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Shingo Machida, Yuki Sada, Daiki Takeda, Shintaro Matsushita, Daisaku Yokoe, Kentaro Uesugi, Akihisa Takeuchi, [Gaku Okuma](https://orcid.org/0000-0002-2997-9166), Tetsuya Yamada

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[Sacrificial carbon fiber-nickel hydroxide core-shell particles for nickel oxide diffusion from pore surfaces to grain interfaces in zirconia ](https://mdr.nims.go.jp/datasets/978733c2-64e1-486d-9520-a5497f0c7864)

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Sacrificial carbon fiber–nickel hydroxide core–shell particles for nickel oxide diffusion from pore surfaces to grain interfaces in zirconiaShingo Machida1*, Daisaku Yokoe1, Yuki Sada2, Masayuki Uesugi2, Akihisa Takeuchi2, Gaku Okuma3, Daiki Takeda4,Shintaro Matsushita5, and Tetsuya Yamada6 1Materials Research and Development Laboratory, Japan Fine Ceramics Center, 2-4-1, Mutsuno, Atsuta-ku, Nagoya, Aichi, 456-8587, Japan2Japan Synchrotron Radiation Research Institute, JASRI/SPring-8, Kouto 1‑1‑1, Sayo, Hyogo 679‑5198, Japan3Research Center for Structural Materials, National Institute for Materials Science, 1‑2‑1 Sengen, Tsukuba, Ibaraki 305‑0047, Japan4Departiment of Chemical and Food, Industrial Research Institute of Ishikawa, 2-1, Kuratsuki, Kanazawa, Ishikawa, 920-8203, Japan5 Department of Mechanical Engineering, Institute of Science Tokyo, 2-12-1-I6-33, Ookayama, Meguro-ku, Tokyo, 152-8550, Japan6 Laboratory for Future Interdisciplinary Research of Science and Technology, Institute of Integrated research, Institute of Science Tokyo, 4259 Nagatsuta, Midori, Yokohama, Kanagawa 226-8503, Japan.*E-mail: shingo.machida@jfcc.or.jp Abstract: The sacrificial use of core–shell particles offers a versatile strategy for generating porous structures while increasing oxide–oxide interfaces within porous materials through diffusion of oxide species from pore surfaces to grain interfaces. In this study, carbon fiber (CF)–nickel hydroxide core–shell particles were combined with zirconia and calcined to produce porous zirconia sintered bodies containing nickel oxide at zirconia grain boundaries. Core–shell particles bearing vertically aligned platy nickel hydroxide on CF surfaces are prepared by a hydrothermal reaction. Upon calcination with zirconia at 1300°C for 12 h, the CF cores were removed by combustion, and the nickel hydroxide shells were converted into tubular nickel oxide hollow particles, whose interiors acted as pores within the zirconia matrix. Complementary analyses—including X-ray diffraction, electron microscopy, elemental mapping, and multiscale synchrotron X-ray computed tomography—revealed that nickel oxide is present in minute amounts, forming thin layers at zirconia grain boundaries and on portions of the pore surfaces. Compared with zirconia sintered without the core–shell particles, the zirconia-core-shell particle system suggests that interfacial diffusion of nickel oxide suppresses zirconia grain growth. These results demonstrate that both the CF core and the nickel hydroxide shell act sacrificially to create interconnected pores while designing additional oxide–oxide interfaces at pore surfaces, thereby generating an increased three-phase oxide–oxide–pore interfacial environment within the sintered bodies.IntroductionThe formation of new interfaces on material surfaces plays a crucial role in both functionalization and protection. Coating pristine surfaces with particles or films creates tailored surface layers whose thickness and interfacial adhesion and stability can be precisely controlled for both fundamental properties and practical application beyond interfacial reactions.1-6 Such design considerations become more critical within pore interiors, where diffusion behavior differs from that on external surfaces. The porous structure increases the number of accessible interfaces and active surfaces. To easily design and control these interfaces, special attention is paid here to the sacrificial use of core–shell particles. Removal of the core typically produces hollow particles defining pore interiors4-6, while the shell components can subsequently diffuse from pore surfaces to grain interfaces, leading to interfacial modification. In this context, the combination of zirconia and nickel oxide serves as a suitable model system. Zirconia is a representative ceramic material that exhibits sintering behavior for forming sintered bodies.7 Alumina also forms typical sintered bodies7, while spinel-type compounds easily develop with various cations.8,9 For simplicity, coloration of the sintered bodies is useful when the amount of components other than zirconia is small and their valence states remain unchanged; in this regard, nickel oxide is a suitable candidate. During heat treatment of the mixed powder compact of zirconia and carbon fiber–nickel hydroxide core–shell particles, the carbon fiber cores are removed, forming pores. This process ensures the presence of nickel oxide, converted from the nickel hydroxide shells, in the pore interiors (Scheme 1). As zirconia sintering proceeds, nickel oxide diffuses from the pore interiors into the grain interfaces within the zirconia porous sintered bodies (Scheme 1). This diffusion suppresses zirconia grain growth, indicating an increased number of interfaces between nickel oxide and zirconia. Based on these observations, possible mechanisms governing interfacial modification are discussed.Scheme 1. Schematic overview of this study.ExperimentalMaterialsNotably, yttria-stabilized zirconia (YSZ) has been widely used to stabilize the crystal phase of zirconia. In addition, YSZ with good sintering behavior is desirable in this study. Therefore, a commercial 3 mol% YSZ powder (TZ-3Y-E, TOSHO Co.) was selected. Powdered carbon fiber was obtained from NEWS Co. As commonly reported10-13, the hydrophilicity of the carbon fiber was improved by sequentially washing it with acetone, nitric acid, and deionized water, followed by drying at 100 °C. For simplicity, this carbon fiber is denoted herein as CF. The nickel chloride hexahydrate (NiCl6・6H2O), nickel oxide, urea, 1 mol/L nitric acid, and acetone were purchased from Wako.Sample preparation First, the hydrothermal conditions for producing suitable core–shell particles were investigated, and corresponding specimens were also prepared in the absence of core particles to compare their morphologies. Second, the conditions required to obtain YSZ sintered bodies were examined, and comparable powder and sintered specimens were prepared for comparison of crystalline phases, as well as pore structures and surfaces.The precipitation of nickel hydroxide on CF was carried out by modifying a previously reported procedure for obtaining well-defined and uniform hydroxide particles via urea hydrolysis.14 The CF (2.5 g) was dispersed in a green aqueous solution (12.5 mL) containing 0.67 mol/L NiCl2·6HO and 1.7 mol/L urea. The resulting slurry was transferred to a Teflon vessel enclosed in a stainless-steel jacket and heated at 150°C for 6 h. After the reaction, the solid product was separated from the resulting slurry by centrifugation at 4000 rpm for 5 min (the difference in dispersibility between CF and the solid product was hardly distinguishable by eye). The collected solid was then washed thoroughly with deionized water and dried at 100 °C. The obtained specimen is denoted as CF@1Ni(OH)2, and this material was used for mixing with YSZ as described below. The resulting supernatant was transparent and colorless, indicating that Ni²⁺ had been fully consumed under the present hydrothermal condition.For comparison, additional preparations using two-fold and four-fold Ni2+ concentrations were performed, and the resulting specimens were denoted as CF@2Ni(OH)2 and CF@4Ni(OH)2, respectively. Furthermore, hydrothermal reactions were also conducted for 6 h or 0.5 h in the absence of CF to obtain specimens denoted as 1Ni(OH)2, 2Ni(OH)2, 4Ni(OH)2, and 1Ni(OH)2-0.5h. Disk-shaped powder compacts were prepared by uniaxial pressing, using 2.0 g of YSZ as the base amount and adjusting the CF content as required. The compacts had a diameter of 25 mm. YSZ and CF were mixed using an agate mortar and pestle until the color of the resulting powder became visibly uniform. The thickness of the compacts increased in proportion to the amount of CF added (approximately 2.5 mm / 0.5 g of CF; the thickness of the YSZ-only compact was 2.5 mm). Thus, uniform mixing of CF with the other powders was achieved relatively easily using this conventional mixing method. When these compacts were calcined at 1300 °C, those containing higher CF contents (exceeding 0.50 g CF / 2.0 g YSZ) were exhibited powder-like surfaces or easily crumbled. Therefore, the CF content was fixed at 0.50 g / 2.0 g YSZ. Nevertheless, the formation of sintered bodies required calcination at 1300 °C for 12 h. The resulting specimen is denoted as YSZ-CF-1300-12. For comparison, YSZ without CF was calcined under the same conditions. The resulting specimen is denoted as YSZ-1300-12. In addition, curved sintered bodies were obtained when heating was performed directly at 1300°C without a prior hold at 800°C for 2 h to ensure complete carbon removal. Accordingly, the optimized heat-treatment condition was determined to be calcination at 1300°C for 12 h with a preliminary hold at 800 °C for 2 h. The heating rate was 10°C / min⁻1, followed by furnace cooling after the heat treatment. Notably, this calcination condition was also applied to the powder compacts of YSZ containing CF@1Ni(OH)2. The amount of CF in CF@1Ni(OH)2 was identical to that used in the CF-only system. For simplicity, the resulting specimen is denoted as YSZ-CF@1Ni(OH)2-1300-12. As mentioned above, because rigid sintered bodies were obtained after calcination at 1300°C for 12 h, the specimens prepared by calcining the powder compacts at 800 °C for 2 h were easily crumbled.For comparison, powder forms of YSZ, YSZ containing CF, and YSZ containing CF@1Ni(OH)2, CF@1Ni(OH)2, nickel oxide (Wako), and nickel oxide (Wako) containing CF were calcined under the conditions described above. In this context, these specimens were obtained by stopping the preliminary hold step mentioned above. The YSZ or CF@1Ni(OH)2 specimens are denoted as p-YSZ sample name-x-y, CF@1Ni(OH)2-x-y, NiO (Wako)-x-y, or NiO(Wako)-CF-x-y, where x and y represent the calcination temperature and time, respectively (p: powder). Notably, a calcination time of 0 indicates that heating was stopped immediately upon reaching the target temperature. In addition, a YSZ–nickel oxide (Wako) powder compact (1:1 weight ratio) containing CF (0.50 g / 2.0 g of the mixed powder) was calcined at 1300°C for 12 h with the preliminary hold, to obtain the specimen denoted as YSZ-CF-NiO (Wako)-1300-12.CharacterizationThe crystal phases of the powdered and sintered specimens were characterized by X-ray diffraction (XRD) using a diffractometer (Empyrean, PANalyical) operated at 40 mA and 45 kV with monochromated Cu Kα radiation. The step size and scan time were 0.01 ° (2θ) and 1.0 s, respectively. Specimen morphologies were examined using field-emission scanning electron microscopy (FE-SEM: SU8000, HITACHI) at an acceleration voltage of 2.0 kV, with an electron beam energy of 2.0 µV and a working distance of 8 mm, used for secondary electron imaging. Prior to observation, the samples were sputter-coated with a 3-nm osmium. Regarding YSZ-CF@1Ni(OH)2-1300-12, the cross-sections of the processed surfaces of the cylindrical specimens described below were examined. For simplicity, FE-SEM images are hereafter referred to as SEM images. Cross-sections of YSZ-CF@1Ni(OH)2-1300-12 were characterized by transmission electron microscopy (TEM; JEM-F200, JEOL) operated at an accelerating voltage of 200 kV. Elemental analyses were conducted during TEM and bright-field scanning TEM (BF-STEM) observations using energy-dispersive X-ray spectroscopy (EDS; JED-2300 SDD, JEOL) at the same accelerating voltage. For cross-sectional preparation, the samples were embedded in epoxy resin and subsequently processed with a focused ion beam (FIB) system (NB5000, Hitachi) operated at 40 kV. Prior to cross-section milling, metallic tungsten, carbon, and platinum were deposited as protective layers.To clarify and visualize the porous structures and of YSZ–CF–NiO-1300-12 and YSZ–CF@1Ni(OH)₂-1300-12, their cross-sections were also characterized using synchrotron radiation multiscale X-ray computed tomography (CT) at beamline BL20XU of the SPring-8 synchrotron radiation facility (Japan)15-17, with an X-ray energy of 30 keV. This multiscale CT system comprises a micro-CT mode (wide field, low resolution) for acquiring the overall structure and a nano-CT mode (narrow field, high resolution, ~100 nm) for detailed structural observation. The voxel sizes of the micro- and nano-CT modes were 0.5 µm and 30 nm, respectively. Sample preparation and measurement conditions, including the X-ray CT setup in the experimental hutch, were identical to those reported in previous studies.15-17 The central portions of the specimens were laser-processed into cylindrical samples with a diameter of 0.85 mm and a height of 5.00 mm, which were then rotated on the sample stage at BL20XU. However, because of the high atomic number of Zr, the upper chipped regions of the cylindrical samples were selected as the observation area. Thus, the CT images correspond to cross-sections of the disk-shaped specimens along their thickness direction (Scheme 2). The three-dimensional (3D) pore structure of YSZ-CF@1Ni(OH)2-1300-12 was reconstructed and visualized from the acquired CT images, which were first binarized and noise-reduced by Gaussian filtering using Avizo.Scheme 2. Schematic illustration of the (a) observation and (b) laser-processed directions of the cylindrical samples prepared from the disk-shaped specimen used for CT measurements. Results and DiscussionFigure 1. XRD patterns of (a) CF, (b) CF@1Ni(OH)2, and (c) 1Ni(OH)2. Figure 1 shows the XRD patterns of CF, CF@1Ni(OH)2, and 1Ni(OH)2. The pattern of CF@1Ni(OH)2 exhibits reflections attributed to α-phase nickel hydroxide along with a halo originating from CF (Figure 1a and b). In the pattern of 1Ni(OH)2, the reflections are broad but can be assigned to α-phase nickel hydroxide (Figure 1c). These results indicate that the hydrothermal condition used for 1Ni(OH)2 yields a single phase of α-phase nickel hydroxide, even in the presence of CF. Figure 2. SEM images of (a) 1Ni(OH)2, (b) an enlarged view of (a), (c) CF, (d) an enlarged view of (c), (e) CF@1Ni(OH)2, (f) an enlarged view of (e), (g) an enlarged view of (f), and (h) 1Ni(OH)2-0.5h. Figure 2 shows the SEM images of 1Ni(OH)2, CF, CF@1Ni(OH)2, and 1Ni(OH)2-0.5h. The image of 1Ni(OH)2 shows spherical particles on which platy particles are vertically aligned, a result consistent with previous reports18-19 describing spherical aggregates composed of platy nickel hydroxide particles (Figure 2a and b). Fiber-shapes do not differ between CF and CF@1Ni(OH)2, whereas vertically aligned platy particles are present on the fiber surfaces of CF@1Ni(OH)2 (Figure 2c-g). Although these vertically aligned platy particles only partially cover the fiber surfaces (Figure 2e–g), this degree of coverage is considered sufficient. These results indicate the successful formation of CF–nickel hydroxide core–shell particles, although the coverage of the nickel hydroxide shell does not change with increasing Ni2+ concentration in the starting solution, as confirmed by the SEM images of CF@2Ni(OH)2 and CF@4Ni(OH)2 (Figure S1a, b, and e-g). This is likely due to the nature of the starting dispersion, in which CF particles form a slurry; therefore, platy particles are scarcely deposited on surface areas where CF particles are in contact with one another. Furthermore, spherical aggregates of vertically aligned platy particles are absent in the images of CF@4Ni(OH)2 (Figure S1e-g). The particle shapes of 2Ni(OH)2 and 4Ni(OH)2 are similar to those observed for CF@2Ni(OH)2 and CF@4Ni(OH)2 (Figure S1c, d, and h). The XRD pattern of 4Ni(OH)2 shows reflections attributed to the β-phase of nickel hydroxide (data not shown). Meanwhile, mechanisms for the formation of spherical aggregates composed of platy nickel hydroxide particles have been proposed18-21, and vertically aligned platy particles on various substrates have also been widely reported.22-26 In this study, such spherical aggregates were already formed after a reaction time of 0.5 h, as shown in the SEM image of 1Ni(OH)2-0.5 (Figure 2h). Therefore, the detailed precipitation behavior and precise coating of vertically aligned platy particles on different materials26 are left for future work. However, as mentioned above, the coverage of the platy nickel hydroxide particles is sufficient (Figure 2e–g) for the mixing and calcination with powdered YSZ described below.Figure 3. XRD patterns of (a) powdered pristine YSZ, (b) p-YSZ-1300-12, (c) p-YSZ-CF-1300-12, (d) YSZ-1300-12, and (e) YSZ-CF-1300-12. Figure 3 shows the XRD patterns of the YSZ specimens. The YSZ powder used in this study exhibits primarily reflections of the tetragonal phase of zirconia, with relatively weak reflections from the monoclinic phase (Figure 3a). The latter phase disappears as the crystallinity of the tetragonal phase increases upon calcination at 1300°C for 12 h, regardless of the presence of CF or the specimen form (Figure 3b–d). This indicates that neither zirconia crystallinity nor crystal phase is affected by the atmosphere generated during carbon combustion27 or by compact formation, whose interior can present a locally reducing environment28, under the experimental conditions employed in this study. The exclusive presence of the tetragonal zirconia phase in all YSZ specimens is also consistent with a previous report.29 As discussed previously, although tetragonal zirconia typically reverts to monoclinic zirconia during cooling in the 1000–800°C range30, the YSZ specimens were insufficiently held within this temperature range during furnace cooling from 1300°C (see Experimental)29, preventing this transformation. Figure 4. XRD patterns of (a) p-YSZ-CF@1Ni(OH)2-1300-12, (b) YSZ-CF@1Ni(OH)2-1300-12 (inset: photograph of this specimen), (c) CF@1Ni(OH)2-800-2, (d) CF@1Ni(OH)2-CF-800-2, (e) CF@1Ni(OH)2-1300-12, (f) CF@1Ni(OH)2-CF-1300-12, (g) NiO(Wako)-1300-12, and (h) NiO(Wako)-CF-1300-12. Open circles indicate the second-strongest reflections of NiO. Figure 4 shows the XRD patterns of the YSZ-CF@1Ni(OH)2-1300-12, CF@1Ni(OH)2, and nickel oxide (Wako) specimens. The patterns, dominated by tetragonal zirconia, do not differ between the powder and compact forms of YSZ-CF@1Ni(OH)2-1300-12 (Figure 4a and b). Although the main reflection of the tetragonal zirconia phase (Figures 3, 4a, and 4b) may overlap with the strongest reflection of NiO, the second-strongest NiO reflection is clearly observed in the XRD patterns of YSZ-CF@1Ni(OH)2-1300-12. In addition, the specimens exhibit green coloration characteristic of nickel compounds, as shown in the inset near Figure 4b. Therefore, nickel oxide is certainly present in YSZ-CF@1Ni(OH)2-1300-12. Furthermore, in addition to the absence of reflections attributable to metallic Ni, the crystalline phase of NiO does not differ among the CF@1Ni(OH)2 specimens (Figure 4a–f). Thus, the nickel hydroxide in CF@1Ni(OH)2 is indeed converted into nickel oxide at temperatures below 800°C without undergoing reduction to metallic Ni31,32 by the atmosphere generated during carbon combustion27 or by the locally reducing environment that may develop inside compacts28 under the experimental conditions employed in this study. Meanwhile, the reflections in the XRD patterns of CF@1Ni(OH)2 specimens are broadened due to the presence of carbon (Figure 4b–f). This behavior is identical to that observed for commercially available nickel oxide (Wako), as shown in the XRD patterns of NiO(Wako)-1300-12 and NiO(Wako)-CF-1300-12 (Figure 4g and h). Thus, this broadening is not intrinsic to CF@1Ni(OH)2, as discussed below.  Figure 5. SEM images of (a) 1Ni(OH)2-800-12, (b) an enlarged view of (a), (c) 1Ni(OH)2-1300-12, (d) an enlarged view of (d), (e) CF@1Ni(OH)2-800-2, (f) an enlarged view of (f), (g) CF@1Ni(OH)2-1300-0, (h) an enlarged view of (g), (i) CF@1Ni(OH)2-1300-12, (j) an enlarged view of (j), (k) powdered pristine YSZ, (l) p-YSZ-1300-12, (m) the cross-section of YSZ-CF@1Ni(OH)2-1300-12, (n) an enlarged view of (n), (o) an enlarged view of (n), and (p) nickel oxide (Wako). Figure 5 shows SEM images of the calcined YSZ, CF@1Ni(OH)2, and nickel oxide (Wako) specimens. In the SEM images of 1Ni(OH)2-800-2 and 1Ni(OH)2-1300-12 (Figure 5a and c), the spherical morphology does not differ from that of 1Ni(OH)2 (Figure 2a). In contrast, the vertically aligned platy particles observed in 1Ni(OH)2 (Figure 2b) disappear, and crystal face–bearing particles appear, increasing in size with increasing calcination temperature (Figure 5c and d). Considering the NiO reflections in the XRD patterns of 1Ni(OH)2-800-2 and 1Ni(OH)2-1300-12 (Figure 4d and f), this particle growth is attributed to grain growth of nickel oxide. In contrast, relatively small platy particles are observed in CF@1Ni(OH)2-800-2 and CF@1Ni(OH)2-1300-0 (Figure 5f and h). Compared with the former, the latter shows tubular particles composed of interconnected platy particles (Figure 5e–h). These tubular particles decrease in CF@1Ni(OH)2-1300-12, and the tube walls consist of particles with ambiguous shapes (Figure 5i and j). Based on the SEM images of CF@1Ni(OH)2 (Figure 2e–g) and the XRD patterns of CF@1Ni(OH)2, CF@1Ni(OH)2-800-2, and CF@1Ni(OH)2-1300-12 (Figures 1a, 4d, and 4f), the nickel hydroxide shell is converted into a nickel oxide shell, resulting in the formation of tubular nickel oxide hollow particles in CF@1Ni(OH)2-1300-0 and CF@1Ni(OH)2-1300-12 (Figure 5g-j). The absence of these tubular shapes in CF@1Ni(OH)2-800-2 (Figure 5e and f) is likely due to the ease of crumbling the nickel oxide shell during calcination or SEM sample preparation, as the shell consists of weakly connected particles formed under relatively short calcination times and low temperatures. With increasing calcination time, the shells tend to collapse, likely due to grain growth of nickel oxide. Meanwhile, grain growth of YSZ particles is also observed after calcination at 1300°C for 12 h, based on the SEM images and XRD patterns of pristine powdered YSZ and p-YSZ-1300-12 (Figures 1a, 1b, 5k, and 5l). Notably, the spherical aggregates of these particles remain unchanged after calcination (Figure S2). In contrast, these distinct CF@1Ni(OH)2- or YSZ-derived particles are absent, and more finite particles appear a as the walls of rod-like pores formed by the shape of CF in the cross-sectional SEM images of YSZ–CF@1Ni(OH)2-1300-12 (Figure 5m–o). Thus, different grain growth behaviors are plausible in this specimen. Given the broadening of NiO diffraction lines in the presence of carbon (Figure 5c–h), the grain growth of platy particles is suppressed to some extent when CF and/or YSZ are present. Notably, the particle sizes and morphologies observed in the CF@1Ni(OH)₂ specimens (Figure 5a–j) are not present in the nickel oxide (Wako) specimen (Figure 5p), confirming that the particles prepared in this study differ distinctly from the commercially available material.Figure 6. BF-STEM image of the cross-section of YSZ-CF@1Ni(OH)2-1300-12 and the corresponding elemental mappings.  Figure 6 shows the BF-STEM image and the corresponding elemental mappings of the cross-section of YSZ–CF@1Ni(OH)2-1300-12. Based on the BF-STEM image and the Zr and Y mappings (Figure 6), the smaller particles observed in the SEM image of YSZ–CF@1Ni(OH)2-1300-12 (Figure 5o) are identified as YSZ. Furthermore, according to the Ni and O mappings (Figure 6) and the XRD patterns of the YSZ–CF@1Ni(OH)2 and CF@1Ni(OH)2 specimens (Figure 4), nickel oxide layers are present at the grain boundaries of YSZ as well as on portions of the pore surfaces. Similar elemental distributions are also observed in another cross-section of YSZ–CF@1Ni(OH)2-1300-12, where the Ni intensity is pronounced in the line mappings along the YSZ grain boundaries (Figure S3). To clarify whether such NiO-containing interfaces are distributed throughout the entire porous structure of YSZ–CF@1Ni(OH)2, the X-ray CT analyses (Figure 7) are helpful, as described below. Figure 7. Micro-CT images of (a) YSZ–CF–NiO-1300-12 and (b) YSZ–CF@1Ni(OH)₂-1300-12, and (c) nano-CT image of the region indicated by the orange circle in (b). 3D images of the pore structure of YSZ–CF@1Ni(OH)₂-1300-12 in the (d) x–y, (e) x–z, and (f) y–z planes. As shown in the XRD patterns (Figure 4a and b) and elemental mappings (Figures 6 and S3), the amount of nickel oxide in YSZ–CF@1Ni(OH)2-1300-12 is minute. Consequently, no discernible contrast is observed in the CT images of YSZ–CF@1Ni(OH)2-1300-12 compared with YSZ–CF-NiO-1300-12 (Figure 7a–c). Considering the atomic numbers of Ni and Zr, the relatively bright regions in the micro-CT images of YSZ–CF-NiO-1300-12 correspond to nickel oxide, whereas no such distinguishable regions appear in either the micro- or nano-CT images of YSZ–CF@1Ni(OH)2-1300-12. However, based on the CT images (Figure 7a–c), in which round fiber cross-sections are predominantly observed, together with the observation and laser-processing directions (Scheme 2), the CF particles (Figure 2c and d) are aligned predominantly in the horizontal direction of the disk-shaped specimens. These fibers, which convert into rod-like pores upon calcination, form interconnected pores throughout the disk specimens, as confirmed by the 3D images of the pore structure of YSZ–CF@1Ni(OH)2-1300-12 (Figure 7d-f). Given the presence of nickel oxide at YSZ grain boundaries and pore surfaces (Figures 6 and S3), together with the green coloration and the NiO reflections (Figure 4), such NiO-containing interfaces are distributed near pore surfaces across the entire porous structure of YSZ–CF@1Ni(OH)2-1300-12. Based on the results described above, the mechanisms underlying the thermal behavior of YSZ–CF@1Ni(OH)₂-1300-12 are proposed below, although the specimens subjected to calcination of the powder compacts at 800°C for 2 h were easily crumbled (see Experimental). Considering the interconnected, rod-like pores observed in the SEM, CT, and 3D images (Figures 5g–j, 5m–o, and 7), as well as the XRD patterns (Figures 1 and 4), the CF–nickel hydroxide core–shell particles are converted into tubular nickel oxide hollow particles, whose interiors act as pores within the YSZ matrix following the removal of CF by combustion during calcination. The tubular nickel oxide walls—formed from the nickel hydroxide shells—remain on the pore surfaces of the YSZ matrix. Given the absence of particles derived from the calcined CF@1Ni(OH)2 specimens on the pore surfaces and pore walls of YSZ–CF@1Ni(OH)2-1300-12 (Figures 5g–j, 5m–o, 6, S3, and 7), it is suggested that the tubular nickel oxide walls diffuse into the YSZ grain interfaces and partially remain as thin nickel oxide layers on the pore surfaces as the calcination temperature and time increase. Thus, the nickel hydroxide shells in the core–shell particles serve as precursors for supplying nickel oxide that diffuses from the pore surfaces into neighboring grain interfaces. Consequently, both the CF core and the nickel hydroxide shell act sacrificially during the calcination of the CF–nickel hydroxide core–shell particles mixed with YSZ. In general, impurities and additional compounds within sintered inorganic materials migrate to grain boundaries.33,34 In this study, because the YSZ particles used in this study originally do not contain nickel oxide as an impurity, it is suggested that components introduced onto zirconia surfaces can also diffuse into grain interfaces concurrently with or subsequent to zirconia sintering. Based on the differences in particle and grain sizes between YSZ-1300-12 and YSZ–CF@1Ni(OH)2-1300-12 (Figures 5l–o, 6, and S3), it is suggested that the diffusion of nickel oxide into YSZ grains suppresses grain growth. Although further investigation using flat-substrate systems35,36 is warranted to simplify the analysis, this interfacial diffusion of nickel oxide within YSZ grains is advantageous for increasing the interfacial area between YSZ and nickel oxide, some of which is exposed on pore surfaces.37,38 This also warrants additional studies involving systems containing nickel oxide within YSZ matrices, for which the YSZ–CF–NiO specimens may serve as promising candidates. Results of such studies will be reported subsequentlyConclusions CF–nickel-hydroxide core–shell particles were successfully employed as sacrificial templates to construct interconnected pore networks and to increase grain boundaries in porous YSZ sintered bodies. Hydrothermal synthesis produced CF particles coated with vertically aligned platy nickel hydroxide particles, which, upon calcination with YSZ, were transformed into tubular nickel oxide hollow particles, while the CF cores were removed to form rod-like interconnected pores. XRD patterns, SEM images, BF-STEM images with elemental mappings, and multiscale synchrotron X-ray CT images revealed that nickel oxide derived from the shells is present in minute amounts, eventually distributed as thin layers at YSZ grain boundaries and partially on pore surfaces. The absence of nickel oxide tubular particles in the obtained microstructure, together with the grain size compared with zirconia sintered without the core–shell particles, indicates that nickel oxide diffuses from pore surfaces into neighboring grain interfaces and suppresses zirconia grain growth. Consequently, both the CF core and the nickel hydroxide shell act sacrificially: the core defines the interconnected pore architecture, while the shell serves as a precursor that supplies nickel oxide diffusing from pore surfaces to grain interfaces, resulting in the formation and expansion of three-phase oxide–oxide–pore interfaces along the pore surfaces.37,38 This dual-sacrificial use of the core–shell components—through their controlled decomposition and diffusion—paves the way for designing porous sintered bodies in which pore structure and interfacial chemistry are co-designed.Acknowledgement This study was performed with the approval of JASRI (Proposal No. 2025A1007 and 2025B1010). This research was supported by Environmental Restoration and Conservation Agency (3RL-2301).Author contributionShingo Machida: conceptualization, data curation, investigation, writing—original draft, project administration, supervision. Daisaku Yokoe: investigation, data curation, writing—review and editing. Yuki Sada: investigation, writing—review and editing. Masayuki Uesugi: investigation, writing—review and editing. Akihisa Takeuchi: investigation, writing—review and editing. Gaku Okuma: writing—review and editing. Daiki Takeda: writing—review and editing. Shintaro Matsushita: writing—review and editing. Tetsuya Yamada: writing—review and editing. Conflicts of InterestThe authors declare no conflict of interest.References1. Le, T. C.; Nguyen, D.-N.; Kaminski, D.; Kolver, T.; Subramanian, P.; Bateman, S. Harnessing Machine Learing for the Design of Surface Coatings: Challenges and Opportunities. ACS Appl. Mater. Interfaces. 2025, 17, 39795-39808.2. Jiang, S.; Dyk, A. V.; Maurice, A.; Bohling, J.; Fasano, D.; Brownell. S. Design Colloidal Particle Morphology and Self-Assembly for Coating Applications. Chem, Soc. Rev. 2017, 46, 3792-3807.3. Ogawa, M. Mesoporous Silica Layer: Preparation and Opportunity. Chem. 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