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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 11 — Jun. 3, 2013
  • pp: 12976–12986
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An intense elastico-mechanoluminescence material CaZnOS:Mn2+ for sensing and imaging multiple mechanical stresses

Jun-Cheng Zhang, Chao-Nan Xu, Sunao Kamimura, Yujin Terasawa, Hiroshi Yamada, and Xusheng Wang  »View Author Affiliations


Optics Express, Vol. 21, Issue 11, pp. 12976-12986 (2013)
http://dx.doi.org/10.1364/OE.21.012976


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Abstract

The elastico-mechanoluminescence (EML) properties of CaZnOS:Mn2+ are investigated. The CaZnOS:Mn2+/epoxy resin composite can simultaneously “feel” (sense) and “see” (image) various types of mechanical stress over a wide energy and frequency range (ultrasonic vibration, impact, friction and compression) as an intense red emission (610 nm) from Mn2+ ions. Further, the accurate linear relation between emission intensity and different stress parameters (intensity, energy and deformation rate) are confirmed. The EML mechanism is explained using a piezoelectrically induced trapped carrier excitation mode. All the results imply that CaZnOS:Mn2+ has potential as a stress probe to sense and image multiple mechanical stresses and decipher the stress intensity distribution.

© 2013 OSA

1. Introduction

The sense of touch of human beings can respond to skin surface mechanical contact and stimuli, including touch, impact, vibration, friction and compression, and can detect the corresponding stress distribution. As the level of sophistication and performance of smart skin technology and humanoid robots increases, great effort has been made to obtain stress sensors that can perceive contact, motion, shape and texture [1

1. O. S. Wolfbeis, “Sensor paints,” Adv. Mater. 20(19), 3759–3763 (2008). [CrossRef]

4

4. H. Yousefa, M. Boukallela, and K. Althoeferb, “Tactile sensing for dexterous in-hand manipulation in robotics-A review,” Sens. Actuators A Phys. 167(2), 171–187 (2011). [CrossRef]

].

Currently stress sensors can be divided into two categories according to the stress feedback signal, i.e. electrical signal and optical signal [4

4. H. Yousefa, M. Boukallela, and K. Althoeferb, “Tactile sensing for dexterous in-hand manipulation in robotics-A review,” Sens. Actuators A Phys. 167(2), 171–187 (2011). [CrossRef]

]. One conventional method that measures and analyzes the electrical characteristics of magneto-resistive, piezoelectric or strain sensitive conducting materials and structures is based on electrodes and physical contact [5

5. Y. Tadesse, S. Priya, H. Stephanou, D. Popa, and D. Hanson, “Piezoelectric actuation and sensing for facial robotics,” Ferroelectrics 345(1), 13–25 (2006). [CrossRef]

7

7. L. Ascari, P. Corradi, L. Beccai, and C. Laschi, “A miniaturized and flexible optoelectronic sensing system for tactile skin,” J. Micromech. Microeng. 17(11), 2288–2298 (2007). [CrossRef]

]. Thus stress sensing is restricted to single point measurements, which is time consuming while producing data with limited spatial resolution, in practice limited to areas of maximally 5 * 5 cm and to planar surfaces [1

1. O. S. Wolfbeis, “Sensor paints,” Adv. Mater. 20(19), 3759–3763 (2008). [CrossRef]

]. Furthermore, it is difficult to measure the stress distribution of a dynamic moving part due to the requirement for physical contact. Optical imaging techniques have been proposed in recent years in order to solve these critical problems. One type of stress sensor using optical data images the contact stress distribution by calculation from the change in shape of the deformable sensor surface obtained using a camera [8

8. N. J. Ferrier and R. W. Brockett, “Reconstructing the shape of a deformable membrane from image data,” Int. J. Robot. Res. 19(9), 795–816 (2000). [CrossRef]

]. However, the analysis is complex and time consuming. Another vision-based approach takes advantage of light emission from photoluminescent [9

9. M. I. J. Stich, S. Nagl, O. S. Wolfbeis, U. Henne, and M. Schaeferling, “A dual luminescent sensor material for simultaneous imaging of pressure and temperature on surfaces,” Adv. Funct. Mater. 18(9), 1399–1406 (2008). [CrossRef]

], electroluminescent [10

10. V. Maheshwari and R. F. Saraf, “High-resolution thin-film device to sense texture by touch,” Science 312(5779), 1501–1504 (2006). [CrossRef] [PubMed]

], piezochromic luminescent [11

11. Y. Sagara and T. Kato, “Mechanically induced luminescence changes in molecular assemblies,” Nat. Chem. 1(8), 605–610 (2009). [CrossRef] [PubMed]

], piezo-phototronic [12

12. Z. L. Wang, “Piezopotential gated nanowire devices: Piezotronics and piezo-phototronics,” Nano Today 5(6), 540–552 (2010). [CrossRef]

], mechanoluminescent (ML) [13

13. J. S. Kim, Y. N. Kwon, and K. S. Sohn, “Dynamic visualization of crack propagation and bridging stress using the mechano-luminescence of SrAl2O4: (Eu,Dy,Nd),” Acta Mater. 51(20), 6437–6442 (2003). [CrossRef]

15

15. C. N. Xu, X. G. Zheng, M. Akiyama, K. Nonaka, and T. Watanabe, “Dynamic visualization of stress distribution mechanoluminescence image,” Appl. Phys. Lett. 76(2), 179–181 (2000). [CrossRef]

] and elastico-mechanoluminescent (EML) [16

16. C. N. Xu, Smart Materials (Wiley, 2002), Vol.1.

] materials and devices. High resolution luminescent images can be efficiently obtained and easily manipulated.

EML materials play a significant role in converting the local contact force into a measurable luminescence signal when mechanical energy is applied [17

17. B. P. Chandra, Y. Rahangdale, M. Ramrakhiani, M. H. Aansari, and Y. K. Sharma, “Satatistical model of mechanoluminescence in crystals,” Cryst. Res. Technol. 27(4), 561–568 (1992). [CrossRef]

]. This type of material has attracted considerable interest due to their potential application as stress sensors and in health monitoring since the EML properties of SrAl2O4:Eu2+ (green) and ZnS:Mn2+ (yellow) were first reported in 1999 [18

18. C. N. Xu, T. Watanabe, M. Akiyama, and X. G. Zheng, “Direct view of stress distribution in solid by mechanoluminescence,” Appl. Phys. Lett. 74(17), 2414–2416 (1999). [CrossRef]

,19

19. C. N. Xu, T. Watanabe, M. Akiyama, and X. G. Zheng, “Artificial skin to sense mechanical stress by visible light emission,” Appl. Phys. Lett. 74(9), 1236–1238 (1999). [CrossRef]

]. To date more than ten kinds of inorganic materials exhibiting excellent EML performance have been successfully developed. These inorganic materials are common silicate, titanate, aluminate and phosphate systems, including Ca2Al2SiO7:Ce3+ (blue) [20

20. M. Akiyama, C. N. Xu, H. Matsui, K. Nonaka, and T. Watanabe, “Recovery phenomenon of mechanoluminescence from Ca2Al2SiO7:Ce by irradiation with ultraviolet light.,” Appl. Phys. Lett. 75(17), 2548–2550 (1999). [CrossRef]

], BaTiO3-CaTiO3:Pr3+ (red) [21

21. X. Wang, C. N. Xu, H. Yamada, K. Nishikubo, and X. G. Zheng, “Electro-mechano-optical conversions in Pr3+-doped BaTiO3-CaTiO3 ceramics,” Adv. Mater. 17(10), 1254–1258 (2005). [CrossRef]

], SrAl2O4:Ce3+(Ho3+) (ultraviolet) [22

22. H. W. Zhang, H. Yamada, N. Terasaki, and C. N. Xu, “Ultraviolet mechanoluminescence from SrAl2O4:Ce and SrAl2O4:Ce,Ho,” Appl. Phys. Lett. 91(8), 081905 (2007). [CrossRef]

], CaMgSi2O7:Eu2+(Dy3+) (green) [23

23. H. W. Zhang, H. Yamada, N. Terasaki, and C. N. Xu, “Green mechanoluminescence of Ca2MgSi2O7:Eu and Ca2MgSi2O7:Eu,Dy,” J. Electrochem. Soc. 155(2), J55–J57 (2008). [CrossRef]

], CaAl2Si2O8:Eu2+ (blue) [24

24. L. Zhang, H. Yamada, Y. Imai, and C. N. Xu, “Observation of elasticoluminescence from CaAl2Si2O8:Eu2+ and its water resistance behavior,” J. Electrochem. Soc. 155(3), J63–J65 (2008). [CrossRef]

], CaYAl3O7:Eu2+ (blue) [25

25. H. W. Zhang, H. Yamada, N. Terasaki, and C. N. Xu, “Blue light emission from stress-activated CaYAl3O7:Eu,” J. Electrochem. Soc. 155(5), J128–J131 (2008). [CrossRef]

] and SrMg2(PO4)2:Eu2+ (purple) [26

26. S. Kamimura, H. Yamada, and C. N. Xu, “Development of new elasticoluminescent material SrMg2(PO4)2:Eu,” J. Lumin. 132(2), 526–530 (2012). [CrossRef]

]. Recently, blue-green emitting oxynitride phosphor BaSi2O2N2:Eu2+ became a new member of the EML family [27

27. L. Zhang, C. N. Xu, and H. Yamada, “Strong mechanoluminescence from oxynitridosilicate phosphors,” Mater. Sci. Eng. 18(15), 212001 (2011).

,28

28. J. Botterman, K. V. D. Eeckhout, I. D. Baere, D. Poelman, and P. F. Smet, “Mechanoluminescence in BaSi2O2N2:Eu,” Acta Mater. 60(15), 5494–5500 (2012). [CrossRef]

]. For the practical application in the field of touch sensors, the EML materials are required to have the characteristics of the intense EML and the ability to sense multi-stress at the same time. To the best of our knowledge, however, only SrAl2O4:Eu2+, one of the most promising EML material, has been reported to demonstrate a prompt and intense luminescent response to several kinds of stress stimulus, such as compression [18

18. C. N. Xu, T. Watanabe, M. Akiyama, and X. G. Zheng, “Direct view of stress distribution in solid by mechanoluminescence,” Appl. Phys. Lett. 74(17), 2414–2416 (1999). [CrossRef]

], stretching [29

29. C. Li, C. N. Xu, Y. Imai, and N. Bu, “Real-time visualisation of the Portevin-Le Chatelier effect with mechanoluminescent-sensing film,” Strain 47(6), 483–488 (2011). [CrossRef]

], bending [14

14. J. S. Kim, Y. N. Kwon, N. Shin, and K. S. Sohn, “Visualization of fractures in alumina ceramics by mechanoluminescence,” Acta Mater. 53(16), 4337–4343 (2005). [CrossRef]

], impact [30

30. X. Fu, H. Yamada, and C. N. Xu, “Property of highly oriented SrAl2O4:Eu film on quartz glass substrates and its potential application in stress sensor,” J. Electrochem. Soc. 156(9), J249–J252 (2009). [CrossRef]

], friction [30

30. X. Fu, H. Yamada, and C. N. Xu, “Property of highly oriented SrAl2O4:Eu film on quartz glass substrates and its potential application in stress sensor,” J. Electrochem. Soc. 156(9), J249–J252 (2009). [CrossRef]

,31

31. N. Terasaki and C. N. Xu, “Mechanoluminescence recording device integrated withphotosensitive material and europium-doped SrAl2O4,” Jpn. J. Appl. Phys. 48(4), 04C150 (2009).

], torsion [16

16. C. N. Xu, Smart Materials (Wiley, 2002), Vol.1.

] and ultrasonic vibration [32

32. T. Zhan, C. N. Xu, O. Fukuda, H. Yamada, and C. Li, “Direct visualization of ultrasonic power distribution using mechanoluminescent film,” Ultrason. Sonochem. 18(1), 436–439 (2011). [CrossRef] [PubMed]

]. Moreover, as the above mentioned, most of EML materials including SrAl2O4:Eu2+ are doped rare-earth elements as the luminescent centers. It is well known that the rapid growth in applications of electronic products that utilize rare-earth elements will lead to their scarcity in the near future with corresponding increases in their prices. Therefore, there is an urgent need to develop novel EML materials with strong luminance and high sensitivity to multi-stress, but without doping rare-earth elements.

In recent years, with the development of the light emitting diode (LED) industry, alkaline earth sulfide and oxysulfide phosphors, such as CaS:Eu2+ (red) [33

33. K. N. Kim, J. M. Kim, K. J. Choi, J. K. Park, and C. H. Kim, “Synthesis, characterization, and luminescent properties of CaS:Eu phosphor,” J. Am. Ceram. Soc. 89(11), 3413–3416 (2006). [CrossRef]

], SrS:Eu2+ (orange) [34

34. C. Chartier, C. Barthou, P. Benalloul, and J. M. Frigerio, “Bandgap energy of SrGa2S4:Eu2+ and SrS:Eu2+,” Electrochem. Solid-State Lett. 9(2), G53–G55 (2006). [CrossRef]

], SrLaGa3S6O:Eu2+ (yellowish-green) [35

35. X. Zhang, J. Zhang, J. Xu, and Q. Su, “Luminescent properties of Eu2+-activated SrLaGa3S6O phosphor,” J. Alloy. Comp. 389(1–2), 247–251 (2005). [CrossRef]

], CaZnOS:Mn2+ (red) [36

36. C. J. Duan, A. C. A. Delsing, and H. T. Hintzen, “Photoluminescence properties of novel red-Emitting Mn2+-activated MZnOS (M=Ca,Ba) phosphors,” Chem. Mater. 21(6), 1010–1016 (2009). [CrossRef]

], CaZnOS:Eu2+ (red) [37

37. T. W. Kuo, W. R. Liu, and T. M. Chen, “High color rendering white light-emitting-diode illuminator using the red-emitting Eu2+-activated CaZnOS phosphors excited by blue LED,” Opt. Express 18(8), 8187–8192 (2010). [CrossRef] [PubMed]

] and Sr8Al12O24S2:Eu2+ (yellowish-orange) [38

38. T. W. Kuo, C. H. Huang, and T. M. Chen, “Novel yellowish-orange Sr8Al12O24S2:Eu2+ phosphor for application in blue light-emitting diode based white LED,” Opt. Express 18(13S2), A231–A236 (2010). [CrossRef] [PubMed]

], have received increasing attention because of their strong absorption in the blue region. However, there has been no investigation of their mechanoluminescence and EML properties except for ZnS:Mn2+ [19

19. C. N. Xu, T. Watanabe, M. Akiyama, and X. G. Zheng, “Artificial skin to sense mechanical stress by visible light emission,” Appl. Phys. Lett. 74(9), 1236–1238 (1999). [CrossRef]

], ZnS:MnTe [39

39. N. M. Rao, D. R. Reddy, B. K. Reddy, and C. N. Xu, “Intense red mechanoluminescence from (ZnS)1-x(MnTe)x,” Phys. Lett. A 372(22), 4122–4126 (2008). [CrossRef]

,40

40. T. Toriyi, Y. Adachi, H. Yamada, Y. Imai, and C. N. Xu, “Enhancement of mechanoluminescence from ZnS:Mn,Te by wet process,” Key Eng. Mater. 388, 301–304 (2009). [CrossRef]

] and ZnS:Cu [41

41. A. Tiwari, S. A. Khan, R. S. Kher, M. Mehta, and S. J. Dhoble, “Effect of capping on the mechanoluminescence of γ-irradiated ZnS:Cu nanophosphors,” J. Lumin. 131(6), 1172–1176 (2011). [CrossRef]

]. Recently the multi-stress sensitive EML characteristics of CaZnOS:Mn2+ phosphor were for the first time discovered in our laboratory. The polar compound CaZnOS was first prepared by Petrova et al. in 2003 [42

42. S. A. Petrova, V. P. Mar’evich, R. G. Zakharov, E. N. Selivanov, V. M. Chumarev, and Y. L. Udoeva, “Crystal structure of zinc calcium oxysulfide,” Dokl. Chem. 393(1–3), 255–258 (2003). [CrossRef]

] and its structure and physical properties were investigated in detail by Clarke et al. in 2007 [43

43. T. Sambrook, C. F. Smura, S. J. Clarke, K. M. Ok, and P. S. Halasyamani, “Structure and physical properties of the polar oxysulfide CaZnOS,” Inorg. Chem. 46(7), 2571–2574 (2007). [CrossRef] [PubMed]

]. Because of its chemical and thermal stability CaZnOS is considered to be good host material for Mn2+ luminescence. Subsequently the photoluminescence properties of CaZnOS:Mn2+ phosphor were reported by Hintzen et al. in 2009 [36

36. C. J. Duan, A. C. A. Delsing, and H. T. Hintzen, “Photoluminescence properties of novel red-Emitting Mn2+-activated MZnOS (M=Ca,Ba) phosphors,” Chem. Mater. 21(6), 1010–1016 (2009). [CrossRef]

]. In this work we have successfully developed rare-earth-ion free CaZnOS:Mn2+ with excellent EML characteristics. The EML indicator CaZnOS:Mn2+ particles were encapsulated into an optically transparent epoxy resin matrix for test purposes. This material can sense various types of mechanical stress, including ultrasonic vibration, impact, friction and compression by the intense EML emission. Moreover, an accurate linear relation between the emission intensity and different stress parameters (such as intensity, energy, and deformation rate) was determined. The EML mechanism is further discussed with respect to the crystal structure and trap levels. All of the results indicate that the investigated CaZnOS:Mn2+ material has great potential as a stress intensity and distribution sensor.

2. Experimental

Polycrystalline phosphors with the composition CaZn1-xMnxOS (0≤x≤0.10) were synthesized using a solid state reaction. The raw materials CaCO3, ZnS and MnCO3 (99.99%) were thoroughly ground, pressed into pellets, and subsequently sintered at 1100 °C for 3 h in an argon atmosphere. The CaZnOS:Mn2+ product was ground and screened through a 20 μm sieve. The EML sensing film with a thickness of 200 μm were prepared by mixing the CaZnOS:Mn2+ screened powder and an optical epoxy resin at a weight ratio of 1:1. For EML characterization under friction and pressure the film was prepared on a plastic disk 25 mm in diameter and 15 mm thick.

The sample crystallization behavior was examined by X-ray diffraction (XRD) (Rint2000, Rigaku Corp.) and field emission scanning electron microscopy (FESEM) using an energy dispersive X-ray spectrometer (JSM-7001F, JEOL Ltd.). Photoluminescence (Photo-L) was recorded with a fluorescence spectrometer (FP-6600, Jasco Co.). Thermoluminescence (ThL) was measured by combining the above mentioned fluorescence spectrometer with an in-house made temperature control unit.

The ultrasonic vibration excited luminescence (Ultrasonic-L) intensity was measured using a photomultiplier tube (R649, Hamamatsu Photonics K.K.) and a photoncounter (C9692, Hamamatsu Photonics K.K.). Ultrasonic-L images were recorded using an intensified charge-coupled device (ICCD) camera (FocusScope SV200-i, Photron Corp.). Ultrasonic waves were generated by a 20 MHz single element piezoelectric transducer (NDK Corp.) and an ultrasonic pulser-receiver (Panametrics-NDT 5072PR, Olympus Corp.). The impact stress-excited luminescence (Impact-L) was excited by a free-falling ZrO2 ball (18 mg). The impact energy could be adjusted from 0 to 54 mJ. The friction-excited luminescence (Tribo-L) was excited using an in-house made friction machine. The compression stress-excited luminescence (Compress-L) was excited with a universal testing machine (RTC-1310A, Orientec Corp.). The Impact-L, Tribo-L and Compress-L intensities were measured using the same photon-counting system that consists of a photomultiplier tube (R649, Hamamatsu Photonics K.K.) and a photoncounter (C3866, Hamamatsu Photonics K.K.). The Ultrasonic-L, Impact-L, Tribo-L and Compress-L measurements were all repeated five times to reduce error. All of the relative standard deviations were less than 10%. The mean values were used for linear fitting. The images and spectra of Impact-L, Tribo-L and Compress-L were recorded by a Nikon D3 camera with a Nikkor 50 mm f1.2 lense and a photon multi-channel analyzer system (PMA-100, Hamamatsu Photonics K.K.). Before measurements of the EML intensity and spectra the specimens were irradiated with 254 nm UV light for 1 min. All measurements except ThL were performed at room temperature.

3. Results and discussion

3.1 Structure, composition and morphology characterization

The XRD patterns of the CaZn1-xMnxOS (0≤x≤0.10) samples were investigated. Figure 1(a)
Fig. 1 (a) XRD patterns of the CaZnOS and CaZn0.997Mn0.003OS samples. (b) EDS spectrum of the CaZn0.997Mn0.003OS pellet sample. (Inset) An FESEM picture of the CaZn0.997Mn0.003OS pellet sample.
shows the XRD patterns of the CaZnOS and CaZn0.997Mn0.003OS samples and the standard PDF card for CaZnOS for comparison. The XRD results reveal that the synthesized CaZn1-xMnxOS (0≤x≤0.10) was composed of a main CaZnOS phase combined with a small amount of impurities, i.e. CaO and CaS. Energy dispersive X-ray spectrometry (EDS) data for the CaZn0.997Mn0.003OS sample depicted in Fig. 1(b) confirm that it is composed of Ca, Zn, O and S. Mn was not detected due to its low content (0.3 at.%). FESEM observation showed that the size of the CaZn0.997Mn0.003OS particles was approximately 1-4 μm and these grains tightly bound to each other, as shown in the inset of Fig. 1(b).

3.2 Elastico-mechanoluminescence (EML) for sensing and imaging multiple mechanical stresses

A multiple stress sensitive film was successfully prepared by the homogeneous dispersion of EML indicator CaZnOS:Mn2+ particles into an optical epoxy resin matrix, as shown in Fig. 2
Fig. 2 Schematic representation of the EML material for sensing and imaging various mechanical stresses.
. The CaZnOS:Mn2+ particles in this film are like a neuron which responds to external stimuli, while the epoxy resin matrix serves as a packaging material to retain the network of neurons, and transmits external mechanical contacts to the CaZnOS:Mn2+ particles. When the stress-sensitive film was subjected to various mechanical stresses (such as ultrasonic vibration, impact, friction and compression) the local contact stress was converted into a measurable EML signal by the CaZnOS:Mn2+ particles, thus the EML image and the luminescence signal could be recorded by both the digital camera and photomultiplier tube (PMT) detector. The recorded EML (including ultrasonic-L, Impact-L, Tribo-L and Compress-L) intensities are modulated on the application of various mechanical stresses to the film, which results in a stress distribution image. This EML film, just like skin, could be distributed over a model surface with an arbitrary shape. Its nondestructive and non-invasive characteristics indicate promising potential as touch sensors or smart skin in a complex structure.

Figures 3
Fig. 3 (a) Ultrasonic-L response signal induced by an ultrasonic output power of 2.94 mW. (Inset) Ultrasonic-L image recorded during ultrasonic vibration. (b) Relationship between Ultrasonic-L intensity and ultrasonic output power.
-6 show the Ultrasonic-L, Impact-L, Tribo-L and Compress-L behaviors, respectively, of CaZnOS:Mn2+ (0.3 at.% Mn2+) under four typical kinds of stress excitation.

Ultrasonic-L was excited by an applied ultrasonic fountain used for medical applications at a frequency of 20 MHz with an output power of no more than 2.94 mW. As shown in Fig. 3(a), an apparent luminescence signal was immediately measured when medical ultrasonic vibration was applied on the film. The Ultrasonic-L intensity increased sharply with increasing time of applied ultrasonic vibration. When the ultrasonic power was turned off the EML luminescence was rapidly attenuated. The Ultrasonic-L image could be directly recorded by an ICCD gray scale camera, although the power of the ultrasonic fountain was weak. A circular luminescent region was observed in the center where the ultrasonic power was applied [inset in Fig. 3(a)]. The diameter of the circular luminescent region was about 4 mm, which is consistent with that of the piezoelectric ceramic patch. The quantitative relationship between Ultrasonic-L intensity and ultrasonic output power is shown in Fig. 3(b). The Ultrasonic-L intensity increased linearly, proportional to the output power of the applied ultrasonic fountain, which implies that the CaZnOS:Mn2+ film can sense ultrasonic vibration and at the same time evaluate the corresponding ultrasonic work power. This proposed technique has potential in low intensity ultrasonic measurements on biological tissues in medical applications. It should be noted that a more intense Ultrasonic-L is obtained when ultrasonic vibrations of greater power (40 kHz, 4 W cm−2) are applied to the film, levels usually used in glass and ceramic vessel cleaning. These results indicate that this EML material may sense ultrasonic vibration with a higher power and wider frequency range.

Figure 4(a)
Fig. 4 (a) Impact-L intensity response induced by an impact energy of 28.2 mJ. (Inset) The Impact-L image. (b) Relationship between Impact-L intensity and impact energy.
illustrates the strong Impact-L response of the CaZnOS:Mn2+ film, in which only one luminescence pulse is obtained corresponding to a single impact. When a ball freely drops onto the CaZnOS:Mn2+ film the Impact-L intensity initially increases with time, until it attains a peak value. The fitting calculation indicates that the decay time of Impact-L is less than 2.3 ms after the collision. The inset in Fig. 4(a) shows the corresponding Impact-L image, in which an intense red gleam is observed. The relationship between Impact-L intensity and impact energy (0-54 mJ) is illuminated in Fig. 4(b). It is clear that the Impact-L intensity increases linearly with impact energy. Correspondingly, the mechanical impact can be remotely monitored using this EML material through detecting the Impact-L intensity.

In addition to successfully sensing ultrasonic vibration and impact energy, the CaZnOS:Mn2+ film can also respond to friction (shear stress). A strong red Tribo-L is emitted in the contact position between the friction rod and film, and no EML trace line was observed by the naked eye [Figs. 5(a)
Fig. 5 (a) and (b) Tribo-L images, and (c) the Tribo-L intensity response induced by mechanical friction with a transparent resin rod of 5 mm diameter. d) Dependence of the Tribo-L intensity on load.
Fig. 6 (a) and (b) Compress-L images with a compressive stress on the CaZnOS:Mn2+ composite disk. (c) Three-dimensional representation of the Compress-L intensity distribution in (b). (d) Compress-L behavior during the reapplication of load. (e) Influence of the deformation rate on the Compress-L intensity.
and 5(b)], i.e. there is no Tribo-L hysteresis phenomenon for CaZnOS:Mn2+, which is different from SrAl2O4:Eu2+ [30

30. X. Fu, H. Yamada, and C. N. Xu, “Property of highly oriented SrAl2O4:Eu film on quartz glass substrates and its potential application in stress sensor,” J. Electrochem. Soc. 156(9), J249–J252 (2009). [CrossRef]

,31

31. N. Terasaki and C. N. Xu, “Mechanoluminescence recording device integrated withphotosensitive material and europium-doped SrAl2O4,” Jpn. J. Appl. Phys. 48(4), 04C150 (2009).

] and BaSi2O2N2:Eu2+ [28

28. J. Botterman, K. V. D. Eeckhout, I. D. Baere, D. Poelman, and P. F. Smet, “Mechanoluminescence in BaSi2O2N2:Eu,” Acta Mater. 60(15), 5494–5500 (2012). [CrossRef]

]. This performance is more suitable for the measurement of dynamic stress distribution. Figure 5(c) shows the Tribo-L intensity response. When friction is applied the Tribo-L intensity increases steeply and attains a peak value, then a periodic oscillation is observed, after which the luminescence intensity gradually decreases as the friction is maintained. The Tribo-L response immediately disappears when the friction force is turned off. The oscillation of Tribo-L has been shown to originate from inadequacies in the machine alignment and the non-uniformity of the sample setting [30

30. X. Fu, H. Yamada, and C. N. Xu, “Property of highly oriented SrAl2O4:Eu film on quartz glass substrates and its potential application in stress sensor,” J. Electrochem. Soc. 156(9), J249–J252 (2009). [CrossRef]

]. Figure 5(d) shows, as expected, that the Tribo-L intensity increases linearly with an increase in the applied friction force, indicating that mechanical friction can also be monitored by the CaZnOS:Mn2+ film.

3.3 Elastico-mechanoluminescence (EML) mechanism of CaZnOS:Mn2+

To elucidate the emission features of luminescence induced by different stresses Impact-L, Tribo-L, Compress-L and Photo-L spectra of CaZnOS:Mn2+ were measured and are compared in Fig. 7
Fig. 7 Impact-L, Tribo-L, Compress-L and Photo-L spectra of CaZnOS:Mn2+.
. Only one broad emission band peaking at about 610 nm, originating from the 4T1(4G) → 6A1(6S) transition of Mn2+ ions, can be observed in each of these EML spectra, which is the same process as is responsible for Photo-L [36

36. C. J. Duan, A. C. A. Delsing, and H. T. Hintzen, “Photoluminescence properties of novel red-Emitting Mn2+-activated MZnOS (M=Ca,Ba) phosphors,” Chem. Mater. 21(6), 1010–1016 (2009). [CrossRef]

]. Furthermore, no red emission was detected in the sample CaZnOS without Mn2+ doping when different stresses were applied. The result also confirms that the Mn2+ ions is the luminescence centers of EML in CaZnOS:Mn2+.

The above results suggest that the CaZnOS:Mn2+ material can simultaneously respond to various mechanical stresses, including ultrasonic vibration, impact, friction and compressive stress, and also indicate the existence of these applied stresses by the intense red light emission from Mn2+ ions. The response to the external stimuli, i.e. EML intensity, is linear with respect to the stress magnitude, energy and deformation rate. This EML material could be coated onto a complex structure to visually reflect and precisely diagnose the dynamic stress and stress distribution as a non-destructive and non-invasive touch sensor, smart skin, or self-diagnosis system.

Now we will discuss the possible principle of the excellent EML properties of the CaZnOS:Mn2+ material. From Figs. 5(c) and 6(c) it is apparent that the Tribo-L and Compress- L intensities decayed during cycles of friction and compression, respectively. This was also the case in the Ultrasonic-L and Impact-L experiments, i.e. the Ultrasonic-L and Impact-L intensities decreased with cycles of ultrasonic vibration and impact, respectively. Interestingly, the EML intensity of CaZnOS:Mn2+ recovered completely after irradiation with 254 nm UV light for 1 min, i.e. EML of CaZnOS:Mn2+ is reproducible. Therefore, CaZnOS:Mn2+ belongs to the so-called defect-controlled group of materials [16

16. C. N. Xu, Smart Materials (Wiley, 2002), Vol.1.

], in which deep energy traps can be filled by UV light irradiation.

In order to provide evidence for deep energy traps the ThL curves of CaZnOS:Mn2+ irradiated with 254 nm UV light for 1 min were measured at four different heating rates, 10, 30, 60 and 90 °C min–1, the results of which are shown in Fig. 8
Fig. 8 Thermoluminescence (ThL) curves of CaZnOS:Mn2+ at the heating rates of 10, 30, 60 and 90 °C min−1.
. There are three peaks in the investigated temperature region (−193 °C to 200 °C), proving the existence of deep energy traps. Among them two peaks (peaks 1 and 2) occurred above room temperature. Based on the Hoogenstraaten method [46

46. W. Hoogenstraaten, “Electron traps in zinc-sulfide phosphors,” Philips Res. Rep. 13(6), 515–693 (1958).

], the depths of these traps were about 0.18 (corresponding to the peak below 0 °C), 0.58 (peak 1) and 0.67 (peak 2) eV, respectively. The trap corresponding to the peaks below 0 °C will be emptied at room temperature because of the shallow depth, while the traps corresponding to peaks 1 and 2 with suitable trap depths could not be thermally activated at room temperature, and will be responsible for the reproducible EML phenomenon. Moreover, the trap levels corresponding to peaks 1 and 2 in the CaZnOS:Mn2+ sample are much higher than that of the SrAl2O4:Eu2+ sample (0.2 ± 0.1 eV) prepared by a solid-state reaction [18

18. C. N. Xu, T. Watanabe, M. Akiyama, and X. G. Zheng, “Direct view of stress distribution in solid by mechanoluminescence,” Appl. Phys. Lett. 74(17), 2414–2416 (1999). [CrossRef]

]. This is the reason why CaZnOS:Mn2+ gives rise to a much shorter EML hysteresis. Due to the existence of CaO and CaS impurity phases in the CaZnOS:Mn2+ material investigated lattice defects associated with Ca, O and S vacancies will form during the high temperature synthesis process. These lattice defects can act as traps of carriers (holes or electrons) in the band gap of CaZnOS:Mn2+. Thus the decrease in EML intensity with reapplication of a stress could be attributed to detrapping of these trapped carriers.

4. Conclusions

In summary, excellent multi-stress sensitive properties of intense Ultrasonic-L, Impact-L, Tribo-L and Compress-L were found for the CaZnOS:Mn2+ material. This stress sensitive EML material can immediately sense and image multiple mechanical stresses due to the 4T1(4G) → 6A1(6S) transition of Mn2+ ions producing strong red emission. Moreover, linear relationships between emission intensity and different stress parameters (such as intensity, energy and deformation rate) were confirmed, which suggests that stress intensity and distribution can be determined from the EML intensity of the CaZnOS:Mn2+ material investigated. The crystal structure and ThL results indicate that the EML properties of CaZnOS:Mn2+ originate from its entirely non-centrosymmetric structure and the existence of carrier traps of appropriate depth. These findings will pave the way to developing novel multi-stress-sensitive EML materials and extend their practical applications.

Acknowledgments

This work was partially supported by the CREST program of JST and the State Scholarship Fund of China (Grant No. 2010626112). We gratefully acknowledge the researchers and technical staff at AIST, particularly Dr. Lin Zhang, Dr. Chenshu Li, Dr. Tianzhuo Zhan, Prof. Nao Terasaki, Ms. Etsuko Kawasaki, for assisting with this work.

References and links

1.

O. S. Wolfbeis, “Sensor paints,” Adv. Mater. 20(19), 3759–3763 (2008). [CrossRef]

2.

V. Maheshwari and R. Saraf, “Tactile devices to sense touch on a par with a human finger,” Angew. Chem. Int. Ed. Engl. 47(41), 7808–7826 (2008). [CrossRef] [PubMed]

3.

P. Brochu and Q. Pei, “Advances in dielectric elastomers for actuators and artificial muscles,” Macromol. Rapid Commun. 31(1), 10–36 (2010). [CrossRef] [PubMed]

4.

H. Yousefa, M. Boukallela, and K. Althoeferb, “Tactile sensing for dexterous in-hand manipulation in robotics-A review,” Sens. Actuators A Phys. 167(2), 171–187 (2011). [CrossRef]

5.

Y. Tadesse, S. Priya, H. Stephanou, D. Popa, and D. Hanson, “Piezoelectric actuation and sensing for facial robotics,” Ferroelectrics 345(1), 13–25 (2006). [CrossRef]

6.

Y. J. Yang, M. Y. Cheng, W. Y. Chang, L. C. Tsao, S. A. Yang, W. P. Shih, F. Y. Chang, S. H. Chang, and K. C. Fan, “An integrated flexible temperature and tactile sensing array using PI-copper films,” Sens. Actuators A Phys. 143(1), 143–153 (2008). [CrossRef]

7.

L. Ascari, P. Corradi, L. Beccai, and C. Laschi, “A miniaturized and flexible optoelectronic sensing system for tactile skin,” J. Micromech. Microeng. 17(11), 2288–2298 (2007). [CrossRef]

8.

N. J. Ferrier and R. W. Brockett, “Reconstructing the shape of a deformable membrane from image data,” Int. J. Robot. Res. 19(9), 795–816 (2000). [CrossRef]

9.

M. I. J. Stich, S. Nagl, O. S. Wolfbeis, U. Henne, and M. Schaeferling, “A dual luminescent sensor material for simultaneous imaging of pressure and temperature on surfaces,” Adv. Funct. Mater. 18(9), 1399–1406 (2008). [CrossRef]

10.

V. Maheshwari and R. F. Saraf, “High-resolution thin-film device to sense texture by touch,” Science 312(5779), 1501–1504 (2006). [CrossRef] [PubMed]

11.

Y. Sagara and T. Kato, “Mechanically induced luminescence changes in molecular assemblies,” Nat. Chem. 1(8), 605–610 (2009). [CrossRef] [PubMed]

12.

Z. L. Wang, “Piezopotential gated nanowire devices: Piezotronics and piezo-phototronics,” Nano Today 5(6), 540–552 (2010). [CrossRef]

13.

J. S. Kim, Y. N. Kwon, and K. S. Sohn, “Dynamic visualization of crack propagation and bridging stress using the mechano-luminescence of SrAl2O4: (Eu,Dy,Nd),” Acta Mater. 51(20), 6437–6442 (2003). [CrossRef]

14.

J. S. Kim, Y. N. Kwon, N. Shin, and K. S. Sohn, “Visualization of fractures in alumina ceramics by mechanoluminescence,” Acta Mater. 53(16), 4337–4343 (2005). [CrossRef]

15.

C. N. Xu, X. G. Zheng, M. Akiyama, K. Nonaka, and T. Watanabe, “Dynamic visualization of stress distribution mechanoluminescence image,” Appl. Phys. Lett. 76(2), 179–181 (2000). [CrossRef]

16.

C. N. Xu, Smart Materials (Wiley, 2002), Vol.1.

17.

B. P. Chandra, Y. Rahangdale, M. Ramrakhiani, M. H. Aansari, and Y. K. Sharma, “Satatistical model of mechanoluminescence in crystals,” Cryst. Res. Technol. 27(4), 561–568 (1992). [CrossRef]

18.

C. N. Xu, T. Watanabe, M. Akiyama, and X. G. Zheng, “Direct view of stress distribution in solid by mechanoluminescence,” Appl. Phys. Lett. 74(17), 2414–2416 (1999). [CrossRef]

19.

C. N. Xu, T. Watanabe, M. Akiyama, and X. G. Zheng, “Artificial skin to sense mechanical stress by visible light emission,” Appl. Phys. Lett. 74(9), 1236–1238 (1999). [CrossRef]

20.

M. Akiyama, C. N. Xu, H. Matsui, K. Nonaka, and T. Watanabe, “Recovery phenomenon of mechanoluminescence from Ca2Al2SiO7:Ce by irradiation with ultraviolet light.,” Appl. Phys. Lett. 75(17), 2548–2550 (1999). [CrossRef]

21.

X. Wang, C. N. Xu, H. Yamada, K. Nishikubo, and X. G. Zheng, “Electro-mechano-optical conversions in Pr3+-doped BaTiO3-CaTiO3 ceramics,” Adv. Mater. 17(10), 1254–1258 (2005). [CrossRef]

22.

H. W. Zhang, H. Yamada, N. Terasaki, and C. N. Xu, “Ultraviolet mechanoluminescence from SrAl2O4:Ce and SrAl2O4:Ce,Ho,” Appl. Phys. Lett. 91(8), 081905 (2007). [CrossRef]

23.

H. W. Zhang, H. Yamada, N. Terasaki, and C. N. Xu, “Green mechanoluminescence of Ca2MgSi2O7:Eu and Ca2MgSi2O7:Eu,Dy,” J. Electrochem. Soc. 155(2), J55–J57 (2008). [CrossRef]

24.

L. Zhang, H. Yamada, Y. Imai, and C. N. Xu, “Observation of elasticoluminescence from CaAl2Si2O8:Eu2+ and its water resistance behavior,” J. Electrochem. Soc. 155(3), J63–J65 (2008). [CrossRef]

25.

H. W. Zhang, H. Yamada, N. Terasaki, and C. N. Xu, “Blue light emission from stress-activated CaYAl3O7:Eu,” J. Electrochem. Soc. 155(5), J128–J131 (2008). [CrossRef]

26.

S. Kamimura, H. Yamada, and C. N. Xu, “Development of new elasticoluminescent material SrMg2(PO4)2:Eu,” J. Lumin. 132(2), 526–530 (2012). [CrossRef]

27.

L. Zhang, C. N. Xu, and H. Yamada, “Strong mechanoluminescence from oxynitridosilicate phosphors,” Mater. Sci. Eng. 18(15), 212001 (2011).

28.

J. Botterman, K. V. D. Eeckhout, I. D. Baere, D. Poelman, and P. F. Smet, “Mechanoluminescence in BaSi2O2N2:Eu,” Acta Mater. 60(15), 5494–5500 (2012). [CrossRef]

29.

C. Li, C. N. Xu, Y. Imai, and N. Bu, “Real-time visualisation of the Portevin-Le Chatelier effect with mechanoluminescent-sensing film,” Strain 47(6), 483–488 (2011). [CrossRef]

30.

X. Fu, H. Yamada, and C. N. Xu, “Property of highly oriented SrAl2O4:Eu film on quartz glass substrates and its potential application in stress sensor,” J. Electrochem. Soc. 156(9), J249–J252 (2009). [CrossRef]

31.

N. Terasaki and C. N. Xu, “Mechanoluminescence recording device integrated withphotosensitive material and europium-doped SrAl2O4,” Jpn. J. Appl. Phys. 48(4), 04C150 (2009).

32.

T. Zhan, C. N. Xu, O. Fukuda, H. Yamada, and C. Li, “Direct visualization of ultrasonic power distribution using mechanoluminescent film,” Ultrason. Sonochem. 18(1), 436–439 (2011). [CrossRef] [PubMed]

33.

K. N. Kim, J. M. Kim, K. J. Choi, J. K. Park, and C. H. Kim, “Synthesis, characterization, and luminescent properties of CaS:Eu phosphor,” J. Am. Ceram. Soc. 89(11), 3413–3416 (2006). [CrossRef]

34.

C. Chartier, C. Barthou, P. Benalloul, and J. M. Frigerio, “Bandgap energy of SrGa2S4:Eu2+ and SrS:Eu2+,” Electrochem. Solid-State Lett. 9(2), G53–G55 (2006). [CrossRef]

35.

X. Zhang, J. Zhang, J. Xu, and Q. Su, “Luminescent properties of Eu2+-activated SrLaGa3S6O phosphor,” J. Alloy. Comp. 389(1–2), 247–251 (2005). [CrossRef]

36.

C. J. Duan, A. C. A. Delsing, and H. T. Hintzen, “Photoluminescence properties of novel red-Emitting Mn2+-activated MZnOS (M=Ca,Ba) phosphors,” Chem. Mater. 21(6), 1010–1016 (2009). [CrossRef]

37.

T. W. Kuo, W. R. Liu, and T. M. Chen, “High color rendering white light-emitting-diode illuminator using the red-emitting Eu2+-activated CaZnOS phosphors excited by blue LED,” Opt. Express 18(8), 8187–8192 (2010). [CrossRef] [PubMed]

38.

T. W. Kuo, C. H. Huang, and T. M. Chen, “Novel yellowish-orange Sr8Al12O24S2:Eu2+ phosphor for application in blue light-emitting diode based white LED,” Opt. Express 18(13S2), A231–A236 (2010). [CrossRef] [PubMed]

39.

N. M. Rao, D. R. Reddy, B. K. Reddy, and C. N. Xu, “Intense red mechanoluminescence from (ZnS)1-x(MnTe)x,” Phys. Lett. A 372(22), 4122–4126 (2008). [CrossRef]

40.

T. Toriyi, Y. Adachi, H. Yamada, Y. Imai, and C. N. Xu, “Enhancement of mechanoluminescence from ZnS:Mn,Te by wet process,” Key Eng. Mater. 388, 301–304 (2009). [CrossRef]

41.

A. Tiwari, S. A. Khan, R. S. Kher, M. Mehta, and S. J. Dhoble, “Effect of capping on the mechanoluminescence of γ-irradiated ZnS:Cu nanophosphors,” J. Lumin. 131(6), 1172–1176 (2011). [CrossRef]

42.

S. A. Petrova, V. P. Mar’evich, R. G. Zakharov, E. N. Selivanov, V. M. Chumarev, and Y. L. Udoeva, “Crystal structure of zinc calcium oxysulfide,” Dokl. Chem. 393(1–3), 255–258 (2003). [CrossRef]

43.

T. Sambrook, C. F. Smura, S. J. Clarke, K. M. Ok, and P. S. Halasyamani, “Structure and physical properties of the polar oxysulfide CaZnOS,” Inorg. Chem. 46(7), 2571–2574 (2007). [CrossRef] [PubMed]

44.

D. Poelman, N. Avci, and P. F. Smet, “Measured luminance and visual appearance of multi-color persistent phosphors,” Opt. Express 17(1), 358–364 (2009). [CrossRef] [PubMed]

45.

J. C. Zhang, X. Wang, X. Yao, C. N. Xu, and H. Yamada, “Strong Elastico-mechanoluminescence in diphase (Ba,Ca)TiO3:Pr3+ with self-assembled sandwich architectures,” J. Electrochem. Soc. 157(12), G269–G273 (2010). [CrossRef]

46.

W. Hoogenstraaten, “Electron traps in zinc-sulfide phosphors,” Philips Res. Rep. 13(6), 515–693 (1958).

47.

R. D. Shannon, “Revised effective ionicradii and systematic studies of interatomic distances in halides and chalcogenides,” Acta Crystallogr. A 32(5), 751–767 (1976). [CrossRef]

OCIS Codes
(040.0040) Detectors : Detectors
(260.3800) Physical optics : Luminescence
(280.5475) Remote sensing and sensors : Pressure measurement

ToC Category:
Sensors

History
Original Manuscript: February 5, 2013
Revised Manuscript: March 30, 2013
Manuscript Accepted: April 5, 2013
Published: May 20, 2013

Citation
Jun-Cheng Zhang, Chao-Nan Xu, Sunao Kamimura, Yujin Terasawa, Hiroshi Yamada, and Xusheng Wang, "An intense elastico-mechanoluminescence material CaZnOS:Mn2+ for sensing and imaging multiple mechanical stresses," Opt. Express 21, 12976-12986 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-11-12976


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References

  1. O. S. Wolfbeis, “Sensor paints,” Adv. Mater.20(19), 3759–3763 (2008). [CrossRef]
  2. V. Maheshwari and R. Saraf, “Tactile devices to sense touch on a par with a human finger,” Angew. Chem. Int. Ed. Engl.47(41), 7808–7826 (2008). [CrossRef] [PubMed]
  3. P. Brochu and Q. Pei, “Advances in dielectric elastomers for actuators and artificial muscles,” Macromol. Rapid Commun.31(1), 10–36 (2010). [CrossRef] [PubMed]
  4. H. Yousefa, M. Boukallela, and K. Althoeferb, “Tactile sensing for dexterous in-hand manipulation in robotics-A review,” Sens. Actuators A Phys.167(2), 171–187 (2011). [CrossRef]
  5. Y. Tadesse, S. Priya, H. Stephanou, D. Popa, and D. Hanson, “Piezoelectric actuation and sensing for facial robotics,” Ferroelectrics345(1), 13–25 (2006). [CrossRef]
  6. Y. J. Yang, M. Y. Cheng, W. Y. Chang, L. C. Tsao, S. A. Yang, W. P. Shih, F. Y. Chang, S. H. Chang, and K. C. Fan, “An integrated flexible temperature and tactile sensing array using PI-copper films,” Sens. Actuators A Phys.143(1), 143–153 (2008). [CrossRef]
  7. L. Ascari, P. Corradi, L. Beccai, and C. Laschi, “A miniaturized and flexible optoelectronic sensing system for tactile skin,” J. Micromech. Microeng.17(11), 2288–2298 (2007). [CrossRef]
  8. N. J. Ferrier and R. W. Brockett, “Reconstructing the shape of a deformable membrane from image data,” Int. J. Robot. Res.19(9), 795–816 (2000). [CrossRef]
  9. M. I. J. Stich, S. Nagl, O. S. Wolfbeis, U. Henne, and M. Schaeferling, “A dual luminescent sensor material for simultaneous imaging of pressure and temperature on surfaces,” Adv. Funct. Mater.18(9), 1399–1406 (2008). [CrossRef]
  10. V. Maheshwari and R. F. Saraf, “High-resolution thin-film device to sense texture by touch,” Science312(5779), 1501–1504 (2006). [CrossRef] [PubMed]
  11. Y. Sagara and T. Kato, “Mechanically induced luminescence changes in molecular assemblies,” Nat. Chem.1(8), 605–610 (2009). [CrossRef] [PubMed]
  12. Z. L. Wang, “Piezopotential gated nanowire devices: Piezotronics and piezo-phototronics,” Nano Today5(6), 540–552 (2010). [CrossRef]
  13. J. S. Kim, Y. N. Kwon, and K. S. Sohn, “Dynamic visualization of crack propagation and bridging stress using the mechano-luminescence of SrAl2O4: (Eu,Dy,Nd),” Acta Mater.51(20), 6437–6442 (2003). [CrossRef]
  14. J. S. Kim, Y. N. Kwon, N. Shin, and K. S. Sohn, “Visualization of fractures in alumina ceramics by mechanoluminescence,” Acta Mater.53(16), 4337–4343 (2005). [CrossRef]
  15. C. N. Xu, X. G. Zheng, M. Akiyama, K. Nonaka, and T. Watanabe, “Dynamic visualization of stress distribution mechanoluminescence image,” Appl. Phys. Lett.76(2), 179–181 (2000). [CrossRef]
  16. C. N. Xu, Smart Materials (Wiley, 2002), Vol.1.
  17. B. P. Chandra, Y. Rahangdale, M. Ramrakhiani, M. H. Aansari, and Y. K. Sharma, “Satatistical model of mechanoluminescence in crystals,” Cryst. Res. Technol.27(4), 561–568 (1992). [CrossRef]
  18. C. N. Xu, T. Watanabe, M. Akiyama, and X. G. Zheng, “Direct view of stress distribution in solid by mechanoluminescence,” Appl. Phys. Lett.74(17), 2414–2416 (1999). [CrossRef]
  19. C. N. Xu, T. Watanabe, M. Akiyama, and X. G. Zheng, “Artificial skin to sense mechanical stress by visible light emission,” Appl. Phys. Lett.74(9), 1236–1238 (1999). [CrossRef]
  20. M. Akiyama, C. N. Xu, H. Matsui, K. Nonaka, and T. Watanabe, “Recovery phenomenon of mechanoluminescence from Ca2Al2SiO7:Ce by irradiation with ultraviolet light.,” Appl. Phys. Lett.75(17), 2548–2550 (1999). [CrossRef]
  21. X. Wang, C. N. Xu, H. Yamada, K. Nishikubo, and X. G. Zheng, “Electro-mechano-optical conversions in Pr3+-doped BaTiO3-CaTiO3 ceramics,” Adv. Mater.17(10), 1254–1258 (2005). [CrossRef]
  22. H. W. Zhang, H. Yamada, N. Terasaki, and C. N. Xu, “Ultraviolet mechanoluminescence from SrAl2O4:Ce and SrAl2O4:Ce,Ho,” Appl. Phys. Lett.91(8), 081905 (2007). [CrossRef]
  23. H. W. Zhang, H. Yamada, N. Terasaki, and C. N. Xu, “Green mechanoluminescence of Ca2MgSi2O7:Eu and Ca2MgSi2O7:Eu,Dy,” J. Electrochem. Soc.155(2), J55–J57 (2008). [CrossRef]
  24. L. Zhang, H. Yamada, Y. Imai, and C. N. Xu, “Observation of elasticoluminescence from CaAl2Si2O8:Eu2+ and its water resistance behavior,” J. Electrochem. Soc.155(3), J63–J65 (2008). [CrossRef]
  25. H. W. Zhang, H. Yamada, N. Terasaki, and C. N. Xu, “Blue light emission from stress-activated CaYAl3O7:Eu,” J. Electrochem. Soc.155(5), J128–J131 (2008). [CrossRef]
  26. S. Kamimura, H. Yamada, and C. N. Xu, “Development of new elasticoluminescent material SrMg2(PO4)2:Eu,” J. Lumin.132(2), 526–530 (2012). [CrossRef]
  27. L. Zhang, C. N. Xu, and H. Yamada, “Strong mechanoluminescence from oxynitridosilicate phosphors,” Mater. Sci. Eng.18(15), 212001 (2011).
  28. J. Botterman, K. V. D. Eeckhout, I. D. Baere, D. Poelman, and P. F. Smet, “Mechanoluminescence in BaSi2O2N2:Eu,” Acta Mater.60(15), 5494–5500 (2012). [CrossRef]
  29. C. Li, C. N. Xu, Y. Imai, and N. Bu, “Real-time visualisation of the Portevin-Le Chatelier effect with mechanoluminescent-sensing film,” Strain47(6), 483–488 (2011). [CrossRef]
  30. X. Fu, H. Yamada, and C. N. Xu, “Property of highly oriented SrAl2O4:Eu film on quartz glass substrates and its potential application in stress sensor,” J. Electrochem. Soc.156(9), J249–J252 (2009). [CrossRef]
  31. N. Terasaki and C. N. Xu, “Mechanoluminescence recording device integrated withphotosensitive material and europium-doped SrAl2O4,” Jpn. J. Appl. Phys.48(4), 04C150 (2009).
  32. T. Zhan, C. N. Xu, O. Fukuda, H. Yamada, and C. Li, “Direct visualization of ultrasonic power distribution using mechanoluminescent film,” Ultrason. Sonochem.18(1), 436–439 (2011). [CrossRef] [PubMed]
  33. K. N. Kim, J. M. Kim, K. J. Choi, J. K. Park, and C. H. Kim, “Synthesis, characterization, and luminescent properties of CaS:Eu phosphor,” J. Am. Ceram. Soc.89(11), 3413–3416 (2006). [CrossRef]
  34. C. Chartier, C. Barthou, P. Benalloul, and J. M. Frigerio, “Bandgap energy of SrGa2S4:Eu2+ and SrS:Eu2+,” Electrochem. Solid-State Lett.9(2), G53–G55 (2006). [CrossRef]
  35. X. Zhang, J. Zhang, J. Xu, and Q. Su, “Luminescent properties of Eu2+-activated SrLaGa3S6O phosphor,” J. Alloy. Comp.389(1–2), 247–251 (2005). [CrossRef]
  36. C. J. Duan, A. C. A. Delsing, and H. T. Hintzen, “Photoluminescence properties of novel red-Emitting Mn2+-activated MZnOS (M=Ca,Ba) phosphors,” Chem. Mater.21(6), 1010–1016 (2009). [CrossRef]
  37. T. W. Kuo, W. R. Liu, and T. M. Chen, “High color rendering white light-emitting-diode illuminator using the red-emitting Eu2+-activated CaZnOS phosphors excited by blue LED,” Opt. Express18(8), 8187–8192 (2010). [CrossRef] [PubMed]
  38. T. W. Kuo, C. H. Huang, and T. M. Chen, “Novel yellowish-orange Sr8Al12O24S2:Eu2+ phosphor for application in blue light-emitting diode based white LED,” Opt. Express18(13S2), A231–A236 (2010). [CrossRef] [PubMed]
  39. N. M. Rao, D. R. Reddy, B. K. Reddy, and C. N. Xu, “Intense red mechanoluminescence from (ZnS)1-x(MnTe)x,” Phys. Lett. A372(22), 4122–4126 (2008). [CrossRef]
  40. T. Toriyi, Y. Adachi, H. Yamada, Y. Imai, and C. N. Xu, “Enhancement of mechanoluminescence from ZnS:Mn,Te by wet process,” Key Eng. Mater.388, 301–304 (2009). [CrossRef]
  41. A. Tiwari, S. A. Khan, R. S. Kher, M. Mehta, and S. J. Dhoble, “Effect of capping on the mechanoluminescence of γ-irradiated ZnS:Cu nanophosphors,” J. Lumin.131(6), 1172–1176 (2011). [CrossRef]
  42. S. A. Petrova, V. P. Mar’evich, R. G. Zakharov, E. N. Selivanov, V. M. Chumarev, and Y. L. Udoeva, “Crystal structure of zinc calcium oxysulfide,” Dokl. Chem.393(1–3), 255–258 (2003). [CrossRef]
  43. T. Sambrook, C. F. Smura, S. J. Clarke, K. M. Ok, and P. S. Halasyamani, “Structure and physical properties of the polar oxysulfide CaZnOS,” Inorg. Chem.46(7), 2571–2574 (2007). [CrossRef] [PubMed]
  44. D. Poelman, N. Avci, and P. F. Smet, “Measured luminance and visual appearance of multi-color persistent phosphors,” Opt. Express17(1), 358–364 (2009). [CrossRef] [PubMed]
  45. J. C. Zhang, X. Wang, X. Yao, C. N. Xu, and H. Yamada, “Strong Elastico-mechanoluminescence in diphase (Ba,Ca)TiO3:Pr3+ with self-assembled sandwich architectures,” J. Electrochem. Soc.157(12), G269–G273 (2010). [CrossRef]
  46. W. Hoogenstraaten, “Electron traps in zinc-sulfide phosphors,” Philips Res. Rep.13(6), 515–693 (1958).
  47. R. D. Shannon, “Revised effective ionicradii and systematic studies of interatomic distances in halides and chalcogenides,” Acta Crystallogr. A32(5), 751–767 (1976). [CrossRef]

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