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Mechanical, spectral, and luminescence properties of ZnS:Mn doped PDMS
Journal of Luminescence 170 (2016) 194–199
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Full Length Article
Mechanical, spectral, and luminescence properties of ZnS:Mn doped PDMS Ross S. Fontenot a,1, Stephen W. Allison b, Kyle J Lynch c, William A. Hollerman a, Firouzeh Sabri c,n a
University of Louisiana at Lafayette, Department of Physics, PO Box 44210, Lafayette, LA 70504, USA Emerging Measurements, Collierville, TN 38017, USA c Department of Physics, University of Memphis, Memphis, TN 38152, USA b
art ic l e i nf o
a b s t r a c t
Article history: Received 9 May 2015 Received in revised form 17 October 2015 Accepted 20 October 2015 Available online 27 October 2015
Zinc sulfide doped with manganese (ZnS:Mn) is one of the brightest triboluminescent materials known and has been studied for a variety of applications. The powder form of this material restricts its safe handling and utilization, which limits the range of applications that can take advantage of its unique properties. In this study, the tribo- and photo-luminescent properties as well as the mechanical properties of ZnS:Mn encapsulated in an inert and optically transparent elastomer – Sylgard 184, have been investigated and fully characterized. ZnS:Mn particles of 8.5 mm diameter were incorporated into the Sylgard 184 polymer matrix prior to the curing stage with increasing amounts targeting a final (mass) concentration of 5%, 15%, and 50%. Additionally, the effect of the ZnS:Mn particles on the overall surface properties of the encapsulating elastomer was investigated and reported here. It was observed that the triboluminescent emission from impact scales with phosphor concentration and was not affected by the encapsulating medium. & 2015 Elsevier B.V. All rights reserved.
Keywords: Triboluminescence Elastomer Polymer Mechanoluminescence Fractoluminescence Impact sensor
1. Introduction Sulfide-based luminescent materials have been known for centuries and zinc sulfide doped with manganese (ZnS:Mn) is one of the oldest and most researched phosphors [1]. While no complete theory exists, today, ZnS:Mn is studied for its use in electroluminescent displays [2,3], phosphor thermometry [4,5,6], triboluminescent applications [4,7–23], biosensing [24], and biological analytics [25]. With its yellow–orange emission peak centered at 585 nm and, a full width at half maximum (FWHM) of 65 nm, ZnS:Mn is one of the brightest triboluminescent materials in the world [9,15]. In 1888, Wiedemann and Schmidt defined triboluminescence (TL) as the emission of light produced when the material undergoes fracture and has since been the object of interest and fascination for many scientists. It has been estimated that 30% of organic crystals and 50% of inorganic crystals are indeed triboluminescent [14]. It is also important to note that luminescence decay time is a sensitive indicator of physical and/or n
Corresponding author. E-mail addresses: [email protected] (S.W. Allison), [email protected] (K. Lynch), [email protected] (F. Sabri). 1 Current address: ASEE Postdoctoral Fellow, Naval Surface Warfare Center, Carderock Division, Code 6301, West Bethesda, MD 20817, USA. http://dx.doi.org/10.1016/j.jlumin.2015.10.047 0022-2313/& 2015 Elsevier B.V. All rights reserved.
chemical change and at times, indicates changes on the atomic level which are not discernible from intensity. Moreover, spectral measurements may be sensed by a change in luminescence decay characteristics [26,27]. More recently, extensive research has been completed using TL as the active element for impact and damage sensors [8,17,18,28– 30]. For these powders to be of practical use for damage detection and shock sensing, they would need to be bright and embedded into an inert and optically accessible structure or composite. Effective incorporation of powders into another matrix is a challenging area and needs to be further developed and more thoroughly investigated. Elastomers, which are from the family of polydimethylsiloxane (PDMS), are flexible, inert, and in some cases optically transparent polymers with tunable chemical, physical, and electrical properties that can be easily doped with specific additives [31–36]. The ability to “dope” these polymers creates a unique and versatile “carrier” for active materials that cannot be handled on their own, such as the TL particles (powders) mentioned earlier. The physical and chemical properties of the combined material can be tuned to deliver the sensitivity of choice by controlling the dopant concentration, among other things. The effect of the triboluminescent particles however, on the physical and chemical properties of the encapsulating polymer need to be thoroughly investigated and
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characterized. In this study, photoluminescent and triboluminescent properties of elastomer-encapsulated ZnS:Mn materials were investigated for the first time. We present the first set of photoluminescence (PL) and TL from Sylagrad-184-encapulsated ZnS: Mn for concentrations of 5, 15, and 50%. The effect of the ZnS:Mn particles on the mechanical, spectral, and surface properties of the elastomer have also been investigated and are presented here.
2. Materials and methods 2.1. Preparation of ZnS:Mn-doped PDMS samples 8.5 mm ZnS:Mn powder (Phosphor Technology Ltd, Lot 19252) was combined with the prepolymer and crosslinker of Sylgard 184 (Dow Corning, Midland, MI) in different mass ratios (5, 15, 50%) prior to the curing stage of the two-part elastomer containing and mixed at a recommended ratio of 10:1 (pre-polymer to cross linker). The slurry was weighed on an Ohaus Pioneer analytical balance and the components (ZnS:Mn powder, prepolymer, and crosslinker) were mixed thoroughly for 3 min and outgassed in a Precision Scientific Model 19 vacuum oven at room temperature. After complete outgassing of the mixture, it was poured into a polished aluminum mold, outgassed one more time, and finally cured at 90 °C for 1 h as recommended by the manufacturers. The samples were then removed from their molds, cut into 1 cm 1 cm squares. ZnS:Mn-doped Sylgard 184 samples with mass concentrations of 5, 15, and 50% were prepared in sheets and cut to specific final geometries (as appropriate for each characterization method) after removal from the mold. 2.2. Photoluminescence: detection, decay, and imaging (5, 15, 50%) To evaluate and characterize the photoluminescent detection and decay of 5, 15, and 50% doped samples, a 405 nm laser diode was directed at each ZnS:Mn/PDMS sample, one at a time, to excite luminescence. An optical fiber situated over the sample collected
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the luminescence and conveyed the light to a photomultiplier detector with an intervening 590 nm bandpass filter. The signal was displayed and digitized by a Tektronix 2012B oscilloscope. The sample was just under the surface of gently heated water in a crock pot. A type K thermocouple was inserted into a slit in the PDMS to measure temperature and confirm the thermal stability. This arrangement is shown in Fig 1. The oscilloscope was cons nected to a laptop computer and analyzed using SigmaPlot software. For imaging measurements, the fiberoptic receiving probe, photomutiplier, and oscilloscope were replaced with a Mighty Scope 5.0M digital microscope by Aven. Video was converted into a series of jpeg image files for subsequent analysis with the laptop. 2.3. Detection of triboluminescence (5, 15, 50%) Using a custom built drop tower (Fig. 2) designed and fabricated by the authors and described in Reference [36], the doped PDMS samples were tested for their triboluminescent properties. The measurement began by placing a 1 cm2 ZnS:Mn PDMS sample on the Plexiglass plate. The sample was positioned each time near the center of the tube. A 130 g steel ball was positioned at a pull pin at a set distance of 42 in. above the material. The pin was pulled and the ball fell and impacted with the sample material producing TL. After each test, the drop tube was removed, the ball cleaned, and a new sample placed in the center of the target area [37]. To determine the triboluminescent yield for a given sample, a United Detector photodiode was positioned under the plexiglass plate 2.25 cm below the sample. A Melles Griot large dynamic range linear amplifier set to a gain of 2 mA was used to increase the signal amplitude. A Tektronix 2024B oscilloscope recorded the signal in single sequence mode with a 250 ms measurement time. s The signals were analyzed using custom LabVIEW program that integrated the area under the curve and calculated the decay time for each particular emission [37,38].
Fig. 1. Photoluminescence decay measurement setup: Schematic diagram of the experimental setup used for measuring the luminescence decay of 5, 15, and 50% ZnS:Mn doped Sylgard 184.
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Fig. 2. Triboluminescence measurement setup: Schematic diagram of the custom built drop tower used to measure the triboluminescent light yield [36] of the 5, 15, and 50% ZnS:Mn doped Sylgard 184 samples.
this concentration yields a seven-fold increase in the total amount of TL emitted.In order to determine how the triboluminescent yield changes with impact energy, the ball drop location was varied between 3 and 42 in. (7.6 and 106.7 cm), i.e., impact energies of 0.1– 1.4 J. The 50% ZnS:Mn-doped PDMS sample was used because this sample emitted the most TL, which would allow measuring the TL emitted light for lower impact energies. The results of this test are shown in Fig. 4. The triboluminescent light yield was normalized to the 6 in. (0.19 J) value, since this is a relative measurement. For comparison purposes, the triboluminescent light yield for 8.5 mm ZnS:Mn powder (1 g of material per impact energy) was added to the graph. Notice that the energy at impact has a significant effect on the amount of TL emitted. More importantly, the effect of the impact energy on ZnS:Mn-PDMS and ZnS:Mn powder is identical. Finally, to further ensure that the light emission from the ZnS: Mn-doped PDMS is originating from the ZnS:Mn, we measured the decay time from each drop. The average decay time for the 5, 15, and 50% ZnS:Mn-doped PDMS was 348.4 718.9, 335.4 7 20.9, and 328.8722.1 ms, respectively. These values are consistent with the decay times for ZnS:Mn previously observed [7,9,39,40]. It should be noted that this slight reduction in decay time for increasing ZnS:Mn is likely due to the increasing light emission. The 5% ZnS: Mn concentration was very dim. As such, the triboluminescent signal was noisy, which slightly lengthens the decay time.
2.4. Surface charge/potential measurements 3.2. Photoluminescence spectra, decay time, and imaging tests The samples underwent non-contact surface potential measurement using the TREK Model 325 Electrostatic Voltmeter attached to an Ardel Kinematic motorized linear stage. Each sample was divided into nine equal sections based on size. The surface potential of each section was measured and the results were manually recorded. 2.5. Contact angle measurements Each sample was placed in a KSV instruments CAM 101 goniometer. A 10 mL drop of water was then manually deposited on each substrate. An image of each sample was then taken using an Imaging Source DMK21F04 camera. The image was then analyzed using KSV instruments' CAM 100 software, which automatically calculated three contact angles: left, right, and mean. The contact angle of each sample was measured three times, at three different locations, for consistency. 2.6. Stress–strain behavior of ZnS:Mn doped PDMS In order to understand the effect of the dopant concentration on the over-all mechanical strength of the polymer, the tensile behavior of the compound behavior was measured at room temperature. The neat and ZnS:Mn-doped PDMS samples were cut into a dog-bone shape and held using Mark-10 G1013 Parallel Jaw Grip and subjected to strain at a rate of 45 mm/min using the Mark-10 model ESM301 test stand with the Mark-10 BG20 force gauge. The data was recorded using the Mark-10 MESURgauge software as force over travel. Measurements were repeated four times for each sample type.
The measured photoluminescent emission spectra for the ZnS:Mndoped PDMS is shown in Fig. 5. Each spectrum was maximized by varying the integration window of the spectrometer. Notice the bright emission from the ZnS:Mn-doped PDMS sample in the inset, which was taken with an iPhone 5 in HDR mode. This yellow emission had a center wavelength of 587 nm, which originates from the Mn2þ transition 6D9/2-6S5/2. The emission from the doped sample matches that from the powdered ZnS:Mn. Next, fluorescence decay measurements were made using a laser diode for excitation. The purpose was to investigate if and how the results might differ from being in the PDMS host. A comparison was made with a coating mixture of ZnS:Mn phosphor powder and a high temperature clear paint, VHT SP115 (Sperex), on an aluminum foil substrate. The 405 nm laser had a pulse duration of 20 ms at 10 Hz. The laser illuminated the sample and the resulting fluorescence was captured by an optical fiber bundle that was conveyed to a photomultiplier detector with an intervening 590 nm band-pass filter. The detector signal was displayed on a digitizing oscilloscope with an USB connection to a laptop computer for data analysis. The signals were fit
3. Results 3.1. Drop tower tests Using the drop tower and custom built LabVIEW VI tools described above, the TL produced from each trial was analyzed. The error in the triboluminescent yield was estimated using the standard deviation of the mean of the five drops. Fig. 3 shows a bar graph comparison of the three ZnS:Mn concentrations. It is evident that the 50% ZnS:Mn doping provides the best (i.e. has the highest light yield) results. In fact,
Fig. 3. Triboluminescent light yields for encapsulated ZnS:Mn: Comparison of the triboluminescent light yields for increasing ZnS:Mn concentrations in elastomer. The error is estimated using the standard deviation of the mean from the five separate drops for each sample type.
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Fig. 4. Dependency of triboluminescent light yield on impact energy: The effect of impact energy on the dependency triboluminescent light yield for a 50% doped sample, as compared to the ZnS:Mn powder only. The error in the triboluminescent yield is estimated using the standard deviation of the mean, while the error in the impact energy was estimated to be 5%.
Fig. 5. Photoluminescent emission spectra for ZnS:Mn-doped PDMS: Photoluminescent emission spectra for the ZnS:Mn-doped PDMS originating from the Mn2 þ transition 6D9/2-6S5/2. The PDMS samples were irradiated using a UV lamp (365 nm). The inset shows the 50% ZnS:Mn-doped PDMS while under UV irradiation.
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where I0 is the initial luminescence amplitude when the light source is terminated. Time is t and decay time is τ. In practice there is usually a background, B, which may originate from several sources such as room lights, detector bias, and oscilloscope DC offset. When the signal is strong and B is negligible, a semilog plot I vs. time (t) will yield a straight line. This is seen in Fig. 6 where also, at long times, the trace curves downward due to a small negative baseline, B. For the analysis, the first 50 ms of the decay was ignored to ensure that the excitation light which could alter the signal, had ceased. Using this approach, the decay time of the luminescence from 15% ZnS:Mn doped PDMS was calculated to be 474 ms and therefore essentially the same as from the ZnS:Mn doped clear paint coating which is 472 ms according to the s analysis. This is a strong indicator that the PDMS does not significantly affect the ZnS:Mn chemistry. Thus, results for temperature dependence and other characteristics of the luminescence derived from pure powder and other coating mixtures are to a first order a guide to what to expect from ZnS:Mn encapsulated in PDMS. Next, luminescence from the same doped sample was recorded for two different temperatures. The overall brightness of the emission decreased gradually, by about 20%, over the limited temperature range of 0–60 °C. This is shown in Fig. 7. An analysis determined there is no temperature dependence of the decay time in this limited temperature range for this transition of this particular phosphor material. This is illustrated by the semilog plots of the two signals in the inset of Fig. 8 where it is seen that the slopes of the curves are the same. Since the results with the 15% doped sample yielded essentially no difference with the powder sample, it is likely that no chemical change of the phosphor takes place. Therefore, it is expected that the same likely holds true for the 5% and 50% samples. Finally, images of laser excited luminescence were recorded for three different concentrations. The setup was similar to Fig. 1 except that the optical fiber was replaced by a 10–200 digital microscope (Aven Mighty Scope 5.0). It was connected to a laptop having image acquisition software. For this experimental setup, the laser light was incident at an angle of about 15°. Digital images in the form of avi files were acquired for the three dopant levels of 5, 15, and 50%. Individual images were selected using the Image J program which also provided the means to determine intensity profile. The intensity of a horizontal scan through each circular image is plotted in Fig. 8. The corresponding images are shown as well. As would be expected, the ZnS: Mn particles embedded in the PDMS act to diffuse and spread the light laterally as it penetrates the samples. The higher the dopant concentration, the greater the peak signal. The peak of the 50% sample is shifted relative to the other two. It may be that the received signal includes significant emission from within the sample. Next, the resulting images were normalized and are depicted in Fig. 10. It is seen from both Figs. 8 and 9 that for a low level of doping, 5%, the laser light
Fig. 6. Comparison of time dependence of ZnS doped PDMS and ZnS/Sperex mixture: decay time of the luminescence from 15% ZnS:Mn doped PDMS essentially the same as the ZnS:Mn doped clear paint coating at 20 °C.
to the following equation using SigmaPlot 11 dynamic fit wizard: I ¼ I 0 e t=τ þB
Fig. 7. Comparison of 15% ZnS:Mn doped PDMS signals at 0 and 60 °C. The semilog inset reveals near identical decay times.
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emission is also brighter versus concentration due to greater number of phosphor particles per unit volume. This is the case when the luminescence is viewed from the same side as the illumination. For the higher density of particles, more laser energy is absorbed per unit volume. Laser energy scattered to the side is more rapidly attenuated than for lower concentrations. Additional testing involved viewing the luminescing sample from the opposite side that was illuminated. The intensity of the lowest concentration sample is brightest since more laser energy penetrates to this side. For the highest concentration, the intensity is weakest. Also for this case it remains that the image of the luminescence spot size is largest for the lowest concentration and smallest for the highest concentration. 3.3. Surface analysis: Kelvin probe and contact angle measurements Fig. 8. Horizontal intensity profile for 5, 15, and 50% doped samples: The higher the dopant concentration, the greater the peak signal.
The surface potential values for the doped and neat (0%) PDMS substrates are very similar suggesting that the ZnS:Mn particles are in fact completely encapsulated by the PDMS, even for the 50% samples. Results are summarized in Table 1. The contact angle measurements confirm full encapsulation of the triboluminescent particles and the surface continues to remain hydrophobic. Table 1 also summarizes the results of multiple contact angle measurements for each sample type and in each case shows negligible dependence on doping concentration levels tested here. 3.4. Mechanical behavior
Fig. 9. Normalized scans show more scattering for lower phosphor concentrations. Low concentration shows most scattering, highest concentration the least.
4 3.5
Stress (N/mm2)
3 2.5
0% 5%
2
15%
1.5
50%
1 0.5 0
0
0.2
0.4
0.6 0.8 Strain (ΔL/Lo)
1
1.2
1.4
Fig. 10. Affect of doping concentration on stress–strain behavior: Comparison of stress–strain profile of doped samples with neat polymer (blue trace) show a gradual increase in material stiffness. At 5% and 15% doping level the material still shows elastomeric behavior. At 50% however material is no longer elastomeric and becomes stiff.
is scattered so as to produce the largest diameter spot size. The laser spot size was observed using a white business card as the target and scaled to the other plots in Fig. 9. The laser diameter is less than the luminescence diameters. Evidently, as ZnS:Mn density increases in the PDMS samples, the scattered light does not have to travel as far for another phosphor particle scattering or absorption event. The
The effect of doping concentration on the mechanical properties of the polymer was explored and is shown in Fig. 10. The room temperature stress (axial)-strain behavior of the material was measured and Young's modulus values are given in Table 1, averaged over three independent runs. The highest concentration of phosphor particles (50%) leads to a compound material with an increased stiffness compared to lower doping levels (5, 15) where a significant degree of elasticity can still be observed when compared to the neat polymer (blue trace). The classic “S” curve seen in elastomers gradually changes from 0 to 15% with a transition to ductile behavior at 50%. The overall rupture load (not shown in the graphs) does not appear to be significantly dependent on the concentration amount for the percentages tested here. Stress–strain behavior for higher concentrations (450%) remains to be investigated but is expected to saturate and, show properties of a brittle material. The effect of the doping concentration on Young's moduli of data from Fig. 10 is given in Table 1 where a strong non-linear dependency of Young's modulus on doping concentration can be seen, ranging 1–5 MPa. The stress–strain behavior and Young's modulus values are expected to have a strong dependence on the particle cluster sizes also. Given that the elastomer is strongly hydrophobic, and, the ZnS:Mn particles are statically charged, clustering does occur during the doping step. In this study however, it is assumed that the distribution of ZnS:Mn in the polymeric mix is uniform with no batch to batch variations. Future studies will Table 1 Affect of doping concentration on: Young's modulus, surface potential, and contact angle readings, of ZnS:Mn doped PDMS samples. Sample type
Contact angle (degrees)
Surface potential (V)
Young's modulus (avg. values)
Neat (0%) Doped 5% ZnS:Mn Doped 15% ZnS:Mn Doped 50% ZnS:Mn
113.5 7 2.0 115.0 7 0.8
56.007 0.11 55.41 70.04
1.05 1.38
113.5 7 1.0
55.45 7 0.05
2.04
114.3 7 0.4
55.60 7 0.00
4.24
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investigate the ZnS:Mn distribution profile and behavior in the polymeric mix and size of clusters formed for each concentration level. [11] [12]
4. Data summary and conclusions The effect of polymer encapsulation on the luminescent properties of ZnS:Mn, one of the brightest triboluminescent materials in the world was thoroughly investigated. For these particles to be of practical sensor use, they would need to be embedded into an inert and optically accessible structure or composite which was the focus of the work presented here. Overall, the luminescent behavior (photo and tribo) of ZnS:Mn powder has not been discernibly affected by incorporating the particles into a clear elastomeric medium. This is indicated by observing the equivalence of the decay time of photo-induced luminescence from doped PDMS samples and the phosphor as mixed into an inert hightemperature-capable paint. The triboluminescent emission from impact scales with phosphor concentration. At low energy impact up to about 0.2 J, the TL is equivalent to the neat powder. At higher impact energies from about 0.6 to about 1 J, this emission is about twice that of the powder. Above that, the emission remains approximately constant. The mechanical and spectral properties of the elastomer were however affected significantly and, strongly dependent on the doping concentration. The work presented here demonstrates the feasibility of ZnS:Mn-based flexible sensors. The results serve as the basis for envisioning impact sensors. By adjusting the PDMS thickness and dopant concentration the mechanical and spectral properties may be controlled as desirable for a given application.
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[26]
Acknowledgments Firouzeh Sabri would like to thank the Memphis Research Consortium Innovation Grants for partial financial support. The authors would also like to thank Victor Gardner White Station high school (MemphisCRESH 2014) for assistance with sample preparation and data acquisition efforts.
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Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Full Length Article
Mechanical, spectral, and luminescence properties of ZnS:Mn doped PDMS Ross S. Fontenot a,1, Stephen W. Allison b, Kyle J Lynch c, William A. Hollerman a, Firouzeh Sabri c,n a
University of Louisiana at Lafayette, Department of Physics, PO Box 44210, Lafayette, LA 70504, USA Emerging Measurements, Collierville, TN 38017, USA c Department of Physics, University of Memphis, Memphis, TN 38152, USA b
art ic l e i nf o
a b s t r a c t
Article history: Received 9 May 2015 Received in revised form 17 October 2015 Accepted 20 October 2015 Available online 27 October 2015
Zinc sulfide doped with manganese (ZnS:Mn) is one of the brightest triboluminescent materials known and has been studied for a variety of applications. The powder form of this material restricts its safe handling and utilization, which limits the range of applications that can take advantage of its unique properties. In this study, the tribo- and photo-luminescent properties as well as the mechanical properties of ZnS:Mn encapsulated in an inert and optically transparent elastomer – Sylgard 184, have been investigated and fully characterized. ZnS:Mn particles of 8.5 mm diameter were incorporated into the Sylgard 184 polymer matrix prior to the curing stage with increasing amounts targeting a final (mass) concentration of 5%, 15%, and 50%. Additionally, the effect of the ZnS:Mn particles on the overall surface properties of the encapsulating elastomer was investigated and reported here. It was observed that the triboluminescent emission from impact scales with phosphor concentration and was not affected by the encapsulating medium. & 2015 Elsevier B.V. All rights reserved.
Keywords: Triboluminescence Elastomer Polymer Mechanoluminescence Fractoluminescence Impact sensor
1. Introduction Sulfide-based luminescent materials have been known for centuries and zinc sulfide doped with manganese (ZnS:Mn) is one of the oldest and most researched phosphors [1]. While no complete theory exists, today, ZnS:Mn is studied for its use in electroluminescent displays [2,3], phosphor thermometry [4,5,6], triboluminescent applications [4,7–23], biosensing [24], and biological analytics [25]. With its yellow–orange emission peak centered at 585 nm and, a full width at half maximum (FWHM) of 65 nm, ZnS:Mn is one of the brightest triboluminescent materials in the world [9,15]. In 1888, Wiedemann and Schmidt defined triboluminescence (TL) as the emission of light produced when the material undergoes fracture and has since been the object of interest and fascination for many scientists. It has been estimated that 30% of organic crystals and 50% of inorganic crystals are indeed triboluminescent [14]. It is also important to note that luminescence decay time is a sensitive indicator of physical and/or n
Corresponding author. E-mail addresses: [email protected] (S.W. Allison), [email protected] (K. Lynch), [email protected] (F. Sabri). 1 Current address: ASEE Postdoctoral Fellow, Naval Surface Warfare Center, Carderock Division, Code 6301, West Bethesda, MD 20817, USA. http://dx.doi.org/10.1016/j.jlumin.2015.10.047 0022-2313/& 2015 Elsevier B.V. All rights reserved.
chemical change and at times, indicates changes on the atomic level which are not discernible from intensity. Moreover, spectral measurements may be sensed by a change in luminescence decay characteristics [26,27]. More recently, extensive research has been completed using TL as the active element for impact and damage sensors [8,17,18,28– 30]. For these powders to be of practical use for damage detection and shock sensing, they would need to be bright and embedded into an inert and optically accessible structure or composite. Effective incorporation of powders into another matrix is a challenging area and needs to be further developed and more thoroughly investigated. Elastomers, which are from the family of polydimethylsiloxane (PDMS), are flexible, inert, and in some cases optically transparent polymers with tunable chemical, physical, and electrical properties that can be easily doped with specific additives [31–36]. The ability to “dope” these polymers creates a unique and versatile “carrier” for active materials that cannot be handled on their own, such as the TL particles (powders) mentioned earlier. The physical and chemical properties of the combined material can be tuned to deliver the sensitivity of choice by controlling the dopant concentration, among other things. The effect of the triboluminescent particles however, on the physical and chemical properties of the encapsulating polymer need to be thoroughly investigated and
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characterized. In this study, photoluminescent and triboluminescent properties of elastomer-encapsulated ZnS:Mn materials were investigated for the first time. We present the first set of photoluminescence (PL) and TL from Sylagrad-184-encapulsated ZnS: Mn for concentrations of 5, 15, and 50%. The effect of the ZnS:Mn particles on the mechanical, spectral, and surface properties of the elastomer have also been investigated and are presented here.
2. Materials and methods 2.1. Preparation of ZnS:Mn-doped PDMS samples 8.5 mm ZnS:Mn powder (Phosphor Technology Ltd, Lot 19252) was combined with the prepolymer and crosslinker of Sylgard 184 (Dow Corning, Midland, MI) in different mass ratios (5, 15, 50%) prior to the curing stage of the two-part elastomer containing and mixed at a recommended ratio of 10:1 (pre-polymer to cross linker). The slurry was weighed on an Ohaus Pioneer analytical balance and the components (ZnS:Mn powder, prepolymer, and crosslinker) were mixed thoroughly for 3 min and outgassed in a Precision Scientific Model 19 vacuum oven at room temperature. After complete outgassing of the mixture, it was poured into a polished aluminum mold, outgassed one more time, and finally cured at 90 °C for 1 h as recommended by the manufacturers. The samples were then removed from their molds, cut into 1 cm 1 cm squares. ZnS:Mn-doped Sylgard 184 samples with mass concentrations of 5, 15, and 50% were prepared in sheets and cut to specific final geometries (as appropriate for each characterization method) after removal from the mold. 2.2. Photoluminescence: detection, decay, and imaging (5, 15, 50%) To evaluate and characterize the photoluminescent detection and decay of 5, 15, and 50% doped samples, a 405 nm laser diode was directed at each ZnS:Mn/PDMS sample, one at a time, to excite luminescence. An optical fiber situated over the sample collected
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the luminescence and conveyed the light to a photomultiplier detector with an intervening 590 nm bandpass filter. The signal was displayed and digitized by a Tektronix 2012B oscilloscope. The sample was just under the surface of gently heated water in a crock pot. A type K thermocouple was inserted into a slit in the PDMS to measure temperature and confirm the thermal stability. This arrangement is shown in Fig 1. The oscilloscope was cons nected to a laptop computer and analyzed using SigmaPlot software. For imaging measurements, the fiberoptic receiving probe, photomutiplier, and oscilloscope were replaced with a Mighty Scope 5.0M digital microscope by Aven. Video was converted into a series of jpeg image files for subsequent analysis with the laptop. 2.3. Detection of triboluminescence (5, 15, 50%) Using a custom built drop tower (Fig. 2) designed and fabricated by the authors and described in Reference [36], the doped PDMS samples were tested for their triboluminescent properties. The measurement began by placing a 1 cm2 ZnS:Mn PDMS sample on the Plexiglass plate. The sample was positioned each time near the center of the tube. A 130 g steel ball was positioned at a pull pin at a set distance of 42 in. above the material. The pin was pulled and the ball fell and impacted with the sample material producing TL. After each test, the drop tube was removed, the ball cleaned, and a new sample placed in the center of the target area [37]. To determine the triboluminescent yield for a given sample, a United Detector photodiode was positioned under the plexiglass plate 2.25 cm below the sample. A Melles Griot large dynamic range linear amplifier set to a gain of 2 mA was used to increase the signal amplitude. A Tektronix 2024B oscilloscope recorded the signal in single sequence mode with a 250 ms measurement time. s The signals were analyzed using custom LabVIEW program that integrated the area under the curve and calculated the decay time for each particular emission [37,38].
Fig. 1. Photoluminescence decay measurement setup: Schematic diagram of the experimental setup used for measuring the luminescence decay of 5, 15, and 50% ZnS:Mn doped Sylgard 184.
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Fig. 2. Triboluminescence measurement setup: Schematic diagram of the custom built drop tower used to measure the triboluminescent light yield [36] of the 5, 15, and 50% ZnS:Mn doped Sylgard 184 samples.
this concentration yields a seven-fold increase in the total amount of TL emitted.In order to determine how the triboluminescent yield changes with impact energy, the ball drop location was varied between 3 and 42 in. (7.6 and 106.7 cm), i.e., impact energies of 0.1– 1.4 J. The 50% ZnS:Mn-doped PDMS sample was used because this sample emitted the most TL, which would allow measuring the TL emitted light for lower impact energies. The results of this test are shown in Fig. 4. The triboluminescent light yield was normalized to the 6 in. (0.19 J) value, since this is a relative measurement. For comparison purposes, the triboluminescent light yield for 8.5 mm ZnS:Mn powder (1 g of material per impact energy) was added to the graph. Notice that the energy at impact has a significant effect on the amount of TL emitted. More importantly, the effect of the impact energy on ZnS:Mn-PDMS and ZnS:Mn powder is identical. Finally, to further ensure that the light emission from the ZnS: Mn-doped PDMS is originating from the ZnS:Mn, we measured the decay time from each drop. The average decay time for the 5, 15, and 50% ZnS:Mn-doped PDMS was 348.4 718.9, 335.4 7 20.9, and 328.8722.1 ms, respectively. These values are consistent with the decay times for ZnS:Mn previously observed [7,9,39,40]. It should be noted that this slight reduction in decay time for increasing ZnS:Mn is likely due to the increasing light emission. The 5% ZnS: Mn concentration was very dim. As such, the triboluminescent signal was noisy, which slightly lengthens the decay time.
2.4. Surface charge/potential measurements 3.2. Photoluminescence spectra, decay time, and imaging tests The samples underwent non-contact surface potential measurement using the TREK Model 325 Electrostatic Voltmeter attached to an Ardel Kinematic motorized linear stage. Each sample was divided into nine equal sections based on size. The surface potential of each section was measured and the results were manually recorded. 2.5. Contact angle measurements Each sample was placed in a KSV instruments CAM 101 goniometer. A 10 mL drop of water was then manually deposited on each substrate. An image of each sample was then taken using an Imaging Source DMK21F04 camera. The image was then analyzed using KSV instruments' CAM 100 software, which automatically calculated three contact angles: left, right, and mean. The contact angle of each sample was measured three times, at three different locations, for consistency. 2.6. Stress–strain behavior of ZnS:Mn doped PDMS In order to understand the effect of the dopant concentration on the over-all mechanical strength of the polymer, the tensile behavior of the compound behavior was measured at room temperature. The neat and ZnS:Mn-doped PDMS samples were cut into a dog-bone shape and held using Mark-10 G1013 Parallel Jaw Grip and subjected to strain at a rate of 45 mm/min using the Mark-10 model ESM301 test stand with the Mark-10 BG20 force gauge. The data was recorded using the Mark-10 MESURgauge software as force over travel. Measurements were repeated four times for each sample type.
The measured photoluminescent emission spectra for the ZnS:Mndoped PDMS is shown in Fig. 5. Each spectrum was maximized by varying the integration window of the spectrometer. Notice the bright emission from the ZnS:Mn-doped PDMS sample in the inset, which was taken with an iPhone 5 in HDR mode. This yellow emission had a center wavelength of 587 nm, which originates from the Mn2þ transition 6D9/2-6S5/2. The emission from the doped sample matches that from the powdered ZnS:Mn. Next, fluorescence decay measurements were made using a laser diode for excitation. The purpose was to investigate if and how the results might differ from being in the PDMS host. A comparison was made with a coating mixture of ZnS:Mn phosphor powder and a high temperature clear paint, VHT SP115 (Sperex), on an aluminum foil substrate. The 405 nm laser had a pulse duration of 20 ms at 10 Hz. The laser illuminated the sample and the resulting fluorescence was captured by an optical fiber bundle that was conveyed to a photomultiplier detector with an intervening 590 nm band-pass filter. The detector signal was displayed on a digitizing oscilloscope with an USB connection to a laptop computer for data analysis. The signals were fit
3. Results 3.1. Drop tower tests Using the drop tower and custom built LabVIEW VI tools described above, the TL produced from each trial was analyzed. The error in the triboluminescent yield was estimated using the standard deviation of the mean of the five drops. Fig. 3 shows a bar graph comparison of the three ZnS:Mn concentrations. It is evident that the 50% ZnS:Mn doping provides the best (i.e. has the highest light yield) results. In fact,
Fig. 3. Triboluminescent light yields for encapsulated ZnS:Mn: Comparison of the triboluminescent light yields for increasing ZnS:Mn concentrations in elastomer. The error is estimated using the standard deviation of the mean from the five separate drops for each sample type.
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Fig. 4. Dependency of triboluminescent light yield on impact energy: The effect of impact energy on the dependency triboluminescent light yield for a 50% doped sample, as compared to the ZnS:Mn powder only. The error in the triboluminescent yield is estimated using the standard deviation of the mean, while the error in the impact energy was estimated to be 5%.
Fig. 5. Photoluminescent emission spectra for ZnS:Mn-doped PDMS: Photoluminescent emission spectra for the ZnS:Mn-doped PDMS originating from the Mn2 þ transition 6D9/2-6S5/2. The PDMS samples were irradiated using a UV lamp (365 nm). The inset shows the 50% ZnS:Mn-doped PDMS while under UV irradiation.
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where I0 is the initial luminescence amplitude when the light source is terminated. Time is t and decay time is τ. In practice there is usually a background, B, which may originate from several sources such as room lights, detector bias, and oscilloscope DC offset. When the signal is strong and B is negligible, a semilog plot I vs. time (t) will yield a straight line. This is seen in Fig. 6 where also, at long times, the trace curves downward due to a small negative baseline, B. For the analysis, the first 50 ms of the decay was ignored to ensure that the excitation light which could alter the signal, had ceased. Using this approach, the decay time of the luminescence from 15% ZnS:Mn doped PDMS was calculated to be 474 ms and therefore essentially the same as from the ZnS:Mn doped clear paint coating which is 472 ms according to the s analysis. This is a strong indicator that the PDMS does not significantly affect the ZnS:Mn chemistry. Thus, results for temperature dependence and other characteristics of the luminescence derived from pure powder and other coating mixtures are to a first order a guide to what to expect from ZnS:Mn encapsulated in PDMS. Next, luminescence from the same doped sample was recorded for two different temperatures. The overall brightness of the emission decreased gradually, by about 20%, over the limited temperature range of 0–60 °C. This is shown in Fig. 7. An analysis determined there is no temperature dependence of the decay time in this limited temperature range for this transition of this particular phosphor material. This is illustrated by the semilog plots of the two signals in the inset of Fig. 8 where it is seen that the slopes of the curves are the same. Since the results with the 15% doped sample yielded essentially no difference with the powder sample, it is likely that no chemical change of the phosphor takes place. Therefore, it is expected that the same likely holds true for the 5% and 50% samples. Finally, images of laser excited luminescence were recorded for three different concentrations. The setup was similar to Fig. 1 except that the optical fiber was replaced by a 10–200 digital microscope (Aven Mighty Scope 5.0). It was connected to a laptop having image acquisition software. For this experimental setup, the laser light was incident at an angle of about 15°. Digital images in the form of avi files were acquired for the three dopant levels of 5, 15, and 50%. Individual images were selected using the Image J program which also provided the means to determine intensity profile. The intensity of a horizontal scan through each circular image is plotted in Fig. 8. The corresponding images are shown as well. As would be expected, the ZnS: Mn particles embedded in the PDMS act to diffuse and spread the light laterally as it penetrates the samples. The higher the dopant concentration, the greater the peak signal. The peak of the 50% sample is shifted relative to the other two. It may be that the received signal includes significant emission from within the sample. Next, the resulting images were normalized and are depicted in Fig. 10. It is seen from both Figs. 8 and 9 that for a low level of doping, 5%, the laser light
Fig. 6. Comparison of time dependence of ZnS doped PDMS and ZnS/Sperex mixture: decay time of the luminescence from 15% ZnS:Mn doped PDMS essentially the same as the ZnS:Mn doped clear paint coating at 20 °C.
to the following equation using SigmaPlot 11 dynamic fit wizard: I ¼ I 0 e t=τ þB
Fig. 7. Comparison of 15% ZnS:Mn doped PDMS signals at 0 and 60 °C. The semilog inset reveals near identical decay times.
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emission is also brighter versus concentration due to greater number of phosphor particles per unit volume. This is the case when the luminescence is viewed from the same side as the illumination. For the higher density of particles, more laser energy is absorbed per unit volume. Laser energy scattered to the side is more rapidly attenuated than for lower concentrations. Additional testing involved viewing the luminescing sample from the opposite side that was illuminated. The intensity of the lowest concentration sample is brightest since more laser energy penetrates to this side. For the highest concentration, the intensity is weakest. Also for this case it remains that the image of the luminescence spot size is largest for the lowest concentration and smallest for the highest concentration. 3.3. Surface analysis: Kelvin probe and contact angle measurements Fig. 8. Horizontal intensity profile for 5, 15, and 50% doped samples: The higher the dopant concentration, the greater the peak signal.
The surface potential values for the doped and neat (0%) PDMS substrates are very similar suggesting that the ZnS:Mn particles are in fact completely encapsulated by the PDMS, even for the 50% samples. Results are summarized in Table 1. The contact angle measurements confirm full encapsulation of the triboluminescent particles and the surface continues to remain hydrophobic. Table 1 also summarizes the results of multiple contact angle measurements for each sample type and in each case shows negligible dependence on doping concentration levels tested here. 3.4. Mechanical behavior
Fig. 9. Normalized scans show more scattering for lower phosphor concentrations. Low concentration shows most scattering, highest concentration the least.
4 3.5
Stress (N/mm2)
3 2.5
0% 5%
2
15%
1.5
50%
1 0.5 0
0
0.2
0.4
0.6 0.8 Strain (ΔL/Lo)
1
1.2
1.4
Fig. 10. Affect of doping concentration on stress–strain behavior: Comparison of stress–strain profile of doped samples with neat polymer (blue trace) show a gradual increase in material stiffness. At 5% and 15% doping level the material still shows elastomeric behavior. At 50% however material is no longer elastomeric and becomes stiff.
is scattered so as to produce the largest diameter spot size. The laser spot size was observed using a white business card as the target and scaled to the other plots in Fig. 9. The laser diameter is less than the luminescence diameters. Evidently, as ZnS:Mn density increases in the PDMS samples, the scattered light does not have to travel as far for another phosphor particle scattering or absorption event. The
The effect of doping concentration on the mechanical properties of the polymer was explored and is shown in Fig. 10. The room temperature stress (axial)-strain behavior of the material was measured and Young's modulus values are given in Table 1, averaged over three independent runs. The highest concentration of phosphor particles (50%) leads to a compound material with an increased stiffness compared to lower doping levels (5, 15) where a significant degree of elasticity can still be observed when compared to the neat polymer (blue trace). The classic “S” curve seen in elastomers gradually changes from 0 to 15% with a transition to ductile behavior at 50%. The overall rupture load (not shown in the graphs) does not appear to be significantly dependent on the concentration amount for the percentages tested here. Stress–strain behavior for higher concentrations (450%) remains to be investigated but is expected to saturate and, show properties of a brittle material. The effect of the doping concentration on Young's moduli of data from Fig. 10 is given in Table 1 where a strong non-linear dependency of Young's modulus on doping concentration can be seen, ranging 1–5 MPa. The stress–strain behavior and Young's modulus values are expected to have a strong dependence on the particle cluster sizes also. Given that the elastomer is strongly hydrophobic, and, the ZnS:Mn particles are statically charged, clustering does occur during the doping step. In this study however, it is assumed that the distribution of ZnS:Mn in the polymeric mix is uniform with no batch to batch variations. Future studies will Table 1 Affect of doping concentration on: Young's modulus, surface potential, and contact angle readings, of ZnS:Mn doped PDMS samples. Sample type
Contact angle (degrees)
Surface potential (V)
Young's modulus (avg. values)
Neat (0%) Doped 5% ZnS:Mn Doped 15% ZnS:Mn Doped 50% ZnS:Mn
113.5 7 2.0 115.0 7 0.8
56.007 0.11 55.41 70.04
1.05 1.38
113.5 7 1.0
55.45 7 0.05
2.04
114.3 7 0.4
55.60 7 0.00
4.24
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investigate the ZnS:Mn distribution profile and behavior in the polymeric mix and size of clusters formed for each concentration level. [11] [12]
4. Data summary and conclusions The effect of polymer encapsulation on the luminescent properties of ZnS:Mn, one of the brightest triboluminescent materials in the world was thoroughly investigated. For these particles to be of practical sensor use, they would need to be embedded into an inert and optically accessible structure or composite which was the focus of the work presented here. Overall, the luminescent behavior (photo and tribo) of ZnS:Mn powder has not been discernibly affected by incorporating the particles into a clear elastomeric medium. This is indicated by observing the equivalence of the decay time of photo-induced luminescence from doped PDMS samples and the phosphor as mixed into an inert hightemperature-capable paint. The triboluminescent emission from impact scales with phosphor concentration. At low energy impact up to about 0.2 J, the TL is equivalent to the neat powder. At higher impact energies from about 0.6 to about 1 J, this emission is about twice that of the powder. Above that, the emission remains approximately constant. The mechanical and spectral properties of the elastomer were however affected significantly and, strongly dependent on the doping concentration. The work presented here demonstrates the feasibility of ZnS:Mn-based flexible sensors. The results serve as the basis for envisioning impact sensors. By adjusting the PDMS thickness and dopant concentration the mechanical and spectral properties may be controlled as desirable for a given application.
[13] [14]
[15] [16] [17] [18] [19]
[20]
[21] [22] [23]
[24]
[25]
[26]
Acknowledgments Firouzeh Sabri would like to thank the Memphis Research Consortium Innovation Grants for partial financial support. The authors would also like to thank Victor Gardner White Station high school (MemphisCRESH 2014) for assistance with sample preparation and data acquisition efforts.
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