Rod-like Grains Research Articles

In situ construction of one-dimensional structures induced by Ba doping in Al2O3-TiO2 ceramics derived from quenched powders

In this work, we propose a strategy for in situ construction of rod-like grains in Al2O3-TiO2 ceramics through Ba doping. Ba-doped Al2O3-13 wt% TiO2 composite powder was initially synthesized using combustion synthesis-air atomization, where feedstocks were melted by the Al-O2 combustion reaction and subsequently atomized and rapidly cooled in air, producing quenched powders rich in metastable phases. Upon heat treatment at 1100℃, the metastable phases transformed into stable forms, accompanied by phase separation and solid solution precipitation. Notably, Ba doping induced the formation of rod-like hollandite grains with preferential growth along the [001] direction. Bulk ceramics were prepared through pressureless sintering, and the sample sintered at 1350℃ exhibited a hardness of 14.14±0.61GPa and a fracture toughness of 5.17±0.49MPa∙m1/2. The toughening behavior of rod-like grains, including crack deflection and pull-out/bridging effects, was observed. This study provides insights into the evolution of quenched oxides and the design of ceramic microstructures.

Ultrasonic spray synthesis, photoelectric properties and photovoltaic performances of chalcogenide CaSnS3

Chalcogenide perovskites are promising lead-free, stable absorber materials for solar cells. This work reports the synthesis of orthorhombic phase pure CaSnS3 thin films by facile low temperature sulfurization of solution-processed CaSnO3 oxide precursors. Structural characterization confirms complete anion exchange to produce crystalline CaSnS3 films with vertically aligned rod-like grains. Optical studies show strong visible light absorption with direct bandgap of 1.72 eV, ideal for photovoltaics. Electrical measurements indicate p-type conductivity with hole concentration of 1.2×1017 cm-3 and mobility around 8 cm2V-1s-1 at room temperature. First-principles DFT calculations corroborate the p-type electronic structure. Prototype CaSnS3 solar cells are fabricated with TiO2 electrode, demonstrating power conversion efficiency of 2.5% under AM1.5G, open-circuit voltage of 0.55 V, short circuit current density of 11.5 mA/cm2 and fill factor of 0.62. The cells also exhibit remarkable ambient shelf stability over 6 months. The comprehensive results validate the photovoltaic potential of these earth abundant, sustainable chalcogenide perovskites synthesized via scalable low-cost solution methods. Further interface engineering can enable enhanced efficiencies.

Properties of Durable Mullite Bodies Manufactured from Waste Clay-Diatomite

Durable mullite bodies have been fabricated using diatom frustules from diatomite powder as the Si source and Al-nitrate as the Al precursor, resulting in fibrous pore morphology. The hard mullite ceramics prepared by mold pressing without additives showed high compressive strength (up to 133 MPa when sintered at 1500 °C). The diatomite-nitrate samples were sintered at three temperatures (1300, 1400, and 1500 °C) for 2 hours. XRPD analysis of the sintered samples showed that the crystalline mineral phases mainly comprise mullite, cristobalite, and corundum. SEM results indicate the presence of rod-like mullite grains measuring 5 µm in length and 500 nm in diameter (aspect ratio 1:10). XRPD analysis of the samples sintered at 1300 °C demonstrated good thermo-mechanical stability and the formation of new hard phases (mullite, corundum, and cristobalite), making the analyzed diatomaceous earth suitable to produce various types of ceramic, construction, and thermal insulating materials.

Surface nanocrystallization mechanism of additive manufactured 316L stainless steel via high-energy shot peening

High-energy shot peening (HESP) is an effective way to achieve surface nanocrystallization modification of additive manufactured metal parts, significantly improving their mechanical properties. Yet, there is still a lack of deep understanding of the surface nanocrystallization evolution mechanism of additive manufactured metals. For that, this study focuses on the surface nanocrystallization mechanism of selective laser melted 316L stainless steel treated by HESP treatment. The detailed microstructural characteristics are investigated via optical microscope (OM) and transmission electron microscopy (TEM). The experimental results show that the matrix layer, the low-strain matrix layer, the elongated ultrafine grain layer, the short rod-like ultrafine grain layer, and the equiaxed nanograin layer occurred in sequence from the matrix to the topmost surface. HESP treatment enables the gradient grain refinement from the matrix with a micrometer structure of about 120 mm to the topmost surface with an average grain size of about 25 nm. The evolution of surface nanocrystallization has resulted from the interaction of high-density dislocation, dislocation tangles (DTS), and twins. This study could provide a deep understanding of the surface nanocrystallization evolution mechanism of additive manufactured metal parts treated by HESP.

Zinc oxide and nickel ferrite nanocomposite as an efficient photocatalyst for crystal violet dye degradation

In the present study, photocatalytic activity of NiFe2O4/ZnO nanocomposite was studied for the degradation of crystal violet dye. Microwave assisted green synthesis method was used for the synthesis of ZnO nanoparticles (NPs). Sol-gel auto-combustion method was used to synthesize NiFe2O4 (NiF) NPs. NiF/ZnO nanocomposite was synthesized through solid-state reaction method. Structural phase was confirmed through X-ray diffraction studies. ZnO NPs were found in wurtzite hexagonal structure whereas cubic spinel structure was found for NiF. NiF/ZnO nanocomposite was observed in both phases. The crystallite size was found to be 31 ± 1, 18 ± 2 and (95 ± 1)/(98 ± 1) nm, for NiF, ZnO and NiF/ZnO nanocomposite, respectively. The optical band gap was determined through UV–Visible spectroscopy and is found to be 1.67 ± 0.03, 2.70 ± 0.08 and 1.70 ± 0.05 eV for NiF, ZnO and NiF/ZnO nanocomposite, respectively. Infrared study of NiF showed the tetrahedral and octahedral band whereas for ZnO the characteristic band was observed. FESEM micrographs of NiF/ZnO nanocomposite showed the formation of agglomerated spherical particles and very less number of rod-like grains indicating the formation of NiF/ZnO nanocomposite. The photocatalytic activity of NiF, ZnO and NiF/ZnO nanocomposite for the degradation of crystal violet dye was studied. It was found that NiF, ZnO and NiF/ZnO nanocomposite showed 89 %, 90.46 % and 87.21 % degradation rate in 60, 120 and 50 min respectively.

Hot-pressing sintering and characterization of a high-strength TiB2-TiN-Al0.3CoCrFeNi cermet composed of ultrafine-grained by in-situ reaction synthesis

The TiB2-TiN-HEA cermet materials were synthesized in-situ reaction hot-pressing sintering at 1450 °C for 1 h, utilizing Ti, BN, and Al0.3CoCrFeNi high entropy alloys (HEA) as starting materials. The thermodynamic characteristics of the reaction system and the phase structure, microstructure, and mechanical properties of Ti-BN, Ti-HEA, BN-HEA, and Ti-BN-HEA were investigated. The results demonstrate that in-situ reaction hot pressing sintering phases include TiN, TiB2, and FCC structural. The synthesis of TiB2 involves the diffusion of B to TiB and the subsequent combination of Ti and B in solution. Conversely, the formation of TiN consists of the diffusion of N to TiN0.3 and Ti2N, as well as the subsequent combination of Ti and N in solution. The TiB2-TiN-HEA cermet exhibits a relative density of 99.1%, Vickers hardness of 18.91 GPa, flexural strength of 721 MPa, and fracture toughness of 7.61 MPa m1/2. The high strength of cermet was due to a combination of factors, including the synergistic effect of high compactness of composites, reinforced interfacial strength with HEA, reinforcement of fine-grains and TiB2 rod-like grains, crack bridging and deflection, and transgranular fracture.

Effect of Thickness on Ferroelectric Properties of Bi3.25La0.75Ti3O12 Thin Films

The pursuit of low-power/low-voltage operation in devices has prompted a keen interest in the mesoscale effects within ferroelectric thin films. The downsizing of ferroelectrics can significantly influence performance; for instance, the remanent polarization and coercive field are susceptible to alterations based on thickness. In this study, randomly oriented Bi3.25La0.75Ti3O12 thin films were fabricated on Pt/Ti/SiO2/Si substrates using the sol–gel method, and SEM observations revealed rod-like grains in all thin films. The investigation delved into the correlation between dielectric and ferroelectric properties with thin film thickness. The thin film exhibited an increased remanent polarization and a reduced coercive electric field. Additionally, the ferroelectric domain structure was scrutinized through PFM, and the resistor properties of the BLT4 thin film were studied, which shows the potential of BLT thin films in non-volatile memory and memristor.

Performance assessment of oxygenated CdS films-based photodetector

CdS films were grown by thermal evaporation on glass substrates. After growth process, samples were oxygenated at 400 °C at various gas pressures for 5 mins employing rapid thermal process. The produced CdS films were used as photodetectors in blue light. X-ray diffraction results revealed that as-deposited CdS films had a wurtzite crystal structure with a satrong preferred orientation along (002) plane. The intensity of (002) peak increased by rising oxygen gas pressure to 2 atm and then decreased with further increase of oxygen gas pressure to 4 atm. Scanning electron microscopy analysis indicated that even though as-deposited CdS thin films included some aligned rod-like grains on an underlaying layer, oxygenation at various gas pressures changed the surface morphology of CdS films. CdS films oxygenated at a gas pressure of 2 atm exhibited the best transmittance value of 80% in the range of 600–1000 nm. It was calculated that band gap increased from 2.42 eV to 2.45 eV as CdS films were oxygenated at a gas pressure of 2 atm. Photoluminescence spectrum of as-deposited CdS films indicated two fundamental peaks located at 530 nm and an interval of 550–700 nm, corresponding to green and deep level emissions, respectively. Consequently, it was attained that CdS films oxygenated at a pressure of 2 atm had the optimized structural, morphological and optical results and therefore, this sample was further employed for photodetecting applications. From photocurrent-time curves, the best photodetecting performance was reached for CdS films-based device oxygenated at a gas pressure of 2 atm including rise time and fall time values of 22 ms and 25 ms, respectively. In addition, the maximum responsivity, detectivity and external quantum efficiency were found to be 23.7 mA/W, 4.75 × 108 Jones and 6.6 for the same photodetector, respectively.

Microstructure and mechanical properties of highly dense Si3N4 ceramics with ZrN–AlN–Re2O3 (Re=Yb, Y, Nd, Ce, La) additives

The Si3N4 ceramics with ternary ZrN–AlN–Re2O3 (Re=Yb, Y, Nd, Ce, La) and ZrN–AlN system as sintering additives were prepared by hot-pressed sintering with vacuum environment at 1750°C-30min–30MPa. Subsequently, the effect of the ternary additives system with 2 wt% Re2O3 on phase transformation, grain boundary phases, microstructure, and mechanical properties of the Si3N4 ceramics were investigated firstly. All samples achieved the high β-Si3N4 phases transformation (≥98.86%), while the grain boundary phases RexZr1-xO(4-x)/2 and Yb2Si2O7 were detected in the sintered Si3N4 ceramics with Re2O3 additives. Additionally, compared with Si3N4 ceramic not contain Re2O3 additives, the Si3N4 ceramics added Re2O3 additives presented the textured microstructure with more elongated rodlike β-Si3N4 grains based on the XRD and SEM results. Consequently, the sample S-Ce with added CeO2 additive shows the highest relative density of 99.85% and optimal fracture toughness of 8.75 ± 0.36 MPa m1/2. The sample S–Nd with added Nd2O3 additive presented a highest bending strength of 1061.78 ± 41.76 MPa. Sample S–Yb with added Yb2O3 additive presented the maximum Vickers hardness of 16.35 ± 0.07 GPa. The results of this study can provide a helpful reference for the preparation of high-performance Si3N4 ceramics.

Effects of MgO-RE2O3 and SiO2-RE2O3 (RE = La, Nd, Gd, Ho and Lu) complex sintering additives on the phase composition, microstructure and mechanical properties of dual-phase Si3N4 ceramics

Dual-phase (α+β)-Si3N4 ceramics with MgO-RE2O3 and SiO2-RE2O3 (RE = La, Nd, Gd, Ho and Lu) complex sintering additives were prepared by SPS at 1650 and 1700 °C. The effect of sintering additives on the phase transition, microstructure evolution and mechanical properties of the Si3N4 ceramics were studied. For the MgO-RE2O3 samples, as the RE radius increased, the phase transition was decelerated, the percentage of rod-like grains and fracture toughness decreased and the hardness showed a hill-shape curve. It is proposed that the effect of RE radius is connected with the segregation behavior of RE cations on the surface of Si3N4 grains. For the SiO2-RE2O3 samples, as the RE radius increased, the phase transition was accelerated and the percentage of rod-like grains increased. It is suggested that the effect of RE radius is related with the liquid phase viscosity and nitrogen solubility.

Achieving ultralow coercive electric field in (Ca1-1.5xGdx)3Ti2O7 ceramics via dual inhibitory effects on oxygen vacancies

(Ca1-1.5xGdx)3Ti2O7 ceramics with ultralow coercive field were obtained by the flowing oxygen sintering combined with donor doping, and their microstructure, dielectric properties ferroelectric properties, and negative piezoelectric properties were studied. The Gd3+ was introduced into A sites of Ca3Ti2O7 ceramics when x is less than 0.75%, but the Gd3+ occupied B sites of Ca3Ti2O7 ceramics when x is above 0.75%. The introduction of Gd3+ suppresses grain growth and leads to a transition from rod-like grain to irregularly shaped grain. The variation of oxygen vacancy induced by the doping of Gd3+ leads to the change of dielectric and ferroelectric properties of (Ca1-1.5xGdx)3Ti2O7 ceramics. The ultralow coercive field of (Ca1-1.5xGdx)3Ti2O7 (x = 0.25%) sample obtained at PUND and DHM modes reached 58.80 kV/cm and 45.76 kV/cm, respectively, and the higher room-temperature remnant polarization (1.797 μC/cm2/PUND and 2.055 μC/cm2/DHM) was achieved simultaneously. These results indicate that the inhibition of oxygen vacancies contributes to realizing the ultralow coercive field in (Ca1-1.5xGdx)3Ti2O7 ceramics. These findings demonstrate that introducing rare-earth ions as donor dopants into the A site of Ca3Ti2O7 ceramics combined with the flowing oxygen sintering is an essential strategy to achieve ultralow coercive field for Ca3Ti2O7-based materials.

Improvement of cutting performance of SiAlON ceramic by texture engineering in turning superalloys

High speed and high efficiency machining of superalloy is a big technological challenge faced by the cutting tool industry in the world. In the present study, cutting performance of nickel-based superalloy was significantly improved by engineering the texture of SiAlON ceramic cutting tools with grains oriented parallel to the cutting edge. Orientation of the rod-like grains played a significant role in the wear resistance of SiAlON ceramic. With the rod-like grains parallel to the cutting edge of the tool, the resistance to notched wear and peeling was much higher, resulting in slow and stable flank and rake wear. The tool life of the textured tool with rod-like grains parallel to the cutting edge was 17% higher than that of the untextured tool. On the other hand, for the tools with the grains perpendicular to the rake or flank face, the friction generated during machining would be parallel to the grain direction, which led to the rapid peeling on the tool surface along the moving direction of the workpiece or the chip flow direction, resulting in rapid tool wear. The preferred orientation of mechanical properties designed by engineering grain orientation provides the possibility to optimize the cutting performance of SiAlON ceramic tool in turning superalloys.

Microstructural evolution during H2 corrosion of Al2O3–SiO2 based refractory aggregates

Hydrogen-based direct reducing iron production (H-DRI) has caused more and more attention in metallurgical industry owing to the near-zero carbon emission. However, the strong reducibility of H2 at elevated temperature can possibly cause serious corrosion to the lining refractories of furnaces. In this study, four types of Al2O3–SiO2 based aggregates (brown corundum, bauxite, mullite and quartz) with various SiO2/Al2O3 mass ratios were utilized for the H2 corrosion test at 1673 K with a soaking time of 8 h. Based on the analysis of weight-loss ratios, phase-composition variation and reduction thermodynamics, the microstructure evolution during H2 corrosion was investigated in detail. The results indicates that the SiO2 impurity of brown corundum was partly reduced by H2 which led to the formation of rod-like or flaky intergranular grains. For bauxite, a porous structure was produced by the accumulation of rod-like or elongated grains owing to the significant reduction of intergranular silicon compounds. Nevertheless, due to the high sintering activity between the Al2O3 products from the reduction of mullite, a high densification structure was realized to effectively prevent the further H2 penetration, hence, mullite inversely obtained the better H2 corrosion resistance than bauxite in spite of its higher S/A ratio. The accumulation of SiO and H2O gases due to the high reduction rate of SiO2 led to the formation of a great deal of SiO2 fiber on the surface of quartz aggregates. The [O] provided by the phase-transformation from α-quartz to cristobalite could be partially captured by the H2 for the reduction, which caused the deterioration of corrosion. With this study, we aim to provide theoretical assistance for the refractory selection applied in hydrogen metallurgy.

The mechanical alignment of dust (MAD)

Context. Observations of polarized light emerging from aligned dust grains are commonly exploited to probe the magnetic field orientation in astrophysical environments. However, the exact physical processes that result in a coherent large-scale grain alignment are still far from being fully constrained. Aims. In this work, we aim to investigate the impact of a gas-dust drift on a microscopic level, potentially leading to a mechanical alignment of fractal dust grains and subsequently to dust polarization. Methods. We scanned a wide range of parameters of fractal dust aggregates in order to statistically analyze the average grain alignment behavior of distinct grain ensembles. In detail, the spin-up efficiencies for individual aggregates were determined utilizing a Monte Carlo approach to simulate the collision, scattering, sticking, and evaporation processes of gas on the grain surface. Furthermore, the rotational disruption of dust grains was taken into account to estimate the upper limit of possible grain rotation. The spin-up efficiencies were analyzed within a mathematical framework of grain alignment dynamics in order to identify long-term stable grain alignment points in the parameter space. Here, we distinguish between the cases of grain alignment in the direction of the gas-dust drift and the alignment along the magnetic field lines. Finally, the net dust polarization was calculated for each collection of stable alignment points per grain ensemble. Results. We find the purely mechanical spin-up processes within the cold neutral medium to be sufficient enough to drive elongated grains to a stable alignment. The most likely mechanical grain alignment configuration is with a rotation axis parallel to the drift direction. Here, roundish grains require a supersonic drift velocity, while rod-like elongated grains can already align for subsonic conditions. We predict a possible dust polarization efficiency in the order of unity resulting from mechanical alignment. Furthermore, a supersonic drift may result in a rapid grain rotation where dust grains may become rotationally disrupted by centrifugal forces. Hence, the net contribution of such a grain ensemble to polarization drastically reduces. In the presence of a magnetic field, the drift velocity required for the most elongated grains to reach a stable alignment is roughly one order of magnitude higher compared to the purely mechanical case. We demonstrate that a considerable fraction of a grain ensemble can stably align with the magnetic field lines and report a possible dust polarization efficiency of 0.6–0.9, indicating that a gas-dust drift alone can provide the conditions required to observationally probe the magnetic field structure. We predict that magnetic field alignment is highly inefficient when the direction of the gas-dust drift and magnetic field lines are perpendicular. Conclusions. Our results strongly suggest that mechanical alignment has to be taken into consideration as an alternative driving mechanism where the canonical radiative torque alignment theory fails to account for the full spectrum of available dust polarization observations.

Multilevel hierarchical super-hydrophobic ceramic membrane for water-in-oil emulsion separation

Hierarchical silicon nitride (Si3N4) membranes were fabricated by catalytic growth of α-Si3N4 nanowires onto the rod-like β-Si3N4 grains. Compared to the bare asymmetric structure of β-Si3N4 membranes, the average pore size reduced from 0.9 µm to 0.67 µm, while the porosity decreased slightly from 47 ± 0.6% to 45.3 ± 1.1%. Inorganic nanoparticles derived from organosilane were used to further modify the membranes. The membranes displayed super-hydrophobic characteristics with a water contact angle of 155 ± 1° due to the multilevel hierarchical structure and the -Si-CH3 low energy terminal groups. The super-hydrophobic membranes were successfully applied for the water-in-oil emulsion separation process. At various driving pressures, the water rejection of the water-in-oil emulsion was higher than 95%. This work's durable super-hydrophobic ceramic membrane has promise for the purification of a variety of oil products.

Physicochemical characterization tungsten oxide modified hydroxyapatite embedded into polylactic acid nanocomposite for biomedical applications

To compensate for the feeble antimicrobial potential of hydroxyapatite (HAp), the compound has been incorporated into different nanocomposites (NCs) in addition to powder and fibrous structure. Hydroxyapatite (HAp)/tungsten oxide/graphene oxide (GO) ternary nanocomposite introduces a significant enhance in the cell viability to 96.7 ± 3% towards the osteoblast cells. The chemical compositions were approved by various analytical techniques such as XRD, FTIR, XPS, and EDS. TEM shows well-defined, rod-like HAp grains. The triple ingredient HAp/WO3/GO nano-composite exhibits an 22.8 nm grain size. FESEM introduces grain shape deviation via changing HAp additives from semi-spherical to needle-like with macro-porous structure within the composite of HAp/WO3/GO. Moreover, compatibility of ternary nanocomposite inside the PLA environment led to decrease in the contact angle of the nanofibrous structure reached 16.5 ± 4.4°. The combination of HAp and organic/inorganic elements might represent a good characterization for the target biological applications. HAp/WO3/GO composite exhibits a significant improvement in anti-microbial activity to be around 85.6 ± 1.6 % and 110.1 ± 2.0 % in for E. coli and S. aureus.

Effects of MgO and Fe2O3 additives on the microstructure and fracture properties of aluminium titanate flexible ceramics

Aluminium titanate (Al2TiO5, AT) flexible ceramics were prepared from Al2O3–TiO2 powder system with MgO and Fe2O3 as additives through solid-state method. The effects of addition level of MgO and Fe2O3 on phase compositions, sintering behavior, microstructure and fracture properties of AT flexible ceramics were systematically investigated. The experimental results show that the introduction of additives can promote the formation of AT and improve the densification by forming solid solution. The addition of MgO could effectively refine AT grains since the formed MgAl2O4 spinel grains could pin at the AT grain boundary and inhibit the growth of AT grains. Conversely, the addition of Fe2O3 could promote the AT grain growth. And the simultaneous addition of MgO and Fe2O3 is beneficial to develop elongated rod-like AT grains. With that, the improved fracture properties can be obtained. Due to pining effect of spinel and better densification, the flexural strength of modified AT flexible ceramics is about 34 times higher than that of virgin. In addition, thanks to the microcracked structure and high grain aspect ratio, events of crack deflection, crack branching, grains pull-out and grains bridging are more likely to occur, leading to an increase in the flexibility by about 133%.

Design strategy of phase and microstructure of Si 3N 4 ceramics with simultaneously high hardness and toughness

Aiming to achieve silicon nitride (Si₃N₄) ceramics with high hardness and high toughness, the relationships among phase composition, microstructure, and mechanical properties of Si₃N₄ ceramics prepared by spark plasma sintering (SPS) at temperatures ranging from 1500 to 1800 ℃ were investigated in this study. Two stages with different phase and microstructure features were observed and summarized. The α–β phase transformation occurs first, and the development and growth of grains lag behind. During the first stage, the average grain size remains basically unchanged, and the hardness maintains at a value of ~20.18±0.26 GPa, despite the β-Si₃N₄ phase fraction increases from 7.67 to 57.34 wt%. Subsequently, the equiaxed grains transform into rod-like grains with a high aspect ratio via the reprecipitation process, resulting in a significant increase in the fracture toughness from 3.36±0.62 to 7.11±0.15 MPa·m^1/2. In the second stage of sintering process, the fraction of β-Si₃N₄ phase increases to 100.00 wt%, and the grain growth also rapidly occurs. Thus, the fracture toughness increases slightly to 7.61±0.42 MPa·m^1/2, but the hardness reduces to 16.80± 0.20 GPa. The current results demonstrate that the phase contents of β-Si₃N₄ and the microstructure shall be carefully tailored to achieve high-performance Si₃N₄ ceramics. Si₃N₄ ceramics with a fine-grained bimodal microstructure, consisting of the main α- and β-phases, can exhibit the optimized combination of hardness and toughness.

Fabrication and modelling of Si3N4 ceramics with radial grain alignment generated through centripetal sinter-forging

• Textured silicon nitride (Si 3 N 4 ) ceramic, with radial grain alignment, was prepared by a novel centripetal sinter-forging using spark plasma heating at 1800 °C in 10 min. • The average values of the included angles between the c -axis of the local Si 3 N 4 grain and radial direction were as low as 16.4° and 11.0° on the section plane perpendicular to the pressing direction and parallel to both the pressing and radial directions, respectively. • A fundamental physical model was created to simulate the texture formation during the 3-dimentional strain reorientation, which revealed the rod-like grain would preferentially rotate toward the center of the sample under the flow motion in centripetal direction during sintering. Silicon nitride (Si 3 N 4 ) based ceramics are one of the most attractive advanced engineering materials, which have been widely used under high-speed rotational operation or for mechanical contacts across a curved surface. In the present study, rotationally symmetric texturing of Si 3 N 4 , with radial grain alignment, was obtained by centripetal sinter-forging (CSF) of a partially sintered sample. The average values of the included angles between the c -axis of the local Si 3 N 4 grain and radial direction were approximately 16.4° and 11.0°, on the section plane perpendicular to the pressing direction, and parallel to both the pressing and radial directions, respectively. The compressive strain in the pressing direction forced the ceramic body to flow towards the central axis, resulting in compressive strain in the tangential direction and tensile strain in the radial direction. A fundamental physical model was created to simulate the grain rotation during the 3-dimentional strain reorientation, which revealed the rod-like grain would preferentially rotate toward the center of the sample under the CSF process. In addition, due to the friction between the sample surface and the pressing punch, the increased shear strain could enhance the Si 3 N 4 grain alignment. Consequently, ceramics with rod-like grains perpendicular to the curved side surface could be anticipated by applying the centripetal forming concept in a controlled manner.

The lamellar preparation of continuous porosity-graded Si3N4 ceramics without obvious interfaces for broadband radome application

Traditional layered broadband radomes are challenging to meet the needs of the high-temperature application, due to thermal mismatch between their layers. In this study, porous Si3N4 ceramics with a novel continuous porosity gradient from 54.7% to 82.4% were achieved by lamellar gel-casting, freeze-drying, and subsequent sintering process. The porosity-graded structure in Si3N4 ceramics shows no failure or cracks after sintering, benefiting from the low shrinkage after sintering at 1900 °C with the rapid sintering rate. The continuous porosity-graded structure without obvious interfaces was architected by ice crystal growth during solidification and the growth of rodlike β-Si3N4 grains across the interfaces during sintering. This approach can be beneficial to designing high wave transmission performance at specific broadband for radome.