Measuring the Process Variability in Triboluminescence Emission Yield for EuD4TEA WILLIAM A. HOLLERMAN, ROSS S. FONTENOT, KAMALA N. BHAT, and MOHAN D. AGGARWAL Europium dibenzoylmethide triethylammonium (EuD4TEA) is one of the brightest known triboluminescent materials. Emission from EuD4TEA can be seen in daylight and has been found to be more than twice as bright compared to inorganic ZnS compounds. Using a custombuilt drop tower, the triboluminescent emission yield for five batches of EuD4TEA was measured. Results show that the measurement variance for the drop tower is less than 9 pct. In addition, no statistically significant batch-to-batch variations in the triboluminescent emission yield were observed for the synthesized EuD4TEA. It can be inferred that the measurement uncertainty for the triboluminescent emission yield between batches is, at most, on the order of a few percent. DOI: 10.1007/s11661-012-1202-9 Ó The Minerals, Metals & Materials Society and ASM International 2012
I.
INTRODUCTION
FOR the last decade, researchers have been investigating triboluminescence (TL) and its potential use as the active element for impact sensors.[1–10] TL is a unique phenomenon that occurs in about 50 pct of known crystals and is the ability to emit light when the crystals are stressed or broken.[11] Most previous TL research centered on zinc sulfide compounds doped with combinations of europium, copper, and lead.[1–10] However, recent work completed on the production of TL from an organic material shows it to be so bright that it can be seen in daylight.[3,12–14] This material is known as europium tetrakis dibenzoylmethide triethylammonium (EuD4TEA) and has 106 pct increase in the triboluminescent emission yield compared to a similar quantity of 7.5-lm-diameter ZnS:Mn powder subjected to low energy impacts.[3] This relative triboluminescent yield for EuD4TEA is an average calculated over five trials in a specially constructed drop tower. Results show the TL emitted for the first drop was more than 3 times the light compared to the ZnS:Mn baseline. The triboliminescent yield decreases by nearly a factor of 3 from the first to the fifth drop. For comparison purposes, the TL yield from the baseline ZnS:Mn is reduced less than 10 pct from the first to the fifth drop.[3] Due to its organic nature, EuD4TEA appears to be more easily damaged by having most of the TL nearly diminished with approxi-
WILLIAM A. HOLLERMAN, Dr. and Mrs. Sammie W. Cosper/ BOSF Endowed Associate Professor of Physics, Department of Physics, University of Louisiana at Lafayette, Lafayette, LA 70504. Contact email:
[email protected] ROSS S. FONTENOT, Ph.D. Student, KAMALA N. BHAT, Assistant Professor of Chemistry, and MOHAN D. AGGARWAL, Chair and Professor of Physics, Department of Physics, Chemistry, and Mathematics, Alabama A&M University, Normal, AL 35762. Manuscript submitted January 10, 2012. Article published online June 12, 2012 4200—VOLUME 43A, NOVEMBER 2012
mately 10 drops. ZnS:Mn, on the other hand, can withstand over 50 drops.[3] The first EuD4TEA was synthesized by Hurt et al.[12] in 1966. Using this method, it was discovered that triboluminescent emission yield was inversely proportional to the chloride ion concentration, which quenches the triboluminescent emission.[15] In 2010, Fontenot et al. replaced the chloride salt with europium (III) nitrate hexahydrate,[16] which caused the triboluminescent emission yield to increase by 82 pct. The quantity of synthesized EuD4TEA also increased since chloride washing was no longer required.[16] In 2011, Fontenot and co-workers investigated the effect of various solvents used to synthesize EuD4TEA on the triboluminescent emission yield.[17,18] These results showed that the triboluminescent emission yield depends on the solvent used to synthesize the EuD4TEA.[17,18] Samples synthesized with acetone produced the largest triboluminescent yield at impact.[17,18] Surprisingly, the use of 200 Proof ethyl alcohol to synthesize EuD4TEA produced a smaller triboluminescent emission yield.[17] Additional research showed that doping EuD4TEA with dimethyl methylphosphonate[14] or morphine[19] also increases the triboluminescent emission yield, raising the potential of this technology to be the active element for impact sensors.[3,12,13] In spite of these advances, how does the triboluminescent emission yield vary for different batches of this material, if solvent quantity and other chemical processing techniques are held constant? An understanding into any potential yield variance could be significant if EuD4TEA is used as the active element for a mass-produced impact sensor. EuD4TEA will need to be synthesized in large quantities in order to meet the demand generated by its potential use in a sensor. Maintaining consistency between various batches requires quality control of the synthesized materials. Quality control is achieved by determining the batchMETALLURGICAL AND MATERIALS TRANSACTIONS A
to-batch variance between the products. To reduce cost, variations in the triboluminescent emission yield by batch need to be small and reproducible in order for EuD4TEA to be used as a practical impact sensor. This article will estimate the batch-to-batch variance for synthesized EuD4TEA. It will also provide an estimate of the triboluminescent measurement uncertainty for the drop tower, which could be a significant fraction of the overall batch-to-batch variance for the synthesized EuD4TEA.
II.
EXPERIMENTAL PROCEDURE
A. Synthesis of EuD4TEA Crystals Five batches of EuD4TEA were synthesized using acetone and ethyl alcohol, as described in Reference 16. These samples were made over many months using different batches of 99.999 pct europium (III) nitrate hexahydrate from Metall Rare Earth Limited (Sheung Shui, New Territories, Hongkong), and dibenzoylmethane (DBM), triethylamine (TEA), laboratory grade acetone, and 200 Proof ethyl alcohol from SigmaAldrich Corp. (St. Louis, MO). The synthesis began by adding 4 mmol of europium (III) nitrate hexahydrate to 25 mL heated 200 Proof ethyl alcohol. To this hot solution, 13 mmol of DBM was added. Once the DBM and europium were completely dissolved into the solution, 14 mmol of TEA was added. The solution was heated until the mixture was clear and then kept aside to cool slowly overnight at ambient temperature. The EuD4TEA crystals that formed were filtered and air dried at room temperature. This same procedure was followed for the synthesis based with acetone; however; in this case, heating was not required as europium and DBM is highly soluble in acetone.
Fig. 1—Schematic diagram of the specially designed drop tower used to measure the triboluminescent integrated light yield of triboluminescent materials.[14,20]
the plexiglass plate 2.25 cm below the sample. A Melles Griot large dynamic range linear amplifier set to a gain of 200 lA increases the signal amplitude. A Tektronix 2024B oscilloscope records the signal in single sequence mode with a 500-ls measurement time. Once the signal is acquired, it is analyzed using the custom LABVIEW** **LABVIEW is a trademark of National Instruments, Austin, TX.
program that integrates the area under the curve and calculates the decay time for the particular emission.[14,18]
B. Measuring the Triboluminescent Emission Yield The relative triboluminescent emission yield from each EuD4TEA batch was measured using a custom-built drop tower designed and built by the authors, as shown in Figure 1.[4,14,20] The test begins by placing a small 0.1 g pile of each sample material on a PLEXIGLAS* plate. The *PLEXIGLAS is a trademark of Rohm & Haas Company, Wilmington, DE.
III.
RESULTS AND DISCUSSION
Table I shows the measured relative triboluminescent emission yields for the five EuD4TEA trial batches synthesized with both acetone and 200 Proof ethyl alcohol. The relative triboluminescent emission yield (Yrel) for a given trial batch is calculated as Yrel ¼
material is arranged so that it is positioned around the center of the tube with minimum height. A 130 g steel ball is positioned on a pull pin at a distance of 1.1 m (42 in.) above the pile. The pin is pulled and the ball falls, producing TL at impact with the sample material. After each drop, the tube is removed, the ball is cleaned, and the sample powder is redistributed near the center of the target area.[14,18] A total of five drops were completed for a given trial batch to determine a statistical average triboluminescent yield. To determine the triboluminescent yield for a given sample, a United Detector photodiode is positioned under METALLURGICAL AND MATERIALS TRANSACTIONS A
Yt Y1
½1
where Yt is the average triboluminescent emission yield measured over five drops for a given trial batch and Y1 is the average triboluminescent emission yield collected over five drops for trial batch number 1. The individual Yrel values shown in Table I are normalized to the average yields measured for the first trial batch of EuD4TEA synthesized with acetone and ethyl alcohol. In other words, the relative triboluminescent emission yields for the first trial batches of EuD4TEA synthesized with acetone and ethyl alcohol were set to one. VOLUME 43A, NOVEMBER 2012—4201
Table I. Relative Triboluminescent Light Yields (Yrel) for EuD4TEA Synthesized with Ethyl Alcohol and Acetone Yrel for EuD4TEA Made with Two Solvents Trial Batch Number
Ethyl Alcohol
Acetone
1 2 3 4 5 Average
1.000 1.083 1.282 1.238 0.896 1.100 ± 0.072
1.000 0.966 1.276 1.365 1.127 1.147 ± 0.077
Table I also shows the average relative triboluminescent yield and its uncertainty calculated over the five trial batches. This uncertainty was estimated using the standard deviation of the mean from the five individual batch results. The average relative triboluminescent yield over the five batches of EuD4TEA synthesized with ethyl alcohol is 1.10 ± 0.07. The corresponding relative yield for five batches of EuD4TEA synthesized with acetone is 1.15 ± 0.08. As a result, there appears to be less than a 7 pct measured batch-to-batch variance in triboluminescent yield for EuD4TEA. The data also show that EuD4TEA samples synthesized with acetone emit slightly more TL than those made with ethyl alcohol. Keep in mind that this 7 pct batch-to-batch yield variance also includes the measurement uncertainty of the drop tower. We need to now estimate the measurement uncertainty of the drop tower to separate these two sources of experimental error. To determine the measurement uncertainty of the drop tower, five additional TL drops were made using EuD4TEA synthesized in trial batch number 2 with ethyl alcohol. Once each TL measurement was completed, the drop plate was cleaned and a new 0.1 g pile of EuD4TEA synthesized during trial batch number 2 was placed in the center. A total of five drops, each taken with a new 0.1 g sample of trial batch number 2 powder, were completed. The average of the relative triboluminescent emission yield (Yrel) for trial batch number 2 drops was 0.99 ± 0.03. The error in these measurements was estimated using the standard deviation of the mean taken over the five drops. During the initial part of this research, as shown in Table I,Yrel for trial batch number 2 was measured to be 1.08, which is about 9 pct larger than the 0.99 average value measured here. For this reason, the measurement uncertainty for the drop tower appears to be less than about 9 pct, which is similar in magnitude to the 7 pct batch-to-batch yield variance for synthesized EuD4TEA. For this reason, no statistically significant batch-to-batch variations in the triboluminescent emission yield were observed for the synthesized EuD4TEA. Additional research needs to be completed to fully understand these phenomena. IV.
CONCLUSIONS
Europium dibenzoylmethide triethylammonium is one of the brightest known triboluminescent materials 4202—VOLUME 43A, NOVEMBER 2012
and is more than twice as bright compared to inorganic ZnS compounds. Using a custom-built drop tower, the triboluminescent emission yield for EuD4TEA was measured. Results show that the measurement uncertainty of the drop tower was found to be less than 9 pct. Conversely, there was only a 7 pct measured batch-to-batch triboluminescent yield variance. Therefore, no statistically significant batch-to-batch variations in the triboluminescent emission yield were observed for the synthesized EuD4TEA. It is likely that the batch-to-batch variance in triboluminescent emission yields is on the order of a few percent. These preliminary results indicate that chemical processes can be controlled with sufficient precision to synthesize reasonable quantities of EuD4TEA. Research to date has shown that EuD4TEA shows significant promise to be used as the active element for future impact sensors. Additional research is needed to fully quantify these results. ACKNOWLEDGMENTS The authors thank Mrs. Sheral Roberson for helping generate the drop tower diagram used in this article. This research was funded, in part, by the NSFRISE Project HRD 0927644, NASA Alabama Space Grant Consortium fellowship under Training Grant NNX10AJ80H, and other grants from the State of Louisiana and federal agencies. REFERENCES 1. R.S. Fontenot, W.A. Hollerman, and S.M. Goedeke: Mater. Lett., 2011, vol. 65, pp. 1108–10. 2. R.S. Fontenot: Master’s Thesis, University of Louisiana at Lafayette, Lafayette, LA, 2010. 3. W.A. Hollerman, R.S. Fontenot, K.N. Bhat, M.D. Aggarwal, C.J. Guidry, and K.M. Nguyen: Opt. Mater., 2012, vol. 34, pp. 1517– 21. 4. S.M. Goedeke, S.W. Allison, F.N. Womack, N.P. Bergeron, and W.A. Hollerman: ‘‘Tribolumininescence and Its Application to Space-Based Damage Sensors,’’ Technical Report, Engineering Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN, 2003. 5. F. Womack, S. Goedeke, N. Bergeron, W. Hollerman, and S. Allison: IEEE Trans. Nucl. Sci., 2004, vol. 51, pp. 1737–41. 6. W.A. Hollerman, S.M. Goedeke, N.P. Bergeron, C.I. Muntele, S.W. Allison, and D. Ila: Nucl. Instrum. Meth. Phys. Res., 2005, vol. B241, pp. 578–82. 7. W.A. Hollerman, S.M. Goedeke, N.P. Bergeron, R.J. Moore, S.W. Allison, and L.A. Lewis: Photonics for Space Environments X, Society of Photo-Optical Instrumentation Engineers, San Diego, CA, 2005, pp. 5897–15. 8. N.P. Bergeron, W.A. Hollerman, S.M. Goedeke, M. Hovater, W. Hubbs, A. Fichum, R.J. Moore, S.W. Allison, and D.L. Edwards: Int. J. Impact Eng., 2006, vol. 33, pp. 91–99. 9. N.P. Bergeron: Master’s Thesis, University of Louisiana at Lafayette, Lafayette, LA, 2006. 10. N.P. Bergeron, W.A. Hollerman, S.M. Goedeke, and R.J. Moore: Int. J. Impact Eng., 2008, vol. 35, pp. 1587–92. 11. A.J. Walton: Adv. Phys., 1977, vol. 26, pp. 887–948. 12. C. Hurt, N. McAvoy, S. Bjorklund, and N. Fillipescu: Nature, 1966, vol. 212, pp. 179–80. 13. M.D. Aggarwal, B.G. Penn, J. Miller, S. Sadate, and A.K. Batra: ‘‘Triboluminescent Materials for Smart Optical Damage Sensors for Space Applications,’’ Technical Report NASA/TM—2008– 215410, 2008, http://ntrs.nasa.gov/search.jsp?R=20080025731. 14. R.S. Fontenot, K.N. Bhat, W.A. Hollerman, and M.D. Aggarwal: J. Lumin., 2012, vol. 132, pp. 1812–18.
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18. K.N. Bhat, R.S. Fontenot, W.A. Hollerman, and M.D. Aggarwal: Int. J. Chem., 2012, vol. 1, pp. 100–18. 19. X.R. Zeng, R.G. Xiong, X.Z. You, and K.K. Cheung: Inorg. Chem. Comm., 2000, vol. 3, pp. 341–44. 20. R.S. Fontenot, W.A. Hollerman, M.D. Aggarwal, K.N. Bhat, and S.M. Goedeke: Measurement, 2012, vol. 45, pp. 431–36.
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