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[Lihong Liu](https://orcid.org/0000-0002-8964-5512), [Jiguang Li](https://orcid.org/0000-0002-5625-7361), [Koji Morita](https://orcid.org/0000-0001-6040-7054), [Byung-Nam Kim](https://orcid.org/0000-0003-4302-462X), [Tohru S. Suzuki](https://orcid.org/0000-0001-9458-6863)

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[Effect of flux on the spark-plasma-sintering (SPS) of translucent YVO<sub>4</sub>:Nd<sup>3+</sup> ceramics: microstructural evolution and optical properties](https://mdr.nims.go.jp/datasets/b0d2f40a-9018-40a0-a358-c52fc5ccf954)

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Effect of flux on the spark-plasma-sintering (SPS) of translucent YVO4:Nd3+ ceramics: microstructural evolution and optical propertiesResearch Article Vol. 33, No. 22 / 3 Nov 2025 / Optics Express 45574Effect of flux on the spark-plasma-sintering(SPS) of translucent YVO4:Nd3+ ceramics:microstructural evolution and optical propertiesLIHONG LIU, JIGUANG LI, KOJI MORITA, BYUNG-NAM KIM, ANDTOHRU S. SUZUKI*Research Center for Electronic and Optical Materials, National Institute for Materials Science, Tsukuba,Ibaraki 305-0047, Japan*Suzuki.tohru@nims.go.jpAbstract: Enhanced optical properties of the (001) texture translucent YVO4:Nd3+ ceramics canbe achieved by adding LiVO3 flux using a spark-plasma-sintering (SPS) technique. YVO4:Nd3+green body oriented along the (001) direction is successfully obtained from well-dispersed andhighly stable YVO4:Nd3+ suspension with suitable amount of LiVO3 flux addition using 1wt%dispersant. The introduction of LiVO3 flux can accelerate the sintering process and promotemore effective densification during sintering. This ultimately leads to improved density of theYVO4:Nd3+ ceramics compared to the sample without flux addition. The enhanced density,caused by the addition of the flux, finally results in the improved transmittance efficiency of thetexture translucent YVO4:Nd3+ ceramics.© 2025 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement1. IntroductionYVO4 is an attractive host material for the rare earth ions, for its wide applications in solidstate media for developing low and medium power micro-chip lasers with laser diode pumpingdue to its high absorption coefficient, high optical transparency in the 400-5000 nm range,large emission cross section, and long fluorescence lifetime [1–4]. Among them, YVO4:Nd3+crystals are especially desirable materials for diode pumped laser systems and deep spacecommunication owing to their superiority over the widely used YAG:Nd3+ for their 2.7 timesgreater cross-section and higher slope efficiency compared with YAG:Nd3+ [5–7]. However, thetraditional single-crystal growth method for producing high quality and large-sized YVO4:Nd3+crystals is costly, and the process is complex and difficult to control. As an alternative approach,the fabrication of transparent YVO4:Nd3 + laser ceramics, which show higher c-cut thermalconductivity than that of YAG as reported by Taira et al. [8], offers a promising route toproduce large-sized YVO4:Nd3+ materials for using in high-power, high-efficiency laser designs.However, fabrication of transparent YVO4:Nd3+ ceramic is quite challenging work due to thelarge birefringent scattering at the grain boundaries caused by its non-cubic crystal structure,along with the high melting point of YVO4 (1810°C) [9]. To minimize light scattering losses atthe grain boundaries of YVO4, in our previous work, [10] aligning the crystal orientation of eachgrain, namely forming texture, was adopted by using strong magnetic field. By designing the(001) texture of YVO4 with aligning the slip casting direction parallel to the magnetic field Bthe SPSed YVO4 gained much higher transmittance than that of non-textured random YVO4.The optical properties, however, are not determined only by one microstructural factor of the(001) texture, but also affected by other microstructural factors of the grain size and porosity[11,12]. Among them, the higher porosity in the sintering bodies would cause the serious lightscattering, resulting in the poor transmittance of the final materials. To achieve high transparencyin ceramics, it is important to eliminate porosity through proper material processing techniques,such as careful powder compaction, optimized sintering parameters, and the use of suitable#574424 https://doi.org/10.1364/OE.574424Journal © 2025 Received 4 Aug 2025; revised 11 Sep 2025; accepted 22 Sep 2025; published 20 Oct 2025https://orcid.org/0000-0001-9458-6863https://doi.org/10.1364/OA_License_v2#VOR-OAhttps://crossmark.crossref.org/dialog/?doi=10.1364/OE.574424&amp;domain=pdf&amp;date_stamp=2025-10-20Research Article Vol. 33, No. 22 / 3 Nov 2025 / Optics Express 45575additives [13–15]. According to our previous works, [10] Y-rich impurity phase was detectedin the YVO4 ceramic after SPS sintering, which amount was increase with the increasing ofsintering temperature. To consider the balance between impurity phase content and bulk density,a sintering temperature of 1300°C was ultimately chosen as the optimal temperature for producingtranslucent YVO4 ceramic, even though some residual pores still exist in the ceramics. Aimingfor even higher density necessitates subjecting the material to higher temperature processing.Nevertheless, this approach can result in the formation of a remarkable amount of impurityphases within the final products.To overcome the drawbacks mentioned above, adding flux may become an effective methodto enhance the density at lower temperature. A flux, characterized by its low melting point,can optimize the sintering process. In other words, introducing a flux can modify the sinteringcharacteristics of the ceramic, making the sintering process more efficient. The flux can reducethe sintering temperature, accelerate particle bonding, and grain growth, thereby minimizing theformation of pores and defects and improving the density of the ceramic [16].In this work, LiVO3 with a melting point of 616 °C, typically used for preparing YVO4 singlecrystal, which demonstrates low toxicity and volatility, [17] was selected as a flux to fabricateYVO4:Nd3+ translucent ceramics. The colloidal technique [18–22] was employed to prepare aYVO4:Nd3+ nanopowder suspension with good dispersion and stability by adding LiVO3 flux.A magnetic field alignment technique [23–25] was proposed and applied using the slip castingmethod to create a (001) textured YVO4:Nd3+ green body. Subsequently, the green body wassintered using the SPS technique, a powerful sintering tool capable of achieving high-densitymaterials at relatively low temperatures, [26,27] to obtain translucent YVO4:Nd3+ ceramics.2. Experimental procedure2.1. Preparation of YVO4:Nd3+ nanopowder suspensionYVO4:Nd3+ suspension was prepared from the nanopowders obtained using the same procedureas described in our previous works [10,28]. First, 15 vol% YVO4:Nd3+ nanopowders weredispersed into distilled water; subsequently, 1 wt% LiVO3 flux (Mitsuwa Chemical Co., Japan)was added to the suspension, followed by the addition of polyelectrolyte (poly(ammonium)acrylateA-6114, Toaghosei Co., Japan) as a dispersion medium to achieve a well-dispersed YVO4:Nd3+suspension. The adding amount of A-6114 was referred to 1wt% mass amount of YVO4:Nd3+ insuspension. During the entire process, the pH value of the YVO4:Nd3+ suspensions was adjustedto> 9 by Tetramethylammonium Hydroxide (TMAH). The suspension was then deagglomeratedusing a homogenizer for 10 minutes, followed by continuous stirring under ultrasonic dispersionfor another 10 minutes.2.2. Fabrication of textured-YVO4 ceramicThe (001) textured-YVO4:Nd3+ green body with flux addition was fabricated using slip castingin a strong magnetic field of 12 T, with the casting direction aligned parallel to the magnetic field.During the process, 4 mL of the prepared suspension was poured into an acryl tube mold with aninner diameter of 10.7 mm, which was fixed onto a porous Al2O3 substrate. The mold assemblywas then placed inside the magnetic field chamber and subjected to the field for 24 hours toallow complete molding and orientation. The resulting slip-cast YVO4:Nd3+ green compactswere subjected to cold isostatic pressing (CIP) at 350 MPa for 10 min and were then densified ina graphite mold using a SPS machine (SPS, LABOX-315, Sinterland Co., Ltd., Japan) at thesintering temperatures of T = 1300 °C with heating rates of 100 °C/min under a constant uniaxialpressure of 90 MPa. The pressure was increased to 90 MPa as the temperature rises from 1000 to1100°C. The temperature during sintering was monitored by measuring the temperature of holeon the mold using an optical pyrometer.Research Article Vol. 33, No. 22 / 3 Nov 2025 / Optics Express 455762.3. Characterization techniquesZeta-potential measurement of the suspension was performed as a function of flux adding amountby using zetasizer nano essentials (Malvern Instruments Ltd., United Kingdom).X-ray diffraction (XRD) analysis of YVO4:Nd3+ green bodies were performed by RINT-TTR-III diffractometer (Rigaku Co., Ltd, Tokyo, Japan, 40 kV 150 mA) using Cu Kα radiation.Electron backscatter diffraction (EBSD, EDAX-TSL OIM EBSD system, EDAX Inc., USA)characterization was conducted for texture analysis, which was performed using a field-emissionscanning electron microscope (JSM7000F, JEOL Ltd., Tokyo, Japan). Orientation ImagingMicroscopy TM (TexSEM Laboratories, Inc., Draper, UT) was used for collecting and analyzingthe EBSD data. Microstructures of the ceramics were observed with a field emission scanningelectron microscope (FE-SEM, model SU-8000, Hitachi Ltd., Tokyo, Japan). Using the SEMmicrographs, the grain size was determined by counting the number of grains. Assuming thegrains to be spherical, the average grain size, d̄, was determined to be 1.225 times the apparentgrain size, which was calculated from the average cross-sectional area per grain [29].The transmittance efficiency in the wavelength range of λ= 400 -1400 nm were measured withthe spectral resolution of 2 nm by using a double-beam spectrophotometer (SolidSpec-3700DUV,Shimadzu) equipped with an integrating sphere.3. Results and discussion3.1. Characterization of YVO4:Nd3+ suspensionTo consider the effect of flux adding amount on the dispersity of YVO4:Nd3+ suspension, zetapotentials of YVO4:Nd3+ suspensions with various LiVO3 adding amounts were measuredboth before and after the addition of flux and A6114, respectively. For comparison, the zetapotential of YVO4:Nd3+ suspension with only A6114 adding is also represented by the darkclose-triangle. As shown in Fig. 1, the suspension exhibits a consistent, small positive zetapotential of approximately 2 mV before the addition of flux and A6114, and then the zetapotentials turn to the negative value of about −30, −40, −9 mV when 0.5 wt%, 1 wt%, and 2wt% of LiVO3 flux were respectively introduced into the suspension. After A6114 was addedinto the suspensions, the zeta potentials exceed> -40 mV for the suspensions containing 0.5 wt%and 1 wt% of LiVO3, a similar value observed in suspension without flux addition. Whereas, thezeta potential of the suspension with 2 wt% LiVO3 is slightly lower (−33 mV) than the others.This because normally, when suitable doping amount of flux was added into the suspension,charged groups adsorbed onto particle surfaces can impart more positive or negative charge tothe particles, thereby increasing the zeta potential and improving stability. However, when thedoping amount exceeds a certain level, adsorption sites on the particle surface become saturated,so additional additives cannot effectively adsorb and instead remain in the solution phase. Thesefree additive molecules may compete with, displace, or “bridge” the molecules already adsorbedon the surface, which reduces the surface charge density. Consequently, the absolute value ofthe zeta potential decreases. Additionally, as the doping amount increases, the ionic strengthof the solution rises, leading to compression of the electrical double layer (Debye layer). Thiscompression reduces the effective transmission of surface charges into the surrounding solution.Thus, even if the actual surface charge remains relatively unchanged, the measured absolutezeta potential appears lower and the particles aggregate together, making rotation difficult andreducing the orientation degree. Although the zeta potential of the suspension with 2 wt% LiVO3is slightly lower than the others, the better colloidal stability can be achieved for the suspensionwith suitable amount of flux addition.Research Article Vol. 33, No. 22 / 3 Nov 2025 / Optics Express 45577Fig. 1. Zeta potentials of YVO4:Nd3+ suspensions with various LiVO3 adding amountsmeasured both before and after the addition of LiVO3 flux and A6114. For comparison, zetapotential of YVO4:Nd3+ suspension without flux addition is also presented (closed blacktriangle).3.2. Characterization of crystalline orientation in YVO4XRD patterns of YVO4:Nd3+ green bodies with the addition of flux fabricated under the strongmagnetic field of 12 T parallel to the slip casting direction are shown in Fig. 2. For comparison,the XRD pattern of the YVO4:Nd3+ green body with the same orientation but without flux, aswell as the XRD pattern of random YVO4:Nd3+ green body with 1 wt% flux addition, are alsopresented. To make data comparison easier, the intensity of peak (112) in Fig. 2 was normalizedto 1, and the intensities of other peaks were normalized relative to the intensity of peak (112)as well. The normalized reflection peak intensities of (004) and (200), which correspond to(001) and a,b-plane of YVO4, respectively, are approximately employed to estimate the degree oforientation. Compared with the random sample, the relative intensity of (004) peak is enhanced,while the intensity (200) peak is correspondingly reduced in all the samples prepared under themagnetic field, indicating the (001) texture development in YVO4:Nd3+ green bodies preparedunder the strong magnetic field both with and without addition of LiVO3 flux. Furthermore,YVO4:Nd3+ samples with the addition of 0.5 wt% and 1 wt% flux exhibit a similar degree oforientation to that of the sample without flux addition. Although the orientation degree is lowerin the sample with the addition of 2 wt% flux, due to the smaller zeta potential value of thesuspension, the orientation in this case is significantly enhanced compared to the non-orientedsample as well. These experimental results suggest that the suitable addition amount of LiVO3flux does not have a detrimental effect on the orientation of the green body.The well-textured green bodies with addition of flux obtained above were then sintered by SPSunder 1300 °C with a fixed dwelling time of 10 min and heating rate of 100 °C/min under thepressure of 90 MPa. In order to examine the effect of flux addition on the shrinkage behaviorof YVO4:Nd3+, the linear shrinkages during the sintering were recorded under the sinteringtemperature, as shown in Fig. 3(a). Before starting to apply the pressure at 1000 °C, as thetemperature increases, the shrinkage occurs at around 850 °C for all the samples containing flux.In contrast, shrinkage starts at about 980 °C for the sample without flux addition, which is about100 °C higher than that of the sample with flux addition. Figure 3(b) shows a comparison ofthe shrinkage rate (dL/dt) evaluated from the displacement L of Fig. 3(a). Samples with theResearch Article Vol. 33, No. 22 / 3 Nov 2025 / Optics Express 45578Fig. 2. XRD patterns of (001) textured-YVO4:Nd3+ green body with various flux additions.For comparison, the XRD patterns of the textured-YVO4:Nd3+ without flux addition and therandom YVO4:Nd3+ with 1 wt% flux addition are also presented.addition of flux exhibit a pronounced shrinkage slope around 925°C, a temperature significantlylower than the corresponding point for the sample without flux addition (which occurs at around1025°C). This discrepancy is a result of the significant impact that flux exerts on the sinteringprocess. That is, flux can form a liquid phase during sintering, it allows particles to rearrangeand bond together at lower temperatures; also, the liquid phase created by flux improves particlemobility, enabling particles to move and densify more effectively during sintering, which canlead to improved densification processing of the sample [30].Texture structures of SPSed-YVO4:Nd3+ with and without added flux were characterized usingthe SEM-EBSD technique. Figures 4 shows the EBSD inverse pole figure (IPF) maps for theYVO4:Nd3+ without and with 1 wt% flux addition. The surface perpendicular to the applyingdirection of the magnetic field was analyzed and colored by the color code on the standardstereographic triangle shown in Fig. 4.In Fig. 4(b), for the sample with 1 wt% flux addition, most grains exhibit red and red-orangecolors, similar in orientation to the sample without flux addition (Fig. 4(a)), suggesting thatwell-orientated (001) planes can be obtained with the addition of 1 wt% flux. This resultdemonstrates that the (001) textured structure can successfully be achieved by controlling theflux addition.The detailed degree of the (001) texture in YVO4:Nd3+ with and without adding flux can beillustrated from the distributions of the tilt angle between the c-axis and the vertical direction.These distributions can be calculated by using the multiples of a random distributions (MRD)from the EBSD data, as shown in Fig. 5. The distribution f MD of the c-axis in the YVO4:Nd3+Research Article Vol. 33, No. 22 / 3 Nov 2025 / Optics Express 45579Fig. 3. (a) Punch displacement L as functions of the sintering temperature T, (b) densificationrate (dL/dt) as a function of the sintering temperature T for the samples fabricated with andwithout LiVO3 flux addition.Fig. 4. EBSD inverse pole figure (IPF) mappings of the SPSed-YVO4:Nd3+ without (a),and with 1 wt% (b) LiVO3 flux addition. Corresponding pole figures were inserted at thebottom of each IPF maps.with and without adding flux is plotted against the tilting angle (θ) between the c-axis and thevertical direction. For samples without and with various flux additions, f MD shows a peak aroundθ = 0°. This suggests that approximately 94.2%, 92.7%, 91.8%, and 61.4% of grains have (001)planes aligned within the tilting angle θ<10° for YVO4:Nd3+ without and with flux additions of0.5%, 1%, and 2% wt, respectively. Although the sample with 2 wt% flux addition shows a lowerdegree of orientation, the results indicate that adding 0.5 and 1 wt% flux to YVO4:Nd3+ has nosignificant detrimental effect on the orientation degree of the samples.Figure 6 gives the microstructures of YVO4:Nd3+ both with and without flux addition. Itis clearly demonstrated that the introduction of flux leads to a corresponding increase in thegrain size of YVO4:Nd3+. As compared with grain size of the sample without flux addition(3.17 µm), for the YVO4:Nd3+ fabricated from the green body with added 0.5 wt% flux, exhibitsResearch Article Vol. 33, No. 22 / 3 Nov 2025 / Optics Express 45580Fig. 5. c-axis distribution in the SPSed-YVO4:Nd3+ without and with various flux additions.Fig. 6. SEM images of (001) textured-YVO4:Nd3+ ceramics fabricated at 1300 °C with aconstant dwelling time of 10 min and a heating rate of 100 °C/min, without flux (a), andwith 0.5 wt% (b), 1 wt% (c), and 2 wt% (d) LiVO3 flux additions.an average grain size of approximately 3.71 µm. Similarly, when the flux addition amount isincreased to 1 wt%, the average grain size further grows to about 4.8 µm. The largest grain size(10.32µm) is achieved in the sample with 2 wt% flux addition. This is because flux increases themobility of atoms or ions within the material, making it easier for them to diffuse and repositionthemselves during sintering. This increased mobility facilitates the grain boundary migrationResearch Article Vol. 33, No. 22 / 3 Nov 2025 / Optics Express 45581Fig. 7. Transmittance efficiencies of the (001) textured-YVO4:Nd3+ ceramics fabricatedwith and without LiVO3 flux addition.and coalescence, finally lead to large grain size of the sample. In addition, the density of thesample can be enhanced by adding flux (Fig. 6), the residual pores at the multiple grain junctionscan be reduced as flux addition. During sintering, flux can form liquid phase which facilitatesparticle rearrangement and bonding, resulting in improved densification. The flux also acts as a“liquid bridge” between particles, promoting better packing and reducing the number of pores inthe sintered material.3.3. Optical properties of (001) textured-YVO4:Nd3+ ceramicsFigure 7 displays the transmittance efficiencies of textured-YVO4:Nd3+ with 1wt% and withoutflux addition in the wavelength range from the visible to near-IR wavelength of λ= 400 -1400nm. Notably, distinct absorption peaks at about the wavelength of 590, 680, 750, and 810 nmcan be observed in both samples owing to the Nd3+ doping. The uneven profile around thewavelength of 900 nm is due to the equipment and does not reflect the properties of the sample.The impact of flux addition on transmittance is significant, the sample with 1 wt% flux additiondisplays markedly higher transmittance compared to the textured sample without flux underidentical sintering conditions, at the whole measurement wavelength range. For YVO4 with anon-cubic crystal structure, transmittance is known to be greatly influenced by microstructuralfactors such as texture, grain size, and porosity. The textured-YVO4:Nd3+ ceramics both with1 wt% and without flux addition exhibit the similar orientation degree, as observed in Fig. 4.Thus, grain size and porosity may the main factors causing the different transmittance in thesetwo samples. Normally, the larger the grain size, the greater the interaction of light within thematerial, resulting in significant effects such as scattering, absorption, multiple reflections, andrefractions, ultimately leading to reduced transmittance. However, YVO4:Nd3+ with 1 wt% fluxaddition, which shows the larger grain size (4.8 µm) than that of the sample without adding flux(3.17 µm), display the higher transmittance, deducing that increased density resulting from fluxaddition is expected to play an important role for increasing the enhanced transmittance efficiency.In addition, the amount of liquid phase generated during sintering with 0.5 wt% flux addition islimited and insufficient to eliminate pores. Consequently, the reduction in light scattering is onlypartial, resulting in a modest improvement in visible transmittance compared to the flux-freesample. In the long-wavelength range, however, the dominant loss mechanism is Mie scattering.Research Article Vol. 33, No. 22 / 3 Nov 2025 / Optics Express 45582Since the pores in this sample remain at the micrometer scale, the scattering intensity remainsnearly unchanged, leading to transmittance values comparable to those of the flux-free sample.In contrast, for the sample with 2 wt% flux addition, the excess liquid phase may form residualLi–V-rich glassy films at grain boundaries, which can introduce absorption or color centers inthe visible wavelength range. Nevertheless, the pores are effectively healed, thereby minimizinglong-wavelength scattering and yielding high near-IR transmittance in this sample.4. ConclusionsEnhanced optical properties of (001) textured translucent YVO4:Nd3+ ceramics can be achievedby introduction of LiVO3 flux using the SPS technique. Similar to the sample without the additionof flux, the introduction of flux with the amount less than 1 wt% to the sample also allows forthe achievement of a well-dispersed suspension and a high degree of orientation in the greenbody, indicating that the addition of LiVO3 flux with suitable amount does not have a detrimentaleffect on the dispersity of the suspension and the orientation of the green body. During theSPS sintering process, the introduction of LiVO3 flux can promote more effective densificationduring sintering via the liquid phase created. This ultimately leads to improved density of theYVO4:Nd3+ ceramics compared to the sample without flux addition, and finally results in theenhanced transmittance of the YVO4:Nd3+ ceramics. These results demonstrate that controllingthe microstructure by introduction suitable amount of flux using a textured structure method is asuitable way to enhance the transmittance of YVO4:Nd3+ ceramics.Funding. Innovative Science and Technology Initiative for Security (JPJ004596).Acknowledgements. The part of this work was financially supported by Innovative Science and TechnologyInitiative for Security, Grant Number JPJ004596, ATLA, Japan.Disclosures. The authors declare no conflicts of interest.Data availability. Data underlying the results presented in this paper are not publicly available at this time but maybe obtained from the authors upon reasonable request.References1. H. K. Yang, H. Choi, B. K. Moon, et al., “Improved luminescent behavior of YVO4: Eu3+ ceramic phosphors by Licontents,” Solid State Sci. 12(8), 1445–1448 (2010).2. T. Taira, A. Mukai, Y. Nozawa, et al., “Single-mode oscillation of laser-diode-pumped Nd : YVO4 microchip lasers,”Opt. 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