Characterization Techniques and Mechanoluminescence Properties of Sr₂SiO₄:Eu²⁺ Phosphor by Solid State Reaction Method

Ishwar Prasad Sahu¹, D. P. Bisen¹, Nameeta Brahme¹, V.K. Patle², Raunak Tamrakar³

¹School of Studies in Physics & Astrophysics, Pt. Ravishankar Shukla University, Raipur (C.G.) - 492010, India

²School of Studies in Computer Science, Pt. Ravishankar Shukla University, Raipur (C.G.)- 492010, India

³Department of Applied Physics, Bhilai Institute of Technology, Durg (C.G.)- 491001, India

*Corresponding Author E-mail: ishwarprasad1986@gmail.com, dpbisen@rediffmail.com

ABSTRACT:

Sr₂SiO₄:Eu²⁺ phosphor was prepared by the solid state reaction method, boric acid (H₃BO₃) was added as flux. The obtained phosphor was characterized using XRD, TEM, SEM techniques. The surface of the prepared phosphor was not found uniform and particles distribution is in nanometer size. The ML intensity of prepared phosphor was increasing linearly with increases of mechanical load.

KEYWORDS: Sr₂SiO₄:Eu²⁺; XRD; TEM; SEM; Mechanoluminescence..

INTRODUCTION:

Luminescence induced during any mechanical action on materials is known as mechanoluminescence (ML). [1] ML can be excited either by grinding, rubbing, cutting, cleaving, shaking, scratching, crushing, compressing, or by impulsive deformation of solids. During the deformation of a solid, a great number of physical processes may occur within very short time intervals, which may excite or stimulate the process of photon emission. ML has been observed in insulators, semiconductors as well as in conductors.[2-3] ML has found various important applications such as impact sensors in spacecrafts (the emission intensity can be used to determine the kinetic energy of impact), fracture sensor, damage sensor, stress sensor etc. Thus, many researchers have been focused on the investigation of phosphors with high ML.[4] Until now, some phosphors with high ML, such as (red phosphor) BaTiO₃–CaTiO₃:Pr, (green phosphor) SrAl₂O₄:Eu, (yellow phosphor) ZnS:Mn, and (blue phosphor) CaYAl₃O₇:Eu etc., have been developed. However, the requirement of application for ML sensors still is not satisfied with the development of ML materials. At the same time, the high stabilities, such as resistance of water, thermal stability are also very important for the application of ML. More ML phosphors with strong ML intensity and high stability are needed. [5-8]

It is well known that silicates have a higher physical and chemical stability after water treatment. Furthermore, Eu²⁺ doped silicates phosphors display various color light. Therefore, in this paper, we investigated the structural characterization of Eu²⁺ doped Sr₂SiO₄ phosphor by the solid state reaction method. The structural characterization is done with the study of XRD, TEM, SEM and Mechanoluminescence properties were also studied.

CHARACTERIZATION:

The phase structure of prepared sample was examined by X-ray powder diffraction (XRD). The XRD pattern has been obtained from Bruker D8 Advanced X-ray powder diffractometer using CuK radiation and the data were collected over the 2θ range 10^o-80^o. The phase structure of the sample was verified with the help of JCPDS file (JCPDS: 17-1630). Particle size of prepared phosphor was determined by Transmission Electron Microscopy (TEM). The surface morphology was examined by the Scanning Electron Microscopy. Mechanoluminescence was monitored by the home made lab system. All measurements were carried out at room temperature.

SAMPLE PREPARATION:

The powder sample of Sr₂SiO₄:Eu²⁺ was prepared by the high temperature solid state reaction method. The starting materials are SrCO₃ (99.90%), SiO₂ (99.99%), and Eu₂O₃ (99.99%)_, all of analytical purity, were employed in this experiment. Boric acid [H₃BO₃ (99.99%)] was added as a flux. Initially, the raw materials were weighed according to the nominal compositions of Sr₂SiO₄:Eu²⁺ phosphor. Then the powders were mixed and milled thoroughly for 2 hour using the agate mortar and pestle. The grinded sample was placed in an alumina crucible and subsequently fired at 1300^oC for 3 hour in a weak reducing atmosphere. The weak reducing atmospheres are generated with the help of activated charcoal.

RESULTS AND DISCUSSION:

XRD Analysis

In order to determine the phase structure, powder XRD analysis has been carried out. The typical XRD patterns of Sr₂SiO₄:Eu²⁺ with that of the standard JCPDS file are shown in Fig.1. Nearly, all the diffraction peaks of the resultant phosphor are consistent with Joint Committee Powder Diffraction Standard data (JCPDS) file (JCPDS: 17-1630). The position and intensity of diffraction peaks of Sr₂SiO₄:Eu²⁺ are well matched with the standard JCPDS file. The space group conditions for all observed XRD patterns were consistent with the orthorhombic space group Pmnb corresponding to the strontium orthosilicate system.

Transmission Electron microscopy (TEM)

Fig. 2 shows the particle size of Sr₂SiO₄:Eu²⁺ phosphor. In Fig. 2 due the high temperature treatment, the agglomeration of powder particles was observed. TEM images shows that the average particle size of Sr₂SiO₄:Eu²⁺ phosphor is in nanometer size.

Scanning Electron Microscopy (SEM)

The surface morphology of the Sr₂SiO₄:Eu²⁺ phosphor is shown in Fig. 3 The surface morphology of Sr₂SiO₄:Eu²⁺ phosphor was not uniform and they aggregated tightly with each other. From the SEM image, it can be observed that the prepared sample consists of particles with different size distribution. In addition, there are some big aggregates existing due to high temperature heat treatment. The SEM results are in good correlation with the TEM studies.

Mechanoluminescence (ML)

The experimental set up used for the impulsive deformation of ML was shown in Fig. 4. The prepared Sr₂SiO₄:Eu²⁺ phosphor was stressed via dropping a load (moving piston) of a particular mass (400 gm) and shape (cylindrical), on the phosphor. To change the impact force the load was dropped from different heights (20 to 50 cm). The sintered phosphor were wrapped in aluminum foil and kept in dark till the ML studies were carried out. RCA 931A photomultiplier tube positioned below the Lucite Plate and connected to the storage oscilloscope (Scientific 300 MHz, SM 340). The output of photomultiplier tube was connected to a storage oscilloscope. In Fig. 4, 1 - Stand; 2 - Pulley; 3- Metallic wire; 4–Load [moving piston (400gm)]; 5-Guiding phosphors; 6-Aluminum foil; 7-Phosphor; 8-Transparent lucite plate; 9-Wooden block; 10-Photomultiplier tube (PMT); 11–Oscilloscope 12-Iron base mounted on a table.

Fig. 4 Experimental setup for ML measurement

Fig. 5(a) shows that the characteristics curve between ML intensity versus time for different heights. The experiment was carried out for a fixed moving piston (400gm) dropped with different heights (h = 20, 30, 40, 50cm). Every time for the ML measurement, the quantity of the powder sample is fixed (8 mg). In this experiment, the maximum ML intensity has been obtained for the 50 cm dropping height. The ML intensity increases linearly with the increases the falling height of the moving piston; i.e., the ML intensity depends upon the impact velocity. The prepared phosphor is not irradiated by any excitation source such as Ultra-Violet, Laser, X-ray, β–rays or γ-rays. Fig. 5(b) shows that the characteristics curve between ML intensity versus impact velocity. These figure shows that the ML intensity increases linearly with the increasing impact velocity of the moving piston [ (where = h, is the different heights)].[9]

Fig. 5(a) ML intensity versus time of Sr₂SiO₄:Eu²⁺ phosphor

Fig. 5(b) ML intensity versus impact velocity of Sr₂SiO₄:Eu²⁺ phosphor

From Fig. 5(b), it can be seen that the linear increase of compressive load can induce the increase of ML intensity, which shows the excellent linear relation. That is, the ML intensity of Sr₂SiO₄:Eu²⁺ is linear proportional to the magnitude of the applied load. Such a ML property of Sr₂SiO₄:Eu²⁺ can provide high sensitivity for smart skin and self diagnosis applications.

The steps involved in the ML emission in prepared phosphor are given below:

1. The moving piston produces piezoelectric field in prepared phosphor because they are non-centrosymmetric in which the piezoelectric field near certain defects centers may be high due to the change in the local structure.

2. The piezoelectric field reduces the trap depth of the carriers.

3. The decrease in trap depth causes transfer of electrons from electron traps to the conduction band.

4. Subsequently, the moving electrons in the conduction band are captured in the excited state, located at the bottom of the conduction band, whereby excited ions are produced.

5. The de-excitation of ions gives rise to the light emission characteristic of the ions.

It was found that the ML intensity is directly proportional to the applied stress and the experimental results indicate that the ML intensity is directly proportional to the square of the applied stress i.e. proportional to the height through which the moving piston is dropped on the samples. Thus the present investigation indicates that the piezo-electrification is responsible to produce ML in prepared phosphor. [10-11]

CONCLUSION:

The Sr₂SiO₄:Eu²⁺ phosphor was prepared by the traditional high temperature solid state reaction method. In the TEM study, due to the high temperature treatment, the agglomeration of powder particles was observed and average particle size of Sr₂SiO₄:Eu²⁺ phosphor is in nanometer size. It should be noted that the dependences between ML intensities and loads are close to linearity, which suggests that these phosphor can be used as sensors to detect the stress of an object.

REFERENCES:

[1] S. Shionoya, W.M. Yen, Phosphor Handbook, CRC Press, New York, 1999.

[2] Y. Lin, Z. Tang, Z. Zhang, C. Nan, J. Alloys Compds. 348 (2003) 76.

[3] Y. Wang, Z. Wang, P. Zhang, Z. Hong, X. Fan, G. Qian, Mater. Lett. 5 (2004) 3308.

[4] H. He, R. Fu, D. Wang, X. Song, Z. Pan, X. Zhao, X. Zhang, and Y. Cao, Mater. Res., 23, (2008) 3288.

[5] B.P. Chandra, C.N. Xu, H. Yamada, X.G. Zheng, Journal of Luminescence 130 (2010) 442–450.

[6] V.K. Chandra, B.P. Chandra, Piyush Jha, Journal of Luminescence 138 (2013) 267–280

[7] Hongwu Zhang, Hiroshi Yamada, Nao Terasaki and Chao-Nan Xu, Japanese Journal of Applied Physics 48 (2009) 04C109.

[8] Hongwu Zhang, Hiroshi Yamada, Nao Terasaki and Chao-Nan Xu, Electrochemical and solid state letters, 10(10) (2007) J129-J131.

[9] Hongwu Zhang, Hiroshi Yamada, Nao Terasaki and Chao-Nan Xu, Thin Solid Films 518, (2009) 610-613.

[10] Hongwu Zhang, Nao Terasaki, Hiroshi Yamada and Chao-Nan Xu, International Journal of Modern Physics B, Vol. 23, Nos. 6 and 7 (2009) 1028-1033.

[11] Hongwu Zhang, Chao-Nan Xu, Nao Terasaki, Hiroshi Yamada, Physics E 42 (2010) 2872-2875.

Received on 12.05.2014 Modified on 23.06.2014

Accepted on 02.07.2014 ©A&V Publications All right reserved

Research J. Science and Tech. 6(3): July- Sept., 2014; Page 147-150