Surface Roughness of the First Yttrium Calcium Oxoborate Crystal 1 ............... 40




Figure A-9: Surface Roughness of the Second Yttrium Calcium Oxoborate Crystal 1 ........... 42

Figure A-10: Surface Roughness of the Second Yttrium Calcium Oxoborate Crystal 2 ......... 42



Figure A-13: Surface Roughness of the Lanthanum Gallium Silicate Crystal 1 ..................... 44



vii


Figure A-17: Surface Roughness of the Lanthanum Gallium Tantalate Crystal 1 .................. 46




Figure B-1: Image of the Aluminum Nitride Crystal 1 ............................................................ 48



Figure B-4: Image of the First Yttrium Calcium Oxoborate Crystal 1 .................................... 51



Figure B-7: Image of the Second Yttrium Calcium Oxoborate Crystal 1 ................................ 54



Figure B-10: Image of the Second Yttrium Calcium Oxoborate Crystal 4 .............................. 57






Figure B-16: Image of the Lanthanum Gallium Silicate Crystal 1 .......................................... 63





viii


Figure B-22: Image of the Lanthanum Gallium Tantalate Crystal 1 ....................................... 69









ix

LIST OF TABLES

Table 3-1: Interplanar Distance and Strain of the Aluminum Nitride Crystal ......................... 12

Table 3-2: Interplanar Distance and Strain of the First Yttrium Calcium Oxoborate

Crystal .............................................................................................................................. 13

Table 3-3: Interplanar Distance and Strain of the Second Yttrium Calcium Oxoborate

Crystal .............................................................................................................................. 14

Table 3-4: Interplanar Distance and Strain of the Lanthanum Gallium Silicate Crystal ......... 15

Table 3-5: Interplanar Distance and Strain of the Lanthanum Gallium Tantalate Crystal ...... 16

Table 3-6: Strain and Stress of the Two Yttrium Calcium Oxoborate Crystals ....................... 21

x

ACKNOWLEDGEMENTS

I would like to thank my advisor Dr. Winfrey who gave me the opportunity working on

this project, as well as the help when I was trying to finish this thesis.

I would appreciate the reader of this thesis, Dr. Motta, who gave me a lot of suggestions

to complete the thesis with more details.

I would also like to thank my parents and friends who supported me during my hardest

time in the past years, I could not be able to finish this thesis without them.

Chapter 1

Introduction to Piezoelectricity

Piezoelectricity is the phenomenon whereby electric dipoles are generated in certain

anisotropic crystals when subjected to mechanical strain [1]. Dimensional change under the

influence of an electric field, which is the converse effect, happens in the same materials.

History of Piezoelectricity

In 1880, Pierre Curie and Jacques Curie summarized their understanding of

pyroelectricity and crystal structure and performed several experiments. The direct piezoelectric

effect, which converts stress to charges, was discovered during their experiments. In 1881,

Gabriel Lippmann mathematically deduced the converse piezoelectric effect, then the converse

effect was experimentally confirmed by Curies.

One of the first applications of the piezoelectric effect was sonar, an ultrasonic submarine

detector based on quartz crystals [2]. By applying a voltage on quartz crystals which were glued

between two steel plates, a 50MHz ultrasonic wave could be generated in the transducer that

allowed to measure the depth of the submarine by timing the return echo. Similar applications

such as microphones or signal filters were also developed and are widely used now.

Only two types of piezoelectric materials were known prior to about 1940, which were

Rochelle salt and quartz [3]. However, in 1941, barium titanate (BaTiO3) was discovered to have

good piezoelectric properties. This was the beginning of the development of the piezoelectric

ceramic materials, which has continued to the present day.

2

Mechanism of Piezoelectricity

Piezoelectric materials belong to a larger class of materials called ferroelectrics. The

molecular structure of a ferroelectric material is oriented, which results in electric separation and

the separated charges forms electric dipoles. There is a critical temperature, called the Curie

temperature, above which the piezoelectric materials dipoles will be free to change. To make the

material piezoelectric, the material should be heated over its Curie temperature under a strong

external electric field, the randomly orientated electric dipole will reorient, such a process called

poling [4]. The dipoles will maintain their orientation after being cooled and the piezoelectric

effect will be apparent, when one either applies a force or an electric field on the material.

When a poled piezoelectric crystal is mechanically strained, the dipole will be changed,

which results in electrical polarization and produces electric charges on the surface of the

material. If electrodes are attached to the surfaces, the generated electric charge can be collected

and used. A schematic diagram is shown in Figure 1-1.

Piezoelectric materials are assumed to be elastic and have linear behavior [6]. It turns out

that at low mechanical stress and low electric fields, piezoelectric materials have a linear response

profile. However, nonlinear effects may happen if high mechanical stresses or high electric fields

are applied to the material. If the nonlinear working region is reached, a new model should be

used to describe the material behavior.

3

Figure 1-1: Schematic representation of the longitudinal (a) direct piezoelectric effect, in which

charges are accumulated with different signs when a negative or positive strain applies, (b)

converse piezoelectric effect, in which an external electrical potential results in a negative or

positive strain of the piezoelectric crystal and (c) shear piezoelectric effect, in which the

deformation is perpendicular to the electric dipoles’ direction resulting in charges accumulating

on the electrodes [5]

Piezoelectric Constants

Several constants are used to describe the behavior of piezoelectric materials [7]:

(1) piezoelectric charge constant, 𝑑, is the polarization generated per unit of mechanical

stress or strain? applied to a piezoelectric material;

(2) piezoelectric voltage constant, 𝑔, is the electric field generated per unit of mechanical

stress applied;

4

(3) dielectric constant, 𝜖, is the dielectric displacement per unit electric field;

(4) elastic compliance, 𝑠, is the strain produced in a piezoelectric material per unit stress

applied;

(5) Young’s modulus, 𝑌, is an indicator of the stiffness of a material.

(6) electromechanical coupling coefficient, 𝑘𝑖𝑗, is an indicator of the effectiveness with

which a piezoelectric material converts electrical energy into mechanical energy or

converts mechanical energy to electrical energy. The index 𝑖𝑗 represents that the

electrodes are applied in the 𝑖 direction and that the strain is in the 𝑗 direction. The

coefficient 𝑘𝑖𝑗 and other piezoelectric constants are related by Equation 1 [6]:

𝑘2 = 𝑑𝑖𝑗𝑔𝑖𝑗𝑌 (1)

Several other constants such as the frequency constant are also used, however, they are

not as significant to piezoelectric behavior as the constants mentioned above.

Piezoelectric Ultrasonic Transducer

Because the piezoelectric materials have such good response to the mechanical force and

electric field, they are good candidates for the detecting technique, and their non-ionizing

character, low cost and high efficiency of ultrasound sensors are among the features that attract

the interest of researchers [5].

A sample design of the transducer is shown in Figure 1-2, where the piezoelectric

element is located between electrodes. A thick layer is attached which is referred to as the

backing. It provides mechanical support and induces damping of the transducer resonance by

allowing acoustic energy to flow by the rear face. On the other side, matching layers are used

between the piezoelectric element and propagation medium which can increase the transfer of

energy from the active layer to the propagation medium.

5

Figure 1-2: Diagram of a Classical Single-Element Transducer [5]

Potential Failure Mode of Piezoelectricity in Reactor

The usage of piezoelectric sensing technique is expanding rapidly. One of the interests is

the nuclear reactor monitoring system. However, there are many challenges to using piezoelectric

materials in the severe environment in the reactor. Two major problems are thermal

depolarization and radiation damage.

Thermal Depolarization

When heating the piezoelectric material to its Curie temperature, the electric dipoles tend

to reorient, and poling will happen under external electric fields. However, if the poled

piezoelectric material is heated above its Curie temperature without an external electric field, the

dipoles will orient randomly to its lowest electric potential which will make the material lose its

piezoelectricity. The thermal stability of these polarized regions strongly influences the

properties. Furthermore, the thermally activated aging may happen on the piezoelectric materials,

6

which the dipoles will tend to revert to a random orientation and degrade the piezoelectricity

before reaching its Curie temperature for a long time [8].

Additional minor issues that piezoelectric materials face at high temperatures include, but

are not limited to:

Thermal instability of the dielectric, piezoelectric and electromechanical properties.

Three effects contribute to the temperature dependence: the decrease in the number of polarizable

particles per unit volume as the temperature increases, which is a direct result of the volume

expansion, the increase of the macroscopic polarizability due to the volume expansion, and the

temperature dependence of the macroscopic polarizability at constant volume [9].

Increased attenuation of acoustic waves with temperature. The attenuation can be

successfully accounted for by a theory proposed by Mason, who has obtained an expression for

the attenuation by considering the effect of strain associated with the wave on the phonons

propagated in different directions in the crystal [10].

Chemical instability: higher temperature increases thermal oscillation which results in the

increase of the free energy of the molecule or atom. For a molecule with free energy higher than

its bonding energy, the molecule will be decomposed. Atoms with high free energy cannot

maintain their position in a crystal, hence defects will be generated by holes and ions in the

crystal. Also, thermal expansion must be considered for component integration in the final device.

Gamma Radiation

When radiation interacts with matter, energy will be deposited into that matter [11].

Gamma radiation and neutron radiation typically can influence piezoelectricity even in high

radiation fields because they are electrically neutral. Other particles, such as alpha particles,

which is the helium-4 nuclei, or beta particles, the electron or positron, have electric charges that

7

will be quickly attenuated when transmitted and lose most of their energy before arriving at the

piezoelectric detector. Ionizing radiation can also cause gradual crystal dielectric loss; charges,

which are induced by radiation, can be trapped near the electrodes and influence the polarization

of the material [12].

When a gamma ray hits an atom, there are three kinds of interaction that can deposit

photon’s energy, which are the photoelectric effect, Compton scattering and pair production [13].

All these three interaction mechanisms will generate free electrons and ions that change the

electric dipole, which directly influence the piezoelectricity. Furthermore, atoms may be activated

by radiation and change the microstructure.

Neutron Radiation

Unlike photons that interact with the atoms’ shell electrons and cause ionization, neutrons

directly interact with nucleus. Neutron interaction includes elastic or inelastic scattering and

change of element by nuclear absorption, reaction or decay, nuclear fission is another mechanism

for heavy nuclei. The changing of isotopes will dope the material or change the local structure

and results in a change or failure of piezoelectric behavior.

Neutron damage to material highly depends on the neutron cross section, in other word,

reaction probability. Very generally, a low energy neutron has much higher cross section

compared to fast neutron, which means lower energy may result in more serious damage.

Chapter 2

Material Selection

The most commonly used piezoelectric transducer is lead zirconium titanate (PZT) due to

its high performance. However, the significant toxicity of lead compounds requires alternative

materials and a lead-free process, which has been researched in recent years [14]. The Curie

temperature of PZT is below 400°𝐶 [15], which means it cannot be used in a high temperature

environment such as that of a nuclear reactor, which is the eventual goal of these sensors. Thus,

new types of materials should be studied and developed to meet the temperature requirement.

The application of piezoelectric detectors in a reactor requires research in the following

areas:

1. Vibration sensing, response of the signal under vibration of the system.

2. Temperature sensing and pressure sensing are performed by measuring the sound

speed of waves in a liquid or in a vessel.

3. Water level detection and structural integrity tests can be analyzed by echo

amplitudes and time-of-flight signals.

4. Radiation intensity sensing can be detected through measuring the leak current across

the high temperature radiation-hard piezoelectric.

5. Also, signal generation from laser and acoustic pulses to the piezoelectric transducer

will be considered.

To achieve the requirement of the usage in high temperature environment, aluminum

nitride, yttrium calcium oxoborate, lanthanum gallium silicate, and lanthanum gallium tantalate

are appeared to be good candidates in vessel sensors and are evaluated prior long-term radiation

studies in this work.

9

Aluminum Nitride

Aluminum nitride (AlN), a hexagonal crystal, is good for surface acoustic wave

applications. It has high piezoelectric constants, which results in good response. The Curie

temperature of AlN is not been reached until 1150°𝐶 in the experiment [16]. A good quality and

large size of bulk material is difficult to grow as single crystal [17].

Yttrium Calcium Oxoborate

Yttrium calcium oxoborate (YCOB) forms as a monoclinic crystal. The Curie

temperature of the YCOB is higher than its melting temperature, which is around 1500°𝐶 [18],

This feature makes the material an excellent candidate for high temperature usage. The high

stability and reliability at about 1000°𝐶 were also confirmed experimentally for a dwell time of 9

hours [17]. A YCOB based sensor has also been developed that can tolerate 1000°𝐶 for vibration

sensing and acoustic emission sensing [19].

Lanthanum Gallium Silicate

Lanthanum gallium silicate crystal CTGS (Ca3TaGa3Si2O14) has been selected. CTGS is

an ordered langasite hexagonal crystal. An experiment shows that the CTGS has no phase

transition until 700°𝐶 [20], which means its Curie temperature is much higher. The stable

piezoelectric properties, electrical resistivity at high temperature [8], makes CTGS a potential

candidate for the high temperature sensing.

10

Lanthanum Gallium Tantalate

Lanthanum gallium tantalate (Langatate, LGT, La3Ga5.5Ta0.5O14) crystal, which is a

hexagonal crystal, also has no phase transition until its melting temperature around 1450°𝐶 [21].

The main application of LGT includes high temperature sensors of surface acoustic waves and

bulk waves.

Goal of the Study

The aim of this study is to fully characterize the candidate materials prior to long term

radiation (on the order of years) to obtain:

1. Surface properties: surface topography;

2. Mechanical properties: modulus and hardness;

3. Microstructure: crystal orientation;

The electrical properties and piezoelectric response will be characterized by another

group.

Chapter 3

Sample Characterization and Surface Topography

Several techniques have been used to characterize the material of intent: An aluminum

nitride crystal, two yttrium calcium oxoborate crystals, a lanthanum gallium silicate crystal and

an lanthanum gallium tantalate crystal.

X-Ray Diffraction

The X-ray diffraction method was used to confirm the structure of the crystal and

determine the crystal orientation, data is shown below. The theory of the technique is given by

Bragg’s law in Equation 2:

2𝑑 sin 𝜃 = 𝑛𝜆 (2)

where 𝑑, 𝜃, 𝑛, 𝜆 represent the interplanar distance, the scattering angle, a positive integer and the

wavelength of the X-ray, respectively. By measuring the angle and intensity of the diffracted

beam, the interplanar spacing between different lattice plane can be calculated, and the crystal

structure can be obtained.

The residual stress has significant influence on the property of the material. Stress is

proportional to strain, which can be calculated from the X-ray diffraction. The peak shift from the

expected plane can be measured and the strain can be calculated by Equation 3:

𝜖 =𝑙 − 𝐿

𝐿 (3)

where 𝜖, 𝑙 and 𝐿 represent the strain, the measured interplanar distance and the original

interplanar distance.

12

The X-ray diffraction measurement (PANalytical 4-Circle XPert3 MRD) was performed

and analyzed (JADE8) in Penn State Material Research Institute. Copper was used to generate X-

rays whose 𝐾𝛼 wavelength is 𝜆 = 1.5406Å.

Figure 3-1: Representative X-Ray Diffraction Pattern of the Aluminum Nitride Crystal

The only aluminum nitride peak can be measured in Figure 3-1 is (0 0 2), which confirms

the crystal is highly oriented. A gold peak can be found in the diffraction pattern, which comes

from the coating on the surface. The lattice parameters of AlN are 𝑎 = 3.1114Å, 𝑐 = 4.9792Å.

The interplanar distance and the calculated strain of AlN are shown in Table 3-1:

Table 3-1: Interplanar Distance and Strain of the Aluminum Nitride Crystal

Direction Measured Distance (Å) Expected Distance (Å) Strain

(0 0 2) 2.4968 2.4896 2.8920E-03

AlN (0 0 2)

Au

13

Figure 3-2: Representative X-Ray Diffraction Pattern of the First Yttrium Calcium Oxoborate

Crystal

The diffraction pattern of YCOB was obtained, as shown in Figure 3-2. The monoclinic

crystal has two peaks: (2 0 0) and (4 0 0), which indicates the orientation of the crystal is in (2 0

0) direction. Noise can be found at the low angle region due to the lack of intensity of the

measured peak. However, the YCOB peaks can be identified with acceptable statistical

significance. The lattice parameters of YCOB are 𝑎 = 8.0778Å, 𝑏 = 16.0220Å, 𝑐 = 3.5343Å.

The interplanar distance and the calculated strain of the YCOB are shown in Table 3-2:

Table 3-2: Interplanar Distance and Strain of the First Yttrium Calcium Oxoborate Crystal


(2 0 0) 3.9686 4.0389 -1.7406E-02

(4 0 0) 1.9820 2.0195 -1.8545E-02

The measured strain from different group of planes have similar value with average 𝜖 =

−1.7975 × 10−2.

YCOB (2 0 0)

YCOB (4 0 0)

14

Figure 3-3: Representative X-Ray Diffraction Pattern of the Second Yttrium Calcium Oxoborate

Crystal

A diffraction pattern of the second YCOB crystal was obtained, as shown in Figure 3-3.

The monoclinic crystal shows four main peaks: (0 2 0), (0 6 0), (0 8 0) and (0 10 0). The (0 4 0)

peak cannot be found, which is expected to appear at about 28°. However, the diffraction pattern

provides enough evidence that the YCOB crystal is oriented to its (0 2 0) direction. Some sub

peaks can also be found close to each of the main peak, which may come from the surface coating

layer. The lattice parameters of YCOB are 𝑎 = 8.0778Å, 𝑏 = 16.0220Å, 𝑐 = 3.5343Å. The

interplanar distance and the calculated strain of the second YCOB are shown in Table 3-3:

Table 3-3: Interplanar Distance and Strain of the Second Yttrium Calcium Oxoborate Crystal


(0 2 0) 7.9948 8.0110 -2.0222E-03

(0 6 0) 2.6762 2.6703 2.1970E-03

(0 8 0) 2.0020 2.0028 -3.7449E-04

(0 10 0) 1.6020 1.6022 -1.2483E-04

The measured strain from different group of planes have been observed with big

variation, the reason will be figured out in the future research.

YCOB (0 2 0)

YCOB (0 6 0)

YCOB (0 8 0)

YCOB (0 10 0)

15

Figure 3-4: Representative X-Ray Diffraction Pattern of the Lanthanum Gallium Silicate Crystal

A diffraction pattern of the CTGS crystal was obtained, as shown in Figure 3-4. The

hexagonal crystal shows two main peaks: (1 1 0) and (2 2 0), which indicates the crystal is

oriented in the (1 1 0) direction. The lattice parameters of CTGS are 𝑎 = 8.1056Å, 𝑐 = 4.9800Å.

The interplanar distance and the calculated strain of the CTGS are shown in Table 3-4:

Table 3-4: Interplanar Distance and Strain of the Lanthanum Gallium Silicate Crystal


(1 1 0) 4.05908 4.05280 1.5495E-03

(2 2 0) 2.02905 2.02640 1.3077E-03

The measured strain from different group of planes have similar value with average

𝜖 = 1.4286 × 10−3.

CTGS (1 1 0)

CTGS (2 2 0)

16

Figure 3-5: Representative X-Ray Diffraction Pattern of the Lanthanum Gallium Tantalate

Crystal

The hexagonal crystal has three main peaks: (1 1 0), (2 2 0) and (3 3 0) which indicates

the crystal is oriented in (1 1 0) direction. The lattice parameter of LGT is 𝑎 = 8.2410Å, 𝑐 =

5.1300Å. The interplanar distance and the calculated strain of the LGT are shown in Table 3-4:

Table 3-5: Interplanar Distance and Strain of the Lanthanum Gallium Tantalate Crystal


(1 1 0) 4.11998 4.12050 -1.2620E-04

(2 2 0) 2.06001 2.06025 -1.1649E-04

(3 3 0) 1.37302 1.37350 -3.4947E-04

The calculated strain from (1 1 0) and (2 2 0) has similar value. However, the strain in (3

3 0) direction is about three times bigger than the other two direction. Further research will be

performed to figure out the reason.

The diffraction patterns obtained show that all these five materials are well oriented

single crystals. The strain of these crystals are also calculated which will be researched deeper in

the future.

LGT (1 1 0)

LGT (2 2 0) LGT (3 3 0)

17

Optical Profilometry

Optical profilometry is a technique that can measure the surface topography by

measuring the movement of interference drift. Because the surface is not flat, the optical path

length between the detection surface and reference plane will be different [22]. By moving the

reference plane’s position, the shifting of the interference fringes can be measured, and the

surface topography can be obtained.

Surface topography images of a certain region of the five crystals are given below. The

size of the diagram shown is 87.7𝜇𝑚 × 87.7𝜇𝑚. The Sa, Sq and Sz in the figures represent the

average roughness, the root mean square (RMS) roughness and the maximum height of the

surface (peak to valley height).

Figure 3-6: Representative Surface Roughness of the Aluminum Nitride Crystal

Occasional shifts can be found in Figure 3-6. Continuous peak region can be found at the

center, with sharp change to the valley region. The average roughness is close to the RMS

roughness, and the roughness is acceptable to do surface coating for further experiment.

18

Figure 3-7: Representative Surface Roughness of the First Yttrium Calcium Oxoborate Crystal

According to Figure 3-7, the upper right area of the region has more shifts than the other

area. Average roughness is close to the RMS roughness, but the peak to valley height is relatively

large compared to the average roughness. It seems the surface provides many supporting points

for surface treatment.

Figure 3-8: Representative Surface Roughness of the Second Yttrium Calcium Oxoborate Crystal

19

According to Figure 3-8, the peak area and the valley area are relative equal with less

shifts, compare to the first YCOB crystal. The roughness value is also acceptable for further

surface treatment.

Figure 3-9: Representative Surface Roughness of the Lanthanum Gallium Silicate Crystal

Shifts can be found in Figure 3-9. Average roughness is close to the RMS roughness, but

the peak to valley height is much greater than the average roughness. There are many dark spots,

which indicate sharp height change happens in those regions.

Figure 3-10: Representative Surface Roughness of the Lanthanum Gallium Tantalate Crystal

20

According to Figure 3-10, many shifts and continuous dark regions can be found all over

the surface, which makes the surface very rough. The average roughness value is small but the

peak to valley value is ten times greater than the average. The crystal has good topography

feature for further experiment.

The surface roughness value and topography images obtained from optical profilometry

for these materials show that the surface of these crystals is good for surface treatment, and the

surface change can be figured out when further experiments are performed. More optical

profilometry diagrams can be found in Appendix A.

Nanoindentation

Nanoindentation is a technique that can measure the mechanical properties of surfaces on

a submicroscopic scale. A small diamond is used as an indenter to apply force to the surface. By

measuring the force-displacement curve, the reduced modules and hardness can be obtained [23].

However, since the surface topography is changed by the indenter, nanoindentation is a

destructive technique.

Figure 3-11: Force-Displacement Curve Obtained by Nanoindentation

21

Figure 3-12: Contact Depth, Modulus and Hardness from Nanoindentation

Sixteen indentations were performed in a small area at the surface of the second YCOB

crystal. The sixteen force-displacement curves are shown in Figure 3-11. By fitting the curves,

the mean contact depth, mean reduced modulus and mean hardness were obtained, as well as their

uncertainty. The reduced modulus 𝐸𝑟 can be converted to the Young’s modulus 𝐸𝑠 with the

diamond indenter tip’s modulus 𝐸𝑖 by Equation 4 [24]:

1

𝐸𝑟=

1 − 𝜈𝑖2

𝐸𝑖+

1 − 𝜈𝑠2

𝐸𝑠 (4)

where ν𝑖 and 𝜈𝑠 represent the Poisson’s ratio of the tip and the crystal.

The material used in the as indenter (Hysitron TI-900) was diamond (𝐸𝑖 = 1140𝐺𝑃𝑎 and

𝜈𝑖 = 0.07). The Poisson’s ratio of YCOB is 0.29 [25], then the Young’s modulus of YCOB is

𝐸𝑠 = 142.5𝐺𝑃𝑎.

The residual stress can be calculated by Equation 5:

𝜎 = 𝐸𝑠𝜖 (5)

where 𝜎, 𝐸𝑠 𝑎𝑛𝑑 𝜖 represent the stress, the Young’s modulus and the strain. Thus, the residual

stress values of the two YCOB are given in Table 3-6:

Table 3-6: Strain and Stress of the Two Yttrium Calcium Oxoborate Crystals

Direction Strain Stress (MPa) Direction Strain Stress (MPa)

(2 0 0) -1.7406E-02 -2480 (0 2 0) -2.0222E-03 -288

(4 0 0) -1.8545E-02 -2643 (0 6 0) 2.1970E-03 313

(0 8 0) -3.7449E-04 -53

(0 10 0) -1.2483E-04 -18

The variation of the values will be researched in the future. However, the Table 3-6

shows that the crystals’ residual stress cannot be ignored.

22

Scanning Electron Microscope and Focused Ion Beam

Scanning electron microscope (SEM) can provide high resolution surface information.

Secondary electrons will be emitted from the surface after the interaction between electrons from

the electron gun and surface atoms. Backscattered electrons may also be detected in the SEM.

Furthermore, the surface atoms may generate characteristic X-rays that can be used for elemental

analysis, which is the energy dispersive spectroscopy (EDS). In addition to X-ray diffraction,

EDS helps to identify the material’s composition, as well as the contamination. However, for the

nonconductive piezoelectric crystal, surface coating, which may change the surface topography,

is required to perform high accuracy EDS spectrum. For all the four crystals except the second

YCOB crystal, their surfaces were coated by platinum, palladium or gold with unknown

thickness. Quantitative EDS analysis cannot be made in this situation; however, qualitative

analysis provides confirmation of the expected elements.

23

Figure 3-13: Representative Image of the Aluminum Nitride Crystal

Figure 3-13 shows that the surface has many scratches, which is the dark region. The

dark hold at the center is a dent that the gold coating layer has been removed. Bubble like

structure on the surface can be found on the non-scratched region. The image gives similar

topography information compare to the optical profilometry diagram on Figure 3-6.

24

Figure 3-14: Representative Energy Spectrum of the Aluminum Nitride Crystal

The EDS spectrum of the AlN confirms the existence of aluminum with gold coating.

The nitrogen peak cannot be found because the expected intensity of the peak is small that is

shaded by the strong peak. The chromium peak measured shows the AlN crystal was prepared in

chromium rich environment. However, the carbon and oxygen peak shows that the surface may

be contaminated by organics.

25

Figure 3-15: Representative Image of the First Yttrium Calcium Oxoborate Crystal

Figure 3-15 shows that the surface has bubbled structures grow on the big grain. The

center region has many layers accumulated, which provides the same topography information as

the optical profilometry.

26

Figure 3-16: Representative Energy Spectrum of the First Yttrium Calcium Oxoborate Crystal

The EDS spectrum of the first YCOB confirms the existence of yttrium, calcium and

oxygen. A small boron peak can be found close to the left edge of the spectrum. Platinum and

palladium, which is commonly used as coating, can be found in the spectrum at the same time.

27

Figure 3-17: Representative Image of the Second Yttrium Calcium Oxoborate Crystal

Figure 3-17 shows that the surface has many continuous grains which makes it smooth.

However, the layered region on the top left and bubbled structure give the supporting points for

further surface treatment. Some sharp changes in height can be found at the center area.

28

Figure 3-18: Representative Energy Spectrum of the Second Yttrium Calcium Oxoborate Crystal

The EDS spectrum of the second YCOB also confirms the existence of yttrium, calcium,

boron and oxygen. The iridium is a layer of conductive coating with thickness 𝑑 = 5.53𝑛𝑚. The

second YCOB crystal is the only crystal that coated in Penn State whose coating thickness can be

given during the characterization. The weight percent shown in the spectrum is the same as the

atomic weight percent in pure YCOB, when the carbon contamination is not considered.

29

Figure 3-19: Representative Image of the Lanthanum Gallium Silicate Crystal

Figure 3-19 shows that most regions of the surface are smooth other than the cliff area at

the center of the image. However, the surface has less bubbled structure compare to the previous

samples. The cliff area shown in the image may be one of the dark spots which obtained in Figure

3-9.

30

Figure 3-20: Representative Energy Spectrum of the Lanthanum Gallium Silicate Crystal

The EDS spectrum of the CTGS confirms the existence of calcium, tantalum, gallium,

silicon and oxygen. The surface was coated by platinum and palladium, which also appear in the

EDS spectrum. Carbon contamination can be found.

31

Figure 3-21: Representative Image of the Lanthanum Gallium Tantalate Crystal

Figure 3-21 shows the local region of the surface is separated into two parts. The left part

of the image is relative smooth with small bubbled structures, while the right part is rough with

many layered structures.

32

Figure 3-22: Representative Energy Spectrum of the Lanthanum Gallium Tantalate Crystal

The EDS spectrum of the LGT confirms the existence of lanthanum, gallium, tantalum,

and oxygen. The surface was coated by platinum and palladium, whose peak can be found in the

spectrum. The gold peak is almost invisible, but the weight percent given in the spectrum shows

the crystal may be coated by a thin layer of gold before.

These SEM images provide the intuitive view of the surface, while the EDS spectrums

provide the element of the crystals. The structures found on the surface confirm the roughness

obtained from the optical profilometry, and the capability for further surface treatment. More

SEM images are given in Appendix B.

Focused ion beam (FIB) is a milling technique which can be installed in the SEM system.

When ion beam hits the surface, local sputtering will be made which removes material. Then,

cross-section can be imaged by SEM.

33

Figure 3-23: Cross-section of Yttrium Calcium Oxoborate

Figure 3-23 is the SEM image which is tilted 52° to the surface normal direction. The

continuous flat region is the cross-section polished by the ion beam, which confirms the expected

single-phase structure of the crystal. The multilayer structure is the bottom of the milling region

which is perpendicular to the cross section and cannot be polished properly. Some contamination

can be found above the surface which is expected have no influence on the material property.

The five crystals were characterized by the X-ray diffraction for structure, the optical

profilometry for topography, the nanoindentation for mechanical parameters and imaged by SEM.

These data will be used after the crystals are irradiated to identify any changes.

Chapter 4

Conclusion

Five crystals were fully characterized to obtain surface topography and crystal

orientations, elemental analysis and the mechanical parameters from nanoindentation:

1. Single crystal orientation was found from the X-ray diffraction for all the five

crystals, as well as the deformation of the crystal lattice. Furthermore, the FIB shows

that the subsurface of the second YCOB crystal is continuous with no visible

boundary. These characterizations data show that the crystals have big grain size,

which is the expected structure that can provide good piezoelectric response.

2. Optical profilometry and SEM images provided surface information, especially the

surface topography.

3. Elemental analysis from EDS qualitatively confirmed the composition of these

materials.

4. Mechanical properties measurement by nanoindentation was only performed on one

of the YCOB crystal because the technique is destructive. Hardness and modulus of

the crystal were obtained and used to calculate the residual stress.

Further work will be performed in post radiation characterization on these materials.

These works will be done in collaboration with other institutions and researchers.

The ultimate goal of the work is to develop the in-vessel reactor sensors, for vibration

sensing, temperature and pressure sensing, structural tests and radiation intensity sensing. The

current work on surface characterizing and bulk crystal characterizing are useful to identify the

changes after the radiation. After these experiments, the development of protective coating layer

on the piezoelectric crystals for reactor monitoring will start.

References

[1] Van Randeraat, Jan, and R. E. Setterington, eds. Piezoelectric ceramics. Gower Publishing

Company, Limited, 1974.

[2] Gallego-Juarez, J. A. "Piezoelectric ceramics and ultrasonic transducers." Journal of Physics

E: Scientific Instruments 22.10 (1989): 804.

[3] Jaffe, Bernard. Piezoelectric ceramics. Vol. 3. Elsevier, 2012.

[4] Sodano, Henry A., Daniel J. Inman, and Gyuhae Park. "A review of power harvesting from

vibration using piezoelectric materials." Shock and Vibration Digest 36.3 (2004): 197-206.

[5] Safari, Ahmad, and E. Koray Akdogan, eds. Piezoelectric and acoustic materials for

transducer applications. Springer Science & Business Media, 2008.

[6] Electronics Engineers. Sonics, and Ultransonics Group. IEEE Transactions on Sonics and

Ultrasonics 31.2 (1984), i.

[7] APC International Ltd. Piezoelectric Ceramics: Principles and Applications. APC

International, 2011.

[8] Zhang, Shujun, and Fapeng Yu. "Piezoelectric materials for high temperature

sensors." Journal of the American Ceramic Society 94.10 (2011): 3153-3170.

[9] Bosman, A. J., and E. E. Havinga. "Temperature dependence of dielectric constants of cubic

ionic compounds." Physical Review 129.4 (1963): 1593.

[10] Mason, Warren P., ed. Physical Acoustics V4B: Principles and Methods. Vol. 4. Elsevier,

2012.

[11] Cember, Herman, Thomas E. Johnson, and Parham Alaei. "Introduction to health

physics." Medical Physics 35.12 (2008): 5959.

36

[12] Sinclair, A. N., and A. M. Chertov. "Radiation endurance of piezoelectric ultrasonic

transducers–A review." Ultrasonics 57 (2015): 1-10.

[13] Lamarsh, John R., and Anthony John Baratta. Introduction to nuclear engineering. Vol. 3.

Upper Saddle River, NJ: Prentice hall, 2001.

[14] Vijaya, M. S. Piezoelectric materials and devices: applications in engineering and

medical sciences. CRC Press, 2016.

[15] Shrout, Thomas R., and Shujun J. Zhang. "Lead-free piezoelectric ceramics: Alternatives for

PZT?." Journal of Electroceramics 19.1 (2007): 113-126.

[16] Lin, Chih-Ming, et al. "Thermally compensated aluminum nitride Lamb wave resonators for

high temperature applications." Applied Physics Letters 97.8 (2010): 083501.

[17] Kim, Kyungrim, et al. "Design, fabrication and characterization of high temperature

piezoelectric vibration sensor using YCOB crystals." Sensors and Actuators A: Physical 178

(2012): 40-48.

[18] Zhang, Shujun, et al. "Characterization of piezoelectric single crystal Y Ca 4 O (BO 3) 3 for

high temperature applications." Applied Physics Letters 92.20 (2008): 202905.

[19] Johnson, Joseph A., et al. "High-temperature (> 1000 C) acoustic emission

sensor." Nondestructive Characterization for Composite Materials, Aerospace Engineering, Civil

Infrastructure, and Homeland Security 2013. Vol. 8694. International Society for Optics and

Photonics, 2013.

[20] Yu, Fapeng, et al. "Investigation of Ca3TaGa3Si2O14 piezoelectric crystals for high

temperature sensors." Journal of applied physics 109.11 (2011): 114103.

[21] Sreenivasulu, G., et al. "Piezoelectric single crystal langatate and ferromagnetic composites:

Studies on low-frequency and resonance magnetoelectric effects." Applied Physics Letters 100.5

(2012): 052901.

37

[22] Vorburger, T. V., and E. C. Teague. "Optical techniques for on-line measurement of surface

topography." Precision Engineering 3.2 (1981): 61-83.

[23] Willems, Guy, et al. "Hardness and Young's modulus determined by nanoindentation

technique of filler particles of dental restorative materials compared with human

enamel." Journal of biomedical materials research 27.6 (1993): 747-755.

[24] Zysset, Philippe K., et al. "Elastic modulus and hardness of cortical and trabecular bone

lamellae measured by nanoindentation in the human femur." Journal of biomechanics 32.10

(1999): 1005-1012.

[25] Loiko, Pavel, et al. "Thermal lensing and multiwatt microchip laser operation of Yb: YCOB

crystals." IEEE Photonics Journal 8.3 (2016): 1-12.

38

Appendix A

Optical Profilometry Diagrams of Piezoelectric Crystals

Figure A-1: Surface Roughness of the Aluminum Nitride Crystal 1


39



40

Figure A-5: Surface Roughness of the First Yttrium Calcium Oxoborate Crystal 1


41



42

Figure A-9: Surface Roughness of the Second Yttrium Calcium Oxoborate Crystal 1


43



44

Figure A-13: Surface Roughness of the Lanthanum Gallium Silicate Crystal 1


45



46

Figure A-17: Surface Roughness of the Lanthanum Gallium Tantalate Crystal 1


47



48

Appendix B

Scanning Electron Microscope Images of Piezoelectric Crystals

Figure B-1: Image of the Aluminum Nitride Crystal 1

49


50


51

Figure B-4: Image of the First Yttrium Calcium Oxoborate Crystal 1

52


53


54

Figure B-7: Image of the Second Yttrium Calcium Oxoborate Crystal 1

55


56


57


58


59


60


61


62


63

Figure B-16: Image of the Lanthanum Gallium Silicate Crystal 1

64


65


66


67


68


69

Figure B-22: Image of the Lanthanum Gallium Tantalate Crystal 1

70


71


72


73


74


75


76


77


surface characterization of piezoelectric sensor materials

Documents