Mechanoluminescent and Phosphorescent Paint Systems for Automotive and Naval Applications

Mechanoluminescent and Phosphorescent Paint Systems for

Automotive and Naval Applications

THESIS

Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University

Srivatsava Krishnan

Graduate Program in Mechanical Engineering

The Ohio State University

2015

Master's Examination Committee:

Dr. Vishnubaba Sundaresan, Advisor

Dr. Ahmet Kahraman

Srivatsava Krishnan

2015

Abstract

Mechanoluminescence (ML) is the emission of light from materials due to mechanical stimulation. Commercially available zinc sulfide phosphors exhibit intense ML during elastic deformations and are a natural fit for applications requiring repeatability and reliability. The motivation for this work is to utilize the intense ML of phosphors and fabricate a ML paint system. The ML paint system is envisioned to be sprayed on to automotive panels that are mechanically actuated to create patterned aesthetic lighting.

Towards this goal, commercially available zinc sulfide phosphor crystals are characterized using experimental techniques to understand the dependence of ML on various physical and chemical properties. Composite coupons of the phosphors impregnated in an elastomeric polymer (PDMS) matrix have been fabricated for characterizing the ML phenomenon. The polymer matrix acts as an efficient medium for transferring stress to dispersed phosphor particulates. ML observed during elastic actuation of these coupons was studied to better understand the nature of the phenomenon, i.e. its dependence on strain/stress and rate of strain/stress. Existence of a threshold strain/stress has been established below which no EML is observed. Above threshold, EML was observed to be non-linearly dependent on and in phase with strain rate. Finite element analyses of the stress transfer occurring within the elastomeric matrix at various strains were also performed. A range stresses on the particle at different applied strains have been predicted and the importance of interfacial binding in stress transfer has been established. ii

Dedication

This thesis is dedicated to my family who have always put my interests ahead of theirs.

iii

Acknowledgments

I would like to acknowledge Honda R&D Americas Inc. and NSF I/UCRC Smart

Vehicle Concept Center for supporting this project. I would particularly like to thank

Nichole Verwys and Duane Detwiler of Honda R&D for providing valuable technical and non-technical inputs and feedback throughout the project.

I would like to thank my advisor Dr. Vishnu Baba Sundaresan for the opportunity to work on this project, the technical guidance and the constant encouragement. I am grateful for the all the knowledge and skills learned through this project. I thank Dr. Ahmet

Kahraman for serving on my Master’s examination committee.

I would also like to acknowledge Profs. Carlos Castro, Prabir Dutta and Yiying Wu for equipment access and use. I thank Hugo Van der Walt for his immense help in running experiments and his stimulating questions and ideas. I am thankful for my lab mates for all the helpful distractions and discussions.

Vita

2013...... B.E. Aeronautical Engineering, Anna

University

2014 to present ...... Graduate Research Associate, Department

of Mechanical and Aerospace Engineering,

The Ohio State University

Fields of Study

Major Field: Mechanical Engineering

Table of Contents

Abstract ...... ii

Dedication ...... iii

Acknowledgments...... iv

Vita ...... v

Fields of Study ...... v

Table of Contents ...... vi

List of Tables ...... ix

List of Figures ...... x

Chapter 1: Introduction ...... 1

1.1. Light emissions mechanisms ...... 1

1.2. Introduction to Mechanoluminescence ...... 3

1.3. Material Selection ...... 6

1.4. Optical activation of ZnS ...... 9

1.5. Electron traps and their contribution to luminescence ...... 11

1.6. Mechanism of Elastico-mechanoluminescence of ZnS:Mn,Cu ...... 14

1.7. Scope of study ...... 18

Chapter 2: Material Synthesis and Characterization ...... 20

2.1. Synthesis of ZnS:Mn nanocrystals ...... 21

2.2. Fabrication of micron-sized particles of ZnS:Mn and ZnS:Mn,Cu ...... 25

2.3. Material Characterization ...... 26

Chapter 3: Characterization of Elastico-Mechanoluminescence ...... 51

3.1. Experimental methods to observe mechanoluminescence ...... 51

3.2. Elastic actuation of composite coupons ...... 56

3.3. Additional information on experimental methods ...... 74

Chapter 4: Understanding Stress Transfer to ML Particles ...... 79

4.1. Model Description ...... 80

4.2. Material Properties ...... 81

4.3. Boundary Conditions ...... 81

4.4. Contact between article and PDMS ...... 83

4.5. Results and Discussions ...... 84

4.6. Summary ...... 91

Chapter 5: Damage Detection of Surface Coatings ...... 93

5.1. Experimental Methods ...... 94

5.2. Experimental Results ...... 97

5.3. Summary ...... 102

vii

Conclusions ...... 104

Contributions to the field ...... 106

References ...... 108

viii

List of Tables

Table 1: List of available phosphors, their source, color of emission and crystal size ..... 27

Table 2: Crystal structure and crystal size of available phosphors...... 50

Table 3: Observance of ML from available phosphors ...... 54

Table 4: Material properties of ZnS and PDMS used in the analyses...... 81

Table 5: Range of stresses on particle at various applied strains ...... 92

List of Figures

Figure 1: Various mechanisms of light emission ...... 2

Figure 2: Cubic and hexagonal structure of ZnS ...... 8

Figure 3: Schematic energy level diagram in ZnS crystals...... 12

Figure 4: Schematic energy level diagram of ZnS:Mn,Cu, ZnS:Cu and ZnS:Mn ...... 17

Figure 5: SEM images of the available phosphors ...... 29

Figure 6: EDS spectra of cubic-ZnS:Mn nanocrystals ...... 30

Figure 7: EDS spectra of hexagonal ZnS:Mn nanocrystals ...... 31

Figure 8: EDS spectra of GL25 phosphor...... 31

Figure 9: EDS spectra of GG13 phosphor ...... 31

Figure 10: EDS spectra of GG25 phosphor ...... 32

Figure 11: Element maps of a cut particle of GG45 ...... 34

Figure 12: Transmittance and Absorbance spectra of the cubic ZnS:Mn nanocrystals .... 36

Figure 13: Transmission and Absorption spectra of GL25 phosphors ...... 38

Figure 14: Fluorescence spectra of cub-ZnS:Mn nanocrystals and GL25 phosphor ...... 40

Figure 15: Schematic of X-Ray diffraction due to two parallel atomic places ...... 42

Figure 16: Schematic of X-Ray diffraction from two parallel atomic places ...... 43

Figure 17: Typical XRD spectra of hexagonal and cubic ZnS crystals ...... 44

Figure 18: XRD spectra of cubic ZnS:Mn nanocrystals ...... 46

Figure 19: XRD spectra of hexagonal ZnS:Mn nanocrystals ...... 48 x

Figure 20: XRD spectra of commercially obtained phosphors ...... 49

Figure 21: Phosphor-PDMS composite coupons showing EML ...... 52

Figure 22: Phosphor-PDMS composite coupons under UV light ...... 53

Figure 23: Experimental setup used to measure EML during elastic actuation ...... 57

Figure 24: Initial conditions of actuation of coupons ...... 59

Figure 25: FFT of a 15Hz discontinuous strain curve ...... 62

Figure 26: Radiance spectra of EML from GG45 phosphor-PDMS coupon...... 63

Figure 27: Luminance against strain rate at different strains for case I ...... 64

Figure 28: Luminance against strain rate at different strains for case II ...... 65

Figure 29: Luminance against maximum strain at different frequencies for case I ...... 67

Figure 30: Luminance against maximum strain at different frequencies for case II ...... 67

Figure 31: Photoresistor circuit used to capture temporal response of EML...... 68

Figure 32: Temporal response of coupon actuated in case I ...... 70

Figure 33: Temporal response of coupon actuated in case II ...... 73

Figure 34: Laser interferometer setup to estimate transfer function of shaker...... 76

Figure 35: Circuit used for obtaining frequency response of the photoresistor ...... 76

Figure 36: Transfer function of shaker ...... 77

Figure 37: Transfer function of photoresistor ...... 78

Figure 38: FEM model of cylindrical PDMS volume enclosing a ZnS particle ...... 80

Figure 39: Longitudinal strain contour of an entire PDMS coupon ...... 82

Figure 40: von Mises stress contours at each applied strain ...... 85

Figure 41: Average stresses within the particle in tie contact ...... 86

Figure 42: Average stresses within the particle in interaction contact ...... 87

Figure 43: EML dependence on stress for initial loading condition I ...... 90

Figure 44: EML dependence on stress for initial loading condition II ...... 91

Figure 45: Experimental setups to create controlled damage and measure luminance. ... 95

Figure 46: Material characterization of SAO-Ey/Dy phosphor ...... 98

Figure 47: Excitation and Emission spectra of SAO-Ey/Dy phosphor...... 99

Figure 48: Dependence of phosphorescence intensity on damage length...... 100

Figure 49: Phosphorescence intensity variation on UV exposure duration...... 101

xii

Chapter 1: Introduction

1.1. Light emissions mechanisms

Generation of light for all day-to-day applications is achieved either by the phenomenon of incandescence or by luminescence. Light emission due to thermal agitation of atoms within a material is called incandescence, and is an inefficient process of light generation as most of the input energy is wasted in heating up the material. Luminescence arises from de-excitation of electrons from a high energy state to a lower energy state which releases energy as electromagnetic radiation or light. Luminescence does not create heat and hence is a much more efficient way of light generation. Energy required to excite electrons from the low energy state (ground state) to high energy state (excited state) can be done in several ways. Figure 1 shows a classification chart of the different light generation mechanisms based on the means of energy input.

Present-day automotive lighting systems for functional and aesthetic applications have large space requirements for housing the lamps. Fixed sizes and shapes of the conventional lighting systems also prohibit simplistic and convenient interior lighting designs in automobiles. A lighting system that can be painted on to surfaces in any required pattern would have none of the constraints of conventional systems thereby greatly aiding

the design process and adding value to the product. The motivation of this thesis is to address this issue through fabrication of a new paintable lighting system that can be sprayed on to any metallic or plastic surfaces of automotive panels in any required pattern.

Figure 1: Various mechanisms of light emission [1]

Utilizing electroluminescence of materials for this purpose seems the first option as the phenomenon is well studied and EL lighting technology is commercially available at very low costs. However, utilizing electroluminescence requires electroding the surface

of the automotive panels underneath the EL material as well as having a transparent electrode coating over the EL material. Electroding of surfaces is usually performed by techniques such as chemical vapor deposition, pulsed laser deposition, sputter coating, etc. in which a layer of conductive material is grown over the required surface. All these methods can prove highly expensive for electroding large areas. Further, a protective insulating coating is required to avoid electric shocks since EL emission requires high AC voltage at high frequencies leading to higher costs. Additionally, automotive paint systems are expected to withstand rigorous thermal cycles without any compromise of build quality and functionality. All these factors prohibit the utilization of electroluminescence for the paintable lighting systems. Hence, for the purpose of a paintable lighting system, triboluminescence or mechanoluminescence is proposed as an alternative luminescence mechanism that does not require electroding and can be cost-effective.

1.2. Introduction to Mechanoluminescence

Luminescence triggered by any mechanical action is called triboluminescence

(TL). Francis Bacon was the first to observe and report the emission of blue light during fracture of sugar crystals in The Advancement of Learning, 1605 [2]. The phenomenon was called triboluminescence, from tribo-rubbing and luminescence- light emission [3].

Another common example of triboluminescence is the emission of blue light when grinding mint candy with teeth [4]. Though the phenomenon of triboluminescence was discovered in the sixteenth century, research on triboluminescence gained momentum only in the

twentieth century with the development of photomultiplier tubes (PMT) in 1930 and its subsequent utilization for quantitative detection and measurement of triboluminescence

[1].

In recent literature, the term triboluminescence is rarely used and has been replaced with the mechanoluminescence (ML), which is more general in definition as it describes the emission of light due to any mechanical action [5]. Mechanical stimuli can include any means of transfer of mechanical energy such as cleaving, scratching, grinding, stretching, rubbing and so on. Mechanoluminescent materials act as transducers and convert the applied mechanical energy into electromagnetic energy i.e. light. The applied mechanical energy can either deform the mechanoluminescent material elastically, plastically or can induce fracture within the material and thereby cause light emission. Based on this, the phenomenon of mechanoluminescence can be classified as elastico-mechanoluminescence

(EML), plastico-mechanoluminescence (PML) and fracto-mechanoluminescence (FML) respectively [5].

In general, most mechanoluminescent materials are crystalline solids, though there are a few examples amorphous materials that show FML. It is estimated that more than

50% of all inorganic crystals and organic macromolecules emit light during fracture or cleaving while only a small percentage of those emit light during elastic or plastic deformations [5-9]. In general, all crystals exhibiting EML and PML emit much higher intensity light during fracture. Several applications utilizing the intense FML emission of

crystals for impact sensors, damage sensors, earthquake sensors, and for visualization of crack propagation [10-17] have been proposed.

Although the intensity of FML emission is higher than EML or PML, the applicability of FML is limited by non-repeatability of emission due to fracturing of crystals. Lighting applications and most sensor applications require repeatability and reliability of energy transduction. FML and PML fail in this aspect whereas EML shows high potential. Since elastic deformations are recoverable, EML materials can be cyclically actuated to produce continuous light emission.

Intense EML of zinc sulfide doped with manganese and copper (ZnS:Mn,Cu) and strontium aluminate doped with europium and dysprosium (SrAl2O4:Eu,Dy) have been utilized by researchers to fabricate lab-scale prototypes of artificial skin for surface stress visualization [13], damage sensors and structural health monitoring systems [15,18], stress recording system [19], real-time visualization and monitoring of stress distribution within solids [20,21], stress fields near crack tip and quasi-dynamic crack-propagation in solids

[14-17].

C.N. Xu et al. [13] have deposited thin solid films of ZnS:Mn on glass substrates using RF magnetron sputtering technique for utilization as an artificial skin for stress visualization. They have studied the dependence of EML intensity on the amount of dopant ions (Mn2+) in the films and have reported the intensity to be maximum at 1.5 wt % of Mn.

They have also reported that films with higher degree of crystallinity showed more intense

EML emission.

Zhang et al [22] have grown thin solid films of ZnS:Mn on a piezoelectric substrates

(PMN-PT) using pulsed laser deposition, and have shown intense mechanoluminescence through actuation of the piezoelectric substrate at ultrasonic frequencies. They have observed linear dependence of ML intensity on both the amplitude and frequency of applied actuation.

Jeong et al [23] have fabricated durable mechanoluminescent composite coupons by impregnating ZnS:Cu phosphor particles in polydimethylsiloxane with high brightness.

They have shown repeatable EML emission during elongation and relaxation of the coupons up to 100,000 cycles with a 35% drop in intensity. A linear dependence of intensity on the elongation and relaxation frequency is reported. They have also reported a blue shift in emission at higher frequencies of operation. Jeong et al have also demonstrated color tuning of EML emission by controlling the weight ratio of different phosphors [24] in the mixture, and using wind energy to create EML emission from micro-pillars of the same phosphor-PDMS composite material [25].

1.3. Material Selection

Materials that show mechanoluminescence also exhibit fluorescence or phosphorescence. The brightest ML materials, ZnS:Mn, ZnS:Cu, SrAl2O4:Eu and

SrAl4O6:Eu,Dy are highly fluorescent or phosphorescent in nature. They are commercially available as electroluminescent phosphors with high brightness and long half-life. It is important to note that mechanoluminescence emission spectra follows similar trends as the

fluorescence/phosphorescence spectra. The peak wavelength (and hence the color) of ML spectra is the same as that of fluorescence/phosphorescence spectra. Strontium aluminate phosphors are popular for their intense brightness, high quantum efficiency and long afterglow. Zinc sulfide phosphors are also designed to have intense brightness and high quantum efficiency but in comparison they have very short afterglow. This makes doped zinc sulfide material an ideal choice for ML paints, as the short afterglow of the phosphorescence will not mask the controlled EML generated from the paint. Further, since

ML spectra follows the phosphorescence spectra, short afterglow of phosphorescence ensures short afterglow of ML, thereby by providing better dynamic response from the ML paint. Hence, the focus of this thesis is primarily limited to doped ZnS phosphors.

Zinc sulfide is an undoped II-IV type wide band gap semiconductor, which occurs as crystalline solid chunks and in powder form. Zinc occurs in group II and sulfur in group

IV and hence ZnS is a II-IV semiconductor. ZnS exists generally in two crystal structures, cubic or zinc blende structure and hexagonal or wurtzite structure as shown in Figure 2. In both structures, the coordination number for both Zn and S is 4 and the coordination geometry is tetrahedral. The cubic structure is the more stable form at low temperatures below 1020oC and hexagonal structure is stable above 1020oC.

Figure 2: Cubic (left) and hexagonal structure (right) of ZnS. Zn-yellow and S-gray atom.

ZnS is a direct band gap semiconductor with a band gap of 3.54eV in cubic crystal structure and 3.91eV in hexagonal crystal structure. Band gap is the energy difference between the lowest possible energy level in the conduction band and the highest possible energy level in the valence band. This implies that 3.54eV of energy (or 3.91eV in the hexagonal form) is required to excite an electron from the highest possible energy level of the valence band to the lowest possible energy level in the conduction band. Since ZnS is a direct band gap semiconductor, it is easily possible for a photon of the required energy to excite an electron to the conduction band. From the Planck-Einstein relation, 퐸 = ℎ휈, where E is the energy of a photon with frequency 휈 and h is the Planck’s constant, a 3.54eV- energy photon would have a frequency of 857 THz or a wavelength of 350nm. For the hexagonal form, the wavelength of photon required for excitation is 317nm. Hence the absorption (or excitation) spectra would have peaks at 350nm and 317nm for cubic and hexagonal structures respectively.

When cubic ZnS crystal is hit with light at 350nm, photons are absorbed to excite electrons from the valence band to the conduction band thereby leaving behind a hole in the valence band. The excited electron is no more attached to one atom or ion, and is now free to move across the crystal within the conduction band. Similarly, the hole is also free to move across the crystal within the valence band. The excited electrons are unstable in the conduction band and they eventually tend to fall back to the valence band, releasing photons. But often, the excited electrons first drop down to lower intermediate energy levels by “non-radiative relaxation” i.e. by releasing vibrational energy. Energy is dissipated to the crystal in very small steps such that the energy release is non-radiative

(not accompanied by photon release). Once the electrons drop down to the intermediate states, they fall back to the valence band by emitting photons (Figure 3a). Since a portion of the energy was released non-radiatively, the energy of emitted photon is lesser than the absorbed photon. Hence the peak wavelength of emission spectra is higher than that of absorption spectra. For undoped ZnS, the emission peak occurs at around 440nm.

1.4. Optical activation of ZnS

Zinc sulfide is often doped with optically active transition metal ions of valence similar to Zn2+ ion such as manganese (Mn2+), copper (Cu2+), aluminum (Al3+) and silver

(Ag+) for engineering optically active materials with emission in the visible range. Zn2+ is substituted within the crystal lattice by the dopant ion, thereby affecting the electronic structure, local crystal structure and the local piezoelectric constant. The size of dopant

ions compared to that of Zn2+ ion influences the alterations to the above mentioned characteristics. The ionic radii of Zn2+ and S2- are 88pm and 170pm (picometer) respectively and the ionic radii of Mn2+, Cu2+, Al3+ and Ag+ are 81pm, 91pm, 67.5pm and

108pm. It can be noted that the substitution dopant ions have very similar ionic radii and valence and hence substitution is feasible without creating too much strain in the crystal lattice.

The impurity (or dopant) ion’s energy levels would be different from the host ion’s energy levels. Since the impurity ion is a localized aberration in the host crystal, the energy band structure of the host is not entirely altered. The energy band structure of the host remains intact near the local region of the impurity ion. The outermost energy levels of impurity ion add as discrete energy levels to the host crystal’s band structure, within the forbidden band gap. Based on the type of impurity, which can be either electron rich or electron deficient, the new discrete energy levels could be donor levels, closer to the conduction band, or acceptor levels, closer to the valence band respectively. Specifically with ZnS, the electron deficient Cu2+, Mn2+, Ni2+ and Ag+ impurity ions create new acceptor levels which are close to the valence band and hence readily accept electrons from higher energy levels. Al3+ impurity ion is electron rich compared to Zn2+ and hence its presence creates donor levels close to the conduction and which readily donate electrons to lower energy levels (Figure 3). Donor levels which hold electrons can also be perceived as electron traps, while acceptor levels can be perceived as hole traps. Electron traps and hole traps are explained further in the next subsection. The discrete acceptor or donor

energy levels are characteristic of the impurity ion and decide the wavelength of luminance emitted from the crystal.

When cubic ZnS crystal doped with electron deficient ions (Cu2+, Mn2+, Ni2+ and

Ag+) is hit with light at 350nm, electron excitation from valence to conduction band occurs, followed by subsequent non-radiative relaxation to intermediate energy levels. This initial part of the process is similar to what happens in undoped crystals. From the intermediate energy levels, the excited electrons fall to the acceptor levels of the dopant ion. This energy drop is lower than the drop to the valence band and hence the emitted photons have longer wavelengths (in the visible range), with peak wavelength of emission characteristic of the acceptor energy level. Doping with Cu2+ ion gives bluish green emission (Figure 3b) with peak at 465nm, with Al2+ gives blue emission (Figure 3c) with peak at 475nm [26] and with Mn2+ gives amber emission (Figure 3d) with peak at 580nm. Co-doping Cu+ and Al3+ results in green emission with peak at 525nm (Figure 3e) [27].

1.5. Electron traps and their contribution to luminescence

Defects in the crystal lattice such as vacancy (or omission) defects and substitutional defects affect the electronic structure in their vicinity as explained above.

Omission of anions from the lattice lowers the electrostatic potential barriers in their close vicinities. This creates a small localized region which often contains most of the excited electrons in the impurity. Excited electrons generated nearby or elsewhere in the crystal can be captured in this small region which prevents the electrons falling back to the ground

state. This small localized region is called an electron trap and can be visualized as a potential well of higher energy than the ground state. Additional energy input is required to raise a trapped electron out of the well to return to the ground state, a process called detrapping of electron trap.

Figure 3: Schematic energy level diagram showing possible electronic transitions in doped and co-doped ZnS crystals. Solid lines represent radiative transitions, dotted lines represent non-radiative transitions.

ZnS synthesized in S2- deficient conditions have a large number of S2- vacancies in the crystal. These omission defects create electron traps which act as shallow donor levels below the conduction band [27]. Similarly, Zn2+ vacancies create a hole traps which act as shallow acceptor levels above the valence band. It is to be noted that the electron and hole traps created by vacancies are referred to as shallow traps because their energy levels are closer to conduction or valence band respectively, and electron or hole trapping is 12

thermodynamically favorable. Comparatively, the donor/acceptor levels created by substitution by Al3+/Cu2+ are further away from conduction/valence band and are hence sometimes referred to as deeper donor/acceptor levels.

It has been found that higher number of filled electron traps increases luminescence emission from phosphor crystals. Detrapping of electrons from filled traps to conduction band requires lesser energy than exciting an electron from the valence band and is much easier. Higher number of excited electrons translates to more photonic emissions i.e., more intense luminescence. However, in a non-irradiated crystal, at room temperature, only a percentage of available electron traps are filled with electrons. Electrons from the valence band can be excited by radiation of appropriate energy to cause trapping within the otherwise empty electron traps. Stray light radiation and thermal energy can provide the energy necessary for filling of the electron traps, but artificial irradiation at the appropriate energy can be done to fill almost all the available electron traps.

Filled electron traps due to vacancy defects have also been shown to play an essential role in the mechanism of EML. Irradiating the crystals with ultraviolet light prior to measurement has been reported to cause an increase in ML intensity by a factor of 1.5, indicating that the filled electron traps contribute to EML mechanism [28].

1.6. Mechanism of Elastico-mechanoluminescence of ZnS:Mn,Cu

There are two established mechanisms that describe EML of ZnS:Mn,Cu crystals

–luminescence induced by moving dislocations and the piezoelectrically induced electron detrapping mechanism.

1.6.1. ML induced by movement of charged dislocations

The moving dislocation mechanism describes plastico-mechanoluminescence

(PML) well. Dislocations refer to defects in the host crystal on the atomic scale, such as substitutional defects, omission defects, etc. PML is observed to occur in pulses as the deformations increases. The number of pulses depend on the deformation rate or strain rate of the crystal, but the amplitude and widths of the pulses do not depend on the strain rate.

The moving dislocations mechanism states that during plastic deformation, charged dislocations of opposite polarity start moving towards opposite surfaces of the crystal. The charge build up on the surface increases the electric field within the crystal as deformation progresses, until a breakdown value when a pulse of electroluminescence is produced. The next cycle of charge build up commences as deformation continues.

This mechanism is reported to explain pulsed PML well, but requires modification to explain EML. Elastic deformations are insufficient to produce enough charge buildup to cause field breakdown and subsequent electroluminescent pulse. The electrostatic interaction between charged dislocations and filled electron traps can be accounted for the

EML emission in doped ZnS crystals. Elastic deformations cause movement of charged

dislocations to opposing surfaces causing buildup of surface charge and electric field within crystal. Generated electric field maybe sufficient enough to detrap electrons from filled electron traps. The detrapped electrons may reach the conduction band, move to vicinity of impurity ions, undergo non-radiative relaxation to intermediate states and drop to acceptor energy levels by emitting photons characteristic of the impurity ions.

A limitation of the moving dislocations mechanism is that it cannot explain EML emission that arises during hydrostatic compression of crystals. The dislocations in the crystal will not be able to move under hydrostatic pressure [29]. Also, this mechanism may not be able to explain EML from nanocrystals since it is more thermodynamically stable for dislocations and stacking faults to move to the surface of the crystals and annihilate.

However, there are reports of existence of dislocations within ZnS nanoparticles [30,31].

1.6.2. Piezoelectrically-induced electron detrapping mechanism

This mechanism, put forth by B.P. Chandra et al [29], identifies two key sources for EML in ZnS:Mn,Cu crystals. The inherent piezoelectricity of the ZnS crystal structure and the probable existence of higher piezoelectric constant in the vicinity of impurity ions are identified as the key sources to EML in ZnS:Mn,Cu. Both cubic and hexagonal crystal structure of ZnS are known to be piezoelectric in nature. This property arises from the non- centrosymmetric arrangement of the atoms in the crystal. Non-centrosymmetric crystal structures lack center of symmetry. Cubic ZnS is known to exist in F4̅3m space group and hexagonal ZnS exists in P6mc space group, both of which are established to have

piezoelectricity. The mechanism of EML of doped ZnS:Mn,Cu can be broken down into the following steps:

1. The elastic deformation of the crystal creates an electric field within the

crystal due to its piezoelectricity.

2. The piezoelectric field can reduce the trap-depth of filled electron traps

thereby detrapping the electrons to the conduction band.

3. The electrons in the conduction band undergo non-radiative relaxation

followed by recombination with holes either in the valence band or with the

holes trapped in the acceptor levels of Cu2+, and the subsequent non-radiative

release of energy.

4. The energy released non-radiatively can be transferred to the Mn2+ ions

5 6 4 causing an intraconfigurational 3d excitation ( A1 – T1) located within the

4 6 band gap of ZnS. The subsequent de-excitation ( T1 – A1) emits amber

radiation characteristic of Mn2+ (Figure 4a).

The mechanism of EML for mono-doped ZnS:Cu is similar, except in step 3, the energy from an electron-hole recombination in the Cu2+ acceptor level is radiated out as a bluish-green photon characteristic of the Cu2+ acceptor level (Figure 4b) . Similarly, for mono-doped ZnS:Mn, in the absence of Cu2+ acceptor levels, the electrons recombine with holes in the acceptor level of Zn2+ omission defects (Figure 4c) . The remainder of the process is similar as explained above.

Figure 4: Schematic energy level diagram for (a) co-doped ZnS:Mn,Cu (b) ZnS:Cu and (c) ZnS:Mn. Solid lines represent radiative transitions and dotted lines represent non- radiative transitions and energy transfer.

EML from ZnS:Mn has been observed for stresses as low as 1 MPa. The

-12 -1 piezoelectric constant (d33) of ZnS in the elastic region is 3.3×10 CN [32,33]. Therefore, the piezoelectric charge density created on the surfaces of the crystal when 1MPa of stress

(휎) is applied will be,

−12 −1 6 −2 −6 2 휌 = 푑33 × 휎 = (3.2 × 10 퐶푁 ) × (10 푁푚 ) = 3.2 × 10 퐶⁄푚

The piezoelectric field near the surface created by this charge density will be,

−6 −2 휌 3.2 × 10 퐶푚 5 −1 퐹 = = −12 −1 −1 = 3.61 × 10 푉푚 휀표 (8.854 × 10 퐶푉 푚 )

where, 휀표 is the permittivity of vacuum. The internal piezoelectric field within the crystal will be one order of magnitude lesser because the dielectric constant or relative permittivity of ZnS is 8.8 i.e.

5 −1 퐹 3.61 × 10 푉푚 4 −1 퐹𝑖푛푡 = = = 4.23 × 10 푉푚 휀푟 8.8

An external piezoelectric field of the order of 105 푉푚−1 will not be sufficient to pull electrons from electron traps and cause impact ionization of Mn2+ ions since electric fields of the order of 107 푉푚−1 and 108 푉푚−1 are required for the two processes respectively. But the observance of EML from ZnS:Mn for stresses as low as 1 MPa suggests that the local electric near the Mn2+ optical centers may be higher. This can be attributed to a higher piezoelectric constant near Mn2+ due to crystal structure distortion.

A limitation of this mechanism is that it does not explain EML from ZnS:Cu or

ZnS:Ag because it builds on the premise that local piezoelectric constant maybe higher near impurity ions and no such effect has been reported for Cu2+ and Ag+ ions.

1.7. Objectives of study

From the mechanisms of EML it can be understood that the phenomenon of EML of ZnS:Cu,Mn phosphors depends on several material properties such as elemental constituents, impurity ion, size of impurity ion, concentration of impurity, size of crystal, crystal structure, degree of crystallinity, crystal structure space group, dielectric constant, piezoelectric constant and so on. The first objective of this thesis work was to understand

the dependence of EML on the size of crystalline particles and crystal structure in particular. The second objective is to understand the dynamic behavior of EML by experimentally measuring EML intensity at different applied strains and strain rates. The third objective is to predict the minimum stress required for EML and understand the stress transfer mechanism from matrix to crystals.

Hence, the second chapter of the thesis discusses the experimental synthesis of

ZnS:Mn nanoparticles in cubic and hexagonal phase and material characterization experiments performed on the synthesized nanoparticles and commercially obtained microparticles of ZnS:Mn and ZnS:Cu,Mn to understand the dependence of EML on material properties. The third chapter will discuss the measurement of EML from commercially obtained ZnS:Mn,Cu phosphor particles. The temporal response of EML emission to applied strain and strain rate is studied and the brightness (or intensity) of EML emission is measured in candelas per sq. meter (cd/m2). Different matrices or binders considered for fabricating the ML paint are also discussed. Fourth chapter will discuss the

Finite Element modeling performed to understand the range of elastic stresses required to observe EML form ZnS:Mn,Cu particles in a matrix. The effect of surface adhesion between the particles and the matrix on the stress transfer is also studied. Additionally, the fifth chapter will discuss the efforts to demonstrate an alternative and relatively simple application of phosphorescent paints for paint damage detection.

Chapter 2: Material Synthesis and Characterization

Elastico-mechanoluminescence of doped-ZnS has been observed to depend on several material factors such as dopant concentration, crystal structure, size of crystals, piezoelectric constant, dielectric constant, etc. But essentially, EML intensity and peak wavelength just depend on the electronic structure or in other words, the energy band structure of the crystal. All the aforementioned factors affect EML through alteration of the local band energy structure and the local crystal structure.

For example, the energy band structure of a perfect crystal is altered not just by omission and impurity defects but also by surface defects. If the crystal size is restricted to the nanoscale, the influence of surface defects become more pronounced because of the higher surface to volume ratio observed with nanoscale structures. Hence the size of the crystal indirectly becomes a factor affecting EML. Similarly, the crystal structure (cubic or hexagonal) decides the interatomic spacing between ions in the lattice and therefore heavily influences the band energy structure of the host crystal itself. This is seen in the difference in band gap of cubic (3.54 eV) and hexagonal (3.91 eV) structures. The arrangement of atoms/ions in the lattice points can also decides the symmetry of the crystal. Non-

centrosymmetric structure is necessary for the existence of piezoelectricity, which has been suggested as the origin of EML of doped-ZnS materials.

The work discussed in this chapter concentrates on identifying the individual influences of the material properties such as crystal size and the crystal structure in particular, on EML emission. Towards this, nanocrystals ZnS doped with manganese were synthesized in the lab in both cubic and hexagonal forms. Commercially available ZnS:Mn phosphor particles in the micron scale were also obtained in the cubic and hexagonal forms.

Characterization experiments were performed to identify the material properties and understand their influence on EML emission.

2.1. Synthesis of ZnS:Mn nanocrystals

There have been conflicting evidences on the observance of EML emission from nanocrystals of ZnS:Mn. C.N. Xu et al. [13] have reported intense orange-yellow emission from nanocrystals of ZnS:Mn thin films deposited on substrates using vapor deposition techniques. B.P. Chandra [29] have reported that the EML emission intensity from nanocrystals of ZnS:Mn to be much higher than from microcrystals. Whereas, Hollerman et al. [34] have reported absence of any form of ML from the ZnS:Mn nanocrystals of average size 5nm.

For the objective of fabricating a ML paint system which consists of particulates of

ML material dispersed in a paint matrix, it is advantageous to have smaller sized ML particulates. Smaller particulates would disperse more uniformly in the paint matrix

offering higher space resolution and a thinner paint build thereby conserving material.

Further, smaller particles offer higher surface area for stress transfer and light transmittance for the same volume and mass of material. Also, nanocrystals of semiconductor materials

(such as ZnS) exhibit quantum-mechanical properties and are called quantum dots. The band gap of the nanocrystal inversely depends on the size of the crystal, thereby offering tuning of light emission by controlling crystal size. These advantages made a strong case for considering synthesis and subsequent utilization of nanocrystals of doped-ZnS materials in fabricating ML paint despite the conflicting reports.

2.1.1. Synthesis of cubic ZnS:Mn nanocrystals

Nanoparticles of ZnS doped with manganese have been synthesized by several researchers through aqueous chemical routes [35-42]. Khosravi et al [41] were among the first to synthesize cubic ZnS:Mn nanocrystals through an aqueous chemical route achieving particle size from 1 to 4nm. They have used zinc chloride, manganese chloride, mercaptoethanol and sodium sulfide as raw materials for the synthesis. Mercaptoethanol acts as surfactant that competes with the growth of ZnS:Mn crystals by adhering on to the surface of the growing crystal thereby limiting its size. They are able to achieve control over particle size by controlling the concentration of mercaptoethanol in the reaction mixture. Increase in surfactant concentration reduced the particle size and increased the band gap energy. They also report the nanocrystals to have cubic structure.

Mercaptoethanol is highly toxic and hence a variation of this method is used for the synthesis of ZnS:Mn nanocrystals in the lab. The wet chemical method carried out by

Porombo et al. [41] and Nazerdeylami et al. [42] has been adopted. In this method, Zinc acetate dihydrate, manganese acetate tetrahydrate, sodium sulfide nonahydrate and polyvinylpyrrolidone (PVP), all obtained from Sigma Aldrich Corp. are used as the raw materials as purchased. PVP acts as surfactant controlling size of nanocrystals.

The synthesis followed a simple double replacement reaction in aqueous solution given by the following flowchart and reaction.

Zn(C H O ) Mn(C H O ) Na S 2 3 2 2 + 2 3 2 2 + 2 + PVP → PVPZnS: Mn + NaC H O 푎푞. 푎푞. 푎푞. 2 3 2

1.925g of PVP was added to 5ml of 1M zinc acetate solution in a beaker under constant stirring. After all the PVP was dissolved, 5ml of 0.01M manganese acetate solution was added dropwise under constant stirring. This gave a 1% Mn:Zn cation ratio in the reaction mixture. To the reaction mixture, 5ml of 0.85M sodium sulfide is added dropwise under vigorous stirring. A white precipitate was formed immediately. The wet

precipitate was then centrifuged at 5000rpm for 10 minutes, after which the liquid was decanted and the precipitate washed thoroughly with deionized water. This centrifuging procedure was repeated thrice to thoroughly remove any sodium salts. The wet precipitate was then dried in vacuum at 60oC. The dried precipitate was then ground with a mortar and pestle to give powdered ZnS:Mn nanocrystals that were used as such for further analysis.

2.1.2. Synthesis of hexagonal ZnS:Mn nanocrystals

Synthesis of hexagonal ZnS:Mn nanocrystals was performed by a variation of the wet chemical method used for cubic phase synthesis. This method was adopted from R.

Viswanatha et al. [43] who have synthesized hexagonal Mn-doped ZnO nanocrystals. The method was modified to synthesize ZnS:Mn. Zinc acetate dihydrate, manganese acetate tetrahydrate, sodium sulfide and polyvinylpyrrolidone were the raw materials for this synthesis as well. The flowchart and reactions for this synthesis is given as follows.

푞푢푒푛푐ℎ Zn(C H O ) + Mn(C H O ) ∆ 50표퐶 2 3 2 2 2 3 2 2 → A → 𝑖푛 𝑖푐푒 퐵 𝑖푛 𝑖푠표푝푟표푝푎푛표푙

B + Na2S + PVP → PVPZnS: Mn + NaC2H3O2 𝑖푛 𝑖푠표푝푟표푝푎푛표푙 24

It can be observed from the reaction equations that the solvent used for this synthesis is isopropanol or isopropyl alcohol (i-PrOH). A 34.6mg of manganese acetate tetrahydrate is first dissolved in 20ml deionized water. This solution is then mixed with

200ml of i-PrOH under vigorous stirring for two hours while kept at room temperature.

2.152g of zinc acetate dihydrate is then added to the solution under stirring and the mixture is heated to 50oC followed by quenching in ice. 3.773g of PVP is subsequently added to the mixture and stirred for a few hours. 58.8ml of 125mM Na2S in i-PrOH is then added dropwise under vigorous stirring resulting in the formation of a yellowish precipitate was formed. The precipitate was then centrifuged at 5000rpm for 10 minutes followed by thorough washing with deionized water. The centrifuging procedure was repeated thrice to remove any dissolved salts. The wet precipitate was then dried in vacuum at 60oC for 10 hours. The dried precipitate was then ground with a mortar and pestle to obtain powdered

ZnS:Mn nanocrystals in the hexagonal form.

2.2. Fabrication of micron-sized particles of ZnS:Mn and ZnS:Mn,Cu

Micron-sized particles of ZnS:Mn and ZnS:Mn,Cu are commercially available as phosphor powders from different sources. ZnS:Mn phosphors exhibiting intense amber fluorescence was obtained from Phosphor Technology Ltd. from United Kingdom. These phosphors are referred to henceforth as GL25, following the product code of the manufacturer. Osram Sylvania GG series ZnS:Mn,Cu phosphors were obtained from

Global Tungsten and Powders Corp. from Towanda, PA. GG13 exhibits intense amber fluorescence, GG25 exhibits green fluorescence and GG45 bluish-green.

ZnS:Mn and ZnS:Mn,Cu micron sized phosphor crystals are generally synthesized by solid state reactions which involve heating sources of zinc, sulfur and the dopant in fixed proportions in furnaces at temperatures above 1000oC. Osram Sylvania’s patent of their yellow ZnS:Mn,Cu electroluminescent phosphors [44] describes the synthesis in detail. Copper sulfate and zinc sulfide are mixed as powders in fixed weight ratios. A chloride flux consisting of barium chloride, magnesium chloride and sodium chloride is also blended with the initial mixture. The admixture is fired in a crucible in air to 1200oC for 5.25 hours. The resulting hexagonal crystalline material is washed to remove excess halides followed by drying at 110oC and milling for 1.5 hours. Optical activation is done by adding manganese carbonate, more Cu as copper sulfate and more Zn as zinc sulfate to the milled powders and firing at 800oC in a crucible in air for 2 hours. After activation, the phosphor is washed with acetic acid, hydrochloric acid and potassium cyanide and then dried at 110oC to give final product.

2.3. Material Characterization

The crystals size and the surface morphology of the synthesized ZnS:Mn nanocrystals and the commercially obtained phosphor powders were studied using

Scanning Electron Microscopy (SEM). The elemental composition of the samples were estimated using Energy Dispersive X-Ray Spectroscopy (EDS) technique. Both SEM and

EDS analysis were performed using a FEI Helios Nanolab 600 Scanning Electron

Microscope. The crystal structure and the crystal size were predicted by performing X-

Ray Diffraction (XRD) analysis of the samples using a Bruker D8 X-Ray Diffractometer.

Fluorescence spectroscopy was performed using a HORIBA FluoroMax-4

Spectrofluorometer and a Shimadzu UV-2501PC UV-Vis Recording Spectrophotometer was used to obtain the absorption spectra.

Table 1 lists the phosphors that were characterized. First, the SEM images are presented, followed by the EDS data, XRD plots and finally the fluorescence spectra.

No Phosphor Source Emission Crystal size 1 Cub - ZnS:Mn Wet chemical synthesis (2.1.1) Amber Nanoscale 2 Hex - ZnS:Mn Wet chemical synthesis (2.1.2) Amber Nanoscale 3 GL25 Phosphor Technology Ltd. UK Amber Micron-scale 4 GG13 GTP Corp. PA Amber Micron-scale 5 GG25 GTP Corp. PA Green Micron-scale 6 GG45 GTP Corp. PA Bluish-green Micron-scale Table 1: List of available phosphors, their source, color of emission and crystal size

2.3.1. Scanning Electron Microscopy

SEM images of the phosphors in Table 1 are presented here in the same order. It can be seen from Figure 5 that the morphology of the ZnS:Mn phosphors synthesized in the lab using the wet chemical synthesis techniques (1 and 2 in Table 1) is different from the morphology of commercially obtained phosphors (3 to 6 in Table 1). The flaky morphology of the synthesized phosphors is because of grinding dried chunks of the

phosphors with mortar and pestle, compared to milling procedure done for the commercial phosphors which yields an almost spherical particles.

Further, upon observation, it is evident that the size of particles of the synthesized phosphors (Sections 2.1.1 and 2.1.2) are in the micron scale and not in the nanoscale as anticipated. It is important to understand that there will be secondary agglomeration of the nanocrystals into lumps held together by the polyvinylpyrrolidone polymer. Hence, the micron scaled particles seen in Figures 5.1 and 5.2 are the agglomerations of nanocrystals held together by PVP. Particle size seen from SEM images does not always correspond to the actual crystal size. The SEM images of the commercially available phosphors (Figure

5.3 to Figure 5.6) show the particle sizes in the micron scale, with highly uniform particle morphology and size distribution. The average particle sized measured is 12µm for GL25,

32µm for GG13, 24µm for GG25 and 21µm for GG45.

Figure 5: SEM images of the phosphors listed in Table 1. .

2.3.2. Energy Dispersive X-Ray Spectroscopy

This technique is similar to SEM, an electron beam hits a small region of interest on the sample, but the x-rays reflected back by the material is collected. The reflected x- rays are characteristic of the element and can be used to identify the elemental composition and elemental mapping of the sample. The EDS data gives the weight percentage and atomic percentage of all detectable elements in the sample which can be used to construct the chemical formula of the material. The EDS spectra plots the counts of x-rays detected against the energy of detected x-rays. Peaks in the spectra give definitive evidence of presence of elements in the sample corresponding to the energy of the peaks.

Figure 6: (a) EDS spectra; (b) Elemental composition (weight % and atomic %) of cubic- ZnS:Mn nanocrystals

Figure 7: (a) EDS spectra; (b) Elemental composition of hex-ZnS:Mn nanocrystals

Figure 8: (a) EDS spectra; (b) Elemental composition of GL25 phosphors

Figure 9: (a) EDS spectra; (b) Elemental composition of GG13 phosphors

Figure 10: (a) EDS spectra; (b) Elemental composition of GG25 phosphors

The EDS elemental composition data shows the presence of Zinc and Sulfur as the main constituents for the synthesized nanoparticles (Figure 6 and Figure 7). Carbon and oxygen detected can be attributed to the polyvinylpyrrolidone surfactant that binds over the nanocrystals and forms agglomerates. Both the cubic and hexagonal samples have been synthesized in sulfur deficient conditions, which can be noted by comparing the atomic percentages of sulfur and zinc. For the cubic-ZnS:Mn nanocrystals, the EDS spectra has markers for trace elements, such as Ti, Ga, Si and Pt (Figure 6). The peaks for these trace elements lie on the peaks of the major elements, and are hence erroneously identified. The weight and atomic percentages for these trace elements were less than 0.1% and some were negative as well, indicating instrument error.

The EDS spectra of GL25 phosphor shows no other element other than Zn and S and the individual elements are present in an almost 1:1 ratio (similar atomic %) implying very few vacancy defects. Comparatively, the GG series phosphors show a higher atomic percentage for Zn than for S, implying presence of sulfur vacancies. Also, importantly,

EDS spectra and data show the presence of aluminum and oxygen in the GG series phosphors. The GG series phosphors have been prepared to have a moisture resistant coating of aluminum oxyhydroxide [45] which is hydrophobic. The ZnS phosphors are coated with aluminum oxyhydroxide through a chemical vapor deposition process to make the phosphors insensitive to atmospheric moisture.

To verify that the detected aluminum and oxygen are from the coating and not constituents or dopants of the phosphor itself, a cut surface of a phosphor particle was imaged. For this purpose, GG45 phosphor particles were ground using a mortar and a pestle for 15 minutes and a sample of the ground particles was imaged. The cut particle and the element maps tracing each element on the particle are shown in Figure 11. It can be clearly seen from the element maps that while zinc and sulfur are found uniformly throughout the particle, aluminum and oxygen are abundant only on the outer surface of the particle. This shows that the aluminum and oxygen previously detected were from the moisture resistant coating. The white traces of aluminum and oxygen found on the cut surface might be due contact and subsequent adherence of the coating from other particles on to the cut surface during grinding with mortar and pestle.

It can be noted that the dopant elements (Mn or Cu) were not detected by EDS for any of the phosphors. This does not necessarily imply ineffective doping. The commercial phosphors GL25 and GG series phosphors show intense fluorescence thereby proving the presence of dopants. Therefore, it can be understood that the resolution of EDS is not

sufficient enough to detect dopant ions. Hence the presence of dopants in the lab synthesized ZnS nanocrystals can only be confirmed by fluorescence spectroscopy.

Figure 11: (a) SEM image, (d) EDS spectra and (b, c, e, f) Element maps of a cut particle of GG45

2.3.3. Absorbance Spectra

The absorbance spectra of a material gives valuable insight into the electronic structure or the energy band structure of a material. As previously explained in Section 1.3, the information regarding various electronic transitions across and within the band gap of the material can be estimated from the absorbance spectra. When a crystal is hit with

photons of a particular energy (or wavelength), they will be absorbed if and only if there are electronic transitions of equivalent energy possible as a consequence of the absorption.

Hence, high absorbance of photons of a particular energy would give information about distinct energy levels and their relative energy difference.

The absorbance spectra of the cub-ZnS:Mn nanocrystals and GL25 phosphor were measured using a Shimadzu UV-2501PC UV-Vis Recording Spectrophotometer.

Absorbance spectra is obtained by hitting the sample with light at different wavelengths and collecting the light transmitted through the material. Percentage of light energy absorbed can be estimated from the measuring the percentage of light energy that was unabsorbed and hence transmitted through the material. Transmittance (T) is defined as the fraction or percentage of incident electromagnetic radiation that is transmitted through a material. Absorbance (A) is defined as the common logarithm of the transmittance and is given by the following formula,

퐴% = 2 − log10(푇%)

The spectrophotometer is capable of measuring only the transmittance from which absorbance has to be calculated. The transmittance and absorbance of the cubic ZnS:Mn nanocrystals at different wavelengths is shown in Figure 12 and Figure 13 shows the transmittance and absorbance for the GL25 phosphors for comparison.

Figure 12: UV/Vis Transmittance and Absorbance spectra of the cubic ZnS:Mn nanocrystals

It is evident from the absorption spectra of cubic ZnS:Mn nanocrystals that there is no absorbance of light having wavelengths higher than 376nm. Hence the longest wavelength that can be absorbed by the crystal is 376nm, which corresponds to radiation with the least energy that can be absorbed. Lowest energy that is absorbed causing excitation of an electron from valence band to the conduction band corresponds to the band gap energy. Therefore, the band gap of the cubic ZnS:Mn nanocrystals is calculated to be

3.3eV (376nm). The peak absorbance at 301nm might correspond to the electron transition from the most-filled energy level in valence band to the most-filled energy level of conduction band (4.12eV). The most-filled energy levels imply energy levels with highest probability of holding electrons within the band. For all practical purposes, this energy gap

of 4.12eV is the band gap of the material, as hitting the sample with photons of 4.12eV would cause the most number of electronic excitations from valence to conduction band.

The sharp peak is hence the result of very narrow electron probability distribution within the valence and conduction bands. Narrow probability distribution could be understood as one certain energy state within the band holding a high number of electrons and immediately adjacent energy states holding substantially lower number of electrons.

The transmittance and absorption spectra for the GL25 phosphors are shown in

Figure 13. It can be seen that the GL25 phosphors have a broad absorption peak centered at 324nm which corresponds to UV light. Broad peak signifies a broader valence and conduction bands with uniform distribution of electron probability function. The broad absorbance peak has a sharp rise at 297nm (4.17eV) and initial sharp fall at around 383nm

(3.24eV) followed by a gradual decrease to 0 at around 610nm. The GL25 phosphor hence can absorb a portion of the visible region as well. Absorption in the visible region might correspond to lower energy intra-bandgap transitions from valence band to Mn2+ impurity states, or within the energy states of Mn2+ ion itself.

Figure 13: Transmission and Absorption spectra of GL25 phosphors

It should also be noticed that the peak absorbance of the GL25 phosphor (54%) is lesser than the cubic ZnS:Mn nanoparticles (77%). This signifies that the valence and conduction bands of the GL25 phosphor have broad and uniform probability distributions compared to the nanocrystals. Hence the band gap energy would correspond to the mid- peak at 324nm i.e. 3.8eV.

2.3.4. Fluorescence spectra

The emission spectra or fluorescence spectra of a material is obtained by hitting the material with radiation at one particular frequency and collecting light emitted by the material at different frequencies. The fluorescence spectra gives us additional information 38

about the energy band structure of the material. While the absorption spectra can give us the band gap of a material, the fluorescence spectra gives us information about energy levels within the bandgap that can hold electrons or holes. The presence of impurity states, vacancy states, and surface defect states are confirmed and their relative position within the band gap is also understood from the fluorescence spectra.

The fluorescence spectra of cub-ZnS:Mn nanocrystals and the GL25 phosphor were obtained using a HORIBA FluoroMax-4 Spectrofluorometer. From their individual absorption spectra, it can be seen that the cubic ZnS:Mn nanocrystals will absorb the maximum amount of incident energy at 301nm and the GL25 phosphors at 325nm. Hence to study fluorescence of these materials, it is wise to excite them at their respective absorption peaks. However, due to constraints of the equipment, both samples were excited at 375nm which was the lowest excitation wavelength possible by the equipment without compromising on the intensity of excitation beam. The fluorescence spectra of cub-

ZnS:Mn nanocrystals and GL25 phosphors are plotted in Figure 14.

Figure 14: Fluorescence spectra of (a) cub-ZnS:Mn nanocrystals and (b) GL25 phosphors

The cubic ZnS:Mn nanocrystals show two emission peaks – a weak emission peak centered at 445nm and a strong emission peak at 606nm. The weak peak at 445nm is characteristic of the cubic ZnS host crystal and can be traced to the blue emission due to electrons falling from sulfur vacancy levels to the valence band or zinc vacancy levels as shown in Figure 3. The strong peak at 606nm corresponds to amber light is characteristic

2+ 4 6 2+ of the Mn impurity ion, more particularly, the T1 – A1 transition within the Mn ion.

When the crystal is hit by photons at 375nm, the electrons get excited from the valence to conduction band. The excited electrons relax to the sulfur vacancy states, from where can follow two paths to ground state. They can either fall directly to the valence band (or zinc vacancy states or surface defect states) thereby emitting blue light at 445nm, or they can transfer the energy by direct impact to the Mn2+ impurity ion, thereby exciting it whose subsequent de-excitation emits amber light at 606nm. Comparing the intensities of the two peaks, we can say that the second route involving direct impact of Mn2+ ions occurs with 40

higher probability. The presence of an emission peak at 606nm also confirms the infusion of the Mn2+ ions into the ZnS crystal without doubt.

The GL25 phosphor has an intense peak centered at 578nm corresponding to yellow light. It can be noted that the intensity of emission from the phosphor is almost 20 times as much as what was obtained from the phosphors. The controlled synthesis of the GL25 and other commercial phosphors ensure high degree of crystallinity, lesser defects and optimized dopant concentration resulting in intense fluorescence. Also, it can be noted that the peak emission wavelength of the nanocrystals (606nm) is higher compared that of

GL25 phosphors. This red shift observed in the nanocrystals has not yet been directly attributed to the quantum confinement effect but it is expected to be a consequence of small crystal size and the presence of surface defect states in the energy band structure [46].

2.3.5. X-Ray Diffraction

The X-Ray diffraction technique is used to study the crystallographic nature of materials. It can provide quantitative information on the type of crystal structure, the crystal lattice constants, size of crystals and the degree of crystallinity. The technique uses the scientific principle that the atomic planes of a crystal cause an incident beam of X-rays to interfere with one another as they leave the crystal. Depending on the angle of incidence relative to the orientation of the atomic planes, constructive or destructive interference occurs. Hence by collecting the x-rays leaving the crystal at a range of angles, it is possible

to fully understand aspects of the crystal structure such as the orientation of planes, atomic spacing and so on, thereby understanding the crystal structure as a whole.

Figure 15: Schematic of X-Ray diffraction due to two parallel atomic places

Figure 15 shows schematic representation of the reflection of two incident X-rays by two adjacent parallel atomic planes. Constructive interference occurs when the two reflected X-rays are in phase. The reflected rays are in phase when the path difference between the two rays is an integer multiple of the wavelength of the x-rays. This is mathematically described as,

퐴퐵 + 퐵퐶 = 푛휆

From Figure 15, by symmetry it can be seen that 퐴퐵 = 퐵퐶. Also from the figure we can deduce 퐴퐵 = 푑푠𝑖푛휃, where d is the spacing between atomic planes. Therefore,

푛휆 = 2푑푠𝑖푛휃

This equation is referred to as the Bragg’s Law which can be used to find the spacing between atomic planes, d, if the angle of incidence at constructive interference and the wavelength of x-rays are known. A simple schematic of an XRD equipment is shown in Figure 16. The XRD equipment is designed with the x-ray source fixed in position and with the beam targeted at the sample to be analyzed. The angle of incidence of the x-ray beam on the sample is referred to as θ. An x-ray detector is also placed in the same plane as the source and the sample mount. The detector moves in a circular arc about the sample mount on the same plane collecting the x-rays reflected from the sample. Also, the sample mount can be moved in x, y and z directions along with rotation about the axis perpendicular to the common plane.

Figure 16: Schematic diagram of an XRD equipment design

As the detector moves along the arc, the reflected x-rays from the sample are collected at each 2θ position. The intensity of the detected radiation will be high at angles where constructive interference occurs and low at angles where destructive interference occurs. Hence the plot of intensity versus 2θ showing peaks at particular 2θ positions will be characteristic of the crystal structure of the sample which can be utilized to identify the same. This is the basis of using X-Ray diffraction to identify crystal structure and crystallographic parameters.

ZnS is known to exist in two crystal structures – cubic and hexagonal. The XRD spectra of ZnS in cubic and hexagonal phases are shown in Figure 17. The peak magnitude is not important as its 2θ location. Cubic phase of ZnS is known to have three major peaks at 28.5o, 47.5o and 56.5o, while hexagonal (or wurtzite) phase shows peaks at 27o, 28.5o,

30.5o, 39.5o, 47.5o, 51.7o, 56.3o.

Figure 17: Typical XRD spectra of Hexagonal (W) and cubic (C) ZnS crystal. [47]

The miller indices of the atomic planes contributing to peaks in the spectra through constructive interference are marked close to each peak. The miller index notation system is used to identify families of parallel atomic planes in 3D space. The indices describing a plane are reciprocal of the intercepts of that plane with the x, y and z-axis within a unit cell.

For example, a plane that is parallel to the yz-plane with an intercept of 1 unit cell dimension on the x-axis is identified by the indices (1 0 0). A plane parallel to z-axis with intercepts of half and one-third the unit cell dimension on x and y axes respectively is identified by the indices (2 3 0). The simple cubic structure can be expected to have lesser number of distinct atomic planes to cause constructive interference than the more complex hexagonal structure. This is corroborated by lesser number peaks for the cubic phase than the hexagonal phase.

The XRD spectra of the lab synthesized ZnS:Mn nanocrystals and the commercially obtained GL25 phosphor and GG series phosphors are presented below. XRD analysis was mainly performed to identify the crystal structure of the material and to understand the dependence of EML on the crystal structure, if any.

Figure 18: Raw XRD spectra and Averaged spectra (inset) of cub-ZnS:Mn nanocrystals

Figure 18 shows the raw data that was obtained for the nanocrystals synthesized as mentioned in Section 2.1.1. The raw data has a low signal to noise ratio and the intensity peaks are not clearly perceivable. This is mainly because the powder has random orientation of crystals and no particular orientation is preferred. Also, the powder sample is coated on vacuum grease so that the powders could be held on the vertical sample.

Vacuum grease is amorphous and can absorb most of the reflected x-rays from the crystals.

Hence, to easily identify the intensity peaks, a moving average filter was performed on the data by which 4 consecutive data points were averaged and plotted as a single data point.

From the averaged data is plotted in Figure 18 the intensity peaks are easily identified. 46

Since the averaged data is inaccurate, the peaks are traced back to the raw data and the 2θ locations are noted down. Comparing with Figure 17, the location of the peaks match those of the cubic crystal structure at 28.5o, 47.5o and 56.5o. Therefore it can be confirmed that the lab synthesis method described in Section 2.1.1 does indeed result in cubic ZnS:Mn.

Further, the widths of the peaks in the XRD spectra depend on the size of crystals, and can be mathematically expressed by Scherrer equation,

퐾휆 휏 = 훽 cos 휃

where 휏 is the mean size of crystals, 퐾 is the shape factor (usually 1), 휆 is the wavelength of x-ray beam, 훽 is the full width at half maximum in radians and 휃 is the incident angle.

By the Scherrer equation it can be seen that as crystal size decreases, the width of the peak increases. This is referred to as peak broadening and is generally observed when the crystal size is in the nanoscale. For the peak at 휃=28.5o, the FWHM value (훽) is

o measured to be 4.2 . The wavelength for the Cu K훼1 x-ray source is 0.154056nm. Hence the crystal size can be calculated to be 2.39nm. Thus, the lab synthesis described in Section

2.1.1 is experimentally proven to yield cubic phase nanocrystals of ZnS:Mn. It should be noted that the addition of dopant ions into the crystal does alter the local crystal structure but, the XRD spectra of the global crystal structure is generally not altered by doping

Figure 19: (a) Raw and (b) Averaged XRD spectra of hex-ZnS:Mn nanocrystals

Figure 19 shows the XRD spectra of the ZnS:Mn nanocrystals synthesized by the method described in Section 2.1.2. The raw data again is found to have a low signal to noise ratio and hence the same moving average filter was performed. The individual peaks are identified from the averaged data and are traced back to the raw data. The 2θ locations of the peaks when compared with Figure 17 indicate that the crystal structure may be hexagonal. Further, the peak broadening phenomenon is also observed confirming nanosized crystals. Since the peaks at 28.5o, 29.25o and 30.13o are very close to each other, their individual broadening results in a single wide coalesced peak seen in the raw data. In summary, the synthesis technique described in Section 2.1.2 has been proved, using XRD 48

analysis, to yield hexagonal phase ZnS:Mn nanocrystals. It is understood that for XRD spectra with higher signal to noise ratio, the powders should be analyzed without being coated on vacuum grease which absorbs most of the reflected x-rays. Hence, analyzing the samples in a horizontal powder XRD equipment is advised for future investigations into crystal structure determination.

Figure 20: XRD spectra of commercially obtained phosphors – GL25 shows hexagonal structure while the GG series show cubic structure.

Figure 20 shows the XRD spectra of commercially obtained phosphors presented in the same order as listed in Table 1. Since the commercially obtained phosphors are synthesized by solid state reactions at very high temperature, the degree of crystallinity for these phosphors is significantly higher than the lab synthesized nanocrystals. This is witnessed in the presence of distinct peaks with much less noise in their XRD spectra.

By comparing the XRD peaks with Figure 17, GL25 phosphor obtained from

Phosphor Technology UK, is found to have hexagonal crystal structure. Sharp peaks imply that the crystals are micron-sized. Scherrer equation is limited to nanocrystals and cannot be used to find size of microcrystals. The GG series phosphors all show cubic crystal structure with peaks close to 28.5o, 47.5o and 56.5o. The peak broadening seen in these three phosphors maybe due to the thin aluminum oxyhydroxide layer on the surface of the crystals. In conclusion, the crystal structure and the crystal size have been estimated from

XRD technique and are summarized in the following table.

Crystal No Phosphor Source Crystal size structure 1 ZnS:Mn Wet chemical synthesis (2.1.1) Cubic Nanoscale 2 ZnS:Mn Wet chemical synthesis (2.1.2) Hexagonal Nanoscale 3 GL25 Phosphor Technology Ltd. UK Hexagonal Micron-scale 4 GG13 GTP Corp. PA Cubic Micron-scale 5 GG25 GTP Corp. PA Cubic Micron-scale 6 GG45 GTP Corp. PA Cubic Micron-scale Table 2: Crystal structure and size of available phosphors.

Chapter 3: Characterization of Elastico-Mechanoluminescence

In the previous chapter, the synthesized and commercially obtained phosphors were characterized to understand their material properties – crystal size, elemental composition, crystal structure and band gap. In this chapter, elastico-mechanoluminescence of the phosphors dispersed in an elastomeric substrate is investigated to develop relationship between strain, strain-rate and luminance. The experimental methods used to demonstrate and measure ML of the phosphors are explained in the following sections.

3.1. Experimental methods to observe mechanoluminescence

Phosphors that are tested for mechanoluminescence are particulates with size in micrometer range. Stress can be applied to these microparticles by either direct load application like crushing between two surfaces, or by indirect load application in which load is applied to a matrix or medium that transfers stress to the microparticles. A direct load application technique – grinding with mortar and pestle, and two indirect techniques involving impregnation of the phosphor particles in an elastomeric matrix were used to observe ML from the phosphors. PDMS (polydimethylsiloxane) is a thermosetting elastomeric polymer that is used to hold the phosphor particles as a matrix. Coupons of

PDMS-phosphor composites were fabricated and tested for ML either by elastic actuation

(elongation by hand or using an electromechanical shaker) of the whole coupon (Figure 21 left) or by striking on the surface with a glass rod (Figure 21 right).

Figure 21: Phosphor-PDMS composite coupons showing EML during elastic actuation (left) and striking with a glass rod (right).

3.1.1. Fabrication of phosphor-PDMS composite coupons

Flexible rectangular coupons of dimensions 40mm×10mm×1.5mm were fabricated by mixing phosphor particles and viscous liquid polydimethylsiloxane elastomer (Sylgard

184 Silicone Elastomer from Sigma Aldrich Corp.) in 7:3 w/w ratio. PDMS elastomer contained base and curing agent in 10:1 w/w ratio. The constituents were thoroughly mixed for 15 minutes followed by cyclic exposure to 25in. Hg vacuum to remove trapped air

bubbles. The viscous liquid mixture was then cured at 150oC for 10 minutes to obtain solid flexible ML coupons. Figure 22 shows a few phosphor-PDMS composite coupons containing different phosphors under UV light showing fluorescence and a SEM image of a cross-section of a flash-frozen composite coupon.

Figure 22: (a) Phosphor-PDMS composite coupons under UV light – from left to right, GG13, GG25 and GG45. (b) SEM image of cross-section of GG45 phosphor-PDMS composite.

3.1.2. Phosphors that show mechanoluminescence

All the available phosphors were analyzed by the three techniques mentioned in the previous subsection for mechanoluminescence emission. Grinding with mortar and pestle is method I, striking or scribing composite coupons with glass rod is method II and elastic

actuation (elongation by hand) of coupons is method III. The results are summarized in the table below.

Crystal Crystal Method I Method II Method III No Phosphor structure size FML EML EML 1 ZnS:Mn Cubic Nanoscale No No No 2 ZnS:Mn Hexagonal Nanoscale No No No 3 GL25 Hexagonal Micron- Yes Yes No 4 GG13 Cubic Micronscale - Yes Yes Yes 5 GG25 Cubic Micronscale - Yes Yes Yes 6 GG45 Cubic Micronscale - Yes Yes Yes Table 3: Observance of ML from availablescale phosphors. Method I – Grinding particulates with mortar and pestle; Method II – scribing with glass rod; Method III – elongation by hand.

The lab synthesized ZnS:Mn nanocrystals having both crystal structures do not show any type of mechanoluminescence, whereas all the phosphors having micron sized crystals show intense FML and EML irrespective of the crystal structure. This evidence of nanocrystals of ZnS:Mn not showing any type of ML corroborates the work of Hollerman et al [34]. To our knowledge, there have been no published literature that reports observance of ML from nanocrystals of ZnS:Mn in the particle form. All reports of observance of ML from ZnS:Mn nanocrystals are limited to nanocrystals in thin solid films having very high order of crystallinity grown using vapor deposition techniques.

The possible reasons for lack of ML emission from ZnS:Mn nanocrystals are stated below:

1. Low degree of crystallinity in nanocrystals. C.N. Xu et al [13] have reported

that solid films having higher degree of crystallinity emitting more intense

EML. From the XRD spectra of the nanocrystals it can be seen the particles

have very low degree of crystallinity since there were no sharp distinct

peaks in the spectra.

2. The nanocrystals show fluorescence peak characteristic of Mn2+ doping,

hence excitation of Mn2+ occurs confirming the presence of dopant ions and

electron traps facilitating non-radiative energy transfer to the Mn2+ ions.

Absence of ML hence can only be attributed to a lower piezoelectric

constant near the dopant ions. The piezoelectric field created therefore is

insufficient to detrap electrons from traps and induce EML.

3. Larger surface area to volume ratio in nanocrystals result in higher influence

of surface defects on the energy band structure of the crystals. Energy states

due to surface defects might facilitate different pathways for excited

electrons to reach ground state without transferring energy to the Mn2+ ions.

Another interesting observation is the GG13 phosphor-PDMS composite coupons showing

EML during scribing with glass rod but not during elastic actuation. The most plausible reason for this is the absence of a hydrophobic coating on GL25 phosphor compared to GG series phosphors. Since PDMS is also hydrophobic, the coating on GG series phosphors can ensure strong binding to PDMS. Such a strong binding can be expected to be lacking

in the case of GL25 phosphor resulting in lesser stress transfer and lower EML emission.

This is explained in Section 4.5 better. The GG series phosphors though show intense

EML during elastic actuation of the composite coupons. In the next section, experiments performed specifically on GG45 phosphor-PDMS coupons are described.

3.2. Elastic actuation of composite coupons

3.2.1. Experimental setup for elastic actuation of coupons

Figure 21 showed EML emission from phosphor-PDMS composite coupons when stretched and relaxed by hand. Though using human muscle energy for mechanically actuating the coupons is very helpful in demonstrating the phenomenon of EML, a more controlled and repeatable mode of actuation is required for measuring EML at different strains and strain rates. Hence, an experimental setup was put together with an electromechanical shaker (K2007E01 Modal Shop Inc.) to induce longitudinal actuation of the coupons and a spectroradiometer (PR655 with MS-75 lens from Photo Research Inc.) to measure various parameters of the emitted light. A dSPACE module was used to feed a sinusoidal voltage input signal to the shaker to induce sinusoidal actuation. Magnitude and rate of applied longitudinal strain were controlled by altering the amplitude and frequency of input voltage signal. The setup was enclosed in a dark chamber to avoid light pollution affecting the EML measurements. Figure 23 shows a schematic and a picture of the experimental setup from top view.

The spectroradiometer collects light from a spot size of 5mm which is focused on the coupons using the MS75 lens. The focal length of the lens is 35.5cm. The spectroradiometer is controlled remotely using a USB cable connected to the same terminal controlling the shaker. The detector array of the spectroradiometer is kept open for a preset exposure time during which the light emitted from the coupon is cumulatively collected by the detectors. Measurements are later averaged out to give luminance per cycle of actuation.

Figure 23: Schematic diagram and picture of experimental setup used to measure EML.

The exposure time for EML measurements was selected to be 6 seconds, which was necessary to capture EML emissions at low strains and strain rates. Such a high exposure time limited the utilization of the spectroradiometer to capture the temporal response of

EML to applied strain. Hence a photoresistor was added to the experimental setup, placed on the other side of the coupon which can simultaneously track the temporal response.

3.2.2. Initial conditions of the coupon

The experimental setup shown in Figure 23 is used to elastically actuate GG45 phosphor-PDMS composite coupons at different strains and strain rates to understand the dependence of EML on each of the parameters. The luminance of the EML emission from the coupons is measured by the PR-655 spectroradiometer kept 35.5mm away. The voltage input to the shaker is a sinusoidal signal of required amplitude and frequency, which results in a sinusoidal strain curve of the same frequency and corresponding amplitude. The maximum applied strain and strain rate are individually varied by controlling the amplitude and frequency of the input signal sent to shaker respectively. Luminance measurements were taken by the spectroradiometer for every variation in the input signal. Strain applied on the coupon was measured by a lead tip attached to the shaker armature. The tip traced the stroke of the armature on paper. The maximum displacement created for each input signal was measured from the trace, using which the maximum strain applied on the coupon was calculated.

Figure 24: Two initial conditions of actuation of coupons based on strain value at midstroke.

The coupons were longitudinally actuated in two different initial conditions – positive pre-strain and zero pre-strain conditions as shown in Figure 24. For convenience, the positive pre-strain condition is referred to as case I and the zero pre-strain condition as case II. In case I, the coupon was initially stretched to a high strain value (30%) and amplitude of actuation was maintained below this value. This ensured the coupon being

positively strained during both forward (push) and backward (pull) strokes of the shaker.

More importantly, it also ensured a continuous strain rate curve.

In case II, the coupon is initially held at zero strain. When the armature moves forward from the initial location and has a negative displacement, the coupon buckles unable to withstand compression. The coupon hence experiences zero longitudinal strain for all negative displacements. When the armature starts moving backward towards zero displacement location, the buckled coupon starts stretching out, but the strain still remains zero. When the armature displacement crosses zero becomes positive the coupon suddenly becomes tensed and the longitudinal strain increases rapidly from zero as a sine curve. This is seen as an abrupt change in the strain rate from zero to maximum as shown in Figure 24.

The reasoning for experimenting case II arose from visual observations of brighter

EML while tugging the coupons by hand. Tugging essentially causes an abrupt increases in strain and strain rate which was suspected to be the reason for brighter emission. Hence the motivation for experimenting the case II was to scientifically record the tugging effect and thereby understand the dependence of EML on strain and strain rate individually.

3.2.3. Strain rate calculations

Understand the dependence of EML on strain rate involves knowledge of strain rate experienced by the coupon during each measurement. For both the initial conditions, the input signal had a sinusoidal variation with time, and hence the displacement and strain

applied to the coupon will also have sinusoidal variation with time. For the case I, the strain can be represented as a function of time as,

푠(푡) = 푆 × sin(2휋휈 × 푡) + 푆푡=0 (3.1)

where S is the maximum strain applied which was measured using a lead probe attached to the shaker armature as explained earlier. Actuation is done such that 푆 < 푆푡=0 where 푆푡=0 is the value of pre-strain – 0.3 or 30% for case I (0% for case II). Strain rate is obtained by differentiating the above equation with time,

푠̇(푡) = 2휋휈 × 푆 cos(2휋휈 × 푡) (3.2)

The maximum strain rate occurs at 푡 = 0 and is given by

푆̇ = 2휋휈 × 푆 (3.3)

The luminance for case I is plotted against this maximum strain rate value. The maximum strain rate i.e. the slope of the strain-time curve at the time=0, can be varied by varying both the amplitude and frequency of the input signal.

On the other hand, a similar approach to estimate strain and strain rate curves cannot be done for case II, since the curves are discontinuous. The discontinuity in strain is essentially the sudden tug felt by the coupon when going from buckled state to strained state. This sudden tug causes the frequency experienced by the coupon to be much higher than frequency of the applied signal (ν). Hence fast fourier transform (FFT) was performed on the discontinuous strain curve and different frequency components that actually make up the strain curve were identified. For example, the FFT obtained for a 15Hz discontinuous strain curve is shown in Figure 25. An effective frequency was estimated by

performing a weighted average of the frequency components identified from FFT. The maximum strain rate was then calculated using this effective frequency 휈푒푓푓 in Equation

3.3 instead of the frequency of applied signal 휈.

A quick observation of Figure 25 reveals that peaks at even multiples of the base frequency of 15Hz, with the amplitude dropping down exponentially. Also, a second exponential series exists involving the odd multiples of 15Hz, with amplitudes comparatively very low and hence contributing very little to the effective frequency value.

The effective frequency for the 15Hz curve was calculated to be 21.06Hz, an increase by a factor of 1.404. This factor of increase was a constant irrespective of the applied frequency since it only depends on the shape of the strain curve.

Figure 25: FFT of a 15Hz discontinuous strain curve. First 6 major peaks are highlighted.

3.2.4. EML measurements using spectroradiometer

Figure 26 shows the radiance spectra of EML from GG45 phosphor-PDMS composite coupon during elastic actuation. Radiance is defined as the power of radiation emitted from a surface per unit solid angle per unit projected area. Figure 26 also shows an image captured by a web-camera fitted to the eyepiece of the spectroradiometer. The black dot in the center is the 5mm OD entrance pupil of the spectroradiometer. Light entering the 5mm aperture falls on a diffraction grating and gets diffracted into components at different wavelengths. The diffracted beam then falls on to a detector. The detector, which has a resolution of 4nm, captures the radiance at each wavelength in steps of 4nm in the visible region (380-780nm). The luminance or brightness in candela per square meter

(cd/m2) is obtained from the radiance spectra.

Figure 26: (Left) Image showing EML of GG45 phosphor-PDMS coupon seen through eyepiece of spectroradiometer. (Right) Radiance spectra of EML shown on left. 63

3.2.5. EML dependence on maximum strain rate

The strain rates calculated as explained in the previous subsection were used to plot the luminance versus strain rate data for the two initial conditions. Maximum strain rate can be varied by changing both amplitude and frequency of the input signal. This is seen in Equation 3.3. But the variation in strain rate was achieved by varying only the frequency of the input signal and not the amplitude constant, thereby ensuring a constant strain.

Hence, the trends seen in the following plots show dependence of luminance on strain rate alone. EML luminance per cycle of actuation (one forward and backward stroke) is plotted against maximum strain rate in Figure 27 and Figure 28 for case I and II respectively.

Figure 27: Luminance against strain rate at different strains for case I 64

In case I (Figure 27), a non-linear dependence of ML intensity on the maximum strain rate is observed for all maximum strains (S). At lower strains, there is practically no

EML emission observable even at moderately high strain rates. The EML emission for

S=26% were detected only by the spectroradiometer and were not visible to human eye.

This may indicate to the presence of a threshold strain/stress value, below which no visible

EML emission occurs.

Figure 28: Luminance against strain rate at different strains for case II

In case II (Figure 28), contrary to case I, EML intensity seems to depend almost linearly on maximum strain rate. The linear trend is observed at all maximum strains, with

higher intensity at higher strains. Further, in case II, only the backward stroke (or negative displacement) is used to create strain in the coupon. Hence the strain values are half of what was created in case I. Despite applying only half the strains, the observed EML intensities are much higher. Such high intensities are mainly due to the occurrence of two bright bursts of light during each cycle that were visibly observed. The two bursts of light correspond to the abrupt changes in the strain rate that occur twice per cycle as seen in

Figure 24. The next subsection on temporal response of EML explains the significance of the bursts of emission observed.

3.2.6. Dependence of EML emission on strain

Figure 29 and Figure 30 show the dependence of EML emission on the maximum applied strain for case I and II respectively. Variations in maximum strain were produced by controlling the amplitude of input voltage signal and keeping the frequency constant.

Since the applied strain is a sine wave, changing the amplitude would implicitly change the slope of the curve at origin i.e. the maximum strain rate. Since the maximum strain rate cannot be kept constant, the trend lines of the luminance vs. strain plots are plotted at the same applied frequency. Also, since the maximum strain rate is variable, it is important to understand the trends seen in the plots below show a coupled dependence on both maximum strain and maximum strain rates. Due to the coupled dependence on strain and strain rate we see trends similar to the strain rate dependence plots (Figure 27 and Figure

28).

Figure 29: Luminance against maximum strain at different applied frequencies for case I.

Figure 30: Luminance against maximum strain at different applied frequencies for case II 67

3.2.7. Photoresistor circuit for temporal response of EML

The temporal response of EML of the coupons were captured by a highly sensitive photoresistor kept 14.75mm away from the coupon. The resistance of the photoresistor drops significantly when exposed to light, from about 55MΩ in darkness to about 160Ω in ambient light. This effect is used to measure the EML intensity, or track the change in EML intensity with time. The photoresistor was powered by a 5V DC supply and was connected in series with a 20kΩ resistor as shown in Figure 31.

Figure 31: Photoresistor circuit used to capture temporal response of EML.

When the incident light intensity increases, the resistance of the photoresistor and hence voltage dropped across the photoresistor decreases. The decrease in resistance also increases the current drawn from the voltage source thereby causing an increase in the voltage dropped across the 20kΩ resistor. Hence measuring the voltage drop across the

20kΩ resistor is more convenient since it directly corresponds to light intensity.

Accordingly, for obtaining the temporal response of EML from the coupon, the voltage drop across the 20kΩ resistor has been measured during actuation of coupon. The terms photoresistor response and photoresistor measurement refer to the voltage drop across the

20kΩ resistor unless mentioned otherwise. The frequency response of the photoresistor is explained in Section 3.3.2.

3.2.8. Temporal response of EML

To obtain the temporal response in both the initial conditions, a sinusoidal input of

5Hz was sent to the shaker as input signal. The photoresistor response is plotted with time along with the strain and strain rate curves to conveniently the track changes in EML intensity with changes in the two parameters at each time instant. The strain curve is built from the sinusoidal input signal with the same frequency and phase. The strain rate curve was obtained by differentiating the strain with time using central difference method. The strain and strain rate curves were scaled to fit within the given axis range. The phase shifts created by the photoresistor and the shaker were taken into account and the strain and strain rate curves were aligned appropriately. The phase shifts were obtained from the transfer function of the shaker and photoresistor which were obtained as explained in Section 3.3.

Figure 32: Temporal response of coupon actuated in case I. Two cycles of the 5Hz actuation are shown. Dotted midline is the zero line only for strain rate curve, and not the strain curve

Figure 32 shows the temporal response of EML for Case I – positive pre-strain condition in which the strain and strain rate curves are expected to be continuous over time.

The plot shows two cycles of 5Hz actuation from 0.6s to 1s. The black trend line is the strain curve, with a high value occurring when the coupon is pulled completely at the end of the backward stroke and low value occurring at the end of the forward stroke. The y- values of strain and strain rate curves are out of scale, only the phase and frequency are accurate. The strain values are always positive for case I and strain rate oscillates about zero.

It can be seen from the figure that two peaks of EML intensity occur in each cycle of actuation. The first peak corresponds to the increase in strain/stress during elongation 70

and the second corresponds to the decrease in strain/stress during relaxation. The peak from relaxation is observed to be less intense than the peak from elongation. This can be explained by observing the state of strain in the crystals when the peaks start to occur. The strain at the start of the second peak (at 0.75s) is higher than the strain at the start of the first peak (at 0.625s). If the number of filled electron traps available for detrapping can be assumed to be indirectly proportional to strain in the crystal at that instant, then the crystal, at the start of the second peak, will have lesser number of filled traps available than at the start of the first peak. Lesser number of filled traps implies fewer detrapped electrons resulting in fewer dopant excitations and subsequently fewer emitted photons and lower intensity. Returning to the unstrained state may refill all or most of the electron traps which then become available for the next cycle. This also explains why the intensity peaks remain constant over consecutive cycles for ZnS based phosphors. Strontium aluminate based phosphor do not show the self-recovery behavior which explains the eventual decay of its

EML intensity with prolonged actuation and the increase in EML intensity after short exposure to UV light. UV light excites electrons from the valence band and fills the empty traps which then become available for EML [48].

To understand the dependence of EML intensity on strain and strain rate, we trace the motion of the coupon with time starting at 0.8s. At this time instant, the strain of the coupon is minimum with the coupon at its most relaxed state and the strain rate is zero. At this instant, the EML intensity is seen decreasing from the previous actuation cycle.

Moving forward in time, the coupon starts being stretched and the strain and strain rate

increase. When the strain goes beyond a particular threshold value, the EML intensity starts increasing rapidly to its peak value. The peak occurs very close to the midstroke location where the strain rate is maximum. Once the peak is attained, the EML intensity decays almost directly proportional to and in phase with the strain rate till the strain rate becomes zero. As the strain rate increases (in the negative direction), EML intensity continues to decay which may be attributed to the crystal having lesser available electron traps in its strained state. As strain decreases, more electron traps become available which are utilized by the increasing strain rate to cause more dopant excitations. This explains the rise of

EML intensity at about 0.95s when strain is zero and strain rate is maximum (in the negative direction). This second peak corresponding to relaxation is triggered by the high negative strain rate.

Figure 33 shows the temporal response of EML for Case II – zero pre-strain condition in which the strain and strain rate curves are expected to be discontinuous over time. The important difference in this case compared to the previous case is that the strain goes to zero and remains zero for half the actuation cycle. It can be seen from the figure that as the strain increases from zero and the strain rate jumps to maximum value from zero at 0.64s, the EML intensity also increases very rapidly. The rapid increase is what is seen visually as a burst of light which can now be attributed mainly to the jump in strain rate.

The decay of the first peak can be explained as for case I. The rise of the second peak occurs when the strain has decreased enough for more electron traps to become filled and

when strain rate is high. It can be seen that the sudden increase in strain rate occurs close to the second peak.

Figure 33: Temporal response of coupon actuated in case II. Two cycles of the 5Hz actuation are shown. Dotted midline is the zero line for both strain and strain rate curves.

Compared to case I, the rise time for both the peaks is much shorter in case II (14ms against 38ms). The time gap between the two peaks is also shorter in case II. These observances could explained by the sudden jump in strain rate in case II. The sudden jump detraps electron traps faster causing increase in intensity to occur rapidly. Additionally, the relative (with first peak) intensity of the second peak is higher in case II. Also, the intensities of both the peaks are higher in case II. This can be explained as follows. In case

II, the strain in the crystal goes to zero, making more electron traps become filled and

available compared to case I, where the strain is always positive and never becomes zero.

Having more filled electron traps available for detrapping explains the higher intense peaks in case II. This is further corroborated by the observance of higher luminance by the spectroradiometer for case II than case I for similar strain rates (Figure 27 and Figure 28).

In summary, it can be understood that the phenomenon of EML is highly dynamic and requires a time-varying strain for emission. The sudden jump in strain rate, which is seen as a sharp peak in strain acceleration, causes a sharp high intensity emission as seen in Figure 33. The existence of a threshold strain/stress value is also established, below which EML is not observed and above which the EML intensity depends on both the strain and strain rate. EML intensity is also observed to be in phase with the strain rate and not the strain. ZnS crystal seems to have more filled electron traps available in the unstrained state than in a strained state. Hence, returning to unstrained state during each actuation cycle can increase the intensity significantly. For future work, utilizing a square or triangular strain curve instead of a sine curve would help confirm the hypotheses made and build a better understanding of EML dependence on strain and strain rate individually.

3.3. Additional information on experimental methods

3.3.1. Frequency response of shaker

To obtain the transfer function of the shaker, a laser interferometer is used to measure the displacement created by the shaker at different frequencies. The setup used to estimate the transfer function of the shaker is shown in Figure 34.

A linear chirp signal (sine wave of constant amplitude and linearly varying frequency) is sent to the shaker and the displacement created by the shaker is measured by the laser interferometer simultaneously. Once the input and output signals were obtained,

MATLAB’s in-built tfestimate function was used to obtain the transfer function of the shaker. Figure 36 shows the shaker input, laser output and the gain of the shaker estimated by tfestimate using Welch’s average method. The gain (푉퐿푎푠푒푟⁄푉푆ℎ푎푘푒푟) of the shaker along with the conversion factor of the laser interferometer (V to displacement in mm) can be used to estimate the displacement created by the shaker for any input signal. Knowing the displacement created, the strain induced on the coupon can be estimated.

It should be noted that the transfer function was obtained for a free boundary condition while the EML measurements are done with the coupon attached to the shaker.

The coupon is essentially a hyperelastic spring with stiffness dependent on the length being actuated. Hence, the transfer function would be different for each condition and length of coupon being strained. However, adding a stiffness element would not affect the phase of the transfer function. Therefore the analysis of EML intensity with strain and strain rate in

Section 3.2.8 will hold well irrespective of the stiffness of the coupon and its initial straining conditions.

Figure 34: Laser interferometer setup to estimate transfer function of shaker.

3.3.2. Frequency response of the photoresistor

Figure 35: Circuit used for obtaining frequency response of the photoresistor

The frequency response of the photoresistor was obtained using a LED of wavelength 512nm which is the ML peak of GG45 phosphor as seen in Figure 26. The

LED was placed at the same distance from the photoresistor as the coupon (14.75mm). A chirp signal of 1V amplitude and 5V DC offset was sent to the LED and the response of the photoresistor was observed as voltage drop across the 20kΩ resistor as shown in Figure

35. MATLAB’s function tfestimate was used to obtain the transfer function of the photoresistor.

Figure 36: Input signal to shaker in V (top left); Output signal from laser interferometer in V (top right) and the magnitude and phase plots of the transfer function. 77

Figure 37: Input signal to LED in V (top left); Voltage drop across resistor VR in V (top right) and the magnitude and phase plots of the transfer function

Chapter 4: Understanding Stress Transfer to ML Particles

Elastico-mechanoluminescence of ZnS:Mn,Cu microcrystals was demonstrated and the dependence of the intensity on strain and strain rate were studied in the previous chapter. Though the results from the experiments helped us understand behavior of EML as a function of strain and strain rate applied to the coupons, an understanding of EML as a direct function of the stresses experienced by the ML particles is required. The experimental results also suggest the existence of a threshold stress value below which

EML is not observed. Hence, to evaluate the stresses transferred to the particles at different strains applied to the PDMS matrix, finite element analyses have been performed.

Jeong et al [23] have supposed that EML was mainly due to compressive stress – compression in the lateral direction during elongation and in the longitudinal direction during relaxation. The motivation for the FEA study conducted was not just to narrow down the on range of stresses experienced by the particle at different strains and but also to distinguish the individual contribution of tensile and compressive stresses to EML intensity. Further, understanding the threshold stress required for EML will help design the matrix for the ML paint better. The FEA model developed can be modified to simulate matrices other than PDMS such as paint binders like polyurethane and lacquer. The strains

required in those matrices to generate threshold stress on the particle can be estimated, which will be an important design parameter of the ML paint system. The FEA model can be used to find the right paint binder that transfers most stress for the least strain.

4.1. Model Description

A model of a ML particle in a cylindrical volume of PDMS was built in Abaqus

FEA software as shown in Figure 38. To save on computational cost and time, only one particle within a small volume of PDMS is modeled. This representative volume element could be repeated over space to obtain the entire composite coupon. The circular ends of the cylinder are subjected to equal longitudinal strains in the opposite directions to simulate the straining of the composite coupons. The size of particle was estimated from SEM images and the size of the PDMS cylinder was selected based on the particle size.

Figure 38: FEM model of cylindrical PDMS volume enclosing a spherical ZnS particle. 80

4.2. Material Properties

The particle was modeled as a sphere with elastic isotropic properties of ZnS

[49,50]. PDMS was modeled as a hyperelastic material described by a second order Odgen model. The Odgen coefficients (µ1, µ2, α1 and α2) estimated experimentally for Sylgard

184 Silicone PDMS cured at 150oC for 15 minutes by T. K. Kim et al [51] have been used in our analysis. These conditions were similar to our composite coupon fabrication conditions explained in Section 3.1.1. Hybrid tetrahedral elements C3D10H are used for the PDMS and regular tetrahedral elements C3D10 are used for the particle. The material parameters used are mentioned in the following table.

Property ZnS Property PDMS Elasticity Elastic Isotropic Hyperelasticity Odgen’s model [51] ρ (Kg/m3) 4079 µ1 (MPa) 0.244339 E (MPa) 98100 µ2 (MPa) 0.0146323 n 0.41 α1 1.01795 α2 3.74094 Table 4: Material properties of ZnS and PDMS used in the analyses.

4.3. Boundary Conditions

The longitudinal strains applied to the boundaries of a composite coupon in the

EML experiments explained in the previous chapter ranged from 10% to 55% depending on the initial loading condition. Since the FEA simulations are performed not for an entire coupon but for a small representative element within the volume of the coupon, it becomes

imperative to understand the volume distribution of longitudinal strain within the coupon.

Hence, an FEA simulation of an entire coupon of PDMS, with dimensions similar to the lab fabricated coupons and applied strain value similar to the experiments, was performed.

The coupon was modeled as a cuboid of hyperelastic PDMS (Odgen model), without any particles impregnated in the volume.

Figure 39: Longitudinal strain contour of PDMS coupon strained by 20% at the boundaries.

Figure 39 shows the longitudinal strain contours of a PDMS coupon strained 20% in the longitudinal (x) direction. For an applied strain of 20%, it was observed that the strain at every longitudinal station was nearly 20% as well. The average longitudinal strain was calculated to be 18.3%. This result validated the assumption that the representative cylindrical element shown in Figure 38 will also undergo strains similar to the strain

applied to the coupon as a whole. Based on this validated assumption, the cylindrical volume shown in Figure 38 was longitudinally strained at 10% to 50% strains in steps of

10%.

4.4. Contact between article and PDMS

The contact between the particle surface and PDMS plays a vital role in stress transfer and hence modeling of the contact required much attention. The extent of adhesion between the particle and the matrix controls the magnitude of stress transferred across the contact. Hence, complete adhesion and lack of adhesion between the surfaces represents the ideal best case and worst case scenarios for stress transfer. Complete adhesion transfers most stress to the particle while stress transfer is the least without any adhesion. A prediction of the range of stresses experienced by the particles at different strains is therefore made by modeling these two extreme contact cases. In Abaqus, the two cases were modeled using the tie type and interaction type of contacts, representing complete adhesion and no adhesion cases respectively. Surfaces in tie contact were completely bound together with strains across the contact being continuous. In the interaction contact, the surfaces were free to slide over and move away from each other but they were not allowed to penetrate one another.

Since the interaction type of contact allows relative sliding between two surfaces, a coefficient of friction can be introduced to control the sliding behavior. Simulations were performed with the coefficient of friction varied from 0.1-0.5 in steps of 0.1. The strain

applied to the coupon at a constant applied strain of 10%. It was observed the shear stresses on the surface of the particle increase with coefficient of friction, but the average maximum stress within the particle remained almost constant. Hence, for simulations at higher strains (10-50%) which are discussed below, friction was set to zero to decrease computational time.

4.5. Results and Discussions

The von Mises stress contours at the Z-plane (or xy plane) passing through the origin of PDMS cylinder is shown at each applied strains in Figure 40. The tie type and the interaction type contact results are shown side-by-side at each applied strain for comparison.

The odd-numbered subfigures show the particle and PDMS surfaces in tie contact, being bound together at all strains. The even-numbered subfigures show the two surfaces in interaction contact, with the PDMS surface free to move away from the particle as the strain increases. This allows the creation of a void between the particle and the matrix, thereby reducing the area of contact to a thin band that runs along the circumference of the particle in the X-plane. The area of this band is calculated to be one-fourth the total surface area of the particle. Since the area of contact is reduced, the stress transferring efficiency also is reduced in the interaction contact.

Figure 40: von Mises stress contours at the Z-cut plane passing through the origin. Tie type contact is shown on the left (1, 3, 5, 7 and 9) and interaction type on the right (2, 4, 6, 8 and 10). Applied strain increases top down from 10% to 50% in steps of 10% 85

The individual stress components at all elements within the particle at each applied strain were collected and averaged. Figure 41 and Figure 42 show the average stresses in the particle at different applied strains for tie and interaction contact respectively.

Longitudinal direction is the x-direction along which strain is applied. The y and z- directions are identical by nature of symmetry of the model and hence only σy is plotted.

The maximum and minimum principal stresses, σ1 and σ2 respectively, are used to define the range of stresses experienced by the particle at different applied strains.

Figure 41: Average stresses within the particle at tie contact with the PDMS matrix.

Figure 42: Average stresses within the particle at interaction contact with the PDMS matrix.

From comparing Figure 41 and Figure 42, it can be observed that, numerically, the tie contact transfers almost five times the stress as the interaction contact. Further, the tension in x-direction contributes most to the stress in the tie contact case while Poisson’s compression in y-direction contributes most in the interaction contact case. It is also interesting to note that practically no longitudinal stress is transferred to the particle in the interaction contact and only the Poisson’s compression is effectively transferred. This is because due to lack of adhesion, the PDMS surface moves away longitudinally creating a void and only a thin band of PDMS is in contact with the particle. This band transfers the compressive Poisson’s stress to the particle in the radial direction which is seen as a 87

negative σy or σz. Additionally for both the cases, σ1 and σ2 are numerically similar to σx and σy which implies that the directions of maximum and minimum principal stresses align with x and y-directions respectively. This is because tension predominantly acts in the longitudinal direction and compression in the radial direction.

Realistically, the two types of contact could represent the interface between the particle and the matrix in freshly fabricated coupons and highly fatigued coupons. In freshly fabricated coupons, the interfacial binding is expected to be strong whereas in highly fatigued coupons the binding is expected to be weak due to wear off. Hence maximum stress transfer occurs in fresh coupons represented by tie contact and much less stress is transferred in fatigued coupons represented by interaction contact. This supports the experimental evidence by Jeong et al. [23] who have observed a 35 % decrease in EML luminance after 100,000 cycles. They have identified wearing off of the interfacial binding and not ZnS crystal fatigue as the reason for the luminance drop.

Assuming the interfacial contact slowly shifts from tie type to interaction type over prolonged actuation, the maximum stress experienced by the particles should over 75% as suggested by the FEA results. Correspondingly the luminance should also drop by at least

75%, since the dependence of luminance on strain has been reported to be linear [] or quadratic [14,29]. Such a huge drop in luminance is not observed because the interaction contact represents a theoretical case where there is no interfacial binding at all. In reality, the PDMS is expected to have partial binding to the particles even in highly fatigued

samples. Therefore, even in highly fatigued coupons, the contribution from longitudinal tension can be expected to be significant, if not higher than Poisson’s compression.

Another interpretation of the two type of contacts involves understanding the interfacial binding between phosphor particle and PDMS. The GG series phosphors were found to have a moisture resistant hydrophobic coating of aluminum oxyhydroxide from the EDS element maps shown in Figure 11. The hydrophobic surface of the particle will readily bind well with PDMS which is also hydrophobic. This ensures good stress transfer efficiency between PDMS and the GG series particle. On the other hand, GL25 phosphor particulates have no hydrophobic coating that can create good binding with PDMS and therefore, the stress transfer efficiency in this case can be expected to be low. This was experimentally observed in 3.1.2. Phosphors that show mechanoluminescence

GG series phosphor-PDMS coupons show intense EML during elastic actuation

(method III), while GL25 phosphor-PDMS coupons shows EML only when scribing with glass rod (method II) and not during elastic actuation. Scribing with glass rod applies direct compressive stress to the particles within the coupon through applied pressure. In this method of load application, the interfacial binding between the particles and PDMS plays no role in stress transfer. In contrast, stress transfer to the particle during elastic actuation of coupons depends largely on the interfacial binding as seen in the FEA simulations. The lack of EML emission during elastic actuation of GL25 phosphor-PDMS coupon hence corroborates the FEA analyses on the large influence of interfacial binding on stress transfer and effectively on the EML emission.

4.5.1. Stress Dependence of EML

The experimental results in Section 3.2.4 that have described the dependence of

EML intensity on the strain applied to the composite coupons and the numerical simulations that have predicted the stresses on a ML particle within the PDMS matrix can be combined together to predict EML dependence on stress. For this purpose, it is assumed that the particles are in complete adhesion to the PDMS matrix and hence the tensile stress from tie contact case is used. A polynomial fit of the average 휎1 stress shown in Figure 41 is made and stresses at different applied strains are obtained from this polynomial fit. The following plots show the EML dependence on stress for the two initial loading conditions.

Figure 43: EML dependence on stress on the ML particles for initial loading condition I

Figure 44: EML dependence on stress on the ML particles for initial loading condition II

The dependence of EML on the average tensile stress on the ML particles is found to follow similar trends as the dependence of EML on the strain applied to the coupons.

This is because the average tensile stress built up within the ML particle depends almost linearly on the strain applied on the coupon as seen in Figure 41. To our knowledge, the dependence of EML from microparticles of ZnS:Mn,Cu on the stress experienced by the microparticles has not been previously reported.

4.6. Summary

In an effort to develop a good understanding of the stress transfer between the matrix and the ML particles and to predict a range of stresses experienced by the particle

that can cause EML, FEA simulations have been performed in Abaqus. A small representative element of a ZnS particle within a cylindrical volume of PDMS was modeled and analyzed at different applied strains. Two different types of contact are modeled to understand the effect of the interfacial binding between the particle and the matrix. The two contacts represent the ideal cases for maximum and minimum stress transfer efficiency of the interface and have been used to predict the range of stresses experienced the particle at various applied strains.

Correlations to real world scenarios for the two contact types are drawn. Complete adhesion contact can represent GG series phosphors particles having hydrophobic surfaces that bind well to PDMS. Whereas, zero adhesion contact can represent GL25 phosphor that does not show EML during elastic actuation. Further, the contact types can also be used to represent freshly prepared coupons with good interfacial binding and highly fatigued coupons with poor interfacial binding. The decrease in EML emission with fatiguing can then be related to a gradual shift from complete adhesion contact to poor or no adhesion contact. The range of stresses that a particle within PDMS might experience is summarized in the following table.

Maximum average stress Minimum average stress Strain (N/mm2 or MPa) (N/mm2 or MPa) 0.1 0.15 0.035 0.2 0.27 0.055 0.3 0.38 0.08 0.4 0.47 0.10 0.5 0.56 0.12 Table 5: Range of stresses on particle at various applied strains 92

Chapter 5: Damage Detection of Surface Coatings

Working towards fabricating a mechanoluminescent paint system led to knowledge about various paint systems currently widely used in the automotive, aerospace, naval and infrastructure sectors. Paints are primarily applied as sacrificial coatings that act as barriers to protect to metals and structural plastics, especially in infrastructure, aerospace and naval structures. A prime example of sacrificial coatings would be corrosion protection coatings that are applied to metallic surfaces of aircrafts, ships, submarines, etc., to prevent corrosion of base metal and increase performance lifetime. Continuous monitoring of these coatings for damages and integrity in mission critical applications mandates frequent maintenance and increased operating cost. Research into damage detection methods have remained specific to the application area and highly influenced by surface and coating material combination. In naval and aerospace applications, various workarounds have been introduced to minimize maintenance and a versatile low-cost automated detection solution has been elusive.

A novel and surprisingly simple solution using phosphors that can be applied to the monitoring of a variety of coating material and surfaces is identified as follows. The approach involves adding strontium aluminate based phosphor particles to the primer layer

underneath the corrosion protection layer for use as damage sensing coating. Strontium aluminate (SAO) doped with europium and dysprosium (SAO-Eu/Dy) has a broad absorption peak centered at around 380 nm and an intense emission peak at 490 nm, and is selected for its long afterglow (6000 minutes), very high quantum efficiency color of emission which is distinctly distinguishable to the naked eye. Damage to the corrosion protection layer exposes the SAO-Eu/Dy layer underneath and enables the detection of damage. Detection of the damage utilizes the long persistent phosphorescence of the SAO-

Eu/Dy layer through in-service imaging and detection methodologies. The phosphor-based approach to detect damage to a corrosion protective coating is simple, quick, nondestructive and not require use of expensive technology for detection or post-processing.

5.1. Experimental Methods

Strontium Aluminate doped with Europium and Dysprosium (Sr4Al14O25:Eu,Dy) was purchased from Sigma Aldrich Corporation as long persistent blue-green phosphor.

The phosphor was characterized using the same experimental techniques discussed in

Chapter 2.

5.1.1. Phosphorescent paint

The phosphorescent paint was fabricated by mixing the phosphor powder in lacquer based Dupli-Color BSP300 automotive clear coat. An 8 in. ×13 in. aluminum panel was initially cleaned using a 200 mesh sandpaper and deionized water. A thin coat of black

enamel was sprayed on the cleaned surface of the aluminum panel. This enamel coat served as the base coat of the paint system and also provided a contrasting background for the blue-green emission from the phosphor. The phosphorescent paint mixture was then sprayed over the base coat using a 1.8mm-tip spray gun operated at 30psi pressure. Another coat of black enamel mimicking the anti-corrosion coat was sprayed over the phosphorescent paint.

5.1.2. Controlled damage

Controlled damage to the top coat (mimicking the anti-corrosion coat) was made in the form of incisions using a scalpel controlled by Sutter Instruments MPC-200 micromanipulator. The painted aluminum panel was segmented into a 6×4 grid of 24 small test panels each of size 1.5 in.×1.5 in.

Figure 45: (a) Incision test setup and (b) schematic of the setup to measure luminance.

Incisions were made in each test panel by first fixing the (x,y) position of the scalpel tip in the plane of panel. The scalpel was then slowly lowered in z direction till the tip touched the top coat of the test panel, which was ascertained with naked eye. Once contact was confirmed, the scalpel tip was further brought down 100 microns, and then moved the required length in x direction. The number and length of incisions in a certain test panel were decided on the position of that panel in the grid. Across the grid, the length of incisions were varied from 5mm to 20mm and number of incisions were varied from 1 to

4. In cases of multiple incisions, the incisions were drawn parallel to each other with a 500- micron gap between one another.

5.1.3. Luminance Measurements

The luminance of emission from the incisions was measured using the PR-655 spectroradiometer kept at a distance of 35.5cm horizontally from the test panel as shown in Figure 45. The entrance pupil, 5mm in diameter with the primary lens (MS-75), was focused on the midpoint of the incisions. The panel were illuminated with a UV source with peak intensity at 365 nm which lies in the broad absorption peak of the phosphor

(Figure 45b). The UV source was kept 7.5 - 8 cm away from the midpoint of the incisions.

After illumination for a constant period of 40 seconds, the UV source was turned off and luminance of the emission was measured immediately. This procedure was carried out for all the test panels. Further, the effect of UV exposure time on luminance was also studied for panels with single incisions.

5.2. Experimental Results

5.2.1. Material Characterization

The morphology and average size of the phosphor particles influence important characteristics such as the surface finish of the coat, minimum coat thickness for sufficient light emission, effective surface area of particles exposed to UV light and the packing factor. SEM was performed to study the morphology of the particles. The average particle size of the phosphor was estimated to be 50 microns from SEM images shown in Figure

46a and 46b.

The powders were used as purchased and were not ground before mixing in clear coat. Milling the powders could be adopted to reduce average particle size enabling denser packing of the particles in the clear coat. Reducing the particle size would also increase effective surface area exposed to light, thereby increasing the absorption of light and luminance output from the coating. These two factors enable thinner coats without sacrificing build or efficiency of the detection layer.

Figure 46c shows the chemical composition of the phosphor obtained using EDS which gives the atomic percentage and weight percentage of each element in the molecule.

Comparing the EDS data with the molecular formula specified by the vendor, there is a

13% variation in the atomic percentage and 14% variation in the weight percentage estimates. Figure 46d shows the XRD peaks for Sr4Al14O25:Eu,Dy. The peaks corresponds to orthorhombic phase which is in good agreement with previous works [54-57].

Figure 46: SEM images (a) as powder and (b) in clear coat (c) EDS spectra and elemental composition (d) XRD spectra of Sr4Al14O25:Eu,Dy

Figure 47: Excitation and Emission spectra of SAO-Ey/Dy

The excitation and emission spectra of Sr4Al14O25:Eu,Dy is shown in Figure 47.

The excitation spectra was obtained for a detection wavelength of 475 nm. A broad excitation peak centered at 380 nm and a sharp peak at 396 nm are observed. Both the peaks were consistently centered at these positions for various detection wavelengths. The emission spectra is obtained for an excitation wavelength of 375 nm which was selected from the excitation spectra. We can observe a strong peak at 490 nm which corresponds to aqua or bluish-green emission. Also, since absorption and excitation spectra follow similar trend, an UV light source with peak intensity lying in the absorption peak region is selected for our luminance experiments.

5.2.2. Luminance measurements

Figure 48 shows the dependence of luminance on the length and number of incisions made on the top coat. It can be observed that the luminance output from the incisions increase as the length of incisions increases. Also, having additional incisions increase the luminance output. This is expected as the exposed area of PL paint is higher with increased length and more incisions. With increased exposed area, more particles within the clear coat are exposed to UV light and hence a higher luminance output is observed from the incisions.

Figure 48: Dependence of luminance on length; n represents number of incisions.

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The aberrations in the observed luminance can be attributed to two factors: residual top coat within incisions and inconsistent depth of incisions. It was observed during incisions that the top coat did not always scrape off but at times got lodged within the incision. Repeated strikes in both directions, while maintaining constant depth, were performed to ensure at least partial removal of residual top coat. Inconsistencies in incision depths crept in due to the ascertaining of contact between scalpel tip and top coat with naked eye.

Figure 49: Luminance variation on UV exposure duration; L represents length of incisions.

UV exposure time also affects the luminance output. The test panels with single incisions were exposed to UV light for varying durations of time from 15 seconds to 40

101

seconds. It is observed from Figure 49 that exposing the incisions to UV light for longer periods of time results in increased luminance output. However, luminance output is expected to reach a steady value as the rate of absorption of energy from UV light and rate of emission energy in the form of visible photons will become equal with time.

5.3. Summary

A novel and simple approach to detect damage and corrosion of the top coat of

Naval and Aerospace paint systems is put forth. Addition of a new phosphorescent paint coat underneath the top coat is suggested as a nondestructive detection technique.

Strontium aluminate doped with europium and dysprosium (Sr4Al14O25:Eu,Dy) is selected for its long persistence time and bluish-green emission. Physical morphology, chemical composition and crystal structure of the phosphor particles are studied using SEM, EDS and XRD characterization techniques. Fluorescence spectroscopy is done to identify the absorption and emission peak wavelengths of the phosphor.

The new paint system with phosphorescent detection coat is sprayed on to an aluminum panel. Controlled damage to the top coat is done in the form of incisions to expose the detection coat underneath. The luminance of light emitted from the incisions after exposure to UV light is measured using a spectroradiometer. The effects of incision length, number of incisions and UV exposure duration on the luminance from incision are studied. It is observed that luminance linearly increases with incision length and number of incisions. This trend is expected because the area of the phosphorescent coat exposed to

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UV light increases with incision length and number of incisions. It is also observed that luminance output initially increases with UV exposure duration and reaches a steady value at higher durations. The results support utilization of luminance output as a tool not only to detect defects but also estimate the size and hence judge the potential of each defect to cause corrosion of the base metal.

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Conclusions

Utilization of elastico-mechanoluminescence of ZnS based phosphors is suggested towards the objective of fabrication of a paintable light source that requires minimal wiring and electroding and can be scaled. The proposed design for an EML-based paintable light source involves ZnS:Mn,Cu phosphor particles thoroughly dispersed in a paint matrix and sprayed on to a metallic panel that is mechanically actuated by piezoelectric wafers or other similar actuators placed underneath the panel. The mechanical actuation creates stress waves that are transferred to the phosphor particles within the paint matrix causing EML emission.

The mechanism of EML of ZnS based phosphors has been discussed and the dependence of EML intensity of ZnS:Mn and ZnS:Mn,Cu on the crystal size, crystal structure, chemical composition and energy band gap has been studied. Both cubic and hexagonal phases of the crystal in the micron scale have been observed to show EML whereas, no EML is observed from both the phases in the nanoscale. Composite coupons of phosphor particles impregnated in PDMS were fabricated and used to understand the dependence of EML intensity on applied strain and strain rate. Cumulative luminance and temporal response results suggest the following:

1. A threshold stress/strain value exists below which there is no EML emission.

104

2. EML intensity is driven by (in phase with) and depends non-linearly on strain

rate.

3. Sudden increase in strain rate also increases EML intensity. In other words, a

peak in strain acceleration causes a peak in EML intensity.

4. Increase in stress was observed to cause higher EML intensity than a decrease

of the same magnitude at the same rate.

FEA simulations were performed resulting in a better understanding of the stress transfer from the matrix to the ML particles. A prediction of the range of stresses on the particle at different applied strains was also made. Two different contact types utilized in the simulations and are correlated to real world cases. Interfacial binding between the particles and PDMS has been established to play a very important role in stress transfer by the simulations and supporting experimental evidence.

An alternative, cost-effective and relatively simple solution to detect damages to paint coatings utilizing the long persistent phosphorescence of strontium aluminate based phosphor has been suggested. A layer of phosphor particulates in clear coat underneath the top coat has been utilized as a detection layer. Intensity of phosphorescence from underneath the damages has been found to depend linearly on size of damage enabling determination the size and extent of damage from intensity measurements.

105

Contributions to the field

A major contribution of this thesis to the field is the understanding of dynamic behavior of EML of ZnS:Mn,Cu in PDMS derived from the temporal response at high time varying strain rates. Most published works on EML of doped ZnS have studied EML from thin solid films, grown by vapor deposition techniques, during impact loading within the elastic range. Comparatively, the EML from ZnS based phosphor micro-particulates has been minimally studied and understood. Therefore, our work improves upon the knowledge from previous studies of EML that have been limited either to capturing temporal responses at very low and constant strain rates in the case of epoxy [14,58-60] or to measuring only the luminance in the case of PDMS [23].

New knowledge gained from this thesis are listed below:

1. The temporal response of EML showed that an increase in stress/strain causes

higher EML intensity than a decrease by the same amount and at the same rate.

2. The inverse dependence of the number of filled available electron traps on the

strain state of the crystal is suggested as the reason for this. In other words, at

lower strains, the crystal has more electron traps filled and available than at

higher strains and vice versa.

3. The self-recovery of EML emission of ZnS:Mn,Cu is then explained by this

inverse dependence. To our knowledge, there has been no previous work

identifying the dependence of number of available electron traps on the strain

state of the crystal as the cause for self-recovery of EML. More experiments are

106

required to conclusively assert this inverse dependence, and hence at this point

of time, it remains a hypothesis.

4. Our FEA simulations that have predicted the range of stresses transferred to the

particle at different strains, and have help gain understanding of the importance

of interfacial binding between particles and matrix have been the first of such

efforts.

5. The dependence of EML intensity on the stress experienced by microparticles

of ZnS:Cu impregnated with PDMS is also reported for the first time to our

knowledge.

These simulations can be extending to predict stress transfer in paint binders such as polyurethane and lacquer which will help in the design of ML paint.

107

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