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Unveiling unusual coloration in amorphous phosphate pigments: a study inspired by the mineral Brazilianite
(2026-12-01)
Onoda, Hiroaki
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Higuchi, Kanata
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Charoonsuk, Thitirat
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Pulphol, Phieraya
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Muanglhua, Rangson
Brazilianite, with the formula NaAl<inf>3</inf>(PO<inf>4</inf>)<inf>2</inf>(OH)<inf>4</inf>, exhibits an intriguing pale yellow to green coloration, a phenomenon not readily explained by the absence of conventional d-block chromophoric metal ions. This study investigates the synthesis of amorphous phosphate-based pigments compositionally analogous to Brazilianite, aiming to replicate and understand the origin of its distinctive coloration through controlled precursor stoichiometry and thermal processing. Sodium, aluminum, and phosphate precursors were precisely mixed and thermally treated. X-ray diffraction (XRD) confirmed the predominantly amorphous nature of the synthesized materials, irrespective of achieving long-range crystalline order analogous to mineral Brazilianite. Despite this, samples processed at intermediate temperatures (e.g., 300–400 °C) exhibited a consistent yellowish hue. This non-conventional coloration, attributed to intrinsic electronic or structural features within the amorphous phosphate network rather than traditional chromophores, represents a significant scientific innovation. This coloration was found to be intrinsically linked to the phosphate network itself. Stability assessments in acidic (0.1 wt% H<inf>2</inf>SO<inf>4</inf>) and basic (0.1 wt% NaOH) environments revealed significant vulnerability, attributed to the facile dissolution of sodium and aluminum phosphate species. Notably, the yellowish coloration persisted across various Na/Al/P compositional ratios, even with systematic variations in aluminum or sodium content. This strongly suggests that the observed color is not critically dependent on a precise Na: Al stoichiometry but is fundamentally governed by the local electronic structure within the phosphate network, potentially involving defect centers or specific P-O-Al/Na linkages. These findings offer valuable insights into designing novel, non-toxic, color-stable pigments where coloration arises from mechanisms beyond traditional transition metal ion incorporation, highlighting the potential role of controlled disorder in phosphate-based materials for sustainable applications.
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Process-structure-property relationships in low-temperature microwave dielectric ceramics: from glass-assisted sintering to cold sintering for 5G/6G devices
(2026-12-01)
Pulphol, Phieraya
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Tang, Ying
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Fang, Liang
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Vittayakorn, Wanwilai
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Sukkha, Usa
With the rapid advancement of wireless communication from 5G to 6G, a pressing need has emerged for microwave dielectric ceramics with excellent performance at reduced processing temperatures, compatible with low-temperature co-fired ceramic technology. This review traces historical milestones and highlights modern design strategies for achieving optimum dielectric constant, ultra-low dielectric loss, and near-zero temperature coefficient of resonant frequency. Special emphasis is placed on recent advances in low-temperature densification routes, including sintering aids, intrinsically low-sintering-temperature ceramic families, and novel techniques like the cold sintering process. This review provides a critical analysis of the performance trade-offs inherent to each strategy, addressing the persistent challenges in achieving ultra-low loss. Furthermore, we highlight the paradigm shift toward a holistic, multifunctional design imperative for 6G systems. Finally, the transformative potential of cross-disciplinary approaches, particularly AI-assisted discovery, and computational modeling, is discussed as a key enabler for accelerating the design of next-generation, high-performance, and sustainable LTCC-compatible materials.
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Cold sintering-assisted low temperature fabrication of dense Ba5Nb4O15 ceramics
(2026-06-08)
Sukkha, Usa
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Teandam, Apichayaporn
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Pakawanit, Phakkhananan
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Kamonpha, Phitsamai
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Vittayakorn, Wanwilai
This study presents a novel approach for fabricating Ba<inf>5</inf>Nb<inf>4</inf>O<inf>15</inf> (BNO) ceramics at low sintering temperatures via the cold sintering process (CSP), using Ba(OH)<inf>2</inf>·8H<inf>2</inf>O (BOH) as a transient liquid phase. CSP was performed under an external pressure of 10MPa with a sintering temperature range of 150°C to 300°C. Optimally, BNO-BOH ceramics achieved a relative density of 93.7 ± 0.43 when sintered at 250°C for 1h. Scanning electron microscopy (SEM) suggested that particle densification occurred via a dissolution-precipitation process, which filled pores and formed necks between particles. The study demonstrates that the residual liquid content is crucial for ceramic densification. Annealing the as-cold sintered BNO-BOH ceramics at 1000°C for 1h successfully eliminates the BaCO<inf>3</inf> secondary phase. Furthermore, dielectric properties of annealed ceramics were also characterized at room temperature from frequency range of 20Hz to 2MHz. The dielectric permittivity is reported to be 39.2 and 0.01 for tanδ at 1.8MHz. The cold sintering process provides an effective strategy to reduce the sintering temperature while achieving high relative density. This method offers a promising alternative for the fabrication of advanced ceramics.
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Application of Heywang model to the apparent PTCR effect in water-adsorbing layered materials
(2026-01-01)
Pulphol, Phieraya
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Sriphan, Saichon
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Chaithaweep, Kanokwan
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Vittayakorn, Naratip
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Maluangnont, Tosapol
Surface proton/hydroxide conduction predominates at ambient conditions prior to levelling off at elevated temperatures due to water evaporation and subsequent loss of charge carriers. This water-induced charge transport results in the “apparent” positive temperature coefficient of resistivity (PTCR) effect. Herein, we show that Heywang model typical of classical ferroelectric PTCR ceramics is applicable to a wide range of water-adsorbing layered materials (0.17–4.76 wt% H<inf>2</inf>O). Several examples include layered alkali titanates with negatively-charge sheets; one van der Waals material (g‒C<inf>3</inf>N<inf>4</inf>) with neutral sheets; and a NiFe layered double hydroxide with positively-charge sheets. The linear log ρ<inf>DC</inf> vs (ε′<inf>DC</inf>T)-1 plots (ρ<inf>DC</inf> = static resistivity, ε′<inf>DC</inf> = static dielectric permittivity, and T temperature) are observed from 25 to 250 °C where resistivity and dielectric permittivity varied up to five orders of magnitude. Using Cs<inf>2</inf>Ti<inf>6</inf>O<inf>13</inf> as a representative sample, the density of acceptor states at the grain boundary N<inf>s</inf> (the exact nature to be elucidated) is ∼1010-1011 cm-2, slightly dependent on the heating/cooling rates (0.5, 2 and 5 °C·min-1). Complex plane analyses show that capacitances at grain/grain boundaries alike are constant regardless of temperatures, but resistances in both cases peak at 150–200 °C. While rigorous theoretical basis is yet to be constructed, the observed linearity suggests that there could be a common foundation between these two classes of PTCR materials which have been treated separately so far.
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Crystal structure, microwave dielectric properties and antenna simulation of low-permittivity Eu2CaMGa4O12(M = Ti, Sn, Zr) garnets
(2025-12-01)
Yang, Yang
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Wang, Zhaohui
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Qiu, Huizi
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Fang, Weishuang
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Tang, Ying
The rattling of cations plays a vital role in energy loss mechanisms and is a fundamental approach for tuning dielectric performance. In this work, phase-pure garnets with the general formula Eu<inf>2</inf>CaMGa<inf>4</inf>O<inf>12</inf> (M = Ti, Sn, Zr) were successfully synthesized. The Eu<inf>2</inf>CaZrGa<inf>4</inf>O<inf>12</inf>, Eu<inf>2</inf>CaSnGa<inf>4</inf>O<inf>12</inf>, and Eu<inf>2</inf>CaTiGa<inf>4</inf>O<inf>12</inf> ceramics were obtained through sintering at 1400°C–1500°C for 6 h. The ceramics sintered at 1450°C exhibited favorable microwave dielectric performance with low ε<inf>r</inf> values of 10.74 (for Sn), 12.34 (for Zr) and 13.66 (for Ti), high Q×f values of 83,500 GHz (for Sn), 72,800 GHz (for Zr) and 59,200 GHz (for Ti), as well as negative τ<inf>f</inf> values of −60.60 ppm/°C (for Sn), −57.20 ppm/°C (for Zr) and −48.20 ppm/°C (for Ti). In Eu<inf>2</inf>CaMGa<inf>4</inf>O<inf>12</inf> (M = Ti, Sn, Zr) ceramics, the “rattling” effect of Eu3+and Ga3+dominated a slight overall structural expansion, which, in turn, led to the higher ε<inf>r</inf> compared to the theoretical prediction (ε<inf>r(C-M)</inf>) and contributed to the negative τ<inf>f</inf>. The variation in the Q × f values of the ceramics exhibited a negative correlation with the FWHM of the A<inf>1g</inf> Raman mode. Furthermore, due to the ultra-low loss of Eu<inf>2</inf>CaSnGa<inf>4</inf>O<inf>12</inf> ceramic, a dielectric resonator antenna was designed. This demonstrates the promising potential of the ceramics for 5G communication applications.
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Titanate Nanosheets/Cellulose Composite Showing Improved Crystallinity and Decreased Water Wettability by Gamma-Irradiation
(2025-11-25)
Tariwong, Yaowaluk
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Pulphol, Phieraya
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Sangtawesin, Tanagorn
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Seriwattanachai, Chaowaphat
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Kanjanaboos, Pongsakorn
While molecularly thin nanosheets have been increasingly studied as functional coatings, their use as a hydrophobic and γ-irradiation-tolerant component in biologically derived matrices is to be demonstrated. Herein, simple dip-coating was employed to fabricate titanate nanosheets/cellulose composites, which were subjected to γ-irradiation up to 50 kGy. Their surface chemistry was evaluated by water contact angle (WCA) measurements and X-ray photoelectron spectroscopy (XPS). Upon irradiation, the WCA of all samples nonmonotonically increased in three stages from ∼29 to 50° (noncoated) and ∼46 to 80° (composite, optimized at ∼1.2 wt %Ti loading, or 0.2 mg·cm–2). The titanium content and the 4+ valence did not change with the dose, suggesting the radiolytic stability. The dual surface modification occurs while cellulose fiber morphology and nanoscale mechanical properties are preserved. The increased WCA at the cellulose-part is explained by the γ-irradiation-induced crystallization according to the increased crystallinity index and improved thermal stability. At the other component, nanosheet coating results in increased surface roughness and diminished water–surface interactions. The latter is deduced from DSC measurements of water evaporation from pristine and 50 kGy-irradiated Cs<inf>0.7</inf>Ti<inf>1.825</inf>O<inf>4</inf>layered crystal-a nanosheet precursor. Our work suggests further exploration of nanosheets with diverse structures and compositions as coatings or fillers, which could find applications in γ-irradiation-sterilized barrier films.
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Surface phosphatization of cerium-lanthanum oxides for catalytically inert white pigments
(2025-11-01)
Onoda, Hiroaki
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Wada, Takuma
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Charoonsuk, Thitirat
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Pulphol, Phieraya
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Muanghlua, Rangson
Cerium dioxide (CeO<inf>2</inf>) is a UV-scattering agent commonly employed in sunscreens but suffers from oxidative catalytic activity, raising concerns for dermal applications. To address this issue, surface passivation via phosphatization has been explored, although prior attempts with CeO<inf>2</inf> alone failed to eliminate its intrinsic yellow hue due to low reactivity with phosphoric acid. In this study, we introduce a novel white pigment synthesized via the phosphoric acid-mediated treatment of CeO<inf>2</inf>–La<inf>2</inf>O<inf>3</inf> mixtures. By co-utilizing lanthanum oxide, which readily forms lanthanum phosphate—a white, inert compound—we achieved enhanced suppression of oxidative activity alongside improved whiteness. The composite materials were systematically characterized via X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), particle size analysis, colorimetry (Lab*), and catalytic activity assays. The results reveal that phosphatization preferentially proceeds at lanthanum sites, forming phosphate-rich surface layers that diminish redox activity while maintaining favorable dispersion and smoothness properties. The pigment shows high acid resistance and negligible photocatalytic activity, indicating its potential as a safe, non-reactive alternative for cosmetic formulations. This work advances the development of rare-earth-based functional pigments via a scalable, low-temperature route.
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High-entropy gallium-based garnet microwave dielectric ceramics with low loss for C-band dielectric resonator antenna application
(2025-11-01)
Wen, Xinyang
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Xiang, Huaicheng
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Gu, Xiaolong
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Pulphol, Phieraya
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Vittayakorn, Naratip
In this work, (Y<inf>0.5</inf> Nd<inf>0.5</inf> Sm<inf>0.5</inf> Eu<inf>0.5</inf> Ho<inf>0.5</inf> Yb<inf>0.5</inf> )Ga<inf>5</inf> O<inf>12</inf> high-entropy ceramic with a cubic garnet structure was designed and synthesized via a solid-state reaction method. Through the high-entropy regulation of the oxygen dodecahedron, the bond strength increases, the cation's bond valence approaches the ideal value, and the structural stability improves, thereby promoting the Q × f value. Low dielectric loss (Q × f = 130,100 GHz) was obtained in the high-entropy ceramic (Y<inf>0.5</inf> Nd<inf>0.5</inf> Sm<inf>0.5</inf> Eu<inf>0.5</inf> Ho<inf>0.5</inf> Yb<inf>0.5</inf> )Ga<inf>5</inf> O<inf>12</inf> , which exhibited a low relative permittivity (ε<inf>r</inf> ) of 12.36, a negative temperature coefficient of resonant frequency (τ<inf>f</inf> ) of −66.37 ppm/°C, and a high flexural strength of 229.68 MPa. Furthermore, a cylindrical dielectric resonator antenna (CDRA) was designed featuring high radiation efficiency (>90 %) and stable gain (5.91–5.97 dBi) within a 255 MHz bandwidth, showing promising application prospects in the C-band. This work indicates that the high-entropy strategy is feasible for the design of high-performance microwave dielectric ceramics.
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Structural and microwave dielectric properties of temperature-stable Ca1-xEuxMo1-xNbxO4 ceramics for dielectric resonator antenna applications
(2025-10-15)
Gu, Xiaolong
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Tang, Ying
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Wu, Jiangtao
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Zhang, Ning
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Zhang, Siying
Ca<inf>1-x</inf>Eu<inf>x</inf>Mo<inf>1-x</inf>Nb<inf>x</inf>O<inf>4</inf> (0.1 ≤ x ≤ 0.7) ceramics were synthesized through the co-substitution of Ca2+ and Mo6+ in CaMoO<inf>4</inf> using Eu3+ and Nb5+. All samples exhibited tetragonal scheelite structures within the range of 0.1 ≤ x ≤ 0.6. However, at x = 0.7, a monoclinic fergusonite structure was observed, as the [Nb/MoO<inf>4</inf>] tetrahedron at the B-site transforms into a [Nb/MoO<inf>6</inf>] octahedron. As the substitution amount increases, ε<inf>r</inf> (11.39–16.40), Q×f (21,950–82,930 GHz), and τ<inf>f</inf> (−47.72 to +8.01 ppm/℃) all show a gradual increase. Notably, the τ<inf>f</inf> value of this system can shift from negative to positive. The structural influence mechanism on the microwave dielectric properties in Ca<inf>1-x</inf>Eu<inf>x</inf>Mo<inf>1-x</inf>Nb<inf>x</inf>O<inf>4</inf> ceramics was clarified through various analytical methods, including ion polarizability, P-V-L chemical bond theory, bond valence theory, density of states (DOS), and electron localization function (ELF). Additionally, a resonant antenna designed and simulated using Ca<inf>0.4</inf>Eu<inf>0.6</inf>Mo<inf>0.4</inf>Nb<inf>0.6</inf>O<inf>4</inf> meets the requirements for satellite communication.
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Synthesis of cerium dioxide-based pigments with co-precipitated phosphate: tuning oxidation catalytic activity for cosmetic and paint applications
(2025-09-01)
Onoda, Hiroaki
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Yamaoka, Kazuto
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Charoonsuk, Thitirat
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Pulphol, Phieraya
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Vittayakorn, Naratip
Cerium oxide, a popular ultraviolet screening agent in cosmetics, is hindered by its oxidation catalytic activity. Seeking an alternative to cerium phosphate, which increases particle size but lacks catalytic activity, we synthesized a novel cerium hydroxide-cerium oxide pigment by coprecipitating a small amount of phosphate. We chose a baseline Ce/P ratio of 10:1 based on preliminary tests showing that a moderate level of phosphate effectively reduces oxidation catalytic activity while still preserving key cerium oxide characteristics. We evaluated its composition, particle size, oxidation catalyst activity, and hue. Coprecipitation and subsequent heating yielded a yellowish pigment containing both cerium oxide and cerium phosphate. Compared to cerium oxide, our pigment exhibited superior smoothness. Additionally, samples prepared under low pH or high phosphate ratios demonstrated reduced oxidation catalytic activity. However, when applied as a pigment in oil paints, its hiding power was diminished. Our findings highlight a promising strategy for mitigating the catalytic drawbacks of cerium oxide while improving pigment properties, yet caution is warranted regarding its efficacy in paint applications. This study underscores the potential of cerium-based compounds in diverse fields, with room for optimization in specific applications. This study underscores the potential of systematically tuning cerium-based compounds for a range of applications, including cosmetics and paints, by exploring how pH and Ce/P ratio influence catalytic activity, coloration, and particle properties.

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