Role of Eu2+ and Dy3+ Concentration in the Persistent Luminescence of Sr2MgSi2O7 Glass-Ceramics

Citation: Fernández-Rodríguez, L.; Balda, R.; Fernández, J.; Durán, A.; Pascual, M.J. Role of Eu2+ and Dy3+ Concentration in the Persistent Luminescence of Sr2MgSi2O7 Glass-Ceramics. Materials 2022,15, 3068. https://doi.org/10.3390/ ma15093068 Academic Editor: Giancarlo C. Righini Received: 30 March 2022 Accepted: 21 April 2022 Published: 23 April 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). materials Article Role of Eu2+ and Dy3+ Concentration in the Persistent Luminescence of Sr2MgSi2O7Glass-Ceramics Laura Fernández-Rodríguez 1, Rolindes Balda 2,3 , Joaquín Fernández 4, Alicia Durán1 and María Jesús Pascual 1,* 1Ceramics and Glass Institute (CSIC), C/Kelsen 5, Campus de Cantoblanco, 28049 Madrid, Spain; [email protected] (L.F.-R.); [email protected] (A.D.) 2Departamento Física Aplicada, Escuela Superior de Ingeniería, Universidad del País Vasco (UPV-EHU), 48013 Bilbao, Spain; [email protected] 3Centro de Física de Materiales, UPV/EHU-CSIC, 20018 San Sebastian, Spain 4Donostia International Physics Center DIPC, 20018 San Sebastian, Spain; [email protected] *Correspondence: [email protected] Abstract: In this study, glass-ceramics based on Sr 2 MgSi 2 O 7 phosphor co-doped with Eu/Dy were obtained from the sintering and crystallisation of glass powders. The glasses were melted in a gas furnace to simulate an industrial process, and the dopant concentration was varied to optimise the luminescence persistence times. The doped parent glasses showed red emission under UV light excitation due to the doping of Eu 3+ ions, while the corresponding glass-ceramics showed persistent blue emission corresponding to the presence of Eu 2+ in the crystalline environment. The dopant concentration had a strong impact on the sintering/crystallisation kinetics affecting the final glassceramic microstructure. The microstructures and morphology of the crystals responsible for the blue emission were observed by scanning electron microscopy–cathodoluminescence. The composition of the crystallised phases and the distribution of rare-earth (RE) ions in the crystals and in the residual glassy phase were determined by X-ray diffraction and energy dispersive X-ray analysis. The emission and persistence of phosphorescence were studied by photoluminescence. Keywords: Sr2MgSi2O7; phosphors; europium; glass-ceramics; persistent luminescence 1. Introduction Rare-earth (RE)-doped glasses and glass-ceramics are considered good matrices for luminescent or light-emitting applications. Some of their well-known applications are LEDs, sensing, solar cells, biomedicine, etc. [1–4]. A wide variety of host materials are used as luminescent compounds, but most known hosts decline when it comes to persistent luminescence. For many decades, copperdoped zinc sulphide [ 5 ] has been the most widely used persistent phosphor; however, its brightness and lifetime were rather low (<1 min) for practical purposes. Currently, the use of ZnS: Cu has decreased in favour of rare-earth-doped aluminates and silicates [6]. Since the mid-1990s, a new generation of persistent luminescent phosphors has been developed and has partially entered the commercial market [ 7 ]. Most research groups focused their attention on strontium aluminate matrices doped with europium and its derivatives SrAl 2 O 4 :Eu 2+ , Dy 3+ [ 8 ]. Eu 2+ -, Dy 3+ -, and Nd 3+ -doped aluminates exhibit blue-centred emission bands [ 9 , 10 ]. These aluminates have a half-life close to 6 h [ 11 , 12 ]. Another group of materials that has recently been investigated is silicates. Silicates were chosen as hosts due to their special properties, such as low cost, ease of preparation, and excellent thermal and chemical stabilities. The M 2 MgSi 2 O 7 (M = Ca, Sr, Ba) family of materials, also called alkaline earth akermanites, plays a similar role to that of MAl 2 O 4 in the aluminate group. The best-known persistent luminescent silicate, Sr 2 MgSi 2 O 7 : Eu 2+ , Dy 3+ was investigated for the first time by Lin et al. in 2001 [ 13 ]. The emission band is Materials 2022,15, 3068. https://doi.org/10.3390/ma15093068 https://www.mdpi.com/journal/materials Materials 2022,15, 3068 2 of 15 centred in the blue and has a half-life of about 10 h when the material is produced by solid-state synthesis [14]. The solid-state reaction is the most common way to prepare crystalline materials based on M 2 MgSi 2 O 7 crystalline materials, but coprecipitation [ 15 ] and combustion [ 16 ] methods have also been successfully applied. In the investigations of Tian et al. [ 17 ], the concentration dependence and energy transfer of Y 2 (MoO 4 ) 3 : Dy 3+ type phosphors were analysed. The mechanism of energy transfers between Dy 3+ ions was studied by several theories, and it was concluded that the electric dipole–dipole interaction between Dy 3+ ions is the main physical mechanism of energy transfers between Dy3+ ions. Jiang et al. [ 18 ] synthesised Ca 2 MgSi 2 O 7 : Eu, Dy, Nd, using the solid-state reaction method, and modified the Dy/Eu ratio, to observe the changes in the luminescent response. The emission intensity at 518 nm and the decay rate were different for phosphors with different Dy/Eu ratios. By slightly increasing the Eu 2+ content with a constant Dy 3+ and Nd 3+ content (Dy/Eu = 1/2 or 1/1), both the emission intensity and afterglow increased. However, when the value of Dy/Eu was higher than 20/7, the afterglow time and emission intensity decreased, which can be attributed to concentration quenching of Eu 2+ . Nevertheless, no persistence studies have been included in their research. Shrivastava et al. [ 19 ] prepared Ca 2 MgSi 2 O 7 : Eu 2+ , Dy 3+ of different Eu/Dy concentration ratios with solid-state reaction. The emission spectra were identical in shape, and the bands differed only in intensities, with the highest observed for Eu/Dy ratio = 0.5/1.5 . The broad emission spectra centred at 510 nm were observed under the ultraviolet excitation of 395 nm, which correspond to Eu 2+ emission. Since the crystal field can greatly affect the electronic states, this suggests that the crystal field does not change much with compositional variations. The intensity of the thermoluminescence signals decreased, and the position of the temperature peak shifted to the upper side with increasing delay time, indicating a reasonable retraction associated with non-first order kinetics. The decay curve showed characteristics of a simple exponential equation, with a decay constant of 4.96 min. He et al. [ 20 ] successfully prepared Sr 2 MgSi 2 O 7 :Eu 2+ , Dy 3+ nanofibers using the electrospinning method. The Sr 2 MgSi 2 O 7 :Eu 2+ , Dy 3+ nanofibers were formed after calcining at 1150 ◦ C for 5 h. The optimum concentration of Eu and Dy co-doping for this investigation was x = 0.03, y = 0.04 in Sr 2-x-y MgSi 2 O 7 :xEu 2+ , yDy 3+ ; the nanofibers had a blue emission peak at 471 nm, attributed to typical Eu 2+ emission, which is made from the 4f 6 5d 1 -4f 7 transition. The co-doped Dy 3+ ion plays an important energy transfer and electron trapping role and can prolong the persistence time of the luminescent Sr 2 MgSi 2 O 7 :Eu 2+ , Dy 3+ nanofibers. However, no data regarding decay or persistence times have been provided in this study. Luminescent emissions in glass-ceramic materials are strongly affected by crystallinity, as well as by microstructure, number of active centres present, matrix composition, crystal field, etc. Wondraczek et al. [ 21 ] investigated (Sr,Ca)–akermanite (Ca,Sr) 2 MgSi 2 O 7 glass-ceramics doped with europium. The red emission of the glass presented a characteristic Eu 3+ band, while the glass-ceramic emitted in blue with the characteristic Eu 2+ band. Scanning electron microscopy–cathode-luminescence (SEM–CL) characterisation indicated that the crystalline phase was associated with Eu 2+ emission, and it was claimed that Eu 2+ is incorporated in the Sr 2+ sites of the akermanite structure, while Eu 3+ accumulates in the intergranular phase of the glass. According to Hölsä’s investigations [ 22 ] on the mechanism of luminescence, the exact role of defects in excitation energy storage is still not well understood. It has been established that defects can form due to charge compensation and preparation conditions. The most significant structural modifications due to defects in the environment of the Eu 2+ luminescent centre were found with the introduction of the strontium vacancy. Electron traps were created by the Eu 2+ and strontium vacancy, as well as by the oxygen vacancy, while strontium, magnesium, and silicon vacancies also created shallow voids in the Materials 2022,15, 3068 3 of 15 material. Electron traps close to the conduction band may contribute to the persistent luminescence efficiency, as they are easily bleached by thermal energy at room temperature. However, traps that are too shallow or too deep can decrease this efficiency. Later, Duan [ 23 ] proposed that, for every oxygen vacancy, there are two associated electrons, and these fully occupy the singlet state, which is created from the valence band states of the host. Furthermore, the gap between oxygen and vacancy narrows on the order of 1–2 eV relative to the Sr 2 MgSi 2 O 7 gap. This means that the incorporation of oxygen vacancies can enhance the persistent luminescence of the material by improving the absorption of ultraviolet light. Eu 2+ ions act as luminescent centres, while Dy 3+ ions act mainly as traps in the host [ 24 ]. The oxygen vacancies and Dy 3+ ions provide the deep traps; the residual conduction band electrons can be trapped after turning off the UV light source. The Dy ions in Sr 2 MgSi 2 O 7 : Eu 2+ , Dy 3+ only act as trapping centres, since Sr 2 MgSi 2 O 7 : Dy 3+ without Eu doping has no luminescent properties [25]. As discussed extensively in a previous paper [ 26 ], glass-ceramics based on Eu/Dydoped Sr 2 MgSi 2 O 7 phosphor were obtained from sintering and crystallisation of glass powders (base composition 55SiO 2 -27SrO-18MgO mol%). Increasing the dopant content resulted in higher stability of the glass against crystallisation. Electric and gas furnaces were used for glass melting. The doped parent glasses showed red emission under UV light excitation, while the corresponding glass-ceramics showed blue emission. The emission spectra of the glass-ceramics indicated features attributable to Eu 2+ and Eu 3+ cations. The Eu/Dy co-doped glass-ceramic provided a lower Eu 3+ associated signal than that of the Eu-doped glass-ceramic. Cathodoluminescence measurements indicated that Eu 2+ emission originated from Sr 2 MgSi 2 O 7 crystals, and Eu 3+ emission from the remaining glassy phase, suggesting that europium reduction occurs in the crystalline phase. Photoluminescence emission spectra showed a main peak at 484 nm, associated with typical T 2g→8 S 7/2 transitions of Eu 2+ under excitation at 390 nm in the glass-ceramics. The presence of Dy 3+ increased persistence in the samples melted in the gas furnace. Dy 3+ ions are generally attributed to the formation of deeper traps and increased persistent luminescence. The results of our latest published research [ 27 ], according to which only glass-ceramics co-doped with Dy3+ showed some persistence, are in agreement with this hypothesis. Analysis of the Eu L 3 -edge XANES spectra of the most persistent sample revealed that the Eu 2+ ratio was ~4% and that Eu 2+ was incorporated in the crystalline phase at a concentration of ~0.16 wt%. A higher amount of Eu 2+ was detected in the co-doped samples treated in a reducing atmosphere, but this did not lead to improvement in the persistence. We suggested that the luminescence mechanism involves the presence of shallow electron traps that are suddenly emptied for temperatures above 100 K. The most persistent luminescence is likely due to an effective concentration of Eu 2+ in the dendritic-shaped crystals, which are formed for this particular composition, and with a suitable level of Sr vacancies, even though Sr vacancies are most likely formed in every sample. The aim of this research was to study the changes in the emission of these Sr 2 MgSi 2 O 7 : Eu 2+ , Dy 3+ glass-ceramics obtained by sintering and crystallisation of glass powders as a function of the dopant concentrations. Glasses of the same base composition (55SiO 2 - 27SrO-18MgO, mol%.) with increasing Eu 2 O 3 and Dy 2 O 3 concentration were melted in a gas furnace, and their corresponding glass-ceramics were obtained after suitable thermal treatment. A complete study of the thermal, structural, and optical properties allows the selection of an optimum dopant level content in order to achieve higher persistence times. 2. Experimental Procedure 2.1. Glasses and Glass-Ceramics Preparation Glasses of the same base composition 55SiO 2 -27SrO-18MgO mol%. doped with different amounts of Eu 2 O 3 and Eu 2 O 3 /Dy 2 O 3 were prepared by melt-quenching. The employed raw materials were SiO 2 sand (Saint-Gobain, 99.6%), SrCO 3 (Alfa Aesar, 97.5%), MgO (PanReac, 98%), Eu 2 O 3 (Alfa Aesar, >99.9%), and Dy 2 O 3 (Alfa Aesar, >99.9%). Batches of 100 g Materials 2022,15, 3068 4 of 15 were mixed and stirred in a Turbula mixer for one hour to achieve homogenisation. The glasses were melted in a gas furnace in air using an alumina–zirconia–silica (AZS; Al 2 O 3 - ZrO 2 -SiO 2 ) crucible. The temperature was maintained at 1300 ◦ C for 15 min, followed by an increase to 1550 ◦ C for 1 h, with final heating to 1600 ◦ C. As soon as the furnace reached 1600 ◦C (without dwell time), the glass was poured into water to obtain a glass frit. The frits were milled in acetone, and the powders were sieved below 20 µ m. Cylindrical pellets (Ø = 2.5 cm) were prepared by pressing the powder into a die (1600 Kp for 3 min) before firing in an electric furnace at 1100 ◦ C (heating and cooling rate 10 ◦ C/min) for 1 min in the air atmosphere. Table 1summarises the studied glass-ceramic samples, whose names indicate that they are glass-ceramic (GC) samples and the dopant content in mol%. The glasses and glass-ceramics were first observed with the naked eye under the light of a UV lamp at 365 nm, which served as an excitation source. All original glass frits showed red emission under the excitation source, and most glass-ceramics showed blue emission under the excitation source. As widely discussed in a previous paper [ 26 ], the blue luminescence is associated with the T 2g→8 S 7/2 transition of Eu 2+ ions in the Sr 2 MgSi 2 O 7 crystals. The table also lists the persistence that the materials showed at room temperature and at 0 ◦ C once the excitation source was switched off. After the blue emission ceased, there was still white emission remaining for longer times, in the case of the higher blue persistence samples—GC-1Eu-0.5Dy and GC-1Eu-1Dy—the white emission reached 152 s and 261 s, respectively [27]. Table 1. Glass-ceramics nomenclature and persistent blue emission information. Measurements were repeated 3–4 times with an error of about 3 s. Sample Name Emission under UV Lamp (365 nm) Persistent Blue Emission at RT (Time) Persistent Blue Emission at 0 ◦C (Time) GC-0.5Eu Blue - - GC-0.25Eu-0.5Dy Blue 22.96 s 102.52 s GC-1Eu-0.5Dy Blue 75.90 s 268.29 s GC-1.2Eu-0.5Dy Blue 39.07 s 146.57 s GC-1.6Eu-0.5Dy Blue 8.39 s 15.57 s GC-1Eu-1Dy Blue 83.33 s 272.17 s GC-2Eu-1Dy Red - - 2.2. Thermal and Structural Characterisation DTA curves were recorded with a SETARAM Setsys Evolution instrument, using glass powder of particle size < 20 µ m with heating rates 2, 5, 10, 20, and 30 ◦ C/min up to 1200 ◦C . An EM 201 side-view hot-stage microscope (HSM) with image analysis and 1750/15 Leica electrical furnace was used to determine the sintering and flow behaviour of the glass powders. Details of the equipment have been reported previously [ 28 ]. Measurements were conducted in air at a heating rate of 10 ◦ C/min on powder samples with particle sizes < 20 µ m. The temperature was measured with a Pt/Rh (6/30) thermocouple placed under the alumina support and in contact with it. The changes in the area of the samples are associated with different processes and correspond to points of viscosity that were previously determined [29]. The density of glass-ceramics was measured according to the Archimedes method using distilled water. The glass-ceramic pellets were milled and sieved to a particle size lower than 60 µ m and initially characterised by X-ray diffraction (Bruker D8 Advance, Massachusetts, USA) in the range of 10–70 ◦ 2 θ , with a step size of 0.02 ◦ , employing CuK α1 radiation ( λ= 1.54056 Å ). Materials 2022,15, 3068 5 of 15 Scanning electron microscopy (SEM) of the glass-ceramic samples was performed on a Hitachi S-3000N microscope, Tokyo, Japan equipped with a vacuum chamber. The instrument is equipped with secondary electron (SE) and backscattering electron (BSE) detectors, as well as an Oxford Instruments energy-dispersive X-ray spectroscopy (EDX) analyser, model INCAx-sight, and allows samples to be inclined at 90◦. Scanning electron microscopy–cathodoluminescence (SEM–CL) of the same selected glass-ceramic samples was performed on a Hitachi S-3000N microscope equipped with a vacuum chamber, on excitation with an electron beam of voltage 15–25 kV and a filament intensity of 100 µ Å. The emission spectra were recorded employing a fiber spectrometer with a charge-coupled device (CCD) through an optical fiber, and the corresponding luminescence photographs, with an ordinary camera. The instrument is equipped with secondary electron (SE) and backscattered electron (BSE) detectors, an energy-dispersive X-ray spectroscope (EDX) Quantax (model XFlash 6I30, Bruker, Massachusetts, USA), and a cathode-luminescence system (CHROMA-CL2 Gatan, Pleaston, CA, USA). Samples were compositionally accurate within the uncertainty in the EDX (~1%). Additionally, spectrally resolved CL measurements were carried out at 80 K on a MONO-CL2 system (Gatan, Pleaston, CA, USA) attached to a field-emission scanning electron microscope (FE-SEM, ZeissLEO 1530, Jena, Germany). Detection was performed with a photomultiplier for panchromatic images, and a Peltier cooled Si-CCD for spectrally resolved images. 2.3. Optical Characterisation The glass-ceramic pellets were firstly observed under UV light (18 W ( λ = 365 nm), I = 0.025 mA/cm2) to check for emission and persistence of phosphorescence. Room temperature emission and excitation spectra, as well as temporal decays, were measured in an Edinburgh FS5 Spectrofluorometer equipped with a 150 W Xenon lamp. The emission was detected by the Hamamatsu R928P photomultiplier. The persistent luminescence decays were obtained after illumination of the samples in the UV (354 nm) with the Xe lamp for 10 min. After that, the illumination was blocked and the luminescence decays were monitored for 600 s with a resolution of 0.5 s. 3. Results and Discussion 3.1. Thermal and Structural Properties The effect of different heating rates on the crystallisation temperatures obtained from DTA curves is illustrated in Figure 1and Table 2for the G-undoped glass. On increasing the heating rate, the nucleation time decreased and, as a result, the crystallisation peaks appeared at a higher temperature. Materials2022,15,xFORPEERREVIEW6of16    Figure1.DTAcurvesoftheundopedglasswithdifferentheatingrates(ø<20μm). Table2.T g ,T x ,andT c oftheundopedglass(ø<20μm)determinedbyDTA. HeatingRateT g (°C)±7T x (°C)±9T c1 (°C)±9T c2 (°C)±9 2°C/min‐8789861082 5°C/min7208919911085 10°C/min72090110061106 20°C/min72290910291129 30°C/min71792810271141 Figure2aandTable3representtheDTAresultsforcompositionsdopedwithdiffer‐ entEu/Dyconcentrationsandaparticlesize<20μm.Theadditionofdopantresultedin ahigherglasstransitiontemperature(T g ).Variationsinthecrystallisationofthesamples wereobservedwhenchangingthenumberofdopants.Intheco‐dopedsamples(with 0.5%Dy),crystallisationwasobservedtoslowdownwithincreasingeuropiumconcentra‐ tion.Intheco‐dopedsamples(with1%Dy),thesameoccurred—i.e.,crystallisationwas slowerasthenumberofeuropiumincreased.Figure2bshowstheHSMresultsforglass powderswithdifferentdopantconcentrations;sinteringandflowtemperaturesaregiven inTable4.Thesofteningtemperature(T S )wasbetween780and840°Cforallsamples.In general,thesamplesbecamesphericalaround1000–1100°C,butsampleG‐1Eu‐0.5Dydid notbecomesphericaluntil1140°C.Allsamplesreachedhalf‐balltemperature(T HB )above 1150°Candthenflowed. Figure 1. DTA curves of the undoped glass with different heating rates (ø < 20 µm). Materials 2022,15, 3068 6 of 15 Table 2. Tg, Tx, and Tcof the undoped glass (ø < 20 µm) determined by DTA. Heating Rate Tg(◦C) ±7 Tx(◦C) ±9 Tc1(◦C) ±9 Tc2(◦C) ±9 2◦C/min - 878 986 1082 5◦C/min 720 891 991 1085 10 ◦C/min 720 901 1006 1106 20 ◦C/min 722 909 1029 1129 30 ◦C/min 717 928 1027 1141 Figure 2a and Table 3represent the DTA results for compositions doped with different Eu/Dy concentrations and a particle size < 20 µ m. The addition of dopant resulted in a higher glass transition temperature (T g ). Variations in the crystallisation of the samples were observed when changing the number of dopants. In the co-doped samples (with 0.5%Dy), crystallisation was observed to slow down with increasing europium concentration. In the co-doped samples (with 1%Dy), the same occurred—i.e., crystallisation was slower as the number of europium increased. Figure 2b shows the HSM results for glass powders with different dopant concentrations; sintering and flow temperatures are given in Table 4. The softening temperature (T S ) was between 780 and 840 ◦ C for all samples. In general, the samples became spherical around 1000–1100 ◦ C, but sample G-1Eu-0.5Dy did not become spherical until 1140 ◦ C. All samples reached half-ball temperature (T HB ) above 1150 ◦ C and then flowed. Materials2022,15,xFORPEERREVIEW7of16    Figure2.(a)DTAand(b)HSMcurvesofG‐undoped,G‐0.25Eu‐0.5Dy,G‐1Eu‐0.5Dy,G‐1.2Eu‐0.5Dy, andG‐1.6Eu‐0.5Dyglasseswithdifferentdopantconcentrations.Heatingrate:10°C/min. Table3.T g ,T x ,andT c ofallglass(ø<20μm)samplesdeterminedwithDTA. SampleT g (°C)±7T x (°C)±9T c (°C)±9 G‐undoped7209011006 G‐0.5Eu721887967 G‐0.25Eu‐0.5Dy727895959 G‐1Eu‐0.5Dy728897960 G‐1.2Eu‐0.5Dy746911989 G‐1.6Eu‐0.5Dy7379221000 G‐1Eu‐1Dy743897961 G‐2Eu‐1Dy7489171000 Table4.Sinteringandflowtemperaturesofglass(ø<20μm)samplesdeterminedwithHSM. SampleT FS (°C) ±10 T MS (°C) ±10 T S (°C) ±10 Sphere(°C) ±10 T HB (°C) ±3 T F (°C) ±3 G‐undoped790910940100011591190 G‐0.5Eu780890920102011901197 G‐0.25Eu‐0.5Dy78010101030109011701190 G‐1Eu‐0.5Dy8208801020110011741200 G‐1.2Eu‐0.5Dy810890990110011601176 G‐1.6Eu‐0.5Dy8409201030109011481159 G‐1Eu‐1Dy8309501100114011601180 G‐2Eu‐1Dy840910948100011841195 ThecombinationofDTAandHSMresultsindicatedthatheattreatmentoftheglass powdersupto1100°Ccanprovidesuitablesintering,crystallisation,andflowstages,thus allowingtheproductionofglass‐ceramicinbulkorenamelformaspossiblefinalprod‐ ucts. Thedensitiesoftheglass‐ceramicsweremeasuredbytheArchimedesmethod,but forthedensityoftheglasssamples,weadoptedthevalueofthemeltsinanelectricfur‐ naceasareference,since,aspreviouslymentionedforthegasfurnace,theglassmeltedin aglassfritform.Theaveragedensityoftheglassmeltedintheelectricfurnacewasabout 3.31g∙cm −3 ,andthedensityofglass‐ceramicsispresentedinTable5.Thetheoreticalden‐ sityofthemaincrystallinephase,Sr 2 MgSi 2 O 7 ,was3.7g∙cm −3 ,whilethehighestdensity obtainedfortheglass‐ceramicswas3.27g∙cm −3 forGC‐1Eu‐0.5Dy.Althoughitwasnot quantified,therewasahighresidualporosityinsomeofthesamples. Figure 2. ( a ) DTA and ( b ) HSM curves of G-undoped, G-0.25Eu-0.5Dy, G-1Eu-0.5Dy, G-1.2Eu-0.5Dy, and G-1.6Eu-0.5Dy glasses with different dopant concentrations. Heating rate: 10 ◦C/min. Table 3. Tg, Tx, and Tcof all glass (ø < 20 µm) samples determined with DTA. Sample Tg(◦C) ±7 Tx(◦C) ±9 Tc(◦C) ±9 G-undoped 720 901 1006 G-0.5Eu 721 887 967 G-0.25Eu-0.5Dy 727 895 959 G-1Eu-0.5Dy 728 897 960 G-1.2Eu-0.5Dy 746 911 989 G-1.6Eu-0.5Dy 737 922 1000 G-1Eu-1Dy 743 897 961 G-2Eu-1Dy 748 917 1000 Materials 2022,15, 3068 7 of 15 Table 4. Sintering and flow temperatures of glass (ø <20 µm) samples determined with HSM. Sample TFS(◦C) ±10 TMS(◦C) ±10 TS(◦C) ±10 Sphere(◦C) ±10 THB(◦C) ±3 TF(◦C) ±3 G-undoped 790 910 940 1000 1159 1190 G-0.5Eu 780 890 920 1020 1190 1197 G-0.25Eu-0.5Dy 780 1010 1030 1090 1170 1190 G-1Eu-0.5Dy 820 880 1020 1100 1174 1200 G-1.2Eu-0.5Dy 810 890 990 1100 1160 1176 G-1.6Eu-0.5Dy 840 920 1030 1090 1148 1159 G-1Eu-1Dy 830 950 1100 1140 1160 1180 G-2Eu-1Dy 840 910 948 1000 1184 1195 The combination of DTA and HSM results indicated that heat treatment of the glass powders up to 1100 ◦ C can provide suitable sintering, crystallisation, and flow stages, thus allowing the production of glass-ceramic in bulk or enamel form as possible final products. The densities of the glass-ceramics were measured by the Archimedes method, but for the density of the glass samples, we adopted the value of the melts in an electric furnace as a reference, since, as previously mentioned for the gas furnace, the glass melted in a glass frit form. The average density of the glass melted in the electric furnace was about 3.31 g · cm −3 , and the density of glass-ceramics is presented in Table 5. The theoretical density of the main crystalline phase, Sr 2 MgSi 2 O 7 , was 3.7 g · cm −3 , while the highest density obtained for the glass-ceramics was 3.27 g · cm −3 for GC-1Eu-0.5Dy. Although it was not quantified, there was a high residual porosity in some of the samples. Table 5. Glass-ceramic densities. Sample Glass-Ceramic Density ±0.01 (g/cm3) GC-undoped 3.11 GC-0.5Eu 3.17 GC-0.25Eu-0.5Dy 3.19 GC-1Eu-0.5Dy 3.27 GC-1.2Eu-0.5Dy 3.26 GC-1.6Eu-0.5Dy 3.19 GC-1Eu-1Dy 3.22 GC-2Eu-1Dy 3.21 3.2. X-ray Diffraction Figure 3shows the X-ray diffraction patterns of some of the glass-ceramics treated at 1100 ◦ C for 1 min. As can be seen in sample GC-undoped and GC-1Eu-0.5Dy, only the Sr 2 MgSi 2 O 7 phase (ICDD: 75–1736) appeared. The increase in the number of dopants, both europium and dysprosium, resulted in the appearance of two small peaks, not corresponding to the akermanite phase. These unidentified peaks are shown in samples GC-1.6Eu-0.5Dy, GC-1Eu-1Dy, and GC-2Eu-1Dy in Figure 3. Materials 2022,15, 3068 8 of 15 Materials2022,15,xFORPEERREVIEW8of16   Table5.Glass‐ceramicdensities. SampleGlass‐CeramicDensity±0.01(g/cm 3 ) GC‐undoped3.11 GC‐0.5Eu3.17 GC‐0.25Eu‐0.5Dy3.19 GC‐1Eu‐0.5Dy3.27 GC‐1.2Eu‐0.5Dy3.26 GC‐1.6Eu‐0.5Dy3.19 GC‐1Eu‐1Dy3.22 GC‐2Eu‐1Dy3.21 3.2.X‐RayDiffraction Figure3showstheX‐raydiffractionpatternsofsomeoftheglass‐ceramicstreatedat 1100°Cfor1min.AscanbeseeninsampleGC‐undopedandGC‐1Eu‐0.5Dy,onlythe Sr 2 MgSi 2 O 7 phase(ICDD:75–1736)appeared.Theincreaseinthenumberofdopants,both europiumanddysprosium,resultedintheappearanceoftwosmallpeaks,notcorre‐ spondingtotheakermanitephase.TheseunidentifiedpeaksareshowninsamplesGC‐ 1.6Eu‐0.5Dy,GC‐1Eu‐1Dy,andGC‐2Eu‐1DyinFigure3.   Figure3.XRDpatternsofglass‐ceramicsamplestreatedat1100°Cfor1min. 3.3.SEMandCL–SEM AnSEManalysisofthepolishedsurfaceofglass‐ceramicsampleswascarriedout. Noincreaseinthesizeofthecrystalswasobserved,whichinmostglass‐ceramicsamples, wereroundedcrystals(2μm)intheSr 2 MgSi 2 O 7 phase.InGC‐1Eu‐0.5Dy,thecrystalsap‐ pearedwithadifferentshape:theygrewinadendriticform,andtheirsizewaslarger(4– 6μm).Figure4showsSEMimagesofGC‐undoped,GC‐1Eu‐0.5Dy,GC‐1Eu‐1Dy,and GC‐2Eu‐1Dy.Itispossibletoappreciatetheverydifferentmicrostructures.IntheGC‐ undoped(Figure4a),onlyonetypeofcrystal(point1)correspondingtotheakermanite phasewasobserved,andthecrystalsweresmallandrounded.Points2and3correspond totheresidualglassyphase.ThesampleGC‐1Eu‐0.5Dy(Figure4b)showeddendriticform crystals(morethan10μm)thatalsocorrespondedtotheakermanitephase(point1).In comparison,inGC‐1Eu‐1Dy(Figure4c)andGC‐2Eu‐1Dy(Figure4d),thereweretwo typesofcrystals;thecrystalscorrespondingtotheakermanitephasewereagainsmall(2 Figure 3. XRD patterns of glass-ceramic samples treated at 1100 ◦C for 1 min. 3.3. SEM and CL–SEM An SEM analysis of the polished surface of glass-ceramic samples was carried out. No increase in the size of the crystals was observed, which in most glass-ceramic samples, were rounded crystals (2 µ m) in the Sr 2 MgSi 2 O 7 phase. In GC-1Eu-0.5Dy, the crystals appeared with a different shape: they grew in a dendritic form, and their size was larger (4–6 µ m). Figure 4shows SEM images of GC-undoped, GC-1Eu-0.5Dy, GC-1Eu-1Dy, and GC-2Eu-1Dy. It is possible to appreciate the very different microstructures. In the GCundoped (Figure 4a), only one type of crystal (point 1) corresponding to the akermanite phase was observed, and the crystals were small and rounded. Points 2 and 3 correspond to the residual glassy phase. The sample GC-1Eu-0.5Dy (Figure 4b) showed dendritic form crystals (more than 10 µ m) that also corresponded to the akermanite phase (point 1). In comparison, in GC-1Eu-1Dy (Figure 4c) and GC-2Eu-1Dy (Figure 4d), there were two types of crystals; the crystals corresponding to the akermanite phase were again small (2 µ m) and rounded (point 2 in both cases). In addition, these glass-ceramics presented elongated white crystals (point 1 in both cases), corresponding to the unidentified phase, the chemical analysis of which is shown in Table 6; this phase was rich in SiO 2 and SrO. Point 3 corresponds to the residual glassy phase. In the sample GC-2Eu-1Dy, these white crystals predominated, although some small crystals of the Sr 2 MgSi 2 O 7 phase could also be observed. These results coincide with those obtained previously by X-ray diffraction. EDX analysis of the original glass frits was carried out to determine if there was alumina or zirconia incorporation from the AZS crucible (Table 7). All glass compositions contained some alumina percentage. The EDX analysis of the previously mentioned points in Figure 4in the glass-ceramic samples is shown in Table 7. Figure 5shows an elemental map of GC-1Eu-1Dy in which it can be seen again how europium and dysprosium are concentrated in the white crystals, coinciding with the EDX analysis of the same sample. In the gray crystals, the elements corresponding to the akermanite phase, strontium, silicon, and magnesium were clearly found. Materials 2022,15, 3068 9 of 15 Materials2022,15,xFORPEERREVIEW9of16   μm)androunded(point2inbothcases).Inaddition,theseglass‐ceramicspresentedelon‐ gatedwhitecrystals(point1inbothcases),correspondingtotheunidentifiedphase,the chemicalanalysisofwhichisshowninTable6;thisphasewasrichinSiO 2 andSrO.Point 3correspondstotheresidualglassyphase.InthesampleGC‐2Eu‐1Dy,thesewhitecrys‐ talspredominated,althoughsomesmallcrystalsoftheSr 2 MgSi 2 O 7 phasecouldalsobe observed.TheseresultscoincidewiththoseobtainedpreviouslybyX‐raydiffraction.  Figure4.SEMimagesof(a)GC‐undoped,(b)GC‐1Eu‐0.5Dy,(c)GC‐1Eu‐1Dy,and(d)GC‐2Eu‐1Dy glass‐ceramics. Table6.Compositionalanalysis(mol%.)ofGC‐Undoped,GC‐1Eu‐0.5Dy,GC‐1Eu‐1Dy,andGC‐ 2Eu‐1Dyglass‐ceramicsasdeterminedbyEDXcorrespondingtothepointsshowninFigure4. SiO 2 SrOMgOAl 2 O 3 Eu 2 O 3 Dy 2 O 3  GC‐Undoped Theoreticalglasscomposition55.027.018.0‐ ‐ ‐ 1Sr 2 MgSi 2 O 7 40.939.517.32.4‐ ‐ 262.121.111.75.1‐ ‐ 360.121.416.81.0‐ ‐ GC‐1Eu‐0.5Dy Theoreticalglasscomposition54.126.617.700.90.5 1Sr 2 MgSi 2 O 7 40.938.117.92.00.50.2 256.620.416.24.01.70.8 GC‐1Eu‐1Dy Theoreticalglasscomposition53.926.417.600.90.9 150.834.96.81.92.72.7 2Sr 2 MgSi 2 O 7 41.139.518.00.80.30.0 362.123.210.42.30.81.0 GC‐2Eu‐1Dy Theoreticalglasscomposition53.426.217.401.90.9 164.026.11.72.14.31.6 2Sr 2 MgSi 2 O 7 39.738.420.01.30.40.0 Figure 4. SEM images of ( a ) GC-undoped, ( b ) GC-1Eu-0.5Dy, ( c ) GC-1Eu-1Dy, and ( d ) GC-2Eu-1Dy glass-ceramics. Table 6. Compositional analysis (mol%.) of GC-Undoped, GC-1Eu-0.5Dy, GC-1Eu-1Dy, and GC-2Eu1Dy glass-ceramics as determined by EDX corresponding to the points shown in Figure 4. SiO2SrO MgO Al2O3Eu2O3Dy2O3 GC-Undoped Theoretical glass composition 55.0 27.0 18.0 - - - 1 Sr2MgSi2O740.9 39.5 17.3 2.4 - - 2 62.1 21.1 11.7 5.1 - - 3 60.1 21.4 16.8 1.0 - - GC-1Eu-0.5Dy Theoretical glass composition 54.1 26.6 17.7 0 0.9 0.5 1 Sr2MgSi2O740.9 38.1 17.9 2.0 0.5 0.2 2 56.6 20.4 16.2 4.0 1.7 0.8 GC-1Eu-1Dy Theoretical glass composition 53.9 26.4 17.6 0 0.9 0.9 1 50.8 34.9 6.8 1.9 2.7 2.7 2 Sr2MgSi2O741.1 39.5 18.0 0.8 0.3 0.0 3 62.1 23.2 10.4 2.3 0.8 1.0 GC-2Eu-1Dy Theoretical glass composition 53.4 26.2 17.4 0 1.9 0.9 1 64.0 26.1 1.7 2.1 4.3 1.6 2 Sr2MgSi2O739.7 38.4 20.0 1.3 0.4 0.0 3 63.8 15.7 14.2 3.4 1.7 0.9