Author’s Accepted Manuscript Fracto-mechanoluminescence in Mn/Cu doped ZnS induced by steel ball and cylindrical piston Piyush Jha, Ayush Khare, Pranav Singh, Gajendra Singh, V.K. Chandra www.elsevier.com/locate/jlumin
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S0022-2313(18)31345-0 https://doi.org/10.1016/j.jlumin.2018.12.025 LUMIN16154
To appear in: Journal of Luminescence Received date: 24 July 2018 Revised date: 25 October 2018 Accepted date: 11 December 2018 Cite this article as: Piyush Jha, Ayush Khare, Pranav Singh, Gajendra Singh and V.K. Chandra, Fracto-mechanoluminescence in Mn/Cu doped ZnS induced by steel ball and cylindrical piston, Journal of Luminescence, https://doi.org/10.1016/j.jlumin.2018.12.025 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Fracto-mechanoluminescence in Mn/Cu doped ZnS induced by steel ball and cylindrical piston Piyush Jha1, Ayush Khare2, Pranav Singh3, Gajendra Singh3, V. K. Chandra4 1
Department of Applied Physics, Raipur Institute of Technology, Chhatauna, Mandir Hasaud, Raipur – 492 101, India
Department of Physics, National Institute of Technology, GE Road, Raipur – 492 010, India 3
Department of Postgraduate Studies and Research in Physics and Electronics, Rani Durgavati University, Jabalpur - 482 001, India
Department of Electrical and Electronics Engineering, Chhatrapati Shivaji Institute of Technology, Shivaji Nagar, Kolihapuri, Durg – 491 001, India
Abstract Herein, we present a comparison of fracto-mechanoluminescence (FML) from ZnS: Mn and ZnS:Cu phosphors prepared by solid state reaction (SSR) route. The FML emitted by two phosphors was compared in terms of its dependence on time and velocity. The FML intensity initially increases with time attains a peak value followed by decrease at still longer times. Peak ML intensities in both the cases are found to linearly increase with impact velocity and total ML intensity linearly increases with mass of the phosphor samples. ML and photoluminescence (PL) spectra of ZnS: Mn and ZnS: Cu phosphors are recorded to lie at 540 nm and 580 nm, respectively.
Keywords: Mechanoluminescence; ZnS: Mn; ZnS: Cu; Phosphors; Sensor *Corresponding author: [email protected]
1 Introduction The history of Mechanoluminescence (ML) is long and interesting . In 1972, Wedgwood  reported about the ML from many substances; such as quartz, diamond and ruby. It was Burke , who made a serious attempt to measure ML spectrum of sugar, but he was unable to photograph the spectrum. Tschugaeff , on the basis of his studies suggested a list of 127 new ML materials. In the first half of nineteenth century, the works on ML were mainly focused on the discovery of new ML materials, but later on emphasis was given on its spectroscopic features also. Because of very short duration and weak intensity of the ML flashes, it was difficult to photograph corresponding spectra. Later in 1922, Longchambon recorded the ML spectra of sugar and other crystals successfully . He crushed approximately 5.0 kg and 25.0 kg of sugar and copper sulfate, respectively and obtained corresponding ML spectra for exposure times of 4 h and even more. ML is a type of luminescence induced by application of stress on non-centrosymmetric crystals [6,7]. Much of the work was undertaken on elastico-mechanoluminescence (EML), plastico-mechanoluminescence (PML) and fracto-mechanoluminescence (FML) [6, 7]. The PML and FML belong to destructive ML, while EML is a non-destructive technique. Actually, before 1950 there were reports only on FML  in which the ML material was used for one time. Since then, a number of mechanisms have been proposed for FML; however, some more study is still needed [9, 10]. In order to design/fabricate fracture sensor, impact sensor, damage sensor, safety management monitoring system, fuse system for army warheads, etc., it is necessary to develop highly intense and cheap phosphors. Xu et al.  reported that 5 at % doping level of manganese (Mn) to ZnS showed the highest triboluminescence (TL) intensity, whereas copper
(Cu) doping level of about 1 at % demonstrated the 2nd highest TL intensity among all inorganic crystals. The ML from ZnS has been associated with its photoluminescence (PL). Nelson in 1926  succeeded in photographing the ML spectrum of four natural sphalerite samples and one commercially produced ZnS: Mn powder. In 1946, Curie and Prost  proposed a mechanism for origin of electroluminescent emission in this crystal. The ML spectra of ZnS doped with a wide variety of activators and co-activators were studied by Orbikat et al. , Meyer and Orbikat  and by Theissen . The ML in crystals and phosphors of ZnS were investigated by Sodomka and Chudasek [17-23]. Scarmozzino  revealed that the pure zinc sulphide was non-centric. Bredhikhin and Shmurak exposed that pure zinc sulphide was mechanoluminescent  and exhibited the lightning lines characteristics of FML. Later, ZnS doped with small percentage of strongly luminescent transition metal ions, such as copper, manganese [26-30] and even silver  has been studied for strong ML under deformation. Fontenot et al.  have studied ML of ZnS: Mn doped PDMS. They used drop tower method for ML excitation. ML was excited by 130 g steel ball. It is found that ML intensity is proportional to impact energy and also found the exponential decay of ML intensity at 0 and 60°C. Leelachao et al.  have investigated stress-induced light emission of ZnS-based phosphors dispersed in polymeric matrix. In their ML set up two striking pins with different tip diameters Φtip of 0.77 and 2.18 mm were used and made from a 6 mm diameter fused silica rod. It is also found that the peak ML intensity of ZnS: Mn is proportional to impact energies. They found that the ML intensity is proportional to impact energy when projectile having small contact area such as steel ball and pin with different tip diameters Φtip of 0.77 and 2.18 mm were
used. The present paper reports comparison of FML from ZnS:Mn and ZnS:Cu phosphors induced by cylindrical piston and steel ball. 2 Experimental techniques The microcrystalline ZnS: Mn0.05 and ZnS:Cu0.01 phosphor samples were prepared through solid state reaction technique (SSRT) using analytical grade ZnS, MnCl2, Cu(CH3COO)2 and 4% NaCl flux as starting materials. The required amount of activator in the form of MnCl2 or CuCl2 was added to the appropriate amount of ZnS. Then, this mixture was heated in an oven at 120°C to evaporate water. After adding 4 % NaCl as flux, the mixture was thoroughly ground using a pestle and a mortar. Consequently, the prepared mixture was transferred to a clean silica boat, which was covered with another boat. The hexagonal ZnS: Mn phosphor was prepared by firing at 1250°C for 1 h in nitrogen atmosphere . After firing, the phosphor was taken out and cooled to room temperature. The synthesized phosphors were cleaned two or three times with distilled water and dried again. After choosing the compact masses of phosphors, they were made of suitable size by grinding and polishing. The sizes of the compact masses of phosphors used in the present ML investigation were 0.4x0.4x0.4 mm3. The X-ray powder diffraction data were collected using a PANalytical 3 kW X’pert Powder – Multifunctional diffractometer. The morphologies of particles were examined using a ZEISS-EVO 18 scanning electron microscope (SEM). The PL spectra of the prepared samples were recorded using Carry eclipse fluorescence spectrophotometer upon excitation with a wavelength of 365 nm. The ML measurements were performed using technique described by us previously . In this technique (Fig.1), the sample was placed on the upper surface of a transparent lucite plate (diameter 2.32 cm) inside the sample holder covered with a thin aluminum foil. The sample holder was fixed using an adhesive
tape to prevent the scattering of crystalline fragments during the impact of the load or cylindrical piston (mass 800g, diameter 2.3 cm) onto the sample. The ML was excited impulsively by dropping a load onto the sample from a fixed height. The luminescence intensity was measured using an RCA-931A photomultiplier tube (PMT) placed just below the lucite plate. The output of the PMT was fed to a storage oscilloscope. The response time of PMT system was nearly 5 μs. The ML spectra were recorded using a constant deviation spectrometer in which the telescopic was replaced by PMT placed near the narrow exit of the spectrometer . The output of the PMT was amplified and monitored in terms of the deflection of a picoammeter. The ML intensity at various wavelengths was recorded by rotating the drum of the spectrometer. At least four samples were studied for each set of observations with an error of ±5%. Same experiment was carried out for steel ball having mass 130g and diameter 3.16 cm. All the experiments were carried out at room temperature. 3 Results and discussion XRD analysis of ZnS: Mn and ZnS: Cu phosphors was done to verify their phase purity and to determine the crystal structure. Hexagonal structures were found for both ZnS: Mn and ZnS: Cu phosphors as shown in Fig. 2 (a) and (b), which agreed well with earlier reports [35, 36, 37]. No other diffraction peak corresponding to any impurity was found in the XRD patterns. The average particle sizes calculated for ZnS: Mn and ZnS: Cu phosphors are found to be 78.3 nm and 56.7 nm, respectively. SEM study was carried out to obtain information about surface morphology (shape and size) of the synthesized phosphors. The SEM images of ZnS: Mn and ZnS: Cu phosphors are presented in Fig. 3 (a) and (b). It is clear from these images that distribution of particles is quite
dense. They appear to be hard and of irregular shape as expected from samples synthesized through SSRT. The snapshots of time dependence of the ML intensity for ZnS:Mn and ZnS:Cu phosphors observed during impulsive excitation are shown in Fig. 4 (a) and (b). The ML intensity rapidly increases with time, reaches maximum at a particular value of time followed by decay at still longer times. It is concluded that the ML intensity of ZnS: Mn phosphor is 2.07 times greater than intensity of ZnS: Cu. Xu et al.  and Majumdar , respectively reported 2.54 times and 2.33 times greater ML intensity of ZnS: Mn phosphor in comparison to ZnS: Cu phosphor. The difference in ML intensity between our studies and earlier studies may be due to the different methods of preparing two phosphors, and devices used for recording the ML. The time dependence of ML intensity for ZnS: Mn and ZnS: Cu phosphor at different impact velocities is illustrated in Fig. 5 (a) and (b), respectively. The ML is produced after the impact of cylindrical piston on the crystal/sample. Like previous case, ML intensity follows the same trend and after remaining maximum for few tenths of a millisecond, it decays. The ZnS:Mn relatively exhibits better ML features as compared to ZnS:Cu phosphor. With increasing impact velocity, the peak in the ML versus time curve increases in intensity and shifts towards shorter time. In other words, tm, the time corresponding to the ML peak shifts towards shorter values with increase in impact velocity. This shows that the deformation rate of the crystal is faster at high velocity. A ZnS: Mn crystal is made of several small crystallites. When crystal is deformed impulsively, the decay time depends on the energy of released electrons from the traps, and these released electrons lead to the excited state levels of Mn2+ due to tunnelling effects. The deexcitation of Mn2+ ions gives rise to ML emission, and the decay duration depends on (i) the combined effect of deformation of several small crystallites (greatly on the exact mechanism of
fracture) and (ii) the shallow and deep traps present in the crystals . The rate of deformation of crystallites is slow during low velocity impact. Thus, ML lasts for several millisecond (longer time), but at higher velocities, the rate of deformation of these crystallites is much faster; hence, ML lasts for few milliseconds (shorter time) [9, 33, 34]. The decay duration time could vary greatly from one experiment to the other. The semilog plots of ML intensity vs. (t−tm) for ZnS:Mn and ZnS:Cu phosphors that provide the ML decay time are depicted in Fig. 6 (a) and (b). It is seen from the figure that initially the ML intensity decreases rapidly followed by a decrease at a slower rate. The decay times for ZnS:Mn and ZnS:Cu phosphors at 140.07, 242.61 and 313.30 cm/s impact velocities are found to be 0.142 ms, 0.086 ms and 0.082 ms and 0.098 ms, 0.061 ms and 0.0555 ms, respectively. With increasing impact velocity, ZnS:Mn and ZnS:Cu phosphors are excited suddenly and then excited Mn and Cu ions suddenly de-excite at a fast rate. So that the decay time for low velocity is more compare to increasing impact velocity. The snapshots of time dependence of the ML intensity for ZnS:Mn and ZnS:Cu phosphors observed during impulsive excitation induced by steel ball of 130g mass are shown in Fig. 7 (a) and (b). The ML intensity also increases linearly increases with time attains a saturation value and decreases with time [30, 40] as observed during impulsive excitation of ML by cylindrical piston. In this case, the decay time for ZnS: Mn is found to be 0.257 ms. Similar results were reported by Fontenot et al.  when they found ML of ZnS:Mn by impact of a steel ball of 130 g mass. The variation of peak ML intensity (Im) with time for ZnS:Mn and ZnS:Cu phosphors is illustrated in Fig. 8. We have conducted this test five times for different velocities (140.0, 198.0, 242.6, 280.1 and 313.2 cm/s) of the cylindrical piston. It is clear from the figure that peak ML
intensity (Im) linearly increases with increasing value of the impact velocity. This observation is explained on the basis that increasing cylindrical piston (having large contact area) velocity creates larger surface area leading to increased ML intensity linearly. Chandra et al.  suggested that for low impact velocity having large contact area of the cylindrical piston the equation of peak ML intensity (Im) and total ML intensity (IT) are given by 
For low impact velocity having small contact area, such as steel ball, the equations of peak ML intensity (Im) and total ML intensity (IT) are expressed as 
rbv02 Im IT
where ‘η’ is the ML efficiency relating the ML intensity and rate of relaxation of surface charges, ‘r’ is the radius of the ball dropped on to the sample, ‘ρ’ is the surface charge density due to the piezo-electrification, b and α are constants, V is the volume of the sample, H is the thickness of the crystal, ξ = 1/τr, where τr is the time-constant for the decrease of impact velocity with time and v0 is the initial impact velocity. Equations (1) and (2) indicate that for low impact velocity, both Im and IT increase linearly with impact velocity of the projectile having larger contact area used to deform the sample. Equations (3) and (4) show that for low impact velocity, both Im and IT should increase quardratically with the impact velocity of the ball used to deform the sample.
It is clear from the Fig.9 that peak ML intensity (Im) and total ML intensity increase linearly with the square of impact velocity of the steel ball. In the case of an impulsive excitation by steel ball having small contact area, increase of strain rate with the impact velocity, the effective volume compressed by the impact also increases linearly with the impact velocity, and thus, the rate of creation of new surfaces increases quadratically with the impact velocity, which is evident from Eq.(3). Similar results are found by Fontenot et al. , Leelachao et al.  and Jha et al. . ]. In the cylindrical piston the weight is distributed over the large surface area as compare to ball piston, so the emission intensity is found greater than the cylindrical piston impact. Total ML intensity (IT) is defined as the area under the ML intensity versus time curve. The change in total ML intensity (IT) on the mass of phosphor samples taken for different studies is depicted through Fig. 10. Total ML intensity linearly increases with increasing mass of the phosphor. It is because larger mass of phosphor offers greater area for impact after piston strikes the sample. In Fig.11 (a) and (b), there is presented the dependence of time (tm) on impact velocity of cylindrical piston for ZnS:Mn and ZnS:Cu phosphors. It is seen that the value of tm decreases with increasing impact velocity of the cylindrical piston. This shows that the deformation rate is slow at low velocities and tm is high, but at higher velocities the deformation rate is high and thus the values of tm are less. This result indicates that after a particular compression, further compression of the phosphor becomes difficult. The ML and PL spectra of ZnS:Mn and ZnS:Cu phosphors are shown in Fig.12 (a) and (b). It is seen from Fig. 12 (a) that ML spectrum centered at 580 nm is almost identical to the PL spectrum. The PL spectrum of ZnS: Mn exhibits the well known orange PL band. The PL
emission obtained from ZnS: Mn is due to the 4T1→
A1 transition of Mn2+ [43, 44]. The ML
and PL spectra come from the same Mn2+ centers. Similar results were reported by Duignan et al. . From Fig.12 (b), it is noticed that the PL spectrum has the green emission due to copper complexes with emission peak centered at 540 nm. The ML spectrum of ZnS: Cu phosphor shift towards shorter wavelength side by 5 nm (535 nm) as compared to the PL spectrum, which is supported by an impulsive excitation induced by steel ball. There are many reports in the literature for a spectral difference between the ML and the PL spectrum of the same compound. Many reasons have been suggested for this difference including pressure-induced changes to Franck-Condon factors during the lifetime of the ML light emission; self-absorption of the ML emission and fracture-induced symmetry changes perturbing the local field of the ML emitting species . In ZnS:Cu, Cu ions do not create much local piezoelectricity in the host material during ML, while the PL emission of ZnS:Cu arises due to the impurity-induced shallow donor state and the t2 state of Cu [46, 47] resulting a slight difference in peak positions of ML and PL spectra of ZnS:Cu phosphor. 4 Conclusions Conventional SSRT has been used for the preparation of ZnS:Mn and ZnS:Cu phosphors. Impulsive excitation technique is used for ML measurements. Upon impact with cylindrical piston or steel ball on ZnS:Mn and ZnS:Cu phosphors, the ML intensity initially increases with time, attains a peak value and then decreases with time. In our case, the ML intensity of ZnS:Mn phosphor is found to be 2.07 times greater than ZnS:Cu phosphor during impulsive excitation by cylindrical piston. The ML intensity in semilog plot against (t−tm) for ZnS:Mn and ZnS:Cu phosphors follow the same pattern. In the beginning, it decays at a faster rate, and later on it decays at a slower rate. The total ML intensity increases linearly with mass of
the phosphor samples. The peak ML intensity increases linearly in case of impact of the cylindrical piston having large contact area and increases quardratically in case of impact of steel ball having small contact area. ML and PL spectra of ZnS:Mn phosphor lies at 580 nm, which reveals that the same center is responsible for luminescence in ZnS:Mn. But, in case of ZnS:Cu phosphor, the ML spectrum is observed to be centered at 540 nm, while PL spectrum peaks at 535 nm. The ML spectrum of ZnS:Cu phosphor shifts towards shorter wavelength side by 5 nm as compared to the PL spectrum due to the impurity-induced shallow donor state and the t2 state of Cu.
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Captions to figures
Fig. 1 Schematic diagram of the experimental arrangement used for measuring time dependence of ML in crystals (1-stand; 2-pulley; 3-metallic wires; 4-load; 5-guiding cylinder; 6aluminium foil; 7-crystals; 8-transparent lucite plate; 9-wooden block; 10-photomultiplier tube; 11-iron base mounted on a table). Fig.2 XRD of (a) ZnS:Mn and (b) ZnS:Cu phosphors. Fig.3 SEM image of (a) ZnS:Mn and (b) ZnS:Cu phosphors. Fig.4 Time dependence of ML intensity during impulsive excitation: (a) ZnS:Mn and (b) ZnS:Cu phosphors for cylindrical piston impact. Fig. 5 Time dependence of the ML intensity for (a) ZnS:Mn and (b) ZnS:Cu phosphors (Curves I, II and III correspond to the impact velocity 140.07,
242.61 and 313.20 cm/s,
respectively). Fig. 6 Semi-log plot of the ML intensity versus (t−tm) for (a) ZnS:Mn and (b) ZnS:Cu phosphors (Curves I, II and III correspond to the impact velocity 140.07, 242.61 and 313.20 cm/s, respectively). Fig.7 Time dependence of ML intensity during impulsive excitation: (a) ZnS:Mn and (b) ZnS:Cu phosphor for steel ball impact. Fig.8 Dependence of peak ML intensity with the impact velocity for (I) ZnS:Cu and (II) ZnS:Mn phosphor for cylindrical piston impact. Fig.9 Dependence of peak ML intensity with the impact velocity for (I) ZnS:Cu and (II) ZnS:Mn phosphor for steel ball impact. Fig.10 Dependence of total ML intensity on the mass of the (I) ZnS:Cu and (II) ZnS:Mn phosphors. Fig.11 Dependence of tm on the impact velocity for (a) ZnS:Mn and (b) ZnS:Cu phosphors. 17
Fig.12 ML and PL spectra of (a) ZnS:Mn and (b) ZnS:Cu phosphors.
Fig. 2 (a) and (b)
Fig. 3 (a) and (b)
Fig.4 (a) and (b)
Fig.5 (a) and (b)
Fig.6 (a) and (b)
Fig. 11 (a) and (b)
Fig.12 (a) and (b)