1. Introduction
Boric oxide, B2O3, acts as one of the most important glass formers and flux materials. Melts with compositions rich in B2O3 exhibit rather high viscosity and tend to the formation of glasses. In crystalline form, on the other hand, borates with various compositions are of exceptional importance due to their interesting linear and nonlinear optical properties [1]. The boron atom usually coordinates with either three or four oxygen atoms forming (BO3)3− or (BO4)5− structural units. Furthermore, these two fundamental units can be arbitrarily combined to form different BxOy structural groups [2]. Among these borates, especially the monoclinic bismuth borate BiB3O6 shows up remarkably large linear and nonlinear optical coefficients [3,4]. Calculations indicate that this can be mainly attributed to the contribution of the (BiO4)5− anionic group [5,6]. For the linear properties (refractive index) this anionic group should act in a similar way in an amorphous environment, i.e., in glass. Combining bismuth oxide with boric oxide thus allows tuning the optical properties in a wide range depending on the composition. Consequently, the properties of glasses of the system Bi2O3-B2O3 have attracted much interest [7].
The trivalent samarium ion (Sm3+) is one of the most important active ions in the RE family (cerium to lutetium) due to its convenient closely lying energy level structure [8], that has been exploited in upconversion processes mainly in low phonon crystalline hosts and rarely in glasses [9-13]. Within the Sm3+ ion energy scheme tricolor visible upconversion processes can take place from the 4G5/2 ® 6H5/2 (green), 4G5/2 ® 6H7/2 (orange) and 4G5/2 ® 6H9/2 (red) electronic transitions. Moreover, Sm3+ doped bismuth-borate glass has high density and radiation hard property. Also it is easy to made, can be produced with low cost and wide range of emission band. Therefore, it is a good candidate for radiation detector and possible to apply high energy and nuclear physics, medical imaging, homeland security and radiation detection. In this work, Sm3+ doped bismuth borate glasses have been synthesized by conventional melt quenching technique and investigate on X-rays luminescence, optical and physical properties of glass samples.
2. Experimental
The compositions of glass are (50 − x) B2O3:50Bi2O3: xSm2O3 (x = 0.0, 0.5, 1.0, 1.5, 2.0, 2.5 mol%). The batch was prepared from the AR grade of Bi2O3, H3BO3 and Sm2O3. The glasses were melted in a high alumina crucible at 1100˚C under normal atmosphere. The molten glass was cast into a stainless steel plate and properly annealed. The glass thus obtained was cut and polished for optical measurement. The density was measured by the Archimedes method using xylene as immersion liquid. Density of xylene at the experimental temperature was found to be 0.863 g/cm3. The corresponding molar volume, Vm, was calculated using the following formula [14]:
(1)
where M is the molecular weight of the multi-component glass system.
The UV-VIS absorption spectra were obtained with a double-beam spectrophotometer (Variance, Cary-50). According to Davis and Mott, the absorption coefficient, a(n), as a function of incident photon energy (hn) for direct and indirect optical transitions is given by [15]:
(2)
where the exponent n = 1/2 for an allowed direct transition, while n = 2 for an allowed indirect transition, a0 is a constant related to the extent of the band tailing, and Eg is the optical band gap energy. The absorption coefficient, a(n), can be determined near the absorption edge of different photon energies for all glass sample. It is well known that for amorphous materials a reasonable fit of Equation (2) with n = 2 is achieved. Therefore, the values of optical band gap energy (Eg) can be determined from the plot of (ahn)1/2 versus photon energy (hn) (Tauc’s plot), for allowed indirect transitions.
Refractive index of these glasses has been calculated by using the relation proposed by Dimitrov et al. [16,17].
(3)
In order to measure the X-ray luminescence of the Sm2O3 doped bismuth borate glass samples at room temperature, X-ray tube (DRGEM Co.) was used and faces of the glass sample were wrapped with several layers of Teflon tape excepting the one for attaching to the optical fiber. Signals from the glass sample by the induced X-ray were measured using a QE65,000 spectrometer (Ocean Optics Co.) The QE65,000 was cooled to −15˚C to reduce thermal noise in the CCD. It was used to plot the X-ray emission spectrum of the glass sample by window based-software [18,19].
3. Result and Discussion
The template is used to format your paper and style the text. All margins, column widths, line spaces, and text fonts are prescribed; please do not alter them. You may note peculiarities. For example, the head margin in this template measures proportionately more than is customary. This measurement and others are deliberate, using specifications that anticipate your paper as one part of the entire proceedings, and not as an independent document. Please do not revise any of the current designations. The measured density of Sm3+ doped bismuth borate glass samples for different Sm2O3 concentrations are shown in Figure 1. As seen in Figure 1, density increase linearly with additional content of Sm2O3 into the network. This indicates that replacing B2O3 by addition of a small amount of Sm2O3 results in the increase of the average molecular weight due to Sm2O3 has a higher relative molecular weight than that of B2O3. Figure 2 shows the variation of the molar volume with Sm2O3 concentration. As shown in Figure 2, the molar volume increased with an increasing of Sm2O3 concentration, because of increasing of non-bridging oxygen (NBOs). The increase of NBOs in the glass structure leads to an increase in average atomic separation. The results obtained indicate that the Sm2O3 oxide enters the glass network as a modifier by occupying the interstitial space in the network and generating the NBOs to the structure. It can also be concluded that the addition of Sm2O3 may accordingly result in an extension of glass network [20].
The absorption spectra of Sm3+ doped bismuth borate glasses in the UV-VIS region at room temperature are shown in Figure 3. It is clearly observed that the absorption intensity of the absorption bands increases with the increase of Sm2O3 concentration. Three absorption bands peaked at 474 nm, 950 nm and 1083 nm were observed. All absorption band spectra are characteristics of Sm3+ doped oxide glasses [21] and the observed absorption bands were assigned to appropriate f-f electronic transitions of Sm3+ ions from the 6H5/2 ground state to (4I13/2 + 4I11/2 + 4M15/2), 6F11/2 and 6F9/2 respectively.
The optical band gap were evaluated by Tauc’s plot using Equation (2) and shown in Figure 4. When increase Sm2O3, bonding defect and non-bridging oxygen were increased. These leads to increase in the degree of
Figure 1. Densities of Sm2O3 doped in bismuth borate glass.
Figure 2. Molar volume of Sm2O3 doped in bismuth borate glass.
Figure 3. Absorption spectra of Sm2O3 doped in bismuth borate glass.
Figure 4. Typical Tauc’s plot of Sm2O3 doped in bismuth borate glass.
localization of electrons there by increasing the donor center in the glass matrix. The increasing presence of donor center, therefore, decreases the optical band gap. As a result of this, the band gap are decreased as shown in Figure 5, for indirect allow transition. The refractive index of these glasses has been calculated by using Equation (3) and show in Figure 6. The result show the refractive index of glasses increased with increasing of Sm2O3 concentration.
Figure 7 showed X-rays luminescence spectra of Sm2O3 doped bismuth borate glasses. The emission wavelength observed at 569 nm, 598 nm, 641 nm and 705 nm The luminescence spectra of the Sm2O3 doped bismuth borate glass were identified as 4G5/2 → 6H5/2 (569 nm), 4G5/2 → 6H7/2 (598 nm), 4G5/2 → 6H9/2 (641 nm) and 4G5/2 → 6H11/2 (705 nm) [22]. The intensity of luminescence was increase with increasing doping concentration.