Luminescent properties of alumina ceramics doped with manganese and magnesium

Luminescent properties of alumina ceramics doped with manganese and magnesium

Optical Materials 91 (2019) 349–354 Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat Lu...

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Optical Materials 91 (2019) 349–354

Contents lists available at ScienceDirect

Optical Materials journal homepage: www.elsevier.com/locate/optmat

Luminescent properties of alumina ceramics doped with manganese and magnesium

T

S.V. Zvonareva,∗, E.I. Frolova,b, K. Yu. Chesnokovc, N.O. Smirnova, V.A. Pankova, V.Y. Churkina a

Ural Federal University, 19 Mira Str., Ekaterinburg, Russia Samara State Technical University, 244 Mologvardeyskaya Str., Samara, Russia c Institute of Solid State Chemistry, Ural Branch, Russian Academy of Sciences, 91 Pervomaiskaya Str., Ekaterinburg, Russia b

A R T I C LE I N FO

A B S T R A C T

Keywords: Alumina ceramic Luminescence Impurity center Concentration dependence High-temperature sintering

Alumina ceramics doped with a different concentration of magnesium and manganese were synthesized at a different annealing temperature and medium. An increase in the dopant concentration leads to an increase in the pulsed cathodoluminescence intensity of impurity centers at 515 nm (Mg) and 678 nm (Mn). The concentration quenching of the luminescence in the 678 nm band occurs for Al2O3:Mn ceramics. The dopants affect the luminescence intensity in the F-center band differently. An increase in the concentration of an impurity from 0.001 to 18.77 wt % in Al2O3:Mn ceramics leads to a 4-fold decrease in the luminescence intensity of the F-center. On the other hand there is a maximum of the luminescence intensity of the F-center for Al2O3:Mg ceramics with a dopant concentration of 1 wt %. The luminescence bands of Cr3+ (687 nm) and most likely F-centers (670 nm) are dominated depending on the excitation energy in photoluminescence spectra of both Al2O3:Mg and Al2O3:Mn ceramics. The low luminescence intensity of the magnesium band (750 nm) is detected upon the excitation in the band of 325 nm. The luminescence bands of Mn2+ (540 nm) and Mn4+ (650 nm) with low intensity are found in the photoluminescence spectrum too.

1. Introduction Studying the role of synthesis parameters, structural condition of the material and mechanisms of charge transfer processes in the establishment of luminescent properties of low-dimensional nonstoichiometric binary and multicomponent oxides is an important fundamental and application-oriented task [1–3]. Among oxides wide band gap insulators applied in numerous devices of modern technology, whose luminescent properties are studied in order to create highly efficient luminescent materials on their basis, are of considerable interest [4,5]. Experimental studies of material optical properties have great importance for better understanding peculiarities of the structural condition and anticipating properties of new functional materials. It is also of particular interest to study the above processes in the materials where cluster defects [6,7] and dopant centers capable of capturing free charge carriers [8–10] can be observed. Changes in the state of the centers population would lead to probabilities redistribution of concurrent processes for electron and hole transfer between defect centers and delocalized zones, and thus, to shifts in luminescent properties of materials. Introduction of various rare earth elements [11–13] and metals



[14,15] in the initial matrix of a material will also cause considerable changes in its luminescent properties. Nowadays oxide systems with rare earth elements [16–18] and metals [19,20] have promising applications in terms of ceramics. Ceramic structures possess higher mechanical strength and stable properties. In addition, ceramics can be used to make samples of a desired shape and size. High-temperature synthesis of luminescent ceramics from nanostructural powders will be accompanied both by grain growth and new phase development. In some cases phosphors based on nanomaterials may have a high light output since the electron-hole pairs of these materials when excited will be produced in small quantities restricted by the size of a nanoparticle. They are at a short distance from one another which increases probability of recombination and intensity of luminescence. Furthermore, the selection rules may fail due to a disturbed translation symmetry of such materials, resulting in additional optical transitions and new emission bands expanding the spectrum of luminescence will develop. Regularities and mechanisms of luminescence in oxide systems with rare earth elements are currently being carefully investigated. Optical properties of binary and multicomponent oxides with metal dopants are poorly studied, in particular matrixes of alumina with magnesium and manganese. Materials based on Mg [21,22] and Mn [23] doped alumina

Corresponding author. E-mail address: [email protected] (S.V. Zvonarev).

https://doi.org/10.1016/j.optmat.2019.03.019 Received 31 October 2018; Received in revised form 5 March 2019; Accepted 13 March 2019 0925-3467/ © 2019 Elsevier B.V. All rights reserved.

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have a potential application in optics and dosimetry. On photo- and cathodoluminescent spectra of the mentioned materials the bands of the maxima of 400 and 500 nm corresponding to various centers of F-type (oxygen vacancies) as well as the bands connected with the impurity ions of Mg (540 nm), Mn (676 nm) and Cr are recorded. Moreover, in such materials complex defects which include both impurity ions and oxygen vacancies can be formed. In addition the position of the luminescent bands changes and a blue shift is observed. In this work we studied luminescent properties of Mg, Mn-doped alumina ceramics with a different impurity concentration synthesized at high-temperature sintering. 2. Materials and methods The initial porous matrixes in the form of compacts were obtained by the method of cold static pressing of α–Al2O3 powder using PRG-150 hydraulic press in metal molds at a pressure varied within the range of 0.5–0.7 GPa. High-purity (99.5%) commercial nanopowder α–Al2O3 (‘VNIIOS NК’, Russia) with the particle size of 10–100 nm was synthetically prepared by an alcoholate method. The compacts were diskshaped with the diameter of 10.1 ± 0.05 mm and 1 ± 0.2 mm thick. The next stage was the heat treatment of compacts at a temperature of 450 °C for 2 h. In order to obtain doped ceramics porous matrixes were immerged into a solution of magnesium and manganese nitrate with a concentration of magnesium within the range of 0.001–6.85 wt % and manganese within the range of 0.001–18.77 wt %. To achieve a uniform dopant propagation by the sample volume the impregnation was carried out for one hour. Ceramics with doped luminescent centers were prepared by annealing both in air in an electric furnace (up to 1500 °C) and under vacuum (0.013 Pa) using high-temperature vacuum electric furnace SNVE 9/18 (up to 1700 °C) for two hours. The X-ray diffraction (XRD) was measured using X'Pert PRO MPD diffractometer in vertical goniometer with CuKα radiation in BraggBrentano geometry and β-filter (Ni) in the secondary beam. A solidstate PIXcel detector was used for recording XRD spectra with the active length of 3.347°. Measurements were carried out at room temperature with 2θ varied in the range from 20° to 144° using a step of 0.026°. The time per one step was 100 s. The effect of Mg and Mn impurities on luminescent properties of alumina ceramic was studied by means of pulse cathodoluminescence (PCL) and photoluminescence (PL) methods. PCL was measured in the spectral region (350–770 nm) by ‘Klavi’ spectrometer upon excitation with an electron beam (the pulse width of 2 ns, the average electron energy of 130 ± 10 keV, the electric current density of 60 А/cm2). PL measurements were carried out on the Perkin Elmer LS-55 luminescent spectrometer with the help of a pulsed xenon lamp as a source of light with the excitation range of 200–515 nm and emission of 430–900 nm applying standard in-built filters. PL measurements were performed both in terms of fluorescence and phosphorescence depending on luminescence intensity of the doped ceramics under study.

Fig. 1. XRD patterns of undoped (a), Mg (b) and Mn-doped (c) alumina ceramics vacuum sintered at 1500°С for 2 h.

second phase formation (MnAl2O4). It should be noted that identical annealing conditions yield the larger amount of Mg spinel compared to Mn spinel. 3.2. Luminescent properties of Al2O3:Mg ceramics It is known [24], that vacuum sintering will lead to the production of oxygen-deficient ceramics. Since carbon is a good reducing agent it is used to increase oxygen vacancy concentration in the course of annealing. A PCL spectrum for alumina ceramics doped with Mg ions and vacuum sintered at the temperature of 1500°С for two hours is given in Fig. 2 compared with pure ceramics. Ceramics with magnesium had the highest possible concentration of impurities (6.85 wt %) when doped in a solution of magnesium nitrates at a room temperature. The high-

3. Results and discussion 3.1. X-ray diffraction analysis XRD patterns of undoped, Mg and Mn-doped alumina ceramics vacuum sintered at 1500°С are given in Fig. 1. According to the data of XRD analysis, the undoped ceramic contained only α-Al2O3. The parameters of crystalline lattice are the following: a = (0.47586 ± 0.00002) nm, c = (1.29933 ± 0.00003) nm. In Mgdoped ceramics AlMg spinel is formed which equals to 23.9% in ceramics vacuum sintered at 1500°С for 2 h preimpregnated in Mg nitrate solution of its maximum concentration (6.85 wt %). High temperature annealing of the ceramics preimpregnated in Mg nitrate solution of its maximum concentration (18.77 wt %) under vacuum also results in the

Fig. 2. PCL spectra of alumina ceramics doped with magnesium synthesized under vacuum at the temperature of 1500°С for 2 h: 1 – pure ceramic, 2 – ceramic doped with Mg (6.85 wt %). 350

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Fig. 3. PCL spectra of Al2O3:Mg ceramics synthesized in air at the temperature of 1500°С for 2 h after impregnation in a solution with different concentration of dopant: 1 – 0.1 wt %, 2 – pregnant solution with magnesium concentration of 6.85 wt %.

Fig. 5. Changes in the PCL intensity at an increasing dopant concentration of the Al2O3:Mg ceramics synthesized under vacuum at the temperature of 1500°С for 2 h at different temperature peaks.

equal concentrations of alumina and magnesium spinel oxygen vacancies. Under small magnesium concentrations the luminescence of the 515 nm band is not registered. To evaluate the effect of dopants on luminescent properties of alumina ceramics, PCL spectra were measured under variation of the dopant ion concentration at annealing temperatures ensuring the maximum luminescence intensity within the emission band of the relevant dopant. Thus, the dependence of the PCL intensity on a magnesium ion concentration for two luminescence bands (F-center and impurity center) was presented for Al2O3:Mg ceramics annealed under vacuum at the temperature of 1500 °C for 2 h (Fig. 5). An insignificant growth of the luminescence intensity is recorded within the dopant emission band under small concentrations of impurities. Approximately a fifteen-fold growth of PCL intensity of the impurity band is observed in the range of dopant concentrations of 0.1–7 wt. %. For Al2O3:Mg ceramics the dopant concentration under which maximum intensity is reached within the emission band of 400 nm is determined. This value is equal to 1 wt %. PL excitation and emission spectra of Al2O3:Mg ceramics are shown in Fig. 6. Two bands can be identified with their maxima of 320 and 420 nm in the excitation spectrum. The 320 nm band is normally associated with excitation of various F-centers in alumina [28], while upon their excitation both various F-centers [29] and ions Cr3+ [30] will luminesce in the 400 nm band. PL emission spectrum under its excitation in these bands contains the two most intense narrow emission bands with the maximum at 670 and 687 nm (Fig. 6b). It is known that chromium ions have a narrow luminescence band in the region of the second emission band (R-line at 693 nm). This band corresponds to the 2E→4A2 electron transition in Cr3+ ion. Furthermore, as it was mentioned above chromium ions are present in the samples under study, as evidenced by the data of PCL spectra. A slight shift of this peak to the short-wave region may occur due to the presence of magnesium and a closely located radiative transition at 1.85 eV. In this band there is no luminescence in the magnesium spinel and the pure nanostructured ceramics synthesized from the α-Al2O3 nanopowder. However, this band was recorded for samples of the (δ + θ) –phases aluminum oxide upon excitation with the wavelength of 175 nm. Since the studied ceramics was synthesized from α-Al2O3 at a temperature of more than 1500 °C, these phases are unlikely to occur.

temperature synthesis would lead to the formation of intrinsic luminescence centers of the alumina ceramic. A luminescence band peaked at 400 nm is well known to occur due to F-centers in aluminum oxide host [25]. The 515 nm emission band is most likely associated with a magnesium dopant center. PCL spectra of ceramics both pure and doped have a luminescent center with the maximum at 693 nm (R-line of Cr3+ impurity ions) [26,27]. Chromium dopant was found in smaller concentrations in the initial alumina nanopowder. The high-temperature synthesis in air will also generate changes in luminescent properties of the ceramic under consideration. Fig. 3 illustrates the PCL spectra of Al2O3:Mg ceramics with different concentrations of the dopant synthesized in air at the same modes as described above. Thus, rise in the concentration of magnesium will provoke a considerable increase in the intensity in the 515 nm band and will have almost no influence on the intensity of the intrinsic band of the alumina luminescence at 400 nm. As it was noted, the emission band at 400 nm is associated with oxygen vacancies of the aluminum oxide (F-center). The 515 nm emission band is associated with oxygen vacancies of the magnesium spinel [23]. It should be noted that for Al2O3:Mg ceramics the annealing in air will lead to a great increase in the PCL intensity of the 515 nm band as compared to the annealing under vacuum. An increase in the intensity of the 400 nm band is almost not registered. An increase in the vacuum annealing temperature will lead to the oxygen vacancy growth of pure alumina ceramics. A luminescence intensity increase of the 400 nm band is not observed for ceramics doped with magnesium (Fig. 4). Under high magnesium concentrations the annealing temperature has no impact on the oxygen vacancy formation of magnesium spinel. In this regard the luminescence of the 400 and 515 nm bands has roughly the same intensity, which is indicative of

3.3. Luminescent properties of Al2O3:Mn ceramics The emission band of F-center for ceramics sintered under vacuum and in air at the temperature of 1500°С for 2 h with different manganese ion concentration is not registered compared to PCL spectra of Al2O3:Mg ceramics. The dopant concentration provides quenching of intrinsic luminescence centers of the matrix. Fig. 7 illustrates the PCL spectra of Al2O3:Mn ceramics with different concentrations of the

Fig. 4. PCL spectra of alumina ceramics with magnesium concentration of 6.85 wt % synthesized for 2 h at different temperatures: 1–1500 °C, 2–1600 °C, 3–1700 °C. 351

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Fig. 6. PL excitation (a) and emission (b) spectra of alumina ceramics with magnesium concentration of 6.85 wt % synthesized under vacuum at the temperature of 1500°С for 2 h.

Fig. 8. PCL spectra of alumina ceramics with manganese concentration of 18.77 wt % synthesized for 2 h at different temperatures: 1–1500 °C, 2–1600 °C, 3–1700 °C.

Fig. 7. PCL spectra of Al2O3:Mn ceramics synthesized in air at the temperature of 1500°С for 2 h after impregnation in a solution with different concentration of dopant: 1 – 0.1 wt %, 2 – pregnant solution with manganese concentration of 18.77 wt %.

dopant synthesized in air. The annealing in air with the same modes of the vacuum sintering does not also contribute to the formation of oxygen vacancies of the alumina (400 nm). However the concentration quenching of the luminescence in the 678 nm band has been observed. Thus, there is no luminescence in the PCL spectrum for ceramics with the maximum concentration of manganese ions. As compared to the annealing under vacuum the annealing in air does not contribute to the formation of Mn4+ ions responsible for the emitting transition 2E → 4A2 by the luminescence in the 678 nm band [31]. These luminescence centers are formed due to the replacement of Al3+ ions with Mn4+ ions in octahedral sites of aluminum oxide structures, which is not the case when annealing takes place in air at high concentrations of dopants. An increase in the sintering temperature for Al2O3:Mn ceramics with high manganese concentrations (more than 1 wt %), unlike magnesium ceramics, results in an increase in the luminescence intensity of a dopant band (Fig. 8). An adverse effect is observed under small dopant concentrations (0.001–0.1 wt %). An increase of the annealing temperature leads to a decrease of luminescence intensity in the 678 nm band and dominance of the chromium luminescence band (693 nm) within PCL spectrum. Unlike Mg doped ceramics, the annealing temperature increase of Mn doped ceramics results in the intensity growth at the 400 nm band. For all manganese concentrations under study the value close to double increase of PCL intensity is observed within the Fcenter emission band. The dependence of the PCL intensity on a manganese ion concentration was presented for Al2O3:Mn ceramics annealed under vacuum at the temperature of 1700 °C for 2 h (Fig. 9). Approximately a fifteen-fold growth of PCL intensity of the impurity band is observed in the range of dopant concentrations of 0.1–7 wt. %. In addition for Al2O3:Mn ceramics there is a saturation of the impurity center emission and a slight intensity decline when reaching maximum value of the impurity concentration for selected doping method at a room

Fig. 9. Changes in the PCL intensity at an increasing dopant concentration of the Al2O3:Mn ceramics synthesized under vacuum at the temperature of 1700°С for 2 h at different temperature peaks.

temperature. It should be noted that magnesium and manganese dopants affect the emission intensity of the intrinsic F-center of the aluminum oxide to different extents. The dopant concentration growth for Al2O3:Mn ceramics leads to a slight decline in the intensity of the Fcenter luminescence. The band of 325 nm is observed in the Al2O3:Mn ceramics excitation spectrum PL (Fig. 10a). This band has the highest intensity also observed in Al2O3:Mg ceramics. In addition the bands with their maxima of 365 and 420 nm are recorded. The PL spectrum of Al2O3:Mn ceramics under wavelength excitation of 325 nm is a similar spectrum of Al2O3:Mg ceramics with a dominant band with a maximum at 670 nm and a less intense band at 687 nm (Fig. 10b). The 720 nm and the 540 nm bands are recorded for Mn-doped ceramic under excitation with wavelength of 365 nm and 420 band respectively unlike ceramics doped with magnesium. Under 422 nm wavelength excitation direct excitation band of Mn2+ ion with the luminescence in the range of 352

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Fig. 10. PL excitation (a) and emission (b) spectra of alumina ceramics with manganese concentration of 18.77 wt % synthesized under vacuum at the temperature of 1500°С for 2 h.

luminescence in the 678 nm band occurs for Al2O3:Mn ceramics. The dopants under study affect the luminescence intensity in the F-center band differently. An increase in the concentration of an impurity from 0.001 to 18.77 wt % in Al2O3:Mn ceramics leads to a 4-fold decrease in the luminescence intensity of the F-center. On the other hand for Al2O3:Mg ceramics with a dopant concentration of 1 wt % there is a maximum luminescence intensity of F-center. The PL spectra measured in ceramics with a various concentration of impurities are found to be in agreement with PCL spectra. The luminescence bands of Cr3+ (687 nm) and most likely F-centers (670 nm) dominate in the PL spectrum of the Al2O3:Mg ceramics depending on the excitation energy. The low luminescence intensity of magnesium band (750 nm) is recorded upon excitation in the band of 325 nm. The bands of 670 and 687 nm also dominate in the PL spectrum of the Al2O3:Mn. The luminescence band of Mn2+ (540 nm) and Mn4+ (650 nm) with low intensity are also found in the PL spectrum. The luminescence intensity of both impurity centers and alumina ones changes when the dopant concentration varies.

Fig. 11. PL spectra (λex = 325 nm) of alumina ceramics synthesized under vacuum at the temperature of 1700°С for 2 h at varying concentration of dopant: 1–10 wt %, 2–1 wt %, 3–0.1 wt %, 4–0.01 wt %.

515–560 nm corresponds to the 6A1→4A1(4G)/4E(4G) transition [15]. The 325 nm wavelength excitation leads to luminescence of Mn4+ ions in the range from 570 to 680 nm [29]. The low luminescence intensity of the 650 nm band as well as the 670 nm band is recorded in PL spectrum. However, the most intense peak at 720 nm for Mn doped alumina ceramics has not been observed. It should be noted that this luminescence band is observed after an intense radiation exposure to a high-energy electron beam as a result of which the luminescence centers are filled. After photo annihilation of this center, the band is not recorded. The luminescence of the 670 nm band has the highest intensity under 325 nm wavelength excitation in the PL spectra. In this regard, this band was selected for analysis of the impurity concentration effect on the luminescence intensity. PL spectra of Al2O3:Mn ceramics synthesized under vacuum at the temperature of 1700°С for 2 h at a varying dopant concentration in the range of 0.01–10 wt % is shown in Fig. 11. An increase in the manganese concentration leads to a significant increase in the luminescence in the region of 650–750 nm. This result shows that the luminescence in this region is associated with Mn4+ ions and increases with an impurity concentration growth.

Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The authors declare the following financial interests/personal relationships which may be considered as potential competing interests. Acknowledgments This work was financially supported by the Russian Science Foundation, project No. 18-72-10082. References [1] T.P. Mokoena, E.C. Linganiso, V. Kumar, H.C. Swart, So-Hye Cho, O.M. Ntwaeaborwa, Up-conversion luminescence in Yb3+-Er3+/Tm3+ co-doped Al2O3-TiO2 nano-composites, J. Colloid Interface Sci. 496 (2017) 87–99 https://doi. org/10.1016/j.jcis.2017.02.018. [2] A. Patra, R.E. Tallman, B.A. Weinstein, Effect of crystal structure and dopant concentration on the luminescence of Cr3+ in Al2O3 nanocrystals, Opt. Mater. 27 (2005) 1396–1401 https://doi.org/10.1016/j.optmat.2004.10.002. [3] E. de Albuquerque Brocchi, R.N.C. de Siqueira, M.S. Motta, F.J. Moura, I.G. Sol_orzano-Naranjo, Reduction reactions applied for synthesizing different nano-structured materials, Mater. Chem. Phys. 140 (2013) 273–283 https://doi. org/10.1016/j.matchemphys.2013.03.034. [4] S.J. Yoon, D.A. Hakeem, K. Park, Synthesis and photoluminescence properties of MgAl2O4:Eu3+ phosphors, Ceram. Int. 42 (2016) 1261–1266 https://doi.org/10. 1016/j.ceramint.2015.09.059. [5] B.S. Choi, O.G. Jeong, J.C. Park, J.W. Kim, S.J. Lee, J.H. Ryu, J.I. Lee, H. Cho, Photoluminescence properties of non-rare earth MgAl2O4:Mn2+ green phosphor for LEDs, J. Ceram. Process. Res. 17 (2016) 778–781. [6] A. Mandowski, How to detect trap cluster systems? Radiat. Meas. 43 (2008) 167–170 https://doi.org/10.1016/j.radmeas.2007.09.018. [7] V. Pagonis, C. Kulp, Simulations of isothermal processes in the semilocalized transition (SLT) model of thermoluminescence (TL), J. Phys. D Appl. Phys. 43

4. Conclusions Mg- and Mn-doped alumina ceramics with a different dopant concentration were synthesized both under vacuum and in air at varying annealing temperatures. PL and PCL spectra have been measured to evaluate the effect of dopant concentration on luminescence both impurity centers and intrinsic centers of alumina centers. An increase in the dopant concentration leads to an increase in the PCL intensity of impurity centers at 515 (Mg) and 678 nm (Mn). The sintering in air contributes to a greater increase in the luminescence intensity of the impurity center in comparison to the sintering under vacuum for Mgdoped alumina ceramics. The concentration quenching of the 353

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