Abstract This work investigates the formation of the tetragonal zirconia (t-ZrO 2 ) phase in ceramics derived from amorphous precursors synthesized from natural zircon sand (ZrSiO 4 ). Amorphous ZrO 2 powders were produced via alkali fusion and ammonia precipitation, followed by the addition of 3 wt.% Bi 2 O 3 or B 2 O 3 as sintering agents (SA). Two calcination–sintering schedules (700 °C/900 °C and 800 °C/1000 °C, 3 h each) were examined. Rietveld refinement showed that SA-free zirconia maintained a phase-pure tetragonal structure under both thermal schedules, with crystallite sizes increasing from ∼13 to ∼28 nm and grain sizes remaining ∼25 nm. Among the SA-assisted samples, only Bi 2 O 3 under the lower temperature schedule preserved full tetragonal retention, yielding ∼16 nm crystallites, ∼33 nm grains, and the highest relative density (≈71.7%). In contrast, B 2 O 3 promoted grain coarsening and multiphase formation. These findings demonstrate the effectiveness of Bi 2 O 3 as a low-temperature sintering aid capable of maintaining nanoscale crystallinity, while highlighting the processing advantages of amorphous zircon-derived precursors for developing cost-efficient tetragonal zirconia without aliovalent dopants.

Polyvinyl alcohol (PVA) is a polymer known for its fluorescence; however, its relatively low exciton formation restricts its potential in optoelectronic devices. This study aims to enhance the optical properties of PVA by incorporating silica fillers (xerogel, amorphous, or crystalline) derived from rice husks, thereby creating silica-dispersed PVA composites. The composites were fabricated by solution casting at a PVA-to-filler mass ratio of 9:1. Structural, absorbance, and fluorescence analyses were performed on the samples. Embedding the different silica polymorphs enhanced photon absorption; moreover, it also improved excitation for the pi-pi* transitions by approximately 7.86, 1.72, and 1.01 times for PVA filled by xerogel, amorphous, and crystalline silica, respectively, compared to unfilled PVA. The composites also displayed a broadened excitation shoulder, enabling a broader excitation wavelength range. This enhancement is caused by the increase in the stiffness and bond strength of the PVA molecules induced by compressive stress resulting from its physical interactions with the filler, as well as B2 defect and hydroxyl dioxasilyrane clusters, which participate in promoting exciton formation. As a result, pi-pi* excitation is boosted, especially in the ultraviolet region, and an increased excitation shoulder intensity is observed between 360 and 425 nm. This higher excitation correlates with a greater emission intensity in the same hierarchical order as the excitation trend. The emission peak position remains relatively constant, with a slight Stokes shift toward the violet region owing to exciton internal conversion. PVA/silica composites also exhibited more complex emission patterns and a lower average lifetime than unfilled PVA.

In this paper, a temperature-dependent storage modulus model for quartz-dispersed polyethylene glycol (PEG) composites is presented. The model is developed by reinterpreting the empirical filler parameter in the Hilmi model as the reinforcement factor of the interphase-modified Halpin-Tsai model, explicitly linking measurable microstructural quantities - filler particle radius R, interphase thickness Ri, and interphase modulus Ei - to the predicted composite storage modulus. Storage modulus data were obtained from dynamic mechanical analysis (DMA) tests in three-point bending mode over a temperature range encompassing the glass transition region of PEG, with varying filler compositions, i.e., 0, 5, 10, and 20 wt.%. The predictions were compared with experimental data to evaluate the ability of the proposed model to capture the combined effects of temperature and filler content. Several model parameters were determined from experimental measurements and values reported in the literature. The results demonstrate good agreement between the model predictions and the temperature- and filler-dependent trends of the storage modulus for low filler compositions. By considering the role of the interphase in the micromechanics formulation, this model provides a more representative predictive framework for analyzing the mechanical behavior of quartz-dispersed PEG composites across a range of temperature and composition conditions.

Eu³ ⁺-doped ZrO₂ (ZrₓEu, x = 0–6 mol%) was synthesized from Indonesian zircon sand via alkaline fusion, co-precipitation of an amorphous ZrO₂ precursor, wet Eu³ ⁺ incorporation, and calcination at 800–1000 °C for 3 h. XRD Rietveld refinement, Raman spectroscopy, FESEM-EDX, and XPS reveal single-phase tetragonal ZrO₂ (t-ZrO₂, P4₂/nmc) across all compositions and temperatures, with no detectable monoclinic or secondary phases. Zener-type grain-boundary pinning by residual amorphous SiO₂ (Si/Zr ≈ 0.29–0.45) is proposed as the dominant stabilization mechanism, maintaining crystallite sizes of 10–38 nm near the Garvie critical threshold, while Eu³ ⁺ provides secondary stabilization via charge-compensating oxygen vacancies. Vegard-type unit-cell volume expansion (64.32 → 64.59 ų) and decreasing tetragonality (c/a: 1.4437 → 1.4374) at 1000 °C confirm Eu³ ⁺ solid-solution incorporation. XPS corroborates exclusive Zr⁴⁺ and Eu³ ⁺ oxidation states and quantifies vacancy-adjacent oxygen growth from 18.3% (x = 0) to 29.1% (x = 6 mol%), while the Si 2p signal assigns residual silicon solely to amorphous SiO₂. Photoluminescence of undoped t-ZrO₂ displays intrinsic F- and F⁺-center emissions at 473 and 497 nm, which are completely quenched upon Eu³ ⁺ doping by resonant host-to-Eu³ ⁺ energy transfer and oxygen-vacancy passivation. The Eu³ ⁺ 4 f emission profiles evolve systematically with Eu content: Among the doped compositions examined by PL (x = 3 and 6 mol%), Zr3Eu exhibits the higher overall ⁵D₀-dominated intensity together with a concentration-exclusive ⁵D₁→⁷F₂ band at 563 nm and dual-site ⁵D₀→⁷F₁ components, while Zr6Eu undergoes concentration quenching. These results demonstrate Indonesian zircon sand as an effective, low-cost precursor for phase-stable t-ZrO₂:Eu³ ⁺, elucidate the coupled roles of amorphous SiO₂ pinning and aliovalent Eu³ ⁺ doping in stabilizing structure, and establish the luminescence as a defect-sensitive probe rather than a phosphor benchmark.

This study investigates the phase stability of nanocrystalline tetragonal zirconia (t-ZrO2) synthesized from natural zircon sand from Central Kalimantan, Indonesia, under prolonged thermal cycling. Zirconia powders were produced via alkali fusion-precipitation, calcined at 800-1000 degrees C for 3 h, and subjected to repeated heat-hold-cool cycling at temperatures up to 1000 degrees C for cumulative durations of up to 50 h. Phase stability was assessed by room-temperature x-ray diffraction after each cycling interval. Complete retention of the tetragonal phase was observed under all investigated conditions, with no tetragonal-to-monoclinic (t -> m) transformation. TEM and SAED confirm persistent tetragonal symmetry and controlled crystallite coarsening from similar to 6 nm after calcination to similar to 24 nm after thermal cycling, remaining below the critical size for martensitic transformation. In contrast, zirconia derived from commercial ZrCl4 exhibits a pronounced t -> m transformation (similar to 62 wt%) under identical conditions, while 3 mol% Y2O3-stabilized zirconia remains fully tetragonal. STEM-EDS analysis of the post-calcined ZA10C sample indicates Si at similar to 10.5 at% with a non-uniform distribution relative to zirconia crystallites, suggesting a tendency for Si to be associated with intercrystallite-related regions rather than uniform lattice incorporation. These results show that phase stability in zircon sand-derived t-ZrO2 is governed by microstructural and kinetic stabilization, likely involving impurity-assisted suppression of crystallite coarsening, rather than classical dopant-induced lattice substitution. This work demonstrates the potential of zircon sand as a sustainable, cost-effective precursor for thermally stable, dopant-free t-ZrO2 for high-temperature ceramic applications.

Abstract This study investigates microstrain changes in zircon (ZrSiO 4 ) powders induced by mechanical milling for 5, 30, and 50 h. The zircon powder was extracted from Puya sand sourced in Kereng Pangi, Central Kalimantan, Indonesia. Characterization was performed using X-ray diffraction (XRD) and Raman spectroscopy. XRD results showed that increasing milling time led to significant broadening of diffraction peaks, with microstrain increasing from 8 × 10 −4 at 5 h to 21 × 10 −4 at 50 h, and crystallite size decreasing from 158 nm to 83 nm. Raman spectroscopy showed a consistent redshift of the dominant Raman band near 1007 cm −1 , along with a noticeable increase in FWHM. These spectral changes suggest enhanced lattice distortion and increased defect density as a result of milling. The observed redshift is attributed to the softening of vibrational modes caused by strain accumulation and disruption of long-range order. The overall results demonstrate that extended milling induces structural disorder in zircon by promoting defect formation, increasing microstrain, decreasing crystallite size, and reducing lattice rigidity.

The crystal and local structures of nanocrystalline undoped and CaO-doped ZrO2 were investigated using high-temperature synchrotron X-ray powder diffraction (XRD) and X-ray absorption spectroscopy (XAS). Nanocrystalline ZrO2 was synthesized via a co-precipitation method, whereas CaO-doped ZrO2 was prepared through mechanochemical wet milling, using CaO derived from natural limestone as the dopant. High-temperature synchrotron XRD analysis showed the transformation of undoped and CaO-doped ZrO2 from an amorphous state to a tetragonal phase, stable up to 1100 °C. The CaO-doped ZrO2 required higher temperatures to achieve a fully tetragonal transformation compared to the undoped sample. At equivalent temperatures, Ca doping induced larger lattice parameters, reduced tetragonality, and slower unit-cell volume contraction. However, CaO-doped ZrO2 with 5.0 mol% CaO dopant concentration following fast cooling at a rate of 50 °C/min induced the formation of minor phases, specifically m-ZrO2 and CaZrO3. Furthermore, in situ extended X-ray absorption fine structure (EXAFS) analysis at 700 and 800 °C revealed that the Ca dopant elongated Zr-OI bonds by substituting Zr4+ with larger Ca2+ ions and forming oxygen vacancies, effectively suppressing atomic vibrations. The results reported here point out the structural adaptability of CaO-doped ZrO2 nanocrystals, reinforcing their suitability for high-temperature applications.

Crystallographic analysis of ZrO 2 powder derived from zircon sand was performed using ex situ conventional X-ray powder diffraction (XRD) and synchrotron extended X-ray absorption fine structure (EXAFS) techniques. The ZrO 2 powder was purified using alkali fusion and co-precipitation methods, with zircon sand as the primary raw material. Phase analysis of the XRD data demonstrated that the as-synthesized powder, after calcination at 1000 °C for 3 h, yielded pure tetragonal ZrO 2 (t-ZrO 2 ). Further structural analysis was conducted using the Rietveld refinement method, which provided the following lattice parameters: a = b = 3.5962(1) Å, c = 5.1953(3) Å, and unit cell volume V = 67.186(6) A 3 . The Zr-O shell in tetragonal ZrO 2 consists of two distinct tetrahedra, designated Zr–O I and Zr–O II . EXAFS analysis of the local atomic structure around the Zr atoms revealed that the bond lengths for Zr-O I and Zr-O II were 2.112(7) and 2.377(4) Å, respectively. These findings provided a valuable understanding of the atomic-level structure of tetragonal ZrO 2 derived from zircon sand.

This study investigates the synthesis of tetragonal zirconia (t-ZrO 2 ) from natural zircon sand via alkali fusion, focusing on the effects of calcination temperature (800, 900, and 1000°C) and holding time (3, 5, 7, and 12 h) on the crystal structure and crystallite size. X-ray diffraction (XRD) analysis revealed that at 800°C, the lattice parameter a and unit cell volume initially decreased with increasing holding time and then increased, indicating a shift from stress relaxation and defect elimination to crystal growth and phase stabilization at longer holding times. In contrast, at 1000°C, both the lattice parameter a and unit cell volume consistently decreased with longer holding times, reflecting enhanced atomic diffusion, reduction of oxygen vacancies, and stabilization of the tetragonal phase. The crystallite size steadily increased with temperature and holding time, ranging from approximately 7 to 17 nm. These results indicate that holding time and temperature play crucial roles in the formation of t-ZrO 2 . Moreover, the crystallite size of t-ZrO 2 showed an increase of about 21% when the holding time was extended from 3 to 12 h at 1000°C. The observed trends reflect the complex interactions between the kinetic effects and thermodynamic stability in shaping the crystal structure and size evolution during alkali fusion synthesis.

Tetragonal zirconia (t-ZrO 2 ) has broad applications for structural ceramics, such as solid oxide fuel cells, electrolysis materials, membranes, and biomedical applications. However, its electronics and magnetics properties are rarely well studied. We carried out spin-polarized calculations to understand the vacancy characteristics in t-ZrO 2 . We apply neutral single Zr vacancy (V Zr ) and O vacancy (V O ) for supercell calculations up to 108 atoms. The calculated band gap of t-ZrO 2 is 3.81 eV, which becomes a narrowing band gap by V Zr . We find that both vacancies systems induce outward relaxations of atoms near vacancies, creating lower symmetry from pristine D 4h to D 2d and C 2v for V Zr and V O cases, respectively. The calculated magnetic moment V Zr is 4 mB due to four electrons of the valence band being occupied in the majority state. Four oxygen electrons of p -orbitals dominate the spin densities near V Zr . In contrast, the magnetic moment in the case of V O is 0 µB. Thus, since they have a high magnetic moment in the case of V Zr , t-ZrO 2 is potentially used as material for diluted magnetic semiconductors.

This study reports the xerogel silica fabrication derived from rice husks via the sol-gel route, which was continued with conventional calcination to produce both amorphous and crystalline phases. The primary objective is to elucidate the relationship between their structural characteristics and optical properties, analyzed by adequate analytical procedures. The xerogel and amorphous phases exhibited many structural defects, characterized by an amorphous framework featuring n-membered rings (n = 3 or 4) and a non-solid morphology alongside an elevated specific surface area, with the xerogel notably enriched in silanol groups. In contrast, the crystalline phase exhibited a well-ordered structure predominantly composed of cristobalite and tridymite, characterized by solid morphology and the lowest structural defects. Structural defects and silanol groups are critical for forming emission centers, leading to significantly higher emission intensity in the xerogel compared to the amorphous and crystalline phases. The xerogel and amorphous phases exhibit maximum emission within the blue spectral region, whereas the crystalline phase shows its peak emission in the green spectral region. The observed shifts in emission peaks are likely influenced by the number of defects that act as emission centers within or on the surface of the samples. Additionally, variations in emission intensity correlate with significant differences in lifetime measurements, recorded at 392.37 ns, 4.08 ns, and 1.24 ns for the xerogel, amorphous, and crystalline phases, respectively.

Abstract Combining amorphous silica (SiO2) particles into polyvinyl alcohol (PVA) results in a composite with unique optical characteristics. This study investigates the effects of SiO2 particles derived from rice husks on PVA’s structure and optical properties. Samples were prepared by blending PVA with SiO2 particles and casting the mixture into composite sheets. FTIR analysis exposed a reduction in hydroxyl groups (−OH) as the SiO2 content increased, designating stronger interactions among the −OH group and the particles through hydrogen bonds. XRD analysis showed increased molecular disorder and amorphousness with higher SiO2 concentrations. These structural changes altered optical properties, increasing the absorption spectrum as SiO2 concentration rose. The direct transition band gap reduced from 5.42 to 5.13 eV, and the indirect transition band gap dropped from 4.65 to 3.52 eV for pure PVA and the composite with 10% SiO2 content, respectively. Urbach energy enlarged from 0.52 to 1.41 eV, and the refractive index rose from 1.93 to 1.98.

This study explores the structural and optical properties of zircon (ZrSiO4) and zirconia (ZrO2) nanopowders using two types of natural Indonesian zircon sands. The former powders were synthesized through dissolution and non-dissolution methods, while the latter powders were synthesized using alkali fusion and co-precipitation techniques. X-ray fluorescence (XRF) analysis revealed a reduction in impurities post-synthesis. X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM) analyses indicated that the dissolution process yielded pure zircon, whereas the non-dissolution method resulted in the presence of minor additional phases. For zirconia, all samples exhibited a tetragonal phase, albeit with varying structures. Optical properties were investigated using UV-Vis spectroscopy, which showed that both materials exhibit high absorption in the ultraviolet region, with slight differences in wavelengths peak. Consequently, zircon and zirconia demonstrated similar band gap energies ranging from 3.76 to 3.95 eV and 3.99 to 4.06 eV, respectively. Photoluminescence testing further revealed identical emission peaks 350 nm for both materials, highlighting their comparable optical characteristics. This study provides valuable understandings into the synthesis and optical properties of zircon and zirconia nanopowders derived from Indonesian zircon sands, emphasizing their potential applications in various optical and electronic fields.

Rice husk is an agricultural waste rich in carbon and silica contents. So far, no studies have been reported that simultaneously transform it into polymorph silica and activated carbon. This study aims to analyze the thermal event and identify the phase, functional groups, and microstructure of the silica and activated carbon derived from a rice husk. Silica precursor was prepared by the sol-gel route followed by calcining at temperatures of 500–1100 °C with a rise of 200 °C. The residue from the silica preparation was transformed into activated carbon through impregnation, followed by carbonation at 650 °C. Thermal analysis results expose that crystallization of silica occurs at around 900 °C, while the formation of active carbon occurs at lower than 832 °C. XRD analysis revealed that silica calcined at 500 and 700 °C is amorphous, while samples calcined at 900 and 1100 °C are crystalline, with the major and minor phases being cristobalite and tridymite, respectively. The FTIR analysis strengthened this analysis, which detected the presence of Si–O–Si bending vibrations in the sample calcined at 900 °C or more, indicating a phase transition from amorphous to crystalline. A morphological change to a denser structure accompanies this phase transformation. On the other hand, processing the rice husk residue resulted in activated carbon with an amorphous structure and highly porous. The formation of activated carbon was confirmed by the FTIR analysis, which detected the appearance of the C = C group, a characteristic of this substance.

Abstract Phase formation and structure examination has been conducted to study the effects of thermal treatment and heating rates on zircon (ZrSiO4) sand-derived zirconia (ZrO2). A preliminary investigation of the elements present in the sand using XRF revealed that Zr was the major element, corresponding to 92.4% of the sample. The zirconia precursor was synthesized through a three-stage process that involves purifying zircon powder, alkali fusion, and co-precipitation method. The zirconia crystal was obtained by heating the sample to 700, 800 and 900 °C with heating rates of 5 and 10 °C/min, respectively. The formation of tetragonal zirconia (t-ZrO2) with space group P42/nmc is shown in a qualitative study of all sample XRD patterns. Furthermore, Rietveld analysis was used to determine crystal structure parameters including lattice parameters and tetragonality. The calcination temperature and heating rate decide the phase, t-ZrO2 crystallite size, and tetragonality. Furthermore, calcining to 700 and 800 °C created t-ZrO2, however, elevated temperature to 900 °C lead to t→m transformation. The t-ZrO2 size increases with temperature between 700 and 900 °C, range of 9 and 15 nm. Meanwhile, the tetragonality of t-ZrO2 tends to decrease due to m-ZrO2 being present.

Abstract Activation energy and crystal growth coefficient of t-ZrO2 (tetragonal zirconia) were determined through an in-situ synchrotron radiation X-ray diffraction (SR-XRD) experiment. The t-ZrO2 precursor was synthesized through (a) purification of zircon sand from Kereng Pangi, Kalimantan to zircon powder, (b) alkali fusion, and (c) co-precipitation. High-temperature SR-XRD experiments were carried out with varying holding times of 1, 2, and 3 hours at 900, 950, and 1000 °C, respectively, where the formation and crystal growth of t-ZrO2 were observed. The diffraction patterns show that t-ZrO2 has formed at 900°C for 1 hour and it grows accordingly. The effect of the heating temperature and holding time on the crystal growth shows that the size of hot crystals increases with increasing temperature and holding time. It is found that the activation energy of t-ZrO2, in general, decreases with holding time, i.e. from 75.3 kJ/mol (3 h) to 57.3 (1h). Meanwhile, Beck’s crystal growth coefficient value is between 0.3 at 900 °C and 0.2 at 1000 °C.

Abstract The structural analysis of the CaO-ZrO2 solid-solutions was carried out using in-situ synchrotron radiation X-ray diffraction (SR-XRD). CaO-ZrO2 solid-solutions with 0, 2.5, and 5 at.% CaO contents were synthesized via a solid-solid ball milling technique. The amorphous ZrO2 powder was synthesized via co-precipitation using ZrCl4 and NH4OH as precursors, while the CaO powder was purified from a local limestone. In-situ SR-XRD measurements were conducted on the CaO-ZrO2 mixtures by independently heating them to 1100°C for different holding times of 0, 12, 24, and 36 minutes using a DHS furnace. Qualitative analysis of all SR-XRD patterns showed that only tetragonal zirconia (t-ZrO2) was present. The crystallite size of tetragonal zirconia ranged from 17 to 22 nm, depending on calcination holding time and CaO addition. Further analysis showed that the tetragonality (c/a) parameter remained relatively constant over the 36-minute holding time. However, the introduction of CaO led to an insignificant reduction in tetragonality, averaging 0.292%.

Abstract This study aims to evaluate titanium’s impact as a coating and doping agent for hydroxyapatite (HAp) material. Ti is typically added to HAp in the medical industry to enhance its biocompatibility and antibacterial properties. HAp was prepared from chicken egg shells and composited with Ti using the solid mixing method. The composite produced with the addition of Ti was proven to increase its antibacterial properties through element characterization. The addition of Ti is able to initiate ion release from HAp and its interaction with the biological environment. The release of titanium ions can also have a direct antibacterial effect. However, this is usually more limited compared to the photocatalytic effect of TiO2.