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Enhanced Transparency through Second Phase Crystallization in BaAl4O7 Scintillating Ceramics Marina Boyer,*,† Salaheddine Alahraché,† Cécile Genevois,† Marina Licheron,† François-Xavier Lefevre,‡ Célia Castro,§ Guillaume Bonnefont,∥ Gael̈ Patton,⊥ Federico Moretti,⊥ Christophe Dujardin,⊥ Guy Matzen,† and Mathieu Allix*,† †

CNRS, CEMHTI UPR 3079, Université Orléans, F-45071 Orléans, France ENSICAEN, UMR 6508 CRISMAT, 6 Boulevard du Maréchal Juin, 14050 Caen Cedex 4, France § Groupe de Physique des Matériaux, UMR CNRS 6634, Université et INSA de Rouen, BP12, 76801 Saint Etienne du Rouvray cedex, France ∥ Laboratoire MATEIS Equipe RI2S, bât L. de Vinci, 21 rue J. Capelle, INSA-Lyon, 69621 Villeurbanne cedex, France ⊥ Institut Lumière Matière, UMR5306, Université Claude Bernard Lyon1-CNRS, bâtiment Kastler, 10 rue Ada Byron 69622 Villeurbanne Cedex, France ‡

S Supporting Information *

ABSTRACT: A series of biphasic (100 − z)BaAl4O7−zBaAl2O4 (0 < z ≤ 45) transparent polycrystalline ceramics have been synthesized by full crystallization from glass process. Despite being composed of two birefringent crystalline phases, these new materials exhibit improved transparency compared to the pure BaAl4O7 ceramic recently reported to show remarkable scintillation properties. Multiscale structural characterizations demonstrate that this transparency enhancement can be explained by the presence of nanometer scale BaAl2O4 crystals which crystallize coherently with the BaAl4O7 matrix. We show that the BaAl2O4 nanostructuration limits the BaAl4O7 growth via an original Zener pinning effect, such decreasing light scattering due to the material birefringence. Interestingly, the BaAl4O7 scintillation properties can be retained in these two-phase transparent ceramics. These new materials, showing easier fabrication process, especially glass forming, could further drive the development of cost-effective scintillators.

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INTRODUCTION Transparent crystalline materials developed for optical and photonic devices such as transparent armors, laser, or medical scintillators are usually synthesized as single crystals. However, although these latter demonstrate high performances related to their structural perfection, they face several major production drawbacks due to their size, shape, doping level, slow production, and cost limitations. Therefore, single crystals would be advantageously replaced by transparent polycrystalline ceramics with remarkable optical performances.1−3 Such transparent polycrystalline ceramics are usually synthesized from powder sintering4 under extreme temperature and pressure conditions (hot isostatic pressing,5−9 vacuum sintering,10−13 spark plasma sintering...14−17) in order to avoid the formation of porosity, acting like scattering centers, and so limiting transparency, during the sintering process.18 A few transparent polycrystalline ceramics were thus reported, most of them being cubic to prevent scattering due to birefringence,19−24 and/or exhibiting nanometric crystallite sizes required in the case of anisotropic structures according to the Rayleigh−Gans−Debye theory.25−27 However, powder sintering also displays major drawbacks given the complex process. An alternative elaboration process, the complete and © 2015 American Chemical Society

congruent crystallization from glass, has recently been proposed to overcome these limitations.28−31 This innovative, simple, and cost-effective synthesis process is especially suited for transparent ceramic elaboration since crystallization from a bulk parent glass can lead to a fully dense polycrystalline ceramic as long as vitrification can first be realized and the densities of the glass and the crystalline phases are close enough to avoid the formation of cracks during crystallization. Then, by judiciously selecting compositions which will lead to ceramics exhibiting cubic symmetry30 or weak birefringent materials,28,31 it is possible to achieve highly transparent ceramics from full glass crystallization. Following this original process, our group has recently reported the elaboration of BaAl4O7,28 the first transparent polycrystalline ceramic synthesized by full crystallization from glass with micrometric grain size and noncubic symmetry. This material exhibits two orthorhombic polymorphs, α or β, depending on the crystallization temperature, with micrometer grain sizes up to 5 μm, both optically transparent in the visible range. Lately, β-BaAl4O7 (high Received: September 24, 2015 Revised: October 30, 2015 Published: November 16, 2015 386

DOI: 10.1021/acs.cgd.5b01374 Cryst. Growth Des. 2016, 16, 386−395

Crystal Growth & Design

Article

laboratory diffractometer (CuKα radiation) equipped with a Vantec-1 linear detector. Data were collected from 8 to 130° (2θ) at room temperature with a 0.008° step size and an acquisition time of 3 s per step. Rietveld refinements were performed using the Jana06 software.38 In situ high-temperature diffraction data were collected every 20 °C from 900 to 1140 °C using an Anton Paar oven chamber (model HTK1200N) from 18 to 31° (2θ) with a 0.025° step size and an acquisition time of 1 s per step. Differential scanning calorimetry (DSC) was performed on a Setaram MULTI HTC 1600 instrument. The glass transition, crystallization, and phase transition temperatures were determined from a 50 mg powder sample, using argon as a purging gas and a platinum crucible, with a heating rate of 10 K/min. Scanning electronic microscopy (SEM) images were obtained on a FEG Carl ZEISS SUPRA 55 microscope. Samples were first optically polished and then annealed at 1050 °C for 1 h to thermally reveal the grain boundaries. As this thermal etching process is realized at a lower temperature and during a much shorter time than the glass crystallization treatment, the effect on the microstructure can be neglected. Transmission electron microscopy (TEM) was used to characterize the nanostructure of the transparent polycrystalline ceramics. The samples were prepared by mechanical polishing with a tripod and inlaid diamond discs until a 50 μm thickness. The foils were finally obtained by argon ion milling (PIPS). TEM images were collected using a Philips CM20 transmission electron microscope. Elemental composition, HRTEM, and STEM micrographs were carried out on a JEOL ARM200F (JEOL Ltd.) operating at 200 kV and Cs corrected on the probe. Scanning transmission electron microscopy−high angle annular dark field (STEM-HAADF) images were acquired with an 8 cm camera length and a 0.1 nm probe size. Elemental compositions were determined by STEM-EDX using a 0.2 nm probe size. Transmittance measurements were collected over the range 200− 2700 nm using a double beam commercial spectrophotometer (Cary 5000) equipped with a Photomultiplier and a PbS photocell for visible and infrared detection, respectively. The beads were optically polished with an automatic polisher on silicon carbide papers up to a final thickness of 1.2 mm. The scintillation properties of 20 eight samples, corresponding to four different compositions and seven annealing duration times, have been investigated. The scintillation yields of the samples were measured with a photomultiplier (PMT) Photonis XP2020Q. The samples were optically coupled with the PMT and covered with Teflon in order to maximize light collection. Data were acquired through a Lecroy LT 372 oscilloscope and processed using a homemade software to extract both pulse height spectra and scintillation decays. Scintillation events were recorded over 12 μs. Results were corrected from photomultiplier spectral response, and the single photoelectron response (SER) method was used for calibration.39 As the absorption probability of high energy gamma photons is limited by the small sample size, we used low energy gamma photons produced by a 241Am radioactive source emitting γ-ray with an energy of 59 keV. This source allows higher absorption compared to the use of the common 137Cs source (662 keV). In addition, the use of the 241Am radioactive source is more representative of X-ray energies used in medical or industrial imaging, which are the potential applications of such ceramics. Thermally stimulated luminescence (TSL) characterization was performed from 90 to 650 K, using a 1 K/s heating rate. Samples were mounted in a liquid nitrogen cooled stage (Linkam HFS600) and irradiated at 85 K using a Philips X-ray tube fitted with a W anode (30 kV). The emitted light was collected using an optical fiber and detected without spectral resolution over the full emission of the samples using an EMI 9789 PMT in current mode. A highly repeatable process for each measurement, including irradiation, light collection and sample placement, enabled intensity comparison among glow curves.

temperature polymorph) was shown to exhibit promising scintillation properties when doped with Eu(II).32 This new scintillator presents remarkable promising performances, especially an emission wavelength of 450 nm which fits well the silicon detectors, a scintillation yield of about 75% of CsI:Tl, a decay of 670 ns and an extremely reduced afterglow which make it a promising alternative to the CsI:Tl single crystal, one of the most currently used scintillator,33−35 for applications requiring a shorter decay time or/and when a low afterglow and bright burn levels are particularly critical parameters. Nevertheless, the vitrification of the BaAl4O7 composition remains difficult to achieve given that the corresponding 1BaO:2Al2O3 ratio is located on the edge of the vitrification domain (reported to range from 33.33% to 37% BaO) of the BaO−Al2O3 binary system.36 An addition of BaO to the nominal composition would thus ease vitrification. Moreover, scintillation properties would benefit from an increase of the material transparency in order to maximize the emission arising from the whole volume. The actual large size of the BaAl4O7 crystallites (several microns) is not favorable for high transparency, given the birefringence of the material which was estimated as 0.01 from DFT calculations.28 In this article, we report the possibility of synthesizing a series of biphasic (100 − z)BaAl4O7−zBaAl2O4 with 0 < z ≤ 45 transparent polycrystalline ceramics using a full crystallization from glass process starting from xBaO−(100 − x)Al2O3, 33.33 ≤ x ≤ 39 compositions. The addition of BaO to the original BaAl4O7 composition not only leads to easier vitrification, but also to materials showing an improved transparency compared to the pure BaAl4O7 ceramic. This result, rather unexpected given the birefringence of both BaAl4O7 and BaAl2O4 crystalline phases, can be explained by the presence of nanometer scale BaAl2O4 crystals growing coherently with BaAl4O7. Their presence limits BaAl 4O 7 crystal growth and thus the corresponding light scattering. Finally, the scintillation properties of these new two-phase transparent polycrystalline ceramics are discussed depending on the composition and crystallization treatment.

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EXPERIMENTAL SECTION

Synthesis Procedure. Two-phase β-BaAl4O7−BaAl2O4 transparent polycrystalline ceramics, based on the general formula xBaO + (100 − x)Al2O3 with x = 33.33, 34, 35, 36, 37, 38, and 39, were obtained by first mixing high purity BaCO3 (99.9%, Strem Chemicals) and Al2O3 powders (99.99%, Alfa Aesar) in an agate mortar with ethanol to maximize the homogeneity of the mixture. The resulting powder was then pressed into pellets and placed in an aerodynamic levitation coupled to a laser heating system. The pressed powder was thus heated up to 2100 °C and then left to homogenize for a few seconds and subsequently quenched down to room temperature at roughly 300 °C/s by shutting off the CO2 lasers.37 The amorphousness of the resulting glass beads was checked by powder X-ray diffraction and transmission electron microscopy (Figure SI1). Full crystallization leading to polycrystalline ceramics was then achieved by a single heat treatment performed in an open air furnace at 1100 °C during 20 h. Regarding europium doped materials, the same protocol was used with the atomic substitution of 0.5% of Ba by Eu (Eu2O3, 99.99%, Strem Chemicals). The samples were then fully crystallized at 1100 °C using various duration time ranging from 30 min to 48 h in order to track scintillation properties evolution (the samples were introduced and taken out of the annealing furnace at high temperature to avoid sample evolution during heating/cooling, especially for short crystallization treatments). Characterization Methods. X-ray powder diffraction (XRPD) analyses were performed using a Bragg−Brentano D8 Advance Bruker 387



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RESULTS AND DISCUSSION Structural Characterizations. A series of seven transparent glasses was prepared based on the general formula xBaO + (100 − x)Al2O3 with x = 33.33, 34, 35, 36, 37, 38, and 39. After being annealed at 1100 °C for 20 h, all the samples showed a high degree of crystallization while retaining most of the glass transparency. A photograph and the corresponding XRPD patterns of the resulting seven ceramics are shown Figure 1a,b, respectively. All the samples seemed to be well

results and the theoretical content according to the initial BaOAl2O3 ratio. The BaAl2O4 content increases up to 43.6 mol % for the 39BaO−61Al2O3 ceramic. It is quite remarkable to notice the high transparency of the resulting polycrystalline ceramics given the presence of two crystalline phases with strong structural anisotropies (BaAl4O7 is orthorhombic and BaAl2O4 is hexagonal). In order to determine the evolution of the different microstructures along the studied system, SEM analyses were performed subsequently to a thermal etching step. Figure 2

Figure 2. SEM patterns of the xBaO−(1 − x)Al2O3 transparent polycrystalline ceramic with x = 33.33 (a) and 34 (b). (c) Representation of the BaAl4O7 grain diameter distribution with standard deviation (σ) calculated from the previous SEM images.

presents two characteristic patterns of these ceramics (x = 33.33 and 34 compositions). All ceramic compositions exhibit large micrometer scale grains. Compositions with x ≥ 34 show the presence of little white particles which will be later assigned to BaAl2O4. The amount of white particles seems to increase with the nominal BaO content. The grain size distribution of the different compositions, obtained from a statistical analysis of the SEM images, is presented on Figure 2c. The pure BaAl4O7 ceramic corresponding to the 33.33 BaO−66.67 Al2O3 sample presents large grains with a mean diameter of 7.2 μm (σ = 4.4 μm). For the 34 BaO−66 Al2O3 composition, the grain size dramatically decreases to 1.4 μm (σ = 0.9 μm). Then, a further increase of the BaO content leads to a slight increase of the grain size, reaching 3.2 μm for the 37 BaO−63 Al2O3 ceramic. Unfortunately, the grain boundaries cannot be distinguished for the ceramics with x ≥ 38, due to an important presence of white particles as a second phase. Therefore, the grain size distribution cannot be determined for these compositions. In order to characterize more accurately the nature of the second phase and its repartition within the BaAl4O7 matrix, a TEM study was performed on ceramic foil preparations. Figure 3a presents a typical TEM micrograph of the 34BaO− 66Al2O3 ceramic composition. This pattern clearly shows the presence of many stacking faults and nanoparticles (in black) located both in the large BaAl4O7 grains and at the grain

Figure 1. (a) Photograph of the xBaO−(100 − x)Al2O3 polycrystalline ceramics. (b) X-ray powder diffraction patterns of the corresponding BaAl4O7−BaAl2O4 materials. The indexation marks correspond to BaAl4O7 (red) and BaAl2O4 (blue). (c) Enlargement of the 17−23° area showing an increase of the BaAl2O4 content in the material when the BaO (x) content is increasing. For easier reading, the diffractograms have been normalized using the intensity of the peak at 2θ = 20.09° (111 reflection of BaAl4O7).

crystallized, and no residual glass was observed on the XRD patterns (the absence of glass phase will be further confirmed by electron microscopy). The 33.33BaO−66.67Al2O3 diffractogram shows only the presence of the BaAl4O7 crystalline phase (ICSD 424775). The other polycrystalline ceramic materials (x ≥ 34) show the presence of a second crystalline phase which can be indexed as BaAl2O4 (ICSD 16845). By increasing the BaO concentration, the BaAl2O4 content clearly extends (Figure 1c). Figure SI2 presents the molar and volume percentage of both BaAl4O7 and BaAl2O4 crystalline phases computed from Rietveld refinements using experimental XRPD 388



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Figure 3. (a) Bright field TEM micrograph of the 34BaO−66Al2O3 transparent polycrystalline ceramic showing the dispersion of BaAl2O4 nanoparticles in the BaAl4O7 matrix. Intragranular nanoparticles appear much smaller (20−30 nm) than the intergranular ones (>100 nm). Black arrows point to stacking faults, and red arrows to a BaAl4O7 grain boundary. (b) STEM-HAADF micrograph and (c) EDX line scan analysis of a single BaAl2O4 nanoparticle lying in a BaAl4O7 grain.

Crystallization Mechanism. The thermal behavior of the 35BaO−65Al2O3 glass composition is presented Figure 4a. The DSC curve is characteristic of all the different glasses based on the xBaO + (100 − x)Al2O3 composition (33.33 ≤ x ≤ 39). No significant difference, apart from a slight increase of glass transition, crystallization, and phase transformation temperatures as a function of the BaO content, are observed for the other compositions. Upon heating, the glass transition can be observed at 915 °C, followed by two exothermic peaks. The first one observed at 975 °C corresponds to the crystallization of the α-BaAl4O7 phase (low temperature polymorph) from the glass, whereas the second peak at 1110 °C corresponds to the α → β phase transition, i.e., the formation into the high temperature polymorph. No extra crystallization peak corresponding to the formation of the BaAl2O4 is visible, which would indicate that BaAl2O4 crystallizes almost simultaneously as BaAl4O7. These results are confirmed by in situ XRPD measurements (Figure 4b). Upon heating, the glass crystallizes at 920 °C to form simultaneously both the α-BaAl4O7 polymorph and the BaAl2O4 phase, followed by the α → β phase transition at 1040 °C, the slight mismatch between the transition temperatures measured either by DSC or in situ XRPD is due to different heating rates during the experiments. No modification of BaAl2O4 could be detected during these experiments. TEM micrographs frequently present BaAl2O4 nanoparticles showing an “in line” organization (Figure 5a). This “in line” crystallization suggests a concerted formation mechanism: as the xBaO−(100 − x)Al2O3 (x ≥ 34) glasses present an excess of BaO in comparison with the pure BaAl4O7 phase, the

boundaries. The intragranular nanoparticles present a spherical shape and a uniform size distribution (about 20−30 nm in diameter), in contrast with the larger intergranular particles which reach up to 100 nm in diameter (Figure SI3). The large size of the intergranular particles can be explained by the fact that they are most probably formed from a non-negligible amount of Ba-enriched residual glass rejected at the BaAl4O7 grain boundaries. Moreover, the diffusion rate is commonly increased at grain boundaries, such enlarging the size of the BaAl2O4 nanoparticles between the BaAl4O7 particles. The chemical compositions of both crystalline phases formed during crystallization were checked by STEM-EDX with a probe size of 0.2 nm (Figure 3b,c). One should note that for STEMHAADF images, the contrast is directly linked to the atomic number Z. Consequently, and contrary to the previous TEM images, the nanoparticles (BaAl2O4) appear with a brighter contrast than the matrix (BaAl4O7). The analysis presented in Figure 3c corresponds to an EDX line scan (3b) through the matrix (in black) and the nanoparticle (in white). The atomic percentages show that the black matrix is composed of 20 atom % of Ba and 80 atom % of Al, which perfectly corresponds to the pure BaAl4O7 phase, whereas the nanoparticles contain 33 atom % of Ba and 67 atom % of Al, which matched very well with the BaAl2O4 composition. These EDX analyses clearly confirm that the polycrystalline ceramics are composed of BaAl4O7 large grains (matrix) and BaAl2O4 nanoparticles. The white amorphization spots visible on the STEM-HAADF micrograph (Figure 3b) appear during the EDX acquisition as the material is submitted to a high irradiation for 30−60 s. 389



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Figure 4. (a) Differential scanning calorimetry (DSC) measurement of the 35BaO−65Al2O3 glass composition. Tg corresponds to the glass transition temperature, Tx to the crystallization temperature of the αBaAl4O7 polymorph, and TT to the α → β phase transition. (b) In situ XRD measurement of a 35BaO−65Al2O3 glass heated up to 1140 °C. Pink, green, and red areas correspond to BaAl2O4, α-BaAl4O7, and βBaAl4O7 respectively.

Figure 5. (a) STEM-HAADF micrograph of “in line” organization of intragranular BaAl2O4 nanoparticles located in a β-BaAl4O7 grain. The image has been recorded for the 34BaO−66Al2O3 transparent polycrystalline ceramic composition. (b) Scheme of the proposed crystallization mechanism illustrating the decrease of the BaAl4O7 crystal growth rate by a Zener pinning effect due to the presence of BaAl2O4 nanoparticles.

crystallization of BaAl4O7 leads to a rejection of BaO into the glass. When the concentration of barium at the grain growth front is high enough, spontaneous crystallization of BaAl2O4 nanoparticles takes place. The resulting microstructure thus shows lines of BaAl2O4 nanocrystals as a consequence of the moving front, leading to “in line” crystallization microstructure. Further crystallization then leads to the embedding of BaAl2O4 nanoparticles in the BaAl4O7 crystalline grain. A scheme summarizing this proposed crystallization mechanism is presented on Figure 5b. An atomic scale resolution HAADF-STEM pattern of a typical BaAl2O4 nanoparticle embedded in a BaAl4O7 grain is shown Figure 6. The crystallographic planes of the BaAl4O7 matrix appear to be extended continuously through the BaAl2O4 nanoparticle, at least over a distance of about 5 nm from the edge of the nanoparticle, demonstrating coherent crystallization of BaAl2O4 in BaAl4O7. This coherent crystallization is possible since only a slight misfit exits between interreticular distances of BaAl2O4 and BaAl4O7 (for both α and β polymorph) phases. Nevertheless, the misfit is large enough to induce stress in the BaAl2O4 lattice as clearly illustrated by the Moiré pattern, i.e., the alternation of bright and dark bands, in the BaAl2O4 nanoparticle. Thus, the epitaxial growth appears to be not perfectly organized from the interface through the center of the nanoparticle. A small distortion of several degrees, depending on the orientation of both crystalline structures, can be found between the planes situated at the center of the

particle and those located at the BaAl4O7/BaAl2O4 interface (Figure SI4). The crystallization of BaAl2O4 nanoparticles induces a wellknown Zener pinning effect;40 i.e., the formation of BaAl2O4 nanoparticles limits BaAl4O7 crystal growth. More precisely, to pursue its crystallization path, a BaAl4O7 grain boundary has to go through the new BaAl2O4 nanoparticle, which requires an excess of energy. In fact, when a growing grain meets a nanoparticle, the part of the growth front in contact with the nanoparticle is removed with an associated energy reduction. To enable further migration of the grain boundary, it is necessary to recreate the missing part of the joint, which costs energy to the system. This energy cost leads to a decrease of the BaAl4O7 growth rate and thus limits the final BaAl4O7 grain size. For volume fractions of the second phase (Fv) lower than 10%,41 the overall pinning force (Pz) of the spherical nanoparticles growing coherently in a given material can be described as Pz = (3Fvγ/2r),40 where γ the grain boundary energy and r the particle radius. The addition of BaO to the original 33.33BaO−66.67Al2O3 composition leads to the formation of small BaAl2O4 nanoparticles growing coherently in BaAl4O7 grains. Regarding the 34BaO−66Al2O3 composition, an important pinning pressure is taking place, which considerably decreases the average BaAl4O7 grain size. For high volume fraction (Fv > 10%), a deviation from the initial theory is expected. In this case an interparticle distance dependent theory is more appropriate and predicts the reduction of the 390



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composition leads to an increase of light scattering until almost opacity for the 39BaO−61Al2O3 composition. Transmittance measurements ranging from 200 to 2700 nm were performed on the different polycrystalline ceramic compositions and are presented in Figure 7a. Straight lines

Figure 6. (a) HRTEM micrograph of the 35BaO−65Al2O3 transparent ceramic showing a typical BaAl2O4 nanoparticle oriented along the [1̅11] zone axis in a β BaAl4O7 grain oriented along the [1̅46] direction. The plan indexations are detailed on the image, using pink color for β BaAl4O7 and blue color for BaAl2O4. (b, c) FFT of the β BaAl4O7 matrix (pink) and the BaAl2O4 nanoparticle (blue) respectively. Details of the matching distances: d(121)αBaAl4O7 = 3.40 Å // d(221̅)βBaAl4O7 = 3.10 Å // d(121̅)BaAl2O4 = 3.19 Å; d(22̅0)αBaAl4O7 = 4.00 Å // d(21̅1)βBaAl4O7 = 3.80 Å // d(11̅2)BaAl2O4 = 3.95 Å; d(301)αBaAl4O7 = 3.23 Å // d(410)βBaAl4O7 = 3.00 Å // d(211)BaAl2O4 = 3.19 Å.

Figure 7. (a) Transmittance of the BaAl4O7−BaAl2O4 polycrystalline ceramics. The transmittance values have been normalized for a thickness of 1 mm. Straight lines correspond to experimental results, and the dot line represents the theoretical transmittance predicted from the Apetz model.25 (b) The evolution of the transmittance at 1200 nm is presented along with the BaAl4O7 average diameter as a function of the BaO content.

pinning force with the nanoparticle volume fraction increase.41 By further increasing the BaO content, the BaAl2O4 nanoparticles become larger (Figure SI3) and the volume fraction exceeds several percent (around 15% for the 35BaO−65Al2O3 composition). In these conditions, the Zener pinning effect still exists, but its intensity is weaker42 and leads to a slight increase of BaAl4O7 grain size as observed from SEM patterns (Figure 2). In fact the increase of the nanoparticle size induces more stress in the structure since the nanoparticle tends to orientate with respect to the matrix, which reduces the coherence between the nanoparticles and the matrix, and consequently dramatically decreases the pinning force.42 Optical Properties. The photograph of the transparent polycrystalline ceramics samples, presented in Figure 1a, shows a clear transparency improvement when the composition evolves from x = 33.33 to x = 36. Remarkably, it appears that contrary to common sense, the presence of a secondary phase does not deteriorate the transparency of the material, as a moderate addition of BaAl2O4 nanoparticles in the BaAl4O7 matrix leads to an improved transparency. However, further increase of the BaO content beyond the 36BaO−64Al2O3

correspond to experimental values obtained with a thickness normalization43 at 1 mm (the experimental thicknesses of the various beads were originally around 1.2 mm). The increase of the BaO content in the nominal composition, corresponding to an increase of the BaAl2O4 content in the BaAl2O4−BaAl4O7 system, clearly affects the transparency, especially in the NIR region. For example, at 1200 nm the transmittance values increase from 40% to 60% from the pure BaAl4O7 (33.33BaO− 66.67Al2O3) material to the 34BaO−66Al2O3 polycrystalline ceramic composition (Figure 7b). In the visible range, the increase of transmittance for a slight addition of BaO appears less important. Then, a larger addition of BaO leads to transparency loss, as shown by the 15% transmittance for the 39BaO−61Al2O3 ceramic at 1200 nm. This transparency evolution can be explained by the microstructure previously determined by electron microscopy. Indeed, for limited BaO addition, the BaAl2O4 nanoparticles are very small (25−50 nm diameter, as shown Figure SI3) compared to the incident wavelength so that they do not 391



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the approximations used during DFT calculations. We used here an extrapolation of the previous equations down to 400 nm, which led to satisfactory results. A first calculation trial was performed by varying the average BaAl4O7 grain size. This step led to correct curve profiles although the overall theoretical transmittance values appeared higher than the experimental data. Actually, the Rs factor was slightly underestimated, meaning that scattering losses related to the sample surface had also to be considered. For example, the samples can indeed show small scratches at the surface or nonperfect parallelism between the two faces, and the small size of the samples probably induces a non-negligible side effect. As these biases cannot be estimated, the Rs value was adjusted in a second RIT calculation trial. The final parameters of the fits are listed in Table 1. The average BaAl4O7 crystallite size values

exhibit scattering. These small nanoparticles have almost no influence on the materials transparency as predicted by the Rayleigh scattering theory.44 Figure 7b represents the mean BaAl4O7 grain size determined from SEM patterns along with transmittance values measured at 1200 nm. It appears that when the average BaAl4O7 grain size decreases, the transmittance increases and vice versa. Actually, in the NIR region, the wavelength is similar to the BaAl4O7 grain size so that important light scattering arises from these birefringent grains. As a consequence, a large decrease of the BaAl4O7 grain size (from 7.2 to 1.4 μm as observed by SEM between the 33.33BaO−66.67Al2O3 and 34BaO−66Al2O3 compositions) limits light scattering due to the birefringent BaAl4O7 grains, and thus increases the overall material transmittance. In the visible region, i.e., for smaller wavelengths, the decrease of light scattering is less important given the larger size of BaAl4O7 grains compared to the incident wavelength. Finally, a large BaO addition leads to larger BaAl2O4 nanoparticles (up to 80 nm as shown on Figure SI3) and BaAl4O7 grains, which induces an increase of light scattering and so to a transmittance decrease as shown for the 39BaO−61Al2O3 composition. These transmittance measurements show that limiting BaAl4O7 grain growth via the controlled presence of nanoparticles (Zener pinning effect) greatly increases the overall polycrystalline ceramic transparency.45,46 As a consequence, in the case of a birefringent polycrystalline ceramic material, the introduction of a second phase such as small particles appears as an interesting process in order to reduce the main phase grain size and so to enhance the overall transmittance of the material. In order to confirm the proposed transparency mechanism in these BaAl4O7−BaAl2O4 polycrystalline ceramics, and so to demonstrate that no other scattering effect than birefringence is taking place in these materials elaborated by full crystallization from glass, we aimed at modeling the transmittance behavior of the developed transparent polycrystalline ceramics. For that purpose, we used a model previously developed by Apetz to describe the real in line transmittance (RIT) of anisotropic polycrystalline α-Al2O3 ceramics.25 This model is based on the Rayleigh−Gans−Debye light scattering theory: RIT = (1 − Rs) exp(− (3π2t/λ20)rΔn2) where Rs corresponds to the specular reflection: Rs = (2R′/(1 + R′)) and R′ = ((n − 1)/(n + 1))2, n is the refractive index, t the sample thickness, r the grain size radius, Δn = (2/3)(n0 − ne) the birefringence, and λ0 the incident wavelength. To apply this model to our BaAl4O7−BaAl2O7 system, it is necessary to consider an overall transmittance combining the three components corresponding to the three BaAl 4 O7 crystallographic directions given the nonisotropy of the BaAl4O7 structure (orthorhombic symmetry). Given that both XRD and electron microscopy experiments did not show preferred orientation of the crystallites in the bulk materials, we used an average transmittance over the main crystallographic axes. Moreover, as BaAl2O4 crystals remain at the nanometer scale (for x < 37 compositions), their effect can be neglected. Thus, the total transmittance can be described as RITtotal = (1/ 3)(RITx + RITy + RITz). The BaAl4O7 refractive index components (nx, ny, and nz) were previously determined between 500 and 4000 nm from density functional theory (DFT) calculations of the dielectric function ε(ω), starting from the structural models established by structure determination from powder diffraction (Figure SI5).28 Below 500 nm, the calculated dielectric function is not reliable enough due to

Table 1. Final Fit Parameters Used in the Adapted Apetz Model to Calculate a Theoretical Real in Line Transmittance in BaAl4O7−BaAl2O4 Polycrystalline Ceramics with xBaO + (100 − x)Al2O3 Nominal Compositions xBaO BaAl4O7 grain diameter (μm) Rs

33.33

34

35

36

37

38

4.40

1.80

2.60

2.70

2.80

3.50

0.22

0.12

0.12

0.16

0.21

0.18

obtained from the simulations appear to be in relatively good agreement with the SEM observations, and the BaAl4O7 size evolution as a function of the BaO content follows the same tendency, i.e., first a decrease of the grain size with an addition of BaO for x ranging from 33.33 to 35 and then an increase for x ≥ 36. As an example of the quality of the fits, Figure 7a shows the good agreement between calculated and experimental RIT values for the 35BaO−65Al2O3 ceramic. Scintillating Properties. In order to confirm the existence of scintillation properties all along the BaAl4O7−BaAl2O4 system and to search for optimum compositions and crystallization annealing conditions, scintillation measurements were performed on the different compositions studied previously. Each composition was crystallized over different durations ranging from 30 min to 48 h. The scintillation light yield values measured on the different BaAl4O7−BaAl2O4 samples annealed for 24 h neither appear to be correlated with the optical transmission measured at the wavelength of light emission (450 nm), nor with the photoluminescence intensity as shown Figure 8a. A drastic dependence between the light yield and room temperature stable traps has previously been described in the literature regarding BaAl4O7 doped with europium.32 In order to evaluate this dependence for the different compositions described in this work, we have investigated the thermally stimulated luminescence (TSL) behavior of various samples (Figure 8b). Most of the traps emptied between 90 and 670 K are present in all samples. However, the BaAl4O7 stoichiometric composition, i.e., without any BaAl2O4, shows several differences regarding the trap amplitude ratios: the most noticeable peculiarities are located between 300 and 650 K. This temperature range corresponds to traps stable at room temperature, which thus affect the scintillation yield. The correlation between light yield and trap amplitude is shown Figure 8a. Considering the negative effect of such stable traps, their identification and subsequent removal are a priority to increase 392



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Finally, scintillation yield optimization was performed by varying both composition and annealing duration (Figure 8c). The results show that although the most competitive performances are observed for the BaAl4O7 stoichiometric composition, the 34BaO−66Al2O3 polycrystalline ceramic crystallized for 2 h could also be an interesting candidate for scintillation applications since it shows improved transparency compared to the pure BaAl4O7 material.

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CONCLUSION BaAl4O7−BaAl2O4 biphasic transparent polycrystalline ceramics have been elaborated based on xBaO + (100 − x)Al2O3 compositions with 33.33 ≤ x ≤ 39, using an innovative full crystallization from glass process which enables fully dense materials to be obtained. Compared to the previously reported BaAl4O7 transparent ceramic showing promising scintillation properties, the increase of the BaO content in the nominal stoichiometric composition leads to easier glass forming and enhanced transparency. These materials show the presence of a secondary phase, BaAl2O4, upon crystallization. BaAl2O4 is shown to crystallize as nanoparticles located mainly inside the BaAl4O7 matrix grains (25−50 nm). Given the small particle size, this nanoscale crystallization does not provide further light scattering. But more interestingly, the nanoparticle presence also limits BaAl4O7 grain growth via a Zener pinning effect. Thus, the BaAl4O7 average grain size drastically decreases from 7.2 μm in the pure BaAl4O7 material (33.33BaO−66.67Al2O3) to 1.4 μm for the biphasic 34BaO−66Al2O3 composition. As a consequence, the light scattering which arises from BaAl4O7 anisotropy is strongly decreased. An increase of 20% of the transmittance compared to the pure BaAl4O7 can thus be obtained. This transparency evolution as a function of the BaAl4O7 grain size was further confirmed by fitting the experimental transmittance curves with an adapted Apetz model. Finally, scintillation properties of transparent polycrystalline ceramics doped with 0.5% of Eu2+ are demonstrated to exist all over the BaAl4O7−BaAl2O4 system. The addition of BaAl2O4 as a secondary phase to the original BaAl4O7 scintillating material enables an easier fabrication process. Therefore, the interesting properties of the 34BaO−66Al2O3 polycrystalline ceramic materials, crystallized during short treatments and showing improved transparency compared to the pure BaAl4O7, could drive the development of further new cost-effective scintillators. It is anticipated that the controlled addition of a secondary phase method, which limits the size of crystalline anisotropic grains, could be applied to a wide range of ceramics for diverse optical/photonic application developments.

Figure 8. (a) Evolution as a function of the BaO content of light yield measured with a 241Am source (the 33.33BaO−66.67Al2O3 composition is used as reference yield (yield = 1)) and integrated TSL between 580 and 640 K (both reported on left axis), and optical transmission measured at 450 nm (right axis) for samples annealing during 24 h. (b) Comparison of TSL from 90 to 670 K for three different sample compositions annealed for 24 h. (c) Light yield characterization of the different measured samples corresponding to various compositions and annealing durations.

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the scintillation performances. For that purpose, we have investigated TSL properties on samples grown either in air or in argon (Figure SI6). Two main differences are observed. First a supplementary trap above 650 K is clearly present for the sample synthesized in argon atmosphere. Second, a general decrease of the TSL amplitude can be noted for the sample grown in air. These results suggest a beneficial role of an oxidizing atmosphere on trap concentration, which could be due to oxygen vacancies filling. Considering that the trap concentration is higher for the samples containing BaAl2O4, a crystallization process in oxidizing atmosphere could lead to an interesting improvement in scintillation properties.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b01374. XRD of the 35BaO−65Al2O3 glass (SI1). Quantification by Rietveld refinement of the BaAl2O4 contents (mol % and vol %) within the biphasic BaAl4O7−BaAl2O4 transparent polycrystalline ceramics (SI2). Normal BaAl2O4 grain size distribution (SI3). Filtered HRTEM image with a 6° misfit between BaAl2O4 nanoparticle and BaAl4O7 matrix (SI4). Expression of the refractive index components of β-BaAl4O7 along the three crystallo393



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graphic (SI5). Comparison of TSL from 90 to 670 K for the 34BaAl-66Al2O3 sample grown in air and in argon (SI6) (PDF)

AUTHOR INFORMATION

Corresponding Authors

*(M.B.) E-mail: [email protected]. *(M.A.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the French ANR for its financial support to the project CrystOG ANR-12-JS08-0002 and the ICMN (Orléans, France) and CME (Orléans, France) laboratories for TEM and PIPS accesses.

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