Annotation of the results obtained in 2022
1. The research on revealing the features of formation of fine-grained Al2O3−RE3+:YAG (RE = Ce; Ce+Gd) composite ceramics within the framework of reactive SPS has been continued. Using pre-optimized consolidation parameters (1425°C/ 30 min), the effect of the external pressure value (30−90 MPa) on the optical and luminescent properties of the composites was examined. Total transmittance is shown to decrease with increasing SPS pressure: in the ranges of ~25−14% and ~19−9% (at λ=800 nm) for 1-mm-thick Ce− and Ce,Gd−samples. The absorption band at 340−360 nm divided into two sub-bands centered at 339−342 and 358−368 nm was found, the probable source of which are cerium centers in Ce4+ the non-luminescent state. Partial replacement of Y3+ ions by larger Gd3+ ions in the dodecahedral sublattice of Ce:(Y,Gd)AG resulted in the shift of Ce3+emission wavelength into red region by 20 nm (from 540 to 560 nm). There was an adaptation effect of energy levels (shift of 5d level to 4f), FWHM increase was ~7 nm. The lifetime of Ce− and Ce,Gd−samples varied in the ranges 63.7−62.1 and 63.8−62.6 nsec, respectively, which is slightly lower than that of Ce:YAG single crystals (65 nsec). The dependences of the external quantum efficiency (EQE) had a pronounced downward trend, the maximum values corresponded to “30 MPa” samples – 80.7% and 72%. Significant decrease of QE values and lifetime reduction for “90 MPa” samples is explained by non-radiative transitions of Ce3+ ions. Probably, it is connected with decrease of 5d1 energy level due to crystal field splitting. At this mode of SPS the distortion of garnet crystal lattice in Al2O3−Ce:YAG as well as presence of several variations of garnet type phase in Al2O3–Ce:(Y,Gd)AG were fixed. According to preliminary measurements under blue LD excitation (λ=450 nm) with 1 W/mm2 power, maximum LE values 264 lm/W (5596 K) and 225 lm/W (5374 K) were achieved on “30 MPa” samples of Al2O3–Ce:YAG and Al2O3–Ce:(Y,Gd)AG composites, respectively. LE and EQE values showed complete correlation for both series. Taking into account the microstructure parameters and luminescent properties, as well as the relatively simple approach, the new materials are viable candidates as phosphor converters for the application in the field of high-brightness white solid-state laser lighting. The results were presented at 3 international conferences (“SFR−2022”, “PMKiFN−2022”, “Yenisei Photonics−2022”); an article was published in the Journal of Advanced Ceramics (IF: 11.5, Q1) [doi:10.26599/JAC.2023.9220735]; and accepted for publication in the journal “Journal of Surface Investigation: X-ray, Synchrotron and Neutron Techniques” (IF: 0.36, Q4). 2. For the first time the method for synthesis of 20−70 wt.% of Al2O3−Ce:YAG ceramics with homogeneous biphasic microstructure was realized using a powder system obtained by controlling the excess of Al3+ ions in (Y,Ce)3Al5O12 at the stage of precursor co-precipitation. The experiments were carried out within the framework of the VS method, without the use of sintering additives. As the excess of Al3+ increased, the multiphase powder deviated from stoichiometric YAG ratio with a tendency to stabilize the YAP phase due to different energy contribution during diffusion of structure-forming Y3+ ions, and some γ−Al2O3. Despite the composition variability of the composite powders, these intermediate compounds can be further transformed into YAG and α−Al2O3 at VS. A series of composites obtained at 1775°C / 10 h showed maximum total transmittance: ~32−28% at 800 nm for 1-mm-thick samples. The composites are characterized by a homogeneous biphasic microstructure, the formation of agglomerates / clusters of Al2O3 grains was not observed. The average grain size of the main phase decreases with increases of the second-phase particles content due to a self-limited growth mechanism takes place, the composites with 30−40 wt.% Al2O3 had similar average values (3.0±0.2 μm). Increasing the Al2O3 content led to a decrease in LE and luminous flux due to the “dilution effect” of Ce3+ luminescent ions in the composite. 40 wt.% Al2O3 contained sample showed an optimum CCT value of 6498 K at 7.1 W/mm2 power density of blue LD, close to standard white light (6500 K). Also, this sample shown a high luminous flux of 1169 lm and LE of 166 lm/W at color rendering index (Ra) of 58.2 (CIE chromaticity coordinates: 0.3062, 0.3778). It is obvious that the development of Al2O3−Ce:YAG phosphors with a high content of thermostable phase is associated with a task of ensuring a high doping level of Ce3+ ions (>> 0.1 at.%). The article with obtained results are under review in the Journal of the American Ceramic Society (IF: 4.19, Q1). 3. In the context of optimization of complex sintering additive MgO / SiO2 content in synthesis of high-doped Al2O3 −RE:YAG (RE3+ = Ce; Ce+Gd; Ce+Pr) composite ceramics, the influence of each of them individually on the example of a 0.5 at.% Ce:YAG model system is revealed. The series of samples were obtained by reactive VS, over stoichiometric content of MgO and TEOS additives varied in the range of 0.04−0.8 wt.%. It was found that when Mg2+ / Si4+ ions dissolve in YAG, their substitution at Y3+ / Al3+ (4) positions is preferable. Addition of MgO leads to significant inhibition of Ce:YAG grain size. At =/> 0.08 wt.%, ceramics with MgO had an average grain size of 7±2 μm, whereas with SiO2 – 20±2 μm. The systems with SiO2 are characterized by more intensive removal of residual porosity and the formation of a grain structure with greater size inhomogeneity. The introduction of Si4+ activates both densifying and non-densifying sintering mechanisms. The highest optical transparency was shown by the samples with 0.6 wt.% sintering additives content. Linear optical transmission coefficient of SiO2− and MgO−containing ceramics reached ~80% and 70% at λ=800 nm, respectively. Depending on the amount of additive, the LE of MgO− and SiO2−containing Ce:YAG ceramics under blue LED excitation varied in the range of 20−70 lm/W and 80−110 lm/W (with a maximum also at 0.6 wt.%). The LE of the comparison sample, Ce:YAG ceramics with 0.08 wt.% MgO + 0.8 wt.% TEOS sintering additives, had intermediate values (~90 lm/W). The similar tendency is also shown by comparing the luminous flux of ceramics under excitation by blue LD. The degradation of phosphor properties with addition of MgO is explained by insufficient absorption of blue light of excitation source due to lower transparency of ceramics, formation of Ce4+, cationic vacancies and other point defects because of change imbalance due to presence of Mg2+. The presence of Si4+, on the contrary, promotes decrease of efficiency of Ce3+→Ce4+ ions transformation due to formation of neutral complexes [Mg2+...Si4+]. The use of complex sintering additive MgO / SiO2 is an alternative approach for the formation of Ce:YAG−based ceramic phosphors with high luminescent properties. The article with obtained results was published in the journal “Ceramics International” (IF: 5.53, Q1) [doi: 10.1016/j.ceramint.2022.11.331].

 

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Annotation of the results obtained in 2020
1. Efficient Al2O3−Ce:YAG and Al2O3−Ce:(Y,Gd)AG ceramic color converters were prepared by reactive vacuum sintering from initial oxides. As one of the possible routes for microstructure optimization, the effect of isothermal holding time was considered. It was revealed only slight microstructural differences for both ball-milled powder systems. The evaluated integral porosity values of green body composition before and after the CIP stage are 55.3±0.1 vol% and 45.6±0.3 vol%, respectively. Ceramic series were sintered at 1765°C for 4, 8, and 12 hours. It was shown that the average grain size of garnet phase in the series of Al2O3– Ce:(Y,Gd)AG increased steadily from 5.0 to 9.8 μm with the increase in dwell time (garnet/alumina ratio ≈ 1.1−1.6). However, in the series of Al2O3–Ce:YAG, the average sizes of constituent phases remained almost the same: alumina stayed in the range of 3.6–4.3 μm, and garnet ranged from 6.9 to 9.1 μm (garnet/alumina ratio ≈ 1.9−2.1). The calculated residual porosity values of Al2O3−Ce:YAG and Al2O3−Ce:(Y,Gd)AG lie within the range of 0.0167−0.0065 vol% and 0.0093−0.0047 vol%, respectively. The observed differences in residual porosity values confirm some positive effect of Gd3+ ions on densification by stretching of the garnet crystal lattice. The highest total transmittance of 48% at 800 nm is shown by the Al2O3−Ce:(Y,Gd)AG ceramic sample sintered at 1765°C for 12 hours. The correlation between microstructural parameters and photoluminescent properties of sintered composite ceramics after combining them with a blue LED was identified. Al2O3−Ce:YAG shows excellent thermal quenching behavior with PL-emission intensities decreasing only by 5−6% from RT to 225°C. The red-shifted emission of Ce3+ in Ce:(Y,Gd)AG-based composite causes worse thermal quenching losses − by ~10 and 40% from RT to 125 and 225°C, respectively. When the incident pump power is 28 mW at an excitation wavelength of 454 nm, the highest LEs of almost 151 lm/W (4526 K) and 133 lm/W (3808 K) were obtained for 1-mm-thick Al2O3−Ce:YAG and Al2O3−Ce:(Y,Gd)AG samples with the garnet/alumina size ratio d=2.1 and 1.6, residual porosity P=0.0065 and 0.0047 vol%, respectively. It was found for both systems that adjusting amount of main scattering centers (Al2O3 grains and residual pores) is an important stage for producing white LEDs with excellent luminous performance [1]. 2. Efficient ceramic color converters Al2O3−Ce:YAG were synthesized by solid-state reactive sintering in vacuum using the initial commercial oxide powders and sintering additives SiO2 and MgO. The effect of cerium content in the garnet host (0.05-0.3 at%) on the structural-phase state, optical and photoluminescent properties of composite ceramic phosphors were investigated comprehensively. The calculated lifetime of Al2O3−Ce:YAG composite ceramics with 0.05, 0.1, 0.2, and 0.3 at% Ce3+ doping was 61.7, 62.1, 63.7, and 63.3 nsec, respectively. At the same time, PLQY gradually decreased with dopant concentration increasing – from 91 to 80 a.u. The highest PL intensity was observed for the 0.1 at% Ce3+ sample due to the optimal ratio of its absorption and radiative recombination rate compared to other samples. The subsequent gradual decrease in the PL intensity (by 7.4% for 0.3 at% Ce3+ doping sample) is explained by the appearance of luminescence concentration quenching due to the non-radiative transfer of the excitation energy between the alloying ions. However, sintered phosphors show excellent thermal quenching behavior − reduction in the PL-emission intensities did not exceed 5% at heating temperatures to 150°C, which can be regarded as almost unchanged. When the incident pump power was 28 mW at an excitation wavelength of 454 nm, the CCT of WLEDs based on the obtained 1.0 and 0.4-mm-thick 0.05-0.3 at% Ce3+-doped phosphors changed in the ranges of 4583-3890 K and 10308-4146 K, respectively (light color range − from “natural” white to “cool” white). More importantly, the CRI decreased drastically as the cerium doping increased, attributed to the inner-filter effect (in this case, the re-absorption of the blue and green light components of Ce:YAG). For example, the CRI values of the 1.0-mm-thick samples with 0.05, 0.1, 0.2, and 0.3 at% Ce3+ content were 61, 53, 37, and 35 at naturally higher values for the 0.4-mm-thick one − 74, 62, 58, and 55, respectively. In comparison with the CCT and CRI parameters, the LE changes were the least significant − the variation of the LE values for the 1.0 and 0.4-mm-thick samples was ±2 and ±12%, respectively. In this case, the obtained Al2O3−Ce:YAG phosphors with 0.05-0.1 at% Ce3+ doping have an optimal balance between high CRI and LE and appropriate CCT values and may find application in high-power and high-brightness natural-WLEDs excited by blue-LED chips [2]. 3. Based on the integral connection between pore size distribution in the bulk and pore section size distribution observed on the slice, accurate model is demonstrated to establish the “intuitive” method for residual porosity assessment of high-density materials as a fraction occupied by the pore sections on the studied slice of the sample. Comparison with other well-known models was carried out. The evaluated difference in the outcomes of different methods applied has been shown to be as high as ~40% based on Al2O3−Ce:YAG and Al2O3−Ce:(Y,Gd)AG composite ceramics taken as model objects. Such a huge variance among the calculated values may leads to incorrect interpretation of the obtained data and demonstrates the importance of the analysis of each certain porosity assessment approach applied when comparing the experimental data from various sources [3-5]. [1] D.Yu. Kosyanov, Xin Liu, A.A. Vornovskikh, A.A. Kosianova, A.M. Zakharenko, A.P. Zavjalov, O.O. Shichalin, V.Yu. Mayorov, V.G. Kuryavyi, Xinglu Qian, Jun Zou, Jiang Li / Al2O3–Ce:YAG and Al2O3–Ce:(Y,Gd)AG composite ceramics for high brightness lighting: Effect of microstructure // Materials Characterization. – 2021. – V. 172. – P. 110883-1−110883-9. – IF: 3.56, Q1. doi:10.1016.j.matchar.2021.110883 [2] D.Yu. Kosyanov, Xin Liu, A.A. Vornovskikh, A.A. Leonov, Wanyuan Li, Jiang Li / Effect of cerium doping on optical and luminescent properties of Al2O3−Ce:YAG composite ceramics // Proceedings of SPIE. – 2021. – V. 11706. – P. 1170618-1−1170618-6. – IF: 0.56. doi:10.1117/12.2582998 [3] D.Yu. Kosyanov, A.P. Zavjalov, A.A. Vornovskikh, A.M. Zakharenko, Xin Liu, Jiang Li / Some approaches for residual porosity estimating // IOP Conference Series: Materials Science and Engineering. – 2021. – V. 1093. – P. 012015-1−012015-4. – IF: 0.6. doi:10.1088/1757-899X/1093/1/012015 [4] D.Yu. Kosyanov, A.P. Zavjalov, A.A. Vornovskikh, A.M. Zakharenko, Xin Liu, Jiang Li / Determination of the bulk fraction of spherical non-uniformities in high-density materials // Journal of Alloys and Compounds. – 2021. – Under Review. – IF: 4.65, Q1. [5] D.Yu. Kosyanov, A.P. Zavjalov, A.A. Vornovskikh, J. Li / Some approaches for residual porosity estimating // International Science And Technology Conference For Youth “Advanced Materials for Engineering and Functional Purposes”: Book of Abstracts. – 2020. – P. 135-136. https://elibrary.ru/item.asp?id=44277448

 

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Annotation of the results obtained in 2021
1. Efficient ceramic color converters Al2O3−Ce:YAG were obtained by reactive vacuum sintering using the initial oxide powders and sintering additives SiO2 and MgO. The effect of cerium content in the garnet host (0.05-0.3 at%) on the structural-phase state of composite ceramic phosphors was investigated comprehensively. It is shown that in the formation of Al2O3−Ce:YAG composites, there is a partial recharge of cerium ions into the non-luminescent state of Ce4+ (for systems with =/> 0.2 at% cerium content), and a parasitic introduction of magnesium ions into the structure of aluminum oxide. This feature results from the combined effect of doping with heterovalent ions and the post-annealing stage in the air during the stabilization of the crystal structure of Mg,Si,Ce:YAG solid solutions. At the same time, a detailed analysis of the microstructure of the composites using the HR SEM, EDX, EBSD, and CLSM methods did not reveal possible spatial variations in the content of cerium ions [1, 4-5]. 2. The formation features of new “fine-grained” Al2O3−RE3+:YAG (RE = Ce; Ce+Gd) ceramic phosphors are revealed within the framework of the reactive SPS approach. The influence of the SPS temperature and the magnitude of the applied pressure are considered as possible ways to control the structural-phase state and microstructure of composites. A series of SPS samples (11.5 wt% Al2O3 phase) were obtained at 1350-1450°C for 15-30 min. at an external pressure of 30-90 MPa. The formation of the Al2O3–Ce:YAG biphasic structure in the samples synthesized at 1400 and 1425°C (15 min / 30 MPa) is shown. The subsequent increase in temperature leads to the disruption of the Ce:YAG phase. At the same time, according to the assessment of the proportions of variants of the garnet solid solutions in Al2O3–Ce:(Y,Gd)AG samples using synchrotron radiation of the [10 0 4] reflection, the completeness of Ce:(Y,Gd)AG formation was recorded only for the “1450°C” sample (a clear single-component structure of the reflection was identified). For the “1450°C” samples, abnormal grain growth with partial melting and the formation of local eutectic zones of the Al2O3–YAG system was shown. Undermelting was predominantly observed at the grains of the Al2O3 phase, which is explained by its lower sintering temperature. Experiments on optimizing the holding time at an optimized SPS temperature (1425°C) showed that exceeding the upper time limit (30 min.) leads to activation of recrystallization processes that are not accompanied by further consolidation of the ceramics. Using previously optimized temperature-time parameters of consolidation (1425°C / 30 min), the influence of the external pressure on the microstructure evolution of composite ceramics is considered in detail. Pure crystalline corundum–garnet Ce-doped and Ce,Gd-codoped biphasic systems with single phases of the corresponding solid solutions were successfully synthesized by reactive SPS at 1425°C / 30 min / 30÷60 MPa, the increase in the crystallinity of the constituent phases with an increase in the external pressure in the indicated range of values was shown. An increase in external pressure is accompanied by inhibition of grain growth; the size distribution becomes narrower with a more pronounced trend for the Al2O3–Ce:YAG composite. As a result, the composite phases of the “90 MPa” samples had an average size of 1 µm and below. A decrease in the relative density of Al2O3–Ce:YAG and Al2O3–Ce:(Y,Gd)AG composite ceramics with an increase in the applied pressure was shown in the range of ≈100–99.1% and 99.1–97.2%, respectively. In the “90 MPa” samples, along with local microswellings, both intra- and intergranular and trans-grain ruptures were revealed - for samples synthesized at a pressure above 60 MPa, the effect of deconsolidating mechanisms at the post-annealing stage is critical. 3. A thorough analysis of various simple models for assessing the materials porosity was carried out, which are traps for researchers, since they do not accurately reflect the intuitive understanding of this issue. Despite the existing developments for the study of grains using the cuboctahedron as a model of their shape to determine volumetric characteristics, based on the data on their cross section by a line, a simple model of spherical pores was used, since there is no requirement for dense filling of space for pores, and there is also no sufficient reason to consider their distribution is lognormal. In the accurate model, the restoration of the pore size distribution in the material's bulk is considered as the inverse Abel transform for the size distribution of pores from the cross-section of the material. The numerical inversion of the Abel transform is based on the division of the volume distribution of pores into a number of fractions. This allows the use of a matrix form that is easily inverted. The numerical concentration of pores in the volume is equal to the numerical concentration of pores on the cross-section plane with the factor (2/π)<1/d>, and the porosity is exactly equal to the fraction of pore cross-sections. Simple models do not use Abel transformations and are considered from the point of view of how the pore size distribution function is restored in the material’s volume. For them, a number of criteria are formulated for comparison with the accurate model. Confocal laser scanning microscopy (CLSM) is often used to assess porosity. The value of the investigated volume in CLSM can reach more than 100 × 100 × 100 µm³ and is usually considered as the total volume containing pores. There is no “given” volume when studying a cross-section, for example, using SEM. However, when constructing an accurate model, an average size (π/2)/<1/d> arises, the multiplication of which by the total area of the studied material’s cross-section acts as a volume for calculating the numerical concentration and porosity V. If the distances between cross-sections in CLSM are greater or less than this average size, then the use of the actual volume as V leads to errors in determining the numerical concentration and porosity of the material in CLSM research [2]. 4. An approach to the use of LiF as a sintering additive was tested on the example of a model system − monophasic YAG − in the context of the developed approach (reactive SPS). The content of the additive varied in the range of 0.1-0.2 wt%. It was shown that LiF leads to grain growth intensification (as an “accelerator” of grain boundary motion), increases the probability of the formation of a bimodal grain size distribution, and, as a consequence, intragranular porosity at the final stage of sintering. Compared to the sintering additive SiO2,MgO, the use of 0.2 wt% LiF resulted in a 10-fold grain growth under optimized reactive SPS conditions. Obviously, this levels the advantage of SPS as one of the basic approaches to obtain fine-grained and nanoceramics. Despite the demonstrated ability to completely evaporate carbon-containing impurities from the bulk of the material, traces of F-enriched phases in the form of inclusions in triple junctions were observed for all synthesized samples. Thus, this approach made it possible to exclude carbon contamination of ceramics; however, even a slight excess of the LiF sintering additive formed new centers of light scattering due to LiF traps [3]. [1] Al2O3–Ce:YAG composite ceramics for high brightness lighting: Cerium doping effect // J. Alloys Compd., 887 (2021) 161486. doi:10.1016/j.jallcom.2021.161486 [2] Determination of the bulk fraction of spherical non-uniformities in high-density materials // Ceram. Int., 47 (2021) 28932–28941. doi:10.1016/j.ceramint.2021.06.266 [3] Reactive SPS of Nd3+:YAG transparent ceramics with LiF sintering additive // Opt. Mater., 119 (2021) 111389. doi:10.1016/j.optmat.2021.111389 [4] Патент на изобретение РФ. Способ получения бифазных керамических люминофоров для белых светодиодов. – Заявл. 19.10.2021; Рег. № 2021130302 [5] Design of high-performance Al2O3−Ce:YAG ceramic converters for white LEDs by optimization of cerium doping // 14th Pacific Rim Conference on Ceramic and Glass Technology. – p. 112. https://ceramics.org/wp-content/uploads/2018/09/PACRIM_GOMD-2021_Abstracts.pdf

 

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