Journal of Luminescence 181 (2017) 467–476

Contents lists available at ScienceDirect

Journal of Luminescence

http://d0022-23

n CorrIsrael.

journal homepage: www.elsevier.com/locate/jlumin

Full Length Article

Quantitative characterization of ZnS:Mn embedded polyurethaneoptical emission in three mechanoluminescent regimes

Nili Persits a,b,n, Abraham Aharoni b,c, Moshe Tur a

a School of Electrical Engineering, Tel Aviv University, Israelb IAI MLM, Be'er Yakov 70350,Israelc Netaco Ltd., Rehovot 76346, Israel

a r t i c l e i n f o

Available online 21 September 2016

Keywords:TriboluminescenceMechanoluminescenceZnS:MnRadiance

x.doi.org/10.1016/j.jlumin.2016.09.01513/& 2016 Elsevier B.V. All rights reserved.

esponding author at: School of Electrical Eng

a b s t r a c t

ZnS:Mn is one of the brightest mechanoluminescent (ML) materials known, which makes it a primecandidate for use in Structural Health Monitoring (SHM) applications. For this work crystalline ZnS:Mnpowder was embedded in a clear polyurethane (PU) matrix. The total emitted energy per unit solidangle per unit area, as well as the average radiance values of different samples are measured with acalibrated camera to determine the concentration and thickness of ZnS:Mn/PU for optimal emission. Atrend is observed where emission from tensile loading appears stronger than that from compressiveloading, and emission from fracture is stronger than that from plastic loading which itself is strongerthan that from elastic loading. Also, the ML emissions by elastic and plastic loadings tend to increasewith material thickness and ZnS:Mn concentration. Both tensile- and bending-induced fracture modeswere tested. While for tensile fracture the emission is approximately independent of thickness andconcentration, the emission accompanying an abrupt bending fracture increases with the ZnS:Mnconcentration. This work presents a first quantitative absolute measurement of ML emission of ZnS:Mn,while also introducing the use of PU as potential host material, that can benefit many SHM and otherapplications.

& 2016 Elsevier B.V. All rights reserved.

1. Introduction

Mechanoluminescence (ML) is the emission of light from cer-tain materials, mostly crystals, induced by mechanical loading.Most mechanoluminescent materials are only fractoluminescent,meaning that they emit light upon fracture. Fractoluminescence isalso commonly referred to as Triboluminescence (TL) [1], thoughTL could also be the result of rubbing or scraping the material. Forthe sake of brevity, the initials ML will be used to denote both thenoun mechanoluminescence and the adjective mechanoluminescent,as was done by previous authors [1–3].

ML materials have been studied in recent years for their po-tential applications in sensing, especially in the field of StructuralHealth Monitoring (SHM) [2]. ML-based sensors offer in-situ, real-time, safe, distributed sensing systems that require no illuminationsource. Applying these sensors in the fields of infrastructure andaerospace could replace or enhance the capabilities of currentSHM methods, such as ultrasound, piezo-electric transducers, andfiber-optic sensors [3].

ineering, Tel Aviv University,

Many common materials are TL: for example, cubed sugar andquartz [4]. Another well-knownML crystal is ZnS:Mn, which has beenwidely researched due to its relatively intense emission, also observedunder elastic and plastic mechanical loading [5]. The emission me-chanism of ZnS:Mn has a piezoelectric origin, which under mechan-ical loads causes changes in the electric field near the Mn2þ centres.The de-excitation of these ions causes ML emission [5,6].

ZnS:Mn has been studied in powder [6], single crystal bulk, andthin-film [7] forms, and has also been incorporated in severalsensor designs, such as in cement-based structures [8] and com-posite materials [9].

There are many envisioned applications in which the ML ma-terial requires embedding in a host material. This study introducesthe possible use of Polyurethane (PU) for this purpose, as it islight-weight, transparent in the ZnS:Mn ML emission band and forthe relative simplicity of sample fabrication. It is, therefore, thepurpose of this work to quantitatively investigate and compare theemission properties of ZnS:Mn/PU in a variety of configurations,using a new radiometric measure, the eneriance, described below.The measured eneriance values provide a standard yardstick al-lowing the comparison of the emission intensity of samples ofdifferent ZnS:Mn concentration and/or thickness.

N. Persits et al. / Journal of Luminescence 181 (2017) 467–476468

The use of drop-towers to test ML materials, while inexpensiveand simple, makes it difficult to separate the materials' emission inthe elastic and plastic regimes. Drop-tower-based techniques in-troduce experimental difficulties in that the surface of impact of-fers limited viewing accessibility. There is also an inevitable needto account for loading reverberations and reflections from the testmaterial container and supports. Here we have used a universaltensile testing machine to provide controlled loading of samples ofdifferent thickness and ZnS:Mn concentration. The resulting MLemission was recorded in three regimes: elastic, plastic and frac-ture. The Yield Stress (YS) and Ultimate Tensile Strength (UTS) ofeach sample type were determined experimentally to ensure thatsample loading is limited to one of the three regimes above. Thesamples were also measured for their optical transmission anduniformity.

Many previously published experiments and designs, seeabove, have mostly utilized Photomultiplier Tubes (PMT) or otherforms of amplified photo-detectors. The output of such detectors,being proportional to the incident photon flux, is difficult to cali-brate in terms of the sample emission, making the comparisonbetween different materials, set-ups, and experiments challen-ging. In this work, the total emitted light energy per unit solidangle per unit area [10], a radiometric quantity which, in the ab-sence of a known name, we term eneriance, as well as the averageradiance are measured with a calibrated camera by the methoddescribed in Refs. [11,12]. The relation between eneriance andradiance is defined by the following equation:

∫⋅

=⋅

[ ]( )

⎡⎣⎢

⎤⎦⎥

⎡⎣⎢

⎤⎦⎥Eneriance

Jsr m

RadianceW

sr mdt s

1ML EventDuration

2 2

Eneriance and radiance are absolute measures of the emission,serving as the basis for comparative evaluation of the ML perfor-mance of different ZnS:Mn/PU configurations, allowing for futurecomparison to the emission levels of other materials in differentset-ups.

The results presented here expand and enhance on the pre-liminary results reported in Ref. [12] (where the term RadianceExposure Product (REP), was used instead of eneriance herein). Inaddition to repeating many of the experiments to enhance theconfidence in the measured values, here we also report on sampleuniformity, on ML emission under compressive loading and onradiance in fractures induced by bending.

The following section describes the ZnS:Mn/PU sample pre-paration and characterization, including a CT-scan, and measure-ment of optical transmittance, YS and UTS. Section 3 details theexperimental setup, procedure and the data analysis. The experi-mental results are presented in Section 4, followed by a discussion.

Fig. 1. “Dog-bone” samples for measurement of (a) YS and UTS; (b) tensile MLemission; and (c) compressive ML emission.

2. Sample preparation and testing

2.1. Sample preparation

Coarse and fine commercial ZnS:Mn powders (supplied byPhosphor Technology Ltd. with proprietary preparation processes),with average grain sizes of 8.5 mm and 2.5 mm, respectively, weretested in powder form using a drop tower, verifying TL emission.The emission was detected with a PMT (Edmond Optics 57-562)and with a CCD camera (Prosilica GE-2040), both mounted withinterference filters in several spectral bandwidths. Decay times of0.3 ms, characteristic to ZnS:Mn, were verified [1,6,13]. The opti-mal filter configuration, marked by the highest gray level pixelvalues, was determined to be in the 550–650 nm filter band,matching that of the expected ZnS:Mn emission wavelength cen-tered around 590 nm [13].

Following the initial testing on the ZnS:Mn powders to verifyits ML performance, they were embedded into clear PU castingresin (Poly-Optic 1410, by Polytek). PU is transparent in the ZnS:Mn ML emission wavelength band, is relatively lightweight, and issuitable for producing samples with controlled powder density.Slab samples, 12�2 cm, of ZnS:Mn/PU were prepared with dif-ferent powder concentration and sample thickness. Since thepresence of ZnS:Mn in the PU gives rise to absorption and scat-tering, the effect of sample thickness was investigated in order todetermine an optimal thickness, which maximizes emission in-tensity and minimizes weight and volume. Coarse powder, whoseemission was more intense in the powder-form tests, was used toprepare samples in nominal thicknesses of: 3 mm, 1.5 mm, and0.75 mm, each in three nominal ZnS:Mn weight concentrations of10%, 20% and 40%. Attempts to prepare ZnS:Mn concentrationsabove 40% failed due to the high viscosity of the mixture thatproved difficult to mold. Thus, nine different sample types weredesigned using the coarse powder, and for each type many slabs,identical by design, were manufactured. To explore the effect ofparticle size on emission intensity, fine powder samples wereprepared in one thickness, 3 mm, and one weight concentration,20%, referred to below as 20F% (the emission results for thesesamples are presented and discussed in the Appendix A). In aneffort to achieve a uniform particle distribution, the ZnS:Mnpowder was continuously and extensively mixed into the PU resinand then injected rapidly into silicone molds. The samples werecured in room temperature for two hours before demolding. Forreference, clear PU samples of the three thickness values abovewere prepared in the same molds.

Samples were machined into a dog-bone shape. Those in-tended for tensile tests, to determine their Yield Stress (YS) andUltimate Tensile Strength (UTS), were shaped according to ASTMstandard D638-10 type IV with a nominal central width of 6 mm(Fig. 1a). Samples intended for tensile ML emission measurementwere machined to a dog-bone shape with a nominal central widthof 13 mm to ensure their fracture occurs far from the holding grips(Fig. 1b) and to provide a larger measurement area. Though manycompression studies are performed on cylinder-shaped samples,this geometry is less convenient for the current optical tests,where a uniform and a relatively large planar emission surface

Fig. 2. CT scan cross-section of ten representative ZnS:Mn/PU samples showing gray level variations with ZnS:Mn particle concentration. The gray level, averaged over thesamples' vertical dimension (blue solid lines), indicates variation in particle concentration across each sample. The labels B, and T indicate the bottom and top of each samplein the mold; the red circles mark conspicuous defects (see text for detail). The samples in the figure, from left to right are: 0.75 mm: 40%, 20%, 10%: 1.5mm: 40%, 20%, 10%,3 mm: 40%, 20%, 10%, 20F%. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 3. (a) Optical transmittance of clear PU (circles) and PU embedded with 10%(squares), 20% (triangles), 20F% (stars) and 40% (asterix) ZnS:Mn concentration. (b):Optical transmittance of 3 mm 10% ZnS:Mn sample over the 400–800 nm wave-length range.

N. Persits et al. / Journal of Luminescence 181 (2017) 467–476 469

facing the camera is preferred. Therefore, samples used for com-pression tests were machined into a dog-bone form specified byASTM standard D638-10 type I for compressive testing with a13 mm nominal central width (Fig. 1c).

2.2. Sample characterization

2.2.1. CT scanRepresentative samples were CT-scanned (YXLON-Modular by

Yxlon Ltd.) to map the ZnS:Mn particle distribution within the PU,and identify possible defects. Fig. 2 presents cross-sections of theCT scan results. The gray scale is related to ZnS:Mn particle den-sity: darker tones correspond to higher particle density (except forthe background which is synthetically set to black). Overlaid onthis image (blue graphs) are the pixel's gray levels averaged overthe vertical dimension of the image, indicating variations in ZnS:Mn particle density across the samples' thickness. The samples inthe figure, from left to right are: 0.75 mm: 40%, 20%, 10%: 1.5 mm:40%, 20%, 10%, 3 mm: 40%, 20%, 10%, 20F%.

While this ZnS:Mn spatial concentration analysis shows a ra-ther homogenous distribution in the samples' bulk, there is evi-dence of ZnS:Mn particle settling, causing local changes in thedensity at the samples' top-most and bottom-most layers (markedT and B respectively in Fig. 2) up to depths of 200 mm. Theseparticle settling effects introduce variations of up to 712% in grayscale values, with a more pronounced effect for the lower ZnS:Mnconcentrations, perhaps due to the corresponding reduction in themixture's viscosity. In addition, rough sample surfaces and defectssuch as air bubbles, some of which are marked in Fig. 2 by redcircles, appear as very bright spots at the top of the 10% ZnS:Mnsamples. These defects are the main cause for the sharp decreasein the average gray scale for these layers. All ML emission testswere performed using the top of the sample, where the particledensity facing the camera is lower.

2.2.2. Optical transmittanceThe optical transmittance of samples with 10%, 20%, 20F% and

40% ZnS:Mn as well as clear PU was measured. For finer thicknessresolution samples with coarse ZnS:Mn powder were machinedinto thicknesses varying from 0.1 mm to 3 mm. The measurementswere made using a spectrophotometer (Perkin Elmer Lambda1050) in the wavelength range 400–800 nm. Fig. 3a presents thetransmittance results for all samples versus thickness at 585 nm,the center of the ZnS:Mn ML emission band. Fig. 3b shows thetransmittance of a 3 mm thick 10% ZnS:Mn over the entire spectralrange. Similar transmittance curves were obtained for the otherZnS:Mn concentrations.

Clear PU samples' transmittance decreased from 80% to 73%with increase in sample thickness from 0.75 to 3 mm, showing aBeer-Law exponential behavior. While the transmittance of theseemingly opaque ZnS:Mn-samples (Fig. 1) is rather low, it is stillresolved by the spectrophotometer. As seen in Fig. 3a, the trans-mittance decreases with the ZnS:Mn concentration and with thesample thickness. Its wavelength dependence, Fig. 3b, shows amonotonous rise in the spectral region of interest.

2.2.3. Yield strength and ultimate tensile strengthEach of the sample types (nine coarse powder, one fine powder

and three clear samples) was tested under tensile loading tillfailure. For improved measurement accuracy strain was measuredin real time with an extensometer (Epsilon model 3542-025M-050-ST) as seen in Fig. 4. Data of strain and applied load wererecorded and with the actual dimensions of each sample, wereused to determine the YS, i.e. elastic limit, and the UTS for eachsample type. Testing was performed for each sample type at a slowtest speed (2 mm/min) to obtain comparative results to those inthe literature, and at a fast test speed (60 mm/min) in order toassess the variation in the material characteristics due to higher

N. Persits et al. / Journal of Luminescence 181 (2017) 467–476470

strain rates, as expected for PU [14]. As is further described in thefollowing, the YS and UTS values determined in these tests serve todefine the loading limits in each step of the three-step loadingsequence used for ML emission measurements.

In general, the stress-strain curves recorded for the pristine PUsamples, as well as for the ZnS:Mn-doped PU, in either test speed,have shown no clear yield point, defined as the point where anincrease in strain causes no increase in stress. Therefore, their YSwas determined according to the 0.2% plastic strain offset yieldcriteria, as is customary for such materials [14,15], and markedYS0.2%.

Fig. 5 presents typical stress-strain curves of 3 mm samplesrecorded at the slow and the fast test speeds. YS0.2% and UTS ofpristine PU samples at the lower test speed were averaged overthe three sample thicknesses and found to be 45, and 53 MPa,respectively. These values match the manufacturer's data andthose reported in Ref. [14]. YS0.2% values for the ZnS:Mn/PU sam-ples were found to increase slightly with decrease in samplethickness, and generally decrease with ZnS:Mn concentration.Results for the 3 mm samples at the slow test speed are: 40, 35, 37and 36 MPa for 10%, 20%, 20F%, and 40% ZnS:Mn concentrations,respectively. YS0.2% for the higher test speed, were lower by some15%. The Young Moduli for the above samples were 2.1, 2, 2.4 and2.3 GPa, respectively, while Young’s Modulus for the pristine PUwas 1.4 GPa.

The UTS values for the ZnS:Mn/PU samples tend to decreaseslightly with ZnS:Mn concentration (except for the 20F% whichshows a similar UTS to that of the pristine material). The stress-strain graphs also show no necking for samples with ZnS:Mnconcentration of 20% and higher. UTS results for the 3 mm samplesat the slow test speed are: 49, 51, 55 and 48 MPa for 10%, 20%,

Fig. 4. Experimental set-up for tensile load testing with an extensometer.

Fig. 5. Typical stress-strain curves for 3 mm samples of different ZnS:Mnconcentration.

20F%, and 40% ZnS:Mn concentrations, respectively. UTS values forthe faster test, were higher by some 20% for all sample thicknessesof 10% and 20% ZnS:Mn concentration, similar in value for the20F% concentration, and somewhat lower for the 40% concentra-tion. The YS0.2% and UTS values obtained at the higher test speedwere used to define the limiting load for the elastic and plasticregimes, respectively.

3. Experimental

All loading tests, both for determining YS and UTS, and for MLemission measurements are conducted with a Zwick/Roell Z100Universal Tensile Testing Machine (UTTM). Prior to each test theactual dimensions of each sample are measured with a digitalcaliper.

For ML emission measurements the sample is mounted in themachine's grips with its top facing a calibrated monochrome CCDcamera (Prosilica GE-2040) [11], and is stressed in the dark. Anf¼50 mm, f/#1.4 lens, with its iris fully open, and a 550–650 nm(the ML emission bandwidth of ZnS:Mn [13]) interference filter isused. The camera is focused onto the front face of the sample. Thecamera is set to capture consecutive 400 ms image frames. Theresulting camera gray scale values are converted into eneriancevalues, Eq. (1), which then serve as the primary, absolute MLemission characterizing parameter. The average ML emission ra-diance is defined here as the eneriance value divided by thetemporal duration of that ML event. Further detail on the mea-surement method, the camera calibration procedure, the camerasetting, and the data analysis are detailed in Ref. [11].

3.1. Experimental method

3.1.1. Eneriance measurement: tensile loadingML emission measurements under tensile loading are per-

formed at three different nominal test speeds: 6, 13 or 25 mm/s.Loads are applied in three separate steps according to the YS0.2%and UTS of each sample type: (a) load to just below the elasticlimit or YS; (b) increase the load to just below the UTS; and(c) increase the load to fracture. In each step the nominal load isreduced by 15–20% to guarantee that the first loading step doesnot exceed the elastic limit and that the second step does notresult in fracture. This procedure ensures separate measurementsfor each loading regime [11]. Fig. 6 shows a schematic illustrationof the loading process and the camera's gray level output. Eachsample type was tested between two and four times at eachloading rate, where each test was conducted on a fresh samplethat had not been previously loaded.

Fig. 6. (a) Schematic illustration of stepped loading, separating the elastic, plasticand fracture ML emission; (b) camera output, or gray level sum, for each loadingstep [11].

Fig. 7. Constraining jig for eneriance measurement under compressive loads.

Fig. 8. Typical ROI regions for (a) tensile measurements and (b) compressivemeasurements.

N. Persits et al. / Journal of Luminescence 181 (2017) 467–476 471

3.1.2. Eneriance measurement: compressive loadingWhile tensile tests were quite straight-forward, compressive

testing proved more challenging; the ZnS:Mn/PU samples are softand deform dramatically under compression, thus preventing ac-curate measurement of the applied load. A jig was manufacturedto constrain the samples and prevent their buckling under theapplied loads (Fig. 7). A window is cut into the front constrainingplate to allow viewing of the ML emission, and a plexiglass plate isplaced to constrain the front face of the sample. ML emissionmeasurements, viewed through the plexiglass, are corrected tocompensate for the attenuation through this window. Eneriancemeasurements were performed on the four types of 3 mm thicksamples at one testing speed (6 mm/s).

3.1.3. Radiance from fracture by bendingAs described above, the average radiance of an ML emission

event is obtained by dividing the eneriance by the time duration ofthe event. The emission duration for elastic and plastic loadingcorresponds to the loading duration as recorded by the UTTM. Theemission duration for fracture, however, is related to the crackpropagation velocity in the material and not the loading duration.Therefore, an independent measurement of the duration ofemission from fracture is required to determine the radiance.

We complemented the measurement for fracture under tensileloading with measurement of the eneriance from a fracture bybending, which was measured to occur significantly faster in thismaterial. This bending-induced fracture more closely resemblesfracture under impact or other sudden events. The top surface ofeach sample is scribed with a knife to seed crack propagation, as isdone when cleaving optical fibers. The sample is then placed in ajig facing the camera. The sample is fractured by abrupt bending

away from the direction of the camera. 3 mm thick samples of allfour ZnS:Mn concentrations were tested. The eneriance is cap-tured with the camera while the emission duration is recordedwith a PMT (Edmond Optics 57-562) and a digital scope (Agilentinfiniium MSO8104A). The measurement was repeated at leastthree times for each sample type (each measurement performedon a new unloaded sample).

3.2. Data analysis

3.2.1. Mechanical loading dataThe average stress on the samples is calculated from the

loading force recorded by the UTTM divided by the samples'measured cross-section dimensions. The duration and magnitudeof the loading increase in each loading step, corresponding to theelastic and plastic regimes, is derived from loading data and timeintervals recorded by the UTTM.

3.2.2. Gray level conversion to eneriance3.2.2.1. ROI selection. The Region of Interest (ROI), defining thepixels from which eneriance values are measured, in tensile testsfor the elastic and plastic regimes is 250 pixels long by 170 pixelswide [11]. In compressive tests the ROI is smaller (at least 170pixels long by 140 pixels wide), limited by the size of the viewingwindow in the constraining jig. Typical ROIs for tensile and com-pressive tests are shown in Fig. 8a and b, respectively. Defectivepixels within the ROI are eliminated from all further calculations.

In fracture events the effective ROI is reduced to include onlypixels whose eneriance exceeds that of plastic emission in thesame sample. An example of the reduced ROI for fracture is pre-sented in Fig. 9.

3.2.2.2. Eneriance measurements for TL (fracture). Due to the safetymargin to avoid fracture during the plastic ML measurement and arelatively long camera frame exposure time, the frame in whichthe fracture event appears also includes contribution from theplastic emission which precedes the fracture. In order to eliminatethe contribution from this plastic emission, the measured TLemission values are reduced by the normalized plastic emissioneneriance/MPa recorded for the same sample, multiplied by theincremental loading used for fracture:

Fig. 9. The ROI for fracture eneriance measurements is reduced to include only pixels with eneriance exceeding the maximal eneriance recorded for plastic emission in thesame sample. An original fracture emission image (left) and the pixels defining the reduced ROI marked in red (right). (For interpretation of the references to color in thisfigure legend, the reader is referred to the web version of this article.)

Fig. 10. Stress-normalized eneriance vs. ZnS:Mn concentration for elastic tensileloading of different sample thicknesses: 0.75 mm – triangle; 1.5 mm – circle; and3 mm – diamond.

Fig. 11. Stress-normalized eneriance vs. ZnS:Mn concentration for plastic tensileloading of different sample thicknesses: 0.75 mm – triangle; 1.5 mm – circle; and3 mm – diamond.

Fig. 12. Eneriance vs. ZnS:Mn concentration for tensile fracture of different samplethicknesses: 0.75 mm – triangle; 1.5 mm – circle; and 3 mm – diamond.

N. Persits et al. / Journal of Luminescence 181 (2017) 467–476472

= −

⋅ ( )

⎛⎝⎜

⎞⎠⎟

⎡⎣ ⎤⎦

Eneriance EnerianceEneriance

MPa

Loading MPa 2

Fracture measuredPlastic

fracture

The emission measurement results show that the contributionof elastic ML emission to the measurements within the plastic MLregime is negligible. While an estimated contribution from theplastic regime is subtracted from fracture emission measurements,no correction is attempted to reduce the recorded emission in theplastic regime by an estimated contribution from the elastic re-gime. This is supported by previous reports [16] on the relativecontribution of the different loading regimes, as is further dis-cussed in Section 5.

Fig. 13. Comparison of stress-normalized eneriance for compressive (empty mar-kers) and tensile (full markers) loading in the elastic (triangle) and plastic (square)regimes vs. ZnS:Mn concentration (note that some of the error bars are too small tobe resolved).

4. Results

This section presents only results for the coarse ZnS:Mn pow-der samples. As the fabrication methods of the ZnS:Mn powders inthis work are unknown, and as the number of grains in a givenvolume varies between the coarse and fine powder samples, adirect comparison of the two grain sizes may be misleading.Emission results for the fine powder samples are, therefore, pre-sented in the Appendix A.

4.1. ML emission under tensile loading

Figs. 10–12 respectively show the eneriance of elastic, plasticand fracture emission under tensile loads. Interestingly, the en-eriance of the different samples is not affected by the loading rate,and these graphs include results from all test speeds. Graph pointsrepresent the average and the error bars are the standard devia-tion of all measurements on each sample type (four to twelve testsper displayed point, each related to the first loading of a freshsample). To allow direct comparison of the emission levels fordifferent sample dimensions, the elastic and plastic emission re-sults are presented in stress-normalize eneriance units: the

eneriance is divided by the increase in stress for the loading stepin each case (J/sr/cm2/MPa). The fracture emission results (Fig. 12)are in eneriance units.

In all tests conducted on samples of pristine PU, no emissionwas detected; the emission captured by the CCD originates en-tirely from the ZnS:Mn particles. Furthermore, the decay timesrecorded during fracture correspond to the decay times of ZnS:Mnas presented in Section 4.5.

In Figs. 10–14, the error bars, representing the standard de-viation for measurements on each sample type, sometime overlap

Fig. 14. Eneriance vs. ZnS:Mn concentration of all regimes in tensile and com-pressive tests for 3 mm samples (note the logarithmic scale).

Table 1Average radiance results for 3 mm samples of different ZnS:Mn concentration ac-cording to their loading regime.

ZnS:Mn [wt%] Average radiance [nW/sr/cm2]

Regime: 10% 20% 40%

Test speed [mm/s] Elastic Plastic Elastic Plastic Elastic Plastic

Nominal Actual

6 3.5 0.51 0.6 1.1 5.0 1.5 3.613 4.8 0.72 1.2 1.2 4.4 1.8 4.625 7.9 0.79 1.0 1.5 5.8 1.2 5.0

Table 2Average radiance for bending fracture results for 3 mm samples of different ZnS:Mnconcentrations.

ZnS:Mn [wt%] 10% 20% 40%

Eneriance [nJ/sr/cm2] 37.1712.7 58.6714.3 67.2717.6Average emission duration [ms] 1.5370.15 1.470.2 1.270.46Average radiance [lW/sr/cm2] 23.976.7 4178.6 58.5713.6

N. Persits et al. / Journal of Luminescence 181 (2017) 467–476 473

results for other sample types. Attributed to uncontrolled differ-ences in slabs of the same type, to be further discussed below,these relatively large spreads potentially prevent us from statisti-cally high confidence conclusions but still allow trends to berecognized.

In the elastic regime, Fig. 10, there is an increase in stress-normalized ML eneriance with ZnS:Mn concentration and withsample thickness. In Fig. 11, plastic stress-normalized ML en-eriance increases with sample thickness but appears to “saturate”with concentration: 20% concentration shows higher ML emissionlevels than 10% but there is no further increase in enhancing theconcentration further to 40%.

Interestingly, for fracture results, the 1.5 mm thick samplesexhibit higher TL eneriance levels than those of 3 mm samples andthere appears to be no significant contribution to the increase inZnS:Mn concentration.

As indicated above, the eneriance of the different samples wasnot affected by the loading rate. Nevertheless, the events' radiancegenerally increases with loading rate as the same amount of en-ergy is released in a shorter period of time, as is further discussedbelow and is consistent with previous work [14].

4.2. ML emission under compressive loading

The stress-normalized eneriance results for compressive load-ing of 3 mm samples are presented in Fig. 13 for the elastic andplastic regimes in comparison to results of tensile loading tests onthe same sample types at the same test speed (6 mm/s). Unlike thetensile ML emission, both the plastic and elastic compressiveemissions increase only slightly with increased ZnS:Mn con-centration from 10% to 20%, but does not increase further for 40%concentration. Except for the plastic tensile ML emission of the10% samples, in all other cases the stress-normalized ML eneriancefor tensile loading is stronger than that of the compressive loading.

4.3. Eneriance results summary

Fig. 14 provides an overall perspective of the comparative en-eriance magnitude for both tensile and compressive loading in allthree regimes. The figure plots the eneriance results for 3mmsamples on a logarithmic scale. This is a comparison of the totalML energy emitted per unit area per unit solid angle withoutconsidering the strain or stress experienced in inducing the signal.Fracture generates by far the highest eneriance, at least an order ofmagnitude stronger than the eneriance for plastic tensile andcompressive loading. The elastic eneriance is typically lowest forboth tensile and compressive loading.

4.4. Radiance for elastic and plastic loading

From the eneriance results and the emission duration we cal-culate the average radiance of the ML emission. Table 1 presentsthe average radiance results for 3 mm samples in different ZnS:Mnconcentrations and different loading speeds in the elastic andplastic loading regimes. In addition to the nominal test speeds, thetable presents the actual test speeds (which are limited in practiceby the acceleration of the UTTM controls) as evaluated from therecorded load vs. time data of the UTTM.

Overall, the radiance increases with ZnS:Mn concentration andis higher for plastic loading than for elastic loading. While theeneriance values are independent of loading rate, the radiance,which is inversely proportional to the emission duration (Eq. (1)),should increase with loading rate. Surprisingly, however, the ra-diance is not proportional to loading rate. Furthermore, in some ofthe samples, the radiance results at the highest loading rate aresmaller than at the medium loading rate. These inconsistencies areconsidered in the discussion section.

4.5. Radiance for fracture by bending

Bending fracture eneriance, total emission durations (rise anddecay), and radiance values are presented in Table 2. Fig. 15 showsa typical temporal form of the emission signal during fracture. Theradiance values are calculated from the eneriance recorded by thecamera and emission duration from the PMT. The fracture averageradiance increases with ZnS:Mn concentration. The eneriance re-sults for fracture by bending were higher than those recordedunder tensile loading. As for the temporal emission signal, itshows an essentially linear rise time, on the order of 30 ms, and adouble exponential decay [18]. The emission duration, comprisingthe rise and decay durations (not to be confused with the decaytime, calculated for 1/e decrease from the peak signal) is estimatedfrom the width of the PMT signal at 5% of its peak. Notably, as thesame ML material is used, the decay time and decay duration,approximately 300 μs and 1.5 ms respectively, are similar for all

Fig. 15. A typical temporal TL emission signal during an abrupt bending fracture(3 mm 20% ZnS:Mn concentration sample).

N. Persits et al. / Journal of Luminescence 181 (2017) 467–476474

samples [1,6,13,18]. However, the emission rise time, and hencethe overall duration, do vary due to the unique fracture propaga-tion in each sample.

5. Discussion

The following paragraphs evaluate the measurement procedureand the compatibility of various ZnS:Mn/PU samples for sensingapplications.

Measurement method attributes:

� Measurement repeatabilityWe note the relatively large variation in the measurement re-sults for repeated testing on the same sample type (it is againemphasized that each test is performed on a fresh, previouslyunloaded independent sample) as evident from the standarddeviation (error bars in Figs. 10 through 14). As the opticaleneriance measurement resolution [11], 70.4 nJ/sr/cm2, is finerthan the recorded measurement uncertainties, we attribute thevariation to uncontrolled deficiencies in sample preparation.ZnS:Mn powder concentration variations within representativesamples (Fig. 2) and molding imperfections, such as air bubblesand surface roughness, were observed. For more repeatablemeasurement results, further experimentation and accompany-ing analysis are required to ensure that the samples are morehomogeneous and free of imperfections. Nevertheless, we feelthat the current results do indicate the correct trends of thedependence of the emission of the ZnS:Mn/PU on concentrationand sample thickness.

� Separability of loading regimes and resulting measurement er-rorsThe recorded real-time loading data confirm that each loadingstep did not exceed the regime boundaries. Nevertheless, limitingthe first two loading steps to slightly below the actual YS0.2% andUTS values, respectively, introduces some elastic regime emissionto the plastic loading measurement and plastic emission to thefracture measurement. The measurements show that in bothcases the contribution from the previous loading regime is rela-tively small. The elastic emission is responsible for less than 10%of the emission in the subsequent plastic regime, while the plasticemission contributes up to 20% to the emission recorded duringfracture. The latter is subtracted from the fracture eneriance (Eq.(2)) so that the overall error experienced is much smaller.The portions of the elastic and plastic emission results in thiswork differ from the results of Alzetta et al. [16]. In the presentwork the elastic emission contribution is some 35% of the plasticemission in the same sample type (Figs. 10, 11 and 14) and thecontribution from fracture is significantly higher than the con-tribution of either. Alzetta reported that 80% of the total emissionoriginates in the plastic regime, only 14% from fracture and 6%from the elastic regime. These differences could be attributed to

several factors: Alzetta et al. used a PMT to record emission froma single crystal of ZnS:Mn excited by bending. In the current studya large surface area of ZnS:Mn/PU with multiple crystals isimaged. The results presented here are normalized to unit sourcearea (stemming from the eneriance units). The normalization tosource area accounts for the significantly larger emission fromfracture, which necessarily occurs over a limited area. Typically, afracture propagates normal to the measurement surface soemission due to fracture originates in a relatively small area(see Fig. 9 with a limited fracture emission area). In contrast,emission due to plastic loading, for example, originates from theentire surface of the material. Therefore the normalization tosource area herein accounts for the significantly larger fractureeneriance, as compared to plastic loading eneriance. A PMTmeasurement, however, integrates the emission over the entirearea of the sample, and, for the same emission performance,presents a larger relative value for the plastic emission.

� Stress concentration on ZnS:Mn particlesAll the stress values considered herein represent the averagestress in the ZnS:Mn/PU matrix, as recorded by the UTTM. TheZnS:Mn particles experience stress concentration due to thedramatic difference in their Young's modulus compared withthat of the PU. A CFD model shows that the local stress acting ona ZnS:Mn sphere embedded in PU is approximately 40% greaterthan the average stress within the matrix [11].Tensile tests:Generally, results show that stress-normalized plastic eneriance(Fig. 11) is between two- to four-fold more intense than stress-normalized elastic eneriance (Fig. 10). The eneriance fromfracture is significantly higher than from elastic or plasticloading (Fig. 14).

� ZnS:Mn concentration:In the elastic and plastic loading regimes, as well as in fracture,the ML eneriance values for coarse ZnS:Mn grain, in samples ofsimilar thickness, increase with ZnS:Mn concentration (Figs. 10–12, Table 2). As expected, these results indicate that the overallML emission energy increases with the number of ZnS:Mngrains in a given volume.

� Sample thickness:Interestingly, there is a nearly proportional increase in stress-normalized ML emission with sample thickness in the elasticand plastic regimes (Figs. 10 and 11). This seems to be incontradiction with the optical transmittance data that showoverwhelming attenuation for material thickness greater thanhalf a millimeter. With an extinction rate of over 99%, thecontribution to the measured eneriance from deeper particles isexpected to be small (Fig. 3a). The absorption of ML emittedwavelengths in ZnS:Mn/PU is small, so the extinction is almostentirely due to scattering [19,20]. Light that was initiallydiverted off the optical axis is partially scattered back towardsthe original propagation direction, replenishing the transmittedlight and reducing the effective extinction rate. The discrepancybetween the transmission measurements and the ML emissionresults may be related to the limited illumination beam cross-section in the transmittance measurement, which would seemto limit the intensity of the multiply scattered light.In the case of fracture, results show no preference for thickersamples (Fig. 12) since an appreciable portion of the emitted lighttravels directly through the gap that opens in the sample withouttraversing the sample material. In addition, due to the particlesettling near the surface farthest from the camera, the emissionfrom that region may be obscured from the camera's line of sight.Compressive tests:The stress-normalized ML emission in compressive loading isgenerally 30–50% weaker than the emission under tensile loading(Fig. 13) as also reported for ZnS:Mn-coated optical fibers

Table 3Normalized eneriance values for 20F% samples according to their loading regime.

Test Normalized Eneriance [pJ/sr/cm2/MPa]

Tensile Elastic 4.874.6Tensile Plastic 1.470.74Compressive Elastic 4.470.6Compressive Plastic 2.470.17Tensile Fracturen 3,0387796

n In units of eneriance [pJ/sr/cm2]

Table 4Average radiance values for 20F% samples according to test speed.

ZnS:Mn [wt%] Average radiance [nW/sr/cm2]Regime: 20F%

Test speed [mm/s] Plastic Elastic

Actual Nominal

N. Persits et al. / Journal of Luminescence 181 (2017) 467–476 475

embedded in cement [8,17]. Note, however, that 10% samplesshow an exception from this general trend as values for tensileand compressive loading are similar in the elastic regime.In comparing the eneriance values themselves (without normal-ization to stress) a similar trend where tensile loading yieldshigher eneriance is observed for the elastic regime (Fig. 14). Theemissions for compressive loading in the plastic regime appear,however, to be significantly higher than the emissions due totensile loading (Fig. 14). This merely reflects the characteristics ofthe PU matrix, a highly plastic material, which flows under largecompressive loads without fracturing so that higher stresses wereapplied as compared to those in tensile loading.Radiance:Radiance, unlike eneriance, provides a quantitative yardstickmore relevant from an engineering standpoint. The radiancevalue of the emission enables to estimate the output power ofdifferent detectors and thus evaluate sensitivity and SNR. Whileour technique only characterizes the average radiance, the peakradiance can be estimated since the temporal signal shape isknown. For the results recorded here, the peak radiance isestimated to be some threefold higher than the average radiance.

� Test speedThe test results do not show a dependence of the eneriance ontesting speed. This indicates that the ML eneriance is only afunction of the strain itself and not the strain rate. The averageradiance, i.e., the eneriance divided by emission durationhowever, is expected to increase with test speed, as is observedfor most (but not all) of the results in Table 1.

� Elastic and plastic regimesUnexpectedly, not all the average radiance values of Table 1exhibit a systematic increase of radiance with increased testspeed. For example, the average radiance for 40% elastic emis-sion actually decreases with test speed. This inconsistency isattributed to ZnS:Mn concentration and distribution variationsof the samples and to the relatively poor time resolution of theUTTM temporal data (0.1 s). An independent temporal measure-ment of the emission duration is required for calculating theaverage radiance in fracture, as describe below.

� FractureEneriance values of emission resulting from the faster bending-induced fracture tests are higher than those obtained fromtensile loading fracture tests. This could be due to differences inthe fracture mechanism. In any case, we believe that the en-eriance measured from fracture by bending is a better model forthe average radiance that can be expected in a practical sensor.The results in Table 2 show that, as can be expected, theradiance is inversely proportional to the fracture duration. Theaverage radiance values range between 20 and 50 mW/sr/cm2,increasing with ZnS:Mn concentration. As peak radiance valuesare higher than the average radiance, and since higher fracturerates are expected in practice, a ZnS:Mn/PU-based sensor ispotentially viable.

3.5 6 0.09 0.154.8 13 0.04 0.317.9 25 0.11 0.30

Table 5Average radiance values for 20F% under fracture bybending.

ZnS:Mn [wt%] 20F%

Eneriance [nJ/sr/cm2] 32.9711.5Average emission duration [ms] 2.8570.38Average radiance [lW/sr/cm2] 11.573.7

6. Conclusions

This study on ZnS:Mn powder embedded in PU successfullymet its two goals: (i) to measure the absolute ML eneriance andaverage radiance under elastic, plastic and fracture loading; and(ii) to determine the preferred ZnS:Mn concentration, grain-size,and material thickness for sensing applications. The absolutemeasurements in terms of eneriance provide for an importantparameter for the design of ZnS:Mn ML-based sensors, and serveas a convenient scale with which the performance of different ML-based components can be compared independently of the setupused for their evaluation.

For the coarse grain size, the higher ZnS:Mn concentrations andthicker material yield stronger ML elastic and plastic emissions.Fracture (TL) emissions are also stronger for the coarser ZnS:Mnbut vary less with ZnS:Mn concentration and material thickness.The measured eneriance values range from an order of 1 nJ/sr/cm2

for the elastic and plastic regimes up to 50 nJ/sr/cm2 for fracture.Furthermore, fracture by bending generated average radiance le-vels in excess of 10 mW/sr/cm2. These results indicate that ZnS:Mnembedded PU is a promising material for ML-based sensors pro-viding high ML emission efficiency, low absorption of emitted MLlight and offering relatively simple sample preparation andhandling.

Acknowledgements

The authors kindly acknowledge the assistance of MLM's ma-terials department and the non-destructive testing lab in setupand operation of the UTTM. The research of M. Tur was supportedby The Israel Science Foundation (grant No. 1380/12).

Appendix A

The 20F% samples exhibit the lowest ML emission results in allregimes. Table 3 presents the normalized eneriance values of thesamples in the elastic and plastic regimes in both tensile andcompressive tests.

Table 4 shows the 20F% radiance values in the elastic andplastic regimes according to test speed:

N. Persits et al. / Journal of Luminescence 181 (2017) 467–476476

Table 5 presents the eneriance, emission duration and averageradiance in fracture induced by abrupt bending:

As seen from these results and the results in Section 4 (Figs. 10–14, Tables 1 and 2), the ML eneriance of 20F% fine grain samples issignificantly lower than the ML eneriance of 20% coarse grainsamples. A previous study [6] showed that the emission risesquadratically with grain size. Considering the average grains sizesused here, one would expect an increase of nearly 12-fold inemission strength for the coarse grain. Approximately such dif-ferences between the emissions from the 20% and the 20F% sam-ples are seen for fracture and plastic loadings (see Figs. 11–14,Table 3) but a significantly smaller difference is observed for theelastic regime (see Figs. 10, 13 and 14, Table 3). This is mostprobably due to their values being only barely resolved from thebackground noise. Additional factors may be at play here, includ-ing the increased scattering in the finer powder material, forwhich the measured transmission is nearly an order of magnitudelower (Fig. 3a), or differences in fabrication processes of thepowder material (which are, unfortunately undisclosed). From apractical perspective, irrespective of the underlying causes for thereduced emission levels, coarse ZnS:Mn powder is clearly prefer-able for sensing applications.

References

[1] J.A. Walton, Adv. Phys. 26 (6) (1977) 887.[2] I. Sage, G. Bourhill, J. Mater. Chem. 11 (2001) 231.[3] D.O. Olawale, T. Dickens, W.G. Sullivan, I.O. Okoli, J. Lumin. 131 (2011) 1407.[4] G.N. Chapman, A.J. Walton, J. Appl. Phys. 54 (10) (1983) 5961.[5] B.P. Chandra, C.N. Xu, H. Yamada, X.G. Zheng, J. Lumin. 130 (3) (2010) 442.[6] R.S. Fontenot, W. a Hollerman, K.N. Bhat, M.D. Aggarwal, J. Theo. App. Phys. 6

(1) (2012) 15.[7] C.N. Xu, T. Watanabe, M. Akiyama, X.G. Zheng, Mat. Res. Bull. 34 (99) (1999)

1491.[8] D.O. Olawale, K. Kliewer, A. Okoye, T.J. Dickens, M.J. Uddin, O.I. Okoli, J. Lumin.

147 (2014) 235.[9] T.J. Dickens, O.I. Okoli, J. Rein. Plas. Comp 30 (22) (2011) 1869.[10] S.P. Parker, McGraw-Hill Dictionary of Scientific and Technical Terms, 5th ed.,

McGraw-Hill, 1997.[11] N. Persits, A Novel Mechanoluminescence-Based Sensor Concept for Structural

Health Monitoring (M.S. Thesis), Tel-Aviv Univ., 2016.[12] N. Persits, A. Aharoni, M. Tur, Proc. SPIE 9655 (2015) 18.[13] W. a Hollerman, Proc. SPIE 5897 (2012).[14] S. Sarva, S. Deschanel, M. Boyce, W. Chen, Polymer 48 (2007) 2208.[15] ISO 527-1:2012: Plastics - Determination of tensile properties.[16] G. Alzetta, I. Chudacek, R. Scramozzino, Phys. Status Solidi A 1 (1970) 775.[17] N. Aich, A. Appalla, N.B. Saleh, P. Ziehl, J. Intellig. Mat. Syst. Struct 24 (14)

(2013) 1714.[18] B.P. Chandra, C.N. Xu, H. Yamada, X.G. Zheng, J. Lumin. 130 (3) (2009) 442.[19] V.D. Mote, Y. Purushotham, B.N. Dole, Ceramica 59 (2013) 395.[20] H. Van Bui, P. Van Ben, H. Nam Nhat, MN- Doped ZnS Nanoparticles Synthe-

sized by a Co–Precipitation Method, 22, 2012, pp. 167–173.